Nok plays an essential role in maintaining the integrity of the outer nuclear layer in the zebrafish retina

Effect of Dync1h1 on Phototransduction Protein Transport and the Development and Maintenance of Photoreceptor Cells in Zebrafish

Purpose

Retinitis pigmentosa (RP) is a hereditary retinal disease characterized by progressive degeneration of photoreceptor cells (PRCs). Identifying potential pathogenic genes and understanding the mechanisms of PRC degeneration are essential for improving diagnosis and treatment. Cytoplasmic dynein 1, responsible for retrograde axonal transport along microtubules, plays critical roles in neuronal function. This study utilized dync1h1-deficient zebrafish to investigate its roles in PRC morphogenesis and degeneration.

Methods

Heterozygous and homozygous dync1h1-deficient zebrafish were confirmed through Sanger sequencing. Morphological changes and retinal phenotypes were assessed through histological analysis. RNA sequencing and bioinformatics were used to explore molecular mechanisms in dync1h1-/- zebrafish, with endoplasmic reticulum (ER) stress and apoptosis pathways validated in vivo.

Results

Dync1h1-/- zebrafish exhibit severe developmental defects, including microphthalmia, disorganized retinal lamination, and PRC apoptosis. In dync1h1+/-, mild but progressive growth retardation and PRC defects were observed. Dync1h1 loss impaired retrograde axonal transport, leading to defects in cilium biogenesis and transport disorder of phototransduction proteins in PRCs. This triggered ER stress, activating BiP-ATF4-CHOP signaling pathway and leading to PRC degeneration.

Conclusions

Dync1h1 is essential for maintaining retrograde axonal transport and proper trafficking of phototransduction proteins in PRCs. Non-resolving ER stress-induced PRC apoptosis is a key factor in DYNC1H1-associated retinal degeneration. This study provides important insights into the precise diagnosis and may help in the development of targeted therapies for retinal degenerative diseases.

Retinal Degeneration Response to Graphene Quantum Dots: Disruption of the Blood-Retina Barrier Modulated by Surface Modification-Dependent DNA Methylation

Graphene quantum dots (GQDs) are used in diverse fields from chemistry-related materials to biomedicines, thus causing their substantial release into the environment. Appropriate visual function is crucial for facilitating the decision-making process within the nervous system. Given the direct interaction of eyes with the environment and even nanoparticles, herein, GQDs, sulfonic acid-doped GQDs (S-GQDs), and amino-functionalized GQDs (A-GQDs) were employed to understand the potential optic neurotoxicity disruption mechanism by GQDs. The negatively charged GQDs and S-GQDs disturbed the response to light stimulation and impaired the structure of the retinal nuclear layer of zebrafish larvae, causing vision disorder and retinal degeneration. Albeit with sublethal concentrations, a considerably reduced expression of the retinal vascular sprouting factor sirt1 through increased DNA methylation damaged the blood-retina barrier. Importantly, the regulatory effect on vision function was influenced by negatively charged GQDs and S-GQDs but not positively charged A-GQDs. Moreover, cluster analysis and computational simulation studies indicated that binding affinities between GQDs and the DNMT1-ligand binding might be the dominant determinant of the vision function response. The previously unknown pathway of blood-retinal barrier interference offers opportunities to investigate the biological consequences of GQD-based nanomaterials, guiding innovation in the industry toward environmental sustainability.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Rapamycin relieves the cataract caused by ablation of Gja8b through stimulating autophagy in zebrafish

Macroautophagy/autophagy is known to be important for intracellular quality control in the lens. GJA8 is a major gap junction protein in vertebrate lenses. Mutations in GJA8 cause cataracts in humans. The well-known cataractogenesis mechanism is that mutated GJA8 leads to abnormal assembly of gap junctions, resulting in defects in intercellular communication among lens cells. In this study, we observed that ablation of Gja8b (a homolog of mammalian GJA8) in zebrafish led to severe defects in organelle degradation, an important cause of cataractogenesis in developing lens. The role of autophagy in organelle degradation in lens remains disputable. Intriguingly, we also observed that ablation of Gja8b induced deficient autophagy in the lens. More importantly, in vivo treatment of zebrafish with rapamycin, an autophagy activator that inhibits MAPK/JNK and MTORC1 signaling, stimulated autophagy in the lens and relieved the defects in organelle degradation, resulting in the mitigation of cataracts in gja8b mutant zebrafish. Conversely, inhibition of autophagy by treatment with the chemical reagent 3-MA blocked these recovery effects, suggesting the important roles of autophagy in organelle degradation in the lens in gja8b mutant zebrafish. Further studies in HLE cells revealed that GJA8 interacted with ATG proteins. Overexpression of GJA8 stimulated autophagy in HLE cells. These data suggest an unrecognized cataractogenesis mechanism caused by ablation of Gja8b and a potential treatment for cataracts by stimulating autophagy in the lens.Abbreviations: 3-MA: 3-methyladenine; ATG: autophagy related; AV: autophagic vacuoles; Dpf: days post fertilization; GJA1: gap junction protein alpha 1; GJA3: gap junction protein alpha 3; GJA8: gap junction protein alpha 8; Hpf: hours post fertilization; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PtdIns3K: class III phosphatidylinositol 3-kinase; WT: wild type.

Zebrafish Crb1, Localizing Uniquely to the Cell Membranes around Cone Photoreceptor Axonemes, Alleviates Light Damage to Photoreceptors and Modulates Cones' Light Responsiveness

The crumbs (crb) apical polarity genes are essential for the development and functions of epithelia. Adult zebrafish retinal neuroepithelium expresses three crb genes (crb1, crb2a, and crb2b); however, it is unknown whether and how Crb1 differs from other Crb proteins in expression, localization, and functions. Here, we show that, unlike zebrafish Crb2a and Crb2b as well as mammalian Crb1 and Crb2, zebrafish Crb1 does not localize to the subapical regions of photoreceptors and Müller glial cells; rather, it localizes to a small region of cone outer segments: the cell membranes surrounding the axonemes. Moreover, zebrafish Crb1 is not required for retinal morphogenesis and photoreceptor patterning. Interestingly, Crb1 promotes rod survival under strong white light irradiation in a previously unreported non--cell-autonomous fashion; in addition, Crb1 delays UV and blue cones' chromatin condensation caused by UV light irradiation. Finally, Crb1 plays a role in cones' responsiveness to light through an arrestin-translocation-independent mechanism. The localization of Crb1 and its functions do not differ between male and female fish. We conclude that zebrafish Crb1 has diverged from other vertebrate Crb proteins, representing a neofunctionalization in Crb biology during evolution.SIGNIFICANCE STATEMENT Apicobasal polarity of epithelia is an important property that underlies the morphogenesis and functions of epithelial tissues. Epithelial apicobasal polarity is controlled by many polarity genes, including the crb genes. In vertebrates, multiple crb genes have been identified, but the differences in their expression patterns and functions are not fully understood. Here, we report a novel subcellular localization of zebrafish Crb1 in retinal cone photoreceptors and evidence for its new functions in photoreceptor maintenance and light responsiveness. This study expands our understanding of the biology of the crb genes in epithelia, including retinal neuroepithelium.

Electron microscopy study of the central retinal fovea in Pied flycatcher: evidence of a mechanism of light energy transmission through the retina

We present unique ultrastructural data on avian retinal cells. Presently and earlier (Zueva et al., 2016) we explored distribution of intermediate filaments (IFs) in retinal cells of the Pied flycatcher (Ficedula hypoleuca, Passeriformes, Aves) in the central foveolar zone. This retinal zone only contains single and double cone photoreceptors. Previously we found that continuous IFs span Müller cells (MC) lengthwise from the retinal inner limiting membrane (ILM) layer up to the outer limiting membrane (OLM) layer. Here we describe long cylindrical bundles of IFs (IFBs) inside the cone inner segments (CIS) adjoining the cone plasma membrane, with these IFBs following along the cone lengthwise, and surrounding the cone at equal spacing one from the other. Double cones form a combined unit, wherein they are separated by their respective plasma membranes. Double cones thus have a common external ring of IFBs, surrounding both cone components. In the layer of cilia, the IFBs that continue into the cone outer segment (COS) follow on to the cone apical tip along the direction of incident light, with single IFs separating from the IFB, touching, and sometimes passing in-between the light-sensitive lamellae of the COS. These new data support our previous hypothesis on the quantum mechanism of light energy propagation through the vertebrate retina (Zueva et al., 2016, 2019).

The extracellular and intracellular regions of Crb2a play distinct roles in guiding the formation of the apical zonula adherens

The transmembrane protein Crumbs (Crb), a key regulator of apical polarity, has a known involvement in establishment of the apical zonula adherens in epithelia, although the precise mechanism remains elusive. The zonula adherens are required to maintain the integrity and orderly arrangement of epithelia. Loss of the zonula adherens leads to morphogenetic defects in the tissues derived from epithelium. In this study, we revealed that the intracellular tail of Crb2a promoted the apical distribution of adherens junctions (AJs) in zebrafish retinal and lens epithelia, but caused assembly into unstable punctum adherens-like adhesion plaques. The extracellular region of Crb2a guided the transformation of AJs from the punctum adherens into stable zonula adherens. Accordingly, a truncated form of Crb2a lacking the extracellular region (Crb2a^ΔEX) could only partially rescue the retinal patterning defects in crb2a null mutant zebrafish (crb2a^m289). By contrast, constitutive over-expression of Crb2a^ΔEX disrupted the integrity of the outer limiting membrane in photoreceptors, which is derived from the zonula adherens of the retinal neuroepithelium. This study demonstrated that both the extracellular region and the intracellular tail of Crb2a are required to guide the formation of the apical zonula adherens.

penner/lgl2 is required for the integrity of the photoreceptor layer in the zebrafish retina

The vertebrate retina is a complex tissue built from multiple neuronal cell types, which develop from a pseudostratified neuroepithelium. These cells are arranged into a highly organized and stereotypic pattern formed by nuclear and plexiform layers. The process of lamination as well as the maturation and differentiation of photoreceptor cells rely on the establishment and maintenance of apico-basal cell polarity and formation of adhesive junctions. Defects in any of these processes can result in impaired vision and are causally related to a variety of human diseases leading to blindness. While the importance of apical polarity regulators in retinal stratification and disease is well established, little is known about the function of basal regulators in retinal development. Here, we analyzed the role of Lgl2, a basolateral polarity factor, in the zebrafish retina. Lgl2 is upregulated in photoreceptor cells and in the retinal pigment epithelium by 72 h post fertilization. In both cell types, Lgl2 is localized basolaterally. Loss of zygotic Lgl2 does not interfere with retinal lamination or photoreceptor cell polarity or maturation. However, knockdown of both maternal and zygotic Lgl2 leads to impaired cell adhesion. As a consequence, severe layering defects occur in the distal retina, manifested by a breakdown of the outer plexiform layer and the outer limiting membrane. These results define zebrafish Lgl2 as an important regulator of retinal lamination, which, given the high degree of evolutionary conservation, may be preserved in other vertebrates, including human.

Summary: Knockdown of penner/lgl2 leads to a breakdown of the outer plexiform layer and the outer limiting membrane in the zebrafish retina due to impaired cell adhesion.

Expression patterns of zebrafish nocturnin genes and the transcriptional activity of the frog nocturnin promoter in zebrafish rod photoreceptors

Purpose

Daily modulation of gene expression is critical for the circadian rhythms of many organisms. One of the modulating mechanisms is based on nocturnin, a deadenylase that degrades mRNA in a circadian fashion. The nocturnin genes are expressed broadly, but their tissue expression patterns differ between mice and the frog Xenopus laevis; this difference suggests that the extent of the regulation of nocturin gene expression varies among species. In this study, we set out to characterize the expression patterns of two zebrafish nocturnin genes; in addition, we asked whether a frog nocturnin promoter has transcriptional activity in zebrafish.

Methods

We used reverse transcription (RT)-PCR, quantitative real-time PCR (qRT-PCR), and rapid amplification of cDNA ends (RACE) analysis to determine whether the nocturnin-a and nocturnin-b genes are expressed in the eye, in situ hybridization to determine the cellular expression pattern of the nocturnin-b gene in the retina, and confocal microscopy to determine the protein expression pattern of the transgenic reporter green fluorescent protein (GFP) driven by the frog nocturnin promoter.

Results

We found that the amino acid sequences of zebrafish nocturnin-a and nocturnin-b are highly similar to those of frog, mouse, and human nocturnin homologs. Only nocturnin-b is expressed in the eye. Within the retina, nocturnin-b mRNA was expressed at higher levels in the retinal photoreceptors layer than in other cellular layers. This expression pattern echoes the restricted photoreceptor expression of nocturnin in the frog. We also found that the frog nocturnin promoter can be specifically activated in zebrafish rod photoreceptors.

Conclusions

The high level of similarities in amino acid sequences of human, mouse, frog, and zebrafish nocturnin homologs suggest these proteins maintain a conserved deadenylation function that is important for regulating retinal circadian rhythmicity. The rod-specific transcriptional activity of the frog nocturnin promoter makes it a useful tool to drive moderate and rod-specific transgenic expression in zebrafish. The results of this study lay the groundwork to study nocturnin-based circadian biology of the zebrafish retina.

Zebrafish methanol exposure causes patterning defects and suppressive cell proliferation in retina

PURPOSE

Methanol exposure have been shown to produce retinal abnormalities and visual dysfunctions in rodents and other mammals developing in utero. In this study, we characterized how methanol affects the retinal development in an ex utero embryonic system, the zebrafish.

METHODS

Zebrafish embryos were raised for 24 hours in fish water supplemented with various concentrations of methanol at 6 hours after fertilisation. The effects of methanol on retinal morphology were assessed by histologic and immunohistochemical analyses.

RESULTS

Zebrafish embryos exposed to moderate (3%) and high (4%) levels of methanol during early embryonic development had a small eye phenotype. Embryos exposed to high (4%) level of methanol had morphological abnormalities of the retinal pigment epithelia and the photoreceptors. Methanol exposure also caused inhibition of cell differentiation and proliferation in the retina at the early developmental stage.

CONCLUSIONS

Low concentrations of methanol affect photoreceptor function but do not disturb retinal morphology. Higher levels of methanol exposure cause retinal patterning defects and a small eye phenotype.

Rainbow Enhancers Regulate Restrictive Transcription in Teleost Green, Red, and Blue Cones

Photoreceptor-specific transcription of individual genes collectively constitutes the transcriptional profile that orchestrates the structural and functional characteristics of each photoreceptor type. It is challenging, however, to study the transcriptional specificity of individual photoreceptor genes because each gene's distinct spatiotemporal transcription patterns are determined by the unique interactions between a specific set of transcription factors and the gene's own cis-regulatory elements (CREs), which remain unknown for most of the genes. For example, it is unknown what CREs underlie the zebrafish mpp5b^ponli (ponli) and crumbs2b (crb2b) apical polarity genes' restrictive transcription in the red, green, and blue (RGB) cones in the retina, but not in other retinal cell types. Here we show that the intronic enhancers of both the ponli and crb2b genes are conserved among teleost species and that they share sequence motifs that are critical for RGB cone-specific transcription. Given their similarities in sequences and functions, we name the ponli and crb2b enhancers collectively rainbow enhancers. Rainbow enhancers may represent a cis-regulatory mechanism to turn on a group of genes that are commonly and restrictively expressed in RGB cones, which largely define the beginning of the color vision pathway.SIGNIFICANCE STATEMENT Dim-light achromatic vision and bright-light color vision are initiated in rod and several types of cone photoreceptors, respectively; these photoreceptors are structurally distinct from each other. In zebrafish, although quite different from rods and UV cones, RGB cones (red, green, and blue cones) are structurally similar and unite into mirror-symmetric pentamers (G-R-B-R-G) by adhesion. This structural commonality and unity suggest that a set of genes is commonly expressed only in RGB cones but not in other cells. Here, we report that the rainbow enhancers activate RGB cone-specific transcription of the ponli and crb2b genes. This study provides a starting point to study how RGB cone-specific transcription defines RGB cones' distinct functions for color vision.

Self-organising aggregates of zebrafish retinal cells for investigating mechanisms of neural lamination

To investigate the cell-cell interactions necessary for the formation of retinal layers, we cultured dissociated zebrafish retinal progenitors in agarose microwells. Within these wells, the cells re-aggregated within hours, forming tight retinal organoids. Using a Spectrum of Fates zebrafish line, in which all different types of retinal neurons show distinct fluorescent spectra, we found that by 48 h in culture, the retinal organoids acquire a distinct spatial organisation, i.e. they became coarsely but clearly laminated. Retinal pigment epithelium cells were in the centre, photoreceptors and bipolar cells were next most central and amacrine cells and retinal ganglion cells were on the outside. Image analysis allowed us to derive quantitative measures of lamination, which we then used to find that Müller glia, but not RPE cells, are essential for this process.

Patterning the cone mosaic array in zebrafish retina requires specification of ultraviolet-sensitive cones

Cone photoreceptors in teleost fish are organized in precise, crystalline arrays in the epithelial plane of the retina. In zebrafish, four distinct morphological/spectral cone types occupy specific, invariant positions within a regular lattice. The cone lattice is aligned orthogonal and parallel to circumference of the retinal hemisphere: it emerges as cones generated in a germinal zone at the retinal periphery are incorporated as single-cell columns into the cone lattice. Genetic disruption of the transcription factor Tbx2b eliminates most of the cone subtype maximally sensitive to ultraviolet (UV) wavelengths and also perturbs the long-range organization of the cone lattice. In the tbx2b mutant, the other three cone types (red, green, and blue cones) are specified in the correct proportion, differentiate normally, and acquire normal, planar polarized adhesive interactions mediated by Crumbs 2a and Crumbs 2b. Quantitative image analysis of cell adjacency revealed that the cones in the tbx2b mutant primarily have two nearest neighbors and align in single-cell-wide column fragments that are separated by rod photoreceptors. Some UV cones differentiate at the dorsal retinal margin in the tbx2b mutant, although they are severely dysmorphic and are eventually eliminated. Incorporating loss of UV cones during formation of cone columns at the margin into our previously published mathematical model of zebrafish cone mosaic formation (which uses bidirectional interactions between planar cell polarity proteins and anisotropic mechanical stresses in the plane of the retinal epithelium to generate regular columns of cones parallel to the margin) reproduces many features of the pattern disruptions seen in the tbx2b mutant.

The cone-dominant retina and the inner ear of zebrafish express the ortholog of CLRN1, the causative gene of human Usher syndrome type 3A

Clarin-1 (CLRN1) is the causative gene in Usher syndrome type 3A, an autosomal recessive disorder characterized by progressive vision and hearing loss. CLRN1 encodes Clarin-1, a glycoprotein with homology to the tetraspanin family of proteins. Previous cell culture studies suggest that Clarin-1 localizes to the plasma membrane and interacts with the cytoskeleton. Mouse models demonstrate a role for the protein in mechanosensory hair bundle integrity, but the function of Clarin-1 in hearing remains unclear. Even less is known of its role in vision, because the Clrn1 knockout mouse does not exhibit a retinal phenotype and expression studies in murine retinas have provided conflicting results. Here, we describe cloning and expression analysis of the zebrafish clrn1 gene, and report protein localization of Clarin-1 in auditory and visual cells from embryonic through adult stages. We detect clrn1 transcripts as early as 24h post-fertilization, and expression is maintained through adulthood. In situ hybridization experiments show clrn1 transcripts enriched in mechanosensory hair cells and supporting cells of the inner ear and lateral line organ, photoreceptors, and cells of the inner retina. In mechanosensory hair cells, Clarin-1 is polarized to the apical cell body and the synapses. In the retina, Clarin-1 localizes to lateral cell contacts between photoreceptors and is associated with the outer limiting membrane and subapical processes emanating from Müller glial cells. We also find Clarin-1 protein in the outer plexiform, inner nuclear and ganglion cell layers of the retina. Given the importance of Clarin-1 function in the human retina, it is imperative to find an animal model with a comparable requirement. Our data provide a foundation for exploring the role of Clarin-1 in retinal cell function and survival in a diurnal, cone-dominant species.

Spatial-temporal expressions of Crumbs and Nagie oko and their interdependence in zebrafish central nervous system during early development

A vast number of apicobasal polarity proteins play essential roles in the polarization and morphogenesis of the neuroepithelia. Crumbs (Crb) type I transmembrane cell-cell adhesion proteins are among these proteins. Five crb genes have been identified in zebrafish. However, their expressional and functional differences during early neural development remain to be fully elucidated. Here, we study the spatial-temporal expression patterns and functions of Crb1, Crb2a, and Crb2b in the central nervous system (CNS) during the neurulation period. We show that: 1, the optic vesicle and undifferentiated retinal neuroepithelium only express Crb2a; 2, Crb1 and Crb2a expressions overlap extensively in the undifferentiated neural tube epithelium; 3, Crb2b expression is the weakest of the three and is restricted to the ventral-most regions of the anterior CNS; and 4, Nok and Crb proteins require each other for their apical localization in neuroepithelium. The commencements of Crb1, Crb2a, and Crb2b expressions follow a spatial-temporal spread from anterior to posterior and from ventral to dorsal and lag behind that of adherens junction components, such as ZO-1 and actin bundles. Genetic and morpholino suppression analyses suggest that in regions where these Crb expressions overlap, they are functionally redundant in maintaining apicobasal polarity of the undifferentiated neuroepithelium.

A functional role of the extracellular domain of Crumbs in cell architecture and apicobasal polarity

Regulated cell shape changes in epithelial cells, which contribute to most organs and tissues, are at the basis of morphogenesis. Crumbs (Crb) is an essential apical determinant controlling epithelial apicobasal polarity. Here we provide evidence for a novel role of Crb apical localisation and stabilisation in controlling cell shape through apical domain organisation and adherens junction positioning. We find that Crb apical stabilisation requires the extracellular domain. In vivo results from Drosophila suggest that the extracellular domain assists Crb apical stabilisation by mediating Crb-Crb interactions at opposing cell membranes. We further confirm Crb-Crb extracellular interactions by showing that the extracellular domain of Crb is sufficient to promote cell aggregation in vitro. Furthermore, we report that Crb apical stabilisation mediated by the extracellular domain is also required for maintenance of Crb apicobasal polarity. Our results provide new insights into the mechanisms of apicobasal polarity and the cellular mechanisms of tissue architecture.

Characterization of transgenic zebrafish lines that express GFP in the retina, pineal gland, olfactory bulb, hatching gland, and optic tectum

Transgenic animals are powerful tools to study gene function invivo. Here we characterize several transgenic zebrafish lines that express green fluorescent protein (GFP) under the control of the LCR(RH2)-RH2-1 or LCR(RH2)-RH2-2 green opsin regulatory elements. Using confocal immunomicroscopy, stereo-fluorescence microscopy, and Western blotting, we show that the Tg(LCR(RH2)-RH2-1:GFP)(pt112) and Tg(LCR(RH2)-RH2-2:GFP)(pt115) transgenic zebrafish lines express GFP in the pineal gland and certain types of photoreceptors. In addition, some of these lines also express GFP in the hatching gland, optic tectum, or olfactory bulb. Some of the expression patterns differ significantly from previously published similar transgenic fish lines, making them useful tools for studying the development of the corresponding tissues and organs. In addition, the variations of GFP expression among different lines corroborate the notion that transgenic expression is often subjected to position effect, thus emphasizing the need for careful verification of expression patterns when transgenic animal models are utilized for research.

Coupling mechanical deformations and planar cell polarity to create regular patterns in the zebrafish retina

The orderly packing and precise arrangement of epithelial cells is essential to the functioning of many tissues, and refinement of this packing during development is a central theme in animal morphogenesis. The mechanisms that determine epithelial cell shape and position, however, remain incompletely understood. Here, we investigate these mechanisms in a striking example of planar order in a vertebrate epithelium: The periodic, almost crystalline distribution of cone photoreceptors in the adult teleost fish retina. Based on observations of the emergence of photoreceptor packing near the retinal margin, we propose a mathematical model in which ordered columns of cells form as a result of coupling between planar cell polarity (PCP) and anisotropic tissue-scale mechanical stresses. This model recapitulates many observed features of cone photoreceptor organization during retinal growth and regeneration. Consistent with the model's predictions, we report a planar-polarized distribution of Crumbs2a protein in cone photoreceptors in both unperturbed and regenerated tissue. We further show that the pattern perturbations predicted by the model to occur if the imposed stresses become isotropic closely resemble defects in the cone pattern in zebrafish lrp2 mutants, in which intraocular pressure is increased, resulting in altered mechanical stress and ocular enlargement. Evidence of interactions linking PCP, cell shape, and mechanical stresses has recently emerged in a number of systems, several of which show signs of columnar cell packing akin to that described here. Our results may hence have broader relevance for the organization of cells in epithelia. Whereas earlier models have allowed only for unidirectional influences between PCP and cell mechanics, the simple, phenomenological framework that we introduce here can encompass a broad range of bidirectional feedback interactions among planar polarity, shape, and stresses; our model thus represents a conceptual framework that can address many questions of importance to morphogenesis.

Genetic ablation of Pals1 in retinal progenitor cells models the retinal pathology of Leber congenital amaurosis

Mutation of the polarity gene Crumbs homolog 1 (CRB1) is responsible for >10% of Leber congenital amaurosis (LCA) cases worldwide; LCA is characterized by early-onset degenerative retinal dystrophy. The role of CRB1 in LCA8 pathogenesis remains elusive since Crb1 mouse mutants, including a null allele, have failed to mimic the early-onset of LCA, most likely due to functional compensation by closely related genes encoding Crb2 and Crb3. Crb proteins form an evolutionarily conserved, apical polarity complex with the scaffolding protein associated with lin-seven 1 (Pals1), also known as MAGUK p55 subfamily member 5 (MPP5). Pals1 and Crbs are functionally inter-dependent in establishing and maintaining epithelial polarity. Pals1 is a single gene in the mouse and human genomes; therefore, we ablated Pals1 to establish a mouse genetic model mimicking human LCA. In our study, the deletion of Pals1 leads to the disruption of the apical localization of Crb proteins in retinal progenitors and the adult retina, validating their mutual interaction. Remarkably, the Pals1 mutant mouse exhibits the critical features of LCA such as early visual impairment as assessed by electroretinogram, disorganization of lamination and apical junctions and retinal degeneration. Our data uncover the indispensible role of Pals1 in retinal development, likely involving the maintenance of retinal polarity and survival of retinal neurons, thus providing the basis for the pathologic mechanisms of LCA8.

Crb apical polarity proteins maintain zebrafish retinal cone mosaics via intercellular binding of their extracellular domains

Cone photoreceptors are assembled by unknown mechanisms into geometrically regular mosaics in many vertebrate species. The formation and maintenance of photoreceptor mosaics are speculated to require differential cell-cell adhesion. However, the molecular basis for this theory has yet to be identified. The retina and many other tissues express Crumbs (Crb) polarity proteins. The functions of the extracellular domains of Crb proteins remain to be understood. Here we report cell-type-specific expression of the crb2a and crb2b genes at the cell membranes of photoreceptor inner segments and Müller cell apical processes in the zebrafish retina. We demonstrate that the extracellular domains of Crb2a and Crb2b mediate a cell-cell adhesion function, which plays an essential role in maintaining the integrity of photoreceptor layer and cone mosaics. Because Crb proteins are expressed in many types of epithelia, the Crb-based cell-cell adhesion may underlie cellular patterning in other epithelium-derived tissues as well.

PALS1 is essential for retinal pigment epithelium structure and neural retina stratification

The membrane-associated palmitoylated protein 5 (MPP5 or PALS1) is thought to organize intracellular PALS1-CRB-MUPP1 protein scaffolds in the retina that are involved in maintenance of photoreceptor-Müller glia cell adhesion. In humans, the Crumbs homolog 1 (CRB1) gene is mutated in progressive types of autosomal recessive retinitis pigmentosa and Leber congenital amaurosis. However, there is no clear genotype-phenotype correlation for CRB1 mutations, which suggests that other components of the CRB complex may influence the severity of retinal disease. Therefore, to understand the physiological role of the Crumbs complex proteins, especially PALS1, we generated and analyzed conditional knockdown mice for Pals1. Small irregularly shaped spots were detected throughout the PALS1 deficient retina by confocal scanning laser ophthalmoscopy and spectral domain optical coherence tomography. The electroretinography a- and b-wave was severely attenuated in the aged mutant retinas, suggesting progressive degeneration of photoreceptors. The histological analysis showed abnormal retinal pigment epithelium structure, ectopic photoreceptor nuclei in the subretinal space, an irregular outer limiting membrane, half rosettes of photoreceptors in the outer plexiform layer, and a thinner photoreceptor synaptic layer suggesting improper photoreceptor cell layering during retinal development. The PALS1 deficient retinas showed reduced levels of Crumbs complex proteins adjacent to adherens junctions, upregulation of glial fibrillary acidic protein indicative of gliosis, and persisting programmed cell death after retinal maturation. The phenotype suggests important functions of PALS1 in the retinal pigment epithelium in addition to the neural retina.

Essential genes for astroglial development and axon pathfinding during zebrafish embryogenesis

The formation of the central nervous system depends on the coordinated development of neural and glial cell types that arise from a common precursor. Using an existing group of zebrafish mutants generated by viral insertion, we performed a "shelf-screen" to identify genes necessary for astroglial development and axon scaffold formation. We screened 274 of 315 viral insertion lines using antibodies that label axons (anti-Acetylated Tubulin) and astroglia (anti-Gfap) and identified 25 mutants with defects in gliogenesis, glial patterning, neurogenesis, and axon guidance. We also identified a novel class of mutants affecting radial glial cell numbers. Defects in astroglial patterning were always associated with axon defects, supporting an important role for axon-glial interactions during axon scaffold development. The genes disrupted in these viral lines have all been identified, providing a powerful new resource for the study of axon guidance, glio- and neurogenesis, and neuron-glial interactions during development of the vertebrate CNS.

Mutations in N-cadherin and a Stardust homolog, Nagie oko, affect cell-cycle exit in zebrafish retina

It has been reported that the loss of apicobasal cell polarity and the disruption of adherens junctions induce hyperplasia in the mouse developing brain. However, it is not fully understood whether hyperplasia is caused by an enhanced cell proliferation, an inhibited neurogenesis, or both. In this study, we found that the ratio of the number of proliferating progenitor cells to the total number of retinal cells increases in the neurogenic stages in zebrafish n-cadherin (ncad) and nagie oko (nok) mutants, in which the apicobasal cell polarity and adherens junctions in the retinal epithelium are disrupted. The cell-cycle progression was not altered in the ncad and nok mutants. Rather, the ratio of the number of cells undergoing neurogenic cell division to the total number of cells undergoing mitosis decreased in the ncad and nok mutant retinas, suggesting that the switching from proliferative cell division to neurogenic cell division was compromised in these mutant retinas. These findings suggest that the inhibition of neurogenesis is a primary defect that causes hyperplasia in the ncad and nok mutant retinas. The Hedgehog-protein kinase A signaling pathway and the Notch signaling pathway regulate retinal neurogenesis in zebrafish. We found that both signaling pathways are involved in the generation of neurogenic defects in the ncad and nok mutant retinas. Taken together, these findings suggest that apicobasal cell polarity and epithelial integrity are essential for retinal neurogenesis in zebrafish.

Restricted localization of ponli, a novel zebrafish MAGUK-family protein, to the inner segment interface areas between green, red, and blue cones

PURPOSE

The inner segments (IS) of the photoreceptors in vertebrates are enriched with polarity scaffold proteins, which maintain the integrity of many tissues by mediating cell-cell adhesion either directly or indirectly. The formation of photoreceptor mosaics may require differential adhesion among different types of photoreceptors. It is unknown whether any polarity proteins are selectively expressed in certain photoreceptors to mediate differential intercellular adhesion, which may be important for photoreceptor patterning. This study was undertaken to identify such polarity proteins.

METHODS

To identify novel MAGUK-family (membrane-associated guanylate kinase) proteins that are similar to Nagie oko (Nok), the authors performed BLAST searches of the zebrafish genome with the Nok amino acid sequence as the query. The coding sequence of one of the identified genes was obtained and verified through RT-PCR and RACE (rapid amplification of cDNA ends). Its protein expression patterns were examined by immunomicroscopy and Western blot analysis. Morpholino knockdown technology was used for loss-of-function analyses.

RESULTS

The authors cloned a novel nok homolog and designated it photoreceptor-layer-nok-like (ponli). Unlike Nok, which is expressed broadly, Ponli is only expressed at the interface areas between the IS of the green, red, and blue cones in differentiated zebrafish retina.

CONCLUSIONS

Ponli is the first identified polarity protein that is not expressed in all types of photoreceptors. Ponli's selective distribution stimulates future investigations on its functions for photoreceptor mosaic formation.

Stepwise maturation of apicobasal polarity of the neuroepithelium is essential for vertebrate neurulation

During vertebrate neurulation, extensive cell movements transform the flat neural plate into the neural tube. This dynamic morphogenesis requires the tissue to bear a certain amount of plasticity to accommodate shape and position changes of individual cells as well as intercellular cohesiveness to maintain tissue integrity and architecture. For most of the neural plate-neural tube transition, cells are polarized along the apicobasal axis. The establishment and maintenance of this polarity requires many polarity proteins that mediate cell-cell adhesion either directly or indirectly. Intercellular adhesion reduces tissue plasticity and enhances tissue integrity. However, it remains unclear how apicobasal polarity is regulated to meet the opposing needs for tissue plasticity and tissue integrity during neurulation. Here, we show that N-Cad/ZO-1 complex-initiated apicobasal polarity is stabilized by the late-onsetting Lin7c/Nok complex after the extensive morphogenetic cell movements in neurulation. Loss of either N-Cad or Lin7c disrupts neural tube formation. Furthermore, precocious overexpression of Lin7c induces multiaxial mirror symmetry in zebrafish neurulation. Our data suggest that stepwise maturation of apicobasal polarity plays an essential role in vertebrate neurulation.

The Crumbs complex: from epithelial-cell polarity to retinal degeneration

The evolutionarily conserved Crumbs protein complex is a key regulator of cell polarity and cell shape in both invertebrates and vertebrates. The important role of this complex in normal cell function is illustrated by the finding that mutations in one of its components, Crumbs, are associated with retinal degeneration in humans, mice and flies. Recent results suggest that the Crumbs complex plays a role in the development of other disease processes that are based on epithelial dysfunction, such as tumorigenesis or the formation of cystic kidneys. Localisation of the complex is restricted to a distinct region of the apical plasma membrane that abuts the zonula adherens in epithelia and photoreceptor cells of invertebrates and vertebrates, including humans. In addition to the core components, a variety of other proteins can be recruited to the complex, depending on the cell type and/or developmental stage. Together with diverse post-transcriptional and post-translational mechanisms that regulate the individual components, this provides an enormous functional diversity and flexibility of the complex. In this Commentary, we summarise findings concerning the organisation and modification of the Crumbs complex, and the conservation of its constituents from flies to mammals. In addition, we discuss recent results that suggest its participation in various human diseases, including blindness and tumour formation.

Intact retinal pigment epithelium maintained by Nok is essential for retinal epithelial polarity and cellular patterning in zebrafish

Within the vertebrate eye, the retinal pigment epithelium (RPE) juxtaposes with the retina, but how the RPE plays a role in retinal morphogenesis remains elusive. It has been shown that the loss of function of the polarity proteins, such as Nagie oko (Nok), disrupts RPE integrity and retinal lamination. However, it is unclear whether or not such defects are caused in a tissue-autonomous manner. Here, by taking advantage of the nok mutation, we have generated a transgenic model to restore the Nok function in the RPE, but not in the retina. With this model, we show that Nok is required for RPE integrity in a tissue-autonomous manner. However, proper retinal epithelial polarity does not require retinal expression of Nok before embryonic photoreceptor genesis; rather, it requires a Nok-mediated intact RPE. Interestingly, sporadic wild-type RPE donor cells are not sufficient to maintain proper retinal polarity. We further show that RPE-mediated retinal epithelial polarity underlies proper patterning of retinal ganglion cells and the cells of the inner nuclear layer. Nevertheless, during embryonic photoreceptor genesis, an intact RPE is not sufficient to maintain retinal epithelial polarity and retinal cellular pattern formation. Our results show that the subcellular architecture and cellular pattern formation of a tissue may be regulated by neighboring tissues through tissue-tissue interactions.

Multiple domains of Stardust differentially mediate localisation of the Crumbs-Stardust complex during photoreceptor development in Drosophila

Drosophila Stardust (Sdt), a member of the MAGUK family of scaffolding proteins, is a constituent of the evolutionarily conserved Crumbs-Stardust (Crb-Sdt) complex that controls epithelial cell polarity in the embryo and morphogenesis of photoreceptor cells. Although apical localisation is a hallmark of the complex in all cell types and in all organisms analysed, only little is known about how individual components are targeted to the apical membrane. We have performed a structure-function analysis of Sdt by constructing transgenic flies that express altered forms of Sdt to determine the roles of individual domains for localisation and function in photoreceptor cells. The results corroborate the observation that the organisation of the Crb-Sdt complex is differentially regulated in pupal and adult photoreceptors. In pupal photoreceptors, only the PDZ domain of Sdt – the binding site of Crb – is required for apical targeting. In adult photoreceptors, by contrast, targeting of Sdt to the stalk membrane, a distinct compartment of the apical membrane between the rhabdomere and the zonula adherens, depends on several domains, and seems to be a two-step process. The N-terminus, including the two ECR domains and a divergent N-terminal L27 domain that binds the multi-PDZ domain protein PATJ in vitro, is necessary for targeting the protein to the apical pole of the cell. The PDZ-, the SH3- and the GUK-domains are required to restrict the protein to the stalk membrane. Drosophila PATJ or Drosophila Lin-7 are stabilised whenever a Sdt variant that contains the respective binding site is present, independently of where the variant is localised. By contrast, only full-length Sdt, confined to the stalk membrane, stabilises and localises Crb, although only in reduced amounts. The amount of Crumbs recruited to the stalk membrane correlates with its length. Our results highlight the importance of the different Sdt domains and point to a more intricate regulation of the Crb-Sdt complex in adult photoreceptor cells.

The genesis of retinal architecture: an emerging role for mechanical interactions?

Patterns in nature have always fascinated human beings. They convey the idea of order, organization and optimization, and, to the enquiring mind, the alluring promise that understanding their building rules may uncover the forces that shaped them. In the retina, two patterns are outstanding: the stacking of cells in layers and, within the layers, the prevalent arrangement of neurons of the same type in orderly arrays, often referred to as mosaics for the crystalline-like order that some can display. Layers and mosaics have been essential keys to our present understanding of retinal circuital organization and function. Now, they may also be a precious guide in our exploration of how the retina is built. Here, we will review studies addressing the mechanisms controlling the formation of retinal mosaics and layers, illustrating common themes and unsolved problems. Among the intricacies of the building process, a world of physical forces is making its appearance. Cells are extremely complex to model as "physical entities", and many aspects of cell mechanotransduction are still obscure. Yet, recent experiments, focusing on the mechanical aspects of growth and differentiation, suggest that adopting this viewpoint will open new ways of understanding retinal formation and novel possibilities to approach retinal pathologies and repair.

Composition and function of the Crumbs protein complex in the mammalian retina

The Crumbs proteins (CRBs) are transmembrane proteins, homologous to Drosophila Crumbs, with a key role in defining the apical membrane domain in photoreceptors as well as in embryonic epithelia. Crumbs proteins are conserved between species and their intracellular domains are involved in organizing a conserved macromolecular protein scaffold with important roles in cell polarity as well as morphogenesis and maintenance of the retina. Mutations in the gene encoding human CRB1, the first one identified out of the three human orthologs, have been associated with a number of retinal dystrophies including Leber amaurosis and retinitis pigmentosa type 12. Although no other mammalian Crumbs complex members as of yet have been associated with retinal degeneration, disruption of different zebrafish and fruitfly orthologs can lead to various retinal defects. The core Crumbs complex localizes apical to the outer limiting membrane, where photoreceptors and Müller glia contact each other. Correct functioning of Crumbs ensures adhesion between these cells by an unknown mechanism. This review summarizes the current view on the composition and function of the Crumbs prsotein complex in the mammalian retina. Recently, a number of new members of the Crumbs protein complex have been identified. These include most members of the membrane palmitoylated protein family (MPP), involved in assembly of macromolecular protein complexes. Some components of the complex are found to exert a function in the photoreceptor synapses and/or at the region of the connecting cilium. Studies using polarized cell cultures or model organisms, like Drosophila and zebrafish, suggest important links of the Crumbs protein complex to several biological processes in the mammalian eye, including retinal patterning, ciliogenesis and vesicular transport.

Have we achieved a unified model of photoreceptor cell fate specification in vertebrates?

How does a retinal progenitor choose to differentiate as a rod or a cone and, if it becomes a cone, which one of their different subtypes? The mechanisms of photoreceptor cell fate specification and differentiation have been extensively investigated in a variety of animal model systems, including human and non-human primates, rodents (mice and rats), chickens, frogs (Xenopus) and fish. It appears timely to discuss whether it is possible to synthesize the resulting information into a unified model applicable to all vertebrates. In this review we focus on several widely used experimental animal model systems to highlight differences in photoreceptor properties among species, the diversity of developmental strategies and solutions that vertebrates use to create retinas with photoreceptors that are adapted to the visual needs of their species, and the limitations of the methods currently available for the investigation of photoreceptor cell fate specification. Based on these considerations, we conclude that we are not yet ready to construct a unified model of photoreceptor cell fate specification in the developing vertebrate retina.

FERM protein EPB41L5 is a novel member of the mammalian CRB-MPP5 polarity complex

Cell polarity is induced and maintained by separation of the apical and basolateral domains through specialized cell-cell junctions. The Crumbs protein and its binding partners are involved in formation and stabilization of adherens junctions. In this study, we describe a novel component of the mammalian Crumbs complex, the FERM domain protein EPB41L5, which associates with the intracellular domains of all three Crumbs homologs through its FERM domain. Surprisingly, the same FERM domain is involved in binding to the HOOK domain of MPP5/PALS1, a previously identified interactor of Crumbs. Co-expression and co-localization studies suggested that in several epithelial derived tissues Epb4.1l5 interacts with at least one Crumbs homolog, and with Mpp5. Although at early embryonic stages Epb4.1l5 is found at the basolateral membrane compartment, in adult tissues it co-localizes at the apical domain with Crumbs proteins and Mpp5. Overexpression of Epb4.1l5 in polarized MDCK cells affects tightness of cell junctions and results in disorganization of the tight junction markers ZO-1 and PATJ. Our results emphasize the importance of a conserved Crumbs-MPP5-EPB41L5 polarity complex in mammals.

Unraveling the genetic complexity of Drosophila stardust during photoreceptor morphogenesis and prevention of light-induced degeneration

Drosophila Stardust, a membrane-associated guanylate kinase (MAGUK), recruits the transmembrane protein Crumbs and the cytoplasmic proteins DPATJ and DLin-7 into an apically localized protein scaffold. This evolutionarily conserved complex is required for epithelial cell polarity in Drosophila embryos and mammalian cells in culture. In addition, mutations in Drosophila crumbs and DPATJ impair morphogenesis of photoreceptor cells (PRCs) and result in light-dependent retinal degeneration. Here we show that stardust is a genetically complex locus. While all alleles tested perturb epithelial cell polarity in the embryo, only a subset of them affects morphogenesis of PRCs or induces light-dependent retinal degeneration. Alleles retaining particular postembryonic functions still express some Stardust protein in pupal and/or adult eyes. The phenotypic complexity is reflected by the expression of distinct splice variants at different developmental stages. All proteins expressed in the retina contain the PSD95, Discs Large, ZO-1 (PDZ), Src homology 3 (SH3), and guanylate kinase (GUK) domain, but lack a large region in the N terminus encoded by one exon. These results suggest that Stardust-based protein scaffolds are dynamic, which is not only mediated by multiple interaction partners, but in addition by various forms of the Stardust protein itself.

Zebrafish dou yan mutation causes patterning defects and extensive cell death in the retina

The size of an organ is largely determined by the number of cells it contains, which in turn is regulated by two opposing processes, cell proliferation and cell death, however, it is generally not clear how cell proliferation and cell death are coordinated during development. Here, we characterize the zebrafish dou yan(mi234) mutation that results in a dramatic reduction of retinal size and a disruption of retinal differentiation and lamination. The retinal size reduction is caused by increased retinal cell death in a non-cell-autonomous manner during early development. The phenotypic defect in dou yan(mi234) arises coincident with the onset of retinal neurogenesis and differentiation. Interestingly, unlike many other small eye mutations, the mutation does not increase the level of cell death in the brain, suggesting that the brain and retina use different mechanisms to maintain cell survival. Identification and further study of the dou yan gene will enhance our understanding of the molecular mechanisms regulating retinal cellular homeostasis, i.e., the balance between cell proliferation and cell death.

Pals1/Mpp5 is required for correct localization of Crb1 at the subapical region in polarized Muller glia cells

Mutations in the human Crumbs homologue-1 (CRB1) gene cause retinal diseases including Leber's congenital amaurosis (LCA) and retinitis pigmentosa type 12. The CRB1 transmembrane protein localizes at a subapical region (SAR) above intercellular adherens junctions between photoreceptor and Müller glia (MG) cells. We demonstrate that the Crb1-/- phenotype, as shown in Crb1-/- mice, is accelerated and intensified in primary retina cultures. Immuno-electron microscopy showed strong Crb1 immunoreactivity at the SAR in MG cells but barely in photoreceptor cells, whereas Crb2, Crb3, Patj, Pals1 and Mupp1 were present in both cell types. Human CRB1, introduced in MG cells in Crb1-/- primary retinas, was targeted to the SAR. RNA interference-induced silencing of the Crb1-interacting-protein Pals1 (protein associated with Lin7; Mpp5) in MG cells resulted in loss of Crb1, Crb2, Mupp1 and Veli3 protein localization and partial loss of Crb3. We conclude that Pals1 is required for correct localization of Crb family members and its interactors at the SAR of polarized MG cells.

Molecular characterization of retinal stem cells and their niches in adult zebrafish

Background

The persistence in adult teleost fish of retinal stem cells that exhibit all of the features of true 'adult stem cells' – self-renewal, multipotency, and the capacity to respond to injury by mitotic activation with the ability to regenerate differentiated tissues – has been known for several decades. However, the specialized cellular and molecular characteristics of these adult retinal stem cells and the microenvironmental niches that support their maintenance in the differentiated retina and regulate their activity during growth and regeneration have not yet been elucidated.

Results

Our data show that the zebrafish retina has two kinds of specialized niches that sustain retinal stem cells: 1) a neuroepithelial germinal zone at the interface between neural retina and ciliary epithelium, called the ciliary marginal zone (CMZ), a continuous annulus around the retinal circumference, and 2) the microenvironment around some Müller glia in the differentiated retina. In the uninjured retina, scattered Müller glia (more frequently those in peripheral retina) are associated with clusters of proliferating retinal progenitors that are restricted to the rod photoreceptor lineage, but following injury, the Müller-associated retinal progenitors can function as multipotent retinal stem cells to regenerate other types of retinal neurons. The CMZ has several features in common with the neurogenic niches in the adult mammalian brain, including access to the apical epithelial surface and a close association with blood vessels. Müller glia in the teleost retina have a complex response to local injury that includes some features of reactive gliosis (up-regulation of glial fibrillary acidic protein, GFAP, and re-entry into the cell cycle) together with dedifferentiation and re-acquisition of phenotypic and molecular characteristics of multipotent retinal progenitors in the CMZ (diffuse distribution of N-cadherin, activation of Notch-Delta signaling, and expression of rx1, vsx2/Chx10, and pax6a) along with characteristics associated with radial glia (expression of brain lipid binding protein, BLBP). We also describe a novel specific marker for Müller glia, apoE.

Conclusion

The stem cell niches that support multi-lineage retinal progenitors in the intact, growing and regenerating teleost retina have properties characteristic of neuroepithelia and neurogenic radial glia. The regenerative capacity of the adult zebrafish retina with its ability to replace lost retinal neurons provides an opportunity to discover the molecular regulators that lead to functional repair of damaged neural tissue.