The autosomal dominantly inherited east Texas bleeding disorder is linked to an A2440G variant in exon 13 of the F5 gene. Affected individuals have normal levels of coagulation factor V (FV) activity, but demonstrate inhibition of global coagulation tests. We demonstrated that the A2440G mutation causes upregulation of an alternatively spliced F5 transcript that results in an in-frame deletion of 702 amino acids of the large activation fragment, the B domain. The approximately 250-kDa FV isoform (FV-short), which can be fully activated by thrombin, is present in all A2440G carriers' plasma (n = 16). FV-short inhibits coagulation through an indirect mechanism by forming a complex with tissue factor pathway inhibitor-α (TFPIα), resulting in an approximately 10-fold increase in plasma TFPIα, suggesting that the TFPIα:FV-short complexes are retained in circulation. The TFPIα:FV-short complexes efficiently inhibit thrombin generation of both intrinsic and extrinsic coagulation pathways. These data demonstrate that the east Texas bleeding disorder-associated F5 A2440G leads to the formation of the TFPIα:FV-short complex, which inhibits activation and propagation of coagulation. IntroductionCoagulation factor V (FV) is a cofactor protein that can both promote and inhibit coagulation (1, 2). Located on 1q24-25, the F5 gene is composed of 25 exons that transcribe a 6.8-kb mRNA (3-7). The translated 330-kDa glycoprotein precursor contains 2,196 amino acids organized into the domain structure A1-A2-B-A3-C1-C2 and is highly homologous to factor VIII (FVIII), sharing 35%-40% identity in the A and C domains (4, 5, 8). Human FV is primarily produced by hepatocytes and circulates in plasma as an intact 330-kDa precursor at a concentration of about 20 nM (7 μg/ml) (9-12). Approximately 20% of the total human FV found in whole blood is stored in platelet α granules in a partially proteolyzed form in conjunction with the binding protein multimerin. This platelet FV derives from the plasma FV pool and is secreted upon platelet activation to create a high, localized concentration of the cofactor at sites of injury (12)(13)(14)(15).The procoagulant cofactor function of FV is primarily dictated by its interaction and cleavage by thrombin and active factor X (FXa). Its cleavage by thrombin is deemed the most biologically important early event in the blood clot formation process (16)(17)(18)(19). As an intact single-chain precursor, FV expresses less than 1% of its potential procoagulant cofactor activity (20). Upon sequential cleavage of Arg-709, Arg-1018, and Arg-1545 by thrombin, the large connecting B domain is removed from the intact FV molecule to produce the heavy chain (A1-A2) and the light chain (A3-C1-C2) that have M r s of 105,000 and 74,000, respectively. The heavy chain and the light chain noncovalently associate in the presence of calcium to produce an active procoagulant cofactor (FVa). FVa and FXa assemble on negatively charged phospholipid (PL) membranes to form the prothrombinase (PTase) complex. The presence of FVa in...

Objective-Plasma high-density lipoproteins (HDL) are potent antiatherogenic and anti-inflammatory particles. However, HDL particles are highly heterogenic in composition, and different HDL-mediated functions can be ascribed to different subclasses of HDL. Only a small HDL population contains apolipoprotein M (ApoM), which is the main plasma carrier of the bioactive lipid mediator sphingosine-1-phosphate (S1P). Vascular inflammation is modulated by S1P, but both proand anti-inflammatory roles have been ascribed to S1P. The goal of this study is to elucidate the role of ApoM and S1P in endothelial anti-inflammatory events related to HDL. Approach and Results-Aortic or brain human primary endothelial cells were challenged with tumor necrosis factor-α (TNF-α) as inflammatory stimuli. The presence of recombinant ApoM-bound S1P or ApoM-containing HDL reduced the abundance of adhesion molecules in the cell surface, whereas ApoM and ApoM-lacking HDL did not. Specifically, ApoM-bound S1P decreased vascular adhesion molecule-1 (VCAM-1) and E-selectin surface abundance but not intercellular adhesion molecule-1. Albumin, which is an alternative S1P carrier, was less efficient in inhibiting VCAM-1 than ApoM-bound S1P. The activation of the S1P receptor 1 was sufficient and required to promote anti-inflammation. Moreover, ApoM-bound S1P induced the rearrangement of the expression of S1P-related genes to counteract TNF-α. Functionally, HDL/ApoM/S1P limited monocyte adhesion to the endothelium and maintained endothelial barrier integrity under inflammatory conditions. Conclusions-ApoM-bound S1P is a key component of HDL and is responsible for several HDL-associated protective functions in the endothelium, including regulation of adhesion molecule abundance, leukocyte-endothelial adhesion, and endothelial barrier. Ruiz et al ApoM-Containing HDL Reduces Vascular Inflammation 119secretion and used by the mature ApoM protein to anchor to the phospholipid surface of HDL. 7,8 Five different membrane-bound G-protein-coupled S1P receptors (S1PRs) are known, and binding of S1P to the receptors activates multiple receptor-specific downstream signaling pathways. In this way, S1P is able to regulate several biological processes, such as immune cell trafficking, angiogenesis, endothelial cell migration, and endothelial barrier function. 9 The role of S1P in the regulation of vascular inflammation has been studied, and contradictory results have been obtained, for example, direct stimulation by S1P is reported to increase the abundance of adhesion molecules, whereas other studies [10][11][12] show that S1P inhibits tumor necrosis factor-α (TNF-α) induction of adhesion molecules, such as E-selectin, ICAM-1, or VCAM-1. 11,13The aim of this study is to characterize the role of S1P in the regulation of human endothelium inflammation taking into account that S1P is mostly bound to ApoM in plasma. Using recombinant human ApoM with or without bound S1P and isolated HDL containing or HDL lacking human ApoM (HDL +ApoM and HDL −ApoM , respectively), w...

The procoagulant function of activated factor V (FVa) is inhibited by activated protein C (APC) through proteolytic cleavages at Arg306, Arg506, and Arg679. The effect of APC is potentiated by negatively charged phospholipid membranes and the APC cofactor protein S. Protein S has been reported to selectively stimulate cleavage at Arg306, an effect hypothesized to be related to reorientation of the active site of APC closer to the phospholipid membrane. To investigate the importance of protein S and phospholipid in the APC-mediated cleavages of individual sites, recombinant FV variants FV(R306Q/R679Q) and FV(R506Q/R679Q) (can be cleaved only at Arg506 and Arg306, respectively) were created. The cleavage rate was determined for each cleavage site in the presence of varied protein S concentrations and phospholipid compositions. In contrast to results on record, we found that protein S stimulated both APC cleavages in a phospholipid composition-dependent manner. Thus, on vesicles containing both phosphatidylserine and phosphatidylethanolamine, protein S increased the rate of Arg306 cleavage 27-fold and that of Arg506 cleavage 5-fold. Half-maximal stimulation was obtained at approximately 30 nm protein S for both cleavages. In conclusion, we demonstrate that APC-mediated cleavages at both Arg306 and Arg506 in FVa are stimulated by protein S in a phospholipid composition-dependent manner. These results provide new insights into the mechanism of APC cofactor activity of protein S and the importance of phospholipid composition.

Essentials FV‐Short, a normal splice isoform of Factor V, binds tissue factor pathway inhibitor (TFPIα) with high affinity.FV‐Short functions as a synergistic TFPIα cofactor with protein S in inhibition of Factor Xa.FV‐Short is much more efficient as TFPIα cofactor than full length FV.TFPIα‐cofactor activity of FV‐Short is lost upon activation of coagulation by thrombin‐mediated cleavage. Background FV‐Short is a normal splice variant of Factor V (FV) having a short B domain, which exposes a high affinity‐binding site for tissue factor pathway inhibitor α (TFPIα). FV‐Short and TFPIα circulate in complex in plasma.ObjectivesThe aim was to elucidate whether FV‐Short affects TFPIα as inhibitor of coagulation FXa and to test whether the TFPIα‐cofactor activity of protein S is influenced by FV‐Short.MethodsRecombinant FV, wild‐type FV‐Short and a FV‐Short thrombin‐cleavage resistant variant were expressed and purified. The influence of FV and FV‐Short variants and/or protein S on the FXa inhibitory activity of TFPIα was monitored both in a purified system and in a plasma‐based thrombin generation assay.Results FV‐Short had intrinsically weak TFPIα‐cofactor activity but with protein S present, FV‐Short yielded efficient inactivation of FXa. Protein S alone did not promote full TFPIα‐activity. Intact FV was inefficient at low protein S concentrations and had 10‐fold lower activity compared to FV‐Short at physiological protein S levels. Activation of FV‐Short by thrombin resulted in the loss of the TFPIα‐cofactor activity. The synergistic TFPIα‐cofactor activity of FV‐Short and protein S was also demonstrated in plasma using a thrombin generation assay.Conclusions FV‐Short and protein S are highly efficient, synergistic cofactors to TFPIα in the regulation of FXa activity, whereas full length FV has lower activity. Our results suggest the formation of an efficient FXa‐inhibitory complex between FV‐Short, TFPIα and protein S on the surface of negatively charged phospholipids.