TRANSMEMBRANE GRASSHOPPER SEMAPHORIN-I PROMOTES AXON OUTGROWTH IN-VIVO

Members of the Semaphorin family of glycoproteins play an important ro le in axonal pathfinding by functioning as inhibitory guidance cues. H ere we provide evidence that a transmembrane form of Semaphorin (Semap horin I), which is expressed by bands of epithelial cells in the devel oping grasshopper limb bud, functions as an attractive/permissive cue for the growth cones of the subgenual organ. In addition, we demonstra te that Semaphorin I is needed for initial axonal outgrowth from the s ubgenual organ. These results are consistent with an alternative funct ion for a transmembrane form of Semaphorin and may explain the previou sly reported arrest of the proximal extension of the subgenual organ g rowth cones in the absence of the Ti1 pioneer pathway.