A dietary non-human sialic acid may facilitate hemolytic-uremic syndrome

Abstract

Hemolytic-uremic syndrome (HUS) is a systemic disease characterized by microvascular endothelial damage, mainly in the gastrointestinal tract and the kidneys. A major cause of HUS is Shiga toxigenic Escherichia coli (STEC) infection. In addition to Shiga toxin, additional STEC virulence factors may contribute to HUS. One is the newly discovered subtilase cytotoxin (SubAB), which is highly toxic to eukaryotic cells, and when injected intraperitoneally into mice causes pathology resembling that associated with human HUS. Recent data show that SubAB exhibits a strong preference for glycans terminating in alpha2-3-linked N-glycolylneuraminic acid (Neu5Gc), a sialic acid that humans are unable to synthesize, because we genetically lack the necessary enzyme. However, Neu5Gc can still be found on human cells due to metabolic incorporation from the diet. Dietary incorporation happens to be highest in human endothelium and to a lesser extent in the intestinal epithelium, the two affected cell types in STEC-induced HUS. Mammalian-derived foods such as red meat and dairy products appear to be the primary source of dietary Neu5Gc. Ironically, these are also common sources of STEC contamination. Taken together, these findings suggest a 'two-hit' process in the pathogenesis of human SubAB-induced disease. First, humans eat Neu5Gc-rich food, leading to incorporation of Neu5Gc on the surfaces of endothelial and intestinal cells. Second, when exposed to a SubAB-producing STEC strain, the toxin produced would be able to bind to the intestinal epithelial cells, perhaps causing acute gastrointestinal symptoms, and eventually damaging endothelial cells in other organs like the kidney, thereby causing HUS.

Figure 1. Binding of subtilase cytotoxin (SubAB) to human intestine and kidney sections is dependent on the presence of α2-3-linked N-glycolylneuraminic acid (Neu5Gc)-containing glycans

(a) Frozen sections of human colon were stained with chicken anti-Neu5Gc or control IgY at 5 μg/ml followed by anti-chicken IgY-HRP (horseradish peroxidase) conjugate, and examined by immunohistochemistry (bar = 100 μm). (b) Similar human colon sections were overlaid with or without 1 μg/ml SubAB or SubAB_A12, and the bound toxin was detected using rabbit anti-SubA and Cy3-labeled goat anti-rabbit IgG, and examined by epifluorescence microscopy (bar = 50 μm). (c) Human colon sections were overlaid first with anti-Neu5Gc or control IgY at 5 μg/ml followed by 1 μg/ml SubAB, and the bound toxin was detected as described in panel b. Background control sections received only rabbit anti-SubAB followed by Cy3-labeled anti-rabbit IgG (Bar = 100 μm). (d) Human kidney sections were overlaid with 1 μg/ml SubAB or SubAB_A12, and in the presence or absence of 10% human or chimpanzee serum, as indicated. Bound toxin was detected as described in panel b (bar 50 μm). (From Byres et al., Figure 3 panels (a-d), see original article for experimental details).

Figure 2. Humans who eat foods rich in the non-human sialic acid, N-glycolylneuraminic acid (Neu5Gc), can become susceptible to food poisoning by the Neu5Gc-preferring subtilase cytotoxin (SubAB) toxin

Dietary intake of Neu5Gc-rich foods, such as red meats and dairy products, can result in metabolic incorporation and display of Neu5Gc on glycoconjugates on the microvascular endothelial cells in the kidney glomeruli and elsewhere, as well as on the surface of the gut epithelial cells. This process can generate high-affinity receptors for SubAB on the cell surface. The very same foods happen to be the commonest source of Shiga toxigenic E. Coli (STEC) bacteria, including those strains capable of producing SubAB. STEC infection caused by consumption of contaminated food will result in toxin production in the gastrointestinal tract. The toxin could then bind to the Neu5Gc-containing receptors on the gut epithelium and cause gastrointestinal symptoms. This may also allow the toxin to be better absorbed systemically, and the lack of competing Neu5Gc-glycoproteins in human serum would then allow maximal binding of SubAB to high-affinity receptors on the endothelium in organs, such as the kidney, thereby triggering hemolytic-uremic syndrome (HUS) (based on Supplementary Figure 1 from Byres et al.12).