Question

Left-right asymmetry is associated with higher levels of the TGF-B family signaling molecule Nodal on the left side of the embryo, relative to the right. What are the influences that led to this asymmetry of Nodal concentration? Include positive feedback, Notch signaling , Sonic hedgehog signaling and Lefty protein in your answer

Answer 1

Nodal is a secretory protein belonging to TGF-B superfamily and is responsible for left-right asymmetry in embryo. Initially, Nodal is present symmetrically on both sides in the embryo but it gets biased towards the left by leftward flow of nodal fluid. Since Nodal can activate its own expression resulting in its amplification via positive feedback loop and therefore concentrating it to the left side of the embryo. Several other factors influence the asymmetry of Nodal concentration on the left side. The establishment of membrane potential at node activates the notch signalling which in turn induces the expression of Nodal in the cells on the left side of embryo. Notch signalling, in combination with the signalling by the protein Sonic hedgehog (Shh) increases the expression of Nodal protein on the left side of the embryo. Nodal signalling is prevented on the right side of the embryo by the protein Lefty. Lefty is an antagonist of Nodal and it blocks the Nodal signalling on the right side of the embryo by preventing Nodal from binding to its receptor. All these factors results in left-sided asymmetry of Nodal in embryo.