cellinjurycelldeath.pdf

CELL INJURY AND CELL DEATH Block: Foundations Block Director: James Proffitt, PhD Session Date: Wednesday, August 07, 2024 Time: 8:00 – 10:00 am Instructor: Margaret Briehl, PhD Department: Pathology Email: Please contact...

CELL INJURY AND CELL DEATH Block: Foundations Block Director: James Proffitt, PhD Session Date: Wednesday, August 07, 2024 Time: 8:00 – 10:00 am Instructor: Margaret Briehl, PhD Department: Pathology Email: Please contact Dr. Proffitt with questions – [email protected] INSTRUCTIONAL METHODS Primary Method: IM10: Independent Learning ☐ Flipped Session Resource Types: RE18: Written or Visual Media (or Digital Equivalent) INSTRUCTIONS Please read lecture objectives and notes prior to attending session. READINGS REQUIRED Reading: Robbins & Cotran: 10E (2021): Chap. 2 [start at ‘Overview of Cellular Responses to Stress and Noxious Stimuli’, stop at ‘Adaptations of Cellular Growth and Differentiation’]. LEARNING OBJECTIVES 1. Explain why an elevated blood level of cardiac troponin is used as an indicator of a myocardial infarct. 2. Differentiate the types of necrosis by their etiology and morphologic features. 3. Summarize the molecular changes occurring in cells exposed to hypoxia and relate these changes to the microscopic appearance of reversibly, and irreversibly, injured cells. 4. Understand the role of glutathione in cells. 5. Describe the mechanism by which cyanide injures cells, and explain why N- acetylcysteine is used to treat acetaminophen poisoning. 6. Summarize key differences between necrosis and apoptosis. CURRICULAR CONNECTIONS Below are the competencies, educational program objectives (EPOs), block goals, disciplines and threads that most accurately describe the connection of this session to the curriculum. Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #1 MK-09: Critical thinking about medical Clinical skills H & I: Acute science and about the diagnosis and Care treatment of disease Block: Foundations | BRIEHL [1 of 9] CELL INJURY AND CELL DEATH Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #2 MK-05: The altered structure and Pathology EBM: Evidence- function (pathology & Based Medicine pathophysiology) of the body/organs in disease CO-01 LO #3 MK-05: The altered structure and Pathology EBM: Evidence- function (pathology & Based Medicine pathophysiology) of the body/organs in disease CO-01 LO #4 MK--03: The molecular, cellular and Histology & EBM: Evidence- biochemical mechanisms of Cellular Biology Based Medicine homeostasis CO-01 LO #5 MK-05: The altered structure and Pathology EBM: Evidence- function (pathology & Based Medicine pathophysiology) of the body/organs in disease CO-02 LO #6 MK-05: The altered structure and Pathology EBM: Evidence- function (pathology & Based Medicine pathophysiology) of the body/organs in disease CONTEXT: This lecture will provide an overview of causes of cell injury and responses of the cell to injury, including various mechanisms and patterns of cell death (apoptosis and the different types of necrosis). Many of the future blocks will entail pathology in which there are cells that are either injured or dying and it will be important that you know how to identify these changes. These lectures begin our examination of the mechanisms of disease, and provide background for the upcoming lectures and labs on inflammation and infarction. The lecture on cell growth, differentiation and adaptation, and later in the block on pharmacology, will also relate to this material, all of which is fundamental to understanding health and disease in all organ systems. The cell biology pre-med basic learning modules provide essential background for these lectures, and you should have completed all of them prior to these sessions. Note that cellular injury often results in specific kinds of cellular accumulations. Some of these are readily identifiable under the microscope and give useful information about disease processes. A review of the most common cellular accumulations has been provided in a pre-med basic learning module. Block: Foundations | BRIEHL [2 of 9] CELL INJURY AND CELL DEATH LECTURE NOTES Normal cells are in a homeostatic "steady state," capable of responding to moderate variations in the local environment Excessive stress (physiologic or pathologic) requires the cell to adapt or suffer injury; irreversible injury results in cell death CAUSES OF CELL INJURY Oxygen deprivation: ischemia or other causes of hypoxia Physical agents: temperature extremes, electrical shock, etc. Chemical agents & drugs: poisons, pollutants, etc. Infectious agents: bacteria, viruses, fungi, parasites, etc. Immunologic reactions: foreign or self-antigens Genetic derangements: lead to biochemical or metabolic disturbances; can alter susceptibility to injury Nutritional imbalances: deficiencies or excess CELL INJURY - GENERAL CAVEATS Reaction to injury varies with the type and severity of the injury and the type of cell Vulnerable cell systems are aerobic respiration (ATP production), integrity of cell membranes, protein synthesis, the cytoskeleton and the integrity of the genetic apparatus Morphological Changes With Reversible and Irreversible Cell Injury Hydropic change or cellular swelling (also called vacuolar degeneration): reflects acute, reversible cell injury and is due to impaired function of membrane ion pumps due to depletion of ATP; grossly, the affected tissue appears swollen with some pallor and increased weight Morphologic changes seen by light microscopy following irreversible cell injury and cell death 1. increased eosinophilia (hypereosinophilia - “red is dead”) 2. nuclear changes (pyknosis, karyorrhexis, karyolysis) Ultrastructural changes seen by electron microscopy following irreversible cell injury and cell death that results in necrosis 1. severe mitochondrial swelling 2. disruption of cell membranes 3. nuclear changes Time sequence from irreversible cell injury to cell death: biochemical changes → light microscope-visible changes → gross morphologic changes Clinical caveat: Intracellular enzymes released from dying cells gain access to the circulation and can be measured as the first sign of injury (e.g., cardiac troponins released from dying myocardial cells can be used to establish a diagnosis of acute myocardial infarction as early as 2 hours after the injury, compared to 4 – Block: Foundations | BRIEHL 12 hours needed for the appearance of morphological changes) [3 of 9] CELL INJURY AND CELL DEATH Cardiac Troponins Part of a protein complex, located along the thin filaments of myofibrils that regulates cardiac muscle contraction Diagnostic tests detect cardiac-specific troponins Proven to be better serum markers of heart damage than other biomarkers of the death of cardiomyocytes (e.g., myoglobin or the MB fraction of creatine kinase, CK-MB) Block: Foundations | BRIEHL [4 of 9] CELL INJURY AND CELL DEATH Types of Necrosis Coagulation necrosis: Occurs with hypoxic death of cells NOTE: hypoxia and ischemia have slightly different meanings; hypoxia is reduced delivery of oxygen to cells and ischemia is the reduced delivery of blood to cells; while ischemia is the most frequent cause of hypoxia, there are additional mechanisms by which tissues can be deprived of oxygen Protein denaturation is the dominant feature Grossly, the infarcted tissue remains firm Microscopically, the outline and architecture of the cells is maintained but they are eosinophilic (i.e., increased redness in H&E sections) and have nuclear changes indicative of cell death (e.g., degradation of the chromatin, known as karyolysis); electron microscopy would show broken cell membranes – one feature that distinguishes necrosis from apoptosis Liquefactive necrosis Occurs with focal infection by bacteria and some fungi due to accumulation of inflammatory cells (neutrophils) Tissue digestion is the dominant feature Grossly, it appears as an abscess (a localized collection of pus buried in a tissue or organ) Microscopically, the tissue is digested and inflammatory cells and debris are present Fat necrosis Special kind of necrosis affecting adipose tissue Occurs when acute pancreatitis or trauma leads to the digestion of fat cell membranes and breakdown of triglycerides stored in the fat cells Grossly, it has a chalky white appearance due to fat saponification – released fatty acids bind calcium to form "soaps" Microscopically, vague outlines of fat cells are present, with basophilic calcium deposits Caseous necrosis Special kind of necrosis whose name is derived from the gross ap- pearance, resembling soft cheese (from the German "käse", meaning cheese) Seen in the central areas of certain granulomas, representing necrotic cell debris; granulomas are focal lesions seen in particular types of chronic inflammation, which include tuberculosis and Valley Fever (causative agent is Coccidioides immitis) – granulomas will be covered in Chronic Inflammation lectures Block: Foundations | BRIEHL [5 of 9] CELL INJURY AND CELL DEATH Microscopically, the dead cells in areas of caseous necrosis do not retain their outlines and appear as granular, eosinophilic debris Fibrinoid necrosis Special kind of necrosis seen in injured blood vessels Microscopically, the tissue has a bright pink, granular appearance resembling fibrin (hence the name), actually made up of a combination of fibrin, plasma proteins and complement components Mechanism begins with injury to the vessel walls (e.g., due to chronic hypertension or deposition of antigen:antibody complexes) followed by increased permeability to fibrinogen and other plasma proteins and focal death of cells in the vessel wall occurring with an acute inflammatory reaction Mechanisms of Cell Injury 1. Depletion of ATP by Ischemic/Hypoxic Injury First injury is to mitochondrial ATP production; this leads to further changes as diagrammed below Ischemia Mitochondrion Decreased oxidative phosphorylation (ATP production) Na+ pumps ATP is produced by Ribosomes shut down anaerobic glycolysis detach Na+, Ca++ and H2O Accumulation of lactic Protein synthesis enter the cell, K+ leaves acid lowers the cell pH is compromised Cell swells Chromatin clumps Lipids accumulate Ischemia causes injury more rapidly than hypoxia since, in addition to oxygen, the cells are deprived of substrates that could be used to generate ATP through anaerobic glycolysis 2. Oxidative Stress Oxidative stress is a state of imbalance between the generation of reactive oxygen species and their removal Reactive oxygen species include the superoxide anion (O2 -) and the hydroxyl radical ( OH), which are free radicals (chemicals with Block: Foundations | BRIEHL [6 of 9] CELL INJURY AND CELL DEATH a single unpaired electron in an outer orbital), and hydrogen peroxide (H2O2) Sources of reactive oxygen species and other radical species: 1. leakage of electrons from the electron transport chain 2. cytochrome P450s, lipoxygenases, NADPH oxidase 3. hydrolysis of water by radiation 4. other types of radicals can be produced during metabolism of exogenous substances (e.g., a carbon-centered radical from metabolism of carbon tetrachloride) Cellular damage from reactive oxygen species: 1. proteins: oxidative modifications, fragmentation or cross- linking lead to loss of enzyme activity or abnormal folding 2. DNA: oxidative modifications, breaks or cross-links lead to mutations 3. cell membranes: autocatalytic lipid peroxidation leads to the loss of membrane function Defenses against reactive oxygen species: 1. non-enzymatic antioxidants (e.g., reduced glutathione is the major intracellular redox buffer and is abbreviated as GSH) 2. control of free iron levels 3. antioxidant enzymes 3. Critical Events for Lethal Cell Injury Cell death occurs subsequent to: 1. Inability to reverse mitochondrial dysfunction even after restoration of oxygenation 2. Profound disturbances of membrane function ATP-dependent pumps maintain cytoplasmic Ca2+ at concentrations