A new model for netrin1 in commissural axon guidance.

Now-classic experiments characterized netrin1 as a major player in commissural axon guidance in the spinal cord. The data suggest a chemotactic model in which netrin1 expression in the floor plate forms a concentration gradient that attracts commissural axons. New research published independently in Neuron and in Nature tests this model by deleting netrin1 specifically in the floor plate. Surprisingly, these conditional mutant mice have no overt commissure defects. The authors report that netrin1 decorates the pial surface of the spinal cord and hindbrain, likely deposited by radial processes of progenitor cells in the ventricular zone. They find that deletion of the cue exclusively in the ventricular zone causes commissural axons to take aberrant trajectories, suggesting a short range, haptotactic guidance mechanism as opposed to chemotaxis. This minireview aims to summarize the classic and the new findings and offer some interpretations of the data.