Contribution of different phospholipid classes to the prothrombin converting capacity of sonicated lipid vesicles

Effect of Refining Degree on the Quality Changes and Lipid Oxidation of Camellia (Camellia oleifera) Oil during Heating

Refining degree has an important influence on the quality of camellia (Camellia oleifera) oil. The deterioration behaviors and lipid oxidation of three kinds of camellia oils, including camellia crude oil (CO), moderate refined oil (MRO), and refined oil (RO), during heating were investigated in this study. The results of deterioration behavior analysis showed that the oxidation degree was RO > CO > MRO. Tocopherol and polyphenolic substances in the oil might help delay oil oxidation. The lipid oxidation results indicated that the heating process had greater effects on CO and MRO than RO; it upregulated neutral lipid content and downregulated phospholipid content in terms of lipid changes and the multiplicity of differences. Glycerophospholipid metabolism was the most remarkable pathway and was important to study the heating process of refined oil. Moderate refining is good for retaining the beneficial lipids in camellia oil. The results of this study would provide a theoretical basis for camellia oil processing.

Stoichiometric analysis reveals a unique phosphatidylserine binding site in coagulation factor X

BACKGROUND

Cellular trauma or activation exposes phosphatidylserine (PS) and the substantially more abundant phospholipid, phosphatidylethanolamine (PE), on the outer layer of the plasma membrane, thereby allowing binding of many blood clotting proteins. We previously proposed the Anything But Choline (ABC) hypothesis to explain how PS and PE synergize to support binding of clotting proteins with gamma-carboxyglutamate (Gla)-rich domains, which posited that each Gla domain binds to a limited number of PS molecules and multiple PE molecules. However, the minimal number of PS molecules required to stably bind a Gla-domain-containing blood clotting protein in the presence of excess PE was unknown.

OBJECTIVE

To test the ABC hypothesis for factor X by determining the threshold binding requirement of PS molecules under conditions of PS-PE synergy.

METHODS

We used surface plasmon resonance to investigate the stoichiometry of factor X binding to nanoscale membrane bilayers (Nanodiscs) of varying phospholipid composition.

RESULTS AND CONCLUSIONS

We quantified 1.05 ± 0.2 PS molecules per bound factor X molecule in Nanodiscs containing a mixture of 10% PS, 60% PE, and 30% phosphatidylcholine. Hence, there appears to be one truly PS-specific binding site per Gla domain, while the remaining membrane binding interactions can be satisfied by PE.

Phosphatidylethanolamine-phosphatidylserine binding synergy of seven coagulation factors revealed using Nanodisc arrays on silicon photonic sensors

Blood coagulation is regulated through protein–protein and protein–lipid interactions that occur at the sub-endothelium following vascular damage. Soluble clotting proteins bind to membrane components in a phosphatidylserine (PS) dependent manner to assemble multi-protein complexes that regulate clot formation; however, PS is of limited abundance physiologically. In this manuscript, we investigate synergy between PS and phosphatidylethanolamine (PE)—a lipid of much higher abundance naturally. Using a label-free, silicon photonic technology, we constructed arrays of Nanodiscs having variable lipid composition and probed the binding interactions of seven different clotting factors with GLA domains that have never been studied in tandem experiments before. The factors studied were prothrombin, activated factor VII, factor IX, factor X, activated protein C, protein S, and protein Z. Equilibrium dissociation constants (K_d) for each coagulation factor binding to Nanodiscs with unique compositions of PE and PS were determined. While all factors showed greater binding affinities in the presence of PS and PE, the most dramatic improvements in binding were observed when PS quantities were lowest. This demonstrates that synergy is effective in promoting coagulation factor binding under physiological lipid compositions, as opposed to the artificially high PS content probed in most in vitro activity studies.

Microparticles formed during storage of red blood cell units support thrombin generation

BACKGROUND

Intact red blood cells (RBCs) appear to support thrombin generation in in vitro models of blood coagulation. During storage of RBC units, biochemical, structural, and physiological changes occur including alterations to RBC membranes and release of microparticles, which are collectively known as storage lesion. The clinical consequences of microparticle formation in RBC units are unclear. This study was performed to assess thrombin generation via the prothrombinase complex by washed RBCs and RBC-derived microparticles as a function of RBC unit age.

METHODS

Well-characterized kinetic and flow cytometric assays were used to quantify and characterize microparticles isolated from leukocyte-reduced RBC units during storage for 42 days under standard blood banking conditions.

RESULTS

Stored RBCs exhibited known features of storage lesion including decreasing pH, cell lysis, and release of microparticles demonstrated by scanning electron microscopy. The rate of thrombin formation by RBC units linearly increased during storage, with the microparticle fraction accounting for approximately 70% of the prothrombinase activity after 35 days. High-resolution flow cytometric analyses of microparticle isolates identified phosphatidylserine-positive RBC-derived microparticles; however, their numbers over time did not correlate with thrombin formation in that fraction.

CONCLUSION

Red blood cell-derived microparticles capable of supporting prothrombinase function accumulate during storage, suggesting an increased potential of transfused units as they age to interact in unplanned ways with ongoing hemostatic processes in injured individuals, especially given the standard blood bank practice of using the oldest units available.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Regulation of tissue factor coagulant activity on cell surfaces

Tissue factor (TF) is a transmembrane glycoprotein and an essential component of the factor VIIa-TF enzymatic complex that triggers activation of the coagulation cascade. Formation of TF-FVIIa complexes on cell surfaces not only trigger the coagulation cascade but also transduce cell signaling via activation of protease-activated receptors. Tissue factor is expressed constitutively on cell surfaces of a variety of extravascular cell types, including fibroblasts and pericytes in and surrounding blood vessel walls and epithelial cells, but is generally absent on cells that come into contact with blood directly. However, TF expression could be induced in some blood cells, such as monocytes and endothelial cells, following an injury or pathological stimuli. Tissue factor is essential for hemostasis, but aberrant expression of TF leads to thrombosis. Therefore, a proper regulation of TF activity is critical for the maintenance of hemostatic balance and health in general. TF-FVIIa coagulant activity at the cell surface is influenced not only by TF protein expression levels but also independently by a variety of mechanisms, including alterations in membrane phospholipid composition and cholesterol content, thiol-dependent modifications of TF allosteric disulfide bonds, and other post-translational modifications of TF. In this article, we critically review the key literature on mechanisms by which TF coagulant activity is regulated at the cell surface in the absence of changes in TF protein levels with specific emphasis on recently published data and provide the authors' perspective on the subject.

Imaging of blood plasma coagulation at supported lipid membranes

The blood coagulation system relies on lipid membrane constituents to act as regulators of the coagulation process upon vascular trauma, and in particular the 2D configuration of the lipid membranes is known to efficiently catalyze enzymatic activity of blood coagulation factors. This work demonstrates a new application of a recently developed methodology to study blood coagulation at lipid membrane interfaces with the use of imaging technology. Lipid membranes with varied net charges were formed on silica supports by systematically using different combinations of lipids where neutral phosphocholine (PC) lipids were mixed with phospholipids having either positively charged ethylphosphocholine (EPC), or negatively charged phosphatidylserine (PS) headgroups. Coagulation imaging demonstrated that negatively charged SiO(2) and membrane surfaces exposing PS (obtained from liposomes containing 30% of PS) had coagulation times which were significantly shorter than those for plain PC membranes and EPC exposing membrane surfaces (obtained from liposomes containing 30% of EPC). Coagulation times decreased non-linearly with increasing negative surface charge for lipid membranes. A threshold value for shorter coagulation times was observed below a PS content of ∼6%. We conclude that the lipid membranes on solid support studied with the imaging setup as presented in this study offers a flexible and non-expensive solution for coagulation studies at biological membranes. It will be interesting to extend the present study towards examining coagulation on more complex lipid-based model systems.

Modulation of prothrombinase assembly and activity by phosphatidylethanolamine

Constituents of platelet membranes regulate the activity of the prothrombinase complex. We demonstrate that membranes containing phosphatidylcholine and phosphatidylethanolamine (PE) bind factor Va with high affinity (K(d) = ∼10 nm) in the absence of phosphatidylserine (PS). These membranes support formation of a 60-70% functional prothrombinase complex at saturating factor Va concentrations. Although reduced interfacial packing does contribute to factor Va binding in the absence of PS, it does not correlate with the enhanced activity of the Xa-Va complex assembled on PE-containing membranes. Instead, specific protein-PE interactions appear to contribute to the effects of PE. In support of this, soluble C6PE binds to recombinant factor Va(2) (K(d) = ∼6.5 μm) and to factor Xa (K(d) = ∼91 μm). C6PE and C6PS binding sites of factor Xa are specific, distinct, and linked, because binding of one lipid enhances the binding and activity effects of the other. C6PE triggers assembly (K(d)(app) = ∼40 nm) of a partially active prothrombinase complex between factor Xa and factor Va(2), compared with K(d)(app) for C6PS ∼2 nm. These findings provide new insights into the possible synergistic roles of platelet PE and PS in regulating thrombin formation, particularly when exposed membrane PS may be limiting.

Molecular determinants of phospholipid synergy in blood clotting

Many regulatory processes in biology involve reversible association of proteins with membranes. Clotting proteins bind to phosphatidylserine (PS) on cell surfaces, but a clear picture of this interaction has yet to emerge. We present a novel explanation for membrane binding by GLA domains of clotting proteins, supported by biochemical studies, solid-state NMR analyses, and molecular dynamics simulations. The model invokes a single “phospho-l-serine-specific” interaction and multiple “phosphate-specific” interactions. In the latter, the phosphates in phospholipids interact with tightly bound Ca²⁺ in GLA domains. We show that phospholipids with any headgroup other than choline strongly synergize with PS to enhance factor X activation. We propose that phosphatidylcholine and sphingomyelin (the major external phospholipids of healthy cells) are anticoagulant primarily because their bulky choline headgroups sterically hinder access to their phosphates. Following cell damage or activation, exposed PS and phosphatidylethanolamine collaborate to bind GLA domains by providing phospho-l-serine-specific and phosphate-specific interactions, respectively.

Effects of apoptosis and lipid peroxidation on T-lymphoblastoid phospholipid-dependent procoagulant activity

BACKGROUND

Coagulation has an absolute requirement for macromolecular complexes to be assembled on a negatively charged phospholipid (PL) surface. Previously, we reported that malignant T-lymphoblastoid cells have the ability to support procoagulant activity (PCA) independently of tissue factor by providing such a surface.

OBJECTIVE

To explore the effect of two pathophysiologic processes, apoptosis and lipid peroxidation (LP), on this PL-dependent PCA.

METHODS

Three different assays for PL-dependent PCA (factor IXa-initiated FXa and thrombin generation and prothrombinase activity) were used to investigate this PCA after exposing three T-lymphoblastoid cell lines to either apoptotic stimuli (1 microM staurosporine) or oxidative stress (4 mm H(2)O(2) and 40 microM CuSO(4)). Surface exposure of anionic PL was measured by flow cytometry using annexin A5(FITC) and an antibody (3G4) specific for native, but not oxidized, phosphatidylserine (PS).

RESULTS AND CONCLUSIONS

Both apoptosis and LP significantly enhanced the PCA of cells, to a level that was greater than that observed following calcium ionophore treatment, suggesting that the increased activity was not solely due to anionic PL exposure. Whereas cells undergoing apoptosis bound both annexin A5(FITC) and 3G4, only annexin A5(FITC) bound to cells undergoing LP. This implies that apoptosis increases PCA by causing the translocation of oxidized/native PS to the outer membrane, whereas LP appears to increase the PCA, possibly due to malondialdehyde adducts altering the net charge on the cell surface, which allows PLs other than PS to participate in thrombin generation.

Phospholipid composition of in vitro endothelial microparticles and their in vivo thrombogenic properties

INTRODUCTION

Microparticles from activated endothelial cells (EMP) are well known to expose tissue factor (TF) and initiate coagulation in vitro. TF coagulant activity is critically dependent on the presence of aminophospholipids, such as phosphatidylserine (PS) and phosphatidylethanolamine (PE), but it is unknown whether or not TF-exposing EMP are enriched in such aminophospholipids. Furthermore, despite the fact that EMP have been reported in several pathological conditions, direct evidence for their (putative) coagulant properties in vivo is still lacking. We investigated the phospholipid composition of endothelial MP (EMP) and their thrombogenic properties in vivo.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVEC; n=3) were incubated with or without interleukin (IL)-1alpha (5 ng/mL; 0-72 h). Phospholipid composition of EMP was determined by high-performance thin layer chromatography. The association between EMP, TF antigen and activity was confirmed in vitro (ELISA, Western blot and thrombin generation). Thrombogenic activity of EMP in vivo was determined in a rat venous stasis model.

RESULTS

Levels of TF antigen increased 3-fold in culture medium of IL-1alpha-treated cells (P<0.0001). This TF antigen was associated with EMP and appeared as a 45-47 kDa protein on Western blot. In addition, EMP from activated cells were enriched in both PS (P<0.0001) and PE (P<0.0001), and triggered TF-dependent thrombin formation in vitro and thrombus formation in vivo. In contrast, EMP from control cells neither initiated coagulation in vitro nor thrombus formation in vivo.

CONCLUSIONS

EMP from activated endothelial cells expose coagulant tissue factor and are enriched in its cofactors PS and PE.

Crystal structure of the bovine lactadherin C2 domain, a membrane binding motif, shows similarity to the C2 domains of factor V and factor VIII

Lactadherin, a glycoprotein secreted by a variety of cell types, contains two EGF domains and two C domains with sequence homology to the C domains of blood coagulation proteins factor V and factor VIII. Like these proteins, lactadherin binds to phosphatidylserine (PS)-containing membranes with high affinity. We determined the crystal structure of the bovine lactadherin C2 domain (residues 1 to 158) at 2.4 A. The lactadherin C2 structure is similar to the C2 domains of factors V and VIII (rmsd of C(alpha) atoms of 0.9 A and 1.2 A, and sequence identities of 43% and 38%, respectively). The lactadherin C2 domain has a discoidin-like fold containing two beta-sheets of five and three antiparallel beta-strands packed against one another. The N and C termini are linked by a disulfide bridge between Cys1 and Cys158. One beta-turn and two loops containing solvent-exposed hydrophobic residues extend from the C2 domain beta-sandwich core. In analogy with the C2 domains of factors V and VIII, some or all of these solvent-exposed hydrophobic residues, Trp26, Leu28, Phe31, and Phe81, likely participate in membrane binding. The C2 domain of lactadherin may serve as a marker of cell surface phosphatidylserine exposure and may have potential as a unique anti-thrombotic agent.

Probing the interaction of coagulation factors with phospholipid vesicle surfaces by surface plasma resonance

The dynamics of the binding of human coagulation factor Xa (FXa) and prothrombin to small unilamellar vesicles (25% phosphatidylserine, 75% phosphatidylcholine) were compared and quantified by Biacore, using two immobilization techniques. The vesicles were either tagged with different molar ratios of cholesterol-DNA and attached on Au chips or fused directly on L1 chips. The diameter in solution was 145 nm, but the more DNA tags/vesicle the more compressed the immobilized vesicles became; with 30 DNA tags the calculated thickness was 88 nm and with 1 DNA tag it was 138 nm. In both models the affinity for the vesicles was higher for the activated coagulation factors than for the corresponding zymogens. FXa and prothrombin had the highest affinities. The affinity was dependent on the vesicle preparation since overall K(D) values were up to 10 times lower for N(2)-dried than for vacuum-dried phospholipids, although with apparently fewer binding sites. However, compression of the vesicles had no effect on the K(D). In contrast, the rate constants were dependent on the number of DNA tags; thus deformation of the vesicles was observed. The k(a) and k(d) for FXa were similar for vesicles attached with 30 DNA tags or fused on the L1 chip but higher with fewer tags and approximately 10 times higher if attached with 1 tag. Thus for controlled kinetic studies immobilized DNA-tagged vesicles should be used.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.

The limits of simulation of the clotting system

OBJECTIVE

To investigate in how far successful simulation of a thrombin generation (TG) curve gives information about the underlying biochemical reaction mechanism.

RESULTS

The large majority of TG curves do not contain more information than can be expressed by four parameters. A limited kinetic mechanism of six reactions, comprising proteolytic activation of factor (F) X and FII, feedback activation of FV, a cofactor function of FVa and thrombin inactivation by antithrombin can simulate any TG curve in a number of different ways. The information content of a TG curve is thus much smaller than the information required to describe a physiologically realistic reaction scheme of TG. Consequently, much of the input information is irrelevant for the output. FVIII deficiency or activation of protein C can, for example, be simulated by a reaction mechanism in which these factors do not occur.

CONCLUSION

A model that comprises not more than six reactions can simulate the same TG curve in a number of possible ways. The possibilities increase exponentially as the model grows more realistic. Successful simulation of experimental data therefore does not validate the underlying assumptions. A fortiori, simulation that is not checked against experimental data lacks any probative force. Simulation can be of use, however, to detect mistaken hypotheses and for parameter estimation in systems with fewer than five free parameters.

Phospholipid composition controls thromboplastin sensitivity to individual clotting factors

BACKGROUND

Tissue factor is the active ingredient in thromboplastin reagents used to perform prothrombin time (PT) clotting tests to monitor oral anticoagulant therapy and to screen for clotting factor deficiencies. Thromboplastins are complex mixtures prepared from extracts of brain or placenta, although newer thromboplastins contain recombinant tissue factor incorporated into phospholipid vesicles. Thromboplastins can vary widely in their sensitivity to reductions in the levels of vitamin K-dependent clotting factors. A system to compensate for this, the International Sensitivity Index (ISI) and International Normalized Ratio (INR), has revolutionized the monitoring of oral anticoagulant therapy. The INR system is also sometimes used to monitor coagulopathies in patients with sepsis or liver failure, applications for which it was not originally designed and for which it has not been rigorously validated.

OBJECTIVES

To better understand thromboplastin performance, we systematically investigated which properties of recombinant thromboplastins influence their sensitivities to changes in the levels of specific clotting factors.

RESULTS

We now report that relative sensitivities to changes in the plasma levels of factors V, VII, X (FV, FVII, FX) and prothrombin are differentially influenced by a recombinant thromboplastin's content of phospholipid and sodium chloride. Furthermore, thromboplastins of similar ISI values may exhibit quite different sensitivities to each of these clotting factors.

CONCLUSIONS

Differing sensitivities of thromboplastin reagents to individual clotting factor levels have implications for monitoring of oral anticoagulant therapy and interpreting results of the PT assay.

Properties of recombinant human thromboplastin that determine the International Sensitivity Index (ISI)

Prothrombin Time (PT) clotting tests are widely used to monitor oral anticoagulation therapy and to screen for clotting factor deficiencies. The active ingredient in PT reagents (thromboplastins) is tissue factor, the integral membrane protein that triggers the clotting cascade through the extrinsic pathway. Several years ago, a system for calibrating and using thromboplastin reagents, known as the International Sensitivity Index (ISI) and the International Normalized Ratio (INR), was developed to standardize monitoring of oral anticoagulant therapy. The ISI/INR method, while revolutionizing the monitoring of coumarin therapy, has been criticized for a number of perceived shortcomings. We have undertaken a series of studies aimed at achieving a detailed understanding of which parameters influence the ISI values of thromboplastin reagents, with an ultimate goal of creating 'designer thromboplastins' whose sensitivities to the various clotting factors can be individually tailored. In this study, we demonstrate that ISI values of thromboplastin reagents based on relipidated, recombinant human tissue factor can be controlled by a combination of changes in the phospholipid content (in particular, the levels of phosphatidylserine and phosphatidylethanolamine) and ionic strength. The sensitivity of a given thromboplastin reagent can be increased (i.e. its ISI value decreased) by decreasing the content of phosphatidylserine and/or increasing the ionic strength. The molar ratio of phospholipid to tissue factor, on the other hand, had essentially no impact on ISI value.

Scott syndrome, a bleeding disorder caused by defective scrambling of membrane phospholipids

Normal quescent cells maintain membrane lipid asymmetry by ATP-dependent membrane lipid transporters, which shuttle different phospholipids from one leaflet to the other against their respective concentration gradients. When cells are challenged, membrane lipid asymmetry can be perturbed resulting in exposure of phosphatidylserine [PS] at the outer cell surface. Translocation of PS from the inner to outer membrane leaflet of activated blood platelets and platelet-derived microvesicles provides a catalytic surface for interacting coagulation factors. This process is dramatically impaired in Scott syndrome, a rare congenital bleeding disorder, underscoring the indispensible role of PS in hemostasis. This also testifies to a defect of a protein-catalyzed scrambling of membrane phospholipids. The Scott phenotype is not restricted to platelets, but can be demonstrated in other blood cells as well. The functional aberrations observed in Scott syndrome have increased our understanding of transmembrane lipid movements, and may help to identify the molecular elements that promote the collapse of phospholipid asymmetry during cell activation and apoptosis.

Importance of protein S and phospholipid for activated protein C-mediated cleavages in factor Va

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.

The omega-loop region of the human prothrombin gamma-carboxyglutamic acid domain penetrates anionic phospholipid membranes

The hydrophobic omega-loop within the prothrombin gamma-carboxyglutamic acid-rich (Gla) domain is important in membrane binding. The role of this region in membrane binding was investigated using a synthetic peptide, PT-(1-46)F4W, which includes the N-terminal 46 residues of human prothrombin with Phe-4 replaced by Trp providing a fluorescent probe. PT-(1-46)F4W and PT-(1-46) bind calcium ions and phospholipid membranes, and inhibit the prothrombinase complex. PT-(1-46)F4W, but not PT-(1-46), exhibits a blue shift (5 nm) and red-edge excitation shift (28 nm) in the presence of phosphatidylserine (PS)-containing vesicles, suggesting Trp-4 is located within the motionally restricted membrane interfacial region. PS-containing vesicles protect PT-(1-46)F4W, but not PT-(1-46), fluorescence from potassium iodide-induced quenching. Stern-Volmer analysis of the quenching of PT-(1-46)F4W in the presence and absence of 80% phosphatidylcholine/20% PS vesicles suggested that Trp-4 is positioned within the membrane and protected from aqueous quenching agents whereas Trp-41 remains solvent-accessible in the presence of PS-containing vesicles. Fluorescence quenching of membrane-bound PT-(1-46)F4W is optimal with 7- and 10-doxyl-labeled lipids, indicating that Trp-4 is inserted 5 to 7 A into the bilayer. This report demonstrates that the omega-loop region of prothrombin specifically interacts with PS-containing membranes within the interfacial membrane region.

Monocytes, but not endothelial cells, downregulate the anticoagulant activity of activated protein C

Activated protein C (APC) is a natural anticoagulant and inhibits thrombin generation by degrading factors Va and VIIIa. We evaluated the ability of APC to inhibit blood coagulation triggered by lipopolysaccharide (LPS)-stimulated [tissue factor (TF)-expressing] human mononuclear cells (MNCs) or umbilical vein endothelial cells (HUVECs). Using a plasma recalcification assay, we found that APC (up to 53.3 nmol/l final concentration) had a poor anticoagulant effect in the presence of LPS-stimulated MNCs, whereas it caused a marked prolongation of clotting time in the presence of LPS-stimulated HUVECs. A poor response to APC was also observed when platelet-free MNCs, monocyte-enriched preparations or the monocytoid cell line U937 were tested. Using a TF-independent (FXa-induced) thrombin generation assay, we demonstrated that both LPS-stimulated and unstimulated MNCs negated the inhibitory activity of APC. Direct determination of FVa activity indicated that MNCs were less efficient than HUVECs in promoting FVa inactivation by APC. Together, our results suggest that MNCs, at variance with HUVECs, protect factor Va from inactivation by APC, probably through the expression of a membrane component not present on endothelial cells. These strengthen the importance of monocytes in fibrin deposition associated with pathological conditions characterized by monocyte recruitment and activation.

Lipid oxidation enhances the function of activated protein C

Although lipid oxidation products are usually associated with tissue injury, it is now recognized that they can also contribute to cell activation and elicit anti-inflammatory lipid mediators. In this study, we report that membrane phospholipid oxidation can modulate the hemostatic balance. Oxidation of natural phospholipids results in an increased ability of the membrane surface to support the function of the natural anticoagulant, activated protein C (APC), without significantly altering the ability to support thrombin generation. Lipid oxidation also potentiated the ability of protein S to enhance APC-mediated factor Va inactivation. Phosphatidylethanolamine, phosphatidylserine, and polyunsaturation of the fatty acids were all required for the oxidation-dependent enhancement of APC function. A subgroup of thrombotic patients with anti-phospholipid antibodies specifically blocked the oxidation-dependent enhancement of APC function. Since leukocytes are recruited and activated at the thrombus or sites of vessel injury, our findings suggest that after the initial thrombus formation, lipid oxidation can remodel the membrane surface resulting in increased anticoagulant function, thereby reducing the thrombogenicity of the thrombus or injured vessel surface. Anti-phospholipid antibodies that block this process would therefore be expected to contribute to thrombus growth and disease.

Lipid translocation across the plasma membrane of mammalian cells

The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase.

The effect of membrane composition on the hemostatic balance

The phospholipid composition requirements for optimal prothrombin activation and factor Va inactivation by activated protein C (APC) anticoagulant were examined. Vesicles composed of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) supported factor Va inactivation relatively well. However, optimal factor Va inactivation still required relatively high concentrations of phosphatidylserine (PS). In addition, at a fixed concentration of phospholipid, PS, and APC, vesicles devoid of PE never attained a rate of factor Va inactivation achievable with vesicles containing PE. Polyunsaturation of any vesicle component also contributed significantly to APC inactivation of factor Va. Thus, PE makes an important contribution to factor Va inactivation that cannot be mimicked by PS. In the absence of polyunsaturation in the other membrane constituents, this contribution was dependent upon the presence of both the PE headgroup per se and unsaturation of the 1,2 fatty acids. Although PE did not affect prothrombin activation rates at optimal PS concentrations, PE reduced the requirement for PS approximately 10-fold. The Km(app) for prothrombin and the Kd(app) for factor Xa-factor Va decreased as a function of increasing PS concentration, reaching optimal values at 10-15% PS in the absence of PE but only 1% PS in the presence of PE. Fatty acid polyunsaturation had minimal effects. A lupus anticoagulant immunoglobulin was more inhibitory to both prothrombinase and factor Va inactivation in the presence of PE. The degree of inhibition of APC was significantly greater and much more dependent on the phospholipid composition than that of prothrombinase. Thus, subtle changes in the phospholipid composition of cells may control procoagulant and anticoagulant reactions differentially under both normal and pathological conditions.

Lipid-protein interactions in blood coagulation

It has long been appreciated that lipids, particularly anionic phospholipids, promote blood coagulation. The last two decades have seen an increasing insight into the kinetic and mechanistic aspects regarding the mode of action of phospholipids in blood coagulation. This essay attempts to review these developments with particular emphasis on the structure of lipid-binding domains of blood coagulation proteins, and the variable effect of phospholipid composition on the interaction with these proteins. Some examples are discussed of how lipid membranes direct the pathway of enzymatic conversions in blood coagulation complexes, also illustrating that the membrane lipid surface is more than an inert platform for the assembly of coagulation factors. Finally, the controlled exposure of procoagulant lipid on the surface of blood cells is shortly reviewed, and an example is discussed of how interference with lipid-protein interactions in blood coagulation may result in pathological phenomena.

Platelet agonists enhance the import of phosphatidylethanolamine into human platelets

It is unknown whether the endocytosis-independent transfer of phospholipids from lipoproteins to platelets is regulated by platelet agonists such as thrombin. The movements of the choline phospholipids phosphatidylcholine and sphingomyelin (labeled with either 14C or the fluorescent pyrenedecanoic acid) between low density lipoproteins and platelets were unaffected by thrombin (0.5 unit/ml). In contrast, thrombin accelerated the import of diacyl phosphatidylethanolamine (PE) and alkenylacyl phosphatidylethanolamine into platelets by about 4-fold. Similarly, thrombin receptor-activating peptide (15 microM), collagen (10 microgram/ml), and ADP (10 microM) enhanced PE uptake. High density lipoprotein particles and egg phosphatidylcholine vesicles were also donors for stimulation of platelet PE import. Part of the [14C]arachidonic acid-labeled PE transferred from low density lipoprotein to platelets activated by thrombin and collagen was metabolized to 14C-eicosanoids. Inhibitors of protein kinase C partially prevented thrombin-induced [14C]PE uptake, while direct activators of protein kinase C increased incorporation of [14C]PE into platelets. Proteinaceous factor(s) recovered in the extracellular medium from ADP- and thrombin-activated platelet suspensions were found to accelerate the transfer of pyrenedecanoic acid-labeled PE between donor and acceptor lipid vesicles. The stimulation of import of ethanolamine phospholipids led to a 2-fold enhancement of the prothrombinase activity of thrombin-activated platelets. Our study demonstrates that physiological platelet stimuli increase specifically the transfer of ethanolamine phospholipids from lipoproteins to platelets through a secretion-dependent mechanism. This might contribute to the increase of procoagulant activity of stimulated platelets.

A chimeric protein C containing the prothrombin Gla domain exhibits increased anticoagulant activity and altered phospholipid specificity

To determine the structural basis of phosphatidylethanolamine (PE)-dependent activated protein C (APC) activity, we prepared a chimeric molecule in which the Gla domain and hydrophobic stack of protein C were replaced with the corresponding region of prothrombin. APC inactivation of factor Va was enhanced 10-20-fold by PE. Protein S enhanced inactivation 2-fold and independently of PE. PE and protein S had little effect on the activity of the chimera. Factor Va inactivation by APC was approximately 5-fold less efficient than with the chimera on vesicles lacking PE and slightly more efficient on vesicles containing PE. The cleavage patterns of factor Va by APC and the chimera were similar, and PE enhanced the rate of Arg506 and Arg306 cleavage by APC but not the chimera. APC and the chimera bound to phosphatidylserine:phosphatidylcholine vesicles with similar affinity (Kd approximately 500 nM), and PE increased affinity 2-3-fold. Factor Va and protein S synergistically increased the affinity of APC on vesicles without PE to 140 nM and with PE to 14 nM, but they were less effective in enhancing chimera binding to either vesicle. In a factor Xa one-stage plasma clotting assay, the chimera had approximately 5 times more anticoagulant activity than APC on PE-containing vesicles. Unlike APC, which showed a 10 fold dependence on protein S, the chimera was insensitive to protein S. To map the site of the PE and protein S dependence further, we prepared a chimera in which residues 1-22 were derived from prothrombin and the remainder were derived from protein C. This protein exhibited PE and protein S dependence. Thus, these special properties of the protein C Gla domain are resident outside of the region normally hypothesized to be critical for membrane interaction. We conclude that the protein C Gla domain possesses unique properties allowing synergistic interaction with factor Va and protein S on PE-containing membranes.

Secretory phospholipases and membrane polishing

Although secretory phospholipase A2 (PLA2) isozymes have been identified in human gestational tissues, their role in homeostasis and pathophysiology during pregnancy has yet to be clearly established. The aims of this brief commentary are: (1) to review recent data concerning the expression of secretory PLA2 isozymes in human gestational tissues; and (2) to present a case for their involvement in regulating the expression of glycerophospholipids in the exoplasmic monolayer of the cell membrane. Three secretory PLA2 isozymes and a secretory PLA2 cell-surface receptor have been identified in human term gestational tissues. In addition to their potential role in the formation of glycerophospholipid-derived metabolites (such as prostaglandins), these isozymes may function to regulate the expression of aminophospholipids on the cell surface. The exposure of aminophospholipids on the cell surface dramatically affects many aspects of cell function. Secreted PLA2 isozymes that display a substrate preference for the negatively charged aminophospholipids (e.g. phosphatidylserine or phosphatidylethanolamine) in the exoplasmic membrane may affect cell function and reactivity via a process of 'membrane polishing', that is, the preferentially removal of aminophospholipids from the exoplasmic leaflet of the cell membranes. By this process, secreted PLA2 isozymes may limit unsolicited cell-surface binding of exogenous proteins, membrane fusion events and recognition by cellular surveillance systems.

Phosphatidylserine exposure and aminophospholipid translocase activity in Rh-deficient erythrocytes

Endogenous phosphatidylserine (PS) exposure and lipid transport activity have been investigated for seven unrelated cases of Rhnull erythrocytes. Endogenous PS exposure was measured by prothrombinase activity. Out of six cases studied, two Rhnull samples exhibited abnormal aminophospholipid exposure, as suggested by the measurement of a lower Km of factor Xa for prothrombin. Aminophospholipid translocase activity was measured through the transbilayer redistribution of spin-labelled analogues of phospholipids. Provided that incubation conditions allow the maintainance of intracellular ATP level, no difference was observed between Rhnull and control erythrocytes, clearly indicating that the aminophospholipid translocase and Rh polypeptides are different molecular species.

Non beta 2-glycoprotein I cofactors for antiphospholipid antibodies

The Antiphospholipid Syndrome is defined by the association between peculiar clinical manifestations, namely arterial and/or venous thrombosis, recurrent abortions and thrombocytopenia, and the antiphospholipid antibodies. These antibodies are directed to plasma proteins bound to anionic phospholipids or other anionic surfaces: so far, beta 2-glycoprotein I is the best known and characterized antiphospholipid 'cofactor' (this issue is specifically treated in other parts of this journal). In recent years, such a role has been reported also for prothrombin, activated Protein C, Protein S, Annexin V, Thrombomodulin, high- and low-molecular weight kininogens. Anti-prothrombin antibodies are detected in approximately 50% of the antiphospholipid-positive patients; conversely, limited data are available regarding the prevalence the other antibodies. 'Cofactors' are necessary for the expression of both the immunological and the functional properties of their respective antiphospholipid antibodies. In particular, the recognition of the calcium-mediated prothrombin/lipid complex by anti-prothrombin antibodies hampers prothrombin activation, thus causing the prolongation of the phospholipid-dependent coagulation reactions. The interaction between antiphospholipid antibodies and natural inhibitors of coagulation such as activated Protein C, its non-enzymatic accessory protein Protein S or Thrombomodulin might increase the risk to develop thromboembolic events. Similarly, the presence of antibodies to surface-bound Annexin V has been hypothesized to play a role in recurrent abortions and fetal deaths. However, to clearly establish whether and which antiphospholipid antibodies represent risk factors for the thromboembolic events of the antiphospholipid syndrome, further studies of their behaviour and properties as well as the identification and characterization of (possibly) other antibodies are required.

Regulatory mechanisms in maintenance and modulation of transmembrane lipid asymmetry: pathophysiological implications

The two leaflets of the plasma membrane of eukaryotic cells differ in lipid composition: the outer leaflet comprises mainly neutral choline containing phospholipids, whereas the aminophospholipids reside almost exclusively in the cytoplasmic leaflet. The importance of transmembrane lipid asymmetry may be judged from the fact that the cell invests energy to maintain this situation for which at least two regulatory mechanisms are held responsible. A translocase, selective for aminophospholipids, acts as an ATP-dependent pump for rapid inward movement of phosphatidylserine (PS) and phosphatidylethanolamine; in addition, a non-selective, but also ATP-dependent pump causes outward movement of phospholipids, be it at a much lower rate compared to the inward transport by the aminophospholipid translocase. These two systems, acting in concert, are thought to be the main players in the maintenance of a dynamic equilibrium of the phospholipids over both membrane leaflets. Dissipation of membrane lipid asymmetry can be elicited in different cell types under a variety of conditions; in particular, platelets upon activation rapidly lose their normal plasma membrane lipid distribution, but also in other blood cells, lipid asymmetry can be lost, be it at a much lower rate and extent than in platelets. A putative protein, referred to as "scramblase' has been described, which requires the continuous presence of elevated intracellular Ca(2+)-levels, to allow a rapid, non-selective and bidirectional transbilayer movement of phospholipids. Although scrambling of lipids does not require ATP as such, preliminary studies suggest the possible involvement of one or more phosphorylated proteins. The most prominent consequence of the loss of phospholipid asymmetry is exposure of PS in the outer leaflet of the plasma membrane. Surface-exposed PS serves several important physiological functions: it promotes assembly of enzyme complexes of the coagulation cascade, it forms a signal for cell-cell recognition, which is important for cell scavenging processes. Surface-exposure of PS is an early phenomenon of apoptosis and appears to be involved in efficient removal of these cells. In addition, PS in the outer leaflet of cells is thought to play a role in cell fusion processes. It may be clear from the foregoing, that the amount of PS present at the cell surface needs to be tightly controlled, and that an impairment of this process leads to either excessive- or diminished exposition of PS which may have several pathophysiological consequences.