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

Comparative study of PRPH2 D2 loop mutants reveals divergent disease mechanism in rods and cones

Mutations in the photoreceptor-specific tetraspanin gene peripherin-2 (PRPH2) lead to widely varying forms of retinal degeneration ranging from retinitis pigmentosa to macular dystrophy. Both inter- and intra-familial phenotypic heterogeneity has led to much interest in uncovering the complex pathogenic mechanisms of PRPH2-associated disease. Majority of disease-causing mutations in PRPH2 reside in the second intradiscal loop, wherein seven cysteines control protein folding and oligomerization. Here, we utilize knockin models to evaluate the role of three D2 loop cysteine mutants (Y141C, C213Y and C150S), alone or in combination. We elucidated how these mutations affect PRPH2 properties, including oligomerization and subcellular localization, and contribute to disease processes. Results from our structural, functional and molecular studies revealed that, in contrast to our understanding from prior investigations, rods are highly affected by PRPH2 mutations interfering with oligomerization and not merely by the haploinsufficiency associated with these mutations. On the other hand, cones are less affected by the toxicity of the mutant protein and significantly reduced protein levels, suggesting that knockdown therapeutic strategies may sustain cone functionality for a longer period. This observation provides useful data to guide and simplify the current development of effective therapeutic approaches for PRPH2-associated diseases that combine knockdown with high levels of gene supplementation needed to generate prolonged rod improvement.

Visual pigment-deficient cones survive and mediate visual signaling despite the lack of outer segments

Rhodopsin and cone opsins are essential for light detection in vertebrate rods and cones, respectively. It is well established that rhodopsin is required for rod phototransduction, outer segment disk morphogenesis, and rod viability. However, the roles of cone opsins are less well understood. In this study, we adopted a loss-of-function approach to investigate the physiological roles of cone opsins in mice. We showed that cones lacking cone opsins do not form normal outer segments due to the lack of disk morphogenesis. Surprisingly, cone opsin-deficient cones survive for at least 12 mo, which is in stark contrast to the rapid rod degeneration observed in rhodopsin-deficient mice, suggesting that cone opsins are dispensable for cone viability. Although the mutant cones do not respond to light directly, they maintain a normal dark current and continue to mediate visual signaling by relaying the rod signal through rod-cone gap junctions. Our work reveals a striking difference between the role of rhodopsin and cone opsins in photoreceptor viability.

Planar polarity in primate cone photoreceptors: a potential role in Stiles Crawford effect phototropism

Human cone phototropism is a key mechanism underlying the Stiles-Crawford effect, a psychophysiological phenomenon according to which photoreceptor outer/inner segments are aligned along with the direction of incoming light. However, such photomechanical movements of photoreceptors remain elusive in mammals. We first show here that primate cone photoreceptors have a planar polarity organized radially around the optical center of the eye. This planar polarity, based on the structure of the cilium and calyceal processes, is highly reminiscent of the planar polarity of the hair cells and their kinocilium and stereocilia. Secondly, we observe under super-high resolution expansion microscopy the cytoskeleton and Usher proteins architecture in the photoreceptors, which appears to establish a mechanical continuity between the outer and inner segments. Taken together, these results suggest a comprehensive cellular mechanism consistent with an active phototropism of cones toward the optical center of the eye, and thus with the Stiles-Crawford effect.

Verschueren et al. expand our understanding of the Stiles-Crawford effect in mammals by using super-high resolution expansion microscopy of the adult macaque eye. They show that cone photoreceptors have a planar polarity organized radially around the optical center of the eye and that Usher proteins establish a mechanical continuity between the outer and inner segments, which sheds light on the Stiles-Crawford effect in this species.

PRPH2-Associated Macular Dystrophy in 4 Family Members with a Novel Mutation

Peripherin-2 (PRPH2) gene variants are a well-established cause of multiple inherited maculopathies including central areolar choroidal dystrophy (CACD) and pattern dystrophy. In this familial case study, we present a 63-year-old proband who presented with visual acuity of 20/63 right eye and 20/100 left eye with a complaint of lowered visual acuity in the left eye for unknown duration. Fundus examination presented with unifocal atrophic lesions bilaterally. Multi-modal imaging was obtained and genetic testing (My Retina Tracker; Blueprint Genetics) was performed. The proband was monoallelic for a novel missense mutation within the PRPH2 gene (Arg203Pro) not previously found in the literature or large databases (gnomAD, ClinVar, and HGMD). Subsequent examination of the proband's mother, older sister (65 years old), younger sister (53), and daughter (35) found the novel mutation to segregate with maculopathy ranging from speckled fundus autofluorescence with EZ disruption and RPE attenuation on spectral domain optical coherence tomography to large unifocal atrophic lesions throughout the macula bilaterally. The purpose of this case report is to add to the literature of PRPH2-associated disease by providing a comprehensive fundus examination of a family with autosomal dominant PRPH2-associated maculopathy diagnosed as central areolar choroidal dystrophy and pattern 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.

Genotype-phenotype associations in a large PRPH2-related retinopathy cohort

Molecular variant interpretation lacks disease gene-specific cohorts for determining variant enrichment in disease versus healthy populations. To address the molecular etiology of retinal degeneration, specifically the PRPH2-related retinopathies, we reviewed genotype and phenotype information obtained from 187 eyeGENE® participants from 161 families. Clinical details were provided by referring clinicians participating in the eyeGENE® Network. The cohort was sequenced for variants in PRPH2. Variant complementary DNA clusters and cohort frequency were compared to variants in public databases to help us to determine pathogenicity by current American College of Medical Genetics and Genomics/Association for Molecular Pathology interpretation criteria. The most frequent variant was c.828+3A>T, which affected 28 families (17.4%), and 25 of 79 (31.64%) variants were novel. The majority of missense variants clustered in the D2 intracellular loop of the peripherin-2 protein, constituting a hotspot. Disease enrichment was noted for 23 (29.1%) of the variants. Hotspot and disease-enrichment evidence modified variant classification for 16.5% of variants. The missense allele p.Arg172Trp was associated with a younger age of onset. To the best of our knowledge, this is the largest patient cohort review of PRPH2-related retinopathy. Large disease gene-specific cohorts permit gene modeling for hotspot and disease-enrichment analysis, providing novel variant classification evidence, including for novel missense variants.

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.

Deletion of the transmembrane protein Prom1b in zebrafish disrupts outer-segment morphogenesis and causes photoreceptor degeneration

Mutations in human prominin 1 (PROM1), encoding a transmembrane glycoprotein localized mainly to plasma membrane protrusions, have been reported to cause retinitis pigmentosa, macular degeneration, and cone-rod dystrophy. Although the structural role of PROM1 in outer-segment (OS) morphogenesis has been demonstrated in Prom1-knockout mouse, the mechanisms underlying these complex disease phenotypes remain unclear. Here, we utilized a zebrafish model to further investigate PROM1's role in the retina. The Prom1 orthologs in zebrafish include prom1a and prom1b, and our results showed that prom1b, rather than prom1a, plays an important role in zebrafish photoreceptors. Loss of prom1b disrupted OS morphogenesis, with rods and cones exhibiting differences in impairment: cones degenerated at an early age, whereas rods remained viable but with an abnormal OS, even at 9 months postfertilization. Immunofluorescence experiments with WT zebrafish revealed that Prph2, an ortholog of the human transmembrane protein peripherin 2 and also associated with OS formation, is localized to the edge of OS and is more highly expressed in the cone OS than in the rod OS. Moreover, we found that Prom1b deletion causes mislocalization of Prph2 and disrupts its oligomerization. We conclude that the variation in Prph2 levels between cones and rods was one of the reasons for the different PROM1 mutation-induced phenotypes of these retinal structures. These findings expand our understanding of the phenotypes caused by PROM1 mutations and provide critical insights into its function.

Prph2 initiates outer segment morphogenesis but maturation requires Prph2/Rom1 oligomerization

The retinal disease gene peripherin 2 (PRPH2) is essential for the formation of photoreceptor outer segments (OSs), where it functions in oligomers with and without its homologue ROM1. However, the precise role of these proteins in OS morphogenesis is not understood. By utilizing a knock-in mouse expressing a chimeric protein comprised of the body of Rom1 and the C-terminus of Prph2 (termed RRCT), we find that the Prph2 C-terminus is necessary and sufficient for the initiation of OSs, while OS maturation requires the body of Prph2 and associated large oligomers. Importantly, dominant-negative physiological and biochemical defects in RRCT heterozygous rods are rescued by removing Rom1, suggesting Rom1 is a regulator for OS formation. Our experiments evaluating Prph2 trafficking show that Rom1 is a key determinant of whether Prph2 complexes utilize conventional versus unconventional (Golgi bypass) secretory pathways to reach the OS. These findings significantly advance our understanding of the molecular underpinnings of OS morphogenesis and particularly the role of Rom1.

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.

Role of RDS and Rhodopsin in Cngb1-Related Retinal Degeneration

Purpose

Rod photoreceptor outer segment (OS) morphogenesis, structural integrity, and proper signal transduction rely on critical proteins found in the different OS membrane domains (e.g., plasma, disc, and disc rim membrane). Among these key elements are retinal degeneration slow (RDS, also known as peripherin-2), rhodopsin, and the beta subunit of the cyclic nucleotide gated channel (CNGB1a), which have been found to interact in a complex. The purpose of this study was to evaluate the potential interplay between these three proteins by examining retinal disease phenotypes in animal models expressing varying amounts of CNGB1a, rhodopsin, and RDS.

Methods

Outer segment trafficking, retinal function, and photoreceptor structure were evaluated using knockout mouse lines.

Results

Eliminating Cngb1 and reducing RDS leads to additive defects in RDS expression levels and rod electroretinogram (ERG) function, (e.g., Cngb1^−/−/rds^+/− versus rds^+/− or Cngb1^−/−) but not to additive defects in rod ultrastructure. These additive effects also manifested in cone function: Photopic ERG responses were significantly lower in the Cngb1^−/−/rds^+/− versus rds^+/− or Cngb1^−/−, suggesting that eliminating Cngb1 can accelerate the cone degeneration that usually presents later in the rds^+/−. This was not the case with rhodopsin; reducing rhodopsin levels in concert with eliminating CNGB1a did not lead to phenotypes more severe than those observed in the Cngb1 knockout alone.

Conclusions

These data support a role for RDS as the core component of a multiprotein plasma membrane-rim-disc complex that has both a structural role in photoreceptor OS formation and maintenance and a functional role in orienting proteins for optimal signal transduction.

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.

Arrestin 1 and Cone Arrestin 4 Have Unique Roles in Visual Function in an All-Cone Mouse Retina

PURPOSE

Previous studies discovered cone phototransduction shutoff occurs normally for Arr1-/- and Arr4-/-; however, it is defective when both visual arrestins are simultaneously not expressed (Arr1-/-Arr4-/-). We investigated the roles of visual arrestins in an all-cone retina (Nrl-/-) since each arrestin has differential effects on visual function, including ARR1 for normal light adaptation, and ARR4 for normal contrast sensitivity and visual acuity.

METHODS

We examined Nrl-/-, Nrl-/-Arr1-/-, Nrl-/-Arr4-/-, and Nrl-/-Arr1-/-Arr4-/- mice with photopic electroretinography (ERG) to assess light adaptation and retinal responses, immunoblot and immunohistochemical localization analysis to measure retinal expression levels of M- and S-opsin, and optokinetic tracking (OKT) to measure the visual acuity and contrast sensitivity.

RESULTS

Study results indicated that Nrl-/- and Nrl-/-Arr4-/- mice light adapted normally, while Nrl-/-Arr1-/- and Nrl-/-Arr1-/-Arr4-/- mice did not. Photopic ERG a-wave, b-wave, and flicker amplitudes followed a general pattern in which Nrl-/-Arr4-/- amplitudes were higher than the amplitudes of Nrl-/-, while the amplitudes of Nrl-/-Arr1-/- and Nrl-/-Arr1-/-Arr4-/- were lower. All three visual arrestin knockouts had faster implicit times than Nrl-/- mice. M-opsin expression is lower when ARR1 is not expressed, while S-opsin expression is lower when ARR4 is not expressed. Although M-opsin expression is mislocalized throughout the photoreceptor cells, S-opsin is confined to the outer segments in all genotypes. Contrast sensitivity is decreased when ARR4 is not expressed, while visual acuity was normal except in Nrl-/-Arr1-/-Arr4-/-.

CONCLUSIONS

Based on the opposite visual phenotypes in an all-cone retina in the Nrl-/-Arr1-/- and Nrl-/-Arr4-/- mice, we conclude that ARR1 and ARR4 perform unique modulatory roles in cone photoreceptors.

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.

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.