Retinal Ganglion Cell Replacement: Current Status and Challenges Ahead

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Retinal organoid light responsivity: current status and future opportunities

The ability to generate human retinas in vitro from pluripotent stem cells opened unprecedented opportunities for basic science and for the development of therapeutic approaches for retinal degenerative diseases. Retinal organoid models not only mimic the histoarchitecture and cellular composition of the native retina, but they can achieve a remarkable level of maturation that allows them to respond to light stimulation. However, studies evaluating the nature, magnitude, and properties of light-evoked responsivity from each cell type, in each retinal organoid layer, have been sparse. In this review we discuss the current understanding of retinal organoid function, the technologies used for functional assessment in human retinal organoids, and the challenges and opportunities that lie ahead.

Mechanism of LSD1 in oxygen-glucose deprivation/reoxygenation-induced pyroptosis of retinal ganglion cells via the miR-21-5p/NLRP12 axis

Background

Retinal ganglion cells (RGCs) are important retinal neurons that connect visual receptors to the brain, and lysine-specific demethylase 1 (LSD1) is implicated in the development of RGCs. This study expounded the mechanism of LSD1 in oxygen-glucose deprivation/reoxygenation (OGD/R)-induced pyroptosis of RGCs.

Methods

Mouse RGCs underwent OGD/R exposure, and then RGC viability was examined using the cell counting kit-8 method. The mRNA levels of Caspase 1, the protein levels of NOD-like receptor family pyrin domain-containing 3 (NLRP3), N-terminal fragment of gasdermin D (GSDMD-N), and cleaved-Caspase1, and the concentrations of interleukin (IL)-1β and IL-18 were respectively examined. Subsequently, LSD1 expression was intervened to explore the underlying effect of LSD1 on OGD/R-induced pyroptosis of RGCs. Afterwards, the enrichments of LSD1 and histone H3 lysine 4 methylation (H3K4me) 1/2 on the microRNA (miR)-21-5p promoter were determined using chromatin-immunoprecipitation assay. And the binding interaction between miR-21-5p and NLRP12 was detected using dual-luciferase and RNA pull-down assays. Finally, the effects of miR-21-5p/NLRP12 on LSD1-mediated pyroptosis of RGCs were verified through functional rescue experiments.

Results

OGD/R treatment increased pyroptosis of RGCs and LSD1 expression. Silencing LSD1 declined levels of Caspase 1 mRNA, NLRP3, GSDMD-N, cleaved-Caspase1, IL-1β, and IL-18 and limited pyroptosis of OGD/R-treated RGCs. Mechanically, LSD1 suppressed miR-21-5p expression via demethylation of H3K4me2 on the miR-21-5p promoter to hamper the binding of miR-21-5p to NLRP12, and thereby increased NLRP12 expression. Silencing miR-21-5p or overexpressing NLRP12 facilitated OGD/R-induced pyroptosis of RGCs.

Conclusion

LSD1-mediated demethylation of H3K4me2 decreased miR-21-5p expression to increase NLRP12 expression, promoting pyroptosis of OGD/R-treated RGCs.

Bioengineering Human Pluripotent Stem Cell-Derived Retinal Organoids and Optic Vesicle-Containing Brain Organoids for Ocular Diseases

Retinal organoids are three-dimensional (3D) structures derived from human pluripotent stem cells (hPSCs) that mimic the retina's spatial and temporal differentiation, making them useful as in vitro retinal development models. Retinal organoids can be assembled with brain organoids, the 3D self-assembled aggregates derived from hPSCs containing different cell types and cytoarchitectures that resemble the human embryonic brain. Recent studies have shown the development of optic cups in brain organoids. The cellular components of a developing optic vesicle-containing organoids include primitive corneal epithelial and lens-like cells, retinal pigment epithelia, retinal progenitor cells, axon-like projections, and electrically active neuronal networks. The importance of retinal organoids in ocular diseases such as age-related macular degeneration, Stargardt disease, retinitis pigmentosa, and diabetic retinopathy are described in this review. This review highlights current developments in retinal organoid techniques, and their applications in ocular conditions such as disease modeling, gene therapy, drug screening and development. In addition, recent advancements in utilizing extracellular vesicles secreted by retinal organoids for ocular disease treatments are summarized.

Ex Vivo Integration of Human Stem Retinal Ganglion Cells into the Mouse Retina

Cell replacement therapies may be key in achieving functional recovery in neurodegenerative optic neuropathies diseases such as glaucoma. One strategy that holds promise in this regard is the use of human embryonic stem cell and induced pluripotent stem-derived retinal ganglion cells (hRGCs). Previous hRGC transplantation studies have shown modest success. This is in part due to the low survival and integration of the transplanted cells in the host retina. The field is further challenged by mixed assays and outcome measurements that probe and determine transplantation success. Thefore, we have devised a transplantation assay involving hRGCs and mouse retina explants that bypasses physical barriers imposed by retinal membranes. We show that hRGC neurites and somas are capable of invading mouse explants with a subset of hRGC neurites being guided by mouse RGC axons. Neonatal mouse retina explants, and to a lesser extent, adult explants, promote hRGC integrity and neurite outgrowth. Using this assay, we tested whether suppmenting cultures with brain derived neurotrophic factor (BDNF) and the adenylate cyclase activator, forskolin, enhances hRGC neurite integration, neurite outgrowth, and integrity. We show that supplementing cultures with a combination BDNF and forskolin strongly favors hRGC integrity, increasing neurite outgrowth and complexity as well as the invasion of mouse explants. The transplantation assay presented here is a practical tool for investigating strategies for testing and optimizing the integration of donor cells into host tissues.

Ectopic insert-dependent neuronal expression of GFAP promoter-driven AAV constructs in adult mouse retina

Direct reprogramming of retinal Müller glia is a promising avenue for replacing photoreceptors and retinal ganglion cells lost to retinal dystrophies. However, questions have recently been raised about the accuracy of studies claiming efficient glia-to-neuron reprogramming in retina that were conducted using GFAP mini promoter-driven adeno-associated virus (AAV) vectors. In this study, we have addressed these questions using GFAP mini promoter-driven AAV constructs to simultaneously overexpress the mCherry reporter and candidate transcription factors predicted to induce glia-to-neuron conversion, in combination with prospective genetic labeling of retinal Müller glia using inducible Cre-dependent GFP reporters. We find that, while control GFAP-mCherry constructs express faithfully in Müller glia, 5 out of 7 transcription factor overexpression constructs tested are predominantly expressed in amacrine and retinal ganglion cells. These findings demonstrate strong insert-dependent effects on AAV-based GFAP mini promoter specificity that preclude its use in inferring cell lineage relationships when studying glia-to-neuron conversion in retina.

Regenerative and restorative medicine for eye disease

Causes of blindness differ across the globe; in higher-income countries, most blindness results from the degeneration of specific classes of cells in the retina, including retinal pigment epithelium (RPE), photoreceptors, and retinal ganglion cells. Advances over the past decade in retinal regenerative medicine have allowed each of these cell types to be produced ex vivo from progenitor stem cells. Here, we review progress in applying these technologies to cell replacement - with the goal of vision restoration in degenerative disease. We discuss the landscape of human clinical trials for RPE transplantation and advanced preclinical studies for other cell types. We also review progress toward in situ repair of retinal degeneration using endogenous progenitor cells. Finally, we provide a high-level overview of progress toward prosthetic ocular vision restoration, including advanced photovoltaic devices, opsin-based gene therapy, and small-molecule photoswitches. Progress in each of these domains is at or near the human clinical-trial stage, bringing the audacious goal of vision restoration within sight.

Solving neurodegeneration: common mechanisms and strategies for new treatments

Across neurodegenerative diseases, common mechanisms may reveal novel therapeutic targets based on neuronal protection, repair, or regeneration, independent of etiology or site of disease pathology. To address these mechanisms and discuss emerging treatments, in April, 2021, Glaucoma Research Foundation, BrightFocus Foundation, and the Melza M. and Frank Theodore Barr Foundation collaborated to bring together key opinion leaders and experts in the field of neurodegenerative disease for a virtual meeting titled "Solving Neurodegeneration". This "think-tank" style meeting focused on uncovering common mechanistic roots of neurodegenerative disease and promising targets for new treatments, catalyzed by the goal of finding new treatments for glaucoma, the world's leading cause of irreversible blindness and the common interest of the three hosting foundations. Glaucoma, which causes vision loss through degeneration of the optic nerve, likely shares early cellular and molecular events with other neurodegenerative diseases of the central nervous system. Here we discuss major areas of mechanistic overlap between neurodegenerative diseases of the central nervous system: neuroinflammation, bioenergetics and metabolism, genetic contributions, and neurovascular interactions. We summarize important discussion points with emphasis on the research areas that are most innovative and promising in the treatment of neurodegeneration yet require further development. The research that is highlighted provides unique opportunities for collaboration that will lead to efforts in preventing neurodegeneration and ultimately vision loss.

The interplay of autophagy and oxidative stress in the pathogenesis and therapy of retinal degenerative diseases

Oxidative stress is mainly caused by intracellular reactive oxygen species (ROS) production, which is highly associated with normal physiological homeostasis and the pathogenesis of diseases, particularly ocular diseases. Autophagy is a self-clearance pathway that removes oxidized cellular components and regulates cellular ROS levels. ROS can modulate autophagy activity through transcriptional and posttranslational mechanisms. Autophagy further triggers transcription factor activation and degrades impaired organelles and proteins to eliminate excessive ROS in cells. Thus, autophagy may play an antioxidant role in protecting ocular cells from oxidative stress. Nevertheless, excessive autophagy may cause autophagic cell death. In this review, we summarize the mechanisms of interaction between ROS and autophagy and their roles in the pathogenesis of several ocular diseases, including glaucoma, age-related macular degeneration (AMD), diabetic retinopathy (DR), and optic nerve atrophy, which are major causes of blindness. The autophagy modulators used to treat ocular diseases are further discussed. The findings of the studies reviewed here might shed light on the development and use of autophagy modulators for the future treatment of ocular diseases.

A bioinspired gelatin-hyaluronic acid-based hybrid interpenetrating network for the enhancement of retinal ganglion cells replacement therapy

Biomaterial-based cell replacement approaches to regenerative medicine are emerging as promising treatments for a wide array of profound clinical problems. Here we report an interpenetrating polymer network (IPN) composed of gelatin-hydroxyphenyl propionic acid and hyaluronic acid tyramine that is able to enhance intravitreal retinal cell therapy. By tuning our bioinspired hydrogel to mimic the vitreous chemical composition and mechanical characteristics we were able to improve in vitro and in vivo viability of human retinal ganglion cells (hRGC) incorporated into the IPN. In vivo vitreal injections of cell-bearing IPN in rats showed extensive attachment to the inner limiting membrane of the retina, improving with hydrogels stiffness. Engrafted hRGC displayed signs of regenerating processes along the optic nerve. Of note was the decrease in the immune cell response to hRGC delivered in the gel. The findings compel further translation of the gelatin-hyaluronic acid IPN for intravitreal cell therapy.

Gene regulatory networks controlling temporal patterning, neurogenesis, and cell-fate specification in mammalian retina

Gene regulatory networks (GRNs), consisting of transcription factors and their target sites, control neurogenesis and cell-fate specification in the developing central nervous system. In this study, we use integrated single-cell RNA and single-cell ATAC sequencing (scATAC-seq) analysis in developing mouse and human retina to identify multiple interconnected, evolutionarily conserved GRNs composed of cell-type-specific transcription factors that both activate genes within their own network and inhibit genes in other networks. These GRNs control temporal patterning in primary progenitors, regulate transition from primary to neurogenic progenitors, and drive specification of each major retinal cell type. We confirm that NFI transcription factors selectively activate expression of genes promoting late-stage temporal identity in primary retinal progenitors and identify other transcription factors that regulate rod photoreceptor specification in postnatal retina. This study inventories cis- and trans-acting factors that control retinal development and can guide cell-based therapies aimed at replacing retinal neurons lost to disease.

Cell-Based Neuroprotection of Retinal Ganglion Cells in Animal Models of Optic Neuropathies

Retinal ganglion cells (RGCs) comprise a heterogenous group of projection neurons that transmit visual information from the retina to the brain. Progressive degeneration of these cells, as it occurs in inflammatory, ischemic, traumatic or glaucomatous optic neuropathies, results in visual deterioration and is among the leading causes of irreversible blindness. Treatment options for these diseases are limited. Neuroprotective approaches aim to slow down and eventually halt the loss of ganglion cells in these disorders. In this review, we have summarized preclinical studies that have evaluated the efficacy of cell-based neuroprotective treatment strategies to rescue retinal ganglion cells from cell death. Intraocular transplantations of diverse genetically nonmodified cell types or cells engineered to overexpress neurotrophic factors have been demonstrated to result in significant attenuation of ganglion cell loss in animal models of different optic neuropathies. Cell-based combinatorial neuroprotective approaches represent a potential strategy to further increase the survival rates of retinal ganglion cells. However, data about the long-term impact of the different cell-based treatment strategies on retinal ganglion cell survival and detailed analyses of potential adverse effects of a sustained intraocular delivery of neurotrophic factors on retina structure and function are limited, making it difficult to assess their therapeutic potential.

Therapeutic effects of mesenchymal stem cells-derived extracellular vesicles' miRNAs on retinal regeneration: a review

Extracellular vesicles (EVs), which consist of microvesicles and exosomes, are secreted from all cells to transform vital information in the form of lipids, proteins, mRNAs and small RNAs such as microRNAs (miRNAs). Many studies demonstrated that EVs' miRNAs have effects on target cells. Numerous people suffer from the blindness caused by retinal degenerations. The death of retinal neurons is irreversible and creates permanent damage to the retina. In the absence of acceptable cures for retinal degenerative diseases, stem cells and their paracrine agents including EVs have become a promising therapeutic approach. Several studies showed that the therapeutic effects of stem cells are due to the miRNAs of their EVs. Considering the effects of microRNAs in retinal cells development and function and studies which provide the possible roles of mesenchymal stem cells-derived EVs miRNA content on retinal diseases, we focused on the similarities between these two groups of miRNAs that could be helpful for promoting new therapeutic techniques for retinal degenerative diseases.

Stem Cell-Based Regeneration and Restoration for Retinal Ganglion Cell: Recent Advancements and Current Challenges

Glaucoma is a group of irreversible blinding eye diseases characterized by the progressive loss of retinal ganglion cells (RGCs) and their axons. Currently, there is no effective method to fundamentally resolve the issue of RGC degeneration. Recent advances have revealed that visual function recovery could be achieved with stem cell-based therapy by replacing damaged RGCs with cell transplantation, providing nutritional factors for damaged RGCs, and supplying healthy mitochondria and other cellular components to exert neuroprotective effects and mediate transdifferentiation of autologous retinal stem cells to accomplish endogenous regeneration of RGC. This article reviews the recent research progress in the above-mentioned fields, including the breakthroughs in the fields of in vivo transdifferentiation of retinal endogenous stem cells and reversal of the RGC aging phenotype, and discusses the obstacles in the clinical translation of the stem cell therapy.

Retinal Organoids: Cultivation, Differentiation, and Transplantation

Retinal organoids (ROs), which are derived from stem cells, can automatically form three-dimensional laminar structures that include all cell types and the ultrastructure of the retina. Therefore, they are highly similar to the retinal structure in the human body. The development of organoids has been a great technological breakthrough in the fields of transplantation therapy and disease modeling. However, the translation of RO applications into medical practice still has various deficiencies at the current stage, including the long culture process, insufficient yield, and great heterogeneity among ROs produced under different conditions. Nevertheless, many technological breakthroughs have been made in transplanting ROs for treatment of diseases such as retinal degeneration. This review discusses recent advances in the development of ROs, improvements of the culture protocol, and the latest developments in RO replacement therapy techniques.

Exploiting hiPSCs in Leber's Hereditary Optic Neuropathy (LHON): Present Achievements and Future Perspectives

More than 30 years after discovering Leber's hereditary optic neuropathy (LHON) as the first maternally inherited disease associated with homoplasmic mtDNA mutations, we still struggle to achieve effective therapies. LHON is characterized by selective degeneration of retinal ganglion cells (RGCs) and is the most frequent mitochondrial disease, which leads young people to blindness, in particular males. Despite that causative mutations are present in all tissues, only a specific cell type is affected. Our deep understanding of the pathogenic mechanisms in LHON is hampered by the lack of appropriate models since investigations have been traditionally performed in non-neuronal cells. Effective in-vitro models of LHON are now emerging, casting promise to speed our understanding of pathophysiology and test therapeutic strategies to accelerate translation into clinic. We here review the potentials of these new models and their impact on the future of LHON patients.

Transplantation of miPSC/mESC-derived retinal ganglion cells into healthy and glaucomatous retinas

Optic neuropathies, including glaucoma, are a group of neurodegenerative diseases, characterized by the progressive loss of retinal ganglion cells (RGCs), leading to irreversible vision loss. While previous studies demonstrated the potential to replace RGCs with primary neurons from developing mouse retinas, their use is limited clinically. We demonstrate successful transplantation of mouse induced pluripotent stem cell (miPSC)/mouse embryonic stem cell (mESC)-derived RGCs into healthy and glaucomatous mouse retinas, at a success rate exceeding 65% and a donor cell survival window of up to 12 months. Transplanted Thy1-GFP+ RGCs were able to polarize within the host retina and formed axonal processes that followed host axons along the retinal surface and entered the optic nerve head. RNA sequencing of donor RGCs re-isolated from host retinas at 24 h and 1 week post-transplantation showed upregulation of cellular pathways mediating axonal outgrowth, extension, and guidance. Additionally, we provide evidence of subtype-specific diversity within miPSC-derived RGCs prior to transplantation.

Ubiquitous Chromatin Modifiers in Congenital Retinal Diseases: Implications for Disease Modeling and Regenerative Medicine

Retinal congenital malformations known as microphthalmia, anophthalmia, and coloboma (MAC) are associated with alterations in genes encoding epigenetic proteins that modify chromatin. We review newly discovered functions of such chromatin modifiers in retinal development and discuss the role of epigenetics in MAC in humans and animal models. Further, we highlight how advances in epigenomic technologies provide foundational and regenerative medicine-related insights into blinding disorders. Combining knowledge of epigenetics and pluripotent stem cells (PSCs) is a promising avenue because epigenetic factors cooperate with eye field transcription factors (EFTFs) to direct PSC fate - a foundation for congenital retinal disease modeling and cell therapy.

Role of the Internal Limiting Membrane in Structural Engraftment and Topographic Spacing of Transplanted Human Stem Cell-Derived Retinal Ganglion Cells

Retinal ganglion cell (RGC) replacement holds potential for restoring vision lost to optic neuropathy. Transplanted RGCs must undergo neuroretinal integration to receive afferent visual signals for processing and efferent transmission. To date, retinal integration following RGC transplantation has been limited. We sought to overcome key barriers to transplanted human stem cell-derived RGC integration. Following co-culture ex vivo on organotypic mouse retinal explants, human RGCs cluster and extend bundled neurites that remain superficial to the neuroretina, hindering afferent synaptogenesis. To enhance integration, we increased the cellular permeability of the internal limiting membrane (ILM). Extracellular matrix digestion using proteolytic enzymes achieved ILM disruption while minimizing retinal toxicity and preserving glial reactivity. ILM disruption is associated with dispersion rather than clustering of co-cultured RGC bodies and neurites, and increased parenchymal neurite ingrowth. The ILM represents a significant obstacle to transplanted RGC connectivity and its circumvention may be necessary for functional RGC replacement.

Organoids and organ chips in ophthalmology

Recent advances have driven the development of stem cell-derived, self-organizing, three-dimensional miniature organs, termed organoids, which mimic different eye tissues including the retina, cornea, and lens. Organoids and engineered microfluidic organ-on-chips (organ chips) are transformative technologies that show promise in simulating the architectural and functional complexity of native organs. Accordingly, they enable exploration of facets of human disease and development not accurately recapitulated by animal models. Together, these technologies will increase our understanding of the basic physiology of different eye structures, enable us to interrogate unknown aspects of ophthalmic disease pathogenesis, and serve as clinically-relevant surrogates for the evaluation of ocular therapeutics. Both the burden and prevalence of monogenic and multifactorial ophthalmic diseases, which can cause visual impairment or blindness, in the human population warrants a paradigm shift towards organoids and organ chips that can provide sensitive, quantitative, and scalable phenotypic assays. In this article, we review the current situation of organoids and organ chips in ophthalmology and discuss how they can be leveraged for translational applications.

Generation of a Transplantable Population of Human iPSC-Derived Retinal Ganglion Cells

Optic neuropathies are a major cause of visual impairment due to retinal ganglion cell (RGC) degeneration. Human induced-pluripotent stem cells (iPSCs) represent a powerful tool for studying both human RGC development and RGC-related pathological mechanisms. Because RGC loss can be massive before the diagnosis of visual impairment, cell replacement is one of the most encouraging strategies. The present work describes the generation of functional RGCs from iPSCs based on innovative 3D/2D stepwise differentiation protocol. We demonstrate that targeting the cell surface marker THY1 is an effective strategy to select transplantable RGCs. By generating a fluorescent GFP reporter iPSC line to follow transplanted cells, we provide evidence that THY1-positive RGCs injected into the vitreous of mice with optic neuropathy can survive up to 1 month, intermingled with the host RGC layer. These data support the usefulness of iPSC-derived RGC exploration as a potential future therapeutic strategy for optic nerve regeneration.

Capacity of Retinal Ganglion Cells Derived from Human Induced Pluripotent Stem Cells to Suppress T-Cells

Retinal ganglion cells (RGCs) are impaired in patients such as those with glaucoma and optic neuritis, resulting in permanent vision loss. To restore visual function, development of RGC transplantation therapy is now underway. Induced pluripotent stem cells (iPSCs) are an important source of RGCs for human allogeneic transplantation. We therefore analyzed the immunological characteristics of iPSC-derived RGCs (iPSC-RGCs) to evaluate the possibility of rejection after RGC transplantation. We first assessed the expression of human leukocyte antigen (HLA) molecules on iPSC-RGCs using immunostaining, and then evaluated the effects of iPSC-RGCs to activate lymphocytes using the mixed lymphocyte reaction (MLR) and iPSC-RGC co-cultures. We observed low expression of HLA class I and no expression of HLA class II molecules on iPSC-RGCs. We also found that iPSC-RGCs strongly suppressed various inflammatory immune cells including activated T-cells in the MLR assay and that transforming growth factor-β2 produced by iPSC-RGCs played a critical role in suppression of inflammatory cells in vitro. Our data suggest that iPSC-RGCs have low immunogenicity, and immunosuppressive capacity on lymphocytes. Our study will contribute to predicting immune attacks after RGC transplantation.

On the Generation and Regeneration of Retinal Ganglion Cells

Retinal development follows a conserved neurogenic program in vertebrates to orchestrate the generation of specific cell types from multipotent progenitors in sequential but overlapping waves. In this program, retinal ganglion cells (RGCs) are the first cell type generated. RGCs are the final output neurons of the retina and are essential for vision and circadian rhythm. Key molecular steps have been defined in multiple vertebrate species to regulate competence, specification, and terminal differentiation of this cell type. This involves neuronal-specific transcription factor networks, regulators of chromatin dynamics and miRNAs. In mammals, RGCs and their optic nerve axons undergo neurodegeneration and loss in glaucoma and other optic neuropathies, resulting in irreversible vision loss. The incapacity of RGCs and axons to regenerate reinforces the need for the design of efficient RGC replacement strategies. Here we describe the essential molecular pathways for the differentiation of RGCs in vertebrates, as well as experimental manipulations that extend the competence window for generation of this early cell type from late progenitors. We discuss recent advances in regeneration of retinal neurons in vivo in both mouse and zebrafish and discuss possible strategies and barriers to achieving RGC regeneration as a therapeutic approach for vision restoration in blinding diseases such as glaucoma.

Intravitreal Co-Administration of GDNF and CNTF Confers Synergistic and Long-Lasting Protection against Injury-Induced Cell Death of Retinal Ganglion Cells in Mice

We have recently demonstrated that neural stem cell-based intravitreal co-administration of glial cell line-derived neurotrophic factor (GDNF) and ciliary neurotrophic factor (CNTF) confers profound protection to injured retinal ganglion cells (RGCs) in a mouse optic nerve crush model, resulting in the survival of ~38% RGCs two months after the nerve lesion. Here, we analyzed whether this neuroprotective effect is long-lasting and studied the impact of the pronounced RGC rescue on axonal regeneration. To this aim, we co-injected a GDNF- and a CNTF-overexpressing neural stem cell line into the vitreous cavity of adult mice one day after an optic nerve crush and determined the number of surviving RGCs 4, 6 and 8 months after the lesion. Remarkably, we found no significant decrease in the number of surviving RGCs between the successive analysis time points, indicating that the combined administration of GDNF and CNTF conferred lifelong protection to injured RGCs. While the simultaneous administration of GDNF and CNTF stimulated pronounced intraretinal axon growth when compared to retinas treated with either factor alone, numbers of regenerating axons in the distal optic nerve stumps were similar in animals co-treated with both factors and animals treated with CNTF only.

Development of Stem Cell Therapies for Retinal Degeneration

Degenerative retinal disease is the major cause of sight loss in the developed world and currently there is a lack of effective treatments. As the loss of vision is directly the result of the loss of retinal cells, effective cell replacement through stem-cell-based therapies may have the potential to treat a great number of retinal diseases whatever their underlying etiology. The eye is an ideal organ to develop cell therapies as it is immune privileged, and modern surgical techniques enable precise delivery of cells to the retina. Furthermore, a range of noninvasive diagnostic tests and high-resolution imaging techniques facilitate the evaluation of any therapeutic intervention. In this review, we evaluate the progress to date of current cell therapy strategies for retinal repair, focusing on transplantation of pluripotent stem-cell-derived retinal pigment epithelium (RPE) and photoreceptor cells.

Recent developments in regenerative ophthalmology

Regenerative medicine (RM) is one of the most promising disciplines for advancements in modern medicine, and regenerative ophthalmology (RO) is one of the most active fields of regenerative medicine. This review aims to provide an overview of regenerative ophthalmology, including the range of tools and materials being used, and to describe its application in ophthalmologic subspecialties, with the exception of surgical implantation of artificial tissues or organs (e.g., contact lens, artificial cornea, intraocular lens, artificial retina, and bionic eyes) due to space limitations. In addition, current challenges and limitations of regenerative ophthalmology are discussed and future directions are highlighted.

Retinal Tissue Bioengineering, Materials and Methods for the Treatment of Glaucoma

BACKGROUND

Glaucoma, a characteristic type of optic nerve degeneration in the posterior pole of the eye, is a common cause of irreversible vision loss and the second leading cause of blindness worldwide. As an optic neuropathy, glaucoma is identified by increasing degeneration of retinal ganglion cells (RGCs), with consequential vision loss. Current treatments only postpone the development of retinal degeneration, and there are as yet no treatments available for this disability. Recent studies have shown that replacing lost or damaged RGCs with healthy RGCs or RGC precursors, supported by appropriately designed bio-material scaffolds, could facilitate the development and enhancement of connections to ganglion cells and optic nerve axons. The consequence may be an improved retinal regeneration. This technique could also offer the possibility for retinal regeneration in treating other forms of optic nerve ailments through RGC replacement.

METHODS

In this brief review, we describe the innovations and recent developments in retinal regenerative medicine such as retinal organoids and gene therapy which are specific to glaucoma treatment and focus on the selection of appropriate bio-engineering principles, biomaterials and cell therapies that are presently employed in this growing research area.

RESULTS

Identification of optimal sources of cells, improving cell survival, functional integration upon transplantation, and developing techniques to deliver cells into the retinal space without provoking immune responses are the main challenges in retinal cell replacement therapies.

CONCLUSION

The restoration of visual function in glaucoma patients by the RGC replacement therapies requires appropriate protocols and biotechnology methods. Tissue-engineered scaffolds, the generation of retinal organoids, and gene therapy may help to overcome some of the challenges in the generation of clinically safe RGCs.

Neuro-protection and neuro-regeneration of the optic nerve: recent advances and future directions

PURPOSE OF REVIEW

Optic neuropathies refer to a collection of diseases in which retinal ganglion cells (RGCs), the specialized neuron of the retina whose axons make up the optic nerve, are selectively damaged. Blindness secondary to optic neuropathies is irreversible as RGCs do not have the capacity for self-renewal and have a limited capacity for self-repair. Numerous strategies are being developed to either prevent further RGC degeneration or replace the cells that have degenerated. In this review, we aim to discuss known limitations to regeneration in central nervous system (CNS), followed by a discussion of previous, current, and future strategies for optic nerve neuroprotection as well as approaches for neuro-regeneration, with an emphasis on developments in the past two years.

RECENT FINDINGS

Neuro-regeneration in the CNS is limited by both intrinsic and extrinsic factors. Environmental barriers to axon regeneration can be divided into two major categories: failure to clear myelin and formation of glial scar. Although inflammatory scars block axon growth past the site of injury, inflammation also provides important signals that activate reparative and regenerative pathways in RGCs. Neuroprotection with neurotrophins as monotherapy is not effective at preventing RGC degeneration likely secondary to rapid clearance of growth factors. Novel approaches involve exploiting different technologies to provide sustained delivery of neurotrophins. Other approaches include application of anti-apoptosis molecules and anti-axon retraction molecules. Although stem cells are becoming a viable option for generating RGCs for cell-replacement-based strategies, there are still many critical barriers to overcome before they can be used in clinical practice. Adjuvant treatments, such as application of electrical fields, scaffolds, and magnetic field stimulation, may be useful in helping transplanted RGCs extend axons in the proper orientation and assist with new synapse formation.

SUMMARY

Different optic neuropathies will benefit from neuro-protective versus neuro-regenerative approaches. Developing clinically effective treatments for optic nerve disease will require a collaborative approach that not only employs neurotrophic factors but also incorporates signals that promote axonogenesis, direct axon growth towards intended targets, and promote appropriate synaptogenesis.

Therapeutic Potential of Mesenchymal Stem Cells and Their Secretome in the Treatment of Glaucoma

Glaucoma represents a group of progressive optic neuropathies characterized by gradual loss of retinal ganglion cells (RGCs), the neurons that conduct visual information from the retina to the brain. Elevated intraocular pressure (IOP) is considered the main reason for enhanced apoptosis of RGCs in glaucoma. Currently used therapeutic agents are not able to repopulate and/or regenerate injured RGCs and, therefore, are ineffective in most patients with advanced glaucoma. Accordingly, several new therapeutic approaches, including stem cell-based therapy, have been explored for the glaucoma treatment. In this review article, we emphasized current knowledge regarding molecular and cellular mechanisms responsible for beneficial effects of mesenchymal stem cells (MSCs) and their secretome in the treatment of glaucoma. MSCs produce neurotrophins and in an exosome-dependent manner supply injured RGCs with growth factors enhancing their survival and regeneration. Additionally, MSCs are able to generate functional RGC-like cells and induce proliferation of retinal stem cells. By supporting integrity of trabecular meshwork, transplanted MSCs alleviate IOP resulting in reduced loss of RGCs. Moreover, MSCs are able to attenuate T cell-driven retinal inflammation providing protection to the injured retinal tissue. In summing up, due to their capacity for neuroprotection and immunomodulation, MSCs and their secretome could be explored in upcoming clinical studies as new therapeutic agents for glaucoma treatment.

Does Retinal Ganglion Cell Loss Precede Visual Field Loss in Glaucoma?

It is often said that substantial retinal ganglion cells are lost before glaucomatous damage is detected by standard automated perimetry. There are 4 key articles referenced to support this belief. To test the hypothesis that the 4 key articles are incorrectly cited, the publications in the first 6 months of 2019 that reference 1 or more of these 4 articles were examined. In particular, the degree to which the quotes from these 2019 publications accurately reflected the evidence in the 4 key articles was assessed. These quotes are inadequately supported by the data, and in some cases even by the conclusions found in the abstracts of the key articles. This is despite several review articles that have questioned the evidence in these key articles. Further, a case can be made that the evidence in the key articles better supports the opposite conclusion. That is, the data suggest that sensitivity loss can be seen on standard automated perimetry before retinal ganglion cells are missing.