Apoptosis Flashcards
Why does programmed cell death occur? •
Harmful cells
• e.g. DNA damaged.
Developmentally defective cells
• e.g. self-antigen B-cells.
Excess cells
• e.g. sculpting of hands during embryonic development (get rid of webbing).
Obsolete cells
• e.g. mammary epithelium at the end of lactation.
Exploitation
• e.g. chemotherapeutic killing of cells.
Necrosis?
Unregulated cell death associated with • trauma • cellular disruption & • INFLAMMATORY response
Apoptosis?
Programmed (regulated) cell death
• controlled disassembly of cellular contents without disruption
&
• NO inflammatory response
Steps associated with necrosis?
- Plasma membrane becomes permeable.
- Cell swelling and rupture.
- Release of proteases leading to auto-digestion.
- Localised inflammation.
Steps associated with apoptosis?
LATENT PHASE
• death pathways are activated but cell stays morphologically the same
EXECUTION PHASE
1. Loss of microvilli and inter-cellular junctions.
2. Cell shrinkage.
3. Loss of plasma membrane asymmetry.
4. Chromatin and nuclear condensation.
5. DNA fragmentation.
6. Membrane bleb formations.
7. Fragmentation into membrane-enclosed apoptotic bodies.
• So, no inflammation as the plasma membrane remains intact
DNA modification can also occur during apoptosis - what can this lead to?
Fragmentation of DNA ladders
• seen in agarose gel
AND
Formation of more “ends”
• labelled with an extra fluorescently-tagged base
Other than necrosis and apoptosis, what are other types of cell death and mechanism?
Apoptosis-like PCD (Programmed Cell Death)
• some, but not all, features of apoptosis
• display of phagocytic recognition molecules BEFORE plasma membrane lyses
Necrosis-like PCD
• variable features of apoptosis BEFORE cell lyses; “Aborted apoptosis”
The fact that other forms of cell death also exist apart from necrosis and apoptosis, what does this suggest?
The fact that these other forms exist suggests a GRADED response of cell death.
4 broad mechanisms of apoptotic cell death?
- Caspase cascade – the executioners
- Death response – death receptors and mitochondria
- Bcl-2 family
- Stopping the death programme
What are caspases and their MoA?
Caspases = Cysteine-dependent ASPartate-directed proteASES
- a cysteine residue in the active site is required for their activity
- they cut proteins after their aspartate residue
- activated by a proteolysis cascade
There are 2 major classes of caspases - what are they and explain it?
Initiator caspases • 2, 9, 8, 10 (TRIGGER APOPTOSIS) • 2 subunits – p20 and P10. • They have an extra targeting subunit: - CARD – Caspase Recruitment Domain - DED – Death Effector Domain (these direct them to a location)
Effector caspases
• 3, 6, 7 (CARRY OUT APOPTOSIS PROCESS)
How do caspases mature?
Procaspases are single-chain polypeptides
• to activate, procaspases undergo proteolytic cleavage to form LARGE and SMALL subunits.
• Initiator caspases also cleave the targeting subunits (DED and CARD)
After cleavage, there is a folding of the 2 large and 2 small chains
• to form an active L2S2 HETEROTETRAMER
Explain the caspases cascade (aka. the main purpose)
Main purpose of the caspase cascade is for:
• amplification
• divergent responses
• regulation
Once apoptosis is triggered, the initiator caspases cleave and activate the effector caspases
What do the effector capcases do?
Execute the APOPTOTIC PROGRAMME
In 2 ways:
- Cleave & INACTIVE proteins/complexes
• e.g. nuclear lamins lead to nuclear breakdown - ACTIVATING enzymes by direct cleavage or cleavage of inhibitors
• e.g. nucleases (CAD)
What are the 2 broad mechanisms for caspase activation?
Death by design
• Receptor-mediated (extrinsic) pathways
Death by default
• Mitochondrial (intrinsic) death pathway
What are death receptors?
(extrinsic pathway)
ALL CELLS have death receptors which consist of:
• EC cysteine domains
• Transmembrane domain
• Cytoplasmic tail (death domain)
What can interact with death receptors?
ADAPTOR PROTEINS can interact with these receptors
• FADD (ACTIVATION)
- +VE regulator
- promotes cell death
- DED + DD
• FLIP (INHIBITION)
- -VE regulator
- inhibits death pathway (allowing for regulation)
- DED + DED
Explain and describe the signalling through the death receptors
- Fas receptor is upregulated when the cell needs to apoptose
- Fas ligand on cytotoxic T-cells binds to the Fas receptor and Fas receptor trimerises
- Trimerised DD domains recruit adaptor proteins such as FADD
- FADD binding causes recruitment and oligomerisation (links monomers to form dimers/trimers/etc.) of procaspase 8 through DED –> FADD DED
- Procaspase 8 + FADD –> DISC (Death-inducing-signalling-complex)
- DISC cross-activates other procaspase 8 molecules
- Active caspase 8 is released to cleave effector caspases
Explain the oligomerisation of procaspase 8
Occurs to allow procaspase 8 for cleavage and activation (as some initiator procaspases have low catalytic activity)
- Initiator procaspases bind to FADD (DED–> DED)
• DED regions bind to DED regions - This brings procaspases into close contact to allow cleavage
- Active initiator caspase 8 tetramers release.
Explain how FLIP functions in relation to the death receptors.
Death receptor activation of procaspase 8 is inhibited by FLIP (negative regulator)
*• FLIP incorporates into the trimer
BUT
it has NO PROTEOLYTIC ACTIVITY and so cannot cleave the other procaspases (competes with procaspase 8)
• It can still bind to the DED regions on FADD however*
Why is Caspase 8 important?
It is the INITIATOR CASPASE
Can go on to activate the other EFFECTOR caspases that then carry out the apopotic process
(onenote!!)
Explain and describe the intrinsic (mitochondrial) pathway
- Cellular stresses
• e.g. lack of GFs, DNA damage (p53) - Loss of mitochondrial membrane potential
- Release of cytochrome C
• and OTHER apoptosis-inducing factors - Stimulation of formation of “apoptosome complex”
What does the Apoptosome (“wheel of death”) consist of?
APAF-1
• Apoptotic Factor 1
Cytochrome C
ATP
Procaspase 9
What do the different parts of the Apoptosome do and how do they relate together?
APAF1 is composed of: • CARD • ATPase and • WD-40 repeats
When cytochrome C binds to WD-40 repeats on APAF-1, it forms a heptamer (the apoptosome) which requires ATP
The CARD domain (also on APAF-1) then binds to CARD on procaspase 9
• so multiple procaspase 9s (x7) can bind to one apoptosome
• proximity of procaspases then allows cross-cleaving
Activated caspase 9 is then released to trigger apoptosis.
What does apoptosis require, determining if this pathway is chosen?
ENERGY - APT
Energy levels in the cell may therefore determine whether death is by
• necrosis (no ATP)
• apoptosis (ATP)
Following on from both the extrinsic & intrinsic pathway, explain the principal mechanisms of apoptosis
Bid links the extrinsic (receptor) and intrinsic (mitochondrial) pathways together
• Caspase 8 from the extrinsic pathway can cleave Bid –> enhances activation of the intrinsic pathway
• Bid releases Cytochrome C from the mitochondrion which triggers the death pathway
Major difference – intrinsic mitochondrial pathway requires energy (ATP)
• Apoptosis always uses intrinsic to some degree so will always use ATP
• Remember that necrosis uses MUCH LESS ATP.
What are the intrinsic modulators of apoptosis?
Bcl-2 family proteins
3 main groups
• ALL contain BH3 domains (required for dimerisation betw. one another)
• SOME contain TM domains
What are the 2 main categories of Bcl-2 proteins?
Anti-apoptotic
• Bcl-2
• Bcl-xL
• Localised in mitochondrial membrane
Pro-apoptotic • Bid • Bad • Bax • Bak • Move between cytosol & mitochondrial membrane
What are the 2 GF pathways associated with anti-apoptotic effects
- Ligand binds –> dimerisation –> cross-phosphorylation –> signal transduction and docking of adapter proteins (e.g. Grb2) to adapt pathway direction (e.g. activating Ras –> MAPK/ERK cascade)
- Another phosphorylation site on tyrosine kinase receptors triggers the PI3-kinase pathway –> cell survival and anti-apoptotic effects.
What does the PI3-kinase pathway lead to?
Cell survival
HENCE
Anti-apoptotic effects
Explain the PI3-kinase pathway
PI3-K = Phosphatidylinositol 3-kinase = a lipid (not protein) kinase
Three main subunits:
• Targeting subunit
• Adapter subunit
• Catalytic subunit
PI3-K phosphorylates PIP2 –> PIP3
• which then binds PKB/Akt (protein kinase B) adapter subunit
• PKB is then recruited to the cell membrane and it is activated - it has anti-apoptotic effects
Explain how PKB has anti-apoptotic effects
PKB has anti-apoptotic effects by:
- Phosphorylating and inactivating Bad (a Bcl-2)
- Phosphorylating and inactivating caspase 9
- Inactivating FOXO (promote expression of apoptosis-promoting genes) transcription factors.
- Other – stimulates ribosome production.
What are 2 models of regulation of apoptosis by Bcl-2 family proteins via. BH3 heterodimerisation
A - PKB/Akt (PI3-K, GF pathways)
B - GF absent
Explain the A model of apoptosis of Bcl-2 family proteins via. BH3 heterodimerisation
Pro-apoptotic proteins (e.g. Bax, Bak) are held INACTIVE in their heterodimers to Bcl-2 and Bcl-xL on the mitochondrial membrane
• by the BH3 domain
As the pro-apoptotic are held in the inactive heterodimers, cell proliferation and survival is promoted
Explain the B model of apoptosis of Bcl-2 family proteins via. BH3 heterodimerisation
GF is ABSENT!!
No PI3-K pathway –> PIP3 not generated –> PKB not recruited –> Bad NOT phosphorylated and so is released from heterodimer with 14-3-3
Bad therefore goes to mitochondrial membrane and binds to Bcl-2 and Bcl-xL
• displacing the pro-apoptotic Bcl-2 family proteins
Bcl-2 family proteins (Bax, Bak) then form a pore in the mitochondrial membrane to allow cytochrome C to escape to induce apoptosis
What can counteract PI3-K?
PTEN (lipid phosphatase)
Counteracts PI3-K signalling and production of PKB
SO
PROMOTES APOPTOSIS
What are IAPs and what do they do?
Inhibitor of Apoptosis Proteins
Regulate programmed cell death by binding to:
• Procaspases - preventing activation
• Caspases - inhibiting activity
ALL extrinsically (as dealing w. caspases)
Summarise the anti-apoptotic pathways
Bcl-2 & Bcl-xL
• via the intrinsic pathway
FLIP & IAPs
• via the extrinsic pathway
Growth factor pathways
• via PI3-K and PKB/Akt
How can cancer cells avoid apoptosis?
Apoptosis regulators as oncogenes or TSGs
Bcl-2
• Oncogene – as overexpression of Bcl-2 results in cancer
PKB/Akt
• Oncogene – as overexpression of PKB leads to increased cell survival and thus cancer
PTEN
• TSG – as inactivation raises the PI3-K pathway and thus allows increased survival of cancer.
