Mechanisms of Disease 2 - Cell Death and Cell Damage Flashcards
What is the purpose of necrosis?
- Removes damaged cells from an organism
- Failure to do so may lead to chronic inflammation.
Why does necrosis cause acute inflammation?
Necrosis causes acute inflammation to clear cell debris via phagocytosis.
What are the causes of necrosis?
- Usually lack of blood supply e.g
- Injury
- Infection
- Cancer
- Infarction
- Inflammation
What is the process of necrosis step by step?
- Result of an injurious agent or event (whole groups of cells are affected)
- Initial events are reversible, later ones are not.
- Lack of oxygen prevents ATP production
- Cells swell due to influx of water (ATP is required for ion pumps to work)
As a result of the cell swelling:
- Changes osmolality of cytoplasm
- Puts more pressure on membrane due to expanded volume.
- Lysosomes rupture – enzymes degrade other organelles and nuclear material haphazardly
- Cellular debris released, triggering inflammation.
What are the three changes in the microscopic appearance of necrosis?
- Nuclear
- cytoplasmic
- biochemical
What are the nuclear changes to a cell during necrosis?
- Chromatin condensation/shrinkage
- Fragmentation of nucleus
- Dissolution of the chromatin by DNAase.
What are the cytoplasmic changes to a cell during necrosis?
- Opacification - protein denaturation and aggregation
- Complete digestion of cells by enzymes causing cell to liquify (liquefactive necrosis)
What are the biochemical changes to a cell during necrosis?
- Release of enzymes such as creatine kinase or lactate dehydrogenase
- Release of other proteins such as myoglobin
Why are the biochemical changes useful to study?
They measure the extent of tissue damage.
Define apoptosis
- Selective process for the deletion of superfluous, infected or transformed cells.
What is apoptosis involved in?
Involved in:
- Embryogenesis
- Metamorphosis
- Normal tissue turnover
- Endocrine-dependent tissue atrophy
- A variety of pathological conditions
Is necrosis reversible?
Initial events are reversible
Later ones are not
Is apoptosis reversible?
No
What are the differences between apoptosis and necrosis?
- Necrosis usually multiple cells at once but apoptosis is very selective.
- Apoptosis requires ATP but necrosis does not.
What is the process of apoptosis step by step?
- Programmed cell death of one or a few cells.
- Events are irreversible and energy (ATP) dependent.
- Cells shrink as the cytoskeleton is disassembled.
- Orderly packaging of organelles and nuclear fragments into membrane bound vesicles.
- New molecules are expressed on vesicle membranes that stimulate phagocytosis without an inflammatory response (so a very clean way of disposing cellular content)
What are the cytoplasmic changes to cells during apoptosis?
- Shrinkage of cell - Organelles packaged into membrane vesicles.
- Cell fragmentation - Membrane bound vesicles bud off.
- Phagocytosis of cell fragments by macrophages and adjacent cells.
- No leakage of cytosolic components.
What are the nuclear changes to cells during apoptosis?
- Nuclear chromatin condenses on nuclear membrane
- DNA cleavage
What are the biochemical changes to a cell during apoptosis?
- Expression of charged sugar molecules on outer surface of cell membranes (recognised by macrophages to enhance phagocytosis)
- Protein cleavage by proteases, caspases.
What is DNA fragmentation?
When you run chromosomal DNA out on agarose gel you get characteristic laddering.
What are some examples of apoptosis?
- Cell death in embryonic hand to form individual fingers
- Apoptosis induced by growth factor deprivation (neuronal death from lack of NGF)
- DNA damage-mediated apoptosis. If DNA is damaged due to radiation or chemo therapeutic agents, p53 accumulates. This arrests the cell cycle enabling the cell to repair the damage. If repair process fails, p53 triggers apoptosis.
- Cell death in tumours causing regression
- Cell death in viral diseases i.e viral hepatitis.
- Cell death induced by cytotoxic T cells
What are some examples of metamorphosis?
- Tadpole’s tail lost by apoptosis
- Interdigital web loss – mouse paw development. Humans too (syndactyly)
What are the balance of signals telling the cell to live or die?
- Survival signals
- Apoptosis signals
What is caspase activation?
Active caspase X activates inactive procaspase Y by cleaving it resulting in a molecule of active caspase Y.
What is the caspase cascade?
- Cleavage of nuclear lamin are nuclear envelope proteins cause the destruction of the nuclear envelope.
- You get signal amplification in the caspase cascade.
- Hundreds of substrates for activated caspases. Substrates fall into most classes of important genes.
What is the effect of caspase activation?
Caspase activation leads to characteristic morphological changes such as:
- shrinkage
- chromatin condensation
- DNA fragmentation
- plasma membrane blebbing.
What is extrinsic apoptosis induced by?
Induced by ligand binding to receptors, causing receptor dimer (or multimer-) isation.
- The ligand binds to its receptor present on the membrane. There is an intermediate protein (Death adaptor) and the caspase (Procaspase-8).
- The Protein domain called Death domain is shared in common between receptor and death adaptor (red block).
- The death effector domain is shared between death adaptor and procaspase-8.
- Similar domains dimerise or multimerise.
What is an example of extrinsic apoptosis?
Tumour Necrosis Factor brings together FAS-associated protein with death domain (FADD) in close proximity on the Tumour necrosis factor receptor which creates an environment in which oligomerisation with other domains is favoured. These death domains (FADD) in turn recruit procaspase-8.
Now a multi protein complex has been assembled which we refer to as Death-inducing signaling complex (DISC). – lots of catalytic domains of procaspase-8 in close proximity to activate themselves. Caspase 8 is produced due to autoproteolysis which gets released into cytoplasm to cause caspase cascade.
What are the molecules involved in cytochrome C-induced apoptosis?
What is cytochrome C-induced apoptosis?
- When cytochrome C is released from mitochondria it binds to its binding domain APAF-1
- bringing molecules together allowing for dimerisation and recruitment of procaspase-9 called “Apoptosome”.
- When procaspase-9 is in close proximity, it can become activated and do autoproteolysis to cleave itself into caspase-9.
How is the release of cytochrome C from the mitochondria regulated?
- A pore made of BCL-2 family of proteins.
- Some are not membrane proteins (some can be cytosolic)
- All have a BH3 domain used to form dimers.
What is an example of BAX and BCL-2?
- BAD is a pro-apoptotic protein that binds more strongly to BCL-2 than BCL-2 does to BAX.
- BAD displaces BCL-2 from the pore allowing cytochrome C to be released.
What is a transcription example of TP53?
- If there is lots of transcription of TP53, more BAX is produced which inserts itself into the membrane forming new pores.
- These cannot be blocked by anti-apoptotic family members.
- Therefore cytochrome C can be released resulting in death.
What is a phosphorylation example of growth factors affecting apoptosis?
Removal of growth factors is an extrinsic apoptosis that works with cytochrome C. Growth factors are a pro survival cell mechanism.
BAD binds to BCL-2 displacing it from the pore allowing cell death. Lots of growth factors will activate Akt/PKB to phosphorylate BAD so it cannot bind to BCL-2 so no cytochrome C is released and no apoptosis.
When you lose growth factors (survival signals) no activation of Akt/PKB so Bad is not phosphorylated and cytochrome C can still be released as BCL-2 cannot bind.
