Slow muscle induction by Hedgehog signalling in vitro

Muscles are composed of several fibre types, the precise combination of whi ch determines muscle function, Whereas neonatal and adult fibre tape is inf luenced by a number of extrinsic factors, such as neural input and muscle l oad, there is little knowledge of how muscle cells are initially determined in the early embryo. In the zebrafish, fibres of the slow twitch class ari se from precociously specified myoblasts that lie close to the midline wher eas the remainder of the myotome differentiates as fast myosin expressing m uscle. In vivo evidence has suggested the Sonic Hedgehog glycoprotein, secr eted from the notochord, controls the formation of slow twitch and fast twi tch muscle fates. sere we describe an in vitro culture system that we have developed to test directly the ability of zebrafish myoblasts to respond to exogenous Sonic Hedgehog peptide. We find that Sonic Hedgehog peptide can control the binary cell fate choice of embryonic zebrafish myoblasts in vit ro. We have also used this culture system to assay the relative activities of different Hedgehog-family proteins and to investigate the possible invol vement of heterotrimeric G-proteins in Hedgehog signal transduction.