Vertebrate laterality, which is manifested by asymmetrically placed organs , depends on asymmetric activation of the Nodal signaling cascade in the left lateral plate mesoderm . In fish, amphibians, and mammals, a cilia-driven leftward flow of extracellular fluid acts upstream of the Nodal cascade [3-6]. The direct target of flow has remained elusive. In Xenopus, flow occurs at the gastrocoel roof plate (GRP) in the dorsal midline of the embryo [4, 7]. The GRP is bordered by a second, bilaterally symmetrical Nodal expression domain . Here we identify the Nodal inhibitor Coco as a critical target of flow. Coco and Xenopus Nodal-related 1 (Xnr1) are coexpressed in the lateralmost ciliated GRP cells. Coco becomes downregulated on the left side of the GRP as a direct readout of flow. Ablation of flow prevented Coco repression, whereas Xnr1 expression was independent of flow. Loss of flow-induced laterality defects were rescued by knockdown of Coco on the left side. Parallel knockdown of Coco and Xnr1 in GRP cells restored laterality defects in flow-impaired embryos, demonstrating that Coco acted through GRP-expressed Xnr1. Coco thus acts as a critical target of flow, suggesting that symmetry is broken by flow-mediated left-asymmetric release of Nodal repression at the midline.
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