Apoptosis

Apoptosis

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  APOPTOSIS Dr. JILSHA
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  OBJECTIVES • Introduction • Historyof apoptosis • Difference between apoptosis and necrosis • Functional significance • Reasons for apoptosis • Mechanism and regulation of apoptosis • Changes in apoptosis • Examples of apoptosis • Detection and recent advances • Applied physiology
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  INTRODUCTION • Apoptosis isdefined as the natural or programmed cell death that occurs under genetic control. • When cells are no longer needed or become a threat to the organism, they undergo PCD or apoptosis. • Apoptosis is a Greek word meaning “falling off”. • Since the cell’s own genes play an active role in its demise, apoptosis can also be called as cell suicide. .
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  HISTORY • German scientistCarl Vogt put forth the principle of apoptosis in 1842.
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  • Walther Flemming,an anatomist described the process of programmed cell death. in 1885.
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  • John Kerrin 1965, while studying tissues using electron microscopy, was able to distinguish apoptosis from traumatic cell death. • In a signal article published in 1972, John F. Kerr, Andrew H. Wyllie and A. R. Curie coined the term “apoptosis”. • Kerr received the Paul Ehrlich and Ludwig Darmstaedter prize on March 14, 2000 for his description of apoptosis.
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  • The 2002Nobel prize in Medicine was awarded to Sydney Brenner, H. Robert Hovitz and John Sulston for their work in identifying genes that control apoptosis.
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  APOPTOSIS vs NECROSIS •Cell death occurs in two ways : necrosis and apoptosis. • Apoptosis should be distinguished from necrosis in which the healthy cells are destroyed by some injury inflicted on the cells. • Since necrosis is caused by some external process, it can also be called as cell murder.
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  FACT About 10 millioncells are produced every day in human body by mitosis. An equal number of cells die by apoptosis. Thus maintaining homeostasis.
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  FUNCTIONAL SIGNIFICANCE • Thepurpose of apoptosis is to remove unwanted cells without causing any stress or damage to the neighboring cells. • It plays a role in cellular homeostasis. • Useful in removal of cell that is damaged beyond repair by toxins or viruses. • It is an essential event during developmental stages and in adults. Therefore, decreased apoptosis has been implicated in the genesis of malignancy and autoimmune diseases.
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  REASONS FOR APOPTOSIS •Withdrawal of positive signals • Growth factors for neurons • IL – 2 • Receipt of negative signals • Increased levels of oxidants within the cell • Damage to DNA by oxidants • Death activators • TNF α • TNF β • Fas ligand
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  MECHANISM • Apoptosis isinitiated by two stimuli: • Extrinsic pathway • Intrinsic pathway • In extrinsic pathway, the stimulus is received from outside of the cell and in intrinsic pathway, the stimulus arises from within the cell. • Both the pathways lead to activation of cysteine proteases in the cell called caspases which triggers apoptosis. • Caspases are synthesized in the cell as precursors named procaspase.
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  FACT There are 12 confirmedcaspases in humans and 10 in mice, carrying out a variety of cellular functions. CASPASES stands for cysteine-aspartic proteases
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  • There arethree types of caspases • Inflammatory caspases : Caspases 1,4 and 5. • Initiator caspases : Caspases 2, 8, 9, 10. • Effector caspases : Caspases 3, 6 and 7. • In the case of apoptosis, the process starts with activation of caspase 8 or 9 which inturn activates caspase 3 & 7 which then activate other caspases resulting in a cascade.
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  REGULATION OF APOPTSIS •Certain intracellular proteins provide signal for the final programmed cell death. • Pro-apoptotic proteins : BAX and BAC • Anti-apoptotic proteins: BCL-2, BCL-X, MCL-1 • Apoptosis depends upon the balance between the pro and anti apoptotic proteins. In a healthy cell, anti-apoptotic proteins binds with pro-apoptotic proteins thereby blocking their action. • If a cell is damaged or it stops receiving survival signals, Bcl – 2 and Bcl –x are blocked in turn. Bax and Bac are then free to punch a series of channels in the mitochondria.
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  INTRINSIC PATHWAY • Alsoknown as mitochondrial pathway. • Internal stimuli may be due to cellular stress, viral infections, increase in the concentration of intracellular oxidants, heat, radiation, nutrient deprivation, hypoxia, increased intracellular calcium concentration, damage to the DNA etc.,
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  INTRINSIC PATHWAY INTERNAL STIMULI Damageto mitochondrial membrane increasing permeability Entry of Cytochrome C into the cytoplasm Cytochrome binds to Apaf – 1 forming apoptosome Apoptosome activates procaspase 9 to caspase 9
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  • Mitochondrial proteinscalled SMACs (Second Mitochondria derived Activator of Caspases) are also released into the cell’s cytosol following increased mitochondrial permeability. • SMAC binds to anti – apoptotic proteins thereby deactivating them and therefore allowing apoptosis to proceed.
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  EXTRINSIC PATHWAY • Theextrinsic pathway of apoptosis begins outside a cell, when conditions in the extracellular environment determine that a cell must die. • External stimuli are various ligands that bind with cell surface to activate apoptosis. • TNF and Fas Ligands are some examples of ligands that activates Caspase 8. • These ligands are called death ligands and they bind with Death receptors like TNFR1. • Ligand binding causes the receptors to cluster and ultimately form a Death Inducing Signalling Complex (DISC). • Upon DISC formation caspase cascade activation takes place.
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  • Activation ofCaspases results in: • Shrinkage of cell • DNA fragmentation • Chromatin condensation(pyknosis)
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  CHANGES IN APOPTOSIS PATHOPHYSIOLOGICAL •Apoptotic cells become round or oval and reduce in size. • Cytoplasm becomes intensely eosinophilic. • Though cytoplasm is reduced, organelles remain almost normal. • Chromatic condensation occurs. • Nuclear fragmentation occurs. • Apoptotic bodies contain compacted organelles. • Phagocytosis of apoptotic bodies occur by macrophages. The phagocytic cells secrete cytokines like IL-10 and TGF-β which inhibit inflammation of neighbouring tissues.
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  BIOCHEMICAL CHANGES • Proteolysisof cytoskeletal proteins. • Cross – linking of protein molecules. • Fragmentation of nuclear chromatin by activation of nuclease. • A glycoprotein – thrombospondin and a phosphoprotein – phosphatidylserine appear on the outer surface of apoptotic bodies for recognition by macrophages.
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  PHYSIOLOGICAL PROCESSES • Theseparation of fingers and toes in a developing human embryo occurs because cells between the digits undergo apoptosis. • Endometrial shedding in menstrual cycles. • Regression of lactating breast after cessation of breastfeeding. • Normal shedding of intestinal epithelium. • Involution of thymus after childhood. • Degeneration and regeneration of neurons within the CNS and formation of synapse. • Regression of duct system during sex differentiation in the foetus.
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  PATHOLOGICAL PROCESSES • Tumorcell death on exposure to chemotherapeutic agents. • Transplant cell death by cytotoxic T cells that cause transplant rejection. • Cell death induced by viral infections. • Cell death induced by radiation, hypoxia. • Degenerative diseases like Alzheimer’s disease, Parkinson’s disease etc.,
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  DETECTION OF APOPTOSIS •DNA fragmentation assay by electrophoresis. • TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) staining. • Demonstration of chromatic condensation by H & E staining. • Estimation of cytosolic cytochrome – C, activated caspase and annexin – V.
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  APPLIED PHYSIOLOGY • Understandingthe concept of apoptosis has promising role in future regenerative medicine. • Disorders with reduced apoptosis: • Cancer • Autoimmune diseases – SLE, Myasthenia gravis • Inflammatory diseases – BA, IBD • Viral infections – Herpes, pox, adenovirus • Disorders with increased apoptosis : • Alzheimer’s disease, Parkinson’s disease • Myocardial infarction, stroke. • AIDS
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  RECENT ADVANCES • Caspaseinhibitors such as N – benzyloxycarbonyl – Val – Ala – Asp fluoromethyl – ketone (Z – VAD) were being investigated for the treatment of neurodegenerative disorders like : • Alzheimer’s disease • ALS • Huntington’s disease • Parkinson’s disease • Another promising compound that is currently under evaluation is Minocycline which is better than other caspase inhibitors with less side effects.
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  SUMMARY • Apoptosis isdefined as the natural or programmed cell death. • The purpose of apoptosis is to remove unwanted cells. • May be physiological or pathological. • There are two pathways – extrinsic and intrinsic pathway. • It is regulated by pre and anti apoptotic proteins. • Several changes occur in the apoptotic cells which is finally phagocytosed by macrophages. • Has a promising role in regenerative medicine.