Block E Part 2: How and why Cells Die Flashcards
During development, what helps determine the size and shapes of limbs?
Carefully orchestrated patterns of cell death
(Lecture 2, Slide 3)
Is cell death random?
No, cell death is programmed
(Lecture 2, Slide 3)
What proteins trigger apoptosis?
Specialised intracellular proteases (called caspases)
(Lecture 2, Slide 5)
How do specialised intracellular proteases trigger apoptosis?
By cleaving specific sequences in numerous proteins inside the cell, bringing about dramatic changes the lead to cell death and engulfment
(Lecture 2, Slide 5)
Why are the specialised intracellular proteases called caspases?
As they have a cysteine at their active site and they cleave their target protein at specific aspartic acids they are called caspases as c for cysteine + asp for aspartic acid
(Lecture 2, Slide 5)
Are caspases active when synthesised in the cell?
No, they are activated only during apoptosis
(Lecture 2, Slide 5)
What are the 2 major types of apoptotic caspases?
Initiator and executioner caspases
(Lecture 2, Slide 5)
What does an initiator caspase contain in its carboxy terminal region?
A protease domain
(Lecture 2, Slide 6)
What does an initiator caspase contain near its amino terminus?
A small protein interaction domain
(Lecture 2, Slide 6)
What form is an initiator caspase initially made in?
An inactive, monomeric form
(Lecture 2, Slide 6)
What is the inactive form of a caspase sometimes called?
Procaspase
(Lecture 2, Slide 6)
What do adaptor proteins assembled by apoptotic signals carry?
Multiple binding sites for the caspase amino-terminal small protein interaction domain
(Lecture 2, Slide 6)
What happens to the initiator caspases upon binding to adaptor proteins?
They dimerize (form a dimer) which activates them, leading to a cleavage of a specific site in their protease domain
(Lecture 2, Slide 6)
What happens to the protease domains of caspases after they are cleaved by the caspase forming a dimer?
They rearrange into a large and small subunit
(Lecture 2, Slide 7)
What form are executioner caspases originally formed as?
Inactive dimers
(Lecture 2, Slide 7)
What happens to an executioner caspase after cleavage at a site in the protease domain by an initiator caspase?
The executioner caspase dimer undergoes an activating conformational change
(Lecture 2, Slide 7)
What do executioner dimers do after undergoing their conformational change?
They cleave a variety of key proteins, leading to the controlled death of the cell
(Lecture 2, Slide 7)
How many proteins have scientists found to be cleaved by caspases during apoptosis?
Over 1,000
(Lecture 2, Slide 8)
What are 4 types of proteins which are cleaved by caspases?
Lamins
Cytoskeleton proteins
Cell-cell adhesion proteins
DNA degrading proteins
(Lecture 2, Slide 8)
What is caspase-activated DNase (CAD)?
A protein in humans encoded by the DFFB gene
(Lecture 2, Slide 9)
What does caspase-activated DNase (CAD) do during apoptosis?
It breaks up DNA
(Lecture 2, Slide 9)
What form is caspase-activated DNase (CAD) usually found in?
Inactive monomer inhibited by iCAD
(Lecture 2, Slide 9)
How do caspases mediate the intracellular proteolytic cascade that apoptosis depends on?
As executioner caspases cleave iCAD, leading to CAD becoming activated and start breaking up DNA
(Lecture 2, Slide 9)
What are the 2 main forms of apoptosis?
Extrinsic pathway
Intrinsic pathway
(Lecture 2, Slide 11)
What cascade occurs in extrinsic apoptosis and where does it begin?
Caspase 8 cascade, beginning outside the cell
(Lecture 2, Slide 11)
What cascade occurs in intrinsic apoptosis?
Caspase 9 mitochondrial pathways
(Lecture 2, Slide 11)
What do both the extrinsic and intrinsic pathways of apoptosis function to do?
Activate the executioner caspases
(Lecture 2, Slide 11)
How is the extrinsic pathway of apoptosis activated?
By extracellular proteins binding to cell surface death receptors
(Lecture 2, Slide 12)
What type of protein complex are death receptors?
Homotrimers
(Lecture 2, Slide 12)
What is a homotrimer?
A structure composing of 3 identical subunits
(Lecture 2, Slide 12)
What are death receptors characterised by?
The presence of a death domain
(Lecture 2, Slide 12)
Are death receptor ligands trimeric?
Yes
(Lecture 2, Slide 12)
How is the fas death receptor activated on the surface of a target cell?
By the fas-ligand on the surface of a cytotoxic T cell lymphocyte binding to the receptor
(Lecture 2, Slide 12)
How do fas ligands lead to several ligand-bound receptor trimers clustering?
Trimeric fas ligands on the surface of a cytotoxic kill T cell interact with trimeric fas receptors on the surface of the target cell, leading to clustering of several ligand-bound receptor trimers
(Lecture 2, Slide 13)
What does clustering of fas receptors activate?
Death domains on the receptor tails
(Lecture 2, Slide 14)
What do death domains activated by receptor clustering interact with?
Similar domains on the adaptor protein FADD (Fas-associated death domain)
(Lecture 2, Slide 14)
What does each FADD (Fas-associated death domain) protein recruit after interacting with death domains?
An initiator caspase (caspase-8)
(Lecture 2, Slide 15)
How do FADD (Fas-associated death domain) proteins recruit caspase-8?
Via a death effector domain on both FADD and the caspase
(Lecture 2, Slide 15)
What does FADD proteins recruiting caspases form?
A death-inducing signalling complex
(DISC)
(Lecture 2, Slide 15)
How do adjacent initiator caspases (caspase 8) in the death-inducing signalling complex (DISC) form an activated protease dimer?
By interacting and cleaving one another
(Lecture 2, Slide 16)
What does the activated protease dimer in the death-inducing signalling complex (DISC) do?
It cleaves itself
(Lecture 2, Slide 16)
What does the activated protease dimer in the death-inducing signalling complex (DISC) cleaving itself result in?
The protease dimer being linked to the death effector domain
(Lecture 2, Slide 16)
What does the activated protease dimer being linked to the death effector domain do?
It stabilises and releases the active protease (caspase) dimer into the cytosol
(Lecture 2, Slide 16)
What does the activated protease dimer activate once released into the cytosol and how does it do this?
Executioner caspases by cleaving them
(Lecture 2, Slide 16)
What do many cells produce to restrain the extrinsic pathway of apoptosis?
Inhibitory proteins
(Lecture 2, Slide 17)
How is the intrinsic pathway of apoptosis activated?
From within the cell, usually in response to a signal such as DNA damage that cannot be repaired, cellular stress or in response to a developmental cue (such as the formation of digits)
(Lecture 2, Slide 18)
What does the intrinsic pathway of apoptosis depend on?
Mitochondria releasing cytochrome c into the cytosol
(Lecture 2, Slide 20)
What causes mitochondria to release cytochrome c?
Intracellular apoptotic stimuli
(Lecture 2, Slide 20)
What does cytochrome c bind to after being released by mitochondria into the cytosol?
Apaf1
(Lecture 2, Slide 20)
What does the binding of cytochrome c to Apaf1 cause?
Apaf1 to unfold partly
(Lecture 2, Slide 20)
What does Apaf1 unfolding partly in response to cytochrome c binding it cause?
A domain to be exposed, which interacts with the same domain in other Apaf1 molecules
(Lecture 2, Slide 20)
How many Apaf1 molecules interacting does it take to form a large ring complex called the apoptosome?
7
(Lecture 2, Slide 20)
How are caspase-9 molecules recruited into the apoptosome and activated ?
The caspase recruitment domain (CARD) in Apaf1 bind similar domains in multiple caspase-9 molecules
(Lecture 2, Slide 21)
What does caspase-9 do once activated?
Cleaves and activates downstream executioner caspases
(Lecture 2, Slide 21)
What are the key regulators of the intrinsic pathway of apoptosis?
The Bcl2 family
(Lecture 2, Slide 22)
How does the mammalian Bcl2 family regulate the intrinsic pathway of apoptosis?
By controlling the release of cytochrome c and other intermembrane mitochondrial proteins into the cytosol
(Lecture 2, Slide 22)
What are the 2 different types of Bcl 2 family proteins and what are their fuctions?
Pro-apoptotic proteins which promote apoptosis by enhancing release of cytochrome c and other intermembrane mitochondrial proteins and anti-apoptotic which inhibit apoptosis by blocking the release
(Lecture 2, Slide 22)
What can pro-apoptotic and anti-apoptotic Bcl2 family proteins form by binding to each other in various combinatons?
Heterodimers
(Lecture 2, Slide 22)
What happens when pro-apoptotic and anti-apoptotic Bcl2 family proteins form heterodimers with each other?
They inhibit each other’s function
(Lecture 2, Slide 22)
What are the 3 different classes of Bcl2 proteins?
Anti-apoptotic
Pro-apoptotic effector
Pro-apoptotic BH3 only
(Lecture 2, Slide 23)
What Bcl-2 homology (BH) domains does the anti-apoptotic Bcl 2 family protein contain?
BH1, BH2, BH3 and BH4
(Lecture 2, Slide 23)
What Bcl-2 homology (BH) domains does the pop-apoptotic effector Bcl2 family protein contain?
BH1, BH2 and BH3
(Lecture 2, Slide 23)
What Bcl-2 homology (BH) domains does the pro-apoptotic BH3-only Bcl2 family protein contain?
BH3 only (clues in the name)
(Lecture 2, Slide 23)
How can complex inter-play occur between the 3 classes of Bcl 2 proteins?
Through the BH3 domain, the only domain they have in common
(Lecture 2, Slide 23)
What happens to the effector Bcl2 family proteins when they are activated by an apoptotic stimulator?
They aggregate on the outer mitochondrial membrane
(Lecture 2, Slide 24)
What is the effector Bcl2 family proteins aggregating on the outer mitochondrial membrane thought to do?
Make a channel which allows the release of cytochrome c and other proteins from the mitochondrial intermembrane space into the cytosol
(Lecture 2, Slide 24)
What do the anti-apoptotic Bcl 2 family proteins do to the pro-apoptotic effector proteins in the absence of an apoptotic stimulus?
They bind to and inhibit them
(Lecture 2, Slide 25)
What do BH3-only proteins do in the presence of an apoptotic stimulator?
They are activated and bind to the anti-apoptotic Bcl 2 family proteins so that they can no longer inhibit the effector Bcl2 family proteins
(Lecture 2, Slide 26)
What does the body need survival factors?
They ensure that individual cells behave for the good or the organism as a whole - by surviving when needed and killing themselves when not
(Lecture 2, Slide 27)
What are 3 ways in which survival factors block apoptosis?
Increase production of anti-apoptotic Bcl2 family proteins
Inactive of pro-apoptotic BH3-only protein
Inactivation of anti-IAPs (counteract the activity of inhibitors of apoptosis proteins)
(Lecture 2, Slide 28)
What happens to a cell after it has been through apoptosis?
The cell and its fragments do not break open and release contents, but instead remain intact and they are phagocytosed
(Lecture 2, Slide 29)
Why are cells that have undergone apoptosis phagocyotsed?
As it does not trigger an inflammatory response
(Lecture 2, Slide 29)
What regulates phagocytosis?
A balance of “eat me” and “don’t eat me” signals displayed on the cell surface
(Lecture 2, Slide 30)
