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APOPTOSIS: An overview Sanjeev Sharma*, Aarti Bhardwaj$, Shalini Jain# and Hariom Yadav# *Animal Genetics and Breeding Division, #Animal Biochemistry Division, National Dairy Research Institute, Karnal-132001, Haryana, India $College of Applied Education and Health Sciences, Meerut, U.P. INTRODUCTION Cell death by injury -Mechanical damage -Exposure to toxic chemicals Cell death by suicide -Internal signals -External signals Conted….. Apoptosis or programmed cell death, is carefully coordinated collapse of cell, protein degradation , DNA fragmentation followed by rapid engulfment of corpses by neighbouring cells. (Tommi, 2002) Essential part of life for every multicellular organism from worms to humans. (Faddy et al.,1992) Apoptosis plays a major role from embryonic development to senescence. Why should a cell commit suicide? Apoptosis is needed for proper development Examples: – The resorption of the tadpole tail – The formation of the fingers and toes of the fetus – The sloughing off of the inner lining of the uterus – The formation of the proper connections between neurons in the brain Apoptosis is needed to destroy cells Examples: – Cells infected with viruses – Cells of the immune system – Cells with DNA damage – Cancer cells What makes a cell decide to commit suicide? Withdrawal of positive signals examples : – growth factors for neurons – Interleukin-2 (IL-2) Receipt of negative signals examples : – increased levels of oxidants within the cell – damage to DNA by oxidants – death activators : • Tumor necrosis factor alpha (TNF-) • Lymphotoxin (TNF-β) • Fas ligand (FasL) Necrosis vs. Apoptosis Necrosis • • • • Cellular swelling Membranes are broken ATP is depleted Cell lyses, eliciting an inflammatory reaction • DNA fragmentation is random, or smeared • In vivo, whole areas of the tissue are affected Apoptosis • • • • Cellular condensation Membranes remain intact Requires ATP Cell is phagocytosed, no tissue reaction • Ladder-like DNA fragmentation • In vivo, individual cells appear affected NECROSIS Vs APOPTOSIS Wilde, 1999 STAGES OF APOPTOSIS Induction of apoptosis related genes, signal transduction Sherman et al., 1997 APOPTOSIS: Morphology organelle membrane blebbing & changes reduction cell mitochondrial leakage shrinkage nuclear fragmentation chromatin condensation Hacker., 2000 APOPTOSIS: Morphological events cell shrinkage organelle reduction mitochondrial leakage chromatin condensation nuclear fragmentation membrane blebbing & changes Blebbing & Apoptotic bodies Bleb The control retained over the cell membrane & cytoskeleton allows intact pieces of the cell to separate for recognition & phagocytosis by MFs Apoptotic body MF MF Caenorhabditis elegans 1090 cells decision to die 131 cells execution ced-3 ced-4 egl-1 ced-9 ces-2 ces-1 engulfment ced-1 ced-2 ced-5 ced-6 ced-7 ced-10 apoptosis degradation nuc-1 Apoptosis: Pathways “Extrinsic Pathway” Death Ligands Death Receptors “Intrinsic Pathway” DNA damage & p53 Mitochondria/ Cytochrome C Initiator Caspase 8 Effector Caspase 3 Initiator Caspase 9 PCD MAJOR PLAYERS IN APOPTOSIS • • • • Caspases Adaptor proteins TNF & TNFR family Bcl-2 family Ligand-induced cell death Ligand FasL TNF TRAIL Receptor Fas (CD95) TNF-R DR4 (Trail-R) Ligand-induced cell death “The death receptors” FasL Ligand-induced trimerization Trail TNF Death Domains Death Effectors Induced proximity of Caspase 8 Activation of Caspase 8 APOPTOSIS: Signaling & Control pathways I Externally driven Apoptotic signals p53 Internally Cytochrome C driven Externally driven Activators of initiator enzymes Initiator caspases mitochondrion 6, 8, 9,12 Execution caspases 2, 3, 7 Apoptosis events Activation APOPTOSIS: Signaling & Control pathways II Externally driven Apoptotic signals p53 Internally Cytochrome C Bcl2 driven Externally driven Inhibitors Activators of initiator enzymes Initiator caspases 6, 8, 9,12 External Survival Internal factors Execution caspases 2, 3, 7 Apoptosis events Inhibitors of apoptosis Inhibition The mitochondrial pathway DNA Fas damage Casp8 p53 Growth factor receptors PI3K Bid Bid Bax Bid Bax Akt casp3 BAD Bcl2 casp9 Apaf1 ATP Cyt.C IAPs casp3 Smac/ DIABLO H2O2 AIF Pollack etal., 2001 REGULATION OF APOPTOSIS Stimuli apoptosis selection of targets (Rich et al., 2000) Apoptosis by conflicting signals that scramble the normal status of cell (Canlon & Raff, 1999) Apoptotic stimulicytokines, death factors (FasL) (Tabibzadeh et al., 1999) DNA breaks p53 is activated arrest cell cycle or activate self destruction (Blaint & Vousden, 2001) Importance of Apoptosis • Important in normal physiology / development – Development: Immune systems maturation, Morphogenesis, Neural development – Adult: Immune privilege, DNA Damage and wound repair. • Excess apoptosis – Neurodegenerative diseases • Deficient apoptosis – Cancer – Autoimmunity DNA DAMAGE p53 The bcl-2 family N BH4 BH3 BH1 Receptor domain Ligand Pore domain formation phosphorylation Raf-1 calcineurin BH2 TM C Membrane anchor Group I Bcl-2 Group II bax Group III Bad bid bik Back P53 & Apoptosis p53 first arrests cell growth between G1 S This allows for DNA repair during delay If the damage is too extensive then p53 induces gene activation leading to apoptosis (programmed cell death) BACK 3 mechanisms of caspase activation a. Proteolytic cleavage e.g. pro-caspase 3 b. Induced proximity, e.g. pro-caspase 8 c. Oligomerization, e.g. cyt c, Apaf-1 & caspase 9 Back Apoptosis signal to kill infected cells Cytolytic lymphocyte/CTL (& natural killer lymphocyte) presents Fas ligand/CD178 on its surface to tell the infected cell to die Fas ligand Externally driven Apoptotic signals Cytochrome c Initiator caspases The immunological synapse holds the cells much tighter together than shown here Fas/ CD95 is the ‘death receptor’ Execution caspases Apoptosis events