Apoptosis Mechanisms of Programmed Cell Death

Apoptosis and Mechanisms of Cell Death

Introduction to Apoptosis

  • Apoptosis is a mechanism for removing cells or tissues that are no longer needed, damaged, or infected.

  • It is a natural process that occurs during development.

Morphological and Biochemical Features of Apoptosis

  • Key features include:

    • Blebbing of Plasma Membrane (PM): The membrane begins to bulge outward.

    • Pyknosis: Refers to the condensation of the nucleus.

    • Golgi Fragmentation: The Golgi apparatus disassembles.

    • DNA Fragmentation: Breakdown of DNA into smaller fragments.

Role of Caspases in Apoptosis

  • Apoptosis is mediated by a family of proteins known as Caspases (Cysteine aspartic acid proteases).

  • Caspases are cysteine proteases that cleave substrates following an aspartate residue.

Activation of Caspases

  • Caspases are produced in an inactive form called procaspases.

  • Caspase Activation: This occurs through proteolytic cleavage.

    • Initiator Caspases: These exist as inactive monomers and cleave the inactive forms of effector caspases.

    • Executioner Caspases: These are responsible for cleaving a variety of cellular proteins, leading to cellular breakdown.

Consequences of Caspase Activation

  • One significant consequence is the cleavage of ICAD (Inhibitor of Caspase-Activated Deoxyribonuclease), which releases CAD (Caspase Activated DNase).

  • CAD acts as an endonuclease that fragments DNA, further confirming apoptosis.

    • i- inhibitory

Pathways of Apoptosis
Extrinsic Pathway of Apoptosis

  • This pathway is initiated by the activation of death receptors on the cell surface.

  • These receptors belong to the Tumor Necrosis Factor (TNF) receptor family and possess:

    • An extracellular domain for ligand binding,

    • A transmembrane domain, and

    • An intracellular death domain that transmits the apoptotic signal.

  • The receptors and ligands function as homotrimers, defined as clusters of three identical molecules that facilitate the binding process and activate downstream signaling pathways.

  • Why is it called the unexposed death effector domain?

  • DDs bind to DDs. DEDs bind to DEDs. Different protein sequences

  • When DISC forms, caspase-8 cleaves

Formation of Reacting Complexes

  • Death receptors recruit adaptor proteins such as FADD (Fas-Associated protein with Death Domain).

  • This recruitment leads to the formation of a Death Inducing Signaling Complex (DISC).

  • Within the DISC, initiator caspases (primarily Caspase 8) are activated.

Intrinsic Pathway of Apoptosis

  • Details and illustrations about the intrinsic pathway can be found in referenced materials (Figure 9.29 in The Biology of Cancer, Garland Science, 2007).

    • Mitochondria important for this- MOMP- mitochondrial outer membrane permeabilization (irreversible process, cell must die within minutes once pathway is triggered. Cytochrome C released quickly)

      • Cytochrome C- releasing out of cell triggers intrinsic pathway (pore formation around outer membrane allows for this. DNA damage can cause this)

      • Binds to Apaf 1 (adaptor-like protein), shape change, forms 7 pin wheel, and activates/recruits multiple caspase-9s, which subsequently leads to the activation of effector caspases that carry out the process of apoptosis.

Regulation by Bcl2 Family of Proteins

  • The Bcl2 family of proteins plays a crucial role in regulating apoptosis:

    • There is a balance between pro-apoptotic and anti-apoptotic members of this family that ultimately determines the cell's fate during apoptosis.

    • BCL- B-cell lymphoma. (Bcl-2) is one of the most well-studied anti-apoptotic proteins in these cancers which promotes cell survival (prevents apoptosis) by inhibiting cytochrome c release from the mitochondria. Inhbbits BAK and BAX, which are pro-apoptotic proteins that promote mitochondrial outer membrane permeabilization and the release of apoptogenic factors.

      • Pro-survival

        • Proteins redundant- their functions can sometimes compensate for one another if one is lost, allowing cells to evade apoptosis even in unfavorable conditions.

      • Pro-apoptosis

        • Can be overexpressed, one inhibiting other

        • BH3- BCL homology3 (BH3) proteins play a critical role in the regulation of apoptosis by promoting pro-apoptotic signaling pathways and antagonizing anti-apoptotic factors, thus tipping the balance toward cell death.

        • BAKs or BAX self associate/self-bind and form pores for Cytochrome C to release

          • Cluster formation

        • P53 can activate BH3 which inhibits BCL2 which promotes the release of pro-apoptotic factors (BAK and BAX) from the mitochondria, further facilitating the apoptotic process.

  • Don't need both BAX or BAK , as either can initiate the process of apoptosis independently, making them functionally interchangeable in the context of mitochondrial permeabilization. But optimal together

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