Iterative inhibition of commissural growth cone exploration, not post-crossing barrier, ensures forward midline navigation through SlitC-PlxnA1 signaling

Summary Sensitization to Slits and Semaphorin (Sema)3B floor plate repellents after midline crossing is thought to be the mechanism expelling commissural axons contralaterally and preventing their back-turning. We studied the role of Slit-C terminal fragment sharing with Sema3B the Plexin (Plxn) A1 receptor, newly implicated in midline guidance. We generated a knock-in mouse strain baring PlxnA1Y1815F mutation altering SlitC but not Sema3B responses and observed recrossing phenotypes. Using fluorescent reporters, we found that Slits and Sema3B form clusters decorating an unexpectedly complex mesh of ramified FP glia basal processes spanning the entire navigation path. Time-lapse analyzes revealed that impaired SlitC sensitivity destabilized axon trajectories by inducing high levels of growth cone exploration from the floor plate entry, increasing risk of aberrant decisions. Thus, FP crossing is unlikely driven by post-crossing sensitization to SlitC. Rather, SlitC limits growth cone plasticity and exploration through reiterated contacts, continuously imposing a straight and forward-directed trajectory.