FGF signaling regulates rod photoreceptor cell maintenance and regeneration in zebrafish

Comparison of the effects of EGF, FGF-b, and NGF on the proliferation, migration, and reprogramming of primary rat Müller cells

Purpose

During the healing process of full-thickness macular holes (FTMHs), the closure and recovery of the hole depend on the migration, proliferation, and activation of Müller cells to promote the closure of holes and restoration of the photosensitive layer. In this study, we investigated the ability of the epidermal growth factor (EGF), fibroblast growth factor-basic (FGF-b), and nerve growth factor (NGF) to influence this process by regulating proliferation, migration, and reprogramming of primary rat Müller cells.

Methods

Cell proliferation was measured using CCK8 [2- (2-Methoxy-4-nitrophenyl)-3- (4-nitrophenyl)-5- (2,4-disulfophenyl)-2H-tetrazolium Sodium Salt] colorimetric assays and EdU [5-Ethynyl-2'-deoxyuridine] assays over 48 h. Cell migration was measured using scratch-wound assays and transwell migration assays over 48 h. In addition, we conducted Western blot assays and immunofluorescence assays on cells that were specially treated for 1, 3, and 5 days for cell reprogramming. The percentage of EdU-positive cells in Nestin-positive have also been tested by co-immunofluorescence (Co-IF) staining.

Results

EGF and FGF-b significantly promoted the proliferation of Müller cells (p < 0.05) at a concentration of 0-50 ng/mL, but NGF did not (p > 0.05), compared to untreated controls. Exogenous FGF-b and EGF promote the reprogramming of primary rat Müller cells, significantly enhancing the neural stem cell marker Nestin after stimulation on the 1st, 3rd, and 5th days, respectively. The expression of Müller cell marker Vimentin was significantly (p < 0.05) reduced during this period compared to the control group. However, there was no significant difference between the NGF and control groups. Furthermore, the EGF group expressed stronger Nestin expression than the SCM group. The Co-IF staining showed that early 50% of activated cells came from newly proliferating cells on the 5th day.

Conclusion

These observations suggest that FGF-b can promote the activation of Müller cells in a short time and enhance the possessive features of neural stem cells, while EGF may act for a longer period of time. This may further the understanding of growth factor therapy in treating FTMHs, and Müller glia may be promising candidates for cell replacement therapy.

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.

An Overview towards Zebrafish Larvae as a Model for Ocular Diseases

Despite the obvious morphological differences in the visual system, zebrafish share a similar architecture and components of the same embryonic origin as humans. The zebrafish retina has the same layered structure and cell types with similar metabolic and phototransduction support as humans, and is functional 72 h after fertilization, allowing tests of visual function to be performed. The zebrafish genomic database supports genetic mapping studies as well as gene editing, both of which are useful in the ophthalmological field. It is possible to model ocular disorders in zebrafish, as well as inherited retinal diseases or congenital or acquired malformations. Several approaches allow the evaluation of local pathological processes derived from systemic disorders, such as chemical exposure to produce retinal hypoxia or glucose exposure to produce hyperglycemia, mimicking retinopathy of prematurity or diabetic retinopathy, respectively. The pathogenesis of ocular infections, autoimmune diseases, or aging can also be assessed in zebrafish larvae, and the preserved cellular and molecular immune mechanisms can be assessed. Finally, the zebrafish model for the study of the pathologies of the visual system complements certain deficiencies in experimental models of mammals since the regeneration of the zebrafish retina is a valuable tool for the study of degenerative processes and the discovery of new drugs and therapies.

Expression of the neuroprotective factors BDNF, CNTF, and FGF-2 in normal and oxygen induced retinopathy

INTRODUCTION

Oxygen-induced retinopathy is a type of retinal pathological neovascularization (NV) disease that leads to vision loss and translates to a significant societal cost. Anti-vascular endothelial growth factor (VEGF) and anti-inflammatory treatments have been widely used in the clinic, but the results have not been entirely satisfactory. It is necessary to explore other treatments for Ischemic retinal diseases.

METHODS

The oxygen-induced retinopathy (OIR) model was induced from P7 to P12 as described. Histology evaluation (HE) and retina flat mounts were checked at P17 to confirm the establishment of the OIR model. Retinal ganglion cell (RGC) degeneration was checked by transmission electron microscopy at P17 to confirm the neurological damage caused by OIR. Western blot analysis was performed at P12, P15, and P17 to study the expression of brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), and fibroblast growth factor 2 (FGF-2) in normal and OIR mice. Comparative analysis of the expressions of BDNF, CNTF, and FGF-2 in normal and OIR mice was performed.

RESULTS

There were many retinal NV and non-perfusion areas in OIR P17. RGCs were degenerated at OIR P17. The expressions of BDNF, CNTF, and FGF-2 gradually increased from P12 to P17 in normal mice and were much higher in OIR mice. The expression curves of BDNF, CNTF, and FGF-2 in the OIR model were inconsistent and did not correlate with each other.

DISCUSSION

This study provides evidence for changes in BDNF, CNTF, and FGF-2 in Oxygen-induced retinopathy.

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.

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.

Jack of all trades, master of each: the diversity of fibroblast growth factor signalling in eye development

A central question in development biology is how a limited set of signalling pathways can instruct unlimited diversity of multicellular organisms. In this review, we use three ocular tissues as models of increasing complexity to present the astounding versatility of fibroblast growth factor (FGF) signalling. In the lacrimal gland, we highlight the specificity of FGF signalling in a one-dimensional model of budding morphogenesis. In the lens, we showcase the dynamics of FGF signalling in altering functional outcomes in a two-dimensional space. In the retina, we present the prolific utilization of FGF signalling from three-dimensional development to homeostasis. These examples not only shed light on the cellular basis for the perfection and complexity of ocular development, but also serve as paradigms for the diversity of FGF signalling.

The Role of Fibroblast Growth Factor (FGF) Signaling in Tissue Repair and Regeneration

Fibroblast growth factors (FGFs) are a large family of secretory molecules that act through tyrosine kinase receptors known as FGF receptors. They play crucial roles in a wide variety of cellular functions, including cell proliferation, survival, metabolism, morphogenesis, and differentiation, as well as in tissue repair and regeneration. The signaling pathways regulated by FGFs include RAS/mitogen-activated protein kinase (MAPK), phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)–protein kinase B (AKT), phospholipase C gamma (PLCγ), and signal transducer and activator of transcription (STAT). To date, 22 FGFs have been discovered, involved in different functions in the body. Several FGFs directly or indirectly interfere with repair during tissue regeneration, in addition to their critical functions in the maintenance of pluripotency and dedifferentiation of stem cells. In this review, we summarize the roles of FGFs in diverse cellular processes and shed light on the importance of FGF signaling in mechanisms of tissue repair and regeneration.

Cellular Mechanisms of FGF-Stimulated Tissue Repair

Growth factors belonging to the FGF family play important roles in tissue and organ repair after trauma. In this review, I discuss the regulation by FGFs of the aspects of cellular behavior important for reparative processes. In particular, I focus on the FGF-dependent regulation of cell proliferation, cell stemness, de-differentiation, inflammation, angiogenesis, cell senescence, cell death, and the production of proteases. In addition, I review the available literature on the enhancement of FGF expression and secretion in damaged tissues resulting in the increased FGF supply required for tissue repair.

Increased interdigitation zone visibility on optical coherence tomography following systemic fibroblast growth factor receptor 1-3 tyrosine kinase inhibitor anticancer therapy

BACKGROUND

To describe ocular adverse events and retinal changes during fibroblast growth factor receptor (FGFR) inhibitor (AZD4547) anticancer therapy.

METHODS

This is a sub-study examining ocular adverse effects from AZD4547 therapy (single-centre, open-label, single arm phase II clinical trial). Comprehensive ocular examinations were performed 3 weekly in 24 patients. Macular optical coherence tomography (OCT) scan (30⁰  × 25⁰ ) was obtained at each visit and OCT parameters [central 1 mm retinal thickness (CRT) and total macular volume in central 6 mm] extracted. OCT scans were subdivided into outer (ELM to RPE) and inner (ELM to ILM) layers to compare outer and inner retinal changes.

RESULTS

In 24 patients, AZD4547 was associated with eyelash elongation (n = 5, 21%) and punctate corneal erosion (n = 2, 8%). One patient developed clinically significant posterior capsular opacification during the study. OCT data were available in 23 patients, retinal changes ranged from an asymptomatic increased visibility of the interdigitation zone (IDZ) (n = 10, 43%) to multilobular subretinal fluid pockets (n = 5, 22%), which was associated with mild visual acuity loss. In a subset of patients (n = 9) with pre-AZD4547 dosing OCT baseline, CRT increased by mean (SD) of 9 (4) μm in those with IDZ change only compared with 64 (38) μm in those with other retinal changes. Retinal changes tended to be bilateral, self-limiting and improved over time without medical intervention.

CONCLUSIONS

The ocular signs and symptoms did not result in dose cessation. Posteriorly, FGFR inhibition leads to outer retinal changes ranging from increased visibility of IDZ to distinct, multiple fluid pockets.

Development and characterization of a chronic photoreceptor degeneration model in adult zebrafish that does not trigger a regenerative response

Zebrafish (Danio rerio) have become a highly-utilized model system in the field of regenerative biology because of their endogenous ability to regenerate many tissues and organs, including the retina. The vast majority of previous research on retinal regeneration in adult zebrafish utilizes acute methodologies for retinal damage. Acute retinal cell death triggers a reactive gliosis response of Müller glia (MG), the resident macroglia of the retina. In addition, each activated MG undergoes asymmetric cell division to produce a neuronal progenitor, which continues to divide and ultimately gives rise to new retinal neurons. Studies using these approaches have uncovered many crucial mechanisms by which MG respond to acute damage. However, they may not adequately mimic the chronic neuronal degeneration observed in many human retinal degenerative diseases. The current study aimed to develop a new long-term, chronic photoreceptor damage and degeneration model in adult zebrafish. Comparing the subsequent cellular responses to that of the commonly-used acute high-intensity model, we found that low, continuous light exposure damaged the outer segments of both rod and cone photoreceptors, but did not result in significant apoptotic cell death, MG gliosis, or MG cell-cycle re-entry. Instead, chronic light nearly completely truncated photoreceptor outer segments and resulted in a recruitment of microglia to the area. Together, these studies present a chronic photoreceptor model that can be performed in a relatively short time frame (21 days), that may lend insight into the cellular events underlying non-regenerative photoreceptor degeneration observed in other model systems.

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.

Photoreceptor ablation following ATP induced injury triggers Müller glia driven regeneration in zebrafish

Retinal regeneration research offers hope to people affected by visual impairment due to disease and injury. Ongoing research has explored many avenues towards retinal regeneration, including those that utilizes implantation of devices, cells or targeted viral-mediated gene therapy. These results have so far been limited, as gene therapy only has applications for rare single-gene mutations and implantations are invasive and in the case of cell transplantation donor cells often fail to integrate with adult neurons. An alternative mode of retinal regeneration utilizes a stem cell population unique to vertebrate retina - Müller glia (MG). Endogenous MG can readily regenerate lost neurons spontaneously in zebrafish and to a very limited extent in mammalian retina. The use of adenosine triphosphate (ATP) has been shown to induce retinal degeneration and activation of the MG in mammals, but whether this is conserved to other vertebrate species including those with higher regenerative capacity remains unknown. In our study, we injected a single dose of ATP intravitreal in zebrafish to characterize the cell death and MG induced regeneration. We used TUNEL labelling on retinal sections to show that ATP caused localised death of photoreceptors and ganglion cells within 24 h. Histology of GFP-transgenic zebrafish and BrdU injected fish demonstrated that MG proliferation peaked at days 3 and 4 post-ATP injection. Using BrdU labelling and photoreceptor markers (Zpr1) we observed regeneration of lost rod photoreceptors at day 14. This study has been undertaken to allow for comparative studies between mammals and zebrafish that use the same specific induction method of injury, i.e. ATP induced injury to allow for direct comparison of across species to narrow down resulting differences that might reflect the differing regenerative capacity. The ultimate aim of this work is to recapitulate pro-neurogenesis Müller glia signaling in mammals to produce new neurons that integrate with the existing retinal circuit to restore vision.

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

Neurodegenerative retinal diseases, such as glaucoma and diabetic retinopathy, involve a gradual loss of neurons in the retina as the disease progresses. Central nervous system neurons are not able to regenerate in mammals, therefore, an often sought after course of treatment for neuronal loss follows a neuroprotective or regenerative strategy. Neuroprotection is the process of preserving the structure and function of the neurons that have survived a harmful insult; while regenerative approaches aim to replace or rewire the neurons and synaptic connections that were lost, or induce regrowth of damaged axons or dendrites. In order to test the neuroprotective effectiveness or the regenerative capacity of a particular agent, a robust experimental model of retinal neuronal damage is essential. Zebrafish are being used more often in this type of study because their eye structure and development is well-conserved between zebrafish and mammals. Zebrafish are robust genetic tools and are relatively inexpensive to maintain. The large array of functional and behavioral tests available in zebrafish makes them an attractive model for neuroprotection studies. Some common insults used to model retinal disease and study neuroprotection in zebrafish include intense light, chemical toxicity and mechanical damage. This review covers the existing retinal neuroprotection and regeneration literature in the zebrafish and highlights their potential for future studies.

miRNAs and Müller Glia Reprogramming During Retina Regeneration

The use of model systems that are capable of robust, spontaneous retina regeneration has allowed for the identification of genetic pathways and components that are required for retina regeneration. Complemented by mouse models in which retina regeneration can be induced after forced expression of key factors, altered chromatin accessibility, or inhibition of kinase/signaling cascades, a clearer picture of the key regulatory events that control retina regeneration is emerging. In all cases, Müller glia (MG) serve as an adult retinal stem cell that must be reprogrammed to allow for regeneration, with the end goal being to understand why regenerative pathways are blocked in mammals, but spontaneous in other vertebrates such as zebrafish. miRNAs have emerged as key gene regulatory molecules that control both development and regeneration in vertebrates. Here, we focus on a small subset of miRNAs that control MG reprogramming during retina regeneration and have the potential to serve as therapeutic targets for treatment of visual disorders and damage.

Leukemia inhibitory factor protects photoreceptor cone cells against oxidative damage through activating JAK/STAT3 signaling

Background

The present study aimed to investigate the protective role of leukemia inhibitory factor (LIF) against oxidative damage in photoreceptor cone cells.

Methods

In vivo, dark-adapted mice were injected with LIF or phosphate-buffered saline (PBS) intravitreously prior to being exposed to 5,000 lux bright light to determine the protective effect of LIF against light damage in cone cells. Oxidative damage to cone cells was analyzed using electroretinograms, immunostaining, Western blotting and reverse transcription quantitative polymerase chain reaction (RT-qPCR). In vitro, 661W cells were pretreated with 5 ng/mL of LIF with or without 50 µM of signal transducer and activator of transcription 3 (STAT3) inhibitor S3I201 for 1 h prior to treatment with 1 mM H₂O₂; cell survival, apoptosis, the oxidative stress index, and the activation of STAT3, extracellular signal-regulated kinase (ERK1/2), and AKT were subsequently determined.

Results

In vivo, light induction damaged the function and morphology of cone cells, and LIF was observed to protect cone cells from this light damage. Moreover, the activation of the Janus tyrosine kinase (JAK)/STAT3 signaling pathway and the subsequent changes in apoptosis and proliferation-related genes were found to be involved in the protective effect of LIF against light-induced retinal damage. In the H₂O₂-induced 661W cell model, H₂O₂ increased cellular apoptosis rates, the expression levels of Bcl-2–associated X-protein (BAX) and cleaved caspase 3, reactive oxygen species (ROS) production, and malondialdehyde content, while decreasing the cell viability, and Bcl-2, superoxide dismutase, catalase, and glutathione peroxidase activity. LIF was observed to block these events; however, the administration of the STAT3 inhibitor S3I201 reversed the beneficial effects of LIF on H₂O₂-triggered apoptosis and ROS production.

Conclusions

In conclusion, the present study suggested that LIF may relieve oxidative damage in cone cells through suppressing apoptosis and oxidative stress by targeting the STAT3 signaling pathway.

Limitations and Promise of Retinal Tissue From Human Pluripotent Stem Cells for Developing Therapies of Blindness

The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches in vitro and in vivo, a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.

Reprogramming Müller Glia to Regenerate Retinal Neurons

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Dual roles of the retinal pigment epithelium and lens in cavefish eye degeneration

Astyanax mexicanus consists of two forms, a sighted surface dwelling form (surface fish) and a blind cave-dwelling form (cavefish). Embryonic eyes are initially formed in cavefish but they are subsequently arrested in growth and degenerate during larval development. Previous lens transplantation studies have shown that the lens plays a central role in cavefish eye loss. However, several lines of evidence suggest that additional factors, such as the retinal pigment epithelium (RPE), which is morphologically altered in cavefish, could also be involved in the eye regression process. To explore the role of the RPE in cavefish eye degeneration, we generated an albino eyed (AE) strain by artificial selection for hybrid individuals with large eyes and a depigmented RPE. The AE strain exhibited an RPE lacking pigment granules and showed reduced expression of the RPE specific enzyme retinol isomerase, allowing eye development to be studied by lens ablation in an RPE background resembling cavefish. We found that lens ablation in the AE strain had stronger negative effects on eye growth than in surface fish, suggesting that an intact RPE is required for normal eye development. We also found that the AE strain develops a cartilaginous sclera lacking boney ossicles, a trait similar to cavefish. Extrapolation of the results to cavefish suggests that the RPE and lens have dual roles in eye degeneration, and that deficiencies in the RPE may be associated with evolutionary changes in scleral ossification.

Biphasic Role of Tgf-β Signaling during Müller Glia Reprogramming and Retinal Regeneration in Zebrafish

Tgf-β signaling is a major antiproliferative pathway governing different biological functions, including cellular reprogramming. Upon injury, Müller glial cells of zebrafish retina reprogram to form progenitors (MGPCs) essential for regeneration. Here, the significance of Tgf-β signaling for inducing MGPCs is explored. Notably, Tgf-β signaling not only performs a pro-proliferative function but also is necessary for the expression of several regeneration-associated, essential transcription factor genes such as ascl1a, lin28a, oct4, sox2, and zebs and various microRNAs, namely, miR-200a, miR-200b, miR-143, and miR-145 during different phases of retinal regeneration. This study also found the indispensable role played by Mmp2/Mmp9 in the efficacy of Tgf-β signaling. Furthermore, the Tgf-β signaling is essential to cause cell cycle exit of MGPCs towards later phases of regeneration. Finally, the Delta-Notch signaling in collaboration with Tgf-β signaling regulates the critical factor, Her4.1. This study provides novel insights into the biphasic roles of Tgf-β signaling in zebrafish during retinal regeneration.

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Tgf-β signaling is essential for retinal progenitor proliferation and cell cycle exit

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pSmad3 binds to 5GC and TIE elements to cause gene activations and repressions

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Tgf-β signaling regulates Zebs and various miRNAs for cellular reprograming

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Translation of Tgf-β signaling requires Mmp2/Mmp9 activity

Demystifying Circalunar and Diel Rhythmicity in Acropora digitifera under Constant Dim Light

Life on earth has evolved under constant environmental changes; in response to these changes, most organisms have developed an endogenous clock that allows them to anticipate daily and seasonal changes and adapt their biology accordingly. Light cycles synchronize biological rhythms and are controlled by an endogenous clock that is entrained by environmental cues. Light is known to play a key role in the biology of symbiotic corals as they exhibit many biological processes entrained by daily light patterns. In this study, we aimed at determining the effect of constant dim light on coral's perception of diel and monthly cycles. Our results show that under constant dim light corals display a loss of rhythmic processes and constant stimuli by light, which initiates signal transduction that results in an abnormal cell cycle, cell proliferation, and protein synthesis. The results emphasize how constant dim light can mask the biological clock of Acropora digitifera.

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Light entrains many biological processes governed by the endogenous clock

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Constant dim light overrides the biological clock of A. digitifera corals

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Artificial light impacts the processes that allow corals to thrive in our oceans

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The increase of artificial light in coastal areas is a growing threat to coral reefs

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.

The expression of fgfr3 in the zebrafish head

Fibroblast growth factor (FGF) signaling is essential for many developmental processes and plays a pivotal role in skeletal homeostasis, regeneration and wound healing. FGF signals through one of five tyrosine kinase receptors: Fgfr1a, -1b, -2, -3, -4. To characterize the expression of zebrafish fgfr3 from the larval stage to adulthood, we used RNAscope in situ hybridization on paraffin sections of the zebrafish head. Our study revealed spatial and temporal distribution of fgfr3 transcript in chondrocytes of the head cartilages, osteoblasts involved in bone formation, ventricular zone of the brain, undifferentiated mesenchymal cells of the skin, and lens epithelium of the eye. In general, the expression pattern of zebrafish fgfr3 is similar to the expression observed in higher vertebrates.

Characterization of the pleiotropic roles of Sonic Hedgehog during retinal regeneration in adult zebrafish

In contrast to the mammalian retina, the zebrafish retina possesses the ability to regenerate. This is primarily accomplished through Müller glial cells, which, upon damage, re-enter the cell cycle to form retinal progenitors. The progenitors continue to proliferate as they migrate to the area of damage and ultimately differentiate into new neurons. The purpose of this study was to characterize the expression and function of Sonic Hedgehog (Shh) during regeneration of the adult zebrafish retina. Expression profiling of Shh pathway genes showed a significant upregulation of expression associated with stages of progenitor proliferation and neuronal differentiation. Activation of Shh signaling during early stages of retinal regeneration using intraocular injections of the recombinant human SHH (SHH-N) resulted in increased Müller cell gliosis, proliferation, and neuroprotection of damaged retinal neurons. Continued activation of Shh resulted in a greater number of differentiated amacrine and ganglion cells in the fully regenerated retina. Conversely, inhibition of Shh signaling using intraocular injections of cyclopamine resulted in decreased Müller glial cell proliferation and a fewer number of regenerated amacrine and ganglion cells. These data suggest that Shh signaling plays pleiotropic roles in proliferation and differentiation during adult zebrafish retinal regeneration.

Fibroblast growth factors: key players in regeneration and tissue repair

Tissue injury initiates a complex repair process, which in some organisms can lead to the complete regeneration of a tissue. In mammals, however, the repair of most organs is imperfect and results in scar formation. Both regeneration and repair are orchestrated by a highly coordinated interplay of different growth factors and cytokines. Among the key players are the fibroblast growth factors (FGFs), which control the migration, proliferation, differentiation and survival of different cell types. In addition, FGFs influence the expression of other factors involved in the regenerative response. Here, we summarize current knowledge on the roles of endogenous FGFs in regeneration and repair in different organisms and in different tissues and organs. Gaining a better understanding of these FGF activities is important for appropriate modulation of FGF signaling after injury to prevent impaired healing and to promote organ regeneration in humans.

Glycolytic reliance promotes anabolism in photoreceptors

Vertebrate photoreceptors are among the most metabolically active cells, exhibiting a high rate of ATP consumption. This is coupled with a high anabolic demand, necessitated by the diurnal turnover of a specialized membrane-rich organelle, the outer segment, which is the primary site of phototransduction. How photoreceptors balance their catabolic and anabolic demands is poorly understood. Here, we show that rod photoreceptors in mice rely on glycolysis for their outer segment biogenesis. Genetic perturbations targeting allostery or key regulatory nodes in the glycolytic pathway impacted the size of the outer segments. Fibroblast growth factor signaling was found to regulate glycolysis, with antagonism of this pathway resulting in anabolic deficits. These data demonstrate the cell autonomous role of the glycolytic pathway in outer segment maintenance and provide evidence that aerobic glycolysis is part of a metabolic program that supports the biosynthetic needs of a normal neuronal cell type.

DOI:http://dx.doi.org/10.7554/eLife.25946.001

Living cells need building materials and energy to grow and carry out their activities. Most cells in the body use sugars like glucose for these purposes. In a process known as glycolysis, cells break down glucose into molecules that are eventually converted to carbon dioxide and water to form the chemical ATP – the cellular currency for energy. Developing cells that have not yet fully specialized, and rapidly dividing cells, like cancer cells, consume large amounts of glucose via aerobic glycolysis (also known as the Warburg effect) as they require high levels of energy and building materials. As cells become more specialized and divide less often, they have a reduced need for building blocks, and adjust their consumption and breakdown of glucose accordingly.

One exception is the photoreceptor cells, found in the light-sensitive part of our eyes. Although these specialized cells do not divide, they still need a lot of energy and building blocks to constantly renew their light-sensing and processing structures, and to capture and convert the information from the environment into signals. Previous research has shown that the eye also uses the Warburg effect. However, until now, it was not known whether the photoreceptors or other cells in the eye carry out this form of glycolysis.

Using genetic tools, Chinchore et al. analysed how the photoreceptor cells in mice used glucose. The experiments demonstrated that the photoreceptors do indeed carry out the Warburg effect. Chinchore et al. further discovered that the Warburg effect is regulated by the same key enzymes and signalling molecules that cancer cells use. This indicates that specialized cells like photoreceptors might choose to retain certain metabolic features of their precursor cells, if they need to.

These findings provide new insight into how photoreceptors use glucose. The next step will be to understand how aerobic glycolysis is regulated in healthy eyes as well as in eyes that are affected by degenerating diseases, which may ultimately lead to new ways of treating blindness.

DOI:http://dx.doi.org/10.7554/eLife.25946.002

Evidence of BrdU-positive retinal neurons after application of an Alpha7 nicotinic acetylcholine receptor agonist

Irreversible vision loss due to disease or age is responsible for a reduced quality of life. The experiments in this study test the hypothesis that the α7 nicotinic acetylcholine receptor agonist, PNU-282987, leads to the generation of retinal neurons in an adult mammalian retina in the absence of retinal injury or exogenous growth factors. Using antibodies against BrdU, retinal ganglion cells, progenitor cells and Müller glia, the results of this study demonstrate that multiple types of retinal cells and neurons are generated after eye drop application of PNU-282987 in adult Long Evans rats in a dose-dependent manner. The results of this study provide evidence that progenitor cells, derived from Müller glia after treatment with PNU-282987, differentiate and migrate to the photoreceptor and retinal ganglion cell layers. If retinas were treated with the alpha7 nAChR antagonist, methyllycaconitine, before agonist treatment, BrdU-positive cells were significantly reduced. As adult mammalian neurons do not typically regenerate or proliferate, these results have implications for reversing vision loss due to neurodegenerative disease or the aging process to improve the quality of life for millions of patients.

Expression of SoxC Transcription Factors during Zebrafish Retinal and Optic Nerve Regeneration

The SoxC transcription factors (Sox4, Sox11, and Sox12) play important roles in the development of the vertebrate eye and retina. However, their expression and function during retinal and optic nerve regeneration remain elusive. In this study, we investigated the expression and possible functions of the SoxC genes after retinal and optic nerve injury in adult zebrafish. We found that among the five SoxC members, Sox11b was strongly induced in BrdU-positive cells in the inner nuclear layer (INL) after retinal injury, and morpholino-mediated Sox11b-knockdown significantly reduced the number of proliferating cells in the INL at 4 days post-injury. After optic nerve lesion, both Sox11a and Sox11b were strongly expressed in retinal ganglion cells (RGCs), and knockdown of both Sox11a and Sox11b inhibited RGC axon regrowth in retinal explants. Our study thus uncovered a novel expression pattern of SoxC family genes after retinal and optic nerve injury, and suggests that they have important functions during retinal and optic nerve regeneration.

Fgf regulates dedifferentiation during skeletal muscle regeneration in adult zebrafish

Fibroblast growth factors (Fgfs) regulate critical biological processes such as embryonic development, tissue homeostasis, wound healing, and tissue regeneration. In zebrafish, Fgf signaling plays an important role in the regeneration of the spinal cord, liver, heart, fin, and photoreceptors, although its exact mechanism of action is not fully understood. Utilizing an adult zebrafish extraocular muscle (EOM) regeneration model, we demonstrate that blocking Fgf receptor function using either a chemical inhibitor (SU5402) or a dominant-negative transgenic construct (dnFGFR1a:EGFP) impairs muscle regeneration. Adult zebrafish EOMs regenerate through a myocyte dedifferentiation process, which involves a muscle-to-mesenchyme transition and cell cycle reentry by differentiated myocytes. Blocking Fgf signaling reduced cell proliferation and active caspase 3 levels in the regenerating muscle with no detectable levels of apoptosis, supporting the hypothesis that Fgf signaling is involved in the early steps of dedifferentiation. Fgf signaling in regenerating myocytes involves the MAPK/ERK pathway: inhibition of MEK activity with U0126 mimicked the phenotype of the Fgf receptor inhibition on both muscle regeneration and cell proliferation, and activated ERK (p-ERK) was detected in injured muscles by immunofluorescence and western blot. Interestingly, following injury, ERK2 expression is specifically induced and activated by phosphorylation, suggesting a key role in muscle regeneration. We conclude that the critical early steps of myocyte dedifferentiation in EOM regeneration are dependent on Fgf signaling.

Inhibition of BMP signaling reduces MMP-2 and MMP-9 expression and obstructs wound healing in regenerating fin of teleost fish Poecilia latipinna

The tail fin of teleost fish responds to amputation by expressing few putative factors that promote scar-free wound healing, which paves the way for restoration of the lost part. Among the factors playing a role in this initial response, bone morphogenetic proteins (BMPs) are crucial. In the current study, we have analyzed the effect of BMP inhibition on wound healing in sailfin molly Poecilia latipinna. The study involved histological assessment of wound epithelium formation, an expression profile of proteins, and gelatinase activity as well as expression in response to BMP signal inhibition. LDN193189, a pharmacological inhibitor of BMP receptor, was administered to experimental fish. Our observations include incomplete wound healing and a significant reduction in the expression of a number of proteins as a result of LDN treatment at 24 h post-amputation. A pronounced effect was also seen on the gelatinases MMP-9 and MMP-2, which showed significantly reduced activities on a zymogram. Reduced expression of these MMPs after inhibitor treatment was also confirmed by western blot and real-time PCR analyses. In view of these results, we confirm that BMP signaling has a definitive role in the early stages of fin regeneration in P. latipinna. The effect of BMP inhibition is especially seen on the expression of MMP-9 and MMP-2, which are very important effectors of tissue remodeling immediately following amputation.

Reactive gliosis in the adult zebrafish retina

In contrast to mammals, zebrafish posses the remarkable ability to regenerate retinal neurons. Damage to the zebrafish retina induces Müller glia to act as stem cells, generating retinal progenitors for regeneration. In contrast, injury in the mammalian retina results in Müller glial reactive gliosis, a characteristic gliotic response that is normally detrimental to vision. Understanding the signaling pathways that determine how Müller glia respond to injury is a critical step toward promoting regeneration in the mammalian retina. Here we report that zebrafish Müller glia exhibit signs of reactive gliosis even under normal regenerative conditions and that cell cycle inhibition increases this response. Persistently reactive Müller glia increase their neuroprotective functions, temporarily saving photoreceptors from a cytotoxic light lesion. However, the absence of a sustained proliferation response results in a significant inhibition of retinal regeneration. Interestingly, when cell cycle inhibition is released, a partial recovery of regeneration is observed. Together, these data demonstrate that zebrafish Müller glia possess both gliotic and regenerative potential.

Development of the Vertebrate Eye and Retina

The mature, functional, and healthy eye is generated by the coordinated regulatory interaction of numerous and diverse developing tissues. The neural retina of the eye must undergo the neurogenesis of multiple retinal cell types in the correct ratios and spatial patterns. This chapter provides an overview of retinal development, and includes a summary of the process of eye organogenesis, a discussion of major principles of retinal neurogenesis, and describes some of the key molecular factors critical for retinal development. Defects in many of these factors underlie diseases of the eye, and an understanding of the process of retinal development will be critical for successful future applications of regenerative therapies for eye disease.

Age-dependent Müller glia neurogenic competence in the mouse retina

The mechanisms limiting neuronal regeneration in mammals and their relationship with reactive gliosis are unknown. Müller glia (MG), common to all vertebrate retinas, readily regenerate neuron loss in some species, but normally not in mammals. However, experimental stimulation of limited mammalian retina regeneration has been reported. Here, we use a mouse retina organ culture approach to investigate the MG responses at different mouse ages. We found that MG undergo defined spatio-temporal changes upon stimulation. In EGF-stimulated juvenile postmitotic retinas, most MG upregulate cell-cycle regulators (Mcm6, Pcna, Ki67, Ccnd1) within 48 h ex vivo; some also express the neurogenic factors Ascl1, Pax6, and Vsx2; up to 60% re-enter the cell cycle, some of which delaminate to divide mostly apically; and the majority cease to proliferate after stimulation. A subpopulation of MG progeny starts to express transcription factors (Ptf1a, Nr4a2) and neuronal (Calb1, Calb2, Rbfox3), but not glial, markers, indicating neurogenesis. BrdU-tracking, genetic lineage-tracing, and transgenic-reporter experiments suggest that MG reprogram to a neurogenic stage and proliferate; and that some MG progeny differentiate into neuronal-like cells, most likely amacrines, no photoreceptors; most others remain in a de-differentiated state. The mouse MG regeneration potential becomes restricted, dependent on the age of the animal, as observed by limited activation of the cell cycle and neurogenic factors. The stage-dependent analysis of mouse MG revealed similarities and differences when compared with MG-derived regeneration in fish and chicks. Therefore, the mouse retina ex vivo approach is a potential assay for understanding and overcoming the limitations of mammalian MG-derived neuronal regeneration. Postmitotic MG in mouse retina ex vivo can be stimulated to proliferate, express neurogenic factors, and generate progeny expressing neuronal or glial markers. This potential regenerative competence becomes limited with increasing mouse age.

Midkine-a protein localization in the developing and adult retina of the zebrafish and its function during photoreceptor regeneration

Midkine is a heparin binding growth factor with important functions in neuronal development and survival, but little is known about its function in the retina. Previous studies show that in the developing zebrafish, Midkine-a (Mdka) regulates cell cycle kinetics in retinal progenitors, and following injury to the adult zebrafish retina, mdka is strongly upregulated in Müller glia and the injury-induced photoreceptor progenitors. Here we provide the first data describing Mdka protein localization during different stages of retinal development and during the regeneration of photoreceptors in adults. We also experimentally test the role of Mdka during photoreceptor regeneration. The immuno-localization of Mdka reflects the complex spatiotemporal pattern of gene expression and also reveals the apparent secretion and extracellular trafficking of this protein. During embryonic retinal development the Mdka antibodies label all mitotically active cells, but at the onset of neuronal differentiation, immunostaining is also localized to the nascent inner plexiform layer. Starting at five days post fertilization through the juvenile stage, Mdka immunostaining labels the cytoplasm of horizontal cells and the overlying somata of rod photoreceptors. Double immunolabeling shows that in adult horizontal cells, Mdka co-localizes with markers of the Golgi complex. Together, these data are interpreted to show that Mdka is synthesized in horizontal cells and secreted into the outer nuclear layer. In adults, Mdka is also present in the end feet of Müller glia. Similar to mdka gene expression, Mdka in horizontal cells is regulated by circadian rhythms. After the light-induced death of photoreceptors, Mdka immuonolabeling is localized to Müller glia, the intrinsic stem cells of the zebrafish retina, and proliferating photoreceptor progenitors. Knockdown of Mdka during photoreceptor regeneration results in less proliferation and diminished regeneration of rod photoreceptors. These data suggest that during photoreceptor regeneration Mdka regulates aspects of injury-induced cell proliferation.

Retinal injury, growth factors, and cytokines converge on β-catenin and pStat3 signaling to stimulate retina regeneration

Müller glia (MG) in the zebrafish retina respond to retinal injury by generating multipotent progenitors for retinal repair. Here, we show that Insulin, Igf-1, and fibroblast growth factor (FGF) signaling components are necessary for retina regeneration. Interestingly, these factors synergize with each other and with heparin-binding EGF-like growth factor (HB-EGF) and cytokines to stimulate MG to generate multipotent progenitors in the uninjured retina. These factors act by stimulating a core set of signaling cascades (Mapk/Erk, phosphatidylinositol 3-kinase [PI3K], β-catenin, and pStat3) that are also shared with retinal injury and exhibit a remarkable amount of crosstalk. Our studies suggest that MG both produce and respond to factors that stimulate MG reprogramming and proliferation following retinal injury. The identification of a core set of regeneration-associated signaling pathways required for MG reprogramming not only furthers our understanding of retina regeneration in fish but also suggests targets for enhancing regeneration in mammals.

Müller glial cell reprogramming and retina regeneration

Müller glia are the major glial component of the retina. They are one of the last retinal cell types to be born during development, and they function to maintain retinal homeostasis and integrity. In mammals, Müller glia respond to retinal injury in various ways that can be either protective or detrimental to retinal function. Although these cells can be coaxed to proliferate and generate neurons under special circumstances, these responses are meagre and insufficient for repairing a damaged retina. By contrast, in teleost fish (such as zebrafish), the response of Müller glia to retinal injury involves a reprogramming event that imparts retinal stem cell characteristics and enables them to produce a proliferating population of progenitors that can regenerate all major retinal cell types and restore vision. Recent studies have revealed several important mechanisms underlying Müller glial cell reprogramming and retina regeneration in fish that may lead to new strategies for stimulating retina regeneration in mammals.

Retinal regeneration is facilitated by the presence of surviving neurons

Teleost fish regenerate their retinas after damage, in contrast to mammals. In zebrafish subjected to an extensive ouabain-induced lesion that destroys all neurons and spares Müller glia, functional recovery and restoration of normal optic nerve head (ONH) diameter take place at 100 days postinjury. Subsequently, regenerated retinas overproduce cells in the retinal ganglion cell (RGC) layer, and the ONH becomes enlarged. Here, we test the hypothesis that a selective injury, which spares photoreceptors and Müller glia, results in faster functional recovery and fewer long-term histological abnormalities. Following this selective retinal damage, recovery of visual function required 60 days, consistent with this hypothesis. In contrast to extensively damaged retinas, selectively damaged retinas showed fewer histological errors and did not overproduce neurons. Extensively damaged retinas had RGC axons that were delayed in pathfinding to the ONH, and showed misrouted axons within the ONH, suggesting that delayed functional recovery following an extensive lesion is related to defects in RGC axons exiting the eye and/or reaching their central targets. The atoh7, fgf8a, Sonic hedgehog (shha), and netrin-1 genes were differentially expressed, and the distribution of hedgehog protein was disrupted after extensive damage as compared with selective damage. Confirming a role for Shh signaling in supporting rapid regeneration, shha(t4) +/- zebrafish showed delayed functional recovery after selective damage. We suggest that surviving retinal neurons provide structural/molecular information to regenerating neurons, and that this patterning mechanism regulates factors such as Shh. These factors in turn control neuronal number, retinal lamination, and RGC axon pathfinding during retinal regeneration.

Müller glia: Stem cells for generation and regeneration of retinal neurons in teleost fish

Adult zebrafish generate new neurons in the brain and retina throughout life. Growth-related neurogenesis allows a vigorous regenerative response to damage, and fish can regenerate retinal neurons, including photoreceptors, and restore functional vision following photic, chemical, or mechanical destruction of the retina. Müller glial cells in fish function as radial-glial-like neural stem cells. During adult growth, Müller glial nuclei undergo sporadic, asymmetric, self-renewing mitotic divisions in the inner nuclear layer to generate a rod progenitor that migrates along the radial fiber of the Müller glia into the outer nuclear layer, proliferates, and differentiates exclusively into rod photoreceptors. When retinal neurons are destroyed, Müller glia in the immediate vicinity of the damage partially and transiently dedifferentiate, re-express retinal progenitor and stem cell markers, re-enter the cell cycle, undergo interkinetic nuclear migration (characteristic of neuroepithelial cells), and divide once in an asymmetric, self-renewing division to generate a retinal progenitor. This daughter cell proliferates rapidly to form a compact neurogenic cluster surrounding the Müller glia; these multipotent retinal progenitors then migrate along the radial fiber to the appropriate lamina to replace missing retinal neurons. Some aspects of the injury-response in fish Müller glia resemble gliosis as observed in mammals, and mammalian Müller glia exhibit some neurogenic properties, indicative of a latent ability to regenerate retinal neurons. Understanding the specific properties of fish Müller glia that facilitate their robust capacity to generate retinal neurons will inform and inspire new clinical approaches for treating blindness and visual loss with regenerative medicine.

The ETS transcription factor Etv1 mediates FGF signaling to initiate proneural gene expression during Xenopus laevis retinal development

Fibroblast growth factor signaling plays a significant role in the developing eye, regulating both patterning and neurogenesis. Members of the Pea3/Etv4-subfamily of ETS-domain transcription factors (Etv1, Etv4, and Etv5) are transcriptional activators that are downstream targets of FGF/MAPK signaling, but whether they are required for eye development is unknown. We show that in the developing Xenopus laevis retina, etv1 is transiently expressed at the onset of retinal neurogenesis. We found that etv1 is not required for eye specification, but is required for the expression of atonal-related proneural bHLH transcription factors, and is also required for retinal neuron differentiation. Using transgenic reporters we show that the distal atoh7 enhancer, which is required for the initiation of atoh7 expression in the Xenopus retina, is responsive to both FGF signaling and etv1 expression. Thus, we conclude that Etv1 acts downstream of FGF signaling to regulate the initiation of neurogenesis in the Xenopus retina.

A novel light damage paradigm for use in retinal regeneration studies in adult zebrafish

Light-induced retinal degeneration (LIRD) is commonly used in both rodents and zebrafish to damage rod and cone photoreceptors. In adult zebrafish, photoreceptor degeneration triggers Müller glial cells to re-enter the cell cycle and produce transient-amplifying progenitors. These progenitors continue to proliferate as they migrate to the damaged area, where they ultimately give rise to new photoreceptors. Currently, there are two widely-used LIRD paradigms, each of which results in varying degrees of photoreceptor loss and corresponding differences in the regeneration response. As more genetic and pharmacological tools are available to test the role of individual genes of interest during regeneration, there is a need to develop a robust LIRD paradigm. Here we describe a LIRD protocol that results in widespread and consistent loss of both rod and cone photoreceptors in which we have combined the use of two previously established LIRD techniques. Furthermore, this protocol can be extended for use in pigmented animals, which eliminates the need to maintain transgenic lines of interest on the albino background for LIRD studies.

Characteristics of rod regeneration in a novel zebrafish retinal degeneration model using N-methyl-N-nitrosourea (MNU)

Primary loss of photoreceptors caused by diseases such as retinitis pigmentosa is one of the main causes of blindness worldwide. To study such diseases, rodent models of N-methyl-N-nitrosourea (MNU)-induced retinal degeneration are widely used. As zebrafish (Danio rerio) are a popular model system for visual research that offers persistent retinal neurogenesis throughout the lifetime and retinal regeneration after severe damage, we have established a novel MNU-induced model in this species. Histology with staining for apoptosis (TUNEL), proliferation (PCNA), activated Müller glial cells (GFAP), rods (rhodopsin) and cones (zpr-1) were performed. A characteristic sequence of retinal changes was found. First, apoptosis of rod photoreceptors occurred 3 days after MNU treatment and resulted in a loss of rod cells. Consequently, proliferation started in the inner nuclear layer (INL) with a maximum at day 8, whereas in the outer nuclear layer (ONL) a maximum was observed at day 15. The proliferation in the ONL persisted to the end of the follow-up (3 months), interestingly, without ongoing rod cell death. We demonstrate that rod degeneration is a sufficient trigger for the induction of Müller glial cell activation, even if only a minimal number of rod cells undergo cell death. In conclusion, the use of MNU is a simple and feasible model for rod photoreceptor degeneration in the zebrafish that offers new insights into rod regeneration.

The zebrafish as a model for complex tissue regeneration

For centuries, philosophers and scientists have been fascinated by the principles and implications of regeneration in lower vertebrate species. Two features have made zebrafish an informative model system for determining mechanisms of regenerative events. First, they are highly regenerative, able to regrow amputated fins, as well as a lesioned brain, retina, spinal cord, heart, and other tissues. Second, they are amenable to both forward and reverse genetic approaches, with a research toolset regularly updated by an expanding community of zebrafish researchers. Zebrafish studies have helped identify new mechanistic underpinnings of regeneration in multiple tissues and, in some cases, have served as a guide for contemplating regenerative strategies in mammals. Here, we review the recent history of zebrafish as a genetic model system for understanding how and why tissue regeneration occurs.

Hippo pathway effector Yap promotes cardiac regeneration

The adult mammalian heart has limited potential for regeneration. Thus, after injury, cardiomyocytes are permanently lost, and contractility is diminished. In contrast, the neonatal heart can regenerate owing to sustained cardiomyocyte proliferation. Identification of critical regulators of cardiomyocyte proliferation and quiescence represents an important step toward potential regenerative therapies. Yes-associated protein (Yap), a transcriptional cofactor in the Hippo signaling pathway, promotes proliferation of embryonic cardiomyocytes by activating the insulin-like growth factor and Wnt signaling pathways. Here we report that mice bearing mutant alleles of Yap and its paralog WW domain containing transcription regulator 1 (Taz) exhibit gene dosage-dependent cardiac phenotypes, suggesting redundant roles of these Hippo pathway effectors in establishing proper myocyte number and maintaining cardiac function. Cardiac-specific deletion of Yap impedes neonatal heart regeneration, resulting in a default fibrotic response. Conversely, forced expression of a constitutively active form of Yap in the adult heart stimulates cardiac regeneration and improves contractility after myocardial infarction. The regenerative activity of Yap is correlated with its activation of embryonic and proliferative gene programs in cardiomyocytes. These findings identify Yap as an important regulator of cardiac regeneration and provide an experimental entry point to enhance this process.

Regulation of Müller glial dependent neuronal regeneration in the damaged adult zebrafish retina

This article examines our current knowledge underlying the mechanisms involved in neuronal regeneration in the adult zebrafish retina. Zebrafish, which has the capacity to regenerate a wide variety of tissues and organs (including the fins, kidney, heart, brain, and spinal cord), has become the premier model system to study retinal regeneration due to the robustness and speed of the response and the variety of genetic tools that can be applied to study this question. It is now well documented that retinal damage induces the resident Müller glia to dedifferentiate and reenter the cell cycle to produce neuronal progenitor cells that continue to proliferate, migrate to the damaged retinal layer and differentiate into the missing neuronal cell types. Increasing our understanding of how these cellular events are regulated and occur in response to neuronal damage may provide critical information that can be applied to stimulating a regeneration response in the mammalian retina. In this review, we will focus on the genes/proteins that regulate zebrafish retinal regeneration and will attempt to critically evaluate how these factors may interact to correctly orchestrate the definitive cellular events that occur during regeneration.

Divergent requirements for fibroblast growth factor signaling in zebrafish maxillary barbel and caudal fin regeneration

The zebrafish maxillary barbel is an integumentary organ containing skin, glands, pigment cells, taste buds, nerves, and endothelial vessels. The maxillary barbel can regenerate (LeClair & Topczewski 2010); however, little is known about its molecular regulation. We have studied fibroblast growth factor (FGF) pathway molecules during barbel regeneration, comparing this system to a well-known regenerating appendage, the zebrafish caudal fin. Multiple FGF ligands (fgf20a, fgf24), receptors (fgfr1-4) and downstream targets (pea3, il17d) are expressed in normal and regenerating barbel tissue, confirming FGF activation. To test if specific FGF pathways were required for barbel regeneration, we performed simultaneous barbel and caudal fin amputations in two temperature-dependent zebrafish lines. Zebrafish homozygous for a point mutation in fgf20a, a factor essential for caudal fin blastema formation, regrew maxillary barbels normally, indicating that the requirement for this ligand is appendage-specific. Global overexpression of a dominant negative FGF receptor, Tg(hsp70l:dn-fgfr1:EGFP)(pd1) completely blocked fin outgrowth but only partially inhibited barbel outgrowth, suggesting reduced requirements for FGFs in barbel tissue. Maxillary barbels expressing dn-fgfr1 regenerated peripheral nerves, dermal connective tissue, endothelial tubes, and a glandular epithelium; in contrast to a recent report in which dn-fgfr1 overexpression blocks pharyngeal taste bud formation in zebrafish larvae (Kapsimali et al. 2011), we observed robust formation of calretinin-positive tastebuds. These are the first experiments to explore the molecular mechanisms of maxillary barbel regeneration. Our results suggest heterogeneous requirements for FGF signaling in the regeneration of different zebrafish appendages (caudal fin versus maxillary barbel) and taste buds of different embryonic origin (pharyngeal endoderm versus barbel ectoderm).

Characterization of multiple light damage paradigms reveals regional differences in photoreceptor loss

Zebrafish provide an attractive model to study the retinal response to photoreceptor apoptosis due to its remarkable ability to spontaneously regenerate retinal neurons following damage. There are currently two widely-used light-induced retinal degeneration models to damage photoreceptors in the adult zebrafish. One model uses constant bright light, whereas the other uses a short exposure to extremely intense ultraviolet light. Although both models are currently used, it is unclear whether they differ in regard to the extent of photoreceptor damage or the subsequent regeneration response. Here we report a thorough analysis of the photoreceptor damage and subsequent proliferation response elicited by each individual treatment, as well as by the concomitant use of both treatments. We show a differential loss of rod and cone photoreceptors with each treatment. Additionally, we show that the extent of proliferation observed in the retina directly correlates with the severity of photoreceptor loss. We also demonstrate that both the ventral and posterior regions of the retina are partially protected from light damage. Finally, we show that combining a short ultraviolet exposure followed by a constant bright light treatment largely eliminates the neuroprotected regions, resulting in widespread loss of rod and cone photoreceptors and a robust regenerative response throughout the retina.

Fgf signaling is required for photoreceptor maintenance in the adult zebrafish retina

Fibroblast growth factors (Fgf) are secreted signaling molecules that have mitogenic, patterning, neurotrophic and angiogenic properties. Their importance during embryonic development in patterning and morphogenesis of the vertebrate eye is well known, but less is known about the role of Fgfs in the adult vertebrate retina. To address Fgf function in adult retina, we determined the spatial distribution of components of the Fgf signaling pathway in the adult zebrafish retina. We detected differential expression of Fgf receptors, ligands and downstream Fgf targets within specific retinal layers. Furthermore, we blocked Fgf signaling in the retina, by expressing a dominant negative variant of Fgf receptor 1 conditionally in transgenic animals. After blocking Fgf signaling we observe a fast and progressive photoreceptor degeneration and disorganization of retinal tissue, coupled with cell death in the outer nuclear layer. Following the degeneration of photoreceptors, a profound regeneration response is triggered that starts with proliferation in the inner nuclear layer. Ultimately, rod and cone photoreceptors are regenerated completely. Our study reveals the requirement of Fgf signaling to maintain photoreceptors and for proliferation during regeneration in the adult zebrafish retina.