Photoreceptor Cell Loss Research Articles

Potential Endogenous Cell Sources for Retinal Regeneration in Vertebrates and Humans: Progenitor Traits and Specialization.

Retinal diseases often cause the loss of photoreceptor cells and, consequently, impairment of vision. To date, several cell populations are known as potential endogenous retinal regeneration cell sources (RRCSs): the eye ciliary zone, the retinal pigment epithelium, the iris, and Müller glia. Factors that can activate the regenerative responses of RRCSs are currently under investigation. The present review considers accumulated data on the relationship between the progenitor properties of RRCSs and the features determining their differentiation. Specialized RRCSs (all except the ciliary zone in low vertebrates), despite their differences, appear to be partially “prepared” to exhibit their plasticity and be reprogrammed into retinal neurons due to the specific gene expression and epigenetic landscape. The “developmental” characteristics of RRCS gene expression are predefined by the pathway by which these cell populations form during eye morphogenesis; the epigenetic features responsible for chromatin organization in RRCSs are under intracellular regulation. Such genetic and epigenetic readiness is manifested in vivo in lower vertebrates and in vitro in higher ones under conditions permissive for cell phenotype transformation. Current studies on gene expression in RRCSs and changes in their epigenetic landscape help find experimental approaches to replacing dead cells through recruiting cells from endogenous resources in vertebrates and humans.

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.

Lysergic acid diethylamide causes mouse retinal damage by up-regulating p-JAK1/p-STAT1

Purpose: Lysergic acid diethylamide (LSD) is a powerful hallucinogen with high potential for abuse. There is far less known about its effects on the retina, especially the underlying mechanisms. This study was to investigate the acute toxicity of LSD on the retina of C57 mice and its mechanisms of action.Methods: C57 mice were treated with LSD at progressively increasing doses (0.2–1.2 mg/kg) intraperitoneally two times daily for 5 days, mice treated with saline served as negative control. Electroretinography (ERG) was used to test the function of the retina. Toluidine blue staining was used to detect the morphology of the retina. Enzyme-linked immunosorbent assay (ELISA) was used to measure the apoptosis-related factors. Real-time PCR and western blot techniques were used to measure expression changes of genes and proteins, respectively.Results: LSD treatment caused retinal damage, as shown by a decrease in ERG response and the loss of photoreceptor cells. LSD treatment also increased apoptosis through up-regulating the expression of p-JAK1/p-STAT1.Conclusions: Our study indicated that intraperitoneal administration of LSD-induced retinal damage of C57 mice, at least partially through regulating the JAK/STAT pathway.

Protective effects of human iPS-derived retinal pigmented epithelial cells on retinal degenerative disease

BackgroundRetinitis pigmentosa (RP) is an inherited retinal disease characterized by progressive loss of photoreceptor cells. This study aim at exploring the effect of retinal pigment epithelium (RPE) derived from human-induced pluripotent stem cell (hiPSC-RPE) on the retina of retinal degeneration 10 (rd10) mice, which are characterized with progressive photoreceptor death.MethodsWe generated RPE from hiPSCs by sequential supplementation with retinal-inducing factors and RPE specification signaling factors. The three-dimensional (3D) spheroid culture method was used to obtain optimal injectable hiPSC-RPE cells. Subretinal space transplantation was conducted to deliver hiPSC-RPE cells into the retina of rd10 mice. Neurotrophic factor secretion from transplanted hiPSC-RPE cells was detected by enzyme-linked immunosorbent assay (ELISA). Immunostaining, Western blotting, electroretinography (ERG), and visual behavior testing were performed to determine the effects of hiPSC-RPE on the retinal visual function in rd10 mice.ResultsOur data demonstrated that hiPSC-RPE cells exhibited classic RPE properties and phenotype after the sequential RPE induction from hiPSCs. hiPSC-RPE cells co-cultured with mouse retinal explants or retinal ganglion cells 5 (RGC5) exhibited decreased apoptosis. The viability and functional properties of hiPSC-RPE cells were enhanced by 3D spheroid culture. Transplanted hiPSC-derived RPE cells were identified by immunostaining with human nuclear antigen staining in the retina of rd10 14 days after subretinal space injection. The pigment epithelium-derived factor level was increased significantly. The expression of CD68, microglial activation marker, reduced after transplantation. The light avoidance behavior and ERG visual function in rd10 mice improved by the transplantation of hiPSC-RPE cells.ConclusionOur findings suggest that injectable hiPSC-RPE cells after 3D spheroid culture can rescue the structure and function of photoreceptors by sub-retinal transplantation, which lay the foundation for future clinical cell therapy to treat RP and other retinal degeneration diseases.

Metabolite therapy guided by liquid biopsy proteomics delays retinal neurodegeneration.

BackgroundNeurodegenerative diseases are incurable disorders caused by progressive neuronal cell death. Retinitis pigmentosa (RP) is a blinding neurodegenerative disease that results in photoreceptor death and progresses to the loss of the entire retinal network. We previously found that proteomic analysis of the adjacent vitreous served as way to indirectly biopsy the retina and identify changes in the retinal proteome.MethodsWe analyzed protein expression in liquid vitreous biopsies from autosomal recessive (ar)RP patients with PDE6A mutations and arRP mice with Pde6ɑ mutations. Proteomic analysis of retina and vitreous samples identified molecular pathways affected at the onset of photoreceptor death. Based on affected molecular pathways, arRP mice were treated with a ketogenic diet or metabolites involved in fatty-acid synthesis, oxidative phosphorylation, and the tricarboxylic acid (TCA) cycle.FindingsDietary supplementation of a single metabolite, ɑ-ketoglutarate, increased docosahexaeonic acid levels, provided neuroprotection, and enhanced visual function in arRP mice. A ketogenic diet delayed photoreceptor cell loss, while vitamin B supplementation had a limited effect. Finally, desorption electrospray ionization mass spectrometry imaging (DESI-MSI) on ɑ-ketoglutarate-treated mice revealed restoration of metabolites that correlated with our proteomic findings: uridine, dihydrouridine, and thymidine (pyrimidine and purine metabolism), glutamine and glutamate (glutamine/glutamate conversion), and succinic and aconitic acid (TCA cycle).InterpretationThis study demonstrates that replenishing TCA cycle metabolites via oral supplementation prolongs retinal function and provides a neuroprotective effect on the photoreceptor cells and inner retinal network.FundingNIH grants [R01EY026682, R01EY024665, R01EY025225, R01EY024698, R21AG050437, P30EY026877, 5P30EY019007, R01EY018213, F30EYE027986, T32GM007337, 5P30CA013696], NSF grant CHE-1734082.

Progressive eye pathology in mucopolysaccharidosis type I mice and effects of enzyme replacement therapy.

Mucopolysaccharidosis type I (MPS I) is a lysosomal storage disorder caused by α-L-iduronidase deficiency, resulting in accumulation of glycosaminoglycans (GAG). Ophthalmological manifestations are common in MPS I patients and often lead to visual impairment. Accumulation of GAG in corneal or retinal tissues reduces vision causing corneal opacity and neurosensory complications. One available treatment for MPS I patients is enzyme replacement therapy (ERT), but the results of such treatment on eye disease are still debatable. Therefore, we aimed to determine the progression of ocular manifestations as well as the effectiveness of intravenous ERT in MPS I. Corneal and retinal analyses were perform in eyes from 2- to 8-month normal and MPS I mice. Some MPS I mice received ERT (1.2 mg/kg of laronidase) every 2 weeks from 6 to 8 months and histological findings were compared with controls. Additionally, cornea from two MPS I patients under ERT were evaluated. Mouse corneal tissues had GAG accumulation early in life. In the retina, we found a progressive loss of photoreceptor cells, starting at 6 months. ERT did not improve or stabilize the histological abnormalities. MPS I patients, despite being on ERT for over a decade, presented GAG accumulation in the cornea, corneal thickening, visual loss and needed corneal transplantation. We provide data on the time course of ocular alteration in MPS I mice. Our results also suggest that ERT is not effective in treating the progressive ocular manifestations in MPS I mice and fails to prevent corneal abnormalities in patients.

Targeting molecular pathways for the treatment of inherited retinal degeneration.

Inherited retinal degeneration is a major cause of incurable blindness characterized by loss of retinal photoreceptor cells. Inherited retinal degeneration is characterized by high genetic and phenotypic heterogeneity with several genes mutated in patients affected by these genetic diseases. The high genetic heterogeneity of these diseases hampers the development of effective therapeutic interventions for the cure of a large cohort of patients. Common cell demise mechanisms can be envisioned as targets to treat patients regardless the specific mutation. One of these targets is the increase of intracellular calcium ions, that has been detected in several murine models of inherited retinal degeneration. Recently, neurotrophic factors that favor the efflux of calcium ions to concentrations below toxic levels have been identified as promising molecules that should be evaluated as new treatments for retinal degeneration. Here, we discuss therapeutic options for inherited retinal degeneration and we will focus on neuroprotective approaches, such as the neuroprotective activity of the Pigment epithelium-derived factor. The characterization of specific targets for neuroprotection opens new perspectives together with many questions that require deep analyses to take advantage of this knowledge and develop new therapeutic approaches. We believe that minimizing cell demise by neuroprotection may represent a promising treatment strategy for retinal degeneration.

PPARG Polymorphisms Are Associated with Unexplained Mild Vision Loss in Patients with Type 2 Diabetes Mellitus.

Objectives To investigate whether the presence of peroxisome proliferator-activated receptor gamma (PPARG) gene polymorphisms is associated with unexplained mild visual impairment (UMVI) in patients with type 2 diabetes mellitus (T2DM). Methods A total of 135 T2DM residents with UMVI and 133 with normal vision (NV; best-corrected visual acuity ≥ 20/25 in both eyes) were enrolled. UMVI was defined as best-corrected visual acuity (BCVA) < 20/25 and ≥ 20/63 in both eyes, with no visual impairment-causing diseases found. Four PPARG gene single-nucleotide polymorphisms (SNPs) (rs3856806, rs1801282, rs709158, and rs10865710) were assessed with the HAPLOVIEW 4.0 software to examine the statistical association of PPARG polymorphisms and UMVI in patients with T2DM. Results Four SNPs qualified the Hardy–Weinberg equilibrium (p > 0.05). The frequency of genotype GC at SNP rs10865710 was significantly higher in the UMVI group than in the NV group (p < 0.001; GG + GC versus CC) (OR = 8.94, 95% CI: 4.90–16.31), whereas genotype CC decreased the risk (OR = 0.07, 95% CI: 0.03–0.14). Genotype TT at SNP rs3856806 was strongly associated with UMVI (p < 0.0001, TT + TC versus CC) (OR = 4.74, 95% CI: 2.68–8.54), whereas genotype CC appeared to be protective for UMVI (OR = 0.55, 95% CI: 0.37–0.82). Conclusions Susceptibilities of PPARG variants may lead to differences in PPARG transcription, result in early function loss of retinal photoreceptor cells, and eventually cause UMVI.

Overexpression of MiR-183/96/182 Triggers Retina-Like Fate in Human Bone Marrow-Derived Mesenchymal Stem Cells (hBMSCs) in Culture.

Retinal degeneration is considered as a condition ensued by different blinding disorders such as retinitis pigmentosa, age-related macular degeneration, and diabetic retinopathy, which can cause loss of photoreceptor cells and also lead to significant vision deficiencies. Although there is no efficient treatment in this domain, transplantation of stem cells has been regarded as a therapeutic approach for retinal degeneration. Thus, the purpose of this study was to analyze the potential of human bone marrow-derived mesenchymal stem cells (hBMSCs) to differentiate into photoreceptor cells via transfection of microRNA (miRNA) in vitro for regenerative medicine purposes. To this end, miR-183/96/182 cluster was transfected into hBMSCs; then, qRT-PCR was performed to measure the expression levels of miR-183/96/182 cluster and some retina-specific neuronal genes such as OTX2, NRL, PKCα, and recoverin. CRX and rhodopsin (RHO) levels were also measured through qRT-PCR and immunocytochemistry, and subsequently, cellular change morphology was detected. The findings showed no changes in the morphology of the given cells, and the expression of the neuroretinal genes such as OTX2, NRL, and PKCα. Moreover, recoverin was upregulated upon miR-183/-96/-182 overexpression in cultured hBMSCs. Ectopic overexpression of the miR-183 cluster could further increase the expression of CRX and RHO at the messenger RNA (mRNA) and protein levels. Furthermore, the data indicated that the miR-183 cluster could serve as a crucial function in photoreceptor cell differentiation. In fact, miRNAs could be assumed as potential targets to exploit silent neuronal differentiation. Ultimately, it was suggested that in vitro overexpression of miR-183 cluster could trigger reprogramming of the hBMSCs to retinal neuron fate, especially photoreceptor cells.

Homozygous Variant in ARL3 Causes Autosomal Recessive Cone Rod Dystrophy.

PurposeCone rod dystrophy (CRD) is a group of inherited retinopathies characterized by the loss of cone and rod photoreceptor cells, which results in poor vision. This study aims to clinically and genetically characterize the segregating CRD phenotype in two large, consanguineous Pakistani families.MethodsFunduscopy, optical coherence tomography (OCT), electroretinography (ERG), color vision, and visual acuity assessments were performed to evaluate the retinal structure and function of the affected individuals. Exome sequencing was performed to identify the genetic cause of CRD. Furthermore, the mutation's effect was evaluated using purified, bacterially expressed ADP-ribosylation factor-like protein 3 (ARL3) and mammalian cells.ResultsFundus photography and OCT imaging demonstrated features that were consistent with CRD, including bull's eye macular lesions, macular atrophy, and central photoreceptor thinning. ERG analysis demonstrated moderate to severe reduction primarily of photopic responses in all affected individuals, and scotopic responses show reduction in two affected individuals. The exome sequencing revealed a novel homozygous variant (c.296G>T) in ARL3, which is predicted to substitute an evolutionarily conserved arginine with isoleucine within the encoded protein GTP-binding domain (R99I). The functional studies on the bacterial and heterologous mammalian cells revealed that the arginine at position 99 is essential for the stability of ARL3.ConclusionsOur study uncovers an additional CRD gene and assigns the CRD phenotype to a variant of ARL3. The results imply that cargo transportation in photoreceptors as mediated by the ARL3 pathway is essential for cone and rod cell survival and vision in humans.

Mice deficient in the lysosomal enzyme palmitoyl-protein thioesterase 1 (PPT1) display a complex retinal phenotype

Neuronal ceroid lipofuscinosis (NCL) type 1 (CLN1) is a neurodegenerative storage disorder caused by mutations in the gene encoding the lysosomal enzyme palmitoyl-protein thioesterase 1 (PPT1). CLN1 patients suffer from brain atrophy, mental and motor retardation, seizures, and retinal degeneration ultimately resulting in blindness. Here, we performed an in-depth analysis of the retinal phenotype of a PPT1-deficient mouse, an animal model of this condition. Reactive astrogliosis and microgliosis were evident in mutant retinas prior to the onset of retinal cell loss. Progressive accumulation of storage material, a pronounced dysregulation of various lysosomal proteins, and accumulation of sequestosome/p62-positive aggregates in the inner nuclear layer also preceded retinal degeneration. At advanced stages of the disease, the mutant retina was characterized by a significant loss of ganglion cells, rod and cone photoreceptor cells, and rod and cone bipolar cells. Results demonstrate that PPT1 dysfunction results in early-onset pathological alterations in the mutant retina, followed by a progressive degeneration of various retinal cell types at relatively late stages of the disease. Data will serve as a reference for future work aimed at developing therapeutic strategies for the treatment of retinal degeneration in CLN1 disease.

Dual ABCA4-AAV Vector Treatment Reduces Pathogenic Retinal A2E Accumulation in a Mouse Model of Autosomal Recessive Stargardt Disease

Autosomal recessive Stargardt disease is the most common inherited macular degeneration in humans. It is caused by mutations in the retina-specific ATP binding cassette transporter A4 (ABCA4) that is essential for the clearance of all-trans-retinal from photoreceptor cells. Loss of this function results in the accumulation of toxic bisretinoids in the outer segment disk membranes and their subsequent transfer into adjacent retinal pigment epithelium (RPE) cells. This ultimately leads to the Stargardt disease phenotype of increased retinal autofluorescence and progressive RPE and photoreceptor cell loss. Adeno-associated virus (AAV) vectors have been widely used in gene therapeutic applications, but their limited cDNA packaging capacity of ∼4.5 kb has impeded their use for transgenes exceeding this limit. AAV dual vectors were developed to overcome this size restriction. In this study, we have evaluated the in vitro expression of ABCA4 using three options: overlap, transplicing, and hybrid ABCA4 dual vector systems. The hybrid system was the most efficient of these dual vector alternatives and used to express the full-length ABCA4 in Abca4-/- mice. The full-length ABCA4 protein correctly localized to photoreceptor outer segments. Moreover, treatment of Abca4-/- mice with this ABCA4 hybrid dual vector system resulted in a reduced accumulation of the lipofuscin/N-retinylidene-N-retinylethanolamine (A2E) autofluorescence in vivo, and retinal A2E quantification supported these findings. These results show that the hybrid AAV dual vector option is both safe and therapeutic in mice, and the delivered ABCA4 transgene is functional and has a significant effect on reducing A2E accumulation in the Abca4-/- mouse model of Stargardt disease.

Tunicamycin-induced photoreceptor atrophy precedes degeneration of retinal capillaries with minimal effects on retinal ganglion and pigment epithelium cells

Endoplasmic reticulum (ER) stress is recognized as a contributing factor to various ocular neurovascular pathologies including retinitis pigmentosa, glaucoma, and diabetic retinopathy (DR). ER stress in particular is implicated in the development of DR, which is significantly influenced by inflammation driven retinal vascular degeneration and dysfunction. Ultimately, loss of vision occurs if left untreated. However, the identity of the target cells and their temporal involvement in diabetes-mediated dysfunction need further investigation. Early diabetes-induced stress in photoreceptor cells is proposed as the driver of inflammatory mediated neurovascular changes during diabetes. Although tunicamycin induced ER stress results in photoreceptor loss, its consequences for retinal vascular degeneration and retinal ganglion (RGC) and pigment epithelium (RPE) cell loss remains unclear. Here we show intravitreal delivery of tunicamycin primarily induced ER stress in photoreceptor cells resulting in their loss by apoptosis. This was concomitant with induced expression of the unfolded protein response marker CHOP in these cells. We also demonstrated significant degeneration of retinal capillaries following the loss of photoreceptor cells with minimal impact on loss of RGC and RPE cells. However, activation of retinal microglial and Muller cells were noticeable. Thus, our data support the notion that ER stress mediated dysfunction and/or loss of photoreceptor cells in response to inflammation and oxidative stress could precede retinal vascular and neuronal dysfunction and degeneration.

Identification of CRB1 mutations in two Chinese consanguineous families exhibiting autosomal recessive retinitis pigmentosa.

Retinitis pigmentosa (RP) is a leading cause of inherited blindness characterized by progressive loss of retinal photoreceptor cells. The present study aimed to identify the causative gene mutations in two Chinese families with autosomal recessive retinitis pigmentosa(arRP). TwoChinese consanguineous arRP families (RP‑2284 and RP‑2360) were recruited in this study, involving totally three affected and 25unaffected members. All the affected members underwent a complete ophthalmic examination, including fundus photography, multifocal electroretinography(ERG) and full field ERG. Exome sequencing was performed on the three RPpatients in the two families, followed by direct Sanger sequencing in all the family members and in 1,260unrelated controls for validation of the mutations identified. Two homozygous missense mutations in the crumbs homolog1 (CRB1) gene, which is known to cause severe retinal dystrophies, were found to be related to the phenotype of the two arRP families. The homozygous missense mutation c.1997T>A in CRB1 was detected in two patients in the RP‑2284 family. The proband in the RP‑2360 family was the only RP patient and was found to carry the novel homozygous missense mutation c.2426A>C in CRB1. The two mutations were heterozygous or absent in the other healthy family members, and they were absent in the 1,260controls. The amino acid changes in the CRB1 protein affected by the two mutations were predicted to be damaging by Polyohen‑2. Our study reported two CRB1 mutations causing arRP in twoChinese families, which expands the CRB1 mutation spectrum of RP in the Chinese population and emphasizes the causative role of CRB1 in RP.

Retinal neuroprotection: current strategies and future directions.

Photoreceptor cell death is the ultimate cause of vision loss in many retinal disorders. Currently, there are no commercially available treatments to prevent photoreceptor cell loss and preserve vision, and there is a critical unmet need for neuroprotective modalities to improve photoreceptor survival in a multitude of retinal disorders. This review summarizes the literature published on this topic in the last 18 months. A plethora of novel therapeutic modalities for photoreceptor neuroprotection have recently been examined in clinical trials for age-related macular degeneration, inherited retinal dystrophies, and macular telangiectasia type 2. These modalities include agents that target the complement pathway, stem cells, gene therapies, and neurotrophic factors. Additionally, improved understanding in the metabolic signals that regulate photoreceptor survival and function may ultimately identify targets for developing novel neuroprotective agents in a multitude of retinal disorders. Retinal neuroprotection is the next frontier in ophthalmic disease, and the discovery of novel neuroprotective strategies will fill a critical unmet need. Although the clinical utility of existing neuroprotective therapies is still quite limited, we are cautiously optimistic in light of the recent successes described in this review as well as other promising developments.

Early detection of cone photoreceptor cell loss in retinitis pigmentosa using adaptive optics scanning laser ophthalmoscopy.

The purpose of the study was to investigate the characteristics of the parafoveal cone density changes in patients with retinitis pigmentosa (RP) using adaptive optics scanning laser ophthalmoscopy (AO-SLO). A total of 14 eyes of RP patients and 10 eyes of control subjects were examined. High-resolution images of cone photoreceptor cells were obtained with a Canon AO-SLO system in the four retinal regions of the superior, inferior, temporal, and nasal areas located 1.0mm from the central fovea. The relationships of cone density with optical coherence tomography (OCT) findings and the visual sensitivity of the static perimetry tests were analyzed in RP patients. The averaged cone densities in RP patients were decreased at 1.0mm eccentricity from the fovea (11,899cells/mm2) compared with those in control subjects (16,647cells/mm2; P < 0.01). The cone density was substantially decreased even in RP patients with an intact interdigitation zone at the examined area (12,865cells/mm2; P < 0.01 vs. controls) and preserved visual sensitivity with > 35dB (13,019cells/mm2; P < 0.001 vs. controls). In RP, cone photoreceptor cell loss occurred in the parafoveal region with a preserved EZ/IZ or visual sensitivity. AO-SLO may be a useful modality to detect early changes of cone photoreceptor cells in RP patients.

Conditional Dicer1 depletion using Chrnb4-Cre leads to cone cell death and impaired photopic vision

Irreversible photoreceptor cell death is a major cause of blindness in many retinal dystrophies. A better understanding of the molecular mechanisms underlying the progressive loss of photoreceptor cells remains therefore crucial. Abnormal expression of microRNAs (miRNAs) has been linked with the aetiology of a number of retinal dystrophies. However, their role during the degenerative process remains poorly understood. Loss of cone photoreceptors in the human macula has the greatest impact on sight as these cells provide high acuity vision. Using a Chrnb4-cre; Dicerflox/flox conditional knockout mouse (Dicer CKO) to delete Dicer1 from cone cells, we show that cone photoreceptor cells degenerate and die in the Dicer-deleted retina. Embryonic eye morphogenesis appeared normal in Dicer CKO mice. Cone photoreceptor abnormalities were apparent by 3 weeks of age, displaying either very short or absent outer segments. By 4 months 50% of cones were lost and cone function was impaired as assessed by electroretinography (ERG). RNAseq analysis of the Dicer CKO retina revealed altered expression of genes involved in the visual perception pathway. These data show that loss of Dicer1 leads to early-onset cone cell degeneration and suggest that Dicer1 is essential for cone photoreceptor survival and homeostasis.

Gene Therapy for Inherited Retinal Degeneration.

Inherited retinal degeneration (IRD), a group of rare retinal diseases that primarily lead to the progressive loss of retinal photoreceptor cells, can be inherited in all modes of inheritance: autosomal dominant (AD), autosomal recessive (AR), X-linked (XL), and mitochondrial. Based on the pattern of inheritance of the dystrophy, retinal gene therapy has 2 main strategies. AR, XL, and AD IRDs with haploinsufficiency can be treated by inserting a functional copy of the gene using either viral or nonviral vectors (gene augmentation). Different types of viral vectors and nonviral vectors are used to transfer plasmid DNA both in vitro and in vivo. AD IRDs with gain-of-function mutations or dominant-negative mutations can be treated by disrupting the mutant allele with (and occasionally without) gene augmentation. This review article aims to provide an overview of ocular gene therapy for treating IRDs using gene augmentation with viral or nonviral vectors or gene disruption through different gene-editing tools, especially with the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system.

CAMP and Photoreceptor Cell Death in Retinal Degeneration.

Inherited retinal degenerations (IRDs) are a genetically heterogeneous group of disorders characterized by the progressive loss of photoreceptor cells. Despite this heterogeneity in the disease-causing mutation, common underlying mechanisms promoting photoreceptor cell death may be present. Dysregulation of photoreceptor cyclic nucleotide signaling may be one such common feature differentiating healthy from diseased photoreceptors. Here we review evidence that elevated retinal cAMP levels promote photoreceptor death and are a common feature of numerous animal models of IRDs. Improving our understanding of how cAMP levels become elevated and identifying downstream effectors may prove important for the development of therapeutics that will be applicable to multiple forms of the disease.

Gene Knockdown in Zebrafish (Danio rerio) as a Tool to Model Photoreceptor Diseases.

Disturbances in the general mRNA metabolism have been recognized as a major defect in a growing number of hereditary human diseases. One prominent example of this disease group is retinitis pigmentosa (RP), characterized by selective loss of photoreceptor cells. RP can be caused by dominant mutations in key factors of the pre-mRNA processing spliceosome. In these cases, the complex events leading to the RP phenotype can only insufficiently be analyzed in animal knockout models due to the essential functions of splice factors. Furthermore knockout animals frequently miss the specific phenotypes caused by knockdown of the respective genes. Here we introduce the zebrafish Danio rerio as a valuable vertebrate model system to study RP and related diseases in knockdown case scenarios.