Abstract 19676: A Molecular Switch Regulating Heart and Pancreatic Development

The mesendoderm is thought to be a population of the intermediate and transient common progenitors prior to the emergence of the me soderm (Me) and the en doderm (En) in vitro ; however, it is unclear whether it exists in vivo , and if so, what are the molecular mechanisms that establish the subsequent Me or En fate of the mesendoderm. Here we show that Mesp1 transiently marks a subset of the epiblast at the p rimitive s treak (PS) initiation site at gastrulation in vivo . It is also evidenced by the cell fate mapping that the progeny of Mesp1 + progenitors gives rise to both the Me and the definitive En, but neither the primitive En nor the ectoderm, suggesting that Mesp1 + cells are the bio-potent mesendodermal progenitors. Mesp1 -fated dorsal foregut En (derived from the definitive En) subsequently contributes exclusively to the Pdx1 + /Foxa2 + progenitors in the p ancreatic b ud (PB), and ultimately to the endocrine cells in the pancreas. RNA-seq and ch romatin- i mmuno p recipitation of Mesp1 -fated cells show that Mesp1 directly binds to the endogenous regulatory elements of mesendodermal and endodermal modulators (e.g. Foxa2 ). Conditional ablation of Foxa2 in the Mesp1 -fated cells results in aberrant PB formation and postnatal death. In situ hybridization shows that ablation of Mesp1 abolishes the majority of mesendodermal modulators while it leads to ectopic expression of endodermal transcription factors in the distal portion of the PS, indicating that transient Mesp1 expression maintains bi-potency of the mesendoderm by inhibiting the endodermal transcription program. We generated chimera embryos by injecting into WT blastocysts the Mesp1 Cre/+ ; Rosa26-EYFP (control) or the Mesp1 Cre/Cre ; Rosa26-EYFP (mutant) ES cell lines (n=3/group). We found that, the mutant ESCs fail to contribute to the cardiac mesoderm, but instead they adopt the endodermal fate evidenced by their increased contribution to the pancreatic endocrine cells, indicating that the inhibition of the endodermal transcription program by Mesp1 in the mesendoderm progenitors is a cell-autonomous effect. Our results demonstrate that Mesp1 marks the bi-potent mesendoderm in vivo and functions as a critical molecular switch between the Me and the En formation via an orchestrating dual molecular mechanism.