PATTERNING OF THE MESODERM IN XENOPUS - DOSE-DEPENDENT AND SYNERGISTIC EFFECTS OF BRACHYURY AND PINTALLAVIS

Widespread expression of the DNA-binding protein Brachyury in Xenopus animal caps causes ectopic mesoderm formation. In this paper, we first show that two types of mesoderm are induced by different concentratio ns of Brachyury. Animal pole explants from embryos injected with low d oses of Xbra RNA differentiate into vesicles containing mesothelial sm ooth muscle and mesenchyme. At higher concentrations semitic muscle is formed. The transition from smooth muscle formation to that of semiti c muscle occurs over a two-fold increase in Brachyury concentration. B rachyury is required for differentiation of notochord in mouse and fis h embryos, but even the highest concentrations of Brachyury do not ind uce this tissue in Xenopus animal caps. Go-expression of Brachyury wit h the secreted glycoprotein noggin does cause notochord formation, but it is difficult to understand the molecular basis of this phenomenon without knowing more about the noggin signal transduction pathway. To overcome this difficulty, we have now tested mesoderm-specific transcr iption factors for the ability to synergize with Brachyury. We find th at coexpression of Pintallavis, but not goosecoid, with Brachyury caus es formation of dorsal mesoderm, including notochord. Furthermore, the effect of Pintallavis, like that of Brachyury, is dose-dependent: a t wo-fold increase in Pintallavis RNA causes a transition from ventral m esoderm formation to that of muscle, and a further two-fold increase i nduces notochord and neural tissue. These results suggest that Pintall avis cooperates with Brachyury to pattern the mesoderm in Xenopus.