Robust Endoplasmic Reticulum-Associated Degradation of Rhodopsin Precedes Retinal Degeneration

Abstract

Rhodopsin is a G protein-coupled receptor essential for vision and rod photoreceptor viability. Disease-associated rhodopsin mutations, such as P23H rhodopsin, cause rhodopsin protein misfolding and trigger endoplasmic reticulum (ER) stress, activating the Unfolded Protein Response (UPR). The pathophysiologic effects of ER stress and UPR activation on photoreceptors are unclear. Here, by examining a P23H rhodopsin knock-in mouse, we found that the UPR IRE1 signaling pathway is strongly activated in misfolded rhodopsin-expressing photoreceptors. IRE1 significantly upregulated ER-associated protein degradation (ERAD), triggering pronounced P23H rhodopsin degradation. Rhodopsin protein loss occurred as soon as photoreceptors developed, preceding photoreceptor cell death. By contrast, IRE1 activation did not affect JNK signaling or rhodopsin mRNA levels. Interestingly, pro-apoptotic signaling from the PERK UPR pathway was also not induced. Our findings reveal that an early and significant pathophysiologic effect of ER stress in photoreceptors is the highly efficient elimination of misfolded rhodopsin protein. We propose that early disruption of rhodopsin protein homeostasis in photoreceptors could contribute to retinal degeneration.

Achromatopsia mutations target sequential steps of ATF6 activation

“…Patient fibroblasts with several of these mutations showed increased sensitivity to ER stressinduced cell death and damage. Diverse environmental insults have been found to trigger ER stress, including hypoxia, infection, inflammation, protein misfolding, and light damage (27)(28)(29)(30)(31)(32)(33). Exposure to these insults during retinal development may contribute to the cone dysfunction and vision loss that arises in children with mutations that compromise ATF6 function.…”

Multiple Mechanisms of Unfolded Protein Response–Induced Cell Death

“…In contrast, other IRE1-mediated signaling events, including c-Jun N-terminal kinase activation or RIDD, are not observed in these photoreceptors. 73 Furthermore, minimal activation of the PERK signaling pathway is seen, with no changes in ATF4, CHOP, or IAP levels in P23H rhodopsin-expressing photoreceptors. 73 These findings reveal that the dominant effect of UPR in photoreceptors of P23H rhodopsin knock-in mice is the induction of ERAD to degrade and clear mutant rhodopsin, which is accomplished through a preferential use of the parts of the UPR regulating ERAD, such as IRE1's induction of XBP1.…”

“…73 IRE1's induction of XBP1s, and the transcriptional up-regulation of ERAD by XBP1s, was seen in photoreceptors expressing P23H rhodopsin. 73 Concomitant with ERAD up-regulation, P23H rhodopsin protein is found to be robustly ubiquitinated and almost entirely degraded in these photoreceptors 73 ( Figure 4A). In contrast, other IRE1-mediated signaling events, including c-Jun N-terminal kinase activation or RIDD, are not observed in these photoreceptors.…”

“…65,66 In P23H rhodopsin knock-in animals, rhodopsin protein degradation occurs as soon as photoreceptors are born, and loss of rhodopsin precedes any photoreceptor cell death ( Figure 4A). 73 Disruption of rhodopsin protein homeostasis by ERAD is likely to be a key trigger for photoreceptor cell death.…”

Rhodopsin Trafficking and Mistrafficking

“…UPR activation induced the expression of chaperones and stimulated ERAD. 140,141 Failure of ERAD causes the UPR to switch from protective to proapoptotic as indicated by increased level of a proapoptotic protein CHOP. CHOP mRNA level was significantly higher in P23H transgenic rats than in WT rats.…”