PATH Quiz 9

Cellular Adaptation (what does it result from?)

Results from more severe physiologic stresses or pathologic stimuli that cause new/altered steady states (ie. hyperplasia, hypertrophy, atrophy, and metaplasia)

When does cell injury occur?

When limits of adaptive response are exceeded, or if the stress is directly harmful (only reversible to a certain point; irreversible injury leads to death)

Adaptations 

Reversible changes in number, size, phenotype, metabolic activity, or functions of cells

Physiologic Adaptations

Response of cells to normal stimulation by hormones or endogenous chemical mediators 

Pathologic Adaptations

Responses to stresses that allow cells to modulate structure and function to accommodate the stress and escape injury

Hypertrophy

Increase in size of cells, resulting in increase in size of organ; Adaptive response of cells with a limited capacity to divide (ex. striated myofibers of skeletal muscle and heart); Can be physiologic (ex. muscle building) or pathologic; Caused by increased functional demand or specific hormonal stimulation (ex. enlargement of uterus during pregnancy); Mechanical and trophic triggers; Eventually reaches limit beyond which cell can support (limits of vascularization, ATP production, or biosynthetic capacity)

Cardiac Hypertrophy

Initiated by activation mechanical sensors (stretch), vasoactive agents (ex. adrenalin), and growth factors (TGF-B); Activation of mechanical sensors themselves induces production of growth factors and agonists; Multiple signal pathways are initiated by these signals (PI3/AKT and GPCRs —> end result is gene transcription) 

Hyperplasia

Increase in number of cells in an organ or tissue, resulting in increased volume of organ or tissue

Physiologic Hyperplasia

Hormonal - increases functional capacity of tissue (ex. proliferation of terminal ductal epithelium of female breast during pregnancy); Compensatory - increases tissue mass after damage or partial resection (ex. proliferation after partial liver resection) 

Pathologic Hyperplasia

Excessive hormonal/growth factor stimulation (ex. endometrial hyperplasia); Important response of connective tissue in wound healing (ex. proliferating fibroblasts and blood vessels); Constitutes fertile soil in which cancerous proliferation may arise (ex. chronic hyperplastic stimuli increases the chances of something going wrong) 

Goiter

Hyperthyroidism; Caused by nutritional deficiences

Atrophy

Reduction/shrinkage in size of cells by loss of cell substance; Accompanied by autophagy (more autophagic vacuoles) 

1. Decreased workload (atrophy of disuse - ex. broken limb in cast)

2. Loss of innervation (denervation atrophy - nerve damage)

3. Diminished blood supply (ischemia)

4. Inadequate nutrition (muscle wasting)

5. Loss of endocrine stimulation 

6. Aging (senile atrophy - happens in the brain)

7. Pressure/tissue compression

8. Decreased protein synthesis and increased protein degradation (via ubiquitin-proteasome pathway) 

Metaplasia

Reversible change in which one mature cell type is replaced by another mature cell type; Cells sensitive to a particular stress are replaced by another cell type better able to withstand the stress; Arises by altered differentiation pathways of stem cells (rather than transdifferation of already differentiated cells); Important protective features of normal epithelium are lost; If stimuli persist, may predispose to malignant transformation 

Barrett’s Esophagus

Chronic acid reflux leads to epithelium of esophagus being replaced with tissue similar to tissue that lines the stomach; Highly likely to develop cancer in that area 

Reversible Cell Injury

Reversible if damaging stimulus is removed and cell can be repaired; Reversible if injurious stress becomes less intense and cell can regain normal homeostatic level; Cellular Swelling and Fatty Change

Irreversible Cell Injury

Persistant or excessive injury leading to loss of membrane function and injury to organelles (ex. mitochondria and nucleus); Characterized by Inability to reverse mitochondrial dysfunction and Profound disturbances in membrane function 

Causes of Cell Injury

Oxygen deprivation (hypoxia vs ischemia); Physical agents (mechanical trauma, extreme temp, atmospheric pressure, radiation, etc); Chemical agents and drugs (pollutants, herbicides, industrial chemicals, drugs); Infectious agents; Immunologic reaction (reactions to endogenous self agents); Genetic derangements; Nutritional imbalances 

Cellular Swelling

Results from inability to maintain ionic and fluid homeostasis; Due to failure of energy-dependent ion pumps in plasma membrane (ex. Na/K ATPase); Causes pallor, turgor, and increased weight of organs; Manifests as small, clear vacuoles in cytoplasm of cells

Fatty Change

Results from hypoxic, toxic, and metabolic injury to cells involved in or dependent on fat metabolism; Seen mainly in hepatocytes and myocardial cells; Manifests as clear, sharply distinguishable lipid vacuoles in cytoplasm 

Necrosis 

Cell death resulting from irreversible injury and displaying characteristic morphologic changes; Inability to maintain membrane integrity; Results in cell swelling (oncosis), leakage of lysosomal enzymes, cytoplasmic changes, increased eosinophilia (Cell looks more pink), loss of cytoplasmic margins, granular/homogenous appearance, myelin figures, and calcification

Necrosis Nuclear Changes

Pyknosis (nuclear shrinkage with DNA condensation); Karyorrhexis (nuclear fragmentation); and Karyolysis (nuclear dissolution) 

Cell Response to Injury Depends on ___

Nature of injury, duration, and severity (ex. small doses of a chemical toxin/brief periods of ischemia may induce irreversible cell injury, while large doses of the same toxin/more prolonged ischemia result in instantaneous cell death 

Consequences of Cell Injury Depend on____

Type, state, and adaptability of the injured cell; Hormonal, nutritional status, and metabolic needs of an individual cell are important 

Most Commonly Affected Cellular Components 

Mitochondria, cell membrane, machinery of protein synthesis and packaging, and DNA

Depletion of ATP

Fundamental cause of necrotic cell death; Can be caused by reduced supply oxygen and nutrients, mitochondrial damage, and the actions of some toxins (ex. Cyanide - disrupts oxidative phosphorylation)

How would a 5-10% reduction in ATP cause cell injury? 

1. Reduced function of active membrane pumps (Na/K ATPase)

2. Reduction in oxidative phosphorylation and a switch to anaerobic glycolysis

3. Failure of Ca2+ pumps and an influx of calcium into the cell

4. Disruption of protein synthesis and detachment of ribosomes from ER 

Mitochondrial Damage

Formation of mitochondrial pores (leads to loss of membrane potential, failure of oxidative phosphorylation, and progressive depletion of ATP); Abnormal oxidative phosphorylation (production of reactive oxygen species); Increased permeability in the outer mitochondrial membrane (leakage of mitochondrial proteins like cytochrome c and the activation of apoptosis)

Loss of Calcium Homeostasis

Cytosolic free calcium is typically maintained at very low concentrations, and most intracellular calcium is sequestered within the mitochondria and ER; Damage results in increased cytosolic calcium which enhances injury by opening mitochondrial pores, activating cytosolic enzymes, and activating apoptosis (calcium can directly activate caspases which initiate apoptosis; changes in mitochondrial permeability can release pro-apoptotic factors like cytochrome c)

Accumulation of Reactive Oxygen Species 

Free radicals have a single, unpaired electron; Unpaired electrons are highly reactive and “attack” adjacent inorganic and organic chemicals including proteins, lipids, carbohydrates, and nucleic acid; These reactions can be auto-catalytic (molecules that react with free radicals and are themselves converted into free radicals); Causes cell damage : chemical and radiation damage, ischemia-reperfusion injury, cellular aging, microbial killing by phagocytes

Generation of Free Radicals

In normal cellular respiration, small amounts of partially-reduced intermediates are produced (superoxide anion, hydrogen peroxide, and hydroxyl ions); Absorption of radiant light (UV light) can hydrolyze water into OH; Inflammation leads to rapid bursts of reactive oxygen species in activated leukocytes; Generated by metabolism of certain chemicals or drugs 

Removal of Free Radicals

Generally unstable and decay spontaneously; Antioxidants can block formation/inactivate (vitamin e and a); Iron and Copper can help generate free radicals and are bound to storage and transport proteins (transferrin and ferritin); Enzymes act as free radical-scavenging systems (ex. superoxidase dismutase; Glutathione peroxidase)

Free Radical Damage

Lipid peroxidation of membranes leads to ROS attacking bonds within fatty acid chains (damages membrane and produces reactive peroxides); Oxidative misfolding of proteins leads to ROS forming cross-links in protein backbone (leads to protein misfolding and increased proteasomal degradation); Lesions in DNA lead to ROS causing single/double stranded b breaks in DNA, crosslinks, and adducts 

Membrane Permeability and Cell Injury

Early loss of selective membrane permeability, leading to overt membrane damage

1. Depletion of ATP

2. Elevations of Ca2+ activating phospholipase

3. ROS and lipid per oxidation

4. Decreased phospholipid synthesis (defective mitochondria) 

5. Increased phospholipid breakdown (Ca2+ activation of phospholipases produces lipid breakdown products which have detergent effect)

6. Cytoskeletal abnormalities can rip apart membrane (Ca2+ activation of proteases)

Mitochondrial Membrane Damage Consequences

Opening of mitochondrial pores; Depletion of ATP and release of pro-apoptotic proteins 

Plasma Membrane Damage Consequences

Results in loss of osmotic balance/influx of fluids and ions, and a loss of cellular contents and important metabolites 

Injury to Lysosomal Membrane Consequences

Results in leakage of lysosomal enzymes including RNases, DNases, proteases, phosphates, and glucosidases; These cells die by necrosis 

Clinical Evaluation of Cell Injury

Leakage of intracellular proteins through damaged cell membranes and ultimately into bloodstream provides a means of detecting tissue-specific injury; Cardiac damage (cardyocytes contain a heart-specific contractile protein called troponin; if troponin is found within the blood, there has been damage to cardiac cells); Liver damage (bile duct epithelium contains enzyme alkaline phosphatase and transaminases, which are liver-specific)

Wrist-Worn Transdermal Troponin-1 Sensor

Cardiac troponins are recorded in people with heart disease (heart attacks); Rather than invasive blood test, wrist-worn sensors provide fast, noninvasive detection 

Repercussion Injury

Restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured, but can also exacerbate the injury and cause cell death; Ischemia-reperfusion injury contributes to tissue damage during myocardial infarction 

3 Possible Contributors to Reperfusion Injury

1. Oxidative Stress (return of oxygen may increase the production of ROS and RNS due to incomplete reduction of oxygen by damaged mitochondria or production of oxidases by WBC and other cells)

2. Intracellular Calcium Overload (return of blood flow brings more calcium to add to influx of calcium that occurs during cell injury)

3. Inflammation (return of blood flow brings inflammatory cells and chemical mediators of inflammation, resulting in recruitment of neutrophils)

Programmed Cell Death

Any form of cell death that occurs in predictable body locations, at predictable developmental periods, and as a part of predetermined development plan of an organism; Belongs to larger group of regulated cell death with tightly controlled signaling cascades and effector mechanisms (ex. deletion of cells within the tail and webbed hands of a tadpole changing into a frog)

Apoptosis

A form of programmed cell death with morphological changes that differ from necrosis; Energy-dependent; Plasma membrane of apoptotic cell remains intact, but is altered to express proteins that bind to receptors on phagocytes; NOT associated with inflammation or regional tissue injury

Apoptosis Morphologic Features

Cell shrinkage, chromatin condensation, blabbing (partitioning) into membrane-bound vesicles of cytosol and organelles (apoptotic bodies) 

Autophagic Death 

Driven by molecular machinery which drives autophagy; Autophagy should be protective to cell, but can result in cell death 

Necroptosis 

Programmed form of cell death showing morphological features similar to necrosis 

Pyroptosis

Form of regulated cell death driven by activation of inflammasome (a cytosolic multiprotein complex which releases important inflammatory cytokines) 

Ferroptosis

Iron-dependent form of non-apoptotic regulated cell death

Caspases

Family of cysteine-dependent aspartate-specific proteases; Critical in cell death, development, inflammation, and immunity; Initially exist as inactive zymogens; Active caspases have 4 domain structures and activation requires processing at specific aspartate residues to remove the N-terminal prodomain; Some have Death Effector Domain (DED) or recruitment domain (CARD)

Caspase-Dependent Forms of Cell Death

Apoptosis and Pyroptosis

Caspase-Independent Forms of Cell Death

Necroptosis and Ferroptosis

4 Groups of Caspases

Initiators - important in apoptosis (CASP 2,8,9,10); Effectors - important in apoptosis (CASP 3,6,7); Inflammatory - important in pyroptosis (CASP 1,4,11,12); Keratinization-relevant (CASP 14) 

Effector Caspases

Require cleavage by other caspases into small and large subunits that assemble into active enzymes, which can cleave substrates such as downstream caspases, cellular structural proteins, and immune molecules, to cause cell death 

Apoptosis in Embryogenesis

Physiologic; Hormone-dependent in adults; Cell deletion in proliferation cells; Death of host cells removed by neutrophils or lymphocytes; Elimination of self-reactive lymphocytes; Cytotoxic T-Cells 

Apoptosis in Pathologic Conditions

Eliminates cells that are genetically altered or irreversibly injured without eliciting a severe host inflammatory reaction; Death by apoptosis is responsible for loss of cells in DNA damage, accumulation of misfolded proteins, cell injury in certain viral infections, and atrophy in parenchymal organs 

Mechanisms of Apoptosis 

Fundamental event is activation of Caspases; Initiated by Mitochondrial Pathway (intrinsic; increased mitochondrial permeability leads to leakage of cytochrome c from damaged mitochondria into cytoplasm) and Death Receptor Pathway (extrinsic; initiated by binding of ligands to death receptors - TNF receptor family)

Release of _____ from the _______ induces apoptosis

Cytochrome C, Mitochondrial Intermembrane Space

BCL2 Family Proteins - 3 Groups

1. Anti-apoptotic (act on mitochondrial membrane to maintain impermeability - BCL2, BCL-X)

2. Pro-apoptotic (when active, oligomerize with the outer mitochondrial membrane and promote permeability - BAX, BAK)

3. Sensors (BH3 only - act as sensors of cellular stress and damage and regulate balance of BCLD and BAX/BAK)

BCL2 Family Proteins 

Humans have over 20 BCL2 proteins, each with at least one of four BH domains (BH1-BH4); Group 1 and Group 2 possess all four domains; Group 3 only possess the BH3 domain and are called BH3-only proteins; Group 3 proteins inhibit the anti-apoptotic Bcl2 proteins and stimulate the pro-apoptotic BAX/BAK

Mitochondrial Pathway of Apoptosis 

Once released into the cytosol, cytochrome c binds to APAF-1 (apoptosis-activating factor 1) forming the apoptosome; The apoptosome is able to bind to caspase-9 (the critical caspase of the mitochondrial pathway); This binding sets off a series of caspase activity by cleaving enzymes which mediates the execution phase of apoptosis; Other proteins released from the mitochondria include Smac/Diable which are also pro-apoptotic. (these proteins prevent normal cytoplasmic mechanisms that function as physiologic inhibitors of apoptosis to permit the initiation of this caspase cascade) 

Death-Receptor-Iniitated Apoptosis

Death receptors are members of the TNF receptor; Death receptors contain a death domain (cytoplasmic domain involved in protein-protein interactions) because it is essential for delivering apoptotic signals; Binding of death receptor to its ligand brings together 3 death receptors and their cytoplasmic death domains; These form a binding site for an adaptor protein called FADD that has its own death domain 

TNFR1 and Fas 

Most well-known death receptor (FasL, Fas ligand, is expressed on T cells that recognize self-antigens) 

FADD

Fas-Activated Death Domain; Adaptor protein with its own death domain; FADD bound to the death receptor activates caspase 8 (10 in humans) called the DISC (death-inducing signaling complex), which initiates an autocatalytic activation of other procaspase 8 and eventually results in activation of executioner caspases 

Execution Phase of Apoptosis

Mitochondrial pathway results in activation of initiator caspase 9; Death receptor pathway results in activation of initiator caspases 8 and 10; Caspases 8 and 10 activate the executioner caspases 3 and 6; Executioner caspases activate DNase by removing inhibitory cytoplasmic proteins and degrade structural components of the nucleus 

Apoptosis and the ER

ER may play a role as second compartment participating in triggering of apoptosis; ER ensures that only properly folded and modified proteins are passed to the Golgi network; Prolonged stress to the ER results in accumulation of misfolded proteins, which activates transcription factors like ATF3, which then stimulate production of chaperone proteins that might attempt to refold the misfolded protein (failure of chaperone proteins results in expression of proapoptotic genes); Chronic ER stress, misfolding of proteins, and apoptosis may contribute to abnormal loss of neurons in neurodegenerative diseases 

Cytotoxic T Cell-Mediated Apoptosis 

In addition to extrinsic and intrinsic pathways, cytotoxic T cells can trigger apoptosis via perforin-dependent/granzyme B-dependent pathway; Activated cytotoxic T-cells inject a pore-forming protein, perforin, and a serine protease, granzyme B, into the cytoplasm of the target viral infected cell; Granzyme B can directly cleave and activate procaspase-3, inducing apoptosis (independent of mitochondria and death receptors) 

Inhibition of Apoptosis by Extracellular Factors

Growth factors and cytokines are inhibitors (IGF-1, VEGF); Two powerful anti-apoptotic signaling pathways are PI-3 kinase-AKT and Raf-MEK-ERK; Activation of AKT in PI3 pathway results in pro-apoptotic BH3-only proteins BAD and BIM, which prevents their localization to the mitochondrial membrane and stimulation of cytochrome c; Akt can also translocate to the nucleus and activate survival-promoting transcription factors that induce expression of anti-apoptotic Bcl-2 proteins; Activated ERK phosphorylates BAD and BIM, which inhibits their pro-apoptotic activity; ERK also phosphorylates caspase 9 and inactivates it

Systemic Lupus Erythematosus

Autoimmune response to nuclear autoantigens creating chronic inflammatory lesions in many parts of the body; During apoptosis, many lupus auto antigens congregate inside the cells and are susceptible to modifications. They are considered foreign, which leads to the accumulation of apoptotic debris and the development of autoimmune responses; Apoptosis and clearance of apoptotic cells are considered key processes in etiology of SLE; Aberrant expression and function of anti and pro-apoptotic factors (BCl-2, BIM, p53) have been associated with development of lupus