Antithrombotic activity of protein S infused without activated protein C in a baboon thrombosis model

The thrombin generation capability of the Chacma baboon (Papio ursinus): implications for haemostatic disease models

Baboon models are often used to investigate haemostatic diseases, such as acquired thrombotic thrombocytopenic purpura or bacterial sepsis-induced disseminated intravascular coagulation, and their potential treatment with novel drugs. Thrombin generation is vital for these models, and an important potential therapeutic target. We investigated the thrombin generation profile of the Chacma baboon (Papio ursinus - a common pre-clinical model) including the effects of sex and ABO blood group. Thrombin generation curves, lag times, peak heights, times-to-peak, velocity indexes and Endogenous Thrombin Potentials (ETPs) of 40 adult Chacma baboons were assessed and compared with normal human plasma, using a low concentration of tissue factor (1 pM) and phospholipids. Reference intervals were calculated, and results compared between O and non-O ABO blood groups, and between males and females. Lag times of all baboons fell within the human reference interval. Most animals (n = 32; 80%) had times-to-peak above, and velocity indexes and peak heights markedly below (n = 27; 68%) the human range. However, 97.5% of baboons had an ETP above the human reference interval, indicating greater overall thrombin generation. ABO blood group had no effect, but males (n = 14; 35%) had less potent thrombin generation than females (n = 26; 65%), with significantly longer lag times (p = 0.0475), lower peak thrombin concentrations (p = 0.0203), and lower ETPs (p = 0.0238). Chacma baboons have greater overall endogenous thrombin generation potentials than humans, which is even more prominent in females. This should be considered when designing future baboon model experiments involving the haemostatic system, or when evaluating novel therapies in these animals.

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.

Interspecies comparison of simultaneous thrombin and plasmin generation

Animal models of hemostasis are often extrapolated to humans; however, only a few studies have compared coagulation and fibrinolysis across species. Simultaneous thrombin (TG) and plasmin (PG) generation is useful to assessing coagulation and fibrinolysis within the same sample. In this study, we performed simultaneous TG and PG analysis in blood plasma samples from humans and 6 species commonly evaluated in pre-clinical research. TG and PG were investigated in male and female donor platelet-poor plasmas (PPP) obtained from 28 healthy humans, 10 baboons, 12 rhesus monkeys, 20 Yorkshire pigs, 20 Sprague-Dawley rats, 10 New Zealand White rabbits and 14 Hartley guinea pigs. The continuous generation of the 7-amino-4-methylcoumarin (AMC) from substrates specific to thrombin or plasmin was monitored. The thrombin and plasmin concentration peak heights (PH) and production rates (PR) were calculated. TG and PG parameters from baboon and rhesus macaque plasma approximated that of humans. The other species differed significantly from both human and non-human primates. For example, swine and rat plasmas demonstrated similar TG, but swine plasmas did not generate plasmin. TG and PG parameters from Guinea pig samples were extremely low, while rabbit plasmas showed variable PG curves demonstrating one or two peaks with low and high PR values, respectively. Correlations between PH and PR values were significant with the exceptions of human PG, baboon TG, rat TG and Guinea pig PG. These findings are informative to pre-clinical animal species selection and optimization of coagulation and fibrinolysis translational research.

Anticoagulant Protein S Targets the Factor IXa Heparin-Binding Exosite to Prevent Thrombosis

OBJECTIVE

PS (protein S) is a plasma protein that directly inhibits the coagulation FIXa (factor IXa) in vitro. Because elevated FIXa is associated with increased risk of venous thromboembolism, it is important to establish how PS inhibits FIXa function in vivo. The goal of this study is to confirm direct binding of PS with FIXa in vivo, identify FIXa amino acid residues required for binding PS in vivo, and use an enzymatically active FIXa mutant that is unable to bind PS to measure the significance of PS-FIXa interaction in hemostasis.

APPROACH AND RESULTS

We demonstrate that PS inhibits FIXa in vivo by associating with the FIXa heparin-binding exosite. We used fluorescence tagging, immunohistochemistry, and protein-protein crosslinking to show in vivo interaction between FIXa and PS. Importantly, platelet colocalization required a direct interaction between the 2 proteins. FIXa and PS also coimmunoprecipitated from plasma, substantiating their interaction in a physiological milieu. PS binding to FIXa and PS inhibition of the intrinsic Xase complex required residues K132, K126, and R170 in the FIXa heparin-binding exosite. A double mutant, K132A/R170A, retained full activity but could not bind to PS. Crucially, Hemophilia B mice infused with FIXa K132A/R170A displayed an accelerated rate of fibrin clot formation compared with wild-type FIXa.

CONCLUSIONS

Our findings establish PS as an important in vivo inhibitor of FIXa. Disruption of the interaction between PS and FIXa causes an increased rate of thrombus formation in mice. This newly discovered function of PS implies an unexploited target for antithrombotic therapeutics.

The Journey of Protein S from an Anticoagulant to a Signaling Molecule

Protein S (PS), a γ-carboxyglutamate-containing serum protein, was unexpectedly discovered in 1977. Soon after its discovery, PS gained the attention of researchers because of its physiological importance, acting as a multifunctional protein at the intersection of blood coagulation, inflammation, and other cellular processes. Protein S functions as an anticoagulant by directly inhibiting procoagulants, such as Factor Xa (FXa), FVa, and FIXa, while also serving as a cofactor for anticoagulants such as Activated Protein C and Tissue Factor Pathway Inhibitor. By associating with C4b binding protein (C4BP), PS has also been shown to minimize the effect of inflammation. Finally, PS promotes efferocytosis through TAM family protein kinase receptors. Mutations in the PS gene cause pathological conditions such as deep vein thrombosis and hereditary ischemia. In this review, we summarize studies regarding the multiple functions of PS.

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

Protein S (PS) is an anticoagulant plasma protein whose deficiency is associated with increased risk of venous thrombosis. PS directly inhibits thrombin generation by the blood coagulation pathways by several mechanisms, including by binding coagulation factors (F) Va and Xa. To identify PS sequences that mediate inhibition of FVa activity, antibodies and synthetic peptides based on PS sequence were prepared and employed in plasma coagulation assays, purified component prothrombinase assays, binding assays, and immunoblots. In the absence of activated protein C, monoclonal antibody (Mab) S4 shortened FXa-induced clotting in normal plasma but not in PS-depleted plasma. Mab S4 also blocked PS inhibition of FVa-dependent prothrombinase activity in purified component assays in the absence or presence of phospholipids and inhibited binding of PS to immobilised FVa. Epitope mapping identified N-terminal region residues 37-67 of PS as this antibody's epitope. A peptide representing PS residues 37-50 inhibited FVa-dependent prothrombinase activity in a non-competitive manner, with 50% inhibition observed at 11 µM peptide, whereas a peptide with a D-amino acid sequence of 37-50 was ineffective. FVa, but not FXa, bound specifically to the immobilised peptide representing residues 37-50, and the peptide inhibited binding of FVa to immobilised PS. These data implicate PS residues 37-50 as a binding site for FVa that mediates, at least in part, the direct inhibition of FVa-dependent procoagulant activity by PS.

Platelet protein S directly inhibits procoagulant activity on platelets and microparticles

Anticoagulant plasma protein S (PS) is essential for maintaining haemostatic balance. About 2.5% of PS is stored in platelets and released upon platelet stimulation. So far, little is known about the functionality and importance of platelet (plt)PS. A platelet-associated protease cleaves plasma-derived (pd)PS and pltPS in the "thrombin-sensitive region", abolishing activated protein C (APC) cofactor activity. However, we showed that cleaved PS retains APC-independent anticoagulant activities ("PS-direct"). To investigate whether pltPS or pdPS exert PS-direct on platelets or platelet-shed microparticles, thrombin and factor (F)Xa generation on unstimulated or stimulated washed platelets and microparticles were measured. Western blotting revealed that pltPS and pdPS bound to washed, stimulated platelets and microparticles, and that pltPS had slower electrophoretic mobility than pdPS. Platelet stimulation in the presence of inhibitory anti-PS antibodies resulted in 2.6 ± 1.6-fold (p<0.0004, n=20) more thrombin generation upon addition of FXa and prothrombin. PltPS exerted PS-direct that was similar to or greater than that of Zn(2+)-containing pdPS and much greater than that of Zn(2+)-deficient pdPS. Findings were confirmed using purified pltPS. Platelet-bound pltPS and microparticle-bound pltPS had similar PS-direct. Finally, platelet stimulation in the presence of inhibitory anti-PS antibodies resulted in 1.5 ± 0.2-fold (p<0.0001, n=11) more FXa generation upon addition of TF/FVIIa and FX. Thus, pltPS inhibits both prothrombinase and extrinsic FXase activities. Neutralising antibodies against APC and TFPI had no effect on the PS-direct of pltPS or pdPS on platelets. This study indicates that pltPS may be an essential pool of PS that counterbalances procoagulant activities on platelets.