Asymmetric distribution of dynamic calcium signals in the node of mouse embryo during left–right axis formation

In the node of mouse embryo, rotational movements of cilia generate an external liquid flow known as nodal flow, which determines left–right asymmetric gene expression. How nodal flow is converted into asymmetric gene expression is still controversial, but the increase of Ca2+ levels in endodermal cells to the left of the node has been proposed to play a role. However, Ca2+ signals inside the node itself have not yet been described. By our optimized Ca2+ imaging method, we were able to observe dynamic Ca2+ signals in the node in live mouse embryos. Pharmacological disruption of Ca2+ signals did not affect ciliary movements or nodal flow, but did alter the expression patterns of the Nodal and Cerl-2 genes. Quantitative analyses of Ca2+ signal frequencies and distributions showed that during left–right axis establishment, formerly symmetric Ca2+ signals became biased to the left side. In iv/iv mutant embryos that showed randomized laterality due to ciliary immotility, Ca2+ signals were found to be variously left-sided, right-sided, or bilateral, and thus symmetric on average. In Pkd2 mutant embryos, which lacked polycystin-2, a Ca2+-permeable cation channel necessary for left–right axis formation, the Ca2+ signal frequency was lower than in wild-type embryos. Our data support a model in which dynamic Ca2+ signals in the node are involved in left–right patterning.

► We found dynamic Ca2+ signals in the node in live mouse embryos. ► Disruption of Ca2+ signals affected asymmetric gene expression. ► During laterality formation, symmetric Ca2+ signals become biased to the left side. ► In iv/iv mutant embryos, Ca2+ signals were symmetric on average. ► In Pkd2 mutant embryos, Ca2+ signal frequency was lower than in wild-type embryos.