Identifying novel roles for the immunoproteasome in the retina.

Abstract

Immunoproteasome is a proteasome sub-type that is known to produce antigenic peptides for MHC class I presentation. However, immunoproteasome is present in the immune-privileged brain and retina and is upregulated with disease in human retina and injury in mouse retina and brain, suggesting functions unrelated to its role in the immune system. The goal of this thesis is to define novel roles for the immunoproteasome in the retina. Potential functions of the immunoproteasome were defined by comparing the stress response of wild-type and knock-out mice missing one (lmp7-/-(L7)) or two (lmp7-/- /mecl-1-/-(L7M1)) of the three immunoproteasome subunits. Aging was used as a model system for chronic stress. Chronic peroxide exposure in cultured retinal pigment epithelial (RPE) cells developed from wild-type mice was used as an additional stress model. In wild-type retinas and RPE cells, upregulation of immunoproteasome was observed in response to both models of chronic stress. To determine the consequence of eliminating immunoproteasome, the retinas and RPE cells from KO mice were examined.L7M1 retina had significantly elevated levels of photoreceptor apoptosis that further increased with age. In addition, L7M1 cell lines were more susceptible to oxidantinduced death. Together these data suggest immunoproteasome is protective against oxidative stress. The localization of immunoproteasome to the outer plexiform layer in wild-type retina suggested a role in retinal function. Electroretinography was used to test the hypothesis that immunoproteasome is required for maintaining normal visual transmission. Data indicated that immunoproteasome-deficient mice had a decreased bipolar cell response as compared to wild-type. Evaluation of several retinal synapse proteins by Western blot revealed no significant difference in protein content across strains. In addition, gross retinal morphology and bipolar cell density were not different. In conclusion, immunoproteasome-deficiency causes a decrease in visual transmission but the mechanism is still unclear. In summary, these data provide compelling evidence that immunoproteasome has a role in retinal stress response, specifically in protecting against oxidative stress. Furthermore, immunoproteasome-deficient mice have a decreased bipolar cell response as measured by ERG. Altogether, data from this thesis strongly support the hypothesis that immunoproteasome has additional functions in the retina that do not involve immune function.