integrins and extracellular matrix in...

Integrins and Extracellular Matrixin Mechanotransduction

Martin Alexander Schwartz

Departments of Microbiology, Cell Biology, and Biomedical Engineering, University of Virginia,Charlottesville, Virginia 22908

Correspondence: [email protected]

Integrins bind extracellular matrix fibrils and associate with intracellular actin filamentsthrough a variety of cytoskeletal linker proteins to mechanically connect intracellular andextracellular structures. Each component of the linkage from the cytoskeleton through theintegrin-mediated adhesions to the extracellular matrix therefore transmits forces that mayderive from both intracellular, myosin-generated contractile forces and forces from outsidethe cell. These forces activate a wide range of signaling pathways and genetic programs tocontrol cell survival, fate, and behavior. Additionally, cells sense the physical properties oftheir surrounding environment through forces exerted on integrin-mediated adhesions.This article first summarizes current knowledge about regulation of cell function by mechan-ical forces acting through integrin-mediated adhesions and then discusses models formechanotransduction and sensing of environmental forces.

The fact that multicellular organisms canresist the wide range of physical forces en-

countered in nature requires appropriate me-chanical connectivity within each tissue. Forexample, skin and other epithelial tissues havekeratin networks that connect across desmo-somes, so that the entire cell layer is mechani-cally integrated (Uitto 2009). Their adhesionto the basement membrane also involves con-nection of cytoplasmic filaments (both actinand intermediate filaments) across the mem-brane to the extracellular matrix (ECM). Ge-netic defects in any of the key componentslead to mechanical fragility and defects suchas blistering of these tissues, severe versions ofwhich are lethal (Aumailley et al. 2006).

A key design feature inherent in all complexorganisms is their ability to modulate mechan-ical strength of tissues to match the forces en-countered. Artery wall thickness is determinedby blood pressure and vessel diameter: The ves-sel wall thickens or thins so that the force perunit of wall is constant (so called, LaPlace’sLaw) (Schiffrin 1992). Arteries also remodel inresponse to changes in fluid flow so that vesseldiameters are well matched to the volume offlowing blood (Schaper 2009). Bone formationand turnover are regulated by weight-bearingexercise such that bone density increases underhigher loads and decreases under lower loading(Robling et al. 2006). This strategy makes forefficient resource allocation because tissues are

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built to be as strong as they need to be, not un-like “on demand” stocking of supplies in ware-houses. But when tissues are subjected to forcesthat are too high, these regulatory mechanismscan also contribute to disease. Over-inflationof the lung triggers ventilator injury, a seriouscomplication for patients on ventilators (Lion-etti et al. 2005). Hypertension is a major riskfactor for atherosclerosis, as well as aneurysm(Hahn and Schwartz 2009; Krishna et al. 2010).

Embryonic development also depends onsensing mechanical forces. Morphogenesis re-quires that each tissue assume its correct shapeand size, specified through complex interac-tions among cell adhesion, cytoskeleton, solu-ble factors, and gene expression programs. It isalmost impossible to envision how cells coulddetermine the macroscopic shape and size ofan organ without input from mechanical forcesexerted on the tissue; certainly organ size orshape are not encoded directly in our genes.Recent evidence supports this view. In Droso-phila embryos, pressure on the anterior foregutand stomodeal primordium from the extendinggerm band appears to activate b-catenin signal-ing and subsequent expression of mesenchymalgenes such as Twist (Farge 2003). Migration ofDrosophila border cells from the anterior regionto the developing oocytes requires nuclear trans-location of the transcription factor Mal-D,which appears to be induced by stretching ofthe cells (Somogyi and Rorth 2004). In mouseembryos, remodeling of the primitive yolk sacvascular plexus requires fluid shear stress fromflowing blood (Lucitti et al. 2007). Correct loop-ing of the outflow tract from the heart alsodepends on the shear stress from blood flowingout of the heart (Yashiro et al. 2007).

It is obvious that the structures that bear theforce should be involved in sensing it. Everycomponent of the mechanical linkages betweenthe ECM, integrins, and the cytoskeleton istherefore a candidate mechanotransducer. In-deed, the participation of many componentsof the ECM-integrin-cytoskeleton linkage inmechanotransduction has been borne out by awide range of experimental data and consider-able insight has been obtained into molecularand biophysical mechanisms. This review will

cover mechanotransduction by each of thesecomponents and their relationship to cellphysiology.

CYTOSKELETAL CONNECTIONS

Formation of adhesions by integrins involvesboth binding to ECM proteins and linkageto the actin cytoskeleton, two processes thatshow highly cooperative behavior (Burridgeand Chrzanowska-Wodnicka 1996). In culture,the adhesions have been divided into severaltypes (Zaidel-Bar et al. 2004; Geiger and Yamada2011). Focal adhesions are large, elongated struc-tures, typically approximately 2 mm wide and3–10 mm long, in which clustered integrinsbind ECM fibrils on the outside of the cell andconnect on the inside to contractile actomyosinstress fiber bundles (Fig. 1). Focal adhesionsdepend on activation of the small GTPase Rhoand can be found distributed across the lowersurface of the cell. Cells also contain focal com-plexes that have similar compositions but aresmaller and circular, typically around 2 mmin diameter. They also connect to actomyosinand are force-dependent but are under activelyprotruding cell edges and require activationof Cdc42 or Rac instead of Rho. Migrating cellsalso contain very small nascent adhesionsthat form right behind protruding lamellipo-dial edges (Choi et al. 2008). These adhesionsare below the spatial resolution of conventionallight microscopes and are myosin-indepen-dent; instead, they require continual rearward-flowing actin, as occurs at active cell edges.Nascent adhesions appear to mature to focalcomplexes on association with actomyosin,whereas focal complexes mature to focal adhe-sions on association with large actin stressfibers. Other types of adhesions such as podo-somes and fibrillar adhesions also link ECMproteins to actin, and some evidence suggeststhey can participate in mechanotransduction(Collin et al. 2008). However, less is knownabout mechanotransduction in these structuresand they will not be discussed further here.

The linkage from integrins to actin is ac-complished by a complex set of interactions(Liu et al. 2000; Wegener and Campbell 2008;

M.A. Schwartz

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Wickstrom et al. 2010; Zaidel-Bar and Geiger2010). The best understood linkage is throughtalin, whose head domain binds sequencesin b integrin cytoplasmic domains, whereasits tail domain binds actin both directly andindirectly through vinculin (Fig. 1A). Integrinscan also associate with F-actin through a-acti-nin, which binds b tails directly as well as bind-ing vinculin; through filamin, which bindsdirectly to both integrin b tails and F-actin;and through the actin-binding protein acto-paxin, which binds paxillin and ILK (Fig. 1B).Integrin b4 can associate with intermediate fil-aments but their relationship to mechanotrans-duction has not been much explored, thus, willnot be discussed further.

CELLULAR RESPONSES TO FORCE

Forces on cells and tissues can be applied fromexternal sources, as in stretching of artery wallsby pressure from the blood, or by the cells’ ownactomyosin (or other motors). Where they havebeen compared, the effects of internal versusexternal forces were similar, indicating thatforce per se across specific structures is what issensed, independent of its origin. This pointhas been shown elegantly with focal adhesions,in which tension from the actin stress fiberscould be replaced by a glass pipet pulling thecell body (Riveline et al. 2001). Stretching cellson an elastic substratum also has effects on theadhesions similar to the effects of stimulation

Fibronectin

IntegrinActin

Linkerprotein

Myosin

Binding

Dragging UnbindingA

B C

Stiff substrate

Dragging

D

Soft substrate

Unbinding

E

Key:

Substrate

Figure 1. A model for stiffness sensing. (A) The lamellipodial edge has actin that is moving backward because of acombination of the force from polymerization pushing against the plasma membrane and pulling force frommyosin further back in the cell. Integrins are bound to fibronectin or other ECM proteins adsorbed to the sub-strate or incorporated into the insoluble matrix, and connect to the actin through linker proteins such as talinand vinculin. (B) Binding of the linker to the actin triggers a rapid increase in tension and, assuming slip bondbehavior, dissociation of the linkage (C). (D–E) On a soft substrate, tension is applied in the same way butdeformation of the substrate allows the fibronectin and integrin to move backward too. Thus, force build-upslows and the interaction is prolonged. One can propose variations on this theme. For example, if linkagesare catch bonds, on soft substrata, the slow onset of tension may lead to a shorter lifetime for the bound state.The central idea is that the clutch mechanism allows the stiffness of the substratum to control the timing of theprotein-protein interactions, which can lead to changes in signaling.

Integrins and Extracellular Matrix in Mechanotransduction

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of myosin activation, for example, focal adhe-sions enlarge and FAK is activated (Hamasakiet al. 1995; Sai et al. 1999). The following dis-cussion therefore does not distinguish betweenendogenous and exogenous forces.

The major response of adhesions to force isstrengthening or reinforcement, in which theadhesions enlarge or recruit new cytoskeletalproteins that help resist the applied force. Focaladhesions only form in the first place when con-nected to highly contractile actin stress fibers,and treatment with myosin inhibitors causestheir rapid disassembly (Chrzanowska-Wod-nicka and Burridge 1996). Focal complexes,though smaller and not associated with largeactin bundles, are also myosin-dependent (Choiet al. 2008). Focal complexes form from smallerso-called nascent adhesions only when theyconnect to the actin-myosin network, and myo-sin inhibitors cause replacement of focal com-plexes with nascent adhesions. Measurementof traction forces exerted by cells on their sub-stratum over a wide range of conditions revealedthat force per unit area of focal adhesion/focalcomplex was constant at approximately 5.5nN/mm2 (Balaban et al. 2001). Changes in ad-hesion size occurred rapidly after altering forces(seconds to minutes), strongly arguing for effi-cient homeostatic control mechanisms (Bala-ban et al. 2001). However, these responses toforces clearly occur at multiple levels and affecteach component of the physical linkage betweenthe ECM and the cytoskeleton. We thereforediscuss each one, starting from outside the celland working in.

ECM

Cells respond to force on integrin-mediated ad-hesions by remodeling the ECM. For example,cyclic stretching of fibroblasts and other celltypes activates expression of genes for collagens(coll), fibronectin (FN), and metalloproteinases(MMPs), and stretched cells assemble a denseECM that is enriched in collagen (Chiquetet al. 2003). Matrix assembly usually occurs ina directional manner according to the appliedforce. Collagen and fibronectin fibrils bothtend to align along the same axis as the force

(Nguyen et al. 2009). Fibronectin assembly itselfis mechanically regulated (Zhong et al. 1998;Gao et al. 2003). Current models propose thatFN initially binds through an integrin, then issubject to tension from actomyosin. Pullingon FN opens folded domains to reveal crypticbinding sites that promote its assembly intofibrils. Stretch-induced FN-FN binding was di-rectly demonstrated (Zhong et al. 1998), as wasextension of FN fibrils under contractile forcefrom cells during matrix assembly (Baneyxet al. 2002). Conversely, inhibiting cell contrac-tility causes loss of the FN matrix. There is alsoevidence that cells actively arrange their fibrillarcollagens in a force-dependent manner (Cantyet al. 2006).

Though less well studied, there are data sug-gesting that assembly of basement membranesconsisting of laminins and type IV collagen isalso sensitive to the mechanics of the envi-ronment (Streuli and Bissell 1990). These dataindicate at least two levels of regulation. First,expression of genes for both ECM proteins andECM-remodeling enzymes (metalloproteinasesand inhibitors thereof, etc.) are turned on oroff by exposure of cells to mechanical stressesor by modulation of actomyosin contractility(Chiquet et al. 2003). In many cases, this regu-lation appears to occur through signaling path-ways downstream of the integrins themselves(Katsumi et al. 2004). Second, the polymeriza-tion or organization of ECM fibrils is influencedmore directly by mechanical forces. Cells ac-tively lay down their own matrix in an orderedfashion, and forces exerted through matrix re-ceptors control these processes (Canty et al.2006). The net result in most cases is productionof ECM that can resist the applied forces (Isen-berg and Tranquillo 2003; Solan et al. 2009).These processes are crucial in vascular smoothmuscle subject to cyclic stretch from pumpingblood, as failure to strengthen would result inaneurysms (Ghorpade and Baxter 1996).

Integrins

There is also evidence that integrins themselvesmay be mechanosensors. As discussed byCampbell and Humphries, integrins undergo

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complex conformational rearrangements thatgovern both affinity for extracellular matrix pro-teins and associations with cytoskeletal proteins(Calvete 2004; Wegener and Campbell 2008;Campbell and Humphries 2010). It would be sur-prising if these conformations were insensitiveto forces between the extracellular ligand bind-ing site and the cytoplasmic domains that bindcytoskeletal proteins. Experimental data, albeitindirect, support the idea that integrin con-formation can be modulated by applied force(Puklin-Faucher et al. 2006; Friedland et al.2009). Application of force to integrin a5b1 isrequired for conversion to a state that can bechemically cross-linked to the FN beneath thecell; inhibition of cell contractility blocks cross-linking but can be rescued by application offorce from fluid shear stress (Friedland et al.2009). This effect was associated with bindingof a5b1 to the synergy site within FN (a secondbinding site within the 10th FN type III repeat,close to but distinct from the RGD sequence inthe ninth type III repeat). These events are alsocorrelated with protein tyrosine phosphory-lation of focal adhesion kinase (see Geiger andYamada 2011), suggesting that integrin confor-mational transitions regulate intracellular sig-naling. Studies of the leukocyte integrin LFA-1(aLb2) also suggest a role for force in integrinconformation (Jin et al. 2004; Zhu et al. 2008).It was proposed based largely on structuraldata and molecular dynamics simulations thatforces that act either parallel or perpendicularto the plasma membrane can induce or stabilizethe high affinity state.

Cytoskeleton

The adhesion-associated actin cytoskeleton ra-pidly remodels in response to changes in force;the best-studied case being adhesion reinfor-cement or strengthening as discussed earlier.These effects in fact represent a set of mecha-nisms that operate on different time frames.Experiments with optical tweezers showedthat adhesions begin to recruit vinculin andincrease their strength within seconds of apply-ing force (Galbraith et al. 2002). Vinculin recrui-tment has now been attributed to the fact that

vinculin-binding sites in the talin tail domainare concealed within bundles ofa helices, whichcan open under tension (del Rio et al. 2009;Campbell and Humphries 2010). At only slightlylonger times, entire adhesions in adherent cellslengthen under applied force, indicating therecruitment of not only vinculin but integrinsand other proteins (Riveline et al. 2001). Ten-sion applied to cells on elastic surfaces alsotriggers an increase in integrin activity (i.e.,affinity) as measured by binding of soluble FNfragments (Katsumi et al. 2005; Thodeti et al.2009). This result implies communicationfrom the bound integrins that bear the force tothe unbound integrins that convert to the highaffinitystate and bind soluble ligands. This signalappeared to be mediated by PI 3-kinase (PI3K)(Katsumi et al. 2005). Integrin activation wasfollowed by new binding to the ECM andenlargement of the adhesions. Thus, it appearsthat the rapid responses to force that involveintegrin and talin conformation, and the sloweractivation of PI3K and unoccupied integrinsmay be related. It is attractive to propose thatforce induces conformational changes in thebound integrins, which triggers FAK activa-tion, which activates PI3K. PI 3-lipids thenmediate activation of unbound integrins fol-lowed by their binding to the ECM to enlargeand strengthen the adhesions. All of these stepshave been observed independently; however, itremains to be tested whether the entire pathwayworks in this manner.

Importantly, adhesion strengthening can-not be universal, because then adhesions wouldalways be very difficult to break. This clearly isnot the case, because cells migrate, which inmany cases involves disassembling or breakingadhesions under force (Ballestrem et al. 2001).Indeed, vinculin recruitment and adhesionstrengthening seen with laser tweezers wereonly observed at cells’ leading edges; quies-cent sides of cells did not show this behavior(Nishizaka et al. 2000). Recent work has shedsome light on the mechanism that controls ad-hesion strengthening versus disassembly underforce. Development of a fluorescence-based ten-sion sensor for vinculin showed that adhesionstrengthening was associated with high force





across this molecule, which was followed by re-cruitment of additional vinculin and enlarge-ment of the adhesion, resulting in decreasedforce per vinculin (Grashoff et al. 2010). How-ever, there was a population of focal adhesionsat the trailing edge of migrating cells whereforce across vinculin was negligible. Theseadhesions showed centripetal sliding, a formof controlled force-dependent disassembly inwhich the whole adhesion appears to movetoward the cell center because of differentialrates of assembly versus disassembly at the adhe-sion’s distal versus proximal ends (Ballestremet al. 2001). It was also shown that stabilizationof adhesions under force requires vinculin.Together, these data suggest that, in adhesionsthat strengthen in response to force, vinculinbears force, whereas in adhesions that disassem-ble, force is transmitted by other linkages. Thenature of the vinculin-independent links andthe pathways that regulate adhesion strengthen-ing versus disassembly are unknown.

SIGNALING PATHWAYS AND GENEEXPRESSION

Application of tension to cells through theirintegrin-mediated adhesions, often by stretch-ing cells on elastic substrata, regulates a widerange of signaling pathways, downstream genesand differentiation programs. These effects areof considerable physiological and pathologicalsignificance because the experimental systemmimics stretching of tissues in vivo. Cardiachypertrophy and remodeling of the smoothmuscle layers in arteries represent two examplesof integrin-dependent processes that are of par-ticular clinical relevance (Brancaccio et al. 2006;Heerkens et al. 2007). Stretching cells triggersactivation of signaling pathways that includeMAP kinases, Rho GTPases, elevated cytoplas-mic calcium, and generation of reactive oxygenamong others, with some variations among celltypes (Chiquet et al. 2003; Orr et al. 2006; Hagaet al. 2007). These events lead to changes in geneexpression for a variety of programs. Matrixremodeling genes are quite prominent, as arecell cycle genes and genes associated with dif-ferentiation toward contractile or mesenchymal

phenotypes. A great many studies have cata-logued the effects of stretch or other forces ongene expression and cellular phenotype (Chi-quet et al. 2003; Orr et al. 2006).

Some of these signaling events are medi-ated by mechanisms associated with adhesionstrengthening. For example, in smooth musclecells and fibroblasts, stretch triggers integrinconversion to the high affinity state, which leadsto increased integrin binding to the ECM (Kat-sumi et al. 2005). These newly bound integrinsthen signal to activate a variety of signalingpathways. One consequence of this mechanismis that, because different integrins signal dif-ferently, the downstream stretch pathways aremodulated by the ECM beneath the cells. Ithas also been proposed that changes in confor-mation of proteins such as talin and vinculinunder force may alter binding sites for signal-ing or adapter proteins, leading to downstreamevents. This has been shown for p130Cas,an important focal adhesion adapter protein(Sawada et al. 2006). Cas becomes a better sub-strate for Src family kinases after stretching cellson elastic substrata or after stretching Cas invitro. Once phosphorylated on tyrosine resi-dues, Cas recruits the SH2 domain adapterprotein Crk, leading to recruitment of GEFs(GTPase exchange factors) and activation ofthe small GTPase Rap1, which is known to acti-vate integrins (Reedquist et al. 2000). Crk canbind other GEFs such as DOCK180 (Hasegawaet al. 1996), suggesting that other small GTPasesmight also be activated through this pathway.

The focal adhesion protein zyxin appearsto mediate an important subset of cellular re-sponses to force. Zyxin is a LIM domain proteinthat binds the Ena/VASP proteins that, in turn,promote actin polymerization by binding thebarbed ends of actin filaments and protect-ing them from capping protein (Renfranz andBeckerle 2002). Zyxin localization to focal ad-hesions shows complex force-dependence. Itslocalization to focal adhesions requires high con-tractility, indeed, it is one of the few proteins thatlocalize strongly to focal adhesions but poorly tofocal complexes (Zaidel-Bar et al. 2003). Zyxin’sdissociation rate from the adhesions increasedwithin seconds of reducing contractile forces,

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suggesting that dissociation could be a relativelydirect response to force, for example, througheffects on protein conformation (Lele et al.2006). However, stretching cells on elastic sub-strata, which presumably applied even higherforces, triggered movement of the zyxin out ofthe focal adhesions and onto actin stress fibersand into the nucleus (Cattaruzza et al. 2004;Yoshigi et al. 2005; Hirata et al. 2008). Ena/VASP proteins translocate between focal adhe-sions and actin filaments together with thezyxin and were required for force-dependent in-creases in actin polymerization at the site ofzyxin/Ena/VASP targeting. In smooth muscle,zyxin moves to the nucleus in response toapplied stretch and antisense oligonucleotidesagainst zyxin altered stretch-induced changesin gene expression in these cells (Cattaruzzaet al. 2004). These data therefore suggest a po-tentially important role for zyxin in transcrip-tional responses to force as well as the betterdocumented regulation of actin polymerization.

Finally, matrix remodeling is a major tar-get for mechanically regulated pathways. Cellsunder high stress generally assemble stiffer orstronger matrices (Isenberg and Tranquillo 2003;Balestrini and Billiar 2006). These changesappear to be due in part to effects of forces ongene expression and in part because of moredirect effects on matrix assembly. For example,as discussed earlier, tension has direct effectson fibronectin conformation and fibrillogenesis(Zhong et al. 1998; Lemmon et al. 2009). Al-though, unlike fibronectin, fibrillar collagensspontaneously and efficiently polymerize insolution, their spatial arrangement in theECM is actively controlled by the cells (Cantyet al. 2006; Nguyen et al. 2009). This effectmay be in part mechanical, with cells physicallypulling collagen into oriented bundles. Directeffects of forces on TGFb1 processing are alsoimportant for ECM assembly in some systems(Munger et al. 1999). TGFb activation requiresits separation from the latency-associated pep-tide (LAP) that maintains it in an inactive state.LAP has an RGD sequence that binds integrinavb6; this interaction mediates force-dependent disruption of the TGFb1-LAP com-plex to release active TGFb1 (Yang et al. 2007)

(see also Sheppard and Munger 2011). TGFb1has numerous effects, including increasing thesynthesis of collagenous matrices and differentia-tion of cells toward more contractile phenotypes(e.g., fibroblasts into myofibroblasts) (Gabbiani2003; Schiller et al. 2004). Activation of TGFbby integrin avb6 is associated with increasedcollagen production and tissue fibrosis in sev-eral disease models, strongly suggesting biolog-ical and perhaps clinical relevance (Nishimura2009).

MATRIX RIGIDITY AND SPREAD AREA

Mechanical forces that originate in the cells’own actomyosin are also modulated by theECM. Cells sense the rigidity of the ECM thatthey are on or in, and adjust the tension thatthey exert accordingly (Choquet et al. 1997;Lo et al. 2000; Saez et al. 2005). Cells exerthigh traction forces on stiff matrices, coincidentwith formation of robust actin stress fibers andfocal adhesions. On softer matrices, less force isexerted, and actin cables and focal adhesions areless well developed. The signaling consequencesof matrix rigidity have been studied extensivelyand in many respects resemble the responses tostretch. For example, FAK phosphorylation,integrin activation, and activity of MAP kinasesand Rho GTPases are all regulated by matrixstiffness (Klein et al. 2009; Michael et al. 2009;Provenzano et al. 2009; Pasapera et al. 2010).These similarities provide additional supportfor the idea that forces across integrins and focaladhesions trigger similar effects whether theforces originate inside or outside the cell.

Matrix rigidity also regulates differentia-tion. Interestingly, mesenchymal stem cells tendto differentiate toward lineages whose stiffnessin vivo matches the stiffness of the artificialenvironment. For example, stiff matrices pro-mote osteogenic differentiation, in keeping withthe rigidity of bone, whereas very soft matri-ces favor neurogenic differentiation in keepingwith the softness of brain (Engler et al. 2006).Matrix stiffness also modulates phenotypes ofcancer cells. Tissue stiffness, determined mainlyby the ECM, increases during progression ofbreast and other (though not all) cancers; breast





cancer cells in stiff 3D matrices show more ag-gressive behavior such as luminal filling, in-vasion, and epithelial-mesenchymal transition(EMT) (Levental et al. 2009; Provenzano et al.2009). By contrast, growing the same cells insoft matrix typical of normal tissue promotesa differentiated, epithelial, less invasive pheno-type. These results strongly suggest that me-chanical consequences of matrix remodelingare a causal factor in cancer progression. Theyare also in keeping with the general notion thatthe mesenchymal phenotype is associated with ahigher force regime than that of epithelial cells.

Finally, it should be noted that tractionforce can be modulated by controlling the areaover which a cell can spread (Reinhart-Kinget al. 2005; Tolic-Norrelykke and Wang 2005).Confining cells to small areas decreases focaladhesions, actin stress fibers, myosin phosphor-ylation, and cell contractility. These changesare also associated with decreased FAK phosphor-ylation. When mesenchymal stem cells weregrown on islands of different sizes, small is-lands favored adipogenic differentiation whereaslarge islands favored osteogenic differentiation(McBeath et al. 2004). Thus, consequences ofaltered cell spreading may occur in part througheffects on the forces across the focal adhesions.

MEDITATIONS ON MECHANISM

The dominant view of mechanotransductionat adhesions and elsewhere is that forces acrossproteins or membranes alter the energy land-scape to cause conformational changes (Orret al. 2006; Vogel 2006). Typically, proteins un-fold to reveal novel sites for binding or phos-phorylation, which alters signaling. This simpleview most likely contains a good deal of truthand appears relevant to numerous situations.However, focal adhesions are dynamic struc-tures whose components are rapidly exchangingeven under ostensibly static conditions (Schles-singer and Geiger 1983; Ballestrem et al. 2001;Lele et al. 2006). Matrix rigidity sensing mustbe more complex.

Although it seems intuitively obvious thathard materials can support more tension thansoft ones, the detailed molecular mechanisms

by which cells sense physical properties of theECM and regulate their contractile forces arelargely unknown. The commonly used materi-als show something close to ideal elastic behav-ior, meaning that force increases in proportionto the displacement (Hooke’s law) (Pelhamand Wang 1997; Tan et al. 2003). Thus, in prin-ciple, cells on more compliant surfaces couldgenerate high tension by pulling the material agreater distance. That this does not happen sug-gests an active sensing mechanism rather thana simple inability to build up high forces. Inthe language of engineering, a stress sensor isunlikely. Indeed, a careful study of rigidity sens-ing using elastic pillars showed that when stiff-ness was varied over a wide range, the forcethat cells exerted increased with stiffness but thedistance cells deformed the pillars was nearlyconstant (Saez et al. 2005). This result would seemto argue for some version of strain sensing.Overall, it would seem that cells actively sensecompliance; doing so implies that they applyforce to the material and “measure” the resul-tant displacement.

One possibility is that the sensing mech-anisms may lie in the “clutch” that governsforce transmission between actin and integrins(Brown et al. 2006; Hu et al. 2007). As describedin more detail by Huttenlocher and Horwitz,actin polymerizing at the leading edge flowsbackward because of the resistance from theplasma membrane (Huttenlocher and Horwitz2011). Myosin-generated tension pulls on theactin and contributes to rearward movement.As it flows over the adhesions, linker proteinssuch as vinculin and talin bind the actin fila-ments and resist the rearward forces, causingmore force to be applied to the plasma mem-brane and leading to forward protrusion. Thereare two interesting and surprising aspects of thissystem. First, talin, vinculin, and other linkerproteins move backward at speeds that are inter-mediate between that of actin and the integrins(which are immobile) (Hu et al. 2007). Theirbinding kinetics must therefore be quite rapid;dissociation rates may also change under force,though to what extent they are slip bonds(increased dissociation under force) or catchbonds (decreased dissociation under force) is

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unknown. Second, the strength of the connec-tion between the actin and the integrins is vari-able, hence the notion of a clutch mechanismthat controls force transmission.

These observations suggest that if the adhe-sions moved backward because of stretching ofthe substratum, the force balance and timingof these interactions would be altered. Forexample, molecules of talin or vinculin couldremain bound to both actin and integrin whilebearing force. The altered timing could shiftthe kinetics of signaling events within the adhe-sions (Fig. 1). Hypothetically, stretched talinmight bind a kinase that triggers downstreamsignals. On rigid surfaces, the interaction wouldbe shorter lived and phosphorylation of sub-strates would be low, whereas on soft surfaces,the interaction would be prolonged and phos-phorylation would be higher. This model ishighly speculative and lacks any direct proof.However, though the details are almost certainlyincorrect, it points toward the types of modelneeded to explain these data, which are essen-tially dynamic. That is, the difference betweensoft and stiff substrates is the rate at whichforces build in response to cell-generated ten-sion. Cells may sense the distance moved in afixed period of time or the time required tomove a fixed distance. But what is sensed mustbe the kinetics of events following applicationof force.

Solving this fascinating problem will requirea detailed understanding of the effects of forceson molecular dynamics within the adhesions.Biophysical studies of molecular events withinthe adhesions, coupled with knowledge of theforces at the molecular level and their effectson protein interactions will be needed to un-ravel the mystery. These types of studies areunderway and a more detailed understandingof mechanisms of force sensing is likely to beachieved in the foreseeable future.

CONCLUSIONS

It would be a major oversimplification to sug-gest that effects of matrix rigidity, externallyapplied strain, modulation of myosin activationand spread area are identical. Indeed, there is

clear evidence to the contrary (Klein et al. 2009).However, there are distinct similarities, whichare strong enough to suggest a unifying themethat reoccurs in multiple biological settings inwhich forces across focal adhesions are altered.Thus, conditions that give rise to high tractionforces, large actin stress fibers and integrin clus-tering into large focal adhesions appear to resultin a specific set of signals that are distinct fromconditions in which traction force is low, actinis organized into smaller bundles, and focaladhesions are smaller. The high traction/largefocal adhesion state requires a high density ofmatrix ligands on or in a stiff matrix and highforces across the adhesions. Thus, the highforce/large focal adhesion state can be shiftedto a low force/small adhesion state by multiplemeans: decreasing activation of myosin in thecells, spread area, matrix rigidity or integrinbinding, and clustering. It therefore seems likelythat focal adhesions themselves are a majordeterminant of the signaling output. The extentof integrin clustering could be an importantvariable that governs signal output. Focal adhe-sion stability could also govern signaling. Smalladhesions are more dynamic (Rottner et al.1999; Choi et al. 2008) and newly formed adhe-sions have high levels of active Src (Schlaepferet al. 1998), high levels of tyrosine phosphoryla-tion (Zamir et al. 1999), and promote high Racand Cdc42 activity (DeMali et al. 2003). By con-trast, large, Rho-dependent adhesions are lessdynamic, and older focal adhesions have lessactive Src, and promote high Rho activity.

Evidence suggests that FAK mediates someof these differences in signaling. For example,FAK2/2 cells cannot distinguish between stiffand soft surfaces during cell migration (Loet al. 2000). Loss of FAK also diminishescells’ ability to modulate cell proliferation inresponse to spread area (Pirone et al. 2006).However, large focal adhesions show otherquantitative differences in composition relativeto focal complexes. For example, as discussedearlier, zyxin is much more abundant in largefocal adhesions, and this localization is force-dependent (Cattaruzza et al. 2004; Zaidel-Bar et al. 2004; Yoshigi et al. 2005; Lele et al.2006; Hirata et al. 2008). By contrast, levels of





phosphotyrosine are higher in small focal com-plexes. These differences are likely to influencesignaling output as well.

That said, it is worth repeating that changesin matrix stiffness, externally applied forces,spread area and myosin activation do not yieldidentical outcomes. There are significant dif-ferences in the kinetics of cell interactions withECM in these different circumstances, as wellas fully independent effects. Changing myosinphosphorylation probably affects tension incell structures other than focal adhesions andactin stress fibers. External stretch applies forcesto structures other than the adhesions and actincables, including the plasma membrane. Themechanisms by which cells sense these aspectsof their environment are beginning to be unrav-eled. At present, it seems likely that differencesin kinetic aspects of adhesion-cytoskeleton dy-namics form the basis for these sensing mecha-nisms. Development of new biophysical tools isbringing these questions into the realm of thesolvable. Understanding these aspects of ad-hesion biology is likely to shed light on diverseproblems from stem cell differentiation and can-cer metastasis to morphogenesis.

ACKNOWLEDGMENTS

I thank Brenton Hoffman for helpful commentson the manuscript. This work was supportedby a USPHS grant ROI 47214 to M.A.S.

REFERENCES

Aumailley M, Has C, Tunggal L, Bruckner-Tuderman L.2006. Molecular basis of inherited skin-blistering disor-ders, and therapeutic implications. Expert Rev Mol Med8: 1–21.

Balaban NQ, Schwartz US, Riveline D, Goichberg P, Tzur G,Sabanay I, Mahula D, Safran S, Beshadsky A, Addadi L,et al. 2001. Force and focal adhesion assembly: A closerelationship studied using elastic micropatterned sub-strates. Nat Cell Biol 3: 466–472.

Balestrini JL, Billiar KL. 2006. Equibiaxial cyclic stretchstimulates fibroblasts to rapidly remodel fibrin. J Biomech39: 2983–2990.

Ballestrem C, Hinz B, Imhof BA, Wehrle-Haller B. 2001.Marching at the front and dragging behind: DifferentialaVb3-integrin turnover regulates focal adhesion behav-ior. J Cell Biol 155: 1319–1332.

Baneyx G, Baugh L, Vogel V. 2002. Fibronectin extensionand unfolding within cell matrix fibrils controlled bycytoskeletal tension. Proc Natl Acad Sci 99: 5139–5143.

Brancaccio M, Hirsch E, Notte A, Selvetella G, Lembo G,Tarone G. 2006. Integrin signalling: The tug-of-war inheart hypertrophy. Cardiovasc Res 70: 422–433.

Brown CM, Hebert B, Kolin DL, Zareno J, Whitmore L,Horwitz AR, Wiseman PW. 2006. Probing the integrin-actin linkage using high-resolution protein velocitymapping. J Cell Sci 119: 5204–5214.

Burridge K, Chrzanowska-Wodnicka M. 1996. Focal adhe-sions, contractility, and signaling. Annu Rev Cell DevBiol 12: 463–518.

Calvete JJ. 2004. Structures of integrin domains and con-certed conformational changes in the bidirectional sig-naling mechanism of aIIbb3. Exp Biol Med (Maywood)229: 732–744.

Campbell I, Humphries M. 2010. Integrin structure, activa-tion, and interactions. Cold Spring Harb Perspect Bioldoi:10.1101/cshperspect.a004994.

Canty EG, Starborg T, Lu Y, Humphries SM, Holmes DF,Meadows RS, Huffman A, O’Toole ET, Kadler KE. 2006.Actin filaments are required for fibripositor-mediatedcollagen fibril alignment in tendon. J Biol Chem 281:38592–38598.

Cattaruzza M, Lattrich C, Hecker M. 2004. Focal adhesionprotein zyxin is a mechanosensitive modulator of geneexpression in vascular smooth muscle cells. Hypertension43: 726–730.

Chiquet M, Renedo AS, Huber F, Fluck M. 2003. Howdo fibroblasts translate mechanical signals into changesin extracellular matrix production? Matrix Biol 22:73–80.

Choi CK, Vicente-Manzanares M, Zareno J, Whitmore LA,Mogilner A, Horwitz AR. 2008. Actin and alpha-actininorchestrate the assembly and maturation of nascent adhe-sions in a myosin II motor-independent manner. Nat CellBiol 10: 1039–1050.

Choquet D, Felsenfeld DP, Sheetz MP. 1997. Extracellularmatrix rigidity causes strengthening of integrin-cyto-skeleton linkages. Cell 88: 39–48.

Chrzanowska-Wodnicka M, Burridge K. 1996. Rho-stimu-lated contractility drives the formation of stress fibersand focal adhesions. J Cell Biol 133: 1403–1415.

Collin O, Na S, Chowdhury F, Hong M, Shin ME, Wang F,Wang N. 2008. Self-organized podosomes are dynamicmechanosensors. Curr Biol 18: 1288–1294.

del Rio A, Perez-Jimenez R, Liu R, Roca-Cusachs P,Fernandez JM, Sheetz MP. 2009. Stretching single talinrod molecules activates vinculin binding. Science 323:638–641.

DeMali KA, Wennerberg K, Burridge K. 2003. Integrin sig-naling to the actin cytoskeleton. Curr Opin Cell Biol 15:572–582.

Engler AJ, Sen S, Sweeney HL, Discher DE. 2006. Matrixelasticity directs stem cell lineage specification. Cell 126:677–689.

Farge E. 2003. Mechanical induction of Twist in the Dro-sophila foregut/stomodeal primordium. Curr Biol 13:1365–1377.

M.A. Schwartz




Friedland JC, Lee MH, Boettiger D. 2009. Mechanicallyactivated integrin switch controls a5b1 function. Sci-ence 323: 642–644.

Gabbiani G. 2003. The myofibroblast in wound healing andfibrocontractive diseases. J Pathol 200: 500–503.

Galbraith CG, Yamada KM, Sheetz MP. 2002. The relation-ship between force and focal complex development. J CellBiol 159: 695–705.

Gao M, Craig D, Lequin O, Campbell ID, Vogel V, SchultenK. 2003. Structure and functional significance ofmechanically unfolded fibronectin type III1 intermedi-ates. Proc Natl Acad Sci 100: 14784–14789.

Geiger B, Yamada KM. 2011. Molecular architecture andfunction of matrix adhesions. Cold Spring Harb PerspectBiol doi:10.1101/cshperspect.a005033.

Ghorpade A, Baxter BT. 1996. Biochemistry and molecularregulation of matrix macromolecules in abdominalaortic aneurysms. Ann N Y Acad Sci 800: 138–150.

Grashoff C, Hoffman BD, Brenner MD, Zhou R, Parsons M,Yang MT, McLean MA, Sligar SG, Chen CS, Ha T, et al.2010. Measuring mechanical tension across vinculinreveals regulation of focal adhesion dynamics. Nature466: 263–266.

Haga JH, Li YS, Chien S. 2007. Molecular basis of the effectsof mechanical stretch on vascular smooth muscle cells.J Biomech 40: 947–960.

Hahn C, Schwartz MA. 2009. Mechanotransduction in vas-cular physiology and atherogenesis. Nat Rev Mol Cell Biol10: 53–62.

Hamasaki K, Mimura T, Furuya H, Morino N, Yamazaki T,Komuro I, Yazaki Y, Nojima Y. 1995. Stretching mesangialcells stimulates tyrosine phosphorylation of focal adhe-sion kinase pp125FAK. Biochem Biophys Res Comm 212:544–549.

Hasegawa H, Kiyokawa E, Tanaka S, Nagashima K, Gotoh N,Shibuya M, Kurata T, Matsuda M. 1996. DOCK180, amajor CRK-binding protein, alters cell morphologyupon translocation to the cell membrane. Mol Cell Biol16: 1770–1776.

Heerkens EH, Izzard AS, Heagerty AM. 2007. Integrins, vas-cular remodeling, and hypertension. Hypertension 49:1–4.

Hirata H, Tatsumi H, Sokabe M. 2008. Mechanical forcesfacilitate actin polymerization at focal adhesions in azyxin-dependent manner. J Cell Sci 121: 2795–2804.

Huttenlocher A, Horwitz AR. 2011. Integrins in cell migra-tion. Cold Spring Harb Perspect Biol doi:10.1101/cshperspect.a005074.

Hu K, Ji L, Applegate KT, Danuser G, Waterman-Storer CM.2007. Differential transmission of actin motion withinfocal adhesions. Science 315: 111–115.

Isenberg BC, Tranquillo RT. 2003. Long-term cyclic dis-tention enhances the mechanical properties of colla-gen-based media-equivalents. Ann Biomed Eng 31:937–949.

Jin M, Andricioaei I, Springer TA. 2004. Conversion be-tween three conformational states of integrin I domainswith a C-terminal pull spring studied with molecular dy-namics. Structure 12: 2137–2147.

Katsumi A, Naoe T, Matsushita T, Kaibuchi K, Schwartz MA.2005. Integrin activation and matrix binding mediate

cellular responses to mechanical stretch. J Biol Chem280: 16546–16549.

Katsumi A, Orr AW, Tzima E, Schwartz MA. 2004. Integrinsin mechanotransduction. J Biol Chem 279: 12001–12004.

Klein EA, Yin L, Kothapalli D, Castagnino P, Byfield FJ, Xu T,Levental I, Hawthorne E, Janmey PA, Assoian RK. 2009.Cell-cycle control by physiological matrix elasticity andin vivo tissue stiffening. Curr Biol 19: 1511–1518.

Krishna SM, Dear AE, Norman PE, Golledge J. 2010.Genetic and epigenetic mechanisms and their possiblerole in abdominal aortic aneurysm. Atherosclerosis doi:101016/jatherosclerosis201002008.

Lele TP, Pendse J, Kumar S, Salanga M, Karavitis J, IngberDE. 2006. Mechanical forces alter zyxin unbindingkinetics within focal adhesions of living cells. J CellPhysiol 207: 187–194.

Lemmon CA, Chen CS, Romer LH. 2009. Cell tractionforces direct fibronectin matrix assembly. Biophys J 96:729–738.

Levental KR, Yu H, Kass L, Lakins JN, Egeblad M, Erler JT,Fong SF, Csiszar K, Giaccia A, Weninger W, et al. 2009.Matrix crosslinking forces tumor progression by enhanc-ing integrin signaling. Cell 139: 891–906.

Lionetti V, Recchia FA, Ranieri VM. 2005. Overview ofventilator-induced lung injury mechanisms. Curr OpinCrit Care 11: 82–86.

Liu S, Calderwood DA, Ginsberg MH. 2000. Integrincytoplasmic domain-binding proteins. J Cell Sci 113:3563–3571.

Lo CM, Wang HB, Dembo M, Wang YL. 2000. Cell move-ment is guided by the rigidity of the substrate. Biophys J79: 144–152.

Lucitti JL, Jones EA, Huang C, Chen J, Fraser SE, DickinsonME. 2007. Vascular remodeling of the mouse yolk sacrequires hemodynamic force. Development 134: 3317–3326.

McBeath R, Pirone DM, Nelson CM, Bhadriraju K, ChenCS. 2004. Cell shape, cytoskeletal tension and RhoA reg-ulate stem cell lineage commitment. Dev Cell 6: 483–495.

Michael KE, Dumbauld DW, Burns KL, Hanks SK, GarciaAJ. 2009. Focal adhesion kinase modulates cell adhesionstrengthening via integrin activation. Mol Biol Cell 20:2508–2519.

Munger JS, Huang X, Kawakatsu H, Griffiths MJD, DaltonSL, Wu J, Pittet JF, Kaminski N, Garat C, Matthay MA,et al. 1999. The integrin avb6 binds and activates latentTGFb1: A mechanism for regulating pulmonary inflam-mation and fibrosis. Cell 96: 319–328.

Nguyen TD, Liang R, Woo SL, Burton SD, Wu C, Almarza A,Sacks MS, Abramowitch S. 2009. Effects of cell seedingand cyclic stretch on the fiber remodeling in an extracel-lular matrix-derived bioscaffold. Tissue Eng Part A 15:957–963.

Nishimura SL. 2009. Integrin-mediated transforminggrowth factor-beta activation, a potential therapeutic tar-get in fibrogenic disorders. Am J Pathol 175: 1362–1370.

Nishizaka T, Shi Q, Sheetz MP. 2000. Position-dependentlinkages of fibronectin- integrin-cytoskeleton. Proc NatlAcad Sci 97: 692–697.





Orr AW, Helmke BP, Blackman BR, Schwartz MA. 2006.Mechanisms of mechanotransduction. Dev Cell 10: 11–20.

Pasapera AM, Schneider IC, Rericha E, Schlaepfer DD,Waterman CM. 2010. Myosin II activity regulates vincu-lin recruitment to focal adhesions through FAK-medi-ated paxillin phosphorylation. J Cell Biol 188: 877–890.

Pelham RJ, Wang YL. 1997. Cell locomotion and focal adhe-sions are regulated by substrate flexibility. Proc Natl AcadSci 94: 13661–13665.

Pirone DM, Liu WF, Ruiz SA, Gao L, Raghavan S, LemmonCA, Romer LH, Chen CS. 2006. An inhibitory role forFAK in regulating proliferation: A link between limitedadhesion and RhoA-ROCK signaling. J Cell Biol 174:277–288.

Provenzano PP, Inman DR, Eliceiri KW, Keely PJ. 2009.Matrix density-induced mechanoregulation of breastcell phenotype, signaling and gene expression througha FAK-ERK linkage. Oncogene 28: 4326–4343.

Puklin-Faucher E, Gao M, Schulten K, Vogel V. 2006. Howthe headpiece hinge angle is opened: New insights into thedynamics of integrin activation. J Cell Biol 175: 349–360.

Reedquist KA, Ross E, Koops EA, Wolthuis RMF, ZwartkruisFJT, vanKooyk Y, Salmon M, Buckley CD, Bos JL. 2000.The small GTPase Rap1 mediates CDS31-induced integ-rin activation. J Cell Biol 148: 1151–1158.

Reinhart-King CA, Dembo M, Hammer DA. 2005. Thedynamics and mechanics of endothelial cell spreading.Biophys J 89: 676–689.

Renfranz PJ, Beckerle MC. 2002. Doing (F/L)PPPPs: EVH1domains and their proline-rich partners in cell polarityand migration. Curr Opin Cell Biol 14: 88–103.

Riveline D, Zamir E, Balaban NQ, Schwartz US, Ishizaki T,Narumiya S, Kam Z, Geiger B, Bershadsky AD. 2001.Focal contacts as mechanosensors: Externally appliedlocal mechanical force induces growth of focal contactsby an mDia1-dependent and ROCK-independent mech-anism. J Cell Biol 153: 1175–1185.

Robling AG, Castillo AB, Turner CH. 2006. Biomechanicaland molecular regulation of bone remodeling. AnnuRev Biomed Eng 8: 455–498.

Rottner K, Hall A, Small JV. 1999. Interplay between Rac andRho in the control of substrate contact dynamics. CurrBiol 9: 640–648.

Saez A, Buguin A, Silberzan P, Ladoux B. 2005. Is themechanical activity of epithelial cells controlled by defor-mations or forces? Biophys J 89: L52–54.

Sai X, Naruse K, Sokabe M. 1999. Activation of pp60(src) iscritical for stretch-induced orienting response in fibro-blasts. J Cell Sci 112: 1365–1373.

Sawada Y, Tamada M, Dubin-Thaler BJ, Cherniavskaya O,Sakai R, Tanaka S, Sheetz MP. 2006. Force sensing bymechanical extension of the Src family kinase substratep130Cas. Cell 127: 1015–1026.

Schaper W. 2009. Collateral circulation: Past and present.Basic Res Cardiol 104: 5–21.

Schiffrin EL. 1992. Reactivity of small blood vessels in hyper-tension: Relation with structural changes. State of the artlecture. Hypertension 19: II1–9.

Schiller M, Javelaud D, Mauviel A. 2004. TGF-beta-inducedSMAD signaling and gene regulation: consequences for

extracellular matrix remodeling and wound healing.J Dermatol Sci 35: 83–92.

Schlaepfer DD, Jones KC, Hunter T. 1998. Multiple Grb2-mediated integrin-stimulated signaling pathways toERK2/mitogen-activated protein kinase: Summation ofboth c-Src- and focal adhesion kinase-initiated tyrosinephosphorylation events. Mol Cell Biol 18: 2571–2585.

Schlessinger J, Geiger B. 1983. The dynamic interrelation-ships of actin and vinculin in cultured cells. Cell Motil3: 399–403.

Sheppard D, Munger J. 2011. Integrin crosstalk/TBG-betaregulation. Cold Spring Harb Perspect Biol doi:10.1101/cshperspect.a005017.

Solan A, Dahl SL, Niklason LE. 2009. Effects of mechanicalstretch on collagen and cross-linking in engineered bloodvessels. Cell Transplant 18: 915–921.

Somogyi K, Rorth P. 2004. Evidence for tension-based reg-ulation of Drosophila MAL and SRF during invasive cellmigration. Dev Cell 7: 85–93.

Streuli CH, Bissell MJ. 1990. Expression of extracellularmatrix components is regulated by substratum. J CellBiol 110: 1405–1415.

Tan JL, Tien J, Pirone DM, Gray DS, Bhadriraju K, Chen CS.2003. Cells lying on a bed of microneedles: An approachto isolate mechanical force. Proc Natl Acad Sci 100:1484–1489.

Thodeti CK, Matthews B, Ravi A, Mammoto A, Ghosh K,Bracha AL, Ingber DE. 2009. TRPV4 channels mediate cy-clic strain-induced endothelial cell reorientation throughintegrin-to-integrin signaling. Circ Res 104: 1123–1130.

Tolic-Norrelykke IM, Wang N. 2005. Traction in smoothmuscle cells varies with cell spreading. J Biomech 38:1405–1412.

Uitto J. 2009. Progress in heritable skin diseases: Trans-lational implications of mutation analysis and pros-pects of molecular therapies�. Acta Derm Venereol 89:228–235.

Vogel V. 2006. Mechanotransduction involving multimodu-lar proteins: converting force into biochemical signals.Annu Rev Biophys Biomol Struct 35: 459–488.

Wegener KL, Campbell ID. 2008. Transmembrane and cyto-plasmic domains in integrin activation and protein-pro-tein interactions (review). Mol Membr Biol 25: 376–387.

Wickstrom SA, Lange A, Montanez E, Fassler R. 2010. TheILK/PINCH/parvin complex: The kinase is dead, longlive the pseudokinase! EMBO J 29: 281–291.

Yang Z, Mu Z, Dabovic B, Jurukovski V, Yu D, Sung J, XiongX, Munger JS. 2007. Absence of integrin-mediatedTGFbeta1 activation in vivo recapitulates the phenotypeof TGFbeta1-null mice. J Cell Biol 176: 787–793.

Yashiro K, Shiratori H, Hamada H. 2007. Haemodynamicsdetermined by a genetic programme govern asymmetricdevelopment of the aortic arch. Nature 450: 285–288.

Yoshigi M, Hoffman LM, Jensen CC, Yost HJ, Beckerle MC.2005. Mechanical force mobilizes zyxin from focal adhe-sions to actin filaments and regulates cytoskeletal rein-forcement. J Cell Biol 171: 209–215.

Zaidel-Bar R, Geiger B. 2010. The switchable integrin adhe-some. J Cell Sci 123: 1385–1388.

Zaidel-Bar R, Ballestrem C, Kam Z, Geiger B. 2003. Earlymolecular events in the assembly of matrix adhesions

M.A. Schwartz




at the leading edge of migrating cells. J Cell Sci 116:4605–4613.

Zaidel-Bar R, Cohen M, Addadi L, Geiger B. 2004. Hierarch-ical assembly of cell-matrix adhesion complexes. BiochemSoc Trans 32: 416–420.

Zamir E, Katz BZ, Aota S, Yamada KM, Geiger B, Kam Z.1999. Molecular diversity of cell-matrix adhesions.J Cell Sci 112: 1655–1669.

Zhong C, Chrzanowska-Wodnicka M, Brown J, Shaub A,Belkin A, Burridge K. 1998. Rho-mediated contractilityexposes a cryptic site in fibronectin and induces fibronec-tin matrix assembly. J Cell Biol 141: 539–551.

Zhu J, Luo BH, Xiao T, Zhang C, Nishida N, Springer TA.2008. Structure of a complete integrin ectodomain in aphysiologic resting state and activation and deactivationby applied forces. Mol Cell 32: 849–861.





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