Left-right asymmetry is formed in the basal bodies of the mouse node cilia in a cilia motility-dependent manner

Laterality of the shapes and arrangements of the visceral organs in mice is determined in the node, a small cavity found at the ventral side of 7.5 dpc (days post coitum) embryos. On the node cells, motile cilia which are tilted toward the posterior side of the embryos show clockwise movement and thus produce fluid flow in the node toward the left side of the embryos. This left-ward flow regulates left/right (L/R) asymmetric gene expressions and L/R asymmetric morphogenesis in later stages. Structurally, node cilia have the characteristics of primary cilia and their basal body (mother centriole) is accompanied by a daughter centriole. Here, to obtain insights into the process of symmetry breaking by node cilia, we investigated whether the structure of the cilia themselves have L/R asymmetry, and found that positions of the daughter centrioles become biased to the right side of the mother centrioles in a stage-dependent manner. We found that this L/R asymmetry of the basal bodies is absent in iv mutant mice, in which node cilia are immotile, suggesting that formation of this L/R asymmetry in the basal bodies requires cilia motility. It has been reported that culturing embryos in a flow chamber with artificial counter-flow, which is toward the opposite direction to the endogenous leftward flow in the node, results in reversed laterality of the visceral organs in later stages. However, we found that applying such artificial counter-flow did not reverse the L/R asymmetry of the basal bodies, and the daughter centrioles were still biased to the right side of the mother centrioles, suggesting that the L/R asymmetry of the basal bodies is formed independently from the direction of the fluid flow in the node and that it is independent from the laterality of the visceral organs. Although the biological significance of this phenomenon is unknown so far, these results suggest that node cilia have a previously unknown mechanism to produce L/R asymmetry in the basal bodies inside the cells in early development, independently from the canonical fluid flow-dependent L/R determining pathway.