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The Semaphorins: A Family of Axonal Guidance Molecules?

During the formation of the vertebrate nervous system axons navigate with remarkable specificity to establish a highly complex network of neuronal connections. Often the initial axonal projections are directed from the outset to their final targets, and few inappropriate pathway choices occur (Gotz and Price, 1994). Only after the basic axonal tracts have been laid down, activity-dependent mechanisms operate to refine the pattern of connectivity (Goodman and Shatz, 1993). Responsible for this remarkable feat of navigation is a sensory structure at the tip of the growing axon, the growth cone, which bears receptors for and interacts with many signals it encounters on the way to its final target. Adhesion and extracellular matrix molecules that promote axonal outgrowth have long been the main focus of interest in attempts to identify the signals governing the innervation of central and peripheral targets (Dodd and Jessell, 1988; Hynes and Lander, 1992). Only recently have repulsive signals been recognized as at least equally important (Dodd and Schuchardt, 1995; Keynes and Cook, 1995a, b). With the cloning of the respective cDNAs, several of these factors have now become amenable to molecular analysis. So far, three families of proteins have emerged as repulsive factors: the netrins (Serafini ef al., 1994). members of the Eph receptor ligand family (Tessier-Lavigne, 1995). and the semaphorid collapsins (Kolodkin et al., 1993; Piischel, 1995; Luo et al., 1993, 1995). The molecular nature of several other repulsive or growth cone-collapsing activities that have been described (Davies et al., 1990; Raper and Kapfhammer, 1990; Pini, 1993; Schwab et al., 1993) is presently unknown and may turn out to represent completely new kinds of molecules.