Novel calcium sensor in the outer hair cells: quantitation of oncomodulin

The parvalbumins are small vertebrate-specific Ca 2+ -binding proteins. The mammalian α -parvalbumin is expressed in diverse cell types, including the inner hair cell, where it is proposed to serve as a cytosolic Ca 2+ buffer. By contrast, the mammalian β -parvalbumin (oncomodulin) has a highly restricted distribution and harbors a 'Ca 2+ -specific' site — properties consistent with Ca 2+ -dependent regulatory behavior. The widespread perception of oncomodulin as an oncofetal protein was recently shattered by our discovery that the protein is present in the organ of Corti. Immunohistochemical data for the rat, mouse and gerbil indicate that expression is in fact confined to the outer hair cell. We have sought to confirm and extend this work through biochemical quantitation studies on organ of Corti from rat, mouse, guinea pig and chinchilla. We find that the protein is expressed at comparably high levels (approximately 0.5 mM) in the outer hair cells in all four species. This finding contrasts sharply with the expression pattern in prenatal and neoplastic settings. We have also extended the immunohistochemical analysis to the guinea pig and performed preliminary analysis of oncomodulin mRNA levels by in situ hydridization. On the basis of our data, we suggest that the outer hair cell is the sole site of physiologically relevant oncomodulin expression. Furthermore, we propose that the protein functions in the efferent control of the electromotile response.