Lecture 34 - Cellular Senescence and Ageing Flashcards
What is Senescence?
Deterioration associated with ageing
Aka Biological ageing
What is the maximum life span?
Give some examples
Maximum life span: maximum number of years that a member of a species has been known to survive
Humans: 120 years
(Jeanne Calment 122 years)
Drosophila: 3 months
Mouse: 3 years
Turtles: 150 years
Trees: >1000 years
Dahlia anemone: non ageing
Which cells supplement and replace the baseline cell population?
Stem cells
What are the various fates of a baseline cell population?
- Proliferation
- Differentiation
- Apoptosis
What are the principal structural targets for cell damage?
Cell membranes
• Plasma membrane
• Organelle membranes
DNA
Proteins
• Structural
• Enzymes
Mitochondria
List some generalised processes that cause cell injury
- Reduced ATP / mitochondria damage
- Loss of intracellular calcium homeostasis
- Disrupted membrane permeability
- Free radicals
What are the protective mechanisms of cells that undergo injury?
Describe the mechanism
Heat shock response genes
• Large group of genes
• Expression of proteins up-regulated during cell stress
Mechanism:
- Cell stress
- Up-regulation of Heat Shock Response genes
- Protection of proteins from stress-related damage
- Cleaning up of damaged proteins from the cell
What is the most critical factor in determining whether cell injury is reversible or irreversible?
Time
After a certain point in time, the injury becomes irreversible
List the chronological morphologic alterations in cell injury
– Reversible cell injury –
- Decreased cell function
– Irreversible injury –
- Biochemical alterations; cell death
- Ultrastructural changes
- Light microscopic changes
- Gross morphologic changes
What changes to cells during cell injury are visible in light microscopy?
Cytoplasmic changes
• Increased eosinophilia
Nuclear changes:
• Pyknosis
• Karyorrhexis
• Karyolysis
What are the hallmarks of the following:
• Reversible injury
• Irreversible injury
1. Reversible injury • Loss of ATP → Failure of the Na/K pump • Anaerobic metabolism → Lactic acid build up • Reduced protein synthesis
- Irreversible injury
• Massive intra-cytoplasmic calcium accumulation
• Enzyme activation (caspases)
What is Autophagy?
When does it occur?
Describe the process
What happens to this process with ageing?
Cell eats its own contents
When:
• A survival mechanism in times of nutrient deprivation
• Recycling of cellular rubbish
- Nutrient deprivation
- Formation of autophagic vacuole containing organelles
- Lysosome fusion with autophagic vacuole
- Degradation of organelles in the vacuole
- Use of nutrients
In ageing:
• This process is not as effective
• There is an accumulation of damaged organelles as the cells get older
What happens when there is inefficient autophagy?
- Accumulation of cellular rubbish
* Increased rate of senescence
What factors lead to cellular ageing (i.e. senescence)?
• Telomere shortening
• Environmental insults
(e.g. Free radicals)
• DNA repair defects
• Abnormal GF signalling
(Insulin, IGF)
Describe Huntington’s chorea, and why there has been no selective pressure to remove this mutation
- Highly penetrant dominant mutation
- Neurodegenerative disease
- Onset at 35.5 years of age
It is only relatively recently that people have been living to this age
There has never been selective pressure to remove this mutation
Discuss the Evolution of Ageing
It is only relatively recently that we have been living this long
Postulate:
“Ageing is the result of late-acting deleterious mutations”, that have previously not been selected against because they were never relevant
Describe mutation accumulation theory in ageing
Genetic diseases should increase in frequency with age
There should be great heterogeneity in deleterious genes between different individuals throughout the entire genome
What are the various causes of cell injury?
- Hypoxia
- Chemical
- Physical
- Nutritional (deficiency, excess)
- Immune
- Infectious
List common chemical insults
How do these cause damage?
1. Poisons: Examples: • Arsenic • Cyanide • Heavy metals
Mechanism:
- Interference with cellular metabolism
- ATP levels drop
- Cell death
- Pharmaceuticals
• Direct effects
• Metabolism into toxic metabolites
e.g. Alcohol
List some Physical insults to cells
What effect do these have on cells?
- Extreme heat
• Coagulative necrosis - Extreme cold
• Coagulative necrosis - Pressure changes
• Cellular disruption
• Contusion - Electrical current
• Breakdown of cellular membrane
Describe the effect of Free radicals on cells
What are the sources?
What is the effect?
How can they be neutralised?
1. Source: • Normal Oxidative phosphorylation occurring within a cell • Absorption of irradiation • Transition metals • Nitrous oxide (an important paracine) • Toxic (acetaldehyde)
- Effect:
Proteins → fragmentation, cross-linkning
Lipids → lipid peroxidation
DNA → single strand breaks
3. Neutralisation: Detoxification enzymes, antioxidants: • Glutathione • Vitamin C • Vitamin E • Beta-carotine
Discuss the role of telomeres in ageing
Compare its level of expression in various cells
Decrease in telomerase expression → replicative senescence
Telomerase:
• Ribonucleoprotein
• Contains template for telomeres
• Brings about the extension of telomeres
Expression: • Germ cells: constant • Stem cells: slow decline • Normal somatic cells: faster decline • Cancer cells: constant (from original point, since it is coming from a somatic cell)
Describe Werner’s syndrome
List the general features
What is the progression?
What is the aetiology?
What is the pathogenesis?
- Premature ageing
- Rare
- Inherited
Progression: • Childhood: normal development • Teenage: stop growing • Adulthood: ageing, more susceptible to cancer, osteoporosis, diabetes, cataracts etc. • Late 40's: death
Aetiology
• WRN gene: encodes a helicase
• Helicase unwinds DNA for replication, DNA repair and transcription
Pathogenesis:
• Not completely known
• Improper DNA repair → rapid accumulation of mutations
• Improper transcription of genes required to maintain vigour or normal function
What is Hutchinson Gilford progeria syndrome?
What is the genetic basis?
What are the symptoms?
Rapid ageing in young children
Extremely rare
Genetic basis:
• Lamin A
• Dominant mutant gene
• Involved in nuclear integrity
Symptoms: • Similar to ageing in older persons • Loss of hair • Thin, transparent skin • Age spots • Osteoporosis • Atherosclerosis
Pathogenesis:
• Not completely clear
• Shorter telomeres
• Gene involved in preventing oxidative damage may be involved
What is the proposed role of Lamin A?
Involvement in nuclear integrity
Describe what things constitute immune insult
- Cytokines
• Altered gene expression
• Altered cellular metabolism
• General stress on cells - Complement
• MAC formation - CTL and NK cells
• Direct attack and killing of cells
Also, decline in immune function as we age
→ especially macrophages
Describe the role of macrophages in ageing
In Sardinia, more men live past 100 that anywhere else in the world
Observations in these men:
• Normal T and B cell decline
• Macrophages have a ‘younger’ phenotype; more active
However, chronic inflammation (macrophage mediated) promotes injury
Discuss nutritional insult
Deficiency: • Interference in normal metabolic pathways Examples: • Protein • Vitamins • Minerals
Excess: • Likewise leads to cellular and tissue alterations that are detrimental Examples: • Fat • Health supplements
Caloric restriction … senescence
… postpones …
One of the most reliable ways to prolong life in lab animals is to simply …
… restrict their calories
Describe the observations in mice who had caloric restriction
What is the proposed mechanism?
In mice, caloric restriction:
• 15% smaller
• Live 50% longer
• Less disease (cancer, atherosclerosis, autoimmune disease)
Compared to mice who eat ad libitum
Mechanism:
• Increased levels of antioxidant molecules
• Interaction of metabolic pathways with the insulin and IGF-1 pathways (?)
Describe the Insulin / IGF-1 signalling pathway
What is the relevance with ageing?
Hypothalamic - pituitary axis
- Growth hormone (GH) releasing hormones (GHRH) released from hypothalamus
- Receptors in anterior pituitary engaged:
• GHRHR: Growth hormone releasing hormone receptors
• Somatostatin receptors
• Growth hormone receptors - Pituitary releases Growth hormones (GH)
- GH acts on GHR on hepatocytes in the liver
- IGF-1 produced (Insulin GF 1) by hepatocytes
- IGF-1 acts on many cells around the body
• Growth is the generalised outcome
Role in ageing:
• Inhibiting this pathway (at any level) leads to extended lifespan
What is the role of the insulin receptor in mouse longevity?
→ IR KO: Lack of insulin receptor in adipose tissue in mouse
Observation:
• Extended longevity
• Protected against age-related obesity
• 18% increase in lifespan
What is the role of the IGF-1 receptor in mouse longevity?
→ IGF+/-
(Mice heterozygous for deletion of IGF-1 receptor gene)
Observations:
• Resistance to oxidative stress
• 33% increased lifespan in females
Describe the general trends in the following parameters as age increases: • Stem cell activity • Mutation accumulation • Disease incidence • Inflammation
Stem cell activity:
• Markedly drops after development
Mutation accumulation:
• Steady accumulation after maturity
Disease incidence:
• Marked increase in later years
Inflammation:
• Steady increase
• Marked increase in later years
Compare ageing in the following cells:
• Short lived
• Long lived
What does this account for?
There is a trade off between accumulation of:
Mutation vs. ROS
Short lived:
• No build up of ROS and Toxic metabolites (because cells are rapidly dividing and thus ROS are diluted out)
• Frequent replacement of the cells from a stem cell progenitor
• Greater accumulation of mutations because the cells are diving so rapidly (errors in DNA replication and cell division)
Long lived:
• Build up of ROS and toxic metabolites (not diluted out in cell division)
• Less accumulation of mutation, because there is less DNA replication and cell division occurring
This accounts for the discrepancies in ways that different organs age
What is inflammageing?
General increase in inflammation observed throughout one’s life
Link between: • Chronic inflammation • DNA repair capacity • Accumulation of ROS • Remodelling
Contributing factors: • Environmental free radicals • Obesity • DNA damage • Immunosenescence • Age and chronic diseases • Years of exposure to inflammatory proteins • Oxidative stress
Describe the factors at play in tissue homeostasis
Baseline population
Stem cell population replenishing baseline population
Baseline population has three fates:
• Proliferation
• Differentiation
• Apoptosis
Why is damage to mitochondria so damaging?
Cell no longer able to produce energy
This has knock on effects on many processes in the cell
For this reason, when there is mitochondrial damage the cell rapidly undergoes apoptosis
Outline the link between the biology of cancer and ageing
Accumulation of DNA damage and genomic instability over time in cells leads to:
a. Arrest / senescence / apoptosis, through gradual loss in telomerase function
→ Ageing, degenerative diseases
or
b. Cancer, through the reactivation of telomerase
On a cellular level, what is seen with ageing?
Build up of damage:
• Organelles
• Proteins
• DNA
Compare proliferative capacity of cells from:
• Newborn
• Centenarian
• Individual with Werner syndrome
Newborn cells have the highest proliferative capacity
Centenarian less than newborn, but still better than those from an individual with Werner syndrome
Compare normal skin cells with those in individuals with Progeria
What is the basis of this?
Progeria:
• Abnormal nuclei
• Different shape
Lamin A seems to be involved with nuclear integrity
What is Polypathology?
What is its significance?
Patient may present with e.g. Diabetes of Rheumatoid arthritis
However, there are many different diseases that have been chronically present and undiagnosed.
Solely treating the pathology that the patient presents with will not have a good outcome in the long run.
Ultimately we are going to have to treat multiple diseases, not just the eg Diabetes or Rheumatoid arthritis
Loss of which enzyme leads to senescence, primarily?
Telomerase
Describe Haldane’s theory of Selection Shadow
1940’s
This theory holds that certain deleterious mutations will not be selected out by natural selection because they only take their effect after replication
Describe Medawar’s theory of Mutation Accumulation
1950’s
Over the course of one’s life mutations accumulation and only very late on in the piece do they take their effect.
These mutations could have some causality in disease
