In the mammalian cochlea, V-shaped hair bundles (rows of actin-based stereocilia) on sensory hair cells convert sound energy into electrical signals. The hair cells display uniform planar polarity, which is necessary for correct sound perception and is controlled by non-canonical Wnt/planar cell polarity (PCP) signalling at the tissue level. But how is the V-shape of hair bundles established? On , Conor Sipe and Xiaowei Lu report that the microtubule motor subunit Kif3a regulates hair cell planar polarisation in mice through both ciliary and non-ciliary mechanisms. They show that Kif3a disruption in the inner ear leads to the absence of the kinocilium (a specialised primary cilium), flattened hair bundle morphology and uncoupling of hair bundle orientation from basal body positioning. Moreover, they report, Kif3a coordinates the planar polarity of hair bundles and hair cell centrioles through localised p21-activated kinase (PAK) activation on the hair cell cortex. These results suggest that Kif3-mediated hair cell intrinsic polarity pathways and PCP signalling converge on PAK to regulate hair cell polarity.
