The rod photoreceptor lineage of teleost fish

Single-cell RNA sequencing reveals the heterogeneity and interactions of immune cells and Müller glia during zebrafish retina regeneration

JOURNAL/nrgr/04.03/01300535-202512000-00031/figure1/v/2025-01-31T122243Z/r/image-tiff Inflammation plays a crucial role in the regeneration of fish and avian retinas. However, how inflammation regulates Müller glia (MG) reprogramming remains unclear. Here, we used single-cell RNA sequencing to investigate the cell heterogeneity and interactions of MG and immune cells in the regenerating zebrafish retina. We first showed that two types of quiescent MG (resting MG1 and MG2) reside in the uninjured retina. Following retinal injury, resting MG1 transitioned into an activated state expressing known reprogramming genes, while resting MG2 gave rise to rod progenitors. We further showed that retinal microglia can be categorized into three subtypes (microglia-1, microglia-2, and proliferative) and pseudotime analysis demonstrated dynamic changes in microglial status following retinal injury. Analysis of cell-cell interactions indicated extensive crosstalk between immune cells and MG, with many interactions shared among different immune cell types. Finally, we showed that inflammation activated Jak1-Stat3 signaling in MG, promoting their transition from a resting to an activated state. Our study reveals the cell heterogeneity and crosstalk of immune cells and MG in zebrafish retinal repair, and may provide valuable insights into future mammalian retina regeneration.

Inhibition of IGF-1Rα affects the differentiation fate of rat optic cup-derived retinal stem cells to retinal ganglion cells in vitro

AIM

To explore the impact of insulin-like growth factor-1 receptor α (IGF-1Rα) on the differentiation fate of optic-cup-derived retinal stem cells (OC-RSCs) into retinal ganglion cells (RGCs) in vitro.

METHODS

OC-RSCs were isolated from optic cups of rats on embryonic day 12.5, and high-purity OC-RSCs were obtained by conditioned culture and passage. Differentiation of OC-RSCs into RGCs under different serum concentrations was examined using flow cytometry, and the serum concentration with high interference with differentiation ratio was selected. Furthermore, the effect of blocking IGF-1Rα on the differentiation of OC-RSCs into RGCs was analyzed through immunocytochemistry and Western blotting.

RESULTS

Immunohistochemical analysis revealed IGF-1Rα was highly expressed in rat embryos at day 12.5. OC-RSCs were isolated and purified, and high-purity OC-RSCs were obtained. When 2.5% serum was administered, the ratio of differentiated RGCs (Thy-1.1 positive) decreased significantly, and the results of immunoblotting also confirmed the blockade of IGF-1Rα reduced Thy-1.1 protein expression.

CONCLUSION

IGF-1Rα blocking can reduce the differentiation of OC-RSCs into RGCs.

New pathways to neurogenesis: Insights from injury-induced retinal regeneration

The vertebrate retina is a tractable system for studying control of cell neurogenesis and cell fate specification. During embryonic development, retinal neurogenesis is under strict temporal regulation, with cell types generated in fixed but overlapping temporal intervals. The temporal sequence and relative numbers of retinal cell types generated during development are robust and show minimal experience-dependent variation. In many cold-blooded vertebrates, acute retinal injury induces a different form of neurogenesis, where Müller glia reprogram into retinal progenitor-like cells that selectively regenerate retinal neurons lost to injury. The extent to which the molecular mechanisms controlling developmental and injury-induced neurogenesis resemble one another has long been unclear. However, a recent study in zebrafish has shed new light on this question, using single-cell multiomic analysis to show that selective loss of different retinal cell types induces the formation of fate-restricted Müller glia-derived progenitors that differ both from one another and from progenitors in developing retina. Here, we discuss the broader implications of these findings, and their possible therapeutic relevance.

Systematic analysis of ocular features and responses of cultured spotted wolffish (Anarhichas minor)

A better understanding of unique anatomical and functional features of the visual systems of teleost fish could provide key knowledge on how these systems influence the health and survival of these animals in both wild and culture environments. We took a systematic approach to assess some of the visual systems of spotted wolffish (Anarhichas minor), a species of increasing importance in North Atlantic aquaculture initiatives. The lumpfish (Cyclopterus lumpus) was included in these studies in a comparative manner to provide reference. Histology, light and electron microscopy were used to study the spatial distribution and occurrence of cone photoreceptor cells and the nature of the retinal tissues, while immunohistochemistry was used to explore the expression patterns of two photoreceptor markers, XAP-1 and XAP-2, in both species. A marine bacterial infection paradigm in lumpfish was used to assess how host-pathogen responses might impact the expression of these photoreceptor markers in these animals. We define a basic photoreceptor mosaic and present an ultrastructural to macroscopic geographical configuration of the retinal pigment tissues in both animals. Photoreceptor markers XAP-1 and XAP-2 have novel distribution patterns in spotted wolffish and lumpfish retinas, and exogenous pathogenic influences can affect the normal expression pattern of XAP-1 in lumpfish. Live tank-side ophthalmoscopy and spectral domain optical coherence tomography (SD-OCT) revealed that normal cultured spotted wolffish display novel variations in the shape of the retinal tissue. These two complementary imaging findings suggest that spotted wolffish harbour unique ocular features not yet described in marine teleosts and that visual function might involve specific retinal tissue shape dynamics in these animals. Finally, extensive endogenous biofluorescence is present in the retinal tissues of both animals, which raises questions about how these animals might use retinal tissue in novel ways for visual perception and/or communication. This work advances fundamental knowledge on the visual systems of two economically important but now threatened North Atlantic teleosts and provides a basic foundation for further research on the visual systems of these animals in health versus disease settings. This work could also be useful for understanding and optimizing the health and welfare of lumpfish and spotted wolffish in aquaculture towards a one health or integrative perspective.

Single-cell RNA sequencing unravels the transcriptional network underlying zebrafish retina regeneration

In the lesioned zebrafish retina, Müller glia produce multipotent retinal progenitors that generate all retinal neurons, replacing lost cell types. To study the molecular mechanisms linking Müller glia reactivity to progenitor production and neuronal differentiation, we used single-cell RNA sequencing of Müller glia, progenitors and regenerated progeny from uninjured and light-lesioned retinae. We discover an injury-induced Müller glia differentiation trajectory that leads into a cell population with a hybrid identity expressing marker genes of Müller glia and progenitors. A glial self-renewal and a neurogenic trajectory depart from the hybrid cell population. We further observe that neurogenic progenitors progressively differentiate to generate retinal ganglion cells first and bipolar cells last, similar to the events observed during retinal development. Our work provides a comprehensive description of Müller glia and progenitor transcriptional changes and fate decisions in the regenerating retina, which are key to tailor cell differentiation and replacement therapies for retinal dystrophies in humans.

A comparative analysis of gene and protein expression in chronic and acute models of photoreceptor degeneration in adult zebrafish

Background: Adult zebrafish are capable of photoreceptor (PR) regeneration following acute phototoxic lesion (AL). We developed a chronic low light (CLL) exposure model that more accurately reflects chronic PR degeneration observed in many human retinal diseases. Methods: Here, we characterize the morphological and transcriptomic changes associated with acute and chronic models of PR degeneration at 8 time-points over a 28-day window using immunohistochemistry and 3'mRNA-seq. Results: We first observed a differential sensitivity of rod and cone PRs to CLL. Next, we found no evidence for Müller glia (MG) gliosis or regenerative cell-cycle re-entry in the CLL model, which is in contrast to the robust gliosis and proliferative response from resident MG in the AL model. Differential responses of microglia between the models was also observed. Transcriptomic comparisons between the models revealed gene-specific networks of PR regeneration and degeneration, including genes that are activated under conditions of chronic PR stress. Finally, we showed that CLL is at least partially reversible, allowing for rod and cone outer segment outgrowth and replacement of rod cell nuclei via an apparent upregulation of the existing rod neurogenesis mechanism. Discussion: Collectively, these data provide a direct comparison of the morphological and transcriptomic PR degeneration and regeneration models in zebrafish.

Knockout of DLIC1 leads to retinal cone degeneration via disturbing Rab8 transport in zebrafish

Retinal photoreceptors execute phototransduction functions and require an efficient system for the transport of materials (e.g. proteins and lipids) from inner segments to outer segments. Cytoplasmic dynein 1 is a minus-end-directed microtubule motor and participates in cargo transport in the cytoplasm. However, the roles of dynein 1 motor in photoreceptor cargo transport and retinal development are still ambiguous. In our present study, the light intermediate chain protein DLIC1 (encoded by dync1li1), links activating adaptors to bind diverse cargos in the dynein 1 motor, was depleted using CRISPR-Cas9 technology in zebrafish. The dync1li1-/- zebrafish displayed progressive degeneration of retinal cone photoreceptors, especially blue cones. The retinal rods were not affected in dync1li1-/- zebrafish. Knockout of DLIC1 resulted in abnormal expression and localization of cone opsins in dync1li1-/- retinas. TUNEL staining suggested that apoptosis was induced after aberrant accumulation of cone opsins in photoreceptors of dync1li1-/- zebrafish. Instead of Rab11 transport, Rab8 transport was disturbed in dync1li1-/- retinas. Our data demonstrate that DLIC1 is required for function maintenance and survival of cone photoreceptors, and hint at an essential role of the cytoplasmic dynein 1 motor in photoreceptor cargo transport.

Tissue stretching is a confounding factor for the evaluation of neurodegeneration in the fast-ageing killifish

The fast-ageing killifish has gained increasing attention as a promising gerontology model to study age-related processes and neurodegeneration. Interestingly, it is the first vertebrate model organism that shows physiological neuron loss at old age in its central nervous system (CNS), including its brain and retina. However, the fact that the killifish brain and retina are ever-growing tissues complicates studying neurodegenerative events in aged fish. Indeed, recent studies showed that the method of tissue sampling, either using sections or whole-organs, has a large effect on the observed cell densities in the fast-expanding CNS. Here, we elaborated on how these two sampling methods affect neuronal counts in the senescent retina and how this tissue grows upon ageing. Analysis of the different retinal layers in cryosections revealed age-dependent reduction in cellular density but evaluation of whole-mount retinas did not detect any neuron loss, as a result of an extremely fast retinal expansion with age. Using BrdU pulse-chase experiments, we showed that the young adult killifish retina mainly grows by cell addition. However, with increasing age, the neurogenic potency of the retina declines while the tissue keeps on growing. Further histological analyses revealed tissue stretching, including cell size increase, as the main driver of retinal growth at old age. Indeed, both cell size and inter-neuronal distance augment with ageing, thereby decreasing neuronal density. All in all, our findings urge the 'ageing science' community to consider cell quantification bias and employ tissue-wide counting methods to reliably quantify neuronal numbers in this unique gerontology model.

Eye flukes (Diplostomum spp) damage retinal tissue and may cause a regenerative response in wild threespine stickleback fish

Fish rely upon vision as a dominant sensory system for foraging, predator avoidance, and mate selection. Damage to the visual system, in particular to the neural retina of the eye, has been demonstrated to result in a regenerative response in captive fish that serve as model organisms (e.g. zebrafish), and this response restores some visual function. The purpose of the present study is to determine whether damage to the visual system that occurs in wild populations of fish also results in a regenerative response, offering a potentially ecologically relevant model of retinal regeneration. Adult threespine stickleback were collected from several water bodies of Iceland, and cryosectioned eye tissues were processed for hematoxylin and eosin staining or for indirect immunofluorescence using cell-specific markers. In many of the samples, eye flukes (metacercariae of Diplostomum spp) were present, frequently between the neural retina and retinal pigmented epithelium (RPE). Damage to the retina and to the RPE was evident in eyes containing flukes, and RPE fragments were observed within fluke bodies, suggesting they had consumed this eye tissue. Expression of a cell proliferation marker was also observed in both retina and RPE, consistent with a proliferative response to the damage. Interestingly, some regions of infected retina displayed "laminar fusions," in which neuronal cell bodies were misplaced within the major synaptic layer of the retina. These laminar fusions are also frequently found in regenerated zebrafish retina following non-parasitic (experimental) forms of retinal damage. The stickleback retina may therefore respond to fluke-mediated damage by engaging in retinal regeneration.

Retina regeneration: lessons from vertebrates

Unlike mammals, vertebrates such as fishes and frogs exhibit remarkable tissue regeneration including the central nervous system. Retina being part of the central nervous system has attracted the interest of several research groups to explore its regenerative ability in different vertebrate models including mice. Fishes and frogs completely restore the size, shape and tissue structure of an injured retina. Several studies have unraveled molecular mechanisms underlying retina regeneration. In teleosts, soon after injury, the Müller glial cells of the retina reprogram to form a proliferating population of Müller glia-derived progenitor cells capable of differentiating into various neural cell types and Müller glia. In amphibians, the transdifferentiation of retinal pigment epithelium and differentiation of ciliary marginal zone cells contribute to retina regeneration. In chicks and mice, supplementation with external growth factors or genetic modifications cause a partial regenerative response in the damaged retina. The initiation of retina regeneration is achieved through sequential orchestration of gene expression through controlled modulations in the genetic and epigenetic landscape of the progenitor cells. Several developmental biology pathways are turned on during the Müller glia reprogramming, retinal pigment epithelium transdifferentiation and ciliary marginal zone differentiation. Further, several tumorigenic pathways and gene expression events also contribute to the complete regeneration cascade of events. In this review, we address the various retinal injury paradigms and subsequent gene expression events governed in different vertebrate species. Further, we compared how vertebrates such as teleost fishes and amphibians can achieve excellent regenerative responses in the retina compared with their mammalian counterparts.

Notch Inhibition Promotes Regeneration and Immunosuppression Supports Cone Survival in a Zebrafish Model of Inherited Retinal Dystrophy

Photoreceptor degeneration leads to irreversible vision loss in humans with retinal dystrophies such as retinitis pigmentosa. Whereas photoreceptor loss is permanent in mammals, zebrafish possesses the ability to regenerate retinal neurons and restore visual function. Following acute damage, Müller glia (MG) re-enter the cell cycle and produce multipotent progenitors whose progeny differentiate into mature neurons. Both MG reprogramming and proliferation of retinal progenitor cells require reactive microglia and associated inflammatory signaling. Paradoxically, in zebrafish models of retinal degeneration, photoreceptor death does not induce the MG to reprogram and regenerate lost cells. Here, we used male and female zebrafish cep290 mutants to demonstrate that progressive cone degeneration generates an immune response but does not stimulate MG proliferation. Acute light damage triggered photoreceptor regeneration in cep290 mutants but cones were only restored to prelesion densities. Using irf8 mutant zebrafish, we found that the chronic absence of microglia reduced inflammation and rescued cone degeneration in cep290 mutants. Finally, single-cell RNA-sequencing revealed sustained expression of notch3 in MG of cep290 mutants and inhibition of Notch signaling induced MG to re-enter the cell cycle. Our findings provide new insights on the requirements for MG to proliferate and the potential for immunosuppression to prolong photoreceptor survival.SIGNIFICANCE STATEMENT Inherited retinal degenerations (IRDs) are genetic diseases that lead to the progressive loss of photoreceptors and the permanent loss of vision. Zebrafish can regenerate photoreceptors after acute injury by reprogramming Müller glia (MG) into stem-like cells that produce retinal progenitors, but this regenerative process fails to occur in zebrafish models of IRDs. Here, we show that Notch pathway inhibition can promote photoreceptor regeneration in models of progressive degeneration and that immunosuppression can prevent photoreceptor loss. These results offer insight into the pathways that promote MG-dependent regeneration and the role of inflammation in photoreceptor degeneration.

Why Has the Ability to Regenerate Following CNS Injury Been Repeatedly Lost Over the Course of Evolution?

While many vertebrates can regenerate both damaged neurons and severed axons in the central nervous system (CNS) following injury, others, including all birds and mammals, have lost this ability for reasons that are still unclear. The repeated evolutionary loss of regenerative competence seems counterintuitive, and any explanation must account for the fact that regenerative competence is lost in both cold-blooded and all warm-blooded clades, that both injury-induced neurogenesis and axonal regeneration tend to be lost in tandem, and that mammals have evolved dedicated gene regulatory networks to inhibit injury-induced glia-to-neuron reprogramming. Here, different hypotheses that have been proposed to account for evolutionary loss of regenerative competence are discussed in the light of new insights obtained into molecular mechanisms that control regeneration in the central nervous system. These include pleiotropic effects of continuous growth, enhanced thyroid hormone signaling, prevention of neoplasia, and improved memory consolidation. Recent evidence suggests that the most compelling hypothesis, however, may be selection for greater resistance to the spread of intra-CNS infections, which has led to both enhanced reactive gliosis and a loss of injury-induced neurogenesis and axonal regeneration. Means of testing these hypotheses, and additional data that are urgently needed to better understand the evolutionary pressures and mechanisms driving loss of regenerative competence, are also discussed.

Zebrafish models of inherited retinal dystrophies

Inherited retinal degenerations (IRDs) cause permanent vision impairment or vision loss due to the death of rod and cone photoreceptors. Animal models of IRDs have been instrumental in providing knowledge of the pathological mechanisms that cause photoreceptor death and in developing successful approaches that could slow or prevent vision loss. Zebrafish models of IRDs represent an ideal model system to study IRDs in a cone-rich retina and to test strategies that exploit the natural ability to regenerate damaged neurons. This review highlights those zebrafish mutants and transgenic lines that exhibit adult-onset retinal degeneration and serve as models of retinitis pigmentosa, cone-rod dystrophy, and ciliopathies.

Differential Responses of Neural Retina Progenitor Populations to Chronic Hyperglycemia

Diabetic retinopathy is a frequent complication of longstanding diabetes, which comprises a complex interplay of microvascular abnormalities and neurodegeneration. Zebrafish harboring a homozygous mutation in the pancreatic transcription factor pdx1 display a diabetic phenotype with survival into adulthood, and are therefore uniquely suitable among zebrafish models for studying pathologies associated with persistent diabetic conditions. We have previously shown that, starting at three months of age, pdx1 mutants exhibit not only vascular but also neuro-retinal pathologies manifesting as photoreceptor dysfunction and loss, similar to human diabetic retinopathy. Here, we further characterize injury and regenerative responses and examine the effects on progenitor cell populations. Consistent with a negative impact of hyperglycemia on neurogenesis, stem cells of the ciliary marginal zone show an exacerbation of aging-related proliferative decline. In contrast to the robust Müller glial cell proliferation seen following acute retinal injury, the pdx1 mutant shows replenishment of both rod and cone photoreceptors from slow-cycling, neurod-expressing progenitors which first accumulate in the inner nuclear layer. Overall, we demonstrate a diabetic retinopathy model which shows pathological features of the human disease evolving alongside an ongoing restorative process that replaces lost photoreceptors, at the same time suggesting an unappreciated phenotypic continuum between multipotent and photoreceptor-committed progenitors.

Decline in Constitutive Proliferative Activity in the Zebrafish Retina with Ageing

It is largely assumed that the teleost retina shows continuous and active proliferative and neurogenic activity throughout life. However, when delving into the teleost literature, one finds that assumptions about a highly active and continuous proliferation in the adult retina are based on studies in which proliferation was not quantified in a comparative way at the different life stages or was mainly studied in juveniles/young adults. Here, we performed a systematic and comparative study of the constitutive proliferative activity of the retina from early developing (2 days post-fertilisation) to aged (up to 3–4 years post-fertilisation) zebrafish. The mitotic activity and cell cycle progression were analysed by using immunofluorescence against pH3 and PCNA, respectively. We observed a decline in the cell proliferation in the retina with ageing despite the occurrence of a wave of secondary proliferation during sexual maturation. During this wave of secondary proliferation, the distribution of proliferating and mitotic cells changes from the inner to the outer nuclear layer in the central retina. Importantly, in aged zebrafish, there is a virtual disappearance of mitotic activity. Our results showing a decline in the proliferative activity of the zebrafish retina with ageing are of crucial importance since it is generally assumed that the fish retina has continuous proliferative activity throughout life.

Retinal Stem/Progenitor Cells Derived From Adult Müller Glia for the Treatment of Retinal Degeneration

Over the past two decades, progress in our understanding of glial function has been revolutionary. Within the retina, a subset of glial cells termed the “Müller glia (MG),” have been demonstrated to play key roles in retinal homeostasis, structure and metabolism. Additionally, MG have also been shown to possess the regenerative capacity that varies across species. In teleost fish, MG respond to injury by reprogramming into stem-like cells capable of regenerating lost tissue. The expression of stem/progenitor cell markers has been demonstrated broadly in mammalian MG, including human MG, but their in vivo regenerative capacity appears evolutionarily limited. Advances in stem cell therapy have progressively elucidated critical mechanisms underlying innate MG reprogramming in teleost fish, which have shown promising results when applied to rodents. Furthermore, when cultured ex vivo, MG from mammals can differentiate into several retina cell types. In this review, we will explore the reparative and regenerative potential of MG in cellular therapy approaches, and outline our current understanding of embryonic retinal development, the stem-cell potential of MG in adult vertebrate retina (including human), and microenvironmental cues that guide MG reprogramming.

Large-scale phenotypic drug screen identifies neuroprotectants in zebrafish and mouse models of retinitis pigmentosa

Retinitis pigmentosa (RP) and associated inherited retinal diseases (IRDs) are caused by rod photoreceptor degeneration, necessitating therapeutics promoting rod photoreceptor survival. To address this, we tested compounds for neuroprotective effects in multiple zebrafish and mouse RP models, reasoning drugs effective across species and/or independent of disease mutation may translate better clinically. We first performed a large-scale phenotypic drug screen for compounds promoting rod cell survival in a larval zebrafish model of inducible RP. We tested 2934 compounds, mostly human-approved drugs, across six concentrations, resulting in 113 compounds being identified as hits. Secondary tests of 42 high-priority hits confirmed eleven lead candidates. Leads were then evaluated in a series of mouse RP models in an effort to identify compounds effective across species and RP models, that is, potential pan-disease therapeutics. Nine of 11 leads exhibited neuroprotective effects in mouse primary photoreceptor cultures, and three promoted photoreceptor survival in mouse rd1 retinal explants. Both shared and complementary mechanisms of action were implicated across leads. Shared target tests implicated parp1-dependent cell death in our zebrafish RP model. Complementation tests revealed enhanced and additive/synergistic neuroprotective effects of paired drug combinations in mouse photoreceptor cultures and zebrafish, respectively. These results highlight the value of cross-species/multi-model phenotypic drug discovery and suggest combinatorial drug therapies may provide enhanced therapeutic benefits for RP patients.

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.

Inflammation Regulates the Multi-Step Process of Retinal Regeneration in Zebrafish

The ability to regenerate tissues varies between species and between tissues within a species. Mammals have a limited ability to regenerate tissues, whereas zebrafish possess the ability to regenerate almost all tissues and organs, including fin, heart, kidney, brain, and retina. In the zebrafish brain, injury and cell death activate complex signaling networks that stimulate radial glia to reprogram into neural stem-like cells that repair the injury. In the retina, a popular model for investigating neuronal regeneration, Müller glia, radial glia unique to the retina, reprogram into stem-like cells and undergo a single asymmetric division to generate multi-potent retinal progenitors. Müller glia-derived progenitors then divide rapidly, numerically matching the magnitude of the cell death, and differentiate into the ablated neurons. Emerging evidence reveals that inflammation plays an essential role in this multi-step process of retinal regeneration. This review summarizes the current knowledge of the inflammatory events during retinal regeneration and highlights the mechanisms whereby inflammatory molecules regulate the quiescence and division of Müller glia, the proliferation of Müller glia-derived progenitors and the survival of regenerated neurons.

Acoustic and Light Selective Behavioral Guidance Systems for Freshwater Fish

The use of non-physical barriers, particularly based on acoustic and luminous stimuli has been historically used to influence the behavior of fish, mainly for fishing purposes. Nowadays, behavioral barriers and guidance systems have been developed, not only to deter movements of fish, but also to promote behavioral responses with the objective of native fish protection, in particular the potamodromous species, reducing their mortality in the hydraulic structures of dams and guiding them towards transposition systems or to replacement habitats in regularized water bodies. This review details the use of acoustic and luminous systems and their evolution in recent years (Scopus 2012–2019) for the development of selective behavioral barriers for fish. We found that recent technologies try to identify new acoustic and luminous sensory ranges. Ambient sound, sound of predators or luminous spectral bands with different wavelengths have been used to selectively stimulate target and non-target species, in order to improve the effectiveness of repulsive/attractive systems for fish. Guidelines for future research in the area are also present.

Cone Photoreceptor Degeneration and Neuroinflammation in the Zebrafish Bardet-Biedl Syndrome 2 (bbs2) Mutant Does Not Lead to Retinal Regeneration

Bardet-Biedl syndrome (BBS) is a heterogeneous and pleiotropic autosomal recessive disorder characterized by obesity, retinal degeneration, polydactyly, renal dysfunction, and mental retardation. BBS results from defects in primary and sensory cilia. Mutations in 21 genes have been linked to BBS and proteins encoded by 8 of these genes form a multiprotein complex termed the BBSome. Mutations in BBS2, a component of the BBSome, result in BBS as well as non-syndromic retinal degeneration in humans and rod degeneration in mice, but the role of BBS2 in cone photoreceptor survival is not clear. We used zebrafish bbs2^–/– mutants to better understand how loss of bbs2 leads to photoreceptor degeneration. Zebrafish bbs2^–/– mutants exhibited impaired visual function as larvae and adult zebrafish underwent progressive cone photoreceptor degeneration. Cone degeneration was accompanied by increased numbers of activated microglia, indicating an inflammatory response. Zebrafish exhibit a robust ability to regenerate lost photoreceptors following retinal damage, yet cone degeneration and inflammation was insufficient to trigger robust Müller cell proliferation. In contrast, high intensity light damage stimulated Müller cell proliferation and photoreceptor regeneration in both wild-type and bbs2^–/– mutants, although the bbs2^–/– mutants could only restore cones to pre-damaged densities. In summary, these findings suggest that cone degeneration leads to an inflammatory response in the retina and that BBS2 is necessary for cone survival. The zebrafish bbs2 mutant also represents an ideal model to identify mechanisms that will enhance retinal regeneration in degenerating diseases.

Conditional and biased regeneration of cone photoreceptor types in the zebrafish retina

A major challenge in regenerative medicine is replacing cells lost through injury or disease. While significant progress has been made, much remains unknown about the accuracy of native regenerative programs in cell replacement. Here, we capitalized on the regenerative capacity and stereotypic retinal organization of zebrafish to determine the specificity with which retinal Müller glial cells replace lost neuronal cell types. By utilizing a targeted genetic ablation technique, we restricted death to all or to distinct cone photoreceptor types (red, blue, or UV-sensitive cones), enabling us to compare the composition of cones that are regenerated. We found that Müller glia produce cones of all types upon nondiscriminate ablation of these photoreceptors, or upon selective ablation of red or UV cones. Pan-ablation of cones led to regeneration of the various cone types in relative abundances that resembled those of nonablated controls, that is, red > green > UV ~ blue cones. Moreover, selective loss of red or UV cones biased production toward the cone type that was ablated. In contrast, ablation of blue cones alone largely failed to induce cone production at all, although it did induce cell division in Müller glia. The failure to produce cones upon selective elimination of blue cones may be due to their low abundance compared to other cone types. Alternatively, it may be that blue cone death alone does not trigger a change in progenitor competency to support cone genesis. Our findings add to the growing notion that cell replacement during regeneration does not perfectly mimic programs of cell generation during development.

Midkine-a functions as a universal regulator of proliferation during epimorphic regeneration in adult zebrafish

Zebrafish have the ability to regenerate damaged cells and tissues by activating quiescent stem and progenitor cells or reprogramming differentiated cells into regeneration-competent precursors. Proliferation among the cells that will functionally restore injured tissues is a fundamental biological process underlying regeneration. Midkine-a is a cytokine growth factor, whose expression is strongly induced by injury in a variety of tissues across a range of vertebrate classes. Using a zebrafish Midkine-a loss of function mutant, we evaluated regeneration of caudal fin, extraocular muscle and retinal neurons to investigate the function of Midkine-a during epimorphic regeneration. In wildtype zebrafish, injury among these tissues induces robust proliferation and rapid regeneration. In Midkine-a mutants, the initial proliferation in each of these tissues is significantly diminished or absent. Regeneration of the caudal fin and extraocular muscle is delayed; regeneration of the retina is nearly completely absent. These data demonstrate that Midkine-a is universally required in the signaling pathways that convert tissue injury into the initial burst of cell proliferation. Further, these data highlight differences in the molecular mechanisms that regulate epimorphic regeneration in zebrafish.

Characterisation of stem and proliferating cells on the retina and lens of loach Misgurnus anguillicaudatus

The eye of the fish has a lifelong persistent neurogenesis unlike eye of mammals, so it's highly interesting to study retinal neurogenesis and its genetic control to give complete knowledge about the cause of this property in fish in comparison to mammals. We performed fluorescent in situ hybridisation for loach Misgurnus anguillicaudatus bmi1, msi1 and sox2 genes, which are used as an indicator of the sites of multipotent stem cells. Proliferating cell nuclear antigen (PCNA), bromodeoxyuridine (BRDU) and KI67 markers were used as indicators of proliferating cells and glial fibrillary acidic protein (GFAP) immunofluorescence was used for detection of the glial property of cells, as well as, immunohistochemistry detected the role of peroxisome proliferator-activated receptor (PPAR)α and γ in retinal neurogenesis. Our results determined that the lens and the retina of loach M. anguillicaudatus contain proliferative and pluripotent stem cells that have both glial and neuroepithelial properties, which add new cells continuously throughout life even without injury-induced proliferation. The PPARα has an essential function in providing energy supply for retinal neurogenesis more than PPARγ.

Midkine-a Is Required for Cell Cycle Progression of Müller Glia during Neuronal Regeneration in the Vertebrate Retina

In the retina of zebrafish, Müller glia have the ability to reprogram into stem cells capable of regenerating all classes of retinal neurons and restoring visual function. Understanding the cellular and molecular mechanisms controlling the stem cell properties of Müller glia in zebrafish may provide cues to unlock the regenerative potential in the mammalian nervous system. Midkine is a cytokine/growth factor with multiple roles in neural development, tissue repair, and disease.

In the retina of zebrafish, Müller glia have the ability to reprogram into stem cells capable of regenerating all classes of retinal neurons and restoring visual function. Understanding the cellular and molecular mechanisms controlling the stem cell properties of Müller glia in zebrafish may provide cues to unlock the regenerative potential in the mammalian nervous system. Midkine is a cytokine/growth factor with multiple roles in neural development, tissue repair, and disease. In midkine-a loss-of-function mutants of both sexes, Müller glia initiate the appropriate reprogramming response to photoreceptor death by increasing expression of stem cell-associated genes, and entering the G₁ phase of the cell cycle. However, transition from G₁ to S phase is blocked in the absence of Midkine-a, resulting in significantly reduced proliferation and selective failure to regenerate cone photoreceptors. Failing to progress through the cell cycle, Müller glia undergo reactive gliosis, a pathological hallmark in the injured CNS of mammals. Finally, we determined that the Midkine-a receptor, anaplastic lymphoma kinase, is upstream of the HLH regulatory protein, Id2a, and of the retinoblastoma gene, p130, which regulates progression through the cell cycle. These results demonstrate that Midkine-a functions as a core component of the mechanisms that regulate proliferation of stem cells in the injured CNS.

SIGNIFICANCE STATEMENT The death of retinal neurons and photoreceptors is a leading cause of vision loss. Regenerating retinal neurons is a therapeutic goal. Zebrafish can regenerate retinal neurons from intrinsic stem cells, Müller glia, and are a powerful model to understand how stem cells might be used therapeutically. Midkine-a, an injury-induced growth factor/cytokine that is expressed by Müller glia following neuronal death, is required for Müller glia to progress through the cell cycle. The absence of Midkine-a suspends proliferation and neuronal regeneration. With cell cycle progression stalled, Müller glia undergo reactive gliosis, a pathological hallmark of the mammalian retina. This work provides a unique insight into mechanisms that control the cell cycle during neuronal regeneration.

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

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

Olfactomedin-like 2 A and B (OLFML2A and OLFML2B) profile expression in the retina of spotted gar (Lepisosteus oculatus) and bioinformatics mining

Olfactomedin-like (OLFML) proteins are members of the olfactomedin domain-containing secreted glycoprotein (OLF) family. OLFML2A and OLFML2B are representative molecules of these glycoproteins. Olfactomedins are critical for the development and functional organization of the nervous system and retina, which is a highly conserved structure in vertebrates, having almost identical anatomical and physiological characteristics in multiple taxa. Spotted gar, a member of the Lepisosteidae family, is a freshwater fish that inhabits rivers, bayous, swamps, and brackish waters. Recently, the complete genome has been sequenced, providing a unique bridge between fish medical models to human biology, making it an excellent animal model. This study was aimed to understanding the evolution OLFML2A and OLFML2B in the retina of spotted gar through looking for the expression of these genes. Spotted gar retina was analyzed with hematoxylin-eosin staining assays to provide an overall view of the retina structure and an immunofluorescence assay to identify OLFML2A and OLFML2B protein expression. A phylogenetic tree was created using the neighbor-joining method. Forces that direct the evolution of the fish genes were tested. Spotted gar retina, as in other vertebrates, is made of several layers. OLFML2A and OLFML2B proteins were detected in the rod and cone photoreceptor layer (PRL), outer nuclear layer (ONL), and inner nuclear layer (INL). Phylogenetic tree analysis confirms the orthology within the OLFML2A gene. Purifying selection is the evolutionary force that directs the OLFML2A genes. OLFML2A genes have a well-conserved function over time and species.

Ciliary genes arl13b, ahi1 and cc2d2a differentially modify expression of visual acuity phenotypes but do not enhance retinal degeneration due to mutation of cep290 in zebrafish

Mutations in the gene Centrosomal Protein 290 kDa (CEP290) result in multiple ciliopathies ranging from the neonatal lethal disorder Meckel-Gruber Syndrome to multi-systemic disorders such as Joubert Syndrome and Bardet-Biedl Syndrome to nonsyndromic diseases like Leber Congenital Amaurosis (LCA) and retinitis pigmentosa. Results from model organisms and human genetics studies, have suggest that mutations in genes encoding protein components of the transition zone (TZ) and other cilia-associated proteins can function as genetic modifiers and be a source for CEP290 pleiotropy. We investigated the zebrafish cep290fh297/fh297 mutant, which encodes a nonsense mutation (p.Q1217*). This mutant is viable as adults, exhibits scoliosis, and undergoes a slow, progressive cone degeneration. The cep290fh297/fh297 mutants showed partial mislocalization of the transmembrane protein rhodopsin but not of the prenylated proteins rhodopsin kinase (GRK1) or the rod transducin subunit GNB1. Surprisingly, photoreceptor degeneration did not trigger proliferation of Müller glia, but proliferation of rod progenitors in the outer nuclear layer was significantly increased. To determine if heterozygous mutations in other cilia genes could exacerbate retinal degeneration, we bred cep290fh297/fh297 mutants to arl13b, ahi1, and cc2d2a mutant zebrafish lines. While cep290fh297/fh297 mutants lacking a single allele of these genes did not exhibit accelerated photoreceptor degeneration, loss of one alleles of arl13b or ahi1 reduced visual performance in optokinetic response assays at 5 days post fertilization. Our results indicate that the cep290fh297/fh297 mutant is a useful model to study the role of genetic modifiers on photoreceptor degeneration in zebrafish and to explore how progressive photoreceptor degeneration influences regeneration in adult zebrafish.

Distribution and density of mixed-input ON bipolar cells of the goldfish (Carassius auratus) during growth

Neurons are continuously produced at different rates and locations in the teleost retina. Goldfish rods are homogeneously distributed and maintain a stable density throughout growth, whereas little is known about their postsynaptic partners. We examined the distribution and density of mixed-input ON bipolar cells (ON mBCs) in 57 goldfish of various sizes by immunolabeling their retinas with an antibody against PKCα and counting PKCα-positive neurons in wholemounts. Cell densities were correlated with morphometric data for the same animals, and the spatial resolution of the ON mBC mosaic was calculated in each case. The distribution of ON mBCs is homogeneous throughout growth. For a 10-fold change in body size (i.e., from 20 to 200 mm), the total number of ON mBCs increases 2.8 times, while retinal area expands around 10 times. As a consequence, the density of ON mBCs in large fish falls to ∼1/3 of that of small animals, and intercellular spacing doubles. The eye and the lens become around three times larger from small to large fish. This causes the retinal magnification factor (and thereby the image projected onto retina) to augment by the same amount. Because the retinal magnification factor rises more than the intercellular spacing in the same animals, the spatial resolution of the ON mBC mosaic improves from 0.8 to 1.4 cycles/degree as the body size increases from 20 to 200 mm. As ON mBCs are mostly rod-driven, our results suggest that the scotopic acuity of the goldfish may improve as the animal grows.

Extensive growth is followed by neurodegenerative pathology in the continuously expanding adult zebrafish retina

The development of effective treatments for age-related neurodegenerative diseases remains one of the biggest medical challenges today, underscoring the high need for suitable animal model systems to improve our understanding of aging and age-associated neuropathology. Zebrafish have become an indispensable complementary model organism in gerontology research, yet their growth-control properties significantly differ from those in mammals. Here, we took advantage of the clearly defined and highly conserved structure of the fish retina to study the relationship between the processes of growth and aging in the adult zebrafish central nervous system (CNS). Detailed morphological measurements reveal an early phase of extensive retinal growth, where both the addition of new cells and stretching of existent tissue drive the increase in retinal surface. Thereafter, and coinciding with a significant decline in retinal growth rate, a neurodegenerative phenotype becomes apparent,-characterized by a loss of synaptic integrity, an age-related decrease in cell density and the onset of cellular senescence. Altogether, these findings support the adult zebrafish retina as a valuable model for gerontology research and CNS disease modeling and will hopefully stimulate further research into the mechanisms of aging and age-related pathology.

Knockout of Nr2e3 prevents rod photoreceptor differentiation and leads to selective L-/M-cone photoreceptor degeneration in zebrafish

Mutations in the photoreceptor cell-specific nuclear receptor gene Nr2e3 increased the number of S-cone photoreceptors in human and murine retinas and led to retinal degeneration that involved photoreceptor and non-photoreceptor cells. The mechanisms underlying these complex phenotypes remain unclear. In the hope of understanding the precise role of Nr2e3 in photoreceptor cell fate determination and differentiation, we generated a line of Nr2e3 knockout zebrafish using CRISPR technology. In these Nr2e3-null animals, rod precursors undergo terminal mitoses but fail to differentiate as rods. Rod-specific genes are not expressed and the outer segment (OS) fails to form. Formation and differentiation of cone photoreceptors is normal. Specifically, there is no increase in the number of UV-cone or S-cone photoreceptors. Laminated retinal structure is maintained. After normal development, L-/M-cones selectively degenerate, with progressive shortening of OS that starts at age 1 month. The amount of cone phototransduction proteins is concomitantly reduced, whereas UV- and S-cones have normal OS lengths even at age 10 months. In vitro studies show Nr2e3 synergizes with Crx and Nrl to enhance rhodopsin gene expression. Nr2e3 does not affect cone opsin expression. Our results extend the knowledge of Nr2e3's roles and have specific implications for the interpretation of the phenotypes observed in human and murine retinas. Furthermore, our model may offer new opportunities in finding treatments for enhanced S-cone syndrome (ESCS) and other retinal degenerative diseases.

Retinal Wnt signaling defect in a zebrafish fetal alcohol spectrum disorder model

Fetal alcohol spectrum disorder caused by prenatal alcohol exposure includes ocular abnormalities (microphthalmia, photoreceptor dysfunction, cataracts). Zebrafish embryos exposed to ethanol from gastrulation through somitogenesis show severe ocular defects, including microphthalmia and photoreceptor differentiation defects. Ethanol-treated zebrafish had an enlarged ciliary marginal zone (CMZ) relative to the retina size and reduced Müller glial cells (MGCs). Ethanol exposure produced immature photoreceptors with increased proliferation, indicating cell cycle exit failure. Signaling mechanisms in the CMZ were affected by embryonic ethanol exposure, including Wnt signaling in the CMZ, Notch signaling and neurod gene expression. Retinoic acid or folic acid co-supplementation with ethanol rescued Wnt signaling and retinal differentiation. Activating Wnt signaling using GSK3 inhibitor (LSN 2105786; Eli Lilly and Co.) restored retinal cell differentiation pathways. Ethanol exposed embryos were treated with Wnt agonist, which rescued Wnt-active cells in the CMZ, Notch-active cells in the retina, proliferation, and photoreceptor terminal differentiation. Our results illustrate the critical role of Wnt signaling in ethanol-induced retinal defects.

Transcripts within rod photoreceptors of the Zebrafish retina

BACKGROUND

The purpose of this study was to identify transcripts of retinal rod photoreceptors of the zebrafish. The zebrafish is an important animal model for vision science due to rapid and tractable development, persistent neurogenesis of rods throughout the lifespan, and capacity for functional retinal regeneration.

RESULTS

Zebrafish rods, and non-rod retinal cells of the xops:eGFP transgenic line, were separated by cell dissociation and fluorescence-activated cell sorting (FACS), followed by RNA-seq. At a false discovery rate of < 0.01, 597 transcripts were upregulated ("enriched") in rods vs. other retinal cells, and 1032 were downregulated ("depleted"). Thirteen thousand three hundred twenty four total transcripts were detected in rods, including many not previously known to be expressed by rods. Forty five transcripts were validated by qPCR in FACS-sorted rods vs. other retinal cells. Transcripts enriched in rods from adult retinas were also enriched in rods from larval and juvenile retinas, and were also enriched in rods sorted from retinas subjected to a neurotoxic lesion and allowed to regenerate. Many transcripts enriched in rods were upregulated in retinas of wildtype retinas vs. those of a zebrafish model for rod degeneration.

CONCLUSIONS

We report the generation and validation of an RNA-seq dataset describing the rod transcriptome of the zebrafish, which is now available as a resource for further studies of rod photoreceptor biology and comparative transcriptomics.

Novel Animal Model of Crumbs-Dependent Progressive Retinal Degeneration That Targets Specific Cone Subtypes

Purpose

Human Crb1 is implicated in some forms of retinal degeneration, suggesting a role in photoreceptor maintenance. Multiple Crumbs (Crb) polarity genes are expressed in vertebrate retina, although their functional roles are not well understood. To gain further insight into Crb and photoreceptor maintenance, we compared retinal cell densities between wild-type and Tg(RH2-2:Crb2b-sfEX/RH2-2:GFP)pt108b transgenic zebrafish, in which the extracellular domain of Crb2b-short form (Crb2b-sfEX) is expressed in the retina as a secreted protein, which disrupts the planar organization of RGB cones (red, green, and blue) by interfering with Crb2a/2b-based cone-cone adhesion.

Methods

We used standard morphometric techniques to assess age-related changes in retinal cell densities in adult zebrafish (3 to 27 months old), and to assess effects of the Crb2b-sfEX transgene on retinal structure and photoreceptor densities. Linear cell densities were measured in all retinal layers in radial sections with JB4-Feulgen histology. Planar (surface) densities of cones were determined in retinal flat-mounts. Cell counts from wild-type and pt108b transgenic fish were compared with both a "photoreceptor maintenance index" and statistical analysis of cell counts.

Results

Age-related changes in retinal cell linear densities and cone photoreceptor planar densities in wild-type adult zebrafish provided a baseline for analysis. Expression of Crb2b-sfEX caused progressive and selective degeneration of RGB cones, but had no effect on ultraviolet-sensitive (UV) cones, and increased numbers of rod photoreceptors.

Conclusions

These differential responses of RGB cones, UV cones, and rods to sustained exposure to Crb2b-sfEX suggest that Crb-based photoreceptor maintenance mechanisms are highly selective.

Anisotropic Müller glial scaffolding supports a multiplex lattice mosaic of photoreceptors in zebrafish retina

Background

The multiplex, lattice mosaic of cone photoreceptors in the adult fish retina is a compelling example of a highly ordered epithelial cell pattern, with single cell width rows and columns of cones and precisely defined neighbor relationships among different cone types. Cellular mechanisms patterning this multiplex mosaic are not understood. Physical models can provide new insights into fundamental mechanisms of biological patterning. In earlier work, we developed a mathematical model of photoreceptor cell packing in the zebrafish retina, which predicted that anisotropic mechanical tension in the retinal epithelium orients planar polarized adhesive interfaces to align the columns as cone photoreceptors are generated at the retinal margin during post-embryonic growth.

Methods

With cell-specific fluorescent reporters and in vivo imaging of the growing retinal margin in transparent juvenile zebrafish we provide the first view of how cell packing, spatial arrangement, and cell identity are coordinated to build the lattice mosaic. With targeted laser ablation we probed the tissue mechanics of the retinal epithelium.

Results

Within the lattice mosaic, planar polarized Crumbs adhesion proteins pack cones into a single cell width column; between columns, N-cadherin-mediated adherens junctions stabilize Müller glial apical processes. The concentration of activated pMyosin II at these punctate adherens junctions suggests that these glial bands are under tension, forming a physical barrier between cone columns and contributing to mechanical stress anisotropies in the epithelial sheet. Unexpectedly, we discovered that the appearance of such parallel bands of Müller glial apical processes precedes the packing of cones into single cell width columns, hinting at a possible role for glia in the initial organization of the lattice mosaic. Targeted laser ablation of Müller glia directly demonstrates that these glial processes support anisotropic mechanical tension in the planar dimension of the retinal epithelium.

Conclusions

These findings uncovered a novel structural feature of Müller glia associated with alignment of photoreceptors into a lattice mosaic in the zebrafish retina. This is the first demonstration, to our knowledge, of planar, anisotropic mechanical forces mediated by glial cells.

Electronic supplementary material

The online version of this article (10.1186/s13064-017-0096-z) contains supplementary material, which is available to authorized users.

Recruitment of Rod Photoreceptors from Short-Wavelength-Sensitive Cones during the Evolution of Nocturnal Vision in Mammals

Vertebrate ancestors had only cone-like photoreceptors. The duplex retina evolved in jawless vertebrates with the advent of highly photosensitive rod-like photoreceptors. Despite cones being the arbiters of high-resolution color vision, rods emerged as the dominant photoreceptor in mammals during a nocturnal phase early in their evolution. We investigated the evolutionary and developmental origins of rods in two divergent vertebrate retinas. In mice, we discovered genetic and epigenetic vestiges of short-wavelength cones in developing rods, and cell-lineage tracing validated the genesis of rods from S cones. Curiously, rods did not derive from S cones in zebrafish. Our study illuminates several questions regarding the evolution of duplex retina and supports the hypothesis that, in mammals, the S-cone lineage was recruited via the Maf-family transcription factor NRL to augment rod photoreceptors. We propose that this developmental mechanism allowed the adaptive exploitation of scotopic niches during the nocturnal bottleneck early in mammalian evolution.

Treatment Paradigms for Retinal and Macular Diseases Using 3-D Retina Cultures Derived From Human Reporter Pluripotent Stem Cell Lines

We discuss the use of pluripotent stem cell lines carrying fluorescent reporters driven by retinal promoters to derive three-dimensional (3-D) retina in culture and how this system can be exploited for elucidating human retinal biology, creating disease models in a dish, and designing targeted drug screens for retinal and macular degeneration. Furthermore, we realize that stem cell investigations are labor-intensive and require extensive resources. To expedite scientific discovery by sharing of resources and to avoid duplication of efforts, we propose the formation of a Retinal Stem Cell Consortium. In the field of vision, such collaborative approaches have been enormously successful in elucidating genetic susceptibility associated with age-related macular degeneration.

Talkin' about my (re)generation: The who of intrinsic retinal stem cells

World-wide, two degenerative retinal diseases, glaucoma and age-related macular degeneration, are estimated to affect more than 12% of individuals over the age of 40 (Tham et al., 2014; Wong et al., 2014). Current therapies can slow progression, but cannot restore lost neurons or vision. Thus, there is increasing interest in developing strategies for therapeutic retinal regeneration. Nearly 50years of research on retinal neurogenesis and regeneration has identified Müller glia as intrinsic retinal stem cells in teleost fish. In the mammalian retina, there is no de novo neurogenesis in adults and only very limited injury-induced regeneration has been induced using exogenous growth factors. The study by (Webster et al., 2017) (Evidence of BrdU Positive Retinal Neurons after Application of an Alpha7 Nicotinic Acetylcholine Receptor Agonist, this issue) is the first to show robust, retinal neurogenesis in an adult, mammalian retina in the absence of overt injury and provides evidence that the source of the new neurons is likely to be the Müller glia. This exciting finding has the potential to be a game-changer in the field of retinal regeneration.

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.

Regulation of Stem Cell Properties of Müller Glia by JAK/STAT and MAPK Signaling in the Mammalian Retina

In humans and other mammals, the neural retina does not spontaneously regenerate, and damage to the retina that kills retinal neurons results in permanent blindness. In contrast to embryonic stem cells, induced pluripotent stem cells, and embryonic/fetal retinal stem cells, Müller glia offer an intrinsic cellular source for regenerative strategies in the retina. Müller glia are radial glial cells within the retina that maintain retinal homeostasis, buffer ion flux associated with phototransduction, and form the blood/retinal barrier within the retina proper. In injured or degenerating retinas, Müller glia contribute to gliotic responses and scar formation but also show regenerative capabilities that vary across species. In the mammalian retina, regenerative responses achieved to date remain insufficient for potential clinical applications. Activation of JAK/STAT and MAPK signaling by CNTF, EGF, and FGFs can promote proliferation and modulate the glial/neurogenic switch. However, to achieve clinical relevance, additional intrinsic and extrinsic factors that restrict or promote regenerative responses of Müller glia in the mammalian retina must be identified. This review focuses on Müller glia and Müller glial-derived stem cells in the retina and phylogenetic differences among model vertebrate species and highlights some of the current progress towards understanding the cellular mechanisms regulating their regenerative response.

Characterization and Evolution of the Spotted Gar Retina

In this study, we characterize the retina of the spotted gar, Lepisosteus oculatus, a ray-finned fish. Gar did not undergo the whole genome duplication event that occurred at the base of the teleost fish lineage, which includes the model species zebrafish and medaka. The divergence of gars from the teleost lineage and the availability of a high-quality genome sequence make it a uniquely useful species to understand how genome duplication sculpted features of the teleost visual system, including photoreceptor diversity. We developed reagents to characterize the cellular organization of the spotted gar retina, including representative markers for all major classes of retinal neurons and Müller glia. We report that the gar has a preponderance of predicted short-wavelength shifted (SWS) opsin genes, including a duplicated set of SWS1 (ultraviolet) sensitive opsin encoding genes, a SWS2 (blue) opsin encoding gene, and two rod opsin encoding genes, all of which were expressed in retinal photoreceptors. We also report that gar SWS1 cones lack the geometric organization of photoreceptors observed in teleost fish species, consistent with the crystalline photoreceptor mosaic being a teleost innovation. Of note the spotted gar expresses both exo-rhodopsin (RH1-1) and rhodopsin (RH1-2) in rods. Exo-rhodopsin is an opsin that is not expressed in the retina of zebrafish and other teleosts, but rather is expressed in regions of the brain. This study suggests that exo-rhodopsin is an ancestral actinopterygian (ray finned fish) retinal opsin, and in teleosts its expression has possibly been subfunctionalized to the pineal gland.

Mismatch of Synaptic Patterns between Neurons Produced in Regeneration and during Development of the Vertebrate Retina

Stereotypic patterns of synaptic connections between neurons underlie the ability of the CNS to perform complex but circuit-specific information processing. Tremendous progress has been made toward advancing our understanding of how circuits are assembled during development, but whether the precision of this process can be recaptured after regeneration of neurons in the damaged CNS remains unclear. Here, we harnessed the endogenous regenerative capacity of the zebrafish retina to reconstruct the circuitry of neurons produced after damage. We tracked the input connectivity of identified bipolar cell (BC) types across stages of retinal development and after BC regeneration. We found that BCs of each type generate a unique and stereotypic wiring pattern with cone photoreceptors by gaining synapses with specific photoreceptor types over time. After selective ablation, the targeted BC types are rapidly reproduced and largely re-establish their characteristic morphological features. The regenerated population connects with appropriate photoreceptor types and establishes the original number of synaptic contacts. However, BC types that normally bias their connectivity in favor of red cones fail to precisely recapture this synaptic partner preference upon regeneration. Furthermore, regenerated BCs succeed in forming synaptic specializations at their axon terminals, but in excess of the usual number. Altogether, we find that regenerated BCs reinstate some, but not all, major features of their stereotypic wiring, suggesting that circuit patterns may be unable to regenerate with the same fidelity as in development.

The two-step development of a duplex retina involves distinct events of cone and rod neurogenesis and differentiation

Unlike in mammals, persistent postembryonic retinal growth is a characteristic feature of fish, which includes major remodeling events that affect all cell types including photoreceptors. Consequently, visual capabilities change during development, where retinal sensitivity to different wavelengths of light (photopic vision), -and to limited photons (scotopic vision) are central capabilities for survival. Differently from well-established model fish, Atlantic cod has a prolonged larval stage where only cone photoreceptors are present. Rods do not appear until juvenile transition (metamorphosis), a hallmark of indirect developing species. Previously we showed that whole gene families of lws (red-sensitive) and sws1 (UV-sensitive) opsins have been lost in cod, while rh2a (green-sensitive) and sws2 (blue-sensitive) genes have tandem duplicated. Here, we provide a comprehensive characterization of a two-step developing duplex retina in Atlantic cod. The study focuses on cone subtype dynamics and delayed rod neurogenesis and differentiation in all cod life stages. Using transcriptomic and histological approaches we show that different opsins disappear in a topographic manner during development where central to peripheral retina is a key axis of expressional change. Early cone differentiation was initiated in dorso-temporal retina different from previously described in fish. Rods first appeared during initiation of metamorphosis and expression of the nuclear receptor transcription factor nr2e3-1, suggest involvement in rod specification. The indirect developmental strategy thus allows for separate studies of cones and rods development, which in nature correlates with visual changes linked to habitat shifts. The clustering of key retinal genes according to life stage, suggests that Atlantic cod with its sequenced genome may be an important resource for identification of underlying factors required for development and function of photopic and scotopic vision.

Identification of Radial Glia Progenitors in the Developing and Adult Retina of Sharks

Neural stem cells give rise to transient progenitors termed neuroepithelial cells (NECs) and radial glial cells (RGCs). RGCs represent the major source of neurons, glia and adult stem cells in several regions of the central nervous system (CNS). RGCs are mostly transient in mammals, but they are widely maintained in the adult CNS of fishes, where they continue to be morphologically similar to RGCs in the mammalian brain and fulfill similar roles as progenitors and guide for migrating neurons. The retina of fishes offers an exceptional model to approach the study of adult neurogenesis because of the presence of constitutive proliferation from the ciliary marginal zone (CMZ), containing NECs, and from adult glial cells with radial morphology (the Müller glia). However, the cellular hierarchies and precise contribution of different types of progenitors to adult neurogenesis remain unsolved. We have analyzed the transition from NECs to RGCs and RGC differentiation in the retina of the cartilaginous fish Scyliorhinus canicula, which offers a particularly good spatial and temporal frame to investigate this process. We have characterized progenitor and adult RGCs by immunohistochemical detection of glial markers as glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). We have compared the emergence and localization of glial markers with that of proliferating cell nuclear antigen (PCNA, a proliferation maker) and Doublecortin (DCX, which increases at early stages of neuronal differentiation). During retinal development, GFAP-immunoreactive NECs located in the most peripheral CMZ (CMZp) codistribute with DCX-immunonegative cells. GFAP-immunoreactive RGCs and Müller cells are located in successive more central parts of the retina and codistribute with DCX- and DCX/GS-immunoreactive cells, respectively. The same types of progenitors are found in juveniles, suggesting that the contribution of the CMZ to adult neurogenesis implies a transition through the radial glia (RG) state.

Retinal development in the gilthead seabream Sparus aurata

The retinal development of the gilthead seabream Sparus aurata has been analysed from late embryonic development to juvenile stages using classical histological and immunohistological methods. Five significant phases were established. Phases 1 and 2 comprise the late embryonic and hatching stages, respectively. The results indicate that during these early stages the retina is composed of a single neuroblastic layer that consists of undifferentiated retinal progenitor cells. Phase 3 (late prolarval stage) is characterized by the emergence of the retinal layers and the appearance of neurochemical profiles in differentiating photoreceptors, amacrine and ganglion cells. Phases 4 and 5 comprise the late larval and juvenile stages. In these stages, all the retinal cell types can be detected immunohistochemically. All the maturational events described are first detected in the central retina and, as development progresses, spread to the rest of the retina following a central-to-peripheral gradient. The results of this study suggest that S. aurata is an altricial teleost species that hatches with a morphologically undifferentiated retina. The most relevant processes involved in retinogenesis occur during the late prolarval stage (phase 3).

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.

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.

Ontogenic retinal changes in three ecologically distinct elopomorph fishes (Elopomorpha:Teleostei) correlate with light environment and behavior

Unlike the mammalian retina, the teleost fish retina undergoes persistent neurogenesis from intrinsic stem cells. In marine teleosts, most cone photoreceptor genesis occurs early in the embryonic and larval stages, and rods are added primarily during and after metamorphosis. This study demonstrates a developmental paradigm in elopomorph fishes in which retinas are rod-dominated in larvae, but undergo periods of later cone genesis. Retinal characteristics were compared at different developmental stages among three ecologically distinct elopomorph fishes—ladyfish (Elops saurus), bonefish (Albula vulpes), and speckled worm eel (Myrophis punctatus). The objectives were to improve our understanding of (1) the developmental strategy in the elopomorph retina, (2) the functional architecture of the retina as it relates to ecology, and (3) how the light environment influences photoreceptor genesis. Photoreceptor morphologies, distributions, and spectral absorption were studied at larval, juvenile, and adult stages. Premetamorphic retinas in all three species are rod-dominated, but the retinas of these species undergo dramatic change over the course of development, resulting in juvenile and adult retinal characteristics that correlate closely with ecology. Adult E. saurus has high rod densities, grouped photoreceptors, a reflective tapetum, and longer-wavelength photopigments, supporting vision in turbid, low-light conditions. Adult A. vulpes has high cone densities, low rod densities, and shorter-wavelength photopigments, supporting diurnal vision in shallow, clear water. M. punctatus loses cones during metamorphosis, develops new cones after settlement, and maintains high rod but low cone densities, supporting primarily nocturnal vision. M. punctatus secondary cone genesis occurs rapidly throughout the retina, suggesting a novel mechanism of vertebrate photoreceptor genesis. Finally, in postsettlement M. punctatus, the continuous presence or absence of visible light modulates rod distribution but does not affect secondary cone genesis, suggesting some degree of developmental plasticity influenced by the light environment.