Programmed Cell Death (18) Flashcards
What are some features of necrosis?
External trigger (non-specific)
Moderate chromatin condensation
Cytoplasmic, mitochondrial and ER swelling
Rupture of the plasma membrane - release of intracellular content, drives inflammation
Collateral damage to surrounding tissues
No genetic programme
No signal transduction inflammatory response
Not programmed cell death (but does overlap)
What are some developmental examples of programmed cell death in plants?
In unisex flowers PCD leads to abortion or developmental arrest: of carpel primordial in male flowers and stamen abortion in female flowers
During sporogenesis PCD occurs in supporting sporophytic tissues - inlacing the anther tapetum and the nucellus in the developing ovules - after meiosis non functional megaspores degenerate
Trichrome development - rapidly dividing protodermal cells, trichome cell selection, mitosis replaced by endoreduplication, genetically controlled branching, death programme activated (holes in monstera genetically programmed)
What are some developmental examples of programmed cell death in animals?
Sculpting tissues → mouse forelimb footplate development
→ almost all larval structures are destroyed during Drosophila metamorphosis, novel structured raised from undifferentiated cells to generate adult structures
Deletion of cells that fail to partner → controls cell number
Elimination of dangerous and abnormal cells e.g. auto reactive lymphocytes
Neural deletion adjusts number of nerve cells → an excess number of neurons made then deleted to target size
What is the overall mechanism of apoptosis?
- Apoptotic signal (multiple signals)
- Assembly of an activating complex with initiator procaspases
- Activation of initiator caspases by proximity, auto cleavage and rearrangement
- Activation of executioner caspases by initiator cause cleavage and rearrangement
→ links with development and inflammation
What are some signals for intrinsic apoptosis?
Intrinsic apoptosis - regulated cell death initiated by multiple signals: marked by mitochondrial outer membrane permeabilisation
Signals: DNA damage, ER stress, ROS burden, replication stress, micro tubular changed, mitotic abnormalities and more
What is involved in intrinsic apoptosis initiation?
- Apoptotic stimulus (sufficiently strong) - encourages channel formation in outer membrane of mitochondria by Bas
→ contents of inner membrane space spill out into cytosol, key component - cytochrome C - Cytochrome C is captured by Apaf-1 (apoptotic protease activating factor-1)
→ Apaf-1 forms a heptameric apoptosome
→ displaces tail revealing CARD (caspase activation and recruitment domain - members of death domain family) - recruits initiator caspase - Initiator caspase 9 proximity activated → caspase cascade leads to apoptosis
How is intrinsic apoptosis imitation regulated?
Pro-apoptotic → BH3-only protein, Bad, Bim, Bid, PUMA, Noxa
→ BH (Bcl-2 homology domain) proteins regulate the release of cytochrome c (and other pro-apoptotic factors) from the mitochondrial inter-membrane space
Anti-apoptotic → Bcl-2 like, Icl-2, Icl-XL
→ Icl-2 inhibits pro-apoptotic family
In a healthy cell → (pro-apoptotic) Bas family proteins circulate between the cytosol (TM domain hidden) and the mt OM (TM unmasked)
→ (anti-apoptotic) Bcl-2 family members heterdimerise with membrane bound Bax family members and inhibit them - stop forming channel
In apoptotic cell → BH3-only proteins inhibit Bcl-2 family proteins - allows membrane oligomerisation of Bas family proteins and channel formation that releases cytochrome c
How do BH3-only proteins inactivate Bcl-2 family members?
Molecular mimicry
The Bcl-2 family members transmembrane domain lies in a hydrophobic groove, rotates 90deg to sit in hydrophobic cleft
BH3 - alpha helix mimics the transmembrane domain
→ BH3 domain displaces the Bcl-2 hydrophobic groove - so can no longer heterdimerise with membrane-bound Bas family members
→ Bad proteins permit Bax protein family channel formation
How are BH3-only proteins regulated?
Post transcriptionally:
Survival stimuli (trophic signals) → phosphorylation on S75 and S99 which promotes phosphorylation of S118 (and S134)
→ disrupts BH3 - in the middle of the domain that inhibits Bcl-2 family members - phosphorylation stops it working
Apoptotic stimuli → methylation of R94 and R96 inhibits phosphorylation of S99
What is the intrinsic apoptosis executioner (effector) caspase cascade?
After CASP9 recruited and activated by proximity - associated with apoptosis
→ activates procaspase 3 to CASP3 - can free CASP9 from CARD - cleaves amyloid beta 4A precursor protein
→ procaspase 6 to CASP6, procaspase 7 to CASP7 - cleaves interleukin-10, lamin inhibitors of apoptosis
Caspase: cysteine-aspartic acid protease
→ chain of executioner caspases activated by cleavage and molecular rearrangement
→ thousands of substrates can be cleaved by caspases during apoptosis (function significance mostly unknown)
What is the role of p53 in intrinsic apoptosis and development?
Causes transcription of PUMA → inhibits Bcl-2 - promotes cytochrome release thus apoptosis
Stressed developmentally → activation of genes leading to PIDDosome
→ proximity: procaspase 2 dimerisation and auto proteolysis activates CASP2 - organogenesis (deletes webbing)
Roles of development and apoptosis integrated
What are the roles of trophic factors in intrinsic apoptosis?
Keeps neighbouring cells alive: trophic signal inhibits Bad - Bad is a Bcl-2 inhibitor thus apoptosis inhibited
e.g. nerve growth factor - when occupies receptor autotransphosphorylation activation - signalling cascade which terminates in AKT (PKB) - phosphorylated Bad inhibiting it
→ p75NTR is NGF receptor - when occupied cells stay alive, deoccupy cells die
→ in adipose tissue NGF upregulated in obese mouse but receptor p75NTR is not - no change in receptor but an increase in occupancy - keeps cell alive, increase in fat tissue
What an example of extrinsic apoptosis?
Death receptor mediated apoptosis
e.g. cytotoxic T cell transmembrane death ligands
What is involved in extrinsic apoptosis building death effector filaments?
Signalling cell CD95L (FASL) binds CD95 (FAS) in responding cell
→ recruits FADD: FAS-associated death domain protein - death domain and death effector domain
→ chains of DED build up - DISC: death inducing signalling complex
→ activates procaspase 8/10 (initiator caspases) - dimerise - activation by proximity
FLIPs: reduce activation by heterodimerising (looks like caspase, lacks enzyme activity)
How are intrinsic and extrinsic apoptosis linked?
In extrinsic apoptosis CD95 and CD95L binding leads to DISC which activates CASP8/10
→ activate Bid by cleavage to generate tBid which inhibits Bcl-2 (pro-apoptotic)
→ also activate pro-apoptotic CASP3, 6, 7 pathway
