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Intrinsic HSC aging
tion in daughter cells that was more asymmetric in young compared to old pairs. Signature-based, daughter stem cells were in general associated to an overall lower amount of ATAC-seq peaks, suggesting a progressive increase in chromatin accessibility upon differentiation.15 Interestingly, both young and aged daughter stem cells preserved open chromatin regions linked to glycolysis and small RhoGTPases signatures. Aging was found to impact prominently on the specific signature of open regions associated to critical signaling pathways in HSC (Wnt sig- naling was associated with aged HSC, while young cells were enriched for VEGF, TGFβ and EGF signaling). In addition, daughter progenitors maintained signatures linked to lipid metabolism and platelet homeostasis in both young and aged cells, while mostly they differed again for open regions linked to specific signaling path- ways (interferon, IFG1 and IL2 were characterizing young cells and TNF was enriched in aged cells).15
Specific chromatin architectural alterations affecting broader alterations have also been detected in aged murine HSC, such as the relative position of chromo- somes. We have observed that chromosome 11 homolog proximity changes upon aging specifically in HSC. This chromosomal rearrangement is dependent on LaminA/C expression and Cdc42 activity.29 Indeed, we demonstrat- ed that upon aging and in chronologically young LaminA/C knock-out HSC, chromosome 11 homolog proximity was altered and this correlated with the func- tional impairment of stem cells. Inhibiting Cdc42 activity could restore LaminA/C expression and localization, and also chromosome 11 homolog proximity. Overall, these findings support the functional relevance of chromatin architecture also for HSC aging and, most importantly, show for the first time that is possible to pharmacologi- cally target the chromatin architecture to rejuvenate aged HSC function.29
Aging and the epigenetic drift
Epigenetic drift is the term that covers all changes that have a general effect on the epigenome and the chromatin architecture.99,100 Recently, an exciting report of the epige- netic drift in murine aging across tissues has been pub- lished. Benayoun et al. showed that the aged-related alter- ations observed in the epigenome and transcriptome land- scape lead to induction of inflammatory responses, mainly interferon related, across different tissues upon aging.100 In humans, the epigenetic drift has been studied at the sin- gle-cell level on peripheral blood mononuclear cells from subjects of different ages.101 In agreement with previous studies performed on bulked murine HSC,27,28 they observed an increment of H3K4me3 and H3K27me3 with age. These variations were reflected in transcriptional alterations and an association of H3K4me3 with DNA breakage and translocation was also observed.102
The fact that the observed epigenetic drift is consistent between mice and humans suggests that it is a conserved phenomenon in mammals. However, its ultimate manifes- tation seems very heterogeneous, even within cells and tissues of the same individual. This has been demonstrat- ed using different approaches such as the generation of a multi-color Hue mouse model as a clonal tracking tool26 and single-cell transcriptomics to study the transcriptional behavior of each clone.26,103 These two studies agreed in identifying (based on transcriptome analysis) distinct clones in aged individuals that have different regulatory
states as a consequence of the epigenetic drift occurred upon aging.26,103
Metabolism and hematopoietic stem cell aging
Hematopoietic stem cells are quiescent during most of their lifespan and their metabolic needs are relatively low. However, one of the aging hallmarks is the dysregulation of metabolic pathways, and this aspect might indeed be critical for HSC function, too. Here we detail some of the key metabolic aspects related to HSC aging (Figure 3).
Effects of oxidative stress and mitochondria homeosta- sis in hematopoietic stem cell aging
It has been shown that artificially altering mitochondrial function can lead to acquiring aging phenotypes in HSC, such as loss of self-renewal and lymphoid/myeloid skew- ing. Sometimes these mitochondrial-related aging pheno- types can be reverted by the addition of redox scavenger compounds [N-acetylcysteine (NAC), rapamycin or MAPK inhibitors]. Normally, rapamycin and calorie restriction can only partially rescue the effects of aging due to intrinsic deteriorated mitochondrial activity (reviewed by Chandel et al.5).
Reactive oxygen species (ROS) are the natural side- product of oxygen-based metabolism and are produced mainly by mitochondria. The effect of ROS in aging has been historically attributed only to the oxidation of DNA, RNA and proteins, but it has an important role in cell sig- naling, too. ROS can be detected by redox sensors that ultimately fire oxidative stress response such as enzymes and transcription factors (reviewed by Bigarella et al.104).
In HSC, high levels of ROS can generate oxidative stress and lead to loss of quiescence in the cells; therefore, ROS levels have to be tightly controlled (reviewed by Bigarella et al.104). Quiescent HSC have a low metabolic rate, which involves low levels of reactive oxygen species (ROS).105 It has been shown in mice that the frequency of HSC with low levels of ROS decreases with age, suggesting that ROS generation/accumulation is a distinctive characteris- tic of aging.106 Increments of ROS levels in adult HSC have consequences that are somewhat similar to the aging phe- notype such as myeloid lineage skewing and defective long-term repopulation activity, as has been demonstrated in the case of FoxO transcription factors depletion in mice.107,108 It is very interesting that high levels of ROS in the FoxO depleted HSC can be rescued by the addition of the antioxidant NAC,108 but only for some weeks.107 The importance of FoxO3 in LT-HSC function goes beyond regulation of ROS levels since it is essential for activation of the autophagy gene program.109
Reactive oxygen species are also augmented in HSC of aged SIRT3 knockout (KO) mice, negatively affecting their function, but not in young SIRT3 KO mice. Intriguingly, overexpression of SIRT3 rescues functional aged-related defects in HSC. SIRT3 is a deacetylase that regulates mito- chondrial protein acetylation in mammalian cells and it is enriched in young compared to aged HSC. Therefore, the role of SIRT3 in the maintenance of proper ROS levels in HSC is essential during aging.110
Mitochondrial homeostasis refers to the correct regula- tion of the mitochondria function within the cell. Studies using “mutator mice” (proof reading mutant for PolγA, the catalytic subunit of mtDNA polymerase that is encoded in the nuclear genome) showed phenotypes indicative of
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