14.1 Apoptosis II

Question 1

What regulates the release of cytochrome c from mitochonrdia?

Answer 1

members of the B-cell leukemia/lymphoma 2 (Bcl-2) protein family.

Question 2

What does Bcl stand for?

Answer 2

B-cell leukemia/lymphoma

Question 3

What are the Bcl-2 homology (BH) domains?

Answer 3

BH1
BH2
BH3
BH4
The BH domains are anti-apoptotic proteins and they crucial for function as deletion of these domains affects survival/apoptosis.

Question 4

Describe the difference between BH proteins that have 4 BH domains and BH proteins that have less than 4 domains

Answer 4

4 domains means they are anti-apoptotic (prevent apoptosis)

Less then 4 domains promote apoptosis

Question 5

What is the effect of pro-apoptotic Bcl-2 proteins on BH123?

Answer 5

The major antagonists of the BH123 signaling proteins are Bcl2 and Bcl-X, which multimerize
with BH123 proteins and thereby prevent their activity.

Question 6

What happens when Bak and Bax are activated

Answer 6

when these proteins are activated, they multimerize and form pores in the mitochondrial membrane, which allows cytochrome c to escape (as well as other proteins)

This triggers the apoptotic signaling pathway through the intrinsic mechanisms

Question 7

What is the difference between Bax and Bak?

Answer 7

Bax is found in the cytoplasm, so it has to translocate to the mitochondrial membrane when it is activated

Bak is always associated w/the mitochondrial membrane and it’s always present and ready to go.

Question 8

What down regulates the pro-apoptotic effects of the BH123 proteins (Bak and Bax)? Why is down regulation important

Answer 8

They are down regulated by the effects of the anti-apoptotic Bcl2 proteins, Bcl2 and Bcl-X.

Bcl2 and Bcl-X are present in order to prevent unintended activation of apoptosis, which could lead to cell death for random reasons. They can also be stimulated by survival factors that prevent programmed cell death that might otherwise occur.

Question 9

How do Bcl2 and Bcl-X act as antagonist to BH123 signaling proteins?

Answer 9

Bcl2 and Bcl-X multimerize w/BH123 proteins and prevent their activity

Bcl2 is associated w/the mitochondrial membrane in order to couteract any accidental apoptotic triggering by Bak.

Question 10

Why does the activity of Bcl2 have to be turned off at some point? What turns it off?

Answer 10

Since Bcl2 is always present at the mitochondrial membrane to prevent any unwanted apoptosis, the activity of Bcl2 has to be turned off when apoptosis is really needed.

A third group of Bcl2 family proteins called BH3 (Bid and Bad) turns off/inactivates Bcl2 at the mitochondrial membrane and then allow Bak to do its thing and promote apoptosis

Question 11

How many domains are in BH3 proteins and what are the two that turn off Bcl2?

Answer 11

Bid and Bad inactivate Bcl2 and they only have one of the BH domains.

Question 12

What are IAP’s? What is their role?

Answer 12

Inhibitors of apoptosis

• Can rescue cells from cell-death
• bind to and inhibit caspases (initiators and executioner)
• prevent accidental apoptosis caused by spontaneous activation of procaspases by inhibiting procaspase activation

Question 13

Why must each cell have at least 1 IAP or one kind of IAP in order to survive?

Answer 13

B/c if it didn’t when one of the apoptotic promoting factors gets started there would be nothing to prevent it from killing off the cell.

Question 14

what is the main structural feature shared by IAP’s

Answer 14

a domain called BIR (baculovirus IAP repeat)

Question 15

How do viruses use IAPs to propagate?

Answer 15

cells that are infected w/a virus will try to go through apoptosis as a way to prevent the spread of the virus in an organism. However, some viruses have developed counter strategies to apoptosis by host cell, such as causing cells to express IAPs

Question 16

In addition to binding and inactivating caspases, what else do IAPs do with caspases?

Answer 16

they might also target caspases for degradation through polyubiquitination

Question 17

How are the functions of IAPs inhibited when cells are supposed to go through cell death?

Answer 17

by inter-membrane mitochondrial proteins called Reaper, Grim, and Hid (in drosophila)

in mammals the protein is called Smac or Diablo

Question 18

What happens when reaper and hid are mutated or overexpressed?

Answer 18

they cause cell death in various tissues.

Question 19

List all anti-IAPs

Answer 19

Reaper
Grim
Hid
Smac/Diablo

Question 20

Gain of function of mutants in anti-IAPs Reaper and grim cause what?

Answer 20

eye defects in flies. Flies will have smaller eyes because many cells have died through apoptotic cell death. This is happening b/c the anti-apoptotic signaling mechanisms of IAPs that are present are overwhelmed by the anti-IAPs, Grim and Reaper

Question 21

Which apoptotic proteins are up regulated as a consequence of p53 gene regulation?

Answer 21

Fas Ligand and proteins Bid and Bax

Question 22

cells that are always programmed to die require what?

Answer 22

their survival requires rescue factors

Question 23

What is an example of cells programmed to die using a rescue factor?

Answer 23

Some cells are always programmed to die: their survival requires rescue factors

A major example of this type of cell survival
signaling is found in the developing nervous system. During fetal development, many nerve
cells are produced and this large number of nerve cells is produced in order to ensure that targeting cells which have to be innervated will be contacted by at least one neuron. The interaction between the nerve cell and a target cell promotes anti-apoptotic signaling pathways in the nerve cell that allow it to continue to live. However, any nerve cell that was produced yet
failed to find an appropriate target cell within some period of time then undergo apoptotic cell death, so they are pruned back, leaving only the neurons that have actually managed to make a connection. This mechanism ensures that every target cell has a neuron associated with it, but that there’s also just exactly one nerve cell for each target cel

Question 24

How do survival factors suppress apoptosis? What are the three mechanisms?

Answer 24

often by regulating activity of members of the Bcl2 family of proteins (mechanism 1)

1) Increased production of the anti-apoptotic Bcl2 protein
2) Inactivation of pro-apoptotic BH3-only Bcl2 protein
3) Inactivation of anti-IAPs

Question 25

What is the Akt cell signaling? How is apoptosis blocked in this signal?

Answer 25

1) Survival factor binds to activated receptor initiation pi-3 kinase signaling pathway
2) Pi-3 Kinase signaling pathway activates Akt kinase
3) Bad is phosphorylated by Akt, and deactivates Bad
4) Deactivation of Bad releases Bcl2 to be functional

Question 26

what is the MAP kinase signaling? How does it block apoptosis?

Answer 26

1) survival factor binds to activated receptor
2) Map-kinase becomes activated and phosphorylates anti-IAP Hid
3) IAP is released from Hid and IAP becomes functional

Question 27

What activates Akt?

Answer 27

Pi-3 kinase signaling pathway

Question 28

What is Akt?

Answer 28

a major regulator of cell survival and cell growth w/many downstream targets like BH3-only protein Bad

(Bad inactivates Bcl2)

Question 29

What happens when Bad is phosphorylated by Akt?

Answer 29

it gets deactivated and this releases Bcl2 to be functional

Question 30

how does the mitogen activated protein in kinase signaling pathway regulate many pro survival pathways?

Answer 30

one way is by phosphorylation of the anti-IAP HID, which inactivates it.

Question 31

How can the extrinsic apoptotic signaling pathway activate the intrinsic pathway?

Answer 31

through activity of BID (a BH3-only protein) that is expressed in an inactive form in the cytoplasm.

BID gets cleaved by initiator caspases, resulting in a smaller active form called tBid.

tBid inactivates Bcl2, and that leads to the release of cytochrome c from mitochondria through the function of proteins like Bak

Question 32

How can p53 cause apoptosis?

Answer 32

p53 can cause the expression of the Fas ligand which binds to death receptor on extrinsic pathway. This can cause apoptosis through autocrine stimulation of the extrinsic apoptotic signaling pathway. This is one way a cell would signal itself to die

Question 33

Anti-apoptotic protein families with 4 BH domains are known as

Answer 33

Bcl2 (4 domains)

Question 34

Pro-apoptotic protein families with 3 domains are known as? What are examples

Answer 34

BH123; Bax and Bak

Question 35

Pro-apoptotic protein with one domain are known as ? what are examples?

Answer 35

BH3; Bad, Bim, Puma, Noxa