An element in the alpha1-tubulin promoter is necessary for retinal expression during optic nerve regeneration but not after eye injury in the adult zebrafish

Photoreceptor loss does not recruit neutrophils despite strong microglial activation

In response to central nervous system (CNS) injury, tissue resident immune cells such as microglia and circulating systemic neutrophils are often first responders. The degree to which these cells interact in response to CNS damage is poorly understood, and even less so, in the neural retina which poses a challenge for high resolution imaging in vivo. In this study, we deploy fluorescence adaptive optics scanning light ophthalmoscopy (AOSLO) to study fluorescent microglia and neutrophils in mice. We simultaneously track immune cell dynamics using label-free phase-contrast AOSLO at micron-level resolution. Retinal lesions were induced with 488 nm light focused onto photoreceptor (PR) outer segments. These lesions focally ablated PRs, with minimal collateral damage to cells above and below the plane of focus. We used in vivo (AOSLO, SLO and OCT) imaging to reveal the natural history of the microglial and neutrophil response from minutes-to-months after injury. While microglia showed dynamic and progressive immune response with cells migrating into the injury locus within 1-day after injury, neutrophils were not recruited despite close proximity to vessels carrying neutrophils only microns away. Post-mortem confocal microscopy confirmed in vivo findings. This work illustrates that microglial activation does not recruit neutrophils in response to acute, focal loss of photoreceptors, a condition encountered in many retinal diseases.

4-Methylbenzylidene camphor induced neurobehavioral toxicity in zebrafish (Danio rerio) embryos

4-Methylbenzylidene camphor (4-MBC) is a widely used organic UV filter in personal care products. Extensive use of 4-MBC and its frequent detection in aquatic ecosystems defile the biota with muscular and neuronal impairments. This study investigates the neurobehavioral toxicity of 4-MBC using Danio rerio as a model organism. Embryos were exposed semi-statically to 4-MBC at 5, 50, and 500 μg/L concentrations for 10-day post fertilization (dpf). Embryos exhibited a significant thigmotaxis and decreased startle touch response with altered expression of nervous system mRNA transcripts on 5 & 10 dpf. Compared to the sham-exposed group, 4-MBC treated larvae exhibited changes in the expression of shha, ngn1, mbp, elavl3, α1-tubulin, syn2a, and gap43 genes. Since ngn1 induction is mediated by shh signaling during sensory neuron specification, the elevated protein expression of NGN1 indicates 4-MBC interference in the sonic hedgehog signaling pathway. This leads to sensory neuron impairment and function such as 'sense' as evident from reduced touch response. In addition, larval brain histology with a reduced number of cells in the Purkinje layer emblazing the defunct motor coordination. Predictive toxicity study also showed a higher affinity of 4-MBC to modeled Shh protein. Thus, the findings of the present work highlighted that 4-MBC is potential to induce developmental neurotoxicity at both behavioral and molecular functional perspectives, and developing D. rerio larvae could be considered as a suitable alternate animal model to assess the neurological dysfunction of organic UV filters.

Microglial depletion after brain injury prolongs inflammation and impairs brain repair, adult neurogenesis and pro-regenerative signaling

The adult zebrafish brain, unlike mammals, has a remarkable regenerative capacity. Although inflammation in part hinders regeneration in mammals, it is necessary for zebrafish brain repair. Microglia are resident brain immune cells that regulate the inflammatory response. To explore the microglial role in repair, we used liposomal clodronate or colony stimulating factor-1 receptor (csf1r) inhibitor to suppress microglia after brain injury, and also examined regeneration in two genetic mutant lines that lack microglia. We found that microglial ablation impaired telencephalic regeneration after injury. Microglial suppression attenuated cell proliferation at the intermediate progenitor cell amplification stage of neurogenesis. Notably, the loss of microglia impaired phospho-Stat3 (signal transducer and activator of transcription 3) and ß-Catenin signaling after injury. Furthermore, the ectopic activation of Stat3 and ß-Catenin rescued neurogenesis defects caused by microglial loss. Microglial suppression also prolonged the post-injury inflammatory phase characterized by neutrophil accumulation, likely hindering the resolution of inflammation. These findings reveal specific roles of microglia and inflammatory signaling during zebrafish telencephalic regeneration that should advance strategies to improve mammalian brain repair.

The regeneration-responsive element careg monitors activation of Müller glia after MNU-induced damage of photoreceptors in the zebrafish retina

In contrast to mammals, zebrafish can regenerate their damaged photoreceptors. This capacity depends on the intrinsic plasticity of Müller glia (MG). Here, we identified that the transgenic reporter careg, a marker of regenerating fin and heart, also participates in retina restoration in zebrafish. After methylnitrosourea (MNU) treatment, the retina became deteriorated and contained damaged cell types including rods, UV-sensitive cones and the outer plexiform layer. This phenotype was associated with the induction of careg expression in a subset of MG until the reconstruction of the photoreceptor synaptic layer. Single-cell RNA sequencing (scRNAseq) analysis of regenerating retinas revealed a population of immature rods, defined by high expression of rhodopsin and the ciliogenesis gene meig1, but low expression of phototransduction genes. Furthermore, cones displayed deregulation of metabolic and visual perception genes in response to retina injury. Comparison between careg:EGFP expressing and non-expressing MG demonstrated that these two subpopulations are characterized by distinct molecular signatures, suggesting their heterogenous responsiveness to the regenerative program. Dynamics of ribosomal protein S6 phosphorylation showed that TOR signaling became progressively switched from MG to progenitors. Inhibition of TOR with rapamycin reduced the cell cycle activity, but neither affected careg:EGFP expression in MG, nor prevented restoration of the retina structure. This indicates that MG reprogramming, and progenitor cell proliferation might be regulated by distinct mechanisms. In conclusion, the careg reporter detects activated MG, and provides a common marker of regeneration-competent cells in diverse zebrafish organs, including the retina.

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.

Regulation of Tubulin Gene Expression: From Isotype Identity to Functional Specialization

Genomes of higher eukaryotes encode a large tubulin gene superfamily consisting of at least six α and six β-tubulin isotypes. While some α and β-tubulin isotypes are ubiquitously expressed, others are cell-type specific. The subset of α and β-tubulins that is expressed in a given cell type is defined transcriptionally. But the precise mechanisms of how cells choose which α and β isotypes to express and at what level remain poorly understood. Differential expression of tubulin isotypes is particularly prominent during development and in specialized cells, suggesting that some isotypes are better suited for certain cell type-specific functions. Recent studies begin to rationalize this phenomenon, uncovering important differences in tubulin isotype behavior and their impact on the biomechanical properties of the microtubule cytoskeleton. I summarize our understanding of the regulation of tubulin isotype expression, focusing on the role of these complex regulatory pathways in building a customized microtubule network best suited for cellular needs.

Visual Function is Gradually Restored During Retina Regeneration in Adult Zebrafish

In comparison to mammals, zebrafish are able to regenerate many organs and tissues, including the central nervous system (CNS). Within the CNS-derived neural retina, light lesions result in a loss of photoreceptors and the subsequent activation of Müller glia, the retinal stem cells. Müller glia-derived progenitors differentiate and eventually restore the anatomical tissue architecture within 4 weeks. However, little is known about how light lesions impair vision functionally, as well as how and to what extent visual function is restored during the course of regeneration, in particular in adult animals. Here, we applied quantitative behavioral assays to assess restoration of visual function during homeostasis and regeneration in adult zebrafish. We developed a novel vision-dependent social preference test, and show that vision is massively impaired early after lesion, but is restored to pre-lesion levels within 7 days after lesion. Furthermore, we employed a quantitative optokinetic response assay with different degrees of difficulty, similar to vision tests in humans. We found that vision for easy conditions with high contrast and low level of detail, as well as color vision, was restored around 7–10 days post lesion. Vision under more demanding conditions, with low contrast and high level of detail, was regained only later from 14 days post lesion onwards. Taken together, we conclude that vision based on contrast sensitivity, spatial resolution and the perception of colors is restored after light lesion in adult zebrafish in a gradual manner.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Faim knockout leads to gliosis and late-onset neurodegeneration of photoreceptors in the mouse retina

Fas Apoptotic Inhibitory Molecule protein (FAIM) is a death receptor antagonist and an apoptosis regulator. It encodes two isoforms, namely FAIM-S (short) and FAIM-L (long), both with significant neuronal functions. FAIM-S, which is ubiquitously expressed, is involved in neurite outgrowth. In contrast, FAIM-L is expressed only in neurons and it protects them from cell death. Interestingly, FAIM-L is downregulated in patients and mouse models of Alzheimer's disease before the onset of neurodegeneration, and Faim transcript levels are decreased in mouse models of retinal degeneration. However, few studies have addressed the role of FAIM in the central nervous system, yet alone the retina. The retina is a highly specialized tissue, and its degeneration has proved to precede pathological mechanisms of neurodegenerative diseases. Here we describe that Faim depletion in mice damages the retina persistently and leads to late-onset photoreceptor death in older mice. Immunohistochemical analyses showed that Faim knockout (Faim^-/- ) mice present ubiquitinated aggregates throughout the retina from early ages. Moreover, retinal cells released stress signals that can signal to Müller cells, as shown by immunofluorescence and qRT-PCR. Müller cells monitor retinal homeostasis and trigger a gliotic response in Faim^-/- mice that becomes pathogenic when sustained. In this regard, we observed pronounced vascular leakage at later ages, which may be caused by persistent inflammation. These results suggest that FAIM is an important player in the maintenance of retinal homeostasis, and they support the premise that FAIM is a plausible early marker for late photoreceptor and neuronal degeneration.

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

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

Distribution of planar cell polarity proteins in the developing avian retina

Planar cell polarity (PCP) is evolutionary conserved and play a critical role in proper tissue development and function. During central nervous system development, PCP proteins exhibit specific patterns of distribution and are indispensable for axonal growth, dendritogenesis, neuronal migration, and neuronal differentiation. The retina constitutes an excellent model in which to study molecular mechanisms involved in neural development. The analysis of the spatiotemporal expression of PCP proteins in this model constitutes an useful histological approach in order to identify possible roles of these proteins in retinogenesis. Immunohistochemical techniques revealed that Frz6, Celsr1, Vangl1, Pk1, Pk3, and Fat1 were present in emerging axons from recently differentiated ganglion cells in the chicken retina. Except for Vangl1, they were also asymmetrically distributed in differentiated amacrine cells. Pk1 and Pk3 were restricted in the outer nuclear layer to the outer segment of photoreceptors. Vangl1 was also located in the cell somata of Müller glia. Given these findings together, the distribution of PCP proteins in the developing chicken retina suggest essential roles in axonal guidance during early retinogenesis and a possible involvement in the establishment of cell asymmetry and maintenance of retinal cell phenotypes.

Müller Glia-Mediated Retinal Regeneration

Müller glia originate from neuroepithelium and are the principal glial cells in the retina. During retinal development, Müller glia are one of the last cell types to be born. In lower vertebrates, such as zebrafish, Müller glia possess a remarkable capacity for retinal regeneration following various forms of injury through a reprogramming process in which endogenous Müller glia proliferate and differentiate into all types of retinal cells. In mammals, Müller glia become reactive in response to damage to protect or to further impair retinal function. Although mammalian Müller glia have regenerative potential, it is limited as far as repairing damaged retina. Lessons learned from zebrafish will help reveal the critical mechanisms involved in Müller glia reprogramming. Progress has been made in triggering Müller glia to reprogram and generate functional neurons to restore vision in mammals indicating that Müller glia reprogramming may be a promising therapeutic strategy for human retinal diseases. This review comprehensively summarizes the mechanisms related to retinal regeneration in model animals and the critical advanced progress made in Müller glia reprogramming in mammals.

Developmental neurotoxicity of low concentrations of bisphenol A and S exposure in zebrafish

The vulnerability to environmental insults is heightened at early stages of development. However, the neurotoxic potential of bisphenol A (BPA) and bisphenol S (BPS) at developmental windows remains unclear. To investigate the mechanisms mediating the developmental neurotoxicity, zebrafish embryos were treated with 0.01, 0.03, 0.01, 0.3, 1 μM BPA/BPS. Also, we used Tg(HuC:GFP) zebrafish to investigate whether BPA/BPS could induce neuron development. The reduction in body length, and increased heart rate were significant in 0.3 and 1 μM BPA/BPS groups. The green fluorescence protein (GFP) intensity increased at 72 hpf and 120 hpf in Tg(HuC:GFP) larvae which was consistent with the increased mRNA expression of elval3 following BPS treatments, an indication of the plausible effect of BPS on embryonic neuron development. Additionally, BPA/BPS treatments elicited hyperactivity and reduced static time in zebrafish larvae, suggesting behavioral alterations. Moreover, qRT-PCR results showed that BPA and BPS could interfere with the normal expression of development-related genes vegfa, wnt8a, and mstn1 at the developmental stages. The expression of neurodevelopment-related genes (ngn1, elavl3, gfap, α1-tubulin, mbp, and gap43) were significantly upregulated in BPA and BPS treatments, except for the remarkable downregulation of mbp and gfap elicited by BPA at 48 (0.03 μM) and 120 hpf (0.3 μM) respectively; ngn1 at 48 hpf for 0.1 μM BPS. Overall, our results highlighted that embryonic exposure to low concentrations of BPA/BPS could be deleterious to the central nervous system development and elicit behavioral abnormalities in zebrafish at developmental stages.

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.

Cellular reprogramming for successful CNS axon regeneration is driven by a temporally changing cast of transcription factors

In contrast to mammals, adult fish display a remarkable ability to fully regenerate central nervous system (CNS) axons, enabling functional recovery from CNS injury. Both fish and mammals normally undergo a developmental downregulation of axon growth activity as neurons mature. Fish are able to undergo damage-induced “reprogramming” through re-expression of genes necessary for axon growth and guidance, however, the gene regulatory mechanisms remain unknown. Here we present the first comprehensive analysis of gene regulatory reprogramming in zebrafish retinal ganglion cells at specific time points along the axon regeneration continuum from early growth to target re-innervation. Our analyses reveal a regeneration program characterized by sequential activation of stage-specific pathways, regulated by a temporally changing cast of transcription factors that bind to stably accessible DNA regulatory regions. Strikingly, we also find a discrete set of regulatory regions that change in accessibility, consistent with higher-order changes in chromatin organization that mark (1) the beginning of regenerative axon growth in the optic nerve, and (2) the re-establishment of synaptic connections in the brain. Together, these data provide valuable insight into the regulatory logic driving successful vertebrate CNS axon regeneration, revealing key gene regulatory candidates for therapeutic development.

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

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

Cell lineage tracing in the retina: Could material transfer distort conclusions?

Recent studies reported the transfer of fluorescent labels between grafted and host cells after transplantation of photoreceptor precursor cells in the mouse retina. While clearly impacting the interpretation of transplantation studies in the retina, the potential impact of material transfer in other experimental paradigms using cell-specific labels remains uncertain. Here, we briefly review the evidence supporting material transfer in transplantation studies and discuss whether it might influence retinal cell lineage tracing experiments in developmental and regeneration studies. We also propose ways to control for the possible confounding occurrence of label exchange in such experiments. Developmental Dynamics 247:10-17, 2018. © 2017 Wiley Periodicals, Inc.

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.

Sleep-Dependent Structural Synaptic Plasticity of Inhibitory Synapses in the Dendrites of Hypocretin/Orexin Neurons

Sleep is tightly regulated by the circadian clock and homeostatic mechanisms. Although the sleep/wake cycle is known to be associated with structural and physiological synaptic changes that benefit the brain, the function of sleep is still debated. The hypothalamic hypocretin/orexin (Hcrt) neurons regulate various functions including feeding, reward, sleep, and wake. Continuous imaging of single neuronal circuits in live animals is vital to understanding the role of sleep in regulating synaptic dynamics, and the transparency of the zebrafish model enables time-lapse imaging of single synapses during both day and night. Here, we use the gephyrin (Gphnb) protein, a central inhibitory synapse organizer, as a fluorescent post-synaptic marker of inhibitory synapses. Double labeling showed that Gphnb-tagRFP and collybistin-EGFP clusters co-localized in dendritic inhibitory synapses. Using a transgenic hcrt:Gphnb-EGFP zebrafish, we showed that the number of inhibitory synapses in the dendrites of Hcrt neurons was increased during development. To determine the effect of sleep on the inhibitory synapses, we performed two-photon live imaging of Gphnb-EGFP in Hcrt neurons during day and night, under light/dark and constant light and dark conditions, and following sleep deprivation (SD). We found that synapse number increased during the night under light/dark conditions but that these changes were eliminated under constant light or dark conditions. SD reduced synapse number during the night, and the number increased during post-deprivation daytime sleep rebound. These results suggest that rhythmic structural plasticity of inhibitory synapses in Hcrt dendrites is independent of the circadian clock and is modulated by consolidated wake and sleep.

MASH1/Ascl1a leads to GAP43 expression and axon regeneration in the adult CNS

Unlike CNS neurons in adult mammals, neurons in fish and embryonic mammals can regenerate their axons after injury. These divergent regenerative responses are in part mediated by the growth-associated expression of select transcription factors. The basic helix-loop-helix (bHLH) transcription factor, MASH1/Ascl1a, is transiently expressed during the development of many neuronal subtypes and regulates the expression of genes that mediate cell fate determination and differentiation. In the adult zebrafish (Danio rerio), Ascl1a is also transiently expressed in retinal ganglion cells (RGCs) that regenerate axons after optic nerve crush. Utilizing transgenic zebrafish with a 3.6 kb GAP43 promoter that drives expression of an enhanced green fluorescent protein (EGFP), we observed that knock-down of Ascl1a expression reduces both regenerative gap43 gene expression and axonal growth after injury compared to controls. In mammals, the development of noradrenergic brainstem neurons requires MASH1 expression. In contrast to zebrafish RGCs, however, MASH1 is not expressed in the mammalian brainstem after spinal cord injury (SCI). Therefore, we utilized adeno-associated viral (AAV) vectors to overexpress MASH1 in four month old rat (Rattus norvegicus) brainstem neurons in an attempt to promote axon regeneration after SCI. We discovered that after complete transection of the thoracic spinal cord and implantation of a Schwann cell bridge, animals that express MASH1 exhibit increased noradrenergic axon regeneration and improvement in hindlimb joint movements compared to controls. Together these data demonstrate that MASH1/Ascl1a is a fundamental regulator of axonal growth across vertebrates and can induce modifications to the intrinsic state of neurons to promote functional regeneration in response to CNS injury.

Different mechanisms regulate expression of zebrafish myelin protein zero (P0) in myelinating oligodendrocytes and its induction following axonal injury

Zebrafish CNS axons regenerate robustly following injury; it is thought that CNS oligodendrocytes contribute to this response by expressing growth-promoting molecules. We characterized the mpz gene, which encodes myelin protein zero and is up-regulated in oligodendroglia following axonal injury. The 2.5-kb mpz mRNA is expressed from a single TATA box promoter. Four independent Tg(mpz:egfp) transgenic zebrafish lines, in which GFP was expressed under the mpz promoter and 10 kb of genomic 5'-flanking sequence, showed transgene expression in CNS oligodendrocytes from larval development through adulthood. Following optic nerve crush injury, the mpz:egfp transgene was strongly up-regulated in oligodendrocytes along the regenerating retinotectal projection, mirroring up-regulation of endogenous mpz mRNA. GFP-expressing oligodendroglia were significantly more abundant in the regenerating optic pathway, resulting from both transgene induction in oligodendroglial precursors and the birth of new cells. Up-regulation of the mpz:egfp transgene was not dependent on axonal regeneration, suggesting that the primary signal may be axonal loss, debris, or microglial infiltration. Deletion experiments indicated that an oligodendroglial enhancer located in the region from -6 to -10 kb with respect to the mpz transcriptional start site is dissociable from the cis-regulatory element mediating the mpz transcriptional response to axonal injury, which is located between -1 and -4 kb. These data show that different mechanisms regulate expression of zebrafish mpz in myelinating oligodendrocytes and its induction following axonal injury. The underlying molecular events could potentially be exploited to enhance axonal repair following mammalian CNS injury. The transgenic lines and cis-regulatory constructs reported here will facilitate identification of the relevant signaling pathways.

Müller glial cell reprogramming and retina regeneration

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

Reprint of: the ciliary marginal zone (CMZ) in development and regeneration of the vertebrate eye

The ciliary marginal zone (CMZ) is a circumferential ring of cells found at the extreme periphery of the maturing and mature neural retina that consists of retinal stem and progenitor cells. It functions to add retinal neurons to the periphery of the neural retina in larval and adult fish, larval frogs, and birds. Additionally, the CMZ may contribute to regeneration of the damaged retina in frogs and fish. In mammals, cells from the ciliary epithelium can be induced to express retinal stem cell-like characteristics in culture but may not comprise a classically defined CMZ.

gdf6a is required for cone photoreceptor subtype differentiation and for the actions of tbx2b in determining rod versus cone photoreceptor fate

Functional vision restoration is within reach via stem cell therapy, but one of the largest obstacles is the derivation of colour-sensitive cone photoreceptors that are required for high-acuity daytime vision. To enhance progress made using nocturnal murine models, we instead utilize cone-rich zebrafish and herein investigate relationships between gdf6a and tbx2b in cone photoreceptor development. Growth/differentiation factor 6a (gdf6a), a bone morphogenetic protein family ligand, is an emerging factor in photoreceptor degenerative diseases. The T-box transcription factor tbx2b is required to specify UV cone photoreceptor fate instead of rod photoreceptor fate. Interactions between these factors in cone development would be unanticipated, considering the discrete phenotypes in their respective mutants. However, gdf6a positively modulates the abundance of tbx2b transcript during early eye morphogenesis, and we extended this conclusion to later stages of retinal development comprising the times when photoreceptors differentiate. Despite this, gdf6a-/- larvae possess a normal relative number of UV cones and instead present with a low abundance of blue cone photoreceptors, approximately half that of siblings (p<0.001), supporting a differential role for gdf6a amongst the spectral subtypes of cone photoreceptors. Further, gdf6a-/- larvae from breeding of compound heterozygous gdf6a+/-;tbx2b+/- mutants exhibit the recessive lots-of-rods phenotype (which also shows a paucity of UV cones) at significantly elevated rates (44% or 48% for each of two tbx2b alleles, χ2 p≤0.007 for each compared to expected Mendelian 25%). Thus the gdf6a-/- background sensitizes fish such that the recessive lots-of-rods phenotype can appear in heterozygous tbx2b+/- fish. Overall, this work establishes a novel link between tbx2b and gdf6a in determining photoreceptor fates, defining the nexus of an intricate pathway influencing the abundance of cone spectral subtypes and specifying rod vs. cone photoreceptors. Understanding this interaction is a necessary step in the refinement of stem cell-based restoration of daytime vision in humans.

Jak/Stat signaling stimulates zebrafish optic nerve regeneration and overcomes the inhibitory actions of Socs3 and Sfpq

The regenerative failure of mammalian optic axons is partly mediated by Socs3-dependent inhibition of Jak/Stat signaling (Smith et al., 2009, 2011). Whether Jak/Stat signaling is part of the normal regenerative response observed in animals that exhibit an intrinsic capacity for optic nerve regeneration, such as zebrafish, remains unknown. Nor is it known whether the repression of regenerative inhibitors, such as Socs3, contributes to the robust regenerative response of zebrafish to optic nerve damage. Here we report that Jak/Stat signaling stimulates optic nerve regeneration in zebrafish. We found that IL-6 family cytokines, acting via Gp130-coupled receptors, stimulate Jak/Stat3 signaling in retinal ganglion cells after optic nerve injury. Among these cytokines, we found that CNTF, IL-11, and Clcf1/Crlf1a can stimulate optic axon regrowth. Surprisingly, optic nerve injury stimulated the expression of Socs3 and Sfpq (splicing factor, proline/glutamine rich) that attenuate optic nerve regeneration. These proteins were induced in a Jak/Stat-dependent manner, stimulated each other's expression and suppressed the expression of regeneration-associated genes. In vivo, the injury-dependent induction of Socs3 and Sfpq inhibits optic nerve regeneration but does not block it. We identified a robust induction of multiple cytokine genes in zebrafish retinal ganglion cells that may contribute to their ability to overcome these inhibitory factors. These studies not only identified mechanisms underlying optic nerve regeneration in fish but also suggest new molecular targets for enhancing optic nerve regeneration in mammals.

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

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

Insm1a-mediated gene repression is essential for the formation and differentiation of Müller glia-derived progenitors in the injured retina

In zebrafish, retinal injury stimulates Müller glia (MG) reprograming; allowing them to generate multipotent progenitors that regenerate damaged cells and restore vision. Recent studies suggest transcriptional repression may underlie these events. To identify these repressors, we compared the transcriptomes of MG and MG-derived progenitors and identified insm1a, a transcriptional repressor exhibiting a biphasic pattern of expression that is essential for retina regeneration. Insm1a was found to suppress ascl1a and its own expression and link injury-dependent ascl1a induction with dickkopf (dkk) suppression, which is necessary for MG dedifferentiation. We also found that Insm1a was responsible for sculpting the zone of injury-responsive MG by suppressing hb-egf_a expression. Finally, we provide evidence that Insm1a stimulates progenitor cell cycle exit by suppressing a genetic program driving progenitor proliferation. Our studies identify Insm1a as a key regulator of retina regeneration and provide a mechanistic understanding of how it contributes to multiple phases of this process.

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.

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.

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.

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.

Genetic dissection of axon regeneration

Axon regeneration has long been studied in vertebrate model organisms and neuronal cultures. Recent development of axon regeneration paradigms in genetic model organisms, such as Caenorhabditis elegans, Drosophila and zebrafish, has opened an exciting field for in vivo functional dissection of regeneration pathways. Studies in these organisms have discovered essential genes and pathways for axon regrowth. The conservation of these genes crossing animal phyla suggests mechanistic relevance to higher organisms. The power of genetic approaches in these organisms makes large-scale genetic and pharmacological screens feasible and can greatly accelerate the mechanistic understanding of axon regeneration.

Transcriptional regulatory regions of gap43 needed in developing and regenerating retinal ganglion cells

Mammals and fish differ in their ability to express axon growth-associated genes in response to CNS injury, which contributes to the differences in their ability for CNS regeneration. Previously we demonstrated that for the axon growth-associated gene, gap43, regions of the rat promoter that are sufficient to promote reporter gene expression in the developing zebrafish nervous system are not sufficient to promote expression in regenerating retinal ganglion cells in zebrafish. Recently, we identified a 3.6-kb gap43 promoter fragment from the pufferfish, Takifugu rubripes (fugu), that can promote reporter gene expression during both development and regeneration. Using promoter deletion analysis, we have found regions of the 3.6-kb fugu gap43 promoter that are necessary for expression in regenerating, but not developing, retinal ganglion cells. Within the 3.6-kb promoter, we have identified elements that are highly conserved among fish, as well as elements conserved among fish, mammals, and birds.

Tuba1a gene expression is regulated by KLF6/7 and is necessary for CNS development and regeneration in zebrafish

We report that knockdown of the alpha1 tubulin isoform Tuba1a, but not the highly related Tuba1b, dramatically impedes nervous system formation during development and RGC axon regeneration following optic nerve injury in adults. Within the tuba1a promoter, a G/C-rich element was identified that is necessary for tuba1a induction during RGC differentiation and optic axon regeneration. KLF6a and 7a, which we previously reported are essential for optic axon regeneration (Veldman et al., 2007), bind this G/C-rich element and transactivate the tuba1a promoter. In vivo knockdown of KLF6a and 7a attenuate regeneration-dependent activation of the endogenous tuba1a and p27 genes. These results suggest tuba1a expression is necessary for CNS development and regeneration and that KLF6a and 7a mediate their effects, at least in part, via transcriptional control of tuba1a promoter activity.

Activating transcription factor 3 (ATF3) expression in the neural retina and optic nerve of zebrafish during optic nerve regeneration

Fish, unlike mammals, can regenerate axons in the optic nerve following optic nerve injury. We hypothesized that using microarray analysis to compare gene expression in fish which had experienced optic nerve lesion to fish which had undergone a similar operation but without optic nerve injury would reveal genes specifically involved in responding to optic nerve injury (including repair), reducing detection of genes involved in the general stress and inflammatory responses. We discovered 120 genes were significantly (minimally two-fold with a P-value < or = 0.05) differentially expressed (up or down) at one or more time point. Among these was ATF3, a member of the cAMP-response element binding protein family. Work by others has indicated that elevated cAMP could be important in axon regeneration. We investigated ATF3 expression further by qRT-PCR, in situ hybridization and immunohistochemistry and found ATF3 expression is significantly upregulated in the ganglion cell layer of the retina, the nerve fiber layer and the optic nerve of the injured eye. The upregulation in retina is detectable by qRT-PCR by 24 h after injury, at which time ATF-3 mRNA levels are 8-fold higher than in retinas from sham-operated fish. We conclude ATF3 may be an important mediator of optic nerve regeneration-promoting gene expression in fish, a role which merits further investigation.

Distribution of carnosine-like peptides in the nervous system of developing and adult zebrafish (Danio rerio) and embryonic effects of chronic carnosine exposure

Carnosine-like peptides (carnosine-LP) are a family of histidine derivatives that are present in the nervous system of various species and that exhibit antioxidant, anti-matrix-metalloproteinase, anti-excitotoxic, and free-radical scavenging properties. They are also neuroprotective in animal models of cerebral ischemia. Although the function of carnosine-LP is largely unknown, the hypothesis has been advanced that they play a role in the developing nervous system. Since the zebrafish is an excellent vertebrate model for studying development and disease, we have examined the distribution pattern of carnosine-LP in the adult and developing zebrafish. In the adult, immunoreactivity for carnosine-LP is specifically concentrated in sensory neurons and non-sensory cells of the olfactory epithelium, the olfactory nerve, and the olfactory bulb. Robust staining has also been observed in the retinal outer nuclear layer and the corneal epithelium. Developmental studies have revealed immunostaining for carnosine-LP as early as 18 h, 24 h, and 7 days post-fertilization in, respectively, the olfactory, corneal, and retinal primordia. These data suggest that carnosine-LP are involved in olfactory and visual function. We have also investigated the effects of chronic (7 days) exposure to carnosine on embryonic development and show that 0.01 microM to 10 mM concentrations of carnosine do not elicit significant deleterious effects. Conversely, treatment with 100 mM carnosine results in developmental delay and compromised larval survival. These results indicate that, at lower concentrations, exogenously administered carnosine can be used to explore the role of carnosine in development and developmental disorders of the nervous system.

Expression of a 12-kb promoter element derived from the zebrafish enolase-2 gene in the zebrafish visual system

We recently cloned the zebrafish neuronal enolase-2 gene and showed that a 12-kb eno2 promoter element was sufficient to drive transgene expression widely in CNS neurons in vivo from 48h post-fertilization through adulthood. The aim of the present study was to establish the expression pattern of the 12-kb eno2 promoter element in the zebrafish visual system. Endogenous eno2 mRNA was detected in the developing retina from 2 days post-fertilization (dpf), and by 12dpf was localized to the retinal ganglion cell, inner and outer nuclear layers. Similar to endogenous eno2, GFP expression in the retina of Tg(eno2:GFP) larvae was first evident at 2dpf, and by 12dpf intense GFP expression was seen in the retinal ganglion cell and photoreceptor layers, with weaker expression in the inner nuclear layer. We identified cell types expressing the eno2 promoter element by using two complementary strategies: (i) double label immunofluorescence analysis of Tg(eno2:GFP) zebrafish, and (ii) generation of double transgenic zebrafish expressing red fluorescent protein under transcriptional control of the 12-kb eno2 promoter and GFP under a rod- or cone-specific promoter. The 12-kb eno2 promoter was expressed in retinal ganglion cells, amacrine cells, including a subset that co-expressed tyrosine hydroxylase, and rod photoreceptors. These data suggest that abnormalities of vision should be sought in transgenic models of diseases generated using this promoter. Owing to the specific expression of fluorescent reporters in neuronal subpopulations, Tg(eno2:GFP) and Tg(eno2:mRFP) zebrafish may be useful for studies of retinal lamination, neuronal differentiation and synapse formation in the visual system.

Highly-restricted, cell-specific expression of the simian CMV-IE promoter in transgenic zebrafish with age and after heat shock

Promoters with high levels of ubiquitous expression are of significant utility in the production of transgenic animals and cell lines. One such promoter is derived from the human cytomegalovirus immediate early (CMV-IE) gene. We sought to ascertain if the simian CMV-IE promoter (sCMV), used extensively in non-mammalian vertebrate research, also directs intense, widespread expression when stably introduced into zebrafish. Analysis of sCMV-driven expression revealed a temporal and spatial pattern not predicted by studies using the hCMV promoter in other transgenic animals or by observations of early F0 embryos expressing injected sCMV-reporter plasmids. Unexpectedly, in transgenic fish produced by both integration of linearized plasmid or Tol2-mediated transgenesis, sCMV promoter expression was generally observed in a small population of cells in telencephalon and spinal cord between days 2 and 7, and was thereafter confined to discrete regions of CNS that included the olfactory bulb, retina, cerebellum, spinal cord, and lateral line. In skeletal muscle, intense transgene expression was not observed until well into adulthood (>2-3 months post-fertilization). One final unexpected characteristic of the sCMV promoter in stable transgenic fish was tissue-specific responsiveness of the promoter to heat shock at both embryonic and adult stages. These data suggest that, in the context of stable transgenesis, the simian CMV-IE gene promoter responds differently to intracellular regulatory forces than other characterized CMV promoters.

Transplantation of mouse embryonic stem cells into the cochlea of an auditory-neuropathy animal model: effects of timing after injury

Application of ouabain to the round window membrane of the gerbil selectively induces the death of most spiral ganglion neurons and thus provides an excellent model for investigating the survival and differentiation of embryonic stem cells (ESCs) introduced into the inner ear. In this study, mouse ESCs were pretreated with a neural-induction protocol and transplanted into Rosenthal's canal (RC), perilymph, or endolymph of Mongolian gerbils either 1-3 days (early post-injury transplant group) or 7 days or longer (late post-injury transplant group) after ouabain injury. Overall, ESC survival in RC and perilymphatic spaces was significantly greater in the early post-injury microenvironment as compared to the later post-injury condition. Viable clusters of ESCs within RC and perilymphatic spaces appeared to be associated with neovascularization in the early post-injury group. A small number of ESCs transplanted within RC stained for mature neuronal or glial cell markers. ESCs introduced into perilymph survived in several locations, but most differentiated into glia-like cells. ESCs transplanted into endolymph survived poorly if at all. These experiments demonstrate that there is an optimal time window for engraftment and survival of ESCs that occurs in the early post-injury period.

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.

Growth and pathfinding of regenerating axons in the optic projection of adult fish

In contrast to mammals, teleost fish are able to regrow severed long-range projection axons in the central nervous system (CNS), leading to recovery of function. The optic projection in teleost fish is used to study neuron-intrinsic and environmental molecular factors that determine successful axon regrowth and navigation through a complex CNS pathway back to original targets. Here we review evidence for regeneration-specific regulation and robust expression of growth- and pathfinding-associated genes in regenerating retinal ganglion cell (RGC) axons of adult fish. The environment of the CNS in fish appears to contain few inhibitory molecules and at the same time a number of promoting molecules for axon regrowth. Finally, some environmental cues that are used as guidance cues for developing RGC axons are also present in continuously growing adult animals. These molecules may serve as guidance cues for the precise navigation of axons from newly generated RGCs in adult animals as well as of regenerating RGC axons after a lesion. The application of new molecular techniques especially to adult zebrafish, is likely to produce new insights into successful axonal regeneration in the CNS of fish and the absence thereof in mammals.

Gene expression analysis of zebrafish retinal ganglion cells during optic nerve regeneration identifies KLF6a and KLF7a as important regulators of axon regeneration

Unlike mammals, teleost fish are able to mount an efficient and robust regenerative response following optic nerve injury. Although it is clear that changes in gene expression accompany axonal regeneration, the extent of this genomic response is not known. To identify genes involved in successful nerve regeneration, we analyzed gene expression in zebrafish retinal ganglion cells (RGCs) regenerating their axons following optic nerve injury. Microarray analysis of RNA isolated by laser capture microdissection from uninjured and 3-day post-optic nerve injured RGCs identified 347 up-regulated and 29 down-regulated genes. Quantitative RT-PCR and in situ hybridization were used to verify the change in expression of 19 genes in this set. Gene ontological analysis of the data set suggests regenerating neurons up-regulate genes associated with RGC development. However, not all regeneration-associated genes are expressed in differentiating RGCs indicating the regeneration is not simply a recapitulation of development. Knockdown of six highly induced regeneration-associated genes identified two, KLF6a and KLF7a, that together were necessary for robust RGC axon re-growth. These results implicate KLF6a and KLF7a as important mediators of optic nerve regeneration and suggest that not all induced genes are essential to mount a regenerative response.

Cellular correlates of proneural and notch-delta gene expression in the regenerating zebrafish retina

Fish can regenerate retinal neurons following ocular injury. Evidence is mounting that astrocytic glia function as inducible, regenerative stem cells in this process, but the underlying molecular events that enable neuronal regeneration are comparatively unclear. In the current study gene array, quantitative real-time PCR,in situhybridization, and immunohistochemical approaches were used to identify, in the damaged retina of adult zebrafish, correlations between transcriptional events and entry into the cell cycle by Müller cells, a type of astrocytic cell present in all vertebrate retinas that is a candidate ‘stem cell’ of regenerated neurons. A proneural gene (achaete-scute homolog 1a,ash1a) and neurogenic components of the Notch signaling pathway, includingnotch3anddeltaA, were implicated. An injury-induced, enhanced expression ofash1awas observed in Müller cells, which is hypothesized to contribute to the transition of these cells, or their cellular progeny, into anotch3-expressing, regenerative progenitor. A model of vertebrate retinal repair is suggested in which damage-induced expression of proneural genes, plus canonical Notch-Delta signaling, could contribute to retinal stem cell promotion and subsequent regenerative neurogenesis.