12. Cell Damage & Cell Death Flashcards
What are the three basic mechanisms of cell death?
1) Necrosis
- Most common cause of cell death. Occurs after stresses such as ischemia, trauma, chemical injury
=> CAUSES:
- lack of blood supply to cells or tissues
- injury,
- infection,
- cancer,
- infarction,
- inflammation
2) Apoptosis
- Programmed cell death. Designed to eliminate unwanted host cells through activation of a co-ordinated, internally programmed series of events effected by a dedicated set of gene products
3) Autophagic cell death
- Autophagy is responsible for the degradation of normal proteins involved in cellular remodeling found during metamorphosis, aging and differentiation
- As well as for the digestion and removal of abnormal proteins that would otherwise accumulate following toxin exposure, cancer, or disease.
- An example is the death of breast cancer cells induced by Tamoxifen.
What are the steps to necrosis?
- Whole groups of cells are affected.
- Result of an injurious agent or event.
- Reversible events proceed irreversible
- Energy deprivation causes changes. (e.g. cells unable to produce ATP because of oxygen deprivation)
- Cells swell due to influx of water (ATP is required for ion pumps to work).
- Haphazard destruction of organelles and nuclear material by enzymes from ruptured lysosomes.
- Cellular debris stimulates an inflammatory cell response
What are the different microscopic changes in necrosis?
NUCLEAR CHANGES:
- Chromatin condensation/shrinkage.
- Fragmentation of nucleus.
- Dissolution of the chromatin by DNAse
CYTOPLASMIC CHANGES:
- Opacification: denaturation of proteins with aggregation.
- Complete digestion of cells by enzymes causing cell to liquify (liquefactive necrosis).
BIOCHEMICAL CHANGES:
- Release of enzymes such as creatine kinase or lactate dehydrogenase
- Release of proteins such as myoglobin
- These biochemical changes are useful in the clinic to measure the extent of tissue damage!
What is the function of necrosis?
- Removes damaged cells from an organism
- Failure to do so may lead to chronic inflammation.
List some functions of apoptosis.
- 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
List 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 (tumour suppressor gene product) accumulates. This arrests the cell cycle enabling the cell repair the damage. If repair process fails, p53 triggers apoptosis.
- Cell death in tumours causing regression
- Cell death in viral diseases (ie viral hepatitis).
- Cell death induced by cytotoxic T cells (ie. Cellular immune rejection or graft vs. host disease).
- Death of neutrophils during an acute inflammatory response
- Death of immune cells (both T and B lymphocytes) after depletion of cytokines as well of death of autoreactive T cells in the developing thymus..
List some factors influencing the balance of life and death at the cellular level.
SURVIVAL:
- cell-cell and/or cell-matrix contacts
- growth factors
- cytokines
APOPTOSIS:
- disruption of cell-cell and/or cell-matrix contacts
- lack of growth factors
- DNA damaging agents
- death domain ligands
Describe 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 growth factors (e.g. IL-3)
- Extracellular signals (e.g. TNF)
- T cell or NK (Natural Killer) (e.g. Granzyme).
Describe caspases (cysteine aspartate-specific proteases).
- Caspases are Cysteine Proteases that play a central role in the initiation of apoptosis.
- Most proteases are synthesised as inactive precursors requiring activation (usually partial digestion by another protease).
Apoptosis is mediated by an intracellular proteolytic cascade. Expand.
1) Caspase is activated. - An inactive procaspase Y is activated by an activated capsapse X. X cleaves the domain off, which deems it active.
2) Caspase X can activate many more caspase Ys, which go on to activate even more molecules of caspase Z. Thus, we have an increased number of effector caspases, known as the caspase cascade.
- active caspase Y cleaves cystolic proteins?
- active caspase Z cleaves nuclear lamin?
What changes does caspase activation cause in the cell?
Caspase activation leads to characteristic morphological changes of the cell such as shrinkage, chromatin condensation, DNA fragmentation and plasma membrane blebbing.
- There are intact organelles in the blebs
- The blebs can dissociate
How can you differentiate different kinds of cell death based on DNA fragmentation?
- You would take a sample of purified DNA from the cell a in question and run it through gel electrophoresis.
- Cells that are normal will have their DNA at one specific band.
- Cells that are apoptosing will have multiple bands of decreasing levels at certain intervals, while cells that are necrosing will have continuously decreasing DNA.
Describe the process of apoptosis,
- Single or few cells selected.
- Programmed cell death.
- Irreversible once initiated.
- Events are energy driven.
- Cells shrink as the cytoskeleton is disassembled.
- Orderly packaging of organelles and nuclear fragments in membrane bound vesicles.
- New molecules expressed on vesicle membranes stimulate phagocytosis, no inflammatory response.
What are the different microscopic changes in apoptosis?
NUCLEAR CHANGES:
- Nuclear chromatin condenses on nuclear membrane.
- DNA cleavage.
CYTOPLASMIC CHANGES:
- 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.
BIOCHEMICAL CHANGES:
1. Expression of charged sugar molecules on outer surface of cell
membranes (recognised by macrophages to enhance phagocytosis)
2. Protein cleavage by proteases, caspaseS
How do we activates the initiator caspases?
- By induced proximity.
For example:
- In response to receptor dimerization upon ligand binding or
- Cytochrome C release from the mitochondria.
