Graphical Abstract Highlights d Floor-plate-derived netrin1 (FP-netrin1) guides commissural axons at long range d FP-netrin1 and Shh collaborate to guide commissural axons in the ventral spinal cord d Both FP-netrin1 and ventricular zone-netrin1 contribute to commissural axon guidance d FP-netrin1 may guide via haptotaxis and/or chemotaxis In Brief Recent studies have queried the role of netrin1 from floor plate (FP-netrin1) in guiding commissural axons. Wu et al. show that, in spinal cord, FP-netrin1 is required and acts at long range to guide commissural axons, collaborating with Shh. SUMMARYAn important model for axon pathfinding is provided by guidance of embryonic commissural axons from dorsal spinal cord to ventral midline floor plate (FP). FP cells produce a chemoattractive activity, comprised largely of netrin1 (FP-netrin1) and Sonic hedgehog (Shh), that can attract the axons at a distance in vitro. netrin1 is also produced by ventricular zone (VZ) progenitors along the axons' route (VZ-netrin1). Recent studies using region-specific netrin1 deletion suggested that FP-netrin1 is dispensable and VZ-netrin1 sufficient for netrin guidance activity in vivo. We show that removing FP-netrin1 actually causes guidance defects in spinal cord consistent with long-range action (i.e., over hundreds of micrometers), and double mutant analysis supports that FP-netrin1 and Shh collaborate to attract at long range. We further provide evidence that netrin1 may guide via chemotaxis or haptotaxis. These results support the model that netrin1 signals at both short and long range to guide commissural axons in spinal cord.

Highlights d Sonic hedgehog (Shh) induces endocytosis of Boc and Ptch1 into Rab5 endosomes d The endocytic adaptor Numb binds to Boc and is required for Boc endocytosis d Numb is required for Shh-mediated axon attraction in vitro and axon guidance in vivo d Binding of Shh to Boc is required for Ptch1 endocytosis and Shh axon attraction Authors Julien Ferent, Fanny Giguè re,

In the developing spinal cord, Sonic hedgehog (Shh) attracts commissural axons toward the floorplate. How Shh regulates the cytoskeletal remodeling that underlies growth cone turning is unknown. We found that Shh-mediated growth cone turning requires the activity of Docks, which are unconventional GEFs. Knockdown of Dock3 and 4, or their binding partner ELMO1 and 2, abolished commissural axon attraction by Shh in vitro. Dock3/4 and ELMO1/2 were also required for correct commissural axon guidance in vivo. Polarized Dock activity was sufficient to induce axon turning, indicating that Docks are instructive for axon guidance. Mechanistically, we show that Dock and ELMO interact with Boc, the Shh receptor, and that this interaction is reduced upon Shh stimulation. Furthermore, Shh stimulation translocates ELMO to the growth cone periphery and activates Rac1. This identifies Dock/ELMO as an effector complex of non-canonical Shh signaling and demonstrates the instructive role of GEFs in axon guidance.