Axon guidance at choice points

Mesocorticolimbic Dopamine Pathways Across Adolescence: Diversity in Development

Mesocorticolimbic dopamine circuity undergoes a protracted maturation during adolescent life. Stable adult levels of behavioral functioning in reward, motivational, and cognitive domains are established as these pathways are refined, however, their extended developmental window also leaves them vulnerable to perturbation by environmental factors. In this review, we highlight recent advances in understanding the mechanisms underlying dopamine pathway development in the adolescent brain, and how the environment influences these processes to establish or disrupt neurocircuit diversity. We further integrate these recent studies into the larger historical framework of anatomical and neurochemical changes occurring during adolescence in the mesocorticolimbic dopamine system. While dopamine neuron heterogeneity is increasingly appreciated at molecular, physiological, and anatomical levels, we suggest that a developmental facet may play a key role in establishing vulnerability or resilience to environmental stimuli and experience in distinct dopamine circuits, shifting the balance between healthy brain development and susceptibility to psychiatric disease.

Nectin spot: a novel type of nectin-mediated cell adhesion apparatus

Nectins are Ca(2+)-independent immunoglobulin (Ig) superfamily cell adhesion molecules constituting a family with four members, all of which have three Ig-like loops at their extracellular regions. Nectins play roles in the formation of a variety of cell-cell adhesion apparatuses. There are at least three types of nectin-mediated cell adhesions: afadin- and cadherin-dependent, afadin-dependent and cadherin-independent, and afadin- and cadherin-independent. In addition, nectins trans-interact with nectin-like molecules (Necls) with three Ig-like loops and other Ig-like molecules with one to three Ig-like loops. Furthermore, nectins and Necls cis-interact with membrane receptors and integrins, some of which are associated with the nectin-mediated cell adhesions, and play roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, and survival, co-operatively with these cell surface proteins. The nectin-mediated cell adhesions are implicated in a variety of diseases, including genetic disorders, neural disorders, and cancers. Of the three types of nectin-mediated cell adhesions, the afadin- and cadherin-dependent apparatus has been most extensively investigated, but the examples of the third type of apparatus independent of afadin and cadherin are recently increasing and its morphological and functional properties have been well characterized. We review here recent advances in research on this type of nectin-mediated cell adhesion apparatus, which is named nectin spot.

Two specific populations of GABAergic neurons originating from the medial and the caudal ganglionic eminences aid in proper navigation of callosal axons

The corpus callosum (CC) plays a crucial role in interhemispheric communication. It has been shown that CC formation relies on the guidepost cells located in the midline region that include glutamatergic and GABAergic neurons as well as glial cells. However, the origin of these guidepost GABAergic neurons and their precise function in callosal axon pathfinding remain to be investigated. Here, we show that two distinct GABAergic neuronal subpopulations converge toward the midline prior to the arrival of callosal axons. Using in vivo and ex vivo fate mapping we show that CC GABAergic neurons originate in the caudal and medial ganglionic eminences (CGE and MGE) but not in the lateral ganglionic eminence (LGE). Time lapse imaging on organotypic slices and in vivo analyses further revealed that CC GABAergic neurons contribute to the normal navigation of callosal axons. The use of Nkx2.1 knockout (KO) mice confirmed a role of these neurons in the maintenance of proper behavior of callosal axons while growing through the CC. Indeed, using in vitro transplantation assays, we demonstrated that both MGE- and CGE-derived GABAergic neurons exert an attractive activity on callosal axons. Furthermore, by combining a sensitive RT-PCR technique with in situ hybridization, we demonstrate that CC neurons express multiple short and long range guidance cues. This study strongly suggests that MGE- and CGE-derived interneurons may guide CC axons by multiple guidance mechanisms and signaling pathways.

Chicken wings and the brachial plexus

OBJECTIVES

Some stages of limb development can now be described in terms of gene sequences and functions. This paper reports on the development of the brachial plexus (BP) in the chick. It also presents a short review on the principles of the peripheral nerve outgrowth.

METHODS

The early development of the brachial plexus of chicken embryos is mapped using immunohistochemistry. This is then analysed in relation to the expression pattern of an axonal guidance gene, Semaphorin3a, by in situ hybridization studies.

RESULTS

The motor axons that innervate the chick wing emerge from the spinal cord in spinal nerves 12-17. These axons grow towards the developing limb and then congregate at its base to form the plexus. In response to unknown cues, these axons rearrange, before emerging in the defined nerve trunks that innervate the limb. The developmental stages of BP morphogenesis described here closely correlate with previous reports with a significant difference of a shorter 'waiting period'.

DISCUSSION

The development of the brachial plexus is now better understood. The waiting period, with more modern techniques, is observed to be shorter than previously reported. The significance of this and the role of the guidance molecule, Semaphorin3a, in this process, are being investigated and the results may have important implications on the management of brachial plexus palsy and other peripheral nerve lesions.

Building Spinal and Brain Commissures: Axon Guidance at the Midline

Commissural circuits are brain and spinal cord connections which interconnect the two sides of the central nervous system (CNS). They play essential roles in brain and spinal cord processing, ensuring left-right coordination and synchronization of information and commands. During the formation of neuronal circuits, all commissural neurons of the central nervous system must accomplish a common task, which is to project their axon onto the other side of the nervous system, across the midline that delineates the two halves of the CNS. How this task is accomplished has been the topic of extensive studies over the last past 20 years and remains one of the best models to investigate axon guidance mechanisms. In the first part of this review, I will introduce the commissural circuits, their general role in the physiology of the nervous system, and their recognized or suspected pathogenic properties in human diseases. In the second part of the review, I will concentrate on two commissural circuits, the spinal commissures and the corpus callosum, to detail the cellular and molecular mechanisms governing their formation, mostly during their navigation at the midline.

The role of nectins in different types of cell-cell adhesion

Mammalian tissues and organs are composed of different types of cells that adhere to each other homotypically (i.e. interactions between cells of the same cell type) or heterotypically (i.e. interactions between different cell types), forming a variety of cellular patterns, including mosaic patterns. At least three types of cell-cell adhesion have been observed: symmetric homotypic, asymmetric homotypic and heterotypic cell adhesions. Cadherins and nectins, which are known cell-cell adhesion molecules, mediate these cell adhesions. Cadherins comprise a family of more than 100 members, but they are primarily involved in homophilic trans-interactions (i.e. interactions between the same cadherin members) between opposing cells. By contrast, the nectin family comprises only four members, and these proteins form both homophilic and heterophilic trans-interactions (i.e. interactions between the same and different nectin members on opposing cells). In addition, heterophilic trans-interactions between nectins are much stronger than homophilic trans-interactions. Because of these unique properties, nectins have crucial roles in asymmetric homotypic cell-cell adhesion at neuronal synapses and in various types of heterotypic cell-cell adhesions. We summarize recent progress in our understanding of the biology of nectins and discuss their roles in heterotypic cell-cell adhesions, whose formation cannot be solely explained by the action of cadherins.

RabGDI controls axonal midline crossing by regulating Robo1 surface expression

BACKGROUND

Axons navigate to their future synaptic targets with the help of choice points, intermediate targets that express axon guidance cues. Once they reach a choice point, axons need to switch their response from attraction to repulsion in order to move on with the next stage of their journey. The mechanisms underlying the change in axonal responsiveness are poorly understood. Commissural axons become sensitive to the repulsive activity of Slits when they cross the ventral midline of the CNS. Responsiveness to Slits depends on surface expression of Robo receptors. In Drosophila, Commissureless (Comm) plays a crucial regulatory role in midline crossing by keeping Robo levels low on precommissural axons. Interestingly, to date no vertebrate homolog of comm has been identified. Robo3/Rig1 has been shown to control Slit sensitivity before the midline, but without affecting Robo1 surface expression.

RESULTS

We had identified RabGDI, a gene linked to human mental retardation and an essential component of the vesicle fusion machinery, in a screen for differentially expressed floor-plate genes. Downregulation of RabGDI by in ovo RNAi caused commissural axons to stall in the floor plate, phenocopying the effect observed after downregulation of Robo1. Conversely, premature expression of RabGDI prevented commissural axons from entering the floor plate. Furthermore, RabGDI triggered Robo1 surface expression in cultured commissural neurons. Taken together, our results identify RabGDI as a component of the switching mechanism that is required for commissural axons to change their response from attraction to repulsion at the intermediate target.

CONCLUSION

RabGDI takes over the functional role of fly Comm by regulating the surface expression of Robo1 on commissural axons in vertebrates. This in turn allows commissural axons to switch from attraction to repulsion at the midline of the spinal cord.

Roles of nectins in cell adhesion, signaling and polarization

Nectins are Ca(2+)-independent immunoglobulin-like cell-cell adhesion molecules which constitute a family of four members. Nectins homophilically and heterophilically trans-interact and cause cell-cell adhesion. This nectin-based cell-cell adhesion plays roles in the organization of adherens junctions in epithelial cells and fibroblasts and synaptic junctions in neurons in cooperation with cadherins. The nectin-based cell-cell adhesion plays roles in the contacts between commissural axons and floor plate cells and in the organization of Sertoli cell-spermatid junctions in the testis, independently of cadherins. Nectins furthermore regulate intracellular signaling through Cdc42 and Rac small G proteins and cell polarization through cell polarity proteins. Pathologically, nectins serve as entry and cell-cell spread mediators of herpes simplex viruses.

Differential expression of neuronal genes in Müller glia in two- and three-dimensional cultures

PURPOSE

Müller glia in the mammalian retina have some stem cell-like characteristics, although their capacity for neurogenesis remains limited both in vivo and in vitro. In vitro studies to date have used traditional two-dimensional (2D) cell culture to assess neuronal differentiation of Müller glia. The purpose of this study was to compare the effects of 2D and three-dimensional (3D) environments on Müller glial gene expression after growth factor stimulation.

METHODS

Conditionally immortalized mouse Müller glia cells (ImM10) were cultured under nonimmortalizing conditions with EGF/FGF2 to generate spheres that were differentiated in vitro on uncoated culture dishes (2D) or encapsulated in self-assembling, RADA-16 peptide hydrogels (3D) under identical media and growth factor supplementation conditions. Gene expression was analyzed using quantitative RT-PCR and immunocytochemistry. Cellular morphology was analyzed with light and confocal microscopy; sphere ultrastructure was analyzed with transmission electron microscopy.

RESULTS

ImM10 Müller cells express numerous genes associated with neural stem cells and retinal progenitors in both normal growth conditions and sphere-forming conditions. When encapsulated in the 3D hydrogel, cells can migrate and send processes into the hydrogel. Many genes associated with neurogenesis, as well as retinal neuron-specific genes, are differentially expressed in 2D and 3D differentiation conditions.

CONCLUSIONS

ImM10 Müller glia upregulate genes characteristic of retinal neurons after growth factor stimulation in vitro, and gene expression patterns are altered in 3D hydrogel cultures.

Forced notch signaling inhibits commissural axon outgrowth in the developing chick central nerve system

Background

A collection of in vitro evidence has demonstrated that Notch signaling plays a key role in the growth of neurites in differentiated neurons. However, the effects of Notch signaling on axon outgrowth in an in vivo condition remain largely unknown.

Methodology/Principal Findings

In this study, the neural tubes of HH10-11 chick embryos were in ovo electroporated with various Notch transgenes of activating or inhibiting Notch signaling, and then their effects on commissural axon outgrowth across the floor plate midline in the chick developing central nerve system were investigated. Our results showed that forced expression of Notch intracellular domain, constitutively active form of RBPJ, or full-length Hes1 in the rostral hindbrain, diencephalon and spinal cord at stage HH10-11 significantly inhibited commissural axon outgrowth. On the other hand, inhibition of Notch signaling by ectopically expressing a dominant-negative form of RBPJ promoted commissural axonal growth along the circumferential axis. Further results revealed that these Notch signaling-mediated axon outgrowth defects may be not due to the alteration of axon guidance since commissural axon marker TAG1 was present in the axons in floor plate midline, and also not result from the changes in cell fate determination of commissural neurons since the expression of postmitotic neuron marker Tuj1 and specific commissural markers TAG1 and Pax7 was unchanged.

Conclusions/Significance

We first used an in vivo system to provide evidence that forced Notch signaling negatively regulates commissural axon outgrowth.

Influence of glial-derived matrix molecules, especially chondroitin sulfates, on neurite growth and survival of cultured mouse embryonic motoneurons

Mechanisms controlling neuronal survival and regeneration play an important role during development, after birth, and under lesion conditions. Isolated embryonic mouse motoneurons have been a useful tool for studying such basic mechanisms. These cultured motoneurons depend on extracellular matrix (ECM) molecules, which are potent mediators of survival and axonal growth and guidance in the CNS and in vitro, exhibiting either attractive or repellent guidance cues. Additionally, ECM proteoglycans and glycoproteins are components of the glial scar acting as a growth barrier for regenerating axons. Compared with CNS axon outgrowth, less is known about the cues that guide motoneurons toward their peripheral targets. Because we are interested in the effects of glial-derived chondroitin sulfate proteoglycans (CSPGs), we have worked out a model system for investigating the influences of glial-derived matrix molecules on motoneuron outgrowth and survival. We used cultured embryonic mouse motoneurons to investigate axon growth effects of matrix molecules produced by the glial-derived cell lines A7, Neu7, and Oli-neu primary astrocytes as well as the immortalized Schwann cell line IMS32. The results indicate that molecules of the ECM, especially chondroitin sulfates, play an important role as axon growth-promoting cues. We could demonstrate a modifying effect of the matrix components on motoneuron survival and caspase3-induced apoptosis.

Nectin-like molecules/SynCAMs are required for post-crossing commissural axon guidance

The Necl/SynCAM subgroup of immunoglobulin superfamily cell adhesion molecules has been implicated in late stages of neural circuit formation. They were shown to be sufficient for synaptogenesis by their trans-synaptic interactions. Additionally, they are involved in myelination, both in the central and the peripheral nervous system, by mediating adhesion between glia cells and axons. Here, we show that Necls/SynCAMs are also required for early stages of neural circuit formation. We demonstrate a role for Necls/SynCAMs in post-crossing commissural axon guidance in the developing spinal cord in vivo. Necl3/SynCAM2, the family member that has not been characterized functionally so far, plays a crucial role in this process. It is expressed by floorplate cells and interacts with Necls/SynCAMs expressed by commissural axons to mediate a turning response in post-crossing commissural axons.

Wiring Olfaction: The Cellular and Molecular Mechanisms that Guide the Development of Synaptic Connections from the Nose to the Cortex

Within the central nervous system, the olfactory system fascinates by its developmental and physiological particularities, and is one of the most studied models to understand the mechanisms underlying the guidance of growing axons to their appropriate targets. A constellation of contact-mediated (laminins, CAMs, ephrins, etc.) and secreted mechanisms (semaphorins, slits, growth factors, etc.) are known to play different roles in the establishment of synaptic interactions between the olfactory epithelium, olfactory bulb (OB) and olfactory cortex. Specific mechanisms of this system (including the amazing family of about 1000 different olfactory receptors) have been also proposed. In the last years, different reviews have focused in partial sights, specially in the mechanisms involved in the formation of the olfactory nerve, but a detailed review of the mechanisms implicated in the development of the connections among the different olfactory structures (olfactory epithelium, OB, olfactory cortex) remains to be written. In the present work, we afford this systematic review: the different cellular and molecular mechanisms which rule the formation of the olfactory nerve, the lateral olfactory tract and the intracortical connections, as well as the few data available regarding the accessory olfactory system. These mechanisms are compared, and the implications of the differences and similarities discussed in this fundamental scenario of ontogeny.

The synaptic remodeling between regenerated perforant pathway and granule cells in slice culture

In order to understand the synaptic remodeling in the course of axonal regeneration, the synaptic remodeling of the perforant path in hippocampus was investigated in the present study with entorhino-hippocampal coculture, DiI DiOlistic assay and transmission electron microscopy. The results showed that the regeneration of the perforant pathway occurred in entorhino-hippocampal slice coculture, and putative synaptic contacts formed between the regenerated fibers and dendritic spines of granule cells. Ultrastructural analysis confirmed the formation of new synaptic contacts. In conclusion, the synaptic formation implicated in the neuroregeneration could integrate into the network in CNS.

Development of glutamate receptors in auditory neurons from long-term organotypic cultures of the embryonic chick hindbrain

We used long-range organotypic cultures of auditory nuclei in the chick hindbrain to test the development of glutamate receptor activity in auditory neurons growing in a tissue environment that includes early deprivation of peripheral glutamatergic input, subsequent to removal of the otocyst. Cultures started at embryonic day (E)5, and lasted from 6 h to 15 days. Neuronal migration, clustering and axonal extension from the nucleus magnocellularis (NM) to the nucleus laminaris (NL) partially resembled events in vivo. However, the distinctive laminar organization of the NL was not observed. Glutamate receptor (GluR) activity was tested with optical recordings of intracellular Ca2+ in the NM. alpha-Amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA)/kainate receptors had Ca2+ responses with a time course similar to that in control slices. Peak amplitude, however, was significantly lower. N-methyl-D-aspartate (NMDA)-mediated Ca2+ responses were higher in 2-day cultures (E5 + 2d) than in E7 explant controls, returning later to control values. Metabotropic GluRs did not elicit Ca2+ responses at standard agonist doses. Blocking NMDA or AMPA/kainate receptors with specific antagonists for 10 days in culture did not limit neuronal survival. Blocking metabotropic GluRs resulted in complete neuronal loss. Thus, ionotropic GluRs are not required for NM neuronal survival. However, their activity during development is affected when neurons grow in an in vitro environment that includes prevention of arrival of peripheral glutamatergic input.

Computational model provides insight into the distinct responses of neurons to chemical and topographical cues

Neuronal cell polarization (i.e., establishment of an axon) and axon guidance are mediated and controlled by mechanical and chemical signals from the environment. Unfortunately, an integrated approach to study cell-substrate interactions in a unified framework incorporating structural and chemical effects of the substrate has been lacking. In this paper, we present a new model combining experimental and computational methods to better understand the distinct behavior of E18 hippocampal neurons in response to topographical vs. immobilized chemical cues. We present results from our coarse-grain physiological computational model that correctly describes previously observed phenomena and predicts behavior that was subsequently tested through new experiments. The model differentiates topographical from chemical cues via a difference in cue spacing in these two substrates. Using the feature size spacing for topographical cues and a minimum step size, governed by the physics of filopodia protrusion, for chemical cues, the model successfully mimics the trend observed in experimental polarization probability for four different topographical feature sizes and constant chemical cue spacing. Our results not only show good agreement with experiments, but also provide novel suggestions for development of substrates for finer control of neuronal cell polarization.

The immunoglobulin-like cell adhesion molecule nectin and its associated protein afadin

Nectins are immunoglobulin-like cell adhesion molecules (CAMs) that compose a family of four members. Nectins homophilically and heterophilically interact in trans with each other to form cell-cell adhesions. In addition, they heterophilically interact in trans with other immunoglobulin-like CAMs. Nectins bind afadin, an actin filament (F-actin)-binding protein, at its cytoplasmic tail and associate with the actin cytoskeleton. Afadin additionally serves as an adaptor protein by further binding many scaffolding proteins and F-actin-binding proteins and contributes to the association of nectins with other cell-cell adhesion and intracellular signaling systems. Nectins and afadin play roles in the formation of a variety of cell-cell junctions cooperatively with, or independently of, cadherins. Cooperation between nectins and cadherins is required for the formation of cell-cell junctions; cadherins alone are not sufficient. Additionally, nectins regulate many other cellular activities (such as movement, proliferation, survival, differentiation, polarization, and the entry of viruses) in cooperation with other CAMs and cell surface membrane receptors.

Cross-talk among integrin, cadherin, and growth factor receptor: roles of nectin and nectin-like molecule

Integrin, cadherin, and growth factor receptor are key molecules for fundamental cellular functions including cell movement, proliferation, differentiation, adhesion, and survival. These cell surface molecules cross-talk with each other in the regulation of such cellular functions. Nectin and nectin-like molecule (Necl) have been identified as cell adhesion molecules that belong to the immunoglobulin superfamily. Nectin and Necl play important roles in the integration of integrin, cadherin, and growth factor receptor at the cell-cell adhesion sites of contacting cells and at the leading edges of moving cells, and thus are also involved in the fundamental cellular functions together with integrin, cadherin, and growth factor receptor. This chapter describes how newly identified cell adhesion molecules, nectin and Necl, modulate the cross-talk among integrin, cadherin, and growth factor receptor and how these integrated molecules act in the regulation of fundamental cellular functions.

Reelin, a guidance signal for the regeneration of the entorhino-hippocampal path

The importance of reelin for cortical lamination in the developing CNS is well established, but its role in nerve fiber growth is not well understood. In this study, hippocampal slices were co-cultured with entorhinal slices of GFP mice, in order to compare the growth of the entorhino-hippocampal path in wild type and reeler mice. On day 6, regenerated fibers were seen to navigate from the entorhinal cortex into the hippocampus. The results showed that in wild type mice, regenerated fibers grew along the molecular layer of the dentate gyrus, and only a few fibers were found to penetrate through the granular layer into the hilus. This specific orientation was similar to the perforant path in vivo. Compared with perforant path regeneration in wild type mice, reeler mice seemed to have lost their specific orientation and proper termination in the hippocampus. Without the guidance signal from reelin, the regenerated fibers left the molecular layers and continued to grow aberrantly, i.e., in the granular layer, hilus, pyramidal layer and even stratum oriens. Particularly in the dentate gyrus, the fibers meandered around the cells in the hilus and resembled a network. The study concludes that reelin also serves as an important guidance signal for neuroregeneration of the perforant path.

Thrombospondin-4 and matrix three-dimensionality in axon outgrowth and adhesion in the developing retina

Thrombospondin-4 (TSP-4), a large pentameric glycoprotein of the extracellular matrix, has been described as a neurite outgrowth-promoting molecule. However, the means by which TSP-4 promotes neurite outgrowth in the developing eye is unclear. Here we show that TSP-4 is present at the appropriate time in development and displays a localization pattern within the developing mouse retina consistent with a role in retinal ganglion cell (RGC) neurite outgrowth. Furthermore, results indicate that while TSP-4 alone does not support adhesion or neurite extension, it enhances the ability of laminins to promote adhesion and neurite outgrowth of embryonic retinal cells. The mechanism of enhancement is, in part, based on the ability of TSP-4 to enhance the three-dimensionality and/or clustering of laminins within the substrate matrix. These results support a model where TSP-4 acts as an organizer of adhesive and axon outgrowth-promoting molecules in the ECM to optimize retinal ganglion cell responses.

The tracing study of developing entorhino-hippocampal pathway

The entorhino-hippocampal pathway is the major excitatory input from neurons of the entorhinal cortex on both ipsilateral and contralateral hippocampus/dentate gyrus. This fiber tract consists of the alvear path, the perforant path and a crossed commissural projection. In this study, the histogenesis and development of the various subsets of the entorhino-hippocampal projection have been investigated. DiI, DiO, Fast Blue tracing and calretinin immunocytochemistry as well as were carried out with pre and postnatal rats at different developmental stages. The alvear path and the commissural pathway start to develop as early as embryonic day E16, while the first perforant afferents reach the stratum lacunosum-moleculare of the hippocampus at E17 and at outer molecular layer of the denate gyrus at postnatal day 2. Retrograde tracing with DiI identifies entorhinal neurons in layer II-IV as the developmental origin of the entorhino-hippocampal pathway. Furthermore, calretinin immunocytochemistry revealed transitory Cajal-Retzius cells in the stratum lacunosum-moleculare of the hippocampus from E16. DiI labeling of entorhinal cortex fibers and combined calretinin-immunocytochemistry reveal a close relationship between Cajal-Retzius cells and entorhinal afferents. This temporal and spatial relationship suggests that Cajal-Retzius cell serves as a guiding cue for entorhinal afferents at early cortical development.

Getting axons onto the right path: the role of transcription factors in axon guidance

The normal function of the nervous system requires that the constituent neurons are precisely 'wired together'. During embryogenesis, each neuron extends an axonal process, which can navigate a considerable distance to its target. Although a number of the receptors and guidance signals that direct axonal growth have been identified, less is known about the transcription factors that regulate the expression of these molecules within the neuron and its environment. This review examines recent studies in vertebrates and Drosophila that address the identity of the transcription factors that either control the repertoire of guidance receptors and signals that permits an axon to take a particular trajectory or act themselves as novel extracellular guidance factors.

Cell adhesion molecule L1 modulates nerve-growth-factor-induced CGRP-IR fiber sprouting

Overexpression of nerve growth factor (NGF) using adenoviruses (Adts) after spinal cord injury induces extensive regeneration and sprouting of calcitonin-gene-related peptide immunoreactive (CGRP-IR) fibers, whereas overexpression of cell adhesion molecules (CAMs) has no effect on the normal distribution of these fibers. Interestingly, co-expression of cell adhesion molecule L1 and NGF significantly decreases (p<0.0001) CGRP-IR fiber sprouting within the spinal cord, when compared to NGF alone. Co-expression of cell adhesion molecules NCAM or N-cadherin had no effect on NGF-induced CGRP-IR fiber sprouting. These data demonstrate that reduced sprouting is specific to L1 co-expression and not other cell adhesion molecules. In vitro studies carried out to address potential mechanisms show that neurite outgrowth over astrocytes overexpressing L1 in the presence of NGF is comparable to controls, indicating that other factors present in vivo might be involved in the L1-mediated reduction in sprouting. One potential factor is semaphorin 3A (sema3A), which mediates growth cone collapse of CGRP-positive axons. Recent studies have shown that L1 is important in sema3A receptor signaling for cortical neurons. In our study, co-expression of sema3A indeed reduces neurite outgrowth from DRG neurons by about 40% on L1-expressing astrocytes. Based on these results, we hypothesize that overexpression of L1 potentiates sema3A signaling resulting in reduced sprouting.

Ephrin A/EphA controls the rostral turning polarity of a lateral commissural tract in chick hindbrain

Most post-crossing commissural axons turn into longitudinal paths to make synaptic connections with their targets. Mechanisms that control their rostrocaudal turning polarity are still poorly understood. We used the hindbrain as a model system to investigate the rostral turning of a laterally located commissural tract, identified as the caudal group of contralateral cerebellar-projecting second-order vestibular neurons (cC-VC). We found that the caudal hindbrain possessed a graded non-permissive/repulsive activity for growing cC-VC axons. This non-permissiveness/repulsion was in part mediated by glycosyl-phosphatidylinositol (GPI)-anchored ephrin A. We further demonstrated that ephrin A2 was distributed in a caudal-high/rostral-low gradient in the caudolateral hindbrain and cC-VC axons expressed EphA receptors. Finally,perturbing ephrin A/EphA signalling both in vitro and in vivo led to rostrocaudal pathfinding errors of post-crossing cC-VC axons. These results suggest that ephrin A/EphA interactions play a key role in regulating the polarity of post-crossing cC-VC axons as they turn into the longitudinal axis.

Longitudinal axon guidance

Our knowledge about molecular mechanisms underlying axon guidance along the antero-posterior axis in contrast to the dorso-ventral axis of the developing nervous system is very limited. During the past two years in vitro and in vivo studies have indicated that morphogens have a role in longitudinal axon guidance. Morphogens are secreted proteins that act in a concentration-dependent manner on susceptible groups of precursor cells and induce their differentiation to a specific cell fate. Thus, gradients of morphogens are responsible for the appropriate patterning of the nervous system during early phases of neural development. Therefore, it was surprising to find that gradients of two of these morphogens, Wnt4 and Shh, can be re-used for longitudinal axon guidance during later stages of nervous system development.

Dual functional activity of semaphorin 3B is required for positioning the anterior commissure

Chemorepulsion by semaphorins plays a critical role during the development of neuronal projections. Although semaphorin-induced chemoattraction has been reported in vitro, the contribution of this activity to axon pathfinding is still unclear. Using genetic and culture models, we provide evidence that both attraction and repulsion by Sema3B, a secreted semaphorin, are critical for the positioning of a major brain commissural projection, the anterior commissure (AC). NrCAM, an immunoglobulin superfamily adhesion molecule of the L1 subfamily, associates with neuropilin-2 and is a component of a receptor complex for Sema3B and Sema3F. Finally, we show that activation of the FAK/Src signaling cascade distinguishes Sema3B-mediated attractive from repulsive axonal responses of neurons forming the AC, revealing a mechanism underlying the dual activity of this guidance cue.

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.

ROBO directs axon crossing of segmental boundaries by suppressing responsiveness to relocalized Netrin

Networks in the CNS consist of neural modules that are connected in a repetitive array. Whereas individual modules contain guidance information along which axons track within the unit, these guidance cues hinder axon extension across module boundaries. We investigated how axons solve this 'boundary problem' by analyzing the longitudinal connections of neuromeres in Drosophila melanogaster. The initial trajectory of the longitudinal axons is guided by Netrin, which is localized on commissural axons by its receptor, Frazzled. The Netrin cue on the commissure of the next segment can act as a barrier to longitudinal axons, inhibiting their extension and misguiding them contralaterally along the commissure. We show that, before reaching the segmental boundary, the longitudinal axons' responsiveness to Netrin presented on the commissure is suppressed by Roundabout (ROBO), through counteracting Gq signaling. The absence of suppression causes the robo phenotype: longitudinal axons project toward the midline, as if running around a roundabout (rotary).

Robo family of proteins exhibit differential expression in mouse spinal cord and Robo-Slit interaction is required for midline crossing in vertebrate spinal cord

The ventral midline of the central nervous system is an important intermediate target where growing commissural axons either cross and project contralaterally or remain on the same side of the body. New studies on mice and humans show that this decision by commissural axons is largely dependent on Slits, extracellular matrix proteins that are widely expressed in the midline of the nervous system, and their receptors, Robos (Long et al. [2004] Neuron 42:213-223; Sabatier et al. [2004] Cell 117:157-169; Jen et al. [2004] Science 304:1509-1513). Here, we show that the Robo family proteins Robo1 and Rig-1 exhibit differential expression patterns on commissural axons as they approach, cross, and leave the midline of the developing mouse spinal cord and demonstrate that Robo1 and Robo2 bind Slit1 and Slit2, but Rig-1 does not. In addition, we show that cultured chick commissural axons are repelled by a source of Slit protein, and the soluble Robo-Fc proteins are capable of neutralizing this repulsion. Finally, we exploit the large size and accessibility of the early chick embryo to analyze the function of Slit/Robo signaling in midline commissural axon guidance, and we demonstrate that the in vivo perturbation of Robo-Slit interaction at the floor plate causes consistent guidance defects of commissural axons during midline crossing. These findings demonstrate the evolutionarily conserved role for Robo-Slit interaction in the control of midline crossing axons in vertebrates.

The Notch-target gene hairy2a impedes the involution of notochordal cells by promoting floor plate fates in Xenopus embryos

We have previously shown that the early Xenopus organiser contains cells equally potent to give rise to notochord or floor plate, and that Notch signalling triggers a binary decision, favouring the floor plate fate at the expense of the notochord. Now, we present evidence that Delta1 is the ligand that triggers the binary switch, which is executed through the Notch-mediated activation of hairy2a in the surrounding cells within the organiser, impeding their involution through the blastopore and promoting their incorporation into the hairy2a+ notoplate precursors (future floor-plate cells) in the dorsal non-involuting marginal zone.

Sonic hedgehog guides commissural axons along the longitudinal axis of the spinal cord

Dorsal commissural axons in the developing spinal cord cross the floor plate, then turn rostrally and grow along the longitudinal axis, close to the floor plate. We used a subtractive hybridization approach to identify guidance cues responsible for the rostral turn in chicken embryos. One of the candidates was the morphogen Sonic hedgehog (Shh). Silencing of the gene SHH (which encodes Shh) by in ovo RNAi during commissural axon navigation demonstrated a repulsive role in post-commissural axon guidance. This effect of Shh was not mediated by Patched (Ptc) and Smoothened (Smo), the receptors that mediate effects of Shh in morphogenesis and commissural axon growth toward the floor plate. Rather, functional in vivo studies showed that the repulsive effect of Shh on postcommissural axons was mediated by Hedgehog interacting protein (Hip).

The Netrin family of guidance factors: emphasis on Netrin-1 signalling

During the development of the nervous system, neurons respond to the coordinated action of a variety of attractive and repulsive signals from the embryonic environment. Netrins form a family of extracellular proteins that regulate the migration of neurons and axonal growth cones. These proteins are bifunctional signals that are chemoattractive for some neurons and chemorepellent for others. Netrins mainly interact with the specific receptors DCC and UNC-5 family. To date, several Netrins have been described in mouse and humans: Netrin-1, -3/NTL2, -4/beta and G-Netrins. Netrin-1 is the most studied member of the family. It is involved in the development many projections of the nervous system. When Netrin-1 interacts with its specific receptors, a cascade of local cytoplasmic events is triggered. Several signal transduction pathways and effector molecules have been implicated in the response to Netrin-1: small Rho-GTPases, MAP-Kinases, second messengers and the Microtubule Associated Protein 1B (MAP1B).

Tissue-engineering approaches for axonal guidance

Owing to the profound impact of nervous system damage, extensive studies have been carried out aimed at facilitating axonal regeneration following injury. Tissue engineering, as an emerging and rapidly growing field, has received extensive attention for nervous system axonal guidance. Numerous engineered substrates containing oriented extracellular matrix molecules, cells or channels have displayed potential of supporting axonal regeneration and functional recovery. Most attempts are focused on seeking new biomaterials, new cell sources, as well as novel designs of tissue-engineered neuronal bridging devices, to generate safer and more efficacious neuronal tissue repairs.

Contacts between the commissural axons and the floor plate cells are mediated by nectins

During development of the central nervous system (CNS), commissural axons grow toward the ventral midline. After crossing the floor plate, they abruptly change their trajectory from the circumferential to the longitudinal axis. The contacts between the commissural axons and the floor plate cells are involved in this axonal guidance, but their mechanisms or structures have not fully been understood. In this study, we found that nectin-1 and -3, immunoglobulin-like cell-cell adhesion molecules, asymmetrically localized at the contact sites between the commissural axons and the floor plate cells, respectively. In vitro perturbation of the endogenous trans-interaction between nectin-1 and -3 caused abnormal fasciculation of the commissural axons and impairment of the contacts, and resulted in failure in longitudinal turns of the commissural axons at the contralateral sites of the rat hindbrain. These results indicate that the contacts between the commissural axons and the floor plate cells are mediated by the hetero-trans-interaction between nectin-1 and -3 and involved in regulation of the trajectory of the commissural axons.

Lower rhombic lip-derived cells undergo transmedian tangential migration followed by radial migration in the chick embryo brainstem

Migration behaviour and fate of cells originated from the lower rhombic lip (LRL) was examined in the chick embryo hindbrain. LRL-derived cells tangentially migrate along the pial surface of the brainstem and form a transient subpial migratory stream. In the initial stages of migration, LRL-derived cells appose each other or axon-like processes, which is indicative of mode of homophilic chain migration and/or axophilic migration. Some LRL-derived cells relocate rostroventrally towards the pontine region, although the majority of them migrate circumferentially to the ventral medulla oblongata. Depending on the stage of generation, LRL-derived cells undergo transmedian migration; late-generated LRL-derived cells preferentially colonize the contralateral brainstem compared with early generated cells. Thus, latecomer neuron precursors may migrate past their predecessors in the migratory stream. When LRL-derived cells leave the subpial migratory stream, they change their migratory direction to a radial one and relocate inwardly, with a profile that resembles a tangential-to-radial change seen in cerebellar granule cell precursors. After they enter the parenchymal region of the brainstem, they exhibited morphological differentiation, and some differentiate into excitatory neurons. The present results suggest that LRL-derived cells migrate across boundaries such as midline or rhombomere, which may facilitate to build up cellular and functional architectures of the hindbrain.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

Transgenic mice expressing F3/contactin from the transient axonal glycoprotein promoter undergo developmentally regulated deficits of the cerebellar function

We have shown that transgenic transient axonal glycoprotein (TAG)/F3 mice, in which the mouse axonal glycoprotein F3/contactin was misexpressed from a regulatory region of the gene encoding the transient axonal glycoprotein TAG-1, exhibit a transient disruption of cerebellar granule and Purkinje cell development [Development 130 (2003) 29]. In the present study we explore the neurobehavioural consequences of this mutation. We report on assays of reproductive parameters (gestation length, litter size and offspring viability) and on somatic and neurobehavioural end-points (sensorimotor development, homing performance, motor activity, motor coordination and motor learning). Compared with wild-type littermates, TAG/F3 mice display delayed sensorimotor development, reduced exploratory activity and impaired motor activity, motor coordination and motor learning. The latter parameters, in particular, were affected also in adult mice, despite the apparent recovery of cerebellar morphology, suggesting that subtle changes of neuronal circuitry persist in these animals after development is complete. These behavioural deficits indicate that the finely coordinated expression of immunoglobulin-like cell adhesion molecules such as TAG-1 and F3/contactin is of key relevance to the functional, as well as morphological maturation of the cerebellum.

The N-terminal and transmembrane domains of Commissureless are necessary for its function and trafficking within neurons

Commissureless (Comm) is a novel transmembrane molecule necessary both for commissural axons to cross the midline of the Drosophila central nervous system and normal synaptogenesis. Comm is able to reduce cell surface levels of Roundabout (Robo), a receptor for the midline repellent Slit, on commissural axons and unknown inhibitors of synaptogenesis expressed on muscle cells. Comm is expressed dynamically and is found at the cell surface and within intracellular vesicles. Comm can bind Robo and when the proteins are co-expressed Robo is found co-localised with Comm intracellularly. Here we show that the ability of Comm to localise intracellularly and hence regulate Robo surface levels requires sequences in both the N-terminal and transmembrane domains. We also show that Comm can dimerise via its N-terminal domain. Furthermore, absence of the Comm N-terminal and transmembrane regions results in the protein being restricted to the neuron soma.

Kinesin superfamily protein 2A (KIF2A) functions in suppression of collateral branch extension

Through interactions with microtubules, the kinesin superfamily of proteins (KIFs) could have multiple roles in neuronal function and development. During neuronal development, postmitotic neurons develop primary axons extending toward targets, while other collateral branches remain short. Although the process of collateral branching is important for correct wiring of the brain, the mechanisms involved are not well understood. In this study, we analyzed kif2a(-/-) mice, whose brains showed multiple phenotypes, including aberrant axonal branching due to overextension of collateral branches. In kif2a(-/-) growth cones, microtubule-depolymerizing activity decreased. Moreover, many individual microtubules showed abnormal behavior at the kif2a(-/-) cell edge. Based on these results, we propose that KIF2A regulates microtubule dynamics at the growth cone edge by depolymerizing microtubules and that it plays an important role in the suppression of collateral branch extension.

Sexual differentiation of projections from the principal nucleus of the bed nuclei of the stria terminalis

The principal nucleus of the bed nuclei of the stria terminalis (BSTp) is sexually dimorphic and participates in several aspects of reproduction. A detailed analysis of its projections revealed that the BSTp provides major inputs to forebrain regions that are sexually dimorphic and contain high densities of neurons that express receptors for sex steroid hormones in a pattern that is remarkably similar to that of the medial amygdaloid nucleus. The BSTp sends its strongest outputs to the periventricular zone of the hypothalamus and innervates structures thought to play important roles in regulating hormone secretion from the anterior pituitary, but it also provides strong inputs to the medial preoptic and ventromedial nuclei of the hypothalamus. The BSTp also sends a strong return projection to the medial nucleus of the amygdala. The projections of the BSTp appear to be more robust in males with striking sex differences observed in most, but not all, major terminal fields. Moreover, various terminal fields appeared to differ in their developmental sensitivity to manipulation of circulating levels of sex steroids during the neonatal period. Thus, the organization of projections from the BSTp suggests that it plays a particularly important role in regulating neuroendocrine function and that neurons in this nucleus may relay olfactory information to the hypothalamus differently in male and female rats. Furthermore, the differential action of sex steroids on the density of afferents from the BSTp in various regions indicates that these hormones exert a target-specific influence on the development of BSTp projections.

Notch steers Drosophila ISNb motor axons by regulating the Abl signaling pathway

The central problem in axon guidance is to understand how guidance signals interact to determine where an axon will grow. Here we investigate a specific axon guidance decision in Drosophila embryos, the sharp inward turn taken by the ISNb motor nerve to approach its muscle targets. We find that this turn requires Notch and its ligand Delta. We show that Delta is expressed on cells adjacent to the ISNb turning point, and we know from previous work that Notch is present on axonal growth cones, suggesting that Delta and Notch might provide a guidance signal to ISNb. To induce the turning of ISNb axons, Notch interacts genetically with multiple components of a signal transduction pathway that includes the Abl tyrosine kinase and its affiliated accessory proteins. In contrast, genetic interaction experiments fail to provide evidence for a major role of the "canonical" Notch/Su(H) signaling pathway in this process. We suggest that the Notch/Abl interaction promotes the turning of ISNb axons by attenuating the Abl-dependent adhesion of ISNb axons to their substratum, thus releasing the axons to respond to attraction from target muscles.

Concentration-dependent actions of glial chondroitin sulfate on the neuritic growth of midbrain neurons

Astrocytes located in two distinct regions of midbrain differ in their neuritic growth support abilities. Midbrain neurons cultured onto astrocyte monolayers from the lateral (L) region develop long and branched neurites while neurons cultured onto astrocyte monolayers from the medial (M) region develop short or no neurites. The extracellular matrix of these astrocytes has an important role in promoting or inhibiting the growth of these neurons. Differences on the compartmental distribution, as well as on the concentration of GAGs of L and M astrocytes, may be related to their differential capacity of supporting neuritic growth. Indeed, enzymatic digestion of heparan sulfate (HS) and chondroitin sulfate (CS) chains also pointed to an important function for GAGs on axon navigation. In order to better characterize the role of CS on the growth of midbrain neurites, we treated L and M astrocyte monolayers with 1 mM of beta-D-xyloside. Under these conditions, astrocytes oversynthesized and secreted CS protein-free chains to the culture medium. M astrocytes had a significant reduction in their neuritic growth-inhibiting ability after xyloside treatment, suggesting a promoting role for soluble CS in neuritic growth. Chondroitin 4-sulfate (CS-4) added in different concentrations to M astrocyte cultures turned this glia into a permissive substrate, acting in a linear way as far as the largest neurite was concerned. However, a U-shaped dose-effect curve on neurite growth resulted from the similar treatment of L astrocytes. These results suggest that glial CS-4 could be involved in the neurite growth modulating properties of midbrain neurons in a complex concentration-dependent way.

Notch activates sonic hedgehog and both are involved in the specification of dorsal midline cell-fates in Xenopus

We analysed the role of Notch signalling during the specification of the dorsal midline in Xenopus embryos. By activating or blocking the pathway we found that Notch expands the floor plate domain of sonic hedgehog and pintallavis and represses the notochordal markers chordin and brachyury, with a concomitant reduction of the notochord size. We propose that within a population of the early organiser with equivalent potential to develop either as notochord or floor plate, Notch activation favours floor plate development at the expense of the notochord, preferentially before mid gastrula. We present evidence that sonic hedgehog down-regulates chordin, suggesting that secreted Sonic hedgehog may be involved or reinforcing the cell-fate switch executed by Notch. We also show that Notch signalling requires Presenilin to modulate this switch.

The function of neuropilin/L1 complex

L1, a cell adhesion molecule of the Ig superfamily (IgCAM) plays a critical role in the formation of neuronal networks. This is reflected by the variety of clinical signs associated with the X-linked recessive neurological disorder that is caused by mutations in the L1 gene. L1 regulates the formation of axon fascicles and promotes neurite outgrowth through interaction with a wide spectrum of binding partners including cell adhesion molecules and extra-cellular matrix components. Here we describe the emerging evidence that indicates, in addition to these well-established functions, that L1 participates in the signaling of a secreted guidance cue of the Semaphorin family, Sema3A. Three types of experimental evidence support L1 as a key component of the Sema3A receptor complex. First, L1-deficient axons do not respond to Sema3A-induced chemorepulsion. Second, L1 and NRP1, the neuropilin responsible for Sema3A binding, associate through their extracellular domains, forming a cell surface heterocomplex. Third, a soluble form of L1 modulates axonal responsiveness to Sema3A, by converting Sema3A chemorepulsion into attraction.

AMIGO, a transmembrane protein implicated in axon tract development, defines a novel protein family with leucine-rich repeats

Ordered differential display identified a novel sequence induced in neurons by the neurite-promoting protein amphoterin. We named this gene amphoterin-induced gene and ORF (AMIGO), and also cloned two other novel genes homologous to AMIGO (AMIGO2 and AMIGO3). Together, these three AMIGOs form a novel family of genes coding for type I transmembrane proteins which contain a signal sequence for secretion and a transmembrane domain. The deduced extracellular parts of the AMIGOs contain six leucine-rich repeats (LRRs) flanked by cysteine-rich LRR NH2- and COOH-terminal domains and by one immunoglobulin domain close to the transmembrane region. A substrate-bound form of the recombinant AMIGO ectodomain promoted prominent neurite extension in hippocampal neurons, and in solution, the same AMIGO ectodomain inhibited fasciculation of neurites. A homophilic and heterophilic binding mechanism is shown between the members of the AMIGO family. Our results suggest that the members of the AMIGO protein family are novel cell adhesion molecules among which AMIGO is specifically expressed on fiber tracts of neuronal tissues and participates in their formation.