Regulation by glycogen synthase kinase-3beta of the arborization field and maturation of retinotectal projection in zebrafish

Microtubule-binding protein MAP1B regulates interstitial axon branching of cortical neurons via the tubulin tyrosination cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial (or collateral) axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs). This method allows for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. These data suggest a cell-autonomous molecular regulation of cortical neuron axon morphology, in which GSK3β can release a MAP1B-mediated brake on interstitial axon branching upstream of the posttranslational tubulin code.

MAP1B Regulates Cortical Neuron Interstitial Axon Branching Through the Tubulin Tyrosination Cycle

Regulation of directed axon guidance and branching during development is essential for the generation of neuronal networks. However, the molecular mechanisms that underlie interstitial axon branching in the mammalian brain remain unresolved. Here, we investigate interstitial axon branching in vivo using an approach for precise labeling of layer 2/3 callosal projection neurons (CPNs), allowing for quantitative analysis of axonal morphology at high acuity and also manipulation of gene expression in well-defined temporal windows. We find that the GSK3β serine/threonine kinase promotes interstitial axon branching in layer 2/3 CPNs by releasing MAP1B-mediated inhibition of axon branching. Further, we find that the tubulin tyrosination cycle is a key downstream component of GSK3β/MAP1B signaling. We propose that MAP1B functions as a brake on axon branching that can be released by GSK3β activation, regulating the tubulin code and thereby playing an integral role in sculpting cortical neuron axon morphology.

β-Catenin-Independent Roles of Wnt/LRP6 Signaling

Wnt/LRP6 signaling is best known for the β-catenin-dependent regulation of target genes. However, pathway branches have recently emerged, including Wnt/STOP signaling, which act independently of β-catenin and transcription. We review here the molecular mechanisms underlying β-catenin-independent Wnt/LRP6 signaling cascades and their implications for cell biology, development, and physiology.

Optic nerve input-dependent regulation of neural stem cell proliferation in the optic tectum of adult zebrafish

Adult neurogenesis attracts broad attention as a possible cure for neurological disorders. However, its regulatory mechanism is still unclear. Therefore, they have been studying the cell proliferation mechanisms of neural stem cells (NSCs) using zebrafish, which have high regenerative potential in the adult brain. The presence of neuroepithelial-type NSCs in the optic tectum of adult zebrafish has been previously reported. In the present study, it was first confirmed that NSCs in the optic tectum decrease or increase in proportion to projection of the optic nerves from the retina. At 4 days after optic nerve crush (ONC), BrdU-positive cells decreased in the optic tectum's operation side. In contrast, at 3 weeks after ONC, BrdU-positive cells increased in the optic tectum's operation side. To study the regulatory mechanisms, they focused on the BDNF/TrkB system as a regulatory factor in the ONC model. It was found that bdnf was mainly expressed in the periventricular gray zone (PGZ) of the optic tectum by using in situ hybridization. Interestingly, expression level of bdnf significantly decreased in the optic tectum at 4 days after ONC, and its expression level tended to increase at 3 weeks after ONC. They conducted rescue experiments using a TrkB agonist and confirmed that decrease of NSC proliferation in the optic tectum by ONC was rescued by TrkB signal activation, suggesting stimuli-dependent regulation of NSC proliferation in the optic tectum of adult zebrafish. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

Neurogenesis of retinal ganglion cells is not essential to visual functional recovery after optic nerve injury in adult zebrafish

Zebrafish central nervous system (CNS) possesses a strong neural regeneration ability to restore visual function completely after optic nerve injury (ONI). However, whether neurogenesis of retinal ganglion cell (RGC) contributes to functional recovery remains controversial. Our quantitative analysis of RGCs in different ONI models showed that almost all RGCs survived in optic nerve crush (ONC) model; while over 90% of RGCs survived in the first 2 weeks with 75% remaining after 7 weeks in optic nerve transection (ONT) model. Retrograde labeling from tectum revealed a surprising regeneration rate, with over 90% and over 50% of RGCs regrowing axons to tectum at the first week in ONC and ONT model respectively. In the latter one, the number of regenerative RGCs after 4 weeks had no significant difference from the control group. As for neurogenesis, newborn RGCs were rarely detected either by double retrograde labeling or BrdU marker. Since few RGCs died, microglia number showed a temporary increase at 3 days post injury (dpi) and a decrease at 14 dpi. Finally, myelin structure within retina kept integrity and optomotor response (OMR) test demonstrated visual functional restoration at 5 weeks post injury (wpi). In conclusion, our results have directly shown that RGC survival and axon regrowth are responsible for functional recovery after ONI in adult zebrafish.

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.

Rewiring the retinal ganglion cell gene regulatory network: Neurod1 promotes retinal ganglion cell fate in the absence of Math5

Retinal progenitor cells (RPCs) express basic helix-loop-helix (bHLH)factors in a strikingly mosaic spatiotemporal pattern, which is thought to contribute to the establishment of individual retinal cell identity. Here, we ask whether this tightly regulated pattern is essential for the orderly differentiation of the early retinal cell types and whether different bHLH genes have distinct functions that are adapted for each RPC. To address these issues, we replaced one bHLH gene with another. Math5 is a bHLH gene that is essential for establishing retinal ganglion cell (RGC) fate. We analyzed the retinas of mice in which Math5 was replaced with Neurod1 or Math3, bHLH genes that are expressed in another RPC and are required to establish amacrine cell fate. In the absence of Math5, Math5Neurod1-KI was able to specify RGCs, activate RGC genes and restore the optic nerve, although not as effectively as Math5. By contrast, Math5Math3-KI was much less effective than Math5Neurod1-KI in replacing Math5. In addition, expression of Neurod1 and Math3 from the Math5Neurod1-KI/Math3-KIallele did not result in enhanced amacrine cell production. These results were unexpected because they indicated that bHLH genes, which are currently thought to have evolved highly specialized functions, are nonetheless able to adjust their functions by interpreting the local positional information that is programmed into the RPC lineages. We conclude that, although Neurod1 and Math3 have evolved specialized functions for establishing amacrine cell fate, they are nevertheless capable of alternative functions when expressed in foreign environments.

Wnt signaling in neural circuit assembly

The Wnt family of secreted proteins plays a crucial role in nervous system wiring. Wnts regulate neuronal positioning, polarization, axon and dendrite development, and synaptogenesis. These diverse roles of Wnt proteins are due not only to the large numbers of Wnt ligands and receptors but also to their ability to signal through distinct signaling pathways in different cell types and developmental contexts. Studies on Wnts have shed new light on novel molecular mechanisms that control the development of complex neuronal connections. This review discusses recent advances on how Wnt signaling influences different aspects of neuronal circuit assembly through changes in gene expression and/or cytoskeletal modulation.

Wnt signaling mediates diverse developmental processes in zebrafish

A combination of forward and reverse genetic approaches in zebrafish has revealed novel roles for canonical Wnt and Wnt/PCP signaling during vertebrate development. Forward genetics in zebrafish provides an exceptionally powerful tool to assign roles in vertebrate developmental processes to novel genes, as well as elucidating novel roles played by known genes. This has indeed turned out to be the case for components of the canonical Wnt signaling pathway. Non-canonical Wnt signaling in the zebrafish is also currently a topic of great interest, due to the identified roles of this pathway in processes requiring the integration of cell polarity and cell movement, such as the directed migration movements that drive the narrowing and lengthening (convergence and extension) of the embryo during early development.

Slit1a inhibits retinal ganglion cell arborization and synaptogenesis via Robo2-dependent and -independent pathways

Upon arriving at their targets, developing axons cease pathfinding and begin instead to arborize and form synapses. To test whether CNS arborization and synaptogenesis are controlled by Slit-Robo signaling, we followed single retinal ganglion cell (RGC) arbors over time. ast (robo2) mutant and slit1a morphant arbors had more branch tips and greater arbor area and complexity compared to wild-type and concomitantly more presumptive presynaptic sites labeled with YFP-Rab3. Increased arborization in ast was phenocopied by dominant-negative Robo2 expressed in single RGCs and rescued by full-length Robo2, indicating that Robo2 acts cell-autonomously. Time-lapse imaging revealed that ast and slit1a morphant arbors stabilized earlier than wild-type, suggesting a role for Slit-Robo signaling in preventing arbor maturation. Genetic analysis showed that Slit1a acts both through Robo2 and Robo2-independent mechanisms. Unlike previous PNS studies showing that Slits promote branching, our results show that Slits inhibit arborization and synaptogenesis in the CNS.

Vesicular glutamate transport at a central synapse limits the acuity of visual perception in zebrafish

The neural circuitry that constrains visual acuity in the CNS has not been experimentally identified. We show here that zebrafish blumenkohl (blu) mutants are impaired in resolving rapid movements and fine spatial detail. The blu gene encodes a vesicular glutamate transporter expressed by retinal ganglion cells. Mutant retinotectal synapses release less glutamate, per vesicle and per terminal, and fatigue more quickly than wild-type in response to high-frequency stimulation. In addition, mutant axons arborize more extensively, thus increasing the number of synaptic terminals and effectively normalizing the combined input to postsynaptic cells in the tectum. This presumably homeostatic response results in larger receptive fields of tectal cells and a degradation of the retinotopic map. As predicted, mutants have a selective deficit in the capture of small prey objects, a behavior dependent on the tectum. Our studies successfully link the disruption of a synaptic protein to complex changes in neural circuitry and behavior.

GSK-3 is activated by the tyrosine kinase Pyk2 during LPA1-mediated neurite retraction

Glycogen synthase kinase-3 (GSK-3) is a multifunctional serine/threonine kinase that is usually inactivated by serine phosphorylation in response to extracellular cues. However, GSK-3 can also be activated by tyrosine phosphorylation, but little is known about the upstream signaling events and tyrosine kinase(s) involved. Here we describe a G protein signaling pathway leading to GSK-3 activation during lysophosphatidic acid (LPA)-induced neurite retraction. Using neuronal cells expressing the LPA(1) receptor, we show that LPA(1) mediates tyrosine phosphorylation and activation of GSK-3 with subsequent phosphorylation of the microtubule-associated protein tau via the G(i)-linked PIP(2) hydrolysis-Ca(2+) mobilization pathway. LPA concomitantly activates the Ca(2+)-dependent tyrosine kinase Pyk2, which is detected in a complex with GSK-3beta. Inactivation or knockdown of Pyk2 inhibits LPA-induced (but not basal) tyrosine phosphorylation of GSK-3 and partially inhibits LPA-induced neurite retraction, similar to what is observed following GSK-3 inhibition. Thus, Pyk2 mediates LPA(1)-induced activation of GSK-3 and subsequent phosphorylation of microtubule-associated proteins. Pyk2-mediated GSK-3 activation is initiated by PIP(2) hydrolysis and may serve to destabilize microtubules during actomyosin-driven neurite retraction.

Expression of zebrafish glutamate receptor δ2 in neurons with cerebellum-like wiring

Mammalian glutamate receptor (GluR) delta2 is selectively expressed in cerebellar Purkinje cells and plays key roles in cerebellar plasticity, motor learning, and neural wiring. Here, we isolated cDNA encoding the zebrafish ortholog of mammalian GluRdelta2. We found that in adult zebrafish brain, glurdelta2 mRNA was expressed not only in cerebellar Purkinje cells, but also in the crest cells of the medial octavolateral nucleus (MON) and the type I neurons of the optic tectum. Immunohistochemical analysis revealed that zebrafish GluRdelta2 proteins were selectively localized in the apical dendrites of these neurons. Interestingly, the crest cells of the MON and the type I neurons of the optic tectum receive large numbers of parallel fiber inputs at the apical dendrites and sensory inputs at the proximal or basal dendrites. These results suggest that the expression of zebrafish GluRdelta2 is selective for cerebellum-like neural wiring with large numbers of parallel fiber inputs.

A novel basic helix–loop–helix (bHLH) transcriptional repressor, NeuroAB, expressed in bipolar and amacrine cells in the chick retina

Basic helix-loop-helix (bHLH) transcription factors are implicated in cell fate determination and differentiation in neurogenesis. We identified a novel chick bHLH transcription factor, NeuroAB. A phylogenetic tree prepared from bHLH sequences suggested that NeuroAB belongs to the BETA3 group in the Atonal-related protein family (ARPs). In situ hybridization and immunostaining indicated that NeuroAB is expressed predominantly in postmitotic bipolar cells and GABAergic amacrine cells in the retina. Reporter and DNA pull down assays indicated that NeuroAB functions as a transcriptional repressor by binding to the E-box sequence, and its activity is modulated by phosphorylation at a specific serine residue that fits the consensus phosphorylation site for glycogen synthase kinase 3beta (GSK3beta). Since members of the BETA3 group possess this consensus site, it is suggested that their activities are commonly regulated by GSK3beta or other kinases bearing the same substrate specificity. We found that the expression of GSK3beta is spatially and temporally regulated in the developing retina; its strong expression was observed in ganglion cells from E8 and a subset of amacrine cells from E12. These findings suggest that NeuroAB is involved in the maturation and maintenance of bipolar cells and GABAergic amacrine cells and regulation by GSK3beta plays an important role in retinogenesis.

Identification of chaperonin CCT gamma subunit as a determinant of retinotectal development by whole-genome subtraction cloning from zebrafish no tectal neuron mutant

Zebrafish no tectal neuron (ntn) mutant obtained by trimethylpsoralen (TMP) mutagenesis showed defects in tectal neuropil formation and small eyes. We carried out whole-genome subtraction between wild-type and mutant zebrafish embryos using the representational difference analysis (RDA) method. Nineteen subtraction products enabled us to construct genetic and physical maps of the ntn region. Direct selection of cDNAs using a YAC clone encompassing the ntn locus and RT-PCR analysis of transcripts identified a 143 bp deletion in the cct3 gene encoding the gamma subunit of chaperonin containing TCP-1 (CCT). Injection of antisense cct3 morpholino oligonucleotides into zebrafish embryos induced characteristic ntn phenotypes including defects in retinal ganglion cell (RGC) differentiation and tectal neuropil formation. Moreover, injection of cct3 mRNA successfully rescued ntn mutant embryos. Our results suggest that RDA is an efficient and widely applicable cloning strategy in zebrafish genetics. The strong expression of the cct3 mRNA started in the entire embryos by 12 hpf and was sustained thereafter, but there were no detectable abnormalities in body patterning and neurogenesis in ntn mutant embryos at 30 hpf. The expression patterns of transcription factor genes ath5 and brn3b that are essential for the development and maintenance of RGCs were indistinguishable between wild-type and ntn mutant embryos, but those of early and late differentiation markers of RGCs, nicotinic acetylcholine receptor beta 3 and zn5, were diminished in mutant embryos. Immunostaining of acetylated tubulin also revealed the impairment of RGC neurite extension. Thus, the ntn mutation of the cct3 gene impaired the differentiation of retinal neuroepithelial cells to RGCs. Similarly, the expression of brn3b was normal in the tectum of ntn mutants, but tectal neuropil formation was abolished. These results suggest that the gamma subunit of chaperonin CCT plays an essential role in retinotectal development.

Seeing More Clearly: Recent Advances in Understanding Retinal Circuitry

Among 10 breakthroughs that Science announced at the end of 2002 was the discovery of a photosensing (melanopsin-containing) retinal ganglion cell (RGC) and its role in entraining the circadian clock. This breakthrough exemplifies the ultimate goal of neuroscience: to understand the nervous system from molecules to behavior. Light-sensing RGCs constitute one of a dozen discrete RGC populations coding various aspects of visual scenes by virtue of their unique morphology, physiology, and coverage of the retina. Interestingly, the function of the melanopsin-containing RGCs in entraining the circadian clock need not involve much retinal processing, making it the simplest form of processing in the retina. This review focuses on recent advances in our understanding of retinal circuitry, visual processing, and retinal development demonstrated by innovative experimental techniques. It also discusses the advantages of using the retina as a model system to address some of the key questions in neuroscience.

Mechanisms of Synapse Assembly and Disassembly

The mechanisms that govern synapse formation and elimination are fundamental to our understanding of neural development and plasticity. The wiring of neural circuitry requires that vast numbers of synapses be formed in a relatively short time. The subsequent refinement of neural circuitry involves the formation of additional synapses coincident with the disassembly of previously functional synapses. There is increasing evidence that activity-dependent plasticity also involves the formation and disassembly of synapses. While we are gaining insight into the mechanisms of both synapse assembly and disassembly, we understand very little about how these phenomena are related to each other and how they might be coordinately controlled to achieve the precise patterns of synaptic connectivity in the nervous system. Here, we review our current understanding of both synapse assembly and disassembly in an effort to unravel the relationship between these fundamental developmental processes.