Multimeric glycoprotein von Willebrand factor (VWF) exhibits a unique triplet structure of individual oligomers, resulting from ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin type 1 motifs 13) cleavage. The faster and slower migrating triplet bands of a given VWF multimer have one shorter or longer N-terminal peptide sequence, respectively. Within this peptide sequence, the A1 domain regulates interaction of VWF with platelet glycoprotein (GP)Ib. Therefore, platelet-adhesive properties of two VWF preparations with similar multimeric distribution but different triplet composition were investigated for differential functional activities. Preparation A was enriched in intermediate triplet bands, whereas preparation B predominantly contained larger triplet bands. Binding studies revealed that preparation A displayed a reduced affinity for recombinant GPIb but an unchanged affinity for collagen type III when compared to preparation B. Under high-shear flow conditions, preparation A was less active in recruiting platelets to collagen type III. Furthermore, when added to blood from patients with von Willebrand disease (VWD), defective thrombus formation was less restored. Thus, VWF forms lacking larger-size triplet bands appear to have a decreased potential to recruit platelets to collagen-bound VWF under arterial flow conditions. By implication, changes in triplet band distribution observed in patients with VWD may result in altered platelet adhesion at high-shear flow.
Prevention and treatment of bleeding in patients suffering from hemophilia A are inconvenient due to repeated intravenous infusions owing to the short half-life of coagulation factor VIII (FVIII) in circulation. Besides (glyco-)pegylation of the FVIII molecule, a bioengineering approach comprises the protein fusion to Fc-immunoglobulin (Ig)G that mediate protection from clearance or degradation via binding to the neonatal Fc receptor. While human-like N-glycosylation of recombinant FVIII is known to be crucial for the clotting factor’s quality and function, the particular glycosylation of the fused Fc portion has not been investigated in detail so far, despite its known impact on Fcγ receptor binding. Here, we analyzed the N-glycosylation of the Fc part of a chimeric FVIII-Fc protein compared to a commercial IgG1 purified from human plasma. Fc parts from both samples were released by enzymatic cleavage and were subsequently separated via sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Corresponding protein bands were referred to PNGase F in-gel digestion in order to release the respective N-glycans. Analysis via matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry revealed structural differences of both N-glycan patterns. Labeling with 2-aminobenzamide (2AB) and analysis via hydrophilic interaction liquid chromatography (HILIC) allowed a quantitative comparison of the respective N-glycosylation. Observed variations in Fc glycosylation of the chimeric FVIII fusion protein and human plasma-derived IgG1, e.g., regarding terminal sialylation, are discussed, focusing on the impact of the clotting factor’s properties, most notably its binding to Fcγ receptors.
N-glycosylated proteins produced in human embryonic kidney 293 (HEK 293) cells often carry terminal N-acetylgalactosamine (GalNAc) and only low levels of sialylation. On therapeutic proteins, such N-glycans often trigger rapid clearance from the patient's bloodstream via efficient binding to asialoglycoprotein receptor (ASGP-R) and mannose receptor (MR). This currently limits the use of HEK 293 cells for therapeutic protein production. To eliminate terminal GalNAc, we knocked-out GalNAc transferases B4GALNT3 and B4GALNT4 by CRISPR/Cas9 in FreeStyle 293-F cells. The resulting cell line produced a coagulation factor VII-albumin fusion protein without GalNAc but with increased sialylation. This glyco-engineered protein bound less efficiently to both the ASGP-R and MR in vitro and it showed improved recovery, terminal half-life and area under the curve in pharmacokinetic rat experiments. By overexpressing sialyltransferases ST6GAL1 and ST3GAL6 in B4GALNT3 and B4GALNT4 knock-out cells, we further increased factor VII-albumin sialylation; for ST6GAL1 even to the level of human plasma-derived factor VII. Simultaneous knock-out of B4GALNT3 and B4GALNT4 and overexpression of ST6GAL1 further lowered factor VII-albumin binding to ASGP-R and MR. This novel glyco-engineered cell line is well-suited for the production of factor VII-albumin and presumably other therapeutic proteins with fully human N-glycosylation and superior pharmacokinetic properties.
1425 Introduction: The characteristic multimer pattern of plasmatic von Willebrand factor (VWF) results from asymmetric cleavage by the processing metalloprotease ADAMTS13 between Y1605/M1606 within the VWF A2 domain. In normal plasma, characteristic species of various multimeric sizes with flanking satellite bands (triplets) encircling the major band on VWF multimer gels are present. The faster and slower migrating bands encompassing a VWF multimer lack one N-terminal fragment or possess an additional N-terminal fragment, respectively. Even though the distribution of VWF satellite bands is significantly altered in some types of von Willebrand disease (VWD) and several commercial VWF concentrates, the impact of triplet structure on VWF function has not been investigated so far. Methods: Four commercially available VWF concentrates were analyzed with respect to ADAMTS13 content as well as VWF multimer- and triplet structure using agarose gel electrophoresis. ADAMTS13 activity was quantified by the fluorescence resonance energy transfer (FRET) assay. VWF zymogram gels were used to test for ADAMTS13 activity. Samples composed of different VWF triplet distribution but comparable VWF multimers were obtained by fractionation of plasmatic VWF using heparin affinity chromatography. VWF affinity to collagen was measured by surface plasmon resonance (SPR). Results: VWF concentrates markedly differed in their content of ADAMTS13 antigen and activity. A higher ADAMTS13 content correlated with an increased portion of the proteolyzed faster migrating VWF triplet band. The degree of VWF proteolysis, i.e. lack of an additional N-terminal fragment, correlated with a decreased collagen binding level measured by SPR. Conclusion: Proteolytic cleavage of N-terminal domains of VWF resulting in a higher content of faster migrating satellite bands affects the function of VWF. The impact of VWF N-terminal domains on collagen binding and potential clinical consequences of enhanced proteolysis in commercial concentrates has to be further evaluated. Disclosures: Schwartz: Octapharma: Employment. Fuchs:Octapharma: Employment. Kannicht:Octapharma: Employment. Solecka:Octapharma: Employment. Kröning:Octapharma: Employment.
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