Apoptosis Mechanisms | Zoology Optional Notes for UPSC PDF Download

Why Do Cells Undergo Apoptosis?

1. Inherent Mechanism: Most cells have an intrinsic apoptosis mechanism integrated into the cell cycle.
2. Cellular Cleanup: Enables the elimination of unnecessary or infected cells from the body.
3. Integral Processes: Essential for normal cell cycle progression, immune system function, embryonic development, and chemically induced cell death.
4. Developmental Role: Crucial in tissue differentiation during development.
5. DNA Defects and Infections: Occurs in response to DNA defects, preventing cells from repairing the damage. Also Targets infected or potentially cancerous cells.
6. Immune System Support: Clears pathogen-specific immune cells after the removal of foreign particles. It Prevents autoimmune diseases by eliminating immune cells reactive against the body.
7. Homeostasis Maintenance: Contributes to body homeostasis by removing old cells, creating space for new ones.

Apoptosis Mechanisms

1. Extrinsic (Death Receptor) Pathway:

   • Initiation: Involves transmembrane receptor-mediated interactions.
   • Participants: Ligands, death receptors (TNF family), death domain.
   • Process: Ligand binding activates adapter proteins, leading to death-inducing signaling complex (DISC) formation and caspase-8 activation.

2. Intrinsic (Mitochondrial) Pathway:

   • Initiation: Non-receptor-mediated processes acting directly on cell targets.
   • Triggers: Positive (radiation, toxins, etc.) and negative (absence of growth factors) signals.
   • Events: Factors induce mitochondrial permeability transition, releasing pro-apoptotic proteins.
   • Formation: Apoptosome, activated caspase-9, and effector caspase-3.

3. Perforin/Granzyme Pathway:

   • Participants: Cytotoxic T lymphocytes, perforin, granzyme A, and granzyme B.
   • Actions: Perforin forms pores, releasing granules containing granzymes.
   • Effects: Granzyme B activates caspase-10, cleaves ICAD, and induces mitochondrial pathway for death signal amplification.
   • Direct Activation: Granzyme B can activate caspase-3 directly.

4. Execution Pathway:

   • Initiation: Triggered by activation of caspases (e.g., caspase-3).
   • Effects: Caspase-3 activates endonucleases and proteases leading to chromatin condensation, cytoskeletal reorganization, and cell disintegration.
   • Outcome: Formation of apoptotic bodies, externalization of phosphatidylserine, facilitating noninflammatory phagocytic recognition.

Execution Pathway and Cellular Outcomes:

1. Caspase Activation: Initiated by caspase-3 activation.
2. Chromatin Condensation: Caspase-activated DNase (CAD) causes chromatin condensation.
3. Cytoskeletal Reorganization: Caspase-3 cleaves gelsolin, disrupting the cytoskeleton.
4. Cell Disintegration:
   • Formation of apoptotic bodies.
   • Externalization of phosphatidylserine for noninflammatory phagocytic recognition

Inhibition of Apoptosis

1. Cell Survival Mechanism:

   • Inhibiting apoptosis disrupts cell death signaling pathways, aiding tumor cells in evading apoptosis.

2. Negative Regulators:

   • Proteins acting as negative regulators include anti-apoptotic factors like IAPs (Inhibitor of Apoptosis) and Bcl-2.

3. IAP Proteins:

   • Human IAP group comprises 8 proteins with a characteristic BIR domain.
   • BIR domain binds to caspases and other apoptosis-related proteins.
   • XIAP, a member, inhibits caspase-9 and caspase-3, preventing their activation.

4. Bcl-2:

   • Governs mitochondrial membrane permeability.
   • Can be pro-apoptotic or anti-apoptotic.
   • Anti-apoptotic Bcl-2 proteins inhibit cytochrome c release, hindering intrinsic apoptosis.

5. Cancer Connection:

   • Cells escaping apoptosis majorly contribute to cancers like leukemia and multiple myeloma.
   • Inhibition of apoptosis leads to immune function loss, and XIAP mutation causes a rare immunodeficiency.

Regulation of Apoptosis

1. Importance of IAPs and Bcl-2:

   • IAPs and Bcl-2 crucially decide whether apoptosis progresses or is inhibited.

2. Extrinsic Pathway Regulation:

   • c-FLIP inhibits FADD and caspase-8, blocking the extrinsic pathway.
   • Toso blocks Fas-induced apoptosis by inhibiting caspase-8 activation in T cells.

3. Intrinsic Pathway Regulation:

   • Bcl-2 family controls mitochondrial membrane permeability.
   • Pro-apoptotic factors like Puma and Noxa counteract anti-apoptotic proteins.
   • Released proteins like Smac inhibit IAPs in the mitochondrial pathway.

Apoptosis Assays

1. Cytomorphological Alteration:

   • H&E-stained tissue sections visualize apoptotic cells.
   • TEM confirms apoptosis with structural characteristics.
   • Changes include electron-dense nucleus, nuclear fragmentation, intact cell membrane, clear vacuoles, and phosphatidylserine externalization.

2. DNA Fragmentation:

   • DNA laddering technique detects endonuclease-cleaved DNA.
   • TUNEL method labels DNA ends for apoptosis detection.
   • Both methods visualize late-stage apoptosis.

3. Caspase Detection:

   • Various caspase activity assays for detecting over 13 caspases.
   • Immunoassays identify cleaved substrates and modifications.
   • Techniques include western blot, immunoprecipitation, and immunohistochemistry.

4. Membrane Alterations:

   • Annexin V detects phosphatidylserine on apoptotic cell membranes.
   • Necrotic cells labeled to avoid false positives.
   • Other dyes visualize membrane changes, like the movement of phosphatidylserine.

5. Whole Mounts:

   • Dyes (acridine orange, Nile blue sulfate, neutral red) visualize embryos or tissues.
   • Acidophilic dyes concentrate in lysosomal and phagocytotic areas.
   • Associated with other assays due to limitations.

6. Mitochondrial Assays:

   • Monitor early-stage intrinsic pathway changes.
   • LSCM provides 3D imaging of apoptotic cells.
   • Measures include mitochondrial permeability, depolarization, redox status, Ca2+ fluxes, and ROS.
   • Changes also occur in necrosis, requiring additional confirmation.

Understanding apoptosis through diverse assays ensures accurate evaluation, given the intricacy of the process and potential overlaps with necrosis. The combination of assays provides a comprehensive view of the dynamic cellular events involved in apoptosis.

Significance, Applications, and Roles of Apoptosis

1. Developmental Role:

   • Original Statement: Apoptosis is essential during development, contributing to tissue and organ formation.
   • Paraphrased: Apoptosis is crucial in development, shaping tissues and organs from an initial mass of tissue.
   • Explanation: Programmed cell death in apoptosis is pivotal for the precise organization of cells during embryonic development.

2. Cell Population Maintenance:

   • Original Statement: Regular removal of old cells aids in producing new cells, maintaining the cell population.
   • Paraphrased: Eliminating aged cells supports continuous cell production, sustaining overall cell numbers.
   • Explanation: Apoptosis facilitates a dynamic cellular turnover, ensuring the renewal of cell populations for optimal functioning.

3. Removal of Damaged Cells:

   • Original Statement: Apoptosis removes redundant and damaged cells, including virus-infected or irreparable ones.
   • Paraphrased: Apoptosis eliminates damaged cells, whether redundant, virus-infected, or beyond repair.
   • Explanation: This process acts as a quality control mechanism, discarding cells with potential harm or malfunction.

4. Immune System Check:

   • Original Statement: The immune system uses apoptosis to check the self-destructive nature of newly formed cells.
   • Paraphrased: Apoptosis is employed by the immune system to ensure newly formed cells are not self-destructive.
   • Explanation: This safeguards against autoimmune diseases by removing immune cells harmful to the body's own cells.

5. Immune Response Regulation:

   • Original Statement: Apoptosis plays a critical role in regulating immune responses, involving cytotoxic cells like T lymphocytes.
   • Paraphrased: Apoptosis is vital in immune response regulation, including the removal of pathogen-specific immune cells.
   • Explanation: It fine-tunes the immune system, eliminating unnecessary immune cells after an infection is cleared.

6. Neuronal Development and Diseases:

   • Original Statement: Apoptosis is involved in removing 50% of neurons in early embryonic development and in forming reproductive parts.
   • Paraphrased: Apoptosis contributes to neuronal removal in embryonic development and reproductive part formation.
   • Explanation: This controlled elimination is crucial for proper neural development, while imbalance leads to neurodegenerative diseases or developmental abnormalities.

7. Disease Implications:

   • Original Statement: Excess apoptosis may result in neurodegenerative diseases, while deficient apoptosis occurs in cancer and autoimmune diseases.
   • Paraphrased: Neurodegenerative diseases may stem from excess apoptosis, while cancer and autoimmune diseases are associated with insufficient apoptosis.
   • Explanation: Maintaining the balance of apoptosis is vital for preventing both excessive cell loss and the survival of potentially harmful cells.

Understanding the multifaceted roles of apoptosis highlights its significance in development, immune response, and disease prevention, emphasizing the delicate balance required for proper cellular function.

Apoptosis and Cancer

1. Genetic Changes in Cancer:

   • Original Statement: Cancer results from genetic changes in a normal cell, requiring evasion of cell death.
   • Paraphrased: Genetic alterations turn normal cells into malignant ones, necessitating escape from programmed cell death.
   • Explanation: Cancer development involves genetic transformations that confer resistance to apoptosis, enabling abnormal cell survival.

2. DNA Damage and Apoptosis Inhibition:

   • Original Statement: Apoptosis is usually triggered by irreparable DNA damage, but damage to apoptotic genes may hinder the process.
   • Paraphrased: DNA damage typically initiates apoptosis, but damage to genes involved in apoptosis can impede the process.
   • Explanation: Genetic changes impacting apoptotic genes can disrupt the cell's ability to undergo programmed cell death.

3. Protein Imbalance and Chemoresistance:

   • Original Statement: Imbalance in pro-apoptotic and anti-apoptotic proteins, like abnormal IAP family expression, can confer resistance to chemotherapy.
   • Paraphrased: Disrupted balance of pro-apoptotic and anti-apoptotic proteins, as seen in abnormal IAP family expression, leads to chemotherapy resistance.
   • Explanation: Altered expression of apoptosis-related proteins, particularly the IAP family, contributes to cancer cells evading cell death induced by chemotherapy.

4. Caspase Dysfunction and Colorectal Cancer Example:

   • Original Statement: Reduced caspase function, as seen in downregulated caspase-9 in stage II colorectal cancer, can result in apoptosis evasion.
   • Paraphrased: Diminished caspase function, exemplified by caspase-9 downregulation in stage II colorectal cancer, hinders apoptosis.
   • Explanation: Dysfunction in caspases, crucial for apoptosis, can contribute to the evasion of programmed cell death in specific cancer stages.

5. Restoration of Apoptotic Signaling as Cancer Treatment:

   • Original Statement: Strategies targeting anti-apoptotic proteins aim to restore apoptotic signaling, potentially eliminating cancer cells.
   • Paraphrased: Treatment approaches addressing anti-apoptotic proteins strive to normalize apoptotic signaling, offering potential for cancer cell elimination.
   • Explanation: Developing therapies that restore the balance in apoptotic signaling pathways presents a promising avenue for cancer treatment.

Apoptosis in Plants

1. Programmed Cell Death in Plants:

   • Original Statement: Apoptosis in plants is termed programmed cell death due to differences in mechanisms and the absence of an immune system.
   • Paraphrased: Programmed cell death is the appropriate term for plant cell death, distinct due to mechanism variances and the lack of an immune system.
   • Explanation: Unlike animals, plants lack an immune system relying on apoptosis, necessitating the use of a different term for their programmed cell death.

2. Control of Plant Programmed Cell Death:

   • Original Statement: Programmed cell death in plants is regulated by oxidative status, phytohormones, and DNA methylation.
   • Paraphrased: Plant programmed cell death is controlled by cellular oxidative status, phytohormones, and DNA methylation.
   • Explanation: Specific factors, including oxidative status and phytohormones, influence the regulated process of programmed cell death in plants.

3. Differences in Mechanism:

   • Original Statement: Plant apoptosis differs from animals; protease proteins replace caspases, inducing unique morphological changes.
   • Paraphrased: Plant programmed cell death contrasts with animal apoptosis; protease proteins induce distinctive morphological changes.
   • Explanation: The mechanisms of cell death vary between plants and animals, involving different proteins and morphological transformations.

4. Morphological Changes in Plant Programmed Cell Death:

   • Original Statement: Morphological changes in plant programmed cell death include vacuolization, cytoplasmic fragmentation, and nuclear alterations.
   • Paraphrased: Plant programmed cell death induces morphological changes like vacuolization, cytoplasmic fragmentation, and nuclear alterations.
   • Explanation: Observable morphological alterations distinguish plant programmed cell death, involving changes in cellular structures.

Understanding the distinctions in apoptosis in cancer and programmed cell death in plants provides insights into the diverse mechanisms governing cell fate in different organisms.