Aging and senescence Flashcards
Definition of aging?
- loss of homeostasis at the cellular level
- increased vulnerability of the organism (to disease)
Why does mortality rate increase with age?
Because vulnerability increases with age
Is aging a disease?
No, it is an increased vulnerability to disease.
How can selective pressure on early life events result in species-specific differences in longevity?
each species evolves based on its environmental niche. Lifespan is just a side-effect
Describe the common pathway involved in lifespan in yeast, worms, flies, and mammals.
mTOR signaling and insuiin/insulin-like pathways. Lifespan extensions are due to single gene mutations.
Because mTOR is a nutrient-sensing pathway effector, a restrictive diet can increase longevity.
Example of lifespan pathway that diverged from worms to mammals?
Worms have Daf-2 insulin-like growth factor receptor, and humans have IGF-1R and GHR
What issues contribute to aging-related vulnerability?
- hormonal changes
- loss of tissue-specific architecture
- decline in immune function and increased inflammation
all due to stem cell loss and cellular damage
Term for cellular mechanisms of aging?
senescence
Hayflick limit?
- normal fibroblasts arrest following a defined number of population doublings
- immortal lines like tumors do not have a limit
What events induce senescence?
- telomere attrition (recognized as DNA damage)
- oncogene conversion
- stress (O2)
Can human cells spontaneously immortalize? Mouse cells?
Human cells cannot spontaneously immortalize, but mouse cells can. Therefore, when culturing mouse cells it is important to choose the right density of cells to work with. At low density, mouse cells senesce. We normally choose middle density cells (3T3) because they maintain normalcy.
What is the effect of viral proteins on human cell immortalization and out of senescence?
adding viral protein induces crisis in cells, leading to apoptosis. Out of the same population, leftover cells will become immortal/tumorigenic.
- SV40T antigen: causes inhibitory binding to p53 and pRb, causes cell transformation
- papilloma virus E6 and E7 proteins: E6 inhibits p53, E7 inhibits pRB
- Li Fraumeni syndrome: heterozygous mutation of p53
Under normal conditions, what is the inhibitor of p53?
Mdm2 binds and inhibits unphosphorylated p53 and targets it to the proteosome, so that p53 is constantly turned over in the absence of DNA damage
Structure and modifications of p53?
tetramer – mutant p53 is dominant.
can be phosphorylated, acetylated, and ubiquitinated to regulate activity and localization
Effect of mono- and poly-ubiquitination on p53?
Monoubiquitination: promotes cytoplasmic localization
Polyubiquitination: promotes degradation
p53 modification that leads to senescence?
phosphorylation of serine 15. Activates p21, a CDKi which inhibits cyclin D and E complexes, leading to G1 arrest (senescence)
Effect when p53 modifications lead it to be upregulated too long?
p53 will cause transcriptional activation of PUMA. This causes apoptotic gene activation: BCL2, mitochondrial transition pore, cytochrome C release from mito, caspase 9 activation
Differences between apoptosis and senescence?
describe telomere attrition
-at 5’ end of lagging strand, DNA pol can’t fill in the 3’ overhang once the RNA primer is removed. This is eventually recognized as DNA damage.
Causes senescence:
- DNA damage response – p53 activation
- DNA damage response protein accumulation: p53BP1, H2AX foci
- telomere-induced damage foci (TIF)
What is T loop formation, and why is it necessary?
T loops are formed to hide the linear overhang so it is not recognized as damage
- TRF1 and TRF2 bind the telomere and allow the 5’ end to invade the 3’ end.
- the telomere cap structure also contains DNA repair proteins Ku70/80 and DNAPK (deletion of Ku in humans causes telomere fusion with other chromosomes)
How do telomeres differ between species?
mice: large, have telomerase activity in somatic cells
human: small, only telomerase in germ cells (present but inactive in somatic cells)
yeast: anchored to nuclear membrane
telomerase structure?
- TERT enzyme protein
- TLC1 template in vitro
What do southern blots of telomeres tell us about senescence?
That telomere length decreases with senescence.
How have we experimentally allowed unlimited growth in human somatic cells?
by introducing telomerase. We see the telomere smear length by southern blotting increases in size.
