apoptosis extrinsic pathway

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Apoptosis — Extrinsic (Death Receptor) Pathway

Death-receptor-mediated vs. mitochondrial-mediated apoptosis pathways

Harrison's Principles of Internal Medicine, 21st Ed.

Overview

The extrinsic pathway (also called the death receptor pathway) initiates apoptosis from outside the cell via specific ligand–receptor interactions at the plasma membrane. It is distinct from the intrinsic (mitochondrial) pathway, though the two can intersect.

Step-by-Step Mechanism

1. Death Receptor Engagement

Death receptors are members of the TNF receptor superfamily that carry a cytoplasmic death domain (DD) — a ~80-amino-acid motif essential for transmitting apoptotic signals. The best-characterized examples:

Receptor	Ligand
Fas (CD95, APO-1)	FasL (CD95L)
TNFR1	TNF-α
DR4 / DR5	TRAIL (TNF-related apoptosis-inducing ligand)

When a death ligand (e.g., FasL) binds its receptor, three or more receptor molecules oligomerize, clustering their cytoplasmic death domains.

2. DISC Formation

The clustered death domains recruit the adaptor protein FADD (Fas-associated death domain protein). FADD contains both a death domain (binds receptor) and a death effector domain (DED) (binds caspase-8/10).

FADD recruits pro-caspase-8 (and/or pro-caspase-10) through DED–DED interactions. This multiprotein complex — receptor + FADD + pro-caspase-8/10 — is the Death-Inducing Signaling Complex (DISC).

3. Caspase-8 Activation

Within the DISC, multiple pro-caspase-8 molecules are brought into close proximity, enabling autocatalytic cleavage and generation of active caspase-8 (an initiator caspase).

4. Effector Caspase Cascade (Execution Phase)

Active caspase-8 directly cleaves and activates the executioner caspases — caspase-3, -6, and -7. These then:

Cleave ICAD (inhibitor of caspase-activated DNase) → releases CAD/DFF40 → causes internucleosomal DNA fragmentation
Proteolyze structural nuclear matrix proteins → nuclear fragmentation
Activate other downstream substrates → membrane blebbing, apoptotic body formation, and phagocytic recognition signals (e.g., phosphatidylserine exposure)

Crosstalk with the Intrinsic Pathway

In some cell types (e.g., hepatocytes, pancreatic β-cells), caspase-8 cleaves the BH3-only protein BID → generating tBID (truncated BID), which translocates to the mitochondria, activates BAX/BAK, and amplifies the apoptotic signal through the intrinsic pathway. This amplification loop is necessary in cells where direct caspase-8 activation alone is insufficient to commit the cell to death.

Key Regulators

Molecule	Role
c-FLIP	Competitive inhibitor; binds pro-caspase-8 and blocks DISC assembly/activation → anti-apoptotic
IAPs (XIAP, survivin, etc.)	Inhibit active caspases-3, -7, and -9
SMAC/DIABLO	Released from mitochondria; neutralizes IAPs → promotes apoptosis

Physiological Roles

Peripheral tolerance: FasL on activated T cells kills self-reactive lymphocytes expressing Fas (activation-induced cell death, AICD)
CTL-mediated killing: Cytotoxic T lymphocytes express FasL to kill virus-infected and tumor target cells (alongside the perforin/granzyme mechanism)
Immune privilege: Tissues like the eye and testis express FasL to induce apoptosis in infiltrating immune cells

Summary Diagram (Extrinsic Pathway)

Death ligand (FasL / TNF / TRAIL)
        ↓
Death receptor (Fas / TNFR1 / DR4-5)  ← oligomerization
        ↓
DISC: FADD + Pro-Caspase-8/10
        ↓ autocatalytic cleavage
Active Caspase-8          ←— inhibited by c-FLIP
     ↙            ↘
tBID (→ mito)    Caspase-3/6/7 (executioners)  ←— inhibited by IAPs
                          ↓
              DNA fragmentation, nuclear collapse,
              membrane blebbing → APOPTOSIS

Sources:

Robbins, Cotran & Kumar — Pathologic Basis of Disease, pp. 59–60
Robbins & Kumar Basic Pathology, block 1
Schwartz's Principles of Surgery, 11th Ed., p. 1644
Harrison's Principles of Internal Medicine, 21st Ed.

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