Function of the two Xenopus Smad4s in early frog development

Signals from the transforming growth factor family members are transmitted in the cell through specific receptor-activated Smads and a common partner Smad4. Two Smad4 genes ( and /10, or smad4 and smad4.2) are isolated from Xenopus, and conflicting data are reported for Smad4/10 actions in mesodermal and neural induction. To further understand the functions of the Smad4s in early frog development, we analyzed their activities in detail. We report that Smad10 is a mutant form of Smad4 that harbors a missense mutation of a conserved arginine to histidine in the MH1 domain. The mutation results in enhanced association of Smad10 with the nuclear transcription corepressor Ski and leads to its neural inducing activity through inhibition of BMP signaling. In contrast to Smad10, both Smad4 and Smad4 enhanced BMP signals in ectodermal explants. Using antisense morpholino oligonucleotides (MOs) to knockdown endogenous Smad4 protein levels, we discovered that Smad4 was required for both activin- and BMP-mediated mesodermal induction in animal caps, while Smad4 affected only the BMP signals. Neither Smad4 was involved directly in neural induction. Expression of Smad4-MO in early frog embryos resulted in reduction of mesodermal markers and defects in axial structures, which were rescued by either Smad4 or Smad4. Smad4-MO induced only minor deficiency at late stages. As Smad4 but not Smad4, is expressed at high levels maternally and during early gastrulation, our data suggest that while Smad4 and Smad4 may have similar activities, they are differentially utilized during frog embryogenesis, with only Smad4 essential for mesoderm induction.