Apoptosis

Overview

• Programmed cell death 
  • requirements
    • ATP
    • functioning caspase enzymes for cellular degradation 
      • break down cytoskeleton
      • degrade DNA
      • endonucleases cut at internucleosomal regions, resulting in segments that are multiples of 180 base pairs 
• Two pathways
  • intrinsic
    • occurs in response to cell injury, loss of stimulatory growth factors, or DNA damage
    • involved in embryogenesis
    • mediated by changes in levels of anti-apoptotic (Bcl-2) and pro-apoptotic (Bax) factors
      • leads to ↑ mitochondrial permeability and release of cytochrome c 
        • release of cytochrome c activates caspase-9
      • Bcl-2 binds to cytochrome c preventing apoptosis and inhibits Apaf-1 (promotes activation of caspases)
        • overexpression of Bcl-2 leads to a ↓ in apoptosis leading to an ↑ of tumorigenesis.
        • p53 degrades Bcl-2 to promote apoptosis when DNA damage is severe
  • extrinsic
    • occurs in response to several external “death” signals
      • FAS ligand binding the FAS death receptor (CD95) 
        • CD95 mediates a cascade of caspase activation via FADD, a death domain-containing adapter protein
      • Also mediated by TNF which acts in a similar fashion to FAS
      • killer CD8 T-cells kill virally infected cells
        • release perforin and granzyme B which damage the cell membrane and activate caspases
• Mechanism
  • caspase cascade
    • regulation
      • pro-apoptotic factors
        • Bax 
          • activated by p53
            • absent in Li-Fraumeni syndrome
              • presents with high frequency of solid tumors
          • antagonizes Bcl-2
          • promotes release of mitochondrial cytochrome c into the cytosol
      • anti-apoptotic factors
        • Bcl-2 
          • prevents cytochrome c from being released and thus inhibits activation of caspase-9
    • caspases – inactive enzymes such as proteases and endonucleases 
      • activated by intrinsic and extrinsic pathways
      • endonuclease activity leads to pyknosis (nuclear condensation)
      • protease activity leads to cytoskeleton breakdown
    • membrane begins to bleb containing cellular fragments
      • apoptotic bodies break off and are phagocytosed by macrophages
• Key principles
  • distinct histological changes
    • cell shrinkage
    • pyknosis
    • eosinophilic cytoplasm
    • membrane blebbing
    • nuclear fragmentation (karyorrhexis)
    • nuclear fading
    • formation of apoptotic bodies
  • lack of inflammation
  • typically involves single or specific groups of cells leaving tissue architecture intact
• Examples
  • menstrual shedding of endometrium
    • apoptosis induced in the endometrium when estrogen and progesterone levels decrease
  • destruction of specific cells during embyrogenesis
    • loss of Mullerian structures in males due to Mullerian inhibititory factor
  • virally infected cells
    • apoptosis induced by cytotoxic T-cells
  • embryogenesis
    • apoptosis induced in skin between fingers