Robo family of proteins exhibit differential expression in mouse spinal cord and Robo-Slit interaction is required for midline crossing in vertebrate spinal cord.

The ventral midline of the central nervous system is an important intermediate target where growing commissural axons either cross and project contralaterally or remain on the same side of the body. New studies on mice and humans show that this decision by commissural axons is largely dependent on Slits, extracellular matrix proteins that are widely expressed in the midline of the nervous system, and their receptors, Robos (Long et al. [2004] Neuron 42:213-223; Sabatier et al. [2004] Cell 117:157-169; Jen et al. [2004] Science 304:1509-1513). Here, we show that the Robo family proteins Robo1 and Rig-1 exhibit differential expression patterns on commissural axons as they approach, cross, and leave the midline of the developing mouse spinal cord and demonstrate that Robo1 and Robo2 bind Slit1 and Slit2, but Rig-1 does not. In addition, we show that cultured chick commissural axons are repelled by a source of Slit protein, and the soluble Robo-Fc proteins are capable of neutralizing this repulsion. Finally, we exploit the large size and accessibility of the early chick embryo to analyze the function of Slit/Robo signaling in midline commissural axon guidance, and we demonstrate that the in vivo perturbation of Robo-Slit interaction at the floor plate causes consistent guidance defects of commissural axons during midline crossing. These findings demonstrate the evolutionarily conserved role for Robo-Slit interaction in the control of midline crossing axons in vertebrates.