Define Apoptosis. Explain its mechanism with suitable example. (IFS 2020, 15 Marks)

Apoptosis

• Apoptosis is defined as the process of programmed cell death that is controlled by genetic programs in the cell.
• This process is characterized by specific morphological changes in the cell such as shrinkage, membrane blebbing, chromatin condensation, and fragmentation into apoptotic bodies.

Mechanism of Apoptosis

1. Initiation Phase

• Apoptosis can be triggered by internal or external signals.
  
  • Intrinsic Pathway: Triggered by internal cellular stress such as DNA damage, oxidative stress, or loss of growth signals.
  • Extrinsic Pathway: Initiated by external signals, usually through the binding of death ligands (e.g., Fas ligand) to cell surface receptors like the Fas receptor.

2. Activation of Caspases

• Central to the apoptotic process is the activation of caspases, a family of cysteine proteases.
  
  • Caspases: These enzymes initiate and execute apoptosis by cleaving specific cellular substrates.
  • There are two major groups of caspases:
    
    • Initiator Caspases (e.g., caspase-8, caspase-9): These are activated first and help activate the executioner caspases.
    • Effector or Executioner Caspases (e.g., caspase-3, caspase-7): These caspases carry out the final events of apoptosis, such as DNA fragmentation and cytoskeletal breakdown.

3. Mitochondrial Involvement

• In the intrinsic pathway, mitochondrial dysfunction plays a key role.
  
  • Mitochondrial membrane permeabilization: The release of cytochrome c from mitochondria into the cytoplasm triggers the activation of caspase-9.
  • Bcl-2 family proteins regulate mitochondrial permeability, with pro-apoptotic proteins (e.g., Bax, Bak) promoting mitochondrial outer membrane permeabilization, while anti-apoptotic proteins (e.g., Bcl-2, Bcl-xL) inhibit this process.

4. Execution Phase

• Following caspase activation, a series of morphological changes occur in the cell:
  
  • Cell Shrinkage: The cell begins to shrink, and its cytoskeleton disassembles.
  • Chromatin Condensation: DNA undergoes condensation and fragmentation.
  • Formation of Apoptotic Bodies: The dying cell breaks into membrane-bound vesicles (apoptotic bodies), which are phagocytosed by surrounding cells without causing inflammation.

Example of Apoptosis in Organisms

• Developmental Process (Tadpole to Frog): In amphibians, apoptosis is responsible for the removal of the tail during the metamorphosis of a tadpole into a frog. The cells of the tail undergo apoptosis, allowing for the proper formation of the adult frog.
• Immune System Regulation: In the immune system, apoptosis eliminates autoreactive T-cells that might attack the body’s own tissues, preventing autoimmune diseases.

Conclusion

Apoptosis is a highly regulated process that is essential for maintaining tissue homeostasis and eliminating unwanted or damaged cells. The mechanisms of apoptosis is crucial for various fields of biology, including developmental biology, immunology, and cancer research.