Cellular Senescence

Question 1

Describe the 2 theories of ageing:

1) Stochastic
2) Programmed

And their issues

Answer 1

Stochastic:
- things you do and things you are exposed to result in altered cell DNA and cell aging…“wear and tear”
- chance error and damage over time –> cellular changes –> aging
- e.g. cell DNA damage, mitochondrial damage, ROS
ISSUES: by chance you’d think we’d see some of these changes early on in some and later in others (there is some variability, but there is a great deal of consistency).
Programmed:
- naturally, over time, cells age and shut down. Nothing you can do about it
- built-in internal clock that programs longevity

Question 2

Cellular Senescence or Replicative Senescence:

• describe experiment
• describe senescence and Hayflick limit

Answer 2

• normal human somatic cells, culture in vitro. Only a finite number of cells divided before they stopped replicating (before this, cell cultures came from cancer lines that kept replicating)
• Limit (Hayflick Limit) of only about 50 or 60 replications before they stopped and were in a terminally non-dividing state (still alive but not replicating). This state is called senescence

Question 3

Telomeres

• what are they?
• why are they important? (2 reasons)
• what happens when DNA replicates?

Answer 3

• short repeat DNA sequences at the end of chromosomes (kind of like aglet on shoelaces). Non-coding, junk DNA.
• they protect the ends of chromosomes: when DNA replicates, ends of chromosomes are vulnerable; also, prevents chromosomes from fusing
• DNA replication: telomeres are shortened with every replication. DNA polymerase cannot completely replicate the 3’ end of the DNA, so 5’ of new DNA is shortened (so there are progressively shorter DNA polymerases every replicative cycle)

Question 4

Consequences of telomere shortening?

- name type of damage response

Answer 4

• incomplete replication of genes adjacent to telomeres
• lack of “capping” - frayed ends of DNA - which looks broken
• potential activation of DNA damage response: non-homologous end-joining pathway: links one broken end to another broken end; ends up long chromosome with 2 centromeres - difficult to replicate and divide –> break further, results in mitotic catastrophe
• Solution: cell puts itself in a state of senescence (p53)

Question 5

Telomerase

• why do we need (for which cells)?
• what does it do?
• what is it?
• is it regulated? why?
• telomerase activity in different cell types?

Answer 5

• there are certain cells that cannot undergo senescence because they are needed
• germ cells; stem cells
• telomerase maintains the length of telomeres
• telomerase is a reverse transcriptase enzyme that adds DNA on the ends of chromosomes
• highly regulated to prevent unnecessary telomere elongation
• telomerase activity varies depending on cell type (it is in stem cells and germ cells, but undetectable in somatic cells)

Question 6

Cancer

• Explain what it cancer does to telomeres and why
• Explain the link between cancer and telomerase
• theory?

Answer 6

Explain: some cancers replicate very quickly, resulting in rapid shortening of telomeres
Link: if cancer cells could only replicate 50 times, they would die out and not take over the body. So…cancer cells reactivate telomerase (reduced telomerase shortening, prevention of cellular senescence, prevention of mitotic catastrophe, results in limitless replicative potential)
Theory: Telomere shortening may be a defence against cancer

Question 7

Aging and cellular senescence experiments:

• experiments in children cells?
• Werner’s syndrome
• contradictory studies

Answer 7

• cells from children undergo more rounds of replication than adults (makes sense)
  Werner’s syndrome (accelerated aging)
• undergo fewer rounds of replication before senescence
  Contradictory studies:
  The connection between replicative senescence and aging is not really clear (RS cannot explain aging completely)
  –> how does in vitro (experimental) process work in vivo?
  –> Werner’s syndrome is not an exact model of aging