Cell-contact-dependent signalling in axon growth and guidance: Eph receptor tyrosine kinases and receptor protein tyrosine phosphatase beta.

The growth and guidance of axons involves the recognition of complex environmental cues by receptor proteins on the surface of the growth cone and their interpretation by cellular machinery, leading to changes in cellular behaviour. Recent advances have demonstrated that the ligands for Eph receptor tyrosine kinases, the ephrins, act as repulsive axon guidance cues, and that Eph receptors are required for correct axonal navigation in vivo. Members of the receptor protein tyrosine phosphatase (RPTP) family also play important roles in axon guidance and growth. RPTP beta and Eph receptors interact with cell-surface-bound ligands, and there is increasing evidence that both transmembrane ephrins and contactin, a ligand for RPTP beta, may possess an intrinsic signalling function. Thus, the cell-contact-dependent interactions between these receptors and ligands may lead to initiation of bidirectional signals that regulate axonal growth and migration.