CELL & MOLECULAR AGEING

define ageing

ageing: complex biological process in which changes at the molecular, cellular and organ levels results in a progressive inevitable and inescapable decrease in the body’s ability to respond appropriately to internal and/ or external stressors

outline lifespan

• different from ageing

• life expectancy is increasing

• increased % elderly in population

• increase in age-related diseases

ageing VS age-related disease

• ageing is not a disease - occurs in every multi-cellular animal

• has universal molecular aetiology

• ageing occurs in the absence of disease

who came up with the traditional theories of ageing

• Galen

• Roger Bacon

• Charles Darwin

outline Galen’s traditional theory of ageing

Galen (AD129-199)

• ageing is not a disease

• death is inevitable as ‘the body deteriorates of itself’ but life could be prolonged

• importance of a healthy youth as the basis for a robust old age incl. diet, walking, wine

outline Roger Bacon’s traditional theory of ageing

Roger Bacon (1220-1292)

• wear and tear theory

• result of abuses and insults to the body - good hygiene may slow process

outline Charles Darwin’s traditional theory of ageing

Charles Darwin (1809-1892)

• programmed mechanism of ageing - biological evolutionary process

• loss of irritability in nervous and muscular tissue

what are the two main categories of modern biological theories of ageing

• programmed theories

  • ageing follows a biological timetable

• damage or error theories

  • environmental assaults to living organisms that induce cumulative damage at various levels

what theories are within the programmed theories of ageing

• programmed longevity

• endocrine theory

• immunological theory

programmed theories of ageing: programmed longevity 

• switching on and off of certain genes 

• changing expression results in genetic instability

programmed theories of ageing: endocrine theory

• biological clocks (e.g. circadian rhythms) act through hormones

programmed theories of ageing: immunological theory

• immune system is programmed to decline » increased vulnerability to infectious disease

• this leads to ageing and death

what theories are within the damage/ error theories of ageing

• wear and tear theory

• rate of living theory

• cross-linking theory

• free radicals theory

• somatic DNA damage theory

damage/ error theories of ageing: wear and tear theory

• cells and tissues have vital parts that wear out resulting in ageing

damage/ error theories of ageing: rate of living theory

• the greater an organism’s rate of oxygen basal metabolism, the shorter its lifespan

damage/ error theories of ageing: cross-linking theory

• accumulation of cross-linked proteins damages cells and tissues

damage/ error theories of ageing: free radicals theory

• superoxide and other free radicals generated cause damage to the macromolecular components of cell (proteins, nucleic acids, lipids, carbohydrates)

outline superoxides 

• superoxides are naturally produced by the body and needed

• however they damage DNA and mitochondria if left in the body for too long 

  • in health, cell has mechanisms to remove superoxides but as you age these mechanisms become less efficient and effective

damage/ error theories of ageing: somatic DNA damage theory

• accumulation of DNA damages results in eventual ageing

• as you age you also accumulate DNA damage

summary of biological theories of ageing

• multiple theories proposed

• however no consensus because many interact with each other in complex ways

what determines the ‘ageing phenotype’

cell and molecular events that contribute to the ageing process and together determine the ‘ageing phenotype’

what does ‘hallmark’ mean in terms of ageing

• cellular and molecular mechanisms that cause ageing

what criteria should each hallmark fulfil

• should manifest during normal ageing

• experimental aggravation should accelerate ageing

• experimental amelioration (improvements) should retard the normal ageing process and hence increase healthy lifespan

what are the cell and molecular hallmarks of ageing (9)

outline genomic instability 

• integrity and stability of DNA is continuously challenged by endogenous e.g. ROS and exogenous agents e.g. UV

• there will be a natural accumulation of genetic damage throughout life - nuclear and mitochondrial DNA

how do DNA alterations result in dysfunctional cells

• DNA alterations may affect essential ageing genes and transcriptional pathways

• this results in dysfunctional cells

• tissue and organismal homeostasis and tissue renewal may be jeopardised

genetically controlled longevity

• heritable component in human longevity - esp. at extreme ages

• large number of genes identified, modification of which affects longevity

• but phenotypic differences in ageing between monozygotic twins

  • 20-30% of ageing is controlled by genetics, the rest is environmental factors

what is an ageing syndrome

Hutchinson-Gilford Progeria (HGPS)

outline HGPS

• rare genetic, fatal disorder with striking features resembling premature ageing

• mutation in LMNA gene

• associated with atherosclerosis, high BP, strokes, angina, heart failure

• usually die by 14.5 years old

outline LMNA gene (mutation) in HGPS

• LMNA encodes for lamin A (encoding nuclear envelope protein)

• defective lamin A protein (truncated) makes the nucleus unstable

• cellular instability appears to lead to the process of premature ageing in progeria

what is an oral implication of progeria

overcrowding of teeth in mandible

what are telomeres 

telomere: repetitive DNA sequences found at either ends of chromosomes that protect the ends of chromosomes

• DNA needs protecting because the ends can fray - telomeres prevent this

outline the change in length of telomeres

• telomeres progressively shorten with each cell division

• telomerase maintains the length of telomeres

  • most mammalian somatic cells do not express telomerase

what kind of enzyme is telomerase

specialised DNA polymerase able to replicate terminal ends of linear DNA

what if telomeres become dysfunctional

• pathological telomere dysfunction accelerates ageing in mice and humans

  • experimental stimulation of telomerase can delay ageing in mice 

across species, do shorter telomeres predict shorter lifespans

no, across species, short telomeres do not necessarily predict short lifespan - humans have shorter telomeres than rodents but have longer lifespans

telomere attrition II

outline epigenetic changes

• chemical changes DNA that does not affect underlying DNA

epigenetic changes: age-associated epigenetic markers

• increase - histone H4K16 acetylation, H4K20 trimethylation or H3K4 trimethylation

• decrease - H3K9 methylation or H3K27 trimethylation

what are the different mechanisms of epigenetic changes

• alterations in DNA methyltransferases

• histone acetylases, deacetylases, methylases and demethylases

• protein complexes implicated in chromatin remodelling 

outline sirtuins

• family of enzymes that has important roles in DNA repair, telomere maintenance and age-related pathological conditions

  • e.g. loss of SIRT6 gene in mice leads to ageing

  • gain of function extends longevity

• class III histone deacetylases 

SIRT6 and links to pathological conditions

what is proteostasis

proteostasis: the process of protein homeostasis - balance of synthesis, folding, degradation

what do Alzheimer’s, Parkinson’s and cataracts have in common

• chronic expression of unfolded, misfolded proteins will begin to aggregate which contributes to the development of some age-related pathologies:

  • Alzheimer’s

  • Parkinson’s

  • cataracts

name the two principal proteolytic systems associated with protein quality control decline with ageing

• autophagy-lysosomal

• ubiquitin-proteasome

what is an experimental example of proteostasis being associated with age-related pathologies 

mutant mice deficient in protein from the heat-shock family exhibit accelerated ageing phenotypes

loss of proteostasis depends on the ______

loss of proteostasis depends on the tissue (so some organs may begin to fail and age before others)

outline deregulated nutrient sensing

• dietary restriction increases lifespan or healthspan in eukaryote species - seen in unicellular and multicellular organisms

• the insulin and IGF-1 signalling (IIS) pathway is the most conserved ageing controlling pathway in evolution

  • decreased nutrient signalling extends longevity

what does Rapamycin do in mice

rapamycin can extend longevity in mice

restricting diet to restrict ____ can improve longevity

restricting diet to restrict mTOR (protein) can improve longevity

light green = improves longevity

define calorie restriction

calorie restriction: the sustained restriction of dietary energy intake compared with the energy requirement for weight maintenance

what does responsible prescription of calorie restriction require

adequate intake of carbohydrate, fat, protein and micronutrients to ensure satisfaction of recommended daily allowance

are there a lot of randomised controlled trials on the effects of calorie restriction on biological ageing in humans

• there are very few

  • expensive

  • time-consuming

  • lots of difficult to control factors

controlling your diet for _ years extends your life for _ year

controlling your diet for 2 years extends your life for 1 year 

outline mitochondrial dysfunction

multiple converging mechanisms:

• accumulation of mutations and deletions in mtDNA (mitochondrial DNA)

• oxidation of mitochondrial proteins by ROS produced in electron transport chain

• destabilisation of macromolecular organisation of respiratory chain (super)complexes

» destabilisation of mitochondria and breakdown of electron transport chain

why does the efficacy of the respiratory chain decrease with age

• efficacy of respiratory chain decreases with age due to:

  • electron leakage

  • reduced ATP production

mitochondrial dysfunction: outline the conventional and current theories of the role of ROS in ageing

• conventional theory: increased ROS production causes progressive mitochondrial deterioration and global cellular damage

• current theory: as age increases, ROS increases to maintain survival until the levels become too high and aggravate age-associated damage

what do ROS trigger

proliferative and survival signals in response to physiological signals and stress conditions

(this is how the current theory of ROS was arrived at - because it can respond to signals it also helps maintain survival to a certain point as we age)

describe the pathway from mitochondrial damage to neurodegeneration

mitochondrial damage » pathologic inflammation » synaptic degeneration » neurodegeneration

• mitochondrial damage initiates ROS production 

• this stimulates pro-inflammatory signals and cytokine production which leads to inflammation

• but there is nothing to attack because the damage is intracellular so it damages synapses instead

• this begins a cascade that results in global neurodegeneration

what conditions are the previous pathway observed in

Alzheimer’s and other neurological disorders

outline the relationship between melatonin and ROS

• melatonin is a neuroprotective hormone that has a largely antioxidant effect

• exclusively synthesised in the mitochondrial matrix

• it soaks up ROS within synapses of neurones

• therefore maintaining melatonin levels via pharmaceuticals or by circadian regulation is an area of therapeutic interest

outline cellular senescence

• irreversible cell-cycle arrest mechanism

• when cells are still living but not proliferating, permanently left the cell cycle therefore protected from dysregulated growth

  • this results in large phenotypic changes

why does cellular senescence result in large phenotypic changes

• cell is no longer in the cell cycle and therefore no longer well regulated

  • no protein level checks, lots of cytokine and growth factors

• DNA coiling is also dysregulated

who was cellular senescence first described by 

Hayflick and Moorhead (1961)

• DNA becomes damaged because of telomere attrition

• therefore cells leave cell cycle permanently to try and protect themselves (cell division is arrested)

  • this is replicative senescence

• protective mechanism for unrestricted growth of damaged cells

why is there an accumulation of senescent cells with ageing

• there is an accumulation of senescent cells with ageing due to:

  • increase in rate of generation of senescent cells

  and/ or

  • decrease in their rate of clearance

state the 5 distinct paracrine mechanisms by which senescent cells could promote tissue dysfunction

• induction of senescence in neighbouring cells (paracrine senescence)

• perturbation of the stem cell niche (causing stem cell dysfunction)

• disruption of extracellular matrix

• induction of aberrant cell differentiation

• stimulation of tissue inflammation (senescence-associated secretory phenotype)

outline stem cell exhaustion

• decrease in cell cycle activity of stem cells with ageing

• consequence of accumulation of DNA damage, overexpression of cell cycle-inhibitory proteins (senescence), telomere shortening

what process declines with age 

haematopoiesis declines with age

» increased incidence of anaemia and myeloid malignancies

outline altered intercellular communication

• inflammaging: inflammation increases with age

  • accumulation of pro-inflammatory tissue damage

  • senescent cells secrete pro-inflammatory cytokines

  • immunosenescence (failure of immune system to clear infectious agents)

—

» impact on cell communication

what type of signalling tends to be deregulated in ageing and why

neurohormonal signalling tends to be deregulated in ageing as inflammatory reactions increase

outline inter-organ coordination in altered intercellular communication

• age related changes in one tissue can lead to ageing-specific deterioration of other tissues

  • e.g. impaired kidney function can increase the risk of heart disease in humans

• however this means that lifespan-extending therapies targeting one tissue can retard the ageing process in other tissues

what is an alternative model to the cell and molecular hallmarks of ageing

the pillars of ageing (2013)

what are the pillars of ageing

intertwined processes that promote ageing

what is a challenge of increasing lifespan/ life expectancy

• while life expectancy continues to rise, healthspan is not keeping pace

• current disease treatment often decreases mortality but does not reverse the decline in overall health

  • elders are sick for longer, often coping with multiple chronic diseases simultaneously

is there a consensus on biological ageing

no, there are many different interlinked theories