Hallmarks of Ageing 2023 PDF

Enemy Within Ageing Hallmarks and Mechanisms of Ageing Dr. Abir Mukherjee CBS [email protected]  Learning Outcomes (1) Describe the hallmarks of ageing (2) Discuss primary, antagonistic and integrative hallmarks of ageing ...

Enemy Within Ageing Hallmarks and Mechanisms of Ageing Dr. Abir Mukherjee CBS [email protected]  Learning Outcomes (1) Describe the hallmarks of ageing (2) Discuss primary, antagonistic and integrative hallmarks of ageing  Hallmarks of Ageing Hallmark criteria: (1) it should manifest during normal ageing (2) its experimental aggravation should accelerate ageing (3) its experimental amelioration should retard the normal ageing process and hence increase healthy lifespan.  The 9 Hallmarks of Ageing 1. Genomic Instability 2. Telomere Attrition DNA/ Chromatin 3. Epigenetic Alterations 4. Loss of Proteostasis 5. Deregulated Nutrient Sensing Metabolic 6. Mitochondrial Dysfunction 7. Cellular Senescence 8. Stem Cell Exhaustion Breakdown 9. Altered Intercellular signalling DNA/ Chromatin Based hallmarks  1. Genomic Instability Endogenous or exogenous factors can produce DNA lesions (errors) DNA lesions can be repaired by a variety of mechanisms. Excessive DNA damage or insufficient DNA repair promotes the ageing process. Somatic mutations accumulate over time Chromosome numbers, aneuploidy also associated with ageing 1. DNA Mutations Factors affecting Genomic Instability 2. Mitochondrial DNA 3. Nuclear Architecture  Damage and Repair Mechanisms DAMAGES REPAIRS BER: base excision repair MMR: mismatch repair HR: homologous recombination ROS: reactive oxygen species NER: nucleotide excision repair TLS: translesion synthesis NHEJ: nonhomologous end-joining SAC: spindle assembly checkpoint  1a. DNA Mutations Deficiencies in DNA repair mechanisms cause accelerated ageing in mice. Similar mechanisms in human progeroid syndromes Werner syndrome Bloom syndrome, xeroderma pigmentosum  1a. DNA Mutations Mechanism of BubR1 action Overexpression of BubR1, a mitotic checkpoint component that ensures accurate segregation of chromosomes (not allowing aneuploidy), in mice increased protection against aneuploidy and cancer Mad3/BubR1 inhibits APC/CCdc20 activity by acting as a pseudosubstrate and/or by extended healthy lifespan mediating Cdc20 ubiquitination and degradation Experimental evidence that artificial reinforcement of nuclear DNA repair mechanisms may delay ageing. Shows biological effect of increased BubR1 expression in muscles Shows increased BubR1 expression in different tissues of transgenic lines Ageing Caused by Reduced DNA Mutation Repair  1b. Mitochondrial DNA Mutations and deletions in aged mtDNA may also contribute to ageing. Major target for ageing-associated somatic mutations: oxidative microenvironment YIKES! lack of protective histones limited efficiency of the mtDNA repair mechanisms. Most mtDNA mutations caused by replication errors early in life, rather than by oxidative damage. Mutations may undergo polyclonal expansion and cause respiratory chain dysfunction in different tissues Multisystem disorders caused by mtDNA mutations partially phenocopy ageing Mice expressing defective mitochondrial DNA polymerase show premature ageing and Proof reduced lifespan.  1c. Nuclear Architecture Defects in the nuclear lamina and lamins can also cause genome instability. Nuclear membrane protein mutations cause accelerated ageing syndromes Hutchinson-Gilford progeria syndrome (HGPS) Nestor-Guillermo progeria syndromes (NGPS) Alterations of the nuclear lamina and Progerin, an aberrant prelamin A isoform, is produced during normal human ageing. Altered gene expression Increased DNA damage A biomarker of ageing Additionally, Lamin B1 levels decline during cell senescence  1c. Nuclear Architecture Stress pathways are elicited by aberrations in the nuclear lamina Activation of p53 Deregulation of the somatotrophic axis Attrition of adult stem cells. Decreasing prelamin A or progerin levels delays the onset of progeroid features and extends life-span in mouse. Restoration of the somatotrophic axis through hormonal treatments or inhibition of NF-kB signaling also extends lifespan in progeroid mice  2. Telomere Attrition Telomeres are particularly susceptible to age-related deterioration. Telomerase and not DNA polymerases can replicate the terminal ends of linear DNA molecules Most mammalian somatic cells do not express telomerase, Therefore progressive and cumulative loss of telomere- protective sequences from chromosome ends. Telomere shortening is observed during normal ageing both in human and in mice Telomere exhaustion explains the limited proliferative capacity of some types of in-vitro cultured cells, replicative senescence, or Hayflick limit. Ectopic expression of telomerase confers immortality to cells without causing oncogenic transformation.  2. Telomere Attrition Telomeres are bound by a characteristic multiprotein complex known as shelterin A main function of this complex is to prevent the access of DNA repair proteins to the telomeres. Telomeres ‘‘repaired’’ as DNA breaks will lead to chromosome fusions. Due to their restricted DNA repair, DNA damage at telomeres is notably Yikes! persistent and highly efficient in inducing senescence and/or apoptosis  2. Telomere Attrition Telomerase deficiency = loss of the regenerative capacity of different tissues premature development of diseases, pulmonary fibrosis, dyskeratosis congenita, and aplastic anemia Deficiencies in shelterin components Telomere uncapping and chromosome fusions Shelterin mutations found in aplastic anemia and dyskeratosis congenita. Loss-of-function models for shelterin components rapid decline of the regenerative capacity, accelerated ageing  2. Telomere Attrition In humans, strong relation between short telomeres and mortality risk, Tamoxifen inducible Mice with shortened or lengthened telomeres exhibit decreased or increased lifespans ageing can be reverted by genetically reactivating telomerase Experimental in aged mice Proof Normal physiological ageing can be delayed by systemic viral transduction of telomerase Telomerase reactivation reverses tissue degeneration in aged telomerase deficient mice. Jaskelioff et al Nature. (2011) 469:102–106 A knock-in allele encoding a 4-hydroxytamoxifen (4- OHT)-inducible telomerase reverse transcriptase- oestrogen receptor (TERT-ER) under transcriptional control of the endogenous TERT promoter. Homozygous TERT-ER mice have short dysfunctional telomeres and sustain increased DNA damage signalling  3. Epigenetic Alteration Multiple enzymatic systems assure the generation and maintenance of epigenetic patterns (DNA methyltransferases, histone acetylases, deacetylases,methylases, and demethylases.)  3. Epigenetic Alteration Histone methylation Deletion of components of histone methylation complexes (for H3K4 and for H3K27) extends longevity in nematodes and flies, respectively. by impinging on DNA repair and genome stability, Mechanism Of action or through transcriptional alterations affecting metabolic or signaling pathways outside of the nucleus.  3a. Sirtuin Action Mammalian Sirtuins can ameliorate various aspects of ageing in mice Transgenic overexpression of mammalian SIRT1 improves aspects of health during ageing but does not increase longevity SIRT1 improves genomic stability and enhanced metabolic efficiency SIRT6 regulates genomic stability, NF-kB signaling, and glucose homeostasis through histone H3K9 deacetylation SIRT6 KO mice exhibit accelerated ageing Transgenic mice overexpressing SIRT6 have a longer lifespan associated with reduced serum IGF-1 and IGF-1 signaling SIRT3 (mitochondrial) mediates beneficial effects of dietary restriction (DR) in longevity, via deacetylation of mitochondrial proteins SIRT3 overexpression improves the regenerative capacity of aged hematopoietic stem cells  3b. DNA Methylation There is an age-associated global hypomethylation, Contrastingly, several loci, including tumor suppressor genes, are hypermethylated with age Cells from patients and mice with progeroid syndromes exhibit DNA methylation patterns and histone modifications that largely recapitulate those found in normal ageing All of these epigenetic defects or epimutations accumulated throughout life may specifically affect the behavior and functionality of stem cells No direct experimental proof that organismal lifespan can be extended by altering Hmmm… patterns of DNA methylation.  3c. Chromatin Remodelling DNA- and histone-modifying enzymes act in concert with heterochromatin protein 1a (HP1a), Polycomb group proteins or the NuRD complex, Their levels are diminished in aged cells These factors determine changes in chromatin architecture, Global heterochromatin loss and redistribution are characteristic features of ageing Flies with loss-of-function mutations in HP1a have a shortened lifespan Bottom Line Overexpression of HP1a extends longevity in flies and delays muscular deterioration  3d. Transcriptional Alterations Ageing is associated with an increase in transcriptional noise and aberrant production and maturation of many mRNAs. Microarray-based comparisons of young and old tissues have identified Ageing DL age-related transcriptional changes. Key components affected in: inflammatory, mitochondrial, lysosomal degradation pathways. Ageing-associated noncoding RNAs Targets components of longevity networks or stem cell behavior. Gain- and loss-of- function studies in fly and worm confirm roles of miRNAs  3e. Reversion of Epigenetic Changes Epigenetic alterations are theoretically reversible Histone deacetylase (HDAC) inhibitors restores physiological H4 acetylation and avoids age-associated memory impairment in mice (Neuroprotective effects) Histone acetyltransferases (HAT) inhibitors also block premature ageing in progeroid mice and extend lifespan HDAC activators may also promote longevity. Resveratrol upregulates SIRT1 activity HDAC Activator HAT HDAC HAT Ac-Histone HDAC Inhibitor Inhibitor ANTI-AGEING/ Physiological Altered AGEING Normal AGEING H4 acetylation H4 acetylation Metabolic Hallmarks  4. Loss of Proteostasis Proteostasis involves mechanisms for the stabilization of correctly folded proteins degradation of proteins by the proteasome or the lysosome. These systems function in a coordinated fashion to restore the structure of misfolded polypeptides remove and degrade misfolded proteins prevent the accumulation of damaged components assure continuous renewal of intracellular proteins. Proteostasis is altered with ageing. Chronic expression of unfolded, misfolded, or aggregated proteins contributes to the development age-related pathologies, Alzheimer’s disease, Parkinson’s disease cataracts.  4. Loss of Proteostasis Proteostasis is the dynamic regulation of a balanced, functional proteome. Proteostasis is achieved by pathways that control the biogenesis, folding, trafficking, and degradation of proteins.  4a. Chaperone-Mediated Protein Folding and Stability The stress-induced synthesis of chaperones is impaired in ageing. Transgenic worms and flies overexpressing chaperones are long-lived. Long-lived mouse strains show upregulation of some heat- shock proteins (chaperones). Activation HSF-1, increases longevity and thermotolerance in nematodes In mammalian cells, deacetylation of HSF-1 by SIRT1 potentiates Hsp70 and downregulation of SIRT1 attenuates the heat-shock response. Pharmacological induction of the heat-shock protein Hsp72 preserves muscle function in mouse models of muscular dystrophy.  4b. Proteolytic Systems: Autophagy Two principal proteolytic systems decline with ageing: the autophagy-lysosomal system ubiquitin-proteasome system Extra copy of LAMP2a (chaperone-mediated autophagy receptor) in transgenic mice blocks decline in autophagic activity and preserve improved hepatic function with ageing. Interventions using chemical inducers of macroautophagy administration of the mTOR inhibitor rapamycin can increase the lifespan of middle-aged mice 4b. Proteolytic Systems: Autophagy  4b. Proteolytic Systems: Proteasome Proteasome Pathway Activation of EGF signaling extends longevity in nematodes by increasing the expression of various components of the ubiquitin-proteasome system Proteasome activity can be enhanced by deubiquitylase inhibitors or proteasome activators. This extends replicative lifespan in yeast. Increased expression of the proteasome subunit RPN-6 by the FOXO transcription factor Proof DAF-16 confers proteotoxic stress resistance and extends lifespan in C. elegans.  5. Deregulated Nutrient Sensing The somatotrophic axis in mammals comprises GHRH from the hypothalamus, GH from the pituitary and IGF1 from the liver. The intracellular signaling pathway of IGF-1, like insulin, which informs cells of the presence of glucose. This is the most conserved ageing-controlling pathway in evolution and targets FOXO family of transcription factors The mTOR complexes, which are also involved in ageing and evolutionarily conserved Mutations that reduce the functions of GH, IGF-1 receptor, insulin receptor, or downstream intracellular effectors such as AKT, mTOR, and FOXO have been linked to longevity. Dietary restriction (DR) increases lifespan or healthspan in all investigated eukaryotes  5a. The Insulin- and IGF-1-Signaling (IIS) Pathway Multiple genetic manipulations that attenuate IIS pathway consistently extend the Insulin/ IGF1 lifespan of worms, flies, and mice. IR/ IGFR Genetic analyses indicate that this pathway mediates part of the beneficial effects of DR on longevity in worms and flies. PIP3 Among the downstream effectors of the IIS pathway, is the transcription factor FOXO. Mouse FOXO1 is required for the tumor suppressive effect of DR. PTEN PI3K PTEN-overexpressing mice, as well as hypomorphic PI3K mice, show an increased AKT longevity. Increased PTEN expression FOXO mTOR downregulates the IIS pathway, increased energy expenditure that is associated with improved mitochondrial oxidative metabolism, AGEING enhanced activity of the brown adipose tissue.  5a. The Insulin- and IGF-1-Signaling Pathway GH and IGF-1 levels decline during normal ageing. Decreased IIS is a characteristic of physiological and accelerated ageing, Extremely low levels of IIS signaling are incompatible with life, Mouse null mutations in the PI3K or AKT are embryonic lethal YET Organisms with a constitutively decreased IIS can survive longer They have lower rates of cell growth and metabolism Thus lower rates of cellular damage. IIS downregulation reflects a defensive response aimed at minimizing cell growth and metabolism in the context of systemic damage Physiologically or pathologically aged organisms decrease IIS in an attempt to extend their lifespan. Question of Supplementation of IGF1 can ameliorate premature ageing in progeroid balance mice with very low levels of IGF-1  5b. Other Nutrient-Sensing Systems mTOR : sensing of high amino acid concentrations; AMPK : senses low-energy states by detecting high AMP levels Sirtuins : sense low-energy states by detecting high NAD+ levels Mice deficient in S6K1 (a main mTORC1 substrate) are long- lived Therefore, the downregulation of mTORC1/S6K1 is the critical mediator of mammalian longevity.  5b. AMPK and SIRT AMPK and sirtuins, act in the opposite direction to IIS and mTOR They signal nutrient scarcity and catabolism. Their upregulation favours healthy ageing. AMPK activation shuts off mTORC1 and thus may mediate lifespan extension SIRT1 can deacetylate and activate the PPARg coactivator 1a (PGC-1a) PGC-1a orchestrates mitochondriogenesis, enhanced antioxidant defenses, and improved fatty acid oxidation. SIRT1 and AMPK can engage in a positive feedback loop, thus connecting both sensors of low-energy states into a unified response Anabolic signaling accelerates ageing and decreased nutrient signaling extends longevity Elixir A pharmacological manipulation that mimics a state of Vitae! limited nutrient availability can extend longevity.  6. Mitochondrial Dysfunction As cells and organisms age, the efficacy of the respiratory chain tends to diminish, thus increasing electron leakage and reducing ATP generation.  6a. Reactive Oxygen Species Progressive mitochondrial dysfunction that occurs with ageing results in increased production of ROS, which in turn causes further mitochondrial deterioration and global cellular damage. Unexpected observations: Increased ROS may prolong lifespan in yeast and worm GM mice that have increased mitochondrial ROS and oxidative damage do not accelerate ageing mice with increased antioxidant defenses do not present an extended life- span genetic manipulations that impair mitochondrial function but do not increase ROS accelerate ageing ROS triggers proliferation and survival in response to physiological signals and stress conditions  6a. Reactive Oxygen Species A Possible Explanation 1. ROS is possibly a stress-elicited survival signal 2. Primary effect of ROS is the activation of compensatory homeostatic responses. 3. With ageing cellular stress and damage increase and the levels of ROS increase in parallel in an attempt to maintain survival. Over a threshold, ROS levels eventually aggravate, rather than alleviate, the age- associated damage 6b. Mitochondrial Integrity and Biogenesis Dysfunctional mitochondria can contribute to ageing independently of ROS, Stress related mitochondrial permeabilization may affect apoptosis signaling Also initiate inflammatory reactions by favoring ROS-mediated and/or permeabilization-facilitated activation of inflammasomes. The reduced efficiency of mitochondrial bioenergetics with ageing may result from multiple mechanisms, including reduced biogenesis of mitochondria 1. Telomere attrition in telomerase-deficient mice, 2. Subsequent p53-mediated repression of PGC-1a and PGC-1b.  6b. Mitochondrial Integrity and Biogenesis Mitochondrial decline also occurs during physiological ageing in wild-type mice and can be partially reversed by telomerase activation. SIRT1 modulates mitochondrial biogenesis through a process involving the transcriptional coactivator PGC-1a and the removal of damaged mitochondria by autophagy. SIRT3, the main mitochondrial deacetylase targets many enzymes involved in energy metabolism, including components of the respiratory chain, tricarboxylic acid cycle, ketogenesis, and fatty acid b-oxidation. SIRT3 may also directly control the rate of ROS production by deacetylating manganese superoxide dismutase, a major mitochondrial antioxidant enzyme. Therefore telomeres and sirtuins may control mitochondrial function and thus play a protective role against age-associated diseases.  6c. Mitohormesis According to this concept, mild toxic treatments trigger beneficial compensatory responses that surpass the repair of the triggering damage and produce an improvement in cellular fitness Thus, mild respiratory deficiencies may increase lifespan Hormetic reactions may trigger a mitochondrial defensive response in situ and even in distant tissues. Metformin and resveratrol are mild mitochondrial poisons that induce a low- energy state characterized by increased AMP levels and activation of AMPK Metformin extends lifespan in C. elegans through the induction of AMPK and the master antioxidant regulator NRF2. Metformin can increase mouse lifespan. Resveratrol and the sirtuin activator SRT1720 protect from metabolic damage and improve mitochondrial respiration in a PGC-1a-dependent fashion Mitochondrial uncoupling can increase lifespan in flies and mice Breakdown Based hallmarks  7. Cellular Senescence Cellular senescence can be defined as a stable arrest of the cell cycle coupled to stereotyped phenotypic changes. This phenomenon was originally described by Hayflick in human fibroblasts serially passaged in culture, caused by telomere shortening  7. Cellular Senescence Other ageing-associated stimuli that trigger senescence Nontelomeric DNA damage and derepression of the INK4/ARF locus, both of which progressively occur with ageing, are also capable of inducing senescence. Senescence-associated b-galactosidase (SABG) to identify senescence in tissues Quantification of SABG and DNA damage in liver show 8% senescent cells in young mice and 17% in very old mice. Similar results were obtained in the skin, lung, and spleen, but no changes were observed in heart, skeletal muscle, and kidney Thus cellular senescence is not a generalized property of all tissues in aged organisms. The accumulation of senescent cells with ageing: increase in the rate of generation of senescent cells and/or a decrease in their rate of clearance.  7. Cellular Senescence It is possible that senescence is a beneficial compensatory response that contributes to rid tissues from damaged and potentially oncogenic cells. This cellular checkpoint, requires an efficient cell replacement system clearance of senescent cells and mobilization of progenitors to re- establish cell numbers. In aged organisms, this turnover system may become inefficient Or may exhaust the regenerative capacity of progenitor cells, Leading to the accumulation of senescent cells that may aggravate the damage and contribute to ageing. Senescent cells show significant alterations in their secretome, Enriched in proinflammatory cytokines and matrix metallo-proteinases This proinflammatory secretome may contribute to ageing. DANGER!  7a. The INK4a/ARF Locus and p53 Excessive mitogenic signaling is most robustly associated to senescence. p16INK4a/Rb and p19ARF/p53 pathways are the major contributor to senescence. The levels of p16INK4a and p19ARF correlate with the chronological age across tissues and across species, p16INK4a and p19ARF are encoded by the INK4a/ARF locus. INK4a/ARF locus is genetically linked to the highest number of age- associated pathologies: cardiovascular diseases, diabetes, glaucoma, and Alzheimer’s  7a. The INK4a/ARF Locus and p53 p16INK4a-induced and p53-induced senescence contribute to physiological ageing. Senescence pathways lead to tumor suppression. Mutant mice with premature ageing due to extensive and persistent damage present dramatic levels of senescence, (BRCA1 KO, HGPS, hypomorphic BubR1) Their progeroid phenotypes are ameliorated by elimination of p16INK4a or p53. Surprisingly mice with a mild and systemic increase in p16INK4a, p19ARF, or p53 tumor suppressors exhibit extended longevity, Also, elimination of p53 aggravates the phenotypes of some progeroid mutant mice. It is possible that with extended damage 1. The regenerative capacity of tissues can be exhausted or saturated, 2. Under these extreme conditions, the p53 andINK4a/ARF responses can become deleterious and accelerate ageing.  8. Stem Cell Exhaustion With ageing the regenerative potential of tissues drop. Hematopoiesis declines with age, resulting in a diminished production of adaptive immune cells —a process termed immuno- senescence A similar attrition of stem cells is seen in all adult stem cell compartments  8. Stem Cell Exhaustion With ageing there is an overall decrease in cell-cycle activity of hematopoietic stem cells (HSCs) Old HSCs undergoes fewer cell divisions than young HSCs, correlates with the accumulation of DNA damage and with overexpression of cell-cycle inhibitory proteins such as p16INK4a Old INK4a KO HSCs exhibit better engraftment capacity and increased cell-cycle activity compared with old wild-type HSCs. Telomere shortening causes stem cell decline with ageing in multiple tissues. Excessive proliferation of stem and progenitor cells can also be deleterious by accelerating the exhaustion of stem cell niches.. Induction of INK4a during ageing and the decrease of serum IGF-1might be a response to preserve the quiescence of stem cells. FGF2 signaling in the aged muscle stem cell niche results in the loss of quiescence and eventually in stem cell depletion and diminished regenerative capacity  8. Stem Cell Exhaustion Cell-intrinsic vs cell-extrinsic pathways Transplantation of muscle-derived stem cells from young mice to progeroid mice extends lifespan and improves degenerative changes Even in tissues in which donor cells are not detected, systemic effects caused by secreted factors. Parabiosis experiments show that the decline in neural and muscle stem cell function in old mice can be reversed by systemic factors from young mice.  9. Altered Intercellular Communication Ageing involves changes at the level of intercellular communication, Neurohormonal signaling is deregulated in ageing as inflammatory reactions increase, immunosurveillance against pathogens and premalignant cells declines The composition of the peri- and extracellular environment changes.  9a. Inflammation Infammageing: Proinflammatory phenotype that accompanies ageing in mammals Multiple causes: Accumulation of proinflammatory tissue damage, Failure of a dysfunctional immune system to effectively clear pathogens Propensity of senescent cells to secrete proinflammatory cytokines Enhanced activation of the NF- kB transcription factor, Occurrence of a defective autophagy response These cause increased production of IL-1b, tumor necrosis factor, and interferons Age-associated inflammation inhibits epidermal stem cell function Concomitantly the function of the adaptive immune system declines This immunosenescence may aggravate the ageing phenotype at the systemic level due to the failure of the immune system to clear infectious agents, infected cells, and cells on the verge of malignant transformation.  9a. Inflammation Overactivation of the NF-kB pathway is a transcriptional signature of ageing Conditional expression of an NF-kB inhibitor in the aged skin of transgenic mice causes the phenotypic rejuvenation of this tissue, as well as the restoration of the transcriptional signature corresponding to young age. A novel link Inflammatory and stress responses activate NF-kB in the hypothalamus Induces a signaling pathway that results in reduced production GnRH GnRH decline contributes to numerous ageing-related changes bone fragility, muscle weakness, skin atrophy, and reduced neurogenesis. GnRH treatment prevents ageing-impaired neurogenesis and slows down ageing development in mice Therefore the hypothalamus may modulate systemic ageing by integrating NF- kB-driven inflammatory responses with GnRH-mediated neuroendocrine effects. 9b. Other Types of Intercellular Communication Ageing-related changes in one tissue can lead to ageing-specific deterioration of other tissues, In addition to inflammatory cytokines, there are other examples of ‘‘contagious ageing’’ or bystander effects Senescent cells induce senescence in neighboring cells via gap-junction-mediated cell-cell contacts and processes involving ROS. The microenvironment contributes to the age-related functional defects of CD4 T cells, Conversely, lifespan-extending manipulations targeting one single tissue can retard the ageing process in other tissues  Functional Interconnections between the Hallmarks of ageing Interventions that Might Extend Human Healthspan