A Zebrafish Model of Retinitis Pigmentosa Shows Continuous Degeneration and Regeneration of Rod Photoreceptors

Models of Photoreceptor Degeneration in Adult Zebrafish

Zebrafish maintain a remarkable ability to regenerate their neural retina following rapid and extensive loss of retinal neurons. This is mediated by Müller glial cells (MG), which re-enter the cell cycle to produce amplifying progenitor cells that eventually differentiate into the lost retinal neurons. For example, exposing adult albino zebrafish to intense light destroys large numbers of rod and cone photoreceptors, which are then restored by MG-mediated regeneration. Here, we describe an updated method for performing these acute phototoxic lesions to adult zebrafish retinas. Next, we contrast this method to a chronic, low light lesion model that results in a more muted and sustained damage to photoreceptors and does not trigger a MG-mediated regeneration response. Thus, these two methods can be used to compare and contrast the genetic and morphological changes associated with acute and chronic methods of photoreceptor degeneration.

Regulating the formation of Müller glia-derived progenitor cells in the retina

We summarize recent findings in different animal models regarding the different cell-signaling pathways and gene networks that influence the reprogramming of Müller glia into proliferating, neurogenic progenitor cells in the retina. Not surprisingly, most of the cell-signaling pathways that guide the proliferation and differentiation of embryonic retinal progenitors also influence the ability of Müller glia to become proliferating Müller glia-derived progenitor cells (MGPCs). Further, the neuronal differentiation of MGPC progeny is potently inhibited by networks of neurogenesis-suppressing genes in chick and mouse models but occurs freely in zebrafish. There are important differences between the model systems, particularly pro-inflammatory signals that are active in mature Müller glia in damaged rodent and chick retinas, but less so in fish retinas. These pro-inflammatory signals are required to initiate the process of reprogramming, but if sustained suppress the potential of Müller glia to become neurogenic MGPCs. Further, there are important differences in how activated Müller glia up- or downregulate pro-glial transcription factors in the different model systems. We review recent findings regarding regulatory cell signaling and gene networks that influence the activation of Müller glia and the transition of these glia into proliferating progenitor cells with neurogenic potential in fish, chick, and mouse model systems.

Generation of Zebrafish Models of Human Retinitis Pigmentosa Diseases Using CRISPR/Cas9-Mediated Gene Editing System

Generating animal models can explore the role of new candidate genes in causing diseases and the pathogenicity of a specific mutation in the underlying genes. These animals can be used to identify new pharmaceutical or genetic therapeutic methods. In the present experiment, we developed a rpe65a knock out (KO) zebrafish as a retinitis pigmentosa (RP) disease model. Using the CRISPR/Cas9 system, the rpe65a gene was KO in zebrafish. Two specific single-guide RNAs (sgRNAs) were designed for the zebrafish rpe65a gene. SgRNAs were cloned into the DR274 plasmid and synthesized using in vitro transcription method. The efficiency of Ribonucleoprotein (synthesized sgRNA and recombinant Cas9) was evaluated by in vitro digestion experiment. Ribonucleoprotein complexes were microinjected into one to four-celled eggs of the TU zebrafish strain. The effectiveness of sgRNAs in KO the target gene was determined using the Heteroduplex mobility assay (HMA) and Sanger sequencing. Online software was used to determine the percent of mosaicism in the sequenced samples. By examining the sequences of the larvae that showed a mobility shift in the HMA method, the presence of indels in the binding region of sgRNAs was confirmed, so the zebrafish model for RP disease established. Zebrafish is an ideal animal model for the functional study of various diseases involving different genes and mutations and used for evaluating different therapeutic approaches in human diseases. This study presents the production of rpe65a gene KO zebrafish models using CRISPR/Cas9 technology. This model can be used in RP pathophysiology studies and preclinical gene therapy experiments.

Cost-effective strategies to knock down genes of interest in the retinas of adult zebrafish

High throughput sequencing has generated an enormous amount of information about the genes expressed in various cell types and tissues throughout the body, and about how gene expression changes over time and in diseased conditions. This knowledge has made targeted gene knockdowns an important tool in screening and identifying the roles of genes that are differentially expressed among specific cells of interest. While many approaches are available and optimized in mammalian models, there are still several limitations in the zebrafish model. In this article, we describe two approaches to target specific genes in the retina for knockdown: cell-penetrating, translation-blocking Vivo-Morpholino oligonucleotides and commercially available lipid nanoparticle reagents to deliver siRNA. We targeted expression of the PCNA gene in the retina of a P23H rhodopsin transgenic zebrafish model, in which rapidly proliferating progenitor cells replace degenerated rod photoreceptors. Retinas collected 48 h after intravitreal injections in adult zebrafish reveal that both Vivo-Morpholinos and lipid encapsulated siRNAs were able to successfully knock down expression of PCNA. However, only retinas injected with Vivo-Morpholinos showed a significant decrease in the formation of P23H rhodopsin-expressing rods, a downstream effect of PCNA inhibition. Surprisingly, Vivo-Morpholinos were able to exit the injected eye and enter the contralateral non-injected eye to inhibit PCNA expression. In this article we describe the techniques, concentrations, and considerations we found necessary to successfully target and inhibit genes through Vivo-Morpholinos and lipid encapsulated siRNAs.

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.

Molecular basis of retinal remodeling in a zebrafish model of retinitis pigmentosa

A hallmark of inherited retinal degenerative diseases such as retinitis pigmentosa (RP) is progressive structural and functional remodeling of the remaining retinal cells as photoreceptors degenerate. Extensive remodeling of the retina stands as a barrier for the successful implementation of strategies to restore vision. To understand the molecular basis of remodeling, we performed analyses of single-cell transcriptome data from adult zebrafish retina of wild type AB strain (WT) and a P23H mutant rhodopsin transgenic model of RP with continuous degeneration and regeneration. Retinas from both female and male fish were pooled to generate each library, combining data from both sexes. We provide a benchmark atlas of retinal cell type transcriptomes in zebrafish and insight into how each retinal cell type is affected in the P23H model. Oxidative stress is found throughout the retina, with increases in reliance on oxidative metabolism and glycolysis in the affected rods as well as cones, bipolar cells, and retinal ganglion cells. There is also transcriptional evidence for widespread synaptic remodeling and enhancement of glutamatergic transmission in the inner retina. Notably, changes in circadian rhythm regulation are detected in cones, bipolar cells, and retinal pigmented epithelium. We also identify the transcriptomic signatures of retinal progenitor cells and newly formed rods essential for the regenerative process. This comprehensive transcriptomic analysis provides a molecular road map to understand how the retina remodels in the context of chronic retinal degeneration with ongoing regeneration.

A comparative analysis of gene and protein expression in chronic and acute models of photoreceptor degeneration in adult zebrafish

Background: Adult zebrafish are capable of photoreceptor (PR) regeneration following acute phototoxic lesion (AL). We developed a chronic low light (CLL) exposure model that more accurately reflects chronic PR degeneration observed in many human retinal diseases. Methods: Here, we characterize the morphological and transcriptomic changes associated with acute and chronic models of PR degeneration at 8 time-points over a 28-day window using immunohistochemistry and 3'mRNA-seq. Results: We first observed a differential sensitivity of rod and cone PRs to CLL. Next, we found no evidence for Müller glia (MG) gliosis or regenerative cell-cycle re-entry in the CLL model, which is in contrast to the robust gliosis and proliferative response from resident MG in the AL model. Differential responses of microglia between the models was also observed. Transcriptomic comparisons between the models revealed gene-specific networks of PR regeneration and degeneration, including genes that are activated under conditions of chronic PR stress. Finally, we showed that CLL is at least partially reversible, allowing for rod and cone outer segment outgrowth and replacement of rod cell nuclei via an apparent upregulation of the existing rod neurogenesis mechanism. Discussion: Collectively, these data provide a direct comparison of the morphological and transcriptomic PR degeneration and regeneration models in zebrafish.

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.

An Overview towards Zebrafish Larvae as a Model for Ocular Diseases

Despite the obvious morphological differences in the visual system, zebrafish share a similar architecture and components of the same embryonic origin as humans. The zebrafish retina has the same layered structure and cell types with similar metabolic and phototransduction support as humans, and is functional 72 h after fertilization, allowing tests of visual function to be performed. The zebrafish genomic database supports genetic mapping studies as well as gene editing, both of which are useful in the ophthalmological field. It is possible to model ocular disorders in zebrafish, as well as inherited retinal diseases or congenital or acquired malformations. Several approaches allow the evaluation of local pathological processes derived from systemic disorders, such as chemical exposure to produce retinal hypoxia or glucose exposure to produce hyperglycemia, mimicking retinopathy of prematurity or diabetic retinopathy, respectively. The pathogenesis of ocular infections, autoimmune diseases, or aging can also be assessed in zebrafish larvae, and the preserved cellular and molecular immune mechanisms can be assessed. Finally, the zebrafish model for the study of the pathologies of the visual system complements certain deficiencies in experimental models of mammals since the regeneration of the zebrafish retina is a valuable tool for the study of degenerative processes and the discovery of new drugs and therapies.

Retina regeneration: lessons from vertebrates

Unlike mammals, vertebrates such as fishes and frogs exhibit remarkable tissue regeneration including the central nervous system. Retina being part of the central nervous system has attracted the interest of several research groups to explore its regenerative ability in different vertebrate models including mice. Fishes and frogs completely restore the size, shape and tissue structure of an injured retina. Several studies have unraveled molecular mechanisms underlying retina regeneration. In teleosts, soon after injury, the Müller glial cells of the retina reprogram to form a proliferating population of Müller glia-derived progenitor cells capable of differentiating into various neural cell types and Müller glia. In amphibians, the transdifferentiation of retinal pigment epithelium and differentiation of ciliary marginal zone cells contribute to retina regeneration. In chicks and mice, supplementation with external growth factors or genetic modifications cause a partial regenerative response in the damaged retina. The initiation of retina regeneration is achieved through sequential orchestration of gene expression through controlled modulations in the genetic and epigenetic landscape of the progenitor cells. Several developmental biology pathways are turned on during the Müller glia reprogramming, retinal pigment epithelium transdifferentiation and ciliary marginal zone differentiation. Further, several tumorigenic pathways and gene expression events also contribute to the complete regeneration cascade of events. In this review, we address the various retinal injury paradigms and subsequent gene expression events governed in different vertebrate species. Further, we compared how vertebrates such as teleost fishes and amphibians can achieve excellent regenerative responses in the retina compared with their mammalian counterparts.

Zebrafish and inherited photoreceptor disease: Models and insights

Photoreceptor dysfunctions and degenerative diseases are significant causes of vision loss in patients, with few effective treatments available. Targeted interventions to prevent or reverse photoreceptor-related vision loss are not possible without a thorough understanding of the underlying mechanism leading to disease, which is exceedingly difficult to accomplish in the human system. Cone diseases are particularly challenging to model, as some popular genetically modifiable model animals are nocturnal with a rod-dominant visual system and cones that have dissimilarities to human cones. As a result, cone diseases, which affect visual acuity, colour perception, and central vision in patients, are generally poorly understood in terms of pathology and mechanism. Zebrafish (Danio rerio) provide the opportunity to model photoreceptor diseases in a diurnal vertebrate with a cone-rich retina which develops many macular degeneration-like pathologies. Zebrafish undergo external development, allowing early-onset retinal diseases to be detected and studied, and many ophthalmic tools are available for zebrafish visual assessment during development and adulthood. There are numerous zebrafish models of photoreceptor disease, spanning the various types of photoreceptor disease (developmental, rod, cone, and mixed photoreceptor diseases) and genetic/molecular cause. In this review, we explore the features of zebrafish that make them uniquely poised to model cone diseases, summarize the established zebrafish models of inherited photoreceptor disease, and discuss how disease in these models compares to the human presentation, where applicable. Further, we highlight the contributions of these zebrafish models to our understanding of photoreceptor biology and disease, and discuss future directions for utilising and investigating these diverse models.

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

The P23H mutation in rhodopsin (Rho), the rod visual pigment, is the most common allele associated with autosomal-dominant retinitis pigmentosa (adRP). The fate of misfolded mutant Rho in rod photoreceptors has yet to be elucidated. We generated a new mouse model, in which the P23H-Rho mutant allele is fused to the fluorescent protein Tag-RFP-T (P23HhRhoRFP). In heterozygotes, outer segments formed, and wild-type (WT) rhodopsin was properly localized, but mutant P23H-Rho protein was mislocalized in the inner segments. Heterozygotes exhibited slowly progressing retinal degeneration. Mislocalized P23HhRhoRFP was contained in greatly expanded endoplasmic reticulum (ER) membranes. Quantification of mRNA for markers of ER stress and the unfolded protein response revealed little or no increases. mRNA levels for both the mutant human rhodopsin allele and the WT mouse rhodopsin were reduced, but protein levels revealed selective degradation of the mutant protein. These results suggest that the mutant rods undergo an adaptative process that prolongs survival despite unfolded protein accumulation in the ER. The P23H-Rho-RFP mouse may represent a useful tool for the future study of the pathology and treatment of P23H-Rho and adRP.

This article has an associated First Person interview with the first author of the paper.

Summary: A mouse line with a knock-in of the human rhodopsin gene altered to contain the P23H mutation and a red fluorescent protein fusion provides a new model for autosomal-dominant retinitis pigmentosa.

CRISPR/Cas9-Mediated Models of Retinitis Pigmentosa Reveal Differential Proliferative Response of Müller Cells between Xenopus laevis and Xenopus tropicalis

Retinitis pigmentosa is an inherited retinal dystrophy that ultimately leads to blindness due to the progressive degeneration of rod photoreceptors and the subsequent non-cell autonomous death of cones. Rhodopsin is the most frequently mutated gene in this disease. We here developed rhodopsin gene editing-based models of retinitis pigmentosa in two Xenopus species, Xenopus laevis and Xenopus tropicalis, by using CRISPR/Cas9 technology. In both of them, loss of rhodopsin function results in massive rod cell degeneration characterized by progressive shortening of outer segments and occasional cell death. This is followed by cone morphology deterioration. Despite these apparently similar degenerative environments, we found that Müller glial cells behave differently in Xenopus laevis and Xenopus tropicalis. While a significant proportion of Müller cells re-enter into the cell cycle in Xenopus laevis, their proliferation remains extremely limited in Xenopus tropicalis. This work thus reveals divergent responses to retinal injury in closely related species. These models should help in the future to deepen our understanding of the mechanisms that have shaped regeneration during evolution, with tremendous differences across vertebrates.

Zebrafish models of inherited retinal dystrophies

Inherited retinal degenerations (IRDs) cause permanent vision impairment or vision loss due to the death of rod and cone photoreceptors. Animal models of IRDs have been instrumental in providing knowledge of the pathological mechanisms that cause photoreceptor death and in developing successful approaches that could slow or prevent vision loss. Zebrafish models of IRDs represent an ideal model system to study IRDs in a cone-rich retina and to test strategies that exploit the natural ability to regenerate damaged neurons. This review highlights those zebrafish mutants and transgenic lines that exhibit adult-onset retinal degeneration and serve as models of retinitis pigmentosa, cone-rod dystrophy, and ciliopathies.

Rhodopsin as a Molecular Target to Mitigate Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of hereditary degenerative diseases affecting 1 of 4000 people worldwide and being the most prevalent cause of visual handicap among working populations in developed countries. These disorders are mainly related to the abnormalities in the rod G protein-coupled receptor (GPCR), rhodopsin reflected in the dysregulated membrane trafficking, stability and phototransduction processes that lead to progressive loss of retina function and eventually blindness. Currently, there is no cure for RP, and the therapeutic options are limited. Targeting rhodopsin with small molecule chaperones to improve the folding and stability of the mutant receptor is one of the most promising pharmacological approaches to alleviate the pathology of RP. This review provides an update on the current knowledge regarding small molecule compounds that have been evaluated as rhodopsin modulators to be considered as leads for the development of novel therapies for RP.

Usher syndrome type 1-associated gene, pcdh15b, is required for photoreceptor structural integrity in zebrafish

Blindness associated with Usher syndrome type 1 (USH1) is typically characterized as rod photoreceptor degeneration, followed by secondary loss of cones. The mechanisms leading to blindness are unknown because most genetic mouse models only recapitulate auditory defects. We generated zebrafish mutants for one of the USH1 genes, protocadherin-15b (pcdh15b), a putative cell adhesion molecule. Zebrafish Pcdh15 is expressed exclusively in photoreceptors within calyceal processes (CPs), at the base of the outer segment (OS) and within the synapse. In our mutants, rod and cone photoreceptor integrity is compromised, with early and progressively worsening abnormal OS disc growth and detachment, in part due to weakening CP contacts. These effects were attenuated or exacerbated by growth in dark and bright-light conditions, respectively. We also describe novel evidence for structural defects in synapses of pcdh15b mutant photoreceptors. Cell death does not accompany these defects at early stages, suggesting that photoreceptor structural defects, rather than overt cell loss, may underlie vision deficits. Thus, we present the first genetic animal model of a PCDH15-associated retinopathy that can be used to understand the aetiology of blindness in USH1.

This article has an associated First Person interview with the first author of the paper.

Summary: We present one of the first genetic animal mutants for PCDH15 that displays a severe, early retinopathy and suggests that zebrafish could be a useful model for PCDH15-associated retinal phenotypes.

Evidence of regional specializations in regenerated zebrafish retina

Adult zebrafish are capable of functional retinal regeneration following damage. A goal of vision science is to stimulate or permit a similar process in mammals to treat human retinal disease and trauma. Ideally such a process would reconstitute the stereotyped, two-dimensional topographic patterns and regional specializations of specific cell types, functionally important for representation of the visual field. An example in humans is the cone-rich fovea, essential for high-acuity color vision. Stereotyped, global topographic patterns of specific retinal cell types are also found in zebrafish, particularly for cone types expressing the tandemly-replicated lws (long wavelength-sensitive) and rh2 (middle wavelength-sensitive) opsins. Here we examine whether regionally specialized patterns of LWS1 and LWS2 cones are restored in regenerated retinas in zebrafish. Adult transgenic zebrafish carrying fluorescent reporters for lws1 and lws2 were subjected to retinal lesions that destroy all neurons but spare glia, via intraocular injection of the neurotoxin ouabain. Regenerated and contralateral control retinas were mounted whole or sectioned, and imaged. Overall spatial patterns of lws1 vs. lws2 opsin-expressing cones in regenerated retinas were remarkably similar to those of control retinas, with LWS1 cones in ventral/peripheral regions, and LWS2 cones in dorsal/central regions. However, LWS2 cones occupied a smaller fraction of regenerated retina, and several cones co-expressed the lws1 and lws2 reporters in regenerated retinas. Local patterns of regenerated LWS1 cones showed modest reductions in regularity. These results suggest that some of the regional patterning information, or the source of such signals, for LWS cone subtypes may be retained by undamaged cell types (Müller glia or RPE) and re-deployed during regeneration.

Loss of Pde6a Induces Rod Outer Segment Shrinkage and Visual Alterations in pde6aQ70X Mutant Zebrafish, a Relevant Model of Retinal Dystrophy

Retinitis pigmentosa (RP) is one of the most common forms of inherited retinal degeneration with 1/4,000 people being affected. The vision alteration primarily begins with rod photoreceptor degeneration, then the degenerative process continues with cone photoreceptor death. Variants in 71 genes have been linked to RP. One of these genes, PDE6a is responsible for RP43. To date no treatment is available and patients suffer from pronounced visual impairment in early childhood. We used the novel zebrafish pde6a^Q70X mutant, generated by N-ethyl-N-nitrosourea at the European Zebrafish Resource Centre, to better understand how PDE6a loss of function leads to photoreceptor alteration. Interestingly, zebrafish pde6a^Q70X mutants exhibited impaired visual function at 5 dpf as evidenced by the decrease in their visual motor response (VMR) compared to pde6a^WT larvae. This impaired visual function progressed with time and was more severe at 21 dpf. These modifications were associated with an alteration of rod outer segment length at 5 and 21 dpf. In summary, these findings suggest that rod outer segment shrinkage due to Pde6a deficiency begins very early in zebrafish, progresses with time. The zebrafish pde6a^Q70X mutant represents an ideal model of RP to screen relevant active small molecules that will block the progression of the disease.

Novel Technique for Retinal Nerve Cell Regeneration with Electrophysiological Functions Using Human Iris-Derived iPS Cells

Regenerative medicine in ophthalmology that uses induced pluripotent stem cells (iPS) cells has been described, but those studies used iPS cells derived from fibroblasts. Here, we generated iPS cells derived from iris cells that develop from the same inner layer of the optic cup as the retina, to regenerate retinal nerves. We first identified cells positive for p75NTR, a marker of retinal tissue stem and progenitor cells, in human iris tissue. We then reprogrammed the cultured p75NTR-positive iris tissue stem/progenitor (H-iris stem/progenitor) cells to create iris-derived iPS (H-iris iPS) cells for the first time. These cells were positive for iPS cell markers and showed pluripotency to differentiate into three germ layers. When H-iris iPS cells were pre-differentiated into neural stem/progenitor cells, not all cells became positive for neural stem/progenitor and nerve cell markers. When these cells were pre-differentiated into neural stem/progenitor cells, sorted with p75NTR, and used as a medium for differentiating into retinal nerve cells, the cells differentiated into Recoverin-positive cells with electrophysiological functions. In a different medium, H-iris iPS cells differentiated into retinal ganglion cell marker-positive cells with electrophysiological functions. This is the first demonstration of H-iris iPS cells differentiating into retinal neurons that function physiologically as neurons.

Zebrafish Models of Photoreceptor Dysfunction and Degeneration

Zebrafish are an instrumental system for the generation of photoreceptor degeneration models, which can be utilized to determine underlying causes of photoreceptor dysfunction and death, and for the analysis of potential therapeutic compounds, as well as the characterization of regenerative responses. We review the wealth of information from existing zebrafish models of photoreceptor disease, specifically as they relate to currently accepted taxonomic classes of human rod and cone disease. We also highlight that rich, detailed information can be derived from studying photoreceptor development, structure, and function, including behavioural assessments and in vivo imaging of zebrafish. Zebrafish models are available for a diversity of photoreceptor diseases, including cone dystrophies, which are challenging to recapitulate in nocturnal mammalian systems. Newly discovered models of photoreceptor disease and drusenoid deposit formation may not only provide important insights into pathogenesis of disease, but also potential therapeutic approaches. Zebrafish have already shown their use in providing pre-clinical data prior to testing genetic therapies in clinical trials, such as antisense oligonucleotide therapy for Usher syndrome.