Cellular pathology: Cell Damage and Cell Death Flashcards
What are the different causes of cell damage/death? Give an example for each cause
- Genetic causes - Abnormal no. of chromosomes (aneuploidy)
- Inflammation - Trauma
- Physical causes - Irradiation
- Traumatic damage - Interruption of blood supply
- Infection - Toxic agents
- Chemical causes - Acids/corrosives
What are the 3 basic mechanisms of cell death?
- Necrosis - Most common cause of cell death
- Apoptosis - Programmed cell death that occurs via activation of a co-ordinated, internally programmed series of events
- Autophagic cell death - Degradation of proteins involved in cellular remodelling found during metamorphasis, aging and degradation. Also involved in removal of abnormal proteins that accumulate during cancer or disease.
What are some of the causes of necrosis?
- Lack of blood supply to cells or tissues - main cause
- Injury
- Infection
- Cancer - tumour growth can compress nearby blood vessels leading to lack of blood supply
- Infarction
- Inflammation
Describe the process of Necrosis
- Cell doesn’t receive oxygen so is unable to produce ATP
- Without ATP ion channels can’t function and so can’t regulate movement of water into/out of cell
- This means lots of water begins to enter cell causing it to swell - organelles also swell
- If swelling continues to an irrevesible level then membrane begins to breakdown and cell disintergrates
- Nucleus disintergrates and lysosomes break down leading tpo release of enzymes
- These enzymes further breakdown cell organelles
Is Necrosis a reversible process?
- Yes - if cell receives oxygen before swelling reaches irreversible point then it’ll make ATP and ion channels will begin to function leading to reversal of swelling
How does necrosis lead to the production of an iflammatory response?
- Cellular debris produced via disintergration of organelles via lysosomal enzymes is detected by nearby healthy cells
- This results in these cells recruiting machrophages and other immune system cells
What changes occur in the nucleus during necrosis?
- Chromatin condensation/shrinkage
- Fragmentation of the nucleus
- Disintergration of the chromation by DNAse 1
What changes occur in the cytoplasm during necrosis?
- Opacification (becomes dark): due to denaturation of proteins
- Complete digestion of cells by enzymes causing cell to liquify (liquefactive necrosis)
What biochemical changes occur during necrosis?
- Release of enzymes e.g. creatine kinase or lactate dehydrogenase
- Release of proteins e.g. myoglobin
How can the biochemical changes that occur to a cell during necrosis be used to asses extent of tissue damage?
- Blood samples can be taken from a person and the levels of enzymes such as creatine kinase can be measured to asses extent of necrosis
what is the function of necrosis?
Removes damaged cells from an organism
What are the functions of apoptosis?
- Selects for and deletes unecessary, infected or transformed cells.
- Involved in:
- Embryogenesis
- Metamorphosis
- Normal tissue turnover
- Endocrine dependent tissue atrophy
Give an example of endocrine dependent tissue atrophy
- After a person stops breast feeding, cells that produce milk are no longer needed and so are destroyed via atrophy
Give some specific examples of apoptosis
- Cell death in the embryonic hand to form individual fingers
- Apoptisis induced by growth factor deprivation
- DNA damage mediated apoptosis
- Cell death in tumours causing regression
- Cell death in viral diseases
- Cell death induced by cytotoxic T cells
- Death of neutrophils during inflammatory response
- Death of immune cells (B and T lymphocytes) after depletion of cytokines
Give an example of apoptosis induced by growth factor deprivation
If neuronal cells aren’t surronded by a sufficient amount of nerve growth factor (NGF) then those neuronal cells will undergo apoptosis
Give an example of apoptosis induced by cytotoxic T cells
Initially cells that make up the graft will release lots of cytokines and interleukins which will trigger apoptosis in nearby tissues
What are the factors that influence a cell that will cause it to survive?
- Growth factors
- Cytokines
- Correct cell-cell or cell-matrix contacts
What are the factors that influence a cell that will cause it to undergo apoptosis?
- Death domain ligands
- DNA damaging agents
- Lack of growth factors
- Disruption of cell-cell or cell-matrix contacts
What are the 2 types of apoptosis?
- Intrinsic apoptosis - caused by intracellular factors
- Extrinsic apoptosis - caused by extracellular factors
What are some examples of things that trigger intrinsic apoptosis?
- DNA damage - triggered by p53
- Interruption of cell cycle
- Inhibition of protein synthesis
- Viral infection
- Change in redox state
Explain how caspases are activated
- Inactive procaspase Y is cleaved near the amino terminal and near the carboxy terminal by active caspase X
- This results in the production of a large and small subunit
- These 2 subunits form a dimer which leads to the formation of an active caspase Y molecule
Explain the process of the caspase cascade
- Active initiator caspase (caspase 8 or 9) will go and active caspase Y molecules
- These caspase Y molecules will themselves go and activate many molecules of caspase Z
What morphological changes occur to cells that undergo apoptosis?
- Shrinkage
- Chromatin condensation
- DNA fragmentation
- Plasma membrane blebbing
When you perform gel electrophoresis on genomic DNA that has been through apoptosis you end up with an ordered assembly of DNA fragments. Why is this?
- This occurs because duting apoptosis only the DNA that is between the nucleosomes are degraded and the nucleosomes remain intact
- This means that all the DNA wrapped around a necleosome is intact
- This means each of the fragments produced represents DNA associated with a nucleosome
When you perform gel electrophoresis on genomic DNA that has been through necrosis you end up with a smear-like pattern. Why is this?
- Due to release of enxymes form lysosomes the nucleosomes are degraded
- This means DNA is no longer wound around the nucleosmes
- Because of this the DNA is able to be cleaved at any point
Explain how ligand-induced receptor dimerization leads to initiator caspase activation
- Tumor necrosis factor (TNF) will bind to its receptor (TNFR) within the cell membrane
- TNFR has a ligand binding domain and a death domain
- Death domain then binds to form dimer with a death adaptor protein (FADD or FAS-Associated protein with Death Domain)
- Death effector domain then binds to procaspase-8
- Multiple TNFRs and FADDs come together and as a result bring multiple procaspase-8s together to form the Death inducing signalling complex (DISC)
- Procaspase-8 molecules can autoproteolyse each other to form active caspase-8
Explain how the release of cytochrome c from mitochondria leads to initiator caspase activation
- Cytochrome c binds to cytochrome c binding domain of APAF-1 protein (Apoptotic protease activation factor 1)
- As well as cytochrome c binding domain, APAF-1 has an APAF domain and a Caspase recruitment domain (CARD)
- Caspase recruitment domain binds to procaspase-9
- Two APAF-1 proteins come together to form the death-inducing complex which brings two procaspase-9 molcules together
- Procaspase-9 molecules can autoproteolyse each other to form active procaspase-9
How do pro-apoptotic members of the bcl-2 family of proteins, such as bax, allow for the release of cytochrome c from mitochondria?
- Bax proteins form homodimers with themselves to form a channel with 6 Bax proteins
- This channel sits between the inner and outer mitochondrial membranes
- This channel has a pore in the middle which cytochrome c is able to move through to allow it to travel from inside the mitochondria to outside
How do anti-apoptotic members of the bcl-2 family of proteins, such as bcl-2, prevent release of cytochrome c from the mitochondria?
- bcl-2 forms a complex with the Bax proteins
- By forming this cmplex bcl-2 blocks the pore formed by the Bax proteins which means cytochrome c is unable to leave the mitochondria
Explain specifically how the pro-apoptopic protein Bad allows for the release of cytochrome c when cytochrome c channel is blocked?
- Bad is able to bind to bcl-2 and this binding menas that bcl-2 is unable to form complex with Bax proteins
- This opens up pore within the Bax protein channel which cytochrome c is able to move through
How does the presence of survival factors prevent apoptosis from occuring?
- Survival factors activate protein kinase B (PKB or Akt)
- Protein kinase B phosphorylates Bad protein which means it’s unable to bind to bcl-2
- This means bcl-2 is stil bound to and blocks the Bax complex so cytochrome c is unable to be released from the mitochondria
- This means cytcochrome c is unable to bind to the APAF-1 protein and form the death-inducing complex
- Without death-inducing complex procaspase-9 molecules aren’t able to come together and activate each other forming active caspase-9
How does expression of Bax via p53 lead to release of cytochrome c and by extension apoptosis?
- Increased expresssion of Bax leads to more Bax protein being present in mitochondria
- This means that more Bax protein complexes can form
- Because more bcl-2 isn’t produced these new complexes aren’t blocked so cytochrome c is able to move through the pore to leave the mitochondria
- Thie eventually leads to cytochrome c forming death-inducing complex with APAF-1 which results in procaspase-9 proteins being brought into close proximity with each other
- This leads to them activating each other forming active caspase-9
