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 embryo nic development of Xenopus laevis were investigated by using a functio nally defective mutant of the BMP-4 receptor (Delta mTFR11), which blo cks the BMP signaling pathway. Injection of Delta mTFR11 RNA into eith er the animal pale area or ventral marginal cells at the two-cell stag e induced a dorsal phenotype in the explant of ventral mesoderm with a nimal pole tissue from stage 10+ embryo, even though the normal fate o f this explant is a ''mesenchymal ball'' containing blood cells. These explants with the dorsal phenotype contained muscle, neural tissue, e ye capsule, and cement gland. Northern blot analysis showed an increas e of cardiac alpha-actin mRNA and a decrease of T alpha-globin mRNA ex pression, providing further evidence of a conversion from ventral to d orsal phenotype. Although injection of Delta mTFR11 RNA did not induce mesoderm in an animal cap culture, the same tissue injected with Delt a mTFR11 RNA can alter the differentiation fate of uninjected ventral mesodermal explant from ventral to dorsal type, suggesting specific in teraction of animal pole tissue and prospective ventral mesoderm in vi vo.