Four hydrophobic amino acids of the factor VIII C2 domain are constituents of both the membrane-binding and von Willebrand factor-binding motifs

Immunogenicity of Therapeutic Biological Modalities - Lessons from Hemophilia A Therapies

The introduction and development of biologics such as therapeutic proteins, gene-, and cell-based therapy have revolutionized the scope of treatment for many diseases. However, a significant portion of the patients develop unwanted immune reactions against these novel biological modalities, referred to as immunogenicity, and no longer benefit from the treatments. In the current review, using Hemophilia A (HA) therapy as an example, we will discuss the immunogenicity issue of multiple biological modalities. Currently, the number of therapeutic modalities that are approved or recently explored to treat HA, a hereditary bleeding disorder, is increasing rapidly. These include, but are not limited to, recombinant factor VIII proteins, PEGylated FVIII, FVIII Fc fusion protein, bispecific monoclonal antibodies, gene replacement therapy, gene editing therapy, and cell-based therapy. They offer the patients a broader range of more advanced and effective treatment options, yet immunogenicity remains the most critical complication in the management of this disorder. Recent advances in strategies to manage and mitigate immunogenicity will also be reviewed.

Dissecting peripheral protein-membrane interfaces

Peripheral membrane proteins (PMPs) include a wide variety of proteins that have in common to bind transiently to the chemically complex interfacial region of membranes through their interfacial binding site (IBS). In contrast to protein-protein or protein-DNA/RNA interfaces, peripheral protein-membrane interfaces are poorly characterized. We collected a dataset of PMP domains representative of the variety of PMP functions: membrane-targeting domains (Annexin, C1, C2, discoidin C2, PH, PX), enzymes (PLA, PLC/D) and lipid-transfer proteins (START). The dataset contains 1328 experimental structures and 1194 AphaFold models. We mapped the amino acid composition and structural patterns of the IBS of each protein in this dataset, and evaluated which were more likely to be found at the IBS compared to the rest of the domains' accessible surface. In agreement with earlier work we find that about two thirds of the PMPs in the dataset have protruding hydrophobes (Leu, Ile, Phe, Tyr, Trp and Met) at their IBS. The three aromatic amino acids Trp, Tyr and Phe are a hallmark of PMPs IBS regardless of whether they protrude on loops or not. This is also the case for lysines but not arginines suggesting that, unlike for Arg-rich membrane-active peptides, the less membrane-disruptive lysine is preferred in PMPs. Another striking observation was the over-representation of glycines at the IBS of PMPs compared to the rest of their surface, possibly procuring IBS loops a much-needed flexibility to insert in-between membrane lipids. The analysis of the 9 superfamilies revealed amino acid distribution patterns in agreement with their known functions and membrane-binding mechanisms. Besides revealing novel amino acids patterns at protein-membrane interfaces, our work contributes a new PMP dataset and an analysis pipeline that can be further built upon for future studies of PMPs properties, or for developing PMPs prediction tools using for example, machine learning approaches.

Stable binding to phosphatidylserine-containing membranes requires conserved arginine residues in tandem C domains of blood coagulation factor VIII

At sites of vascular damage, factor VIII (fVIII) is proteolytically activated by thrombin and binds to activated platelet surfaces with activated factor IX (fIXa) to form the intrinsic "tenase" complex. Previous structural and mutational studies of fVIII have identified the C1 and C2 domains in binding to negatively charged membrane surfaces through β-hairpin loops with solvent-exposed hydrophobic residues and a ring of positively charged basic residues. Several hemophilia A-associated mutations within the C domains are suggested to disrupt lipid binding, preventing formation of the intrinsic tenase complex. In this study, we devised a novel platform for generating recombinant C1, C2, and C1C2 domain constructs and performed mutagenesis of several charged residues proximal to the putative membrane binding region of each C domain. Binding measurements between phosphatidylserine (PS)-containing lipid membrane surfaces and fVIII C domains demonstrated an ionic strength dependence on membrane binding affinity. Mutations to basic residues adjacent to the surface-exposed hydrophobic regions of C1 and C2 differentially disrupted membrane binding, with abrogation of binding occurring for mutations to conserved arginine residues in the C1 (R2163) and C2 (R2320) domains. Lastly, we determined the X-ray crystal structure of the porcine fVIII C2 domain bound to o-phospho-L-serine, the polar headgroup of PS, which binds to a basic cleft and makes charge-charge contact with R2320. We conclude that basic clefts in the fVIII C domains bind to PS-containing membranes through conserved arginine residues via a C domain modularity, where each C domain possesses modest electrostatic-dependent affinity and tandem C domains are required for high affinity binding.

Structural insights into blood coagulation factor VIII: Procoagulant complexes, membrane binding, and antibody inhibition

Advances in structural studies of blood coagulation factor VIII (FVIII) have provided unique insight into FVIII biochemistry. Atomic detail models of the B domain-deleted FVIII structure alone and in complex with its circulatory partner, von Willebrand factor (VWF), provide a structure-based rationale for hemophilia A-associated mutations which impair FVIII stability and increase FVIII clearance rates. In this review, we discuss the findings from these studies and their implications toward the design of a recombinant FVIII with improved circulatory half-life. Additionally, we highlight recent structural studies of FVIII bound to inhibitory antibodies that have refined our understanding of FVIII binding to activated platelet membranes and formation of the intrinsic tenase complex. The combination of bioengineering and structural efforts to understand FVIII biochemistry will improve therapeutics for treating hemophilia A, either through FVIII replacement therapeutics, immune tolerance induction, or gene therapy approaches.

In-Silico Characterization of von Willebrand Factor Bound to FVIII

Factor VIII belongs to the coagulation cascade and is expressed as a long pre-protein (mature form, 2351 amino acids long). FVIII is deficient or defective in hemophilic A patients, who need to be treated with hemoderivatives or recombinant FVIII substitutes, i.e., biologic drugs. The interaction between FVIII and von Willebrand factor (VWF) influences the pharmacokinetics of FVIII medications. In vivo, full-length FVIII (FL-FVIII) is secreted in a plasma-inactive form, which includes the B domain, which is then proteolyzed by thrombin protease activity, leading to an inactive plasma intermediate. In this work, we analyzed through a computational approach the binding of VWF with two structure models of FVIII (secreted full-length with B domain, and B domain-deleted FVIII). We included in our analysis the atomic model of efanesoctocog alfa, a novel and investigational recombinant FVIII medication, in which the VWF is covalently linked to FVIII. We carried out a structural analysis of VWF/FVIII interfaces by means of protein–protein docking, PISA (Proteins, Interfaces, Structures and Assemblies), and protein contact networks (PCN) analyses. Accordingly, our computational approaches to previously published experimental data demonstrated that the domains A3-C1 of B domain-deleted FVIII (BDD-FVIII) is the preferential binding site for VWF. Overall, our computational approach applied to topological analysis of protein–protein interface can be aimed at the rational design of biologic drugs other than FVIII medications.

Protein residue network analysis reveals fundamental properties of the human coagulation factor VIII

Hemophilia A is an X-linked inherited blood coagulation disorder caused by the production and circulation of defective coagulation factor VIII protein. People living with this condition receive either prophylaxis or on-demand treatment, and approximately 30% of patients develop inhibitor antibodies, a serious complication that limits treatment options. Although previous studies performed targeted mutations to identify important residues of FVIII, a detailed understanding of the role of each amino acid and their neighboring residues is still lacking. Here, we addressed this issue by creating a residue interaction network (RIN) where the nodes are the FVIII residues, and two nodes are connected if their corresponding residues are in close proximity in the FVIII protein structure. We studied the characteristics of all residues in this network and found important properties related to disease severity, interaction to other proteins and structural stability. Importantly, we found that the RIN-derived properties were in close agreement with in vitro and clinical reports, corroborating the observation that the patterns derived from this detailed map of the FVIII protein architecture accurately capture the biological properties of FVIII.

Prediction of hemophilia A severity using a small-input machine-learning framework

Hemophilia A is a relatively rare hereditary coagulation disorder caused by a defective F8 gene resulting in a dysfunctional Factor VIII protein (FVIII). This condition impairs the coagulation cascade, and if left untreated, it causes permanent joint damage and poses a risk of fatal intracranial hemorrhage in case of traumatic events. To develop prophylactic therapies with longer half-lives and that do not trigger the development of inhibitory antibodies, it is essential to have a deep understanding of the structure of the FVIII protein. In this study, we explored alternative ways of representing the FVIII protein structure and designed a machine-learning framework to improve the understanding of the relationship between the protein structure and the disease severity. We verified a close agreement between in silico, in vitro and clinical data. Finally, we predicted the severity of all possible mutations in the FVIII structure – including those not yet reported in the medical literature. We identified several hotspots in the FVIII structure where mutations are likely to induce detrimental effects to its activity. The combination of protein structure analysis and machine learning is a powerful approach to predict and understand the effects of mutations on the disease outcome.

Endothelial Dysfunction, Atherosclerosis, and Increase of von Willebrand Factor and Factor VIII: A Randomized Controlled Trial in Swine

It is well known that high von Willebrand factor (VWF) and factor VIII (FVIII) levels are associated with an increased risk of cardiovascular disease. It is still debated whether VWF and FVIII are biomarkers of endothelial dysfunction and atherosclerosis or whether they have a direct causative role. Therefore, we aimed to unravel the pathophysiological pathways of increased VWF and FVIII levels associated with cardiovascular risk factors. First, we performed a randomized controlled trial in 34 Göttingen miniswine. Diabetes mellitus (DM) was induced with streptozotocin and hypercholesterolemia (HC) via a high-fat diet in 18 swine (DM + HC), while 16 healthy swine served as controls. After 5 months of follow-up, FVIII activity (FVIII:C) was significantly higher in DM + HC swine (5.85 IU/mL [5.00-6.81]) compared with controls (4.57 [3.76-5.40], p = 0.010), whereas VWF antigen (VWF:Ag) was similar (respectively 0.34 IU/mL [0.28-0.39] vs. 0.34 [0.31-0.38], p = 0.644). DM + HC swine had no endothelial dysfunction or atherosclerosis during this short-term follow-up. Subsequently, we performed a long-term (15 months) longitudinal cohort study in 10 Landrace-Yorkshire swine, in five of which HC and in five combined DM + HC were induced. VWF:Ag was higher at 15 months compared with 9 months in HC (0.37 [0.32-0.42] vs. 0.27 [0.23-0.40], p = 0.042) and DM + HC (0.33 [0.32-0.37] vs. 0.25 [0.24-0.33], p = 0.042). Both long-term groups had endothelial dysfunction compared with controls and atherosclerosis after 15 months. In conclusion, short-term hyperglycemia and dyslipidemia increase FVIII, independent of VWF. Long-term DM and HC increase VWF via endothelial dysfunction and atherosclerosis. Therefore, VWF seems to be a biomarker for advanced cardiovascular disease.

Molecular determinants of the factor VIII/von Willebrand factor complex revealed by BIVV001 cryo-electron microscopy

Interaction of factor VIII (FVIII) with von Willebrand factor (VWF) is mediated by the VWF D'D3 domains and thrombin-mediated release is essential for hemostasis after vascular injury. VWF-D'D3 mutations resulting in loss of FVIII binding are the underlying cause of von Willebrand disease (VWD) type 2N. Furthermore, the FVIII-VWF interaction has significant implications for the development of therapeutics for bleeding disorders, particularly hemophilia A, in which endogenous VWF clearance imposes a half-life ceiling on replacement FVIII therapy. To understand the structural basis of FVIII engagement by VWF, we solved the structure of BIVV001 by cryo-electron microscopy to 2.9 Å resolution. BIVV001 is a bioengineered clinical-stage FVIII molecule for the treatment of hemophilia A. In BIVV001, VWF-D'D3 is covalently linked to an Fc domain of a B domain-deleted recombinant FVIII (rFVIII) Fc fusion protein, resulting in a stabilized rFVIII/VWF-D'D3 complex. Our rFVIII/VWF structure resolves BIVV001 architecture and provides a detailed spatial understanding of previous biochemical and clinical observations related to FVIII-VWF engagement. Notably, the FVIII acidic a3 peptide region (FVIII-a3), established as a critical determinant of FVIII/VWF complex formation, inserts into a basic groove formed at the VWF-D'/rFVIII interface. Our structure shows direct interaction of sulfated Y1680 in FVIII-a3 and VWF-R816 that, when mutated, leads to severe hemophilia A or VWD type 2N, respectively. These results provide insight on this key coagulation complex, explain the structural basis of many hemophilia A and VWD type 2N mutations, and inform studies to further elucidate how VWF dissociates rapidly from FVIII upon activation.

Hydrogen-Deuterium Exchange Mass Spectrometry Identifies Activated Factor IX-Induced molecular Changes in Activated Factor VIII

Hydrogen-deuterium exchange mass spectrometry (HDX-MS) was employed to gain insight into the changes in factor VIII (FVIII) that occur upon its activation and assembly with activated factor IX (FIXa) on phospholipid membranes. HDX-MS analysis of thrombin-activated FVIII (FVIIIa) revealed a marked increase in deuterium incorporation of amino acid residues along the A1-A2 and A2-A3 interface. Rapid dissociation of the A2 domain from FVIIIa can explain this observation. In the presence of FIXa, enhanced deuterium incorporation at the interface of FVIIIa was similar to that of FVIII. This is compatible with the previous finding that FIXa contributes to A2 domain retention in FVIIIa. A2 domain region Leu631-Tyr637, which is not part of the interface between the A domains, also showed a marked increase in deuterium incorporation in FVIIIa compared with FVIII. Deuterium uptake of this region was decreased in the presence of FIXa beyond that observed in FVIII. This implies that FIXa alters the conformation or directly interacts with this region in FVIIIa. Replacement of Val634 in FVIII by alanine using site-directed mutagenesis almost completely impaired the ability of the activated cofactor to enhance the activity of FIXa. Surface plasmon resonance analysis revealed that the rates of A2 domain dissociation from FVIIIa and FVIIIa-Val634Ala were indistinguishable. HDX-MS analysis showed, however, that FIXa was unable to retain the A2 domain in FVIIIa-Val634Ala. The combined results of this study suggest that the local structure of Leu631-Tyr637 is altered by FIXa and that this region contributes to the cofactor function of FVIII.

Discordance between platelet-supported and vesicle-supported factor VIII activity in the presence of anti-C2 domain inhibitory antibodies

BACKGROUND

We recently reported that factor VIII (FVIII) binds to a macromolecular complex including fibrin on thrombin-stimulated platelets and that two antibodies against FVIII diminish platelet-supported FVIII activity more than vesicle-supported activity. The C2 domain of FVIII is known to bind to phospholipid membrane and also binds fibrin.

OBJECTIVES

We asked whether the degree of inhibition by anti-C2 antibodies would show differences between platelet-supported and the standard activated partial thromboplastin time (aPTT) assay.

METHODS

We evaluated the inhibition by a well-defined panel of monoclonal anti-C2 domain antibodies encompassing the major epitopes of the C2 domain. Activity was measured in an activated platelet time (aPT) assay containing fresh, density gradient-purified human platelets.

RESULTS

The aPT exhibited a log-linear relationship between FVIII and time to fibrin formation over a 4-log range, encompassing 0.01% to 100% plasma FVIII. Nine of 10 mAbs inhibited 89% to 96% of FVIII activity, whereas mAb F85 did not. There was no correlation between the degree of inhibition in the aPTT-based assay and the platelet assay. In particular, four mAbs did not inhibit the aPTT assay, yet inhibited 90% of platelet-based activity. Residual FVIII activity in purified-protein assays, relying on platelets, correlated with the aPT assay.

CONCLUSIONS

The degree of FVIII impairment by some inhibitor antibodies is substantially different on platelet membranes vs synthetic vesicles. Thus, current inhibitor assays may underestimate the frequency of significant inhibitors, and a platelet-based assay may more accurately assess bleeding risk.

Unique surface-exposed hydrophobic residues in the C1 domain of factor VIII contribute to cofactor function and von Willebrand factor binding

BACKGROUND

The identity of the amino acid regions of factor VIII (FVIII) that contribute to factor IXa (FIXa) and von Willebrand factor (VWF) binding has not been fully resolved. Previously, we observed that replacing the FVIII C1 domain for the one of factor V (FV) markedly reduces VWF binding and cofactor function. Compared to the FV C1 domain, this implies that the FVIII C1 domain comprises unique surface-exposed elements involved in VWF and FIXa interaction.

OBJECTIVE

The aim of this study is to identify residues in the FVIII C1 domain that contribute to VWF and FIXa binding.

METHODS

Structures and primary sequences of FVIII and FV were compared to identify surface-exposed residues unique to the FVIII C1 domain. The identified residues were replaced with alanine residues to identify their role in FIXa and VWF interaction. This role was assessed employing surface plasmon resonance analysis studies and enzyme kinetic assays.

RESULTS

Five surface-exposed hydrophobic residues unique to the FVIII C1 domain, ie, F2035, F2068, F2127, V2130, I2139 were identified. Functional analysis indicated that residues F2068, V2130, and especially F2127 contribute to VWF and/or FIXa interaction. Substitution into alanine of the also surface-exposed V2125, which is spatially next to F2127, affected only VWF binding.

CONCLUSION

The surface-exposed hydrophobic residues in C1 domain contribute to cofactor function and VWF binding. These findings provide novel information on the fundamental role of the C1 domain in FVIII life cycle.

The 3.2 Å structure of a bioengineered variant of blood coagulation factor VIII indicates two conformations of the C2 domain

BACKGROUND

Coagulation factor VIII represents one of the oldest protein-based therapeutics, serving as an effective hemophilia A treatment for half a century. Optimal treatment consists of repeated intravenous infusions of blood coagulation factor VIII (FVIII) per week for life. Despite overall treatment success, significant limitations remain, including treatment invasiveness, duration, immunogenicity, and cost. These issues have inspired research into the development of bioengineered FVIII products and gene therapies.

OBJECTIVES

To structurally characterize a bioengineered construct of FVIII, termed ET3i, which is a human/porcine chimeric B domain-deleted heterodimer with improved expression and slower A2 domain dissociation following proteolytic activation by thrombin.

METHODS

The structure of ET3i was characterized with X-ray crystallography and tandem mass spectrometry-based glycoproteomics.

RESULTS

Here, we report the 3.2 Å crystal structure of ET3i and characterize the distribution of N-linked glycans with LC-MS/MS glycoproteomics. This structure shows remarkable conservation with the human FVIII protein and provides a detailed view of the interface between the A2 domain and the remaining FVIII structure. With two FVIII molecules in the crystal, we observe two conformations of the C2 domain relative to the remaining FVIII structure. The improved model and stereochemistry of ET3i served as a scaffold to generate an improved, refined structure of human FVIII. With the original datasets at 3.7 Å and 4.0 Å resolution, this new structure resulted in improved refinement statistics.

CONCLUSIONS

These improved structures yield a more confident model for next-generation engineering efforts to develop FVIII therapeutics with longer half-lives, higher expression levels, and lower immunogenicity.

Single-molecule measurement and bioinformatics analysis suggest a preferred orientation of human coagulation factor VIII on hydrophobic interfaces

Investigating the adsorption behavior of coagulation proteins on interfaces will contribute to better understating blood clotting and to the development of biocompatible materials. In this work, atomic force microscopy (AFM)-based peakforce quantitative nanomechanical mapping (PF-QNM) was combined with bioinformatics tool to study the adsorption and orientation of coagulation factor VIII (FVIII) on both hydrophilic and hydrophobic interfaces by the height and mechanical measurement of single protein molecules. We found that interfacial hydrophilicity/hydrophobicity greatly influence the heights and Young's modulus of individual proteins. Compared to on the hydrophilic mica surface, FVIII proteins appear bigger vertical sizes while similar lateral sizes on the HOPG surface. The water accessible surface area analysis indicate stronger apolar properties C1 and C2 domains than others, suggesting a preferred orientation through the strong hydrophobic interactions between HOPG and the hydrophobic residues interface of the protein domains. These results provide novel insights on the adsorption and binding mechanism of the FVIII on cell membrane and will be helpful for the design of anticoagulant materials.

A model for hydrophobic protrusions on peripheral membrane proteins

With remarkable spatial and temporal specificities, peripheral membrane proteins bind to biological membranes. They do this without compromising solubility of the protein, and their binding sites are not easily distinguished. Prototypical peripheral membrane binding sites display a combination of patches of basic and hydrophobic amino acids that are also frequently present on other protein surfaces. The purpose of this contribution is to identify simple but essential components for membrane binding, through structural criteria that distinguish exposed hydrophobes at membrane binding sites from those that are frequently found on any protein surface. We formulate the concepts of protruding hydrophobes and co-insertability and have analysed more than 300 families of proteins that are classified as peripheral membrane binders. We find that this structural motif strongly discriminates the surfaces of membrane-binding and non-binding proteins. Our model constitutes a novel formulation of a structural pattern for membrane recognition and emphasizes the importance of subtle structural properties of hydrophobic membrane binding sites.

Peripheral membrane proteins bind cellular membranes transiently, and are otherwise soluble proteins. As the interaction between proteins and membranes happens at cellular interfaces they are naturally involved in important interfacial processes such as recognition, signaling and trafficking. Commonly their binding sites are also soluble, and their binding mechanisms poorly understood. This complicates the elaboration of conceptual and quantitative models for peripheral membrane binding and makes binding site prediction difficult. It is therefore of great interest to discover traits that are common between these binding sites and that distinguishes them from other protein surfaces. In this work we identify simple and general structural features that facilitate membrane recognition by soluble proteins. We show that these motifs are highly over-represented on peripheral membrane proteins.

Molecular simulation studies of human coagulation factor VIII C domain-mediated membrane binding

The C-terminal C domains of activated coagulation factor VIII (FVIIIa) are essential to membrane binding of this crucial coagulation cofactor protein. To provide an overall membrane binding mechanism for FVIII, we performed simulations of membrane binding through coarsegrained molecular dynamics simulations of the C1 and C2 domain, and the combined C-domains (C1+C2). We found that the C1 and C2 domain have different membrane binding properties. The C1 domain uses hydrophobic spikes 3 and 4, of its total of four spikes, as major loops to bind the membrane, whereas all four of its hydrophobic loops of the C2 domain appear essential for membrane binding. Interestingly, in the C1+C2 system, we observed cooperative binding of the C1 and C2 domains such that all four C2 domain spikes bound first, after which all four loops of the C1 domain inserted into the membrane, while the net binding energy was higher than that of the sum of the isolated C domains. Several residues, mutations of which are known to cause haemophilia A, were identified as key residues for membrane binding. In addition to these known residues, we identified residues from the C1 and C2 domains, which are involved in the membrane binding process, that have not been reported before as a cause for haemophilia A, but which contribute to overall membrane binding and which are likely candidates for novel causative missense mutations in haemophilia A.

Factor VIII/V C-domain swaps reveal discrete C-domain roles in factor VIII function and intracellular trafficking

Factor VIII C-domains are believed to have specific functions in cofactor activity and in interactions with von Willebrand factor. We have previously shown that factor VIII is co-targeted with von Willebrand factor to the Weibel-Palade bodies in blood outgrowth endothelial cells, even when factor VIII carries mutations in the light chain that are associated with defective von Willebrand factor binding. In this study, we addressed the contribution of individual factor VIII C-domains in intracellular targeting, von Willebrand factor binding and cofactor activity by factor VIII/V C-domain swapping. Blood outgrowth endothelial cells were transduced with lentivirus encoding factor V, factor VIII or YFP-tagged C-domain chimeras, and examined by confocal microscopy. The same chimeras were produced in HEK293-cells for in vitro characterization and chemical foot-printing by mass spectrometry. In contrast to factor VIII, factor V did not target to Weibel-Palade bodies. The chimeras showed reduced Weibel-Palade body targeting, suggesting that this requires the factor VIII C1–C2 region. The factor VIII/V-C1 chimera did not bind von Willebrand factor and had reduced affinity for activated factor IX, whereas the factor VIII/V-C2 chimera showed a minor reduction in von Willebrand factor binding and normal interaction with activated factor IX. This suggests that mainly the C1-domain carries factor VIII-specific features in assembly with von Willebrand factor and activated factor IX. Foot-printing analysis of the chimeras revealed increased exposure of lysine residues in the A1/C2- and C1/C2-domain interface, suggesting increased C2-domain mobility and disruption of the natural C-domain tandem pair orientation. Apparently, this affects intracellular trafficking, but not extracellular function.

The C1 and C2 domains of blood coagulation factor VIII mediate its endocytosis by dendritic cells

The development of inhibitory antibodies to therapeutic factor VIII is the major complication of replacement therapy in patients with hemophilia A. The first step in the initiation of the anti-factor VIII immune response is factor VIII interaction with receptor(s) on antigen-presenting cells, followed by endocytosis and presentation to naïve CD4⁺ T cells. Recent studies indicate a role for the C1 domain in factor VIII uptake. We investigated whether charged residues in the C2 domain participate in immunogenic factor VIII uptake. Co-incubation of factor VIII with BO2C11, a monoclonal C2-specific immunoglobulin G, reduced factor VIII endocytosis by dendritic cells and presentation to CD4⁺ T cells, and diminished factor VIII immunogenicity in factor VIII-deficient mice. The mutation of basic residues within the BO2C11 epitope of C2 replicated reduced in vitro immunogenic uptake, but failed to prevent factor VIII immunogenicity in mice. BO2C11 prevents factor VIII binding to von Willebrand factor, thus potentially biasing factor VIII immunogenicity by perturbing its half-life. Interestingly, a factor VIII^Y1680C mutant, that does not bind von Willebrand factor, demonstrated unaltered endocytosis by dendritic cells as well as immunogenicity in factor VIII-deficient mice. Co-incubation of factor VIII^Y1680C with BO2C11, however, resulted in decreased factor VIII immunogenicity in vivo In addition, a previously described triple C1 mutant showed decreased uptake in vitro, and reduced immunogenicity in vivo, but only in the absence of endogenous von Willebrand factor. Taken together, the results indicate that residues in the C1 and/or C2 domains of factor VIII are implicated in immunogenic factor VIII uptake, at least in vitro Conversely, in vivo, the binding to endogenous von Willebrand factor masks the reducing effect of mutations in the C domains on factor VIII immunogenicity.

Structure of the Human Factor VIII C2 Domain in Complex with the 3E6 Inhibitory Antibody

Blood coagulation factor VIII is a glycoprotein cofactor that is essential for the intrinsic pathway of the blood coagulation cascade. Inhibitory antibodies arise either spontaneously or in response to therapeutic infusion of functional factor VIII into hemophilia A patients, many of which are specific to the factor VIII C2 domain. The immune response is largely parsed into "classical" and "non-classical" inhibitory antibodies, which bind to opposing faces cooperatively. In this study, the 2.61 Å resolution structure of the C2 domain in complex with the antigen-binding fragment of the 3E6 classical inhibitory antibody is reported. The binding interface is largely conserved when aligned with the previously determined structure of the C2 domain in complex with two antibodies simultaneously. Further inspection of the B factors for the C2 domain in various X-ray crystal structures indicates that 3E6 antibody binding decreases the thermal motion behavior of surface loops in the C2 domain on the opposing face, thereby suggesting that cooperative antibody binding is a dynamic effect. Understanding the structural nature of the immune response to factor VIII following hemophilia A treatment will help lead to the development of better therapeutic reagents.

Engineering less immunogenic and antigenic FVIII proteins

The development of neutralizing antibodies against blood coagulation factor VIII (FVIII), referred to clinically as "inhibitors", is the most challenging and deleterious adverse event to occur following intravenous infusions of FVIII to treat hemophilia A. Inhibitors occlude FVIII surfaces that must bind to activated phospholipid membranes, the serine proteinase factor IXa, and other components of the 'intrinsic tenase complex' in order to carry out its important role in accelerating blood coagulation. Inhibitors develop in up to one of every three patients, yet remarkably, a substantial majority of severe hemophilia A patients, who circulate no detectable FVIII antigen or activity, acquire immune tolerance to FVIII during initial infusions or else after intensive FVIII therapy to overcome their inhibitor. The design of less immunogenic FVIII proteins through identification and modification ("de-immunization") of immunodominant T-cell epitopes is an important goal. For patients who develop persistent inhibitors, modification of B-cell epitopes through substitution of surface-exposed amino acid side chains and/or attachment of bulky moieties to interfere with FVIII attachment to antibodies and memory B cells is a promising approach. Both experimental and computational methods are being employed to achieve these goals. Future therapies for hemophilia A, as well as other monogenic deficiency diseases, are likely to involve administration of less immunogenic proteins in conjunction with other novel immunotherapies to promote a regulatory cellular environment promoting durable immune tolerance.

Membrane Interaction of the Factor VIIIa Discoidin Domains in Atomistic Detail

A recently developed membrane-mimetic model was applied to study membrane interaction and binding of the two anchoring C2-like discoidin domains of human coagulation factor VIIIa (FVIIIa), the C1 and C2 domains. Both individual domains, FVIII C1 and FVIII C2, were observed to bind the phospholipid membrane by partial or full insertion of their extruding loops (the spikes). However, the two domains adopted different molecular orientations in their membrane-bound states; FVIII C2 roughly was positioned normal to the membrane plane, while FVIII C1 displayed a multitude of tilted orientations. The results indicate that FVIII C1 may be important in modulating the orientation of the FVIIIa molecule to optimize the interaction with FIXa, which is anchored to the membrane via its γ-carboxyglutamic acid-rich (Gla) domain. Additionally, a structural change was observed in FVIII C1 in the coiled main chain leading the first spike. A tight interaction with one lipid per domain, similar to what has been suggested for the homologous FVa C2, is characterized. Finally, we rationalize known FVIII antibody epitopes and the scarcity of documented hemophilic missense mutations related to improper membrane binding of FVIIIa, based on the prevalent nonspecificity of ionic interactions in the simulated membrane-bound states of FVIII C1 and FVIII C2.

Platelet binding sites for factor VIII in relation to fibrin and phosphatidylserine

Thrombin-stimulated platelets expose very little phosphatidylserine (PS) but express binding sites for factor VIII (fVIII), casting doubt on the role of exposed PS as the determinant of binding sites. We previously reported that fVIII binding sites are increased three- to sixfold when soluble fibrin (SF) binds the αIIbβ3 integrin. This study focuses on the hypothesis that platelet-bound SF is the major source of fVIII binding sites. Less than 10% of fVIII was displaced from thrombin-stimulated platelets by lactadherin, a PS-binding protein, and an fVIII mutant defective in PS-dependent binding retained platelet affinity. Therefore, PS is not the determinant of most binding sites. FVIII bound immobilized SF and paralleled platelet binding in affinity, dependence on separation from von Willebrand factor, and mediation by the C2 domain. SF also enhanced activity of fVIII in the factor Xase complex by two- to fourfold. Monoclonal antibody (mAb) ESH8, against the fVIII C2 domain, inhibited binding of fVIII to SF and platelets but not to PS-containing vesicles. Similarly, mAb ESH4 against the C2 domain, inhibited >90% of platelet-dependent fVIII activity vs 35% of vesicle-supported activity. These results imply that platelet-bound SF is a component of functional fVIII binding sites.

Factor VIII Interacts with the Endocytic Receptor Low-density Lipoprotein Receptor-related Protein 1 via an Extended Surface Comprising "Hot-Spot" Lysine Residues

Lysine residues are implicated in driving the ligand binding to the LDL receptor family. However, it has remained unclear how specificity is regulated. Using coagulation factor VIII as a model ligand, we now study the contribution of individual lysine residues in the interaction with the largest member of the LDL receptor family, low-density lipoprotein receptor-related protein (LRP1). Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and SPR interaction analysis on a library of lysine replacement variants as two independent approaches, we demonstrate that the interaction between factor VIII (FVIII) and LRP1 occurs over an extended surface containing multiple lysine residues. None of the individual lysine residues account completely for LRP1 binding, suggesting an additive binding model. Together with structural docking studies, our data suggest that FVIII interacts with LRP1 via an extended surface of multiple lysine residues that starts at the bottom of the C1 domain and winds around the FVIII molecule.

The 1.7 Å X-ray crystal structure of the porcine factor VIII C2 domain and binding analysis to anti-human C2 domain antibodies and phospholipid surfaces

The factor VIII C2 domain is essential for binding to activated platelet surfaces as well as the cofactor activity of factor VIII in blood coagulation. Inhibitory antibodies against the C2 domain commonly develop following factor VIII replacement therapy for hemophilia A patients, or they may spontaneously arise in cases of acquired hemophilia. Porcine factor VIII is an effective therapeutic for hemophilia patients with inhibitor due to its low cross-reactivity; however, the molecular basis for this behavior is poorly understood. In this study, the X-ray crystal structure of the porcine factor VIII C2 domain was determined, and superposition of the human and porcine C2 domains demonstrates that most surface-exposed differences cluster on the face harboring the "non-classical" antibody epitopes. Furthermore, antibody-binding results illustrate that the "classical" 3E6 antibody can bind both the human and porcine C2 domains, although the inhibitory titer to human factor VIII is 41 Bethesda Units (BU)/mg IgG versus 0.8 BU/mg IgG to porcine factor VIII, while the non-classical G99 antibody does not bind to the porcine C2 domain nor inhibit porcine factor VIII activity. Further structural analysis of differences between the electrostatic surface potentials suggest that the C2 domain binds to the negatively charged phospholipid surfaces of activated platelets primarily through the 3E6 epitope region. In contrast, the G99 face, which contains residue 2227, should be distal to the membrane surface. Phospholipid binding assays indicate that both porcine and human factor VIII C2 domains bind with comparable affinities, and the human K2227A and K2227E mutants bind to phospholipid surfaces with similar affinities as well. Lastly, the G99 IgG bound to PS-immobilized factor VIII C2 domain with an apparent dissociation constant of 15.5 nM, whereas 3E6 antibody binding to PS-bound C2 domain was not observed.

Changes in the Factor VIII C2 domain upon membrane binding determined by hydrogen–deuterium exchange MS

Blood coagulation Factor VIII binds to a membrane in order to function as a cofactor for Factor IXa, preventing haemophilia. The present study indicates that membrane-binding peptides of Factor VIII are largely protected from water exposure, indicating that they become immersed in the membrane.

Phosphatidylserine-mediated cellular signaling

Phosphatidylserine (PS), a phospholipid with a negatively charged head group, is an important constituent of eukaryotic membranes. Rather than being a passive component of cellular membranes, PS plays an important role in a number of signaling pathways. Signaling is mediated by proteins that are recruited and/or activated by PS in one of two ways: via domains that stereospecifically recognize the head group, or by electrostatic interactions with membranes that are rich in PS and therefore display negative surface charge. Such interactions are key to both intracellular and extracellular signaling cascades. PS, exposed extracellularly, is instrumental in triggering blood clotting and also serves as an "eat me" signal for the clearance of apoptotic cells. Inside the cell, a number of pathways depend of PS; these include kinases, small GTPases and fusogenic proteins. This review will discuss the generation and distribution of PS, current methods of phospholipid visualization within live cells, as well as the current understanding of the role of PS in both extracellular and intracellular signaling events.

Factor VIII C1 domain spikes 2092-2093 and 2158-2159 comprise regions that modulate cofactor function and cellular uptake

The C1 domain of factor VIII (FVIII) has been implicated in binding to multiple constituents, including phospholipids, von Willebrand factor, and low-density lipoprotein receptor-related protein (LRP). We have previously described a human monoclonal antibody called KM33 that blocks these interactions as well as cellular uptake by LRP-expressing cells. To unambiguously identify the apparent "hot spot" on FVIII to which this antibody binds, we have employed hydrogen-deuterium exchange mass spectrometry. The results showed that KM33 protects FVIII regions 2091-2104 and 2157-2162 from hydrogen-deuterium exchange. These comprise the two C1 domain spikes 2092-2093 and 2158-2159. Spike 2092-2093 has been demonstrated recently to contribute to assembly with lipid membranes with low phosphatidylserine (PS) content. Therefore, spike 2158-2159 might serve a similar role. This was assessed by replacement of Arg-2159 for Asn, which introduces a motif for N-linked glycosylation. Binding studies revealed that the purified, glycosylated R2159N variant had lost its interaction with antibody KM33 but retained substantial binding to von Willebrand factor and LRP. Cellular uptake of the R2159N variant was reduced both by LRP-expressing U87-MG cells and by human monocyte-derived dendritic cells. FVIII activity was virtually normal on membranes containing 15% PS but reduced at low PS content. These findings suggest that the C1 domain spikes 2092-2093 and 2158-2159 together modulate FVIII membrane assembly by a subtle, PS-dependent mechanism. These findings contribute evidence in favor of an increasingly important role of the C1 domain in FVIII biology.

Structure of the factor VIII C2 domain in a ternary complex with 2 inhibitor antibodies reveals classical and nonclassical epitopes

The factor VIII C2 domain is a highly immunogenic domain, whereby inhibitory antibodies develop following factor VIII replacement therapy for congenital hemophilia A patients. Inhibitory antibodies also arise spontaneously in cases of acquired hemophilia A. The structural basis for molecular recognition by 2 classes of anti-C2 inhibitory antibodies that bind to factor VIII simultaneously was investigated by x-ray crystallography. The C2 domain/3E6 FAB/G99 FAB ternary complex illustrates that each antibody recognizes epitopes on opposing faces of the factor VIII C2 domain. The 3E6 epitope forms direct contacts to the C2 domain at 2 loops consisting of Glu2181-Ala2188 and Thr2202-Arg2215, whereas the G99 epitope centers on Lys2227 and also makes direct contacts with loops Gln2222-Trp2229, Leu2261-Ser2263, His2269-Val2282, and Arg2307-Gln2311. Each binding interface is highly electrostatic, with positive charge present on both C2 epitopes and complementary negative charge on each antibody. A new model of membrane association is also presented, where the 3E6 epitope faces the negatively charged membrane surface and Arg2320 is poised at the center of the binding interface. These results illustrate the potential complexities of the polyclonal anti-factor VIII immune response and further define the "classical" and "nonclassical" types of antibody inhibitors against the factor VIII C2 domain.

Replacing the factor VIII C1 domain with a second C2 domain reduces factor VIII stability and affinity for factor IXa

Factor VIII (FVIII) consists of a heavy chain (A1(a1)A2(a2)B domains) and light chain ((a3)A3C1C2 domains). To gain insights into a role of the FVIII C domains, we eliminated the C1 domain by replacing it with the homologous C2 domain. FVIII stability of the mutant (FVIIIC2C2) as measured by thermal decay at 55 °C of FVIII activity was markedly reduced (~11-fold), whereas the decay rate of FVIIIa due to A2 subunit dissociation was similar to WT FVIIIa. The binding affinity of FVIIIC2C2 for phospholipid membranes as measured by fluorescence resonance energy transfer was modestly lower (~2.8-fold) than that for WT FVIII. Among several anti-FVIII antibodies tested (anti-C1 (GMA8011), anti-C2 (ESH4 and ESH8), and anti-A3 (2D2) antibody), only ESH4 inhibited membrane binding of both WT FVIII and FVIIIC2C2. FVIIIa cofactor activity measured in the presence of each of the above antibodies was examined by FXa generation assays. The activity of WT FVIIIa was inhibited by both GMA8011 and ESH4, whereas the activity of FVIIIC2C2 was inhibited by both the anti-C2 antibodies, ESH4 and ESH8. Interestingly, factor IXa (FIXa) binding affinity for WT FVIIIa was significantly reduced in the presence of GMA8011 (~10-fold), whereas the anti-C2 antibodies reduced FIXa binding affinity of FVIIIC2C2 variant (~4-fold). Together, the reduced stability plus impaired FIXa interaction of FVIIIC2C2 suggest that the C1 domain resides in close proximity to FIXa in the FXase complex and contributes a critical role to FVIII structure and function.

Molecular orientation of factor VIIIa on the phospholipid membrane surface determined by fluorescence resonance energy transfer

F (Factor) VIIIa binds to phospholipid membranes during formation of the FXase complex. Free thiols from cysteine residues of isolated FVIIIa A1 and A2 subunits and the A3 domain of the A3C1C2 subunit were labelled with PyMPO maleimide {1-(2-maleimidylethyl)-4-[5-(4-methoxyphenyl)-oxazol-2-yl]pyridinium methanesulfonate} or fluorescein (fluorescence donors). Double mutations of the A3 domain (C2000S/T1872C and C2000S/D1828C) were also produced to utilize Cys(1828) and Cys(1872) residues for labelling. Labelled subunits were reacted with complementary non-labelled subunits to reconstitute FVIIIa. Octadecylrhodamine incorporated into phospholipid vesicles was used as an acceptor for distance measurements between FVIII residues and membrane surface by fluorescence resonance energy transfer. The results of the present study indicate that a FVIII axis on a plane that intersects the approximate centre of each domain is orientated with a tilt angle of ~30-50° on the membrane surface. This orientation predicted the existence of contacts mediated by residues 1713-1725 in the A3 domain in addition to a large area of contacts within the C domains. FVIII variants where Arg(1719) or Arg(1721) were mutated to aspartate showed a >40-fold reduction in membrane affinity. These results identify possible orientations for FVIIIa bound to the membrane surface and support a new interaction between the A3 domain and the membrane probably mediated in part by Arg(1719) and Arg(1721).

Domain organization of membrane-bound factor VIII

Factor VIII (FVIII) is the blood coagulation protein which when defective or deficient causes for hemophilia A, a severe hereditary bleeding disorder. Activated FVIII (FVIIIa) is the cofactor to the serine protease factor IXa (FIXa) within the membrane-bound Tenase complex, responsible for amplifying its proteolytic activity more than 100,000 times, necessary for normal clot formation. FVIII is composed of two noncovalently linked peptide chains: a light chain (LC) holding the membrane interaction sites and a heavy chain (HC) holding the main FIXa interaction sites. The interplay between the light and heavy chains (HCs) in the membrane-bound state is critical for the biological efficiency of FVIII. Here, we present our cryo-electron microscopy (EM) and structure analysis studies of human FVIII-LC, when helically assembled onto negatively charged single lipid bilayer nanotubes. The resolved FVIII-LC membrane-bound structure supports aspects of our previously proposed FVIII structure from membrane-bound two-dimensional (2D) crystals, such as only the C2 domain interacts directly with the membrane. The LC is oriented differently in the FVIII membrane-bound helical and 2D crystal structures based on EM data, and the existing X-ray structures. This flexibility of the FVIII-LC domain organization in different states is discussed in the light of the FVIIIa-FIXa complex assembly and function.

Conservative mutations in the C2 domains of factor VIII and factor V alter phospholipid binding and cofactor activity

Factor VIII and factor V share structural homology and bind to phospholipid membranes via tandem, lectin-like C domains. Their respective C2 domains bind via 2 pairs of hydrophobic amino acids and an amphipathic cluster. In contrast, the factor V-like, homologous subunit (Pt-FV) of a prothrombin activator from Pseudonaja textilis venom is reported to function without membrane binding. We hypothesized that the distinct membrane-interactive amino acids of these proteins contribute to the differing membrane-dependent properties. We prepared mutants in which the C2 domain hydrophobic amino acid pairs were changed to the homologous residues of the other protein and a factor V mutant with 5 amino acids changed to those from Pt-FV (FV(MTTS/Y)). Factor VIII mutants were active on additional membrane sites and had altered apparent affinities for factor X. Some factor V mutants, including FV(MTTS/Y), had increased membrane interaction and apparent membrane-independent activity that was the result of phospholipid retained during purification. Phospholipid-free FV(MTTS/Y) showed increased activity, particularly a 10-fold increase in activity on membranes lacking phosphatidylserine. The reduced phosphatidylserine requirement correlated to increased activity on resting and stimulated platelets. We hypothesize that altered membrane binding contributes to toxicity of Pt-FV.

Functional mapping of factor VIII C2 domain

The factor VIII (FVIII) is a cofactor of the coagulation cascade. The FVIII C2 domain is a critical domain that participates in the interactions with the von Willebrand factor and the phospholipidic surfaces. To assess the importance of each residue of this domain in the maintenance of the structure and the function of FVIII, a number (n=139) of mutants were generated by substituting the original residues, from Ser2173 to Gly2325, by an alanine. Mutants were built within a complete B domain-deleted FVIII and expressed in COS-1 cells. Mutant antigen levels and procoagulant activities were measured. Two in silico analyses, a sliding average procedure and an analysis of the mutation energy cost were conducted in parallel on the FVIII structure. Both results were in agreement with the functional data, and illustrated the benefit of using such strategies prior to targeting specific residues in the aim of generating active recombinant molecules. The functional assays identify the residues that are important to maintaining the structure of the C2 domain, mainly those forming β-sheet, and those that can afford substitution, establishing a detailed functional relation with the available crystallographic data. This study provided a comprehensive functional mapping of the FVIII C2 domain and discussed the implication of specific residues in respect to the maintenance in the activity and structure stability, the efficiency in secretion, the binding to phospholipids and the formation of epitope.

Membrane-binding properties of the Factor VIII C2 domain

Factor VIII functions as a cofactor for Factor IXa in a membrane-bound enzyme complex. Membrane binding accelerates the activity of the Factor VIIIa-Factor IXa complex approx. 100000-fold, and the major phospholipid-binding motif of Factor VIII is thought to be on the C2 domain. In the present study, we prepared an fVIII-C2 (Factor VIII C2 domain) construct from Escherichia coli, and confirmed its structural integrity through binding of three distinct monoclonal antibodies. Solution-phase assays, performed with flow cytometry and FRET (fluorescence resonance energy transfer), revealed that fVIII-C2 membrane affinity was approx. 40-fold lower than intact Factor VIII. In contrast with the similarly structured C2 domain of lactadherin, fVIII-C2 membrane binding was inhibited by physiological NaCl. fVIII-C2 binding was also not specific for phosphatidylserine over other negatively charged phospholipids, whereas a Factor VIII construct lacking the C2 domain retained phosphatidyl-L-serine specificity. fVIII-C2 slightly enhanced the cleavage of Factor X by Factor IXa, but did not compete with Factor VIII for membrane-binding sites or inhibit the Factor Xase complex. Our results indicate that the C2 domain in isolation does not recapitulate the characteristic membrane binding of Factor VIII, emphasizing that its role is co-operative with other domains of the intact Factor VIII molecule.

A membrane-interactive surface on the factor VIII C1 domain cooperates with the C2 domain for cofactor function

Factor VIII binds to phosphatidylserine (PS)-containing membranes through its tandem, lectin-homology, C1 and C2 domains. However, the details of C1 domain membrane binding have not been delineated. We prepared 4 factor VIII C1 mutations localized to a hypothesized membrane-interactive surface (Arg2090Ala/Gln2091Ala, Lys2092Ala/Phe2093Ala, Gln2042Ala/Tyr2043Ala, and Arg2159Ala). Membrane binding and cofactor activity were measured using membranes with 15% PS, mimicking platelets stimulated by thrombin plus collagen, and 4% PS, mimicking platelets stimulated by thrombin. All mutants had at least 10-fold reduced affinities for membranes of 4% PS, and 3 mutants also had decreased apparent affinity for factor X. Monoclonal antibodies against the C2 domain produced different relative impairment of mutants compared with wild-type factor VIII. Monoclonal antibody ESH4 decreased the Vmax for all mutants but only the apparent membrane affinity for wild-type factor VIII. Monoclonal antibody BO2C11 decreased the Vmax of wild-type factor VIII by 90% but decreased the activity of 3 mutants more than 98%. These results identify a membrane-binding face of the factor VIII C1 domain, indicate an influence of the C1 domain on factor VIII binding to factor X, and indicate that cooperation between the C1 and C2 domains is necessary for full activity of the factor Xase complex.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

Immunogenicity and pharmacokinetic studies of recombinant factor VIII containing lipid cochleates

Replacement therapy using recombinant factor VIII (rFVIII) is currently the most common therapy for hemophilia A, a bleeding disorder caused by the deficiency of FVIII. However, 15-30% of patients develop inhibitory antibodies against administered rFVIII, which complicates the therapy. Encapsulation or association of protein with lipidic structures can reduce this immune response. Previous studies developed and characterized rFVIII-containing phosphatidylserine (PS) cochleate cylinders using biophysical techniques. It was hypothesized that these structures may provide a reduction in immunogenicity while avoiding the rapid clearance by the reticuloendothelial system (RES) previously observed with liposomal vesicles of similar composition. This study investigated in vivo behavior of the cochleates containing rFVIII including immunogenicity and pharmacokinetics in hemophilia A mice. The rFVIII-cochleate complex significantly reduced the level of inhibitory antibody developed against rFVIII following intravenous (i.v.) administration. Pharmacokinetic modeling allowed assessment of in vivo release kinetics. Cochleates acted as a delayed release delivery vehicle with an input peak of cochleates showed limited RES uptake and associated rFVIII displayed a similar disposition to the free protein upon release from the structure. Incomplete disassociation from the complex limits systemic availability of the protein. Further formulation efforts are warranted to regulate the rate and extent of release of rFVIII from cochleate complexes.

Phosphatidylserine reduces immune response against human recombinant Factor VIII in Hemophilia A mice by regulation of dendritic cell function

A major clinical complication in the treatment of Hemophilia A using exogenously administered recombinant Factor VIII (FVIII) is the development of neutralizing antibodies. It has been shown previously that FVIII complexed with phosphatidylserine (PS) reduces the development of total and neutralizing antibody titers in hemophilic mice. The effect of complexation of FVIII with PS upon dendritic cell (DC) uptake, maturation and processing, T-cell proliferation and cytokine secretion profiles was investigated. Flow cytometric studies of DC showed that PS inhibited the up-regulation of cell surface co-stimulatory markers (CD86 and CD40). PS reduced T-cell proliferation and significantly increased levels of TGF-β and IL-10 but reduced secretion of IL-6 and IL-17 compared to controls. The data suggest that PS reduces immunogenicity of FVIII by regulating dendritic cell maturation and subsequent T-lymphocyte activity through modulation of cytokine secretion. A possible mechanism for PS-mediated induction of FVIII tolerance is discussed.

Regulation of the protein C anticoagulant and antiinflammatory pathways

Protein C is a vitamin K-dependent anticoagulant serine protease zymogen in plasma which upon activation by the thrombin-thrombomodulin complex down-regulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. In addition to its anticoagulant function, activated protein C (APC) also binds to endothelial protein C receptor (EPCR) in lipid-rafts/caveolar compartments to activate protease- activated receptor 1 (PAR-1) thereby eliciting antiinflammatory and cytoprotective signaling responses in endothelial cells. These properties have led to FDA approval of recombinant APC as a therapeutic drug for severe sepsis. The mechanism by which APC selects its substrates in the anticoagulant and antiinflammatory pathways is not well understood. Recent structural and mutagenesis data have indicated that basic residues of three exposed surface loops known as 39-loop (Lys-37, Lys-38, and Lys-39), 60-loop (Lys-62, Lys- 63, and Arg-67), and 70-80-loop (Arg-74, Arg-75, and Lys-78) (chymotrypsin numbering) constitute an anion binding exosite in APC that interacts with the procoagulant cofactors Va and VIIIa in the anticoagulant pathway. Furthermore, two negatively charged residues on the opposite side of the active-site of APC on a helical structure have been demonstrated to determine the specificity of the PAR-1 recognition in the cytoprotective pathway. This article will review the mechanism by which APC exerts its proteolytic function in two physiologically inter-related pathways and how the structure- function insights into determinants of the specificity of APC interaction with its substrates in two pathways can be utilized to tinker with the structure of the molecule to obtain APC derivatives with potentially improved therapeutic profiles.

Factor VIII lacking the C2 domain retains cofactor activity in vitro

Factor (F) VIII consists of a heavy chain (A1A2B domains) and light chain (A3C1C2 domains). The activated form of FVIII, FVIIIa, functions as a cofactor for FIXa in catalyzing the membrane-dependent activation of FX. Whereas the FVIII C2 domain is believed to anchor FVIIIa to the phospholipid surface, recent x-ray crystal structures of FVIII suggest that the C1 domain may also contribute to this function. We constructed a FVIII variant lacking the C2 domain (designated DeltaC2) to characterize the contributions of the C1 domain to function. Binding affinity of the DeltaC2 variant to phospholipid vesicles as measured by energy transfer was reduced approximately 14-fold. However, the activity of DeltaC2 as measured by FXa generation and one-stage clotting assays retained 76 and 36%, respectively, of the WT FVIII value. Modest reductions ( approximately 4-fold) were observed in the functional affinity of DeltaC2 FVIII for FIXa and rates of thrombin activation. On the other hand, deletion of C2 resulted in significant reductions in FVIIIa stability ( approximately 3.6-fold). Thrombin generation assays showed peak thrombin and endogenous thrombin potential were reduced as much as approximately 60-fold. These effects likely result from a combination of the intermolecular functional defects plus reduced protein stability. Together, these results indicate that FVIII domains other than C2, likely C1, make significant contributions to membrane-binding and membrane-dependent function.

Phosphatidylinositol containing lipidic particles reduces immunogenicity and catabolism of factor VIII in hemophilia a mice

Factor VIII (FVIII) is an important cofactor in blood coagulation cascade. It is a multidomain protein that consists of six domains, NH2-A1-A2-B-A3-C1-C2-COOH. The deficiency or dysfunction of FVIII causes hemophilia A, a life-threatening bleeding disorder. Replacement therapy using recombinant FVIII (rFVIII) is the first line of therapy, but a major clinical complication is the development of inhibitory antibodies that abrogate the pharmacological activity of the administered protein. FVIII binds to anionic phospholipids (PL), such as phosphatidylinositol (PI), via lipid binding region within the C2 domain of FVIII. This lipid binding site not only consists of immunodominant epitopes but is also involved in von Willebrand factor binding that protects FVIII from degradation in vivo. Thus, we hypothesize that FVIII-PL complex will influence immunogenicity and catabolism of FVIII. The biophysical studies showed that PI binding did not alter conformation of the protein but improved intrinsic stability as measured by thermal denaturation studies. ELISA studies confirmed the involvement of the C2 domain in binding to PI containing lipid particles. PI binding prolonged the in vivo circulation time and reduced catabolism of FVIII in hemophilia A mice. FVIII-PI complex reduced inhibitor development in hemophilia A mice following intravenous and subcutaneous administration. The data suggest that PI binding reduces catabolism and immunogenicity of FVIII and has potential to be a useful therapeutic approach for hemophilia A.

Mild hemophilia A

Mild hemophilia A (HA), defined by clinical features and factor VIII coagulant activity (FVIII:C) between 0.05 and 0.40 IU mL(-1), is characteristically distinct from severe HA. Indeed, although the molecular characterization of mild HA has permitted the identification of specific underlying mutations, its clinical phenotype is strikingly different from that of patients with a severe FVIII defect, where spontaneous hemorrhages or recurrent joint bleeding are usual manifestations. With aging, mild HA patients may develop complications (i.e. cancers and cardiovascular disorders), the management of which may prove challenging due to the concomitant bleeding tendency. Furthermore, the development of inhibitors provides an additional major complication in these patients, because it increases the severity of the bleeding phenotype and complicates their management. Standard management of mild HA includes the use of desmopressin and antifibrinolytic agents for minor bleeding episodes or surgical procedures, whilst major bleeding or surgery requires replacement therapy with FVIII concentrates. As regards treatment of patients with inhibitors, bypassing agents (i.e. activated prothrombin complex concentrates and recombinant activated FVII) have proven effective in the treatment of bleeding episodes, but as there are insufficient data to determine the optimal approach to immune tolerance induction in this group of patients, their optimal management remains controversial. Rituximab is a newer, promising therapeutic option for inhibitor eradication in such patients. Many aspects concerning mild HA remain to be clarified, including the molecular basis, the natural history and the optimal diagnostic and therapeutic strategies. Only large prospective studies will shed light on this condition.