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Introduction to Apoptosis

• What is Apoptosis?
  • Definition: A form of programmed cell death that helps eliminate unneeded cells in the body.
  • Occurrence: Naturally occurs during various life stages (development, maintenance of tissues, removal of damaged cells).
  • Inducers: Triggered by DNA damaging agents, toxins, immune responses, and other stress factors.
  • Evolutionary Aspect: Conserved process across species (from worms to humans).

Role of Apoptosis

• Developmental Role:
  • Critical for morphogenesis (e.g., formation of fingers, neural connections).
• Tissue Maintenance:
  • Regulates cell number and tissue size by eliminating excess cells (e.g., lymphocytes post-infection).
  • Facilitates tissue remodeling.
• Protection:
  • Removes old or damaged cells (e.g., red blood cells with DNA damage).

Key Characteristics of Apoptosis

• Commitment to Death: Once committed, intracellular cascades are activated to coordinate cell death.
• Cellular Changes:
  • Cell condensation, rounding, blebbing of the membrane.
  • Nuclear breakdown and DNA fragmentation.
• Phagocytosis: Macrophages engulf apoptotic bodies, avoiding inflammation.

Morphological and Biochemical Changes

• Typical Features:
  • Cell blebbing and loss of membrane asymmetry (e.g., translocation of phosphatidylserine).
  • DNA fragmentation visible via gel electrophoresis (DNA laddering).

Pathways of Apoptosis

• Intrinsic Pathway:
  • Triggered by internal signals (e.g., DNA damage).
  • Mitochondria release cytochrome c, leading to caspase activation.
• Extrinsic Pathway:
  • Triggered by external signals (death receptors).
  • Involves initiator caspases (e.g., Caspase 8, Caspase 9) that activate executioner caspases (Caspase 3, 6, 7).

Caspase Cascade

• Caspases: Proteases crucial for apoptosis.
  • Initiator Caspases: Activate executioner caspases and form large complexes.
  • Executioner Caspases: Induce cell death by widespread protein cleavage.

Function of Mitochondria in Apoptosis

• Role in Intrinsic Pathway:
  • Cytochrome c release triggers apoptosome formation and caspase activation.
• Regulation: Balances the decision for cell survival or death.

Apoptosis in Disease

• Implications:
  • Excess Apoptosis: Contributes to conditions like heart attack or stroke.
  • Insufficient Apoptosis: Linked to autoimmune diseases (due to failure of lymphocyte death) and cancer (abnormal cell proliferation).

Apoptosis and Cancer

• Resistance to Apoptosis:
  • Cancer cells often upregulate anti-apoptotic proteins, evading typical cell death mechanisms.
  • Imbalance between cell growth and death leads to tumor progression and metastasis.

Contrast with Necrosis and Autophagy

• Necrosis:
  • Accidental, uncontrolled cell death often causing inflammation.
• Autophagy:
  • Regulated degradation of cellular components, crucial for cell survival under stress.
  • Cross-talk with apoptosis; can function as a survival method under nutrient deprivation.

Summary of Key Points

• Apoptosis is essential for normal development and tissue homeostasis.
• It is mediated by caspases through specific pathways.
• Understanding apoptosis is critical in addressing cancer therapies and other diseases.

Further Reading

• Alberts et al., Molecular Biology of the Cell 6th Edition, Chapters 18 and 13.