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Wei-Chieh Chiang, Heike Kroeger, Carissa Messah, Douglas Yasumura, Michael T Matthes, Sanae Sakami, Krzysztof Palczewski, Matthew M LaVail, Jonathan H Lin; Endoplasmic Reticulum-Associated Degradation (ERAD) of Mutant Rod Opsin Disrupts Photoreceptor Protein Homeostasis During Retinal Degeneration. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1742.
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© ARVO (1962-2015); The Authors (2016-present)
Rod opsin is a G-protein coupled receptor exclusively expressed by rod photoreceptors. Over 150 mutations in rod opsin have been identified in hereditary forms of retinal degeneration. The P23H rod opsin mutation leads to opsin protein misfolding, increased ER stress, and the activation of ER stress-induced Unfolded Protein Response (UPR) signal transduction network. The role of ER stress and UPR signaling in rod cells is poorly understood. Here, we investigated the function of UPR activation in P23H knock-in (P23H-KIn) mice that recapitulate the gene dosage and retinal degeneration disease phenotype seen in patients carrying P23H opsin alleles.
The rate of retinal degeneration in P23H-KIn homozygous mice was determined by measuring the thickness of the retinal outer nuclear layer at postnatal days 4, 6, 10, 15, 20, 30, 45, 60, and 90. P23H-KIn mice were crossed with Xbp1-Venus (ERAI) reporter mice for monitoring IRE1 pathway activation in retinas expressing P23H opsin.
Through analysis of P23H-KIn;Xbp1-Venus mice, we find that rod cells expressing P23H opsin strongly activate the IRE1 signal transduction pathway of the UPR to generate XBP1 transcription factor. IRE1-XBP1 signaling upregulates many genes required for ER-associated degradation (ERAD) of misfolded proteins in the ER. We find that ERAD components physically associate with P23H opsin in photoreceptors leading to pronounced P23H opsin ubiquitination, and rapid degradation. By contrast to heterologous cell culture studies of misfolded opsin, virtually no misfolded opsin accumulates within photoreceptors as ER-retained aggregates. Consistent with efficient elimination of P23H opsin from the ER of photoreceptors, we find no increase in proapoptotic Chop, induced by persistent ER protein aggregation. Surprisingly, despite ERAD induction and rapid P23H opsin protein degradation, total cellular levels of ubiquitinated proteins markedly increase during retinal degeneration.
Our findings show that IRE1 branch of the UPR promotes robust ERAD of P23H opsin in photoreceptors. We propose that increased buildup of damaged, ubiquitinated proteins arises as a consequence of extensive ERAD of P23H opsin, and disruption of cellular protein homeostasis could lead to rod photoreceptor cell death and retinal degeneration.
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