Senescence

Question 1

When can senescence occur?

Answer 1

G1, G2

Question 2

How does quiescence differ to senescence?

Answer 2

Quiescence is growth arrest in G0 due to starvation of growth factors or contact inhibition. It is reversible, unlike senescence.

Question 3

What is antagonistic pleitrophy of senescence?

Answer 3

It is a beneficial, tumour suppressing function in early life. But it becomes procancerous in the elderly

Question 4

What is the role of senescence in wound healing?

Answer 4

senescent cells will secrete PDGF-AA which induces myofibroblast differentiation leading to accelerated wound closure

Question 5

What effect does removal of p16 positive senescent cells have on wound healing?

Answer 5

It delays the time for wound closure

Question 6

What are SASP proteins?

Answer 6

senescence associated secretory phenotype proteins. These are pro-inflammatory cytokines, chemokines, growth factors and proteases

Question 7

What is the effect of SASP protein autocrine Vs paracrine signalling?

Answer 7

Autocrine acts to reinforce senescence.

Paracrine has an effect on the microenvironment and neighbouring cells.

Question 8

What SASP protein is secreted by senescent cells that aids in invasion and metastasis?

Answer 8

Matrix metalloproteases

Also VEGF

Question 9

What is a good way to mark senescent cells in mice? How do these mice respond if these cells are lost?

Answer 9

p16INK4a positive cells

clearance of these cells delays ageing associated disorders

Question 10

What procancerous traits do SASP proteins have

Answer 10

Degradation of the ECM facilitates invasion, induces EMT, angiogenic factors promote neo-angiogenesis.
SASP establishes immunosuppressive environment that promotes tumour growth.

Question 11

Which cancer is associated with SASP proteins?

Answer 11

Hepatocellular carcinoma

Question 12

What is SAB?

Answer 12

a marker for senescence: senescence associated B-galactosidase. At a pH of 6, it will turn the cells blue if they express B-galactosidase.

Question 13

Is senescence a dominant or recessive trait?

Answer 13

Dominant

Question 14

What is the effect of injecting polyA+ RNA from senescent human fibroblasts into proliferating cells? What does this suggest?

Answer 14

It inhibits the synthesis of DNA, the cell becomes quiescent suggesting that senescence is a dominant cellular trait./

Question 15

What cell cycle inhibitor is required for the induction of senescence?

Answer 15

p21CIP1

Question 16

What is shelterin?

Answer 16

a complex of six telomere specific proteins. They form a lariet like structure which tucks the ends of the telomeres.

Question 17

What is the result of losing TTAGGG telomere repeat sequences?

Answer 17

Results in the loss of shelterin complex’s and activation of DNA damage response signal

Question 18

Why do telomeres shorten over time?

Answer 18

The lagging strand requires primers before it can undergo 5’ to 3’ DNA synthesis. So at the end of the telomere on the lagging strand there will be an RNA primer. When this is removed there is no 3’ OH group to allow for the synthesis of DNA. So a small portion of the telomere is lost.

Question 19

What are the two critical componenets of telomerase and what role do they have?

Answer 19

hTERT is the catalytic subunit - acts as a reverse transcriptase
hTR acts as a template RNA

Question 20

how does telomerase maintain the length of telomeres

Answer 20

Telomerase recognises tip of telomeres. RNA template (hTR) within the enzyme used to elongate the parental strand in the 5’ to 3’ direction (requires hTERT reverse transcription). It adds additional repeats which is recognised by DNA pol alpha which creates an RNA primer for the lagging strand. DNA pol alpha can then fill the gap using the 3’OH group to begin DNA synthesis.

Question 21

Do somatic cells have high or low telomerase activity?

Answer 21

Low/no telomerase activity. They lack hTERT, hTR is constitutive.

Question 22

How do cancer cells maintain telomeres?

Answer 22

Reactivation of telomerase or ALT (recombinational mechanism)

Question 23

How does a cell respond to shortening of telomeres?

Answer 23

Recognised as DNA damage which leads to the activation of ATM/ATR (p53 dependent damage response)

Question 24

What is the result of introducing oncogenic Ras into fibroblast cell culture?

Answer 24

Induces cellular senescence (oncogene induced senescence)

Question 25

What are some triggers of senescence?

Answer 25

Oncogenic signalling, ionizing radiation, oxidative stress, genotoxic stress, lack of nutrients,

Question 26

What are the two major pathways that induce senescence?

Answer 26

1) activation of the p53 pathway which induces p21CIP1. This prevents cyclin/cdk complexes from phosphorylating pRB which activates the RB pathway.
2) RB activation independently of p53. Upregulation of p16INK4A inhibits cyclinD/cdk4/6 complex’s that would normally phosphorylate RB and inactivate it.

Question 27

What is geroconversion?

Answer 27

The conversion of a reversible cell cycle arrest into cellular senescence. It requires mTOR activity in the context of activated p53. Sustained mTOR activity in the presence of active p53 results in geroconversion.