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Tropical parasites: Glycome giveaway

Schistosomiasis, an often chronic illness with huge socioeconomic impact, affects hundreds of millions of people worldwide. Adult S. mansoni worms can live in their human hosts for many years and release hundreds of eggs daily. Egg-derived glycoproteins are known to modulate the host immune response but may also provoke inflammation and damage of the host liver. Glycan and glycoconjugate antigens reach the host circulation and can be detected with anti-carbohydrate antibodies to diagnose infection. Searching for novel glycoconjugate markers in the urine of infected individuals, Crina Balog et al. were surprised to identify aberrantly O-glycosylated peptides derived from human Apolipoprotein C-III (Apo C-III). Reporting in Molecular & Cellular Proteomics, they demonstrate the power of mass spectrometry (MS) to characterize unknown O-glycopeptides, and reveal that the host glycome is altered as a result of S. mansoni infection.

Urine samples from individuals with and without S. mansoni infection were fractionated and analyzed using matrix-assisted laser desorption/ionization–time of flight (MALDI-ToF) MS. Several signals were detected in the spectra of infected individuals only. In individual spectra, the presence of a series of glycopeptides was revealed by mass differences that corresponded to monosaccharides. Tandem MS was used to analyze the fractions of interest, and glycan-specific ions were detected. The fragmentation data indicated a core 2 type O-glycosylation, and the authors used knowledge of this type of modification to assist in defining the motifs.

Based on the glycan compositions, which were highly fucosylated, the authors deduced the masses of the peptide backbones. Furthermore, they specifically purified the glycopeptides from urine and used several separations and MS analyses to identify the peptide sequences. These were truncated versions of one another, but searching their sequences against a database of Schistosoma proteins produced no matches. Instead, full homology with human Apo C-III was detected. This protein usually carries a core 1 mucin type O-glycosylation, with either zero, one or two sialic acids (Apo C-III₀, C-III₁ and C-III₂), very different to the highly fucosylated structures detected in the infected urine.

The authors next looked for the aberrantly glycosylated Apo C-III in the serum of infected individuals, by analyzing the full-length glycoforms. None of these carried the fucosylated elements detected on the urinary peptides, but the ratio of Apo C-III₀, C-III₁ and C-III₂ was substantially altered in infected individuals. The significance of the fucosylated Apo C-III peptides in schistosomiasis is not yet clear. Nor is it obvious whether the peptides are taken up by the parasites and modified by the schistosomal glycosylation machinery, or derive from altered liver glycosylation activity as a consequence of infection. Further work will identify whether this marker is specific to Schistosoma infection, or associated with other liver disorders. Nevertheless, this study demonstrates the power of the omics approach to characterize novel and unexpected markers of disease.

Author: Emma Leah