L8, Modulatory mechanisms

Question 1

Loosely describe how a mechanism might extend lifespan:

Answer 1

A. Reducing activity of processes that impair health and function with age
B. Increasing activities that protect against aging

Question 2

How are modulatory mechanisms for aging identified? (x3)

Answer 2

• Gene expression profiling by microarray analysis of long-lived mutant or DR treated worms, flies and mice
• Determining downstream components or processes controlled by the IIS/TOR network important for aging
• Experimental testing of candidate genes and mechanisms in model organisms

Question 3

Evolutionarily conserved modulatory mechanisms for extended lifespan (x6):

Answer 3

• Reduced oxidative damage to macromolecules -> FRTA
• Cellular detoxification
• Activation of autophagy
• Reduced protein translation
• Protection of proteins by chaperones
• Mitochondrial dynamics

Question 4

Summarise FRTA:

Answer 4

• Molecular damage contributes to ageing in most organisms and in various ways; however, doesn’t normally determine life/healthspan
• Antioxidant defence may contribute
• Many damage pathways present; not necessarily all relevant
• Additional forms of molecular damage may contribute to aging

Question 5

How was cellular detoxification implicated as a modulatory mechanism in aging?

Answer 5

• Microarray analysis of long-lived daf-2 mutants -> upregulated genes for cellular detox.
• Cytochrome P450s (CYPs)
• Glutathione S-transferases (GSTs)
• Short chain dehydrogenases/reductases (SDRs)
• UDP-glucoronosyltransferases (UGTs)
• -> These 4 enzyme classes act to dispose of toxic endobiotic or xenobiotic compounds

Question 6

Broad spectrum detoxification theory of aging: (Green theory)

Answer 6

• The cell is under constant threat from metabolic waste products and xenobiotics
• Suggesting smooth ER works a a cellular filter, deploying phase I and II metabolism to mobilise and excrete these mainly lipophilic toxins
• -> Clearing cell of molecular rubbish, preventing molecular damage and aging

Question 7

Evaluating evidence for Green (detox) theory of aging:

Answer 7

• Experimentally increasing expression of xenobiotic detoxification genes using drugs or transgenes
• e.g. Flies selected for resistant to xenobiotic DDT or overexpression transgene encoding cytochrome P450
• -> Although both interventions increased DDT resistance, neither increased lifespan

Question 8

What is autophagy:

Answer 8

• Present in all eukaryotic cells; evolutionarily conserved
• Ubiquitous catabolic process; bulk degradation of cytoplasmic components through lysosomal pathway
• -> engulfment of part of cytoplasm inside double membrane vesicles (autophagosomes)
• Autophagosomes then fuse with lysosomes -> degrade cytoplasmic cargo, recycling of products
• Turnover of most long-lived proteins, macromolecules, biological membranes and whole organelles is mediated in this way

Question 9

How is autophagy involved in aging: (+ Positive mechanisms)

Answer 9

• Elevated levels of autophagy considered beneficial for prevention of aging -> increased rates of removal of damaged molecules and organelles
• Reduced TOR activity elevates autophagy
• Lifespan extension due to DR and reduced IIS in worms requires activation of autophagy, as well as by rapamycin in flies

Question 10

Negative mechanisms for autophagy and aging:

Answer 10

• Downregulation of autophagy in worms shortens the lifespan of long-lived daf-2 mutants
• Downregulation of autophagy blocks rapamycin-mediated lifespan extension

Question 11

Points against autophagy for mediating lifespan:

Answer 11

• In worms, daf-16 mutation blocks daf-2 longevity but does not reduce autophagy levels
• Reduced expression of Atg5 does not shorten lifespan of flies
• Excessive autophagy both in flies and worms reduces lifespan

Question 12

Sexual dimorphism in autophagy of C.elegans:

Answer 12

• Autophagy promotes aging in a gender-specific manner in C.elegans
• In hemaphrodites, reproductive programme promotes visceral aging; IIS -> high autophagy, yolk production -> old
• In males, moderate autophagy

Question 13

Overview and evidence for protein translation role in aging:

Answer 13

• Reduced protein translation can also have an antiaging effect, but the mechanism is unclear
• Control of translation is downstream of TOR signalling pathway
• Mutations in genes encoding ribosomal proteins, S6K, or translational initiation factors can all extend lifespan in worms and flies
• Rapamycin represses translational via targets of TORC1

Question 14

Potential mechanisms for protein translation involvement in aging:

Answer 14

• Reduced global protein synthesis may result in
• Fewer but better quality proteins via reduced burden on the protein repair and degradation machinery
• Preferential translation of a subset of proteins involved in lifespan extension

Question 15

DR effect on mitochondrial biogenesis:

See FC

Answer 15

• DR increases the activity of factors crucial for mitochondrial biogenesis and respiration
• -> SIRT1 stimulates PGC1a -> increased transcription of genes for mitochondrial biogenesis and respiration
• -> 4E-BP-> upregulation of translation of genes encoding respiratory components in Drosophila
• -> SKN-1/NRF2 which enhances mitochondria respiration in C.elegans

Question 16

Effect of TOR inhibition on mitochondria:

Answer 16

• Inhibition of TOR selectively upregulates translation of mitochondrial components and reduces general translation via regulation of 4E-BP
• Positively regulating
• Not yet fully understood