Time course analysis of gene expression during light-induced photoreceptor cell death and regeneration in albino zebrafish

Light-induced degeneration and microglial response in the retina of an epibenthonic pigmented teleost: age-dependent photoreceptor susceptibility to cell death

Constant intense light causes apoptosis of photoreceptors in the retina of albino fish. However, very few studies have been performed on pigmented species. Tench (Tinca tinca) is a teleost inhabiting dimly lit environments that has a predominance of rods within the photoreceptor layer. To test the hypothesis that constant high intensity light can result in retinal damage in such pigmented epibenthonic teleost species, photodegeneration of the retina was investigated in the larvae and in juveniles of tench to assess whether any damage may also be dependent on fish age. We exposed both groups of animals to 5 days of constant darkness, followed by 4 days of constant 20,000 lx light, and then by 6 days of recovery in a 14 h light:10 h dark cycle. The results showed that the retina of the larvae group exhibited abundant photoreceptor cell apoptosis during the time of exposition to intense light, whereas that of juveniles was indifferent to it. Damaged retinas showed a strong TUNEL signal in photoreceptor nuclei, and occasionally a weak cytoplasmic TUNEL signal in Müller glia. Specific labelling of microglial cells with Griffonia simplicifolia lectin (GSL) histochemistry revealed that photoreceptor cell death alerts microglia in the degenerating retina, leading to local proliferation, migration towards the injured outer nuclear layer (ONL), and enhanced phagocytosis of photoreceptor debris. During the first days of intense light treatment, Müller cells phagocytosed dead photoreceptor cells but, once microglial cells became activated, there was a progressive increase in the phagocytic capacity of the microglia.

Spectral-domain optical coherence tomography as a noninvasive method to assess damaged and regenerating adult zebrafish retinas

PURPOSE

These experiments assessed the ability of spectral-domain optical coherence tomography (SD-OCT) to accurately represent the structural organization of the adult zebrafish retina and reveal the dynamic morphologic changes during either light-induced damage and regeneration of photoreceptors or ouabain-induced inner retinal damage.

METHODS

Retinas of control dark-adapted adult albino zebrafish were compared with retinas subjected to 24 hours of constant intense light and recovered for up to 8 weeks or ouabain-damaged retinas that recovered for up to 3 weeks. Images were captured and the measurements of retinal morphology were made by SD-OCT, and then compared with those obtained by histology of the same eyes.

RESULTS

Measurements between SD-OCT and histology were very similar for the undamaged, damaged, and regenerating retinas. Axial measurements of SD-OCT also revealed vitreal morphology that was not readily visualized by histology.

CONCLUSIONS

SD-OCT accurately represented retinal lamination and photoreceptor loss and recovery during light-induced damage and subsequent regeneration. SD-OCT was less accurate at detecting the inner nuclear layer in ouabain-damaged retinas, but accurately detected the undamaged outer nuclear layer. Thus, SD-OCT provides a noninvasive and quantitative method to assess the morphology and the extent of damage and repair in the zebrafish retina.

HB-EGF is necessary and sufficient for Müller glia dedifferentiation and retina regeneration

Müller glia (MG) dedifferentiation into a cycling population of multipotent progenitors is crucial to zebrafish retina regeneration. The mechanisms underlying MG dedifferentiation are unknown. Here we report that heparin-binding epidermal-like growth factor (HB-EGF) is rapidly induced in MG residing at the injury site and that pro-HB-EGF ectodomain shedding is necessary for retina regeneration. Remarkably, HB-EGF stimulates the formation of multipotent MG-derived progenitors in the uninjured retina. We show that HB-EGF mediates its effects via an EGFR/MAPK signal transduction cascade that regulates the expression of regeneration-associated genes, like ascl1a and pax6(b). We also uncover an HB-EGF/Ascl1a/Notch/hb-egf(a)-signaling loop that helps define the zone of injury-responsive MG. Finally, we show that HB-EGF acts upstream of the Wnt/β-catenin-signaling cascade that controls progenitor proliferation. These data provide a link between extracellular signaling and regeneration-associated gene expression in the injured retina and suggest strategies for stimulating retina regeneration in mammals.

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.

Using the Tg(nrd:egfp)/albino zebrafish line to characterize in vivo expression of neurod

In this study, we used a newly-created transgenic zebrafish, Tg(nrd:egfp)/albino, to further characterize the expression of neurod in the developing and adult retina and to determine neurod expression during adult photoreceptor regeneration. We also provide observations regarding the expression of neurod in a variety of other tissues. In this line, EGFP is found in cells of the developing and adult retina, pineal gland, cerebellum, olfactory bulbs, midbrain, hindbrain, neural tube, lateral line, inner ear, pancreas, gut, and fin. Using immunohistochemistry and in situ hybridization, we compare the expression of the nrd:egfp transgene to that of endogenous neurod and to known retinal cell types. Consistent with previous data based on in situ hybridizations, we show that during retinal development, the nrd:egfp transgene is not expressed in proliferating retinal neuroepithelium, and is expressed in a subset of retinal neurons. In contrast to previous studies, nrd:egfp is gradually re-expressed in all rod photoreceptors. During photoreceptor regeneration in adult zebrafish, in situ hybridization reveals that neurod is not expressed in Müller glial-derived neuronal progenitors, but is expressed in photoreceptor progenitors as they migrate to the outer nuclear layer and differentiate into new rod photoreceptors. During photoreceptor regeneration, expression of the nrd:egfp matches that of neurod. We conclude that Tg(nrd:egfp)/albino is a good representation of endogenous neurod expression, is a useful tool to visualize neurod expression in a variety of tissues and will aid investigating the fundamental processes that govern photoreceptor regeneration in adults.

Light-induced photoreceptor degeneration in the retina of the zebrafish

Exposure of the zebrafish retina to intense light is a noninvasive method to elicit the selective degeneration of photoreceptors. In zebrafish, photoreceptor degeneration is followed by robust photoreceptor regeneration from stem cells that are intrinsic to the teleost retina. Two recent light-lesioning methods have been developed, each with specific advantages. The first involves a prolonged period of dark adaptation followed by exposure to high-intensity halogen lighting at ∼3,000-20,000 lux for 3-4 days. This causes widespread degeneration of rod and cone cells in the dorsal and central regions of the retina. The second method uses ultrahigh-intensity lighting at intensities greater than 120,000 lux, with an exposure time of 30 min. This causes degeneration of rod and cone cells within a relatively narrow naso-temporal band in the central retina. The advantages of the first (lower light intensity) technique are the widespread destruction of photoreceptors and the lower cost of equipment. The advantages of the second (higher light intensity) technique are the elimination of the prolonged dark adaptation and short duration of the exposure, thereby allowing experiments to be conducted more rapidly.

Application of Cre-loxP recombination for lineage tracing of adult zebrafish retinal stem cells

The Cre-loxP recombination system is widely used as a genetic tool to achieve conditional gene expression and for lineage tracing. Though extensively used in mice, this technology has only recently been applied to zebrafish. Here we describe Cre-loxP methodology for conditional expression of transgenes in zebrafish and their use in lineage tracing Müller glia as they undergo cellular reprogramming and proliferation to repair damaged retinal circuitry following mechanical injury. This methodology can be used for conditional gene expression and lineage tracing at any stage of development and in any cell type.

Teleost fish as a model system to study successful regeneration of the central nervous system

Traumatic brain injury and spinal cord injury are devastating conditions that may result in death or long-term disability. A promising strategy for the development of effective cell replacement therapies involves the study of regeneration-competent organisms. Among this group, teleost fish are distinguished by their excellent potential to regenerate nervous tissue and to regain function after injury to the central nervous system. In this chapter, we summarize our current understanding of the cellular processes that mediate this regenerative potential, and we show that several of these processes are shared with the normal development of the intact central nervous system; we describe how the spontaneous self-repair of the teleostean central nervous system leads to functional recovery, at physiological and behavioral levels; we discuss the possible function of molecular factors associated with the degenerative and regenerative processes after injury; and, finally, we speculate on evolutionary aspects of adult neurogenesis and neuronal regeneration, and on how a better understanding of these aspects could catalyze the development of therapeutic strategies to overcome the regenerative limits of the mammalian CNS.

In vivo electroporation of morpholinos into the adult zebrafish retina

Many devastating inherited eye diseases result in progressive and irreversible blindness because humans cannot regenerate dying or diseased retinal neurons. In contrast, the adult zebrafish retina possesses the robust ability to spontaneously regenerate any neuronal class that is lost in a variety of different retinal damage models, including retinal puncture, chemical ablation, concentrated high temperature, and intense light treatment. Our lab extensively characterized regeneration of photoreceptors following constant intense light treatment and inner retinal neurons after intravitreal ouabain injection. In all cases, resident Müller glia re-enter the cell cycle to produce neuronal progenitors, which continue to proliferate and migrate to the proper retinal layer, where they differentiate into the deficient neurons. We characterized five different stages during regeneration of the light-damaged retina that were highlighted by specific cellular responses. We identified several differentially expressed genes at each stage of retinal regeneration by mRNA microarray analysis. Many of these genes are also critical for ocular development. To test the role of each candidate gene/protein during retinal regeneration, we needed to develop a method to conditionally limit the expression of a candidate protein only at times during regeneration of the adult retina. Morpholino oligos are widely used to study loss of function of specific proteins during the development of zebrafish, Xenopus, chick, mouse, and tumors in human xenografts. These modified oligos basepair with complementary RNA sequence to either block the splicing or translation of the target RNA. Morpholinos are stable in the cell and can eliminate or "knockdown" protein expression for three to five days. Here, we describe a method to efficiently knockdown target protein expression in the adult zebrafish retina. This method employs lissamine-tagged antisense morpholinos that are injected into the vitreous of the adult zebrafish eye. Using electrode forceps, the morpholino is then electroporated into all the cell types of the dorsal and central retina. Lissamine provides the charge on the morpholino for electroporation and can be visualized to assess the presence of the morpholino in the retinal cells. Conditional knockdown in the retina can be used to examine the role of specific proteins at different times during regeneration. Additionally, this approach can be used to study the role of specific proteins in the undamaged retina, in such processes as visual transduction and visual processing in second order neurons.

Stab wound injury of the zebrafish telencephalon: a model for comparative analysis of reactive gliosis

Reactive glia, including astroglia and oligodendrocyte progenitors (OPCs) are at the core of the reaction to injury in the mammalian brain with initially beneficial and later partially adverse functions such as scar formation. Given the different glial composition in the adult zebrafish brain with radial ependymoglia but no parenchymal astrocytes, we examined the glial response to an invasive stab wound injury model in the adult zebrafish telencephalon. Strikingly, already a few days after injury the wound was closed without any scar tissue. Similar to mammals, microglia cells reacted first and accumulated close to the injury site, while neither GFAP+ radial ependymoglia nor adult OPCs were recruited to the injury site. Moreover, OPCs failed to increase their proliferation after this injury, while the number of proliferating GFAP+ glia was increased until 7 days after injury. Importantly, neurogenesis was also increased after injury, generating additional neurons recruited to the parenchyma which survived for several months. Thus, these data suggest that the specific glial environment in the adult zebrafish telencephalon is not only permissive for long-term neuronal survival, but avoids scar formation. Invasive injury in the adult zebrafish telencephalon may therefore provide a useful model to untangle the molecular mechanisms involved in these beneficial glial reactions.

FGF signaling regulates rod photoreceptor cell maintenance and regeneration in zebrafish

Fgf signaling is required for many biological processes involving the regulation of cell proliferation and maintenance, including embryonic patterning, tissue homeostasis, wound healing, and cancer progression. Although the function of Fgf signaling is suggested in several different regeneration models, including appendage regeneration in amphibians and fin and heart regeneration in zebrafish, it has not yet been studied during zebrafish photoreceptor cell regeneration. Here we demonstrate that intravitreal injections of FGF-2 induced rod precursor cell proliferation and photoreceptor cell neuroprotection during intense light damage. Using the dominant-negative Tg(hsp70:dn-fgfr1) transgenic line, we found that Fgf signaling was required for homeostasis of rod, but not cone, photoreceptors. Even though fgfr1 is expressed in both rod and cone photoreceptors, we found that Fgf signaling differentially affected the regeneration of cone and rod photoreceptors in the light-damaged retina, with the dominant-negative hsp70:dn-fgfr1 transgene significantly repressing rod photoreceptor regeneration without affecting cone photoreceptors. These data suggest that rod photoreceptor homeostasis and regeneration is Fgf-dependent and that rod and cone photoreceptors in adult zebrafish are regulated by different signaling pathways.

Ascl1a/Dkk/beta-catenin signaling pathway is necessary and glycogen synthase kinase-3beta inhibition is sufficient for zebrafish retina regeneration

Key to successful retina regeneration in zebrafish are Müller glia (MG) that respond to retinal injury by dedifferentiating into a cycling population of retinal progenitors. Although recent studies have identified several genes involved in retina regeneration, the signaling mechanisms underlying injury-dependent MG proliferation have remained elusive. Here we report that canonical Wnt signaling controls the proliferation of MG-derived retinal progenitors. We found that injury-dependent induction of Ascl1a suppressed expression of the Wnt signaling inhibitor, Dkk, and induced expression of the Wnt ligand, Wnt4a. Genetic and pharmacological inhibition of Wnt signaling suppressed injury-dependent proliferation of MG-derived progenitors. Remarkably, in the uninjured retina, glycogen synthase kinase-3β (GSK-3β) inhibition was sufficient to stimulate MG dedifferentiation and the formation of multipotent retinal progenitors that were capable of differentiating into all major retinal cell types. Importantly, Ascl1a expression was found to contribute to the multipotential character of these progenitors. Our data suggest that Wnt signaling and GSK-3β inhibition, in particular, are crucial for successful retina regeneration.

The rod photoreceptor lineage of teleost fish

The retinas of postembryonic teleost fish continue to grow for the lifetime of the fish. New retinal cells are added continuously at the retinal margin, by stem cells residing at the circumferential germinal zone. Some of these retinal cells differentiate as Müller glia with cell bodies that reside within the inner nuclear layer. These glia retain some stem cell properties in that they carry out asymmetric cell divisions and continuously generate a population of transit-amplifying cells--the rod photoreceptor lineage--that are committed to rod photoreceptor neurogenesis. These rod progenitors progress through a stereotyped sequence of changes in gene expression as they continue to divide and migrate to the outer nuclear layer. Now referred to as rod precursors, they undergo terminal mitoses and then differentiate as rods, which are inserted into the existing array of rod and cone photoreceptors. The rod lineage displays developmental plasticity, as rod precursors can respond to the loss of rods through increased proliferation, resulting in rod replacement. The stem cells of the rod lineage, Müller glia, respond to acute damage of other retinal cell types by increasing their rate of proliferation. In addition, the Müller glia in an acutely damaged retina dedifferentiate and become multipotent, generating new, functional neurons. This review focuses on the cells of the rod lineage and includes discussions of experiments over the last 30 years that led to their identification and characterization, and the discovery of the stem cells residing at the apex of the lineage. The plasticity of cells of the rod lineage, their relationships to cone progenitors, and the applications of this information for developing future treatments for human retinal disorders will also be discussed.

Retinal proliferation response in the buphthalmic zebrafish, bugeye

The zebrafish retina regenerates in response to acute retinal lesions, replacing damaged neurons with new neurons. In this study we test the hypothesis that chronic stress to inner retinal neurons also triggers a retinal regeneration response in the bugeye zebrafish. Mutations in the lrp2 gene in zebrafish are associated with a progressive eye phenotype (bugeye) that models several risk factors for human glaucoma including buphthalmos (enlarged eyes), elevated intraocular pressure (IOP), and upregulation of genes related to retinal ganglion cell pathology. The retinas of adult bugeye zebrafish showed high rates of ongoing proliferation which resulted in the production of a small number of new retinal neurons, particularly photoreceptors. A marker of mechanical cell stress, Hsp27, was strongly expressed in inner retinal neurons and glia of bugeye retinas. The more enlarged eyes of individual bugeye zebrafish showed disrupted retinal lamination, and a persistent reduced density of neurons in the ganglion cell layer (GCL), although total numbers of GCL neurons were higher than in control eyes. Despite the presence of a proliferative response to damage, the adult bugeye zebrafish remained behaviorally blind. These findings suggest the existence of an unsuccessful regenerative response to a persistent pathological condition in the bugeye zebrafish.

The loss of vacuolar protein sorting 11 (vps11) causes retinal pathogenesis in a vertebrate model of syndromic albinism

PURPOSE

To establish the zebrafish platinum mutant as a model for studying vision defects caused by syndromic albinism diseases such as Chediak-Higashi syndrome, Griscelli syndrome, and Hermansky-Pudlak syndrome (HPS).

METHODS

Bulked segregant analysis and candidate gene sequencing revealed that the zebrafish platinum mutation is a single-nucleotide insertion in the vps11 (vacuolar protein sorting 11) gene. Expression of vps11 was determined by RT-PCR and in situ hybridization. Mutants were analyzed for pigmentation defects and retinal disease by histology, immunohistochemistry, and transmission electron microscopy.

RESULTS

Phenocopy and rescue experiments determined that a loss of Vps11 results in the platinum phenotype. Expression of vps11 appeared ubiquitous during zebrafish development, with stronger expression in the developing retina and retinal pigmented epithelium (RPE). Zebrafish platinum mutants exhibited reduced pigmentation in the body and RPE; however, melanophore development, migration, and dispersion occurred normally. RPE, photoreceptors, and inner retinal neurons formed normally in zebrafish platinum mutants. However, a gradual loss of RPE, an absence of mature melanosomes, and the subsequent degradation of RPE/photoreceptor interdigitation was observed.

CONCLUSIONS

These data show that Vps11 is not necessary for normal retinal development or initiation of melanin biosynthesis, but is essential for melanosome maturation and healthy maintenance of the RPE and photoreceptors.

Microarray analysis of XOPS-mCFP zebrafish retina identifies genes associated with rod photoreceptor degeneration and regeneration

PURPOSE

XOPS-mCFP transgenic zebrafish experience a continual cycle of rod photoreceptor development and degeneration throughout life, making them a useful model for investigating the molecular determinants of rod photoreceptor regeneration. The purpose of this study was to compare the gene expression profiles of wild-type and XOPS-mCFP retinas and identify genes that may contribute to the regeneration of the rods.

METHODS

Adult wild-type and XOPS-mCFP retinal mRNA was subjected to microarray analysis. Pathway analysis was used to identify biologically relevant processes that were significantly represented in the dataset. Expression changes were verified by RT-PCR. Selected genes were further examined during retinal development and in adult retinas by in situ hybridization and immunohistochemistry and in a transgenic fluorescent reporter line.

RESULTS

More than 600 genes displayed significant expression changes in XOPS-mCFP retinas compared with expression in wild-type controls. Many of the downregulated genes were associated with phototransduction, whereas upregulated genes were associated with several biological functions, including cell cycle, DNA replication and repair, and cell development and death. RT-PCR analysis of a subset of these genes confirmed the microarray

RESULTS

Three transcription factors (sox11b, insm1a, and c-myb), displaying increased expression in XOPS-mCFP retinas, were also expressed throughout retinal development and in the persistently neurogenic ciliary marginal zone.

CONCLUSIONS

This study identified numerous gene expression changes in response to rod degeneration in zebrafish and further suggests a role for the transcriptional regulators sox11b, insm1a, and c-myb in both retinal development and rod photoreceptor regeneration.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Investigating the genetics of visual processing, function and behaviour in zebrafish

Over the past three decades, the zebrafish has been proven to be an excellent model to investigate the genetic control of vertebrate embryonic development, and it is now also increasingly used to study behaviour and adult physiology. Moreover, mutagenesis approaches have resulted in large collections of mutants with phenotypes that resemble human pathologies, suggesting that these lines can be used to model diseases and screen drug candidates. With the recent development of new methods for gene targeting and manipulating or monitoring gene expression, the range of genetic modifications now possible in zebrafish is increasing rapidly. Combined with the classical strengths of the zebrafish as a model organism, these advances are set to substantially expand the type of biological questions that can be addressed in this species. In this review, we outline how the potential of the zebrafish can be harvested in the context of eye development and visual function. We review recent technological advances used to study the formation of the eyes and visual areas of the brain, visual processing on the cellular, subcellular and molecular level, and the genetics of visual behaviour in vertebrates.

Conditional gene expression and lineage tracing of tuba1a expressing cells during zebrafish development and retina regeneration

The tuba1a gene encodes a neural-specific α-tubulin isoform whose expression is restricted to the developing and regenerating nervous system. By using zebrafish as a model system for studying CNS regeneration, we recently showed that retinal injury induces tuba1a gene expression in Müller glia that reentered the cell cycle. However, because of the transient nature of tuba1a gene expression during development and regeneration, it was not possible to trace the lineage of the tuba1a-expressing cells with a reporter directly under the control of the tuba1a promoter. To overcome this limitation, we generated tuba1a:CreER(T2) and β-actin2:loxP-mCherrry-loxP-GFP double transgenic fish that allowed us to label tuba1a-expressing cells conditionally and permanently via ligand-induced recombination. During development, recombination revealed transient tuba1a expression in not only neural progenitors but also cells that contribute to skeletal muscle, heart, and intestine. In the adult, recombination revealed tuba1a expression in brain, olfactory neurons, and sensory cells of the lateral line, but not in the retina. After retinal injury, recombination showed tuba1a expression in Müller glia that had reentered the cell cycle, and lineage tracing indicated that these cells are responsible for regenerating retinal neurons and glia. These results suggest that tuba1a-expressing progenitors contribute to multiple cell lineages during development and that tuba1a-expressing Müller glia are retinal progenitors in the adult.

Toward a better understanding of human eye disease insights from the zebrafish, Danio rerio

Visual impairment and blindness is widespread across the human population, and the development of therapies for ocular pathologies is of high priority. The zebrafish represents a valuable model organism for studying human ocular disease; it is utilized in eye research to understand underlying developmental processes, to identify potential causative genes for human disorders, and to develop therapies. Zebrafish eyes are similar in morphology, physiology, gene expression, and function to human eyes. Furthermore, zebrafish are highly amenable to laboratory research. This review outlines the use of zebrafish as a model for human ocular diseases such as colobomas, glaucoma, cataracts, photoreceptor degeneration, as well as dystrophies of the cornea and retinal pigmented epithelium.

Investigating regeneration and functional integration of CNS neurons: lessons from zebrafish genetics and other fish species

Zebrafish possess a robust, innate CNS regenerative ability. Combined with their genetic tractability and vertebrate CNS architecture, this ability makes zebrafish an attractive model to gain requisite knowledge for clinical CNS regeneration. In treatment of neurological disorders, one can envisage replacing lost neurons through stem cell therapy or through activation of latent stem cells in the CNS. Here we review the evidence that radial glia are a major source of CNS stem cells in zebrafish and thus activation of radial glia is an attractive therapeutic target. We discuss the regenerative potential and the molecular mechanisms thereof, in the zebrafish spinal cord, retina, optic nerve and higher brain centres. We evaluate various cell ablation paradigms developed to induce regeneration, with particular emphasis on the need for (high throughput) indicators that neuronal regeneration has restored sensory or motor function. We also examine the potential confound that regeneration imposes as the community develops zebrafish models of neurodegeneration. We conclude that zebrafish combine several characters that make them a potent resource for testing hypotheses and discovering therapeutic targets in functional CNS regeneration. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Ascl1a regulates Müller glia dedifferentiation and retinal regeneration through a Lin-28-dependent, let-7 microRNA signalling pathway

Unlike mammals, teleost fish mount a robust regenerative response to retinal injury that culminates in restoration of visual function. This regenerative response relies on dedifferentiation of Müller glia into a cycling population of progenitor cells. However, the mechanism underlying this dedifferentiation is unknown. Here, we report that genes encoding pluripotency factors are induced following retinal injury. Interestingly, the proneural transcription factor, Ascl1a, and the pluripotency factor, Lin-28, are induced in Müller glia within 6 h following retinal injury and are necessary for Müller glia dedifferentiation. We demonstrate that Ascl1a is necessary for lin-28 expression and that Lin-28 suppresses let-7 microRNA (miRNA) expression. Furthermore, we demonstrate that let-7 represses expression of regeneration-associated genes such as, ascl1a, hspd1, lin-28, oct4, pax6b and c-myc. hspd1, oct4 and c-myc(a) exhibit basal expression in the uninjured retina and let-7 may inhibit this expression to prevent premature Müller glia dedifferentiation. The opposing actions of Lin-28 and let-7 miRNAs on Müller glia differentiation and dedifferentiation are similar to that of embryonic stem cells and suggest novel targets for stimulating Müller glia dedifferentiation and retinal regeneration in mammals.

The inhibitor of phagocytosis, O-phospho-L-serine, suppresses Müller glia proliferation and cone cell regeneration in the light-damaged zebrafish retina

The damaged zebrafish retina replaces lost neurons through a regenerative response that initiates with the asymmetric cell division of Müller glia to produce neuronal progenitor cells that proliferate and migrate to the damaged retinal layer, where they differentiate into the lost neuronal cell types. Because Müller glia are known to phagocytose apoptotic retinal cells during development, we tested if Müller glia engulfed apoptotic rod cell bodies in light-damaged retinas. After 24h of constant intense light, damaged retinas revealed both a strong nuclear TUNEL signal in photoreceptors and a weak cytoplasmic TUNEL signal in Müller glia, although Müller glial apoptosis is not observed in the light-damaged retina. Light damage of a rod-specific transgenic reporter line, Tg(XlRho:EGFP)(fl1), resulted in some Müller glia containing both TUNEL signal and EGFP, which indicated that this subset of Müller glia engulfed apoptotic photoreceptor cell bodies. To determine if phagocytosis induced the Müller glial proliferative response in the light-damaged retina, we utilized O-phospho-l-serine (L-SOP), a molecule that mimics the phosphatidylserine head group and partially blocks microglial phagocytosis of apoptotic cells. Intravitreal injection of L-SOP immediately prior to beginning constant intense light treatment: i) did not significantly reduce light-induced photoreceptor cell death, ii) significantly reduced the number of PCNA-positive Müller glia, and iii) significantly reduced the number of cone photoreceptors in the regenerated retina relative to control retinas. Because L-SOP is also a specific group III metabotropic glutamate receptor (mGluR) agonist, we also tested if the more potent specific group III agonist, L-2-amino-4-phosphonobutyrate (L-AP4), the specific group III antagonist (RS)-α-Methylserine-O-phosphate (MSOP) or the specific group I antagonist, L-2-amino-3-phophonopropanoic acid (L-AP3) affected Müller glial proliferation. We found no changes with any of these factors compared to control retinas, revealing that metabotropic glutamate receptors were not necessary in the Müller glia proliferative response. Furthermore, ascl1a and stat3 expression were unaffected in either the L-SOP or MSOP-injected retinas relative to controls, suggesting L-SOP disrupts Müller glia proliferation subsequent to or in parallel with ascl1a and stat3 activation. This implies that at least one signaling mechanism, in addition to the process disrupted by L-SOP, is required to activate Müller glia proliferation in the light-damaged retina.

A novel model of retinal ablation demonstrates that the extent of rod cell death regulates the origin of the regenerated zebrafish rod photoreceptors

The adult zebrafish retina continuously produces rod photoreceptors from infrequent Müller glial cell division, yielding neuronal progenitor cells that migrate to the outer nuclear layer and become rod precursor cells that are committed to differentiate into rods. Retinal damage models suggested that rod cell death induces regeneration from rod precursor cells, whereas loss of any other retinal neurons activates Müller glia proliferation to produce pluripotent neuronal progenitors that can generate any other neuronal cell type in the retina. We tested this hypothesis by creating two transgenic lines that expressed the E. coli nitroreductase enzyme fused to EGFP (NTR-EGFP) in only rods. Treating transgenic adults with metronidazole resulted in two rod cell death models. First, killing all rods throughout the Tg(zop:nfsB-EGFP)(nt19) retina induced robust Müller glial proliferation, which yielded clusters of neuronal progenitor cells. In contrast, ablating only a subset of rods across the Tg(zop:nfsB-EGFP)(nt20) retina led to rod precursor, but not Müller glial, cell proliferation. We propose that two different criteria determine whether rod cell death will induce a regenerative response from the Müller glia rather than from the resident rod precursor cells in the ONL. First, there must be a large amount of rod cell death to initiate Müller glia proliferation. Second, the rod cell death must be acute, rather than chronic, to stimulate regeneration from the Müller glia. This suggests that the zebrafish retina possesses mechanisms to quantify the amount and timing of rod cell death.

Regenerative medicine for retinal diseases: activating endogenous repair mechanisms

The retina is subject to degenerative diseases that often lead to significant visual impairment. Non-mammalian vertebrates have the remarkable ability to replace neurons lost through damage. Fish, and to a limited extent birds, replace lost neurons by the dedifferentiation of Müller glia to a progenitor state followed by the replication of these neuronal progenitor cells. Over the past five years, studies have investigated whether regeneration can be stimulated in the mouse and rat retina. Several groups have reported that at least some types of neurons can be regenerated in the mammalian retina in vivo or in vitro, and that the regeneration of neurons can be stimulated using growth factors, transcription factors or subtoxic levels of excitatory amino acids. These recent results suggest that some part of the regenerative program that occurs in non-mammalian vertebrates remains in the mammalian retina, and could provide a basis to develop new strategies for retinal repair in patients with retinal degenerations.

Pax6a and Pax6b are required at different points in neuronal progenitor cell proliferation during zebrafish photoreceptor regeneration

The light-damaged zebrafish retina results in the death of photoreceptor cells and the subsequent regeneration of the missing rod and cone cells. Photoreceptor regeneration initiates with asymmetric Müller glial cell division to produce neuronal progenitor cells, which amplify, migrate to the outer nuclear layer (ONL), and differentiate into both classes of photoreceptor cells. In this study, we examined the role of the Pax6 protein in regeneration. In zebrafish, there are two Pax6 proteins, one encoded by the pax6a gene and the other encoded by the pax6b gene. We intravitreally injected and electroporated morpholinos that were complementary to either the pax6a or pax6b mRNA to knockdown the translation of the corresponding protein. Loss of Pax6b expression did not affect Müller glial cell division, but blocked the subsequent first cell division of the neuronal progenitors. In contrast, the paralogous Pax6a protein was required for later neuronal progenitor cell divisions, which maximized the number of neuronal progenitors. Without neuronal progenitor cell amplification, proliferation of resident ONL rod precursor cells, which can only regenerate rods, increased inversely proportional to the number of INL neuronal progenitor cells. This confirmed that Müller glial-derived neuronal progenitor cells are necessary to regenerate cones and that distinct mechanisms selectively regenerate rod and cone photoreceptors. This work also defines distinct roles for Pax6a and Pax6b in regulating neuronal progenitor cell proliferation in the adult zebrafish retina and increases our understanding of the molecular pathways required for photoreceptor cell regeneration.

The zebrafish galectin Drgal1-l2 is expressed by proliferating Müller glia and photoreceptor progenitors and regulates the regeneration of rod photoreceptors

PURPOSE

The purpose of this study was to identify secreted proteins in the retina of the adult zebrafish that are induced by the selective death of photoreceptors and to test experimentally the function of these proteins during the regeneration of photoreceptors.

METHODS

Induced selective death of photoreceptors in the retina of the adult zebrafish was combined with in situ hybridization and immunocytochemistry to identify the induced cellular expression of the secreted beta-galactoside binding protein Galectin 1-like 2 (Drgal1-L2). Electroporation of morpholino oligonucleotides was used to knock down protein synthesis, and regenerated photoreceptors were counted in control and experimental retinas after labeling with cell type-specific RNA probes.

RESULTS

Expression analysis and immunocytochemistry showed that Drgal1-L2 is induced de novo by photoreceptor death and is synthesized by microglia and proliferating Müller glia and their mitotic progeny. Knockdown of Drgal1-L2 expression in Müller glia results in reduced regeneration of rod photoreceptors without affecting injury-induced proliferation or the regeneration of cone photoreceptors.

CONCLUSIONS

Based on these data, the authors conclude that Drgal1-L2 is induced by photoreceptor cell death and secreted by stem cells and neuronal progenitors and that it regulates the regeneration of rod photoreceptors. Drgal1-L2 is the first secreted factor shown to regulate aspects of regenerative neurogenesis in the teleost retina.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Time course Analysis of Gene expression patterns in ZebrafIsh Eye during Optic Nerve Regeneration

It is well-established that neurons in the adult mammalian central nervous system (CNS) are terminally differentiated and, if injured, will be unable to regenerate their connections. In contrast to mammals, zebrafish and other teleosts display a robust neuroregenerative response. Following optic nerve crush (ONX), retinal ganglion cells (RGC) regrow their axons to synapse with topographically correct targets in the optic tectum, such that vision is restored in ~21 days. What accounts for these differences between teleostean and mammalian responses to neural injury is not fully understood. A time course analysis of global gene expression patterns in the zebrafish eye after ONX can help to elucidate cellular and molecular mechanisms that contribute to a successful neuroregeneration. To define different phases of regeneration after ONX, alpha tubulin 1 ( tuba1) and growth-associated protein 43 ( gap43), markers previously shown to correspond to morphophological events, were measured by real time quantitative PCR (qPCR). Microarray analysis was then performed at defined intervals (6 hours, 1, 4, 12, and 21 days) post-ONX and compared to SHAM. Results show that optic nerve damage induces multiple, phase-related transcriptional programs, with the maximum number of genes changed and highest fold-change occurring at 4 days. Several functional groups affected by optic nerve regeneration, including cell adhesion, apoptosis, cell cycle, energy metabolism, ion channel activity, and calcium signaling, were identified. Utilizing the whole eye allowed us to identify signaling contributions from the vitreous, immune and glial cells as well as the neural cells of the retina. Comparisons between our dataset and transcriptional profiles from other models of regeneration in zebrafish retina, heart and fin revealed a subset of commonly regulated transcripts, indicating shared mechanisms in different regenerating tissues. Knowledge of gene expression patterns in all components of the eye in a model of successful regeneration provides an entry point for functional analyses, and will help in devising hypotheses for testing normal and toxic regulatory factors.

Neural regeneration and cell replacement: a view from the eye

Neuronal degenerations in the retina are leading causes of blindness. Like most other areas of the CNS, the neurons of the mammalian retina are not replaced following degeneration. However, in nonmammalian vertebrates, endogenous repair processes restore neurons very efficiently, even after complete loss of the retina. We describe the phenomenon of retinal regeneration in nonmammalian vertebrates and attempts made in recent years to stimulate similar regenerative processes in the mammalian retina. In addition, we review the various strategies employed to replace lost neurons in the retina and the recent use of stem cell technologies to address problems of retinal repair.

Genetic evidence for shared mechanisms of epimorphic regeneration in zebrafish

In a microarray-based gene profiling analysis of Müller glia-derived retinal stem cells in light-damaged retinas from adult zebrafish, we found that 2 genes required for regeneration of fin and heart tissues in zebrafish, hspd1 (heat shock 60-kDa protein 1) and mps1 (monopolar spindle 1), were up-regulated. Expression of both genes in the neurogenic Müller glia and progenitors was independently verified by quantitative reverse transcriptase PCR and in situ hybridization. Functional analysis of temperature-sensitive mutants of hspd1 and mps1 revealed that both are necessary for Müller glia-based cone photoreceptor regeneration in adult zebrafish retina. In the amputated fin, hspd1 is required for the induction of mesenchymal stem cells and blastema formation, whereas mps1 is required at a later step for rapid cell proliferation and outgrowth. This temporal sequence of hspd1 and mps1 function is conserved in the regenerating retina. Comparison of gene expression profiles from regenerating zebrafish retina, caudal fin, and heart muscle revealed additional candidate genes potentially implicated in injury-induced epimorphic regeneration in diverse zebrafish tissues.

Cellular expression of midkine-a and midkine-b during retinal development and photoreceptor regeneration in zebrafish

In the retina of adult teleosts, stem cells are sustained in two specialized niches: the ciliary marginal zone (CMZ) and the microenvironment surrounding adult Müller glia. Recently, Müller glia were identified as the regenerative stem cells in the teleost retina. Secreted signaling molecules that regulate neuronal regeneration in the retina are largely unknown. In a microarray screen to discover such factors, we identified midkine-b (mdkb). Midkine is a highly conserved heparin-binding growth factor with numerous biological functions. The zebrafish genome encodes two distinct midkine genes: mdka and mdkb. Here we describe the cellular expression of mdka and mdkb during retinal development and the initial, proliferative phase of photoreceptor regeneration. The results show that in the embryonic and larval retina mdka and mdkb are expressed in stem cells, retinal progenitors, and neurons in distinct patterns that suggest different functions for the two molecules. Following the selective death of photoreceptors in the adult, mdka and mdkb are coexpressed in horizontal cells and proliferating Müller glia and their neurogenic progeny. These data reveal that Mdka and Mdkb are signaling factors present in the retinal stem cell niches in both embryonic and mature retinas, and that their cellular expression is actively modulated during retinal development and regeneration.

A zebrafish retinal graded photochemical stress model

INTRODUCTION

In order to develop a model for investigating the genes that contribute to retinal degeneration, we examined the early graded photochemical stress response in the adult zebrafish (Danio rerio) retina and investigated the role of an NMDA inhibitor, thiokynurenate.

METHODS

Following intravitreal injection of rose bengal (6 or 12 mg/mL), light (37x10(3) or 83x10(3) lx) was directed onto the central retina with and without 400 nM thiokynurenate. Histologic and electron microscopic analysis was performed at 2 and 4 h and gene expression analysis was carried out at 2, 4 and 6 h.

RESULTS

Light and electron microscopy demonstrated a graded photochemical response in photoreceptor, nuclear, and ganglion cell layer thickness. Increased vacuolation of the inner plexiform layer was also observed. The inhibitor produced a distinct lesion pattern. Cellular stress genes were elevated in low and high lesions, while some homeobox gene expression was reduced with thiokynurenate.

DISCUSSION

The phenotypic and genetic changes observed from this model can serve as a basis for understanding the pathology of retinal oxidative and cellular stress. These changes may aid our understanding of aging and macular degeneration.

CNTF induces photoreceptor neuroprotection and Müller glial cell proliferation through two different signaling pathways in the adult zebrafish retina

Ciliary neurotrophic factor (CNTF) acts in several processes in the vertebrate retina, including neuroprotection of photoreceptors in the stressed adult retina and regulation of neuronal progenitor cell proliferation during retinal development. However, the signaling pathway it utilizes (Jak/Stat, MAPK, or Akt) in these processes is ambiguous. Because dark-adapted albino zebrafish exhibit light-induced rod and cone cell death and subsequently regenerate the lost photoreceptor cells, zebrafish should be a useful model to study the role of CNTF in both neuroprotection and neuronal progenitor cell proliferation. We therefore investigated the potential roles of CNTF in both the undamaged and light-damaged adult zebrafish retinas. Intraocular injection of CNTF suppressed light-induced photoreceptor cell death, which then failed to exhibit the regeneration response that is marked by proliferating Müller glia and neuronal progenitor cells. Inhibiting the MAPK signaling pathway, but neither the Stat3 nor Akt pathways, significantly reduced the CNTF-mediated neuroprotection of light-induced photoreceptor cell death. Intraocular injection of CNTF into non-light-treated (undamaged) eyes mimicked constant intense light treatment by increasing Stat3 expression in Müller glia followed by increasing the number of proliferating Müller glia and neuronal progenitors. Knockdown of Stat3 expression in the CNTF-injected non-light-treated retinas significantly reduced the number of proliferating Müller glia, while coinjection of CNTF with either MAPK or Akt inhibitors did not inhibit the CNTF-induced Müller glia proliferation. Thus, CNTF utilizes a MAPK-dependant signaling pathway in neuroprotection of light-induced photoreceptor cell death and a Stat3-dependant signaling pathway to stimulate Müller glia proliferation.

Stimulation of neural regeneration in the mouse retina

Müller glia can serve as a source of new neurons after retinal damage in both fish and birds. Investigations of regeneration in the mammalian retina in vitro have provided some evidence that Müller glia can proliferate after retinal damage and generate new rods; however, the evidence that this occurs in vivo is not conclusive. We have investigated whether Müller glia have the potential to generate neurons in the mouse retina in vivo by eliminating ganglion and amacrine cells with intraocular NMDA injections and stimulating Müller glial to re-enter the mitotic cycle by treatment with specific growth factors. The proliferating Müller glia dedifferentiate and a subset of these cells differentiated into amacrine cells, as defined by the expression of amacrine cell-specific markers Calretinin, NeuN, Prox1, and GAD67-GFP. These results show for the first time that the mammalian retina has the potential to regenerate inner retinal neurons in vivo.

Identification of the molecular signatures integral to regenerating photoreceptors in the retina of the zebra fish

Investigating neuronal and photoreceptor regeneration in the retina of zebra fish has begun to yield insights into both the cellular and molecular means by which this lower vertebrate is able to repair its central nervous system. However, knowledge about the signaling molecules in the local microenvironment of a retinal injury and the transcriptional events they activate during neuronal death and regeneration is still lacking. To identify genes involved in photoreceptor regeneration, we combined light-induced photoreceptor lesions, laser-capture microdissection of the outer nuclear layer (ONL) and analysis of gene expression to characterize transcriptional changes for cells in the ONL as photoreceptors die and are regenerated. Using this approach, we were able to characterize aspects of the molecular signature of injured and dying photoreceptors, cone photoreceptor progenitors, and microglia within the ONL. We validated changes in gene expression and characterized the cellular expression for three novel, extracellular signaling molecules that we hypothesize are involved in regulating regenerative events in the retina.

Characterization of Müller glia and neuronal progenitors during adult zebrafish retinal regeneration

The adult zebrafish retina exhibits a robust regenerative response following light-induced photoreceptor cell death. This response is initiated by the Müller glia proliferating in the inner nuclear layer (INL), which gives rise to neuronal progenitor cells that continue to divide and migrate to the outer nuclear layer (ONL), where they differentiate into rod and cone photoreceptors. We previously conducted a microarray analysis of retinal gene expression at 16, 31, 51, 68, and 96 h of constant intense-light treatment to identify genes and their corresponding proteins that may be involved in the generation and proliferation of the neuronal progenitor cells. We examined the expression of two candidate transcription factors, Pax6 and Ngn1, and one candidate transgene, olig2:EGFP, in the regenerating light-damaged retina. We compared the temporal and spatial expression patterns of these markers relative to PCNA (proliferating cell nuclear antigen), an established marker for proliferating cells in the zebrafish retina, and the Tg(gfap:EGFP) nt11 transgenic line that specifically labels Müller glial cells. We found that Müller glial cells dedifferentiate during regeneration, based on the loss of cell-specific markers such as GFAP (glial fibrillary acidic protein) and glutamine synthetase following their reentry into the cell cycle to produce neuronal progenitors. Pax6 expression was first detected in the proliferating neuronal progenitors by 51 h of constant light treatment, which is significantly after the Müller glia first reenter the cell cycle after 31h of light. This suggests that Pax6 expression increases in neuronal progenitors, rather than in the proliferating Müller glia. EGFP expression from the olig2 promoter was first detected by 68 h of constant light treatment in the dedifferentiated Müller glia, with Pax6 expressed in the closely associated proliferating neuronal progenitors migrating to the ONL. Both Pax6 and olig2 expression persisted until 3 days post-light treatment, when the neuronal progenitors begin differentiating into new rod and cone photoreceptors. Ngn1 protein expression was initially detected in proliferating neuronal progenitors at 68 h of light treatment. However, Ngn1 expression persisted in a subset of the INL nuclei until 17 days post-light treatment. Using the Tg(gfap:EGFP) nt11 transgenic line, Ngn1 was localized to the Müller glial nuclei that were reestablished following the regenerative response. These markers, therefore, can be used to identify different cell types at particular stages of retinal regeneration: neuronal progenitor formation, proliferation, and the reestablishment of the Müller glia cells. These markers will be important to further characterize the regeneration response in other retinal damage models and to elucidate the defects associated with mutants and morphants that disrupt the regeneration response.

Genetic dissection reveals two separate pathways for rod and cone regeneration in the teleost retina

Development of therapies to treat visual system dystrophies resulting from the degeneration of rod and cone photoreceptors may directly benefit from studies of animal models, such as the zebrafish, that display continuous retinal neurogenesis and the capacity for injury-induced regeneration. Previous studies of retinal regeneration in fish have been conducted on adult animals and have relied on methods that cause acute damage to both rods and cones, as well as other retinal cell types. We report here the use of a genetic approach to study progenitor cell responses to photoreceptor degeneration in the larval and adult zebrafish retina. We have compared the responses to selective rod or cone degeneration using, respectively, the XOPS-mCFP transgenic line and zebrafish with a null mutation in the pde6c gene. Notably, rod degeneration induces increased proliferation of progenitors in the outer nuclear layer (ONL) and is not associated with proliferation or reactive gliosis in the inner nuclear layer (INL). Molecular characterization of the rod progenitor cells demonstrated that they are committed to the rod photoreceptor fate while they are still mitotic. In contrast, cone degeneration induces both Müller cell proliferation and reactive gliosis, with little change in proliferation in the ONL. We found that in both lines, proliferative responses to photoreceptor degeneration can be observed as 7 days post fertilization (dpf). These two genetic models therefore offer new opportunities for investigating the molecular mechanisms of selective degeneration and regeneration of rods and cones.

Inhibition of Müller glial cell division blocks regeneration of the light-damaged zebrafish retina

The adult zebrafish retina possesses a robust regenerative response. In the light-damaged retina, Müller glial cell divisions precede regeneration of rod and cone photoreceptors. Neuronal progenitors, which arise from the Müller glia, continue to divide and use the Müller glial cell processes to migrate to the outer nuclear layer and replace the lost photoreceptors. We tested the necessity of Müller glial cell division for photoreceptor regeneration. As knockdown tools were unavailable for use in the adult zebrafish retina, we developed a method to conditionally inhibit the expression of specific proteins by in vivo electroporation of morpholinos. We determined that two separate morpholinos targeted against the proliferating cell nuclear antigen (PCNA) mRNA reduced PCNA protein levels. Furthermore, injection and in vivo electroporation of PCNA morpholinos immediately prior to starting intense light exposure inhibited both Müller glial cell proliferation and neuronal progenitor marker Pax6 expression. PCNA knockdown additionally resulted in decreased expression of glutamine synthetase in Müller glia and Müller glial cell death, while amacrine and ganglion cells were unaffected. Finally, histological and immunological methods showed that long-term effects of PCNA knockdown resulted in decreased numbers of Müller glia and the failure to regenerate rod photoreceptors, short single cones, and long single cones. These data suggest that Müller glial cell division is necessary for proper photoreceptor regeneration in the light-damaged zebrafish retina and are consistent with the Müller glia serving as the source of neuronal progenitor cells in regenerating teleost retinas.

Age-related cone abnormalities in zebrafish with genetic lesions in sonic hedgehog

PURPOSE

Sonic hedgehog (Shh) signaling is essential for photoreceptor differentiation and retinal cell survival in embryonic zebrafish. The study was conducted to determine whether adult heterozygous carriers of mutant alleles for the shh gene display retinal abnormalities.

METHODS

Retinal cryosections from young, middle-aged, and senescent wild-type and sonic-you(+/-) (syu(+/-)) zebrafish were probed with retinal cell type-specific markers. Contralateral retinal flatmounts from these fish, and from adult albino zebrafish subjected to light-induced photoreceptor damage followed by regeneration, were hybridized with blue cone opsin cRNA for quantitative analysis of the blue cone pattern. Retinal expression of shh mRNA was measured by quantitative RT-PCR.

RESULTS

Regions of cone loss and abnormal cone morphology were observed in the oldest syu(+/-) zebrafish, although no other retinal cell type was affected. This phenotype was age-related and genotype-specific. Cone distribution in the oldest syu(+/-) zebrafish was predominantly random, as assessed by measuring the short-range pattern, whereas that of wild-type fish and the younger syu(+/-) zebrafish was statistically regular. A measure of long-range pattern revealed atypical cone aggregation in the oldest syu(+/-) zebrafish. The light-treated albino zebrafish displayed random cone patterns immediately after light toxicity, but showed cone aggregation on regeneration. Retinas from the syu(+/-) fish showed reduced expression of shh mRNA compared with those of wild-type siblings.

CONCLUSIONS

The syu(+/-) zebrafish presents a model for the study of hereditary age-related cone abnormalities. The syu(+/-) retinas most likely experience progressive cone photoreceptor loss, accompanied by cone regeneration. Shh signaling may be required to maintain cone viability throughout life.

Ganglion cell regeneration following whole-retina destruction in zebrafish

The retinas of adult teleost fish can regenerate neurons following injury. The current study provides the first documentation of functional whole retina regeneration in the zebrafish, Danio rerio, following intraocular injection of the cytotoxin, ouabain. Loss and replacement of laminated retinal tissue was monitored by analysis of cell death and cell proliferation, and by analysis of retina-specific gene expression patterns. The spatiotemporal process of retinal ganglion cell (RGC) regeneration was followed through the use of selective markers, and was found to largely recapitulate the spatiotemporal process of embryonic ganglion cell neurogenesis, over a more protracted time frame. However, the re-expression of some ganglion cell markers was not observed. The growth and pathfinding of ganglion cell axons was evaluated by measurement of the optic nerve head (ONH), and the restoration of normal ONH size was found to correspond to the time of recovery of two visually-mediated behaviors. However, some abnormalities were noted, including overproduction of RGCs, and progressive and excessive growth of the ONH at longer recovery times. This model system for whole-retina regeneration has provided an informative view of the regenerative process.

Lengsin expression and function during zebrafish lens formation

A zebrafish ortholog of human lengsin was identified by EST analysis of an adult lens cDNA library. During zebrafish development, lengsin transcription is first detected at 24 h post-fertilization (hpf). Immunolocalization, using polyclonal antiserum generated against a Lengsin bacterial fusion protein, detects lens-specific protein in whole-mount embryos at 30 hpf. Lengsin expression in zebrafish follows the temporal expression of the alphaA- alphaB1- and betaB1-crystallin proteins in the lens. At 72 hpf, Lengsin is localized to a subpopulation of differentiating secondary fiber cells, while no expression is detected in the lens epithelial cells or central lens fibers. In the adult lens, Lengsin is restricted to a narrow band of cortical fibers and co-localizes with actin at the lateral faces of these interdigitating cells. Stable transgenic lines, using a 3 kb lengsin genomic fragment to regulate EGFP expression, recapitulate the Lengsin temporal and spatial expression patterns. Lengsin function in zebrafish lens formation was examined by antisense morpholino-mediated translation and mRNA splice inhibition. At 72 hpf, the lengsin morphant lenses are reduced in size and exhibit separations within the cortex due to defects in secondary fiber morphogenesis. The location of the morphant lens defects correlates with the Lengsin protein localization at this age. These results demonstrate Lengsin is required for proper fiber cell differentiation by playing roles in either cell elongation or the establishment of cell interactions.

The proneural basic helix-loop-helix gene ascl1a is required for retina regeneration

Unlike mammals, teleost fish can regenerate an injured retina, restoring lost visual function. Little is known of the molecular events that underlie retina regeneration. We previously found that in zebrafish, retinal injury stimulates Müller glia to generate multipotent alpha1-tubulin (alpha1T) and pax6-expressing progenitors for retinal repair. Here, we report the identification of a critical E-box in the alpha1T promoter that mediates transactivation by achaete-scute complex-like 1a (ascl1a) during retina regeneration. More importantly, we show that ascl1a is essential for retina regeneration. Within 4 h after retinal injury, ascl1a is induced in Müller glia. Knockdown of ascl1a blocks the induction of alpha1T and pax6 as well as Müller glial proliferation, consequently preventing the generation of retinal progenitors and their differentiated progeny. These data suggest ascl1a is required to convert quiescent Müller glia into actively dividing retinal progenitors, and that ascl1a is a key regulator in initiating retina regeneration.

A web based resource characterizing the zebrafish developmental profile of over 16,000 transcripts

Using a spotted 65-mer oligonucleotide microarray, we have characterized the developmental expression profile from mid-gastrulation (75% epiboly) to 5 days post-fertilization (dpf) for >16,000 unique transcripts in the zebrafish genome. Microarray profiling data sets are often immense, and one challenge is validating the results and prioritizing genes for further study. The purpose of the current study was to address such issues, as well as to generate a publicly available resource for investigators to examine the developmental expression profile of any of the over 16,000 zebrafish genes on the array. On the chips, there are 16,459 printed spots corresponding to 16,288 unique transcripts and 172 beta-actin (AF025305) spots spatially distributed throughout the chip as a positive control. We have collected 55 microarray gene expression profiling results from various zebrafish laboratories and created a Perl/CGI-based software tool (http://serine.umdnj.edu/approximately ouyangmi/cgi-bin/zebrafish/profile.htm) for researchers to look for the expression patterns of their gene of interest. Users can search for their genes of interest by entering the accession numbers or the nucleotide sequences and the expression profiling will be reported in the form of expression intensities versus time-course graphical displays. In order to validate this web tool, we compared 74 genes' expression results between our web tool and the in situ hybridization results from Thisse et al. [Thisse, B., Heyer, V., Lux, A., Alunni, A., Degrave, A., Seiliez, I., Kirchner, J., Parkhill, J.-P., Thisse, C., 2004. Spatial and temporal expression of the zebrafish genome by large-scale in situ hybridization screening. Meth. Cell. Biol. 77, 505-519] as well as those reported by Mathavan et al. [Mathavan, S., Lee, S.G., mark, A., Miller, L.D., Murthy, K.R., Tong, Y., Wu, Y.L., Lam, S.H., Yang, H., Ruan, Y., Korzh, V., Gong, Z., Liu, E.T., Lufkin, T., 2005. Transcriptome analysis of zebrafish embryogenesis using microarrays. PLoS Genet. 1, 260-276]. The comparison indicates that our microarray-derived expression patterns are 80% and 75% in agreement with the in situ database (Thisse et al., 2004) and previously published microarray data (Mathavan et al., 2005), respectively. Those genes that conflict between our web tool and the in situ database either have high sequence similarity with other genes or the in situ probes are not reliable. Among those genes that disagree between our web tool and those reported by Mathavan et al. (2005), 93% of the genes are in agreement between our web tool and the in situ database, indicating our web tool results are quite reliable. Thus, this resource provides a user-friendly web based platform for researchers to determine the developmental profile of their gene of interest and to prioritize genes identified in microarray analyses by their developmental expression profile.

Generation and characterization of transgenic zebrafish lines using different ubiquitous promoters

Two commonly used promoters to ubiquitously express transgenes in zebrafish are the Xenopus laevis elongation factor 1 alpha promoter (XlEef1a1) and the zebrafish histone variant H2A.F/Z (h2afv) promoter. Recently, transgenes utilizing these promoters were shown to be silenced in certain adult tissues, particularly the central nervous system. To overcome this limitation, we cloned the promoters of four zebrafish genes that likely are transcribed ubiquitously throughout development and into the adult. These four genes are the TATA box binding protein gene, the taube nuss-like gene, the eukaryotic elongation factor 1-gamma gene, and the beta-actin-1 gene. We PCR amplified approximately 2.5 kb upstream of the putative translational start site of each gene and cloned each into a Tol2 expression vector that contains the EGFP reporter transgene. We used these four Tol2 vectors to independently generate stable transgenic fish lines for analysis of transgene expression during development and in the adult. We demonstrated that all four promoters drive a very broad pattern of EGFP expression throughout development and the adult. Using the retina as a well-characterized component of the CNS, all four promoters appeared to drive EGFP expression in all neuronal and non-neuronal cells of the adult retina. In contrast, the h2afv promoter failed to express EGFP in the adult retina. When we examined EGFP expression in the various cells of the blood cell lineage, we observed that all four promoters exhibited a more heterogenous expression pattern than either the XlEef1a1 or h2afv promoters. While these four ubiquitous promoters did not express EGFP in all the adult blood cells, they did express EGFP throughout the CNS and in broader expression patterns in the adult than either the XlEef1a1 or h2afv promoters. For these reasons, these four promoters will be valuable tools for expressing transgenes in adult zebrafish.