Lecture 7: Apoptosis Flashcards
What are meant by external signals?
- Activate via “death receptors”
- Detect presence of EC death signals, in response, ignite the intrinsic apoptotic machinery
- Death receptors activate apoptosis independent of p53
- 30 members total:
a. cysteine rich EC domains
b. cytoplasmic death domain (DD)
How do “death receptors” function?
- FasL: binds 3 Fas molecules
- Clustering of death domains
- Adapter protein FADD (Mort1) binds via DD = DISC
- FADD also contains DED, which binds to analogous domain in pro-caspase 8 (FLICE, MACH)
- DED = CARD
- Pro-caspase 8 drives own activation via self-cleavage
- Caspase 8 activates down-stream effector caspases (eg caspase 3) committing cell to apoptosis
What is meant by DD
death domain
What is meant by FADD
Fas associated death domain
What is meant by DISC
Death inducing signalling complex
What is meant by DED
Death effector domain
What is meant by CARD
Caspase recruitment domain
What are some details about death receptors
- membrane bound FasL = 39kDA
- Proteolytically cleaved by MMPs
- Soluble form (26kDa) sFasL
- Both forms can self-associate and trimerise after it can bind with receptor Fas
What are the inhibitors of Fas?
- FAP-1 (Fas-associated phosphatase-1) inhibits Fas export to cell surface
- SODD silencer of TNRF1 death domain. Prevents trimerization
- FLIP inhibits FLICE (=caspase 8)
What is the signalling cascade of death receptors?
- TNF prod mainly by activated macrophages and T-cells in response to infection
- TNFR1: activation of TFs, pro-inflammatory and immunomodulatory genes
- Adapter protein TRADD (TNFR associated DD) binds via DD
- FADD or RIP bind to TRADD via DD
- FADD activation leads to apoptosis if other signals blocked = context dependent
- TNFR1-TRADD-FADD-Caspase 8 = DISC
What are other triggers of apoptosis
DNA damage
Metabolite deprivation
Physical damage
Heat shock
Hypoxia
What are the two types of apoptosis via caspases
Caspase-dependent
Caspase-independent
What is the apoptosome?
What is the process of apoptosis via cytochrome c?
- Pro-apoptotic signals triggers mitochondria to release cytochrome c and Smac/DIABLO
- Cytochrome c and
Apaf-1 hybridise (using dATP) - Procaspase 9 (initiator procaspase) binds and it enters the apoptosome
- Executioner procaspases (3, 6, & 7) become caspases, cleaving death substrates (ICAD, Lamin, Vimentin, Actin).
- This can be inhibited by IAPs (Smac/Diablo derivatives)
What are caspases?
- Cysteine aspartyl-specific proteases
- absolute specificity for aspartic acid in the P-1 position [x/x/x/asp/y/]
- cascade = disassembly of cell
- synthesised as inactive pro-enzyme (zymogen), activated by apoptosis trigger (proteolytic cleavage), & subsequent formation of a dimer