Hematopoietic stem cell aging is associated with functional decline and delayed cell cycle progression

The molecular mechanisms underlying hematopoietic stem cell (HSC) aging remain to be elucidated. In this study, we investigated age-related changes in the functional and phenotypic properties of murine HSCs. Consistent with previous studies, we found that the number and frequency of CD34^−/lowc-Kit⁺Sca-1⁺lineage marker⁻ (CD34⁻KSL) cells, a highly enriched HSC population, significantly increased in old mice, though their repopulating ability was reduced. Continuous bromodeoxyuridine labeling revealed a significant delay in the cell cycle progression of CD34⁻KSL cells in old mice. This delay was also observed in young recipients transplanted with whole bone marrow cells from old mice. When cultured in vitro, CD34⁻KSL cells from old mice showed a greater capacity to give rise to primitive CD48⁻KSL cells with reduced HSC activity. Gene expression profiling identified age-related changes in the expression of several cell cycle regulatory genes, including p21/Cdkn1a and p18/Cdkn2c. These results support the notion that HSC aging is largely regulated by an intrinsic genetic program.