Platelet adhesion via glycoprotein IIb integrin is critical for atheroprogression and focal cerebral ischemia: an in vivo study in mice lacking glycoprotein IIb

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

The GPVI-Fc fusion protein Revacept improves cerebral infarct volume and functional outcome in stroke

OBJECTIVES

We examined the effect of Revacept, an Fc fusion protein which is specifically linked to the extracellular domain of glycoprotein VI (GPVI), on thrombus formation after vessel wall injury and on experimental stroke in mice.

BACKGROUND

Several antiplatelet drugs for the treatment of myocardial infarction or ischemic stroke with potent anti-ischemic effects have been developed, but all incur a significant risk of bleeding.

METHODS

Platelet adhesion and thrombus formation after endothelial injury was monitored in the carotid artery by intra-vital fluorescence microscopy. The morphological and clinical consequences of stroke were investigated in a mouse model with a one hour-occlusion of the middle cerebral artery.

RESULTS

Thrombus formation was significantly decreased after endothelial injury by 1 mg/kg Revacept i.v., compared to Fc only. 1 mg/kg Revacept i.v. applied in mice with ischemic stroke immediately before reperfusion significantly improved functional outcome, cerebral infarct size and edema compared to Fc only. Also treatment with 10 mg/kg rtPA was effective, and functional outcome was similar in both treatment groups. The combination of Revacept with rtPA leads to increased reperfusion compared to treatment with either agent alone. In contrast to rtPA, however, there were no signs of increased intracranial bleeding with Revacept. Both rtPA and Revacept improved survival after stroke compared to placebo treatment. Revacept and vWF bind to collagen and Revacept competitively prevented the binding of vWF to collagen.

CONCLUSIONS

Revacept reduces arterial thrombus formation, reduces cerebral infarct size and edema after ischemic stroke, improves functional and prognostic outcome without intracranial bleeding. Revacept not only prevents GPVI-mediated, but probably also vWF-mediated platelet adhesion and aggregate formation. Therefore Revacept might be a potent and safe tool to treat ischemic complications of stroke.

Platelets in atherosclerosis and thrombosis

Rupture of an atherosclerotic plaque exposes a thrombogenic matrix, which instantly triggers platelet tethering and activation. We here delineate the sequence of events during arterial thrombus formation and dissect the specific role of the various platelet receptors in this process. We also discuss the interplay of platelets with circulating immune cells, which support arterial thrombosis by fibrin formation in a process that involves extracellular nucleosomes. In the second part of this chapter we describe the role of platelets in atherosclerotic lesion formation. Platelets adhere to the dysfunctional endothelium early during atherogenesis. They contain a large machinery of proinflammatory molecules, which can be released upon their activation. This prepares the ground for subsequent leukocyte recruitment and infiltration, and boosts the inflammatory process of the arterial wall. Together, platelets play a critical role in both acute and chronic processes of the vascular wall, which makes them an attractive target for pharmacological strategies to treat arterial thrombosis and, potentially, also atheroprogression.

The Inflammatory Chemokine CXC Motif Ligand 16 Triggers Platelet Activation and Adhesion Via CXC Motif Receptor 6–Dependent Phosphatidylinositide 3-Kinase/Akt Signaling

Rationale:

The recently discovered chemokine CXC motif ligand 16 (CXCL16) is highly expressed in atherosclerotic lesions and is a potential pathogenic mediator in coronary artery disease.

Objective:

The aim of this study was to test the role of CXCL16 on platelet activation and vascular adhesion, as well as the underlying mechanism and signaling pathway.

Methods and Results:

Reverse-transcriptase polymerase chain reaction, Western blotting, confocal microscopy, and flow cytometry revealed that CXCL16-specific receptor, CXC motif receptor 6, is highly expressed in platelets. According to flow cytometry and confocal microscopy, stimulation of platelets with CXCL16 induced platelet degranulation, integrin α IIb β 3 activation, and shape change. CXCL16 increased Akt phosphorylation (Thr 308 /Ser 473 ), an effect abrogated by phosphatidylinositide 3-kinase inhibitors wortmannin (100 nmol/L) and LY294002 (25 µmol/L). The phosphatidylinositide 3-kinase inhibitors and Akt inhibitor SH-6 (20 µmol/L) further diminished CXCL16-induced platelet activation. CXCL16-mediated platelet degranulation, integrin α IIb β 3 activation, and Akt phosphorylation were blunted in platelets lacking CXCL16-specific receptor CXC motif receptor 6. CXCL16-induced platelet activation was abrogated in Akt1- or Akt2-deficient platelets. CXCL16 enhanced platelet adhesion to endothelium in vitro after high arterial shear stress (2000 −s ) and to injured vascular wall in vivo after carotid ligation. CXCL16-induced stimulation of platelet adhesion again was prevented by phosphatidylinositide 3-kinase and Akt inhibitors. Apyrase and antagonists of platelet purinergic receptors P 2 Y 1 (MRS2179, 100 µmol/L) and especially P 2 Y 12 (Cangrelor, 10 µmol/L) blunted CXCL16-triggered platelet activation as well as CXCL16-induced platelet adhesion under high arterial shear stress in vitro and after carotid ligation in vivo.

Conclusions:

The inflammatory chemokine CXCL16 triggers platelet activation and adhesion via CXC motif receptor 6–dependent phosphatidylinositide 3-kinase/Akt signaling and paracrine activation, suggesting a decisive role for CXCL16 in linking vascular inflammation and thrombo-occlusive diseases.

High platelet reactivity on clopidogrel therapy correlates with increased coronary atherosclerosis and calcification: a volumetric intravascular ultrasound study

OBJECTIVES

This study sought to evaluate the relationship between platelet reactivity and atherosclerotic burden in patients undergoing percutaneous coronary intervention (PCI) with pre-intervention volumetric intravascular ultrasound (IVUS) imaging.

BACKGROUND

Atherosclerosis progresses by the pathologic sequence of subclinical plaque rupture, thrombosis, and healing. In this setting, increased platelet reactivity may lead to more extensive arterial thrombosis at the time of plaque rupture, leading to a more rapid progression of the disease. Alternatively, abnormal vessel wall biology with advanced atherosclerosis is known to enhance platelet reactivity. Therefore, it is possible that by either mechanism, increased platelet reactivity may be associated with greater atherosclerotic burden.

METHODS

This study included patients who underwent PCI with pre-intervention IVUS imaging and platelet reactivity functional assay (P2Y(12) reaction units) performed >16 h after PCI, after the stabilization of clopidogrel therapy (administered before PCI). Platelet reactivity >230 P2Y(12) reaction units defined high on-treatment platelet reactivity (HPR).

RESULTS

Among 335 patients (mean age 65.0 years, 71% men), there were 109 patients with HPR (32.5%) and 226 without HPR (67.5%), with HPR being associated with diabetes and chronic renal insufficiency. By IVUS analysis, patients with HPR had significantly greater target lesion calcium lengths, calcium arcs, and calcium indexes. Furthermore, patients with HPR tended to have longer lesions and greater volumetric dimensions, indicating higher plaque volume, larger total vessel volume, and also greater luminal volume, despite similar plaque burden. By multivariate analysis controlling for baseline clinical variables, HPR was the single consistent predictor of all IVUS parameters examined, including plaque volume, calcium length, and calcium arc.

CONCLUSIONS

Increased platelet reactivity on clopidogrel treatment, defined as >230 P2Y(12) reaction units, is associated with greater coronary artery atherosclerotic disease burden and plaque calcification.

Contribution of endothelial progenitor cells to neovascularization (Review)

Endothelial progenitor cells (EPCs) are a cell population mobilized from bone marrow into the peripheral circulation and recruited into sites of vessel injury to participate in blood vessel formation in both physiological and pathological conditions. Due to the lack of unique surface markers and different isolation methods, EPCs represent heterogeneous cell populations including cells of myeloid or endothelial origin. Evidence suggests that EPCs play a critical role in postnatal blood vessel formation and vascular homeostasis and provide a promising therapy for vascular disease. However, the mechanisms by which EPCs participate in new vessel formation are still incompletely understood. We review the process of EPCs in neovascularization including EPC mobilization, migration, adhesion and effect on new vessel formation, in an attempt to better understand the underlying mechanisms and to provide potential effective management for the treatment of patients with vascular disease.

Fractalkine is expressed in early and advanced atherosclerotic lesions and supports monocyte recruitment via CX3CR1

Fractalkine (CX3CL1, FKN) is expressed in the inflamed vascular wall and absence of FKN reduces atherogenesis. Whether FKN is expressed throughout all stages of atherosclerotic disease and whether it directly contributes to monocyte recruitment to atherosclerotic lesions is not known. We collected human atherosclerotic plaque material and blood samples from patients with carotid artery disease undergoing endarterectomy. Plaques were analyzed by immunohistochemistry and qPCR. We found that FKN is expressed at all stages of atherosclerotic lesion formation, and that the number of FKN-expressing cells positively correlates with the number of CX3CR1-positive cells in human carotid artery plaques. In the circulation, soluble FKN levels are significantly elevated in the presence of high-grade (sub-occlusive) stenosis. To determine the role of the FKN-CX3CR1 axis for monocyte adhesion in vivo we then performed intravital videofluorescence microscopy of the carotid artery in ApoE(-/-) mice. Notably, FKN-CX3CR1 interactions are critical for recruitment of circulating monocytes to the injured atherosclerotic vascular wall. Thus, this chemokine dyad could represent an attractive target for anti-atherosclerotic strategies.

Intracellular cyclophilin A is an important Ca(2+) regulator in platelets and critically involved in arterial thrombus formation

Platelet adhesion and aggregation play a critical role in primary hemostasis. Uncontrolled platelet activation leads to pathologic thrombus formation and organ failure. The decisive central step for different processes of platelet activation is the increase in cytosolic Ca(2+) activity ([Ca(2+)](i)). Activation-dependent depletion of intracellular Ca(2+) stores triggers Ca(2+) entry from the extracellular space. Stromal interaction molecule 1 (STIM1) has been identified as a Ca(2+) sensor that regulates store-operated Ca(2+) entry through activation of the pore-forming subunit Orai1, the major store-operated Ca(2+) entry channel in platelets. In the present study, we show for the first time that the chaperone protein cyclophilin A (CyPA) acts as a Ca(2+) modulator in platelets. CyPA deficiency strongly blunted activation-induced Ca(2+) mobilization from intracellular stores and Ca(2+) influx from the extracellular compartment and thus impaired platelet activation substantially. Furthermore, the phosphorylation of the Ca(2+) sensor STIM1 was abrogated upon CyPA deficiency, as shown by immunoprecipitation studies. In a mouse model of arterial thrombosis, CyPA-deficient mice were protected against arterial thrombosis, whereas bleeding time was not affected. The results of the present study identified CyPA as an important Ca(2+) regulator in platelets, a critical mechanism for arterial thrombosis.

Platelets selectively enhance lymphocyte adhesion on subendothelial matrix under arterial flow conditions

Platelet adhesion at sites of cardiovascular injury may facilitate leukocyte deposition. We asked if and how platelets enhance lymphocyte adhesion on different subendothelial matrix protein (SEMP)-coated surface at arterial shear stress. Hirudinised whole blood was subjected to an arterial shear rate (500 s(-1)) in a Cone and Plate(let) analyser (CPA) for 5 minutes using plates coated with bovine serum albumin (BSA), collagen, fibrinogen, von Willebrand factor (vWF), or fibronectin. Platelet and lymphocyte adhesion were monitored by CPA and flow cytometry. Exposure of blood to collagen, fibrinogen, and vWF-coated surfaces induced platelet activation. The most marked effect was seen with collagen-coating, which markedly enhanced the adhesion of all lymphocyte subpopulations compared to BSA-coating. Fibrinogen-coating supported both T and NK cell adhesion, while vWF-coated surface only enhanced NK cell deposition. In contrast, fibronectin enhanced neither platelet activation nor lymphocyte adhesion. Moreover, platelets preferentially facilitated adhesion of large CD4(+) and CD8(+) T cells and NK cells, and of small B cells. Enhanced cell adhesion of larger lymphocytes was associated with elevated platelet conjugation and higher lymphocyte expression of PSGL-1, Mac-1, and CD40L. The enhancement of lymphocyte adhesion was totally platelet-dependent, and was abolished in platelet-depleted blood. Moreover, blockade of the platelet adhesion molecules P-selectin, GPIIb/IIIa, and CD40L attenuated platelet-dependent lymphocyte deposition. In conclusion, platelets support lymphocyte adhesion on SEMP-coated surfaces under arterial shear. The enhancement is selective for large T and NK cells and small B cells.

eNOS protects from atherosclerosis despite relevant superoxide production by the enzyme in apoE mice

Background

All three nitric oxide synthase (NOS) isoforms are expressed in atherosclerotic plaques. NOS enzymes in general catalyse NO production. However, under conditions of substrate and cofactor deficiency, the enzyme directly catalyse superoxide formation. Considering this alternative chemistry, the effects of NOS on key events in spontaneous hyperlipidemia driven atherosclerosis have not been investigated yet. Here, we evaluate how endothelial nitric oxide synthase (eNOS) modulates leukocyte/endothelial- (L/E) and platelet/endothelial- (P/E) interactions in atherosclerosis and the production of nitric oxide (NO) and superoxide by the enzyme.

Principal Findings

Intravital microscopy (IVM) of carotid arteries revealed significantly increased L/E-interactions in apolipoproteinE/eNOS double knockout mice (apoE^−/−/eNOS^−/−), while P/E-interactions did not differ, compared to apoE^−/−. eNOS deficiency increased macrophage infiltration in carotid arteries and vascular cell adhesion molecule-1 (VCAM-1) expression, both in endothelial and smooth muscle cells. Despite the expression of other NOS isoforms (inducible NOS, iNOS and neuronal NOS, nNOS) in plaques, Electron Spin Resonance (ESR) measurements of NO showed significant contribution of eNOS to total circulating and vascular wall NO production. Pharmacological inhibition and genetic deletion of eNOS reduced vascular superoxide production, indicating uncoupling of the enzyme in apoE^−/− vessels.

Conclusion

Overt plaque formation, increased vascular inflammation and L/E- interactions are associated with significant reduction of superoxide production in apoE^−/−/eNOS^−/− vessels. Therefore, lack of eNOS does not cause an automatic increase in oxidative stress. Uncoupling of eNOS occurs in apoE^−/− atherosclerosis but does not negate the enzyme's strong protective effects.

FSP-1 Silencing in Bone Marrow Cells Suppresses Neointima Formation in Vein Graft

Rationale:

Fibroblast-specific protein 1 (FSP-1) plays multiple roles in promoting cell proliferation and motility. Increased FSP-1 expression in smooth muscle cells (SMCs) has been associated with their enhanced proliferation.

Objective:

To study how FSP-1 contributes to neointima formation of vein grafts.

Methods:

Arteriovenous grafts were created in wild-type or FSP-1–GFP mice (green fluorescent protein expression regulated by FSP-1 promoter). The effects of FSP-1 on bone marrow (BM) cell migration and on SMC proliferation were studied in vivo and in vitro.

Results:

On creation of a vein graft, there was rapid deposition of platelets on the denuded surface leading to secretion of the chemokine stromal cell–derived factor-1α (SDF-1α). This was followed by recruitment of BM-derived cells expressing the SDF-1α receptor CXCR4; homing of FSP-1–positive cells was found to be dependent on platelet-derived SDF-1α. FSP-1 was expressed in 8% of the BM cells, and 20% of these express CD45; 85% of FSP-1–positive cells express CD11b. We found that the FSP-1–positive cells migrated into the vein graft in a Rac-1–dependent fashion. FSP-1 expression was also found to stimulate proliferation of SMCs through a MEK5-ERK5 signaling pathway that can be suppressed by a dominant-negative Rac1. Consequently, knocking down FSP-1 expression in BM cells prevented neointimal formation.

Conclusions:

BM-derived FSP-1 + cells enhance neointima formation through an increase in transendothelial invasion with stimulation of SMC proliferation. The Rac1 and ERK5 signaling cascade mediate FSP-1–induced responses in SMCs and BM cells. This novel pathophysiology suggests a new therapeutic target, FSP-1, for preventing the development of neointima in vein grafts.

Platelets in atherosclerosis

Beyond obvious functions in haemostasis and thrombosis, platelets are considered to be essential in proinflammatory surroundings such as atherosclerosis, allergy, rheumatoid arthritis and even cancer. In atherosclerosis, platelets facilitate the recruitment of inflammatory cells towards the lesion sites and release a plethora of inflammatory mediators, thereby enriching and boosting the inflammatory milieu. Platelets do so by interacting with endothelial cells, circulating leukocytes (monocytes, neutrophils, dendritic cells, T-cells) and progenitor cells. This cross-talk enforces leukocyte activation, adhesion and transmigration. Furthermore, platelets are known to function in innate host defense through the release of antimicrobial peptides and the expression of pattern recognition receptors. In severe sepsis, platelets are able to trigger the formation of neutrophil extracellular traps (NETs), which bind and clear pathogens. The present antiplatelet therapies that target key pathways of platelet activation and aggregation therefore hold the potential to modulate platelet-derived immune functions by reducing cellular interactions of platelets with other immune components and by reducing the secretion of inflammatory proteins into the milieu. The objective of this review is to update and discuss the current perceptions of the platelet immune constituents and their prospect as therapeutic targets in an atherosclerotic setting.

The PPAR-Platelet Connection: Modulators of Inflammation and Potential Cardiovascular Effects

Historically, platelets were viewed as simple anucleate cells responsible for initiating thrombosis and maintaining hemostasis, but clearly they are also key mediators of inflammation and immune cell activation. An emerging body of evidence links platelet function and thrombosis to vascular inflammation. peroxisome proliferator-activated receptors (PPARs) play a major role in modulating inflammation and, interestingly, PPARs (PPARβ/δ and PPARγ) were recently identified in platelets. Additionally, PPAR agonists attenuate platelet activation; an important discovery for two reasons. First, activated platelets are formidable antagonists that initiate and prolong a cascade of events that contribute to cardiovascular disease (CVD) progression. Dampening platelet release of proinflammatory mediators, including CD40 ligand (CD40L, CD154), is essential to hinder this cascade. Second, understanding the biologic importance of platelet PPARs and the mechanism(s) by which PPARs regulate platelet activation will be imperative in designing therapeutic strategies lacking the deleterious or unwanted side effects of current treatment options.

How platelets safeguard vascular integrity

The haemostatic role of platelets was established in the 1880s by Bizzozero who observed their ability to adhere and aggregate at sites of vascular injury. It was only some 80 years later that the function of platelets in maintaining the structural integrity of intact blood vessels was reported by Danielli. Danielli noted that platelets help preserve the barrier function of endothelium during organ perfusion. Subsequent studies have demonstrated further that platelets are continuously needed to support intact mature blood vessels. More recently, platelets were shown to safeguard developing vessels, lymphatics, as well as the microvasculature at sites of leukocyte infiltration, including inflamed organs and tumours. Interestingly, from a mechanistic point of view, the supporting role of platelets in these various vessels does not necessarily involve the well-understood process of platelet plug formation but, rather, may rely on secretion of the various platelet granules and their many active components. The present review focuses on these nonconventional aspects of platelet biology and function by presenting situations in which platelets intervene to maintain vascular integrity and discusses possible mechanisms of their actions. We propose that modulating these newly described platelet functions may help treat haemorrhage as well as treat cancer by increasing the efficacy of drug delivery to tumours.

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

EMMPRIN (CD147/basigin) mediates platelet-monocyte interactions in vivo and augments monocyte recruitment to the vascular wall

BACKGROUND

Platelets play a central role in hemostasis, in inflammatory diseases such as atherosclerosis, and during thrombus formation following vascular injury. Thereby, platelets interact intensively with monocytes and enhance their recruitment to the vascular wall.

OBJECTIVES

To investigate the role of the extracellular matrix metalloproteinase inducer (EMMPRIN) in platelet-monocyte interactions.

METHODS AND RESULTS

Isolated human monocytes were perfused in vitro over firmly adherent platelets to allow investigation of the role of EMMPRIN in platelet-monocyte interactions under flow conditions. Monocytes readily bound to surface-adherent platelets. Both antibody blockade and gene silencing of monocyte EMMPRIN substantially attenuated firm adhesion of monocytes to platelets at arterial and venous shear rates. In vivo, platelet interactions with the murine monocyte cell line ANA-1 were significantly decreased when ANA-1 cells were pretreated with EMMPRIN-silencing small interfering RNA prior to injection into wild-type mice. Using intravital microscopy, we showed that recruitment of EMMPRIN-silenced ANA-1 to the injured carotid artery was significantly reduced as compared with control cells. Further silencing of EMMPRIN resulted in significantly fewer ANA-1-platelet aggregates in the mouse circulation as determined by flow cytometry. Finally, we identified glycoprotein (GP)VI as a critical corresponding receptor on platelets that mediates interaction with monocyte EMMPRIN. Thus, blocking of GPVI inhibited the effect of EMMPRIN on firm monocyte adhesion to platelets under arterial flow conditions in vitro, and abrogated EMMPRIN-mediated platelet-monocyte aggregate formation in vivo.

CONCLUSIONS

EMMPRIN supports platelet-monocyte interactions and promotes monocyte recruitment to the arterial wall. Therefore, EMMPRIN might represent a novel target to reduce vascular inflammation and atherosclerotic lesion development.

Endothelial cell-borne platelet bridges selectively recruit monocytes in human and mouse models of vascular inflammation

AIMS

Cells of the monocyte lineage are the most abundant inflammatory cells found in atherosclerotic lesions. Dominance of the inflammatory infiltrate by monocytes indicates that there is a disease-driven mechanism supporting their selective recruitment. Previous studies have demonstrated that interactions between endothelial cells (ECs) and platelets may promote monocyte recruitment. In this study, we sought to expand on this knowledge using a complex coculture model of the diseased vessel wall.

METHODS AND RESULTS

Using primary human cells in an in vitro flow-based adhesion assay, we found that secretory arterial smooth muscle cells (SMCs), cocultured with ECs, promote preferential recruitment of monocytes from blood in a TGF-β1-dependent manner. Approximately 85% of leucocytes recruited to the endothelium were CD14(+). Formation of adhesive platelet bridges on ECs was essential for monocyte recruitment as platelet removal or inhibition of adhesion to the ECs abolished monocyte recruitment. Monocytes were recruited from flow by platelet P-selectin and activated by EC-derived CC chemokine ligand 2 (CCL2), although the presentation of CCL2 to adherent monocytes was dependent upon platelet activation and release of CXC chemokine ligand 4 (CXCL4). In an intravital model of TGF-β1-driven vascular inflammation in mice, platelets were also necessary for efficient leucocyte recruitment to vessels of the microcirculation in the cremaster muscle.

CONCLUSIONS

In this study, we have demonstrated that stromal cells found within the diseased artery wall may promote the preferential recruitment of monocytes and this is achieved by establishing a cascade of interactions between SMCs, ECs, platelets, and monocytes.

Antiplatelet activity of white and pink Nelumbo nucifera Gaertn flowers

Nelumbo nucifera Gaertn (Nelumbonaceae) a plant used in Ayurvedic medicine (common name: lotus), is a perennial, large and rhizomatous aquatic herb most prevalent in South India. Preliminary phytochemical screening of both white and pink Nelumbo nucifera flowers revealed the presence of phytochemical constituents (flavonoids, alkaloids, phenols etc,). Hence, an attempt has been made to screen the effect of Nelumbo nucifera flowers (both types) on platelet aggregation. The antiplatelet activity of hydroethanolic extract of both types of flowers was studied using platelet-rich plasma in different concentrations (100-500µg/ml). Both white and pink Nelumbo nucifera flower extracts showed dose-dependent effective antiplatelet activity with maximum activity at 500µg/ml concentration; prevention of platelet aggregation was 50% of that achieved with standard aspirin. Furthermore, the antiplatelet activity of white flowers was relatively high (p<0.05; ANOVA) compared to pink flowers. In conclusion, these observations suggest that both varieties of Nelumbo nucifera flower extracts exert different levels of inhibitory action on platelets in vitro (secretion and platelet aggregation suppression) due to differences in phytochemical content (alkaloids, flavonoids, phenols, tannins, phytosteroids, glycosides and saponins).

Impact of glycoprotein VI and platelet adhesion on atherosclerosis--a possible role of fibronectin

Glycoprotein VI (GPVI) mediates binding of platelets to subendothelial collagen during acute arterial thrombosis. GPVI interactions with the activated atherosclerotic vascular endothelium during early atherosclerosis, however, are not well understood. In ApoE-/- mice, platelet adhesion to atherosclerotic arteries was increased, as measured by intravital microscopy. This platelet adhesion was significantly inhibited by IV injection of GPVI-Fc (1 mg/kg body weight). Atherosclerosis in ApoE-/- mice was attenuated both after 7 and 10 weeks of treatment with the anti-GPVI antibody JAQ1 (2 mg/kg body weight i.p. twice weekly). Binding of GPVI-Fc (1 mg/kg IV) occurred to deeper layers, but also to the luminal site of plaques in atherosclerotic rabbits, but not to the vessel wall of healthy littermates. Gene transfer of GPVI-Fc to the carotid vascular wall significantly attenuated athero-progression and endothelial dysfunction in atherosclerotic rabbits in vivo. Specific binding of the soluble GPVI receptor (GPVI-Fc) to fibronectin was found in vitro to coated ELISA plates. Platelet adhesion to fibronectin was significantly inhibited both by GPVI-Fc and by the anti-GPVI antibody 5C4 ex vivo in flow chamber experiments. GPVI plays a role in platelet adhesion to atherosclerotic endothelium in the absence of plaque rupture. Inhibition of GPVI both via GPVI-Fc and anti-GPVI-antibodies results in protection against atherosclerosis in both cholesterol-fed rabbits and ApoE-/- mice. This novel mechanism of GPVI-mediated platelet adhesion-possibly via fibronectin-could relevantly contribute to platelet-triggered atheroprogression.

High-density lipoproteins, platelets and the pathogenesis of atherosclerosis

1. Prospective and interventional studies demonstrate an inverse relationship between plasma high-density lipoprotein (HDL)-cholesterol and the incidence of coronary artery disease. Although the atheroprotective effects of HDL are usually attributed to the reverse cholesterol transport, in which HDL shuttles cholesterol from cells in the arterial wall to the liver, other mechanisms are also under investigation. 2. Platelets are involved in both the initiation and progression of atherosclerotic lesions. In addition, the formation of thrombi over ruptured atherosclerotic plaques results in the narrowing or complete occlusion of coronary arteries. Current experimental evidence suggests that HDL may exert antiplatelet effects and thereby counteract the development of atherothrombotic vascular disease. 3. In vitro studies show that HDL inhibits agonist-stimulated platelet aggregation, fibrinogen binding, granule secretion and liberation of thromboxane A(2). Inhibitory effects of HDL are mediated, in part, by scavenger receptor type B1 and/or the apolipoprotein E receptor apoER2/LRP8 and are linked to the induction of intracellular signalling cascades encompassing stimulation of protein kinase C, cytoplasmatic alkalization and generation of nitric oxide. 4. Populational studies demonstrate that there is an inverse association between plasma HDL levels and recurrent venous thromboembolism. In addition, HDL-cholesterol has been identified as an independent predictor of acute platelet thrombus formation. The administration of reconstituted HDL particles in humans attenuates ex vivo platelet activation. 5. The present review summarizes recent advances in understanding HDL-platelet interactions and discusses the potential use of HDL-like particles in the therapy of thrombosis.

Platelet interleukin-1alpha drives cerebrovascular inflammation

White blood cell infiltration across an activated brain endothelium contributes to neurologic disease, including cerebral ischemia and multiple sclerosis. Identifying mechanisms of cerebrovascular activation is therefore critical to our understanding of brain disease. Platelet accumulation in microvessels of ischemic mouse brain was associated with endothelial activation in vivo. Mouse platelets expressed interleukin-1alpha (IL-1alpha), but not IL-1beta, induced endothelial cell adhesion molecule expression (ICAM-1 and VCAM-1), and enhanced the release of CXC chemokine CXCL1 when incubated with primary cultures of brain endothelial cells from wild-type or IL-1alpha/beta-deficient mice. A neutralizing antibody to IL-1alpha (but not IL-1beta) or application of IL-1 receptor antagonist inhibited platelet-induced endothelial activation by more than 90%. Platelets from IL-1alpha/beta-deficient mice did not induce expression of adhesion molecules in cerebrovascular endothelial cells and did not promote CXCL1 release in vitro. Conditioned medium from activated platelets induced an IL-1alpha-dependent activation of mouse brain endothelial cells and supported the transendothelial migration of neutrophils in vitro. Thus, we have identified platelets as a key source of IL-1alpha and propose that platelet activation of brain endothelium via IL-1alpha is a critical step for the entry of white blood cells, major contributors to inflammation-mediated injury in the brain.

Leukocyte-endothelial interactions in inflammation

At sites of inflammation, infection or vascular injury local proinflammatory or pathogen-derived stimuli render the luminal vascular endothelial surface attractive for leukocytes. This innate immunity response consists of a well-defined and regulated multi-step cascade involving consecutive steps of adhesive interactions between the leukocytes and the endothelium. During the initial contact with the activated endothelium leukocytes roll along the endothelium via a loose bond which is mediated by selectins. Subsequently, leukocytes are activated by chemokines presented on the luminal endothelial surface, which results in the activation of leukocyte integrins and the firm leukocyte arrest on the endothelium. After their firm adhesion, leukocytes make use of two transmigration processes to pass the endothelial barrier, the transcellular route through the endothelial cell body or the paracellular route through the endothelial junctions. In addition, further circulating cells, such as platelets arrive early at sites of inflammation contributing to both coagulation and to the immune response in parts by facilitating leukocyte-endothelial interactions. Platelets have thereby been implicated in several inflammatory pathologies. This review summarizes the major mechanisms and molecules involved in leukocyte-endothelial and leukocyte-platelet interactions in inflammation.

Atherogenesis and atherothrombosis--focus on diabetes mellitus

Diabetes mellitus represents a major cause of cardiovascular morbidity and mortality in developed countries, and atherothrombosis accounts for most deaths among patients with diabetes mellitus. Atherothrombosis is defined as atherosclerotic lesion disruption with superimposed thrombus formation. As a long-term, progressive disease process, atherosclerosis often results in an acute atherothrombotic event through plaque rupture and formation of a platelet-rich thrombus. The principal clinical manifestations of atherothrombosis are sudden cardiac death, myocardial infarction, ischaemic stroke, and peripheral arterial ischaemia comprising both intermittent claudication and critical limb ischaemia. Atherosclerosis is the leading cause of morbidity and mortality in the industrialized world, and diabetes mellitus magnifies the risk of cardiovascular events. In addition to the well-known microvascular complications of diabetes mellitus - such as nephropathy, retinopathy and neuropathy - the risk of macrovascular complications affecting the large conduit arteries markedly increases in patients with diabetes mellitus.

Structural and therapeutic insights from the species specificity and in vivo antithrombotic activity of a novel alphaIIb-specific alphaIIbbeta3 antagonist

We previously reported on a novel compound (Compound 1; RUC-1) identified by high-throughput screening that inhibits human alphaIIbbeta3. RUC-1 did not inhibit alphaVbeta3, suggesting that it interacts with alphaIIb, and flexible ligand/rigid protein molecular docking studies supported this speculation. We have now studied RUC-1's effects on murine and rat platelets, which are less sensitive than human to inhibition by Arg-Gly-Asp (RGD) peptides due to differences in the alphaIIb sequences contributing to the binding pocket. We found that RUC-1 was much less potent in inhibiting aggregation of murine and rat platelets. Moreover, RUC-1 potently inhibited fibrinogen binding to murine platelets expressing a hybrid alphaIIbbeta3 receptor composed of human alphaIIb and murine beta3, but not a hybrid receptor composed of murine alphaIIb and human beta3. Molecular docking studies of RUC-1 were consistent with the functional data. In vivo studies of RUC-1 administered intraperitoneally at a dose of 26.5 mg/kg demonstrated antithrombotic effects in both ferric chloride carotid artery and laser-induced microvascular injury models in mice with hybrid halphaIIb/mbeta3 receptors. Collectively, these data support RUC-1's specificity for alphaIIb, provide new insights into the alphaIIb binding pocket, and establish RUC-1's antithrombotic effects in vivo.

Disruption of SEMA4D ameliorates platelet hypersensitivity in dyslipidemia and confers protection against the development of atherosclerosis

OBJECTIVE

In dyslipidemic states, platelets become hyperreactive, secreting molecules that promote atherosclerosis. We have shown that the semaphorin family member, sema4D (CD100), is expressed on the surface of platelets and proposed that its role includes promoting thrombus growth by binding to nearby platelets and endothelial cells, both of which express sema4D receptors. Here we tested the hypothesis that deleting sema4D will attenuate the adverse consequences of dyslipidemia on platelets and the vessel wall.

METHODS AND RESULTS

Platelet function and atherosclerotic lesion formation were measured in LDLR(-/-) and sema4D(-/-)LDLR(-/-) mice after 6 months on a high-fat diet. All of the mice developed the dyslipidemia expected on this diet in the absence of functional LDL receptors. However, when compared to LDLR(-/-) mice, sema4D(-/-) LDLR(-/-) mice had reduced lipid deposition in the descending aorta, a 6-fold decrease in the frequency of arterial occlusion and a reduction to near wild-type levels in the accumulation of platelets after injury. These differences were retained ex vivo, with a marked decrease in platelet accumulation on collagen under flow and in platelet aggregation.

CONCLUSIONS

These results show that loss of sema4D expression reduces the platelet hyperactivity otherwise found in dyslipidemia, and confers protection against the development of atherosclerosis.

Intercellular Communication in Atherosclerosis

Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.

Atherosclerosis proceeds independently of thrombin-induced platelet activation in ApoE-/- mice

Platelet activation has long been postulated to contribute to the development of atherosclerotic plaques, although the mechanism by which this might occur remains unknown. Thrombin is a potent platelet activator and transfusion of thrombin-activated platelets into mice increases plaque formation, suggesting that thrombin-induced platelet activation might contribute to platelet-dependent atherosclerosis. Platelets from protease-activated receptor 4-deficient (Par4-/-) mice fail to respond to thrombin. To determine whether thrombin-activated platelets play a necessary role in a model of atherogenesis, we compared plaque formation and progression in Par4+/+ and Par4-/- mice in the atherosclerosis-prone apolipoprotein E-deficient (ApoE-/-) background. Littermate Par4+/+ and Par4-/- mice, all ApoE-/-, were placed on a Western diet (21% fat, 0.15% cholesterol) for 5 or 10 weeks. The percent of aortic lumenal surface covered by plaques in Par4+/+ and Par4-/- mice was not different at either time point (2.2+/-0.3% vs. 2.5+/-0.2% and 5.1+/-0.4% vs. 5.6+/-0.4% after 5 and 10 weeks, respectively). Further, no differences were detected in the cross-sectional area of plaques measured at the aortic root (1.53+/-0.17 vs. 1.66+/-0.16x10(5)microm(2) and 12.56+/-1.23 vs. 13.03+/-0.55x10(5)microm(2) after 5 and 10 weeks, respectively). These findings indicate that thrombin-mediated platelet activation is not required for the early development of atherosclerotic plaques in the ApoE-/- mouse model and suggest that, if platelet activation is required for plaque formation under these experimental conditions, platelet activators other than thrombin suffice.

Species differences in small molecule binding to alpha IIb beta 3 are the result of sequence differences in 2 loops of the alpha IIb beta propeller

Compared with human platelets, rodent platelets are less responsive to peptides and peptidomimetics containing an arginine-glycine-aspartic acid (RGD) motif. Using chimeric human-rat alphaIIbbeta3 molecules, we found that this difference in Arg-Gly-Asp-Ser (RGDS) sensitivity was the result of amino acid substitutions at residues 157, 159, and 162 in the W3:4-1 loop and an Asp-His replacement at residue 232 in the W4:4-1 loop of the alphaIIb beta propeller. Introducing the entire rat W3:4-1 and W4:4-1 loops into human alphaIIbbeta3 also decreased the inhibitory effect of the disintegrins, echistatin and eristostatin, and the alphaIIbbeta3 antagonists, tirofiban and eptifibatide, on fibrinogen binding, whereas the specific point mutations did not. This suggests that RGDS interacts with alphaIIb in a different manner than with these small molecules. None of these species-based substitutions affected the ability of alphaIIbbeta3 to interact with RGD-containing macromolecules. Thus, human von Willebrand factor contains an RGD motif and binds equally well to adenosine diphosphate-stimulated human and rodent platelets, implying that other motifs are responsible for maintaining ligand binding affinity. Many venoms contain RGD-based toxins. Our data suggest that these species amino acids differences in the alphaIIb beta-propeller represent an evolutionary response by rodents to maintain hemostasis while concurrently protecting against RGD-containing toxins.

Molecular mechanisms of thrombus formation in ischemic stroke: novel insights and targets for treatment

In ischemic stroke, treatment options are limited. Therapeutic thrombolysis is restricted to the first few hours after stroke, and the utility of current platelet aggregation inhibitors, including GPIIb/IIIa receptor antagonists, and anticoagulants is counterbalanced by the risk of intracerebral bleeding complications. Numerous attempts to establish neuroprotection in ischemic stroke have been unfruitful. Thus, there is strong demand for novel treatment strategies. Major advances have been made in understanding the molecular functions of platelet receptors such as glycoprotein Ib (GPIb) and GPVI and their downstream signaling pathways that allow interference with their function. Inhibition of these receptors in the mouse stroke model of transient middle cerebral artery occlusion prevented infarctions without increasing the risk of intracerebral bleeding. Similarly, it is now clear that the intrinsic coagulation factor XII (FXII) and FXI play a functional role in thrombus formation and stabilization during stroke: their deficiency or blockade protects from cerebral ischemia without overtly affecting hemostasis. Based on the accumulating evidence that thrombus formation and hemostasis are not inevitably linked, new concepts for prevention and treatment of ischemic stroke may eventually emerge without the hazard of severe bleeding complications. This review discusses recent advances related to antithrombotic strategies in experimental stroke research.

Reduced Atherosclerotic Lesions in P2Y 1 /Apolipoprotein E Double-Knockout Mice

Background— The P2Y 1 receptor plays a key role in arterial thrombosis and is widely expressed in many cell types involved in atherosclerosis. The aim of this study was to evaluate its potential involvement in the development of atherosclerotic lesions.

Methods and Results— Apolipoprotein E–deficient ( ApoE −/− ) and P2Y 1 −/− / ApoE −/− mice were maintained on regular chow for 17 or 30 weeks before analysis of atherosclerotic lesions. At 17 weeks, lesions in the aortic sinus and entire aorta were smaller in P2Y 1 −/− / ApoE −/− compared with those in ApoE −/− animals. At 30 weeks, the aortic sinus lesions in P2Y 1 −/− / ApoE −/− mice were still diminished in size and displayed reduced inflammation, reflected by decreased macrophage infiltration and diminished VCAM-1 immunostaining, compared with those in ApoE −/− mice. They also had a lower smooth muscle cell content. Unexpectedly, bone marrow transplantation showed that the absence of the P2Y 1 receptor in blood cells only led to no significant modification of the lesion compared with control ApoE −/− reconstituted animals. Conversely, the absence of the P2Y 1 receptor except in blood cells resulted in a reduction in lesion size similar to that in control P2Y 1 −/− / ApoE −/− reconstituted mice, pointing to a role of non–hematopoietic-derived P2Y 1 receptors, most likely the endothelial or smooth muscle cell P2Y 1 receptors. In addition, although this was not statistically significant, plasma cholesterol levels were consistently decreased in P2Y 1 −/− animals, suggesting that a modification of lipid metabolism could be responsible for the observed phenotype.

Conclusion— The P2Y 1 receptor contributes to atherosclerosis, primarily through its role in non–hematopoietic-derived cells.

Effect of curcumin on the adhesion of platelets to brain microvascular endothelial cells in vitro

AIM

To determine whether curcumin prevents the adhesion of platelets to brain microvascular endothelial cells (BMECs) cultured in vitro.

METHODS

[3H]Adenine- labeled platelets were incubated with BMECs to investigate the role of curcumin in the adhesion of platelets to BMECs. The number of platelets adhering to the BMECs monolayer was determined by liquid scintillation spectroscopy. The thrombin-induced expression of platelets P-selectin, glycoprotein IIb (GPIIb), and glycoprotein IIIa (GPIIIa) on the cell surface, was measured by flow cytometry. P-selectin mRNA levels of BMECs were determined by RT-PCR. The TNF-alpha- induced expressions of P-selectin and E-selectin on the surface of BMECs were determined by Western blotting.

RESULTS

The adhesion between thrombin-activated platelets and normal BMECs, and that of TNF-alpha-activated BMECs and normal platelets were significantly increased, and this increase could be inhibited by curcumin (30-240 micromol/L) in a concentration-dependant manner. The platelets activated with thrombin and BMECs stimulated by TNF-alpha demonstrated an upregulated expressions of P-selectin and E-selectin, and this increase, when pretreated with curcumin for 30 min, could be restrained dose dependently. Curcumin also inhibited the increase of the GPIIb/GPIIIa expression of thrombinactivated platelets in a concentration-dependent manner.

CONCLUSION

Curcumin can inhibit the platelets to BMECs. This effect may be related to the decreased expressions of P-selectin, E-selectin, and GPIIb/GPIIIa on platelets and BMECs.

Contribution of cyclooxygenase-1 to thromboxane formation, platelet-vessel wall interactions and atherosclerosis in the ApoE null mouse

OBJECTIVE

Prostaglandin and thromboxane (TXA(2)) generation is increased in atherosclerosis. Studies with selective inhibitors attribute the enhanced prostacyclin (PGI(2)) generation to both cyclooxygenase-1 (COX-1) and COX-2 whereas the increased TXA(2) generation reflects platelet COX-1 expression. However, TXA(2) formation remains elevated in patients with cardiovascular disease on doses of aspirin that fully suppress platelet COX-1, suggesting other tissue sources for TXA(2) formation. Disruption of the thromboxane receptor gene suppresses the development of atherosclerosis. Notwithstanding this, the role of COX-1 in atherosclerosis is unclear, as it is widely distributed and contributes to a number of products, including those that potentially contribute to the resolution of inflammation.

METHODS AND RESULTS

We examined the role of COX-1 on prostaglandin generation, development of atherosclerosis and platelet-vessel wall interactions in the apoE(-/-) murine model by disrupting the COX-1 gene. ApoE(-/-)/COX-1(+/+), ApoE(-/-)/COX-1(+/-) and ApoE(-/-)/COX-1(-/-), were administered a 1% cholesterol diet for 8 weeks. Stable urinary metabolites of PGI(2) and TXA(2), which were markedly increased in the ApoE(-/-)/COX-1(+/+) were reduced by disruption of COX-1. Deletion of one or both copies of the COX-1 gene suppressed lesion formation. Assessment of platelet-vessel wall interactions by intravital microscopy showed a significant decrease in firm adhesion of platelets in the apoE/COX-1 double knockout (DKO).

CONCLUSION

COX-1 contributes to the enhanced formation of both PGI(2) and TXA(2) in atherosclerosis, and to the development of the disease. Non-platelet sources of COX-1 and TXA(2) that are inaccessible to standard doses of aspirin may contribute to the development of atherosclerosis.

Platelet membrane fluidity and Na+/K+ ATPase activity in acute stroke

Stroke is a consequence of a reduction in cerebral blood flow but the mechanisms involved in the production of ischemic damage are complex and probably not fully known. It is hypothesized that alterations in platelet membrane fluidity are directly related to the severity of the stroke as measured by the National Institute of Health Stroke Scale (NIHSS). Thus, the aim of the present study was to investigate Na+/K+ ATPase activity and platelet membrane fluidity, measured by fluorescent probes TMA-DPH and DPH in patients affected by ischemic stroke and controls in order to identify, if any, chemical-physical and/or functional modifications associated with cerebral ischemic damage. Patients were divided into three groups according to the presence of vascular risk factors (Diabetes Mellitus, Hypertension and Smoking) in order to evaluate the possible influence of each risk factor on the NIHSS score and both Na+/K+ ATPase activity and platelet membrane fluidity. Data showed a significant decrease in both Na+/K+ ATPase activity and platelet fluidity values in patients compared to controls. Moreover, all three groups showed a negative significant correlation between NIHSS and Na+/K+ ATPase activity and a positive significant correlation between NIHSS, TMA-DPH and DPH. In conclusion, the present data point out that alterations in the platelet membrane's chemical-physical (decreased fluidity) and functional properties (reduced Na+/K+ ATPase activity) rose proportionally with NIHSS increase. These modifications and their interaction with some vascular risk factors might be involved in the pathogenesis of ischemic damage development.

Inflammation induces hemorrhage in thrombocytopenia

The role of platelets in hemostasis is to produce a plug to arrest bleeding. During thrombocytopenia, spontaneous bleeding is seen in some patients but not in others; the reason for this is unknown. Here, we subjected thrombocytopenic mice to models of dermatitis, stroke, and lung inflammation. The mice showed massive hemorrhage that was limited to the area of inflammation and was not observed in uninflamed thrombocytopenic mice. Endotoxin-induced lung inflammation during thrombocytopenia triggered substantial intra-alveolar hemorrhage leading to profound anemia and respiratory distress. By imaging the cutaneous Arthus reaction through a skin window, we observed in real time the loss of vascular integrity and the kinetics of skin hemorrhage in thrombocytopenic mice. Bleeding-observed mostly from venules-occurred as early as 20 minutes after challenge, pointing to a continuous need for platelets to maintain vascular integrity in inflamed microcirculation. Inflammatory hemorrhage was not seen in genetically engineered mice lacking major platelet adhesion receptors or their activators (alphaIIbbeta3, glycoprotein Ibalpha [GPIbalpha], GPVI, and calcium and diacylglycerol-regulated guanine nucleotide exchange factor I [CalDAG-GEFI]), thus indicating that firm platelet adhesion was not necessary for their supporting role. While platelets were previously shown to promote endothelial activation and recruitment of inflammatory cells, they also appear indispensable to maintain vascular integrity in inflamed tissue. Based on our observations, we propose that inflammation may cause life-threatening hemorrhage during thrombocytopenia.

Platelets modulate atherogenesis and progression of atherosclerotic plaques via interaction with progenitor and dendritic cells

Platelets not only play a role in the late complications of atherosclerosis, but are also essential in its initiation, interacting with endothelial cells and leukocytes. Platelet adhesion to injured or atherosclerotic vessels is critical for the initiation of atherosclerotic lesion formation in vivo. Increasing evidence has recently highlighted the role of progenitor cells in inflammation, atherogenesis, and atheroprogression. Recruitment of progenitor and dendritic cells to sites of vascular injury is poorly understood so far. Both human progenitor and dendritic cells significantly adhere to platelets, indicating that platelets adherent to collagen or to endothelial cells can serve as a bridging mechanism directing circulating progenitor and dendritic cells to sites of impaired vasculature. Moreover, platelets regulate differentiation of progenitor cells to endothelial cells or macrophages and foam cells and modulate essential functions of dendritic cells, including their activation, differentiation and apoptosis in vitro. This review describes recent findings on platelet interaction with progenitor cells or dendritic cells and discusses potential consequences of this interaction in atherosclerosis.

Platelet-lymphocyte cross-talk

Platelets and lymphocytes reciprocally regulate mutual functions, i.e., platelet-lymphocyte cross-talk. The heterotypic interactions have emerged as important regulatory mechanisms in the pathophysiological processes of thrombosis, inflammation, immunity, and atherosclerosis. Platelets influence lymphocyte function via direct cell-cell contact and/or soluble mediators. Hence, platelets enhance adhesion and cell migration of T(H), T cytolytic (T(C)), NK, and B cells. Platelets affect other functional aspects of lymphocyte subpopulations in a complex manner. They may attenuate cytokine secretion and immunosuppressive responses of T(H) cells and enhance T(C) cell proliferation and cytotoxicity. Platelets promote isotype shifting and antibody production of B cells but ameliorate cytolytic activity of NK cells. On the other hand, lymphocytes can also regulate platelet aggregation and secretion, as well as the effector cell function of platelets in immune defense. The two cell types collaborate in transcellular phospholipid metabolism, CD40-CD40 ligand-mediated intercellular signaling, and their involvements in atherogenesis. The research perspectives of platelet-lymphocyte cross-talk have also been addressed.

Platelet adhesion receptors do not modulate infarct volume after a photochemically induced stroke in mice

Photochemically induced cerebral infarction has been considered a clinically relevant model for ischemic stroke. We evaluated various transgenic mice to study the role of platelet adhesion molecules in this model. Infarction to the sensorimotoric cortex was induced by erythrosin B and laser light. Infarct volumes were calculated from triphenyltetrazolium chloride stained brain slices. Thrombus formation and vessel leakage were observed in vivo by multiphoton microscopy. Mice mutant in VWF, GPIbalpha, beta3 integrin, and P-selectin did not show any significant differences in infarct volume compared to wild type (WT). This is in contrast to the intraluminal middle cerebral artery occlusion model in which alphaIIbbeta3 integrin, GPIbalpha, and P-selectin are known to modulate infarct size. Multiphoton microscopy showed that small, non-occlusive embolizing platelet thrombi formed in the photochemically injured brains. Massive vessel leakage was observed within 25 min of laser injury. Interestingly, we observed a significant increase in infarct size with aging, accordant with heightened fragility of the blood brain barrier (BBB) in older mice. This model of photochemically induced stroke is closer to a BBB injury model than a thrombotic stroke model in which platelets and their adhesion molecules are crucial. This model will be useful to study mechanisms regulating BBB permeability.

Chemokine Fractalkine Mediates Leukocyte Recruitment to Inflammatory Endothelial Cells in Flowing Whole Blood

Background— The membrane-bound chemokine fractalkine (CX 3 CL1) is expressed on various cell types such as activated endothelial cells and has been implicated in the inflammatory process of atherosclerosis. The aim of the present study was to dissect the role of fractalkine in leukocyte recruitment to inflamed endothelium under arterial shear forces.

Methods and Results— With the use of immunofluorescence and laminar flow assays, the present study shows that human umbilical vein endothelial cells stimulated with tumor necrosis factor-α and interferon-γ abundantly express CX 3 CL1 and promote substantial leukocyte accumulation under arterial flow conditions. In the presence of high shear, firm adhesion of leukocytes to inflamed endothelial cells is reduced by ≈40% by a function-blocking anti-fractalkine antibody or by an antibody directed against the fractalkine receptor (CX 3 CR1). With the use of intravital video-fluorescence microscopy we demonstrate that inhibition of fractalkine signaling attenuates leukocyte adhesion to the atherosclerotic carotid artery of apolipoprotein E–deficient mice, which suggests that the CX 3 CL1-CX 3 CR1 axis is critically involved in leukocyte adhesion to inflamed endothelial cells under high shear forces both in vitro and in vivo. Surprisingly, platelets were strictly required for fractalkine-induced leukocyte adhesion at high shear rates. Correspondingly, specific inhibition of platelet adhesion to inflamed endothelial cells also significantly reduced leukocyte accumulation. We show that both soluble and membrane-bound fractalkine induces platelet degranulation and subsequent surface expression of P-selectin, which thereby promotes direct platelet-leukocyte interaction.

Conclusion— Fractalkine expressed by inflamed endothelial cells triggers P-selectin exposure on adherent platelets, which thereby initiates the local accumulation of leukocytes under arterial shear, an essential step in the development of atherosclerotic lesions.

Platelets induce differentiation of human CD34+ progenitor cells into foam cells and endothelial cells

Recruitment of human CD34+ progenitor cells toward vascular lesions and differentiation into vascular cells has been regarded as a critical initial step in atherosclerosis. Previously we found that adherent platelets represent potential mediators of progenitor cell homing besides their role in thrombus formation. On the other hand, foam cell formation represents a key process in atherosclerotic plaque formation. To investigate whether platelets are involved in progenitor cell recruitment and differentiation into endothelial cells and foam cells, we examined the interactions of platelets and CD34+ progenitor cells. Cocultivation experiments showed that human platelets recruit CD34+ progenitor cells via the specific adhesion receptors P-selectin/PSGL-1 and beta1- and beta2-integrins. Furthermore, platelets were found to induce differentiation of CD34+ progenitor cells into mature foam cells and endothelial cells. Platelet-induced foam cell generation could be prevented partially by HMG coenzyme A reductase inhibitors via reduction of matrix metalloproteinase-9 (MMP-9) secretion. Finally, agonists of peroxisome proliferator-activated receptor-alpha and -gamma attenuated platelet-induced foam cell generation and production of MMP-9. The present study describes a potentially important mechanism of platelet-induced foam cell formation and generation of endothelium in atherogenesis and atheroprogression. The understanding and modulation of these mechanisms may offer new treatment strategies for patients at high risk for atherosclerotic diseases.