Calpain and PARP activation during photoreceptor cell death in P23H and S334ter rhodopsin mutant rats

The role of epigenetic changes in the pathology and treatment of inherited retinal diseases

Elucidation of the cellular changes that occur in degenerating photoreceptors of people with inherited retinal diseases (IRDs) has been a focus for many research teams, leading to numerous theories on how these changes affect the cell death process. What is clearly emerging from these studies is that there are common denominators across multiple models of IRD, regardless of the underlying genetic mutation. These common markers could open avenues for broad neuroprotective therapeutics to prevent photoreceptor loss and preserve functional vision. In recent years, the role of epigenetic modifications contributing to the pathology of IRDs has been a particular point of interest, due to many studies noting changes in these epigenetic modifications, which coincide with photoreceptor cell death. This review will discuss the two broad categories of epigenetic changes, DNA methylation and histone modifications, that have received particular attention in IRD models. We will review the altered epigenetic regulatory events that are believed to contribute to cell death in IRDs and discuss the therapeutic potential of targeting these alterations.

Inherited Retinal Degeneration: Towards the Development of a Combination Therapy Targeting Histone Deacetylase, Poly (ADP-Ribose) Polymerase, and Calpain

Inherited retinal degeneration (IRD) represents a diverse group of gene mutation-induced blinding diseases. In IRD, the loss of photoreceptors is often connected to excessive activation of histone-deacetylase (HDAC), poly-ADP-ribose-polymerase (PARP), and calpain-type proteases (calpain). Moreover, the inhibition of either HDACs, PARPs, or calpains has previously shown promise in preventing photoreceptor cell death, although the relationship between these enzyme groups remains unclear. To explore this further, organotypic retinal explant cultures derived from wild-type mice and rd1 mice as a model for IRD were treated with different combinations of inhibitors specific for HDAC, PARP, and calpain. The outcomes were assessed using in situ activity assays for HDAC, PARP, and calpain, immunostaining for activated calpain-2, and the TUNEL assay for cell death detection. We confirmed that inhibition of either HDAC, PARP, or calpain reduced rd1 mouse photoreceptor degeneration, with the HDAC inhibitor Vorinostat (SAHA) being most effective. Calpain activity was reduced by inhibition of both HDAC and PARP whereas PARP activity was only reduced by HDAC inhibition. Unexpectedly, combined treatment with either PARP and calpain inhibitors or HDAC and calpain inhibitors did not produce synergistic rescue of photoreceptors. Together, these results indicate that in rd1 photoreceptors, HDAC, PARP, and calpain are part of the same degenerative pathway and are activated in a sequence that begins with HDAC and ends with calpain.

Decitabine improves MMS-induced retinal photoreceptor cell damage by targeting DNMT3A and DNMT3B

Introduction

Retinitis pigmentosa (RP) is a group of neurodegenerative retinopathies causing blindness due to progressive and irreversible photoreceptor cell death. The alkylating agent methyl methanesulfonate (MMS) can induce selective photoreceptor cell death, which is used to establish RP animal models. MMS induces DNA base damage by adding alkyl groups to DNA, and epigenetic modifications influence DNA damage response. Here, we aimed to explore the relationship between DNA methylation and DNA damage response in dying photoreceptors of RP.

Methods

The mouse RP model was established by a single intraperitoneal injection of MMS. The retinal structure and function were assessed by H&E, OCT, TUNEL, and ERG at several time points. The expression of DNA methylation regulators was assessed by qPCR and Western blot. DNMT inhibitor 5-aza-dC was applied to inhibit the activity of DNA methyltransferases and improve the retinal photoreceptor damage.

Results

The outer nuclear layer (ONL) and IS/OS layer were significantly thinner and the retinal function was impaired after MMS treatment. The cell death was mainly located in the ONL. The retinal damage induced by MMS was accompanied by hyperexpression of DNMT3A/3B. The application of DNMT inhibitor 5-aza-dC could suppress the expression level of DNMT3A/3B, resulting in the remission of MMS-induced photoreceptor cell damage. The ONL and IS/OS layers were thicker than that of the control group, and the retinal function was partially restored. This protective effect of 5-aza-dC was associated with the down-regulated expression of DNMT3A/3B.

Conclusion

These findings identified a functional role of DNMT3A/3B in MMS-induced photoreceptor cell damage and provided novel evidence to support DNMTs as potential therapeutic targets in retinal degenerative diseases.Graphical Abstract.

Role of Nuclear NAD+ in Retinal Homeostasis

The retina is one of the most metabolically active tissues and maintenance of metabolic homeostasis is critical for retinal function. Nicotinamide adenine dinucleotide (NAD+) is a cofactor that is required for key processes, including the electron transport chain, glycolysis, fatty acid oxidation, and redox reactions. NAD+ also acts as a co-substrate for enzymes involved in maintaining genomic DNA integrity and cellular homeostasis, including poly-ADP ribose polymerases (PARPs) and Sirtuins. This review highlights the importance of NAD+ in the retina, including the role of enzymes involved in NAD+ production in the retina and how NAD+-consuming enzymes may play a role in disease pathology. We also suggest a cell death pathway that may be common in multiple models of photoreceptor degeneration and highlight the role that NAD+ likely plays in this process. Finally, we explore future experimental approaches to enhance our understanding of the role of NAD+ in the retina.

Inhibition of the MAPK/c-Jun-EGR1 Pathway Decreases Photoreceptor Cell Death in the rd1 Mouse Model for Inherited Retinal Degeneration

Retinitis pigmentosa (RP) is a group of inherited retinal dystrophies that typically results in photoreceptor cell death and vision loss. Here, we explored the effect of early growth response-1 (EGR1) expression on photoreceptor cell death in Pde6b^rd1 (rd1) mice and its mechanism of action. To this end, single-cell RNA-seq (scRNA-seq) was used to identify differentially expressed genes in rd1 and congenic wild-type (WT) mice. Chromatin immunoprecipitation (ChIP), the dual-luciferase reporter gene assay, and western blotting were used to verify the relationship between EGR1 and poly (ADP-ribose) polymerase-1 (PARP1). Immunofluorescence staining was used to assess PARP1 expression after silencing or overexpression of EGR1. Photoreceptor cell death was assessed using the TUNEL assay following silencing/overexpression of EGR1 or administration of MAPK/c-Jun pathway inhibitors tanzisertib and PD98059. Our results showed differential expression of ERG1 in rd1 and WT mice via scRNA-seq analysis. The ChIP assay demonstrated EGR1 binding to the PARP1 promoter region. The dual-luciferase reporter gene assay and western blotting results revealed that EGR1 upregulated PARP1 expression. Additionally, the TUNEL assay showed that silencing EGR1 effectively reduced photoreceptor cell death. Similarly, the addition of tanzisertib and PD98059 reduced the expression of c-Jun and EGR1 and decreased photoreceptor cell death. Our study revealed that inhibition of the MAPK/c-Jun pathway reduced the expression of EGR1 and PARP1 and prevented photoreceptor cell death. These results highlight the importance of EGR1 for photoreceptor cell death and identify a new avenue for therapeutic interventions in RP.

Transcriptome Sequencing Reveals Tgf-β-Mediated Noncoding RNA Regulatory Mechanisms Involved in DNA Damage in the 661W Photoreceptor Cell Line

Transforming growth factor β (Tgf-β), a pleiotropic cytokine, can enhance DNA repair in various cells, including cancer cells and neurons. The noncoding regulatory system plays an important role in Tgf-β-mediated biological activities, whereas few studies have explored its role in DNA damage and repair. In this study, we suggested that Tgf-β improved while its inhibitor LSKL impaired DNA repair and cell viability in UV-irradiated 661W cells. Moreover, RNA-seq was carried out, and a total of 106 differentially expressed (DE)-mRNAs and 7 DE-lncRNAs were identified between UV/LSKL and UV/ctrl 661W cells. Gene ontology and Reactome analysis confirmed that the DE-mRNAs were enriched in multiple DNA damaged- and repair-related biological functions and pathways. We then constructed a ceRNA network that included 3 lncRNAs, 19 miRNAs, and 29 mRNAs with a bioinformatics prediction. Through RT-qPCR and further functional verification, 2 Tgf-β-mediated ceRNA axes (Gm20559-miR-361-5p-Oas2/Gbp7) were further identified. Gm20559 knockout or miR-361-5p mimics markedly impaired DNA repair and cell viability in UV-irradiated 661W cells, which confirms the bioinformatics results. In summary, this study revealed that Tgf-β could reduce DNA damage in 661W cells, provided a Tgf-β-associated ceRNA network for DNA damage and repair, and suggested that the molecular signatures may be useful candidates as targets of treatment for photoreceptor pathology.

Integrative RNA-seq and ATAC-seq analyses of phosphodiesterase 6 mutation-induced retinitis pigmentosa

PURPOSE

Inhibition of poly-ADP-ribose polymerase 1 (PARP1) could relieve phosphodiesterase 6 mutation-induced retinitis pigmentosa (RP). However, the mechanism related to PARP1 overexpression in the RP has not been clarified. We attempted to explore the potential mechanism related to PARP1 regulating RP.

METHODS

ATAC-seq and RNA-seq were performed for retina tissues of C3H and rd1 mice. The differentially expressed genes (DEGs) were identified, followed by the construction of PARP1-DEG co-expression and protein-protein interaction (PPI) networks. Gene ontology-biological process and pathway enrichment of DEGs were performed by clusterProfiler software. The overlapped genes that might play regulatory roles in PARP1 expression were mined by integrated analysis of RNA-seq and ATAC-seq data.

RESULTS

A total of 1061 DEGs were identified between C3H and rd1 group. Co-expression network was constructed with 313 PARP1-gene co-expression pairs. The down-regulated DEGs were closely related to visual perception and light stimulus-related biological process, while the up-regulated DEGs were significantly enriched in phototransduction and PPAR signaling pathway. PPI network was constructed with 202 nodes and 375 edges, which was clustered into 3 modules. Module 1 genes were closely related to detection of light stimulus, visual perception related biological process and phototransduction pathway (involved with Gnat1/Guca1b/Gnat2/Sag/Pde6g). By integrated analysis of the RNA-seq and ATAC-seq, the overlapped up-regulated genes were Asxl3 and Nyap2, while the down-regulated genes were Tmem136 and Susd3.

CONCLUSION

Gnat1 may play a key role in RP development by interacting with PARP1. Susd3 may play a regulatory role in PARP1 expression and affect RP formation.

Primary and Secondary Cone Cell Death Mechanisms in Inherited Retinal Diseases and Potential Treatment Options

Inherited retinal diseases (IRDs) are a leading cause of blindness. To date, 260 disease-causing genes have been identified, but there is currently a lack of available and effective treatment options. Cone photoreceptors are responsible for daylight vision but are highly susceptible to disease progression, the loss of cone-mediated vision having the highest impact on the quality of life of IRD patients. Cone degeneration can occur either directly via mutations in cone-specific genes (primary cone death), or indirectly via the primary degeneration of rods followed by subsequent degeneration of cones (secondary cone death). How cones degenerate as a result of pathological mutations remains unclear, hindering the development of effective therapies for IRDs. This review aims to highlight similarities and differences between primary and secondary cone cell death in inherited retinal diseases in order to better define cone death mechanisms and further identify potential treatment options.

Reduced nuclear NAD+ drives DNA damage and subsequent immune activation in the retina

Mutations in NMNAT1, a key enzyme involved in the synthesis of NAD+ in the nucleus, lead to an early onset severe inherited retinal degeneration (IRD). We aimed to understand the role of nuclear NAD+ in the retina and to identify the molecular mechanisms underlying NMNAT1-associated disease, using a mouse model that harbors the p.V9M mutation in Nmnat1 (Nmnat1V9M/V9M). We identified temporal transcriptional reprogramming in the retinas of Nmnat1V9M/V9M mice prior to retinal degeneration, which begins at 4 weeks of age, with no significant alterations in gene expression at 2 weeks of age and over 2600 differentially expressed genes by 3 weeks of age. Expression of the primary consumer of NAD+ in the nucleus, PARP1, an enzyme involved in DNA damage repair and transcriptional regulation, as well as 7 other PARP family enzymes, was elevated in the retinas of Nmnat1V9M/V9M. This was associated with elevated levels of DNA damage, PARP-mediated NAD+ consumption, and migration of Iba1+/CD45+ microglia/macrophages to the subretinal space in the retinas of Nmnat1V9M/V9M mice. These findings suggest that photoreceptor cells are especially sensitive to perturbation of genome homeostasis, and that PARP-mediated cell death may play a role in other genetic forms of IRDs, and potentially other forms of neurodegeneration.

Programmed Non-Apoptotic Cell Death in Hereditary Retinal Degeneration: Crosstalk between cGMP-Dependent Pathways and PARthanatos?

Programmed cell death (PCD) is a highly regulated process that results in the orderly destruction of a cell. Many different forms of PCD may be distinguished, including apoptosis, PARthanatos, and cGMP-dependent cell death. Misregulation of PCD mechanisms may be the underlying cause of neurodegenerative diseases of the retina, including hereditary retinal degeneration (RD). RD relates to a group of diseases that affect photoreceptors and that are triggered by gene mutations that are often well known nowadays. Nevertheless, the cellular mechanisms of PCD triggered by disease-causing mutations are still poorly understood, and RD is mostly still untreatable. While investigations into the neurodegenerative mechanisms of RD have focused on apoptosis in the past two decades, recent evidence suggests a predominance of non-apoptotic processes as causative mechanisms. Research into these mechanisms carries the hope that the knowledge created can eventually be used to design targeted treatments to prevent photoreceptor loss. Hence, in this review, we summarize studies on PCD in RD, including on apoptosis, PARthanatos, and cGMP-dependent cell death. Then, we focus on a possible interplay between these mechanisms, covering cGMP-signaling targets, overactivation of poly(ADP-ribose)polymerase (PARP), energy depletion, Ca2+-permeable channels, and Ca2+-dependent proteases. Finally, an outlook is given into how specific features of cGMP-signaling and PARthanatos may be targeted by therapeutic interventions.

Stimulation of α7 nAChR leads to regeneration of damaged neurons in adult mammalian retinal disease models

The adult mammal lacks the ability to regenerate neurons lost to retinal damage or disease in a meaningful capacity. However, previous studies from this laboratory have demonstrated that PNU-282987, an α7 nicotinic acetylcholine receptor agonist, elicits a robust neurogenic response in the adult murine retina. With eye drop application of PNU-282987, Müller glia cells re-enter the cell cycle and produce progenitor-like cells that can differentiate into various types of retinal neurons. In this study, we analyzed the regenerative capability of PNU-282987 in two retinal disease models and identified the source of newly regenerated neurons. Wild-type mice and mice with a transgenic Müller-glia lineage tracer were manipulated to mimic loss of retinal cells associated with glaucoma or photoreceptor degeneration. Following treatment with PNU-282987, the regenerative response of retinal neurons was quantified and characterized. After onset of photoreceptor degeneration, PNU-282987 was able to successfully regenerate both rod and cone photoreceptors. Quantification of this response demonstrated significant regeneration, restoring photoreceptors to near wild-type density. In mice that had glaucoma-like conditions induced, PNU-282987 treatment led to a significant increase in retinal ganglion cells. Retrograde labeling of optic nerve axon fibers demonstrated that newly regenerated axons projected into the optic nerve. Lineage tracing analysis demonstrated that these new neurons were derived from Müller glia. These results demonstrate that PNU-282987 can induce retinal regeneration in adult mice following onset of retinal damage. The ability of PNU-282987 to regenerate retinal neurons in a robust manner offers a new direction for developing novel and potentially transformative treatments to combat neurodegenerative disease.

Retinal Inflammation, Cell Death and Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are a group of retinal disorders that cause progressive and severe loss of vision because of retinal cell death, mainly photoreceptor cells. IRDs include retinitis pigmentosa (RP), the most common IRD. IRDs present a genetic and clinical heterogeneity that makes it difficult to achieve proper treatment. The progression of IRDs is influenced, among other factors, by the activation of the immune cells (microglia, macrophages, etc.) and the release of inflammatory molecules such as chemokines and cytokines. Upregulation of tumor necrosis factor alpha (TNFα), a pro-inflammatory cytokine, is found in IRDs. This cytokine may influence photoreceptor cell death. Different cell death mechanisms are proposed, including apoptosis, necroptosis, pyroptosis, autophagy, excessive activation of calpains, or parthanatos for photoreceptor cell death. Some of these cell death mechanisms are linked to TNFα upregulation and inflammation. Therapeutic approaches that reduce retinal inflammation have emerged as useful therapies for slowing down the progression of IRDs. We focused this review on the relationship between retinal inflammation and the different cell death mechanisms involved in RP. We also reviewed the main anti-inflammatory therapies for the treatment of IRDs.

Efficient Ocular Delivery of VCP siRNA via Reverse Magnetofection in RHO P23H Rodent Retina Explants

The use of synthetic RNA for research purposes as well as RNA-based therapy and vaccination has gained increasing importance. Given the anatomical seclusion of the eye, small interfering RNA (siRNA)-induced gene silencing bears great potential for targeted reduction of pathological gene expression that may allow rational treatment of chronic eye diseases in the future. However, there is yet an unmet need for techniques providing safe and efficient siRNA delivery to the retina. We used magnetic nanoparticles (MNPs) and magnetic force (Reverse Magnetofection) to deliver siRNA/MNP complexes into retinal explant tissue, targeting valosin-containing protein (VCP) previously established as a potential therapeutic target for autosomal dominant retinitis pigmentosa (adRP). Safe and efficient delivery of VCP siRNA was achieved into all retinal cell layers of retinal explants from the RHO P23H rat, a rodent model for adRP. No toxicity or microglial activation was observed. VCP silencing led to a significant decrease of retinal degeneration. Reverse Magnetofection thus offers an effective method to deliver siRNA into retinal tissue. Used in combination with retinal organotypic explants, it can provide an efficient and reliable preclinical test platform of RNA-based therapy approaches for ocular diseases.

Fluorescent detection of PARP activity in unfixed tissue

Poly-ADP-ribose-polymerase (PARP) relates to a family of enzymes that can detect DNA breaks and initiate DNA repair. While this activity is generally seen as promoting cell survival, PARP enzymes are also known to be involved in cell death in numerous pathologies, including in inherited retinal degeneration. This ambiguous role of PARP makes it attractive to have a simple and fast enzyme activity assay, that allows resolving its enzymatic activity in situ, in individual cells, within complex tissues. A previously published two-step PARP activity assay uses biotinylated NAD+ and streptavidin labelling for this purpose. Here, we used the fluorescent NAD+ analogues ε-NAD+ and 6-Fluo-10-NAD+ to assess PARP activity directly on unfixed tissue sections obtained from wild-type and retinal degeneration-1 (rd1) mutant retina. In standard UV microscopy ε-NAD+ incubation did not reveal PARP specific signal. In contrast, 6-Fluo-10-NAD+ resulted in reliable detection of in situ PARP activity in rd1 retina, especially in the degenerating photoreceptor cells. When the 6-Fluo-10-NAD+ based PARP activity assay was performed in the presence of the PARP specific inhibitor olaparib, the activity signal was completely abolished, attesting to the specificity of the assay. The incubation of live organotypic retinal explant cultures with 6-Fluo-10-NAD+, did not produce PARP specific signal, indicating that the fluorescent marker may not be sufficiently membrane-permeable to label living cells. In summary, we present a new, rapid, and simple to use fluorescence assay for the cellular resolution of PARP activity on unfixed tissue, for instance in complex neuronal tissues such as the retina.

Inhibition of PARP activity improves therapeutic effect of ARPE-19 transplantation in RCS rats through decreasing photoreceptor death

Photoreceptor (PR) dysfunction or death is the key pathological change in retinal degeneration (RD). The death of PRs might be due to a primary change in PRs themselves or secondary to the dysfunction of the retinal pigment epithelium (RPE). Poly(ADP-ribose) polymerase (PARP) was reported to be involved in primary PR death, but whether it plays a role in PR death secondary to RPE dysfunction has not been determined. To clarify this question and develop a new therapeutic approach, we studied the changes in PAR/PARP in the RCS rat, a RD model, and tested the effect of PARP intervention when given alone or in combination with RPE cell transplantation. The results showed that poly(ADP-ribosyl)ation of proteins was increased in PRs undergoing secondary death in RCS rats, and this result was confirmed by the observation of similar changes in sodium iodate (SI)-induced secondary RD in SD rats. The increase in PAR/PARP was highly associated with increased apoptotic PRs and decreased visual function, as represented by lowered b-wave amplitudes on electroretinogram (ERG). Then, as we expected, when the RCS rats were treated with subretinal injection of the PARP inhibitor PJ34, the RD process was delayed. Furthermore, when PJ34 was given simultaneously with subretinal ARPE-19 cell transplantation, the therapeutic effects were significantly improved and lasted longer than those of ARPE-19 or PJ34 treatment alone. These results provide a potential new approach for treating RD.

An in vitro Model of Human Retinal Detachment Reveals Successive Death Pathway Activations

Purpose

was to create an in vitro model of human retinal detachment (RD) to study the mechanisms of photoreceptor death.

Methods

Human retinas were obtained through eye globe donations for research purposes and cultivated as explants. Cell death was investigated in retinas with (control) and without retinal pigment epithelium (RPE) cells to mimic RD. Tissues were studied at different time points and immunohistological analyses for TUNEL, Cleaved caspase3, AIF, CDK4 and the epigenetic mark H3K27me3 were performed. Human and monkey eye globes with retinal detachment served as controls.

Results

The number of TUNEL-positive cells, compared between 1 and 7 days, increased with time in both retinas with RPE (from 1.2 ± 0.46 to 8 ± 0.89, n = 4) and without RPE (from 2.6 ± 0.73 to 16.3 ± 1.27, p < 0.014). In the group without RPE, cell death peaked at day 3 (p = 0.014) and was high until day 7. Almost no Cleaved-Caspase3 signal was observed, whereas a transient augmentation at day 3 of AIF-positive cells was observed to be about 10-fold in comparison to the control group (n = 2). Few CDK4-positive cells were found in both groups, but significantly more in the RD group at day 7 (1.8 ± 0.24 vs. 4.7 ± 0.58, p = 0.014). The H3K27me3 mark increased by 7-fold after 5 days in the RD group (p = 0.014) and slightly decreased at day 7 and was also observed to be markedly increased in human and monkey detached retina samples.

Conclusion

AIF expression coincides with the first peak of cell death, whereas the H3K27me3 mark increases during the cell death plateau, suggesting that photoreceptor death is induced by different successive pathways after RD. This in vitro model should permit the identification of neuroprotective drugs with clinical relevance.

Photoreceptor Degeneration in Pro23His Transgenic Rats (Line 3) Involves Autophagic and Necroptotic Mechanisms

Photoreceptor death contributes to 50% of irreversible vision loss in the western world. Pro23His (P23H) transgenic albino rat strains are widely used models for the most common rhodopsin gene mutation associated with the autosomal dominant form of retinitis pigmentosa. However, the mechanism(s) by which photoreceptor death occurs are not well understood and were the principal aim of this study. We first used electroretinogram recording and optical coherence tomography to confirm the time course of functional and structural loss. Electroretinogram analyses revealed significantly decreased rod photoreceptor (a-wave), bipolar cell (b-wave) and amacrine cell responses (oscillatory potentials) from P30 onward. The cone-mediated b-wave was also decreased from P30. TUNEL analysis showed extensive cell death at P18, with continued labeling detected until P30. Focused gene expression arrays indicated activation of, apoptosis, autophagy and necroptosis in whole retina from P14-18. However, analysis of mitochondrial permeability changes (ΔΨm) using JC-1 dye, combined with immunofluorescence markers for caspase-dependent (cleaved caspase-3) and caspase-independent (AIF) cell death pathways, indicated mitochondrial-mediated cell death was not a major contributor to photoreceptor death. By contrast, reverse-phase protein array data combined with RIPK3 and phospho-MLKL immunofluorescence indicated widespread necroptosis as the predominant mechanism of photoreceptor death. These findings highlight the complexity of mechanisms involved in photoreceptor death in the Pro23His rat model of degeneration and suggest therapies that target necroptosis should be considered for their potential to reduce photoreceptor death.

Intravitreal administration of adalimumab delays retinal degeneration in rd10 mice

Retinitis pigmentosa (RP) is a group of inherited retinal dystrophies characterized by the progressive and irreversible loss of vision. We previously found that intraperitoneal administration of Adalimumab, a monoclonal anti-TNFα antibody, slowed down retinal degeneration in the murine model of RP, the rd10 mice. The aims of this study were to improve its neuroprotective effect and to deepen understanding of the molecular mechanisms involved in this effect. We analyzed (i) the in vitro effect of Adalimumab on the TNFα-mediated cell death in retinal cells; (ii) the effect of a single intravitreal injection of Adalimumab on retinal degeneration in rd10 mice at postnatal day (P) 23. In vitro studies showed that TNFα induced caspase and poly ADP ribose polymerase (PARP) activation, downregulation of (kinase receptor-interacting protein 1) RIPK1 and upregulation of RIPK3 in retinal cells. Adalimumab reduced cell death probably through the inhibition of caspase 3 activation. In vivo studies suggested that PARP and NLRP3 inflammasome are mainly activated and to a lesser extent caspase-dependent mechanisms in rd10 retinas at P23. Necroptosis seems to be inhibited by the downregulation of RIPK1. Adalimumab prevented from retinal degeneration without affecting caspase -dependent mechanisms but decreasing PARP activation, microglia activation as well as NLRP3 inflammasome.

Drug repurposing studies of PARP inhibitors as a new therapy for inherited retinal degeneration

The enzyme poly-ADP-ribose-polymerase (PARP) has important roles for many forms of DNA repair and it also participates in transcription, chromatin remodeling and cell death signaling. Currently, some PARP inhibitors are approved for cancer therapy, by means of canceling DNA repair processes and cell division. Drug repurposing is a new and attractive aspect of therapy development that could offer low-cost and accelerated establishment of new treatment options. Excessive PARP activity is also involved in neurodegenerative diseases including the currently untreatable and blinding retinitis pigmentosa group of inherited retinal photoreceptor degenerations. Hence, repurposing of known PARP inhibitors for patients with non-oncological diseases might provide a facilitated route for a novel retinitis pigmentosa therapy. Here, we demonstrate and compare the efficacy of two different PARP inhibitors, BMN-673 and 3-aminobenzamide, by using a well-established retinitis pigmentosa model, the rd1 mouse. Moreover, the mechanistic aspects of the PARP inhibitor-induced protection were also investigated in the present study. Our results showed that rd1 rod photoreceptor cell death was decreased by about 25-40% together with the application of these two PARP inhibitors. The wealth of human clinical data available for BMN-673 highlights a strong potential for a rapid clinical translation into novel retinitis pigmentosa treatments. Remarkably, we have found that the efficacy of 3 aminobenzamide was able to decrease PARylation at the nanomolar level. Our data also provide a link between PARP activity with the Wnt/β-catenin pathway and the major intracellular antioxidant concentrations behind the PARP-dependent retinal degeneration. In addition, molecular modeling studies were integrated with experimental studies for better understanding of the role of PARP1 inhibitors in retinal degeneration.

mTOR may interact with PARP-1 to regulate visible light-induced parthanatos in photoreceptors

BACKGROUND

Excessive light exposure is a detrimental environmental factor that plays a critical role in the pathogenesis of retinal degeneration. However, the mechanism of light-induced death of retina/photoreceptor cells remains unclear. The mammalian/mechanistic target of rapamycin (mTOR) and Poly (ADP-ribose) polymerase-1 (PARP-1) have become the primary targets for treating many neurodegenerative disorders. The aim of this study was to elucidate the mechanisms underlying light-induced photoreceptor cell death and whether the neuroprotective effects of mTOR and PARP-1 inhibition against death are mediated through apoptosis-inducing factor (AIF).

METHODS

Propidium iodide (PI)/Hoechst staining, lentiviral-mediated short hairpin RNA (shRNA), Western blot analysis, cellular fraction separation, plasmid transient transfection, laser confocal microscopy, a mice model, electroretinography (ERG), and hematoxylin-eosin (H & E) staining were employed to explore the mechanisms by which rapamycin/3-Aminobenzamide (3AB) exert neuroprotective effects of mTOR/PARP-1 inhibition in light-injured retinas.

RESULTS

A parthanatos-like death mechanism was evaluated in light-injured 661 W cells that are an immortalized photoreceptor-like cell line that exhibit cellular and biochemical feature characteristics of cone photoreceptor cells. The death process featured over-activation of PARP-1 and AIF nuclear translocation. Either PARP-1 or AIF knockdown played a significantly protective role for light-damaged photoreceptors. More importantly, crosstalk was observed between mTOR and PARP-1 signaling and mTOR could have regulated parthanatos via the intermediate factor sirtuin 1 (SIRT1). The parthanatos-like injury was also verified in vivo, wherein either PARP-1 or mTOR inhibition provided significant neuroprotection against light-induced injury, which is evinced by both structural and functional retinal analysis. Overall, these results elucidate the mTOR-regulated parthanatos death mechanism in light-injured photoreceptors/retinas and may facilitate the development of novel neuroprotective therapies for retinal degeneration diseases.

CONCLUSIONS

Our results demonstrate that inhibition of the mTOR/PARP-1 axis exerts protective effects on photoreceptors against visible-light-induced parthanatos. These protective effects are conducted by regulating the downstream factors of AIF, while mTOR possibly interacts with PARP-1 via SIRT1 to regulate parthanatos. Video Abstract Schematic diagram of mTOR interacting with PARP-1 to regulate visible light-induced parthanatos. Increased ROS caused by light exposure penetrates the nuclear membrane and causes nuclear DNA strand breaks. PARP-1 detects DNA breaks and synthesizes PAR polymers to initiate the DNA repair system that consumes a large amount of cellular NAD+. Over-production of PAR polymers prompts the release of AIF from the mitochondria and translocation to the nucleus, which leads to parthanatos. Activated mTOR may interact with PARP-1 via SIRT1 to regulate visible light-induced parthanatos.

Calpain Activation Is the Major Cause of Cell Death in Photoreceptors Expressing a Rhodopsin Misfolding Mutation

The majority of mutations in rhodopsin (RHO) cause misfolding of the protein and has been linked to degeneration of photoreceptor cells in the retina. A lot of attention has been set on targeting ER stress for the development of new therapies for inherited retinal degeneration caused by mutations in the RHO gene. Nevertheless, the cell death pathway activated by RHO misfolded protein is still debated. In this study, we analyzed the retina of the knock-in mouse expressing the P23H misfolded mutant RHO. We found persistent unfolded protein response (UPR) during degeneration. Interestingly, long-term stimulation of the PERK branch of ER stress had a protective effect by phosphorylating nuclear factor erythroid 2-related factor 2 (NRF2) transcription factor, associated with antioxidant responses. Otherwise, we provide evidence that increased intracellular calcium and activation of calpains strongly correlated with rod photoreceptor cell death. By blocking calpain activity, we significantly decreased the activation of caspase-7 and apoptosis-inducing factor (AIF), two cell death effectors, and cell demise, and effectively protected the retina from degeneration caused by the P23H dominant mutation in RHO.

The Spectral-Domain Optical Coherence Tomography Findings Associated with the Morphological and Electrophysiological Changes in a Rat Model of Retinal Degeneration, Rhodopsin S334ter-4 Rats

Purpose

To characterize the spectral-domain optical coherence tomography (SD-OCT) findings of the rhodopsin S334ter transgenic rats (line 4) in relation to the morphologic and electroretinographic features.

Materials and Methods

Rhodopsin S334ter transgenic rats (line 4) were employed as a model of retinal degeneration. The Sprague-Dawley (SD) rats were used as a wild-type control. SD-OCT (Micron IV®; Phoenix Research Labs, Pleasanton, CA, USA) was performed on the S334ter rats (line 4) from postnatal days (P) 13−110. The longitudinal changes of the SD-OCT images were analyzed both qualitatively and quantitatively in comparison to those of SD rats. The SD-OCT images were also compared to the histological and electron microscopic findings from examination performed on P 22, 36, and 61. Full field combined rod and cone electroretinography (ERG) was performed and the relationship between the thickness of the retinal sublayers and the amplitudes of the a- and b-waves was further analyzed.

Results

The photoreceptor inner and outer segment layer became diffusely hyperreflective in the SD-OCT images of the S334ter rats; these findings were not observed in the SD rats. This hyperreflective change corresponded to the degenerated inner and outer segments and the accumulation of the extracellular vesicles in the interphotoreceptor matrix. Quantitatively, the retinal outer sublayer and the photoreceptor sublayer in the S334ter rats became progressively thinner in comparison to those in the SD rats; the difference was statistically significant. The amplitudes of both the a- and b-waves on ERG were severely deteriorated in the S334ter rats.

Discussion

The SD-OCT images in the S334ter rats noninvasively provided information regarding the pathological changes in the photoreceptors and the longitudinal changes of both qualitative and quantitative changes during retinal degeneration in the S334ter rats (line 4). The pathological features of the photoreceptor inner and outer segments can be detected on SD-OCT as diffuse hyperreflective changes in the photoreceptor layer.

Cell Death Pathways in Mutant Rhodopsin Rat Models Identifies Genotype-Specific Targets Controlling Retinal Degeneration

Retinitis pigmentosa (RP) is a group of inherited neurological disorders characterized by rod photoreceptor cell death, followed by secondary cone cell death leading to progressive blindness. Currently, there are no viable treatment options for RP. Due to incomplete knowledge of the molecular signaling pathways associated with RP pathogenesis, designing therapeutic strategies remains a challenge. In particular, preventing secondary cone photoreceptor cell loss is a key goal in designing potential therapies. In this study, we identified the main drivers of rod cell death and secondary cone loss in the transgenic S334ter rhodopsin rat model, tested the efficacy of specific cell death inhibitors on retinal function, and compared the effect of combining drugs to target multiple pathways in the S334ter and P23H rhodopsin rat models. The primary driver of early rod cell death in the S334ter model was a caspase-dependent process, whereas cone cell death occurred though RIP3-dependent necroptosis. In comparison, rod cell death in the P23H model was via necroptotic signaling, whereas cone cell loss occurred through inflammasome activation. Combination therapy of four drugs worked better than the individual drugs in the P23H model but not in the S334ter model. These differences imply that treatment modalities need to be tailored for each genotype. Taken together, our data demonstrate that rationally designed genotype-specific drug combinations will be an important requisite to effectively target primary rod cell loss and more importantly secondary cone survival.

Antioxidant effects of Lycium barbarum polysaccharides on photoreceptor degeneration in the light-exposed mouse retina

We assessed the neuroprotective effects of Lycium barbarum Polysaccharides (LBP) on photoreceptor degeneration and the mechanisms involved in oxidative stress in light-exposed mouse retinas. Mice were given a gavage of LBP (150 mg/kg or 300 mg/kg) or phosphate buffered saline (PBS) for 7 days before exposure to light (5000 lx for 24 h). We found that LBP significantly improved the electroretinography (ERG) amplitudes of the a- and b-waves that had been attenuated by light exposure. In addition, changes caused by light exposure including photoreceptor cell loss, nuclear condensation, an increased number of mitochondria vacuoles, outer membrane disc swelling and cristae fractures were distinctly ameliorated by LBP. LBP treatment also significantly prevented the generation of reactive oxygen species (ROS) compared with PBS treatment. The levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and thioredoxin reductase (TrxR1) mRNA were decreased in PBS-treated mice compared with controls but increased remarkably in LBP-treated mice. The mRNA levels of the DNA repair gene Poly (ADP-ribose) polymerase (PARP14) was increased in PBS-treated mice but decreased significantly in the LBP-treated mice. Our findings indicate that pretreatment with LBP effectively protected photoreceptor cells against light-induced retinal damage probably through the up-regulation of the antioxidative genes Nrf2 and TrxR1, the elimination of oxygen free radicals, and the subsequent reduction in the mitochondrial reaction to oxidative stress and enhancement in antioxidant capacity. In addition, the decreased level of PARP14 mRNA in LBP-treated mice also indicated a protective effect of LBP on delaying photoreceptor in the light-damaged retina.

A review of the immunologic findings observed in retinitis pigmentosa

Most patients suffering from retinitis pigmentosa (RP) inherit the disorder; however, the immune-pathologic features associated with this disease have yet to be extensively studied. Six reports correlate antiretinal immune activity with vision deterioration in RP patients. Some of these patients have sporadic RP that occurs in excess of expected gene segregation during inheritance. The hypothesis that a primary immune-mediated disease process occurs in this sporadic group is supported by significant associations of RP with autoimmune endocrinopathies and other immune-related conditions or factors; however, no immunologic difference regarding RP family history is reported in the peripheral blood studies of RP patients. Twenty-one percent to 51% of RP patients display antiretinal antibodies, whereas 19-58% have antiretinal lymphocyte reactivity to retinal extract, and 60-85% have activated T cells. Mutations in animal models of RP have been shown to cause endoplasmic reticulum stress that may initiate immunopathology for genetic RP, but oxidative stress also encourages immune cytotoxicity. In addition, necrotic cell death is evident, which promotes inflammatory conditions. We review mechanisms and evidence for an occult inflammation in genetic RP and examine reports of efficacy in retarding RP progression with anti-inflammatory agents in clinical trials.

Characterization of photoreceptor degeneration in the rhodopsin P23H transgenic rat line 2 using optical coherence tomography

Purpose

To characterize the optical coherence tomography (OCT) appearances of photoreceptor degeneration in the rhodopsin P23H transgenic rat (line 2) in relation to the histological, ultrastructural, and electroretinography (ERG) findings.

Materials and methods

Homozygous rhodopsin P23H transgenic albino rats (line 2, very-slow degeneration model) were employed. Using OCT (Micron IV^®; Phoenix Research Labs, Pleasanton, CA, USA), the natural course of photoreceptor degeneration was recorded from postnatal day (P) 15 to P 287. The OCT images were qualitatively observed by comparing them to histological and ultrastructural findings at P 62 and P 169. In addition, each retinal layer was quantitatively analyzed longitudinally during degeneration, compared it to that observed in wild type Sprague-Dawley (SD) rats. The relationships between the ERG (full-field combined rod-cone response, 3.0 cds/m² stimulation) findings and OCT images were also analyzed.

Results

In the qualitative study, the two layers presumably corresponding to the photoreceptor inner segment ellipsoid zone (EZ) and interdigitation zone (IZ) were identified in the P23H rat until PN day 32. However, the photoreceptor inner and outer segment (IS/OS) layer became diffusely hyperreflective on OCT after P 46, and the EZ and IZ zones could no longer be identified on OCT. In contrast, in the SD rats, the EZ and IZ were clearly distinguished until at least P 247. The ultrastructural study showed partial disarrangements of the photoreceptor outer segment discs in the P23H rats at P 62, although a light-microscopic histological study detected almost no abnormality in the outer segment. In the quantitative study, the outer retinal layer including the outer plexiform layer (OPL) and the outer nuclear layer (ONL) became significantly thinner in the P23H rats than in the SD rats after P 71. The thickness of the IS/OS layer was maintained in the P23H rats until P 130, and it became statistically thinner than in the SD rats at P 237. The longitudinal attenuation in the amplitude of the a- and b-waves of ERG was significantly correlated with the thickness of the combined OPL and ONL but not with that of the IS/OS layer.

Conclusion

OCT showed the degenerated photoreceptor IS/OS layer in rhodopsin P23H transgenic rats (line 2) as a diffuse hyperreflective zone, even in the early stage, with the partially disarranged and destabilized OS discs recognizable by ultrastructural assessment but not by a histological study. The amplitude of the a- and b-waves mainly depends on the thickness of the OPL and ONL layer rather than the thickness of the photoreceptor IS/OS layer in P23H rats.

Proteomic analyses of brain tumor cell lines amidst the unfolded protein response

Brain tumors such as high grade gliomas are among the deadliest forms of human cancers. The tumor environment is subject to a number of cellular stressors such as hypoxia and glucose deprivation. The persistence of the stressors activates the unfolded proteins response (UPR) and results in global alterations in transcriptional and translational activity of the cell. Although the UPR is known to effect tumorigenesis in some epithelial cancers, relatively little is known about the role of the UPR in brain tumors. Here, we evaluated the changes at the molecular level under homeostatic and stress conditions in two glioma cell lines of differing tumor grade. Using mass spectrometry analysis, we identified proteins unique to each condition (unstressed/stressed) and within each cell line (U87MG and UPN933). Comparing the two, we find differences between both the conditions and cell lines indicating a unique profile for each. Finally, we used our proteomic data to identify the predominant pathways within these cells under unstressed and stressed conditions. Numerous predominant pathways are the same in both cell lines, but there are differences in biological and molecular classifications of the identified proteins, including signaling mechanisms, following UPR induction; we see that relatively minimal proteomic alterations can lead to signaling changes that ultimately promote cell survival.

Phenotypic characterization of P23H and S334ter rhodopsin transgenic rat models of inherited retinal degeneration

We produced 8 lines of transgenic (Tg) rats expressing one of two different rhodopsin mutations in albino Sprague-Dawley (SD) rats. Three lines were generated with a proline to histidine substitution at codon 23 (P23H), the most common autosomal dominant form of retinitis pigmentosa in the United States. Five lines were generated with a termination codon at position 334 (S334ter), resulting in a C-terminal truncated opsin protein lacking the last 15 amino acid residues and containing all of the phosphorylation sites involved in rhodopsin deactivation, as well as the terminal QVAPA residues important for rhodopsin deactivation and trafficking. The rates of photoreceptor (PR) degeneration in these models vary in proportion to the ratio of mutant to wild-type rhodopsin. The models have been widely studied, but many aspects of their phenotypes have not been described. Here we present a comprehensive study of the 8 Tg lines, including the time course of PR degeneration from the onset to one year of age, retinal structure by light and electron microscopy (EM), hemispheric asymmetry and gradients of rod and cone degeneration, rhodopsin content, gene dosage effect, rapid activation and invasion of the outer retina by presumptive microglia, rod outer segment disc shedding and phagocytosis by the retinal pigmented epithelium (RPE), and retinal function by the electroretinogram (ERG). The biphasic nature of PR cell death was noted, as was the lack of an injury-induced protective response in the rat models. EM analysis revealed the accumulation of submicron vesicular structures in the interphotoreceptor space during the peak period of PR outer segment degeneration in the S334ter lines. This is likely due to the elimination of the trafficking consensus domain as seen before as with other rhodopsin mutants lacking the C-terminal QVAPA. The 8 rhodopsin Tg lines have been, and will continue to be, extremely useful models for the experimental study of inherited retinal degenerations.

The molecular and cellular basis of rhodopsin retinitis pigmentosa reveals potential strategies for therapy

Inherited mutations in the rod visual pigment, rhodopsin, cause the degenerative blinding condition, retinitis pigmentosa (RP). Over 150 different mutations in rhodopsin have been identified and, collectively, they are the most common cause of autosomal dominant RP (adRP). Mutations in rhodopsin are also associated with dominant congenital stationary night blindness (adCSNB) and, less frequently, recessive RP (arRP). Recessive RP is usually associated with loss of rhodopsin function, whereas the dominant conditions are a consequence of gain of function and/or dominant negative activity. The in-depth characterisation of many rhodopsin mutations has revealed that there are distinct consequences on the protein structure and function associated with different mutations. Here we categorise rhodopsin mutations into seven discrete classes; with defects ranging from misfolding and disruption of proteostasis, through mislocalisation and disrupted intracellular traffic to instability and altered function. Rhodopsin adRP offers a unique paradigm to understand how disturbances in photoreceptor homeostasis can lead to neuronal cell death. Furthermore, a wide range of therapies have been tested in rhodopsin RP, from gene therapy and gene editing to pharmacological interventions. The understanding of the disease mechanisms associated with rhodopsin RP and the development of targeted therapies offer the potential of treatment for this currently untreatable neurodegeneration.

Involvement of Innate Immune System in Late Stages of Inherited Photoreceptor Degeneration

Retinitis pigmentosa (RP) is a group of inherited retinal degenerations that lead to progressive vision loss. Many mutations in 60 different genes have been shown to cause RP. Given the diversity of genes and mutations that cause RP, corrective gene therapy approaches currently in development may prove both time-consuming and cost-prohibitive for treatment of all forms of RP. An alternative approach is to find common biological pathways that cause retinal degeneration in various forms of RP, and identify new molecular targets. With this goal, we analyzed the retinal transcriptome of two non-allelic forms of RP in dogs, rcd1 and xlpra2, at clinically relevant advanced stages of the two diseases. Both diseases showed very similar trends in changes in gene expression compared to control normal dogs. Pathway analysis revealed upregulation of various components of the innate immune system in both diseases, including inflammasome and complement pathways. Our results show that the retinal transcriptome at advanced stages of RP is very similar to that of other retinal degenerative diseases such as age-related macular degeneration and diabetic retinopathy. Thus, drugs and therapeutics already in development for targeting these retinopathies may also prove useful for the treatment of many forms of RP.

The role of the ER stress-response protein PERK in rhodopsin retinitis pigmentosa

Mutations in rhodopsin, the light-sensitive protein of rod cells, are the most common cause of dominant retinitis pigmentosa (RP), a type of inherited blindness caused by the dysfunction and death of photoreceptor cells. The P23H mutation, the most frequent single cause of RP in the USA, causes rhodopsin misfolding and induction of the unfolded protein response (UPR), an adaptive ER stress response and signalling network that aims to enhance the folding and degradation of misfolded proteins to restore proteostasis. Prolonged UPR activation, and in particular the PERK branch, can reduce protein synthesis and initiate cell death through induction of pro-apoptotic pathways. Here, we investigated the effect of pharmacological PERK inhibition on retinal disease process in the P23H-1 transgenic rat model of retinal degeneration. PERK inhibition with GSK2606414A led to an inhibition of eIF2α phosphorylation, which correlated with reduced ERG function and decreased photoreceptor survival at both high and low doses of PERK inhibitor. Additionally, PERK inhibition increased the incidence of inclusion formation in cultured cells overexpressing P23H rod opsin, and increased rhodopsin aggregation in the P23H-1 rat retina, suggesting enhanced P23H misfolding and aggregation. In contrast, treatment of P23H-1 rats with an inhibitor of eIF2α phosphatase, salubrinal, led to improved photoreceptor survival. Collectively, these data suggest the activation of PERK is part of a protective response to mutant rhodopsin that ultimately limits photoreceptor cell death.

Calpain-5 gene expression in the mouse eye and brain

Objective

Our objective was to characterize CAPN5 gene expression in the mouse central nervous system. Mouse brain and eye sections were probed with two high-affinity RNA oligonucleotide analogs designed to bind CAPN5 RNA and one scramble, control oligonucleotide. Images were captured in brightfield.

Results

CAPN5 RNA probes were validated on mouse breast cancer tumor tissue. In the eye, CAPN5 was expressed in the ganglion cell, inner nuclear and outer nuclear layers of the retina. Signal could not be detected in the ciliary body or the iris because of the high density of melanin. In the brain, CAPN5 was expressed in the granule cell layers of the hippocampus and cerebellum. There was scattered expression in pons. The visual cortex showed faint signal. Most signal in the brain was in a punctate pattern.

Temporal progression of PARP activity in the Prph2 mutant rd2 mouse: Neuroprotective effects of the PARP inhibitor PJ34

Peripherin (peripherin/rds) is a membrane-associated protein that plays a critical role in the morphogenesis of rod and cone photoreceptor outer segments. Mutations in the corresponding PRPH2 gene cause different types of retinal dystrophies characterized by a loss of photoreceptors. Over activation of poly-ADP-ribose polymerase (PARP) was previously shown to be involved in different animal models for hereditary retinal dystrophies. This includes the rd2 mouse, which suffers from a human homologous mutation in the PRPH2 gene. In the present study, we show that increased retinal PARP activity and poly-ADP-ribosylation of proteins occurs before the peak of rd2 photoreceptor degeneration. Inhibition of PARP activity with the well-characterized PARP inhibitor PJ34 decreased the levels of poly-ADP-ribosylation and photoreceptor cell death. These results suggest a causal involvement of PARP in photoreceptor degeneration caused by peripherin mutations and highlight the possibility to use PARP inhibition for the mutation-independent treatment of hereditary retinal dystrophies.

Calpain-5 Expression in the Retina Localizes to Photoreceptor Synapses

Purpose

We characterize calpain-5 (CAPN5) expression in retinal and neuronal subcellular compartments.

Methods

CAPN5 gene variants were classified using the exome variant server, and RNA-sequencing was used to compare expression of CAPN5 mRNA in the mouse and human retina and in retinoblastoma cells. Expression of CAPN5 protein was ascertained in humans and mice in silico, in mouse retina by immunohistochemistry, and in neuronal cancer cell lines and fractionated central nervous system tissue extracts by Western analysis with eight antibodies targeting different CAPN5 regions.

Results

Most CAPN5 genetic variation occurs outside its protease core; and searches of cancer and epilepsy/autism genetic databases found no variants similar to hyperactivating retinal disease alleles. The mouse retina expressed one transcript for CAPN5 plus those of nine other calpains, similar to the human retina. In Y79 retinoblastoma cells, the level of CAPN5 transcript was very low. Immunohistochemistry detected CAPN5 expression in the inner and outer nuclear layers and at synapses in the outer plexiform layer. Western analysis of fractionated retinal extracts confirmed CAPN5 synapse localization. Western blots of fractionated brain neuronal extracts revealed distinct subcellular patterns and the potential presence of autoproteolytic CAPN5 domains.

Conclusions

CAPN5 is moderately expressed in the retina and, despite higher expression in other tissues, hyperactive disease mutants of CAPN5 only manifest as eye disease. At the cellular level, CAPN5 is expressed in several different functional compartments. CAPN5 localization at the photoreceptor synapse and with mitochondria explains the neural circuitry phenotype in human CAPN5 disease alleles.

Controlled delivery of tauroursodeoxycholic acid from biodegradable microspheres slows retinal degeneration and vision loss in P23H rats

Successful drug therapies for treating ocular diseases require effective concentrations of neuroprotective compounds maintained over time at the site of action. The purpose of this work was to assess the efficacy of intravitreal controlled delivery of tauroursodeoxycholic acid (TUDCA) encapsulated in poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres for the treatment of the retina in a rat model of retinitis pigmentosa. PLGA microspheres (MSs) containing TUDCA were produced by the O/W emulsion-solvent evaporation technique. Particle size and morphology were assessed by light scattering and scanning electronic microscopy, respectively. Homozygous P23H line 3 rats received a treatment of intravitreal injections of TUDCA-PLGA MSs. Retinal function was assessed by electroretinography at P30, P60, P90 and P120. The density, structure and synaptic contacts of retinal neurons were analyzed using immunofluorescence and confocal microscopy at P90 and P120. TUDCA-loaded PLGA MSs were spherical, with a smooth surface. The production yield was 78%, the MSs mean particle size was 23 μm and the drug loading resulted 12.5 ± 0.8 μg TUDCA/mg MSs. MSs were able to deliver the loaded active compound in a gradual and progressive manner over the 28-day in vitro release study. Scotopic electroretinografic responses showed increased ERG a- and b-wave amplitudes in TUDCA-PLGA-MSs-treated eyes as compared to those injected with unloaded PLGA particles. TUDCA-PLGA-MSs-treated eyes showed more photoreceptor rows than controls. The synaptic contacts of photoreceptors with bipolar and horizontal cells were also preserved in P23H rats treated with TUDCA-PLGA MSs. This work indicates that the slow and continuous delivery of TUDCA from PLGA-MSs has potential neuroprotective effects that could constitute a suitable therapy to prevent neurodegeneration and visual loss in retinitis pigmentosa.

Lycium barbarum polysaccharides attenuates N-methy-N-nitrosourea-induced photoreceptor cell apoptosis in rats through regulation of poly (ADP-ribose) polymerase and caspase expression

ETHNOPHARMACOLOGICAL RELEVANCE

Lycium barbarum L., popularly known as "Goji berry", a classic of Traditional Chinese Medicine has long been used to treat ocular diseases and cardiovascular diseases. Recently, the photoreceptor cell protection of Lycium barbarum polysaccharides (LBP), a water extract from Lycium barbarum L. has received more attention. The present study was designed to investigate the effect of LBP on N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell apoptosis, and the involvement of the poly (ADP-ribose) polymerase (PARP) and caspase.

MATERIALS AND METHODS

Photoreceptor cell injury was induced in male Sprague-Dawley rats by an intraperitoneal injection of MNU 60mg/kg. Seven days prior to MNU injection, LBP were intragastrical administered daily, rats were sacrificed at 24h and 7 days after MNU injection. Retinal morphologies, photoreceptor cells apoptosis, and protein expression were evaluated at 24h and 7 days after MNU injection.

RESULTS

Morphologically, the outer nuclear layer was well preserved in the LBP-treated rat retinas throughout the experimental period. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-digoxigenin nick-end labeling (TUNEL) assays showed that LBP could significantly suppress the loss of photoreceptor cells, as determined by the photoreceptor cell ratio at the central retina 24h and 7 days after MNU administration. Western-blot analysis demonstrated the expression levels of procaspase-9, -7, -3 and cleaved caspase-9, -7, -3 were upregulated, and PARP were downregulated both 24h and 7 days after MNU injection. LBP treatment significantly decreased protein levels of procaspase and cleaved caspase, increased the level of PARP and cleaved PARP on 24h and 7 days.

CONCLUSIONS

LBP inhibits MNU-induced rat photoreceptor cell apoptosis and protects retinal structure via the regulation of the expressions of PARP and caspase.

Efficacy of PARP inhibition in Pde6a mutant mouse models for retinitis pigmentosa depends on the quality and composition of individual human mutations

Retinitis pigmentosa (RP), an inherited blinding disease, is caused by a variety of different mutations that affect retinal photoreceptor function and survival. So far there is neither effective treatment nor cure. We have previously shown that poly(ADP-ribose)polymerase (PARP) acts as a common and critical denominator of cell death in photoreceptors, qualifying it as a potential target for future therapeutic intervention. A significant fraction of RP-causing mutations affect the genes for the rod photoreceptor phosphodiesterase 6A (PDE6A) subunit, but it is not known whether they all engage the same death pathway. Analysing three homozygous point mutations (Pde6a R562W, D670G, and V685M) and one compound heterozygous Pde6a^V685M/R562W mutation in mouse models that match human RP patients, we demonstrate excessive activation of PARP, which correlated in time with the progression of photoreceptor degeneration. The causal involvement of PARP activity in the neurodegenerative process was confirmed in organotypic retinal explant cultures treated with the PARP-selective inhibitor PJ34, using different treatment time-points and durations. Remarkably, the neuroprotective efficacy of PARP inhibition correlated inversely with the strength of the genetically induced insult, with the D670G mutant showing the best treatment effects. Our results highlight PARP as a target for neuroprotective interventions in RP caused by PDE6A mutations and are a first attempt towards personalized, genotype-matched therapy development for RP. In addition, for each of the different mutant situations, our work identifies windows of opportunity for an optimal treatment regimen for further in vivo experimentation and possibly clinical studies.

Erythropoietin Slows Photoreceptor Cell Death in a Mouse Model of Autosomal Dominant Retinitis Pigmentosa

PURPOSE

To test the efficacy of systemic gene delivery of a mutant form of erythropoietin (EPO-R76E) that has attenuated erythropoietic activity, in a mouse model of autosomal dominant retinitis pigmentosa.

METHODS

Ten-day old mice carrying one copy of human rhodopsin with the P23H mutation and both copies of wild-type mouse rhodopsin (hP23H RHO+/-,mRHO+/+) were injected into the quadriceps with recombinant adeno-associated virus (rAAV) carrying either enhanced green fluorescent protein (eGFP) or EpoR76E. Visual function (electroretinogram) and retina structure (optical coherence tomography, histology, and immunohistochemistry) were assessed at 7 and 12 months of age.

RESULTS

The outer nuclear layer thickness decreased over time at a slower rate in rAAV.EpoR76E treated as compared to the rAAV.eGFP injected mice. There was a statistically significant preservation of the electroretinogram at 7, but not 12 months of age.

CONCLUSIONS

Systemic EPO-R76E slows death of the photoreceptors and vision loss in hP23H RHO+/-,mRHO+/+ mice. Treatment with EPO-R76E may widen the therapeutic window for retinal degeneration patients by increasing the number of viable cells. Future studies might investigate if co-treatment with EPO-R76E and gene replacement therapy is more effective than gene replacement therapy alone.

Dominant and recessive mutations in rhodopsin activate different cell death pathways

Mutations in rhodopsin (RHO) are a common cause of retinal dystrophy and can be transmitted by dominant or recessive inheritance. Clinical symptoms caused by dominant and recessive mutations in patients and animal models are very similar but the molecular mechanisms leading to retinal degeneration may differ. We characterized three murine models of retina degeneration caused by either Rho loss of function or expression of the P23H dominant mutation in Rho. Rho loss of function is characterized by activation of calpains and apoptosis-inducing factor (Aif) in dying photoreceptors. Retinas bearing the P23H dominant mutations activate both the calpain-Aif cell death pathway and ER-stress responses that together contribute to photoreceptor cell demise. In vivo treatment with the calpastatin peptide, a calpain inhibitor, was strongly neuroprotective in mice lacking Rho while photoreceptor survival in retinas expressing the P23H dominant mutation was more affected by treatment with salubrinal, an inhibitor of the ER-stress pathway. The further reduction of photoreceptor cell demise by co-treatment with calpastatin and salubrinal suggests co-activation of the calpain and ER-stress death pathways in mice bearing dominant mutations in the Rho gene.

NLRP3 inflammasome activation drives bystander cone photoreceptor cell death in a P23H rhodopsin model of retinal degeneration

The molecular signaling leading to cell death in hereditary neurological diseases such as retinal degeneration is incompletely understood. Previous neuroprotective studies have focused on apoptotic pathways; however, incomplete suppression of cell death with apoptosis inhibitors suggests that other mechanisms are at play. Here, we report that different signaling pathways are activated in rod and cone photoreceptors in the P23H rhodopsin mutant rat, a model representing one of the commonest forms of retinal degeneration. Up-regulation of the RIP1/RIP3/DRP1 axis and markedly improved survival with necrostatin-1 treatment highlighted necroptosis as a major cell-death pathway in degenerating rod photoreceptors. Conversely, up-regulation of NLRP3 and caspase-1, expression of mature IL-1β and IL-18 and improved cell survival with N-acetylcysteine treatment suggested that inflammasome activation and pyroptosis was the major cause of cone cell death. This was confirmed by generation of the P23H mutation on an Nlrp3-deficient background, which preserved cone viability. Furthermore, Brilliant Blue G treatment inhibited inflammasome activation, indicating that the 'bystander cell death' phenomenon was mediated through the P2RX7 cell-surface receptor. Here, we identify a new pathway in cones for bystander cell death, a phenomenon important in development and disease in many biological systems. In other retinal degeneration models different cell-death pathways are activated, which suggests that the particular pathways that are triggered are to some extent genotype-specific. This also implies that neuroprotective strategies to limit retinal degeneration need to be customized; thus, different combinations of inhibitors will be needed to target the specific pathways in any given disease.

Unfolded protein response-induced dysregulation of calcium homeostasis promotes retinal degeneration in rat models of autosomal dominant retinitis pigmentosa

The molecular mechanism of autosomal dominant retinitis pigmentosa (ADRP) in rats is closely associated with a persistently activated unfolded protein response (UPR). If unchecked, the UPR might trigger apoptosis, leading to photoreceptor death. One of the UPR-activated cellular signaling culminating in apoptotic photoreceptor cell death is linked to an increase in intracellular Ca(2+). Therefore, we validated whether ADRP retinas experience a cytosolic Ca(2+) overload, and whether sustained UPR in the wild-type retina could promote retinal degeneration through Ca(2+)-mediated calpain activation. We performed an ex vivo experiment to measure intracellular Ca(2+) in ADRP retinas as well as to detect the expression levels of proteins that act as Ca(2+) sensors. In separate experiments with the subretinal injection of tunicamycin (UPR inducer) and a mixture of calcium ionophore (A231278) and thapsigargin (SERCA2b inhibitor) we assessed the consequences of a sustained UPR activation and increased intracellular Ca(2+) in the wild-type retina, respectively, by performing scotopic ERG, histological, and western blot analyses. Results of the study revealed that induced UPR in the retina activates calpain-mediated signaling, and increased intracellular Ca(2+) is capable of promoting retinal degeneration. A significant decline in ERG amplitudes at 6 weeks post treatment was associated with photoreceptor cell loss that occurred through calpain-activated CDK5-pJNK-Csp3/7 pathway. Similar calpain activation was found in ADRP rat retinas. A twofold increase in intracellular Ca(2+) and up- and downregulations of ER membrane-associated Ca(2+)-regulated IP3R channels and SERCA2b transporters were detected. Therefore, sustained UPR activation in the ADRP rat retinas could promote retinal degeneration through increased intracellular Ca(2+) and calpain-mediated apoptosis.

Anthocyanin can arrest the cone photoreceptor degeneration and act as a novel treatment for retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of heterogeneous inherited retinal diseases that is characterized by primary death rod photoreceptors and the secondary loss of cones. The degeneration of cones causes gradual constriction of visual fields, leaving the central islands that are eventually snuffed out. Studies indicate that the hyperoxia causes oxidative damage in the retina and contributes to the cone death of RP. Moreover, abundant reactive oxidative species (ROS) which are generated in cones may result in mitochondria membrane depolarization, which has been ascribed a central role in the apoptotic process and has been proposed to act as a forward feeding loop for the activation of downstream cascades. Anthocyanin is a potent antioxidant which has been evidenced to be able to counteract oxidative damages, scavenge surplus ROS, and rectify abnormities in the apoptotic cascade. Taken together with its ability to attenuate inflammation which also contributes to the etiology of RP, it is reasonable to hypothesize that the anthocyanin could act as a novel therapeutic strategy to retard or prevent cone degeneration in RP retinas, particularly if the treatment is timed appropriately and delivered efficiently. Future pharmacological investigations will identify the anthocyanin as an effective candidate for PR therapy and refinements of that knowledge would ignite the hope of restoring the visual function in RP patients.

Long time remodeling during retinal degeneration evaluated by optical coherence tomography, immunocytochemistry and fundus autofluorescence

PURPOSE

To characterize the relationship between fundus autofluorescence (FAF), Optical Coherence Tomography (OCT) and immunohistochemistry (IHC) over the course of chronic retinal degeneration in the P23H rat.

METHODS

Homozygous albino P23H rats, Sprague-Dawley (SD) rats as controls and pigmented Long Evans (LE) rats were used. A Spectralis HRA OCT system was used for scanning laser ophthalmoscopy (SLO) imaging OCT and angiography. To determine FAF, fluorescence was excited using diode laser at 488 nm. A fast retina map OCT was performed using the optic nerve as a landmark. IHC was performed to correlate with the findings of OCT and FAF changes.

RESULTS

During the course of retinal degeneration, the FAF pattern evolved from some spotting at 2 months old to a mosaic of hyperfluorescent dots in rats 6 months and older. Retinal thicknesses progressively diminished over the course of the disease. At later stages of degeneration, OCT documented changes in the retinal layers, however, IHC better identified the cell loss and remodeling changes. Angiography revealed attenuation of the retinal vascular plexus with time.

CONCLUSION

We provide for the first time a detailed long-term analysis of the course of retinal degeneration in P23H rats using a combination of SLO and OCT imaging, angiography, FAF and IHC. Although, the application of noninvasive methods enables longitudinal studies and will decrease the number of animals needed for a study, IHC is still an essential tool to identify retinal changes at the cellular level.

Targeting Caspase-12 to Preserve Vision in Mice With Inherited Retinal Degeneration

PURPOSE

The unfolded protein response is known to contribute to the inherited retinal pathology observed in T17M rhodopsin (T17M) mice. Recently it has been demonstrated that the endoplasmic reticulum stress-associated caspase-12 is activated during progression of retinal degeneration in different animal models. Therefore, we wanted to explore the role of caspase-12 in the mechanism of retinopathy in T17M mice and determine if inhibiting apoptosis in this way is a viable approach for halting retinal degeneration.

METHODS

One, two-, and three-month-old C57BL6/J, caspase-12-/-, T17M, and T17M caspase-12-/- mice were analyzed by scotopic ERG, spectral-domain optical coherence tomography (SD-OCT), histology, quantitative (q)RT-PCR, and Western blot of retinal RNA and protein extracts. Calpain and caspase-3/7 activity assays were measured in postnatal (P) day 30 retinal extracts.

RESULTS

Caspase-12 ablation significantly prevented a decline in the a- and b-wave ERG amplitudes in T17M mice during three months, increasing the amplitudes from 232% to 212% and from 160% to 138%, respectively, as compared to T17M retinas. The SD-OCT results and photoreceptor row counts demonstrated preservation of retinal structural integrity and postponed photoreceptor cell death. The delay in photoreceptor cell death was due to significant decreases in the activity of caspase-3/7 and calpain, which correlated with an increase in calpastatin expression.

CONCLUSIONS

We validated caspase-12 as a therapeutic target, ablation of which significantly protects T17M photoreceptors from deterioration. Although the inhibition of apoptotic activity alone was not sufficient to rescue T17M photoreceptors, in combination with other nonapoptotic targets, caspase-12 could be used to treat inherited retinopathy.

Natural Compounds from Saffron and Bear Bile Prevent Vision Loss and Retinal Degeneration

All retinal disorders, regardless of their aetiology, involve the activation of oxidative stress and apoptosis pathways. The administration of neuroprotective factors is crucial in all phases of the pathology, even when vision has been completely lost. The retina is one of the most susceptible tissues to reactive oxygen species damage. On the other hand, proper development and functioning of the retina requires a precise balance between the processes of proliferation, differentiation and programmed cell death. The life-or-death decision seems to be the result of a complex balance between pro- and anti-apoptotic signals. It has been recently shown the efficacy of natural products to slow retinal degenerative process through different pathways. In this review, we assess the neuroprotective effect of two compounds used in the ancient pharmacopoeia. On one hand, it has been demonstrated that administration of the saffron constituent safranal to P23H rats, an animal model of retinitis pigmentosa, preserves photoreceptor morphology and number, the capillary network and the visual response. On the other hand, it has been shown that systemic administration of tauroursodeoxycholic acid (TUDCA), the major component of bear bile, to P23H rats preserves cone and rod structure and function, together with their contact with postsynaptic neurons. The neuroprotective effects of safranal and TUDCA make these compounds potentially useful for therapeutic applications in retinal degenerative diseases.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

Retinal Cell Death Caused by Sodium Iodate Involves Multiple Caspase-Dependent and Caspase-Independent Cell-Death Pathways

Herein, we have investigated retinal cell-death pathways in response to the retina toxin sodium iodate (NaIO₃) both in vivo and in vitro. C57/BL6 mice were treated with a single intravenous injection of NaIO₃ (35 mg/kg). Morphological changes in the retina post NaIO₃ injection in comparison to untreated controls were assessed using electron microscopy. Cell death was determined by TdT-mediated dUTP-biotin nick end labeling (TUNEL) staining. The activation of caspases and calpain was measured using immunohistochemistry. Additionally, cytotoxicity and apoptosis in retinal pigment epithelial (RPE) cells, primary retinal cells, and the cone photoreceptor (PRC) cell line 661W were assessed in vitro after NaIO₃ treatment using the ApoToxGlo™ assay. The 7-AAD/Annexin-V staining was performed and necrostatin (Nec-1) was administered to the NaIO₃-treated cells to confirm the results. In vivo, degenerating RPE cells displayed a rounded shape and retracted microvilli, whereas PRCs featured apoptotic nuclei. Caspase and calpain activity was significantly upregulated in retinal sections and protein samples from NaIO₃-treated animals. In vitro, NaIO₃ induced necrosis in RPE cells and apoptosis in PRCs. Furthermore, Nec-1 significantly decreased NaIO₃-induced RPE cell death, but had no rescue effect on treated PRCs. In summary, several different cell-death pathways are activated in retinal cells as a result of NaIO₃.