Stepwise intrinsic mechanism of apoptotic induction (IAS 2022/7.5 Marks)

Introduction

Apoptosis, also known as programmed cell death, is a highly regulated process that plays a crucial role in maintaining tissue homeostasis and eliminating damaged or unnecessary cells. The intrinsic mechanism of apoptotic induction involves a series of steps that are initiated within the cell itself in response to various internal signals.

Stepwise Intrinsic Mechanism of Apoptotic Induction

• Triggering of the Intrinsic Pathway
  
  • The intrinsic apoptotic pathway is initiated by internal cellular stress signals like DNA damage, oxidative stress, or other forms of cellular injury.
  • These signals lead to the activation of pro-apoptotic proteins within the cell.
• Mitochondrial Dysfunction
  
  • The key event in the intrinsic pathway is mitochondrial dysfunction, which is triggered by cellular stress.
  • Damaged mitochondria release proteins that are normally contained within the mitochondria, such as cytochrome c, into the cytoplasm.
• Release of Cytochrome C
  
  • Cytochrome c is released from the mitochondria into the cytosol.
  • This release is regulated by pro-apoptotic proteins of the Bcl-2 family, such as Bax and Bak, which form pores in the mitochondrial membrane, allowing cytochrome c to escape.
• Formation of the Apoptosome
  
  • Once in the cytoplasm, cytochrome c binds to the adaptor protein Apaf-1 (apoptotic protease-activating factor-1).
  • This binding causes the formation of the apoptosome, a complex that activates caspase-9, the initiator caspase in apoptosis.
• Activation of Caspase Cascade
  
  • Activated caspase-9 cleaves and activates downstream effector caspases, such as caspase-3, -6, and -7.
  • These effector caspases dismantle the cell by cleaving various cellular components, such as structural proteins, and activating nucleases that degrade DNA.
• DNA Fragmentation
  
  • As a result of the activation of caspases, DNases such as CAD (caspase-activated DNase) are activated, leading to the fragmentation of the DNA into smaller fragments.
  • This is a hallmark of apoptosis, as the cell's genetic material is systematically degraded.
• Phagocytosis of the Apoptotic Cell
  
  • The cell membrane undergoes blebbing, and the cell eventually breaks apart into smaller apoptotic bodies.
  • These apoptotic bodies are recognized and engulfed by neighboring cells or macrophages, preventing an inflammatory response.

Conclusion

The stepwise intrinsic mechanism of apoptotic induction is a highly regulated process that ensures the efficient elimination of unwanted or damaged cells. Understanding the molecular pathways involved in apoptosis is crucial for developing therapeutic strategies for diseases characterized by dysregulated cell death, such as cancer and neurodegenerative disorders.