Lecture 12.

• Cell Death.

  - Process of cells ceasing to function or survive.
• Why do Cells Die?

  - Faulty cells that stay without serving its function = has negative effects on organism

  - Not enough nutrients, damage to cells.
• 3 Main Cell Death Mechanisms.

  - Type I and II = controlled

  - Type III = uncontrolled.

  
• Type 1: Apoptosis.

  - Controlled cell death vital for organism health

  - Without apoptosis = old, damaged cells remain

  - Occurs automatically at end of cell lifespan.

  
• How does it occur?

  - Shrinkage > blebbing > cytoskeleton detaches from membrane > helps in disassembly of the cell.
• Blebbing.

  - Membrane protrusions formed during apoptotic cell shrinkage.
• Physiological Relevance - During Development.

  - Apoptosis involved in morphogenesis & tissue remodelling

  - E.g. tadpole tail removed by apoptosis at time of its metamorphosis into a frog

  - E.g. individualisation of digits that occurs in many animals through removal of inter-digital webs

  - Eliminates unwanted cells.

  
• Morphogenesis.

  - Developmental process involving cell shape changes.
• Tissue Remodelling.

  - Elimination of organs and tissue only useful during development.
• Neurodegenerative Diseases & Organ Failure.

  - Conditions linked to excessive apoptosis in neurons & organs.
• Proliferative Diseases.

  - Diseases like cancer associated with defective apoptosis.
• Induction Factors.

  - Triggers for apoptosis

  - E.g. DNA damage, hypoxia, oxidative stress, death receptor ligands.

  
• Hallmarks of Apoptosis.

  - Cellular & nuclear shrinkage

  - Disassembly into apoptotic bodies

  - Chromatin condensation & DNA fragmentation

  - Mitochondrial membrane permeabilisation

  - Cytochrome-c release

  - Caspases cascade activation.

  
• Apoptotic Bodies.

  - Membrane-bound fragments resulting from cell disassembly.
• Caspases.

  - Proteins that execute apoptosis through a cascade.
• Extrinsic Apoptosis.

  - Cell death initiated ligand binding to death receptor

  - Leading to cell death.

  
• Intrinsic Apoptosis.

  - Cell death regulated by internal mitochondrial signals

  - Can control death process

  - Cytochrome c release = permeable membrane.

  
• Cytochrome c release - how does it work?

  - (1) carcinogen/mutagen causing DNA damage

  - (2) p53 detects DNA damage

  - (3) releases BAK

  - (4) triggers mitochondrial membrane permeability

  - (5) cytochrome c release

  - (6) joins caspase 9 (proteins that trigger apoptosis - "initiator caspase")

  - (7) caspase 3 ("executioner caspase").

  
• Is Apoptosis a Slow Process? True or false.

  - True, highly controlled process.
• DNA damage can trigger both the extrinsic and intrinsic cell death pathway. True or false.

  - True, extrinsic pathway is triggered by external signals e.g. binding of death ligands to death receptors

  - This binding activates a cascade that leads to activation of caspases > cell death.
• Extrinsic Pathway.

  - Triggered by external signals like death ligands.
• Death Receptor Ligands.

  - Molecules that bind to receptors triggering apoptosis.
• Death Ligands.

  - Bind to death receptors, activating apoptosis.
• Changes in the Plasma Membrane Externalisation of Phosphatidylserine.

  - Indicator of apoptosis

  - Marks cells for phagocytosis.

  
• Mitochondrial Membrane Potential Loss.

  - Precedes cytochrome c release into cytosol.

  
• Reliance on Intracellular Proteolytic Cascade.

  - Mediated by caspases

  - Caspase 9 = breaks bond in caspase 3 (activate it) > breaks bonds in processes that trigger apoptosis

  - Activated caspase-3 cleaves various cellular substrates > leading to morphological and biochemical changes

  - E.g. DNA fragmentation, cell shrinkage & membrane blebbing.

  
• Caspase-9.

  - Activates caspase-3 by cleaving its bonds.
• Caspase-3.

  - Cleaves substrates causing cell death changes.
• Cell-surface (FAS) Death Receptors.

  - Activate the extrinsic pathway.

  
• Bcl-2 (B-cell lymphoma 2) Protein Family & their Control of Apoptosis.

  - Regulates apoptosis; includes pro- and anti-apoptotic members.

  
• Bcl-2 Anti-apoptotic Mechanism.

  - Inhibits cell death.

  
• Bcl-2 Pro-apoptotic Mechanism.

  - Bax

  - Bak

  - Both trigger Cytochrome c release.

  
• Bax.

  - Pro-apoptotic protein that promotes cytochrome c release

  - Bound to mitochondrial outer membrane.
• Bak.

  - Pro-apoptotic protein that translocates to mitochondria

  - Located in cytosol, moves upon apoptotic signalling.
• Complexity of Bcl-2 Regulation.

  
• IAP Family.

  - Inhibitors of apoptosis proteins that bind caspases and inhibit them.

  
• IAP Mechanism.

  - Proposed model for IAPs and anti-IAPs controlling apoptosis.

  
• Correlation of IAPs Expression and Patient Survival in Different Types of Cancers.

  - Global Prognostic Values of IAPs for patient survival

  - IAPs differed in prognostic values among different cancers

  - Higher IAPs expression = shorter survival.

  
• Summary.

  - Mechanism of programmed cell death

  - Destroy and replace unneeded cells

  - Morphology: cellular & nuclear shrinkage, chromatin condensation, DNA fragmentation, apoptotic bodies

  - Activation = intrinsic/extrinsic

  - Regulation = Bcl-2 & IAP.

  
• Type II Cell Death.

  - Autophagic cell death, distinct from apoptosis

  - E.g. cytotoxic T killer cells.