19.1 Mechanisms of apoptosis

Cells have a built-in self-destruct program called apoptosis. It's a controlled demolition that neatly dismantles the cell without harming its neighbors. This process is crucial for keeping tissues in balance, removing old or damaged cells, and shaping organs during development.

Apoptosis can be triggered from inside or outside the cell. Internal stress or DNA damage sets off a chain reaction centered on mitochondria. External death signals activate receptors on the cell surface. Both paths converge on protein-cutting enzymes called caspases that systematically disassemble the cell's components.

Apoptosis Pathways and Mechanisms

Intrinsic vs extrinsic apoptosis pathways

Intrinsic (mitochondrial) pathway

This pathway fires when the cell detects internal problems like DNA damage, oxidative stress, or growth factor withdrawal. The decision to die or survive depends on a tug-of-war between members of the Bcl-2 protein family:

• Pro-apoptotic members (Bax, Bak) oligomerize and form pores in the outer mitochondrial membrane, allowing cytochrome c to leak into the cytoplasm.
• Anti-apoptotic members (Bcl-2, Bcl-xL) bind Bax and Bak to keep them inactive, preventing cytochrome c release.
• A third group, the BH3-only proteins (e.g., Bid, Bim, Bad), act as sensors of cellular stress. They either activate Bax/Bak directly or neutralize the anti-apoptotic proteins.

Once cytochrome c is released, it binds the adaptor protein Apaf-1. Apaf-1 then oligomerizes and recruits procaspase-9, forming a large wheel-shaped complex called the apoptosome. The apoptosome activates caspase-9, which in turn cleaves and activates the effector caspases (caspase-3, -6, -7).

Extrinsic (death receptor) pathway

This pathway starts at the cell surface when extracellular death ligands (TNF, FasL, TRAIL) bind their corresponding death receptors (TNF receptor, Fas/CD95, TRAIL receptors). Here's how the signal propagates:

1. Ligand binding causes the receptor's intracellular death domains to cluster.
2. The adaptor protein FADD is recruited to these death domains.
3. FADD then binds procaspase-8 (and procaspase-10), assembling the death-inducing signaling complex (DISC).
4. Within the DISC, procaspase-8 molecules are brought into close proximity, triggering their auto-activation.
5. Active caspase-8 directly cleaves and activates effector caspases (caspase-3, -7).

Cross-talk between pathways: Caspase-8 can also cleave the BH3-only protein Bid, generating truncated Bid (tBid). tBid translocates to mitochondria and activates Bax/Bak, linking the extrinsic pathway into the intrinsic pathway. This amplification loop is especially important in cell types where DISC signaling alone isn't strong enough to trigger full apoptosis.

Role of caspases in apoptosis

Caspases (cysteine-aspartic proteases) are the central executioners of apoptosis. They cleave their substrates after aspartate residues and exist as inactive zymogens (procaspases) until activated.

They fall into two functional groups:

• Initiator caspases (caspase-8, -9, -10) sit at the top of the cascade. They're activated by recruitment into large signaling platforms (the DISC or the apoptosome), where proximity-induced dimerization triggers their activity.
• Effector caspases (caspase-3, -6, -7) are activated by initiator caspases and carry out the actual demolition work. They cleave hundreds of cellular substrates to produce the hallmark features of apoptosis.

Key substrates of effector caspases and what their cleavage accomplishes:

• ICAD (inhibitor of caspase-activated DNase): Cleavage releases CAD, the endonuclease that fragments DNA between nucleosomes.
• Cytoskeletal proteins (actin, fodrin, vimentin): Cleavage causes cell shrinkage and membrane blebbing.
• Nuclear lamins: Cleavage dismantles the nuclear envelope, contributing to chromatin condensation and nuclear fragmentation.

This cascade design provides both amplification (each initiator caspase activates many effector caspases) and regulation (multiple checkpoints where the process can be blocked).

Morphological changes during apoptosis

Apoptosis produces a distinctive set of morphological features that distinguish it from necrosis. Recognizing these differences matters for identifying cell death type in experiments and tissue sections.

• Cell shrinkage: Effector caspases cleave cytoskeletal proteins, and ion channel dysregulation causes water loss. The cytoplasm becomes denser and organelles compact together. This contrasts sharply with necrosis, where cells swell and lyse.
• Membrane blebbing and apoptotic body formation: The plasma membrane bulges outward, pinching off into membrane-bound fragments called apoptotic bodies. These packages contain organelles and nuclear fragments, keeping cellular contents sealed away from surrounding tissue.
• Chromatin condensation and nuclear fragmentation: Chromatin condenses along the nuclear periphery (pyknosis), and the nucleus breaks into discrete fragments (karyorrhexis).
• DNA fragmentation: CAD cleaves DNA between nucleosomes (~180 bp intervals), producing a characteristic "DNA ladder" pattern visible on agarose gel electrophoresis. This organized pattern distinguishes apoptosis from the random, smeared DNA degradation seen in necrosis.
• Phosphatidylserine (PS) exposure: PS normally sits on the inner leaflet of the plasma membrane. During apoptosis, it flips to the outer leaflet, serving as an "eat me" signal that flags the cell for phagocytosis by macrophages and neighboring cells.

The net result: apoptotic cells are cleared quickly and quietly, without releasing their contents into the tissue and without triggering inflammation.

Apoptosis in tissue homeostasis

Apoptosis balances cell proliferation with cell death to maintain constant cell numbers. The lining of your small intestine, for example, completely renews every 3-5 days, with apoptosis eliminating old enterocytes at the villus tip as new ones are generated in the crypts.

Developmental roles:

• Sculpts tissue architecture during embryogenesis (e.g., regression of interdigital webs to separate fingers, palate fusion)
• Eliminates transient structures that are no longer needed

Damage control:

• Removes cells with irreparable DNA damage or dangerous mutations before they can become cancerous
• Destroys virus-infected cells to limit viral spread

Immune regulation:

• Deletes self-reactive T cells in the thymus and self-reactive B cells in the bone marrow, establishing self-tolerance
• Terminates immune responses by eliminating activated lymphocytes once an infection is cleared

When apoptosis goes wrong:

Understanding where apoptosis is dysregulated in a disease often points directly toward therapeutic strategies, such as using BH3 mimetics to reactivate apoptosis in cancer cells or caspase inhibitors to limit cell death after ischemic injury.