Recent studies support the existence of a common progenitor for the cardiac and endothelial cell lineages, but the underlying transcriptional networks responsible for specification of these cell fates remain unclear. Here we demonstrated that Ets-related protein 71 (Etsrp71), a newly discovered ETS family transcription factor, was a novel downstream target of the homeodomain protein, Nkx2-5. Using genetic mouse models and molecular biological techniques, we demonstrated that Nkx2-5 binds to an evolutionarily conserved Nkx2-5 response element in the Etsrp71 promoter and induces the Etsrp71 gene expression in vitro and in vivo. Etsrp71 was transiently expressed in the endocardium/endothelium of the developing embryo (E7.75-E9.5) and was extinguished during the latter stages of development. Using a gene disruption strategy, we found that Etsrp71 mutant embryos lacked endocardial/endothelial lineages and were nonviable. Moreover, using transgenic technologies and transcriptional and chromatin immunoprecipitation (ChIP) assays, we further established that Tie2 is a direct downstream target of Etsrp71. Collectively, our results uncover a novel functional role for Nkx2-5 and define a transcriptional network that specifies an endocardial/endothelial fate in the developing heart and embryo.cardiac progenitor cells ͉ endocardium ͉ Etsrp71 ͉ Tie2 ͉ cardiac development

Abstract-Stem and progenitor cell populations occupy a specialized niche and are consequently exposed to hypoxic as well as oxidative stresses. We have previously established that the multidrug resistance protein Abcg2 is the molecular determinant of the side population (SP) progenitor cell population. We observed that the cardiac SP cells increase in number more than 3-fold within 3 days of injury. Transcriptome analysis of the SP cells isolated from the injured adult murine heart reveals increased expression of cytoprotective transcripts. Overexpression of Abcg2 results in an increased ability to consume hydrogen peroxide and is associated with increased levels of ␣-glutathione reductase protein expression. Importantly, overexpression of Abcg2 also conferred a cell survival benefit following exposure to hydrogen peroxide. To further examine the molecular regulation of the Abcg2 gene, we demonstrated that hypoxia-inducible factor (HIF)-2␣ binds an evolutionary conserved HIF-2␣ response element in the murine Abcg2 promoter. [1][2][3][4] This strategy defines a rare population of progenitor cells that can adopt alternative fates in permissive environments. [1][2][3] We have verified that the ability of SP cells to efflux Hoechst 33342 dye is dependent on the expression of Abcg2, which is a member of the family of ATP-binding cassette (ABC) transporters. 3,5,6 Although the functional role of the ABC transporters remains ill-defined, 7 we established that Abcg2 was able to confer the SP phenotype in a striated muscle cell line. 3 Stem and progenitor cell populations, including SP cells, are exposed to environmental stress by virtue of their physical location. Although oxidative stress attributable to unchecked levels of free radical-derived reactive oxygen species (ROS) can damage DNA, proteins, and lipids, 8 oxidative stress caused by modestly increased ROS can activate specific signal transduction pathways, leading to either senescence or apoptosis. 9 Previous transcriptome analyses of embryonic, hematopoietic, and neural stem cells revealed a common signature of gene expression in these stem cell populations. This profile includes transcripts that function as cytoprotective factors to provide resistance against environmental stress. 10 -12 Recent studies that examined circulating, bloodderived endothelial progenitor cells reveal enrichment for the expression of genes encoding for antioxidative factors that reduce sensitivity toward ROS-induced cell death. 13 Regulation of cytoprotective factors during injury states would be beneficial for survival and expansion of stem and progenitor cell populations.Members of the hypoxia-inducible factor (HIF) family are activated by multiple environmental stimuli. HIF-1␣, a master regulator for hypoxia-inducible gene expression, regulates gene expression to promote energy production as well as oxygen delivery in response to hypoxia. 14 -16 HIF-2␣, also known as endothelial PAS domain protein 1 (EPAS1), has many similarities with HIF-1␣. [17][18][19] However, several mole...

Until now the allantois has not been considered as a hematopoietic organ. Here we report experimental evidence demonstrating the in situ emergence of both hematopoietic and endothelial precursors in the avian allantoic bud. When the prevascularized allantoic bud from a quail embryo was grafted in the coelom of a chicken host, hematopoietic and endothelial cells later were found in the bone marrow of the host. Because the graft was located at a distance from the limb bud, these cells could reach the bone marrow only by the circulatory pathway. This blood-borne seeding may be accomplished by distinct hematopoietic and endothelial precursors, or by hemangioblasts, the postulated common precursors of these two lineages; we consider the latter interpretation more likely. We also show by reverse transcription-PCR that the allantois region expresses very early the GATA genes involved in hematopoiesis and some ␤-globin chain genes.

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