Evidence for the regulation of left-right asymmetry in Ciona intestinalis by ion flux

Vertebrate embryos develop distinct left-right asymmetry under the control of a conserved pathway involving left-sided deployment of the nodal and Pitx2 genes. The mechanism that initiates asymmetric expression of these genes is less clear, with cilia, ion flux, and signalling molecules all implicated. Vertebrates share the chordate phylum with urochordates such as the sea squirt Ciona intestinalis. We have explored the role of ion flux in regulating left-right asymmetry in Ciona, using an assay in which perturbation of left-sided Ci-Pitx expression provides a read-out for the disruption of asymmetry. Our data show that omeprazole, which specifically inhibits H(+)K(+)ATPase activity, disrupts asymmetry in Ciona. The vertebrate H(+)K(+)ATPase is composed of two subunits, alpha and beta. We identified one Ciona beta ortholog and two Ciona alpha orthologs of the vertebrate H(+)K(+)ATPase genes, and show that one of these is expressed in dorsal and ventral embryonic midline cells shortly before the activation of left-sided Ci-Pitx expression. Furthermore, we show that omeprazole exerts its effect on asymmetry at this point in development, and additionally implicate K+ channels in the regulation of asymmetry in Ciona. These experiments demonstrate a role for ion flux in the regulation of asymmetry in Ciona, and show a conserved, ancestral role for the H(+)K(+)ATPase ion pump in this process.