Simple and Efficient Isolation of Hematopoietic Stem Cells from H2K-zFP Transgenic Mice

Bmi-1-green fluorescent protein-knock-in mice reveal the dynamic regulation of bmi-1 expression in normal and leukemic hematopoietic cells

The ability to self-renew is essential for all kinds of stem cells regardless of tissue type. One of the best candidate genes involved in conferring self-renewal capacity is Bmi-1, which has been proven to be essential for the maintenance of both normal adult hematopoietic and leukemia stem cells, as well as adult neural stem cells. To investigate the possible role of Bmi-1 in other cell types that also self-renew, we generated Bmi-1-green fluorescent protein (GFP)-knock-in mice, in which GFP was expressed under the endogenous transcriptional regulatory elements of the Bmi-1 gene. Using these targeted reporter mice, we demonstrated that Bmi-1 is expressed in hematopoietic stem cells (HSCs) at its highest levels and downregulated upon commitment to differentiation. An in vivo reconstitution assay revealed that the frequency of HSCs was 1/16 in Bmi-1high c-kit+ lin -Sca-1+ bone marrow (BM) cells and 1/49 in Bmi-1 high lin- BM cells, suggesting that Bmi-1 may serve as a marker for normal HSCs. In murine leukemia models induced by P210BCR/ABL or TEL/PDGFbetaR + AML1/ETO, Bmi-1 was not overexpressed in leukemic HSCs, despite the increase in the HSC numbers. Bmi-1 was expressed at its highest levels in undifferentiated leukemia cells. Furthermore, in several other nonhematopoietic tissues, cells could be separated into distinct subpopulations with differential Bmi-1 expression. Thus, these mice allow for the isolation of viable Bmi-1-expressing cells and have the potential to become a useful tool for understanding the role of Bmi-1 in normal and cancer stem cells in multiple tissue types. Disclosure of potential conflicts of interest is found at the end of this article.

Stem cell fate analysis revisited: interpretation of individual clone dynamics in the light of a new paradigm of stem cell organization

Many experimental findings on heterogeneity, flexibility, and plasticity of tissue stem cells are currently challenging stem cell concepts that assume a cell intrinsically predefined, unidirectional differentiation program. In contrast to these classical concepts, nonhierarchical self-organizing systems provide an elegant and comprehensive alternative to explain the experimental data. Here we present the application of such a self-organizing concept to quantitatively describe the hematopoietic stem cell system. Focusing on the analysis of individual-stem-cell fates and clonal dynamics, we particularly discuss implications of the theoretical results on the interpretation of experimental findings. We demonstrate that it is possible to understand hematopoietic stem cell organization without assumptions on unidirectional developmental hierarchies, preprogrammed asymmetric division events or other assumptions implying the existence of a predetermined stem cell entity. The proposed perspective, therefore, changes the general paradigm of thinking about stem cells.