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

The TGF-beta signal Nodal is the major effector of left-right axis development. In mice, Nodal forms heterodimers with Gdf1 and is restricted to the left side by Cerl2/Dand5. Nodal expression then propagates up the lateral plate mesoderm (LPM) by autoinduction, while Lefty1 inhibition maintains it on the left. Studies in zebrafish have suggested some parallels, but also differences, between the modes of left-right patterning in mouse and zebrafish. To address these discrepancies, we generated single and double zebrafish mutants for spaw (the Nodal ortholog), dand5 (the Cerl2 ortholog) 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 inducing target gene expression, and contrary to previous results, formed heterodimers. Loss of Dand5 or Lefty1 caused bilateral spaw expression, similar to mouse mutants, and premature expression and propagation of spaw in the LPM. Finally, Lefty1 inhibition of Nodal activity could be replaced by uniform exposure to a 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.