CELl cycle Feature
Cell Cycle 10:14, 2246-2247; July 15, 2011; © 2011 Landes Bioscience
Dynamic regulation of hematopoietic stem cell cycling Hitoshi Takizawa and Markus G. Manz* Division of Hematology; University Hospital Zürich; Zürich, Switzerland
To maintain blood homeostasis, large amounts of cells need to be produced continuously. Moreover, during hematopoietic challenges, blood cell production needs to be enhanced several-fold. Such robust regenerative capacity is ensured by lifelong self-renewing bone marrow hematopoietic stem cells (HSCs) and their progeny, highly proliferative hematopoietic progenitors. As hematopoiesis can recover even after several days of cell cycletargeting damage, it has been assumed that at least some HSCs are deeply quiescent. However, there was little experimental evidence on divisional frequency of HSCs. Moreover, how HSCs turnover is possibly changed upon hematopoietic system challenges, such as bleeding, infection and inflammation, as well as upon aging and, finally, how HSC divisional history influences subsequent HSC division and hematopoietic potential are, thus far, not well studied.
followed by a fraction of cells switching to quiescence thereafter? Two observations may explain these findings: (1) BrdU, possibly via cellular injury, can drive HSCs into cell cycle and thus influences read-out2,4 and (2) a small fraction of non-dividing HSCs was likely missed in previous experiments.4 Thus, BrdU labeling and retention should be interpreted with caution. Furthermore, experimental analysis of BrdU-labeled cells leads to cell death, and thus does not allow further functional cellular evaluations. To overcome these technical limitations, transgenic animal models with regulated expression of green fluorescent protein-labeled Histon 2B protein (H2BGFP) were recently employed, allowing isolation of LRCs and subsequent testing of HSC potential.3,5 In vivo experiments demonstrated that some phenotypically defined HSCs divide frequently, while some cells remained undivided over five months. If slow division would be a fixed state, these cells would only divide five times in a mouse lifetime. Moreover, serial transplantation revealed that actively cycling HSCs had limited self-renewal and blood production capacity, while dormant HSCs maintained lifelong hematopoiesis. The data suggested that HSC potential is closely associated with dormancy and decreases with divisional activity, and that dormant HSC represent a “reservoir” HSC population, rarely used in steady-state. In parallel to these studies, we employed 5-(and-6)-carboxyfluorescein succinimidyl ester (CFSE), a cytoplasmatic protein labeling dye that is equally distributed into daughter cells, to observe HSC divisional behavior in vivo. In contrast to previous studies, we demonstrated that HSCs with lifelong self-renewal
capacity are contained in both the fastand slow-cycling fractions at any given time. Beyond that, we showed that the fast- and slow-cycling status is not a permanently fixed state, but cells with HSC phenotype reversibly switch cycling status even under steady-state conditions.4 HSC Cycling upon Hematopoietic Challenge HSCs divide rapidly upon limiting dilution transplantation into HSC-depleted recipients.4,6 In more natural situations, where HSCs and the progenitor pool are in a relative steady-state, it is established that hematopoietic progenitor cells respond to respective reactive lineage differentiation cues, while the impact on HSC cycling was not clear. Recent studies now provided evidence that, upon infection, increased interferons (IFNs) activate HSC from dormancy.7-9 In addition, we have provided in vivo evidence that lipopolysaccharide (LPS) challenge, resembling gram-negative bacterial infection, drives most dormant HSC into cycle.4 While IFNs seem to directly act on IFN receptor-expressing HSCs, the mechanism of action of Tolllike receptor (TLR) 4 agonists on HSCs still needs to be determined. Interestingly, TLRs are expressed on BM-resident and blood-trafficking hematopoietic progenitors and possibly HSCs, indicating that some direct action might be expected.10,11
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HSC Cycling in Steady-State Homeostasis In a pioneering work, the turnover rate of HSCs in steady-state was determined by monitoring in vivo incorporation of the DNA analog 5-bromo-2'-deoxyuridine (BrdU) into immunophenotypically enriched HSC populations. From these observations, it was calculated that all HSCs enter cell cycle within three weeks and divide rather homogeneously.1 An elaboration of this approach, in vivo labeling for 2 weeks and subsequent BrdU retention, revealed that some cells retain their label for months (label-retaining cells, LRCs).2,3 These observations posed a problem: why do most HSCs get labeled within two weeks, i.e., are actively cycling,
Hypothesis for an Evolutionary Shaped Model of Intrinsic and Extrinsic Regulation of HSC Cycling Based on our observations, we propose a dynamically fluctuating HSC cycling
*Correspondence to: Markus G. Manz; Email:
[email protected] Submitted: 04/21/11; Accepted: 04/26/11 DOI: 10.4161/cc.10.14.16197 Comment on: Takizawa H, et al. J Exp Med 2011; 208:273–84. 2246
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Volume 10 Issue 14
CELl cycle Feature
CELl cycle Feature
Figure 1. An evolutionary shaped model of intrinsic and extrinsic regulation of HSC cycling. HSC cycling activity fluctuates in steady state. Upon demand of blood production, an extrinsic drive activates most HSCs into cycle, while, opposite to this, intrinsic drive, determined and increased by divisional history, inactivates HSC to prevent their exhaustion.
model in steady state, where the turnover of HSCs would naturally be similar to end of life. This would assure protection of the dormant HSC fraction from cell cycle-damaging events and would allow at the same time overall homogenous use of HSC resources during life. This model is in line with a linear correlation of telomere shortening in human HSCs with aging.12 Also, it would be in line with an increased risk for the whole HSC population to accumulate genetic events, promoting clonal HSC diseases, such as myelodysplasia and myeloid leukemia in the aged
population. Steady-state cycling differences of HSCs at any given time in this model would reflect the broad variation of possibilities, but not a static separation of HSCs in distinct classes with different divisional kinetics.4,13 Inreased cycling of most HSC during hematopoietic challenges via external cues then might have developed as an advantage to recruit dormant HSCs to better cope with blood demand for the necessary time. Broad activation, however, also poses a risk to loose HSCs due to genetic vulnerability and differentiation. Thus, a mechanism
to “drive” HSC quiescence that increases strength with frequency of total divisions needs to be established. This might be achieved by down-modulation of sensitivity to external stimulation and upregulation of sensitivity to quiescence signals. Indeed, we observed that phenotypically defined HSC populations with extensive proliferative history, i.e., HSCs from aged mice and HSCs that have divided massively after transplantation, subsequently increase quiescence in steady-state environments, likely as an effort to prevent HSC exhaustion4 (Fig. 1). Detailed understanding of the molecular mechanisms of HSC cycling and hibernation in steady state, upon hematopoietic challenge and during aging will provide not only basic knowledge on HSC biology, but might also open new avenues to therapeutically interfere with steady-state and reactive hematopoiesis as well as with leukemia initiating cell populations.
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References
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