PRCD is essential for high-fidelity photoreceptor disc formation

Abstract

Progressive rod-cone degeneration (PRCD) is a small protein residing in the light-sensitive disc membranes of the photoreceptor outer segment. Until now, the function of PRCD has remained enigmatic despite multiple demonstrations that its mutations cause blindness in humans and dogs. Here, we generated a PRCD knockout mouse and observed a striking defect in disc morphogenesis, whereby newly forming discs do not properly flatten. This leads to the budding of disc-derived vesicles, specifically at the site of disc morphogenesis, which accumulate in the interphotoreceptor matrix. The defect in nascent disc flattening only minimally alters the photoreceptor outer segment architecture beyond the site of new disc formation and does not affect the abundance of outer segment proteins and the photoreceptor’s ability to generate responses to light. Interestingly, the retinal pigment epithelium, responsible for normal phagocytosis of shed outer segment material, lacks the capacity to clear the disc-derived vesicles. This deficiency is partially compensated by a unique pattern of microglial migration to the site of disc formation where they actively phagocytize vesicles. However, the microglial response is insufficient to prevent vesicular accumulation and photoreceptors of PRCD knockout mice undergo slow, progressive degeneration. Taken together, these data show that the function of PRCD is to keep evaginating membranes of new discs tightly apposed to each other, which is essential for the high fidelity of photoreceptor disc morphogenesis and photoreceptor survival.

Loss of PRCD alters number and packaging density of rhodopsin in rod photoreceptor disc membranes

“…These models also show that retina lacking PRCD develop an abundance of extracellular vesicles (EV) containing rhodopsin, which phagocytic microglia cannot manage to clear properly 34,35 . Spencer et al demonstrates that discs lacking PRCD do not form properly and that prior to flattening, membrane bulges containing rhodopsin separate into EV in the interphotoreceptor space 34 . We were able to identify several examples of EV in our model, but they often appear in the same areas as fixation artifacts and were also observed in WT retina.…”

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

“…The Cys2Tyr mutation results in mislocalization of PRCD from the OS to the ONL where it is actively degraded [109]. In the PRs of mice with homozygous Prcd tm1Vya mutations, loss of PRCD results in the formation of bulging discs that do not properly flatten and in the accumulation of extracellular vesicles that originate at the OS base [112]. Interestingly, mutant PRs are able to form membrane discs and the distribution of OS proteins and light response do not appear to be perturbed.…”

“…While activated microglia infiltrate the interphotoreceptor space to remove extracellular vesicles and debris, removal is insufficient and PRs undergo a very slow degeneration. Homozygous Prcd tm1Vya mice show only 36% ONL loss at 17 months [112] while in Prcd tm1(KOMP)Mbp homozygotes a similar loss is observed at 30 weeks of age [110]. Both models are knockout alleles that target the 5 end of Prcd but are on different genetic backgrounds.…”

Measurement of Diurnal Variation in Rod Outer Segment Length In Vivo in Mice With the OCT Optoretinogram

“…Despite a substantial body of research, [14][15][16][17][18][19][20][21][22][23][24][25] the cellular and molecular mechanisms governing outer segment renewal and disc shedding remain an ongoing topic of investigation. [26][27][28][29][30][31][32] Given the importance of the mouse as a model organism for the investigation of the mechanisms of ocular function and disease, [33][34][35][36][37] it would be valuable to be able to noninvasively measure rod outer segment (ROS) renewal in vivo in the mouse.…”