The outer segments of photoreceptors are specialized sensory cilia crucial for light detection. Any disruption that alters outer segment morphology can impair photoreceptor function and therefore vision. Progressive rod-cone degeneration (PRCD) is an integral membrane protein exclusively present in the photoreceptor OS with an unknown function. Multiple mutations in PRCD are linked with retinitis pigmentosa. The most common PRCD mutation observed in both human and multiple dog breeds, PRCD-C2Y, lacks the lipid modification \"palmitoylation,\" which is crucial for protein stability and trafficking to the OS. Previous studies including ours show impaired disc morphogenesis and rhodopsin distributions in the absence of PRCD, but the precise role of PRCD in maintaining OS structure and function remains unclear. In this chapter, we discuss the potential role of PRCD in the maintenance of photoreceptor OS structural and functional integrity.

Progressive rod-cone degeneration (PRCD) is a photoreceptor outer segment (OS) disc-specific protein essential for maintaining OS structures while contributing to rhodopsin packaging densities and distribution in disc membranes. Previously, we showed PRCD undergoing palmitoylation at the sole cysteine (Cys2), where a mutation linked with retinitis pigmentosa (RP) in humans and dogs demonstrates the importance of palmitoylation for protein stability and trafficking to the OS. We demonstrate a mutation, in the polybasic region (PBR) of PRCD (Arg17Cys) linked with RP where an additional lipidation is observed through acyl-RAC. Immunolocalization of transiently expressed R17C in hRPE1 cells depicts similar characteristics to wild-type PRCD; however, a double mutant lacking endogenous palmitoylation at Cys2Tyr with Arg17Cys is comparable to the C2Y protein as both aggregate, mislocalized to the subcellular compartments within the cytoplasm. Subretinal injection of PRCD mutant constructs followed by electroporation in murine retina exhibit mislocalization in the inner segment. Despite being additionally lipidated and demonstrating strong membrane association, the mutation in the PBR affects protein stability and localization to the OS. Acylation within the PBR alone neither compensates for protein stability nor trafficking, revealing defects in the PBR likely lead to dysregulation of PRCD protein associated with blinding diseases.

The purpose of this study was to establish spectral domain optical coherence tomography (SD-OCT) assessment data in well-established canine models of inherited retinal dystrophies: PDE6B-rod-cone dysplasia 1 (RCD1: early onset retinitis pigmentosa), PRCD-progressive rod-cone degeneration (PRCD: late onset retinitis pigmentosa), CNGB3-achromatopsia, and RPE65-Leber congenital amaurosis (LCA). High resolution SD-OCT images of the retina were acquired from both eyes in 5 planes: temporal; superotemporal; superior; nasal; and inferior in adult dogs with: RCD1 (n = 4 dogs, median age: 1.5 yrs); PRCD (n = 2, 4.3 yrs); LCA (n = 3, 5.2 yrs); achromatopsia (n = 3, 4.2 yrs); and wild types (wt, n = 6, 5.5 yrs). Total, inner and outer retinal thicknesses and ellipsoid zone were analyzed. In selected animals, histomorphometric evaluations were performed. In dogs with RCD1, PRCD, and LCA, the thickness of the outer retina was, compared to wt, significantly decreased (p ≤ 0.02) in all OCT imaging planes, and in superotemporal and inferior imaging planes in dogs with achromatopsia. No significant thinning was observed in inner retina thickness in any disease model except in the inferior imaging plane in dogs with RCD1. Dogs with RCD1, PRCD, and LCA had significantly more areas with disrupted ellipsoid zone in the presumed area centralis than wt (p ≤ 0.001). OCT findings provide baseline information for research of retinal dystrophies using these canine models.

PRCD (progressive rod-cone degeneration) is a small ~6 kDa protein with unknown function that specifically resides in photoreceptor discs and interacts with rhodopsin. PRCD\'s discovery resulted from decades-long study of a canine retinal disease called progressive rod-cone degeneration which is one of the most frequent causes of blindness in dogs characterized by the slow, progressive death of rod photoreceptors followed by cones. A series of genetic studies eventually mapped the disease to a single point mutation in a novel gene which was then named Prcd. Highlighting the importance of this gene, this and several other mutations have been identified in human patients suffering from retinitis pigmentosa. In this review, we highlight what is currently known about PRCD protein, including the etiology and pathology of the retinal disease caused by its mutation, the protein\'s trafficking, localization, and biochemical characterization.

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.

Progressive rod-cone degeneration (PRCD) is a photoreceptor outer segment (OS) disc-specific protein with unknown function that is associated with retinitis pigmentosa (RP). The most common mutation in PRCD linked with severe RP phenotype is substitution of the only cysteine to tyrosine (C2Y). In this study, we find that PRCD is post-translationally modified by a palmitoyl lipid group at the cysteine residue linked with RP. Disrupting PRCD palmitoylation either chemically or by genetically eliminating the modified cysteine dramatically affects the stability of PRCD. Furthermore, in vivo electroporation of PRCD C2Y mutant in the mouse retina demonstrates that the palmitoylation of PRCD is important for its proper localization in the photoreceptor OS. Mutant PRCD C2Y was found in the inner segment in contrast to normal localization of WT PRCD in the OS. Our results also suggest that zDHHC3, a palmitoyl acyltransferase (PAT), catalyzes the palmitoylation of PRCD in the Golgi compartment. In conclusion, we find that the palmitoylation of PRCD is crucial for its trafficking to the photoreceptor OS and mislocalization of this protein likely leads to RP-related phenotypes.

Retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration. Mutations of the PRCD gene are associated with RP in both dogs and humans. To date, four distinct PRCD mutations have been reported worldwide. Here we report the clinical phenotype of another patient with PRCD mutations, carrying the known p.R18X mutation and a novel missense mutation, p.P25T. This mutation affects a highly conserved amino acid, is predicted to be damaging by several prediction tools, and was not found in the public databases or in 115 ethnically-matched control individuals. The phenotype of this patient resembles that of previously reported patients with PRCD mutations, including bull\'s eye maculopathy, which appears to be a hallmark of the PRCD-induced phenotype. PRCD encodes for a 54 amino acids long protein with unknown function. The first 20 amino acids appear to encode for a signal peptide (SP), suggesting that PRCD is a secreted protein. To study PRCD secretion, C-terminally myc-tagged PRCD was expressed in cultured cells. Cells and conditioned media were analyzed by Western blot. PRCD was found in both cell extracts and media. However, a truncated PRCD protein lacking the first 20 amino acids was present only in cell extracts and not in media, confirming that PRCD extracellular secretion is mediated by its N-terminal SP. To characterize the secretory pathway of PRCD, various pharmacological agents which interfere with transport of proteins through the ER and Golgi to the plasma membrane were used. PRCD secretion was significantly inhibited by all tested pharmacological agents, confirming that it is secreted through the classic ER/Golgi-dependent secretory pathway. We tested the effect of two mutations on the PRCD protein, and found that p.C2Y, but not p.P25T, affects protein stability, and that neither mutation affects secretion. Our data suggest that PRCD functions as a secreted protein. These findings shed a new light on PRCD function and the etiology of RP.