The tumor suppressor SMAD4/DPC4 is essential for epiblast proliferation and mesoderm induction in mice

Members of the transforming growth factor (TGF)-beta superfamily have been shown to play a variety of important roles in embryogenesis, including dorsal and ventral mesoderm induction. The tumor suppressor SMAD4, also known as DPC4, is believed to be an essential factor that mediates TGF-beta signals. To explore functions of SMAD4 in development, we have mutated it by truncating its functional C-domain. We show that Smad4 is expressed ubiquitously during murine embryogenesis. Mice heterozygous for the Smad4(ex8/+) mutation are developmentally normal, whereas homozygotes die between embryonic day 6.5 (E6.5) and 8.5. All Smad4(ex8/ex8) mutants are developmentally delayed at E6 and show little or no elongation in the extraembryonic portion of late egg cylinder stage embryos. Consistent with this, cultured Smad4(ex8/ex8) blastocyst outgrowths suffer cellular proliferation defects and fail to undergo endoderm differentiation. Although a portion of mutant embryos at E8.5 show an increase in the embryonic ectoderm and endoderm, morphological and molecular analyses indicate that they do not form mesoderm. Altogether, these data demonstrate that SMAD4-mediated signals are required for epiblast proliferation, egg cylinder formation, and mesoderm induction.