Sonic hedgehog induces response of commissural axons to Semaphorin repulsion during midline crossing

Pathfinding axons change responses to guidance cues at intermediate targets. During midline crossing, spinal cord commissural axons acquire responsiveness to class 3 Semaphorins and Slits, which regulate their floor plate exit and restrict their post-crossing trajectory into a longitudinal pathway. We found that Sonic Hedgehog (Shh) could activate the repulsive response of pre-crossing axons to Semaphorins. Blocking Shh function with a monoclonal antibody to Shh, 5E1, in 'open-book' explants or by expressing a dominant-negative form of Patched-1, Ptch1(Delta loop2), or a Smoothened (Smo) shRNA construct in commissural neurons resulted in severe guidance defects, including stalling and knotting inside the floor plate, recrossing, randomized anterior-posterior projection and overshooting after crossing, reminiscent of Neuropilin-2 mutant embryos. Enhancing protein kinase A activity in pre-crossing axons diminished Shh-induced Semaphorin repulsion and caused profound midline stalling and overshooting/wandering of post-crossing axons. Therefore, a morphogen, Shh, can act as a switch of axon guidance responses.