From the initial stages of axon outgrowth to the formation

The establishment of accurate neuronal connectivity is a sequential process involving directed axon initiation, growth cone pathfinding, target selection and finally synapse formation. During each of these stages, neuronal growth cones are guided by many mechanisms, including both attractive and repulsive guidance molecules (reviewed by Goodman, 1996; Tessier-Lavigne and Goodman, 1996). Several gene families contain members that have been implicated in growth cone guidance, and it is becoming clear that guidance decisions are influenced by a balance of attractive and repulsive signals recognized by migrating growth cones (Stoeckli, 1997; Winberg et al., 1998). Proteins belonging to the semaphorin gene family have been demonstrated to function in both vertebrate and invertebrate nervous systems to mediate axon pathfinding and target selection (Culotti and Kolodkin, 1996; Mark et al., 1997). This large family of guidance molecules, comprising both transmembrane and secreted glycoproteins, is characterized by a conserved ~500 amino acid extracellular semaphorin (sema) domain. The first functionally characterized semaphorins were grasshopper semaphorin I (gSema I, formerly Fasciclin IV), a transmembrane semaphorin shown to be essential for correct neuronal pathfinding in the developing grasshopper limb bud (Kolodkin et al., 1992) and Collapsin-1 (Coll-1), a secreted semaphorin in chick shown to collapse dorsal root ganglion growth cones in vitro (Luo et al., 1993). Since then as many as 30 semaphorins, which can be subdivided into at least 7 structurally distinct classes, have been identified in many animal species from worms to mammals (Mark et al., 1997). Secreted semaphorins are characterized by the conserved sema domain, an immunoglobulin (Ig) domain C-terminal to the sema domain and, in vertebrates, a carboxy terminal basic domain. Results from a variety of systems have demonstrated that these proteins can function as inhibitory neuronal guidance cues (reviewed by Mark et al., 1997). Evidence implicating secreted semaphorins in chemorepulsion came initially from the finding that chick Coll-1 can cause the collapse of chick dorsal root ganglion growth cones in vitro (Luo et al., 1993). This collapse can direct growth cones to turn away from a source of Coll-1 by inducing a localized reorganization of the actin cytoarchitecture within the growth cone (Fan et al., 1993; Fan and Raper, 1995). Later studies showed that the mammalian orthologues of chick Coll-1, human Sema III/mouse Sem D, could act as repellents to pattern sensory and motor axon projections in the spinal cord and brain (Messersmith et al., 1995; Behar et al., 1996; Puschel et al., 1996; Tanelian et al., 1997; Taniguchi et al., 1997; Varela-Echavarria et al., 1997). Further evidence for an inhibitory function for secreted semaphorins comes from invertebrate studies, where Drosophila semaphorin 2a (dSema 2a) has been shown to function as a selective target-derived cue capable of inhibiting synaptic arborization formation (Matthes et al., 1995; Winberg et al., 1998; the original D-Sema II has been renamed dSema 2a, C. Goodman, personal communication). Taken together, the 2007 Development 126, 2007-2019 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV8593