The Cholangiocyte Glycocalyx Stabilizes the ‘Biliary HCO3- Umbrella': An Integrated Line of Defense against Toxic Bile Acids

Abstract

Background: Destruction of cholangiocytes is the hallmark of chronic cholangiopathies such as primary biliary cirrhosis. Under physiologic conditions, cholangiocytes display a striking resistance to the high, millimolar concentrations of toxic bile salts present in bile. We recently showed that a ‘biliary HCO₃^- umbrella', i.e. apical cholangiocellular HCO₃^- secretion, prevents cholangiotoxicity of bile acids, and speculated on a role for extracellular membrane-bound glycans in the stabilization of this protective layer. This paper summarizes published and thus far unpublished evidence supporting the role of the glycocalyx in stabilizing the ‘biliary HCO₃^- umbrella' and thus preventing cholangiotoxicity of bile acids. Key Messages: The apical glycocalyx of a human cholangiocyte cell line and mouse liver sections were visualized by electron microscopy. FACS analysis was used to characterize the surface glycan profile of cultured human cholangiocytes. Using enzymatic digestion with neuraminidase the cholangiocyte glycocalyx was desialylated to test its protective function. Using lectin assays, we demonstrated that the main N-glycans in human and mouse cholangiocytes were sialylated biantennary structures, accompanied by high expression of the H-antigen (α_1-2 fucose). Apical neuraminidase treatment induced desialylation without affecting cell viability, but lowered cholangiocellular resistance to bile acid-induced toxicity: both glycochenodeoxycholate and chenodeoxycholate (pK_a ≥4), but not taurochenodeoxycholate (pK_a <2), displayed cholangiotoxic effects after desialylation. A 24-hour reconstitution period allowed cholangiocytes to recover to a pretreatment bile salt susceptibility pattern. Conclusion: Experimental evidence indicates that an apical cholangiocyte glycocalyx with glycosylated mucins and other glycan-bearing membrane glycoproteins stabilizes the ‘biliary HCO₃^- umbrella', thus aiding in the protection of human cholangiocytes against bile acid toxicity.