Retinal transplant immunology and advancements.

Several gaps and barriers remain for transplanting stem cells into the eye to treat ocular disease, especially diseases of the retina. While the eye has historically been considered immune privileged, recent thinking has identified the immune system as both a barrier and an opportunity for eye stem cell transplantation. Recent approaches leveraging scaffolds or cloaking have been considered in other tissues beyond immune suppression. This perspective paper outlines approaches for transplantation and proposes opportunities to overcome barriers of the immune system in stem cell transplantation in the eye.

Visual Deficits and Diagnostic and Therapeutic Strategies for Neurofibromatosis Type 1: Bridging Science and Patient-Centered Care.

Neurofibromatosis type 1 (NF1) is an inherited autosomal dominant disorder primarily affecting children and adolescents characterized by multisystemic clinical manifestations. Mutations in neurofibromin, the protein encoded by the Nf1 tumor suppressor gene, result in dysregulation of the RAS/MAPK pathway leading to uncontrolled cell growth and migration. Neurofibromin is highly expressed in several cell lineages including melanocytes, glial cells, neurons, and Schwann cells. Individuals with NF1 possess a genetic predisposition to central nervous system neoplasms, particularly gliomas affecting the visual pathway, known as optic pathway gliomas (OPGs). While OPGs are typically asymptomatic and benign, they can induce visual impairment in some patients. This review provides insight into the spectrum and visual outcomes of NF1, current diagnostic techniques and therapeutic interventions, and explores the influence of NF1-OPGS on visual abnormalities. We focus on recent advancements in preclinical animal models to elucidate the underlying mechanisms of NF1 pathology and therapies targeting NF1-OPGs. Overall, our review highlights the involvement of retinal ganglion cell dysfunction and degeneration in NF1 disease, and the need for further research to transform scientific laboratory discoveries to improved patient outcomes.

Cold-induced FOXO1 nuclear transport aids cold survival and tissue storage.

Cold-induced injuries severely limit opportunities and outcomes of hypothermic therapies and organ preservation, calling for better understanding of cold adaptation. Here, by surveying cold-altered chromatin accessibility and integrated CUT&Tag/RNA-seq analyses in human stem cells, we reveal forkhead box O1 (FOXO1) as a key transcription factor for autonomous cold adaptation. Accordingly, we find a nonconventional, temperature-sensitive FOXO1 transport mechanism involving the nuclear pore complex protein RANBP2, SUMO-modification of transporter proteins Importin-7 and Exportin-1, and a SUMO-interacting motif on FOXO1. Our conclusions are supported by cold survival experiments with human cell models and zebrafish larvae. Promoting FOXO1 nuclear entry by the Exportin-1 inhibitor KPT-330 enhances cold tolerance in pre-diabetic obese mice, and greatly prolongs the shelf-life of human and mouse pancreatic tissues and islets. Transplantation of mouse islets cold-stored for 14 days reestablishes normoglycemia in diabetic mice. Our findings uncover a regulatory network and potential therapeutic targets to boost spontaneous cold adaptation.

Stem cell sources and characterization in the development of cell-based products for treating retinal disease: An NEI Town Hall report.

National Eye Institute recently issued a new Strategic Plan outlining priority research areas for the next 5 years. Starting cell source for deriving stem cell lines is as an area with gaps and opportunities for making progress in regenerative medicine, a key area of emphasis within the NEI Strategic Plan. There is a critical need to understand how starting cell source affects the cell therapy product and what specific manufacturing capabilities and quality control standards are required for autologous vs allogeneic stem cell sources. With the goal of addressing some of these questions, in discussion with the community-at-large, NEI hosted a Town Hall at the Association for Research in Vision and Ophthalmology annual meeting in May 2022. This session leveraged recent clinical advances in autologous and allogeneic RPE replacement strategies to develop guidance for upcoming cell therapies for photoreceptors, retinal ganglion cells, and other ocular cell types. Our focus on stem cell-based therapies for RPE underscores the relatively advanced stage of RPE cell therapies to patients with several ongoing clinical trials. Thus, this workshop encouraged lessons learned from the RPE field to help accelerate progress in developing stem cell-based therapies in other ocular tissues. This report provides a synthesis of the key points discussed at the Town Hall and highlights needs and opportunities in ocular regenerative medicine.

Characterizing the rod pathway in cone-dominated thirteen-lined ground squirrels.

AII-amacrine cells (AIIs) are widely accepted as a critical element of scotopic pathways mediating night vision in the mammalian retina and have been well-characterized in rod-dominant mice, rabbits, and non-human primates. The rod pathway is characteristic of all mammalian eyes, however, the anatomic and physiologic role of AIIs and the rod pathways in cone dominant thirteen-lined ground squirrels (TLGS) is limited. Here, we employed both immunohistochemistry and electrophysiological approaches to investigate the morphology of AIIs and functional aspects of the rod pathway in TLGS. In all TLGS retinas examined, putative AIIs were calretinin-positive and exhibited connections to rod bipolar cells with decreased cell density and expanded arborization. Notably, AIIs retained connections with each other via gap junctions labeled with Connexin36. Comparisons between single photoreceptor recordings and full-field electroretinograms revealed scotopic ERG responses were mediated by both rods and cones. Thus, the components of the rod pathway are conserved in TLGS and rod signals traverse the retina in these cone-dominant animals. AIIs are sparsely populated, matching the diminished rod and rod bipolar cell populations compared to rod-dominant species. The infrequent distribution and lateral spacing of AII's indicate that they probably do not play a significant role in cone signaling pathways that encode information at a finer spatial scale. This contrasts with the mouse retina, where they significantly contribute to cone signaling pathways. Therefore, the AII's original function is likely that of a 'rod' amacrine cell, and its role in cone pathways in the mouse retina might be an adaptive feature stemming from its rod dominance.

AMPK modulation ameliorates dominant disease phenotypes of CTRP5 variant in retinal degeneration.

Late-onset retinal degeneration (L-ORD) is an autosomal dominant disorder caused by a missense substitution in CTRP5. Distinctive clinical features include sub-retinal pigment epithelium (RPE) deposits, choroidal neovascularization, and RPE atrophy. In induced pluripotent stem cells-derived RPE from L-ORD patients (L-ORD-iRPE), we show that the dominant pathogenic CTRP5 variant leads to reduced CTRP5 secretion. In silico modeling suggests lower binding of mutant CTRP5 to adiponectin receptor 1 (ADIPOR1). Downstream of ADIPOR1 sustained activation of AMPK renders it insensitive to changes in AMP/ATP ratio resulting in defective lipid metabolism, reduced Neuroprotectin D1(NPD1) secretion, lower mitochondrial respiration, and reduced ATP production. These metabolic defects result in accumulation of sub-RPE deposits and leave L-ORD-iRPE susceptible to dedifferentiation. Gene augmentation of L-ORD-iRPE with WT CTRP5 or modulation of AMPK, by metformin, re-sensitize L-ORD-iRPE to changes in cellular energy status alleviating the disease cellular phenotypes. Our data suggests a mechanism for the dominant behavior of CTRP5 mutation and provides potential treatment strategies for L-ORD patients.

Epithelial phenotype restoring drugs suppress macular degeneration phenotypes in an iPSC model.

Age-related Macular Degeneration (AMD), a blinding eye disease, is characterized by pathological protein- and lipid-rich drusen deposits underneath the retinal pigment epithelium (RPE) and atrophy of the RPE monolayer in advanced disease stages - leading to photoreceptor cell death and vision loss. Currently, there are no drugs that stop drusen formation or RPE atrophy in AMD. Here we provide an iPSC-RPE AMD model that recapitulates drusen and RPE atrophy. Drusen deposition is dependent on AMD-risk-allele CFH(H/H) and anaphylatoxin triggered alternate complement signaling via the activation of NF-κB and downregulation of autophagy pathways. Through high-throughput screening we identify two drugs, L-745,870, a dopamine receptor antagonist, and aminocaproic acid, a protease inhibitor that reduce drusen deposits and restore RPE epithelial phenotype in anaphylatoxin challenged iPSC-RPE with or without the CFH(H/H) genotype. This comprehensive iPSC-RPE model replicates key AMD phenotypes, provides molecular insight into the role of CFH(H/H) risk-allele in AMD, and discovers two candidate drugs to treat AMD.

Establishing the ground squirrel as a superb model for retinal ganglion cell disorders and optic neuropathies.

Retinal ganglion cell (RGC) death occurs after optic nerve injury due to acute trauma or chronic degenerative conditions such as optic neuropathies (e.g., glaucoma). Currently, there are no effective therapies to prevent permanent vision loss resulting from RGC death, underlining the need for research on the pathogenesis of RGC disorders. Modeling human RGC/optic nerve diseases in non-human primates is ideal because of their similarity to humans, but has practical limitations including high cost and ethical considerations. In addition, many retinal degenerative disorders are age-related making the study in primate models prohibitively slow. For these reasons, mice and rats are commonly used to model RGC injuries. However, as nocturnal mammals, these rodents have retinal structures that differ from primates - possessing less than one-tenth of the RGCs found in the primate retina. Here we report the diurnal thirteen-lined ground squirrel (TLGS) as an alternative model. Compared to other rodent models, the number and distribution of RGCs in the TLGS retina are closer to primates. The TLGS retina possesses ~600,000 RGCs with the highest density along the equatorial retina matching the location of the highest cone density (visual streak). TLGS and primate retinas also share a similar interlocking pattern between RGC axons and astrocyte processes in the retina nerve fiber layer (RNFL). In addition, using TLGS we establish a new partial optic nerve injury model that precisely controls the extent of injury while sparing a portion of the retina as an ideal internal control for investigating the pathophysiology of axon degeneration and RGC death. Moreover, in vivo optical coherence tomography (OCT) imaging and ex vivo microscopic examinations of the retina in optic nerve injured TLGS confirm RGC loss precedes proximal axon degeneration, recapitulating human pathology. Thus, the TLGS retina is an excellent model, for translational research in neurodegeneration and therapeutic neuroprotection.

Direct-Coupled Electroretinogram (DC-ERG) for Recording the Light-Evoked Electrical Responses of the Mouse Retinal Pigment Epithelium.

The retinal pigment epithelium (RPE) is a specialized monolayer of cells strategically located between the retina and the choriocapillaris that maintain the overall health and structural integrity of the photoreceptors. The RPE is polarized, exhibiting apically and basally located receptors or channels, and performs vectoral transport of water, ions, metabolites, and secretes several cytokines. In vivo noninvasive measurements of RPE function can be made using direct-coupled ERGs (DC-ERGs). The methodology behind the DC-ERG was pioneered by Marmorstein, Peachey, and colleagues using a custom-built stimulation recording system and later demonstrated using a commercially available system. The DC-ERG technique uses glass capillaries filled with Hank's buffered salt solution (HBSS) to measure the slower electrical responses of the RPE elicited from light-evoked concentration changes in the subretinal space due to photoreceptor activity. The prolonged light stimulus and length of the DC-ERG recording make it vulnerable to drift and noise resulting in a low yield of useable recordings. Here, we present a fast, reliable method for improving the stability of the recordings while reducing noise by using vacuum pressure to reduce/eliminate bubbles that result from outgassing of the HBSS and electrode holder. Additionally, power line artifacts are attenuated using a voltage regulator/power conditioner. We include the necessary light stimulation protocols for a commercially available ERG system as well as scripts for analysis of the DC-ERG components: c-wave, fast oscillation, light peak, and off response. Due to the improved ease of recordings and rapid analysis workflow, this simplified protocol is particularly useful in measuring age-related changes in RPE function, disease progression, and in the assessment of pharmacological intervention.

Regulation of phagolysosomal activity by miR-204 critically influences structure and function of retinal pigment epithelium/retina.

MicroRNA-204 (miR-204) is expressed in pulmonary, renal, mammary and eye tissue, and its reduction can result in multiple diseases including cancer. We first generated miR-204-/- mice to study the impact of miR-204 loss on retinal and retinal pigment epithelium (RPE) structure and function. The RPE is fundamentally important for maintaining the health and integrity of the retinal photoreceptors. miR-204-/- eyes evidenced areas of hyper-autofluorescence and defective photoreceptor digestion, along with increased microglia migration to the RPE. Migratory Iba1+ microglial cells were localized to the RPE apical surface where they participated in the phagocytosis of photoreceptor outer segments (POSs) and contributed to a persistent build-up of rhodopsin. These structural, molecular and cellular outcomes were accompanied by decreased light-evoked electrical responses from the retina and RPE. In parallel experiments, we suppressed miR-204 expression in primary cultures of human RPE using anti-miR-204. In vitro suppression of miR-204 in human RPE similarly showed abnormal POS clearance and altered expression of autophagy-related proteins and Rab22a, a regulator of endosome maturation. Together, these in vitro and in vivo experiments suggest that the normally high levels of miR-204 in RPE can mitigate disease onset by preventing generation of oxidative stress and inflammation originating from intracellular accumulation of undigested photoreactive POS lipids. More generally, these results implicate RPE miR-204-mediated regulation of autophagy and endolysosomal interaction as a critical determinant of normal RPE/retina structure and function.

Clinical-grade stem cell-derived retinal pigment epithelium patch rescues retinal degeneration in rodents and pigs.

Considerable progress has been made in testing stem cell-derived retinal pigment epithelium (RPE) as a potential therapy for age-related macular degeneration (AMD). However, the recent reports of oncogenic mutations in induced pluripotent stem cells (iPSCs) underlie the need for robust manufacturing and functional validation of clinical-grade iPSC-derived RPE before transplantation. Here, we developed oncogenic mutation-free clinical-grade iPSCs from three AMD patients and differentiated them into clinical-grade iPSC-RPE patches on biodegradable scaffolds. Functional validation of clinical-grade iPSC-RPE patches revealed specific features that distinguished transplantable from nontransplantable patches. Compared to RPE cells in suspension, our biodegradable scaffold approach improved integration and functionality of RPE patches in rats and in a porcine laser-induced RPE injury model that mimics AMD-like eye conditions. Our results suggest that the in vitro and in vivo preclinical functional validation of iPSC-RPE patches developed here might ultimately be useful for evaluation and optimization of autologous iPSC-based therapies.

iPSCs from a Hibernator Provide a Platform for Studying Cold Adaptation and Its Potential Medical Applications.

Hibernating mammals survive hypothermia (<10°C) without injury, a remarkable feat of cellular preservation that bears significance for potential medical applications. However, mechanisms imparting cold resistance, such as cytoskeleton stability, remain elusive. Using the first iPSC line from a hibernating mammal (13-lined ground squirrel), we uncovered cellular pathways critical for cold tolerance. Comparison between human and ground squirrel iPSC-derived neurons revealed differential mitochondrial and protein quality control responses to cold. In human iPSC-neurons, cold triggered mitochondrial stress, resulting in reactive oxygen species overproduction and lysosomal membrane permeabilization, contributing to microtubule destruction. Manipulations of these pathways endowed microtubule cold stability upon human iPSC-neurons and rat (a non-hibernator) retina, preserving its light responsiveness after prolonged cold exposure. Furthermore, these treatments significantly improved microtubule integrity in cold-stored kidneys, demonstrating the potential for prolonging shelf-life of organ transplants. Thus, ground squirrel iPSCs offer a unique platform for bringing cold-adaptive strategies from hibernators to humans in clinical applications. VIDEO ABSTRACT.

Primary Cilium-Mediated Retinal Pigment Epithelium Maturation Is Disrupted in Ciliopathy Patient Cells.

Primary cilia are sensory organelles that protrude from the cell membrane. Defects in the primary cilium cause ciliopathy disorders, with retinal degeneration as a prominent phenotype. Here, we demonstrate that the retinal pigment epithelium (RPE), essential for photoreceptor development and function, requires a functional primary cilium for complete maturation and that RPE maturation defects in ciliopathies precede photoreceptor degeneration. Pharmacologically enhanced ciliogenesis in wild-type induced pluripotent stem cells (iPSC)-RPE leads to fully mature and functional cells. In contrast, ciliopathy patient-derived iPSC-RPE and iPSC-RPE with a knockdown of ciliary-trafficking protein remain immature, with defective apical processes, reduced functionality, and reduced adult-specific gene expression. Proteins of the primary cilium regulate RPE maturation by simultaneously suppressing canonical WNT and activating PKCδ pathways. A similar cilium-dependent maturation pathway exists in lung epithelium. Our results provide insights into ciliopathy-induced retinal degeneration, demonstrate a developmental role for primary cilia in epithelial maturation, and provide a method to mature iPSC epithelial cells for clinical applications.

A basis for comparison: sensitive authentication of stem cell derived RPE using physiological responses of intact RPE monolayers.

The retinal pigment epithelium (RPE) is a monolayer of highly specialized cells that help maintain the chemical composition of its surrounding subretinal and choroidal extracellular spaces. Retinal cells (photoreceptors in particular), RPE, and choroidal endothelial cells together help ensure a homeostatically stable metabolic environment with exquisitely sensitive functional responses to light. Aging and disease of the RPE impairs its supportive functions contributing to the progressive loss of photoreceptors and vision. The prevalence of RPE associated retinal degenerations has prompted researchers to develop new therapies aimed at replacing the affected RPE with induced pluripotent stem cell (iPSC) or embryonic stem cell (ESC) derived RPE. Despite recent attempts to characterize stem cell derived RPE and to truly authenticate RPE for clinical applications, there remains a significant unmet need to explore the heterogeneity resulting from donor to donor variation as well as the variations inherent in the current processes of cell manufacture. Additionally, it remains unknown whether the starting cell type influences the resulting RPE phenotype following reprogramming and differentiation. To address these questions, we performed a comprehensive evaluation (genomic, structural, and functional) of 15 iPSC derived RPE originating from different donors and tissues and compiled a reference data set for the authentication of iPSC-derived RPE and RPE derived from other stem cell sources.

Voltage-sensitive conductances increase the sensitivity of rod photoresponses following pigment bleaching.

KEY POINTS: Following substantial bleaching of the visual pigment, the desensitization of the rod photovoltage is not as substantial as the desensitization of the rod outer segment photocurrent. The block of cation conductances during the internal dialysis of Cs+ further desensitizes the photovoltage thereby eliminating its difference in desensitization with the rod outer segment photocurrent. Bleached visual pigment produced an acceleration of the rod photovoltage with respect to the outer segment photocurrent, which is eliminated upon internal dialysis of Cs+ . ABSTRACT: A majority of our visual experience occurs during the day when a substantial fraction of the visual pigment in our photoreceptor cells is bleached. Under these conditions it is widely believed that rods are saturated and do not contribute substantially to downstream signalling. However, behavioural experiments on subjects with only rod function reveals that these individuals unexpectedly retain substantial vision in daylight. We sought to understand this discrepancy by characterizing the sensitivity of rod photoresponses following exposure to bright bleaching light. Measurements of the rod outer segment photocurrent in transgenic mice, which have only rod function, revealed the well-studied reduction in the sensitivity of rod photoresponses following pigment bleaching. However, membrane voltage measurements showed that the desensitization of the photovoltage was considerably less than that of the outer segment photocurrent following equivalent pigment bleaching. This discrepancy was largely eliminated during the blockade of cation channels due to the internal dialysis of Cs+ , which increased the bleach-induced desensitization of the photovoltage and slowed its temporal characteristics. Thus, sensitization of the photovoltage by rod inner segment conductances appears to extend the operating range of rod phototransduction following pigment bleaching.

In Pursuit of Authenticity: Induced Pluripotent Stem Cell-Derived Retinal Pigment Epithelium for Clinical Applications.

: Induced pluripotent stem cells (iPSCs) can be efficiently differentiated into retinal pigment epithelium (RPE), offering the possibility of autologous cell replacement therapy for retinal degeneration stemming from RPE loss. The generation and maintenance of epithelial apical-basolateral polarity is fundamental for iPSC-derived RPE (iPSC-RPE) to recapitulate native RPE structure and function. Presently, no criteria have been established to determine clonal or donor based heterogeneity in the polarization and maturation state of iPSC-RPE. We provide an unbiased structural, molecular, and physiological evaluation of 15 iPSC-RPE that have been derived from distinct tissues from several different donors. We assessed the intact RPE monolayer in terms of an ATP-dependent signaling pathway that drives critical aspects of RPE function, including calcium and electrophysiological responses, as well as steady-state fluid transport. These responses have key in vivo counterparts that together help determine the homeostasis of the distal retina. We characterized the donor and clonal variation and found that iPSC-RPE function was more significantly affected by the genetic differences between different donors than the epigenetic differences associated with different starting tissues. This study provides a reference dataset to authenticate genetically diverse iPSC-RPE derived for clinical applications. SIGNIFICANCE: The retinal pigment epithelium (RPE) is essential for maintaining visual function. RPE derived from human induced pluripotent stem cells (iPSC-RPE) offer a promising cell-based transplantation therapy for slowing or rescuing RPE-induced visual function loss. For effective treatment, iPSC-RPE must recapitulate the physiology of native human RPE. A set of physiologically relevant functional assays are provided that assess the polarized functional activity and maturation state of the intact RPE monolayer. The present data show that donor-to-donor variability exceeds the tissue-to-tissue variability for a given donor and provides, for the first time, criteria necessary to identify iPSC-RPE most suitable for clinical application.

A multiplex high-throughput gene expression assay to simultaneously detect disease and functional markers in induced pluripotent stem cell-derived retinal pigment epithelium.

There is continuing interest in the development of lineage-specific cells from induced pluripotent stem (iPS) cells for use in cell therapies and drug discovery. Although in most cases differentiated cells show features of the desired lineage, they retain fetal gene expression and do not fully mature into "adult-like" cells. Such cells may not serve as an effective therapy because, once implanted, immature cells pose the risk of uncontrolled growth. Therefore, there is a need to optimize lineage-specific stem cell differentiation protocols to produce cells that no longer express fetal genes and have attained "adult-like" phenotypes. Toward that goal, it is critical to develop assays that simultaneously measure cell function and disease markers in high-throughput format. Here, we use a multiplex high-throughput gene expression assay that simultaneously detects endogenous expression of multiple developmental, functional, and disease markers in iPS cell-derived retinal pigment epithelium (RPE). We optimized protocols to differentiate iPS cell-derived RPE that was then grown in 96- and 384-well plates. As a proof of principle, we demonstrate differential expression of eight genes in iPS cells, iPS cell-derived RPE at two different differentiation stages, and primary human RPE using this multiplex assay. The data obtained from the multiplex gene expression assay are significantly correlated with standard quantitative reverse transcription-polymerase chain reaction-based measurements, confirming the ability of this high-throughput assay to measure relevant gene expression changes. This assay provides the basis to screen for compounds that improve RPE function and maturation and target disease pathways, thus providing the basis for effective treatments of several retinal degenerative diseases.

Processing of S-cone signals in the inner plexiform layer of the mammalian retina.

Color information is encoded by two parallel pathways in the mammalian retina. One pathway compares signals from long- and middle-wavelength sensitive cones and generates red-green opponency. The other compares signals from short- and middle-/long-wavelength sensitive cones and generates blue-green (yellow) opponency. Whereas both pathways operate in trichromatic primates (including humans), the fundamental, phylogenetically ancient color mechanism shared among most mammals is blue-green opponency. In this review, we summarize the current understanding of how signals from short-wavelength sensitive cones are processed in the primate and nonprimate mammalian retina, with a focus on the inner plexiform layer where bipolar, amacrine, and ganglion cell processes interact to facilitate the generation of blue-green opponency.

Flow of energy in the outer retina in darkness and in light.

Structural features of neurons create challenges for effective production and distribution of essential metabolic energy. We investigated how metabolic energy is distributed between cellular compartments in photoreceptors. In avascular retinas, aerobic production of energy occurs only in mitochondria that are located centrally within the photoreceptor. Our findings indicate that metabolic energy flows from these central mitochondria as phosphocreatine toward the photoreceptor's synaptic terminal in darkness. In light, it flows in the opposite direction as ATP toward the outer segment. Consistent with this model, inhibition of creatine kinase in avascular retinas blocks synaptic transmission without influencing outer segment activity. Our findings also reveal how vascularization of neuronal tissue can influence the strategies neurons use for energy management. In vascularized retinas, mitochondria in the synaptic terminals of photoreceptors make neurotransmission less dependent on creatine kinase. Thus, vasculature of the tissue and the intracellular distribution of mitochondria can play key roles in setting the strategy for energy distribution in neurons.