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Intrinsic HSC aging
Metabolic alterations and impaired autophagy
Hematopoietic stem cells are characterized by having a low metabolic rate, being essentially glycolytic while qui- escent.5,31 Upon activation, young HSC change towards a more oxidative metabolism that can be reverted when they return to quiescence (reviewed by Verovskaya et al.32). However, in aged HSC, basal metabolism is shifted to a higher level of oxidative metabolism (reviewed by Verovskaya et al.32), which increases reactive oxidative species (ROS) leading to oxygen related-stress, accompa- nied by impaired regenerative potential (reviewed by Chandel et al.5). Metabolic stress can fire autophagy in the cell, which is a “housekeeping" mechanism involved in self-degrading components of the cell in response to a spe- cific stress.33 The process consists of enclosing organelles or portions of the cytosol within double membrane vesicles, which later fuse with the lysosome, where the degradation takes place.33,34 Autophagy deregulation has been related with aging and with age-related diseases such as cancer.34,35
Altered proteostasis
Proteostasis or protein homeostasis is described as the situation of balanced levels of protein biogenesis and degradation.36 Proteostasis is regulated by several mecha- nisms such as autophagy, the ubiquitination proteasome system (UPS), the unfolded protein control system (UPR), and the proper levels of protein translation.37 As men- tioned above, autophagy is impaired with age, but also UPS and UPR levels are reduced leading to a situation of protein stress that contributes to loss of regenerative potential of aged HSC.36,38,39
as immunosenescence, skin fragility and osteoporosis. However, these syndromes recapitulate only some physi- ological aging characteristics, but not all.46
In the case of HSC, work performed on mice deficient in DNA repair proteins shows an expected reduction in HSC repopulation potential together with a decrease in their self-renewal capacity, driving a reduction of the whole HSC pool.31,47,48 As one of the most evident pheno- types occurring upon chronological aging is the expansion of HSC, the phenotype observed in mice deficient in DNA repair proteins sharply contrast with physiological HSC aging. More recently, it was elegantly shown by genetic barcoding and clonal functional evaluation of young and aged HSC followed by induced pluripotent stem cells (iPSC) generations and re-differentiation that, despite the heterogeneity of the aged HSC pool, several functional aspects of HSC aging could be reversed to a young-like state. These data strongly support the view that accumu- lations of DNA mutations in genes critical for hematopoiesis would not be the principal mechanism for the aged-dependent functional decline of HSC,49 although they might contribute to clonality.22,23
Alterations in intrinsic signaling pathways
γH2A has been historically interpreted as being a marker of DNA damage in general, and in HSC in particular, since it is one of the first signals occurring at the damaged site and it is needed for subsequent DNA repair.48,51 Aged HSC present with increased numbers of γH2A foci compared to young HSC.48 Therefore, it was logical to conclude that there is deficiency in DNA repair in old HSC because of the persistence of γH2A. Interestingly, γH2A foci have been demonstrated to be not only markers of double strand breaks (DSB). γH2A also signals DNA replication stress (i.e. replication fork collapse) and it accumulates at rDNA in old quiescent HSC. Furthermore, in aged murine HSC, γH2A cannot be removed by the specific phos- phatase (PP4c) as efficiently as it is removed in young HSC because PP4c is mis-localized.21
Signaling pathways such as TGF1-β, Notch, NF-kB or
Wnt have an essential role in modulating the response of
the hematopoietic system within the hematopoietic
niche, tightly regulating the balance between quiescence
and differentiation.40-43 These pathways are intrinsically
altered in aged HSC, with a consequent effect on their function.15,28,44,45
The hallmarks of intrinsic HSC aging cover a very diverse group of biological characteristics that might nev- ertheless be all interconnected. Here, we will focus on some of these intrinsic molecular aspects, that are implied in driving functional and phenotypical alterations of HSC upon aging.
Is DNA damage a driver of HSC aging?
DNA damage and genomic instability are thought to be primary causes of mutation accumulation upon aging, leading to cancer or premature aging of tissues.19
Here we summarize what is known about the effects of DNA damage on aging of HSC under a different perspec- tive, with consideration of new data generated by state- of-the-art techniques.
From DNA damage to hematopoietic clonality
There are premature aging syndromes, such as Werner or Bloom syndrome, in which the cause of the disease is the accumulation of mutations due to an inefficient DNA damage repair. The accumulated mutations in these syn- dromes lead to different kinds of cancer to arise at early age together with some premature aging phenotypes such
Moreover, murine young and old HSC have been demonstrated to be equally efficient in repairing DNA under induced DNA damage.20,21,52 HSC make this possible by having the main DNA repair pathways attenuated while quiescent and reactivating them once they re-enter the cell cycle.20 In addition, when young and old HSC are subjected to DNA damage, they lack G1-S arrest and their apoptosis rate increases in order to minimize the acquisi- tion of mutations.52
In spite of the ability of HSC to avoid mutation accumu- lation through age, a series of point mutations have been found in humans to be predominant with age, as is the case of mutations in the epigenetic regulators DNMT3, TET2 and ASXL1, among others.53-56 These mutations appear frequently in healthy older individuals and they are also found over-represented in MDS and AML patients.23 It can be hypothesized that these mutations confer a com- petitive advantage that leads to clonal hematopoiesis. This is supported by some studies in the murine hematopoietic system in which the loss of Dnmt3a or Tet2 enhances the self-renewal potential of HSC.57 DNMT3a and TET2 are epigenetic modifiers: DNMT3s catalyze DNA methyla- tion (mC) and TET2 oxidizes mC to hydroximethyl-C,
Still, mutations occur in every tissue and they are shown to accumulate with age. It has been calculated that somat- ic mutations in human HPSC occur and accumulate in life in a linear rate of 14bp base substitutions per year, which is similar to or even lower than other tissues.50
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