Zebrafish semaphorin Z1b inhibits growing motor axons in vivo

scMultiome analysis identifies embryonic hindbrain progenitors with mixed rhombomere identities

Rhombomeres serve to position neural progenitors in the embryonic hindbrain, thereby ensuring appropriate neural circuit formation, but the molecular identities of individual rhombomeres and the mechanism whereby they form has not been fully established. Here, we apply scMultiome analysis in zebrafish to molecularly resolve all rhombomeres for the first time. We find that rhombomeres become molecularly distinct between 10hpf (end of gastrulation) and 13hpf (early segmentation). While the embryonic hindbrain transiently contains alternating odd- versus even-type rhombomeres, our scMultiome analyses do not detect extensive odd versus even molecular characteristics in the early hindbrain. Instead, we find that each rhombomere displays a unique gene expression and chromatin profile. Prior to the appearance of distinct rhombomeres, we detect three hindbrain progenitor clusters (PHPDs) that correlate with the earliest visually observed segments in the hindbrain primordium that represent prospective rhombomere r2/r3 (possibly including r1), r4, and r5/r6, respectively. We further find that the PHPDs form in response to Fgf and RA morphogens and that individual PHPD cells co-express markers of multiple mature rhombomeres. We propose that the PHPDs contain mixed-identity progenitors and that their subdivision into individual rhombomeres requires the resolution of mixed transcription and chromatin states.

scMultiome analysis identifies embryonic hindbrain progenitors with mixed rhombomere identities

Rhombomeres serve to position neural progenitors in the embryonic hindbrain, thereby ensuring appropriate neural circuit formation, but the molecular identities of individual rhombomeres and the mechanism whereby they form have not been fully established. Here we apply scMultiome analysis in zebrafish to molecularly resolve all rhombomeres for the first time. We find that rhombomeres become molecularly distinct between 10hpf (end of gastrulation) and 13hpf (early segmentation). While the mature hindbrain consists of alternating odd- versus even-type rhombomeres, our scMultiome analyses do not detect extensive odd versus even characteristics in the early hindbrain. Instead, we find that each rhombomere displays a unique gene expression and chromatin profile. Prior to the appearance of distinct rhombomeres, we detect three hindbrain progenitor clusters (PHPDs) that correlate with the earliest visually observed segments in the hindbrain primordium and that represent prospective rhombomere r2/r3 (possibly including r1), r4 and r5/r6, respectively. We further find that the PHPDs form in response to Fgf and RA morphogens and that individual PHPD cells co-express markers of multiple mature rhombomeres. We propose that the PHPDs contain mixed-identity progenitors and that their subdivision into individual mature rhombomeres requires resolution of mixed transcription and chromatin states.

Long-Range Optogenetic Control of Axon Guidance Overcomes Developmental Boundaries and Defects

Axons connect neurons together, establishing the wiring architecture of neuronal networks. Axonal connectivity is largely built during embryonic development through highly constrained processes of axon guidance, which have been extensively studied. However, the inability to control axon guidance, and thus neuronal network architecture, has limited investigation of how axonal connections influence subsequent development and function of neuronal networks. Here, we use zebrafish motor neurons expressing a photoactivatable Rac1 to co-opt endogenous growth cone guidance machinery to precisely and non-invasively direct axon growth using light. Axons can be guided over large distances, within complex environments of living organisms, overriding competing endogenous signals and redirecting axons across potent repulsive barriers to construct novel circuitry. Notably, genetic axon guidance defects can be rescued, restoring functional connectivity. These data demonstrate that intrinsic growth cone guidance machinery can be co-opted to non-invasively build new connectivity, allowing investigation of neural network dynamics in intact living organisms.

Attractant and repellent cues cooperate in guiding a subset of olfactory sensory axons to a well-defined protoglomerular target

Olfactory sensory axons target well-defined intermediate targets in the zebrafish olfactory bulb called protoglomeruli well before they form odorant receptor-specific glomeruli. A subset of olfactory sensory neurons are labeled by expression of the or111-7:IRES:GAL4 transgene whose axons terminate in the central zone (CZ) protoglomerulus. Previous work has shown that some of these axons misproject to the more dorsal and anterior dorsal zone (DZ) protoglomerulus in the absence of Netrin 1/Dcc signaling. In search of additional cues that guide these axons to the CZ, we found that Semaphorin 3D (Sema3D) is expressed in the anterior bulb and acts as a repellent that pushes them towards the CZ. Further analysis indicates that Sema3D signaling is mediated through Nrp1a, while Nrp2b also promotes CZ targeting but in a Sema3D-independent manner. nrp1a, nrp2b and dcc transcripts are detected in or111-7 transgene-expressing neurons early in development and both Nrp1a and Dcc act cell-autonomously in sensory neurons to promote accurate targeting to the CZ. dcc and nrp1a double mutants have significantly more DZ misprojections than either single mutant, suggesting that the two signaling systems act independently and in parallel to direct a specific subset of sensory axons to their initial protoglomerular target.

Genome-wide analysis reveals NRP1 as a direct HIF1α-E2F7 target in the regulation of motorneuron guidance in vivo

In this study, we explored the existence of a transcriptional network co-regulated by E2F7 and HIF1α, as we show that expression of E2F7, like HIF1α, is induced in hypoxia, and because of the previously reported ability of E2F7 to interact with HIF1α. Our genome-wide analysis uncovers a transcriptional network that is directly controlled by HIF1α and E2F7, and demonstrates both stimulatory and repressive functions of the HIF1α -E2F7 complex. Among this network we reveal Neuropilin 1 (NRP1) as a HIF1α-E2F7 repressed gene. By performing in vitro and in vivo reporter assays we demonstrate that the HIF1α-E2F7 mediated NRP1 repression depends on a 41 base pairs ‘E2F-binding site hub’, providing a molecular mechanism for a previously unanticipated role for HIF1α in transcriptional repression. To explore the biological significance of this regulation we performed in situ hybridizations and observed enhanced nrp1a expression in spinal motorneurons (MN) of zebrafish embryos, upon morpholino-inhibition of e2f7/8 or hif1α. Consistent with the chemo-repellent role of nrp1a, morpholino-inhibition of e2f7/8 or hif1α caused MN truncations, which was rescued in TALEN-induced nrp1a^hu10012 mutants, and phenocopied in e2f7/8 mutant zebrafish. Therefore, we conclude that repression of NRP1 by the HIF1α-E2F7 complex regulates MN axon guidance in vivo.

Genome wide expression profiling during spinal cord regeneration identifies comprehensive cellular responses in zebrafish

BACKGROUND

Among the vertebrates, teleost and urodele amphibians are capable of regenerating their central nervous system. We have used zebrafish as a model to study spinal cord injury and regeneration. Relatively little is known about the molecular mechanisms underlying spinal cord regeneration and information based on high density oligonucleotide microarray was not available. We have used a high density microarray to profile the temporal transcriptome dynamics during the entire phenomenon.

RESULTS

A total of 3842 genes expressed differentially with significant fold changes during spinal cord regeneration. Cluster analysis revealed event specific dynamic expression of genes related to inflammation, cell death, cell migration, cell proliferation, neurogenesis, neural patterning and axonal regrowth. Spatio-temporal analysis of stat3 expression suggested its possible function in controlling inflammation and cell proliferation. Genes involved in neurogenesis and their dorso-ventral patterning (sox2 and dbx2) are differentially expressed. Injury induced cell proliferation is controlled by many cell cycle regulators and some are commonly expressed in regenerating fin, heart and retina. Expression pattern of certain pathway genes are identified for the first time during regeneration of spinal cord. Several genes involved in PNS regeneration in mammals like stat3, socs3, atf3, mmp9 and sox11 are upregulated in zebrafish SCI thus creating PNS like environment after injury.

CONCLUSION

Our study provides a comprehensive genetic blue print of diverse cellular response(s) during regeneration of zebrafish spinal cord. The data highlights the importance of different event specific gene expression that could be better understood and manipulated further to induce successful regeneration in mammals.

Activity-dependent competition regulates motor neuron axon pathfinding via PlexinA3

The role of electrical activity in axon guidance has been extensively studied in vitro. To better understand its role in the intact nervous system, we imaged intracellular Ca(2+) in zebrafish primary motor neurons (PMN) during axon pathfinding in vivo. We found that PMN generate specific patterns of Ca(2+) spikes at different developmental stages. Spikes arose in the distal axon of PMN and were propagated to the cell body. Suppression of Ca(2+) spiking activity in single PMN led to stereotyped errors, but silencing all electrical activity had no effect on axon guidance, indicating that an activity-based competition rule regulates this process. This competition was not mediated by synaptic transmission. Combination of PlexinA3 knockdown with suppression of Ca(2+) activity in single PMN produced a synergistic increase in the incidence of pathfinding errors. However, expression of PlexinA3 transcripts was not regulated by activity. Our results provide an in vivo demonstration of the intersection of spontaneous electrical activity with the PlexinA3 guidance molecule receptor in regulation of axon pathfinding.

Ccdc80-l1 Is involved in axon pathfinding of zebrafish motoneurons

Axon pathfinding is a subfield of neural development by which neurons send out axons to reach the correct targets. In particular, motoneurons extend their axons toward skeletal muscles, leading to spontaneous motor activity. In this study, we identified the zebrafish Ccdc80 and Ccdc80-like1 (Ccdc80-l1) proteins in silico on the basis of their high aminoacidic sequence identity with the human CCDC80 (Coiled-Coil Domain Containing 80). We focused on ccdc80-l1 gene that is expressed in nervous and non-nervous tissues, in particular in territories correlated with axonal migration, such as adaxial cells and muscle pioneers. Loss of ccdc80-l1 in zebrafish embryos induced motility issues, although somitogenesis and myogenesis were not impaired. Our results strongly suggest that ccdc80-l1 is involved in axon guidance of primary and secondary motoneurons populations, but not in their proper formation. ccdc80-l1 has a differential role as regards the development of ventral and dorsal motoneurons, and this is consistent with the asymmetric distribution of the transcript. The axonal migration defects observed in ccdc80-l1 loss-of-function embryos are similar to the phenotype of several mutants with altered Hedgehog activity. Indeed, we reported that ccdc80-l1 expression is positively regulated by the Hedgehog pathway in adaxial cells and muscle pioneers. These findings strongly indicate ccdc80-l1 as a down-stream effector of the Hedgehog pathway.

Semaphorin-PlexinD1 signaling limits angiogenic potential via the VEGF decoy receptor sFlt1

Sprouting angiogenesis expands the embryonic vasculature enabling survival and homeostasis. Yet how the angiogenic capacity to form sprouts is allocated among endothelial cells (ECs) to guarantee the reproducible anatomy of stereotypical vascular beds remains unclear. Here we show that Sema-PlxnD1 signaling, previously implicated in sprout guidance, represses angiogenic potential to ensure the proper abundance and stereotypical distribution of the trunk's segmental arteries (SeAs). We find that Sema-PlxnD1 signaling exerts this effect by antagonizing the proangiogenic activity of vascular endothelial growth factor (VEGF). Specifically, Sema-PlxnD1 signaling ensures the proper endothelial abundance of soluble flt1 (sflt1), an alternatively spliced form of the VEGF receptor Flt1 encoding a potent secreted decoy. Hence, Sema-PlxnD1 signaling regulates distinct but related aspects of angiogenesis: the spatial allocation of angiogenic capacity within a primary vessel and sprout guidance.

Nogo-B receptor is essential for angiogenesis in zebrafish via Akt pathway

Our previous work has shown that axon guidance gene family Nogo-B and its receptor (NgBR) are essential for chemotaxis and morphogenesis of endothelial cells in vitro. To investigate NogoB-NgBR function in vivo, we cloned the zebrafish ortholog of both genes and studied loss of function in vivo using morpholino antisense technology. Zebrafish ortholog of Nogo-B is expressed in somite while expression of zebrafish NgBR is localized in intersomitic vessel (ISV) and axial dorsal aorta during embryonic development. NgBR or Nogo-B knockdown embryos show defects in ISV sprouting in the zebrafish trunk. Mechanistically, we found that NgBR knockdown not only abolished its ligand Nogo-B-stimulated endothelial cell migration but also reduced the vascular endothelial growth factor (VEGF)-stimulated phosphorylation of Akt and vascular endothelial growth factor-induced chemotaxis and morphogenesis of human umbilical vein endothelial cells. Further, constitutively activated Akt (myristoylated [myr]Akt) or human NgBR can rescue the NgBR knockdown umbilical vein endothelial cell migration defects in vitro or NgBR morpholino-caused ISV defects in vivo. These data place Akt at the downstream of NgBR in both Nogo-B- and VEGF-coordinated sprouting of ISVs. In summary, this study identifies the in vivo functional role for Nogo-B and its receptor (NgBR) in angiogenesis in zebrafish.

Position fine-tuning of caudal primary motoneurons in the zebrafish spinal cord

In zebrafish embryos, each myotome is typically innervated by three primary motoneurons (PMNs): the caudal primary (CaP), middle primary (MiP) and rostral primary (RoP). PMN axons first exit the spinal cord through a single exit point located at the midpoint of the overlying somite, which is formed beneath the CaP cell body and is pioneered by the CaP axon. However, the placement of CaP cell bodies with respect to corresponding somites is poorly understood. Here, we determined the early events in CaP cell positioning using neuropilin 1a (nrp1a):gfp transgenic embryos in which CaPs were specifically labeled with GFP. CaP cell bodies first exhibit an irregular pattern in presence of newly formed corresponding somites and then migrate to achieve their proper positions by axonogenesis stages. CaPs are generated in excess compared with the number of somites, and two CaPs often overlap at the same position through this process. Next, we showed that CaP cell bodies remain in the initial irregular positions after knockdown of Neuropilin1a, a component of the class III semaphorin receptor. Irregular CaP position frequently results in aberrant double exit points of motor axons, and secondary motor axons form aberrant exit points following CaP axons. Its expression pattern suggests that sema3ab regulates the CaP position. Indeed, irregular CaP positions and exit points are induced by Sema3ab knockdown, whose ectopic expression can alter the position of CaP cell bodies. Results suggest that Semaphorin-Neuropilin signaling plays an important role in position fine-tuning of CaP cell bodies to ensure proper exit points of motor axons.

Novel mutations affecting axon guidance in zebrafish and a role for plexin signalling in the guidance of trigeminal and facial nerve axons

In zebrafish embryos, the axons of the posterior trigeminal (Vp) and facial (VII) motoneurons project stereotypically to a small number of target muscles derived from the first and second branchial arches (BA1, BA2). Use of the Islet1 (Isl1)-GFP transgenic line enabled precise real-time observations of the growth cone behaviour of the Vp and VII motoneurons within BA1 and BA2. Screening for N-ethyl-N-nitrosourea-induced mutants identified seven distinct mutations affecting different steps in the axonal pathfinding of these motoneurons. The class 1 mutations caused severe defasciculation and abnormal pathfinding in both Vp and VII motor axons before they reached their target muscles in BA1. The class 2 mutations caused impaired axonal outgrowth of the Vp motoneurons at the BA1-BA2 boundary. The class 3 mutation caused impaired axonal outgrowth of the Vp motoneurons within the target muscles derived from BA1 and BA2. The class 4 mutation caused retraction of the Vp motor axons in BA1 and abnormal invasion of the VII motor axons in BA1 beyond the BA1-BA2 boundary. Time-lapse observations of the class 1 mutant, vermicelli (vmc), which has a defect in the plexin A3 (plxna3) gene, revealed that Plxna3 acts with its ligand Sema3a1 for fasciculation and correct target selection of the Vp and VII motor axons after separation from the common pathways shared with the sensory axons in BA1 and BA2, and for the proper exit and outgrowth of the axons of the primary motoneurons from the spinal cord.

Analysis of zebrafish sidetracked mutants reveals a novel role for Plexin A3 in intraspinal motor axon guidance

One of the earliest guidance decisions for spinal cord motoneurons occurs when pools of motoneurons orient their growth cones towards a common, segmental exit point. In contrast to later events, remarkably little is known about the molecular mechanisms underlying intraspinal motor axon guidance. In zebrafish sidetracked (set) mutants, motor axons exit from the spinal cord at ectopic positions. By single-cell labeling and time-lapse analysis we show that motoneurons with cell bodies adjacent to the segmental exit point properly exit from the spinal cord, whereas those farther away display pathfinding errors. Misguided growth cones either orient away from the endogenous exit point, extend towards the endogenous exit point but bypass it or exit at non-segmental, ectopic locations. Furthermore, we show that sidetracked acts cell autonomously in motoneurons. Positional cloning reveals that sidetracked encodes Plexin A3, a semaphorin guidance receptor for repulsive guidance. Finally, we show that sidetracked (plexin A3) plays an additional role in motor axonal morphogenesis. Together, our data genetically identify the first guidance receptor required for intraspinal migration of pioneering motor axons and implicate the well-described semaphorin/plexin signaling pathway in this poorly understood process. We propose that axonal repulsion via Plexin A3 is a major driving force for intraspinal motor growth cone guidance.

Semaphorin 6C expression in innervated and denervated skeletal muscle

Semaphorins are secreted or transmembrane proteins important for axonal guidance and for the structuring of neuronal systems. Semaphorin 6C, a transmembrane Semaphorin, has growth cone collapsing activity and is expressed in adult skeletal muscle. In the present study the expression of Semaphorin 6C mRNA and immunoreactivity has been compared in innervated and denervated mouse hind-limb and hemidiaphragm muscles. Microscopic localization of immunoreactivity was studied in innervated and denervated rat skeletal muscle. The results show that Semaphorin 6C mRNA expression and immunoreactivity on Western blots are down-regulated following denervation. The mRNA of Semaphorin 6C as well as immunoreactivity determined by Western blots are expressed in extrasynaptic as well as perisynaptic regions of muscle. Immunohistochemical studies, however, show Semaphorin 6C-like immunoreactivity to be concentrated at neuromuscular junctions. The results suggest a role for Semaphorin 6C in neuromuscular communication.

PlexinA3 restricts spinal exit points and branching of trunk motor nerves in embryonic zebrafish

The pioneering primary motor axons in the zebrafish trunk are guided by multiple cues along their pathways. Plexins are receptor components for semaphorins that influence motor axon growth and path finding. We cloned plexinA3 in zebrafish and localized plexinA3 mRNA in primary motor neurons during axon outgrowth. Antisense morpholino knock-down led to substantial errors in motor axon growth. Errors comprised aberrant branching of primary motor nerves as well as additional exit points of axons from the spinal cord. Excessively branched and supernumerary nerves were found in both ventral and dorsal pathways of motor axons. The trunk environment and several other types of axons, including trigeminal axons, were not detectably affected by plexinA3 knock-down. RNA overexpression rescued all morpholino effects. Synergistic effects of combined morpholino injections indicate interactions of plexinA3 with semaphorin3A homologs. Thus, plexinA3 is a crucial receptor for axon guidance cues in primary motor neurons.

Proteoglycans as modulators of axon guidance cue function

Organizing a functional neuronal network requires the precise wiring of neuronal connections. In order to find their correct targets, growth cones navigate through the extracellular matrix guided by secreted and membrane-bound molecules of the slit, netrin, ephrin and semaphorin families. Although many of these axon guidance molecules are able to bind to heparan sulfate proteoglycans, the role of proteoglycans in regulating axon guidance cue function is only now beginning to be understood. Recent developmental studies in a wide range of model organisms have revealed a crucial role for heparan sulfate proteoglycans as modulators of key signaling pathways in axon guidance. In addition, emerging evidence indicates an essential role for chondroitin sulfate proteoglycans in modifying the guidance function of semaphorins. It is becoming increasingly clear that extracellular matrix molecules, rather than just constituting a structural scaffold, can critically influence axon guidance cue function in development, and may continue to do so in the injured adult nervous system.

Vertebrate MAX-1 is required for vascular patterning in zebrafish

During embryogenesis, stereotypic vascular patterning requires guidance cues from neighboring tissues. However, key molecules involved in this process still remain largely elusive. Here, we report molecular cloning, expression, and functional studies of zebrafish max-1, a homolog of Caenorhabditis elegans max-1 that has been implicated in motor neuron axon guidance. During early embryonic development, zebrafish max-1 is specifically expressed in subsets of neuronal tissues, epithelial cells, and developing somites through which vascular endothelial cells migrate from large ventral axial vessels to form stereotypic intersegmental blood vessels (ISV). Blocking zebrafish max-1 mRNA splicing by morpholino injection led to aberrant ISV patterning, which could be rescued by injection of either C. elegans or zebrafish max-1 mRNA. Analysis of motor neurons in the same region showed normal neuronal axon pathfinding. Further studies suggested that the ISV defect caused by max-1 knockdown could be partially rescued by overexpression of ephrinb3 and that max-1 was involved in mediating membrane localization of ephrin proteins, which have been shown to provide guidance cues for endothelial cell migration. Our findings therefore suggest that max-1, acting upstream of the ephrin pathway, is critically required in vascular patterning in vertebrate species.

Sema3a1 guides spinal motor axons in a cell- and stage-specific manner in zebrafish

In order for axons to reach their proper targets, both spatiotemporal regulation of guidance molecules and stepwise control of growth cone sensitivity to guidance molecules is required. Here, we show that, in zebrafish, Sema3a1, a secreted class 3 semaphorin, plays an essential role in guiding the caudal primary (CaP) motor axon that pioneers the initial region of the motor pathway. The expression pattern of Sema3a1 suggests that it delimits the pioneer CaP axons to the initial, common pathway via a repulsive action, but then CaP axons become insensitive to Sema3a1 beyond the common pathway. Indeed, nrp1a, which probably encodes a component of the Sema3a1 receptor, is specifically expressed by CaP during the early part of its outgrowth but not during later stages when extending into sema3a1-expressing muscle cells. To examine this hypothesis directly,expression of sema3a1 and/or nrp1a was manipulated in several ways. First, antisense knockdown of Sema3a1 induced CaP axons to branch excessively, stall and/or follow aberrant pathways. Furthermore,dynamic analysis showed they extended more lateral filopodia and often failed to pause at the horizontal myoseptal choice point. Second, antisense knockdown of Nrp1a and double knockdown of Nrp1a/Sema3a1 induced similar outgrowth defects in CaP. Third, CaP axons were inhibited by focally misexpressed sema3a1 along the initial common pathway but not along their pathway beyond the common pathway. Thus, as predicted, Sema3a1 is repulsive to CaP axons in the common region of the pathway, but not beyond the common pathway. Fourth, induced ubiquitous overexpression of sema3a1 caused the CaP axons but not the other primary motor axons to follow aberrant pathways. These results suggest that the repulsive response to Sema3a1 of the primary motor axons along the common pathway is both cell-type specific and dynamically regulated, perhaps via regulation of nrp1a.

Tenascin-C is involved in motor axon outgrowth in the trunk of developing zebrafish

Motor axons in the trunk of the developing zebrafish exit from the ventral spinal cord in one ventral root per hemisegment and grow on a common path toward the region of the horizontal myoseptum, where they select their specific pathways. Tenascin-C, a component of the extracellular matrix, is concentrated in this choice region. Adaxial cells and other myotomal cells express tenascin-C mRNA, suggesting that these cells are the source of tenascin-C protein. Overexpressing an axon repellent fragment containing the cysteine-rich region and the epidermal growth factor-like repeats of tenascin-C led to retarded growth of ventral motor nerves between their spinal exit point and the horizontal myoseptum. Injection of a protein fragment containing the same part of tenascin-C also induced slower growth of motor nerves. Conversely, knock down of tenascin-C protein resulted in abnormal lateral branching of ventral motor nerves. In the zebrafish unplugged mutant, in which axons display pathfinding defects in the region of the horizontal myoseptum, tenascin-C immunoreactivity was not detectable in this region, indicating an abnormal extracellular matrix in unplugged. We conclude that tenascin-C is part of a specialized extracellular matrix in the region of the horizontal myoseptum that influences the growth of motor axons.

Neuropilin-1a is involved in trunk motor axon outgrowth in embryonic zebrafish

Neuropilin-1, a receptor for axon-repellent semaphorins and vascular endothelial growth factor (VEGF), functions both in angiogenesis and axon growth. Here, we show strong expression of neuropilin-1a in primary motor neurons in the trunk of embryonic zebrafish. Reducing the expression of neuropilin-1a using antisense morpholino oligonucleotides induced aberrant branching of motor nerves, additional exit points of motor nerves from the spinal cord, and migration of neurons out of the spinal cord along the motor axon pathway in a dose-dependent manner. These phenotypes could be partially rescued by co-injecting neuropilin-1a mRNA. Other axons in the spinal cord and head appeared unaffected by the morpholino treatment. In addition, neuropilin-1a morpholino treatment disturbed normal formation of blood vessels in the trunk of 24 hours postfertilization embryos, as shown by microangiography. Morpholinos to VEGF also disturbed formation of blood vessels but did not affect motor axons, indicating that correct formation of blood vessels is not needed for the growth of primary motor axons. Morpholinos to the semaphorin 3A homologs semaphorin 3A1 and semaphorin 3A2 also had no effect on motor axon growth. However, combined injections of neuropilin-1a morpholino, at a concentration that did not elicit axonal aberrations when injected alone, with VEGF, semaphorin 3A1, or semaphorin 3A2 morpholinos synergistically increased the proportion of embryos showing aberrant motor axon growth. Thus, neuropilin-1a in primary motor neurons may integrate signals from several ligands and is needed for proper segmental growth of primary motor nerves in zebrafish.

Developmental expression of sema3G, a novel zebrafish semaphorin

The semaphorins are a large, evolutionarily conserved family of signaling molecules with broad functions during development. The class 3 semaphorins are a subclass of secreted semaphorins found in vertebrates. There have been six class 3 semaphorins identified to date (sema3A to sema3F) and some have been shown to function in axon guidance and cardiovascular development. However, the functions of many class 3 semaphorins and their potential interactions in vivo are still not well understood. As a step toward understanding the actions of all class 3 semaphorins in vivo, we have cloned and analyzed the developmental expression pattern of a novel zebrafish class 3 semaphorin, sema3H [corrected] sema3H [corrected] is expressed in a dynamic pattern throughout the first 3 days of development. It is expressed in the adaxial cells of the somite during somitogenesis. In the brain, sema3H [corrected] is expressed in cell clusters in the midbrain and diencephalon, and is expressed in the telencephalon in close proximity to the olfactory epithelium. sema3H [corrected] also is expressed in the pharyngeal arches, the pectoral fin bud, and the developing pronephros. These results provide a basis for studying how expression of multiple semaphorins could be essential for aspects of early development.

Semaphorin signaling guides cranial neural crest cell migration in zebrafish

Cranial neural crest cells (NCCs) migrate into the pharyngeal arches in three primary streams separated by two cranial neural crest (NC)-free zones. Multiple tissues have been implicated in the guidance of cranial NCC migration; however, the signals provided by these tissues have remained elusive. We investigate the function of semaphorins (semas) and their receptors, neuropilins (nrps), in cranial NCC migration in zebrafish. We find that genes of the sema3F and sema3G class are expressed in the cranial NC-free zones, while nrp2a and nrp2b are expressed in the migrating NCCs. sema3F/3G expression is expanded homogeneously in the head periphery through which the cranial NCCs migrate in lzr/pbx4 mutants, in which the cranial NC streams are fused. Antisense morpholino knockdown of Sema3F/3G or Nrp2 suppresses the abnormal cranial NC phenotype of lzr/pbx4 mutants, demonstrating that aberrant Sema3F/3G-Nrp2 signaling is responsible for this phenotype and suggesting that repulsive Sema3F/3G-Npn2 signaling normally contributes to the guidance of migrating cranial NCCs. Furthermore, global over-expression of sema3Gb phenocopies the aberrant cranial NC phenotype of lzr/pbx4 mutants when endogenous Sema3 ligands are knocked down, consistent with a model in which the patterned expression of Sema3 ligands in the head periphery coordinates the migration of Nrp-expressing cranial NCCs.

Semaphorin 3A displays a punctate distribution on the surface of neuronal cells and interacts with proteoglycans in the extracellular matrix

Secreted semaphorins are essential for neural development and continue to be expressed in subpopulations of adult neurons, where they subserve as yet unknown functions. We employed functional myc- and GFP-tagged Sema3A proteins to obtain insight in the localization of Sema3A in neuronal cells. Sema3A localized to both axons and dendrites of cortical neurons. GFP-Sema3A exhibited a characteristic punctate distribution on the surface of Neuro-2a cells, localized to migratory pathways of cultured cells, and co-localized with and induced clustering of its receptor component neuropilin-1. Treatment with excess glycosaminoglycans and chondroitinase ABC resulted in the removal of cell surface Sema3A. Heparin enhanced Sema3A's binding to neuropilin-1-expressing cells and potentiated its growth cone collapsing activity. Together, these results indicate that association with proteoglycans in the extracellular matrix of neuronal cells plays an important role in the localization of the chemorepulsive guidance cue Sema3A, and that this interaction may enhance its biological activity.

Semaphorin-plexin signaling guides patterning of the developing vasculature

Major vessels of the vertebrate circulatory system display evolutionarily conserved and reproducible anatomy, but the cues guiding this stereotypic patterning remain obscure. In the nervous system, axonal pathways are shaped by repulsive cues provided by ligands of the semaphorin family that are sensed by migrating neuronal growth cones through plexin receptors. We show that proper blood vessel pathfinding requires the endothelial receptor PlexinD1 and semaphorin signals, and we identify mutations in plexinD1 in the zebrafish vascular patterning mutant out of bounds. These results reveal the fundamental conservation of repulsive patterning mechanisms between axonal migration in the central nervous system and vascular endothelium during angiogenesis.

Somite polarity and segmental patterning of the peripheral nervous system

The analysis of the outgrowth pattern of spinal axons in the chick embryo has shown that somites are polarized into anterior and posterior halves. This polarity dictates the segmental development of the peripheral nervous system: migrating neural crest cells and outgrowing spinal axons traverse exclusively the anterior halves of the somite-derived sclerotomes, ensuring a proper register between spinal axons, their ganglia and the segmented vertebral column. Much progress has been made recently in understanding the molecular basis for somite polarization, and its linkage with Notch/Delta, Wnt and Fgf signalling. Contact-repulsive molecules expressed by posterior half-sclerotome cells provide critical guidance cues for axons and neural crest cells along the anterior-posterior axis. Diffusible repellents from surrounding tissues, particularly the dermomyotome and notochord, orient outgrowing spinal axons in the dorso-ventral axis ('surround repulsion'). Repulsive forces therefore guide axons in three dimensions. Although several molecular systems have been identified that may guide neural crest cells and axons in the sclerotome, it remains unclear whether these operate together with considerable overall redundancy, or whether any one system predominates in vivo.

PlexinA4 is necessary as a downstream target of Islet2 to mediate Slit signaling for promotion of sensory axon branching

Slit is a secreted protein known to repulse the growth cones of commissural neurons. By contrast, Slit also promotes elongation and branching of axons of sensory neurons. The reason why different neurons respond to Slit in different ways is largely unknown. Islet2 is a LIM/homeodomain-type transcription factor that specifically regulates elongation and branching of the peripheral axons of the primary sensory neurons in zebrafish embryos. We found that PlexinA4, a transmembrane protein known to be a co-receptor for class III semaphorins, acts downstream of Islet2 to promote branching of the peripheral axons of the primary sensory neurons. Intriguingly, repression of PlexinA4 function by injection of the antisense morpholino oligonucleotide specific to PlexinA4 or by overexpression of the dominant-negative variant of PlexinA4 counteracted the effects of overexpression of Slit2 to induce branching of the peripheral axons of the primary sensory neurons in zebrafish embryos, suggesting involvement of PlexinA4 in the Slit signaling cascades for promotion of axonal branching of the sensory neurons. Colocalized expression of Robo, a receptor for Slit2, and PlexinA4 is observed not only in the primary sensory neurons of zebrafish embryos but also in the dendrites of the pyramidal neurons of the cortex of the mammals, and may be important for promoting the branching of either axons or dendrites in response to Slit, as opposed to the growth cone collapse.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

Motor axon migration: a long way to go

Coordinated motor behaviors rely on a maze-like network of axonal connections between motoneurons and the body musculature. Since the time of the Renaissance, scientists have been fascinated by the question of how such complex, yet stereotypic, connectivity arises during embryogenesis. Here, we review the long journey of motor axons, which traverse diverse territories along predetermined routes riddled with a wealth of divergent pathway options. We conclude that motor axon migration occurs in a stepwise manner, with each step being controlled by local guidance cues, that motor axons rely on some of the same cues that also control axon migration within the CNS, and that, due to species-specific anatomical variations, the cell types providing such local cues may vary. Although studies of motor axon migration have not yet resulted in a complete understanding of the cellular and molecular mechanisms regulating this process, we have come a long way since the days of da Vinci.

Semaphorin3a1 regulates angioblast migration and vascular development in zebrafish embryos

Semaphorins are a large family of secreted and cell surface molecules that guide neural growth cones to their targets during development. Some semaphorins are expressed in cells and tissues beyond the nervous system suggesting the possibility that they function in the development of non-neural tissues as well. In the trunk of zebrafish embryos endothelial precursors (angioblasts) are located ventral and lateral to the somites. The angioblasts migrate medially and dorsally along the medial surface of the somites to form the dorsal aorta just ventral to the notochord. Here we show that in zebrafish Sema3a1 is involved in angioblast migration in vivo. Expression of sema3a1 in somites and neuropilin 1, which encodes for a component of the Sema3a receptor, in angioblasts suggested that Sema3a1 regulates the pathway of the dorsally migrating angioblasts. Antisense knockdown of Sema3a1 inhibited the formation of the dorsal aorta. Induced ubiquitous expression of sema3a1 in hsp70:(gfp)sema3a1(myc) transgenic embryos inhibited migration of angioblasts ventral and lateral to the somites and retarded development of the dorsal aorta, resulting in severely reduced blood circulation. Furthermore, analysis of cells that express angioblast markers following induced expression of sema3a1 or in a mutant that changes the expression of sema3a1 in the somites confirmed these results. These data implicate Sema3a1, a guidance factor for neural growth cones, in the development of the vascular system.

Integrin antagonists affect growth and pathfinding of ventral motor nerves in the trunk of embryonic zebrafish

Integrins are thought to be important receptors for extracellular matrix (ECM) components on growing axons. Ventral motor axons in the trunk of embryonic zebrafish grow in a midsegmental pathway through an environment rich in ECM components. To test the role of integrins in this process, integrin antagonists (the disintegrin echistatin in native and recombinant form, as well as the Arg-Gly-Asp-Ser peptide) were injected into embryos just prior to axon outgrowth at 14-16 h postfertilization (hpf). All integrin antagonists affected growth of ventral motor nerves in a similar way and native echistatin was most effective. At 24 hpf, when only the three primary motor axons per trunk hemisegment had grown out, 80% (16 of 20) of the embryos analyzed had abnormal motor nerves after injection of native echistatin, corresponding to 19% (91 of 480) of all nerves. At 33 hpf, when secondary motor axons were present in the pathway, 100% of the embryos were affected (24 of 24), with 20% of all nerves analyzed (196 of 960) being abnormal. Phenotypes comprised abnormal branching (64% of all abnormal nerves) and truncations (36% of all abnormal nerves) of ventral motor nerves at 24 hpf and mostly branching of the nerves at 33 hpf (94% of all abnormal nerves). Caudal branches were at least twice as frequent as rostral branches. Surrounding trunk tissue and a number of other axon fascicles were apparently not affected by the injections. Thus integrin function contributes to both growth and pathfinding of axons in ventral motor nerves in the trunk of zebrafish in vivo.

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.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

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.

Multiple functions of LIM domain-binding CLIM/NLI/Ldb cofactors during zebrafish development

The crucial involvement of CLIM/NLI/Ldb cofactors for the exertion of the biological activity of LIM homeodomain transcription factors (LIM-HD) has been demonstrated. In this paper we show that CLIM cofactors are widely expressed during zebrafish development with high protein levels in specific neuronal cell types where LIM-HD proteins of the Isl class are synthesized. The overexpression of a dominant-negative CLIM molecule (DN-CLIM) that contains the LIM interaction domain (LID) during early developmental stages of zebrafish embryos results in an impairment of eye and midbrain-hindbrain boundary (MHB) development and disturbances in the formation of the anterior midline. On a cellular level we show that the outgrowth of peripheral but not central axons from Rohon Beard (RB) and trigeminal sensory neurons is inhibited by DN-CLIM overexpression. We demonstrate a further critical role of CLIM cofactors for axonal outgrowth of motor neurons. Additionally, DN-CLIM overexpression causes an increase of Isl-protein expression levels in specific neuronal cell types, likely due to a protection of the DN-CLIM/LIM-HD complex from proteasomal degradation. Our results demonstrate multiple roles of the CLIM cofactor family for the development of entire organs, axonal outgrowth of specific neurons and protein expression levels.

Repellent guidance of regenerating optic axons by chondroitin sulfate glycosaminoglycans in zebrafish

We analyzed the role of chondroitin sulfate (CS) glycosaminoglycans, putative inhibitors of axonal regeneration in mammals, in the regenerating visual pathway of adult zebrafish. In the adult, CS immunoreactivity was not detectable before or after an optic nerve crush in the optic nerve and tract but was constitutively present in developing and adult nonretinorecipient pretectal brain nuclei, where CSs may form a boundary preventing regenerating optic fibers from growing into these inappropriate locations. Enzymatic removal of CSs by chondroitinase ABC after optic nerve crush significantly increased the number of animals showing erroneous growth of optic axons into the nonretinorecipient magnocellular superficial/posterior pretectal nucleus (83% vs 42% in controls). In vitro, a substrate border of CSs, but not heparan sulfates, strongly repelled regenerating retinal axons from adult zebrafish. We conclude that CSs contribute to repellent axon guidance during regeneration of the optic projection in zebrafish.

Wiring the zebrafish: axon guidance and synaptogenesis

Many zebrafish mutants have specific defects in axon guidance or synaptogenesis, particularly in the retinotectal and motor systems. Several mutants have now been characterized in detail and/or cloned. A combination of genetic studies, in vivo imaging and new techniques for misexpressing genes or blocking their function promises to reveal the molecules and principles that govern wiring of the vertebrate nervous system.

Antibody to the HNK-1 glycoepitope affects fasciculation and axonal pathfinding in the developing posterior lateral line nerve of embryonic zebrafish

The HNK-1 glycoepitope, carried by many cell recognition molecules, is present in the developing posterior lateral line nerve and on other primary axons of zebrafish. To elucidate the function of HNK-1 in vivo, the antibody 412 to HNK-1 was injected into zebrafish embryos at 16 h post fertilization (hpf). The injected antibody bound specifically to axons carrying HNK-1. This treatment selectively affected the growth of either one or both posterior lateral line nerves in 39% of the experimental cases (13 of 33 animals), which was significantly more (P<0.0002) than in uninjected, vehicle injected, and non-immune IgG injected controls (1.2% of the animals; one of 85 animals), as assessed at 27 or 33 hpf. Other HNK-1 immunoreactive nerves, such as the ventral motor nerves were unaffected, indicating that antibody binding per se did not interfere with axon growth. The primordium of the posterior lateral line was not affected in its caudal migration and in depositing differentiating neuromasts along the trunk, showing that injections did not retard development and that initial formation of lateral line organs is probably independent of contact with nerve fibers. We suggest that the HNK-1 glycoepitope is an important modulator of embryonic nerve growth.

Zebrafish deadly seven functions in neurogenesis

In a genetic screen, we isolated a mutation that perturbed motor axon outgrowth, neurogenesis, and somitogenesis. Complementation tests revealed that this mutation is an allele of deadly seven (des). By creating genetic mosaics, we demonstrate that the motor axon defect is non-cell autonomous. In addition, we show that the pattern of migration for some neural crest cell populations is aberrant and crest-derived dorsal root ganglion neurons are misplaced. Furthermore, our analysis reveals that des mutant embryos exhibit a neurogenic phenotype. We find an increase in the number of primary motoneurons and in the number of three hindbrain reticulospinal neurons: Mauthner cells, RoL2 cells, and MiD3cm cells. We also find that the number of Rohon-Beard sensory neurons is decreased whereas neural crest-derived dorsal root ganglion neurons are increased in number supporting a previous hypothesis that Rohon-Beard neurons and neural crest form an equivalence group during development. Mutations in genes involved in Notch-Delta signaling result in defects in somitogenesis and neurogenesis. We found that overexpressing an activated form of Notch decreased the number of Mauthner cells in des mutants indicating that des functions via the Notch-Delta signaling pathway to control the production of specific cell types within the central and peripheral nervous systems.

Function of Neurolin (DM-GRASP/SC-1) in guidance of motor axons during zebrafish development

Neurolin (zf DM-GRASP), a transmembrane protein with five extracellular immunoglobulin domains, is expressed by secondary but not primary motoneurons during zebrafish development. The spatiotemporally restricted expression pattern suggests that Neurolin plays a role in motor axon growth and guidance. To test this hypothesis, we injected zebrafish embryos with function-blocking Neurolin antibodies. In injected embryos, secondary motor axons form a broadened bundle along the common path and ectopic branches leave the common path at right angles. Moreover, the formation of the ventral and the rostral projection of secondary motor axons is inhibited during the second day of development. Pathfinding errors, resulting in secondary motor axons growing through ectopic regions of the somites, occur along the common path and in the dorsal and rostral projection. Our data are compatible with the view that Neurolin is involved in the recognition of guidance cues and acts as a receptor on secondary motor axons. Consistent with this idea is the binding pattern of a soluble Neurolin-Fc construct showing that putative ligands are distributed along the common path, the ventral projection, and in the area where the rostral projection develops.

Control of motor axon guidance in the zebrafish embryo

The zebrafish neuromuscular system has been an exemplary model for studying motor axon guidance since its detailed characterization almost two decades ago. In particular, characterization and detailed analysis has focused on the development and axogenesis of early developing primary motoneurons. During the first day of development, neuromuscular connections are limited to three primary motoneurons per spinal cord hemisegment innervating three discreet myotome territories. Observations of dye labeled primary motor axons in living embryos revealed that axogenesis is highly stereotyped with each primary motor axon extending along specific pathways and displaying particular characteristics. Exploiting the unique attributes of zebrafish, notably the ability to analyze motoneurons in living embryos and the capability to induce mutations, has allowed a comprehensive cellular, molecular and genetic approach to discerning the mechanisms that control the formation of neuromuscular connectivity. Knowledge gained from this body of work not only relates to zebrafish, but to vertebrate axon guidance in general.

Semaphorin function in the developing invertebrate peripheral nervous system

Different members of the semaphorin family of secreted and transmembrane guidance molecules play important and diverse roles during neuronal development. Within the developing grasshopper limb bud, two semaphorins are expressed in relatively non-overlapping and distinct expression patterns. The establishment of the tibial sensory projection within the limb bud relies on the combinatorial action of both semaphorins. In this review, we describe the function of the two semaphorins in axonal guidance and propose that a hierarchy of cues guide sensory neurons in the developing peripheral nervous system.Key words: semaphorin, axon guidance, grasshopper, peripheral nervous system, review.

Chondroitin sulfates affect the formation of the segmental motor nerves in zebrafish embryos

Chondroitin sulfates have been implicated in the promotion and in the inhibition of axon growth. In the zebrafish embryo, chondroitin sulfates are present at the interface of the somites and the notochord where spinal motor axons extend ventrally to establish the midsegmental ventral motor nerves. Injection of chondroitinase ABC prior to motor axon outgrowth effectively removed all chondroitin sulfate immunoreactivity and induced abnormal axonal outgrowth in many (39%) of the ventral motor nerves. The most common abnormality was the formation of side branches, approximately half of which extended posteriorly, the others anteriorly. The effect was specific to the removal of chondroitin sulfates, since injections of vehicle solution or of heparinase III did not affect the ventral motor nerves. Electron microscopic examination demonstrated that the injections caused no damage to spinal cord, somite, and notochord. This suggests that chondroitin sulfates normally constrain the outgrowth of the ventral motor nerves. Consistent with this hypothesis, injections of soluble chondroitin sulfates, either as a mixture or individually, led to truncated or missing ventral motor nerves. Truncations were most frequent after injection of chondroitin sulfate-B (up to 23%) while chondroitin sulfate-A had a lesser, and chondroitin sulfate-C no apparent, effect.

Laser-induced gene expression in specific cells of transgenic zebrafish

Over the past few years, a number of studies have described the generation of transgenic lines of zebrafish in which expression of reporters was driven by a variety of promoters. These lines opened up the real possibility that transgenics could be used to complement the genetic analysis of zebrafish development. Transgenic lines in which the expression of genes can be regulated both in space and time would be especially useful. Therefore, we have cloned the zebrafish promoter for the inducible hsp70 gene and made stable transgenic lines of zebrafish that express the reporter green fluorescent protein gene under the control of a hsp70 promoter. At normal temperatures, green fluorescent protein is not detectable in transgenic embryos with the exception of the lens, but is robustly expressed throughout the embryo following an increase in ambient temperature. Furthermore, we have taken advantage of the accessibility and optical clarity of the embryos to express green fluorescent protein in individual cells by focussing a sublethal laser microbeam onto them. The targeted cells appear to develop normally: cells migrate normally, neurons project axons that follow normal pathways, and progenitor cells divide and give rise to normal progeny cells. By generating other transgenic lines in which the hsp70 promoter regulates genes of interest, it should be possible to examine the in vivo activity of the gene products by laser-inducing specific cells to express them in zebrafish embryos. As a first test, we laser-induced single muscle cells to make zebrafish Sema3A1, a semaphorin that is repulsive for specific growth cones, in a hsp70-sema3A1 transgenic line of zebrafish and found that extension by the motor axons was retarded by the induced muscle.