Computational analysis of Asp519 and Glu665 mutations of coagulation factor FVIIIa: Implications for enhanced binding affinity of A2-domain.

The A2-domain of blood coagulation factor VIIIa is non-covalently bound to the A1 and A3 domains via weak intermolecular interactions. Functional instability due to rapid dissociation of A2-domain from the active FVIII in blood presents a major hurdle for the therapeutic applications of FVIIIa to treat Hemophilia-A. To identify the ideal hot-spot residues at the interface of A2 and A1/A3 domains that could enhance the structural stability of FVIIIa, we performed a comprehensive computational mutagenesis study of two A2-domain residues, Asp519 and Glu665, that interface the A1 and A3-domains. Each residue was mutated to 15 uncharged amino-acids and the mutant structures were refined by MD simulations. Based on the estimated relative binding affinities of mutant structures, we predict that the mutation of Asp519 to Leu, Gln, Thr, Val and the mutation of Glu665 to Val, Ile, Met, Asn and Trp enhance the A2-domain binding affinity by more than 20kcal/mol, compared to the WT structure. We anticipate that these predictions will be valuable for enzymatic studies towards the rational design of FVIIIa synthetic constructs with improved A2-domain binding affinity.

Coagulation factor VIIa-mediated protease-activated receptor 2 activation leads to ß-catenin accumulation via the AKT/GSK3ß pathway and contributes to breast cancer progression.

Cell migration and invasion are very characteristic features of cancer cells that promote metastasis, which is one of the most common causes of mortality among cancer patients. Emerging evidence has shown that coagulation factors can directly mediate cancer-associated complications either by enhancing thrombus formation or by initiating various signaling events leading to metastatic cancer progression. It is well established that, apart from its distinct role in blood coagulation, coagulation factor FVIIa enhances aggressive behaviors of breast cancer cells, but the underlying signaling mechanisms still remain elusive. To this end, we investigated FVIIa's role in the migration and invasiveness of the breast cancer cell line MDA-MB-231. Consistent with previous observations, we observed that FVIIa increased the migratory and invasive potential of these cells. We also provide molecular evidence that protease-activated receptor 2 activation followed by PI3K-AKT activation and GSK3ß inactivation is involved in these processes and that ß-catenin, a well known tumor-regulatory protein, contributes to this signaling pathway. The pivotal role of ß-catenin was further indicated by the up-regulation of its downstream targets cyclin D1, c-Myc, COX-2, MMP-7, MMP-14, and Claudin-1. ß-Catenin knockdown almost completely attenuated the FVIIa-induced enhancement of breast cancer migration and invasion. These findings provide a new perspective to counteract the invasive behavior of breast cancer, indicating that blocking PI3K-AKT pathway-dependent ß-catenin accumulation may represent a potential therapeutic approach to control breast cancer.

Asymmetric processing of mutant factor X Arg386Cys reveals differences between intrinsic and extrinsic pathway activation.

Alterations in coagulation factor X (FX) activation, mediated by the extrinsic VIIa/tissue factor (FVIIa/TF) or the intrinsic factor IXa/factor VIIIa (FIXa/FVIIIa) complexes, can result in hemorrhagic/prothrombotic tendencies. However, the molecular determinants involved in substrate recognition by these enzymes are poorly defined. Here, we investigated the role of arginine 386 (chymotrypsin numbering c202), a surface-exposed residue on the FX catalytic domain. The naturally occurring FX386Cys mutant and FX386Ala variant were characterized. Despite the unpaired cysteine, recombinant (r)FX386Cys was efficiently secreted (88.6±21.3% of rFXwt) and possessed normal clearance in mice. rFX386Cys was also normally activated by FVIIa/TF and displayed intact amidolytic activity. In contrast, rFX386Cys activation by the FIXa/FVIIIa complex was 4.5-fold reduced, which was driven by a decrease in the kcat (1.6∗10(-4) s(-1) vs 5.8∗10(-4) s(-1), rFXwt). The virtually unaltered Km (70.6 nM vs 55.6nM, rFXwt) suggested no major alterations in the FX substrate exosite. Functional assays in plasma supplemented with rFX386Cys indicated a remarkable reduction in the thrombin generation rate and thus in coagulation efficiency. Consistently, the rFX386Ala variant displayed similar biochemical features suggesting that global changes at position 386 impact the intrinsic pathway activation. These data indicate that the FXArg386 is involved in FIXa/FVIIIa-mediated FX activation and help in elucidating the bleeding tendency associated with the FX386Cys in a rare FX deficiency case. Taking advantage of the unpaired cysteine, the rFX386Cys mutant may be efficiently targeted by thiol-specific ligands and represent a valuable tool to study FX structure-function relationships both in vitro and in vivo.

Role of hydrophobic mutations on the binding affinity and stability of blood coagulation factor VIIIa: a computational molecular dynamics and free-energy analysis.

Factor VIIIa is a non-covalently bound hetero-trimer among A1, A2 and A3-C1-C2 domains and an essential co-factor for factor IXa enzyme during proteolytic activation of factor X zymogen. The relatively weak interactions between A2 and the interface A1/A3 domains dampen the functional stability of FVIIIa in plasma and results in rapid degradation. We studied the mutational effect of three charged residues (Asp519, Glu665 and Asp666) to several hydrophobic residues by molecular dynamics simulations. Analysis of the binding free energy by MM-PBSA and MM-GBSA methods shows that the mutation of Asp519 and Glu665 residues to either Val or Ala enhance the A2 domain binding affinity in agreement with the experimental site-specific mutagenesis data. Mutation of Asp666 to Val, Tyr, Met and Phe showed largest improvement in the A2-domain binding among the eight hydrophobic mutants studied. Our studies suggest that the enrichment of hydrophobic interactions in the buried surface regions of A2 domain plays crucial role in improving the overall stability of FVIIIa.

Combining mutations that modulate inter-subunit interactions and proteolytic inactivation enhance the stability of factor VIIIa.

FVIIIa is labile due to the dissociation of A2 subunit. Previously, we introduced hydrophobic mutations at select A1/A2/A3 subunit interfaces yielding more stable FVIII(a) variants. Separately we showed that altering the sequence flanking the primary FXa cleavage site in FVIIIa (Arg336) yielded reduced rates of proteolytic inactivation of FVIIIa. In this study we prepared the FXa-cleavage resistant mutant (336(P4-P3')562) combined with mutations of Ala108Ile, Asp519Val/Glu665Val or Ala108Ile/Asp519Val/Glu665Val and examined the effects of these combinations relative to FVIII thermal stability, rates of FVIIIa decay and proteolytic inactivation of FVIIIa by FXa. Thermal decay rates for 336(P4-P3')562/Ala108Ile, 336(P4-P3')562/Asp519Val/Glu665Val, and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ~2- to 5-fold as compared with wild-type (WT) primarily reflecting the effects of the A domain interface mutations. FVIIIa decay rates for 336(P4-P3')562/Asp519Val/Glu665Val and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants were reduced by ~25 fold, indicating greater stability than the control Asp519Val/Glu665Val variant (~14-fold). Interestingly, 336(P4-P3')562/Asp519Val/Glu665Val and 336(P4-P3')562/Ala108Ile/Asp519Val/Glu665Val variants showed reduced FXa-inactivation rates compared with the 336(P4-P3')562 control (~4-fold), suggesting A2 subunit destabilisation is a component of proteolytic inactivation. Thrombin generation assays using the combination variants were similar to the Asp519Val/Glu665Val control. These results indicate that combining multiple gain-of-function FVIII mutations yields FVIII variants with increased stability relative to a single type of mutation.

A3 domain region 1803-1818 contributes to the stability of activated factor VIII and includes a binding site for activated factor IX.

A recent chemical footprinting study in our laboratory suggested that region 1803-1818 might contribute to A2 domain retention in activated factor VIII (FVIIIa). This site has also been implicated to interact with activated factor IX (FIXa). Asn-1810 further comprises an N-linked glycan, which seems incompatible with a role of the amino acids 1803-1818 for FIXa or A2 domain binding. In the present study, FVIIIa stability and FIXa binding were evaluated in a FVIII-N1810C variant, and two FVIII variants in which residues 1803-1810 and 1811-1818 are replaced by the corresponding residues of factor V (FV). Enzyme kinetic studies showed that only FVIII/FV 1811-1818 has a decreased apparent binding affinity for FIXa. Flow cytometry analysis indicated that fluorescent FIXa exhibits impaired complex formation with only FVIII/FV 1811-1818 on lipospheres. Site-directed mutagenesis revealed that Phe-1816 contributes to the interaction with FIXa. To evaluate FVIIIa stability, the FVIII/FV chimeras were activated by thrombin, and the decline in cofactor function was followed over time. FVIII/FV 1803-1810 and FVIII/FV 1811-1818 but not FVIII-N1810C showed a decreased FVIIIa half-life. However, when the FVIII variants were activated in presence of FIXa, only FVIII/FV 1811-1818 demonstrated an enhanced decline in cofactor function. Surface plasmon resonance analysis revealed that the FVIII variants K1813A/K1818A, E1811A, and F1816A exhibit enhanced dissociation after activation. The results together demonstrate that the glycan at 1810 is not involved in FVIII cofactor function, and that Phe-1816 of region 1811-1818 contributes to FIXa binding. Both regions 1803-1810 and 1811-1818 contribute to FVIIIa stability.

Factor VIIIa A2 subunit shows a high affinity interaction with factor IXa: contribution of A2 subunit residues 707-714 to the interaction with factor IXa.

Factor (F) VIIIa forms a number of contacts with FIXa in assembling the FXase enzyme complex. Surface plasmon resonance was used to examine the interaction between immobilized biotinylated active site-modified FIXa, and FVIII and FVIIIa subunits. The FVIIIa A2 subunit bound FIXa with high affinity (Kd = 3.9 ± 1.6 nm) that was similar to the A3C1C2 subunit (Kd = 3.6 ± 0.6 nm). This approach was used to evaluate a series of baculovirus-expressed, isolated A2 domain (bA2) variants where alanine substitutions were made for individual residues within the sequence 707-714, the C-terminal region of A2 thought to be FIXa interactive. Three of six bA2 variants examined displayed 2- to 4-fold decreased affinity for FIXa as compared with WT bA2. The variant bA2 proteins were also tested in two reconstitution systems to determine activity and affinity parameters in forming FXase and FVIIIa. Vmax values for all variants were similar to the WT values, indicating that these residues do not affect cofactor function. All variants showed substantially greater increases in apparent Kd relative to WT in reconstituting the FXase complex (8- to 26-fold) compared with reconstituting FVIIIa (1.3- to 6-fold) suggesting that the mutations altered interaction with FIXa. bA2 domain variants with Ala replacing Lys(707), Asp(712), and Lys(713) demonstrated the greatest increases in apparent Kd (17- to 26-fold). These results indicate a high affinity interaction between the FVIIIa A2 subunit and FIXa and show a contribution of several residues within the 707-714 sequence to this binding.

Variable contributions of basic residues forming an APC exosite in the binding and inactivation of factor VIIIa.

Basic residues contained in the 39-, 60-, and 70-80-loops of activated protein C (APC) comprise an exosite that contributes to the binding and subsequent proteolytic inactivation of factor (F) VIIIa. Surface plasmon resonance (SPR) showed that WT APC bound to FVIII light chain (LC) and the FVIIIa A1/A3C1C2 dimer with equivalent affinity (Kd = 525 and 546 nM, respectively). These affinity values may reflect binding interactions to the acidic residue-rich a1 and a3 segments adjacent to A1 domain in the A1/A3C1C2 and A3 domain in LC, respectively. Results from SPR, using a panel of APC exosite variants where basic residues were mutated, in binding to immobilized FVIIIa A1/A3C1C2 or LC indicated ~4-10-fold increases in the Kd values relative to WT for several of the variants including Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, and Arg67Ala. On the other hand, a number of APC variants including Lys38Ala, Lys62Ala, and Lys78Ala showed little if any change in binding affinity to the FVIII substrates. FXa generation assays and Western blotting, used to monitor rates of FVIIIa inactivation and proteolysis at the primary cleavage site in the cofactor (Arg(336)), respectively, showed marked rate reductions relative to WT for the Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, Arg67Ala, and Arg74Ala variants. Furthermore, kinetic analysis monitoring FVIIIa inactivation by APC variants at varying FVIIIa substrate concentration showed ~2.6-4.4-fold increases in Km values relative to WT. These results show a variable contribution of basic residues comprising the APC exosite, with significant contributions from Lys39, Arg67, and Arg74 to forming a FVIIIa-interactive site.

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).

P3-P3' residues flanking scissile bonds in factor VIII modulate rates of substrate cleavage and procofactor activation by thrombin.

Thrombin-catalyzed activation of factor VIII (FVIII) occurs through proteolysis at three P1 Arg residues: Arg(372) and Arg(740) in the FVIII heavy chain and Arg(1689) in the FVIII light chain. Cleavage at the latter two sites is relatively fast compared with cleavage at Arg(372), which appears to be rate-limiting. Examination of the P3-P3' residues flanking each P1 site revealed that those sequences at Arg(740) and Arg(1689) are more optimal for thrombin cleavage than at Arg(372), suggesting these sequences may impact reaction rates. Recombinant FVIII variants were prepared with mutations swapping scissile bond flanking sequences in the heavy chain individually and in combination with a second swap or with a P1 point mutation. Rates of generation of A1 and A3-C1-C2 subunits were determined by Western blotting and correlated with rates of cleavage at Arg(372) and Arg(1689), respectively. Rates of thrombin cleavage at Arg(372) were increased ~10- and ~3-fold compared with that of wild-type FVIII when it was replaced with P3-P3' residues flanking Arg(740) and Arg(1689), respectively, and these values paralleled increased rates of A2 subunit generation and procofactor activation. Positioning of more optimal residues flanking Arg(372) abrogated the need for initial cleavage at Arg(740) to facilitate this step. These results show marked changes in cleavage rates correlate with the extent of cleavage-optimal residues flanking the scissile bond and modulate the mechanism for procofactor activation.

Sequences flanking Arg336 in factor VIIIa modulate factor Xa-catalyzed cleavage rates at this site and cofactor function.

Factor (F)VIII can be activated to FVIIIa by FXa following cleavages at Arg(372), Arg(740), and Arg(1689). FXa also cleaves FVIII/FVIIIa at Arg(336) and Arg(562) resulting in inactivation of the cofactor. These inactivating cleavages occur on a slower time scale than the activating ones. We assessed the contributions to cleavage rate and cofactor function of residues flanking Arg(336), the primary site yielding FVIII(a) inactivation, following replacement of these residues with those flanking the faster-reacting Arg(740) and Arg(372) sites and the slower-reacting Arg(562) site. Replacing P4-P3' residues flanking Arg(336) with those from Arg(372) or Arg(740) resulted in ∼4-6-fold increases in rates of FXa-catalyzed inactivation of FVIIIa, which paralleled the rates of proteolysis at Arg(336). Examination of partial sequence replacements showed a predominant contribution of prime residues flanking the scissile bonds to the enhanced rates. Conversely, replacement of this sequence with residues flanking the slow-reacting Arg(562) site yielded inactivation and cleavage rates that were ∼40% that of the WT values. The capacity for FXa to activate FVIII variants where cleavage at Arg(336) was accelerated due to flanking sequence replacement showed marked reductions in peak activity, whereas reducing the cleavage rate at this site enhanced peak activity. Furthermore, plasma-based thrombin generation assays employing the variants revealed significant reductions in multiple parameter values with acceleration of Arg(336) cleavage suggesting increased down-regulation of FXase. Overall, these results are consistent with a model of competition for activating and inactivating cleavages catalyzed by FXa that is modulated in large part by sequences flanking the scissile bonds.

Increasing hydrophobicity or disulfide bridging at the factor VIII A1 and C2 domain interface enhances procofactor stability.

Factor VIII (FVIII) consists of a heavy (A1A2B domains) and light chain (A3C1C2 domains), whereas the contiguous A1A2 domains are separate subunits in the cofactor, FVIIIa. FVIII x-ray structures show close contacts between A1 and C2 domains. To explore the role of this region in FVIII(a) stability, we generated a variant containing a disulfide bond between A1 and C2 domains by mutating Arg-121 and Leu-2302 to Cys (R121C/L2302C) and a second variant with a bulkier hydrophobic group (A108I) to better occupy a cavity between A1 and C2 domains. Disulfide bonding in the R121C/L2302C variant was >90% efficient as judged by Western blots. Binding affinity between the A108I A1 and A3C1C2 subunits was increased ∼3.7-fold in the variant as compared with WT as judged by changes in fluorescence of acrylodan-labeled A1 subunits. FVIII thermal and chemical stability were monitored following rates of loss of FVIII activity at 57 °C or in guanidinium by factor Xa generation assays. The rate of decay of FVIIIa activity was monitored at 23 °C following activation by thrombin. Both R121C/L2302C and A108I variants showed up to ∼4-fold increases in thermal stability but minimal improvements in chemical stability. The purified A1 subunit of A108I reconstituted with the A3C1C2 subunit showed an ∼4.6-fold increase in thermal stability, whereas reconstitution of the variant A1 with a truncated A3C1 subunit showed similar stability values as compared with WT A1. Together, these results suggest that altering contacts at this A1-C2 junction by covalent modification or increasing hydrophobicity increases inter-chain affinity and functionally enhances FVIII stability.

The role of P4-P3' residues flanking Arg336 in facilitating activated protein C-catalyzed cleavage and inactivation of factor VIIIa.

INTRODUCTION: Activated protein C (APC) inactivates factor VIIIa (FVIIIa) through cleavages at Arg336 in the A1 subunit and Arg562 in the A2 subunit. Proteolysis at Arg336 occurs 25-fold faster than at Arg562. Replacing residues flanking Arg336 en bloc with the corresponding residues surrounding Arg562 markedly reduced the rate of cleavage at Arg336, indicating a role for these residues in the catalysis mechanism. MATERIALS AND METHODS: To assess the contributions of individual P4-P3' residues flanking the Arg336 site to cleavage efficiency, point mutations were made based upon those flanking Arg562 of FVIIIa (Pro333Val, Gln334Asp, Leu335Gln, Met337Gly, Lys338Asn, Asn339Gln) and selected residues flanking Arg506 of FVa (Leu335Arg, and Lys338Ile). APC-catalyzed inactivation of the FVIII variants and cleavage of FVIIIa subunits were monitored by FXa generation assays and Western blotting. RESULTS: Specific activity values of the variants were 60-135% of the wild type (WT) value. APC-catalyzed rates of cleavage at Arg336 remained similar to WT for the Pro333Val and Lys338Ile variants and was modestly increased for the Asn339Gln variant; while rates were reduced ~2-3-fold for the Gln334Asp, Leu335Gln, Leu335Arg, and Lys338Asn variants, and 5-fold for the Met337Gly variant. Rates for cofactor inactivation paralleled cleavage at the A1 site. APC slowly cleaves Arg372 in FVIII, a site responsible for procofactor activation. Using FVIII as substrate for APC, the Met337Gly variant yielded significantly greater activation compared with WT FVIII. CONCLUSIONS: These results show that individual P4-P3' residues surrounding Arg336 are in general more favorable to cleavage than those surrounding the Arg562 site.

[Enhancing effect of porcine coagulation factor VIII A1 and A3 domains on secretion of post-translationally spliced human/porcine hybrid coagulation factor VIII].

Low levels of coagulation factor VIII (fVIII) protein expression caused by its inefficient secretion and the over-sized fVIII gene affect the transgene-based gene therapy for hemophilia A adversely. Our previous study demonstrated that intein-mediated protein trans-splicing for delivery of the fVIII gene with a dual-vector system could improve secretion of post-translationally spliced fVIII by light chain in cis. In this study, a human/porcine hybrid fVIII (HP-fVIII) containing replaced A1 and A3 domains of porcine fVIII was investigated for secretion and activity of the spliced HP-fVIII after intein-based dual-vector delivery of the HP-fVIII gene. A pair of expression plasmids comprising intein-fused HP-fVIII heavy and light chains were constructed and transiently co-transfected into COS-7 cells. The spliced HP-fVIII and bio-activity in culture media were quantitatively analyzed by ELISA and Coatest method respectively. The intracellular splicing of HP-fVIII was detected by Western blotting. The results showed that in the culture supernatant of cells co-transfected with HP-fVIII, the amount and activity of spliced HP-fVIII were significantly higher than those of spliced hfVIII secreted from the cells co-transfected with human fVIII [(184+/-34 ng/mL) vs (48+/-12) ng/mL, P<0.01; (1.18+/-0.22) IU/mL vs (0.31+/-0.10) IU/mL, P<0.01], demonstrating the dramatically enhancing effect of porcine A1 and A3 domains on the secretion of intein-spliced HP-fVIII. The spliced HP-fVIII protein and its activity were also detected in the supernatant from combined cells separately transfected with intein-fused HP-fVIII heavy and light chain genes, indicating that the intein-mediated HP-fVIII splicing was independent of cellular mechanism and could occur outside the cell after the secretion of precursor proteins. Additionally, an intracellularly spliced HP-fVIII band was found with a molecular weight similar to human fVIII protein, confirming the HP-fVIII splicing. These results provided experimental basis for ongoing study using intein-based dual adeno-associated virus (AAV) vector to transfer HP-fVIII gene in animal models.

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.

Activated protein C cofactor function of protein S: a critical role for Asp95 in the EGF1-like domain.

Protein S has an established role in the protein C anticoagulant pathway, where it enhances the factor Va (FVa) and factor VIIIa (FVIIIa) inactivating property of activated protein C (APC). Despite its physiological role and clinical importance, the molecular basis of its action is not fully understood. To clarify the mechanism of the protein S interaction with APC, we have constructed and expressed a library of composite or point variants of human protein S, with residue substitutions introduced into the Gla, thrombin-sensitive region (TSR), epidermal growth factor 1 (EGF1), and EGF2 domains. Cofactor activity for APC was evaluated by calibrated automated thrombography (CAT) using protein S-deficient plasma. Of 27 variants tested initially, only one, protein S D95A (within the EGF1 domain), was largely devoid of functional APC cofactor activity. Protein S D95A was, however, gamma-carboxylated and bound phospholipids with an apparent dissociation constant (Kd(app)) similar to that of wild-type (WT) protein S. In a purified assay using FVa R506Q/R679Q, purified protein S D95A was shown to have greatly reduced ability to enhance APC-induced cleavage of FVa Arg306. It is concluded that residue Asp95 within EGF1 is critical for APC cofactor function of protein S and could define a principal functional interaction site for APC.

Fucosylated chondroitin sulfate inhibits plasma thrombin generation via targeting of the factor IXa heparin-binding exosite.

Depolymerized holothurian glycosaminoglycan (DHG) is a fucosylated chondroitin sulfate with antithrombin-independent antithrombotic properties. Heparin cofactor II (HCII)-dependent and -independent mechanisms for DHG inhibition of plasma thrombin generation were evaluated. When thrombin generation was initiated with 0.2 pM tissue factor (TF), the half maximal effective concentration (EC(50)) for DHG inhibition was identical in mock- or HCII-depleted plasma, suggesting a serpin-independent mechanism. In the presence of excess TF, the EC(50) for DHG was increased 13- to 27-fold, suggesting inhibition was dependent on intrinsic tenase (factor IXa-factor VIIIa) components. In factor VIII-deficient plasma supplemented with 700 pM factor VIII or VIIIa, and factor IX-deficient plasma supplemented with plasma-derived factor IX or 100 pM factor IXa, the EC(50) for DHG was similar. Thus, cofactor and zymogen activation did not contribute to DHG inhibition of thrombin generation. Factor IX-deficient plasma supplemented with mutant factor IX(a) proteins demonstrated resistance to DHG inhibition of thrombin generation [factor IX(a) R233A > R170A > WT] that inversely correlated with protease-heparin affinity. These results replicate the effect of these mutations with purified intrinsic tenase components, and establish the factor IXa heparin-binding exosite as the relevant molecular target for inhibition by DHG. Glycosaminoglycan-mediated intrinsic tenase inhibition is a novel antithrombotic mechanism with physiologic and therapeutic applications.

Identification of plasmin-interactive sites in the light chain of factor VIII responsible for proteolytic cleavage at Lys36.

We have recently reported that plasmin likely associates with the factor VIII light chain to proteolyze at Lys36 within the A1 domain. In this study, we determined that the rate of plasmin-catalyzed inactivation on the forms of factor VIIIa containing A1-(1-336) and 1722A3C1C2, reflecting Lys36 cleavage, was reduced by approximately 60%, compared with those containing 1649A3C1C2 and 1690A3C1C2. SDS-PAGE analysis revealed that Lys36 cleavage of factor VIIIa with 1722A3C1C2 was markedly slower than those with 1649A3C1C2 and 1690A3C1C2. Surface plasmon resonance-based assays, using active site-modified anhydro-plasmin (Ah-plasmin) showed that 1722A3C1C2 bound to Ah-plasmin with an approximately 3-fold lower affinity than 1649A3C1C2 or 1690A3C1C2 (Kd, 176, 68.2, and 60.3 nM, respectively). Recombinant A3 bound to Ah-plasmin (Kd, 44.2 nM), whereas C2 failed to bind, confirming the presence of a plasmin-binding site within N terminus of A3. Furthermore, the Glu-Gly-Arg active site-modified factor IXa also blocked 1722A3C1C2 binding to Ah-plasmin by approximately 95%, supporting the presence of another plasmin-binding site overlapping the factor IXa-binding site in A3. In keeping with a major contribution of the lysine-binding sites in plasmin for interaction with the factor VIII light chain, analysis of the A3 sequence revealed two regions involving clustered lysine residues in 1690-1705 and 1804-1818. Two peptides based on these regions blocked 1649A3C1C2 binding to Ah-plasmin by approximately 60% and plasmin-catalyzed Lys36 cleavage of factor VIIIa with A1-(1-336) by approximately 80%. Our findings indicate that an extended surface, centered on residues 1690-1705 and 1804-1818 within the A3 domain, contributes to a unique plasmin-interactive site that promotes plasmin docking during cofactor inactivation by cleavage at Lys36.

The factor VIIIa C2 domain (residues 2228-2240) interacts with the factor IXa Gla domain in the factor Xase complex.

Factor VIIIa functions as a cofactor for factor IXa in the phospholipid surface-dependent activation of factor X. Both the C2 domain of factor VIIIa and the Gla domain of factor IXa are involved in phospholipid binding and are required for the activation of factor X. In this study, we have examined the close relationship between these domains in the factor Xase complex. Enzyme-linked immunosorbent assay-based and surface plasmon resonance-based assays in the absence of phospholipid showed that Glu-Gly-Arg active site-modified factor IXa bound to immobilized recombinant C2 domain (rC2) dose-dependently (Kd = 108 nm). This binding ability was optimal under physiological conditions. A monoclonal antibody against the Gla domain of factor IXa inhibited binding by approximately 95%, and Gla domainless factor IXa failed to bind to rC2. The addition of monoclonal antibody or rC2 with factor VIIIa inhibited factor IXa-catalyzed factor X activation in the absence of phospholipid. Inhibition was not evident, however, in similar experiments in the absence of factor VIIIa, indicating that the C2 domain interacted with the Gla domain of factor IXa. A fragment designated C2-(2182-2259), derived from V8 protease-cleaved rC2, bound to Glu-Gly-Arg active site-modified factor IXa. Competitive assays, using overlapping synthetic peptides encompassing residues 2182-2259, demonstrated that peptide 2228-2240 significantly inhibited both this binding and factor Xa generation, independently of phospholipid. Our results indicated that residues 2228-2240 in the factor VIIIa C2 domain constitutes an interactive site for the Gla domain of factor IXa. The findings provide the first evidence for an essential role for this interaction in factor Xase assembly.

Dissociation of activated protein C functions by elimination of protein S cofactor enhancement.

Activated protein C (APC) plays a critical anticoagulant role in vivo by inactivating procoagulant factor Va and factor VIIIa and thus down-regulating thrombin generation. In addition, APC bound to the endothelial cell protein C receptor can initiate protease-activated receptor-1 (PAR-1)-mediated cytoprotective signaling. Protein S constitutes a critical cofactor for the anticoagulant function of APC but is not known to be involved in regulating APC-mediated protective PAR-1 signaling. In this study we utilized a site-directed mutagenesis strategy to characterize a putative protein S binding region within the APC Gla domain. Three single amino acid substitutions within the APC Gla domain (D35T, D36A, and A39V) were found to mildly impair protein S-dependent anticoagulant activity (<2-fold) but retained entirely normal cytoprotective activity. However, a single amino acid substitution (L38D) ablated the ability of protein S to function as a cofactor for this APC variant. Consequently, in assays of protein S-dependent factor Va proteolysis using purified proteins or in the plasma milieu, APC-L38D variant exhibited minimal residual anticoagulant activity compared with wild type APC. Despite the location of Leu-38 in the Gla domain, APC-L38D interacted normally with endothelial cell protein C receptor and retained its ability to trigger PAR-1 mediated cytoprotective signaling in a manner indistinguishable from that of wild type APC. Consequently, elimination of protein S cofactor enhancement of APC anticoagulant function represents a novel and effective strategy by which to separate the anticoagulant and cytoprotective functions of APC for potential therapeutic gain.