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.

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.

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.

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).

Molecular mechanisms of growth cone guidance: stop and go?

Evidence from pathfinding studies in both vertebrates and invertebrates indicates that growth cones are not guided by simple stop or go signals. Rather, the navigation of growth cones through the preexisting tissue is controlled by a continuous integration of both positive and negative cues. The path taken by an axon is determined by the continuously changing situation encountered by the growth cone at any given site along the trajectory of the axon to the target. The signals derived from interactions of surface molecules with these cues provided by the environment of the growth cone are constantly changing both temporally and spatially. Therefore, each growth cone encounters a unique set of guidance cues directing it to its specific target, thus allowing for the tremendous complexity required for the guidance of millions of axons in the developing nervous system.

Phenotype of mice lacking functional Deleted in colorectal cancer (Dcc) gene

The DCC (Deleted in colorectal cancer) gene was first identified as a candidate for a tumour-suppressor gene on human chromosome 18q. More recently, in vitro studies in rodents have provided evidence that DCC might function as a receptor for the axonal chemoattractant netrin-1. Inactivation of the murine Dcc gene caused defects in axonal projections that are similar to those observed in netrin-1-deficient mice but did not affect growth, differentiation, morphogenesis or tumorigenesis in mouse intestine. These observations fail to support a tumour-suppressor function for Dcc, but are consistent with the hypothesis that DCC is a component of a receptor for netrin-1.

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.

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

Growth cones and neurites of chicken dorsal root ganglia neurons cultured on laminin, Ng-CAM, or axonin-1 exhibit substratum-dependent morphology and growth patterns which are accompanied by distinctive distributions of axonin-1 and Ng-CAM in the growth cone membrane. On either Ng-CAM or axonin-1 substratum, both Ng-CAM and axonin-1 were depleted from some areas of the apical growth cone membrane. In contrast, on laminin, both axonin-1 and Ng-CAM remained randomly distributed. Removal of axonin-1 from growth cones resulted in a blockage of neurite outgrowth on both Ng-CAM and axonin-1 substrata, indicating that in these neurons axonin-1 cooperates with Ng-CAM in the activation of axon growth. Based on these results possible molecular models for cooperation between axonin-1 and Ng-CAM on the growth cone are discussed.

Neuroserpin, an axonally secreted serine protease inhibitor

We have identified and chromatographically purified an axonally secreted glycoprotein of CNS and PNS neurons. Several peptides derived from it were microsequenced. Based on these sequences, a fragment of the corresponding cDNA was amplified and used as a probe to isolate a full length cDNA from a chicken brain cDNA library. Because the deduced amino acid sequence qualified the protein as a novel member of the serpin family of serine protease inhibitors, we called it neuroserpin. Analysis of the primary structural features further characterized neuroserpin as a heparin-independent, functional inhibitor of a trypsin-like serine protease. In situ hybridization revealed a predominantly neuronal expression during the late stages of neurogenesis and in the adult brain in regions which exhibit synaptic plasticity. Thus, neuroserpin might function as an axonally secreted regulator of the local extracellular proteolysis involved in the reorganization of the synaptic connectivity during development and synapse plasticity in the adult.

Axonin-1, Nr-CAM, and Ng-CAM play different roles in the in vivo guidance of chick commissural neurons

Immunoglobulin/fibronectin type III-like cell adhesion molecules have been implicated in axon pathfinding based on their expression pattern in the developing nervous system and on their complex interactions described in vitro. The present in vivo study demonstrates that interactions by two of these molecules, axonin-1 on commissural growth cones and Nr-CAM on floor plate cells, are required for accurate pathfinding at the midline. When axonin-1 or Nr-CAM interactions were perturbed, many commissural axons failed to cross the midline and turned instead along the ipsilateral floor plate border. In contrast, though perturbation of Ng-CAM produced a defasciculation of the commissural neurites, it did not affect their guidance across the floor plate.

Distribution of TAG-1/axonin-1 in fibre tracts and migratory streams of the developing mouse nervous system

The axonal cell adhesion molecule, TAG-1/axonin-1, stimulates axonal growth and supports neurite fasciculation in vitro. Using a polyclonal antiserum raised against chick axonin-1, which shares 75% of its sequence with TAG-1 of the rat, we have mapped the distribution of TAG-1/axonin-1 throughout the developing nervous system of the mouse. Although absent from proliferating neuroepithelia and from non-neuronal cells, immunoreactivity for TAG-1/axonin-1 is expressed by stage-specific subpopulations of differentiating neurons from embryonic day 10 to postnatal day 15. It stains their axons and the surface of their parent somata during the early phases of axogenesis. In agreement with a putative role of TAG-1/axonin-1 as an axon-bound growth substrate, immunoreactivity is found in developing spinal and cranial nerves, in corticothalamic projections, as well as in subsets of fasciculating long projecting tracts of the central nervous system, such as the dorsal funiculi of the spinal cord, the lateral olfactory and optic tracts, the fasciculus retroflexus, and the predorsal bundle. High levels of immunoreactivity characterise the development of the cerebellar molecular layer, the corpus callosum, anterior and hippocampal commissure, and of crossed projections in the spinal cord and at several levels of the brainstem. Intense immunoreactivity in fine collaterals of cutaneous afferents, including their growth cones that are in contact with the embryonic skin, suggests a role of TAG-1/axonin-1 in target recognition. While staining is weak on the somata of radially migrating neurons such as cortical neurons and cerebellar granule cells, strong immunoreactivity is associated with neural somata and processes of the three tangential migrations that form the precerebellar nuclei, indicating a possible involvement of TAG-1/axonin-1 in contacts between these neurons and the processes they migrate upon.

Cell-cell adhesion by homophilic interaction of the neuronal recognition molecule axonin-1

The axonal surface glycoprotein axonin-1, which occurs both as a glycosyl-phosphatidylinositol-anchored membrane-bound form and a secreted form, promotes neurite outgrowth and is thought to be involved in axon-guidance mechanisms in the developing nervous system. Recently, we have demonstrated that the neurite-outgrowth-promoting activity of axonin-1, presented as a substratum for cultured neurons, is mediated by a heterophilic interaction with the axonal glycoprotein neuronglia cell-adhesion molecule (Ng-CAM). Here we present evidence for homophilic (like-like) binding among axonin-1 molecules. Axonin-1 was heterologously expressed in myeloma cells. Clonal cell lines, with exposed membrane-bound axonin-1 at their surface, formed large multicellular aggregates. Incubations of transfected and parental myeloma cells, under a series of different conditions, revealed homophilic axonin-1/axonin-1 interactions across the intermembrane space as the molecular mechanism promoting stable cell-cell contacts. Using structural and functional characterisation, recombinant axonin-1 was very similar to native axonin-1, suggesting that homophilic axonin-1 interactions are also established in neurons. The capability of axonin-1 to interact with both Ng-CAM and other axonin-1 molecules might contribute to the formation of macromolecular networks at contact sites of growth cones and axons, comprising molecules of both membranes, and thus represent a mechanism for regulating neurite outgrowth and pathfinding.

cDNA cloning, structural features, and eucaryotic expression of human TAG-1/axonin-1

Axonal surface glycoproteins, composed of repeated immunoglobulin-like and fibronectin-type-III(FNIII)-like domains, mediate adhesion between axons or between axons and non-neuronal cells or extracellular matrix proteins. Several representatives of this group promote neurite outgrowth, when presented as substratum to neurons in culture, and have been implicated in axonal guidance mechanisms. TAG-1 and axonin-1 are presumptive species homologues of the rat and the chick, respectively; together with F11/F3, they form a subgroup of Ig/FNIII-like molecules containing a glycosyl-PtdIns membrane anchor. Recent reports on tumor suppressor genes encoding Ig-like and FNIII-like sequences prompted us to isolate the human homologue to TAG-1 and axonin-1. Polymerase chain reaction (PCR) primers were designed to regions conserved in both TAG-1 and axonin-1 using deoxyinosine at ambiguous positions. An expected 1000-bp fragment was obtained from cDNA derived from adult human cerebellum. Using this PCR fragment as a probe, several clones were isolated from a human fetal brain cDNA library. Nucleotide sequence analysis of a full-length clone, as expected, revealed a high degree of similarity to rat TAG-1 (91% identity) and chicken axonin-1 (75% identity) at the amino acid level. The encoded protein was then transiently expressed in monkey COS1 cells, and a stable mouse myeloma cell line was established expressing human TAG-1/axonin-1. The transfected COS1 and myeloma cells showed immunoreactivity on the cell surface with polyclonal anti-(chicken axonin-1) serum. On Western blots, the same antibodies recognized the recombinant protein migrating slightly slower on SDS/PAGE than chicken axonin-1. A comparison of chicken and human brain-tissue proteins by Western-blot analysis revealed a similar apparent molecular mass difference between the two species, which might be due to three additional N-glycosylation sites present on human TAG-1/axonin-1. Immunostaining of cryostat sections of embryonic retinas with polyclonal anti-(axonin-1) serum showed similar expression patterns in chicken and human samples at corresponding developmental stages. An additional shared feature of human TAG-1/axonin-1, rat TAG-1 and chick axonin-1 is their attachment to the cell membrane with a glycosyl-PtdIns anchor.

The axonally secreted cell adhesion molecule, axonin-1. Primary structure, immunoglobulin-like and fibronectin-type-III-like domains and glycosyl-phosphatidylinositol anchorage

Axonin-1 is an axon-associated cell adhesion molecule (AxCAM) of the chicken, which promotes neurite outgrowth by interaction with the AxCAM L1(G4) of the neuritic membrane. Here we report the cloning and sequence determination of a cDNA encoding axonin-1. Peptides generated by enzymatic cleavage showed similarity to the AxCAM F11. Degenerated polymerase chain reaction (PCR) primers were designed and an axonin-1 fragment was amplified from mRNA of embryonic retina. Screening of a cDNA library from embryonic brain resulted in the isolation of a 4.0-kb cDNA insert with an open reading frame of 3108 nucleotides. The deduced polypeptide of 1036 amino acids includes a putative hydrophobic N-terminal signal sequence of 23 or 25 amino acids and a C-terminal hydrophobic sequence of 29 amino acids which is suggestive of sequences serving as signal for the attachment of a glycosyl-phosphatidylinositol (glycosyl-PtdIns) anchor. The putative mature form of axonin-1 comprises six immunoglobulin-like repeats, followed by four fibronectin-type III repeats. Axonin-1 exhibits 75% amino acid identity with the AxCAM TAG-1 of the rat, suggesting that it is the chicken homologue of TAG-1. Like TAG-1, axonin-1 is glycosyl-PtdIns-anchored to the neuronal membrane; in contrast to TAG-1, it does not exhibit an Arg-Gly-Asp sequence.

Neurite outgrowth on immobilized axonin-1 is mediated by a heterophilic interaction with L1(G4)

Axonin-1 is an axon-associated cell adhesion molecule with dualistic expression, one form being glycophosphatidylinositol-anchored to the axonal membrane, the other secreted from axons in a soluble form. When presented as a substratum for neuronal cultures it strongly promotes neurite outgrowth from chicken embryonic dorsal root ganglia neurons. In this study, the axon-associated cell adhesion molecule G4, which is identical with Ng-CAM and 8D9, and homologous or closely related to L1 of the mouse and NILE of the rat, was investigated with respect to a receptor function for axonin-1. Using fluorescent microspheres with covalently coupled axonin-1 or L1(G4) at their surface we showed that these proteins bind to each other. Within the sensitivity of this microsphere assay, no interaction of axonin-1 with itself could be detected. Axonin-1-coated microspheres also bound to the neurites of cultured dorsal root ganglia neurons. This interaction was exclusively mediated by L1(G4), as indicated by complete binding suppression by monovalent anti-L1(G4) antibodies. The interaction between neuritic L1(G4) and immobilized axonin-1 was found to mediate the promotion of neurite growth on axonin-1, as evidenced by the virtually complete arrest of neurite outgrowth in the presence of anti-L1(G4) antibodies. Convincing evidence has recently been presented that neurite growth on L1(8D9) is mediated by the homophilic binding of neuritic L1(G4) (1989. Neuron. 2: 1597-1603). Thus, both L1(G4)- and axonin-1-expressing axons may serve as "substrate pathways" for the guidance of following axons expressing L1(G4) into their target area. Conceivably, differences in the concentration of axonin-1 and L1(G4), and/or modulatory influences on their specific binding parameters in leading pathways and following axons could represent elements in the control of axonal pathway selection.

The axonally secreted protein axonin-1 is a potent substratum for neurite growth

Axonin-1 is a neuronal glycoprotein occurring both as a membrane-bound and a secreted form. Membrane-bound axonin-1 is predominantly located in membranes of developing nerve fiber tracts and has recently been characterized as a cell adhesion molecule; the soluble form is secreted from axons and accumulates in the cerebrospinal fluid and the vitreous fluid of the eye. In the present study, we addressed the question as to whether secreted axonin-1 was released in a functionally competent form and we found that it strongly promotes neurite outgrowth when presented to neurons as an immobilized substratum. Neurite lengths elaborated by embryonic dorsal root ganglia neurons on axonin-1 were similar to those on the established neurite-promoting substrata L1 and laminin. Fab fragments of axonin-1 antibodies completely inhibited neurite growth on axonin-1, but not on other substrata. In soluble form, axonin-1 had an anti-adhesive effect, as revealed by perturbation of neurite fasciculation. In view of their structural similarity, we conclude that secreted and membrane-bound axonin-1 interact with the same growth-promoting neuritic receptor. The fact that secreted axonin-1 is functionally active, together with our previous findings that it is secreted from an internal cellular pool, suggests a functional dualism between membrane-bound and secreted axonin-1 at the site of secretion, which is most likely the growth cone. The secretion of adhesion molecules could represent a powerful and rapidly acting regulatory element of growth cone-neurite interactions in the control of neurite elongation, pathway selection, and possibly target recognition.

A homologue of the axonally secreted protein axonin-1 is an integral membrane protein of nerve fiber tracts involved in neurite fasciculation

Axonin-1 is a glycoprotein that is released from axons of cultured neurons (Stoeckli, E. T., P. F. Lemkin, T. B. Kuhn, M. A. Ruegg, M. Heller, and P. Sonderegger. 1989. Eur. J. Biochem. 180:249-258). It has recently been purified from the ocular vitreous fluid of the chicken embryo (Ruegg, M. A., E. T. Stoeckli, T. B. Kuhn, M. Heller, R. Zuellig, and P. Sonderegger. 1989. EMBO (Eur. Mol. Biol. Organ.) J. 8:55-63). Immunohistochemistry localized axonin-1 prevalently in developing nerve fiber tracts. The presence of anti-axonin-1 Fab fragments during axon growth in vitro resulted in antibody binding to the axonal surfaces and in a marked perturbation of the fasciculation pattern. Hence, a fraction of axonin-1 is associated with axonal membranes and, by operational criteria, qualifies as a cell adhesion molecule. The major proportion of membrane-associated axonin-1 co-solubilized with the integral membrane proteins. By physico-chemical, immunological, and protein-chemical criteria, the integral membrane form was found to be highly similar to soluble axonin-1. In common with a number of other cell adhesion molecules, both soluble and membrane-bound axonin-1 express the L2/HNK-1 and the L3 epitopes. Radioactive pulse-chase and double-labeling experiments revealed that the released form was not derived from the membrane-bound form by shedding from the membrane surface, but directly secreted from an intracellular pool. Due to its high degree of similarity to the membrane-associated form and the presence of the L2/HNK-1 and L3 epitopes, reported to be ligands in adhesive cell interactions, adhesive properties are postulated for secreted axonin-1. As a soluble adhesive protein, it may function as a regulator of cell adhesion around its most likely site of secretion, the growth cone.

Identification of proteins secreted from axons of embryonic dorsal-root-ganglia neurons

Secretion of proteins from the growth cone has been implicated in axon growth and synapse formation and might be involved in the transmission of a variety of axon-derived regulatory signals during neurogenesis. In order to identify axonally secreted proteins, dorsal-root-ganglia neurons from chicken embryos were cultured in a compartmentalized cell culture system that allows separate access to neuronal cell somas and axons. The proteins synthesized by the neurons were metabolically labeled by addition of [35S]methionine to the compartment containing the cell somas; the proteins released from the axons were harvested from the culture medium of the axonal compartment. Two-dimensional gel electrophoresis revealed two axonally secreted proteins with apparent molecular mass of 132-140 kDa and 54-60 kDa; they were termed axonin-1 and axonin-2, respectively. Both axonins were found to be secreted from a variety of neuronal cell cultures, but not from any of the nonneuronal cultures investigated, and hence might be neuron-specific. Virtual absence of these proteins from the axonal protein pattern suggests constitutive secretion. The information acquired on coordinates and spot morphology of these proteins in two-dimensional gel electrophoresis provides a useful assay for their purification.

Purification of axonin-1, a protein that is secreted from axons during neurogenesis

Using selective metabolic labelling in a compartmental cell culture system two proteins, denoted axonin-1 and axonin-2, were found to be secreted by axons of dorsal root ganglia neurons from chicken embryos. Based on its characteristic coordinates and spot morphology in two-dimensional gel electrophoresis, axonin-1 was detected in the cerebrospinal fluid and the vitreous fluid, axonin-1 was purified 476-fold to homogeneity by a four-step chromatographic procedure. The identity of the purified protein as axonin-1 was confirmed by immunological methods. Axonin-1 is a glycoprotein that subdivides into at least 16 immunologically similar isoelectric variants; their molecular weight range extends from 132 to 140 kd and their pI range from 5.3 to 6.2. In the vitreous fluid of the embryo, axonin-1 could first be detected on the embryonic day 5 and highest concentrations were measured during the second half of embryonic life; in the vitreous fluid of the adult chicken, concentrations were approximately 20 times lower. The early onset of secretion and the time course of expression suggest a role for axonin-1 in the development of the nervous system.