Cerberus-Nodal-Lefty-Pitx signaling cascade controls left-right asymmetry in amphioxus.
Li G., Liu X., Xing C., Zhang H., Shimeld SM., Wang Y.
Many bilaterally symmetrical animals develop genetically programmed left-right asymmetries. In vertebrates, this process is under the control of Nodal signaling, which is restricted to the left side byNodalantagonists Cerberus and Lefty. Amphioxus, the earliest diverging chordate lineage, has profound left-right asymmetry as a larva. We show thatCerberus,Nodal,Lefty, and their target transcription factorPitxare sequentially activated in amphioxus embryos. We then address their function by transcription activator-like effector nucleases (TALEN)-based knockout and heat-shock promoter (HSP)-driven overexpression. Knockout ofCerberusleads to ectopic right-sided expression ofNodal,Lefty, andPitx, whereas overexpression ofCerberusrepresses their left-sided expression. Overexpression of Nodal in turn repressesCerberusand activatesLeftyandPitxectopically on the right side. We also show Lefty repressesNodal, whereasPitxactivatesNodalThese data combine in a model in which Cerberus determines whether the left-sided gene expression cassette is activated or repressed. These regulatory steps are essential for normal left-right asymmetry to develop, as when they are disrupted embryos may instead form two phenotypic left sides or two phenotypic right sides. Our study shows the regulatory cassette controlling left-right asymmetry was in place in the ancestor of amphioxus and vertebrates. This includes the Nodal inhibitors Cerberus and Lefty, both of which operate in feedback loops withNodaland combine to establish asymmetricPitxexpression.CerberusandLeftyare missing from most invertebrate lineages, marking this mechanism as an innovation in the lineage leading to modern chordates.