Design, manufacturing and testing of a green non-isocyanate polyurethane prosthetic heart valve

The sole effective treatment for most patients with heart valve disease is valve replacement by implantation of mechanical or biological prostheses. However, mechanical valves represent high risk of thromboembolism, and biological prostheses are prone to early degeneration. In this work, we aim to determine the potential of novel environmentally-friendly non-isocyanate polyurethanes (NIPUs) for manufacturing synthetic prosthetic heart valves. Polyhydroxyurethane (PHU) NIPUs are synthesized via an isocyanate-free route, tested in vitro, and used to produce aortic valves. PHU elastomers reinforced with a polyester mesh show mechanical properties similar to native valve leaflets. These NIPUs do not cause hemolysis. Interestingly, both platelet adhesion and contact activation-induced coagulation are strongly reduced on NIPU surfaces, indicating low thrombogenicity. Fibroblasts and endothelial cells maintain normal growth and shape after indirect contact with NIPUs. Fluid-structure interaction (FSI) allows modeling of the ideal valve design, with minimal shear stress on the leaflets. Injection-molded valves are tested in a pulse duplicator and show ISO-compliant hydrodynamic performance, comparable to clinically-used bioprostheses. Poly(tetrahydrofuran) (PTHF)-NIPU patches do not show any evidence of calcification over a period of 8 weeks. NIPUs are promising sustainable biomaterials for the manufacturing of improved prosthetic valves with low thrombogenicity.

Ultra-high throughput-based screening for the discovery of antiplatelet drugs affecting receptor dependent calcium signaling dynamics

Distinct platelet activation patterns are elicited by the tyrosine kinase-linked collagen receptor glycoprotein VI (GPVI) and the G-protein coupled protease-activated receptors (PAR1/4) for thrombin. This is reflected in the different platelet Ca2+ responses induced by the GPVI agonist collagen-related peptide (CRP) and the PAR1/4 agonist thrombin. Using a 96 well-plate assay with human Calcium-6-loaded platelets and a panel of 22 pharmacological inhibitors, we assessed the cytosolic Ca2+ signaling domains of these receptors and developed an automated Ca2+ curve algorithm. The algorithm was used to evaluate an ultra-high throughput (UHT) based screening of 16,635 chemically diverse small molecules with orally active physicochemical properties for effects on platelets stimulated with CRP or thrombin. Stringent agonist-specific selection criteria resulted in the identification of 151 drug-like molecules, of which three hit compounds were further characterized. The dibenzyl formamide derivative ANO61 selectively modulated thrombin-induced Ca2+ responses, whereas the aromatic sulfonyl imidazole AF299 and the phenothiazine ethopropazine affected CRP-induced responses. Platelet functional assays confirmed selectivity of these hits. Ethopropazine retained its inhibitory potential in the presence of plasma, and suppressed collagen-dependent thrombus buildup at arterial shear rate. In conclusion, targeting of platelet Ca2+ signaling dynamics in a screening campaign has the potential of identifying novel platelet-inhibiting molecules.

Endothelium-mediated regulation of platelet activation: Involvement of multiple protein kinases

The endothelial regulation of platelet activity is incompletely understood. Here we describe novel approaches to find molecular pathways implicated on the platelet-endothelium interaction. Using high-shear whole-blood microfluidics, employing coagulant or non-coagulant conditions at physiological temperature, we observed that the presence of human umbilical vein endothelial cells (HUVEC) strongly suppressed platelet adhesion and activation, via the collagen receptor glycoprotein VI (GPVI) and the PAR receptors for thrombin. Real-time monitoring of the cytosolic Ca2+ rises in the platelets indicated no major improvement of inhibition by prostacyclin or nitric oxide. Similarly under stasis, exposure of isolated platelets to HUVEC reduced the Ca2+ responses by collagen-related peptide (CRP-XL, GPVI agonist) and thrombin (PAR agonist). We then analyzed the label-free phosphoproteome of platelets (three donors), exposed to HUVEC, CRP-XL, and/or thrombin. High-resolution mass spectrometry gave 5463 phosphopeptides, corresponding to 1472 proteins, with good correlation between biological and technical replicates (R > .86). Stringent filtering steps revealed 26 regulatory pathways (Reactome) and 143 regulated kinase substrates (PhosphoSitePlus), giving a set of protein phosphorylation sites that was differentially (44) or similarly (110) regulated by HUVEC or agonist exposure. The differential regulation was confirmed by stable-isotope analysis of platelets from two additional donors. Substrate analysis indicated major roles of poorly studied protein kinase classes (MAPK, CDK, DYRK, STK, PKC members). Collectively, these results reveal a resetting of the protein phosphorylation profile in platelets exposed to endothelium or to conventional agonists and to endothelium-promoted activity of a multi-kinase network, beyond classical prostacyclin and nitric oxide actors, that may contribute to platelet inhibition.

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

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

Role of SHP2 (PTPN11) in glycoprotein VI-dependent thrombus formation: Improved platelet responsiveness by the allosteric drug SHP099 in Noonan syndrome patients

INTRODUCTION

The protein tyrosine phosphatase SHP2 (PTPN11) is a negative regulator of glycoprotein VI (GPVI)-induced platelet signal under certain conditions. Clinical trials with derivatives of the allosteric drug SHP099, inhibiting SHP2, are ongoing as potential therapy for solid cancers. Gain-of-function mutations of the PTPN11 gene are observed in part of the patients with the Noonan syndrome, associated with a mild bleeding disorder. Assessment of the effects of SHP2 inhibition in platelets from controls and Noonan syndrome patients.

MATERIALS AND METHODS

Washed human platelets were incubated with SHP099 and stimulated with collagen-related peptide (CRP) for stirred aggregation and flow cytometric measurements. Whole-blood microfluidics assays using a dosed collagen and tissue factor coating were performed to assess shear-dependent thrombus and fibrin formation. Effects on clot formation were evaluated by thromboelastometry.

RESULTS

Pharmacological inhibition of SHP2 did not alter GPVI-dependent platelet aggregation under stirring, but it enhanced integrin αIIbβ3 activation in response to CRP. Using whole-blood microfluidics, SHP099 increased the thrombus buildup on collagen surfaces. In the presence of tissue factor and coagulation, SHP099 increased thrombus size and reduced time to fibrin formation. Blood from PTPN11-mutated Noonan syndrome patients, with low platelet responsiveness, after ex vivo treatment with SHP099 showed a normalized platelet function. In thromboelastometry, SHP2 inhibition tended to increase tissue factor-induced blood clotting profiles with tranexamic acid, preventing fibrinolysis.

CONCLUSION

Pharmacological inhibition of SHP2 by the allosteric drug SHP099 enhances GPVI-induced platelet activation under shear conditions with a potential to improve platelet functions of Noonan syndrome patients.

Tumor-educated platelet blood tests for Non-Small Cell Lung Cancer detection and management

Liquid biopsy approaches offer a promising technology for early and minimally invasive cancer detection. Tumor-educated platelets (TEPs) have emerged as a promising liquid biopsy biosource for the detection of various cancer types. In this study, we processed and analyzed the TEPs collected from 466 Non-small Cell Lung Carcinoma (NSCLC) patients and 410 asymptomatic individuals (controls) using the previously established thromboSeq protocol. We developed a novel particle-swarm optimization machine learning algorithm which enabled the selection of an 881 RNA biomarker panel (AUC 0.88). Herein we propose and validate in an independent cohort of samples (n = 558) two approaches for blood samples testing: one with high sensitivity (95% NSCLC detected) and another with high specificity (94% controls detected). Our data explain how TEP-derived spliced RNAs may serve as a biomarker for minimally-invasive clinical blood tests, complement existing imaging tests, and assist the detection and management of lung cancer patients.

Differential Regulation of GPVI-Induced Btk and Syk Activation by PKC, PKA and PP2A in Human Platelets

Bruton's tyrosine kinase (Btk) and spleen tyrosine kinase (Syk) are major signaling proteins in human platelets that are implicated in atherothrombosis and thrombo-inflammation, but the mechanisms controlling their activities are not well understood. Previously, we showed that Syk becomes phosphorylated at S297 in glycoprotein VI (GPVI)-stimulated human platelets, which limits Syk activation. Here, we tested the hypothesis that protein kinases C (PKC) and A (PKA) and protein phosphatase 2A (PP2A) jointly regulate GPVI-induced Btk activation in platelets. The GPVI agonist convulxin caused rapid, transient Btk phosphorylation at S180 (pS180↑), Y223 and Y551, while direct PKC activation strongly increased Btk pS180 and pY551. This increase in Btk pY551 was also Src family kinase (SFK)-dependent, but surprisingly Syk-independent, pointing to an alternative mechanism of Btk phosphorylation and activation. PKC inhibition abolished convulxin-stimulated Btk pS180 and Syk pS297, but markedly increased the tyrosine phosphorylation of Syk, Btk and effector phospholipase Cγ2 (PLCγ2). PKA activation increased convulxin-induced Btk activation at Y551 but strongly suppressed Btk pS180 and Syk pS297. PP2A inhibition by okadaic acid only increased Syk pS297. Both platelet aggregation and PLCγ2 phosphorylation with convulxin stimulation were Btk-dependent, as shown by the selective Btk inhibitor acalabrutinib. Together, these results revealed in GPVI-stimulated platelets a transient Syk, Btk and PLCγ2 phosphorylation at multiple sites, which are differentially regulated by PKC, PKA or PP2A. Our work thereby demonstrated the GPVI-Syk-Btk signalosome as a tightly controlled protein kinase network, in agreement with its role in atherothrombosis.

Age- and gender-matched controls needed for platelet-based biomarker studies

Not available.

Detection and localization of early- and late-stage cancers using platelet RNA

Cancer patients benefit from early tumor detection since treatment outcomes are more favorable for less advanced cancers. Platelets are involved in cancer progression and are considered a promising biosource for cancer detection, as they alter their RNA content upon local and systemic cues. We show that tumor-educated platelet (TEP) RNA-based blood tests enable the detection of 18 cancer types. With 99% specificity in asymptomatic controls, thromboSeq correctly detected the presence of cancer in two-thirds of 1,096 blood samples from stage I-IV cancer patients and in half of 352 stage I-III tumors. Symptomatic controls, including inflammatory and cardiovascular diseases, and benign tumors had increased false-positive test results with an average specificity of 78%. Moreover, thromboSeq determined the tumor site of origin in five different tumor types correctly in over 80% of the cancer patients. These results highlight the potential properties of TEP-derived RNA panels to supplement current approaches for blood-based cancer screening.

Quantitative and qualitative changes in platelet traits of sunitinib-treated patients with renal cell carcinoma in relation to circulating sunitinib levels: a proof-of-concept study

Background

Tyrosine kinase inhibitors (TKIs), such as sunitinib, are used for cancer treatment, but may also affect platelet count and function with possible hemostatic consequences. Here, we investigated whether patient treatment with the TKI sunitinib affected quantitative and qualitative platelet traits as a function of the sunitinib level and the occurrence of bleeding.

Methods

Blood was collected from 20 metastatic renal cell carcinoma (mRCC) patients before treatment, and at 2 weeks, 4 weeks and 3 months after sunitinib administration. We measured blood cell counts, platelet aggregation, and concentrations of sunitinib as well as its N-desethyl metabolite in plasma, serum and isolated platelets. Progression of disease (PD) and bleeding were monitored after 3 months.

Results

In sunitinib-treated mRCC patients, concentrations of (N-desethyl-)sunitinib in plasma and serum were highly correlated. In the patients’ platelets the active metabolite levels were relatively increased as compared to sunitinib. On average, a sustained reduction in platelet count was observed on-treatment, which was significantly related to the inhibitor levels in plasma/serum. Principal component and correlational analysis showed that the (N-desethyl-)sunitinib levels in plasma/serum were linked to a reduction in both platelet count and collagen-induced platelet aggregation. The reduced aggregation associated in part with reported bleeding, but did not correlate to PD.

Conclusion

The sunitinib-induced reduction in quantitative and qualitative platelet traits may reflect the effective sunitinib levels in the patient. These novel results may serve as a proof-of-principle for other TKI-related drugs, where both platelet count and functions are affected, which could be used for therapeutic drug monitoring.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09676-0.

MicroRNA-26b Attenuates Platelet Adhesion and Aggregation in Mice

Platelets are key regulators of haemostasis, making platelet dysfunction a major driver of thrombosis. Numerous processes that determine platelet function are influenced by microRNAs (miRs). MiR-26b is one of the highest-expressed miRs in healthy platelets, and its expression in platelets is changed in a diseased state. However, the exact effect of this miR on platelet function has not been studied yet. In this study, we made use of a whole-body knockout of miR-26b in ApoE-deficient mice in order to determine its impact on platelet function, thrombus formation and platelet signalling both ex vivo and in vivo. We show that a whole-body deficiency of miR-26b exacerbated platelet adhesion and aggregation ex vivo. Additionally, in vivo, platelets adhered faster, and larger thrombi were formed in mice lacking miR-26b. Moreover, isolated platelets from miR-26b-deficient mice showed a hyperactivated Src and EGFR signalling. Taken together, we show here for the first time that miR-26b attenuates platelet adhesion and aggregation, possibly through Src and EGFR signalling.

Protein C or Protein S deficiency associates with paradoxically impaired platelet‐dependent thrombus and fibrin formation under flow

Background

Low plasma levels of protein C or protein S are associated with venous thromboembolism rather than myocardial infarction. The high coagulant activity in patients with thrombophilia with a (familial) defect in protein C or S is explained by defective protein C activation, involving thrombomodulin and protein S. This causes increased plasmatic thrombin generation.

Objective

Assess the role of platelets in the thrombus‐ and fibrin‐forming potential in patients with familial protein C or protein S deficiency under high‐shear flow conditions.

Patients/Methods

Whole blood from 23 patients and 15 control subjects was perfused over six glycoprotein VI–dependent microspot surfaces. By real‐time multicolor microscopic imaging, kinetics of platelet thrombus and fibrin formation were characterized in 49 parameters.

Results and Conclusion

Whole‐blood flow perfusion over collagen, collagen‐like peptide, and fibrin surfaces with low or high GPVI dependency indicated an unexpected impairment of platelet activation, thrombus phenotype, and fibrin formation but unchanged platelet adhesion, observed in patients with protein C deficiency and to a lesser extent protein S deficiency, when compared to controls. The defect extended from diminished phosphatidylserine exposure and thrombus contraction to delayed and suppressed fibrin formation. The mechanism was thrombomodulin independent, and may involve negative platelet priming by plasma components.

Temporal Roles of Platelet and Coagulation Pathways in Collagen- and Tissue Factor-Induced Thrombus Formation

In hemostasis and thrombosis, the complex process of thrombus formation involves different molecular pathways of platelet and coagulation activation. These pathways are considered as operating together at the same time, but this has not been investigated. The objective of our study was to elucidate the time-dependency of key pathways of thrombus and clot formation, initiated by collagen and tissue factor surfaces, where coagulation is triggered via the extrinsic route. Therefore, we adapted a microfluidics whole-blood assay with the Maastricht flow chamber to acutely block molecular pathways by pharmacological intervention at desired time points. Application of the technique revealed crucial roles of glycoprotein VI (GPVI)-induced platelet signaling via Syk kinase as well as factor VIIa-induced thrombin generation, which were confined to the first minutes of thrombus buildup. A novel anti-GPVI Fab EMF-1 was used for this purpose. In addition, platelet activation with the protease-activating receptors 1/4 (PAR1/4) and integrin αIIbβ3 appeared to be prolongedly active and extended to later stages of thrombus and clot formation. This work thereby revealed a more persistent contribution of thrombin receptor-induced platelet activation than of collagen receptor-induced platelet activation to the thrombotic process.

Effects of Platelet Agonists and Priming on the Formation of Platelet Populations

Platelets from healthy donors display heterogeneity in responsiveness to agonists. The response thresholds of platelets are controlled by multiple bioactive molecules, acting as negatively or positively priming substances. Higher circulating levels of priming substances adenosine and succinate, as well as the occurrence of hypercoagulability, have been described for patients with ischaemic heart disease. Here, we present an improved methodology of flow cytometric analyses of platelet activation and the characterisation of platelet populations following activation and priming by automated clustering analysis.Platelets were treated with adenosine, succinate, or coagulated plasma before stimulation with CRP-XL, 2-MeSADP, or TRAP6 and labelled for activated integrin α_IIbβ₃ (PAC1), CD62P, TLT1, CD63, and GPIX. The Super-Enhanced Dmax subtraction algorithm and 2% marker (quadrant) setting were applied to identify populations, which were further defined by state-of-the-art clustering techniques (tSNE, FlowSOM).Following activation, five platelet populations were identified: resting, aggregating (PAC1 + ), secreting (α- and dense-granules; CD62P + , TLT1 + , CD63 + ), aggregating plus α-granule secreting (PAC1 + , CD62P + , TLT1 + ), and fully active platelet populations. The type of agonist determined the distribution of platelet populations. Adenosine in a dose-dependent way suppressed the fraction of fully activated platelets (TRAP6 > 2-MeSADP > CRP-XL), whereas succinate and coagulated plasma increased this fraction (CRP-XL > TRAP6 > 2-MeSADP). Interestingly, a subset of platelets showed a constant response (aggregating, secreting, or aggregating plus α-granule secreting), which was hardly affected by the stimulus strength or priming substances.

The Analysis of Platelet-Derived circRNA Repertoire as Potential Diagnostic Biomarker for Non-Small Cell Lung Cancer

Tumor-educated Platelets (TEPs) have emerged as rich biosources of cancer-related RNA profiles in liquid biopsies applicable for cancer detection. Although human blood platelets have been found to be enriched in circular RNA (circRNA), no studies have investigated the potential of circRNA as platelet-derived biomarkers for cancer. In this proof-of-concept study, we examine whether the circRNA signature of blood platelets can be used as a liquid biopsy biomarker for the detection of non-small cell lung cancer (NSCLC). We analyzed the total RNA, extracted from the platelet samples collected from NSCLC patients and asymptomatic individuals, using RNA sequencing (RNA-Seq). Identification and quantification of known and novel circRNAs were performed using the accurate CircRNA finder suite (ACFS), followed by the differential transcript expression analysis using a modified version of our thromboSeq software. Out of 4732 detected circRNAs, we identified 411 circRNAs that are significantly (p-value < 0.05) differentially expressed between asymptomatic individuals and NSCLC patients. Using the false discovery rate (FDR) of 0.05 as cutoff, we selected the nuclear receptor-interacting protein 1 (NRIP1) circRNA (circNRIP1) as a potential biomarker candidate for further validation by reverse transcription-quantitative PCR (RT-qPCR). This analysis was performed on an independent cohort of platelet samples. The RT-qPCR results confirmed the RNA-Seq data analysis, with significant downregulation of circNRIP1 in platelets derived from NSCLC patients. Our findings suggest that circRNAs found in blood platelets may hold diagnostic biomarkers potential for the detection of NSCLC using liquid biopsies.

Inhibition of Phosphodiesterase 3A by Cilostazol Dampens Proinflammatory Platelet Functions

OBJECTIVE

platelets possess not only haemostatic but also inflammatory properties, which combined are thought to play a detrimental role in thromboinflammatory diseases such as acute coronary syndromes and stroke. Phosphodiesterase (PDE) 3 and -5 inhibitors have demonstrated efficacy in secondary prevention of arterial thrombosis, partially mediated by their antiplatelet action. Yet it is unclear whether such inhibitors also affect platelets' inflammatory functions. Here, we aimed to examine the effect of the PDE3A inhibitor cilostazol and the PDE5 inhibitor tadalafil on platelet function in various aspects of thromboinflammation. Approach and results: cilostazol, but not tadalafil, delayed ex vivo platelet-dependent fibrin formation under whole blood flow over type I collagen at 1000 s-1. Similar results were obtained with blood from Pde3a deficient mice, indicating that cilostazol effects are mediated via PDE3A. Interestingly, cilostazol specifically reduced the release of phosphatidylserine-positive extracellular vesicles (EVs) from human platelets while not affecting total EV release. Both cilostazol and tadalafil reduced the interaction of human platelets with inflamed endothelium under arterial flow and the release of the chemokines CCL5 and CXCL4 from platelets. Moreover, cilostazol, but not tadalafil, reduced monocyte recruitment and platelet-monocyte interaction in vitro.

CONCLUSIONS

this study demonstrated yet unrecognised roles for platelet PDE3A and platelet PDE5 in platelet procoagulant and proinflammatory responses.

Galectin-1 and platelet factor 4 (CXCL4) induce complementary platelet responses in vitro

Galectin-1 (gal-1) is a carbohydrate-binding lectin with important functions in angiogenesis, immune response, hemostasis and inflammation. Comparable functions are exerted by platelet factor 4 (CXCL4), a chemokine stored in the α-granules of platelets. Previously, gal-1 was found to activate platelets through integrin αIIbβ3. Both gal-1 and CXCL4 have high affinities for polysaccharides, and thus may mutually influence their functions. The aim of this study was to investigate a possible synergism of gal-1 and CXCL4 in platelet activation. Platelets were treated with increasing concentrations of gal-1, CXCL4 or both, and aggregation, integrin activation, P-selectin and phosphatidyl serine (PS) exposure were determined by light transmission aggregometry and by flow cytometry. To investigate the influence of cell surface sialic acid, platelets were treated with neuraminidase prior to stimulation. Gal-1 and CXCL4 were found to colocalize on the platelet surface. Stimulation with gal-1 led to integrin αIIbβ3 activation and to robust platelet aggregation, while CXCL4 weakly triggered aggregation and primarily induced P-selectin expression. Co-incubation of gal-1 and CXCL4 potentiated platelet aggregation compared with gal-1 alone. Whereas neither gal-1 and CXCL4 induced PS-exposure on platelets, prior removal of surface sialic acid strongly potentiated PS exposure. In addition, neuraminidase treatment increased the binding of gal-1 to platelets and lowered the activation threshold for gal-1. However, CXCL4 did not affect binding of gal-1 to platelets. Taken together, stimulation of platelets with gal-1 and CXCL4 led to distinct and complementary activation profiles, with additive rather than synergistic effects.

Platelet calcium signaling by G-protein coupled and ITAM-linked receptors regulating anoctamin-6 and procoagulant activity

Most agonists stimulate platelet Ca²⁺ rises via G-protein coupled receptors (GPCRs) or ITAM-linked receptors (ILRs). Well studied are the GPCRs stimulated by the soluble agonists thrombin (PAR1, PAR4), ADP (P2Y₁, P2Y₁₂), and thromboxane A₂ (TP), signaling via phospholipase (PLC)β isoforms. The platelet ILRs glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2), and FcγRIIa are stimulated by adhesive ligands or antibody complexes and signal via tyrosine protein kinases and PLCγ isoforms. Marked differences exist between the GPCR- and ILR-induced Ca²⁺ signaling in: (i) dependency of tyrosine phosphorylation; (ii) oscillatory versus continued Ca²⁺ rises by mobilization from the endoplasmic reticulum; and (iii) smaller or larger role of extracellular Ca²⁺ entry via STIM1/ORAI1. Co-stimulation of both types of receptors, especially by thrombin (PAR1/4) and collagen (GPVI), leads to a highly enforced Ca²⁺ rise, involving mitochondrial Ca²⁺ release, which activates the ion and phospholipid channel, anoctamin-6. This highly Ca²⁺-dependent process causes swelling, ballooning, and phosphatidylserine expression, establishing a unique platelet population swinging between vital and necrotic (procoagulant 'zombie' platelets). Additionally, the high Ca²⁺ status of procoagulant platelets induces a set of additional events: (i) Ca²⁺ dependent cleavage of signaling proteins and receptors via calpain and ADAM isoforms; (ii) microvesiculation; (iii) enhanced coagulation factor binding; and (iv) fibrin-coat formation involving transglutaminases. Given the additive roles of GPCR and ILR in Ca²⁺ signal generation, high-throughput screening of biomolecules or small molecules based on Ca²⁺ flux measurements provides a promising way to find new inhibitors interfering with prolonged high Ca²⁺, phosphatidylserine expression, and hence platelet procoagulant activity.

Mild hyperlipidemia in mice aggravates platelet responsiveness in thrombus formation and exploration of platelet proteome and lipidome

Hyperlipidemia is a well-established risk factor for cardiovascular diseases. Millions of people worldwide display mildly elevated levels of plasma lipids and cholesterol linked to diet and life-style. While the prothrombotic risk of severe hyperlipidemia has been established, the effects of moderate hyperlipidemia are less clear. Here, we studied platelet activation and arterial thrombus formation in Apoe^-/- and Ldlr^-/- mice fed a normal chow diet, resulting in mildly increased plasma cholesterol. In blood from both knockout mice, collagen-dependent thrombus and fibrin formation under flow were enhanced. These effects did not increase in severe hyperlipidemic blood from aged mice and upon feeding a high-fat diet (Apoe^-/- mice). Bone marrow from wild-type or Ldlr^-/- mice was transplanted into irradiated Ldlr^-/- recipients. Markedly, thrombus formation was enhanced in blood from chimeric mice, suggesting that the hyperlipidemic environment altered the wild-type platelets, rather than the genetic modification. The platelet proteome revealed high similarity between the three genotypes, without clear indication for a common protein-based gain-of-function. The platelet lipidome revealed an altered lipid profile in mildly hyperlipidemic mice. In conclusion, in Apoe^-/- and Ldlr^-/- mice, modest elevation in plasma and platelet cholesterol increased platelet responsiveness in thrombus formation and ensuing fibrin formation, resulting in a prothrombotic phenotype.

Vascular protective effect of aspirin and rivaroxaban upon endothelial denudation of the mouse carotid artery

While in recent trials the dual pathway inhibition with aspirin plus rivaroxaban has shown to be efficacious in patients with atherosclerotic cardiovascular disease, little is known about the effects of this combination treatment on thrombus formation and vascular remodelling upon vascular damage. The aim of this study was to examine the effects of aspirin and/or rivaroxaban on injury-induced murine arterial thrombus formation in vivo and in vitro, vessel-wall remodelling, and platelet-leukocyte aggregates. Temporary ligation of the carotid artery of C57BL/6 mice, fed a western type diet, led to endothelial denudation and sub-occlusive thrombus formation. At the site of ligation, the vessel wall stiffened and the intima-media thickened. Aspirin treatment antagonized vascular stiffening and rivaroxaban treatment led to a positive trend towards reduced stiffening. Local intima-media thickening was antagonized by both aspirin or rivaroxaban treatment. Platelet-leukocyte aggregates and the number of platelets per leukocyte were reduced in aspirin and/or rivaroxaban treatment groups. Furthermore, rivaroxaban restricted thrombus growth and height in vitro. In sum, this study shows vascular protective effects of aspirin and rivaroxaban, upon vascular injury of the mouse artery.

Platelet-primed interactions of coagulation and anticoagulation pathways in flow-dependent thrombus formation

In haemostasis and thrombosis, platelet, coagulation and anticoagulation pathways act together to produce fibrin-containing thrombi. We developed a microspot-based technique, in which we assessed platelet adhesion, platelet activation, thrombus structure and fibrin clot formation in real time using flowing whole blood. Microspots were made from distinct platelet-adhesive surfaces in the absence or presence of tissue factor, thrombomodulin or activated protein C. Kinetics of platelet activation, thrombus structure and fibrin formation were assessed by fluorescence microscopy. This work revealed: (1) a priming role of platelet adhesion in thrombus contraction and subsequent fibrin formation; (2) a surface-independent role of tissue factor, independent of the shear rate; (3) a mechanism of tissue factor-enhanced activation of the intrinsic coagulation pathway; (4) a local, suppressive role of the anticoagulant thrombomodulin/protein C pathway under flow. Multiparameter analysis using blood samples from patients with (anti)coagulation disorders indicated characteristic defects in thrombus formation, in cases of factor V, XI or XII deficiency; and in contrast, thrombogenic effects in patients with factor V-Leiden. Taken together, this integrative phenotyping approach of platelet–fibrin thrombus formation has revealed interaction mechanisms of platelet-primed key haemostatic pathways with alterations in patients with (anti)coagulation defects. It can help as an important functional add-on whole-blood phenotyping.

Native, Intact Glucagon-Like Peptide 1 Is a Natural Suppressor of Thrombus Growth Under Physiological Flow Conditions

OBJECTIVE

In patients with diabetes mellitus, increased platelet reactivity predicts cardiac events. Limited evidence suggests that DPP-4 (dipeptidyl peptidase 4) influences platelets via GLP-1 (glucagon-like peptide 1)-dependent effects. Because DPP-4 inhibitors are frequently used in diabetes mellitus to improve the GLP-1-regulated glucose metabolism, we characterized the role of DPP-4 inhibition and of native intact versus DPP-4-cleaved GLP-1 on flow-dependent thrombus formation in mouse and human blood. Approach and Results: An ex vivo whole blood microfluidics model was applied to approach in vivo thrombosis and study collagen-dependent platelet adhesion, activation, and thrombus formation under shear-flow conditions by multiparameter analyses. In mice, in vivo inhibition or genetic deficiency of DPP-4 (Dpp4^-/-), but not of GLP-1-receptors (Glp1r^-/-), suppressed flow-dependent platelet aggregation. In human blood, GLP-1(7-36), but not DPP-4-cleaved GLP-1(9-36), reduced thrombus volume by 32% and impaired whole blood thrombus formation at both low/venous and high/arterial wall-shear rates. These effects were enforced upon ADP costimulation and occurred independently of plasma factors and leukocytes. Human platelets did not contain detectable levels of GLP-1-receptor transcripts. Also, GLP-1(7-36) did not inhibit collagen-induced aggregation under conditions of stirring or stasis of platelets, pointing to a marked flow-dependent role.

CONCLUSIONS

Native, intact GLP-1 is a natural suppressor of thrombus growth under physiological flow conditions, with DPP-4 inhibition and increased intact GLP-1 suppressing platelet aggregation under flow without a main relevance of GLP-1-receptor on platelets.

Comparative Analysis of Microfluidics Thrombus Formation in Multiple Genetically Modified Mice: Link to Thrombosis and Hemostasis

Genetically modified mice are indispensable for establishing the roles of platelets in arterial thrombosis and hemostasis. Microfluidics assays using anticoagulated whole blood are commonly used as integrative proxy tests for platelet function in mice. In the present study, we quantified the changes in collagen-dependent thrombus formation for 38 different strains of (genetically) modified mice, all measured with the same microfluidics chamber. The mice included were deficient in platelet receptors, protein kinases or phosphatases, small GTPases or other signaling or scaffold proteins. By standardized re-analysis of high-resolution microscopic images, detailed information was obtained on altered platelet adhesion, aggregation and/or activation. For a subset of 11 mouse strains, these platelet functions were further evaluated in rhodocytin- and laminin-dependent thrombus formation, thus allowing a comparison of glycoprotein VI (GPVI), C-type lectin-like receptor 2 (CLEC2) and integrin α₆β₁ pathways. High homogeneity was found between wild-type mice datasets concerning adhesion and aggregation parameters. Quantitative comparison for the 38 modified mouse strains resulted in a matrix visualizing the impact of the respective (genetic) deficiency on thrombus formation with detailed insight into the type and extent of altered thrombus signatures. Network analysis revealed strong clusters of genes involved in GPVI signaling and Ca²⁺ homeostasis. The majority of mice demonstrating an antithrombotic phenotype in vivo displayed with a larger or smaller reduction in multi-parameter analysis of collagen-dependent thrombus formation in vitro. Remarkably, in only approximately half of the mouse strains that displayed reduced arterial thrombosis in vivo, this was accompanied by impaired hemostasis. This was also reflected by comparing in vitro thrombus formation (by microfluidics) with alterations in in vivo bleeding time. In conclusion, the presently developed multi-parameter analysis of thrombus formation using microfluidics can be used to: (i) determine the severity of platelet abnormalities; (ii) distinguish between altered platelet adhesion, aggregation and activation; and (iii) elucidate both collagen and non-collagen dependent alterations of thrombus formation. This approach may thereby aid in the better understanding and better assessment of genetic variation that affect in vivo arterial thrombosis and hemostasis.

High-throughput elucidation of thrombus formation reveals sources of platelet function variability

In combination with microspotting, whole-blood microfluidics can provide high-throughput information on multiple platelet functions in thrombus formation. Based on assessment of the inter- and intra-subject variability in parameters of microspot-based thrombus formation, we aimed to determine the platelet factors contributing to this variation. Blood samples from 94 genotyped healthy subjects were analyzed for conventional platelet phenotyping: i.e. hematologic parameters, platelet glycoprotein (GP) expression levels and activation markers (24 parameters). Furthermore, platelets were activated by ADP, CRP-XL or TRAP. Parallel samples were investigated for whole-blood thrombus formation (6 microspots, providing 48 parameters of adhesion, aggregation and activation). Microspots triggered platelet activation through GP Ib-V-IX, GPVI, CLEC-2 and integrins. For most thrombus parameters, inter-subject variation was 2-4 times higher than the intra-subject variation. Principal component analyses indicated coherence between the majority of parameters for the GPVI-dependent microspots, partly linked to hematologic parameters, and glycoprotein expression levels. Prediction models identified parameters per microspot that were linked to variation in agonist-induced α_IIbβ₃ activation and secretion. Common sequence variation of GP6 and FCER1G, associated with GPVI-induced α_IIbβ₃ activation and secretion, affected parameters of GPVI-and CLEC-2-dependent thrombus formation. Subsequent analysis of blood samples from patients with Glanzmann thrombasthenia or storage pool disease revealed thrombus signatures of aggregation-dependent parameters that were subject-dependent, but not linked to GPVI activity. Taken together, this high-throughput elucidation of thrombus formation revealed patterns of inter-subject differences in platelet function, which were partly related to GPVI-induced activation and common genetic variance linked to GPVI, but also included a distinct platelet aggregation component.

Tyrosine Kinase Inhibitor Pazopanib Inhibits Platelet Procoagulant Activity in Renal Cell Carcinoma Patients

Pazopanib is an angiostatic tyrosine kinase inhibitor (TKI) presently used for cancer treatment, particularly in patients with renal cell carcinoma (RCC). This treatment can be accompanied by mild bleeding as an adverse effect. Given the role of protein tyrosine kinases in platelet activation processes, we investigated whether and how pazopanib can affect platelet functions in purified systems and during treatment of advanced RCC patients. In isolated platelets from healthy volunteers, pazopanib dose-dependently reduced collagen-induced integrin activation and secretion, as well as platelet aggregation. Pazopanib addition diminished glycoprotein (GP) VI-dependent tyrosine phosphorylation of multiple platelet proteins, including the tyrosine kinase Syk. Furthermore, pazopanib inhibited GPVI-induced Ca²⁺ elevation, resulting in reduced exposure of the procoagulant phospholipid phosphatidylserine (PS). Upon perfusion of control blood over a collagen surface, pazopanib inhibited thrombus size as well as PS exposure. Blood samples from 10 RCC patients were also analyzed before and after 14 days of pazopanib treatment as monotherapy. This treatment caused an overall lowering in platelet count, with 3 out of 10 patients experiencing mild bleeding. Platelets isolated from pazopanib-treated patients showed a significant lowering of PS exposure upon activation. In addition, platelet procoagulant activity was inhibited in thrombi formed under flow conditions. Control experiments indicated that higher pazopanib concentrations were required to inhibit GPVI-mediated PS exposure in the presence of plasma. Together, these results indicated that pazopanib suppresses GPVI-induced platelet activation responses in a way partly antagonized by the presence of plasma. In treated cancer patients, pazopanib effects were confined to a reduction in GPVI-dependent PS exposure. Together with the reduced platelet count, this may explain the mild bleeding tendency observed in pazopanib-treated patients.

Acquired platelet antagonism: off-target antiplatelet effects of malignancy treatment with tyrosine kinase inhibitors

Platelets can contribute to tumor progression and metastasis. Cancer patients are at increased risk of thrombosis, and advanced stages of cancer are associated with thrombocytosis or increased platelet reactivity. Tyrosine kinase inhibitors (TKIs) are widely used as a targeted strategy for cancer treatment, with the aim of prolonging progression-free survival of the patients. Because of their broad kinase target spectrum, most TKIs inevitably have off-target effects. Platelets rely on tyrosine kinase activity for their activation. Frequently observed side effects are lowering of platelet count and inhibition of platelet functions, whether or not accompanied by an increased bleeding risk. In this review, we aim to give insights into: (i) 38 TKIs that are currently used for the treatment of different types of cancer, either on the market or in clinical trials; (ii) how distinct TKIs can inhibit activation mechanisms in platelets; and (iii) the clinical consequences of the antiplatelet effects of TKI treatment. For several TKIs, the knowledge on affinity for their targets does not align with the published effects on platelets and reported bleeding events. This review should raise awareness of the potential antiplatelet effects of several TKIs, which will be enhanced in the presence of antithrombotic drugs.

Exploration of the platelet proteome in patients with early-stage cancer

Platelets play an important role in tumor growth and, at the same time, platelet characteristics are affected by cancer presence. Therefore, we investigated whether the platelet proteome harbors differentially expressed proteins associated with early-stage cancer. For this proof-of-concept study, patients with early-stage lung (n = 8) or head of pancreas cancer (n = 4) were included, as were healthy sex- and age-matched controls for both subgroups. Blood samples were collected from controls and from patients before surgery. Furthermore, from six of the patients, a second sample was collected two months after surgery. NanoLC-MS/MS-based proteomics of gel-fractionated platelet proteins was used for comparative spectral count analyses of patients to controls and before to after surgery samples. The total platelet proteome dataset included 4384 unique proteins of which 85 were significantly (criteria Fc > 1.5 and p < 0.05) changed in early-stage cancer compared to controls. In addition, the levels of 81 platelet proteins normalized after tumor resection. When filtering for the most discriminatory proteins, we identified seven promising platelet proteins associated with early-stage cancer. In conclusion, this pioneering study on the platelet proteome in cancer patients clearly identifies platelets as a new source of candidate protein biomarkers of early-stage cancer.

BIOLOGICAL SIGNIFICANCE

Currently, most blood-based diagnostics/biomarker research is performed in serum or plasma, while the content of blood cells is usually neglected. It is known that especially blood platelets, which are the main circulating pool of many bioactive proteins, such as growth factors, chemokines, and cytokines, are a potentially rich source of biomarkers. The current study is the first to measure the effect of early-stage cancer on the platelet proteome of patients. Our study demonstrates that the platelet proteome of patients with early-stage lung or head of pancreas cancer differs considerably compared to that of healthy individuals of matched sex and age. In addition, the platelet proteome of cancer patients normalized after surgical resection of the tumor. Exploiting platelet proteome differences linked to both tumor presence and disease status, we were able to demonstrate that the platelet proteome can be mined for potential biomarkers of cancer.

Sunitinib uptake inhibits platelet function in cancer patients

BACKGROUND

Sunitinib is an oral tyrosine kinase inhibitor used for cancer treatment. Patients treated with sunitinib are at higher bleeding risk. As tyrosine kinases are essential for platelet signalling, the effects of sunitinib on platelet function in vitro and in cancer patients on treatment were investigated.

PATIENTS AND METHODS

Blood samples were collected from eight healthy volunteers and eight patients diagnosed with metastatic renal cell cancer (RCC) before and 2 weeks on treatment with sunitinib. Platelets from 15 additional healthy individuals were preincubated with sunitinib or vehicle to perform in vitro experiments. Immunofluorescence imaging, western blotting, light transmission aggregometry, whole blood perfusion over collagen, flow cytometry and ELISA were performed.

RESULTS

Confocal microscopy indicated that platelets sequester sunitinib in vitro and in patients. In platelets from healthy controls, tyrosine phosphorylation was inhibited by sunitinib. Also, sunitinib dose dependently reduced collagen- and ADP-induced aggregation, collagen-dependent thrombus formation and collagen-induced secretion of platelet-derived growth factor and β-thromboglobulin. In blood from RCC patients before treatment, thrombus formation and procoagulant activity under flow were 47% and 80% higher than in healthy controls. After 14 d of sunitinib treatment, platelet count was moderately, but significantly decreased (from 243 to 144 × 10(9)/l). At the same time, collagen-induced platelet aggregation as well as thrombus formation and phosphatidylserine exposure under flow were significantly reduced (by 45%, 16% and 61%, respectively).

CONCLUSIONS

Sunitinib uptake by platelets inhibits collagen receptor-induced aggregation and thrombus formation via reduction of protein tyrosine phosphorylation and α-granule secretion. This dysfunction may contribute to the higher bleeding tendency observed in sunitinib-treated patients.

Survival protein anoctamin-6 controls multiple platelet responses including phospholipid scrambling, swelling, and protein cleavage

Scott syndrome is a rare bleeding disorder, characterized by altered Ca(2+)-dependent platelet signaling with defective phosphatidylserine (PS) exposure and microparticle formation, and is linked to mutations in the ANO6 gene, encoding anoctamin (Ano)6. We investigated how the complex platelet phenotype of this syndrome is linked to defective expression of Anos or other ion channels. Mice were generated with heterozygous of homozygous deficiency in Ano6, Ano1, or Ca(2+)-dependent KCa3.1 Gardos channel. Platelets from these mice were extensively analyzed on molecular functions and compared with platelets from a patient with Scott syndrome. Deficiency in Ano1 or Gardos channel did not reduce platelet responses compared with control mice (P > 0.1). In 2 mouse strains, deficiency in Ano6 resulted in reduced viability with increased bleeding time to 28.6 min (control 6.4 min, P < 0.05). Platelets from the surviving Ano6-deficient mice resembled platelets from patients with Scott syndrome in: 1) normal collagen-induced aggregate formation (P > 0.05) with reduced PS exposure (-65 to 90%); 2) lowered Ca(2+)-dependent swelling (-80%) and membrane blebbing (-90%); 3) reduced calpain-dependent protein cleavage (-60%); and 4) moderately affected apoptosis-dependent PS exposure. In conclusion, mouse deficiency of Ano6 but not of other channels affects viability and phenocopies the complex changes in platelets from hemostatically impaired patients with Scott syndrome.

Platelet CD40 Exacerbates Atherosclerosis by Transcellular Activation of Endothelial Cells and Leukocytes

OBJECTIVE

Beyond their eminent role in hemostasis and thrombosis, platelets are recognized as mediators of inflammation. Platelet cluster of differentiation 40 (CD40) ligand (CD40L and CD154) plays a key role in mediating platelet-induced inflammation in atherosclerosis. CD40, the receptor for CD40L, is present on platelets; however, the role of CD40 on this cell type is until now undefined.

APPROACH AND RESULTS

We found that in both mice and humans, platelet CD40 mediates the formation of platelet-leukocyte aggregates and the release of chemokine (C-X-C motif) ligand 4. Leukocytes were also less prone to adhere to CD40-deficient thrombi. However, platelet CD40 was not involved in platelet aggregation. Activated platelets isolated from Cd40(-/-)Apoe(-/-) mice adhered less to the endothelium upon injection into Apoe(-/-) mice when compared with CD40-sufficient platelets. Furthermore, lack of CD40 on injected platelets led to reduced leukocyte recruitment to the carotid artery as assayed by intravital microscopy. This was accompanied by a decrease in endothelial vascular cell adhesion molecule-1, platelet endothelial cell adhesion molecule, VE-cadherin, and P-selectin expression. To investigate the effect of platelet CD40 in atherosclerosis, Apoe(-/-) mice received thrombin-activated Apoe(-/-) or Cd40(-/-)Apoe(-/-) platelets every 5 days for 12 weeks, starting at the age of 17 weeks, when atherosclerotic plaques had already formed. When compared with mice that received Apoe(-/-) platelets, those receiving Cd40(-/-)Apoe(-/-) platelets exhibited a >2-fold reduction in atherosclerosis. Plaques of mice receiving CD40-deficient platelets were less advanced, contained less macrophages, neutrophils, and collagen, and displayed smaller lipid cores.

CONCLUSIONS

Platelet CD40 plays a crucial role in inflammation by stimulating leukocyte activation and recruitment and activation of endothelial cells, thereby promoting atherosclerosis.

Rate-limiting roles of the tenase complex of factors VIII and IX in platelet procoagulant activity and formation of platelet-fibrin thrombi under flow

The importance of factor Xa generation in thrombus formation has not been studied extensively so far. Here, we used mice deficient in either factor VIII or factor IX to determine the role of platelet-stimulated tenase activity in the formation of platelet-fibrin thrombi on collagen. With tissue factor present, deficiency in factor VIII or IX markedly suppressed thrombus growth, fibrin formation and platelet procoagulant activity (phosphatidylserine exposure). In either case, residual fibrin formation was eliminated in the absence of tissue factor. Effects of factor deficiencies were antagonized by supplementation of the missing coagulation factor. In wild-type thrombi generated under flow, phosphatidylserine-exposing platelets bound (activated) factor IX and factor X, whereas factor VIII preferentially co-localized at sites of von Willebrand factor binding. Furthermore, proteolytic activity of the generated activated factor X and thrombin was confined to the sites of phosphatidylserine exposure. With blood from a hemophilia A or B patient, the formation of platelet-fibrin thrombi was greatly delayed and reduced, even in the presence of high concentrations of tissue factor. A direct activated factor X inhibitor, rivaroxaban, added to human blood, suppressed both thrombin and fibrin formation. Together, these data point to a potent enforcement loop in thrombus formation due to factor X activation, subsequent thrombin and fibrin generation, causing activated factor X-mediated stimulation of platelet phosphatidylserine exposure. This implies that the factor VIII/factor IX-dependent stimulation of platelet procoagulant activity is a limiting factor for fibrin formation under flow conditions, even at high tissue factor concentrations.

Factor XI regulates pathological thrombus formation on acutely ruptured atherosclerotic plaques

OBJECTIVE

Coagulation factor XI is proposed as therapeutic target for anticoagulation. However, it is still unclear whether the antithrombotic properties of factor XI inhibitors influence atherosclerotic disease and atherothrombosis. Our aim is to investigate whether factor XI antisense oligonucleotides could prevent thrombus formation on acutely ruptured atherosclerotic plaques.

APPROACH AND RESULTS

Atherosclerotic plaques in the carotid arteries of Apoe(-/-) mice were acutely ruptured using ultrasound. The subsequent thrombus formation was visualized and quantified by intravital microscopy and immunohistochemistry. Mice were pretreated with either factor XI antisense or nonsense oligonucleotides (50 mg/kg) to lower factor XI plasma levels. A tail bleeding assay was used to determine the safety. On plaque rupture, initial platelet adhesion and platelet plug formation were not impaired in animals treated with factor XI antisense oligonucleotides. However, the ensuing thrombus formation and fibrin deposition were significantly lower after 5 to 10 minutes (P<0.05) in factor XI antisense oligonucleotide-treated animals without inducing a bleeding tendency. Furthermore, thrombi from antisense-treated animals were less stable than thrombi from placebo-treated animals. Moreover, macrophage infiltration and collagen deposition were lower in the carotid arteries of factor XI antisense-treated animals. No neutrophils were present.

CONCLUSIONS

Factor XI antisense oligonucleotides safely prevent thrombus formation on acutely ruptured atherosclerotic plaques in mice. Furthermore, perturbed carotid arteries from factor XI antisense-treated animals show a less severe inflammatory response.

Factor XII regulates the pathological process of thrombus formation on ruptured plaques

OBJECTIVE

Atherothrombosis is the main cause of myocardial infarction and ischemic stroke. Although the extrinsic (tissue factor-factor VIIa [FVIIa]) pathway is considered as a major trigger of coagulation in atherothrombosis, the role of the intrinsic coagulation pathway via coagulation FXII herein is unknown. Here, we studied the roles of the extrinsic and intrinsic coagulation pathways in thrombus formation on atherosclerotic plaques both in vivo and ex vivo.

APPROACH AND RESULTS

Plaque rupture after ultrasound treatment evoked immediate formation of subocclusive thrombi in the carotid arteries of Apoe(-/-) mice, which became unstable in the presence of structurally different FXIIa inhibitors. In contrast, inhibition of FVIIa reduced thrombus size at a more initial stage without affecting embolization. Genetic deficiency in FXII (human and mouse) or FXI (mouse) reduced ex vivo whole-blood thrombus and fibrin formation on immobilized plaque homogenates. Localization studies by confocal microscopy indicated that FXIIa bound to thrombi and fibrin particularly in luminal-exposed thrombus areas.

CONCLUSIONS

The FVIIa- and FXIIa-triggered coagulation pathways have distinct but complementary roles in atherothrombus formation. The tissue factor-FVIIa pathway contributes to initial thrombus buildup, whereas FXIIa bound to thrombi ensures thrombus stability.

Antithrombotic potential of blockers of store-operated calcium channels in platelets

OBJECTIVE

Platelet Orai1 channels mediate store-operated Ca(2+) entry (SOCE), which is required for procoagulant activity and arterial thrombus formation. Pharmacological blockage of these channels may provide a novel way of antithrombotic therapy. Therefore, the thromboprotective effect of SOCE blockers directed against platelet Orai1 is determined.

METHODS AND RESULTS

Candidate inhibitors were screened for their effects on SOCE in washed human platelets. Tested antagonists included the known compounds, SKF96365, 2-aminoethyl diphenylborate, and MRS1845 and the novel compounds, Synta66 and GSK-7975A. The potency of SOCE inhibition was in the order of Synta66>2-aminoethyl diphenylborate>GSK-7975A>SKF96365>MRS1845. The specificity of the first 3 compounds was verified with platelets from Orai1-deficient mice. Inhibitory activity on procoagulant activity and high-shear thrombus formation was assessed in plasma and whole blood. In the presence of plasma, all 3 compounds suppressed platelet responses and restrained thrombus formation under flow. Using a murine stroke model, arterial thrombus formation was provoked in vivo by transient middle cerebral artery occlusion. Postoperative administration of 2-aminoethyl diphenylborate markedly diminished brain infarct size.

CONCLUSIONS

Plasma-soluble SOCE blockers such as 2-aminoethyl diphenylborate suppress platelet-dependent coagulation and thrombus formation. The platelet Orai1 channel is a novel target for preventing thrombotic events causing brain infarction.

Intravital imaging of thrombus formation in small and large mouse arteries: experimentally induced vascular damage and plaque rupture in vivo

Intravital fluorescence microscopy is increasingly used to measure experimental arterial thrombosis in large and small arteries of mice in vivo. This chapter describes protocols for applying this technology to detect and measure thrombi formed by: (1) ultrasound-induced rupture of an atherosclerotic plaque in the carotid artery of adult Apoe (-/-) mice; (2) FeCl(3) or ligation in the carotid artery of nonatherosclerotic mice; and (3) FeCl(3) in the mesenteric venules and arterioles of young mice. In addition, we describe a protocol using two-photon laser scanning microscopy for intraluminal scanning of thrombi formed in the carotid artery. These approaches provide important information that cannot be obtained with ex vivo methods and thus are likely to lead to new insights into the complex process of thrombosis.

Role of newly formed platelets in thrombus formation in rat after clopidogrel treatment: comparison to the reversible binding P2Y₁₂ antagonist ticagrelor

Platelet P2Y₁₂ receptors play an important role in arterial thrombosis by stimulating thrombus growth. Both irreversibly (clopidogrel) and reversibly binding (ticagrelor, AZD6140) P2Y₁₂ antagonists are clinically used for restricted periods, but possible differences in platelet function recovery after drug cessation have not been investigated. We treated WKY rats with a single, high dose of 200 mg/kg clopidogrel or 40 mg/kg ticagrelor. Blood was collected at different time points after treatment. Flow cytometry confirmed full platelet protection against ADP-induced αIIbβ₃ activation shortly after clopidogrel or ticagrelor treatment. At later time points after clopidogrel treatment, a subpopulation of juvenile platelets appeared that was fully responsive to ADP. Addition of ticagrelor to clopidogrel-treated blood reduced αIIbβ₃ activation of the unprotected platelets. In contrast, at later time points after ticagrelor treatment, all platelets gradually lost their protection against ADP activation. Perfusion experiments showed abolishment of thrombus formation shortly after clopidogrel or ticagrelor treatment. Thrombus formation on collagen was determined under high shear flow conditions. At later time points, large thrombi formed in the clopidogrel but not in the ticagrelor group, and unprotected, juvenile platelets preferentially incorporated into the formed thrombi. However, platelets from both groups were still similarly reduced in assays of whole blood aggregation. Conclusively, recovery of rat platelet function after ticagrelor differs mechanistically from that after clopidogrel. This difference is masked by conventional platelet aggregation methods, but is revealed by thrombus formation measurement under flow. Juvenile platelets formed at later time points after clopidogrel treatment promoted thrombus formation.

Signaling role of CD36 in platelet activation and thrombus formation on immobilized thrombospondin or oxidized low-density lipoprotein

BACKGROUND AND OBJECTIVE

Platelets abundantly express glycoprotein CD36 with thrombospondin-1 (TSP1) and oxidized low-density lipoprotein (oxLDL) as proposed ligands. How these agents promote platelet activation is still poorly understood.

METHODS AND RESULTS

Both TSP1 and oxLDL caused limited activation of platelets in suspension. However, immobilized TSP1 and oxLDL, but not LDL, strongly supported platelet adhesion and spreading with a major role of CD36. Platelet spreading was accompanied by potent Ca(2+) rises, and resulted in exposure of P-selectin and integrin activation, all in a CD36-dependent manner with additional contributions of α(IIb) β(3) and ADP receptor stimulation. Signaling responses via CD36 involved activation of the protein tyrosine kinase Syk. In whole blood perfusion, co-coating of TSP1 or oxLDL with collagen enhanced thrombus formation at high-shear flow conditions, with increased expression on platelets of activated α(IIb) β(3), P-selectin and phosphatidylserine, again in a CD36-dependent way.

CONCLUSIONS

Immobilized TSP1 and oxLDL activate platelets partly via CD36 through a Syk kinase-dependent Ca(2+) signaling mechanism, which enhances collagen-dependent thrombus formation under flow. These findings provide novel insight into the role of CD36 in hemostasis.

Roles of platelet STIM1 and Orai1 in glycoprotein VI- and thrombin-dependent procoagulant activity and thrombus formation

In platelets, STIM1 has been recognized as the key regulatory protein in store-operated Ca(2+) entry (SOCE) with Orai1 as principal Ca(2+) entry channel. Both proteins contribute to collagen-dependent arterial thrombosis in mice in vivo. It is unclear whether STIM2 is involved. A key platelet response relying on Ca(2+) entry is the surface exposure of phosphatidylserine (PS), which accomplishes platelet procoagulant activity. We studied this response in mouse platelets deficient in STIM1, STIM2, or Orai1. Upon high shear flow of blood over collagen, Stim1(-/-) and Orai1(-/-) platelets had greatly impaired glycoprotein (GP) VI-dependent Ca(2+) signals, and they were deficient in PS exposure and thrombus formation. In contrast, Stim2(-/-) platelets reacted normally. Upon blood flow in the presence of thrombin generation and coagulation, Ca(2+) signals of Stim1(-/-) and Orai1(-/-) platelets were partly reduced, whereas the PS exposure and formation of fibrin-rich thrombi were normalized. Washed Stim1(-/-) and Orai1(-/-) platelets were deficient in GPVI-induced PS exposure and prothrombinase activity, but not when thrombin was present as co-agonist. Markedly, SKF96365, a blocker of (receptor-operated) Ca(2+) entry, inhibited Ca(2+) and procoagulant responses even in Stim1(-/-) and Orai1(-/-) platelets. These data show for the first time that: (i) STIM1 and Orai1 jointly contribute to GPVI-induced SOCE, procoagulant activity, and thrombus formation; (ii) a compensating Ca(2+) entry pathway is effective in the additional presence of thrombin; (iii) platelets contain two mechanisms of Ca(2+) entry and PS exposure, only one relying on STIM1-Orai1 interaction.

Platelet hyperreactivity and a prothrombotic phenotype in mice with a gain-of-function mutation in phospholipase Cgamma2

BACKGROUND

Agonist-induced platelet activation involves different signaling pathways leading to the activation of phospholipase C (PLC) beta or PLCgamma2. Activated PLC produces inositol 1,4,5-trisphosphate and diacylglycerol, which trigger Ca(2+) mobilization and the activation of protein kinase C, respectively. PLCbeta is activated downstream of Gq-coupled receptors for soluble agonists with only short interaction times in flowing blood. In contrast, PLCgamma2 becomes activated downstream of receptors that interact with immobilized ligands such as the collagen receptor glycoprotein (GP) VI or activated integrins.

OBJECTIVE AND METHODS

We speculated that PLCgamma2 activity might be optimized for sustained but submaximal signaling to control relatively slow platelet responses. To test this hypothesis, we analyzed platelets from mice heterozygous for a gain-of-function mutation in the Plcg2 gene (Plcg2(Ali5/+)).

RESULTS

Plcg2(Ali5/+) platelets showed enhanced Ca(2+) mobilization, integrin activation, granule secretion and phosphatidylserine exposure upon GPVI or C-type lectin-like receptor-2 stimulation. Furthermore, integrin alpha(IIb)beta(3) outside-in signaling was markedly enhanced in the mutant platelets, as shown by accelerated spreading on different matrices and faster clot retraction. These defects translated into virtually unlimited thrombus formation on collagen under flow in vitro and a prothrombotic phenotype in vivo.

CONCLUSIONS

These results demonstrate that the enzymatic activity of PLCgamma2 is tightly regulated to ensure efficient but limited platelet activation at sites of vascular injury.

Stabilizing role of platelet P2Y(12) receptors in shear-dependent thrombus formation on ruptured plaques

BACKGROUND

In most models of experimental thrombosis, healthy blood vessels are damaged. This results in the formation of a platelet thrombus that is stabilized by ADP signaling via P2Y(12) receptors. However, such models do not predict involvement of P2Y(12) in the clinically relevant situation of thrombosis upon rupture of atherosclerotic plaques. We investigated the role of P2Y(12) in thrombus formation on (collagen-containing) atherosclerotic plaques in vitro and in vivo, by using a novel mouse model of atherothrombosis.

METHODOLOGY

Plaques in the carotid arteries from Apoe(-/-) mice were acutely ruptured by ultrasound treatment, and the thrombotic process was monitored via intravital fluorescence microscopy. Thrombus formation in vitro was assessed in mouse and human blood perfused over collagen or plaque material under variable conditions of shear rate and coagulation. Effects of two reversible P2Y(12) blockers, ticagrelor (AZD6140) and cangrelor (AR-C69931MX), were investigated.

PRINCIPAL FINDINGS

Acute plaque rupture by ultrasound treatment provoked rapid formation of non-occlusive thrombi, which were smaller in size and unstable in the presence of P2Y(12) blockers. In vitro, when mouse or human blood was perfused over collagen or atherosclerotic plaque material, blockage or deficiency of P2Y(12) reduced the thrombi and increased embolization events. These P2Y(12) effects were present at shear rates >500 s(-1), and they persisted in the presence of coagulation. P2Y(12)-dependent thrombus stabilization was accompanied by increased fibrin(ogen) binding.

CONCLUSIONS/SIGNIFICANCE

Platelet P2Y(12) receptors play a crucial role in the stabilization of thrombi formed on atherosclerotic plaques. This P2Y(12) function is restricted to high shear flow conditions, and is preserved in the presence of coagulation.

Complementary roles of platelets and coagulation in thrombus formation on plaques acutely ruptured by targeted ultrasound treatment: a novel intravital model

BACKGROUND

Atherothrombosis is a major cause of cardiovascular events. However, animal models to study this process are scarce.

OBJECTIVES

We describe the first murine model of acute thrombus formation upon plaque rupture to study atherothrombosis by intravital fluorescence microscopy.

METHODS

Localized rupture of an atherosclerotic plaque in a carotid artery from Apoe(-/-) mice was induced in vivo using ultrasound. Rupture of the plaque and formation of localized thrombi were verified by two-photon laser scanning microscopy (TPLSM) in isolated arteries, and by immunohistochemistry. The thrombotic reaction was quantified by intravital fluorescence microscopy.

RESULTS

Inspection of the ultrasound-treated plaques by histochemistry and TPLSM demonstrated local damage, collagen exposure, luminal thrombus formation as well as intra-plaque intrusion of erythrocytes and fibrin. Ultrasound treatment of healthy carotid arteries resulted in endothelial damage and limited platelet adhesion. Real-time intravital fluorescence microscopy demonstrated rapid platelet deposition on plaques and formation of a single thrombus that remained subocclusive. The thrombotic process was antagonized by thrombin inhibition, or by blocking of collagen or adenosine diphosphate receptor pathways. Multiple thrombi were formed in 70% of mice lacking CD40L.

CONCLUSIONS

Targeted rupture of murine plaques results in collagen exposure and non-occlusive thrombus formation. The thrombotic process relies on platelet activation as well as on thrombin generation and coagulation, and is sensitive to established and novel antithrombotic medication. This model provides new possibilities to study atherothrombosis in vivo.

Role of murine integrin alpha2beta1 in thrombus stabilization and embolization: contribution of thromboxane A2

Platelets stably interact with collagen via glycoprotein (GP)VI and alpha2beta1integrin. With alpha2-null mice, we investigated the role of alpha2beta1 in thrombus formation and stability in vivo and in vitro. Using a FeCl(3)-induced thrombosis model, in arteries from alpha2-null mice smaller thrombi were formed with more embolization compared to vessels from wild-type mice. Aspirin treatment of wild-type mice causes similar effects, while the thromboxane A(2) analogue U46619 was borderline effective in suppressing the embolisation in alpha2-null mice. In vitro, perfusion of alpha2-null blood over collagen resulted in formation of thrombi that were smaller and looser in appearance, regardless of the presence or absence of coagulation. Aspirin treatment or blockage of thromboxane receptors provoked embolus formation in wildtype blood, while U46619 normalized thrombus formation in blood from alpha2-null mice. We conclude that integrin alpha2beta1 plays a role in stabilizing murine thrombi, likely by enhancing GPVI activation and thromboxane A(2) release. The increased embolization in alpha2-null mice may argue against the use of alpha2beta1 integrin inhibitors for antithrombotic therapy.

Segregation of platelet aggregatory and procoagulant microdomains in thrombus formation: regulation by transient integrin activation

OBJECTIVE

Platelets play a dual role in thrombosis by forming aggregates and stimulating coagulation. We investigated the commitment of platelets to these separate functions during collagen-induced thrombus formation in vitro and in vivo.

METHODS AND RESULTS

High-resolution 2-photon fluorescence microscopy revealed that in thrombus formation under flow, fibrin(ogen)-binding platelets assembled into separate aggregates, whereas distinct patches of nonaggregated platelets exposed phosphatidylserine. The latter platelet population had inactivated alphaIIb beta3 integrins and displayed increased binding of coagulation factors. Coated platelets, expressing serotonin binding sites, were not identified as a separate population. Thrombin generation and coagulation favored the transformation to phosphatidylserine-exposing platelets with inactivated integrins and reduced adhesion. Prolonged tyrosine phosphorylation in vitro resulted in secondary downregulation of active alphaIIb beta3.

CONCLUSIONS

These results lead to a new spatial model of thrombus formation, in which aggregated platelets ensure thrombus stability, whereas distinct patches of nonaggregated platelets effectuate procoagulant activity and generate thrombin and fibrin. Herein, the hemostatic activity of a developing thrombus is determined by the balance in formation of proaggregatory and procoagulant platelets. This balance is influenced by antiplatelet and anticoagulant medication.

Imaging collagen in intact viable healthy and atherosclerotic arteries using fluorescently labeled CNA35 and two-photon laser scanning microscopy

We evaluated CNA35 as a collagen marker in healthy and atherosclerotic arteries of mice after both ex vivo and in vivo administration and as a molecular imaging agent for the detection of atherosclerosis. CNA35 conjugated with fluorescent Oregon Green 488 (CNA35/OG488) was administered ex vivo to mounted viable muscular (uterine), elastic (carotid), and atherosclerotic (carotid) arteries and fresh arterial rings. Two-photon microscopy was used for imaging. CNA35/OG488 labeling in healthy elastic arteries was compared with collagen type I, III, and IV antibody labeling in histologic sections. For in vivo labeling experiments, CNA35/OG488 was injected intravenously in C57BL6/J and apolipoprotein E(-/-) mice. Ex vivo CNA35/OG488 strongly labeled collagen in the tunica adventitia, media, and intima of muscular arteries. In healthy elastic arteries, tunica adventitia was strongly labeled, but labeling in tunica media and intima was prevented by endothelium and elastic laminae. Histology confirmed the affinity of CNA35 for type I, III, and IV collagen in arteries. Strong CNA35/OG488 labeling was found in atherosclerotic plaques. In vivo applied CNA35/OG488 minimally labeled the tunica intima of healthy carotid arteries. Atherosclerotic plaques in apolipoprotein E(-/-) mice exhibited large uptake. CNA35/OG488 imaging in organs revealed endothelium as a limiting barrier for in vivo uptake. CNA35/OG488 is a good molecular imaging agent for atherosclerosis.

Dual Role of Platelet Protein Kinase C in Thrombus Formation

Protein kinase C (PKC) isoforms regulate many platelet responses in a still incompletely understood manner. Here we investigated the roles of PKC in the platelet reactions implicated in thrombus formation as follows: secretion aggregate formation and coagulation-stimulating activity, using inhibitors with proven activity in plasma. In human and mouse platelets, PKC regulated aggregation by mediating secretion and contributing to alphaIIbbeta3 activation. Strikingly, PKC suppressed Ca(2+) signal generation and Ca(2+)-dependent exposure of procoagulant phosphatidylserine. Furthermore, under coagulant conditions, PKC suppressed the thrombin-generating capacity of platelets. In flowing human and mouse blood, PKC contributed to platelet adhesion and controlled secretion-dependent thrombus formation, whereas it down-regulated Ca(2+) signaling and procoagulant activity. In murine platelets lacking G(q)alpha, where secretion reactions were reduced in comparison with wild type mice, PKC still positively regulated platelet aggregation and down-regulated procoagulant activity. We conclude that platelet PKC isoforms have a dual controlling role in thrombus formation as follows: (i) by mediating secretion and integrin activation required for platelet aggregation under flow, and (ii) by suppressing Ca(2+)-dependent phosphatidylserine exposure, and consequently thrombin generation and coagulation. This platelet signaling protein is the first one identified to balance the pro-aggregatory and procoagulant functions of thrombi.

The Glycoprotein VI-Phospholipase Cγ2 Signaling Pathway Controls Thrombus Formation Induced by Collagen and Tissue Factor In Vitro and In Vivo

Objective— Both collagen and tissue factor can be initiating factors in thrombus formation. We investigated the signaling pathway of collagen-induced platelet activation in interaction with tissue factor–triggered coagulation during the thrombus-forming process.

Methods and Results— In murine blood flowing over collagen, platelet exposure of phosphatidylserine and procoagulant activity, but not adhesion, completely relied on each of the following signaling modules: glycoprotein VI (GPVI), FcR γ-chain, Src kinases, adaptor protein LAT, and phospholipase Cγ2 (PLCγ2). On flow in the presence of tissue factor, these signaling components were essential for platelet aggregation and greatly enhanced fibrin clot formation. Collagen-stimulated thrombin generation relied on the presence and activity of GPVI, FcR γ-chain, Src kinase, LAT, and PLCγ2. The physiological importance of this GPVI pathway was shown in a FeCl 3 -induced in vivo murine thrombosis model. In both venules and arterioles, signaling through GPVI, FcR γ-chain, and Src kinases enhanced the formation of phosphatidylserine-exposing and fibrin-rich thrombi.

Conclusions— The GPVI-PLCγ2 activation pathway regulates collagen-dependent coagulation in venous and arterial thrombus formation.