Platelet receptor-mediated factor X activation by factor IXa. High-affinity factor IXa receptors induced by factor VIII are deficient on platelets in Scott syndrome

Binding of coagulation factor IXa to procoagulant platelets revisited: Low affinity and interactions with other factors

Binding of activated factor IX (fIXa) to the phosphatidylserine-expressing procoagulant platelets is a critical step in blood coagulation, which is necessary for the membrane-dependent intrinsic tenase complex assembly and factor X activation. However, the nature and parameters of the fIXa binding sites on the procoagulant platelet surface remain unclear. We used flow cytometry to elucidate the quantitative details of the fluorescently labeled fIXa binding to gel-filtered activated platelets. FIXa bound to the procoagulant platelet subpopulation only, with the parameters (maximal number of binding sites at 58900 ± 3400, Kd at 1000 ± 170 nM) similar to binding observed with phospholipid vesicles. No specific high-affinity binding sites for fIXa were detected, and binding proceeded similarly for different methods of procoagulant platelet production (thrombin, thrombin receptor activation peptide, collagen-related peptide, their combinations, or calcium ionophore A23187). Factor VIII, known to form a high affinity complex with fIXa, enhanced fIXa binding to platelets. In contrast, only competition effects were observed for factor X, which binds fIXa with much lower affinity. Unexpectedly, fIXa itself, fIX, and prothrombin also dose-dependently enhance fIXa binding at concentrations below 1000 nM, suggesting the formation of membrane-bound fIXa dimers and fIXa-prothrombin complexes on platelets. These findings provide a novel perspective on the fIXa binding site on procoagulant platelets, which does not have any major differences from pure phospholipid-based model membranes, exhibits inherently low affinity (3-5 orders of magnitude below the physiologically relevant fIXa concentration) but is significantly enhanced by its cofactor VIII, and regulated by previously unknown membrane interactions.

Comprehensive functional characterization of a novel ANO6 variant in a new patient with Scott syndrome

BACKGROUND

Scott syndrome is a mild platelet-type bleeding disorder, first described in 1979, with only 3 unrelated families identified through defective phosphatidylserine (PS) exposure and confirmed by sequencing. The syndrome is distinguished by impaired surface exposure of procoagulant PS on platelets after stimulation. To date, platelet function and thrombin generation in this condition have not been extensively characterized.

OBJECTIVES

Genetic and functional studies were undertaken in a consanguineous family with a history of excessive bleeding of unknown cause.

METHODS

A targeted gene panel of known bleeding and platelet genes was used to identify possible genetic variants. Platelet phenotyping, flow adhesion, flow cytometry, whole blood and platelet-rich plasma thrombin generation, and specialized extracellular vesicle measurements were performed.

RESULTS

We detected a novel homozygous frameshift variant, c.1943del (p.Arg648Hisfs∗23), in ANO6 encoding Anoctamin 6, in a patient with a bleeding history but interestingly with normal ANO6 expression. Phenotyping of the patient's platelets confirmed the absence of PS expression and procoagulant activity but also revealed other defects including reduced platelet δ granules, reduced ristocetin-mediated aggregation and secretion, and reduced P-selectin expression after stimulation. PS was absent on spread platelets, and thrombi formed over collagen at 1500/s. Reduced thrombin generation was observed in platelet-rich plasma and confirmed in whole blood using a new thrombin generation assay.

CONCLUSION

We present a comprehensive report of a patient with Scott syndrome with a novel frameshift variant in AN06, which is associated with no platelet PS exposure and markedly reduced thrombin generation in whole blood, explaining the significant bleeding phenotype observed.

SLAPSHOT reveals rapid dynamics of extracellularly exposed proteome in response to calcium-activated plasma membrane phospholipid scrambling

To facilitate our understanding of the often rapid and nuanced dynamics of extracellularly exposed proteomes during signaling events, it is important to devise robust workflows affording fast time resolution without biases and confounding factors. Here, we present Surface-exposed protein Labeling using PeroxidaSe, H2O2, and Tyramide-derivative (SLAPSHOT), to label extracellularly exposed proteins in a rapid, sensitive, and specific manner, while preserving cellular integrity. This experimentally simple and flexible method utilizes recombinant soluble APEX2 peroxidase that is applied to cells, thus circumventing biological perturbations, tedious engineering of tools and cells, and labeling biases. APEX2 neither requires metal cations for activity nor contains disulfide bonds, conferring versatility for a wide spectrum of experimental setups. We applied SLAPSHOT followed by quantitative mass spectrometry-based proteomics analysis to examine the immediate and extensive cell surface expansion and ensuing restorative membrane shedding upon the activation of Scott syndrome-linked TMEM16F, a ubiquitously expressed calcium-dependent phospholipid scramblase and ion channel. Time-course data ranging from one to thirty minutes of calcium stimulation using wild-type and TMEM16F deficient cells revealed intricate co-regulation of known protein families, including those in the integrin and ICAM families. Crucially, we identified proteins that are known to reside in intracellular organelles, including ER, as occupants of the freshly deposited membrane, and mitovesicles as an abundant component and contributor to the extracellularly exposed proteome. Our study not only provides the first accounts of the immediate consequences of calcium signaling on the extracellularly exposed proteome, but also presents a blueprint for the application of SLAPSHOT as a general approach for monitoring extracellularly exposed protein dynamics.

Discordance between platelet-supported and vesicle-supported factor VIII activity in the presence of anti-C2 domain inhibitory antibodies

BACKGROUND

We recently reported that factor VIII (FVIII) binds to a macromolecular complex including fibrin on thrombin-stimulated platelets and that two antibodies against FVIII diminish platelet-supported FVIII activity more than vesicle-supported activity. The C2 domain of FVIII is known to bind to phospholipid membrane and also binds fibrin.

OBJECTIVES

We asked whether the degree of inhibition by anti-C2 antibodies would show differences between platelet-supported and the standard activated partial thromboplastin time (aPTT) assay.

METHODS

We evaluated the inhibition by a well-defined panel of monoclonal anti-C2 domain antibodies encompassing the major epitopes of the C2 domain. Activity was measured in an activated platelet time (aPT) assay containing fresh, density gradient-purified human platelets.

RESULTS

The aPT exhibited a log-linear relationship between FVIII and time to fibrin formation over a 4-log range, encompassing 0.01% to 100% plasma FVIII. Nine of 10 mAbs inhibited 89% to 96% of FVIII activity, whereas mAb F85 did not. There was no correlation between the degree of inhibition in the aPTT-based assay and the platelet assay. In particular, four mAbs did not inhibit the aPTT assay, yet inhibited 90% of platelet-based activity. Residual FVIII activity in purified-protein assays, relying on platelets, correlated with the aPT assay.

CONCLUSIONS

The degree of FVIII impairment by some inhibitor antibodies is substantially different on platelet membranes vs synthetic vesicles. Thus, current inhibitor assays may underestimate the frequency of significant inhibitors, and a platelet-based assay may more accurately assess bleeding risk.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Evidence of maternal platelet activation, excessive thrombin generation, and high amniotic fluid tissue factor immunoreactivity and functional activity in patients with fetal death

OBJECTIVE

Fetal death can lead to disseminated intravascular coagulation or fetal death syndrome. However, currently it is not clear what are the changes in the coagulation system in patients with a fetal death without the fetal death syndrome. This study was undertaken to determine: (1) whether fetal death in the absence of fetal death syndrome is associated with changes in hemostatic markers in maternal plasma and amniotic fluid; and (2) whether maternal hypertension or placental abruption are associated with further changes in the hemostatic profile of these patients.

METHODS

A cross-sectional study included the following: (1) determination of changes in markers of coagulation and platelet activation in patients with a normal pregnancy (n = 71) and patients with fetal demise (FD) without disseminated intravascular coagulation (n = 65); (2) determination of the amniotic fluid (AF)-tissue factor concentration and activity, as well as the concentrations of thrombin-antithrombin III (TAT) complexes in patients with a normal pregnancy (n = 25) and those with a FD (n = 36) who underwent amniocentesis. Plasma and AF concentrations of TAT complexes and TF (an index of thrombin generation), as well as maternal plasma concentrations of sCD40L (a marker of platelet activation), tissue factor pathway inhibitor (TFPI) and prothrombin fragments (PF) 1 + 2 (also an indicator of in vivo thrombin generation) were measured by ELISA. TF and TFPI activity were measured using chromogenic assays.

RESULTS

(1) patients with FD without hypertension had a higher median maternal plasma sCD40L concentration than normal pregnant women (P < 0.001); (2) patients with FD had a higher median maternal plasma TAT III complexes than women with a normal pregnancy (P < 0.001); (3) the median AF-TF concentration and activity were higher in the FD group than in the normal pregnancy group (P < 0.001 for both); (4) patients with preeclampsia and FD had a higher median maternal plasma immunoreactive TF concentration than both normotensive patients with FD and women with normal pregnancies (P < 0.001 and P = 0.001, respectively); (5) the median plasma TF activity was higher in patients with preeclampsia and FD than that of women with normal pregnancies (P = 0.003); (6) among patients with a FD, those with placental abruption had a higher median AF-TAT complexes concentration than those without abruption (P = 0.0004).

CONCLUSIONS

Our findings indicate that: (1) mothers with a FD have evidence of increased in vivo thrombin generation and platelet activation than women with normal pregnancies; (2) patients with a FD and hypertension had a higher degree of TF activation than those with fetal death but without hypertension; (3) the AF of women with a FD had a higher median TF concentration and activity than that of normal pregnant women. AF can be a potential source for tissue factor and it participates in the development of fetal death syndrome in patients with a retained dead fetus.

Lactadherin blocks thrombosis and hemostasis in vivo: correlation with platelet phosphatidylserine exposure

BACKGROUND

Platelet membrane phosphatidylserine (PS) is considered to be essential for hemostasis and thrombosis, but the in vivo topography of platelet PS has not been characterized. We hypothesized that platelet PS exposure would be identified on adherent platelets at the site of vascular injury and that blockade of PS would impede hemostasis and thrombosis.

OBJECTIVE

To localize and estimate the extent of platelet PS exposure and evaluate the impact of PS blockade in vivo.

METHODS

Lactadherin, a PS-binding milk protein, was utilized together with annexin V to detect both partial and complete membrane PS exposure on platelets in a mouse model of thrombosis and to evaluate the functional need for PS. Preliminary experiments were performed with synthetic membranes and with purified platelets.

RESULTS

The number of lactadherin-binding sites on synthetic membranes was proportional to PS content, whereas annexin V required a threshold of 2.5-8% PS. Approximately 95% of thrombin-stimulated platelets exposed PS, but the quantity was below the threshold for annexin V binding at physiologic Ca(2+) concentrations. In mice, most adherent and aggregated platelets on the walls of ferric chloride-treated mesenteric veins exposed low levels of PS, rather than having complete exposure. In mice, blockade of PS with lactadherin inhibited platelet prothrombinase and factor Xase activity, and prolonged tail bleeding time and the time to carotid artery thrombosis.

CONCLUSIONS

In vivo PS exposure contributes to both hemostasis and thrombosis. In this model of vascular injury, most platelets exhibit partial rather than complete PS exposure.

PAR-1-stimulated factor IXa binding to a small platelet subpopulation requires a pronounced and sustained increase of cytoplasmic calcium

We previously reported that only a subpopulation of PAR-1-stimulated platelets binds coagulation factor IXa, since confirmed by other laboratories. Since calcium changes have been implicated in exposure of procoagulant aminophospholipids, we have now examined calcium fluxes in this subpopulation by measuring fluorescence changes in Fura Red/AM-loaded platelets following PAR-1 stimulation. While fluorescence changes in all platelets indicated calcium release from internal stores and influx of external calcium, a subpopulation of platelets displayed a pronounced increase in calcium transients by 15 s and positive factor IXa binding by 2 min, with calcium transients sustained for 45 min. Pretreatment of platelets with Xestospongin C to inhibit IP3-mediated dense tubule calcium release, and the presence of impermeable calcium channel blockers nifedipine, SKF96365, or LaCl3, inhibited PAR-1-induced development of a subpopulation with pronounced calcium transients, factor IXa binding, and platelet support of FXa generation, suggesting the importance of both release of calcium from internal stores and influx of extracellular calcium. When platelets were stimulated in EDTA for 5-20 min before addition of calcium, factor IXa binding sites developed on a smaller subpopulation but with unchanged rate, indicating sustained opening of calcium channels and continued availability of signaling elements required for binding site exposure. While pretreatment of platelets with 100 microM BAPTA/AM (Kd 160 nM) had minimal effects, 100 microM 5,5'-dimethylBAPTA/AM (Kd 40 nM) completely inhibited the appearance and function of the platelet subpopulation, indicating the importance of minor increases of cytoplasmic calcium. We conclude that PAR-1-stimulated development of factor IXa binding sites in a subpopulation of platelets is dependent upon release of calcium from internal stores leading to sustained and pronounced calcium transients.

Disruption of Protein-Membrane Binding and Identification of Small-Molecule Inhibitors of Coagulation Factor VIII

Factor VIII is a critical member of the blood coagulation cascade. It binds to the membrane surfaces of activated platelets at the site of vascular injury via a highly specific interaction between factor VIII's carboxy-terminal C2 domain and their phosphatidylserine-rich lipid bilayer. We have identified small-molecule inhibitors of factor VIII's membrane binding activity that have IC50 values as low as 2.5 microM. This interaction is approximately 10(3)-fold tighter than that of free o-phospho-L-serine. These compounds also inhibit factor VIII-dependent activation of factor X, indicating that disruption of membrane lipid binding leads to inhibition of the intrinsic coagulation pathway. The tightest binding inhibitor is specific and does not prevent membrane binding by the closely related coagulation factor V. These results indicate that this and related compounds may be used as leads to develop novel antithrombotic agents.

Fibrin stimulates platelets to increase factor VIIIa binding site expression

Factor (F)VIII functions as an enzymatic cofactor on the membranes of stimulated platelets. However, thrombin stimulates platelets to express only a small number of binding sites for FVIII. We wished to determine whether molecules that are likely to be present in a developing thrombus stimulate platelets to up-regulate FVIII binding site expression. Flow cytometry was utilized to measure binding of fluorescein-labeled FVIIIa to activated platelets and a FXase assay was utilized to measure platelet-dependent function. Various agonists as well as normal and mutant fibrinogens and fibrin were evaluated as co-stimuli. Thrombin-stimulated platelets expressed 214 +/- 67 binding sites for thrombin-activated FVIII (FVIIIa) and none of the established soluble agonists enhanced binding site exposure. However, the presence of 5 micro g mL(-1) fibrin increased the number of FVIIIa binding sites/platelet three- to eight-fold (1470 +/- 130, range 600-1800) with a parallel increase in platelet-based FXase assay. Binding site up-regulation was not stimulated by fibrinogen and was blocked by inhibitors of GPIIbIIIa. Mutant fibrin lacking the gamma-chain C-terminal four residues was ineffective while fibrin with altered RGD sequences did stimulate expression of FVIIIa binding sites indicating that co-stimulation is mediated by the fibrin gamma-chain termini. Fibrin-enhanced expression of FVIIIa binding sites was not supported by D364H fibrin, which does not aggregate normally, and was blocked by the GPRP peptide, which inhibits fibrin polymerization. Polymerized fibrin can function as a platelet co-stimulus, up-regulating expression of binding sites for FVIIIa.

A Subpopulation of Platelets Responds to Thrombin- or SFLLRN-stimulation with Binding Sites for Factor IXa

Strong agonists cause platelets to expose a procoagulant surface supporting the assembly of two important coagulation enzyme complexes. Equilibrium binding has determined the density of high affinity saturable factor IXa binding sites to be 500-600 sites/platelet. We have now used flow cytometry to visualize the binding of factor IX and IXa to thrombin- or SFLLRN-activated platelets. Concentrations of these agonists that are half-maximal or maximal in kinetic studies resulted in only a small subpopulation (4-20%) of platelets binding factor IX or IXa with the density of binding sites for factor IX being about half of that for factor IXa, consistent with previous equilibrium binding studies. A small subpopulation (5 +/- 1.5%) of platelets stimulated with either agonist also exposed annexin V binding sites, and this subpopulation of platelets also bound factor IXa. Annexin V decreased factor IXa binding in the presence or absence of factor VIIIa, and factor IXa could also decrease annexin V binding on some platelets indicating a common binding site in agreement with previous studies. All platelets binding factor IXa were positive for glycoprotein IX, at the same glycoprotein IX surface density as seen in platelets negative for factor IXa binding. These studies refine the results from equilibrium binding studies and suggest that, on average, only a small subpopulation (approximately 10%) of PAR 1-stimulated platelets expose approximately 6000 factor IXa binding sites/platelet.

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.

The protein kinase C inhibitor RO318220 potentiates thrombin-stimulated platelet-supported prothrombinase activity

Prothrombinase activity was tested on thrombin- and SFLLRN-activated platelets treated with RO318220, a potent inhibitor of protein kinase C. RO318220 completely inhibited platelet dense and alpha-granule secretion at a concentration of 20 microM but had no effect on prothrombinase activity in the presence of excess factor Va (20 nM). This indicates that protein kinase C activity and agonist-initiated secretion are not necessary for the development of a procoagulant surface. Treatment with 75 to 150 microM RO318220 potentiated platelet-supported thrombin generation up to 280% of control platelets with no change in Kd appFXa. Treated with increasing concentrations of RO318220, an increasing proportion of thrombin-stimulated platelets bound annexin V with decreasing binding sites per platelet. A lower mean forward scatter (FSC-H) of platelets treated with RO318220 suggested platelet vesiculation as a result of RO318220 treatment; however, 100 microM calpeptin pretreatment eliminated the decrease in FSC-H without affecting either the increase in platelets positive for annexin V binding, the decrease in binding sites per platelet, or the 3-fold increase in prothrombinase activity. Thus, RO318220 appears to increase prothrombinase activity by increasing platelet responsiveness to thrombin rather than by inducing platelet vesiculation. This suggests that RO318220 inhibits a signaling molecule within a negative regulatory pathway that governs platelet procoagulant surface changes.

The assembly of the factor X-activating complex on activated human platelets

Platelet membranes provide procoagulant surfaces for the assembly and expression of the factor X-activating complex and promote the proteolytic activation and assembly of the prothrombinase complex resulting in normal hemostasis. Recent studies from our laboratory and others indicate that platelets possess specific, high-affinity, saturable, receptors for factors XI, XIa, IX, IXa, X, VIII, VIIIa, V, Va and Xa, prothrombin, and thrombin. Studies described in this review support the hypothesis that the factor X-activating complex on the platelet surface consists of three receptors (for the enzyme, factor IXa; the substrate, factor X; and the cofactor, factor VIIIa), the colocalization of which results in a 24 million-fold acceleration of the rate of factor X activation. Whether the procoagulant surface of platelets is defined exclusively by procoagulant phospholipids, or whether specific protein receptors exist for the coagulant factors and proteases, is currently unresolved. The interaction between coagulation proteins and platelets is critical to the maintenance of normal hemostasis and is pathogenetically important in human disease.

Structural and functional characterization of platelet receptor-mediated factor VIII binding

Optimal rates of factor X (FX) activation require occupancy of receptors for factor IXa (FIXa), factor VIII (FVIII), and FX on the activated platelet surface. The presence of FVIII and FX increases 5-fold the affinity of FIXa for the surface of activated platelets, and the presence of FVIII or FVIIIa generates a high affinity, low capacity specific FX-binding site on activated platelets. We have now examined the effects of FX and active site-inhibited FIXa (EGR-FIXa) on the binding of both FVIII and FVIIIa to activated platelets and show the following: (a) von Willebrand factor inhibits FVIII binding (K(i) = 0.54 nM) but not FVIIIa binding; (b) thrombin and the thrombin receptor activation peptide (SFLLRN amide) are the most potent agonists required for FVIII-binding site expression, whereas ADP is inert; (c) FVa does not compete with FVIIIa or FVIII for functional platelet-binding sites; and (d) Annexin V is a potent inhibitor of FVIIIa binding (IC(50) = 10 nM) to activated platelets. The A2 domain of FVIII significantly increases the affinity and stoichiometry of FVIIIa binding to platelets and contributes to the stability of the FX-activating complex. Both FVIII and FVIIIa binding were specific, saturable, and reversible. FVIII binds to specific, high affinity receptors on activated platelets (n = 484 +/- 59; K(d) = 3.7 +/- 0.31 nM) and FVIIIa interacts with an additional 300-500 sites per platelet with enhanced affinity (K(d) = 1.5 +/- 0.11 nM). FVIIIa binding to activated platelets in the presence of FIXa and FX is closely coupled with rates of F-X activation. The presence of EGR-FIXa and FX increases both the number and the affinity of binding sites on activated platelets for both FVIII and FVIIIa, emphasizing the validity of a three-receptor model in the assembly of the F-X-activating complex on the platelet surface.

Procoagulant expression in platelets and defects leading to clinical disorders

Hemostasis is a result of interactions between fibrillar structures in the damaged vessel wall, soluble components in plasma, and cellular elements in blood represented mainly by platelets and platelet-derived material. During formation of a platelet plug at the damaged vessel wall, factors IXa and VIIIa form the "tenase" complex, leading to activation of factor X on the surface of activated platelets. Subsequently, factors Xa and Va form the "prothrombinase" complex, which catalyzes the formation of thrombin from prothrombin, leading to fibrin formation. An enhanced expression of negatively charged phosphatidylserine in the outer membrane leaflet resulting from a breakdown of the phospholipid asymmetry is essential for the formation of the procoagulant surface. An ATP-driven and inward-acting aminophospholipid "translocase" and a "floppase" counterbalancing this have been postulated to maintain the dynamic state of phospholipid asymmetry. A phospholipid-nonspecific "scramblase," believed to be responsible for the fast breakdown of the asymmetry during cell activation, has recently been isolated from erythrocytes, cloned, and characterized. An intracellular calcium-binding segment and one or more thioesterified fatty acids are probably of importance for calcium-induced activation of this transporter protein. Cytosolic calcium ions also activate the calcium-dependent protease calpain associated with shedding of microvesicles from the transformed platelet membrane. These are shed with a procoagulant surface and with surface-exposed P-selectin from the alpha-granules. Theoretically, therefore, microvesicles can be involved in both coagulation and inflammation. Scott syndrome is probably caused by a defect in the activation of an otherwise normal scramblase, resulting in a relatively severe bleeding tendency. In Stormorken syndrome, the patients demonstrate a spontaneous surface expression of aminophospholipids. Activated platelets and the presence of procoagulant microvesicles have been demonstrated in several clinical conditions, such as thrombotic and idiopathic thrombocytopenia, disseminated intravascular coagulation, and HIV-1 infection, and have been found to be associated with fibrin in thrombosis. Procoagulant microvesicles may also be formed from other cells as a result of apoptosis.

Use of surface plasmon resonance for studies of protein-protein and protein-phospholipid membrane interactions. Application to the binding of factor VIII to von Willebrand factor and to phosphatidylserine-containing membranes

The surface plasmon resonance phenomenon is used for real time measurements of protein-protein and protein-membrane interactions. In the present study two surface plasmon resonance-based binding assays permitting study of the interaction of coagulation factor VIII (fVIII) with von Willebrand factor (vWf) and phospholipid have been developed. These interactions of fVIII are required for maintenance of fVIII concentration in circulation and for the assembly of the functional factor Xase complex, respectively. With these binding assays, the role of the light chain (LCh) in fVIII binding to vWf and to immobilized phospholipid monolayers and intact vesicles containing 25% phosphatidylserine (PS) and 4% PS was examined. The finding that Kd of LCh binding to vWf (3.8 nM) is 9.5 times higher than that of fVIII (0.4 nM), indicates that the heavy chain (HCh) is required for the maximal affinity of fVIII for vWf. In contrast, affinities of LCh for 25/75 PS/phosphatidylcholine (PC) monolayers and 4/76/20 PSPC-phosphatidylethanolamine (PE) vesicles are similar to that of fVIII, indicating that LCh is solely responsible for these interactions. It was also examined how removal of the acidic region affects the binding affinity of the remaining part of LCh for vWf and phospholipid. It was demonstrated that the loss of the LCh acidic region upon thrombin cleavage leads to an 11 and 160-fold increase in the dissociation rate constant (k(off) value) and a 165 and 1500-fold increase in the Kd value of the binding of fVIII fragment A3-C1-C2 to vWf compared to that of LCh and fVIII, respectively. In contrast, the binding affinity of A3-C1-C2 for PS-containing membranes was 8-11-fold higher than that of LCh. Possible conformational change(s) in C2 domain upon removal of the acidic region were studied using anti-fVIII monoclonal antibody NMC-VIII/5 with an epitope within the C2 domain of LCh as a probe. The determined lower binding affinity of A3-C1-C2 for NMC-VIII/5 immobilized to a sensor chip than that of LCh, indicates that these conformational changes do occur.

The second epidermal growth factor-like domain of human factor IXa mediates factor IXa binding to platelets and assembly of the factor X activating complex

Factor IXa binding to the activated platelet surface is required for efficient catalysis of factor X activation. Platelets possess a specific binding site for factor IXa, occupancy of which has been correlated with rates of factor X activation. However, the specific regions of the factor IXa molecule that are critical to this interaction have not yet been fully elucidated. To assess the importance of the second epidermal growth factor (EGF2) domain of factor IXa for platelet binding and catalysis, a chimeric protein (factor IXa(Xegf2)) was created by replacement of the EGF2 domain of factor IX with that of factor X. Competition binding experiments showed 2 different binding sites on activated platelets (approximately 250 each/platelet): (1) a specific factor IXa binding site requiring the intact EGF2 domain; and (2) a shared factor IX/IXa binding site mediated by residues G(4)-Q(11) within the Gla domain. In kinetic studies, the decreased V(max) of factor IXa(Xegf2) activation of factor X on the platelet surface (V(max) 2. 90 +/- 0.37 pM/min) versus normal factor IXa (37.6 +/- 0.15 pM/min) was due to its decreased affinity for the platelet surface (K(d) 64.7 +/- 3.9 nM) versus normal factor IXa (K(d) 1.21 +/- 0.07 nM), resulting in less bound enzyme (functional complex) under experimental conditions. The hypothesis that the binding defects of factor IXa(Xegf2) are the cause of the kinetic perturbations is further supported by the normal k(cat) of bound factor IXa(Xegf2) (1701 min(-)(1)) indicating (1) an intact catalytic site and (2) the normal behavior of bound factor IXa(Xegf2). The EGF2 domain is not a cofactor binding site since the mutant shows a normal rate enhancement upon the addition of cofactor. Thus, the intact EGF2 domain of factor IXa is critical for the formation of the factor X activating complex on the surface of activated platelets.

Activation of factor VIII by thrombin increases its affinity for binding to synthetic phospholipid membranes and activated platelets

Membrane-bound thrombin-activated factor VIII (fVIIIa) functions as a cofactor for factor IXa in the factor Xase complex. We found that binding of heterotrimeric fVIIIa (A1.A2.A3-C1-C2) to synthetic vesicles with a physiologic content of 4% phosphatidylserine (PS), 76% phosphatidylcholine, and 20% phosphatidylethanolamine occurs with a 10-fold higher affinity than that of factor VIII (fVIII). The increased affinity of fVIIIa for PS-containing membranes resulted from the reduced rate of fVIIIa dissociation from the vesicles compared with that of fVIII. Similar affinities of A3-C1-C2, A1.A2. A3-C1-C2, and A3-C1-C2.heavy chain for interaction with PS-containing membranes demonstrate that removal of the light chain (LCh) acidic region by thrombin is responsible for these increased affinities of fVIIIa and its derivatives. Similar kinetic parameters of fVIII and its LCh and C2 domain for binding to PS-containing membranes and to activated platelets indicated that the C2 domain is entirely responsible for the interaction of fVIII with membranes. We conclude that the increased fVIIIa affinity for PS-containing membranes is a result of conformational change(s) within the C2 domain upon removal of the acidic region of the LCh. This conclusion is based on the finding that binding of the monoclonal antibody ESH8 to the C2 domain, which is known to prevent this conformational transition, resulted in fVIIIa binding to PS/phosphatidylcholine/phosphatidylethanolamine vesicles (4/76/20) with a lower affinity similar to that of fVIII. In addition, stabilization of the low affinity binding conformation of the C2 domain of fVIIIa by this antibody led to an inhibition of the fVIIIa activity in the factor X activation complex.

Expression of Proteins Controlling Transbilayer Movement of Plasma Membrane Phospholipids in the B Lymphocytes From a Patient With Scott Syndrome

Scott syndrome is a rare inherited bleeding disorder in which platelets and other blood cells fail to promote normal assembly of the membrane-stabilized proteases of the plasma coagulation system. The defect in Scott blood cells is known to reflect inability to mobilize phosphatidylserine from inner plasma membrane leaflet to the cell surface in response to an elevation of Ca2+ at the endofacial surface. To gain insight into the molecular basis of this membrane defect, we examined the expression in Scott cells of plasma membrane proteins that have been implicated to participate in the accelerated transbilayer movement of plasma membrane PL. By both reverse transcriptase-polymerase chain reaction (RT-PCR) and functional assay, the level of expression of the multidrug resistance (MDR)1 and MDR3 P-glycoproteins in immortalized B-lymphoblast cell lines from the patient with Scott syndrome were indistinguishable from matched cell lines derived from normal controls. Whereas the plasma membrane of Scott cells are insensitive to activation of the plasma membrane PL scramblase pathway, it had been shown that PL scramblase protein isolated from detergent-solubilized Scott erythrocytes exhibits normal function when incorporated into proteoliposomes (Stout JG, Basse F, Luhm RA, Weiss HJ, Wiedmer T, Sims PJ: J Clin Invest 99:2232, 1997). Consistent with this finding in Scott erythrocytes, we found that Scott lymphoblasts expressed normal levels of PL scramblase mRNA and protein, and that the deduced sequence of PL scramblase in Scott cells is identical to that of normal controls. These data suggest that the defect in Scott syndrome is related either to aberrant posttranslational processing of the PL scramblase polypeptide or to a defect or deficiency in an unknown cofactor that is required for normal expression of plasma membrane PL scramblase function in situ, or alternatively, reflects the presence of a detergent-dissociable inhibitor of this pathway.

© 1998 by The American Society of Hematology.

Expression of Proteins Controlling Transbilayer Movement of Plasma Membrane Phospholipids in the B Lymphocytes From a Patient With Scott Syndrome

Scott syndrome is a rare inherited bleeding disorder in which platelets and other blood cells fail to promote normal assembly of the membrane-stabilized proteases of the plasma coagulation system. The defect in Scott blood cells is known to reflect inability to mobilize phosphatidylserine from inner plasma membrane leaflet to the cell surface in response to an elevation of Ca2+ at the endofacial surface. To gain insight into the molecular basis of this membrane defect, we examined the expression in Scott cells of plasma membrane proteins that have been implicated to participate in the accelerated transbilayer movement of plasma membrane PL. By both reverse transcriptase-polymerase chain reaction (RT-PCR) and functional assay, the level of expression of the multidrug resistance (MDR)1 and MDR3 P-glycoproteins in immortalized B-lymphoblast cell lines from the patient with Scott syndrome were indistinguishable from matched cell lines derived from normal controls. Whereas the plasma membrane of Scott cells are insensitive to activation of the plasma membrane PL scramblase pathway, it had been shown that PL scramblase protein isolated from detergent-solubilized Scott erythrocytes exhibits normal function when incorporated into proteoliposomes (Stout JG, Basse F, Luhm RA, Weiss HJ, Wiedmer T, Sims PJ: J Clin Invest 99:2232, 1997). Consistent with this finding in Scott erythrocytes, we found that Scott lymphoblasts expressed normal levels of PL scramblase mRNA and protein, and that the deduced sequence of PL scramblase in Scott cells is identical to that of normal controls. These data suggest that the defect in Scott syndrome is related either to aberrant posttranslational processing of the PL scramblase polypeptide or to a defect or deficiency in an unknown cofactor that is required for normal expression of plasma membrane PL scramblase function in situ, or alternatively, reflects the presence of a detergent-dissociable inhibitor of this pathway.

© 1998 by The American Society of Hematology.

Impaired redistribution of aminophospholipids with distinctive cell shape change during Ca2+-induced activation of platelets from a patient with Scott syndrome

We have investigated phospholipid redistribution, membrane vesicle shedding, shape change, and granule release following A23187 activation of platelets from a patient with Scott syndrome, characterized by impaired transmembrane migration of phosphatidylserine (PS) accompanied by haemorrhagic complications, and two of her children. Electron spin resonance spectroscopy measurement of phospholipids redistribution showed that the internalization of PS was unaffected by the disorder but, after activation, PS exposure was significantly reduced in platelets from the homozygous-type patient. Vesicle shedding was also reduced in these platelets. However, the slow redistribution of phosphatidylcholine was similar to that observed in normal platelets. When treated with calpeptin, platelets from the homozygous-type patient, unlike normal or heterozygous Scott syndrome platelets, showed a smoothly rounded shape without filopods after activation. Following A23187 activation of normal platelets, filopod formation was consecutive to the re-exposition of aminophospholipids on the outer leaflet of the plasma membrane, and the existence of a floppase (outward aminoPLs translocase) has been suggested. In homozygous Scott syndrome platelets the deficiency in PS re-exposition, the absence of filopod formation, and low vesicle shedding are correlated with each other, and argue in favour of a disruption of the proposed floppase activity.

Impaired Ca2+-Induced Tyrosine Phosphorylation and Defective Lipid Scrambling in Erythrocytes From a Patient With Scott Syndrome: A Study Using an Inhibitor for Scramblase That Mimics the Defect in Scott Syndrome

Scott syndrome is an hereditary bleeding disorder characterized by a deficiency in platelet procoagulant activity. Unlike normal blood cells, Scott platelets, as well as erythrocytes and lymphocytes, are strongly impaired in their ability to scramble their membrane phospholipids when challenged with Ca2+. In normal cells this collapse of membrane asymmetry leads to surface exposure of phosphatidylserine. Here we report that Scott erythrocytes show an apparent defect in tyrosine phosphorylation on treatment with Ca2+-ionophore. Diminished tyrosine phosphorylation was also apparent in activated Scott platelets, but much less pronounced than observed in red blood cells. On the other hand, tyrosine phosphorylation profiles observed in Scott red blood cell ghosts after sealing in the presence of adenosine triphosphate (ATP) were indistinguishable from those obtained from normal ghosts. Several observations argue in favor of a mechanism in which tyrosine phosphorylation in red blood cells is facilitated by, rather than required for scrambling of membrane lipids. Staurosporin blocks tyrosine phosphorylation in normal red blood cells, but does not inhibit the lipid scrambling process. White ghosts from normal erythrocytes, resealed in the absence of ATP, exhibit Ca2+-induced lipid scrambling without tyrosine phosphorylation. A selective inhibitor of Ca2+-induced lipid scrambling also showed an apparent inhibition of tyrosine phosphorylation in ionophore-treated normal red blood cells, similar to that observed in Scott erythrocytes. While this inhibitor also suppressed Ca2+-induced lipid scrambling in ghosts that were sealed in the presence of ATP, it did not inhibit tyrosine kinase activity. We conclude that the apparent deficiency in tyrosine phosphorylation in Scott cells is an epiphenomenon, possibly associated with a defect in phospholipid scrambling, but not causal to this defect.

Impaired Ca2+-Induced Tyrosine Phosphorylation and Defective Lipid Scrambling in Erythrocytes From a Patient With Scott Syndrome: A Study Using an Inhibitor for Scramblase That Mimics the Defect in Scott Syndrome

Scott syndrome is an hereditary bleeding disorder characterized by a deficiency in platelet procoagulant activity. Unlike normal blood cells, Scott platelets, as well as erythrocytes and lymphocytes, are strongly impaired in their ability to scramble their membrane phospholipids when challenged with Ca2+. In normal cells this collapse of membrane asymmetry leads to surface exposure of phosphatidylserine. Here we report that Scott erythrocytes show an apparent defect in tyrosine phosphorylation on treatment with Ca2+-ionophore. Diminished tyrosine phosphorylation was also apparent in activated Scott platelets, but much less pronounced than observed in red blood cells. On the other hand, tyrosine phosphorylation profiles observed in Scott red blood cell ghosts after sealing in the presence of adenosine triphosphate (ATP) were indistinguishable from those obtained from normal ghosts. Several observations argue in favor of a mechanism in which tyrosine phosphorylation in red blood cells is facilitated by, rather than required for scrambling of membrane lipids. Staurosporin blocks tyrosine phosphorylation in normal red blood cells, but does not inhibit the lipid scrambling process. White ghosts from normal erythrocytes, resealed in the absence of ATP, exhibit Ca2+-induced lipid scrambling without tyrosine phosphorylation. A selective inhibitor of Ca2+-induced lipid scrambling also showed an apparent inhibition of tyrosine phosphorylation in ionophore-treated normal red blood cells, similar to that observed in Scott erythrocytes. While this inhibitor also suppressed Ca2+-induced lipid scrambling in ghosts that were sealed in the presence of ATP, it did not inhibit tyrosine kinase activity. We conclude that the apparent deficiency in tyrosine phosphorylation in Scott cells is an epiphenomenon, possibly associated with a defect in phospholipid scrambling, but not causal to this defect.

Inhibition of arterial thrombosis by recombinant annexin V in a rabbit carotid artery injury model

BACKGROUND

The procoagulant effect of anionic phospholipid may play a major role in the development of arterial thrombosis.

METHODS AND RESULTS

Annexin V, a calcium-dependent anionic-phospholipid-binding protein, was expressed and isolated from Escherichia coli and its antithrombotic effect examined in a rabbit carotid artery thrombosis model. A partially occlusive thrombus was formed in the left carotid artery by application of electric current to produce an approximately 50% occlusion of the lumen. After the current was discontinued, flow ceased completely within 42+/-12 minutes (n=6) because of continuing platelet/fibrin thrombus formation. When annexin V was given at doses of 2.8 to 16.6 microg x kg(-1) x min(-1) for a period of 180 minutes, starting at the time the current was stopped, there was a dose-dependent inhibition of thrombus formation. At a dose of 5.6 microg x kg(-1) x min(-1), blood flow remained patent throughout the infusion and for an additional 60 minutes after the infusion was stopped. In addition, there was a decrease in thrombus weight (16+/-7.4 versus 2.0+/-1.0 g), (125)I-fibrin deposition (approximately 45% reduction, P<.001), and (111)In-labeled platelet accumulation (approximately 43% reduction, P<.001). Prior mixing of annexin V with phosphatidylserine micelles abolished the antithrombotic effect of annexin V, whereas mixing with phosphatidylcholine micelles had no effect. The antithrombotic effect of annexin V was not associated with bleeding tendency, as judged by the amount of blood absorbed in a gauze pad placed in a surgical incision extending to the muscle tissue and by the standard template bleeding time.

CONCLUSIONS

These observations support a potentially important role for anionic phospholipid exposure in platelets in arterial thrombosis, and inhibition of this activity could be a novel target for therapy in coronary thrombosis and stroke and after angioplasty.

Partial activation of the factor VIIIa-factor IXa enzyme complex by dihexanoic phosphatidylserine at submicellar concentrations

Phosphatidylserine (PS)-containing membranes increase the kcat of the factor VIIIa-factor IXa enzyme complex by more than 1000-fold. While PS supports specific, high-affinity membrane binding of factor VIIIa and factor IXa, it is not known whether PS is the lipid that activates the membrane-bound complex. It is also not known whether PS or other activating lipids must reside in the two-dimensional membrane matrix for efficacy. We have found that submicellar concentrations of dihexanoic phosphatidylserine (C6PS) increase the activity of the factor VIIIa-factor IXa complex in a biphasic manner with half-maximal concentrations of 0.2 and 1.6 mM while the micelle-forming concentration is 4.0 mM. Increased cleavage of factor X at 0.25 mM C6PS was due to a 25-fold enhancement of the kcat and a 30-fold increase in the affinity of factor VIIIa for factor IXa. C6 phosphatidylethanolamine and C6 phosphatidic acid, but not C6 phosphatidylcholine, also accelerated the Xase complex, indicating that kcat enhancement has less structural specificity than membrane binding. Submicellar C6PS enhanced activity of factor IXa in the absence of factor VIIIa, but the effect was due to a decreased KM rather than an increased kcat. These results suggest that activation of the factor VIIIa-factor IXa complex can result from binding of individual C6PS molecules or small aggregates in the absence of a membrane bilayer. They provide a model system in which the phospholipid-induced activation may be distinguished from membrane-binding of the enzyme complex.

Scott syndrome erythrocytes contain a membrane protein capable of mediating Ca2+-dependent transbilayer migration of membrane phospholipids

Phospholipid (PL) scramblase is a plasma membrane protein that mediates accelerated transbilayer migration of PLs upon binding Ca2+, facilitating rapid mobilization of phosphatidylserine to the cell surface upon elevation of internal Ca2+. In patients with Scott syndrome, a congenital bleeding disorder related to defective expression of membrane coagulant activity, circulating blood cells show decreased cell surface exposure of phosphatidylserine at elevated cytosolic [Ca2+], implying an underlying defect or deficiency of PL scramblase. To gain insight into the molecular basis of this disorder, we compared PL scramblase in Scott erythrocyte membranes to those of normal controls. Whereas membranes of Scott cells were unresponsive to Ca2+-induced activation of PL scramblase at neutral pH, apparently normal PL scramblase activity was induced at pH < 6.0. After extraction with octylglucoside, a membrane protein was isolated from the Scott cells which exhibited normal PL scramblase activity when reconstituted in vesicles with exogenous PLs. Like PL scramblase from normal erythrocytes, PL scramblase from Scott erythrocytes was maximally activated either by addition of Ca2+ (at pH 7.4) or by acidification to pH < 6.0, and similar apparent affinities for Ca2+ and rates of transbilayer transfer of PLs were observed. This suggests that the defect in Scott syndrome is related to an altered interaction of Ca2+ with PL scramblase on the endofacial surface of the cell membrane, due either to an intrinsic constraint upon the protein preventing interaction with Ca2+ in situ, or due to an unidentified inhibitor or cofactor in the Scott cell that is dissociated by detergent.

Platelet Prothrombinase Activity and Intracellular Calcium Responses in Patients With Storage Pool Deficiency, Glycoprotein IIb-IIIa Deficiency, or Impaired Platelet Coagulant Activity — A Comparison With Scott Syndrome

The procoagulant activity of platelets induced by collagen, thrombin, and collagen plus thrombin, measured as their capacity to promote the conversion of prothrombin to thrombin in the presence of factors Va and Xa, was studied in patients with α, αδ, and δ storage pool deficiency (SPD), thrombasthenia, and in two new patients with isolated defects in platelet coagulant activity, and compared with that in Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in Scott syndrome, is an impairment in collagen plus thrombin–induced prothrombinase activity in the absence of added factor Va. In one of these patients this may be caused by an abnormality in platelet α-granule factor V distinct from that described for factor V Quebec, αδ-SPD, or α-SPD (gray platelet syndrome). Prothrombinase activity in response to all agonists was impaired in δ-SPD and was associated with an inability of these platelets to maintain elevated intracellular calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired prothrombinase activation in δ-SPD platelets were corrected by the addition of adenosine diphosphate (ADP) after stimulation. These findings suggest that secreted ADP may play an important role in the generation of prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of ADP which can contribute to calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and prothrombinase responses were also observed in thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased prothrombinase activity in all but one atypical patient, was seen in stirred collagen plus thrombin-activated platelets. Although the parallel alterations in these responses in thrombasthenia, as in SPD, further show the close association between the generation of prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a Scott syndrome defect in normal platelets may provide a new approach to antithrombotic therapy.

Platelet Prothrombinase Activity and Intracellular Calcium Responses in Patients With Storage Pool Deficiency, Glycoprotein IIb-IIIa Deficiency, or Impaired Platelet Coagulant Activity — A Comparison With Scott Syndrome

The procoagulant activity of platelets induced by collagen, thrombin, and collagen plus thrombin, measured as their capacity to promote the conversion of prothrombin to thrombin in the presence of factors Va and Xa, was studied in patients with α, αδ, and δ storage pool deficiency (SPD), thrombasthenia, and in two new patients with isolated defects in platelet coagulant activity, and compared with that in Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in Scott syndrome, is an impairment in collagen plus thrombin–induced prothrombinase activity in the absence of added factor Va. In one of these patients this may be caused by an abnormality in platelet α-granule factor V distinct from that described for factor V Quebec, αδ-SPD, or α-SPD (gray platelet syndrome). Prothrombinase activity in response to all agonists was impaired in δ-SPD and was associated with an inability of these platelets to maintain elevated intracellular calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired prothrombinase activation in δ-SPD platelets were corrected by the addition of adenosine diphosphate (ADP) after stimulation. These findings suggest that secreted ADP may play an important role in the generation of prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of ADP which can contribute to calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and prothrombinase responses were also observed in thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased prothrombinase activity in all but one atypical patient, was seen in stirred collagen plus thrombin-activated platelets. Although the parallel alterations in these responses in thrombasthenia, as in SPD, further show the close association between the generation of prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a Scott syndrome defect in normal platelets may provide a new approach to antithrombotic therapy.

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.

Factor X bound to the surface of activated human platelets is preferentially activated by platelet-bound factor IXa

Factor X is a zymogen in the blood coagulation system which is activated by the serine protease, factor IXa, in a reaction that is promoted by the presence of stimulated platelets. We have shown previously that platelets possess a binding site for factor IXa, the occupancy of which is correlated with the rate of factor X activation (Ahmad et al., 1989b,c). Similarly, we have described a different binding site on the surface of activated platelets to which the substrate for this reaction, factor X, can bind (see the accompanying paper). This "zymogen binding site" is of moderate affinity and is relatively nonspecific; apparently shared to some degree by factor X and other vitamin K-dependent proteins, most notably prothrombin. We have found that prothrombin fragment 1 not only is able to displace factor X from this platelet binding site but also possesses the ability to inhibit the platelet-dependent activation of factor X. We have developed two mathematical models for the activation of factor X by platelet-bound factor IXa. The first model assumes that factor X is activated in a manner that is totally unrelated to the presumptive zymogen binding site, whereas the second model requires factor X to first bind to this site before it may interact with platelet-bound factor IXa and become activated. Within the context of each of these models, we have evaluated three mechanisms by which prothrombin fragment 1 may inhibit factor X activation. The data presented herein are most consistent with the precept that platelet-bound factor X is activated by platelet-bound factor IXa (kcat approximately 0.0011 s-1) in an explicitly two-dimensional reaction (Km.2D approximately 230 molecules per platelet). Prothrombin fragment 1 is believed to disrupt this reaction by competing with factor X for the zymogen binding site (Ki approximately 470 nM) and, to a lesser degree, by displacing factor IXa from its binding site (Ki approximately 7 microM). These findings suggest that platelet-bound zymogen factor X represents a kinetically important pool of substrate that is preferentially activated on the surface of activated platelets.

The role of the second growth-factor domain of human factor IXa in binding to platelets and in factor-X activation

To study the structural requirements for factor IXa binding to platelets, we have carried out equilibrium binding studies with human factor IXa after replacing the second epidermal growth factor (EGF) domain by the corresponding polypeptide region of factor X. The chimeric protein, factor IX(Xegf2), and the wild-type, factor IXwt, produced in embryonic kidney cells 293 were radiolabelled with 125I and activated with factor XIa. Direct binding studies with thrombin-activated platelets showed normal stoichiometry and affinity of binding of factor IXawt in the presence of factor VIIIa (2 units/ml) and factor X (1.5 microM). However, under similar experimental conditions, factor IXa(Xegf2) was bound to a smaller number of sites (396 sites/platelet) with decreased affinity, i.e. a dissociation constant (Kd) of 1.4 nM, compared with normal factor IXa, factor IXaN (558 sites/platelet; Kd 0.67 nM), or factor IXawt (590 sites/platelet; Kd 0.61 nM). The concentrations of factor IXaN and factor IXawt required for half-maximal rates of factor-X activation were 0.63 nM and 0.7 nM, indicating a close correspondence of the Kd,app. for binding of factor IXawt to the factor-X activating complex on activated platelets to the Kd obtained in equilibrium binding studies. In contrast, kinetic parameters for factor-X activation by factor IXa(Xegf2) showed a decreased affinity (Kd 1.5 nM), in agreement with results of binding studies. These studies with factor IX(Xegf2) suggest that the EGF-2 domain may be important for specific high-affinity factor IXa binding to platelets in the presence of factor VIIIa and factor X.

Calreticulin, an antithrombotic agent which binds to vitamin K-dependent coagulation factors, stimulates endothelial nitric oxide production, and limits thrombosis in canine coronary arteries

Coagulation Factor IX/IXa has been shown to bind to cellular surfaces, and Factor IXa expresses its procoagulant activity by assembling into the intrinsic Factor X activating complex (Factors IXa/VIIIa/X), which also forms on membrane surfaces. This led us to identify cellular proteins which bind Factor IX/IXa; an approximately 55-kDa polypeptide was purified to homogeneity from bovine lung extracts based on its capacity to bind 125I-Factor IX in a dose-dependent and saturable manner. From protein sequence data of the amino terminus and internal peptides, the approximately 55-kDa polypeptide was identified as calreticulin, a previously identified intracellular calcium-binding protein. Recombinant calreticulin bound vitamin K-dependent coagulation factors, 125I-Factor IX, 125I-Factor X, and 125I-prothrombin (Kd values of approximately 2.7, 3.2, and 8.3 nM, respectively), via interaction with its C-domain, although it did not affect the coagulant properties of these proteins. 125I-Calreticulin also bound to endothelial cells in vitro (Kd approximately 7.4 nM), and mouse infusion studies showed an initial rapid phase of clearance in which calreticulin could be localized on the vascular endothelium. Exposure of endothelial cells to calreticulin led to dose-dependent, immediate, and sustained increase in the production of nitric oxide, as measured using a porphyrinic microsensor. In a canine electrically induced thrombosis model, intracoronary infusion of calreticulin (n = 7) prevented occlusion of the left circumflex coronary artery in a dose-dependent manner compared with vehicle-treated controls (n = 5). These results indicate that calreticulin interacts with the endothelium to stimulate release of nitric oxide and inhibit clot formation.

Production and characterization of transformed B-lymphocytes expressing the membrane defect of Scott syndrome

Scott syndrome is a bleeding disorder associated with an isolated defect in expression of membrane coagulant activity by stimulated platelets. This defect represents a decrease in platelet membrane binding sites for coagulation factors Va and VIIIa, reflecting diminished surface exposure of phosphatidylserine (PS). To gain insight into the cellular and genetic basis for this disorder, B-lymphocytes from a patient with Scott syndrome and from normal donors were immortalized by EBV-transformation, and tested for their capacity to expose plasma membrane PS in response to the Ca2+ ionophore, A23187. Upon incubation with A23187, EBV-lymphoblasts derived from normal donors consistently induced surface expression of PS in > 70% of all cells, as detected by membrane association of the PS-binding proteins, factor Va or annexin V. PS exposure in these cells was maximal after 5 min, and saturated at < 100 microM external free [Ca2+]. By contrast, < 30% of Scott syndrome lymphoblasts exposed PS, and saturation was not observed at > 1 mM external free [Ca2+]. Single-cell clones derived from the Scott lymphoblasts all exhibited a diminished response to A23187 comparable with that of the parental cells, suggesting that all lymphocytes from this patient share this membrane abnormality. Hybridomas prepared by fusion of Scott lymphoblasts with the myeloma cell line UC-LUC showed responses to Ca2+ ionophore comparable to those observed for normal lymphoblasts and for hybridomas prepared by fusion of normal lymphoblasts with UC-LUC. This correction of the Scott abnormality suggests possible complementation of an aberrant gene(s) responsible for this disorder.

High-affinity, specific factor IXa binding to platelets is mediated in part by residues 3-11

To identify the amino acids in the Gla domain that mediate factor IXa binding to human platelets, we have used chimeric molecules and point mutations in the Gla domain of recombinant factor IX, based on molecular modeling using the coordinates of the Gla domain of bovine prothrombin, which reveals two surface structures whose sequences differ among factor IX, factor X, and factor VII. Binding to thrombin-activated platelets of factor IXa in the presence of factor VIIIa (2 units/mL) and factor X (1.5 microM) revealed a stoichiometry of approximately 550 sites per platelet with a Kd of approximately 0.65 nM compared with a Kd of approximately 2.5 nM in the absence of factor VIIIa and factor X. In contrast, mutations of factor IX to factor X residues at positions 4 and 5 or at positions 9, 10, and 11 results in decreases in the number of sites and affinity of factor IXa binding in the presence or absence of factor VIIIa and factor X. A chimera consisting of the Gla domain of factor VII with factor IX residues at positions 33, 34, 35, 39, and 40 displayed abnormal factor IXa binding and a decreased Vmax and a normal Km for factor X activation, and the replacement of amino acid residues 3-10 with those of factor IX restored normal binding and factor X activation kinetics to this chimeric protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial-dependent procoagulant and anticoagulant mechanisms. Recent advances in understanding

Modulation of endothelial cell coagulant function is one of a group of changes common to many cytokine-mediated events. Changes that 1) cause migration of leukocytes, 2) increase vascular permeability, and 3) increase the thrombotic potential occur at atherosclerotic arterial branch points, in tumor vasculature, and at sites of inflammation. Regulation of procoagulant activity on the luminal surface of the vessel is crucial and is achieved by presentation of a predominantly anticoagulant surface on the endothelium. Inflammatory mediators can cause a decrease in the expression of the anticoagulant mechanisms and up-regulation of the procoagulant tissue factor. However, under these conditions very little tissue factor is exposed to the blood; instead it is sequestered under the endothelium and presumably becomes exposed only when significant vascular damage is present. Inhibition of intravascular coagulation by factor IXai without impairment of extravascular hemostasis suggests that when tissue factor concentrations are low, the continued generation of factor Xa is dependent on the presence of factor IXa. The demonstration that the blockade of factor IXa is selective for prevention of intravascular thrombus formation suggests a new means for managing intravascular thrombosis without altering the normal hemostatic mechanisms.

Active site-blocked factor IXa prevents intravascular thrombus formation in the coronary vasculature without inhibiting extravascular coagulation in a canine thrombosis model

To assess the contribution of Factor IX/IXa, to intravascular thrombosis, a canine coronary thrombosis model was studied. Thrombus formation was initiated by applying current to a needle in the circumflex coronary artery. When 50% occlusion of the vessel developed, the current was stopped and animals received an intravenous bolus of either saline, bovine glutamyl-glycyl-arginyl-Factor IXa (IXai), a competitive inhibitor of Factor IXa assembly into the intrinsic Factor X activation complex, bovine Factor IX, or heparin. Animals receiving saline or Factor IX developed coronary occlusion due to a fibrin/platelet thrombus in 70 +/- 11 min. In contrast, infusion of IXai prevented thrombus formation completely (greater than 180 min) at doses of 460 and 300 micrograms/kg, and partially blocked thrombus formation at 150 micrograms/kg. IXai attenuated the accumulation of 125I-fibrinogen/fibrin at the site of the thrombus by approximately 67% (P less than 0.001) and resulted in approximately 26% decrease in serotonin release from platelets in coronary sinus (P less than 0.05). Hemostatic variables in animals receiving IXai, remained within normal limits. Animals given heparin in a concentration sufficient to prevent occlusive thrombosis had markedly increased bleeding, whereas heparin levels that maintained extravascular hemostasis did not prevent intracoronary thrombosis. This suggests that Factor IX/IXa can contribute to thrombus formation, and that inhibition of IXa participation in the clotting mechanism blocks intravascular thrombosis without impairing extravascular hemostasis.