Expression patterns of the cell adhesion molecule Nr-CAM during histogenesis of the chick nervous system

Antisense oligonucleotides targeting alternative splicing of Nrcam exon 10 suppress neurite outgrowth of ganglion sensory neurons in vitro

Neuron-glial-related cell adhesion molecule (NrCAM) is a neuronal cell adhesion molecule that has been shown to be involved in several cellular processes in the peripheral nervous system, including neurite outgrowth. We recently reported that alternative splicing of Nrcam mRNA at exon 10 in the dorsal root ganglion (DRG) contributes to the peripheral mechanism of neuropathic pain. Specially, Nrcam antisense oligonucleotides (ASO) targeting Nrcam exon 10, attenuated neuropathic pain hypersensitivities in mice. Here, we investigated the effect of Nrcam ASO on neurite outgrowth of DRG neurons in vitro. By immunostaining DRG neurons with different DRG markers, Nrcam ASO significantly reduced neurite lengths in neurofilament 200-, calcitonin gene-related peptide and isolectin B4-positive neurons in primary DRG neuronal culture. Moreover, Nrcam ASO activates epidermal growth factor receptor, which may mediate the effect of Nrcam ASO on neurite outgrowth of cultured DRG neurons. These results provide evidence that Nrcam ASO suppresses neurite outgrowth in DRG neurons by regulating alternative splicing of Nrcam gene at exon 10 and activation of epidermal growth factor receptor signaling, indicating the differential roles of NrCAM variants/isoforms in neurite outgrowth.

Dorsal commissural axon guidance in the developing spinal cord

Commissural axons have been a key model system for identifying axon guidance signals in vertebrates. This review summarizes the current thinking about the molecular and cellular mechanisms that establish a specific commissural neural circuit: the dI1 neurons in the developing spinal cord. We assess the contribution of long- and short-range signaling while sequentially following the developmental timeline from the birth of dI1 neurons, to the extension of commissural axons first circumferentially and then contralaterally into the ventral funiculus.

Xenopus laevis neuronal cell adhesion molecule (nrcam): plasticity of a CAM in the developing nervous system

Neuron-glial-related cell adhesion molecule (NRCAM) is a neuronal cell adhesion molecule of the L1 immunoglobulin superfamily, which plays diverse roles during nervous system development including axon growth and guidance, synapse formation, and formation of the myelinated nerve. Perturbations in NRCAM function cause a wide variety of disorders, which can affect wiring and targeting of neurons, or cause psychiatric disorders as well as cancers through abnormal modulation of signaling events. In the present study, we characterize the Xenopus laevis homolog of nrcam. Expression of Xenopus nrcam is most abundant along the dorsal midline throughout the developing brain and in the outer nuclear layer of the retina.

Bilaterally symmetric populations of chicken dI1 (commissural) axons cross the floor plate independently of each other

Axons use temporal and directional guidance cues at intermediate targets to set the rate and direction of growth towards their synaptic targets. Our recent studies have shown that disrupting the temporal guidance process, by unilaterally accelerating the rate at which spinal dI1 (commissural) axons grow, resulted in turning errors both in the ventral spinal cord and after crossing the floor plate. Here we investigate a mechanistic explanation for these defects: the accelerated dI1 axons arrive in the ventral spinal cord before necessary fasciculation cues from incoming dI1 axons from the opposite side of the spinal cord. The identification of such an interaction would support a model of selective fasciculation whereby the pioneering dI1 axons serve as guides for the processes of the bilaterally symmetrical population of dI1 neurons. To test this model, we first developed the ability to “double” in ovo electroporate the embryonic chicken spinal cord to independently manipulate the rate of growth of the two bilateral populations of dI1 axons. Second, we examined the requirement for a putative bilateral interaction by unilaterally ablating the dI1 population in cultured explants of chicken embryonic spinal cord. Surprisingly, we find no evidence for a bilateral dI1 axon interaction, rather dI1 axons appear to project independently of each other.

The role of NrCAM in neural development and disorders--beyond a simple glue in the brain

NrCAM is a neuronal cell adhesion molecule of the L1 family of immunoglobulin super family. It plays a wide variety of roles in neural development, including cell proliferation and differentiation, axon growth and guidance, synapse formation, and the formation of the myelinated nerve structure. NrCAM functions in cell adhesion and modulates signaling pathways in neural development through multiple molecular interactions with guidance and other factors. Alterations in NrCAM structure/expression are associated with psychiatric disorders such as autism and drug addiction and with tumor progression. The mechanisms of NrCAM participation in development and how these might be perturbed in disorders are reviewed.

Novel distribution of cluster of differentiation 200 adhesion molecule in glial cells of the peripheral nervous system of rats and its modulation after nerve injury

This study examined CD200 expression in different peripheral nerves and ganglia. Intense CD200 immunoreactivity was consistently localized in unmyelinated nerve fibers as opposed to a faint immunostaining in the myelinated nerve fibers. By light microscopy, structures resembling the node of Ranvier and Schmidt-Lanterman incisures in the myelinated nerve fibers displayed CD200 immunoreactivity. Ultrastructural study revealed CD200 expression on the neurilemma of Schwann cells whose microvilli and paranodal loops at the node of Ranvier were immunoreactive. The CD200 immunoexpression was also localized in the satellite glial cells of sensory and autonomic ganglia and in the enteric glial cells. Double labeling of CD200 with specific antigens of satellite glia or Schwann cells in the primary cultures of dorsal root ganglia had shown a differential expression of CD200 in the peripheral glial cells. The existence of CD200 in glial cells in the peripheral nervous system (PNS) was corroborated by the expression of CD200 mRNA and protein in a rat Schwann cell line RSC96. Using the model of crush or transected sciatic nerve, it was found that CD200 expression was attenuated or diminished at the site of lesion. A remarkable feature, however, was an increase in incidence of CD200-labelled Schmidt-Lanterman incisures proximal to the injured site at 7 days postlesion. Because CD200 has been reported to impart immunosuppressive signal, we suggest that its localization in PNS glial cells may play a novel inhibitory role in immune homeostasis in both normal and pathological conditions.

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

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

Expression and involvement of gicerin, a cell adhesion molecule, in the development of chick optic tectum

Gicerin is a cell adhesion molecule belonging to the immunoglobulin superfamily. It has both a homophilic binding activity and a heterophilic binding activity to neurite outgrowth factor (NOF) a molecule belonging to the laminin family. We have reported many studies on the heterophilic activity of gicerin and NOF, but the function of its homophilic binding activity in vivo had been unclear. In the retina, gicerin is expressed in retinal ganglion cells only when they extend neurites to the optic tectum. In this report we have found that gicerin is also transiently expressed in the optic tectum during this time. First, cell aggregation assays were used to show that gicerin expressed in the optic tectum displays homophilic binding activity. Then, explant cultures of embryonic day 6 chick optic tectum on gicerin-Fc chimeric protein-coated dishes and NOF-coated dishes were carried out. It was found that gicerin-gicerin homophilic interactions promoted cell migration, whereas heterophilic interactions with NOF induced neurite formation. Furthermore, when anti-gicerin antibodies were injected in order to examine the effect of gicerin protein in the formation of the tectal layer in ovo, cell migration was strongly inhibited. These data suggest that homophilic interaction of gicerin participates in the migration of neural cells during the layer formation and plays a crucial role in the organization of the optic tectum.

Dynamic change of neural cell adhesion molecule polysialylation on human neuroblastoma (IMR-32) and rat pheochromocytoma (PC-12) cells during growth and differentiation

Polysialic acid (PSA) is a regulatory epitope of neural cell adhesion molecule (NCAM) in homophilic adhesion of neural cells mediated by NCAM, is also known to be re-expressed in several human tumors, thus serves as an oncodevelopmental antigen. In this study, using a recently developed ultrasensitive chemical method in addition to immunochemical methods, growth stage-dependent and retinoic acid (RA)-induced differentiation-dependent changes of PSA expression in human neuroblastoma (IMR-32) and rat pheochromocytoma (PC-12) cells were analyzed both qualitatively and quantitatively. Both IMR-32 and PC-12 cells expressed PSA on NCAM, and the level of PSA expressed per unit weight of cells increased with post-inoculation incubation time. The most prominent feature was seen at the full confluence stage. RA induced neuronal differentiation in both IMR-32 and CP-12 cells that paralleled the change in the PSA level. Chemical analysis revealed the presence of NCAM glycoforms differing in the degree of polymerization (DP) of oligo/polysialyl chains, whose DP was smaller than 40. DP distribution of PSA was different between the cell lines and was changed by the growth stage and the RA treatment. Thus DP analysis of PSA is important in understanding both mechanism and biological significance of its regulated expression.

Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development

The structurally related cell adhesion molecules L1 and Nr-CAM have overlapping expression patterns in cerebellar granule cells. Here we analyzed their involvement in granule cell development using mutant mice. Nr-CAM-deficient cerebellar granule cells failed to extend neurites in vitro on contactin, a known ligand for Nr-CAM expressed in the cerebellum, confirming that these mice are functionally null for Nr-CAM. In vivo, Nr-CAM-null cerebella did not exhibit obvious histological defects, although a mild size reduction of several lobes was observed, most notably lobes IV and V in the vermis. Mice deficient for both L1 and Nr-CAM exhibited severe cerebellar folial defects and a reduction in the thickness of the inner granule cell layer. Additionally, anti-L1 antibodies specifically disrupted survival and maintenance of Nr-CAM-deficient granule cells in cerebellar cultures treated with antibodies. The combined results indicate that Nr-CAM and L1 play a role in cerebellar granule cell development, and suggest that closely related molecules in the L1 family have overlapping functions.

Nr-CAM expression in the developing mouse nervous system: ventral midline structures, specific fiber tracts, and neuropilar regions

Nr-CAM is a member of the L1 subfamily of cell adhesion molecules (CAMs) that belong to the immunoglobulin superfamily. To explore the role of Nr-CAM in the developing nervous system, we prepared specific antibodies against both chick and mouse Nr-CAM using recombinant Fc fusion proteins of chick Nr-CAM and mouse Nr-CAM, respectively. First, we show the specificity of the new anti-chick Nr-CAM antibody compared with a previously employed antibody using the expression patterns of Nr-CAM in the chick spinal cord and floor plate and on commissural axons, where Nr-CAM has been implicated in axon guidance. Using the anti-mouse Nr-CAM antibody, we then studied the expression patterns of Nr-CAM in the developing mouse nervous system along with the patterns of two related CAMs, L1, which labels most growing axons, and TAG-1, which binds to Nr-CAM and has a more restricted distribution. Major sites that are positive for Nr-CAM are specialized glial formations in the ventral midline, including the floor plate in the spinal cord, the hindbrain and midbrain, the optic chiasm, and the median eminence in the forebrain. Similar to what is seen in the chick spinal cord, Nr-CAM is expressed on crossing fibers as they course through these areas. In addition, Nr-CAM is found in crossing fiber pathways, including the anterior commissure, corpus callosum, and posterior commissure, and in nondecussating pathways, such as the lateral olfactory tract and the habenulointerpeduncular tract. Nr-CAM, for the most part, is colocalized with TAG-1 in all of these systems. Based on in vitro studies indicating that the Nr-CAM-axonin-1/TAG-1 interaction is involved in peripheral axonal growth and guidance in the spinal cord [Lustig et al. (1999) Dev Biol 209:340-351; Fitzli et al. (2000) J Cell Biol 149:951-968], the expression patterns described herein implicate a role for this interaction in central nervous system axon growth and guidance, especially at points of decussation. Nr-CAM also is expressed in cortical regions, such as the olfactory bulb. In the hippocampus, however, TAG-1-positive areas are segregated from Nr-CAM-positive areas, suggesting that, in neuropilar regions, Nr-CAM interacts with molecules other than TAG-1.

Longitudinal elongation of primary afferent axons in the dorsal funiculus of the chick embryo spinal cord

The longitudinal elongation of primary afferent axons (PAAs) in the dorsal funiculus of chick embryo spinal cord was examined using a lipophilic tracer, DiI and immunohistochemistry. The earliest developing PAAs in the brachial segments invaded the spinal cord around embryonic day (E) 3.5. Thereafter, they elongated both rostrally and caudally in the presumptive dorsal funiculus, with frequent contacts between pre-existing axons and later arriving growth cones. By E4, the PAAs had elongated 3 segments both rostrally and caudally. In the course of their longitudinal elongation, the PAAs shifted their trajectory dorsally within the dorsal funiculus. By E6-6.5, the PAAs had extended as far as 10 segments rostrally and 6 segments caudally in the dorsal funiculus, and collaterals began to enter the dorsal horn. By E9, the PAAs extended up to 13 segments rostrally and 7 segments caudally, and collaterals reached the ventral spinal cord. During their longitudinal course, the PAAs shifted their trajectory medially within the dorsal funiculus.

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.

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.

Nr-CAM promotes neurite outgrowth from peripheral ganglia by a mechanism involving axonin-1 as a neuronal receptor

Nr-CAM is a neuronal cell adhesion molecule (CAM) belonging to the immunoglobulin superfamily that has been implicated as a ligand for another CAM, axonin-1, in guidance of commissural axons across the floor plate in the spinal cord. Nr-CAM also serves as a neuronal receptor for several other cell surface molecules, but its role as a ligand in neurite outgrowth is poorly understood. We studied this problem using a chimeric Fc-fusion protein of the extracellular region of Nr-CAM (Nr-Fc) and investigated potential neuronal receptors in the developing peripheral nervous system. A recombinant Nr-CAM-Fc fusion protein, containing all six Ig domains and the first two fibronectin type III repeats of the extracellular region of Nr-CAM, retains cellular and molecular binding activities of the native protein. Injection of Nr-Fc into the central canal of the developing chick spinal cord in ovo resulted in guidance errors for commissural axons in the vicinity of the floor plate. This effect is similar to that resulting from treatment with antibodies against axonin-1, confirming that axonin-1/Nr-CAM interactions are important for guidance of commissural axons through a spatially and temporally restricted Nr-CAM positive domain in the ventral spinal cord. When tested as a substrate, Nr-Fc induced robust neurite outgrowth from dorsal root ganglion and sympathetic ganglion neurons, but it was not effective for tectal and forebrain neurons. The peripheral but not the central neurons expressed high levels of axonin-1 both in vitro and in vivo. Moreover, antibodies against axonin-1 inhibited Nr-Fc-induced neurite outgrowth, indicating that axonin-1 is a neuronal receptor for Nr-CAM on these peripheral ganglion neurons. The results demonstrate a role for Nr-CAM as a ligand in axon growth by a mechanism involving axonin-1 as a neuronal receptor and suggest that dynamic changes in Nr-CAM expression can modulate axonal growth and guidance during development.

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.

Cell adhesion molecules regulate guidance of dorsal root ganglion axons in the marginal zone and their invasion into the mantle layer of embryonic spinal cord

In order to elucidate the mechanisms regulating the projections of dorsal root ganglion (DRG) axons in the dorsal funiculus and invasion into target regions in the mantle layer (prospective gray matter) of the spinal cord, we examined the interactions between DRG axons and spinal cord. DRG neurons were dissociated from chick embryos and cultured for 1-2 days on cryostat sections of the spinal cord at embryonic day 5 (E5) or at E9. E5 and E9 DRG neurons extended neurites onto both marginal zone (prospective white matter) and mantle layer (prospective gray matter) of the spinal cord, suggesting that both of these regions are permissive for neurite growth. When E5 DRG neurites approached cryosections of E5 spinal cord from outside, most of them ran in the marginal zone without invading the mantle layer. In contrast, about half of E9 DRG neurites entered the mantle layer after crossing the marginal zone of E9 spinal cord. These growth patterns of DRG neurites on spinal marginal zone and mantle layer are similar to the pathway formation of DRG axons at comparable stages in vivo; DRG axons run exclusively in the prospective dorsal funiculus before E6, and enter the mantle layer (prospective dorsal horn) to reach the target regions by E9. Perturbation of functions of Ng-CAM, Nr-CAM, and axonin-1/SC2 by adding the specific antibodies in the culture medium increased the ratio of DRG neurites entering the mantle layer of E5 spinal cord, suggesting that these cell adhesion molecules are involved in keeping DRG neurites in the marginal zone. Taken together with the expression of Ng-CAM, Nr-CAM, and axonin-1/SC2, these CAMs on DRG axons may regulate the guidance of these axons in the marginal zone before E6, and the subsequent decrease in the relative levels of these CAMs might allow DRG axons to invade the target mantle layer.

Tissue-specific expression of the L1 cell adhesion molecule is modulated by the neural restrictive silencer element

The cell adhesion molecule L1 mediates neurite outgrowth and fasciculation during embryogenesis and mutations in its gene have been linked to a number of human congenital syndromes. To identify DNA sequences that restrict expression of L1 to the nervous system, we isolated a previously unidentified segment of the mouse L1 gene containing the promoter, the first exon, and the first intron and examined its activity in vitro and in vivo. We found that a neural restrictive silencer element (NRSE) within the second intron prevented expression of L1 gene constructs in nonneural cells. For optimal silencing of L1 gene expression by the NRSE-binding factor RE-1-silencing transcription factor (REST)/NRSF, both the NRSE and sequences in the first intron were required. In transgenic mice, an L1lacZ gene construct with the NRSE generated a neurally restricted expression pattern consistent with the known pattern of L1 expression in postmitotic neurons and peripheral glia. In contrast, a similar construct lacking the NRSE produced precocious expression in the peripheral nervous system and ectopic expression in mesenchymal derivatives of the neural crest and in mesodermal and ectodermal cells. These experiments show that the NRSE and REST/NRSF are important components in restricting L1 expression to the embryonic nervous system.

Barx2, a new homeobox gene of the Bar class, is expressed in neural and craniofacial structures during development

Homeobox genes are regulators of place-dependent morphogenesis and play important roles in controlling the expression patterns of cell adhesion molecules (CAMs). To identify proteins that bind to a regulatory element common to the genes for two neural CAMs, Ng-CAM and L1, we screened a mouse cDNA expression library with a concatamer of the sequence CCATTAGPyGA and found a new homeobox gene, which we have called Barx2. The homeodomain encoded by Barx2 is 87% identical to that of Barx1, and both genes are related to genes at the Bar locus of Drosophila melanogaster. Barx1 and Barx2 also encode an identical stretch of 17 residues downstream of the homeobox; otherwise, they share no appreciable homology. In vitro, Barx2 stimulated activity of an L1 promoter construct containing the CCATTAGPyGA motif but repressed activity when this sequence was deleted. Localization studies showed that expression of Barx1 and Barx2 overlap in the nervous system, particularly in the telencephalon, spinal cord, and dorsal root ganglia. Barx2 was also prominently expressed in the floor plate and in Rathke's pouch. During craniofacial development, Barx1 and Barx2 showed complementary patterns of expression: whereas Barx1 appeared in the mesenchyme of the mandibular and maxillary processes, Barx2 was observed in the ectodermal lining of these tissues. Intense expression of Barx2 was observed in small groups of cells undergoing tissue remodeling, such as ectodermal cells within indentations surrounding the eye and maxillo-nasal groove and in the first branchial pouch, lung buds, precartilagenous condensations, and mesenchyme of the limb. The localization data, combined with Barx2's dual function as activator and repressor, suggest that Barx2 may differentially control the expression of L1 and other target genes during embryonic development.

Induction of neurite outgrowth through contactin and Nr-CAM by extracellular regions of glial receptor tyrosine phosphatase beta

Receptor protein tyrosine phosphatase beta (RPTPbeta) is expressed as soluble and receptor forms with common extracellular regions consisting of a carbonic anhydrase domain (C), a fibronectin type III repeat (F), and a unique region called S. We showed previously that a recombinant Fc fusion protein with the C domain (beta C) binds to contactin and supports neuronal adhesion and neurite growth. As a substrate, betaCFS was less effective in supporting cell adhesion, but it was a more effective promoter of neurite outgrowth than betaCF. betaS had no effect by itself, but it potentiated neurite growth when mixed with betaCF. Neurite outgrowth induced by betaCFS was inhibited by antibodies against Nr-CAM and contactin, and these cell adhesion molecules formed a complex that bound betaCFS. NIH-3T3 cells transfected to express betaCFS on their surfaces induced neuronal differentiation in culture. These results suggest that binding of glial RPTPbeta to the contactin/Nr-CAM complex is important for neurite growth and neuronal differentiation.

Molecular characterization of E587 antigen: an axonal recognition molecule expressed in the goldfish central nervous system

The E587 antigen (Ag) is a 200-Kd membrane glycoprotein originally identified by a monoclonal antibody on new and regenerating retinal ganglion cell axons in the adult goldfish. We report the isolation of cDNAs encoding the E587 Ag and identify it as a member of the L1 family of cell adhesion molecules (CAMs). The predicted amino acid sequence of E587 Ag shows an approximately equal identity (40%) to mouse L1, chick neuron-glia CAM, and chick neuron-glia-related CAM. Although the overall similarity is low, there is a high conservation of structural domains and specific sequence motifs. Wholemount in situ hybridizations were performed on goldfish between 34 hours and 3 days postfertilization (pf). A dramatic increase in E587 Ag mRNA was observed between 34 and 48 hours pf. The expression of E587 Ag mRNA in neurons shortly precedes axonogenesis. A marked decrease in expression occurs by 3 days pf, when the axonal scaffold has already been established. Wholemount immunohistochemistry on embryos demonstrates expression of E587 Ag on all major tracts. E587 Ag is absent from mature retinal ganglion cell axons, but its expression is induced by optic nerve transection. A corresponding induction of E587 Ag mRNA in retinal ganglion cells is shown by in situ hybridization. Furthermore, E587 Ag mRNA was detected in the optic nerve, which suggests that nonneuronal cells also express this molecule. E587 Ag was previously shown to promote retinal axon fasciculation and outgrowth in young fish and to mediate axon-glial interactions in vitro. The expression pattern and developmental regulation of E587 Ag in the central nervous system, its reexpression in retinal ganglion cells following optic nerve transection, and its relation to the L1 family indicate that E587 Ag functions as a cell recognition molecule important during axonal growth and regeneration.

BEN as a presumptive target recognition molecule during the development of the olivocerebellar system

It has been shown previously that in the chick embryo the cell adhesion molecule BEN/SC1/DM-GRASP is expressed by neurons in the inferior olive (IO) and by their terminal axonal arbors in the cerebellar cortex, the climbing fibers (Porquié et al., 1992b). Here, new information on the expression of BEN during the formation of the olivocerebellar projection adds the important notion that BEN is also expressed by the cerebellar targets of inferior olivary axons, Purkinje cells (PCs) and deep nuclear neurons. This expression is transient, starting at E7-E8 and vanishing shortly after hatching. More importantly, BEN expression is restricted to precise subsets of IO neurons and PCs. In the cerebellar cortex, BEN-immunoreactive (BEN-IR) structures are not found randomly but are distributed according to a reproducible pattern of parasagittal stripes. A maximum of four distinct sagittal stripes is found in each lobule, along the whole rostrocaudal extent of the cerebellum. Moreover, BEN-expressing stripes belong to two classes; one contains BEN-IR climbing fibers terminating on BEN-IR PCs and the other, more frequent class is solely composed of BEN-IR climbing fibers. Organotypic cultures of isolated cerebella have shown that the expression of BEN in the IO and in the cerebellum arise independently, probably because of an intrinsic developmental program. Thus, the cell adhesion molecule BEN meets all criteria for a recognition molecule involved in the formation of the olivocerebellar projection.

Synergistic neurite-outgrowth promoting activity of two related axonal proteins, Bravo/Nr-CAM and G4/Ng-CAM in chicken retinal explants

In the developing chicken retina, optic fibres migrating to the tectum express on their surfaces several cell adhesion molecules, including Bravo/Nr-CAM and G4/Nr-CAM and G4/Ng-CAM. We have previously described differential distribution along the retinotectal projection and differential modulation by environmental cues for Bravo and G4 and here we further compare the characteristics of these immunoglobulin superfamily molecules. From day 6 of embryonic development (E6) to 20 (E20), Bravo and G4 were found to coexist in the retinal optic fibre layer. However, while G4 staining was confined to that layer, as development proceeded Bravo staining spread to plexiform layers and some radial structures of the retina. G4 displayed a dose-dependent neurite-outgrowth promoting activity for E6 retinal explants, while Bravo did not support neurite growth. Surprisingly, when the retinal explants were grown on mixtures of the two molecules, a much more vigorous growth of neurites was seen, revealing a synergistic effect. We propose that Bravo and G4, as well as other axonal surface molecules, affect axonal growth in different ways when they are present in combination than when they are alone.

Binding between the neural cell adhesion molecules axonin-1 and Nr-CAM/Bravo is involved in neuron-glia interaction

Neural cell adhesion molecules of the immunoglobulin superfamily mediate cellular interactions via homophilic binding to identical molecules and heterophilic binding to other family members or structurally unrelated cell-surface glycoproteins. Here we report on an interaction between axonin-1 and Nr-CAM/Bravo. In search for novel ligands of axonin-1, fluorescent polystyrene microspheres conjugated with axonin-1 were found to bind to peripheral glial cells from dorsal root ganglia. By antibody blockage experiments an axonin-1 receptor on the glial cells was identified as Nr-CAM. The specificity of the interaction was confirmed with binding studies using purified axonin-1 and Nr-CAM. In cultures of dissociated dorsal root ganglia antibodies against axonin-1 and Nr-CAM perturbed the formation of contacts between neurites and peripheral glial cells. Together, these results implicate a binding between axonin-1 of the neuritic and Nr-CAM of the glial cell membrane in the early phase of axon ensheathment in the peripheral nervous system.

Zebrafish neurons express two L1-related molecules during early axonogenesis

Partial clones coding for two L1-related genes, zebrafish L1.1 and L1.2, were isolated from a zebrafish embryonic cDNA library. The homology analysis, based on the deduced amino acid sequences of L1.1 and L1.2, revealed that the two molecules are most closely related to each other and to mouse L1. Analysis by in situ hybridization revealed that during embryonic development of the nervous system the L1.1 and L1.2 messages are restricted to postmitotic neurons and that the onset of expression correlates with the initiation of axonogenesis. L1.1 is expressed by all known classes of neurons, consistent with an important general function during axonal outgrowth. Most of the neurons also express L1.2. However, L1.2 either is undetectable or is expressed at very low levels in the neurons of the olfactory placodes, anterior lateral line/acoustic ganglia complex, posterior lateral line ganglion, and in late developing hindbrain neurons. In the spinal cord, L1.2 message is detected only in a subpopulation of Rohon-Beard cells. This suggests the possibility that different levels of L1.2 expression may serve to distinguish different populations of neurons and their axons.

Axonal localisation of the CAM-like tyrosine phosphatase CRYP alpha: a signalling molecule of embryonic growth cones

Migrating embryonic growth cones require multiple, membrane-associated signalling molecules to monitor and respond to guidance cues. Here we present the first evidence that vertebrate cell adhesion molecule-like protein tyrosine phosphatases are likely to be components of this signalling system. CRYP alpha, the gene for an avian cell adhesion molecule-like phosphatase, is strongly expressed in the embryonic nervous system. In this study we have immunolocalised the protein in the early chick embryo and demonstrated its predominant localisation in axons of the central and peripheral nervous systems. This location suggests that the major, early role of the enzyme is in axonal development. In a study of sensory neurites in culture, we furthermore show that this phosphatase localises in migrating growth cones, within both the lamellipodia and filopodia. The dependence of growth cone migration on both cell adhesion and signalling through phosphotyrosine turnover, places the cell adhesion molecule-like CRYP alpha phosphatase in a position to be a regulator of these processes.

A family of molecules related to collapsin in the embryonic chick nervous system

Signaling molecules with either attractive or repulsive effects on specific growth cones are likely to play a role in guiding axons to their appropriate targets. A chick brain glycoprotein, collapsin, has been shown to be a good candidate for a repulsive guidance cue. We report here the discovery of four new molecules related to collapsin in chick brains. All contain a semaphorin domain. One is structurally very similar to collapsin but is only 50% identical in its amino acid sequence. We have named it collapsin-2. The collapsin-related genes exhibit distinct but overlapping patterns of mRNA expression in the developing spinal cord and the developing visual system. This family of collapsin-related molecules could potentially act as repulsive cues toward specific neuronal populations.

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.

Developmental changes in epitope accessibility as an indicator of multiple states of an immunoglobulin-like neural cell adhesion molecule

Cell surface molecules with restricted spatial and temporal distributions are good candidates for mediators of the cell-cell interactions that are necessary for the development of the nervous system. A monoclonal antibody (MAb 23A7) was produced that selectively and transiently labeled a limited subset of axons in the chick embryo spinal cord. Determination of the N-terminal amino acid sequence and immunoprecipitation experiments demonstrated that the 23A7 antigen is identical to Bravo/Nr-CAM, a previously described cell adhesion molecule with immunoglobulin-like domains (E.J. de la Rosa, J.F. Kayyem, J.M. Roman, Y.-D. Stierhof, W.J. Dreyer, and U. Schwartz [1989] J. Cell Biol. 111:3087-3096; M. Grumet, V. Mauro, M.P. Goon, G.M. Edelman, and B.A. Cunningham [1991] J. Cell Biol. 113:1399-1412). The temporal distribution of the 23A7 antigen is unusual in that, immunohistochemically, MAb 23A7 binding greatly decreases after 7 days of development, whereas Western blot analysis indicates increasing levels of the antigen until 17 days of development. In contrast, an antiserum against purified Nr-CAM, which also binds only to the 23A7 antigen, labels nearly all the axons in the tissue throughout all the later stages of development. These anomalous observations are apparently not the result of differential sensitivity of the 23A7 epitope to fixation, the use of suboptimal concentrations of the MAb, or selective MAb binding to a subset of Bravo/Nr-CAM molecules produced by alternative splicing of the transcript or by posttranslational modification. These findings could indicate the existence of multiple states of Bravo/Nr-CAM, which during development, vary in the accessibility of their extracellular domains to the MAb. This suggests the existence of multiple conformation or aggregation states of this cell adhesion molecule, each of which might be performing a different function.

Expression of four immunoglobulin superfamily adhesion molecules (L1, Nr-CAM/Bravo, neurofascin/ABGP, and N-CAM) in the developing mouse spinal cord

To identify cell adhesion molecules (CAMs) expressed by mammalian motoneurons, we applied the polymerase chain reaction to a murine motor neuron-like cell line, NSC-34. Using primers derived from a group of L1-related CAMs, we cloned two alternatively spliced forms of mouse L1, which differ by a 12-base-pair insert, plus putative murine orthologs of the chicken cell adhesion molecules Nr-CAM/Bravo and neurofascin. All four mRNAs are expressed in NSC-34 cells, but only neurofascin and the insert-minus form of L1 are expressed in its neuroblastoma parent, N18TG2. Analysis of RNA in neonatal tissues reveals expression largely restricted to the brain and spinal cord. In situ hybridization histochemistry of spinal cord shows that motoneurons express L1, Nr-CAM, and neurofascin as well as N-CAM. L1 and N-CAM RNAs are detected throughout the period studied (from embryonic day [E]11 to postnatal day [P]28), whereas Nr-CAM is expressed only at early ages (< E15) and neurofascin is predominantly expressed postnatally. Moreover, each CAM is expressed by distinct subsets of neighboring cells and at distinct times. For example, Nr-CAM mRNA is present in floor plate cells of embryonic spinal cord, whereas neurofascin is expressed by a subset of glia postnatally. Finally, we show that each CAM has a distinct spatiotemporal pattern of expression in dorsal root ganglia.

Clustering and co-localization of immunogold double labelled neural cell adhesion molecule isoforms in chick forebrain

A quantitative immunoelectron microscopic study was carried out in the intermediate portion of hyperstriatum ventrale of chick forebrain (a region exhibiting learning-related morphological plasticity) in order to examine the fine distribution of the neural cell adhesion molecule (N-CAM). Antibodies against alpha 2,8 polysialic acid (PSA), characteristic for embryonic N-CAM, and those against the protein backbones of all N-CAM isoforms were sequentially labelled (post-embedding) with 5- and 15-nm gold particles, and binding to their epitopes was analyzed using the statistics of point processes. The results showed that: (1) PSA tends to form clusters (up to 200 nm in size), in contrast to the protein backbones of all N-CAM isoforms, and (2) the two tissue bound antibodies are co-localized spatially in clusters of sizes up to 100 nm. The data suggest that there is a spatial sub-cellular domain enriched in both PSA and protein isoforms of N-CAM.

Induction of axonal growth by heterophilic interactions between the cell surface recognition proteins F11 and Nr-CAM/Bravo

F11 and Nr-CAM/Bravo are two axon-associated glycoproteins belonging to different subgroups of the immunoglobulin superfamily. In this report we have investigated the interaction of both proteins using neurite outgrowth and binding assays. Antibody blocking experiments demonstrate that neurite extension of tectal cells on immobilized F11 is mediated by Nr-CAM/Bravo. Binding studies further reveal a direct heterophilic interaction between F11 and Nr-CAM/Bravo. This activity can be mapped to the amino-terminal second or third immunoglobulin-like domain within F11 with domain-specific monoclonal antibodies and deletion mutant proteins expressed on COS cells. Furthermore, perturbation experiments with domain-specific monoclonal antibodies demonstrate that this region is required for adhesion and neurite extension.