Tracking <i>clusterin</i> expression in hematopoietic stem cells reveals their heterogeneous composition across the life span

Hematopoietic stem cells (HSCs) exhibit significant age-related phenotypic and functional alterations. Although single-cell technologies have elucidated age-related compositional changes, prospective identification of aging-associated HSC subsets has remained challenging. In this study, using clusterin (Clu)-green fluorescent protein (GFP) reporter mice, we demonstrated that Clu expression faithfully marks age-associated myeloid/platelet-biased HSCs throughout life. Clu-GFP expression clearly segregates a novel age-associated HSC subset that overlaps with but is distinct from those previously identified using antibodies against aging maker proteins or reporter systems of aged HSC signature genes. Clu-positive (Clu+) HSCs emerge as a minor population in the fetus and progressively expand with age. Clu+ HSCs display not only an increased propensity for myeloid/platelet-biased differentiation but also a unique behavior in the bone marrow, favoring self-renewal over differentiation into downstream progenitors. In contrast, Clu-negative (Clu-) HSCs exhibit lineage-balanced differentiation, which predominates in the HSC pool during development but becomes underrepresented as aging progresses. Both subsets maintain long-term self-renewal capabilities even in aged mice but contribute differently to hematopoiesis. The predominant expansion of Clu+ HSCs largely drives the age-related changes observed in the HSC pool. Conversely, Clu- HSCs preserve youthful functionality and molecular characteristics into old age. Consequently, progressive changes in the balance between Clu+ and Clu- HSC subsets account for HSC aging. Our findings establish Clu as a novel marker for identifying aging-associated changes in HSCs and provide a new approach that enables lifelong tracking of the HSC aging process.