Retinal remodeling during retinal degeneration

Virtual Human Retina: Simulating Neural Signalling, Degeneration, and Responses to Electrical Stimulation

INTRODUCTION

Current brain-based visual prostheses pose significant challenges impeding adoption such as the necessarily complex surgeries and occurrence of more substantial side effects due to the sensitivity of the brain. This has led to much effort toward vision restoration being focused on the more approachable part of the brain - the retina. Here we introduce a novel, parameterized simulation platform that enables study of human retinal degeneration and optimization of stimulation strategies. The platform bears immense potential for patient-specific tailoring and serves to enhance artificial vision solutions for individuals with visual impairments.

MATERIAL AND METHOD

Our virtual retina is developed using the software package, NEURON. This virtual retina platform supports large-scale simulations of over 10,000 neurons whilst upholding strong biological plausibility with multiple important visual pathways and detailed network properties. The comprehensive three-dimensional model includes photoreceptors, horizontal cells, bipolar cells, amacrine cells, and midget and parasol retinal ganglion cells, with comprehensive network connectivity across various eccentricities (1 mm to 5 mm from the fovea) in the human retina. The model is constructed using electrophysiology, immunohistology, and optical coherence tomography imaging data from healthy and degenerate human retinas. We validated our model by replicating numerous experimental observations from human and primate retina, with a particular focus on retinal degeneration.

RESULT

We simulated interactions between diseased retinas and state-of-the-art retinal implants, shedding light on the limitations of commercial retinal prostheses. Our results suggested that appropriate stimulation settings with intraretinal prototype devices could leverage network-mediated activation to achieve activation mosaics more alike that of the retina's response to natural light, promoting the prospect of more naturalistic vision. Our study additionally highlights the importance of controlling inhibitory circuits in the retinal network to induce functionally relevant retinal activity.

CONCLUSION

This study demonstrates the potential of this software package and highlights its utility as a valuable tool for engineers, scientists, and clinicians in the design and optimisation of retinal stimulation devices for both research and educational applications.

Aligning Visual Prosthetic Development With Implantee Needs

Purpose

Visual prosthetics are a promising assistive technology for vision loss, yet research often overlooks the human aspects of this technology. While previous studies focus on the perceptual experiences or attitudes of implant recipients (implantees), a systematic account of how current implants are being used in everyday life is still lacking.

Methods

We interviewed six recipients of the most widely used visual implants (Argus II and Orion) and six leading researchers in the field. Through thematic analyses, we explored the daily usage of these implants by implantees and compared their responses to the expectations of researchers. We also sought implantees' input on desired features for future versions, aiming to inform the development of the next generation of implants.

Results

Although implants are designed to facilitate various daily activities, we found that implantees use them less frequently than researchers expect. This discrepancy primarily stems from issues with usability and reliability, with implantees finding alternative methods to accomplish tasks, reducing the need to rely on the implant. For future implants, implantees emphasized the desire for improved vision, smart integration, and increased independence.

Conclusions

Our study reveals a significant gap between researcher expectations and implantee experiences with visual prostheses. Although limited by access to a small population of implantees, this study highlights the importance of focusing future research on usability and real-world applications.

Translational Relevance

This retrospective qualitative study advocates for a better alignment between technology development and implantee needs to enhance clinical relevance and practical utility of visual prosthetics.

prominin-1-null Xenopus laevis develop subretinal drusenoid-like deposits, cone-rod dystrophy and RPE atrophy

Prominin-1 (PROM1) variants are associated with inherited, non-syndromic vision loss. We used CRISPR/Cas9 to induce prom1-null mutations in Xenopus laevis and then tracked retinal disease progression from the ages of 6 weeks to 3 years. We found that prom1-null-associated retinal degeneration in frogs was age-dependent and involved retinal pigment epithelium (RPE) dysfunction preceding photoreceptor degeneration. Before photoreceptor degeneration occurred, aging prom1-null frogs developed larger and increasing numbers of cellular debris deposits in the subretinal space and outer segment layer, which resembled subretinal drusenoid deposits (SDDs) in their location, histology and representation as seen by color fundus photography and optical coherence tomography (OCT). Evidence for an RPE origin of these deposits included infiltration of pigment granules into the deposits, thinning of the RPE as measured by OCT, and RPE disorganization as measured by histology and OCT. The appearance and accumulation of SDD-like deposits and RPE thinning and disorganization in our animal model suggests an underlying disease mechanism for prom1-null-mediated blindness that involves death and dysfunction of the RPE preceding photoreceptor degeneration, instead of direct effects upon photoreceptor outer segment morphogenesis, as was previously hypothesized.

Alterations of the Ganglion Cell Complex in Age-Related Macular Degeneration: An Amish Eye Study Analysis

PURPOSE

To compare the ganglion cell complex (GCC) thickness in eyes with age-related macular degeneration (AMD) vs healthy controls in an elderly Amish population.

DESIGN

Prospective cross-sectional study.

METHODS

This is a post hoc analysis of the family-based prospective study of Amish subjects. Study subjects underwent imaging with the Cirrus HD-OCT (Carl Zeiss Meditec Inc) using a macular cube protocol of 512 × 128 scans (128 horizontal B-scans, each comprising 512 A-scans) over a 6 mm × 6 mm region centered on the fovea. The ganglion cell analysis algorithm calculated the GCC thickness by segmenting the outer boundaries of the retinal nerve fiber layer (RNFL) and inner plexiform layer (IPL) in all B-scans of the volume, with the region between these boundaries representing the combined thickness of the ganglion cell layer (GCL) and the IPL. A number of parameters were used to evaluate the GCC thickness: the average GCC thickness, minimum (lowest GCC thickness at a single meridian crossing the elliptical annulus), and sectoral (within each of 6 sectoral areas: superior, superotemporal, superonasal, inferior, inferonasal, and inferotemporal). The stage of AMD was graded on color fundus photographs in accordance with the Beckman Initiative for Macular Research classification system.

RESULTS

Of 1339 subjects enrolled in the Amish eye study, a total of 1294 eyes of 1294 subjects had all required imaging studies of sufficient quality and were included in the final analysis. Of these, 798 (62%) were female. Following age adjustment, the average GCC thickness was significantly (P < .001) thinner in AMD subjects (73.71 ± SD; 13.77 µm) compared to normals (77.97 ± 10.42 µm). An independent t test showed that the early AMD (75.03 ± 12.45 µm) and late AMD (61.64 ± 21.18 µm) groups (among which eyes with geographic atrophy [GA] had the lowest thickness, of 58.10 ± 20.27 µm) had a statistically significant lower GCC thickness compared to eyes without AMD. There was no significant differences in average GCC thickness between early AMD and intermediate AMD (76.36 ± 9.25 µm) eyes.

CONCLUSIONS

The GCC thickness in AMD eyes is reduced compared to normal eyes; however, the relationship is complex, with the greatest reduction in late AMD eyes (particularly eyes with GA) but no difference between early and intermediate AMD eyes.

Retinal Connectomics: A Review

The retina is an ideal model for understanding the fundamental rules for how neural networks are constructed. The compact neural networks of the retina perform all of the initial processing of visual information before transmission to higher visual centers in the brain. The field of retinal connectomics uses high-resolution electron microscopy datasets to map the intricate organization of these networks and further our understanding of how these computations are performed by revealing the fundamental topologies and allowable networks behind retinal computations. In this article, we review some of the notable advances that retinal connectomics has provided in our understanding of the specific cells and the organization of their connectivities within the retina, as well as how these are shaped in development and break down in disease. Using these anatomical maps to inform modeling has been, and will continue to be, instrumental in understanding how the retina processes visual signals.

Prominin-1 null Xenopus laevis develop subretinal drusenoid-like deposits, cone-rod dystrophy, and RPE atrophy

Mutations in the PROMININ-1 (PROM1) gene are associated with inherited, non-syndromic vision loss. Here, we used CRISPR/Cas9 to induce truncating prom1-null mutations in Xenopus laevis to create a disease model. We then tracked progression of retinal degeneration in these animals from the ages of 6 weeks to 3 years old. We found that retinal degeneration caused by prom1-null is age-dependent and likely involves death or damage to the retinal pigment epithelium (RPE) that precedes photoreceptor degeneration. As prom1-null frogs age, they develop large cellular debris deposits in the subretinal space and outer segment layer which resemble subretinal drusenoid deposits (SDD) in their location, histology, and representation in color fundus photography and optical coherence tomography (OCT). In older frogs, these SDD-like deposits accumulate in size and number, and they are present before retinal degeneration occurs. Evidence for an RPE origin of these deposits includes infiltration of pigment granules into the deposits, thinning of RPE as measured by OCT, and RPE disorganization as measured by histology and OCT. The appearance and accumulation of SDD-like deposits and RPE thinning and disorganization in our animal model suggests an underlying disease mechanism for prom1-null mediated blindness of death and dysfunction of the RPE preceding photoreceptor degeneration, instead of direct effects upon photoreceptor outer segment morphogenesis, as was previously hypothesized.

Targeting miR-181a/b in retinitis pigmentosa: implications for disease progression and therapy

BACKGROUND

Retinitis pigmentosa (RP) is a genetically heterogeneous group of degenerative disorders causing progressive vision loss due to photoreceptor death. RP affects other retinal cells, including the retinal pigment epithelium (RPE). MicroRNAs (miRs) are implicated in RP pathogenesis, and downregulating miR-181a/b has shown therapeutic benefit in RP mouse models by improving mitochondrial function. This study investigates the expression profile of miR-181a/b in RPE cells and the neural retina during RP disease progression. We also evaluate how miR-181a/b downregulation, by knocking out miR-181a/b-1 cluster in RPE cells, confers therapeutic efficacy in an RP mouse model and explore the mechanisms underlying this process.

RESULTS

Our findings reveal distinct expression profiles, with downregulated miR-181a/b in RPE cells suggesting a protective response and upregulated miR-181a/b in the neural retina indicating a role in disease progression. We found that miR-181a/b-2, encoded in a separate genomic cluster, compensates for miR-181a/b-1 ablation in RPE cells at late time points. The transient downregulation of miR-181a/b in RPE cells at post-natal week 6 (PW6) led to improved RPE morphology, retarded photoreceptor degeneration and decreased RPE aerobic glycolysis.

CONCLUSIONS

Our study elucidates the underlying mechanisms associated with the therapeutic modulation of miR-181a/b, providing insights into the metabolic processes linked to its RPE-specific downregulation. Our data further highlights the impact of compensatory regulation between miR clusters with implications for the development of miR-based therapeutics.

Specific photoreceptor cell fate pathways are differentially altered in NR2E3-associated diseases

Mutations in NR2E3, a gene encoding an orphan nuclear transcription factor, cause two retinal dystrophies with a distinct phenotype, but the precise role of NR2E3 in rod and cone transcriptional networks remains unclear. To dissect NR2E3 function, we performed scRNA-seq in the retinas of wildtype and two different Nr2e3 mouse models that show phenotypes similar to patients carrying NR2E3 mutations. Our results reveal that rod and cone populations are not homogeneous and can be separated into different sub-classes. We identify a previously unreported cone pathway that generates hybrid cones co-expressing both cone- and rod-related genes. In mutant retinas, this hybrid cone subpopulation is more abundant and includes a subpopulation of rods transitioning towards a cone cell fate. Hybrid photoreceptors with high misexpression of cone- and rod-related genes are prone to regulated necrosis. Overall, our results shed light on the role of NR2E3 in modulating photoreceptor differentiation towards cone and rod fates and explain how different mutations in NR2E3 lead to distinct visual disorders in humans.

A metabotropic glutamate receptor agonist enhances visual signal fidelity in a mouse model of retinitis pigmentosa

Many inherited retinal diseases target photoreceptors, which transduce light into a neural signal that is processed by the downstream visual system. As photoreceptors degenerate, physiological and morphological changes to retinal synapses and circuitry reduce sensitivity and increase noise, degrading visual signal fidelity. Here, we pharmacologically targeted the first synapse in the retina in an effort to reduce circuit noise without sacrificing visual sensitivity. We tested a strategy to partially replace the neurotransmitter lost when photoreceptors die with an agonist of receptors that ON bipolars cells use to detect glutamate released from photoreceptors. In rd10 mice, which express a photoreceptor mutation that causes retinitis pigmentosa (RP), we found that a low dose of the mGluR6 agonist l-2-amino-4-phosphonobutyric acid (L-AP4) reduced pathological noise induced by photoreceptor degeneration. After making in vivo electroretinogram recordings in rd10 mice to characterize the developmental time course of visual signal degeneration, we examined effects of L-AP4 on sensitivity and circuit noise by recording in vitro light-evoked responses from individual retinal ganglion cells (RGCs). L-AP4 decreased circuit noise evident in RGC recordings without significantly reducing response amplitudes, an effect that persisted over the entire time course of rod photoreceptor degeneration. Subsequent in vitro recordings from rod bipolar cells (RBCs) showed that RBCs are more depolarized in rd10 retinas, likely contributing to downstream circuit noise and reduced synaptic gain, both of which appear to be ameliorated by hyperpolarizing RBCs with L-AP4. These beneficial effects may reduce pathological circuit remodeling and preserve the efficacy of therapies designed to restore vision.

Long-Term Porcine Retina Explants as an Alternative to In Vivo Experimentation

Purpose

The porcine retina represents an optimal model system to study treatment approaches for inherited retinal dystrophies owing to close anatomical similarities to the human retina, including a cone enriched visual streak. The aim of this work was to establish a protocol to keep explants in culture for up to 28 days with good morphological preservation.

Methods

Two to four retina explants per eye were obtained from the central part of the retina and transferred onto a membrane insert with the photoreceptors facing down. Different medium compositions using Neurobasal-A medium containing 100 or 450 mg/dL glucose and combinations of fetal calf serum, B-27 with or without insulin and N-2 were tested. We developed a tissue quality score with robust markers for different retinal cell types (protein kinase C alpha, peanut agglutinin and 4',6-diamidino-2-phenylindol).

Results

Retinae were kept until 28 days with only little degradation. The best results were attained using Neurobasal-A medium containing 100 mg/dL glucose supplemented with B-27 containing insulin and N-2. For an easy preparation process, it is necessary to minimize transport time and keep the eyes on ice until dissected. Heat-mediated decontamination by the butcher has to be avoided.

Conclusions

Using a standardized protocol, porcine retina explants represent an easy to handle intermediate model between in vitro and in vivo experimentation. This model system is robustly reproducible and contributes to the implementation of the 3R principle to minimize animal experimentation.

Translational Relevance

This model can be used to test future therapeutic approaches for inherited retinal dystrophies.

Histologic and Immunohistochemical Characterization of GA-Like Pathology in the Rat Subretinal Sodium Iodate Model

Purpose

Geographic atrophy (GA) is an advanced form of dry age-related macular degeneration with multifactorial etiology and no well-established treatment. A model recapitulating the hallmarks would serve as a key to understanding the underlying pathologic mechanisms better. In this report, we further characterized our previously reported subretinal sodium iodate model of GA.

Methods

Retinal degeneration was induced in rats (6-8 weeks old) by subretinal injections of NaIO3 as described previously. Animals were sacrificed at 3, 8 and 12 weeks after injection and eyes were fixed or cryopreserved. Some choroids were processed as flatmounts while other eyes were cryopreserved, sectioned, and immunolabeled with a panel of antibodies. Finally, some eyes were prepared for transmission electron microscopic (TEM) analysis.

Results

NaIO3 subretinal injection resulted in a well-defined focal area of retinal pigment epithelium (RPE) degeneration surrounded by viable RPE. These atrophic lesions expanded over time. RPE morphologic changes at the border consisted of hypertrophy, multilayering, and the possible development of a migrating phenotype. Immunostaining of retinal sections demonstrated external limiting membrane descent, outer retinal tubulation (ORT), and extension of Müller cells toward RPE forming a glial membrane in the subretinal space of the atrophic area. TEM findings demonstrated RPE autophagy, cellular constituents of ORT, glial membranes, basal laminar deposits, and defects in Bruch's membrane.

Conclusions

In this study, we showed pathologic features of a rodent model resembling human GA in a temporal order through histology, immunofluorescence, and TEM analysis and gained insights into the cellular and subcellular levels of the GA-like phenotypes.

Translational Relevance

Despite its acute nature, the expansion of atrophy and the GA-like border in this rat model makes it ideal for studying disease progression and provides a treatment window to test potential therapeutics for GA.

Exacerbated response to oxidative stress in the Retinitis Pigmentosa CerklKD/KO mouse model triggers retinal degeneration pathways upon acute light stress

The retina is particularly vulnerable to genetic and environmental alterations that generate oxidative stress and cause cellular damage in photoreceptors and other retinal neurons, eventually leading to cell death. CERKL (CERamide Kinase-Like) mutations cause Retinitis Pigmentosa and Cone-Rod Dystrophy in humans, two disorders characterized by photoreceptor degeneration and progressive vision loss. CERKL is a resilience gene against oxidative stress, and its overexpression protects cells from oxidative stress-induced apoptosis. Besides, CERKL contributes to stress granule-formation and regulates mitochondrial dynamics in the retina. Using the CerklKD/KO albino mouse model, which recapitulates the human disease, we aimed to study the impact of Cerkl knockdown on stress response and activation of photoreceptor death mechanisms upon light/oxidative stress. After acute light injury, we assessed immediate or late retinal stress response, by combining both omic and non-omic approaches. Our results show that Cerkl knockdown increases ROS levels and causes a basal exacerbated stress state in the retina, through alterations in glutathione metabolism and stress granule production, overall compromising an adequate response to additional oxidative damage. As a consequence, several cell death mechanisms are triggered in CerklKD/KO retinas after acute light stress. Our studies indicate that Cerkl gene is a pivotal player in regulating light-challenged retinal homeostasis and shed light on how mutations in CERKL lead to blindness by dysregulation of the basal oxidative stress response in the retina.

Metabolic changes and retinal remodeling in Heterozygous CRX mutant cats (CRXRDY/+)

CRX is a transcription factor essential for normal photoreceptor development and survival. The CRXRdy cat has a naturally occurring truncating mutation in CRX and is a large animal model for dominant Leber congenital amaurosis. This study investigated retinal remodeling that occurs as photoreceptors degenerate. CRXRdy/+ cats from 6 weeks to 10 years of age were investigated. In vivo structural changes of retinas were analyzed by fundus examination, confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography. Histologic analyses included immunohistochemistry for computational molecular phenotyping with macromolecules and small molecules. Affected cats had a cone-led photoreceptor degeneration starting in the area centralis. Initially there was preservation of inner retinal cells such as bipolar, amacrine and horizontal cells but with time migration of the deafferented neurons occurred. Early in the process of degeneration glial activation occurs ultimately resulting in formation of a glial seal. With progression the macula-equivalent area centralis developed severe atrophy including loss of retinal pigmentary epithelium. Microneuroma formation occured in advanced stages as more marked retinal remodeling occurred. This study indicates that retinal degeneration in the CrxRdy/+ cat retina follows the progressive, phased revision of retina that have been previously described for retinal remodeling. These findings suggest that therapy dependent on targeting inner retinal cells may be useful in young adults with preserved inner retinas prior to advanced stages of retinal remodeling and neuronal cell loss.

Electronic Retinal Prostheses

Retinal prostheses are a promising means for restoring sight to patients blinded by photoreceptor atrophy. They introduce visual information by electrical stimulation of the surviving inner retinal neurons. Subretinal implants target the graded-response secondary neurons, primarily the bipolar cells, which then transfer the information to the ganglion cells via the retinal neural network. Therefore, many features of natural retinal signal processing can be preserved in this approach if the inner retinal network is retained. Epiretinal implants stimulate primarily the ganglion cells, and hence should encode the visual information in spiking patterns, which, ideally, should match the target cell types. Currently, subretinal arrays are being developed primarily for restoration of central vision in patients impaired by age-related macular degeneration (AMD), while epiretinal implants-for patients blinded by retinitis pigmentosa, where the inner retina is less preserved. This review describes the concepts and technologies, preclinical characterization of prosthetic vision and clinical outcomes, and provides a glimpse into future developments.

A Dhdds K42E knock-in RP59 mouse model shows inner retina pathology and defective synaptic transmission

Retinitis pigmentosa (RP) defines a group of hereditary progressive rod-cone degenerations that exhibit a common phenotype caused by variants in over 70 genes. While most variants in the dehydrodolichyl diphosphate synthase (DHDDS) gene result in syndromic abnormalities, some variants cause non-syndromic RP (RP59). DHDDS encodes one subunit of the enzyme cis-prenyltransferase (CPT), which is required for the synthesis of dolichol (Dol), that is a necessary protein glycosylation cofactor. We previously reported the creation and initial characterization of a knock-in (KI) mouse model harboring the most prevalent RP59-associated DHDDS variant (K42E) to understand how defects in DHDDS lead to retina-specific pathology. This model exhibited no profound retinal degeneration, nor protein N-glycosylation defects. Here, we report that the Dol isoprenylogue species in retina, liver, and brain of the K42E mouse model are statistically shorter than in the corresponding tissues of age-matched controls, as reported in blood and urine of RP59 patients. Retinal transcriptome analysis demonstrated elevation of many genes encoding proteins involved in synaptogenesis and synaptic function. Quantitative retinal cell layer thickness measurements demonstrated a significant reduction in the inner nuclear layer (INL) and total retinal thickness (TRT) beginning at postnatal (PN) ∼2 months, progressively increasing to PN 18-mo. Histological analysis revealed cell loss in the INL, outer plexiform layer (OPL) disruption, and ectopic localization of outer nuclear layer (ONL) nuclei into the OPL of K42E mutant retinas, relative to controls. Electroretinograms (ERGs) of mutant mice exhibited reduced b-wave amplitudes beginning at PN 1-mo, progressively declining through PN 18-mo, without appreciable a-wave attenuation, relative to controls. Our results suggest that the underlying cause of DHDDS K42E variant driven RP59 retinal pathology is defective synaptic transmission from outer to inner retina.

Probing the Contribution of Vertical Processing Layers of the Retina to White-Noise Electrical Stimulation Responses

Optimal stimulus parameters for epiretinal prostheses have been investigated by analyzing retinal ganglion cell (RGC) spiking responses to white-noise electrical stimulation, through a spike-triggered average (STA) analysis technique. However, it is currently unknown as to activation of which retinal cells contribute to features of the STA. We conducted whole-cell patch clamping recordings in ON and OFF RGCs in response to white-noise epiretinal electrical stimulation by using different inhibitors of synaptic transmission in a healthy retina. An mGluR6 agonist, L-AP4, was firstly used to selectively block the output of photoreceptors (PRs) to ON bipolar cells (BCs). We subsequently fully blocked all synaptic inputs to RGCs using a combination of pharmacological agents. Our data shows that PRs dominate the ability of ON RGCs to integrate electrical pulses and form a unique STA shape, while BCs do not contribute in any way. In addition, our results demonstrate that the ability of OFF RGCs to integrate pulses is consistently impaired after blocking the PR to ON BC pathway. We hypothesise that the mechanisms underlying this co-effect are related to the narrow field AII amacrine cells connecting ON and OFF pathways.Clinical Relevance-Recent retinal studies recorded mirror-inverted STAs in ON and OFF retinal pathways, thus raising the possibility of designing a stimulation approach that can differentially activate ON and OFF pathways with electrical stimulation. However, the detailed contribution of three major retinal cell layers in forming characteristic STAs is still unclear. It is of great clinical relevance to investigate the isolated contribution of PRs to the electrically driven STA since PRs progressively degenerate in the course of retinal disease.

SpikeSEE: An energy-efficient dynamic scenes processing framework for retinal prostheses

Intelligent and low-power retinal prostheses are highly demanded in this era, where wearable and implantable devices are used for numerous healthcare applications. In this paper, we propose an energy-efficient dynamic scenes processing framework (SpikeSEE) that combines a spike representation encoding technique and a bio-inspired spiking recurrent neural network (SRNN) model to achieve intelligent processing and extreme low-power computation for retinal prostheses. The spike representation encoding technique could interpret dynamic scenes with sparse spike trains, decreasing the data volume. The SRNN model, inspired by the human retina's special structure and spike processing method, is adopted to predict the response of ganglion cells to dynamic scenes. Experimental results show that the Pearson correlation coefficient of the proposed SRNN model achieves 0.93, which outperforms the state-of-the-art processing framework for retinal prostheses. Thanks to the spike representation and SRNN processing, the model can extract visual features in a multiplication-free fashion. The framework achieves 8 times power reduction compared with the convolutional recurrent neural network (CRNN) processing-based framework. Our proposed SpikeSEE predicts the response of ganglion cells more accurately with lower energy consumption, which alleviates the precision and power issues of retinal prostheses and provides a potential solution for wearable or implantable prostheses.

Bioengineering strategies for restoring vision

Late-stage retinal degenerative disease involving photoreceptor loss can be treated by optogenetic therapy, cell transplantation and retinal prostheses. These approaches aim to restore light sensitivity to the retina as well as visual perception by integrating neuronal responses for transmission to the cortex. In age-related macular degeneration, some cell-based therapies also aim to restore photoreceptor-supporting tissue to prevent complete photoreceptor loss. In the earlier stages of degeneration, gene-replacement therapy could attenuate retinal-disease progression and reverse loss of function. And gene-editing strategies aim to correct the underlying genetic defects. In this Review, we highlight the most promising gene therapies, cell therapies and retinal prostheses for the treatment of retinal disease, discuss the benefits and drawbacks of each treatment strategy and the factors influencing whether functional tissue is reconstructed and repaired or replaced with an electronic device, and summarize upcoming technologies for enhancing the restoration of vision.

Chromatin Remodeling Enzyme Snf2h Is Essential for Retinal Cell Proliferation and Photoreceptor Maintenance

Chromatin remodeling complexes are required for many distinct nuclear processes such as transcription, DNA replication, and DNA repair. However, the contribution of these complexes to the development of complex tissues within an organism is poorly characterized. Imitation switch (ISWI) proteins are among the most evolutionarily conserved ATP-dependent chromatin remodeling factors and are represented by yeast Isw1/Isw2, and their vertebrate counterparts Snf2h (Smarca5) and Snf2l (Smarca1). In this study, we focused on the role of the Snf2h gene during the development of the mammalian retina. We show that Snf2h is expressed in both retinal progenitors and post-mitotic retinal cells. Using Snf2h conditional knockout mice (Snf2h cKO), we found that when Snf2h is deleted, the laminar structure of the adult retina is not retained, the overall thickness of the retina is significantly reduced compared with controls, and the outer nuclear layer (ONL) is completely missing. The depletion of Snf2h did not influence the ability of retinal progenitors to generate all the differentiated retinal cell types. Instead, the Snf2h function is critical for the proliferation of retinal progenitor cells. Cells lacking Snf2h have a defective S-phase, leading to the entire cell division process impairments. Although all retinal cell types appear to be specified in the absence of the Snf2h function, cell-cycle defects and concomitantly increased apoptosis in Snf2h cKO result in abnormal retina lamination, complete destruction of the photoreceptor layer, and consequently, a physiologically non-functional retina.

Chromatin remodeling enzyme Snf2h is essential for retinal cell proliferation and photoreceptor maintenance

Chromatin remodeling complexes are required for many distinct nuclear processes such as transcription, DNA replication and DNA repair. However, how these complexes contribute to the development of complex tissues within an organism is poorly characterized. Imitation switch (ISWI) proteins are among the most evolutionarily conserved ATP-dependent chromatin remodeling factors and are represented by yeast Isw1/Isw2, and their vertebrate counterparts Snf2h (Smarca5) and Snf2l (Smarca1). In this study, we focused on the role of the Snf2h gene during development of the mammalian retina. We show that Snf2h is expressed in both retinal progenitors and post-mitotic retinal cells. Using Snf2h conditional knockout mice ( Snf2h cKO), we found that when Snf2h is deleted the laminar structure of the adult retina is not retained, the overall thickness of the retina is significantly reduced compared with controls, and the outer nuclear layer (ONL) is completely missing. Depletion of Snf2h did not influence the ability of retinal progenitors to generate all of the differentiated retinal cell types. Instead, Snf2h function is critical for proliferation of retinal progenitor cells. Cells lacking Snf2h have a defective S-phase, leading to the entire cell division process impairments. Although, all retinal cell types appear to be specified in the absence of Snf2h function, cell cycle defects and concomitantly increased apoptosis in Snf2h cKO result in abnormal retina lamination, complete destruction of the photoreceptor layer and, consequently, in a physiologically non-functional retina.

Outer retinal degeneration in a non-human primate model using temporary intravitreal tamponade with N-methyl-N-nitrosourea in cynomolgus monkeys

Objective:The main objective of this study was to induce and evaluate drug-dose-dependent outer retinal degeneration in cynomolgus monkeys by application of N-methyl-N-nitrosourea (MNU).Approach:Intravitreal temporary tamponade induced outer retinal degeneration with MNU solutions (2-3 mg ml^-1) after vitrectomy in five cynomolgus monkeys. Optical coherence tomography (OCT), fundus autofluorescence (FAF), full-field electroretinography (ffERG), and visual evoked potentials (VEP) were performed at baseline and weeks 2, 6, and 12 postoperatively. At week 12, OCT angiography, histology, and immunohistochemistry were performed.Main results:Outer retinal degeneration was observed in four monkeys, especially in the peripheral retina. Anatomical and functional changes occurred at week 2 and persisted until week 12. FAF images showed hypoautofluorescence dots, similar to AF patterns seen in human retinitis pigmentosa. Hyperautofluorescent lesions in the pericentral area were also observed, which corresponded to the loss of the ellipsoid zone on OCT images. OCT revealed thinning of the outer retinal layer adding to the loss of the ellipsoid zone outside the vascular arcade. Histological findings confirmed that the abovementioned changes resulted from a gradual loss of photoreceptors from the perifovea to the peripheral retina. In contrast, the inner retina, including ganglion cell layers, was preserved. Functionally, a decrease or extinction of scotopic ffERGs was observed, which indicated rod-dominant loss. Nevertheless, VEPs were relatively preserved.Significance:Therefore, we can conclude that temporary exposure to intravitreal MNU tamponade after vitrectomy induces rod-dominant outer retinal degeneration in cynomolgus monkeys, especially in the peripheral retina.

Loss of Müller cell glutamine synthetase immunoreactivity is associated with neuronal changes in late-stage retinal degeneration

Introduction

A hallmark of photoreceptor degenerations is progressive, aberrant remodeling of the surviving retinal neurons and glia following photoreceptor loss. The exact relationship between neurons and glia remodeling in this late stage of retinal degeneration, however, is unclear. This study assessed this by examining Müller cell dysfunction via glutamine synthetase immunoreactivity and its spatial association with retinal neuron subpopulations through various cell markers.

Methods

Aged Rd1 mice retinae (P150 - P536, n = minimum 5 per age) and control heterozygous rd1 mice retinae (P536, n = 5) were isolated, fixed and cryosectioned. Fluorescent immunolabeling of glutamine synthetase was performed and retinal areas quantified as having low glutamine synthetase immunoreactivity if proportion of labeled pixels in an area was less than two standard deviations of the mean of the total retina. Other Müller cell markers such as Sox9 and Glial fibrillary acidic protein along with neuronal cell markers Calbindin, Calretinin, recoverin, Protein kinase C-α, Glutamic acid decarboxylase 67, and Islet-1 were then quantified within areas of low and normal synthetase immunoreactivity.

Results

Glutamine synthetase immunoreactivity was lost as a function of age in the rd1 mouse retina (P150 - P536). Immunoreactivity of other Müller cell markers, however, were unaffected suggesting Müller cells were still present in these low glutamine synthetase immunoreactive regions. Glutamine synthetase immunoreactivity loss affected specific neuronal populations: Type 2, Type 8 cone, and rod bipolar cells, as well as AII amacrine cells based on reduced recoverin, protein kinase Ca and parvalbumin immunoreactivity, respectively. The number of cell nuclei within regions of low glutamine synthetase immunoreactivity was also reduced suggesting possible neuronal loss rather than reduced cell marker immunoreactivity.

Conclusion

These findings further support a strong interplay between glia-neuronal alterations in late-stage degeneration and highlight a need for future studies and consideration in intervention development.

Current perspective on retinal remodeling: Implications for therapeutics

The retinal degenerative diseases retinitis pigmentosa and age-related macular degeneration are a leading cause of irreversible vision loss. Both present with progressive photoreceptor degeneration that is further complicated by processes of retinal remodeling. In this perspective, we discuss the current state of the field of retinal remodeling and its implications for vision-restoring therapeutics currently in development. Here, we discuss the challenges and pitfalls retinal remodeling poses for each therapeutic strategy under the premise that understanding the features of retinal remodeling in totality will provide a basic framework with which therapeutics can interface. Additionally, we discuss the potential for approaching therapeutics using a combined strategy of using diffusible molecules in tandem with other vision-restoring therapeutics. We end by discussing the potential of the retina and retinal remodeling as a model system for more broadly understanding the progression of neurodegeneration across the central nervous system.

Potential therapeutic strategies for photoreceptor degeneration: the path to restore vision

Photoreceptors (PRs), as the most abundant and light-sensing cells of the neuroretina, are responsible for converting light into electrical signals that can be interpreted by the brain. PR degeneration, including morphological and functional impairment of these cells, causes significant diminution of the retina's ability to detect light, with consequent loss of vision. Recent findings in ocular regenerative medicine have opened promising avenues to apply neuroprotective therapy, gene therapy, cell replacement therapy, and visual prostheses to the challenge of restoring vision. However, successful visual restoration in the clinical setting requires application of these therapeutic approaches at the appropriate stage of the retinal degeneration. In this review, firstly, we discuss the mechanisms of PR degeneration by focusing on the molecular mechanisms underlying cell death. Subsequently, innovations, recent developments, and promising treatments based on the stage of disorder progression are further explored. Then, the challenges to be addressed before implementation of these therapies in clinical practice are considered. Finally, potential solutions to overcome the current limitations of this growing research area are suggested. Overall, the majority of current treatment modalities are still at an early stage of development and require extensive additional studies, both pre-clinical and clinical, before full restoration of visual function in PR degeneration diseases can be realized.

Simulating the impact of photoreceptor loss and inner retinal network changes on electrical activity of the retina

Objective.A major reason for poor visual outcomes provided by existing retinal prostheses is the limited knowledge of the impact of photoreceptor loss on retinal remodelling and its subsequent impact on neural responses to electrical stimulation. Computational network models of the neural retina assist in the understanding of normal retinal function but can be also useful for investigating diseased retinal responses to electrical stimulation.Approach.We developed and validated a biophysically detailed discrete neuronal network model of the retina in the software package NEURON. The model includes rod and cone photoreceptors, ON and OFF bipolar cell pathways, amacrine and horizontal cells and finally, ON and OFF retinal ganglion cells with detailed network connectivity and neural intrinsic properties. By accurately controlling the network parameters, we simulated the impact of varying levels of degeneration on retinal electrical function.Main results.Our model was able to reproduce characteristic monophasic and biphasic oscillatory patterns seen in ON and OFF neurons during retinal degeneration (RD). Oscillatory activity occurred at 3 Hz with partial photoreceptor loss and at 6 Hz when all photoreceptor input to the retina was removed. Oscillations were found to gradually weaken, then disappear when synapses and gap junctions were destroyed in the inner retina. Without requiring any changes to intrinsic cellular properties of individual inner retinal neurons, our results suggest that changes in connectivity alone were sufficient to give rise to neural oscillations during photoreceptor degeneration, and significant network connectivity destruction in the inner retina terminated the oscillations.Significance.Our results provide a platform for further understanding physiological retinal changes with progressive photoreceptor and inner RD. Furthermore, our model can be used to guide future stimulation strategies for retinal prostheses to benefit patients at different stages of disease progression, particularly in the early and mid-stages of RD.

Quantitative characterisation of ipRGCs in retinal degeneration using a computation platform for extracting and reconstructing single neurons in 3D from a multi-colour labeled population

Light has a profound impact on mammalian physiology and behavior. Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin, rendering them sensitive to light, and are involved in both image-forming vision and non-image forming responses to light such as circadian photo-entrainment and the pupillary light reflex. Following outer photoreceptor degeneration, the death of rod and cone photoreceptors results in global re-modeling of the remnant neural retina. Although ipRGCs can continue signaling light information to the brain even in advanced stages of degeneration, it is unknown if all six morphologically distinct subtypes survive, or how their dendritic architecture may be affected. To answer these questions, we generated a computational platform-BRIAN (Brainbow Analysis of individual Neurons) to analyze Brainbow labeled tissues by allowing objective identification of voxels clusters in Principal Component Space, and their subsequent extraction to produce 3D images of single neurons suitable for analysis with existing tracing technology. We show that BRIAN can efficiently recreate single neurons or individual axonal projections from densely labeled tissue with sufficient anatomical resolution for subtype quantitative classification. We apply this tool to generate quantitative morphological information about ipRGCs in the degenerate retina including soma size, dendritic field size, dendritic complexity, and stratification. Using this information, we were able to identify cells whose characteristics match those reported for all six defined subtypes of ipRGC in the wildtype mouse retina (M1-M6), including the rare and complex M3 and M6 subtypes. This indicates that ipRGCs survive outer retinal degeneration with broadly normal morphology. We additionally describe one cell in the degenerate retina which matches the description of the Gigantic M1 cell in Humans which has not been previously identified in rodent.

Bipolar cell targeted optogenetic gene therapy restores parallel retinal signaling and high-level vision in the degenerated retina

Optogenetic gene therapies to restore vision are in clinical trials. Whilst current clinical approaches target the ganglion cells, the output neurons of the retina, new molecular tools enable efficient targeting of the first order retinal interneurons, the bipolar cells, with the potential to restore a higher quality of vision. Here we investigate retinal signaling and behavioral vision in blind mice treated with bipolar cell targeted optogenetic gene therapies. All tested tools, including medium-wave opsin, Opto-mGluR6, and two new melanopsin based chimeras restored visual acuity and contrast sensitivity. The best performing opsin was a melanopsin-mGluR6 chimera, which in some cases restored visual acuities and contrast sensitivities that match wild-type animals. Light responses from the ganglion cells were robust with diverse receptive-field types, inferring elaborate inner retinal signaling. Our results highlight the potential of bipolar cell targeted optogenetics to recover high-level vision in human patients with end-stage retinal degenerations.

A chimeric Mela(CTmGluR6) optogenetic tool has the potential to restore vision and signaling in a mouse model of degenerative retinal disease.

Glial remodeling and choroidal vascular pathology in eyes from two donors with Choroideremia

Choroideremia (CHM) is a recessive, X-linked disease that affects 1 in 50,000 people worldwide. CHM causes night blindness in teenage years with vision loss progressing over the next two to three decades. While CHM is known to cause progressive loss of retinal pigment epithelial (RPE) cells, photoreceptors and choroidal vessels, little attention has been given to retinal glial changes in eyes with CHM. In addition, while choroidal loss has been observed clinically, no histopathologic assessment of choroidal loss has been done. We investigated glial remodeling and activation as well as choriocapillaris changes and their association with RPE loss in postmortem eyes from two donors with CHM. Eyes were fixed and cryopreserved or the retina and choroid/RPE were processed as flatmounts with a small piece cut for transmission electron microscopy. A dense glial membrane, made up of vimentin and GFAP double-positive cells, occupied the subretinal space in the area of RPE and photoreceptor loss of both eyes. The membranes did not extend into the far periphery, where RPE and photoreceptors were viable. A glial membrane was also found on the vitreoretinal surface. Transmission electron microscopy analysis demonstrated prominence and disorganization of glial cells, which contained exosome-like vesicles. UEA lectin demonstrated complete absence of choriocapillaris in areas with RPE loss while some large choroidal vessels remained viable. In the far periphery, where the RPE monolayer was intact, choriocapillaris appeared normal. The extensive glial remodeling present in eyes with CHM should be taken into account when therapies such as stem cell replacement are considered as it could impede cells entering the retina. This gliosis would also need to be reversed to some extent for Müller cells to perform their normal homeostatic functions in the retina. Future studies investigating donor eyes as well as clinical imaging from carriers or those with earlier stages of CHM will prove valuable in understanding the glial changes, which could affect disease progression if they occur early. This would also provide insights into the progression of disease in the photoreceptor/RPE/choriocapillaris complex, which is crucial for identifying new treatments and finding the windows for treatment.

Implications of Neural Plasticity in Retinal Prosthesis

Retinal degenerative diseases such as retinitis pigmentosa cause a progressive loss of photoreceptors that eventually prevents the affected person from perceiving visual sensations. The absence of a visual input produces a neural rewiring cascade that propagates along the visual system. This remodeling occurs first within the retina. Then, subsequent neuroplastic changes take place at higher visual centers in the brain, produced by either the abnormal neural encoding of the visual inputs delivered by the diseased retina or as the result of an adaptation to visual deprivation. While retinal implants can activate the surviving retinal neurons by delivering electric current, the unselective activation patterns of the different neural populations that exist in the retinal layers differ substantially from those in physiologic vision. Therefore, artificially induced neural patterns are being delivered to a brain that has already undergone important neural reconnections. Whether or not the modulation of this neural rewiring can improve the performance for retinal prostheses remains a critical question whose answer may be the enabler of improved functional artificial vision and more personalized neurorehabilitation strategies.

Cellular and molecular alterations in neurons and glial cells in inherited retinal degeneration

Multiple gene mutations have been associated with inherited retinal dystrophies (IRDs). Despite the spectrum of phenotypes caused by the distinct mutations, IRDs display common physiopathology features. Cell death is accompanied by inflammation and oxidative stress. The vertebrate retina has several attributes that make this tissue vulnerable to oxidative and nitrosative imbalance. The high energy demands and active metabolism in retinal cells, as well as their continuous exposure to high oxygen levels and light-induced stress, reveal the importance of tightly regulated homeostatic processes to maintain retinal function, which are compromised in pathological conditions. In addition, the subsequent microglial activation and gliosis, which triggers the secretion of pro-inflammatory cytokines, chemokines, trophic factors, and other molecules, further worsen the degenerative process. As the disease evolves, retinal cells change their morphology and function. In disease stages where photoreceptors are lost, the remaining neurons of the retina to preserve their function seek out for new synaptic partners, which leads to a cascade of morphological alterations in retinal cells that results in a complete remodeling of the tissue. In this review, we describe important molecular and morphological changes in retinal cells that occur in response to oxidative stress and the inflammatory processes underlying IRDs.

Robust cone-mediated signaling persists late into rod photoreceptor degeneration

Rod photoreceptor degeneration causes deterioration in the morphology and physiology of cone photoreceptors along with changes in retinal circuits. These changes could diminish visual signaling at cone-mediated light levels, thereby limiting the efficacy of treatments such as gene therapy for rescuing normal, cone-mediated vision. However, the impact of progressive rod death on cone-mediated signaling remains unclear. To investigate the fidelity of retinal ganglion cell (RGC) signaling throughout disease progression, we used a mouse model of rod degeneration (Cngb1neo/neo). Despite clear deterioration of cone morphology with rod death, cone-mediated signaling among RGCs remained surprisingly robust: spatiotemporal receptive fields changed little and the mutual information between stimuli and spiking responses was relatively constant. This relative stability held until nearly all rods had died and cones had completely lost well-formed outer segments. Interestingly, RGC information rates were higher and more stable for natural movies than checkerboard noise as degeneration progressed. The main change in RGC responses with photoreceptor degeneration was a decrease in response gain. These results suggest that gene therapies for rod degenerative diseases are likely to prolong cone-mediated vision even if there are changes to cone morphology and density.

Preferential modulation of individual retinal ganglion cells by electrical stimulation

OBJECTIVE

Retinal prostheses have been able to recover partial vision in blind patients with retinal degeneration by electrically stimulating surviving cells in the retina, such as retinal ganglion cells (RGCs), but the restored vision is limited. This is partly due to non-preferential stimulation of all RGCs near a single stimulating electrode, which include cells that conflict in their response properties and their contribution to the vision process. Our study proposes a stimulation strategy to preferentially stimulate individual RGCs based on their temporal electrical receptive fields (tERFs).

APPROACH

We recorded the responses of RGCs using whole-cell current-clamp and demonstrated the stimulation strategy, first using intracellular stimulation, then via extracellular stimulation.

MAIN RESULTS

We successfully reconstructed the tERFs according to the RGC response to Gaussian white noise current stimulation. The characteristics of the tERFs were extracted and compared according to the morphological and light response types of the cells. By re-delivering stimulation trains that are composed of the tERFs obtained from different cells, we could target individual RGCs as the cells showed lower activation thresholds to their own tERFs.

SIGNIFICANCE

This proposed stimulation strategy implemented in the next generation of recording and stimulating retinal prostheses may improve the quality of artificial vision.

A systematic comparison of optogenetic approaches to visual restoration

During inherited retinal degenerations (IRDs), vision is lost due to photoreceptor cell death; however, a range of optogenetic tools have been shown to restore light responses in animal models. Restored response characteristics vary between tools and the neuronal cell population to which they are delivered: the interplay between these is complex, but targeting upstream neurons (such as retinal bipolar cells) may provide functional benefit by retaining intraretinal signal processing. In this study, our aim was to compare two optogenetic tools: mammalian melanopsin (hOPN4) and microbial red-shifted channelrhodopsin (ReaChR) expressed within two subpopulations of surviving cells in a degenerate retina. Intravitreal adeno-associated viral vectors and mouse models utilising the Cre/lox system restricted expression to populations dominated by bipolar cells or retinal ganglion cells and was compared with non-targeted delivery using the chicken beta actin (CBA) promoter. In summary, we found bipolar-targeted optogenetic tools produced faster kinetics and flatter intensity-response relationships compared with non-targeted or retinal-ganglion-cell-targeted hOPN4. Hence, optogenetic tools of both mammalian and microbial origins show advantages when targeted to bipolar cells. This demonstrates the advantage of bipolar-cell-targeted optogenetics for vision restoration in IRDs. We therefore developed a bipolar-cell-specific gene delivery system employing a compressed promoter with the potential for clinical translation.

The role of motor proteins in photoreceptor protein transport and visual function

BACKGROUND

Rods and cones are photoreceptor neurons in the retina that are required for visual sensation in vertebrates, wherein the perception of vision is initiated when these neurons respond to photons in the light stimuli. The photoreceptor cell is structurally studied as outer segments (OS) and inner segments (IS) where proper protein sorting, localization, and compartmentalization are critical for phototransduction, visual function, and survival. In human retinal diseases, improper protein transport to the OS or mislocalization of proteins to the IS and other cellular compartments could lead to impaired visual responses and photoreceptor cell degeneration that ultimately cause loss of visual function.

RESULTS

Therefore, studying and identifying mechanisms involved in facilitating and maintaining proper protein transport in photoreceptor cells would help our understanding of pathologies involving retinal cell degeneration in inherited retinal dystrophies, age-related macular degeneration, and Usher Syndrome.

CONCLUSIONS

Our mini-review will discuss mechanisms of protein transport within photoreceptors and introduce a novel role for an unconventional motor protein, MYO1C, in actin-based motor transport of the visual chromophore Rhodopsin to the OS, in support of phototransduction and visual function.

Retinal Cell Transplantation, Biomaterials, and In Vitro Models for Developing Next-generation Therapies of Age-related Macular Degeneration

Retinal pigment epithelium (RPE) cells grown on a scaffold, an RPE patch, have potential to ameliorate visual impairment in a limited number of retinal degenerative conditions. This tissue-replacement therapy is suited for age-related macular degeneration (AMD), and related diseases. RPE cells must be transplanted before the disease reaches a point of no return, represented by the loss of photoreceptors. Photoreceptors are specialized, terminally differentiated neurosensory cells that must interact with RPE's apical processes to be functional. Human photoreceptors are not known to regenerate. On the RPE's basal side, the RPE transplant must induce the reformation of the choriocapillaris, thereby re-establishing the outer blood-retinal barrier. Because the scaffold is positioned between the RPE and choriocapillaris, it should ideally degrade and be replaced by the natural extracellular matrix that separates these tissues. Besides biodegradable, the scaffolds need to be nontoxic, thin enough to not affect the focal length of the eye, strong enough to survive the transplant procedure, yet flexible enough to conform to the curvature of the retina. The challenge is patients with progressing AMD treasure their remaining vision and fear that a risky surgical procedure will further degrade their vision. Accordingly, clinical trials only treat eyes with severe impairment that have few photoreceptors to interact with the transplanted patch. Although safety has been demonstrated, the cell-replacement mechanism and efficacy remain difficult to validate. This review covers the structure of the retina, the pathology of AMD, the limitations of cell therapy approaches, and the recent progress in developing retinal therapies using biomaterials.

Retinal Organoids and Retinal Prostheses: An Overview

Despite the progress of modern medicine in the last decades, millions of people diagnosed with retinal dystrophies (RDs), such as retinitis pigmentosa, or age-related diseases, such as age-related macular degeneration, are suffering from severe visual impairment or even legal blindness. On the one hand, the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and the progress of three-dimensional (3D) retinal organoids (ROs) technology provide a great opportunity to study, understand, and even treat retinal diseases. On the other hand, research advances in the field of electronic retinal prosthesis using inorganic photovoltaic polymers and the emergence of organic semiconductors represent an encouraging therapeutical strategy to restore vision to patients at the late onset of the disease. This review will provide an overview of the latest advancement in both fields. We first describe the retina and the photoreceptors, briefly mention the most used RD animal models, then focus on the latest RO differentiation protocols, carry out an overview of the current technology on inorganic and organic retinal prostheses to restore vision, and finally summarize the potential utility and applications of ROs.

The Visual Cortical Responses to Sinusoidal Transcorneal Electrical Stimulation

Retinal stimulation has become a widely utilized approach to restore visual function for individuals with retinal degenerative diseases. Although the rectangular electrical pulse is the primary stimulus waveform used in retinal neuromodulation, it remains unclear whether alternate waveforms may be more effective. Here, we used the optical intrinsic signal imaging system to assess the responses of cats' visual cortex to sinusoidal electrical stimulation through contact lens electrode, analyzing the response to various stimulus parameters (frequency, intensity, pulse width). A comparison between sinusoidal and rectangular stimulus waveform was also investigated. The results indicated that the optimal stimulation frequency for sinusoidal electrical stimulation was approximately 20 Hz, supporting the hypothesis that low-frequency electrostimulation induces more responsiveness in retinal neurons than high-frequency electrostimulation in case of sinusoidal stimulation. We also demonstrated that for low-frequency retinal neuromodulation, sinusoidal pulses are more effective than rectangular ones. In addition, we found that compared to current intensity, the effect of the sinusoidal pulse width on cortical responses was more prominent. These results suggested that sinusoidal electrical stimulation may provide a promising strategy for improved retinal neuromodulation in clinical settings.

Late-stage rescue of visually guided behavior in the context of a significantly remodeled retinitis pigmentosa mouse model

Patients with progressive neurodegenerative disorder retinitis pigmentosa (RP) are diagnosed in the midst of ongoing retinal degeneration and remodeling. Here, we used a Pde6b-deficient RP gene therapy mouse model to test whether treatment at late disease stages can halt photoreceptor degeneration and degradative remodeling, while sustaining constructive remodeling and restoring function. We demonstrated that when fewer than 13% of rods remain, our genetic rescue halts photoreceptor degeneration, electroretinography (ERG) functional decline and inner retinal remodeling. In addition, in a water maze test, the performance of mice treated at 16 weeks of age or earlier was indistinguishable from wild type. In contrast, no efficacy was apparent in mice treated at 24 weeks of age, suggesting the photoreceptors had reached a point of no return. Further, remodeling in the retinal pigment epithelium (RPE) and retinal vasculature was not halted at 16 or 24 weeks of age, although there appeared to be some slowing of blood vessel degradation. These data suggest a novel working model in which restoration of clinically significant visual function requires only modest threshold numbers of resilient photoreceptors, halting of destructive remodeling and sustained constructive remodeling. These novel findings define the potential and limitations of RP treatment and suggest possible nonphotoreceptor targets for gene therapy optimization.

Comparison of the qualitative and quantitative optical coherence tomographic features between sudden acquired retinal degeneration syndrome and normal eyes in dogs

OBJECTIVE

To quantitatively and qualitatively characterize the retinal optical coherence tomographic features of sudden acquired retinal degeneration syndrome (SARDS) and SARDS suspect dogs.

ANIMALS STUDIED

Fourteen SARDS affected dogs, 11 age-, breed-, and sex-matched control dogs, and two SARDS suspect dogs.

PROCEDURES

Spectral-domain optical coherence tomography (OCT) images were used to evaluate the quantitative features, including thickness, intereye asymmetry, and longitudinal changes in retinal layer thickness and the qualitative features, including retinal architecture and vitreous haze.

RESULTS

Mean outer retinal layer thickness (ORT), outer nuclear layer thickness (ONL), and photoreceptor layer thickness (PRL) were significantly lower in the SARDS group, whereas mean inner retinal layer thickness was significantly higher in the SARDS group than in the control group. While thickness values of all retinal layers did not differ significantly between paired eyes in each group, the absolute intereye asymmetries in the ORT (p < .0001), ONL (p = .008), and PRL (p < .0001) were significantly higher in the SARDS group than in the control group. Some SARDS patients and SARDS suspects had a greater PRL than the control group, and serial OCT evaluation showed an increase in PRL in one SARDS suspect. Vitreous haze severity was greater in the SARDS group than in the control group (vitreous relative intensity, p = .030).

CONCLUSIONS

We described the OCT features of SARDS patients and suspects. In particular, PRL thickening in the SARDS suspects might indicate an early change in SARDS. Although further studies are needed, this finding might provide new insights into the pathogenesis of SARDS.

Retinal damage related to high-intensity light-emitting diode exposure: An in vivo study

INTRODUCTION

The objective of this investigation was to evaluate the effects of high-intensity light-emitting diode (LED) light from a curing device on the retinas of Wistar rats.

METHODS

Six male Wistar rats were used, and their ocular structures were the focus of this study. During the photostimulation of each animal, the right eye of the animal, considered the control sample, was covered with a removable polyvinyl chloride cap, and the contralateral eye, the experimental sample, was exposed to high-intensity LED light, 3200 mW/cm² (VALO Ortho; Ultradent Products, South Jordan, Utah) for 144 seconds from a distance of 30 cm. The animals were exposed to the LED light 3 times on the same day to investigate if any acute inflammatory changes in the retina occurred. Seven days after the photostimulation sessions, the animals were anesthetized and perfused with paraformaldehyde solution. After which, the eyes were resected and processed histologically. The histologic sections were analyzed stereologically and histomorphometrically to measure the parameters of the retina under investigation.

RESULTS

There was a statistically significant increase in total retinal volume in the experimental group because of the increased volume of the ganglion cell layers, inner plexiform layers, outer nuclear layers, and the cone and rod extensions. There was no statistically significant difference in terms of density. However, there was a statistically significant increase in the nuclear area of the cells in all the studied layers in the group exposed to high-intensity LED light. In addition, hyperchromatic cells that are suggestive of pyknosis were observed.

CONCLUSIONS

An acute but short protocol of exposure of high-intensity LED light to the eye caused morphometric alterations in the retinal structures, specifically in the nuclear area of the photosensitive cells.

Characterization and staging of outer plexiform layer development in human retina and retinal organoids

The development of the first synapse of the visual system between photoreceptors and bipolar cells in the outer plexiform layer (OPL) of the human retina is critical for visual processing but poorly understood. By studying the maturation state and spatial organization of photoreceptors, depolarizing bipolar cells, and horizontal cells in the human fetal retina, we establish a pseudo-temporal staging system for OPL development that we term OPL-Stages 0 to 4. This was validated through quantification of increasingly precise subcellular localization of Bassoon to the OPL with each stage (p<0.0001). By applying these OPL staging criteria to human retinal organoids (HROs) derived from human embryonic and induced pluripotent stem cells, we observed comparable maturation from OPL-Stage 0 at day 100 in culture up to OPL-Stage 3 by day 160. Quantification of presynaptic protein localization confirmed progression from OPL-Stage 0 to 3 (p<0.0001). Overall, this study defines stages of human OPL development through mid-gestation and establishes HROs as a model system that recapitulates key aspects of human photoreceptor-bipolar cell synaptogenesis in vitro.

A Computational Model of Phosphene Appearance for Epiretinal Prostheses

Retinal neuroprostheses are the only FDA-approved treatment option for blinding degenerative diseases. A major outstanding challenge is to develop a computational model that can accurately predict the elicited visual percepts (phosphenes) across a wide range of electrical stimuli. Here we present a phenomenological model that predicts phosphene appearance as a function of stimulus amplitude, frequency, and pulse duration. The model uses a simulated map of nerve fiber bundles in the retina to produce phosphenes with accurate brightness, size, orientation, and elongation. We validate the model on psychophysical data from two independent studies, showing that it generalizes well to new data, even with different stimuli and on different electrodes. Whereas previous models focused on either spatial or temporal aspects of the elicited phosphenes in isolation, we describe a more comprehensive approach that is able to account for many reported visual effects. The model is designed to be flexible and extensible, and can be fit to data from a specific user. Overall this work is an important first step towards predicting visual outcomes in retinal prosthesis users across a wide range of stimuli.

Functional Availability of ON-Bipolar Cells in the Degenerated Retina: Timing and Longevity of an Optogenetic Gene Therapy

Degenerative diseases of the retina are responsible for the death of photoreceptors and subsequent loss of vision in patients. Nevertheless, the inner retinal layers remain intact over an extended period of time, enabling the restoration of light sensitivity in blind retinas via the expression of optogenetic tools in the remaining retinal cells. The chimeric Opto-mGluR6 protein represents such a tool. With exclusive ON-bipolar cell expression, it combines the light-sensitive domains of melanopsin and the intracellular domains of the metabotropic glutamate receptor 6 (mGluR6), which naturally mediates light responses in these cells. Albeit vision restoration in blind mice by Opto-mGluR6 delivery was previously shown, much is left to be explored in regard to the effects of the timing of the treatment in the degenerated retina. We performed a functional evaluation of Opto-mGluR6-treated murine blind retinas using multi-electrode arrays (MEAs) and observed long-term functional preservation in the treated retinas, as well as successful therapeutical intervention in later stages of degeneration. Moreover, the treatment decreased the inherent retinal hyperactivity of the degenerated retinas to levels undistinguishable from healthy controls. Finally, we observed for the first time micro electroretinograms (mERGs) in optogenetically treated animals, corroborating the origin of Opto-mGluR6 signalling at the level of mGluR6 of ON-bipolar cells.

Genetically engineered stem cell-derived retinal grafts for improved retinal reconstruction after transplantation

ESC/iPSC-retinal sheet transplantation, which supplies photoreceptors as well as other retinal cells, has been shown to be able to restore visual function in mice with end-stage retinal degeneration. Here, by introducing a novel type of genetically engineered mouse ESC/iPSC-retinal sheet with reduced numbers of secondary retinal neurons but intact photoreceptor cell layer structure, we reinforced the evidence that ESC/iPSC-retinal sheet transplantation can establish synaptic connections with the host, restore light responsiveness, and reduce aberrant retinal ganglion cell spiking in mice. Furthermore, we show that genetically engineered grafts can substantially improve the outcome of the treatment by improving neural integration. We speculate that this leads to reduced spontaneous activity in the host which in turn contributes to a better visual recovery.

Outer Retinal Cell Replacement: Putting the Pieces Together

Retinal degenerative diseases (RDDs) affecting photoreceptors (PRs) are one of the most prevalent sources of incurable blindness worldwide. Due to a lack of endogenous repair mechanisms, functional cell replacement of PRs and/or retinal pigmented epithelium (RPE) cells are among the most anticipated approaches for restoring vision in advanced RDD. Human pluripotent stem cell (hPSC) technologies have accelerated development of outer retinal cell therapies as they provide a theoretically unlimited source of donor cells. Human PSC-RPE replacement therapies have progressed rapidly, with several completed and ongoing clinical trials. Although potentially more promising, hPSC-PR replacement therapies are still in their infancy. A first-in-human trial of hPSC-derived neuroretinal transplantation has recently begun, but a number of questions regarding survival, reproducibility, functional integration, and mechanism of action remain. The discovery of biomaterial transfer between donor and PR cells has highlighted the need for rigorous safety and efficacy studies of PR replacement. In this review, we briefly discuss the history of neuroretinal and PR cell transplantation to identify remaining challenges and outline a stepwise approach to address specific pieces of the outer retinal cell replacement puzzle.

Analysis of imaging biomarkers and retinal nerve fiber layer thickness in RPGR-associated retinitis pigmentosa

Purpose

To investigate multimodal retinal imaging characteristics including the retinal nerve fiber layer (RNFL) thickness in patients with RPGR-associated retinitis pigmentosa (RP).

Methods

This cross-sectional case–control study included 17 consecutive patients (median age, 21 years) with RPGR-associated RP who underwent retinal imaging including optical coherence tomography (OCT), short-wavelength fundus autofluorescence (AF) imaging, and RNFL scans centered on the optic disc. RNFL thickness was manually segmented and compared to clinical and imaging parameters including the transfoveal ellipsoid zone (EZ) width, the horizontal diameter of the macular hyperautofluorescent ring. RNFL thickness was compared to 17 age- and sex-matched controls.

Results

In patients with RPGR-associated RP, the EZ width (R² = 0.65), the central hyperautofluorescent ring on AF images (R² = 0.72), and visual acuity (R² = 0.68) were negatively correlated with age. In comparison to controls, a significantly (p < 0.0001) increased global RNFL thickness was identified in RPGR-associated RP, which was, however, less pronounced in progressed disease as indicated by the EZ width or the diameter of the central hyperautofluorescent ring.

Conclusions

This study describes retinal characteristics in patients with RPGR-associated RP including a pronounced peripapillary RNFL thickness compared to healthy controls. These results contribute to the knowledge about imaging biomarkers in RP, which might be of interest for therapeutic approaches such as gene replacement therapies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00417-021-05233-w.

Matrix metalloproteinase-9 (MMP-9) and tissue inhibitor of metalloproteinases 1 (TIMP-1) are localized in the nucleus of retinal Müller glial cells and modulated by cytokines and oxidative stress

Matrix metalloproteinases (MMPs) are involved in the pathology of numerous inflammatory retinal degenerations, including retinitis pigmentosa (RP). Our previous work revealed that intravitreal injections with tissue inhibitor of metalloproteinases 1 (TIMP-1) reduce the progression of rod cell death and inhibit cone cell remodeling that involves reactive gliosis in retinal Müller glial cells (MGCs) in rodent models. The underlying cellular and molecular mechanisms of how TIMP-1 functions in the retina remain to be resolved; however, MGCs are involved in structural homeostasis, neuronal cell survival and death. In the present study, MMP-9 and TIMP-1 expression patterns were investigated in a human MGC line (MIO-M1) under inflammatory cytokine (IL-1β and TNF-α) and oxidative stress (H₂O₂) conditions. First, both IL-1β and TNF-α, but not H₂O_2, have a mild in vitro pro-survival effect on MIO-M1 cells. Treatment with either cytokine results in the imbalanced secretion of MMP-9 and TIMP-1. H₂O₂ treatment has little effect on their secretion. The investigation of their intracellular expression led to interesting observations. MMP-9 and TIMP-1 are both expressed, not only in the cytoplasm, but also inside the nucleus. None of the treatments alters the MMP-9 intracellular distribution pattern. In contrast to MMP-9, TIMP-1 is detected as speckles. Intracellular TIMP-1 aggregation forms in the cytoplasmic area with IL-1β treatment. With H₂O₂ treatments, the cell morphology changes from cobbles to spindle shapes and the nuclei become larger with increases in TIMP-1 speckles in an H₂O₂ dose-dependent manner. Two TIMP-1 cell surface receptors, low density lipoprotein receptor-related protein-1 (LRP-1) and cluster of differentiation 82 (CD82), are expressed within the nucleus of MIO-M1 cells. Overall, these observations suggest that intracellular TIMP-1 is a target of proinflammatory and oxidative insults in the MGCs. Given the importance of the roles for MGCs in the retina, the functional implication of nuclear TIMP-1 and MMP-9 in MGCs is discussed.

Monitoring Cortical Response and Electrode-Retina Impedance Under Epiretinal Stimulation in Rats

Retinal prosthesis can restore partial vision in patients with retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Epiretinal prosthesis is one of three therapeutic approaches, which received regulatory approval several years ago. The thresholds of an epiretinal stimulation is partly determined by the size of the physical gap between the electrode and the retina after implantation. Precise positioning of epiretinal stimulating electrode array is still a challenging task. In this study, we demonstrate an approach to positioning epiretinal prostheses for an optimal response at the cortical output by monitoring both the impedance at the electrode-retina interface and the evoked-potential at the cortical level. We implanted a single-channel electrode on the epiretinal surface in adult rats, acutely, guided by both the impedance at the electrode-retina interface and by electrically evoked potentials (EEPs) in the visual cortex during retinal stimulation. We observe that impedance monotonously increases with decreasing electrode-retina distance, but that the strongest cortical responses were achieved at intermediate impedance levels. When the electrode penetrates the retina, the impedance keeps increasing. The effect of stimulation on the retina changes from epiretinal paradigm to intra-retinal paradigm and a decrease in cortical activation is observed. It is found that high impedance is not always favorable to elicit best cortical responses. Histopathological results showed that the electrode was placed at the intra-retinal space at high impedance value. These results show that monitoring impedance at the electrode-retina interface is necessary but not sufficient in obtaining strong evoked-potentials at the cortical level. Monitoring the cortical EEPs together with the impedance can improve the safety of implantation as well as efficacy of stimulation in the next generation of retinal implants.

Classifying Retinal Degeneration in Histological Sections Using Deep Learning

Purpose

Artificial intelligence (AI) techniques are increasingly being used to classify retinal diseases. In this study we investigated the ability of a convolutional neural network (CNN) in categorizing histological images into different classes of retinal degeneration.

Methods

Images were obtained from a chemically induced feline model of monocular retinal dystrophy and split into training and testing sets. The training set was graded for the level of retinal degeneration and used to train various CNN architectures. The testing set was evaluated through the best architecture and graded by six observers. Comparisons between model and observer classifications, and interobserver variability were measured. Finally, the effects of using less training images or images containing half the presentable context were investigated.

Results

The best model gave weighted-F1 scores in the range 85% to 90%. Cohen kappa scores reached up to 0.86, indicating high agreement between the model and observers. Interobserver variability was consistent with the model-observer variability in the model's ability to match predictions with the observers. Image context restriction resulted in model performance reduction by up to 6% and at least one training set size resulted in a model performance reduction of 10% compared to the original size.

Conclusions

Detecting the presence and severity of up to three classes of retinal degeneration in histological data can be reliably achieved with a deep learning classifier.

Translational Relevance

This work lays the foundations for future AI models which could aid in the evaluation of more intricate changes occurring in retinal degeneration, particularly in other types of clinically derived image data.

Neuromodulation using electroosmosis

Objective.Our laboratory has proposed chemical stimulation of retinal neurons using exogenous glutamate as a biomimetic strategy for treating vision loss caused by photoreceptor (PR) degenerative diseases. Although our previousin-vitrostudies using pneumatic actuation indicate that chemical retinal stimulation is achievable, an actuation technology that is amenable to microfabrication, as needed for anin-vivoimplantable device, has yet to be realized. In this study, we sought to evaluate electroosmotic flow (EOF) as a mechanism for delivering small quantities of glutamate to the retina. EOF has great potential for miniaturization.Approach.An EOF device to dispense small quantities of glutamate was constructed and its ability to drive retinal output tested in anin-vitropreparation of PR degenerate rat retina.Main results.We built and tested an EOF microfluidic system, with 3D printed and off-the-shelf components, capable of injecting small volumes of glutamate in a pulsatile fashion when a low voltage control signal was applied. With this device, we produced excitatory and inhibitory spike rate responses in PR degenerate rat retinae. Glutamate evoked spike rate responses were also observed to be voltage-dependent and localized to the site of injection.Significance.The EOF device performed similarly to a previously tested conventional pneumatic microinjector as a means of chemically stimulating the retina while eliminating the moving plunger of the pneumatic microinjector that would be difficult to miniaturize and parallelize. Although not implantable, the prototype device presented here as a proof of concept indicates that a retinal prosthetic based on EOF-driven chemical stimulation is a viable and worthwhile goal. EOF should have similar advantages for controlled dispensing of charged neurochemicals at any neural interface.