Mechanisms of Disease II - Cell Death and Cell Damage Flashcards
What is the function of necrosis
Removes damaged cells from an organism
Failure to do so may lead to chronic inflammation
Necrosis causes acute inflammation to clear cell debris via phagocytosis
What causes necrosis
Usually lack of blood supply as a result of:
- injury
- infection
- cancer
- infarction
- inflammation
Describe the step-by-step process of necrosis
- Result of an injurious agent or event.
- 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).
- Lysosomes rupture; enzymes degrade other organelles and nuclear material hapzardly
- Cellular debris released, triggering inflammation
What are the nuclear changes in necrosis
- Chromatin condensation/shrinkage.
- Fragmentation of nucleus.
- Dissolution of the chromatin by DNAse
What are the cytoplasmic changes in necrosis
- Opacification: protein denaturation & aggregation.
- Complete digestion of cells by enzymes causing cell to liquify (liquefactive necrosis).
What are the biochemical changes in necrosis
- Release of enzymes such as creatine kinase or lactate dehydrogenase
- Release of other proteins such as myoglobin
These biochemical changes are useful in the clinic to measure the extent of tissue damage!
What is Apoptosis and what are its functions
Selective process for the deletion of superfluous, infected or transformed cells.
Involved in:
- Embryogenesis
- Metamorphosis
- Normal tissue turnover
- Endocrine-dependent tissue atrophy
- A variety of pathological conditions
Describe the step-by-step process of Apoptosis
- 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.
What are the cytoplasmic changes of Apoptosis
- Shrinkage of cell. Organelles packaged into membrane vesicles.
- Cell fragmentation. Membrane bound vesicles bud off.
- Phagocytosis of cell fragments by macrophage and adjacent cell.
- No leakage of cytosolic components.
What are the nuclear changes in Apoptosis
- Nuclear chromatin condenses on nuclear membrane.
- DNA cleavage.
What are the biochemical changes in Apoptosis
- Expression of charged sugar molecules on outer surface of cell membranes (recognised by macrophages to enhance phagocytosis)
- Protein cleavage by proteases, caspases
Describe how normal, apoptic and necrotic DNA would look like in DNA fragmentation
Normal - No banding
Apoptic - distinct bands visible
Necrotic - Smear like banding pattern
What are some examples of Apoptosis
Metamorphosis - Tadpole’s tail is lost
Interdigital web loss in mouse paw development
What are the two types of apoptosis
Intrinsic:
DNA damage – p53-dependent pathway
Interruption of the cell cycle
Inhibition of protein synthesis
Viral Infection
Change in redox state
Extrinsic:
Withdrawal of survival factors e.g. mitogens
Extracellular signals (e.g. TNF)
T cell or NK (Natural Killer) (e.g. Granzyme).
What are caspases
Caspases are cysteine protease
Caspases are the point of convergence for causes of apoptosis.
They form an activation cascade, where one cleaves and activates the next (analogous to kinase cascades)
Describe caspase activation
Inactive procaspase-Y is cleaved at 2 sites by active caspase. This yeilds an active caspase-Y
Describe the caspase cascade
Initiator caspases (8&9) few molecules, cleave processes into their active form.
They then continue cleaving more procaspases to form a cascade. The newly formed caspases are called effectors
What is the effect of caspase activation
Caspase activation leads to characteristic morphological changes, such as shrinkage, chromatin condensation, DNA fragmentation and plasma membrane blebbing.
How is the initiator caspase activated
Initiator caspases activate themselves when in close proximity
Activation, therefore, means bringing initiator caspases together
How is the extrinsic apoptosis pathway activated
Induced by ligand binding to receptors, causing receptor dimer- (or multimer-) isation
Explain the Extrinsic Apoptosis pathway
When an extrinsic factor binds to its receptor, it brings death domains closer together and creates an environment for death adapters to join them. This leads to an increase in death effectors, which allows procaspase-8 to join them as well. The proximity of these components enables them to undergo auto-proteolysis in that environment, which activates the caspase.
How is the intrinsic pathway activated
Induced by cytochrome c released from mitochondria
Note: growth factor withdrawl (extrinsic apoptosis) an exception that uses cytochrome c
What is cytochrome c
Mitochondrial matrix protein
Known for many years to be released in response to oxidative stress by a “permeability transition”
Any inducers of the permeability transition also eventually induce apoptosis.
Explain the intrinsic Apoptosis pathway
Cytochrome C binds its receptor which brings together caspase recruitment domains together. This causes procaspase 9 to oligomerise with the domain and is brought near each other. They undergo auto-proteolysis which activates it.
How is the release of cytochrome c regulated from the mitochondria
A family of proteins called the BCL-2 family form a pore on the mitochondrial membrane which allows cytochrome c to leave via it.
These proteins can either be pro or anti apoptotic.
Explain how the Bax protein works
Bax proteins form a pore which allows cytochrome c to pass through. this is pro-apoptotic
Explain how the Bax + BCL-2 protein works
BCL-2 binds to the spore opening and stops cytochrome c from leaving.
Anti-apoptic
Explain how the Bax + BCL-2 + BAD protein works
BAD binds to BCL-2 preventing BCL-2 to bind to the pore.
Pro-apoptic
What regulates the BCL-2 family proteins
Transcription factors and growth factors
How does TP53 cause apoptosis
Transcription driven by TP53 causes more pores to be inserted into the membrane.
Causes more cytochrome c to leave causing apoptosis
How do Growth factors regulate BCL-2 family proteins
Growth factors cause phosphorylation of BAD which stops it from binding BCL-2 which means the pores are blocked so no cytochrome c release.
