mRNA expression of netrin-1, an axon guidance protein in chick and rat embryos.
The guidance of axons to their targets in developing neurons is believed to be mediated by diffusible chemotropic factors secreted by target cells. In vertebrates, commissural axons pioneer a circumferential pathway to the floor plate at the ventral midline of the embryonic spinal cord. Floor plate cells secrete a diffusible factor called 'netrin', that promotes the outgrowth of axons in the embryonic chick brain. The cloning of cDNAs encoding netrin showed that it is homologous to UNC-6, a laminin-related protein which is required for the circumferential migration of cells and axons in Caenorhabditis elegans. The differential expression of the netrin-1 gene was examined by slot blot analysis in various chick embryonic tissues, especially in the embryonic brain at various developmental stages. The netrin-1 transcript was most strongly expressed in the brain at the early developmental stages of E3 to E7 that corresponds to the time of emergence and full generation of commissural axons in the chick brain. In order to study whether netrin-1 can act as a global cue to guide all circumferentially migrating axons in the CNS, the localization of netrin-1 mRNA expression in the rat embryos was examined by in situ hybridization. Netrin-1 mRNA was detected in the neuroepithelial cells of the ventral midline along the entire rostrocaudal axis of the E18 rat embryo. These results suggest that netrin-1 functions as a global guidance cue for ventrally directed circumferential migrations toward the midline at all axial levels where the floor plate is found.