rapamycin) shown in mice may be mediated by their autophagy‐inducing properties.40 Thus, loss of mechanisms controlling protein homeostasis, especially autophagy, may be involved in ageing, and there is growing interest in measuring these mechanisms as markers of biological age.27
Metabolic dysfunction
Major metabolic changes during ageing are insulin resistance, body composition modifications (increase in visceral fat mass and decrease in skeletal muscle mass), and decline in both sex steroids and hormones of the somatotrophic axis. This axis comprises the growth hormone and insulin‐like growth factor (IGF‐1), which shares a downstream intracellular pathway with insulin, thereby signalling nutrient abundance and anabolism. Interestingly, genetically driven reductions in the functions of GH, IGF‐1 receptors, insulin receptors, and their intracellular effectors (including mammalian target of rapamycin, mTOR) are associated with longevity in humans and model organisms.41,42 More comprehensively, insulin/IGF‐1 axis activity decreases during ageing, whereas its constitutive impairment extends longevity. This paradox may be understood if we consider intense trophic and anabolic activity as an accelerator of ageing, and the downregulation of insulin/IGF‐1 and mTOR pathways as defensive responses against systemic damage during ageing, aimed at reducing cell growth and metabolism.15,43
Furthermore, both dietary restriction and drugs that mimic it (e.g. rapamycin) were shown to increase lifespan in animal models (including mice) through the effect on the insulin/IGF‐1 pathway, emphasising the role of deregulated nutrient‐sensing in the biology of ageing.44,45 Of note, mTOR inhibitors are currently approved and used as immunosuppressive drugs for recipients of organ transplants. Nevertheless, given their side effects, their net effect on human ageing remains to be determined. Metformin is also seen as a potential anti‐ageing drug due to its positive effect on deregulated nutrient sensing and mitochondrial dysfunction, DNA damage, and inflammation.46
Cell senescence
Cellular senescence is a state of stable arrest of the cell cycle coupled to phenotypic changes, including the production of several molecules (especially matrix metalloproteases and pro‐inflammatory cytokines) collectively known as the senescence‐associated secretory phenotype (SASP),47 that contributes to senescence spreading among others cells, inflammation, and tissue dysfunction. Of note, a better understanding of molecular mechanisms of cell senescence led to a more complex picture of a multi‐step progressive and dynamic phenomenon rather than a static endpoint.48
As cited earlier, cellular senescence was originally described by Hayflick in human fibroblasts in vitro 21; this phenomenon, called replicative senescence, is now known to be caused by telomere shortening. Nevertheless, cell senescence can be triggered by other stimuli during ageing, notably non‐telomeric DNA damage and excessive mitogenic signalling, particularly by the p16Ink4a tumour‐suppressor protein upon epigenetic de‐repression of the ink4/ark locus.49 p16Ink4a positively correlates with age in various tissues in mice and human skin.50,51 In a meta‐analysis of 372 genome‐wide association studies (GWASs) aiming at identifying susceptibility polymorphisms for age‐associated diseases, the ink4/ark locus was linked to the greatest number of diseases, including Alzheimer’s, cardiovascular diseases, cancer, and type 2 diabetes.52 Furthermore, the number of cells expressing p16Ink4a in muscular fat correlates with muscle strength and walking performance.53
Since the number of senescent cells is positively associated with age, it has been postulated that cell senescence contributes to ageing. However, this phenomenon may also be seen as a mechanism to prevent the propagation of damaged and potentially oncogenic cells and to trigger their elimination by the immune system. Tissue dysfunction and ageing could then only be explained by an impaired turnover of cells due to reduced clearance of senescent cells and/or reduced regeneration by progenitor cells. As described previously for free radicals, this dual role of cell senescence in ageing can reconcile apparently contradictory effects of experimental modulation of p16Ink4a activity on health and lifespan in mice.15,54‐56
Interestingly, the selective elimination of senescent cells attenuates age‐related deterioration of several organs and extends lifespan in mice,57 and the potential anti‐ageing effects of senolytic drugs is an intense area of research.58
Stem cell exhaustion
Stem cells are present in all tissues and organs after development and regenerate damaged tissues throughout life. The repair and regenerative potential of many tissues declines with ageing due to functional attrition in several stem cell compartments (e.g. hematopoietic, neural, mesenchymal, and intestinal epithelial stem cells, as well as satellite cells in muscles).59 Stem cell exhaustion is seen as an integrative consequence of several components of biological ageing discussed in this chapter, including DNA damage, epigenetic alterations, telomere shortening, cellular senescence, and mitochondrial dysfunction.15 Hematopoietic stem cells are probably the most‐studied (partly because they are more easily available), and stem cell exhaustion is thought to be a major driver of immunosenescence (see the next section).
As mentioned earlier, reduced regeneration of tissues by progenitor cells may theoretically contribute to tissue dysfunction and, thus, to the ageing process. However, stem cell exhaustion is difficult to measure before the onset of its clinical consequences, and evidence is scarce for a contribution of this phenomenon to ageing. It is worth mentioning that transplantation of muscle‐derived stem cells from young mice extends the health‐ and lifespan of progeroid mice, even in tissues with undetectable donor cells, suggesting that they may be beneficial through secreted factors.60
Immunosenescence
The term immunosenescence is not restricted to senescence (as a component of biological ageing described earlier) of immune cells but encompasses all immune system modifications that characterise ageing. Components of biological ageing discussed in this chapter are probably synergistic in reducing functions of several physiologic systems, thus contributing to different phenotypes of ageing. Among these physiologic systems, we propose here to emphasise the role of the immune system for two reasons.
First, a major feature of the biology of ageing is chronic low‐grade inflammation (also called inflammageing 61), thought to contribute to age‐related diseases and ageing phenotypes.62 Known causes of this inflammation are other components of biological ageing (mitochondrial dysfunction, cell senescence, loss of proteostasis, epigenetic alterations) and extrinsic factors (chronic infections and changes in microbiota).63 Thus, one could argue that inflammation is more of a bystander consequence than a phenomenon that contributes to ageing. Despite associations between higher levels of inflammatory cytokines, age‐related diseases, and mortality, it is still somewhat unclear how (and even whether) chronic low‐grade inflammation contributes to the ageing process. Nevertheless, ageing mice lacking proteins of the inflammasome pathway exhibit less decline in immune, physical, and cognitive functions than wild‐type mice,64,65 and this pathway is involved in Alzheimer’s disease pathophysiology.66 In addition, if we consider inflammation as a common link between several components of biological ageing and phenotypes of ageing, it becomes an interesting target for intervention. In line with this, drugs specifically designed to block pro‐inflammatory cytokines, such as interleukin‐1β, have reduced the incidence of age‐related diseases in humans.67,68
Second, as the function of the immune system is to maintain homeostasis, it is easy to conceive that defects in this system will exacerbate detrimental effects of other components of biological ageing by failure to eliminate not only pathogens but also pre‐malignant cells, senescent cells, and misfolded proteins. Thus, its contribution
