Search

Deficiency or dysfunction of von Willebrand factor (VWF) is associated with a bleeding disorder known as von Willebrand disease (VWD). The clinical manifestations of VWD are heterogeneous, and are in part dictated by the structural or functional defects of VWF. The tools to control bleeding in VWD are dominated by VWF concentrates, desmopressin and antifibrinolytic therapy. In view of these treatments being considered as effective, it is surprising that quality‐of‐life studies consistently demonstrate a significant mental and physical burden in VWD patients, particularly in women. Apparently, the current weaponry to support the management of VWD is insufficient to fully address the needs of the patients. It is important therefore to continue to search for innovative treatment options which could better serve the VWD patients. In this short review, two of such options are discussed in more detail: emicizumab to correct for the deficiency of factor VIII (FVIII), and the pegylated aptamer BT200 to increase endogenous levels of the VWF/FVIII complex. [ABSTRACT FROM AUTHOR]

Novel therapies for hemophilia, including non‐factor replacement and in vivo gene therapy, are showing promising results in the clinic, including for patients having a history of inhibitor development. Here, we propose a novel therapeutic approach for hemophilia based on llama‐derived single‐domain antibody fragments (sdAbs) able to restore hemostasis by inhibiting the antithrombin (AT) anticoagulant pathway. We demonstrated that sdAbs engineered in multivalent conformations were able to block efficiently AT activity in vitro, restoring the thrombin generation potential in FVIII‐deficient plasma. When delivered as a protein to hemophilia A mice, a selected bi‐paratopic sdAb significantly reduced the blood loss in a model of acute bleeding injury. We then packaged this sdAb in a hepatotropic AAV8 vector and tested its safety and efficacy profile in hemophilic mouse models. We show that the long‐term expression of the bi‐paratopic sdAb in the liver is safe and poorly immunogenic, and results in sustained correction of the bleeding phenotype in hemophilia A and B mice, even in the presence of inhibitory antibodies to the therapeutic clotting factor. Synopsis: This study presents a novel therapeutic approach for hemophilia A and B, with and without inhibitors. By using single‐domain antibodies targeting the anticoagulant antithrombin, a sustained correction of the in vivo bleeding phenotype was achieved following protein or AAV8‐based gene therapy. Bi‐paratopic single domain antibodies (sdAbs) have been generated that neutralized the anticoagulant protein antithrombin.Neutralization of antithrombin normalized thrombin generation in hemophilic plasma, irrespective of the presence of inhibitory antibodies.Liver expression of a bivalent sdAb restored clotting in hemophilia A and in hemophilia B mice, even in the presence of anti‐FIX antibodies. [ABSTRACT FROM AUTHOR]

Background & Aims In addition to their function in thrombosis and haemostasis, platelets play an important role in the stimulation of liver regeneration. It has been suggested that platelets deliver mitogenic cargo to the regenerating liver, and accumulation of platelets in the regenerating liver has been demonstrated. We studied kinetics of platelet influx in the regenerating liver and investigated the signal that initiates platelet influx. Methods We visualized platelets in the liver remnant after partial hepatectomy in mice using intravital microscopy and assessed liver regeneration by examination of liver/body weight ratio and the number of proliferating hepatocytes examined by immunohistochemistry. Results We demonstrated rapid but transient platelet influx into the liver remnant after a partial liver resection. Liver regeneration in thrombocytopenic mice was substantially impaired as evidenced by a reduced liver-to-body weight ratio and decreased numbers of proliferating hepatocytes at day 3 compared to mice with normal platelet counts. In contrast, liver regeneration was only mildly impaired when thrombocytopaenia was induced 2 hours after partial liver resection. Platelet influx into the liver remnant was virtually absent in the presence of an antibody to von Willebrand factor ( VWF) suggesting that VWF release from liver sinusoidal endothelial cells mediates platelet influx. Additionally, liver regeneration in mice deficient in VWF was markedly impaired. Conclusions A rapid but transient VWF-dependent platelet influx into the liver remnant drives platelet-mediated liver regeneration. [ABSTRACT FROM AUTHOR]

C4b-binding protein (C4BP) is a protein acting as a complement inhibitor and a carrier protein for anticoagulant protein S. Previously, we reported that the in vivo clearance of C4BP involves CD91, and that a CD91-interactive site overlaps the heparin-binding site within C4BP α-chains . Here, we investigated the C4BP-CD91 interaction in more detail. Binding of C4BP to CD91 was unaffected by protein S, which associates with C4BP β-chain. Second, mutagenesis of cationic residues within C4BP α-chains impaired CD91 binding, reducing the affinity of triple mutant C4BPα/R39Q-R64Q-R66Q by 20-fold (Kd= 10 nM versus 214 nM for wild-type and mutant C4BP, respectively). Accordingly, intracellular degradation of this mutant by CD91-expressing cells was reduced to levels of CD91-deficient cells. Moreover, C4BPα/R39Q-R64Q-R66Q displayed a 3-fold prolonged survival compared to normal C4BP in in vivo clearance experiments. Since these residues also contribute to heparin binding, we explored the role of heparin-sulfate proteoglycans (HSPG) in the endocytosis of C4BP. The absence of HSPG was associated with a near complete absence of cell binding and intracellular degradation of C4BP. Apparently, the cellular uptake of C4BP depends on both HSPG and CD91, involving interactions with positively charged residues within C4BP α-chain. [ABSTRACT FROM AUTHOR]

Protein A (Spa) is a surface-associated protein of Staphylococcus aureus best known for its ability to bind to the Fc region of IgG. Spa also binds strongly to the Fab region of the immunoglobulins bearing VH3 heavy chains and to von Willebrand factor (vWF). Previous studies have suggested that the protein A–vWF interaction is important in S. aureus adherence to platelets under conditions of shear stress. We demonstrate that Spa expression is sufficient for adherence of bacteria to immobilized vWF under low fluid shear. The full length recombinant Ig-binding region of protein A, Spa-EDABC, fused to glutathione- S-transferase (GST), bound recombinant vWF in a dose-dependent and saturable fashion with half maximal binding of about 30 nm in immunosorbent assays. Full length-Spa did not bind recombinant vWF A3 domain but displayed binding to recombinant vWF domains A1 and D′-D3 (half maximal binding at 100 nm and 250 nm, respectively). Each recombinant protein A Ig-binding domain bound to the A1 domain in a similar manner to the full length-Spa molecule (half maximal binding 100 nm). Amino acid substitutions were introduced in the GST-SpaD protein at sites known to be involved in IgG Fc or in VH3 Fab binding. Mutants altered in residues that recognized IgG Fc but not those that recognized VH3 Fab had reduced binding to vWF A1 and D′-D3. This indicated that both vWF regions recognized a region on helices I and II that overlapped the IgG Fc binding site. [ABSTRACT FROM AUTHOR]

Activation and limited proteolysis of factor VIII have been investigated with respect to the role of the heavy-chain region LysT13-Arg740. The kinetics of factor VIII activation have been analyzed in a system consisting of human factor VIII, factor IXa, factor X, phospholipids, and thrombin or factor Xa. Plasma-derived factor VIII is activated by thrombin with a second-order rate constant of 3.3±0.3×106 M-1 s-1, which proved to be slightly higher than for activation by factor Xa. The second-order rate constant of activation by thrombin of plasma-derived factor VIII in the presence of a monoclonal antibody against the sequence Lys713-Arg740 is markedly reduced. The same result was obtained for activation by thrombin and factor Xa of factor VIII with a deletion including the sequence Lys713-Arg740, des-(713–1637) factor VIII. This suggests that the region Lys713-Arg740 promotes factor VIII activation by both thrombin and factor Xa. Since factor VIII activation is associated with proteolysis, cleavage of factor VIII heavy and light chains was analyzed quantitatively. These studies indicated that heavy-chain cleavage of des-(713–1637)-factor VIII is similar to that of plasma-derived factor VIII. In contrast, cleavage of the light chain of des-(713–1637)-factor VIII is clearly reduced. Furthermore, the second-order rate constant (0.2±0.1×106 M-1 s-1) of des-(713–1637)-factor VIII light-chain cleavage by thrombin was reduced tenfold compared with that of plasma-derived factor VIII. Proteolysis by factor Xa yielded similar results. The rate of des-(713–1637)-factor VIII light chain cleavage by thrombin is similar to that of isolated light-chain, but isolated light-chain is cleaved by factor Xa 20-fold more efficiently than the light chain in des (713–1637)-factor VIII. We conclude that activation of factor VIII by both thrombin and factor Xa is closely associated with light-chain cleavage. Furthermore, within the factor VIII heterodimer, the heavy-chain sequence Lys713-Arg740 promotes both activation and light-chain proteolysis. [ABSTRACT FROM AUTHOR]