Apoptosis Flashcards
Necrosis
Cell swelling Organelle disruption Plasma membrane rupture Inflammatory Externally caused damage-> trauma and disease
Apoptosis definition
Programmed cell death
Non inflammatory
Often physiological-> extension of limbs, formation of digits! counter selection of immature B/T cells, ultimate response to DNA damage
Ultimate response to DNA damage
Morphological changes in apoptosis
1) onset-> shrinkage of cell-> condensation of cytoplasm
-> shrinkage of nucleus-> nuclear chromatin condensation in to delineated masses against nuclear membranes
2) nucleus fragments (karyorrhexis), cell detaches from surrounding tissue, outlines become convoluted and form extensions
3) blebbing-> extensions separate and plasma membrane seals to form separate membrane around detached solid cellular material-> apoptotic bodies
4) phagocytosis of apoptotic bodies
If not phagocytosed -> undergo degradation-> secondary necrosis
Intrinsic apoptosis pathway
Activated by withdrawal of survival signals, often as a consequence of cell damage -> events that effect mitochondrial integrity
Recruited during extrinsic pathway activation by Bid
Leads to cytochrome C realise from mitochondria, a necessary protein component of mitochondrial electron transport chain
Stimuli of intrinsic apoptosis
O2 deprivation DNA damage Viral infection Hypoxia Oncogene activation Growth factor withdrawal Cell injury Certain steroids Inability to maintain low Ca levels
Intrinsic pathway apoptosis, factors released from mitochondria
Outer mitochondrial membrane becomes permeable-> relase of apoptogenic factors
- Cytochrome C-> binds Apaf (pro apoptotic protease activating factor)-> complex then binds caspase 9 (initiator)-> forms an apoptosome-> activates execution caspase by zymogen cleavage
- SMAC/DIABLO
- pro-caspases
- AIF
- endo G
External apoptosis pathway
Stimulation of death receptors by ligands
-> TNFa and deprivation of growth factors
Or fas ligand
Death receptor pathway
Death receptors-> subset of TNF-1 receptors -> 3 cysteine rich domains and homologous intracellular domain
Fas/CD95, TNF-R1 and DR3 form a trimer-> binds TNF-1 or other death ligand on the external domain and adaptor proteins internally
Activated TNF-R complex forms the scaffolding for binding two molecules of pro caspase 8
Death by fas in the death receptor pathway
1) fas L acts as a ligand for fas-> digomerisation of its R on binding-> associated with clustering of the death domains and binding of FADD
2) FADD binds pro caspase 8 via its death effector domain
3) forms a complex of fas, FADD, and procaspase 8-> death inducing signalling complex
4) self cleavage of pro caspase 8-> caspase 8
5) caspase 8 activates effector caspases
Caspas 8 effects
An initiator caspase
Activates execution caspases 3,6,7
Also cleaves Bcl-2(Bid)-> activates the mitochondrial integritity (intrinsic) pathway-> interacts with BAX-> cytochrome C relase-> binds APAF with dATP and pro caspase 9-> cleaves pro caspase 3 and 7
-> stress pathway
Fas
=CD95=APO-1
Type 1 membrane protein
Monoclonal antibodies cytoxic for human cell lines isolated-> recognised cell surface proteins Fas
PC60 T cell lymphoma-> specifically kills cells expressing fas
Death by TNFR-1 in the death receptor pathway
Various other death domain containing proteins can form once the receptor is activated
TNF-R is activated by TNFa-> binds either TRAF or TRADD
TRAF expression is controlled by survial signals (NFuB)-> binding signals stress mediated inflammatory signals->activation of MAPKK and JNKS-> RIP dependent activation of IKK and NFaB
Loss of TRAF expression favours TRADD binding-> recruits FADD for caspase 8
Caspases
Cysteine proteases that cleave peptide bonds next to an Aspartate residue
Present as pro caspases-> activated by proteolytic cleavage
Initiator caspases-> cleave other caspases
Execution caspases-> cleave cellular protiens involved in maintianing cellular integrity and endonucleases CAD
Caspase substrates
Cytoplasmic-> beta catenin, IP3R Nuclear-> lamins, SRF Protein kinases-> PKC isoforms PRK2 Other signalling protiens-> pro-interleukin, Ras, GAP Cell cycle-> Rb, p21, p27 Apoptotic mediators-> pro caspases
BCL-2 family
Decision makers that integrate prodeath and anti death signals
Originally identified in follicular lymphoma
Led-9 homologue
Extends cell survival
> 15 members
Anti apoptotic Bcl
Bcl-2
Bcl-X-> regulates outer mitochondrial membrane integrity
Bcl-w
Mol-1
Localise predominantly in outer mitochondrial membrane
Complex with APAF-> inhibit apoptosome formation
Heterodimerase with BAK and BAX-> block pore formation
Sequester and inhibit BH-3 only pro apoptotic factors
Viral homologues-> EBV, BHRF-1
Proapoptoic Bcl
Activated by conformational change or multimerisation
Channel forming-> BAX, BAK, BOK-> associate with VDACS-> release of apoptogenic factors
BH3-> activated by dephosphorlyation, cleavage,due novo expression-> activation of multi domain pro apoptotic factors-> dimerise-> form and ion channel that promotes cyto c release
-> inactivate anti apoptotic factors
Bid-> activates BAX and binds Bcl-2-> frees APAF
Inhibitors of apoptosis
Eight human protiens Expression up regulated by NFaB Inhibit/ubiquylating caspases Down regulated by phosphorylation IAP kinase stimulated by activated fas R
PI3K dependent survival pathway
Activated by binding of growth factors
-> activates AKT-> results in phosphorylation of the pro apoptotic BAD-> inactivated.
Fork head phosphorylation-> deactivation of PI3K-> normally causes production of fas ligand
Clearance of apoptotic cells
By phagocytes
Macrophages recognise apoptotic cells-> phosphatidylserine, calrecticulin, thrombospondin
Ingest apoptotic cells by phagocytosis
Release anti inflam-> TGFb1-> resolves inflammation and promotes tissue repair
Too much apoptosis
Neurodegeneration
Atherosclerosis
Multiple sclerosis
AIDS-> HIV infects CD4+ lymphocytes-> increased susceptibility to apoptosis-> progressive loss of CD4 T cells
Too little apoptosis
Autoimmunity
Chronic inflammation
Cancer-> Bcl-2 over expressed
-> B cell lymphomas, breast cancers, prostate, colorectal
