Spatiotemporal expression of cochlin in the inner ear of rats during postnatal development

Cochlin (encoded by COCH) constitutes 70% of non-collagenous protein in the inner ear, and the expression of cochlin is highly specific to the inner ear. Eleven missense mutation and one in-frame deletion have been reported in the COCH gene, causing hereditary progressive sensorineural hearing loss and vestibular dysfunction, DFNA9. These data imply that cochlin should bear an essential and crucial role in the inner ear function. However, the role of cochlin has not been fully clarified. We have investigated the spatiotemporal expression of cochlin in the inner ear of rats during postnatal development to better understand the functional role of cochlin. By immunohistochemistry, cochlin expression was faint in the cochlea and vestibule on the 6th day after birth (DAB6). At DAB70, strong expression of cochlin was detected in the spiral limbus and spiral ligament within the cochlea, and in the stromata of the maculae of otolithic organs and crista ampullaris within the vestibule. Immunoreactivity for cochlin increased during the postnatal development. Western blot analysis also showed an increase in the expression of cochlin isoforms. Furthermore, the dominant isoform of cochlin expressed changed from p63s to p40s between DAB24 and DAB70. These results suggest that the expression of cochlin may be related to the maturation of inner ear function, and the change in isoforms of cochlin expressed will provide important insight into the understanding of both cochlin function and formation of cochlin isoforms. This is the first to report about the spatiotemporal expression of cochlin in the developing rat inner ear.