Gap junctions in the inner ear: comparison of distribution patterns in different vertebrates and assessement of connexin composition in mammals

The distribution and size of gap junctions (GJ) in the sensory epithelia of the inner ear have been examined in a reptile (gecko), birds (chicken and owl), and mammals (mouse, guinea pig, gerbil, and bat), and the connexin composition of GJs in the mammalian inner ear has been assessed. Freeze fracture revealed a common pattern of GJ distribution in auditory and vestibular sensory epithelia in the different vertebrate classes. In all these tissues, GJs are numerous, often occupying more than 25% of the plasma membrane area of supporting cells and sometimes composed of more than 100,000 channels. Screening for 12 members of the connexin family in the mammalian inner ear by RT-PCR, Western blotting, and immunohistochemistry revealed four connexin isotypes, cx26, cx30, cx31, and cx43, in the cochlea and three, cx26, cx30, and cx43, in the vestibular organs. With antibodies characterised for their specificity, cx26 and cx30 colocalised in supporting cells of the organ of Corti, in the basal cell region of the stria vascularis, and in type 1 fibrocytes of the spiral ligament. No other connexin was detected in these regions. Cx31 was localised among type 2 fibrocytes below the spiral prominence, a region where cx30 was not expressed and cx26 expression appeared to be low. Cx43 was detected only in the region of "tension fibrocytes" lining the inner aspect of the otic capsule. This suggests separate functional compartments in the cochlea. In addition to cx26 and cx30, cx43 was detected in supporting cells of the vestibular sensory epithelia. Where cx26 and cx30 were colocalised, double immunogold labelling of thin sections showed both cx26 and cx30 evenly distributed in individual GJ plaques, a pattern consistent with the presence of heteromeric connexons. Coimmunoprecipitation of cochlear membrane proteins solubilised with a procedure that preserves the oligomeric structure of connexons confirmed the presence of heteromeric cx26/cx30 connexons. Heteromeric cx26/cx30 connexons may be unique to the inner ear, which could be one factor underlying the non-syndromic character of the deafness caused by mutations in cx26.

Connexins and gap junctions in the inner ear

Mutations in the genes for three different isotypes of the gap junction channel protein connexin are associated with deafness. This indicates an important role for gap junctions in auditory function and provides an opportunity to explore structure-function relationships in the connexin molecule. We have been examining the distribution of gap junctions and the pattern of connexin expression in the mature inner ear and during development, and the effect of specific mutations on the processing and functionality of the expressed connexin proteins in an in vitro system.

Gap junctions and connexin expression in the inner ear

Several different recessive mutations in the connexin26 (Cx26; beta 2) gene have been associated with non-syndromic hereditary deafness. This suggests gap junctions are important to cochlear function. Numerous large gap junctions are present between adjacent supporting cells in both the vestibular and auditory sensory epithelia of the mature inner ear. In vestibular organs, Cx26 is highly expressed, but antibodies of Cx32 (beta 1) also label the supporting cells. In the organ of Corti of the cochlea, Cx26 is the predominant connexin isoform; neither Cx32 nor Cx43 (alpha 1) can be detected by immunohistochemistry. One role for gap junctions between supporting cells may be to provide a pathway for the rapid removal of ions away from the region of the sensory cells during transduction in order to maintain sensitivity. In the cochlea gap junctions are also associated with the basal cells of the stria vascularis, an ion-transporting epithelium that maintains a positive electrical potential in the potassium-rich endolymph fluid which bathes the apical surfaces of the sensory 'hair' cells and which is crucial for auditory transduction. Gap junctions are present between fibrocytes in the spiral ligament that underlies the stria vascularis, and between these fibrocytes and strial basal cells. During cochlear development, the initial formation and subsequent increase in size and number of gap junctions in the stria vascularis coincides with the initial generation and rise of the endocochlear potential. This and other evidence suggests that one role of gap junctions in the cochlea is to provide a pathway for passage of ions to maintain endolymph and, thus, auditory acuity. Mutations to Cx26 could, therefore, disrupt this ion circulation, resulting in deafness.