Sci. STKE, 21 December 2004
GUIDANCE CUES Converting Attraction to Repulsion
Growing axons need to follow precise paths to make the proper connections. Kantor et al. investigated the mechanism by which class 5 semaphorins, specifically Sema5A, control migration of neurons of the fasciculus retroflexus (FR), a diencephalic fiber tract of axons from the habenula nucleus (Hb). Sema5A is a transmembrane semaphorin with thrombospondin-1 repeats (TSR) in the extracellular domain. Using diencephalic explants from rats, the authors found that FR axon migration was disrupted by function-blocking antibodies against Sema5A; the axons did not reach their targets and frequently crossed into the adjacent area called prosomere 2. In a membrane stripe assay in which Hb neurons were allowed to grow across cell membranes from HEK 293 cells transfected with either GFP (green fluorescent protein) or Sema5A, the neurons showed a preference for growing across the Sema5A substrate, which suggests that Sema5A acted as an attractive guidance cue. Hb neurons avoided membranes from cells expressing a version of Sema5A that was forced to form pentamers, revealing a repulsive effect of this clustered form of Sema5A. The attractive effect of Sema5A was lost if heparan sulfate proteoglycans (HSPGs) were diminished on the Hb neurons by either heparinase treatment or inhibition of HSPG biosynthesis. Sema5A coprecipitated with the HSPG syndecan-3, suggesting a direct interaction. In a more natural version of the membrane stripe assay using membranes from the prosomere 2 region, which is enriched in Sema5A, the Hb neurons avoided those membranes and function-blocking antibodies against Sema5A blocked this effect, suggesting a repulsive signal. Binding of Sema5A to the prosomere 2 region was not inhibited by heparinase or proteinase K, suggesting that another class of proteoglycan may be important for the repulsive signal. Indeed, binding to the prosomere 2 region was decreased if the tissue was treated with chondroitinase ABC. When plated onto a chondroitin sulfate proteoglycan (CSPG) surface, Hb axons fasciculated into bundles, whereas this was not observed when the neurons were plated on bovine serum albumin. In the membrane stripe assay, coating the filters with CSPG and then membranes from the Sema5A-expressing HEK 293 cells resulted in the Hb neurons avoiding the Sema5A-containing membranes. Thus, in the prosomere region, the presence of CSPG with Sema5A results in a repulsive signal, and in the FR neurons, HSPG and Sema5A are attractive. This may be how Sema5A can contribute to both fasciculation and guidance: the FR neurons fasciculate (attraction to each other mediated by Sema5A and HSPG) and avoid aberrantly crossing the prosomere region (repulsion mediated by Sema5A and CSPG).
D. B. Kantor, O. Chivatakarn, K. L. Peer, S. F. Oster, M. Inatani, M. J. Hansen, J. G. Flanagan, Y. Yamaguchi, D. W. Sretavan, R. J. Giger, A. K. Kolodkin, Semaphorin 5A is a bifunctional axon guidance cue regulated by heparan and chondroitin sulfate proteoglycans. Neuron 44, 961-975 (2004). [Online Journal]
Citation: Converting Attraction to Repulsion. Sci. STKE 2004, tw453 (2004).
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