It has been postulated that as we age, accumulated damage causes stem cells to die by apoptosis. This could lead to a diminished stem cell pool and consequently a reduced organ regeneration potential that contributes to somatic senescence. Hematopoietic stem cells have evolved many mechanisms to cope with their exposure to toxins during life. Cell surface transporters and anti-toxic enzymes are highly expressed in hematopoietic stem cells. If toxins do get the opportunity to damage the DNA of stem cells then the cell is more likely to die by apoptosis than attempt DNA repair and risk an error.
Summarised below are our results from an investigation of the frequency, phenotype, cell cycle status and repopulation potential (in young recipients) of C57BL6 side population (SP) cells from mice with a range of ages. The absolute frequency of SP cells increases with age (Figure-A). The proportion of the lineage negative, Sca-1+, c-kit+ (KLS) cell population that is an SP stem cell increases from ~1% to over 30% during the murine lifetime (red bars in Figure-B). These SP cells from older mice have a reduced 4-month competitive repopulation potential when compared to SP cells from younger mice but contain a similarly low proportion of phenotypically-defined mature cells (blue bars in Figure-B) and have a similar cell cycle profile and progenitor cell output (2% of 3 x 96 well plates for each). SP cells from older mice contained a higher proportion of SP cells with the highest efflux ability (61 vs 414 days, p=<0.001, n=6) Engrafted cells derived from old SP cells 4 months previously still displayed an increased SP frequency when compared to engrafted cells derived from SP cells of young mice.
Hence, more progenitors or committed cells have not gained the SP ability; rather this difference in SP distribution reflects an age-dependent change in hematopoietic stem cell biology that is independent of the microenvironment. Specifically, the proportion of stem and progenitor cells (KLS) that is a stem cell (SP fraction of KLS) increases with age.
We hypothesize that this may be a progressive enrichment of primitive cells over time via selection. As we age, accumulative damage to hematopoietic stem and progenitor cells causes more cells to die by apoptosis. It may be that the stem/progenitor cells with the lowest hoechst efflux ability are most susceptible to damage and hence most likely to die by apoptosis. Since the HSCs with the highest efflux of hoechst are thought to be the most primitive, it may be that there is an enrichment of primitive cells. This could account for the increased SP proportion observed within KLS cells. As there may be cells with ABC/G2 activity that is undetectable via the SP technique, selection of cells with a higher pump activity could also explain the increased SP frequency we observed. This hypothetical mechanism would also be independent of microenvirinment.
In summary, we surmise that HSCs have a mechanism that copes with cellular damage while compensating for the reduced cellular output of HSCs with age by increasing the absolute number of HSCs.