L5, Mitochondria and Telomeres in aging Flashcards
1
Q
Mitochondrial dysfunction with age:
Enzyme example in mice
A
- Rate of mtDNA mutation is 100-1000x higher than nucleus
- Mitochondrial function declines with age
- Lots of conflict opinions about mitochondrial involvement and FRT in field
- Lon protease declines with age in sedentary mice
- Lon protease = mt matrix protein which degrades misshapen proteins
- However, there is not a vicious cycle in place (mutation cycle)
2
Q
Mice engineered with proof-reading deficient PolgA:
A
- 2007 study in mice
- PolgA= catalytic subunit of mtDNA polymerase
- Expecting more mtDNA mutations to arise -> increased rate of aging
- Results showed increased pathologies, increased aging side effects (greying, hearing loss, cardiomyopathy etc)
- Shorter lifespan
- Effect was prevented by endurance exercise -> known to stimulate mitochondrial biogenesis
- Further study: mitochondrial point mutations found not to limit natural lifespan of mice, but deletions do
3
Q
Quality control of mitochondrial proteostasis (3 levels):
Grouped by severity of damage
A
- Minor damage: Managed at molecular level by chaperones and proteases -> protein folding and turnover, e.g. reduced Lon protease
- IM damage I (Organellar QC): mitochondrial fusion causes content mixing, SIMH (e.g. stress-induced mt hyperfusion)
- IM damage II (Organellar QC): Mitochondrial fission -> segregation and mitophagy (cf. Lysosomes, lipofuscin)
- Major damage: Managed on cellular level -> Apoptosis (MOMP, cytochrome c release, cellular turnover)
4
Q
Hormesis:
A
- Dose-response phenomenon
- Low dose: stimulation, high dose: inhibition (or vice versa)
- First observed in tracheal cilia under different doses of strong oxidants
5
Q
Mitochondrial hormesis under mild stress:
A
- Mild mitochondrial stress promoting cellular health
- Stress signalling (retrograde response / ETC disruption, exercise or DR induced metabolic stress) -> upregulation of stress coping mechanisms in nucleus -> promotes gene stability, energy mobilisation, autophagy and mitophagy upregulation, TOR inhibition
- Process appears to be inhibited by antioxidants
6
Q
Signalling in rapidly growing cells (AP in mitochondria):
(In mitochondria)
A
- Rapidly growing cells limited by ATP production -> actively inhibiting mitophagy to maximise mitochondrial ATP production
- This process maximises early growth and reproduction, but permits the persistence of damaged mitochondria
- In this case, mitophagy is inhibited via ROS-dependent activation of insulin signalling
7
Q
Role of telomeres in lifespan of a chromosome:
A
- Protect against uneven chromosome segregation and cancer
- Chromosomes lose telomeric DNA with each division -> arrested when sufficiently eroded -> Hayflick limit
- Telomerase activated in many cancers to evade this (85-90% of malignant biopsies are telomerase positive)
8
Q
Stem cell depletion argument:
A
- Debunked
- Idea that telomere erosion causes stem cells to senesce -> no rejuvenation
9
Q
Evidence against stem cell depletion argument:
A
- Mice have constitutively active telomerase (and thus long telomeres)
- Humans have repressed telomerase
- Mice don’t get stem cell depletion yet they have hugely shorter lifespans
- -> telomere shortening and replicative senescence unlikely cause of aging
10
Q
Purported role of replicative senescence in cancer suppression:
Links to evolutionary theories
A
- Theory that the evolution of homeotherms with increased metabolic rate and thus DNA damage/telomere erosion due to ROS production -> cancer
- Selective pressure to upregulate replicative senescence to root out more frequent damaged cells
11
Q
Mammalian ancestors vs aquatic poikilotherms and telomeres:
A
- Mammalian ancestors had human type with short telomeres and repressed telomerase
- Aquatic poikilotherms had short telomeres but expressed telomerase
- -> telomerase repression likely evolved to protect against tumours
12
Q
Telomeres in humans:
A
- Short telomeres associated with CVD (not causative)
- Short telomeres generally speaking, with repressed telomerase
- No correlation/causative link found between telomere length and frailty or CVD (West of Scotland Coronary Prevention Study)
13
Q
Telomerase gene therapy in mice:
A
- 2021 paper investigating intranasal and injectable gene therapy route
- Used cytomegalovirus virus to transfect mice with TERT
- Lifespan extended considerably (max lifespan 29 mths vs 40)
14
Q
Cell senescence: How is it induced, hallmarks?
A
- Stress or damage induced
- Irreversible, non-dividing state
- Cell still able to metabolise and express genes
- Larger with increased lysosomal mass
- Express p16INIK4a (cyclin-dependent kinase inhibitor; tumor suppression)
- May be labelled using SA-beta-gal
15
Q
Key initiators of cell senescence:
A
- Telomere erosion
- Oncogene overexpression
- ROS-mediated DNA damage
- Mitochondrial dysfunction
- Inflammation
- -> DDR -> p53 activated -> G1 arrest
16
Q
SASP:
A
- Senescence-associated secretory phenotype -> noxious cell microenvironment
- Varies between cell types but proinflammatory cytokines most common, e.g. GM-CSF (link to B301 L9)
- Various effectors, including EMT, angiogenesis (via VEGF), MMPs, cell proliferation (via GROs)
- Also: chemotherapy resistance and stem cell renewal/differentiation
- Excellent example of hormesis and AP; detrimental effect is late onset and chronic
- Associated with various inflammation-related pathologies (atherosclerosis, liver cirrhosis, osteoarthtritis etc)
17
Q
Treating senescence in humans and mice:
A
- Mouse study: killing senescent cells improved health and lifespan, used a p16INK4a targeting transgene
- Human clinical trial: Senolytic agents (dasatinib and quercetin) -> removing senescent cells, diabetic theoretical kidney disease treatment
18
Q
Parabiosis in mice: (Findings)
A
- Milieu, not stem cells, limits muscle regeneration in old mice
- Isochronic young: good tissue regeneration
- Heterochronic: ‘ ‘
- Isochronic old: impaired regeneration
19
Q
Application of membrane pacemaker theory to mitochondria:
A
- Likelihood of mt membrane lipid peroxidation correlates inversely with lifespan
- ROS nearly all produced in mitochondria
20
Q
Mouse type telomeres: What are they and why did they likely evolve?
A
- Constitutively active telomerase and long telomeres
- Possibly trade-off; evolving resistance to oxidative damage instead of foregoing replicative capacity -> longer telomeres to cope with erosion
21
Q
Correlations across species: Telomeres, oxidants and lifespan
A
- Telomerase expression correlates negatively with mass
- Telomere length correlates negatively with lifespan
- Species with a high extrinsic mortality may have reverted to long telomeres to limit somatic maintenance costs and boost cell stemness, to cope with injuries
22
Q
Why may cell senescence have evolved, AP effect later in life:
A
- Beneficial short term efects; stop damaged cells from dividing -> prevents cancer
- Evidence for positive role in tissue repair (resolving fibrosis)
- However, detrimental chronic effect; noxious cell microenvironment, disruption of tissue integrity, promoting chronic inflammation -> cancer promotion
23
Q
List 7 diseases associated with cell senescence:
A
- Glaucoma
- Atherosclerosis
- Liver cirrhosis
- Glomerulosclerosis
- Type 2 diabetes
- Sarcopenia
- Osteoarthritis
24
Q
Human blood biomarkers of mortality risk: (x4)
A
- Orosomucoid
- Albumin
- VLDL size
- Citrate
25
Q
+ Skeletal muscle studies in humans: Mitochondrial aging
A
- Decline in activity of mitochondrial enzymes (e.g. citrate synthase)
- Decrease in respiratory capacity per mitochondria
- Increased ROS production
- Reduced phosphocreatine recovery time (in vivo measurement of mitochondrial respiratory capacity)
26
Q
+ How is mitochondrial biogenesis signalled?
A
- Endurance exercise -> mitochondrial biogenesis
- Largely coordinated by PCG-1a
- This regulates the activity of several TFs including NRF1 and NRF2, TFAM
- Increasing PGC-1a levels in mice was sufficient to stall age-related sarcopenia
27
Q
+ Mitochondrial UPR and longevity
A
- Stress response pathway; initially observed in conditions of mit genome depletion or accumulation of misfolded proteins in mitochondria
- Involves transcriptional changes affecting vast range of mitochondrial processes (chaperone proteins, ROS defenses, metabolism, regulation of iron sulfur cluster assembly, modulation of innate immune response)
- Largely studied in nematode worms -> long-lived mitochondrial mutants appear to require activation of UPR^mt (not observed in other mutants e.g. IIS mutants)
