Loading
What changed between revision 1 and revision 2, compared over the article's markdown source with its frontmatter — so a change to status, evidence or reversibility shows up as the edit it is.
Older · revision 1
Import· initial corpus import, not an edit
Initial import of content/cellular-senescence.md — the filesystem corpus, unchanged. Not an edit.
13.3 kB
Revision 1
Revision 2
lines 7–13 → 7–136 unchanged lines not shown
categories: ["longevity"]categories: ["longevity"]tags: ["aging", "senescence", "sasp", "cell biology", "cancer", "mechanisms"]tags: ["aging", "senescence", "sasp", "cell biology", "cancer", "mechanisms"]summary: "A stable arrest of cell division, triggered by telomere attrition or stress, in which the arrested cell stays metabolically active and secretes inflammatory factors."summary: "A stable arrest of cell division, triggered by telomere attrition or stress, in which the arrested cell stays metabolically active and secretes inflammatory factors."updated: "2026-07-27"updated: "2026-10-29"humanEvidence: "The arrest was first described in cultured human fibroblasts and the markers appear in aged human tissue; every result showing that clearing senescent cells extends life is from mice, and randomized senolytic trials have not yet shown clinical benefit."humanEvidence: "The arrest was first described in cultured human fibroblasts and the markers appear in aged human tissue; every result showing that clearing senescent cells extends life is from mice, and randomized senolytic trials have not yet shown clinical benefit."issues: ["The Unity Biotechnology phase 2 osteoarthritis result is stated without a source."]issues: ["The Unity Biotechnology phase 2 osteoarthritis result is stated without a source."]------lines 26–61 → 26–10812 unchanged lines not shown
}}`````` **Cellular senescence** is a stable exit from the cell cycle in which a cell stops dividing but does not die, remains metabolically active, resists apoptosis, and alters what it secretes. It is one of the [[hallmarks-of-aging]] and one of the clearest cases of antagonistic pleiotropy in mammalian biology: the same arrest that prevents damaged cells from becoming tumours degrades the tissue around them when senescent cells accumulate with age.**Cellular senescence** is a stable exit from the cell cycle in which a cell stops dividing but doesnot die: it remains metabolically active, resists apoptosis and changes what it secretes. It is oneof the [[hallmarks-of-aging]] and one of the clearest cases of antagonistic pleiotropy in mammalianbiology. The same arrest that prevents damaged cells from becoming tumours degrades the surroundingtissue when senescent cells accumulate with age. ## Discovery and definition## Discovery and definition Leonard Hayflick and Paul Moorhead reported in 1961 that normal human diploid fibroblasts divide a limited number of times in culture and then stop, contradicting the then-standard belief that vertebrate cells were immortal under good conditions.[^hayflick1961] The limit is now attributed largely to progressive [[telomeres-and-telomerase|telomere shortening]], but replicative exhaustion turned out to be only one route into the state.Leonard Hayflick and Paul Moorhead reported in 1961 that normal human diploid fibroblasts divide alimited number of times in culture and then stop, contradicting the then-standard belief thatvertebrate cells were immortal under good conditions.[^hayflick1961] The limit is now attributedlargely to progressive [[telomeres-and-telomerase|telomere shortening]], but replicative exhaustionturned out to be only one route into the state. Senescence has no single definitive marker, which is a persistent methodological problem. Investigators typically require several features together: absence of proliferation markers, expression of the cyclin-dependent kinase inhibitors p16INK4a or p21CIP1, senescence-associated β-galactosidase activity detected at pH 6, loss of nuclear Lamin B1, and evidence of a persistent DNA damage response.[^dimri1995] Each of these appears in non-senescent contexts. p16 expression, in particular, can be induced in macrophages as a reversible physiological response, which confounds any attempt to quantify senescent burden in tissue by p16 alone.[^hall2017]Senescence has no single definitive marker, a persistent methodological problem. Investigatorstypically require several features together: absence of proliferation markers, expression of thecyclin-dependent kinase inhibitors p16INK4a or p21CIP1, senescence-associated β-galactosidaseactivity detected at pH 6, loss of nuclear Lamin B1, and evidence of a persistent DNA damageresponse.[^dimri1995] Each of these also appears in cells that are not senescent. p16 expression inparticular can be induced in macrophages as a reversible physiological response, which confounds anyattempt to quantify senescent burden in tissue by p16 alone.[^hall2017] > [!note] Terminology> [!note] Terminology> "Senescence" in this article means the cellular state. It is unrelated in mechanism to organismal senescence — the whole-body decline the word denotes in demography and evolutionary biology — although the two are often conflated in popular coverage.> "Senescence" in this article means the cellular state. It is mechanistically unrelated to> organismal senescence, the whole-body decline the word denotes in demography and evolutionary> biology, although popular coverage often conflates the two. ## Triggers## Triggers Replicative senescence follows telomere attrition and the DNA damage response it provokes at uncapped chromosome ends. Oncogene-induced senescence is triggered by aberrant proliferative signalling; Serrano and colleagues showed in 1997 that expressing oncogenic *RAS* in normal human cells drives them into arrest rather than transformation, establishing senescence as a tumour-suppressive barrier.[^serrano1997] Stress-induced premature senescence follows oxidative damage, [[mitochondrial-dysfunction|mitochondrial]] impairment, irradiation or [[proteostasis|proteotoxic]] stress without telomere involvement. Therapy-induced senescence is a common outcome of cytotoxic chemotherapy and contributes to the long-term functional deficits seen in cancer survivors.Replicative senescence follows telomere attrition and the DNA damage response it provokes atuncapped chromosome ends. Oncogene-induced senescence is triggered by aberrant proliferativesignalling. In 1997 Serrano and colleagues showed that expressing oncogenic *RAS* in normal humancells drives them into arrest instead of transformation, establishing senescence as a barrieragainst tumours.[^serrano1997] Stress-induced premature senescence follows oxidative damage,[[mitochondrial-dysfunction|mitochondrial]] impairment, irradiation or [[proteostasis|proteotoxic]]stress without any telomere involvement. Therapy-induced senescence is a common outcome of cytotoxicchemotherapy and contributes to the long-term functional deficits seen in cancer survivors. Senescence also has programmed roles outside pathology. Two independent groups reported in 2013 that transient senescent populations appear at defined sites during mammalian embryonic development, including the apical ectodermal ridge, where they shape tissue patterning before being cleared by macrophages.[^munoz2013] Senescence is therefore not a failure mode that evolution overlooked; it is a used mechanism whose persistence in aged tissue is the problem.Senescence also has programmed roles outside pathology. In 2013 two independent groups reported thattransient senescent populations appear at defined sites during mammalian embryonic development,including the apical ectodermal ridge, where they shape tissue patterning before macrophages clearthem.[^munoz2013] Senescence is therefore a mechanism with normal uses, not something evolutionoverlooked; the harm comes from its persistence in aged tissue. ## The senescence-associated secretory phenotype## The senescence-associated secretory phenotype The feature that makes senescent cells matter beyond their own arrest is the senescence-associated secretory phenotype, or SASP: a program of secreted interleukins, chemokines, growth factors, and matrix-remodelling proteases characterized by Coppé, Campisi and colleagues in 2008 at the [[buck-institute|Buck Institute]] and Lawrence Berkeley National Laboratory.[^coppe2008] SASP composition varies with the trigger and the cell type; there is no canonical SASP. Cytosolic DNA sensing through the cGAS–STING pathway is one of its principal upstream activators, which links the secretory program to genome instability and to leaked mitochondrial DNA.Senescent cells matter beyond their own arrest because of the senescence-associated secretoryphenotype, or SASP. This program of secreted interleukins, chemokines, growth factors andmatrix-remodelling proteases was characterized by Coppé, Campisi and colleagues in 2008 at the[[buck-institute|Buck Institute]] and Lawrence Berkeley National Laboratory.[^coppe2008] SASPcomposition varies with the trigger and the cell type, and there is no canonical SASP. Cytosolic DNAsensing through the cGAS-STING pathway is one of its principal upstream activators, which links thesecretory program to genome instability and to leaked mitochondrial DNA. The SASP reinforces arrest in the secreting cell, recruits immune cells to clear it, and induces senescence in neighbours — a paracrine spread that lets a small number of senescent cells alter a large volume of tissue. Its chronic version is one of the main proposed sources of [[inflammaging|sterile chronic inflammation]] in old animals, and it impairs the function of nearby progenitors, connecting senescence to [[stem-cell-exhaustion|stem cell exhaustion]]. Transplanting a modest number of senescent cells into young mice is sufficient to produce measurable physical dysfunction that spreads beyond the injection site.[^xu2018]The SASP reinforces arrest in the secreting cell, recruits immune cells to clear it, and inducessenescence in neighbouring cells. Through this paracrine spread a small number of senescent cellscan alter a large volume of tissue. The chronic form of the SASP is one of the main proposed sourcesof [[inflammaging|sterile chronic inflammation]] in old animals, and it impairs the function ofnearby progenitors, which connects senescence to [[stem-cell-exhaustion|stem cell exhaustion]].Transplanting a modest number of senescent cells into young mice is enough to produce measurablephysical dysfunction that spreads beyond the injection site.[^xu2018] ## Beneficial roles## Beneficial roles Senescent cells are not simply debris. Beyond tumour suppression and development, they participate in wound healing: in mice, senescent fibroblasts and endothelial cells appearing at wound sites secrete PDGF-AA and accelerate closure, and animals unable to mount that response heal more slowly.[^demaria2014] Senescence of activated hepatic stellate cells limits liver fibrosis in mouse models. Indiscriminate clearance is therefore not obviously good. Work using genetic ablation of p16-high cells has reported that removing certain populations — including liver sinusoidal endothelial cells — causes tissue damage rather than rejuvenation.[^grosse2020]Senescent cells are more than debris. Beyond tumour suppression and development, they take part inwound healing. In mice, senescent fibroblasts and endothelial cells that appear at wound sitessecrete PDGF-AA and accelerate closure, and animals unable to mount that response heal moreslowly.[^demaria2014] Senescence of activated hepatic stellate cells limits liver fibrosis in mousemodels. Indiscriminate clearance is therefore not obviously good. Studies using genetic ablation ofp16-high cells have reported that removing certain populations, including liver sinusoidalendothelial cells, causes tissue damage instead of rejuvenation.[^grosse2020] ## From cell-culture curiosity to drug target## From cell-culture curiosity to drug target For four decades senescence was treated as an artifact of culture with uncertain relevance in vivo. Two developments changed that. First, reliable in vivo markers and reporter mice made senescent cells visible in tissue. Second, transgenic systems allowed their selective killing.For four decades senescence was treated as an artifact of culture whose relevance in vivo wasuncertain. Two developments changed that. Reliable in vivo markers and reporter mice made senescentcells visible in tissue, and transgenic systems allowed them to be killed selectively. ```timeline```timeline[[lines 68–85 → 115–1626 unchanged lines not shown
]]`````` The transgenic results — first in a progeroid strain, then in normally aged animals — established that senescent cells are causally involved in mouse aging phenotypes rather than merely present.[^baker] That finding motivated the search for drugs with the same effect, which became the field of [[senolytics]]. Progress from there has been slower than the mouse work implied. Early human studies have been small, open-label, and focused on surrogate measures such as senescent-cell burden in fat biopsies; a phase 2 trial of a locally injected senolytic for osteoarthritis run by Unity Biotechnology did not beat placebo, and a randomized trial of intermittent dasatinib plus quercetin in postmenopausal women missed its primary bone-turnover endpoint.[^farr2024] Senomorphics, which suppress the SASP without killing the cell, are the alternative approach; [[rapamycin]] and JAK inhibitors both blunt SASP output in animal models.The transgenic results, first in a progeroid strain and then in normally aged animals, establishedthat senescent cells are causally involved in mouse aging phenotypes and not merely present.[^baker]That finding prompted the search for drugs with the same effect, which became the field of[[senolytics]]. Progress since has been slower than the mouse work implied. Early human studies havebeen small and open-label, focused on surrogate measures such as senescent-cell burden in fatbiopsies. A phase 2 trial of a locally injected senolytic for osteoarthritis run by UnityBiotechnology did not beat placebo, and a randomized trial of intermittent dasatinib plus quercetinin postmenopausal women missed its primary bone-turnover endpoint.[^farr2024] The alternativeapproach uses senomorphics, which suppress the SASP without killing the cell; [[rapamycin]] and JAKinhibitors both blunt SASP output in animal models. > [!debate] The disagreement> [!debate] The disagreement> Whether senescent cells are a primary driver of aging or one downstream consequence among many is unsettled. Clearance experiments show they are causal for specific phenotypes in mice. They do not show that senescence sits upstream of the other hallmarks, and the mouse strains used are not a straightforward model of human aging.> It is unsettled whether senescent cells are a primary driver of aging or one downstream> consequence among many. Clearance experiments show that they cause specific phenotypes in mice.> They do not show that senescence sits upstream of the other hallmarks, and the mouse strains used> are not a straightforward model of human aging. ## Open problems## Open problems Quantifying senescent burden in living humans is not currently possible with any validated assay, which blocks the obvious trial design: measure burden, remove cells, measure again, follow outcomes. The NIH's Cellular Senescence Network was created in part to build a tissue atlas that would support such measures, and defining senescence in a way that is specific enough for [[aging-biomarkers|biomarker]] use remains its central task.No validated assay can currently quantify senescent burden in living humans. That blocks the obvioustrial design: measure burden, remove cells, measure again, follow outcomes. The NIH's CellularSenescence Network was created partly to build a tissue atlas that would support such measures, andits central task remains defining senescence specifically enough for [[aging-biomarkers|biomarker]]use. Heterogeneity is the deeper issue. Senescent cells derived from different tissues by different triggers share arrest but differ in secretome, in the anti-apoptotic pathways they depend on, and probably in whether they help or harm. A senolytic that kills one subtype may leave another untouched, which is a plausible explanation for the gap between mouse results and human trial outcomes. Whether the state is truly irreversible is also in question: [[partial-reprogramming|transient reprogramming]] can restore proliferative capacity to senescent cells in culture, which suggests the arrest is enforced epigenetically rather than written into the genome.The deeper issue is heterogeneity. Senescent cells derived from different tissues by differenttriggers share the arrest but differ in secretome, in the anti-apoptotic pathways they depend on,and probably in whether they help or harm. A senolytic that kills one subtype may leave anotheruntouched, a plausible explanation for the gap between mouse results and human trial outcomes.Whether the state is truly irreversible is also in question.[[partial-reprogramming|Transient reprogramming]] can restore proliferative capacity to senescentcells in culture, which suggests that the arrest is enforced epigenetically and not written into thegenome. The clinical question that follows is which diseases to target. Senescent-cell involvement has been argued for idiopathic pulmonary fibrosis, osteoarthritis, diabetic kidney disease, atherosclerosis and Alzheimer's disease, and small early-phase studies exist in several of these. The [[geroscience-hypothesis|geroscience]] version of the argument is more ambitious: that clearing senescent cells would delay several such conditions at once, and so would demonstrate a change in the aging process rather than in one disease. Testing that claim requires an outcome measure the field does not yet have, and in its absence senescence therapeutics are assessed one indication at a time on ordinary clinical endpoints rather than on [[biological-age]] or [[healthspan]].The clinical question that follows is which diseases to target. Senescent-cell involvement has beenargued for idiopathic pulmonary fibrosis, osteoarthritis, diabetic kidney disease, atherosclerosisand Alzheimer's disease, and small early-phase studies exist in several of these. The[[geroscience-hypothesis|geroscience]] version of the argument goes further: clearing senescentcells would delay several such conditions at once and so demonstrate a change in the aging process,not in one disease. Testing that claim requires an outcome measure the field does not yet have.Until one exists, senescence therapeutics are assessed one indication at a time on ordinary clinicalendpoints rather than on [[biological-age]] or [[healthspan]]. ## See also## See also removed, struck through added, underlinedLine numbers count the serialised markdown of each revision, frontmatter included.
90 lines are the same in both revisions, and 45 of them are not shown. Neither revision here is necessarily the current article — read Cellular senescence for the text this wiki stands behind today.