Manipulation of hematopoietic cell development by expression of transcription factors

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We have been proposing a hypothesis that the primitive hematopoietic cell lineage diverges directly from lateral mesoderm whereas the definitive hematopoietic cell lineage originates from the endothelial cell lineage which is another descendant of mesoderm. In this project, we analyzed expression and function of transcription factors which are thought to play important roles in hematopoietic cell development by using an in vitro differentiation system of mouse ES cells.We produced ES cell clones which revealed activity of the Gatal and Gata2 genes by expressing a GFP transgene as a marker. Expression of the Gata2 gene was already detected in a fraction of the earliest lateral mesoderm cells developed in a culture of the ES cells (day 3). The Gata2-expressing mesoderm cells purified by FACS from day 4 cultures exclusively contained primitive erythroid colony-forming cells (CFU-EryP) as well as stromal cell-dependent progenitors for definitive hematopoietic cells. On the other hand, expr … More ession of the Gatal gene in mesoderm cells became detectable after 4 days of ES cell differentiation. CFU-EryP was highly enriched in the Gata1-expressing mesoderm cell fraction. Mesoderm cells which have not started expression of the Gata1 gene by day 4 contained high frequency of endothelial cell progenitors. Endothelial cells derived from the progenitors possessed a potential to give rise to definitive hematopoietic cells.We next introduced an estrogen receptor/Cre-mediated inducible Tal1 transgene into an ES cell clone which is deficient for the endogenous Tall gene. By rescuing Tal1 expression in various stages of ES cell differentiation, we demonstrated that mesoderm cells require Tal1 at a period between day 3 and day 4 for the development of not only CFU-EryP but also hematogenic endothelial cells. Furthermore, overexpression of Tal1 in wild type ES cells resulted in an increase of Gata1-expressing mesoderm cells. The results collectively suggest that Tal1 and Gata2 are the prerequisite for the initiation of hematopoietic cell lineages. Less