Development of spinal neurons and tracts in the zebrafish embryo

Molecular regulation of axon termination in mechanosensory neurons

Spatially and temporally accurate termination of axon outgrowth, a process called axon termination, is required for efficient, precise nervous system construction and wiring. The mechanosensory neurons that sense low-threshold mechanical stimulation or gentle touch have proven exceptionally valuable for studying axon termination over the past 40 years. In this Review, we discuss progress made in deciphering the molecular and genetic mechanisms that govern axon termination in touch receptor neurons. Findings across model organisms, including Caenorhabditis elegans, Drosophila, zebrafish and mice, have revealed that complex signaling is required for termination with conserved principles and players beginning to surface. A key emerging theme is that axon termination is mediated by complex signaling networks that include ubiquitin ligase signaling hubs, kinase cascades, transcription factors, guidance/adhesion receptors and growth factors. Here, we begin a discussion about how these signaling networks could represent termination codes that trigger cessation of axon outgrowth in different species and types of mechanosensory neurons.

Electrical Synapses Mediate Embryonic Hyperactivity in a Zebrafish Model of Fragile X Syndrome

Although hyperactivity is associated with a wide variety of neurodevelopmental disorders, the early embryonic origins of locomotion have hindered investigation of pathogenesis of these debilitating behaviors. The earliest motor output in vertebrate animals is generated by clusters of early-born motor neurons (MNs) that occupy distinct regions of the spinal cord, innervating stereotyped muscle groups. Gap junction electrical synapses drive early spontaneous behavior in zebrafish, prior to the emergence of chemical neurotransmitter networks. We use a genetic model of hyperactivity to gain critical insight into the consequences of errors in motor circuit formation and function, finding that Fragile X syndrome model mutant zebrafish are hyperexcitable from the earliest phases of spontaneous behavior, show altered sensitivity to blockade of electrical gap junctions, and have increased expression of the gap junction protein Connexin 34/35. We further show that this hyperexcitable behavior can be rescued by pharmacological inhibition of electrical synapses. We also use functional imaging to examine MN and interneuron (IN) activity in early embryogenesis, finding genetic disruption of electrical gap junctions uncouples activity between mnx1 + MNs and INs. Taken together, our work highlights the importance of electrical synapses in motor development and suggests that the origins of hyperactivity in neurodevelopmental disorders may be established during the initial formation of locomotive circuits.

Polystyrene nanoplastics induced size-dependent developmental and neurobehavioral toxicities in embryonic and juvenile zebrafish

Because of widespread environmental contamination, there is growing concern that nanoplastics may pose a risk to humans and the environment. Due to their small particle size, nanoplastics may cross the blood-nerve barrier and distribute within the nervous system. The present study systematically investigated the uptake/distribution and developmental/neurobehavioral toxicities of different sizes (80, 200, and 500 nm) of polystyrene nanoplastics (PS) in embryonic and juvenile zebrafish. The results indicate that all three sizes of PS could cross the chorion, adsorb by the yolk, and distribute into the intestinal tract, eye, brain, and dorsal trunk of zebrafish, but with different patterns. The organ distribution and observed developmental and neurobehavioral effects varied as a function of PS size. Although all PS exposures induced cell death and inflammation at the cellular level, only exposures to the larger PS resulted in oxidative stress. Meanwhile, exposure to the 80 nm PS increased the expression of neural and optical-specific mRNAs. Collectively, these studies indicate that early life-stage exposures to PS adversely affect zebrafish neurodevelopment and that the observed toxicities are influenced by particle size.

Studying zebrafish nervous system structure and function in health and disease with electron microscopy

Zebrafish (Danio rerio) is a well-established model for studying the nervous system. Findings in zebrafish often inform studies on human diseases of the nervous system and provide crucial insight into disease mechanisms. The functions of the nervous system often rely on communication between neurons. Signal transduction is achieved via release of signaling molecules in the form of neuropeptides or neurotransmitters at synapses. Snapshots of membrane dynamics of these processes are imaged by electron microscopy. Electron microscopy can reveal ultrastructure and thus synaptic processes. This is crucial both for mapping synaptic connections and for investigating synaptic functions. In addition, via volumetric electron microscopy, the overall architecture of the nervous system becomes accessible, where structure can inform function. Electron microscopy is thus of particular value for studying the nervous system. However, today a plethora of electron microscopy techniques and protocols exist. Which technique is most suitable highly depends on the research question and scope as well as on the type of tissue that is examined. This review gives an overview of the electron microcopy techniques used on the zebrafish nervous system. It aims to give researchers a guide on which techniques are suitable for their specific questions and capabilities as well as an overview of the capabilities of electron microscopy in neurobiological research in the zebrafish model.

Fmrp regulates neuronal balance in embryonic motor circuit formation

Motor behavior requires the balanced production and integration of a variety of neural cell types. Motor neurons are positioned in discrete locations in the spinal cord, targeting specific muscles to drive locomotive contractions. Specialized spinal interneurons modulate and synchronize motor neuron activity to achieve coordinated motor output. Changes in the ratios and connectivity of spinal interneurons could drastically alter motor output by tipping the balance of inhibition and excitation onto target motor neurons. Importantly, individuals with Fragile X syndrome (FXS) and associated autism spectrum disorders often have significant motor challenges, including repetitive behaviors and epilepsy. FXS stems from the transcriptional silencing of the gene Fragile X Messenger Ribonucleoprotein 1 (FMR1), which encodes an RNA binding protein that is implicated in a multitude of crucial neurodevelopmental processes, including cell specification. Our work shows that Fmrp regulates the formation of specific interneurons and motor neurons that comprise early embryonic motor circuits. We find that zebrafish fmr1 mutants generate surplus ventral lateral descending (VeLD) interneurons, an early-born cell derived from the motor neuron progenitor domain (pMN). As VeLD interneurons are hypothesized to act as central pattern generators driving the earliest spontaneous movements, this imbalance could influence the formation and long-term function of motor circuits driving locomotion. fmr1 embryos also show reduced expression of proteins associated with inhibitory synapses, including the presynaptic transporter vGAT and the postsynaptic scaffold Gephyrin. Taken together, we show changes in embryonic motor circuit formation in fmr1 mutants that could underlie persistent hyperexcitability.

Early life stage transient aristolochic acid exposure induces behavioral hyperactivity but not nephrotoxicity in larval zebrafish

Aristolochic acids (AA) are nitrophenanthrene carboxylic acids found in plants of the Aristolochiaceae family. Humans are exposed to AA by deliberately taking herbal medicines or unintentionally as a result of environmental contamination. AA is notorious for its nephrotoxicity, however, fewer studies explore potential neurotoxicity associated with AA exposure. The developing nervous system is vulnerable to xenobiotics, and pregnant women exposed to AA may put their fetuses at risk. In the present study, we used the embryonic zebrafish model to evaluate the developmental neurotoxicity associated with AA exposure. At non-teratogenic concentrations (≤ 4 µM), continuous AA exposure from 8 to 120 hours post fertilization (hpf) resulted in larval hyperactivity that was characterized by increased moving distance, elevated activity and faster swimming speeds in several behavioral assays. Further analysis revealed that 8-24 hpf is the most sensitive exposure window for AA-induced hyperactivity. AA exposures specifically increased motor neuron proliferation, increased apoptosis in the eye, and resulted in cellular oxidative stress. In addition, AA exposures increased larval eye size and perturbed the expression of vision genes. Our study, for the first time, demonstrates that AA is neurotoxic to the developmental zebrafish with a sensitive window distinct from its well-documented nephrotoxicity.

Microtubule organization of vertebrate sensory neurons in vivo

Dorsal root ganglion (DRG) neurons are the predominant cell type that innervates the vertebrate skin. They are typically described as pseudounipolar cells that have central and peripheral axons branching from a single root exiting the cell body. The peripheral axon travels within a nerve to the skin, where free sensory endings can emerge and branch into an arbor that receives and integrates information. In some immature vertebrates, DRG neurons are preceded by Rohon-Beard (RB) neurons. While the sensory endings of RB and DRG neurons function like dendrites, we use live imaging in zebrafish to show that they have axonal plus-end-out microtubule polarity at all stages of maturity. Moreover, we show both cell types have central and peripheral axons with plus-end-out polarity. Surprisingly, in DRG neurons these emerge separately from the cell body, and most cells never acquire the signature pseudounipolar morphology. Like another recently characterized cell type that has multiple plus-end-out neurites, ganglion cells in Nematostella, RB and DRG neurons maintain a somatic microtubule organizing center even when mature. In summary, we characterize key cellular and subcellular features of vertebrate sensory neurons as a foundation for understanding their function and maintenance.

Single-Cell Reconstruction of Emerging Population Activity in an Entire Developing Circuit

Animal survival requires a functioning nervous system to develop during embryogenesis. Newborn neurons must assemble into circuits producing activity patterns capable of instructing behaviors. Elucidating how this process is coordinated requires new methods that follow maturation and activity of all cells across a developing circuit. We present an imaging method for comprehensively tracking neuron lineages, movements, molecular identities, and activity in the entire developing zebrafish spinal cord, from neurogenesis until the emergence of patterned activity instructing the earliest spontaneous motor behavior. We found that motoneurons are active first and form local patterned ensembles with neighboring neurons. These ensembles merge, synchronize globally after reaching a threshold size, and finally recruit commissural interneurons to orchestrate the left-right alternating patterns important for locomotion in vertebrates. Individual neurons undergo functional maturation stereotypically based on their birth time and anatomical origin. Our study provides a general strategy for reconstructing how functioning circuits emerge during embryogenesis. VIDEO ABSTRACT.

The HMG box transcription factors Sox1a and b specify a new class of glycinergic interneurons in the spinal cord of zebrafish embryos

Specification of neurons in the spinal cord relies on extrinsic and intrinsic signals, which in turn are interpreted by expression of transcription factors. V2 interneurons develop from the ventral aspects of the spinal cord. We report here a novel neuronal V2 subtype, named V2s, in zebrafish embryos. Formation of these neurons depends on the transcription factors sox1a and sox1b. They develop from common gata2a/gata3 dependent precursors co-expressing markers of V2b and V2s interneurons. Chemical blockage of Notch signaling causes a decrease of V2s and an increase of V2b cells. Our results are consistent with the existence of at least two types of precursors arranged in a hierarchical manner in the V2 domain. V2s neurons grow long ipsilateral descending axonal projections with a short branch at the ventral midline. They acquire a glycinergic neurotransmitter type during the second day of development. Unilateral ablation of V2s interneurons causes a delay in touch-provoked escape behavior suggesting that V2s interneurons are involved in fast motor responses.

Differences in the morphology of spinal V2a neurons reflect their recruitment order during swimming in larval zebrafish

Networks of neurons in spinal cord generate locomotion. However, little is known about potential differences in network architecture that underlie the production of varying speeds of movement. In larval zebrafish, as swimming speed increases, Chx10-positive V2a excitatory premotor interneurons are activated from ventral to dorsal in a topographic pattern that parallels axial motoneuron recruitment. Here, we examined whether differences in the morphology and synaptic output of V2a neurons reflect their recruitment order during swimming. To do so, we used in vivo single-cell labeling approaches to quantify the dorsoventral distribution of V2a axonal projections and synapses. Two different classes of V2a neurons are described, cells with ascending and descending axons and cells that are only descending. Among the purely descending V2a cells, more dorsal cells project longer distances than ventral ones. Proximally, all V2a neurons have axonal distributions that suggest potential connections to cells at and below their own soma positions. At more distal locations, V2a axons project dorsally, which creates a cumulative intersegmental bias to dorsally located spinal neurons. Assessments of the synapse distribution of V2a cells, reported by synaptophysin expression, support the morphological observations and also demonstrate that dorsal V2a cells have higher synapse densities proximally. Our results suggest that V2a cells with more potential output to spinal neurons are systematically engaged during increases in swimming frequency. The findings help explain patterns of axial motoneuron recruitment and set up clear predictions for future physiological studies examining the nature of spinal excitatory network connectivity as it relates to movement intensity.

Connexin 35b expression in the spinal cord of Danio rerio embryos and larvae

Electrical synapses are expressed prominently in the developing and mature nervous systems. Unlike chemical synapses, little is known about the developmental role of electrical synapses, reflecting the limitations imposed by the lack of selective pharmacological blockers. At a molecular level, the building blocks of electrical synapses are connexin proteins. In this study, we report the expression pattern for neuronally expressed connexin 35b (cx35b), the zebrafish orthologue of mammalian connexin (Cx) 36. We find that cx35b is expressed at the time of neural induction, indicating a possible early role in neural progenitor cells. Additionally, cx35b localizes to the ventral spinal cord during embryonic and early larval stages. We detect cx35b mRNA in secondary motor neurons (SMNs) and interneurons. We identified the premotor circumferential descending (CiD) interneuron as one interneuron subtype expressing cx35b. In addition, cx35b is present in other ventral interneurons of unknown subtype(s). This early expression of cx35b in SMNs and CiDs suggests a possible role in motor network function during embryonic and larval stages.

Six1 is a key regulator of the developmental and evolutionary architecture of sensory neurons in craniates

BACKGROUND

Various senses and sensory nerve architectures of animals have evolved during adaptation to exploit diverse environments. In craniates, the trunk sensory system has evolved from simple mechanosensory neurons inside the spinal cord (intramedullary), called Rohon-Beard (RB) cells, to multimodal sensory neurons of dorsal root ganglia (DRG) outside the spinal cord (extramedullary). The fish and amphibian trunk sensory systems switch from RB cells to DRG during development, while amniotes rely exclusively on the DRG system. The mechanisms underlying the ontogenic switching and its link to phylogenetic transition remain unknown.

RESULTS

In Xenopus, Six1 overexpression promoted precocious apoptosis of RB cells and emergence of extramedullary sensory neurons, whereas Six1 knockdown delayed the reduction in RB cell number. Genetic ablation of Six1 and Six4 in mice led to the appearance of intramedullary sensory neuron-like cells as a result of medial migration of neural crest cells into the spinal cord and production of immature DRG neurons and fused DRG. Restoration of SIX1 expression in the neural crest-linage partially rescued the phenotype, indicating the cell autonomous requirements of SIX1 for normal extramedullary sensory neurogenesis. Mouse Six1 enhancer that mediates the expression in DRG neurons activated transcription in Xenopus RB cells earlier than endogenous six1 expression, suggesting earlier onset of mouse SIX1 expression than Xenopus during sensory development.

CONCLUSIONS

The results indicated the critical role of Six1 in transition of RB cells to DRG neurons during Xenopus development and establishment of exclusive DRG system of mice. The study provided evidence that early appearance of SIX1 expression, which correlated with mouse Six1 enhancer, is essential for the formation of DRG-dominant system in mice, suggesting that heterochronic changes in Six1 enhancer sequence play an important role in alteration of trunk sensory architecture and contribute to the evolution of the trunk sensory system.

The first mecp2-null zebrafish model shows altered motor behaviors

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder and one of the most common causes of mental retardation in affected girls. Other symptoms include a rapid regression of motor and cognitive skills after an apparently early normal development. Sporadic mutations in the transcription factor MECP2 has been shown to be present in more than 90% of the patients and several models of MeCP2-deficient mice have been created to understand the role of this gene. These models have pointed toward alterations in the maintenance of the central nervous system rather than its development, in line with the late onset of the disease in humans. However, the exact functions of MeCP2 remain difficult to delineate and the animal models have yielded contradictory results. Here, we present the first mecp2-null allele mutation zebrafish model. Surprisingly and in contrast to MeCP2-null mouse models, mecp2-null zebrafish are viable and fertile. They present nonetheless clear behavioral alterations during their early development, including spontaneous and sensory-evoked motor anomalies, as well as defective thigmotaxis.

Activation of Wnt signaling using lithium chloride: inquiry-based undergraduate laboratory exercises

Zebrafish provide researchers and students alike with an excellent model of vertebrate nervous system development due to a high degree of conserved developmental mechanisms and transparent embryos that develop in synchrony. In these laboratory exercises, undergraduate students explore cell biological concepts while performing hypothesis-driven novel research utilizing methodologies such as immunofluorescence, confocal microscopy, image analysis, pharmacology, and basic statistics. In the first block of exercises, students perform anti-acetylated tubulin (anti-AT) immunofluorescence, identify spinal tracts and neuronal subtypes, and perform conventional and confocal microscopy. Building on knowledge acquired in the first block of exercises, during the second block, students subsequently perform pharmacological activation of Wnt signaling through lithium chloride treatments, and assess nervous system integrity through anti-AT immunofluorescence. Students perform various quantitative methods and apply statistics to determine outcomes of Wnt activation. In their final laboratory report, students contextualize their results with foundations of molecular mechanisms of nervous system development. In sum, these exercises offer undergraduate students a model of independent research at the graduate level.

Centrosome movements in vivo correlate with specific neurite formation downstream of LIM homeodomain transcription factor activity

Neurons must develop complex structure to form proper connections in the nervous system. The initiation of axons in defined locations on the cell body and their extension to synaptic targets are critical steps in neuronal morphogenesis, yet the mechanisms controlling axon formation in vivo are poorly understood. The centrosome has been implicated in multiple aspects of neuronal morphogenesis; however, its function in axon development is under debate. Conflicting results from studies of centrosome function in axonogenesis suggest that its role is context dependent and underscore the importance of studying centrosome function as neurons develop in their natural environment. Using live imaging of zebrafish Rohon-Beard (RB) sensory neurons in vivo, we discovered a spatiotemporal relationship between centrosome position and the formation of RB peripheral, but not central, axons. We tested centrosome function by laser ablation and found that centrosome disruption inhibited peripheral axon outgrowth. In addition, we show that centrosome position and motility are regulated by LIM homeodomain transcription factor activity, which is specifically required for the development of RB peripheral axons. Furthermore, we show a correlation between centrosome mislocalization and ectopic axon formation in bashful (laminin alpha 1) mutants. Thus, both intrinsic transcription factor activity and extracellular cues can influence centrosome position and axon formation in vivo. This study presents the first positive association between the centrosome and axon formation in vivo and suggests that the centrosome is important for differential neurite formation in neurons with complex axonal morphologies.

Midline crossing is not required for subsequent pathfinding decisions in commissural neurons

Background

Growth cone navigation across the vertebrate midline is critical in the establishment of nervous system connectivity. While midline crossing is achieved through coordinated signaling of attractive and repulsive cues, this has never been demonstrated at the single cell level. Further, though growth cone responsiveness to guidance cues changes after crossing the midline, it is unclear whether midline crossing itself is required for subsequent guidance decisions in vivo. In the zebrafish, spinal commissures are initially formed by a pioneer neuron called CoPA (Commissural Primary Ascending). Unlike in other vertebrate models, CoPA navigates the midline alone, allowing for single-cell analysis of axon guidance mechanisms.

Results

We provide evidence that CoPA expresses the known axon guidance receptors dcc, robo3 and robo2. Using loss of function mutants and gene knockdown, we show that the functions of these genes are evolutionarily conserved in teleosts and that they are used consecutively by CoPA neurons. We also reveal novel roles for robo2 and robo3 in maintaining commissure structure. When midline crossing is prevented in robo3 mutants and dcc gene knockdown, ipsilaterally projecting neurons respond to postcrossing guidance cues. Furthermore, DCC inhibits Robo2 function before midline crossing to allow a midline approach and crossing.

Conclusions

Our results demonstrate that midline crossing is not required for subsequent guidance decisions by pioneer axons and that this is due, in part, to DCC inhibition of Robo2 function prior to midline crossing.

Trimethyltin chloride (TMT) neurobehavioral toxicity in embryonic zebrafish

Trimethyltin chloride (TMT) is a neurotoxicant that is widely present in the aquatic environment, primarily from the manufacture of PVC plastic, but few studies have evaluated aquatic neurotoxicity. We have examined TMT dose-dependent malformation and neurobehavioral toxicity in the embryonic zebrafish model. Exposure of embryos to TMT (0-10 μM) from 48 to 72 hours post fertilization (hpf) elicited a concentration-related increase (0-100%) in malformation incidence with an EC(25) of 5.55 μM. TMT also significantly modulated the frequency of tail flexion, the earliest motor behavior observed in developing zebrafish, and the ability to respond to a mechanical tail touch. Exposure to 5 μM TMT from 48 to 72 hpf modulated the photomotor response at 4 and 5 days post fertilization and significantly promoted apoptosis in the tail. Our study demonstrates the morphological and behavioral sensitivity of the developing zebrafish to TMT and establishes a platform for future identification of the affected pathways and chemical modulators of TMT toxicity.

Transgenic zebrafish recapitulating tbx16 gene early developmental expression

We describe the creation of a transgenic zebrafish expressing GFP driven by a 7.5 kb promoter region of the tbx16 gene. This promoter segment is sufficient to recapitulate early embryonic expression of endogenous tbx16 in the presomitic mesoderm, the polster and, subsequently, in the hatching gland. Expression of GFP in the transgenic lines later in development diverges to some extent from endogenous tbx16 expression with the serendipitous result that one line expresses GFP specifically in commissural primary ascending (CoPA) interneurons of the developing spinal cord. Using this line we demonstrate that the gene mafba (valentino) is expressed in CoPA interneurons.

In vivo imaging of cell behaviors and F-actin reveals LIM-HD transcription factor regulation of peripheral versus central sensory axon development

Background

Development of specific neuronal morphology requires precise control over cell motility processes, including axon formation, outgrowth and branching. Dynamic remodeling of the filamentous actin (F-actin) cytoskeleton is critical for these processes; however, little is known about the mechanisms controlling motile axon behaviors and F-actin dynamics in vivo. Neuronal structure is specified in part by intrinsic transcription factor activity, yet the molecular and cellular steps between transcription and axon behavior are not well understood. Zebrafish Rohon-Beard (RB) sensory neurons have a unique morphology, with central axons that extend in the spinal cord and a peripheral axon that innervates the skin. LIM homeodomain (LIM-HD) transcription factor activity is required for formation of peripheral RB axons. To understand how neuronal morphogenesis is controlled in vivo and how LIM-HD transcription factor activity differentially regulates peripheral versus central axons, we used live imaging of axon behavior and F-actin distribution in vivo.

Results

We used an F-actin biosensor containing the actin-binding domain of utrophin to characterize actin rearrangements during specific developmental processes in vivo, including axon initiation, consolidation and branching. We found that peripheral axons initiate from a specific cellular compartment and that F-actin accumulation and protrusive activity precede peripheral axon initiation. Moreover, disruption of LIM-HD transcriptional activity has different effects on the motility of peripheral versus central axons; it inhibits peripheral axon initiation, growth and branching, while increasing the growth rate of central axons. Our imaging revealed that LIM-HD transcription factor activity is not required for F-actin based protrusive activity or F-actin accumulation during peripheral axon initiation, but can affect positioning of F-actin accumulation and axon formation.

Conclusion

Our ability to image the dynamics of F-actin distribution during neuronal morphogenesis in vivo is unprecedented, and our experiments provide insight into the regulation of cell motility as neurons develop in the intact embryo. We identify specific motile cell behaviors affected by LIM-HD transcription factor activity and reveal how transcription factors differentially control the formation and growth of two axons from the same neuron.

Cabin1 expression suggests roles in neuronal development

Nervous system assembly and function depends on precise regulation of developmental gene expression. Cabin1, an essential gene in developing mice, is enriched in regions of the developing zebrafish central nervous system (CNS). Cabin1 is a repressor of MEF2- (myocyte enhancer factor 2) and calcineurin-mediated transcription in the immune system, but its function in the CNS during development is unknown. We identified Cabin1 from a library of genes enriched in developing neurons and determined the temporal and spatial expression of Cabin1 mRNA during CNS development. We found Cabin1 mRNA expression in the developing brain at times correlated with later aspects of neuronal differentiation. In some regions of the CNS Cabin1 expression overlaps with regions that also express proteins known to interact with Cabin1: MEF2 and/or calcineurin. We suggest that Cabin1 could act as a regulator of MEF2 and calcineurin activity in the developing nervous system, given their roles in neuronal differentiation and synaptic refinement.

Development of motor rhythms in zebrafish embryos

The nervous system can generate rhythms of various frequencies; on the low-frequency side, we have the circuits regulating circadian rhythms with a 24-h period, while on the high-frequency side we have the motor circuits that underlie flight in a hummingbird. Given the ubiquitous nature of rhythms, it is surprising that we know very little of the cellular and molecular mechanisms that produce them in the embryos and of their potential role during the development of neuronal circuits. Recently, zebrafish has been developed as a vertebrate model to study the genetics of neural development. Zebrafish offer several advantages to the study of nervous system development including optical and electrophysiological analysis of neuronal activity even at the earliest embryonic stages. This unique combination of physiology and genetics in the same animal model has led to insights into the development of neuronal networks. This chapter reviews work on the development of zebrafish motor rhythms and speculates on birth and maturation of the circuits that produce them.

Muscle contractions guide rohon-beard peripheral sensory axons

Multiple molecular cues guide neuronal axons to their targets during development. Previous studies in vitro have shown that mechanical stimulation also can affect axon growth; however, whether mechanical force contributes to axon guidance in vivo is unknown. We investigated the role of muscle contractions in the guidance of zebrafish peripheral Rohon-Beard (RB) sensory axons in vivo. We analyzed several mutants that affect muscle contraction through different molecular pathways, including a new mutant allele of the titin a (pik) gene, mutants that affect the hedgehog signaling pathway, and a nicotinic acetylcholine receptor mutant. We found RB axon defects in these mutants, the severity of which appeared to correlate with the extent of muscle contraction loss. These axons extend between the muscle and skin and normally have ventral trajectories and repel each other on contact. RB peripheral axons in muscle mutants extend longitudinally instead of ventrally, and the axons fail to repel one another on contact. In addition, we showed that limiting muscle movements by embedding embryos in agarose caused similar defects in peripheral RB axon guidance. This work suggests that the mechanical forces generated by muscle contractions are necessary for proper sensory axon pathfinding in vivo.

Multiple regulatory elements mediating neuronal-specific expression of zebrafish sodium channel gene, scn8aa

Zebrafish scn8aa sodium channels mediate the majority of sodium conductance, which is essential for the embryonic locomotor activities. Here, we investigated the transcriptional regulation of scn8aa in developing zebrafish embryos by constructing a GFP reporter driven by a 15-kb fragment of scn8aa gene designed as scn8aa:GFP. GFP expression patterns of scn8aa:GFP temporally and spatially recapitulated the expression of endogenous scn8aa mRNA during zebrafish embryonic development, with one exception in the inner nuclear layer of the retina. Three novel elements, along with an evolutionarily conserved element shared with mouse SCN8A, modulated neuronal-specific expression of scn8aa. The deletion of each positive element reduced the expression levels in neurons without inducing ectopic GFP expression in non-neuronal cells. Our results demonstrate that these four regulatory elements function cooperatively to enhance scn8aa expression in the zebrafish nervous system.

The development of motor coordination in Drosophila embryos

We used non-invasive muscle imaging to study the onset of motor activity and emergence of coordinated movement in Drosophila embryos. Earliest movements are myogenic, and neurally controlled muscle contractions first appear with the onset of bursting activity 17 hours after egg laying. Initial episodes of activity are poorly organised and coordinated crawling sequences only begin to appear after a further hour of bursting. Thus, network performance improves during this first period of activity. The embryo continues to exhibit bursts of crawling-like sequences until shortly before hatching, while other reflexes also mature. Bursting does not begin as a reflex response to sensory input but appears to reflect the onset of spontaneous activity in the motor network. It does not require GABA-mediated transmission, and, by using a light-activated channel to excite the network, we demonstrate activity-dependent depression that may cause burst termination.

A novel conserved evx1 enhancer links spinal interneuron morphology and cis-regulation from fish to mammals

Spinal interneurons are key components of locomotor circuits, driving such diverse behaviors as swimming in fish and walking in mammals. Recent work has linked the expression of evolutionarily conserved transcription factors to key features of interneurons in diverse species, raising the possibility that these interneurons are functionally related. Consequently, the determinants of interneuron subtypes are predicted to share conserved cis-regulation in vertebrates with very different spinal cords. Here, we establish a link between cis-regulation and morphology of spinal interneurons that express the Evx1 homeodomain transcription factor from fish to mammals. Using comparative genomics, and complementary transgenic approaches, we have identified a novel enhancer of evx1, that includes two non-coding elements conserved in vertebrates. We show that pufferfish evx1 transgenes containing this enhancer direct reporter expression to a subset of spinal commissural interneurons in zebrafish embryos. Pufferfish, zebrafish and mouse evx1 downstream genomic enhancers label selectively Evx1(+) V0 commissural interneurons in chick and rat embryos. By dissecting the zebrafish evx1 enhancer, we identify a role for a 25 bp conserved cis-element in V0-specific gene expression. Our findings support the notion that spinal interneurons shared between distantly related vertebrates, have been maintained in part via the preservation of highly conserved cis-regulatory modules.

Zebrafish V2 cells develop into excitatory CiD and Notch signalling dependent inhibitory VeLD interneurons

The vertebrate spinal cord contains distinct classes of cells that form at precise dorsal-ventral locations and express specific combinations of transcription factors. In amniotes, V2 cells develop in the ventral spinal cord, just dorsal to motoneurons. All V2 cells develop from the same progenitor domain and hence are initially molecularly identical. However, as they start to become post-mitotic and differentiate they subdivide into two intermingled molecularly-distinct subpopulations of cells, V2a and V2b cells. Here we show that the molecular identities of V2a and V2b cells are conserved between zebrafish and amniotes. In zebrafish, these two cell types both develop into interneurons with very similar morphologies, but while V2a cells become excitatory Circumferential Descending (CiD) interneurons, V2b cells become inhibitory Ventral Lateral Descending (VeLD) interneurons. In addition, we demonstrate that Notch signalling is required for V2 cells to develop into V2b cells. In the absence of Notch signalling, all V2b cells develop as V2a cells.

alx, a zebrafish homolog of Chx10, marks ipsilateral descending excitatory interneurons that participate in the regulation of spinal locomotor circuits

Recent molecular genetic studies suggest that the expression of transcription factors in the developing spinal cord helps determine the morphological and physiological properties of neurons. Using the zebrafish preparation, we have examined the properties of neurons marked by alx, a zebrafish homolog of mammalian Chx10. We performed morphological and physiological studies using transgenic zebrafish expressing fluorescent reporter constructs in cells that had at any time point expressed alx (alx neurons). Our data reveal that zebrafish alx neurons are all ipsilateral descending neurons that are positive for vesicular glutamate transporter 2, suggesting that they are glutamatergic excitatory interneurons. Patch recordings show that earlier-born neurons are active during stronger movements such as escapes and fast swimming (strong movement class), whereas later-born ones are involved in sustained weak swimming (weak movement class). Paired recordings between alx neurons and motoneurons show that neurons of the strong movement class make frequent monosynaptic excitatory connections onto motoneurons. Thus, neurons of this class are likely premotor interneurons that regulate motoneuron activity during escapes and fast swimming. We also show the existence of a monosynaptic connection between an alx neuron of the weak movement class and a motoneuron. Collectively, our data demonstrate that alx marks ipsilateral descending neurons that are involved in the regulation of motoneuron activity during forms of locomotion, such as escape and swimming.

Zebrafish bashful/laminin-alpha 1 mutants exhibit multiple axon guidance defects

Laminin is known to provide a highly permissive substratum and in some cases directional information for axon outgrowth in vitro. However, there is still little known about laminin function in guiding axons in vivo. We investigated the axon guidance role of laminin-alpha1 in the developing zebrafish nervous system. Analysis of zebrafish bashful (bal)/laminin-a1 mutants revealed multiple functions for laminin-alpha1 in the outgrowth and guidance of central nervous system (CNS) axons. Most CNS axon pathways are defective in bal embryos. Some axon types, including retinal ganglion cell axons, early forebrain axons, and hindbrain reticulospinal axons, make specific pathfinding errors, suggesting laminin-alpha1 is required for directional decisions. Other axon tracts are defasciculated or not fully extended in bal embryos, suggesting a function for laminin-alpha1 in regulating adhesion or providing a permissive substratum for growth. In addition, some neurons have excessively branched axons in bal, indicating a potential role for laminin-alpha1 in branching. In contrast to CNS axons, most peripheral axons appear normal in bal mutants. Our results, thus, reveal important and diverse functions for laminin-alpha1 in guiding developing axons in vivo.

Cloning of full-length zebrafish dcc and expression analysis during embryonic and early larval development

Members of the DCC family play key roles in axon guidance in vertebrates as well as invertebrates. In zebrafish, only a short partial sequence of the dcc gene has been reported to date. Here, we report the cloning of full-length zebrafish dcc. Zebrafish DCC shares the typical structure of the DCC subgroup of the immunoglobulin superfamily, consisting of four immunoglobulin and six fibronectin-type III repeats in the extracellular domain, a single transmembrane domain, and an intracellular domain with three conserved motifs. As a first step toward studying the function of dcc, we analyzed its sequence and characterized its expression pattern during embryonic and larval development. dcc is expressed highly in brain and spinal cord, but distinct staining was also observed in developing pectoral fins, pancreas, intestine, and heart. Thus, dcc may play roles not only in axon guidance, but in morphogenesis and functioning of these organs as well.

Central and peripheral axon branches from one neuron are guided differentially by Semaphorin3D and transient axonal glycoprotein-1

For multiple axons from one neuron to extend in different directions to unique targets, the growth cones of each axon must have distinct responses to guidance cues. However, the mechanisms by which separate axon branches are guided along different pathways are mainly unknown. Zebrafish Rohon-Beard (R-B) sensory neurons extend central axon branches in the spinal cord and peripheral axons to the epidermis. To investigate the differential guidance mechanisms of the central versus peripheral R-B axon branches, we used live-growth cone imaging in vivo combined with manipulation of individual guidance molecules. We show that a semaphorin expressed at the dorsal spinal cord midline, Semaphorin3D (Sema3D), may act to repel the peripheral axons out of the spinal cord. Sema3D knock-down reduces the number of peripheral axons. Remarkably, Sema3D ectopic expression repels and induces branching of peripheral axons in vivo but has no effect on central axons from the same neurons. Conversely, central axons require a growth-promoting molecule, transient axonal glycoprotein-1 (TAG-1), to advance, whereas peripheral axons do not. After TAG-1 knock-down, central growth cones display extensive protrusive activity but make little forward advance. TAG-1 knock-down has no effect on the motility or advance of peripheral growth cones. These experiments show how Sema3D and TAG-1 regulate the motility and behavior of growth cones extending in their natural in vivo environment and demonstrate that two different axon branches from one neuron respond differently to guidance cues in vivo.

Expression of PKC in the developing zebrafish, Danio rerio

Protein kinase C (PKC) is a family of enzymes involved in a wide range of biological functions. We investigated the expression of PKC-positive cells in zebrafish embryos and larvae within the first week of development to determine the developmental profile of PKC-containing cells. Our other goal was to determine if PKC alpha was associated with Rohon-Beard neurons during the first 5 days of development, when they are reported to undergo apoptosis. First, we confirmed the specificity of the antibodies by Western blotting zebrafish brain homogenates with anti-PKC and anti-PKC alpha, and detected single protein bands of approximately 78-82 kDa in size. Immunohistochemistry showed that several types of neurons were labeled, including neurons in the trigeminal ganglia, the dorsal spinal cord, and the dorsal root ganglia. Double-labeling with anti-PKC alpha and both anti-Islet-1 and zn12 confirmed the identity of the PKC-positive cells in the brain as trigeminal neurons, and in the spinal cord as Rohon-Beard cells. Some Rohon-Beard cells were labeled with anti-PKC alpha up to 7 days post fertilization (dpf). We performed TUNEL labeling and found no correlation between TUNEL-labeled and PKC alpha-labeled Rohon-Beard cells, suggesting that PKC alpha is not involved in Rohon-Beard apoptosis. Only approximately 40% of the approximately 130 Rohon-Beard cells at 24 h postfertilization (hpf) were positively labeled for PKC. Mauthner cells were labeled by anti-PKC, but not anti-PKC alpha, suggesting that the major form of PKC within these cells was not PKC alpha.

Engrailed-1 expression marks a primitive class of inhibitory spinal interneuron

Studies in chicks and mice have suggested that transcription factors mark functional subtypes of interneurons in the developing spinal cord. We used genetic, morphological, and physiological studies to test this proposed association in zebrafish. We found that Engrailed-1 expression uniquely marks a class of ascending interneurons, called circumferential ascending (CiA) interneurons, with ipsilateral axonal projections in both motor and sensory regions of spinal cord. These cells express the glycine transporter 2 gene and are the only known ipsilateral interneurons positive for this marker of inhibitory transmission. Patch recordings show that the CiA neurons are rhythmically active during swimming. Pairwise recordings from the CiA interneurons and postsynaptic cells reveal that the Engrailed-1 neurons produce monosynaptic, strychnine-sensitive inhibition of dorsal sensory interneurons and also inhibit more ventral neurons, including motoneurons and descending interneurons. We conclude that Engrailed-1 expression marks a class of inhibitory interneuron that seems to provide all of the ipsilateral glycinergic inhibition in the spinal cord of embryonic and larval fish. Individual Engrailed-1-positive cells are multifunctional, playing roles in both sensory gating and motor pattern generation. This primitive cell type may have given rise to several, more specialized glycinergic inhibitory interneurons in birds and mammals. Our data support the view that the subdivision of spinal cord into different regions by transcription factors defines a primitive functional organization of spinal interneurons that formed a developmental and evolutionary foundation on which more complex systems were built.

Steps during the development of the zebrafish locomotor network

This review summarizes recent data from our lab concerning the development of motor activities in the developing zebrafish. The zebrafish is a leading model for studies of vertebrate development because one can obtain a large number of transparent, externally and rapidly developing embryos with motor behaviors that are easy to assess (e.g. for mutagenic screens). The emergence of embryonic motility was studied behaviorally and at the cellular level. The embryonic behaviors appear sequentially and include an early, transient period of spontaneous, alternating tail coilings, followed by responses to touch, and swimming. Patch clamp recording in vivo revealed that an electrically coupled network of a subset of spinal neurons generates spontaneous tail coiling, whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming and requires input from the hindbrain. Swimming becomes sustained in larvae once serotonergic neuromodulatory effects are integrated. We end with a brief overview of the genetic tools available for the study of the molecular determinants implicated in locomotor network development in the zebrafish. Combining genetic, behavioral and cellular experimental approaches will advance our understanding of the general principles of locomotor network assembly and function.

Axonogenesis in the medaka embryonic brain

In order to know the general pattern of axonogenesis in vertebrates, we examined axonogenesis in the embryonic brain of a teleost fish, medaka (Oryzias latipes), and the results were compared with previous studies in zebrafish and mouse. The axons and somata were stained immunocytochemically using antibodies to a cell surface marker (HNK-1) and acetylated tubulin and visualized by retrograde and anterograde labeling with a lipophilic dye. The fiber systems developed correlating with the organization of the longitudinal and transverse subdivisions of the embryonic brain. The first axons extended from the synencephalic tegmentum, forming the first fiber tract (fasciculus longitudinalis medialis) in the ventral longitudinal zone of the neural rod, 38 hours after fertilization. In the neural tube, throughout the entire brain two pairs of longitudinal fiber systems, one ventral series and one dorsal or intermediate series, and four pairs of transverse fiber tracts in the rostral brain were formed sequentially during the first 16 hours of axon production. In one of the dorsal longitudinal tracts, its branch retracted and disappeared at later stages. One of the transverse tracts was found to course in the telencephalon and hypothalamus. The overall pattern of the longitudinal fiber systems in medaka brain is similar to that in mouse, but apparently different from that in zebrafish. We propose that a ventral tract reported in zebrafish partially belongs to the dorsal fiber system, and that the longitudinal fiber systems in all vertebrate brains pass through a common layout defined by conserved genetic and developmental programs.

Neurotransmitter properties of spinal interneurons in embryonic and larval zebrafish

Many classes of spinal interneurons in zebrafish have been described based on morphology, but their transmitter phenotypes are largely unknown. Here we combine back-filling or genetic labeling of spinal interneurons with in situ staining for markers of neurotransmitter phenotypes, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD) for GABAergic neurons. Neurons positive for VGLUT include the commissural CoPA, MCoD, UCoD, and some of the CoSA neurons. The CiD interneurons, which have ipsilateral descending axons, were also VGLUT-positive, as were the ventrally located VeMe interneurons, whose descending axonal trajectory has not been clearly revealed. Cells positive for GLYT2 include the commissural CoLAs as well as some of the CoBL and CoSA neurons. The CiA cells were the only GLYT2-positive cells with an ipsilateral axon. Cells staining for GAD included, most notably, the dorsal longitudinal ascending (DoLA) and KA interneurons. Our approach allowed us to define the likely transmitter phenotypes of most of the known classes of spinal interneurons. These data provide a foundation for understanding the functional organization of the spinal networks in zebrafish.

Distribution of prospective glutamatergic, glycinergic, and GABAergic neurons in embryonic and larval zebrafish

Zebrafish are an excellent model for studies of the functional organization of neuronal circuits, but little is known regarding the transmitter phenotypes of the neurons in their nervous system. We examined the distribution in spinal cord and hindbrain of neurons expressing markers of transmitter phenotype, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD65/67) for GABAergic neurons. All three markers were expressed in a large domain in the dorsal two-thirds of spinal cord, with additional, more ventral expression domains for VGLUT2 and GAD/GABA. In the large dorsal domain, dual in situ staining showed that GLYT2-positive cells were intermingled with VGLUT2 cells, with no dual-stained neurons. Many of the neurons in the dorsal expression domain that were positive for GABA markers at embryonic stages were also positive for GLYT2, suggesting that the cells might use both GABA and glycine, at least early in their development. The intermingling of neurons expressing inhibitory and excitatory markers in spinal cord contrasted markedly with the organization in hindbrain, where neurons expressing a particular marker were clustered together to form stripes that were visible running from rostral to caudal in horizontal sections and from dorsomedial to ventrolateral in cross sections. Dual labeling showed that the stripes of neurons labeled with one transmitter marker alternated with stripes of cells labeled for the other transmitter phenotypes. The differences in the distribution of excitatory and inhibitory neurons in spinal cord versus hindbrain may be tied to differences in their patterns of development and functional organization.

Knockdown of the survival motor neuron (Smn) protein in zebrafish causes defects in motor axon outgrowth and pathfinding

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by a loss of alpha motoneurons in the spinal cord. SMA is caused by low levels of the ubiquitously expressed survival motor neuron (Smn) protein. As it is unclear how low levels of Smn specifically affect motoneurons, we have modeled SMA in zebrafish, a vertebrate model organism with well-characterized motoneuron development. Using antisense morpholinos to reduce Smn levels throughout the entire embryo, we found motor axon-specific pathfinding defects. Reduction of Smn in individual motoneurons revealed that smn is acting cell autonomously. These results show for the first time, in vivo, that Smn functions in motor axon development and suggest that these early developmental defects may lead to subsequent motoneuron loss.

From cells to circuits: development of the zebrafish spinal cord

The ability of an animal to carry out its normal behavioral repertoire requires generation of an enormous diversity of neurons and glia. The relative simplicity of the spinal cord makes this an especially attractive part of the nervous system for addressing questions about the development of vertebrate neural specification and function. The last decade has witnessed an explosion in our understanding of spinal cord development and the functional interactions among spinal cord neurons and glia. Cellular, genetic, molecular, physiological and behavioral studies in zebrafish have all been important in providing insights into questions that remained unanswered by studies from other vertebrate model organisms. This is the case because many zebrafish spinal neurons can be individually identified and followed over time in living embryos and larvae. In this review, we discuss what is currently known about the cellular, genetic and molecular mechanisms involved in specifying distinct cell types in the zebrafish spinal cord and how these cells establish the functional circuitry that mediates particular behaviors. We start by describing the early signals and morphogenetic movements that form the nervous system, and in particular, the spinal cord. We then provide an overview of the cell types within the spinal cord and describe how they are specified and patterned. We begin ventrally with floor plate and proceed dorsally, through motoneurons and oligodendrocytes, interneurons, astrocytes and radial glia, spinal sensory neurons and neural crest. We next describe axon pathfinding of spinal neurons. Finally, we discuss the roles of particular spinal cord neurons in specific behaviors.

Nicotinic receptors mediate changes in spinal motoneuron development and axonal pathfinding in embryonic zebrafish exposed to nicotine

We show that transient exposure of embryonic zebrafish to nicotine delays the development of secondary spinal motoneurons. Furthermore, there is a long-lasting alteration in axonal pathfinding in secondary motoneurons that is not ameliorated by drug withdrawal. These effects of nicotine were reversed by mammalian nicotinic receptor antagonists. Coupled with these changes is a long-term alteration in swimming behavior. Our results show that transient embryonic exposure to nicotine leads to long-lasting effects on the vertebrate nervous system. These results also demonstrate that the zebrafish is a useful model to examine the effects of nicotine specifically, and drugs of abuse in general, on the development of the CNS in vertebrates.

Rapid in vivo labeling of identified zebrafish neurons

We report a simple and rapid method to label individual neurons in live zebrafish embryos and to examine their gene expression profiles. Injection of plasmid DNA encoding an alpha-tubulin promotor driving GFP expression results in mosaic embryos containing a limited number of GFP-positive neurons. Labeled neurons express GFP in their soma and axon, providing the opportunity to analyze pathfinding behaviors of identified neurons in vivo. Moreover, the presence of only a small subset of GFP tagged neurons permits the rapid anatomical identification of these neurons based on soma position and axonal trajectory. Analysis of injected embryos reveals that most, if not all, spinal cord cell types and many other neuronal cell types elsewhere in the nervous system can be GFP tagged. Finally, by combining GFP labeling of individual neurons with fluorescent in situ hybridization, we demonstrate the potential of this method to elucidate gene expression patterns at single cell resolution.

Neurogenin1 defines zebrafish cranial sensory ganglia precursors

Cells delaminate from epithelial placodes to form sensory ganglia in the vertebrate head. We describe the formation of cranial neurogenic placodes in the zebrafish, Danio rerio, using bHLH transcription factors as molecular markers. A single neurogenin gene, neurogenin1 (ngn1), is required for the development of all zebrafish cranial ganglia, which contrasts with other described vertebrates. Expression of ngn1 delineates zebrafish ganglionic placodes, including trigeminal, lateral line, and epibranchial placodes. In addition, ngn1 is expressed in a subset of cells within the otic vesicle that will delaminate to form the octaval (statoacoustic) ganglion. The trigeminal placode is the first to differentiate, and forms just lateral and adjacent to the neural crest. Expression of ngn1 is transient and prefigures expression of a related bHLH transcription factor, neuroD. Interfering with ngn1 function using a specific antisense morpholino oligonucleotide blocks differentiation of all cranial ganglia but not associated glial cells. Lateral line sensory neuromasts develop independently of ngn1 function, suggesting that two derivatives of lateral line placodes, ganglia and migrating primordia, are under separate genetic control.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.

spiel-ohne-grenzen/pou2mediates regional competence to respond to Fgf8 during zebrafish early neural development

Neural patterning of the vertebrate brain starts within the ectoderm during gastrulation and requires the activity of organizer cell populations in the neurectoderm. One such organizer is located at the prospective midbrain-hindbrain boundary (MHB) and controls development of the midbrain and the anterior hindbrain via the secreted signaling molecule Fgf8. However, little is known about how the ability of neural precursors to respond to Fgf8 is regulated. We have studied the function of the zebrafish spiel-ohne-grenzen (spg) gene in early neural development. Genetic mapping and molecular characterization presented in the accompanying paper revealed that spg mutations disrupt the pou2 gene, which encodes a POU domain transcription factor that is specifically expressed in the MHB primordium, and is orthologous to mammalian Oct3/Oct4. We show that embryos homozygous for spg/pou2 have severe defects in development of the midbrain and hindbrain primordium. Key molecules that function in the formation of the MHB, such as pax2.1, spry4, wnt1, her5, eng2 and eng3, and in hindbrain development, such as krox20, gbx2, fkd3 and pou2, are all abnormal in spg mutant embryos. By contrast, regional definition of the future MHB in the neuroectoderm by complementary expression of otx2 and gbx1, before the establishment of the complex regulatory cascade at the MHB, is normal in spg embryos. Moreover, the Fgf8 and Wnt1 signaling pathways are activated normally at the MHB but become dependent on spg towards the end of gastrulation. Therefore, spg plays a crucial role both in establishing and in maintaining development of the MHB primordium. Transplantation chimeras show that normal spg function is required within the neuroectoderm but not the endomesoderm. Importantly, gain-of-function experiments by mRNA injection of fgf8 and pou2 or Fgf8 bead implantations, as well as analysis of spg-ace double mutants show that spg embryos are insensitive to Fgf8, although Fgf receptor expression and activity of the downstream MAP kinase signaling pathway appear intact. We suggest that spg/pou2 is a transcription factor that mediates regional competence to respond to Fgf8 signaling.

A zebrafish homologue of deleted in colorectal cancer (zdcc) is expressed in the first neuronal clusters of the developing brain

DCC (deleted in colon cancer), Neogenin and UNC-5 are all members of the immunoglobulin superfamily of transmembrane receptors which are believed to play a role in axon guidance by binding to their ligands, the Netrin/UNC-40 family of secreted molecules (Cell. Mol. Life Sci. 56 (1999) 62; Curr. Opin. Genet. Dev. 7 (1997) 87). Although zebrafish homologues of the Netrin family of secreted molecules have been reported, to date there has been no published description of zebrafish DCC homologues (Mol. Cell. Neurosci. 9 (1997) 293; Mol. Cell. Neurosci. 11 (1998) 194; Mech. Dev. 62 (1997) 147). We report here the expression pattern of a zebrafish dcc (zdcc) homologue during the initial period of neurogenesis and axon tract formation within the developing central nervous system. Between 12 and 33 h post-fertilisation zdcc is expressed in a dynamic spatiotemporal pattern in all major subdivisions of the central nervous system. Double-labelling for zdcc and the post-mitotic neuronal marker HNK-1 revealed that subpopulations of neurons within the first nuclei of the zebrafish brain express zdcc. These results support our previous observation that patterning of neuronal clusters in the zebrafish brain occurs early in development (Dev. Biol. 229 (2001) 271).

Synchronization of an embryonic network of identified spinal interneurons solely by electrical coupling

There is a need to understand the mechanisms of neural synchronization during development because correlated rhythmic activity is thought to be critical for the establishment of proper connectivity. The relative importance of chemical and electrical synapses for synchronization of electrical activity during development is unclear. We examined the activity patterns of identified spinal neurons at the onset of motor activity in zebrafish embryos. Rhythmic activity appeared early and persisted upon blocking chemical neurotransmission but was abolished by inhibitors of gap junctions. Paired recordings revealed that active spinal neurons were electrically coupled and formed a simple network of motoneurons and a subset of interneurons. Thus, the earliest spinal central pattern generator consists of synchronously active, electrically coupled neurons.

Buffering intracellular calcium disrupts motoneuron development in intact zebrafish embryos

Numerous studies, performed mainly on dissociated cells, have shown that calcium signals have a role during different stages of neuronal development. However, the actions of calcium during neuronal development in vivo remain to be established. The present study has investigated the role of intracellular calcium signals during development of motoneurons in the spinal cord of intact zebrafish embryos. Loading blastomeres of early embryos with either the calcium buffer BAPTA or the calcium reporter dye Calcium Green, was shown to disrupt motoneuron development in the spinal cord of embryos at 24 h postfertilisation. Loading the calcium buffer BAPTA, at an intracellular concentration of 1 mM, into the blastomeres of early embryos did not alter the resting levels of intracellular calcium, but significantly dampened transient rises in intracellular calcium in the cells of later stage embryos. Loading cells with 1 mM BAPTA significantly decreased the number of motoneurons present in the spinal cord at 24 h, indicating that calcium signals are important for normal motoneuron differentiation. Furthermore, in those BAPTA-filled cells that did adopt a motoneuron cell fate, axogenesis was found to be inhibited, suggestive of a role for calcium signalling in neurite initiation. This work provides evidence that calcium signals are necessary at several stages of motoneuron development in vivo.

Cloning and expression of three zebrafish roundabout homologs suggest roles in axon guidance and cell migration

We report the cloning and expression patterns of three novel zebrafish Roundabout homologs. The Roundabout (robo) gene encodes a transmembrane receptor that is essential for axon guidance in Drosophila and Robo family members have been implicated in cell migration. Analysis of extracellular domains and conserved cytoplasmic motifs shows that zebrafish Robo1 and Robo2 are orthologs of mammalian Robo1 and Robo2, respectively, while zebrafish Robo3 is likely to be an ortholog of mouse Rig-1. The three zebrafish robos are expressed in distinct but overlapping patterns during embryogenesis. They are highly expressed in the developing nervous system, including the olfactory system, visual system, hindbrain, cranial ganglia, spinal cord, and posterior lateral line primordium. They are also expressed in several nonneuronal tissues, including somites and fin buds. The timing and patterns of expression suggest roles for zebrafish robos in axon guidance and cell migration. Wiley-Liss, Inc.