3. Mitochondrial Metabolism
Despite the high preference of mitochondria for glycolysis, recent studies have also highlighted the importance of mitochondrial respiration to HSC for proliferation and maintenance [
48,
49,
50]. We have recently shown that HSCs have a relatively high number of mitochondria, which are not completely inactive [
51]. Indeed, mitochondrial membrane potential (ΔΨ
mt) is high in HSCs, although ATP production or intracellular ROS levels are low [
52,
53]. The higher complex II: complex V ratio gives rise to high ΔΨ
mt in HSCs due to limited coupling of the electron transport chain (ETC), which supports the idea that mitochondrial complex II is pivotal for both HSC maintenance and the prevention of the aging process [
53]. Indeed, inhibition of complex II reduces the in vitro colony-replating capacity of HSCs [
53], and genetic mutation of
mev-1, a subunit of the succinate dehydrogenase cytochrome b enzyme, which is a component of complex II, leads to oxygen hypersensitivity and premature aging of HSCs [
54]. Studies of
C. elegans uncovered that the mutated or silenced components of ETC or the ATP synthase can markedly extend [
55,
56,
57] or reduce lifespan [
58]. Although these varying experimental results must eventually be resolved, it is clear that imbalances in ETC activity are closely linked to the overall survival of the organism. Interestingly, a recent paper showed that ΔΨ
mt is a source of heterogeneity in old HSCs, with a prevalent fraction of low ΔΨ
mt in aged HSCs. Enhancement of ΔΨ
mt by mitoquinol (Mito-Q), a mitochondrial-targeted coenzyme-Q10 [
59], successfully increased ΔΨ
mt of old HSCs and ameliorated or prevented onset of aging phenotypes [
60].
HSCs are mainly dormant but can become highly active on demand, either to maintain hematopoietic homeostasis by replenishing matured/immature hematopoietic cells, or to respond to situations of emergency, such as infection or blood loss [
61]. This shift requires a metabolic switch from glycolysis to mitochondrial oxidative phosphorylation, which is precisely regulated by various signaling pathways. The mammalian TOR (mTOR) pathway is a key regulator of cellular and mitochondrial metabolism. mTOR directly controls the mitochondrial oxidative function through a YY1–PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) transcriptional complex [
62]. Defects of
Tuberous sclerosis complex subunit 1 (
TSC1), the major negative regulator for mTOR [
63], lead to increased mitochondrion biogenesis and accumulation of ROS. Blockade of ROS activity in vivo restores these HSC defects, demonstrating that the TSC-mTOR pathway controls the quiescence and on-demand functions of HSCs by repressing ROS production [
64,
65].
HSCs exhibit low AKT/mTOR activity, but, upon stress, upregulation of this pathway drives dormant HSCs toward activation [
64,
66]. Interestingly, the dysregulation of AKT/mTOR signaling correlates with the aging process in HSCs [
67]. Experimental evidence has shown that mTOR activation is involved in HSC aging, as well as that rapamycin treatment restores HSC potential and prolongs the lifespan of mice [
68]. mTOR activity is higher in HSCs from elder mice than younger mice, and mTOR activation, through conditional deletion of
TSC1 in the HSCs of young mice, mimics the phenotype of HSCs from aged mice; similarly, in older mice, rapamycin restores the self-renewal capacity of HSCs and, importantly, correlates with increased life span [
68].
ASXL1 is frequently mutated in age-related clonal hematopoiesis. Its mutation activates the AKT/mTOR pathway, causing aberrant cell cycle progression in the HSC compartment and provoking HSC dysfunction. This is associated with mitochondrial activation, elevated ROS levels, and increased DNA damage, leading to age-associated phenotypes, such as myeloid-biased differentiation, hypocellular bone marrow, and increased frequency, of phenotypic LT-HSCs. Inhibition of the AKT/mTOR pathway can partially rescue these phenotypes, suggesting its involvement in the enhanced aging of the hematopoietic system [
69].
A similar phenotype is observed in wild-type p53-induced phosphatase 1 (WIP1), which is highly expressed in HSCs but decreases with age.
WIP1-deficient (
WIP1−/−) mice exhibit multiple aging-like phenotypes in HSCs, including declines in reconstitution ability and HSC expansion. Mechanistically, their impaired regenerative capacity is due to a p53-mediated differentiation defect, whereas increasing numbers of
WIP1−/− HSCs are associated with mTOR-mediated cell cycle progression of HSCs [
70]. Notably, experimental results have shown that aged HSCs have higher mTOR [
71] activity levels, as well as that its inhibitor rapamycin can restore the self-renewal of aged HSCs, an effect which can be translated to human HSCs [
72].
Recent advances have demonstrated that epigenetic, transcriptional, and post-transcriptional mechanisms also control the quiescence of HSCs, which are maintained in a paused state that allows for rapid activation [
73]. Mitochondrial activity modifies the epigenetic state of cells affecting their aging process [
74]. Citric acid, generated by the tricarboxylic acid (TCA) cycle in the mitochondria, modulates histone acetylation and gene expression through its conversion to acetyl-CoA [
74]. Mitochondrial fatty acid oxidation (FAO) also generates acetyl-CoA for histone modification in HSCs [
75].
Sirtuins are a family of protein deacetylases, which regulate the mitochondrial metabolic checkpoint in stem cells, and they are key regulators of stem cell aging [
21,
76]. SIRT3 plays a critical role in the mitochondria, where it deacetylates two critical lysine residues on SOD2 to promote the antioxidative activity. Brown and colleagues have demonstrated that SIRT3 is highly enriched in HSCs, as well as is suppressed with aging [
77]. Although SIRT3 has no effect on HSCs maintenance or tissue homeostasis at a young age under homeostatic conditions, it is essential under stress or in old age. Indeed, SIRT3 loss induces HSC quiescence and compromises regenerative capacity in old mice [
77].
SIRT1 is a key regulator of HSCs self-renewal and lineage specification under homeostasis. Interestingly, Ghaffari’s group has shown that loss of
SIRT1 causes anemia and myeloid expansion at the expense of the lymphoid compartment, overlapping features with aged HSCs. SIRT1 plays a role in HSCs homeostasis by targeting FOXO3, a longevity transcription factor and mitochondrial ROS regulator [
78].
Another key regulator of metabolism is nicotinamide adenine dinucleotide (NAD
+). Decreased levels of NAD
+ are associated with cancer, metabolic disorders, and physiological and accelerated aging processes [
79,
80,
81]. Supplementation of nicotinamide riboside (NR), a NAD
+ precursor, significantly improved lifespan and health span in model of aged-related disease, such as ataxia–telangiectasia mutation (ATM), thanks to the improvement of both DNA damage repair and mitophagy [
71,
82]. Murine models of ATM loss show defects in DNA damage repair associated with mitochondrial dysfunction [
83] and loss of hematopoietic stem cell (HSC) potential [
23]. NR treatment caused significant alterations in lineage commitment of HSCs with enhanced lymphoid potential [
84].