Most of the hematopoietic stem cells (HSCs) within the bone marrow (BM) show quiescent state with a low mitochondrial membrane potential (ΔΨ). In contrast, upon stress hematopoiesis, HSCs actively start to divide. However, the underlying mechanism for the initiation of HSC division still remains unclear. To elucidate the mechanism underlying the transition of cell cycle state in HSCs, we analyzed the change of mitochondria in HSCs after BM suppression induced by 5-fluoruracil (5-FU). We found that HSCs initiate cell division after exhibiting enhanced ΔΨ as a result of increased intracellular Ca level. Although further activation of Ca-mitochondria pathway led to loss of HSCs after cell division, the appropriate suppression of intracellular Ca level by exogenous adenosine or Nifedipine, a Ca channel blocker, prolonged cell division interval in HSCs, and simultaneously achieved both cell division and HSC maintenance. Collectively, our results indicate that the Ca-mitochondria pathway induces HSC division critically to determine HSC cell fate.

STEM CELLS 2006;24:86 -94

No abstract

Many types of organ-specific stem cells have been recently shown to exhibit a side population (SP) phenotype based on their ability to efflux Hoechst 33342 dye. Because stem cells from corneal epithelium reside in the basal layer of the limbal epithelium, the purpose of this study was to examine whether the limbal epithelium contains SP cells. The ATP-binding cassette transporter Bcrp1/ABCG2 is reported to contribute to the SP phenotype in cells from several diverse sources. Here we show data from fluorescence-activated cell sorting and real-time quantitative RT-PCR analysis showing that harvested limbal epithelial cells contain SP cells expressing ABCG2. Immunofluorescence revealed that a portion of limbal epithelial basal cells expressed ABCG2. Data indicate that ABCG2 positive limbal epithelial cells are putative corneal epithelial stem cells.

Hematopoietic stem cells (HSCs) and HSC-independent progenitors are generated from hemogenic endothelium. Yokomizo et al. show that Hlf expression distinguishes nascent HSCs from HSC-independent progenitors. HSC specification, regulated by the Evi-1/Hlf axis, is activated only within Hlf+ nascent hematopoietic clusters.

Highlights d Thpo facilitates Mk-lineage differentiation through mitochondrial activation in HSCs d Mitochondria-rich HSCs exhibit Mk-lineage differentiation d Mitochondria-associated pSTAT3 is involved in mitochondria activation via Thpo signaling

Highlights d G 0 marker (G 0 M) discriminates between dormant (G 0 M high ) and active (G 0 M low ) HSCs d Active (G 0 M low ) HSCs exhibit higher CDK4/6 activity than dormant (G 0 M high ) HSCs d [Ca 2+ ] c high HSCs have higher bone marrow reconstitution ability than [Ca 2+ ] c low HSCs d Upregulation of [Ca 2+ ] c enhances bone marrow reconstitution ability of HSCs

Recent identification of platelet/megakaryocyte-biased hematopoietic stem/repopulating cells requires revision of the intermediate pathway for megakaryopoiesis. Here, we show a unipotent megakaryopoietic pathway bypassing the bipotent megakaryocyte/erythroid progenitors (biEMPs). Cells purified from mouse bone marrow by CD42b (GPIba) marking were demonstrated to be unipotent megakaryocytic progenitors (MKPs) by culture and transplantation. A subpopulation of freshly isolated CD411 cells in the lineage Sca1 1 cKit 1 (LSK) fraction (subCD41 1 LSK) differentiated only into MKP and mature megakaryocytes in culture. Although CD411 LSK cells as a whole were capable of differentiating into all myeloid and lymphoid cells in vivo, they produced unipotent MKP, mature megakaryocytes, and platelets in vitro and in vivo much more efficiently than Flt3 1 CD41 2 LSK cells, especially at the early phase after transplantation. In single cell polymerase chain reaction and thrombopoietin (TPO) signaling analyses, the MKP and a fraction of CD41 1 LSK, but not the biEMP, showed the similarities in mRNA expression profile and visible TPO-mediated phosphorylation. On increased demand of platelet production after 5-FU treatment, a part of CD41 1 LSK population expressed CD42b on the surface, and 90% of them showed unipotent megakaryopoietic capacity in single cell culture and predominantly produced platelets in vivo at the early phase after transplantation. These results suggest that the CD41 1 CD42b 1 LSK are straightforward progenies of megakaryocytes/plateletbiased stem/repopulating cells, but not progenies of biEMP. Consequently, we show a unipotent/highly biased megakaryopoietic pathway interconnecting stem/repopulating cells and mature megakaryocytes, the one that may play physiologic roles especially in emergency megakaryopoiesis.