XENOPUS SMAD8 ACTS DOWNSTREAM OF BMP-4 TO MODULATE ITS ACTIVITY DURING VERTEBRATE EMBRYONIC PATTERNING

Bone morphogenetic proteins (BMPs) participate in the development of n early all organs and tissues, BMP signaling is mediated by specific Sm ad proteins, Smad1 and/or Smad5, which undergo serine phosphorylation in response to BMP-receptor activation and are then translocated to th e nucleus where they modulate transcription of target genes. We have i dentified a distantly related member of the Xenopus Smad family, Smad8 , which lacks the C-terminal SSXS phosphorylation motif present in oth er Smads, and which appears to function in the BMP signaling pathway. During embryonic development, the spatial pattern of expression of Sma d8 mirrors that of BMP-4. We show that an intact BMP signaling pathway is required for its expression. Overexpression of Smad8 in Xenopus em bryos phenocopies the effect of blocking BMP-4 signaling, leading to i nduction of a secondary axis on the ventral side of intact embryos and to direct neural induction in ectodermal explants. Furthermore, Smad8 can block BMP-4-mediated induction of ventral mesoderm-specific gene expression in ectodermal explants. Overexpression of Smad8 within dors al cells, however, causes patterning defects that are distinct from th ose reported in BMP-4-deficient embryos, suggesting that Smad8 may int eract with additional signaling pathways. Indeed, overexpression of Sm ad8 blocks expression of Xbra in whole animals, and partially blocks a ctivin signaling in animal caps. In addition, Smad8 inhibits involutio n of mesodermal cells during gastrulation, a phenotype that is not obs erved following blockade of activin or BMPs in Xenopus. Together, thes e results are consistent with the hypothesis that Smad8 participates i n a negative feedback loop in which BMP signaling induces the expressi on of Smad8, which then functions to negatively modulate the amplitude or duration of signaling downstream of BMPs and, possibly, downstream of other transforming growth factor-beta (TGF-beta) family ligands.