Molecular cloning and functional expression of the potassium-dependent sodium-calcium exchanger from human and chicken retinal cone photoreceptors

Light causes a rapid lowering of cytosolic free calcium in the outer segmen ts of both retinal rod and cone photoreceptors. This light-induced lowering of calcium is caused by extrusion via a Na-Ca exchanger located in the rod and cone outer segment plasma membrane and plays a key role in the process of light adaptation. The Na-Ca exchanger in retinal rod outer segment was shown earlier to be a novel Na-Ca+K exchanger (NCKX), and its cDNA was obta ined by molecular cloning from several mammalian species. On the other hand , the proper identity of the retinal cone Na-Ca exchanger, in terms of both functional characteristics (e.g., requirement for and transport of potassi um) and molecular identity, has not yet been elucidated. Here, we report th e molecular cloning, intraretinal localization by in situ hybridization, an d initial functional characterization of the chicken and human cone-specifi c Na-Ca exchangers. In addition we report the chicken rod-specific NCKX. We identified NCKX transcripts in both human and chicken cones and observed s trong potassium-dependent Na-Ca exchange activity after heterologous expres sion of human and chicken cone NCKX cDNAs in cultured insect cells. In situ hybridization in chicken retina showed abundant rod NCKX transcripts only in rod photoreceptors, whereas abundant cone NCKX transcripts were found in most, if not all, cone photoreceptors and also in a subpopulation of retin al ganglion cells. A detailed comparison with the previously described reti nal rod and brain NCKX cDNAs is presented.