Platelet-and plasma-derived factor Va (FVa) serve essential cofactor roles in prothrombinase-catalyzed thrombin generation. Platelet-derived FV/Va, purified from Triton X-100 platelet lysates was composed of a mixture of polypeptides ranging from ϳ40 to 330 kDa, mimicking those visualized by Western blotting of platelet lysates and releasates with anti-FV antibodies. The purified, platelet-derived protein expressed significant cofactor activity such that thrombin activation led to only a 2-3-fold increase in cofactor activity yet expression of a specific activity identical to that of purified, plasma-derived FVa. Physical and functional differences between the two cofactors were identified. Purified, platelet-derived FVa was 2-3-fold more resistant to activated protein C-catalyzed inactivation than purified plasma-derived FVa on the thrombin-activated platelet surface. The heavy chain subunit of purified, plateletderived FVa contained only a fraction (ϳ10 -15%) of the intrinsic phosphoserine present in the plasma-derived FVa heavy chain and was resistant to phosphorylation at Ser 692 catalyzed by either casein kinase II or thrombin-activated platelets. MALDI-TOF mass spectrometric analyses of tryptic digests of platelet-derived FV peptides detected an intact heavy chain uniquely modified on Thr 402 with an N-acetylglucosamine or N-acetylgalactosamine, whereas Ser 692 remained unmodified. N-terminal sequencing and MALDI-TOF analyses of plateletderived FV/Va peptides identified the presence of a fulllength heavy chain subunit, as well as a light chain subunit formed by cleavage at Tyr 1543 rather than Arg 1545 accounting for the intrinsic levels of cofactor activity exhibited by native platelet-derived FVa. These collective data are the first to demonstrate physical differences between the two FV cofactor pools and support the hypothesis that, subsequent to its endocytosis by megakaryocytes, FV is modified to yield a platelet-derived cofactor distinct from its plasma counterpart.Factor Va (FVa), 1 a heterodimeric protein composed of heavy chain (105 kDa) and light chain (74 kDa) subunits, is formed by limited proteolysis of factor V (FV) (1). In normal hemostasis, FVa functions as a non-enzymatic cofactor of the prothrombinase complex, which consists of a 1:1 stoichiometric and Ca 2ϩ -dependent complex of the serine protease factor Xa and FVa, bound to the membrane of appropriately activated platelets, and catalyzes the proteolytic conversion of prothrombin to thrombin (2). When incorporated into the prothrombinase complex, the catalytic activity of factor Xa is increased by approximately 5 orders of magnitude, and FVa contributes substantially to this increase (3). Removal of FVa from the prothrombinase complex results in a 10,000-fold decrease in the rate of thrombin generation (3), the physiologic effect of which is demonstrated in the bleeding diatheses expressed by FV-deficient individuals (4 -8)Factor V circulates in two pools in whole blood. The majority (75-80%) is found in the plasma as an inactive, singl...

Summary. Gas6 (encoded by growth arrest-specific gene 6) is a vitamin-K dependent protein highly homologous to coagulation protein S that is secreted from platelet a-granules and has recently been demonstrated to participate in platelet thrombus formation. The current study evaluated the contribution of each of the three known Gas6 receptors (Axl, Sky and Mer) in human and mouse platelet function. Flow cytometry analyses confirmed that all three receptors are present on both human and mouse platelets. Pre-incubation of human platelets with either an anti-Gas6 antibody or blocking antibodies to Sky or Mer inhibited platelet aggregation and degranulation responses to both ADP and the PAR-1 activating peptide, SFLLRN, by more than 80%. In contrast, a stimulatory anti-Axl antibody increased activation responses to these agonists, suggesting a potentiating role for Gas6 in platelet activation. Moreover, in a mouse model of thrombosis, administration of Gas6 or Sky blocking antibodies resulted in a decrease in thrombus weight similar to clopidogrel but, unlike clopidogrel, produced no increase in template bleeding. Thus, Gas6 enhances platelet degranulation and aggregation responses through its known receptors, promoting platelet activation and mediating thrombus formation such that its inhibition prevents thrombosis without increasing bleeding.

To cite this article: Gould WR, Simioni P, Silveira JR, Tormene D, Kalafatis M, Tracy PB. Megakaryocytes endocytose and subsequently modify human factor V in vivo to form the entire pool of a unique platelet-derived cofactor. J Thromb Haemost 2005; 3: 450-6.See also Hoffman M. One more way that mice and men are different. This issue, pp 448-9.Summary. Factor Va (FVa), derived from plasma or released from stimulated platelets, is the essential cofactor in thrombin production catalyzed by the prothrombinase complex. Plasma-derived factor V (FV) is synthesized in the liver. The source(s) of the platelet-derived cofactor remains in question. We identified a patient homozygous for the FV Leiden mutation, who received a liver transplant from a homozygous wildtype FV donor. Eighteen days post-transplant, phenotypic analysis of the patient's platelet-derived FV indicated that the platelets were acquiring wild-type FV, consistent with the temporal differentiation of megakaryocytes and subsequent platelet production. Nine months post-transplant, the plateletderived FV pool consisted entirely of wild-type FV. Consequently, megakaryocyte endocytosis of plasma-derived FV must account for the entire platelet-derived pool, because blood-borne platelets cannot bind or endocytose FV. Subsequent to this endocytic process, the patient's platelet-derived FV was cleaved to a partially active cofactor, and rendered resistant to phosphorylation catalyzed by a platelet-associated kinase, and hence less susceptible to activated protein C-catalyzed inactivation. These data provide the first in vivo demonstration of an endocytosed plasma protein undergoing intracellular modifications that alter its function. This process results in the sequestration of active FVa within the platelet compartment, poised for immediate action subsequent to release from platelets at a site of injury.

Coronary heart disease (CHD) is the leading cause of mortality and morbidity in the United States. Currently, there are approximately 12 million Americans with CHD, which is most frequently caused by atherosclerosis. The thrombotic complications of atherosclerosis, such as acute coronary syndrome and ischemic stroke, can be fatal and those who survive such events have a far greater risk of future cardiovascular events. This huge medical need cries out for improved novel anticoagulants, antiplatelet agents, and profibrinolytic agents. These agents will successfully respond to the medical need by providing safe, effective, and easily administered treatments that have little, if any, drug and food interactions and that require minimal monitoring. The currently approved antiplatelet agent, clopidogrel, has satisfied some of these requirements and has played a large role in expanding the antithrombotic market over the past few years. New antithrombotics approaching the marketplace, such as the prodrug thrombin inhibitor ximelagatran, have promise in expanding the antithrombotic market further. Over the past two decades, the pharmaceutical industry has mounted a huge effort to develop antithrombotics that function by inhibiting key enzymes positioned at "higher" levels of the coagulation system. Direct inhibitors of factor Xa, which may provide a better safety and efficacy profile than currently available agents, appear to be the next major class of antithrombotic agents poised to take the pharmaceutical industry one step closer to delivering the ideal antithrombotic agent. This review focuses on recent innovations in the discovery and development of potent parenteral and oral direct factor Xa inhibitors.

factors Xa and IIa synergize to reduce thrombus weight and thrombin generation in vivo and in vitro. J Thromb Haemost 2006; 4: 834-41. Summary. Background: Many compounds currently in development for treatment of thrombotic disorders demonstrate high specificity for single targets of blood coagulation such as factor Xa (FXa) or thrombin. Aim: The aim of this study is to determine if inhibition of both FXa and thrombin by simultaneous administration of PD0313052 and argatroban, respectively, synergistically increases the effect of either drug alone in vivo and in vitro. Methods and Results: Analyses of thrombin generation from combined inhibition in human plasma using statistical methods of Bliss independence identified a synergistic reduction in thrombin production 30% lower than predicted by simple additivity. The greatest synergy occurred at concentrations of each compound below their individual IC 50 values. In a rabbit arterio-venous shunt model (RAV) of thrombosis, co-administration of PD0313052 and argatroban reduced thrombus weight (TW) to a much greater degree than expected by additivity alone producing a synergistic decrease of 45% over the level predicted by additivity. Analyses of thrombin generation in plasma samples from the RAV also demonstrated 38% synergy ex vivo. Furthermore, at plasma concentrations with the greatest synergistic effect, no increase in bleeding or appreciable change in prothrombin time, activated partial thromboplastin time, or activated clotting time was observed, but thrombus weight reduction was greater than twofold higher than that expected from simple additivity. Conclusions: These results demonstrate a significant synergistic antithrombotic effect of combining low doses of PD0313052 and argatroban and support the hypothesis that simultaneous targeting of multiple coagulation enzymes may offer an improved therapeutic index in the prevention and treatment of thrombosis.