Wnt ligands signal in a cooperative manner to promote foregut organogenesis.

Endoderm-mesenchyme cross-talk is a central process in the development of foregut-derived organs. How signaling pathways integrate the activity of multiple ligands to guide organ development is poorly understood. We show that two Wnt ligands, Wnt2 and Wnt7b, cooperatively induce Wnt signaling without affecting the stabilization of the Wnt canonical effector β-catenin despite it being necessary for Wnt2-Wnt7b cooperativity. Wnt2-Wnt7b cooperation is specific for mesenchymal cell lineages and the combined loss of Wnt2 and Wnt7b leads to more severe developmental defects in the lung than loss of Wnt2 or Wnt7b alone. High-throughput small-molecule screens and biochemical assays reveal that the Pdgf pathway is required for cooperative Wnt2-Wnt7b signaling. Inhibition of Pdgf signaling in cell culture reduces Wnt2-Wnt7b cooperative signaling. Moreover, inhibition of Pdgf signaling in lung explant cultures results in decreased Wnt signaling and lung smooth-muscle development. These data suggest a model in which Pdgf signaling potentiates Wnt2-Wnt7b signaling to promote high levels of Wnt activity in mesenchymal progenitors that is required for proper development of endoderm-derived organs, such as the lung.