Loading
Old version — revision 2
This is a fixed snapshot of Autophagy, saved by Import. It is not edited and it is not updated; the article may have changed since.
Edit summary: Re-import of content/autophagy.md — the file changed after the initial import. Not an edit.
Read the current versionWhat this revision changedAll 2 revisions
The lysosomal pathway by which cells digest their own damaged components, and the process through which most known lifespan-extending interventions appear to act.
Autophagy is the process by which a cell delivers its own cytoplasmic contents (misfolded proteins, damaged organelles, invading bacteria, surplus lipid) to the lysosome for degradation and recycling. It is the bulk arm of protein quality control and the only route by which a cell can dispose of a structure too large for the proteasome. Disabled macroautophagy was added as a separate entry to the Hallmarks of aging in 2023. In invertebrates, interfering with autophagy genes abolishes the lifespan extension produced by most known geroprotective interventions.
Macroautophagy, the form usually meant by the unqualified term, engulfs cargo in a double-membraned vesicle that then fuses with a lysosome. Microautophagy takes up small volumes of cytosol by invaginating the lysosomal membrane directly. Chaperone-mediated autophagy selects individual proteins carrying a KFERQ-like motif: HSC70 recognizes the motif and delivers the protein to the lysosomal receptor LAMP2A, where it is unfolded and translocated directly across the membrane. Only mammals and some other vertebrates have this third form, and it declines steeply with age in rodent liver.
Macroautophagy begins when the ULK1 kinase complex nucleates an isolation membrane, or phagophore. Lipid reaches the site from ATG9-containing vesicles and from the class III phosphatidylinositol 3-kinase complex built around VPS34 and Beclin-1. Two ubiquitin-like conjugation cascades then act on the growing membrane. The first builds an ATG12-ATG5-ATG16L1 complex; the second uses that complex as its ligase to attach LC3 to the lipid phosphatidylethanolamine in the phagophore membrane. Lipidated LC3 recruits cargo receptors and drives membrane expansion, and it remains the standard experimental marker of autophagosome formation.
The closed autophagosome travels along microtubules and fuses with a lysosome. Acid hydrolases degrade the contents, and permeases return amino acids, fatty acids and nucleosides to the cytosol. The whole cycle takes minutes.
De Duve saw autophagy in electron micrographs and named it, but for three decades the process resisted study because it lacked genetic handles. Ohsumi made it tractable. By blocking vacuolar proteases in yeast so that autophagic bodies accumulated visibly, he created a screenable phenotype and identified the core machinery, which turned out to be conserved from yeast to humans.1
The nutrient-sensing network that governs growth also controls autophagy. mTORC1, active when amino acids and growth factors are abundant, phosphorylates ULK1 and ATG13 and holds initiation off. It also phosphorylates TFEB, the transcription factor that drives lysosomal and autophagy gene expression, which keeps TFEB out of the nucleus. AMPK, active when the AMP:ATP ratio rises, does the opposite on both counts. Starvation, exercise and Rapamycin therefore act on the same switch from different directions, which is why dietary restriction and mTOR inhibition produce overlapping transcriptional signatures.
Bulk autophagy is non-specific, but under normal conditions most autophagy is selective. Cargo is ubiquitinated and recognized by receptors such as p62/SQSTM1, NBR1, optineurin and NDP52, which bind both the tag and LC3 on the forming membrane. Named subtypes include mitophagy for damaged mitochondria, aggrephagy for protein aggregates, lipophagy for lipid droplets, ferritinophagy for iron stores, and xenophagy for intracellular pathogens. Mitophagy is the subtype most directly tied to aging, and the PINK1-Parkin route that governs it is mutated in familial early-onset Parkinson's disease.
Selectivity matters to aging biology for a specific reason. Adult stem cells depend on autophagy to clear damaged mitochondria and protein aggregates that would otherwise be partitioned asymmetrically at division. In mice, loss of autophagy in haematopoietic and muscle stem cells produces a prematurely aged compartment that is myeloid-biased or regenerates poorly, a direct mechanistic link to stem cell exhaustion. In macrophages, failure to degrade cytosolic DNA and damaged organelles activates inflammasome signalling, one of the proposed sources of chronic sterile inflammation.
The strongest evidence that autophagy is more than incidental to aging comes from epistasis. In C. elegans, autophagy genes were shown to be required for the lifespan extension of the long-lived insulin-signalling mutants that Cynthia Kenyon's laboratory characterized, and later for the extension produced by dietary restriction and by TOR inhibition.23 The pattern holds in Drosophila. Gain-of-function experiments in mice point the same way. Animals engineered to express extra ATG5, and animals carrying a Beclin-1 point mutation that frees it from inhibition by BCL2, both show elevated basal autophagy, better metabolic health and extended lifespan.45
The epistasis experiments show that autophagy is required, not that it is sufficient. Finding that an intervention's effect depends on autophagy does not show that raising autophagy alone would reproduce the effect, and the mouse gain-of-function experiments alter the pathway from conception, not in an already-aged animal. Drugs promoted on autophagy grounds, such as Metformin and the TAME trial acting through AMPK and NAD⁺ precursors acting through sirtuin-dependent regulation of the machinery, have not been shown to raise autophagic activity in human tissue at the doses used.
Why this mattersMost candidate longevity interventions share autophagy as a mechanism. Rapamycin, dietary restriction, exercise, spermidine and several NAD⁺-related manipulations all raise autophagic activity in model systems. That is evidence either that autophagy is the shared final pathway or that autophagy is easy to raise and easy to measure.
Human data are much weaker. Exercise induces autophagy in mouse muscle, and mice unable to mount that response lose the metabolic benefits of exercise. Spermidine, a polyamine that induces autophagy across species and is abundant in some fermented foods, extends lifespan in yeast, flies, worms and mice and is associated with lower mortality in observational human cohorts; randomized trials of supplementation in older adults have not shown clear cognitive benefit. At least one well-funded longevity company, Retro Biosciences, names autophagy as an explicit programme focus.
More autophagy is not uniformly better. Established tumours depend on autophagy to survive nutrient stress, which is why autophagy inhibitors such as hydroxychloroquine have been trialled as cancer adjuvants, the reverse of the geroprotective logic. Monoallelic loss of BECN1 is found in some breast and ovarian cancers, so the pathway suppresses tumours in one phase and supports them in another, a pattern it shares with cellular senescence. Variants in ATG16L1 predispose to Crohn's disease through impaired handling of intestinal bacteria rather than through any failure of recycling.
A measurement problem limits every human claim in this article. Autophagy is a flux, not a level. A high LC3-II signal can mean rapid autophagosome formation or blocked degradation, and telling the two apart requires adding a lysosomal inhibitor, which cannot be done in a living person. No assay of autophagic flux has been qualified as an aging biomarker, so the field's human evidence rests on downstream functional endpoints instead of on the pathway itself. As a result, the central claim, that raising autophagy in an aged human would slow the processes described by the geroscience hypothesis, is at present untestable rather than merely untested.
paperTsukada, M., Ohsumi, Y. "Isolation and characterization of autophagy-defective mutants of Saccharomyces cerevisiae." FEBS Letters, 1993. ↩
paperMeléndez, A. et al. "Autophagy genes are essential for dauer development and life-span extension in C. elegans." Science, 2003.↩Worms, not mammals; the experiment shows autophagy genes are required for the lifespan extension, not that raising autophagy alone extends life.
paperHansen, M. et al. "A role for autophagy in the extension of lifespan by dietary restriction in C. elegans." PLoS Genetics, 2008. ↩
paperPyo, J.-O. et al. "Overexpression of Atg5 in mice activates autophagy and extends lifespan." Nature Communications, 2013. ↩
paperFernández, Á. F. et al. "Disruption of the beclin 1–BCL2 autophagy regulatory complex promotes longevity in mice." Nature, 2018.↩The mice carry a knock-in mutation present from conception, so the result does not test raising autophagy in an already-aged animal.