Dual branch-promoting and branch-repelling actions of Slit/Robo signaling on peripheral and central branches of developing sensory axons.

Secreted Slit proteins signal through Robo receptors and negatively regulate axon guidance and cell migration, but in vertebrates, Slit proteins can also stimulate branching and elongation of sensory axons and cortical dendrites in vitro. Here, we show that this branching activity is required for developing peripheral sensory arbors in vivo, because trigeminal sensory branching above the eye is reduced in Slit2;Slit3 double or Slit1,2,3 triple mutants. A similar phenotype is observed in Robo1;Robo2 double mutants, implicating Robo receptors in mediating this activity. Interestingly, by studying the central projection of sensory neurons in the spinal cord, we discovered that Slit ligands are also required for proper guidance of sensory branches during bifurcation but through a different cellular mechanism. In Slit1;Slit2 or Robo1;Robo2 double mutant mice, sensory axons enter the spinal cord prematurely because of the loss of an inhibitory guidance function on one of the daughter branches of each afferent during bifurcation. Together, our studies reveal that Slit/Robo signaling contributes to patterning both the peripheral and central branches of sensory neurons but via distinct positive branching and negative guidance actions, respectively.