BONE MORPHOGENETIC PROTEIN-2 IN THE EARLY DEVELOPMENT OF XENOPUS-LAEVIS

The temporal and spatial transcription patterns of the Xenopus laevis Bone morphogenetic protein 2 (BMP-2) gene have been investigated. Unli ke the closely related BMP-4 gene, the BMP-2 gene is strongly transcri bed during oogenesis. Besides some enrichment within the animal half, maternal BMP-2 transcripts are ubiquitously distributed in the early c leavage stage embryos but rapidly decline during gastrulation. Zygotic transcription of this gene starts during early neurulation and transc ripts are subsequently localized to neural crest cells, olfactory plac odes, pineal body and heart anlage. Microinjection of BMP-2 RNA into t he two dorsal blastomeres of 4-cell stage embryos leads to ventralizat ion of developing embryos. This coincides with a decrease of transcrip ts from dorsal marker genes (beta-tubulin, alpha-actin) but not from v entral marker genes (alpha-globin). BMP-2 overexpression inhibits tran scription of the early response gene XFD-1, a fork head/HNF-3 related transcription factor expressed in the dorsal lip, but stimulates trans cription of the posterior/ventral marker gene Xhox3, a member of the h elix-turn-helix family. Activin A incubated animal caps from BMP-2 RNA injected embryos show transcription of ventral but an inhibition of d orsal marker genes; thus, BMP-2 overrides the dorsalizing activity of activin A. The results demonstrate that BMP-2 overexpression exerts ve ry similar effects as have previously been described for BMP-4, and th ey suggest that BMP-2 may act already as a maternal factor in ventral mesoderm formation.