Conserved regulation of Nodal-mediated left-right patterning in zebrafish and mouse

Nodal is the major effector of left-right axis development. In mice, Nodal forms heterodimers with Gdf1 and is inhibited by Cerl2/Dand5 at the node, and by Lefty1 in the lateral plate mesoderm (LPM). Studies in zebrafish have suggested some parallels, but also differences, between left-right patterning in mouse and zebrafish. To address these discrepancies, we generated single and double zebrafish mutants for; southpaw; (; spaw; , the; Nodal; ortholog),; dand5; and; lefty1; , and performed biochemical and activity assays with Spaw and Vg1/Gdf3 (the Gdf1 ortholog). Contrary to previous findings,; spaw; mutants failed to initiate; spaw; expression in the LPM, and asymmetric heart looping was absent, similar to mouse; Nodal; mutants. In blastoderm assays, Vg1 and Spaw were interdependent for target gene induction, and contrary to previous results, formed heterodimers. Loss of Dand5 or Lefty1 caused bilateral; spaw; expression, similar to mouse mutants, and Lefty1 was replaceable with a uniform Nodal signaling inhibitor. Collectively, these results indicate that Dand5 activity biases Spaw-Vg1 heterodimer activity to the left, Spaw around Kupffer's vesicle induces the expression of; spaw; in the LPM and global Nodal inhibition maintains the left bias of Spaw activity, demonstrating conservation between zebrafish and mouse mechanisms of left-right patterning.