Continuous renewal of the axonal pathway sensor apparatus by insertion of new sensor molecules into the growth cone membrane

TAG-1 Multifunctionality Coordinates Neuronal Migration, Axon Guidance, and Fasciculation

Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges during development because their cell bodies reside within the central nervous system (CNS) and their axons project to various targets in the body periphery. The molecular mechanisms that contain MN somata within the spinal cord while allowing their axons to exit the CNS and navigate to their final destinations remain incompletely understood. We find that the MN cell surface protein TAG-1 anchors MN cell bodies in the spinal cord to prevent their emigration, mediates motor axon fasciculation during CNS exit, and guides motor axons past dorsal root ganglia. TAG-1 executes these varied functions in MN development independently of one another. Our results identify TAG-1 as a key multifunctional regulator of MN wiring that coordinates neuronal migration, axon fasciculation, and axon guidance.

Suter et al. demonstrate that the motor neuron cell surface molecule TAG-1 confines motor neurons to the central nervous system, promotes motor axon fasciculation, and steers motor axons past inappropriate targets. This study highlights how a single cell adhesion molecule coordinates multiple steps in neuronal wiring through partially divergent mechanisms.

Regulation of plasma membrane expansion during axon formation

Here, will review current evidence regarding the signaling pathways and mechanisms underlying membrane addition at sites of active growth during axon formation. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 170-180, 2018.

Pharmacological, Cell, and Gene Therapy Strategies to Promote Spinal Cord Regeneration

In this review, recent studies using pharmacological treatment, cell transplantation, and gene therapy to promote regeneration of the injured spinal cord in animal models will be summarized. Pharmacological and cell transplantation treatments generally revealed some degree of effect on the regeneration of the injured ascending and descending tracts, but further improvements to achieve a more significant functional recovery are necessary. The use of gene therapy to promote repair of the injured nervous system is a relatively new concept. It is based on the development of methods for delivering therapeutic genes to neurons, glia cells, or nonneural cells. Direct in vivo gene transfer or gene transfer in combination with (neuro)transplantation (ex vivo gene transfer) appeared powerful strategies to promote neuronal survival and axonal regrowth following traumatic injury to the central nervous system. Recent advances in understanding the cellular and molecular mechanisms that govern neuronal survival and neurite outgrowth have enabled the design of experiments aimed at viral vector-mediated transfer of genes encoding neurotrophic factors, growth-associated proteins, cell adhesion molecules, and antiapoptotic genes. Central to the success of these approaches was the development of efficient, nontoxic vectors for gene delivery and the acquirement of the appropriate (genetically modified) cells for neurotransplantation. Direct gene transfer in the nervous system was first achieved with herpes viral and E1-deleted adenoviral vectors. Both vector systems are problematic in that these vectors elicit immunogenic and cytotoxic responses. Adeno-associated viral vectors and lentiviral vectors constitute improved gene delivery systems and are beginning to be applied in neuroregeneration research of the spinal cord. Ex vivo approaches were initially based on the implantation of genetically modified fibroblasts. More recently, transduced Schwann cells, genetically modified pieces of peripheral nerve, and olfactory ensheathing glia have been used as implants into the injured spinal cord.

New insights into the roles of the contactin cell adhesion molecules in neural development

In vertebrates, the contactin (CNTN) family of neural cell recognition molecules includes six related cell adhesion molecules that play non-overlapping roles in the formation and maintenance of the nervous system. CNTN1 and CNTN2 are the prototypical members of the family and have been involved, through cis- and trans-interactions with distinct cell adhesion molecules, in neural cell migration, axon guidance, and the organization of myelin subdomains. In contrast, the roles of CNTN3-6 are less well characterized although the generation of null mice and the recent identification of a common extracellular binding partner have considerably advanced our grasp of their physiological roles in particular as they relate to the wiring of sensory tissues. In this review, we aim to present a summary of our current understanding of CNTN functions and give an overview of the challenges that lie ahead in understanding the roles these proteins play in nervous system development and maintenance.

The specific targeting of guidance receptors within neurons: who directs the directors?

Guidance molecules present in both axonal and dendritic growth cones mediate neuronal responses to extracellular cues thereby ensuring correct neurite pathfinding and development of the nervous system. Little is known though about the mechanisms employed by neurons to deliver these receptors, specifically and efficiently, to the extending growth cone. A deeper understanding of this process is crucial if guidance receptors are to be manipulated to promote nervous system repair. Studies in other polarised cells, notably epithelial, have elucidated fundamental routes to the intracellular segregation of molecules mediated by endosomal pathways. Due to their extreme complexity and specialisation, neurons appear to have built upon these generic systems to evolve sophisticated trafficking networks. A striking feature is the axon initial segment which acts like a valve to tightly regulate the flux of molecules both entering and leaving the axon. Once in the growth cone, further controls operate to enhance the retention or rejection, as appropriate, of membrane receptors. We discuss the current state of knowledge regarding the intracellular trafficking of axon guidance receptors and how this relates to their developmental roles. We highlight the various facets still to be properly elucidated and by building on existing data regarding neuronal polarity and intracellular sorting mechanisms suggest ways to fill these gaps.

Identification of a developmentally regulated pathway of membrane retrieval in neuronal growth cones

The growth-cone plasma membrane constantly reconfigures during axon navigation and upon target recognition. The identity and regulation of the membrane pathway(s) participating in remodeling of the growth-cone surface remain elusive. Here, we identify a constitutive, high-capacity plasma-membrane-recycling activity in the axonal growth cones, which is mediated by a novel bulk endocytic pathway that is mechanistically related to macropinocytosis. This pathway generates large compartments at sites of intense actin-based membrane ruffling through the actions of phosphatidylinositol 3-kinase, the small GTPase Rac1 and the pinocytic chaperone Pincher. At early developmental stages, bulk endocytosis is the primary endocytic pathway for rapid retrieval of the growth-cone plasma membrane. At later stages, during the onset of synaptogenesis, an intrinsic program of maturation leads to downregulation of basal bulk endocytosis and the emergence of depolarization-induced synaptic-vesicle exo-endocytosis. We propose that the control of bulk membrane retrieval contributes to the homeostatic regulation of the axonal plasma membrane and to growth-cone remodeling during axonal outgrowth. In addition, we suggest that the downregulation of bulk endocytosis during synaptogenesis might contribute to the preservation of synaptic-vesicle specificity.

Disorganized microtubules underlie the formation of retraction bulbs and the failure of axonal regeneration

Axons in the CNS do not regrow after injury, whereas lesioned axons in the peripheral nervous system (PNS) regenerate. Lesioned CNS axons form characteristic swellings at their tips known as retraction bulbs, which are the nongrowing counterparts of growth cones. Although much progress has been made in identifying intracellular and molecular mechanisms that regulate growth cone locomotion and axonal elongation, a comprehensive understanding of how retraction bulbs form and why they are unable to grow is still elusive. Here we report the analysis of the morphological and intracellular responses of injured axons in the CNS compared with those in the PNS. We show that retraction bulbs of injured CNS axons increase in size over time, whereas growth cones of injured PNS axons remain constant. Retraction bulbs contain a disorganized microtubule network, whereas growth cones possess the typical bundling of microtubules. Using in vivo imaging, we find that pharmacological disruption of microtubules in growth cones transforms them into retraction bulb-like structures whose growth is inhibited. Correspondingly, microtubule destabilization of sensory neurons in cell culture induces retraction bulb formation. Conversely, microtubule stabilization prevents the formation of retraction bulbs and decreases axonal degeneration in vivo. Finally, microtubule stabilization enhances the growth capacity of CNS neurons cultured on myelin. Thus, the stability and organization of microtubules define the fate of lesioned axonal stumps to become either advancing growth cones or nongrowing retraction bulbs. Our data pinpoint microtubules as a key regulatory target for axonal regeneration.

The role of cytoplasmic serine residues of the cell adhesion molecule L1 in neurite outgrowth, endocytosis, and cell migration

1. The cell adhesion molecule L1 has been implicated in adhesion and migration of cells, in axon growth, guidance, and fasciculation, in myelination and synaptic plasticity. The cytoplasmic domain of neuronal L1 is highly conserved between species and has been shown to be phosphorylated at serine and tyrosine residues. 2. To investigate the significance of L1 serine phosphorylation, mutants of L1 were generated in which ser-1152, ser-1181, ser-1204, and ser-1248 were exchanged for leucine and rat B35 neuroblastoma cells were stably transfected with the L1-cDNA constructs. 3. Neurite outgrowth on poly-L-lysine (PLL) as substrate was determined either with or without differentiation into a neuronal phenotype with dbcAMP. In addition, antibody-induced endocytosis and cell migration were examined. 4. Our observations indicate that phosphorylation of single serine residues of the cytoplasmic domain of L1 contributes to neurite outgrowth through different mechanisms. Neurite growth is increased when ser-1152 or ser-1181 is replaced by a non-phosphorylatable leucine and decreased when ser-1204 or ser-1248 is mutated to leucine. Furthermore, mutation of ser-1181 to leucine results in strongly enhanced antibody-induced endocytosis of L1 and also in enhanced cell migration.

Calsyntenin-1 docks vesicular cargo to kinesin-1

We identified a direct interaction between the neuronal transmembrane protein calsyntenin-1 and the light chain of Kinesin-1 (KLC1). GST pulldowns demonstrated that two highly conserved segments in the cytoplasmic domain of calsyntenin-1 mediate binding to the tetratricopeptide repeats of KLC1. A complex containing calsyntenin-1 and the Kinesin-1 motor was isolated from developing mouse brain and immunoelectron microscopy located calsyntenin-1 in association with tubulovesicular organelles in axonal fiber tracts. In primary neuronal cultures, calsyntenin-1-containing organelles were aligned along microtubules and partially colocalized with Kinesin-1. Using live imaging, we showed that these organelles are transported along axons with a velocity and processivity typical for fast axonal transport. Point mutations in the two kinesin-binding segments of calsyntenin-1 significantly reduced binding to KLC1 in vitro, and vesicles bearing mutated calsyntenin-1 exhibited a markedly altered anterograde axonal transport. In summary, our results indicate that calsyntenin-1 links a certain type of vesicular and tubulovesicular organelles to the Kinesin-1 motor.

Inhibition of sphingolipid synthesis affects kinetics but not fidelity of L1/NgCAM transport along direct but not transcytotic axonal pathways

Glycosphingolipids are constituents of lipid rafts which might function in sorting apical and axonal cargoes in the trans-Golgi network. In fact, two GPI-linked proteins, Thy1 and PrPC, require lipid raft lipids for sorting to the axon. It was previously shown that inhibition of glycosphingolipid synthesis by FumonisinB1 (FB1) impairs axon outgrowth but not axon specification, leading to the hypothesis that formation of axonally-targeted vesicles is coupled to sphingolipid synthesis. Since the axonal cell adhesion molecule L1/NgCAM can partition into membrane rafts biochemically, we asked whether correct targeting to the axon is FB1-sensitive, similarly to GPI-linked proteins. We previously showed that cultured hippocampal neurons use more than one trafficking pathway to the axon: a transcytotic pathway and a direct pathway. We show here that reducing raft lipid levels does not disrupt axonal targeting of L1/NgCAM along either pathway. Unexpectedly, FB1 selectively slowed the kinetics of surface expression of a truncated NgCAM using the direct pathway, but not of NgCAM using the transcytotic pathway. Therefore, the formation and/or transport of a subset of axonally-targeted vesicles are coupled to sphingolipid synthesis. Our results yield a mechanism for the axon outgrowth defect observed in FB1.

Transcytosis of NgCAM in epithelial cells reflects differential signal recognition on the endocytic and secretory pathways

NgCAM is a cell adhesion molecule that is largely axonal in neurons and apical in epithelia. In Madin-Darby canine kidney cells, NgCAM is targeted to the apical surface by transcytosis, being first inserted into the basolateral domain from which it is internalized and transported to the apical domain. Initial basolateral transport is mediated by a sequence motif (Y₃₃RSL) decoded by the AP-1B clathrin adaptor complex. This motif is a substrate in vitro for tyrosine phosphorylation by p60src, a modification that disrupts NgCAM's ability to interact with clathrin adaptors. Based on the behavior of various NgCAM mutants, it appears that after arrival at the basolateral surface, the AP-1B interaction site is silenced by phosphorylation of Tyr₃₃. This slows endocytosis and inhibits basolateral recycling from endosomes, resulting in NgCAM transcytosis due to a cryptic apical targeting signal in its extracellular domain. Thus, transcytosis of NgCAM and perhaps other membrane proteins may reflect the spatial regulation of recognition by adaptors such as AP-1B.

Efficient axonal localization of alphaherpesvirus structural proteins in cultured sympathetic neurons requires viral glycoprotein E

Pseudorabies virus (PRV) glycoprotein E (gE) is a type I viral membrane protein that facilitates the anterograde spread of viral infection from the peripheral nervous system to the brain. In animal models, a gE-null mutant infection spreads inefficiently from presynaptic neurons to postsynaptic neurons (anterograde spread of infection). However, the retrograde spread of infection from post- to presynaptic neurons remains unaffected. Here we show that gE is required for wild-type localization of viral structural proteins in axons of infected neurons. During a gE-null PRV infection, a subset of viral glycoproteins, capsids, and tegument proteins enter and localize to the axon inefficiently. This defect is most obvious in the distal axon and growth cones. However, axonal entry and localization of other viral membrane proteins and endogenous cellular proteins remains unaffected. Neurons infected with gE-null mutants produce wild-type levels of viral structural proteins and infectious virions in the cell body. Our results indicate that reduced axonal targeting of viral structural proteins is a compelling explanation for the lack of anterograde spread in neural circuits following infection by a gE-null mutant.

The development of a long, coiled, optic nerve in the stalk-eyed fly Cyrtodiopsis whitei

In the stalk-eyed fly Cyrtodiopsis whitei (Diopsidae; Diptera), the relatively long optic nerve develops within the tight lumen of a very short eyestalk. Axonal growth is generally considered in terms of path finding, selective fasciculation, and towing. Physical forces that are necessary for axon lengthening are generated either by the growth cone or by the growth of surrounding tissues. Therefore, it is surprising to encounter a loosely coiled nerve apparently lacking any attachments that could allow for pull, or towing, of the nerve. In this study, we used histological sections and whole-mount preparations to confirm that the optic nerve of the stalk-eyed fly indeed elongates without the external application of tension to the nerve. Secondly, we examined the distribution of cytoskeletal elements and selected proteins that may be involved in axon extension. Staining against the vesicle fusion proteins SNAP-24 and SNAP-25 consistently results in stronger staining in the rapidly extending optic nerve than in a control nerve, suggesting a possible role of these proteins in the extension process. On a gross morphological level, SNAP-24/25 as well as the cytoskeletal elements actin and tubulin are uniformly distributed throughout the lengths of the growing nerve, suggesting that nerve elongation is distributed rather than localized. Finally, we identified glia as a possible source for tension within the nerve bundle. Glia proliferate rapidly in the optic nerve but not in the control nerve. Much work continues to focus on the growth of axons in culture, but this study is one of the few that considers the dynamics of nerve bundle extension as a whole.

Molecular motors and mechanisms of directional transport in neurons

Intracellular transport is fundamental for neuronal morphogenesis, function and survival. Many proteins are selectively transported to either axons or dendrites. In addition, some specific mRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamily participate in selective transport by using adaptor or scaffolding proteins to recognize and bind cargoes. The molecular components of RNA-transporting granules have been identified, and it is becoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins. Here we discuss the molecular mechanisms of directional axonal and dendritic transport with specific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

Uncovering multiple axonal targeting pathways in hippocampal neurons

Neuronal polarity is, at least in part, mediated by the differential sorting of membrane proteins to distinct domains, such as axons and somata/dendrites. We investigated the pathways underlying the subcellular targeting of NgCAM, a cell adhesion molecule residing on the axonal plasma membrane. Following transport of NgCAM kinetically, surprisingly we observed a transient appearance of NgCAM on the somatodendritic plasma membrane. Down-regulation of endocytosis resulted in loss of axonal accumulation of NgCAM, indicating that the axonal localization of NgCAM was dependent on endocytosis. Our data suggest the existence of a dendrite-to-axon transcytotic pathway to achieve axonal accumulation. NgCAM mutants with a point mutation in a crucial cytoplasmic tail motif (YRSL) are unable to access the transcytotic route. Instead, they were found to travel to the axon on a direct route. Therefore, our results suggest that multiple distinct pathways operate in hippocampal neurons to achieve axonal accumulation of membrane proteins.

Critical calpain-dependent ultrastructural alterations underlie the transformation of an axonal segment into a growth cone after axotomy of cultured Aplysia neurons

The transformation of a stable axonal segment into a motile growth cone is a critical step in the regeneration of amputated axons. In earlier studies we found that axotomy of cultured Aplysia neurons leads to a transient and local elevation of the free intracellular Ca2+ concentration, resulting in calpain activation, localized proteolysis of submembranal spectrin, and, eventually, growth cone formation. Moreover, inhibition of calpain by calpeptin prior to axotomy inhibits growth cone formation. Here we investigated the mechanisms by which calpain activation participates in the transformation of an axonal segment into a growth cone. To that end we compared the ultrastructural alterations induced by axotomy performed under control conditions with those caused by axotomy performed in the presence of calpeptin, using cultured Aplysia neurons as a model. We identified the critical calpain-dependent cytoarchitectural alterations that underlie the formation of a growth cone after axotomy. Calpain-dependent processes lead to restructuring of the neurofilaments and microtubules to form an altered cytoskeletal region 50-150 microm proximal to the tip of the transected axon in which vesicles accumulate. The dense pool of vesicles forms in close proximity to a segment of the plasma membrane along which the spectrin membrane skeleton has been proteolyzed by calpain. We suggest that the rearrangement of the cytoskeleton forms a transient cellular compartment that traps transported vesicles and serves as a locus for microtubule polymerization. We propose that this cytoskeletal configuration facilitates the fusion of vesicles with the plasma membrane, promoting the extension of the growth cone's lamellipodium. The growth process is further supported by the radial polymerization of microtubules from the growth cone's center.

Two distinct mechanisms target membrane proteins to the axonal surface

We have investigated the trafficking of two endogenous axonal membrane proteins, VAMP2 and NgCAM, in order to elucidate the cellular events that underlie their polarization. We found that VAMP2 is delivered to the surface of both axons and dendrites, but preferentially endocytosed from the dendritic membrane. A mutation in the cytoplasmic domain of VAMP2 that inhibits endocytosis abolished its axonal polarization. In contrast, the targeting of NgCAM depends on sequences in its ectodomain, which mediate its sorting into carriers that preferentially deliver their cargo proteins to the axonal membrane. These observations show that neurons use two distinct mechanisms to polarize proteins to the axonal domain: selective retention in the case of VAMP2, selective delivery in the case of NgCAM.

Thyroid hormone regulates TAG-1 expression in the developing rat brain

TAG-1 is a member of the immunoglobulin superfamily of cell adhesion molecules thought to play important roles in neuronal differentiation and the establishment of connectivity during brain development. Because these are processes also affected by hypothyroidism, we studied the effects of thyroid hormone deprivation and administration on TAG-1 expression in the developing rat brain. By in situ hybridization, immunohistochemistry and Western blotting we found that TAG-1 RNA and protein levels are upregulated in the hypothyroid brain. From embryonic day 20 to postnatal day (P) 15, elevated TAG-1 RNA was found in several areas including the cerebral cortex, hippocampus and olfactory bulb. In agreement with this, TAG-1 protein was overexpressed in the major fibre tracts arising from these structures, including the corpus callosum, anterior and hippocampal commissures and lateral olfactory tract. A similar overexpression of TAG-1 by hypothyroidism was detected in the cerebellum, but starting only at P15. In all cases, elevation of TAG-1 RNA and protein expression could be reversed by thyroid hormone treatment. These results show that the deregulation of TAG-1 might contribute to the alterations caused by the lack of thyroid hormone during brain development.

Topographic restriction of TAG-1 expression in the developing retinotectal pathway and target dependent reexpression during axon regeneration

TAG-1, a glycosylphosphatidyl inositol (GPI)-anchored protein of the immunoglobulin (Ig) superfamily, exhibits an unusual spatiotemporal expression pattern in the fish visual pathway. Using in situ hybridization and new antibodies (Abs) against fish TAG-1 we show that TAG-1 mRNA and anti-TAG-1 staining is restricted to nasal retinal ganglion cells (RGCs) in 24- to 72-h-old zebrafish embryos and in the adult, continuously growing goldfish retina. Anti-TAG-1 Abs selectively label nasal RGC axons in the nerve, optic tract, and tectum. Axotomized RGCs reexpress TAG-1, which occurs as late as 12 days after optic nerve lesion, when regenerating RGC axons arrive in the tectum, suggesting TAG-1 reexpression is target contact-dependent. Accordingly, TAG-1 reexpression ceases upon interruption of the regenerating projection by a second lesion. The topographic restriction of TAG-1 expression and its target dependency during regeneration suggests that TAG-1 might play a role in the retinotopic organization and restoration of the retinotectal pathway.

Plasma membrane recycling and flow in growing neurites

Axonal growth requires insertion of newly synthesized membrane components into the plasmalemma. Imbalance between exocytotic membrane addition and endocytic retrieval at specific axonal sites may lead to the bulk plasma membrane flow along the axon and, thus, contribute to the renewal of plasma membrane components. By using fluorescent lipid analogs incorporated into the plasma membrane, we determined the sites of membrane internalization in growing Xenopus embryo neurons. Vectorial traffic of endocytic membranes from the distal axon to the cell body was observed, suggesting bulk retrieval of plasma membrane at the growth cone. No long-range axonal transport of membrane material internalized at the cell body or along the axon was detected. In addition, we measured the rate of plasma membrane flow in Xenopus neurites. Axonal plasma membrane was found to flow anterogradely with the rate equal to approximately 30% of the rate of neurite elongation. Our results suggest that the "growth from the tip" pattern of neurite elongation may coexist with transport of new membrane components along plasmalemma by anterograde membrane flow.

Recycling of the cell adhesion molecule L1 in axonal growth cones

The cell adhesion molecule (CAM) L1 plays crucial roles in axon growth in vitro and in the formation of major axonal tracts in vivo. It is generally thought that CAMs link extracellular immobile ligands with retrogradely moving actin filaments to transmit force that pulls the growth cone forward. However, relatively little is known about the fate of CAMs that have been translocated into the central (C)-domain of the growth cone. We have shown previously that L1 is preferentially endocytosed at the C-domain. In the present study, we further analyze the subcellular distribution of endocytic organelles containing L1 at different time points and demonstrate that internalized L1 is transported into the peripheral (P)-domain of growth cones advancing via an L1-dependent mechanism. Internalized L1 is found in vesicles positioned along microtubules, and the centrifugal transport of these L1-containing vesicles is dependent on dynamic microtubules in the P-domain. Furthermore, we show that endocytosed L1 is reinserted into the plasma membrane at the leading edge of the P-domain. Monitoring recycled L1 reveals that it moves retrogradely on the cell surface into the C-domain. In contrast, the growth cone advancing independently of L1 internalizes and recycles L1 within the C-domain. For the growth cone to advance, the leading edge needs to establish strong adhesive interactions with the substrate while attachments at the rear are released. Recycling L1 from the C-domain to the leading edge provides an effective way to create asymmetric L1-mediated adhesion and therefore would be critical for L1-based growth cone motility.

The role of selective transport in neuronal protein sorting

To assess whether selective microtubule-based vesicle transport underlies the polarized distribution of neuronal proteins, we expressed green fluorescent protein- (GFP-) tagged chimeras of representative axonal and dendritic membrane proteins in cultured hippocampal neurons and visualized the transport of carrier vesicles containing these proteins in living cells. Vesicles containing a dendritic protein, transferrin receptor (TfR), were preferentially transported into dendrites and excluded from axons. In contrast, vesicles containing the axonal protein NgCAM (neuron-glia cell adhesion molecule) were transported into both dendrites and axons. These data demonstrate that neurons utilize two distinct mechanisms for the targeting of polarized membrane proteins, one (for dendritic proteins) based on selective transport, the other (for axonal proteins) based on a selectivity "filter" that occurs downstream of transport.

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.

Use of lipophilic dyes in studies of axonal pathfinding in vivo

Fluorescent lipophilic dyes are an ideal tool to study axonal pathfinding. Because these dyes do not require active axonal transport for their spreading, they can be used in fixed tissue. Here, we describe the method we have used to study the molecular mechanisms of commissural axon pathfinding in the embryonic chicken spinal cord in vivo. Based on in vitro studies, different families of molecules had been suggested to play a role in the guidance of developing axons. In order to test their function in vivo, we used the commissural neurons that are located at the dorsolateral border of the chicken spinal cord as a model system [Stoeckli and Landmesser (1995) Neuron 14:1165-1179]. Axonin-1, NgCAM, and NrCAM, three members of the immunoglobulin (Ig) superfamily of cell adhesion molecules (CAMs), were shown to be important for the correct growth pattern of commissural axons. We studied the effect of perturbations of specific CAM/CAM interactions by injection of function-blocking antibodies into the central canal of the spinal cord in ovo. After 2 days, the embryos were sacrificed and fluorescent tracers, such as Fast-DiI, were used to visualize commissural axons, and thus, to analyze their response to these perturbations in two different types of fixed preparations: transverse vibratome sections and whole-mount preparations of the spinal cord. Both pathfinding errors and defasciculation of axons were observed as a result of the perturbation of CAM/CAM interactions.

Changes in membrane trafficking and actin dynamics during axon formation in cultured hippocampal neurons

Neurons begin to polarize when one of the neurites becomes the axon. Hippocampal neurons in cell culture have a sharp transition between their unpolarized and polarized stage revealed by the rapid growth of the future axon. Recent progress shows that both a cytoplasmic membrane flow and actin dynamics govern axon formation, and thereby initial neuronal polarization. We here review these mechanisms, evaluate their physiological role, and show similarities to the transient polarization of migrating fibroblasts. Finally, we present a model how actin dynamics and vectorial membrane flow may interact to achieve axon formation.

The role of cell adhesion molecules in synaptic plasticity and memory

Studies in the past few years suggest that cell adhesion molecules may play signaling as well as structural roles at adult synapses during plasticity. The observation that many adhesion molecules are expressed both pre-synaptically and post-synaptically raises the possibility that information about synaptic activity might simultaneously be communicated to both sides of the synapse, circumventing the need for distinct anterograde and retrograde messengers.

Ultrastructural observations on the expression of axonin-1: implications for the fasciculation of sensory axons during axonal outgrowth into the chick hindlimb

To help understand how axons interact as they grow into the developing chick hindlimb, we used electron microscopy in conjunction with immunoperoxidase staining for the cell adhesion molecule axonin-1 to label sensory axons. The results showed that sensory axons travel together in bundles, tightly apposed to one another. In contrast, motoneuron axons are more widely spaced, although motoneuron axons situated at the perimeter of sensory axon bundles are found in close contact with neighboring sensory axons. Sensory growth cones and lamellipodia tend to be located centrally within the bundles, with several lamellipodia typically being found stacked together. Strikingly, regions of close axonal apposition are accompanied by axonin-1 expression, suggesting that such contacts are indeed adhesive. Taken together, these observations suggest that groups of sensory axons of a similar age grow together, with some of the older sensory axons fasciculating along motoneuron axons and younger sensory axons later fasciculating along older sensory axons. Axons situated at the periphery of sensory bundles are typically partly labelled, such that axonin-1 is expressed on membranes apposing other labelled axons but not on those facing unlabelled axons or unlabelled Schwann cells. Thus, axonin-1 appears to become redistributed within the membranes of axons growing into the limb, as it does on cultured neurons. In contrast, the neuron-glia cell adhesion molecule (NgCAM), which binds heterophilically to axonin-1, appears uniformly distributed on even those axons that would have an asymmetric distribution of axonin-1. Thus, the localization of axonin-1 strongly suggests that it plays an important role in sensory axon fasciculation, but the relative contributions of its interactions with various potential ligands are unclear. Finally, we found that some sensory growth cones have lamellipodia that are spread over considerable expanses. This suggests that although fasciculation is important in sensory axon guidance, sensory axons may also explore the local environment.

GPI-anchored human placental alkaline phosphatase has a nonpolarized distribution on the cell surface of mouse cerebellar granule neurons in vitro

The glycosyl phosphatidylinositol (GPI) lipid anchor, which directs GPI-anchored proteins to the apical cell surface in certain polarized epithelial cell types, has been proposed to act as an axonal protein targeting signal in neurons. However, as several GPI-anchored proteins have been found on both the axonal and somatodendritic cell-surface domains of a variety of neuronal cell types, the role of the GPI anchor in protein localization to the axon remains unclear. To begin to address the role of the GPI anchor in neuronal protein localization, we used a replication-incompetent retroviral vector to express a model GPI-anchored protein, human placental alkaline phosphatase (hPLAP), in early postnatal mouse cerebellar granule neurons developing in vitro. Purified granule neurons were cultured in large mitotically active cellular reaggregates to allow retroviral infection of undifferentiated, proliferating granule neuron precursors. To more easily visualize hPLAP localization during the sequence of differentiation of single postmitotic granule neurons, reaggregates were dissociated following infection, plated as high-density monolayers, and maintained for 1-9 days under serum-free culture conditions. As we previously demonstrated for uninfected granule neurons developing in monolayer culture, hPLAP-expressing granule neurons likewise developed in vitro through a series of discrete temporal stages highly similar to those observed in situ. hPLAP-expressing granule neurons first extended either a single neurite or two axonal processes, and subsequently attained a mature, well-polarized morphology consisting of multiple short dendrites and one or two axons that extended up to 3 mm across the culture substratum. hPLAP was expressed uniformly on the entire cell surface at each stage of granule neuron differentiation. Thus, it appears that the GPI anchor is not sufficient to confer axonal localization to an exogenous GPI-anchored protein expressed in a well-polarized primary neuronal cell type in vitro; other signals, such as those present in the extracellular domain of these proteins, may be necessary for the polarized targeting or retention of axon-specific GPI-anchored proteins.

Distribution of neurotransmitter secretion in growing axons

Neurotransmitter secretion from the nerve terminal is mediated by the fusion of synaptic vesicles with the plasma membrane. It is generally believed that neurotransmitter release in mature synapses is localized to the presynaptic nerve terminals. To probe the topology of neurotransmitter secretion along developing axons in culture, we recorded membrane currents from myocytes manipulated into contact with axons. At the early stages of growth, exocytotic events were detected along the axon as well as at the growth cone. At the later stages of growth, neurotransmitter secretion adopted the form of a smooth proximodistal gradient, with the highest level of activity at the growth cone region. Our results reveal the existence of a previously undetected early stage of axonal growth and suggest developmental regulation in the pattern of neurotransmitter secretion along the growing nerve processes.

A diffusion barrier maintains distribution of membrane proteins in polarized neurons

The asymmetric distribution of proteins to distinct domains in the plasma membrane is crucial to the function of many polarized cells. In epithelia, distinct apical and basolateral surfaces are maintained by tight junctions that prevent diffusion of proteins and lipids between the two domains. Polarized neurons maintain axonal and somatodendritic plasma membrane domains without an obvious physical barrier. Indeed, the artificial lipid Dil encounters no diffusion barrier at the presumptive domain boundary, the axon hillock. By measuring the lateral mobility of membrane proteins using optical tweezers, we show here that some membrane proteins exhibit markedly reduced mobility in the initial segment of the axon. Disruption of F-actin and low levels of dimethyl sulphoxide (DMSO) abolish this diffusion barrier and lead to redistribution of membrane markers that had previously been polarized. Immobilization in the initial segment may reflect, at least in part, differential tethering to cytoskeletal components. Therefore, the ability to maintain a polarized distribution of membrane proteins depends on a specialized domain at the initial segment of the axon, which restricts lateral mobility and serves as a new type of diffusion barrier that acts in the absence of cell-cell contact.

Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis

In mature neurons, synaptic vesicles continuously recycle within the presynaptic nerve terminal. In developing axons which are free of contact with a postsynaptic target, constitutive membrane recycling is not localized to the nerve terminal; instead, plasma membrane components undergo cycles of exoendocytosis throughout the whole axonal surface (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). Here we demonstrate that in Xenopus neurons ACh secretion is mediated by vesicles which recycle locally within the axon. Similar to neurotransmitter release at the presynaptic nerve terminal, ACh secretion along the axon could be elicited by the action potential or by hypertonic solutions. We found that the parameters of neurotransmitter secretion at the nerve terminal and at the middle axon were strikingly similar. These results lead us to conclude that, as in the case of the presynaptic nerve terminal, synaptic vesicles involved in neurotransmitter release along the axon contain a complement of proteins for vesicle docking and Ca2+-dependent fusion. Taken together, our results support the idea that, in developing axons, the rudimentary machinery for quantal neurotransmitter secretion is distributed throughout the whole axonal surface. Maturation of this machinery in the process of synaptic development would improve the fidelity of synaptic transmission during high-frequency stimulation of the presynaptic cell.

Neurite fasciculation mediated by complexes of axonin-1 and Ng cell adhesion molecule

Neural cell adhesion molecules composed of immunoglobulin and fibronectin type III-like domains have been implicated in cell adhesion, neurite outgrowth, and fasciculation. Axonin-1 and Ng cell adhesion molecule (NgCAM), two molecules with predominantly axonal expression exhibit homophilic interactions across the extracellular space (axonin- 1/axonin-1 and NgCAM/NgCAM) and a heterophilic interaction (axonin-1-NgCAM) that occurs exclusively in the plane of the same membrane (cis-interaction). Using domain deletion mutants we localized the NgCAM homophilic binding in the Ig domains 1-4 whereas heterophilic binding to axonin-1 was localized in the Ig domains 2-4 and the third FnIII domain. The NgCAM-NgCAM interaction could be established simultaneously with the axonin-1-NgCAM interaction. In contrast, the axonin-1-NgCAM interaction excluded axonin-1/axonin-1 binding. These results and the examination of the coclustering of axonin-1 and NgCAM at cell contacts, suggest that intercellular contact is mediated by a symmetric axonin-12/NgCAM2 tetramer, in which homophilic NgCAM binding across the extracellular space occurs simultaneously with a cis-heterophilic interaction of axonin-1 and NgCAM. The enhanced neurite fasciculation after overexpression of NgCAM by adenoviral vectors indicates that NgCAM is the limiting component for the formation of the axonin-12/NgCAM2 complexes and, thus, neurite fasciculation in DRG neurons.

Dynamics of axonal microtubules regulate the topology of new membrane insertion into the growing neurites

Nerve growth depends on the delivery of cell body-synthesized material to the growing neuronal processes. The cellular mechanisms that determine the topology of new membrane addition to the axon are not known. Here we describe a technique to visualize the transport and sites of exocytosis of cell body- derived vesicles in growing axons. We found that in Xenopus embryo neurons in culture, cell body-derived vesicles were rapidly transported all the way down to the growth cone region, where they fused with the plasma membrane. Suppression of microtubule (MT) dynamic instability did not interfere with the delivery of new membrane material to the growth cone region; however, the insertion of vesicles into the plasma membrane was dramatically inhibited. Local disassembly of MTs by focal application of nocodazole to the middle axonal segment resulted in the addition of new membrane at the site of drug application. Our results suggest that the local destabilization of axonal MTs is necessary and sufficient for the delivery of membrane material to specific neuronal sites.

Adenoviral vector-mediated gene delivery to injured rat peripheral nerve

Although much progress has been made, current treatments of peripheral nerve damage mostly result in only partial recovery. Local production of neurite outgrowth-promoting molecules, such as neurotrophins and/or cell adhesion molecules, at the site of damage may be used as a new means to promote the regeneration process. We have now explored the ability of an adenoviral vector encoding the reporter gene LacZ (Ad-LacZ) to direct the expression of a foreign gene to Schwann cells of intact and crushed rat sciatic nerves. Infusion of 8 x 10(7) PFU Ad-LacZ in the intact sciatic nerve resulted in the transduction of many Schwann cells with high levels of transgene expression lasting at least up to 12 days following viral vector administration. The efficacy of adenoviral vector delivery to a crushed nerve was investigated using three strategies. Injection of the adenoviral vector at the time of, or immediately after, a crush resulted in the transduction of only a few Schwann cells. Administration of the adenoviral vector the day after the crush resulted in the transduction of a similar number of Schwann cells 5 days after administration, as observed in uncrushed nerves. Regenerating nerve fibers were closely associated with beta-galactosidase-positive Schwann cells, indicating that the capacity of transduced Schwann cells to guide regenerating fibers was not altered. These results imply that the expression of growth-promoting proteins through adenoviral vector-mediated gene transfer may be a realistic option to promote peripheral nerve regeneration.

A neuronal form of the cell adhesion molecule L1 contains a tyrosine-based signal required for sorting to the axonal growth cone

The neural cell adhesion molecule L1, which is present on axons and growth cones, plays a crucial role in the formation of major axonal tracts such as the corticospinal tract and corpus callosum. L1 is preferentially transported to axons and inserted in the growth cone membrane. However, how L1 is sorted to axons remains unclear. Tyr1176 in the L1 cytoplasmic domain is adjacent to a neuron-specific alternatively spliced sequence, RSLE (Arg-Ser-Leu-Glu). The resulting sequence of YRSLE conforms to a tyrosine-based consensus motif (YxxL) for sorting of integral membrane proteins into specific cellular compartments. To study a possible role of the YRSLE sequence in L1 sorting, chick DRG neurons were transfected with human L1 cDNA that codes for full-length L1 (L1FL), a non-neuronal form of L1 that lacks the RSLE sequence (L1DeltaRSLE), mutant L1 with a Y1176A substitution (L1Y1176A), or L1 truncated immediately after the RSLE sequence (L1DeltaC77). L1FL and L1DeltaC77, both of which possess the YRSLE sequence, were expressed in the axonal growth cone and to a lesser degree in the cell body. In contrast, expression of both L1DeltaRSLE and L1Y1176A was restricted to the cell body and proximal axonal shaft. We also found that L1DeltaRSLE and L1Y1176A were integrated into the plasma membrane in the cell body after missorting. These data demonstrate that the neuronal form of L1 carries the tyrosine-based sorting signal YRSLE, which is critical for sorting L1 to the axonal growth cone.

The polarized sorting of membrane proteins expressed in cultured hippocampal neurons using viral vectors

One model of neuronal polarity (Dotti and Simons, 1990) proposes that neurons and polarized epithelia use similar mechanisms to sort membrane proteins. To explore this hypothesis, we used viral vectors to express proteins in cultured neurons and assessed their distribution using quantitative immunofluorescence microscopy. Basolateral epithelial proteins were polarized to dendrites; more significantly, mutations of sequences required for their basolateral targeting in epithelia also disrupted dendritic targeting. Unexpectedly, apical proteins were not polarized to axons but were expressed at roughly equal amounts in dendrites and axons. These data provide strong evidence that targeting of basolateral and dendritic proteins depends on common mechanisms. In contrast, the sorting of proteins to the axon requires signals that are not present in apical proteins.

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.

Neural cell recognition molecule L1: from cell biology to human hereditary brain malformations

The neural cell recognition molecule L1 is a member of the immunoglobulin superfamily implicated in embryonic brain development. L1 is engaged in complex extracellular interactions, with multiple binding partners on cell surfaces and in the extracellular matrix. It is the founder of a neural group of related cell surface receptors that share with L1 a highly conserved cytoplasmic domain that associates with the cytoskeleton. Phenotypic analyses of human patients with mutations in the L1 gene and characterizations of L1-deficient mice suggest that L1 is important for embryonic brain histogenesis, in particular the development of axon tracts.

Axon guidance at choice points

The common theme in many recent axonal pathfinding studies, both in vertebrates and invertebrates, is the demonstration of the importance of a balance between positive and negative cues. The integration of multiple and often opposing molecular interactions at each site along the axon's trajectory, especially at choice points, helps to fine tune the directional response of its growth cone, which continuously samples its environment for guidance cues. The dynamic regulation of the receptors for such cues, in response to extrinsic signals, also enhances the behavioral repertoire of growth cones at different points along their trajectory. Some of the molecules identified as being important for axon guidance at choice points are conserved between invertebrates and vertebrates (e.g. Robo and netrin), whereas other molecules have been identified, so far, only in invertebrates (e.g. Comm) or vertebrates (e.g. axonin-1 and NrCAM).

Neuronal polarity: vectorial cytoplasmic flow precedes axon formation

Axon formation in multipolar neurons is believed to depend on the existence of precise sorting mechanisms for axonal membrane and membrane-associated proteins. Conclusive evidence in living neurons, however, is lacking. In the present study, we use light and video microscopy to address this issue directly. We show that axon formation is preceded by the appearance in one of the multiple neurites of (1) a larger growth cone, (2) a higher amount and greater transport of membrane organelles, (3) polarized delivery of TGN-derived vesicles, (4) a higher concentration of mitochondria and peroxisomes, (5) a higher concentration of a cytosolic protein, and (6) a higher concentration of ribosomes. These results provide evidence for the involvement of bulk cytoplasmic flow as an early determinant of neuronal morphological polarization. Molecular sorting events would later trigger the establishment of functional polarity.

Cell biology of polysialic acid

The unusual carbohydrate polysialic acid (PSA), attached uniquely to neural cell adhesion molecule (NCAM) through a developmentally regulated process, modulates neural cell interactions. Major advances in the past two years have increased our understanding of PSA biosynthesis and regulation. Of particular interest is the cloning of the genes encoding polysialyltransferases (PSTs) and the finding that a single enzyme is able to confer polysialylation to NCAM. The electrical activity of neurons and transmembrane signalling are probably major players in controlling both PSA biosynthesis and its expression at the cell surface. A direct causal relationship between PSA expression and activity-induced synaptic plasticity has been reported.

Mechanisms of neuronal polarity

The mechanisms that permit neurons to establish axons and dendrites involve an interplay between a cell's genetic program and signals in its environment. Recent experiments have identified some of the important extracellular molecules that regulate dendritic development and have furthered our understanding of the endogenous cell biological mechanisms that underlie protein sorting. Some of the signaling pathways that allow extracellular cues to regulate neuronal morphogenesis are also being elucidated.

Adenoviral vector-directed expression of neurotrophin-3 in rat dorsal root ganglion explants results in a robust neurite outgrowth response

The neurotrophins are a family of proteins that promote neuronal survival and neurite outgrowth during development and can also enhance the regeneration of injured adult neurons. The local and continuous delivery of these proteins at the site of injury is problematic, since this requires repeated intraparenchymal injections or the use of invasive canula-micropump devices. In the present study we report the generation and characterization of an adenoviral vector for a member of the neurotrophins, neurotrophin-3 (Ad-NT-3). Using Ad-NT-3, we examined the expression and biological activity of NT-3 in dorsal root ganglia (DRG) explant cultures. Gene transfer with Ad-NT-3 results in the synthesis of genuine NT-3 and in a dosage-dependent neurite outgrowth response in DRG explants. Transduction of DRG explants with a viral vector dosage of 5 x 10(5) to 5 x 10(6) plaque-forming units induced the formation of a dense halo of neurites comparable to outgrowth observed following the addition of 100 ng/mL exogenous NT-3. In addition, a single infection with Ad-NT-3 produced biologically active NT-3 for at least 20 days in culture, as evidenced by continued neurite extension. This indicates that adenoviral vector-mediated expression of NT-3 results in high-level production of biologically active NT-3 and could therefore be used as a strategy to promote the regeneration of injured peripheral and central nerve projections.

Adenoviral vector-mediated gene expression in the nervous system of immunocompetent Wistar and T cell-deficient nude rats: preferential survival of transduced astroglial cells in nude rats

In the present paper, we examined the effect of the adenoviral vector dosage, the role of T cells, and the influence of the presence of replication-competent adenovirus (RCA) in adenoviral vector stocks, on the efficacy of adenoviral vector-directed transgene expression in the facial nucleus of immunocompetent Wistar and athymic nude rats. A small number of motor neurons and glial cells was transduced at low dosages of viral vector (1 x 10(6) pfu) and in the absence of RCA, and transgene-expressing cells persisted throughout the 3-week period of observation. Intraparenchymal infusion of 2 x 10(7) pfu of a recombinant adenoviral vector free of RCA was required for optimal transduction of facial motor neurons. In Wistar rats, a biphasic immune response occurred at higher dosages of the vector (5 x 10(6) and 2 x 10(7) pfu) that was characterized by early infiltration of macrophages and the occurrence of T cells during the second week after injection of the vector. The immune response was associated with the loss of transduced neural cells. In nude rats, administration of an adenoviral vector free of RCA resulted in a macrophage response comparable to that in the Wistar rat and long-term survival of transduced astroglial cells. However, transduced motor neurons degenerated according to a similar time course as observed in Wistar rats. Small amounts of RCA (2 x 10(5) pfu) injected with 2 x 10(7) pfu recombinant viral vector particles resulted in an accelerated T cell response and a rapid elimination of transduced cells within 1 week in Wistar rats, whereas in nude rats transgene expression continued during this period. Taken together, these observations suggest that at the high viral vector loads necessary to achieve optimal transduction of the facial nucleus, T cells play a role in the degeneration of adenoviral vector-transduced astroglial cells. The adverse effects on neurons appear to be due to the observed inflammatory response or to direct adenoviral vector toxicity.

Cell adhesion molecules NgCAM and axonin-1 form heterodimers in the neuronal membrane and cooperate in neurite outgrowth promotion

The axonal surface glycoproteins neuronglia cell adhesion molecule (NgCAM) and axonin-1 promote cell-cell adhesion, neurite outgrowth and fasciculation, and are involved in growth cone guidance. A direct binding between NgCAM and axonin-1 has been demonstrated using isolated molecules conjugated to the surface of fluorescent microspheres. By expressing NgCAM and axonin-1 in myeloma cells and performing cell aggregation assays, we found that NgCAM and axonin-1 cannot bind when present on the surface of different cells. In contrast, the cocapping of axonin-1 upon antibody-induced capping of NgCAM on the surface of CV-1 cells coexpressing NgCAM and axonin-1 and the selective chemical cross-linking of the two molecules in low density cultures of dorsal root ganglia neurons indicated a specific and direct binding of axonin-1 and Ng-CAM in the plane of the same membrane. Suppression of the axonin-1 translation by antisense oligonucleotides prevented neurite outgrowth in dissociated dorsal root ganglia neurons cultured on an NgCAM substratum, indicating that neurite outgrowth on NgCAM substratum requires axonin-1. Based on these and previous results, which implicated NgCAM as the neuronal receptor involved in neurite outgrowth on NgCAM substratum, we concluded that neurite outgrowth on an NgCAM substratum depends on two essential interactions of growth cone NgCAM: a trans-interaction with substratum NgCAM and a cis-interaction with axonin-1 residing in the same growth cone membrane.