Identification of a photoreceptor-specific mRNA encoded by the gene responsible for retinal degeneration slow (rds)

LONG-TERM FOLLOW-UP OF PRPH2 -ASSOCIATED RETINAL DYSTROPHY

PURPOSE

To report a 13-year follow-up of Peripherin-2 (PRPH2) -associated retinal dystrophy.

METHODS

A 54-year-old female patient presented with decreased vision and mild metamorphopsia in both eyes since the age of 40 years. A complete evaluation was performed using multimodal imaging techniques.

RESULTS

At presentation, fundus examination revealed multiple irregular pisciform flecks in the posterior pole sparing the peripapillary area in both eyes, as well as some mildly atrophic zones in the perifoveal area. The mildly atrophic areas evolved and merged into a central atrophic zone in the following 10 years, leading to a decreased vision of <20/400 in both eyes. The genetic molecular diagnosis revealed a mutation in PRPH2/RDS gene (NM_000322.4:c.421T>C ( p .Tyr141His)). Based on genetics, imaging, and clinical findings, a diagnosis of multifocal pattern dystrophy simulating Stargardt disease 1/fundus flavimaculatus was evoked. Her mother was found to have the same gene mutation, with multiple irregular pisciform flecks in the posterior pole associated with central areolar choroidal dystrophy.

CONCLUSION

This report demonstrated the 13-year progression of multifocal pattern dystrophy simulating Stargardt disease 1/fundus flavimaculatus in a patient with a pathogenic variant of the PRPH2/RDS gene (NM_000322.4:c.421T>C ( p .Tyr141His)).

Recent advances in the understanding of cilia mechanisms and their applications as therapeutic targets

The primary cilium is a single immotile microtubule-based organelle that protrudes into the extracellular space. Malformations and dysfunctions of the cilia have been associated with various forms of syndromic and non-syndromic diseases, termed ciliopathies. The primary cilium is therefore gaining attention due to its potential as a therapeutic target. In this review, we examine ciliary receptors, ciliogenesis, and ciliary trafficking as possible therapeutic targets. We first discuss the mechanisms of selective distribution, signal transduction, and physiological roles of ciliary receptors. Next, pathways that regulate ciliogenesis, specifically the Aurora A kinase, mammalian target of rapamycin, and ubiquitin-proteasome pathways are examined as therapeutic targets to regulate ciliogenesis. Then, in the photoreceptors, the mechanism of ciliary trafficking which takes place at the transition zone involving the ciliary membrane proteins is reviewed. Finally, some of the current therapeutic advancements highlighting the role of large animal models of photoreceptor ciliopathy are discussed.

Utilization of the retinal organoid model to evaluate the feasibility of genetic strategies to ameliorate retinal disease(s)

Organoid models have quickly become a popular research tool to evaluate novel therapeutics on 3-D recapitulated tissue. This has enabled researchers to use physiologically relevant human tissue in vitro to augment the standard use of immortalized cells and animal models. Organoids can also provide a model when an engineered animal cannot recreate a specific disease phenotype. In particular, the retinal research field has taken advantage of this burgeoning technology to provide insight into inherited retinal disease(s) mechanisms and therapeutic intervention to ameliorate their effects. In this review we will discuss the use of both wild-type and patient-specific retinal organoids to further gene therapy research that could potentially prevent retinal disease(s) progression. Furthermore, we will discuss the pitfalls of current retinal organoid technology and present potential solutions that could overcome these hurdles in the near future.

A Potential Neuroprotective Role for Pyruvate Kinase 2 in Retinal Degeneration

Retinitis pigmentosa (RP) is an inherited disorder that results in vision impairment that specific therapeutic strategies are not available. However, it is widely regarded that the cGMP system, including cGMP and its interactor cGMP-dependent protein kinase (PKG), acts as a crucial effector during retinal degeneration. We have previously identified a list of cGMP-PKG-dependent genes in the context of RP, and in this study, we further validated one of the targets, namely, pyruvate kinase 2 (PKM2), and investigated the potential role of PKM2 for the photoreceptors' well-being during RP. With the aid of organotypic retinal explant cultures, we pharmacologically manipulated the PKM2 activities in different RP mouse models via the addition of TEPP-46 (a PKM2 activator) and found that activation of PKM2 alleviates the progress of photoreceptor death in the rd10 mouse model. This observation provides supportive evidence that PKM2 may serve as a novel potential molecular target in RP.

Preclinical Models of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the name for a group of phenotypically-related heritable retinal degenerative disorders. Many genes have been implicated as causing variants of RP, and while the clinical phenotypes are remarkably similar, they may differ in age of onset, progression, and severity. Common inheritance patterns for specific genes connected with the development of the disorder include autosomal dominant, autosomal recessive, and X-linked. Modeling the disease in animals and other preclinical systems offers a cost-conscious, ethical, and time-efficient method for studying the disease subtypes. The history of RP models is briefly examined, and both naturally occurring and transgenic preclinical models of RP in many different organisms are discussed. Syndromic forms of RP and models thereof are reviewed as well.

PRPH2 mutation update: In silico assessment of 245 reported and 7 novel variants in patients with retinal disease

Mutations in PRPH2, encoding peripherin-2, are associated with the development of a wide variety of inherited retinal diseases (IRDs). To determine the causality of the many PRPH2 variants that have been discovered over the last decades, we surveyed all published PRPH2 variants up to July 2020, describing 720 index patients that in total carried 245 unique variants. In addition, we identified seven novel PRPH2 variants in eight additional index patients. The pathogenicity of all variants was determined using the ACMG guidelines. With this, 107 variants were classified as pathogenic, 92 as likely pathogenic, one as benign, and two as likely benign. The remaining 50 variants were classified as variants of uncertain significance. Interestingly, of the total 252 PRPH2 variants, more than half (n = 137) were missense variants. All variants were uploaded into the Leiden Open source Variation and ClinVar databases. Our study underscores the need for experimental assays for variants of unknown significance to improve pathogenicity classification, which would allow us to better understand genotype-phenotype correlations, and in the long-term, hopefully also support the development of therapeutic strategies for patients with PRPH2-associated IRD.

Photoreceptor Discs: Built Like Ectosomes

The light-sensitive outer segment organelle of the vertebrate photoreceptor cell is a modified cilium filled with hundreds of flattened 'disc' membranes that provide vast light-absorbing surfaces. The outer segment is constantly renewed with new discs added at its base every day. This continuous process is essential for photoreceptor viability. In this review, we describe recent breakthroughs in the understanding of disc morphogenesis, with a focus on the molecular mechanisms responsible for initiating disc formation from the ciliary membrane. We highlight the discoveries that this mechanism evolved from an innate ciliary process of releasing small extracellular vesicles, or ectosomes, and that both disc formation and ectosome release rely on the actin cytoskeleton.

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.

Genetic Types of Inheritance of Retinal Pigment Abiotrophy in Patients Living in “Closed” Communities in the Republic of Buryatia

Retinal pigment abiotrophy is a hereditary disease with a primary diffuse lesion of photoreceptors and the retinal pigment epithelium, in which characteristic functional changes and a typical picture of the fundus with pigment bone bodies are noted. Retinal pigment abiotrophy is the most common disease of all dystrophies of the retina.

Aim. To do a clinical genealogical analysis of the type of inheritance of pigmentary retinitis in residents of Tarbagatai region of the Republic of Buryatia.

Material and methods. At the first stage of the study, a round between the residents of Tarbagatai region of the Republic of Buryatia was conducted by interviewing and questioning. At the second stage, people with suspected pigment retinitis and their blood relatives underwent a full ophthalmologic examination to confirm the diagnosis, and a thorough collection of genealogical information was carried out to determine the type of inheritance.

Results. 20 patients were allocated from 12 families of 1237 interviewed people aged 15 to 76 years living in a closed community in the territory of Buryatia in the Tarbagatai region, 12 of them with a presumptive diagnosis of retinitis pigmentosa, with signs of Usher syndrome (retinitis pigmentosa + sensorineural hearing loss) – 8 people. In general, the analysis of 20 pedigrees with an established type of inheritance in 89.9 % of cases revealed an autosomal recessive type; in 10.1 % – autosomal dominant type of retinitis pigmentosa.

Conclusion. A complete collection of genealogical history is the basis for the diagnosis of hereditary eye diseases.

Mouse germ line mutations due to retrotransposon insertions

Transposable element (TE) insertions are responsible for a significant fraction of spontaneous germ line mutations reported in inbred mouse strains. This major contribution of TEs to the mutational landscape in mouse contrasts with the situation in human, where their relative contribution as germ line insertional mutagens is much lower. In this focussed review, we provide comprehensive lists of TE-induced mouse mutations, discuss the different TE types involved in these insertional mutations and elaborate on particularly interesting cases. We also discuss differences and similarities between the mutational role of TEs in mice and humans.

Golgi bypass of ciliary proteins

Primary cilia represent small, yet distinct compartments of the plasma membrane. They are speculated to exercise chemo- and mechanosensory functions and to serve as signaling hubs for crucial pathways such as the Wnt and hedgehog cascades. It is therefore necessary that specific integral membrane proteins, in particular sensors and receptors, are sorted to the cilium and not to the surrounding somatic plasma membrane upon being synthesized at the rough endoplasmic reticulum. Apparently no singular "zip code" for the primary cilium exists but rather several ciliary targeting signals whose biochemical and cell biological implications are just about being unravelled. Among the better understood proteins residing in the primary cilium is polycystin-2 which is mutated in patients suffering from autosomal-dominant polycystic kidney disease. A special case in the context of this review concerns the connecting cilium which serves as the trafficking pathway for proteins involved in visual sensation of retinal photoreceptor cells. In order to efficiently capture photons, the photopigments are organized in discs or membrane invaginations. Mutations in certain proteins involved in these processes lead to retinal degeneration and ultimately to blindness. One example is peripherin/rds which is mutated in the rds (retinal degeneration slow) mouse. The trafficking of peripherin/rds from the inner to the outer segment of photoreceptor cells by way of the connecting cilium also seems to diverge at the Golgi apparatus, and the routes of polycystin-2 and peripherin/rds may represent paradigms of ciliary proteins for the type IV pathway of unconventional protein "secretion". This review is part of a special issue of Seminars in Cell and Developmental Biology edited by Walter Nickel and Catherine Rabouille.

Two Novel Missense Mutations in the Peripherin/RDS Gene in two Unrelated French Patients with Autosomal Dominant Retinitis Pigmentosa

Purpose

To report the identification of two novel RDS mutations in the peripherin/RDS gene of two unrelated French patients affected by autosomal dominant retinitis pigmentosa (ADRP).

Methods

Fifty-eight unrelated patients affected by ADRP were analyzed. Our diagnostic criteria for RP were bilateral fundus involvement, concentric depression of the visual field and severe involvement on electroretinogram. Transmission of the trait was unambiguous. Our strategy was to analyze the coding sequence of the gene using a combination of single-strand conformation polymorphism (SSCP) and direct sequence analysis of the exons of the gene. Exons that displayed conformational polymorphisms were sequenced on an automated DNA sequencer.

Results

The sequence analyses revealed two previously unreported missense mutations: Cys165Tyr and Phe211Leu in exons 1 and 2, respectively. None of the 70 controls analyzed carried these base changes. Cosegregation of the base substitution with the disease could be tested in both families presenting the Cys165Tyr and Phe211Leu mutations.

Conclusions

Several lines of evidence support the idea that these base substitutions are disease-causing mutations. To the best of our knowledge, no peripherin/RDS gene analysis has been previously reported in ADRP in France.

Photoreceptor discs form through peripherin-dependent suppression of ciliary ectosome release

Visual signal transduction occurs on the surface of membrane discs stacked inside the ciliary outer segment of photoreceptor cells. Salinas et al. show that discs are formed from ciliary ectosomes whose release is blocked by the protein peripherin/RDS. This explains how photoreceptors transform their primary cilia into the light-sensing outer segment organelle.

The primary cilium is a highly conserved organelle housing specialized molecules responsible for receiving and processing extracellular signals. A recently discovered property shared across many cilia is the ability to release small vesicles called ectosomes, which are used for exchanging protein and genetic material among cells. In this study, we report a novel role for ciliary ectosomes in building the elaborate photoreceptor outer segment filled with hundreds of tightly packed “disc” membranes. We demonstrate that the photoreceptor cilium has an innate ability to release massive amounts of ectosomes. However, this process is suppressed by the disc-specific protein peripherin, which enables retained ectosomes to be morphed into discs. This new function of peripherin is performed independently from its well-established role in maintaining the high curvature of disc edges, and each function is fulfilled by a separate part of peripherin’s molecule. Our findings explain how the outer segment structure evolved from the primary cilium to provide photoreceptor cells with vast membrane surfaces for efficient light capture.

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.

Structural and molecular bases of rod photoreceptor morphogenesis and disease

The rod cell has an extraordinarily specialized structure that allows it to carry out its unique function of detecting individual photons of light. Both the structural features of the rod and the metabolic processes required for highly amplified light detection seem to have rendered the rod especially sensitive to structural and metabolic defects, so that a large number of gene defects are primarily associated with rod cell death and give rise to blinding retinal dystrophies. The structures of the rod, especially those of the sensory cilium known as the outer segment, have been the subject of structural, biochemical, and genetic analysis for many years, but the molecular bases for rod morphogenesis and for cell death in rod dystrophies are still poorly understood. Recent developments in imaging technology, such as cryo-electron tomography and super-resolution fluorescence microscopy, in gene sequencing technology, and in gene editing technology are rapidly leading to new breakthroughs in our understanding of these questions. A summary is presented of our current understanding of selected aspects of these questions, highlighting areas of uncertainty and contention as well as recent discoveries that provide new insights. Examples of structural data from emerging imaging technologies are presented.

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.

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.

Founder Effect of a c.828+3A&gt;T Splice Site Mutation in Peripherin 2 (PRPH2) Causing Autosomal Dominant Retinal Dystrophies

IMPORTANCE

Screening for splice site mutation c.828+3A>T in the peripherin 2 (PRPH2) gene should be a high priority in families with highly variable retinal dystrophies. The correction of missplicing is a potential therapeutic target.

OBJECTIVE

To determine the prevalence, genetic origin, and molecular mechanism of a donor c.828+3A>T mutation in the PRPH2 (peripherin 2, retinal degeneration slow) gene in individuals with retinal dystrophies.

DESIGN, SETTING, AND PARTICIPANTS

Case-control study that took place at the University of Texas Health Science Center, the University of Iowa, and the Retina Foundation of the Southwest, from January 1, 1987, to August 1, 2014, including affected individuals from 200 families with a diagnosis of autosomal dominant retinitis pigmentosa, 35 families with unspecified macular dystrophies, and 116 families with pattern dystrophy. Participants were screened for the c.828+3A>T mutation by restriction-enzyme digest, single-strand conformational polymorphism screening, or bidirectional sequencing. Haplotypes of polymorphic markers flanking the PRPH2 locus and sequence variants within the gene were determined by denaturing gel electrophoresis or automated capillary-based cycle sequencing. The effect of the splice site mutation on the PRPH2 transcript was analyzed using NetGene2, a splice prediction program and by the reverse transcription polymerase chain reaction of illegitimate transcripts from peripheral white blood cells.

MAIN OUTCOMES AND MEASURES

Results of testing for splice site mutation, haplotypes, and alternate transcripts.

RESULTS

The PRPH2 mutation was found in 97 individuals of 19 independently ascertained families with a clinical diagnosis of retinitis pigmentosa, macular dystrophy, and/or pattern dystrophy. All affected individuals also shared a rare haplotype of approximately 644 kilobase pairs containing the c.828+3A>T mutation, which extends from the short tandem repeat polymorphism D6S282 to c.1013G>A (rs434102, a single-nucleotide polymorphism) in exon 3 of PRPH2, suggesting this mutation is from a common ancestor and is a founder mutation. It has a prevalence of 2% in families diagnosed as having autosomal dominant retinitis pigmentosa and 10% in families with variable clinical diagnosis of pattern, macular, and retinal dystrophies. Individuals with the c.828+3A>T mutation expressed a PRPH2 transcript not found in control participants and that was consistent with abnormal splicing.

CONCLUSIONS AND RELEVANCE

The PRPH2 c.828+3A>T splice site mutation is a frequent cause of inherited retinal dystrophies and is owing to the founder effect. The likely cause of disease is the missplicing of the PRPH2 message that results in a truncated protein product. Identifying the genetic etiology assists in more accurate management and possible future therapeutic options.

Light regulates the ciliary protein transport and outer segment disc renewal of mammalian photoreceptors

The outer segment (OS) of the rod photoreceptor is a light-sensing cilium containing ~1,000 membrane-bound discs. Each day, discs constituting the distal tenth of the OS are shed, whereas nascent discs are formed at the base of the OS through the incorporation of molecules transported from the inner segment. The mechanisms regulating these processes remain elusive. Here, we show that rhodopsin preferentially enters the OS in the dark. Photoexcitation of post-Golgi rhodopsins retains them in the inner segment. Disc-rim protein peripherin2/rds enters the OS following a rhythm complementary to that of rhodopsin. Light-dark cycle-regulated protein trafficking serves as a mechanism to segregate rhodopsin-rich and peripherin2/rds-rich discs into alternating stacks, which are flanked by characteristic cytoplasmic pockets. This periodic cytostructure divides the OS into approximately ten fractions, each containing discs synthesized in a single day. This mechanism may explain how the rod photoreceptor balances the quantity of discs added and removed daily.

Rhodopsin Trafficking and Mistrafficking: Signals, Molecular Components, and Mechanisms

Rhodopsin is a seven-transmembrane G protein-coupled receptor (GPCR) and is the main component of the photoreceptor outer segment (OS), a ciliary compartment essential for vision. Because the OSs are incapable of protein synthesis, rhodopsin must first be synthesized in the inner segments (ISs) and subsequently trafficked across the connecting cilia to the OSs where it participates in the phototransduction cascade. Rapid turnover of the OS necessitates a high rate of synthesis and efficient trafficking of rhodopsin to the cilia. This cilia-targeting mechanism is shared among other ciliary-localized GPCRs. In this review, we will discuss the process of rhodopsin trafficking from the IS to the OS beginning with the trafficking signals present on the protein. Starting from the endoplasmic reticulum and the Golgi apparatus within the IS, we will cover the molecular components assisting the biogenesis and the proper sorting. We will also review the confirmed binding and interacting partners that help target rhodopsin toward the connecting cilium as well as the cilia-localized components which direct proteins into the proper compartments of the OS. While rhodopsin is the most critical and abundant component of the photoreceptor OS, mutations in the rhodopsin gene commonly lead to its mislocalization within the photoreceptors. In addition to covering the trafficking patterns of rhodopsin, we will also review some of the most common rhodopsin mutants which cause mistrafficking and subsequent death of photoreceptors. Toward the goal of understanding the pathogenesis, three major mechanisms of aberrant trafficking as well as putative mechanisms of photoreceptor degeneration will be discussed.

Gene therapy for PRPH2-associated ocular disease: challenges and prospects

The peripherin-2 (PRPH2) gene encodes a photoreceptor-specific tetraspanin protein called peripherin-2/retinal degeneration slow (RDS), which is critical for the formation and maintenance of rod and cone outer segments. Over 90 different disease-causing mutations in PRPH2 have been identified, which cause a variety of forms of retinitis pigmentosa and macular degeneration. Given the disease burden associated with PRPH2 mutations, the gene has long been a focus for preclinical gene therapy studies. Adeno-associated viruses and compacted DNA nanoparticles carrying PRPH2 have been successfully used to mediate improvement in the rds(-/-) and rds(+/-) mouse models. However, complexities in the pathogenic mechanism for PRPH2-associated macular disease coupled with the need for a precise dose of peripherin-2 to combat a severe haploinsufficiency phenotype have delayed the development of clinically viable genetic treatments. Here we discuss the progress and prospects for PRPH2-associated gene therapy.

Multimodal imaging of adult-onset foveomacular vitelliform dystrophy

Adult-onset foveomacular vitelliform dystrophy (AOFVD) is a clinically heterogeneous maculopathy that may mimic other conditions and be difficult to diagnose. It is characterized by late onset, slow progression and high variability in morphologic and functional alterations. Diagnostic evaluation should include careful ophthalmoscopy and imaging studies. The typical ophthalmoscopic findings are bilateral, asymmetric, foveal or perifoveal, yellow, solitary, round to oval elevated subretinal lesions, often with central pigmentation. The lesions characteristically demonstrate increased autofluorescence and hypofluorescent lesions surrounded by irregular annular hyperfluorescence on fluorescein angiography. Optical coherence tomography studies demonstrate homogenous or heterogeneous hyperreflective material between the retinal pigment epithelium and the neurosensory retina. The visual prognosis is generally favorable, but visual loss can occur from chorioretinal atrophy and choroidal neovascularization.

Transgenic expression of constitutively active RAC1 disrupts mouse rod morphogenesis

PURPOSE

Dominant-active RAC1 rescues photoreceptor structure in Drosophila rhodopsin-null mutants, indicating an important role in morphogenesis. This report assesses the morphogenetic effect of activated RAC1 during mammalian rod photoreceptor development using transgenic mice that express constitutively active (CA) RAC1.

METHODS

Transgenic mice were generated by expressing CA RAC1 under control of the Rhodopsin promoter, and morphological features of the photoreceptors were evaluated by histology, immunohistochemistry, and transmission electron microscopy. Function was evaluated by electroretinography. Potential protein partners of CA RAC1 were identified by co-immunoprecipitation of retinal extracts.

RESULTS

Constitutively active RAC1 expression in differentiating rods disrupted outer retinal lamination as early as postnatal day (P)6, and many photoreceptor cell nuclei were displaced apically into the presumptive subretinal space. These photoreceptors did not develop normal inner and outer segments and had abnormal placement of synaptic elements. Some photoreceptor nuclei were also mislocalized into the inner nuclear layer. Extensive photoreceptor degeneration was subsequently observed in the adult animal. Constitutively active RAC1 formed a complex with the polarity protein PAR6 and with microtubule motor dynein in mouse retina. The normal localization of the PAR6 complex was disrupted in CA RAC1-expressing rod photoreceptors.

CONCLUSIONS

Constitutively active RAC1 had a profound negative effect on mouse rod cell viability and development. Rod photoreceptors in the CA RAC1 retina exhibited a defect in polarity and migration. Constitutively active RAC1 disrupted rod morphogenesis and gave a phenotype resembling that found in the Crumbs mutant. PAR6 and dynein are two potential downstream effectors that may be involved in CA RAC1-mediated defective mouse photoreceptor morphogenesis.

An unconventional secretory pathway mediates the cilia targeting of peripherin/rds

It is unclear how unconventional secretion interplays with conventional secretion for the normal maintenance and renewal of membrane structures. The photoreceptor sensory cilium is recognized for fast membrane renewal, for which rhodopsin and peripherin/rds (P/rds) play critical roles. Here, we provide evidence that P/rds is targeted to the cilia by an unconventional secretion pathway. When expressed in ciliated hTERT-RPE1 human cell line, P/rd is localized to cilia. Cilium trafficking of P/rds was sustained even when the Golgi functions, including trans-Golgi-mediated conventional secretion, were inhibited by the small molecules brefeldin A, 30N12, and monensin. The unconventional cilia targeting of P/rds is dependent on COPII-mediated exit from the ER, but appears to be independent of GRASP55-mediated secretion. The regions in the C-terminal tail of P/rds are essential for this unconventional trafficking. In the absence of the region required for cilia targeting, P/rds was prohibited from entering the secretory pathways and was retained in the Golgi apparatus. A region essential for this Golgi retention was also found in the C-terminal tail of P/rds and supported the cilia targeting of P/rds mediated by unconventional secretion. In ciliated cells, including bovine and Xenopus laevis rod photoreceptors, P/rds was robustly sensitive to endoglycosidase H, which is consistent with its bypassing the medial Golgi and traversing the unconventional secretory pathway. Because rhodopsin is known to traffic through conventional secretion, this study of P/rds suggests that both conventional secretion and unconventional secretion need to cooperate for the renewal of the photoreceptor sensory cilium.

Proteasome overload is a common stress factor in multiple forms of inherited retinal degeneration

Inherited retinal degenerations, caused by mutations in over 100 individual genes, affect approximately 2 million people worldwide. Many of the underlying mutations cause protein misfolding or mistargeting in affected photoreceptors. This places an increased burden on the protein folding and degradation machinery, which may trigger cell death. We analyzed how these cellular functions are affected in degenerating rods of the transducin γ-subunit (Gγ1) knockout mouse. These rods produce large amounts of transducin β-subunit (Gβ1), which cannot fold without Gγ1 and undergoes intracellular proteolysis instead of forming a transducin βγ-subunit complex. Our data revealed that the most critical pathobiological factor leading to photoreceptor cell death in these animals is insufficient capacity of proteasomes to process abnormally large amounts of misfolded protein. A decrease in the Gβ1 production in Gγ1 knockout rods resulted in a significant reduction in proteasomal overload and caused a striking reversal of photoreceptor degeneration. We further demonstrated that a similar proteasomal overload takes place in photoreceptors of other mutant mice where retinal degeneration has been ascribed to protein mistargeting or misfolding, but not in mice whose photoreceptor degenerate as a result of abnormal phototransduction. These results establish the prominence of proteasomal insufficiency across multiple degenerative diseases of the retina, thereby positioning proteasomes as a promising therapeutic target for treating these debilitating conditions.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Pattern dystrophy with high intrafamilial variability associated with Y141C mutation in the peripherin/RDS gene and successful treatment of subfoveal CNV related to multifocal pattern type with anti-VEGF (ranibizumab) intravitreal injections

OBJECTIVE

To identify disease causing mutation in three generations of a Swiss family with pattern dystrophy and high intrafamilial variability of phenotype. To assess the effect of intravitreal ranibizumab injections in the treatment of subfoveal choroidal neovascularization associated with pattern dystrophy in one patient.

METHODS

Affected family members were ascertained for phenotypic and genotypic characterization. Ophthalmic evaluations included fundus photography, autofluorescence imaging, optical coherence tomography, and International Society for Clinical Electrophysiology of Vision standard full-field electroretinography. When possible family members had genetic testing. The proband presented with choroidal neovascularization and had intravitreal injections as needed according to visual acuity and optical coherence tomography.

RESULTS

Proband had a multifocal type pattern dystrophy, and his choroidal neovascularization regressed after four intravitreal injections. The vision improved from 0.8 to 1.0, and optical coherence tomography showed complete anatomical restoration. A butterfly-shaped pattern was observed in her cousin, whereas a fundus pulverulentus pattern was seen in a second cousin. Aunt had a multifocal atrophic appearance, simulating geographic atrophy in age-related macular degeneration. The Y141C mutation was identified in the peripherin/RDS gene and segregated with disease in the family.

CONCLUSION

This is the first report of marked intrafamilial variation of pattern dystrophy because of peripherin/RDS Y141C mutation. Intravitreal ranibizumab injections might be a valuable treatment for associated subfoveal choroidal neovascularization.

A single valine residue plays an essential role in peripherin/rds targeting to photoreceptor outer segments

Peripherin/retinal degeneration slow (rds) is an integral membrane protein specifically localized to the light-sensing organelle of the photoreceptor cell, the outer segment. Within the outer segment, peripherin is found at the edges of photoreceptor discs, where it plays a critical role in disc morphogenesis and maintenance. Peripherin loss or mutations are often associated with severe forms of visual impairments. Like all other resident outer segment proteins, peripherin is synthesized in the photoreceptor cell body and subsequently transported to the outer segment. In an effort to further examine peripherin’s delivery to outer segments, we undertook a careful examination of its targeting sequence. Using a fluorescently labeled reporter expressed in the rods of transgenic tadpoles, we narrowed peripherin’s targeting sequence to ten amino acids within its C-terminal tail. This small stretch of amino acid residues is both necessary and sufficient for outer segment targeting. We also conducted alanine scanning of all residues within this sequence and found that only a single residue, valine at position 332, is essential for outer segment targeting. This valine is conserved in all species and its mutation is sufficient to completely abrogate the targeting of full-length peripherin in mouse rods.

AAV-mediated gene therapy in mouse models of recessive retinal degeneration

In recent years, more and more mutant genes that cause retinal diseases have been detected. At the same time, many naturally occurring mouse models of retinal degeneration have also been found, which show similar changes to human retinal diseases. These, together with improved viral vector quality allow more and more traditionally incurable inherited retinal disorders to become potential candidates for gene therapy. Currently, the most common vehicle to deliver the therapeutic gene into target retinal cells is the adenoassociated viral vector (AAV). Following delivery to the immuno-privileged subretinal space, AAV-vectors can efficiently target both retinal pigment epithelium and photoreceptor cells, the origin of most retinal degenerations. This review focuses on the AAV-based gene therapy in mouse models of recessive retinal degenerations, especially those in which delivery of the correct copy of the wild-type gene has led to significant beneficial effects on visual function, as determined by morphological, biochemical, electroretinographic and behavioral analysis. The past studies in animal models and ongoing successful LCA2 clinical trials, predict a bright future for AAV gene replacement treatment for inherited recessive retinal diseases.

Protective gene expression changes elicited by an inherited defect in photoreceptor structure

Inherited defects in retinal photoreceptor structure impair visual transduction, disrupt relationship with the retinal pigment epithelium (RPE), and compromise cell viability. A variety of progressive retinal degenerative diseases can result, and knowledge of disease etiology remains incomplete. To investigate pathogenic mechanisms in such instances, we have characterized rod photoreceptor and retinal gene expression changes in response to a defined insult to photoreceptor structure, using the retinal degeneration slow (rds) mouse model. Global gene expression profiling was performed on flow-sorted rds and wild-type rod photoreceptors immediately prior and subsequent to times at which OSs are normally elaborated. Dysregulated genes were identified via microarray hybridization, and selected candidates were validated using quantitative PCR analyses. Both the array and qPCR data revealed that gene expression changes were generally modest and dispersed amongst a variety of known functional networks. Although genes showing major (>5-fold) differential expression were identified in a few instances, nearly all displayed transient temporal profiles, returning to WT levels by postnatal day (P) 21. These observations suggest that major defects in photoreceptor cell structure may induce early homeostatic responses, which function in a protective manner to promote cell viability. We identified a single key gene, Egr1, that was dysregulated in a sustained fashion in rds rod photoreceptors and retina. Egr1 upregulation was associated with microglial activation and migration into the outer retina at times subsequent to the major peak of photoreceptor cell death. Interestingly, this response was accompanied by neurotrophic factor upregulation. We hypothesize that activation of Egr1 and neurotrophic factors may represent a protective immune mechanism which contributes to the characteristically slow retinal degeneration of the rds mouse model.

Longitudinal assessment of retinal structure and function reveals a rod-cone degeneration in a guinea pig model initially presented as night blind

We have previously reported a naturally occurring retinopathy in a population of guinea pigs, where the affected animals presented a defect of the rod-mediated vision. The purpose of this study was to investigate if the mutants were affected with a stationary or degenerative retinopathy and to identify the cellular origin of this unique disorder. Electroretinogram (ERG) [postnatal day 1 (P1) to P450], light (LM) and electron microscopy (EM) [P5, P150, P450], and immunohistochemistry [P30, P150, P450] were evaluated from normal and mutant animals. Irrespective of age, the scotopic ERGs of mutants could only be evoked by bright flashes, and the resulting ERGs were of photopic waveform. Interestingly, the amplitude of the cone and the rod/cone a-waves was always of smaller amplitude in mutants, but this difference tended to decrease with age. In contrast, the b-waves were of larger amplitude than normal in photopic ERGs obtained prior to age 25 (days) and prior to age 10 for rod/cone ERGs. LM revealed, in mutants, an absence of the outer segment layer (OSL) with a reduction in the outer nuclear layer (ONL) thickness. EM disclosed the presence of cone outer segment (OS) while no rod OS could be evidenced. Immunohistochemistry revealed the presence of rhodopsin, both cone opsins as well as normal synaptophysin immunoreactivity. Finally, neither the retinal structure nor the function in the mutants achieved normal development. Results suggest that mutant animals are suffering from a degenerative retinal disorder that affects the structure and function of rods and cones.

Systematic screening of BEST1 and PRPH2 in juvenile and adult vitelliform macular dystrophies: a rationale for molecular analysis

PURPOSE

To evaluate a genetic approach of BEST1 and PRPH2 screening according to age of onset, family history, and Arden ratio in patients with juvenile vitelliform macular dystrophy (VMD2) or adult-onset vitelliform macular dystrophy (AVMD), which are characterized by autofluorescent deposits.

DESIGN

Clinical, electrophysiologic, and molecular retrospective study.

PARTICIPANTS

The database of a clinic specialized in genetic sensory diseases was screened for patients with macular vitelliform dystrophy. Patients with an age of onset less than 40 years were included in the VMD2 group (25 unrelated patients), and patients with an age of onset more than 40 years were included in the AVMD group (19 unrelated patients).

METHODS

Clinical, fundus photography, and electro-oculogram (EOG) findings were reviewed. Mutation screening of BEST1 and PRPH2 genes was systematically performed.

MAIN OUTCOME MEASURES

Relevance of age of onset, family history, and Arden ratio were reviewed.

RESULTS

Patients with VMD2 carried a BEST1 mutation in 60% of the cases. Seven novel mutations in BEST1 (p.V9L, p.F80V, p.I73V, p.R130S, pF298C, pD302A, and p.179delN) were found. Patients with VMD2 with a positive family history or a reduced Arden ratio carried a BEST1 mutation in 70.5% of cases and in 83% if both criteria were fulfilled. Patients with AVMD carried a PRPH2 mutation in 10.5% of cases and did not carry a BEST1 mutation. The probability of finding a PRPH2 mutation increased in the case of a family history (2/5 patients). Electro-oculogram was normal in 3 of 15 patients with BEST1 mutations and reduced in the 3 patients with PRPH2 mutations.

CONCLUSIONS

Age of onset is a major criterion to distinguish VMD2 from AVMD. Electro-oculogram is not as relevant because decreased or normal Arden ratios have been associated with mutations in both genes and diseases. A positive family history increased the probability of finding a mutation. BEST1 screening should be recommended to patients with an age of onset less than 40 years, and PRPH2 screening should be recommended to patients with an age of onset more than 40 years. For an onset between 30 and 40 years, PRPH2 can be screened if no mutation has been detected in BEST1.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Animal models of retinal disease

Diseases of the retina are the leading causes of blindness in the industrialized world. The recognition that animals develop retinal diseases with similar traits to humans has led to not only a dramatic improvement in our understanding of the pathogenesis of retinal disease but also provided a means for testing possible treatment regimes and successful gene therapy trials. With the advent of genetic and molecular biological tools, the association between specific gene mutations and retinal signs has been made. Animals carrying natural mutations usually in one gene now provide well-established models for a host of inherited retinal diseases, including retinitis pigmentosa, Leber congenital amaurosis, inherited macular degeneration, and optic nerve diseases. In addition, the development of transgenic technologies has provided a means by which to study the effects of these and novel induced mutations on retinal structure and function. Despite these advances, there is a paucity of suitable animal models for complex diseases, including age-related macular degeneration (AMD) and diabetic retinopathy, largely because these diseases are not caused by single gene defects, but involve complex genetics and/or exacerbation through environmental factors, epigenetic, or other modes of genetic influence. In this review, we outline in detail the available animal models for inherited retinal diseases and how this information has furthered our understanding of retinal diseases. We also examine how transgenic technologies have helped to develop our understanding of the role of isolated genes or pathways in complex diseases like AMD, diabetes, and glaucoma.

Light-evoked responses of the retinal pigment epithelium: changes accompanying photoreceptor loss in the mouse

Mutations in genes expressed in the retinal pigment epithelium (RPE) underlie a number of human inherited retinal disorders that manifest with photoreceptor degeneration. Because light-evoked responses of the RPE are generated secondary to rod photoreceptor activity, RPE response reductions observed in human patients or animal models may simply reflect decreased photoreceptor input. The purpose of this study was to define how the electrophysiological characteristics of the RPE change when the complement of rod photoreceptors is decreased. To measure RPE function, we used an electroretinogram (dc-ERG)-based technique. We studied a slowly progressive mouse model of photoreceptor degeneration (Prph(Rd2/+)), which was crossed onto a Nyx(nob) background to eliminate the b-wave and most other postreceptoral ERG components. On this background, Prph(Rd2/+) mice display characteristic reductions in a-wave amplitude, which parallel those in slow PIII amplitude and the loss of rod photoreceptors. At 2 and 4 mo of age, the amplitude of each dc-ERG component (c-wave, fast oscillation, light peak, and off response) was larger in Prph(Rd2/+) mice than predicted by rod photoreceptor activity (Rm(P3)) or anatomical analysis. At 4 mo of age, the RPE in Prph(Rd2/+) mice showed several structural abnormalities including vacuoles and swollen, hypertrophic cells. These data demonstrate that insights into RPE function can be gained despite a loss of photoreceptors and structural changes in RPE cells and, moreover, that RPE function can be evaluated in a broader range of mouse models of human retinal disease.

Deciphering the structure and function of Als2cr4 in the mouse retina

PURPOSE

The role of Als2cr4 (amyotrophic lateral sclerosis 2 [juvenile] chromosome region, candidate 4; also known as hypothetical protein FLJ33282) in the mouse retina was determined by characterizing the molecular structure, cellular interacting partners, and potential biochemical functions. Previous in situ hybridization and gene expression profiles show that the mRNAs encoding Als2cr4 are abundant in the eye, hippocampus, cerebellum, and olfactory bulb.

METHODS

From predicted antigenic epitopes of Als2cr4, two novel antibodies were developed to examine protein expression and morphologic localization in retinas from light-adapted and dark-adapted mice by immunohistochemistry, immunoblot analysis, and immunoelectron microscopy, and then immunoprecipitation was performed to identify interacting proteins by mass spectroscopy.

RESULTS

Peptide antibodies with Als2cr4 antigenic epitopes from either the amino- or carboxyl terminus were characterized with Als2cr4 recombinant proteins and peptide competition assays. Als2cr4 is a 45-kDa insoluble protein, highly enriched in retina, and localizes to photoreceptor outer segments, ciliary complex, and horizontal cells in the outer plexiform layer. Immunoelectron microscopy for Als2cr4 verified its expression in the discs of photoreceptor outer segments. Immunoprecipitation and mass spectroscopy identified eight potential interacting partners: vimentin, actin, myosin Va, myosin VI, myosin X, myosin XIV, kinesin 1, Als2cr4, and lamin B-1.

CONCLUSIONS

Als2cr4 is a novel protein, with a probable tetraspanin-like membrane structure, that is localized in photoreceptors and in the postsynaptic outer plexiform layer and that interacts with cytoskeletal proteins. Als2cr4 may be involved in membrane transport between the photoreceptor inner and outer segments and may be a key component in maintaining the structural integrity of the outer segment.

Neuroprotection of photoreceptors by direct delivery of erythropoietin to the retina of the retinal degeneration slow mouse

The primary objectives of this study were to determine if erythropoietin (EPO) is neuroprotective to the photoreceptors in the retinal degeneration slow (rds) mouse in the absence of an increase in hematocrit and to determine if deglycosylated EPO (DEPO) is less neuroprotective. We performed subretinal injections of 10U EPO, DEPO or hyperglycosylated EPO (HEPO) in postnatal day 7 rds mice. Whole eye EPO levels were quantified by ELISA at specified time points post-injection. TUNEL analysis, hematocrit, and immunohistochemistry were performed at postnatal day 20. Half of the amount of EPO measured immediately after injection was detected less than 1 h later. Twenty four hours later, EPO levels were 1000 times lower than the amount originally detected. Uninjected rds mice contained 36 +/- 2 TUNEL-positive cells/mm retina and PBS injected mice contained 17 +/- 3 TUNEL-positive cells/mm retina. EPO, DEPO, and HEPO treated rds retinas contained 5 +/- 2, 9 +/- 2, and 3 +/- 1 TUNEL-positive cells/mm retina, respectively. The hematocrit was 43% in control and 41% in treated rds mice Previous studies have shown neuroprotection of the retina by treatment with as little as 24-39 mU EPO/mg total protein in the eye. In this study, we detected 40 mU/mg EPO in the eye 11 h after injection of 10 U EPO. Treatment with all forms of EPO tested was neuroprotective to the photoreceptors without a concomitant increase in hematocrit.

Naturally occurring animal models with outer retina phenotypes

Naturally occurring and laboratory generated animal models serve as powerful tools with which to investigate the etiology of human retinal degenerations, especially retinitis pigmentosa (RP), Leber congenital amaurosis (LCA), cone dystrophies (CD) and macular degeneration (MD). Much progress has been made in elucidating gene defects underlying disease, in understanding mechanisms leading to disease, and in designing molecules for translational research and gene-based therapy to interfere with the progression of disease. Key to this progress has been study of naturally occurring murine and canine retinal degeneration mutants. This article will review the history, phenotypes and gene defects of select animal models with outer retina (photoreceptor and retinal pigment epithelium) degeneration phenotypes.

Analysis of protein expression and compartmentalization in retinal neurons using serial tangential sectioning of the retina

The progress in understanding visual signal transduction in vertebrate photoreceptors, arguably the best studied G protein-mediated signal transduction cascade in modern biology, was facilitated by the unique anatomy of rod photoreceptors. Held only by thin connected cilia, rod outer segments can be readily separated from the rest of the retina simply by shaking, and then purified by gradient centrifugation. The availability of such an efficient procedure of rod outer segment purification not only previously facilitated the identification of many principal visual signaling proteins located in this cellular compartment, but it is also currently being exploited in proteomics studies. In this paper, we describe a simple and inexpensive technique that allows for the quantitative analysis of protein expression within different subcellular compartments of photoreceptors, and could also be used for studying protein expression in the secondary retinal neurons. This technique is based on the Western blot analysis of the protein content of serial sections obtained by tangential sectioning of flat-mounted frozen retinas from mouse and rat, and it could serve as a way to validate proteomic data, similar to the way the quantitative RT-PCR technique is used for validation of gene-microarray data. To demonstrate the utility of this technique, we have determined the expression profiles in normal mouse retina of several signaling, energy-producing, and chaperone proteins, which were recently identified in bovine rod photoreceptors by mass spectrometry.

Membrane frizzled-related protein is necessary for the normal development and maintenance of photoreceptor outer segments

A 4 base pair deletion in a splice donor site of the Mfrp (membrane-type frizzled-related protein) gene, herein referred to as Mfrprd6/rd6, is predicted to lead to the skipping of exon 4 and photoreceptor degeneration in retinal degeneration 6 (rd6) mutant mice. Little, however, is known about the function of the protein or how the mutation causes the degenerative retinal phenotype. Here we examine ultrastructural changes in the retina of Mfrprd6/rd6 mice to determine the earliest effects of the mutation. We also extend the reported observations of the expression pattern of the dicistronic Mfrp/C1qtnf5 message and the localization of these and other retinal pigment epithelium (RPE) and retinal proteins during development and assess the ability of RPE cells to phagocytize outer segments (OSs) in mutant and wild-type (WT) mice. At the ultrastructural level, OSs do not develop normally in Mfrprd6/rd6 mutants. They are disorganized and become progressively shorter as mutant mice age. Additionally, there are focal areas in which there is a reduction of apical RPE microvilli. At P25, the rod electroretinogram (ERG) a-wave of Mfrprd6/rd6 mice is reduced in amplitude by ~50% as are ERG components generated by the RPE. Examination of beta-catenin localization and Fos and Tcf-1 expression, intermediates of the canonical Wnt pathway, showed that they were not different between mutant and WT mice, suggesting that MFRP may operate through an alternative pathway. Finally, impaired OS phagocytosis was observed in Mfrprd6/rd6 mice both in standard ambient lighting conditions and with bright light exposure when compared to WT controls.

Mutant prominin 1 found in patients with macular degeneration disrupts photoreceptor disk morphogenesis in mice

Familial macular degeneration is a clinically and genetically heterogeneous group of disorders characterized by progressive central vision loss. Here we show that an R373C missense mutation in the prominin 1 gene (PROM1) causes 3 forms of autosomal-dominant macular degeneration. In transgenic mice expressing R373C mutant human PROM1, both mutant and endogenous PROM1 were found throughout the layers of the photoreceptors, rather than at the base of the photoreceptor outer segments, where PROM1 is normally localized. Moreover, the outer segment disk membranes were greatly overgrown and misoriented, indicating defective disk morphogenesis. Immunoprecipitation studies showed that PROM1 interacted with protocadherin 21 (PCDH21), a photoreceptor-specific cadherin, and with actin filaments, both of which play critical roles in disk membrane morphogenesis. Collectively, our results identify what we believe to be a novel complex involved in photoreceptor disk morphogenesis and indicate a possible role for PROM1 and PCDH21 in macular degeneration.