Shared cell adhesion molecule (CAM) homology domains point to CAMs signalling via FGF receptors

A number of cell adhesion molecules (CAMs) promote neurite outgrowth following transfection and expression in a variety of monolayer cells. We have shown that N-cadherin, L1 and some isoforms of NCAM can stimulate neurite outgrowth from PC12 cells and primary neurons following transfection and expression at physiologically relevant levels in NIH-3T3 cells. A number of observations suggest that these CAMs stimulate neurite outgrowth by activating a convergent second messenger pathway in neurons rather than by modulating adhesion per se, and that an early or initial step in the pathway involves activation of a tyrosine kinase. The observation that the fibroblast growth factor receptor (FGFR) contains an evolutionarily conserved sequence with homology to the above CAMs (the CAM homology domain-CHD) points to the possibility that CAMs might interact with, and signal via, FGFR tyrosine kinases. This hypothesis has been substantiated by a number of independent experimental tests. We present a speculative model in which the evolutionary conservation of a pair of complementary binding motifs can account for a direct binding interaction between FGFR and the above three CAMs.