functional and morphological impact of er stress on...

Functional and MorphologicalImpact of ER Stress onMitochondriaKAYLEEN VANNUVEL, PATRICIA RENARD, MARTINE RAES, AND THIERRY ARNOULD*

Laboratory of Biochemistry and Cellular Biology, URBC-NARILIS (NAmur Research Institute for LIfe Sciences),

University of Namur (FUNDP), Namur, Belgium

Over the past years, knowledge and evidence about the existence of crosstalks between cellular organelles and their potential effects onsurvival or cell death have been constantly growing. More recently, evidence accumulated showing an intimate relationship betweenendoplasmic reticulum (ER) and mitochondria. These close contacts not only establish extensive physical links allowing exchange of lipidsand calcium but they can also coordinate pathways involved in cell life and death. It is now obvious that ER dysfunction/stress and unfoldedprotein response (UPR) as well as mitochondria play major roles in apoptosis. However, while the effects of major ER stress on cell deathhave been largely studied and reviewed, it becomes more and more evident that cells might regularly deal with sublethal ER stress, acondition that does not necessarily lead to cell death but might affect the function/activity of other organelles such as mitochondria. In thisreview, we will particularly focus on these new, interesting and intriguing metabolic and morphological events that occur during the earlyadaptative phase of the ER stress, before the onset of cell death, and that remain largely unknown. Relevance and implication of thesemitochondrial changes in response to ER stress conditions for human diseases such as type II diabetes and Alzheimer’s disease will also beconsidered.

J. Cell. Physiol. 228: 1802–1818, 2013. � 2013 Wiley Periodicals, Inc.

The ER, a major protein folding platform

The endoplasmic reticulum (ER) is one of the largestintracellular organelles represented by a continuousmembranous network that extends throughout the cytoplasmand that is continuous with the nuclear envelope (Lavoieand Paiement, 2008). The ER fulfils multiple cellular functionssuch as synthesis, folding and transport of Golgi, lysosomal,secretory and cell-surface proteins (Berridge, 2002;Groenendyk and Michalak, 2005). ER is also involved in thesynthesis of N-linked oligosaccharides and in the first steps ofN-glycosylation, as well as in glycosyl phosphatidyl inositol(GPI) anchoring. In addition, the ER also participates in calciumstorage (Brostrom and Brostrom, 1990; Koch, 1990), lipidmetabolism, and, in certain cell types such as hepatocytes,drug detoxification (Cribb et al., 2005; Neve and Ingelman-Sundberg, 2010).

Most of the Golgi, lysosomal, secreted and plasmamembrane proteins first enter into the ER through the Sec61translocon co-translationally, which means that they haveto fold properly within the lumen of the ER before beingaddressed to the appropriate compartments in the cell. Proteinfolding in the ER is mediated and monitored by ER-residentchaperones such as GRP78 (glucose-regulated protein 78, alsoknown as BiP), GRP94 (glucose-regulated protein 94), thatprevent the aggregation of newly synthesised proteins andhelp them to fold and assemble correctly. Calnexin andcalreticulin exert the same function, targeting more particularlyN-glycosylated proteins (Cabral et al., 2001; Hebertet al., 2005). Protein folding requires several factors such asATP, calcium and an oxidizing environment allowing the activityof PDIs (Protein Disulfide Isomerases), a family of foldasesoptimising disulfide bond formation in maturing proteins(Gaut and Hendershot, 1993; Pollard et al., 1998; Tu andWeissman, 2004).

Therefore, the ER constitutes a ‘protein folding factory’that imposes a strictly regulated quality control, ensuring thatonly perfectly folded, assembled and functional proteins aredelivered to their appropriate destinations. To do so,

ER homeostasis is essential for both cell function andsurvival.

It is thus understandable that many physiological and/orenvironmental perturbations including alterations in calciumhomeostasis, redox changes (Delic et al., 2012), accumulationof misfolded and aggregating proteins (Ellgaard andHelenius, 2001), elevated secretory protein synthesis, glucosedeprivation (Ikesugi et al., 2006; Auf et al., 2010; Badiolaet al., 2011), altered N-glycosylation (Olivari andMolinari, 2007), cholesterol overload (Kedi et al., 2009; Liet al., 2009), ischaemia (Bilecova-Rabajdova et al., 2010) andviral infection (Zhang and Wang, 2012), might cause an ERstress (defined as any cellular state in which the folding capacityof the ER is overwhelmed owing to an increase in protein loadand/or disruption of the folding capacity (Berridge, 2002)).Such an ER stress triggers the activation of an evolvedevolutionary well conserved among higher eukaryotes(Hollien, 2013) and adaptative signalling pathway, called theunfolded protein response (UPR). The activation of thispathway increases the amount of ER membrane, chaperonesand protein foldases, attenuates the rate of general translationand activates ER-associated degradation (ERAD) to eliminateor limit the consequences of accumulating unsalvageableunfolded polypeptides (Schroder, 2008).

Contract grant sponsor: University of Namur (FUNDP).

*Correspondence to: Prof. Thierry Arnould, University of Namur(FUNDP), rue de Bruxelles, 61, 5000 Namur, Belgium.E-mail: [email protected]

Manuscript Received 21 February 2013Manuscript Accepted 4 March 2013

Accepted manuscript online in Wiley Online Library(wileyonlinelibrary.com): 29 April 2013.DOI: 10.1002/jcp.24360

MINI-REVIEW 1802J o u r n a l o fJ o u r n a l o f

CellularPhysiologyCellularPhysiology

� 2 0 1 3 W I L E Y P E R I O D I C A L S , I N C .

Major actors in the unfolded protein response (UPR)

The accumulation of unfolded proteins is detected bytransmembrane sensors/receptors at the ER membrane thatinitiate a transduction cascade known as the UPR. Thisresponse is intended first to cause a general arrest of proteintranslation and to induce a set of specific nuclear genes torestore the folding capacity of the ER (Ron and Walter, 2007).However, when the stress is prolonged, or the adaptiveresponse failing, UPR eventually induces cell death by apoptosis(Rasheva and Domingos, 2009).

Three ER-resident transmembrane proteins have beenidentified as sensors of ER stress: inositol-requiring kinase 1(IRE1), protein kinase RNA-like endoplasmic reticulum kinase(PERK) and activating transcription factor 6 (ATF6) (for arecent comprehensive review see Hetz, 2012). The activationof all three signalling pathways of the UPR depends on thedissociation of these ER-resident proteins from the abundantchaperone BiP (immunoglobulin heavy chain-binding protein)/GRP78. BiP/GRP78, a member of the heat shock 70 proteinfamily, is one of the most highly expressed ER residentchaperones (Hamman et al., 1998). BiP binds to hydrophobicdomains of proteins and helps preventing protein misfoldingduring translocation into the ER, a process known to bedependent on energy as BiP is an ATPase with a N-terminalATPase domain and a C-terminal substrate-binding domain(Hamman et al., 1998; Schroder and Kaufman, 2005).

In resting cells, BiP is associated with the lumenal domain ofIRE1, PERK and ATF6 and no signal is transmitted. When theER is overwhelmed by the accumulation of unfolded proteins,BiP preferentially associates with them and dissociates fromthe three receptor/sensor proteins (Bertolotti et al., 2000).Both IRE1 and PERK, free to homodimerise, get activated byauto-transphosphorylation, whereas ATF6moves to the Golgi,where it is released after cleavage in the cytosol andtranslocates into the nucleus (Shen and Prywes, 2004).

Although the three UPR branches of the UPR signallingpathway are simultaneously activated upon ER stress, thebehaviour of each of these branches varies markedly in timeafter the onset of the stress (Lin et al., 2007). The PERKpathway is the first to be activated followed by the activation ofthe ATF6 and the IRE1 pathways, respectively (Rutkowski andKaufman, 2004).

PERK is a type I serine threonine transmembrane proteinkinase which, upon release from BiP, dimerises and promotesits autophosphorylation and activation, reducing the general(cap- or eIF2a-dependent) translation by phosphorylatingserine-51 of the a subunit of eIF2. eIF2a phosphorylationinhibits delivery of the initiator methionyl-tRNAi

Met to theribosome, resulting in a general inhibition of protein translation(Harding et al., 1999), thereby relieving the ER. Paradoxically,phosphorylation of eIF2a also preferentially favours thetranslation of selective mRNAs that contain an internalribosomal entry site (IRES) (an inhibitory upstream openreading frame within their 50 untranslated region). The bestexample is the activation of the transcription factor 4 gene(ATF4), encoding a cAMP response element binding protein(C/EBP) family member (Ameri and Harris, 2008). The ATF4mRNA, 278 nucleotides in length, contains two upstream openreading frames (uORFs), uORF1 and uORF2. The first oneencodes a short polypeptide and the second one is largerbut overlaps out of frame with the first nucleotides of theATF4-coding region, being a negative-acting element in ATF4translational control. In non-stressed cells, ribosomes scanningdownstream of uORF1 will reinitiate translation at the nextcoding region, uORF2. After translation of uORF2, ribosomesdissociate from the ATF4 mRNA. During ER stress, phos-phorylated eIF2a reduces the eIF2-GTP levels.When the levelsof eIF2-GTP/Met-tRNAi

Met complex are low, the time required

for the scanning ribosome to become competent to reinitiatetranslation increases. Because of this delayed reinitiation, someribosomes would bypass the inhibitory uORF2. During theinterval between the uORF2 initiation codon and the ATF4-coding region, scanning ribosomes would have sufficient timeto reacquire eIF2-GTP/Met-tRNAi

Met and initiate ATF4translation (Lu et al., 2004; Vattem and Wek, 2004; Wek andCavener, 2007). ATF4 is a transcription factor regulating theexpression of both anti- and pro-apoptotic genes, such as theCHOP-10 (C/EBP homologous protein 10), GADD34 and ATF3genes involved in the integrated stress response, leading eitherto a pro-survival gene expression program promoting cellsurvival and stress resistance (Hu et al., 2012) or to a cell deathprogram, depending on the intensity and duration of the ERstress (Meares et al., 2011).

Cleavage of ATF6 also follows fairly rapidly after exposureto stress. ATF6 is an ER type-II transmembrane protein with aN-terminal cytoplasmic region containing a bZIP and DNAtranscription activation domain and a C-terminal lumenalregion that senses ER stress (Hai et al., 1989; Haze et al., 1999;Wild et al., 2004). Once UPR activated, ATF6, freed from BiP,translocates to the Golgi apparatus where residentproteins (site 1 and site 2 proteases) cleave ATF6, generating a50–60 kDa cytosolic bZIP-containing fragment (ATF6 (p50))that migrates into the nucleus and acts as a pro-survivaltranscription factor to regulate the expression of genesencoding proteins such as BiP/GRP78, GRP94, protein disulfideisomerases, XBP-1, CHOP-10/GADD153 (growth arrest andDNA damage 153) and P58IPK (Ye et al., 2000; Okadaet al., 2002).

Finally, the full activation of the IRE1 (a dual enzyme thatdisplays a Ser/Thr kinase domain and an endoribonucleasedomain) pathway is delayed due to the activation of the ATF4and ATF6 pathways (Yoshida et al., 2001, 2003). This delayoccurs because XBP1 mRNA, the substrate of the IRE1endonuclease domain, is expressed at low levels in unstressedcells. Indeed, the synthesis of XBP1 mRNA is upregulated byATF6 as a consequence of the ER stress. Upon activation, IRE1initiates the unconventional splicing (removal of a 26nucleotide intron) of XBP1 mRNA (Lee et al., 2002; Uemuraet al., 2009), generating a spliced variant (sXBP1) that functionsas a transcriptional activator of genes involved in ER expansion,protein maturation, folding and export from the ER, but alsoexport for degradation of misfolded proteins in the cytosol(Yoshida et al., 2001). IRE1 is also able to assemble a complexsignalling platform involved in the life-to-death transition (Hetzand Glimcher, 2009), as it will be discussed here under.

These three UPR pathways restore homeostasis in the cellfacing changing physiological or environmental conditions,preserving ER protein folding efficiency. However, if theprosurvival pathway of the UPR is insufficient, stress cannot beresolved and ER-stress related apoptosis is triggered, mostlikely to protect the multicellular organism from cell,dysfunction due to the overload of un/misfolded proteins.

ER Stress-Induced Cell Death

Both mitochondria-dependent and -independent cell deathpathways have been described to mediate apoptosis inresponse to an ER stress (Fig. 1). The mechanisms that wouldmediate this switch between beneficial UPR restoring ERfolding capacity and apoptosis could involve CHOP-10(CHOP-10 being a target of ATF4, ATF6 and IRE1), theactivation of JNK (downstream of an IRE1 complex signallingplatform illustrated in Fig. 1), the cleavage of Bap31, theactivation of calpains and of the still debated proteases such ascaspase-12 (in rodents) and caspase-4 (in humans). As thesemechanisms have been previously reviewed (Kim et al., 2006;Szegezdi et al., 2006; Martinez et al., 2010; Minamino and

JOURNAL OF CELLULAR PHYSIOLOGY

E N D O P L A S M I C R E T I C U L U M A N D M I T O C H O N D R I A I N T E R A C T I O N S 1 8 0 3

Kitakaze, 2010), we will summarise them and point out theinvolvement of mitochondria, when evidenced. Of note,although morphological modifications of the mitochondrialnetwork may be involved in the processes of cell death, theeffects of an ER stress on the mitochondrial morphology willnot be included in this section, but will be discussed in the lastpart of this review.

ER mitochondria-independent cell death pathways

Caspase-12 and caspase-4. Caspases are required forapoptosis, and certain members of this family of cysteineproteases are associated with the ER. In rodents, caspase-12 isan ER-membrane pro-apoptotic cysteine protease specificallyactivated upon prolonged UPR. Several pathways seem to

Fig. 1. Mitochondria-independent or mitochondria-dependent cell death pathways in response to an ER stress. Accumulation of unfolded ormisfolded proteins in the ER, induction of ER stress by physiological cellular stress or pharmacological ER stress inducers lead to thedissociation of BiP from the three ER stress transmembrane proteins: PERK, ATF6 and IRE1. Prolonged ER stress leads to several cell deathpathways that are either dependent or independent on mitochondria. Left side of the dashed line: Mitochondria-independent cell deathpathway after an ER stress. In response to a severe and prolonged ER stress, several pathways leading to cell death can be activated. ER stressinducers can lead to the translocation of the cytosolic caspase 7 at the surface of the ER to cleave the pro-caspase-12 generating the activecaspase-12. Active caspase-12 induces apoptosis by subsequent activation of caspase-9 and -3. IRE1, an ER-resident transmembrane enzymewith both kinase and endoribonuclease (RNase) activity, forms homodimers and is phosphorylated by JIK allowing the recruitment of TRAF2.This complex recruits, in turn, ASK1 that phosphorylates JNK. Activation of JNK induces phosphorylation and translocation of Bim leading tothe subsequent activation of the caspase-12. In melanoma cell lines, the expression of the APR-2 protein results in the activation of an ERstress leading to the release of Ca2þ from the ER and the activation of the PERK/ATF4/CHOP-10/Bim pathway resulting in the activation ofcaspase-12, caspase-9 and capase-3 axis. BAX/BAK proteins may also initiate apoptosis from the ER surface. ER stress induces conformationalchanges and oligomerisation of BAX/BAK at the ER surface, leading to the release of Ca2þ from the ER that activates calpain enzymes in thecytosol. Activated calpains then cleave the pro-caspase-12 to form the active caspase-12 that triggers apoptosis via activation of caspase-9and -3. Right side of the dashed line: Mitochondria-dependent cell death pathway in response to ER stress. The phosphorylation of IRE1 leadsto the phosphorylation of JNK (a process mediated by TRAF2 and ASK1) that can activate p53 transcription factor. Induction of Bimexpression in response to p53 leads to the formation of the BAX/BAK pore on the outer mitochondrial membrane, releasing cytochrome cleading to the formation of the apoptosome and the subsequent activation of the canonical apoptosis pathway. BAP31, an ER transmembraneprotein, can also be cleaved by caspase-8 after a prolonged ER stress, generating the p20 fragment of BAP31 that triggers the release ofCa2þ from the ER via the IP3Rs. Calcium is next taken by mitochondrial calcium uniporter (MCU) and the increase in the mitochondrialmatrix calcium concentration leads to the release of the cytochrome c via BAX/BAK proteins and the subsequent formation of theapoptosome triggering apoptosis. BAP31 can also physically interact with FIS1 to connect the ER and mitochondria. This complex BAP31-FIS1 could also recruit the active caspase-8, leading to the cleavage of BAP31, and the subsequent activation of apoptosis.


1804 V A N N U V E L E T A L .

involve the activation of caspase-12 in cell apoptosis after anER-induced stress. One of them triggers apoptosis by an ERstress-specific caspase cascade that leads to the activation ofcaspase-9 and -3 in a cytochrome c and Apaf-1 independentmanner (Morishima et al., 2002; Rao et al., 2002a). Indeed, Raoet al. showed that a treatment of cells with ER stressors, such asthapsigargin (an inhibitor of the SERCA pumps) or brefeldin-A(an inhibitor of the anterograde protein transport from the ERto the Golgi apparatus), induced the translocation of cytosoliccaspase-7 to the ER surface to associate with caspase-12 andcleave its pro-domain to generate active caspase-12. Activecaspase-12, in turn, can specifically cleave and activate caspase-9 leading to the activation of caspase-3, the major effectorcaspase (Rao et al., 2001).

Caspase-12 activation can also be triggered by a TNFreceptor-associated-factor-2 (TRAF2) and IRE-1-dependentmechanism (Urano et al., 2000). Indeed, in mammalian cells, JIK,a c-Jun-N-terminal inhibitory kinase binds to IRE1, andphosphorylates the protein allowing the recruitment ofTRAF2. This complex, in turn, recruits apoptosis signal-regulating kinase 1 (ASK1), a mitogen-activated protein kinasekinase kinase (MAPKKK) upstream of p38 MAPKs and JNK,(c-Jun N-terminal kinase) (Nishitoh et al., 1998). Therecruitment of TRAF2 at the JIK–IRE1 complexes would alsoallow the release of pro-caspase-12 from TRAF2 at the ER,which is required for the activation of pro-caspase-12 and thesubsequent caspase-9, -3, -7 cascade activation (Uranoet al., 2000; Yoneda et al., 2001; Matsuzawa et al., 2002;Nishitoh et al., 2002; Kim et al., 2009b).

In addition, in resting cells, it seems that BiP forms a complexwith caspase-7 and -12, at the ER surface, preventing cleavageand release of caspase-12 from the ER (Rao et al., 2002b).However, while in humans, a similar role for caspase-4 has beenproposed (as caspase-12 in humans has been silenced bymutations during evolution) (Yukioka et al., 2008; Matsuzakiet al., 2010; Yamamuro et al., 2011), the role of capase-12 in ERstress-induced apoptosis remains debated as Nakagawa et al.reported that caspase-12�/� mouse embryonic fibroblasts(MEFs) are more resistant to ER stress-inducing molecules(Nakagawa et al., 2000), while resistance of these cells tothapsigargin was not observed by others (Saleh et al., 2006).JNK. Moreover, the c-Jun NH2-terminal kinase JNK is also

activated when cells are exposed to multiple forms of stress,and this signalling pathway has been implicated as a mediator ofstress-induced apoptosis (Davis, 2000). Morishima et al.showed, in C2C12 murine myoblasts, that activation of JNKcan also induce phosphorylation and translocation of Bim, aBH-3 only protein from the Bcl-2 family, to the ER. Thisactivation and translocation of Bim could be an important steptowards the activation of caspase-12 and of the cytochromec and Apaf-1-independent apoptotic pathway (Lei andDavis, 2003; Morishima et al., 2004).CHOP-10. CHOP-10/GADD153, a key transcription

factor belonging to the C/EBP family members activated byUPR, is also known to upregulate the expression of bim.Indeed, it was shown that induction of autophagy, after growthfactor withdrawal in hematopoietic cells, can lead to apoptosisvia the transcriptional induction of Bim protein in a CHOP-dependent manner (Altman et al., 2009). Selimovic et al.confirmed the role of a mitochondria-independent apoptoticpathway via the PERK/ATF4/CHOP-10/Bim cascade in mela-noma cells, where the induction of the apoptosis relatedprotein-2 (APR-2) results in the activation of ER stress leadingto the release of Ca2þ, activation of the PERK/ATF4/CHOP-10pathway, triggering the subsequent Bim cascade. The latterleads to the activation of caspase-12 and subsequentactivation of caspase-9 and -3 (Selimovic et al., 2011). ThisCHOP-10/GADD153-dependent transcriptional activation ofbim would be critical for ER stress-induced apoptosis in

different cell lines, both in vitro and in vivo (Puthalakathet al., 2007).Calpains. Bcl-2 family proteins play essential roles in

regulating apoptosis. Whereas some members of the Bcl-2family antagonise cell death as Bcl-2 and Bcl-XL, others exhibitpro-apoptotic activities. Bax and Bak are the pro-apoptoticmembers of the Bcl-2 family that are ubiquitously expressed. Inhealthy cells, these proteins reside in the cytosol (Bax) or onmitochondria (Bak) and are kept in an inactive form. When acell is committed to apoptosis, Bax and Bak undergoconformational changes and assemble into oligomericcomplexes in the mitochondrial outer membrane (Eret al., 2006; Oh et al., 2010). This process induces the release ofcytochrome c from mitochondria to the cytosol. Cytochromec then binds to Apaf-1 leading to the cleavage of pro-caspase-9into active caspase-9. This process leads to the formation of theapoptosome followed by the subsequent activation of caspase-3 and -7 (Gross et al., 1998; Griffiths et al., 1999; Lindstenet al., 2000; Wei et al., 2001). Although numerous studiesfocused on how proteins of the Bcl-2 family induce apoptosisby targeting the mitochondria, evidences exist that Bcl-2proteins are also located at the ER surface (Krajewskiet al., 1993; Akao et al., 1994; Lithgow et al., 1994). In additionto their roles in mitochondria, Bax/Bak proteins may also beinvolved in initiating apoptosis from the ER surface. Indeed inNIH3T3 fibroblasts treated with tunicamycin or thapsigargin,quickly elevated intracellular calcium levels were observed andthis was followed by the accumulation of active cytosolic non-caspase proteases, calpains (Tan et al., 2006). Moreovercalpains have also been implicated in cleavage of Bcl-2 familymembers including Bax (Wood et al., 1998; Cao et al., 2003).The hypothesis raised that ER-stress activated Bax mightpromote calcium release from the ER by blocking the inhibitoryrole of Bcl-XL on IP3R mediated calcium release that couldsubsequently activate calpains, that activate pro-caspase-12 totrigger apoptosis by the activation of caspase-9 (Tan et al.,2006).

ER mitochondria-dependent cell death pathway

ER-stress can also trigger apoptosis by mitochondria-dependent pathways (Wang and Youle, 2009; Ola et al., 2011;Estaquier et al., 2012) by mechanisms that might share actorswith mitochondria-independent cell death pathways (Fig. 1).CHOP-10. This is the case for the ER stress-induced cell

death modulator CHOP-10/GADD153 as Chop-10�/� MEFscells are more resistant to ER stress-induced apoptosis(Zinszner et al., 1998). Its expression is induced to highlevels by the three major regulators of the UPR as Ddit3, thegene encoding CHOP-10, is not only a target gene ofPERK/eIF2a/ATF4 pathway (Mak et al., 2008), of ATF6 (Gotohet al., 2002), but is also regulated by IRE1 (Takayanagiet al., 2013). Indeed, in the context of ER stress, it wasdemonstrated that both the induction of CHOP-10 expression(Marciniak et al., 2004), as well as its phosphorylation by ap38-ASK1-dependent mechanism leading to full activation ofthe factor is observed (Hao et al., 2006). Mechanistically,among the numerous genes regulated by UPR and CHOP-10/GADD153, several such as Bcl-2, GADD34 (encoding a proteinphosphatase 1 (PP1) that relieves the general translationinhibition by targeting phosphorylated eIF2a) (Brushet al., 2003; Boyce and Yuan, 2006), endoplasmic reticulumoxidoreductin 1 (ERO1b) (Gess et al., 2003) and tribbles-related protein 3 (TRB3) (Ohoka et al., 2005), might play a rolein CHOP-10/GADD153-induced apoptosis (reviewed bySzegezdi et al., 2006). Among the various mechanismsreported, it seems that a lower expression of the anti-apoptotic protein Bcl-2 (CHOP-10/GADD153 acting as arepressor for the bcl2 gene promoter) associated with a



depletion of glutathione, one of the major intracellularantioxidants, could represent crucial mechanisms in CHOP-10/GADD153-induced cell death (McCullough et al., 2001).

Bim. The UPR stress sensors, when activated, promote thetranscriptional activity of ATF4, p53 and CHOP-10 (Fig. 1)(Gorman et al., 2012; Hetz, 2012). As already explained hereabove, Bim is upregulated after an ER stress via two differentpathways; the APR-2/PERK/ATF4/CHOP-10 pathway(Selimovic et al., 2011) and the IRE1/TRAF2/ASK1/JNK/p53pathway (Han et al., 2010) (Fig. 1). The upregulation andactivation by JNK-mediated phoshorylation (Lei andDavis, 2003) of the BH3-only protein, Bim, lead to theoligomerisation of Bax and/or Bak within the mitochondriaouter membrane with the subsequent release of cytochromec and caspase-dependent apoptosis (Kim et al., 2009a).

Bap31. Bap31 is an ER transmembrane protein acting as aneffector of cell death in response to prolonged ER stress(Breckenridge et al., 2003). In non-stressed cells, Bap31 bindsto nascent membrane proteins in transit between ER and cis-Golgi, and forms a complex with the pro-caspase-8 and theantiapoptotic proteins Bcl-2 or Bcl-XL at the membrane of theER (Ng et al., 1997). During apoptosis signalling, Bap31 issubject to early cleavage by initiator caspase-8. This cleavage ofBap31 generates a p20 fragment that causes a release of Ca2þ

from the ER via the IP3Rs, resulting in an uptake of Ca2þ bymitochondria (see functional crosstalks between ER andmitochondria below). These events lead to the release ofcytochrome c from mitochondria and activation of mito-chondria-dependent apoptosis (Ng et al., 1997; Nguyenet al., 2000). Notice that ER-stress induced mitochondria-dependent apoptosis can also involve changes in the fusion/fission processes, as explained in the Impact of an ER Stress onMitochondrial Morphology and Bioenergetics Section.

The ER–Mitochondria Interactions

As spatial and temporal compartmentalisation is a key featurefor eukaryotic cellular signalling, especially because organellepositioning seems to be crucial for both organellar and cellularfunction/signalling (Yadav et al., 2009; Dupin and Etienne-Manneville, 2011; Pous and Codogno, 2011), it was previouslyassumed that organelles like mitochondria, lysosomes andperoxisomes were individual entities scattered in a randomfashion through the cytosol. There is now accumulatingevidence that mitochondria for instance, form specific reticularand/or branched networks in the cytosol, that interactintimately in a dynamic way at some particular locations,comparable to the ER network. Moreover, both mitochondrialand endoplasmic reticula are now well known to interactthrough well identified molecular mechanisms as nicelyreviewed by de Brito and Scorrano (2010).

Endoplasmic reticulum morphology and dynamics

ER forms a unique continuous but highly dynamic complexmembranous network composed of structurally andfunctionally different subdomains such as nuclear envelope andperipheral ER comprising rough (ribosomes coated) and thesmooth ER, but also interconnected tubules and sheets, thelatter including the nuclear envelope (Lin et al., 2012). Inaddition, it has been shown that GTP hydrolysis is required forER rearrangements/remodelling (Hu et al., 2009) as well assome cytoskeleton-binding proteins such as CLIMP63(Shibata et al., 2010), VAP-B/Nir3 (Amarilio et al., 2005) andp22 (Andrade et al., 2004). While CLIMP63, an integral ERprotein, would anchor ER to microtubules to maintain thespatial distribution of the ER network (Shibata et al., 2010),VAP-B/Nir3 is known to link the ER membranes to themicrotubule network leading to gross remodelling of the ER

(Amarilio et al., 2005). p22, a myristoylated EF-hand containingprotein would mediate the effect of calcium on the control ofER morphology (Andrade et al., 2004). Furthermore, whileproteins belonging to either reticulons and or DP1/Yop1families would control the tubular shape of the ER (Shibataet al., 2008), atlastins, dynamin-related GTPases, would play arole in branching (Orso et al., 2009). However, we are stillmissing a comprehensive view of the molecular machinery thatmediates ER-shape, movements and spatial distribution inmammalian cells (Terasaki et al., 1986; Lee and Chen, 1988;Waterman-Storer and Salmon, 1998; Hu et al., 2009).

Mitochondria morphology and dynamics

Mitochondria are cytoplasmic organelles essential to a widerange of fundamental functions in eukaryotic cells including theATP production via the Oxidative PHOSphorylation(OXPHOS) (reviewed in Duchen, 2004). In addition to be the‘power house’ of the cell, mitochondria also participate in theregulation of intracellular calcium concentration, control theb-oxidation of fatty acids, play a role in the metabolism of someamino acids and of steroids, in urea and phospholipid synthesis.Hence, the maintenance of an appropriate mitochondrialfunctional status is critical for cell survival. To achieve this goal,mitochondria adopt very heterogeneous morphologies(ranging from small spheres/vesicles to long and intercon-nected/branched reticular networks) in different cell types(Bakeeva et al., 1978; Lea et al., 1994; Ogata andYamasaki, 1997), at different stages of differentiation ordevelopment (Chen et al., 2003), according to variousmetabolic conditions and energy needs (Rossignol et al., 2004;Benard and Rossignol, 2008), and even for a particular cell typein response to various stimuli (Reis et al., 2012) or during thedifferent phases of the cell cycle (Barni et al., 1996; Kennadyet al., 2004; Arakaki et al., 2006).

While mitochondria were initially thought to be staticindividual organelles, several live cell microscopy studiesconducted in the 1990s, revealed that mitochondrialmorphology is much more dynamic than initially thought(Bereiter-Hahn, 1990; Rizzuto et al., 1998; Logan andLeaver, 2000; Collins et al., 2002; Jakobs et al., 2003).Mitochondria are now perceived as a highly dynamic organellethat continuously fuses, divides and moves within cells in amanner that is mainly dependent on microtubules and/ormicrofilaments (Anesti and Scorrano, 2006; Boldogh andPon, 2007). These dynamic morphological changes areessential not only for the maintenance of mitochondrial DNAor respiratory activity (Gilkerson et al., 2000; Margineantuet al., 2002; Lee et al., 2009), but also for the control of cellularprocesses such as calcium signalling (Szabadkai et al., 2004), celldifferentiation during embryonic development (Chenet al., 2003) or the regulation of apoptosis (Parone andMartinou, 2006; Landes and Martinou, 2011; Fang et al., 2012;Faccenda et al., 2013).

Indeed, on one hand, mitochondrial fission contributes toeliminate damaged mitochondria through mitophagy (specificautophagy of mitochondria) (Twig et al., 2008; Twig andShirihai, 2011; Park et al., 2012; Thomas and Jacobson, 2012)and to localise properly mitochondria in response to localdemands for ATP (Bravo et al., 2011). On the other hand,mitochondrial fusion facilitates the exchange of vital compo-nents and/or mtDNA between different mitochondria toensure the maintenance of functional mitochondria (Chenet al., 2007, 2010; Nakada et al., 2009). Both mitochondrialfusion and fission are tightly controlled processes that requireseveral high weight molecular GTPases that are highlyevolutionary conserved from yeast to mammals (Benard andKarbowski, 2009): mitofusins, anchored in the OMM (OuterMitochondrial Membrane); Opa1, a dynamin-related GTPase



located in the IMM (Inner Mitochondrial Membrane); and Drp1(Dynamin-related protein 1), the master regulator of mito-chondrial division in most eukaryotic organisms (Otera andMihara, 2011). Drp1 facilitates fission of mitochondria via itsrecruitment on mitochondria by Fis1 (Fission homolog 1), asmall tail-anchored protein present in the outer membrane(Yoon et al., 2003). However, another protein, mitochondrialfission factor (Mff), was recently discovered as an essentialfactor in mitochondrial recruitment of Drp1, suggesting thatFis1 could be dispensable for mitochondrial fission (Oteraet al., 2010).

Mitochondrial fusion machinery and regulation

Mitochondrial fusion involves the tethering of two adjacentmitochondria followed by the fusion of the two outer and twoinner mitochondrial membranes. In mammals, proteinsdescribed to control the outer and inner mitochondrialmembrane fusion processes show distinct mitochondrialsublocalisations: mitofusin 1 and 2 (MFN1 and MFN2) arelocated on the outer mitochondrial membrane and facilitatethe outer membrane fusion, while Opa1 is located in the innermitochondrial membrane and regulates the inner membranefusion (Song et al., 2009). MFN1 and MFN2 were identified asthe mammalian orthologues of the Drosophila gene fuzzy onion(Fzo), the first gene discovered to mediate mitochondrialfusion (Santel and Fuller, 2001). Mitofusins represent a class ofhighly conserved mitochondrial transmembrane GTPases(Santel and Fuller, 2001) with a N-terminal GTPase domain,two regions of hydrophobic heptad-repeat coiled-coil motifs,HR1 and HR2, and two transmembrane domains between HR1and HR2, near the C-terminal domain (Santel and Fuller, 2001;Santel et al., 2003). Because HR1 (together with the GTPasedomain) and HR2 lie at the opposite sides of the transmem-brane domain, both are exposed to the cytosol. This specificmembrane topology of MFN1 and MFN2 make them goodcandidates involved in outer membrane fusion. Indeed, theHR2 domain is important in tethering of two adjacentmitochondria through a dimeric antiparallel coiled-coil struc-ture that can result from either a homotypic (MFN1-MFN1) ora heterotypic (MFN1-MFN2) interaction (Koshiba et al., 2004).While both mitofusins facilitate OMM fusion, they exertcomplementary and slightly different roles in mitochondrialfusion. Indeed, MFN1 is more efficient in promotingmitochondrial fusion than MFN2 (Ishihara et al., 2004) andMFN1 (but not MFN2) is necessary to induce mitochondrialfusion driven by Opa1 (Cipolat et al., 2004). MFN2 seems topreferentially participate in later steps of mitochondrial fusion(Eura et al., 2003; Santel et al., 2003; Ishihara et al., 2004), butthe protein would be essential for the regulation of a number ofcellular processes, related or not to its primary function asmitochondria-shaping protein (de Brito and Scorrano, 2008b).Indeed, MFN2 also controls the shape of the ER and tethers itto mitochondria (de Brito and Scorrano, 2008a). Finally, MFN2mutations can cause early-onset of axonal Charcot–Marie–Tooth disease (with an apparent recessive inheritance) and inCharcot–Marie–Tooth type IIa peripheral neuropathy (Calvoet al., 2009; Polke et al., 2011). Thus, MFN2 seems to have abroader spectrum of functions than MFN1.

The third mitochondrial fusion protein, Opa1, is a memberof the dynamin-related protein GTPase family (Smirnovaet al., 1998). This protein consists of a N-terminal mitochon-drial targeting sequence (MTS) followed by two consecutivehydrophobic segments, a coiled-coil domain, a GTPase domain,a middle domain, and a C-terminal coiled-coil domain that maycorrespond to a GTPase effector domain (GED) (Pittset al., 2004; Zhu et al., 2004). In human cells, there are eightOpa1 splice variants (Delettre et al., 2001) that are allsynthesised as precursor proteins with the MTS and the

following hydrophobic domains responsible for insertion intothe IMM. During the import of Opa1 in the matrix, theN-terminal matrix-targeting signal (MTS) is cleaved by themitochondrial processing peptidase (MPP) to form themature Opa1 isoform (L-isoform) (Ishihara et al., 2006). TheseL-isoforms undergo further processing events at two distinctsites, S1 and S2, generating shorter isoforms. Recently, severalstudies have shown that a combination of long and short Opa1isoforms is required for mitochondrial fusion. Moreover, theloss of long isoforms in response to mitochondrial dysfunctioncharacterised by low mitochondrial ATP production, dissipa-tion of themembrane potential across the innermembrane andintegration of pro-apoptotic stimuli induces cleavage of Opa1by theOma1 protease (that has an overlapping activity with them-AAA proteases, that are a conserved class of ATP-dependent proteases that mediate the degradation ofmembrane proteins in bacteria, mitochondria and chloro-plasts), leading to an increased mitochondrial fragmentation(Duvezin-Caubet et al., 2006; Baricault et al., 2007; Griparicet al., 2007; Ehses et al., 2009). Processing of Opa1 is thus veryimportant for the regulation of mitochondrial morphology.Indeed, it is well established that the L-isoform has amitochondrial fusion stimulating activity, a feature that is lostafter the proteolytic cleavage into the S-isoform (Ishiharaet al., 2006). In addition to its role in mitochondrial fusion,Opa1 is also important in maintaining normal cristae structurein the inner mitochondrial membrane by forming oligomers(formed by the soluble form of Opa1 present in the intermembrane mitochondrial space and the IMM transmembraneform of Opa1) that participate in the formation andmaintenance of cristae junction in a way that needs to befurther explored (Griparic et al., 2004; Wasilewski andScorrano, 2009).

Mitochondrial fission machinery and regulation

Mitochondrial fission is essential to ensure mitochondrialgrowth and division but also to help to clear old or damagedmitochondria from the cell through an autophagic processcalled mitophagy (Twig and Shirihai, 2011). However,mitochondrial fission is a well-regulated process and increasedor unregulated mitochondrial fission can cause a heteroge-neous population of mitochondria with non-uniform DNAdistribution, altered capacity to generate ATP, increasedproduction of reactive oxygen species and confering increasedsensitivity of cells to apoptosis (Yaffe, 1999; Paroneet al., 2008). The most studied proteins involved inmitochondrial fission are the dynamin-related protein 1 (Drp1)and fission homolog I (Fis1). These proteins are also found inperoxisomes and regulate peroxisomal fission (Kochet al., 2003, 2005). Drp1 is a cytosolic protein with aN-terminalGTPase domain, a dynamin-like middle domain and a GEDlocated in the C-terminal region (Zhu et al., 2004). Drp1mainlylocalises in the cytosol while during mitochondrial fission, theprotein is recruited from the cytosol to the fission sites ofmitochondria by the dynein/dynactin complex of the cellfacilitating its relocalisation on mitochondria (Varadiet al., 2004).

Another key component of the mammalian mitochondrialfissionmachinery is Fis1. Fis1 is a small, 17-kDa, integral proteinof the OMM with a TPR-like domain that mediates protein–protein interactions and a C-terminal transmembrane region(James et al., 2003; Suzuki et al., 2003). Through this TPR-likedomain, Fis1 recruits Drp1 on mitochondria and facilitates thefission of the organelle by a self-assembled structure that wrapsaround the mitochondrial tubules and constrict them by aGTP-hydrolysis-dependent mechanism (Yoon et al., 2001,2003; Ingerman et al., 2005). However, this mechanism isprobably not the only one involved in mitochondria fission as,



in Drosophila melanogaster, a recent high-throughput screeningapproach using siRNAs allowed the identification of Mff a tail-anchored protein affecting the fission of mitochondria andperoxisomes (Otera et al., 2010). It was demonstrated that thisprotein is also localised in the OMM (as Fis1), but Mff is notlocalised in the same complex and would fulfil differentfunctions in the fission process. Indeed, Mff could act as atransient receptor for Drp1 to mediate the initialconstriction process of the mitochondrial membrane whileFis1 would function downstream by modulating the assemblyof the fission complex and the subsequent squeezing andsevering process (Gandre-Babbe and Van, 2008; Oteraet al., 2010).

More recently, Friedman et al. (2011) brought to light a newfunction in the ER–mitochondrial contact sites (see below),both in yeast and in mammalian cells, by showing that ERtubules cross over and wrap around mitochondria leading to amitochondrial membrane constriction and a reduction oftheir diameter by almost 30%, followed by a mitochondrialdivision. Moreover, these authors also demonstrated that theER–mitochondrial contacts mark positions of mitochondrialfission independently of either Drp1 or Mff, by a mechanismthat remains to be elucidated (Friedman et al., 2011).

About 50 years ago, a first report based on electronmicroscopy demonstrated the close proximity of the ER andmitochondria in fish gills (Copeland and Dalton, 1959). Othertechniques like subcellular fractionation also allowed tohighlight the existence of ER–mitochondria juxtapositions(Area-Gomez et al., 2009; Cerqua et al., 2010). Subsequentstudies demonstrated that mitochondria cannot synthesisetheir own phospholipids, which are synthesised and trans-ferred from the ER to the inner and outer membranes ofmitochondria (Sauner and Levy, 1971). In fact, experiments ledby Rizzuto et al. (1998) in living cells with the ER andmitochondria labelled with GFP have demonstrated thatphysical interactions between the two organelles do exist.More recently, electron tomography showed that ER andmitochondria are adjoined by tethers of about 10 nm long atthe smooth ER and 25 nm at the rough ER (Csordaset al., 2006). Great attention is now paid to these close physicalinteractions between ER and mitochondria. It is thought thatthese associations could play essential roles in several cellularfunctions, including transport of lipids, calcium signalling andcellular survival or death. Indeed, a major discovery in yeastSaccharomyces cerevisiae was made when, to uncover compo-nents involved in mitochondria/ER junctions, a screen formutants that could be complemented by a synthetic protein ledto the identification of the Mmm1/Mdm10/Mdm12/Mdm34complex as a molecular tether between ER and mitochondria.The tethering complex is composed of proteins resident ofboth ER and mitochondria and functionally connected tophospholipid biosynthesis and calcium-signalling genes (Korn-mann et al., 2009). As the signalling mechanisms that controlthe contacts between ER and mitochondria have been recentlyreviewed (Liesa et al., 2009; Scott and Youle, 2010; Palmeret al., 2011), these will not be detailed here. In the next section,we will focus on the molecular actors and processes thatcharacterise the interaction sites between mitochondria andER.

Currently, the close contact and juxtaposition sites throughwhich mitochondria and ER communicate are referred asmitochondria-associated membrane (MAM) and theirfunctional roles are mainly described in the collaborativeproduction of lipids and calcium homeostasis (Picciniet al., 1998; Stone and Vance, 2000; Berridge, 2002). Indeed,close associations between both organelles enhance theinterorganelle phospholipid transport (Vance, 1990, 2008;Voelker, 1990; Ardail et al., 1991) but also the transport ofother lipids such as cholesterol (Hayashi and Su, 2003;

Fujimoto et al., 2012) and sphingolipids supporting themetabolism of ceramide in cell cycle, cell differentiation andapoptosis (Stiban et al., 2008). In addition to promote lipidtransfer, MAMs are also involved in Ca2þ ions exchange thatregulate several biological processes such as folding of newlysynthesised proteins by chaperones in the ER, production ofATP during the Krebs cycle reactions via the regulation ofseveral dehydrogenases (pyruvate dehydrogenase, isocitratedehydrogenase and oxoglutarate dehydrogenase) (Dentonet al., 1988) and the activation of Ca2þ-dependent enzymesthat activate cell death pathways (Berridge, 2002). Recently,several ER or mitochondria bound proteins have been shownto play an important role in maintaining the close relationshipbetween ER and mitochondria and, hence, have also beeninvolved in the formation of calcium channel enhancing transferof calcium from ER to mitochondria. These proteins includethe ER resident Ca2þ channel IP3 receptor, the mitochondrialvoltage-dependent anion channel, the chaperones Grp75 andsigma-1R, the sorting protein PACS-2, the ERMES complex andthemitofusin MFN2, that will be detailed in the next paragraphs(Fig. 2).

IP3R–VDAC interaction and calcium homeostasis

The protein content of the ERmembrane reflects its importantrole in Ca2þ signalling as demonstrated by the presence ofreceptors responsible for releasing Ca2þ in response to inputsignals; the inositol 1,4,5-triphosphate (IP3) receptors andryanodine receptors (RyRs). Currently, three isoforms of RyRs(RyR 1, 2 and 3) and of IP3Rs (IP3R I, II and III) have beenreported to form very large complexes at the surface of the ERmembrane. IP3Rs are ligand-gated channels that function inreleasing Ca2þ from ER stores in response to the binding ofagonists to cell surface receptors and production of inositol-1,4,5-phosphate (IP3), a second messenger (Patel et al., 1999;Patterson et al., 2004; van Rossum et al., 2004). Type-3 IP3receptors are enriched at the MAMs and the silencing of theexpression of these receptors significantly reduces mito-chondrial Ca2þ concentration in CHO cells (Mendeset al., 2005). Moreover, it was demonstrated that transmissionof Ca2þ from ER to mitochondria leads to apoptosis byinducing the formation of the permeability transition pore(Deniaud et al., 2008). So, these studies showed a physical andfunctional link between the release of Ca2þ from ER and itsuptake by mitochondria. In addition, accumulating evidenceindicates that another complex does act as an importantmitochondrial effector that mediates transport of ions andCa2þ across the OMM: the voltage-dependent anionchannels, VDACs (Bathori et al., 2006). VDAC, also calledmitochondrial porin, forms a very abundant large voltage-gatedpore in the OMM at the ER–mitochondria contacts(Szabadkai et al., 2006). Studies on the role of VDAC in thetransmission of Ca2þ between ER and mitochondria showedthat the over-expression of VDAC1 in HeLa cells increases theefficacy of the existing contacts between the two organelles,thus enhancing the mitochondrial Ca2þ concentration (Rapizziet al., 2002). More recently, it was demonstrated that VDAC1is physically and indirectly interacting with the type-1 IP3receptor through glucose-regulated protein 75 (Grp75), amolecular chaperone (Szabadkai et al., 2006). Even if Grp75 hasbeen extensively studied for its role in mitochondrial proteinimport (Mizzen et al., 1991; Wadhwa et al., 2002), this studyshowed that Grp75 is a key linker in the functional couplingbetween IP3R1 at the ER and VDAC1 on mitochondria(Szabadkai et al., 2006). Moreover, it was shown that theover-expression of the cytosolic Grp75 enhances the IP3-induced mitochondrial Ca2þ accumulation by stabilizing theinteractions between these two proteins (Szabadkaiet al., 2006).



ERMES complex

Because MAMs are enriched in enzymes involved inphospholipid biosynthesis, these structures were supposed tobe involved in lipid transfer between ER and mitochondria.Indeed, in S. cerevisiae, Kornmann et al. (2009) identified a novelprotein complex acting as a molecular tether between ER andmitochondria, referred as the ER–mitochondria encounterstructures (ERMES) complex. ERMES includes fourcomponents: Mmm1, an ER-resident integral membraneprotein; Mdm10, the b-barrel mitochondrial outer membraneprotein; Mdm34, an outer membrane protein and Mdm12, acytosolic protein (Kornmann et al., 2009). Two of these ERMEScomponents, Mmm1 and Mdm12, were reported to contain asynaptotagmin-like, mitochondrial and lipid-binding protein(SMP) domain that is present in a large number of othereukaryotic-membrane-associated proteins (Lee andHong, 2006). Recently, it was shown that the SMP domainbelongs to the tubular lipid-binding proteins (TULIP) domainsuperfamily, present in a large group of proteins that bind lipidsand other hydrophobic ligands within a central and tubularcavity (Kopec et al., 2010). This discovery suggests a possiblerole of the ERMES complex in cellular phospholipid trafficbetween ER and mitochondria. Besides its role in phospholipid

trafficking, ERMES would also be involved in Ca2þ transferfrom the ER tomitochondria. This assumption is based onmassspectrometry analyses of purified ERMES complexes thatidentified the Ca2þ-binding Miro GTPase Gem1 as an integralcomponent of the ERMES complex (Kornmann et al., 2011).Gem1 is a Rho GTPase anchored in the mitochondrial outermembrane with a large cytosolic domain containing 2 GTPasedomains separated by a pair of Ca2þ-binding EF hands(Kornmann et al., 2011). These authors showed that both thefirst GTPase domain and the Ca2þ-binding domain of Gem1regulate the physical association between Gem1 and otherERMES components while the second GTPase domain ofGem1 seems to influence the ERMES activity, acting as aregulatory domain.

PACS2

PACS2 is another multifunctional protein that regulates theER–mitochondria axis in controlling the balance betweencellular homeostasis and apoptosis. Indeed Simmen et al.(2005) showed that besides being necessary for the intimateassociation of mitochondria with the ER, PACS2 depletion withsiRNAs, in MCF7, HeLa and A7 melanoma cells, induces aBAP31-dependent mitochondrial fragmentation and its

Fig. 2. ER–mitochondria interface and interactions. IP3R1–VDAC interactions through the GRP75 protein form physical and functional linksbetween the ER and mitochondria. These complexes mediate the transport of Ca2þ from the ER to the mitochondria. Complex ERMES inyeast are molecular effectors that tether ER–mitochondria by five components; Mmm 1, Mdm 12, Mdm 34, Mdm10 and Gem1. Thesecomplexes also participate in phospholipids and Ca2þ movement between ER and mitochondria. PACS2 is a multifunctional protein, anchoredin the ER membranes, that tethers the ER with mitochondria. It binds to the ER chaperones such as calnexin. Mitofusins 1 and 2 (MFN1/MFN2)are other proteins that facilitate the physical tethering between ER and mitochondria. These mitochondria-shaping proteins that regulatemitochondria dynamics are located on mitochondria and MFN2 is also enriched at the MAMs, the interface between ER and mitochondria, inthe ER. ER-MFN2 forms hetero- or homodimers with MFN1 or MFN2 on mitochondria, respectively, keeping both organelles in close contactsfor Ca2þ transfers and control of calcium homeostasis. Sigma-1 receptor localises at the MAMs. Induction of ER stress, redistributes Sig-1Rfrom the MAM to the IP3R3 leading to the release of Ca2þ from the ER and subsequent uptake by mitochondria.



physical dissociation from the ER. Moreover, the disruption ofthe interaction between the two organelles induces ER stressand these authors also showed that when the ER stress cannotbe resolved, apoptosis is initiated via PACS2 that translocatesfrom the ER to mitochondria and promotes the formation oftBid and the release of cytochrome c with subsequentactivation of caspase-3 (Simmen et al., 2005). ER chaperones,and more particularly the Ca2þ-binding chaperones such ascalreticulin, calnexin and BiP, were also found to be retained atthe MAMs (Hayashi and Su, 2007; Myhill et al., 2008). PACS2could also play a role in the retention of the chaperones inMAMs as PACS2 does interact with the non-phoshorylatedform of calnexin, retaining the protein in the ER at the MAMs(Myhill et al., 2008).

Sigma-1 receptor

Hayashi and Su identified a new role for the Sigma-1 receptoras a novel ‘ligand-operated’ chaperone that specifically targetsthe MAMs. The sigma receptor is a non-opioid receptor thatspecifically localises at the ER–mitochondrion interface(Hayashi and Su, 2007; Hayashi and Su, 2010). Two subtypes ofthis receptor do exist: Sigma-1 and Sigma-2 (Hellewellet al., 1994). Sig-1 R is known to bind benzomorphans, steroidsand other psychotropic drugs (Su et al., 1988; Snyder andLargent, 1989). However, Sig-1R would also form a Ca2þ-sensitive chaperone machinery with BiP/Grp78, the major ERchaperone (Hayashi and Su, 2007). The interaction betweenthese two proteins would occur at the MAMs at physiologicalCa2þ concentrations (between 0.5 and 1.0mM). Induction of aER stress caused by thapsigargin or tunicamycin in CHO cellsleads to a decrease in Ca2þ concentration in the ER, causing arapid disassembly of the BiP/Sig-1R complex, redistributingSig-1R from MAMs to the IP3R3, a translocation processleading to prolonged calcium signalling into mitochondria(Hayashi and Su, 2007; Tsai et al., 2009).

Mitofusins

Recently, de Brito and Scorrano identified another protein thatfacilitates the physical association between the ER andmitochondria. Indeed, the mitochondria-shaping protein,mitofusin-2 (MFN2) was shown to regulate the contactsbetween the two organelles (de Brito and Scorrano, 2008a). Inthis study, it was shown that MFN2 not only localises onmitochondria but is also enriched at MAMs, in the ER. Morespecifically, MFN2 seems to regulate the morphology of the ERand also its tethering with mitochondria. ER-MFN2 would thusinteract with MFN1 and MFN2 on mitochondria to formhomo- or heterodimeric bridges between the two organelles,keeping them in close contact for Ca2þ homeostasis (de Britoand Scorrano, 2008a,b). This assumption is supported by theobservation that when the distance between ER andmitochondria increases in MEFs and HeLa cells lacking MFN2,mitochondrial Ca2þ uptake is impaired (de Brito andScorrano, 2008a).

Impact of an ER Stress on Mitochondrial Morphologyand Bioenergetics

Because these two organelles are tightly bound, it is thus likelythat an ER stress does affect and modify the morphology andthe bioenergetics activity of the mitochondrial population.However, while numerous studies report these interactions inthe context of apoptosis (Rao et al., 2004; Rodriguezet al., 2011), some more recent studies also show that asublethal ER stress, probably of physiological relevancefrequently encountered, might affect the biology of mito-chondria (Fig. 3).

Several evidence indicate that an ER stress, caused byprimary (genetic) or secondary (environmental) factors,activates the UPR that could resolve the stress. However,beyond a certain degree of ER damage, this response triggersapoptotic pathways (Rasheva and Domingos, 2009). Thegeneral mechanisms that link the UPR and apoptosis are wellcharacterised and described in the ER Stress-Induced CellDeath Section (Fig. 1), but the metabolic events that occurduring the early adaptative phase of the ER stress, before theonset of cell death, remain largely unknown. However, a recentstudy showed that, at the very early stage of ER stress, the ERand mitochondrial networks are more physically connectedthan before the stress, favouring the transfer of calciumbetween the ER and mitochondria. This increased mitochon-drial Ca2þ uptake enhances a localised increase in mitochon-drial metabolism, providing energetic substrates for the cellularadaptative response. In particular, several parameters ofmitochondrial metabolism are enhanced in these conditionssuch as the ATP production, oxygen consumption andmitochondrial transmembrane potential (Bravo et al., 2011). Itwas also elegantly demonstrated that an ER stress that triggersthe global inhibition of protein synthesis by the PERK pathwaycan also be accompanied by the induction of Lon expression, amitochondrial protease that controls the assembly and/ordegradation of cytochrome c oxidase (COX) (Margineantuet al., 2002; Venkatesh et al., 2012). The over-expression ofLon protease seems to prevent mitochondrial dysfunction byincreasing the assembly and stabilisation of COX II into a COXI-based complex and by reducing the abundance of COX IVand V (Hori et al., 2002) (ER-stress 1 in Fig. 3). However, thedetailed mechanisms underlying the protective effect of Lonunder ER stress remain to be elucidated.

Much more is known about the morphologic changes ofmitochondria during an ER stress that induces apoptosis even ifthe timing of certain remodelling events remains controversial.It is also unclear whether these morphological changes arenecessary early events required for the release of pro-apoptotic factors or are simply downstream effects(Breckenridge et al., 2003; Hom et al., 2007; Bhavyaet al., 2012). Several studies suggest that mitochondrialremodelling and fission observed in apoptosis are the cause ofcytochrome c release and subsequent activation of cell deathpathways (Yuan et al., 2007; Faccenda et al., 2013). On theother hand, opposite results demonstrated that cytochromec release in apoptosis was not caused by mitochondrial swellingand fission, but that changes in mitochondrial morphologywould be the consequence of the release of cytochrome c (Gaoet al., 2001; Arnoult et al., 2005). Indeed a sustained ER stresscan induce morphological changes of the mitochondrialnetwork and it seems that Ca2þ is a key regulator in theactivation of apoptosis (Nicotera and Orrenius, 1998). Severalstudies now reveal that physical interactions between ER andmitochondria facilitate the transfer of Ca2þ and activateapoptosis (see reviews) (Chami et al., 2008; Pinton et al., 2008;Grimm, 2012). Moreover, apoptosis is very often accompaniedby fragmentation of the mitochondrial network resulting in apunctate distribution (Frank et al., 2001; Hom et al., 2007) evenif the mechanisms involved are not fully understood.

For instance, caspase-8 cleavage of Bap31 at the ER couldalso trigger the fragmentation of mitochondria. Indeed, thep20Bap31 fragment not only causes a release of Ca2þ from theER but also induces an early fragmentation of the mitochondrialreticular network in different cell lines incubated withthapsigargin (Breckenridge et al., 2003) or staurosporin (Franket al., 2001). Moreover, fragmentation of mitochondria inresponse to ER stress was demonstrated to be dependent onthe recruitment of Drp1, the dynamin-related protein thatmediates fission of the OMM (Breckenridge et al., 2003).Several studies showed that mitochondrial reticular networks,



known to be physically associated with ER at some connectingjunctions, undergo controlled fission during apoptosis(Fannjiang et al., 2004; Lee et al., 2004; Sugioka et al., 2004;Hom et al., 2007). This process is evolutionarily conserved andrecently, it was reported that Fis1 requires the ER gateway toinduce apoptosis (Alirol et al., 2006). Fis1 is an OMM proteinevenly distributed on the surface of mitochondria (Jameset al., 2003) that recruits Drp1 to the mitochondrial membranesurface (Stojanovski et al., 2004). Recent work described atransfer of apoptosis signals, frommitochondria to the ER, backto mitochondria through Fis1–Bap31 complexes that span themitochondria–ER interface and would serve as a platform toactivate the initiator pro-caspase-8 (Fig. 1) (Iwasawaet al., 2011). The Fis1–Bap31 interaction does already exist innon-apoptotic cells, showing that these two proteins physicallybridge the mitochondria and the ER (Iwasawa et al., 2011). Thecurrent model connecting ER and mitochondria in apoptosistriggered by ER stress is based on a crosstalk initiated by Fis1on the mitochondria targeting the ER through the interactionbetween Fis1 and Bap31 (ARCosomes). This complex wouldnext recruit pro-capase-8 to the platform, leading to thecleavage of Bap31 generating a pro-apoptotic fragment

(p20Bap31) that, in turn, would trigger a release of Ca2þ fromthe ER, feeding back the mitochondria and increasing themitochondrial matrix calcium concentration (Iwasawaet al., 2011). These studies suggest that the ER–mitochondriainterface and tight tethers between the ER and mitochondria,known for a long time to physically associate both organelles(de Brito and Scorrano, 2008a; Friedman et al., 2011) could playa functional role in the apoptosis triggered by ER stress.

Hom et al. observed this phenomenon by exposing normalrat liver cell line Clone 9 to thapsigargin, a SR/ER Ca2þ-ATPase(SERCA) inhibitor, leading to a rapid and reversiblefragmentation of mitochondria caused by increased Ca2þ inmitochondria. However, if the incubation with thapsigargin isprolonged, this causes a depletion of the ER Ca2þ and asubsequent mitochondrial Ca2þ uptake leading to anirreversible fragmentation of the mitochondria (Homet al., 2007). The massive and prolonged accumulation of Ca2þ

into the mitochondrial matrix can impair mitochondrialrespiration and ATP production as the electrochemicalgradient across the mitochondrial inner membrane is reduced(Bianchi et al., 2004; Deniaud et al., 2008). As a consequence,the mitochondrial inner-membrane permeability is modified

Fig. 3. Impact of an ER stress on mitochondrial morphology and bioenergetics. ER and mitochondria are two tightly bound organelles.Induction of an ER stress does affect both the morphology and the bioenergetics of mitochondria. (1) The ER stress activates the PERK/ATF4pathway that is accompanied by the induction of expression of Lon protease that controls the assembly and/or the degradation of cytochromec oxidase subunits in mitochondria (quality control). (2) Several proteins, such as Bcl-2, Bax, Bak, BIK and NIX, localise at the ER–mitochondria interface. BAX/BAK proteins promote rapid transfer of Ca2þ to mitochondria followed by the accumulation of Ca2þ inmitochondria. In these conditions, apoptosis then results from subsequent swelling of mitochondria leading to the release of pro-apoptoticfactors such as Smac/Diablo, AIF and cytochrome c. (3) BIK, an integral protein of the ERmembrane, stimulates the Ca2þ release from the ERand its subsequent uptake by mitochondria, triggering accumulation of Ca2þ, loss of mitochondrial transmembrane potential and release ofcytochrome c. Cytochrome c released from mitochondria can recruit Drp1 on mitochondria and mediates mitochondrial fragmentation.(4) NIX, a pro-apoptotic Bcl-2 family member, localises on both mitochondria and ER, and triggers Ca2þ transfer between organelles leadingto the release of cytochrome c and activation of caspase cascade leading to apoptosis. (5) ER stress induces translocation of PKCd to the ER inan Abl1-dependent manner, forming a complex at the ER membrane. This PKCd–Abl1 complex next translocates to mitochondria triggeringa JNK-dependent ER-stress-induced apoptosis by the recruitment of Bax/Bak followed by the release of cytochrome c. (6) Bid, a pro-apoptoticBH3-only protein, is cleaved upon ER stress. Cleavage of Bid into tBid can be mediated by caspase-2 or caspase-8 leading to the subsequentcleavage and enhanced activation of caspase-2. Both pathways then induce apoptosis via the mitochondria-dependent or mitochondria-independent activation of caspase-3. (7) Finally, ER stress can induce mitochondrial fragmentation and mitophagy by the PERK/ATF4dependent-pathway. ATF4 acting as a transcription factor for the gene encoding Parkin that is recruited at the mitochondrial membrane byPINK1 that is stabilised on the OMM of damaged mitochondria (that display lower mitochondrial membrane potential). The PINK1–Parkincomplex next ubiquitinates several proteins leading to the recruitment of p62 that triggers the activation of mitophagy, a specific autophagythat targets damaged mitochondria for degradation.



and influx of water causing mitochondrial swelling (and finallydisruption of the integrity of the OMM) is observed (Petronilliet al., 1999). Physical rupture of mitochondria would thusfacilitate the release of a series of pro-apoptotic proteins suchas cytochrome c, Smac/Diablo, AIF, endonuclease G andpro-caspases into the cytosol (Susin et al., 1999; Du et al., 2000;Li et al., 2001; Donovan and Cotter, 2004; Pradelli et al., 2010).Several proteins localised at the tethers between ER andmitochondria are also involved in the transfer of Ca2þ betweenthe 2 organelles and in the control of mitochondrial membranepermeability, such as Bcl-2, Bax and Bak, BIK and Nix (Nuttet al., 2002). Indeed, the over-expression of pro-apoptoticproteins such as Bax and Bak promotes a rapid depletion of theER Ca2þ store, subsequent accumulation of Ca2þ intomitochondria followed by the release of cytochrome c andDNA fragmentation (ER-stress 2 in Fig. 3) (Nutt et al., 2002).However, these effects can be blocked and/or prevented by theover-expression of the anti-apoptotic protein Bcl-2 (Nuttet al., 2002; James et al., 2003; Oakes et al., 2003; Scorranoet al., 2003). Interestingly, both Bcl-2 and Bcl-XL were alsoshown to block mitochondrial fission and cell death inmammalian cells and in yeast (Fannjiang et al., 2004; Konget al., 2005). Moreover, Bax was found to translocate toscission foci on mitochondria during the initial stages ofapoptosis and also to colocalise with two proteins involved inthe regulation of mitochondrial morphology, Drp1 and MFN2(Karbowski et al., 2002). Germain et al. (2002) showed thatBIK, a BH3-only protein, is an integral protein of the ERmembrane that stimulates the Ca2þ release from the ER,triggers a loss of mitochondrial transmembrane potential aswell as the remodelling of mitochondrial cristae that mobilisescytochrome c stores upon fragmentation of mitochondriamediated by Drp1 (ER-stress 3 in Fig. 3). A second event,independent of Drp1, is caused by a combined activity of BIKand NOXA, another BH3-only protein, resulting in confor-mational changes in BAX on mitochondria, a strong release ofcytochrome c and the activation of the canonical caspasecascade (Germain et al., 2005; Mathai et al., 2005).

Also, Nip3-like protein X (NIX), another pro-apoptoticBcl-2 family member was shown to control the ER/SR calciumconcentration in cardiac myocytes (Diwan et al., 2009). Indeed,NIX localises both on mitochondria and sarcoplasmic reticu-lum (SR) and promotes propagation of SR-mitochondriacalcium transfer leading to the activation of caspases andapoptosis (ER-stress 4 in Fig. 3) (Diwan et al., 2009).WhenNIXis targeted to mitochondria and SR, it activates two canonicalpathways that originate from either mitochondria or SR. It wasalready shown that mitochondria-targeted NIX causesapoptosis by permeabilizing the OMM with Bax and Bak,leading to the release of cytochrome c, apoptosome formation,caspase-3 activation and cell death (Yussman et al., 2002;Galvez et al., 2006). However, the new discovered function forNIX protein is related to the SR/ER calcium overload, a signalrecognised by the cell as an environmental stimulus leading to alocal release of calcium between junctional ‘hot spots’ withmitochondria (Diwan et al., 2009). Calcium released by SR isthen taken up by a mitochondrial uniporter, resulting in anintramitochondrial calcium overload triggering the formationof the permeability transition pore, loss of mitochondrialmembrane potential, mitochondrial swelling and release ofpro-apoptotic proteins in the cytosol. The general collapse ofmitochondrial membrane potential leads to ATP depletion inthe cell that generates a failure in the energy flow causingcellular swelling and membrane rupture and, finally, cell death(Diwan et al., 2009).

More recently, it was shown that shortly after induction ofER stress, protein kinase C delta (PKC d, a member of the PKCfamily) translocates to the ER in a process dependent on Abl1, anon-receptor tyrosine kinase, forming a complex at the ER

membrane (ER-stress 5 in Fig. 3) (Qi and Mochly-Rosen, 2008).This PKCd–Abl1 complex further translocates to mitochon-dria where it triggers JNK-dependent ER-stress-inducedapoptosis recruiting Bax and Bak on mitochondria resulting inrelease of cytochrome c (Qi and Mochly-Rosen, 2008).Upstream of the recruitment of Bax and Bak on mitochondria,Bid, another BH3-only protein and pro-apoptotic molecule,was found to be cleaved upon ER stress (ER-stress 6 in Fig. 3)(Upton et al., 2008; Uchibayashi et al., 2011). Indeed, ER stresswas shown to induce proteolytic activation of Bid, a processmediated by caspase-2 (Upton et al., 2008). However, anotherstudy suggested that caspase-8 is activated after an ER stress inretinal ganglion cell, leading to the cleavage of Bid causing thesubsequent activation of caspase-2 (Uchibayashi et al., 2011).The debate is still open to determine whether caspase-2 or -8acts to cleave Bid to initiate apoptosis. In conclusion, even if wedo not fully understand the UPR-induced apoptosis and howER stress-mediated events are linked to the actual executers ofprogrammed cell death, it is now accepted that mitochondriaand/or ER stress as well as inter-organelle crosstalks (in termsof both physical and biochemical interactions) are veryimportant in cell fate (cell death or cell survival).

Besides triggering directly or indirectly apoptosis, ER stresscan also induce mitochondrial stress, resulting in loss ofmitochondrial membrane potential, fragmentation ofmitochondrial network and subsequent specific autophagy ofmitochondria, referred as mitophagy (Tolkovsky, 2009;Narendra and Youle, 2011; Rambold and Lippincott-Schwartz, 2011; Springer and Kahle, 2011; Jin and Youle, 2012).Briefly, damaged mitochondria can be recognised to beprocessed by mitophagy through the voltage-sensitive kinasePink1 (Matsuda and Tanaka, 2010; Narendra et al., 2010).Under normal circumstances, Pink1 is continuously degradedon mitochondria. In response to mitochondrial membranedepolarisation during excessive mitochondrial fission orcellular/ER stresses, Pink1 is stabilised on the OMM (Jinet al., 2010; Matsuda et al., 2010; Narendra et al., 2010). Theaccumulation of Pink1 then facilitates the recruitment ofParkin, an E3 ligase, on mitochondria where it ubiquitinatesseveral proteins such as MFN1/2 and VDAC (Gegg et al., 2010;Ziviani and Whitworth, 2010; Chan et al., 2011). Theaccumulation of ubiquitin-modifications leads to the recruit-ment of p62 triggering the subsequent activation of theautophagosomal degradation of damaged mitochondria(Okatsu et al., 2010). ER stress can also induce mitophagy ofdamaged mitochondria by the activation of the PERK–ATF4pathway of the UPR (ER-stress 7 in Fig. 3) (Bouman et al., 2011).Indeed, increased expression of Parkin was observed inresponse to the activation of ATF4 during UPR. The promoterof the gene encoding Parkin contains a CREB/ATF4 binding site,thereby increasing its expression in response to ER stress(Bouman et al., 2011). However, the exact role of Parkin incommunication between ER stress and mitophagy has not yetbeen experimentally demonstrated in all cell types.

Pathophysiological Consequences of ER Stress andMitochondrial Dysfunction Cross-Talk

Importantly, ER-stress-induced apoptosis is associated with avariety of diseases, including neurodegenerative andcardiovascular diseases and diabetes (Harding and Ron, 2002;Lindholm et al., 2006; Minamino and Kitakaze, 2010).

Type II diabetes

Increasing evidence suggests that mitochondrial dysfunctionand ER stress response are causative of insulin resistance andtype II diabetes (Lim et al., 2009; Rieusset, 2011; Leem andKoh, 2012). Type II diabetes is a complex metabolic disorder



associating peripheral insulin resistance and altered insulinsecretion by pancreatic beta cells (Kahn et al., 2006). Theprevalence of diabetes for all age-groups worldwide wasestimated to be 2.8% in 2000 and 4.4% in 2030. The totalnumber of people with diabetes is projected to rise from 171million in 2000 to 366 million in 2030 (Wild et al., 2004). It iswell established that type II diabetes is associated with ERstress (Ozcan et al., 2004) and oxidative stress-associatedmitochondrial dysfunction (Lowell and Shulman, 2005). Inresponse to ER stress, the dialog between ER andmitochondriathat is initiated and the three branches of UPR that areactivated lead to b-cell apoptosis (Eizirik et al., 2008; Backet al., 2009; Allagnat et al., 2010; Papa, 2012). The PERKpathway supports b-cells in solving the ER stress and re-establishing the ER homeostasis by attenuating proteinsynthesis via the phosphorylation of eIF2a (Back et al., 2009).However, excessive PERK activation leads to down-regulationof Mcl-1 protein expression, an anti-apoptotic protein of theBcl-2 family, and activates CHOP-10/GADD153, contributingto the mitochondrial pathway leading to b-cell death (Allagnatet al., 2011). In addition, when the IRE1 a pathway is activated,IRE1a recruits TNF receptor-associated factor 2(TRAF2), anadaptor molecule that activates the ASK1/JNK pathway (Fig. 1).JNK, in turn, phosphorylates/activates c-jun and also upregu-lates the expression of DRP5 (BH3-only sensitiser). Thisprotein can further bind directly to Bcl-2 pro-survival familymembers such as Bcl-XL and also initiate subsequent apoptoticcascades including release of mitochondrial pro-apoptoticfactors and caspase-3 activation (Yin et al., 2005; Gurzovet al., 2009). This pathway leading to the over-expression ofDRP5 can be antagonised by another AP-1 transcription factordimer that contains JunB (Kaneko et al., 2003; Hu et al., 2006;Gurzov et al., 2008, 2010). Finally, it was also shown that theCa2þ transfer between ER and mitochondria also activatesapoptotic signals in b-cells resulting from the hyperactivationof calpain-2 (Huang et al., 2010).

Alzheimer’s disease

Alzheimer’s disease (AD) is the most commonneurodegenerative disorder of the late life characterised by adegeneration of the central nervous system associated withprogressive memory impairment, accumulation of senileplaques and neurofibrillary tangles causing massive loss ofneurons resulting in dementia. A neurofibrillary tangle is aneuronal inclusion composed of paired helical filaments of thehyperphosphorylated protein TAU, a microtubule bindingprotein (Brion, 2006; Iqbal and Novak, 2006). Senile plaquesare extracellular deposits consisting of a dense core offibrillous 40–42 amino acids long peptides, the amyloid b (Ab)peptides, surrounded by dystrophic neurites and microglia(Iwatsubo, 1998; Gouras et al., 2000). The Ab peptide isderived from the larger b-amyloid precursor protein (b-APP)by the consecutive action of two enzymes, the b- and theg-secretase which is composed of presenilin-1 and 2, nicastrin,anterior pharynx-defective 1 (APH-1) and presenilin enhancer2 (PEN 2) (Storey and Cappai, 1999; De Strooper et al., 2012).The major risk factors for developing AD are aging and familyhistory. Mutations in the gene encoding b-APP cause theproduction and accumulation of the longer A b42 peptide andlead to some forms of familial AD (Goate et al., 1991;Goate, 2006). Moreover, mutations in presenilin 1 andpresenilin 2 also cause familial AD and are associated withincreased Ab plaques (Borchelt et al., 1996; Scheuneret al., 1996). Mutations in these three genes account for about5% of all cases of AD. While genetic mutations in b-APP andpresenilins are responsible for the accumulation of Ab infamilial AD, the causative factors for accumulation of Ab insporadic AD, which represents the majority of AD cases, are

not well understood. This raises the possibility that, in theabsence of genetic mutations affecting b-APP and presenilins,factors that affect pathways of generation or clearance of Abmay also alter the level of this peptide, causing sporadic formsof AD. Recent studies have shown an involvement of ER stressand disturbed calcium homeostasis in AD (LaFerla, 2002;Hoozemans et al., 2005, 2009; Lee do et al., 2010; Casas-Tintoet al., 2011; Ho et al., 2012). Indeed, in postmortem braintissues from AD patients, a significant increase in the levels ofthe ER stress markers such as phospho-PERK, phospho-eIF2a,phospho-IRE1a, XBP1, BiP/GRP78 and also the mediator ofcell death CHOP-10 has been observed (Kudo et al., 2002;Milhavet et al., 2002; Lee do et al., 2010; Casas-Tintoet al., 2011), suggesting that a prolonged activation of the UPRcould be involved in the neurodegenerative process in AD. Inhuman neuroblastoma cells overexpressing either wild-typeAPP or muted APP, it was shown that both the UPR activationand the A b42 peptide production were higher in APP mutantcells. Moreover, ER stress toxicity was also the highest in theAPP mutant cells suggesting a role of A b42 peptide in theincreased sensitivity of neuroblastoma cells to ER stress(Chafekar et al., 2008). It has also been proposed that Ab candirectly mediate ER stress response and apoptosis. Ab42accumulation in the ER induces the release on calcium from theER leading to the subsequent activation cytosolic calpains thatcleave and activate the ER caspase-12 in primary corticalneurons in mice (Nakagawa and Yuan, 2000). Moreover,caspase-12�/� neurons were shown to be partially resistant toAb-peptide-induced cell death (Nakagawa et al., 2000). Morerecently, the same role in Ab-induced cell death was describedfor the homologue of caspase-12 in human cells (humanneuroblastoma SK-N-SH cells and human carcinoma HeLacells), caspase-4 (Hitomi et al., 2004). Increased caspase-4might elevate the vulnerability of neurons to apoptosis, andtherefore would be involved in the pathogenesis of AD. It hasalso been shown that reduction of ER calcium release canpartially attenuate Ab-peptide neurotoxicity in rat primarycortical neurons (Suen et al., 2003). Finally, mitochondrialdisturbance was also shown to be involved in AD. Indeed,decreased COX activity (Ojaimi and Byrne, 2001), mtDNAmutations (Krishnan et al., 2007; Mancuso et al., 2008), loss ofmembrane potential (Abramov et al., 2004), depletion in ATP(Keil et al., 2004) and release of cytochrome c (Kim et al., 2002;Aleardi et al., 2005) and Smac (Yin et al., 2002) aremitochondrial dysfunction that trigger the onset of neuronalapoptosis in AD. Besides the evidence that demonstrates theinvolvement of ER and mitochondrial dysfunction in ADpathogenesis (Takuma et al., 2005), it was recently shown thatcrosstalks existing between ER and mitochondria are involvedin familial AD. Indeed, it was discovered that presenilin 1 and 2are not distributed homogeneously in the ER, but rather arehighly enriched in the MAM compartment, along with othercomponents of the g-secretase complex (Area-Gomezet al., 2009). The possibility that the g-secretase complex mayreside at the ER–mitochondria interface could explain theobserved transport of Ab-peptides in mitochondria (HanssonPetersen et al., 2008). Moreover, one of the functions of theMAMs is to mediate ER–mitochondria calcium transfer(Csordas and Hajnoczky, 2009; Hayashi et al., 2009) that couldbe impaired if presenilins mutations described for familial ADhave an effect on the MAMs tethering ER and mitochondria.However, many questions remain regarding the interplaybetween ER and mitochondria in the pathogenesis of AD.

In conclusion, during the past decade, several studies havethus demonstrated that one or more branches of the UPRseem important in the pathogenesis of several diseases such astype II diabetes and insulin resistance (Rieusset, 2011; Leemand Koh, 2012), neurodegenerative diseases (AD) (Pereiraet al., 2004; Takuma et al., 2005), (Pereira et al., 2004; Takuma



et al., 2005) Parkinson’s disease (Cali et al., 2011) oramyotrophic lateral sclerosis (Lautenschlaeger et al., 2012) andcardiovascular diseases (Minamino and Kitakaze, 2010). It isnow obvious that interactions between the ER and otherorganelles such as mitochondria are very important and thatabnormalities of one or both of the organelle are involved inthese pathologies.

Conclusion

For a long time, most of the cellular organelles were classicallyconsidered as separate and independent entities with specificcellular and metabolic functions. This point of view has changedover the last years as organelles are now recognised as highlydynamic entities forming inter-connected networks. This isparticularly well illustrated at the molecular level for theintricate crosstalk that exists between the ER andmitochondria. Not only do physical links tether these twoorganelles but more and more evidence now reveal thatdysfunction of one organelle can affect functions of the otherone leading or not to cell death and possibly contributing tosome major chronic diseases.

In this review, besides the strong and well-studiedmechanisms linking ER stress and UPR to apoptosis, we alsosummarised the recent advances on the impact of a non-lethalER-stress, a situation that might be regularly and physiologicallyencountered by a cell, on the biology of mitochondria.Unravelling the impact of ER stress on both bioenergetics andmorphology of mitochondria certainly deserves moreattention as it could pave the way for a better understanding ofseveral chronic disorders.

Acknowledgments

The authors thank Michel Savels for his contribution to thefigure layout and the University of Namur for the financialsupport.

Literature Cited

Abramov AY, Canevari L, Duchen MR. 2004. Beta-amyloid peptides induce mitochondrialdysfunction and oxidative stress in astrocytes and death of neurons through activation ofNADPH oxidase. J Neurosci 24:565–575.

Akao Y,Otsuki Y, Kataoka S, Ito Y, Tsujimoto Y. 1994.Multiple subcellular localization of bcl-2: Detection in nuclear outer membrane, endoplasmic reticulum membrane, andmitochondrial membranes. Cancer Res 54:2468–2471.

Aleardi AM, Benard G, Augereau O, Malgat M, Talbot JC, Mazat JP, Letellier T, Dachary-Prigent J, Solaini GC, Rossignol R. 2005. Gradual alteration of mitochondrial structure andfunction by beta-amyloids: Importance of membrane viscosity changes, energydeprivation, reactive oxygen species production, and cytochrome c release. J BioenergBiomembr 37:207–225.

Alirol E, James D, Huber D, Marchetto A, Vergani L, Martinou JC, Scorrano L. 2006. Themitochondrial fission protein hFis1 requires the endoplasmic reticulum gateway to induceapoptosis. Mol Biol Cell 17:4593–4605.

Allagnat F, Christulia F, Ortis F, Pirot P, Lortz S, Lenzen S, Eizirik DL, Cardozo AK. 2010.Sustained production of spliced X-box binding protein 1 (XBP1) induces pancreatic betacell dysfunction and apoptosis. Diabetologia 53:1120–1130.

Allagnat F, Cunha D, Moore F, Vanderwinden JM, Eizirik DL, Cardozo AK. 2011. Mcl-1downregulation by pro-inflammatory cytokines and palmitate is an early eventcontributing to beta-cell apoptosis. Cell Death Differ 18:328–337.

Altman BJ, Wofford JA, Zhao Y, Coloff JL, Ferguson EC, Wieman HL, Day AE, Ilkayeva O,Rathmell JC. 2009. Autophagy provides nutrients but can lead to Chop-dependentinduction of Bim to sensitize growth factor-deprived cells to apoptosis. Mol Biol Cell20:1180–1191.

Amarilio R, Ramachandran S, Sabanay H, Lev S. 2005. Differential regulation of endoplasmicreticulum structure through VAP-Nir protein interaction. J Biol Chem 280:5934–5944.

Ameri K, Harris AL. 2008. Activating transcription factor 4. Int J BiochemCell Biol 40:14–21.Andrade J, Zhao H, Titus B, Timm Pearce S, Barroso M. 2004. The EF-hand Ca2þ-binding

protein p22 plays a role in microtubule and endoplasmic reticulum organization anddynamics with distinct Ca2þ-binding requirements. Mol Biol Cell 15:481–496.

Anesti V, Scorrano L. 2006. The relationship between mitochondrial shape and function andthe cytoskeleton. Biochim Biophys Acta 1757:692–699.

Arakaki N, Nishihama T, Owaki H, Kuramoto Y, Suenaga M, Miyoshi E, Emoto Y, Shibata H,Shono M, Higuti T. 2006. Dynamics of mitochondria during the cell cycle. Biol Pharm Bull29:1962–1965.

Ardail D, Lerme F, Louisot P. 1991. Involvement of contact sites in phosphatidylserine importinto liver mitochondria. J Biol Chem 266:7978–7981.

Area-gomez E, De Groof AJ, Boldogh I, Bird TD, Gibson GE, Koehler CM, YuWH, Duff KE,Yaffe MP, Pon LA, Schon EA. 2009. Presenilins are enriched in endoplasmic reticulummembranes associated with mitochondria. Am J Pathol 175:1810–1816.

Arnoult D, Grodet A, Lee YJ, Estaquier J, Blackstone C. 2005. Release of OPA1 duringapoptosis participates in the rapid and complete release of cytochrome c and subsequentmitochondrial fragmentation. J Biol Chem 280:35742–35750.

Auf G, Jabouille A, Guerit S, Pineau R, Delugin M, Bouchecareilh M, Magnin N, Favereaux A,Maitre M, Gaiser T, Von Deimling A, Czabanka M, Vajkoczy P, Chevet E, Bikfalvi A,Moenner M. 2010. Inositol-requiring enzyme 1alpha is a key regulator of angiogenesis andinvasion in malignant glioma. Proc Natl Acad Sci USA 107:15553–15558.

Back SH, Scheuner D, Han J, Song B, Ribick M, Wang J, Gildersleeve RD, Pennathur S,Kaufman RJ. 2009. Translation attenuation through eIF2alpha phosphorylation preventsoxidative stress and maintains the differentiated state in beta cells. Cell Metab 10:13–26.

Badiola N, Penas C, Minano-molina A, Barneda-zahonero B, Fado R, Sanchez-opazo G,Comella JX, Sabria J, Zhu C, Blomgren K, Casas C, Rodriguez-alvarez J. 2011. Induction ofER stress in response to oxygen-glucose deprivation of cortical cultures involves theactivation of the PERK and IRE-1 pathways and of caspase-12. Cell Death Dis 2:e149.

Bakeeva LE, Chentsov Yu S, Skulachev VP. 1978. Mitochondrial framework (reticulummitochondriale) in rat diaphragm muscle. Biochim Biophys Acta 501:349–369.

Baricault L, Segui B, Guegand L, Olichon A, Valette A, Larminat F, Lenaers G. 2007. OPA1cleavage depends on decreased mitochondrial ATP level and bivalent metals. Exp Cell Res313:3800–3808.

Barni S, Sciola L, Spano A, Pippia P. 1996. Static cytofluorometry and fluorescencemorphology of mitochondria and DNA in proliferating fibroblasts. Biotech Histochem71:66–70.

Bathori G, Csordas G, Garcia-perez C, Davies E, Hajnoczky G. 2006. Ca2þ-dependentcontrol of the permeability properties of the mitochondrial outer membrane and voltage-dependent anion-selective channel (VDAC). J Biol Chem 281:17347–17358.

Benard G, Karbowski M. 2009. Mitochondrial fusion and division: Regulation and role in cellviability. Semin Cell Dev Biol 20:365–374.

Benard G, Rossignol R. 2008. Ultrastructure of the mitochondrion and its bearing onfunction and bioenergetics. Antioxid Redox Signal 10:1313–1342.

Bereiter-hahn J. 1990. Behavior of mitochondria in the living cell. Int Rev Cytol 122:1–63.Berridge MJ. 2002. The endoplasmic reticulum: A multifunctional signaling organelle. Cell

Calcium 32:235–249.Bertolotti A, Zhang Y, Hendershot LM,HardingHP, RonD. 2000. Dynamic interaction of BiP

and ER stress transducers in the unfolded-protein response. Nat Cell Biol 2:326–332.Bhavya BC, Indira D, Seervi M, Joseph J, Sobhan PK, Mathew KA, Varghese S, Santhoshkumar

TR. 2012. Endoplasmic reticulum-targeted Bcl-2 inhibitable mitochondrial fragmentationinitiates ER stress-induced cell death. Adv Exp Med Biol 749:83–95.

Bianchi K, Rimessi A, Prandini A, Szabadkai G, Rizzuto R. 2004. Calcium and mitochondria:Mechanisms and functions of a troubled relationship. Biochim Biophys Acta 1742:119–131.

Bilecova-Rabajdova M, Urban P, Maslankova J, Vesela J, Marekova M. 2010. Analysis ofchanges in pro (Gadd153) and anti apoptotic (Grp78) gene expression after ischemic-reperfusion injury of the small intestine. Prague Med Rep 111:249–256.

Boldogh IR, Pon LA. 2007. Mitochondria on the move. Trends Cell Biol 17:502–510.Borchelt DR, Thinakaran G, Eckman CB, LeeMK, Davenport F, Ratovitsky T, Prada CM, Kim

G, Seekins S, Yager D, Slunt HH, Wang R, Seeger M, Levey AI, Gandy SE, Copeland NG,Jenkins NA, Price DL, Younkin SG, Sisodia SS. 1996. Familial Alzheimer’s disease-linkedpresenilin 1 variants elevate Abeta1-42/1-40 ratio in vitro and in vivo. Neuron 17:1005–1013.

Bouman L, Schlierf A, Lutz AK, Shan J, Deinlein A, Kast J, Galehdar Z, Palmisano V, PatengeN,Berg D, Gasser T, Augustin R, Trumbach D, Irrcher I, Park DS, Wurst W, Kilberg MS,Tatzelt J, Winklhofer KF. 2011. Parkin is transcriptionally regulated by AT F4: Evidence foran interconnection between mitochondrial stress and ER stress. Cell Death Differ18:769–782.

BoyceM, Yuan J. 2006. Cellular response to endoplasmic reticulum stress: A matter of life ordeath. Cell Death Differ 13:363–373.

Bravo R, Vicencio JM, Parra V, Troncoso R, Munoz JP, Bui M, Quiroga C, Rodriguez AE,Verdejo HE, Ferreira J, Iglewski M, Chiong M, Simmen T, Zorzano A, Hill JA, RothermelBA, Szabadkai G, Lavandero S. 2011. Increased ER-mitochondrial coupling promotesmitochondrial respiration and bioenergetics during early phases of ER stress. J Cell Sci124:2143–2152.

Breckenridge DG, Stojanovic M, Marcellus RC, Shore GC. 2003. Caspase cleavage productof BAP31 induces mitochondrial fission through endoplasmic reticulum calcium signals,enhancing cytochrome c release to the cytosol. J Cell Biol 160:1115–1127.

Brion JP. 2006. Immunological demonstration of tau protein in neurofibrillary tangles ofAlzheimer’s disease. J Alzheimers Dis 9:177–185.

Brostrom CO, Brostrom MA. 1990. Calcium-dependent regulation of protein synthesis inintact mammalian cells. Annu Rev Physiol 52:577–590.

Brush MH, Weiser DC, Shenolikar S. 2003. Growth arrest and DNA damage-inducibleprotein GADD34 targets protein phosphatase 1 alpha to the endoplasmic reticulum andpromotes dephosphorylation of the alpha subunit of eukaryotic translation initiationfactor 2. Mol Cell Biol 23:1292–1303.

Cabral CM, Liu Y, Sifers RN. 2001. Dissecting glycoprotein quality control in the secretorypathway. Trends Biochem Sci 26:619–624.

Cali T, Ottolini D, Brini M. 2011. Mitochondria, calcium, and endoplasmic reticulum stress inParkinson’s disease. Biofactors 37:228–240.

Calvo J, Funalot B, Ouvrier RA, Lazaro L, Toutain A, DeMas P, Bouche P, Gilbert-DussardierB, Arne-Bes MC, Carriere JP, Journel H, Minot-Myhie MC, Guillou C, Ghorab K, Magy L,Sturtz F, Vallat JM, Magdelaine C. 2009. Genotype-phenotype correlations in Charcot-Marie-Tooth disease type 2 caused by mitofusin 2 mutations. Arch Neurol 66:1511–1516.

Cao X, Deng X, May WS. 2003. Cleavage of Bax to p18 Bax accelerates stress-inducedapoptosis, and a cathepsin-like protease may rapidly degrade p18 Bax. Blood 102:2605–2614.

Casas-Tinto S, Zhang Y, Sanchez-Garcia J, Gomez-Velazquez M, Rincon-Limas DE,Fernandez-Funez P. 2011. The ER stress factor XBP1s prevents amyloid-betaneurotoxicity. Hum Mol Genet 20:2144–2160.

Cerqua C, Anesti V, Pyakurel A, Liu D, Naon D, Wiche G, Baffa R, Dimmer KS, Scorrano L.2010. Trichoplein/mitostatin regulates endoplasmic reticulum-mitochondriajuxtaposition. EMBO Rep 11:854–860.

Chafekar SM, Zwart R, Veerhuis R, Vanderstichele H, Baas F, Scheper W. 2008. IncreasedAbeta1-42 production sensitizes neuroblastoma cells for ER stress toxicity. CurrAlzheimer Res 5:469–474.

Chami M, Oules B, Szabadkai G, Tacine R, Rizzuto R, Paterlini-Brechot P. 2008. Role ofSERCA1 truncated isoform in the proapoptotic calcium transfer from ER to mitochondriaduring ER stress. Mol Cell 32:641–651.

Chan NC, Salazar AM, Pham AH, Sweredoski MJ, Kolawa NJ, Graham RL, Hess S, Chan DC.2011. Broad activation of the ubiquitin-proteasome system by Parkin is critical formitophagy. Hum Mol Genet 20:1726–1737.



Chen H, Detmer SA, Ewald AJ, Griffin EE, Fraser SE, Chan DC. 2003. Mitofusins Mfn1 andMfn2 coordinately regulate mitochondrial fusion and are essential for embryonicdevelopment. J Cell Biol 160:189–200.

Chen H, Mccaffery JM, Chan DC. 2007. Mitochondrial fusion protects againstneurodegeneration in the cerebellum. Cell 130:548–562.

Chen H, Vermulst M, Wang YE, Chomyn A, Prolla TA, Mccaffery JM, Chan DC. 2010.Mitochondrial fusion is required for mtDNA stability in skeletal muscle and tolerance ofmtDNA mutations. Cell 141:280–289.

Cipolat S, Martins de Brito O, Dal Zilio B, Scorrano L. 2004. OPA1 requires mitofusin 1 topromote mitochondrial fusion. Proc Natl Acad Sci USA 101:15927–15932.

Collins TJ, Berridge MJ, Lipp P, Bootman MD. 2002. Mitochondria are morphologically andfunctionally heterogeneous within cells. EMBO J 21:1616–1627.

Copeland DE, Dalton AJ. 1959. An association between mitochondria and the endoplasmicreticulum in cells of the pseudobranch gland of a teleost. J Biophys Biochem Cytol 5:393–396.

Cribb AE, Peyrou M, Muruganandan S, Schneider L. 2005. The endoplasmic reticulum inxenobiotic toxicity. Drug Metab Rev 37:405–442.

Csordas G, Hajnoczky G. 2009. SR/ER-mitochondrial local communication: Calcium andROS. Biochim Biophys Acta 1787:1352–1362.

Csordas G, Renken C, Varnai P, Walter L, Weaver D, Buttle KF, Balla T, Mannella CA,Hajnoczky G. 2006. Structural and functional features and significance of the physicallinkage between ER and mitochondria. J Cell Biol 174:915–921.

Davis RJ. 2000. Signal transduction by the JNK group of MAP kinases. Cell 103:239–252.de BritoOM, Scorrano L. 2008a. Mitofusin 2 tethers endoplasmic reticulum tomitochondria.Nature 456:605–610.

de Brito OM, Scorrano L. 2008b. Mitofusin 2: A mitochondria-shaping protein with signalingroles beyond fusion. Antioxid Redox Signal 10:621–633.

de Brito OM, Scorrano L. 2010. An intimate liaison: Spatial organization of the endoplasmicreticulum-mitochondria relationship. EMBO J 29:2715–2723.

de Strooper B, Iwatsubo T, Wolfe MS. 2012. Presenilins and gamma-Secretase: Structure,Function, and Role in Alzheimer Disease. Cold Spring Harb Perspect Med 2:a006304.

Delettre C, Griffoin JM, Kaplan J, Dollfus H, Lorenz B, Faivre L, Lenaers G, Belenguer P,Hamel CP. 2001. Mutation spectrum and splicing variants in the OPA1 gene. Hum Genet109:584–591.

Delic M, Rebnegger C, Wanka F, Puxbaum V, Haberhauer-Troyer C, Hann S, KollenspergerG, Mattanovich D, Gasser B. 2012. Oxidative protein folding and unfolded proteinresponse elicit differing redox regulation in endoplasmic reticulum and cytosol of yeast.Free Radic Biol Med 52:2000–2012.

Deniaud A, Sharaf el Dein O, Maillier E, Poncet D, Kroemer G, Lemaire C, Brenner C. 2008.Endoplasmic reticulum stress induces calcium-dependent permeability transition,mitochondrial outer membrane permeabilization and apoptosis. Oncogene 27:285–299.

Denton RM, Rutter GA, Midgley PJ, Mccormack JG. 1988. Effects of Ca2þ on the activities ofthe calcium-sensitive dehydrogenases within the mitochondria of mammalian tissues. JCardiovasc Pharmacol 12:S69–S72.

Diwan A, Matkovich SJ, Yuan Q, Zhao W, Yatani A, Brown JH, Molkentin JD, Kranias EG,Dorn GW II. 2009. Endoplasmic reticulum-mitochondria crosstalk in NIX-mediatedmurine cell death. J Clin Invest 119:203–212.

Donovan M, Cotter TG. 2004. Control of mitochondrial integrity by Bcl-2 family membersand caspase-independent cell death. Biochim Biophys Acta 1644:133–147.

Du C, Fang M, Li Y, Li L, Wang X. 2000. Smac, a mitochondrial protein that promotescytochrome c-dependent caspase activation by eliminating IAP inhibition. Cell 102:33–42.

Duchen MR. 2004. Mitochondria in health and disease: Perspectives on a newmitochondrialbiology. Mol Aspects Med 25:365–451.

Dupin I, Etienne-Manneville S. 2011. Nuclear positioning: Mechanisms and functions. Int JBiochem Cell Biol 43:1698–1707.

Duvezin-Caubet S, Jagasia R, Wagener J, Hofmann S, Trifunovic A, Hansson A, Chomyn A,Bauer MF, Attardi G, Larsson NG, NeupertW, Reichert AS. 2006. Proteolytic processingof OPA1 links mitochondrial dysfunction to alterations in mitochondrial morphology.J Biol Chem 281:37972–37979.

Ehses S, Raschke I, Mancuso G, Bernacchia A, Geimer S, Tondera D, Martinou JC,Westermann B, Rugarli EI, Langer T. 2009. Regulation of OPA1 processing andmitochondrial fusion by m-AAA protease isoenzymes and OMA1. J Cell Biol 187:1023–1036.

Eizirik DL, Cardozo AK, Cnop M. 2008. The role for endoplasmic reticulum stress indiabetes mellitus. Endocr Rev 29:42–61.

Ellgaard L, Helenius A. 2001. ER quality control: Towards an understanding at the molecularlevel. Curr Opin Cell Biol 13:431–437.

Er E, Oliver L, Cartron PF, Juin P, Manon S, Vallette FM. 2006. Mitochondria as the target ofthe pro-apoptotic protein Bax. Biochim Biophys Acta 1757:1301–1311.

Estaquier J, Vallette F, Vayssiere JL, Mignotte B. 2012. The mitochondrial pathways ofapoptosis. Adv Exp Med Biol 942:157–183.

Eura Y, Ishihara N, Yokota S, Mihara K. 2003. Two mitofusin proteins, mammalianhomologues of FZO, with distinct functions are both required for mitochondrial fusion.J Biochem 134:333–344.

Faccenda D, Tan CH, Seraphim A, Duchen MR, Campanella M. 2013. IF(1) limits theapoptotic-signalling cascade by preventing mitochondrial remodelling. Cell Death Differ.doi: 10.1038/cdd 2012.163 [Epub ahead of print].

Fang L, Hemion C, Goldblum D, Meyer P, Orgul S, Frank S, Flammer J, Neutzner A. 2012.Inactivation of MARCH5 prevents mitochondrial fragmentation and interferes with celldeath in a neuronal cell model. PLoS ONE 7:e52637.

Fannjiang Y, ChengWC, Lee SJ, Qi B, Pevsner J, Mccaffery JM, Hill RB, Basanez G, HardwickJM. 2004. Mitochondrial fission proteins regulate programmed cell death in yeast. GenesDev 18:2785–2797.

Frank S, Gaume B, Bergmann-Leitner ES, LeitnerWW, Robert EG, Catez F, Smith CL, YouleRJ. 2001. The role of dynamin-related protein 1, a mediator of mitochondrial fission, inapoptosis. Dev Cell 1:515–525.

Friedman JR, Lackner LL, West M, Dibenedetto JR, Nunnari J, Voeltz GK. 2011. ER tubulesmark sites of mitochondrial division. Science 334:358–362.

FujimotoM,Hayashi T, Su TP. 2012. The role of cholesterol in the association of endoplasmicreticulum membranes with mitochondria. Biochem Biophys Res Commun 417:635–639.

Galvez AS, Brunskill EW,Marreez Y, Benner BJ, Regula KM, Kirschenbaum LA, Dorn GW II.2006. Distinct pathways regulate proapoptotic Nix and BNip3 in cardiac stress. J BiolChem 281:1442–1448.

Gandre-Babbe S, Van Der Bliek AM. 2008. The novel tail-anchored membrane protein Mffcontrols mitochondrial and peroxisomal fission in mammalian cells. Mol Biol Cell19:2402–2412.

Gao W, Pu Y, Luo KQ, Chang DC. 2001. Temporal relationship between cytochrome crelease and mitochondrial swelling during UV-induced apoptosis in living HeLa cells. J CellSci 114:2855–2862.

Gaut JR, Hendershot LM. 1993. The modification and assembly of proteins in theendoplasmic reticulum. Curr Opin Cell Biol 5:589–595.

Gegg ME, Cooper JM, Chau KY, Rojo M, Schapira AH, Taanman JW. 2010. Mitofusin 1 andmitofusin 2 are ubiquitinated in a PINK1/parkin-dependent manner upon induction ofmitophagy. Hum Mol Genet 19:4861–4870.

Germain M, Mathai JP, Shore GC. 2002. BH-3-only BIK functions at the endoplasmicreticulum to stimulate cytochrome c release from mitochondria. J Biol Chem 277:18053–18060.

Germain M, Mathai JP, Mcbride HM, Shore GC. 2005. Endoplasmic reticulum BIK initiatesDRP1-regulated remodelling of mitochondrial cristae during apoptosis. EMBO J 24:1546–1556.

Gess B, Hofbauer KH,Wenger RH, Lohaus C, Meyer HE, Kurtz A. 2003. The cellular oxygentension regulates expression of the endoplasmic oxidoreductase ERO1-Lalpha. Eur JBiochem 270:2228–2235.

Gilkerson RW,MargineantuDH,Capaldi RA, Selker JM. 2000. Mitochondrial DNAdepletioncauses morphological changes in the mitochondrial reticulum of cultured human cells.FEBS Lett 474:1–4.

Goate A. 2006. Segregation of a missense mutation in the amyloid beta-protein precursorgene with familial Alzheimer’s disease. J Alzheimers Dis 9:341–347.

Goate A, Chartier-Harlin MC, Mullan M, Brown J, Crawford F, Fidani L, Giuffra L, Haynes A,Irving N, James L, Mant A., Newton P, Rooke K, Roques P, Talbot C, Pericak-Vance M,Roses A, Williamson R, Rossor M, Owen M, Hardy J. 1991. Segregation of a missensemutation in the amyloid precursor protein gene with familial Alzheimer’s disease. Nature349:704–706.

Gorman AM, Healy SJ, Jager R, Samali A. 2012. Stress management at the ER: Regulators ofER stress-induced apoptosis. Pharmacol Ther 134:306–316.

Gotoh T, Oyadomari S, Mori K, Mori M. 2002. Nitric oxide-induced apoptosis in RAW264.7macrophages is mediated by endoplasmic reticulum stress pathway involving ATF6 andCHOP. J Biol Chem 277:12343–12350.

Gouras GK, Tsai J, Naslund J, Vincent B, Edgar M, Checler F, Greenfield JP, Haroutunian V,Buxbaum JD, XuH,Greengard P, RelkinNR. 2000. Intraneuronal Abeta42 accumulation inhuman brain. Am J Pathol 156:15–20.

Griffiths GJ, Dubrez L, Morgan CP, Jones NA,Whitehouse J, Corfe BM, Dive C, Hickman JA.1999. Cell damage-induced conformational changes of the pro-apoptotic protein Bak invivo precede the onset of apoptosis. J Cell Biol 144:903–914.

Grimm S. 2012. The ER-mitochondria interface: The social network of cell death. BiochimBiophys Acta 1823:327–334.

Griparic L, Van Der Wel NN, Orozco IJ, Peters PJ, Van Der Bliek AM. 2004. Loss of theintermembrane space protein Mgm1/OPA1 induces swelling and localized constrictionsalong the lengths of mitochondria. J Biol Chem 279:18792–18798.

Griparic L, Kanazawa T, Van Der Bliek AM. 2007. Regulation of the mitochondrial dynamin-like protein Opa1 by proteolytic cleavage. J Cell Biol 178:757–764.

Groenendyk J, Michalak M. 2005. Endoplasmic reticulum quality control and apoptosis. ActaBiochim Pol 52:381–395.

Gross A, Jockel J, Wei MC, Korsmeyer SJ. 1998. Enforced dimerization of BAX results in itstranslocation, mitochondrial dysfunction and apoptosis. EMBO J 17:3878–3885.

Gurzov EN, Ortis F, Bakiri L, Wagner EF, Eizirik DL. 2008. JunB inhibits ER stress andapoptosis in pancreatic beta cells. PLoS ONE 3:e3030.

Gurzov EN, Ortis F, Cunha DA, Gosset G, Li M, Cardozo AK, Eizirik DL. 2009. Signaling byIL-1betaþ IFN-gamma and ER stress converge onDP5/Hrk activation: A novel mechanismfor pancreatic beta-cell apoptosis. Cell Death Differ 16:1539–1550.

Gurzov EN, Germano CM, Cunha DA, Ortis F, Vanderwinden JM, Marchetti P, Zhang L,Eizirik DL. 2010. p53 up-regulated modulator of apoptosis (PUMA) activation contributesto pancreatic beta-cell apoptosis induced by proinflammatory cytokines and endoplasmicreticulum stress. J Biol Chem 285:19910–19920.

Hai TW, Liu F, Coukos WJ, Green MR. 1989. Transcription factor ATF cDNA clones: Anextensive family of leucine zipper proteins able to selectively form DNA-bindingheterodimers. Genes Dev 3:2083–2090.

Hamman BD, Hendershot LM, Johnson AE. 1998. BiP maintains the permeability barrier ofthe ER membrane by sealing the lumenal end of the translocon pore before and early intranslocation. Cell 92:747–758.

Han J, Goldstein LA, HouW, Gastman BR, Rabinowich H. 2010. Regulation of mitochondrialapoptotic events by p53-mediated disruption of complexes between antiapoptotic Bcl-2members and Bim. J Biol Chem 285:22473–22483.

Hansson Petersen CA, Alikhani N, Behbahani H,Wiehager B, Pavlov PF, Alafuzoff I, LeinonenV, Ito A,Winblad B, Glaser E, Ankarcrona M. 2008. The amyloid beta-peptide is importedinto mitochondria via the TOM import machinery and localized to mitochondrial cristae.Proc Natl Acad Sci USA 105:13145–13150.

Hao W, Takano T, Guillemette J, Papillon J, Ren G, Cybulsky AV. 2006. Induction ofapoptosis by the Ste20-like kinase SLK, a germinal center kinase that activates apoptosissignal-regulating kinase and p38. J Biol Chem 281:3075–3084.

Harding HP, Ron D. 2002. Endoplasmic reticulum stress and the development of diabetes:A review. Diabetes 51:S455–S461.

Harding HP, Zhang Y, Ron D. 1999. Protein translation and folding are coupled by anendoplasmic-reticulum-resident kinase. Nature 397:271–274.

Hayashi T, Su TP. 2003. Sigma-1 receptors (sigma(1) binding sites) form raft-likemicrodomains and target lipid droplets on the endoplasmic reticulum: Roles inendoplasmic reticulum lipid compartmentalization and export. J Pharmacol Exp Ther306:718–725.

Hayashi T, Su TP. 2007. Sigma-1 receptor chaperones at the ER-mitochondrion interfaceregulate Ca(2þ) signaling and cell survival. Cell 131:596–610.

Hayashi T, Su TP. 2010. Cholesterol at the endoplasmic reticulum: Roles of the sigma-1receptor chaperone and implications thereof in human diseases. Subcell Biochem 51:381–398.

Hayashi T, Rizzuto R, Hajnoczky G, Su TP. 2009. MAM: More than just a housekeeper.Trends Cell Biol 19:81–88.

Haze K, Yoshida H, Yanagi H, Yura T, Mori K. 1999. Mammalian transcription factor ATF6 issynthesized as a transmembrane protein and activated by proteolysis in response toendoplasmic reticulum stress. Mol Biol Cell 10:3787–3799.

Hebert DN, Garman SC, Molinari M. 2005. The glycan code of the endoplasmic reticulum:Asparagine-linked carbohydrates as protein maturation and quality-control tags. TrendsCell Biol 15:364–370.

Hellewell SB, Bruce A, Feinstein G, Orringer J, WilliamsW, BowenWD. 1994. Rat liver andkidney contain high densities of sigma 1 and sigma 2 receptors: Characterization by ligandbinding and photoaffinity labeling. Eur J Pharmacol 268:9–18.



Hetz C. 2012. The unfolded protein response: Controlling cell fate decisions under ER stressand beyond. Nat Rev Mol Cell Biol 13:89–102.

Hetz C, Glimcher LH. 2009. Fine-tuning of the unfolded protein response: Assembling theIRE1alpha interactome. Mol Cell 35:551–561.

Hitomi J, Katayama T, Eguchi Y, Kudo T, Taniguchi M, Koyama Y, Manabe T, Yamagishi S,Bando Y, Imaizumi K, Tsujimoto Y, Tohyama M. 2004. Involvement of caspase-4 inendoplasmic reticulum stress-induced apoptosis and Abeta-induced cell death. J Cell Biol165:347–356.

Ho YS, Yang X, Lau JC, Hung CH,Wuwongse S, ZhangQ,Wang J, Baum L, So KF, Chang RC.2012. Endoplasmic reticulum stress induces tau pathology and forms a vicious cycle:Implication in Alzheimer’s disease pathogenesis. J Alzheimers Dis 28:839–854.

Hollien J. 2013. Evolution of the unfolded protein response. Biochim Biophys Acta. doi: pii:S0167-4889 (13) 00031-1.10.1016/J.bbamcr 2013.01.016 [Epub ahead of print].

Hom JR, Gewandter JS, Michael L, Sheu SS, Yoon Y. 2007. Thapsigargin induces biphasicfragmentation of mitochondria through calcium-mediated mitochondrial fission andapoptosis. J Cell Physiol 212:498–508.

Hoozemans JJ, Veerhuis R, Van Haastert ES, Rozemuller JM, Baas F, Eikelenboom P, ScheperW. 2005. The unfolded protein response is activated in Alzheimer’s disease. ActaNeuropathol 110:165–172.

Hoozemans JJ, Van Haastert ES, Nijholt DA, Rozemuller AJ, Eikelenboom P, Scheper W.2009. The unfolded protein response is activated in pretangle neurons in Alzheimer’sdisease hippocampus. Am J Pathol 174:1241–1251.

Hori O, Ichinoda F, Tamatani T, Yamaguchi A, Sato N, Ozawa K, Kitao Y, Miyazaki M,Harding HP, Ron D, Tohyama M, Stern DM, Ogawa S. 2002. Transmission of cell stressfrom endoplasmic reticulum tomitochondria: Enhanced expression of Lon protease. J CellBiol 157:1151–1160.

Hu P, Han Z, Couvillon AD, Kaufman RJ, Exton JH. 2006. Autocrine tumor necrosis factoralpha links endoplasmic reticulum stress to the membrane death receptor pathwaythrough IRE1alpha-mediated NF-kappaB activation and down-regulation of TRAF2expression. Mol Cell Biol 26:3071–3084.

Hu J, Shibata Y, Zhu PP, Voss C, Rismanchi N, PrinzWA, Rapoport TA, Blackstone C. 2009.A class of dynamin-likeGTPases involved in the generation of the tubular ER network. Cell138:549–561.

Hu J, Dang N, Menu E, De Bryune E, Xu D, Van Camp B, Van Valckenborgh E, VanderkerkenK. 2012. Activation of ATF4 mediates unwanted Mcl-1 accumulation by proteasomeinhibition. Blood 119:826–837.

Huang CJ, Gurlo T, Haataja L, Costes S, Daval M, Ryazantsev S, Wu X, Butler AE, Butler PC.2010. Calcium-activated calpain-2 is a mediator of beta cell dysfunction and apoptosis intype 2 diabetes. J Biol Chem 285:339–348.

Ikesugi K, Mulhern ML, Madson CJ, Hosoya K, Terasaki T, Kador PF, Shinohara T. 2006.Induction of endoplasmic reticulum stress in retinal pericytes by glucose deprivation. CurrEye Res 31:947–953.

Ingerman E, Perkins EM, Marino M, Mears JA, Mccaffery JM, Hinshaw JE, Nunnari J. 2005.Dnm1 forms spirals that are structurally tailored to fit mitochondria. J Cell Biol 170:1021–1027.

Iqbal K, Novak M. 2006. From tangles to tau protein. Bratisl Lek Listy 107:341–342.Ishihara N, Eura Y, Mihara K. 2004. Mitofusin 1 and 2 play distinct roles in mitochondrial

fusion reactions via GTPase activity. J Cell Sci 117:6535–6546.IshiharaN, Fujita Y,Oka T,Mihara K. 2006. Regulation ofmitochondrial morphology through

proteolytic cleavage of OPA1. EMBO J 25:2966–2977.Iwasawa R, Mahul-Mellier AL, Datler C, Pazarentzos E, Grimm S. 2011. Fis1 and Bap31 bridge

the mitochondria-ER interface to establish a platform for apoptosis induction. EMBO J30:556–568.

Iwatsubo T. 1998. Abeta42, presenilins, and Alzheimer’s disease. Neurobiol Aging 19:S11–S13.

Jakobs S, Martini N, Schauss AC, Egner A, Westermann B, Hell SW. 2003. Spatial andtemporal dynamics of budding yeast mitochondria lacking the division component Fis1p. JCell Sci 116:2005–2014.

James DI, Parone PA, Mattenberger Y, Martinou JC. 2003. hFis1, a novel component of themammalian mitochondrial fission machinery. J Biol Chem 278:36373–36379.

Jin SM, Youle RJ. 2012. PINK1- and Parkin-mediatedmitophagy at a glance. J Cell Sci 125:795–799.

Jin SM, Lazarou M, Wang C, Kane LA, Narendra DP, Youle RJ. 2010. Mitochondrialmembrane potential regulates PINK1 import and proteolytic destabilization by PARL. JCell Biol 191:933–942.

Kahn SE, Hull RL, Utzschneider KM. 2006. Mechanisms linking obesity to insulin resistanceand type 2 diabetes. Nature 444:840–846.

Kaneko M, Niinuma Y, Nomura Y. 2003. Activation signal of nuclear factor-kappa B inresponse to endoplasmic reticulum stress is transduced via IRE1 and tumor necrosisfactor receptor-associated factor 2. Biol Pharm Bull 26:931–935.

Karbowski M, Lee YJ, Gaume B, Jeong SY, Frank S, Nechushtan A, Santel A, Fuller M, SmithCL, Youle RJ. 2002. Spatial and temporal association of Bax with mitochondrial fissionsites, Drp1, and Mfn2 during apoptosis. J Cell Biol 159:931–938.

Kedi X, Ming Y, YongpingW, Yi Y, Xiaoxiang Z. 2009. Free cholesterol overloading inducedsmooth muscle cells death and activated both ER- and mitochondrial-dependent deathpathway. Atherosclerosis 207:123–130.

Keil U, Bonert A, Marques CA, Scherping I, Weyermann J, Strosznajder JB, Muller-Spahn F,Haass C, Czech C, Pradier L, MullerWE, Eckert A. 2004. Amyloid beta-induced changes innitric oxide production and mitochondrial activity lead to apoptosis. J Biol Chem279:50310–50320.

Kennady PK, Ormerod MG, Singh S, Pande G. 2004. Variation of mitochondrial size duringthe cell cycle: A multiparameter flow cytometric and microscopic study. Cytometry A62:97–108.

KimHS, Lee JH, Lee JP, Kim EM, Chang KA, Park CH, Jeong SJ,WittendorpMC, Seo JH, ChoiSH, Suh YH. 2002. Amyloid beta peptide induces cytochrome C release from isolatedmitochondria. Neuroreport 13:1989–1993.

Kim SJ, Zhang Z, Hitomi E, Lee YC, Mukherjee AB. 2006. Endoplasmic reticulum stress-induced caspase-4 activation mediates apoptosis and neurodegeneration in INCL. HumMol Genet 15:1826–1834.

Kim H, Tu HC, Ren D, Takeuchi O, Jeffers JR, Zambetti GP, Hsieh JJ, Cheng EH. 2009a.Stepwise activation of BAX and BAK by tBID, BIM, and PUMA initiates mitochondrialapoptosis. Mol Cell 36:487–499.

Kim SM, Park HS, Jun DY, Woo HJ, Woo MH, Yang CH, Kim YH. 2009b. Mollugin inducesapoptosis in human Jurkat T cells through endoplasmic reticulum stress-mediatedactivation of JNK and caspase-12 and subsequent activation of mitochondria-dependentcaspase cascade regulated by Bcl-xL. Toxicol Appl Pharmacol 241:210–220.

Koch GL. 1990. The endoplasmic reticulum and calcium storage. BioEssays 12:527–531.

Koch A, Thiemann M, Grabenbauer M, Yoon Y, Mcniven MA, Schrader M. 2003. Dynamin-like protein 1 is involved in peroxisomal fission. J Biol Chem 278:8597–8605.

Koch A, Yoon Y, Bonekamp NA, Mcniven MA, Schrader M. 2005. A role for Fis1 in bothmitochondrial and peroxisomal fission in mammalian cells. Mol Biol Cell 16:5077–5086.

Kong D, Xu L, Yu Y, Zhu W, Andrews DW, Yoon Y, Kuo TH. 2005. Regulation ofCa2þ-induced permeability transition by Bcl-2 is antagonized by Drpl and hFis1. Mol CellBiochem 272:187–199.

Kopec KO, Alva V, Lupas AN. 2010. Homology of SMP domains to the TULIP superfamily oflipid-binding proteins provides a structural basis for lipid exchange between ER andmitochondria. Bioinformatics 26:1927–1931.

Kornmann B, Currie E, Collins SR, Schuldiner M, Nunnari J,Weissman JS,Walter P. 2009. AnER-mitochondria tethering complex revealed by a synthetic biology screen. Science325:477–481.

Kornmann B, Osman C, Walter P. 2011. The conserved GTPase Gem1 regulatesendoplasmic reticulum-mitochondria connections. Proc Natl Acad Sci USA 108:14151–14156.

Koshiba T, Detmer SA, Kaiser JT, Chen H, Mccaffery JM, Chan DC. 2004. Structural basis ofmitochondrial tethering by mitofusin complexes. Science 305:858–862.

Krajewski S, Tanaka S, Takayama S, Schibler MJ, Fenton W, Reed JC. 1993. Investigation ofthe subcellular distribution of the bcl-2 oncoprotein: Residence in the nuclear envelope,endoplasmic reticulum, and outer mitochondrial membranes. Cancer Res 53:4701–4714.

Krishnan KJ, Greaves LC, Reeve AK, Turnbull DM. 2007. Mitochondrial DNAmutations andaging. Ann NY Acad Sci 1100:227–240.

Kudo T, Katayama T, Imaizumi K, Yasuda Y, Yatera M, Okochi M, Tohyama M, Takeda M.2002. The unfolded protein response is involved in the pathology of Alzheimer’s disease.Ann NY Acad Sci 977:349–355.

LaFerla FM. 2002. Calcium dyshomeostasis and intracellular signalling in Alzheimer’s disease.Nat Rev Neurosci 3:862–872.

Landes T, Martinou JC. 2011. Mitochondrial outer membrane permeabilization duringapoptosis: The role of mitochondrial fission. Biochim Biophys Acta 1813:540–545.

Lautenschlaeger J, Prell T, Grosskreutz J. 2012. Endoplasmic reticulum stress and the ERmitochondrial calcium cycle in amyotrophic lateral sclerosis. Amyotroph Lateral Scler13:166–177.

Lavoie C, Paiement J. 2008. Topology of molecular machines of the endoplasmic reticulum: Acompilation of proteomics and cytological data. Histochem Cell Biol 129:117–128.

Lea PJ, Temkin RJ, FreemanKB, Mitchell GA, Robinson BH. 1994. Variations inmitochondrialultrastructure and dynamics observed by high resolution scanning electron microscopy(HRSEM). Microsc Res Tech 27:269–277.

Lee C, Chen LB. 1988. Dynamic behavior of endoplasmic reticulum in living cells. Cell 54:37–46.

Lee I, Hong W. 2006. Diverse membrane-associated proteins contain a novel SMP domain.FASEB J 20:202–206.

LeeDo Y, Lee KS, Lee HJ, Kim doH, Noh YH, Yu K, Jung HY, Lee SH, Lee JY, Youn YC, JeongY, Kim DK, Lee WB, Kim SS. 2010. Activation of PERK signaling attenuates Abeta-mediated ER stress. PLoS ONE 5:e10489.

Lee K, TirasophonW, Shen X, Michalak M, Prywes R, Okada T, Yoshida H, Mori K, KaufmanRJ. 2002. IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavagemerge to regulate XBP1 in signaling the unfolded protein response. Genes Dev 16:452–566.

Lee YJ, Jeong SY, Karbowski M, Smith CL, Youle RJ. 2004. Roles of the mammalianmitochondrial fission and fusion mediators Fis1, Drp1, and Opa1 in apoptosis. Mol BiolCell 15:5001–5011.

Lee TH, Mun JY, Han SM, Yoon G, Han SS, Koo HS. 2009. DIC-1 over-expression enhancesrespiratory activity in Caenorhabditis elegans by promoting mitochondrial cristaeformation. Genes Cells 14:319–327.

Leem J, Koh EH. 2012. Interaction between mitochondria and the endoplasmic reticulum:Implications for the pathogenesis of type 2 diabetes mellitus. Exp Diabetes Res2012:242984.

Lei K, Davis RJ. 2003. JNK phosphorylation of Bim-related members of the Bcl2 familyinduces Bax-dependent apoptosis. Proc Natl Acad Sci USA 100:2432–2437.

Li LY, Luo X, Wang X. 2001. Endonuclease G is an apoptotic DNase when released frommitochondria. Nature 412:95–99.

Li Q, Liu Z, Guo J, Chen J, Yang P, Tian J, Sun J, Zong Y, Qu S. 2009. Cholesterol overloadingleads to hepatic L02 cell damage through activation of the unfolded protein response. Int JMol Med 24:459–464.

Liesa M, Palacin M, Zorzano A. 2009. Mitochondrial dynamics in mammalian health anddisease. Physiol Rev 89:799–845.

Lim JH, Lee HJ, Ho Jung M, Song J. 2009. Coupling mitochondrial dysfunction to endoplasmicreticulum stress response: A molecular mechanism leading to hepatic insulin resistance.Cell Signal 21:169–177.

Lin JH, Li H, Yasumura D, Cohen HR, Zhang C, Panning B, Shokat KM, Lavail MM, Walter P.2007. IRE1 signaling affects cell fate during the unfolded protein response. Science318:944–949.

Lin S, Sun S, Hu J. 2012. Molecular basis for sculpting the endoplasmic reticulum membrane.Int J Biochem Cell Biol 44:1436–1443.

Lindholm D, Wootz H, Korhonen L. 2006. ER stress and neurodegenerative diseases. CellDeath Differ 13:385–392.

Lindsten T, Ross AJ, King A, Zong WX, Rathmell JC, Shiels HA, Ulrich E, Waymire KG,Mahar P, Frauwirth K, Chen Y, Wei M, Eng VM, Adelman DM, Simon MC, Ma A, GoldenJA, Evan G, Korsmeyer SJ, Macgregor GR, Thompson CB. 2000. The combined functionsof proapoptotic Bcl-2 family members bak and bax are essential for normal developmentof multiple tissues. Mol Cell 6:1389–1399.

Lithgow T, Van Driel R, Bertram JF, Strasser A. 1994. The protein product of the oncogenebcl-2 is a component of the nuclear envelope, the endoplasmic reticulum, and the outermitochondrial membrane. Cell Growth Differ 5:411–417.

Logan DC, Leaver CJ. 2000. Mitochondria-targeted GFP highlights the heterogeneity ofmitochondrial shape, size and movement within living plant cells. J Exp Bot 51:865–871.

Lowell BB, Shulman GI. 2005. Mitochondrial dysfunction and type 2 diabetes. Science307:384–387.

Lu PD, Harding HP, Ron D. 2004. Translation reinitiation at alternative open reading framesregulates gene expression in an integrated stress response. J Cell Biol 167:27–33.

Mak BC,WangQ, LaschingerC, LeeW, RonD, HardingHP, Kaufman RJ, ScheunerD, AustinRC, Mcculloch CA. 2008. Novel function of PERK as a mediator of force-inducedapoptosis. J Biol Chem 283:23462–23472.

Mancuso M, Orsucci D, Siciliano G, Murri L. 2008. Mitochondria, mitochondrial DNA andAlzheimer’s disease. What comes first? Curr Alzheimer Res 5:457–468.



Marciniak SJ, Yun CY, Oyadomari S, Novoa I, Zhang Y, Jungreis R, Nagata K, Harding HP,Ron D. 2004. CHOP induces death by promoting protein synthesis and oxidation in thestressed endoplasmic reticulum. Genes Dev 18:3066–3077.

Margineantu DH, Gregory Cox W, Sundell W, Sherwood SW, Beechem JM, Capaldi RA.2002. Cell cycle dependent morphology changes and associated mitochondrial DNAredistribution in mitochondria of human cell lines. Mitochondrion 1:425–435.

Martinez JA, Zhang Z, Svetlov SI, Hayes RL, Wang KK, Larner SF. 2010. Calpain and caspaseprocessing of caspase-12 contribute to the ER stress-induced cell death pathway indifferentiated PC12 cells. Apoptosis 15:1480–1493.

Mathai JP, Germain M, Shore GC. 2005. BH3-only BIK regulates BAX, BAK-dependentrelease of Ca2þ from endoplasmic reticulum stores and mitochondrial apoptosis duringstress-induced cell death. J Biol Chem 280:23829–23836.

Matsuda N, Tanaka K. 2010. Uncovering the roles of PINK1 and parkin in mitophagy.Autophagy 6:952–954.

MatsudaN, Sato S, Shiba K,Okatsu K, SaishoK, Gautier CA, SouYS, Saiki S, kawajiri S, Sato F,Kimura M, Komatsu M, Hattori N, Tanaka K. 2010. PINK1 stabilized by mitochondrialdepolarization recruits Parkin to damaged mitochondria and activates latent Parkin formitophagy. J Cell Biol 189:211–221.

Matsuzaki S, Hiratsuka T, Kuwahara R, Katayama T, Tohyama M. 2010. Caspase-4 is partiallycleaved by calpain via the impairment of Ca2þ homeostasis under the ER stress.Neurochem Int 56:352–356.

Matsuzawa A, Nishitoh H, Tobiume K, Takeda K, Ichijo H. 2002. Physiological roles ofASK1-mediated signal transduction in oxidative stress- and endoplasmic reticulumstress-induced apoptosis: Advanced findings from ASK1 knockout mice. Antioxid RedoxSignal 4:415–425.

Mccullough KD, Martindale JL, Klotz LO, Aw TY, Holbrook NJ. 2001. Gadd153 sensitizescells to endoplasmic reticulum stress by down-regulating Bcl2 and perturbing the cellularredox state. Mol Cell Biol 21:1249–1259.

Meares GP, Mines MA, Beurel E, Eom TY, Song L, Zmijewska AA, Jope RS. 2011. Glycogensynthase kinase-3 regulates endoplasmic reticulum (ER) stress-induced CHOP expressionin neuronal cells. Exp Cell Res 317:1621–1628.

Mendes CC, Gomes DA, Thompson M, Souto NC, Goes TS, Goes AM, Rodrigues MA,Gomez MV, Nathanson MH, Leite MF. 2005. The type III inositol 1,4,5-trisphosphatereceptor preferentially transmits apoptotic Ca2þ signals into mitochondria. J Biol Chem280:40892–40900.

Milhavet O, Martindale JL, Camandola S, Chan SL, Gary DS, Cheng A, Holbrook NJ, MattsonMP. 2002. Involvement of Gadd153 in the pathogenic action of presenilin-1 mutations.J Neurochem 83:673–681.

Minamino T, Kitakaze M. 2010. ER stress in cardiovascular disease. J Mol Cell Cardiol48:1105–1110.

Mizzen LA, Kabiling AN, Welch WJ. 1991. The two mammalian mitochondrial stressproteins, grp 75 and hsp 58, transiently interact with newly synthesized mitochondrialproteins. Cell Regul 2:165–179.

Morishima N, Nakanishi K, Takenouchi H, Shibata T, Yasuhiko Y. 2002. An endoplasmicreticulum stress-specific caspase cascade in apoptosis. Cytochrome c-independentactivation of caspase-9 by caspase-12. J Biol Chem 277:34287–34294.

Morishima N, Nakanishi K, Tsuchiya K, Shibata T, Seiwa E. 2004. Translocation of Bim to theendoplasmic reticulum (ER) mediates ER stress signaling for activation of caspase-12during ER stress-induced apoptosis. J Biol Chem 279:50375–50381.

Myhill N, Lynes EM,Nanji JA, BlagoveshchenskayaAD, Fei H, Carmine SimmenK, Cooper TJ,Thomas G, Simmen T. 2008. The subcellular distribution of calnexin is mediated by PACS-2. Mol Biol Cell 19:2777–2788.

Nakada K, Sato A, Hayashi J. 2009. Mitochondrial functional complementation inmitochondrial DNA-based diseases. Int J Biochem Cell Biol 41:1907–1913.

Nakagawa T, Yuan J. 2000. Cross-talk between two cysteine protease families. Activation ofcaspase-12 by calpain in apoptosis. J Cell Biol 150:887–894.

Nakagawa T, ZhuH,MorishimaN, Li E, Xu J, Yankner BA, Yuan J. 2000. Caspase-12mediatesendoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-beta. Nature403:98–103.

Narendra DP, Youle RJ. 2011. Targeting mitochondrial dysfunction: Role for PINK1 andParkin in mitochondrial quality control. Antioxid Redox Signal 14:1929–1938.

Narendra DP, Jin SM, Tanaka A, Suen DF, Gautier CA, Shen J, Cookson MR, Youle RJ. 2010.PINK1 is selectively stabilized on impaired mitochondria to activate Parkin. PLoS Biol 8:e1000298.

Neve EP, Ingelman-Sundberg M. 2010. Cytochrome P450 proteins: Retention anddistribution from the endoplasmic reticulum. Curr Opin Drug Discov Devel 13:78–85.

Ng FW, Nguyen M, Kwan T, Branton PE, Nicholson DW, Cromlish JA, Shore GC. 1997. p28Bap31, a Bcl-2/Bcl-XL- and procaspase-8-associated protein in the endoplasmic reticulum.J Cell Biol 139:327–338.

Nguyen M, Breckenridge DG, Ducret A, Shore GC. 2000. Caspase-resistant BAP31 inhibitsfas-mediated apoptotic membrane fragmentation and release of cytochrome c frommitochondria. Mol Cell Biol 20:6731–6740.

Nicotera P, Orrenius S. 1998. The role of calcium in apoptosis. Cell Calcium 23:173–180.NishitohH, SaitohM,Mochida Y, Takeda K,NakanoH, RotheM,MiyazonoK, IchijoH. 1998.ASK1 is essential for JNK/SAPK activation by TRAF2. Mol Cell 2:389–395.

Nishitoh H, Matsuzawa A, Tobiume K, Saegusa K, Takeda K, Inoue K, Hori S, Kakizuka A,Ichijo H. 2002. ASK1 is essential for endoplasmic reticulum stress-induced neuronal celldeath triggered by expanded polyglutamine repeats. Genes Dev 16:1345–1355.

Nutt LK, Pataer A, Pahler J, Fang B, Roth J, Mcconkey DJ, Swisher SG. 2002. Bax and Bakpromote apoptosis by modulating endoplasmic reticular and mitochondrial Ca2þ stores. JBiol Chem 277:9219–9225.

Oakes SA, Opferman JT, Pozzan T, Korsmeyer SJ, Scorrano L. 2003. Regulation ofendoplasmic reticulum Ca2þ dynamics by proapoptotic BCL-2 family members. BiochemPharmacol 66:1335–1340.

Ogata T, Yamasaki Y. 1997. Ultra-high-resolution scanning electron microscopy ofmitochondria and sarcoplasmic reticulum arrangement in human red, white, andintermediate muscle fibers. Anat Rec 248:214–223.

Oh KJ, Singh P, Lee K, Foss K, Lee S, Park M, Aluvila S, Kim RS, Symersky J,Walters DE. 2010.Conformational changes in BAK, a pore-forming proapoptotic Bcl-2 family member, uponmembrane insertion and direct evidence for the existence of BH3-BH3 contact interfacein BAK homo-oligomers. J Biol Chem 285:28924–28937.

OhokaN, Yoshii S, Hattori T, Onozaki K, Hayashi H. 2005. TRB3, a novel ER stress-induciblegene, is induced via ATF4-CHOP pathway and is involved in cell death. EMBO J 24:1243–1255.

Ojaimi J, Byrne E. 2001. Mitochondrial function and Alzheimer’s disease. Biol Signals Recept10:254–262.

Okada T, Yoshida H, Akazawa R, Negishi M, Mori K. 2002. Distinct roles of activatingtranscription factor 6 (ATF6) and double-stranded RNA-activated protein kinase-like

endoplasmic reticulum kinase (PERK) in transcription during the mammalian unfoldedprotein response. Biochem J 366:585–594.

Okatsu K, Saisho K, Shimanuki M, Nakada K, Shitara H, Sou YS, Kimura M, Sato S, Hattori N,Komatsu M, Tanaka K, Matsuda N. 2010. p62/SQSTM1 cooperates with Parkin forperinuclear clustering of depolarized mitochondria. Genes Cells 15:887–900.

OlaMS, NawazM, AhsanH. 2011. Role of Bcl-2 family proteins and caspases in the regulationof apoptosis. Mol Cell Biochem 351:41–58.

Olivari S, Molinari M. 2007. Glycoprotein folding and the role of EDEM1, EDEM2 and EDEM3in degradation of folding-defective glycoproteins. FEBS Lett 581:3658–3664.

Orso G, Pendin D, Liu S, Tosetto J, Moss TJ, Faust JE, Micaroni M, Egorova A, Martinuzzi A,Mcnew JA, Daga A. 2009. Homotypic fusion of ER membranes requires the dynamin-likeGTPase atlastin. Nature 460:978–983.

Otera H, Mihara K. 2011. Discovery of the membrane receptor for mitochondrial fissionGTPase Drp1. Small GTPases 2:167–172.

Otera H, Wang C, Cleland MM, Setoguchi K, Yokota S, Youle RJ, Mihara K. 2010. Mff is anessential factor for mitochondrial recruitment of Drp1 during mitochondrial fission inmammalian cells. J Cell Biol 191:1141–1158.

Ozcan U, Cao Q, Yilmaz E, Lee AH, Iwakoshi NN, Ozdelen E, Tuncman G, Gorgun C,Glimcher LH, Hotamisligil GS. 2004. Endoplasmic reticulum stress links obesity, insulinaction, and type 2 diabetes. Science 306:457–461.

Palmer CS, Osellame LD, Stojanovski D, Ryan MT. 2011. The regulation of mitochondrialmorphology: Intricate mechanisms and dynamic machinery. Cell Signal 23:1534–1545.

Papa FR. 2012. Endoplasmic reticulum stress, pancreatic beta-cell degeneration, anddiabetes. Cold Spring Harb Perspect Med 2:a007666.

Park SJ, Shin JH, Kim ES, Jo YK, Kim JH, Hwang JJ, Kim JC, Cho DH. 2012. Mitochondrialfragmentation caused by phenanthroline promotes mitophagy. FEBS Lett 586:4303–4310.

Parone PA, Martinou JC. 2006. Mitochondrial fission and apoptosis: An ongoing trial.Biochim Biophys Acta 1763:522–530.

Parone PA, Da Cruz S, Tondera D, Mattenberger Y, James DI, Maechler P, Barja F, MartinouJC. 2008. Preventingmitochondrial fission impairs mitochondrial function and leads to lossof mitochondrial DNA. PLoS ONE 3:e3257.

Patel S, Joseph SK, Thomas AP. 1999. Molecular properties of inositol 1,4,5-trisphosphatereceptors. Cell Calcium 25:247–264.

Patterson RL, Boehning D, Snyder SH. 2004. Inositol 1,4,5-trisphosphate receptors as signalintegrators. Annu Rev Biochem 73:437–465.

Pereira C, Ferreiro E, Cardoso SM, De Oliveira CR. 2004. Cell degeneration induced byamyloid-beta peptides: Implications for Alzheimer’s disease. J Mol Neurosci 23:97–104.

Petronilli V, Miotto G, Canton M, Brini M, Colonna R, Bernardi P, Di Lisa F. 1999. Transientand long-lasting openings of the mitochondrial permeability transition pore can bemonitored directly in intact cells by changes in mitochondrial calcein fluorescence.Biophys J 76:725–734.

Piccini M, Vitelli F, Bruttini M, Pober BR, Jonsson JJ, Villanova M, Zollo M, Borsani G, BallabioA, Renieri A. 1998. FACL4, a new gene encoding long-chain acyl-CoA synthetase 4, isdeleted in a family with Alport syndrome, elliptocytosis, and mental retardation.Genomics 47:350–358.

Pinton P, Giorgi C, Siviero R, Zecchini E, Rizzuto R. 2008. Calcium and apoptosis:ER-mitochondria Ca2þ transfer in the control of apoptosis. Oncogene 27:6407–6418.

Pitts KR, Mcniven MA, Yoon Y. 2004. Mitochondria-specific function of the dynamin familyprotein DLP1 is mediated by its C-terminal domains. J Biol Chem 279:50286–50294.

Polke JM, Laura M, Pareyson D, Taroni F, Milani M, Bergamin G, Gibbons VS, Houlden H,Chamley SC, Blake J, Devile C, Sandford R, Sweeney MG, Davis MB, Reilly MM. 2011.Recessive axonal Charcot-Marie-Tooth disease due to compound heterozygousmitofusin2 mutations. Neurology 77:168–173.

Pollard MG, Travers KJ, Weissman JS. 1998. Ero1p: A novel and ubiquitous protein with anessential role in oxidative protein folding in the endoplasmic reticulum. Mol Cell 1:171–182.

Pous C, Codogno P. 2011. Lysosome positioning coordinates mTORC1 activity andautophagy. Nat Cell Biol 13:342–344.

Pradelli LA, Beneteau M, Ricci JE. 2010. Mitochondrial control of caspase-dependent and-independent cell death. Cell Mol Life Sci 67:1589–1597.

Puthalakath H, O’reilly LA, Gunn P, Lee L, Kelly PN, Huntington ND, Hughes PD, MichalakEM, Mckimm-Breschkin J, Motoyama N, Gotoh T, Akira S, Bouillet P, Strasser A. 2007. ERstress triggers apoptosis by activating BH3-only protein Bim. Cell 129:1337–1349.

Qi X, Mochly-Rosen D. 2008. The PKCdelta–Abl complex communicates ER stress to themitochondria—An essential step in subsequent apoptosis. J Cell Sci 121:804–813.

Rambold AS, Lippincott-Schwartz J. 2011. Mechanisms of mitochondria and autophagycrosstalk. Cell Cycle 10:4032–4038.

Rao RV, Hermel E, Castro-Obregon S, Del Rio G, Ellerby LM, Ellerby HM, Bredesen DE.2001. Coupling endoplasmic reticulum stress to the cell death program. Mechanism ofcaspase activation. J Biol Chem 276:33869–33874.

Rao RV, Castro-Obregon S, Frankowski H, Schuler M, Stoka V, Del Rio G, Bredesen DE,Ellerby HM. 2002a. Coupling endoplasmic reticulum stress to the cell death program. AnApaf-1-independent intrinsic pathway. J Biol Chem 277:21836–21842.

Rao RV, Peel A, Logvinova A, Del Rio G, Hermel E, Yokota T, Goldsmith PC, Ellerby LM,Ellerby HM, Bredesen DE. 2002b. Coupling endoplasmic reticulum stress to the cell deathprogram: Role of the ER chaperone GRP78. FEBS Lett 514:122–128.

Rao RV, Ellerby HM, Bredesen DE. 2004. Coupling endoplasmic reticulum stress to the celldeath program. Cell Death Differ 11:372–380.

Rapizzi E, Pinton P, Szabadkai G,Wieckowski MR, Vandecasteele G, Baird G, Tuft RA, FogartyKE, Rizzuto R. 2002. Recombinant expression of the voltage-dependent anion channelenhances the transfer of Ca2þ microdomains to mitochondria. J Cell Biol 159:613–624.

Rasheva VI, Domingos PM. 2009. Cellular responses to endoplasmic reticulum stress andapoptosis. Apoptosis 14:996–1007.

Reis Y, Bernardo-Faura M, Richter D,Wolf T, Brors B, Hamacher-Brady A, Eils R, Brady NR.2012. Multi-parametric analysis and modeling of relationships between mitochondrialmorphology and apoptosis. PLoS ONE 7:e28694.

Rieusset J. 2011. Mitochondria and endoplasmic reticulum: Mitochondria-endoplasmicreticulum interplay in type 2 diabetes pathophysiology. Int J Biochem Cell Biol 43:1257–1262.

Rizzuto R, Pinton P, CarringtonW, Fay FS, Fogarty KE, Lifshitz LM, Tuft RA, Pozzan T. 1998.Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2þ

responses. Science 280:1763–1766.Rodriguez D, Rojas-Rivera D, Hetz C. 2011. Integrating stress signals at the endoplasmicreticulum: The BCL-2 protein family rheostat. Biochim Biophys Acta 1813:564–574.

Ron D, Walter P. 2007. Signal integration in the endoplasmic reticulum unfolded proteinresponse. Nat Rev Mol Cell Biol 8:519–529.

Rossignol R, Gilkerson R, Aggeler R, Yamagata K, Remington SJ, Capaldi RA. 2004. Energysubstrate modulates mitochondrial structure and oxidative capacity in cancer cells.Cancer Res 64:985–993.



Rutkowski DT, Kaufman RJ. 2004. A trip to the ER: Coping with stress. Trends Cell Biol14:20–28.

Saleh M, Mathison JC, Wolinski MK, Bensinger SJ, Fitzgerald P, Droin N, Ulevitch RJ, GreenDR, Nicholson DW. 2006. Enhanced bacterial clearance and sepsis resistance in caspase-12-deficient mice. Nature 440:1064–1068.

Santel A, Fuller MT. 2001. Control of mitochondrial morphology by a humanmitofusin. J CellSci 114:867–874.

Santel A, Frank S, Gaume B, Herrler M, Youle RJ, Fuller MT. 2003. Mitofusin-1 protein is agenerally expressed mediator of mitochondrial fusion in mammalian cells. J Cell Sci116:2763–2774.

Sauner MT, Levy M. 1971. Study of the transfer of phospholipids from the endoplasmicreticulum to the outer and inner mitochondrial membranes. J Lipid Res 12:71–75.

Scheuner D, Eckman C, Jensen M, Song X, Citron M, Suzuki N, Bird TD, Hardy J, Hutton M,KukullW, Larson E, Levy-Lahad E, Viitanen M, Peskind E, Poorkaj P, Schellenberg G, TanziR,WascoW, Lannfelt L, SelkoeD, Younkin S. 1996. Secreted amyloid beta-protein similarto that in the senile plaques of Alzheimer’s disease is increased in vivo by the presenilin 1and 2 and APP mutations linked to familial Alzheimer’s disease. Nat Med 2:864–870.

Schroder M. 2008. Endoplasmic reticulum stress responses. Cell Mol Life Sci 65:862–894.Schroder M, Kaufman RJ. 2005. ER stress and the unfolded protein response. Mutat Res

569:29–63.Scorrano L, Oakes SA, Opferman JT, Cheng EH, Sorcinelli MD, Pozzan T, Korsmeyer SJ.

2003. BAX and BAK regulation of endoplasmic reticulum Ca2þ: A control point forapoptosis. Science 300:135–139.

Scott I, Youle RJ. 2010. Mitochondrial fission and fusion. Essays Biochem 47:85–98.Selimovic D, Ahmad M, El-Khattouti A, Hannig M, Haikel Y, Hassan M. 2011. Apoptosis-

related protein-2 triggers melanoma cell death by a mechanism including bothendoplasmic reticulum stress and mitochondrial dysregulation. Carcinogenesis 32:1268–1278.

Shen J, Prywes R. 2004. Dependence of site-2 protease cleavage of ATF6 on prior site-1protease digestion is determined by the size of the luminal domain of ATF6. J Biol Chem279:43046–43051.

Shibata Y, Voss C, Rist JM, Hu J, Rapoport TA, PrinzWA, Voeltz GK. 2008. The reticulon andDP1/Yop1p proteins form immobile oligomers in the tubular endoplasmic reticulum. J BiolChem 283:18892–18904.

Shibata Y, Shemesh T, Prinz WA, Palazzo AF, Kozlov MM, Rapoport TA. 2010. Mechanismsdetermining the morphology of the peripheral ER. Cell 143:774–788.

Simmen T, Aslan JE, Blagoveshchenskaya AD, Thomas L, Wan L, Xiang Y, Feliciangeli SF,Hung CH, Crump CM, Thomas G. 2005. PACS-2 controls endoplasmic reticulum-mitochondria communication and Bid-mediated apoptosis. EMBO J 24:717–729.

Smirnova E, ShurlandDL, Ryazantsev SN, VanDer Bliek AM. 1998. A human dynamin-relatedprotein controls the distribution of mitochondria. J Cell Biol 143:351–358.

Snyder SH, Largent BL. 1989. Receptor mechanisms in antipsychotic drug action: Focus onsigma receptors. J Neuropsychiatry Clin Neurosci 1:7–15.

Song Z, Ghochani M, Mccaffery JM, Frey TG, Chan DC. 2009. Mitofusins and OPA1 mediatesequential steps in mitochondrial membrane fusion. Mol Biol Cell 20:3525–3532.

Springer W, Kahle PJ. 2011. Regulation of PINK1-Parkin-mediated mitophagy. Autophagy7:266–278.

Stiban J, Caputo L, Colombini M. 2008. Ceramide synthesis in the endoplasmic reticulum canpermeabilize mitochondria to proapoptotic proteins. J Lipid Res 49:625–634.

Stojanovski D, Koutsopoulos OS, Okamoto K, Ryan MT. 2004. Levels of human Fis1 at themitochondrial outer membrane regulate mitochondrial morphology. J Cell Sci 117:1201–1210.

Stone SJ, Vance JE. 2000. Phosphatidylserine synthase-1 and -2 are localized tomitochondria-associated membranes. J Biol Chem 275:34534–34540.

Storey E, Cappai R. 1999. The amyloid precursor protein of Alzheimer’s disease and theAbeta peptide. Neuropathol Appl Neurobiol 25:81–97.

Su TP, London ED, Jaffe JH. 1988. Steroid binding at sigma receptors suggests a link betweenendocrine, nervous, and immune systems. Science 240:219–221.

Suen KC, Lin KF, ElyamanW, So KF, Chang RC, Hugon J. 2003. Reduction of calcium releasefrom the endoplasmic reticulum could only provide partial neuroprotection against beta-amyloid peptide toxicity. J Neurochem 87:1413–1426.

Sugioka R, Shimizu S, Tsujimoto Y. 2004. Fzo1, a protein involved in mitochondrial fusion,inhibits apoptosis. J Biol Chem 279:52726–52734.

Susin SA, Lorenzo HK, Zamzami N, Marzo I, Snow BE, Brothers GM, Mangion J, Jacotot E,Costantini P, Loeffler M, Larochette N, Goodlett DR, Aebersold R, Siderovski DP,Penninger JM, Kroemer G. 1999. Molecular characterization of mitochondrial apoptosis-inducing factor. Nature 397:441–446.

Suzuki M, Jeong SY, Karbowski M, Youle RJ, Tjandra N. 2003. The solution structure ofhuman mitochondria fission protein Fis1 reveals a novel TPR-like helix bundle. J Mol Biol334:445–458.

Szabadkai G, Simoni AM, Chami M, Wieckowski MR, Youle RJ, Rizzuto R. 2004.Drp-1-dependent division of the mitochondrial network blocks intraorganellar Ca2þ

waves and protects against Ca2þ-mediated apoptosis. Mol Cell 16:59–68.Szabadkai G, Bianchi K, Varnai P, De Stefani D,Wieckowski MR, CavagnaD, Nagy AI, Balla T,

Rizzuto R. 2006. Chaperone-mediated coupling of endoplasmic reticulum andmitochondrial Ca2þ channels. J Cell Biol 175:901–911.

Szegezdi E, Logue SE, Gorman AM, Samali A. 2006. Mediators of endoplasmic reticulumstress-induced apoptosis. EMBO Rep 7:880–885.

Takayanagi S, Fukuda R, Takeuchi Y, Tsukada S, Yoshida K. 2013. Gene regulatory networkof unfolded protein response genes in endoplasmic reticulum stress. Cell StressChaperones 18:11–23.

Takuma K, Yan SS, Stern DM, Yamada K. 2005. Mitochondrial dysfunction, endoplasmicreticulum stress, and apoptosis in Alzheimer’s disease. J Pharmacol Sci 97:312–316.

Tan Y, Dourdin N,WuC, De Veyra T, Elce JS, Greer PA. 2006. Ubiquitous calpains promotecaspase-12 and JNK activation during endoplasmic reticulum stress-induced apoptosis.J Biol Chem 281:16016–16024.

Terasaki M, Chen LB, Fujiwara K. 1986. Microtubules and the endoplasmic reticulum arehighly interdependent structures. J Cell Biol 103:1557–1568.

Thomas KJ, Jacobson MR. 2012. Defects in mitochondrial fission protein dynamin-relatedprotein 1 are linked to apoptotic resistance and autophagy in a lung cancer model. PLoSONE 7:e45319.

Tolkovsky AM. 2009. Mitophagy. Biochim Biophys Acta 1793:1508–1515.Tsai SY, Hayashi T, Mori T, Su TP. 2009. Sigma-1 receptor chaperones and diseases. Cent

Nerv Syst Agents Med Chem 9:184–189.Tu BP, Weissman JS. 2004. Oxidative protein folding in eukaryotes: Mechanisms and

consequences. J Cell Biol 164:341–346.Twig G, Shirihai OS. 2011. The interplay between mitochondrial dynamics and mitophagy.

Antioxid Redox Signal 14:1939–1951.

Twig G, Elorza A, Molina AJ, MohamedH,Wikstrom JD,Walzer G, Stiles L, Haigh SE, Katz S,Las G, Alroy J, Wu M, Py BF, Yuan J, Deeney JT, Corkey BE, Shirihai OS. 2008. Fission andselective fusion govern mitochondrial segregation and elimination by autophagy. EMBO J27:433–446.

Uchibayashi R, Tsuruma K, Inokuchi Y, Shimazawa M, Hara H. 2011. Involvement of Bid andcaspase-2 in endoplasmic reticulum stress- and oxidative stress-induced retinal ganglioncell death. J Neurosci Res 89:1783–1794.

Uemura A,OkuM,Mori K, Yoshida H. 2009. Unconventional splicing of XBP1mRNAoccursin the cytoplasm during the mammalian unfolded protein response. J Cell Sci 122:2877–2886.

Upton JP, Austgen K, Nishino M, Coakley KM, Hagen A, Han D, Papa FR, Oakes SA. 2008.Caspase-2 cleavage of BID is a critical apoptotic signal downstream of endoplasmicreticulum stress. Mol Cell Biol 28:3943–3951.

Urano F, Wang X, Bertolotti A, Zhang Y, Chung P, Harding HP, Ron D. 2000. Coupling ofstress in the ER to activation of JNK protein kinases by transmembrane protein kinaseIRE1. Science 287:664–666.

Van Rossum DB, Patterson RL, Kiselyov K, Boehning D, Barrow RK, Gill DL, Snyder SH.2004. Agonist-induced Ca2þ entry determined by inositol 1,4,5-trisphosphaterecognition. Proc Natl Acad Sci USA 101:2323–2327.

Vance JE. 1990. Phospholipid synthesis in amembrane fraction associatedwithmitochondria.J Biol Chem 265:7248–7256.

Vance JE. 2008. Phosphatidylserine and phosphatidylethanolamine in mammalian cells: Twometabolically related aminophospholipids. J Lipid Res 49:1377–1387.

Varadi A, Johnson-Cadwell LI, Cirulli V, Yoon Y, Allan VJ, Rutter GA. 2004. Cytoplasmicdynein regulates the subcellular distribution of mitochondria by controlling therecruitment of the fission factor dynamin-related protein-1. J Cell Sci 117:4389–4400.

Vattem KM, Wek RC. 2004. Reinitiation involving upstream ORFs regulates ATF4 mRNAtranslation in mammalian cells. Proc Natl Acad Sci USA 101:11269–11274.

Venkatesh S, Lee J, Singh K, Lee I, Suzuki CK. 2012. Multitasking in the mitochondrion by theATP-dependent Lon protease. Biochim Biophys Acta 1823:56–66.

Voelker DR. 1990. Characterization of phosphatidylserine synthesis and translocation inpermeabilized animal cells. J Biol Chem 265:14340–14346.

Wadhwa R, Taira K, Kaul SC. 2002. An Hsp70 family chaperone, mortalin/mthsp70/PBP74/Grp75: What, when, and where? Cell Stress Chaperones 7:309–316.

Wang C, Youle RJ. 2009. The role of mitochondria in apoptosis� . Annu Rev Genet 43:95–118.

Wasilewski M, Scorrano L. 2009. The changing shape of mitochondrial apoptosis. TrendsEndocrinol Metab 20:287–294.

Waterman-Storer CM, Salmon ED. 1998. Endoplasmic reticulum membrane tubules aredistributed by microtubules in living cells using three distinct mechanisms. Curr Biol8:798–806.

Wei MC, Zong WX, Cheng EH, Lindsten T, Panoutsakopoulou V, Ross AJ, Roth KA,Macgregor GR, Thompson CB, Korsmeyer SJ. 2001. Proapoptotic BAX and BAK: Arequisite gateway to mitochondrial dysfunction and death. Science 292:727–730.

Wek RC, Cavener DR. 2007. Translational control and the unfolded protein response.Antioxid Redox Signal 9:2357–2371.

Wild S, Roglic G, Green A, Sicree R, King H. 2004. Global prevalence of diabetes: Estimatesfor the year 2000 and projections for 2030. Diabetes Care 27:1047–1053.

Wood DE, Thomas A, Devi LA, Berman Y, Beavis RC, Reed JC, Newcomb EW. 1998. Baxcleavage is mediated by calpain during drug-induced apoptosis. Oncogene 17:1069–1078.

Yadav S, Puri S, Linstedt AD. 2009. A primary role for Golgi positioning in directed secretion,cell polarity, and wound healing. Mol Biol Cell 20:1728–1736.

Yaffe MP. 1999. The machinery of mitochondrial inheritance and behavior. Science283:1493–1497.

Yamamuro A, Kishino T, Ohshima Y, Yoshioka Y, Kimura T, Kasai A, Maeda S. 2011.Caspase-4 directly activates caspase-9 in endoplasmic reticulum stress-induced apoptosisin SH-SY5Y cells. J Pharmacol Sci 115:239–243.

Ye J, Rawson RB, Komuro R, Chen X, Dave UP, Prywes R, BrownMS, Goldstein JL. 2000. ERstress induces cleavage of membrane-bound ATF6 by the same proteases that processSREBPs. Mol Cell 6:1355–1364.

Yin KJ, Lee JM, Chen SD, Xu J, Hsu CY. 2002. Amyloid-beta induces Smac release viaAP-1/Bim activation in cerebral endothelial cells. J Neurosci 22:9764–9770.

Yin KJ, Kim GM, Lee JM, He YY, Xu J, Hsu CY. 2005. JNK activation contributes to DP5induction and apoptosis following traumatic spinal cord injury. Neurobiol Dis 20:881–889.

Yoneda T, Imaizumi K, Oono K, Yui D, Gomi F, Katayama T, TohyamaM. 2001. Activation ofcaspase-12, an endoplastic reticulum (ER) resident caspase, through tumor necrosis factorreceptor-associated factor 2-dependent mechanism in response to the ER stress. J BiolChem 276:13935–13940.

Yoon Y, Pitts KR, Mcniven MA. 2001. Mammalian dynamin-like protein DLP1 tubulatesmembranes. Mol Biol Cell 12:2894–2905.

Yoon Y, Krueger EW, Oswald BJ, Mcniven MA. 2003. The mitochondrial protein hFis1regulates mitochondrial fission in mammalian cells through an interaction with thedynamin-like protein DLP1. Mol Cell Biol 23:5409–5420.

Yoshida H, Matsui T, Yamamoto A, Okada T, Mori K. 2001. XBP1mRNA is induced by ATF6and spliced by IRE1 in response to ER stress to produce a highly active transcription factor.Cell 107:881–891.

Yoshida H, Matsui T, Hosokawa N, Kaufman RJ, Nagata K, Mori K. 2003. A time-dependentphase shift in the mammalian unfolded protein response. Dev Cell 4:265–271.

Yuan H, Gerencser AA, Liot G, Lipton SA, Ellisman M, Perkins GA, Bossy-Wetzel E. 2007.Mitochondrial fission is an upstream and required event for bax foci formation in responseto nitric oxide in cortical neurons. Cell Death Differ 14:462–471.

Yukioka F, Matsuzaki S, Kawamoto K, Koyama Y, Hitomi J, Katayama T, Tohyama M. 2008.Presenilin-1 mutation activates the signaling pathway of caspase-4 in endoplasmicreticulum stress-induced apoptosis. Neurochem Int 52:683–687.

Yussman MG, Toyokawa T, Odley A, Lynch RA,Wu G, Colbert MC, Aronow BJ, Lorenz JN,DornGW II. 2002. Mitochondrial death proteinNix is induced in cardiac hypertrophy andtriggers apoptotic cardiomyopathy. Nat Med 8:725–730.

Zhang L, Wang A. 2012. Virus-induced ER stress and the unfolded protein response. FrontPlant Sci 3:293.

Zhu PP, Patterson A, Stadler J, Seeburg DP, Sheng M, Blackstone C. 2004. Intra- andintermolecular domain interactions of the C-terminal GTPase effector domain of themultimeric dynamin-like GTPase Drp1. J Biol Chem 279:35967–35974.

Zinszner H, KurodaM,WangX, BatchvarovaN, Lightfoot RT, Remotti H, Stevens JL, RonD.1998. CHOP is implicated in programmed cell death in response to impaired function ofthe endoplasmic reticulum. Genes Dev 12:982–995.

Ziviani E, Whitworth AJ. 2010. How could Parkin-mediated ubiquitination of mitofusinpromote mitophagy? Autophagy 6:660–662.



functional and morphological impact of er stress on...

Documents