A 4-year-old boy is brought to the emergency department with a 3-day history of severe, crampy abdominal pain and grossly bloody diarrhea. His parents report that he attended a neighborhood cookout one week ago where he consumed a hamburger. Initial laboratory studies reveal severe thrombocytopenia, microangiopathic hemolytic anemia with schistocytes on peripheral smear, and an elevated serum creatinine level. The child's condition is caused by a bacterial exotoxin. Arrange the following steps in the precise molecular pathogenesis of this toxin in sequential chronological order, from initial host cell interaction to final cellular consequence.
- 1Toxin B-subunit binding to globotriaosylceramide (Gb3) receptors on glomerular and intestinal endothelial cells.
- 2Internalization of the toxin via receptor-mediated endocytosis and retrograde transport to the endoplasmic reticulum.
- 3Cleavage and release of the catalytically active A-subunit into the host cell cytoplasm.
- 4Removal of a specific adenine nucleobase from the 28S rRNA of the 60S ribosomal subunit.
- 5Arrest of host cell protein synthesis leading to cellular apoptosis and microthrombi formation.
Answer
The correct sequence of Shiga-like toxin pathogenesis begins with extracellular binding to Gb3 receptors, followed by endocytosis and retrograde transport, translocation of the active A-subunit into the cytoplasm, depurination of the 28S rRNA, and finally, the cessation of protein synthesis leading to cell death.
The correct sequence describes the established pathophysiological cascade of Shiga-like toxin (produced by Enterohemorrhagic Escherichia coli). The toxin is an AB5 multimeric exotoxin. First, the five B-subunits bind to globotriaosylceramide (Gb3) receptors, which are highly expressed on the surface of intestinal mucosa and renal glomerular endothelial cells. Following binding, the entire toxin complex is internalized via receptor-mediated endocytosis and undergoes retrograde transport through the Golgi network to the endoplasmic reticulum. Within the endoplasmic reticulum, the catalytically active A-subunit is cleaved and translocated into the host cell cytoplasm. The A-subunit then acts as an N-glycosidase, specifically removing an adenine nucleobase from the 28S ribosomal RNA of the 60S ribosomal subunit. This depurination prevents the binding of elongation factor-1 (EF-1), thereby completely halting host cell protein synthesis. The sudden arrest in translation leads to endothelial cell apoptosis, exposing subendothelial collagen and prompting the widespread formation of platelet-rich microthrombi characteristic of Hemolytic Uremic Syndrome.
Step-by-Step Solution
Key Concept
Pathogenesis of Shiga and Shiga-like toxins via Gb3 receptor binding and 60S ribosomal subunit inactivation.