Characterization of RPE65 and RDH12, two enzymes associated with retinal dystrophy and retinoid processing

Phototransduction in vertebrate vision is mediated by visual pigments composed of opsin apoproteins covalently attached to a light-sensitive chromophore, 11-cis retinal. Absorption of light isomerizes 11- cis retinal to all-trans retinal, initiating a signal transduction cascade. A complex set of enzyme reactions occurring in the retinal pigment epithelium and the retina is responsible for the synthesis and regeneration of the chromophore and is termed the visual cycle. Several forms of inherited retinal degeneration and dysfunction manifest as a result of mutations in the genes associated with visual cycle function, such as RPE65 and RDH12.;RPE65 is essential for the synthesis of 11-cis retinal and was recently confirmed to function as the visual cycle isomerase. Cone photoreceptors have been proposed to possess an exclusive chromophore regenerative pathway. Using a monoclonal antibody approach, we have now mapped antigenic determinants of the protein surface, shown that RPE65 is not expressed in cone cells, and confirmed that RPE65 is associated with the visual cycle enzyme RDH5.;Rdh12 has an in vitro activity and localization profile that made it an excellent candidate to serve as the all-trans retinal reductase of the visual cycle. Immunochemical analysis localized RDH12 protein to the photoreceptor inner segments and outer nuclear layer in both humans and mice, suggesting an equivalent physiological role for RDH12 in both species. However, analysis of the phenotype of Rdh12-deficient mice revealed no differences in histology, retinoid processing or electroretinogram response compared to wild-type. Rdh12-deficient mice did show a decreased ability to reduce all-trans retinal and 11-cis retinal as measured by in vitro activity assays of retinal homogenates.;These findings suggest that RDH12 function in mice does not directly contribute to visual cycle function. Instead, a critical function of RDH12 is likely the reduction of retinaldehydes that exceed the reductive capacity of the photoreceptor outer segment and gain access to the inner segments in conditions of high illumination. The study of these genes is important not only to gain a better understanding of visual cycle mechanism, but also to elucidate mechanisms of pathogenesis and to develop targeted forms of therapeutic intervention.