(Received for publication, July 17, 1996, and in revised form, January 28, 1997)
From the Cell & Molecular Biology Program and
¶ Department of Biochemistry, University of Vermont, College
of Medicine, Burlington, Vermont 05405 and the
Department of
Pathology, Yale University, School of Medicine,
New Haven, Connecticut 06536
At sites of vascular injury thrombin is generated via prothrombinase, a stoichiometric (1:1), Ca2+-dependent, and membrane-bound complex consisting of the nonenzymatic cofactor factor Va and the serine protease factor Xa. While the importance of anionic platelet membrane phospholipids in regulating thrombin generation is well recognized, the identification of regulatory protein receptors has eluded investigators. This study reports the first description of a human platelet membrane protein that regulates prothrombinase complex assembly and function. Direct platelet-protein binding studies indicated that, although required, platelet-bound factor Va alone is insufficient to mediate factor Xa binding, and that factor Va and factor Xa bind to discrete sites on activated platelets for which expression is independently regulated as a function of the agonist concentration. When specific monoclonal antibodies against effector cell protease receptor-1 (EPR-1, a 65-kDa membrane receptor for factor Xa) were used in Western blotting, immunohistochemical staining, and/or flow cytometric analyses, activated platelets and their precursors, megakaryocytes, were shown to express EPR-1. These results were confirmed by reverse transcription-polymerase chain reaction of mRNA extracted from megakaryocyte-like cell lines. Additional flow cytometric studies demonstrated that a platelet-bound factor Va/factor Xa complex precluded binding of the anti-EPR-1 antibody, B6, to activated platelets by ~50%. Likewise, the anti-EPR-1 antibody was shown to inhibit prothrombinase-catalyzed thrombin generation on activated platelets in a dose- and platelet donor-dependent manner, indicating that platelet-expressed EPR-1 mediates factor Xa assembly into the prothrombinase complex. These collective data indicate that both EPR-1 and membrane-bound factor Va are required to mediate factor Xa binding to the activated platelet to form a functional prothrombinase complex.
Platelets, through their adhesive properties in concert with their ability to contribute to and regulate thrombin generation, are central to the (patho)physiological processes of hemostasis, thrombosis, and atherosclerosis. In normal hemostasis, subsequent to adherence at a site of vascular injury, platelets play a pivotal role by localizing, regulating, and modulating catalytic activities within the vascular system, which result in thrombin generation and the formation of a platelet-fibrin plug at the injured site. Activated platelets promote the catalysis of two sequential, homologous, and Ca2+-dependent reactions in coagulation: the proteolytic conversion of factor X to factor Xa catalyzed by the intrinsic tenase complex, a membrane-bound complex consisting of the nonenzymatic cofactor factor VIIIa and the serine protease factor IXa; and the proteolytic conversion of prothrombin to thrombin catalyzed by the prothrombinase complex, a membrane-bound complex consisting of the nonenzymatic cofactor factor Va and the serine protease factor Xa (1). The high degree of homology between the intrinsic tenase and prothrombinase complexes is underscored by the observations that the serine proteases factor IXa and factor Xa (2) and the procofactors factor VIII and factor V (3) are structurally homologous, as well as the observations that both factor VIIIa (4) and factor Va (5) are essential to complex assembly and function by virtue of their membrane-binding capabilities to form at least part of the receptor for their respective serine proteases at the human platelet surface.
Binding of the protein components of each enzymatic complex to an
appropriate membrane surface is an obligate requirement for
physiologically relevant catalysis (1). It is well known that synthetic
phospholipid vesicles support the assembly of both complexes with
functional activities equal to those provided by the complexes bound to
activated platelets (6). However, activated platelets appear to express
high affinity (1 × 10
9 M), saturable
binding sites that are specific for the various protein components (5,
7-15). To date, studies detailing the direct binding of factor VIIIa
to platelets have not been performed. Furthermore, since platelets
contain a large reserve of factor V, which is released and activated to
factor Va subsequent to thrombin-induced platelet activation (16),
direct factor Va binding measurements are difficult to interpret. In
contrast, extensive studies have been performed detailing the binding
of factor IXa (7-9) and factor Xa (13-15) to the activated platelet surface. Ahmad and colleagues have demonstrated that factor IXa binds
to approximately 600 specific binding sites/activated platelet (7).
Prothrombin and factor X, at a 450-fold molar excess, minimally
displaced factor IXa binding (15% and 35%, respectively) (7). Similar
results have been obtained in analyses of factor Xa binding to
platelets. In these studies, factor X, factor IXa, and prothrombin did
not compete with factor Xa for platelet binding sites (13). These
combined studies strongly suggest that factor IXa and factor Xa
interact with discrete binding sites on thrombin-activated platelets.
Previous studies from our laboratory using analyses of kinetics of
prothrombin activation with both normal and factor V-deficient human
platelets indicate that both factors Va and Xa bind to ~3,000 functional sites on the surface of thrombin-activated platelets (17)
through formation of a 1:1 stoichiometric complex, governed by a
Kd 10
10 M and
expressing a kcat/site = 30 s
1. Furthermore, factor Xa binding is absolutely
dependent upon prebound factor Va (15, 17) indicating that factor Va
forms at least part of the receptor for factor Xa at the activated
platelet membrane surface.
Altieri and colleagues (18-20) have demonstrated that subpopulations
of peripheral blood leukocytes express
EPR-1,1 a 65-kDa membrane-spanning receptor
for factor Xa. EPR-1 expressed at the leukocyte surface appears to bind
factor Xa specifically in a Ca2+-dependent
reaction (n = 150,000, Kd 10-30
nM) (18). More recent studies have shown that EPR-1 is also
expressed by human endothelial cells and smooth muscle cells (21), as
well as cells expressed during Hodgkin's disease (22). Since factor Xa
binding to EPR-1 expressed by these cell populations occurs independently of factor Va, this binding interaction appears
to be important in initiating the intracellular signaling responses that are important in cell proliferation (20, 21) and perhaps the
immune response (23).
Most recently we have shown that EPR-1 plays an important role in mediating prothrombinase-catalyzed thrombin generation at the membrane surface of pericytes derived from normal, human brain microvessels (24). In the current study, we demonstrate that expression of EPR-1 plays an important role in regulating prothrombinase complex assembly and function on the surface of activated platelets. Based on published data from several groups of investigators, we propose a model for assembly of the functional complex on the activated platelet membrane in which the platelet receptor for factor Xa assembly into the prothrombinase complex consists minimally of EPR-1, membrane-bound factor Va, and the surrounding phospholipid environment.
L--Phosphatidyl-L-serine
(bovine brain) (PS), L-
-phosphatidylcholine (egg yolk)
(PC), and Arg-Gly-Asp-Ser (RGDS) peptide were purchased from
Sigma. HEPES was purchased from J.T. Baker. Recrystallized bovine serum albumin was from ICN ImmunoBiologicals. The
fluorescent
-thrombin inhibitor dansylarginine
N-(3-ethyl-1,5-pentanediyl)amide (DAPA) was obtained from
Haematologic Technologies. Phospholipid vesicles composed of 75% (w/w)
PC and 25% (w/w) PS were prepared as described (25).
Coagulation proteins
were isolated from human fresh-frozen plasma as described previously
(26). Protein purity was assessed by sodium dodecyl
sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) prior to and
following reduction with -mercaptoethanol as described by Laemmli
(27), and visualized using Coomasie Blue R-250. Protein concentrations
were determined by absorbance at 280 nm using the following molecular
weights and extinction coefficients,
280 nm1%,1 cm: factor V, 330,000, 9.6; factor Xa, 50,000, 11.6; prothrombin, 72,000, 14.2; thrombin,
37,000, 17.4; and mouse IgG, 150,000, 14.0. Factor Xa, the specific
monoclonal antibody directed against human factor V/Va,
-HFV#2, and
a nonspecific mouse IgG2a were labeled with 125I by the
IODOGEN transfer technique as described previously (26).
Washed platelets were routinely prepared from anticoagulated whole human blood as detailed previously (28). Platelets were suspended and maintained in 5 mM HEPES-buffered Tyrode's solution (0.137 M NaCl, 2.7 mM KCl, 12 mM NaHCO2, 0.36 mM NaH2PO4, 1 mM MgCl2, 2 mM CaCl2, 5 mM dextrose), pH 7.4, containing 0.35% bovine serum albumin (HTA). In some instances platelets were metabolically inhibited with 30 mM 2-deoxy-D-glucose, 10 mM D-gluconic acid lactone, and 50 µg/ml antimycin A (30 min, 37 °C) to deplete ATP stores as described previously (28).
Megakaryocytes were isolated from human bone marrow aspirates as described previously (29). The megakaryocyte-like cell lines CMK (30), Mo7 (31), HEL (32), and CHRF-288 (33) were cultured as detailed.
Anti-EPR-1 Monoclonal AntibodiesMonoclonal antibodies specific for EPR-1 (B6, 2E1, and 9D4), were generously provided by Dr. Dario C. Altieri (Yale University, New Haven, CT). B6 was provided as an ascites fluid, while 2E1 and 9D4 were supplied as purified immunoglobulins. Their production has been described elsewhere (18, 34). Even though the specificity of these antibodies for EPR-1 has been established (34, 35), their lack of reactivity with plasma- or platelet-derived factor V/Va was confirmed unequivocally using both enzyme-linked immunosorbent assay (36) and Western blotting technologies.
Prothrombinase Activity MeasurementsProthrombin activation mixtures consisting of purified components were monitored to determine how increasing concentrations of thrombin as agonist affected platelet factor Va release as well as expression of platelet prothrombinase activity. Subsequent to platelet isolation and washing, platelet suspensions (2 × l08/ml) were incubated with varying concentrations of thrombin (2 min, 22 °C, without stirring) followed by addition of the thrombin inhibitor, DAPA (3 µM) to quench the reaction. Expression of platelet prothrombinase activity was monitored as described in detail previously (17, 37). Briefly, subsequent to thrombin-induced platelet activation, equimolar concentrations of factor Va and factor Xa (10 nM) were added and allowed to incubate for 2 min. The reaction was initiated by the addition of 1.39 µM prothrombin. The initial rate of thrombin generation was monitored continuously using the reversible thrombin inhibitor DAPA (3 µM), which displays enhanced fluorescence upon binding to thrombin (38).
Alternatively, platelet releasates were prepared in analogous protocols, and released and activated platelet factor Va was separated from the platelets and assayed for cofactor activity as described in detail previously (39). Briefly, aliquots of platelet factor Va releasates (50 µl containing less than 1 nM factor Va) were assayed for cofactor activity using 20 µM PCPS, 5 nM factor Xa, 1.39 µM prothrombin, and 3 µM DAPA in 20 mM HEPES, 0.15 M NaCl, 5 mM Ca2+, pH 7.4. The factor Va concentration in each sample was determined by comparing the initial rate of thrombin generation with the initial rates obtained using known concentrations of factor Va.
For anti-EPR-1 antibody inhibition studies, washed platelets (1 × 108/ml in HTA containing 2 mM Ca2+) were preincubated with buffer and various dilutions of B6 ascites (1:50-1:10), or a nonspecific mouse ascites (30 min, 22 °C). Platelets were then activated with thrombin (30 nM, 2 min, 22 °C) and assayed immediately using 0.1 nM factor Xa and 1.39 µM prothrombin. No exogenous factor Va was added so that platelet-released factor Va was the sole factor Va source. Since small reaction volumes were used, platelet prothrombinase activity (rate of thrombin generated) was determined by discontinuous, two-stage assay as described previously (40).
CMK and CHRF-288 cells (1 × 105/ml in 20 mM HEPES, 0.15 M NaCl, 5 mM Ca2+, 0.1% bovine serum albumin, pH 7.4) were assayed for prothrombinase activity using 5 nM each factor Va and factor Xa and 1.39 µM prothrombin. The initial rate of thrombin generation was again determined by discontinuous, two-stage assays (40).
Radiolabeled Ligand Binding AssaysLigand binding assays
were used to determine how increasing concentration of thrombin as
agonist affected factor Va and factor Xa binding to the activated
platelet surface. Again platelets (1 × l08/ml in HTA
containing 5 mM Ca2+) were activated with
thrombin as detailed above. Factor Va binding to the activated platelet
surface was determined in the presence of 10 nM each added
factor Va and factor Xa, and 1.39 µM prothrombin using
100 nM 125I--HFV#2, which in separate
experiments was determined to be a saturating concentration. Likewise,
direct 125I-factor Xa binding (10 nM) was
determined in the presence of 10 nM factor Va and 1.39 µM prothrombin. Nonspecific binding/entrapment was
determined using an equimolar concentration of an
125I-labeled isotype-matched monoclonal antibody (IgG2a),
or using a 50 M excess of unlabeled factor Xa or 10 mM EDTA. Specific binding of the various ligands was
determined as described previously using a centrifugation through oil
technique (37). Similar protocols were used to assess factor Va and
factor Xa binding to HEL cells.
For SDS-PAGE, platelets,
megakaryocytes, and megakaryocyte-like cell lines were solubilized with
62.5 mM Tris-HCl, 2% SDS, 10% glycerol, 5%
-mercaptoethanol, 20 mM EDTA, and 0.001% bromphenol blue, pH 6.8, to yield final concentrations for platelets (1 × 109/ml) and for megakaryocytes (1 × 107/ml) and megakaryocyte-like cells (1 × 107/ml). Following heating (95 °C, 3 min), SDS-PAGE
analysis was performed on 5-15% gradient slab gels as described by
Laemmli (27). Subsequent to electrophoresis, Western blotting analyses were performed as described previously (39). Either B6 (1:50 dilution),
2E1 (25 µg/ml), or nonspecific mouse IgG or ascites (at comparable
amounts) was used as primary antibody. Detection of EPR-1 was performed
by enhanced chemiluminescence (Western Blot Chemiluminescence Detection
Kit, DuPont NEN) by exposure of blots to Kodak Scientific Imaging film
(X-Omat) developed in a Kodak M35A X-Omat processor.
Total RNA was extracted from HEL or CMK cells using the
guanidinium-isothiocyanate method (41). The following EPR-1
sequence-specific oligonucleotides were used for RT-PCR: forward T5/27
(nucleotides 161-184, 5-CGTGTGGAGAACGTGACAGATGTG-3
) and reverse
G5/27 (nucleotides 739-718, 5
-AATGTGTCTGGACCTCATGTTG-3
) (42).
Typically total RNA (
5 µg) was heated for 10 min at 68 °C and
then added to the reverse transcription reaction mix, which contained
50 mM Tris, pH 8.3, 75 mM KCl, 3 mM
MgCl2, 10 mM dithiothreitol, 0.5 mM
dNTPs, 0.8 unit/µl RNase inhibitor (RNasin, Promega Corp., Madison,
WI), and 10 units/µl Moloney murine leukemia virus reverse
transcriptase (Life Technologies, Inc.) in a final volume of 50 µl.
The reaction was incubated at 37 °C for 60 min, and then the entire
volume was diluted in a PCR reaction mixture containing supplemental KCl, gelatin, and sequence-specific oligonucleotides. The final PCR
reaction conditions in a 100-µl volume were 25 mM Tris,
pH 8.3, 50 mM KCl, 1.5 mM MgCl2,
0.01% gelatin, 5 mM dithiothreitol, 0.25 mM
dNTPs, 1.0 µM each forward and reverse primer, and 5 units of Taq polymerase (Perkin Elmer). The PCR reaction
mixture, minus the polymerase, was initially heated at 98 °C for 5 min and then at 54 °C for 5 min. Taq polymerase was added
and amplification was carried out in a Perkin Elmer Thermal Cycler
programmed to denature at 98 °C for 1.5 min, to anneal at 54 °C
for 1.5 min, and to chain extend at 72 °C for 3 min. The step-cycle
reaction was repeated 30 times with a 7-min final chain extension at
72 °C. PCR products were analyzed on a 1.25% agarose gel by
ethidium bromide staining. Sequencing of gel-purified DNA products
using sequence-specific primers was performed using the ABI PRISM Dye Teminator Cycle Sequencing Core Kit (Perkin Elmer) according to the
manufacturer's directions.
Paraffin-embedded bone marrow
sections (5 µm) were deparaffinized and rehydrated through a graded
alcohol series. Sections were incubated with 2%
H2O2/methanol (30 min, 22 °C), and then boiled in 0.01 M citrate (pH 6.0) (5 min). After rinsing in
H2O and phosphate-buffered saline (PBS), the sections were
incubated with 10% normal horse serum (20 min, 22 °C), followed by
the anti-EPR-1 monoclonal antibody 9D4 (5 µg/ml, 14 h, 4 °C)
(20, 34, 35). After washes in PBS, the sections were incubated with a
biotinylated horse anti-mouse IgG (30 min, 22 °C) (Vector
Laboratories, Burlingame, CA). After washes in PBS, the sections were
then incubated with streptavidin-biotin-conjugated peroxidase (30 min,
22 °C). The sections were washed with PBS, and positive
immunostaining was detected using 3,3-diaminobenzidine.
Metabolically inhibited or thrombin-activated platelets (5 × 107/ml, 50 µl) were incubated with B6 or nonspecific ascites (5 µl, 45 min, 22 °C). Thrombin activated platelets were initially preincubated with the RGDS peptide (1 mM, 1 h, 37 °C) to prevent thrombin-induced aggregation. The platelets were washed by dilution in HTA (1:10) followed by centrifugation (1000 × g, 3 min), and then incubated with 50 µl of goat anti-mouse IgG phycoerythrin (PE) conjugate (1:160 dilution, 45 min, 22 °C). The platelets were again washed in HTA followed by incubation with 50 µl of nonspecific mouse IgG (10 min, 22 °C), and an anti-glycoprotein IIb/IIIa monoclonal antibody, HP1-1D, conjugated to fluorescein isothiocyanate (FITC) (333 ng, 45 min, 22 °C). The platelets were washed three times and resuspended in HTA. Fluorescence from 1 × 104 platelets was analyzed on a Coulter EPICS Elite Flow cytometer (Hialeah, FL).
Initial experiments were done
to determine how the binding of factor Va and factor Xa to human
platelets was affected by increasing concentrations of thrombin (0-200
nM) as a platelet agonist. Because platelets contain stores
of factor V/Va, which contribute significantly to prothrombinase
complex assembly and function (16), a radioiodinated monoclonal
antibody directed against the light chain of human factor Va
(-HFV#2), which does not affect complex assembly and function and
binds with equal avidity to both platelet-released and plasma forms of
factor Va, was used to indirectly monitor factor Va/platelet binding
interactions. Factor Xa was radioiodinated directly. Incubation of
excess 125I-
-HFV#2 with platelets, with or without
increasing concentrations of thrombin as agonist, and in the presence
of saturating, equimolar concentrations of factor Va and factor Xa (10 nM), indicated that little, if any, factor Va bound to
unactivated platelets, whereas maximal binding (~6000
molecules/platelet) was achieved following platelet activation with 0.5 nM thrombin (Fig. 1A). As might
be expected, factor Va binding to platelets correlated directly with its release from
-granule stores (Fig. 1B). However, the
amount of platelet-derived cofactor released was not sufficient to
saturate the available platelet binding sites (data not shown), which
required addition of exogenous plasma factor Va as seen in Fig.
1A. In marked contrast to observations made with factor Va
binding, 20 times more thrombin (10 nM) was required to
elicit a platelet activation state resulting in maximal
125I-factor Xa binding (~2800 molecules/platelet; Fig.
1A) in the presence of saturating concentrations of factor
Va. These data indicate that saturating the surface of an activated
platelet with factor Va alone is not sufficient to mediate a subsequent factor Xa/platelet binding interaction. Furthermore, when maximum binding of each ligand was achieved, approximately twice as much factor
Va as factor Xa was bound. Since kinetic studies indicate that a
functional enzyme consists of a 1:1 complex (17), only 50% of the
bound factor Va molecules will function in this capacity. As would be
expected, a factor Xa/platelet binding interaction correlated most
closely with the functional expression of thrombin-induced platelet
prothrombinase activity (Fig. 1C) achieved with saturating concentrations of both ligands. These combined data indicate that platelet activation is absolutely required for the assembly and function of the prothrombinase complex at the human platelet surface and that the extent of factor Va and factor Xa binding is regulated independently by the agonist concentration used to effect activation, as well as the "level" of activation achieved (Fig. 1). We
hypothesize that their independent binding interactions are most likely
mediated through discrete platelet membrane receptors.
Effects of thrombin-induced platelet
activation on prothrombinase complex assembly and function. Washed
human platelets (2 × 108/ml), suspended in HTA
containing 5 mM CaCl2 (pH 7.4), were activated with thrombin (0-200 nM, 2 min, 22 °C), followed by the
addition of 3 µM DAPA to inhibit added thrombin.
A, in parallel, yet separate reaction mixtures, the binding
of factor Va, as assessed using 125I-HFV#2 (100 nM,
) and 125I-factor Xa (10 nM,
), to the platelet surface were directly compared as a function of
the concentration of thrombin used to activate the platelets as
detailed under "Experimental Procedures." All binding measurements
were made with prothrombin present at 1.39 µM and factor
Va and factor Xa (labeled or unlabeled) present at 10 nM
each. B, factor Va binding (
) was also compared with functional platelet factor Va release (
) from
-granules as
detailed under "Experimental Procedures." C, factor Xa
binding (
) was directly compared with expression of platelet
prothrombinase activity (
), i.e. rate of thrombin
generated/platelet at saturating enzyme (10 nM factor Va
and factor Xa) and a plasma concentration of substrate (1.39 µM prothrombin) as detailed previously (17, 40) and under
"Experimental Procedures." Thrombin generation was monitored
continuously by enhanced fluorescence of DAPA-thrombin complex. All
data represent the mean ± S.E. (n = 8 different
platelet donors).
Activated Platelets and Their Precursors Express EPR-1 at Their Membrane Surface
Based on the observations detailed above
indicating that factor Va alone was not sufficient to support the
binding of factor Xa to the activated platelet membrane, we
hypothesized that activated platelets may express EPR-1, or perhaps an
EPR-1-like molecule, which serves to further regulate prothrombinase
complex assembly and function. Two specific anti-EPR-1 monoclonal
antibodies, B6 and 2E1, detected a 65-kDa protein by Western blotting
of platelet lysates derived from multiple donors, as well as from
lysates of isolated megakaryocytes (Fig. 2A)
and various megakaryocytic cell lines (Fig. 2B). In all
instances blotting analyses with non-immune/nonspecific antibodies
indicated that only the 65-kDa protein band could be attributed to
specific anti-EPR-1 antibody binding. On a per cell basis, CMK, Mo7,
and HEL cells appear to express similar amounts of EPR-1, whereas much
less of the 65-kDa protein was detected in CHRF-288 cell lysates (Fig.
2B).
The Western blotting data obtained with HEL cells complement those of
Altieri (42), who demonstrated the presence of EPR-1 mRNA
transcripts in HEL cells. Likewise, as shown in Fig. 3,
we confirm the expression of an EPR-1 transcript in HEL cells and demonstrate its presence in CMK cells using RT-PCR with specific EPR-1
primers. A band of the expected molecular size (579 base pairs) was
amplified from total mRNA extracted from both cell lines, but not
in control incubations in the absence of template cDNA. The
cDNA sequences of the PCR products shown were identical to that of
the cloned EPR-1 transcript (data not shown). Our ability to
demonstrate the coexpression of EPR-1 mRNA and protein in
megakaryocyte-like cell lines is consistent with the
immunohistochemical staining of normal human bone marrow with a third
specific anti-EPR-1 monoclonal antibody, demonstrating that
megakaryocytes and platelets were the only cell types to stain
positively in situ (Fig. 4).
The expression of EPR-1 on the surface of thrombin-activated platelets
was demonstrated using flow cytometric analyses (Fig. 5,
A and B). Platelet activation was required for B6
immunoreactivity, since metabolically inhibited platelets, which could
not be activated by thrombin addition (Fig. 5A) exhibited
dramatically reduced binding of the anti-EPR-1 antibody (<10%) when
compared with thrombin-activated platelets (Fig. 5B).
Two-color fluorescence was used to demonstrate that the anti-EPR-1
antibody was binding to platelets by analyzing the coexpression of the
platelet-specific marker, glycoprotein IIb/IIIa (43). The requirement
of platelet activation for EPR-1 membrane expression is consistent,
with EPR-1 potentially playing an important role in prothrombinase
complex assembly.
EPR-1 expression on activated platelets analyzed by flow cytometric techniques. Metabolically inhibited (A) and thrombin-activated (B) platelet suspensions were analyzed by two-color flow cytometry to measure the coexpression of glycoprotein IIb/IIIa (FITC) and EPR-1 (PE) as detailed under "Experimental Procedures." Horizontal axes, FITC fluorescence related to specific binding of the anti-glycoprotein IIb/IIIa monoclonal antibody, HP1-1D (333 ng). Vertical axes, PE fluorescence related to specific binding of the anti-EPR-1 monoclonal antibody, B6 (1:10 dilution). In each scatter plot, the upper right quadrant represents positive staining for both EPR-1 and GPIIb/IIIa, the lower right quadrant represents positive staining for GPIIb/IIIa only, the upper left quadrant represents positive staining for EPR-1 only, and the lower left quadrant represents double negatives. The values shown in the upper left quadrant of each scatter plot indicate the number of PE (B6) positive events. In some experiments (C), activated platelets were incubated with 20 nM each factor Va and factor Xa prior to the addition of B6.
Platelet-expressed EPR-1 Regulates Prothrombinase Complex Assembly and Function
Additional flow cytometric analyses demonstrated that preincubation of activated platelets with equimolar concentrations of factors Va and Xa (20 nM) inhibited a B6/platelet interaction by 50% (Fig. 5C), suggesting that B6 and a preformed prothrombinase complex share a common EPR-1 epitope. This hypothesis is consistent with the observations of Ambrosini and Altieri (35), who epitope-mapped B6 to the factor Xa binding site on EPR-1 expressed by transfected Chinese hamster ovary cells and human umbilical vein endothelial cells.
A role for platelet EPR-1 expression in regulating factor Xa binding and the function of the platelet prothrombinase complex was confirmed by addition of B6 to platelets prior to their thrombin-induced activation and assay for prothrombinase activity. In the absence of added plasma factor Va (i.e. relying on platelet-released factor Va) and the presence of limiting factor Xa (0.1 nM), the expression of platelet prothrombinase activity was inhibited by B6 in a dose-dependent manner (Table I), whereas control antibodies were without affect. The inhibitory effect of B6 on the expression of the activity of the prothrombinase complex was variable and dependent on the platelet donor. Furthermore, 100% inhibition was never achieved. Our inability to effect total inhibition of platelet prothrombinase activity with B6 is most likely due to competition of a factor Va/factor Xa complex for EPR-1 since flow cytometric analyses clearly demonstrated that the preformed complex effectively precludes, thereby limiting, anti-EPR-1 antibody binding (see Fig. 5C).
|
An EPR-1 contribution to prothrombinase complex assembly and function
was supported further by kinetic analyses of prothrombin activation on
various megakaryocytic cell lines. These data indicate that CMK cells
generate thrombin at an initial rate equal to 2.6 ± 0.72 × 108 molecules thrombin/min/cell, whereas the initial rate
of thrombin generation expressed by CHRF-288 cells equals 3.6 ± 0.35 × 106 molecules thrombin/min/cell. Assuming that
the prothrombinase complex assembled on these cell lines expresses the
same catalytic efficiency/site as that expressed by platelets (30 s1) (17), then CMK cells possess approximately 70 times
more sites/cell than CHRF-288 cells (1.4 × 105
versus 2.0 × 103 sites/cell,
respectively). While it is difficult to quantify proteins by Western
blotting analyses, comparison of the blots of the megakaryocytic cells
(see Fig. 2B) indicates that CMK cells express substantially
more EPR-1 than CHRF-288 cells, on a per cell basis, which is
consistent with the functional data. Likewise, HEL cells, which also
expressed substantially more EPR-1 than CHRF-288 cells by Western
blotting analyses, were shown to bind 1.2 ± 0.05 × 105 molecules of 125I-factor Xa in the presence
of added factor Va (data not shown). Thus, HEL cells appear to express
EPR-1 sites/cell at a level nearly identical to that of CMK cells and
approximately 60 times greater than that of CHRF-288 cells.
Collectively, these data indicate that activated human platelets and their precursors express EPR-1 on their membrane surface. Functional and immunological studies demonstrate that this platelet molecule forms at least part of a membrane receptor for factor Xa, which mediates its assembly in the prothrombinase complex. These observations document the first description of a membrane protein, which regulates coagulant enzyme complex assembly on the surface of human activated platelets.
Our data, when considered with that of other investigators (12-15), suggest that EPR-1 is most likely only one component of a complex platelet receptor for the prothrombinase complex. We also observed that saturation of the platelet factor Va binding sites, in the absence of factor Xa, inhibited B6 binding 45% (data not shown) indicating that platelet-bound factor Va may sterically hinder a B6 interaction with platelet EPR-1. Interestingly, Nesheim and colleagues (44) have detected a 65-kDa protein in platelet lysates using Sepharose-immobilized factor Va. These data, combined with our observations, suggest that platelet EPR-1 and bound factor Va may form a heterodimer that functions as a factor Xa receptor. Furthermore, several studies using synthetic vesicles of defined phospholipid content have demonstrated that anionic phospholipids are important for both the assembly and function of the prothrombinase complex (45, 46). The exposure of anionic phospholipids, primarily phosphatidylserine, subsequent to agonist-induced platelet activation has likewise been correlated to the expression of prothrombinase activity at the platelet surface (47). In support of this notion, Jenny and Mann (10) demonstrated that the anionic phospholipid-binding protein, annexin V, inhibited assembly and function of the intrinsic tenase and prothrombinase complexes on the activated platelet surface. However, cofactor-cofactor competition studies indicated that the binding of factor Va and factor VIIIa to activated platelets showed more specificity than on synthetic phospholipid bilayers (10), suggesting that activated platelets may express specific factor Va and factor VIIIa receptors as well.
More recently, London and colleagues (48) demonstrated that annexin V differentially inhibited factor IXa binding and function on activated platelets when compared with a model system comprising synthetic PC vesicles containing PS. In addition, in the absence of factor VIIIa, the affinity of factor IXa for the activated platelet surface was 25-fold higher than its affinity for synthetic phospholipid bilayers, suggesting that components in addition to phospholipids in the activated platelet membrane govern factor VIIIa-independent factor IXa binding. Studies done in the presence of annexin V indicated that synthetic phospholipid bilayers did not mimic activated platelets with regard to binding of factor IXa, as determined by equilibrium binding studies and kinetic studies of factor X activation. Their collected data are consistent with the existence of a binding site for factor IXa on the activated platelet surface comprising both phospholipid and protein moieties. These combined observations indicate that other membrane components, in addition to anionic phospholipids, regulate procoagulant protein binding to cellular membrane surfaces.
Model of Platelet Activation-dependent Prothrombinase Complex Assembly and FunctionWhile it has been clearly shown
that factor Va functions as at least part of the receptor for factor Xa
on platelets (5, 17), our current functional binding studies indicate
that prebound factor Va alone is not sufficient to mediate factor Xa
binding to the activated platelet membrane. The current study also
demonstrates that expression of EPR-1 or an EPR-1-like molecule by
activated platelets is involved in the assembly of a functional
prothrombinase complex at their membrane surface. We propose a
hypothetical model for the expression, assembly, and function of the
platelet prothrombinase complex receptor, which is consistent with all
known data (12-15, 17) (Fig. 6). In this model, neither
factor Va nor factor Xa binds to unactivated human platelets.
Subsequent to agonist-induced activation, binding of each ligand is
distinct and can be correlated with the extent of platelet activation.
A subthreshold level of activation results in release of functional
factor Va from -granules, which parallels expression of platelet
membrane binding sites (i.e. a putative receptor) for either
platelet-released or plasma forms of factor Va. Even when maximal
factor Va binding is achieved, the bound factor Va is not competent to
support factor Xa binding. Rather, factor Xa binding is only achieved
with additional platelet activation such that EPR-1, in proximity to
bound factor Va, is expressed in a conformation where it is able to
support a factor Xa binding interaction. Furthermore, only one-half of
the bound factor Va molecules interact with EPR-1 to form a competent
factor Xa binding site. Factor Xa binding to the activated platelet is regulated further by, in addition to EPR-1 and bound factor Va, anionic
phospholipid made available subsequent to platelet activation. Thus,
the expression of a supramolecular membrane receptor for the
prothrombinase complex by platelets regulates their ability to
effectively generate thrombin and effect formation of a platelet-fibrin clot at the site of vascular injury.
This work would not have been possible without the generous donation of anti-EPR-1 monoclonal antibodies by Dr. Dario C. Altieri (Yale University). We thank Dr. Barbara Grant (University of Vermont) for providing freshly isolated human megakaryocytes and megakaryocytic cell lines, and Laurie O'Rourke for technical assistance. We thank Gary Nelson, Medical Illustrator (University of Vermont), for the computer-generated model.