Hematopoietic Stem Cells Increase Quiescence during Aging

Introduction: Definitive hematopoietic stem cells (HSCs) sustain blood production from fetal development throughout life. In mice, most of steady state, young adult HSCs are in the G0 phase of cell cycle (quiescence), and are estimated to divide roughly once a month. Daily hematopoietic production is thus mainly sustained by highly proliferative downstream hematopoietic progenitor cells (HPCs). Aged haematopoiesis was demonstrated to be distinct from young haematopoiesis in various aspects such as i) a shift from lymphopoiesis to myelopoiesis, ii) functional decline of HSCs (self-renewal, homing), and iii) HSCs pool expansion. While several studies attempted to address whether changes in HSCs turnover during aging can account for the distinct aging associated phenotype and function, it remained to be determined whether aged HSCs overall cycle more or less frequently than young HSCs.

Methods: To construct data-based, quantitative models, we measured turnover rates and compartment sizes of populations of HSCs, HSPCs and granulopoiesis/granulocytes, i.e. a post-mitotic mature hematopoietic linage with a short half-life. We examined four age groups: 3 week, 2 month, 1 year and 2 year old mice. Mice in each group were i.p. injected every 4 hours with 1 mcg EdU up to a maximum time of 48 hours. HSC, HSPC and granulopoiesis/granoulocyte compartment sizes and snapshot cell-cycle analysis was performed by FACS at multiple sampling points in BM and peripheral blood (PB), respectively. Based on this data, we built a mathematical model of HSC turn-over and HSPC differentiation during ageing. Moreover, we evaluated HSC cycling by CFSE dilution in steady-state transplantation experiments (as described before; Takizawa et al., J Exp Med 2011).

Results: In line with previous reports, the HSCs compartment size gradually increased with age from 3wk old mice to 2 year old mice. In sharp contrast, cycling activity of HSCs as determined by EdU incorporation decreased gradually and significantly with increasing age. This was driven by decreased activation from the quiescent state, while the time that actively cycling HSCs require to progress through cell-division remains constant with age. Multipotent Progenitor (MPP) cycling showed a non-significant trend towards slower turn-over. These results were confirmed by complementary CFSE-dilution experiments. Mathematical modeling of HSC proliferation and differentiation revealed a higher probability of self-renewing divisions in 3 week old mice as compared to 2 month, 1 and 2 year old mice, with the latter both having nearly equal chances of self-renewing versus differentiating divisions.

Conclusions: Our data clarifies the long-standing question, how the HSC pool increases with age. Instead of an increase in active cycling, an increase in HSC quiescence is responsible for the increased size of the HSCs pool in aging.