Molecular mechanisms regulating the growth and differentiation of hematopoietic stem cells

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(1) Stem cell factor (SCF) has crucial roles in proliferation, survival and differentiation of hematopoietic stem cells through binding to c-Kit receptor (KIT). We made a series of 22 KIT mutants, in which Tyr (Y) residue was substituted to Phe (F) in the cytoplasmic domain, and introduced into BAF3 cells. On stimulation with SCF, BAF3 expressing KIT^<WT>(WT) showed cell migration and Ca^<2+> mobilization. Among 22YF mutants, Y567F and Y719F showed significantly reduced cell migration and Ca^<2+> mobilization. Analysis on signaling cascades suggested that Y567-mediated Src family kinase (SFK) activation led to Ca^<2+> influx and migration, and that P38MAPkinase (P38MAPK) and Erk1/2 were also regulated by Y567/SFK and involved in cell migration. Also, Y719-mediated PI3K pathway was suggested to be involved in the migration. These results indicate that two major KIT signaling pathways lead to cell migration, one is Y567-SFK-p38MAPK-Erk and another is Y719-PI3 kinase.(2) Thrombopoietin (T … More PO) and its receptor c-mpl play crucial roles in growth and megakaryocytic differentiation of hematopoietic stem cells. We found that Ras activation was involved in thrombopoietin (TPO)-induced megakaryocytic differentiation. Furthermore, we demonstrated that GATA-1 activities was required for the Ras-mediated megakaryocytic differentiation, and that GATA-1 activities were regulated negatively by the direct interaction with other lineage-specific transcription factors, PU.1 and c-Myb.(3) AIM-1 belongs to an Aurora/Ipl1 serine threonine kinase family, and is supposed to play key roles in mitosis. In human hematopoietic cells, expression of AIM-1 was restrictedly observed at G2/M phase of cell cycle. In contrast, AIM-1 was continuously repressed during megakaryocytic polyploidization. Supplement of AIM-1 activities by the induced expression of wild-type AIM-1 canceled TPA-induced polyploidization of K562 cells, and the suppression of AIM-1 activities by dominant-negative AIM-1 led to polyploidization. These results suggested that down-regulation of AIM-1 at M phase may be involved in abortive mitosis and polyploid formation of megakaryocytes. Less