Identification and functional characterization of PTK7 ligands in Xenopus laevis

Abstract

Wnt signaling pathways are evolutionary conserved and regulate numerous processes in both developing and adult organisms. The canonical Wnt signaling pathway regulates cell proliferation and differentiation through changes in gene transcription. Non-canonical or Planar cell polarity (PCP) Wnt signaling pathway regulates cell polarization and migration through changes in the cytoskeleton organization. These pathways have both shared players like Wnt, Frizzled (Fz) and Disheveled (Dsh) and pathway-specific players. However, it is still unclear, how a cell distinguishes between different Wnt pathways. PTK7 (Protein tyrosine kinase 7) is a regulator of the PCP Wnt signaling pathway, which is required for neural tube closure and inner ear hair cell polarity in vertebrates. Several intracellular interaction partners of PTK7 have been described. For example, it has been shown that PTK7 recruits Dsh to the plasma membrane to regulate Xenopus neural crest migration, but upstream signals or PTK7 ligands have so far not been identified. Here we show that PTK7 can interact with canonical Wnt ligands through Frizzled, suggesting that PTK7 may also regulate canonical Wnt signaling. Indeed, PTK7 can inhibit canonical Wnt signaling in both Xenopus and human cell culture. Furthermore, epistasis experiments show that PTK7 inhibits canonical Wnt signaling upstream of Dsh on the level of Wnt/Fz level. The knock-down of PTK7 activates canonical Wnt signaling. The neural tube closure defects induced by knock-down of PTK7 are partially rescued by the inhibition of canonical Wnt signaling, further confirming the inhibitory effect of PTK7 on canonical Wnt pathway. In addition to suppression of canonical Wnt signaling, PTK7 induces an ATF2-mediated transcription in Xenopus, indicating that it activates non-canonical Wnt signaling. In summary, we suggest that PTK7 in Xenopus promotes non-canonical Wnt signaling by inhibiting canonical Wnt signaling through the interaction with Wnt and Fz.