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

hPSC-based treatment of retinal diseases - Current progress and challenges

Degenerative retinal diseases, such as age-related macular degeneration (AMD) and inherited retinal diseases (IRDs), cause visual impairment due to irreversible damage to the retinal pigment epithelium (RPE) and photoreceptor cells (PRCs). Currently, no definitive treatment exists. However, cell-based therapies using induced pluripotent stem cells (iPSCs) or embryonic stem cells (ESCs) offer potential solutions for restoring damaged retinal cells. This review summarizes recent advances in RPE and PRC transplantation, highlighting the benefits of each approach. For RPE transplantation, we focus on the outcomes of clinical studies involving three formulations: RPE sheets, RPE suspensions, and RPE strips. In the context of PRC transplantation, we trace the progress from fetal retinal transplantation to the latest studies. Additionally, we discuss our recent clinical work with retinal sheet transplantation and genome-edited retinal organoid sheets, which aim to improve functional integration by reducing bipolar cells in grafts. Finally, with the overall safety of the regenerative cell-based therapies demonstrated in past clinical applications, we explore future prospects for these therapies.

A review of the 661W cell line as a tool to facilitate treatment development for retinal diseases

Retinal diseases encompass a diverse group of disorders that affect the structure and function of the retina, leading to visual impairment and, in some cases, irreversible vision loss. The investigation of retinal diseases is crucial for understanding their underlying mechanisms, identifying potential therapeutic targets, and developing effective treatments. The use of in vitro cell models has become instrumental in advancing our knowledge of these disorders, but given that these conditions usually affect retinal neuronal cell types, access to appropriate cell models can be potentially challenging. Among the available in vitro cell models, the 661W cone-like cell line has emerged as a valuable tool for studying various retinal diseases, ranging from monogenic conditions, such as inherited retinal diseases, to complex conditions such as age-related macular degeneration (AMD), diabetic retinopathy, amongst others. Developed from immortalized murine photoreceptor cells, and freely available for academics from its creator, the 661W cell line has offered visual scientists and clinicians around the world a reliable and well-characterised platform for investigating disease pathogenesis, exploring disease-specific molecular signatures, and evaluating potential therapeutic interventions. This review aims to provide an overview of the 661W cell line and its applications in the study of both inherited and acquired retinal diseases. By examining the applications and limitations of this unique cell line, we may gain valuable insights into its contributions in unravelling the complexities of retinal diseases and its potential impact on the development of novel treatments for these diseases.

PARP inhibition preserves cone photoreceptors in rd2 retina

The rd2 mouse model, characterized by a mutation in the Prph2 gene, exhibits abnormal development of photoreceptor outer segments, resulting in progressive retinal degeneration. While the correlation between poly-ADP-ribose polymerase (PARP) activity and the degeneration of rod photoreceptors is established in the rd2 model, the specific mechanism driving cone degeneration in this model remains unclear. Furthermore, it is yet to be determined whether inhibiting PARP activity can effectively impede the degeneration of cone photoreceptors in this context. We demonstrated that PARP inhibitors Olaparib, BMN-673, and 3-aminobenzamide (3AB), effectively reduced photoreceptor cell loss in the rd2 retina. Notably, rd2 retinas exhibited decreased cone density, but treatment with PARP inhibitors significantly protected cone photoreceptors. The PARP inhibitors, particularly BMN-673, demonstrated a significant protective effect as evidenced by increased rhodopsin expression within the outer segment and a concurrent decrease in Müller cell activity indicated by GFAP expression. The treatment also resulted in significant changes for markers of oxidative stress, such as glutathione (GSH), and oxidized glutathione (GSSG). Notably, the administration of PARP inhibitors also reduced CD9 expression (extracellular vesicle marker), which were significantly increased within the outer nuclear layer (ONL) in the rd2 retinas. Among PARP inhibitors, BMN-673 demonstrated the highest efficacy in preserving photoreceptors, particularly benefiting cone cells.

Vision Loss Reduction with Avacincaptad Pegol for Geographic Atrophy: A 12-Month Post Hoc Analysis of the GATHER1 and GATHER2 Trials

PURPOSE

To evaluate the impact of reduction in geographic atrophy (GA) lesion growth on visual acuity in the GATHER trials using categorical outcome measures.

DESIGN

Randomized, double-masked, sham-controlled phase 3 trials.

PARTICIPANTS

Aged ≥50 years with noncenter point-involving GA and best-corrected visual acuity (BCVA) of 25 to 80 ETDRS letters in the study eye.

METHODS

GATHER1 consisted of 2 parts. In part 1, 77 patients were randomized 1:1:1 to avacincaptad pegol (ACP) 1 mg, ACP 2 mg, and sham. In part 2, 209 patients were randomized 1:2:2 to ACP 2 mg, ACP 4 mg, and sham. In GATHER2, patients were randomized 1:1 to ACP 2 mg (n = 225) and sham (n = 223). A post hoc analysis of 12-month data for pooled ACP 2 mg and sham groups is reported.

MAIN OUTCOME MEASURES

Proportion of study eyes that experienced ≥10-, ≥15-, or ≥20-BCVA ETDRS letter loss from baseline to month 12; time-to-event analysis of persistent vision loss of ≥10, ≥15, or ≥20 BCVA letters from baseline at ≥2 consecutive visits over 12 months; proportion of study eyes with BCVA loss to a level below driving eligibility threshold at month 12 among those eligible to drive at baseline.

RESULTS

Lower proportions of study eyes experienced ≥10-, ≥15-, or ≥20-BCVA letter loss from baseline over 12 months with ACP 2 mg (11.6%, 4.0%, and 1.6%, respectively) versus sham (14.1%, 7.6%, and 4.5%, respectively). There was a reduction in the risk of persistent loss of ≥15 BCVA ETDRS letters with ACP 2 mg (3.4%) versus sham (7.8%) through 12 months. A lower proportion of study eyes treated with ACP 2 mg reached the threshold for driving ineligibility versus sham by 12 months.

CONCLUSIONS

Treatment with ACP 2 mg delayed the risk of progression to persistent vision loss (i.e., ≥10-, ≥15-, and ≥20-BCVA letter loss or BCVA loss to a level below driving eligibility threshold) versus sham over 12 months.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Deprivation of visual input alters specific subset of inhibitory neurons and affect thalamic afferent terminals in V1 of rd1 mouse

Retinitis Pigmentosa (RP) is a heterogenous group of inherited disorder, and its progression not only affects the retina but also the primary visual cortex. This manifests imbalances in the excitatory and inhibitory neurotransmission. Here, we investigated if changes in cortical functioning is linked to alterations in GABAergic population of neurons and its two important subsets, somatostatin (SST) and parvalbumin (PV) neuron in rd1 model of retinal degeneration (RD). We demonstrate marked decrease in the proportion of SST neurons in different layers of cortex whereas PV neurons were less affected. Moreover, we found reduced expression of glutamatergic thalamic afferents (VGLUT2) due to lack of visual activity. These results suggest PV neurons are likely recruited by the cortical circuitry to increase the inhibitory drive and compensate the disrupted inhibition-excitation balance. However, reduced SST expression perhaps results in weakening of stimulus selectivity. Delineating their functional role during RD will provide insights for acquisition of high-resolution vision thereby improving current state of vision restoration.

The deubiquitinase Otud7b suppresses cone photoreceptor degeneration in mouse models of retinal degenerative diseases

Primary and secondary cone photoreceptor death in retinal degenerative diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP), leads to severe visual impairment and blindness. Although the cone photoreceptor protection in retinal degenerative diseases is crucial for maintaining vision, the underlying molecular mechanisms are unclear. Here, we found that the deubiquitinase Otud7b/Cezanne is predominantly expressed in photoreceptor cells in the retina. We analyzed Otud7b-/- mice, which were subjected to light-induced damage, a dry AMD model, or were mated with an RP mouse model, and observed increased cone photoreceptor degeneration. Using RNA-sequencing and bioinformatics analysis followed by a luciferase reporter assay, we found that Otud7b downregulates NF-κB activity. Furthermore, inhibition of NF-κB attenuated cone photoreceptor degeneration in the light-exposed Otud7b-/- retina and stress-induced neuronal cell death resulting from Otud7b deficiency. Together, our findings suggest that Otud7b protects cone photoreceptors in retinal degenerative diseases by modulating NF-κB activity.

The Mongolian gerbil as an advanced model to study cone system physiology

In this work, we introduce a diurnal rodent, the Mongolian gerbil (Meriones unguiculatus) (MG) as an alternative to study retinal cone system physiology and pathophysiology in mice. The cone system is of particular importance, as it provides high-acuity and color vision and its impairment in retinal disorders is thus especially disabling. Despite their nocturnal lifestyle, mice are currently the most popular animals to study cone-related diseases due to the high availability of genetically modified models. However, the potential for successful translation of any cone-related results is limited due to the substantial differences in retinal organization between mice and humans. Alternatively, there are diurnal rodents such as the MG with a higher retinal proportion of cones and a macula-like specialized region for improved visual resolution, the visual streak. The focus of this work was the evaluation of the MG's cone system functionality using full-field electroretinography (ERG), together with a morphological assessment of its retinal/visual streak organization via angiography, optical coherence tomography (OCT), and photoreceptor immunohistochemistry. We found that rod system responses in MGs were comparable or slightly inferior to mice, while in contrast, cone system responses were much larger, more sensitive, and also faster than those in the murine counterparts, and in addition, it was possible to record sizeable ON and OFF ERG components. Morphologically, MG cone photoreceptor opsins were evenly distributed throughout the retina, while mice show a dorsoventral M- and S-opsin gradient. Additionally, each cone expressed a single opsin, in contrast to the typical co-expression of opsins in mice. Particular attention was given to the visual streak region, featuring a higher density of cones, elongated cone and rod outer segments (OSs), and an increased thickness of the inner and outer retinal layers in comparison to peripheral regions. In summary, our data render the MG a supreme model to investigate cone system physiology, pathophysiology, and to validate potential therapeutic strategies in that context.

GRF2 Is Crucial for Cone Photoreceptor Viability and Ribbon Synapse Formation in the Mouse Retina

Using constitutive GRF1/2 knockout mice, we showed previously that GRF2 is a key regulator of nuclear migration in retinal cone photoreceptors. To evaluate the functional relevance of that cellular process for two putative targets of the GEF activity of GRF2 (RAC1 and CDC42), here we compared the structural and functional retinal phenotypes resulting from conditional targeting of RAC1 or CDC42 in the cone photoreceptors of constitutive GRF2KO and GRF2WT mice. We observed that single RAC1 disruption did not cause any obvious morphological or physiological changes in the retinas of GRF2WT mice, and did not modify either the phenotypic alterations previously described in the retinal photoreceptor layer of GRF2KO mice. In contrast, the single ablation of CDC42 in the cone photoreceptors of GRF2WT mice resulted in clear alterations of nuclear movement that, unlike those of the GRF2KO retinas, were not accompanied by electrophysiological defects or slow, progressive cone cell degeneration. On the other hand, the concomitant disruption of GRF2 and CDC42 in the cone photoreceptors resulted, somewhat surprisingly, in a normalized pattern of nuclear positioning/movement, similar to that physiologically observed in GRF2WT mice, along with worsened patterns of electrophysiological responses and faster rates of cell death/disappearance than those previously recorded in single GRF2KO cone cells. Interestingly, the increased rates of cone cell apoptosis/death observed in single GRF2KO and double-knockout GRF2KO/CDC42KO retinas correlated with the electron microscopic detection of significant ultrastructural alterations (flattening) of their retinal ribbon synapses that were not otherwise observed at all in single-knockout CDC42KO retinas. Our observations identify GRF2 and CDC42 (but not RAC1) as key regulators of retinal processes controlling cone photoreceptor nuclear positioning and survival, and support the notion of GRF2 loss-of-function mutations as potential drivers of cone retinal dystrophies.

Control of protein and lipid composition of photoreceptor outer segments-Implications for retinal disease

Vision is arguably our most important sense, and its loss brings substantial limitations to daily life for affected individuals. Light is perceived in retinal photoreceptors (PRs), which are highly specialized neurons subdivided into several compartments with distinct functions. The outer segments (OSs) of photoreceptors represent highly specialized primary ciliary compartments hosting the phototransduction cascade, which transforms incoming light into a neuronal signal. Retinal disease can result from various pathomechanisms originating in distinct subcompartments of the PR cell, or in the retinal pigment epithelium which supports the PRs. Dysfunction of primary cilia causes human disorders known as "ciliopathies", in which retinal disease is a common feature. This chapter focuses on PR OSs, discussing the mechanisms controlling their complex structure and composition. A sequence of tightly regulated sorting and trafficking events, both upstream of and within this ciliary compartment, ensures the establishment and maintenance of the adequate proteome and lipidome required for signaling in response to light. We discuss in particular our current understanding of the role of ciliopathy proteins involved in multi-protein complexes at the ciliary transition zone (CC2D2A) or BBSome (BBS1) and how their dysfunction causes retinal disease. While the loss of CC2D2A prevents the fusion of vesicles and delivery of the photopigment rhodopsin to the ciliary base, leading to early OS ultrastructural defects, BBS1 deficiency results in precocious accumulation of cholesterol in mutant OSs and decreased visual function preceding morphological changes. These distinct pathomechanisms underscore the central role of ciliary proteins involved in multiple processes controlling OS protein and lipid composition.

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.

Roles of Histone Acetyltransferases and Deacetylases in the Retinal Development and Diseases

The critical role of epigenetic modification of histones in maintaining the normal function of the nervous system has attracted increasing attention. Among these modifications, the level of histone acetylation, modulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), is essential in regulating gene expression. In recent years, the research progress on the function of HDACs in retinal development and disease has advanced remarkably, while that regarding HATs remains to be investigated. Here, we overview the roles of HATs and HDACs in regulating the development of diverse retinal cells, including retinal progenitor cells, photoreceptor cells, bipolar cells, ganglion cells, and Müller glial cells. The effects of HATs and HDACs on the progression of various retinal diseases are also discussed with the highlight of the proof-of-concept research regarding the application of available HDAC inhibitors in treating retinal diseases.

Gene-agnostic therapeutic approaches for inherited retinal degenerations

Inherited retinal diseases (IRDs) are associated with mutations in over 250 genes and represent a major cause of irreversible blindness worldwide. While gene augmentation or gene editing therapies could address the underlying genetic mutations in a small subset of patients, their utility remains limited by the great genetic heterogeneity of IRDs and the costs of developing individualised therapies. Gene-agnostic therapeutic approaches target common pathogenic pathways that drive retinal degeneration or provide functional rescue of vision independent of the genetic cause, thus offering potential clinical benefits to all IRD patients. Here, we review the key gene-agnostic approaches, including retinal cell reprogramming and replacement, neurotrophic support, immune modulation and optogenetics. The relative benefits and limitations of these strategies and the timing of clinical interventions are discussed.

Increased H3K27 trimethylation contributes to cone survival in a mouse model of cone dystrophy

Inherited retinal diseases (IRDs) are a heterogeneous group of blinding disorders, which result in dysfunction or death of the light-sensing cone and rod photoreceptors. Despite individual IRDs (Inherited retinal disease) being rare, collectively, they affect up to 1:2000 people worldwide, causing a significant socioeconomic burden, especially when cone-mediated central vision is affected. This study uses the Pde6c^cpfl1 mouse model of achromatopsia, a cone-specific vision loss IRD (Inherited retinal disease), to investigate the potential gene-independent therapeutic benefits of a histone demethylase inhibitor GSK-J4 on cone cell survival. We investigated the effects of GSK-J4 treatment on cone cell survival in vivo and ex vivo and changes in cone-specific gene expression via single-cell RNA sequencing. A single intravitreal GSK-J4 injection led to transcriptional changes in pathways involved in mitochondrial dysfunction, endoplasmic reticulum stress, among other key epigenetic pathways, highlighting the complex interplay between methylation and acetylation in healthy and diseased cones. Furthermore, continuous administration of GSK-J4 in retinal explants increased cone survival. Our results suggest that IRD (Inherited retinal disease)-affected cones respond positively to epigenetic modulation of histones, indicating the potential of this approach in developing a broad class of novel therapies to slow cone degeneration.