Ventral spinal cord inhibition of neurite outgrowth from embryonic rat dorsal root ganglia

Netrin-1 as A neural guidance protein in development and reinnervation of the larynx

Neural guidance proteins participate in motor neuron migration, axonal projection, and muscle fiber innervation during development. One of the guidance proteins that participates in axonal pathfinding is Netrin-1. Despite the well-known role of Netrin-1 in embryogenesis of central nervous tissue, it is still unclear how the expression of this guidance protein contributes to primary innervation of the periphery, as well as reinnervation. This is especially true in the larynx where Netrin-1 is upregulated within the intrinsic laryngeal muscles after nerve injury and where blocking of Netrin-1 alters the pattern of reinnervation of the intrinsic laryngeal muscles. Despite this consistent finding, it is unknown how Netrin-1 expression contributes to guidance of the axons towards the larynx. Improved knowledge of Netrin-1's role in nerve regeneration and reinnervation post-injury in comparison to its role in primary innervation during embryological development, may provide insights in the search for therapeutics to treat nerve injury. This paper reviews the known functions of Netrin-1 during the formation of the central nervous system and during cranial nerve primary innervation. It also describes the role of Netrin-1 in the formation of the larynx and during recurrent laryngeal reinnervation following nerve injury in the adult.

A robust and reproducible human pluripotent stem cell derived model of neurite outgrowth in a three-dimensional culture system and its application to study neurite inhibition

The inability of neurites to grow and restore neural connections is common to many neurological disorders, including trauma to the central nervous system and neurodegenerative diseases. Therefore, there is need for a robust and reproducible model of neurite outgrowth, to provide a tool to study the molecular mechanisms that underpin the process of neurite inhibition and to screen molecules that may be able to overcome such inhibition. In this study a novel in vitro pluripotent stem cell based model of human neuritogenesis was developed. This was achieved by incorporating additional technologies, notably a stable synthetic inducer of neural differentiation, and the application of three-dimensional (3D) cell culture techniques. We have evaluated the use of photostable, synthetic retinoid molecules to promote neural differentiation and found that 0.01 μM EC23 was the optimal concentration to promote differentiation and neurite outgrowth from human pluripotent stem cells within our model. We have also developed a methodology to enable quick and accurate quantification of neurite outgrowth derived from such a model. Furthermore, we have obtained significant neurite outgrowth within a 3D culture system enhancing the level of neuritogenesis observed and providing a more physiological microenvironment to investigate the molecular mechanisms that underpin neurite outgrowth and inhibition within the nervous system. We have demonstrated a potential application of our model in co-culture with glioma cells, to recapitulate aspects of the process of neurite inhibition that may also occur in the injured spinal cord. We propose that such a system that can be utilised to investigate the molecular mechanisms that underpin neurite inhibition mediated via glial and neuron interactions.

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Development of a robust, novel neurite outgrowth assay from human pluripotent stem cell derived neural cell aggregates.

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Synthetic retinoids induce neural differentiation of pluripotent stem cells to a greater extent than natural ATRA.

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Neurospheres cultured on a 3D scaffold provide a more physiologically relevant model of neurite outgrowth.

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Suppression of neurite outgrowth by glioma cells in 3D enables the study of neurite inhibitory mechanisms in the glial scar.

Neuroanatomical correlation of the House-Brackmann grading system in the microsurgical treatment of vestibular schwannoma

Avoidance of facial nerve injury is one of the major goals of vestibular schwannoma (VS) surgery because functional deficits of the facial nerve can lead to physical, cosmetic, and psychological consequences for patients. Clinically, facial nerve function is assessed using the House-Brackmann grading scale, which also allows physicians to track the progress of a patient's facial nerve recovery. Because the facial nerve is a peripheral nerve, it has the ability to regenerate, and the extent of its functional recovery depends largely on the location and nature of its injury. In this report, the authors first describe the facial nerve anatomy, the House-Brackmann grading system, and factors known to be predictors of postoperative facial nerve outcome. The mechanisms and pathophysiology of facial nerve injury during VS surgery are then discussed, as well as factors affecting facial nerve regeneration after surgery.

Locally released small (non-protein) ninhydrin-reacting molecules underlie developmental differences of cultured medullary versus spinal dorsal horn neurons

Neurons located in the trigeminal subnucleus caudalis (Vc) play crucial roles in pain and sensorimotor functions in the orofacial region. Because of many anatomical and functional similarities with the spinal dorsal horn (SDH), Vc has been termed the medullary dorsal horn--analogous to the SDH. Here, we report that when compared with embryonic SDH neurons in culture, neurons isolated from the Vc region showed significantly slower growth, lower glutamate receptor activity, and more cells undergoing cell death. SDH neuron development was inhibited in co-cultures of SDH and Vc tissues while Vc neuron development was promoted by co-culture with SDH tissues. Furthermore, we identified that small (non-protein) ninhydrin-reacting molecules purified from either embryonic or post-natal Vc-conditioned medium inhibited neuronal growth whereas ninhydrin-reacting molecules from SDH-conditioned medium promoted neuronal growth. These findings suggest the involvement of locally released factors in the region-specific regulation of neuronal development in Vc and SDH, central nervous system regions playing critical roles in pain, and point to novel avenues for investigating central nervous system regionalization and for designing therapeutic approaches to manage neurodegenerative diseases and pain.

Type III neuregulin 1 regulates pathfinding of sensory axons in the developing spinal cord and periphery

Sensory axons must develop appropriate connections with both central and peripheral targets. Whereas the peripheral cues have provided a classic model for neuron survival and guidance, less is known about the central cues or the coordination of central and peripheral connectivity. Here we find that type III Nrg1, in addition to its known effect on neuron survival, regulates axon pathfinding. In type III Nrg1(-/-) mice, death of TrkA(+) nociceptive/thermoreceptive neurons was increased, and could be rescued by Bax elimination. In the Bax and type III Nrg1 double mutants, axon pathfinding abnormalities were seen for TrkA(+) neurons both in cutaneous peripheral targets and in spinal cord central targets. Axon guidance phenotypes in the spinal cord included penetration of axons into ventral regions from which they would normally be repelled by Sema3A. Accordingly, sensory neurons from type III Nrg1(-/-) mice were unresponsive to the repellent effects of Sema3A in vitro, which might account, at least in part, for the central projection phenotype, and demonstrates an effect of type III Nrg1 on guidance cue responsiveness in neurons. Moreover, stimulation of type III Nrg1 back-signaling in cultured sensory neurons was found to regulate axonal levels of the Sema3A receptor neuropilin 1. These results reveal a molecular mechanism whereby type III Nrg1 signaling can regulate the responsiveness of neurons to a guidance cue, and show that type III Nrg1 is required for normal sensory neuron survival and axon pathfinding in both central and peripheral targets.

Axon guidance in the spinal cord: choosin' by exclusion

A critical step in the formation of correct patterns of sensory inputs to the spinal cord is the guidance of specific subsets of sensory axons to their appropriate target regions. Yoshida et al. demonstrate in this issue of Neuron that a repulsive interaction between plexinA1 and sema6C/6D prevents the growth of proprioceptive sensory axons into the superficial laminae of the dorsal horn where cutaneous sensory axons terminate.

Estrogen increases sensory nociceptor neuritogenesis in vitro by a direct, nerve growth factor-independent mechanism

Estrogen affects many aspects of the nervous system, including pain sensitivity and neural regulation of vascular function. We have shown that estrogen elevation increases sensory nociceptor innervation of arterioles in Sprague-Dawley rat mammary gland, external ear and mesentery, suggesting widespread effects on sensory vasodilatory innervation. However, it is unclear whether estrogen elicits nociceptor hyperinnervation by promoting target release of neurotrophic factors, or by direct effects on sensory neurons. To determine if estrogen may promote axon sprouting by increasing release of target-derived diffusible factors, dorsal root ganglia explants were co-cultured with mesenteric arterioles for 36 h in the absence or presence of 17beta-estradiol (E2). Mesenteric arteriolar target substantially increased neurite outgrowth from explanted ganglia, but estrogen had no effect on outgrowth, suggesting that estrogen does not increase the availability of trophic proteins responsible for target-induced neurite outgrowth. To assess the direct effects of estrogen, dissociated neonatal dorsal root ganglion neurons were cultured for 3 days in the absence or presence of E2 and nerve growth factor (NGF; 1-10 ng/mL), and immunostained for the nociceptor markers peripherin or calcitonin gene-related peptide. NGF increased neuron size, survival and numbers of neurons with neurites, but did not affect neurite area per neuron. Estrogen did not affect neuron survival, size or numbers of neurons with neurites, but did increase neurite area per neuron. The effects of these agents were not synergistic. We conclude that estrogen exerts direct effects on nociceptor neurons to promote axon outgrowth, and this occurs through an NGF-independent mechanism.

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.

Gradients and growth cone guidance of grasshopper neurons

The generation of a functional nervous system is dependent on precise pathfinding of axons during development. This pathfinding is directed by the distribution of local and long-range guidance cues, the latter of which are believed to be distributed in gradients. Gradients of guidance cues have been associated with growth cone function for over a hundred years. However, little is known about the mechanisms used by growth cones to respond to these gradients, in part owing to the lack of identifiable gradients in vivo. In the developing grasshopper limb, two gradients of the semaphorin Sema-2a are necessary for correct neuronal pathfinding in vivo. The gradients are found in regions where growth cones make critical steering decisions. Observations of different growth cone behaviors associated with these gradients have provided some insights into how growth cones respond to them. Growth cones appear to respond more faithfully to changes in concentration, rather than absolute levels, of Sema-2a expression, whereas the absolute levels may regulate growth cone size.

Target-independent specification of proprioceptive sensory neurons

Previous studies in the chick embryo have shown that sensory neurons fail to innervate muscle in the absence of motor neurons. Instead, motor neuron deletion causes more sensory axons to project to the skin. We used this experimental paradigm to determine when sensory neurons are specified to become proprioceptive afferents. Experimental embryos were treated with either saline or exogenous neurotrophin-3 (NT-3) to promote the survival of proprioceptive afferents. In saline-treated embryos, motor neuron deletion caused an increase in sensory neuron apoptosis on the deleted side, an effect reversed by NT3. Motor neuron deletion also eliminated the sartorious muscle nerve, as previously reported. In NT3-treated embryos, this altered nerve pattern was accompanied by the enlargement of the adjacent cutaneous nerve. These embryos were further analyzed by using immunohistochemistry for trkC (a receptor for NT3) retrograde and transganglionic labeling. Our results show that, following motor neuron deletion, more trkC+ afferents project in cutaneous nerves on the deleted side of NT3-treated embryos. Transganglionic labeling demonstrated that at least some of these neurons made spinal projections that are typical of proprioceptive afferents. These results therefore indicate that the proprioceptive phenotype is specified prior to target innervation and that these neurons can retain their identity despite projecting to inappropriate (cutaneous) targets.

Somatotopic organization of the facial nucleus is disrupted after lesioning and regeneration of the facial nerve: the histological representation of synkinesis

OBJECTIVE

After facial nerve repair, involuntary movement of part of the face during voluntary movement of another part of the face is common. We describe an animal model of facial nerve lesion, repair, and regeneration that demonstrates abnormal organization of the facial nucleus; this model may be used to study synkinesis.

METHODS

In 18 rats, the facial nerve was cut completely, proximal to the parotid gland, and immediately sutured end-to-end. After a period of regeneration of 1 to 10 months, retrograde fluorescence labeling of the distal branches of the facial nerve was performed. The distribution of the tracers in the facial nucleus was assessed in both the lesioned animals and in a nonlesioned group (n = 20).

RESULTS

In the control animals, muscle groups were somatotopically represented in the facial nucleus. After lesioning, repair, and regeneration, the somatotopy of the facial nucleus was disrupted. Axons projected from the facial nucleus to incorrect peripheral muscle groups, and aberrant branches were observed to simultaneously innervate different subdivisions of the facial nerve. The numbers of aberrant axons and branches did not change significantly during periods of regeneration ranging from 1 to 10 months.

CONCLUSION

Our model provides a clear demonstration of the failure of adult facial nerve axons to make correct connections with their distal targets during regeneration. This model may be used to assess strategies aimed at minimizing synkinesis and, by assessing histology together with behavior, provides a more robust model than those previously described.

Local nonpermissive and oriented permissive cues guide vestibular axons to the cerebellum

Information that originates from peripheral sensory organs is conveyed by axons of cephalic sensory cranial ganglia connecting the sensory organs to appropriate central targets in the brain. Thus, the establishment of correct axonal projections by sensory afferents is one of the most important issues in neural development. Previously, we examined the development of the vestibular nerve that originates from the VIIIth ganglion using a flat whole-mount preparation of the rat hindbrain and developed an in vitro, culture preparation that can recapitulate vestibular nerve development (Tashiro, Y., Endo, T., Shirasaki, R., Miyahara, M., Heizmann, C. W. and Murakami, F. (2000) J. Comp. Neurol. 417, 491–500). Both in vivo and in vitro, the ascending branch of the VIIIth ganglion projecting to the cerebellum reaches the base of the cerebellar primordium and starts to splay out towards the rhombic lip, apparently avoiding the ventral metencephalon. We now examine the nature of cues that guide vestibulocerebellar axons by applying various manipulations to the flat whole-mount in vitro preparation. Our observations suggest that local nonpermissive cues and oriented cues play a pivotal role in the guidance of vestibular axons to their central target.

Analysis of the L1-deficient mouse phenotype reveals cross-talk between Sema3A and L1 signaling pathways in axonal guidance

In humans, defects of the corticospinal tract have been attributed to mutations in the gene encoding L1 CAM, a phenotype that is reproduced in L1-deficient mice. Using coculture assays, we report that Sema3A secreted from the ventral spinal cord repels cortical axons from wild-type but not from L1-deficient mice. L1 and neuropilin-1 (NP-1) form a stable complex, and their extracellular domains can directly associate. Thus, L1 is a component of the Sema3A receptor complex, and L1 mutations may disrupt Sema3A signaling in the growth cone, leading to guidance errors. Addition of soluble L1Fc chimeric molecules does not restore Sema3A responsiveness of L1-deficient axons; instead, it converts the repulsion of wild-type axons into an attraction, further supporting a function for L1 in the Sema3A transducing pathways within the growth cone.

Ectopic adenoviral vector-directed expression of Sema3A in organotypic spinal cord explants inhibits growth of primary sensory afferents

Sema3A (Sema III, SemD, collapsin-1) can induce neuronal growth cone collapse and axon repulsion of distinct neuronal populations. To study Sema3A function in patterning afferent projections into the developing spinal cord, we employed the recombinant adenoviral vector technique in embryonic rat spinal cord slices. Virus solution was injected in the dorsal aspect of organotypic spinal cord cultures with segmentally attached dorsal root ganglia (sc-DRG). In cultures grown in the presence of nerve growth factor (NGF), injected either with the control virus AdCMVLacZ or with vehicle only, afferent innervation patterns were similar to those of control. However, unilateral injection of AdCMVSema3A/AdCMVLacZ in sc-DRG slices revealed a strong inhibitory effect on NGF-dependent sensory afferent growth. Ectopic Sema3A in the dorsal spinal cord, the target area of NGF-responsive DRG fibers in vivo, created an exclusion zone for these fibers and as a result they failed to reach and innervate their appropriate target zones. Taken together, gain of Sema3A function in the dorsal aspect of sc-DRG cultures revealed a dominant inhibitory effect on NGF-dependent, nociceptive sensory DRG afferents, an observation in line with the model proposed by E. K. Messersmith et al. (1995, Neuron 14, 949-959), suggesting that Sema3A secreted by spinal cord cells can act to repel central sensory fibers during the formation of lamina-specific connections in the spinal cord.

Semaphorin III can induce death in sensory neurons

Semaphorin III has been described to function as a guidance molecule directing growing axons to their target. However, its effect on the neuron cell body has not been characterized. Semaphorin III has a highly dynamic expression pattern, which generally corroborates a chemorepellent guidance function, but also suggests additional functions, different from axon guidance. A number of studies show that some sensory neurons are eliminated, while their axons are still pathfinding. In this study we have investigated whether Semaphorin III also influences the survival of sensory neurons. We here present evidence that Semaphorin III can function in vitro as selective death factor for NGF-dependent sensory neurons. Semaphorin III induces a type of cell death that is characterized by slow onset, cell body shrinking, nuclear condensation, and TUNEL-positive staining of dying neurons. These are all hallmarks of neuronal apoptosis. We also show evidence that neurons can modulate the response to Semaphorin III. The novel function described here may also be relevant in vivo, contributing to active elimination of neurons during development or after injury.

Collapsin-1/semaphorin D is a repellent for chick ganglion of Remak axons

Chick collapsin-1/human semaphorin III/mouse semaphorin D is believed to guide the extension of specific axons by a repellent mechanism. Here we examine its role in the guidance of axons of the ganglion of Remak (Remak) in the developing chick intestine. Early in embryogenesis Remak axons extend parallel to, but do not enter, the intestine when collapsin-1 is expressed in the adjacent rectal wall. Remak axons later penetrate the peripheral portions of the rectal wall when collapsin-1 expression retreats from the outer muscle layer to the more internal submucosal and mucosal layers of the rectum. Extension of Remak neurites is repelled in vitro by rectum explants and also by 293T cells expressing collapsin-1. The rectal chemorepellent activity is blocked by anti-collapsin-1 antibodies. Our results suggest that collapsin-1 may help prevent Remak axons from projecting into the intestinal wall at early developmental times and later restricts Remak axon trajectories to the outer part of the intestinal muscle layer.

Molecular cloning, expression, and activity of zebrafish semaphorin Z1a

Semaphorins/collapsins are a large family of secreted and cell surface molecules that are thought to guide growth cones to their targets. Although some members are clearly repulsive to specific growth cones in vitro, the in vivo role of many of these molecules in vertebrate embryos is still unclear. As a first step towards clarifying the in vivo role of semaphorins/collapsins, we analyzed semaZ1a in the simple and well-characterized zebrafish embryo. SemaZ1a is a secreted molecule that is highly homologous to Sema III/D/collapsin-1, and it can collapse chick dorsal root ganglion growth cones in vitro. It is expressed in highly specific patterns within the developing embryo, which suggests that it influences outgrowth by a variety of growth cones including those of the posterior lateral line ganglion. Consistent with this hypothesis, the peripherally extending growth cones of posterior lateral line neurons retract and partially collapse during normal outgrowth.

Vertebrate slit, a secreted ligand for the transmembrane protein roundabout, is a repellent for olfactory bulb axons

The olfactory bulb plays a central role in olfactory information processing through its connections with both peripheral and cortical structures. Axons projecting from the olfactory bulb to the telencephalon are guided by a repulsive activity in the septum. The molecular nature of the repellent is not known. We report here the isolation of vertebrate homologs of the Drosophila slit gene and show that Slit protein binds to the transmembrane protein Roundabout (Robo). Slit is expressed in the septum whereas Robo is expressed in the olfactory bulb. Functionally, Slit acts as a chemorepellent for olfactory bulb axons. These results establish a ligand-receptor relationship between two molecules important for neural development, suggest a role for Slit in olfactory bulb axon guidance, and reveal the existence of a new family of axon guidance molecules.

Slit2-Mediated chemorepulsion and collapse of developing forebrain axons

Diffusible chemorepellents play a major role in guiding developing axons toward their correct targets by preventing them from entering or steering them away from certain regions. Genetic studies in Drosophila revealed a novel repulsive guidance system that prevents inappropriate axons from crossing the CNS midline; this repulsive system is mediated by the Roundabout (Robo) receptor and its secreted ligand Slit. In rodents, Robo and Slit are expressed in the spinal cord and Slit can repel spinal motor axons in vitro. Here, we extend these findings into higher brain centers by showing that Robo1 and Robo2, as well as Slit1 and Slit2, are often expressed in complementary patterns in the developing forebrain. Furthermore, we show that human Slit2 can repel olfactory and hippocampal axons and collapse their growth cones.

Semaphorins III and IV repel hippocampal axons via two distinct receptors

The semaphorins are the largest family of repulsive axon guidance molecules. Secreted semaphorins bind neuropilin receptors and repel sensory, sympathetic and motor axons. Here we show that CA1, CA3 and dentate gyrus axons from E15-E17 mouse embryo explants are selectively repelled by entorhinal cortex and neocortex. The secreted semaphorins Sema III and Sema IV and their receptors Neuropilin-1 and −2 are expressed in the hippocampal formation during appropriate stages. Sema III and Sema IV strongly repel CA1, CA3 and dentate gyrus axons; entorhinal axons are only repelled by Sema III. An antibody against Neuropilin-1 blocks the repulsive action of Sema III and the entorhinal cortex, but has no effect on Sema IV-induced repulsion. Thus, chemorepulsion plays a role in axon guidance in the hippocampus, secreted semaphorins are likely to be responsible for this action, and the same axons can be repelled by two distinct semaphorins via two different receptors.

Tissues exhibiting inhibitory [correction of inhibiory] and repulsive activities during the initial stages of neurite outgrowth from the dorsal root ganglion in the chick embryo

To elucidate the mechanisms underlying the projection of dorsal root ganglion (DRG) axons into the dorsal root entry zone in the dorsolateral region of the spinal cord, we examined tissue interactions which affect neurite outgrowth from DRG. We cultured explants or dissociated cells of DRG from embryonic day 4 (E4) chick embryos in combination with E3 spinal cord, notochord, and dermomyotome in three-dimensional collagen gels. The ventral spinal cord, notochord, and dermomyotome, which are located close to the initial projection pathway of DRG but do not receive direct innervation, strongly inhibited DRG neurite outgrowth and repelled DRG neurites. These inhibitory/repulsive cues appear diffusible in nature, because this activity was observed in the absence of direct contacts between tissue explants and DRG neurites. Furthermore, in heterochronic cultures, E9 DRG lost its responsiveness to inhibitory/repulsive factors from E3 ventral spinal cord, while retaining responsiveness to E3 notochord and dermomyotome, suggesting that the E3 ventral spinal cord may secrete a different inhibitory/repulsive signal than notochord and dermomyotome. Putative inhibitory/repulsive signals secreted from tissues along the axonal pathway may serve to guide growing DRG axons to the dorsal root entry zone.

Zebrafish semaphorin Z1a collapses specific growth cones and alters their pathway in vivo

The semaphorin/collapsin gene family encodes secreted and transmembrane proteins several of which can repulse growth cones. Although the in vitro activity of Semaphorin III/D/Collapsin 1 is clear, recent analyses of two different strains of semaphorin III/D/collapsin 1 knockout mice have generated conflicting findings. In order to clarify the in vivo action of this molecule, we analyzed sema Z1a, a zebrafish homolog of semaphorin III/D/collapsin 1. The expression pattern of sema Z1a suggested that it delimited the pathway of the growth cones of a specific set of sensory neurons, the posterior ganglion of the lateral line, in zebrafish. To examine the in vivo action of this molecule, we analyzed (1) the pathways followed by lateral line growth cones in mutants in which the expression of sema Z1a is altered in an interesting way, (2) response of lateral line growth cones to exogenous Sema Z1a in living embryos, and (3) the pathway followed by lateral line growth cones when Sema Z1a is misexpressed by cells along their normal route. The results suggest that a repulsive action of Sema Z1a helps guide the growth cones of the lateral line along their normal pathway.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

Sensory axons are guided by local cues in the developing dorsal spinal cord

During development, different classes of sensory neurons establish distinctive central projections within the spinal cord. Muscle spindle afferents (Ia fibers) grow ventrally through the dorsal horn to the ventral cord, whereas cutaneous sensory collaterals remain confined to the dorsal horn. We have studied the nature of the cues used by Ia fibers in establishing their characteristic projections within the dorsal horn. An organotypic culture preparation of embryonic chicken spinal cord and sensory ganglia was used to test the influence of ventral spinal cord and local cues within the dorsal spinal cord on the growing Ia afferents. When the ventral half of the spinal cord was replaced with an inverted duplicate dorsal half, Ia fibers entering through the dorsal columns still grew ventrally within the host dorsal horn. After the fibers entered the duplicate dorsal half, they continued growing in the same direction. With respect to the duplicate dorsal tissue, this was in an opposite, ventral-to-dorsal, direction. In both cases, however, Ia collaterals remained confined to the medial dorsal laminae. Restriction to these laminae was maintained even when the fibers had to change their direction of growth to stay within them. These results show that cues from the ventral cord are not required for the development of correct Ia projections within the dorsal horn. Local, rather than long-range directional, cues appear to determine the pattern of these projections. When the ventral half of the spinal cord was left intact but sensory axons were forced to enter the dorsal gray matter growing rostrally or caudally, their collateral axons grew in random directions, further showing the absence of directional cues even when the ventral cord was present. Taken together, these observations suggest that Ia fibers are guided by local positional cues that keep them confined to the medial gray matter within the dorsal horn, but their direction of growth is determined primarily by their orientation and position as they enter the dorsal gray matter.

Axon guidance to and from choice points

Significant progress has been made recently in understanding axon guidance to and from choice points. Netrins have been shown to function as conserved midline chemoattractants in vertebrates and insects, and receptors for netrins and semaphorins/collapsins have been identified. More evidence has accumulated that repulsion plays a key role in guidance, including the involvement of the ephrin/Eph receptor system in contact repulsion.

Semaphorin III can repulse and inhibit adult sensory afferents in vivo

During development, semaphorins (collapsin, fasciclin) mediate repulsive and inhibitory guidance of neurons. Semaphorin III, a secretable member of this family, is expressed by the ventral spinal cord at the time corresponding to projection of sensory afferents from the dorsal root ganglion (DRG) into the spinal cord. The inhibitory effect of E14 ventral cord is active only on nerve growth factor (NGF)-responsive sensory afferents (small-diameter A-delta and C fibers subserving sensations of temperature and pain). Similarly, COS cells secreting recombinant semaphorin III are able to selectively repel DRG afferents whose growth is stimulated by NGF and not NT-3. However, it is not known whether these molecules can exert a functional role in the fully developed adult peripheral nervous system. In this study, we demonstrated that gene gun transfection and production of semaphorin III in corneal epithelial cells in adult rabbits in vivo can cause repulsion of established A-delta and C fiber trigeminal sensory afferents. In addition, it is shown that, following epithelial wounding and denervation, semaphorin III is able to inhibit collateral nerve sprouts from innervating the reepithelialized tissue. These findings are significant in that they provide direct evidence that small-diameter adult sensory neurons retain the ability to respond to semaphorin III. In addition, the corneal gene gun technique may be generally used to study the in vivo effects of neural growth modulators by quantifying the amount of sensory nerve growth.

Border controls at the mammalian spinal cord: late-surviving neural crest boundary cap cells at dorsal root entry sites may regulate sensory afferent ingrowth and entry zone morphogenesis

Boundary caps (BCs) form when neural crest cells, migrating ventrally alongside the neural tube, arrest at sites where axons will enter and exit. However, nothing is known of their subsequent fate and functions. We have found late-surviving neural crest BC cell clusters at proximal dorsal root entry sites throughout rat spinal cord development. Sensory afferents cross BCs to enter the spinal cord, while exiting astrocyte processes, destined to form the dorsal root entry zone (DREZ) after birth, are temporarily stalled in their vicinity. To test whether contact with BC cells influences neurite outgrowth from dorsal root ganglia, neurons were cultured on embryonic dorsal root/spinal cord cryosections. Neurites that entered CNS territory preferentially extended over BC cells. Thus, BC cells could be instrumental in regulating afferent ingrowth and DREZ morphogenesis in mammalian spinal cord development.

Neuropilin is a semaphorin III receptor

The semaphorin family contains a large number of phylogenetically conserved proteins and includes several members that have been shown to function in repulsive axon guidance. Semaphorin III (Sema III) is a secreted protein that in vitro causes neuronal growth cone collapse and chemorepulsion of neurites, and in vivo is required for correct sensory afferent innervation and other aspects of development. The mechanism of Sema III function, however, is unknown. Here, we report that neuropilin, a type I transmembrane protein implicated in aspects of neurodevelopment, is a Sema III receptor. We also describe the identification of neuropilin-2, a related neuropilin family member, and show that neuropilin and neuropilin-2 are expressed in overlapping, yet distinct, populations of neurons in the rat embryonic nervous system.

Neuropilin is a receptor for the axonal chemorepellent Semaphorin III

Extending axons in the developing nervous system are guided to their targets through the coordinate actions of attractive and repulsive guidance cues. The semaphorin family of guidance cues comprises several members that can function as diffusible axonal chemorepellents. To begin to elucidate the mechanisms that mediate the repulsive actions of Collapsin-1/Semaphorin III/D (Sema III), we searched for Sema III-binding proteins in embryonic rat sensory neurons by expression cloning. We report that Sema III binds with high affinity to the transmembrane protein neuropilin, and that antibodies to neuropilin block the ability of Sema III to repel sensory axons and to induce collapse of their growth cones. These results provide evidence that neuropilin is a receptor or a component of a receptor complex that mediates the effects of Sema III on these axons.

Embryonic expression pattern of H218, a G-protein coupled receptor homolog, suggests roles in early mammalian nervous system development

Heterologous expression studies employing mammalian cell tissue culture techniques and in vivo studies of lower eukaryotes suggest that G-protein coupled receptors may play critical roles in regulating early stages of vertebrate nervous system development. Previous work suggests that H218, a rat G-protein coupled receptor homolog, could serve such a role. Most importantly, northern blot data indicate that whole brain H218 mRNA levels are highest during embryogenesis. In the present studies we raised, affinity-purified and characterized several anti-H218, polyclonal antisera and immunohistochemically mapped the expression of H218 during the early stages of rat embryonic nervous system development. The resulting data indicate that H218 is preferentially expressed in young, differentiating neuronal cell bodies and axons. Moreover, the expression is temporally regulated such that highest H218 levels are found in neuronal cell bodies during their early stages of differentiation and in axons during their outgrowth. Therefore, we propose that H218 signal transduction may widely participate in the regulation of some of the first steps in neuronal differentiation including axon outgrowth.

Surround repulsion of spinal sensory axons in higher vertebrate embryos

We have tested whether the orientation of axons sprouting from bipolar dorsal root ganglion neurons is influenced by diffusible cues from surrounding tissues. Surface ectoderm, dermomyotome, and notochord exert strong chemorepulsion on axons growing in collagen gels, operating at separations beyond those found in vivo and active in cocultures of chick and mouse tissues. Basal and alar plates of the neural tube are devoid of activity, as is the posterior-half-sclerotome, which repels in a contact-dependent manner. When ganglia are sandwiched between dermomyotome and notochord placed at a distance, axon growth is channeled in a bipolar trajectory. These results show that gradients of diffusible repulsion molecules flanking axon pathways can generate linear patterns of axon growth. We suggest that such "surround repulsion" may function generally, in concert with contact-dependent guidance mechanisms, to guide axons in the developing nervous system.

A sensory axon repellent secreted from ventral spinal cord explants is neutralized by antibodies raised against collapsin-1

During embryogenesis, different subclasses of sensory neurons extend central projections to specific locations in the spinal cord. Muscle and cutaneous afferents initially project to the same location in the dorsal cord. Later, specific muscle afferents leave other afferents behind and project into the ventral cord. Previous studies have shown that ventral spinal cord explants secrete a repellent for sensory neurites. We now find that antibodies to collapsin-1 neutralize this repellent activity. Additional data suggest that all afferents respond to collapsin-1 when they are first confined to the dorsal cord, but that ventrally projecting muscle afferents become collapsin-1 insensitive as they project into the ventral cord. Our results suggest that the transient dorsal expression of collapsin-1 prevents all efferents from entering the cord early and sustained ventral expression prevents dorsally terminating afferents from entering the ventral cord later.

Cues intrinsic to the spinal cord determine the pattern and timing of primary afferent growth

We have used organotypic cultures of embryonic rat spinal cord and dorsal root ganglia (DRG) to study the development of central projections of primary sensory afferent axons that express calcitonin gene-related peptide (CGRP). In vivo, small- and medium-diameter CGRP-positive primary afferents terminate in laminae I, II, and V of the spinal cord and do not enter the ventral horn. A similar pattern of CGRP-positive axonal projections was observed in spinal cord slices of Day 16 embryos (E16) maintained in culture for 6 days. Both intact and dissociated DRG neurons showed the same pattern of central arborization, indicating that complex intercellular interactions between DRG neurons are not required for laminar specific targeting. Furthermore, targeting to the dorsal horn and avoidance of the ventral horn was observed in isolated dorsal and ventral hemicords, suggesting that separate mechanisms mediate the avoidance of CGRP-positive axons from the ventral horn and the elaboration of the afferent arbors within the dorsal horn. CGRP-positive afferents can grow into the dorsal horn only during a brief time window. Cultures of age-matched (isochronic) DRG and spinal cord from E14, E16, and E18 animals showed the characteristic pattern of CGRP-positive axon arborization, while cultures from E20 and neonatal animals did not. Heterochronic cultures indicate that it is the age of the spinal cord, and not the age of the DRG, that determines the ability of the CGRP-positive afferents to arborize within the dorsal horn. Together these results demonstrate that cues intrinsic to the spinal cord can direct sensory projections to appropriate locations in the spinal cord.

The molecular biology of axon guidance

Neuronal growth cones navigate over long distances along specific pathways to find their correct targets. The mechanisms and molecules that direct this pathfinding are the topics of this review. Growth cones appear to be guided by at least four different mechanisms: contact attraction, chemoattraction, contact repulsion, and chemorepulsion. Evidence is accumulating that these mechanisms act simultaneously and in a coordinated manner to direct pathfinding and that they are mediated by mechanistically and evolutionarily conserved ligand-receptor systems.

Growth cones and the cues that repel them

Neuronal growth cones establish appropriate connections with their targets during development by responding to both positive and negative guidance cues. The importance of repulsive and inhibitory cues in pathfinding and target selection has now been firmly established at the cellular and molecular levels. Observations in vitro have demonstrated developmentally significant repulsive interactions among various neuronal populations, providing the basis for molecular and functional characterization of several families of molecules that can mediate these guidance events. Analysis of both the expression and function of these molecules in vivo suggests how they, together with positive guidance cues, participate in the dynamic process of growth-cone guidance during both development and axonal regeneration.

Chemosuppression of retinal axon growth by the mouse optic chiasm

To determine whether diffusible guidance cues direct retinal axon growth and divergence at the optic chiasm, we cocultured mouse retinal and chiasm explants in collagen gels. The chiasm reduced retinal neurite lengths and numbers, but did not affect commissural or pontine neurite growth. This reduction in growth was equal for all retinal quadrants and occurred without reorienting the direction of neurite extension. The floor plate, another midline guidance locus, also suppressed retinal neurite outgrowth, whereas cortex or cerebellum explants did not. Growth suppression was not mediated by netrin-1, which instead enhanced retinal neurite extension. We propose that chemosuppression may be a general guidance mechanism that acts in intermediate targets to prime growth cones to perceive other, more specific cues.

Novel gene families involved in neural pathfinding

Recent studies have uncovered a bifunctional role of the diffusible axon guidance molecules netrin-1 and UNC-6 in that they attract some axons and steer others away simultaneously. Homology screens have extended the semaphorin and collapsin family to include at least 20 members, in both secreted and membrane-bound forms. Eph-related receptors and their membrane-bound ligands, the function of which have only been characterized poorly to date, have been added to the growing number of molecules involved in axon guidance and fasciculation.

Cranial motor axons respond differently to the floor plate and sensory ganglia in collagen gel co-cultures

Within the developing chick hindbrain, motor neurons differentiate in columns on either side of the ventral midline floor plate. Along the rostrocaudal axis, populations of motor neurons are organized segmentally with the trigeminal (V) and facial (VII) nuclei occupying successive pairs of rhombomeres. To reach their targets, motor axons follow stereotyped pathways. Branchiomotor and visceral motor axons of the Vth and VIIth nerves first project in a dorsal (lateral) direction away from the floor plate and towards the nerve exit point located in the alar plate of the even-numbered rhombomere of the pair. Having exited the hindbrain, axons grow in association with the cranial sensory ganglia before branchiomotor axons enter the branchial arches. We have investigated some of the factors that might guide cranial motor axons using a three-dimensional collagen gel culture system. When explants of hindbrain basal plate containing trigeminal or facial motor neurons were co-cultured with floor plate explants, axon outgrowth from the side facing the floor plate was inhibited in a manner consistent with chemorepulsion. When basal plate explants that contained an exit point were cultured alone, motor axons grew to the exit point and then stopped. When basal plate explants were co-cultured with trigeminal ganglia, motor outgrowth was increased in comparison with that in control cultures, suggesting a trophic influence. The findings presented here indicate that motor pathways are elaborated due to a progression of signals to which the growth cones respond in sequence.

Positional cues that are strictly localized in the telencephalon induce preferential growth of mitral cell axons

In mice, mitral cells are the major efferent neurons of the main olfactory bulb and elongate axons into a very narrow part of the telencephalon to form a fiber bundle referred to as the lateral olfactory tract (LOT). To clarify the mechanisms responsible for guidance of mitral cell axons along this particular pathway, we co-cultured mouse embryo main olfactory bulbs with the telencephalons, and analyzed the pathways taken by mitral cell axons. Ingrowth of mitral cell axons into the telencephalon was observed in those co-cultures in which the olfactory bulbs had been exactly combined to their normal pathway (the LOT position) of the telencephalon. The axons grew preferentially along the LOT position, and formed a LOT-like fiber bundle. When the olfactory bulbs were grafted at positions apart from their normal pathway, however, no mitral cell axons grew into the telencephalon. Neocortical fragments combined with the telencephalon projected fibers into the telencephalon in random directions. These results suggest that the LOT position of the telencephalon offers a guiding pathway for mitral cell axons and that guiding cues for mitral cell axons are extremely localized.

Functions of netrins and semaphorins in axon guidance

Neuronal growth cones respond to both contact-mediated and chemotropic guidance cues; these cues can be either attractive or repulsive. This past year has seen further characterization of two gene families implicated in long-range chemoattraction and chemorepulsion: the netrins and the semaphorins. Analysis of invertebrate members of these gene families demonstrates in vivo how netrins play multiple roles in axonal guidance in Caenorhabditis elegans, how specific domains of the netrin molecule confer attractive and repulsive guidance cues, and how semaphorins can function to generate neuromuscular specificity.

Semaphorins: mediators of repulsive growth cone guidance

During development, neuronal growth cones encounter a variety of guidance cues while mediating axon path finding, target recognition and synapse formation. It is clear that repulsive guidance mechanisms play an essential role in these processes. The semaphorin gene family, which is conserved from invertebrates to mammals, includes members that mediate repulsive guidance. Molecular and cellular analysis of this gene family is providing insight into how inhibitory cues function during neurodevelopment.