Na,K-ATPase 2 and Ncx4a regulate zebrafish left-right patterning

A conserved molecular cascade involving Nodal signaling that patterns the laterality of the lateral mesoderm in vertebrates has been extensively studied, but processes involved in the initial break of left-right (LR) symmetry are just beginning to be explored. Here we report that Na,K-ATPase 2 and Ncx4a function upstream of Nodal signaling to regulate LR patterning in zebrafish. Knocking down Na,K-ATPase 2 and Ncx4a activity in dorsal forerunner cells (DFCs), which are precursors of Kupffer's vesicle (KV), is sufficient to disrupt asymmetric gene expression in the lateral plate mesoderm and randomize the placement of internal organs, indicating that the activity of Na,K-ATPase 2 and Ncx4a in DFCs/KV is crucial for LR patterning. High-speed videomicroscopy and bead implantation experiments show that KV cilia are immobile and the directional fluid flow in KV is abolished in Na,K-ATPase 2 and Ncx4a morphants, suggesting their essential role in KV ciliary function. Furthermore, we found that intracellular Ca²⁺ levels are elevated in Na,K-ATPase 2 and Ncx4a morphants and that the defects in ciliary motility, KV fluid flow and placement of internal organs induced by their knockdown could be suppressed by inhibiting the activity of Ca²⁺/calmodulin-dependent protein kinase II. Together, our data demonstrate that Na,K-ATPase 2 and Ncx4a regulate LR patterning by modulating intracellular calcium levels in KV and by influencing cilia function, revealing a previously unrecognized role for calcium signaling in LR patterning.