FLOOR PLATE DERIVED NETRIN-1 IS AN INSTRUCTIVE LONG-RANGE GUIDANCE CUE FOR COMMISSURAL AXONS IN THE EMBRYONIC SPINAL CORD

The distribution and functional significance of long-range axon guidance cues remain controversial. Netrin-1 is essential for commissural axon extension to the ventral midline of the neural tube. It remains unclear, however, to what extent netrin-1 protein functions as a long-range or short-range cue in the embryonic spinal cord, nor has it been determined if netrin-1 protein has an instructive function in vivo that directs axon migration, or is merely permissive and required for axon extension. Here, we address how netrin-1 protein is distributed in the developing spinal cord and how that distribution influences axon guidance. In early embryonic chick spinal cord, only floor plate cells express NTN1, yet the distribution of netrin-1 protein extends ~200 μm dorsal of the floor plate, exemplary of a long-range cue. In the embryonic mouse spinal cord, NTN1 is expressed by cells in the floor plate and ventral ventricular zone. We show that selective deletion of NTN1 from floor plate cells in mouse, flattened the gradient of netrin-1 protein within ~200 μm of the midline and altered commissural axon trajectories across the same distance. In gain-of-function studies, we demonstrate that ectopic expression to change the distribution of netrin-1 in the embryonic spinal cord is sufficient to redirect commissural axons. These findings reveal that netrin-1 secreted by floor plate cells is distributed as a long-range cue in embryonic chick and mouse spinal cord, and demonstrate that the distribution of netrin-1 protein instructs the direction of commissural axon extension.