Comment on Balazs et al, page 2317
A study by Balazs and colleagues in this issue of Blood begins to fill a significant gap in our abilities to analyze the biology of hematopoietic stem cells.
Stem cells in mammalian tissues such as gut, epidermis, and blood are responsible for the robust generation of mature cells throughout adult life, requiring an ability to correctly balance self-renewal and differentiation cell fate choices, a process that is poorly understood. Unraveling stem cell regulatory mechanisms depends on the ability to identify these rare cells and to isolate them for functional analyses. Alternatively, as in the gut and epidermal systems, the positions of stem cells within complex tissue architectures have provided a “roadmap” for mechanistic studies. In the hematopoietic system, this has been difficult, although the hematopoietic stem cells (HSCs) can be isolated to near homogeneity using a variety of flow cytometric approaches. However, essentially all of these strategies require complex combinations of cell-surface (or other) markers, none of which are specific to HSCs. An “explicit” HSC marker has, to date, eluded identification. Building on results from HSC gene expression analyses, Balazs and colleagues show that endothelial protein C receptor (EPCR) is an “explicit” marker that can be used alone to isolate HSC from murine bone marrow. EPCR expression is quantitatively distributed in a small subset of bone marrow cells. The authors show that the very highest levels of expression correlate with the ability of the purified cells to reconstitute hematopoiesis after transplantation into radiation-ablated recipient animals. Furthermore, this population is not enriched for the more numerous committed progenitor cells that can produce myeloid and erythroid progeny in vitro. These progenitors are, however, enriched in populations that express intermediate levels of EPCR. Such a quantitative distribution of this molecule within the hematopoietic hierarchy mirrors quantitative differences in the ability to efflux dyes, perhaps suggesting a similar gradient in transporter pump expression levels. Importantly, all bone marrow cells that are HSCs express high EPCR levels, and it appears that the converse is true as well. The availability of an explicit set of HSC markers will facilitate answers to a number of outstanding questions in HSC biology. First, these markers should facilitate a more precise definition of the bone marrow niches within which HSCs reside. Second, it should be possible to dissect the regulatory mechanisms that ensure a high level of EPCR expression in HSCs and progressively lower expression as more committed progeny are produced. Third, the actual role of EPCR in HSC biology is likely to be interesting. Fourth, the ability to isolate HSCs to near homogeneity using a single marker will certainly be a significant advantage. Recently, specific combinatorial expression patterns of the signaling lymphocytic activation molecule (SLAM) receptor family members have also defined distinct stages in the HSC and progenitor cell hierarchy.1 It will be interesting to use these together with EPCR in efforts to further define and understand hematopoietic development. ▪