Role of Wnt signaling and planar cell polarity in left-right asymmetry.

Wnt signaling plays essential roles in multiple steps of left-right (L-R) determination in development. First, canonical Wnt signaling is required to form the node, where L-R symmetry breaking takes place. Secondly, planar cell polarity (PCP) driven by non-canonical Wnt signaling polarizes node cells along the anterio-posterior (A-P) axis and provides the tilt of rotating cilia at the node, which generate the leftward fluid flow. Thus, reciprocal expression of Wnt5a/5b and their inhibitors Sfrp1, 2, 5 generates a gradient of Wnt5 activity along the embryo's anterior-posterior (A-P) axis. This polarizes cells at the node, by placing PCP core proteins on the anterior or posterior side of each node cell. Polarized PCP proteins subsequently induce asymmetric organization of microtubules along the A-P axis, which is thought to push the centrally localized basal body toward the posterior side of a node cell. Motile cilia that extend from the posteriorly-shifted basal body is tilted toward the posterior side of the embryo. Thirdly, canonical-Wnt signaling regulates the level and expansion of Nodal activity and establishes L-R asymmetric Nodal activity at the node, the first molecular asymmetry in the mouse embryo. Overall, both canonical and non-canonical Wnt signalings are essential for L-R symmetry breaking.