Apoptosis pathways

Apoptosis Intro - The Body's Tidy Takedown\n\n* Definition: ATP-dependent, genetically programmed cell death involving a coordinated cascade of caspases. It's a neat, single-cell process with no inflammation, unlike necrosis.\n* Physiologic Examples:\n - Embryogenesis (e.g., removal of interdigital webs).\n - Endometrial shedding during the menstrual cycle.\n - Removing self-reactive lymphocytes.\n* Pathologic Examples:\n - Viral infections (e.g., cytotoxic T-cell killing of infected cells).\n - DNA damage or misfolded protein accumulation.\n - Atrophy in parenchymal organs after duct obstruction.\n\n\n\n> ⭐ Key Distinction: Unlike necrosis, apoptosis does not trigger an inflammatory response because cell contents are contained within apoptotic bodies and cleared by phagocytes.

Intrinsic Pathway - Mitochondrial Mayhem

• Triggered by internal insults: DNA damage (p53 mediated), misfolded proteins (ER stress), or loss of growth factors.
• Regulated by the BCL-2 family of proteins, which control mitochondrial permeability.
  • Anti-apoptotic (pro-survival): BCL-2, BCL-xL. They prevent leakage. 📌 BCL-2 = Cancer Loves to 2 Live.
  • Pro-apoptotic: BAX, BAK. When activated, they dimerize and form pores in the outer mitochondrial membrane.
• Cellular stress inactivates BCL-2/BCL-xL, allowing BAX/BAK to create channels.
• This ↑ permeability allows cytochrome c to escape into the cytosol.
• In the cytosol, cytochrome c binds to APAF-1 (Apoptotic Peptidase Activating Factor 1), forming the apoptosome, which activates initiator Caspase-9.

⭐ Follicular lymphoma is often driven by a t(14;18) translocation, which causes overexpression of the anti-apoptotic BCL-2 protein, preventing cancer cells from undergoing apoptosis.

Extrinsic Pathway - Death's Doorbell

• Trigger: Initiated by extracellular signals. Ligands from other cells bind to "death receptors" on the target cell surface.
• Key Receptors & Ligands:
  • Fas (CD95) receptor binds to Fas Ligand (FasL).
  • TNF Receptor 1 (TNFR1) binds to Tumor Necrosis Factor-α (TNF-α).

⭐ Exam Favorite: Cytotoxic T-lymphocytes (CTLs) kill virus-infected cells and tumor cells by expressing FasL on their surface. This binds to Fas on the target cell, triggering apoptosis and forming a core part of immune surveillance.

• The key initiator caspase for this pathway is Caspase-8.

Execution & Cleanup - Taking Out the Trash

• Common Pathway: Both intrinsic and extrinsic pathways converge on activating executioner caspases (e.g., Caspase-3, 6).
• Execution Phase:
  • Caspases dismantle the cytoskeleton & activate endonucleases to degrade DNA.
  • The cell condenses and buds off into membrane-bound apoptotic bodies.
• Cleanup Crew:
  • Apoptotic bodies display “eat-me” signals (e.g., phosphatidylserine).
  • These are rapidly engulfed by phagocytes (e.g., macrophages).

⭐ Unlike necrosis, apoptosis is an orderly, non-inflammatory process because cell contents are packaged and cleared before they can leak out and incite a reaction.

High‑Yield Points - ⚡ Biggest Takeaways

• Apoptosis is an ATP-dependent programmed cell death that eliminates cells without significant inflammation.
• The intrinsic (mitochondrial) pathway is controlled by the Bcl-2 family, leading to cytochrome c release.
• The extrinsic (death receptor) pathway is triggered by Fas-FasL or TNF-α binding.
• Both pathways converge to activate executioner caspases (e.g., Caspase-3), the key enzymes of apoptosis.
• Morphologic hallmarks include cell shrinkage, chromatin condensation, and formation of apoptotic bodies.