Effects of Retinoid Treatment on Cochlear Development, Connexin Expression and Hearing Thresholds in Mice

Mutations in GJB2, gene coding for connexin 26 (Cx26), and GJB6, gene coding for connexin 30 (Cx30), are the most common genetic defects causing non-syndromic hereditary hearing loss. We previously reported that overexpression of Cx26 completely rescues the hearing in a mouse model of human GJB6 null mutations. The results suggest that therapeutic agents up-regulating the expression of Cx26 may potentially be a novel treatment for non-syndromic hereditary deafness caused by Cx30 null mutations. Retinoids are a family of vitamin A derivatives that exert broad and profound effects on cochlear protein expression including connexins. They are readily available and already utilized as therapeutic agents for recurrent otitis media and hearing loss due to noise exposure. In this study, we characterized the expression of Cx26 and Cx30 in the postnatal inner ear by different retinoids including retinyl palmitate (RP), the main source of vitamin A in over-the-counter (OTC) supplements, retinyl acetate (RAc) which is an isomer of RP, and all-trans-retinoic acid (ATRA), the most active retinoid derivative. The results revealed ATRA significantly increased cochlear Cx26 expression and improved hearing in Cx30 knockout (KO) mice by 10 dB suggesting its potential benefits as a therapeutic agent. In contrast, RP selectively reduced cochlear Cx30 expression and did not improve hearing thresholds at the dosages we tested.