Cell Injury and its Adaptation part 1

Cell Injury I

1. Know the definitions of etiology, pathogenesis, free radical, ischemia, hypoxia, and hydropic change.

a. Etiology: the cause of disease
b. Pathogenesis: the mechanisms of development of disease
c. Free radical: atoms or molecules with a single unpaired electron in the outer orbit.  Frequently derived from oxygen.  Very unstable and very reactive and will combine readily with both inorganic and organic molecules.
d. Ischemia: loss of blood supply due to impeded arterial flow or reduced venous drainage
i. Ischemia is a common cause of hypoxia, but not the only cause.
ii. Ischemia is frequently caused by arterial blockage resulting in myocardial infarct or blockage of cerebral arteries in stroke.
e. Hypoxia: oxygen deficiency, caused by ischemic events, inabinlity to oxygenate blood (respiratory failure), or loss of blood’s ability to carry oxygen effectively (anemia)
f. Hydropic change:

2. Know the causes of cell injury or stress.
a. Hypoxia
b. Chemicals/drugs: by direct injury of a cell compartment or metabolic event or a resulting toxic metabolite or by-product
c. Microbiologic agents: infection
d. Immunologic agents: immune reactions (anaphylaxis) or complexes (autoimmune disease)
e. Genetic defects: from abnormal chromosome number affecting entire body to single point mutations that alter a single protein
f. Nutritional: both deficiencies and excesses
g. Aging: genetically programmed or progressive wear and tear

3. Understand the 4 general principles that guide our understanding of the mechanisms of disease.
a. Cell response to injury depends on the type, duration, and dose of the agent or event.
b. Cell responses are cell-specific and highly variable among different tissue types
c. 4 common subcellular targets of injury:
i. Cell membranes: cell volume and permeability loss
ii. Mitochondria: ATP production
iii. Endoplasmic Reticulum: protein synthesis
iv. Genetic apparatus: nucleic acid damage (DNA/RNA)
d. 3 common biochemical events in most forms of injury:
i. Loss of calcium homeostasis
1. Cell utilizes Ca2+, Mg2+-ATPase to maintain intracellular calcium levels that are much lower (0.1 μM) than that of the extracellular environment (1.3 mM).
2. Mitochondrial and ER calcium stores also help maintain the lower cytosolic calcium levels.
3. When membranes are damaged, extracellular calcium enters the cytosol resulting in release of both the mitochondrial and ER stores of calcium as well.
4. Elevated cytosolic calcium then activates a number of enzymes that damage membrane components, DNA, and deplete cellular ATP.
5. Depletion shuts down the membrane Na+, K+-ATPase that drives transport of sodium from the cell.
6. With increased concentrations of intracellular sodium, water enters the cell and selective membrane permeability is lost.
ii. ATP depletion
1. ATPase activity is increased in the presence of high cytosolic calcium levels.
2. Mitochondrial damage may lead to lowered production of ATP, which is essential for protein synthesis, membrane transport, and lipid turnover.
3. Mitochondria receive pyruvate, which is oxidized to acetyl CoA and finally to CO2 and H2O by the CAC.
a. Produces FADH2 and NADH, which at the inner mitochondrial membrane provide the protons that are pumped via the ETS.
b. Proton gradient formed provides the energy for the generation of ATP.
c. Usually produces 36 ATP molecules.

4. When oxygen supply decreases, cells with glycogen stores can revert to temporary dependence on the glycolytic pathway of ATP production under anaerobic conditions.
a. Elevated AMP and low oxygen increase the activity of PFK and LDH to convert glucose to pyruvate and pyruvate to lactic acid.
b. Usually produces 2 ATP molecules.
c. Liability of decreasing cellular pH as lactic acid accumulates.  Lowered pH can also damage cell components and enzyme activity.
iii. Generation of oxygen-derived free radicals
1. Normal oxidation-reduction reactions in the cell (respiration)
a. The addition of 4 electrons to molecular oxygen reduces it to water.
b. If the electron additions occur one at a time, toxic intermediate species (superoxide anion, hydrogen peroxide, and hydroxyl radical) are formed.
i. Hydroxyl radical is the most toxic.
4. Understand the mechanisms for free radical formation.
a. Presence of transition metals
i. Intracellular oxidases can generate free radicals in the presence of transition metals such as copper and iron.
ii. In the presence of superoxide anion, ferric iron (Fe3+) will accept an electron to form molecular oxygen and ferrous iron (Fe2+).
iii. The reduced ferrous iron and hydrogen peroxide produce the hydroxyl radical, hydroxyl ion, and ferric ion.
b. UV light, X-rays, and ionizing radiation hydrolyze water into hydroxyl and H free radicals.
c. Phagocytic cells generate free radicals to kill microbes.
d. Metabolism of exogenous chemicals also creates free radicals.

5. Know the 3 cell targets for free radicals and the type of damage caused.
a. DNA damage: single strand breaks in DNA may be detected by the cell. p53 signals for a prolonged G1 phase to allow repair or apoptosis if damage is severe
b. Protein cross-linking: alters enzyme activity and membrane potential
c. Lipid peroxidation of membranes (most important).  Free radicals attack double bonds in unsaturated fatty acids of membrane lipids (123º angles) by removing H from the carbons adjacent to the double bond.
i. This straightens the 123º bend and breakage occurs.

6. Chemical injury
a. 2 mechanisms:
i. Direct combination: chemicals may directly combine with an organelle or molecule
1. Ex: taxol or vincristine that directly binds to tubulin of microtubules to either stabilize or dissociate those structures preventing mitosis
ii. Conversion: chemical is converted to a reactive and sometimes more toxic metabolite by the cell’s own metabolic machinery
1. Ex: carbon tetrachloride and the P450 system of oxidases in the SER of the liver
a. P450 oxidases convert CCl4 to CCl3•, which then attacks surrounding phospholipid molecules.
b. The free radical form is more toxic than the original molecule.
c. The common substance acetaminophen is treated similarly.
d. Rapid breakdown of the ER proceeds with swelling of the SER and loss of ribosomes from the RER with concomitant loss of protein synthesis.
e. Without production of apolipoprotein, triglycerides accumulate and the liver becomes “fatty.”
f. The products of lipid peroxidation continue to damage membranes and, if the insult continues, selective permeability is lost and the cell swells.
g. This is can be and is often irreversible.

7. Understand the events in ischemia and hypoxic injury.
a. When blood supply is lost, tissue oxygen levels fall and this slows the production of ATP by oxidative phosphorylation.
b. Glycolytic pathways are upregulated, lowering intracellular pH due to accumulation of lactic acid.
c. The loss of ATP reduces the activity of both Na+, K+-ATPase and Ca2+, Mg2+-ATPase.
d. Increased concentrations of intracellular sodium cause movement of water into the cytoplasm and the cell swells.
e. Increased calcium activates damaging enzymes.  This acute cellular swelling is reversible.
f. Mitochondrial changes and damage ensues with swelling of the matrix and loss of cristae.  As cell swelling continues, the RER becomes distended and ribosomes are shed with loss of protein synthesis.
g. Calcium-induced proteases and phospholipase attack the proteins and lipids of the cell membrane and dissociate the cytoskeleton from the membrane resulting in membrane blebs and loss of microvilli.
h. Cell junctions and other cell-to-cell contacts are damaged.  Endonucleases attack the DNA of the nucleus and chromatin condenses with nuclei becoming pyknotic.

8. Why does reperfusion have the potential to cause further injury?
a. The sudden input of oxygen and white blood cells may promote free radical damage.
b. The increase of oxygen in an area of injury promotes continued free radical formation with products of the initial injury.
c. WBCs will also produce free radicals in the process of their normal phagocytic activity.

9. Ultrastructural changes in injured cells:
a. Mitochondrial changes: swelling with loss of cristae due to loss of ATP and increased water content.
i. May also fill with flocculent material or collect dense deposits of calcium and debris.
ii. May become extremely dense and shrink (calcification).
b. Membrane damage: loss of membrane ion transport pumps as a result of the loss of ATP, lytic effects resulting in leakage and loss of membrane polarity.
i. Other abnormal features include blebbing, bizarre microvilli shapes, and total loss of microvilli.
ii. Loss of cell-to-cell contacts and tight junction integrity is also common.
c. Lysosomes: More secondary lysosomes seen.  Autophagosomes may become apparent.  Residual bodies remain.
d. RER and SER: Swelling occurs in both with influx of water.
i. Loss of associated ribosomes occurs in RER.
ii. Exposure to certain drugs can cause in increase in the volume of SER.
e. Cytoskeleton: Proteases liberated by increased calcium may damage microfilaments, microtubules, and intermediate filaments.
f. Nuclear changes: Chromatin usually condenses near the periphery of the nucleus in injured cells, and the nucleus may become indented.
i. Pyknosis: with continued stress injury that is irreversible, the nucleus becomes very small and very condensed throughout
ii. Karyolysis: initial loss of density in the nucleus secondary to endonuclease activity
iii. Karyorrhexis: process where the nucleus fragments before disappearing in the dead cell

10. Understand the subcellular changes that account for hydropic change (cell swelling) seen at the light microscope level.
a. Initially, clear vacuoles are observed in the cytoplasm and cell-to-cell contacts appear to loosen as more water enters the cell and its organelles.
b. A loss of cytoplasmic basophilia is directly related to loss of ribosomes from RER and eosinophilic inclusions in the cytoplasm may be formed by aggregations of intermediate filaments.
c. Nuclear changes reflect the rearrangement of condensed chromatin.
d. In cells that metabolize fat, fatty change is the most common change detected in reversible injury.