2018 – EOMESODERMIN GOVERNS THE HEMOGENIC COMPETENCE OF MURINE YOLK-SAC MESODERMAL PROGENITORS

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The gene regulatory networks that coordinate hematopoietic commitment in the developing murine yolk-sac (YS) remain ill-defined. Here we report that the T-box transcription factor Eomesodermin (Eomes) is transiently expressed in mesodermal progenitors that generate virtually all YS hematopoietic and endothelial cells. Using an embryonic stem cell (ESC) differentiation system, we find that Eomes activity is essential for the production of primitive erythrocytes and definitive hematopoietic progenitors but dispensable for the development of endothelial cells. Bulk RNA-seq and single-cell-RNA-seq experiments demonstrate that in the absence of Eomes function Flk-1+ hematovascular mesoderm is specified but upon further differentiation it is diverted towards an endothelial rather than hematopoietic fate. Utilizing ESC reporter lines, we show that Eomes is expressed prior to both SCL and Runx1 during mesoderm patterning. Interestingly, Eomes activity is dispensable for the expression of SCL but is essential for the normal development of Runx1+ hemogenic endothelium (HE). ATAC-Seq experiments reveal that Eomes governs the accessibility of numerous hematopoietic enhancers that SCL normally utilizes to specify the hematopoietic fate. Finally, ChIP-seq experiments suggest that Eomes coordinates the development of hemogenic competent mesoderm in the context of Activin/Nodal and Tead-Yap signalling at mesodermal stages of development. Collectively, these experiments demonstrate that Eomes sits at the top of the transcriptional hierarchy, functioning upstream of Runx1 expression and SCL functional activity, to promote hemogenic competence of the YS mesodermal lineage. These results 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.