Patterning of the Dorsal-Ventral Axis in Echinoderms: Insights into the Evolution of the BMP-Chordin Signaling Network
Figure 11
A model for morphogen gradient formation in the sea urchin embryo.
(A) chordin and bmp2/4 are both expressed in the ventral ectoderm downstream of Nodal while the presumed Chordin protease Tolloid/BMP1 [83], the BMP2/4 receptor alk3/6, and glypican 5 are expressed widely throughout the ectoderm at blastula stages. BMP2/4 ligands expressed on the ventral side of the embryo are immediately complexed with Chordin and cannot bind to their receptors. Therefore, on the ventral side of the embryo, no free BMP2/4 ligand can bind to BMP receptors since an excess of Chordin is able to constantly reform Chordin/BMP2/4 complexes. (B) The Chordin/BMP2/4 complexes produced on the ventral side of the embryo can nevertheless diffuse towards the dorsal side of the embryo where Tolloid/BMP1 releases free BMP2/4 ligands that, in the absence of Chordin, can bind to their receptors and trigger Smad1/5/8 phosphorylation and nuclearization. BMP signaling on the dorsal side up-regulates expression of glypican 5, which may facilitate BMP2/4 mobility and BMP2/4 binding to its receptor, thereby reinforcing BMP2/4 signaling via a positive feedback loop.