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The Journal of Neuroscience, April 1, 2000, 20(7):2638-2648

The Transmembrane Protein Semaphorin 6A Repels Embryonic Sympathetic Axons

Xiao-Mei Xu1, Daniel A. Fisher2, 3, Lijuan Zhou2, Fletcher A. White2, Sheldon Ng1, William D. Snider2, 3, and Yuling Luo1

1 Exelixis Pharmaceuticals, Inc., South San Francisco, California 94080, 2 Department of Neurology, Center for the Study of Nervous System Injury, Washington University School of Medicine, St. Louis, Missouri 63110, and 3 Neuroscience Center, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599

Semaphorin 6A (Sema6A) (previously named Semaphorin VIa) is the originally described member of the vertebrate semaphorin class 6, a group of transmembrane semaphorins homologous to the insect semaphorin class 1. Although Sema-1a (previously named semaphorin I) has been implicated in axon guidance in insects, the function of Sema6A is currently unknown. We have expressed the extracellular domain of Sema6A in mammalian cells as either a monomeric or a dimeric fusion protein and tested for potential axon guidance effects on two populations of embryonic neurons in growth cone collapse and collagen matrix chemorepulsion assays. Sema6A was observed to induce growth cone collapse of sympathetic neurons with an EC50 of ~200 pM, although a 10-fold higher (EC50 of ~2 nM) concentration was necessary to induce growth cone collapse of dorsal root ganglion neurons. The activity of Sema6A is likely to depend on protein dimerization or oligomerization. Although Sema6A mRNA is expressed in complex patterns during embryonic development, it is strikingly absent from sympathetic ganglia. Sema6A is, however, expressed in areas avoided by sympathetic axons and in areas innervated by sympathetics, but before their arrival. Our results demonstrate that transmembrane semaphorins, like the secreted ones, can act as repulsive axon guidance cues. Our findings are consistent with a role for Sema6A in channeling sympathetic axons into the sympathetic chains and controlling the temporal sequence of sympathetic target innervation.

Key words: semaphorin; growth cone collapse; repulsion; sympathetic development; plexin; axon guidance

Copyright © 2000 Society for Neuroscience  0270-6474/00/2072638-11$05.00/0




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