Abstract
Mounting evidence accumulates that bone marrow (BM) contains a population of pluripotent stem cells (PSC) that give rise to long-term repopulating hematopoietic stem cells (HSC). Recently we identified in murine BM a homogenous population of rare (∼0.01% of BMMNC) and very small (2–4 μm) Sca-1+ lin− CD45− cells that express by RQ-PCR and immunhistochemistry markers of PSC such as SSEA-1, Oct-4, Nanog and Rex-1 and highly express Rif-1 telomerase protein (Leukemia2006;20,857–869). Direct electronmicroscopical analysis revealed that these cells display several features typical for primary epiblast-derived embryonic stem cells such as i) a large nuclei surrounded by a narrow rim of cytoplasm, and ii) open-type chromatin (euchromatin). In co-cultures with C2C12 murine sarcoma supportive feeder-layer, these cells grow spheres that are composed of immature CXCR4+SSEA-1+Oct-4+ cells that have large nuclei containing euchromatin and, if plated into cultures promoting tissue differentiation, show pluripotency and expand into cells from all three germ-cell layers. Based on this, we called them very small embryonic like (VSEL) stem cells. However, VSEL isolated freshly from the BM do not posses immediate hematopoietic activity - they neither grow hematopoietic colonies nor radioprotect lethally irradiated recipients. Recently, however, we noticed that if plated over supportive OP9 cell line, these CD45− VSEL give rise to colonies of CD45+CD41+Gr-1−Ter119− cells where their phenotype resembles those of the earliest hematopoietic cells that are derived in vitro from established embryonic cell lines. This hematopoietic differentiation of VSEL was accompanied by upregulation of mRNA for several genes regulating hematopoiesis (e.g. PU-1, c-myb, LMO2, Ikaros). More importantly, the CD45+CD41−Gr-1−Ter119− cells expanded from VSEL isolated from GFP+ mice if transplanted into wild-type animals protected them from lethal irradiation and differentiated in vivo into all major hematopoietic lineages (e.g., Gr-1+, B220+ and CD3+ cells). This hematopoietic activity was maintained after transplantation into secondary recipients. Based on this, we postulate that i) VSEL are PSC that give rise to HSC and ii) that CD45+ cells may derive from a CD45− population. Thus we propose that VSEL are a population of BM-residing PSC that may give rise to long-term engrafting hematopoietic stem cells.
Enforced Activation of STAT5A Facilitates the Generation of Embryonic Stem-Derived Hematopoietic Stem Cells That Contribute to Hematopoiesis In Vivo
