Evaluation of fucosylated receptors for Cholera toxin in the human small intestine

Abstract

Cholera toxin (CT) produced by Vibrio cholerae is the causative agent for the diarrheal disease cholera. Cholera is yearly afflicting millions and is estimated to kill over 100 000 people every year. In this thesis I aimed to better understand the role of noncanonical CT receptors, e.g. receptors other than the glycolipid GM1. Epidemiological studies have found a link between cholera severity and blood group indicating that histo-blood group antigens (HBGAs) could play a role as receptors for CT. The work presented in this thesis shows that CT readily binds to the HBGA Lewis X on cells and on some cells CTB binding correlates with the level of Lewis X. Furthermore, we show that other fucosylated glycans such as Lewis Y, A/BLewis Y and 2´-fucosyllactose (found in human breast milk) readily inhibit CT binding to cell lines and primary cells from human small intestine. In contrast, sialylated or non-fucosylated glycans did not show any inhibitory effect on CT binding to human cell lines indicating a fucose-dependent binding. This was further confirmed in blocking studies using long synthetic polymers displaying glucose, fucose, galactose or a mix of the latter two. Functional evaluation identified that the fucose-binding lectin AAL completely blocked the effect of CT, but so could the galactose-binding lectin PNA. The galactose-fucose polymers yielded a partial inhibition of CT intoxication of human small intestinal enteroids whereas GM1 glycan completely blocked the effect of CT. Hence, fucosylated glycans are involved in attachment of CT to the intestinal wall. However, if this binding assists or counteracts subsequent internalization by other receptors carrying terminal galactoses remains to be determined. Importantly, these receptors can be other glycans than GM1 as this thesis show GM1-independent CT-mediated intoxication.