Ectopic semaphorin-1a functions as an attractive guidance cue for developing peripheral neurons.

Transmembrane and secreted glycoproteins of the semaphorin family are typically classified as inhibitory neuronal guidance molecules. However, although chemorepulsive activity has been demonstrated for several semaphorin family members, little is known about the function of the numerous transmembrane semaphorins identified to date. Here we demonstrated that the extracellular semaphorin domain of a transmembrane semaphorin, semaphorin-1a, could actively perturb axon pathfinding in vivo when presented homogenously as a recombinant freely soluble factor. When ectopic overexpression was limited to defined epithelial regions, semaphorin-1a could directly steer axons by acting as an attractive guidance molecule.