A recent study dedicated to the analysis of O-linked glycans on VWF Small molecule library molecular weight revealed some interesting details regarding these glycans [21]. First, about one quarter of the T-antigen structures contained di-sialyl structures, indicating that terminal Gal or GalNAc residues are capped with two rather than one sialic acid. Second, a small portion of the O-linked carbohydrates (∼0.8% of all

glycans corresponding to 1 per 10 monomers) is characterized by the presence of ABO blood group structure. Inhibition of the enzyme GlcNAc phosphotransferase, which is responsible for the attachment of the precursor N-glycan structure to the protein backbone, results in a complete inhibition of initial dimerization of VWF protomers and subsequent

targeting to the Golgi [22]. Thus, N-linked glycosylation is indeed an important process to facilitate the production of multimerized VWF molecules. The notion that mutation of the asparagine residue at position 2546 is associated with severe VWD (type 3) supports this view. By contrast, expression of VWF in CHO-cells permitting selective suppression 3-MA concentration of O-linked glycosylation allowed the secretion of fully multimerized VWF molecules [23]. Apparently, O-linked glycosylation is of less relevance for the assembly and secretion of VWF multimers. Whether and how glycan structures affect MCE VWF function is unclear. Opposite results regarding ristocetin-dependent activity of VWF treated with sialidases and other carbohydrate-removing enzymes have been reported (for review see [24]). Of course, these data should be interpreted with care, because not only VWF–platelet interactions but VWF-ristocetin interactions can also be affected upon treatment with these enzymes. This complication was avoided upon testing of sialidase-treated VWF in perfusion assays using different adhesive surfaces [25,26]. Enhanced platelet

adhesion was observed in these studies, indicating that the sialic acid residues negatively modulate VWF-platelet interactions. Interestingly, hypo-sialylation of VWF may occur under some pathological conditions, for instance, in precapillary pulmonary hypertension and upon exposure to sialidase activity in the circulation following micobiological infection [27,28]. Of note, hypo-sialylated VWF molecules are rapidly cleared via ASGPR [28–30], thereby preventing the presence of too high levels of overly-active VWF molecules under such conditions. Sialylation of its carbohydrates also affects the susceptibility of VWF to proteolytic degradation. Whereas the presence of sialic acids protects against degradation by unidentified plasma proteases, it renders the molecule more sensitive to degradation by the VWF cleaving protease ADAMTS13 [31,32].