Examkin

Question

Difficulty: Very hardInfectious Disease Mechanisms

A 45-year-old agricultural worker presents to the emergency department with a 2-day history of progressively worsening jaw stiffness, difficulty swallowing, and painful, uncontrollable muscle spasms in his back. Vital signs show a heart rate of 112/min and blood pressure of 165/95 mm Hg. Physical examination is notable for severe trismus, risus sardonicus, and opisthotonos. He reports stepping on a rusty nail in a barn two weeks ago and has not received any vaccinations since early childhood. The pathogenic mechanism underlying this patient's condition is driven by a potent protein exotoxin. Arrange the following steps in the correct chronological sequence to describe the molecular pathogenesis of this exotoxin, from initial neuronal contact to its ultimate synaptic effect.

  1. 1Binding of the heavy chain carboxyl-terminal domain to specific polysialogangliosides on the presynaptic membrane of peripheral lower motor neurons.
  2. 2Internalization into endosomes and microtubule-dependent retrograde axonal transport toward the anterior horn of the spinal cord.
  3. 3Exocytosis into the synaptic cleft and subsequent transcytosis into presynaptic terminals of adjacent inhibitory interneurons.
  4. 4Acidification of the endosome driving translocation of the light chain into the cytosol, followed by zinc-endopeptidase cleavage of synaptobrevin (VAMP).
  5. 5Irreversible failure of synaptic vesicle fusion, preventing the exocytosis of glycine and gamma-aminobutyric acid (GABA) into the synapse.

Answer

The correct sequence begins with peripheral ganglioside binding, followed by retrograde axonal transport, transcytosis into inhibitory interneurons, cleavage of synaptobrevin, and finally the blockade of GABA and glycine release.
The pathogenesis of Clostridium tetani begins with the binding of its exotoxin (tetanospasmin) to polysialogangliosides on peripheral motor neurons, followed by retrograde axonal transport to the spinal cord. It then undergoes transcytosis into inhibitory interneurons (Renshaw cells). Within these cells, the light chain translocates into the cytosol and cleaves the SNARE protein synaptobrevin. This irreversibly blocks the release of the inhibitory neurotransmitters GABA and glycine, culminating in the unchecked motor neuron firing and spastic paralysis characteristic of tetanus.

Step-by-Step Solution

1
Analyze the clinical vignette to identify the infectious pathogen and the disease.
The patient has tetanus (trismus, risus sardonicus, opisthotonos) caused by Clostridium tetani following a puncture wound.
Identifying the disease is the prerequisite for determining the specific exotoxin (tetanospasmin) involved.
2
Determine the structural nature of the responsible exotoxin.
Tetanospasmin is an A-B exotoxin with a heavy (binding) chain and a light (catalytic) chain.
Understanding its biphasic structure helps delineate how it enters cells and exerts its intracellular effects.
3
Map the anatomical movement of the toxin from the peripheral wound to the central nervous system.
The toxin binds peripherally, undergoes retrograde transport to the spinal cord, and crosses the synapse into inhibitory Renshaw cells.
Tetanus uniquely causes central (spinal) disinhibition, which differentiates its pathway from toxins that act strictly at the peripheral neuromuscular junction.
4
Identify the molecular target and the ultimate cellular consequence.
The light chain acts as a zinc metalloproteinase to cleave synaptobrevin, blocking the release of inhibitory neurotransmitters (GABA and glycine).
This molecular blockade directly explains the unchecked alpha motor neuron firing and the spasticity seen clinically.

Key Concept

The pathogenesis of tetanospasmin involves peripheral binding, retrograde axonal transport to the central nervous system, and subsequent cleavage of SNARE proteins in inhibitory interneurons, resulting in spastic paralysis.
Rate this question

Topics

Infectious Disease Mechanisms

More questions from this topic