Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (-1, -2, -3, -5, -6, -7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

The proper development and maturation of neuronal circuits require precise migration of component neurons from their birthplace (germinal zone) to their final positions. Little is known about the effects of aberrant neuronal position on the functioning of organized neuronal groups, especially in mammals. Here, we investigated the formation and properties of brainstem respiratory neurons in looptail (Lp) mutant mice in which facial motor neurons closely apposed to some respiratory neurons fail to migrate due to loss of function of the Wnt/Planar Cell Polarity (PCP) protein Vangl2. Using calcium imaging and immunostaining on embryonic hindbrain preparations, we found that respiratory neurons constituting the embryonic parafacial oscillator (e-pF) settled at the ventral surface of the medulla in Vangl2Lp/+ and Vangl2Lp/Lp embryos despite the failure of tangential migration of its normally adjacent facial motor nucleus. Anatomically, the e-pF neurons were displaced medially in Lp/+ embryos and rostro-medially Lp/Lp embryos. Pharmacological treatments showed that the e-pF oscillator exhibited characteristic network properties in both Lp/+ and Lp/Lp embryos. Furthermore, using hindbrain slices, we found that the other respiratory oscillator, the preBötzinger complex, was also anatomically and functionally established in Lp mutants. Importantly, the displaced e-pF oscillator established functional connections with the preBötC oscillator in Lp/+ mutants. Our data highlight the robustness of the developmental processes that assemble the neuronal networks mediating an essential physiological function.

During hindbrain development, facial branchiomotor neurons (FBM neurons) migrate from medial rhombomere (r) 4 to lateral r6. In zebrafish, mutations in planar cell polarity genes celsr2 and frizzled3a block caudal migration of FBM neurons. Here, we investigated the role of cadherins Celsr1-3, and Fzd3 in FBM neuron migration in mice. In Celsr1 mutants (knock-out and Crash alleles), caudal migration was compromised and neurons often migrated rostrally into r2 and r3, as well as laterally. These phenotypes were not caused by defects in hindbrain patterning or neuronal specification. Celsr1 is expressed in FBM neuron precursors and the floor plate, but not in FBM neurons. Consistent with this, conditional inactivation showed that the function of Celsr1 in FBM neuron migration was non-cell autonomous. In Celsr2 mutants, FBM neurons initiated caudal migration but moved prematurely into lateral r4 and r5. This phenotype was enhanced by inactivation of Celsr3 in FBM neurons and mimicked by inactivation of Fzd3. Furthermore, Celsr2 was epistatic to Celsr1. These data indicate that Celsr1-3 differentially regulate FBM neuron migration. Celsr1 helps to specify the direction of FBM neuron migration, whereas Celsr2 and 3 control its ability to migrate.

The transmembrane protein Van gogh-like 2 (Vangl2) is a component of the non-canonical Wnt/Planar Cell Polarity (PCP) signaling pathway, and is required for tangential migration of facial branchiomotor neurons (FBMNs) from rhombomere 4 (r4) to r5–r7 in the vertebrate hindbrain. Since vangl2 is expressed throughout the zebrafish hindbrain, it might also regulate motor neuron migration in other rhombomeres. We tested this hypothesis by examining whether migration of motor neurons out of r2 following ectopic hoxb1b expression was affected in vangl2− (trilobite) mutants. Hoxb1b specifies r4 identity, and when ectopically expressed transforms r2 to an "r4-like" compartment. Using time-lapse imaging, we show that GFP-expressing motor neurons in the r2/r3 region of a hoxb1b-overexpressing wild-type embryo migrate along the anterior-posterior (AP) axis. Furthermore, these cells express prickle1b (pk1b), a Wnt/PCP gene that is specifically expressed in FBMNs and is essential for their migration. Importantly, GFP-expressing motor neurons in the r2/r3 region of hoxb1b-overexpressing trilobite mutants and pk1b morphants often migrate, even though FBMNs in r4 of the same embryos fail to migrate longitudinally (tangentially) into r6 and r7. These observations suggest that tangentially migrating motor neurons in the anterior hindbrain (r1–r3) can use mechanisms that are independent of vangl2 and pk1b functions. Interestingly, analysis of tri; val double mutants also suggests a role for vangl2-independent factors in neuronal migration, since the valentino mutation partially suppresses the trilobite mutant migration defect. Together, the hoxb1b and val experiments suggest that multiple mechanisms regulate motor neuron migration along the AP axis of the zebrafish hindbrain.

Newborn neurons migrate extensively in the radial and tangential directions to organize the developing vertebrate nervous system. We show here that mutations in zebrafish trilobite (tri) that affect gastrulation-associated cell movements also eliminate tangential migration of motor neurons in the hindbrain. In the wild-type hindbrain, facial (nVII) and glossopharyngeal (nIX) motor neurons are induced in rhombomeres 4 and 6, respectively, and migrate tangentially into r6 and r7 (nVII), and r7 (nIX). In all three tri alleles examined, although normal numbers of motor neurons are induced, nVII motor neurons are found exclusively in r4, and nIX-like motor neurons are found exclusively in r6. The migration of other neuronal and non-neuronal cell types is unaffected in tri mutants. Rhombomere formation and the development of other hindbrain neurons are also unaffected in tri mutants. Furthermore, tangential neuronal migration occurs normally in the gastrulation mutant knypek, indicating that the trilobite neuron phenotype does not arise non-specifically from aberrant gastrulation-associated movements. We conclude that trilobite function is specifically required for two types of cell migration that occur at different stages of zebrafish development.

Interactions between a neuron and its environment play a major role in neuronal migration. We show here that the cell adhesion molecule Transient Axonal Glycoprotein (Tag1) is necessary for the migration of the facial branchiomotor neurons (FBMNs) in the zebrafish hindbrain. In tag1 morphant embryos, FBMN migration is specifically blocked, with no effect on organization or patterning of other hindbrain neurons. Furthermore, using suboptimal morpholino doses and genetic mutants, we found that tag1, lamininα1 (lama1) and stbm, which encodes a transmembrane protein Vangl2, exhibit pairwise genetic interactions for FBMN migration. Using time-lapse analyses, we found that FBMNs are affected similarly in all three single morphant embryos, with an inability to extend protrusions in a specific direction, and resulting in the failure of caudal migration. These data suggest that tag1, lama1 and vangl2 participate in a common mechanism that integrates signaling between the FBMN and its environment to regulate migration.

Background

How axon guidance signals regulate growth cone behavior and guidance decisions in the complex in vivo environment of the central nervous system is not well understood. We have taken advantage of the unique features of the zebrafish embryo to visualize dynamic growth cone behaviors and analyze guidance mechanisms of axons emerging from a central brain nucleus in vivo.

Results

We investigated axons of the nucleus of the medial longitudinal fascicle (nucMLF), which are the first axons to extend in the zebrafish midbrain. Using in vivo time-lapse imaging, we show that both positive axon-axon interactions and guidance by surrounding tissue control initial nucMLF axon guidance. We further show that two guidance molecules, transient axonal glycoprotein-1 (TAG-1) and laminin-α1, are essential for the initial directional extension of nucMLF axons and their subsequent convergence into a tight fascicle. Fixed tissue analysis shows that TAG-1 knockdown causes errors in nucMLF axon pathfinding similar to those seen in a laminin-α1 mutant. However, in vivo time-lapse imaging reveals that while some defects in dynamic growth cone behavior are similar, there are also defects unique to the loss of each gene. Loss of either TAG-1 or laminin-α1 causes nucMLF axons to extend into surrounding tissue in incorrect directions and reduces axonal growth rate, resulting in stunted nucMLF axons that fail to extend beyond the hindbrain. However, defects in axon-axon interactions were found only after TAG-1 knockdown, while defects in initial nucMLF axon polarity and excessive branching of nucMLF axons occurred only in laminin-α1 mutants.

Conclusion

These results demonstrate how two guidance cues, TAG-1 and laminin-α1, influence the behavior of growth cones during axon pathfinding in vivo. Our data suggest that TAG-1 functions to allow growth cones to sense environmental cues and mediates positive axon-axon interactions. Laminin-α1 does not regulate axon-axon interactions, but does influence neuronal polarity and directional guidance.

The Gli family of zinc-finger transcription factors mediates Hedgehog (Hh) signaling in all vertebrates. However, their roles in ventral neural tube patterning, in particular motor neuron induction, appear to have diverged across species. For instance, cranial motor neurons are essentially lost in zebrafish detour (gli1−) mutants, whereas motor neuron development is unaffected in mouse single gli and some double gli knockouts. Interestingly, the expression of some Hh-regulated genes (ptc1, net1a, gli1) is mostly unaffected in the detour mutant hindbrain, suggesting that other Gli transcriptional activators may be involved. To better define the roles of the zebrafish gli genes in motor neuron induction and in Hh-regulated gene expression, we examined these processes in you-too (yot) mutants, which encode dominant repressor forms of Gli2 (Gli2DR), and following morpholino-mediated knockdown of gli1, gli2, and gli3 function. Motor neuron induction at all axial levels was reduced in yot (gli2DR) mutant embryos. In addition, Hh target gene expression at all axial levels except in rhombomere 4 was also reduced, suggesting an interference with the function of other Glis. Indeed, morpholino-mediated knockdown of Gli2DR protein in yot mutants led to a suppression of the defective motor neuron phenotype. However, gli2 knockdown in wild-type embryos generated no discernable motor neuron phenotype, while gli3 knockdown reduced motor neuron induction in the hindbrain and spinal cord. Significantly, gli2 or gli3 knockdown in detour (gli1−) mutants revealed roles for Gli2 and Gli3 activator functions in ptc1 expression and spinal motor neuron induction. Similarly, gli1 or gli3 knockdown in yot (gli2DR) mutants resulted in severe or complete loss of motor neurons, and of ptc1 and net1a expression, in the hindbrain and spinal cord. In addition, gli1 expression was greatly reduced in yot mutants following gli3, but not gli1, knockdown, suggesting that Gli3 activator function is specifically required for gli1 expression. These observations demonstrate that Gli activator function (encoded by gli1, gli2, and gli3) is essential for motor neuron induction and Hh-regulated gene expression in zebrafish.

The zebrafish embryo is an excellent system for studying dynamic processes such as cell migration during vertebrate development. Dynamic analysis of neuronal migration in the zebrafish hindbrain has been hampered by morphogenetic movements in vivo, and by the impermeability of embryos. We have applied a recently reported technique of embryo explant culture to the analysis of neuronal development and migration in the zebrafish hindbrain. We show that hindbrain explants prepared at the somitogenesis stage undergo normal morphogenesis for at least 14 h in culture. Importantly, several aspects of hindbrain development such as patterning, neurogenesis, axon guidance, and neuronal migration are largely unaffected, inspite of increased cell death in explanted tissue. These results suggest that hindbrain explant culture can be employed effectively in zebrafish to analyze neuronal migration and other dynamic processes using pharmacological and imaging techniques.

Embryonic morphogenesis is driven by a suite of cell behaviours, including coordinated shape changes, cellular rearrangements and individual cell migrations, whose molecular determinants are largely unknown. In the zebrafish, Dani rerio, trilobite mutant embryos have defects in gastrulation movements1–4 and posterior migration of hindbrain neurons5. Here, we have used positional cloning to demonstrate that trilobite mutations disrupt the transmembrane protein Strabismus (Stbm)/Van Gogh (Vang), previously associated with planar cell polarity (PCP) in Drosophila melanogaster6,7, and PCP and canonical Wnt/β-catenin signalling in vertebrates8,9. Our genetic and molecular analyses argue that during gastrulation, trilobite interacts with the PCP pathway without affecting canonical Wnt signalling. Furthermore, trilobite may regulate neuronal migration independently of PCP molecules. We show that trilobite mediates polarization of distinct movement behaviours. During gastrulation convergence and extension movements, trilobite regulates mediolateral cell polarity underlying effective intercalation and directed dorsal migration at increasing velocities. In the hindbrain, trilobite controls effective migration of branchiomotor neurons towards posterior rhombomeres. Mosaic analyses show trilobite functions cell-autonomously and non-autonomously in gastrulae and the hindbrain. We propose Trilobite/Stbm mediates cellular interactions that confer directionality on distinct movements during vertebrate embryogenesis.

Failure of Notch signaling in zebrafish mind bomb (mib) mutants results in a neurogenic phenotype where an overproduction of early differentiating neurons is accompanied by the loss of later-differentiating cell types. We have characterized in detail the hindbrain phenotype of mib mutants. Hindbrain branchiomotor neurons (BMNs) are reduced in number but not missing in mib mutants. In addition, BMN clusters are frequently fused across the midline in mutants. Mosaic analysis indicates that the BMN patterning and fusion defects in the mib hindbrain arise non–cell autonomously. Ventral midline signaling is defective in the mutant hindbrain, in part due to the differentiation of some midline cells into neural cells. Interestingly, while early hindbrain patterning appears normal in mib mutants, subsequent rhombomere-specific gene expression is completely lost. The defects in ventral midline signaling and rhombomere patterning are accompanied by an apparent loss of neuroepithelial cells in the mutant hindbrain. These observations suggest that, by regulating the differentiation of neuroepithelial cells into neurons, Notch signaling preserves a population of non-neuronal cells that are essential for maintaining patterning mechanisms in the developing neural tube

The cranial motor neurons innervate muscles that control eye, jaw, and facial movements of the vertebrate head and parasympathetic neurons that innervate certain glands and organs. These efferent neurons develop at characteristic locations in the brainstem, and their axons exit the neural tube in well-defined trajectories to innervate target tissues. This review is focused on a subset of cranial motor neurons called the branchiomotor neurons, which innervate muscles derived from the branchial (pharyngeal) arches. First, the organization of the branchiomotor pathways in zebrafish, chick, and mouse embryos will be compared, and the underlying axon guidance mechanisms will be addressed. Next, the molecular mechanisms that generate branchiomotor neurons and specify their identities will be discussed. Finally, the caudally directed or tangential migration of facial branchiomotor neurons will be examined. Given the advances in the characterization and analysis of vertebrate genomes, we can expect rapid progress in elucidating the cellular and molecular mechanisms underlying the development of these vital neuronal networks.