Specific N-linked glycosylation sites modulate synthesis and secretion of von Willebrand factor.

We examined the role that N-linked glycans play in the synthesis and expression of von Willebrand Factor (VWF). Blocking the addition of N-linked glycans (NLGs) or inhibiting initial glycan processing prevented secretion of VWF. To determine whether specific glycosylation sites were important, the 16 VWF N-linked glycosylation sites were mutated followed by expression in HEK293T cells. Four NLG mutants affected VWF expression: N99Q (D1 domain), N857Q (D' domain), N2400Q (B1 domain), and N2790Q (CK domain) either abolished or reduced secretion of VWF and this was confirmed by metabolic labeling. Multimer analysis of mutant N2790Q cell lysate revealed an increase in VWF monomers, which was also observed when the isolated CK domain was expressed with N2790 mutated. Immunofluorescence microscopy showed that mutants N99Q, N857Q, and N2790Q were primarily retained within the ER, producing only few pseudo Weibel-Palade bodies over longer time periods compared with wtVWF. All the variants also showed an increase in free thiol reactivity. This was greatest with N857Q and D4-C2 NLG mutants, which had approximately 6-fold and 3- to 4-fold more free thiol reactivity than wtVWF. These data provide further evidence of the critical role that individual N-linked glycans play in determining VWF synthesis and expression.