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During mouse gastrulation pluripotent epiblast cells traverse the primitive streak (PS) and diversify into mesoderm and endoderm subtypes. The T-box transcription factor Eomesodermin (Eomes) is expressed during PS formation and is critical for the development of the cardiac mesoderm and definitive endoderm lineages. The earliest wave of progenitors to traverse the posterior PS give rise to extraembryonic mesoderm (ExM) that differentiates into the endothelium and hematopoietic progenitors of the yolk-sac (YS). How a specific subset of ExM becomes committed to a hematopoietic fate remains unclear. Here we report that Eomes is transiently expressed in Flk-1+ ExM progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell differentiation model of YS hematopoiesis, we find Eomes activity is essential for the production of primitive erythrocytes and pro-definitive hematopoietic progenitors but dispensable for the development of endothelial cells. Single-cell RNA-seq experiments demonstrate that in the absence of Eomes function Flk-1hi/PdgfRa- hematoendothelial progenitors are specified but fail to transition into hematopoietic cells. Strikingly, Eomes regulates Runx1 expression in the hemogenic endothelial (HE) lineage. Furthermore, ATAC-seq experiments demonstrate that the accessibility of enhancers, that SCL normally utilizes to specify primitive blood cells, are governed by Eomes functional activity. ChIP-seq experiments suggest that Eomes coordinates the development of hematopoietic mesoderm in the context of Activin/Nodal and Tead-Yap signalling. Finally, the potencies of Eomes+ progenitors that arise during gastrulation remain ill-defined; therefore, I additionally generate two temporally regulatable Eomes.CreERT2 lines that will be used for clonal lineage tracing experiments in the future. Collectively, these results demonstrate that Eomes promotes hemogenic competence of the YS mesodermal lineage by regulating Runx1 expression and SCL functional activity at the outset of gastrulation. These findings suggest that hemogenic competence is endowed earlier during murine embryogenesis than previously appreciated and have widespread implications for the generation of HE from pluripotent stem cell sources. |
