FAST-1 is a key maternal effector of mesoderm inducers in the early Xenopus embryo

We have examined the role of the maternally encoded transcription factor FA ST-1 in the establishment of the mesodermal transcriptional program in Xeno pus embryos, FAST-1 has been shown to associate with Smad2 and Smad4, trans ducers of TGF beta superfamily signals, in response to stinulation by sever al TGF beta superfamily ligands, The FAST-1/Smad2/Smad4 complex binds and a ctivates a 50 bp activin responsive element identified in the promoter of t he meso-endodermal marker Mix.2. We have now used three complementary appro aches to demonstrate that FAST-1 is a central regulator of mesoderm inducti on by ectopic TGF beta superfamily ligands and during endogenous patterning : ectopic expression of mutationally activated FAST-1, ectopic expression o f dominant inhibitory FAST-1, and injection of a blocking antibody specific for FAST-1. Expression of constitutively transcriptionally active FAST-1 fusion protein (FAST-VP16(A)) in prospective ectoderm can directly induce the same set of general and dorsal mesodermal genes, as well as some endodermal genes, as are induced by activin or Vg1, In intact embryos, this construct can induce secondary axes similar to those induced by activin or Vg1, Conversely, exp ression of a FAST-1-repressor fusion (FAST-En(R)) in prospective ectoderm b locks induction of mesodermal genes by activin, while expression of FAST-En (R) in intact embryos prevents general/dorsal mesodermal gene expression an d axial development, Injection of a blocking antibody specific for FAST-1 p revents induction of mesodermal response genes by activin or Vg1, but not b y FGF. In intact embryos, this antibody can prevent the expression of early mesodermal markers and inhibit axis formation, demonstrating that FAST-1 i s a necessary component of the first steps in the specification of mesoderm .