The Nodal Inhibitor Coco Is a Critical Target of Leftward Flow in Xenopus

Vertebrate laterality, which is manifested by asymmetrically placed organs [ 1], depends on asymmetric activation of the Nodal signaling cascade in the left lateral plate mesoderm [ 2]. 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 [ 8]. 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. ► Nodal-expressing cells represent the somitic GRP and harbor unpolarized cilia ► GRP Nodal expression patterns are independent of leftward flow ► Coco is a critical target of flow on the left side of the GRP ► The left-sided Nodal cascade is induced by flow-mediated release of Coco repression