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

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.

Cellular and Molecular Mechanisms of Pathogenesis Underlying Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are congenital retinal degenerative diseases that have various inheritance patterns, including dominant, recessive, X-linked, and mitochondrial. These diseases are most often the result of defects in rod and/or cone photoreceptor and retinal pigment epithelium function, development, or both. The genes associated with these diseases, when mutated, produce altered protein products that have downstream effects in pathways critical to vision, including phototransduction, the visual cycle, photoreceptor development, cellular respiration, and retinal homeostasis. The aim of this manuscript is to provide a comprehensive review of the underlying molecular mechanisms of pathogenesis of IRDs by delving into many of the genes associated with IRD development, their protein products, and the pathways interrupted by genetic mutation.

The Role of Peripherin-2/ROM1 Complexes in Photoreceptor Outer Segment Disc Morphogenesis

The light-sensitive outer segment organelle of photoreceptor cells contains a stack of hundreds of flat, disc-shaped membranes called discs. The rims of these discs contain a photoreceptor-specific tetraspanin protein peripherin-2 (also known as rds or PRPH2). Mutations in the PRPH2 gene lead to a wide variety of inherited retinal degenerations in humans. The vast majority of these mutations occur within a large, intradiscal loop of peripherin-2, known as the D2 loop. The D2 loop mediates well-established intermolecular interactions of peripherin-2 molecules among themselves and a homologous protein ROM1. These interactions lead to the formation of large, highly ordered oligomers. In this chapter, we discuss the supramolecular organization of peripherin-2/ROM1 complexes and their contribution to the process of outer segment disc morphogenesis and enclosure.

Ca2+ and Mg2+ Influence the Thermodynamics of Peptide-Membrane Interactions

Accurate quantitative estimates of protein-membrane interactions are critical to studies of membrane proteins. Here, we demonstrate that thermodynamic analyses based on current hydropathy scales do not account for the significant and experimentally determined effects that Ca2+ or Mg2+ have on protein-membrane interactions. We examined distinct modes of interaction (interfacial partitioning and folding and transmembrane insertion) by studying three highly divergent peptides: Bid-BH3 (derived from apoptotic regulator Bid), peripherin-2-derived prph2-CTER, and the cancer-targeting pH-Low-Insertion-Peptide (pHLIP). Fluorescence experiments demonstrate that adding 1-2 mM of divalent cations led to a substantially more favorable bilayer partitioning and insertion, with free energy differences of 5-15 kcal/mol.

Determinants shaping the nanoscale architecture of the mouse rod outer segment

The unique membrane organization of the rod outer segment (ROS), the specialized sensory cilium of rod photoreceptor cells, provides the foundation for phototransduction, the initial step in vision. ROS architecture is characterized by a stack of identically shaped and tightly packed membrane disks loaded with the visual receptor rhodopsin. A wide range of genetic aberrations have been reported to compromise ROS ultrastructure, impairing photoreceptor viability and function. Yet, the structural basis giving rise to the remarkably precise arrangement of ROS membrane stacks and the molecular mechanisms underlying genetically inherited diseases remain elusive. Here, cryo-electron tomography (cryo-ET) performed on native ROS at molecular resolution provides insights into key structural determinants of ROS membrane architecture. Our data confirm the existence of two previously observed molecular connectors/spacers which likely contribute to the nanometer-scale precise stacking of the ROS disks. We further provide evidence that the extreme radius of curvature at the disk rims is enforced by a continuous supramolecular assembly composed of peripherin-2 (PRPH2) and rod outer segment membrane protein 1 (ROM1) oligomers. We suggest that together these molecular assemblies constitute the structural basis of the highly specialized ROS functional architecture. Our Cryo-ET data provide novel quantitative and structural information on the molecular architecture in ROS and substantiate previous results on proposed mechanisms underlying pathologies of certain PRPH2 mutations leading to blindness.

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.

The GARP Domain of the Rod CNG Channel's β1-Subunit Contains Distinct Sites for Outer Segment Targeting and Connecting to the Photoreceptor Disk Rim

Vision begins when light is captured by the outer segment organelle of photoreceptor cells in the retina. Outer segments are modified cilia filled with hundreds of flattened disk-shaped membranes. Disk membranes are separated from the surrounding plasma membrane, and each membrane type has unique protein components. The mechanisms underlying this protein sorting remain entirely unknown. In this study, we investigated the outer segment delivery of the rod cyclic nucleotide-gated (CNG) channel, which is located in the outer segment plasma membrane, where it mediates the electrical response to light. Using Xenopus and mouse models of both sexes, we now show that the targeted delivery of the CNG channel to the outer segment uses the conventional secretory pathway, including protein processing in both ER and Golgi, and requires preassembly of its constituent α1 and β1 subunits. We further demonstrate that the N-terminal glutamic acid-rich protein (GARP) domain of CNGβ1 contains two distinct functional regions. The glutamic acid-rich region encodes specific information targeting the channel to rod outer segments. The adjacent proline-enriched region connects the CNG channel to photoreceptor disk rims, likely through an interaction with peripherin-2. These data reveal fine functional specializations within the structural domains of the CNG channel and suggest that its sequestration to the outer segment plasma membrane requires an interaction with peripherin-2.SIGNIFICANCE STATEMENT Neurons and other differentiated cells have a remarkable ability to deliver and organize signaling proteins at precise subcellular locations. We now report that the CNG channel, mediating the electrical response to light in rod photoreceptors, contains two specialized regions within the N terminus of its β-subunit: one responsible for delivery of this channel to the ciliary outer segment organelle and another for subsequent channel sequestration into the outer segment plasma membrane. These findings expand our understanding of the molecular specializations used by neurons to populate their critical functional compartments.

Impairments of Photoreceptor Outer Segments Renewal and Phototransduction Due to a Peripherin Rare Haplotype Variant: Insights from Molecular Modeling

BACKGROUND

Retinitis pigmentosa punctata albescens (RPA) is a particular form of retinitis pigmentosa characterized by childhood onset night blindness and areas of peripheral retinal atrophy. We investigated the genetic cause of RPA in a family consisting of two affected Egyptian brothers with healthy consanguineous parents.

METHODS

Mutational analysis of four RPA causative genes was realized by Sanger sequencing on both probands, and detected variants were subsequently genotyped in their parents. Afterwards, found variants were deeply, statistically, and in silico characterized to determine their possible effects and association with RPA.

RESULTS

Both brothers carry three missense PRPH2 variants in a homozygous condition (c.910C > A, c.929G > A, and c.1013A > C) and two promoter variants in RHO (c.-26A > G) and RLBP1 (c.-70G > A) genes, respectively. Haplotype analyses highlighted a PRPH2 rare haplotype variant (GAG), determining a possible alteration of PRPH2 binding with melanoregulin and other outer segment proteins, followed by photoreceptor outer segment instability. Furthermore, an altered balance of transcription factor binding sites, due to the presence of RHO and RLBP1 promoter variants, might determine a comprehensive downregulation of both genes, possibly altering the PRPH2 shared visual-related pathway.

CONCLUSIONS

Despite several limitations, the study might be a relevant step towards detection of novel scenarios in RPA etiopathogenesis.

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.

Syntaxin 3 is essential for photoreceptor outer segment protein trafficking and survival

Trafficking of photoreceptor membrane proteins from their site of synthesis in the inner segment (IS) to the outer segment (OS) is critical for photoreceptor function and vision. Here we evaluate the role of syntaxin 3 (STX3), in trafficking of OS membrane proteins such as peripherin 2 (PRPH2) and rhodopsin. Photoreceptor-specific Stx3 knockouts [Stx3 ^f/f(iCre75) and Stx3 ^f/f(CRX-Cre) ] exhibited rapid, early-onset photoreceptor degeneration and functional decline characterized by structural defects in IS, OS, and synaptic terminals. Critically, in the absence of STX3, OS proteins such as PRPH2, the PRPH2 binding partner, rod outer segment membrane protein 1 (ROM1), and rhodopsin were mislocalized along the microtubules to the IS, cell body, and synaptic region. We find that the PRPH2 C-terminal domain interacts with STX3 as well as other photoreceptor SNAREs, and our findings indicate that STX3 is an essential part of the trafficking pathway for both disc (rhodopsin) and rim (PRPH2/ROM1) components of the OS.

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.

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).

The Late Endosomal Pathway Regulates the Ciliary Targeting of Tetraspanin Protein Peripherin 2

Peripherin 2 (PRPH2) is a tetraspanin protein concentrated in the light-sensing cilium (called the outer segment) of the vertebrate photoreceptor. The mechanism underlying the ciliary targeting of PRPH2 and the etiology of cone dystrophy caused by PRPH2 mutations remain elusive. Here we show that the late endosome (LE) is the main waystation that critically sorts newly synthesized PRPH2 to the cilium. PRPH2 is expressed in the luminal membrane of the LE. We delineate multiple C-terminal motifs of PRPH2 that distinctively regulate its LE and ciliary targeting through ubiquitination and binding to ESCRT (Endosomal Sorting Complexes Required for Transport) component Hrs. Using the newly developed TetOn-inducible system in transfected male and female mouse cones in vivo, we show that the entry of nascent PRPH2 into the cone outer segment can be blocked by either cone dystrophy-causing C-terminal mutations of PRPH2, or by short-term perturbation of the LE or recycling endosomal traffic. These findings open new avenues of research to explore the biological role of the LE in the biosynthetic pathway and the etiology of cone dystrophy caused by PRPH2 mutations and/or malfunctions of the LE.SIGNIFICANCE STATEMENT Peripherin 2 (PRPH2) is a tetraspanin protein abundantly expressed in the light-sensing cilium, the outer segment, of the vertebrate photoreceptor. The mechanism underlying the ciliary transport of PRPH2 is unclear. The present study reveals a novel ciliary targeting pathway, in which the newly synthesized PRPH2 is first targeted to the lumen of the late endosome (LE) en route to the cilia. We deciphered the protein motifs and the machinery that regulates the LE trafficking of PRPH2. Using a novel TetOn-inducible system in transfected mouse cones, we showed that the LE pathway of PRPH2 is critical for its outer segment expression. A cone dystrophy-causing mutation impairs the LE and ciliary targeting of PRPH2, implicating the relevance of LE to cone/macular degenerative diseases.

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.

The Role of the Prph2 C-Terminus in Outer Segment Morphogenesis

Peripherin 2 (also known as RDS/Prph2) is localized to the rims of rod and cone outer segment (OS) discs. The C-terminus of Prph2 is a critical functional domain, but its exact role is still unknown. In this mini review, we describe work on the Prph2 C-terminus, highlighting its role as a regulator of protein trafficking, membrane curvature, ectosome secretion, and membrane fusion. Evidence supports a role for the Prph2 C-terminus in these processes and demonstrates that it is necessary for the initiation of OS morphogenesis.

Peripherin diverts ciliary ectosome release to photoreceptor disc morphogenesis

Molday and Goldberg discuss work by Salinas et al. describing formation of the light-sensing outer segment organelle of photoreceptor cells.

Formation of membrane discs in photoreceptor cells requires evagination of its ciliary plasma membrane by an unknown molecular mechanism. Salinas et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201608081) show that peripherin (also known as peripherin-2 or peripherin-2/rds) diverts membrane traffic to photoreceptor disc formation by inhibiting ectosome release from the cilium.