LOCALIZATION OF ORGAN OF CORTI PROTEIN-II IN THE ADULT AND DEVELOPINGGERBIL COCHLEA

The distribution of organ of Corti protein II (OCP-II) was assessed in the developing and mature gerbil cochlea by light and electron micros copic immunohistochemistry. In the adult cochlea, OCP-II was expressed only in certain epithelial cells which included all supporting cells of the organ of Corti, inner and outer sulcus cells and interdental ce lls. Inner and outer hair cells lacked immunoreactivity. The highest g old particle labeling density was seen overlying intracellular regions devoid of organelles. In the developing inner ear, OCP-II was first d etected at 2 days after birth (DAB) with the strongest staining in imm ature Deiters, inner phalangeal and pillar cells. Immunostaining inten sity increased gradually in cells lying laterally and medially to the more centrally located supporting cells and reached adult levels in al l reactive cell types around 18 DAB. The results demonstrate conclusiv ely that OCP-II is a cytosolic protein and fail to support its role as a transcription factor postulated on the basis of its homology with p 15 or a role in the control of the cell cycle as suggested by its near -identity with p19(Skp1), a cyclin A/CDK2-associated protein. The cont inued high level of expression in the mature cochlea argues against OC P-II's involvement in regulating the development and differentiation o f epithelial cells. The protein's unique distribution and its gradual increase in expression prior to and during the onset and maturation of hearing, however, support its potential function in the recycling of K+ effluxed from hair cells and neurons back to endolymph.