Apoptosis

Cheatsheet Content

What is Apoptosis? Apoptosis (Programmed Cell Death): A highly regulated and genetically controlled process of cell dismantling and removal without causing inflammation. Essential for development, tissue homeostasis, and elimination of damaged/infected cells. Distinguished from necrosis , which is uncontrolled cell death due to injury, leading to inflammation. Key Morphological Changes Cell shrinkage and rounding Chromatin condensation (pyknosis) and fragmentation (karyorrhexis) Nuclear fragmentation Blebbing of the plasma membrane Formation of apoptotic bodies (membrane-bound vesicles containing cellular content) Phagocytosis of apoptotic bodies by macrophages or neighboring cells Biochemical Hallmarks Activation of Caspases (cysteine-aspartic proteases) DNA fragmentation into $\approx 180-200$ base-pair multiples (laddering pattern on gel electrophoresis) Externalization of phosphatidylserine to the outer leaflet of the plasma membrane (detected by Annexin V binding) Loss of mitochondrial transmembrane potential Major Apoptotic Pathways 1. Extrinsic Pathway (Death Receptor Pathway) Initiated by binding of extracellular death ligands to specific cell surface death receptors. Death Receptors: Members of the TNF receptor superfamily (e.g., Fas/CD95, TNFR1, DR4, DR5). Ligands: FasL, TNF-$\alpha$, TRAIL. Mechanism: Ligand binding induces trimerization of death receptors. Recruitment of adaptor proteins (e.g., FADD for Fas, TRADD for TNFR1) via death domains (DD). Formation of the Death-Inducing Signaling Complex (DISC) . DISC recruits and activates initiator caspases ( Caspase-8 or Caspase-10 ). Activated initiator caspases cleave and activate executioner caspases (Caspase-3, -6, -7). Type I cells: Directly activate executioner caspases via DISC (e.g., lymphocytes). Type II cells: Require mitochondrial amplification loop (Bid cleavage) to activate intrinsic pathway (e.g., hepatocytes). 2. Intrinsic Pathway (Mitochondrial Pathway) Triggered by intracellular stress signals (e.g., DNA damage, growth factor withdrawal, ER stress, oxidative stress). Regulated by the Bcl-2 protein family . Bcl-2 Family Members: Anti-apoptotic: Bcl-2, Bcl-XL, Mcl-1 (prevent MOMP). Pro-apoptotic (BH3-only): Bad, Bid, Bim, Noxa, Puma (induce MOMP). Pro-apoptotic (multi-domain): Bax, Bak (form pores in mitochondrial membrane). Mechanism: Stress signals activate BH3-only proteins. BH3-only proteins inhibit anti-apoptotic Bcl-2 proteins and/or directly activate Bax/Bak. Activated Bax and Bak oligomerize and insert into the outer mitochondrial membrane, causing Mitochondrial Outer Membrane Permeabilization (MOMP) . Release of pro-apoptotic factors from the intermembrane space into the cytosol (e.g., Cytochrome c , Smac/Diablo, Omi/HtrA2). Cytochrome c binds to Apaf-1, causing Apaf-1 oligomerization. Formation of the Apoptosome (Apaf-1/Cytochrome c/dATP complex). Apoptosome recruits and activates initiator caspase ( Caspase-9 ). Activated Caspase-9 cleaves and activates executioner caspases (Caspase-3, -6, -7). Caspases: The Executioners Caspases are a family of cysteine proteases that cleave target proteins after aspartic acid residues. Synthesized as inactive pro-enzymes (zymogens). Activated by proteolytic cleavage. Initiator Caspases: Caspase-2, -8, -9, -10. Activate executioner caspases. Executioner (Effector) Caspases: Caspase-3, -6, -7. Cleave critical cellular proteins leading to morphological changes and DNA fragmentation. Targets include: PARP (DNA repair), Lamin (nuclear envelope), ICAD (inhibitor of CAD, freeing CAD to cleave DNA). Regulation of Apoptosis IAPs (Inhibitors of Apoptosis Proteins): Bind to and inhibit caspases (e.g., XIAP, cIAP1/2). Can be neutralized by mitochondrial proteins like Smac/Diablo and Omi/HtrA2. Decoy Receptors: Soluble or membrane-bound receptors that bind death ligands but lack signaling domains, thus preventing activation of death receptors. FLIP (FLICE-like Inhibitory Protein): Homolog of Caspase-8 that lacks protease activity. Competes with Caspase-8 for binding to FADD in the DISC, inhibiting extrinsic pathway. Role in Disease Too little apoptosis: Cancer (e.g., mutations in p53, overexpression of anti-apoptotic Bcl-2). Autoimmune diseases (e.g., SLE - failure to remove autoreactive lymphocytes). Too much apoptosis: Neurodegenerative diseases (e.g., Alzheimer's, Parkinson's, Huntington's). Ischemic injury (e.g., stroke, myocardial infarction). AIDS (CD4+ T cell depletion).