Circadian proteome changes in photoreceptor outer segments

The goal of this study was to better understand the molecular mechanisms of light-induced photoreceptor cell death. Apoptotic photoreceptor cell death is the cause of several common retinal diseases (e.g. age related macular degeneration, retinitis pigmentosa). Light-induced photoreceptor cell degeneration in laboratory animals is one of the experimental models for studying such human retinal diseases. It begins with the absorption of light by rhodopsin in the photoreceptor outer segments (OS) which eventually leads to apoptotic photoreceptor cell death but the exact molecular mechanisms are yet to be elucidated. To gain insight into these mechanisms we investigated the light-induced and circadian changes in the OS proteome using a quantitative proteomic method called proteolytic 18O labeling. The first study investigated the changes of OS proteome upon intense light exposure. Rapigest, acid cleavable detergent, was used to solubilize OS proteins for tryptic digestion and 18O labeling. The quantities of 11 proteins were found to differ by more than 2-fold upon light exposure. Eight proteins were phototransduction proteins and seven of these were altered to reduce efficiency or quench phototransduction during light exposure. In contrast, the amount of rhodopsin kinase was reduced by 2-fold after light exposure, suggesting attenuation in the mechanism of quenching phototransduction. Our findings suggest that this reduction may be a contributing factor to light-induced photoreceptor cell death. The second study investigated OS proteome differences between midnight and noon. This study was designed to elucidate the mechanisms of circadian dependent susceptibility to light induced damage, since the susceptibility is greater at night than daytime. OS proteins were fractionated by 1D-SDS-PAGE prior to 18O labeling. Most OS protein amounts remained unchanged at night versus daytime. The most notable difference was the lower amount of anti-stress proteins (crystallin aB, and peroxiredoxin 1) at midnight, suggesting the link with the greater nighttime OS susceptibility to light. Our studies create a foundation for understanding the mechanisms of light-induced photoreceptor cell death.