Hedgehog signalling is required for maintenance of myf5 and myoD expression and timely terminal differentiation in zebrafish adaxial myogenesis

Hedgehog proteins have been implicated in the control of myogenesis in the medial vertebrate somite. In the mouse, normal epaxial expression of the my ogenic transcription factor gene myf5 is dependent on Sonic hedgehog. Here we examine in zebrafish the interaction between Hedgehog signals, the expre ssion of myoD family genes, including the newly cloned zebrafish myf5, and slow myogenesis. We show that Sonic hedgehog is necessary for normal expres sion of both myf5 and myoD in adaxial slow muscle precursors, but not in la teral paraxial mesoderm. Expression of both genes is initiated normally in rostral presomitic mesoderm in sonic you mutants, which lack all Sonic hedg ehog. Similar initiation continues during tailbud outgrowth when the cells forming caudal somites are generated. However, adaxial cells in sonic you e mbryos are delayed in terminal differentiation and caudal adaxial cells fai l to maintain myogenic regulatory factor expression. Despite these defects, other signals are able to maintain, or reinitiate, some slow muscle develo pment in sonic you mutants. In the cyclops mutant, the absence of floorplat e-derived Tiggywinkle hedgehog and Sonic hedgehog has no discernible effect on slow adaxial myogenesis. Similarly, the absence of notochord-derived So nic hedgehog and Echidna hedgehog in mutants lacking notochord delays, but does not prevent, adaxial slow muscle development. In contrast, removal of both Sonic hedgehog and a floorplate signal, probably Tiggywinkle hedgehog, from the embryonic midline in cyclops;sonic you double mutants essentially abolishes slow myogenesis. We conclude that several midline signals, likel y to be various Hedgehogs, collaborate to maintain adaxial slow myogenesis in the zebrafish embryo. Moreover, the data demonstrate that, in the absenc e of this required Hedgehog signalling, expression of myf5 and myoD is insu fficient to commit cells to adaxial myogenesis. (C) 2001 Academic Press.