Functional Studies Reveal New Mechanisms for Deafness Caused by Connexin Mutations

Objective: Connexin26 (Cx26) and Cx30 are the major protein subunits forming gap junction (GJ) intercellular channels in the cochlea. Mutations in these 2 Cxs are the major cause of non-syndromic early childhood deafness in humans. The underlying mechanism for cochlear abnormality is unclear. Here, we used targeted Cx30 gene deletion (Cx30(-/-)) mice to investigate molecular mechanisms responsible for Cx mutation-linked deafness. Our hypothesis is that specific loss of GJ-mediated biochemical coupling in the cochlea is sufficient to cause deafness. Study Design: We compared: (1) expression of major cochlear GJ protein subunits, Cx26 and Cx30; and (2) biochemical coupling among cochlear supporting cells in the cochleae of wild type and Cx30(-/-) mice. Methods: Immunolabeling was used to examine the expression of the remaining Cx protein expression in the cochlea of Cx30(-/-) mice. We also used a fluorescent dye diffusion assay performed on a novel flattened cochlear preparation to examine GJ-mediated metabolite transfer among cochlear supporting cells. Results: Estimation of the residual ionic conductance indicated that considerable intercellular ionic coupling remained in the cochlea of Cx30(-/-) mice. Direct measurement of GJ-mediated biochemical coupling showed that the transfer of metabolites among cochlear supporting cells in Cx30(-/-) mice was severely reduced. Conclusion: Our data support that deficiency in GJ-mediated biochemical coupling is sufficient to cause Cx mutation-linked deafness.