Randomization of left-right asymmetry due to loss of nodal cilia generating leftward flow of extraembryonic fluid in mice lacking KIF3B motor protein

Microtubule-dependent motor, murine KIF3B, was disrupted by gene targeting. The null mutants did not survive beyond midgestation, exhibiting growth re tardation, pericardial sac ballooning, and neural tube disorganization. Pro minently, the left-right asymmetry was randomized in the heart loop and the direction of embryonic turning, lefty-2 expression was either bilateral or absent. Furthermore, the node lacked monocilia while the basal bodies were present. Immunocytochemistry revealed KIF3B localization in wildtype nodal cilia. Video microscopy showed that these cilia were motile and generated a leftward flow. These data suggest that KIF3B is essential for the left-ri ght determination through intraciliary transportation of materials for cili ogenesis of motile primary cilia that could produce a gradient of putative morphogen along the left-right axis in the node.