Cell death Flashcards
Autophagy
Autophagy is a cell survival mechanism that aims to derive energy by eliminating defective organelles, very rarely will autophagy result in cell death.
Give some different mechanisms of cell injury
Loss of cell membrane integrity: breakdown of concentration gradients of ions and metabolites
Decreased ATP concentration: rapid shutdown of homeostatic pathways
Protein unfolding and protein aggregation
Generation of ROS: activates multiple signalling pathways, kinases, phosphatases, NF-kB, phospholipases
Protein trafficking defects
Cation imbalance (Na+, Ca2+, H+)
Loss of integrity of the genome
Mitochondrial dysfunction: increase in membrane permeability, leads to H+ leakage which dissipates the proton-motive force and acidification of the cytosol
How are organelles recycled in autophagy?
Autophagosomes are generated and fuse with a lysosome containing cellular material. This is a autophagolysosome, the products can be recycled for ATP production and protein synthesis.
What is apoptosis?
Apoptosis is ‘programmed cell death’ directed by expression of specific genes. It does not contribute to inflammation.
Signs of apoptosis
Membrane blebbing
Cell shrinkage
Nuclear fragmentation
Chromatin condensation
Chromosomal DNA fragmentation
Global mRNA decay
Intrinsic apoptosis mechanism
Activated by intracellular signals generated when cells are stressed and depends on the release of proteins from the intermembrane space of mitochondria. Or can be initiated by developmental programming / lack of specific growth factor.
Extrinsic apoptosis mechanism
The extrinsic pathway is activated by extracellular ligands binding to cell-surface death receptors, which leads to the formation of the death-inducing signaling complex (DISC).
Caspase activation
Two pathways of extrinsic apoptosis
The binding of TNF-alpha to TNFR1 has been shown to initiate the pathway that leads to caspase activation
The Fas-Fas ligand-mediated model The interaction between Fas and FasL results in the formation of the death-inducing signaling complex (DISC), which contains the FADD, caspase-8 and caspase-10.
Caspase 8 cleaves caspase 3 which cleaves I-CAD the inhibitor of CAD which is released into the nucleus to cleave DNA
Describe how caspases cause apoptosis
Caspases cleave key enzymes involved in DNA repair and replication as well as key structural proteins that maintain nuclear architecture.
PARP: The ability of the enzyme PARP (poly (ADP-ribose) polymerase) to repair DNA damage is prevented following cleavage of PARP by caspase-3.
DNA topoisomerase II is a nuclear enzyme essential for DNA replication and repair. Caspases can inactivate this enzyme leading to DNA damage.
Lamins are intra-nuclear proteins that maintain the shape of the nucleus and mediate interactions between chromatin and the nuclear membrane. Degradation of lamins by caspase 6 results in chromatin condensation and nuclear fragmentation.
Why may some tissues express FAS-L
Confers immune privilege to the organ - immune cells killed that are nearby
What caspase degrades lamins?
Caspase 6
What caspase cleaves PARP?
Caspase 3
What caspase cleaves I-CAD
Caspase 3 (cleaved by caspase 8)
What is necrosis?
Necrosis is a sequence of morphological changes that follow cell death following irreversible injury in living tissue.
What does necrosis promote?
Inflammation
What is pyknosis?
Shrunken basophilic nucleus (less visible chromatin)
What is Karyorrhexis?
More basophilic chromatin, fragmented nucleus (appears almost granular)
What is Karyolysis?
Loss of basophilic chromatin - appears to have no nucleus
What happens when macrophages interact with apoptotic cells?
When macrophages interact with apoptotic cells (bind with CD-14/ICAM-3 and PS interactions). Macrophage expression is altered, production of anti-inflammatory cytokines (TGF-B1) and down-regulation of inflammatory cytokines.
What happens when macrophages interact with necrotic cells?
When macrophages interact with necrotic cells via opsonin complement receptor interactions there is upregulation of pro-inflammatory cytokines (IL-1B, IL-8)
Why do apoptotic cells not promote inflammation but necrotic ones do?
Blebbing leads to the hiding of endogenous DAMPs, apoptotic cells are phagocytosed.
Necrotic membrane lysis leads to the display and/or release of these intracellular molecules. Exposure of certain intracellular molecules (DAMPs) can trigger inflammation via innate immune cells.
What is pyroptosis?
Pyroptosis is carried out in a deliberate and highly self-programmed process that generally
involves innate immune cells and activation of inflammasomes.
How is the inflammasome activated?
TLR signalling leads to NFKB stimulations, leads to transcription of a large number of genes in response to inflammatory stimuli
These mRNA are translated into protein for pro-IL-1B and caspases.
Proteins could be secreted, but must be first cleaved by active cysteine protease (caspase).
Inactive NLRP3 inflammasome activated - leads to cleavage of caspases - and cleavages and release of IL-1b and IL-18.
Leads to pyroptosis
What pathogens is the inflammasome specifically important for?
Important for intracellular pathogens
What caspases does the inflammasome activate?
1,4 and 5
What is produced by the inflammasome that leads to pyroptosis?
Pore forming gasdermins
What is cellular senescence?
‘permanent’ arrest of the cell division cycle.
It is associated with hyperactivated secretion of pro-inflammatory factors that affect a range of patho-physiological processes such as impaired wound healing, cancer and aging.
