Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor–related protein

Thrombin generation on vascular cells in the presence of factor VIII and/or emicizumab

BACKGROUND

The effect of factor VIII (FVIII) or emicizumab on thrombin generation is usually assessed in assays using synthetic phospholipids. Here, we assessed thrombin generation at the surface of human arterial cells (aortic endothelial cells [hAECs] and aortic vascular smooth muscle cells [hVSMCs]).

OBJECTIVES

To explore the capacity of hAECs (resting or stimulated) and hVSMCs to support thrombin generation by FVIII or emicizumab.

METHODS

Primary hVSMCs and hAECs were analyzed for tissue factor (TF)-activity and antigen, phosphatidylserine (PS)-exposure, tissue factor pathway inhibitor (TFPI)-content and thrombomodulin expression. Cells were incubated with FVIII-deficient plasma spiked with FVIII, emicizumab, activated prothrombin complex concentrate (APCC) or combinations thereof.

RESULTS

TF activity and PS-exposure were present on both hVSMCs and hAECs. In contrast, thrombomodulin and TFPI were expressed on hAECs, while virtually lacking on hVSMCs, confirming the procoagulant nature of hVSMCs. Tumor necrosis factor α-mediated stimulation of hAECs increased not only TF antigen, TF activity, and PS-exposure but also TFPI and thrombomodulin expression. As expected, FVIII and emicizumab promoted thrombin generation on nonstimulated hAECs and hVSMCs, with more thrombin being generated on hVSMCs. Unexpectedly, FVIII and emicizumab increased thrombin generation to a lesser extent on stimulated hAECs compared with nonstimulated hAECs. Finally, adding emicizumab to FVIII did not further increase thrombin generation, whereas the addition of emicizumab to APCC resulted in exaggerated thrombin generation.

CONCLUSION

Tumor necrosis factor stimulation of hAECs increases both pro- and anticoagulant activity. Unexpectedly, the increased anticoagulant activity is sufficient to limit both FVIII- and emicizumab-induced thrombin generation. This protective effect disappears when emicizumab is combined with APCC.

Significance of Preexisting Vector Immunity and Activation of Innate Responses for Adenoviral Vector-Based Therapy

An adenoviral (AdV)-based vector system is a promising platform for vaccine development and gene therapy applications. Administration of an AdV vector elicits robust innate immunity, leading to the development of humoral and cellular immune responses against the vector and the transgene antigen, if applicable. The use of high doses (10¹¹-10¹³ virus particles) of an AdV vector, especially for gene therapy applications, could lead to vector toxicity due to excessive levels of innate immune responses, vector interactions with blood factors, or high levels of vector transduction in the liver and spleen. Additionally, the high prevalence of AdV infections in humans or the first inoculation with the AdV vector result in the development of vector-specific immune responses, popularly known as preexisting vector immunity. It significantly reduces the vector efficiency following the use of an AdV vector that is prone to preexisting vector immunity. Several approaches have been developed to overcome this problem. The utilization of rare human AdV types or nonhuman AdVs is the primary strategy to evade preexisting vector immunity. The use of heterologous viral vectors, capsid modification, and vector encapsulation are alternative methods to evade vector immunity. The vectors can be optimized for clinical applications with comprehensive knowledge of AdV vector immunity, toxicity, and circumvention strategies.

F9 missense mutations impairing factor IX activation are associated with pleiotropic plasma phenotypes

BACKGROUND

Circulating dysfunctional factor IX (FIX) might modulate distribution of infused FIX in hemophilia B (HB) patients. Recurrent substitutions at FIX activation sites (R191-R226, >300 patients) are associated with variable FIX activity and antigen (FIXag) levels.

OBJECTIVES

To investigate the (1) expression of a complete panel of missense mutations at FIX activation sites and (2) contribution of F9 genotypes on the FIX pharmacokinetics (PK).

METHODS

We checked FIX activity and antigen and activity assays in plasma and after recombinant expression of FIX variants and performed an analysis of infused FIX PK parameters in patients (n = 30), mostly enrolled in the F9 Genotype and PK HB Italian Study (GePKHIS; EudraCT ID2017-003902-42).

RESULTS

The variable FIXag amounts and good relation between biosynthesis and activity of multiple R191 variants results in graded moderate-to-mild severity of the R191C>L>P>H substitutions. Recombinant expression may predict the absence in the HB mutation database of the benign R191Q/W/K and R226K substitutions. Equivalent changes at R191/R226 produced higher FIXag levels for R226Q/W/P substitutions, as also observed in p.R226W female carrier plasma. Pharmacokinetics analysis in patients suggested that infused FIX Alpha distribution and Beta elimination phases positively correlated with endogenous FIXag levels. Mean residence time was particularly prolonged (79.4 h, 95% confidence interval 44.3-114.5) in patients (n = 7) with the R191/R226 substitutions, which in regression analysis were independent predictors (β coefficient 0.699, P = .004) of Beta half-life, potentially prolonged by the increasing over time ratio between endogenous and infused FIX.

CONCLUSIONS

FIX activity and antigen levels and specific features of the dysfunctional R191/R226 variants may exert pleiotropic effects both on HB patients' phenotypes and substitutive treatment.

Half-life-extended recombinant coagulation factor IX-albumin fusion protein is recycled via the FcRn-mediated pathway

The neonatal Fc receptor (FcRn) has a pivotal role in albumin and IgG homeostasis. Internalized IgG captured by FcRn under acidic endosomal conditions is recycled to the cell surface where exocytosis and a shift to neutral pH promote extracellular IgG release. Although a similar mechanism is proposed for FcRn-mediated albumin intracellular trafficking and recycling, this pathway is less well defined but is relevant to the development of therapeutics exploiting FcRn to extend the half-life of short-lived plasma proteins. Recently, a long-acting recombinant coagulation factor IX–albumin fusion protein (rIX-FP) has been approved for the management of hemophilia B. Fusion to albumin potentially enables internalized proteins to engage FcRn and escape lysosomal degradation. In this study, we present for the first time a detailed investigation of the FcRn-mediated recycling of albumin and the albumin fusion protein rIX-FP. We demonstrate that following internalization via FcRn at low pH, rIX-FP, like albumin, is detectable within the early endosome and rapidly (within 10–15 min) traffics into the Rab11+ recycling endosomes, from where it is exported from the cell. Similarly, rIX-FP and albumin taken up by fluid-phase endocytosis at physiological pH traffics into the Rab11+ recycling compartment in FcRn-positive cells but into the lysosomal compartment in FcRn-negative cells. As expected, recombinant factor IX (without albumin fusion) and an FcRn interaction–defective albumin variant localized to the lysosomal compartments of both FcRn-expressing and nonexpressing cells. These results indicate that FcRn-mediated recycling via the albumin moiety is a mechanism for the half-life extension of rIX-FP observed in clinical studies.

LDL receptor-related protein 1 contributes to the clearance of the activated factor VII-antithrombin complex

Essentials Factor VIIa is cleared principally as a complex with antithrombin. Enzyme/serpin complexes are preferred ligands for the scavenger-receptor LRP1. Factor VIIa/antithrombin but not factor VIIa alone is a ligand for LRP1. Macrophage-expressed LRP1 contributes to the clearance of factor VIIa/antithrombin.

SUMMARY

Background Recent findings point to activated factor VII (FVIIa) being cleared predominantly (± 65% of the injected protein) as part of a complex with the serpin antithrombin. FVIIa-antithrombin complexes are targeted to hepatocytes and liver macrophages. Both cells lines abundantly express LDL receptor-related protein 1 (LRP1), a scavenger receptor mediating the clearance of protease-serpin complexes. Objectives To investigate whether FVIIa-antithrombin is a ligand for LRP1. Methods Binding of FVIIa and pre-formed FVIIa-antithrombin to purified LRP1 Fc-tagged cluster IV (rLRP1-cIV/Fc) and to human and murine macrophages was analyzed. FVIIa clearance was determined in macrophage LRP1 (macLRP1)-deficient mice. Results Solid-phase binding assays showed that FVIIa-antithrombin bound in a specific, dose-dependent and saturable manner to rLRP1-cIV/Fc. Competition experiments with human THP1 macrophages indicated that binding of FVIIa but not of FVIIa-antithrombin was reduced in the presence of annexin-V or anti-tissue factor antibodies, whereas binding of FVIIa-antithrombin but not FVIIa was inhibited by the LRP1-antagonist GST-RAP. Additional experiments revealed binding of both FVIIa and FVIIa-antithrombin to murine control macrophages. In contrast, no binding of FVIIa-antithrombin to macrophages derived from macLRP1-deficient mice could be detected. Clearance of FVIIa-antithrombin but not of active site-blocked FVIIa was delayed 1.5-fold (mean residence time of 3.3 ± 0.1 h versus 2.4 ± 0.2 h) in macLRP1-deficient mice. The circulatory presence of FVIIa was prolonged to a similar extent in macLRP1-deficient mice and in control mice. Conclusions Our data show that FVIIa-antithrombin but not FVIIa is a ligand for LRP1, and that LRP1 contributes to the clearance of FVIIa-antithrombin in vivo.

Engineered Tissue Inhibitor of Metalloproteinases-3 Variants Resistant to Endocytosis Have Prolonged Chondroprotective Activity

Tissue inhibitor of metalloproteinases-3 (TIMP-3) is a central inhibitor of matrix-degrading and sheddase families of metalloproteinases. Extracellular levels of the inhibitor are regulated by the balance between its retention on the extracellular matrix and its endocytic clearance by the scavenger receptor low density lipoprotein receptor-related protein 1 (LRP1). Here, we used molecular modeling to predict TIMP-3 residues potentially involved in binding to LRP1 based on the proposed LRP1 binding motif of 2 lysine residues separated by about 21 Å and mutated the candidate lysine residues to alanine individually and in pairs. Of the 22 mutants generated, 13 displayed a reduced rate of uptake by HTB94 chondrosarcoma cells. The two mutants (TIMP-3 K26A/K45A and K42A/K110A) with lowest rates of uptake were further evaluated and found to display reduced binding to LRP1 and unaltered inhibitory activity against prototypic metalloproteinases. TIMP-3 K26A/K45A retained higher affinity for sulfated glycosaminoglycans than K42A/K110A and exhibited increased affinity for ADAMTS-5 in the presence of heparin. Both mutants inhibited metalloproteinase-mediated degradation of cartilage at lower concentrations and for longer than wild-type TIMP-3, indicating that their increased half-lives improved their ability to protect cartilage. These mutants may be useful in treating connective tissue diseases associated with increased metalloproteinase activity.

In silico designing of hyper-glycosylated analogs for the human coagulation factor IX

N-glycosylation is a process during which a glycan moiety attaches to the asparagine residue in the N-glycosylation consensus sequence (Asn-Xxx-Ser/Thr), where Xxx can be any amino acid except proline. Introduction of a new N-glycosylation site into a protein backbone leads to its hyper-glycosylation, and may improve the protein properties such as solubility, folding, stability, and secretion. Glyco-engineering is an approach to facilitate the hyper-glycosylation of recombinant proteins by application of the site-directed mutagenesis methods. In this regard, selection of a suitable location on the surface of a protein for introduction of a new N-glycosylation site is a main concern. In this work, a computational approach was conducted to select suitable location(s) for introducing new N-glycosylation sites into the human coagulation factor IX (hFIX). With this aim, the first 45 residues of mature hFIX were explored to find out suitable positions for introducing either Asn or Ser/Thr residues, to create new N-glycosylation site(s). Our exploration lead to detection of five potential positions, for hyper-glycosylation. For each suggested position, an analog was defined and subjected for N-glycosylation efficiency prediction. After generation of three-dimensional structures, by homology-based modeling, the five designed analogs were examined by molecular dynamic (MD) simulations, to predict their stability levels and probable structural distortions caused by amino acid substitutions, relative to the native counterpart. Three out of five suggested analogs, namely; E15T, K22N, and R37N, reached equilibration state with relatively constant Root Mean Square Deviation values. Additional analysis on the data obtained during MD simulations, lead us to conclude that, R37N is the only qualified analog with the most similar structure and dynamic behavior to that of the native counterpart, to be considered for further experimental investigations.

Selective disruption of heparin and antithrombin-mediated regulation of human factor IX

BACKGROUND

Interaction with antithrombin and heparin regulates distribution, activity, and clearance of factor IXa (FIXa). Hemophilia B prophylaxis targets plasma FIX levels > 1% but neglects extravascular FIX, which colocalizes with antithrombin-heparan sulfate.

OBJECTIVE

Combined mutagenesis of FIX was undertaken to selectively disrupt heparin- and antithrombin-mediated regulation of the protease.

METHODS

Human FIX alanine substitutions in the heparin (K126A and K132A) and antithrombin (R150A) exosites were characterized with regard to coagulant activity, plasma thrombin generation, antithrombin inhibition, and plasma half-life.

RESULTS

Single or combined (K126A/R150A or K132A/R150A) exosite mutations variably reduced coagulant activity relative to wild-type (WT) for FIX (27-91%) and FIXa (25-91%). Double mutation in the heparin exosite (K126A/K132A and K126A/K132A/R150A) markedly reduced coagulant activity (7-21%) and plasma TG. In contrast to coagulant activity, FIX K126A (1.8-fold), R150 (1.6-fold), and K132A/R150A (1.3-fold) supported increased tissue factor-initiated plasma TG, while FIX K132A and K126A/R150A were similar to WT. FIXa K126A/R150A and K132A/R150A (1.5-fold) demonstrated significantly increased FIXa-initiated TG, while FIXa K132A, R150A, and K126A (0.8-0.9-fold) were similar to WT. Dual mutations in the heparin exosite or combined mutations in both exosites synergistically reduced the inhibition rate for antithrombin-heparin. The half-life of FIXa WT in FIX-deficient plasma was remarkably lengthy (40.9 ±1.4 min) and further prolonged for FIXa R150A, K126A/R150A, and K132A/R150A (> 2 h).

CONCLUSION

Selective disruption of exosite-mediated regulation by heparin and antithrombin can be achieved with preserved or enhanced thrombin generation capacity. These proteins should demonstrate enhanced therapeutic efficacy for hemophilia B.

Factor IXa as a target for anticoagulation in thrombotic disorders and conditions

From acute coronary syndrome (ACS) to the prevention of cardioembolic events in patients with atrial fibrillation and thrombosis of mechanical heart valves, there is a quest to develop a new generation of anticoagulants. Perhaps the 'holy grail' of antithrombotic therapy is not only a drug that will prevent coagulation without promoting bleeding but also an anticoagulant that is easily reversible should the clinical need arise. Further, an optimally designed anticoagulant would have broad applications to include arterial, venous, hybrid conditions (atrial flutter and fibrillation) and nonbiological materials. Factor (F)IXa plays a pivotal part in tissue factor (TF)-mediated thrombin generation, and therefore represents a potentially promising target for drug development. FIXa activity has been targeted by multiple modalities, including oral inhibitors, RNA aptamers, monoclonal antibodies and synthetic active-site-blocking competitive inhibitors. Herein, we summarize the biochemistry of FIXa as it applies to thrombotic disorders and conditions, as well as the evolution of targeted therapies.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

RNA-Seq reveals dynamic changes of gene expression in key stages of intestine regeneration in the sea cucumber Apostichopus japonicus. [corrected]

BACKGROUND

Sea cucumbers (Holothuroidea; Echinodermata) have the capacity to regenerate lost tissues and organs. Although the histological and cytological aspects of intestine regeneration have been extensively studied, little is known of the genetic mechanisms involved. There has, however, been a renewed effort to develop a database of Expressed Sequence Tags (ESTs) in Apostichopus japonicus, an economically-important species that occurs in China. This is important for studies on genetic breeding, molecular markers and special physiological phenomena. We have also constructed a library of ESTs obtained from the regenerative body wall and intestine of A. japonicus. The database has increased to ~30000 ESTs.

RESULTS

We used RNA-Seq to determine gene expression profiles associated with intestinal regeneration in A. japonicus at 3, 7, 14 and 21 days post evisceration (dpe). This was compared to profiles obtained from a normally-functioning intestine. Approximately 5 million (M) reads were sequenced in every library. Over 2400 up-regulated genes (>10%) and over 1000 down-regulated genes (~5%) were observed at 3 and 7dpe (log2Ratio ≥ 1, FDR ≤ 0.001). Specific "Go terms" revealed that the DEGs (Differentially Expressed Genes) performed an important function at every regeneration stage. Besides some expected pathways (for example, Ribosome and Spliceosome pathway term), the "Notch signaling pathway," the "ECM-receptor interaction" and the "Cytokine-cytokine receptor interaction" were significantly enriched. We also investigated the expression profiles of developmental genes, ECM-associated genes and Cytoskeletal genes. Twenty of the most important differentially expressed genes (DEGs) were verified by Real-time PCR, which resulted in a trend concordance of almost 100% between the two techniques.

CONCLUSION

Our studies demonstrated dynamic changes in global gene expression during intestine regeneration and presented a series of candidate genes and enriched pathways that contribute to intestine regeneration in sea cucumbers. This provides a foundation for future studies on the genetics/molecular mechanisms associated with intestine regeneration.

Differential regulation of extracellular tissue inhibitor of metalloproteinases-3 levels by cell membrane-bound and shed low density lipoprotein receptor-related protein 1

Tissue inhibitor of metalloproteinases-3 (TIMP-3) plays a key role in regulating extracellular matrix turnover by inhibiting matrix metalloproteinases (MMPs), adamalysins (ADAMs), and adamalysins with thrombospondin motifs (ADAMTSs). We demonstrate that levels of this physiologically important inhibitor can be regulated post-translationally by endocytosis. TIMP-3 was endocytosed and degraded by a number of cell types including chondrocytes, fibroblasts, and monocytes, and we found that the endocytic receptor low density lipoprotein receptor-related protein-1 (LRP-1) plays a major role in TIMP-3 internalization. However, the cellular uptake of TIMP-3 significantly slowed down after 10 h due to shedding of LRP-1 from the cell surface and formation of soluble LRP-1 (sLRP-1)-TIMP-3 complexes. Addition of TIMP-3 to HTB94 human chondrosarcoma cells increased the release of sLRP-1 fragments of 500, 215, 160, and 110 kDa into the medium in a concentration-dependent manner, and all of these fragments were able to bind to TIMP-3. TIMP-3 bound to sLRP-1, which was resistant to endocytosis, retained its inhibitory activity against metalloproteinases. Extracellular levels of sLRP-1 can thus increase the half-life of TIMP-3 in the extracellular space, controlling the bioavailability of TIMP-3 to inhibit metalloproteinases.

Shear stress is required for the endocytic uptake of the factor VIII-von Willebrand factor complex by macrophages

BACKGROUND

Low-density lipoprotein (LDL) receptor family members contribute to the cellular uptake of factor VIII. How von Willebrand factor fits into this endocytic pathway has remained poorly understood.

OBJECTIVES

It has been suggested that macrophages contribute to the clearance of the factor VIII (FVIII)-von Willebrand factor (VWF) complex. We now assessed the mechanisms of uptake employing human monocyte-derived macrophages.

METHODS

A confocal microscopy study was employed to study the uptake by monocyte-derived macrophages of a functional green fluorescent FVIII-GFP derivative in the presence and absence of VWF.

RESULTS

The results revealed that FVIII-GFP is internalized by macrophages. We found that FVIII-GFP co-localizes with LDL receptor-related protein (LRP), and that the LRP antagonist Receptor Associated Protein (RAP) blocks the uptake of FVIII-GFP. However, FVIII-GFP was not detected in the macrophages in the presence of VWF, suggesting that the FVIII-VWF complex is not internalized by these cells at all. Apart from static conditions, we also investigated the effect of shear stress on the uptake of FVIII-GFP in presence of VWF. Immunofluorescence studies demonstrated that VWF does not block endocytosis of FVIII-GFP under flow conditions. Moreover, VWF itself was also internalized by the macrophages. Strikingly, in the presence of RAP, endocytosis of FVIII-GFP and VWF was inhibited.

CONCLUSION

The results show that shear stress is required for macrophages to internalize both constituents of the FVIII-VWF complex.

Coagulation factor IX mediates serotype-specific binding of species A adenoviruses to host cells

Human species A adenoviruses (HAdVs) comprise three serotypes: HAdV-12, -18, and -31. These viruses are common pathogens and cause systemic infections that usually involve the airways and/or intestine. In immunocompromised individuals, species A adenoviruses in general, and HAdV-31 in particular, cause life-threatening infections. By combining binding and infection experiments, we demonstrate that coagulation factor IX (FIX) efficiently enhances binding and infection by HAdV-18 and HAdV-31, but not by HAdV-12, in epithelial cells originating from the airways or intestine. This is markedly different from the mechanism for HAdV-5 and other human adenoviruses, which utilize coagulation factor X (FX) for infection of host cells. Surface plasmon resonance experiments revealed that the affinity of the HAdV-31 hexon-FIX interaction is higher than that of the HAdV-5 hexon-FX interaction and that the half-lives of these interactions are profoundly different. Moreover, both HAdV-31-FIX and HAdV-5-FX complexes bind to heparan sulfate-containing glycosaminoglycans (GAGs) on target cells, but binding studies utilizing cells expressing specific GAGs and GAG-cleaving enzymes revealed differences in GAG dependence and specificity between these two complexes. These findings add to our understanding of the intricate infection pathways used by human adenoviruses, and they may contribute to better design of HAdV-based vectors for gene and cancer therapy. Furthermore, the interaction between the HAdV-31 hexon and FIX may also serve as a target for antiviral treatment.

Factor IXa as a target for pharmacologic inhibition in acute coronary syndrome

Anticoagulant therapy, combined with platelet-directed inhibitors, represents a standard-of-care in the management of patients with acute coronary syndrome, particularly those who require percutaneous coronary interventions. While a vast clinical experience, coupled with large clinical trials have collectively provided guidance, an optimal anticoagulant drug and applied strategy, defined as one that reduces thrombotic and hemorrhagic events consistently, with minimal off-target effects and active control of systemic anticoagulation according to patient and clinical-setting specific need, remains at large. An advancing knowledge of coagulation, hemostasis, and thrombosis suggests that factor IXa, a protease that governs thrombin generation in common thrombotic disorders may represent a prime target for pharmacologic inhibition.

Principles of Anticoagulation and New Therapeutic Agents in Atrial Fibrillation

Anticoagulation is required in cardiac arrhythmias, specifically atrial fibrillation (AF) and atrial flutter, to reduce the risk of thromboembolism. Principles of anticoagulation of both AF and atrial flutter are similar because the location and nature of the arrhythmias are similar. Approximately 2 million people in the United States are affected by AF, and the prevalence is expected to exceed 10 million by the year 2050. Warfarin is known to reduce stroke risk by 68% in patients with AF and is the most effective agent for this indication, although it is not without risk. Antithrombotic therapy with antiplatelets or anticoagulants is recommended for most patients with AF. This review discusses the principles of anticoagulation and the mechanism of action, pharmacologic profile, and phase of development of the therapeutic agents used as anticoagulants.

Generation of a novel factor IX with augmented clotting activities in vitro and in vivo

BACKGROUND

Hemophilia B is an X-linked inherited disorder caused by the lack of functional factor IX (FIX). Currently, treatment of hemophilia B is performed by intravenous infusion of plasma-derived or recombinant FIX.

OBJECTIVE

In an effort to reduce factor usage and cost, we investigated the potential use of FIX variants with enhanced specific clotting activity.

METHODS

Seven recombinant FIX variants using alanine replacement were generated and assayed for their activity in vitro and in vivo.

RESULTS

One variant containing three substitutions (V86A/E277A/R338A, FIX-Triple) exhibited 13-fold higher specific clotting activity and a 10-fold increased affinity for human FVIIIa compared with FIX-wild-type (FIX-WT) and was thus investigated systematically in vivo. Liver-specific FIX-Triple gene expression following hydrodynamic plasmid delivery revealed a 3.5-fold higher specific clotting activity compared with FIX-WT. Human FIX-Triple and FIX-WT knock-in mice were generated and it was confirmed that FIX-Triple has 7-fold higher specific clotting activity than FIX-WT under normal physiological conditions. Protein infusion of FIX-Triple into hemophilia B mice resulted in greater improvement of hemostasis than that achieved with FIX-WT. Moreover, tail-vein administration of a serotype 8 recombinant Adeno-associated vector (AAV8) expressing either FIX-WT or FIX-Triple in hemophilia B mice demonstrated a 7-fold higher specific clotting activity of FIX-Triple than FIX-WT.

CONCLUSIONS

Our results indicate that the FIX-Triple variant exhibits significantly enhanced clotting activity relative to FIX-WT due to tighter binding to FVIIIa, as demonstrated both in vitro and in vivo. Therefore, FIX-Triple is a good candidate for further evaluation in protein replacement therapy as well as gene-based therapeutic strategies.

The binding sites for the very low density lipoprotein receptor and low-density lipoprotein receptor-related protein are shared within coagulation factor VIII

Coagulation factor VIII (FVIII) is a ligand for two members of the low-density lipoprotein receptor family, low-density lipoprotein receptor-related protein (LRP) and low-density lipoprotein receptor, which cooperate in regulating clearance of FVIII from the circulation. This study was aimed to explore the mechanism of interaction of FVIII with very low density lipoprotein receptor (VLDLR), another member of the family, and map receptor-binding sites. Binding of plasma-derived FVIII and its fragments to recombinant soluble ectodomain of VLDLR (sVLDLR) was studied in solid-phase and surface plasmon resonance assays. Full-length FVIII and its light chain bound to sVLDLR with similar affinities (KD = 114 +/- 14 and 95 +/- 11 nmol/l, respectively); in contrast, exposure of high-affinity VLDLR-binding site within the heavy chain (KD = 30 +/- 2 nmol/l) required proteolytic cleavage by thrombin. The VLDLR-binding sites within heavy and light chains were mapped to the A2 domain residues 484-509 and the A3-C1 fragment, based on the inhibitory effects of anti-A2 monoclonal antibody 413 and anti-A3-C1 antibody fragment scFv KM33, respectively, previously shown to inhibit FVIII/LRP interaction. Soluble ligand-binding fragment of VLDLR inhibited activation of factor X by the intrinsic Xase in purified system. In cell culture, a higher Xase activity was associated with wild-type human embryonic kidney cells compared with transfected cells that express VLDLR on the cell surface. We conclude that the binding sites for VLDLR and LRP within FVIII overlap and the A2 site becomes exposed upon physiological activation of FVIII. A functional role of FVIII/VLDLR interaction may be related to regulation of intrinsic Xase activity.

Advancement in antithrombotics for stroke prevention in atrial fibrillation

The focus of this review is the evolving field of antithrombotic drug therapy for stroke prevention in patients with atrial fibrillation (AF). The current standard of therapy includes warfarin, acenocoumarol and phenprocoumon which have proven efficacy by reducing stroke by 68% against placebo. However, a narrow therapeutic index, wide variation in metabolism, and numerous food and drug interactions have limited their clinical application to only 50% of the indicated population. Newer agents such as direct thrombin inhibitors, factor Xa inhibitors, factor IX inhibitors, tissue factor inhibitors and a novel vitamin K antagonist are being developed to overcome the limitations of current agents. The direct thrombin inhibitor dabigatran is farthest along in development. Further clinical trial testing, and eventual incorporation into clinical practice will depend on safety, efficacy and cost. Development of a novel vitamin K antagonist with better INR control will challenge the newer mechanistic agents in their quest to replace the existing vitamin K antagonists. Till then, the large unfilled gap to replace conventional agents remains open. This review will assess all these agents, and compare their mechanism of action, stage of development and pharmacologic profile.

Functional duplication of ligand-binding domains within low-density lipoprotein receptor-related protein for interaction with receptor associated protein, alpha2-macroglobulin, factor IXa and factor VIII

The low-density lipoprotein receptor-related protein (LRP) binds a range of proteins including receptor associated protein (RAP), activated alpha2-macroglobulin (alpha2M*), factor IXa (FIXa), and factor VIII (FVIII) light chain. The binding is mediated by the complement-type repeats, which are clustered in four distinct regions within LRP. Cluster II of 8 repeats (CR3-10) and cluster IV of 11 repeats (CR21-31) have been implicated in ligand-binding. Previous studies have aimed to identify the cluster II repeats involved in binding alpha2M* and RAP. We now evaluated the binding to RAP, alpha2M*, FIXa and FVIII light chain of triplicate repeat-fragments of not only clusters II but also of cluster IV. Employing surface plasmon resonance analysis, we found that most efficient ligand-binding was displayed by the repeats within region CR4-8 of cluster II and within region CR24-28 of cluster IV. Whereas the binding to RAP could be attributed to two consecutive repeats (CR5-6, CR26-27), combinations of three repeats showed most efficient binding to FIXa (CR6-8, CR26-28), FVIII light chain (CR5-7, CR6-8, CR24-26), and alpha2M* (CR4-6, CR24-26). The results imply that there is an internal functional duplication of complement-type repeats within LRP resulting in two clusters that bind the same ligands.

Factor IXa inhibitors as novel anticoagulants

Currently available anticoagulants are limited by modest therapeutic benefits, narrow clinical applications, increased bleeding risk, and drug-induced thrombophilia. Because factor IX plays a pivotal role in tissue factor (TF)-mediated thrombin generation, it may represent a promising target for drug development. Several methods of attenuating factor IX activity, including monoclonal antibodies, synthetic active site-blocked competitive inhibitors, oral inhibitors, and RNA aptamers, have undergone investigation. This review summarizes present knowledge of factor IX inhibitors with emphasis on biology, pharmacology, preclinical data, and early-phase clinical experience in humans.

Haemophilia A: molecular insights

Haemophilia A is the most common inherited bleeding disorder caused by defects in the

Clin Chem Lab Med 2007;45:450–61.

Proteolytic cleavage of factor VIII heavy chain is required to expose the binding-site for low-density lipoprotein receptor-related protein within the A2 domain

BACKGROUND

Low-density lipoprotein receptor-related protein (LRP) is an endocytic receptor that contributes to the clearance of coagulation factor (F) VIII from the circulation. Previously, we have demonstrated that region Glu(1811)-Lys(1818) within FVIII light chain constitutes an important binding region for this receptor. We have further found that FVIII light chain and intact FVIII are indistinguishable in their LRP-binding affinities. In apparent contrast to these observations, a second LRP-binding region has been identified within A2 domain region Arg(484)-Phe(509) of FVIII heavy chain.

OBJECTIVE

In this study, we addressed the relative contribution of FVIII heavy chain in binding LRP.

METHODS AND RESULTS

Surface plasmon resonance analysis unexpectedly showed that FVIII heavy chain poorly associated to the receptor. The binding to LRP was, however, markedly enhanced upon cleavage of the heavy chain by thrombin. The A2 domain, purified from thrombin-activated FVIII, also showed efficient binding to LRP. Competition studies employing a recombinant antibody fragment demonstrated that region Arg(484)-Phe(509) mediates the enhanced LRP binding after thrombin cleavage.

CONCLUSIONS

We propose that LRP binding of non-activated FVIII is mediated via the FVIII light chain while in activated FVIII both the heavy and light chain contribute to LRP binding.

Identification of Coagulation Factor VIII A2 Domain Residues Forming the Binding Epitope for Low-Density Lipoprotein Receptor-Related Protein,

Regulation of the coagulation factor VIII (fVIII) level in circulation involves a hepatic receptor low-density lipoprotein receptor-related protein (LRP). One of two major LRP binding sites in fVIII is located within the A2 domain (A2), likely exposed within the fVIII complex with von Willebrand factor and contributing to regulation of fVIII via LRP. This work aimed to identify A2 residues forming its LRP-binding site, previously shown to involve residues 484-509. Isolated A2 was subjected to alanine-scanning mutagenesis followed by expression of a set of mutants in a baculovirus system. In competition and surface plasmon resonance assays, affinities of A2 mutants K466A, R471A, R484A, S488A, R489A, R490A, H497A, and K499A for LRP were found to be decreased by 2-4-fold. This correlated with 1.3-1.5-fold decreases in the degree of LRP-mediated internalization of the mutants in cell culture. Combining these mutations into pairs led to cumulative effects, i.e., 7-13-fold decrease in affinity for LRP and 1.6-2.2-fold decrease in the degree of LRP-mediated internalization in cell culture. We conclude that the residues mentioned above play a key role in formation of the A2 binding epitope for LRP. Experiments in mice revealed an approximately 4.5 times shorter half-life for A2 in the circulation in comparison with that of fVIII. The half-lives of A2 mutant R471A/R484A or A2 co-injected with receptor-associated protein, a classical ligand of LRP, were prolonged by approximately 1.9 and approximately 3.5 times, respectively, compared to that of A2. This further confirms the importance of the mutated residues for interaction of A2 with LRP and suggests the existence of an LRP-dependent mechanism for removing A2 as a product of dissociation of activated fVIII from the circulation.

Adenovirus binding to blood factors results in liver cell infection and hepatotoxicity

Adenoviruses (Ad) are efficient vehicles for gene delivery in vitro and in vivo. Therefore, they are a promising tool in gene therapy, particularly in the treatment of cancer and cardiovascular diseases. However, preclinical and clinical studies undertaken during the last decade have revealed a series of problems that limit both the safety and efficacy of Ad vectors, specifically after intravenous application. Major obstacles to clinical use include innate toxicity and Ad sequestration by nontarget tissues. The factors and mechanisms underlying these processes are poorly understood. The majority of intravenously injected Ad particles are sequestered by the liver, which in turn causes an inflammatory response characterized by acute transaminitis and vascular damage. Here, we describe a novel pathway that is used by Ad for infection of hepatocytes and Kupffer cells upon intravenous virus application in mice. We found that blood factors play a major role in targeting Ad vectors to hepatic cells. We demonstrated that coagulation factor IX and complement component C4-binding protein can bind the Ad fiber knob domain and provide a bridge for virus uptake through cell surface heparan sulfate proteoglycans and low-density lipoprotein receptor-related protein. An Ad vector, Ad5mut, which contained mutations in the fiber knob domain ablating blood factor binding, demonstrated significantly reduced infection of liver cells and liver toxicity in vivo. This study contributes to a better understanding of adenovirus-host interactions for intravenously applied vectors. It also provides a rationale for novel strategies to target adenovirus vector to specific tissues and to reduce virus-associated toxicity after systemic application.

The apoE isoform binding properties of the VLDL receptor reveal marked differences from LRP and the LDL receptor

Apolipoprotein E (apoE) associates with lipoproteins and mediates their interaction with members of the LDL receptor family. ApoE exists as three common isoforms that have important distinct functional and biological properties. Two apoE isoforms, apoE3 and apoE4, are recognized by the LDL receptor, whereas apoE2 binds poorly to this receptor and is associated with type III hyperlipidemia. In addition, the apoE4 isoform is associated with the common late-onset familial and sporadic forms of Alzheimer's disease. Although the interaction of apoE with the LDL receptor is well characterized, the specificity of other members of this receptor family for apoE is poorly understood. In the current investigation, we have characterized the binding of apoE to the VLDL receptor and the LDL receptor-related protein (LRP). Our results indicate that like the LDL receptor, LRP prefers lipid-bound forms of apoE, but in contrast to the LDL receptor, both LRP and the VLDL receptor recognize all apoE isoforms. Interestingly, the VLDL receptor does not require the association of apoE with lipid for optimal recognition and avidly binds lipid-free apoE. It is likely that this receptor-dependent specificity for various apoE isoforms and for lipid-free versus lipid-bound forms of apoE is physiologically significant and is connected to distinct functions for these receptors.

LDL Receptor–Related Protein and the Vascular Wall

LDL receptor–related protein 1 (LRP1) is highly expressed in the vascular wall and is mainly associated with macrophages and vascular smooth muscle cells (VSMCs). Overexpression of LRP1 in atherosclerotic lesions has been demonstrated in several animal models and human lesions. Clinical studies have suggested a relation between alterations in LRP1 expression and coronary heart disease. Indeed, it has been demonstrated that LRP1 gene expression is increased in blood mononuclear cells from patients with coronary obstruction and that the LRP1 mRNA-protein expression ratio is altered in coronary patients. Taken together, these results seem to suggest that LRP1 may be a pivotal receptor in the etiology of atherosclerosis. Our group has contributed to the elucidation of the physiopathologic role of LRP1 in the vascular wall by demonstrating that LRP1-mediated, matrix-retained LDL internalization could be crucial for VSMC–foam cell formation, that LRP1 is upregulated by lipid during human atherosclerotic lesion progression, and that LRP1-mediated aggregated LDL uptake causes the prothrombotic transformation of the vascular wall. Therefore, LRP1 seems to play a pathologic function during atherosclerotic lesion progression; however, LRP1 also seems to be essential for embryonic development and for the maintenance of vascular integrity. The protective effect of LRP1 in the vessel wall seems to be mainly due to its role in controlling certain signaling pathways. In this review, we will focus on the description of the main physiopathologic functions of LRP1 in the vascular wall.

Effects of genetic fusion of factor IX to albumin on in vivo clearance in mice and rabbits

Individuals with haemophilia B require replacement therapy with recombinant or plasma-derived coagulation factor IX (fIX). More benefit per injected dose might be obtained if fIX clearance could be slowed. The contribution of overall size to fIX clearance was explored, using genetic fusion to albumin. Recombinant murine fIX (MIX), and three proteins with C-terminal epitope tags were expressed in HEK 293 cells: tagged MIX (MIXT), tagged mouse serum albumin (MSAT) and MFUST, in which MIX and MSAT were fused in a single polypeptide chain. Proteins MFUST and MIXT were two- to threefold less active in clotting assays than MIX. In mice, the area under the clearance curve (AUC) was reduced for MFUST compared with MSAT or plasma-derived MSA (pd-MSA); the terminal catabolic half-life (t(0.5)) did not differ amongst the three proteins. Two minutes after injection, >40% of the injected MFUST was found in the liver, compared with <10% of either MSAT or pd-MSA. In rabbits, the AUC for MFUST was reduced compared to MIXT, MSAT, or pd-MSA, while the t(0.5) of the fusion protein fell between that of MIXT and MSAT or pd-MSA. Similar results were obtained with non-radioactive fused or non-fused recombinant human fIX in fIX knockout mice. The clearance behaviour of the fusion protein thus more closely resembled that of fIX than that of albumin despite a modest increase in terminal half-life, suggesting that fIX-specific interactions that are important in determining clearance were maintained in spite of the increased size of the fusion protein.

The Low Density Lipoprotein Receptor-related Protein Is a Motogenic Receptor for Plasminogen Activator Inhibitor-1

Although plasminogen activator inhibitor-1 (PAI-1) is known to stimulate cell migration, little is known about underlying mechanisms. We show that both active and inactive (e.g. cleaved) PAI-1 can activate the Jak/Stat signaling system and stimulate cell migration in chemotaxis, haptotaxis, chemokinesis, and wound healing assays. Moreover, antibodies to the LDL receptor-related protein (LRP) and an LRP antagonist (RAP) blocked these motogenic effects of PAI-1, while a PAI-1 mutant that did not bind to LRP failed to activate the Jak/Stat signaling pathway or to stimulate cell migration. PAI-1 had no chemotactic effect on LRP-deficient cells. These results indicate that LRP is a signaling molecule, that it mediates the migration-promoting activity of PAI-1, and that this activity does not require intact, biologically active PAI-1. Activation of this LRP-dependent signaling pathway by PAI-1 may begin to explain how the inhibitor stimulates cell migration in a variety of normal and pathological processes.

Diverse role of LDL receptor-related protein in the clearance of proteases and in signaling

The low density lipoprotein receptor-related protein (LRP) is a large endocytic receptor that participates in several biological pathways and plays prominent roles in lipoprotein metabolism and in the catabolism of proteinases involved in coagulation and fibrinolysis. LRP also mediates the cellular entry of certain viruses and toxins and facilitates the activation of various lysosomal enzymes. Deletion of the LRP gene in mice is lethal, confirming an important role for this receptor in development, although its exact function in development is still not known. In addition to its role in the endocytosis of numerous ligands, recent studies are emerging that describe a signaling role for this receptor as well.

Elevated plasma factor VIII in a mouse model of low-density lipoprotein receptor-related protein deficiency

It has been established that low-density lipoprotein receptor-related protein (LRP) is involved in the cellular uptake and degradation of coagulation factor VIII (FVIII) in vitro. To address the physiologic role of LRP in regulating plasma FVIII in vivo, we used cre/loxP-mediated conditional LRP- deficient mice (MX1cre(+)LRP(flox/flox)). Upon inactivation of the LRP gene, MX1cre(+)LRP(flox/flox) mice had significantly higher plasma FVIII as compared with control LRP(flox/flox) mice (3.4 and 2.0 U/mL, respectively; P <.001). Elevated plasma FVIII levels in MX1cre(+)LRP(flox/flox) mice coincided with increased plasma von Willebrand factor (VWF) (2.0 and 1.6 U/mL for MX1cre(+)LRP(flox/flox) and control LRP(flox/flox) mice, respectively; P <.05). Elevation of plasma FVIII and VWF persisted for at least 6 weeks after inactivation of the LRP gene. Upon comparing plasma FVIII and VWF in individual mice, we observed an increase of the FVIII/VWF ratio in MX1cre(+)LRP(flox/flox) mice as compared with control LRP(flox/flox) mice. Administration of either a vasopressin analog or an endotoxin resulted in increased plasma VWF, but not FVIII. In clearance experiments, MX1cre(+)LRP(flox/flox) mice displayed a 1.5-fold prolongation of FVIII mean residence time. Adenovirus-mediated overexpression of the 39-kDa receptor-associated protein (RAP) in normal mice resulted in a 3.5-fold increase of plasma FVIII. These data confirm that the regulation of plasma FVIII in vivo involves a RAP-sensitive mechanism. Surprisingly, plasma FVIII in MX1cre(+)LRP(flox/flox) mice increased 2-fold after RAP gene transfer. We propose that RAP-sensitive determinants other than hepatic LRP contribute to the regulation of plasma FVIII in vivo.

The future of recombinant coagulation factors

Hemophilias A and B are X chromosome-linked bleeding disorders, which are mainly treated by repeated infusions of factor (F)VIII or FIX, respectively. In the present review, we specify the limitations in expression of recombinant (r)FVIII and summarize the bioengineering strategies that are currently being explored for constructing novel rFVIII molecules characterized by high efficiency expression and improved functional properties. We present the strategy to prolong FVIII lifetime by disrupting FVIII interaction with its clearance receptors and demonstrate how construction of human-porcine FVIII hybrid molecules can reduce their reactivity towards inhibitory antibodies. While the progress in improving rFIX is impeded by low recovery rates, the authors are optimistic that the efforts of basic science may ultimately lead to higher efficiency of replacement therapy of both hemophilias A and B.

Residues Phe342-Asn346 of activated coagulation factor IX contribute to the interaction with low density lipoprotein receptor-related protein

When blood coagulation factor IX is converted to activated factor IX (factor IXa), it develops enzymatic activity and exposes the binding sites for both activated factor VIII and the endocytic receptor low density lipoprotein receptor-related protein (LRP). In the present study we investigated the interaction between factor IXa and LRP in more detail, using an affinity-purified soluble form of LRP (sLRP). Purified sLRP and full-length LRP displayed similar binding to factor IXa. An anti-factor IX monoclonal antibody CLB-FIX 13 inhibited factor IXa.sLRP complex formation. Both the antibody and a soluble recombinant fragment of LRP (i.e. cluster IV) interfered with factor IXa amidolytic activity, suggesting that the antibody and LRP share similar binding regions near the active site of factor IXa. Next, a panel of recombinant factor IXa variants with amino acid replacements in the surface loops bordering the active site was tested for binding to antibody CLB-FIX 13 and sLRP in a solid phase binding assay. Factor IXa variants with mutations in the region Phe(342)-Asn(346), located between the active site of factor IXa and factor VIII binding helix, showed reduced binding to both antibody CLB-FIX 13 and sLRP. Surface plasmon resonance analysis revealed that the variant with Asn(346) replaced by Asp displayed slower association to sLRP, whereas the variant with residues Phe(342)-Tyr(345) replaced by the corresponding residues of thrombin showed faster dissociation. Recombinant soluble LRP fragment cluster IV inhibited factor IXa-mediated activation of factor X with IC(50) values of 5 and 40 nm in the presence and absence of factor VIII, respectively. This inhibition thus seems to occur via two mechanisms: by interference with factor IXa.factor VIIIa complex assembly and by direct inhibition of factor IXa enzymatic activity. Accordingly, we propose that LRP may function as a regulator of blood coagulation.

Low density lipoprotein receptor-related protein and factor IXa share structural requirements for binding to the A3 domain of coagulation factor VIII

Low-density lipoprotein receptor-related protein (LRP) is an endocytic receptor that binds multiple distinct ligands, including blood coagulation factor VIII (FVIII). FVIII is a heterodimeric multidomain protein that consists of a heavy chain (domains A1, a1, A2, a2, and B) and a light chain (domains a3, A3, C1, and C2). Both chains contribute to high-affinity interaction with LRP. One LRP-interactive region has previously been located in the C2 domain, but its affinity is low in comparison with that of the entire FVIII light chain. We now have compared a variety of FVIII light chain derivatives with the light chain of its homolog FVa for LRP binding. In surface plasmon resonance studies employing LRP cluster II, the FVa and FVIII light chains proved different in that only FVIII displayed high-affinity binding. Because the FVIII a3-A3-C1 fragment was effective in associating with LRP, this region was explored for structural elements that are exposed but not conserved in FV. Competition studies using synthetic peptides suggested that LRP binding involves the FVIII-specific region Lys(1804)-Ala(1834) in the A3 domain. In line with this observation, LRP binding was inhibited by a recombinant antibody fragment that specifically binds to the FVIII sequence Glu(1811)-Lys(1818). The role of this sequence in LRP binding was further tested using a FVIII/FV chimera in which sequence Glu(1811)-Lys(1818) was replaced with the corresponding sequence of FV. Although this chimera still displayed residual binding to LRP cluster II, its affinity was reduced. This suggests that multiple sites in FVIII contribute to high-affinity LRP binding, one of which is the FVIII A3 domain region Glu(1811)-Lys(1818). This suggests that LRP binding to the FVIII A3 domain involves the same structural elements that also contribute to the assembly of FVIII with FIXa.

The low-density lipoprotein receptor gene family: a cellular Swiss army knife?

The low-density lipoprotein receptor gene family is an evolutionarily conserved group of cell-surface receptors produced by mammals and other organisms. Initially thought to be endocytic receptors that mediate the uptake of lipoproteins, recent findings have shown that these receptors have other roles in a range of cellular processes. Among other activities, members of this family act as signal transducers in neuronal migration processes, regulate synaptic plasticity or control vitamin homeostasis. Such multifunctionality is achieved by interaction with diverse cell-surface proteins including glycolipid-anchored receptors, G-protein-coupled receptors and ion channels. Here, we review the molecular interactions of this protein family with other cell-surface proteins that provide specificity and versatility - a versatility that may be reminiscent of a cellular Swiss army knife.

Expression of factor VIII in recombinant and transgenic systems

Deficiency in a coagulation factor VIII (FVIII) causes a genetic disorder hemophilia A, which is treated by repeated infusions of expensive FVIII products. Recombinant FVIII (rFVIII), the culmination of years of extensive international research, is an important alternative to plasma-derived FVIII (pdFVIII) and is considered to have a higher margin of safety. Advances in biotechnology allowed production of rFVIII at industrial scale, which significantly improved treatment of hemophilia A patients. We review the contemporary methods used for FVIII expression in mammalian cell culture systems and discuss the factors responsible for insufficient recoveries of rFVIII, such as inefficient accumulation of FVIII mRNA in the cell, complexity of the mechanisms of FVIII secretion, and instability of secreted FVIII. The approaches to improve the yield of rFVIII in cell culture systems include genetic engineering of B-domain-deleted FVIII, introduction of introns into FVIII cDNA constructs for more efficient processing and accumulation of FVIII mRNA, and introduction of mutations into chaperone-binding sites of FVIII to improve its secretion. Design of FVIII with prolonged half-life in vivo is considered as another promising direction in improving rFVIII protein and efficiency of hemophilia A therapy. As an alternative to expression of rFVIII in cell culture systems, we discuss production of rFVIII in transgenic animals, where high levels of rFVIII have been successfully secreted into milk. We also pay attention to the major limitations of this approach, such as safety issues associated with potential transmission of animal pathogens. Finally, we present a brief characterization of commercial recombinant FVIII products currently available on the market for hemophilia A treatment.

The alpha -chains of C4b-binding protein mediate complex formation with low density lipoprotein receptor-related protein

C4b-binding protein (C4BP) is a heparin-binding protein that participates in both the complement and hemostatic system. We investigated the interaction between C4BP and low density lipoprotein receptor-related protein (LRP), an endocytic receptor involved in the catabolism of various heparin-binding proteins. Both plasma-derived C4BP and recombinant C4BP consisting of only its alpha-chains (rC4BPalpha) bound efficiently to immobilized LRP, as determined by surface plasmon resonance analysis. Complementary, two distinct fragments of LRP, i.e. clusters II and IV, both associated to immobilized rC4BPalpha, and binding could be inhibited by the LRP antagonist receptor-associated protein. Further analysis showed that association of rC4BPalpha to LRP was inhibited by heparin or by anti-C4BP antibody RU-3B9, which recognizes the heparin-binding region of the C4BP alpha-chains. In cellular degradation experiments, LRP-expressing fibroblasts effectively degraded (125)I-labeled rC4BPalpha, whereas their LRP-deficient counterparts displayed a 4-fold diminished capacity of degrading (125)I-rC4BPalpha. Finally, initial clearance of C4BP in mice was significantly delayed upon co-injection with receptor-associated protein. In conclusion, our data demonstrate that the alpha-chains of C4BP comprise a binding site for LRP. We propose that LRP mediates at least in part the catabolism of C4BP and, as such, may regulate C4BP participation in complement and hemostatic processes.

Haemophilia A: effects of inhibitory antibodies on factor VIII functional interactions and approaches to prevent their action

Factor VIII (FVIII) is an essential component of the intrinsic pathway of blood coagulation. Normal functioning of FVIII requires its interactions with other components of the coagulation cascade. In the circulation, it exists as a complex with von Willebrand factor (vWF). Upon activation by thrombin or activated factor X (FXa), activated FVIII (FVIIIa) functions as a cofactor for the serine protease factor IXa. Their complex assembled on the phospholipid surface activates FX to FXa, which consequently participates in formation of thrombin, the key protease of the coagulation cascade. Genetic deficiency in FVIII results in a coagulation disorder haemophilia A, which is treated by infusions of FVIII products. Approximately 25-30% of patients develop antibodies inhibiting FVIII activity (FVIII inhibitors). The major epitopes of inhibitors are located within the A2, C2 and A3 domains of the FVIII molecule. The inhibitory effects of antibodies are manifested at various stages of the FVIII functional pathway, including FVIII binding to vWF, activation of FVIII by thrombin, and FVIIIa incorporation into the Xase complex. We summarize the current knowledge of the FVIII sites involved in interaction with its physiological ligands and different classes of inhibitory antibodies and describe their inhibitory mechanisms. We outline the strategies aimed to overcome the effects of inhibitory antibodies such as development of human/porcine FVIII molecules, resistant to inhibitors. We also discuss approaches to modulate the antibody response, as well as efforts to develop a long-term immunotolerance to FVIII protein.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Cell surface heparan sulfate proteoglycans participate in factor VIII catabolism mediated by low density lipoprotein receptor-related protein

We have demonstrated previously that catabolism of a coagulation factor VIII (fVIII) from its complex with von Willebrand factor (vWf) is mediated by low density lipoprotein receptor-related protein (LRP) (Saenko, E. L., Yakhyaev, A. V., Mikhailenko, I., Strickland, D. K., and Sarafanov, A. G. (1999) J. Biol. Chem. 274, 37685-37692). In the present study, we found that this process is facilitated by cell surface heparan sulfate proteoglycans (HSPGs). This was demonstrated by simultaneous blocking of LRP and HSPGs in model cells, which completely prevented fVIII internalization and degradation from its complex with vWf. In contrast, the selective blocking of either receptor had a lesser effect. In vivo studies of clearance of (125)I-fVIII-vWf complex in mice also demonstrated that the simultaneous blocking of HSPGs and LRP led to a more significant prolongation of fVIII half-life (5.5-fold) than blocking of LRP alone (3.5-fold). The cell culture and in vivo experiments revealed that HSPGs are also involved in another, LRP-independent pathway of fVIII catabolism. In both pathways, HSPGs act as receptors providing the initial binding of fVIII-vWf complex to cells. We demonstrated that this binding occurs via the A2 domain of fVIII, since A2, but not other portions of fVIII or isolated vWf, strongly inhibited cell surface binding of fVIII-vWf complex, and the affinities of A2 and fVIII-vWf complex for the cells were similar. The A2 site involved in binding to heparin was localized to the region 558-565, based on the ability of the corresponding synthetic peptide to inhibit A2 binding to heparin, used as a model for HSPGs.