SEMA3A regulates developing sensory projections in the chicken spinal cord.

The present study explores the role of SEMA3A (collapsin-1) in the temporal and spatial regulation of developing sensory projections in the chick spinal cord. During development, SEMA3A mRNA (SEMA3A) is first expressed throughout the spinal gray matter, but disappears from the dorsal region when small caliber (trkA(+)) sensory axon collaterals first grow into the dorsal horn. In explant cultures of spinal cord segments with attached sensory ganglia, the spatial extent of SEMA3A expression varied in different explants, but in each case the growth of trkA(+) sensory collaterals was largely excluded from areas of SEMA3A expression. To test if SEMA3A had a direct effect on sensory axon growth, we injected recombinant protein into the explants before placing them in culture. Increased levels of SEMA3A substantially reduced the ingrowth of trkA(+) axons, whereas trkC(+) axon collaterals were not affected. Consistent with the insensitivity of trkC(+) collaterals to SEMA3A, these collaterals did not express neuropilin-1, a receptor for SEMA3A. The inhibitory effects of SEMA3A on trkA(+) axons within the spinal cord suggests that the fall in SEMA3A expression in the dorsal horn may contribute to the initiation of growth of these axons into gray matter. In addition, the observation that trkA(+) axons frequently grew close to but rarely over areas of SEMA3A expression suggests that semaphorin may act principally as a short-range guidance cue within the spinal cord.