Turning the fate of reprogramming cells from retinal disorder to regeneration by Pax6 in newts

The neuroimmune interface in retinal regeneration

Retinal neurodegeneration leads to irreversible blindness due to the mammalian nervous system's inability to regenerate lost neurons. Efforts to regenerate retina involve two main strategies: stimulating endogenous cells to reprogram into neurons or transplanting stem-cell derived neurons into the degenerated retina. However, both approaches must overcome a major barrier in getting new neurons to grow back down the optic nerve and connect to appropriate visual targets in environments that differ significantly from developmental conditions. While immune privilege has historically been associated with the central nervous system, an emerging literature highlights the active role of immune cells in shaping neurodegeneration and regeneration. This review explores the neuroimmune interface in retinal repair, dissecting how immune interactions influence glial reprogramming, transplantation outcomes, and axonal regeneration. By integrating insights from regenerative species with mammalian models, we highlight novel immunomodulatory strategies to optimize retinal regeneration.

One-step Cre-loxP organism creation by TAx9

The creation of organisms with Cre-loxP conditional gene recombination systems often faces challenges, particularly when creating the initial (F0) generation with both a Cre recombinase and a DNA site flanked by loxP elements (floxed site). The primary reason is that it is difficult to synthesize a single plasmid with both the Cre gene and the floxed site, since Cre-mediated recombination spontaneously occurs when the plasmid is amplified in Escherichia coli bacterial cells. Here, we introduce an artificial nucleic acid sequence TATATATATATATATATA, named TAx9, that enables the integration of both the Cre gene and the floxed site into a single plasmid. TAx9 effectively blocks spontaneous Cre-mediated recombination in E. coli cells. Using this system, we created an F0 generation of transgenic newts and CRISPR-Cas9 knock-in mice with tissue-specific Cre recombination triggered by tamoxifen. TAx9 technology will be a powerful strategy for creating organisms capable of conditional genetic modification in the F0 generation, accelerating various life science research by reducing the time and cost for ultimately establishing and maintaining lines of genetically modified organisms.

Using Small Molecules to Reprogram RPE Cells in Regenerative Medicine for Degenerative Eye Disease

The main purpose of regenerative medicine for degenerative eye diseases is to create cells to replace lost or damaged ones. Due to their anatomical, genetic, and epigenetic features, characteristics of origin, evolutionary inheritance, capacity for dedifferentiation, proliferation, and plasticity, mammalian and human RPE cells are of great interest as endogenous sources of new photoreceptors and other neurons for the degrading retina. Promising methods for the reprogramming of RPE cells into retinal cells include genetic methods and chemical methods under the influence of certain low-molecular-weight compounds, so-called small molecules. Depending on the goal, which can be the preservation or the replacement of lost RPE cells and cellular structures, various small molecules are used to influence certain biological processes at different levels of cellular regulation. This review discusses the potential of the chemical reprogramming of RPE cells in comparison with other somatic cells and induced pluripotent stem cells (iPSCs) into neural cells of the brain and retina. Possible mechanisms of the chemically induced reprogramming of somatic cells under the influence of small molecules are explored and compared. This review also considers other possibilities in using them in the treatment of retinal degenerative diseases based on the protection, preservation, and support of survived RPE and retinal cells.

Development of a ZRS Reporter System for the Newt (Cynops pyrrhogaster) During Terrestrial Limb Regeneration

BACKGROUND

Newts, a type of urodele amphibian, offer remarkable insights into regenerative medicine due to their extraordinary tissue regeneration capabilities-a challenging feat in humans. During limb regeneration of adult newts, fascinating cellular and molecular processes are revealed, including scarless healing, de-differentiation of mature cells, and regeneration of limbs and digits. Sonic hedgehog (Shh), crucial for vertebrate limb development, is regulated by the zone of polarizing activity regulatory sequence (ZRS) in the limb bud zone of polarizing activity (ZPA). The metamorphosed (terrestrial) newt can reactivate Shh during regeneration, facilitating proper limb patterning. Cell types capable of regulating the ZRS in metamorphosed newts remain unknown. The identification of such cell types provides invaluable insight into novel regenerative mechanisms.

OBJECTIVE

In this study, we developed the first newt ZRS reporter.

METHODS

We isolated and characterized the newt ZRS enhancer (nZRS), identifying conserved DNA binding sites. Several binding sites with medical relevance were conserved in the newt ZRS. In functional analysis, we developed a system composed of a transgenic nZRS reporter newt and a new newt anti-Shh antibody, which allowed Shh monitoring during limb regeneration.

RESULTS

We identified a group of Schwann cells capable of ZRS reporter and Shh protein expression during terrestrial limb regeneration.

CONCLUSIONS

This system provides a valuable in vivo approach for future genetic studies of patterning during limb regeneration.

Cell Sources for Retinal Regeneration: Implication for Data Translation in Biomedicine of the Eye

The main degenerative diseases of the retina include macular degeneration, proliferative vitreoretinopathy, retinitis pigmentosa, and glaucoma. Novel approaches for treating retinal diseases are based on cell replacement therapy using a variety of exogenous stem cells. An alternative and complementary approach is the potential use of retinal regeneration cell sources (RRCSs) containing retinal pigment epithelium, ciliary body, Müller glia, and retinal ciliary region. RRCSs in lower vertebrates in vivo and in mammals mostly in vitro are able to proliferate and exhibit gene expression and epigenetic characteristics typical for neural/retinal cell progenitors. Here, we review research on the factors controlling the RRCSs' properties, such as the cell microenvironment, growth factors, cytokines, hormones, etc., that determine the regenerative responses and alterations underlying the RRCS-associated pathologies. We also discuss how the current data on molecular features and regulatory mechanisms of RRCSs could be translated in retinal biomedicine with a special focus on (1) attempts to obtain retinal neurons de novo both in vivo and in vitro to replace damaged retinal cells; and (2) investigations of the key molecular networks stimulating regenerative responses and preventing RRCS-related pathologies.

Newtic1 Is a Component of Globular Structures That Accumulate along the Marginal Band of Erythrocytes in the Limb Blastema of Adult Newt, Cynops pyrrhogaster

In adult newts, when a limb is amputated, a mesenchymal cell mass called the blastema is formed on the stump, where blood vessels filled with premature erythrocytes, named polychromatic normoblasts (PcNobs), elongate. We previously demonstrated that PcNobs in the blastema express an orphan gene, Newtic1, and that they secrete growth factors such as BMP2 and TGFβ1 into the surrounding tissues. However, the relationship between Newtic1 expression and growth factor secretion was not clear since Newtic1 was thought to encode a membrane protein. In this study, we addressed this issue using morphological techniques and found that the Newtic1 protein is a component of globular structures that accumulate at the marginal band in the cytoplasm along the equator of PcNobs. Newtic1-positive (Newtic1(+)) globular structures along the equator were found only in PcNobs with a well-developed marginal band in the blastema. Newtic1(+) globular structures were associated with microtubules and potentially incorporated TGFβ1. Based on these observations, we propose a hypothesis that the Newtic1 protein localizes to the membrane of secretory vesicles that primarily carry TGFβ1 and binds to microtubules, thereby tethering secretory vesicles to microtubules and transporting them to the cell periphery as the marginal band develops.

The latent dedifferentiation capacity of newt limb muscles is unleashed by a combination of metamorphosis and body growth

Newts can regenerate their limbs throughout their life-span. Focusing on muscle, certain species of newts such as Cynops pyrrhogaster dedifferentiate muscle fibers in the limb stump and mobilize them for muscle creation in the regenerating limb, as they grow beyond metamorphosis. However, which developmental process is essential for muscle dedifferentiation, metamorphosis or body growth, is unknown. To address this issue, we tracked muscle fibers during limb regeneration under conditions in which metamorphosis and body growth were experimentally shifted along the axis of development. Our results indicate that a combination of metamorphosis and body growth is necessary for muscle dedifferentiation. On the other hand, ex vivo tracking of larval muscle fibers revealed that newt muscle fibers have the ability to dedifferentiate independently of metamorphosis and body growth. These results suggest that newt muscle fibers have an intrinsic ability to dedifferentiate, but that metamorphosis and body growth are necessary for them to exhibit this hidden ability. Presumably, changes in the extracellular environment (niche) during developmental processes allow muscle fibers to contribute to limb regeneration through dedifferentiation. This study can stimulate research on niches as well as gene regulation for dedifferentiation, contributing to a further understanding of regeneration and future medical applications.

Comparative Biology of Vertebrate Retinal Regeneration: Restoration of Vision through Cellular Reprogramming

The regenerative capacity of the vertebrate retina varies substantially across species. Whereas fish and amphibians can regenerate functional retina, mammals do not. In this perspective piece, we outline the various strategies nonmammalian vertebrates use to achieve functional regeneration of vision. We review key differences underlying the regenerative potential across species including the cellular source of postnatal progenitors, the diversity of cell fates regenerated, and the level of functional vision that can be achieved. Finally, we provide an outlook on the field of engineering the mammalian retina to replace neurons lost to injury or disease.

Pigment Epithelia of the Eye: Cell-Type Conversion in Regeneration and Disease

Pigment epithelial cells (PECs) of the retina (RPE), ciliary body, and iris (IPE) are capable of altering their phenotype. The main pathway of phenotypic switching of eye PECs in vertebrates and humans in vivo and/or in vitro is neural/retinal. Besides, cells of amphibian IPE give rise to the lens and its derivatives, while mammalian and human RPE can be converted along the mesenchymal pathway. The PECs’ capability of conversion in vivo underlies the lens and retinal regeneration in lower vertebrates and retinal diseases such as proliferative vitreoretinopathy and fibrosis in mammals and humans. The present review considers these processes studied in vitro and in vivo in animal models and in humans. The molecular basis of conversion strategies in PECs is elucidated. Being predetermined onto- and phylogenetically, it includes a species-specific molecular context, differential expression of transcription factors, signaling pathways, and epigenomic changes. The accumulated knowledge regarding the mechanisms of PECs phenotypic switching allows the development of approaches to specified conversion for many purposes: obtaining cells for transplantation, creating conditions to stimulate natural regeneration of the retina and the lens, blocking undesirable conversions associated with eye pathology, and finding molecular markers of pathology to be targets of therapy.

Deciphering the Retinal Epigenome during Development, Disease and Reprogramming: Advancements, Challenges and Perspectives

Retinal neurogenesis is driven by concerted actions of transcription factors, some of which are expressed in a continuum and across several cell subtypes throughout development. While seemingly redundant, many factors diversify their regulatory outcome on gene expression, by coordinating variations in chromatin landscapes to drive divergent retinal specification programs. Recent studies have furthered the understanding of the epigenetic contribution to the progression of age-related macular degeneration, a leading cause of blindness in the elderly. The knowledge of the epigenomic mechanisms that control the acquisition and stabilization of retinal cell fates and are evoked upon damage, holds the potential for the treatment of retinal degeneration. Herein, this review presents the state-of-the-art approaches to investigate the retinal epigenome during development, disease, and reprogramming. A pipeline is then reviewed to functionally interrogate the epigenetic and transcriptional networks underlying cell fate specification, relying on a truly unbiased screening of open chromatin states. The related work proposes an inferential model to identify gene regulatory networks, features the first footprinting analysis and the first tentative, systematic query of candidate pioneer factors in the retina ever conducted in any model organism, leading to the identification of previously uncharacterized master regulators of retinal cell identity, such as the nuclear factor I, NFI. This pipeline is virtually applicable to the study of genetic programs and candidate pioneer factors in any developmental context. Finally, challenges and limitations intrinsic to the current next-generation sequencing techniques are discussed, as well as recent advances in super-resolution imaging, enabling spatio-temporal resolution of the genome.

Skin Wound Healing of the Adult Newt, Cynops pyrrhogaster: A Unique Re-Epithelialization and Scarless Model

In surgical and cosmetic studies, scarless regeneration is an ideal method to heal skin wounds. To study the technologies that enable scarless skin wound healing in medicine, animal models are useful. However, four-limbed vertebrates, including humans, generally lose their competency of scarless regeneration as they transit to their terrestrial life-stages through metamorphosis, hatching or birth. Therefore, animals that serve as a model for postnatal humans must be an exception to this rule, such as the newt. Here, we evaluated the adult newt in detail for the first time. Using a Japanese fire-bellied newt, Cynops pyrrhogaster, we excised the full-thickness skin at various locations on the body, and surveyed their re-epithelialization, granulation or dermal fibrosis, and recovery of texture and appendages as well as color (hue, tone and pattern) for more than two years. We found that the skin of adult newts eventually regenerated exceptionally well through unique processes of re-epithelialization and the absence of fibrotic scar formation, except for the dorsal-lateral to ventral skin whose unique color patterns never recovered. Color pattern is species-specific. Consequently, the adult C. pyrrhogaster provides an ideal model system for studies aimed at perfect skin wound healing and regeneration in postnatal humans.

Advantages of nanofibrous membranes for culturing of primary RPE cells compared to commercial scaffolds

PURPOSE

Dysfunction of the retinal pigment epithelium (RPE) causes numerous forms of retinal degeneration. RPE replacement is a modern option to save vision. We aimed to test the results of transplanting cultured RPEs on biocompatible membranes.

METHODS

We cultivated porcine primary RPE cells isolated from cadaver eyes from the slaughterhouse on two types of membranes: commercial polyester scaffolds Transwell (Corning Inc., Kenneburg, ME, USA) with 0.4 µm pore size and prepared Poly (L-lactide-co-DL-lactide) (PDLLA) nanofibrous membranes with an average pore size of 0.4 µm.

RESULTS

Five types of assays were used for the analysis: immunocytochemistry (ICC), phagocytosis assay, Western blotting, real-time qPCR (RT-qPCR) and electron microscopy. RT-qPCR demonstrated that RPEs cultured on nanofibrous membranes have higher expressions of BEST1 (bestrophin 1), RLBP1 (retinaldehyde-binding protein 1), RPE65 (retinal pigment epithelium-specific 65 kDa protein), PAX6 (transcription factor PAX6), SOX9 (transcription factor SOX9), DCT (dopachrome tautomerase) and MITF (microphthalmia-associated transcription factor). ICC of the RPEs cultured on nanofibrous membranes showed more intensive staining of markers such as BEST1, MCT1 (monocarboxylate transporter 1), Na⁺ /K⁺ ATPase, RPE65 and acetylated tubulin in comparison with commercial ones. Additionally, the absence of α-SMA proved the stability of the RPE polarization state and the absence of epithelial-to-mesenchymal transition. RPE possessed high phagocytic activity. Electron microscopy of both membranes confirmed a confluent layer of RPE cells and their genuine morphological structure, which was comparable to native RPEs.

CONCLUSIONS

Retinal pigment epitheliums cultured on polylactide nanofibrous membranes improved the final quality of the cell product by having better maturation and long-term survival of the RPE monolayer compared to those cultured on commercial polyester scaffolds. PDLLA-cultured RPEs are a plausible source for the replacement of non-functioning RPEs during cell therapy.

Ecological Risks Due to Immunotoxicological Effects on Aquatic Organisms

The immunotoxic effects of some anthropogenic pollutants on aquatic organisms are among the causes of concern over the presence of these pollutants in the marine environment. The immune system is part of an organism's biological defense necessarily for homeostasis. Thus, the immunotoxicological impacts on aquatic organisms are important to understand the effects of pollutant chemicals in the aquatic ecosystem. When aquatic organisms are exposed to pollutant chemicals with immunotoxicity, it results in poor health. In addition, aquatic organisms are exposed to pathogenic bacteria, viruses, parasites, and fungi. Exposure to pollutant chemicals has reportedly caused aquatic organisms to show various immunotoxic symptoms such as histological changes of lymphoid tissue, changes of immune functionality and the distribution of immune cells, and changes in the resistance of organisms to infection by pathogens. Alterations of immune systems by contaminants can therefore lead to the deaths of individual organisms, increase the general risk of infections by pathogens, and probably decrease the populations of some species. This review introduced the immunotoxicological impact of pollutant chemicals in aquatic organisms, including invertebrates, fish, amphibians, and marine mammals; described typical biomarkers used in aquatic immunotoxicological studies; and then, discussed the current issues on ecological risk assessment and how to address ecological risk assessment through immunotoxicology. Moreover, the usefulness of the population growth rate to estimate the immunotoxicological impact of pollution chemicals was proposed.

Transcriptome Profiling of Embryonic Retinal Pigment Epithelium Reprogramming

The plasticity of human retinal pigment epithelium (RPE) has been observed during proliferative vitreoretinopathy, a defective repair process during which injured RPE gives rise to fibrosis. In contrast, following injury, the RPE of the embryonic chicken can be reprogrammed to regenerate neural retina in a fibroblast growth factor 2 (FGF2)-dependent manner. To better explore the mechanisms underlying embryonic RPE reprogramming, we used laser capture microdissection to isolate RNA from (1) intact RPE, (2) transiently reprogrammed RPE (t-rRPE) 6 h post-retinectomy, and (3) reprogrammed RPE (rRPE) 6 h post-retinectomy with FGF2 treatment. Using RNA-seq, we observed the acute repression of genes related to cell cycle progression in the injured t-rRPE, as well as up-regulation of genes associated with injury. In contrast, the rRPE was strongly enriched for mitogen-activated protein kinase (MAPK)-responsive genes and retina development factors, confirming that FGF2 and the downstream MAPK cascade are the main drivers of embryonic RPE reprogramming. Clustering and pathway enrichment analysis was used to create an integrated network of the core processes associated with RPE reprogramming, including key terms pertaining to injury response, migration, actin dynamics, and cell cycle progression. Finally, we employed gene set enrichment analysis to suggest a previously uncovered role for epithelial-mesenchymal transition (EMT) machinery in the initiation of embryonic chick RPE reprogramming. The EMT program is accompanied by extensive, coordinated regulation of extracellular matrix (ECM) associated factors, and these observations together suggest an early role for ECM and EMT-like dynamics during reprogramming. Our study provides for the first time an in-depth transcriptomic analysis of embryonic RPE reprogramming and will prove useful in guiding future efforts to understand proliferative disorders of the RPE and to promote retinal regeneration.

The retinal pigment epithelium: Development, injury responses, and regenerative potential in mammalian and non-mammalian systems

Diseases that result in retinal pigment epithelium (RPE) degeneration, such as age-related macular degeneration (AMD), are among the leading causes of blindness worldwide. Atrophic (dry) AMD is the most prevalent form of AMD and there are currently no effective therapies to prevent RPE cell death or restore RPE cells lost from AMD. An intriguing approach to treat AMD and other RPE degenerative diseases is to develop therapies focused on stimulating endogenous RPE regeneration. For this to become feasible, a deeper understanding of the mechanisms underlying RPE development, injury responses and regenerative potential is needed. In mammals, RPE regeneration is extremely limited; small lesions can be repaired by the expansion of adjacent RPE cells, but large lesions cannot be repaired as remaining RPE cells are unable to functionally replace lost RPE tissue. In some injury paradigms, RPE cells proliferate but do not regenerate a morphologically normal monolayer, while in others, proliferation is pathogenic and results in further disruption to the retina. This is in contrast to non-mammalian vertebrates, which possess tremendous RPE regenerative potential. Here, we discuss what is known about RPE formation during development in mammalian and non-mammalian vertebrates, we detail the processes by which RPE cells respond to injury, and we describe examples of RPE-to-retina and RPE-to-RPE regeneration in non-mammalian vertebrates. Finally, we outline barriers to RPE-dependent regeneration in mammals that could potentially be overcome to stimulate a regenerative response from the RPE.

Potential Endogenous Cell Sources for Retinal Regeneration in Vertebrates and Humans: Progenitor Traits and Specialization

Retinal diseases often cause the loss of photoreceptor cells and, consequently, impairment of vision. To date, several cell populations are known as potential endogenous retinal regeneration cell sources (RRCSs): the eye ciliary zone, the retinal pigment epithelium, the iris, and Müller glia. Factors that can activate the regenerative responses of RRCSs are currently under investigation. The present review considers accumulated data on the relationship between the progenitor properties of RRCSs and the features determining their differentiation. Specialized RRCSs (all except the ciliary zone in low vertebrates), despite their differences, appear to be partially "prepared" to exhibit their plasticity and be reprogrammed into retinal neurons due to the specific gene expression and epigenetic landscape. The "developmental" characteristics of RRCS gene expression are predefined by the pathway by which these cell populations form during eye morphogenesis; the epigenetic features responsible for chromatin organization in RRCSs are under intracellular regulation. Such genetic and epigenetic readiness is manifested in vivo in lower vertebrates and in vitro in higher ones under conditions permissive for cell phenotype transformation. Current studies on gene expression in RRCSs and changes in their epigenetic landscape help find experimental approaches to replacing dead cells through recruiting cells from endogenous resources in vertebrates and humans.

Model systems for regeneration: salamanders

Salamanders have been hailed as champions of regeneration, exhibiting a remarkable ability to regrow tissues, organs and even whole body parts, e.g. their limbs. As such, salamanders have provided key insights into the mechanisms by which cells, tissues and organs sense and regenerate missing or damaged parts. In this Primer, we cover the evolutionary context in which salamanders emerged. We outline the varieties of mechanisms deployed during salamander regeneration, and discuss how these mechanisms are currently being explored and how they have advanced our understanding of animal regeneration. We also present arguments about why it is important to study closely related species in regeneration research.

Novel erythrocyte clumps revealed by an orphan gene Newtic1 in circulating blood and regenerating limbs of the adult newt

The newt, a group of urodele amphibians, has outstanding ability to repeatedly regenerate various body parts, even in the terrestrial life-stage. In this animal, when the limb is amputated, a cell mass named the blastema appears on the stump and eventually gives rise to a new functional limb. Erythrocytes (red blood cells) in most non-mammalian vertebrates, including the newt, preserve their nucleus throughout their life-span, although physiological roles of such nucleated erythrocytes, other than oxygen delivery, are not known. Here we report novel behavior of erythrocytes in the newt. We identified an orphan gene Newtic1, whose transcripts significantly increased in the blastema. Newtic1 was expressed in a subset of erythrocytes that formed a novel clump (EryC). EryC formed a complex with monocytes and was circulating throughout the body. When the limb was amputated, EryCs were newly generated in the stump and accumulated into a distal portion of the growing blastema. Our data suggested that the newt erythrocytes carried multiple secretory molecules including growth factors and matrix metalloproteases, and were capable of delivering these molecules into the blastema as a form of EryCs. This study provides insight into regulations and roles of nucleated erythrocytes, that are independent of oxygen delivery.

Implications of a Multi-Step Trigger of Retinal Regeneration in the Adult Newt

The newt is an amazing four-limbed vertebrate that can regenerate various body parts including the retina. In this animal, when the neural retina (NR) is removed from the eye by surgery (retinectomy), both the NR and the retinal pigment epithelium (RPE) eventually regenerate through the process of reprogramming and proliferation of RPE cells. Thus far, we have pursued the onset mechanism of adult newt retinal regeneration. In this study, using an in vitro system, we found that both mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK and β-catenin were involved in cell cycle re-entry of RPE cells. MEK-ERK signaling activity in RPE cells was strengthened by retinectomy, and nuclear translocation of β-catenin in RPE cells was induced by attenuation of cell–cell contact, which was promoted by incision of the RPE or its treatment with ethylene glycol tetraacetic acid (EGTA). EGTA is a Ca²⁺ chelator that disrupts cadherin-mediated cell–cell adhesion. Reinforcement of MEK-ERK signaling activity was a prerequisite for nuclear translocation of β-catenin. These results suggest that retinectomy followed by attenuation of cell–cell contact may trigger cell cycle re-entry of RPE cells. This study, together with our previous findings concerning the proliferation and multipotency of adult newt RPE cells, provides insight into the mechanism of the multi-step trigger in which the onset of retinal regeneration in the adult newt is rigorously controlled.

Report on the National Eye Institute Audacious Goals Initiative: Replacement of Retinal Ganglion Cells from Endogenous Cell Sources

This report emerges from a workshop convened by the National Eye Institute (NEI) as part of the "Audacious Goals Initiative" (AGI). The workshop addressed the replacement of retinal ganglion cells (RGCs) from exogenous and endogenous sources, and sought to identify the gaps in our knowledge and barriers to progress in devising cellular replacement therapies for diseases where RGCs die. Here, we briefly review relevant literature regarding common diseases associated with RGC death, the genesis of RGCs in vivo, strategies for generating transplantable RGCs in vitro, and potential endogenous cellular sources to regenerate these cells. These topics provided the clinical and scientific context for the discussion among the workshop participants and are relevant to efforts that may lead to therapeutic approaches for replacing RGCs. This report also summarizes the content of the workshop discussion, which focused on: (1) cell sources for RGC replacement and regeneration, (2) optimizing integration, survival, and synaptogenesis of new RGCs, and (3) approaches for assessing the outcomes of RGC replacement therapies. We conclude this report with a summary of recommendations, based on the workshop discussions, which may guide vision scientists seeking to develop therapies for replacing RGCs in humans.

Cellular and Molecular Preconditions for Retinal Pigment Epithelium (RPE) Natural Reprogramming during Retinal Regeneration in Urodela

Many regeneration processes in animals are based on the phenomenon of cell reprogramming followed by proliferation and differentiation in a different specialization direction. An insight into what makes natural (in vivo) cell reprogramming possible can help to solve a number of biomedical problems. In particular, the first problem is to reveal the intrinsic properties of the cells that are necessary and sufficient for reprogramming; the second, to evaluate these properties and, on this basis, to reveal potential endogenous sources for cell substitution in damaged tissues; and the third, to use the acquired data for developing approaches to in vitro cell reprogramming in order to obtain a cell reserve for damaged tissue repair. Normal cells of the retinal pigment epithelium (RPE) in newts (Urodela) can change their specialization and transform into retinal neurons and ganglion cells (i.e., actualize their retinogenic potential). Therefore, they can serve as a model that provides the possibility to identify factors of the initial competence of vertebrate cells for reprogramming in vivo. This review deals mainly with the endogenous properties of native newt RPE cells themselves and, to a lesser extent, with exogenous mechanisms regulating the process of reprogramming, which are actively discussed.