Plasma protein S residues 37-50 mediate its binding to factor Va and inhibition of blood coagulation

Pleiotropic anticoagulant functions of protein S, consequences for the clinical laboratory. Communication from the SSC of the ISTH

Hereditary deficiencies of protein S (PS) increase the risk of thrombosis. However, assessing the plasma levels of PS is complicated by its manifold physiological interactions, while the large inter-individual variability makes it problematic to establish reliable cut-off values. PS has multiple physiological functions, with only two appearing to have significant anticoagulant properties: the activated protein C (APC) and tissue factor pathway inhibitor alpha (TFPIα) cofactor activities. Current clinical laboratory investigations for deficiency in PS function rely only on the APC-dependent activity. This communication presents an argument for reclassifying the qualitative PS deficiencies to differentiate the two major anticoagulant functions of PS. Reliable assays are necessary for accurate evaluation of PS function when making a specific diagnosis of PS deficiency based on the anticoagulant phenotype alone. This report emphasizes the pleiotropic anticoagulant functions of PS and presents evidence-based recommendations for their implementation in the clinical laboratory.

Anticoagulant protein S-New insights on interactions and functions

Protein S is a critical regulator of coagulation that functions as a cofactor for the activated protein C (APC) and tissue factor pathway inhibitor (TFPI) pathways. It also has direct anticoagulant functions, inhibiting the intrinsic tenase and prothrombinase complexes. Through these functions, protein S regulates coagulation during both its initiation and its propagation phases. The importance of protein S in hemostatic regulation is apparent from the strong association between protein S deficiencies and increased risk for venous thrombosis. This is most likely because both APC and TFPIα are inefficient anticoagulants in the absence of any cofactors. The detailed molecular mechanisms involved in protein S cofactor functions remain to be fully clarified. However, recent advances in the field have greatly improved our understanding of these functions. Evidence suggests that protein S anticoagulant properties often depend on the presence of synergistic cofactors and the formation of multicomponent complexes on negatively charged phospholipid surfaces. Their high affinity binding to negatively charged phospholipids helps bring the anticoagulant proteins to the membranes, resulting in efficient and targeted regulation of coagulation. In this review, we provide an update on protein S and how it functions as a critical hemostatic regulator.

The roles of factor Va and protein S in formation of the activated protein C/protein S/factor Va inactivation complex

BACKGROUND

Activated protein C (APC)-mediated inactivation of factor (F)Va is greatly enhanced by protein S. For inactivation to occur, a trimolecular complex among FVa, APC, and protein S must form on the phospholipid membrane. However, direct demonstration of complex formation has proven elusive.

OBJECTIVES

To elucidate the nature of the phospholipid-dependent interactions among APC, protein S, and FVa.

METHODS

We evaluated binding of active site blocked APC to phospholipid-coated magnetic beads in the presence and absence of protein S and/or FVa. The importance of protein S and FV residues were evaluated functionally.

RESULTS

Activated protein C alone bound weakly to phospholipids. Protein S mildly enhanced APC binding to phospholipid surfaces, whereas FVa did not. However, FVa together with protein S enhanced APC binding (>14-fold), demonstrating formation of an APC/protein S/FVa complex. C4b binding protein-bound protein S failed to enhance APC binding, agreeing with its reduced APC cofactor function. Protein S variants (E36A and D95A) with reduced APC cofactor function exhibited essentially normal augmentation of APC binding to phospholipids, but diminished APC/protein S/FVa complex formation, suggesting involvement in interactions dependent upon FVa. Similarly, FVaNara (W1920R), an APC-resistant FV variant, also did not efficiently incorporate into the trimolecular complex as efficiently as wild-type FVa. FVa inactivation assays suggested that the mutation impairs its affinity for phospholipid membranes and with protein S within the complex.

CONCLUSIONS

FVa plays a central role in the formation of its inactivation complex. Furthermore, membrane proximal interactions among FVa, APC, and protein S are essential for its cofactor function.

Measurement of plasma and platelet tissue factor pathway inhibitor, factor V and Protein S in people with haemophilia

INTRODUCTION

Tissue factor pathway inhibitor (TFPI) is a naturally occurring anticoagulant found in plasma, where it circulates bound to lipoproteins, factor V (FV) or Protein S (PS), and in platelets. Therapeutic agents targeting TFPI are under development for the treatment of haemophilia A and haemophilia B.

AIM

To begin to understand how TFPI, FV and PS interact to modulate haemophilia bleeding.

METHODS

Plasma and platelet antigen concentrations of these factors were determined in 73 people with haemophilia A and 18 with haemophilia B. Using multiple regression models, these were compared to the same analytes measured in 224 male blood donors.

RESULTS

There were no differences in plasma or platelet TFPI, FV or PS concentrations between haemophilia types or severities. However, compared to blood donors, people with haemophilia had approximately one-third lower plasma PS, 9% lower plasma TFPIα, 50% higher platelet FV and 26% lower platelet Protein S.

CONCLUSION

Together, the presented data suggest that individuals with haemophilia may have a compensatory procoagulant response of both plasma and platelet proteins to the decreased concentrations of FVIII or FIX.

Factor V has an anticoagulant cofactor activity that targets the early phase of coagulation

Tissue factor pathway inhibitor (TFPI), the main inhibitor of initiation of coagulation, exerts an important anticoagulant role through the factor Xa (FXa)-dependent inhibition of tissue factor/factor VIIa. Protein S is a TFPI cofactor, enhancing the efficiency of FXa inhibition. TFPI can also inhibit prothrombinase assembly by directly interacting with coagulation factor V (FV), which has been activated by FXa. Because full-length TFPI associates with FV in plasma, we hypothesized that FV may influence TFPI inhibitory function. Using pure component FXa inhibition assays, we found that although FV alone did not influence TFPI-mediated FXa inhibition, it further enhanced TFPI in the presence of protein S, resulting in an ∼8-fold reduction in K_i compared with TFPI alone. A FV variant (R709Q/R1018Q/R1545Q, FV^ΔIIa) that cannot be cleaved/activated by thrombin or FXa also enhanced TFPI-mediated inhibition of FXa ∼12-fold in the presence of protein S. In contrast, neither activated FV nor recombinant B-domain-deleted FV could enhance TFPI-mediated inhibition of FXa in the presence of protein S, suggesting a functional contribution of the B domain. Using TFPI and protein S variants, we show further that the enhancement of TFPI-mediated FXa inhibition by protein S and FV depends on a direct protein S/TFPI interaction and that the TFPI C-terminal tail is not essential for this enhancement. In FXa-catalyzed prothrombin activation assays, both FV and FV^ΔIIa (but not activated FV) enhanced TFPI function in the presence of protein S. These results demonstrate a new anticoagulant (cofactor) function of FV that targets the early phase of coagulation before prothrombinase assembly.