Abstract
A subset of cardiac progenitors display multipotency to give rise to major cardiovascular lineages during cardiogenesis. In addition, we have recently identified Nkx2-5+ endocardial cells in the outflow and atria that contribute to definitive hematopoietic cells in vivo, which represents phylogenetically conserved pathway of cardio-vasculo-hematopoietic differentiation. Here, we have examined the molecular mechanism underlying the endocardial hematopoiesis. Immunostaining and colony assay suggest that Nkx2-5 mutant mice showed significant decrease in CD31+/41+ hemogenic endocardial cells, while lineage tracing on Nkx2-5-null background revealed that Nkx2-5-derived endocardial cells are present. These data indicate that Nkx2-5 is required not for the formation of the Nkx2-5-derived endocardium but for the hemogenic activity of the endocardium. Notch signaling is also active in cushion endocardium and its mutant embryos show a similar defect with Nkx2-5 mutants as previously reported. Inspired by these similarities between the function of Nkx2-5 and Notch signaling during cardiogenesis, we next investigated the impact of Notch activation on the endocardial hematopoiesis. In vivo forced activation of Notch in the developing heart using Nkx2-5-Cre mice led to massive increase in the endocardial hematopoiesis. As Nkx2-5 binding sites are found in the regulatory region of Notch1, RBPjk, and Jag1 genes, we tested whether Notch regulates endocardial hematopoiesis downstream of Nkx2-5. Surprisingly, Notch activation restored the defective endocardial hematopoiesis as well as endocardial cushion formation in Nkx2-5 knockout mice, suggesting that Notch signaling mediates the Nkx2-5-dependent endocardial hematopoiesis and cushion development. Drosophila dorsal vessel and lymph gland are closely related and regulated by tinman and notch. Therefore, Nkx2-5-Notch regulatory pathway represents phylogenetically conserved fundamental mechanism of cardio-vasculo-hematopoietic differentiation pathway.
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