M-opsin protein degradation is inhibited by MG-132 in Rpe65 ^-/- retinal explant culture

Purpose: The 65 kDa retinal pigment epithelium-specific protein, RPE65, is an essential enzyme for 11-cis-retinal synthesis in the eye. Mutations of the RPE65 gene in humans result in severe vision loss, and Rpe65 ^-/- mice show early cone photoreceptor degeneration. We used an explant culture system to evaluate whether posttranslational downregulation of M-opsin protein in Rpe65 ^-/- mice is caused by proteolytic degradation. Methods: The eyes of three-week-old Rpe65 ^-/- mice were incubated in culture medium. Western blot analysis was used to evaluate the level of M-opsin protein, and immunofluorescence was used for protein localization. The transcriptional level of M-opsin was evaluated with real-time reverse-transcriptase-PCR. Results: Degradation of the M-opsin protein in Rpe65 ^-/- mouse retina was inhibited by the proteasome inhibitor MG-132 but not by the lysosomal inhibitor pepstatin A and E64d. 9-cis-retinal, used as an analog of 11-cis-retinal, increased Mopsin protein but did not increase M-opsin mRNA. Moreover, 9-cis-retinal did not change the transcriptional levels of photoreceptor specific genes. Conclusions: Our data suggest that M-opsin protein was degraded through a proteasome pathway and that M-opsin degradation was suppressed with 9-cis-retinal treatment in Rpe65 ^-/- mice to some extent.