Biology Of Ageing Flashcards
Define ageing
Process of change in properties of material occurring over a period of time - oxford dictionary
Collection of changes that render human beings progressively more likely to die
Decline of biological functions and of organisms ability to adapt to metabolic stress
Many age related changes appear in the fourth decade but some can be as early as age 10 eg hearing loss
Medical classification of age
Prenatal life - fertilisation to 40 weeks Birth Neonate - newborn to week 2 Infancy - week 3 to 1 year Childhood - 2-9 in females and 2-12 males Prepubertal - 10+ females 13+ males Adolescence - 6 years following puberty Adulthood - 20-65 Senescence - 65+
WHO age classification
Young old - 60-74
Old old - 75-84
Oldest old - 85+
Centenarians - 100+
Functional reserve capacity purpose
Most systems have spare capacity to prevent system failure - reserved to have more than the level required to maintain homeostasis
Theories of ageing
Deterministic - programmed ageing where genes continue to turn on ageing similar to development. Evidence - species specific lifespans, twin studies, limited cell division, fast ageing syndromes like progeria. Gerontogenes. Gershon&Gershon observed built in programme in genome activated at certain stage in organisms life cycle leading to death via self destruct and autoregulatory mechanisms.
Non-deterministic - random damage accumulation from wear and tear. Protein damage and autophagy. Energy metabolism and ageing. Free radical theory. DNA damage theory.
Evolutionary theories - force of natural selection declines with age. Antagonistic pleiotrophy. Disposable soma.
Species specific lifespan evidence for deterministic ageing
Evidence supporting the deterministic ageing theory.
Species specific max lifespan correlates with How long it takes to reach sexual maturity - maturity is genetically programmed so may also be ageing is too.
Twin studies evidence for deterministic ageing
Danish cohort study 1870-1910
24% variation in twins lifespan accounted for by genetics - McGue et al 1993.
Genetic influences more obvious in later life
After age 70 up to 50% of variation in twins lifespan was accounted for by genetics suggesting genetics do not determine ageing ! It’s the interaction with the environment that determines this
Limited cell division evidence for deterministic ageing
Hayflick limit
Cells in culture can divide only a set number of times suggesting genetic influences mean they must age
Senescent cells are in cell cycle arrest, but remain metabolically active and display gene expression patterns typically express p16INK4a
can be subject to morphological changes, reduced strength, secrete pro-oncotic factors increasing cancer risk, secrete MMPs degrading tissues and lack of cell division reduced tissue wound repair.
How long it takes to reach the hayflick limit correlates with max lifespan - Rickleffs&Finch 1996.
Against this theory - End regions of chromosomes - telomeres are also lost with cell divisions gradually making them less stable as the telomeres get shorter suggesting genetics determine ageing. Telomeres are lost because during cell division DNA polymerase can’t make new DNA to the ends of the chromosomes resulting in some telomere region being lost. ageing of senescence cells can’t be programmed by this as there’s no gene for telomerase, is only due to imperfect copying of DNA.
Unlikely to function in post mitotic organisms like c elegans or drosophila
Fast ageing syndrome - Werner’s syndrome
Is a premature ageing disease
Symptoms - premature white hair, reduced skin suppleness, cataracts, diabetes, osteoporosis, vascular disease, cancer.
In vitro fibroblasts typically show abbreviated cellular lifespan in culture and in vivo affected tissues contain division contentment cells
WRN gene mutation on chromosome 8(recessive) in gene for recQ ATP dependent Helicase which unwinds DNA in replication, protein synthesis and DNA repair. Enhances telomere loss and causes ageing of tissues
Mutation causes dysfunctional Helicase stopping DNA synthesis meaning cells reach senescence early.
However this isn’t a model of programmed ageing as it’s still due to poor DNA copying, repair and new protein production.
Gerontogenes
Genes which effect lifespan when mutated or expression is altered eg C elegans age-1 gene increases duration of expression causes max lifespan from 31 to 58 days.
Other genes - daf genes in worm also increase lifespan. Does this by mutation stopping normal effects of insulin and worms think they’re starving.
In drosophila fly additional genes increase antioxidants and lifespan by 30%. Extra antioxidants reduce free radical damage.
Johnson 2005 and cutler 2005
All gerontogenes increase resistance to physical stressors like free radicals, UV light etc. Suggest these environmental factors can drive ageing.
Programmed ageing evidence summary
No good evidence as of yet
Genes are important in build up of damage counteracted by genetically regulated mechanisms
Genes involved are all those involved in cell maintenance (70% all genome) - hayflick limit prevented by re expression of telomerase gene, progeria - Re express Helicase gene, gerontogenes - additional genes and protective agents against stress damage
Evolutionary ageing theories
Natural selection - only beneficial traits are selected for survival but senescence isn’t beneficial so why does it occur
Adaptive theory - programmed ageing. Age for a reason and are designed to age. Similar to genetics controlling development. Prevents overcrowding, increases generation turnover, aiding evolutionary change, but there aren’t enough old to contribute to overcrowding, and turnover of generations depends on rate of reproduction not death.
Non- adaptive theory- non programmed suggest ageing is passive result of inability to better itself and withstand deteriorative processes. Has no purpose or benefit. Defect of organism design. Late acting gene theory and antagonistic pleiotropy say ageing is adverse side effect of function ie reproduction. Disposable soma theory is organisms can’t withstand deteriorative process and soma (body cells) are sacrificed to maintain germ cells
Late acting gene theory - Peter Medawar 1952
Ageing as by product of natural selection- force of selection declines with age
Probability of reproduction changes with age - increases from birth to adulthood and decreases due to probability of death from external causes.
Therefore the greatest contribution to create new generation comes from young organisms so deleterious mutations expressed during reproductive phase are severely selected against. While mutations in later life are neutral to selection as genes are already transmitted to next generation and so these aren’t removed. Concept of selection shadow where older ages may permit accumulation of late acting mutations as hazardous genes persist and build up. Eg Huntington disease or apolipoprotein E4 for Alzheimer’s and CV disease
George Williams 1957 antagonistic pleiotropy hypothesis
Pleiotropy - one gene influences two or more unrelated traits
Builds on late acting gene theory some genes may benefit in younger life but be detrimental in later life. These are favoured in selection when reproductive but may have bad effects later on. Small benefits to reproduction such as the gene for colourful long feathers in peacock males may be favoured over large deleterious effects such as escaping prey or poor camouflage. Oestrogen in early life is necessary for reproduction but in later life in linked to cancer. Sickle trait protects from malaria early on but late can cause haemorrhage and organ failure, Huntingtin gene increases fertility in early life but causes cognitive and movement disorders later.
Disposable soma hypothesis
Maintenance of germline at expense of soma cells is observed in humans and all species. Germline is kept separate from soma.
August Weismann 1891
Soma cells may be sacrificed to maintain germ cells - Tom kirkwood 1979
By this theory species in hazardous environments should have poor somatic cell maintenance - and when lifespan increases a decrease in fertility would be seen. Supporting this - cell maintenance correlates with max lifespan per species and long lived species are less fertile - could be confounding factors involved.
Examples of ROS/RNS
Radicals
Non - radicals - hydrogen peroxide, nitric oxide etc
OIL RIG
Oxidation is loss
Reduction is gain of electrons
Sources of ROS/RNS - superoxide
Variety of cell sources including mitochondria, NADPH, oxidases (NOXs), coupled/uncoupled nitric oxide synthase NOS etc
Main source of superoxide from mitochondria during oxidative phosphorylation - many free electrons, H ions and oxygen molecules for H2O2 formation. This is influenced by PO2. Forms 0.1-0.2% all superoxide. Major site of ROS generation is complex III in basal mitochondria. ROS are removed by cytosolic and mitochondrial ROS scavenging systems. In pathological conditions backflow of electrons in complex I also increases ROS generation.
NOS reactions
Coupled NOS forms NO important for vasodilation, uncoupled NOS forms O2- which decreases NO bioavailability by reacting and forming ONOO-.
BH4 is essential for NOS activity
Reduced BH4 or L arginine uncouples NO synthesis from NADPH consumption to generate superoxide
Vit C stabilises BH4 and increases levels in endothelial cells promoting eNOS coupling.
constitutive Antioxidant systems
Cells contain constitutive antioxidant systems to detoxify ROS eg superoxide dismutase to detoxify superoxide.
SOD catalyses reaction converting SO to hydrogen peroxide and oxygen. SOD1 Found In cytosol and outer mitochondrial memb. SOD2 In mitochondrial matrix and SOD3 In cytosol. Catalase CAT removes hydrogen peroxide to water and oxygen.
Cellular glutathione GSH synthesis within cells is abundant and is responsible for reducing oxidised proteins and can detoxify ROS through selenium containing enzyme glutathione peroxidase GPx
Peroxiredoxin and thioredoxin systems can reverse protein oxidation.
Inducible antioxidant systems
Nrf2 nuclear factor E2 related factor 2 defence pathway - regulates transcription of hundreds of cytoprotective genes.
Target genes - GSH related genes like cystine transporter xCT, GCLM, heme oxygenase 1 (HO-1), NADPH quinone oxidoreductase (NQO1).
Mitochondrial function in health and age
Healthy - more ATP and less ROS produced
Unhealthy/aged cells - less ATP and more ROS generation
Why are mitochondria susceptible to oxidative damage
Mitochondrial organelles make the majority of ATP and free radicals in mammalian cells.
MtDNA susceptible to mutation - encodes 37 genes, 2rRNA, 22 tRNA, 13 ETC proteins (complex I, III, IV and V)
Highly mutable - lacks histone proteins, few repair enzymes, circular DNA with few introns.
also damaged mitochondria aren’t degraded so continue to produce more ROS and damage healthy proteins.
The more mitochondrial dysfunction, the more ROS generation and spread of mutation there is and mutation also increases with age
ROS underlying ageing controversies
ROS production increased in mice and DNA mutations also increased
Mice without ROS antioxidant pathways had much shorter lifespan however still showed the same amount of DNA damage suggesting ROS not responsible for the damage shown
Causes of mitochondrial swelling may be from
Oxidative damage or could be independent of ROS formation as same morphology seen in ROS normal and excessive cells
Consequences of mitochondrial dysfunction
Decline in function increases ROS production in lipid membranes and transmits to neighbouring cells
Necrosis occurs from lack of respiration and insufficient ATP production - inflammation and enzyme release including degrading enzymes which cause more damage to neighbouring cells
lifespan is shortened
There is a gender difference in DNA damage seen in monkeys - more prevalent in males, females are better at protecting from damage?
No direct evidence so far so may be result of other processes rather than causal.
MnSOD2 knockout mice
Lack mitochondrial SOD but have cytoplasmic SOD
Show reduced life span and reduced activity in complex I and III
Polg exonuclease mutant mice
Hereto and homozygous mice
More mutant errors seen
Limited evidence suggesting accumulation of mitochondrial mutations reduced lifespan - no effect seen in heterozygous and young mouse survival rate not much different despite mutation rate.
Likely multiple contributors to ageing, just mitochondrial insufficiency alone isn’t enough to drive ageing
Dietary coenzyme q 10 and vit E And p66shc mice for mitochondrial function
Coenzyme Q10 transfers electrons from complex I And II to iii lowering oxidative stress and reducing superoxide generation. Increases lifespan in c elegans mustangs for MeV-1.
P66shc mutant mice reduce H2O2 production and increases lifespan
Beneficial mitochondrial DNA mutations in humans
More likely that centenarians like gene for. Complex V, increasing lifespan and resistance to disease and more efficient production of ATP
Gene for complex I - mt5178 Increases longevity, more antioxidant
Gene for complex iii seems to do the same
Role of ROS in insulin receptor kinase activation
Ligand interaction leads to production of superoxide and hydrogen peroxide involved in autophosphorylation and activation of receptor kinase
ROS excess consequences
Protein modification, lipid perocidation, DNA damage
Lipid peroxidation - hydroxyl radicals attack lipid membranes particularly PUFAS resulting in lipid peroxyl radicals and further lipid damage. Termination occurs when radical species react together or terminated by vitamin E and C.
DNA damage - hydroxyl radicals or aldehydes can induce DNA damage particularly mitochondrial DNA.
Protein modification - can occur during ROS mediated cell signalling or result in abnormal function
Oxidative damage theory of ageing
In health there is equilibrium between ROS generation and removal by endogenous defences - redox homeostasis, But with age this shifts favouring higher ROS production and decreases antioxidant defences leading to DNA damage and cellular dysfunction
Metabolic rate is thought to be inversely proportional to lifespan - higher metabolic mammals like mice have much shorter lifespan than lower metabolic animals like elephants
Markers of oxidative damage also correlate with lifespan and antioxidant capacity - higher capacity increasing lifespan
With age these decline. However this isn’t necessarily the whole story and many other factors may also be involved
Not all free radical evidence is supporting - glutathione levels of the naked mole rat are roughly similar to mice however mice live 2-3 years while NMR live 25-30 years. Some antioxidant markers thought to be more important than others and it may not be the actual molecule, but the ability to diffuse to and from cells and interact and bind (it’s coproteins) that are of importance!
Ageing population
Ageing population are those in which proportion of elderly people is increasing
Difference between growth of older population and population ageing
Less developed countries are ‘younger’ and more developed countries are ‘older’
Caused by:
Declining fertility: started in Europe/north America and spreading to rest of world
Increased life expectancy: Not many young people because of fertility decline, but there was big birth cohorts that occurred before the fertility decline and those cohorts are ageing
Migration?
Implications of ageing populations
Slower economic growth (demographic divided)
Health and healthcare (mortality, morbidity/disability)
Intergenerational social and family support
Labour force participation and retirement
Pension and old-age security
Nations growing old before growing rich: dependent on economic systems and institutions, policies that channel intergenerational flows
Why are we living longer
Gains in life expectancy reflect achievements at older ages Medical and technological advancements Rapid mortality declines at oldest ages Causes of death contributing to decline Mostly CV disease Cancer Respiratory diseases and infections
Population health is measured by
Life expectancy: average number of years remaining to be lived
Healthy life expectancy: Average number of years to be lived without disease
Active (disability-free) life expectancy: Average number of years to be lived without disability
How long will we live in future?
Depends on how you measure it: as an average maximum life span or by individual life span
Mortality has been improved the longer humans have been around
But we will spend a good portion of our lives disabled when bodily function starts to decline (compression of morbidity theory)
Morbidity is where disability starts
With life extension lifespan increases, but disability starts at the same time (spend more time disabled)
With a shift to the right you live longer and get sick later
Compression of morbidity (most ideal scenario) you are disabled for a shorter amount of time.
No compression of morbidity means growing numbers of chronically ill and disabled elderly, creating increased burden on health systems
Proteostasis
The maintenance of proteome homeostasis
Proteome homeostasis is the sum total of:
Protein synthesis (translation)
Post-translational processing and transport
Folding
Assembly and disassembly into macromolecular complexes
Stability and clearance
Achieved by an integrated network of several hundred proteins: Molecular chaperones: Heat shock proteins (HSPs) that prevent protein misfolding and aggregation
The ubiquitin-proteasome system (UPS) and autophagy: major pathways of protein degradation function in the cell
Proteotoxicity
The adverse effects of damaged or misfolded proteins on the cell
Most cells have a sophisticated proteolytic apparatus which ensures rapid elimination of altered proteins (implies altered proteins are deleterious (toxic) to cell survival)
The causes behind the formation of these altered proteins are:
Mutation and biosynthetic errors (can happen at any point in life)
Post-synthetic damage by ROS/RNS, reactive aldehydes and glycating agents
Protein misfolding
Incomplete proteolysis
Protein modification by deleterious agents
Protein carbonylation
This is the irreversible, non-enzymatic modification of proteins
Reactive carbonyl compounds are molecules with highly reactive carbonyl (C=O) groups
Introduced into proteins by a variety of oxidative pathways:
Direct oxidation of proteins by ROS yields highly reactive carbonyl derivatives resulting either from
Oxidation of side chains of Lys, Arg, Pro or Thr residues (metals are catalysts)
Cleavage of peptide bonds by the alpha-amidation pathway or by oxidation of glutamyl residues
Indirect oxidation through conjugation by reactive species:
Lipid peroxidation
Glycation
Lipid peroxidation
formation of ALEs (advanced lipid peroxidation end products)
ALEs are a class of covalent adducts which are generated by the non-enzymatic reaction of reactive carbonyl compounds (RCCs), produced by lipid peroxidation and lipid metabolism with the free amino groups of cellular and tissue proteins. Oxidative stress involved in mechanism of formation.
Lipid peroxidation is the oxidative deterioration of polyunsaturated lipids
Polyunsaturated fatty acids (PUFAs) are the building blocks of biological lipids, and comprise the membranes that surround cells and organelles
The lipid hydroperoxide formed by lipid peroxidation is important because this product can fragment to yield reactive intermediates: toxic reactive aldehydes (RCCs)
Note 1: Proteasomes are abundant in young people, and destroy misfolded proteins
Note 2: Aggregates are not good as they inhibit proteasome
Protein glycation
Formation of AGEs (Advanced glycosylation end products)
AGEs are a class of covalently modified proteins (adducts) generated through a nonenzymatic reaction between reducting sugars and free amino groups of cellular and tissue proteins. Oxidative stress involved in mechanism of formation.
Non-enzymatic glycosylation: the Maillard Reaction
Spontaneous chemical reaction between sugars and protein, increased by ROS
Maillard reaction is one of the most important chemical reactions taking place during thermal processing of food by frying, roasting or baking
Any reactive aldehyde or ketone can react non-enzymatically with amino groups present on proteins, nucleic acids and aminolipids, forming cross-links
Malliard reaction
Non-oxidative pathway
Takes weeks/months of rearrangement to form AGE
Forms glucosepane
Glucosepane: major collagen crosslink between collagen fibres, causes changes in structural protein and disorders in extracellular matrix
Oxidative pathway I: fast
Schiff base/Amadori product oxidised by ROS -> fragmentation -> RCCs
RCCs + protein (free amino group) -> (fast formation of) protein-RCC adduct (AGE)
Glucose auto-oxidation
Autooxidation by ROS
Forms RCCs which react with NH2 to form AGE (CML, carboxymethyl lysine)
Methylglyoxal
Methylglyoxal is a RCC that leads to the formation of AGEs
Excessive glycolysis might promote protein glycation via increased MG
Formed from glyceraldehyde-3-phosphate (G3P) and dihidroxyactone phosphate (DHAP) which are two normal glycolytic intermediates
Routes of formation are from glucose and fructose, MG is a major cause of secondary complication of diabetes
Glyoxalase
MG is detoxified by the glyoxalase system into D-lactate Increased levels of MG and MG-derived AGE and dysfunction of glyoxalase system linked to several age-related health problems i.e.: Diabetes CVD Cancer Disorders of the central NS Overexpression of glyoxalase in C. elegans Decreased MG-mediated protein expression Decreased mitochondrial dysfunction Decreased ROS production Increased lifespan Glyoxalase activity decreases with age
Origin of AGEs
Sugar consumption
Glucose is the least reactive of common biological sugars
Only 0.25% of glucose exists in reactive chain form (aldehyde group available for reaction, In cyclic structure group unavailable)
But because glucose is typically consumed in high quantities, it poses a threat
Galactose
Fructose (fruit sugar)
Ribose (in RNA)
Exogenous (dietary AGEs)
Present in food and drink
Role of AGEs/AGLs in ageing
Lower plasma AGEs predicted increased survival in human data
2 consequences of AGE accumulation
Protein aggregates
Leads to proteins becoming dysfunctional, damaging cells and overwhelm/inhibit proteolytic apparatus
They interact with normal proteins particularly via cross-links
Random protein aggregation, protein is dysfunctional
Cannot be broken down; accumulate and damage cells
Proteolytic apparatus (proteasomes and lysosomes) having a difficulty coping with cross-linked structures
Cross-linked structures can inhibit proteasome activity, leading to accumulation of more cross-linked proteins
Cellular signalling pathways
AGEs may activate intracellular signals through several receptor/non-receptor mediated mechanisms
Leads to increased ROS and increased inflammatory cytokines
Receptors of AGEs
MSR (macrophage scavenger receptor) and AGER (age-specific receptor)
Responsible for maintaining AGE homeostasis through regulation of their degradation and removal
Overexpression of AGER enhances AGE binding and degradation and suppresses RAGE-mediated ROS generation and inflammatory response in mouse mesangial cells
RAGE (receptor for AGE)
Distinct from scavenger receptors
Promotes oxidant-stress-dependent NF-kB activation and inflammatory gene expression
“Inflammageing”: many age-related pathologies involve an inflammatory condition which includes ROS generation
Autophagy decreases with age, cellular ability to degrade protein-AGEs via AGE-R1 becomes compromised
Some AGEs and ALEs have the same structure, since they arise from common precursors e.g. carboxymethyl lysine (CML) which is generated by glyoxal, which in turn is formed by both lipid and sugar oxidative degradation pathways.
Proteotoxicity and ageing
AGEs contribute to: AD Atherosclerosis Arteriosclerosis Bone fragility Skin wrinkling Renal failure AGEs increase with age Diabetic complications resemble premature ageing (but earlier in diabetes) Cataract Vascular disease Retinopathy Neuropathy Skin changes Kidney failure
Accumulation of altered proteins in ageing
Breakdown of proteostasis contributes to ageing and age-related pathologies:
Proteasome and autophagic system can be damaged by ROS and glycation agents
Damage can also affect the chaperone proteins which normally assist in correct protein folding
Manifests as:
Lipofuscin (skin pigment): age spots on skin and organs
Yellow-brown granular material that accumulates progressively over time in the lysosomes of postmitotic cells
Produced mainly by peroxidation of unsaturated FA in complex with cross-linked proteins
Metals (mercury, Al, Fe, Cu and/or Zn) present in lipofuscins
Ageing: proteostasis dysfunction = poor removal of oxidised proteins, which accumulate causing tissue damage and deposition of lipofuscin
alpha-crystallin: cataracts (crystalline aggregates in eye lens)
Cartilage protein, collaged: hip joint surface, arteries (atheromas)
Normal cartilage/collagen proteins:
The shape of these proteins are important for function
In AGE accumulation the protein strands randomly cross-link and become stiff and irregular
Bad in hip joints
Bad in arteries - makes them stiff and contributes to formation of atheromas
Protein aggregates in age-related neuropathies:
alpha synuclein: Lewy bodies in Parkinson’s disease
amyloid plaques and tau tangles in Alzheimer’s disease
Prions in Creutzfeldt-Jacob disease
Modulation of AGEs
Calorie restriction
Deoxyglucose (non-metabolisable) (decrease glucose levels)
Carnosine (Beta-alanyl-L-histidine) (readily glycated by RCCs)
mTOR (mammalian target of rapamycin) inhibition (decreases glycolysis, stimulates mito function)
AGE cross-link breaker improved heart function in aged dogs (increased stroke volume and end-diastolic (ventricular) volume after 4 week treatment
What is calorie restriction
Dietary restriction of energy intake by 30-40% normal calories but with adequate nutrition: normal levels of vitamins, minerals, essential amino acids and fats must be maintained.
Only intervention consistently increases life expectancy
Calorie restriction
Episodes from human past - Denmark 1917 WW1: adequate intake of whole grain cereals, veg and milk reduced mortality by 34%
Norway WW2: whole grain cereals, veg, potatoes and fish reduced mortality by 30%
Animal models had 30-50% fewer calories, normal vitamins, minerals and essential aas and fats showed 30% increase in maximum lifespan even from Middle Ages models.
Postpones hypertension, cancers, immune system dysfunction, kidney pathology, brain ageing, cataract formation, diabetes.
Rhesus monkeys had 30% CR and showed increase in 20+ years of lifespan
Human calorie restriction trial
Redman et al 2011
Comprehensive assessment of long term effect of reducing intake of energy
25% restriction in non obese humans
6 month results show reduced body weight and fat mass and body temp, insulin and CV risk by 28%
Disadvantages of calorie restriction
Lower growth rate and smaller maximum body size achieved
Delayed puberty
Lower fertility
Compromised thermogenesis
Calorie restriction mechanisms for lifespan
Advanced glycation end products and oxidative stress and the mitochondrion
Glycation end products - advanced glycation end products AGE is reduced by calorie restriction
Lower human plasma AGE predicts increased survival.
Oxidative stress also increases with age but CR Changes ROS activity, free radical generation, antioxidant defences, mitochondria production etc. CR means there is little NADPH and all is used to drive ETC making ATP and inner membrane becomes more permeable to H ions and membrane potential is reduced (polarised) therefore less ROS and heat is produced. Increases cellular lifespan. Also generate less free radicals per Oxygen atom used in the ETC.
Molecular mechanisms involved in effect of Calorie restriction on longevity
Decrease in calorie in diet activates systems involved in:
More efficient metabolism
Higher protection against cellular damage
Activation of remodelling mechanisms
Less efficient metabolism and synthetic pathways are blocked
Mitochondrial gene expression changes in CR. Increasing SIRT1 expression due to high NAD:NADH ratio. SIRT1 is silent info regulator decreases apoptosis enhances glucose production, lipid metabolism and fat mobilisation, angiogenesis and decreases tumour formation.
Increasing sir2(animal SIRT1 equivalent) increases lifespan.
CR mice with SIRT1 knockout DO NOT live longer. Activation of this gene therefore important for longer survival.
SIRT1 can be activated by drugs and resveratrol - natural polyphenol.
PGC-1alpha increases due to high AMP:ATP ratio due to little excess of ATP. AMP activates AMP kinase and inhibits TOR as part of signalling pathway increasing gene expression and mitochondrial biogenesis. More mitochondria mean less electron leak forming ROS. Metabolic rate doesn’t alter in CR.
SIRT1 and PGC1 are linked and activate together.
Nutrient sensing pathways and calorie restriction
Nutrients activate systems to increase cell growth and storage of nutrients - IGF1 increases cell division and size and insulin increases uptake and storage of glucose by cells.
Calorie restriction reduces these molecules and so reduces these effects. This has unexpected benefits.
IGF1 knockout in animal models increases lifespan show 23% average increase
Low IGF1 induced expression of mitochondrial antioxidant. Low signalling promotes translocation of FOXO3a to nucleus (normally inhibited by IGF1) where it induces MnSOD and GPx glutathione peroxidases which promotes removal of hydrogen peroxide. Enhances MnSOD lowers superoxide levels and peroxynitrite formation. also associated with reduced apoptosis.
Define intrinsic and stressed ageing
Intrinsic (deterministic) - time dependent functional decline despite optimal conditions often observable in laboratory animals and on a cellular levels
Stressed (non deterministic) - physiological or functional decline in wild organisms reflecting exposure to stress or inactivity etc.
Potentially -
Animals die because of intrinsic ageing repair mechanisms being unable to keep up with molecular damage.
Evidence for genetic involvement in ageing
Heritability
Family studies
Progeria syndromes
Animal studies of age
Progeria
Extremely rare genetic condition causing advanced ageing at early ages
Symptoms closely relate to ageing including wrinkles, hair loss, delayed growth.
Have normal development up to 18 months and suddenly stop gaining weight and display stunted height.
Progeria becomes more severe as they age - average life expectancy is 12 years old
Affects the nuclear membrane disrupting normal nuclear architecture leading to DNA damage and impaired replication
Is a defective splicing of pro-lamin A meaning a sticky membrane is produced instead of smooth.
Humans show effects on CV ageing and function but mice do not - exploring these differences may give insight into ageing and the disease.
Define fragility
Poor resilience to external stressors
Results from ageing related declines across physiological systems
Vulnerability to adverse outcomes
C. Elegans and DAF2/16
C. Elegans can arrest their development in adverse conditions halting ageing to survive.
DAF2 is a gene similar to the insulin gene. A mutation in this gene prolongs their lifespan, DAF16 is also important in lifespan - DAF2 is dependent on DAF16. If DAF16 isn’t present lifespan decreases as it’s a transcriptional regulator for multiple DNA sites involved in antioxidant pathways, antimicrobial pathways and protective pathways DAF2 switches this pathway off so when mutated does not block DAF16 and therefore extends the animals lifespan.
Sirtuins and DAF16 pathways
Over expression of sir2 extends lifespan in yeast, worm, flies.
SIRT1 is the mammalian homologue but effect is controversial in mammals
Common pathway - sir2, HSP And JNK all act through the same DAF16 Pathway
DAF16 mammalian analogue is the FOXO regulatory element.
Regulates multiple downstream genes which effect longevity
These pathways are conserved across many species of worm, flies and rodents.
Rapamycin and insulin pathway interference
Inhibits mTOR which interacts with the insulin pathway - conserved across many species : yeast, nematodes, flies, mice, human progeria syndromes.
Interference with the insulin pathway can extend the lifespan of those affected
Changes seen in liver, heart when administered
Restricting diet increases lifespan this is thought to have a similar effect.
Growth hormone and lifespan
Mammalian target as GH links diet growth pathways
In Ames and snell Dwarf mice loss of function mutations in prop1 and pit1 genes reducing trophic hormones like GH, prolactin, IGF1 etc. Loss of the GH means no calorie restriction response occurred suggesting GH mediates link between two pathways
Pathways related to ageing
Decreasing insulin IGF1 signalling increases stress resistance and lifespan
TOR decreased signalling causes increased lifespan increasing autophagy, and decreased protein translation.
Mitochondrial function severe dysfunction causes decreased lifespan but modestly decreased function has been shown to increase lifespan
Sirtuins can increase or decrease lifespan in different contexts
Calorie restriction causes increased lifespan when optimally done.
Defective Mitochondrial polymerase gamma
In mouse causes weight loss, alopecia, osteoporosis, cardiomyopathy, hypogonadism but mice don’t show neurological disease degeneration which is why they cannot apply them to human studies whereas rats have a more similar pathways to humans.
Drosophila ageing defects
Mitochondrial dysfunction POLG gene causes premature ageing
Mito
Longevity and fragility inheritability
25% heritable
How do cells count division
Two categories
Telomere erosion and telomere independent mechanisms
Telomeres and telomerase
Structures present at the natural end of linear chromosomes enabling it to behave differently from double stranded DNA break in the genome
Functions:
Stop natural ends fusing with other chromosomes
Stop ends activating genome damage checkpoints
Prevent loss of sequence by exonuclease attack
Deal with end replication problem
With each cell division telomeres erode meaning every replication is progressively shorter.
Telomeres prevents the actual DNA being shortened, telomerase is a reverse transcriptase that adds TTAGGG onto chromosome ends to maintain the telomeres.
hTR is the RNA template from TTAGGG synthesis. hTERT is the catalytic protein subunit.
Shelterins are protein complexes that protect telomeres in eukaryotes as well as regulating telomerase activity.
GWAS also regulates telomere elongation
Telomere length is shorter when there is: cell division, nuclease action, chemical damage, DNA replication
And is longer when there is: telomerase expression, recombination between telomeres repeats, shelterins.
Telomerase expression is highest in germ line cells, haemopoietic and intestinal villus stem cells and in 85-90% malignancies.
Low in mortal primary cells and many differentiated human cells and fibroblasts.
Telomere erosion and senescence
In fibroblasts telomerase is not present and they show telomerase erosion when cultured
However if treated with hTERT mRNA by viral transfection cells become telomerase positive and no longer show senescence - cellular immortality
There’s also indirect evidence in vivo recording proliferative cell history
Telomere length declines with age in several human tissues and length may be associated with age related disease but this is difficult to test
Twin studies show strong associations with leukocyte telomere length and increased fragility independent of age. However mixed results with all fragility, longevity, immune functions studied in humans.
Many short lived animals age and die with long telomeres such as mice, c elegans, zebra fish and drosophila don’t even have telomeres contradicting this evidence.
Experimental deletion of telomere repair mechanisms in these models does cause premature ageing phenotypes to emerge however!
Inherited telomere syndromes
Mostly Alzheimer’s disorders
11 human genes to date code for telomerase components - TERC, TERT, DKC1 etc. And telomerase bonding proteins like TINF2, POT1
Disorders are varied - include pulmonary fibrosis, myelodysplastic syndrome, dyskeratosis congenita
Telomeres and ageing
Still unknown if telomeres are a cause or byproduct of ageing - shortening can directly contribute to aspects of ageing, be a consequence of disease and progression and set up a cycle interacting with other disease processes
Telomeres, senescence and DDR
Telomere erosion leads to dysfunction - uncapped telomeres
Leads to chronic DNA damage response DDR at telomeres
Can visualise with telomere stains and markers like 53BP1
TIF assay for senescent cells allows detection of cells in vivo.
Thought to be Dependent on p53 Activation, p21 accumulation and p16 activation
Characteristics of senescent cells
Prevents replication of damaged DNA Profound chromatin changes Increased expression of cell cycle inhibitor p16INK4a Replication arrest Other secretome changes Resistance to apoptosis
How to senescent cells cause age related pathologies
Can impact organ function by non dividing - many tissues require division as part of function such as immune system
Adult stem cell renewal declines with age affecting tissue function and regenerative ability
Senescent cells also influence local tissue microenvironment via altered gene expression - MGP up regulation in VMSC promotes vascular calcification and elevated CVD risk
Show elevated range of proteins - senescence associated secretory phenotype SASP. Proteins include cytokines, ECM metabolising professes
SASP is caused by prolonged DDR and needs key mutations to occur like mutant RAS. Doesn’t occur in cells made senescent by forced overexpression of p16INK4a
Senolytics
Screening for agents which start apoptosis in senescent cells like dasatinib and quercetin or flavonoid fisetin.
These agents can go everywhere including crossing blood brain barrier
Human trials in senolytics need to assess long term outcomes
Targets senescent cells
Rescues age related osteoporosis in mice
Why did telomere driven senescence evolve
Antagonistic pleiotropy - systems have alternative benefits that outweigh these downfalls
Adaptive theory - Accumulation of senescent cells and contribution to age related pathologies are non-selected late life deleterious effects.
Evolved as tumour suppressor mechanism to allow large long loved species to live long enough to reproduce ?
Cancer and cell proliferation
Cancer is disease of uncontrolled unlimited cell division
Somatic genetic disease driven by accumulation of genetic changes
Main hallmarks - cell division, angiogenesis, invasion etc.
Cell division necessary to create the diseases
Early hallmark of CV disease
Hypertension caused by increased endothelial cell dysfunction no longer generating NO in response to ACh and so acetylcholine stimulates smooth muscle cells directly and contracts the muscle creating faster blood flow
Atherosclerotic plaque types
Vulnerable - thin vessel wall and small lumen increases blood pressure until vessel wall thickens becoming stabilised or ruptures and thrombosis forms.
Stabilised - lumen size reduced but thick vessel wall means is unlikely to rupture. If the thrombus doesn’t clear or occluded entire vessel then blood flow is prevented leading to MI etc.
Evolution of fatty streak
Hypercholesterolemia
Adherence of monocytes to arterial endothelium
Penetration of monocytes into artery
Oxidation, aggregation our immune complexing occurs and smooth muscle vascular tone alters
Phenotypic modulation, expression of scavenger receptors, inhibition of modified LDL
fatty streak made up mainly of cholesterol rich foam cells.
