1 - Cell Injury and Cell Death

Question 1

Why does cell injury occur

Answer 1

If cells are exposed to a mild injurious agent/stress (change in environ or increase in demand) they may be able to adapt but if the agent exceeds the cells adaptive capacity cell injury will occur. Cell injury may be reversible or not and lead to cell death

Question 2

What are 6 causes of cell injury?

Answer 2

1. Physical injury - trauma, heat, radiation, decrease O2,
2. Chemical injury - pH, free radicals, O2 changes, poison
3. Biological injury - factors released from micro-organisms, factors released from IS, infection, lack of growth factors
4. Immunological Injury - hypersensitivity due to autoimmune reactions
5. Genetic Derangements - inherited genetic malformation i.e. congenital malformations or change in genetics that influences how susceptible the cell is to injury
6. Nutritional Imbalances - protein or calorie deficiencies, anorexia, excess lipid or cholesterol leading to atherosclerosis. It can also alter how cells respond to injury

Question 3

Summary (flow diagram) of the effects of stress and cell injury on individual cells

Answer 3

Injury/ Stress Stimuli
> Decrease in ATP = Loss of energy dependent functions
> Membrane Damage > Mitochondria =
Decrease in ATP and cell death
> Lysosome =
Enzymatic digestion of cellular components
> Plasma =
Loss of cellular components
> Increase in intracellular Ca = Protein break down and DNA damage
> Increase in reactive oxygen species = Protein breakdown and DNA damage

Question 4

Does cell injury impact one cell component?

Answer 4

No, when injured/stressed a stimulus often targets and affects several cellular components rather than one isolated part of the cell. This is known as synergistic events. All diseases involve some type of cell injury

Question 5

How does a decrease in ATP production affect cells and how does it occur?

Answer 5

1. Occurs in cell injury by a decrease in O2, damage to enzymes in the cytoplasm and damage to mitochondria
2. • Decrease in ATP available to enzymes the repair DNA and proteins
     - Less ATP to ATP driven membrane ion pump; important to control the ionic/osmotic homeostasis in cells and organelles. If these pumps are less active then water and Na will accumulate in the cells/organelle causing swelling. Intracellular Ca will increase causing damage to cellular components by signalling destructive enzymes
     - Less ATP for protein synthesis and causes ribosomes ro detach from the ER

Question 6

How does damage to cell membranes occur and how does it affect cells?

Answer 6

Damaged by ROS/free radicals, low oxygen/hypoxia, membrane targeting bacterial toxins, failure of the Ca pump

1. Plasma:
   - Loss of cellular components (proteins, enzymes, RNA), loss of osmotic balance due to solutes moving and in/outflux of ions
   - transmembrane pumps injured OR function inhibited by drop in ATP causing disruption to osmotic balance
2. Lysomes:
   - Leakage of enzymes into cytoplasm and digestion of cellular components (autolysis) causing the cell to become red and homogenous
3. Decrease in membrane phospholipids as phospholipase are activated during cell injury as well as a decrease in ATP decreasing lipid synthesis
4. Mitochondria:
   - Damage results in the formation of non-selective high conductance channels in the inner membrane i.e. the Mitochondria Permeability Transition and this removes the transmembrane potential needed for ox phos to occur.
   - allows leakage of cytochrome c into the cytosol which primes the cell for apoptosis (its loss from the mito also causes and decrease in ox phos and so ATP production)

Question 7

How does an increase in cytosolic Ca occur and affect cells?

Answer 7

May be release from extracellular, ER, mitochondria. Increase in cell due to a decrease in activity of the membrane Ca pump. Destructive enzymes are then activated

1. ATPases - further depleting ATP
2. Phospholipase’s - damage to lipid components in membrane causing more Ca release)
3. Proteases - membrane and cytoskeletal breakdown
4. Endonucleases - damage DNA

Question 8

What are free radicals/ROS?

Answer 8

Is a secondary form of injury in cells exposed to stressors
They are reactive molecules with an unpaired free electron in the outer orbit which release energy through reactions with adjacent molecules. An accumulation of these causes oxidative stress.
They are generated by radiation, metabolism of chemicals, O2 toxicity, inflammation, repurfusion (lack blood and suddenly comes back) and normal metabolism

Question 9

Hyperplasia

Answer 9

When glandular epithelium needs to do more work so increases the cell number

Question 10

What do cells undergoing autolysis look like?

Answer 10

Red and homogenous due to denatured proteins and chewed up DNA as are destroyed by own cells enzymes

Question 11

Acute Inflammation

Answer 11

The body’s NON-SPECIFIC response to cell injury

Question 12

How do free radicals cause injury to cells?

Answer 12

1. Attack double bonds in unsaturated fatty acids so disturbs membranes (membrane potential/cellular components
2. Oxidises amino acids side chains so decreases enzyme function
3. Reacts with thymine to cause DNA damage
   This initial damage leads to autocatalytic reactions where molecules they react with are converted in to ROS themselves and chain of damage is propagated

Question 13

How to protect against free radicals?

Answer 13

Cells have defensive mechanisms in place to prevent injury from ROS’s. Antioxidants (Vit E, A, C…) and enzymes such as catalase.

Question 14

How does damage to proteins occurs and how does it affect cells?

Answer 14

Can be injured directly by free radicals or increase in Ca dependent proteases, but also by a decrease in protein synthesis by depletion of ATP. May also be damaged by GLYCATION as occurs in diabetes. Damage to cytoskeletal proteins may detach the membrane from the cell/cytoskeleton

Question 15

How does damage to DNA occur and affect cells?

Answer 15

Ionising radiation (X/gamma rays) - break chemical and DNA bonds
UV radiation - structural changes in DNA bases which may lead to cell death (sunburn)
Chemicals, ROS, genetic causes, nutritional deficiencies of B12/folic acid. 
Cells have evolved repair enzymes as DNA is constantly damaged

Question 16

Signalling pathways initiate the cells response to injury by sensing the degree of damage. This is often accompanied by the activation of … AND …

Answer 16

Heat Shock Factors and Stress Kinases

Question 17

What are heat shock factors?

Answer 17

Heat shock factors are transcription factors that induce expression of heat shock proteins which are molecular chaperones; they bind to damaged proteins, help refold and hold the protein together until they can be repaired.

Question 18

Stress Kinases?

Answer 18

For example p38 MAP kinase and Jun N-Terminal Kinase.
These initiate phosphorylation signalling cascades that co-ordinate and amplify (more enzymes involved at each stage) a cell’s response to damage. Specific damage activates specific cascades.
- p53; DNA damage and STOPS cell division to allow repair or will induce suicide
- BMF; damage to actin cytoskeleton
- BIM; damaged microtubules
- Bad; inadequate stimulation by growth factors

Question 19

When may a cell show an adaptive response?

Answer 19

Mild injurious agent or mild stress - increased workload, altered environment, chronic irritant. Adapt to allow the cell to continue to function without being injured despite the stress or change in enviorment

Question 20

What are the 3 adaptive responses?

Answer 20

Hypertrophy/hyperplasia, Atrophy and Metaplasia

Question 21

Hypertrophy/hyperplasia

Answer 21

Hypertrophy refers to an increase in the SIZE of cells, hyperplasia is an increase in the number of cells especially in glandular epithelium in order to make more substance. Is in a response to increased functional demand and stimulation. Intracellular signalling pathways are stimulated and activate lots of cellular genes. Hyperplasia involves the division of cells as well as STEM cells that move into the adapting tissue from BONE MARROW

Question 22

Atrophy

Answer 22

Due to decrease in functional demand, nutrients, growth factors, stimulation through nerves and results in cell SHRINKAGE (atrophy). Involves proteolytic systems like lysosomes (autophagy - cellular components digested) and a cellular destruction machine called ubiquitin-proteasome pathway

Question 23

Metaplasia

Answer 23

Reversibly changing from one cell type to another when a type of cell is better able to withstand the adverse environment.
E.G. oesophagus exposed to ^ gastric acid then the stratified epithelium will be replaced by reprogrammed stem cells into glandular epithelium to better withstand the environment
Results from reprogramming the stem cells via signals of growth factors and ECM components. The reprogrammed stem cells then differentiate along a new pathway via complex molecular cascades

Question 24

When does cell death occur

Answer 24

Injury/stress is too severe or persist too long and the cell has an inability to reverse damage.

Question 25

Name and describe the means in which a cell with irreversible injury would die if the injury was overwhelming and had little energy

Answer 25

Necrosis/oncosis
Occurs when cells die passively with no energy as there is overwhelming damage.
The cell releases damaging enzymes resulting in enzymatic digestion and LEAKAGE of cellular components as there is often lysosomal membrane rupture (autolysis) resulting in leakage of lysosomal enzymes into the EXTRAcellular space which will cause an INFLAMMATION RESPONSE
Common if the mitochondria is damaged or there is a lack of oxygen (need to make energy)
This process often affects large numbers of cells at a time
Messy, UNCONTROLLED process that is always PATHOLOGICAL
Nuclei often show fragmentation of chromatin, and fading of chromatin
Cytoplasm is FEATURELESS due to denaturation and digestion of proteins
Cells lose structure (homogenous shape), eosinophilic, (red) due to proteins
Energy must be invested into cleaning up the mess
Necrotic cells usually fragment and debris removed by phagocytosis - if not can cause the precipitation of calcium causing dystrophic calcification

Question 26

Describe programmed cell death

Answer 26

Apoptosis
Genetically programmed and controlled cell death
Requires a lot of energy which isn’t available when cells are greatly injured
As it requires a lot of energy the first step is the destruction of ATP consuming processes like DNA repair enzymes
Common after DNA damage, when there is a loss of growth factors, mild hypoxia, accumulation of damaged proteins
ALSO used not during cell injury but to eliminate wrongly placed and unwanted cells during embryogenesis, self reactive immune cells, cells infected with virus
Chromatin is cleaved and condenses, and membrane blebs containing cytosolic contents and organelles break off the cells and are phagocytosed
NO cellular contents leak into ECSpace and so no inflammation response is activated
Caspaze enzymes mediate process by proteolytic cleavage removing an inhibitory domain. Executioner cascades digest cellular proteins and activate other enzymes
Appearance = membrane blebs, shrinkage of nucleus and condensation (pyknosis)

Question 27

How many pathways are there for apoptosis initiation?

Answer 27

2. Cause activation of a cascade of upstream and downstream caspases. Both of these pathways feed into a cascade of caspase enzymes called executioner caspases (kill cells)