Distinct neurogenic potential in the retinal margin and the pars plana of mammalian eye

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.

Insights on the Regeneration Potential of Müller Glia in the Mammalian Retina

Müller glia, the major glial cell types in the retina, maintain retinal homeostasis and provide structural support to retinal photoreceptors. They also possess regenerative potential that might be used for retinal repair in response to injury or disease. In teleost fish (such as zebrafish), the Müller glia response to injury involves reprogramming events that result in a population of proliferative neural progenitors that can regenerate the injured retina. Recent studies have revealed several important mechanisms for the regenerative capacity of Müller glia in fish, which may shed more light on the mechanisms of Müller glia reprogramming and regeneration in mammals. Mammalian Müller glia can adopt stem cell characteristics, and in response to special conditions, be persuaded to proliferate and regenerate, although their native regeneration potential is limited. In this review, we consider the work to date revealing the regenerative potential of the mammalian Müller glia and discuss whether they are a potential source for cell regeneration therapy in humans.

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.

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.

Variations in the proliferative activity of the peripheral retina correlate with postnatal ocular growth in squamate reptiles

The retina is a complex, multilayered tissue responsible for the perception of visual stimuli from the environment. Contrary to mammals, the capacity for postnatal eye growth in fish and amphibians, and to a lower extent in birds, is coordinated with a progenitor population residing in the ciliary marginal zone (CMZ) at the retinal peripheral margin. However, little is known about embryonic retinogenesis and postnatal retinal growth in squamates (lizards, snakes), despite their exceptional array of ecologies and ocular morphologies. Here, we address this gap by performing the first large‐scale study assessing both ontogenetic and adult changes in the stem/progenitor activity of the squamate peripheral retina. Our study reveals for the first time that squamates exhibit a source of proliferating progenitors persisting post embryogenesis in a newly identified retinociliary junction anteriorly adjacent to the retina. This region is strikingly similar to the vertebrate CMZ by its peripheral location and pseudostratified nature, and shares a common pattern of slow‐cycling cells, spatial differentiation gradient, and response to postnatal ocular growth. Additionally, its proliferative activity varies considerably among squamate species, in correlation with embryonic and postnatal differences in eye size and growth. Together our data indicate that squamates possess a proliferative peripheral retina that acts as a source of progenitors to compensate, at least in part, for postnatal ocular growth. Our findings also highlight the remarkable variation in activity and location of vertebrate retinal progenitors, indicating that the currently accepted scenario of reduced CMZ activity over the course of evolution is too simplistic.

ATN-161 as an Integrin α5β1 Antagonist Depresses Ocular Neovascularization by Promoting New Vascular Endothelial Cell Apoptosis

BACKGROUND ATN-161 (Ac-PHSCN-NH2), an antagonist of integrin α5β1, has shown an important influence in inhibiting tumor angiogenesis and metastasis of other tumor types. However, the mechanism of action of ATN-161 and whether it can inhibit ocular neovascularization (NV) are unclear. This study investigated the role of ATN-161 in regulating ocular angiogenesis in mouse models and explored the underlying signaling pathway. MATERIAL AND METHODS An oxygen-induced retinopathy (OIR) mouse model and a laser-induced choroidal neovascularization (CNV) mouse model were used to test integrin a5b1 expression and the effect of ATN-161 on ocular NV by immunofluorescence staining, Western blot analysis, and flat-mount analysis. The activation of nuclear factor-κB (NF-κB), matrix metalloproteinase-2/9 (MMP-2/9), and cell apoptosis were detected by immunofluorescence staining, Western blot, real-time RT-PCR, and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL). The cell proliferation was detected by BrdU labeling. RESULTS In OIR and CNV mice, the protein expression level of integrin α5β1 increased compared with that in age-matched controls. The mice given ATN-161 had significantly reduced retinal neovascularization (RNV) and CNV. Blocking integrin a5b1 by ATN-161 strongly inhibited nuclear factor-κB (NF-κB) activation and matrix metalloproteinase-2/9 (MMP-2/9) expression and promoted cell apoptosis, but the effect of ATN-161 on proliferation in CNV mice was indirect and required the inhibition of neovascularization. Inhibiting NF-κB activation by ammonium pyrrolidinedithiocarbamate (PDTC) reduced RNV and promoted cell apoptosis in ocular NV. CONCLUSIONS Blocking integrin α5β1 by ATN-161 reduced ocular NV by inhibiting MMP-2/MMP-9 expression and promoting the cell apoptosis of ocular NV.

Proliferative Cells Isolated from the Adult Human Peripheral Retina only Transiently Upregulate Key Retinal Markers upon Induced Differentiation

Purpose/Aim: The adult human retina has limited regenerative potential, and severe injury will result in permanent damage. Lower vertebrates handle retinal injury by activating neural stem cells (NSCs) in the ciliary marginal zone (CMZ). Müller glia-like cells expressing markers of NSCs are also present in the peripheral retina (PR) of the adult human eye, leading to the hypothesis that a CMZ-like zone might exists also in humans. In order to shed further light on this hypothesis we investigated the in vitro differentiation potential of proliferative cells isolated from the adult human PR towards a retinal phenotype.

MATERIALS AND METHODS

Proliferative cells were isolated from the peripheral retina of human eyes (n = 6) within 24 to 48 hours post mortem and further expanded for 2 or 3 passages before being differentiated for 1-3 weeks. Gene expression was analyzed by microarray and qRT-PCR analysis, while protein expression was identified by immunocytochemistry.

RESULTS

A high density of cells co-staining with markers for progenitor cells and Müller glia was found in situ in the PR. Cells isolated from this region and cultured adherently showed fibrillary processes and were positive for the immature marker Nestin and the glial marker GFAP, while a few co-expressed PAX6. After 7 days of differentiation, there was a transient upregulation of early and mature photoreceptor markers, including NRL, CRX, RHO and RCVRN, as well as the Müller cell and retinal pigmented epithelium (RPE) marker CRALBP, and the early RPE marker MITF. However, the expression of all these markers dropped from Day 14 and onwards.

CONCLUSIONS

Upon exposure of proliferating cells from the adult human PR to differentiating conditions in culture, there is a widespread change in morphology and gene expression, including the upregulation of key retinal markers. However, this upregulation is only transient and decreases after 14 days of differentiation.

Bipotent progenitors as embryonic origin of retinal stem cells

In lower vertebrates, retinal stem cells (RSCs) capable of producing all retinal cell types are a resource for retinal tissue growth throughout life. However, the embryonic origin of RSCs remains largely elusive. Using a Zebrabow-based clonal analysis, we characterized the RSC niche in the ciliary marginal zone of zebrafish retina and illustrate that blood vessels associated with RSCs are required for the maintenance of actively proliferating RSCs. Full lineage analysis of RSC progenitors reveals lineage patterns of RSC production. Moreover, in vivo lineage analysis demonstrates that these RSC progenitors are the direct descendants of a set of bipotent progenitors in the medial epithelial layer of developing optic vesicles, suggesting the involvement of the mixed-lineage states in the RSC lineage specification.

Retinal Degeneration and Regeneration-Lessons From Fishes and Amphibians

PURPOSE OF REVIEW

Retinal degenerative diseases have immense socio-economic impact. Studying animal models that recapitulate human eye pathologies aids in understanding the pathogenesis of diseases and allows for the discovery of novel therapeutic strategies. Some non-mammalian species are known to have remarkable regenerative abilities and may provide the basis to develop strategies to stimulate self-repair in patients suffering from these retinal diseases.

RECENT FINDINGS

Non-mammalian organisms, such as zebrafish and Xenopus, have become attractive model systems to study retinal diseases. Additionally, many fish and amphibian models of retinal cell ablation and cell lineage analysis have been developed to study regeneration. These investigations highlighted several cellular sources for retinal repair in different fish and amphibian species. Moreover, major differences in repair mechanisms have been reported in these animal models.

SUMMARY

This review aims to emphasize first on the importance of zebrafish and Xenopus models in studying the pathogenesis of retinal diseases and, second, on the different modes of regeneration processes in these model organisms.

Müller glial cell-dependent regeneration of the neural retina: An overview across vertebrate model systems

Retinal dystrophies are a major cause of blindness for which there are currently no curative treatments. Transplantation of stem cell‐derived neuronal progenitors to replace lost cells has been widely investigated as a therapeutic option. Another promising strategy would be to trigger self‐repair mechanisms in patients, through the recruitment of endogenous cells with stemness properties. Accumulating evidence in the past 15 year0s has revealed that several retinal cell types possess neurogenic potential, thus opening new avenues for regenerative medicine. Among them, Müller glial cells have been shown to be able to undergo a reprogramming process to re‐acquire a stem/progenitor state, allowing them to proliferate and generate new neurons for repair following retinal damages. Although Müller cell–dependent spontaneous regeneration is remarkable in some species such as the fish, it is extremely limited and ineffective in mammals. Understanding the cellular events and molecular mechanisms underlying Müller cell activities in species endowed with regenerative capacities could provide knowledge to unlock the restricted potential of their mammalian counterparts. In this context, the present review provides an overview of Müller cell responses to injury across vertebrate model systems and summarizes recent advances in this rapidly evolving field. Developmental Dynamics 245:727–738, 2016. © 2015 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc.

The present review provides an overview of Müller cell responses to injury across vertebrate model systems and summarizes recent advances in this rapidly evolving field.

Loss of ATOH8 Increases Stem Cell Features of Hepatocellular Carcinoma Cells

BACKGROUND & AIMS

Levels of atonal homolog 8 (ATOH8) are reduced in 48% of hepatitis B virus-associated hepatocellular carcinoma cells (HCCs). ATOH8 downregulation is associated with loss of tumor differentiation, indicating an effect mediated by cancer stem cells. We investigated the effects of loss of ATOH8 in human hepatocellular carcinoma (HCC) cells and cell lines.

METHODS

HCC and adjacent nontumor tissues were collected, from 2001 through 2012, from 242 patients undergoing hepatectomy at Sun Yat-Sen University Cancer Center in China; 83% of HCCs were associated with hepatitis B virus (HBV) infection. CD133(+) cells were isolated from tumor tissues by flow cytometry. Experiments were performed in HBV-positive and HBV-negative HCC cell lines, the immortalized liver cell line LO2, and 8 other HCC cell lines. ATOH8 was expressed from lentiviral vectors in PLC8024 and Huh7 cells; levels were knocked down with small interfering RNAs in QSG7701 cells. Cells carrying empty vectors were used as controls. Gene regulation by ATOH8 was assessed in mobility shift and luciferase reporter assays. Cells were analyzed in proliferation, foci formation, and colony formation assays. The tumorigenic and chemo-resistant potential of cells were investigated by assessing growth of xenograft tumors in immunocompromised mice. Metastatic features of cells were assessed in Matrigel invasion assays and wound healing analyses.

RESULTS

Levels of ATOH8 mRNA were reduced by more than 4-fold, compared to nontumor tissues, in 118 of 242 HCC samples (48.8%). Patients with tumor reductions in ATOH8 had significantly shorter times of disease-free survival (mean, 41.4 months) than patients with normal tissue levels (mean, 52.6 months). ATOH8 expression was reduced in HepG2, Huh7, PLC8024 and CRL8064 HCC cells, as well as CD133(+) cells isolated from human HCC samples. Transgenic expression of ATOH8 in HCC cell lines significantly reduced proliferation and foci colony formation, as well as their invasive and migratory abilities. Transgenic expression of ATOH8 reduced the ability of HBV-positive PLC8024 cells to form tumors in mice, compared to control cells. Cells with ATOH8 knockdown formed xenograft tumors more rapidly, in more mice, than control cells. ATOH8 repressed transcription of stem-cell associated genes including OCT4, NANOG, and CD133. Knockdown of ATOH8 in CD133-negative QSG7701 cells caused them to express CD133; acquire self-renewal, differentiation, chemo-resistance properties; form more xenograft tumors in mice; and generate induced pluripotent stem cells (based on staining for alkaline phosphatase and their ability to form embryoid bodies and teratomas). Alternatively, expression of ATOH8 in PLC8024 and Huh7 cells significantly reduced the numbers of cells expressing CD133, and increased the chemo-sensitivity of Huh7 cells to 5-fluorouracil (5-FU) and cisplatin, in vitro and in mice.

CONCLUSIONS

ATOH8 appears to be a tumor suppressor that induces stem-cell features and chemoresistance in HCC cells. Strategies to restore its levels and activities might be developed to treat patients with liver cancer.

T-box transcription regulator Tbr2 is essential for the formation and maintenance of Opn4/melanopsin-expressing intrinsically photosensitive retinal ganglion cells

Opsin 4 (Opn4)/melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) play a major role in non-image-forming visual system. Although advances have been made in understanding their morphological features and functions, the molecular mechanisms that regulate their formation and survival remain unknown. Previously, we found that mouse T-box brain 2 (Tbr2) (also known as Eomes), a T-box-containing transcription factor, was expressed in a subset of newborn RGCs, suggesting that it is involved in the formation of specific RGC subtypes. In this in vivo study, we used complex mouse genetics, single-cell dye tracing, and behavioral analyses to determine whether Tbr2 regulates ipRGC formation and survival. Our results show the following: (1) Opn4 is expressed exclusively in Tbr2-positive RGCs; (2) no ipRGCs are detected when Tbr2 is genetically ablated before RGC specification; and (3) most ipRGCs are eliminated when Tbr2 is deleted in established ipRGCs. The few remaining ipRGCs display abnormal dendritic morphological features and functions. In addition, some Tbr2-expressing RGCs can activate Opn4 expression on the loss of native ipRGCs, suggesting that Tbr2-expressing RGCs may serve as a reservoir of ipRGCs to regulate the number of ipRGCs and the expression levels of Opn4.

Bone morphogenetic proteins and secreted frizzled related protein 2 maintain the quiescence of adult mammalian retinal stem cells

Rare retinal stem cells (RSCs) within the ciliary epithelium at the retinal margin of the adult mouse and human eyes can divide in vitro in the absence of growth factors to generate clonal, self-renewing spheres which can generate all the retinal cell types. Since no regenerative properties are seen in situ in the adult mammalian eye, we sought to determine the factors that are involved in the repression of endogenous RSCs. We discovered that factors secreted by the adult lens and cornea block the proliferation of adult RSCs in vitro. Bone morphogenetic protein (BMP)2, BMP4, and secreted frizzled related protein 2 were identified as principal effectors of the anti-proliferative effects on RSCs. As a similar induced quiescence was observed in vitro on both mouse and human RSCs, targeting these molecules in vivo may reactivate RSCs directly in situ in the eyes of the blind.

Advances in repairing the degenerate retina by rod photoreceptor transplantation

Despite very different aetiologies, age-related macular degeneration (AMD) and most inherited retinal disorders culminate in the same final common pathway, loss of the light-sensitive photoreceptors. There are few clinical treatments and none can reverse the loss of vision. Photoreceptor replacement by transplantation is proposed as a broad treatment strategy applicable to all degenerations. The past decade has seen a number of landmark achievements in this field, which together provide strong justification for continuing investigation into photoreceptor replacement strategies. These include proof of principle for restoring vision by rod-photoreceptor transplantation in mice with congenital stationary night blindness and advances in stem cell biology, which have led to the generation of complete optic structures in vitro from embryonic stem cells. The latter represents enormous potential for generating suitable and renewable donor cells with which to achieve the former. However, there are still challenges presented by the degenerating recipient retinal environment that must be addressed as we move to translating these technologies towards clinical application.

The potential of stem cell research for the treatment of neuronal damage in glaucoma

Stem cell research offers a wide variety of approaches for the advancement of our understanding of basic mechanisms of neurodegeneration and tissue regeneration and for the discovery and development of new therapeutic strategies to prevent and restore neuronal cell loss. Similar to most other regions of our central nervous system, degenerative diseases of the retina lead to the loss of neurons, which are not replaced. Recent work in animals has provided proof-of-concept evidence for the restoration of photoreceptor cells by cell transplantation and neuronal cell replacement by regeneration from endogenous cell sources. However, efficient therapeutic prevention of neuronal cell loss has not been achieved. Moreover, successful cell replacement of retinal neurons in humans, including that of ganglion cells, remains a major challenge. Future successes in the discovery and translation of neuroprotective drug and gene therapies and of cell-based regenerative therapies will depend on a better understanding of the underlying disease pathomechanisms. Existing stem cell and cell-reprogramming technologies offer the potential to generate human retina cells, to develop specific human-cell-based retina disease models, and to open up novel therapeutic strategies. Further, we might glean substantial knowledge from species that can or cannot regenerate their neuronal retina, in the search for new therapeutic approaches. Thus, stem cell research will pave the way toward clinical translation. In this review, I address some of the major possibilities presently on offer and speculate about the power of stem cell research to gain further insights into the pathomechanisms of retinal neurodegeneration (with special emphasis on glaucoma) and to advance our therapeutic options.