Requirement of 19K form of Sonic hedgehog for induction of distinct ventral cell types in CNS explants.

The identity and patterning of ventral cell types in the vertebrate central nervous system depends on cell interactions. For example, induction of a specialized population of ventral midline cells, the floor plate, appears to require contact-mediated signalling by the underlying notochord, whereas diffusible signals from the notochord and floor plate can induce ventrolaterally positioned motor neurons. Sonic hedgehog (Shh), a vertebrate hedgehog-family member, is processed to generate two peptides (M(r) 19K and 26/27K) which are secreted by both of these organizing centres. Moreover, experiments in a variety of vertebrate embryos, and in neural explants in vitro, indicate that Shh can mediate floor-plate induction. Here we have applied recombinant Shh peptides to neural explants in serum-free conditions. High concentrations of Shh bound to a matrix induce floor plate and motor neurons, and addition of Shh to the medium leads to dose-dependent induction of motor neurons. All inducing activity resides in a highly conserved amino-terminal peptide (M(r) 19K). Moreover, antibodies that specifically recognize this peptide block induction of motor neurons by the notochord. We propose that Shh acts as a morphogen to induce distinct ventral cell types in the vertebrate central nervous system.