The homeobox gene Pitx2: mediator of asymmetric left-right signaling in vertebrate heart and gut looping

Left-right asymmetry in vertebrates is controlled by activities emanating f rom the left lateral plate. How these signals get transmitted to the formin g organs is not known. A candidate mediator in mouse, frog and zebrafish em bryos is the homeobox gene Pitx2, It is asymmetrically expressed in the lef t lateral plate mesoderm, tubular heart and early gut tube. Localized Pitx2 expression continues when these organs undergo asymmetric looping morphoge nesis, Ectopic expression of Xnr1 in the right lateral plate induces Pitx2 transcription in Xenopus. Misexpression of Pitx2 affects situs and morpholo gy of organs. These experiments suggest a role for Pitx2 in promoting loopi ng of the linear heart and gut.