L13 : C. elegans Signalling Pathways that Control Ageing Flashcards
What is ageing?
Progressive degenerative process that results in declined physiological function and cellular integrity
Occurs in most but not all animal species
What are two animals with exception to ageing?
Hydra vulgaris
Seas anemones
What are some diseases commonly associated with ageing?
- Cancer
- Cardiovascular disease
- Macular degeneration (affecting eyesight)
- Neurodegeneration
- Type II diabetes
- Urinary incontinence
What is C. elegans?
- Caenorhabditis elegans
- Microbivorous terrestrial nematode (roundworm)
- ~1.2 mm long
Sexes:
- Hermaphrodites (XX) can self fertilise
- Males (XO) rarer, required for cross fertilisation
Why is C. elegans a convenient model for studying ageing?
- Short life span (2-3 weeks)
- Exhibits clear signs of ageing
- Has fully sequenced genome (~97 mil bases, ~20k genes)
- Simple anatomy and well-characterised genetics
What signs of ageing are observed in C. elegans?
- Reduced fertility, feeding, movement
- Increased molecular damage
- Deterioration of organs like intestine or gonad
What determines maximum lifespan of species? Examples?
- Humans = ~110 yrs
- Chimpanzee = ~60 yrs
- Common ancestor only ~5-6 mil yrs ago
- Drosophila = ~3 mths
- Ant queens = ~28 yrs
Longevity is a genetically determined trait
How can a classic genetics approach be used to study ageing?
- Isolate mutants with altered ageing rates
- Map, clone, and sequence genes involved
- Identify proteins and biochemical pathways that influence lifespan
- Use this to understand mechanisms of ageing
Are long or short lived mutants more informative in ageing research?
Long lived
Short lived may be sick or have other health issues unrelated to ageing
What are some key discovered related to long-lived C. e mutants?
1983:
Micheal Klass isolated first long-lived mutants using mutagenesis (ethyl methane sulphonate)
1988:
- Identified age-1 (hx546) mutation
- Increasing mean lifespan by 65% and maintaining normal development/ movement
1993:
- Discovered daf-2 mutants
- Double life span and affect dauer larva formation
2007:
- age-1 null mutants showed 10-fold increase in mean/max lifespan
What are dauer larvae in C. e and why do they form?
Developmentally arrested, alternative third-stage larvae
Highly stress resistant and can survive up to 70 days
Form in response to:
- High population density
- Scarce food
- High temperatures
What are key characteristics of dauer larvae?
- Non feeding
- Buccal cavity is sealed
- Survuve on stored food granules - Reduced movement and metabolic activity
3 Considered non-ageinga
- Length of time spent in dauer stage does not affect post-dauer adult lifespan
What are the types of daf mutations?
Daf (dauer abnormal):
Mutants have abnormal dauer formation
Daf-c (dauer constitutive):
Mutants form dauers even in non-dauer inducing conditions
Daf-d (dauer defective):
Mutants cannot form dauers at all
Briefly describe gene regulation of ageing and dauer formation?
daf genes form a complex, branched signalling pathway
Roles include regulating both dauer formation and lifespan
daf-c genes: daf-2 and age-1
daf-d gene: daf-16
How does daf-2 and age-1 contribute in the daf signalling pathway?
daf-2:
- Most mutations are temperature sensitive
- Daf-c at non-permissive
- Development to long-lived adults at permissive
age-1:
- All age-1 mutants are long-lived
- Only severe age-1 alleles are Daf-c
How does daf-16 contribute in the daf signalling pathway?
- daf-16 and daf-16/daf-2 mutants not long lived
- daf-16 (-) suppresses daf-2 life extension
- daf-16 promotes longevity
What is the hypothesis regarding daf-2 and adult longevity in C.e?
Loss of function in daf-2 thought to activate the dauer longevity programme in adults, leading to increased lifespan
Two distinct longevity programmes in C.e?
Dauers
Normal adults
EXPAND
Are daf genes in C.e relevant in humans? Examples?
daf genes have human homologues that function in similar signalling pathways
age-1: encodes catalytic subunit of phosphatidylinositol 3-kinase (PI3K
daf-2: homologous to the insulin/IGF-1 receptor
daf-16: homologous to FOXO-class forkhead TFs
These 3 homologues interact in humans, suggesting conserved mechanisms for regulating metabolism, stress resistance, and longevity
How does the daf-2 pathway regulate daf-16?
- 40 insulin-like ligands resulting in activation of DAF-2 (insulin/IGF receptor)
- Protein kinase signalling cascade
- Ser/Thr kinases attach phosphate groups to DAF-16
- Inactivates DAF-16 through cytoplasmic retention
- Prevents access to nucleus, so unable to activate longevity genes
Insulin/IGF-1 signalling inhibits longevity by inactivating DAF-16
How can the effect of insulin/IGF-1 on DAF-16 be visualised in C.e?
- Tagging DAF-16 with GFP
- Optical transparency of C.e allows researchers to observe whether DAF-16 is
- Direct visualisation of effect on localisation and activity
- Cytoplasm (inactive)
- Nucleus (active)
Where is the DAF-2 pathway acting to promote longevity? Experiment ?
Took daf-16/daf-2 double mutant and expressed WT daf-16 in single tissues
- Nervous system and muscle: showed little increase in lifespan
- Intestine showed large increase
Note: worm intestine combines roles of intestine, liver, adipose tissue
Is insulin/IGF-1 signalling a conserved regulator of ageing across species?
Evolutionary conserved pathway that regulates lifespan in multiple organisms
Considered a public determinant of ageing (operates broadly across species)
What do studies in dwarf humans suggest about ageing?
Study of 99 Ecuadorian individuals with growth hormone receptor deficiency (Laron syndrome)
- Not longer-lived
- Showed reduced risk of age-related diseases (eg. cancer, TII diabetes)
