老化

Ageing — Grokipedia Fact-checked by Grok 3 months ago Ageing Ara Eve Leo Sal 1x Ageing is the time-dependent functional decline that affects most living organisms, manifesting as a progressive deterioration of physiological integrity, increased vulnerability to death , and diminished capacity for survival and reproduction . [1] [2] This process arises from the accumulation of molecular and cellular damage across multiple biological systems, rather than a deliberate genetic program, as supported by empirical observations in diverse species where interventions reducing damage extend lifespan. [3] Key defining characteristics include the hallmarks of ageing , a framework identifying twelve interconnected mechanisms: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis , disabled macroautophagy, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence , stem cell exhaustion, altered intercellular communication, chronic inflammation, and dysbiosis.01377-0) From an evolutionary perspective, ageing is a non-adaptive by-product of natural selection favoring traits that enhance early-life fitness at the expense of late-life maintenance, as articulated in theories such as mutation accumulation, antagonistic pleiotropy, and disposable soma. [3] 00186-5.pdf) Empirical evidence from model organisms like nematodes, flies, and mice demonstrates that genetic or environmental manipulations—such as caloric restriction or targeted gene edits—can modulate these hallmarks and extend healthy lifespan, underscoring causal links between damage accumulation and functional decline.01377-0) Controversies persist regarding the relative primacy of specific drivers, with debates over whether certain changes (e.g., mitochondrial dysfunction) are causal or correlative, but longitudinal studies and interventions consistently affirm that ageing accelerates age-related diseases like cancer, neurodegeneration, and cardiovascular pathology through shared mechanistic pathways. [4] Demographically, ageing drives global shifts, with median ages rising due to declining fertility and improved early-life survival , imposing challenges on healthcare systems through heightened prevalence of frailty and multimorbidity in later decades. [5] While biomedical research targets these hallmarks for interventions—yielding modest extensions in healthspan via drugs like metformin or senolytics in preclinical models—fundamental limits imposed by thermodynamic entropy and evolutionary trade-offs suggest that radical lifespan extension remains constrained by biological realities, prioritizing empirical validation over speculative narratives. [6] [7] Biological Basis Hallmarks of Ageing The hallmarks of aging constitute a framework identifying key cellular and molecular drivers of the aging process. Initially outlined in a 2013 review by López-Otín et al., the model was updated in 2023 to encompass twelve hallmarks, grouped into primary (causes of cellular damage), antagonistic (responses to such damage that become deleterious), and integrative (manifestations leading to organismal dysfunction).01377-0) These features meet three criteria: they increase with chronological age, experimentally accelerating them hastens aging phenotypes, and interventions targeting them can ameliorate aging outcomes.01377-0) The framework emphasizes interconnected causality, where primary damage elicits antagonistic responses that, if dysregulated, propagate integrative effects culminating in frailty and disease susceptibility.01377-0) Primary hallmarks represent the initial sources of molecular harm. Genomic instability arises from accumulated DNA damage due to replication errors, environmental mutagens, and repair deficiencies, leading to mutations and chromosomal aberrations that impair cellular function over time.01377-0) Telomere attrition involves progressive shortening of chromosome ends with each cell division, eventually triggering replicative senescence or apoptosis when critically short.01377-0) Epigenetic alterations encompass changes in DNA methylation, histone modifications, and chromatin remodeling that disrupt gene expression patterns, such as hypermethylation of promoter regions silencing tumor suppressors or repair genes.01377-0) Loss of proteostasis refers to declining protein quality control, including impaired chaperone activity, ubiquitin-proteasome dysfunction, and macroautophagy failure, resulting in toxic aggregate accumulation like amyloid fibrils.01377-0) Antagonistic hallmarks emerge as adaptive countermeasures to primary damage but promote pathology when chronic. Disabled macroautophagy denotes impaired autophagic clearance of damaged organelles and proteins, exacerbating proteostasis collapse and metabolic dysregulation; for instance, knockout of autophagy genes like Atg5 in mice shortens lifespan.01377-0) Deregulated nutrient sensing involves hyperactivation of pathways like mTOR and insulin/IGF-1 signaling or hypoactivity of AMPK and sirtuins, skewing resource allocation toward growth over maintenance, as evidenced by lifespan extension via caloric restriction or rapamycin.01377-0) Mitochondrial dysfunction features reduced bioenergetic efficiency, increased reactive oxygen species production, and mtDNA mutations, forming a feedback loop with nuclear genome instability.01377-0) Cellular senescence entails irreversible cell cycle arrest in response to stress, with senescent cells secreting pro-inflammatory factors (SASP) that propagate tissue dysfunction, though partial clearance via senolytics extends healthspan in models.01377-0) Integrative hallmarks integrate upstream processes into systemic decline. Stem cell exhaustion manifests as diminished regenerative capacity from depleted pools and impaired self-renewal, driven by accumulated damage and niche alterations, contributing to sarcopenia and immune decline.01377-0) Altered intercellular communication includes disrupted endocrine signaling and paracrine effects, such as elevated inflammaging cytokines, fostering a pro-pathogenic milieu.01377-0) Chronic inflammation (inflammaging) involves persistent low-grade immune activation without resolution, linking to multiple pathologies via NF-κB pathway overdrive.01377-0) Dysbiosis denotes gut microbiome shifts toward pro-inflammatory taxa, impairing barrier integrity and metabolite production, with fecal transplants in aged models restoring youthful profiles and mitigating frailty.01377-0) This expanded model underscores aging as a multifaceted, pleiotropic process amenable to targeted interventions, though causal hierarchies remain under investigation.01377-0) Cellular and Molecular Mechanisms Cellular and molecular mechanisms of ageing encompass a network of interconnected processes that drive the accumulation of damage and functional decline at the subcellular level. These mechanisms, often categorized as hallmarks of ageing, include primary causes such as genomic instability, telomere attrition, and epigenetic alterations; antagonistic responses like loss of proteostasis , deregulated nutrient sensing, mitochondrial dysfunction, and cellular senescence ; and integrative manifestations such as stem cell exhaustion and altered intercellular communication.01377-0) Evidence from model organisms and human studies indicates these processes are conserved across species, with interventions targeting them extending lifespan in yeast, worms, flies, and mice by up to 50% in some cases. [1] Genomic instability results from the progressive accumulation of DNA damage from sources including reactive oxygen species (ROS), replication errors, and environmental mutagens, coupled with declining repair efficiency via pathways like nucleotide excision repair and non-homologous end joining . [1] In humans, mutations in DNA repair genes, as seen in disorders like xeroderma pigmentosum , accelerate ageing phenotypes, with somatic mutation rates increasing exponentially after age 60.01377-0) Telomere attrition, driven by incomplete replication and oxidative stress , shortens chromosome ends by 50-100 base pairs per cell division , triggering replicative senescence when telomeres reach a critical length of about 5-7 kb in humans. [1] Telomerase deficiency in somatic cells exacerbates this, though rare telomerase activation in cancers underscores its dual role. [8] Epigenetic alterations involve aberrant DNA methylation patterns, histone modifications, and chromatin remodeling, leading to altered gene expression; for instance, global hypomethylation and promoter hypermethylation of tumor suppressor genes increase with age, correlating with a 20-30% loss in methylation fidelity by age 70.01377-0) Loss of proteostasis manifests as impaired protein folding and degradation, with chaperone expression declining and aggregates like amyloid forming; autophagy defects, evident in Atg-deficient mice, reduce clearance and shorten lifespan by 25%. [1] Deregulated nutrient sensing, via hyperactive mTOR or insulin/IGF-1 pathways, promotes anabolism over maintenance, while caloric restriction activates AMPK and sirtuins to mitigate this, extending mouse lifespan by 30-40%. [9] Mitochondrial dysfunction arises from mtDNA mutations accumulating at rates 10-17 times higher than nuclear DNA, elevating ROS production and impairing bioenergetics ; heteroplasmy levels above 60-80% trigger dysfunction, as observed in mitochondrial diseases mimicking premature ageing. [9] Cellular senescence , induced by persistent DNA damage or oncogene activation, enforces permanent G1 arrest via p53 /p21 and p16 /Rb pathways, accompanied by the senescence-associated secretory phenotype (SASP) that propagates inflammation through IL-6, IL-8, and chemokines . [10] Senescent cell burden doubles in human tissues by middle age , and senolytics like dasatinib plus quercetin reduce it, improving physical function in aged mice. [10] Stem cell exhaustion reflects diminished s