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 concentrations of Brachyury. Animal pole explants from embryos injected with low doses of Xbra RNA differentiate into vesicles containing mesothelial smooth muscle and mesenchyme. At higher concentrations somitic muscle is formed. The transition from smooth muscle formation to that of somitic muscle occurs over a two-fold increase in Brachyury concentration. Brachyury is required for differentiation of notochord in mouse and fish embryos, but even the highest concentrations of Brachyury do not induce this tissue in Xenopus animal caps. Co-expression of Brachyury with 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 transcription factors for the ability to synergize with Brachyury. We find that co-expression of Pintallavis, but not goosecoid, with Brachyury causes formation of dorsal mesoderm, including notochord. Furthermore, the effect of Pintallavis, like that of Brachyury, is dose-dependent: a two-fold increase in Pintallavis RNA causes a transition from ventral mesoderm formation to that of muscle, and a further two-fold increase induces notochord and neural tissue. These results suggest that Pintallavis cooperates with Brachyury to pattern the mesoderm in Xenopus.