Platelet endothelial cell adhesion molecule-1 is a negative regulator of platelet-collagen interactions

Coronary microthrombi in the failing human heart: the role of von Willebrand factor and PECAM-1

The recognition of microthrombi in the heart microcirculation has recently emerged from studies in COVID-19 decedents. The present study investigated the ultrastructure of coronary microthrombi in heart failure (HF) due to cardiomyopathies that are unrelated to COVID-19 infection. In addition, we have investigated the role of von Willebrand factor (VWF) and PECAM-1 in microthrombus formation. We used electron microscopy to investigate the occurrence of microthrombi in patients with HF due to dilated (DCM, n = 7), inflammatory (MYO, n = 6) and ischemic (ICM, n = 7) cardiomyopathy and 4 control patients. VWF and PECAM-1 was studied by quantitative immunohistochemistry and Western blot. In comparison to control, the number of microthrombi was increased 7-9 times in HF. This was associated with a 3.5-fold increase in the number of Weibel-Palade bodies (WPb) in DCM and MYO compared to control. A fivefold increase in WPb in ICM was significantly different from control, DCM and MYO. In Western blot, VWF was increased twofold in DCM and MYO, and more than threefold in ICM. The difference between ICM and DCM and MYO was statistically significant. These results were confirmed by quantitative immunohistochemistry. Compared to control, PECAM-1 was by approximatively threefold increased in all groups of patients. This is the first study to demonstrate the occurrence of microthrombi in the failing human heart. The occurrence of microthrombi is associated with increased expression of VWF and the number of WPb, being more pronounced in ICM. These changes are likely not compensated by increases in PECAM-1 expression.

Restraining of glycoprotein VI- and integrin α2β1-dependent thrombus formation by platelet PECAM1

The platelet receptors, glycoprotein VI (GPVI) and integrin α2β1 jointly control collagen-dependent thrombus formation via protein tyrosine kinases. It is unresolved to which extent the ITIM (immunoreceptor tyrosine-based inhibitory motif) receptor PECAM1 and its downstream acting protein tyrosine phosphatase PTPN11 interfere in this process. Here, we hypothesized that integrin α2β1 has a co-regulatory role in the PECAM1- and PTPN11-dependent restraint of thrombus formation. We investigated platelet activation under flow on collagens with a different GPVI dependency and using integrin α2β1 blockage. Blood was obtained from healthy subjects and from patients with Noonan syndrome with a gain-of-function mutation of PTPN11 and variable bleeding phenotype. On collagens with decreasing GPVI activity (types I, III, IV), the surface-dependent inhibition of PECAM1 did not alter thrombus parameters using control blood. Blockage of α2β1 generally reduced thrombus parameters, most effectively on collagen IV. Strikingly, simultaneous inhibition of PECAM1 and α2β1 led to a restoration of thrombus formation, indicating that the suppressing signaling effect of PECAM1 is masked by the platelet-adhesive receptor α2β1. Blood from 4 out of 6 Noonan patients showed subnormal thrombus formation on collagen IV. In these patients, effects of α2β1 blockage were counterbalanced by PECAM1 inhibition to a normal phenotype. In summary, we conclude that the suppression of GPVI-dependent thrombus formation by either PECAM1 or a gain-of-function of PTPN11 can be overruled by α2β1 engagement.

CD31 as a probable responding and gate-keeping protein of the blood-brain barrier and the risk of Alzheimer's disease

Several studies have shown that an abnormal vascular-immunity link could increase Alzheimer's disease (AD) risk; however, the mechanism is unclear. CD31, also named platelet endothelial cell adhesion molecule (PECAM), is a surface membrane protein of both endothelial and immune cells and plays important roles in the interaction between the vascular and immune systems. In this review, we focus on research regarding CD31 biological actions in the pathological process that may contribute to AD based on the following rationales. First, endothelial, leukocyte and soluble forms of CD31 play multi-roles in regulating transendothelial migration, increasing blood-brain barrier (BBB) permeability and resulting in neuroinflammation. Second, CD31 expressed by endothelial and immune cells dynamically modulates numbers of signaling pathways, including Src family kinases, selected G proteins, and β-catenin which in turn affect cell-matrix and cell-cell attachment, activation, permeability, survival, and ultimately neuronal cell injury. In endothelia and immune cells, these diverse CD31-mediated pathways act as a critical regulator in the immunity-endothelia-brain axis, thereby mediating AD pathogenesis in ApoE4 carriers, which is the major genetic risk factor for AD. This evidence suggests a novel mechanism and potential drug target for CD31 in the background of genetic vulnerabilities and peripheral inflammation for AD development and progression.

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Objective

Implantable cardiovascular therapeutic devices (CTD) including stents, percutaneous heart valves and ventricular assist devices, while lifesaving, impart supraphysiologic shear stress to platelets resulting in thrombotic and bleeding device-related coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of platelet-derived microparticles (PDMPs). Here, we test the hypothesis that shear-generated PDMPs manifest phenotypical heterogeneity of their morphology and surface expression of platelet receptors, and modulate platelet hemostatic function.

Approach and Results

Human gel-filtered platelets were exposed to continuous shear stress and sonication. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation in plasma was measured by optical aggregometry. We demonstrate that platelet exposure to shear stress promotes notable alterations in platelet morphology and ejection of several distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the differential remodeling of platelet receptors with PDMPs expressing significantly higher levels of both adhesion (α _IIb β ₃ , GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist-evoked receptors (P ₂ Y ₁₂ & PAR1). Shear-mediated PDMPs have a bidirectional effect on platelet hemostatic function, promoting thrombin generation and inhibiting platelet aggregation induced by collagen and ADP.

Conclusions

Shear-generated PDMPs demonstrate phenotypic heterogeneity as to morphologic features and defined patterns of surface receptor alteration, and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Whole genome sequence analysis of platelet traits in the NHLBI Trans-Omics for Precision Medicine (TOPMed) initiative

Platelets play a key role in thrombosis and hemostasis. Platelet count (PLT) and mean platelet volume (MPV) are highly heritable quantitative traits, with hundreds of genetic signals previously identified, mostly in European ancestry populations. We here utilize whole genome sequencing (WGS) from NHLBI's Trans-Omics for Precision Medicine initiative (TOPMed) in a large multi-ethnic sample to further explore common and rare variation contributing to PLT (n = 61 200) and MPV (n = 23 485). We identified and replicated secondary signals at MPL (rs532784633) and PECAM1 (rs73345162), both more common in African ancestry populations. We also observed rare variation in Mendelian platelet-related disorder genes influencing variation in platelet traits in TOPMed cohorts (not enriched for blood disorders). For example, association of GP9 with lower PLT and higher MPV was partly driven by a pathogenic Bernard-Soulier syndrome variant (rs5030764, p.Asn61Ser), and the signals at TUBB1 and CD36 were partly driven by loss of function variants not annotated as pathogenic in ClinVar (rs199948010 and rs571975065). However, residual signal remained for these gene-based signals after adjusting for lead variants, suggesting that additional variants in Mendelian genes with impacts in general population cohorts remain to be identified. Gene-based signals were also identified at several genome-wide association study identified loci for genes not annotated for Mendelian platelet disorders (PTPRH, TET2, CHEK2), with somatic variation driving the result at TET2. These results highlight the value of WGS in populations of diverse genetic ancestry to identify novel regulatory and coding signals, even for well-studied traits like platelet traits.

Mice lacking PECAM-1 and Ceacam1 have an aberrant platelet and thrombus phenotype

The immunoglobulin (Ig)-immunoreceptor tyrosine-based inhibitory motif (ITIM) bearing receptors, PECAM-1 and CEACAM1 have been shown net negative regulators of platelet-collagen interactions and hemi-ITAM signalling pathways. In this study, a double knockout (DKO) mouse was developed with deleted PECAM-1 and CEACAM1 to study their combined contribution in platelet activation by glycoprotein VI, C-type lectin-like receptor 2 (CLEC-2), protease activated receptor PAR-4, ADP purinergic receptors and thromboxane receptor TP A2 pathways. Additionally, their collective contribution was examined in thrombus formation under high shear and microvascular thrombosis using in vivo models. DKO platelets responded normally to ADP purinergic receptors and TP A2 pathway. However, DKO platelets released significantly higher amounts of P-selectin compared to hyper-responsive Pecam-1-/- or Ceacam1-/- versus wild-type (WT) upon stimulation with collagen related peptide or rhodocytin. Contrastingly, DKO platelets released increased amounts of P-selectin upon stimulation with PAR-4 agonist peptide or thrombin but not Pecam-1-/-, Ceacam1-/- or WT platelets. Blockade of phospholipase C (PLC) or Rho A kinase revealed that DKO platelets enhanced alpha granule release via PAR-4/Gαq/PLC signalling without crosstalk with Src/Syk or G12/13 signalling pathways. This DKO model showed a significant increase in thrombus formation compared to the hyper-responsive Ceacam1-/- or Pecam-1-/- versus WT phenotype. DKO platelets have similar glycoprotein surface expression compared to Pecam-1-/-, Ceacam1-/- and WT platelets. PECAM-1 and CEACAM1 work in concert to negatively regulate hemiITAM signalling, platelet-collagen interactions and PAR-4 Gαq protein coupled signalling pathways. Both PECAM-1 and CEACAM1 are required for negative regulation of platelet activation and microvascular thrombosis in vivo.

Targeting platelet inhibition receptors for novel therapies: PECAM-1 and G6b-B

While current oral antiplatelet therapies benefit many patients, they deregulate the hemostatic balance leaving patients at risk of systemic side-effects such as hemorrhage. Dual antiplatelet treatment is the standard approach, combining aspirin with P2Y₁₂ blockers. These therapies mainly target autocrine activation mechanisms (TxA₂, ADP) and, more recently, the use of thrombin or thrombin receptor antagonists have been added to the available approaches. Recent efforts to develop new classes of anti-platelet drugs have begun to focus on primary platelet activation pathways such as through the immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor GPVI/FcRγ-chain complex. There are already encouraging results from targeting GPVI, with reduced aggregation and smaller arterial thrombi, without major bleeding complications, likely due to overlapping activation signaling pathways with other receptors such as the GPIb-V-IX complex. An alternative approach to reduce platelet activation could be to inhibit this signaling pathway by targeting the inhibitory pathways intrinsic to platelets. Stimulation of endogenous negative modulators could provide more specific inhibition of platelet function, but is this feasible? In this review, we explore the potential of the two major platelet immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing inhibitory receptors, G6b-B and PECAM-1, as antithrombotic targets.

Assessment of a pro-healing stent in an animal model of early neoatherosclerosis

Background: Neoatherosclerosis represents an accelerated manifestation of atherosclerosis in nascent neointima after stenting, associated with adverse events. We investigated whether improved reendothelialization using RGD-coated stents results in diminished vascular permeability and reduced foam cell formation compared to standard DES in atherosclerotic rabbits. Methods and Results: Neointimal foam cell formation was induced in rabbits (n = 7). Enhanced endothelial integrity in RGD-coated stents resulted in decreased vascular permeability relative to DES, which was further confirmed by SEM and TEM. Cell culture experiments examined the effect of everolimus on endothelial integrity. Increasing concentrations of everolimus resulted in a dose-dependent decrease of endothelial cell junctions and foam cell transformation of monocytes, confirming the relevance of endothelial integrity in preventing permeability of LDL. Conclusion: Incomplete endothelial integrity was confirmed as a key factor of neointimal foam cell formation following stent implantation. Pro-healing stent coatings may facilitate reendothelialization and reduce the risk of neoatherosclerosis.

Effect of quinolinic acid - A uremic toxin from tryptophan metabolism - On hemostatic profile in rat and mouse thrombosis models

PURPOSE

We aimed to determine the effect of quinolinic acid (QA) on hemostasis in rat and mouse models of thrombosis.

MATERIAL AND METHODS

Wistar rats (male, n = 72) received QA dissolved in drinking water in doses of 3, 10, 30 mg/kg or pure drinking water (vehicle control group -VEH) for 14 days. On the 14^th day of the experiment the effect of QA on hemostasis was evaluated using electrically induced arterial thrombosis model. The following parameters were measured: thrombus weight, hematology, thromboelastometric (ROTEM) parameters, TXA₂ and 6-keto-PGF_1α concentration, coagulation and fibrinolytic markers activity and concentration. GFP mice (male, n = 30) were assigned to the group receiving QA (30 mg/kg) or VEH for 14 days and to the group receiving: single intravenous dose of QA (30 mg/kg) or VEH or the same dose of QA and anti-CD31 (platelet endothelial cell adhesion molecule-1, PECAM-1) antibody conjugated with Alexa Fluor 647. The effect of QA on hemostasis was evaluated in the model of laser-induced injury of mesentery vein using intravital confocal microscopy.

RESULTS

Administering QA for 14 days resulted in a divergent, depending on dose, increase in concentration of active form of tPA and PAI-1 and concentration of total PAI-1 and PAP complexes in rats' plasma. In turn, administering QA for 14 days in mice revealed its prothrombotic activity, while single-dose IV administration revealed its antithrombotic activity, through the up-regulation of PECAM-1 expression.

CONCLUSIONS

We demonstrated the first evidence for the opposite biological effects of QA on hemostasis in rat and mouse thrombosis models.

CD31 Expression Determines Redox Status and Chemoresistance in Human Angiosarcomas

Purpose: Angiosarcomas are soft tissue sarcomas with endothelial differentiation and vasoformative capacity. Most angiosarcomas show strong constitutive expression of the endothelial adhesion receptor CD31/PECAM-1 pointing to an important role of this molecule. However, the biological function of CD31 in angiosarcomas is unknown.Experimental Design: The expression levels of CD31 in angiosarcoma cells and its effects on cell viability, colony formation, and chemoresistance were evaluated in human angiosarcoma clinical samples and in cell lines through isolation of CD31^high and CD31^low cell subsets. The redox-regulatory CD31 function linked to YAP signaling was determined using a CD31-blocking antibody and siRNA approach and was further validated in CD31-knockout endothelial cells.Results: We found that most angiosarcomas contain a small CD31^low cell population. CD31^low cells had lost part of their endothelial properties and were more tumorigenic and chemoresistant than CD31^high cells due to more efficient reactive oxygen species (ROS) detoxification. Active downregulation of CD31 resulted in loss of endothelial tube formation, nuclear accumulation of YAP, and YAP-dependent induction of antioxidative enzymes. Addition of pazopanib, a known enhancer of proteasomal YAP degradation resensitized CD31^low cells for doxorubicin resulting in growth suppression and induction of apoptosis.Conclusions: Human angiosarcomas contain a small aggressive CD31^low population that have lost part of their endothelial differentiation programs and are more resistant against oxidative stress and DNA damage due to intensified YAP signaling. Our finding that the addition of YAP inhibitors can resensitize CD31^low cells toward doxorubicin may aid in the rational development of novel combination therapies to treat angiosarcomas. Clin Cancer Res; 24(2); 460-73. ©2017 AACR.

Improved Antithrombotic Function of Oriented Endothelial Cell Monolayer on Microgrooves

Achievement of an endothelial cell (EC) monolayer (re-endothelialization) on the vascular implant surface with competent and functioning features is critical for long-term safety after implantation. Oriented EC monolayer is beneficial to improve endothelial function such as enhanced athero-resistant property. However, the information about antithrombotic property of oriented EC monolayer is limited. Here, we used the microgrooved polydimethylsiloxane substrates to guide EC orientation and obtain oriented EC monolayer. The effects of anisotropic topography on EC behaviors and antithrombotic function of the EC monolayer were then evaluated. Our data demonstrated that ECs responded to grooves in a size-dependent way as shown in oriented cell cytoskeleton and nuclei, enhanced directed migration, and overall velocity. Furthermore, compared to the EC monolayer on the flat surface, the oriented EC monolayer formed on the grooved substrates exhibited improved antithrombotic capability as indicated by higher expression of functional related genes, production of prostacyclin and tissue plasminogen activator, and prolonged activated coagulation time. The improvement of antithrombotic function was especially notable on the smaller-size groove. These findings reveal the responses of ECs to varisized topography and antithrombotic function of the oriented EC monolayer, providing insights into optimal design of vascular implants.

ITIM receptors: more than just inhibitors of platelet activation

Since their discovery, immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptors have been shown to inhibit signaling from immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors in almost all hematopoietic cells, including platelets. However, a growing body of evidence has emerged demonstrating that this is an oversimplification, and that ITIM-containing receptors are versatile regulators of platelet signal transduction, with functions beyond inhibiting ITAM-mediated platelet activation. PECAM-1 was the first ITIM-containing receptor identified in platelets and appeared to conform to the established model of ITIM-mediated attenuation of ITAM-driven activation. PECAM-1 was therefore widely accepted as a major negative regulator of platelet activation and thrombosis for many years, but more recent findings suggest a more complex role for this receptor, including the facilitation of α_IIbβ₃-mediated platelet functions. Since the identification of PECAM-1, several other ITIM-containing platelet receptors have been discovered. These include G6b-B, a critical regulator of platelet reactivity and production, and the noncanonical ITIM-containing receptor TREM-like transcript-1, which is localized to α-granules in resting platelets, binds fibrinogen, and acts as a positive regulator of platelet activation. Despite structural similarities and shared binding partners, including the Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, knockout and transgenic mouse models have revealed distinct phenotypes and nonredundant functions for each ITIM-containing receptor in the context of platelet homeostasis. These roles are likely influenced by receptor density, compartmentalization, and as-yet unknown binding partners. In this review, we discuss the diverse repertoire of ITIM-containing receptors in platelets, highlighting intriguing new functions, controversies, and future areas of investigation.

CEACAM2 positively regulates integrin αIIbβ3-mediated platelet functions

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) is an Ig-ITIM superfamily member that regulates integrin αIIbβ3 function. We hypothesized that its twin protein, CEACAM2, exerts a similar physiologic role in murine platelets. CEACAM2-deficient mice (Cc2-/-) displayed prolonged tail bleeding times and increased volume of blood loss. Cc2-/- platelets have moderate integrin αIIbβ3-mediated functional defects with impaired kinetics of platelet spreading on fibrinogen and type I collagen and delayed kinetics in the retraction of fibrin clots in vitro. This functional integrin αIIbβ3 defect could not be attributed to altered integrin αIIbβ3 expression. Cc2-/- platelets displayed normal 'inside-out' signaling properties as demonstrated by normal agonist-induced binding of soluble fluorescein isothiocyanate (FITC)-fibrinogen and JON/A antibody binding. This data provides direct evidence that disruption of CEACAM2 induces a moderate integrin αIIbβ3-mediated platelet function defect, and that CEACAM2 is essential to maintain a normal integrin αIIbβ3-mediated platelet function.

Pulmonary hypertension and vascular remodeling in mice exposed to crystalline silica

Background

Occupational and environmental exposure to crystalline silica may lead to the development of silicosis, which is characterized by inflammation and progressive fibrosis. A substantial number of patients diagnosed with silicosis develop pulmonary hypertension. Pulmonary hypertension associated with silicosis and with related restrictive lung diseases significantly reduces survival in affected subjects. An animal model of silicosis has been described previously however, the magnitude of vascular remodeling and hemodynamic effects of inhaled silica are largely unknown. Considering the importance of such information, this study investigated whether mice exposed to silica develop pulmonary hypertension and vascular remodeling.

Methods

C57BL6 mice were intratracheally injected with either saline or crystalline silica at doses 0.2 g/kg, 0.3 g/kg and 0.4 g/kg and then studied at day 28 post-exposure. Pulmonary hypertension was characterized by changes in right ventricular systolic pressure and lung histopathology.

Results

Mice exposed to saline showed normal lung histology and hemodynamic parameters while mice exposed to silica showed increased right ventricular systolic pressure and marked lung pathology characterized by a granulomatous inflammatory reaction and increased collagen deposition. Silica-exposed mice also showed signs of vascular remodeling with pulmonary artery muscularization, vascular occlusion, and medial thickening. The expression of pro-inflammatory genes such as TNF-α and MCP-1 was significantly upregulated as well as the expression of the pro-remodeling genes collagen type I, fibronectin and the metalloproteinases MMP-2 and TIMP-1. On the other hand, the expression of several vasculature specific genes involved in the regulation of endothelial function was significantly attenuated.

Conclusions

We characterized a new animal model of pulmonary hypertension secondary to pulmonary fibrosis induced by crystalline silica. Our data suggest that silica promotes the damage of the pulmonary vasculature through mechanisms that might involve endothelial dysfunction, inflammation, and vascular remodeling.

Advances in our understanding of mechanisms of venous thrombus resolution

Traditionally, venous thrombosis has been seen as the consequence of a regulated cascade of proteolytic steps leading to the polymerization of fibrinogen and fibrin crosslinking that is facilitated by platelets. A new view of thrombosis is providing a more integrated concept, with components of the vascular wall contributing to the vascular remodeling of thrombosis. Angiogenesis and inflammation are two key mechanisms that safeguard venous thrombus resolution and restitution of vascular patency after thrombosis. Disturbance of these processes leads to thrombus persistence and has potentially severe consequences for affected patients. Examples for clinical conditions associated with recurrent or persisting venous thrombosis are post-thrombotic syndrome or chronic thromboembolic pulmonary hypertension. Recently, studies using animal models of venous thrombosis have contributed to a better understanding of thrombus non-resolution that will eventually lead to modification of current treatment concepts. For example, recent data suggest that innate immunity is involved in the modification of thrombosis.

Kaempferol suppresses collagen-induced platelet activation by inhibiting NADPH oxidase and protecting SHP-2 from oxidative inactivation

Reactive oxygen species (ROS) generated upon collagen stimulation act as second messengers to propagate various platelet-activating events. Among the ROS-generating enzymes, NADPH oxidase (NOX) plays a prominent role in platelet activation. Thus, NOX has been suggested as a novel target for anti-platelet drug development. Although kaempferol has been identified as a NOX inhibitor, the influence of kaempferol on the activation of platelets and the underlying mechanism have never been investigated. Here, we studied the effects of kaempferol on NOX activation, ROS-dependent signaling pathways, and functional responses in collagen-stimulated platelets. Superoxide anion generation stimulated by collagen was significantly inhibited by kaempferol in a concentration-dependent manner. More importantly, kaempferol directly bound p47(phox), a major regulatory subunit of NOX, and significantly inhibited collagen-induced phosphorylation of p47(phox) and NOX activation. In accordance with the inhibition of NOX, ROS-dependent inactivation of SH2 domain-containing protein tyrosine phosphatase-2 (SHP-2) was potently protected by kaempferol. Subsequently, the specific tyrosine phosphorylation of key components (Syk, Vav1, Btk, and PLCγ2) of collagen receptor signaling pathways was suppressed by kaempferol. Kaempferol also attenuated downstream responses, including cytosolic calcium elevation, P-selectin surface exposure, and integrin-αIIbβ3 activation. Ultimately, kaempferol inhibited platelet aggregation and adhesion in response to collagen in vitro and prolonged in vivo thrombotic response in carotid arteries of mice. This study shows that kaempferol impairs collagen-induced platelet activation through inhibition of NOX-derived ROS production and subsequent oxidative inactivation of SHP-2. This effect suggests that kaempferol has therapeutic potential for the prevention and treatment of thrombovascular diseases.

CEA a thrombus CAM: CEACAM2, a twin of CEACAM1?

In this issue of Blood, Alshahrani et al demonstrate that carcinoembryonic antigen-related cell adhesion molecule 2 (CEACAM2) is expressed on platelets and negatively regulates the collagen receptor glycoprotein (GP)VI-FcRγ chain and C-type lectin-like receptor 2 (CLEC-2)-mediated platelet activation.

CEACAM2 negatively regulates hemi (ITAM-bearing) GPVI and CLEC-2 pathways and thrombus growth in vitro and in vivo

Carcinoembryonic antigen-related cell adhesion molecule-2 (CEACAM2) is a cell-surface glycoprotein expressed on blood, epithelial, and vascular cells. CEACAM2 possesses adhesive and signaling properties mediated by immunoreceptor tyrosine-based inhibitory motifs. In this study, we demonstrate that CEACAM2 is expressed on the surface and in intracellular pools of platelets. Functional studies of platelets from Ceacam2(-/-)-deficient mice (Cc2(-/-)) revealed that CEACAM2 serves to negatively regulate collagen glycoprotein VI (platelet) (GPVI)-FcRγ-chain and the C-type lectinlike receptor 2 (CLEC-2) signaling. Cc2(-/-) platelets displayed enhanced GPVI and CLEC-2-selective ligands, collagen-related peptide (CRP), collagen, and rhodocytin (Rhod)-mediated platelet aggregation. They also exhibited increased adhesion on type I collagen, and hyperresponsive CRP and CLEC-2-induced α and dense granule release compared with wild-type platelets. Furthermore, using intravital microscopy to ferric chloride (FeCl3)-injured mesenteric arterioles and laser-induced injury of cremaster muscle arterioles, we herein show that thrombi formed in Cc2(-/-) mice were larger and more stable than wild-type controls in vivo. Thus, CEACAM2 is a novel platelet immunoreceptor that acts as a negative regulator of platelet GPVI-collagen interactions and of ITAM receptor CLEC-2 pathways.

Platelet endothelial cell adhesion molecule-1 inhibits platelet response to thrombin and von Willebrand factor by regulating the internalization of glycoprotein Ib via AKT/glycogen synthase kinase-3/dynamin and integrin αIIbβ3

OBJECTIVE

Platelet endothelial cell adhesion molecule-1 (PECAM-1) regulates platelet response to multiple agonists. How this immunoreceptor tyrosine-based inhibitory motif-containing receptor inhibits G protein-coupled receptor-mediated thrombin-induced activation of platelets is unknown.

APPROACH AND RESULTS

Here, we show that the activation of PECAM-1 inhibits fibrinogen binding to integrin αIIbβ3 and P-selectin surface expression in response to thrombin (0.1-3 U/mL) but not thrombin receptor-activating peptides SFLLRN (3×10(-7)-1×10(-5) mol/L) and GYPGQV (3×10(-6)-1×10(-4) mol/L). We hypothesized a role for PECAM-1 in reducing the tethering of thrombin to glycoprotein Ibα (GPIbα) on the platelet surface. We show that PECAM-1 signaling regulates the binding of fluorescein isothiocyanate-labeled thrombin to the platelet surface and reduces the levels of cell surface GPIbα by promoting its internalization, while concomitantly reducing the binding of platelets to von Willebrand factor under flow in vitro. PECAM-1-mediated internalization of GPIbα was reduced in the presence of both EGTA and cytochalasin D or latrunculin, but not either individually, and was reduced in mice in which tyrosines 747 and 759 of the cytoplasmic tail of β3 integrin were mutated to phenylalanine. Furthermore, PECAM-1 cross-linking led to a significant reduction in the phosphorylation of glycogen synthase kinase-3β Ser(9), but interestingly an increase in glycogen synthase kinase-3α pSer(21). PECAM-1-mediated internalization of GPIbα was reduced by inhibitors of dynamin (Dynasore) and glycogen synthase kinase-3 (CHIR99021), an effect that was enhanced in the presence of EGTA.

CONCLUSIONS

PECAM-1 mediates internalization of GPIbα in platelets through dual AKT/protein kinase B/glycogen synthase kinase-3/dynamin-dependent and αIIbβ3-dependent mechanisms. These findings expand our understanding of how PECAM-1 regulates nonimmunoreceptor signaling pathways and helps to explains how PECAM-1 regulates thrombosis.

Reactive oxygen species play a critical role in collagen-induced platelet activation via SHP-2 oxidation

AIMS

The collagen-stimulated generation of reactive oxygen species (ROS) regulates signal transduction in platelets, although the mechanism is unclear. The major targets of ROS include protein tyrosine phosphatases (PTPs). ROS-mediated oxidation of the active cysteine site in PTPs abrogates the PTP catalytic activity. The aim of this study was to elucidate whether collagen-induced ROS generation leads to PTP oxidation, which promotes platelet stimulation.

RESULTS

SH2 domain-containing PTP-2 (SHP-2) is oxidized in platelets by ROS produced upon collagen stimulation. The oxidative inactivation of SHP-2 leads to the enhanced tyrosine phosphorylation of spleen tyrosine kinase (Syk), Vav1, and Bruton's tyrosine kinase (Btk) in the linker for the activation of T cells signaling complex, which promotes the tyrosine phosphorylation-mediated activation of phospholipase Cγ2 (PLCγ2). Moreover, we found that, relative to wild-type platelets, platelets derived from glutathione peroxidase 1 (GPx1)/catalase double-deficient mice showed enhanced cellular ROS levels, oxidative inactivation of SHP-2, and tyrosine phosphorylation of Syk, Vav1, Btk, and PLCγ2 in response to collagen, which subsequently led to increased intracellular calcium levels, degranulation, and integrin αIIbβ3 activation. Consistent with these findings, GPx1/catalase double-deficiency accelerated the thrombotic response in FeCl3-injured carotid arteries.

INNOVATION

The present study is the first to demonstrate that SHP-2 is targeted by ROS produced in collagen-stimulated platelets and suggests that a novel mechanism for the regulation of platelet activation by ROS is due to oxidative inactivation of SHP-2.

CONCLUSION

We conclude that collagen-induced ROS production leads to SHP-2 oxidation, which promotes platelet activation by upregulating tyrosine phosphorylation-based signal transduction.

Treatment with a histone deacetylase inhibitor, valproic acid, is associated with increased platelet activation in a large animal model of traumatic brain injury and hemorrhagic shock

BACKGROUND

We have previously shown that resuscitation with fresh frozen plasma (FFP) in a large animal model of traumatic brain injury (TBI) and hemorrhagic shock (HS) decreases the size of the brain lesion, and that addition of a histone deacetylase inhibitor, valproic acid (VPA), provides synergistic benefits. In this study, we hypothesized that VPA administration would be associated with a conservation of platelet function as measured by increased platelet activation after resuscitation.

MATERIALS AND METHODS

Ten swine (42-50 kg) were subjected to TBI and HS (40% blood loss). Animals were left in shock for 2 h before resuscitation with either FFP or FFP+VPA (300 mg/kg). Serum levels of platelet activation markers transforming growth factor beta, CD40 L, P-selectin, and platelet endothelial cell adhesion molecule (PECAM) 1 were measured at baseline, postresuscitation, and after a 6-h observation period. Platelet activation markers were also measured in the brain whole cell lysates and immunohistochemistry.

RESULTS

Circulating P-selectin levels were significantly higher in the FFP+VPA group compared with the FFP alone group (70.85±4.70 versus 48.44±7.28 ng/mL; P<0.01). Likewise, immunohistochemistry data showed elevated P-selectin in the VPA treatment group (22.30±10.39% versus 8.125±3.94%, P<0.01). Serum sCD40L levels were also higher in the FFP+VPA group (3.21±0.124 versus 2.38±0.124 ng/mL; P<0.01), as was brain sCD40L levels (1.41±0.15 versus 1.22±0.12 ng/mL; P=0.05). Circulating transforming growth factor beta levels were elevated in the FFP+VPA group, but this did not reach statistical significance (11.20±1.46 versus 8.09±1.41 ng/mL; P=0.17). Brain platelet endothelial cell adhesion molecule 1 levels were significantly lower in the FFP+VPA group compared with the FFP group (5.22±2.00 pg/mL versus 7.99±1.13 pg/mL; P=0.03).

CONCLUSIONS

In this clinically relevant large animal model of combined TBI+HS, the addition of VPA to FFP resuscitation results in an early upregulation of platelet activation in the circulation and the brain. The previously observed neuroprotective effects of VPA may be due to a conservation of platelet function as measured by a higher platelet activation response after resuscitation.

Platelet endothelial cell adhesion molecule 1 deficiency misguides venous thrombus resolution

PECAM-1 deficiency misguides venous thrombus resolution. PECAM-1 cell surface shedding occurs at the site of venous thrombosis.

Association of single nucleotide polymorphisms in the gene encoding platelet endothelial cell adhesion molecule-1 with the risk of myocardial infarction: a systematic review and meta-analysis

BACKGROUND

Single nucleotide polymorphisms (SNPs) within the platelet endothelial cell adhesion molecule-1 (PECAM-1) gene have been proposed as predisposing factors for myocardial infarction (MI) but published reports have given conflicting findings.

OBJECTIVE

The present study aimed to clarify the association between SNPs in PECAM-1 and MI using a meta-analysis of published studies.

METHODS

Medline, HuGE Navigator and SCOPUS Library databases were searched to identify case-control studies which examined the association of SNPs in PECAM-1 and MI. Data were extracted using standardized methods. Combined odds ratios (OR) with 95% confidence intervals (CI) for the association of SNPs with MI were calculated using a random effect approach and under additive, dominant and recessive models of inheritance.

RESULTS

A total of 7 studies comprising 3886 cases and 4097 controls fulfilled the inclusion criteria. Three SNPs in PECAM-1 were investigated, namely rs668 (Leu125Val), rs12953 (Ser563Asn) and rs1131012 (Arg670Gly). The GG genotype of rs1131012 was associated with a reduced risk of MI under a recessive (OR: 0.81; 95%CI: 0.69-0.94; p=0.010), but not additive and dominant models (p>0.05). This association was robust in sensitivity analyses and not subject to heterogeneity. No significant association was detected between rs668 and rs12953 with MI under any of the inheritance models.

CONCLUSION

The results of the current meta-analysis suggest that homozygous polymorphic genotype (GG) of the rs1131012 SNP may confer protection against MI. The impact of this variant on the expression and function of PECAM-1 needs to be elucidated in future investigations.

Absence of platelet endothelial cell adhesion molecule 1, PECAM-1/CD31, in vivo increases resistance to Salmonella enterica serovar Typhimurium in mice

PECAM-1/CD31 is known to regulate inflammatory responses and exhibit pro- and anti-inflammatory functions. This study was designed to determine the functional role of PECAM-1 in susceptibility to murine primary in vivo infection with Salmonella enterica serovar Typhimurium and in in vitro inflammatory responses of peritoneal macrophages. Lectin profiling showed that cellular PECAM-1 and recombinant human PECAM-1-Ig chimera contain high levels of mannose sugars and N-acetylglucosamine. Consistent with this carbohydrate pattern, both recombinant human and murine PECAM-1-Ig chimeras were shown to bind S. Typhimurium in a dose-dependent manner in vitro. Using oral and fecal-oral transmission models of S. Typhimurium SL1344 infection, PECAM-1(-/-) mice were found to be more resistant to S. Typhimurium infection than wild-type (WT) C57BL/6 mice. While fecal shedding of S. Typhimurium was comparable in wild-type and PECAM-1(-/-) mice, the PECAM-1-deficient mice had lower bacterial loads in systemic organs such as liver, spleen, and mesenteric lymph nodes than WT mice, suggesting that extraintestinal dissemination was reduced in the absence of PECAM-1. This reduced bacterial load correlated with reduced tumor necrosis factor (TNF), interleukin-6 (IL-6), and monocyte chemoattractant protein (MCP) levels in sera of PECAM-1(-/-) mice. Following in vitro stimulation of macrophages with either whole S. Typhimurium, lipopolysaccharide (LPS) (Toll-like receptor 4 [TLR4] ligand), or poly(I·C) (TLR3 ligand), production of TNF and IL-6 by PECAM-1(-/-) macrophages was reduced. Together, these results suggest that PECAM-1 may have multiple functions in resistance to infection with S. Typhimurium, including binding to host cells, extraintestinal spread to deeper tissues, and regulation of inflammatory cytokine production by infected macrophages.

Megakaryocyte-specific deletion of the protein-tyrosine phosphatases Shp1 and Shp2 causes abnormal megakaryocyte development, platelet production, and function

The SH2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2 have been implicated in regulating signaling from a variety of platelet and megakaryocyte receptors. In this study, we investigate the functions of Shp1 and Shp2 in megakaryocytes and platelets. Megakaryocyte/platelet (MP)-specific deletion of Shp1 in mice resulted in platelets being less responsive to collagen-related peptide due to reduced GPVI expression and signaling via the Src family kinase (SFK)-Syk-PLCγ2 pathway, and fibrinogen due to reduced SFK activity. By contrast, deletion of Shp2 in the MP lineage resulted in macrothrombocytopenia and platelets being hyper-responsive to anti-CLEC-2 antibody and fibrinogen. Shp1- and Shp2-deficient megakaryocytes had partial blocks at 2N/4N ploidy; however, only the latter exhibited reduced proplatelet formation, thrombopoietin, and integrin signaling. Mice deficient in both Shp1 and Shp2 were severely macrothrombocytopenic and had reduced platelet surface glycoprotein expression, including GPVI, αIIbβ3, and GPIbα. Megakaryocytes from these mice were blocked at 2N/4N ploidy and did not survive ex vivo. Deletion of the immunoreceptor tyrosine-based inhibition motif-containing receptor G6b-B in the MP lineage phenocopied multiple features of Shp1/2-deficient mice, suggesting G6b-B is a critical regulator of Shp1 and Shp2. This study establishes Shp1 and Shp2 as major regulators of megakaryocyte development, platelet production, and function.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

The importance of the endothelium in atherothrombosis and coronary stenting

Deployment of drug-eluting stents instead of bare-metal stents has dramatically reduced restenosis rates, but rates of very late stent thrombosis (>1 year postimplantation) have increased. Vascular endothelial cells normally provide an efficient barrier against thrombosis, lipid uptake, and inflammation. However, endothelium that has regenerated after percutaneous coronary intervention is incompetent in terms of its integrity and function, with poorly formed cell junctions, reduced expression of antithrombotic molecules, and decreased nitric oxide production. Delayed arterial healing, characterized by poor endothelialization, is the primary cause of late (1 month-1 year postimplantation) and very late stent thrombosis following implantation of drug-eluting stents. Impairment of vasorelaxation in nonstented proximal and distal segments of stented coronary arteries is more severe with drug-eluting stents than bare-metal stents, and stent-induced flow disturbances resulting in complex spatiotemporal shear stress can also contribute to increased thrombogenicity and inflammation. The incompetent endothelium leads to late stent thrombosis and the development of in-stent neoatherosclerosis. The process of neoatherosclerosis occurs more rapidly, and more frequently, following deployment of drug-eluting stents than bare-metal stents. Improved understanding of vascular biology is crucial for all cardiologists, and particularly interventional cardiologists, as maintenance of a competently functioning endothelium is critical for long-term vascular health.

JAM-A protects from thrombosis by suppressing integrin αIIbβ3-dependent outside-in signaling in platelets

Mounting evidence suggests that agonist-initiated signaling in platelets is closely regulated to avoid excessive responses to injury. A variety of physiologic agonists induce a cascade of signaling events termed as inside-out signaling that culminate in exposure of high-affinity binding sites on integrin α(IIb)β(3). Once platelet activation has occurred, integrin α(IIb)β(3) stabilizes thrombus formation by providing agonist-independent "outside-in" signals mediated in part by contractile signaling. Junctional adhesion molecule A (JAM-A), a member of the cortical thymocyte marker of the Xenopus (CTX) family, was initially identified as a receptor for a platelet stimulatory mAb. Here we show that JAM-A in resting platelets functions as an endogenous inhibitor of platelet function. Genetic ablation of Jam-A in mice enhances thrombotic function of platelets in vivo. The absence of Jam-A results in increase in platelet aggregation ex vivo. This gain of function is not because of enhanced inside-out signaling because granular secretion, Thromboxane A2 (TxA2) generation, as well as fibrinogen receptor activation, are normal in the absence of Jam-A. Interestingly, integrin outside-in signaling such as platelet spreading and clot retraction is augmented in Jam-A-deficient platelets. We conclude that JAM-A normally limits platelet accumulation by inhibiting integrin outside-in signaling thus preventing premature platelet activation.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Regulation of platelet biology by platelet endothelial cell adhesion molecule-1

Platelet endothelial cell adhesion molecule-1 (PECAM-1), an immunoreceptor tyrosine-based inhibitory motif containing receptor, plays diverse and apparently contradictory roles in regulating the response of platelets to stimuli; inhibiting platelet response to immunoreceptor tyrosine-based activation motif and G protein-coupled receptor signalling following stimulation with collagen, adenosine diphosphate, and thrombin, as well as enhancing integrin outside-in signalling. These dual, and opposing, roles suggest an important and complex role for PECAM-1 in orchestrating platelet response to vascular damage. Indeed, during thrombus formation, the influence of PECAM-1 on the multiple signalling pathways combines leading to a relatively large inhibitory effect on thrombus formation.

Connexin 37 limits thrombus propensity by downregulating platelet reactivity

BACKGROUND

Formation of platelet plug initiates hemostasis after vascular injury and triggers thrombosis in ischemic disease. However, the mechanisms leading to the formation of a stable thrombus are poorly understood. Connexins comprise a family of proteins that form gap junctions enabling intercellular coordination of tissue activity, a process termed gap junctional intercellular communication.

METHODS AND RESULTS

In the present study, we show that megakaryocytes and platelets express connexin 37 (Cx37). Deletion of the Cx37 gene in mice shortens bleeding time and increases thrombus propensity. Aggregation is increased in murine Cx37(-/-) platelets or in murine Cx37(+/+) and human platelets treated with gap junction blockers. Intracellular microinjection of neurobiotin, a Cx37-permeant tracer, revealed gap junctional intercellular communication in platelet aggregates, which was impaired in Cx37(-/-) platelets and in human platelets exposed to gap junction blockers. Finally, healthy subjects homozygous for Cx37-1019C, a prognostic marker for atherosclerosis, display increased platelet responses compared with subjects carrying the Cx37-1019T allele. Expression of these polymorphic channels in communication-deficient cells revealed a decreased permeability of Cx37-1019C channels for neurobiotin.

CONCLUSIONS

We propose that the establishment of gap junctional communication between Cx37-expressing platelets provides a mechanism to limit thrombus propensity. To our knowledge, these data provide the first evidence incriminating gap junctions in the pathogenesis of thrombosis.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

PECAM-1 functions as a negative regulator of laminin-induced platelet activation

SUMMARY BACKGROUND

Interaction of resting platelets with exposed components of the subendothelial matrix is an important early activating event that takes place at sites of vascular injury. Platelet responses to collagen are mediated by integrin alpha(2)beta(1) and the glycoprotein (GP)VI-Fc receptor (FcR) gamma-chain complex, whereas platelet activation by laminin is mediated by the related integrin, alpha(6)beta(1), and similarly requires signaling through GPVI-FcR gamma-chain.

OBJECTIVE

Because the cell adhesion and signaling receptor PECAM-1 has previously been shown to dampen collagen-induced platelet activation, we sought to determine whether PECAM-1 might similarly regulate platelet activation by laminin.

METHODS/RESULTS

We found that PECAM-1 became tyrosine phosphorylated on its cytoplasmic immunoreceptor tyrosine-based inhibitory motifs following adhesion of either human or murine platelets to immobilized laminin. Whereas the presence or absence of PECAM-1 had no effect on either the rate or extent of platelet adhesion or spreading on laminin, PECAM-1 inhibited laminin-induced phosphorylation of GPVI-FcR gamma-chain immunoreceptor tyrosine-based activation motifs (ITAMs) and activation of its downstream effector, Syk kinase, and suppressed granule secretion.

CONCLUSIONS

Taken together, these data are consistent with previous findings in platelets and other blood and vascular cells that PECAM-1 functions by modulating ITAM-mediated signaling pathways that amplify cellular activation.

Phosphorylation of leukocyte PECAM and its association with detergent-resistant membranes regulate transendothelial migration

Leukocyte migration across the endothelial lining is a critical step in the body's response to infection and inflammation. The homophilic interaction between endothelial PECAM and leukocyte PECAM is essential for this process. The molecular events that are triggered in the endothelial cell by PECAM engagement have been well characterized; however, the function of leukocyte PECAM remains to be elucidated. To study this, we first blocked leukocyte transmigration using anti-PECAM Ab and then specifically activated leukocyte PECAM. This was sufficient to overcome the block and promote transmigration, suggesting an active signaling role for leukocyte PECAM. Consistent with this, we found that ligation of leukocyte PECAM induces phosphorylation of two tyrosine residues on its cytoplasmic tail. By performing RNA interference-rescue experiments, we demonstrate that these phosphorylation events are indispensable for transendothelial migration. Finally, we show that leukocyte PECAM translocates to a detergent-resistant membrane (DRM) during transmigration. PECAM localized in DRMs displays reduced phosphorylation and does not support transmigration. Together, these data support a model whereby engagement of leukocyte PECAM induces its transient tyrosine phosphorylation and induction of downstream signals that drive transmigration. These signals are then downregulated following PECAM translocation to DRMs.

The anti-inflammatory actions of platelet endothelial cell adhesion molecule-1 do not involve regulation of endothelial cell NF-kappa B

PECAM-1 is a cell adhesion and signaling receptor that is expressed on many hematopoietic cells and at endothelial cell-cell junctions. Accumulating evidence from a number of in vitro and in vivo model systems suggests that PECAM-1 suppresses cytokine production and vascular permeability induced by a wide range of inflammatory stimuli. In several of these models of inflammatory disease, endothelial, and not leukocyte or platelet, PECAM-1 conferred protection against inflammatory insult. However, the mechanism by which endothelial PECAM-1 functions as an anti-inflammatory protein is poorly understood. It was recently suggested that PECAM-1 exerts its anti-inflammatory effects in endothelial cells by inhibiting the activity of NF-kappaB, a proinflammatory transcription factor. To confirm and extend these observations, we examined the effect of engaging, cross-linking, or expressing PECAM-1 on NF-kappaB activation in a variety of human cells. PECAM-1 had no effect on the phosphorylation of the NF-kappaB inhibitory protein, IkappaBalpha; on the nuclear translocation of NF-kappaB; on the suppression of cytokine-induced transcriptional activation of an NF-kappaB luciferase reporter plasmid; or on the cytokine-stimulated upregulation of ICAM-1, an NF-kappaB target gene, in endothelial cells. Taken together, these studies strongly suggest that the anti-inflammatory actions of PECAM-1 in endothelial cells are not likely to involve its regulation of NF-kappaB.

PECAM-1 expression and activity negatively regulate multiple platelet signaling pathways

Platelet endothelial cell adhesion molecule-1 (PECAM-1) inhibits platelet response to collagen and may also inhibit two other major platelet agonists ADP and thrombin although this has been less well explored. We hypothesized that the combined effect of inhibiting these three platelet activating pathways may act to significantly inhibit thrombus formation. We demonstrate a negative relationship between PECAM-1 surface expression and platelet response to cross-linked collagen related peptide (CRP-XL) and ADP, and an inhibitory effect of PECAM-1 clustering on platelet response to CRP-XL, ADP and thrombin. This combined inhibition of multiple signaling pathways results in a marked reduction in thrombus formation.

Mechanisms of PECAM-1-mediated cytoprotection and implications for cancer cell survival

Defects in apoptotic pathways can promote cancer development and cause cancers to become resistant to chemotherapy. The cell adhesion and signaling molecule PECAM-1 has been shown to potently suppress apoptosis in a variety of cellular systems. PECAM-1 expression has been reported on a variety of human malignancies-especially hematopoietic and vascular cell cancers-but the significance of this expression has not been fully explored. The ability of PECAM-1 to inhibit apoptosis makes it an attractive candidate as a molecule that may promote cancer development and/or confer resistance to chemotherapeutic treatment. The exact mechanisms by which PECAM-1 mediates its cytoprotection have not been fully defined, but its anti-apoptotic effects have been shown to require both homophilic binding and intracellular signaling via its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. In this review, we will discuss the data regarding PECAM-1's anti-apoptotic effects and ways in which this cytoprotection may be clinically relevant to the development and/or treatment of hematologic malignancies that express this vascular cell-specific surface molecule.

Plasma fibronectin depletion enhances platelet aggregation and thrombus formation in mice lacking fibrinogen and von Willebrand factor

We previously showed that platelet aggregation and thrombus formation occurred in mice lacking both fibrinogen (Fg) and von Willebrand factor (VWF) and that plasma fibronectin (pFn) promoted thrombus growth and stability in injured arterioles in wild-type mice. To examine whether pFn is required for Fg/VWF-independent thrombosis, we generated Fg/VWF/conditional pFn triple-deficient (TKO; Cre+, Fnflox/flox, Fg/VWF−/−) mice and littermate control (Cre−, Fnflox/flox, Fg/VWF−/−) mice. Surprisingly, TKO platelet aggregation was not abolished, but instead was enhanced in both heparinized platelet-rich plasma and gel-filtered platelets. This enhancement was diminished when TKO platelets were aggregated in pFn-positive control platelet-poor plasma (PPP), whereas aggregation was enhanced when control platelets were aggregated in pFn-depleted TKO PPP. The TKO platelet aggregation can be completely inhibited by our newly developed mouse anti–mouse β3 integrin antibodies but was not affected by anti–mouse GPIbα antibodies. Enhanced platelet aggregation was also observed when heparinized TKO blood was perfused in collagen-coated perfusion chambers. Using intravital microscopy, we further showed that thrombogenesis in TKO mice was enhanced in both FeCl3-injured mesenteric arterioles and laser-injured cremaster arterioles. Our data indicate that pFn is not essential for Fg/VWF-independent thrombosis and that soluble pFn is probably an important inhibitory factor for platelet aggregation.

CEACAM1 negatively regulates platelet-collagen interactions and thrombus growth in vitro and in vivo

Carcinoembryonic antigen cell adhesion molecule-1 (CEACAM1) is a surface glycoprotein expressed on various blood cells, epithelial cells, and vascular cells. CEACAM1 possesses adhesive and signaling properties mediated by its intrinsic immunoreceptor tyrosine-based inhibitory motifs that recruit SHP-1 protein-tyrosine phosphatase. In this study, we demonstrate that CEACAM1 is expressed on the surface and in intracellular pools of platelets. In addition, CEACAM1 serves to negatively regulate signaling of platelets by collagen through the glycoprotein VI (GPVI)/Fc receptor (FcR)–γ-chain. ceacam1−/− platelets displayed enhanced type I collagen and GPVI-selective ligand, collagen-related peptide (CRP), CRP-mediated platelet aggregation, enhanced platelet adhesion on type I collagen, and elevated CRP-mediated alpha and dense granule secretion. Platelets derived from ceacam1−/− mice form larger thrombi when perfused over a collagen matrix under arterial flow compared with wild-type mice. Furthermore, using intravital microscopy to ferric chloride-injured mesenteric arterioles, we show that thrombi formed in vivo in ceacam1−/− mice were larger and were more stable than those in wild-type mice. GPVI depletion using monoclonal antibody JAQ1 treatment of ceacam1−/− mice showed a reversal in the more stable thrombus growth phenotype. ceacam1−/− mice were more susceptible to type I collagen–induced pulmonary thromboembolism than wild-type mice. Thus, CEACAM1 acts as a negative regulator of platelet-collagen interactions and of thrombus growth involving the collagen GPVI receptor in vitro and in vivo.

Monocytes downregulate the early stage of collagen-induced platelet activation by a PECAM-1-dependent mechanism

BACKGROUND

Blood vessel damage results in exposure of the subendothelial matrix, to which platelets adhere. Monocytes are recruited and activated at the site of injury.

OBJECTIVES

Here we studied the effect of monocytes on platelet activation induced by exposure to fibrillar collagen.

METHODS

Washed platelets and isolated monocytes (100/1) were coincubated with type I collagen in static adhesion conditions or in suspension. Platelet activation was assessed by measuring RANTES production and alpha-granule secretion. Platelet adherence on immobilized collagen was analyzed by fluorescence confocal microscopy. Cell-cell contacts were prevented by incubating platelets and monocytes in transwell coculture dishes. Experiments were also performed in the presence of soluble recombinant platelet endothelial cell adhesion molecule-1 (PECAM-1) or of antibodies to PECAM-1.

RESULTS

Unexpectedly, unstimulated monocytes limited the initial phase of platelet activation by fibrillar collagen. In adhesion conditions, monocytes reduced the secretion by platelets of the inflammatory chemokine RANTES and of beta-thromboglobulin and the formation of platelet aggregates. The inhibitory effect of monocytes on platelet activation required direct cell-cell contacts between platelets and monocytes. Monocytes also inhibited collagen-induced platelet activation in suspension conditions as assessed by the reduction of P-selectin exposure and RANTES secretion. A recovery of platelet responses was observed in the presence of soluble PECAM-1 and of PECAM-1.3 Fab, indicating that PECAM-1 is involved in monocyte-triggered downregulation of platelet reactivity.

CONCLUSIONS

Our data provide the first evidence that unstimulated monocytes limit the initial phase of platelet activation by collagen via a mechanism that is, at least in part, PECAM-1-dependent.

Downregulation of platelet responsiveness upon contact with LDL by the protein-tyrosine phosphatases SHP-1 and SHP-2

OBJECTIVE

The sensitivity of platelets to aggregating agents increases when low-density lipoprotein (LDL) binds to apolipoprotein E receptor 2' (apoER2'), triggering activation of p38MAPK and formation of thromboxane A2. LDL signaling is terminated by PECAM-1 through recruitment and activation of the Ser/Thr protein phosphatase PP2A, but platelets remain unresponsive to LDL when PECAM-1 activation disappears. We report a second mechanism that halts LDL signaling and in addition lowers platelet responsiveness to aggregating agents.

METHODS AND RESULTS

After a first stimulation with LDL, platelets remain unresponsive to LDL for 60 minutes, despite normal apoER2' activation by a second dose of LDL. A possible cause is persistent activation of the tyrosine phosphatases SHP-1 and SHP-2, which may not only block a second activation of p38MAPK, PECAM-1, and PP2A by LDL but also seem to reduce aggregation by TRAP, collagen, and ADP.

CONCLUSION

These findings reveal that p38MAPK phosphorylation and platelet activation by LDL are suppressed by two mechanisms: (1) short activation of PECAM-1/PP2A, and (2) prolonged activation of SHP-1 and SHP-2. Activation of SHP-1 and SHP-2 is accompanied by reduced responsiveness to aggregating agents, which--if present in vivo--would make LDL an aggregation inhibitor during prolonged contact with platelets.

G6b-B inhibits constitutive and agonist-induced signaling by glycoprotein VI and CLEC-2

Platelets play an essential role in wound healing by forming thrombi that plug holes in the walls of damaged blood vessels. To achieve this, platelets express a diverse array of cell surface receptors and signaling proteins that induce rapid platelet activation. In this study we show that two platelet glycoprotein receptors that signal via an immunoreceptor tyrosine-based activation motif (ITAM) or an ITAM-like domain, namely the collagen receptor complex glycoprotein VI (GPVI)-FcR γ-chain and the C-type lectin-like receptor 2 (CLEC-2), respectively, support constitutive (i.e. agonist-independent) signaling in a cell line model using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay that can detect low level activation of phospholipase Cγ (PLCγ). Constitutive and agonist signaling by both receptors is dependent on Src and Syk family kinases, and is inhibited by G6b-B, a platelet immunoglobulin receptor that has two immunoreceptor tyrosine-based inhibitory motifs in its cytosolic tail. Mutation of the conserved tyrosines in the two immunoreceptor tyrosine-based inhibitory motifs prevents the inhibitory action of G6b-B. Interestingly, the inhibitory activity of G6b-B is independent of the Src homology 2 (SH2)-domain containing tyrosine phosphatases, SHP1 and SHP2, and the inositol 5′-phosphatase, SHIP. Constitutive signaling via Src and Syk tyrosine kinases is observed in platelets and is associated with tyrosine phosphorylation of GPVI-FcR γ-chain and CLEC-2. We speculate that inhibition of constitutive signaling through Src and Syk tyrosine kinases by G6b-B may help to prevent unwanted platelet activation.

An alternatively spliced isoform of PECAM-1 is expressed at high levels in human and murine tissues, and suggests a novel role for the C-terminus of PECAM-1 in cytoprotective signaling

The Ig-ITIM family member PECAM-1 is expressed in vascular and endothelial cells, and its functions include suppression of mitochondria-dependent apoptosis. Previous studies have identified distinct PECAM-1 cytoplasmic domain splice variants at the mRNA, but not protein, level. Several relatively abundant mRNA isoforms lack exon 15 (Delta15) and would theoretically encode a protein with a truncated cytoplasmic domain and a unique C-terminal sequence. Using a novel rabbit polyclonal antibody that specifically recognizes Delta15 PECAM-1, we found that the Delta15 PECAM-1 isoform was expressed in human tissues, including brain, testes and ovary. This isoform was also expressed on the cell surface of human platelets, human umbilical vein endothelial cells (HUVECs) and the Jurkat T-cell leukemia, human erythroleukemia (HEL) and U937 histiocytic lymphoma cell lines. Furthermore, murine platelets and lung lysates demonstrated abundant amounts of exon-15-deficient PECAM-1. Functional studies revealed that Delta15 PECAM-1 retains both its homophilic binding capacity and its ability to signal by means of its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. Delta15 PECAM-1 was unable, however, to protect against apoptosis induced by overexpression of Bax or treatment with the chemotherapy agent etoposide. These studies suggest a novel role for the PECAM-1 C-terminus in cytoprotective signaling and highlight a need for further characterization of expression of PECAM-1 isoforms in normal and malignant tissues.

Future innovations in anti-platelet therapies

Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti-platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet-specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti-platelet agents. In this review, the mechanisms of platelet regulation and current anti-platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti-platelet drug development are discussed.

Novel therapeutic targets at the platelet vascular interface

Platelet activation in vivo can be part of the hemostatic response to injury or a pathological response to disease. In either setting, platelets adhere to the vessel wall and to each other, forming a closely packed mass interspersed with fibrin. Recent studies have identified new molecules on the platelet surface and within platelets that support and regulate thrombus growth and stability, ensuring that platelet accumulation after injury is sufficient to stop bleeding, but not so exuberant that vascular occlusion occurs. An understanding of how this balance is achieved helps to illuminate the events of platelet activation and, at the same time, provides potential targets for new classes of antiplatelet agents.