Found throughout the animal kingdom, the semaphorins usually guide growth cones by selective repulsion. They are especially important in forcing “turns” when an axon must change direction. Semaphorin-1, for example, is a transmembrane protein that is expressed in a band of epithelial cells in the developing insect limb. This protein appears to inhibit the growth cones of the Ti1 sensory neurons from moving forward, thus causing them to turn (Figure 1; Kolodkin et al. 1992, 1993). In Drosophila, semaphorin-2 is secreted by a single large thoracic muscle. In this way, the thoracic muscle prevents itself from being innervated by inappropriate axons (Matthes et al. 1995).
Kolodkin, A. L., D. J. Matthes and C. S. Goodman. 1993. The semaphorin genes encode a family of transmembrane and secreted growth cone guidance molecules. Cell 75: 1389–1399.
Kolodkin, A. L., D. J. Matthes, T. P. O’Connor, N. H. Patel, D. Bentley and C. S. Goodman. 1992. Fasciclin IV: Sequence, expression, and function during growth cone guidance in the grasshopper embryo. Neuron 9: 831–845.
Matthes, D. J., H. Sink, A. L. Kolodkin and C. S. Goodman. 1995. Semaphorin II can function as a selective inhibitor of specific synaptic arborizations. Cell 81: 631–639.