Retinal Degeneration and Regeneration-Lessons From Fishes and Amphibians

Regeneration from three cellular sources and ectopic mini-retina formation upon neurotoxic retinal degeneration in Xenopus

Regenerative abilities are not evenly distributed across the animal kingdom. The underlying modalities are also highly variable. Retinal repair can involve the mobilization of different cellular sources, including ciliary marginal zone (CMZ) stem cells, the retinal pigmented epithelium (RPE), or Müller glia. To investigate whether the magnitude of retinal damage influences the regeneration modality of the Xenopus retina, we developed a model based on cobalt chloride (CoCl2 ) intraocular injection, allowing for a dose-dependent control of cell death extent. Analyses in Xenopus laevis revealed that limited CoCl2 -mediated neurotoxicity only triggers cone loss and results in a few Müller cells reentering the cell cycle. Severe CoCl2 -induced retinal degeneration not only potentializes Müller cell proliferation but also enhances CMZ activity and unexpectedly triggers RPE reprogramming. Surprisingly, reprogrammed RPE self-organizes into an ectopic mini-retina-like structure laid on top of the original retina. It is thus likely that the injury paradigm determines the awakening of different stem-like cell populations. We further show that these cellular sources exhibit distinct neurogenic capacities without any bias towards lost cells. This is particularly striking for Müller glia, which regenerates several types of neurons, but not cones, the most affected cell type. Finally, we found that X. tropicalis also has the ability to recruit Müller cells and reprogram its RPE following CoCl2 -induced damage, whereas only CMZ involvement was reported in previously examined degenerative models. Altogether, these findings highlight the critical role of the injury paradigm and reveal that three cellular sources can be reactivated in the very same degenerative model.

Cost-effective strategies to knock down genes of interest in the retinas of adult zebrafish

High throughput sequencing has generated an enormous amount of information about the genes expressed in various cell types and tissues throughout the body, and about how gene expression changes over time and in diseased conditions. This knowledge has made targeted gene knockdowns an important tool in screening and identifying the roles of genes that are differentially expressed among specific cells of interest. While many approaches are available and optimized in mammalian models, there are still several limitations in the zebrafish model. In this article, we describe two approaches to target specific genes in the retina for knockdown: cell-penetrating, translation-blocking Vivo-Morpholino oligonucleotides and commercially available lipid nanoparticle reagents to deliver siRNA. We targeted expression of the PCNA gene in the retina of a P23H rhodopsin transgenic zebrafish model, in which rapidly proliferating progenitor cells replace degenerated rod photoreceptors. Retinas collected 48 h after intravitreal injections in adult zebrafish reveal that both Vivo-Morpholinos and lipid encapsulated siRNAs were able to successfully knock down expression of PCNA. However, only retinas injected with Vivo-Morpholinos showed a significant decrease in the formation of P23H rhodopsin-expressing rods, a downstream effect of PCNA inhibition. Surprisingly, Vivo-Morpholinos were able to exit the injected eye and enter the contralateral non-injected eye to inhibit PCNA expression. In this article we describe the techniques, concentrations, and considerations we found necessary to successfully target and inhibit genes through Vivo-Morpholinos and lipid encapsulated siRNAs.

Molecular Signatures Integral to Natural Reprogramming in the Pigment Epithelium Cells after Retinal Detachment in Pleurodeles waltl

The reprogramming of retinal pigment epithelium (RPE) cells into retinal cells (transdifferentiation) lies in the bases of retinal regeneration in several Urodela. The identification of the key genes involved in this process helps with looking for approaches to the prevention and treatment of RPE-related degenerative diseases of the human retina. The purpose of our study was to examine the transcriptome changes at initial stages of RPE cell reprogramming in adult newt Pleurodeles waltl. RPE was isolated from the eye samples of day 0, 4, and 7 after experimental surgical detachment of the neural retina and was used for a de novo transcriptome assembly through the RNA-Seq method. A total of 1019 transcripts corresponding to the differently expressed genes have been revealed in silico: the 83 increased the expression at an early stage, and 168 increased the expression at a late stage of RPE reprogramming. We have identified up-regulation of classical early response genes, chaperones and co-chaperones, genes involved in the regulation of protein biosynthesis, suppressors of oncogenes, and EMT-related genes. We revealed the growth in the proportion of down-regulated ribosomal and translation-associated genes. Our findings contribute to revealing the molecular mechanism of RPE reprogramming in Urodela.

Cellular and Molecular Triggers of Retinal Regeneration in Amphibians

Understanding the mechanisms triggering the initiation of retinal regeneration in amphibians may advance the quest for prevention and treatment options for degenerating human retina diseases. Natural retinal regeneration in amphibians requires two cell sources, namely retinal pigment epithelium (RPE) and ciliary marginal zone. The disruption of RPE interaction with photoreceptors through surgery or injury triggers local and systemic responses for retinal protection. In mammals, disease-induced damage to the retina results in the shutdown of the function, cellular or oxidative stress, pronounced immune response, cell death and retinal degeneration. In contrast to retinal pathology in mammals, regenerative responses in amphibians have taxon-specific features ensuring efficient regeneration. These include rapid hemostasis, the recruitment of cells and factors of endogenous defense systems, activities of the immature immune system, high cell viability, and the efficiency of the extracellular matrix, cytoskeleton, and cell surface remodeling. These reactions are controlled by specific signaling pathways, transcription factors, and the epigenome, which are insufficiently studied. This review provides a summary of the mechanisms initiating retinal regeneration in amphibians and reveals its features collectively directed at recruiting universal responses to trauma to activate the cell sources of retinal regeneration. This study of the integrated molecular network of these processes is a prospect for future research in demand biomedicine.

Use of vivo-morpholinos for gene knockdown in the postnatal shark retina

Work in the catshark Scyliorhinus canicula has shown that the evolutionary origin of postnatal neurogenesis in vertebrates is earlier than previously thought. Thus, the catshark can serve as a model of interest to understand postnatal neurogenic processes and their evolution in vertebrates. One of the best characterized neurogenic niches of the catshark CNS is found in the peripheral region of the retina. Unfortunately, the lack of genetic tools in sharks limits the possibilities to deepen in the study of genes involved in the neurogenic process. Here, we report a method for gene knockdown in the juvenile catshark retina based on the use of Vivo-Morpholinos. To establish the method, we designed Vivo-Morpholinos against the proliferation marker PCNA. We first evaluated the possible toxicity of 3 different intraocular administration regimes. After this optimization step, we show that a single intraocular injection of the PCNA Vivo-Morpholino decreases the expression of PCNA in the peripheral retina, which leads to reduced mitotic activity in this region. This method will help in deciphering the role of other genes potentially involved in postnatal neurogenesis in this animal model.

Transcriptome Changes in Retinal Pigment Epithelium Post-PNU-282987 Treatment Associated with Adult Retinal Neurogenesis in Mice

PNU-282987, a selective alpha7 nicotinic acetylcholine receptor agonist, has previously been shown to have both neurogenic and broad regenerative effects in the adult murine retina. The objective of this study was to assay the molecular mechanism by which PNU-282987 promotes the production of Muller-derived progenitor cells through signaling via the resident retinal pigment epithelium. These Muller-derived progenitor cells generate a myriad of differentiated neurons throughout the retina that have previously been characterized by morphology. Herein, we demonstrate that topical application of PNU-282987 stimulates production of functional neurons as measured by electroretinograms. Further, we examine the mechanism of how this phenomenon occurs through activation of this atypical receptor using a transcriptomic approach isolated retinal pigment epithelium activated by PNU-282987 and in whole retina. We provide evidence that PNU-282987 causes a bi-modal signaling event in which early activation primes the retina with an inflammatory response and developmental signaling cues, followed by an inhibition of gliotic mechanisms and a decrease in the immune response, ending with upregulation of genes associated with specific retinal neuron generation. Taken together, these data provide evidence that PNU-282987 activates the retinal pigment epithelium to signal to Muller glia to produce Muller-derived progenitor cells, which can differentiate into new, functional neurons in adult mice. These data not only increase our understanding of how adult mammalian retinal regeneration can occur, but also provide therapeutic promise for treating functional vision loss.

Activation of adult mammalian retinal stem cells in vivo via antagonism of BMP and sFRP2

Background

The adult mammalian retina does not have the capacity to regenerate cells lost due to damage or disease. Therefore, retinal injuries and blinding diseases result in irreversible vision loss. However, retinal stem cells (RSCs), which participate in retinogenesis during development, persist in a quiescent state in the ciliary epithelium (CE) of the adult mammalian eye. Moreover, RSCs retain the ability to generate all retinal cell types when cultured in vitro, including photoreceptors. Therefore, it may be possible to activate endogenous RSCs to induce retinal neurogenesis in vivo and restore vision in the adult mammalian eye.

Methods

To investigate if endogenous RSCs can be activated, we performed combinatorial intravitreal injections of antagonists to BMP and sFRP2 proteins (two proposed mediators of RSC quiescence in vivo), with or without growth factors FGF and Insulin. We also investigated the effects of chemically-induced N-methyl-N-Nitrosourea (MNU) retinal degeneration on RSC activation, both alone and in combination withthe injected factors. Further, we employed inducible Msx1-Cre^ERT2 genetic lineage labeling of the CE followed by stimulation paradigms to determine if activated endogenous RSCs could migrate into the retina and differentiate into retinal neurons.

Results

We found that in vivo antagonism of BMP and sFRP2 proteins induced CE cells in the RSC niche to proliferate and expanded the RSC population. BMP and sFRP2 antagonism also enhanced CE cell proliferation in response to exogenous growth factor stimulation and MNU-induced retinal degeneration. Furthermore, Msx1-Cre^ERT2 genetic lineage tracing revealed that CE cells migrated into the retina following stimulation and/or injury, where they expressed markers of mature photoreceptors and retinal ganglion cells.

Conclusions

Together, these results indicate that endogenous adult mammalian RSCs may have latent regenerative potential that can be activated by modulating the RSC niche and hold promise as a means for endogenous retinal cell therapy to repair the retina and improve vision.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02630-0.

An insight on established retinal injury mechanisms and prevalent retinal stem cell activation pathways in vertebrate models

Implementing different tools and injury mechanisms in multiple animal models of retina regeneration, researchers have discovered the existence of retinal stem/progenitor cells. Although they appear to be distributed uniformly across the vertebrate lineage, the reparative potential of the retina is mainly restricted to lower vertebrates. Regenerative repair post-injury requires the creation of a proliferative niche, vital for proper stem cell activation, propagation, and lineage differentiation. This seems to be lacking in mammals. Hence, in this review, we first discuss the many forms of retinal injuries that have been generated using animal models. Next, we discuss how they are utilized to stimulate regeneration and mimic eye disease pathologies. The key to driving stem cell activation in mammals relies on the information we can gather from these models. Lastly, we present a brief update about the genes, growth factors, and signaling pathways that have been brought to light using these models.

Transplantation Efficacy of Human Ciliary Epithelium Cells from Fetal Eye and Lin-ve Stem Cells from Umbilical Cord Blood in the Murine Retinal Degeneration Model of Laser Injury

A number of degenerative conditions affecting the neural retina including age-related macular degeneration have no successful treatment, resulting in partial or complete vision loss. There are a number of stem cell replacement strategies for recovery of retinal damage using cells from variable sources. However, literature is still deficit in the comparison of efficacy of types of stem cells. The purpose of the study was to compare the therapeutic efficacy of undifferentiated cells, i.e., lineage negative stem cells (Lin-ve SC) with differentiated neurosphere derived from ciliary epithelium (CE) cells on retinal markers associated with laser-induced retinal injury. Laser-induced photocoagulation was carried out to disrupt Bruch’s membrane and retinal pigmented epithelium in C57BL/6 mouse model. Lineage negative cells were isolated from human umbilical cord blood, whereas neurospheres were derived from CE of post-aborted human eyeballs. The cells were then transplanted into subretinal space to study their effect on injury. Markers of neurotropic factors, retina, apoptosis, and proliferation were analyzed after injury and transplantation. mRNA expression was also analyzed by real-time polymerase chain reaction at 1 week, and 3-month immunohistochemistry was evaluated at 1-week time point. CE cell transplantation showed enhanced differentiation of rods and retinal glial cells. However, Lin-ve cells exerted paracrine-dependent modulation of neurotrophic factors, which is possibly mediated by antiapoptotic and proliferative effects. In conclusion, CE transplantation showed superior regenerative outcome in comparison to Lin-ve SC for rescue of artificially injured rodent retinal cells. It is imperative that this source for transplantation may be extensively studied in various doses and additional retinal degeneration models for prospective clinical applications.

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 immune response is a critical regulator of zebrafish retinal pigment epithelium regeneration

Loss of the retinal pigment epithelium (RPE) because of dysfunction or disease can lead to blindness in humans. Harnessing the intrinsic ability of the RPE to self-repair is an attractive therapeutic strategy; however, mammalian RPE is limited in its regenerative capacity. Zebrafish possess tremendous intrinsic regenerative potential in ocular tissues, including the RPE, but little is known about the mechanisms driving RPE regeneration. Here, utilizing transgenic and mutant zebrafish lines, pharmacological manipulations, transcriptomics, and imaging analyses, we identified elements of the immune response as critical mediators of intrinsic RPE regeneration. After genetic ablation, the RPE express immune-related genes, including leukocyte recruitment factors such as interleukin 34 We demonstrate that macrophage/microglia cells are responsive to RPE damage and that their function is required for the timely progression of the regenerative response. These data identify the molecular and cellular underpinnings of RPE regeneration and hold significant potential for translational approaches aimed toward promoting a pro-regenerative environment in mammalian RPE.

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.

Let-7, Lin28 and Hmga2 Expression in Ciliary Epithelium and Retinal Progenitor Cells

Purpose

Ciliary epithelium (CE) of adult mammalian eyes contains quiescent retinal progenitor/stem cells that generate neurospheres in vitro and differentiate into retinal neurons. This ability doesn't evolve efficiently probably because of regulatory mechanisms, such as microRNAs (miRNAs) that control pluripotent, progenitor, and differentiation genes. Here we investigate the presence of Let-7 miRNAs and its regulator and target, Lin28 and Hmga2, in CE cells from neurospheres, newborns, and adult tissues.

Methods

Newborn and adult rats CE cells were dissected into pigmented and nonpigmented epithelium (PE and NPE). Newborn PE cells were cultured with growth factors to form neurospheres and we analyzed Let-7, Lin28a, and Hmga2 expression. During the neurospheres formation, we added chemically modified single-stranded oligonucleotides designed to bind and inhibit or mimic endogenous mature Let-7b and Let-7c. After seven days in culture, we analyzed neurospheres size, number and expression of Let-7, Lin28, and Hmga2.

Results

Let-7 miRNAs were expressed at low rates in newborn CE cells with significant increase in adult tissues, with higher levels on NPE cells, that does not present the stem cells reprogramming ability. The Lin28a and Hmga2 protein and transcripts were more expressed in newborns than adults cells, opposed to Let-7. Neurospheres presented higher Lin28 and Hmga2 expression than newborn and adult, but similar Let-7 than newborns. Let-7b inhibitor upregulated Hmga2 expression, whereas Let-7c mimics upregulated Lin28 and downregulated Hmga2.

Conclusions

This study shows the dynamic of Lin28-Let-7-Hmga regulatory axis in CE cells. These components may develop different roles during neurospheres formation and postnatal CE cells.

Microfluidic and Microscale Assays to Examine Regenerative Strategies in the Neuro Retina

Bioengineering systems have transformed scientific knowledge of cellular behaviors in the nervous system (NS) and pioneered innovative, regenerative therapies to treat adult neural disorders. Microscale systems with characteristic lengths of single to hundreds of microns have examined the development and specialized behaviors of numerous neuromuscular and neurosensory components of the NS. The visual system is comprised of the eye sensory organ and its connecting pathways to the visual cortex. Significant vision loss arises from dysfunction in the retina, the photosensitive tissue at the eye posterior that achieves phototransduction of light to form images in the brain. Retinal regenerative medicine has embraced microfluidic technologies to manipulate stem-like cells for transplantation therapies, where de/differentiated cells are introduced within adult tissue to replace dysfunctional or damaged neurons. Microfluidic systems coupled with stem cell biology and biomaterials have produced exciting advances to restore vision. The current article reviews contemporary microfluidic technologies and microfluidics-enhanced bioassays, developed to interrogate cellular responses to adult retinal cues. The focus is on applications of microfluidics and microscale assays within mammalian sensory retina, or neuro retina, comprised of five types of retinal neurons (photoreceptors, horizontal, bipolar, amacrine, retinal ganglion) and one neuroglia (Müller), but excludes the non-sensory, retinal pigmented epithelium.

DNA demethylation is a driver for chick retina regeneration

Cellular reprogramming resets the epigenetic landscape to drive shifts in transcriptional programmes and cell identity. The embryonic chick can regenerate a complete neural retina, after retinectomy, via retinal pigment epithelium (RPE) reprogramming in the presence of FGF2. In this study, we systematically analysed the reprogramming competent chick RPE prior to injury, and during different stages of reprogramming. In addition to changes in the expression of genes associated with epigenetic modifications during RPE reprogramming, we observed dynamic changes in histone marks associated with bivalent chromatin (H3K27me3/H3K4me3) and intermediates of the process of DNA demethylation including 5hmC and 5caC. Comprehensive analysis of the methylome by whole-genome bisulphite sequencing (WGBS) confirmed extensive rearrangements of DNA methylation patterns including differentially methylated regions (DMRs) found at promoters of genes associated with chromatin organization and fibroblast growth factor production. We also identified Tet methylcytosine dioxygenase 3 (TET3) as an important factor for DNA demethylation and retina regeneration, capable of reprogramming RPE in the absence of exogenous FGF2. In conclusion, we demonstrate that injury early in RPE reprogramming triggers genome-wide dynamic changes in chromatin, including bivalent chromatin and DNA methylation. In the presence of FGF2, these dynamic modifications are further sustained in the commitment to form a new retina. Our findings reveal active DNA demethylation as an important process that may be applied to remove the epigenetic barriers in order to regenerate retina in mammals.

ABBREVIATIONS

bp: Base pair; DMR: Differentially methylated region; DMC: Differentially methylated cytosines; GFP: Green fluorescent protein; PCR: Polymerase chain reaction. TET: Ten-eleven translocation; RPE: retinal pigment epithelium.

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.

Update on Müller glia regenerative potential for retinal repair

Retinal regeneration efficiency from Müller glia varies tremendously among vertebrate species, being extremely limited in mammals. Efforts towards the identification of molecular mechanisms underlying Müller cell proliferative and neurogenic potential should help finding strategies to awake them and ensure regeneration in mammals. We provide here an update on the most recent and original progresses made in the field. These include remarkable discoveries regarding (i) unprecedented cross-species comparison of Müller cell transcriptome using single-cell technologies, (ii) the identification of new strategies to promote both the proliferative and the neurogenic potential of mammalian Müller cells, (iii) the role of the epigenome in regulating Müller glia plasticity, (iv) miRNA-based regulatory mechanisms of Müller cell response to injury, and (v) the influence of inflammatory signals on the regenerative process.

Glia-Mediated Regenerative Response Following Acute Excitotoxic Damage in the Postnatal Squamate Retina

The retina is a complex tissue responsible for both detection and primary processing of visual stimuli. Although all vertebrate retinas share a similar, multi-layered organization, the ability to regenerate individual retinal cells varies tremendously, being extremely limited in mammals and birds when compared to anamniotes such as fish and amphibians. However, little is yet known about damage response and regeneration of retinal tissues in "non-classical" squamate reptiles (lizards, snakes), which occupy a key phylogenetic position within amniotes and exhibit unique regenerative features in many tissues. Here, we address this gap by establishing and characterizing a model of excitotoxic retinal damage in bearded dragon lizard (Pogona vitticeps). We particularly focus on identifying, at the cellular and molecular level, a putative endogenous cellular source for retinal regeneration, as diverse self-repair strategies have been characterized in vertebrates using a variety of retinal injury and transgenic models. Our findings reveal for the first time that squamates hold the potential for postnatal retinal regeneration following acute injury. Although no changes occur in the activity of physiologically active progenitors recently identified at the peripheral retinal margin of bearded dragon, two distinct successive populations of proliferating cells at central retina respond to neurotoxin treatment. Following an initial microglia response, a second source of proliferating cells exhibit common hallmarks of vertebrate Müller glia (MG) activation, including cell cycle re-entry, dedifferentiation into a progenitor-like phenotype, and re-expression of proneural markers. The observed lizard glial responses, although not as substantial as in anamniotes, appear more robust than the absent or neonatal-limited regeneration reported without exogenous stimulation in other amniotes. Altogether, these results help to complete our evolutionary understanding of regenerative potential of the vertebrate retina, and further highlight the major importance of glial cells in retinal regeneration. Furthermore, our work offers a new powerful vertebrate model to elucidate the developmental and evolutionary bases of retinal regeneration within amniotes. Such new understanding of self-repair mechanisms in non-classical species endowed with regenerative properties may help designing therapeutic strategies for vertebrate retinal diseases.

P2X7 and A2A receptor endogenous activation protects against neuronal death caused by CoCl2 -induced photoreceptor toxicity in the zebrafish retina

Injured retinas in mammals do not regenerate and heal with loss of function. The adult retina of zebrafish self-repairs after damage by activating cell-intrinsic mechanisms, which are regulated by extrinsic signal interactions. Among relevant regulatory extrinsic systems, purinergic signaling regulates progenitor proliferation during retinogenesis and regeneration and glia proliferation in proliferative retinopathies. ATP-activated P2X7 (P2RX7) and adenosine (P1R) receptors are involved in the progression of almost all retinopathies leading to blindness. Here, we examined P2RX7 and P1R participation in the retina regenerative response induced by photoreceptor damage caused by a specific dose of CoCl₂ . First, we found that treatment of uninjured retinas with a potent agonist of P2RX7 (BzATP) provoked photoreceptor damage and mitotic activation of multipotent progenitors. In CoCl₂ -injured retinas, blockade of endogenous extracellular ATP activity on P2RX7 caused further neurodegeneration, Müller cell gliosis, progenitor proliferation, and microglia reactivity. P2RX7 inhibition in injured retinas also increased the expression of lin28a and tnfα genes, which are related to multipotent progenitor proliferation. Levels of hif1α, vegf3r, and vegfaa mRNA were enhanced by blockade of P2RX7 immediately after injury, indicating hypoxic like damage and endothelial cell growth and proliferation. Complete depletion of extracellular nucleotides with an apyrase treatment strongly potentiated cell death and progenitor proliferation induced with CoCl₂ . Blockade of adenosine P1 and A_2A receptors (A_2A R) had deleterious effects and deregulated normal timing for progenitor and precursor cell proliferation following photoreceptor damage. ATP via P2RX7 and adenosine via A_2A R are survival extracellular signals key for retina regeneration in zebrafish.

Rebuilding the Retina: Prospects for Müller Glial-mediated Self-repair

Retinal degeneration is a leading cause of untreatable blindness in the industrialised world. It is typically irreversible and there are few curative treatments available. The use of stem cells to generate new retinal neurons for transplantation purposes has received significant interest in recent years and is beginning to move towards clinical trials. However, such approaches are likely to be most effective for relatively focal areas of repair. An intriguing complementary approach is endogenous self-repair. Retinal cells from the ciliary marginal zone (CMZ), retinal pigment epithelium (RPE) and Müller glial cells (MG) have all been shown to play a role in retinal repair, typically in lower vertebrates. Among them, MG have received renewed interest, due to their distribution throughout (centre to periphery) the neural retina and their potential to re-acquire a progenitor-like state following retinal injury with the ability to proliferate and generate new neurons. Triggering these innate self-repair mechanisms represents an exciting therapeutic option in treating retinal degeneration. However, these cells behave differently in mammalian and non-mammalian species, with a considerably restricted potential in mammals. In this short review, we look at some of the recent progress made in our understanding of the signalling pathways that underlie MG-mediated regeneration in lower vertebrates, and some of the challenges that have been revealed in our attempts to reactivate this process in the mammalian retina.

Rewiring the Regenerated Zebrafish Retina: Reemergence of Bipolar Neurons and Cone-Bipolar Circuitry Following an Inner Retinal Lesion

We previously reported strikingly normal morphologies and functional connectivities of regenerated retinal bipolar neurons (BPs) in zebrafish retinas sampled 60 days after a ouabain-mediated lesion of inner retinal neurons (60 DPI) (McGinn et al., 2018). Here we report early steps in the birth of BPs and formation of their dendritic trees and axonal arbors during regeneration. Adult zebrafish were subjected to ouabain-mediated lesion that destroys inner retinal neurons but spares photoreceptors and Müller glia, and were sampled at 13, 17, and 21 DPI, a timeframe over which plexiform layers reemerge. We show that this timeframe corresponds to reemergence of two populations of BPs (PKCα+ and nyx::mYFP+). Sequential BrdU, EdU incorporation reveals that similar fractions of PKCα+ BPs and HuC/D+ amacrine/ganglion cells are regenerated concurrently, suggesting that the sequence of neuronal production during retinal regeneration does not strictly match that observed during embryonic development. Further, accumulation of regenerated BPs appears protracted, at least through 21 DPI. The existence of isolated, nyx::mYFP+ BPs allowed examination of cytological detail through confocal microscopy, image tracing, morphometric analyses, identification of cone synaptic contacts, and rendering/visualization. Apically-projecting neurites (=dendrites) of regenerated BPs sampled at 13, 17, and 21 DPI are either truncated, or display smaller dendritic trees when compared to controls. In cases where BP dendrites reach the outer plexiform layer (OPL), numbers of dendritic tips are similar to those of controls at all sampling times. Further, by 13-17 DPI, BPs with dendritic tips reaching the outer nuclear layer (ONL) show patterns of photoreceptor connections that are statistically indistinguishable from controls, while those sampled at 21 DPI slightly favor contacts with double cone synaptic terminals over those of blue-sensitive cones. These findings suggest that once regenerated BP dendrites reach the OPL, normal photoreceptor connectomes are established, albeit with some plasticity. Through 17 DPI, some basally-projecting neurites (=axons) of regenerated nyx::mYFP+ BPs traverse long distances, branch into inappropriate layers, or appear to abruptly terminate. These findings suggest that, after a tissue-disrupting lesion, regeneration of inner retinal neurons is a dynamic process that includes ongoing genesis of new neurons and changes in BP morphology.

Dual regulation of lin28a by Myc is necessary during zebrafish retina regeneration

Cellular reprogramming leading to induction of Muller glia-derived progenitor cells (MGPCs) with stem cell characteristics is essential for zebrafish retina regeneration. Although several regeneration-specific genes are characterized, the significance of MGPC-associated Mycb induction remains unknown. Here, we show that early expression of Mycb induces expression of genes like ascl1a, a known activator of lin28a in MGPCs. Notably, mycb is simultaneously activated by Ascl1a and repressed by Insm1a in regenerating retina. Here, we unravel a dual role of Mycb in lin28a expression, both as an activator through Ascl1a in MGPCs and a repressor in combination with Hdac1 in neighboring cells. Myc inhibition reduces the number of MGPCs and abolishes normal regeneration. Myc in collaboration with Hdac1 inhibits her4.1, an effector of Delta-Notch signaling. Further, we also show the repressive role of Delta-Notch signaling on lin28a expression in post-injured retina. Our studies reveal mechanistic understanding of Myc pathway during zebrafish retina regeneration, which could pave way for therapeutic intervention during mammalian retina regeneration.

In Vivo Evaluation of PAX6 Overexpression and NMDA Cytotoxicity to Stimulate Proliferation in the Mouse Retina

Retinal degenerative diseases, due to the lack of regeneration systems and self-renewable cells, often lead to visual impairment. Pax6 is a pleiotropic transcription factor and its expression level determines self-renewal status or differentiation of retinal cells. Here, we investigated the fate of simultaneous induction of retinal ganglion cell death and Pax6 overexpression in retro-differentiation of retinal cells and their commitment to re-enter into the cell cycle. Induction of acute retinal ganglion cell death and generation of mouse experimental model was performed by N-methyl D-aspartic acid (NMDA) injection. Recombinant AAV2 virus harboring PAX6 cDNA and reporter gene was injected into untreated and model mouse eyes. Histological analyses, including IHC and retinal flatmounts immunostaining were performed. The number of Ki67+ cells was clearly increased in model mice, presumably due to NMDA treatment and regardless of Pax6 over-expression. Unlike previous studies, Ki67+ cells were found in GCL layer and interestingly ONL cells expressed Sox2 stemness marker after NMDA cytotoxicity. The potential of retinal cells for robust Ki67 expression, after injury, and expression of Sox2, confirmed their intrinsic plasticity and made a vivid prospect for retinal regenerative medicine.

Regulation of neuronal and photoreceptor cell differentiation by Wnt signaling from iris-derived stem/progenitor cells of the chick in flat vs. Matrigel-embedding cultures

Previously we developed a simple culture method of the iris tissues and reported novel properties of neural stem/progenitor-like cells in the iris tissues of the chick and pig. When the iris epithelium or connective tissue (stroma) was treated with dispase, embedded in Matrigel, and cultured, neuronal cells extended from the explants within 24 h of culture, and cells positively stained for photoreceptor cell markers were also observed within a few days of culturing. In ordinary flat tissue culture conditions, explants had the same differentiation properties to those in tissue environments. Previously, we suggested that iris neural stem/progenitor cells are simply suppressed from neuronal differentiation within tissue, and that separation from the tissue releases the cells from this suppression mechanism. Here, we examined whether Wnt signaling suppressed neuronal differentiation of iris tissue cells in tissue environments because the lens, which has direct contact with the iris, is a rich source of Wnt proteins. When the Wnt signaling activator 6-bromoindirubin-3'-oxime (BIO) was administered to Matrigel culture, neuronal differentiation was markedly suppressed, but cell proliferation was not affected. When Wnt signaling inhibitors, such as DKK-1 and IWR-1, were applied to the same culture, they did not have any effect on cell differentiation and proliferation. However, when the inhibitors were applied to flat tissue culture, cells with neural properties emerged. These results indicate that the interaction of iris tissue with neighboring tissues and the environment regulates the stemness nature of iris tissue cells, and that Wnt signaling is a major factor.

Histone Deacetylase-Mediated Müller Glia Reprogramming through Her4.1-Lin28a Axis Is Essential for Retina Regeneration in Zebrafish

Histone deacetylases (Hdacs) play significant roles in cellular homeostasis and tissue differentiation. Hdacs are well characterized in various systems for their physiological and epigenetic relevance. However, their significance during retina regeneration remains unclear. Here we show that inhibition of Hdac1 causes a decline in regenerative ability, and injury-dependent regulation of hdacs is essential for regulating regeneration-associated genes like ascl1a, lin28a, and repressors like her4.1 at the injury site. We show selective seclusion of Hdac1 from the proliferating Müller glia-derived progenitor cells (MGPCs) and its upregulation in the neighboring cells. Hdacs negatively regulate her4.1, which also represses lin28a and essential cytokines to control MGPCs proliferation. Interestingly, Hdacs' inhibition reversibly blocks regeneration through the repression of critical cytokines and other regeneration-specific genes, which is also revealed by whole-retina RNA sequence analysis. Our study shows mechanistic understanding of the Hdac pathway during zebrafish retina regeneration.

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Hdac1, along with other Hdacs, is a key regulator of retina regeneration in zebrafish

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Hdacs regulate MGPCs' formation through Her4.1/Lin28a/let-7 miRNA axis

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Hdacs' inhibition reversibly blocks MGPCs' proliferation and retina regeneration

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Hdacs/Her4.1 interplay regulates essential cytokines during retina regeneration

Retinal histogenesis in an altricial avian species, the zebra finch (Taeniopygia guttata, Vieillot 1817)

Comparative developmental studies have shown that the retina of altricial fish and mammals is incompletely developed at birth, and that, during the first days of life, maturation proceeds rapidly. In contrast, precocial fish and mammals are born with fully differentiated retinas. Concerning birds, knowledge about retinal development is generally restricted to a single order of precocial birds, Galliformes, due to the fact that both the chicken and the Japanese quail are considered model systems. However, comparison of embryonic pre-hatchling retinal development between altricial and precocial birds has been poorly explored. The purpose of this study was to examine the morphogenesis and histogenesis of the retina in the altricial zebra finch (Taeniopygia guttata, Vieillot 1817) and compare the results with those from previous studies in the precocial chicken. Several maturational features (morphogenesis of the optic vesicle and optic cup, appearance of the first differentiated neurons, the period in which the non-apical cell divisions are observable, and the emergence of the plexiform layers) were found to occur at later stages in the zebra finch than in the chicken. At hatching, the retina of T. guttata showed the typical cytoarchitecture of the mature tissue, although features of immaturity were still observable, such as a ganglion cell layer containing many thick cells, very thin plexiform layers, and poorly developed photoreceptors. Moreover, abundant mitotic activity was detected in the entire retina, even in the regions where the layering was complete. The circumferential marginal zone was very prominent and showed abundant mitotic activity. The partially undifferentiated stage of maturation at hatching makes the T. guttata retina an appropriate model with which to study avian postnatal retinal neurogenesis.

Conditional Chemogenetic Ablation of Photoreceptor Cells in Xenopus Retina

Xenopus is an attractive model system for regeneration studies, as it exhibits an extraordinary regenerative capacity compared to mammals. It is commonly used to study body part regeneration following amputation, for instance of the limb, the tail, or the retina. Models with more subtle injuries are also needed for human degenerative disease modeling, allowing for the study of stem cell recruitment for the regeneration of a given cellular subtype. We present here a model to ablate photoreceptor cells in the Xenopus retina. This method is based on the nitroreductase/metronidazole (NTR/MTZ) system, a combination of chemical and genetic tools, allowing for the conditional ablation of targeted cells. This type of approach establishes Xenopus as a powerful model to study cellular regeneration and stem cell regulation.

Opportunities for CRISPR/Cas9 Gene Editing in Retinal Regeneration Research

While retinal degeneration and disease results in permanent damage and vision loss in humans, the severely damaged zebrafish retina has a high capacity to regenerate lost neurons and restore visual behaviors. Advancements in understanding the molecular and cellular basis of this regeneration response give hope that strategies and therapeutics may be developed to restore sight to blind and visually-impaired individuals. Our current understanding has been facilitated by the amenability of zebrafish to molecular tools, imaging techniques, and forward and reverse genetic approaches. Accordingly, the zebrafish research community has developed a diverse array of research tools for use in developing and adult animals, including toolkits for facilitating the generation of transgenic animals, systems for inducible, cell-specific transgene expression, and the creation of knockout alleles for nearly every protein coding gene. As CRISPR/Cas9 genome editing has begun to revolutionize molecular biology research, the zebrafish community has responded in stride by developing CRISPR/Cas9 techniques for the zebrafish as well as incorporating CRISPR/Cas9 into available toolsets. The application of CRISPR/Cas9 to retinal regeneration research will undoubtedly bring us closer to understanding the mechanisms underlying retinal repair and vision restoration in the zebrafish, as well as developing therapeutic approaches that will restore vision to blind and visually-impaired individuals. This review focuses on how CRISPR/Cas9 has been integrated into zebrafish research toolsets and how this new tool will revolutionize the field of retinal regeneration research.

Restoration of Dendritic Complexity, Functional Connectivity, and Diversity of Regenerated Retinal Bipolar Neurons in Adult Zebrafish

Adult zebrafish (Danio rerio) are capable of regenerating retinal neurons that have been lost due to mechanical, chemical, or light damage. In the case of chemical damage, there is evidence that visually mediated behaviors are restored after regeneration, consistent with recovery of retinal function. However, the extent to which regenerated retinal neurons attain appropriate morphologies and circuitry after such tissue-disrupting lesions has not been investigated. Adult zebrafish of both sexes were subjected to intravitreal injections of ouabain, which destroys the inner retina. After retinal regeneration, cell-selective markers, confocal microscopy, morphometrics, and electrophysiology were used to examine dendritic and axonal morphologies, connectivities, and the diversities of each, as well as retinal function, for a subpopulation of regenerated bipolar neurons (BPs). Although regenerated BPs were reduced in numbers, BP dendritic spreads, dendritic tree morphologies, and cone-bipolar connectivity patterns were restored in regenerated retinas, suggesting that regenerated BPs recover accurate input pathways from surviving cone photoreceptors. Morphological measurements of bipolar axons found that numbers and types of stratifications were also restored; however, the thickness of the inner plexiform layer and one measure of axon branching were slightly reduced after regeneration, suggesting some minor differences in the recovery of output pathways to downstream partners. Furthermore, ERG traces from regenerated retinas displayed waveforms matching those of controls, but with reduced b-wave amplitudes. These results support the hypothesis that regenerated neurons of the adult zebrafish retina are capable of restoring complex morphologies and circuitry, suggesting that complex visual functions may also be restored.SIGNIFICANCE STATEMENT Adult zebrafish generate new retinal neurons after a tissue-disrupting lesion. Existing research does not address whether regenerated neurons of adults successfully reconnect with surrounding neurons and establish complex morphologies and functions. We report that, after a chemical lesion that ablates inner retinal neurons, regenerated retinal bipolar neurons (BPs), although reduced in numbers, reconnected to undamaged cone photoreceptors with correct wiring patterns. Regenerated BPs had complex morphologies similar to those within undamaged retina and a physiological measure of photoreceptor-BP connectivity, the ERG, was restored to a normal waveform. This new understanding of neural connectivity, morphology, and physiology suggests that complex functional processing is possible within regenerated adult retina and offers a system for the future study of synaptogenesis during adult retinal regeneration.

Stem cells and genome editing: approaches to tissue regeneration and regenerative medicine

Understanding the basis of regeneration of each tissue and organ, and incorporating this knowledge into clinical treatments for degenerative tissues and organs in patients, are major goals for researchers in regenerative biology. Here we provide an overview of current work, from high-regeneration animal models, to stem cell-based culture models, transplantation technologies, large-animal chimeric models, and programmable nuclease-based genome-editing technologies. Three-dimensional culture generating organoids, which represents intact tissue/organ identity including cell fate and morphology are getting more general approaches in the fields by taking advantage of embryonic stem cells, induced pluripotent stem cells and adult stem cells. The organoid culture system potentially has profound impact on the field of regenerative medicine. We also emphasize that the large animal model, in particular pig model would be a hope to manufacture humanized organs in in vivo empty (vacant) niche, which now potentially allows not only appropriate cell fate identity but nearly the same property as human organs in size. Therefore, integrative and collaborative researches across different fields might be critical to the aims needed in clinical trial.