The hedgehog-PKA pathway regulates two distinct steps of the differentiation of retinal ganglion cells: the cell-cycle exit of retinoblasts and their neuronal maturation

Requirement of a novel gene, drish, in the zebrafish retinal ganglion cell and primary motor axon development

BACKGROUND

During neurogenesis, growing axons must navigate through the complex extracellular environment and make correct synaptic connections for the proper functioning of neural circuits. The mechanisms underlying the formation of functional neural networks are still only partially understood.

RESULTS

Here we analyzed the role of a novel gene si:ch73-364h19.1/drish in the neural and vascular development of zebrafish embryos. We show that drish mRNA is expressed broadly and dynamically in multiple cell types including neural, glial, retinal progenitor and vascular endothelial cells throughout the early stages of embryonic development. To study Drish function during embryogenesis, we generated drish genetic mutant using CRISPR/Cas9 genome editing. drish loss-of-function mutant larvae displayed defects in early retinal ganglion cell, optic nerve and the retinal inner nuclear layer formation, as well as ectopic motor axon branching. In addition, drish mutant adults exhibited deficient retinal outer nuclear layer and showed defective light response and locomotory behavior. However, vascular patterning and blood circulation were not significantly affected.

CONCLUSIONS

Together, these data demonstrate important roles of zebrafish drish in the retinal ganglion cell, optic nerve and interneuron development and in spinal motor axon branching.

Molecular mechanisms controlling vertebrate retinal patterning, neurogenesis, and cell fate specification

This review covers recent advances in understanding the molecular mechanisms controlling neurogenesis and specification of the developing retina, with a focus on insights obtained from comparative single cell multiomic analysis. We discuss recent advances in understanding the mechanisms by which extrinsic factors trigger transcriptional changes that spatially pattern the optic cup (OC) and control the initiation and progression of retinal neurogenesis. We also discuss progress in unraveling the core evolutionarily conserved gene regulatory networks (GRNs) that specify early- and late-state retinal progenitor cells (RPCs) and neurogenic progenitors and that control the final steps in determining cell identity. Finally, we discuss findings that provide insight into regulation of species-specific aspects of retinal patterning and neurogenesis, including consideration of key outstanding questions in the field.

Spatial regulation of amacrine cell genesis by Semaphorin 3f

PURPOSE

The axonal projections of retinal ganglion cells (RGCs) of the eye are topographically organized so that spatial information from visual images is preserved. This retinotopic organization is established during development by secreted morphogens that pattern domains of transcription factor expression within naso-temporal and dorso-ventral quadrants of the embryonic eye. Poorly understood are the downstream signaling molecules that generate the topographically organized retinal cells and circuits. The secreted signaling molecule Semaphorin 3fa (Sema3fa) belongs to the Sema family of molecules that provide positional information to developing cells. Here, we test a role for Sema3fa in cell genesis of the temporal zebrafish retina.

METHODS

We compare retinal cell genesis in wild type and sema3fa CRISPR zebrafish mutants by in situ hybridization and immunohistochemistry.

RESULTS

We find that mRNAs for sema3fa and known receptors, neuropilin2b (nrp2b) and plexina1a (plxna1a), are expressed by progenitors of the temporal, but not nasal zebrafish embryonic retina. In the sema3fa^ca304/ca304 embryo, initially the domains of expression for atoh7 and neurod4, transcription factors necessary for the specification of RGCs and amacrine cells, respectively, are disrupted. Yet, post-embryonically only amacrine cells of the temporal retina are reduced in numbers, with both GABAergic and glycinergic subtypes affected.

CONCLUSIONS

These data suggest that Sema3fa acts early on embryonic temporal progenitors to control in a spatially-dependent manner the production of amacrine cells, possibly to allow the establishment of neural circuits with domain-specific functions. We propose that spatially restricted extrinsic signals in the neural retina control cell genesis in a domain-dependent manner.

Banp regulates DNA damage response and chromosome segregation during the cell cycle in zebrafish retina

Btg3-associated nuclear protein (Banp) was originally identified as a nuclear matrix-associated region (MAR)-binding protein and it functions as a tumor suppressor. At the molecular level, Banp regulates transcription of metabolic genes via a CGCG-containing motif called the Banp motif. However, its physiological roles in embryonic development are unknown. Here, we report that Banp is indispensable for the DNA damage response and chromosome segregation during mitosis. Zebrafish banp mutants show mitotic cell accumulation and apoptosis in developing retina. We found that DNA replication stress and tp53-dependent DNA damage responses were activated to induce apoptosis in banp mutants, suggesting that Banp is required for regulation of DNA replication and DNA damage repair. Furthermore, consistent with mitotic cell accumulation, chromosome segregation was not smoothly processed from prometaphase to anaphase in banp morphants, leading to a prolonged M-phase. Our RNA- and ATAC-sequencing identified 31 candidates for direct Banp target genes that carry the Banp motif. Interestingly, a DNA replication fork regulator, wrnip1, and two chromosome segregation regulators, cenpt and ncapg, are included in this list. Thus, Banp directly regulates transcription of wrnip1 for recovery from DNA replication stress, and cenpt and ncapg for chromosome segregation during mitosis. Our findings provide the first in vivo evidence that Banp is required for cell-cycle progression and cell survival by regulating DNA damage responses and chromosome segregation during mitosis.

In order for a cell to divide, it must progress through a series of carefully controlled steps known as the cell cycle. First, the cell replicates its DNA and both copies get segregated to opposite ends. The cell then splits into two and each new cell receives a copy of the duplicated genetic material. If any of the stages in the cell cycle become disrupted or mis-regulated this can lead to uncontrolled divisions that may result in cancer.

Researchers have often used a structure within the eye known as the retina to study the cell cycle in zebrafish and other animals as cells in the retina rapidly divide in a highly controlled manner. A protein called Banp is known to help stop tumors from growing in humans and mice, but its normal role in the body, particularly the cell cycle, has remained unclear. To investigate, Babu et al. studied the retina of mutant zebrafish that were unable to make the Banp protein.

The experiments revealed that two stress responses indicating DNA damage or defects in copying DNA were active in the retinal cells of the mutant zebrafish. This suggested that Banp allows cell to progress through the cell cycle by repairing any DNA damage that may arise during replication. Banp does this by activating the gene for another protein called Wrnip1. Babu et al. also found that Banp helps segregate the two copies of DNA during cell division by promoting the activation of two other proteins called Cenpt and Ncapg. Further experiments identified 31 genes that were directly regulated by Banp.

These findings demonstrate that Banp is required for zebrafish cells to be able to accurately copy their DNA and divide in to two new cells. In the future, the work of Babu et al. will provide a useful resource to investigate how tumors grow and spread around the body, and may contribute to the development of new treatments for cancer.

Application of Small Molecules in the Central Nervous System Direct Neuronal Reprogramming

The lack of regenerative capacity of neurons leads to poor prognoses for some neurological disorders. The use of small molecules to directly reprogram somatic cells into neurons provides a new therapeutic strategy for neurological diseases. In this review, the mechanisms of action of different small molecules, the approaches to screening small molecule cocktails, and the methods employed to detect their reprogramming efficiency are discussed, and the studies, focusing on neuronal reprogramming using small molecules in neurological disease models, are collected. Future research efforts are needed to investigate the in vivo mechanisms of small molecule-mediated neuronal reprogramming under pathophysiological states, optimize screening cocktails and dosing regimens, and identify safe and effective delivery routes to promote neural regeneration in different neurological diseases.

Compensatory engulfment and Müller glia reactivity in the absence of microglia

Microglia are known for important phagocytic functions in the vertebrate retina. Reports also suggest that Müller glia have phagocytic capacity, though the relative levels and contexts in which this occurs remain to be thoroughly examined. Here, we investigate Müller glial engulfment of dying cells in the developing zebrafish retina in the presence and absence of microglia, using a genetic mutant in which microglia do not develop. We show that in normal conditions clearance of dying cells is dominated by microglia; however, Müller glia do have a limited clearance role. In retinas lacking intact microglial populations, we found a striking increase in the engulfment load assumed by the Müller glia, which displayed prominent cellular compartments containing apoptotic cells, several of which localized with the early phagosome/endosome marker Rab5. Consistent with increased engulfment, lysosomal staining was also increased in Müller glia in the absence of microglia. Increased engulfment load led to evidence of Müller glia reactivity including upregulation of gfap but did not trigger cell cycle re-entry by differentiated Müller glia. Our work provides important insight into the phagocytic capacity of Müller glia and the ability for compensatory functions and downstream effects. Therefore, effects of microglial deficiency or depletion on other glial cell types should be well-considered in experimental manipulations, in neurodegenerative disease, and in therapeutic approaches that target microglia. Our findings further justify future work to understand differential mechanisms and contexts of phagocytosis by glial cells in the central nervous system, and the significance of these mechanisms in health and disease.

Mechanisms underlying microglial colonization of developing neural retina in zebrafish

Microglia are brain-resident macrophages that function as the first line of defense in brain. Embryonic microglial precursors originate in peripheral mesoderm and migrate into the brain during development. However, the mechanism by which they colonize the brain is incompletely understood. The retina is one of the first brain regions to accommodate microglia. In zebrafish, embryonic microglial precursors use intraocular hyaloid blood vessels as a pathway to migrate into the optic cup via the choroid fissure. Once retinal progenitor cells exit the cell cycle, microglial precursors associated with hyaloid blood vessels start to infiltrate the retina preferentially through neurogenic regions, suggesting that colonization of retinal tissue depends upon the neurogenic state. Along with blood vessels and retinal neurogenesis, IL34 also participates in microglial precursor colonization of the retina. Altogether, CSF receptor signaling, blood vessels, and neuronal differentiation function as cues to create an essential path for microglial migration into developing retina.

The immune system is comprised of many different cells which protect our bodies from infection and other illnesses. The brain contains its own population of immune cells called microglia. Unlike neurons, these cells form outside the brain during development. They then travel to the brain and colonize specific regions like the retina, the light-sensing part of the eye in vertebrates.

It is poorly understood how newly formed microglia migrate to the retina and whether their entry depends on the developmental state of nerve cells (also known as neurons) in this region. To help answer these questions, Ranawat and Masai attached fluorescent labels that can be seen under a microscope to microglia in the embryos of zebrafish. Developing zebrafish are transparent, making it easy to trace the fluorescent microglia as they travel to the retina and insert themselves among its neurons.

Ranawat and Masai found that blood vessels around the retina act as a pathway that microglia move along. Once they reach the retina, the microglia remain attached and only enter the retina at sites where brain cells are starting to mature in to adult neurons. Further experiments showed that microglia fail to infiltrate and colonize the retina when blood vessels are damaged or neuron maturation is blocked.

These findings reveal some of the key elements that guide microglia to the retina during development. However, further work is needed to establish the molecular and biochemical processes that allow microglia to attach to blood vessels and detect when cells in the retina are starting to mature.

LIM Homeobox 4 (lhx4) regulates retinal neural differentiation and visual function in zebrafish

LIM homeobox 4 (LHX4) is expressed in the photoreceptors (PRs) of the outer nuclear layer (ONL) and bipolar cells (BCs) of the inner nuclear layer (INL) in mouse and chicken retina. It regulates the subtype-specific development of rod BCs and cone BCs in the mouse retina. However, no report has been published on its expression and function in the zebrafish retina. In this study, we assessed the expression of Lhx4 using in situ hybridization (ISH) technique and explored its role in zebrafish (Danio rerio) retinal development via morpholino (MO) technology. We found that the expression of lhx4 in the zebrafish retina begins 48 h post-fertilization (hpf) and is continuously expressed in the ONL and INL. A zebrafish model constructed with lhx4 knockdown in the eyes through vivo-MO revealed that: lhx4 knockdown inhibits the differentiation of Parvalbumin⁺ amacrine cells (ACs) and Rhodopsin⁺ rod photoreceptors (RPs), enhances the expression of visual system homeobox 2 (vsx2); and damages the responses of zebrafish to light stimulus, without affecting the differentiation of OFF-BCs and rod BCs, and apoptosis in the retina. These findings reveal that lhx4 regulates neural differentiation in the retina and visual function during zebrafish embryonic development.

Human Organoids for the Study of Retinal Development and Disease

Recent advances in stem cell engineering have led to an explosion in the use of organoids as model systems for studies in multiple biological disciplines. Together with breakthroughs in genome engineering and the various omics, organoid technology is making possible studies of human biology that were not previously feasible. For vision science, retinal organoids derived from human stem cells allow differentiating and mature human retinal cells to be studied in unprecedented detail. In this review, we examine the technologies employed to generate retinal organoids and how organoids are revolutionizing the fields of developmental and cellular biology as they pertain to the retina. Furthermore, we explore retinal organoids from a clinical standpoint, offering a new platform with which to study retinal diseases and degeneration, test prospective drugs and therapeutic strategies, and promote personalized medicine. Finally, we discuss the range of possibilities that organoids may bring to future retinal research and consider their ethical implications.

Transcriptome analysis of the zebrafish atoh7-/- Mutant, lakritz, highlights Atoh7-dependent genetic networks with potential implications for human eye diseases

Expression of the bHLH transcription protein Atoh7 is a crucial factor conferring competence to retinal progenitor cells for the development of retinal ganglion cells. Several studies have emerged establishing ATOH7 as a retinal disease gene. Remarkably, such studies uncovered ATOH7 variants associated with global eye defects including optic nerve hypoplasia, microphthalmia, retinal vascular disorders, and glaucoma. The complex genetic networks and cellular decisions arising downstream of atoh7 expression, and how their dysregulation cause development of such disease traits remains unknown. To begin to understand such Atoh7-dependent events in vivo, we performed transcriptome analysis of wild-type and atoh7 mutant (lakritz) zebrafish embryos at the onset of retinal ganglion cell differentiation. We investigated in silico interplays of atoh7 and other disease-related genes and pathways. By network reconstruction analysis of differentially expressed genes, we identified gene clusters enriched in retinal development, cell cycle, chromatin remodeling, stress response, and Wnt pathways. By weighted gene coexpression network, we identified coexpression modules affected by the mutation and enriched in retina development genes tightly connected to atoh7. We established the groundwork whereby Atoh7-linked cellular and molecular processes can be investigated in the dynamic multi-tissue environment of the developing normal and diseased vertebrate eye.

Transcriptome Analysis of Small Molecule-Mediated Astrocyte-to-Neuron Reprogramming

Chemical reprogramming of astrocytes into neurons represents a promising approach to regenerate new neurons for brain repair, but the underlying mechanisms driving this trans-differentiation process are not well understood. We have recently identified four small molecules – CHIR99021, DAPT, LDN193189, and SB431542 – that can efficiently reprogram cultured human fetal astrocytes into functional neurons. Here we employ the next generation of RNA-sequencing technology to investigate the transcriptome changes during the astrocyte-to-neuron (AtN) conversion process. We found that the four small molecules can rapidly activate the hedgehog signaling pathway while downregulating many glial genes such as FN1 and MYL9 within 24 h of treatment. Chemical reprogramming is mediated by several waves of differential gene expression, including upregulation of hedgehog, Wnt/β-catenin, and Notch signaling pathways, together with downregulation of TGF-β and JAK/STAT signaling pathways. Our gene network analyses reveal many well-connected hub genes such as repulsive guidance molecule A (RGMA), neuronatin (NNAT), neurogenin 2 (NEUROG2), NPTX2, MOXD1, JAG1, and GAP43, which may coordinate the chemical reprogramming process. Together, these findings provide critical insights into the molecular cascades triggered by a combination of small molecules that eventually leads to chemical conversion of astrocytes into neurons.

Developmental expression patterns of protein kinase A catalytic subunits in zebrafish

Protein kinase A (PKA), also known as cAMP dependent protein kinase, is an essential component of many signaling pathways, many of which regulate key developmental processes. Inactive PKA is a tetrameric holoenzyme, comprised of two catalytic (PRKAC), and two regulatory subunits. Upon cAMP binding, the catalytic subunits are released and thereby activated. There are multiple isoforms of PKA catalytic subunits, but their individual roles are not well understood. In order to begin studying their roles in zebrafish development, it is first necessary to identify the spatial and temporal expression profiles for each prkac subunit. Here we evaluate the expression profiles for the four zebrafish prkacs: prkacαa, αb, βa, and βb, at key developmental time points: 24, 48 and 72 h post fertilization. We show that zebrafish prkacs are expressed throughout the developing nervous system, each showing unique expression patterns. This body of work will inform future functional studies into the roles of PKA during development.

Intracellular Calcium Mobilization Is Required for Sonic Hedgehog Signaling

Graded Shh signaling across fields of precursor cells coordinates patterns of gene expression, differentiation, and morphogenetic behavior as precursors form complex structures, such as the nervous system, the limbs, and craniofacial skeleton. Here we discover that intracellular calcium mobilization, a process tightly controlled and readily modulated, regulates the level of Shh-dependent gene expression in responding cells and affects the development of all Shh-dependent cell types in the zebrafish embryo. Reduced expression or modified activity of ryanodine receptor (RyR) intracellular calcium release channels shifted the allocation of Shh-dependent cell fates in the somitic muscle and neural tube. Mosaic analysis revealed that RyR-mediated calcium mobilization is required specifically in Shh ligand-receiving cells. This work reveals that RyR channels participate in intercellular signal transduction events. As modulation of RyR activity modifies tissue patterning, we hypothesize that alterations in intracellular calcium mobilization contribute to both birth defects and evolutionary modifications of morphology.

All in the family: proneural bHLH genes and neuronal diversity

Proneural basic Helix-Loop-Helix (bHLH) proteins are required for neuronal determination and the differentiation of most neural precursor cells. These transcription factors are expressed in vastly divergent organisms, ranging from sponges to primates. Here, we review proneural bHLH gene evolution and function in the Drosophila and vertebrate nervous systems, arguing that the Drosophila gene atonal provides a useful platform for understanding proneural gene structure and regulation. We also discuss how functional equivalency experiments using distinct proneural genes can reveal how proneural gene duplication and divergence are interwoven with neuronal complexity.

Ectopic Phosphorylated Creb Marks Dedifferentiated Proximal Tubules in Cystic Kidney Disease

Ectopic cAMP signaling is pathologic in polycystic kidney disease; however, its spatiotemporal actions are unclear. We characterized the expression of phosphorylated Creb (p-Creb), a target and mediator of cAMP signaling, in developing and cystic kidney models. We also examined tubule-specific effects of cAMP analogs in cystogenesis in embryonic kidney explants. In wild-type mice, p-Creb marked nephron progenitors (NP), early epithelial NP derivatives, ureteric bud, and cortical stroma; p-Creb was present in differentiated thick ascending limb of Henle, collecting duct, and stroma; however, it disappeared in mature NP-derived proximal tubules. In Six2cre;Frs2αFl/Fl mice, a renal cystic model, ectopic p-Creb stained proximal tubule-derived cystic segments that lost the differentiation marker lotus tetragonolobus lectin. Furthermore, lotus tetragonolobus lectin-negative/p-Creb-positive cyst segments (re)-expressed Ncam1, Pax2, and Sox9 markers of immature nephron structures and dedifferentiated proximal tubules after acute kidney injury. These dedifferentiation markers were co-expressed with p-Creb in renal cysts in Itf88 knockout mice subjected to ischemia and Six2cre;Pkd1Fl/Fl mice, other renal cystogenesis models. 8-Br-cAMP addition to wild-type embryonic kidney explants induced proximal tubular cystogenesis and p-Creb expression; these effects were blocked by co-addition of protein kinase A inhibitor. Thus p-Creb/cAMP signaling is appropriate in NP and early nephron derivatives, but disappears in mature proximal tubules. Moreover, ectopic p-Creb expression/cAMP signaling marks dedifferentiated proximal tubular cystic segments. Furthermore, proximal tubules are predisposed to become cystic after cAMP stimulation.

Tet-mediated DNA hydroxymethylation regulates retinal neurogenesis by modulating cell-extrinsic signaling pathways

DNA hydroxymethylation has recently been shown to play critical roles in regulating gene expression and terminal differentiation events in a variety of developmental contexts. However, little is known about its function during eye development. Methylcytosine dioxygenases of the Tet family convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), an epigenetic mark thought to serve as a precursor for DNA demethylation and as a stable mark in neurons. Here, we report a requirement for Tet activity during zebrafish retinal neurogenesis. In tet2-/-;tet3-/- mutants, retinal neurons are specified but most fail to terminally differentiate. While differentiation of the first born retinal neurons, the retinal ganglion cells (RGCs), is less affected in tet2-/-;tet3-/- mutants than other retinal cell types, the majority of RGCs do not undergo terminal morphogenesis and form axons. Moreover, the few photoreceptors that differentiate in tet2-/-;tet3-/- mutants fail to form outer segments, suggesting that Tet function is also required for terminal morphogenesis of differentiated retinal neurons. Mosaic analyses revealed a surprising cell non-autonomous requirement for tet2 and tet3 activity in facilitating retinal neurogenesis. Through a combination of candidate gene analysis, transcriptomics and pharmacological manipulations, we identified the Notch and Wnt pathways as cell-extrinsic pathways regulated by tet2 and tet3 activity during RGC differentiation and morphogenesis. Transcriptome analyses also revealed the ectopic expression of non-retinal genes in tet2-/-;tet3-/- mutant retinae, and this correlated with locus-specific reduction in 5hmC. These data provide the first evidence that Tet-dependent regulation of 5hmC formation is critical for retinal neurogenesis, and highlight an additional layer of complexity in the progression from retinal progenitor cell to differentiated retinal neuron during development of the vertebrate retina.

Mechanical stimulation promote the osteogenic differentiation of bone marrow stromal cells through epigenetic regulation of Sonic Hedgehog

Mechanical unloading leads to bone loss and disuse osteoporosis partly due to impaired osteoblastogenesis of bone marrow stromal cells (BMSCs). However, the underlying molecular mechanisms of this phenomenon are not fully understood. In this study, we demonstrated that cyclic mechanical stretch (CMS) promotes osteoblastogenesis of BMSCs both in vivo and in vitro. Besides, we found that Hedgehog (Hh) signaling pathway was activated in this process. Inhibition of which by either knockdown of Sonic hedgehog (Shh) or treating BMSCs with Hh inhibitors attenuated the osteogenic effect of CMS on BMSCs, suggesting that Hh signaling pathway acts as an endogenous mediator of mechanical stimuli on BMSCs. Furthermore, we demonstrated that Shh expression level was regulated by DNA methylation mechanism. Chromatin Immunoprecipitation (ChIP) assay showed that DNA methyltransferase 3b (Dnmt3b) binds to Shh gene promoter, leading to DNA hypermethylation in mechanical unloading BMSCs. However, mechanical stimulation down-regulates the protein level of Dnmt3b, results in DNA demethylation and Shh expression. More importantly, we found that inhibition of Dnmt3b partly rescued bone loss in HU mice by mechanical unloading. Our results demonstrate, for the first time, that mechanical stimulation regulates osteoblastic genes expression via direct regulation of Dnmt3b, and the therapeutic inhibition of Dnmt3b may be an efficient strategy for enhancing bone formation under mechanical unloading.

Requirement of Smad4 from Ocular Surface Ectoderm for Retinal Development

Microphthalmia is characterized by abnormally small eyes and usually retinal dysplasia, accounting for up to 11% of the blindness in children. Right now there is no effective treatment for the disease, and the underlying mechanisms, especially how retinal dysplasia develops from microphthalmia and whether it depends on the signals from lens ectoderm are still unclear. Mutations in genes of the TGF-β superfamily have been noted in patients with microphthalmia. Using conditional knockout mice, here we address the question that whether ocular surface ectoderm-derived Smad4 modulates retinal development. We found that loss of Smad4 specifically on surface lens ectoderm leads to microphthalmia and dysplasia of retina. Retinal dysplasia in the knockout mice is caused by the delayed or failed differentiation and apoptosis of retinal cells. Microarray analyses revealed that members of Hedgehog and Wnt signaling pathways are affected in the knockout retinas, suggesting that ocular surface ectoderm-derived Smad4 can regulate Hedgehog and Wnt signaling in the retina. Our studies suggest that defective of ocular surface ectoderm may affect retinal development.

Characterization of primary cilia during the differentiation of retinal ganglion cells in the zebrafish

Background

Retinal ganglion cell (RGC) differentiation in vivo is a highly stereotyped process, likely resulting from the interaction of cell type-specific transcription factors and tissue-derived signaling factors. The primary cilium, as a signaling hub in the cell, may have a role during this process but its presence and localization during RGC generation, and its contribution to the process of cell differentiation, have not been previously assessed in vivo.

Methods

In this work we analyzed the distribution of primary cilia in vivo using laser scanning confocal microscopy, as well as their main ultrastructural features by transmission electron microscopy, in the early stages of retinal histogenesis in the zebrafish, around the time of RGC generation and initial differentiation. In addition, we knocked-down ift88 and elipsa, two genes with an essential role in cilia generation and maintenance, a treatment that caused a general reduction in organelle size. The effect on retinal development and RGC differentiation was assessed by confocal microscopy of transgenic or immunolabeled embryos.

Results

Our results show that retinal neuroepithelial cells have an apically-localized primary cilium usually protruding from the apical membrane. We also found a small proportion of sub-apical cilia, before and during the neurogenic period. This organelle was also present in an apical position in neuroblasts during apical process retraction and dendritogenesis, although between these stages cilia appeared highly dynamic regarding both presence and position. Disruption of cilia caused a decrease in the proliferation of retinal progenitors and a reduction of neural retina volume. In addition, retinal histogenesis was globally delayed albeit RGC layer formation was preferentially reduced with respect to the amacrine and photoreceptor cell layers.

Conclusions

These results indicate that primary cilia exhibit a highly dynamic behavior during early retinal differentiation, and that they are required for the proliferation and survival of retinal progenitors, as well as for neuronal generation, particularly of RGCs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-016-0064-z) contains supplementary material, which is available to authorized users.

The apical complex protein Pals1 is required to maintain cerebellar progenitor cells in a proliferative state

Through their biased localization and function within the cell, polarity complex proteins are necessary to establish the cellular asymmetry required for tissue organization. Well-characterized germinal zones, mitogenic signals and cell types make the cerebellum an excellent model for addressing the crucial function of polarity complex proteins in the generation and organization of neural tissues. Deletion of the apical polarity complex protein Pals1 in the developing cerebellum results in a remarkably undersized cerebellum with disrupted layers in poorly formed folia and strikingly reduced granule cell production. We demonstrate that Pals1 is not only essential for cerebellum organogenesis, but also for preventing premature differentiation and thus maintaining progenitor pools in cerebellar germinal zones, including cerebellar granule neuron precursors in the external granule layer. In the Pals1 mouse mutants, the expression of genes that regulate the cell cycle was diminished, correlating with the loss of the proliferating cell population of germinal zones. Furthermore, enhanced Shh signaling through activated Smo cannot overcome impaired cerebellar cell generation, arguing for an epistatic role of Pals1 in proliferation capacity. Our study identifies Pals1 as a novel intrinsic factor that regulates the generation of cerebellar cells and Pals1 deficiency as a potential inhibitor of overactive mitogenic signaling.

Reactive gliosis in the adult zebrafish retina

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

Loss of DDB1 Leads to Transcriptional p53 Pathway Activation in Proliferating Cells, Cell Cycle Deregulation, and Apoptosis in Zebrafish Embryos

DNA damage-binding protein 1 (DDB1) is a large subunit of the heterodimeric DDB complex that recognizes DNA lesions and initiates the nucleotide excision repair process. DDB1 is also a component of the CUL4 E3 ligase complex involved in a broad spectrum of cellular processes by targeted ubiquitination of key regulators. Functions of DDB1 in development have been addressed in several model organisms, however, are not fully understood so far. Here we report an ENU induced mutant ddb1 allele (ddb1^m863) identified in zebrafish (Danio rerio), and analyze its effects on development. Zebrafish ddb1 is expressed broadly, both maternally and zygotically, with enhanced expression in proliferation zones. The (ddb1^m863 mutant allele affects the splice acceptor site of exon 20, causing a splicing defect that results in truncation of the 1140 amino acid protein after residue 800, lacking part of the β-propeller domain BPC and the C-terminal helical domain CTD. ddb1^m863 zygotic mutant embryos have a pleiotropic phenotype, including smaller and abnormally shaped brain, head skeleton, eyes, jaw, and branchial arches, as well as reduced dopaminergic neuron groups. However, early forming tissues develop normally in zygotic ddb1^m863 mutant embryos, which may be due to maternal rescue. In ddb1^m863 mutant embryos, pcna-expressing proliferating cell populations were reduced, concurrent with increased apoptosis. We also observed a concomitant strong up-regulation of transcripts of the tumor suppressor p53 (tp53) and the cell cycle inhibitor cdkn1a (p21a/b^CIP1/WAF1) in proliferating tissues. In addition, transcription of cyclin genes ccna2 and ccnd1 was deregulated in ddb1^m863 mutants. Reduction of p53 activity by anti-sense morpholinos alleviated the apoptotic phenotype in ddb1^m863 mutants. These results imply that Ddb1 may be involved in maintaining proper cell cycle progression and viability of dividing cells during development through transcriptional mechanisms regulating genes involved in cell cycle control and cell survival.

Development of the Vertebrate Eye and Retina

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

Keeping an eye on SOXC proteins

The formation of a mature, functional eye requires a complex series of cell proliferation, migration, induction among different germinal layers, and cell differentiation. These processes are regulated by extracellular cues such as the Wnt/BMP/Hh/Fgf signaling pathways, as well as cell intrinsic transcription factors that specify cell fate. In this review article, we provide an overview of stages of embryonic eye morphogenesis, extrinsic and intrinsic factors that are required for each stage, and pediatric ocular diseases that are associated with defective eye development. In addition, we focus on recent findings about the roles of the SOXC proteins in regulating vertebrate ocular development and implicating SOXC mutations in human ocular malformations.

Activation of Wnt/β-catenin signaling in Muller glia protects photoreceptors in a mouse model of inherited retinal degeneration

The canonical Wnt/β-catenin ("Wnt") pathway is an essential signaling cascade in the embryonic central nervous system (CNS) that regulates neuronal differentiation and survival. Loss of Wnt signaling in developing and adult tissue has been implicated in numerous CNS diseases, but the precise role of Wnt in regulating neuronal survival, and how its absence could lead to disease, is not understood. In this study, we investigated the effect of Wnt activation on neuronal survival in the adult retina, and identified cellular and molecular mediators. Pan-retinal Wnt signaling activation using Wnt3a induced functional and morphological rescue of photoreceptor neurons in the rd10 mouse model of retinal degeneration. Furthermore, Wnt activation using constitutively active β-catenin specifically targeted to Muller glia increased photoreceptor survival and reduced markers of glial and neuronal remodeling. Wnt-induced photoreceptor protection was associated with elevated levels of the prosurvival protein Stat3, and was reduced by shRNA-mediated knock-down of Stat3, indicating cross-talk between survival pathways. Therefore, these data increase our understanding of the role of Wnt signaling in the retina, and identify radial Muller glia as important cellular mediators of Wnt activity.

Notch signaling differentially regulates Atoh7 and Neurog2 in the distal mouse retina

Notch signaling regulates basic helix-loop-helix (bHLH) factors as an evolutionarily conserved module, but the tissue-specific mechanisms are incompletely elucidated. In the mouse retina, bHLH genes Atoh7 and Neurog2 have distinct functions, with Atoh7 regulating retinal competence and Neurog2 required for progression of neurogenesis. These transcription factors are extensively co-expressed, suggesting similar regulation. We directly compared Atoh7 and Neurog2 regulation at the earliest stages of retinal neurogenesis in a broad spectrum of Notch pathway mutants. Notch1 and Rbpj normally block Atoh7 and Neurog2 expression. However, the combined activities of Notch1, Notch3 and Rbpj regulate Neurog2 patterning in the distal retina. Downstream of the Notch complex, we found the Hes1 repressor mediates Atoh7 suppression, but Hes1, Hes3 and Hes5 do not regulate Neurog2 expression. We also tested Notch-mediated regulation of Jag1 and Pax6 in the distal retina, to establish the appropriate context for Neurog2 patterning. We found that Notch1;Notch3 and Rbpj block co-expression of Jag1 and Neurog2, while specifically stimulating Pax6 within an adjacent domain. Our data suggest that Notch signaling controls the overall tempo of retinogenesis, by integrating cell fate specification, the wave of neurogenesis and the developmental status of cells ahead of this wave.

Nel positively regulates the genesis of retinal ganglion cells by promoting their differentiation and survival during development

For correct functioning of the nervous system, the appropriate number and complement of neuronal cell types must be produced during development. However, the molecular mechanisms that regulate the production of individual classes of neurons are poorly understood. In this study, we investigate the function of the thrombospondin-1-like glycoprotein, Nel (neural epidermal growth factor [EGF]-like), in the generation of retinal ganglion cells (RGCs) in chicks. During eye development, Nel is strongly expressed in the presumptive retinal pigment epithelium and RGCs. Nel overexpression in the developing retina by in ovo electroporation increases the number of RGCs, whereas the number of displaced amacrine cells decreases. Conversely, knockdown of Nel expression by transposon-mediated introduction of RNA interference constructs results in decrease in RGC number and increase in the number of displaced amacrine cells. Modifications of Nel expression levels do not appear to affect proliferation of retinal progenitor cells, but they significantly alter the progression rate of RGC differentiation from the central retina to the periphery. Furthermore, Nel protects RGCs from apoptosis during retinal development. These results indicate that Nel positively regulates RGC production by promoting their differentiation and survival during development.

The Hedgehog signaling pathway in ovarian teratoma is stimulated by Sonic Hedgehog which induces internalization of Patched

The Hedgehog-Gli (Hh-Gli) signaling pathway was examined in ovarian dermoids, which show characteristics of both tumors and developmental malformations. Dermoids are classified as mature teratomas that present differentiation into various tissues, mostly epidermal elements such as glands, multilayered epithelium, hair follicles and occasionally bone and cartilage. Their development is attributed to aberrant meiosis of germinal cells within the ovary. We showed activation of the Hh-Gli signaling in ovarian dermoid primary cultures. Cyclopamine treatment slows down cell proliferation, while the Sonic Hedgehog (Shh) protein stimulates cell proliferation and induces internalization of the Patched (Ptch) protein, which accumulates in the form of granules in the cytoplasm, colocalized with the Shh protein. Cyclopamine treatment decreases Gli1 localization in the nucleus compared to non-treated cells. Based on our observations, the mechanism of Hedgehog activation in the ovarian dermoids could be the ligand-dependent autocrine pathway, which can also be stimulated by paracrine signals.

Zebrafish Cxcr4a determines the proliferative response to Hedgehog signalling

The Hedgehog (Hh) pathway plays dual roles in proliferation and patterning during embryonic development, but the mechanism(s) that distinguish the mitogenic and patterning activities of Hh signalling are not fully understood. An additional level of complexity is provided by the observation that Hh signalling can both promote and inhibit cell proliferation. One model to account for this apparent paradox is that Hh signalling primarily regulates cell cycle kinetics, such that activation of Hh signalling promotes fast cycling and an earlier cell cycle exit. Here we report that activation of Hh signalling promotes endodermal cell proliferation but inhibits proliferation in neighbouring non-endodermal cells, suggesting that the cell cycle kinetics model is insufficient to account for the opposing proliferative responses to Hh signalling. We show that expression of the chemokine receptor Cxcr4a is a critical parameter that determines the proliferative response to Hh signalling, and that loss of Cxcr4a function attenuates the transcription of cell cycle regulator targets of Hh signalling without affecting general transcriptional targets. We show that Cxcr4a inhibits PKA activity independently of Hh signalling, and propose that Cxcr4a enhances Hh-dependent proliferation by promoting the activity of Gli1. Our results indicate that Cxcr4a is required for Hh-dependent cell proliferation but not for Hh-dependent patterning, and suggest that the parallel activation of Cxcr4a is required to modulate the Hh pathway to distinguish between patterning and proliferation.

Compartmentalization of vertebrate optic neuroephithelium: external cues and transcription factors

The vertebrate eye is a laterally extended structure of the forebrain. It develops through a series of events, including specification and regionalization of the anterior neural plate, evagination of the optic vesicle (OV), and development of three distinct optic structures: the neural retina (NR), optic stalk (OS), and retinal pigment epithelium (RPE). Various external signals that act on the optic neuroepithelium in a spatial- and temporal-specific manner control the fates of OV subdomains by inducing localized expression of key transcription factors. Investigating the mechanisms underlying compartmentalization of these distinct optic neuroepithelium-derived tissues is therefore not only important from the standpoint of accounting for vertebrate eye morphogenesis, it is also helpful for understanding the fundamental basis of fate determination of other neuroectoderm- derived tissues. This review focuses on the molecular signatures of OV subdomains and the external factors that direct the development of tissues originating from the OV.

Transcriptional mechanisms of developmental cell cycle arrest: problems and models

Metazoans begin their life as a single cell. Then, this cell enters a more or less protracted period of active cell proliferation, which can be considered as the default cellular state. A crucial event, the developmental cell cycle exit, occurs thereafter. This phenomenon allows for differentiation to happen and regulates the final size of organs and organisms. Its control is still poorly understood. Herein, we review some transcriptional mechanisms of cell cycle exit in animals, and propose to use cellular conveyor belts as model systems for its study. We finally point to evidence that suggests that the mechanisms of developmental cell cycle arrest may have to be maintained in adult tissues.

Ascl1-induced neuronal differentiation of P19 cells requires expression of a specific inhibitor protein of cyclic AMP-dependent protein kinase

cAMP-dependent protein kinase (PKA) plays a critical role in nervous system development by modulating sonic hedgehog and bone morphogenetic protein signaling. In the current studies, P19 embryonic carcinoma cells were neuronally differentiated by expression of the proneural basic helix-loop-helix transcription factor Ascl1. After expression of Ascl1, but prior to expression of neuronal markers such as microtubule associated protein 2 and neuronal β-tubulin, P19 cells demonstrated a large, transient increase in both mRNA and protein for the endogenous protein kinase inhibitor (PKI)β. PKIβ-targeted shRNA constructs both reduced the levels of PKIβ expression and blocked the neuronal differentiation of P19 cells. This inhibition of differentiation was rescued by transfection of a shRNA-resistant expression vector for the PKIβ protein, and this rescue required the PKA-specific inhibitory sequence of the PKIβ protein. PKIβ played a very specific role in the Ascl1-mediated differentiation process as other PKI isoforms were unable to rescue the deficit conferred by shRNA-mediated knockdown of PKIβ. Our results define a novel requirement for PKIβ and its inhibition of PKA during neuronal differentiation of P19 cells.

The Wnt signaling pathway protects retinal ganglion cell 5 (RGC-5) cells from elevated pressure

The Wnt pathway is an essential signaling cascade that regulates survival and differentiation in the retina. We recently demonstrated that retinal ganglion cells (RGCs) have constitutively active Wnt signaling in vivo. However, the role of Wnt in RGC viability or function is unknown. In this study, we investigated whether Wnt protects the retinal ganglion cell line RGC-5 from elevated pressure, oxidative stress, and hypoxia injuries. Expression of RGC marker genes in the RGC-5 cultures was confirmed by immunocytochemistry and PCR. We demonstrated that the Wnt3a ligand significantly reduced pressure-induced caspase activity in RGC-5 cells (n = 5, P = 0.03) and decreased the number of TUNEL-positive cells (n = 5, P = 0.0014). Notably, Wnt3a-dependent protection was reversed by the Wnt signaling inhibitor Dkk1. In contrast, Wnt3a did not protect RGC-5 cells from oxidative stress or hypoxia. Furthermore, Wnt3a significantly increased growth factor expression in the presence of elevated pressure but not in the presence of oxidative stress and hypoxia. These results indicate that Wnt3a induces injury-specific survival pathways in RGC-5 cells, potentially by upregulating neuroprotective growth factors. Therefore, activation of the Wnt pathway by Wnt3a could be investigated further as a tool to develop novel molecular therapeutic strategies for the prevention of RGC death in retinal disease.

Core fucosylation is required for midline patterning during zebrafish development

Complex carbohydrates represent one of the most polymorphic classes of macromolecules, but their functions during embryonic development remain poorly defined. Herein, we show that knockdown of FucT8, the fucosyltransferase responsible for adding an α1,6 fucosyl residue to the core region of N-linked oligosaccharides, results in defective midline patterning during zebrafish development. Reduced FucT8 expression leads to mild cyclopia, small forebrains, U-shaped somites, among other midline patterning defects. One of the principal FucT8 substrates was identified as Apolipoprotein B (ApoB), the major scaffold protein that is responsible for assembly and secretion of lipoprotein particles in vertebrates. In Drosophila, lipoprotein particles are thought to facilitate cell signaling by serving as a transport vehicle for lipid-modified cell signaling proteins, such as hedgehog. In this regard, knockdown of ApoB expression in zebrafish embryos leads to similar midline patterning defects as those seen in FucT8 morphant embryos. Furthermore, preliminary studies suggest that ApoB facilitates Sonic hedgehog signaling during zebrafish development, analogous to the function of lipoprotein particles during hedgehog signaling in Drosophila.

Genetics of photoreceptor degeneration and regeneration in zebrafish

Zebrafish are unique in that they provide a useful model system for studying two critically important problems in retinal neurobiology, the mechanisms responsible for triggering photoreceptor cell death and the innate stem cell–mediated regenerative response elicited by this death. In this review we highlight recent seminal findings in these two fields. We first focus on zebrafish as a model for studying photoreceptor degeneration. We summarize the genes currently known to cause photoreceptor degeneration, and we describe the phenotype of a few zebrafish mutants in detail, highlighting the usefulness of this model for studying this process. In the second section, we discuss the several different experimental paradigms that are available to study regeneration in the teleost retina. A model outlining the sequence of gene expression starting from the dedifferentiation of Müller glia to the formation of rod and cone precursors is presented.

In the absence of Sonic hedgehog, p53 induces apoptosis and inhibits retinal cell proliferation, cell-cycle exit and differentiation in zebrafish

Background

Sonic hedgehog (Shh) signaling regulates cell proliferation during vertebrate development via induction of cell-cycle regulator gene expression or activation of other signalling pathways, prevents cell death by an as yet unclear mechanism and is required for differentiation of retinal cell types. Thus, an unsolved question is how the same signalling molecule can regulate such distinct cell processes as proliferation, cell survival and differentiation.

Methodology/Principal Findings

Analysis of the zebrafish shh^−/− mutant revealed that in this context p53 mediates elevated apoptosis during nervous system and retina development and interferes with retinal proliferation and differentiation. While in shh^−/− mutants there is activation of p53 target genes and p53-mediated apoptosis, an increase in Hedgehog (Hh) signalling by over-expression of dominant-negative Protein Kinase A strongly decreased p53 target gene expression and apoptosis levels in shh^−/− mutants. Using a novel p53 reporter transgene, I confirm that p53 is active in tissues that require Shh for cell survival. Proliferation assays revealed that loss of p53 can rescue normal cell-cycle exit and the mitotic indices in the shh^−/− mutant retina at 24, 36 and 48 hpf. Moreover, generation of amacrine cells and photoreceptors was strongly enhanced in the double p53^−/−shh^−/− mutant retina suggesting the effect of p53 on retinal differentiation.

Conclusions

Loss of Shh signalling leads to the p53-dependent apoptosis in the developing nervous system and retina. Moreover, Shh-mediated control of p53 activity is required for proliferation and cell cycle exit of retinal cells as well as differentiation of amacrine cells and photoreceptors.

Loss of adenomatous polyposis coli (apc) results in an expanded ciliary marginal zone in the zebrafish eye

The distal region of neural retina (ciliary marginal zone [CMZ]) contains stem cells that produce non-neural and neuronal progenitors. We provide a detailed gene expression analysis of the eyes of apc mutant zebrafish where the Wnt/beta-catenin pathway is constitutively active. Wnt/beta-catenin signaling leads to an expansion of the CMZ accompanied by a central shift of the retinal identity gene sox2 and the proneural gene atoh7. This suggests an important role for peripheral Wnt/beta-catenin signaling in regulating the expression and localization of neurogenic genes in the central retina. Retinal identity genes rx1 and vsx2, as well as meis1 and pax6a act upstream of Wnt/beta-catenin pathway activation. Peripheral cells that likely contain stem cells can be identified by the expression of follistatin, otx1, and axin2 and the lack of expression of myca and cyclinD1. Our results introduce the zebrafish apc mutation as a new model to study signaling pathways regulating the CMZ.

Thermodynamic analysis of protein kinase A Ialpha activation

Thermodynamic analysis of protein kinase A (PKA) Ialpha activation was performed using Quantum 3.3.0 docking software and a Gaussian 03W quantum mechanical computational package. Expected stacking interactions between adenine of 3':5'-AMP and aromatic moieties of amino acids were taken into account by means of MP2/6-31G(d) IPCM (isodensity polarizable continuum model) computations (epsilon = 4.0). It is demonstrated that thermodynamically favorable agonist-induced PKA Ialpha activation is mediated by two processes. First, 3':5'-AMP binding is accompanied by structural changes leading to a thermodynamically favorable regulatory subunit conformation, which is hardly realized in the absence of the ligand (DeltaG degrees (R) = -23.9 +/- 8.2 kJ/mol). Second, 3':5'-AMP affinity to the regulatory subunit conformation observed after agonist-induced PKA Ialpha activation is higher than that to inactive holoenzyme complex (DeltaG degrees (3':5'-AMP) = -28.1 +/- 9.7 kJ/mol). ATP is capable of docking into the 3':5'-AMP-binding site B of the regulatory subunit complexed with the catalytic one, resulting in inhibition of kinase activation. True constants of 3':5'-AMP binding to PKA Ialpha holoenzyme were found to be 60 and 57 microM for the regulatory subunit domains A and B, respectively. These constants, unlike the binding equilibrium constant determined using established experimental techniques and ranging from 15 nM to 2.9 microM, are proved to be direct measures of 3':5'-AMP-PKA Ialpha binding affinity. Their values are in a reasonable agreement with the changes in 3':5'-AMP concentration in the cell (2-55 microM) and account for PKA Ialpha activation in response to adequate stimuli.

Activated T-cells inhibit neurogenesis by releasing granzyme B: rescue by Kv1.3 blockers

There is a great need for pharmacological approaches to enhance neural progenitor cell (NPC) function particularly in neuroinflammatory diseases with failed neuroregeneration. In diseases such as multiple sclerosis and stroke, T-cell infiltration occurs in periventricular zones where NPCs are located and is associated with irreversible neuronal loss. We studied the effect of T-cell activation on NPC functions. NPC proliferation and neuronal differentiation were impaired by granzyme B (GrB) released by the T-cells. GrB mediated its effects by the activation of a Gi-protein-coupled receptor leading to decreased intracellular levels of cAMP and subsequent expression of the voltage-dependent potassium channel, Kv1.3. Importantly, blocking channel activity with margatoxin or blocking its expression reversed the inhibitory effects of GrB on NPCs. We have thus identified a novel pathway in neurogenesis. The increased expression of Kv1.3 in pathological conditions makes it a novel target for promoting neurorestoration.

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.

Progenitor cell maturation in the developing vertebrate retina

Progenitor cells in the developing retina initially divide so that each division produces two cells that divide again. Subsequently, progenitor cells change their mode of division so that one or both cells produced by a division can withdraw from the mitotic cycle and differentiate. We asked how these two progenitor cell stages differ molecularly and what controls the switch in the mode of division. We show that early preneurogenic progenitor cells express the transcription factor, Sox2, and the Notch ligand, Delta1. More mature neurogenic progenitor cells express Sox2 and the bHLH transcription factor, E2A, and not Delta1. Notch signaling maintains progenitor cells in the preneurogenic state. Sonic hedgehog expressed by newly differentiating cells initiates maturation of progenitor cells from preneurogenic to neurogenic at the neurogenic front, possibly by down-regulating Delta1 expression. Our results show that the preneurogenic-to-neurogenic transition is a highly organized unidirectional step made in unison by neighboring cells.

Neurog2 controls the leading edge of neurogenesis in the mammalian retina

In the mammalian retina, neuronal differentiation begins in the dorso-central optic cup and sweeps peripherally and ventrally. While certain extrinsic factors have been implicated, little is known about the intrinsic factors that direct this process. In this study, we evaluate the expression and function of proneural bHLH transcription factors during the onset of mouse retinal neurogenesis. Dorso-central retinal progenitor cells that give rise to the first postmitotic neurons express Neurog2/Ngn2 and Atoh7/Math5. In the absence of Neurog2, the spread of neurogenesis stalls, along with Atoh7 expression and RGC differentiation. However, neurogenesis is eventually restored, and at birth Neurog2 mutant retinas are reduced in size, with only a slight increase in the retinal ganglion cell population. We find that the re-establishment of neurogenesis coincides with the onset of Ascl1 expression, and that Ascl1 can rescue the early arrest of neural development in the absence of Neurog2. Together, this study supports the hypothesis that the intrinsic factors Neurog2 and Ascl1 regulate the temporal progression of retinal neurogenesis by directing overlapping waves of neuron formation.

Analyzing retinal axon guidance in zebrafish

How neuronal connections are established during development is one of the most fascinating questions in the field of neurobiology. The zebrafish retinotectal system offers distinct advantages for studying axon guidance in an in vivo context. Its accessibility and the larva's transparency not only allow its direct visualization, but also facilitate experimental manipulations to address the mechanisms of its development. Here we describe methods for labeling and visualizing retinal axons in vivo, including transient expression of DNA constructs, injection of lipophilic dyes, and time-lapse imaging. We describe in detail the available transgenic lines for marking retinal ganglion cells (RGCs); a protocol for very precise lipophilic dye labeling; and a protocol for single cell electroporation of RGCs. We then describe several approaches for perturbing the retinotectal system, including morpholino or DNA injection; localized heat shock to induce misexpression of genes; a comprehensive list of known retinotectal mutants; and a detailed protocol for RGC transplants to test cell autonomy. These methods not only provide new ways for examining how retinal axons are guided by their environment, but also can be used to study other axonal tracts in the living embryo.

Analysis of the retina in the zebrafish model

The zebrafish is one of the leading models for the analysis of the vertebrate visual system. A wide assortment of molecular, genetic, and cell biological approaches is available to study zebrafish visual system development and function. As new techniques become available, genetic analysis and imaging continue to be the strengths of the zebrafish model. In particular, recent developments in the use of transposons and zinc finger nucleases to produce new generations of mutant strains enhance both forward and reverse genetic analysis. Similarly, the imaging of developmental and physiological processes benefits from a wide assortment of fluorescent proteins and the ways to express them in the embryo. The zebrafish is also highly attractive for high-throughput screening of small molecules, a promising strategy to search for compounds with therapeutic potential. Here we discuss experimental approaches used in the zebrafish model to study morphogenetic transformations, cell fate decisions, and the differentiation of fine morphological features that ultimately lead to the formation of the functional vertebrate visual system.

Expanded progenitor populations, vitreo-retinal abnormalities, and Müller glial reactivity in the zebrafish leprechaun/patched2 retina

Background

The roles of the Hedgehog (Hh) pathway in controlling vertebrate retinal development have been studied extensively; however, species- and context-dependent findings have provided differing conclusions. Hh signaling has been shown to control both population size and cell cycle kinetics of proliferating retinal progenitors, and to modulate differentiation within the retina by regulating the timing of cell cycle exit. While cell cycle exit has in turn been shown to control cell fate decisions within the retina, a direct role for the Hh pathway in retinal cell fate decisions has yet to be established in vivo.

Results

To gain further insight into Hh pathway function in the retina, we have analyzed retinal development in leprechaun/patched2 mutant zebrafish. While lep/ptc2 mutants possessed more cells in their retinas, all cell types, except for Müller glia, were present at identical ratios as those observed in wild-type siblings. lep/ptc2 mutants possessed a localized upregulation of GFAP, a marker for 'reactive' glia, as well as morphological abnormalities at the vitreo-retinal interface, where Müller glial endfeet terminate. In addition, analysis of the over-proliferation phenotype at the ciliary marginal zone (CMZ) revealed that the number of proliferating progenitors, but not the rate of proliferation, was increased in lep/ptc2 mutants.

Conclusion

Our results indicate that Patched2-dependent Hh signaling does not likely play an integral role in neuronal cell fate decisions in the zebrafish retina. ptc2 deficiency in zebrafish results in defects at the vitreo-retinal interface and Müller glial reactivity. These phenotypes are similar to the ocular abnormalities observed in human patients suffering from Basal Cell Naevus Syndrome (BCNS), a disorder that has been linked to mutations in the human PTCH gene (the orthologue of the zebrafish ptc2), and point to the utility of the lep/ptc2 mutant line as a model for the study of BCNS-related ocular pathologies. Our findings regarding CMZ progenitor proliferation suggest that, in the zebrafish retina, Hh pathway activity may not affect cell cycle kinetics; rather, it likely regulates the size of the retinal progenitor pool in the CMZ.