Molecular mechanisms of gap junction assembly in the cochlea

Hereditary hearing loss affects about 1 per 1000 children. Mutations in GJB2 and GJB6, which encode connexins 26 and 30 (Cx26 and Cx30) involved in cochlear homeostasis, cause autosomal syndromic and non-syndromic hearing loss. In cochlear non-sensory supporting cells, Cx26 and Cx30 form two types of homomeric and heteromeric gap junctions. Deciphering the assembly mechanisms of these channels in situ should represent an advance in understanding the pathogenic significance of many of these mutations. Connexin trafficking and delivery to the gap junction plaque usually occurs from the peripheral non-junctional membrane region, called perinexus, and requires the actin network. Combining immunolabelling and organotypic in vitro assay, we found that Cx30 homomeric and Cx26/Cx30 heteromeric channels exhibit distinct assembly mechanisms in the cochlea. When expressed as homomeric channels, Cx30 preferentially associates with the actin network in the perinexus and strongly relies on it for gap junction plaque assembly. In contrast, we found that Cx26/Cx30 heteromeric gap junction plaques are devoid of perinexus and associated actin network, and resist to actin-depolymerizating drug. This supports that Cx26/Cx30 oligomers could be directly delivered from the interior of the cell to the junctional plaque. Altogether, our data provide a novel insight in homomeric and heteromeric gap junction plaque assembly in the cochlea.