Netrinl establishes multiple boundaries for axon growth in the developing spinal cord

The canonical model for netrinl function proposed that it acted as a long-range chemotropic axon guidance cue. In the developing spinal cord, floor-plate (FP)-derived netrinl was thought to act as a diffusible attractant to draw commissural axons to the ventral midline. However, our recent studies have shown that netrinl is dispensable in the FP for axon guidance. We have rather found that netrinl acts locally: netrinl is produced by neural progenitor cells (NPCs) in the ventricular zone (VZ), and deposited on the pial surface as a haptotactic adhesive substrate that guides Dec(+) axon growth. Here, we further demonstrate that this netrinl pial-substrate has an early role orienting pioneering spinal axons, directing them to extend ventrally. However, as development proceeds, commissural axons choose to grow around a boundary of netrinl expressing cells in VZ, instead of continuing to extend alongside the netrinl pial-substrate in the ventral spinal cord. This observation suggests netrinl may supply a more complex activity than pure adhesion, with netrinl-expressing cells also supplying a growth boundary for axons. Supporting this possibility, we have observed that additional domains of netrinl expression arise adjacent to the dorsal root entry zone (DREZ) in E12.5 mice that are also required to sculpt axonal growth. Together, our studies suggest that netrinl provides hederal boundaries: a local growth substrate that promotes axon extension, while also preventing local innervation of netrinlexpressing domains.