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.

Identification of novel pro-survival factors in the brain is paramount for developing neuroprotective therapies. The multifunctional S100 family proteins have important roles in many human diseases and are also upregulated by brain injury. However, S100 functions in the nervous system remain unclear. Here we show that the S100A4 protein, mostly studied in cancer, is overexpressed in the damaged human and rodent brain and released from stressed astrocytes. Genetic deletion of S100A4 exacerbates neuronal loss after brain trauma or excitotoxicity, increasing oxidative cell damage and downregulating the neuroprotective protein metallothionein I þ II. We identify two neurotrophic motifs in S100A4 and show that these motifs are neuroprotective in animal models of brain trauma. Finally, we find that S100A4 rescues neurons via the Janus kinase/STAT pathway and, partially, the interleukin-10 receptor. Our data introduce S100A4 as a therapeutic target in neurodegeneration, and raise the entire S100 family as a potentially important factor in central nervous system injury.

The S100A4 protein belongs to the S100 family of vertebrate-specific proteins possessing both intra-and extracellular functions. In the nervous system, high levels of S100A4 expression are observed at sites of neurogenesis and lesions, suggesting a role of the protein in neuronal plasticity. Extracellular oligomeric S100A4 is a potent promoter of neurite outgrowth and survival from cultured primary neurons; however, the molecular mechanism of this effect has not been established. Here we demonstrate that oligomeric S100A4 increases the intracellular calcium concentration in primary neurons. We present evidence that both S100A4-induced Ca 2؉ signaling and neurite extension require activation of a cascade including a heterotrimeric G protein(s), phosphoinositide-specific phospholipase C, and diacylglycerol-lipase, resulting in Ca 2؉ entry via nonselective cation channels and via T-and L-type voltage-gated Ca 2؉ channels. We demonstrate that S100A4-induced neurite outgrowth is not mediated by the receptor for advanced glycation end products, a known target for other extracellular S100 proteins. However, S100A4-induced signaling depends on interactions with heparan sulfate proteoglycans at the cell surface. Thus, glycosaminoglycans may act as coreceptors of S100 proteins in neurons. This may provide a mechanism by which S100 proteins could locally regulate neuronal plasticity in connection with brain lesions and neurological disorders.The S100 family is a group of vertebrate-specific Ca 2ϩ -binding proteins with a highly conserved primary structure possessing both intra-and extracellular functions. Most S100 family members, including S100A4, are antiparallelly packed homodimers stabilized by disulfide bridges (reviewed in references 8 and 9). Intracellularly, S100 proteins are involved in a variety of processes, including the regulation of cytoskeletal dynamics, Ca 2ϩ homeostasis, and cell proliferation and differentiation. Importantly, some S100 proteins can also be secreted, form oligomers owing to the nonreducing conditions of the environment, and exert their effects acting at the cell surface (10; 43; reviewed in reference 20). A plasma membrane target for S100B and S100A12, the receptor for advanced glycation end products (RAGE), has been identified on inflammatory and neural cells (14). However, RAGE is probably not the sole receptor for members of the S100 family, since the effects of extracellular S100A12 and S100B proteins can be observed in cells lacking RAGE (32), and some of these effects are RAGE independent in cells expressing the receptor (37).The S100A4 (also termed Mts1) gene was isolated from tumor cells (11,40), where its expression increased the ability of the tumor to metastasize. S100A4 has also been detected in healthy tissues, particularly in the nervous system. In both the brain and spinal cord, S100A4 expression appears in astrocytes shortly after the start of myelination, with the highest level observed in the areas in which neurogenesis takes place and in regions possessing high plasticit...

NCAM plays a key role in neural development and plasticity-mediating cell adhesion and differentiation mainly through homophilic binding. Until recently, attempts to modulate neuronal differentiation and plasticity through NCAM have been impeded by the absence of small synthetic agonists mimicking homophilic interactions of NCAM. We show here that a peptide, P2, corresponding to a 12-amino acid sequence localized in the FG loop of the second Ig module of NCAM, binds to the first Ig module, which is the natural binding partner of the second Ig module, with an apparent K d of 4.7 ؎ 0.9 ؋ 10 ؊6 M. P2 inhibits cell aggregation and induces neurite outgrowth from hippocampal neurons, maximal neuritogenic effect being obtained at a concentration of 0.8 M. The neuritogenic effect was inhibited by preincubation of P2 with the recombinant NCAM-IgI. Both the length of P2 and the basic amino acid residues at the N and C termini are important for its neuritogenic activity. Treatment of hippocampal cultures with P2 results in induction of phosphorylation of the mitogen-activated protein kinases ERK1 and ERK2. Thus, P2 is a potent mimetic of NCAM, and therefore, an attractive compound for the development of drugs for the treatment of neurodegenerative diseases.The neural cell adhesion molecule (NCAM) 1 is a cell surface glycoprotein that belongs to the Ig superfamily. NCAM is primarily expressed in neurons, glial cells, and skeletal muscle. Alternative splicing of mRNA transcribed from a single gene generates three major NCAM isoforms: NCAM-A (180 kDa), NCAM-B (140 kDa), and NCAM-C (120 kDa). These isoforms have identical extracellular parts consisting of five N-terminal Ig-like modules (IgI-IgV) followed by two fibronectin type III modules (1). The transmembrane isoforms, NCAM-A and NCAM-B, have intracellular parts of different sizes, whereas NCAM-C is linked to the cell membrane via a glycosylphosphatidylinositol anchor. NCAM is known to mediate Ca 2ϩ -independent cell-cell and cell-substratum adhesion via homophilic (NCAM binding to NCAM) and heterophilic (e.g. NCAM binding to heparin/heparan sulfate proteoglycans) interactions, respectively (2-4). NCAM homophilic binding was originally reported to depend on a reciprocal interaction involving IgIII modules of two opposing NCAM molecules (5, 6). Later, an interaction involving all five Ig modules was also suggested (7). A double reciprocal binding between the recombinant IgI and IgII modules was demonstrated by means of surface plasmon resonance analysis (8), and recently, the three-dimensional structure of the IgI and IgII modules of rat NCAM was determined by both NMR spectroscopy and x-ray crystallography, resulting in the identification of homophilic binding sites in the IgI and IgII modules (9, 10).Neurons have been shown to respond with an increased neurite outgrowth to both glycosylphosphatidylinositol-linked and transmembrane isoforms of NCAM expressed on the surface of non-neuronal cells (11). NCAM homophilic binding has been suggested to activate the fibroblast g...

The neural cell adhesion molecule (NCAM), and the growthassociated 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.