Two papers in this issue shed light on the establishment and functions of planar cell polarity (PCP). The PCP pathway, which regulates cell orientation within the plane of an epithelium, was first described in Drosophila:activated Frizzled (Fz) receptors trigger the redistribution of core group PCP proteins, which include Dishevelled (Dvl) and Flamingo (Fmi); these then specify the orientation of cells and associated bristles. Frank Laski and colleagues (see p. 1789) have identified a new component of the PCP pathway - a Drosophila homologue of the actin depolymerization factor cofilin called Twinstar (Tsr) - that they find is required for Fz and Fmi redistribution during PCP establishment. What does this tell us about the mechanisms that govern the redistribution of core proteins? The authors propose that Tsr-dependent actin reorganisation is triggered by a gradient of activated Fz across the cell, leading to the asymmetric accumulation of Fz and other core proteins that might be either stabilised by actin filaments or transported via an actin-dependent pathway. Asymmetric Fz then signals further reorganisation of actin filaments into future bristles. The authors discuss other factors that might be required for actin cytoskeleton reorganisation in PCP, including the small GTPase Rho.
The PCP pathway is conserved, and homologous pathways have been described in Xenopus (where it regulates convergent extension), in zebrafish,and recently in the mouse (where it orientates cochlea sensory hairs). On p. 1767, Anthony Wynshaw-Boris and colleagues show that in the mouse, Dvl homologues - Dvl1 and Dvl2 - regulate a coordinated lengthening and narrowing of the neural plate,which strongly resembles Xenopus convergent extension. Furthermore,the authors introduced into the mouse a Dvl2 transgene carrying a point mutation identical to the Dsh1 allele that abolishes the PCP pathway in the fly. Following this, Dvl2 was no longer able to regulate convergent extension or the polarity of cochlea sensory hair cells,demonstrating remarkable conservation between PCP in the fly and convergent extension in the mouse.