PTK7: a cell polarity receptor with multiple facets.

PTK7 is a tyrosine kinase receptor implicated in planar cell polarity, a process with multiple implications at the cellular and organism levels. Loss of function of PTK7 leads to profound morphogenetic defects in the mouse, such as neural tube defects, misorientation of stereocilia in the inner ear, and impaired polarized cell movements. The planar cell polarity pathway is classically assigned to a non-canonical Wnt pathway, which does not rely on b-catenin transcriptional activity. We recently revealed that PTK7 is implicated in b-catenin-dependent developmental processes in mammalian and Xenopus systems. Based on data recently obtained by our group as well as others, we discuss how PTK7 could be involved in canonical and non-canonical Wnt pathways, and which implications are expected from these data in physiology and physiopathology.