XENOPUS NODAL-RELATED GENE THAT ACTS IN SYNERGY WITH NOGGIN TO INDUCECOMPLETE SECONDARY AXIS AND NOTOCHORD FORMATION

Using a paracrine assay to screen for signaling proteins that could re specify ectodermal tissue, we isolated a Xenopus gene related to the m ouse gene nodal, a member of the TGF beta superfamily, The gene is exp ressed in three regions in the early Xenopus embryo: first in the gast rula organizer, then in two stripes of cells flanking the posterior no tochord in late neurulae, and finally in lateral plate mesoderm restri cted to the left side of tailbud-stage embryos, Ectopic expression of the gene induces muscle formation in ectodermal explants and partial s econdary axes in whole embryos. Together with noggin, another secreted protein also present in the organizer, it induces notochord formation in ectodermal explants and complete secondary axes in whole embryos, These results suggest that the nodal-related gene may act together wit h noggin to induce axial pattern during gastrulation and also may play a role in left-right asymmetry generation in the post-gastrula embryo .