Homophilic NCAM interactions interfere with L1 stimulated neurite outgrowth

Impact of neural cell adhesion molecule deletion on regeneration after mouse spinal cord injury

The neural cell adhesion molecule (NCAM) plays important functional roles in development of the nervous system. We investigated the influence of a constitutive ablation of NCAM on the outcome of spinal cord injury. Transgenic mice lacking NCAM (NCAM-/-) were subjected to severe compression injury of the lower thoracic spinal cord using wild-type (NCAM+/+) littermates as controls. According to the single-frame motion analysis, the NCAM-/- mice showed reduced locomotor recovery in comparison to control mice at 3 and 6 weeks after injury, indicating an overall positive impact of NCAM on recovery after injury. Also the Basso Mouse Scale score was lower in NCAM-/- mice at 3 weeks after injury, whereas at 6 weeks after injury the difference between genotypes was not statistically significant. Worse locomotor function was associated with decreased monoaminergic and cholinergic innervation of the spinal cord caudal to the injury site and decreased axonal regrowth/sprouting at the site of injury. Astrocytic scar formation at the injury site, as assessed by immunohistology for glial fibrillary acidic protein at and around the lesion site was increased in NCAM-/- compared with NCAM+/+ mice. Migration of cultured monolayer astrocytes from NCAM-/- mice was reduced as assayed by scratch wounding. Numbers of Iba-1 immunopositive microglia were not different between genotypes. We conclude that constitutive NCAM deletion in young adult mice reduces recovery after spinal cord injury, validating the hypothesized beneficial role of this molecule in recovery after injury.

NCAM-mimetic, FGL peptide, restores disrupted fibroblast growth factor receptor (FGFR) phosphorylation and FGFR mediated signaling in neural cell adhesion molecule (NCAM)-deficient mice

Neural cell adhesion molecule (NCAM) is a membrane-bound glycoprotein expressed on the surface of neuronal and glial cells. Previous in vitro studies have demonstrated that NCAM promotes neuronal functions largely via three main interaction partners: the fibroblast growth factor receptor (FGFR), a member of Src family of tyrosine kinases, Fyn and Raf1 kinase which all activate different intracellular signaling pathways. The objective was to clarify, which signaling pathways are being disrupted in NCAM knockout mice and whether FGL peptide is able to restore observed disruptions. Therefore we compared the levels of phosphorylation of FGFR1, Src kinase Fyn, Raf1 kinase, MAP kinases, Akt kinase and calcium/calmodulin-dependent kinases II and IV (CaMKII and CaMKIV) in the hippocampus of NCAM knockout mice to their wild-type littermates. The data of our study show that mice constitutively deficient in all isoforms of NCAM have decreased basal phosphorylation levels of FGFR1 and CaMKII and CaMKIV. Furthermore, NCAM-mimetic, FGL peptide, is found to be able to restore FGFR1, CaMKII and CaMKIV phosphorylation levels and thereby mimic the interactions of NCAM at this receptor in NCAM deficient mice. Also, we found that Fyn(Tyr530), Raf1, MAP kinases and Akt kinase phosphorylation in adult animals is not affected by NCAM deficiency but interestingly, we found an over-expression of another cell adhesion molecule L1. We conclude that in NCAM deficient mice FGFR1-dependent signaling is disrupted and it can be restored by FGL peptide.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

Genetic analysis of an overlapping functional requirement for L1- and NCAM-type proteins during sensory axon guidance in Drosophila

L1- and NCAM-type cell adhesion molecules represent distinct protein families that function as specific receptors for different axon guidance cues. However, both L1 and NCAM proteins promote axonal growth by inducing neuronal tyrosine kinase activity and are coexpressed in subsets of axon tracts in arthropods and vertebrates. We have studied the functional requirements for the Drosophila L1- and NCAM-type proteins, Neuroglian (Nrg) and Fasciclin II (FasII), during postembryonic sensory axon guidance. The rescue of the Neuroglian loss-of-function (LOF) phenotype by transgenically expressed L1- and NCAM-type proteins demonstrates a functional interchangeability between these proteins in Drosophila photoreceptor pioneer axons, where both proteins are normally coexpressed. In contrast, the ectopic expression of Fasciclin II in mechanosensory neurons causes a strong enhancement of the axonal misguidance phenotype. Moreover, our findings demonstrate that this functionally redundant specificity to mediate axon guidance has been conserved in their vertebrate homologs, L1-CAM and NCAM.

Post-training administration of a synthetic peptide ligand of the neural cell adhesion molecule, C3d, attenuates long-term expression of contextual fear conditioning

The neural cell adhesion molecule (NCAM) plays a key role in synaptic plasticity and memory formation. We have recently developed a synthetic peptide, termed C3d, which, through the binding to the first, N-terminal immunoglobulin-like (Ig) module in the extracellular portion of NCAM, has been shown to promote neurite outgrowth and synapse formation in vitro, and to interfere with passive avoidance memory in rats in vivo. In this study, we investigated whether the i.c.v. administration of C3d, either 5.5 h after or 2 days before training, could be effective to modulate the strength at which emotional memory for aversive situations is established into a long-term memory. The effects of the peptide were evaluated in adult male Wistar rats trained in the contextual fear conditioning task. The results indicated that C3d significantly reduced the subsequent long-term retention of the conditioned fear response when administered 5.5 h post-training, as indicated by retention tests performed 2-3 and 7 days post-training. However, this treatment failed to influence conditioning for this task when injected 2 days pre-training. Additional experiments showed that C3d did not influence the emotional or locomotor behaviour of the animals, when tested in the open field task. Furthermore, hippocampal levels of microtubule-associated protein 2 (MAP2), Synaptophysin and NCAM were found unchanged when evaluated by enzyme-linked immunosorbent assay in crude synaptosomal preparations 2 days after peptide i.c.v. injection. Therefore, post-training injection of this synthetic peptide was efficient to attenuate the strength at which memory for contextual fear conditioning was enduringly stored, whilst it did not affect the acquisition of new memories. In addition to further support the view that NCAM is critically involved in memory consolidation, the current findings suggest that the NCAM IgI module is a potential target for the development of therapeutic drugs capable to reduce the cognitive impact induced by exposure to intensive stress experiences.

The role of neural cell adhesion molecules in plasticity and repair

Repair and functional recovery after brain injury critically depends on structural and functional plasticity of preserved neuronal networks. A striking feature of brain structures where tissue reorganization and plasticity occur is a strong expression of the polysialylated neural cell adhesion molecule (PSA-NCAM). An important role of this molecule in various aspects of neuronal and synaptic plasticity has been revealed by many studies. Recently, a new mechanism has been elucidated whereby PSA-NCAM may contribute to signalling mediated by the neurotrophic factor BDNF, thereby sensitizing neurons to this growth factor. This mechanism was shown to be important for activity-induced synaptic plasticity and for the survival and differentiation of cortical neurons. A cross-talk between these molecules may, thus, reveal a key factor for properties of structural plasticity and in particular could mediate the activity-dependent aspects of synaptic network remodeling. Animal models have been developed to assess the role of these molecules in functional recovery after lesions.

Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.