The Carbohydrate Moiety of Factor V Modulates Inactivation by Activated Protein C

Comprehensive N- and O-glycosylation mapping of human coagulation factor V

BACKGROUND/OBJECTIVE

Coagulation factor V (FV), a multidomain glycoprotein, is an essential cofactor in the blood clotting cascade. FV deficiency is a rare bleeding disorder that results in poor clotting after an injury or surgery. The only treatment for the disease is infusions of fresh frozen plasma and blood platelets. Glycosylation affects the biological activity, pharmacokinetics, immunogenicity, and in vivo clearance rate of proteins in the plasma. The glycan profile of FV, as well as how it affects the activity, stability, and immunogenicity, remains unknown.

METHODS

In this study, we comprehensively mapped the glycosylation patterns of human plasma-derived FV by combining multienzyme digestion, hydrophilic interaction chromatography enrichment of glycopeptides, and alternated fragmentation mass spectrometry analysis.

RESULTS/CONCLUSION

A total of 57 unique N-glycopeptides and 51 O-glycopeptides were identified, which were categorized into 40 N-glycan and 17 O-glycan compositions. Such glycosylation details are fundamental for future functional studies and therapeutics development. In addition, the established methodology can be readily applied to analyze glycosylation patterns of proteins with more than 2000 amino acids.

The anticoagulant function of coagulation factor V

Almost two decades ago an anticoagulant function of factor V (FV) was discovered, as an anticoagulant cofactor for activated protein C (APC). A natural mutant of FV in which the R506 inactivation site was mutated to Gln (FVLeiden) was inactivated slower by APC, but also could not function as anticoagulant cofactor for APC in the inactivation of activated factor VIII (FVIIIa). This mutation is prevalent in populations of Caucasian descent, and increases the chance of thrombotic events in carriers. Characterisation of the FV anticoagulant effect has elucidated multiple properties of the anticoagulant function of FV: 1) Cleavage of FV at position 506 by APC is required for anticoagulant function. 2) The C-terminal part of the FV B domain is required and the B domain must have an intact connection with the A3 domain of FV. 3) FV must be bound to a negatively charged phospholipid membrane. 4) Protein S also needs to be present. 5) FV acts as a cofactor for inactivation of both FVa and FVIIIa. 6) The prothrombotic function of FVLeiden is a function of both reduced APC cofactor activity and resistance of FVa to APC inactivation. However, detailed structural and mechanistic properties remain to be further explored.

Syndromes of Thrombosis and Hypercoagulability: Congenital and Acquired Thrombophilias

This article stresses the common hereditary and acquired blood protein defects associated with thrombosis. The most common of the hereditary defects apear to be APC-R, SPS, antithrombin, protein C, and protein S deficiency, and the most common acquired defects are anticardiolipin antibodies and the lupus anticoagulant (antiphospholipid antibodies). Therefore, these are the defects that should first be looked for in an individual with unexplained thrombosis. If these more common defects are not found, then the rarer defects including HC II, plasminogen or TPA deficiency, dysfibrinogenemia, el evated PAI-1 and hyperhomocysteinemia should be sought. The importance of finding these defects has significant impli cations for therapy of the individual patient and for institutions of family studies to identify, inform, and possibly treat others at risk. It is expected that as knowledge of hemostasis expands, more hereditary and acquired defects, such as elevated lipopro tein (a) or defects of extrinsic (tissue factor) pathway inhibitor (EPI, TFPI), may be associated with enhanced risks of throm bosis. Finally, it must be recalled that a diagnosis of thrombo sis, like that of anemia, is only a generic and partial diagnosis; just as in the anemic patient, the etiology must be clearly de fined. Only in this manner can cost-effective and appropriate therapy for both primary treatment and secondary prevention be designed. In addition, the demonstration of a hereditary defect will allow primary prevention in afflicted family mem bers by allowing the choice of appropriate therapy.

Direct anticoagulant activity of protein S-C4b binding protein complex in Heerlen heterozygotes and normals

BACKGROUND

Plasma protein S normally circulates free (40%) or complexed with C4b-binding protein (PS-C4BP); only free protein S is a cofactor for activated protein C during factor (F) Va inactivation. Protein S-Heerlen lacks a carbohydrate group, leading to low plasma free protein S levels, but normal levels of PS-C4BP.

OBJECTIVES

Because protein S-Heerlen is not associated with thrombosis, we investigated whether PS-C4BP is directly anticoagulant in plasma and whether PS-Heerlen-C4BP has enhanced direct anticoagulant activity.

METHODS

An assay for protein S direct activity was applied to Heerlen-heterozygous plasmas. Free and complexed protein S were repeatedly isolated from normal and Heerlen-heterozygous plasmas and tested for direct anticoagulant activity in prothrombinase assays and in plasma.

RESULTS

Heerlen-heterozygous plasmas were deficient in free and total protein S antigen but had normal to high protein S direct anticoagulant activity. Purified Heerlen-heterozygous PS-C4BP was 7-fold more potent than normal PS-C4BP in inhibiting full prothrombinase activity, and 22-fold more potent in inhibiting prothrombin activation in the absence of FVa; it also specifically prolonged plasma clotting times 14-fold more than normal PS-C4BP. Heerlen-heterozygous PS-C4BP did not compete for limiting phospholipids any better than normal PS-C4BP. However, ligand blots and surface plasmon resonance studies showed that Heerlen-heterozygous PS-C4BP bound more avidly to FXa than did normal PS-C4BP (apparent Kd = 4.3 nm vs. 82 nm).

CONCLUSIONS

Plasma-derived PS-C4BP has direct anticoagulant activity in plasma and in purified systems. Enhanced direct activity of PS-Heerlen-C4BP may compensate for low free protein S levels and low cofactor activity in individuals with protein S-Heerlen.

Factor V I359T: a novel mutation associated with thrombosis and resistance to activated protein C

We report a kindred in which two siblings suffered spontaneous venous thromboses in the second decade of life. Further investigation showed reduced coagulation factor V (FV) activity and activated protein C resistance (APCR) ratio but no other thrombophilic abnormalities. The reduction in APCR ratio persisted in a modified APCR assay in which FV activity was normalized between test and control plasmas. Analysis of the FV gene showed that the thrombotic individuals had a complex genotype that included two novel point mutations c.529G>T and c.1250T>C resulting in FV E119X and FV I359T substitutions inherited on different alleles. Individuals in the kindred with FV E119X or FV I359T substitutions alone were asymptomatic. We suggest that the FV I359T substitution confers pro-thrombotic risk and APCR, but that this is only clinically manifest when co-inherited with the FV E119X allele. The FV I359T substitution creates a new consensus sequence for N-linked glycosylation within the FV heavy chain and we speculate that this abnormal glycosylation may disrupt activated protein C-mediated proteolysis of the variant FV and FVa.

Neutral glycosphingolipid-dependent inactivation of coagulation factor Va by activated protein C and protein S

To test whether neutral glycosphingolipids can serve as anticoagulant cofactors, the effects of incorporation of neutral glycosphingolipids into phospholipid vesicles on anticoagulant and procoagulant reactions were studied. Glucosylceramide (GlcCer), lactosylceramide (LacCer), and globotriaosylceramide (Gb(3)Cer) in vesicles containing phosphatidylserine (PS) and phosphatidylcholine (PC) dose dependently enhanced factor Va inactivation by the anticoagulant factors, activated protein C (APC) and protein S. Addition of GlcCer to PC/PS vesicles enhanced protein S-dependent APC cleavage in factor Va at Arg-506 by 13-fold, whereas PC/PS vesicles alone minimally affected protein S enhancement of this reaction. Incorporation into PC/PS vesicles of GlcCer, LacCer, or Gb(3)Cer, but not galactosylceramide or globotetraosylceramide, dose dependently prolonged factor Xa-1-stage clotting times of normal plasma in the presence of added APC without affecting baseline clotting times in the absence of APC, showing that certain neutral glycosphingolipids enhance anticoagulant but not procoagulant reactions in plasma. Thus, certain neutral glycosphingolipids (e.g. GlcCer, LacCer, and Gb(3)Cer) can enhance anticoagulant activity of APC/protein S by mechanisms that are distinctly different from those of phospholipids alone. We speculate that under some circumstances certain neutral glycosphingolipids either in lipoprotein particles or in cell membranes may help form antithrombotic microdomains that might enhance down-regulation of thrombin by APC in vivo.

Importance of individual activated protein C cleavage site regions in coagulation factor V for factor Va inactivation and for factor Xa activation

Activated protein C (APC) cleavage of Factor Va (FVa) at residues R506 and R306 correlates with its inactivation. APC resistance and increased thrombotic risk are due to the mutation R506Q in Factor V (FV). To study the effects of individual cleavages in FVa by APC and the importance of regions near the cleavage sites, the following recombinant (r) human FVs were prepared and purified: wild-type, Q306-rFV, Q506-rFV, and Q306Q506-rFV. All had similar time courses for thrombin activation. Q506-rFVa was cleaved by APC at R306 and was moderately resistant to APC in plasma-clotting assays and in prothrombinase assays measuring FVa residual activity, in agreement with studies of purified plasma-derived Q506-FVa. Q306-rFVa was cleaved by APC at R506 and gave a low APC-resistance ratio similar to Q506-rFVa in clotting assays, whereas unactivated Q306-rFV gave a near-normal APC-resistance ratio. When FVa residual activity was measured after long exposure to APC, Q306-rFVa was inactivated by only < or = 40% under conditions where Q506-rFVa was inactivated > 90%, supporting the hypothesis that efficient inactivation of normal FVa by APC requires cleavage at R306. In addition, the heavy chain of Q306-rFVa was cleaved at R506 much more rapidly than activity was lost, suggesting that FVa cleaved at only R506 is partially active. Under the same conditions, Q306Q506-rFVa lost no activity and was not cleaved by APC. Therefore, cleavage at either R506 or R306 appears essential for significant inactivation of FVa by APC. Modest loss of activity, probably due to cleavage at R679, was observed for the single site rFVa mutants, as evidenced by a second phase of inactivation. Q306Q506-rFVa had a low activity-to-antigen ratio of 0.50-0.77, possibly due to abnormal Factor Xa (FXa) binding. Furthermore, Q306Q506-rFV was very resistant to cleavage and activation by FXa. Q306Q506-rFV appeared to bind FXa and inhibit FXa's ability to activate normal FV. Thus, APC may downregulate FV/Va partly by impairing FXa-binding sites upon cleavage at R306 and R506. This study shows that R306 is the most important cleavage site for normal efficient inactivation of FVa by APC and supports other studies suggesting that regions near R306 and R506 provide FXa-binding sites and that FVa cleaved at only R506 retains partial activity.

Upregulation of the antithrombotic protein C pathway at birth

Serious thrombotic complications occur in sick neonates, while healthy infants have a very low risk of thrombosis. To better understand the regulation of physiological anticoagulation at birth, components of the protein C pathway were measured in cord plasma samples from 14 full-term healthy newborns and in samples from 10 adult controls. Although zymogen protein C was significantly reduced in cord plasma (mean +/- SEM in cord vs. adult sample 37 +/- 1.4% vs. 90 +/- 5.5%, p < 0.0001), levels of the active enzyme, activated protein C (APC), were not (119 +/- 20% vs. 75 +/- 12%, p = 0.0762). Relative to the protein C level, cord plasmas had a 5.2-fold higher APC level (p < 0.01). The APC increase was partially due to slower inactivation of APC in cord plasma (half-life for APC 50 min in cord plasma vs. 27 minutes in adult plasma). Increased sensitivity of factor V to inactivation by APC in cord plasma was observed since the activated partial thromboplastin time-based APC sensitivity ratio was significantly increased for cord vs. adult plasma samples (2.28 +/- 0.09 versus 1.97 +/- 0.03, p < 0.01). Thus, despite low zymogen protein C, the protein C pathway in newborns seems to be functionally well developed and at an activated stage at birth.

The Structure and Function of Murine Factor V and Its Inactivation by Protein C

Factor V (FV) is a central regulator of hemostasis, serving both as a critical cofactor for the prothrombinase activity of factor Xa and the target for proteolytic inactivation by the anticoagulant, activated protein C (APC). To examine the evolutionary conservation of FV procoagulant activity and functional inactivation by APC, we cloned and sequenced the coding region of murine FV cDNA and generated recombinant wild-type and mutant murine FV proteins. The murine FV cDNA encodes a 2,183-amino acid protein. Sequence comparison shows that the A1-A3 and C1-C2 domains of FV are highly conserved, demonstrating greater than 84% sequence identity between murine and human, and 60% overall amino acid identity among human, bovine, and murine FV sequences. In contrast, only 35% identity among all three species is observed for the poorly conserved B domain. The arginines at all thrombin cleavage sites and the R305 and R504 APC cleavage sites (corresponding to amino acid residues R306 and R506 in human FV) are invariant in all three species. Point mutants were generated to substitute glutamine at R305, R504, or both (R305/R504). Wild-type and all three mutant FV recombinant proteins show equivalent FV procoagulant activity. Single mutations at R305 or R504 result in partial resistance of FV to APC inactivation, whereas recombinant murine FV carrying both mutations (R305Q/R504Q) is nearly completely APC resistant. Thus, the structure and function of FV and its interaction with APC are highly conserved across mammalian species.

The Structure and Function of Murine Factor V and Its Inactivation by Protein C

Factor V (FV) is a central regulator of hemostasis, serving both as a critical cofactor for the prothrombinase activity of factor Xa and the target for proteolytic inactivation by the anticoagulant, activated protein C (APC). To examine the evolutionary conservation of FV procoagulant activity and functional inactivation by APC, we cloned and sequenced the coding region of murine FV cDNA and generated recombinant wild-type and mutant murine FV proteins. The murine FV cDNA encodes a 2,183-amino acid protein. Sequence comparison shows that the A1-A3 and C1-C2 domains of FV are highly conserved, demonstrating greater than 84% sequence identity between murine and human, and 60% overall amino acid identity among human, bovine, and murine FV sequences. In contrast, only 35% identity among all three species is observed for the poorly conserved B domain. The arginines at all thrombin cleavage sites and the R305 and R504 APC cleavage sites (corresponding to amino acid residues R306 and R506 in human FV) are invariant in all three species. Point mutants were generated to substitute glutamine at R305, R504, or both (R305/R504). Wild-type and all three mutant FV recombinant proteins show equivalent FV procoagulant activity. Single mutations at R305 or R504 result in partial resistance of FV to APC inactivation, whereas recombinant murine FV carrying both mutations (R305Q/R504Q) is nearly completely APC resistant. Thus, the structure and function of FV and its interaction with APC are highly conserved across mammalian species.

Syndromes of thrombosis and hypercoagulability. Congenital and acquired causes of thrombosis

Blood coagulation protein and platelet defects are now known to account for up to ninety percent of unexplained venous thrombosis and up to seventy percent of unexplained arterial thrombotic or ischemic events. This article summarizes the common and uncommon blood protein and platelet defects which should be suspected, and searched for, in patients with such events. Defining such defects will have major impact on secondary prevention and duration of antithrombotic therapy in the afflicted patient and impact on primary prevention for identified family members in those harboring hereditary defects.

Platelet-Derived Factor Va/VaLeiden Cofactor Activities Are Sustained on the Surface of Activated Platelets Despite the Presence of Activated Protein C

We investigated the role of the thrombin-activated platelet in modulating the rate and extent of activated protein C (APC)-catalyzed inactivation of platelet-derived factor Va and factor VaLeiden. Platelet-derived factor Va and factor VaLeiden were inactivated by APC at near identical rates; however, complete inactivation of the cofactors was never achieved. Greater residual cofactor activity remained when using thrombin-activated platelets compared with that observed with synthetic phospholipid vesicles and platelet-derived microparticles, suggesting that thrombin-activated platelets protect the cofactors from APC-catalyzed inactivation. This apparent protection was not due to (1) an insufficient number of membrane binding sites for APC or factor Va; (2) the destruction of these sites; or (3) the presence of a platelet-associated APC inhibitor. Results from a plasma-based clotting assay (with or without APC) with platelets or PCPS vesicles added to induce clot formation indicated that, even in the presence of high concentrations of APC, platelets offered protection of the cofactor by delaying cleavage at Arg506. This resulted in incomplete proteolysis of the heavy chain, suggesting that platelets can also protect plasma-derived factor Va from APC-catalyzed inactivation. However, additional experiments indicated that the plasma-derived cofactor, bound to thrombin-activated platelets, was completely inactivated by APC, suggesting that the plasma and platelet-derived cofactor pools represent different substrates for APC. Collectively, these results indicate that platelets sustain procoagulant events by providing a membrane surface that delays cofactor inactivation and by releasing a cofactor molecule that displays an APC resistant phenotype. Thus, at sites of arterial injury, the factor VLeidenmutation may not as readily predict arterial thrombosis, because the normal and variant platelet-derived cofactors are equally resistant to APC at the activated platelet surface.