Which caspase is considered the primary executioner caspase in the apoptotic pathway?

Explanation

## Caspases in Apoptosis: The Executioner Role **Key Point:** Caspase-3 is the **final executioner caspase** that cleaves downstream substrates and directly executes apoptosis, regardless of which pathway (extrinsic or intrinsic) initiated the cascade. ### Caspase Classification and Function **Initiator Caspases** (upstream, pathway-specific): - **Caspase-8**: activated in the extrinsic (death receptor) pathway - **Caspase-9**: activated in the intrinsic (mitochondrial) pathway - Both activate downstream executioner caspases **Executioner Caspases** (downstream, final effectors): - **Caspase-3** (primary) - **Caspase-6, Caspase-7** (secondary) - Cleave structural proteins (PARP, actin, lamin), activate CAD/DFF45, fragment DNA, and cause apoptotic body formation ### Apoptotic Pathway Convergence ```mermaid flowchart TD A[Death receptor ligand]:::outcome --> B[Caspase-8 activation]:::action C[Mitochondrial stress]:::outcome --> D[Caspase-9 activation]:::action B --> E[Caspase-3 activation]:::action D --> E E --> F[Cleavage of PARP, lamin, actin]:::action E --> G[CAD/DFF activation]:::action G --> H[DNA fragmentation]:::outcome F --> I[Apoptotic body formation]:::outcome ``` **Mnemonic:** **EPIC** — **E**xtrinsic pathway → Caspase-**8** → **I**ntrinsic pathway → Caspase-**9** → **P**rimary executioner Caspase-**3** **High-Yield:** Caspase-3 is the **convergence point** of both apoptotic pathways. Its activation is the point of no return — once caspase-3 is activated, apoptosis proceeds to completion. Many cancer drugs (e.g., etoposide, doxorubicin) work by triggering caspase-3 activation. **Clinical Pearl:** Defects in caspase-3 or inhibition of caspase-3 (by viral proteins like CrmA or cellular inhibitors like XIAP) prevent apoptosis and can lead to cancer or autoimmune disease.