Switching repulsion to attraction: Changing responses to slit during transition in mesoderm migration

Slit is secreted by cells at the midline of the central nervous system, where it binds to Roundabout (Robo) receptors and functions as a potent repellent. We found that migrating mesodermal cells in vivo respond to Slit as both an attractant and a repellent and that Robe receptors are required for both functions. Mesoderm cells expressing Robe receptors initially migrate away from Slit at the midline. A few hours after migration, these same cells change their behavior and require Robe to extend toward Slit-expressing muscle attachment sites. Thus, Slit functions as a chemoattractant to provide specificity for muscle patterning.