Platelets in atherothrombosis: lessons from mouse models

Gastrodia elata Blume and Zanthoxylum schinifolium Siebold & Zucc Mixed Extract Suppress Platelet Aggregation and Thrombosis

Background and objectives: Blood vessel thrombosis causes blood circulation disorders, leading to various diseases. Currently, various antiplatelet and anticoagulant drugs, such as aspirin, warfarin, heparin, and non-vitamin K antagonist oral anticoagulants (NOACs), are used as the major drugs for the treatment of a wide range of thrombosis. However, these drugs have a side effect of possibly causing internal bleeding due to poor hemostasis when taken for a long period of time. Materials and Methods: Gastrodia elata Blume (GE) and Zanthoxylum schinifolium Siebold & Zucc (ZS) are known to exhibit hemostatic and antiplatelet effects as traditional medicines that have been used for a long time. In this study, we investigated the effect of a mixed extract of GE and ZS (MJGE09) on platelet aggregation and plasma coagulation. Results: We found that MJGE09 inhibited collagen-and ADP-induced platelet aggregation in vitro. In addition, collagen- and ADP-induced platelet aggregation were also inhibited in a dose-dependent manner on the platelets of mice that were orally administered MJGE09 ex vivo. However, compared with aspirin, MJGE09 did not prolong the rat tail vein bleeding time in vivo and did not show a significant effect on the increase in the prothrombin time (PT) and activated partial thromboplastin time (aPTT). Conclusions: These results suggest that MJGE09 can be used as a potential anticoagulant with improved antithrombotic efficacy.

NBEAL2 (Neurobeachin-Like 2) Is Required for Retention of Cargo Proteins by α-Granules During Their Production by Megakaryocytes

Objective- Human and mouse megakaryocytes lacking NBEAL2 (neurobeachin-like 2) produce platelets where α-granules lack protein cargo. This cargo is mostly megakaryocyte-synthesized, but some proteins, including FGN (fibrinogen), are endocytosed. In this study, we examined the trafficking of both types of cargo within primary megakaryocytes cultured from normal and NBEAL2-null mice, to determine the role of NBEAL2 in α-granule maturation. We also examined the interaction of NBEAL2 with the granule-associated protein P-selectin in human megakaryocytes and platelets. Approach and Results- Fluorescence microscopy was used to compare uptake of labeled FGN by normal and NBEAL2-null mouse megakaryocytes, which was similar in both. NBEAL2-null cells, however, showed decreased FGN retention, and studies with biotinylated protein showed rapid loss rather than increased degradation. Intracellular tracking via fluorescence microscopy revealed that in normal megakaryocytes, endocytosed FGN sequentially associated with compartments expressing RAB5 (Ras-related protein in brain 5), RAB7 (Ras-related protein in brain 7), and P-selectin, where it was retained. A similar initial pattern was observed in NBEAL2-null megakaryocytes, but then FGN passed from the P-selectin compartment to RAB11 (Ras-related protein in brain 11)-associated endosomes before release. Megakaryocyte-synthesized VWF (Von Willebrand factor) was observed to follow the same route out of NBEAL2-null cells. Immunofluorescence microscopy revealed intracellular colocalization of NBEAL2 with P-selectin in human megakaryocytes, proplatelets, and platelets. Native NBEAL2 and P-selectin were coimmunoprecipitated from platelets and megakaryocytes. Conclusions- NBEAL2 is not required for FGN uptake by megakaryocytes. NBEAL2 is required for the retention of both endocytosed and megakaryocyte-synthesized proteins by maturing α-granules, and possibly by platelet-borne granules. This function may involve interaction of NBEAL2 with P-selectin.

Platelet MEKK3 regulates arterial thrombosis and myocardial infarct expansion in mice

MAPKs play important roles in platelet activation. However, the molecular mechanisms by which MAPKs are regulated in platelets remain largely unknown. Real-time polymerase chain reaction and western blot data showed that MEKK3, a key MAP3K family member, was expressed in human and mouse platelets. Then, megakaryocyte/platelet-specific MEKK3-deletion (MEKK3^-/- ) mice were developed to elucidate the platelet-related function(s) of MEKK3. We found that agonist-induced aggregation and degranulation were reduced in MEKK3^-/- platelets in vitro. MEKK3 deficiency significantly impaired integrin αIIbβ3-mediated inside-out signaling but did not affect the outside-in signaling. At the molecular level, MEKK3 deficiency led to severely impaired activation of extracellular signal-regulated kinases 1/2 (ERK1/2) and c-Jun NH₂-terminal kinase 2 but not p38 or ERK5. In vivo, MEKK3^-/- mice showed delayed thrombus formation following FeCl₃-induced carotid artery injury. Interestingly, the tail bleeding time was normal in MEKK3^-/- mice. Moreover, MEKK3^-/- mice had fewer microthrombi, reduced myocardial infarction (MI) size, and improved post-MI heart function in a mouse model of MI. These results suggest that MEKK3 plays important roles in platelet MAPK activation and may be used as a new effective target for antithrombosis and prevention of MI expansion.

Pretreatment Inflammatory Indexes as Prognostic Predictors for Survival in Colorectal Cancer Patients Receiving Neoadjuvant Chemoradiotherapy

This study was to evaluate the prognostic value of pretreatment inflammatory indexes including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), lymphocyte-to-monocyte ratio (LMR), and systemic immune-inflammation index (SII) in colorectal cancer (CRC) patients receiving neoadjuvant chemoradiotherapy (CRT). We enrolled 98 eligible CRC patients and divided them into high or low NLR, PLR, LMR, and SII groups according to their median index value, respectively. Univariate and multivariate analysis were performed to identify the potential predictors of progression-free survival (PFS) and overall survival (OS). In the univariate analysis, ECOG performance status, distant metastasis, NLR, PLR, LMR, and SII were found to be significantly associated with PFS and OS. In the multivariate analysis, ECOG performance status, distant metastasis, and NLR were identified to be independent predictors of PFS (HR 2.487, p = 0.012; HR 2.422, p = 0.042; HR 2.243, p = 0.034, respectively), and OS (HR 2.237, p = 0.018; HR 2.757, p = 0.020; HR 2.336, p = 0.017, respectively). The results of our study revealed that ECOG performance status, distant metastasis and NLR were independent prognostic factors of PFS and OS in CRC patients receiving neoadjuvant CRT.

Refinement of Mouse Protocols for the Study of Platelet Thromboembolic Responses In Vivo

Mouse models of thromboembolism are frequently used to investigate platelet function in vivo and, according to European Union (EU) legislation, must be conducted in the context of replacement, refinement and reduction. We have previously developed a refined real-time mouse model of thromboembolism as an alternative to models of thromboembolic mortality which inflict considerable pain and suffering. Real-time monitoring involves infusion of radiolabelled platelets into the circulation of anaesthetized mice, and platelet aggregation is measured as increases in platelet-associated counts in the pulmonary vasculature following injection of platelet agonists. This gives a definitive data set on the tissue localization and extent of platelet activation. We developed an additional, more simplistic alternative to mortality models based on blood microsampling which entails the measurement of circulating platelet counts following agonist stimulation. Blood microsamples were collected from the tail vein of anaesthetized mice at three different time points leading to a reduction in animal numbers. Platelet counts significantly dropped 1 minute after stimulation with collagen or thrombin and were restored over 10 minutes. These results correlate with those obtained via real-time monitoring and were confirmed by immunohistochemistry. Pre-treatment of mice with aspirin significantly inhibited the decrease in platelet counts following collagen. These data suggest that blood microsampling may be implemented as a simplistic refined alternative to mortality models of thromboembolism when specialized monitoring equipment, or use of radioactive isotopes for real-time monitoring, which remains the ‘gold standard’, is not feasible. Microsampling refines and reduces animal procedures in compliance with current EU legislation.

Comparison of two murine models of thrombosis induced by atherosclerotic plaque injury

Arterial thrombosis occurs at sites of erosion or rupture of atherosclerotic vascular lesions. To better study the pathophysiology of this complex phenomenon, there is a need for animal models of localised thrombosis at sites of atherosclerotic lesions with closer resemblance to the human pathology as compared to commonly used thrombosis models in healthy vessels. In the present study, we describe and compare a new model of thrombosis induced by atherosclerotic plaque rupture in the carotid artery from ApoE-/- mice using a suture needle to a milder model of ultrasound-induced plaque injury. Needle injury induces atherosclerotic plaque rupture with exposure of plaque material and formation of a thrombus that is larger, nearly occlusive and more stable as compared to that formed by application of ultrasounds. These two models have common features such as the concomitant involvement of platelet activation, thrombin generation and fibrin formation, which translates into sensitivity toward both antiplatelet drugs and anticoagulants. On the other hand, they display differences with respect to the role of the platelet collagen receptor GPVI, the plaque rupture model being less sensitive to its inhibition as compared to the ultrasound-induced injury, which may be related to the amount of thrombin generated. These models represent an improvement as compared to models in healthy vessels and may help identify specific plaque triggers of thrombosis. They should therefore be useful to evaluate new antithrombotic targets.

Clinical implications of pretreatment inflammatory biomarkers as independent prognostic indicators in prostate cancer

OBJECTIVES

Research on the relationship between inflammatory biomarkers and malignant tumors has become a hotspot. Many studies have demonstrated that neutrophil-lymphocyte ratio (NLR), platelet-lymphocyte ratio (PLR), and red blood cell distribution width (RDW) could act as independent prognostic indicators for several solid tumors. This study aimed to evaluate the clinical implications of pretreatment inflammatory biomarkers, including NLR, PLR, and RDW as independent prognostic indicators in prostate cancer (PCa).

METHODS

A total of 226 PCa patients who were diagnosed at our institution from 2011 to 2016 were analyzed retrospectively. We compared the clinicopathological features, survival curves, and prognosis of the PCa patients between the high and low groups according to the cutoffs of NLR, PLR, and RDW.

RESULTS

The pretreatment NLR, PLR, and RDW values were significantly higher in the patients with PCa than those in the controls (P<.05). Increased NLR and PLR values were significantly associated with high risk of progression, including higher Gleason scores, cell proliferation antigen 67 (Ki-67) indexes, and prostate-specific antigen (PSA) levels (P<.05), whereas an elevated RDW was only associated with an older age. An increased NLR was correlated with both overall survival (OS) (P=.025) and disease-free survival (DFS) (P=.017). In addition, a higher PLR only showed a significantly worse DFS (P=.040). Pretreatment NLR was an independent prognostic indicator of DFS.

CONCLUSIONS

The pretreatment NLR and PLR might be beneficial to predict the progression and prognosis of PCa. Furthermore, NLR was more effective than PLR acting as an independent prognostic indicator for PCa.

Platelet-derived CXCL4 regulates neutrophil infiltration and tissue damage in severe acute pancreatitis

Platelets are known to play an important role in acute pancreatitis (AP) via promotion of neutrophil accumulation, although mechanisms behind platelet-dependent accumulation of neutrophils in the pancreas remain elusive. Platelets contain a wide spectrum of different pro-inflammatory compounds, such as chemokines. CXCL4 (platelet factor 4) is one of the most abundant chemokine in platelets, and we hypothesized that CXCL4 might be involved in platelet-dependent accumulation of neutrophils in the inflamed pancreas. The aim of this study was to examine the role of CXCL4 in severe AP. Pancreatitis was provoked by infusion of taurocholate into the pancreatic duct or by intraperitoneal administration of L-arginine in C57BL/6 mice. Animals were treated with an antibody against platelets or CXCL4 before induction of pancreatitis. Plasma and lung levels of CXCL2, CXCL4, and interleukin (IL)-6 were determined by use of enzyme-linked immunosorbent assay. Flow cytometry was used to examine surface expression of macrophage-1 (Mac-1) on neutrophils. Plasma was obtained from healthy individuals (controls) and patients with AP. Challenge with taurocholate increased plasma levels of CXCL4, and depletion of platelets markedly reduced plasma levels of CXCL4 indicating that circulating levels of CXCL4 are mainly derived from platelets in AP. Inhibition of CXCL4 reduced taurocholate-induced neutrophil recruitment, IL-6 secretion, edema formation, amylase release, and tissue damage in the pancreas. However, immunoneutralization of CXCL4 had no effect on CXCL2-evoked neutrophil expression of Mac-1 or chemotaxis in vitro, suggesting an indirect effect of CXCL4 on neutrophil recruitment in AP. Targeting CXCL4 significantly attenuated plasma and lung levels of CXCL2, which is a potent neutrophil chemoattractant, and inhibition of the CXCL2 receptor attenuated neutrophil infiltration and tissue damage in the inflamed pancreas. A significant role of CXCL4 was confirmed in an alternate model of AP induced by L-arginine challenge. Moreover, patients with AP had significantly increased plasma levels of CXCL4 compared with healthy controls. These findings' results suggest that platelet-derived CXCL4 is a potent stimulator of neutrophil accumulation in AP and that this is mediated via generation of CXCL2 in the inflamed pancreas. We conclude that CXCL4 plays an important role in pancreatic inflammation and that targeting CXCL4 might be a useful way to ameliorate tissue damage in AP.

Clinical significance of pre-operative neutrophil lymphocyte ratio and platelet lymphocyte ratio as prognostic factors for patients with colorectal cancer

The present study aimed to investigate the independent prognostic values of the pre-operative neutrophil lymphocyte ratio (NLR) and platelet lymphocyte ratio (PLR) in patients with colorectal cancer (CRC). The present study retrospectively analyzed the data of 216 patients with CRC from a single hospital. The clinicopathological characteristics of the patients were compared and prognostic factors were evaluated. NLR and PLR were associated with tumor differentiation status and the tumor diameter, respectively, and PLR was also associated with the primary tumor classification (T classification). Furthermore, NLR and PLR were positively associated with each other (R2=0.5368; P<0.0001). Univariate analyses indicated that stage II and III patients with a high NLR (≥4.98; P<0.001) or PLR (≥246.36; P<0.001) possessed a significantly poorer 5-year OS rate compared with those with a low NLR or PLR. Post-operative adjuvant chemotherapy improved the 5-year OS rate in patients with a high NLR or PLR. Multivariate analyses indicated that NLR and PLR were independent prognostic factors [NLR, relative risk (RR)=4.074 and P<0.001; PLR, RR=2.029 and P=0.029] in patients with CRC, and were associated with the T classification, lymph node metastasis and post-operative adjuvant chemotherapy response of patients. Additionally, the area under the curve (AUC) was 0.748 for NLR (95% CI, 0.684-0.804; P<0.0001) and 0.690 for PLR (95% CI, 0.623-0.751; P<0.0001). The RR and AUC indicated that NLR was the superior predictive factor in patients with CRC. In conclusion, the pre-operative NLR and PLR were significant independent prognostic factors in patients with CRC, and NLR was more effective as a prognostic marker compared with PLR. Adjuvant chemotherapy appeared to be more effective in CRC patients with a higher NLR or PLR.

Efficacy and Safety of Platelet Glycoprotein Receptor Blockade in Aged and Comorbid Mice With Acute Experimental Stroke

BACKGROUND AND PURPOSE

Despite the medical and socioeconomic effect of ischemic stroke and extensive preclinical research, treatment options for ischemic stroke are limited. We recently identified and characterized essential steps of thrombus formation in stroke and demonstrated that inhibition of the platelet glycoprotein (GP) receptors Ib and VI, but not IIb/IIIa, protects young and healthy mice from ischemic neurodegeneration. Whether these findings translate to the clinic remains unclear. Considering that the typical stroke patient is elderly with comorbidity, we aimed to analyze the efficacy and safety of novel preclinical antithrombotics in adult and comorbid mice with acute experimental stroke.

METHODS

We subjected adult, healthy, atherosclerotic (Ldlr(-/-)), diabetic (streptozotocin treated), and hypertensive (RenTgMK) mice to a 60-minute transient middle cerebral artery occlusion. Animals were pretreated with anti-GPVI antibodies or treated 1 hour after stroke induction with anti-GPIb or anti-GPIIb/IIIa antigen-binding fragments, respectively. Isotype treatment served as control. Twenty-four hours after transient middle cerebral artery occlusion, we visually assessed the intracerebral hemorrhage rate and measured infarct volumes (using 2,3,5-triphenyltetrazolium chloride-stained brain slices) and functional outcome (using Bederson and grip-test scores).

RESULTS

GPIb and GPVI inhibition protected the mice from ischemic stroke without increasing bleeding complications. In contrast, GPIIb/IIIa inhibition was not protective but increased the intracerebral hemorrhage rate.

CONCLUSIONS

Inhibition of early steps of thrombus formation protects adult and comorbid mice from ischemic stroke. The use of clinically meaningful mouse strains might improve the translation of preclinical stroke research to the clinic.

Critical off-target effects of the widely used Rac1 inhibitors NSC23766 and EHT1864 in mouse platelets

BACKGROUND

Platelet aggregation at sites of vascular injury is essential for normal hemostasis, but may also cause pathologic vessel occlusion. Rho GTPases are molecular switches that regulate essential cellular processes, and they have pivotal functions in the cardiovascular system. Rac1 is an important regulator of platelet cytoskeletal reorganization, and contributes to platelet activation. Rac1 inhibitors are thought to be beneficial in a wide range of therapeutic settings, and have therefore been tested in vivo for a variety of disorders. Two small-molecule inhibitors, NSC23766 and EHT1864, have been characterized in different cell types, demonstrating high specificity for Rac1 and Rac, respectively.

OBJECTIVES

To analyze the specificity of NSC23766 and EHT1864.

METHODS

Platelet function was assessed in mouse wild-type and Rac1-deficient platelets by the use of flow cytometric analysis of cellular activation and aggregometry. Platelet spreading was analyzed with differential interference contrast microscopy, and activation of effector molecules was analyzed with biochemical approaches.

RESULTS

NSC23766 and EHT1864 showed strong and distinct Rac1-independent effects at 100 μm in platelet function tests. Both inhibitors induced Rac1-specific inhibition of platelet spreading, but also markedly impaired agonist-induced activation of Rac1(-/-) platelets. Furthermore, glycoprotein Ib-mediated signaling was dramatically inhibited by NSC23766 in both wild-type and Rac1-deficient platelets. Importantly, these inhibitors directly affected the activation of the Rac1 effectors p21-activated kinase (PAK)1 and PAK2.

CONCLUSIONS

Our results reveal critical off-target effects of NSC23766 and EHT1864 at 100 μm in mammalian cells, raising questions about their utility as specific Rac1/Rac inhibitors in biochemical studies at these concentrations and possibly as therapeutic agents.

EMILIN2 regulates platelet activation, thrombus formation, and clot retraction

Thrombosis, like other cardiovascular diseases, has a strong genetic component, with largely unknown determinants. EMILIN2, Elastin Microfibril Interface Located Protein2, was identified as a candidate gene for thrombosis in mouse and human quantitative trait loci studies. EMILIN2 is expressed during cardiovascular development, on cardiac stem cells, and in heart tissue in animal models of heart disease. In humans, the EMILIN2 gene is located on the short arm of Chromosome 18, and patients with partial and complete deletion of this chromosome region have cardiac malformations. To understand the basis for the thrombotic risk associated with EMILIN2, EMILIN2 deficient mice were generated. The findings of this study indicate that EMILIN2 influences platelet aggregation induced by adenosine diphosphate, collagen, and thrombin with both EMILIN2-deficient platelets and EMILIN2-deficient plasma contributing to the impaired aggregation response. Purified EMILIN2 added to platelets accelerated platelet aggregation and reduced clotting time when added to EMILIN2-deficient mouse and human plasma. Carotid occlusion time was 2-fold longer in mice with platelet-specific EMILIN2 deficiency, but stability of the clot was reduced in mice with both global EMILIN2 deficiency and with platelet-specific EMILIN2 deficiency. In vitro clot retraction was markedly decreased in EMILIN2 deficient mice, indicating that platelet outside-in signaling was dependent on EMILIN2. EMILIN1 deficient mice and EMILIN2:EMILIN1 double deficient mice had suppressed platelet aggregation and delayed clot retraction similar to EMILIN2 mice, but EMILIN2 and EMILIN1 had opposing affects on clot retraction, suggesting that EMILIN1 may attenuate the effects of EMILIN2 on platelet aggregation and thrombosis. In conclusion, these studies identify multiple influences of EMILIN2 in pathophysiology and suggest that its role as a prothrombotic risk factor may arise from its effects on platelet aggregation and platelet mediated clot retraction.

Mice lacking the SLAM family member CD84 display unaltered platelet function in hemostasis and thrombosis

Background

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes that safeguard vascular integrity by forming thrombi at sites of vascular injury. Although the early events of thrombus formation—platelet adhesion and aggregation—have been intensively studied, less is known about the mechanisms and receptors that stabilize platelet-platelet interactions once a thrombus has formed. One receptor that has been implicated in this process is the signaling lymphocyte activation molecule (SLAM) family member CD84, which can undergo homophilic interactions and becomes phosphorylated upon platelet aggregation.

Objective

The role of CD84 in platelet physiology and thrombus formation was investigated in CD84-deficient mice.

Methods and Results

We generated CD84-deficient mice and analyzed their platelets in vitro and in vivo. Cd84^−/− platelets exhibited normal activation and aggregation responses to classical platelet agonists. Furthermore, CD84 deficiency did not affect integrin-mediated clot retraction and spreading of activated platelets on fibrinogen. Notably, also the formation of stable three-dimensional thrombi on collagen-coated surfaces under flow ex vivo was unaltered in the blood of Cd84^−/− mice. In vivo, Cd84^−/− mice exhibited unaltered hemostatic function and arterial thrombus formation.

Conclusion

These results show that CD84 is dispensable for thrombus formation and stabilization, indicating that its deficiency may be functionally compensated by other receptors or that it may be important for platelet functions different from platelet-platelet interactions.

Platelets: still a therapeutical target for haemostatic disorders

Platelets are cytoplasmatic fragments from bone marrow megakaryocytes present in blood. In this work, we review the basis of platelet mechanisms, their participation in syndromes and in arterial thrombosis, and their potential as a target for designing new antithrombotic agents. The option of new biotechnological sources is also explored.

The functional role of platelets in the regulation of angiogenesis

Functionally, platelets are primarily recognized as key regulators of thrombosis and hemostasis. Upon vessel injury, the typically quiescent platelet interacts with subendothelial matrix to regulate platelet adhesion, activation and aggregation, with subsequent induction of the coagulation cascade forming a thrombus. Recently, however, newly described roles for platelets in the regulation of angiogenesis have emerged. Platelets possess an armory of pro- and anti-angiogenic proteins, which are actively sequestered and highly organized in α-granule populations. Platelet activation facilitates their release, eliciting potent angiogenic responses through mechanisms that appear to be tightly regulated. In conjunction, the release of platelet-derived phospholipids and microparticles has also earned merit as synergistic regulators of angiogenesis. Consequently, platelets have been functionally implicated in a range of angiogenesis-dependent processes, including physiological roles in wound healing, vascular development and blood/lymphatic vessel separation, whilst facilitating aberrant angiogenesis in a range of diseases including cancer, atherosclerosis and diabetic retinopathy. Whilst the underlying mechanisms are only starting to be elucidated, significant insights have been established, suggesting that platelets represent a promising therapeutic strategy in diseases requiring angiogenic modulation. Moreover, anti-platelet therapies targeting thrombotic complications also exert protective effects in disorders characterized by persistent angiogenesis.

Hydrogen sulfide-releasing aspirin derivative ACS14 exerts strong antithrombotic effects in vitro and in vivo

OBJECTIVE

Hydrogen sulfide (H(2)S)-releasing NSAIDs exert potent anti-inflammatory effects beyond classical cyclooxygenase inhibition. Here, we compared the platelet inhibitory effects of the H(2)S-releasing aspirin derivative ACS14 with its mother compound aspirin to analyze additional effects on platelets.

METHODS AND RESULTS

In platelets of mice fed with ACS14 for 6 days (50 mg/kg per day), not only arachidonic acid-induced platelet aggregation but also ADP-dependent aggregation was decreased, an effect that was not observed with an equimolar dose of aspirin (23 mg/kg per day). ACS14 led to a significantly longer arterial occlusion time after light-dye-induced endothelial injury as well as decreased thrombus formation after ferric chloride-induced injury in the carotid artery. Bleeding time was not prolonged compared with animals treated with equimolar doses of aspirin. In vitro, in human whole blood, ACS14 (25-500 µmol/L) inhibited arachidonic acid-induced platelet aggregation, but compared with aspirin additionally reduced thrombin receptor-activating peptide-, ADP-, and collagen-dependent aggregation. In washed human platelets, ACS14 (500 µmol/L) attenuated αIIbβ3 integrin activation and fibrinogen binding and increased intracellular cAMP levels and cAMP-dependent vasodilator-stimulated phosphoprotein (VASP) phosphorylation.

CONCLUSIONS

The H(2)S-releasing aspirin derivative ACS14 exerts strong antiaggregatory effects by impairing the activation of the fibrinogen receptor by mechanisms involving increased intracellular cyclic nucleotides. These additional antithrombotic properties result in a more efficient inhibition of thrombus formation in vivo as achieved with aspirin alone.

Coronary artery remodeling in a model of left ventricular pressure overload is influenced by platelets and inflammatory cells

Left ventricular hypertrophy (LVH) is usually accompanied by intensive interstitial and perivascular fibrosis, which may contribute to arrhythmogenic sudden cardiac death. The mechanisms underlying the development of cardiac fibrosis are incompletely understood. To investigate the role of perivascular inflammation in coronary artery remodeling and cardiac fibrosis during hypertrophic ventricular remodeling, we used a well-established mouse model of LVH (transverse aortic constriction [TAC]). Three days after pressure overload, macrophages and T lymphocytes accumulated around and along left coronary arteries in association with luminal platelet deposition. Consistent with these histological findings, cardiac expression of IL-10 was upregulated and in the systemic circulation, platelet white blood cell aggregates tended to be higher in TAC animals compared to sham controls. Since platelets can dynamically modulate perivascular inflammation, we investigated the impact of thrombocytopenia on the response to TAC. Immunodepletion of platelets decreased early perivascular T lymphocytes' accumulation and altered subsequent coronary artery remodeling. The contribution of lymphocytes were examined in Rag1^−/− mice, which displayed significantly more intimal hyperplasia and perivascular fibrosis compared to wild-type mice following TAC. Collectively, our studies support a role of early perivascular accumulation of platelets and T lymphocytes in pressure overload-induced inflammation.

Genetic variants in platelet factor 4 modulate inflammatory and platelet activation biomarkers

BACKGROUND

African Americans suffer from higher prevalence and severity of atherosclerosis compared with whites, highlighting racial and ethnic disparities in cardiovascular disease. Previous studies have pointed to the role of vascular inflammation and platelet activation in the formation of atherosclerotic lesions.

METHODS AND RESULTS

We explored the role of genetic variation in 4 chemokine/chemokine receptor genes (CX3CR1, CX3CL1, CXCR3, and PF4) on systemic inflammation and platelet activation serum biomarkers (fractalkine, platelet P-selectin, platelet factor 4 [PF4], and tumor necrosis factor-α). In total, 110 single nucleotide polymorphisms were tested among 1042 African Americans and 763 whites. The strongest association with serum PF4 levels was observed for rs168449, which was significant in both racial groups (P value: African Americans=0.0017, whites=0.014, combined=1.2 × 10(-4)), and remained significant after permutation-based multiple corrections (P(c) value: combined=0.0013). After accounting for the effect of rs168449, we identified another significant single nucleotide polymorphism (rs1435520), suggesting a second independent signal regulating serum PF4 levels (conditional P value: African Americans=0.02, whites=0.02). Together, these single nucleotide polymorphisms explained 0.98% and 1.23% of serum PF4 variance in African Americans and whites, respectively. Additionally, in African Americans, we found an additional PF4 variant (rs8180167), uncorrelated with rs168449 and rs1435520, associated with serum tumor necrosis factor-α levels (P=0.008, P(c)=0.048).

CONCLUSIONS

Our study highlights the importance of PF4 variants in the regulation of platelet activation (PF4) and systemic inflammation (tumor necrosis factor-α) serum biomarkers.

Assessment of platelet aggregation responses in vivo in the mouse

Platelet aggregation responses are conventionally assessed in cuvette-based systems using either isolated platelets or whole blood. Unfortunately, in vitro aggregometry poorly predicts in vivo functionality, since mediators derived from the vascular endothelium are major regulators of platelet function. There is a need, therefore, for functional assays that assess platelet responsiveness in vivo in the presence of an intact and functional vascular endothelium. We have developed methodology for monitoring aggregation responses of freely circulating radiolabelled platelets using external detection probes in the anaesthetised mouse. Intravenous injection of platelet agonists induces reversible, dose-dependent aggregation responses that are sensitive to anti-platelet therapies and modification of the vascular endothelium. The technique provides a means of determining the effects of pharmacological and genetic manipulation upon platelet function in vivo.

Arterial thrombosis--insidious, unpredictable and deadly

The formation of blood clots--thrombosis--at sites of atherosclerotic plaque rupture is a major clinical problem despite ongoing improvements in antithrombotic therapy. Progress in identifying the pathogenic mechanisms regulating arterial thrombosis has led to the development of newer therapeutics, and there is general anticipation that these treatments will have greater efficacy and improved safety. However, major advances in this field require the identification of specific risk factors for arterial thrombosis in affected individuals and a rethink of the 'one size fits all' approach to antithrombotic therapy.

Occlusive thrombi arise in mammals but not birds in response to arterial injury: evolutionary insight into human cardiovascular disease

Mammalian platelets are small, anuclear circulating cells that form tightly adherent, shear-resistant thrombi to prevent blood loss after vessel injury. Platelet thrombi that form in coronary and carotid arteries also underlie common vascular diseases such as myocardial infarction and stroke and are the target of drugs used to treat these diseases. Birds have high-pressure cardiovascular systems like mammals but generate nucleated thrombocytes rather than platelets. Here, we show that avian thrombocytes respond to many of the same activating stimuli as mammalian platelets but are unable to form shear-resistant aggregates ex vivo. Avian thrombocytes are larger than mammalian platelets, spread less efficiently on collagen, and express much lower levels of the α(₂b)β₃ integrin required for aggregate formation, features predicted to make thrombocyte aggregates less resistant than platelets are to the high fluid shear forces of the arterial vasculature. In vivo carotid vessel injury stimulates the formation of occlusive platelet thrombi in mice but not in the size- and flow-matched carotid artery of the Australian budgerigar. These studies indicate that unique physical and molecular features of mammalian platelets enable them to form shear-resistant arterial thrombi, an essential element in the pathogenesis of human cardiovascular diseases.

Genome-wide RNA-seq analysis of human and mouse platelet transcriptomes

Inbred mice are a useful tool for studying the in vivo functions of platelets. Nonetheless, the mRNA signature of mouse platelets is not known. Here, we use paired-end next-generation RNA sequencing (RNA-seq) to characterize the polyadenylated transcriptomes of human and mouse platelets. We report that RNA-seq provides unprecedented resolution of mRNAs that are expressed across the entire human and mouse genomes. Transcript expression and abundance are often conserved between the 2 species. Several mRNAs, however, are differentially expressed in human and mouse platelets. Moreover, previously described functional disparities between mouse and human platelets are reflected in differences at the transcript level, including protease activated receptor-1, protease activated receptor-3, platelet activating factor receptor, and factor V. This suggests that RNA-seq is a useful tool for predicting differences in platelet function between mice and humans. Our next-generation sequencing analysis provides new insights into the human and murine platelet transcriptomes. The sequencing dataset will be useful in the design of mouse models of hemostasis and a catalyst for discovery of new functions of platelets. Access to the dataset is found in the "Introduction."

Hyperglycemia-induced cerebral hematoma expansion is mediated by plasma kallikrein

Hyperglycemia is associated with increased hematoma expansion and worse clinical outcomes following intracerebral hemorrhage. We demonstrate that cerebral hematoma expansion triggered by intracerebral infusion of autologous blood is increased in diabetic rats and mice, and this response is ameliorated by plasma kallikrein (PK) inhibition and deficiency, respectively. Both diabetes and hyperglycemia induced in nondiabetic rats increase hematoma expansion following intracerebral injection of purified PK, a response not observed with bradykinin, plasmin, or tissue plasminogen activator. This response is rapid, prevented by co-injection of the glycoprotein VI (GPVI) agonist convulxin, and mimicked by GPVI inhibition or deficiency. We show that PK binding to collagen and PK-mediated inhibition of collagen-induced platelet aggregation is enhanced by hyperglycemia. Hyperosmotic mannitol also increases hematoma expansion induced by blood and PK, and increases PK-mediated inhibition of platelet aggregation. These findings suggest that hyperglycemia increases cerebral hematoma expansion by PK-mediated osmotic-sensitive inhibition of hemostasis.

Proteolysis of platelet receptors in humans and other species

In the past 5 years, metalloproteinase-mediated ectodomain shedding of platelet receptors has emerged as a new mechanism for modulating platelet function. By regulating surface expression of the platelet-specific receptors, glycoprotein (GP)VI that binds collagen, and GPIbalpha (the major ligand-binding subunit of the GPIb-IX-V complex) that binds von Willebrand factor (VWF) and other procoagulant and proinflammatory ligands, shedding not only irreversibly downregulates GPVI/GPIbalpha function, but generates proteolytic fragments that might be unique biomarkers or modulators in plasma. This is potentially significant because GPVI and GPIbalpha are involved in initiating thrombotic diseases such as heart attack and stroke, as well as autoimmune diseases where anti-platelet antibodies result in thrombocytopenia. Altered expression levels of GPIbalpha/GPVI are associated with both thrombotic propensity and platelet aging, suggesting an additional role in platelet clearance. Although emerging data are elucidating molecular mechanisms underlying GPIbalpha/GPVI shedding, evidence for the functional consequences of shedding in vivo, either clinically or in animal models, is far more limited. Here we consider recent published evidence for GPVI or GPIbalpha shedding in humans, nonhuman primates and mice, and whether conservation of sheddase cleavage sites across species points to a functional role for metalloproteolytic shedding in vivo.

Atherosclerosis and thrombosis: insights from large animal models

Atherosclerosis and its thrombotic complications are responsible for remarkably high numbers of deaths. The combination of in vitro, ex vivo, and in vivo experimental approaches has largely contributed to a better understanding of the mechanisms underlying the atherothrombotic process. Indeed, different animal models have been implemented in atherosclerosis and thrombosis research in order to provide new insights into the mechanisms that have already been outlined in isolated cells and protein studies. Yet, although no model completely mimics the human pathology, large animal models have demonstrated better suitability for translation to humans. Indeed, direct translation from mice to humans should be taken with caution because of the well-reported species-related differences. This paper provides an overview of the available atherothrombotic-like animal models, with a particular focus on large animal models of thrombosis and atherosclerosis, and examines their applicability for translational research purposes as well as highlights species-related differences with humans.

Refinement, reduction and replacement approaches to in vivo cardiovascular research

In this review, the justification and benefits of refinement, reduction and replacement (3Rs) approaches to cardiovascular research are examined using the field of platelet biology and arterial thrombosis as an example. Arterial thrombosis is a platelet-driven condition and platelets are regulated by autologous signals, but also by external factors such as the vascular endothelium. In vitro assays using isolated platelets therefore poorly reflect in vivo platelet function and human disease. As a consequence, animal models, including mouse models, are frequently used. In particular, models of thromboembolic mortality have been successfully employed to determine the role of the vascular endothelium in regulating platelet function and thrombosis in vivo. Such models raise both scientific and ethical concerns and have recently been refined permitting the use of fewer mice at a lower severity level. These refinements have been scientifically beneficial in permitting analysis of the development and progression of thrombotic diseases and in improving our understanding of the role of the vascular endothelium in regulating platelet function and thrombosis. For many, the ultimate goal in 3Rs-driven science is replacement of animal models with non-animal alternatives; this is exemplified, in the platelet field, by the development of in vitro flow systems. The development of 3Rs approaches to cardiovascular research is shown to have led to improved scientific models. Further characterization and use of these models will likely contribute to increased understanding of thrombotic disease processes and facilitate drug development in the cardiovascular field.

The anti-platelet effects of apocynin in mice are not mediated by inhibition of NADPH oxidase activity

Apocynin, or a (myelo)peroxidase-derived product thereof, is a powerful inhibitor of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Apocynin has also been shown to prevent aggregation of platelets in response to agonists such as collagen and thrombin. The aims of this study were to establish whether NADPH oxidase activity is required for aggregation of murine platelets to collagen and other agonists and whether the anti-aggregatory effects of apocynin are due to an inhibitory action against this enzyme. Washed platelets were isolated from male C57BL6 (wild-type), Nox2-deficient (Nox2(-/y )), and p47phox-deficient (p47phox(-/-)) mice for assessment of aggregation and NADPH oxidase subunit (Nox2, p47phox) expression. Collagen and U46619 elicited aggregation of murine platelets, and these responses were inhibited by apocynin at concentrations ≥100 μM. By contrast, aggregations to a direct protein kinase C activator, phorbol-12,13-dibutyrate, were insensitive to apocynin. Immunoblotting of platelet protein homogenates from wild-type mice with anti-Nox2 or p47phox antibodies revealed strong bands at 58 and 50 kDa, respectively. While expression of these immunoreactive bands was greatly diminished in platelets from Nox2(-/y ) and p47phox(-/-) mice, collagen still elicited aggregations that were similar to those observed in platelets from wild-types. Moreover, apocynin was an equally effective inhibitor of aggregation in platelets from all three mouse strains. In conclusion, these data suggest that NADPH oxidase-derived reactive oxygen species play no role in the aggregation response of washed murine platelets to collagen. Thus, our observation that apocynin is a powerful inhibitor of platelet aggregation raises further questions about the selectivity of this drug as an NADPH oxidase inhibitor.

Arterial thrombosis: relevance of a model with two levels of severity assessed by histologic, ultrastructural and functional characterization

BACKGROUND

We previously described a model of laser-induced thrombosis in mesenteric arterioles with superficial and deep levels of injury producing a transient thrombus resolving within 2 min and a larger almost occlusive thrombus, respectively. Both types of lesion were sensitive to platelet GPIIb-IIIa and P2Y(12) inhibition, whereas only deep injuries were sensitive to thrombin blockade.

OBJECTIVE

The aim of the present study was to use histologic methods and electron and intravital microscopy to characterize the lesions and thrombi and to extend our knowledge of the sensitivity of this model to genetic and pharmacologic inhibition.

RESULTS

A superficial injury was found to detach the endothelial cells and expose a collagen III- and IV-rich subendothelium where platelets could adhere. Tissue factor and fibrin were not detected. Deeper penetration of the external elastic lamina occurred in deep injuries, with exposure of collagen I, III and IV. Here the thrombus was composed of platelets exhibiting a decreasing gradient of degranulation from the deepest lesion area to the surface. Fibrin was found close to the most activated platelets. Consistently, glycoprotein VI (GPVI)-collagen and GPIb-von Willebrand factor (VWF) interactions were found to be critical in superficial injuries. After deep lesion, thrombus formation was modestly reduced in GPVI-immunodepleted mice and still strongly inhibited in VWF(-/-) mice. Combined hirudin infusion and GPVI depletion further inhibited thrombosis after deep injury.

CONCLUSIONS

This study confirms the feasibility of inducing arterial thrombosis with distinct levels of severity and establishes the central roles of collagen and VWF in thrombus formation after superficial injury. Collagen, VWF and thrombin all appear to contribute to thrombosis after deep arterial lesion.

Imaging of structural changes in endothelial cells and thrombus formation at the site of FeCl(3)-induced injuries in mice cremasteric arteries

AIM

We investigated thrombus formation at the site of functional injury to endothelial cells by FeCl(3).

METHODS

After preparation of cremasteric arteries of mice, controlled endothelial injury was induced by application of FeCl(3). Endothelial cells were rendered fluorescent by addition of FITC (fluorescein isothiocyanate)-labeled isolectin B4. Circulating platelets and leukocytes were made fluorescent by rhodamine 6G. Three-dimensional (3D) growth of thrombi was visualized in real time. Effects of aspirin and clopidogrel pre-treatments on the growth of thrombi were investigated in vivo as well as in an ex vivo flow chamber system.

RESULTS

Endothelial cells were tightly bound to each other to protect local thrombus formation. Platelets started to adhere to endothelial cells when FeCl(3) was applied. Three-dimensional growth of thrombi, which takes 10.6+/-7.5 minutes for complete occlusion in control, can be visualized with our imaging system. Aspirin pre-treatment at the dose tested did not influence either endothelial injury or platelet thrombus growth, while clopidogrel pretreatment significantly inhibited 3D growth and prolonged occlusion time up to 64.6+/-25.3 minutes (100 mg/kg). A similar inhibiting effect of clopidogrel was reproduced in ex vivo flow chamber experiments.

CONCLUSIONS

We have developed an in vivo system to detect thrombus formation after functional damage to the endothelium.

Can eccentric arterial plaques alone cause flow stagnation points and favour thrombus incorporation?

We have used an experimental model of aorta stenosis, with a Plexiglas plug, simulating a stable atheromatous plaque that promotes local turbulence and thrombosis. With animal survival of more than 24 h, we followed the partial fibrinolysis of the thrombus as well as its posterior organization and incorporation to the arterial wall as a neointima for up to 30 days. The mushroom plug form permitted the development of recirculation and stasis areas around it, favouring this evolution. Despite noted limitations, this study demonstrates that thrombus incorporation can contribute to plaque extension, as it can promote recirculation and stasis areas.

Platelet receptors and signaling in the dynamics of thrombus formation

Hemostasis and pathological thrombus formation are dynamic processes that require a co-ordinated series of events involving platelet membrane receptors, bidirectional intracellular signals, and release of platelet proteins and inflammatory substances. This review aims to summarize current knowledge in the key steps in the dynamics of thrombus formation, with special emphasis on the crucial participation of platelet receptors and signaling in this process. Initial tethering and firm adhesion of platelets to the exposed subendothelium is mediated by glycoprotein (GP) Ib/IX/V complex and collagen receptors, GP VI and alpha(2)beta(1) integrin, in the platelet surface, and by VWF and fibrillar collagen in the vascular site. Interactions between these elements are largely influenced by flow and trigger signaling events that reinforce adhesion and promote platelet activation. Thereafter, soluble agonists, ADP, thrombin, TxA(2), produced/released at the site of vascular injury act in autocrine and paracrine mode to amplify platelet activation and to recruit circulating platelets to the developing thrombus. Specific interactions of these agonists with their G-protein coupled receptors generate inside-out signaling leading to conformational activation of integrins, in particular alpha(IIb)beta(3), increasing their ligand affinity. Binding of alpha(IIb)beta(3) to its ligands, mainly fibrinogen, supports processes such as clot retraction and platelet aggregation. Stabilization of thrombi is supported by the late wave of signaling events promoted by close contact between aggregated platelets. The best known contact-dependent signaling is outside-in signaling through alphaIb beta(3), but new ones are being clarified such as those mediated by interaction of Eph receptors with ephrins, or by Sema 4D and Gas-6 binding to their receptors. Finally, newly identified mechanisms appear to control thrombus growth, including back-shifting of activated integrins and actuation of compensatory molecules such as ESAM or PECAM-1. The expanding knowledge of thrombotic disease is expected to translate into the development of new drugs to help management and prevention of thrombosis.

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.

The role of the endocannabinoid system in atherosclerosis

Our current understanding of the pathophysiology of atherosclerosis suggests a prominent role for immune responses from its initiation through its complications. Given the increasing prevalence of cardiovascular risk factors worldwide, there is an urgent need to better understand the underlying mechanisms to improve current treatment protocols. A growing body of evidence suggests that endocannabinoid signalling plays a critical role in the pathogenesis of atherogenesis and its clinical manifestations. Blocking CB(1) receptors has been shown to mediate not only weight reduction, but also several cardiometabolic effects in rodents and humans, indicating a potential relevance for the process of atherosclerosis. Activation of CB(2) receptors with Delta(9)-tetrahydrocannabinol (THC) has been shown to inhibit atherosclerotic plaque progression in mice, mainly by inhibiting macrophage recruitment. Endocannabinoids released from endothelial cells, macrophages or platelets, reduce hypertension in rodents, a major risk factor for atherosclerosis. In addition, anandamide inhibits inflammatory gene expression in endothelial cells, and consequently monocyte adhesion. Conversely, endocannabinoids might also mediate pro-atherosclerotic effects by inducing platelet activation. In conclusion, the precise role of the endocannabinoid system during atherosclerosis is not yet understood. Whether increased endocannabinoid signalling is associated with disease progression and increased risk of acute thrombotic events remains to be determined.

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.

Cannabinoid receptors in acute and chronic complications of atherosclerosis

Atherosclerosis is a chronic inflammatory disease that is the primary cause of myocardial infarction and stroke, which occur after sudden thrombotic occlusion of an artery. A growing body of evidence suggests that cannabinoid signalling plays a fundamental role in atherosclerosis development and its clinical manifestations. Thus, CB2 receptors are protective in myocardial ischaemia/reperfusion and implicated in the modulation of chemotaxis, which is crucial for the recruitment of leukocytes during inflammation. Delta-9-Tetrahydrocannabinol (THC)-mediated activation has been shown to inhibit atherosclerotic plaque progression in a CB2 dependent manner. Although CB1 and CB2 expression has been reported on platelets, their involvement in thrombus formation is still controversial. While several reports suggest that CB1 receptors may have a relevant role in neuroprotection after ischaemic stroke, recent studies show the protective effects in various forms of neuroprotection are not related to CB1 stimulation, and a protective role of CB1 blockade has also been reported. In addition, vascular and myocardial CB1 receptors contribute to the modulation of blood pressure and heart rate. It is tempting to suggest that pharmacological modulation of the endocannabinoid system is a potential novel therapeutic strategy in the treatment of atherosclerosis. For these purposes, it is important to better understand the complex mechanisms of endocannabinoid signalling and potential consequences of its pharmacological modulation, as it may have both pro- and anti-atherosclerotic effects.

Vascular bed-specific thrombosis

Hemostasis represents a finely tuned balance between procoagulant and anticoagulant forces. An imbalance of these forces may lead to clinically significant disease, including arterial, venous and/or microvascular thrombosis. The vast majority of hypercoagulable states are associated with local thrombus formation. The goal of this review is to discuss the mechanisms underlying site-specific thrombosis.

Targeting Platelets in Acute Experimental Stroke

Background— Ischemic stroke is a frequent and serious disease with limited treatment options. Platelets can adhere to hypoxic cerebral endothelial cells by binding of their glycoprotein (GP) Ib receptor to von Willebrand factor. Exposure of subendothelial matrix proteins further facilitates firm attachment of platelets to the vessel wall by binding of collagen to their GPVI receptor. In the present study, we addressed the pathogenic role of GPIb, GPVI, and the aggregation receptor GPIIb/IIIa in experimental stroke in mice.

Methods and Results— Complete blockade of GPIbα was achieved by intravenous injection of 100 μg Fab fragments of the monoclonal antibody p0p/B to mice undergoing 1 hour of transient middle cerebral artery occlusion. At 24 hours after transient middle cerebral artery occlusion, cerebral infarct volumes were assessed by 2,3,5-triphenyltetrazolium chloride staining. In mice treated with anti-GPIbα Fab 1 hour before middle cerebral artery occlusion, ischemic lesions were reduced to ≈40% compared with controls (28.5±12.7 versus 73.9±17.4 mm 3 , respectively; P <0.001). Application of anti-GPIbα Fab 1 hour after middle cerebral artery occlusion likewise reduced brain infarct volumes (24.5±7.7 mm 3 ; P <0.001) and improved the neurological status. Similarly, depletion of GPVI significantly diminished the infarct volume but to a lesser extent (49.4±19.1 mm 3 ; P <0.05). Importantly, the disruption of early steps of platelet activation was not accompanied by an increase in bleeding complications as revealed by serial magnetic resonance imaging. In contrast, blockade of the final common pathway of platelet aggregation with anti-GPIIb/IIIa F(ab) 2 fragments had no positive effect on stroke size and functional outcome but increased the incidence of intracerebral hemorrhage and mortality after transient middle cerebral artery occlusion in a dose-dependent manner.

Conclusions— Our data indicate that the selective blockade of key signaling pathways of platelet adhesion and aggregation has a different impact on stroke outcome and bleeding complications. Inhibition of early steps of platelet adhesion to the ischemic endothelium and the subendothelial matrix may offer a novel and safe treatment strategy in acute stroke.

Flavonoids inhibit the platelet TxA(2) signalling pathway and antagonize TxA(2) receptors (TP) in platelets and smooth muscle cells

AIMS

Flavonoids may affect platelet function by several mechanisms, including antagonism of TxA(2) receptors (TP). These TP are present in many tissues and modulate different signalling cascades. We explored whether flavonoids affect platelet TP signalling, and if they bind to TP expressed in other cell types.

METHODS

Platelets were treated with flavonoids, or other selected inhibitors, and then stimulated with U46619. Similar assays were performed in aspirinized platelets activated with thrombin. Effects on calcium release were analysed by fluorometry and changes in whole protein tyrosine phosphorylation and activation of ERK 1/2 by Western blot analysis. The binding of flavonoids to TP in platelets, human myometrium and TPalpha- and TPbeta-transfected HEK 293T cells was explored using binding assays and the TP antagonist (3)H-SQ29548.

RESULTS

Apigenin, genistein, luteolin and quercetin impaired U46619-induced calcium mobilization in a concentration-dependent manner (IC(50) 10-30 microm). These flavonoids caused a significant impairment of U46619-induced platelet tyrosine phosphorylation and of ERK 1/2 activation. By contrast, in aspirin-treated platelets all these flavonoids, except quercetin, displayed minor effects on thrombin-induced calcium mobilization, ERK 1/2 and total tyrosine phosphorylation. Finally, apigenin, genistein and luteolin inhibited by >50% (3)H-SQ29548 binding to different cell types.

CONCLUSIONS

These data further suggest that flavonoids may inhibit platelet function by binding to TP and by subsequent abrogation of downstream signalling. Binding of these compounds to TP occurs in human myometrium and in TP-transfected HEK 293T cells and suggests that antagonism of TP might mediate the effects of flavonoids in different tissues.

Platelet adhesion receptors and their ligands in mouse models of thrombosis

Platelet adhesion and aggregation at sites of vascular injury are two key events in hemostasis and thrombosis. Because of exciting advances in genetic engineering, the mouse has become an important and frequently used model to unravel the molecular mechanisms underlying the multistep process leading to the formation of a stable platelet plug. In gene-targeted mice, the crucial importance of platelet adhesion receptors such as glycoprotein Ib alpha or the alphaIIb beta3 integrin has been confirmed and further clarified. Their absence leads to highly impaired thrombus formation, independent of the model used to induce vascular injury. In contrast, the relative contribution of other receptors, such as glycoprotein VI, or of various platelet ligands may be regulated by the severity of injury, the type of vessel injured, and the signaling pathways that are generated. Murine models have also helped improve understanding of the second wave of events that leads to stabilization of the platelet aggregate. Despite the current limitations due to lack of standardization and the virtual absence of thrombosis models in diseased vessels, there is no doubt that the mouse will play a key role in the discovery and characterization of the next generation of antithrombotic agents. This review focuses on key findings about the molecular mechanisms supporting hemostasis and thrombosis that have been obtained with genetically engineered mouse models deficient in various platelet adhesion receptors and ligands. Combination of these models with sophisticated methods allowing direct visualization of platelet-vessel wall interactions after injury greatly contributed to recent advances in the field.