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Difficulty: MediumInfectious Disease Mechanisms

A 28-year-old aid worker recently returned from a disaster relief mission in Southeast Asia. She presents to the emergency department with severe, voluminous watery diarrhea and muscle cramps that began 12 hours ago. Her vital signs show a blood pressure of 86/50 mmHg, heart rate of 124/min, and a respiratory rate of 22/min. Physical examination reveals deeply sunken eyes, dry mucous membranes, and delayed capillary refill. Stool is liquid, pale, and flecked with small amounts of mucus. She is aggressively rehydrated with intravenous crystalloids. The pathogen responsible for this patient's condition exerts its effect through a specific multi-step cellular cascade.

Arrange the following events in the correct chronological order to describe the cellular mechanism of action of the primary enterotoxin produced by the most likely causative organism.

  1. 1The pentameric B subunit binds to the GM1 ganglioside receptor on the apical surface of the enterocyte.
  2. 2The toxin is internalized via endocytosis and undergoes retrograde transport to the endoplasmic reticulum.
  3. 3The A1 subunit catalyzes the ADP-ribosylation of the stimulatory G-protein (Gs) alpha subunit.
  4. 4Adenylate cyclase becomes constitutively activated, causing a rapid accumulation of intracellular cyclic AMP (cAMP).
  5. 5Cystic fibrosis transmembrane conductance regulator (CFTR) channels open, leading to a massive efflux of chloride ions and water.

Answer

The correct sequence is: binding to the GM1 receptor, endocytosis and retrograde transport, ADP-ribosylation of the Gs alpha subunit, constitutive activation of adenylate cyclase, and finally the opening of CFTR channels causing chloride and water efflux.
The correct sequence begins with the cholera toxin's pentameric B subunit binding to the GM1 ganglioside receptor on the apical surface of intestinal epithelial cells. Following attachment, the toxin is internalized via endocytosis and undergoes retrograde transport through the Golgi network to the endoplasmic reticulum. There, the enzymatically active A1 subunit is released into the cytosol. The A1 subunit then catalyzes the ADP-ribosylation of the stimulatory G-protein (Gs) alpha subunit, permanently locking it in the active GTP-bound state. This leads to the constitutive activation of adenylate cyclase and a subsequent rapid accumulation of intracellular cAMP. Finally, the elevated cAMP levels activate protein kinase A, which phosphorylates and opens CFTR chloride channels, resulting in a massive efflux of chloride ions and water into the intestinal lumen. This cascade directly produces the 'rice-water' secretory diarrhea characteristic of cholera.

Step-by-Step Solution

1
Identify the clinical syndrome and likely pathogen from the vignette.
The patient's presentation of severe, voluminous 'rice-water' diarrhea and profound dehydration after a disaster relief mission is classic for cholera, caused by Vibrio cholerae.
Recognizing the pathogen is necessary to determine the specific virulence factor and its mechanism of action.
2
Determine the primary virulence factor responsible for the symptoms.
The severe secretory diarrhea is mediated by the cholera toxin (CT), an AB5-type enterotoxin.
Understanding the toxin's classification helps in recalling its specific cellular target and mechanism.
3
Sequence the cellular mechanism of action of the cholera toxin chronologically.
The sequence is: 1) B subunit binds GM1, 2) Toxin undergoes endocytosis/retrograde transport, 3) A1 subunit ADP-ribosylates Gs alpha, 4) Adenylate cyclase is constitutively activated, 5) CFTR channels open causing fluid efflux.
This step-by-step cascade directly answers the question's requirement to order the mechanism of action.

Key Concept

The pathophysiological mechanism of cholera toxin involves binding to the GM1 ganglioside receptor, retrograde transport, ADP-ribosylation of the Gs alpha subunit, increased intracellular cAMP, and subsequent CFTR-mediated chloride and water secretion.
Estimated Time:1m 30s
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Infectious Disease Mechanisms

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