Nodal ligands are essential for the patterning of chordate embryos. Genetic evidence indicates that EGF-CFC factors are required for Nodal signaling, but the molecular basis for this requirement is unknown. We have investigated the role of Cripto, an EGF-CFC factor, in Nodal signaling. We find that Cripto interacts with the type I receptor ALK4 via the conserved CFC motif in Cripto. Cripto interaction with ALK4 is necessary both for Nodal binding to the ALK4/ActR-IIB receptor complex and for Smad2 activation by Nodal. We also find that Nodal can inhibit BMP signaling by a Cripto-independent mechanism. Inhibition appears to be mediated by heterodimerization between Nodal and BMPs, indicating that antagonism between Nodal and BMPs can occur at the level of dimeric ligand production.
Bibliographical noteFunding Information:
We thank doctors Liliana Attisano, Ken Cho, Hiroshi Hamada, Joan Massagué, Elizabeth J. Robertson, Alexander F. Schier, Michael M. Shen, and Jeffrey L. Wrana for cDNA constructs. We thank Charles Farnsworth, Sandrine Faure, Tracy Keller, Kwang-Youl Lee, Michelle A. Lee, Maria Aleida Leza, Minoru Watanabe, and Stefan Wawersik for valuable comments and technical advice, and Fredrik O. Vannberg for excellent sequencing services. This work was supported by grants from NICHD and by a gift from the Yamanouchi Foundation to M.W.