Ageing and senescence

Question 1

what is senescence?

Answer 1

Senescence is the age-dependent decline in vital physiological functions

Question 2

what 2 factors cause senescence?

Answer 2

1. wear and tear
   - e.g. decline in C. elegans muscle function, teeth worn off in elephants
2. genetics
   - e.g. salmon die as soon as they lay eggs
   - e.g. species difference in ageing such as elephant and mouse: gestation of elephant has finished at 21 months, which is halfway through a mouse’s life

Question 3

what is the disposable soma theory?

Answer 3

Natural selection tunes life history of the organism so that sufficient resources are invested in maintaining the repair mechanisms that prevent ageing, at least until that organism has reproduced and cared for its young
- As soon as an individual cannot increase number, or the chance of survival of its offspring any further, there is no natural selection against decline/ageing in that individual
- If genes/mechanisms exist that increase the reproductive success in early life stages but are deleterious once progeny is independent, they will be selected for
- Genetic programs drive ageing

Question 4

what are the 3 senescence factors?

Answer 4

1. metabolism
2. reactive oxygen species (ROS)
3. DNA damage

Question 5

what 3 factors increase the lifespan of an organism?

Answer 5

1. dietary restriction
2. environmental stresses
3. signals from the somatic gonad

Question 6

what is the rate of living theory?

Answer 6

if an organism has a high metabolism, it has a short lifespan
- e.g. mice have fast metabolism and short life, elephants have slow metabolism and long life
- link between rate of metabolism and age - this is especially clear in cold-blooded animals like drosophila where low temperatures slow metabolism and increase life span

Question 7

what evidence is there for the rate of living theory?

Answer 7

Cold-blooded animals like Drosophila can live longer at low temperatures as it slows their metabolism
- If you compare lifespan in drosophila at 29 and 18C, they live at least 3 times as long at low temperature, where metabolism is slowed

Question 8

what is the ROS theory of ageing?

Answer 8

high metabolism produces ROS and oxidative damage:
- The superoxide radical is central in the ROS theory of ageing
- Superoxide is an oxygen molecule that contains an extra unpaired electron and makes the oxygen highly reactive and can therefore oxidise cellular components and leads to oxidative damage.

Question 9

how is the ROS theory of ageing limited?

Answer 9

It seems radicals may even be anti-aging
- In C. elegans, treatment of worm with compound that leads to the production of oxygen radicals like paraquat or juglone (known ROS generators), causes increased life span, the opposite of what would be expected.
- Importantly by using antioxidants in addition to the ROS producers it could be shown that the effect is really due to ROS rather than some unknown effect of these compounds
- Similarly it has been shown that glucose restriction in C. elegans can extend life span by inducing mitochondrial respiration and increasing oxidative stress.

Question 10

what evidence suggests that the ROS theory of ageing is true?

Answer 10

longevity genes (long life promoting genes) are inducing genes that increase resistance to ROS

Question 11

how does dose have an impact on the ROS theory of ageing?

Answer 11

very high doses of ROS forming chemicals will be ageing promoters, but low levels may be harmless and even beneficial

Question 12

how has DNA damage been implicated in ageing?

Answer 12

many genetic disorders which have accelerated ageing express mutations in DNA repair mechanisms:
- Werner syndrome: RecQ DNA helicase DNA repair/ replication/ chromosome segregation causes premature ageing
- Cockayne syndrome: group 8 excision-repair cross-complementing protein=> DNA repair
- Hutchinson Guilford: LAMINA mutations=> nuclear architecture and mitosis altered

Question 13

what is the DNA damage theory of ageing?

Answer 13

Ageing may be caused by defects in genome maintenance and DNA repair
- In non-replicating cells unrepaired DNA damage may accumulate and cause aging (in dividing cells it will lead to mutations)

Question 14

how is it certain that mutation doesn’t cause ageing, and instead it is defect DNA repair?

Answer 14

Mice with a high mutation rate do not age quicker
- Perhaps NAD depletion? via excessive PARP activation?
- Therefore damage, not mutation, must be the cause

Question 15

what is Poly (ADP-ribose) polymerase (PARP)?

Answer 15

Poly (ADP-ribose) polymerase (PARP) is an enzyme that responds to damaged DNA, and uses NAD as a cofactor to do this
- PARP creates senescent cells

Question 16

how are senescent cells induced?

Answer 16

Unrepaired DNA in a non-dividing cell can cause a cell to become senescent

Question 17

how may senescent cells drive ageing?

Answer 17

1. Senescent cells may get “in the way” of normal stem cell-based repair
2. senescent cells may promote an ageing phenotype by secreting proinflammatory mediators called SASPs (senescence-associated secretory phenotype)
   - they therefore cause inflammation and block normal cell replacement, leading to ageing

Question 18

what evidence is there that senescent cells drive ageing?

Answer 18

• A mouse was genetically engineered where senescent cells could be killed/removed specifically.
• The wildtype mouse is clearly aged, whereas the engineered mouse that had its senescent cells removed looks much better

Question 19

how is dietary restriction thought to increase lifespan?

Answer 19

• The life extending effects of caloric restriction were already discovered in the 1930s.
• This has now been confirmed in all model organisms, it was also shown that it is not simply due to a reduced metabolism as a result of the reduce intake
• as calorie intake increases, lifespan decreases

Question 20

how are environmental stresses such as ROS thought to increase lifespan?

Answer 20

Normal ROS levels not an issue, as ROS alerts a cell to a problem->allows activation of counter-measures: “Fire-alarm”
- High level of antioxidants causes ROS to be suppressed: “Fire-alarm OFF”

A certain amount of ROS is optimal to promote protection
- ROS may not be dangerous unless they go over a certain level
- Hormesis: low level insults may activate protective mechanisms

Question 21

how have forward genetics been used in model organisms to understand ageing?

Answer 21

Found mutants that affect aging, clone the gene, see what is does:
- Screen for short lived mutants - tricky….is it aging?
- Screen for long lived mutants
- Developmental models C. elegans, Drosophila have led the way - short life span/isogenic strains/cheap/genomics
- Found mutants that live longer than usual
- Genetic analysis has identified mutations that affect life span and this has provided a molecular handle on the crucial genes

Question 22

what 3 genetic pathways were identified to be implicated in ageing?

Answer 22

1. IGF (insulin-like growth factor) pathway – negative regulators of age
   - Block to prevent ageing
2. TOR pathway – negative regulators of age
   - Block to prevent ageing
3. Sirtuins – positive regulators of age
   - Activate to prevent ageing

Question 23

what is IGF (insulin-like growth factor) signalling?

Answer 23

• IGF signalling via DAF2 (IGF receptor) inhibits DAF16 (FOXO)

Question 24

how was IGF signalling discovered?

Answer 24

IGF signalling discovered C. elegans:
- C. elegans can dauer state to survive adverse conditions such as low food
- Normal life: 25 days, dauer can last 60 days, under insulin/IGF1 control
- A particular combination of age1 alleles can extend lifespan by 10-fold
- If IGF signalling is lost, they enter dauer state and live longer

Question 25

what is FOXO?

Answer 25

FOXO (DAF16) is a transcription factor which regulates genes that increase resistance to stressors and promote longevity

Question 26

how is IGF signalling implicated in ageing?

Answer 26

- Mutations in Insulin/IGF1 pathway double lifespan (flies are in reproductive diapause) linked to resistance to oxidative stress - Female mice with a mutation in IGF1R (IGF1&2 receptor) live 33% longer If IGF is mutated, FOXO is no longer inhibited, resulting in anti-ageing

Question 27

what is the result of IGF signalling vs IGF mutation?

Answer 27

IGF signalling shortens life span, as it inhibits FOXO - IGF signalling promotes ageing mutations in IGF promote anti-ageing factors, as FOXO is no longer inhibited

Question 28

what human data supports that FOXO is anti-ageing and IGF signalling is ageing?

Answer 28

- GWAS data identified FOXO1 and FOXO3A, AKT and IGF1 receptor variants that have been linked in Human longevity in multiple cohort studies - Lifespan extension by dietary restriction is mostly (mice) or partly mediated by blocking IGF signalling (Worms Fly) - In mice, growth hormone receptor mutant mice (downregulating IGF), dietary restriction does not extend life

Question 29

what is downstream of FOXO/DAF16?

Answer 29

1. FOXO downregulates Insulin-like genes=> nonautonomous effects - FOXO is an anti-ageing transcription factor 2. Antioxidant genes such as superoxide dismutase, metallothionine, catalase, glutathione S-transferase => ROS theory FOXO leads to increased resistance against ROS 3. Metabolic genes: apolipoprotein genes (downregulated), glyoxylate-cycle genes, and genes involved in amino acid turnover 4. Chaperones: particularly small heat-shock protein genes 5. Antibacterial genes: Blockage of autophagy limits lifespan of long-lived DAF2/IGFR mutants - Autophagy extends lifespan

Question 30

what is TOR kinase?

Answer 30

The TOR kinase is a major amino-acid and nutrient sensor that stimulates growth and blocks salvage pathways such as autophagy when food is plentiful

Question 31

what is the TOR signalling loop?

Answer 31

salvage: - Stress activates TSC, TSC blocks TOR - This then blocks S6K1 and activates 4E-BP => “salvage” pathway active to inhibit protein synthesis and cell growth, inhibit ageing Wasteful: - Growth Factors and amino acids block TSCs which normally block TOR - TOR kinase now active, 4E-BP inactive, S6K active=> “wasteful” pathways active to promote cell growth, protein synthesis but also promoting aging

Question 32

how do IGF and TOR interact? how does this affect ageing?

Answer 32

- Insulin is a growth factor that “talks” to TOR via PI3K & Akt, thereby inactivating TSC1/TSC2 and activating TOR - So blocking insulin signalling could inactivate TOR and cause “longevity pathways” to be activated - blocking IGF and TOR is therefore anti-ageing

Question 33

what is rapamcyin?

Answer 33

blocker of TOR which extends lifespan - upregulates FoxA and 4E-BP to increase longevity pathway - downregulates S6K to decrease ageing pathway rapamycin given to an ageing mouse provides longevity and reduces its rate of ageing

Question 34

what are sirtuins?

Answer 34

Sirtuins are NAD+-dependent protein deacetylases overexpression of Sirtuins extends lifespan in yeast, worms and flies

Question 35

how are sirtuins implicated in ageing?

Answer 35

overexpression of Sirtuins extends lifespan in yeast, worms and flies: - In C. elegans, Sir-2.1 activates FOXO/Daf16, and mitochondrial unfolded protein response (UPR-mt) - UPR degrades unfolded proteins to reduce accumulation of damage in the cell - Sirtuins upregulate this so are anti-ageing - Sir proteins act as Insulin-independent activators of Daf16/FOXO - Sir has direct interaction with FOXO - anti-ageing - In mammals: many isoforms, SIRT6 has been linked to longevity in male mice, as it acts to downregulate IGF Sir promotes FOXO and inhibits IGF, so Sir is anti-ageing adam * Downregulates IGF *Upregulates FOXO *Upregulates UPR

Question 36

what is resveratol?

Answer 36

Resveratrol, the proposed anti-ageing compound in red wine (and even more in Japanese knotweed) activates Sir proteins - promotes Sirtuins to increase longevity

Question 37

how are mitochondrial mutations thought to increase lifespan?

Answer 37

- Sirtuins (which require NAD) lead to activation FOXO and in addition to a protective mitochondrial response called the mitochondrial unfolded protein (UPR) response - This fits with the fact that certain mitochondrial mutations have also been found to extend lifespan - similar to activation of SIRT- mitochondrial mutations will lead to the mitochondrial unfolded protein response (UPR-mt) which by an unknown mechanism protects against ageing and increases lifespan

Question 38

what is the NAD-depletion theory of ageing?

Answer 38

- NAD is an essential cofactor for Sirtuins - NAD levels decline with age - Unrepaired DNA damage might lead to NAD depletion via poly ADP ribose polymerase (PARP) activation - DNA damage results in NAD being mopped up for repair

Question 39

how do signals from the somatic gonad increase lifespan?

Answer 39

- In C. elegans, removal of entire reproductive system does not extend lifespan, but when only germ cells are removed animals live 60% longer - Somatic gonads extend lifespan, their effects counteracted by germline cells - It can extend the lifespan of IGFR/DAF2 mutants even further - Steroidal hormones (dafachronic acid) and DAF12 Nuclear hormone receptor are involved in this, they appear to act on DAF16/FOXO but in a different manner than insulin