Much remains unknown about the signals that induce early mesoderm to initiate hematopoietic differentiation. Here we show that endoglin (Eng), a receptor for the TGFβ superfamily, identifies all cells with hematopoietic fate in the early embryo. These arise in an Eng+Flk1+ mesodermal precursor population at E7.5, a cell fraction also endowed with endothelial potential. In Eng knockout embryos, hematopoietic colony activity and numbers of CD71+Ter119+ erythroid progenitors were severely reduced. This coincided with severely reduced expression of embryonic globin and key BMP target genes including the hematopoietic regulators Scl, Gata1, Gata2 and Msx-1. To interrogate molecular pathways active in the earliest hematopoietic progenitors, we applied transcriptional profiling to sorted cells from E7.5 embryos. Eng+Flk-1+ progenitors co-expressed TGFβ and BMP receptors and target genes. Furthermore, Eng+Flk-1+ cells presented high levels of phospho-SMAD1/5, indicating active TGFβ and/or BMP signaling. Remarkably, under hematopoietic serum-free culture conditions, hematopoietic outgrowth of endoglin-expressing cells was dependent on TGFβ superfamily ligands: BMP4, BMP2, or TGF-β1. These data demonstrate that the E+F+ fraction at E7.5 represents mesodermal cells competent to respond to TGFb1, BMP4, or BMP2, shaping their hematopoietic development, and that endoglin is a critical regulator in this process by modulating TGF/BMP signaling.
Overall design: E7.5 pooled embryos (25 litters; 300 embryos approximately) were dissected and 3,000 cells were sorted in triplicate for Eng-Flk1-, Eng-Flk1+, Eng+Flk1+, and Eng+Flk1- fractions. Microarray results were analyzed with GeneSpring GX software.
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