Opsin gene expression during early and late phases of retinal degeneration in rds mice

Opsin mRNA levels, opsin synthetic rates and localization of opsin were studied throughout the photoreceptor's life span in the rds mice. Mutant mice 11 days to 11 months old were investigated. Opsin mRNA levels were studied by means of northern blot analysis. Opsin synthesis was measured by incorporation of [35S]methionine into newly synthesized opsin in vitro. Distribution of opsin in the retina was determined by immunoelectron microscopy. Opsin mRNA was detected in young as well as old retinas, and opsin synthesis could be detected at early phases of degeneration but not in late phases. The absence of opsin synthesis in older rds mice might be due to translational down-regulation or some other defect in the capacity to synthesize opsin. In young mice, opsin was detected in the subretinal space in opsin-laden vesicular membranes: such membranes were absent from retinas of older mice. This disappearance parallels the cessation of opsin synthesis and the consequent failure to deliver opsin to the subretinal space in retinas from older mice. Immunochemical analysis revealed the presence of small amounts of opsin in all retinas up to 11 months of age. Immunoelectron microscopy localized the residual opsin, mostly to the plasma membrane which envelops the nuclei and synaptic terminals. These opsin molecules might be a consequence of very low levels of opsin synthesis, too low to be detected by our assays, or may have been synthesized at an earlier age and retained in the plasma membrane of the old mutant photoreceptors.