Apoptosis

Types of Cell Death

  • Cell death by injury
    • Mechanical damage
    • Exposure to toxic chemicals
  • Cell death by suicide
    • External signals
    • Internal signals

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

Why Should A Cell Commit Apoptosis?

  • Apoptosis is needed for proper development
    • Embryonic Morphogenesis
    • 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
    • Regulation of cell viability by hormones and growth factors (most cells die if they fail to receive survival signals from other cells)
  • Apoptosis is needed to destroy cells 
    • Cells infected with viruses
    • Cells of the immune system
    • Cells with DNA damage
    • Cancer cells

What Makes a Cell Decide to Commit To Apoptosis?

  • 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-α) 
      • Lymphotoxin (TNF-β) 
      • Fas ligand (FasL)

Caspases

  • Caspases: specialized proteases that mediate apoptosis
  • Protease with cysteine at active sites
  • Cleave  substrates at specific aspartic acids
  • Synthesized as procaspases and activated by other caspases

Caspase Activation Amplification Cascade

  1. An inhibitor of a DNAse: leads to fragmentation of DNA
  2. Nuclear Lamina: leads to fragmentation of nucleus
  3. Other Cytoskeletal Associated Proteins: leads to disruption of cytoskeleton and cell fragmentation
  4. Additional Caspases

Main Pathways Regulating Caspase Activation During Apoptosis

  • Intrinsic Pathway- Mitochondrial Mediated Major Pathway in Mammalian Cells 
    • Outer Mitochondrial Membrane Permeabilization (MOMP)
    • Release of Cytochrome C from Mitochondrial Intermembrane Space into Cytosol
    • Apoptosome formation: Activation of Initiator Caspase 
    • Effector caspases activated
  • Extrinsic Pathway- Signaling through Death Receptors
    • Ligand Bound Death Receptors
    • Adaptor Protein Association
    • Initiator Caspase Recruitment and Activation
    • Used by Immune System

Critical Regulators of Cell Death

  • Bcl-2 Family– Regulate whether MOMP Occurs
    • Anti-Apoptotic Factors: death inhibitors 
    • Function to inhibit MOMPs (Mitochondrial Outer Membrane Permeabilization) by pro apoptotic factors (aka death activators)
    • Pro-Apoptotic Factors: death activators 
    • Bind and inhibit death inhibitors
    • Directly cause MOMP to stimulate release of cytochrome C (BAX AND BAK)
  • IAP Family  (Inhibitor of Apoptosis)
    • Bind Procaspases prevent activation
    • Bind Caspases and inhibit Activity

Survival Factors

  • When cells are deprived of survival factors, they activate programmed cell death pathway
  • Survival factors usually act by binding to cell surface receptors 
    • Turn on signaling pathways that suppress cell death program
    • Some increase production of Bcl2