Modulating the sialic acid content of endothelial cells affects their adhesion and barrier permeability

The crossing of the endothelial cell layer and the glycocalyx that shields it can be considered the final barrier for substances trying to pass from the blood into the surrounding tissue. Due to their terminal position on glycan structures, sialic acids constitute the initial point of contact encountered by invading substances or bacteria. In this study, we investigated the influence of varying sialic acid levels on endothelial cell adhesion and barrier permeability. Furthermore, the successful desialylation as well as the potential for resialylation of two natively sialylated proteins - podocalyxin and integrin beta-1 - were investigated in detail. Desialylation of the cells was achieved either by sialidase-treatment or by knocking out the key enzyme of the sialic acid biosynthesis - the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). We could confirm a sialic acid deficiency after both approaches, which was more pronounced in the GNE-KO cells than in the sialidase-treated cells. Furthermore, a reduced barrier permeability and a negative effect on adhesion were observed. Partial resialylation of GNE-KO cells was achieved either by supplementation with N-acetylmannosamine (ManNAc) or with N-acetylneuraminic acid (Neu5Ac); however, the effects on adhesion and permeability were not fully compensated thereby.