Slit proteins are not dominant chemorepellents for olfactory tract and spinal motor axons.

Members of the Slit family are large extracellular glycoproteins that may function as chemorepellents in axon guidance and neuronal cell migration. Their actions are mediated through members of the Robo family that act as their receptors. In vertebrates, Slit causes chemorepulsion of embryonic olfactory tract, spinal motor, hippocampal and retinal ganglion cell axons. Since Slits are expressed in the septum and floor plate during the period when these tissues cause chemorepulsion of olfactory tract and spinal motor axons respectively, it has been proposed that Slits function as guidance cues. We have tested this hypothesis in collagen gel co-cultures using soluble Robo/Fc chimeras, as competitive inhibitors, to disrupt Slit interactions. We find that the addition of soluble Robo/Fc has no effect on chemorepulsion of olfactory tract and spinal motor axons when co-cultured with septum or floor plate respectively. Thus, we conclude that although Slits are expressed in the septum and floor plate, their proteins do not contribute to the major chemorepulsive activities emanating from these tissues which cause repulsion of olfactory tract and spinal motor axons.