The planar cell polarity (PCP) pathway controls the establishment of polarity within the plane of a sheet of cells. PCP was initially characterized in Drosophila, where it controls the arrangement of hair bristles and photoreceptors in the eye (reviewed in Maung and Jenny, 2011). In vertebrates, PCP regulates convergent extension (CE, a process by which a tissue narrows along one axis and lengthens along a perpendicular one), closure of the neural tube, hair orientation and inner ear development, among others (reviewed in Seifert and Mlodzik, 2007). Studies in Drosophila identified a core group of PCP genes including Frizzled (Fz), Flamingo (Fmi), Van Gogh (Vang), Dishevelled (Dsh), Prickle (Pk) and Diego (Dgo), whose products become asymmetrically localized in the cell upon initiation of PCP (reviewed Maung and Jenny, 2011). Subsequent studies in vertebrates have shown that many of these PCP genes are conserved.
Unlike in Drosophila, where the upstream signal for the PCP pathway has not been defined, in vertebrates, a number of so-called 'non-canonical' WNTs have been shown to have roles in PCP processes. WNT5B and WNT11 are both required for CE during gastrulation, and WNT5A physically and genetically interacts with VANGL2 in the inner ear and the developing limb bud (Heisenberg et al, 2000; Rauch et al, 1997; Qian et al, 2007; Gao et al, 2011). WNT ligand can be bound by one of a number of FZD receptors or the single pass transmembrane proteins RYK or ROR, depending on context (reviewed in Green et al, 2008; Fradkin et al, 2010). Although the downstream pathway is not well established, vertebrate PCP signaling appears to work at least in part through DVL, DAAM1 and small GTPases to remodel the actin cytoskeleton (reviewed in Lai et al, 2009; Gao et al, 2012).
