Initial tract formation in the vertebrate brain

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

Astroglial exosome HepaCAM signaling and ApoE antagonization coordinates early postnatal cortical pyramidal neuronal axon growth and dendritic spine formation

Developing astroglia play important roles in regulating synaptogenesis through secreted and contact signals. Whether they regulate postnatal axon growth is unknown. By selectively isolating exosomes using size-exclusion chromatography (SEC) and employing cell-type specific exosome reporter mice, our current results define a secreted astroglial exosome pathway that can spread long-range in vivo and stimulate axon growth of cortical pyramidal neurons. Subsequent biochemical and genetic studies found that surface expression of glial HepaCAM protein essentially and sufficiently mediates the axon-stimulating effect of astroglial exosomes. Interestingly, apolipoprotein E (ApoE), a major astroglia-secreted cholesterol carrier to promote synaptogenesis, strongly inhibits the stimulatory effect of astroglial exosomes on axon growth. Developmental ApoE deficiency also significantly reduces spine density of cortical pyramidal neurons. Together, our study suggests a surface contact mechanism of astroglial exosomes in regulating axon growth and its antagonization by ApoE, which collectively coordinates early postnatal pyramidal neuronal axon growth and dendritic spine formation.

Zebrafish as a systems toxicology model for developmental neurotoxicity testing

The developing brain is extremely sensitive to many chemicals. Exposure to neurotoxicants during development has been implicated in various neuropsychiatric and neurological disorders, including autism spectrum disorder, attention deficit hyperactive disorder, schizophrenia, Parkinson's disease, and Alzheimer's disease. Although rodents have been widely used for developmental neurotoxicity testing, experiments using large numbers of rodents are time-consuming, expensive, and raise ethical concerns. Using alternative non-mammalian animal models may relieve some of these pressures by allowing testing of large numbers of subjects while reducing expenses and minimizing the use of mammalian subjects. In this review, we discuss some of the advantages of using zebrafish in developmental neurotoxicity testing, focusing on central nervous system development, neurobehavior, toxicokinetics, and toxicodynamics in this species. We also describe some important examples of developmental neurotoxicity testing using zebrafish combined with gene expression profiling, neuroimaging, or neurobehavioral assessment. Zebrafish may be a systems toxicology model that has the potential to reveal the pathways of developmental neurotoxicity and to provide a sound basis for human risk assessments.

Robo2--slit and Dcc--netrin1 coordinate neuron axonal pathfinding within the embryonic axon tracts

In the embryonic vertebrate brain, early born neurons establish highly stereotyped embryonic axonal tracts along which the neuronal interconnections form. To understand the mechanism underlying neuron axonal pathfinding within the embryonic scaffold of axon tracts, we studied zebrafish anterior dorsal telencephalic (ADt) neuron development. While previous studies suggest the ADt neuronal axons extend along a commissural tract [anterior commissure (AC)] and a descending ipsilateral tract [supraoptic tract (SOT)], it is unclear whether individual ADt neuronal axons choose specific projection paths at the intersection between the AC and the SOT. We labeled individual ADt neurons using a forebrain-specific promoter to drive expression of fluorescent proteins. We found the ADt axonal projection patterns were heterogeneous and correlated with their soma positions. Our results suggest that cell intrinsic differences along the dorsal ventral axis of the telencephalon regulate the axonal projection choices. Next, we determined that the guidance receptors roundabout2 (Robo2) and deleted in colorectal cancer (Dcc) were differentially expressed in the ADt neurons. We showed that knocking down Robo2 function by injecting antisense morpholino oligonucleotides abolished the ipsilateral SOT originating from the ADt neurons. Knocking down Dcc function did not prevent formation of the AC and the SOT. In contrast, the AC was specifically reduced when Netrin1 function was knocked down. Further mechanistic studies suggested that Robo2 responded to the repellent Slit signals and suppressed the attractive Netrin signals. These findings demonstrate how Robo2-Slit and Dcc-Netrin coordinate the axonal projection choices of the developing neurons in the vertebrate forebrain.

Robo1 and Robo2 have distinct roles in pioneer longitudinal axon guidance

Pioneer longitudinal axons grow long distances parallel to the floor plate and precisely maintain their positions using guidance molecules released from the floor plate. Two receptors, Robo1 and Robo2, are critical for longitudinal axon guidance by the Slit family of chemorepellents. Previous studies showed that Robo1(-/-);2(-/-) double mutant mouse embryos have disruptions in both ventral and dorsal longitudinal tracts. However, the role of each Robo isoform remained unclear, because Robo1 or 2 single mutants have mild or no errors. Here we utilized a more sensitive genetic strategy to reduce Robo levels for determining any separate functions of the Robo1 and 2 isoforms. We found that Robo1 is the predominant receptor for guiding axons in ventral tracts and prevents midline crossing. In contrast, Robo2 is the main receptor for directing axons within dorsal tracts. Robo2 also has a distinct function in repelling neuron cell bodies from the floor plate. Therefore, while Robo1 and 2 have some genetic overlap to cooperate in guiding longitudinal axons, each isoform has distinct functions in specific longitudinal axon populations.

Secondary neurogenesis and telencephalic organization in zebrafish and mice: a brief review

Most zebrafish neurodevelopmental studies have focused on the embryo, which is characterized by primary neurogenesis of mostly transient neurons. Secondary neurogenesis becomes dominant in the hatching larva, when major brain parts are established and begin to differentiate. This developmental period allows for a comparative analysis of zebrafish brain organization with amniotes at equivalent stages of neurogenesis. Within a particular time window, the early forebrains of mice (Embyronic stage [E] 12.5/13.5 days [d]) and zebrafish (3 d) reveal highly comparable expression patterns of genes involved in neurogenesis, for example proneural and other transcription factors (Neurogenin1, NeuroD, Mash1/Zashla and Pax6). Further topological correspondences are seen in the expression of LIM and homeobox genes, such as Lhx6/7, Tbr2 and Dlx2a. When this analysis is extended to gamma-aminobutyric acid/glutamic acid decarboxylase (GABA/GAD) cell patterns during this critical time window, an astonishing degree of similarity between the two species is again seen, for example regarding the presence of GABA/GAD cells in the subpallium, with the pallium only starting to be invaded by such cells from the subpallium. Furthermore, the expression of proneural and other genes correlates with GABA cell patterns (e.g. Mash1/Zash1a gene expression in GABA-positive and Neurogenin1/NeuroD in GABA-negative telencephalic regions) in mice and zebrafish. Data from additional vertebrates, such as Xenopus, are also highly consistent with this analysis. Therefore, the vertebrate forebrain appears to undergo a phylotypic stage of secondary neurogenesis, characterized by regionally separated GABAergic (inhibitory) versus glutamatergic (excitatory) cell production sites, which are obscured later in development by tangential migration. This period is highly advantageous for molecular neuroanatomical cross-species comparisons.

Development of the early axon scaffold in the rostral brain of the chick embryo

The arrangement of the early nerve connections in the embryonic vertebrate brain follows a well-conserved pattern, forming the early axon scaffold. The early axon tracts have been described in a number of anamniote species and in mouse, but a detailed analysis in chick is lacking. We have used immunostaining, axon tracing and in situ hybridisation to analyse the development of the early axon scaffold in the embryonic chick brain in relation to the neuromeric organisation of the brain. The first tract to be formed is the medial longitudinal fascicle (MLF), shortly followed by the tract of the postoptic commissure to pioneer the ventral longitudinal tract system. The MLF was found to originate from three different populations of neurones located in the diencephalon. Neurones close to the dorsal midline of the mesencephalon establish the descending tract of the mesencephalic nucleus of the trigeminus. Their axons pioneer the lateral longitudinal tract. At later stages, the tract of the posterior commissure emerges in the caudal pretectum as the first transversal tract. It is formed by dorsally projecting axons from neurones located in the ventral pretectum, and by ventrally projecting axons from neurones located in the dorsal pretectum. The organisation of neurones and axons in the chick brain is similar to that described in the mouse, though tracts form in a different order and appear more clearly distinguished than in the mammalian model.

The Enhancer of split transcription factor Her8a is a novel dimerisation partner for Her3 that controls anterior hindbrain neurogenesis in zebrafish

Background

Neurogenesis control and the prevention of premature differentiation in the vertebrate embryo are crucial processes, allowing the formation of late-born cell types and ensuring the correct shape and cytoarchitecture of the brain. Members of the Hairy/Enhancer of Split (Hairy/E(spl)) family of bHLH-Orange transcription factors, such as zebrafish Her3, 5, 9 and 11, are implicated in the local inhibition of neurogenesis to maintain progenitor pools within the early neural plate. To better understand how these factors exert their inhibitory function, we aimed to isolate some of their functional interactors.

Results

We used a yeast two-hybrid screen with Her5 as bait and recovered a novel zebrafish Hairy/E(spl) factor - Her8a. Using phylogenetic and synteny analyses, we demonstrate that her8a evolved from an ancient duplicate of Hes6 that was recently lost in the mammalian lineage. We show that her8a is expressed across the mid- and anterior hindbrain from the start of segmentation. Through knockdown and misexpression experiments, we demonstrate that Her8a is a negative regulator of neurogenesis and plays an essential role in generating progenitor pools within rhombomeres 2 and 4 - a role resembling that of Her3. Her8a co-purifies with Her3, suggesting that Her8a-Her3 heterodimers may be relevant in this domain of the neural plate, where both proteins are co-expressed. Finally, we demonstrate that her8a expression is independent of Notch signaling at the early neural plate stage but that SoxB factors play a role in its expression, linking patterning information to neurogenesis control. Overall, the regulation and function of Her8a differ strikingly from those of its closest relative in other vertebrates - the Hes6-like proteins.

Conclusions

Our results characterize the phylogeny, expression and functional interactions involving a new Her factor, Her8a, and highlight the complex interplay of E(spl) proteins that generates the neurogenesis pattern of the zebrafish early neural plate.

Longitudinal axons are guided by Slit/Robo signals from the floor plate

Longitudinal axons grow long distances along precise pathways to connect major CNS regions. However, during embryonic development, it remains largely undefined how the first longitudinal axons choose specific positions and grow along them. Here, we review recent evidence identifying a critical role for Slit/Robo signals to guide pioneer longitudinal axons in the embryonic brain stem. These studies indicate that Slit/Robo signals from the floor plate have dual functions: to repel longitudinal axons away from the ventral midline, and also to maintain straight longitudinal growth. These dual functions likely cooperate with other guidance cues to establish the major longitudinal tracts in the brain.

Wiring the brain: the biology of neuronal guidance

The mammalian brain is the most complex organ in the body. It controls all aspects of our bodily functions and interprets the world around us through our senses. It defines us as human beings through our memories and our ability to plan for the future. Crucial to all these functions is how the brain is wired in order to perform these tasks. The basic map of brain wiring occurs during embryonic and postnatal development through a series of precisely orchestrated developmental events regulated by specific molecular mechanisms. Below we review the most important features of mammalian brain wiring derived from work in both mammals and in nonmammalian species. These mechanisms are highly conserved throughout evolution, simply becoming more complex in the mammalian brain. This fascinating area of biology is uncovering the essence of what makes the mammalian brain able to perform the everyday tasks we take for granted, as well as those which give us the ability for extraordinary achievement.

Netrin-DCC, Robo-Slit, and heparan sulfate proteoglycans coordinate lateral positioning of longitudinal dopaminergic diencephalospinal axons

Longitudinal axons provide connectivity between remote areas of the nervous system. Although the molecular determinants driving commissural pathway formation have been well characterized, mechanisms specifying the formation of longitudinal axon tracts in the vertebrate nervous system are largely unknown. Here, we study axon guidance mechanisms of the longitudinal dopaminergic (DA) diencephalospinal tract. This tract is established by DA neurons located in the ventral diencephalon and is thought to be involved in modulating locomotor activity. Using mutant analysis as well as gain of function and loss of function experiments, we demonstrate that longitudinal DA axons navigate by integrating long-range signaling of midline-derived cues. Repulsive Robo2/Slit signaling keeps longitudinal DA axons away from the midline. In the absence of repulsive Robo2/Slit function, DA axons are attracted toward the midline by DCC (deleted in colorectal cancer)/Netrin1 signaling. Thus, Slit-based repulsion counteracts Netrin-mediated attraction to specify lateral positions of the DA diencephalospinal tract. We further identified heparan sulfate proteglycans as essential modulators of DA diencephalospinal guidance mechanisms. Our findings provide insight into the complexity of positioning far-projecting longitudinal axons and allow us to provide a model for DA diencephalospinal pathfinding. Simultaneous integrations of repulsive and attractive long-range cues from the midline act in a concerted manner to define lateral positions of DA longitudinal axon tracts.

Gsk3beta/PKA and Gli1 regulate the maintenance of neural progenitors at the midbrain-hindbrain boundary in concert with E(Spl) factor activity

Neuronal production in the midbrain-hindbrain domain (MH) of the vertebrate embryonic neural tube depends on a progenitor pool called the ;intervening zone' (IZ), located at the midbrain-hindbrain boundary. The progressive recruitment of IZ progenitors along the mediolateral (future dorsoventral) axis prefigures the earlier maturation of the MH basal plate. It also correlates with a lower sensitivity of medial versus lateral IZ progenitors to the neurogenesis inhibition process that maintains the IZ pool. This role is performed in zebrafish by the E(Spl) factors Her5 and Her11, but the molecular cascades cooperating with Her5/11, and those accounting for their reduced effect in the medial IZ, remain unknown. We demonstrate here that the kinases Gsk3beta and cAMP-dependent protein kinase A (PKA) are novel determinants of IZ formation and cooperate with E(Spl) activity in a dose-dependent manner. Similar to E(Spl), we show that the activity of Gsk3beta/PKA is sensed differently by medial versus lateral IZ progenitors. Furthermore, we identify the transcription factor Gli1, expressed in medial IZ cells, as an antagonist of E(Spl) and Gsk3beta/PKA, and demonstrate that the neurogenesis-promoting activity of Gli1 accounts for the reduced sensitivity of medial IZ progenitors to neurogenesis inhibitors and their increased propensity to differentiate. We also show that the expression and activity of Gli1 in this process are, surprisingly, independent of Hedgehog signaling. Together, our results suggest a model in which the modulation of E(Spl) and Gsk3beta/PKA activities by Gli1 underlies the dynamic properties of IZ maintenance and recruitment.

Pioneer longitudinal axons navigate using floor plate and Slit/Robo signals

Longitudinal axons transmit all signals between the brain and spinal cord. Their axon tracts through the brain stem are established by a simple set of pioneer axons with precise trajectories parallel to the floor plate. To identify longitudinal guidance mechanisms in vivo, the overall role of floor plate tissue and the specific roles of Slit/Robo signals were tested. Ectopic induction or genetic deletion of the floor plate diverted longitudinal axons into abnormal trajectories. The expression patterns of the diffusible cues of the Slit family were altered in the floor plate experiments, suggesting their involvement in longitudinal guidance. Genetic tests of Slit1 and Slit2, and the Slit receptors Robo1 and Robo2 were carried out in mutant mice. Slit1;Slit2 double mutants had severe longitudinal errors, particularly for ventral axons, including midline crossing and wandering longitudinal trajectories. Robo1 and Robo2 were largely genetically redundant, and neither appeared to specify specific tract positions. However, combined Robo1 and Robo2 mutations strongly disrupted each pioneer tract. Thus, pioneer axons depend on long-range floor plate cues, with Slit/Robo signaling required for precise longitudinal trajectories.

Pax6 guides a relay of pioneer longitudinal axons in the embryonic mouse forebrain

We have characterized a system of early neurons that establish the first two major longitudinal tracts in the embryonic mouse forebrain. Axon tracers and antibody labels were used to map the axon projections in the thalamus from embryonic days 9.0-12, revealing several distinct neuron populations that contributed to the first tracts. Each of the early axon populations first grew independently, pioneering a short segment of new tract. However, each axon population soon merged with other axons to form one of only two shared longitudinal tracts, both descending: the tract of the postoptic commissure (TPOC), and, in parallel, the stria medullaris. Thus, the forebrain longitudinal tracts are pioneered by a relay of axons, with distinct axon populations pioneering successive segments of these pathways. The extensive merging of tracts suggests that axon-axon interactions are a major guidance mechanism for longitudinal axons. Several axon populations express tyrosine hydroxylase, identifying the TPOC as a major pathway for forebrain dopaminergic projections. To start a genetic analysis of pioneer axon guidance, we have identified the transcription factor Pax6 as critical for tract formation. In Pax6 mutants, both longitudinal tracts failed to form due to errors by every population of early longitudinal axons. Taken together, these results have identified potentially important interactions between series of pioneer axons and the Pax6 gene as a general regulator of longitudinal tract formation in the forebrain.

Local inhibition guides the trajectory of early longitudinal tracts in the developing chick brain

During development of the chick central nervous system, the trajectories of the descending medial and lateral longitudinal fascicles (MLF and LLF) are pioneered by axons originating from the interstitial nucleus of Cajal (INC) and the mesencephalic trigeminal nucleus (MTN), respectively. Both tracts cross rhombomere 1 at two specific locations in the basal plate. In this study, we have investigated the molecular properties of these crossing points and find that they are permissive regions situated in an otherwise inhibitory boundary region. We show that the dorsal part of rhombomere 1 is inhibitory for the growth of both MTN and INC axons. Ventrally, MLF and LLF axons are repelled from the midline by Slit proteins. Our results reveal the existence of a new repulsive/inhibitory mechanism for axons in the alar plate in addition to the ventral repulsion by Slit proteins. This suggests a model where MLF and LLF axons are channeled longitudinally within the neural tube by both dorsal and ventral constraints.

Dynamics of neural crest-derived cell migration in the embryonic mouse gut

Neural crest-derived cells that form the enteric nervous system undergo an extensive migration from the caudal hindbrain to colonize the entire gastrointestinal tract. Mice in which the expression of GFP is under the control of the Ret promoter were used to visualize neural crest-derived cell migration in the embryonic mouse gut in organ culture. Time-lapse imaging revealed that GFP(+) crest-derived cells formed chains that displayed complicated patterns of migration, with sudden and frequent changes in migratory speed and trajectories. Some of the leading cells and their processes formed a scaffold along which later cells migrated. To examine the effect of population size on migratory behavior, a small number of the most caudal GFP(+) cells were isolated from the remainder of the population. The isolated cells migrated slower than cells in large control populations, suggesting that migratory behavior is influenced by cell number and cell-cell contact. Previous studies have shown that neurons differentiate among the migrating cell population, but it is unclear whether they migrate. The phenotype of migrating cells was examined. Migrating cells expressed the neural crest cell marker, Sox10, but not neuronal markers, indicating that the majority of migratory cells observed did not have a neuronal phenotype.

Her5 acts as a prepattern factor that blocks neurogenin1 and coe2 expression upstream of Notch to inhibit neurogenesis at the midbrain-hindbrain boundary

Neurogenesis in both vertebrates and invertebrates is tightly controlled in time and space involving both positive and negative regulators. We report here that the bHLH factor Her5 acts as a prepattern gene to prevent neurogenesis in the anlage of the midbrain/hindbrain boundary in the zebrafish neural plate. This involves selective suppression of both neurogenin1(ngn1) and coe2 mRNA expression in a process that is independent of Notch signalling, and where inhibition of either ngn1or coe2 expression is sufficient to prevent neuronal differentiation across the midbrain-hindbrain boundary. A ngn1 transgene faithfully responds to Her5 and deletion analysis of the transgene identifies an E-box in a ngn1 upstream enhancer to be required for repression by Her5. Together our data demonstrate a role of Her5 as a prepattern factor in the spatial definition of proneural domains in the zebrafish neural plate, in a manner similar to its Drosophila homologue Hairy.

Selective control of neuronal cluster size at the forebrain/midbrain boundary by signaling from the prechordal plate

Within the vertebrate embryonic neural plate, the first neuronal clusters often differentiate at the border of patterning identities. Whether the information inherent in the intersection of patterning identities alone controls all aspects of neuronal cluster development (location, identity, and size) is unknown. Here, we focus on the cluster of the medial longitudinal fascicle (nMLF) and posterior commissure (nPC), located at the forebrain/midbrain (fore/mid) boundary, to address this issue. We first identify expression of the transcription factor Six3 as a common and distinct molecular signature of nMLF and nPC neurons in zebrafish, and we use this marker to monitor mechanisms controlling the location and number of nMLF/nPC neurons. We demonstrate that six3 expression is induced at the fore/mid boundary in pax2.1/no-isthmus and smoothened/slow muscle omitted mutants, where identities adjacent to the six3 cluster are altered; however, in these mutants, the subpopulation of six3-positive cells located within the mispatterned territory is reduced. These results show that induction of the six3 cluster is triggered by the information derived from the intersection in patterning identities alone, whereas correct cluster size depends, in a modular manner, on the identities themselves. The size of the six3 cluster is also controlled independently of neural tube patterning: we demonstrate that the prechordal plate (PCP) is impaired in mixer/bonnie and clyde mutants and that this phenotype secondarily results in an increased production of six3-positive cells at the fore/mid boundary, without correlatively affecting patterning in this area. Thus, a signaling process originating from the PCP distinguishes between neural patterning and the control of six3 cluster size at the fore/mid junction in vivo. Together, our results suggest that a combination of patterning-related and -unrelated mechanisms specifically controls the size of individual early neuronal clusters within the anterior neural plate.

bHLH transcription factor Her5 links patterning to regional inhibition of neurogenesis at the midbrain-hindbrain boundary

The midbrain-hindbrain (MH) domain of the vertebrate embryonic neural plate displays a stereotypical profile of neuronal differentiation, organized around a neuron-free zone ('intervening zone', IZ) at the midbrain-hindbrain boundary (MHB). The mechanisms establishing this early pattern of neurogenesis are unknown. We demonstrate that the MHB is globally refractory to neurogenesis, and that forced neurogenesis in this area interferes with the continued expression of genes defining MHB identity. We further show that expression of the zebrafish bHLH Hairy/E(spl)-related factor Her5 prefigures and then precisely delineates the IZ throughout embryonic development. Using morpholino knock-down and conditional gain-of-function assays, we demonstrate that Her5 is essential to prevent neuronal differentiation and promote cell proliferation in a medial compartment of the IZ. We identify one probable target of this activity, the zebrafish Cdk inhibitor p27Xic1. Finally, although the her5 expression domain is determined by anteroposterior patterning cues, we show Her5 does not retroactively influence MH patterning. Together, our results highlight the existence of a mechanism that actively inhibits neurogenesis at the MHB, a process that shapes MH neurogenesis into a pattern of separate neuronal clusters and might ultimately be necessary to maintain MHB integrity. Her5 appears as a partially redundant component of this inhibitory process that helps translate early axial patterning information into a distinct spatiotemporal pattern of neurogenesis and cell proliferation within the MH domain.

Induction and patterning of neuronal development, and its connection to cell cycle control

Nervous tissue is derived from early embryonic ectoderm, which also gives rise to epidermal derivatives such as skin. The progression from naive ectoderm to differentiated postmitotic neurons involves multiple steps, two of which are crucial in shaping the final neurogenesis pattern. First, is the identification of the neural plate by the process of neural induction. Second, is the selection of a restricted number of sites within the neural plate where neurogenesis, the process leading to final differentiation of neural precursors, is initiated. Recent findings point to the existence of positive inducers of the neural state, whereas, neurogenesis initiation sites appear to be largely defined by inhibition. However, both neural induction and the initiation of neurogenesis appear to be connected to cell cycle control systems that govern whether stem cell maintenance and cell proliferation, or cell specification and differentiation, take place.

Axon routing at the optic chiasm after enzymatic removal of chondroitin sulfate in mouse embryos

The effects of removing chondroitin sulfate from chondroitin sulfate proteoglycan molecules on guidance of retinal ganglion cell axons at the optic chiasm were investigated in a brain slice preparation of mouse embryos of embryonic day 13 to 15. Slices were grown for 5 hours and growth of dye-labeled axons was traced through the chiasm. After continuous enzymatic digestion of the chondroitin sulfate proteoglycans with chondroitinase ABC, which removes the glycosaminoglycan chains, navigation of retinal axons was disrupted. At embryonic day 13, before the uncrossed projection forms in normal development, many axons deviated from their normal course, crossing the midline at aberrant positions and invading the ventral diencephalon. In slices from embryonic day 14 embryos, axons that would normally form the uncrossed projection at this stage failed to turn into the ipsilateral optic tract. In embryonic day 15 slices, enzyme treatment caused a reduction of the uncrossed projection that develops at this stage. Growth cones in enzyme-treated slices showed a significant increase in the size both before and after they crossed the midline. This indicates that responses of retinal axons to guidance signals at the chiasm have changed after removal of the chondroitin sulfate epitope. We concluded that the chondroitin sulfate moieties of the proteoglycans are involved in patterning the early phase of axonal growth across the midline and at a later stage controlling the axon divergence at the chiasm.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Molecular cloning of Zcoe2, the zebrafish homolog of Xenopus Xcoe2 and mouse EBF-2, and its expression during primary neurogenesis

Xcoe2 is a recently identified HLH transcription factor of the Xenopus primary neurogenesis pathway, which is necessary downstream of Neurogenin to stabilize neuroblast determination (Dubois, L. et al., 1998. Curr. Biol. 8, 199-209). We report here the embryonic expression pattern of Zcoe2, its zebrafish homolog. As observed for Xcoe2, Zcoe2 is strongly expressed in a subset of the neurogenin1- (ngn1-) positive primary neuroblasts of the spinal cord. In the anterior neural plate, in contrast, Zcoe2 is expressed earlier and more widely than ngn1. This pattern is strongly maintained in the presumptive mesencephalon and rhombomeres 1-4 until the 2-3-somite stage. This expression of Zcoe2 in the brain anlage calls for a re-analysis in zebrafish of the functional relationship demonstrated in Xenopus between Coe2 and Neurogenin factors. At later stages, Zcoe2 is expressed in early forming neurons of the anterior brain and is a marker of the olfactory placodes.

Zebrafish TrkC1 and TrkC2 receptors define two different cell populations in the nervous system during the period of axonogenesis

We identified previously five distinct trk genes in the zebrafish. The structures of two of these, TrkC1 and TrkC2, are most similar to mammalian TrkC. Detailed sequence comparisons reported here indicate that although the similarities to TrkC are greatest in those regions of the extracellular domain implicated in ligand binding, the two sequences also differ significantly in these regions. Whole-mount in situ hybridization experiments in the early embryo revealed full-length trkC1 but no trkC2 transcripts in the cranial ganglia and in a subset of Rohon-Beard neurons. At the same time, full-length trkC2 but no trkC1 transcripts were detected laterally in the spinal cord, in the caudal hindbrain, in reticulospinal neurons of rhombomeres 4, 5, and 6, and in the midbrain. Both types of transcripts were expressed in clusters of cells in the dorsal telencephalon and the nucleus of the tract of the postoptic commissure. These results suggest distinct functions of trkC1 and trkC2 in nervous system development. The expression patterns define two different neuronal populations in the zebrafish.

Regeneration in the developing optic nerve: correlating observations in the opossum to other mammalian systems

Regeneration of severed axons within the central nervous system of adult mammals does not normally occur with any degree of success. During development, however, newly forming projections must send axons to distant sites and form appropriate connections with their targets: successful regeneration has been observed during this critical period. The opossum central nervous system develops during early postnatal life and has provided a useful experimental model to investigate this specialized mode of axonal regeneration in mammals. The presence of a clear decision point at the optic chiasm has also provided a useful site at which to investigate the navigational capacity of retinal ganglion cells regenerating along the optic nerve during this critical period. Regeneration failure occurs as the central nervous system progresses from this permissive, developing state to a mature, non-permissive adult state. Studies into the behaviour of glial and neuronal elements around this transition period can help elucidate some of the factors that need to be overcome if regeneration is ever to become successful in adult mammals. The regeneration characteristics of a lesioned projection are dependent upon its developmental stage and are also related to the proximity of axotomy along its pathway. A system of staging is proposed to correlate observations in the opossum optic nerve to other mammalian systems.

B-50/growth-associated protein-43, a marker of neural development in Xenopus laevis

To study the regulation and function of the growth-associated protein B-50/growth-associated protein-43 (mol. wt 43,000) in Xenopus laevis, B-50/growth-associated protein-43 complementary DNAs were isolated and characterized. The deduced amino acid sequence revealed potential functional domains of Xenopus B-50/growth-associated protein-43 that may be involved in G-protein interaction, membrane-binding, calmodulin-binding and protein kinase C phosphorylation. The expression of B-50/growth-associated protein-43 at the RNA and protein level during development was investigated using the Xenopus complementary DNA and the monoclonal B-50/growth-associated protein-43 antibody NM2. The antibody NM2 recognized the gene product on western blot and in whole-mount immunocytochemistry of Xenopus embryos. Moreover, visualization of the developmentally regulated appearance of B-50/growth-associated protein-43 immunoreactivity showed that this mode of detection may be used to monitor axonogenesis under various experimental conditions. In the adult Xenopus, XB-50/growth-associated protein-43 messenger RNA was shown to be expressed at high levels in brain, spinal cord and eye using northern blotting. The earliest expression detected on northern blot was at developmental stage 13 with poly(A) RNA. By whole-mount immunofluorescence, applying the confocal laser scanning microscope, the protein was first detected in embryos from stage 20, where it was expressed in the developing trigeminal ganglion. Also later in development the expression of the B-50/growth-associated protein-43 gene was restricted to the nervous system in Xenopus, as was previously found for the mouse. In conclusion, we find that XB-50/growth-associated protein-43 is a good marker to study the development of the nervous system in Xenopus laevis.

Expression of glial fibrillary acidic protein and its relation to tract formation in embryonic zebrafish (Danio rerio)

To address possible roles of glial cells during axon outgrowth in the vertebrate central nervous system, we investigated the appearance and distribution of the glial-specific intermediate filament, glial fibrillary acidic protein (GFAP), during early embryogenesis of the zebrafish (Danio rerio). Immunopositive cells first appear at 15 hours, which is at the time of, or slightly before, the first axon outgrowth in the brain. Immunopositive processes are not initially present in a pattern that prefigures the location of the first tracts but rather are distributed widely as endfeet adjacent to the pia, overlying most of the surface of the brain with the exception of the dorsal and ventral midline. The first evidence for a specific association of immunopositive cells with the developing tracts is observed at 24 hours in the hindbrain, where immunopositive processes border axons in the medial longitudinal fasciculus. By 48 hours, immunopositive processes have disappeared from most of the subpial lamina and are found exclusively in association with tracts and commissures in three forms: endfeet, radially oriented processes, and tangentially oriented processes parallel to axons. This last form is particularly prominent in the transverse plane of the hindbrain, where they define the boundaries between rhombomeres. These results suggest that glial cells contribute to the development and organization of the central nervous system by supporting early axon outgrowth in the subpial lamina and by forming boundaries around tracts and between neuromeres. The results are discussed in relation to previous results on neuron-glia interactions and possible roles of glial cells in axonal guidance.

Initial tract formation in the brain of the chick embryo: selective expression of the BEN/SC1/DM-GRASP cell adhesion molecule

This study reports the spatio-temporal pattern of BEN expression (a molecule of the immunoglobulin superfamily) during early stages of the first axonal tract formation, in the fore- and midbrain of chick embryos [Hamburger and Hamilton (HH) stages 12-22]. The expression of BEN has been analysed using immunohistochemistry and non-radioactive in situ hybridization. Furthermore, double labelling experiments (combining anti-class III beta-tubulin, a pan-neuronal marker, and anti-BEN antibodies) have been carried out to determine whether BEN is expressed by all first axonal tracts. The first neurons expressing BEN appear around stage HH13-14, in the caudal diencephalon. They belong to the interstitial nucleus of Cajal, and their axons are the first components of the medial longitudinal fasciculus. By HH14, two other early axonal tracts appear: the tract of the postoptic commissure and the descending root of the mesencephalic nucleus of the trigeminal nerve. Only the latter expresses BEN. At later stages of development numerous new axonal tracts appear in the telencephalic, diencephalic and mesencephalic domains. Only a few of them (the fourth nerve, the lemniscus lateralis, the tectobulbar and habenulopeduncular tracts) express BEN. In all BEN positive systems, the cell bodies, axons and growth cones are uniformly labelled by the antibody. We have found that none of the early axonal tracts grows preferentially at interneuromeric boundaries. Moreover, each tract is formed by several thin fascicles rather than a single one. The expression of BEN is transient and disappears shortly before hatching. These results suggest that BEN may serve to promote axonal outgrowth of precise neuronal systems involved in 'axonal scaffolding'.