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HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
From the Division of Haematology, Hanson Centre for
Cancer Research, IMVS, Adelaide; the Department of Medicine, Australian
Centre for Blood Diseases, Monash University, Box Hill Hospital,
Victoria, Australia; and the Department of Biochemistry, University of
Cambridge, United Kingdom.
The functional importance of platelet endothelial cell adhesion
molecule-1 (PECAM-1/CD31) in platelets is unclear. Because PECAM-1
represents a newly assigned immunoglobulin-ITIM superfamily member
expressed on the surface of platelets, it was hypothesized that it may
play an important regulatory role in modulating ITAM-bearing receptors
such as collagen (GP)VI receptor and Fc Platelet adhesion to subendothelial matrix proteins
plays a central role in both hemostatic and thrombotic
processes.1 Collagen represents an important thrombogenic
component of the subendothelium, with collagen types I, III, and VI
identified as major components of the vessel wall. Platelet-collagen
interactions are mediated by several platelet surface receptors,
including integrin Biochemical and functional studies in human and mouse platelets have
demonstrated that (GP)VI physically couples collagen stimulation to the
phosphorylation of immunoreceptor tyrosine-based activation motif
(ITAM)-bearing FcR Although considerable progress has been made concerning the molecular
mechanisms by which platelets are activated, little is known about
inhibitory mechanisms that regulate platelet adhesion under
physiological blood flow conditions. A potential candidate inhibitory
receptor is platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31). We have previously demonstrated that the PECAM-1 cytoplasmic domain contains consensus sequences typical of
immunoreceptor tyrosine-based inhibitory motifs (ITIM) that, when
phosphorylated, serve as docking sites for SH2-containing
protein-tyrosine phosphatases.11-13 The PECAM-1 ITIM
motifs have been shown to recruit and activate protein-tyrosine
phosphatases SHP-1 and SHP-2 in response to a variety of stimuli,
including integrin To test whether PECAM-1 functions as a physiological regulator of
collagen-mediated platelet activation, we applied 2 different experimental approaches. First, a recombinant human
PECAM-1-immunoglobulin chimera containing the complete extracellular
domain of PECAM-1 was used to selectively activate PECAM-1, thereby
mimicking homophilic ligand interactions. In the second approach,
platelets derived from PECAM-1-deficient mice were used to examine
collagen-induced platelet activation. By applying these distinct but
complementary approaches, we examined the role of PECAM-1 in collagen-
and CRP-induced platelet aggregation and secretion and during platelet
thrombus formation on type I fibrillar collagen under physiological
flow conditions. These studies have defined a key role for PECAM-1 in
negatively regulating collagen-induced platelet activation.
Materials and antibodies
Platelet aggregation studies
Wild-type C57/BL6 and PECAM-1-deficient C57/BL6 age- and sex-matched mice were halothane asphyxiated and bled by cardiac puncture into 0.1 vol. 3.8% (vol/vol) trisodium citrate. Anticoagulated whole blood was then centrifuged at 200g for 15 minutes at room temperature and PRP isolated. Platelet counts of PRP were performed and normalized (0.5 × 106 platelets/µL) for platelet cell number by dilution with Ringer citrate-dextrose buffer (108 mM NaCl, 38 mM KCl, 1.7 mM NaHCO3, 21.2 mM sodium citrate, 27.8 mM glucose, and 1.1 mM MgCl2, pH 7.4). Collagen (30 µg/mL) and CRP (10 µg/mL) -induced aggregation were performed as described above. 5-Hydroxytryptamine platelet secretion assay PECAM-1+/+ and PECAM-1 / murine PRP
was loaded with 0.5 µCi/mL 5-[3H]HT for 1 hour at
37°C. Platelets were isolated and washed as described above and
stimulated with CRP (5 µg/mL) or thrombin (1 U/mL) for 2 minutes.
Platelets were pelleted by centrifugation at 13 000 rpm for 5 minutes,
and the level of 5-[3H]HT released into the supernatant
was determined by scintillation spectrometry. 5-Hydroxytryptamine
(5-HT) release is expressed as a percentage of total tissue content
after subtraction of release under nonstimulated conditions.
Analysis of mural platelet adhesion and thrombus formation under flow Platelet adhesion and platelet thrombus formation under flow was performed as previously described.18,19 Rectangular glass microcapillary tubes (dimensions 0.1 × 1.0 × 100 mm, H×W×L) (microslides; Vitro Dynamics, Rockaway, NJ) were coated with type I fibrillar collagen (2.5 mg/mL) overnight at 4°C. In some experiments, citrated human whole blood was pre-incubated with Fc RIIA-blocking antibody IV.3 Fab (10 µg/mL) and recombinant human
PECAM-1-immunoglobulin chimera (0-100 µg/mL) for 10 minutes. Blood
was perfused through collagen-coated microslides at a wall shear rate
of 150 seconds1 and 600 seconds1 for 5 minutes. Nonadherent cells were removed by perfusion of modified Tyrode
buffer (10 mM HEPES, 12 mM NaHCO3, pH 7.4, 137 mM NaCl, 2.7 mM KCl, 5 mM glucose) through the microcapillary tubes, and adherent
erythrocytes were removed through lysis with 1% (vol/vol) ammonium
oxalate. Thrombi were lysed in 1% (vol/vol) Triton X-100 and collected
for analysis of platelet lactate dehydrogenase content (U/mL) using the
Unimate 3 lactate dehydrogenase assay kit (Hoffman-LaRoche, Basel,
Switzerland). The accuracy of this technique for quantitation of
platelet content was confirmed by confocal imaging of duplicated
thrombi samples.
In studies examining murine thrombus formation under flow, platelets
were fluorescently labeled by incubating citrated whole blood with the
fluorescent probe, DiOC6 (1 µM) for 10 minutes at room
temperature. Blood was perfused through the collagen-coated microcapillary tubes at a wall shear rate of 1800 seconds Immunoprecipitation and immunoblotting After platelet stimulation, reactions were terminated by the addition of an equal volume of Triton lysis buffer (15 mM HEPES, pH 7.4, containing 145 mM NaCl, 0.1 mM MgCl2, 10 mM EGTA, 2 mM sodium orthovanadate, 0.2 mM leupeptin, 1 mM phenylmethylsulfonyl fluoride, and 1% [vol/vol] Triton X-100). Cell suspensions were mixed on a nutator for 1 hour at 4°C, then centrifuged at 13 000 rpm for 15 minutes at 4°C. Triton-soluble supernatants were separated and precleared twice with 50 µL 50% Protein G-Sepharose beads by mixing for 15 minutes at 4°C. Precleared supernatants were incubated overnight with 10 µg anti-PECAM-1.3 monoclonal IgG1 antibody followed by the addition of 50 µL 50% Protein G-Sepharose beads for 1 hour at 4°C. Protein-antibody complexes were washed 5 times with immunoprecipitation buffer (50 mM Tris, pH 7.4, containing 150 mM NaCl and 1% [vol/vol] Triton X-100), eluted in 30 µL SDS reducing buffer, and boiled for 10 minutes. Eluted proteins were electrophoresed on a 10% SDS-PAGE gel and transferred to polyvinylidene difluoride (PVDF) membrane by semidry Western blotting. PVDF membranes were blocked by incubation for 1 hour at room temperature in blocking buffer (20 mM Tris, pH 7.4, containing 3% [wt/vol] bovine serum albumin and 0.05% [vol/vol] Tween 20), then probed with either HRP-conjugated 4G10 anti-phosphotyrosine antibody (1 µg/mL), 4G10, or polyclonal anti-human PECAM-1 antibody, SEW16 (1 µg/mL) for 2 hours at room temperature. Membranes were washed for more than 1 hour with Tris-buffered saline (TBS; 20 mM Tris, pH 7.4, containing 150 mM NaCl and 0.05% [vol/vol] Tween 20); where appropriate, membranes were incubated with HRP-conjugated secondary antibody diluted 1:10 000 in TBS. Membranes were then washed for more than 1 hour with TBS and developed with the enhanced chemiluminescence detection system.Statistical analysis Significant differences were detected using Student t test and one-way analysis of variance, using the Prism software program (GraphPAD Software for Science, San Diego, CA).
Aggregation-dependent and -independent mechanisms of PECAM-1 tyrosine phosphorylation Human platelets contain at least 2 known ITAM-associated receptors including FcRIIA with an intrinsic ITAM motif in its cytoplasmic domain and the collagen receptor (GP)VI, constitutively associated with
the ITAM-bearing signaling molecule, FcR- chain.20-22
In the context of platelets, PECAM-1 represents a newly assigned immunoglobulin-ITIM-bearing receptor that may serve a negative regulatory role to dampen the activation-dependent pathways induced by
collagen receptors, including (GP)VI and an additional cross-linking receptor, FcRIIA. In our initial studies, we examined whether the
activation of protein tyrosine kinase-dependent signaling pathways by
collagen and CRP induces tyrosine phosphorylation of the PECAM-1
cytoplasmic domain. As shown in Figure
1A, stimulation of platelets with soluble
collagen resulted in the time-dependent tyrosine phosphorylation of
PECAM-1, which was maximal at 2 minutes. PECAM-1 tyrosine
phosphorylation was observed over a range of collagen concentrations
(1-20 µg/mL) (data not shown). These characteristics of induction of
tyrosine phosphorylation of PECAM-1 by collagen are similar to those
recently reported by Cicmil et al.23 Using CRP to
stimulate the (GP)VI-coupled signaling pathway, tyrosine phosphorylation of PECAM-1 was evident by 30 seconds of stimulation (Figure 1B). These results suggest that the activation of platelets by
collagen or by CRP induces tyrosine phosphorylation of PECAM-1. An
important issue is whether (GP)VI-induced PECAM-1 tyrosine phosphorylation is downstream of integrin
 IIb 3 activation. Given that there has
been some controversy in the literature about the mechanisms of
induction of PECAM-1 tyrosine phosphorylation, we decided to use a
number of IIb3 blockers to examine
whether tyrosine phosphorylation of PECAM-1 occurred in an
aggregation-dependent and -independent mechanism.11,23 We
and others have used RGDW (Arg-Gly-Asp-Trp) as an
integrin IIb3 blocker with different effects. In these experiments, washed platelets were pre-incubated and
stirred in an aggregometer cuvette for 10 minutes at 37°C in the
presence or absence of either 0.5 mM RGDW peptide, 20 µg/mL c7E3 Fab
(Reopro), or 500 nM Aggrastat before CRP stimulation. In our initial
studies, we observed that RGDW had minimal effect on reducing the
tyrosine phosphorylation of PECAM-1 on CRP-induced platelet aggregation
(Figure 1C). This was in contrast to c7E3 Fab and Aggrastat inhibitors
that showed a major reduction in the tyrosine phosphorylation of
PECAM-1 but did not completely block the tyrosine phosphorylation of
PECAM-1 on CRP-induced platelet aggregation (Figure 1C). The RDGW
CRP-treated samples showed the presence of microaggregates, but not in
the c7E3 Fab and Aggrastat-treated samples, indicating the potency of
inhibitors, c7E3 Fab fragment (an inhibitor of integrins
IIb3 and
v3), and Aggrastat (an inhibitor specific
for integrin IIb3). These results
indicate the importance of integrin IIb3
activation and aggregation in the induction of the tyrosine
phosphorylation of PECAM-1; however, the residual tyrosine
phosphorylation indicates a (GP)VI-induced aggregation-independent
mechanism of tyrosine phosphorylation of PECAM-1.
Recombinant human PECAM-1-immunoglobulin chimera induces tyrosine phosphorylation of PECAM-1 To define the signaling properties of the recombinant human PECAM-1-immunoglobulin chimera, we performed dose-response and time-course experiments of recombinant human PECAM-1-immunoglobulin chimera stimulation of IV.3-treated washed platelets under stirred conditions in the aggregometer. Platelets were lysed, PECAM-1 was immunoprecipitated, and anti-phosphotyrosine content was determined by Western blotting. As shown in Figure 2A, we observed a weak induction in the tyrosine phosphorylation of PECAM-1, which was evident by 1 minute of stimulation.
This is not surprising given that in previous immunological studies, it
is observed that for inhibitory signaling to occur, co-aggregation
mechanisms with simultaneous engagement of the ITAM-containing receptor
with the ITIM-containing receptor leads to synergy with increased
tyrosine phosphorylation of the inhibitory coreceptor, recruitment of
protein-tyrosine phosphates (PTPs), and negative modulation of
ITAM-dependent signaling cascades.24 By applying this
concept to platelets, we compared the tyrosine phosphorylation profiles
of PECAM-1 following a time course of CRP stimulation alone and with
simultaneous engagement of collagen (GP)VI receptor and PECAM-1 with
CRP peptide and recombinant human PECAM-1-immunoglobulin chimera. In
these experiments, Fc Regulation of collagen- and CRP-induced platelet aggregation by PECAM-1-PECAM-1 interactions To explore the functional importance of PECAM-1 interactions on collagen and collagen-related triple-helical peptide-induced platelet aggregation, a recombinant human PECAM-1-immunoglobulin chimeric protein encompassing the 6 extracellular immunoglobulin domains of human PECAM-1 and the Fc portion of human IgG was used as a physiological PECAM-1 homophilic ligand. As controls, a mutant K89A form of recombinant human PECAM-1-immunoglobulin chimera was included that prevents homophilic PECAM-1 interactions, and human IgG was used to ensure there was no contribution by FcRIIA activation. Further,
in all these experiments, platelets were pretreated with IV.3 Fab
fragments to block FcRIIA activation. Previous studies have
validated the use of a recombinant PECAM-1-immunoglobulin chimeric
protein for functional studies in preference to anti-PECAM-1 antibodies
because antibody recognition may not necessarily mimic a natural ligand
interaction.16 To examine the effect of PECAM-1 interactions in modulating collagen-induced platelet aggregation, washed platelets or PRP (data not shown) were pre-incubated with either
recombinant human PECAM-1-immunoglobulin chimera (0-100 µg/mL) in
the presence of IV.3 Fab (10 µg/mL) followed by the induction of
collagen-induced platelet aggregation. As shown in Figure
3A, recombinant human
PECAM-1-immunoglobulin chimera (white bars) induced a moderate
dose-dependent decrease in collagen-induced platelet aggregation
(22% ± 2.1% at 100 µg/mL recombinant human PECAM-1-immunoglobulin; P < .0001). This decrease in
collagen-induced platelet aggregation was consistently observed over a
range of collagen concentrations (40-100 µg/mL;
P < .0001). In control experiments, no effect was
observed even at 100 µg/mL recombinant human K89A
PECAM-1-immunoglobulin chimera (black bars) or human IgG (gray bars)
(Figure 3B). Using a similar approach, we examined the effect of
PECAM-1 interactions in modulating CRP-induced platelet aggregation. As
shown in Figure 4A, a significant
dose-dependent reduction (46% ± 1.7% at 100 µg/mL recombinant
human PECAM-1-immunoglobulin; P < .0001) (white bars) in
CRP-induced platelet aggregation after the induction of
PECAM-1-PECAM-1 interactions was observed. This decrease in
CRP-induced platelet aggregation was consistently observed over a range
of CRP concentrations (20-100 µg/mL; P < .0001). In
control experiments, no inhibitory effect was observed with 100 µg/mL
recombinant human K89A PECAM-1-immunoglobulin chimera (black bars) or
human IgG (gray bars) (Figure 4B).
PECAM-1-deficient platelets demonstrate enhanced collagen- and CRP-induced platelet aggregation and secretion responses To further investigate the role of PECAM-1 in platelet function, we examined the collagen- and CRP-induced platelet aggregation responses of age- and sex-matched wild-type (PECAM-1+/+) and PECAM-1/ platelets. As shown in Figure
5, PECAM-1-deficient platelets showed
enhancement in both collagen- and CRP-induced platelet aggregation
responses. In these experiments, aggregation in response to soluble
collagen (30 µg/mL) was not as robust with mouse platelets as with
human platelets. However, this enhancement in platelet aggregation was
observed for both collagen- and CRP-induced platelet aggregation over a
wide range of doses (20-100 µg/mL collagen and 5-30 µg/mL CRP)
(data not shown). This increase in aggregation in the absence of
PECAM-1 did not result in global enhancement in platelet function
because adenosine diphosphate-induced platelet aggregation was normal,
consistent with previous reports15 (data not shown).
Evaluation of platelet secretion responses using a 5-HT secretion assay
revealed that PECAM-1-deficient platelets showed enhanced secretion
with CRP but not thrombin stimulation (Figure
6). Collectively, these results suggest
that PECAM-1 serves to negatively regulate the ITAM-bearing pathway
mediated by collagen (GP)VI-FcR -chain platelet interactions.
Regulation of human platelet thrombus formation on collagen under flow by PECAM-1-PECAM-1 interactions To investigate the functional importance of PECAM-1 in regulating thrombus formation on a collagen matrix under flow conditions, we performed in vitro flow studies as described in "Materials and methods." Whole blood was pretreated with varying concentrations of recombinant human PECAM-1-immunoglobulin chimera in the presence of FcRIIA-blocking antibody IV.3 Fab (10 µg/mL) for 10 minutes, before perfusion through collagen-coated microslides at wall shear rates of 150 seconds1 and 600 seconds1. As
shown in Figure 7, pre-incubation of
human PECAM-1-immunoglobulin chimera induced a dose-dependent
reduction in platelet thrombus formation at a wall shear rate of 150 seconds1. This effect was also observed at a wall shear
rate of 600 seconds1 (data not shown). This reduction
could not be attributed to complexed IgG alone because pre-incubation
of whole blood with up to 100 µg/mL human IgG had no effect on
platelet thrombus formation (data not shown).
PECAM-1-deficient platelets exhibit enhanced thrombus formation on collagen under physiological conditions of flow To further investigate the role of PECAM-1 in modulating platelet thrombus formation on immobilized collagen, in vitro flow studies were performed with blood obtained from either wild-type (PECAM-1+/+) or PECAM-1-deficient (PECAM-1/) mice. In initial studies, blood was perfused
through collagen-coated microcapillary tubes at a wall shear rate of
1800 seconds1, and thrombi were imaged after 5 minutes of
blood perfusion by confocal microscopy. Volumetric analysis of
individual thrombi revealed that platelets from PECAM-1-deficient mice
formed significantly larger thrombi (178% ± 54%;
P < .05) than wild-type platelets (Figure
8A). To determine whether this increase
in thrombus volume was attributed to differences in thrombus growth or
stability, real-time analysis of thrombus formation was performed. For
these studies, growing thrombi were imaged by confocal microscopy at various time points during blood perfusion (0.5, 1.0, 1.5, 2.0, 3.0, 4.0, and 5.0 minutes). As demonstrated in Figure 8B, the increase in
thrombus volume observed in PECAM-1/ platelets occurred
in a time-dependent manner. This increase in thrombus volume in the
absence of PECAM-1 appeared to result from enhanced thrombus growth
because at no stage were thrombi from either PECAM-1+/+ or
PECAM-1/ platelets observed to be unstable (Figure 8C).
Collectively, these results demonstrate that PECAM-1 serves as a
negative physiological regulator of thrombus formation on immobilized
collagen by inhibiting thrombus growth.
These studies demonstrate a key role for PECAM-1 in negatively regulating collagen-induced platelet activation. Using 2 distinct but complementary approaches, we have examined the functional role of PECAM-1 in the context of collagen- and CRP-induced platelet aggregation and secretion and platelet thrombus formation under flow conditions. Our study demonstrates that in both human and mouse models, PECAM-1 acts as a negative regulator of both collagen and CRP-induced platelet aggregation (Figures 3, 4, and 5), CRP-induced dense granule secretion (Figure 6) and platelet thrombus formation on collagen under flow (Figures 7 and 8). Our study provides a key role for a naturally occurring inhibitory receptor, PECAM-1 that serves as a negative regulator of platelet-collagen interactions under physiological flow conditions (Figure 8A-C). Previous studies in B cells have demonstrated that PECAM-1 is able to
inhibit ITAM-mediated receptor activation events.14 Platelets contain 2 ITAM-bearing receptors, the (GP)VI-associated FcR
Although PECAM-1 was originally identified as expressed in platelets more than 10 years ago, its functional importance has been unclear. In previous studies, it was shown on platelet activation and spreading that PECAM-1 redistributes toward the platelet granulomere and that a subset of PECAM-1 remains at points of platelet-platelet contact.25 These PECAM-1 molecules at sites of platelet contact did not appear to play a role in platelet cohesion, as evidenced by the fact that Glanzmann thrombasthenia platelets do not aggregate despite having a full complement of PECAM-1.25 Because no patients have been reported with qualitative or quantitative defects in PECAM-1, the availability of primary PECAM-1-deficient platelets provided us with an opportunity to directly test its involvement in platelet function. Initial assessment of PECAM-1-deficient mice demonstrated that megakaryocyte and platelet production were normal and that no abnormality was detected in adenosine diphosphate-induced platelet aggregation.15 PECAM-1-deficient mice display a prolonged bleeding time that has been attributed to a vascular defect rather than a platelet abnormality because irradiation of PECAM-1-deficient mice and reconstitution of the hematopoietic cell compartment containing PECAM-1-positive platelets did not correct the prolonged bleeding time.26 Our study demonstrates that in the absence of PECAM-1, platelets are
hyperresponsive to collagen and (GP)VI-selective agonist CRP but not to
thrombin. These findings suggest that PECAM-1 primarily acts as a
regulator of a tyrosine kinase-dependent pathway and not a
G-protein-coupled pathway (Figures 5, 6). Although our studies provide
evidence that PECAM-1 acts as a negative regulator of ITAM-associated
collagen (GP)VI-FcR A recent study by Pasquet et al28 demonstrates that platelets derived from moth-eaten viable mice with a catalytically inactive protein-tyrosine phosphatase, SHP-1 (which retains approximately 20% normal activity), show hypo-responsiveness rather than hyper-responsiveness to (GP)VI-induced signaling and may, in fact, potentiate activation through (GP)VI. These data could be complicated by the residual 20% normal SHP-1 activity; therefore, further studies on me/me mice, which are completely devoid of SHP-1, will be required to define the role of SHP-1 in (GP)VI-induced signaling events. These studies appear to conflict with a potential role for SHP-1 in PECAM-1-inhibitory signaling to down-modulate platelet collagen interactions. An alternative hypothesis we have proposed is that PECAM-1 may exert a negative regulatory role on (GP)VI-induced signaling by SHP-2 and not SHP-1. SHP-2 has been implicated in both positive and negative signaling pathways, depending on the cell type and the signaling complexes formed.29 Additional studies are required to more precisely define this inhibitory signaling mechanism in platelets. Our studies have also shown an important role for PECAM-1 in regulating
thrombus formation on immobilized collagen under flow. Adhesion to
collagen under rapid blood flow conditions requires an initial
vWF-GPIb/IX/V-mediated platelet tethering that then allows integrin
In conclusion, our studies demonstrate that PECAM-1 elicits a negative
regulatory role in collagen-platelet interactions, particularly
involving ITAM-associated collagen (GP)VI receptor-Fc
We thank Prof Peter Newman for supplying anti-PECAM-1 antibodies and recombinant human PECAM-1-immunoglobulin chimeras. We thank Dr Chris Buckley for supplying the mutant K89A human PECAM-1-immunoglobulin chimera. We also thank Dr Tak Mak and Dr Gordon Duncan (Amgen Institute, University of Toronto, Ontario, Canada) for kindly providing the PECAM-1-deficient mice.
Submitted December 22, 2000; accepted May 8, 2001.
Supported by National Heart Foundation grant G00A 0517 (D.E.J.) and by National Health and Medical Research Council of Australia grant 129700 (D.E.J.). D.E.J. is the recipient of an NHMRC RD Wright Fellowship.
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: Denise E. Jackson, Division of Haematology, Hanson Centre for Cancer Research, IMVS, Frome Road, Adelaide; Australia 5000; e-mail: denise.jackson{at}imvs.sa.gov.au.
1. Savage B, Almus-Jacobs F, Ruggeri ZM. Specific synergy of multiple substrate-receptor interactions in platelet thrombus formation under flow. Cell. 1998;94:657-666[CrossRef][Medline] [Order article via Infotrieve]. 2. Ruggeri ZM. Mechanisms initiating platelet thrombus formation. Thromb Haemost. 1997;78:611-616[Medline] [Order article via Infotrieve]. 3. Watson SP, Gibbins J. Collagen receptor signaling in platelets: extending the role of the ITAM. Immunol Today. 1998;19:260-265[CrossRef][Medline] [Order article via Infotrieve].
4.
Clemetson JM, Polgar J, Magnenat E, Wells TN, Clemetson KJ.
The platelet collagen receptor glycoprotein VI is a member of the immunoglobulin superfamily closely related to Fc |
Top
Abstract
Introduction
) (solid line) and PECAM-1
)
(dashed line) platelets represented as the mean ± SEM from an
experiment performed using the blood of 4 individual mice. (C)
Representative thrombi were reconstructed in 3-D using Voxblast image
analysis software package (Vaytek). The upper panel represents an
oblique view of the thrombi, illustrating thrombi surface coverage, and
the lower panel represents a cross-section of the thrombi, illustrating
thrombi height. All confocal images were taken from planes equidistant
from the microcapillary inlet.