Spatio-temporal distribution of cellular retinoid binding protein gene transcripts in the developing and the adult cochlea. Morphological and functional consequences in CRABP- and CRBPI-null mutant mice

The expression patterns of the mouse cellular retinoid binding protein gene s were investigated by in situ hybridization analysis in the inner ear from 10.5 days post coitum (dpc) up to the adult stage. The cellular retinoic a cid binding protein II (CRABPII) and cellular retinol binding protein I (CR BPI) were present in a widespread and abundant pattern in cochlear structur es during embryogenesis. Expression of the cellular retinoic acid binding p rotein I (CRABPI) is restricted during development in Kolliker's organ whil st cellular retinol binding protein II (CRBPII) is only visible after birth with a ubiquitous distribution in most regions of the cochlea including ne rvous components. No CRABP or CRBP transcripts were observed in the auditor y receptors. Morphological observations of CRBPI- and CRABPI/CRABPII-null m utant fetus at 18.5 dpc do not show any structural modification at the leve l of the organ of Corti. Furthermore, electrophysiological tests performed by measuring distorsion-product otoacoustic emissions and auditory brainste m evoked responses did not present significant alteration of the auditory f unction for the different types of mutants. The expression of retinoid bind ing proteins in cochlear structures during embryogenesis could suggest impo rtant roles for these proteins during ontogenesis and morphogenesis of the inner ear. Despite these observations, morphological and functional data fr om mutant mice did not present obvious modifications of the cochlear struct ures and auditory thresholds. It is therefore unlikely that CRABPs and CRBP I are directly involved in development of the cochlea and hair cell differe ntiation.