A truncated bone morphogenetic protein 4 receptor alters the fate of ventral mesoderm to dorsal mesoderm: roles of animal pole tissue in the development of ventral mesoderm.

The biological effects of endogenous bone morphogenetic protein 4 (BMP-4), a member of the transforming growth factor beta family, on embryonic development of Xenopus laevis were investigated by using a functionally defective mutant of the BMP-4 receptor (delta mTFR11), which blocks the BMP signaling pathway. Injection of delta mTFR11 RNA into either the animal pole area or ventral marginal cells at the two-cell stage induced a dorsal phenotype in the explant of ventral mesoderm with animal pole tissue from stage 10+ embryo, even though the normal fate of this explant is a "mesenchymal ball" containing blood cells. These explants with the dorsal phenotype contained muscle, neural tissue, eye capsule, and cement gland. Northern blot analysis showed an increase of cardiac alpha-actin mRNA and a decrease of T alpha-globin mRNA expression, providing further evidence of a conversion from ventral to dorsal phenotype. Although injection of delta mTFR11 RNA did not induce mesoderm in an animal cap culture, the same tissue injected with delta mTFR11 RNA can alter the differentiation fate of uninjected ventral mesodermal explant from ventral to dorsal type, suggesting specific interaction of animal pole tissue and prospective ventral mesoderm in vivo.