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From the Department of Pathology,*
Yale University
School of Medicine, New Haven, Connecticut; the Blood Research
Institute,
Blood Center of Southeastern
Wisconsin, Milwaukee, Wisconsin; and the AMGEN
Institute,
University of Toronto, Toronto,
Ontario, Canada
| Abstract |
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| Introduction |
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In addition to its adhesive functions, PECAM-1 has been shown to be a participant in signaling pathways.3,8,11,14 After engagement of PECAM-1, integrin affinity changes have been noted on platelets and lymphocytes,15-19 and changes in intracellular calcium localization have been documented.20 PECAM-1-mediated signaling is thought to occur, in part, via its cytoplasmic ITIM (immunoregulatory tyrosine inhibitory motif) domain. This domain is known to mediate binding of signaling and adapter molecules having one or tandem SH2 domains, when the tyrosine residues residing in the PECAM-1 ITIM domain are phosphorylated.3,11,12 Specifically, the phosphatase SHP-2 has been shown to bind to tyrosine-phosphorylated PECAM-1.19,21 Recently we have found that in addition to its interactions with SHP-2, PECAM-1 can serve as a reservoir for and a modulator of ß-catenin, binding tyrosine-phosphorylated ß-catenin, and, if PECAM-1 is tyrosine phosphorylated, bringing ß-catenin into close proximity to SHP-2, facilitating dephosphorylation of the bound ß-catenin.22
These observations prompted the generation of PECAM-1-deficient mice. The animals were found to be viable and exhibited an abnormal transit of polymorphonuclear leukocytes across vascular basement membranes as their only demonstrable phenotype.23 Other recent investigations using vital microscopy techniques have revealed delays in leukocyte transmigration in PECAM-1-deficient mice.24
In this manuscript we report further analyses of the PECAM-1-deficient mice. Specifically, we have demonstrated a prolongation in bleeding time in PECAM-1-deficient mice. Furthermore, we have shown that this phenotype persists when the PECAM-1-deficient mice are engrafted with wild-type hematopoietic precursors.
| Materials and Methods |
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Platelet, RBC, and WBC counts were performed on blood collected by retroorbital bleeding with a heparinized capillary tube. Two hundred microliters of blood was immediately transferred to Eppendorf tubes containing K3 EDTA (2.0 mg/ml). Samples were diluted in saline and counted on a Baker System 9110+CP Hematology Analyzer (Biochem Immunosystems, Allentown, PA) or sent to a commercial laboratory (ANTECH Diagnostics, Farmingdale, NY) for analysis.
Bleeding times were assessed using an adaptation of the method used by Kung et al.25 Deep anesthesia was induced with metofane gas. The mice were then secured into a tabletop holder, with their tails taped downward and perpendicular to their bodies. After being pulled through a 1.5-mm-diameter template, the tails were transected with a scalpel blade and bled onto a Whatman filter paper. The filter was dabbed to the wound every 30 seconds without disrupting the forming clot. Any blood dripping during the 30-second intervals was allowed to drop freely onto the filter. The experiment was continued until bleeding stopped completely (wild type and heterozygous). The bleeding of PECAM-1-deficient animals was stopped by cauterization at 20 minutes to prevent hypovolemic shock.
Hematopoietic precursor engraftment of PECAM-1-deficient and wild-type mice was performed as described.26 Recipient mice were irradiated (500 Rads twice, separated by 2 hours) using a cesium 127 irradiator. Donor animals were euthanized by cervical dislocation. Femur, tibia, and iliac crest were aseptically removed and cleaned free of muscle, and the ends were cut off with a scalpel blade. The marrow was flushed into a 50-ml tube, using a 22-gauge needle attached to a 5-ml syringe containing 10% fetal bovine serum in 1x phosphate-buffered saline. Cells from multiple animals were pooled, filtered through a 40-µm cell strainer (Beckton Dickinson, Franklin New Jersey), washed, and counted. Marrow precursor cells were diluted to 2.5 x 106/200 µl and injected retroorbitally into each irradiated recipient deficient or wild-type mouse. After 30 days recipients were tested for platelet number and bleeding time.
Statistics (averages, standard deviations, Students t-test) were performed on a Power Macintosh 9600/300 computer using Excel 4.0. Statistical differences were assessed by one-way analysis of variance and the Student-Newman-Keuls method for post hoc analysis, with SigmaStat software (Jandel Corporation, San Rafael, CA).
| Results |
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As previously described,19 PECAM-1 expression in circulating blood elements is not detectable in PECAM-1 homozygous null mice (-/-, KO), whereas it is present in reduced amounts in heterozygotes (+/-, Het) compared to wild-type (+/+, WT) littermates. The presence or absence of PECAM-1 expression was confirmed by immunofluorescence of tissue sections (data not shown).
PECAM-1 Null Mice Exhibit Prolonged Bleeding Times
During our harvests of blood for FACS analysis of phenotype we
noticed a difference (prolongation) in bleeding times of some of the
mice. When the FACS data were correlated with this observation we
determined that the animals exhibiting prolonged bleeding times were
the PECAM-1-deficient (-/-) mice. This finding prompted us to perform
standardized bleeding times on all of our litters. Bleeding times were
performed in a blinded fashion at two sites by separate investigators
over a 2-year period. At the Yale University School of Medicine the
mice wre bled at 3 weeks of age. In all of the mice studied to date (60
mice of varying ages spanning a 2-year period), all of the homozygous
null (-/-, KO) mice exhibited a prolonged bleeding time of more than
20 minutes (the test was arbitrarily stopped at this time point to
avoid possible death due to hypovolemic shock). The heterozygous
animals(+/-, Het) exhibited bleeding times similar to those of
wild-type (+/+, WT) littermates (11 ± 1.5 minutes
versus 12 ± 1.4 minutes) (Figure 1)
. Repeat bleedings of the same mice
yielded similar results. At the Blood Research Institute at the Blood
Center of Southeastern Wisconsin, mice were bled at 89 weeks of age.
Similar to the findings noted at Yale University, the homozygous null
(-/-, KO) mice exhibited prolonged bleeding times (16.6 ± 3.2
minutes) compared to wild-type littermates (12.1 ± 6.9 minutes)
(n = 15, P = 0.03). In addition,
when the area of blood deposited on the filter paper was analyzed, a
significant increase in area was noted for the homozygous null (-/-,
KO) mice (1097.2 ± 390.7 mm2) compared to
wild-type littermates (689.1 ± 382.3 mm2)
(n = 15, P = 0.01). Differences
in the bleeding times obtained at Yale University and the Blood
Research Center are most likely the result of subtle differences in
technique of particular individuals. It should be noted, however, that
at both sites the knockout animals exhibited prolonged bleeding times
compared to wild-type mice.
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In light of recent findings suggesting that modulation of platelet
PECAM-1 interactions can influence platelet
function,16,27-30
we embarked on a stem cell therapy
approach to attempt a correction of the prolonged bleeding time and to
gain a better understanding of the role of PECAM-1 roles in thrombosis.
As illustrated in Figure 3, A and B
,
engraftment of wild-type bone marrow stem cells in marrow-ablated
PECAM-1-deficient mice resulted in reconstitution of the bone marrow
with PECAM-1-positive hematopoietic precursors and circulating blood
elements. Platelet counts obtained from these engrafted mice were
essentially indistinguishable from those of wild-type mice (1.03
x 106
± 1.21 x 105
versus 8.2 x 105
± 2.25 x
105, respectively) (Figure 4B)
. Surprisingly, bleeding times in
these mice remained prolonged (more than 20 minutes) (Figure 4A)
,
despite the presence of PECAM-1-positive platelets (Figure 3B)
.
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Taken together, these data suggest that the prolonged bleeding in PECAM-1-deficient animals is due to a vascular rather than a platelet defect.
| Discussion |
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Our observation of prolonged bleeding times in PECAM-1-deficient mice and data implicating PECAM-1 as a participant in platelet/platelet and platelet/endothelial associations16,28-33 prompted us to study these interactions in the PECAM-1-deficient mice. In our initial attempt to characterize this deficiency, we assessed platelet numbers and morphological characteristics and noted no major differences between PECAM-1-deficient and wild-type mice. In this study, our approach has been to quantify differences in bleeding time. Wild-type mice reconstituted with PECAM-1-deficient marrow, PECAM-1-deficient mice reconstituted with wild-type marrow, and wild-type mice reconstituted with wild-type marrow hematopoietic precursors were generated and used to further characterize the PECAM-1-deficient phenotype.
The techniques used to measure tail bleeding time vary and can often contribute to disparate findings between laboratories.34 We achieved the most consistent results by using a variation of the methods described by Kung et al,25 combined with principles outlined by the template method developed by Mielke,35 which uses a pressure cuff and standardized incisions when bleeding times in humans are taken. We anesthetized the animals and normalized the tail transection diameters, using a 1.5-mm template. The tail was positioned downward and perpendicular to the animals body, corresponding the application of a pressure cuff in humans34 and contributing less to inconsistent clotting kinetics due to collapsing capillaries.36 To minimize investigator bias and unrecognized differences in technique, bleeding times were performed in a blinded fashion at two distinct sites by different investigators over a long time period. In all instances the homozygous null mice were found to exhibit significantly prolonged bleeding times.
PECAM-1 on the surface of platelets has been shown to act as a
costimulatory agonist receptor capable of modulating integrin function
in human platelets during adhesion and aggregation.16
PECAM-1 expression is not limited to the surface of platelets, as
2030% is contained in the
-granules.37
Furthermore,
thrombin-activated degranulated platelets show increased PECAM-1
expression at the surface. Blocking studies using a monoclonal antibody
against PECAM-1 inhibited ADP-, collagen-, and epinephrine-induced
platelet aggregation. However, pooled PECAM-1-deficient and wild-type
platelets aggregate at nearly the same rate after ADP
stimulation.23
This apparent disparity may be explained by
the following: 1) Recent studies have shown that hemodynamic forces as
well as substrate characteristics influence platelet adhesion pathways,
further addressing the need for more physiologically relevant
assays.38,39
2) Increased expression or surface
redistribution of PECAM-1 expression concomitantly on platelets and
endothelial cells may, in a thrombogenic microenvironment, augment
adhesion and potentiate signal transduction through platelet/platelet
as well as through heterotypic interactions with endothelial
cells.40,41
Furthermore, Rosenblum et al showed in
vivo that two different antibodies directed against PECAM-1
delayed platelet aggregation after light/dye energy transferring,
nondenuding injury to endothelium.28-30
However,
platelets harvested from mice after systemic infusion with the same
antibodies and activated with arachidonate and ADP did not exhibit
delayed aggregation.29
Taken together, these data implicate a potential role for PECAM-1 on the endothelium in mediating signaling cascades and adhesive functions during thrombosis. Bombeli et al have demonstrated that adhesion molecules on endothelial cells can act to tether secreted or soluble plasma proteins, facilitating activated platelet adhesion and further aggregation.42 Our finding that bleeding time characteristics remain with the recipient animal after bone marrow engraftment is consistent with a potential role for PECAM-1 on the endothelium, mediating signaling cascades and/or adhesive functions during thrombosis. Elucidation of the role(s) of PECAM-1 in modulating thrombotic phenomena will require further investigation.39
We expected to correct the bleeding time anomaly in PECAM-1-deficient mice through reconstitution with wild-type bone marrow stem cells and to induce abnormal bleeding times in wild-type mice through reconstitution with PECAM-1-deficient bone marrow stem cells. However, although platelet numbers remained approximately equivalent, the bleeding time of each recipient prevailed. These results suggest that PECAM-1 on the endothelium and not necessarily on platelets is important for normal thrombotic phenomena in the periphery. Previous studies have shown that PECAM-1 on endothelial cells plays a role in the transmigration of hematopoietic progenitor cells through the vasculature and that PECAM-1 can undergo heterophilic interactions with other molecules.43-47 The roles that endothelial PECAM-PECAM and endothelial PECAM-heterophilic interactions play in these processes and in modulating the bleeding time in the periphery remain to be determined.
Taken together, our data and published reports4,7- 9,14,15,43-45,47-52 suggest a role of PECAM-1 on endothelial cells in signal transduction, heterophilic/heterotypic interactions, and modulation of thrombosis in the peripheral vasculature. Further studies are needed to further elucidate the mechanism(s) involved in these processes.
| Footnotes |
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Supported in part by U.S. Public Health Service grants R37-HL-28373, RO1-HL-51018, and PO1-DK-38979 to J. A. M. and RO1-HL-40926 to J. P. N.
Accepted for publication April 20, 2000.
| References |
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