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Het extrinsieke apoptotische pad
De extrinsieke apoptotische route wordt geïnitieerd wanneer extracellulaire dood-inducerende signalen, zoals specifieke cytokines, de doodsreceptoren…
The extrinsic apoptotic pathway is activated through extracellular death signals from immune cells.
These immune cells recognize dangerous cells, such as diseased ones, through homotrimeric ligands that bind to death receptors on the target cell membrane.
The death receptors belong to the tumor necrosis factor or TNF superfamily, which includes TNF receptor 1 and the Fas receptor.
In the Fas signaling pathway, the Fas ligand expressed on the surface of immune cells binds to Fas receptor on the target cell, causing activation of the receptor death domain.
The activated death domain recruits an adaptor protein, Fas-associated death domain, which then binds to initiator procaspase-8 to form the death-inducing signaling complex or DISC.
Inside DISC, procaspase-8 molecules dimerize and self-cleave to form active caspase-8, which is released into the cytosol.
Caspase-8 activates executioner procaspase-3 to form caspase-3 that cleaves cellular proteins, resulting in apoptosis.
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Q1: How do immune cells activate the extrinsic apoptotic pathway?
Immune cells such as natural killer cells and cytotoxic T-lymphocytes recognize dangerous cells through homotrimeric ligands that bind to death receptors on the target cell membrane. Death receptors belong to the TNF superfamily, including TNF receptor 1 and Fas receptor. This binding activates the receptor death domain, initiating the extrinsic apoptotic pathway to eliminate cancerous or virus-infected cells.
Q2: What is the death-inducing signaling complex and how does it form?
The death-inducing signaling complex (DISC) forms when the Fas ligand binds to the Fas receptor, activating the death domain. The activated death domain recruits the adaptor protein FADD (Fas-associated death domain), which then binds to initiator procaspase-8. Inside DISC, procaspase-8 molecules dimerize and self-cleave to form active caspase-8, which is released into the cytosol to continue the apoptotic cascade.
Q3: How does caspase-8 lead to cell death in the extrinsic pathway?
Active caspase-8 released from DISC activates executioner procaspase-3 to form caspase-3. Caspase-3 then cleaves cellular proteins, resulting in apoptosis. This cascade of protease activation ensures rapid and efficient cell death once the extrinsic pathway is triggered by death receptor signaling.
Q4: Why is the extrinsic apoptotic pathway important during T-cell development?
During T-cell development in the thymus, immature T-cells (thymocytes) undergo screening to determine whether they bind self-proteins. If a T-cell binds to self-proteins, it is signaled for apoptosis through the extrinsic pathway. This process eliminates self-reactive T-cells and prevents autoimmune responses that could damage the body's own tissues.
Q5: What role does c-FLIP play in regulating the extrinsic apoptotic pathway?
c-FLIP (FLICE inhibitory protein) is a master anti-apoptotic regulator that binds to FADD and caspase-8, preventing the formation of the death-inducing signaling complex (DISC). By blocking DISC assembly, c-FLIP inhibits the extrinsic apoptotic pathway and allows cells to survive even when death signals are present.
Q6: What is the difference between natural killer cells and cytotoxic T-lymphocytes in apoptosis?
Natural killer cells are critical in innate immune response and recognize altered cells without prior sensitization. Cytotoxic T-lymphocytes are associated with adaptive immune response and target specific foreign antigens. Both cell types activate the extrinsic apoptotic pathway in target cells, but they differ in how they recognize threats and the specificity of their responses.
Q7: How does the extrinsic pathway differ from other forms of programmed cell death?
The extrinsic apoptotic pathway is triggered by extracellular death signals from immune cells binding to death receptors. This contrasts with the intrinsic apoptotic pathway, which is activated by internal cellular stress. Understanding both pathways and other forms like autophagic cell death provides comprehensive insight into how cells undergo programmed death through different mechanisms.