Rom-1 is required for rod photoreceptor viability and the regulation of disk morphogenesis

A new mouse model for PRPH2 pattern dystrophy exhibits functional compensation prior and subsequent to retinal degeneration

Mutations in PRPH2 are a relatively common cause of sight-robbing inherited retinal degenerations (IRDs). Peripherin-2 (PRPH2) is a photoreceptor-specific tetraspanin protein that structures the disk rim membranes of rod and cone outer segment (OS) organelles, and is required for OS morphogenesis. PRPH2 is noteworthy for its broad spectrum of disease phenotypes; both inter- and intra-familial heterogeneity have been widely observed and this variability in disease expression and penetrance confounds efforts to understand genotype-phenotype correlations and pathophysiology. Here we report the generation and initial characterization of a gene-edited animal model for PRPH2 disease associated with a nonsense mutation (c.1095:C>A, p.Y285X), which is predicted to truncate the peripherin-2 C-terminal domain. Young (P21) Prph2Y285X/WT mice developed near-normal photoreceptor numbers; however, OS membrane architecture was disrupted, OS protein levels were reduced, and in vivo and ex vivo electroretinography (ERG) analyses found that rod and cone photoreceptor function were each severely reduced. Interestingly, ERG studies also revealed that rod-mediated downstream signaling (b-waves) were functionally compensated in the young animals. This resiliency in retinal function was retained at P90, by which time substantial IRD-related photoreceptor loss had occurred. Altogether, the current studies validate a new mouse model for investigating PRPH2 disease pathophysiology, and demonstrate that rod and cone photoreceptor function and structure are each directly and substantially impaired by the Y285X mutation. They also reveal that Prph2 mutations can induce a functional compensation that resembles homeostatic plasticity, which can stabilize rod-derived signaling, and potentially dampen retinal dysfunction during some PRPH2-associated IRDs.

The Formation and Renewal of Photoreceptor Outer Segments

The visual system is essential for humans to perceive the environment. In the retina, rod and cone photoreceptor neurons are the initial sites where vision forms. The apical region of both cone and rod photoreceptors contains a light-sensing organelle known as the outer segment (OS), which houses tens of thousands of light-sensitive opsins. The OSs of photoreceptors are not static; they require rhythmic renewal to maintain normal physiological functions. Disruptions in OS renewal can lead to various genetic disorders, such as retinitis pigmentosa (RP). Understanding the patterns and molecular mechanisms of photoreceptor OS renewal remains one of the most intriguing topics in visual biology. This review aims to elucidate the structure of photoreceptor OSs, the molecular mechanisms underlying photoreceptor OS renewal, and the retinal diseases resulting from defects in this renewal process. Additionally, we will explore retinal diseases related to photoreceptor OS renewal and potential therapeutic strategies, concluding with a discussion on future research directions for OS renewal.

The role of syntaxins in retinal function and health

The soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE) superfamily plays a pivotal role in cellular trafficking by facilitating membrane fusion events. These SNARE proteins, including syntaxins, assemble into complexes that actively facilitate specific membrane fusion events. Syntaxins, as integral components of the SNARE complex, play a crucial role in initiating and regulating these fusion activities. While specific syntaxins have been extensively studied in various cellular processes, including neurotransmitter release, autophagy and endoplasmic reticulum (ER)-to-Golgi protein transport, their roles in the retina remain less explored. This review aims to enhance our understanding of syntaxins' functions in the retina by shedding light on how syntaxins mediate membrane fusion events unique to the retina. Additionally, we seek to establish a connection between syntaxin mutations and retinal diseases. By exploring the intricate interplay of syntaxins in retinal function and health, we aim to contribute to the broader comprehension of cellular trafficking in the context of retinal physiology and pathology.

Restoring retinal polyunsaturated fatty acid balance and retina function by targeting ceramide in AdipoR1-deficient mice

Mutations in the adiponectin receptor 1 gene (AdipoR1) lead to retinitis pigmentosa and are associated with age-related macular degeneration. This study explores the effects of AdipoR1 gene deficiency in mice, revealing a striking decline in ω3 polyunsaturated fatty acids (PUFA), an increase in ω6 fatty acids, and elevated ceramides in the retina. The AdipoR1 deficiency impairs peroxisome proliferator-activated receptor α signaling, which is crucial for FA metabolism, particularly affecting proteins associated with FA transport and oxidation in the retina and retinal pigmented epithelium. Our lipidomic and proteomic analyses indicate changes that could affect membrane composition and viscosity through altered ω3 PUFA transport and synthesis, suggesting a potential influence of AdipoR1 on these properties. Furthermore, we noted a reduction in the Bardet-Biedl syndrome proteins, which are crucial for forming and maintaining photoreceptor outer segments that are PUFA-enriched ciliary structures. Diminution in Bardet-Biedl syndrome-proteins content combined with our electron microscopic observations raises the possibility that AdipoR1 deficiency might impair ciliary function. Treatment with inhibitors of ceramide synthesis led to substantial elevation of ω3 LC-PUFAs, alleviating photoreceptor degeneration and improving retinal function. These results serve as the proof of concept for a ceramide-targeted strategy to treat retinopathies linked to PUFA deficiency, including age-related macular degeneration.

Identification and cellular localization in Xenopus laevis photoreceptors of three Peripherin-2 family members, Prph2, Rom1 and Gp2l, which arose from gene duplication events in the common ancestors of jawed vertebrates

Rod and cone photoreceptors are named for the distinct morphologies of their outer segment organelles, which are either cylindrical or conical, respectively. The morphologies of the stacked disks that comprise the rod and cone outer segments also differ: rod disks are completely sealed and are discontinuous from the plasma membrane, while cone disks remain partially open to the extracellular space. These morphological differences between photoreceptor types are more prominent in non-mammalian vertebrates, whose cones typically possess a greater proportion of open disks and are more tapered in shape. In mammals, the tetraspanin prph2 generates and maintains the highly curved disk rim regions by forming extended oligomeric structures with itself and a structurally similar paralog, rom1. Here we determined that in addition to these two proteins, there is a third Prph2 family paralog in most non-mammalian vertebrate species, including X. laevis: Glycoprotein 2-like protein or "Gp2l". A survey of multiple genome databases revealed a single invertebrate Prph2 'pro-ortholog' in Amphioxus, several echinoderms and in a diversity of protostomes indicating an ancient divergence from other tetraspanins. Based on phylogenetic analysis, duplication of the vertebrate predecessor likely gave rise to the Gp2l and Prph2/Rom1 clades, with a further duplication distinguishing the Prph2 and Rom1 clades. Mammals have lost Gp2l and their Rom1 has undergone a period of accelerated evolution such that it has lost several features that are retained in non-mammalian vertebrate Rom1. Specifically, Prph2, Gp2l and non-mammalian Rom1 encode proteins with consensus N-linked glycosylation and outer segment localization signals; mammalian rom1 lacks these motifs. We determined that X. laevis gp2l is expressed exclusively in cones and green rods, while X. laevis rom1 is expressed exclusively in rods, and prph2 is present in both rods and cones. The presence of three Prph2-related genes with distinct expression patterns as well as the rapid evolution of mammalian Rom1, may contribute to the more pronounced differences in morphology between rod and cone outer segments and rod and cone disks observed in non-mammalian versus mammalian vertebrates.

ROM1 is redundant to PRPH2 as a molecular building block of photoreceptor disc rims

Visual signal transduction takes place within a stack of flattened membranous 'discs' enclosed within the light-sensitive photoreceptor outer segment. The highly curved rims of these discs, formed in the process of disc enclosure, are fortified by large hetero-oligomeric complexes of two homologous tetraspanin proteins, PRPH2 (a.k.a. peripherin-2 or rds) and ROM1. While mutations in PRPH2 affect the formation of disc rims, the role of ROM1 remains poorly understood. In this study, we found that the knockout of ROM1 causes a compensatory increase in the disc content of PRPH2. Despite this increase, discs of ROM1 knockout mice displayed a delay in disc enclosure associated with a large diameter and lack of incisures in mature discs. Strikingly, further increasing the level of PRPH2 rescued these morphological defects. We next showed that disc rims are still formed in a knockin mouse in which the tetraspanin body of PRPH2 was replaced with that of ROM1. Together, these results demonstrate that, despite its contribution to the formation of disc rims, ROM1 can be replaced by an excess of PRPH2 for timely enclosure of newly forming discs and establishing normal outer segment structure.

ROM1 is redundant to PRPH2 as a molecular building block of photoreceptor disc rims

Visual signal transduction takes place within a stack of flattened membranous "discs" enclosed within the light-sensitive photoreceptor outer segment. The highly curved rims of these discs, formed in the process of disc enclosure, are fortified by large hetero-oligomeric complexes of two homologous tetraspanin proteins, PRPH2 (a.k.a. peripherin-2 or rds) and ROM1. While mutations in PRPH2 affect the formation of disc rims, the role of ROM1 remains poorly understood. In this study, we found that the knockout of ROM1 causes a compensatory increase in the disc content of PRPH2. Despite this increase, discs of ROM1 knockout mice displayed a delay in disc enclosure associated with a large diameter and lack of incisures in mature discs. Strikingly, further increasing the level of PRPH2 rescued these morphological defects. We next showed that disc rims are still formed in a knockin mouse in which the tetraspanin body of PRPH2 was replaced with that of ROM1. Together, these results demonstrate that, despite its contribution to the formation of disc rims, ROM1 can be replaced by an excess of PRPH2 for timely enclosure of newly forming discs and establishing normal outer segment structure.

Photoreceptor disc incisures form as an adaptive mechanism ensuring the completion of disc enclosure

The first steps of vision take place within a stack of tightly packed disc-shaped membranes, or 'discs', located in the outer segment compartment of photoreceptor cells. In rod photoreceptors, discs are enclosed inside the outer segment and contain deep indentations in their rims called 'incisures'. The presence of incisures has been documented in a variety of species, yet their role remains elusive. In this study, we combined traditional electron microscopy with three-dimensional electron tomography to demonstrate that incisures are formed only after discs become completely enclosed. We also observed that, at the earliest stage of their formation, discs are not round as typically depicted but rather are highly irregular in shape and resemble expanding lamellipodia. Using genetically manipulated mice and frogs and measuring outer segment protein abundances by quantitative mass spectrometry, we further found that incisure size is determined by the molar ratio between peripherin-2, a disc rim protein critical for the process of disc enclosure, and rhodopsin, the major structural component of disc membranes. While a high perpherin-2 to rhodopsin ratio causes an increase in incisure size and structural complexity, a low ratio precludes incisure formation. Based on these data, we propose a model whereby normal rods express a modest excess of peripherin-2 over the amount required for complete disc enclosure in order to ensure that this important step of disc formation is accomplished. Once the disc is enclosed, the excess peripherin-2 incorporates into the rim to form an incisure.

Photoreceptor disc incisures form as an adaptive mechanism ensuring the completion of disc enclosure

The first steps of vision take place within a stack of tightly packed disc-shaped membranes, or "discs", located in the outer segment compartment of photoreceptor cells. In rod photoreceptors, discs are enclosed inside the outer segment and contain deep indentations in their rims called "incisures". The presence of incisures has been documented in a variety of species, yet their role remains elusive. In this study, we combined traditional electron microscopy with three-dimensional electron tomography to demonstrate that incisures are formed only after discs become completely enclosed. We also observed that, at the earliest stage of their formation, discs are not round as typically depicted but rather are highly irregular in shape and resemble expanding lamellipodia. Using genetically manipulated mice and frogs and measuring outer segment protein abundances by quantitative mass spectrometry, we further found that incisure size is determined by the molar ratio between peripherin-2, a disc rim protein critical for the process of disc enclosure, and rhodopsin, the major structural component of disc membranes. While a high perpherin-2 to rhodopsin ratio causes an increase in incisure size and structural complexity, a low ratio precludes incisure formation. Based on these data, we propose a model whereby normal rods express a modest excess of peripherin-2 over the amount required for complete disc enclosure in order to ensure that this important step of disc formation is accomplished. Once the disc is enclosed, the excess peripherin-2 incorporates into the rim to form an incisure.

Cryo-EM structures of peripherin-2 and ROM1 suggest multiple roles in photoreceptor membrane morphogenesis

Mammalian peripherin-2 (PRPH2) and rod outer segment membrane protein 1 (ROM1) are retina-specific tetraspanins that partake in the constant renewal of stacked membrane discs of photoreceptor cells that enable vision. Here, we present single-particle cryo-electron microscopy structures of solubilized PRPH2-ROM1 heterodimers and higher-order oligomers. High-risk PRPH2 and ROM1 mutations causing blindness map to the protein-dimer interface. Cysteine bridges connect dimers forming positive-curved oligomers, whereas negative-curved oligomers were observed occasionally. Hexamers and octamers exhibit a secondary micelle that envelopes four carboxyl-terminal helices, supporting a potential role in membrane remodeling. Together, the data indicate multiple structures for PRPH2-ROM1 in creating and maintaining compartmentalization of photoreceptor cells.

Retinitis Pigmentosa: Progress in Molecular Pathology and Biotherapeutical Strategies

Retinitis pigmentosa (RP) is genetically heterogeneous retinopathy caused by photoreceptor cell death and retinal pigment epithelial atrophy that eventually results in blindness in bilateral eyes. Various photoreceptor cell death types and pathological phenotypic changes that have been disclosed in RP demand in-depth research of its pathogenic mechanism that may account for inter-patient heterogeneous responses to mainstream drug treatment. As the primary method for studying the genetic characteristics of RP, molecular biology has been widely used in disease diagnosis and clinical trials. Current technology iterations, such as gene therapy, stem cell therapy, and optogenetics, are advancing towards precise diagnosis and clinical applications. Specifically, technologies, such as effective delivery vectors, CRISPR/Cas9 technology, and iPSC-based cell transplantation, hasten the pace of personalized precision medicine in RP. The combination of conventional therapy and state-of-the-art medication is promising in revolutionizing RP treatment strategies. This article provides an overview of the latest research on the pathogenesis, diagnosis, and treatment of retinitis pigmentosa, aiming for a convenient reference of what has been achieved so far.

Rare and common variants in ROM1 and PRPH2 genes trans-modify Stargardt/ABCA4 disease

Over 1,500 variants in the ABCA4 locus cause phenotypes ranging from severe, early-onset retinal degeneration to very late-onset maculopathies. The resulting ABCA4/Stargardt disease is the most prevalent Mendelian eye disorder, although its underlying clinical heterogeneity, including penetrance of many alleles, are not well-understood. We hypothesized that a share of this complexity is explained by trans-modifiers, i.e., variants in unlinked loci, which are currently unknown. We sought to identify these by performing exome sequencing in a large cohort for a rare disease of 622 cases and compared variation in seven genes known to clinically phenocopy ABCA4 disease to cohorts of ethnically matched controls. We identified a significant enrichment of variants in 2 out of the 7 genes. Moderately rare, likely functional, variants, at the minor allele frequency (MAF) <0.005 and CADD>25, were enriched in ROM1, where 1.3% of 622 patients harbored a ROM1 variant compared to 0.3% of 10,865 controls (p = 2.41E04; OR 3.81 95% CI [1.77; 8.22]). More importantly, analysis of common variants (MAF>0.1) identified a frequent haplotype in PRPH2, tagged by the p.Asp338 variant with MAF = 0.21 in the matched general population that was significantly increased in the patient cohort, MAF 0.25, p = 0.0014. Significant differences were also observed between ABCA4 disease subgroups. In the late-onset subgroup, defined by the hypomorphic p.Asn1868Ile variant and including c.4253+43G>A, the allele frequency for the PRPH2 p.Asp338 variant was 0.15 vs 0.27 in the remaining cohort, p = 0.00057. Known functional data allowed suggesting a mechanism by which the PRPH2 haplotype influences the ABCA4 disease penetrance. These associations were replicated in an independent cohort of 408 patients. The association was highly statistically significant in the combined cohorts of 1,030 cases, p = 4.00E-05 for all patients and p = 0.00014 for the hypomorph subgroup, suggesting a substantial trans-modifying role in ABCA4 disease for both rare and common variants in two unlinked loci.

Photoreceptor Disc Enclosure Is Tightly Controlled by Peripherin-2 Oligomerization

Mutations in the PRPH2 gene encoding the photoreceptor-specific protein PRPH2 (also known as peripherin-2 or rds) cause a broad range of autosomal dominant retinal diseases. Most of these mutations affect the structure of the light-sensitive photoreceptor outer segment, which is composed of a stack of flattened "disc" membranes surrounded by the plasma membrane. The outer segment is renewed on a daily basis in a process whereby new discs are added at the outer segment base and old discs are shed at the outer segment tip. New discs are formed as serial membrane evaginations, which eventually enclose through a complex process of membrane remodeling (completely in rods and partially in cones). As disc enclosure proceeds, PRPH2 localizes to the rims of enclosed discs where it forms oligomers which fortify the highly curved membrane structure of these rims. In this study, we analyzed the outer segment phenotypes of mice of both sexes bearing a single copy of either the C150S or the Y141C PRPH2 mutation known to prevent or increase the degree of PRPH2 oligomerization, respectively. Strikingly, both mutations increased the number of newly forming, not-yet-enclosed discs, indicating that the precision of disc enclosure is regulated by PRPH2 oligomerization. Without tightly controlled enclosure, discs occasionally over-elongate and form large membranous "whorls" instead of disc stacks. These data show that the defects in outer segment structure arising from abnormal PRPH2 oligomerization are manifested at the stage of disc enclosure.SIGNIFICANCE STATEMENT The light-sensitive photoreceptor outer segment contains a stack of flattened "disc" membranes that are surrounded, or "enclosed," by the outer segment membrane. Disc enclosure is an adaptation increasing photoreceptor light sensitivity by facilitating the diffusion of the second messenger along the outer segment axes. However, the molecular mechanisms by which photoreceptor discs enclose within the outer segment membrane remain poorly understood. We now demonstrate that oligomers of the photoreceptor-specific protein peripherin-2, or PRPH2, play an active role in this process. We further propose that defects in disc enclosure because of abnormal PRPH2 oligomerization result in major structural abnormalities of the outer segment, ultimately leading to loss of visual function and cell degeneration in PRPH2 mutant models and human patients.

Identification of Novel EYS Mutations by Targeted Sequencing Analysis

Purpose: Retinitis pigmentosa (RP) is an inherited and progressive degenerative retinal disease that often results in severe vision loss and blindness. However, mutations in known RP disease genes account for only 60% of RP cases, indicating that there are additional pathogenic mutations are yet to be identified. We aimed to identify the causative mutations in the eyes shut homolog (EYS) gene in a cohort of Chinese RP and rod-cone dystrophy families. Materials and Methods: Targeted next-generation sequencing was applied to identify novel mutations in these patients. Candidate variants were evaluated using bioinformatics tools. Mutations were confirmed by Sanger sequencing. Results: We identified eight heterozygous mutations in the EYS gene in the four probands, including a novel frameshift deletion mutation, c.8242_8243del (p.L2748fs); a novel insertion mutation, c.5802_5803insT (p.I1935YfsX6); a novel splicing mutation, c.1300-1G>A; two heterozygous stop-gain mutations, c.1750G>T (p.E584X) and c.8805C>A (p.Y2935X); and three novel missense mutations, c.8269G>A (p.V2757I), c.2545C>T (p.R849C) and c.7506C>A (p.S2502R). Only c.8805C>A had been reported previously in RP patients. None of these mutations were present in 1000 control individuals. Conclusions: We identified seven novel mutations in the EYS gene, expanding the mutational specra of EYS in Chinese patients with RP and rod-cone dystrophy.

ROM1 contributes to phenotypic heterogeneity in PRPH2-associated retinal disease

Peripherin 2 (PRPH2) is a retina-specific tetraspanin protein essential for the formation of rod and cone photoreceptor outer segments (OS). Patients with mutations in PRPH2 exhibit severe retinal degeneration characterized by vast inter- and intra-familial phenotypic heterogeneity. To help understand contributors to this within-mutation disease variability, we asked whether the PRPH2 binding partner rod OS membrane protein 1 (ROM1) could serve as a phenotypic modifier. We utilized knockin and transgenic mouse models to evaluate the structural, functional and biochemical effects of eliminating one allele of Rom1 (Rom1+/-) in three different Prph2 models which mimic human disease: C213Y Prph2 (Prph2C/+), K153Del Prph2 (Prph2K/+) and R172W (Prph2R172W). Reducing Rom1 in the absence of Prph2 mutations (Rom1+/-) had no effect on retinal structure or function. However, the effects of reducing Rom1 in the presence of Prph2 mutations were highly variable. Prph2K/+/Rom1+/- mice had improved rod and cone function compared with Prph2K/+ as well as amelioration of K153Del-associated defects in PRPH2/ROM1 oligomerization. In contrast, Prph2R172W/Rom1+/- animals had worsened rod and cone function and exacerbated retinal degeneration compared with Prph2R172W animals. Removing one allele of Rom1 had no effect in Prph2C/+. Combined, our findings support a role for non-pathogenic ROM1 null variants in contributing to phenotypic variability in mutant PRPH2-associated retinal degeneration. Since the effects of Rom1 reduction are variable, our data suggest that this contribution is specific to the type of Prph2 mutation.

Photoreceptor Disc Enclosure Occurs in the Absence of Normal Peripherin-2/rds Oligomerization

Mutations in the peripherin-2 gene (PRPH2, also known as rds) cause a heterogeneous range of autosomal dominant retinal diseases. PRPH2 encodes a photoreceptor-specific tetraspanin protein, PRPH2, that is a main structural component of the photoreceptor outer segment. PRPH2 distributes to the rims of outer segment disc membranes as they undergo the process of disc membrane enclosure. Within these rims, PRPH2 exists in homo-oligomeric form or as a hetero-oligomer with another tetraspanin protein, ROM1. While complete loss of PRPH2 prevents photoreceptor outer segment formation, mutations affecting the state of its oligomerization, including C150S, C213Y and Y141C, produce outer segment structural defects. In this study, we addressed whether any of these mutations also affect disc enclosure. We employed recently developed methodology for ultrastructural analysis of the retina, involving tissue processing with tannic acid, to assess the status of disc enclosure in knockin mouse models bearing either one or two alleles of the C150S, C213Y and Y141C PRPH2 mutations. While varying degrees of outer segment structural abnormalities were observed in each of these mouse models, they contained both newly forming “open” discs and mature “enclosed” discs. These data demonstrate that normal PRPH2 oligomerization is not essential for photoreceptor disc enclosure.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

The Interplay between Peripherin 2 Complex Formation and Degenerative Retinal Diseases

Peripherin 2 (Prph2) is a photoreceptor-specific tetraspanin protein present in the outer segment (OS) rims of rod and cone photoreceptors. It shares many common features with other tetraspanins, including a large intradiscal loop which contains several cysteines. This loop enables Prph2 to associate with itself to form homo-oligomers or with its homologue, rod outer segment membrane protein 1 (Rom1) to form hetero-tetramers and hetero-octamers. Mutations in PRPH2 cause a multitude of retinal diseases including autosomal dominant retinitis pigmentosa (RP) or cone dominant macular dystrophies. The importance of Prph2 for photoreceptor development, maintenance and function is underscored by the fact that its absence results in a failure to initialize OS formation in rods and formation of severely disorganized OS membranous structures in cones. Although the exact role of Rom1 has not been well studied, it has been concluded that it is not necessary for disc morphogenesis but is required for fine tuning OS disc size and structure. Pathogenic mutations in PRPH2 often result in complex and multifactorial phenotypes, involving not just photoreceptors, as has historically been reasoned, but also secondary effects on the retinal pigment epithelium (RPE) and retinal/choroidal vasculature. The ability of Prph2 to form complexes was identified as a key requirement for the development and maintenance of OS structure and function. Studies using mouse models of pathogenic Prph2 mutations established a connection between changes in complex formation and disease phenotypes. Although progress has been made in the development of therapeutic approaches for retinal diseases in general, the highly complex interplay of functions mediated by Prph2 and the precise regulation of these complexes made it difficult, thus far, to develop a suitable Prph2-specific therapy. Here we describe the latest results obtained in Prph2-associated research and how mouse models provided new insights into the pathogenesis of its related diseases. Furthermore, we give an overview on the current status of the development of therapeutic solutions.

Multistep peripherin-2/rds self-assembly drives membrane curvature for outer segment disk architecture and photoreceptor viability

Rod and cone photoreceptor outer segment (OS) structural integrity is essential for normal vision; disruptions contribute to a broad variety of retinal ciliopathies. OSs possess many hundreds of stacked membranous disks, which capture photons and scaffold the phototransduction cascade. Although the molecular basis of OS structure remains unresolved, recent studies suggest that the photoreceptor-specific tetraspanin, peripherin-2/rds (P/rds), may contribute to the highly curved rim domains at disk edges. Here, we demonstrate that tetrameric P/rds self-assembly is required for generating high-curvature membranes in cellulo, implicating the noncovalent tetramer as a minimal unit of function. P/rds activity was promoted by disulfide-mediated tetramer polymerization, which transformed localized regions of curvature into high-curvature tubules of extended lengths. Transmission electron microscopy visualization of P/rds purified from OS membranes revealed disulfide-linked tetramer chains up to 100 nm long, suggesting that chains maintain membrane curvature continuity over extended distances. We tested this idea in Xenopus laevis photoreceptors, and found that transgenic expression of nonchain-forming P/rds generated abundant high-curvature OS membranes, which were improperly but specifically organized as ectopic incisures and disk rims. These striking phenotypes demonstrate the importance of P/rds tetramer chain formation for the continuity of rim formation during disk morphogenesis. Overall, this study advances understanding of the normal structure and function of P/rds for OS architecture and biogenesis, and clarifies how pathogenic loss-of-function mutations in P/rds cause photoreceptor structural defects to trigger progressive retinal degenerations. It also introduces the possibility that other tetraspanins may generate or sense membrane curvature in support of diverse biological functions.

Retinal proteomic evaluation of rats following streptozotocin‑injection using shotgun proteomics

It is important to elucidate how retinal stimulation leads to retinal protection and dysfunction. The current study aimed to identify factors that are up‑ and downregulated in the retinas of streptozotocin (STZ)‑induced diabetic rats with acute retinal dysfunction. Retinal function was measured and changes in protein expressions were determined using electroretinograms (ERGs) and liquid chromatography/mass spectroscopy‑based shotgun proteomics, respectively. The results revealed that the plasma glucose levels of STZ rats were markedly higher when compared with normal rats. Furthermore, levels of a‑waves, b‑waves and oscillatory potential amplitudes on ERGs in STZ rats were decreased compared with healthy animals. With use of shotgun proteomics, 391 proteins were identified in the retinas of normal rats and 541 proteins were found in the retinas of STZ rats. Of the 560 proteins identified in rat retinas, 372 (66.4%) were present in both normal and STZ rats. Of these, 19 (3.39%) were unique to normal rats and 169 (30.1%) were unique to STZ rats. Gene Ontology analysis was performed on the candidate proteins that were differentially regulated in the retinas of STZ rats and focused on those classified as 'protein binding', which serve important roles in retinal neurodegeneration. The results revealed an excessive expression of retinol‑binding protein 1 (RBP1) and a negative expression of rod outer segment membrane protein 1 (Rom-1) in the retinas of STZ rats. Therefore, retinal function may be decreased with STZ‑induced injury, and expressions of Rom‑1 and RBP1 may be altered in the retinas of STZ rats.

Functional Genomics of the Retina to Elucidate its Construction and Deconstruction

The retina is the light sensitive part of the eye and nervous tissue that have been used extensively to characterize the function of the central nervous system. The retina has a central position both in fundamental biology and in the physiopathology of neurodegenerative diseases. We address the contribution of functional genomics to the understanding of retinal biology by reviewing key events in their historical perspective as an introduction to major findings that were obtained through the study of the retina using genomics, transcriptomics and proteomics. We illustrate our purpose by showing that most of the genes of interest for retinal development and those involved in inherited retinal degenerations have a restricted expression to the retina and most particularly to photoreceptors cells. We show that the exponential growth of data generated by functional genomics is a future challenge not only in terms of storage but also in terms of accessibility to the scientific community of retinal biologists in the future. Finally, we emphasize on novel perspectives that emerge from the development of redox-proteomics, the new frontier in retinal biology.

Cellular mechanisms of hereditary photoreceptor degeneration - Focus on cGMP

The cellular mechanisms underlying hereditary photoreceptor degeneration are still poorly understood, a problem that is exacerbated by the enormous genetic heterogeneity of this disease group. However, the last decade has yielded a wealth of new knowledge on degenerative pathways and their diversity. Notably, a central role of cGMP-signalling has surfaced for photoreceptor cell death triggered by a subset of disease-causing mutations. In this review, we examine key aspects relevant for photoreceptor degeneration of hereditary origin. The topics covered include energy metabolism, epigenetics, protein quality control, as well as cGMP- and Ca²⁺-signalling, and how the related molecular and metabolic processes may trigger photoreceptor demise. We compare and integrate evidence on different cell death mechanisms that have been associated with photoreceptor degeneration, including apoptosis, necrosis, necroptosis, and PARthanatos. A special focus is then put on the mechanisms of cGMP-dependent cell death and how exceedingly high photoreceptor cGMP levels may cause activation of Ca²⁺-dependent calpain-type proteases, histone deacetylases and poly-ADP-ribose polymerase. An evaluation of the available literature reveals that a large group of patients suffering from hereditary photoreceptor degeneration carry mutations that are likely to trigger cGMP-dependent cell death, making this pathway a prime target for future therapy development. Finally, an outlook is given into technological and methodological developments that will with time likely contribute to a comprehensive overview over the entire metabolic complexity of photoreceptor cell death. Building on such developments, new imaging technology and novel biomarkers may be used to develop clinical test strategies, that fully consider the genetic heterogeneity of hereditary retinal degenerations, in order to facilitate clinical testing of novel treatment approaches.

Dissecting the transcriptome landscape of the human fetal neural retina and retinal pigment epithelium by single-cell RNA-seq analysis

The developmental pathway of the neural retina (NR) and retinal pigment epithelium (RPE) has been revealed by extensive research in mice. However, the molecular mechanisms underlying the development of the human NR and RPE, as well as the interactions between these two tissues, have not been well defined. Here, we analyzed 2,421 individual cells from human fetal NR and RPE using single-cell RNA sequencing (RNA-seq) technique and revealed the tightly regulated spatiotemporal gene expression network of human retinal cells. We identified major cell classes of human fetal retina and potential crucial transcription factors for each cell class. We dissected the dynamic expression patterns of visual cycle- and ligand-receptor interaction-related genes in the RPE and NR. Moreover, we provided a map of disease-related genes for human fetal retinal cells and highlighted the importance of retinal progenitor cells as potential targets of inherited retinal diseases. Our findings captured the key in vivo features of the development of the human NR and RPE and offered insightful clues for further functional studies.

Late-onset pattern macular dystrophy mimicking ABCA4 and PRPH2 disease is caused by a homozygous frameshift mutation in ROM1

ROM1 (retinal outer segment membrane protein 1) is a 351-amino acid integral membrane protein on Chromosome 11q, with high structural similarity to PRPH2/RDS. Localized at the rims of photoreceptor outer segments (OSs), it is required for the maintenance of OS structure. Here, we describe a case with a phenotypic manifestation of a homozygous single-base pair deletion, c.712delC (p.Leu238Cysfs*78) in the ROM1 gene, resulting in early termination at exon 2. The variant was detected by whole-exome sequencing (WES) in a 63-yr-old Caucasian woman with late-onset pattern macular dystrophy. Notably, although the phenotype resembles those caused by pathogenic variants in ABCA4 or RDS/PRPH2, no pathogenic variants in these, or any other plausible candidate genes, were identified by WES. Clinical features include the presence of hyperautofluorescent flecks, relative sparing of the central macula, and preserved visual acuity. Reduced visual sensitivity was detected among flecked regions in the retina; however, full-field electroretinogram testing revealed no generalized cone dysfunction. The described first case of the complete loss of ROM1 protein function in the retina suggests its sufficiency for late-onset macular dystrophy. ROM1 and PRPH2 pattern macular dystrophies exhibit phenotype overlap, which may be attributable to their shared role in maintenance of the photoreceptor outer segment structure.

Oligomerization of Prph2 and Rom1 is essential for photoreceptor outer segment formation

Mutations in peripherin 2 (PRPH2, also known as Rds), a tetraspanin protein found in photoreceptor outer segments (OSs), cause retinal degeneration ranging from rod-dominant retinitis pigmentosa (RP) to cone-dominant macular dystrophy (MD). Understanding why some Prph2 mutants affect rods while others affect cones remains a critical unanswered question. Prph2 is essential for OS structure and function and exhibits a very specific pattern of oligomerization with its homolog Rom1. Non-covalent Prph2/Rom1 homo- and hetero-tetramers assemble into higher-order covalently linked complexes held together by an intermolecular disulfide bond at Prph2-C150/Rom1-C153. Here we disrupt this crucial bond using a C150S-Prph2 knockin mouse line to study the role of Prph2 higher-order complex formation. We find that C150S-Prph2 traffics to the OS, interacts with Rom1 and forms non-covalent tetramers, but alone cannot support normal OS structure and function. However, C150S-Prph2 supports the initiation or elaboration of OS disc structures, and improves rod OS ultrastructure in the presence of wild-type (WT) Prph2 (i.e. Prph2C150S/+ versus Prph2+/-). Prph2C150S/+ animals exhibit haploinsufficiency in rods, but a dominant-negative phenotype in cones, suggesting cones have a different requirement for large Prph2 complexes than rods. Importantly, cone but not rod function can be improved by the addition of one Prph2Y141C allele, a mutation responsible for pattern dystrophy owing to the extra cysteine. Combined these findings show that covalently linked Prph2 complexes are essential for OS formation, but not for Prph2 targeting to the OS, and that cones are especially sensitive to having a broad distribution of Prph2 complex types (i.e. tetramers and large complexes).

Photoreceptor Cilia and Retinal Ciliopathies

Photoreceptors are sensory neurons designed to convert light stimuli into neurological responses. This process, called phototransduction, takes place in the outer segments (OS) of rod and cone photoreceptors. OS are specialized sensory cilia, with analogous structures to those present in other nonmotile cilia. Deficient morphogenesis and/or dysfunction of photoreceptor sensory cilia (PSC) caused by mutations in a variety of photoreceptor-specific and common cilia genes can lead to inherited retinal degenerations (IRDs). IRDs can manifest as isolated retinal diseases or syndromic diseases. In this review, we describe the structure and composition of PSC and different forms of ciliopathies with retinal involvement. We review the genetics of the IRDs, which are monogenic disorders but genetically diverse with regard to causality.

Genetic rescue models refute nonautonomous rod cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited neurodegenerative disease, in which the death of mutant rod photoreceptors leads secondarily to the non-cell autonomous death of cone photoreceptors. Gene therapy is a promising treatment strategy. Unfortunately, current methods of gene delivery treat only a fraction of diseased cells, yielding retinas that are a mosaic of treated and untreated rods, as well as cones. In this study, we created two RP mouse models to test whether dying, untreated rods negatively impact treated, rescued rods. In one model, treated and untreated rods were segregated. In the second model, treated and untreated rods were diffusely intermixed, and their ratio was controlled to achieve low-, medium-, or high-efficiency rescue. Analysis of these mosaic retinas demonstrated that rescued rods (and cones) survive, even when they are greatly outnumbered by dying photoreceptors. On the other hand, the rescued photoreceptors did exhibit long-term defects in their outer segments (OSs), which were less severe when more photoreceptors were treated. In summary, our study suggests that even low-efficiency gene therapy may achieve stable survival of rescued photoreceptors in RP patients, albeit with OS dysgenesis.

Taking Stock of Retinal Gene Therapy: Looking Back and Moving Forward

Over the past 20 years, there has been tremendous progress in retinal gene therapy. The safety and efficacy results in one early-onset severe blinding disease may lead to the first gene therapy drug approval in the United States. Here, we review how far the field has come over the past two decades and speculate on the directions that the field will take in the future.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

An inducible amphipathic helix within the intrinsically disordered C terminus can participate in membrane curvature generation by peripherin-2/rds

Peripherin-2/rds is required for biogenesis of vertebrate photoreceptor outer segment organelles. Its localization at the high-curvature rim domains of outer segment disk membranes suggests that it may act to shape these structures; however, the molecular function of this protein is not yet resolved. Here, we apply biochemical, biophysical, and imaging techniques to elucidate the role(s) played by the protein's intrinsically disordered C-terminal domain and an incipient amphipathic α-helix contained within it. We investigated a deletion mutant lacking only this α-helix in stable cell lines and Xenopus laevis photoreceptors. We also studied a soluble form of the full-length ∼7-kDa cytoplasmic C terminus in cultured cells and purified from Escherichia coli The α-helical motif was not required for protein biosynthesis, tetrameric subunit assembly, tetramer polymerization, localization at disk rims, interaction with GARP2, or the generation of membrane curvature. Interestingly, however, loss of the helical motif up-regulated membrane curvature generation in cellulo, introducing the possibility that it may regulate this activity in photoreceptors. Furthermore, the incipient α-helix (within the purified soluble C terminus) partitioned into membranes only when its acidic residues were neutralized by protonation. This suggests that within the context of full-length peripherin-2/rds, partitioning would most likely occur at a bilayer interfacial region, potentially adjacent to the protein's transmembrane domains. In sum, this study significantly strengthens the evidence that peripherin-2/rds functions directly to shape the high-curvature rim domains of the outer segment disk and suggests that the protein's C terminus may modulate membrane curvature-generating activity present in other protein domains.

Rom1 converts Y141C-Prph2-associated pattern dystrophy to retinitis pigmentosa

Mutations in peripherin 2 (PRPH2), also known as retinal degeneration slow/RDS, lead to various retinal degenerations including retinitis pigmentosa (RP) and macular/pattern dystrophy (MD/PD). PRPH2-associated disease is often characterized by a phenotypic variability even within families carrying the same mutation, raising interest in potential modifiers. PRPH2 oligomerizes with its homologue rod outer segment (OS) membrane protein 1 (ROM1), and non-pathogenic PRPH2/ROM1 mutations, when present together, lead to digenic RP. We asked whether ROM1 could modify the phenotype of a PRPH2 mutation associated with a high degree of intrafamilial phenotypic heterogeneity: Y141C. In vitro, Y141C-Prph2 showed signs of retention in the endoplasmic reticulum (ER), however co-expression with Rom1 rescued this phenotype. In the heterozygous Y141C knockin mouse model (Prph2Y/+), Y141C-Prph2 and Rom1 formed abnormal complexes but were present at normal levels. Abnormal complexes were eliminated in the absence of Rom1 (Prph2Y/+/Rom1-/-) and total Prph2 levels were reduced to those found in the haploinsufficient Prph2+/- RP model. The biochemical changes had functional and structural consequences; while Prph2Y/+ animals exhibited a cone-rod electroretinogram defect, Prph2Y/+/Rom1-/- animals displayed a rod-dominant phenotype and OSs similar to those seen in the Prph2+/-. These data show that ablation of Rom1 results in the conversion of an MD/PD phenotype characterized by cone functional defects and the formation of abnormal Prph2/Rom1 complexes to an RP phenotype characterized by rod-dominant functional defects and reductions in total Prph2 protein. Thus one method by which ROM1 may act as a disease modifier is by contributing to the large variability in PRPH2-associated disease phenotypes.

Photoreceptors at a glance

Retinal photoreceptor cells contain a specialized outer segment (OS) compartment that functions in the capture of light and its conversion into electrical signals in a process known as phototransduction. In rods, photoisomerization of 11-cis to all-trans retinal within rhodopsin triggers a biochemical cascade culminating in the closure of cGMP-gated channels and hyperpolarization of the cell. Biochemical reactions return the cell to its 'dark state' and the visual cycle converts all-trans retinal back to 11-cis retinal for rhodopsin regeneration. OS are continuously renewed, with aged membrane removed at the distal end by phagocytosis and new membrane added at the proximal end through OS disk morphogenesis linked to protein trafficking. The molecular basis for disk morphogenesis remains to be defined in detail although several models have been proposed, and molecular mechanisms underlying protein trafficking are under active investigation. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight our current understanding of photoreceptor structure, phototransduction, the visual cycle, OS renewal, protein trafficking and retinal degenerative diseases.

Glaucoma related Proteomic Alterations in Human Retina Samples

Glaucoma related proteomic changes have been documented in cell and animal models. However, proteomic studies investigating on human retina samples are still rare. In the present work, retina samples of glaucoma and non-glaucoma control donors have been examined by a state-of-the-art mass spectrometry (MS) workflow to uncover glaucoma related proteomic changes. More than 600 proteins could be identified with high confidence (FDR < 1%) in human retina samples. Distinct proteomic changes have been observed in 10% of proteins encircling mitochondrial and nucleus species. Numerous proteins showed a significant glaucoma related level change (p < 0.05) or distinct tendency of alteration (p < 0.1). Candidates were documented to be involved in cellular development, stress and cell death. Increase of stress related proteins and decrease of new glaucoma related candidates, ADP/ATP translocase 3 (ANT3), PC4 and SRFS1-interacting protein 1 (DFS70) and methyl-CpG-binding protein 2 (MeCp2) could be documented by MS. Moreover, candidates could be validated by Accurate Inclusion Mass Screening (AIMS) and immunostaining and supported for the retinal ganglion cell layer (GCL) by laser capture microdissection (LCM) in porcine and human eye cryosections. The workflow allowed a detailed view into the human retina proteome highlighting new molecular players ANT3, DFS70 and MeCp2 associated to glaucoma.

The K153Del PRPH2 mutation differentially impacts photoreceptor structure and function

Peripherin 2 (Prph2) is a photoreceptor tetraspanin, and deletion of codon 153 (K153Δ) leads to retinitis pigmentosa, pattern dystrophy, and fundus flavimaculatus in the same family. To study this variability, we generated a K153Δ-Prph2 knockin mouse. K153Δ-Prph2 cannot form the complexes required for outer segment formation, and in cones cannot interact with its binding partner rod outer segment membrane protein 1. K153Δ causes dominant defects in rod and cone function; however, rod but not cone ultrastructure is improved by the presence of K153Δ-Prph2. Likewise, supplementation of K153Δ heterozygotes with WT-Prph2 results in structural but not functional improvements. These results support the idea that mutations may differentially affect Prph2's role as a structural component, and its role as a functional protein key for organizing membrane domains for cellular signalling. These roles may be different in rods and cones, thus contributing to the phenotypic heterogeneity that characterizes diseases associated with Prph2 mutations.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Varying the GARP2-to-RDS Ratio Leads to Defects in Rim Formation and Rod and Cone Function

PURPOSE

The beta subunit of the rod cyclic nucleotide gated channel B1 (CNGB1) contains a proline/glutamic acid-rich N-terminal domain (GARP), which is also present in rods as a non-membrane-bound protein (GARP1/2). GARP2 and CNGB1 bind to retinal degeneration slow (RDS), which is present in the rims of rod and cone outer segment (OS) layers. Here we focus on the importance of RDS/GARP complexes in OS morphogenesis and stability.

METHODS

Retinal structure, function, and biochemistry were assessed in GARP2-Tg transgenic mice crossed onto rds+/+, rds+/-, and rds-/- genetic backgrounds.

RESULTS

GARP2 expression decreased in animals with reduced RDS levels. Overexpression of GARP2 led to abnormalities in disc stacking in GARP2-Tg/rds+/+ and the accumulation of abnormal vesicular structures in GARP2-Tg/rds+/- OS, as well as alterations in RDS-ROM-1 complex formation. These abnormalities were associated with diminished scotopic a- and b-wave amplitudes in GARP2-Tg mice on both the rds+/+ and rds+/- backgrounds. In addition, severe defects in cone function were observed in GARP2-Tg mice on all RDS backgrounds.

CONCLUSIONS

Our results indicate that overexpression of GARP2 significantly exacerbates the defects in rod function associated with RDS haploinsufficiency and leads to further abnormalities in OS ultrastructure. These data also suggest that GARP2 expression in cones can be detrimental to cones. RDS/GARP interactions remain under investigation but are critical for both OS structure and function.

Preventive Role of Tetraspanin CD9 in Systemic Inflammation of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is frequently associated with extrapulmonary complications, including cardiovascular disease, diabetes, and osteoporosis. Persistent, low-grade, systemic inflammation underlies these comorbid disorders. Tetraspanins, which have a characteristic structure spanning the membrane four times, facilitate lateral organization of molecular complexes and thereby form tetraspanin-enriched microdomains that are distinct from lipid rafts. Recent basic research has suggested a preventive role of tetraspanin CD9 in COPD. CD9-enriched microdomains negatively regulate LPS-induced receptor formation by preventing CD14 from accumulating into the rafts, and decreased CD9 in macrophages enhances inflammation in mice. Mice doubly deficient in CD9 and a related tetraspanin, CD81, show pulmonary emphysema, weight loss, and osteopenia, a phenotype akin to human COPD. A therapeutic approach to up-regulating CD9 in macrophages might improve the clinical course of patients with COPD with comorbidities.

Retinal remodeling in human retinitis pigmentosa

Retinitis Pigmentosa (RP) in the human is a progressive, currently irreversible neural degenerative disease usually caused by gene defects that disrupt the function or architecture of the photoreceptors. While RP can initially be a disease of photoreceptors, there is increasing evidence that the inner retina becomes progressively disorganized as the outer retina degenerates. These alterations have been extensively described in animal models, but remodeling in humans has not been as well characterized. This study, using computational molecular phenotyping (CMP) seeks to advance our understanding of the retinal remodeling process in humans. We describe cone mediated preservation of overall topology, retinal reprogramming in the earliest stages of the disease in retinal bipolar cells, and alterations in both small molecule and protein signatures of neurons and glia. Furthermore, while Müller glia appear to be some of the last cells left in the degenerate retina, they are also one of the first cell classes in the neural retina to respond to stress which may reveal mechanisms related to remodeling and cell death in other retinal cell classes. Also fundamentally important is the finding that retinal network topologies are altered. Our results suggest interventions that presume substantial preservation of the neural retina will likely fail in late stages of the disease. Even early intervention offers no guarantee that the interventions will be immune to progressive remodeling. Fundamental work in the biology and mechanisms of disease progression are needed to support vision rescue strategies.

PRPH2/RDS and ROM-1: Historical context, current views and future considerations

Peripherin 2 (PRPH2), also known as RDS (retinal degeneration slow) is a photoreceptor specific glycoprotein which is essential for normal photoreceptor health and vision. PRPH2/RDS is necessary for the proper formation of both rod and cone photoreceptor outer segments, the organelle specialized for visual transduction. When PRPH2/RDS is defective or absent, outer segments become disorganized or fail to form entirely and the photoreceptors subsequently degenerate. Multiple PRPH2/RDS disease-causing mutations have been found in humans, and they are associated with various blinding diseases of the retina such as macular degeneration and retinitis pigmentosa, the vast majority of which are inherited dominantly, though recessive LCA and digenic RP have also been associated with RDS mutations. Since its initial discovery, the scientific community has dedicated a considerable amount of effort to understanding the molecular function and disease mechanisms of PRPH2/RDS. This work has led to an understanding of how the PRPH2/RDS molecule assembles into complexes and functions as a necessary part of the machinery that forms new outer segment discs, as well as leading to fundamental discoveries about the mechanisms that underlie OS biogenesis. Here we discuss PRPH2/RDS-associated research and how experimental results have driven the understanding of the PRPH2/RDS protein and its role in human disease.

Transcription factor PRDM8 is required for rod bipolar and type 2 OFF-cone bipolar cell survival and amacrine subtype identity

Retinal bipolar (BP) cells mediate the earliest steps in image processing in the visual system, but the genetic pathways that regulate their development and function are incompletely known. We identified PRDI-BF1 and RIZ homology domain containing 8 (PRDM8) as a highly conserved transcription factor that is abundantly expressed in mouse retina. During development and in maturity, PRDM8 is expressed strongly in BP cells and a fraction of amacrine and ganglion cells. To determine whether Prdm8 is essential to BP cell development or physiology, we targeted the gene in mice. Prdm8(EGFP/EGFP) mice showed nonprogressive b-wave deficits on electroretinograms, consistent with compromised BP cell function or circuitry resembling the incomplete form of human congenital stationary night blindness (CSNB). BP cell specification was normal in Prdm8(EGFP/EGFP) retina as determined by VSX2(+) cell numbers and retinal morphology at postnatal day 6. BP subtype differentiation was impaired, however, as indicated by absent or diminished expression of BP subtype-specific markers, including the putative PRDM8 regulatory target PKCα (Prkca) and its protein. By adulthood, rod bipolar (RB) and type 2 OFF-cone bipolar (CB) cells were nearly absent from Prdm8-null mice. Although no change was detected in total amacrine cell (AC) numbers, increased PRKCA(+) and cholinergic ACs and decreased GABAergic ACs were seen, suggesting an alteration in amacrine subtype identity. These findings establish that PRDM8 is required for RB and type 2 OFF-CB cell survival and amacrine subtype identity, and they present PRDM8 as a candidate gene for human CSNB.

The Y141C knockin mutation in RDS leads to complex phenotypes in the mouse

Mutations in the photoreceptor-specific gene peripherin-2 (PRPH-2, also known as retinal degeneration slow/RDS) cause incurable retinal degeneration with a high degree of phenotypic variability. Patient phenotypes range from retinitis pigmentosa to various forms of macular and pattern dystrophy. Macular and pattern dystrophy in particular are associated with complex, poorly understood disease mechanisms, as severe vision loss is often associated both with defects in the photoreceptors, as well as the choroid and retinal pigment epithelium (RPE). Since there is currently no satisfactory model to study pattern dystrophy disease mechanisms, we generated a knockin mouse model expressing an RDS pattern dystrophy mutation, Y141C. Y141C mice exhibited clinical signs similar to those in patients including late-onset fundus abnormalities characteristic of RPE and choroidal defects and electroretinogram defects. Ultrastructural examination indicated that disc formation was initiated by the Y141C protein, but proper sizing and alignment of discs required wild-type RDS. The biochemical mechanism underlying these abnormalities was tied to defects in the normal process of RDS oligomerization which is required for proper RDS function. Y141C-RDS formed strikingly abnormal disulfide-linked complexes which were localized to the outer segment (OS) where they impaired the formation of proper OS structure. These data support a model of pattern dystrophy wherein a primary molecular defect occurring in all photoreceptors leads to secondary sequellae in adjacent tissues, an outcome which leads to macular vision loss. An understanding of the role of RDS in the interplay between these tissues significantly enhances our understanding of RDS-associated pathobiology and our ability to design rational treatment strategies.

Peripherin-2 couples rhodopsin to the CNG channel in outer segments of rod photoreceptors

Outer segments (OSs) of rod photoreceptors are cellular compartments specialized in the conversion of light into electrical signals. This process relies on the light-triggered change in the intracellular levels of cyclic guanosine monophosphate, which in turn controls the activity of cyclic nucleotide-gated (CNG) channels in the rod OS plasma membrane. The rod CNG channel is a macromolecular complex that in its core harbors the ion-conducting CNGA1 and CNGB1a subunits. To identify additional proteins of the complex that interact with the CNGB1a core subunit, we applied affinity purification of mouse retinal proteins followed by mass spectrometry. In combination with in vitro and in vivo co-immunoprecipitation and fluorescence resonance energy transfer (FRET), we found that the tetraspanin peripherin-2 links CNGB1a to the light-detector rhodopsin. Using immunoelectron microscopy, we found that this peripherin-2/rhodopsin/CNG channel complex localizes to the contact region between the disk rims and the plasma membrane. FRET measurements revealed that the fourth transmembrane domain (TM4) of peripherin-2 is required for the interaction with rhodopsin. Quantitatively, the binding affinity of the peripherin-2/rhodopsin interaction was in a similar range as that observed for rhodopsin dimers. Finally, we demonstrate that the p.G266D retinitis pigmentosa mutation found within TM4 selectively abolishes the binding of peripherin-2 to rhodopsin. This finding suggests that the specific disruption of the rhodopsin/peripherin-2 interaction in the p.G266D mutant might contribute to the pathophysiology in affected persons.

Initiation of rod outer segment disc formation requires RDS

Rod outer segment (OS) morphogenesis involves assembly of flattened discs circumscribed by a hairpin-like rim, however, the role of the rim and rim proteins such as retinal degeneration slow (RDS) and its homologue rod OS membrane protein-1 (ROM-1) in this process remains unclear. Here we show that without RDS, no disc/OS formation occurs, while without rhodopsin, small OS structures form containing aligned nascent discs. In the absence of both rhodopsin and RDS, RDS-associated degeneration is slowed, and ROM-1 is stabilized and trafficked to the OS. These animals (rho-/-/rds-/-) exhibit OSs slightly better than those lacking only RDS, but still without signs of disc formation. These results clearly demonstrate that OS morphogenesis is initiated by RDS-mediated rim formation, a process ROM-1 cannot recapitulate, with subsequent disc growth mediated by rhodopsin. The critical role of RDS in this process helps explain why photoreceptors are so sensitive to varied RDS levels, and why mutations in RDS cause debilitating retinal disease.

Eyes underground: regression of visual protein networks in subterranean mammals

Regressive evolution involves the degeneration of formerly useful structures in a lineage over time, and may be accompanied by the molecular decay of phenotype-specific genes. The mammalian eye has repeatedly undergone degeneration in taxa that occupy dim-light environments including subterranean habitats. Here we assess whether a decrease in the amount of light that reaches the retina is associated with increased regression of retinal genes, whether the phototransduction and visual cycle pathways degrade in a predictable pattern, and if the timing of retinal gene loss is associated with the entrance of mammalian lineages into subterranean environments. Sequence data were obtained from the publically available genomes of the Cape golden mole (Chrysochloris asiatica), naked mole-rat (Heterocephalus glaber) and star-nosed mole (Condylura cristata) for 65 genes associated with phototransduction, the visual cycle, and other retinal functions. Gene sequences were inspected for inactivating mutations and, when present, pseudogene sequences were compared to sequences from subaerial outgroup species. To test whether retinal degeneration is correlated with historical entrances into subterranean environments, estimated dates of retinal gene inactivation were compared to the fossil record and phylogenetic inferences of ancestral fossoriality. Our results show that (1) lower levels of light available to the retina correspond with an increase in the number of retinal pseudogenes, (2) retinal protein networks generally degrade in a predictable manner, although the extensive loss of cone phototransduction genes in Heterocephalus raises further questions regarding SWS1-cone monochromacy versus functional rod monochromacy in this species, and (3) inactivation dates of retinal genes usually post-date inferred entrances into subterranean habitats.

Rim formation is not a prerequisite for distribution of cone photoreceptor outer segment proteins

Retinal degeneration slow (RDS/PRPH2) is critical for the formation of the disc/lamella rim in photoreceptor outer segments (OSs), but plays a different role in rods vs. cones. Without RDS, rods fail to form OSs, however, cones lacking RDS (in the rds(-/-)/Nrl(-/-)) exhibit balloon-like OSs devoid of lamellae. We show that distribution of most proteins in the lamella and PM domains is preserved even in the absence of RDS, rim, and lamella structures. However, the rim protein prominin-1 exhibits altered trafficking and OS localization, suggesting that proper targeting and distribution of rim proteins may require RDS. Our ultrastructural studies show that in cones, OS formation is initiated by the growth of opsin-containing membrane with RDS-mediated rim formation as a secondary step. This is directly opposite to rods and significantly advances our understanding of the role of the rim in cone OS morphogenesis. Furthermore, our results suggest that the unique folded lamella architecture of the cone OS may maximize density or proximity of phototransduction proteins, but is not required for OS function or for protein distribution and retention in different membrane domains.

Modeling the flexural rigidity of rod photoreceptors

In vertebrate eyes, the rod photoreceptor has a modified cilium with an extended cylindrical structure specialized for phototransduction called the outer segment (OS). The OS has numerous stacked membrane disks and can bend or break when subjected to mechanical forces. The OS exhibits axial structural variation, with extended bands composed of a few hundred membrane disks whose thickness is diurnally modulated. Using high-resolution confocal microscopy, we have observed OS flexing and disruption in live transgenic Xenopus rods. Based on the experimental observations, we introduce a coarse-grained model of OS mechanical rigidity using elasticity theory, representing the axial OS banding explicitly via a spring-bead model. We calculate a bending stiffness of ∼10(5) nN⋅μm2, which is seven orders-of-magnitude larger than that of typical cilia and flagella. This bending stiffness has a quadratic relation to OS radius, so that thinner OS have lower fragility. Furthermore, we find that increasing the spatial frequency of axial OS banding decreases OS rigidity, reducing its fragility. Moreover, the model predicts a tendency for OS to break in bands with higher spring number density, analogous to the experimental observation that transgenic rods tended to break preferentially in bands of high fluorescence. We discuss how pathological alterations of disk membrane properties by mutant proteins may lead to increased OS rigidity and thus increased breakage, ultimately contributing to retinal degeneration.

Membrane curvature generation by a C-terminal amphipathic helix in peripherin-2/rds, a tetraspanin required for photoreceptor sensory cilium morphogenesis

Vertebrate vision requires photon absorption by photoreceptor outer segments (OSs), structurally elaborate membranous organelles derived from non-motile sensory cilia. The structure and function of OSs depends on a precise stacking of hundreds of membranous disks. Each disk is fully (as in rods) or partially (as in cones) bounded by a rim, at which the membrane is distorted into an energetically unfavorable high-curvature bend; however, the mechanism(s) underlying disk rim structure is (are) not established. Here, we demonstrate that the intrinsically disordered cytoplasmic C-terminus of the photoreceptor tetraspanin peripherin-2/rds (P/rds) can directly generate membrane curvature. A P/rds C-terminal domain and a peptide mimetic of an amphipathic helix contained within it each generated curvature in liposomes with a composition similar to that of OS disks and in liposomes generated from native OS lipids. Association of the C-terminal domain with liposomes required conical phospholipids, and was promoted by membrane curvature and anionic surface charge, results suggesting that the P/rds C-terminal amphipathic helix can partition into the cytosolic membrane leaflet to generate curvature by a hydrophobic insertion (wedging) mechanism. This activity was evidenced in full-length P/rds by its induction of small-diameter tubulovesicular membrane foci in cultured cells. In sum, the findings suggest that curvature generation by the P/rds C-terminus contributes to the distinctive structure of OS disk rims, and provide insight into how inherited defects in P/rds can disrupt organelle structure to cause retinal disease. They also raise the possibility that tethered amphipathic helices can function for shaping cellular membranes more generally.

17β-estradiol eye drops protect the retinal ganglion cell layer and preserve visual function in an in vivo model of glaucoma

Neuroprotection in glaucoma as a curative strategy complementary to current therapies to lower intraocular pressure (IOP) is highly desirable. This study was designed to investigate neuroprotection by 17β-estradiol (E2) to prevent retinal ganglion cell (RGC) death in a glaucoma model of surgically elevated IOP in rats. We found that daily treatment with E2-containing eye drops resulted in significant E2 concentration in the retina with concomitant profound neuroprotective therapeutic benefits, even in the presence of continually elevated IOP. The number of apoptotic cells in the RGC layer was significantly decreased in the E2-treated group, when compared to the vehicle-treated controls. Deterioration in visual acuity in these animals was also markedly prevented. Using mass spectrometry-based proteomics, beneficial changes in the expression of several proteins implicated in the maintenance of retinal health were also found in the retina of E2-treated animals. On the other hand, systemic side effects could not be avoided with the eye drops, as confirmed by the measured high circulating estrogen levels and through the assessment of the uterus representing a typical hormone-sensitive peripheral organ. Collectively, the demonstrated significant neuroprotective effect of topical E2 in the selected animal model of glaucoma provides a clear rationale for further studies aiming at targeting E2 into the eye while avoiding systemic E2 exposure to diminish undesirable off-target side effects.