Title: Tracking Sonic hedgehog with green fluorescent protein during patterning of the mouse neural tube
Description: Sonic hedgehog (Shh) plays a critical role in the patterning of developing spinal cord. Here, Shh released by the notochord induces all ventral cell identities by a concentration-dependent mechanism. In order to gain insight into the mechanism of Shh action, I have aimed at examining the distribution of the Shh signal in conjunction with the neural patterning process. To directly observe the Shh signal in the context of neural patterning in vivo , I have utilized a genetically engineered mouse that produces fluorescently labeled Shh ligands from the endogenous locus. My analysis shows that while ligand is distributed apically as a long-range gradient across target cells, a coincident time-based gradient occurs at the ventral midline. The gradients are Shh-specific, restricted by ligand-based feedback mechanisms and require appropriate ligand lipidation for formation. In addition, sub-cellular analysis suggests that Shh ligand trafficks within the neural target field in close association with an apical-to-basal microtubule scaffold, accumulating apically at the basal body. These data link ligand trafficking to intracellular signal transduction at the cilia/basal body juncture. Further, they suggest that local, temporal changes in Shh levels may promote the induction of distinct ventral cell identities.