Role of oxidative stress and telomerase on haematopoietic stem and progenitor cell ageing

Abstract

Ageing is associated with the functional decline of both haematopoietic stem and endothelial progenitor cells (HSPCs and EPCs), leading to an imbalance between cellular damage and repair. Telomeres are the end caps of chromosomes that maintain chromosomal integrity and shorten with age. Telomerase is the enzyme responsible for telomere replication. The absence of telomerase leads to premature ageing. Oxidative stress as well as metabolic stress and short telomeres are key contributors to the manifestation of different age-related diseases. However, the exact effect of these factors on HSPCs and EPCs is not clear. The effects of metabolic stress were studied by the addition of different glucose concentrations to low passage (early) cells in culture. Metabolic stress impaired the growth of EPCs and CD34+ HSPCs. There was no change in telomerase enzyme activity under metabolic stress in CD34+ HSPCs. However, metabolic stress upregulated the metabolic co-activator PGC-1α in EPCs but not in CD34+ HSPCs. The effects of oxidative stress were investigated by incubating peripheral blood EPCs, and cord blood CD34+ expanded HSPCs under 40% O2 in culture. While early EPCs show resistance to oxidative stress, CD34+ HSPCs showed impaired growth and differentiation potential. This impairment was associated with increased telomerase activity, no changes to TERT or TERC expression, and maintenance of telomere length. Oxidative stress limited CD34+ HSPC myeloid differentiation. In particular, CD15+ granulocytes were more sensitive to oxidative stress than CD14+ monocytes. Furthermore, CD15+ granulocytes reduced the expression of TERC during myeloid differentiation. In contrast to CD34+ HSPCs under growth conditions, there was no increase in telomerase activity during myeloid differentiation under oxidative stress. To investigate the effects of ageing in vivo with telomerase dysfunction, HSPCs from bone marrow of aged telomerase deficient TERT-/- and TERC-/- first generation mice were studied. Ageing resulted in the accumulation of Lineage-Sca-1+CKit+ stem cells and CFU-GM colonies in wild type mice. TERT-/- mice without telomere shortening showed a normal phenotype at young age (1.5-7.5 months) and augmented ageing of bone marrow with increased age (22 months). On the other hand, TERC-/- mice with short telomeres led to a premature ageing bone marrow phenotype, even at young ages (8-12 months). Interestingly, both TERT-/- and TERC-/- showed more erythroid progenitor colonies. Furthermore, short- (7 months) and long-term (16 months) dietary restrictions ameliorated the ageing bone marrow phenotype. Together, the data presented demonstrates the damaging effect of oxidative and metabolic stress on humans in early EPCs and CD34+ HSPCs. In a mouse model, normal ageing disrupted HSPCs. Telomerase deficiency augmented normal ageing, whilst short telomeres appear to be a major determinant of ageing. These ageing phenotypes in mice can be ameliorated by dietary restriction.