Cell injury, inflammation and healing

Question 1

Cellular response to stress and injury

Answer 1

1) adapt to it
2) become injured. Cell injury may be reversible or irreversible, the latter of which will lead to cell death. Cell death may either be necrotic or apoptotic.

Question 2

Causes of cell injury

Answer 2

1) biological factors: pathogens (bacteria and viruses) 2) Immunological factors: cytokines (cytokine storm), sepsis, autoimmune diseases. 3) Nutritional: Vit D deficiency leading to rickets, folate deficiencies, vit V (scurvy). 4) Chemical: ROS, irradiation 5) Physical: Trauma, heat 6) Genetic: genetic abnormalities that produce misfolded proteins etc.

Question 3

Effects of cell stress and injury on cellular components

Answer 3

These effects often occur simultaneously, but they can also propogate each other.

• Decrease in cellular ATP
• Membrane damage
• Increases in intracellular calcium
• Reactive oxygen species.

Question 4

ATP depletion (Effects of injury)

Answer 4

ATP depletion can occur as a result of hypoxic injury, damage to enzymes or membrane injury for example to the mitochondria.

Loss of enzyme function: ATP is a common cofactor and provides energy for reactions to occur (that would otherwise not have neough activation energy to occur). Loss of ATP may prevent function of enzymes that repair damaged DNA and proteins.

ATP-dependent ion pumps: The sodium/potassium ATPase requies a constant source of ATP to maintain membane potentials. Loss of ATp leads to potassium efflux and sodium influx.

The influx of sodium causes the increase in water movement into the cell via osmosis.

Decreases in calcium pumps also leads to an influx of calcium into the cytosol, which will active destructive calcium-dependent enzymes (like caspases)

Decreased ATP will also reduce protein synthesis

Question 5

Membrane damage (3 types)

Answer 5

Plasma membrane

Gaps in the membrane leads to loss of ionic gradients, influx of water, loss of enzymes, proteins, ribonucleic acids, and other cellular components

Lysosomal membrane

Leads to autolysis, whereby hydrolic enzymes are released into the cytosol and digest cellular contents (protein, DNA, lipids)

Mitochondrial membrane

The formation of nonselective high conductance channels will occur in the inner mitochondrial membranes. This will

a) causes a loss of the transmembrane potential and cytochrome C required for oxidative phosphorylation
b) The loss of cytochrome C results in activation of intracellular apoptotic cascades,

Question 6

Increased intracellular calcium

Answer 6

Loss of ATP can cause ATP-dependent calcium pumps to stop working, meaning they are unable to sequester calcium into the endoplasmic reticulum or into the mitochondria. As well as unable to remove calcium inot the extracellular space.

Lipid peroxidation can also cause extracellular calcium to come into the cell.

Calcium with a potent second messanger and cofactor. it plays a role int he activation of apoptotic cascades via caspase enzymes. It activates ATPases, phospholipases, proteases, and endonucleuses. This perpetuates membrane and protein damage.

Question 7

Free radicals

Answer 7

These can be generated through irradiation, metabolism of chemicals and drugs, oxygen toxicity, inflammation, and reperfusion.

Once generated, free radicals can attack double bonds in unsaturated fatty acids (causing lipid peroxidation).

They can oxidise side chains of proteins, inducing enzymatic/protein damage.

And lastly, they can react with thymine to cause DNA damage.

Question 8

Signalling pathways activated by stress or injury (2)

Answer 8

Heat shock factors

Heat shock factors are transcription factor sthat induce the transcription and translation of heat shock proteins. These proteins are molecular chaperones that assist the folding and repair of damaged proteins.

Stress enzymes

The P38 MAP kinase and Jun N-terminal kinase enzymes initiate phosphorylation cascades.

P53 is activated by DNA damage. I twill arrest cell cyling and commit the cell to suicide if damage isn’t repaired.

BMF (actin cytoskeleton damage), Bim (microtubule damage), Bad (cell stress due to inadequate stimulation by growth factors)

Question 9

Types of cell adaption to stress

Answer 9

When the stress of injury to a cell is mild, often the cells can adapt to this pressure whilst maintaining its function (perhaps at a different level or intensity or type than before).

Examples of morphological and functional adaptations that cells can take to deal with changes in demand:

Hyperplasia: Stem cells divide, functional cells divide, or stem cells migrate to area, causing an increase in cellular numbers. Occurs when there is increased functional demand.

Hypertrophy: is when the cells increase in size, also in response to an increase in functional demand.

Atrophy: when the size of the cell shrinks, can be used to decrease metabolic demands and oxygen consumation. In a response to reduced functional demand.

Metaplasia: Is when the functionof the cell changes; cell differentiates into a different type. Can be done through signalling pathways acting on stem cells to induce reprogramming.

Question 10

Necrosis vs apoptosis

Answer 10

Necrosis

Passive form of cell death caused by trauma or other pathological conditions.

it is a proinflammatory process, that doens’t require ATP, and is unregulated. it commonly affects many cells in the vacinity of injury. Membranes are ruptured so that cellular contents psill out and induce inflammatory responses, lysosomal membrane rupture will also induce autolysis. Leading to the appearance of a featureless cytoplasm and faded or fragmented chromatin.

Apoptosis

Is a regulated, and not always pathological ATP-dependent process. The cell has been committed to suicide, and although often has been badly damaged, it still retains enough ATP and functionality to perform the regulated suicide program.

Can be extrinsically (death receptors) or intrinsically (mitochondrially) activated depending on the stimulus.

It is non-inflammatory becuase the membrane give soff blebs with cellular contents that are absorbed by neighbouring cells or surrounding macrophages, and so aren’t exposed to the immune system.

Question 11

Inflammation

Answer 11

Inflammation is the first bodily repsonse to injury. It is intertwined with cellular injury and healing. It is a dynamic process that changes with time, and has a generic/stereotyped progression of events irrespective of the tissue involved.

Question 12

Triggers of inflammation

Answer 12

• Blunt trauma
• Foreign bodies
• Pathogens (virus, bacteria, microbial toxins)
• immune reactions (hypersensitivity)
• tissue necrosis

Question 13

Acute inflammation stages

Answer 13

1. Triggering
2. Vascular pemeability and blood flow changes
3. Endothelial cell signalling changes, gene changes, adhesion molecule regulation
4. Neutrophil signalling and gene expression changes, they will adhere to endothelial cells and migrate into tissues
5. neutrophil activation, survival, function, and death.
6. inflammation may either resolve at this point or become chronic whereby other immune cells (leukocytes) are recruited.

Question 14

Importance of endothelial cells in acute inflammation

Answer 14

Endothelial cells are considered the gate-keepers of inflammation. They sense the damage, and have gene expression change sin repsonse to them, which leads to the upregulation of adhesive molecules that regulate the entry of inflammatory cells into the area of injury.

They sense damage through alpha-TNF binding to TNF receptors. This activates NFkB transcription factors, which go on to activate other genes leading to neutrophil adhesion and passage into tissues.

Question 15

Inflammation-associated gene expression and signaling changes in endothelial cells.

Answer 15

There are many many gene expression changes that occur; some of the important ones are -

• Increase in adhesion molecules: To allow recruitment of neutrophils
• Decrease in cytoskeletal stabilizers: To allow relaxation of tight junctions so that neutrophils can extravasate out of blood vessels and into tissues.
• Increase in anti-apoptotic molecules: blood cells often damaged or in harsh environments, but you want to keep them alive.
• Increase in cytokine and chemokines: To attract neutrophils
• Increase in coagulation factors: as blood vessels are often damaged in injury
• Increase in pro-angiogenesis factors: help repair damaged blood vessels.

Question 16

Blood flow and permeability changes

Answer 16

• Increase blood flow: hyperaemia (responsible for reddening of inflammed area) blood flow increases to allow more nutrients and WBH to reach site of injury
• Blood vessels dilate: This allows the increased blood flow, but it also decreases flow velocity, which helps neutrophils to adhere onto endothelial cells.
• Vascular permeability increases: spaces between endothelial cells increases to alow neutrophil extravasation, but also the release of fluid and protein to form exudate. This is a type of oedema, it stimulates lymph drainage which can tak epathogens to immunologically active lymph nodes to stimulate immune responses. it can brings in firbin (protein from blood) to create fibrous mesh which can restrict pathogen infection (I think) the fibrous exudate can also bring in antimicrobial factors and antibodies.

Question 17

Neutrophil adhesion and migration into tissues

Answer 17

• Resident macrophages in a tissue respond to a pathogen/activation by injury by secreting cytokines such as IL-1, aTNF, and chemokines.
• These cytokine and chemokine factors cause activation of the endothelial cells, which will have gene expression changes as a result. The chemokines like C5 also form a chemotactic gradient which attacts leukocytes to thsi endothelial site
• The neutrophils first undergo ‘rolling’ with weak adhesion molecules through the formation of transitory bonds. These weak transitory bonds are formed from the interaction of P and E selectins on endothelial cells with carbohydrates on glycoproteins on the leukocyte surface like siayl-lewis x.
• Following this, the cytokines cause activation of the leukocytes, which causes high-affinity integrins to be expressed (LFA-1) which bind with ICAM-1/2 on endothelial cells. This forms a strong bond that then stops the the neutrophil rolling.
• PECAMs then help the neutrophil extravasate into the tissues.

Question 18

Neutrophils killing potential

Answer 18

Neutrophils use non-specifc mechanisms of action.

When oxygen is present, neutrophils can generate ROS and free radicals to damage pathogens.

When oxygen isn’t present they can secrete damaging enzymes such as lysozyme, lactoferrin, and defensins.

Question 19

Neutrophil survivability in hypoxia

Answer 19

Neutrophils are very good at surviving hypoxia.

Hypoxia activates the “Hypoxia inducible factors system”, which in turn activates the NFkB activity. This leads to an increase in pro-survival target trascripts that promote survival of the neutrophils.

Often in normal oxygen the neutrophils will actually undergo apoptosis.

Question 20

Inflammatin over time - acute to chronic

Answer 20

If acute inflammation with neutrophils as first responders doesn’t resolve the problem, then other leukocyte types will be activated and thus start chronic inflammation. Other leukocytes such as macrophages or lymphocytes.

Oedema peaks in the first day with neutrophils steadily rising till the first day too. Neutrophils then decline and macrophages increase by day 2 (peak).

Question 21

The role of macrophages

Answer 21

Macrophages are known as monocytes whilst they circulate in the blood. Once they migrate into the tissue they become macrophages. These cells have greater phagocytic potential than neutrophils.

Macrophages also link acute inflamation to chronic inflammation by functioning as antigen presenting cells. After phagotising pathogens they will put their antigens ion their MHC molecules on their surface membranes. They are taken up my lymphatics and delivered to the lymph nodes where they function as antigen presenting cells to T cells and B cells.

Question 22

Inflammation vs immune response

Answer 22

They use many of the same cells.

The immune responses and inflammation co-exist and co-operate when the trigger for the inflammaiton is infection. They are connected by complex molecular signals.

Question 23

Granuloma formation

Answer 23

A specific example of chronic inflammation.

Has a necrotic core, with macrophage aggregates that form epithelioid macrophages and may fuse to give giant cells (good for phagocytosing very large particulate matter). Surrounding these is a collar of lymphocytes that are reacting to the core and releasing many signals to active and promote survival of the macrophages.

Question 24

Clinical manifestations of inflammation - systemic effects

Answer 24

Pyrexia (fever): Caused by the inflammatory mediates Il-1 and aTNF that are released into the circulation.

Leucocytosis: increase in production of leukocytes from the bone marrow (can be measured in blood)

Acute phase proteins from the liver and an increase in glucocorticoid steroid hormones.

Question 25

Inflammation in pathology

Answer 25

If inflammation is triggered inappropriately or not properly controlled then it can have harmful effects:

Rheumatoid arthritis, atherosclerosis, hypersensitivity reactions to insect bites, drugs, and toxins.

Hyper immune reactions - cytokine storms.

Question 26

After inflammation

Answer 26

Inflammation helps to resolve dmaage and clear debri - setting the tissue up for the healing process. Many of the cells or signalling moelcules used in inflammatory processes can also be co-opted to be used in repair.

After this the healing process is started, whereby the derbi is cleaned up and the original structures rebuilt through a mix of regeneration and repair.

Question 27

Healing process

Answer 27

Repair occurs if stem cells can’t divide or aren’t present. There is organisation of the tissue into granulation tissue, which eventaully forms a non-functional fibrous scar.

If stem cells are present and can divide, then regeneration occurs. this is when tissue structure and function is reestablished. This will end with returning some or all of the previous function.