BMP-4 Regulates the Dorsal–Ventral Differences in FGF/MAPKK-Mediated Mesoderm Induction inXenopus

AbstractRecent studies onXenopusdevelopment have revealed an increasingly complex array of inductive, prepatterning, and competence signals that are necessary for proper mesoderm formation. In this study, we establish that fibroblast growth factor (FGF) signals through mitogen-activated protein kinase kinase (MAPKK) to induce mesodermal gene expression. We demonstrate that a partially activated form of MAPKK restores expression of the mesodermal genesXcad-3andXbra,eliminated by the dominant-negative FGF receptor (ΔFGFR). Similar to the results reported earlier with ΔFGFR, expression of a dominant-negative form of MAPKK (MAPKKD) preferentially eliminates the dorsal expression ofXcad-3andXbra.We tested whether the regional localization of bone morphogenetic protein-4 (BMP-4) could explain why both MAPKKD and ΔFGFR eliminate the dorsal and not the ventral expression ofXcad-3andXbra.We show that ectopic expression of BMP-4 is sufficient to maintain the dorsal expression ofXcad-3andXbrain embryos containing ΔFGFR and that expression of a dominant-negative BMP receptor reduces the dorsal–ventral differences in ΔFGFR embryos. These results indicate that regional localization of BMP-4 is responsible for the dorsal–ventral asymmetry in FGF/MAPKK-mediated mesoderm induction