THE TERATOGENIC VERATRUM ALKALOID CYCLOPAMINE INHIBITS SONIC HEDGEHOGSIGNAL-TRANSDUCTION

The steroidal alkaloid cyclopamine produces cyclopia and holoprosencep haly when administered to gastrulation-stage amniote embryos. cyclopam ine-induced malformations in chick embryos are associated with interru ption of Sonic hedgehog (Shh)-mediated dorsoventral patterning of the neural tube and somites. Cell types normally induced in the ventral ne ural tube by Shh are either absent or appear aberrantly at the ventral midline after cyclopamine treatment, while dorsal cell types normally repressed by Shh appear ventrally. Somites in cyclopamine-treated emb ryos show Pax7 expression throughout, indicating failure of sclerotome induction. Cyclopamine at concentrations of 20-100 nM blocks the resp onse of neural plate explants to recombinant Shh-N in a dose-dependent manner. Similar concentrations have no effect on the post-translation al modification of Shh by cholesterol in transfected COS-1 cells. Comp arison of the effects of cyclopamine to those of the holoprosencephaly -inducing cholesterol synthesis inhibitor AY-9944 shows that cyclopami ne does not induce malformations by interfering with cholesterol metab olism. Although AY-9944 does not interrupt Shh signaling in ovo, it bl ocks the response to Shh-N in explants cultured without an exogenous c holesterol source. As predicted by current models of the regulation of cholesterol metabolism, the response to Shh-N in AY-9944-treated expl ants is restored by providing exogenous cholesterol, However, exogenou s cholesterol does not restore Shh signaling in cyclopamine-treated ex plants. These findings suggest that cyclopamine-induced teratogenesis is due to a more direct antagonism of Shh signal transduction.