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categories: ["longevity"]categories: ["longevity"]tags: ["aging", "protein folding", "chaperones", "proteasome", "neurodegeneration", "mechanisms"]tags: ["aging", "protein folding", "chaperones", "proteasome", "neurodegeneration", "mechanisms"]summary: "The age-related failure of the chaperone, proteasome and lysosomal systems that keep proteins correctly folded and turned over, and its link to aggregation diseases."summary: "The age-related failure of the chaperone, proteasome and lysosomal systems that keep proteins correctly folded and turned over, and its link to aggregation diseases."updated: "2026-07-28"updated: "2026-10-29"issues: ["Lecanemab and donanemab approvals are stated without a source."]issues: ["Lecanemab and donanemab approvals are stated without a source."]------ lines 25–31 → 25–3611 unchanged lines not shown
}}`````` **Proteostasis collapse** is the progressive age-related failure of the systems that fold proteins correctly, refold them when they are damaged, and destroy them when they cannot be repaired. Proteostasis — protein homeostasis — is maintained by a network of several hundred chaperones, the ubiquitin–proteasome system, and lysosomal degradation, working under the control of inducible stress responses.[^balch2008] Its loss is a primary hallmark in the [[hallmarks-of-aging]] framework and the mechanistic link between aging and the major neurodegenerative diseases.**Proteostasis collapse** is the progressive age-related failure of the systems that fold proteinscorrectly, refold them when they are damaged, and destroy them when they cannot be repaired.Proteostasis, or protein homeostasis, is maintained by a network of several hundred chaperones, theubiquitin-proteasome system and lysosomal degradation, working under the control of inducible stressresponses.[^balch2008] Its loss is a primary hallmark in the [[hallmarks-of-aging]] framework andthe mechanistic link between aging and the major neurodegenerative diseases. ## The proteostasis network## The proteostasis network lines 35–82 → 40–1653 unchanged lines not shown
{ "year": "1972", "title": "Sequence determines structure", "text": "Christian Anfinsen receives the Nobel Prize in Chemistry for showing that a protein's amino acid sequence encodes its folded conformation, framing misfolding as a solvable problem." }, { "year": "1972", "title": "Sequence determines structure", "text": "Christian Anfinsen receives the Nobel Prize in Chemistry for showing that a protein's amino acid sequence encodes its folded conformation, framing misfolding as a solvable problem." }, { "year": "2004", "title": "Ubiquitin system recognized", "text": "Aaron Ciechanover, Avram Hershko and Irwin Rose share the Nobel Prize in Chemistry for the discovery of ubiquitin-mediated protein degradation." }, { "year": "2004", "title": "Ubiquitin system recognized", "text": "Aaron Ciechanover, Avram Hershko and Irwin Rose share the Nobel Prize in Chemistry for the discovery of ubiquitin-mediated protein degradation." }, { "year": "2008", "title": "Proteostasis named", "text": "Balch, Morimoto, Dillin and Kelly propose 'proteostasis' for the integrated network and argue it is a druggable target across folding diseases." }, { "year": "2008", "title": "Proteostasis named", "text": "Balch, Morimoto, Dillin and Kelly propose 'proteostasis' for the integrated network and argue it is a druggable target across folding diseases." }, { "year": "2009–2010", "title": "Collapse mapped in a model organism", "text": "Work in C. elegans locates the loss of folding capacity in early adulthood and shows that hundreds of proteins become insoluble with age." } { "year": "2009-2010", "title": "Collapse mapped in a model organism", "text": "Work in C. elegans locates the loss of folding capacity in early adulthood and shows that hundreds of proteins become insoluble with age." }]]`````` A newly made polypeptide must reach one conformation out of an enormous number of possible ones, in a cytoplasm containing several hundred milligrams of protein per millilitre. Molecular chaperones — the HSP70 and HSP90 families, the small heat-shock proteins, and the chaperonin complex that folds actin and tubulin — bind exposed hydrophobic surfaces and prevent inappropriate association while folding proceeds. Only part of the network does folding work; the rest handles triage and disposal. Predicting a native structure from sequence, the task [[ai-protein-design|machine-learning models]] now perform well, is a different problem from predicting how a chain reaches that structure or how often it fails to, which is what the network exists to manage.A newly made polypeptide must reach one conformation out of an enormous number of possible ones, ina cytoplasm containing several hundred milligrams of protein per millilitre. Molecular chaperonesbind exposed hydrophobic surfaces and prevent inappropriate association while folding proceeds; theyinclude the HSP70 and HSP90 families, the small heat-shock proteins, and the chaperonin complex thatfolds actin and tubulin. Only part of the network does folding work, and the rest handles triage anddisposal. [[ai-protein-design|Machine-learning models]] now predict a native structure from sequencewell, but that is a different problem from predicting how a chain reaches the structure or how oftenit fails to, which is what the network exists to manage. Two degradation systems handle proteins that cannot be rescued. The ubiquitin–proteasome system tags substrates with polyubiquitin chains through a cascade of activating, conjugating and ligase enzymes, and feeds them into the 26S proteasome, which unfolds and cleaves them. Lysosomal degradation handles bulk cargo and organelles by [[autophagy]], and handles individual proteins bearing a specific pentapeptide motif through chaperone-mediated autophagy, in which HSC70 delivers substrates to the lysosomal receptor LAMP2A for direct translocation.Two degradation systems deal with proteins that cannot be rescued. The ubiquitin-proteasome systemtags substrates with polyubiquitin chains through a cascade of activating, conjugating and ligaseenzymes and feeds them into the 26S proteasome, which unfolds and cleaves them. Lysosomaldegradation takes bulk cargo and organelles by [[autophagy]]. It also takes individual proteinsbearing a specific pentapeptide motif through chaperone-mediated autophagy, in which HSC70 deliverssubstrates to the lysosomal receptor LAMP2A for direct translocation. Three inducible stress responses supervise the network in different compartments: the cytosolic heat-shock response, governed by the transcription factor HSF1; the unfolded protein response of the endoplasmic reticulum, running through IRE1, PERK and ATF6; and the mitochondrial unfolded protein response, which couples proteostasis to [[mitochondrial-dysfunction|mitochondrial]] state.Three inducible stress responses supervise the network in different compartments. The cytosolicheat-shock response is governed by the transcription factor HSF1. The unfolded protein response ofthe endoplasmic reticulum runs through IRE1, PERK and ATF6. The mitochondrial unfolded proteinresponse couples proteostasis to [[mitochondrial-dysfunction|mitochondrial]] state. ## What collapses, and when## What collapses, and when The most striking result on timing came from *Caenorhabditis elegans*, where the capacity to buffer misfolded proteins falls sharply in early adulthood rather than degrading gradually across life. Morimoto's group showed that this collapse coincides with reproductive maturity and precedes any visible aging phenotype, suggesting proteostasis capacity is actively downregulated once reproduction is under way rather than simply worn out.[^benzvi2009] Long-lived insulin/IGF-1 signalling mutants of the kind [[cynthia-kenyon|Cynthia Kenyon's]] laboratory characterized delay that collapse, and the FOXO-family transcription factor their longevity depends on drives expression of chaperones and other network components. A companion finding is that hundreds of proteins become detergent-insoluble in aging worms, well beyond the handful associated with named aggregation diseases.[^david2010]The most striking result on timing came from *Caenorhabditis elegans*, in which the capacity tobuffer misfolded proteins falls sharply in early adulthood instead of degrading gradually acrosslife. Morimoto's group showed that this collapse coincides with reproductive maturity and precedesany visible aging phenotype, which suggests that proteostasis capacity is actively turned down oncereproduction is under way and does not simply wear out.[^benzvi2009] Long-lived insulin/IGF-1signalling mutants of the kind [[cynthia-kenyon|Cynthia Kenyon's]] laboratory characterized delaythe collapse, and the FOXO-family transcription factor on which their longevity depends drivesexpression of chaperones and other network components. A companion finding is that hundreds ofproteins become detergent-insoluble in aging worms, far more than the handful associated with namedaggregation diseases.[^david2010] In mammals the picture is less crisp but consistent in direction. HSF1-driven induction of chaperones in response to heat or oxidative stress weakens with age, and that response is the mechanism usually invoked for [[heat-and-cold-exposure|sauna bathing]], where the human mortality data are observational. Proteasome activity declines in several tissues, though not uniformly. Chaperone-mediated autophagy declines markedly with age in rodent liver, and restoring LAMP2A expression in old mice preserved proteolytic activity and hepatic function.[^zhang2008] That experiment is one of the cleaner demonstrations that the decline is not merely correlated with aging but contributes to organ dysfunction.In mammals the picture is less crisp but points the same way. HSF1-driven induction of chaperones inresponse to heat or oxidative stress weakens with age; that response is the mechanism usuallyinvoked for [[heat-and-cold-exposure|sauna bathing]], where the human mortality data areobservational. Proteasome activity declines in several tissues, though not uniformly.Chaperone-mediated autophagy declines markedly with age in rodent liver, and restoring LAMP2Aexpression in old mice preserved proteolytic activity and hepatic function.[^zhang2008] Thatexperiment is one of the cleaner demonstrations that the decline contributes to organ dysfunctionand is not merely correlated with aging. > [!note] Terminology> [!note] Terminology> "Collapse" describes the abrupt loss of buffering capacity seen in invertebrate models. In mammals the change is gradual and tissue-specific, and the word is used loosely. Neither usage implies that all proteostasis functions fail together.> "Collapse" describes the abrupt loss of buffering capacity seen in invertebrate models. In mammals> the change is gradual and tissue-specific, and the word is used loosely. Neither usage implies> that all proteostasis functions fail together. ## Aggregation and neurodegeneration## Aggregation and neurodegeneration The clinical face of proteostasis failure is protein aggregation in the nervous system. Amyloid-β and tau in [[alzheimers-disease|Alzheimer's disease]], α-synuclein in Parkinson's disease and Lewy body dementia, TDP-43 in most amyotrophic lateral sclerosis and frontotemporal dementia, and polyglutamine-expanded huntingtin in Huntington's disease all form ordered assemblies that spread between cells in a templated, prion-like manner. Age is by a wide margin the largest risk factor for all of them, which is the argument for treating them as manifestations of an aging process rather than as independent diseases — the core claim of the [[geroscience-hypothesis|geroscience hypothesis]].Proteostasis failure shows up clinically as protein aggregation in the nervous system. Amyloid-β andtau in [[alzheimers-disease|Alzheimer's disease]], α-synuclein in Parkinson's disease and Lewy bodydementia, TDP-43 in most amyotrophic lateral sclerosis and frontotemporal dementia, andpolyglutamine-expanded huntingtin in Huntington's disease all form ordered assemblies that spreadbetween cells in a templated, prion-like manner. Age is by a wide margin the largest risk factor forall of them. That is the argument for treating them as manifestations of an aging process and not asindependent diseases, the core claim of the [[geroscience-hypothesis|geroscience hypothesis]]. Whether the aggregates themselves are the toxic species is unsettled. Soluble oligomers correlate better with neuronal dysfunction than mature fibrils in several systems, and inclusion body formation may sequester damaging material rather than cause damage. The therapeutic record is sobering: decades of amyloid-clearing programmes produced two anti-amyloid antibodies, lecanemab and donanemab, approved in the United States in the 2020s. Both slow cognitive decline modestly, at the cost of amyloid-related imaging abnormalities including brain swelling and microhaemorrhage. The size and clinical meaning of that benefit remain contested among neurologists.Whether the aggregates themselves are the toxic species is unsettled. In several systems, solubleoligomers correlate better with neuronal dysfunction than mature fibrils do, and inclusion bodiesmay sequester damaging material instead of causing damage. The therapeutic record is sobering.Decades of amyloid-clearing programmes produced two anti-amyloid antibodies, lecanemab anddonanemab, approved in the United States in the 2020s. Both slow cognitive decline modestly, at thecost of amyloid-related imaging abnormalities that include brain swelling and microhaemorrhage, andneurologists still dispute the size and clinical meaning of the benefit. ## Long-lived proteins## Long-lived proteins Not all proteins are replaced. Crystallins in the eye lens are synthesized before birth and never turned over, which is why they accumulate racemized and glycated residues across a lifetime and eventually scatter light as cataract. Nuclear pore complex scaffold proteins, some histones, and elastin and collagen in the extracellular matrix persist for years to decades.[^toyama2013] For this fraction of the proteome, no amount of chaperone capacity helps; the damage is chemical, spontaneous, and irreversible without replacement of the whole structure.Not every protein is replaced. Crystallins in the eye lens are synthesized before birth and neverturned over, so they accumulate racemized and glycated residues across a lifetime and eventuallyscatter light as cataract. Nuclear pore complex scaffold proteins, some histones, and elastin andcollagen in the extracellular matrix persist for years to decades.[^toyama2013] For this fraction ofthe proteome no amount of chaperone capacity helps: the damage is chemical and spontaneous, and itcannot be reversed without replacing the whole structure. This category is a distinctive problem for aging biology because it is not a regulatory failure that can be corrected by restoring a signalling pathway. Advanced glycation end-products cross-link long-lived extracellular matrix proteins and stiffen arteries and skin; breaking those cross-links pharmacologically has been attempted and has not produced a durable clinical result.This category poses a distinct problem for aging biology because it is not a regulatory failure thatrestoring a signalling pathway could correct. Advanced glycation end-products cross-link long-livedextracellular matrix proteins and stiffen arteries and skin. Drugs to break those cross-links havebeen tried and have not produced a durable clinical result. ## Interventions and evidence## Interventions and evidence Interventions divide into three groups. Raising folding capacity: HSF1 activators and heat-shock protein co-inducers, of which arimoclomol is the best-known example — it failed a phase 3 trial in ALS before being approved in the United States for a rare lysosomal storage disease. Raising degradation capacity: proteasome activation extends lifespan under proteotoxic stress in nematodes, and enhancing autophagy with [[rapamycin]], spermidine or [[caloric-restriction|dietary restriction]] improves outcomes in mouse models of aggregation disease. Reducing the load: antisense oligonucleotides and RNA interference that lower production of the aggregating protein, an approach now in clinical use for transthyretin amyloidosis and in trials for Huntington's disease.Interventions fall into three groups. The first raises folding capacity, with HSF1 activators andheat-shock protein co-inducers; the best-known example, arimoclomol, failed a phase 3 trial in ALSbefore being approved in the United States for a rare lysosomal storage disease. The second raisesdegradation capacity. Proteasome activation extends lifespan under proteotoxic stress in nematodes,and enhancing autophagy with [[rapamycin]], spermidine or[[caloric-restriction|dietary restriction]] improves outcomes in mouse models of aggregationdisease. The third reduces the load, using antisense oligonucleotides and RNA interference to lowerproduction of the aggregating protein, an approach now in clinical use for transthyretin amyloidosisand in trials for Huntington's disease. Comparative biology offers a suggestive parallel. Naked mole-rats maintain higher proteasome activity and more accurate translation than mice, and their proteins are more resistant to unfolding — one of several traits invoked in explanations of [[negligible-senescence|negligible senescence]].[^perez2009nmr] Human embryonic and [[induced-pluripotent-stem-cells|induced pluripotent stem cells]] likewise sustain unusually high proteasome activity, which falls on differentiation. That observation connects proteostasis to [[stem-cell-exhaustion|stem cell decline]] and to [[epigenetic-reprogramming|reprogramming]], since restoring a more pluripotent-like state also restores proteostatic capacity in culture.Comparative biology offers a suggestive parallel. Naked mole-rats maintain higher proteasomeactivity and more accurate translation than mice, and their proteins resist unfolding better, one ofseveral traits invoked to explain [[negligible-senescence|negligible senescence]].[^perez2009nmr]Human embryonic and [[induced-pluripotent-stem-cells|induced pluripotent stem cells]] likewisesustain unusually high proteasome activity, which falls on differentiation. The observation connectsproteostasis to [[stem-cell-exhaustion|stem cell decline]] and to[[epigenetic-reprogramming|reprogramming]], since restoring a more pluripotent-like state alsorestores proteostatic capacity in culture. ## Open problems## Open problems There is no validated way to measure proteostatic capacity in a living human. Chaperone transcript levels, proteasome activity in blood cells, and circulating aggregate species have all been proposed and none has been qualified as an [[aging-biomarkers|aging biomarker]]. Without such a measure, the hypothesis that boosting proteostasis would slow aging cannot be tested except through disease endpoints in populations already sick.No validated method measures proteostatic capacity in a living human. Chaperone transcript levels,proteasome activity in blood cells and circulating aggregate species have all been proposed, andnone has been qualified as an [[aging-biomarkers|aging biomarker]]. Without such a measure, thehypothesis that boosting proteostasis would slow aging can be tested only through disease endpointsin populations that are already sick. The direction of causality between proteostasis and the other hallmarks is also unresolved. Failing proteostasis produces damaged proteins that trigger [[cellular-senescence|senescence]] and [[inflammaging|inflammatory signalling]]; senescent cells and inflammation in turn impair chaperone induction. Nor is more degradation obviously better: excessive autophagy and excessive proteasome activity both cause pathology, and the network's set points appear to be tuned rather than maximized. Whether an intervention could raise capacity in old tissue without disturbing that tuning is untested in any mammal.The direction of causality between proteostasis and the other hallmarks is also unresolved. Failingproteostasis produces damaged proteins that trigger [[cellular-senescence|senescence]] and[[inflammaging|inflammatory signalling]], and senescent cells and inflammation in turn impairchaperone induction. Nor is more degradation obviously better. Excessive autophagy and excessiveproteasome activity both cause pathology, and the network's set points appear to be tuned, notmaximized. No one has tested in any mammal whether an intervention could raise capacity in oldtissue without disturbing that tuning. ## See also## See also removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
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