Subcellular localization of mutant P23H rhodopsin in an RFP fusion knock-in mouse model of retinitis pigmentosa

A Y178C rhodopsin mutation causes aggregation and comparatively severe retinal degeneration

Rhodopsin is the light-activated G protein-coupled receptor that initiates vision in photoreceptor cells of the retina. Numerous mutations in rhodopsin promote receptor misfolding and aggregation, causing autosomal dominant retinitis pigmentosa, a progressive retinal degenerative disease. The mechanism by which these mutations cause photoreceptor cell death, and the role aggregation plays in this process is still unclear. We recently demonstrated with the P23H and G188R rhodopsin mutants that the severity of aggregation observed in vitro is also reflected in vivo and impacts the rate of retinal degeneration. A Y178C rhodopsin mutant was investigated here to determine if this relationship applies broadly among mutations that cause misfolding and aggregation of the receptor. In vitro characterization indicated the Y178C rhodopsin mutant exhibits similar properties to the more severely aggregating G188R rhodopsin mutant, where the mutant is mislocalized to the endoplasmic reticulum in HEK293 cells and form aggregates that cannot be rescued by treatment with the retinoid 9-cis retinal. Despite these similarities in vitro, the Y178C rhodopsin mutant promoted a more severe retinal degeneration compared to the G188R mutant in vivo in mice. Aggregates of the Y178C rhodopsin mutant labeled by the dye PROTEOSTAT were morphologically similar to those formed by both the P23H and G188R rhodopsin mutants. There was, however, significantly greater photoreceptor cell death occurring independently of PROTEOSTAT-labeled aggregates in mice expressing the Y178C rhodopsin mutant compared to those expressing either the P23H or G188R rhodopsin mutants. Here, we demonstrate that PROTEOSTAT-labeled aggregates are not the sole cause of photoreceptor cell death promoted by the Y178C rhodopsin mutation in vivo, and there may be alternate aggregate forms contributing to cell death in these mice.

Ciliopathy-associated protein, CEP290, is required for ciliary necklace and outer segment membrane formation in retinal photoreceptors

The most common genetic cause of the childhood blinding disease Leber Congenital Amaurosis is mutation of the ciliopathy gene CEP290. Though studied extensively, the photoreceptor-specific roles of CEP290 remain unclear. Using advanced microscopy techniques, we investigated the sub-ciliary localization of CEP290 and its role in mouse photoreceptors during development. CEP290 was found throughout the connecting cilium between the microtubules and membrane, with nine-fold symmetry. In the absence of CEP290 ciliogenesis occurs, but the connecting cilium membrane is aberrant, and sub-structures, such as the ciliary necklace and Y-links, are defective or absent throughout the mid to distal connecting cilium. Transition zone proteins AHI1 and NPHP1 were abnormally restricted to the proximal connecting cilium in the absence of CEP290, while others like NPHP8 and CEP89 were unaffected. Although outer segment disc formation is inhibited in CEP290 mutant retina, we observed large numbers of extracellular vesicles. These results suggest roles for CEP290 in ciliary membrane structure, outer segment disc formation and photoreceptor-specific spatial distribution of a subset of transition zone proteins, which collectively lead to failure of outer segment formation and photoreceptor degeneration.

P23H rhodopsin aggregation in the ER causes synaptic protein imbalance in rod photoreceptors

Rod photoreceptor neurons in the retina detect scotopic light through the visual pigment rhodopsin (Rho) in their outer segments (OS). Efficient Rho trafficking to the OS through the inner rod compartments is critical for long-term rod health. Given the importance of protein trafficking to the OS, less is known about the trafficking of rod synaptic proteins. Furthermore, the subcellular impact of Rho mislocalization on rod synapses (i.e., "spherules") has not been investigated. In this study we used super-resolution and electron microscopies, along with proteomics, to perform a subcellular analysis of Rho synaptic mislocalization in P23H-Rho-RFP mutant mice. We discovered that mutant P23H-Rho-RFP protein mislocalized in distinct ER aggregations within the spherule cytoplasm, which we confirmed with AAV overexpression. Additionally, we found synaptic protein abundance differences in P23H-Rho-RFP mice. By comparison, Rho mislocalized along the spherule plasma membrane in WT and rd10 mutant rods, in which there was no synaptic protein disruption. Throughout the study, we also identified a network of ER membranes within WT rod presynaptic spherules. Together, our findings indicate that photoreceptor synaptic proteins are sensitive to ER dysregulation.

CoCas9 is a compact nuclease from the human microbiome for efficient and precise genome editing

The expansion of the CRISPR-Cas toolbox is highly needed to accelerate the development of therapies for genetic diseases. Here, through the interrogation of a massively expanded repository of metagenome-assembled genomes, mostly from human microbiomes, we uncover a large variety (n = 17,173) of type II CRISPR-Cas loci. Among these we identify CoCas9, a strongly active and high-fidelity nuclease with reduced molecular size (1004 amino acids) isolated from an uncultivated Collinsella species. CoCas9 is efficiently co-delivered with its sgRNA through adeno associated viral (AAV) vectors, obtaining efficient in vivo editing in the mouse retina. With this study we uncover a collection of previously uncharacterized Cas9 nucleases, including CoCas9, which enriches the genome editing toolbox.

Aggregation of rhodopsin mutants in mouse models of autosomal dominant retinitis pigmentosa

Mutations in rhodopsin can cause it to misfold and lead to retinal degeneration. A distinguishing feature of these mutants in vitro is that they mislocalize and aggregate. It is unclear whether or not these features contribute to retinal degeneration observed in vivo. The effect of P23H and G188R misfolding mutations were examined in a heterologous expression system and knockin mouse models, including a mouse model generated here expressing the G188R rhodopsin mutant. In vitro characterizations demonstrate that both mutants aggregate, with the G188R mutant exhibiting a more severe aggregation profile compared to the P23H mutant. The potential for rhodopsin mutants to aggregate in vivo was assessed by PROTEOSTAT, a dye that labels aggregated proteins. Both mutants mislocalize in photoreceptor cells and PROTEOSTAT staining was detected surrounding the nuclei of photoreceptor cells. The G188R mutant promotes a more severe retinal degeneration phenotype and greater PROTEOSTAT staining compared to that promoted by the P23H mutant. Here, we show that the level of PROTEOSTAT positive cells mirrors the progression and level of photoreceptor cell death, which suggests a potential role for rhodopsin aggregation in retinal degeneration.

Structural and functional rescue of cones carrying the most common cone opsin C203R missense mutation

An arginine to cysteine substitution at amino acid position 203 (C203R) is the most common missense mutation in human cone opsin. Linked to color blindness and blue cone monochromacy (BCM), C203 is involved in a crucial disulfide bond required for proper folding. It has previously been postulated that expression of mutant C203R cone opsin exerts a toxic effect on cone photoreceptors, similar to some well-characterized missense mutations in rhodopsin that lead to protein misfolding. In this study, we generated and characterized a BCM mouse model carrying the equivalent C203R mutation (Opn1mwC198R Opn1sw-/-) to investigate the disease mechanism and develop a gene therapy approach for this disorder. Untreated Opn1mwC198R Opn1sw-/- cones phenocopied affected cones in human patients with the equivalent mutation, exhibiting shortened or absent cone outer segments and loss of function. We determined that gene augmentation targeting cones specifically yielded robust rescue of cone function and structure when Opn1mwC198R Opn1sw-/- mice were treated at early ages. Importantly, treated cones displayed elaborated outer segments and replenished expression of crucial cone phototransduction proteins. Interestingly, we were unable to detect OPN1MWC198R mutant opsin at any age. We believe this is the first proof-of-concept study exploring the efficacy of gene therapy in BCM associated with a C203R mutation.

Super-resolution mapping in rod photoreceptors identifies rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient phototransduction and vision. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane, at the surface, in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

Mapping rhodopsin trafficking in rod photoreceptors with quantitative super-resolution microscopy

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient long-term phototransduction. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

Network biology analysis of P23H rhodopsin interactome identifies protein and mRNA quality control mechanisms

Rhodopsin is essential for phototransduction, and many rhodopsin mutations cause heritable retinal degenerations. The P23H rhodopsin variant generates a misfolded rhodopsin protein that photoreceptors quickly target for degradation by mechanisms that are incompletely understood. To gain insight into how P23H rhodopsin is removed from rods, we used mass spectrometry to identify protein interaction partners of P23H rhodopsin immunopurified from RhoP23H/P23H mice and compared them with protein interaction partners of wild-type rhodopsin from Rho+/+ mice. We identified 286 proteins associated with P23H rhodopsin and 276 proteins associated with wild-type rhodopsin. 113 proteins were shared between wild-type and mutant rhodopsin protein interactomes. In the P23H rhodopsin protein interactome, we saw loss of phototransduction, retinal cycle, and rhodopsin protein trafficking proteins but gain of ubiquitin-related proteins when compared with the wild-type rhodopsin protein interactome. In the P23H rhodopsin protein interactome, we saw enrichment of gene ontology terms related to ER-associated protein degradation, ER stress, and translation. Protein-protein interaction network analysis revealed that translational and ribosomal quality control proteins were significant regulators in the P23H rhodopsin protein interactome. The protein partners identified in our study may provide new insights into how photoreceptors recognize and clear mutant rhodopsin, offering possible novel targets involved in retinal degeneration pathogenesis.

First person - Michael Robichaux

First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping early-career researchers promote themselves alongside their papers. Michael Robichaux is first author on ‘ Subcellular localization of mutant P23H rhodopsin in an RFP fusion knock-in mouse model of retinitis pigmentosa’, published in DMM. Michael conducted the research described in this article while a postdoctoral fellow in Ted Wensel's lab at Baylor College of Medicine, Houston, TX, USA. He is now an assistant professor at West Virginia University, Morgantown, WV, USA, investigating the intricate subcellular processes in photoreceptor neurons that enable vision.