CAM-FGF receptor interactions: a model for axonal growth.

A number of experimental paradigms have been used to demonstrate that NCAM, N-cadherin, and L1 stimulate axonal growth. The molecular basis of this response has been extensively studied and a range of agents that inhibit neurite outgrowth stimulated by the above CAMs, but not integrins, have now been identified. These studies pointed to the activation of a tyrosine kinase-PLCgamma cascade as being important for the neurite outgrowth responses stimulated by all three CAMs, and this was substantiated by the identification of agents that could activate the cascade and mimic the growth response. In this review we will suggest that the neurite growth response stimulated by these CAMs is mediated by activation of the fibroblast growth factor receptor (FGFR) in neurons and that this results in the recruitment and activation of PLCgamma via interactions of its SH2 domain with the activated receptor. In this context the key events downstream from activation of PLCgamma required for neurite growth appear to be the conversion of diacylglycerol (DAG) to arachidonic acid (AA) via DAG lipase activity, followed by an increased influx of calcium into the neurons. The evolutionary conservation of putative binding motifs between the above CAMs and the FGFR suggests that activation of the FGFR-PLCgamma cascade by the CAMs might involve a direct CAM-FGFR interaction. The identification of the binding motifs also allows for predictions to be made concerning whether other CAMs might directly interact with the FGFR.