Connexins and gap junctions in the inner ear--it's not just about K⁺ recycling

Abstract

Normal development, function and repair of the sensory epithelia in the inner ear are all dependent on gap junctional intercellular communication. Mutations in the connexin genes GJB2 and GJB6 (encoding CX26 and CX30) result in syndromic and non-syndromic deafness via various mechanisms. Clinical vestibular defects, however, are harder to connect with connexin dysfunction. Cx26 and Cx30 proteins are widely expressed in the epithelial and connective tissues of the cochlea, where they may form homomeric or heteromeric gap junction channels in a cell-specific and spatiotemporally complex fashion. Despite the study of mutant channels and animal models for both recessive and dominant autosomal deafness, it is still unclear why gap junctions are essential for auditory function, and why Cx26 and Cx30 do not compensate for each other in vivo. Cx26 appears to be essential for normal development of the auditory sensory epithelium, but may be dispensable during normal hearing. Cx30 appears to be essential for normal repair following sensory cell loss. The specific modes of intercellular signalling mediated by inner ear gap junction channels remain undetermined, but they are hypothesised to play essential roles in the maintenance of ionic and metabolic homeostasis in the inner ear. Recent studies have highlighted involvement of gap junctions in the transfer of essential second messengers between the non-sensory cells, and have proposed roles for hemichannels in normal hearing. Here, we summarise the current knowledge about the molecular and functional properties of inner ear gap junctions, and about tissue pathologies associated with connexin mutations.

Fig. 1

Expression patterns of Cx26 and Cx30 in epithelial and connective tissues of the cochlea. a A section of the cochlear apical turn of a P30 mouse showing the key sensory and non-sensory tissues involved in auditory sensory transduction. b Higher magnification of the organ of Corti showing the inner hair cells and outer hair cells separated by the tunnel 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 and Claudius’ cells. Immunofluorescent labelling of the organ of Corti (c) for Cx26 (green) and Cx30 (red) reveals a differential pattern of the connexins. 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 of the cochlear lateral wall (d), most gap junction plaques are double-labelled for Cx26 and Cx30. anf auditory nerve fibres, bc basal cell, Cc Claudius’ cell, Dc Deiters’ cell, ec endothelial cell, fc fibrocyte, Hc Hensen’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 pillar cell, Rm Reissner’s membrane, sg spiral ganglion, sl spiral ligament, sv stria vascularis, sm scala media, tm tectorial membrane. Scale bars 20 μm

Fig. 2

Cx26 and Cx30 play specific roles in development, membrane function and epithelial repair of the cochlea. a, b Light microscopy images 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 and spaces of Nuel are absent. b In comparison, in a juvenile wild-type mouse, the organ of Corti has elongated Deiters’ cells and pillar cells, and a fully patent tunnel of Corti (*). c, d Freeze-fracture images of Deiters’ cell membranes in mice. Freeze-fracture reveals gap junction plaques as clusters of particles in the membrane. Each particle represents an individual channel (connexon). c In Cx30-null animals, plaques of the gap junctions between adjacent Deiters’ cells consist of only a few channels, and they are dispersed. d In wild-type animals, the plaques are extremely large (consisting of thousands of channels) and occupy a significant proportion of the membrane area. e, f Transmission electron micrographs of cochlear tissues. e Following loss of outer hair cells from a Cx30-null mouse, the Deiters’ cells remain columnar in shape, the tunnel of Corti is open (*), and the reticular lamina is displaced towards the pillar cells (arrow). f In a wild-type mouse treated systemically with kanamycin and bumetanide to cause loss of inner and outer hair cells, the Deiters’ cells have expanded to fill the gaps left behind by the hair cells. Cells with characteristics of Deiters’ cells have migrated between the outer pillar cells so that the tunnel of Corti appears filled (*). Dc Deiters’ cell, oC organ of Corti, Rm Reissner’s membrane, sg spiral ganglion, sv stria vascularis, Tc tectal cell. Scale bars (a, b, e, f) 10 μm, (c, d) 0.1 μm