Stem cell exhaustion

Stem cell exhaustion is the progressive failure of adult stem and progenitor cells to maintain and repair the tissues they serve. It is classified as an integrative hallmark in the Hallmarks of aging framework, meaning it is understood as a consequence of upstream damage that becomes a cause of visible decline in its own right. The term is slightly misleading: in several tissues the number of stem cells stays constant or rises with age while their per-cell function falls.

§Blood

The haematopoietic system is the best-characterized case because its stem cells can be purified, counted, and functionally tested by transplantation. Aged mouse haematopoietic stem cells are more numerous than young ones but reconstitute an irradiated recipient less well, and they produce a skewed output: more myeloid cells, fewer lymphoid.¹ The human correlate is a shrinking naive T-cell pool, weakened responses to new vaccines, and thymic involution that begins in childhood and is largely complete by middle age.

§Clonal hematopoiesis

The most consequential discovery in this area is clonal hematopoiesis of indeterminate potential, or CHIP: the expansion of a single mutant stem cell clone to a detectable fraction of blood cells in people with no blood disorder. Two 2014 studies identified it in large sequencing datasets, found that it becomes common after age 60, and showed that carriers face elevated risks of haematological malignancy and, unexpectedly, of death from cardiovascular causes.²³ The driver mutations cluster in a small set of genes, chiefly DNMT3A, TET2, ASXL1, JAK2, TP53 and PPM1D — mostly epigenetic regulators and DNA damage response genes.

The cardiovascular association is thought to run through inflammation: macrophages descended from a TET2-mutant clone secrete more interleukin-1β and interleukin-6, accelerating atherosclerotic plaque development in mice. That connects CHIP directly to Inflammaging and offers one of the few cases where a specific somatic mutation in an aged human is linked to a specific inflammatory mechanism and a specific disease.

Whole-genome sequencing of individual stem cell colonies has since shown that the change is not confined to people with detectable CHIP. Clonal diversity in human haematopoiesis stays high through midlife and then falls sharply after about age 70, with the blood of the very old produced by a small number of expanded clones.⁴ The proximate cause appears to be a shift in the balance between drift and selection rather than a sudden loss of stem cells.

By the numbers

CHIP is detectable by standard sequencing in a minority of people in their sixties and in a substantially larger fraction over 70. With ultra-sensitive assays, some degree of clonal expansion can be found in nearly everyone past middle age, which makes the clinical threshold a matter of convention rather than biology.

§Muscle

Skeletal muscle repair depends on satellite cells, quiescent progenitors under the basal lamina of each fibre. Their number declines modestly with age; their capacity to activate, proliferate and fuse declines more. Some geriatric satellite cells lose quiescence not by activating but by entering senescence, a switch associated with de-repression of the p16INK4a locus.⁵ Sarcopenia — the loss of muscle mass and strength with age — is the visible outcome, and it is the single functional decline most responsive to resistance training.

§Gut, skin and pigment

Intestinal epithelium turns over every few days, driven by Lgr5-expressing crypt base columnar cells supported by neighbouring Paneth cells. Aged mouse intestinal stem cells form organoids less efficiently, and both the stem cells and their niche contribute. A day of fasting improves their function in young and old mice through a shift to fatty acid oxidation, which is one of the more direct demonstrations that dietary restriction acts on a stem compartment.⁶

Hair greying is the most visible instance of the hallmark. It results from incomplete maintenance of melanocyte stem cells in the hair follicle bulge, which are progressively lost rather than merely inactivated.⁷

§Nervous system

Whether the adult human brain generates new neurons at all remains disputed. Two studies published in 2018 reached opposite conclusions from human hippocampal tissue, one finding that neurogenesis becomes undetectable in adults and the other that it persists into old age; subsequent work has reported abundant immature neurons in healthy adults with a sharp decline in Alzheimer's disease. The disagreement turns substantially on tissue fixation and antibody specificity. Rodent hippocampal neurogenesis clearly declines with age. Whether that finding carries over to humans is precisely what the conflicting human studies leave open.

§Niche versus cell-intrinsic causes

Heterochronic experiments separate the two contributions. Transplanting old haematopoietic stem cells into young recipients does not restore balanced lineage output, indicating that blood stem cell aging is largely cell-intrinsic and written into the epigenome. Exposing old muscle to a young systemic environment through heterochronic parabiosis substantially restores satellite cell activation, indicating that muscle stem cell aging is substantially extrinsic.⁸ Both results are from mice, and the parabiosis literature has since become entangled in disputes over which circulating factor, if any, is responsible.

Intrinsic causes include accumulated DNA damage, telomere attrition in high-turnover compartments, epigenetic drift, declining Autophagy that leaves damaged organelles to be inherited at division, and mitochondrial impairment. Niche causes include fibrosis and matrix stiffening, altered Wnt and Notch signalling, marrow adiposity, and the secretory output of accumulated senescent cells.

§Interventions

Haematopoietic stem cell transplantation is a mature therapy, but it replaces a compartment rather than rejuvenating one, and it carries conditioning toxicity that makes it unsuitable for anything short of life-threatening disease. Among experimental approaches, senolytic clearance improves muscle stem cell function in aged and progeroid mice, and transient reprogramming restores regenerative capacity in mouse muscle and other tissues. Neither has been shown to improve regeneration in an aged human.

Unproven clinics

A large international market sells "stem cell therapy" for aging, joint pain and neurological disease, typically using autologous adipose-derived cells that have not been shown to engraft or differentiate. Regulators including the US Food and Drug Administration have taken enforcement action against several such providers. Three patients suffered severe and permanent bilateral vision loss after intravitreal injection of adipose-derived cells at one Florida clinic, a case series reported in the New England Journal of Medicine in 2017.⁹

§Open problems

The framing question is whether stem cell decline is a cause worth targeting or a readout of everything upstream of it. If aged stem cells fail mainly because their epigenomes have drifted, reprogramming is the appropriate intervention. If they fail mainly because the niche has become fibrotic and inflamed, the target is the niche. If clonal selection is the dominant process in blood, then the goal is not to restore stem cell function but to prevent the expansion of clones with a fitness advantage — a problem closer to cancer prevention than to regeneration, and one that no current intervention addresses.

§See also

• Hallmarks of aging
• Cellular senescence
• Parabiosis and young blood
• Partial reprogramming
• Induced pluripotent stem cells
• Inflammaging

§References