Endoplasmic reticulum‐associated degradation of mutant rhodopsin disrupts protein homeostasis in photoreceptor cells during retinal degeneration (558.3)

Abstract

Rhodopsin is a G‐protein coupled receptor that is expressed exclusively by rod photoreceptors of the retina and is essential for phototransduction and vision. Over 150 missense mutations in rhodopsin have been identified in the hereditary forms of retinal degeneration. Many of these missense mutations, such as P23H mutation, cause rhodopsin protein misfolding and ultimately lead to rod cell death and vision loss via poorly understood mechanisms. Here, we examined a P23H knock‐in mouse model that exactly recapitulates the gene dosage and retinal degeneration disease phenotype seen in human families carrying P23H rhodopsin alleles. In these retinas, we find that photoreceptors immediately target P23H rhodopsin for elimination as soon as it is produced. Through analysis of P23H knock‐in mice crossed with Xbp1‐Venus reporter mice, we find that photoreceptors expressing P23H rhodopsin strongly activate the IRE1 branch of the UPR. IRE1 activation leads to pronounced upregulation of ER‐associated protein degradation in photoreceptors, pronounced ubiquitination of P23H rhodopsin, and its rapid degradation. By contrast to heterologous cell culture studies of misfolded rhodopsin, virtually no misfolded rhodopsin protein accumulates within photoreceptors as ER‐retained aggregates. Consistent with the efficient elimination of P23H rhodopsin from the ER of photoreceptors, we find no increase in proapoptotic Chop, induced by persistent ER protein aggregation. Surprisingly, despite the induction of ERAD and rapid elimination of P23H rhodopsin, total cellular levels of ubiquitylated proteins markedly increase during retinal degeneration. Our findings show that the IRE1 branch of the UPR promotes robust ERAD of P23H rhodopsin in photoreceptors. We suggest that the increased buildup of damaged, ubiquitylated proteins arises as a consequence of ERAD of P23H rhodopsin, and disruption of cellular protein homeostasis leads to rod photoreceptor cell death and retinal degeneration.