L12 Apoptosis Flashcards
(19 cards)
definition and properties of apoptosis
programmed cell death
highly complex and orchestrated mechanism
characterised by cell shrinkage; cytoplasmic membrane blebbing (bubbling out of membrane); formation of apoptotic body; dna fragmentation and chromatin condensation
energy-dependent cascade of molecular events
activation of a set of cysteine proteases called cascases
no inflammation
definition and function of necrosis
accidental cell death
no energy expenditure
cell swelling; formation of cytoplasmic vacuoles; release of cytoplasmic contents
leads to inflammation
difference between apoptosis and necrosis
apoptosis: formation of blebbing > cell breaks apart into several apoptotic bodies > phagocytosed; no inflammation
necrosis: cell swells > plasma membrane rupture; cellular and nuclear lysis causes inflammation
what are caspases
asparate-directed cysteine proteases that play a key role in the initiation and execution of apoptosis
(not all caspases involved in apoptosis)
types of caspases in apoptosis
initiator caspase: first one to be activated; contain CARD (caspase activation and recruitment domain) and DED (death effector domain)
executioner caspase: the ones that break down the cell; activated after initiator caspases are triggered
how are caspases activated
caspases started as inactive precursors called procaspases
procaspase cleaved into large and small subunit > form heterodimer > two heterodimers come together to make active tetramer
extrinsic pathway of apoptosis
death signals bind to death receptors that are part of tumour necrosis factor (TNF) receptor gene superfamily
receptors all have cysteine-rich extracellular domain and cytoplasmic death domain (DD)
DD helps receptor connect with other proteins in cell > chain reaction > activates initiator caspases > executioner caspases turned on to carry out cell death
how does receptor mediated caspase activation at DISC works
DISC: death-inducing signalling complex
ligand (FasL, TNF-alpha, TRAIL) bind to death receptor (Fas, TNFR1, DR5) > conformational change in receptor > recruit adaptor proteins like FADD and TRADD > recruit procaspase 8 > activate to form caspase 8 > cleave and activate caspases 3, 6, 7 > apoptosis
intrinsic pathway of apoptosis
triggers by non-receptor signals like dna damage, oxidative stress etc > signals act directly on targets within the cell; mitochondrial initiated events
leads to loss of mitochondrial transmembrane potential > opening of mitochondria permeability transition pore > release two groups of pro-apoptotic proteins into cytosol
what are the two groups of pro-apoptotic proteins released into cytosol
- cytochrome c, Smac/DIABLO: activates caspase
- AIF, endonuclease G and CAD: late event - dna fragmentation
how is mitochondrial membrane compromised
intrinsic apoptotic pathway hinges on the balance of activity between pro and anti apoptotic members of the Bcl2 superfamily > act to regulate permeability of mitochondrial membrane
examples of anti-apoptotic proteins and their structure
Bcl-2, Bbl-x, Mcl-1, A1
TM: transmembrane domain to anchor to mitochondrial domain
BH1 and BH2: death repressor domain and pore formation
BH3: death domain and gating function
BH4: death repressor domain
examples of pro apoptotic and their structure
Bax subfamily:
- Bax, Bak, Bok
- only have BH1, BH2 and BH3 and TM
BH3-only subfamily:
- Bik, Hrk, Bid (TM missing), Bcl-x8 (contains BH4)
- only have BH3 and TM
examples of BH3 only pro apoptotic proteins
BIK, BAD, BIM and PUMA
examples of multi domain pro apoptotic proteins
BAX, BAK, BOK, BOO
how is mitochondrial membrane permeability controlled
BH123/Bax family activated by apoptotic stimuli > form oligomeric structures > create pores for other proteins to exit mitochondria
if got active anti apoptotic Bcl-2 protein > prevents oligomerisation of BH123 > no pore formation > no release of pro apoptotic factor s
what happens during mitochondria-mediated caspase activation at apoptosome
stress or damage inside cell triggers apoptosis
proteins like Bax and Bak form pores in membrane > cytochrome c leaks out into cytosol > binds to Apaf-1 > form apoptosome > procaspase 9 binds to apoptosome > apoptosome cleaves it into active caspase 9 > activates caspase 3 and 7 > carry out cell death
how is apoptosis inhibited
by inhibitors of apoptosis proteins (IAPs), by blocking caspases
IAPs bind to active caspases to stop them from cutting up the cell
some IAPs like Livin tag pro apoptotic proteins like Smac/DIABLO for destruction using ubiquitin
when is caspase 8 and 9 activated
extrinsic: 8
intrinsic: 9
