REVIEW

Connexins and gap junctions in the inner ear – it’s notjust about K+ recycling

Daniel J. Jagger & Andrew Forge

Received: 22 July 2014 /Accepted: 6 October 2014 /Published online: 9 November 2014# The Author(s) 2014. This article is published with open access at Springerlink.com

Abstract Normal development, function and repair of thesensory epithelia in the inner ear are all dependent on gapjunctional intercellular communication. Mutations in theconnexin genes GJB2 and GJB6 (encoding CX26 andCX30) result in syndromic and non-syndromic deafness viavarious mechanisms. Clinical vestibular defects, however, areharder to connect with connexin dysfunction. Cx26 and Cx30proteins are widely expressed in the epithelial and connectivetissues of the cochlea, where they may form homomeric orheteromeric gap junction channels in a cell-specific and spa-tiotemporally complex fashion. Despite the study of mutantchannels and animal models for both recessive and dominantautosomal deafness, it is still unclear why gap junctions areessential for auditory function, and why Cx26 and Cx30 donot compensate for each other in vivo. Cx26 appears to beessential for normal development of the auditory sensoryepithelium, but may be dispensable during normal hearing.Cx30 appears to be essential for normal repair followingsensory cell loss. The specific modes of intercellular signal-ling mediated by inner ear gap junction channels remainundetermined, but they are hypothesised to play essential rolesin the maintenance of ionic and metabolic homeostasis in theinner ear. Recent studies have highlighted involvement of gapjunctions in the transfer of essential second messengers be-tween the non-sensory cells, and have proposed roles forhemichannels in normal hearing. Here, we summarise thecurrent knowledge about the molecular and functional prop-erties of inner ear gap junctions, and about tissue pathologiesassociated with connexin mutations.

Keywords Gap junction . Connexin . Inner ear . Cochlea .

Deafness . Vestibular

Introduction

The vertebrate inner ear consists of the hearing organ, thecochlea, and the vestibular system that detects changes inhead position thereby contributing to maintenance of balance.Inner ear function is dependent on tightly controlled ionicenvironments, in particular for potassium ions (K+), whichare the main charge carrier for sensory transduction. The innerear contains two major fluid spaces, which are separated fromeach other by tight junction barriers. Scala media in thecochlea (Fig. 1a) and the vestibular semi-circular canals arefilled with endolymph, which is rich in K+ but poor in Na+,thus more resembling an intracellular fluid. The remainingextracellular spaces within the inner ear contain perilymph,which has a high Na+ and low K+ concentration. The essentialdriving force for sensory transduction in the mammalian co-chlea (but not in the vestibular system) is provided by the highpositive potential of the endolymph (ca. 80 mV), theendocochlear potential (EP) (Wangemann 2006). In the co-chlea, the ionic composition of the endolymph and the EP aregenerated and maintained by the stria vascularis, a stratifiedion-transporting tissue that lines the lateral wall of the cochlearduct. The sensory epithelia contain the mechano-sensory re-ceptors, known as hair cells, each of which is surrounded bynon-sensory supporting cells (Fig. 1b). Supporting cellsthroughout all vertebrate classes are extensively coupled bylarge (>10-μm2) gap junctions (Forge et al. 2003a,b), some ofwhich are amongst the largest in the body containing severalhundred thousand connexon channels. The interconnection ofmost cells within the inner ear via gap junctions is thought toplay a role in fluid homeostasis and/or intercellular signalling(Kikuchi et al. 2000a; Zhao et al. 2006).

D. J. Jagger (*) :A. ForgeUCL Ear Institute, University College London, 332 Gray’s Inn Road,London WC1X 8EE, UKe-mail: d.jagger@ucl.ac.uk

A. Forgee-mail: a.forge@ucl.ac.uk

Cell Tissue Res (2015) 360:633–644DOI 10.1007/s00441-014-2029-z

Gap junctions in the inner Ear

Based on ultra-structural studies, two gap junction net-works—the gap junction system of the epithelial tissue andthat of the connective tissue—have been described within themammalian cochlear duct (Kikuchi et al. 1995) and the ves-tibular system (Kikuchi et al. 1994). The epithelial gap junc-tion system connects the supporting cells of the sensory epi-thelia and bordering epithelial cells. In the cochlea, gap junc-tions are present between the supporting cells of the organ ofCorti (Fig. 1c) and the root cells whose processes penetrate thespiral ligament (Jagger and Forge 2013). No gap junction

plaques have been identified between hair cells andsupporting cells. The syncytial nature of supporting cells andthe segregation of hair cells from this functional unit has beenconfirmed by electrophysiology (Santos-Sacchi and Dallos1983; Oesterle and Dallos 1990) and dye tracer studies incochlear slices (Jagger and Forge 2006; Taylor et al. 2012;Forge et al. 2013). The gap junction system of inner earconnective tissues comprises the cells of the ion-transportingepithelium, various types of fibrocytes, and mesenchymalcells that line the scala vestibuli. In the cochlear duct, gapjunctions are present between the fibrocytes of the spirallimbus and of the spiral ligament (Fig. 1d). Numerous gap

Fig. 1 Expression patterns of Cx26 and Cx30 in epithelial and connec-tive tissues of the cochlea. aA section of the cochlear apical turn of a P30mouse showing the key sensory and non-sensory tissues involved inauditory sensory transduction. b Higher magnification of the organ ofCorti showing the inner hair cells and outer hair cells separated by thetunnel of Corti, formed by the inner pillar cells and outer pillar cells.Adjacent to the outer hair cells are the Deiters’ cells, Hensen’s cells andClaudius’ cells. Immunofluorescent labelling of the organ of Corti (c) forCx26 (green) and Cx30 (red) reveals a differential pattern of theconnexins. Nuclei are stained using DAPI.Within the Deiters’ cell region,

there are mostly Cx30-labelled gap junction plaques, whereas double-labelled plaques are evident elsewhere. In the connective tissue region ofthe cochlear lateral wall (d), most gap junction plaques are double-labelled for Cx26 and Cx30. anf auditory nerve fibres, bc basal cell, CcClaudius’ cell, Dc Deiters’ cell, ec endothelial cell, fc fibrocyte, HcHensen’s cell, ic intermediate cell, ihc inner hair cell, ipc inner pillar cell,mc marginal cell, oC organ of Corti, ohc outer hair cell, opc outer pillarcell, Rm Reissner’s membrane, sg spiral ganglion, sl spiral ligament, svstria vascularis, sm scala media, tm tectorial membrane. Scale bars 20 μm

634 Cell Tissue Res (2015) 360:633–644

junctions have been found between adjacent basal cells of thestria vascularis, between basal cells and intermediate cells, andbetween basal cells and fibrocytes of the spiral ligament. No gapjunctions have been identified between the intermediate and themarginal cells of the stria vascularis, nor between adjacent mar-ginal cells (Kikuchi et al. 1995; Forge et al. 2003b), suggestingthat these cells are isolated from a functional unit formed byintermediate cells, basal cells and fibrocytes of the spiral liga-ment. Dye-coupling experiments in cochlear slices have con-firmed the specific connectivity of spiral ligament fibrocytes withbasal/intermediate cells, and the functional isolation of this unitfrom the marginal cells (Kelly et al. 2011).

Connexin expression in the inner ear

The predominant connexins in the mammalian inner ear areCx26 and Cx30 (see Fig. 1c, d), which are present in cellscomprising the epithelial and connective tissue gap junctionsystems of the cochlea and vestibule (Kikuchi et al. 1994,1995; Lautermann et al. 1998; Ahmad et al. 2003; Forge et al.2003b; Sun et al. 2005; Jagger and Forge 2006; Liu et al.2009). Furthermore, double immunofluorescence revealedoverlapping labelling patterns for Cx26 and Cx30, suggestinga co-localisation of the two connexin isoforms within somegap junction plaques in the inner ear (Lautermann et al. 1998;Forge et al. 2003c; Sun et al. 2005; Liu et al. 2009). This hasbeen supported by co-immunoprecipitation of Cx26 andCx30 (Ahmad et al. 2003; Forge et al. 2003b) and byimmuno-gold labelling of thin sections, where Cx26 andCx30 were evenly distributed within the gap junctionplaques (Forge et al. 2003b).

Cx31 is apparently confined to the spiral limbus and aparticular population of fibrocytes in the spiral ligament andsupra-strial zone, where it gradually decreases from base toapex (Xia et al. 2000; Lopez-Bigas et al. 2002; Forge et al.2003b). In the rat cochlea, weak immunostaining of Cx43 hasbeen reported between supporting cells in the organ of Corti,the stria vascularis and spiral limbus (Suzuki et al. 2003). Thisis contrary to the postnatal mouse, where Cx43 is confined totype III fibrocytes lining the inside of the bony wall, and thebone of the otic capsule, with no expression in the organ ofCorti (Forge et al. 2003b; Cohen-Salmon et al. 2004). In thespiral ligament of the adult guinea pig, Cx43 immunofluores-cence has been localised within the type III fibrocytes, specif-ically to intercellular plaques and intracellular structures con-sistent with a classical trafficking pathway (Kelly et al. 2012).Cx29 is expressed in the Schwann cells surrounding theauditory and vestibular myelinated fibres of ganglion neurons(Eiberger et al. 2006; Tang et al. 2006). In the avian inner ear,the chicken orthologs to the mammalian Cx30, Cx26, andCx43 have been identified as major connexin isoforms(Nickel et al. 2006). Interestingly, chicken Cx30 is exclusively

expressed in inner ear tissues and is present in all cells com-prising the gap junction networks of the sensory and iontransporting epithelia of the cochlear duct and utricle (Helleret al. 1998; Nickel et al. 2006).

Inner ear gap junctions and K+ homeostasis

The serial arrangement of gap junction networks within twodistinct cochlear tissues has prompted the theory that gapjunctions form the structural pathways for K+ re-circulationwithin cochlear and vestibular tissues (Kikuchi et al. 2000b;Wangemann 2002; Jagger and Forge 2013). In this model, K+

ions exiting hair cells during auditory transduction are si-phoned from the extracellular perilymph by supporting cellsimmediately adjacent to hair cells. The ions are then relayedradially via the epithelial gap junction network to the connec-tive tissue gap junction network, where they may be taken upby fibrocytes of the spiral ligament or the vestibular connec-tive tissue and secreted back into the endolymph by themarginal cells of the stria vascularis or the vestibular darkcells. This model is derived largely from indirect evidence,often in the form of protein expression data, and still requires asubstantial amount of support from direct physiological evi-dence. The importance of buffering K+ in perilymph is clear,however; depolarisation-induced damage to hair cells andneurons would lead to permanent hearing impairment.

A homeostatic model can be extended from the “spatial K+

buffering” observed in glial cell networks in the brain (Kofujiand Newman 2004). Here, the extensive cell syncytium acts asa K+ sink, allowing redistribution of the K+ flux to regions ofinactivity. The extensive connexin expression in the epithelialand connective tissue networks along the cochlear partitionreveals a substantial buffering volume. The K+-buffering the-ories have a number of potential problems, not least theinability of individual connexins (either Cx26 or Cx30) tocompensate for the loss of the other (Cohen-Salmon et al.2002; Teubner et al. 2003). Both Cx26 and Cx30 homotypicchannels can transfer K+ ions (Valiunas et al. 1999; Mantheyet al. 2001), and so should ably manage K+ when expressedalone in the cochlea. Acceptance of the K+ recirculation modelhas been limited by the lack of evidence of K+ transfer fromthe epithelial gap junction network to the connectivetissue gap junction network. However, our recent workdemonstrated the existence of weakly rectifying K+ cur-rents through the basolateral processes of root cells,which were likely mediated via Kir4.1 channels(Jagger et al. 2010). Furthermore, the Kir4.1 channelsare co-localized with aquaporin channels (Eckhard et al.2012), suggesting that large K+ fluxes could occurwithout disturbance of the hydrostatic microenvironmentof the spiral ligament.

Cell Tissue Res (2015) 360:633–644 635

The biophysical characteristics of cochlear gap junctions

The properties of cochlear gap junctions have been deter-mined in native cochlear tissue, using combinations ofin vivo and ex vivo preparations, and in transfected cell linesexpressing Cx26 and/or Cx30. Homotypic Cx26 orhomotypic Cx30 channels have been studied extensively incell culture systems using the double patch clamp technique(Dahl et al. 1996; Valiunas et al. 1999; Beltramello et al. 2003,2005). Considering the high sequence homology betweenCx26 and Cx30 (Dahl et al. 1996), the properties of thesechannel types are surprisingly distinct, particularly in terms ofconductance and voltage gating. Unfortunately, there are few,if any, patch clamp data from cell lines co-transfected withCx26 and Cx30. However, the voltage-dependence and gatingproperties of gap junction channels in small groups ofsupporting cells have been determined. The voltage-dependence of gap junctions in Hensen’s cells could begrouped into four distinct types (Zhao 2000). These gatingresponses showed varying degrees of polarity-dependent orpolarity-independent rectification. The observation of exten-sive asymmetric voltage gating points to a complexity in gapjunctional coupling which argues in favour of channels com-posed of non-homotypic connexin types. In single recordings, itis possible that multiple channel types contribute to the ob-served complex responses. Channels between adjacent Deiters’cells may also allow rectification of current flow (Zhao 2000).There is evidence from patch clamp recordings that the gapjunctional conductance between cochlear supporting cells canbe modulated by intrinsic factors such as nitric oxide (Blasitset al. 2000) or turgor pressure (Zhao and Santos-Sacchi 1998).

The molecular selectivity of cochlear gap junctions

The molecular selectivity of cochlear gap junctions may be ofequal importance to their electrophysiological properties.There is increasing evidence that intercellular fluxes of secondmessengers such as inositol phosphates and Ca2+ ions mayregulate cochlear physiology. Indeed, the impaired transfer ofthe Ca2+-mobilizingmolecule inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] has been suggested as a cause of recessive deafnessdue to a specific Cx26 mutation (Beltramello et al. 2005). Themolecular selectivity of cochlear gap junctions has been ex-plored extensively using dye transfer techniques. As in othertissues, these channels have the ability to select betweenmolecules of different molecular weight and/or charge.Native cochlear gap junctions, in common with other Cx30-containing junctions (Eiberger et al. 2001; Forge et al. 2003a;Sun et al. 2005), often show a preference for positivelycharged species over negatively charged ones, and restrictthe movement of molecules with a high molecular weight.In studies utilising a cochlear slice preparation (Forge et al.

2003a; Jagger and Forge 2006; Taylor et al. 2012), matureDeiters’ cells (supporting cells immediately adjacent to outerhair cells; Fig. 1b) allowed intercellular transfer of neurobiotin(molecular weight 287 Da, charge +1) but resisted the passageof Lucifer yellow (molecular weight 443 Da, charge −2).However, in Hensen’s cells (supporting cells at lateral borderof organ of Corti), the passage of Lucifer yellow is lessrestricted. In the outer sulcus region of the adult guinea pigcochlea, Lucifer yellow transfers freely via gap junctionsbetween root cells (Jagger et al. 2010).

The differential permeability to Lucifer yellow supports avarying expression of Cx26 and Cx30 in the organ of Corti, asdemonstrated in Fig. 1c, and numerous other immuno-localization studies (Lautermann et al. 1998; Forge et al.2003b; Sun et al. 2005). In various species, there is a higherexpression of Cx30 in Deiters’ cells compared to Hensen’scells (Sun et al. 2005; Jagger and Forge 2006; Zhao and Yu2006; Liu et al. 2009; Forge et al. 2013). In addition, there aredramatic postnatal changes in the permeability properties ofgap junctions in the organ of Corti. There is a rapid increase ofdye-transfer between Deiters’ cells in rats during the firstpostnatal week, concomitant with increasing connexinimmuno-reactivity (Jagger and Forge 2006). Though there isfree transfer of Lucifer yellow between Deiters’ cells at P8,this likely reflects an immature phenotype, as this transfer isno longer apparent after the onset of hearing (>P12, seeabove). It is interesting to note that, contrary to the restrictedtransfer of Lucifer yellow between supporting cells of theorgan of Corti, gap junctions between supporting cells in theavian inner ear are highly permeable for large anionic dyes(Nickel et al. 2006).

Although dye transfer experiments provide useful descrip-tions of gap junction characteristics, it is perhaps more perti-nent to study the movement of endogenous messengers suchas Ca2+ and inositol phosphates. These agents present a great-er technical challenge as they themselves influence the phys-iology of the cells being studied. Gap junction-mediated Ca2+

waves have been monitored in HEK-293 cells expressingCx26 and/or Cx30 (Sun et al. 2005). Ca2+ waves appear tospread significantly faster through groups of cells expressingheteromeric Cx26/Cx30 channels compared to groups of cellsexpressing homomeric Cx26 or homomeric Cx30 channels.Such differences have yet to be confirmed in functionallymature native cochlear tissue. In supporting cells of immaturecochlear cultures, injection of Ins (1,4,5)P3 into a single cellinitiated Ca2+ waves through neighbouring cells (Beltramelloet al. 2005), suggesting an innate permeability of these gapjunctions to this molecule. Of note is the observation thathomomeric Cx26 channels in HeLa cells allow the passageof myo-inositol and various inositol polyphosphates, butheteromeric Cx26/Cx32 channels are much more selective,even distinguishing between isoforms of inositol triphosphate(Ayad et al. 2006). Comparable results have been observed for

636 Cell Tissue Res (2015) 360:633–644

heteromeric Cx26/Cx30 channels (He et al. 2006). Gap junc-tions within the cochlear sensory epithelium of immature miceare permeable to fluorescent analogues of D-glucose (Changet al. 2008), pointing to a role for connexins in the transport ofenergy substrates. Future studies must consider variations inconnexin composition of channels throughout the maturecochlea, as these are all likely to have quite distinct properties.The distinct localisation of Cx30 to gap junctions betweenDeiters’ cells (Fig. 1c) suggests that larger anionic signals inparticular will not easily transfer within that organisationalcompartment. Also, whilst much energy has been spent defin-ing the physiology of the supporting cells in the organ ofCorti, it is just as important to understand the contribution ofthe connective tissue gap junction network in the lateral wallto normal hearing (Kelly et al. 2011), and how its dysfunctionleads to cochlear pathology.

Do connexin hemichannels contribute to inner earfunction?

A condition generally required for unpaired connexon channelsto act as autonomous hemichannels is exposure to an extracellu-lar medium largely devoid of divalent cations, particularly Ca2+

(Bennett et al. 2003). Whilst it is generally advisable to approachthe area of hemichannels and their likely contribution to normaltissue homeostasis with some caution (Spray et al. 2006), thecochlea presents an unusual situation in which certain aspects ofepithelial cells are exposed to an extracellular fluid which maytheoretically permit hemichannel activation under normal cir-cumstances. Endolymph within scala media is characterised asa fluid unusually rich in K+ (but low in Na+), but which also hasan unusually lowCa2+ concentration,measured as approximately20 μM in adult rats (Bosher and Warren 1978). The apical polesof some supporting cells are permanently exposed to the endo-lymph, thus raising the possibility of permanent activation ofhemichannels. This has been adopted as the explanation forobserved spontaneous ATP release from early postnatal cochlearcultures in vitro (Tritsch et al. 2007; Anselmi et al. 2008;Majumder et al. 2010; Schutz et al. 2010). Released ATP isproposed subsequently to activate purinergic receptors on thesupporting cell luminal surface, which encourage the release ofCa2+ from intracellular stores in an IP3-dependent manner.Homomeric Cx26 gap junction channels in these immaturetissues may then mediate intercellular transfer of Ca2+ and IP3to instigate the regenerative propagation of intercellular Ca2+

waves, thus providing a system for long-distance signalling.This complex interplay between gap junctions, hemichan-

nels and purinergic receptors has been purported to playessential roles in the development of the cochlea (Dale2008), and may explain observed spontaneous activity withinthe pre-hearing auditory nerve via the purinergic activation ofimmature hair cells (Tritsch et al. 2007; Johnson et al. 2011). This

same mechanism has been proposed as a mediator of damagesignals within the hearing cochlea (Gale et al. 2004). ATP hasbeen measured at significant concentrations within the endo-lymph during normal hearing, and it rises during noise exposure(Munoz et al. 1995).Whether hemichannel-dependent purinergicsignalling between supporting cells (and possibly hair cells)endures in vivo after hearing onset remains unproven, thoughATP release has been reported from isolated adult cochlea, andthis release was inhibited by raised Ca2+ or gap junction blockers(Zhao et al. 2005). Similarly, isolated adult supporting cells arereported to allow uptake of large anionic molecules in Ca2+-freeconditions and this is reduced by physiological Ca2+ levels orapplied gap junction blockers (Zhao 2005).

Connexin mutations and deafness

The importance of gap junctional communication for auditoryfunction has been highlighted by the discoveries that muta-tions in GJB2 (coding for CX26), GJB6 (CX30) and GJB3(CX31) may all cause hereditary hearing loss (Rabionet et al.2002; Lee and White 2009; Xu and Nicholson 2013).Connexin mutations are associated with autosomal recessiveand dominant hearing loss, whose phenotypes are mostlyconfined to the inner ear (non-syndromic) but can occur withother clinical features (syndromic), in particular skin disor-ders. Despite the genetic heterogeneity of non-syndromicautosomal recessive deafness (DFNB), a single locus on chro-mosome 13q11-12, DFNB1, accounts for up to 50 % of thistype of hearing loss (Kenneson et al. 2002; Snoeckx et al.2005). The gene responsible for DFNB1 has been identified asGJB2 (Kelsell et al. 1997). Around 100, mostly recessive,mutations have been characterised in the GJB2 gene, includ-ing splice, nonsense, missense and frame-shift mutations (seehttp://www.crg.es/deafness). Further complications may alsoarise following the identification of pathogenic mutationsoutside the coding region of GJB2 (Matos et al. 2007). Witha carrier frequency of 2–4 %, the most common mutation inEuropean and North American populations is a deletion of asingle guanine nucleotide, known as 35delG, which results ina frame-shift and the subsequent premature termination ofprotein translation. The majority of recessive Cx26 mutationsstudied to date do not form functional channels in recombinantexpression systems, partly owing to impaired assembly ofconnexons, impaired targeting to the plasma membrane, orreduced protein stability (Martin et al. 1999; D’Andrea et al.2002; Thonnissen et al. 2002; Oshima et al. 2003). However,several Cx26 mutations have been reported to form functionalgap junction channels, albeit with reduced electrical couplingand impaired permeability for dye tracers (D’Andrea et al.2002; Wang et al. 2003; Skerrett et al. 2004; Bicego et al.2006). A subset of Cx26 mutant channels with amino acidsubstitutions at the second transmembrane domain (V84L,

Cell Tissue Res (2015) 360:633–644 637

A88S and V95M) did not significantly affect electrical cou-pling, but impaired the transfer of larger molecules such as Ins(1,4,5)P3 (Beltramello et al. 2005; Zhang et al. 2005).

Several rare missense mutations inGJB2 have been detect-ed in families with autosomal dominant inheritance (DFNA3)(Denoyelle et al. 1998; Feldmann et al. 2005). These muta-tions primarily affect amino acids within the extracellularloops and result in impaired electrical coupling and dye trans-fer (Marziano et al. 2003; Chen et al. 2005; Piazza et al. 2005;Deng et al. 2006). In addition, dominant Cx26 mutations, inparticular those that interfere with intracellular trafficking(Thomas et al. 2004), may be associated with various skindisorders (Richard et al. 1998, 2002; Maestrini et al. 1999;Heathcote et al. 2000; Uyguner et al. 2002). Currently, fourdeafness-causing recessive mutations at the DFNB1 locushave been reported. One encompasses the full DFNB1 locus,including bothGJB2 and GJB6 genes (Feldmann et al. 2009).Two of them truncate the GJB6 gene without affecting GJB2(del Castillo et al. 2002, 2005). Another deletion removes a131-kb fragment in the DFNB1 regionwithout affecting eitherGJB2 or GJB6 (Wilch et al. 2006), which supports the hy-pothesis that the deletions remove a regulatory element nec-essary for the expression of CX26 and/or CX30 in the innerear. In fact, additional reports indicate that the expression ofthe GJB2 allele in cis with either of the two deletions thattruncate GJB6 is dramatically reduced or switched off(Common et al. 2005; Rodriguez-Paris and Schrijver 2009).In addition, a missense mutation inGJB6 affecting the amino-terminal of CX30 (T5M) is associated with non-syndromicautosomal dominant (DFNA3) middle to high-frequencyhearing impairment with late onset (Grifa et al. 1999).Unlike CX30 mutations associated with skin disease, T5Mmutants formed electrically coupled channels but showedimpaired permeability for dye tracer and Ins (1,4,5)P3(Common et al. 2003; Zhang et al. 2005; Schutz et al. 2010).

Mutations in the gene encoding connexin 31 (GJB3) havebeen detected in two different disorders: deafness (Xia et al.1998; Liu et al. 2000) and erythrokeratodermia variabilis(Richard et al. 1998). Mutations linked to non-syndromic,dominant deafness (DFNA2) are concentrated at the secondextracellular loop and do not form functional channels (Heet al. 2005). The mild to moderate hearing loss is of late onsetand, consistent with the Cx31 expression pattern (Xia et al.2000), preferentially affects high frequencies (Xia et al. 1998).Autosomal recessive mutations located within the third trans-membrane domain are associated with non-syndromic mod-erate to profound hearing loss (Liu et al. 2000).

Insights and controversies arising from animal models

In agreement with the clinical phenotype of connexin muta-tions in humans, the deletion of Cx26 (Cohen-Salmon et al.

2002; Kudo et al. 2003) or Cx30 (Teubner et al. 2003) in theinner ear of mice results in severe hearing impairment (be-tween 30–100 dB) shortly after the onset of hearing. Themechanism by which Cx30 deletion causes hearing impair-ment continues to be the subject of debate (see below). Theablation of Cx31 from the inner ear of mice does not result inmorphological or functional defects (Plum et al. 2001).Differences between murine (Elfgang et al. 1995) and humanCx31 (Abrams et al. 2006) in their ability to form heteromeric/heterotypic gap junction channels may contribute to the dis-parities in the phenotype between mice and humans. In addi-tion, while normal auditory function may not be compromisedby the loss of Cx31, it would be interesting to study theirsusceptibility to noise and aging, especially as the absence offunctional Cx31 channels is associated with progressive hear-ing loss. Conflicting reports have been presented for Cx29-deficient mice. While one study has found no effect of thedeletion of Cx29 on auditory function (Eiberger et al. 2006),another using a different knockout mouse model detecteddemyelination of spiral ganglion neurons, which may beresponsible for the prolonged latency and distortion in audi-tory brainstem responses, and higher sensitivity to noise dam-age observed in ∼50 % of these animals (Tang et al. 2006).

In a mouse model for recessive Cx26-related deafness, theneonatal lethality of Cx26 knockout mice was overcome byrestricting the deletion of Cx26 to the epithelial gap junctionsystem (Cohen-Salmon et al. 2002). The inner ear of homo-zygous mice developed normally, but by postnatal day 14,soon after the hearing onset, apoptosis of supporting cellsaround the inner hair cells was observed, extending later tothe outer hair cells and their supporting cells. It has beensuggested that the death of supporting cells surrounding theinner hair cells is caused by oxidative stress owing to theinterference of accumulated K+ with the removal of the neu-rotransmitter glutamate from the extracellular space (Cohen-Salmon et al. 2002). However, the specific properties of Cx26that are required for hearing are called into question by theobservation that its replacement by Cx32 results in no obviouscochlear deficits (Degen et al. 2011). In another mouse model,the function of Cx26 is inhibited by expressing the humanR75W-Cx26 mutation, which is associated with dominantlyinherited hearing loss (Kudo et al. 2003). The dominant-negative effect of this missense mutation on the function ofgap junction channels in vitro has been confirmed by electro-physiology and dye transfer approaches that demonstrated gapjunctional communication was not only inhibited throughhomotypic R75W-Cx26 channels but also throughheteromeric gap junction channels also containing wild-typeCx26 or Cx30 (Richard et al. 1998; Marziano et al. 2003). At2 weeks of age, R75W-Cx26mutant mice displayed deformi-ties of both the tunnel of Corti and the supporting cells (seeFig. 2a from our own studies of these mice) compared to thosein wild-type mice (Fig. 2b). In the mutant mice, supporting

638 Cell Tissue Res (2015) 360:633–644

cells fail to undergo essential maturation processes thatoccur just prior to the onset of hearing. These animalsdisplayed profound increases of hearing thresholds (Kudoet al. 2003). By 7 weeks in R75W-Cx26 mice, outer haircells had degenerated, whereas inner hair cells were stillpresent. No apparent effect was detected in the striavascularis, which was confirmed by a normal endocochlearpotential. These findings suggest that the pathogenicity ofR75W-Cx26 is confined to the epithelial gap junctionsystem of the cochlea. The developmental defects ob-served in R75W-Cx26 mutants were comparable to thosein three mouse models with conditional deletions of Cx26(Wang et al. 2009), further highlighting key roles thatCx26 plays in the differentiation of cells in the organ ofCorti before the onset of hearing.

The genetic deletion of Cx30 from both the sensory epitheli-um and the connective tissues of the inner ear results in the failureto generate an endocochlear potential in 2-week old homozygousmutant (“Cx30-null”) mice despite apparently normal inner eardevelopment and normal endolymphatic K+ concentrations(Teubner et al. 2003). From postnatal day 18 onwards, theapoptosis of hair cells was observed. In adult Cx30-null mice,the endolymphatic K+ concentration is also decreased, furthercontributing to the profound loss of hearing sensitivity. A subse-quent study of these mutant mice reported that the key cellularcomponents of the stria vascularis were all present and of appar-ently normal appearance (Cohen-Salmon et al. 2007). However,the unusual presence of serum proteins within stria vascularis,andmarked abnormalities in the fine structure of strial capillaries,suggested the loss of endocochlear potential was caused bydisruptions of the capillary endothelial barrier and a resultingintra-strial electrical shunt.

The question remains as to why both Cx26 and Cx30 arenecessary for the maintenance of function in the cochlea. Oneexplanation could be that the deletion of one connexin geneaffects the expression of the other, resulting in an insufficientnumber of functional gap junctions in the cochlea. Althoughthe immuno-cytochemical expression patterns of the remain-ing connexin are unchanged in mice with targeted deletions ofCx26 or Cx30 (Cohen-Salmon et al. 2002; Kudo et al. 2003;Teubner et al. 2003; Ahmad et al. 2007; Forge et al. 2013),Cx30 knock-out mice have Cx26 protein levels ∼25 % that ofcontrols (measured by western blotting) despite unchangedCx26 mRNA levels (Ahmad et al. 2007). A comparabledecrease of Cx26 protein is also observed in a hearing-impaired knock-in mouse model of the Cx30-T5M mutation(Schutz et al. 2010). Together, these observations mightsuggest homotypic Cx26 channels are more unstablethan heteromeric Cx26/Cx30 channels, which may leadto an insufficient number of functional gap junctionchannels. Indeed, in Cx30-null mice, the size and num-ber of gap junction plaques between certain supportingcells is greatly reduced compared with those in normal

animals (Fig. 2c, d). This reduced gap junction sizebetween Deiters’ cells was reflected by decreased dyetransfer between those cells in Cx30-null mice (Forgeet al. 2013). However, between Hensen’s cells (whereCx26 is more highly expressed, Fig. 1d), plaque sizeswere comparable between the genotypes, and dye trans-fer appeared unaffected. Lending further support to thehypothesis of reduced channel number is the observa-tion that the over-expression of Cx26 restores hearing inCx30-deficient mice (Ahmad et al. 2007).

It would seem, therefore, that Cx30 can be replaced byexogenous Cx26 to restore normal hearing, and that the biophys-ical properties of cochlear gap junctions are ultimately unimpor-tant. Recently, a novel knock-outmousewas developed to furtherinvestigate the importance of Cx30 in normal auditory function.In the Cx30Δ/Δ model, Cx30 protein is completely absent andCx26 protein is decreased, but to only ∼50 % of normal levels(Boulay et al. 2013). These mice hear normally, provoking aconclusion that Cx30 is dispensable for normal hearing. Further,the authors propose that dominant GJB6 deafness mutationsmanifest themselves via their negative effects on heteromericchannels comprising normal Cx26 and mutated Cx30, andlarge deletions of GJB6 affect GJB2 expression. These ob-servations in mice are consistent with those in humans wherelarge deletions of GJB3 may not result in hearing impair-ment (see above).

In work from our own laboratories employing the Cx30-null mouse model, we found that supporting cells were unableto carry out normal repair processes (Forge et al. 2013).Following hair cell loss in these mutant animals, thesupporting cells fail to expand into the spaces left behind bythe hair cells (Fig. 2e), and do not undertake the radial migra-tion into the tunnel of Corti that occurs in normal animalspharmacologically induced to lose hair cells (Fig. 2f).Interestingly, during ototoxic hair cell loss in the chickencochlea (“basilar papilla”), there is a decreased expression ofCx43 within supporting cells, a change associated with a lossof directional intercellular signalling (Jagger et al., in press, JNeuroscience). Furthermore, when remaining Cx30-containing gap junctions are blocked pharmacologically, nor-mal repair processes are halted.

One remaining question that currently available mousemodels have failed to answer is why specific human connexindeafness mutations result in hearing impairments that advanceat different rates (Bitner-Glindzicz 2002). Some patients areprofoundly deaf from birth (and presumably prenatally), yetothers lose their hearing progressively over several years. Asignificant challenge to our field is to create biologically andclinically relevant models, which (1) can be used to attain abetter understanding of the underlying pathologies occur-ring in human connexin deafness, and (2) can beexploited in order to develop effective ameliorativetreatments to restore hearing.

Cell Tissue Res (2015) 360:633–644 639

Fig. 2 Cx26 and Cx30 play specific roles in development, membranefunction and epithelial repair of the cochlea. a, b Light microscopyimages of cochlear tissues. a In the basal turn of a 2-week old R75W-Cx26 mouse, the essential sensory and non-sensory tissues are present,but on closer inspection the organ of Corti appears compact (a’). Deiters’cells are short and intercellular spaces such as the tunnel of Corti andspaces of Nuel are absent. b In comparison, in a juvenile wild-typemouse, the organ of Corti has elongated Deiters’ cells and pillar cells,and a fully patent tunnel of Corti (*). c, d Freeze-fracture images ofDeiters’ cell membranes in mice. Freeze-fracture reveals gap junctionplaques as clusters of particles in the membrane. Each particle representsan individual channel (connexon). c In Cx30-null animals, plaques of thegap junctions between adjacent Deiters’ cells consist of only a few

channels, and they are dispersed. d In wild-type animals, the plaquesare extremely large (consisting of thousands of channels) and occupy asignificant proportion of the membrane area. e, f Transmission electronmicrographs of cochlear tissues. e Following loss of outer hair cells froma Cx30-null mouse, the Deiters’ cells remain columnar in shape, thetunnel of Corti is open (*), and the reticular lamina is displaced towardsthe pillar cells (arrow). f In a wild-type mouse treated systemically withkanamycin and bumetanide to cause loss of inner and outer hair cells, theDeiters’ cells have expanded to fill the gaps left behind by the hair cells.Cells with characteristics of Deiters’ cells have migrated between theouter pillar cells so that the tunnel of Corti appears filled (*). Dc Deiters’cell, oC organ of Corti, Rm Reissner’s membrane, sg spiral ganglion, svstria vascularis, Tc tectal cell. Scale bars (a, b, e, f) 10 μm, (c, d) 0.1 μm

640 Cell Tissue Res (2015) 360:633–644

Conclusions

Building on the insights into functional properties of inner eargap junctions and their mutations, often gained from studies inexpression systems and immature cochlear cultures, the focusmust now be turned to mature native tissues and their endog-enous metabolites and second messengers. Increasingly re-fined animal models of connexin deletion will help us betterrecapitulate human inherited hearing loss within the laborato-ry setting. This will enable us to unravel the complexities ofgap junctional communication in the inner ear and to betterunderstand cochlear pathologies that result from connexinmutations. Great challenges lie ahead, not least deliveringtranslational therapies within the clinical setting.

Acknowledgments Work in our laboratories is supported by the Bio-technology and Biological Sciences Research Council, the Medical Re-search Council, the Rosetrees Trust, and Action on Hearing Loss. We aregrateful to recent members of our laboratories who have contributed toour work on this area, particularly to John Kelly, Ruth Taylor and ReginaNickel. We apologise to colleagues whose work could not be cited owingto space restrictions.

Open Access This article is distributed under the terms of the CreativeCommons Attribution License which permits any use, distribution, andreproduction in any medium, provided the original author(s) and thesource are credited.

References

Abrams CK, Freidin MM, Verselis VK, Bargiello TA, Kelsell DP,Richard G, Bennett MV, Bukauskas FF (2006) Properties of humanconnexin 31, which is implicated in hereditary dermatological dis-ease and deafness. Proc Natl Acad Sci U S A 103:5213–5218

Ahmad S, Chen S, Sun J, Lin X (2003) Connexins 26 and 30 are co-assembled to form gap junctions in the cochlea of mice. BiochemBiophys Res Commun 307:362–368

Ahmad S, Tang W, Chang Q, Qu Y, Hibshman J, Li Y, Sohl G, Willecke K,Chen P, Lin X (2007) Restoration of connexin26 protein level in thecochlea completely rescues hearing in a mouse model of humanconnexin30-linked deafness. ProcNatl Acad Sci USA104:1337–1341

Anselmi F, Hernandez VH, Crispino G, Seydel A, Ortolano S, Roper SD,Kessaris N, Richardson W, Rickheit G, Filippov MA, Monyer H,Mammano F (2008) ATP release through connexin hemichannels andgap junction transfer of second messengers propagate Ca2+ signalsacross the inner ear. Proc Natl Acad Sci U S A 105:18770–18775

Ayad WA, Locke D, Koreen IV, Harris AL (2006) Heteromeric, but nothomomeric, connexin channels are selectively permeable to inositolphosphates. J Biol Chem 281:16727–16739

Beltramello M, Bicego M, Piazza V, Ciubotaru CD, Mammano F,D’Andrea P (2003) Permeability and gating properties of humanconnexins 26 and 30 expressed in HeLa cells. BiochemBiophys ResCommun 305:1024–1033

Beltramello M, Piazza V, Bukauskas FF, Pozzan T, Mammano F (2005)Impaired permeability to Ins (1,4,5)P3 in a mutant connexin under-lies recessive hereditary deafness. Nat Cell Biol 7:63–69

Bennett MV, Contreras JE, Bukauskas FF, Saez JC (2003) New roles forastrocytes: gap junction hemichannels have something to commu-nicate. Trends Neurosci 26:610–617

Bicego M, Beltramello M, Melchionda S, Carella M, Piazza V, Zelante L,Bukauskas FF, Arslan E, Cama E, Pantano S, Bruzzone R, D’AndreaP, Mammano F (2006) Pathogenetic role of the deafness-relatedM34T mutation of Cx26. Hum Mol Genet 15:2569–2587

Bitner-Glindzicz M (2002) Hereditary deafness and phenotyping inhumans. Br Med Bull 63:73–94

Blasits S, Maune S, Santos-Sacchi J (2000) Nitric oxide uncouples gapjunctions of supporting Deiters cells from Corti’s organ. PflugersArch 440:710–712

Bosher SK, Warren RL (1978) Very low calcium content of cochlearendolymph, an extracellular fluid. Nature 273:377–378

Boulay AC, del Castillo FJ, Giraudet F, Hamard G, Giaume C, Petit C,Avan P, Cohen-Salmon M (2013) Hearing is normal withoutconnexin30. J Neurosci 33:430–434

Chang Q, Tang W, Ahmad S, Zhou B, Lin X (2008) Gap junctionmediated intercellular metabolite transfer in the cochlea is compro-mised in connexin30 null mice. PLoS ONE 3:e4088

Chen Y, Deng Y, Bao X, Reuss L, Altenberg GA (2005) Mechanism ofthe defect in gap-junctional communication by expression of aconnexin 26 mutant associated with dominant deafness. FASEB J19:1516–1518

Cohen-Salmon M, Ott T, Michel V, Hardelin JP, Perfettini I, Eybalin M,Wu T, Marcus DC, Wangemann P, Willecke K, Petit C (2002)Targeted ablation of connexin26 in the inner ear epithelial gapjunction network causes hearing impairment and cell death. CurrBiol 12:1106–1111

Cohen-SalmonM,Maxeiner S,KrugerO, TheisM,WilleckeK, Petit C (2004)Expression of the connexin43- and connexin45-encoding genes in thedeveloping and mature mouse inner ear. Cell Tissue Res 316:15–22

Cohen-Salmon M, Regnault B, Cayet N, Caille D, Demuth K, HardelinJP, Janel N, Meda P, Petit C (2007) Connexin30 deficiency causesinstrastrial fluid-blood barrier disruption within the cochlear striavascularis. Proc Natl Acad Sci U S A 104:6229–6234

Common JE, Di WL, Davies D, Galvin H, Leigh IM, O’TooleEA, Kelsell DP (2003) Cellular mechanisms of mutantconnexins in skin disease and hearing loss. Cell CommunAdhes 10:347–351

Common JE, Bitner-Glindzicz M, O’Toole EA, Barnes MR, Jenkins L,Forge A, Kelsell DP (2005) Specific loss of connexin 26 expressionin ductal sweat gland epithelium associated with the deletion muta-tion del (GJB6-D13S1830). Clin Exp Dermatol 30:688–693

Dahl E, Manthey D, Chen Y, Schwarz HJ, Chang YS, Lalley PA,Nicholson BJ, Willecke K (1996) Molecular cloning and functionalexpression of mouse connexin-30, a gap junction gene highlyexpressed in adult brain and skin. J Biol Chem 271:17903–17910

Dale N (2008) Dynamic ATP signalling and neural development. JPhysiol 586:2429–2436

D’Andrea P, Veronesi V, Bicego M, Melchionda S, Zelante L, Di Iorio E,Bruzzone R, Gasparini P (2002) Hearing loss: frequency and func-tional studies of the most common connexin26 alleles. BiochemBiophys Res Commun 296:685–691

Degen J, Schutz M, Dicke N, Strenzke N, Jokwitz M, Moser T, WilleckeK (2011) Connexin32 can restore hearing in connexin26 deficientmice. Eur J Cell Biol 90:817–824

del Castillo I, Villamar M, Moreno-Pelayo MA, del Castillo FJ, AlvarezA, Telleria D, Menendez I, Moreno F (2002) A deletion involvingthe connexin 30 gene in nonsyndromic hearing impairment. N EnglJ Med 346:243–249

del Castillo FJ et al (2005) A novel deletion involving the connexin-30gene, del (GJB6-d13s1854), found in trans with mutations in theGJB2 gene (connexin-26) in subjects with DFNB1 non-syndromichearing impairment. J Med Genet 42:588–594

Deng Y, Chen Y, Reuss L, Altenberg GA (2006) Mutations of connexin26 at position 75 and dominant deafness: essential role of argininefor the generation of functional gap-junctional channels. Hear Res220:87–94

Cell Tissue Res (2015) 360:633–644 641

Denoyelle F, Lina-Granade G, Plauchu H, Bruzzone R, Chaib H, Levi-Acobas F, Weil D, Petit C (1998) Connexin 26 gene linked to adominant deafness. Nature 393:319–320

Eckhard A, Gleiser C, Rask-Andersen H, Arnold H, Liu W, Mack A,Muller M, Lowenheim H, Hirt B (2012) Co-localisation of K(ir)4.1and AQP4 in rat and human cochleae reveals a gap in water channelexpression at the transduction sites of endocochlear K(+) recyclingroutes. Cell Tissue Res 350:27–43

Eiberger J, Degen J, Romualdi A, Deutsch U, Willecke K, Sohl G (2001)Connexin genes in the mouse and human genome. Cell CommunAdhes 8:163–165

Eiberger J, Kibschull M, Strenzke N, Schober A, Bussow H, Wessig C,Djahed S, Reucher H, Koch DA, Lautermann J, Moser T,Winterhager E,Willecke K (2006) Expression pattern and functionalcharacterization of connexin29 in transgenic mice. Glia 53:601–611

Elfgang C, Eckert R, Lichtenberg-Frate H, Butterweck A, Traub O, KleinRA, Hulser DF, Willecke K (1995) Specific permeability and selec-tive formation of gap junction channels in connexin-transfectedHeLa cells. J Cell Biol 129:805–817

Feldmann D, Denoyelle F, Blons H, Lyonnet S, Loundon N, Rouillon I,Hadj-Rabia S, Petit C, Couderc R, Garabedian EN, Marlin S (2005)The GJB2 mutation R75Q can cause nonsyndromic hearing lossDFNA3 or hereditary palmoplantar keratoderma with deafness. AmJ Med Genet A 137:225–227

Feldmann D, Le Marechal C, Jonard L, Thierry P, Czajka C, Couderc R,Ferec C, Denoyelle F, Marlin S, Fellmann F (2009) A new largedeletion in the DFNB1 locus causes nonsyndromic hearing loss. EurJ Med Genet 52:195–200

Forge A, Marziano NK, Casalotti SO, Becker DL, Jagger D (2003a) Theinner ear contains heteromeric channels composed of cx26 and cx30and deafness-related mutations in cx26 have a dominant negativeeffect on cx30. Cell Commun Adhes 10:341–346

Forge A, Becker D, Casalotti S, Edwards J, Marziano N, Nevill G(2003b) Gap junctions in the inner ear: comparison of distributionpatterns in different vertebrates and assessment of connexin compo-sition in mammals. J Comp Neurol 467:207–231

Forge A, Jagger DJ, Kelly JJ, Taylor RR (2013) Connexin30-mediatedintercellular communication plays an essential role in epithelialrepair in the cochlea. J Cell Sci 126:1703–1712

Gale JE, Piazza V, Ciubotaru CD, Mammano F (2004) A mechanism forsensing noise damage in the inner ear. Curr Biol 14:526–529

Grifa A, Wagner CA, D’Ambrosio L, Melchionda S, Bernardi F, Lopez-Bigas N, Rabionet R, Arbones M, Monica MD, Estivill X, ZelanteL, Lang F, Gasparini P (1999) Mutations in GJB6 cause nonsyn-dromic autosomal dominant deafness at DFNA3 locus. Nat Genet23:16–18

He LQ, Liu Y, Cai F, Tan ZP, Pan Q, Liang DS, Long ZG,Wu LQ, HuangLQ, Dai HP, Xia K, Xia JH, Zhang ZH (2005) Intracellular distri-bution, assembly and effect of disease-associated connexin 31 mu-tants in HeLa cells. Acta Biochim Biophys Sin 37:547–554

He L, Ayad WA, Harris AL (2006) Expression and functional analysis ofdeafness-associated heteromeric connexin channels. 46th AnnualMeeting of The American Society for Cell Biology, San Diego, p358

Heathcote K, Syrris P, Carter ND, Patton MA (2000) A connexin 26mutation causes a syndrome of sensorineural hearing loss andpalmoplantar hyperkeratosis (MIM 148350). J MedGenet 37:50–51

Heller S, Sheane CA, Javed Z, Hudspeth AJ (1998) Molecular markersfor cell types of the inner ear and candidate genes for hearingdisorders. Proc Natl Acad Sci U S A 95:11400–11405

Jagger DJ, Forge A (2006) Compartmentalized and signal-selective gapjunctional coupling in the hearing cochlea. J Neurosci 26:1260–1268

Jagger DJ, Forge A (2013) The enigmatic root cell - emerging rolescontributing to fluid homeostasis within the cochlear outer sulcus.Hear Res 303:1–11

Jagger DJ, Nevill G, Forge A (2010) The Membrane Properties ofCochlear Root Cells are Consistent with Roles in PotassiumRecirculation and Spatial Buffering. J Assoc Res Otolaryngol 11:435–448

Johnson SL, Eckrich T, Kuhn S, Zampini V, Franz C, Ranatunga KM,Roberts TP, Masetto S, Knipper M, Kros CJ, Marcotti W (2011)Position-dependent patterning of spontaneous action potentials inimmature cochlear inner hair cells. Nat Neurosci 14:711–717

Kelly JJ, Forge A, Jagger DJ (2011) Development of gap junctionalintercellular communication within the lateral wall of the rat co-chlea. Neuroscience 180:360–369

Kelly JJ, Forge A, Jagger DJ (2012) Contractility in type III cochlearfibrocytes is dependent on non-muscle myosin II and intercellulargap junctional coupling. J Assoc Res Otolaryngol 13:473–484

Kelsell DP, Dunlop J, Stevens HP, Lench NJ, Liang JN, Parry G, MuellerRF, Leigh IM (1997) Connexin 26 mutations in hereditary non-syndromic sensorineural deafness. Nature 387:80–83

Kenneson A, Van Naarden BK, Boyle C (2002) GJB2 (connexin 26)variants and nonsyndromic sensorineural hearing loss: a HuGEreview. Genet Med 4:258–274

Kikuchi T, Adams JC, Paul DL, Kimura RS (1994) Gap junction systemsin the rat vestibular labyrinth: immunohistochemical and ultrastruc-tural analysis. Acta Otolaryngol 114:520–528

Kikuchi T, Kimura RS, Paul DL, Adams JC (1995) Gap junctions in therat cochlea: immunohistochemical and ultrastructural analysis. AnatEmbryol (Berl) 191:101–118

Kikuchi T, Kimura RS, Paul DL, Takasaka T, Adams JC (2000a) Gapjunction systems in the mammalian cochlea. Brain Res Brain ResRev 32:163–166

Kikuchi T, Adams JC, Miyabe Y, So E, Kobayashi T (2000b) Potassiumion recycling pathway via gap junction systems in the mammaliancochlea and its interruption in hereditary nonsyndromic deafness.Med Electron Microsc 33:51–56

Kofuji P, Newman EA (2004) Potassium buffering in the central nervoussystem. Neuroscience 129:1045–1056

Kudo T, Kure S, Ikeda K, Xia AP, Katori Y, Suzuki M, Kojima K,Ichinohe A, Suzuki Y, Aoki Y, Kobayashi T, Matsubara Y (2003)Transgenic expression of a dominant-negative connexin26 causesdegeneration of the organ of Corti and non-syndromic deafness.Hum Mol Genet 12:995–1004

Lautermann J, ten Cate WJ, Altenhoff P, Grummer R, Traub O, Frank H,Jahnke K, Winterhager E (1998) Expression of the gap-junctionconnexins 26 and 30 in the rat cochlea. Cell Tissue Res 294:415–420

Lee JR, White TW (2009) Connexin-26 mutations in deafness and skindisease. Expert Rev Mol Med 11:e35

Liu XZ, Xia XJ, Xu LR, Pandya A, Liang CY, Blanton SH, Brown SD,Steel KP, Nance WE (2000) Mutations in connexin31 underlierecessive as well as dominant non-syndromic hearing loss. HumMol Genet 9:63–67

Liu W, Bostrom M, Kinnefors A, Rask-Andersen H (2009) Uniqueexpression of connexins in the human cochlea. Hear Res 250:55–62

Lopez-Bigas N, ArbonesML, Estivill X, Simonneau L (2002) Expressionprofiles of the connexin genes, Gjb1 and Gjb3, in the developingmouse cochlea. Mech Dev 119(Suppl 1):S111–S115

Maestrini E, Korge BP, Ocana-Sierra J, Calzolari E, Cambiaghi S,Scudder PM, Hovnanian A, Monaco AP, Munro CS (1999) Amissense mutation in connexin26, D66H, causes mutilating kerato-derma with sensorineural deafness (Vohwinkel’s syndrome) in threeunrelated families. Hum Mol Genet 8:1237–1243

Majumder P, Crispino G, Rodriguez L, Ciubotaru CD, Anselmi F, PiazzaV, Bortolozzi M, Mammano F (2010) ATP-mediated cell-cell sig-naling in the organ of Corti: the role of connexin channels.Purinergic Signal 6:167–187

Manthey D, Banach K, Desplantez T, Lee CG, Kozak CA, Traub O,Weingart R, Willecke K (2001) Intracellular domains of mouse

642 Cell Tissue Res (2015) 360:633–644

connexin26 and −30 affect diffusional and electrical properties ofgap junction channels. J Membr Biol 181:137–148

Martin PE, Coleman SL, Casalotti SO, Forge A, Evans WH (1999)Properties of connexin26 gap junctional proteins derived from mu-tations associated with non-syndromal heriditary deafness. HumMol Genet 8:2369–2376

Marziano NK, Casalotti SO, Portelli AE, Becker DL, Forge A (2003)Mutations in the gene for connexin 26 (GJB2) that cause hearingloss have a dominant negative effect on connexin 30. Hum MolGenet 12:805–812

Matos TD, Caria H, Simoes-Teixeira H, Aasen T, Nickel R, Jagger DJ,O’Neill A, Kelsell DP, Fialho G (2007) A novel hearing-loss-relatedmutation occurring in the GJB2 basal promoter. J Med Genet 44:721–725

Munoz DJ, Thorne PR, Housley GD, Billett TE, Battersby JM (1995)Extracellular adenosine 5’-triphosphate (ATP) in the endolymphaticcompartment influences cochlear function. Hear Res 90:106–118

Nickel R, Becker D, Forge A (2006) Molecular and functional character-ization of gap junctions in the avian inner ear. J Neurosci 26:6190–6199

Oesterle EC, Dallos P (1990) Intracellular recordings from supporting cells inthe guinea pig cochlea: DC potentials. J Neurophysiol 64:617–636

Oshima A, Doi T, Mitsuoka K, Maeda S, Fujiyoshi Y (2003) Roles ofMet-34, Cys-64, and Arg-75 in the assembly of human connexin 26.Implication for key amino acid residues for channel formation andfunction. J Biol Chem 278:1807–1816

Piazza V, Beltramello M, Menniti M, Colao E, Malatesta P, Argento R,Chiarella G, Gallo LV, Catalano M, Perrotti N, Mammano F,Cassandro E (2005) Functional analysis of R75Q mutation in thegene coding for Connexin 26 identified in a family with nonsyn-dromic hearing loss. Clin Genet 68:161–166

Plum A, Winterhager E, Pesch J, Lautermann J, Hallas G, Rosentreter B,Traub O, Herberhold C, Willecke K (2001) Connexin31-deficiencyin mice causes transient placental dysmorphogenesis but does notimpair hearing and skin differentiation. Dev Biol 231:334–347

Rabionet R, Lopez-Bigas N, Arbones ML, Estivill X (2002) Connexinmutations in hearing loss, dermatological and neurological disor-ders. Trends Mol Med 8:205–212

Richard G, White TW, Smith LE, Bailey RA, Compton JG, Paul DL,Bale SJ (1998) Functional defects of Cx26 resulting from a hetero-zygous missense mutation in a family with dominant deaf-mutismand palmoplantar keratoderma. Hum Genet 103:393–399

Richard G, Rouan F, Willoughby CE, Brown N, Chung P, Ryynanen M,Jabs EW, Bale SJ, DiGiovanna JJ, Uitto J, Russell L (2002)Missense mutations in GJB2 encoding connexin-26 cause the ecto-dermal dysplasia keratitis-ichthyosis-deafness syndrome. Am JHum Genet 70:1341–1348

Rodriguez-Paris J, Schrijver I (2009) The digenic hypothesis unraveled: theGJB6 del (GJB6-D13S1830) mutation causes allele-specific loss ofGJB2 expression in cis. Biochem Biophys Res Commun 389:354–359

Santos-Sacchi J, Dallos P (1983) Intercellular communication in thesupporting cells of the organ of Corti. Hear Res 9:317–326

Schutz M, Scimemi P, Majumder P, De Siati RD, Crispino G, RodriguezL, BortolozziM, Santarelli R, Seydel A, Sonntag S, InghamN, SteelKP, Willecke K, Mammano F (2010) The human deafness-associated connexin 30 T5M mutation causes mild hearing lossand reduces biochemical coupling among cochlear non-sensorycells in knock-in mice. Hum Mol Genet

Skerrett IM, Di WL, Kasperek EM, Kelsell DP, Nicholson BJ (2004)Aberrant gating, but a normal expression pattern, underlies therecessive phenotype of the deafness mutant Connexin26M34T.FASEB J 18:860–862

Snoeckx RL et al (2005) GJB2 mutations and degree of hearing loss: amulticenter study. Am J Hum Genet 77:945–957

Spray DC, Ye ZC, Ransom BR (2006) Functional connexin “hemichan-nels”: a critical appraisal. Glia 54:758–773

Sun J, Ahmad S, Chen S, Tang W, Zhang Y, Chen P, Lin X (2005)Cochlear gap junctions coassembled from Cx26 and 30 show fasterintercellular Ca2+ signaling than homomeric counterparts. Am JPhysiol Cell Physiol 288:C613–C623

Suzuki T, Takamatsu T, Oyamada M (2003) Expression of gapjunction protein connexin43 in the adult rat cochlea: compar-ison with connexin26. J Histochem Cytochem 51:903–912

Tang W, Zhang Y, Chang Q, Ahmad S, Dahlke I, Yi H, Chen P,Paul DL, Lin X (2006) Connexin29 is highly expressed incochlear Schwann cells, and it is required for the normaldevelopment and function of the auditory nerve of mice. JNeurosci 26:1991–1999

Taylor RR, Jagger DJ, Forge A (2012) Defining the cellular environmentin the organ of Corti following extensive hair cell loss: a basis forfuture sensory cell replacement in the cochlea. PLoS ONE 7:e30577

Teubner B, Michel V, Pesch J, Lautermann J, Cohen-Salmon M, Sohl G,Jahnke K, Winterhager E, Herberhold C, Hardelin JP, Petit C,Willecke K (2003) Connexin30 (Gjb6)-deficiency causes severehearing impairment and lack of endocochlear potential. Hum MolGenet 12:13–21

Thomas T, Telford D, Laird DW (2004) Functional domain mapping andselective trans-dominant effects exhibited by Cx26 disease-causingmutations. J Biol Chem 279:19157–19168

Thonnissen E, Rabionet R, Arbones ML, Estivill X, Willecke K, Ott T(2002) Human connexin26 (GJB2) deafness mutations affect thefunction of gap junction channels at different levels of proteinexpression. Hum Genet 111:190–197

Tritsch NX, Yi E, Gale JE, Glowatzki E, Bergles DE (2007) The origin ofspontaneous activity in the developing auditory system. Nature 450:50–55

Uyguner O, Tukel T, Baykal C, Eris H, Emiroglu M, Hafiz G, GhanbariA, Baserer N, Yuksel-Apak M, Wollnik B (2002) The novel R75Qmutation in the GJB2 gene causes autosomal dominant hearing lossand palmoplantar keratoderma in a Turkish family. Clin Genet 62:306–309

ValiunasV,MantheyD,Vogel R,WilleckeK,Weingart R (1999)Biophysicalproperties of mouse connexin30 gap junction channels studied intransfected human HeLa cells. J Physiol 519(Pt 3):631–644

Wang HL, Chang WT, Li AH, Yeh TH, Wu CY, Chen MS, Huang PC(2003) Functional analysis of connexin-26 mutants associated withhereditary recessive deafness. J Neurochem 84:735–742

Wang Y, Chang Q, Tang W, Sun Y, Zhou B, Li H, Lin X (2009) Targetedconnexin26 ablation arrests postnatal development of the organ ofCorti. Biochem Biophys Res Commun 385:33–37

Wangemann P (2002) K(+) cycling and its regulation in the cochlea andthe vestibular labyrinth. Audiol Neurootol 7:199–205

Wangemann P (2006) Supporting sensory transduction: cochlear fluidhomeostasis and the endocochlear potential. J Physiol 576:11–21

Wilch E, Zhu M, Burkhart KB, Regier M, Elfenbein JL, Fisher RA,Friderici KH (2006) Expression of GJB2 and GJB6 is reduced in anovel DFNB1 allele. Am J Hum Genet 79:174–179

Xia JH, Liu CY, Tang BS, Pan Q, Huang L, Dai HP, Zhang BR, Xie W,Hu DX, Zheng D, Shi XL,Wang DA, Xia K, Yu KP, Liao XD, FengY, Yang YF, Xiao JY, Xie DH, Huang JZ (1998) Mutations in thegene encoding gap junction protein beta-3 associated with autoso-mal dominant hearing impairment. Nat Genet 20:370–373

Xia AP, Ikeda K, Katori Y, Oshima T, Kikuchi T, Takasaka T (2000)Expression of connexin 31 in the developing mouse cochlea.Neuroreport 11:2449–2453

Xu J, Nicholson BJ (2013) The role of connexins in ear and skinphysiology - functional insights from disease-associated mutations.Biochim Biophys Acta 1828:167–178

Zhang Y, Tang W, Ahmad S, Sipp JA, Chen P, Lin X (2005) Gapjunction-mediated intercellular biochemical coupling in cochlearsupporting cells is required for normal cochlear functions. ProcNatl Acad Sci U S A 102:15201–15206

Cell Tissue Res (2015) 360:633–644 643

Zhao HB (2000) Directional rectification of gap junctional voltage gatingbetween dieters cells in the inner ear of guinea pig. Neurosci Lett296:105–108

Zhao HB (2005) Connexin26 is responsible for anionic molecule perme-ability in the cochlea for intercellular signalling and metaboliccommunications. Eur J Neurosci 21:1859–1868

Zhao HB, Santos-Sacchi J (1998) Effect of membrane tension on gapjunctional conductance of supporting cells in Corti’s organ. J GenPhysiol 112:447–455

Zhao HB, Yu N (2006) Distinct and gradient distributions of connexin26and connexin30 in the cochlear sensory epithelium of guinea pigs. JComp Neurol 499:506–518

Zhao HB, Yu N, Fleming CR (2005) Gap junctional hemichannel-mediated ATP release and hearing controls in the inner ear. ProcNatl Acad Sci U S A 102:18724–18729

Zhao HB, Kikuchi T, Ngezahayo A, White TW (2006) Gapjunctions and cochlear homeostasis. J Membr Biol 209:177–186

644 Cell Tissue Res (2015) 360:633–644