Intracellular signaling by the neural cell adhesion molecule

Expression of ICAM-1, ICAM-2, NCAM-1 and VCAM-1 by human synovial cells exposed to Borrelia burgdorferi in vitro

The interaction of resident tissue cells with migratory inflammatory cells is essential for the recruitment of immune effector cells to inflammatory sites. The sustained expression of adhesion molecules in the synovium of patients with chronic Lyme arthritis seems to contribute to this chronic inflammation. Whether cell adhesion molecules influence the early steps of Borreliosis is unclear. Therefore, we examined the expression of ICAM-1, ICAM-2, VCAM-1 and NCAM-1 in synovial cells exposed to two different Borrelia burgdorferi sensu stricto strains Geho and B31. The mRNA expression of ICAM-1, ICAM-2, VCAM-1 and NCAM-1 was not changed in synovial cells exposed to B31. Whereas ICAM-2 and VCAM-1 was upregulated, NCAM-1 mRNA was downregulated and ICAM-1 mRNA was unchanged by strain Geho. The ICAM-1 protein expression on the synovial cell surface was downregulated by both strains. Differential regulation of adhesion molecule mRNA, and subsequent high turnover or elevated shedding from the cell membrane may contribute to early pathogenesis in Lyme arthritis.

Growth-associated protein GAP-43 and L1 act synergistically to promote regenerative growth of Purkinje cell axons in vivo

Neuronal expression of growth-associated protein 43 (GAP-43) and the cell adhesion molecule L1 has been correlated with CNS axonal growth and regeneration, but it is not known whether expression of these molecules is necessary for axonal regeneration to occur. We have taken advantage of the fact that Purkinje cells do not express GAP-43 or L1 in adult mammals or regenerate axons into peripheral nerve grafts to test the importance of these molecules for axonal regeneration in vivo. Transgenic mice were generated in which Purkinje cells constitutively express L1 or both L1 and GAP-43 under the Purkinje cell-specific L7 promoter, and regeneration of Purkinje cell axons into peripheral nerve grafts implanted into the cerebellum was examined. Purkinje cells expressing GAP-43 or L1 showed minor enhancement of axonal sprouting. Purkinje cells expressing both GAP-43 and L1 showed more extensive axonal sprouting and axonal growth into the proximal portion of the graft. When a predegenerated nerve graft was implanted into double-transgenic mice, penetration of the graft by Purkinje cell axonal sprouts was strongly enhanced, and some axons grew along the entire intracerebral length of the graft (2.5-3.0 mm) and persisted for several months. The results demonstrate that GAP-43 and L1 coexpressed in Purkinje cells can act synergistically to switch these regeneration-incompetent CNS neurons into a regeneration-competent phenotype and show that coexpression of these molecules is a key regulator of the regenerative ability of intrinsic CNS neurons in vivo.

Tissue expression of alternatively spliced GFRα1, NCAM and RET isoforms and the distinct functional consequence of ligand-induced activation of GFRα1 isoforms

Glial-cell-line-derived neurotrophic factor (GDNF) exerts its effect through a multi-component receptor system consisting of GFRalpha1, RET and NCAM. Two highly homologous alternatively spliced GFRalpha1 isoforms (GFRalpha1a and GFRalpha1b) have previously been identified. In this study, isoform specific real-time PCR assays were used to quantify the expression levels of GFRalpha1, RET and NCAM isoforms in murine embryonic and adult tissues. The expression levels of GFRalpha1b were found to be comparable to that of GFRalpha1a in peripheral tissues. However, GFRalpha1a was the predominant isoform expressed in the whole brain. The co-expressions of GFRalpha1 and the co-receptors were developmentally regulated and differentially expressed in some tissues. Microarray analyses of GFRalpha1 isoforms transfected cells stimulated with NTN showed distinct and non-overlapping gene profiles. These observations are consistent with the emerging view that the combinatorial interactions of the spliced isoforms of GFRalpha, RET and NCAM may contribute to the pleiotropic biological responses.

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.

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.

Glial cell line-derived neurotrophic factor-induced signaling in Schwann cells

Glial cell line-derived neurotrophic factor (GDNF), a known survival factor for neurons, has recently been shown to stimulate the migration of Schwann cells (SCs) and to enhance myelination. GDNF exerts its biological effects by activating the Ret tyrosine kinase in the presence of glycosylphosphatidylinositol-linked receptor, GDNF family receptor (GFR) alpha1. In Ret-negative cells, the alternative transmembrane coreceptor is the 140-kDa isoform of neural cell adhesion molecule (NCAM) associated with a non-receptor tyrosine kinase Fyn. We confirmed that GDNF, GFRalpha1 and NCAM are expressed in neonatal rat SCs. We found that GDNF induces an increase in the partitioning of NCAM and heparan sulfate proteoglycan agrin into lipid rafts and that heparinase inhibits GDNF-signaling in SCs. In addition to activation of extracellular signal-regulated kinases, and phosphorylation of cAMP response element binding protein, we found that cAMP-dependent protein kinase A and protein kinase C are involved in GDNF-mediated signaling in SCs. Although GDNF did not promote the differentiation of purified SCs into the myelinating phenotype, it enhanced myelination in neuron-SC cocultures. We conclude that GDNF utilizes NCAM signaling pathways to regulate SC function prior to myelination and at early stages of myelin formation.

P75 neurotrophin receptor regulates expression of neural cell adhesion molecule 1

Our recent transcriptome profiling studies suggest that presenilin 1 (PS1) regulates expression of neural cell adhesion molecule (Ncam1) through p75 neurotrophin receptor. To better understand regulation of Ncam1 transcript and protein levels by p75, we performed a series of in vitro and in vivo experiments. The combined results suggest that p75 receptor is required for both resting and NGF-induced Ncam1 expression. Activation of TrkA receptors alone does not upregulate Ncam1. The normal Ncam1 expression depends on the relative ratio of TrkA and p75 receptors, and p75 extracellular domain is necessary for baseline Ncam1 expression. NGF-induced Ncam1 expression is dependent on the presence of an intact palmitoylation site within p75 receptor. Finally, we show that the expression of Ncam1 is altered in brains of two transgenic mouse lines that express familial Alzheimer's disease (FAD)-linked PS1 variants, suggesting that expression of dominantly inherited mutant PS1 genes interferes with the normal Ncam1 expression via the p75 signaling pathway.

Retrovirally transduced NCAM140 facilitates neuronal fate choice of hippocampal progenitor cells

Neural cell adhesion molecule (NCAM) influences proliferation and differentiation of neuronal cells. However, only a little is known about the downstream effects of NCAM signalling, such as alterations in gene transcription, which are associated with cell fate choice. To examine whether NCAM plays a role in cell fate choice during hippocampal neurogenesis, we performed a gain-of-function study, using a retroviral vector which contained full-length NCAM140 cDNA and the marker gene EGFP, and found that NCAM140 promoted neurogenesis by activating proneural transcription activators with concurrent inhibition of gliogenesis. The enhanced transcript levels of proneural transcription factors in NCAM140-transduced cells were down-regulated by treatment of the cells with mitogen-activated protein kinase kinase (MEK) inhibitor PD098059. Overall, these findings suggest that NCAM140 may facilitate hippocampal neurogenesis via regulation of proneurogenic transcription factors in an extracellular signal-regulated kinase (ERK)-dependent manner.

Signal transduction pathways implicated in neural recognition molecule L1 triggered neuroprotection and neuritogenesis

The signal transduction pathways involved in adhesion molecule L1-triggered neuritogenesis and neuroprotection were investigated using the extracellular domain of mouse or human L1 in fusion with the Fc portion of human immunoglobulin G or L1 purified from mouse brain by affinity chromatography. Substrate L1-triggered neuritogenesis and neuroprotection depended on distinct but also overlapping signal transduction pathways and on the expression of L1 at the neuronal cell surface. PI3 kinase inhibitors, Src family kinase inhibitors as well as mitogen-activated protein kinase kinase inhibitors reduced both L1-triggered neuritogenesis and neuroprotection. In contrast, fibroblast growth factor receptor inhibitors, a protein kinase A inhibitor, and an inhibitor of cAMP-mediated signal transduction pathways, blocked neuritogenesis, but did not affect L1-triggered neuroprotection. Proteolytic cleavage of L1 or its interaction partners is necessary for both L1-mediated neuritogensis and neuroprotection. Furthermore, L1-triggered neuroprotection was found to be associated with increased phosphorylation of extracellular signal-regulated kinases 1/2, Akt and Bad, and inhibition of caspases. These observations suggest possibilities of differentially targeting signal transduction pathways for L1-dependent neuritogenesis and neuroprotection.

Tyrosine 734 of NCAM180 interferes with FGF receptor-dependent signaling implicated in neurite growth

The cytoplasmic domain of the neural cell adhesion molecule (NCAM) contains multiple phosphorylation sites. We report here that in addition to serine and threonine residues a tyrosine of the NCAM180 isoform is phosphorylated as shown by phosphoamino acid analysis. Exchange of the only cytoplasmic tyrosine at position 734 of human NCAM180 (NCAM180-Y734F) to phenylalanine resulted in increased neurite outgrowth of NCAM180-Y734F transfected B35 neuroblastoma cells compared to NCAM180-wt transfectants on poly-L-lysine as substrate. As demonstrated by inhibitor studies the increased neurite outgrowth was due to higher FGF receptor 1 and ERK1 activity in NCAM180-Y734F cells, indicating that tyrosine residue 734 plays a role in signal transduction mediated by the FGF receptor. On an NCAM expressing monolayer of COS-7 cells the Y734F mutation also influences FGF receptor 1 dependent neurite outgrowth, but under these conditions additional mechanisms seem to be responsible for the increased neurite length observed for NCAM180-Y734F transfected cells.

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.

Regulators of neurite outgrowth: role of cell adhesion molecules

Neuronal differentiation is a fundamental event in the development of the nervous system as well as in the regeneration of damaged nervous tissue. The initiation and guidance of a neurite are accomplished by positive (permissive or attractive), negative (inhibitory or repulsive), or guiding (affecting the advance of the growth cone) signals from the extracellular space. The signals may arise from either the extracellular matrix (ECM) or the surface of other cells, or be diffusible secreted factors. Based on this classification, we briefly describe selected positive, negative, and guiding signaling cues focusing on the role of cell adhesion molecules (CAMs). CAMs not only regulate cell-cell and cell-ECM adhesion "mechanically," they also trigger intracellular signaling cascades launching neurite outgrowth. Here, we describe the structure, function, and signaling of three key CAMs found in the nervous system: N-cadherin and two Ig-CAMs, L1 and the neural cell adhesion molecule NCAM.

Cytoplasmic domain of NCAM 180 reduces NCAM-mediated neurite outgrowth

The neural cell adhesion molecule (NCAM) is one of the best-characterized cell adhesion molecules of the immunoglobulin superfamily. In the nervous system, NCAM is involved in cell migration, axon fasciculation and in neurite outgrowth. Neurite outgrowth is mediated by homophilic NCAM-NCAM interactions. Alternative splicing generates three major isoforms of NCAM differing in their intracellular portion. Two of them, NCAM 180 and NCAM 140, are transmembrane proteins with large intracellular domains. The present study is concerned with novel details of the intracellular domains of NCAM 140 and NCAM 180. We expressed these NCAM isoforms consisting only of the transmembrane and intracellular domains (without extracellular domains) in PC12 cells and elaborated their function in neurite outgrowth assays. Our data demonstrate that membrane-associated NCAM 180 interferes with neurite outgrowth, whereas membrane-associated NCAM 140 promotes neurite outgrowth.

A cell-adhesion pathway regulates intercellular communication during Dictyostelium development

Cell adhesion molecules play an important physical role in shaping the structure of multicellular organisms. Recent studies show that they also play a role in intracellular and intercellular signaling. We describe a cell adhesion pathway that is mediated by the intercellular communication genes comC, lagC, and lagD during Dictyostelium development. Disruptions of these genes result in strains that are unable to generate spores when developed in a pure population but are capable of sporulation when developed in chimerae with wild-type cells. In contrast, any pair-wise chimera of the three mutants fails to form spores. We postulate that the wild-type cells supply the mutant cells with a signal that partially rescues their sporulation. We also propose that the three mutants are deficient in the production of that signal, suggesting that the three genes function in one signaling pathway. In support of that notion, the mutant cells share common non-cell-autonomous prespore and prestalk-specific defects and a common pattern of developmental progression and regression. We provide transcriptional and functional evidence for a network in which comC inhibits lagC and activates lagD expression, lagC and lagD are mutually inductive, and the cell adhesion gene lagC is the terminal node in this signaling network.

The neural cell adhesion molecule is associated with major components of the cytoskeleton

The neural cell adhesion molecule (NCAM) is a member of the immunoglobulin superfamily. Two of the three major isoforms (NCAM 140 and NCAM 180) are transmembrane glycoproteins, which differ in their intracellular domains. The present study is concerned with the identification of novel intracellular binding partners of NCAM. We expressed and purified both cytoplasmic domains of NCAM. Using ligand affinity chromatography followed by peptide mass fingerprinting, we could identify several novel binding partners of the cytoplasmic domains of NCAM 140 and 180. We present data that alpha- and beta-tubulin as well as alpha-actinin 1 are associated with both NCAM 140 and 180. In contrast, beta-actin, tropomyosin, microtubuli-associated protein MAP 1A, and rhoA-binding kinase-alpha preferentially bind to NCAM 180. Furthermore, we demonstrate that inhibition of rhoA-binding kinase-alpha stimulates neurite outgrowth independently from NCAM.