Redefining the Role of Netrin1 as an Axon Guidance Cue in the Developing Spinal Cord

Localized diffusible chemotropic signals are canonical sources of guidance information for axons extending towards their synaptic targets. Equally important, but less well understood, are the contact-dependent regional boundaries that provide either permissive or non-permissive substrates for axon growth. Classic work has demonstrated the importance of netrin1 as a floor plate chemoattractant for commissural axons in the developing spinal cord; subsequent studies in different systems have suggested that netrin1 also has short-range guidance activities. Here, I have further analyzed the role of netrin1 in the spinal cord and find that netrin1 mediates short-range growth boundaries that guide many additional classes of spinal axons. All spinal axons grow precisely around the ventricular zone (VZ), without innervating it, suggesting that the edge of the VZ represents a growth boundary. Multiple lines of evidence suggest that this boundary is mediated by netrin1: first, netrin1 is expressed by neural progenitors in the VZ and transported to the progenitor end feet at the pial surface; second, neurofilament (NF)+ axons initiate oriented growth coincident with the dorsal boundary of VZ-netrin1; third, several axons project aberrantly into the VZ in the absence of netrin1. This phenotype is observed only when netrin1 is ablated from the VZ, not the FP. Moreover, the selective ablation of netrin1 from the VZ is sufficient to locally reshape the trajectories of NF+ axons. Our studies have shown that netrin1 mediates the growth boundary acting primarily through the Dcc receptor and that netrin1 accumulates in axons in a Dcc dependent manner. Taken together, our data suggest that netrin1 establishes local “hederal” boundaries that provide an adhesive substrate while also preventing local innervation