Role of Goosecoid, Xnot and Wnt antagonists in the maintenance of the notochord genetic programme in Xenopus gastrulae

The Xenopus trunk organiser recruits neighbouring tissues into secondary tr unk axial and paraxial structures and itself differentiates into notochord. The inductive properties of the trunk organiser are thought to be mediated by the secretion of bone morphogenetic protein (BMP) antagonists. Ectopic repression of BMP signals on the ventral side is sufficient to mimic the in ductive properties of the trunk organiser. Resultant secondary trunks conta in somite and neural tube, but no notochord. We show that inhibition of BMP signalling is sufficient for the initiation of the trunk organiser genetic programme at the onset of gastrulation. Duri ng late gastrulation, however, this programme is lost, due to an invasion o f secreted Wnts from neighbouring tissues. Maintenance of this programme re quires co-repression of BMP and Wnt signalling within the presumptive notoc hord region. To shed light on the molecular cascade that leads to the repre ssion of the Wnt pathway, we looked for individual organiser genes whose ov erexpression could complement the inhibition of BMT signalling to promote n otochord formation in the secondary trunks. Two genes, gsc and Xnot, were t hus identified and shown to act in different ways. Xnot acts as a transcrip tional repressor within the mesodermal region. Gsc acts in deeper vegetal c ells, where it regulates Frzb expression to maintain Xnot expression in the neighbouring notochord territory. These results suggest that, during gastrulation, the necessary repression o f Wnt/beta -catenin signalling in notochord precursors is achieved by the a ction of secreted inhibitors, such as Frzb, emitted by gsc-expressing dorsa l vegetal cells.