Neural Cell Adhesion Molecule Homophilic Binding Research Articles

A peptide derived from a trans‐homophilic binding site in neural cell adhesion molecule induces neurite outgrowth and neuronal survival

The neural cell adhesion molecule (NCAM) plays a key role in neural development, regeneration, and synaptic plasticity. The crystal structure of a fragment of NCAM comprising the three N-terminal immunoglobulin (Ig)-like modules indicates that the first and second Ig modules bind to each other, thereby presumably mediating dimerization of NCAM molecules expressed on the same cell surface (cis-interactions), whereas the third Ig module, through interactions with the first or second Ig module, mediates interactions between NCAM molecules expressed on the surface of opposing cells (trans-interactions). We have designed a new potent peptide ligand of NCAM, termed plannexin, based on a discontinuous sequence in the second NCAM Ig module that represents a homophilic binding site for an opposing third Ig module. The peptide was found by surface plasmon resonance analysis to bind the third NCAM Ig module. It promoted survival of cultured cerebellar granule neurons (CGNs) and also induced neurite extension in cultures of dopaminergic neurons and CGNs; the latter effect was shown to be dependent on NCAM expression, indicating that plannexin mimics the neuritogenic effect of homophilic NCAM binding.

Insight into the structural mechanism of the bi-modal action of an NCAM mimetic, the C3 peptide

C3, a synthetic peptide binding to the Ig1 module of the neural cell adhesion molecule (NCAM) has previously been identified and shown to inhibit NCAM homophilic binding and NCAM-mediated activation of the fibroblast growth factor (FGF) receptor (FGFR). However, C3 can also stimulate signalling on its own in a way similar to NCAM. Here we show that in the absence of NCAM, C3 can bind and activate FGFR, whereas in the presence of NCAM, C3 inhibits the NCAM-stimulated FGFR activation without activating FGFR on its own. Several competing models of FGFR activation by NCAM have been previously proposed. In one of them, the FGFR Ig2–Ig3 modules are involved in binding to NCAM, whereas in another – the FGFR “acid box” region mediates the interaction. The bi-modal effect of C3 can be explained in the context of the former model and is not consistent with the latter, thus providing evidence in support of the former model.

Stimulation of choline acetyltransferase by C3d, a neural cell adhesion molecule ligand

Septal cholinergic neurons project to the hippocampus and release acetylcholine, a neurotransmitter involved in learning and memory. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing acetylcholine. Promoting ChAT activity and acetylcholine release can lead to new treatments for neurodegenerative diseases with cholinergic deficits, such as Alzheimer's disease. We present evidence that the synthetic molecule C3d, which is a peptide mimetic of the neural cell adhesion molecule (NCAM), promotes ChAT activity in cultures of rat embryonic septal neurons. Our data demonstrate that ChAT activity triggered by C3d is dependent on the fibroblast growth factor receptor (FGFR) and the mitogen-activated protein kinase (MAPK) pathway. C3d did not affect the number of cholinergic neurons in culture, indicating that NCAM homophilic binding enhances ChAT activity, without affecting cholinergic cell survival. In conclusion, the NCAM mimetic peptide C3d promotes ChAT activity in septal neurons through FGFR and MAPK. These findings are relevant to the design of new strategies aimed at stimulating cholinergic function and improving cognition in disorders such as Alzheimer's disease.

NCAM Mimetic Peptides: An Update

The neural cell adhesion molecule (NCAM) is involved in multiple, relatively low affinity interactions with itself and with other cell surface receptors and growth factors. Its cytoplasmic domains do not posses any intrinsic enzymatic activity, which makes it difficult to develop reliable pharmacological tools interfering with NCAM functions. Recent progress in our understanding of the structural basis of NCAM-mediated cell adhesion and signaling has allowed a structure-based design of NCAM mimetic peptides. Using this approach, a number of peptides termed P2, P1-B, P-3-DE and P-3-G, whose sequences contain one or several NCAM homophilic binding sites involved in NCAM binding to itself, have been identified. By means of NMR titration analysis and molecular modeling, a number of peptides derived from NCAM and targeting NCAM heterophilic ligands, such as the fibroblast growth factor receptor and heparan sulfate proteoglycans, (HSPG) have been identified. The FGL, dekaCAM, FRM/EncaminA, BCL, EncaminC and EncaminE peptides, all target the FGF receptor, whereas the heparin-binding peptide HBP targets HSPG. Moreover, a number of NCAM-binding peptides have been identified employing screening of combinatorial peptide libraries. The C3 and NBP10 peptides target the first Ig module, whereas the ENFIN2 and ENFIN11 peptides target fibronectin type III (FN3) modules of NCAM. A number of NCAM mimetics can induce neurite outgrowth and exhibit neuroprotective and synaptic plasticity modulating properties in vitro and in vivo, making them attractive pharmacological tools suitable for drug development for the treatment of neurodegenerative disorders and impaired memory.

Neural cell adhesion molecule‐180‐mediated homophilic binding induces epidermal growth factor receptor (EGFR) down‐regulation and uncouples the inhibitory function of EGFR in neurite outgrowth

The neural cell adhesion molecule (NCAM) plays important roles in neuronal development, regeneration, and synaptic plasticity. NCAM homophilic binding mediates cell adhesion and induces intracellular signals, in which the fibroblast growth factor receptor plays a prominent role. Recent studies on axon guidance in Drosophila suggest that NCAM also regulates the epidermal growth factor receptor (EGFR) (Molecular and Cellular Neuroscience, 28, 2005, 141). A possible interaction between NCAM and EGFR in mammalian cells has not been investigated. The present study demonstrates for the first time a functional interaction between NCAM and EGFR in mammalian cells and investigates the molecular mechanisms underlying this interaction. First, NCAM and EGFR are shown to play opposite roles in neurite outgrowth regulation in cerebellar granular neurons. The data presented indicate that negative regulation of EGFR is one of the mechanisms underlying the neuritogenic effect of NCAM. Second, it is demonstrated that expression of the NCAM-180 isoform induces EGFR down-regulation in transfected cells and promotes EGFR down-regulation induced by EGF stimulation. It is demonstrated that the mechanism underlying this NCAM-180-induced EGFR down-regulation involves increased EGFR ubiquitination and lysosomal EGFR degradation. Furthermore, NCAM-180-mediated EGFR down-regulation requires NCAM homophilic binding and interactions of the cytoplasmic domain of NCAM-180 with intracellular interaction partners, but does not require NCAM-mediated fibroblast growth factor receptor activation.

GAP‐43 regulates NCAM‐180‐mediated neurite outgrowth

The neural cell adhesion molecule (NCAM), and the growth-associated protein (GAP-43), play pivotal roles in neuronal development and plasticity and possess interdependent functions. However, the mechanisms underlying the functional association of GAP-43 and NCAM have not been elucidated. In this study we show that (over)expression of GAP-43 in PC12E2 cells and hippocampal neurons strongly potentiates neurite extension, both in the absence and in the presence of homophilic NCAM binding. This potentiation is crucially dependent on the membrane association of GAP-43. We demonstrate that phosphorylation of GAP-43 by protein kinase C (PKC) as well as by casein kinase II (CKII) is important for the NCAM-induced neurite outgrowth. Moreover, our results indicate that in the presence of GAP-43, NCAM-induced neurite outgrowth requires functional association of NCAM-180/spectrin/GAP-43, whereas in the absence of GAP-43, the NCAM-140/non-receptor tyrosine kinase (Fyn)-associated signaling pathway is pivotal. Thus, expression of GAP-43 presumably acts as a functional switch for NCAM-180-induced signaling. This suggests that under physiological conditions, spatial and/or temporal changes of the localization of GAP-43 and NCAM on the cell membrane may determine the predominant signaling mechanism triggered by homophilic NCAM binding: NCAM-180/spectrin-mediated modulation of the actin cytoskeleton, NCAM-140-mediated activation of Fyn, or both.

Induction of apoptosis in human salivary gland tumor cells by anti-NCAM antibody

Neural cell adhesion molecule (NCAM) is a type of cell surface glycoprotein and a member of the immunoglobulin superfamily. It has been reported that NCAM may be associated with perineural invasion by malignant salivary gland tumors such as adenoid cystic carcinoma. We have previously demonstrated that NCAM is constitutively expressed in the human salivary gland tumor cell line HSG, in vitro. In the present study, we have aimed to clarify the hypothesis that NCAM-mediated inhibition of salivary gland tumor proliferation is caused by homophilic binding and involves the prevention of signal transduction for perineural invasion using HSG cells. NCAM mRNA and protein expression was found to decrease in a dose-dependent manner upon treatment with the anti-NCAM antibody (MAb NCAM) for 24 h. The MTT assay showed a significant reduction in the number of viable HSG cells. Confocal laser microscopy showed that HSG cells underwent apoptosis after treatment with MAb NCAM. The activation of caspases 3, 7 and 9 was observed in HSG cells after treatment with MAb NCAM, thus confirming that apoptosis was induced by the activated caspases. Apaf-1 activity was also detected in HSG cells in a dose-dependent manner after treatment with MAb NCAM. The up-regulation of TGF-beta1-mediated NCAM expression appeared to lead to the activation of homophilic NCAM binding, further accelerating HSG cell proliferation. In addition, the localization of NCAM in adenoid cystic carcinomas (ACCs) was examined using an immunohistochemical method. NCAM was slightly to moderately positive in 9 of 13 cases (69.2%) of ACC. These findings suggest that NCAM is associated not only with a cell-to-cell adhesion mechanism, but also with tumorigenesis, including growth, development and perineural invasion in human salivary gland tumors.

Structural biology of NCAM homophilic binding and activation of FGFR

In this review, we analyse the structural basis of the homophilic interactions of the neural cell adhesion molecule (NCAM) and the NCAM-mediated activation of the fibroblast growth factor receptor (FGFR). Recent structural evidence suggests that NCAM molecules form cis-dimers in the cell membrane through a high affinity interaction. These cis-dimers, in turn, mediate low affinity trans-interactions between cells via formation of either one- or two-dimensional 'zippers'. We provide evidence that FGFR is probably activated by NCAM very differently from the way by which it is activated by FGFs, reflecting the different conditions for NCAM-FGFR and FGF-FGFR interactions. The affinity of FGF for FGFR is approximately 10(6) times higher than that of NCAM for FGFR. Moreover, in the brain NCAM is constantly present on the cell surface in a concentration of about 50 microm, whereas FGFs only appear transiently in the extracellular environment and in concentrations in the nanomolar range. We discuss the structural basis for the regulation of NCAM-FGFR interactions by two molecular 'switches', polysialic acid (PSA) and adenosine triphosphate (ATP), which determine whether NCAM acts as a signalling or an adhesion molecule.

Triple Effect of Mimetic Peptides Interfering with Neural Cell Adhesion Molecule Homophilic Cis Interactions

The neural cell adhesion molecule (NCAM) is pivotal in neural development, regeneration, and learning. Here we characterize two peptides, termed P1-B and P2, derived from the homophilic binding sites in the first two N-terminal immunoglobulin (Ig) modules of NCAM, with regard to their effects on neurite extension and adhesion. To evaluate how interference of these mimetic peptides with NCAM homophilic interactions in cis influences NCAM binding in trans, we employed a coculture system in which PC12-E2 cells were grown on monolayers of fibroblasts with or without NCAM expression and the rate of neurite outgrowth subsequently was analyzed. P2, but not P1-B, induced neurite outgrowth in the absence of NCAM binding in trans. When PC12-E2 cells were grown on monolayers of NCAM-expressing fibroblasts, the effect of both P1-B and P2 on neurite outgrowth was dependent on peptide concentrations. P1-B and P2 acted as conventional antagonists, agonists, and reverse agonists of NCAM at low, intermediate, and high peptide concentrations, respectively. The demonstrated in vitro triple pharmacological effect of mimetic peptides interfering with the NCAM homophilic cis binding will be valuable for the understanding of the actions of these mimetics in vivo.

Neural cell adhesion molecule function is regulated by metalloproteinase-mediated ectodomain release

The neural cell adhesion molecule (NCAM) is involved in development of the nervous system, in brain plasticity associated with learning and memory, and in neuronal regeneration. NCAM regulates these processes by influencing cell adhesion, cell migration, and neurite outgrowth. NCAM activates intracellular signaling upon homophilic NCAM binding, and this is a prerequisite for NCAM-stimulated neurite outgrowth. NCAM is synthesized in three main membrane-bound isoforms, NCAM-120, NCAM-140, and NCAM-180. Soluble forms of NCAM in blood and cerebrospinal fluid have also been found, although the functional significance of these forms remains unclear. In this report, we demonstrate that NCAM can be released from primary hippocampal neurons in culture. The release was enhanced by application of ATP and inhibited by the metalloproteinase inhibitor BB-3103. ATP also induced metalloproteinase-dependent release of all three major NCAM isoforms from NCAM-transfected fibroblastoid L-cells. In this model system, the extracellular ATP-binding site of NCAM was shown not to be necessary for ATP-induced NCAM release. Furthermore, inhibition of serine, cysteine, and aspartic proteinases could not prevent ATP-induced down-regulation of NCAM in L-cells, suggesting that NCAM is cleaved directly by a metalloproteinase. Aggregation of hippocampal neurons in culture was increased in the presence of the metalloproteinase inhibitor GM 6001, consistent with a metalloproteinase-dependent shedding of NCAM occurring in these cells. Moreover, NCAM-dependent neurite outgrowth was significantly reduced by application of GM 6001. Taken together, these results suggest that membrane-bound NCAM can be cleaved extracellularly by a metalloproteinase and that metalloproteinase-dependent shedding of NCAM regulates NCAM-mediated neurite outgrowth.

Neural Cell Adhesion Molecule (N-CAM) Homophilic Binding Mediated by the Two N-terminal Ig Domains Is Influenced by Intramolecular Domain-Domain Interactions

The mechanism by which the neural cell adhesion molecule, N-CAM, mediates homophilic interactions between cells has been variously attributed to an isologous interaction of the third immunoglobulin (Ig) domain, to reciprocal binding of the two N-terminal Ig domains, or to reciprocal interactions of all five Ig domains. Here, we have used a panel of recombinant proteins in a bead binding assay, as well as transfected and primary cells, to clarify the molecular mechanism of N-CAM homophilic binding. The entire extracellular region of N-CAM mediated bead aggregation in a concentration- and temperature-dependent manner. Interactions of the N-terminal Ig domains, Ig1 and Ig2, were essential for bead binding, based on deletion and mutation experiments and on antibody inhibition studies. These findings were largely in accord with aggregation experiments using transfected L cells or primary chick brain cells. Additionally, maximal binding was dependent on the integrity of the intramolecular domain-domain interactions throughout the extracellular region. We propose that these interactions maintain the relative orientation of each domain in an optimal configuration for binding. Our results suggest that the role of Ig3 in homophilic binding is largely structural. Several Ig3-specific reagents failed to affect N-CAM binding on beads or on cells, while an inhibitory effect of an Ig3-specific monoclonal antibody is probably due to perturbations at the Ig2-Ig3 boundary. Thus, it appears that reciprocal interactions between Ig1 and Ig2 are necessary and sufficient for N-CAM homophilic binding, but that maximal binding requires the quaternary structure of the extracellular region defined by intramolecular domain-domain interactions.

An NCAM‐derived FGF‐receptor agonist, the FGL‐peptide, induces neurite outgrowth and neuronal survival in primary rat neurons

The Neural Cell Adhesion Molecule (NCAM) plays a crucial role in development of the central nervous system regulating cell migration, differentiation and synaptogenesis. NCAM mediates cell-cell adhesion through homophilic NCAM binding, subsequently resulting in activation of the fibroblast growth factor receptor (FGFR). NCAM-mediated adhesion leads to activation of various intracellular signal transduction pathways, including the Ras-mitogen activated protein kinase (MAPK) and the phosphatidylinositol-3-kinase (PI3K)-Akt pathways. A synthetic peptide derived from the second fibronectin type III module of NCAM, the FGL peptide, binds to and induces phosphorylation of FGFR without prior homophilic NCAM binding. We here present evidence that this peptide is able to mimic NCAM heterophilic binding to the FGFR by inducing neuronal differentiation as reflected by neurite outgrowth through a direct interaction with FGFR in primary cultures of three different neuronal cell types all expressing FGFR subtype 1: dopaminergic, hippocampal and cerebellar granule neurons. Moreover, we show that the FGL peptide promotes neuronal survival upon induction of cell death in the same three cell types. The effects of the FGL peptide are shown to depend on activation of FGFR and the MAPK and PI3K intracellular signalling pathways, all three kinases being necessary for the effects of FGL on neurite outgrowth and neuronal survival.

Backbone dynamics of the first, second, and third immunoglobulin modules of the neural cell adhesion molecule (NCAM).

The neural cell adhesion molecule (NCAM) is a cell surface multimodular protein, which plays an important role in cell-cell adhesion by homophilic (NCAM-NCAM) and heterophilic (NCAM-non-NCAM molecules) binding. In the present study, the backbone dynamics of the first three immunoglobulin-like (Ig) modules of NCAM have been investigated by NMR spectroscopy. Ig1, Ig2, and Ig3 share low sequence identity but possess the same fold and have very similar three-dimensional structures. (15)N longitudinal and transverse relaxation rates and heteronuclear NOEs have been measured and subsequently analyzed by the axial symmetric Lipari-Szabo modelfree formalism to characterize fast (pico- to nanosecond) and slow (micro- to millisecond) motions in the three protein modules. We found that backbone motions of residues located in the beta-strand regions are generally restricted, while increased flexibility is observed in turns and loops. In all three modules, residues located in the segments connecting the C- and D-strand plus residues located in the segment connecting the E- and F-strand show significant chemical exchange on the micro- to millisecond time scale. In addition, a number of residues with small chemical exchange contribution seem to form contiguous regions in the beta sheets, suggesting that these motions might be correlated. Only few residues in the homophilic binding sites in the NCAM Ig1 and Ig2 modules show increased flexibility, indicating that the Ig1-Ig2-mediated NCAM homophilic binding does not depend on the local backbone mobility of the interacting modules.

NCAM mimetic peptides: Pharmacological and therapeutic potential.

The neural cell adhesion molecule (NCAM) plays an important role in neuronal differentiation and synaptic plasticity, making it an attractive target for the development of drugs for the treatment of neurodegenerative disorders. NCAM binds to itself (homophilic binding) and to a series of counter-receptors, including the fibroblast growth factor receptor (FGFR), other adhesion molecules, and various extracellular matrix components (heterophilic binding). By means of combinatorial chemistry and based on the unraveling of the structure of NCAM, it has been possible to develop a number of peptides that mimic NCAM homophilic binding. These peptides interfere with cell adhesion and promote differentiation and cell survival. Recently, a peptide mimicking the heterophilic binding to FGFR has also been identified. It binds and activates the receptor, thereby modulating neurite extension and synaptic plasticity.

唾液腺腺様嚢胞癌の増殖・進展メカニズムに関する研究―ＮＣＡＭの発現とその機能について―

Neural cell adhesion molecule (NCAM) is one cell surface glycoprotein and a member of the immunoglobulin superfamily. It has been reported that NCAM may be associated with perineural invasion by malignant glandular tumors, such as bile duct carcinoma or gallbladder carcinoma. However, it remains unclear whether NCAM is really associated with the development of salivary gland tumors. Previous studies have demonstrated that NCAM is constitutively expressed in the human salivary gland tumor cell line HSG in vitro. In this study, molecular mechanisms of perineural invasion were examined using HSG cells in which NCAM-mediated inhibition of salivary gland tumor proliferation is caused by homophilic binding. In addition, the localization of NCAM in adenoid cystic carcinomas (ACCs) was examined immunohistochemically. The levels of NCAM mRNA and protein expression were found to decrease in a dose-dependent manner on treatment with anti-NCAM antibody. The MTT assay revealed a significant reduction in the number of viable HSG cells. Confocal laser microscopy showed that HSG cells undergo apoptosis in response to treatment with anti-NCAM antibody. The activation of caspases 3, 7, and 9 was observed in HSG cells after treatment with anti-NCAM antibody, thus confirming the induction of activated caspase-mediated apoptosis. The up-regulation of TGF-β1-mediated NCAM expression seemed to lead to the activation of homophilic NCAM binding, further accelerating HSG cell proliferation. Immunohistochemical examinations revealed that NCAM was slightly to moderately positive in 9 of 13 cases (69.2%) of ACC.These findings suggest that NCAM is associated not only with a cell-to-cell adhesion mechanism but also with tumorigenesis, including growth, development, and perineural invasion in human ACCs.

A Synthetic Peptide Ligand of Neural Cell Adhesion Molecule (NCAM), C3d, Promotes Neuritogenesis and Synaptogenesis and Modulates Presynaptic Function in Primary Cultures of Rat Hippocampal Neurons

The neural cell adhesion molecule (NCAM) plays a key role in morphogenesis of the nervous system and in remodeling of neuronal connections accompanying regenerative and cognitive processes. Recently, a new synthetic ligand of NCAM, the C3-peptide, which binds to the NCAM IgI module, has been identified by means of combinatorial chemistry (Rønn, L. C. B, Olsen, M., Ostergaard, S., Kiselyov, V., Berezin, V., Mortensen, M. T., Lerche, M. H., Jensen, P. H., Soroka, V., Saffell, J. L., Doherty, P., Poulsen, F. M., Bock, E., Holm, A., and Saffells, J. L. (1999) Nat. Biotechnol. 17, 1000-1005). In vitro, the dendrimeric form of C3, termed C3d, disrupts NCAM-mediated cell adhesion, induces neurite outgrowth, and triggers intracellular signaling cascades similar to those activated by homophilic NCAM binding. The peptide may therefore be expected to regulate regeneration and synaptic plasticity. Here we demonstrate that in primary cultures of hippocampal neurons: 1) C3d induces a sustained neuritogenic response, the neuritogenic activity of the compound being dependent on the dose, starting time, and duration of peptide application; 2) the peptide triggers the neuritogenic response by forming an adhesive substratum necessary for NCAM-mediated neurite formation and elongation; 3) C3d promotes synapse formation; and 4) C3d modulates the presynaptic function, causing a transient increase of the function at low (2 and 5 microm) doses and a reduction when applied at a higher concentration (10 microm). The effect of the peptide is dependent on the activation of the fibroblast growth factor receptor. We suggest that C3d may constitute a useful lead for the development of compounds for treatment of various neurodegenerative disorders.

Intracellular signaling by the neural cell adhesion molecule.

Cell adhesion molecules are known to play far more complex roles than mechanically attaching one cell to an adjacent cell or to components of the extracellular matrix. Thus, important roles for cell adhesion molecules in the regulation of intracellular signaling pathways have been revealed. In this review, we discuss the present knowledge about signaling pathways activated upon homophilic binding of the neural cell adhesion molecule (NCAM). Homophilic NCAM binding leads to activation of a signal transduction pathway involving Ca2+ through activation of the fibroblast growth factor receptor, and to activation of the mitogen-activated protein kinase pathway. In addition, cyclic adenosine monophosphate and protein kinase A are involved in NCAM-mediated signaling. Among these pathways the possibility exists of cross talk or convergence, of which different possible mediators have been suggested. Finally, several downstream effector molecules leading to NCAM-mediated cellular endpoints have been demonstrated, including transcription factors and regulators of the cytoskeleton.

Identification of a neuritogenic ligand of the neural cell adhesion molecule using a combinatorial library of synthetic peptides.

The neural cell adhesion molecule (NCAM) plays a key role in neural development, regeneration, and learning. In this study, we identified a synthetic peptide-ligand of the NCAM Ig1 module by combinatorial chemistry and showed it could modulate NCAM-mediated cell adhesion and signal transduction with high potency. In cultures of dissociated neurons, this peptide, termed C3, stimulated neurite outgrowth by activating a signaling pathway identical to that activated by homophilic NCAM binding. A similar effect was shown for the NCAM Ig2 module, the endogenous ligand of NCAM Ig1. By nuclear magnetic resonance spectroscopy, the C3 binding site in the NCAM Ig1 module was mapped and shown to be different from the binding site of the NCAM Ig2 module. The C3 peptide may prove useful as a lead in development of therapies for neurodegenerative disorders, and the C3 binding site of NCAM Ig1 may represent a target for discovery of nonpeptide drugs.

Neural cell adhesion molecule (N-CAM) domains and intracellular signaling pathways involved in the inhibition of astrocyte proliferation.

The neural cell adhesion molecule (N-CAM) inhibits astrocyte proliferation in vitro and in vivo, and this effect is partially reversed by the glucocorticoid antagonist RU-486. The present studies have tested the hypothesis that N-CAM-mediated inhibition of astrocyte proliferation is caused by homophilic binding and involves the activation of glucocorticoid receptors. It was observed that all N-CAM Ig domains inhibited astrocyte proliferation in parallel with their ability to influence N-CAM binding. The proliferation of other N-CAM-expressing cells also was inhibited by the addition of N-CAM. In contrast, the proliferation of astrocytes from knockout mice lacking N-CAM was not inhibited by added N-CAM. These findings support the hypothesis that it is binding of soluble N-CAM to N-CAM on the astrocyte surface that leads to decreased proliferation. Signaling pathways stimulated by growth factors include activation of mitogen-activated protein (MAP) kinase. Addition of N-CAM inhibited MAP kinase activity induced by basic fibroblast growth factor in astrocytes. In accord with previous findings that RU-486 could partially prevent the proliferative effects of N-CAM, inhibition of MAP kinase activity by N-CAM was reversed by RU-486. The ability of N-CAM to inhibit astrocyte proliferation was unaffected, however, by agents that block the ability of N-CAM to promote neurite outgrowth. Together, these findings indicate that homophilic N-CAM binding leads to inhibition of astrocyte proliferation via a pathway involving the glucocorticoid receptor and that the ability of N-CAM to influence astrocyte proliferation and neurite outgrowth involves different signal pathways.

The neural cell adhesion molecule (N-CAM) inhibits proliferation in primary cultures of rat astrocytes.

Cell proliferation is a key primary process during neural development and also plays an important role in the regenerative response of neural tissue to injury. It has been reported that glial cell proliferation is, at least in part, controlled by a neuronal signal, possibly involving cell surface molecules. We report here that the addition of purified rat neural cell adhesion molecule (N-CAM) to primary cultures of rat forebrain astrocytes inhibits their proliferation. This inhibitory effect can be elicited in cultures grown in chemically defined serum-free medium or in medium that had been supplemented with growth factors. Polyclonal antibodies to N-CAM or their Fab' fragments elicited a similar inhibitory effect. The magnitude of the inhibitory effect of N-CAM was dependent on cell density: it was maximal at low cell densities and weakened progressively as cells approached confluency. Synthetic peptides with sequences identical to a putative homophilic binding region of N-CAM mimicked the effect of purified N-CAM, while peptides of the same length and amino acid composition but with a randomized sequence did not. The addition of N-CAM antisense oligonucleotides to primary astrocyte cultures for 48 h resulted in reduced levels of N-CAM expression. After N-CAM levels on astrocytes were diminished by this treatment, the antiproliferative effect of N-CAM added to the medium was significantly reduced. The combined results suggest that N-CAM homophilic binding may be involved in the control of glial cell proliferation.