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Explain apoptosis, it's mechanism

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I now have comprehensive content from Robbins Cotran Pathologic Basis of Disease and other sources. Let me compile a thorough, well-organized answer.

Apoptosis — Definition and Mechanism

Apoptosis (Greek: "falling off," as leaves from a tree) is a form of programmed cell death in which cells activate intrinsic enzymes that systematically degrade their own nuclear DNA and cytoplasmic proteins. Unlike necrosis, it is an energy-dependent, organized process that clears unwanted cells without triggering inflammation.

Key Morphologic Features

Morphologic features of apoptosis — H&E, electron microscopy, blebbing, nuclear fragmentation, and caspase-3 activation
Fig. A — Morphologic features of apoptosis. (A) Apoptotic epidermal cell with eosinophilic cytoplasm and condensed nucleus. (B) EM showing peripheral chromatin crescents and fragmented nuclei. (C) Blebbing/apoptotic bodies, nuclear fragmentation (DAPI), and caspase-3 activation (red immunofluorescence). — Robbins & Cotran Pathologic Basis of Disease
FeatureDescription
Cell shrinkageCell volume decreases; cytoplasm becomes dense and eosinophilic (opposite of necrosis)
Chromatin condensationChromatin aggregates peripherally under the nuclear membrane — the hallmark of apoptosis
Nuclear fragmentationNucleus breaks into 2 or more fragments
Membrane blebbingSurface blebbing followed by fragmentation into apoptotic bodies (membrane-bound, contain organelles ± nuclear fragments)
PhagocytosisApoptotic bodies expose "eat-me" signals (e.g., phosphatidylserine) → rapidly engulfed by macrophages
No inflammationCell contents never leak into surrounding tissue, so no inflammatory response is triggered

Central Executioners — Caspases

Apoptosis is mediated by caspases — cysteine proteases that cleave substrates after aspartic acid residues. They exist as inactive procaspases and are activated by:
  1. Enzymatic cleavage by another caspase
  2. Focal autocatalytic accumulation of inactive forms
The process has two phases:
  • Initiation phase — specific initiator caspases (caspase-8 or -9) become active
  • Execution phase — executioner caspases (caspase-3, -6, -7) mediate cellular fragmentation

Two Main Pathways

Apoptosis pathways — mitochondrial (intrinsic) and death receptor (extrinsic), converging on executioner caspases
Fig. B — Mechanisms of apoptosis. Left: Mitochondrial (intrinsic) pathway. Right: Death receptor (extrinsic) pathway. Both converge on executioner caspases (3, 6). — Robbins & Cotran Pathologic Basis of Disease

1. Mitochondrial (Intrinsic) Pathway

The predominant pathway in most physiologic and pathologic apoptosis.
Trigger signals:
  • Loss of growth factors / survival signals
  • DNA damage (e.g., radiation, chemotherapy)
  • Oxidative stress, hypoxia
  • Accumulation of misfolded proteins (ER stress)
Mechanism — step by step:
  1. Stress sensors activate BH3-only proteins (BAD, BIM, BID, PUMA, NOXA) — these are the molecular sensors of cellular damage.
  2. BH3-only proteins activate the pro-apoptotic effectors BAX and BAK, which oligomerize and insert into the outer mitochondrial membrane, forming pores.
  3. This increases mitochondrial outer membrane permeability → cytochrome c and other pro-apoptotic proteins (Smac/DIABLO) leak into the cytosol.
  4. Cytochrome c binds APAF-1 (apoptosis-activating factor-1) → forms the apoptosome (a heptameric wheel-shaped complex).
  5. The apoptosome recruits and activates caspase-9 (the initiator caspase).
  6. Active caspase-9 cleaves and activates executioner caspases-3 and -7 → cell death.
  7. Smac/DIABLO neutralizes IAPs (inhibitor of apoptosis proteins), removing the brake on caspase activation.
Regulators — the BCL-2 family:
GroupMembersAction
Anti-apoptoticBCL-2, BCL-XL, MCL-1Maintain outer mitochondrial membrane integrity; block cytochrome c release
Pro-apoptotic effectorsBAX, BAKPermeabilize outer mitochondrial membrane
BH3-only sensorsBAD, BIM, BID, PUMA, NOXAActivate BAX/BAK; neutralize BCL-2/BCL-XL
The ratio of pro-apoptotic to anti-apoptotic BCL-2 family members determines whether a cell lives or dies. BCL-2 overexpression (seen in follicular B-cell lymphoma due to t(14;18)) blocks apoptosis and is a key oncogenic mechanism.

2. Death Receptor (Extrinsic) Pathway

Triggered by extracellular signals — important in immune regulation and cytotoxic T lymphocyte (CTL)-mediated killing.
Trigger signals:
  • FasL (Fas ligand) binding Fas (CD95)
  • TNF binding TNFR1
Mechanism — step by step:
  1. Death ligands (FasL, TNF) bind their death receptors (Fas/CD95, TNFR1) on the cell surface.
  2. Receptor trimerization → cytoplasmic death domains recruit the adaptor protein FADD (Fas-associated death domain).
  3. FADD recruits procaspase-8 → forms the DISC (death-inducing signaling complex).
  4. Procaspase-8 undergoes autocatalytic cleavage → active caspase-8.
  5. Caspase-8 directly activates executioner caspases-3 and -6 → cell death.
  6. In some cell types, caspase-8 also cleaves BID (a BH3-only protein) → truncated BID (tBID) enters mitochondria, amplifying the intrinsic pathway (cross-talk).
Inhibitor: FLIP (FLICE-like inhibitory protein) — binds procaspase-8 and blocks FADD binding, acting as a decoy that inhibits this pathway. Some viruses produce FLIP homologs to evade immune-mediated apoptosis.

Execution Phase

Both pathways converge on caspase-3, which:
  • Activates a cytoplasmic DNase (CAD) that cleaves nuclear DNA into nucleosome-sized fragments (the "ladder" on gel electrophoresis)
  • Degrades structural nuclear proteins (lamins) → nuclear collapse
  • Breaks down cytoskeletal proteins → membrane blebbing
  • Activates enzymes for final breakdown of cellular components

Physiologic vs. Pathologic Apoptosis

ContextExamples
PhysiologicEmbryonic development (tissue sculpting); elimination of self-reactive lymphocytes; post-immune response contraction; endometrial shedding
PathologicDNA damage beyond repair; viral infections; CTL killing of infected/tumor cells; duct obstruction (pancreas, kidney)

Clinical Relevance

  • Too little apoptosis → cancer (BCL-2 overexpression in follicular lymphoma), autoimmunity (failure to delete self-reactive T cells)
  • Too much apoptosis → neurodegeneration (Alzheimer's, Parkinson's), ischemia-reperfusion injury, AIDS (CD4+ T cell depletion)
  • Therapeutic targeting: BH3 mimetics (e.g., venetoclax/ABT-199, a BCL-2 inhibitor) are used clinically in CLL and AML to restore apoptosis in cancer cells
Sources: Robbins & Cotran Pathologic Basis of Disease; Robbins & Kumar Basic Pathology; Sleisenger & Fordtran's Gastrointestinal and Liver Disease
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