Members of the TGFB superfamily of growth factors play a central role in the establishment of axial pattern in the early vertebrate embryo. We have used the Xenopus embryo as a model system to identify a novel maternal transcription factor, FAST-1, that is a key regulator of the specification of the axial mesodermal gene program by TGFB ligands during early embryogenesis. We have also established that FAST-1 is regulated by direct interaction with Smads, a recently identified class of TGFB signal transducers. We will now examine: 1) how FAST-1 contributes to patterning of early embryonic gene expression 2) how FAST-1 is regulated, and 3) how the endogenous spatial and temporal patterns of Smad activation are regulated in the embryo. FAST-1 is necessary for the expression of a wide variety of early mesodermal genes, but we do not understand the contribution of transcriptional regulation by FAST-1 to specific patterns of endogenous gene expression. We will use transgenic analysis of FAST-1 regulated genomic reporters to investigate the role of FAST-1 in the regulation of complex patterns of mesodermal gene expression in response to endogenous inducing signals. Correct developmental patterning depends as much on the competence of embryonic cells to respond to specific signals as it does on the correct localization of the signals themselves. We have identified several ways in which the competence of embryonic cells to respond to inducing signals is regulated. One involves the regulation of FAST-Smad complexes at the start of gastrulation, a second involves promoter-specific regulation of responsiveness to activated FAST-1 after the end of gastrulation, a third involves interaction between FGF and TGFB signals in the pre-gastrula embryo. We plan to determine the mechanistic basis for these various mechanisms for competence regulation to clarify the fundamental problem of how single intercellular signals can regulate different developmental events as embryogenesis progresses. Although we know that Smads are important for early embryonic patterning, we do not know where and when these regulators are active in early development. We have developed a new approach to the study of the endogenous state of activation of Smads that is applicable to early embryos. Investigation of the regulation of endogenous Smad activity will allow us to critically test a variety of hypotheses concerning the role of TGFBs in early development.
