Planar polarization of node cells determines the rotational axis of node cilia.

Rotational movement of the node cilia generates a leftward fluid flow in the mouse embryo because the cilia are posteriorly tilted. However, it is not known how anterior-posterior information is translated into the posterior tilt of the node cilia. Here, we show that the basal body of node cilia is initially positioned centrally but then gradually shifts toward the posterior side of the node cells. Positioning of the basal body and unidirectional flow were found to be impaired in compound mutant mice lacking Dvl genes. Whereas the basal body was normally positioned in the node cells of Wnt3a(-/-) embryos, inhibition of Rac1, a component of the noncanonical Wnt signalling pathway, impaired the polarized localization of the basal body in wild-type embryos. Dvl2 and Dvl3 proteins were found to be localized to the apical side of the node cells, and their location was polarized to the posterior side of the cells before the posterior positioning of the basal body. These results suggest that posterior positioning of the basal body, which provides the posterior tilt to node cilia, is determined by planar polarization mediated by noncanonical Wnt signalling.