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 ce ntral nervous system depends on cell interactions(1). For example, ind uction of a specialized population of ventral midline cells, the Boor plate, appears to require contact-mediated signalling by the underlyin g notochord, whereas diffusible signals from the notochord and floor p late can induce ventrolaterally positioned motor neurons. Sonic hedgeh og (Shh), a vertebrate hedgehog-family member, is processed to generat e two peptides (M(r) 19K and 26/27K) which are secreted by both of the se organizing centres(2,30). Moreover, experiments in a variety of ver tebrate embryos(3-5), and in neural explants in vitro(5), indicate tha t Shh can mediate floor-plate induction. Here we have applied recombin ant Shh peptides to neural explants in serum-free conditions. High con centrations of Shh bound to a matrix induce floor plate and motor neur ons, and addition of Shh to the medium leads to dose-dependent inducti on of motor neurons. All inducing activity resides in a highly conserv ed amino-terminal peptide (M(r) 19K). Moreover, antibodies that specif ically 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.