Interpretation of BMP signaling in early Xenopus development.

Very little is known about how the extracellular binding of a morphogen is transduced to the nucleus of a cell in a concentration-related way, enabling cells to interpret their position in a concentration gradient. Here, we have analyzed when and how Xenopus embryo cells perceive and interpret a BMP signal. Dissociated embryo cells are exposed for short times to different concentrations of BMP4. We find that cells are already competent to receive a BMP4 signal at the blastula stage. They phosphorylate Smad1 very rapidly and express downstream genes less than half an hour after exposure to BMP. However, Smad1 is present in the nucleus even in the absence of BMP. To quantitate intracellular signaling after BMP exposure, we have constructed a chimeric type I receptor that registers BMP signaling as the intranuclear migration of Smad2, and as the transcription of Smad2 downstream genes. The combination of the chimeric receptor and GFP-Smad2 makes it possible to follow the transduction of BMP signaling to the nucleus. From our results, we conclude that an extracellular BMP concentration is interpreted by the steady state nuclear concentration of phosphorylated Smad1.