Regulation of hematopoietic stem cell aging in vivo by a distinct genetic element

Abstract

Until recently, stem cells were thought to be endowed with unlimited self-renewal capacity and, thus, assumed exempt from aging. But accumulating evidence over the past decade compellingly argues that a measurable and progressive replicative impairment in the hematopoietic, intestinal, and muscle stem cell activity exists from adulthood to old age, resulting in a decline in stem cell function and rendering stem cell aging as the possible link between cellular aging and organismal aging. By using a previously uncharacterized congenic animal model to study genetic regulation of hematopoietic stem cell aging, we have demonstrated definitively that a locus on murine chromosome 2 regulates hematopoietic stem cell aging. In addition to demonstrating that hematopoietic stem cell aging is regulated by a distinct genetic element, experimental evidence links the response of hematopoietic stem cells to DNA double-strand breaks to cellular aging, suggesting DNA integrity influences stem cell aging.

Fig. 1.

Significant linkage of a locus on murine chr. 2 to HSC aging. Linkage between 120 and 170 Mbp on chr. 2 to the difference in the number of HSCs (CAFC day 35) between aged C57BL/6 and DBA/2 animals is shown. The values for suggestive and significant linkage to the trait along chr. 2 are given, as well as the interval covered in the congenic line (130–180 Mbp) and the 95% confidence interval for linkage (135–151 Mbp).

Fig. 2.

Decreased frequency of primitive hematopoietic cells (CAFC day 28 and 35) in BM of 24-month-old B6.D2 chr. 2 animals compared with the frequency in C57BL/6 BM. CAFC assays with BM cells from 2- and 24-month-old C57BL/6 and B6.D2 (chr. 2) animals were performed as described in Materials and Methods. *, P < 0.05. BM from two aged animals for each genotype was analyzed in six individual CAFC assays per animal.

Fig. 3.

Competitive disadvantage of congenic hematopoietic stem cells in aged animals. (A) Schematic diagram of the competitive repopulation assay. (B) Equal numbers (1 × 10⁶ each) of B6.SJL (BoyJ) competitor and either C57BL/6 (group 1) or B6.D2 (chr.2) (group 2) BM cells were transplanted into lethally irradiated B6.SJL recipients, and the animals subsequently aged up to 10 months. Chimerism in peripheral blood was determined by flow cytometry at 1 month, 3–4 months, 5–7 months, and 10 months after transplantation. *, P < 0.05. C57BL/6 donor: n = 18 at 1 month, n = 30 at 3–4 months, n = 13 at 5–7 months, and n = 5 at 10–11 months. B6.D2 (chr. 2) donor: n = 6 for 1 month, n = 6 for 3–4 months, n = 5 for 5–7 months, and n = 5 for 10 months.

Fig. 4.

Radiation susceptibility of congenic stem cells as a surrogate assay for stem cell aging. (A) Schematic diagram of the competitive repopulation assay used for these analyses. (B) Tri-Con and B6.D2 (chr. 2) congenic animals were irradiated by 0, 1, or 2 Gy 2 weeks before BM harvest. Equal numbers of Tri-Con competitor and B6.D2 (chr. 2) donor BM cells (1 × 10⁶ each) were transplanted into lethally irradiated B6.SJL (BoyJ) recipients and the animals subsequently aged up to 20 weeks. Chimerism in peripheral blood was determined at 8, 12, and 20 weeks after transplantation by flow cytometry based on the Ly5.1/Ly5.2 mismatch of the transplanted BM cells. n = 4 for each dose.