Expression pattern of mammalian cochlea outer hair cell (OHC) mRNA: Screening of a rat OHC cDNA library

The aim of this study was to characterize the mRNA content of mammalian coc hlear outer hair cells (OHCs) and to search for specific genes possibly inv olved in their unique properties. Indeed, OHCs, which feature high-frequenc y electromotility, are responsible for the exquisite sensitivity and freque ncy selectivity of the cochlea, Damage to these cells, which occurs in vari ous conditions, causes a reduction in the cochlear sensitivity by about 50 dB and the alteration of frequency discrimination. Total RNA was extracted from about 2000 mechanically dissociated OHCs, and a polymerase chain react ion (PCR) amplified cDNA library was constructed. The presence of the alpha -9 acetylcholine receptor subunit, preferentially expressed in OHCs, was fo und by direct PCR amplification of the library. A systematic sequencing of 218 clones showed 78% known genes, 11% EST-related sequences, and 11% unkno wn genes. The known-gene group was characterized by two main features: a la rge proportion (55%) of mitochondrial transcripts and an abundance in calci um-binding proteins, such as calmodulin and calbindin, for which expression has already been demonstrated in OHCs. Another protein, the oncomodulin re cently shown to be OHC specific, was also found, and its mRNA expression wa s confirmed by irt situ hybridization, Among the 24 unknown genes, 7 were e xpressed in a restricted pattern, including one expressed in cochlea and sp leen and, to a lesser extent, in lungs.