Cardiac differentiation in Xenopus is initiated by mespa

Aims Future cardiac repair strategies will require a profound understanding of the principles underlying cardiovascular differentiation. Due to its extracorporal and rapid development, Xenopus laevis provides an ideal experimental system to address these issues in vivo. Whereas mammalian MesP1 is currently regarded as the earliest marker for the cardiovascular system, several MesP1-related factors from Xenopus - mespa, mespb and mespo - have been assigned only to somitogenesis so far. We therefore analysed these genes comparatively for potential contributions to cardiogenesis. Methods and Results RNA in situ hybridisations revealed a novel anterior expression domain exclusively occupied by mespa during gastrulation, which precedes the prospective heart field. Correspondingly, when overexpressed mespa most strongly induced cardiac markers in vivo as well as ex vivo. Transference to murine embryonic stem cells (ES) cells and subsequent FACS analyses for Flk-1 and TroponinI confirmed the high potential of mespa as cardiac inducer. In vivo, Morpholino based knockdown of mespa protein led to a dramatic loss of procardiac and sarcomeric markers, which could be rescued either by mespa itself or human MesP1, but neither by mespb nor mespo. Epistatic analysis positioned mespa upstream of mespo and mespb, and revealed positive autoregulation for mespa at the time of its induction. Conclusions Our findings contribute to the understanding of conserved events initiating vertebrate cardiogenesis. We identify mespa as functional amphibian homolog of mammalian MesP1. These results will enable the dissection of cardiac specification from the very beginning in the highly versatile Xenopus system.