Different extracellular domains of the neural cell adhesion molecule (N-CAM) are involved in different functions.

Abstract

Abstract. The neural cell adhesion molecule (N-CAM) engages in diverse functional roles in neural cell interactions. Its extracellular part consists of five Ig-like domains and two fibronectin type III homologous (type III) repeats. To investigate the functional properties of the different structural domains of the molecule in cell interactions and signal transduction to the cell interior, we have synthesized, in a bacterial expression system, the individual domains and tandem sets of individual domains as protein fragments. These protein fragments were tested for their capacity to influence adhesion and spreading of neuronal cell bodies, promote neurite outgrowth, and influence cellular migration patterns from cerebellar microexplants in vitro. Ig-like domains I and II and the combined type III repeats I-II were most efficient for adhesion of neuronal cell bodies, when coated as substrates. Neurite outgrowth was best on the substrate-coated combined type III repeats I-II, followed by the combined Ig-like domains I-V and Ig-like domain I. Spreading of neuronal cell bodies was best on substrate-coated combined type III repeats I-II, followed by Ig-like domain I and the combined Ig-like domains I-V. The cellular migration pattern from cerebellar microexplant cultures plated on a mixture of laminin and poly-L-lysine was modified by Ig-like domains I, III, and IV, while Ig-like domains II and V and the combined type III repeats I-II did not show significant modifications, when added as soluble fragments. Outgrowth of astrocytic processes from the explant core was influenced only by Ig-like domain I. Metabolism of inositol phosphates was strongly increased by Ig-like domain I and less by the Ig-like domains II, III, IV, and V, and not influenced by the combined type HI repeats I-II. Intracellular concentrations of Ca 2 § and pH values were increased only by the Ig-like domains I and II. Intracellular levels of cAMP and GMP were not influenced by any protein fragment. These experiments indicate that different domains of N-CAM subserve different functional roles in cell recognition and signal transduction, and are functionally competent without nervous system-derived carbohydrate structures.

Homophilic adhesion mediated by the neural cell adhesion molecule involves multiple immunoglobulin domains.

“…Our data are also in accord with a variety of results (2,18,19,21) that suggest that Ig III has a special role in N-CAM binding and indicate that this effect is due to the ability of Ig III to interact with itself. Our results and those from cellular transfection studies (17, 18, 21, from a recent report (23) There is no evidence that the carbohydrates play a direct role in N-CAM binding, although the ability of polysialic acid to modulate binding (15) is prominent. It should be pointed out that none of our recombinant domains is glycosylated whereas Ig III, IV, and V are probably glycosylated in the intact molecule in vivo (2,9).…”

“…This study, however, did not reveal which of the domains specifically interact with the others. Another transfection study using only N-CAM constructs indicated that mutations in Ig III had a significant effect on N-CAM binding and showed that peptides corresponding to a short region of Ig III could inhibit adhesion (18,21,22 (23).…”

Transmembrane neural cell-adhesion molecule (NCAM), but not glycosyl-phosphatidylinositol-anchored NCAM, down-regulates secretion of matrix metalloproteinases.

“…Thus, the neurite-outgrowth response to NCAM can be abolished by pertussis toxin, an inhibitor of G proteins, and this response is strongly reduced by Ca2+ channel blockers (24). Triggering of cell-surface NCAM by NCAM antibodies (25) as well as by various NCAM fragments (26) can modulate intracellular levels of inositol phosphates, Ca2+, and pH. The fact that expression of both NCAM isoforms mediated cell aggregation, whereas only NCAM-B expression decreased secreted metalloproteinase activity, indicates that the presumed signal for this down-regulation may be transduced by NCAM-B itself.…”

The Fibronectin Domains of the Neural Adhesion Molecule TAX-1 Are Necessary and Sufficient for Homophilic Binding

“…Most members of the neural adhesion subfamily have been shown to possess both such adhesive and axon outgrowth activities in vitro (5,11). In various molecules studied, these activities have been mapped to both the IgC2 and FNIII or independently on either one of the two domains (17,(22)(23)(24)(25).…”