Mutations in the gene encoding the gap junction protein connexin26 (Cx26) are responsible for the autosomal recessive isolated deafness, DFNB1, which accounts for half of the cases of prelingual profound hereditary deafness in Caucasian populations. To date, in vivo approaches to decipher the role of Cx26 in the inner ear have been hampered by the embryonic lethality of the Cx26 knockout mice. To overcome this difficulty, we performed targeted ablation of Cx26 specifically in one of the two cellular networks that it underlies in the inner ear, namely, the epithelial network. We show that homozygous mutant mice, Cx26(OtogCre), have hearing impairment, but no vestibular dysfunction. The inner ear developed normally. However, on postnatal day 14 (P14), i.e., soon after the onset of hearing, cell death appeared and eventually extended to the cochlear epithelial network and sensory hair cells. Cell death initially affected only the supporting cells of the genuine sensory cell (inner hair cell, IHC), thus suggesting that it could be triggered by the IHC response to sound stimulation. Altogether, our results demonstrate that the Cx26-containing epithelial gap junction network is essential for cochlear function and cell survival. We conclude that prevention of cell death in the sensory epithelium is essential for any attempt to restore the auditory function in DFNB1 patients.

Stria vascularis of the cochlea generates the endocochlear potential and secretes K(+). K(+) is the main charge carrier and the endocochlear potential the main driving force for the sensory transduction that leads to hearing. Stria vascularis consists of two barriers, marginal cells that secrete potassium and basal cells that are coupled via gap junctions to intermediate cells. Mice lacking the KCNJ10 (Kir4.1) K(+) channel in strial intermediate cells did not generate an endocochlear potential. Endolymph volume and K(+) concentration ([K(+)]) were reduced. These studies establish that the KCNJ10 K(+) channel provides the molecular mechanism for generation of the endocochlear potential in concert with other transport pathways that establish the [K(+)] difference across the channel. KCNJ10 is also a limiting pathway for K(+) secretion.

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Immunolocalization studies of mouse cochlea and vestibular end-organ were performed to study the expression pattern of pendrin, the protein encoded by the Pendred syndrome gene (PDS), in the inner ear. The protein was restricted to the areas composed of specialized epithelial cells thought to play a key role in regulating the composition and resorption of endolymph. In the cochlea, pendrin was abundant in the apical membrane of cells in the spiral prominence and outer sulcus cells (along with their root processes). In the vestibular end-organ, pendrin was found in the transitional cells of the cristae ampullaris, utriculi, and sacculi as well as in the apical membrane of cells in the endolymphatic sac. Pdsknockout (Pds )/) ) mice were found to lack pendrin immunoreactivity in all of these locations. Histological studies revealed that the stria vascularis in Pds )/) mice was only two-thirds the thickness seen in wildtype mice, with the strial marginal cells showing irregular shapes and sizes. Functional studies were also performed to examine the role of pendrin in endolymph homeostasis. Using double-barreled electrodes placed in both the cochlea and the utricle, the endocochlear potential and endolymph potassium concentration were measured in wild-type and Pdsmice. Consistent with the altered strial morphology, the endocochlear potential in Pds )/) mice was near zero and did not change during anoxia. On the other hand, the endolymphatic potassium concentration in Pds )/) mice was near normal in the cochlea and utricle. Together, these results suggest that pendrin serves a key role in the functioning of the basal and/ or intermediate cells of the stria vascularis to maintain the endocochlear potential, but not in the potassium secretory function of the marginal cells.