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Nuclear reprogramming is an innovative advance in cell biology. An important research initiative in this field is cell fusion-mediated nuclear reprogramming, wherein the nuclei of somatic cells, such as thymocytes, are initialized through cell fusion with embryonic stem cells (ESCs). However, hybrid cells obtained through cell fusion between ESCs and thymocytes failed to contribute to the embryo proper when injected into blastocysts, which suggested that there are fundamental defects in such hybrid cells. Here, we performed side-by-side comparative analyses of the in vitro growth and differentiation capacities of ESCs and ESC-T hybrid cells. We found that the hybrid cells were larger and proliferated more slowly than the ESCs in 2i/LIF medium. Upon in vitro induction of differentiation, hybrid cells gave rise to cells of the three germ layers. Under culture conditions for hematopoietic differentiation, hybrid cells successively differentiated into lateral mesodermal cells, hemogenic endothelial cells, and various types of hematopoietic cells, including erythroid, myeloid, and lymphoid cells, although T cell maturation in the CD4/CD8 double-negative fraction was delayed. These results verified the multi-lineage differentiation capacity of ESC-T hybrid cells. The minimal contribution of hybrid cells to chimeric embryos may be due to their slow growth.

Platelets are essential for blood circulation and coagulation. Previous study indicated that overexpression of Gata2 in differentiated mouse embryonic stem cells (ESCs) resulted in robust induction of megakaryocytes (Mks). To evaluate platelet production capacity of the Gata2-induced ESC-derived Mks, we generated iGata2-ESC line carrying the doxycycline-inducible Gata2 expression cassette. When doxycycline was added to day 5 hemogenic endothelial cells in the in vitro differentiation culture of iGata2-ESCs, c-Kit(-)Tie2(-)CD41(+) Mks were predominantly generated. These iGata2-ESC-derived Mks efficiently produced CD41(+)CD42b(+)CD61(+) platelets and adhered to fibrinogen-coated glass coverslips in response to thrombin stimulation. Transmission electron microscopy analysis demonstrated that the iGata2-ESC-derived platelets were discoid-shaped with α-granules and an open canalicular system, but were larger than peripheral blood platelets in size. These results demonstrated that an enforced expression of Gata2 in late HECs of differentiated ESCs efficiently promotes megakaryopoiesis followed by platelet production. This study provides valuable information for ex vivo platelet production from human pluripotent stem cells in future.

The generation of mouse hematopoietic stem cells from hemogenic endothelial cells (HECs) in the aorta/gonad/mesonephros region of developing embryos requires a zinc finger transcription factor Gata2. In the previous study, an enforced expression of Gata2 in vitro promoted the production of HECs from mesodermal cells differentiated from mouse embryonic stem cells (ESCs). Our research group has previously demonstrated that the enforced expression of Gata2 in ESC-derived HECs enhances erythroid and megakaryocyte differentiation and inhibits macrophage differentiation. However, the manner in which the multiple functions of Gata2 are regulated remains unclear. Mouse ESCs differentiate into various types of hematopoietic cells when cocultured with OP9 stromal cells (OP9 system). Using this system and the inducible gene cassette exchange system, which facilitates the establishment of ESCs carrying inducible transgenes under an identical gene expression regulatory unit, the domain-specific functions of Gata2 were systematically dissected in this study. We determined that the N-terminal (amino acid 1-110) region of Gata2 was an erythroid-inducing region, both the middle (amino acid 111-200) and C-terminal (amino acid 413-480) regions were megakaryocyte-inducing regions. Furthermore, the present data strongly suggest that intramolecular antagonistic interactions between each of these regions fine-tune the biological functions of Gata2.

WNT/β-CATENIN signaling promotes the hematopoietic/endothelial differentiation of human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs). The transient addition of a GSK3β inhibitor (GSKi) has been shown to facilitate in vitro endothelial cell differentiation from hESCs/hiPSCs. Since hematopoietic and endothelial cells are derived from common progenitors (hemogenic endothelial progenitors [HEPs]), we examined the effect of transient GSKi treatment on hematopoietic cell differentiation from hiPSCs. We found that transient GSKi treatment at the start of hiPSC differentiation induction altered the gene expression profile of the cells. Multiple CDX/HOX genes, which are expressed in the posterior mesoderm of developing embryos, were significantly upregulated by GSKi treatment. Further, inclusion of the GSKi in a serum- and stroma-free culture with chemically defined medium efficiently induced HEPs, and the HEPs gave rise to various lineages of hematopoietic and endothelial cells. Therefore, transient WNT/β-CATENIN signaling triggers the activation of the CDX/HOX pathway, which in turn confers hemogenic posterior mesoderm identity to differentiating hiPSCs. These data enhance our understanding of human embryonic hematopoietic/endothelial cell development and provide a novel in vitro system for inducing the differentiation of hematopoietic cells from hiPSCs.