Lecture 1: Disease, Cell Injury & Immune System (MT 2024) PDF

MedST, VetST and NST Part IB - Biology of Disease MT 2024 Professor Adrian Liston Lectures 1-4 Lecture 1: Disease, Cell Injury and the Immune System Pathology is the study of disease and injury, a disturbance of homeostasis occurring whenever there is damage...

MedST, VetST and NST Part IB - Biology of Disease MT 2024 Professor Adrian Liston Lectures 1-4 Lecture 1: Disease, Cell Injury and the Immune System Pathology is the study of disease and injury, a disturbance of homeostasis occurring whenever there is damage to cells. In this course we will study the causes of disease (aetiology), how diseases develop (pathogenesis) and what happens to diseased cells and tissues. The causes of death The proximal causes of death are systems failures in any of the three critical systems: brain, heart and lungs. While systems failure in other organs can cause death, this is due to the failure to support one or more of these three critical systems. E.g. diabetes (failure in the endocrine system) can cause death when it leads to sufficient levels of disturbance in glucose homeostasis that the brain, heart or lungs fail. Distal factors of death are the original problems that caused this systems failure, such as infection, cancer or injury. The bar graph opposite shows the World Health Organisation (WHO) top 10 causes of death globally and is dominated by ischaemic heart disease and stroke, diseases of aging populations. Failures in brain (#2, #7), heart (#1), and lungs (#3, #4, #6) figure prominently in the list, showing the proximal importance of these systems. Major distal factors of death include: Inflammatory diseases (#1, #2, #3, #7, #9) Infectious diseases (#4, #5, #8) Cancers (#6) ➔ Subjects of this course Cell injury Virchow proposed that cell injury is the basis of disease. Cell injury can be inflicted by; extremes of oxygen tension or pH; lack of ATP; exposure to toxins, drugs and chemical agents (xenobiotics); cold and heat; prolonged deprivation of vital nutrients; trauma; and aging. Cells can also be injured through infection by bacteria, fungi, parasites and viruses. Injury due to infection may be the direct result of xenobiotics produced by the infectious agent itself, or may be initiated or accelerated by the tissue reaction to infection. The processes of acute and chronic inflammation that may be successful in killing invading organisms may also injure cells 1 of the infected host. We will see that the immune system plays a major role in many of the common causes of death outlined above. Cellular and biochemical targets of cell injury Numerous injurious stimuli target common cellular systems/components and elicit similar cell responses. Common cellular targets include: Mitochondria, the organelle responsible for ATP generation, Cell membranes, damage to which disturbs ionic and osmotic homeostasis of the cell and internal organelles, The cytoskeleton, Protein synthesis, The genetic competence of the cell. HARMFUL STIMULUS DNA DAMAGE, êATP MEMBRANE CYTOSKELETAL éROS UNFOLDED PROTEIN DAMAGE DAMAGE ACCUMULATION Membrane damage Membrane damage DNA damage Failure of most Mitochondria Plasma Lysosome Protein/enzyme damage synthetic and membrane degradative cell processes Cell Death (apoptosis or Loss of cell Enzymatic digestion CELL DEATH necrosis) components of cell components (mainly apoptosis) CELL DEATH (mainly necrosis) Oxidative stress Many types of injury result in oxidative stress and the cell responds by activating common pathways. Cells are dependent upon oxygen for energy and most cells have cytoprotective ROS systems that protect against free radical injury. Free and Cell radicals Injurythat have unpaired are molecules electrons (e.g. O2 reactive oxygen species (ROS)) and Oxidative enzymes in: mitochondria nitric oxide (NO). ER They are created by Inflammation cytosol plasma membrane ionising radiation and Radiation Xenobiotics Reactive also by many Reperfusion O2 H2O2 OH oxygen injury Superoxide Hydrogen Hydroxyl species xenobiotics. They are peroxide radical the normal by- products of reactions Damage caused by ROS Removal of free radicals by catalysed by many DNA oxidationà mutations/breaks Glutathione oxidase enzymes. Protein oxidation à enzyme activity/folding Catalase Fatty acid oxidationà disrupts plasma membrane SOD Free radicals have and organelles 2 short half-lives but are highly reactive, causing strand scission in nucleic acid and disruption of protein (enzyme) structure. They damage lipid membranes creating additional free radicals. Neutrophils and macrophages use ROS and nitric oxide to good effect to kill invading microorganisms, but this may also damage host cells. When tissue is re-oxygenated after periods of hypoxia (oxygen shortage) free radicals are generated resulting in reperfusion injury, a dramatic destruction of the endothelium of small blood vessels carrying the renewed blood flow. This encourages recruitment of neutrophils, which may cause further damage, and platelets, and thrombin which seal off the blood supply, the process of thrombosis. Impaired energy homeostasis (cellular starvation/suffocation) Loss of control of energy production also has major consequences for the cell. In the presence of oxygen ATP is generated by oxidative phosphorylation and in it’s absence by the glycolytic pathway. Reduced oxygen supply causes depletion of ATP and leads to; Reduced activity of the membrane Na+/K+ pump (Na+ accumulates in the cell and K+ is lost). Water accumulates causing ER dilation and cell swelling Glycolysis increases, lactic acid is produced and pH is reduced resulting in decreased cellular enzyme activity Influx of Ca+ leads to increased activity of intracellular proteases, phospolipases, endonucleases and ATPases Ribosome detachmentConsequence and loss of protein ofsynthesis loss of occurs ATP Mitochondria (reduced oxygen supply) oxidative phosphorylation ATP ribosome Na+/K+ pump Anaerobic glycolysis detachment Glycogen Lactic pH influx Ca+ protein acid H2O, Na+ synthesis Efflux K+ Clumping of nuclear chromatin ER swelling Cellular swelling Blebbing NECROSIS Cell responses to stress and injury Cells are in dialogue with their environment and can adapt to cope with changing situations (stress or injury) to maintain homeostasis within limits. When those limits are reached, cells will die. However often cells will enter the cell death pathway well before stress levels reach the limits imposed. If cell death occurs in large enough numbers within an organ, this can cause organ failure and death. This is a particular risk during infection or inflammation. Why would the system include stress responses to infection or inflammation that are strong enough to cause organismal death? The answer is that cells are operating semi-autonomously, each making an individual decision on how to respond to the stress level experienced. This 3 decision-making process is influenced by the type of injury, its duration and its severity, the type of cell, its capacity to adapt, and its genetic makeup. Because of this, there are degrees of inflammation that can be tolerated for a short period of time in a healthy host, providing a benefit in some contexts, while the same degree of inflammation, if given for an extended period of time or in an unhealthy host would cause organ failure and death. i.e., the organism does not use a hard limit for the appropriate immune response to make. Instead, when driving immunity, the body integrates information from multiple sources to make a probability-based decision that provides the most beneficial outcome at a population level over evolutionary time. The cellular response to stress When cells are stressed, for example by NORMAL CELL increased demand, they can adapt. Stress (HOMEOSTASIS) Recovery Adaptation is often reversible and cells return Recovery Injury REVERSIBLE to normal when the stimulus is removed. ADAPTATION INJURY Adaptation may involve an increase in the CELL DAMAGE Unable to Transient or size (hypertrophy) or number (hyperplasia) adapt mild injury Severe of individual cells. Cells may adapt by injury reducing their complexity (cell atrophy). In IRREVERSIBLE cell atrophy cell volume diminishes over INJURY several hours or days through reduction of the CELL DEATH complexity of the cytoplasm. This is usually APOPTOSIS NECROSIS achieved by digestion of cellular proteins by the proteasome system, backed up by autophagy. In autophagy organelles are encapsulated by intracytoplasmic membranes and digested by fusion with lysosomes. If cells are unable to adapt they will suffer cell damage that may be reversible or irreversible. Reversible injury may be seen as cell swelling or fatty deposits. Cell swelling is commonly caused by Na+/K+-ATPase (sodium/potassium pump) shut down, leading to an influx of Na+ and hence H2O into the cell/mitochondria. Cell adaptation (protective responses to stress) 4 The heat shock response This is a fundamental reaction to injury of all The Heat Shock Response hsp40 living cells. Many stresses will cause hsp70 Heat Shock Factor monomer heat shock proteins bind dissociation of cytosolic proteins called heat bound by heat shock proteins hsp90 unfolded protein shock factors (HSFs) from cytoplasmic HEAT HSF-1 hsp70 complexes that normally keep them inactive. hsp90 They translocate to the nucleus and hsp40 HSF-1 suppress transcription of many genes but TRIMER HSF trimer enters nucleus activate transcription of heat shock and activates transcriptio n preconditioning of hsp genes proteins (HSPs). HSPs are chaperone transcription p pp hsp70 etc proteins that help refold partially denatured proteins thereby allowing the cell to maintain HSE function under stress conditions. HSPs are hsp70 hsp90 Nucleus responsible for preconditioning, where hsp40 cells exposed to minor injury become resistant to more major stresses. The unfolded protein response Protein synthesis in the endoplasmic reticulum (ER) is challenging. In an active cell protein concentration in the ER can reach 100mg/ml, a concentration at which unwanted precipitation and aggregation of proteins can occur unless proteins are correctly folded and chaperoned. The unfolded protein response (UPR) ensures that the rate of protein synthesis does not exceed the cell’s capacity to complete the folding process. The UPR activates signalling cascades that increase synthesis of folding chaperones, enhance proteasomal protein degradation and slow down protein translation. The UPR is usually reversible and is part of a process termed ‘host cell shut-down’. Cell shut-down is a primitive reversible response to injury and is initiated within minutes. RNA and DNA synthesis is suppressed and many enzyme-catalysed reactions are inhibited. The stress kinase pathways These are activated by a wide range of factors (osmotic stress, oxidative stress, heat, UV, DNA cleavage) and activate heterodimeric transcription factors (e.g. AP1) that re-programme transcription. Important examples include Jun N-terminal Kinase (JNK)/stress-activated protein kinase pathway (SAPK) pathway 5  P38 kinase pathway These modulate decisions that are determined by combinations of factors rather than having a determining role. For example the final outcome may be protein synthesis shutdown, necrosis or apoptosis, depending on additional factors pertaining to the cell at the time. Cell death (failure responses to stress) If homeostasis is restored the cell can recover however if injury is too severe or persistent then a ‘point of no return’ can be reached and the cell will die, often by NECROSIS but also by APOPTOSIS. Apoptosis Apoptosis is a programmed death process that requires the cell to retain control over its own energy metabolism. This is a common process in tissue development and repair. Apoptotic cells lose contact with surrounding cells and shrink in volume. The membranes bleb and bud and the cell may fragment into multiple membrane bounded subcellular bodies. Many proteins critical for cell function are cleaved by selective, site-specific proteases (caspases) and the surface of the dying cell exposes signals that ensure immediate phagocytosis by neighbouring cells including macrophages. Apoptosis is referred to as a ‘silent death’ and rarely induces inflammation. Necrosis Necrosis is an uncontrolled cell death. In necrosis there is a loss of cell volume homeostasis and cellular swelling and rupture of internal and plasma membranes occurs. Intracellular contents leak into the extracellular space and can reach the bloodstream. Some of these components are chemotactic for neutrophils and elicit an acute inflammatory reaction. Neutrophil activation may itself cause local cell injury. Necrosis is a poorly controlled process that tends to spread and involve sheets or groups of adjacent cells. Pyroptosis, ferroptosis, NETosis, etc There are multiple additional forms of death that are like apoptosis, in that they are programmed, but also like necrosis, in that they are inflammatory. These are initiated when the body is deliberately killing a cell in a manner that recruits an immune response. Pyroptosis is amongst the most inflammatory events a cell can undertake, releasing IL1β. How does the system work? There are multiple different sources of stress, triggering multiple different pathways. Cells need to make a decision as to what the appropriate response is: adaptation or death. Different sources of stress will have different optimal responses, for example stress due to environmental stress would often be best served by adaptation while stress due to intracellular infection would often 6 best be served by cell death. Yet this optimal decision is not always taken, because the cells are operating (and making decisions based on) incomplete information. The information available to cells is the type of stress (oxidative stress, DNA damage, unfolded protein stress, etc) and the amount of stress (how much each pathway is activated), but they do not have a direct ability to identify the source of the stress. Cellular decision making is improved by integrating contextual cues. For example, the same amount of unfolded protein stress could result in a different response depending of the presence/absence of activated pathways sensing viral infection. Cells need to make the decision in a probabilistic manner. This means in some occasions cells with make non-optimal decisions, leading to pathogenic outcomes. Consider whether the following responses to stress could be considered optimal or non-optimal: - Cell death following low oxygen levels due to vascular injury - DNA damage repair following high-dose mutagen exposure - Muscle hypertrophy following prolonged lactate exposure - Fibroblast hyperplasia during viral infection The Biology of Disease Course Your course continues with lectures on the innate and adaptive immune systems. We will discuss how the immune system works to protect us from infection and what happens when it makes inappropriate responses. You will see examples of the major groups of infectious pathogens, how they interact with the host and how the host reacts to them. You will examine the vascular response to injury, ischemia, infarction and atherosclerosis, major causes of mortality. Finally, the core course concludes with lectures covering tissue growth and cancer, again a major cause of death. Although taught in distinct blocks, you will see continual links made between the various course modules. Pathology involves many interacting components. 7