Clustering and functional cooperation of Ng-CAM and axonin-1 in the substratum-contact area of growth cones

SynCAMs - From axon guidance to neurodevelopmental disorders

Many cell adhesion molecules are located at synapses but only few of them can be considered synaptic cell adhesion molecules in the strict sense. Besides the Neurexins and Neuroligins, the LRRTMs (leucine rich repeat transmembrane proteins) and the SynCAMs/CADMs can induce synapse formation when expressed in non-neuronal cells and therefore are true synaptic cell adhesion molecules. SynCAMs (synaptic cell adhesion molecules) are a subfamily of the immunoglobulin superfamily of cell adhesion molecules. As suggested by their name, they were first identified as cell adhesion molecules at the synapse which were sufficient to trigger synapse formation. They also contribute to myelination by mediating axon-glia cell contacts. More recently, their role in earlier stages of neural circuit formation was demonstrated, as they also guide axons both in the peripheral and in the central nervous system. Mutations in SynCAM genes were found in patients diagnosed with autism spectrum disorders. The diverse functions of SynCAMs during development suggest that neurodevelopmental disorders are not only due to defects in synaptic plasticity. Rather, early steps of neural circuit formation are likely to contribute.

Calsyntenin1-mediated trafficking of axon guidance receptors regulates the switch in axonal responsiveness at a choice point

Axon guidance at choice points depends on the precise regulation of guidance receptors on the growth cone surface. Upon arrival at the intermediate target or choice point, a switch from attraction to repulsion is required for the axon to move on. Dorsal commissural (dI1) axons crossing the ventral midline of the spinal cord in the floor plate represent a convenient model for the analysis of the molecular mechanism underlying the switch in axonal behavior.

We identified a role of Calsyntenin1 in the regulation of vesicular trafficking of guidance receptors in dI1 axons at choice points. In cooperation with RabGDI, Calsyntenin1 shuttles Rab11-positive vesicles containing Robo1 to the growth cone surface in a precisely regulated manner. In contrast, Calsyntenin1-mediated trafficking of Frizzled3, a guidance receptor in the Wnt pathway, is independent of RabGDI. Thus, tightly regulated insertion of guidance receptors, which is required for midline crossing and the subsequent turn into the longitudinal axis, is achieved by specific trafficking.

The SynCAM synaptic cell adhesion molecules are involved in sensory axon pathfinding by regulating axon-axon contacts

Synaptic cell adhesion molecules (SynCAMs) are crucial for synapse formation and plasticity. However, we have previously demonstrated that SynCAMs are also required during earlier stages of neural circuit formation because SynCAM1 and SynCAM2 (also known as CADM1 and CADM2, respectively) are important for the guidance of post-crossing commissural axons. In contrast to the exclusively homophilic cis-interactions reported by previous studies, our previous in vivo results suggested the existence of heterophilic cis-interactions between SynCAM1 and SynCAM2. Indeed, as we show here, the presence of homophilic and heterophilic cis-interactions modulates the interaction of SynCAMs with trans-binding partners, as observed previously for other immunoglobulin superfamily cell adhesion molecules. These in vitro findings are in agreement with results from in vivo studies, which demonstrate a role for SynCAMs in the formation of sensory neural circuits in the chicken embryo. In the absence of SynCAMs, selective axon-axon interactions are perturbed resulting in aberrant pathfinding of sensory axons.

BACE1 activity regulates cell surface contactin-2 levels

Background

Although BACE1 is a major therapeutic target for Alzheimer’s disease (AD), potential side effects of BACE1 inhibition are not well characterized. BACE1 cleaves over 60 putative substrates, however the majority of these cleavages have not been characterized. Here we investigated BACE1-mediated cleavage of human contactin-2, a GPI-anchored cell adhesion molecule.

Results

Our initial protein sequence analysis showed that contactin-2 harbors a strong putative BACE1 cleavage site close to its GPI membrane linker domain. When we overexpressed BACE1 in CHO cells stably transfected with human contactin-2, we found increased release of soluble contactin-2 in the conditioned media. Conversely, pharmacological inhibition of BACE1 in CHO cells expressing human contactin-2 and mouse primary neurons decreased soluble contactin-2 secretion. The BACE1 cleavage site mutation 1008MM/AA dramatically impaired soluble contactin-2 release. We then asked whether contactin-2 release induced by BACE1 expression would concomitantly decrease cell surface levels of contactin-2. Using immunofluorescence and surface-biotinylation assays, we showed that BACE1 activity tightly regulates contactin-2 surface levels in CHO cells as well as in mouse primary neurons. Finally, contactin-2 levels were decreased in Alzheimer’s disease brain samples correlating inversely with elevated BACE1 levels in the same samples.

Conclusion

Our results clearly demonstrate that mouse and human contactin-2 are physiological substrates for BACE1. BACE1-mediated contactin-2 cleavage tightly regulates the surface expression of contactin-2 in neuronal cells. Given the role of contactin-2 in cell adhesion, neurite outgrowth and axon guidance, our data suggest that BACE1 may play an important role in these physiological processes by regulating contactin-2 surface levels.

Analysis of cell-cell contact mediated by Ig superfamily cell adhesion molecules

Cell-cell adhesion is a fundamental requirement for all multicellular organisms. The calcium-independent cell adhesion molecules of the immunoglobulin superfamily (IgSF-CAMs) represent a major subgroup. They consist of immunoglobulin folds alone or in combination with other protein modules, often fibronectin type-III folds. More than 100 IgSF-CAMs have been identified in vertebrates and invertebrates. Most of the IgSF-CAMs are cell surface molecules that are membrane-anchored either by a single transmembrane segment or by a glycosylphosphatidylinositol (GPI) anchor. Some of the IgSF-CAMs also occur in soluble form, e.g., in the cerebrospinal fluid or in the vitreous fluid of the eye, due to naturally occurring cleavage of the GPI anchor or the membrane-proximal peptide segment. Some IgSF-CAMs, such as NCAM, occur in various forms that are generated by alternative splicing. This unit contains a series of protocols that have been used to study the function of IgSF-CAMs in vitro and in vivo.

Neural circuit formation in the cerebellum is controlled by cell adhesion molecules of the Contactin family

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) have been implicated in neural circuit formation in both the peripheral and the central nervous system. Several recent studies highlight a role of the Contactin group of IgSF CAMs in cerebellar development, in particular in the development of granule cells. Granule cells are the most numerous type of neurons in the nervous system and by forming a secondary proliferative zone in the cerebellum they provide an exception to the rule that neuronal precursors proliferate in the ventricular zone. Granule cells express Contactin-2, Contactin-1, and Contactin-6 in a sequential manner. Contactins are required for axon guidance, fasciculation, and synaptogenesis, and thus affect multiple steps in neural circuit formation in the developing cerebellum.

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

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

Hereditary sensory neuropathy type 1 is caused by the accumulation of two neurotoxic sphingolipids

HSAN1 is an inherited neuropathy found to be associated with several missense mutations in the SPTLC1 subunit of serine palmitoyltransferase (SPT). SPT catalyzes the condensation of serine and palmitoyl-CoA, the initial step in the de novo synthesis of sphingolipids. Here we show that the HSAN1 mutations induce a shift in the substrate specificity of SPT, which leads to the formation of the two atypical deoxy-sphingoid bases (DSBs) 1-deoxy-sphinganine and 1-deoxymethyl-sphinganine. Both metabolites lack the C(1) hydroxyl group of sphinganine and can therefore neither be converted to complex sphingolipids nor degraded. Consequently, they accumulate in the cell, as demonstrated in HEK293 cells overexpressing mutant SPTLC1 and lymphoblasts of HSAN1 patients. Elevated DSB levels were also found in the plasma of HSAN1 patients and confirmed in three groups of HSAN1 patients with different SPTLC1 mutations. The DSBs show pronounced neurotoxic effects on neurite formation in cultured sensory neurons. The neurotoxicity co-occurs with a disturbed neurofilament structure in neurites when cultured in the presence of DSBs. Based on these observations, we conclude that HSAN1 is caused by a gain of function mutation, which results in the formation of two atypical and neurotoxic sphingolipid metabolites.

The neural adhesion molecule TAG-1 modulates responses of sensory axons to diffusible guidance signals

When the axons of primary sensory neurons project into the embryonic mammalian spinal cord, they bifurcate and extend rostrocaudally before sending collaterals to specific laminae according to neuronal subclass. The specificity of this innervation has been suggested to be the result both of differential sensitivity to chemorepellants expressed in the ventral spinal cord and of the function of Ig-like neural cell adhesion molecules in the dorsal horn. The relationship between these mechanisms has not been addressed. Focussing on the pathfinding of TrkA+ NGF-dependent axons, we demonstrate for the first time that their axons project prematurely into the dorsal horn of both L1 and TAG-1 knockout mice. We show that axons lacking TAG-1, similar to those lacking L1, are insensitive to wild-type ventral spinal cord(VSC)-derived chemorepellants, indicating that adhesion molecule function is required in the axons, and that this loss of response is explained in part by loss of response to Sema3A. We present evidence that TAG-1 affects sensitivity to Sema3A by binding to L1 and modulating the endocytosis of the L1/neuropilin 1 Sema3A receptor complex. However, TAG-1 appears to affect sensitivity to other VSC-derived chemorepellants via an L1-independent mechanism. We suggest that this dependence of chemorepellant sensitivity on the functions of combinations of adhesion molecules is important to ensure that axons project via specific pathways before extending to their final targets.

Axonin-1/TAG-1 is required for pathfinding of granule cell axons in the developing cerebellum

BACKGROUND

Neural development consists of a series of steps, including neurogenesis, patterning, cell migration, axon guidance, and finally, synaptogenesis. Because all these steps proceed in a constantly changing environment, functional gene analyses during development have to take time into account. This is quite challenging, however, as loss-of-function approaches based on classic genetic tools do not allow for the precise temporal control that is required for developmental studies. Gene silencing by RNA interference (RNAi) in combination with the chicken embryo or with cultured embryos opens new possibilities for functional gene analysis in vivo. Axonin-1/TAG-1 is a cell adhesion molecule of the immunoglobulin superfamily with a well defined temporal and spatial expression pattern in the developing vertebrate nervous system. Axonin-1/TAG-1 was shown to promote neurite outgrowth in vitro and to be required for commissural and sensory axon pathfinding in vivo.

RESULTS

To knock down axonin-1 in a temporally and spatially controlled manner during development of the nervous system, we have combined RNAi with the accessibility of the chicken embryo even at late stages of development. Using ex ovo RNAi, we analyzed the function of axonin-1/TAG-1 in cerebellar development. Axonin-1 is expressed in postmitotic granule cells while they extend their processes, the parallel fibers. In the absence of axonin-1 these processes still extend but no longer in a parallel manner to each other or to the pial surface of the cerebellum.

CONCLUSION

Axonin-1/TAG-1 is required for the navigation, but not for the elongation, of granule cell processes in the developing cerebellum in vivo.

The SALM family of adhesion-like molecules forms heteromeric and homomeric complexes

Synaptic adhesion-like molecules (SALMs) are a newly discovered family of adhesion molecules that play roles in synapse formation and neurite outgrowth. The SALM family is comprised of five homologous molecules that are expressed largely in the central nervous system. SALMs 1-3 contain PDZ-binding domains, whereas SALMs 4 and 5 do not. We are interested in characterizing the interactions of the SALMs both among the individual members and with other binding partners. In the present study, we focused on the interactions formed by the five SALM members in rat brain and heterologous cells. In brain, we found that SALMs 1-3 strongly co-immunoprecipitated with each other, whereas SALMs 4 and 5 did not, suggesting that SALMs 4 and 5 mainly form homomeric complexes. In heterologous cells transfected with SALMs, co-immunoprecipitation studies showed that all five SALMs form heteromeric and homomeric complexes. We also determined if SALMs could form trans-cellular associations between transfected heterologous cells. Both SALMs 4 and 5 formed homophilic, but not heterophilic associations, whereas no trans associations were formed by the other SALMs. The ability of SALM4 to form trans interactions is due to its extracellular N terminus because chimeras of SALM4 N terminus and SALM2 C terminus can form trans interactions, whereas chimeras of SALM2 N terminus and SALM4 C terminus cannot. Co-culture experiments using HeLa cells and rat hippocampal neurons expressing the SALMs showed that SALM4 is recruited to points of contact between the cells. In neurons, these points of contact were seen in both axons and dendrites.

Semaphorin6A acts as a gate keeper between the central and the peripheral nervous system

Background

During spinal cord development, expression of chicken SEMAPHORIN6A (SEMA6A) is almost exclusively found in the boundary caps at the ventral motor axon exit point and at the dorsal root entry site. The boundary cap cells are derived from a population of late migrating neural crest cells. They form a transient structure at the transition zone between the peripheral nervous system (PNS) and the central nervous system (CNS). Ablation of the boundary cap resulted in emigration of motoneurons from the ventral spinal cord along the ventral roots. Based on its very restricted expression in boundary cap cells, we tested for a role of Sema6A as a gate keeper between the CNS and the PNS.

Results

Downregulation of Sema6A in boundary cap cells by in ovo RNA interference resulted in motoneurons streaming out of the spinal cord along the ventral roots, and in the failure of dorsal roots to form and segregate properly. PlexinAs interact with class 6 semaphorins and are expressed by both motoneurons and sensory neurons. Knockdown of PlexinA1 reproduced the phenotype seen after loss of Sema6A function both at the ventral motor exit point and at the dorsal root entry site of the lumbosacral spinal cord. Loss of either PlexinA4 or Sema6D function had an effect only at the dorsal root entry site but not at the ventral motor axon exit point.

Conclusion

Sema6A acts as a gate keeper between the PNS and the CNS both ventrally and dorsally. It is required for the clustering of boundary cap cells at the PNS/CNS interface and, thus, prevents motoneurons from streaming out of the ventral spinal cord. At the dorsal root entry site it organizes the segregation of dorsal roots.

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.

Photolithographic generation of protein micropatterns for neuron culture applications

Standard positive photoresist techniques were adapted to generate micropatterns of proteins on glass and oxide surfaces. Both lift-off and plasma-etching techniques were used to transfer the photoresist pattern into a layer of covalently immobilised protein. The surface properties of the areas adjacent to the patterns were altered by chemical surface modification. Using a combination of the lift-off and the etching process complementary patterns of two different proteins were generated. The biochemical and biological functionality of the protein patterns were assessed by immunostaining and by investigating the outgrowth of neurites from neurons plated on the patterned substrates. The investigated patterning processes are compatible with microstructuring and thin film processes, and may be used to generate functional surfaces for sensor and neuron culture applications.

The role of endocytic l1 trafficking in polarized adhesion and migration of nerve growth cones

Motility of the nerve growth cone is highly dependent on its dynamic interactions with the microenvironment mediated by cell adhesion molecules (CAMs). These adhesive interactions can be spatially regulated by changing the density and avidity of CAMs on the growth cone. Previous studies have shown that L1, a member of the immunoglobulin superfamily of CAMs, is endocytosed at the central domain of the growth cone followed by centrifugal vesicular transport and reinsertion into the plasma membrane of the leading edge. The present paper focuses on the functional significance of endocytic L1 trafficking in dorsal root ganglia neurons in vitro. We demonstrate that the rate of L1-based neurite growth has a positive correlation with the amount of endocytosed L1 in the growth cone, whereas stimulation of neurite growth via an N-cadherin-dependent mechanism does not increase L1 endocytosis. A growth cone that migrates on an L1 substrate exhibits a steep gradient of L1-mediated adhesion (strong adhesion at the growth cone's leading edge and weak adhesion at the central domain). This gradient of L1 adhesion is attenuated after inhibition of L1 endocytosis in the growth cone by intracellular loading of a function-blocking antibody against alpha-adaptin, a subunit of the clathrin-associated AP-2 adaptor. Inhibition of L1 endocytosis by this antibody also decreased the rate of L1-dependent growth cone migration. These results indicate that the growth cone actively translocates CAMs to create spatial asymmetry in adhesive interactions with its environment and that this spatial asymmetry is important for growth cone migration.

Affinity capture of proteins from solution and their dissociation by contact printing

Biological experiments at the solid/liquid interface, in general, require surfaces with a thin layer of purified molecules, which often represent precious material. Here, we have devised a method to extract proteins with high selectivity from crude biological sample solutions and place them on a surface in a functional, arbitrary pattern. This method, called affinity-contact printing (alphaCP), uses a structured elastomer derivatized with ligands against the target molecules. After the target molecules have been captured, they are printed from the elastomer onto a variety of surfaces. The ligand remains on the stamp for reuse. In contrast with conventional affinity chromatography, here dissociation and release of captured molecules to the substrate are achieved mechanically. We demonstrate this technique by extracting the cell adhesion molecule neuron-glia cell adhesion molecule (NgCAM) from tissue homogenates and cell culture lysates and patterning affinity-purified NgCAM on polystyrene to stimulate the attachment of neuronal cells and guide axon outgrowth.

Distinct subpopulations of sensory afferents require F11 or axonin-1 for growth to their target layers within the spinal cord of the chick

Dorsal root ganglion neurons project axons to specific target layers in the gray matter of the spinal cord, according to their sensory modality. Using an in vivo approach, we demonstrate an involvement of the two immunoglobulin superfamily cell adhesion molecules axonin-1/TAG-1 and F11/F3/contactin in subpopulation-specific sensory axon guidance. Proprioceptive neurons, which establish connections with motoneurons in the ventral horn, depend on F11 interactions. Nociceptive fibers, which target to layers in the dorsal horn, require axonin-1 for pathfinding. In vitro NgCAM and NrCAM were shown to bind to both axonin-1 and F11. However, despite this fact and despite their ubiquitous expression in the spinal cord, NgCAM and NrCAM are selective binding partners for axonin-1 and F11 in sensory axon guidance. Whereas nociceptive pathfinding depends on NgCAM and axonin-1, proprioceptive fibers require NrCAM and F11.

Adhesion proteins for a tight neuron-electrode contact

The neural cell adhesion molecules axonin-1 and NgCAM have been genetically engineered and covalently immobilized on glass and silicon oxide surfaces in their correct orientation. Surfaces treated with these adhesion molecules were used as substrates for culturing dorsal root ganglion neurons. The cleft between the neuron cell membrane and the surface was determined using fluorescence interference contrast (FLIC) microscopy. For comparison, cell--material distances on laminin, RGDC, polylysine and amino-terminated surfaces were measured. When the neurons grow on axonin-1 the cell--surface distance is at a minimum (37 nm) probably because the glycocalyx hinders a closer contact. A selective treatment of extracellular electrodes with axonin-1 could be used to improve the cell-material contact and thus increase extracellularly recorded signals.

The involvement of axonin-1/SC2 in mediating notochord-derived chemorepulsive activities for dorsal root ganglion neurites

Previous studies have suggested that the developing notochord secretes diffusible axon guidance molecules that repel dorsal root ganglion (DRG) neurites (R. Keynes et al., 1997, Neuron 18, 889-897; K. Nakamoto and T. Shiga, 1998, Dev. Biol. 202, 304-314). Neither notochord-derived chemorepellents nor their receptors on DRG neurites are, however, known. Here we investigated whether cell adhesion molecules (CAMs) of the immunoglobulin/fibronectin type III subfamily present on DRG neurites, including axonin-1/SC2, N-CAM, Ng-CAM, and Nr-CAM, are required for mediating the notochord-derived chemorepulsion. Using collagen gel cocultures of DRGs and notochord explants, we found that an antibody against axonin-1/SC2 diminished the effects of the chemorepulsive activity from the notochord, whereas antibodies against N-CAM, Ng-CAM, and Nr-CAM had no effect. We further showed that the removal of glycosylphosphatidylinositol-anchored cell surface molecules, including axonin-1/SC2, from DRG neurites diminished the effects of the notochord-derived chemorepulsive activity to an extent similar to that of treatment with the anti-axonin-1/SC2 antibody. These results suggest that axonin-1/SC2 expressed on DRG neurites may be involved in mediating the notochord-derived chemorepulsive activity.

A direct interaction of axonin-1 with NgCAM-related cell adhesion molecule (NrCAM) results in guidance, but not growth of commissural axons

An interaction of growth cone axonin-1 with the floor-plate NgCAM-related cell adhesion molecule (NrCAM) was shown to play a crucial role in commissural axon guidance across the midline of the spinal cord. We now provide evidence that axonin-1 mediates a guidance signal without promoting axon elongation. In an in vitro assay, commissural axons grew preferentially on stripes coated with a mixture of NrCAM and NgCAM. This preference was abolished in the presence of anti-axonin-1 antibodies without a decrease in neurite length. Consistent with these findings, commissural axons in vivo only fail to extend along the longitudinal axis when both NrCAM and NgCAM interactions, but not when axonin-1 and NrCAM or axonin-1 and NgCAM interactions, are perturbed. Thus, we conclude that axonin-1 is involved in guidance of commissural axons without promoting their growth.

The crystal structure of the ligand binding module of axonin-1/TAG-1 suggests a zipper mechanism for neural cell adhesion

We have determined the crystal structure of the ligand binding fragment of the neural cell adhesion molecule axonin-1/TAG-1 comprising the first four immunoglobulin (Ig) domains. The overall structure of axonin-1(Ig1-4) is U-shaped due to contacts between domains 1 and 4 and domains 2 and 3. In the crystals, these molecules are aligned in a string with adjacent molecules oriented in an anti-parallel fashion and their C termini perpendicular to the string. This arrangement suggests that cell adhesion by homophilic axonin-1 interaction occurs by the formation of a linear zipper-like array in which the axonin-1 molecules are alternately provided by the two apposed membranes. In accordance with this model, mutations in a loop critical for the formation of the zipper resulted in the loss of the homophilic binding capacity of axonin-1.

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.

The neural cell adhesion molecules L1 and NCAM-180 act in different steps of neurite outgrowth

The formation of neurocircuitry depends on the control of neurite outgrowth that, in turn, can be divided into two processes: nerve growth cone protrusion and neurite extension. It has long been known that the neural cell adhesion molecules L1 and NCAM-180 promote neurite outgrowth, but how they function in growth cones is unclear. We addressed the roles of L1 and NCAM-180 in neurite outgrowth by using microscale chromophore-assisted laser inactivation (micro-CALI) of these proteins to perturb their functions at precise times in single growth cones of embryonic chick dorsal root ganglion neurons grown in culture. Micro-CALI of L1 causes neurite retraction after a 10 min lag period but does not affect growth cone protrusion. In contrast, micro-CALI of NCAM-180 causes rapid growth cone retraction but does not affect neurite extension. The simultaneous inactivation of both these molecules resulted in both distinct effects that were segregated in time. The behavior of growth cones after these micro-CALI treatments resemble the drug-induced perturbation of microtubules for L1 and F-actin for NCAM-180. These findings suggest distinct roles in the growth cone for L1 and NCAM-180 in different steps of neurite outgrowth: L1 functions in neurite extension,whereas NCAM-180 functions in growth cone protrusion.

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.

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.

Cis-activation of L1-mediated ankyrin recruitment by TAG-1 homophilic cell adhesion

Neural cell adhesion molecules (CAMs) of the immunoglobulin (Ig) superfamily mediate not only cell aggregation but also growth cone guidance and neurite outgrowth. In this study we demonstrate that two neural CAMs, L1-CAM and TAG-1, induce the homophilic aggregation of Drosophila S2 cells but are unable to interact with each other when expressed on different cells (trans-interaction). However, immunoprecipitations from cells co-expressing L1-CAM and TAG-1 showed a strong cis-interaction between the two molecules in the plane of the plasma membrane. TAG-1 is linked to the membrane by a glycosylphosphatidylinositol (GPI) anchor and therefore is unable to directly interact with cytoplasmic proteins. In contrast, L1-CAM-mediated homophilic cell adhesion induces the selective recruitment of the membrane skeleton protein ankyrin to areas of cell contact. Immunolabeling experiments in which S2 cells expressing TAG-1 were mixed with cells co-expressing L1-CAM and TAG-1 demonstrated that the homophilic interaction between TAG-1 molecules results in the cis-activation of L1-CAM to bind ankyrin. This TAG-1-dependent recruitment of the membrane skeleton provides an example of how GPI-anchored CAMs are able to transduce signals to the cytoplasm. Furthermore, such interactions might ultimately result in the recruitment and the activation of other signaling molecules at sites of cell contacts.

Dissection of complex molecular interactions of neurofascin with axonin-1, F11, and tenascin-R, which promote attachment and neurite formation of tectal cells

Neurofascin is a member of the L1 subgroup of the Ig superfamily that promotes axon outgrowth by interactions with neuronal NgCAM-related cell adhesion molecule (NrCAM). We used a combination of cellular binding assays and neurite outgrowth experiments to investigate mechanisms that might modulate the interactions of neurofascin. In addition to NrCAM, we here demonstrate that neurofascin also binds to the extracellular matrix glycoprotein tenascin-R (TN-R) and to the Ig superfamily members axonin-1 and F11. Isoforms of neurofascin that are generated by alternative splicing show different preferences in ligand binding. While interactions of neurofascin with F11 are only slightly modulated, binding to axonin-1 and TN-R is strongly regulated by alternatively spliced stretches located in the NH2-terminal half, and by the proline-alanine-threonine-rich segment. In vitro neurite outgrowth and cell attachment assays on a neurofascin-Fc substrate reveal a shift of cellular receptor usage from NrCAM to axonin-1, F11, and at least one additional protein in the presence of TN-R, presumably due to competition of the neurofascin- NrCAM interaction. Thereby, F11 binds to TN-R of the neurofascin/TN-R complex, but not to neurofascin, whereas axonin-1 is not able to bind directly to the neurofascin/TN-R complex as shown by competition binding assays. In conclusion, these investigations indicate that the molecular interactions of neurofascin are regulated at different levels, including alternative splicing and by the presence of interacting proteins.

Neural cell adhesion molecule L1: relating disease to function

Neural cell adhesion molecules of the immunoglobulin superfamily are important components of the network of guidance cues and receptors that govern axon growth and guidance during development. For neural cell adhesion molecule L1, the combined application of human genetics, knockout mouse technology, and cell biology is providing fundamental insight into the role of L1 in mediating neuronal differentiation. Disease-causing mutations as well as mouse models of L1 disruption can now be used to examine the relevance of L1 binding specificities and signal transduction pathways that have been observed in vitro.

Adhesion molecules and inherited diseases of the human nervous system

Mutations in the human genes for the adhesion molecules Po, L1, and merosin cause severe abnormalities in nervous system development. Po and merosin are required for normal myelination in the nervous system, and L1 is essential for development of major axon pathways such as the corticospinal tract and corpus callosum. While mutations that lead to a loss of the adhesive function of these molecules produce severe phenotypes, mutations that disrupt intracellular signals or intracellular interactions are also deleterious. Geneticists have found that more than one clinical syndrome can be caused by mutations in each of these adhesion molecules, confirming that these proteins are multifunctional. This review focuses on identifying common mechanisms by which mutations in adhesion molecules alter neural development.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Functional cooperation of beta1-integrins and members of the Ig superfamily in neurite outgrowth induction

Neurite outgrowth is a central aspect of the ontogenetic formation of neural networks and is regulated by distinct groups of cell surface molecules. One protein involved in neurite elongation and fasciculation is the neural Ig superfamily member F11/contactin. We have shown previously that F11 promotes neurite extension of chick tectal neurons by interaction with the tectal receptor NrCAM, a member of the L1 subgroup of the Ig superfamily. By contrast, it does not induce outgrowth of retinal neurons despite the fact that these cells also express NrCAM, suggesting that in retinal cells the F11-NrCAM interaction alone is not sufficient to induce neurite extension. In this report we present a novel image analysis procedure to quantify neurite outgrowth and use it to demonstrate that F11 enhances the fibronectin-induced outgrowth response of embryonic retinal neurons. We reveal that NrCAM is the neuronal receptor mediating the enhanced outgrowth of retinal neurons, whereas the related F11-binding molecule NgCAM is not involved. Furthermore, we provide evidence that a beta1-integrin may represent the fibronectin-dependent receptor that cooperates indirectly with the F11-NrCAM pathway. Our results support the concept of a combinatorial labeling of cells in nervous system histogenesis by different classes of cell surface proteins, in particular by integrins and molecules of the Ig superfamily.

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.

Neural cell adhesion molecule L1: signaling pathways and growth cone motility

The neural cell adhesion molecule L1 plays a key role in nervous system development including neuronal migration, neurite growth, and axonal fasciculation. L1 is expressed on most developing axons, and homophilic binding of L1 molecules on adjacent axons is likely to play a key role in axon extension. It is now well documented that a number of second-messenger systems are involved in L1-stimulated neurite growth in vitro. However, it is unclear how L1 homophilic or heterophilic binding trigger signals that regulate the mechanical forces that produce axon extension. In this report, we will review recent advances in understanding L1-associated signals, L1 interactions with the cytoskeleton, and the molecular mechanisms underlying growth cone motility.

Interference with axonin-1 and NrCAM interactions unmasks a floor-plate activity inhibitory for commissural axons

Axonin-1 and NrCAM were previously shown to be involved in the in vivo guidance of commissural growth cones across the floor plate of the embryonic chicken spinal cord. To further characterize their role in axon pathfinding, we developed a two-dimensional coculture system of commissural and floor-plate explants in which it was possible to study the behavior of growth cones upon floor-plate contact. Although commissural axons readily entered the floor plate under control conditions, perturbations of either axonin-1 or NrCAM interactions prevented the growth cones from entering the floor-plate explants. The presence of antiaxonin-1 resulted in the collapse of commissural growth cones upon contact with the floor plate. The perturbation of NrCAM interactions also resulted in an avoidance of the floor plate, but without inducing growth-cone collapse. Therefore, axonin-1 and NrCAM are crucial for the contact-mediated interaction between commissural growth cones and the floor plate, which in turn is required for the proper guidance of the axons across the ventral midline and their subsequent rostral turn into the longitudinal axis.

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.

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.

Intracellular signaling is changed after clustering of the neural cell adhesion molecules axonin-1 and NgCAM during neurite fasciculation

Neural cell adhesion molecules of the immunoglobulin/fibronectin type III family on axons have been implicated in promotion of neurite outgrowth, fasciculation, and the mediation of specific cell adhesion. The present study demonstrates that two of these molecules on dorsal root ganglion neurons are associated with distinct protein kinases, axonin-1 with the src-related nonreceptor tyrosine kinase fyn and NgCAM with a casein kinase II-related activity and a serine/ threonine kinase related to S6 kinase. When neurites grew without contacts involving axonin-1 and NgCAM, strong fyn kinase activity was associated with axonin-1, whereas the NgCAM-associated kinase activities were low. Clustering of axonin-1 with NgCAM induced by the formation of cell-cell contacts correlated with a reduction of the axonin-1-associated fyn activity and an increased phosphorylation of NgCAM by the associated casein kinase II-related activity. Thus, axonin-1 and NgCAM trigger distinctive intracellular signals during in vitro differentiation depending on their state of association.

Structure/function relationships of axon-associated adhesion receptors of the immunoglobulin superfamily

Evidence is accumulating that axonal members of the Ig superfamily (IgSF) interact in a complex manner with other axonal Ig-like proteins and with proteins of the extracellular matrix. Studies investigating the structure/function relationships of these proteins have highlighted the importance of Ig-like domains near the amino terminus (N-proximal) as both necessary and sufficient for homophilic and heterophilic binding. Although efforts have been made in the past year to correlate the structure and neurite-outgrowth-promoting ability of axonal IgSF members, this work is still at an early stage.

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

BACKGROUND

Growth cones at the tips of growing axons move along predetermined pathways to establish synaptic connections between neurons and their distant targets. To establish their orientation, growth cones continuously sample for, and respond to, guidance information provided by cell surfaces and the extracellular matrix. To identify specific guidance cues, growth cones have sensor molecules on their surface, which are expressed differentially during the temporospatial progress of axon outgrowth, at levels that depend on the pattern of neural activity. However, it has not been elucidated whether a change in gene expression can indeed change the molecular composition and, hence, the function of the sensor apparatus of growth cones.

RESULTS

We have constructed adenoviral gene transfer vectors of the chicken growth cone sensor molecules axonin-1 and Ng-CAM. Using these vectors, we initiated the expression of axonin-1 and Ng-CAM in rat dorsal root ganglia explants during ongoing neurite outgrowth. Using specific surface immunodetection at varying time points after infection, we found that axonin-1 and Ng-CAM are transported directly to the growth cone and inserted exclusively in the growth cone membrane and not in the axolemma of the axon shaft. Furthermore, we found that axonin-1 and Ng-CAM do not diffuse retrogradely, suggesting that the sensor molecules are integrated into multimolecular complexes in the growth cone.

CONCLUSIONS

During axon outgrowth, the pathway sensor apparatus of the growth cone is continuously updated by newly synthesized sensor molecules that originate directly from the transcription/translation machinery. Changes in the expression of sensor molecules may have a direct impact, therefore, on the exploratory function of the growth cone.