Pinhead antagonizes Admp to promote notochord formation

Dorsoventral patterning of a vertebrate embryo critically depends on the activity of Smad1 that mediates signaling by BMP proteins, anti-dorsalizing morphogenetic protein (Admp), and their antagonists. Pinhead (Pnhd), a cystine-knot-containing secreted protein, is expressed in the ventrolateral mesoderm during Xenopus gastrulation; however, its molecular targets and signaling mechanisms have not been fully elucidated. Our mass spectrometry-based screen of the gastrula secretome identified Admp as Pnhd-associated protein. We show that Pnhd binds Admp and inhibits its ventralizing activity by reducing Smad1 phosphorylation and its transcriptional targets. Importantly, Pnhd depletion further increased phospho-Smad1 levels in the presence of Admp. Furthermore, Pnhd synergized with Chordin and a truncated BMP4 receptor in the induction of notochord markers in ectoderm cells, and Pnhd-depleted embryos displayed notochord defects. Our findings suggest that Pnhd binds and inactivates Admp to promote notochord development. We propose that the interaction between Admp and Pnhd refines Smad1 activity gradients during vertebrate gastrulation.