Age-associated differences in human hematopoietic stem cell proliferation control

Hematopoietic stem cells (HSCs) comprise a functionally and molecularly heterogeneous population of cells that collectively maintain the lifelong production of mature blood cells. Known developmental changes in their properties include an early postnatal switch from a rapidly cycling high self-renewal state to a quiescent state with an overall reduced self-renewal potential. Additional age-associated alterations in mouse HSC properties have been predicted but these have remained poorly explored in human HSCs. Of recent interest has been the finding that most healthy humans of advancing age possess enlarged clones of normal blood cells marked by somatic mutations in genes commonly involved in hematopoietic malignancies. Together, these findings suggest that altered regulation of HSC cycling may be a feature that develops with advancing age in humans. To examine this possibility, an investigation of the HSC and progenitor compartments in healthy human donors aged 0-69 years was initiated. A first characterization of cells within the HSC-enriched subset characterized by a CD34+CD38-CD45RA-CD90+CD49f+ (CD49f+) phenotype showed that these cells are consistently present from birth to old age at similarly low frequencies. Functional assays showed their long-term and lympho-myeloid differentiation capacity in vitro and in vivo (in transplanted immunodeficient mice) to be similar but indicated a possible increased in vivo supportive requirement with age. Kinetic analyses of individually tracked CD49f+ cells further revealed a strong and progressive aging-related delay in completing their first and subsequent divisions in vitro that was exacerbated when the growth factor stimulus was reduced. Development of a method for simultaneous cell-cycle staging and multiplexed molecular analysis of single CD49f+ cells traced this delay to a mitogen-sensitive G1 elongation which was also evident at slightly later stages of hematopoietic cell differentiation. This delay appeared related to a reduced immediate growth factor-induced activation of AKT and β-catenin obtained in adult cells. These findings point to a newly identified intrinsic and pervasive, aging-related alteration in specific early signaling intermediates that are required to drive G1 progression in HSCs (and their early progeny) and lay the foundation for further analyses of how this regulatory change may impact the acquisition of other aging-related phenotypes in the hematopoietic system.Medicine, Faculty ofExperimental Medicine, Division ofMedicine, Department ofGraduat