Lectures 5&6 - Apoptosis Flashcards
What is apoptosis
a form of cell death that occurs in multicellular organisms. It is orderly and precisely controlled.
Why do metazoans need apoptosis?
need a body plan that requires tissue shaping,
eliminate infected cells
eliminate cells that do not retain genomic integrity
Among survival and apoptosis which one is the default pathway
apoptosis
Why and when is apoptosis important?
1- During development.
Sculpting structures, e.g. digits.
2- During immune system shaping.
Eliminating self-reacting cells.
3- Deleting damaged cells
Virus infected cells.
Sunburn, Radiation.
Genetic abnormality.
4- Eliminating misplaced cells
Cancer.
Autoproliferative disorders.
What is cell death classified according to?
1- Its regulation (regulated vs non-regulated) (some halfway regulated).
2- Its function.
3- Morphological changes.
4- Biochemical changes
How many types of cell death are there? give examples
14 :
necrosis, apoptosis
Describe the intrinsic pathway of apoptosis - upstream of MMP loss
Apoptosis occurs by signals from within the cell itself e.g. DNA damage
DNA damage leads to p53 activation, this leads to activation of Bax
Bax makes a pore in the mitochondrial membrane which leads to the leakage of molecules, inducing a caspase cascade ending with the cleavage of particular proteins that mean there is DNA fragmentation, protein cleavage, nuclear membrane loss and organelle breakdown
This ultimately leads to apoptosis
Describe the Type I extrinsic pathway of apoptosis - upstream of MMP loss
Extrinsic stimuli e.g. Trail, Fas etc. Lead to death receptor activation and this leads to caspase 8 activation, this initiates caspase 6 etc. and then same pathway as intrinsic from here
Describe the Type II extrinsic pathway of apoptosis - upstream of MMP loss
Activation of caspase 8 also leads to activation of tBid which is a faster version (6hrs) of what Bax does (takes 16-24) (so cuts mitochondria)
What is the core of apoptotic pathways
(cystein aspartic proteases): caspases
Describe the activation of caspases
proteolytic cleavage:
Procaspase get cleaved and then associated into a heterotetreamer
Name the different initiator and effector caspases
Initiator:
caspase-8
caspase-9
caspase-10
Effector:
caspase-3
caspase-6
caspase-7
Describe the process of how Effector caspases keep cleaving/activating initiator caspases
initial trigger -> initiator caspases -> Effector caspases ->
-> back to initiator caspases
AND
-> Substrates -> apoptosis
Describe the structure of the DISC (Death inducing signal complex), and what the different parts are referred to as
Fas receptor:
the “trigger”
made up of extracellular domain ligand binding and death domain
FADD:
the “adaptor”
made up of the death domain and a death effector domain
Pro-caspase 8:
the “Killer”
made up of 2 death effector domains and a catalytic domain
Explain how The DISC responsible for caspase activation via the “extrinsic pathway”
Fas ligand brought by another cell, and binds to Fas receptor
Pro-caspase 8 is weak as bound to death effector domains
caspase 8 becomes active
Describe how cytochrome c (the alarm signal) works in intrinsic apoptosis
When there’s a hole in mitochondria: Cyc C is released and binds to APAF-1 protein creating an open conformation with the CARD domain exposed, and this binds to other CARD domains making a wheel (wheel of death)
Caspase 9 binds to the wheels of death this is now called the apoptosome
Once caspase 9 is activated it causes a caspase cascade - activates effector caspases (caspases 3,6 and 7) and they chop up substrates and the cell dies
What is a key event in the initiation of apoptosis
Release of mitochondrial contents such as calcium and cytochrome C
How does bax induce apoptosis
by forming pores in the mitochondrial membrane
Explain how pores in the mitochondrial membrane are made during apoptosis (simple)
A tunnel is created on the mitochondrial membrane by BH3 proteins forming a tunnel and inserting themselves into the membrane
Describe properties of the BH3 family
24 members
very sticky - they are inclined to bind to each other
If Bax family of proteins dominate and allow pore forming complexes, the cell will die.
Bcl-2 family proteins bind to Bax family to restrain them.
Sensitizer BH3 proteins fine tune survival-apoptosis balance
Explain how pores in the mitochondrial membrane are made during apoptosis (complex)
BAX and BAK directly permeabilize mitochondrial membrane
BID and BIM activate BAX and BAK
(BID,BIM PUMA NOXA bind to BAX or BAK to expose them and change conformation, unbinds and leave open for a small time (“hit & run”) if in this time there’s more binding of more BAX/BAK, this eventually creates a pore )
BCL-2 and BCL-xL sequester BAX, BAK, BID and BIM
What two events must take place to activate mitochondrial membrane permeabilization
All the pro-survival members of the BCL-2 family are inhibited
BAX and BAK are activated by the activator BH3-only proteins
BH3-only pro-apoptotic molecules are inactivated by what?
Survival signals
What is bmf sequestrated by?
Cytoskeleton complex - if cytoskeleton can’t function it will be released and will do its job to create open BAX or open BAC
