Use of cytometry technology for the study of stem cell biology

Hematopoietic stem cells (HSCs) are best-studied stem cells and they have contributed a great deal for the development of stem cell biology. HSCs reside in a bone marrow niche in a non-dividing state from which they occasionally are aroused to undergo cell division. Using highly purified HSCs by flow cytometry, we have recently shown that cytokine treatment of HSCs led to polarization of the lipid raft marker GM1 ganglioside as well as to phosphorylation of Akt and relocation of FOXO3a to the cytoplasm. Lipid raft clustering induced by cytokines is essential for HSC re-entry into the cell cycle. Furthermore, based on a hypothesis that signals from the niche inhibit LRC and induce hibernation in HSCs, we screened candidate niche signals for inhibition of LRC. Among niche signals examined, transforming growth factor-ß (TGF-ß) efficiently inhibits LRC and induces p57^Kip2 expression, leading to HSC hibernation ex vivo. These data establish the role of TGF-ß as a niche signal in control of HSC hibernation and provide important clues to identify stem cell niche in bone marrow.