PTK7 localization and protein stability is affected by canonical Wnt ligands

Protein tyrosine kinase 7 (PTK7) is an evolutionarily conserved transmembrane receptor with important roles in embryonic development and disease. Originally identified as a gene upregulated in colon cancer, it was later shown to regulate planar cell polarity (PCP) and directional cell movement. PTK7 is a Wnt co-receptor; however, its role in Wnt signaling remains controversial. Here, we find evidence that places PTK7 at the intersection of canonical and non-canonical Wnt signaling pathways. In presence of canonical Wnt ligands PTK7 is subject to caveolin-mediated endocytosis, while it is unaffected by non-canonical Wnt ligands. PTK7 endocytosis is dependent on the presence of the PTK7 co-receptor Fz7 (also known as Fzd7) and results in lysosomal degradation of PTK7. As we previously observed that PTK7 activates non-canonical PCP Wnt signaling but inhibits canonical Wnt signaling, our data suggest a mutual inhibition of canonical and PTK7 Wnt signaling. PTK7 likely suppresses canonical Wnt signaling by binding canonical Wnt ligands thereby preventing their interaction with Wnt receptors that would otherwise support canonical Wnt signaling. Conversely, if canonical Wnt proteins interact with the PTK7 receptor, they induce its internalization and degradation.