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Difficulty: HardInfectious Disease Mechanisms

A 45-year-old construction worker presents to the emergency department with a 2-day history of worsening jaw stiffness, difficulty swallowing, and painful muscle spasms in his back. He sustained a deep puncture wound to his foot from a rusty nail 10 days ago and has not received any immunizations in the past 15 years. Physical examination reveals severe trismus and a sustained spasm of the facial muscles (risus sardonicus). The patient is diagnosed with tetanus and admitted to the intensive care unit. Arrange the following pathophysiological events in the correct chronological sequence to describe the molecular mechanism of the primary exotoxin responsible for this patient's condition, beginning immediately after its secretion in the contaminated wound.

  1. 1Binding of the toxin's carboxyl-terminal heavy chain to specific gangliosides on the presynaptic membrane of local motor nerve terminals.
  2. 2Internalization into an endocytic vesicle and retrograde axonal transport along the motor neuron toward the spinal cord.
  3. 3Trans-synaptic migration from the somatodendritic region of the motor neuron into an adjacent presynaptic inhibitory interneuron (Renshaw cell).
  4. 4Translocation of the light chain into the interneuron cytosol and zinc-dependent endopeptidase cleavage of synaptobrevin (VAMP).
  5. 5Failure of synaptic vesicle fusion, resulting in the permanent blockade of gamma-aminobutyric acid (GABA) and glycine exocytosis.

Answer

The correct sequence begins with the heavy chain binding to the peripheral motor nerve, followed by retrograde axonal transport to the spinal cord. The toxin then undergoes trans-synaptic migration into inhibitory interneurons, where its light chain cleaves synaptobrevin, ultimately blocking GABA and glycine release.
Tetanospasmin is an A-B exotoxin produced by Clostridium tetani. The cascade begins peripherally when the carboxyl-terminal domain of the heavy chain (B subunit) binds to specific gangliosides (e.g., GT1b) on presynaptic motor nerve terminals. Following internalization, the toxin is transported retrogradely within endocytic vesicles along the motor axon to the neuronal cell body in the spinal cord. It is then released from the motor neuron dendrites and undergoes trans-synaptic migration into adjacent presynaptic inhibitory interneurons (Renshaw cells). Once inside the inhibitory interneuron, endosome acidification triggers the translocation of the light chain (A subunit) into the cytosol. The light chain is a zinc-dependent endopeptidase that specifically cleaves synaptobrevin (VAMP), a crucial SNARE protein. The destruction of synaptobrevin permanently prevents the fusion of synaptic vesicles, blocking the exocytosis of the inhibitory neurotransmitters GABA and glycine. This loss of inhibition leads to the unchecked firing of motor neurons, manifesting clinically as spastic paralysis, trismus, and risus sardonicus.

Step-by-Step Solution

1
Identify the pathogen and its virulence factor from the clinical presentation.
The patient has tetanus, caused by Clostridium tetani, which produces the exotoxin tetanospasmin.
Understanding the specific toxin is required to accurately trace its pathophysiological pathway.
2
Determine the initial cellular interaction of the toxin.
Tetanospasmin binds via its heavy chain to ganglioside receptors on peripheral motor nerve terminals.
Like many A-B toxins, the B (binding) subunit dictates tissue tropism.
3
Trace the movement of the toxin from the periphery to the central nervous system.
The toxin is internalized and utilizes retrograde axonal transport to reach the anterior horn of the spinal cord.
Tetanus toxin acts centrally, unlike botulinum toxin which acts at the peripheral neuromuscular junction.
4
Establish the exact target cell within the central nervous system.
The toxin crosses the synaptic cleft (trans-synaptic migration) into presynaptic inhibitory interneurons, such as Renshaw cells.
The clinical presentation of spastic paralysis is due to the loss of central inhibition.
5
Determine the molecular mechanism of action inside the target cell.
The light chain acts as a zinc metalloproteinase to cleave synaptobrevin (VAMP), preventing the exocytosis of GABA and glycine.
This final biochemical step completes the cascade, resulting in the unregulated firing of motor neurons.

Key Concept

Molecular pathogenesis of Clostridium tetani exotoxin (tetanospasmin)
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Topics

Infectious Disease Mechanisms

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