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Lecture 1: Cell Injury
Introduction:
Pathology: The causes of disease and the changes in cells, tissues, and organs associated with development of disease.
Etiology: The origin of a disease, including the underlying causes and modifying factors.
Example: Infection, toxin, genetic mutation, hypoxia, ischemia, abnormal immune reactions, nutritional imbalances, physical agents.
Pathogenesis: Steps in disease development.
Example: Biochemical changes or structural changes
Cellular Responses:
Homeostasis: The steady state of internal physical and chemical conditions! The key idea is that there is NO STRESS.
Adaptation: A new steady state that preserves viability and function. Occurs when a cell becomes stressed so it must adjust shape, function, or size to stay alive.
Reversible Injury: This is when the cell undergoes stress but if the stress is removed, the cell can revert to homeostasis.
Irreversible Injury: This is when the cell undergoes severe and progressive stress that cannot be reversed, leading to cell death,
Types of Cellular Stress
Oxidative Stress
Oxidative stress is characterized by cellular damage caused by the accumulation of reactive oxygen species (ROS).
Reactive Oxygen Species: Unstable molecules that damage DNA, proteins, and membranes .
Damage caused in DNA: ROS can react with DNA bases causing mutations and breaks.
Damage caused in Proteins: ROS can induce crosslinks between protein molecules, impairing their function.
Damage caused in Membranes: ROS can directly oxidize amino acid side chains within proteins, altering their structure and function
ROS are also called “free radicals” and they can come from radiation, toxins, inflammation, or ischemia. The levels of these free radicals are determined by rates of production and removal. Note that ROS’s in small amounts can be used for signaling but too many cause injury indicating they are tightly regulated.
Levels of ROS:
Free radicals may be removed enzymatically or by antioxidants such as glutathione, peroxidase, and catalase.
Free radicals may be increased through injuries which either increases the rate of production or decreases clearance of ROS.
Causes of Oxidative Stress:
Chemical Injury
Radiation
Hypoxia
Cellular Aging
Tissue injury caused by inflammatory cells
Ischemia-reperfusion injury
ER Stress & UPR
Recall: Endoplasmic Reticulum: A network of membrane-bound tubes and sacs in eukaryotic cells involved in the synthesis and transport of molecules.
Rough ER: Makes and folds proteins for secretion or membrane use.
Smooth ER: Makes lipids, detoxifies chemicals, and stores calcium.
In short, the ER folds newly made proteins and ensures only properly folded proteins move on.
Injuries to the ER can lead to problems with protein folding. Abnormalities that increase the production of misfolded proteins or reduce the ability to eliminate them may also cause problems. High levels of misfolded protein can also trigger apoptosis via the mitochondrial intrinsic pathway
Protein misfolding within cells may cause disease by creating a deficiency of an essential protein (loss of function), by inducing apoptosis, or by gaining a toxic function (gain of function)
Misfolded proteins in the ER activate the unfolded protein response (UPR) via sensors such as IRE1. UPR then increases chaperone expression, reduces protein synthesis, and increases protein degradation to reduce the load of misfolded proteins.
UPR is helpful short-term but prolonged activation is harmful.
The Ubiquitin Proteasome System is the main system the cell uses to clear misfolded proteins. It tags proteins using ubiquitin ligases and then sends them to the proteasome to be degraded (the proteasome recognizes the ubiquitinated proteins, unfolds them, removes the ubiquitin tags, and then breaks them down into small peptides). This prevents the accumulation of toxic protein aggregates so the disruption of this system plays a key role in disease such as neurodegenerative diseases.
The UPR tries to control things and rebalance them by stopping the influx of new proteins and promoting degradation pathways. The UPS is actually the destroyer of the proteins. UPR is a stress response system (sensing + signaling). UPS is a degradation machinery (actual destruction of proteins).
Disruption of Calcium Homeostasis
Calcium (Ca2+) is an important second messenger and a source of cell injury. Normally, intracellular Ca2+ is kept at low concentrations while extracellular Ca is kept high.
Injuries such as ischemia and certain toxins may cause high concentrations of intracellular Ca2+ which can disrupt multiple signaling pathways and activate enzymes such as proteases and phospholipases that damage cellular components such as the plasma membrane and cytoskeleton.
Cellular Adaptations to Stress
Adaptations are reversible changes in the number, size, phenotype, metabolic activity, or functions of cells to help survive stress
Physiologic Adaptations: A normal response to hormonal stimulation or increased demand of mechanical stress.
Pathologic Adaptation: An abnormal response to stimuli which allows cells to adapt their structure and function to escape injury BUT at an expense.
Hypertropy: An increase in cell and organ size in response to increased workload, induced by growth factors. This occurs in tissues incapable of cell division.
Key features:
Bigger cells, no increase in cell number.
Can be physiologic (normal adaptation) or pathologic (disease-related).
May progress to injury if stress is chronic/maladaptive.
Examples:
Physiologic Example: Enlargement of the uterus during pregnancy (combination of hypertrophy and hyperplasia).
Pathologic Example: Hypertrophy of the heart
Hyperplasia: An increase in cell numbers in response to hormones and growth factors. Occurs in tissues whose cells can divide or contain stem cells.
Key features:
A form of cell proliferation.
Often occurs with hypertrophy under the same stimulus.
Can be physiologic or pathologic.
Examples:
Physiologic:
Hormonal: breast epithelium proliferation during puberty & pregnancy.
Compensatory: liver regeneration after partial resection.
Pathologic:
Endometrial hyperplasia (from unopposed estrogen).
Benign prostatic hyperplasia (BPH, from androgen/estrogen imbalance).
Atrophy: A decrease in cell and organ size due to pathological and physiologic causes including:
Decreased workload (disuse)
Loss of innervation (the disconnection of nerves from their target muscles, causing the muscle to weaken and shrink)
Diminished blood supply
Poor nutrition
Loss of endocrine stimulation
Aging
Atrophy results from a combination of decreased protein synthesis and increased UPS. It is also accomplished by increased autophagy
If atrophy worsens, affected cells may pass a threshold and undergo apoptosis.
Metaplasia: A change in one adult cell type to another adult cell type. This arises from reprogramming of stem cells rather than phenotypic change. Metaplasia comes with a high risk of malignant transformation; if the stimulus that causes metaplasia continues it can lead to the development of cancer.
Examples:
Respiratory epithelium of smokers: normal ciliated columnar become stratified squamous epithelium.
Chronic gastric reflux: normal stratified squamous epithelium of the lower esophagus undergo metaplastic transformation to gastric or intestinal-type columnar epithelium
Mechanisms of Cell Injury and Cell Death
Causes of Injuries:
The cellular response to injuries, which usually results from functional and biochemical abnormalities in one or more essential cellular components, depends on:
The type of injury
The duration of the injury
The severity of the injury
The type of cell (including its metabolic state, adaptability, and genetic makeup).
For example, neurons tolerate very short ischemia compared to skeletal muscle.
There are two types of cell injury:
Reversible Cell Injury: Injury that cells can recover from IF the damaging stimulus is removed.
Cellular Swelling
Fatty Changes
Irreversible Cell Injury: The inability of a cell to restore mitochondrial (ATP) function even after the stimulus is removed. This leads to altered structure and loss of function of plasma and intracellular membranes. DNA and chromatin structure becomes damaged beyond repair.
Mechanisms of Injury
Mitochondrial Dysfunction and Damage
Mitochondria are the main source of ATP, they control apoptosis through the release of cytochrome-c, and they generate ROS.
Causes of Mitochondrial Dysfunction & Damage:
Hypoxia, Ischemia, Mitochondrial toxicant, Radiation, High intracellular calcium.
These injuries lead to:
Decreased ATP generation. The lack of energy causes Na+/K+ pumps to fail which leads to cellular swelling and dilation of the ER overall causing the synthesis of proteins to decrease.
Due to the lack of ATP production, the cell switches to anaerobic glycolysis which leads to lactic acid accumulation, decreased intracellular pH, and decreased activity of many cellular enzymes.
Prolonged ATP depletion causes disruption in the structure of the protein synthetic apparatus leading to prolonged decrease in synthesis of proteins.
Formation of mitochondrial permeability transition pore leads to loss of mitochondrial membrane potential and pH changes also leading to no ATP production
Mitochondrial fragmentations
Production of ROS
Membrane Damage
Membrane injury happens as a result of the earlier stresses discussed (ATP depletion, calcium overload, ROS, and direct toxins). Once the plasma, mitochondrial, and lysosomal membranes are damaged the injury becomes irreversible.
Mitochondrial membranes
Damage leads to the formation of MPTP (mitochondrial permeability transition pore causing a loss of membrane potential which prevents ATP production. This triggers the release of cytochrome-c & pro-apoptotic proteins leading to apoptosis.
Plasma membrane
A loss of osmotic balance causes an uncontrolled influx of Ca²⁺, water, Na⁺ leading to the leakage of cellular contents (enzymes, proteins, metabolites spill) which triggers inflammation.
Identify that at this point, the membrane is not SWOLLEN (reversible) and it RUPTURED (irreversible)
Lysosomal membrane
Damage causes leakage of hydrolytic enzymes (DNases, RNases, proteases, lipases).
KEY:
Reversible injury = mitochondrial swelling + decreased ATP + cell swelling (but membranes still intact).
Irreversible injury = Mitochondria can’t make ATP even after reperfusion, MPTP opens, Cytochrome-c leaks out.
DNA Damage:
DNA damage can arise from exposure to radiation, chemotherapeutic agents, ROS, and mutations. If any of these issues occur, the DNA can repair or it can induce apoptosis to avoid malignation. This decision is up to p53.
P53: A tumor suppressor gene that plays a crucial role in preventing cancer. It is a checkpoint in the cell cycle. TP53 (the gene for p53) is mutated in more than 50% of tumors.
Mild DNA Damage: p53 halts the cell cycle to allow DNA repair to occur before replication.
Excessive DNA Damage: p53 triggers apoptosis through the mitochondrial intrinsic pathway.
Consequences of Injury
Intracellular Depositions: Refers to the abnormal intracellular accumulations caused by the inefficient removal and degradation of material, the excessive production of substances, or the deposition of an abnormal endogenous substance.
Fatty change (steatosis): triglycerides pile up in liver cells (alcoholism, obesity).
Cholesterol: foam cells in atherosclerosis.
Proteins: misfolded or excess proteins (e.g., amyloid, neurodegeneration).
Glycogen: seen in metabolic disorders (diabetes, glycogen storage disease).
Pigments: colored substances that may be exogenous such as carbon or endogenous such as melanin.
Extracellular Depositions: Cells can also leave their “garbage” outside, in the form of calcium deposits.
Dystrophic calcification:
Ca²⁺ deposits in dead/damaged tissue.
Classic examples: TB granulomas, atherosclerotic plaques, necrotic myocardium.
Metastatic calcification:
Ca²⁺ deposits in normal tissues due to systemic hypercalcemia.
Causes: ↑PTH, bone destruction, vitamin D intoxication, renal failure.
Cellular Aging
Aging is the progressive decline of physiologic, cellular, and molecular homeostatic mechanisms after the reproductive years. It is one of the strongest independent risk factors for many chronic diseases
Aging is a consequence of alterations in genes and signaling pathways that are evolutionarily conserved from yeast to mammals. Due to imperfect DNA repair, mutations accumulate over time and those that are deleterious in those pathways contribute to cellular aging.
Mechanisms:
DNA Damage & Mutation Accumulation:
Continuous exposure to ROS, toxins, replication errors.
Defects accumulate in both nuclear DNA and mitochondrial DNA.
Causes telomere dysfunction & cellular senescence and mitochondrial dysfunction.
Telomere Shortening
Telomeres are short, repeated sequences of DNA at the end of chromosomes. They ensure complete replication of chromosome ends while protecting the ends from fusion and degradation.
Telomerase maintains telomere length in germ cell and stem cells BUT NOT somatic cells.
Telomeres progressively shorten with each division of somatic cells and when they completely erode, the ends are recognized as broken DNA leading to cell cycle arrest.
In many cancer cells, telomerase is reactivated
Telomeropathies: inherited telomerase defects → aplastic anemia, pulmonary fibrosis, premature graying, skin/nail changes.
Altered Signaling Pathways
Environmental Stressors
Can shift the balance toward pro-aging or anti-aging cellular signals.
Insulin/IGF-1 Pathway
Normal role: promotes growth and metabolism
Chronic activation: accelerates aging and age-related decline.
mTOR Pathway
Acts as a nutrient and growth signal sensor.
Overactivation: linked to shortened lifespan.
Calorie restriction (≈30% less intake without malnutrition):
Decreases IGF-1 and mTOR activity.
Shown to consistently prolong lifespan in multiple species.
Persistent Low-Level Inflammation ("Inflammaging")
Accumulation of damaged proteins, lipids, and organelles triggers innate immunity.
Leads to chronic, smoldering inflammation.
Contributes to age-related diseases:
Atherosclerosis
Diabetes
Neurodegenerative disorders
Aging can be slowed through exercise and physical activity. It can also be slowed through a caloric restriction (without causing malnutrition). On the other hand, aging can be accelerated by stress.
Mechanisms of Cell Death:
Necrosis: A form of cell death characterized by irreversible damage and disintegration of cellular components. It can be characterized by:
Cytoplasmic Changes:
Glassy, homogeneous appearance (protein denaturation).
Vacuolated cytoplasm.
Plasma/organelle membranes break down.
Swollen mitochondria (large amorphous deposits)
Lysosomal rupture → enzymes digest cytoplasm
Intracytoplasmic “myelin figures” (whorled phospholipid masses).
Nuclear Changes:
Pyknosis = DNA condensation, nuclear shrinkage.
Karyorrhexis = nuclear fragmentation.
Karyolysis = fading/dissolution of nucleus (DNA digested).
Apoptosis: Programmed pathway by which cells degrade their own nuclear DNA and nuclear and cytoplasmic proteins causing cellular and nuclear fragmentation and chromatin condensation
Chromatin condensation (hallmark).
Nuclear & cytoplasmic fragmentation.
Membrane blebs form apoptotic bodies.
Plasma membrane stays intact (so no leakage).
Phagocytosed quickly → no inflammation.
Intrinsic (Mitochondrial) Pathway of Apoptosis
Most common pathway for apoptosis.
Controlled by the Bcl-2 family
Characterized by a release of pro-apoptotic proteins from the mitochondria
Triggered by internal stressors:
DNA damage
Misfolded proteins
Oxidative stress (ROS)
Growth factor withdrawal
Cytosolic Cytochrome-c leads to the activation of caspase cascade.
Extrinsic (Death Receptor) Pathway of Apoptosis
Triggered by external signals (often from the immune system).
This pathway is controlled by death receptors (TNF family and Fas)
When the ligand binds, the receptors cross-link via the death domain and bind adaptor proteins leading to the activation of caspase cascade.
Caspase Cascade (Convergence Point)
Both intrinsic and extrinsic pathways converge here.
Caspases are cysteine proteases that cut proteins at aspartate residues.
Effects of caspases:
Degrade nuclear proteins causing DNA fragmentation.
Break down cytoskeletal proteins leading to shrinkage and blebbing.
Package contents into apoptotic bodies.
Signal to phagocytes for clearance which avoids inflammation.
Other Mechanisms of Cell Death:
Pyroptosis: Inflammasome-mediated cell death.
Necroptosis: Induced by TNF with necrotic and apoptotic features
Ferroptosis: Dependent of cellular iron levels
Autophagy is a crucial cellular process involving the degradation and recycling of cellular components. This occurs when a cell forms an autophagosome around a damaged organelle or misfolded protein, which then fuses with a lysosome for degradation.
The type of cell death that occurs depends on the type of injury:
Injury and Death operate as a continuum:
Examples of Cellular Injury & Consequences
Hypoxia & Ischemia
Cells are highly dependent on a continuous supply of oxygen to maintain aerobic metabolism and generate ATP. Both hypoxia and ischemia deprive cells of oxygen, but they are not the same.
In hypoxia, oxygen delivery is reduced, yet blood flow is maintained. Causes include anemia, respiratory failure, or carbon monoxide poisoning. Under hypoxic conditions, cells can still carry out anaerobic glycolysis as a temporary survival mechanism.
In ischemia, blood flow itself is obstructed, which means oxygen and other nutrients cannot reach the tissue. Because substrates for glycolysis are also cut off, ischemia produces much more severe and rapid injury than hypoxia.
As a result of oxygen deprivation, cells experience ATP depletion. Without ATP:
The sodium-potassium pump fails, leading to sodium and water influx, cell swelling, and endoplasmic reticulum dilation.
Intracellular pH decreases due to lactic acid accumulation, impairing enzyme activity.
Reactive oxygen species (ROS) production increases, damaging proteins, lipids, and DNA.
Protein synthesis falls, and organelles such as the ER and mitochondria begin to swell and fail.
The plasma membrane is compromised, triggering inflammation.
Cells that do not die immediately may activate compensatory survival mechanisms under low-oxygen conditions, but if hypoxia or ischemia is prolonged or severe, irreversible injury develops. The typical outcome of severe ischemia is necrosis, as the cell cannot maintain structural or functional integrity.
Ischemia–Reperfusion Injury
Interestingly, when blood flow is restored to ischemic tissues, it can paradoxically worsen cell injury instead of rescuing the tissue. This is known as ischemia–reperfusion injury.
During reperfusion:
A burst of reactive oxygen species is generated as oxygen suddenly re-enters the tissue, causing oxidative damage.
Large amounts of calcium flood into cells, further disturbing calcium homeostasis and activating degradative enzymes.
The inflammatory response that was primed during ischemia now accelerates, as leukocytes infiltrate the damaged tissue and release more ROS and proteases.
Clinically, reperfusion injury is extremely important because it contributes to the severity of damage following myocardial infarction and cerebral ischemia (stroke). Therapies designed to restore blood flow must be balanced with strategies to limit oxidative and inflammatory injury.
Chemical and Toxic Injury
Cells can also be damaged by exposure to toxins and toxicants. These can act through two broad mechanisms:
Direct-acting toxins: These substances cause damage immediately by binding directly to cellular proteins or organelles.
Mercury poisoning (e.g., from seafood) occurs because mercury binds to sulfhydryl groups on cell membrane proteins. This disrupts ATP-dependent ion transport, increases membrane permeability, and results in cell injury.
Chemotherapy drugs often act directly on DNA, causing breaks or cross-linking that trigger apoptosis.
Latent toxins (indirect-acting): These compounds require metabolic activation, usually in the liver, to be converted into toxic metabolites.
A classic example is acetaminophen (Tylenol) overdose. Under normal doses, acetaminophen is detoxified by conjugation pathways. At very high doses, these pathways are overwhelmed, and the drug is metabolized by the cytochrome P-450 system into a highly reactive metabolite. This metabolite depletes glutathione (a protective antioxidant) and binds covalently to proteins, leading to massive hepatocellular necrosis.