Thrombin-stimulated Platelets Research Articles

Essential role of CD38 in platelet aggregation through the PKC-mediated internalization and activation.

CD38 is a multifunctional enzyme with a potent Ca2+ mobilizing effect, cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). Here, we aimed to demonstrate the role of CD38 in platelets via protein kinase C (PKC)-mediated internalization and activation. Mouse platelets were used in this study. Thrombin, an agonist of platelet function, provoked a prompt and long-lasting increase in intracellular Ca2+ concentration ([Ca2+]i), resulting from an interplay of multifold Ca2+ mobilizing messengers.The signaling pathway was delineated using different inhibitors and techniques such as platelet aggregation assay, intracellular calcium measurements, immunoprecipitation, immunoblotting, and flow cytometry. We observed a sequential formation of cADPR and NAADP through CD38 activation by PKC of non-muscle myosin heavy chain IIA (MHCIIA), resulting in phospholipase C (PLC) activation in the thrombin-stimulated platelets. These findings reveal that PKC is fundamental in activating CD38 and elicits a physiological response in the murine platelets. PKC is involved in many signaling pathways. Specifically, PKC is involved in the internalization of CD38 via MHCIIA in CD38+/+ wild-type (WT) and CD38-/- knockout mice (KO). CD38 generates calcium-mobilizing agents that act on specific receptors of the calcium stores. Calcium triggered platelet aggregation while serving as a secondary messenger.

Pictolysin-III, a Hemorrhagic Type-III Metalloproteinase Isolated from Bothrops pictus (Serpentes: Viperidae) Venom, Reduces Mitochondrial Respiration and Induces Cytokine Secretion in Epithelial and Stromal Cell Lines.

From the venom of the Bothrops pictus snake, an endemic species from Peru, we recently have described toxins that inhibited platelet aggregation and cancer cell migration. In this work, we characterize a novel P-III class snake venom metalloproteinase, called pictolysin-III (Pic-III). It is a 62 kDa proteinase that hydrolyzes dimethyl casein, azocasein, gelatin, fibrinogen, and fibrin. The cations Mg2+ and Ca2+ enhanced its enzymatic activity, whereas Zn2+ inhibited it. In addition, EDTA and marimastat were also effective inhibitors. The amino acid sequence deduced from cDNA shows a multidomain structure that includes a proprotein, metalloproteinase, disintegrin-like, and cysteine-rich domains. Additionally, Pic-III reduces the convulxin- and thrombin-stimulated platelet aggregation and in vivo, it has hemorrhagic activity (DHM = 0.3 µg). In epithelial cell lines (MDA-MB-231 and Caco-2) and RMF-621 fibroblast, it triggers morphological changes that are accompanied by a decrease in mitochondrial respiration, glycolysis, and ATP levels, and an increase in NAD(P)H, mitochondrial ROS, and cytokine secretion. Moreover, Pic-III sensitizes to the cytotoxic BH3 mimetic drug ABT-199 (Venetoclax) in MDA-MB-231 cells. To our knowledge, Pic-III is the first SVMP reported with action on mitochondrial bioenergetics and may offer novel opportunities for promising lead compounds that inhibit platelet aggregation or ECM-cancer-cell interactions.

Activated Platelets Mediate Monocyte Killing of Klebsiella pneumoniae.

Platelets are known for essential activities in hemostasis and for their important contribution to protection against infectious pathogens. Klebsiella pneumoniae is an opportunistic pathogen widely known to cause nosocomial infections. Recently, hypervirulent strains of K. pneumoniae have been emerging, which can cause severe infections in immunocompetent individuals. Combined with the increase in antibiotic resistance, it is important to understand how K. pneumoniae affects components of the immune system. We studied the interactions of human platelets with several K. pneumoniae strains (the wild type encapsulated strain, and a nonencapsulated mutant). Thrombin-stimulated whole human and mouse blood significantly inhibited bacterial growth compared to unstimulated whole blood. Furthermore, we investigated the effect of K. pneumoniae on platelet activation. Both strains induced significant increase in activation of both unstimulated and thrombin-stimulated human platelets. Additionally, only the nonencapsulated mutant increased aggregation of platelets in response to ADP. K. pneumoniae killing assays were then performed with washed platelets in the presence or absence of thrombin. Surprisingly, washed platelets failed to exhibit any effects on the growth of K. pneumoniae. We further explored the impact of platelets on monocyte-mediated killing of K. pneumoniae. Importantly, we found that activated platelets significantly enhanced monocyte-mediated killing of K. pneumoniae. This effect was likely due to the formation of platelet-monocyte aggregates in blood upon thrombin stimulation. Overall, this study highlights the role of platelets in mediating a protective response against K. pneumoniae and reinforces the importance of platelets in modulating leukocyte behavior.

Activation of Hippo Pathway Proteins in Thrombin-Stimulated Platelets and Regutates Platelets Activation in Vitro

To investigate the phosphorylation levels of Hippo pathway proteins in thrombin stimulated platelets and to explore its effects on platelet activation. The phosphorylation levels of Hippo pathway proteins - Mammalian STE20-like kinase 1/2 (MST1/2), Nuclear Dbf2 related kinase 1/2 (NDR1/2) and Mps one binder 1(MOB1) in human thrombin stimulated platelets was detected by Western blot. The effect of MST1/2 inhibitor XMU-MP-1 on platelet aggregation was detected by Platelet Aggregometer. The effect of XMU-MP-1 on platelet integrin αIIbβ3 activation and CD62p expression was detected by flow cytometry. The effect of XMU-MP-1 on the "outside-in" signal of platelet integrin was detected by blood clot retraction test. The effects of XMU-MP-1 on platelet Hippo pathway proteins and p38 phosphorylation levels was detected by Western blot. The phosphorylation levels of MST1/2, NDR1/2 and MOB1 were significantly increased in thrombin activated platelets; XMU-MP-1 inhibited thrombin-induced platelet aggregation and αIIbβ3 activation, but did not affect α-granules release and clot retraction. In addition, thrombin induced phosphorylation of the Hippo proteins were decreased in XMU-MP-1 treated platelets, while the phosphorylation of p38 was not affected. In thrombin stimulated platelets, Hippo pathway proteins were activated and contributed to platelets activation.

Inside the Mechanism of Action of Three Pyrazole Derivatives in Human Platelets and Endothelial Cells

In the effort to obtain multitarget compound interfering with inflammation, oxidative stress, and tumorigenesis, we synthesized a small library of pyrazole compounds, selecting 4a, 4f, and 4g as the most noteworthy being IC50 against platelet ROS production induced by thrombin of about 10 µM. The in vitro antioxidant potential of the three molecules was evaluated, and since they show a remarkable antioxidative activity, their effect on several parameter indicative of oxidative status and on the efficiency of the aerobic metabolism was tested. The three molecules strongly inhibit superoxide anion production, lipid peroxidation, NADPH oxidase activity and almost restore the oxidative phosphorylation efficiency in thrombin-stimulated platelet, demonstrating a protective effect against oxidative stress. This effect was confirmed in endothelial cell in which 4a, 4f, and 4g show an interesting inhibition activity on H2O2-stimulated EA.hy926 cells. At last, antiproliferative activity of 4a, 4f, and 4g was submitted to a large screening at the NCI. The molecules show interesting anticancer activity, among them the most remarkable is 4g able to strongly inhibit the proliferation of both solid tumor and leukemia cells lines. In conclusion, all the three newly synthetized pyrazoles show remarkable antioxidant and antiproliferative effect worthy of further study.

Fatty acid oxidation fuels agonist-induced platelet activation and thrombus formation: Targeting β-oxidation of fatty acids as an effective anti-platelet strategy.

Platelet mitochondria possess remarkable plasticity for oxidation of energy substrates, where metabolic dependency on glucose or fatty acids is higher than glutamine. Since platelets metabolize nearly the entire pool of glucose to lactate rather than fluxing through mitochondrial tricarboxylic acid cycle, we posit that majority of mitochondrial ATP, which is essential for platelet granule secretion and thrombus formation, is sourced from oxidation of fatty acids. We performed a comprehensive analysis of bioenergetics and function of stimulated platelets in the presence of etomoxir, trimetazidine and oxfenicine, three pharmacologically distinct inhibitors of β-oxidation. Each of them significantly impaired oxidative phosphorylation in unstimulated as well as thrombin-stimulated platelets leading to a small but consistent drop in ATP level in activated cells due to a lack of compensation from glycolytic ATP. Trimetazidine and oxfenicine attenuated platelet aggregation, P-selectin externalization and integrin αIIb β3 activation. Both etomoxir and trimetazidine impeded agonist-induced dense granule release and platelet thrombus formation on collagen under arterial shear. The effect of inhibitors on platelet aggregation and dense granule release was dose- and incubation time- dependent with significant inhibition at higher doses and prolonged incubation times. Neither of the inhibitors could protect mice from collagen-epinephrine-induced pulmonary embolism or prolong mouse tail bleeding times. However, mice pre-administered with etomoxir, trimetazidine and oxfenicine were protected from ferric chloride-induced mesenteric thrombosis. In conclusion, β-oxidation of fatty acids sustains ATP level in stimulated platelets and is therefore essential for energy-intensive agonist-induced platelet responses. Thus, fatty acid oxidation may constitute an attractive therapeutic target for novel antiplatelet agents.

Tumor-Educated Platelet Extracellular Vesicles: Proteomic Profiling and Crosstalk with Colorectal Cancer Cells.

Platelet-cancer cell interactions modulate tumor metastasis and thrombosis in cancer. Platelet-derived extracellular vesicles (EVs) can contribute to these outcomes. We characterized the medium-sized EVs (mEVs) released by thrombin-stimulated platelets of colorectal cancer (CRC) patients and healthy subjects (HS) on the capacity to induce epithelial-mesenchymal transition (EMT)-related genes and cyclooxygenase (COX)-2(PTGS2), and thromboxane (TX)B2 production in cocultures with four colorectal cancer cell lines. Platelet-derived mEVs were assessed for their size distribution and proteomics signature. The mEV population released from thrombin-activated platelets of CRC patients had a different size distribution vs. HS. Platelet-derived mEVs from CRC patients, but not from HS, upregulated EMT marker genes, such as TWIST1 and VIM, and downregulated CDH1. PTGS2 was also upregulated. In cocultures of platelet-derived mEVs with cancer cells, TXB2 generation was enhanced. The proteomics profile of mEVs released from activated platelets of CRC patients revealed that 119 proteins were downregulated and 89 upregulated vs. HS. We show that mEVs released from thrombin-activated platelets of CRC patients have distinct features (size distribution and proteomics cargo) vs. HS and promote prometastatic and prothrombotic phenotypes in cancer cells. The analysis of platelet-derived mEVs from CRC patients could provide valuable information for developing an appropriate treatment plan.

Loss of zinc transporters ZIP1 and ZIP3 augments platelet reactivity in response to thrombin and accelerates thrombus formation in vivo.

Zinc (Zn2+) is considered as important mediator of immune cell function, thrombosis and haemostasis. However, our understanding of the transport mechanisms that regulate Zn2+ homeostasis in platelets is limited. Zn2+ transporters, ZIPs and ZnTs, are widely expressed in eukaryotic cells. Using mice globally lacking ZIP1 and ZIP3 (ZIP1/3 DKO), our aim was to explore the potential role of these Zn2+ transporters in maintaining platelet Zn2+ homeostasis and in the regulation of platelet function. While ICP-MS measurements indicated unaltered overall Zn2+ concentrations in platelets of ZIP1/3 DKO mice, we observed a significantly increased content of FluoZin3-stainable free Zn2+, which, however, appears to be released less efficiently upon thrombin-stimulated platelet activation. On the functional level, ZIP1/3 DKO platelets exhibited a hyperactive response towards threshold concentrations of G protein-coupled receptor (GPCR) agonists, while immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor agonist signalling was unaffected. This resulted in enhanced platelet aggregation towards thrombin, bigger thrombus volume under flow ex vivo and faster in vivo thrombus formation in ZIP1/3 DKO mice. Molecularly, augmented GPCR responses were accompanied by enhanced Ca2+ and PKC, CamKII and ERK1/2 signalling. The current study thereby identifies ZIP1 and ZIP3 as important regulators for the maintenance of platelet Zn2+ homeostasis and function.

Role of Glycogen Mobilization and Mitochondrial Bioenergetics in Platelet Function, Hemostasis, and Thrombosis

Platelets are one of the most metabolically active cells in the bloodstream. They show metabolic flexibility, switching between glycolysis and oxidative phosphorylation (OxPhos) depending on oxygen tension and the availability of substrates. Platelets also have metabolizable glycogen granules that contribute to the energy pool upon activation. Our goal was to investigate platelet energy metabolism under normal physiological conditions to better understand the relative roles of the major ATP-generating processes (glycolysis, OxPhos, and glycogenolysis). To address the role of glycogen, we used 2 glycogen phosphorylase inhibitors and tested their effects on standard platelet functions e.g., secretion, aggregation, clot contraction, and thrombus formation under shear. We also measured the glycogen levels in resting and activated platelets and the presence of inhibitors. Consistent with previous studies, glycogen was utilized when platelets are activated with a ~25% decrease upon thrombin-mediated activation. Both inhibitors blocked the use of glycogen and led to a 3-fold increase in glycogen compared to controls. These data suggest the active turnover of glycogen in platelets. Our inhibitor studies indicated that glycogen is dispensable for low-energy processes like aggregation but contributed to secretion, clot contraction, and thrombus formation under shear. The inhibition was rescued by either increasing external glucose or bypassing glycolysis with added pyruvate. This study shows that the glycogen granules are metabolically active and contribute to the energy pool upon platelet activation; this pool is important for high-energy platelet functions like clot contraction. Deletion of platelet glucose transporters (GLUT1 and GLUT3) revealed an essential role of glucose metabolism in hemostasis in vivo. Glucose can power platelets via glycolysis alone or in combination with OxPhos (in the mitochondria). We sought to probe the relative importance of platelet mitochondrial bioenergetics in hemostasis and thrombosis. Previously used mitochondrial inhibitors (antimycin, oligomycin) are toxic and cannot be used for in vivo studies, thus we developed two novel mouse models with altered mitochondrial function using a platelet-specific deletion of TFAM and QPC. TFAM (Transcription Factor A Mitochondrial), is essential for the maintenance, transcription, and translation of mitochondrial DNA. Its deletion is expected to disrupt platelet mitochondrial DNA, which encodes 13 subunits of OxPhos. QPC is a subunit of ubiquinol-cytochrome c reductase complex III. Its deletion is expected to disrupt Complex III, which is part of the platelet mitochondrial respiratory chain. We confirmed TFAM deletion using Western blot analysis. Using the Seahorse Analyzer, we confirmed that deleting TFAM disrupted platelet mitochondrial bioenergetics in both resting and thrombin-stimulated platelets as seen as a ~40% decrease in Oxygen Consumption Rate (OCR). We also observed a significant decrease in ATP production indicating the contribution of OxPhos to ATP generation. Aggregation was not affected but in vitro clot contraction and thrombus formation under shear were defective (~58% increase in lag time, ~30% increase in AUC, and ~60% decrease in surface area coverage). Both the KO animals showed an increased tail-bleeding time, an increased occlusion time in the FeCl3 carotid injury model, and delayed hemostasis in a jugular puncture injury model with significantly higher rebleeding indicating that mitochondrial bioenergetics is important for clot stability. Using two novel mouse models with dysfunctional mitochondrial bioenergetics, we show that OxPhos is dispensable for low-energy demanding platelet functions such as aggregation but is important for secretion, clot contraction, hemostasis, and thrombosis (Table 1). Platelets show considerable plasticity in energy metabolism using both glycolysis and oxidative phosphorylation. Using mouse models and inhibitors, we show the relative importance of these energy-producing processes and the fuels and metabolic pathway choices made by platelets at different functional stages. Improving our understanding of platelet metabolism and the relative contributions of each pathway in the future could lead to a better understanding of how metabolic disorders can affect thrombotic risk in diabetic and obese patients. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Inhibition of Hypoxia-Inducible Factor Prolyl-Hydroxylase Modulates Platelet Function.

Hypoxia-inducible factor-1α (HIF-1α) involves in redox reactions. Considering the role of reactive oxygen species (ROS) in platelet function, whether it regulates platelet function remains unclear. Using an inhibitor of HIF prolyl-hydroxylase, IOX-2, we intend to investigate its effect on platelet function. Human platelets were treated with IOX-2 (0, 10, 25, and 50 μM) followed by analysis of platelet aggregation, granule secretion, receptor expression, platelet spreading, or clot retraction. Additionally, IOX-2 (10 mg/kg) was injected intraperitoneally into mice to measure tail bleeding time and arterial thrombosis. IOX-2 significantly inhibited collagen-related peptide (CRP; 0.25 μg/mL) or thrombin (0.03 U/mL)-induced platelet aggregation and ATP release dose dependently without affecting P-selectin expression and the surface levels of glycoprotein (GP)Ibα, GPVI, or αIIbβ3. In addition, IOX-2-treated platelets presented significantly decreased spreading on fibrinogen or collagen and clot retraction. Moreover, IOX-2 administration into mice significantly impaired the in vivo hemostatic function of platelets and arterial thrombus formation without affecting the number of circulating platelets and coagulation factors (FVIII and FIX). Further, IOX-2 significantly upregulated HIF-1α in platelets, decreased ROS generation, and downregulated NOX1 expression. Finally, IOX-2 increased the phosphorylation level of VASP (Ser157/239), and inhibited the phosphorylation of p38 (Thr180/Tyr182), ERK1/2 (Thr202/Tyr204), AKT (Thr308/Ser473), and PKCδ (Thr505) in CRP- or thrombin-stimulated platelets. In conclusion, inhibition of HIF prolyl-hydroxylase modulates platelet function and arterial thrombus formation, possibly through upregulation of HIF-1α expression and subsequent inhibition of ROS generation, indicating that HIF-1α might be a novel target for the treatment of thrombotic disorders.

Abstract 12496: Plasma Growth Factors and Hormones Drive Constitutive Translation of a Broad-Spectrum Proteome in Circulating Platelets via Stimulation of Translation Initiation Signal Pathways, Necessary for Rapid Hemostasis

Introduction: The ability of anucleate platelets to translate nascent protein from their inherited mRNAs was first demonstrated in 1967. Later studies supported these findings, but translation quickly ceased in washed ex vivo platelets, leading to a general presumption that platelet translation is not robust, and thus of limited biological significance. Agonist stimulation promotes platelet protein translation, and recent profiling studies confirmed >6000 inherited mRNAs in polysomes, associated with inducible translation of cognate protein products in thrombin-stimulated human platelets. However, relevance of post-activation translation - a slow, rate-limiting step in cell response - to rapid hemostasis is not clear. Platelets express the requisite components and show broad ribosome occupancy of mRNAs under resting conditions. Hypothesis: Plasma-borne growth factors/hormones (GFH) drive constitutive translation in circulating platelets to modulate reactivity. Methods: We employed metabolic labeling of newly synthesized proteins by bio-orthogonal non-canonical amino acid tagging in plasma-starved human platelets fed back with autologous plasma or isolated GFH (VEGF, IGF-1, insulin, EGF, HMGB1), coupled with translation initiation signaling and functional tests. Results: We found that platelets undergo constitutive translation of a broad-spectrum translatome dependent on plasma or GFH exposure. Platelets freshly isolated from platelet-rich plasma showed homeostatic activation of translation initiation signaling, evidenced by phosphorylation of p38/ERK upstream kinases, essential intermediate MNK1/2, and downstream effectors eIF4E/4E-BP1. Plasma starvation led to a steady loss of phosphorylation of this pathway, which was fully restored within 5 min stimulation by either plasma or most GFH. Inhibition of translation with puromycin or cycloheximide suppressed thrombin-induced integrin GPIIb/IIIa activation, P-selectin exposure, and platelet aggregation in vitro , and delayed platelet clot retraction. Transfusion of puromycin or MNK inhibitor eFT-508 resulted in delayed hemostasis in a tail bleed assay in mice. Conclusions: Constitutive plasma GFH-driven translation supports hemostatic platelet function.

Platelet-Mediated NET Release Amplifies Coagulopathy and Drives Lung Pathology During Severe Influenza Infection.

The influenza A virus (IAV) causes a respiratory tract infection with approximately 10% of the population infected by the virus each year. Severe IAV infection is characterized by excessive inflammation and tissue pathology in the lungs. Platelet and neutrophil recruitment to the lung are involved in the pathogenesis of IAV, but the specific mechanisms involved have not been clarified. Using confocal intravital microscopy in a mouse model of IAV infection, we observed profound neutrophil recruitment, platelet aggregation, neutrophil extracellular trap (NET) production and thrombin activation within the lung microvasculature in vivo. Importantly, deficiency or antagonism of the protease-activated receptor 4 (PAR4) reduced platelet aggregation, NET production, and neutrophil recruitment. Critically, inhibition of thrombin or PAR4 protected mice from virus-induced lung tissue damage and edema. Together, these data imply thrombin-stimulated platelets play a critical role in the activation/recruitment of neutrophils, NET release and directly contribute to IAV pathogenesis in the lung.

Acetyl-CoA carboxylase inhibition alters tubulin acetylation and aggregation in thrombin-stimulated platelets

Abstract Introduction Acetyl-CoA carboxylase (ACC), the first enzyme regulating lipid synthesis, promotes thrombus formation by increasing platelet phospholipid content. Inhibition of its activity decreases lipogenesis and increases the content in acetyl-CoA which can serve as a substrate for protein acetylation. This posttranslational modification plays a key role in the regulation of platelet aggregation, via tubulin acetylation. Purpose To demonstrate that ACC inhibition may affect platelet functions via an alteration of lipid content and/or tubulin acetylation. Methods Platelets were treated 2 hours with CP640.186, a pharmacological ACC inhibitor, prior to thrombin stimulation. Platelet functions were assessed by aggregometry and flow cytometry. Lipogenesis was measured via 14C-acetate incorporation into lipids. Lipidomics analysis was carried out on the commercial Lipidyzer platform. Protein phosphorylation and acetylation were evaluated by western blot. Results Treatment with CP640.186 drastically decreased platelet lipogenesis. However, the quantitative lipidomics analyses showed that preincubation with the compound did not affect global platelet lipid content. Interestingly, this short-term ACC inhibition was sufficient to increase tubulin acetylation level, at basal state and after thrombin stimulation. It was associated with an impaired platelet aggregation, in response to low thrombin concentration, while granules secretion was not affected. Mechanistically, we highlighted a decrease in Rac1 activity, associated with a reduced phosphorylation of its downstream effector PAK2. Surprisingly, actin cytoskeleton was not impacted but we evidenced a significant decrease in ROS production which could result from a decreased NOX2 activity. Conclusion Pharmacological ACC inhibition decreases platelet aggregation upon thrombin stimulation. The mechanism depends on increased tubulin acetylation, with subsequent alteration of the Rac1/PAK2/NOX2 signaling pathway Funding Acknowledgement Type of funding sources: Other. Main funding source(s): Fonds pour la formation à la Recherche dans l'Industrie et l'Agriculture (FRIA)

The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

AbstractTyrosine phosphorylation of extracellular proteins is observed in cell cultures and in vivo, but little is known about the functional roles of tyrosine phosphorylation of extracellular proteins. Vertebrate lonesome kinase (VLK) is a broadly expressed secretory pathway tyrosine kinase present in platelet α-granules. It is released from platelets upon activation and phosphorylates substrates extracellularly. Its role in platelet function, however, has not been previously studied. In human platelets, we identified phosphorylated tyrosines mapped to luminal or extracellular domains of transmembrane and secreted proteins implicated in the regulation of platelet activation. To determine the role of VLK in extracellular tyrosine phosphorylation and platelet function, we generated mice with a megakaryocyte/platelet-specific deficiency of VLK. Platelets from these mice are normal in abundance and morphology but have significant changes in function both in vitro and in vivo. Resting and thrombin-stimulated VLK-deficient platelets exhibit a significant decrease in several tyrosine phosphobands. Results of functional testing of VLK-deficient platelets show decreased protease-activated receptor 4–mediated and collagen-mediated platelet aggregation but normal responses to adenosine 5′-diphosphate. Dense granule and α-granule release are reduced in these platelets. Furthermore, VLK-deficient platelets exhibit decreased protease-activated receptor 4–mediated Akt (S473) and Erk1/2 (T202/Y204) phosphorylation, indicating altered proximal signaling. In vivo, mice lacking VLK in megakaryocytes/platelets display strongly reduced platelet accumulation and fibrin formation after laser-induced injury of cremaster arterioles compared with control mice but with normal bleeding times. These studies show that the secretory pathway tyrosine kinase VLK is critical for stimulus-dependent platelet activation and thrombus formation, providing the first evidence that a secreted protein kinase is required for normal platelet function.

A Functional Interplay Between Pyk2 and Src Family Kinases Links Ca2+ Signaling To Protein Tyrosine Phosphorylation In Thrombin-Stimulated Platelets

IntroductionThrombin and other soluble agonists promote platelet activation by binding to G-protein-coupled receptors, which typically recruit phospholipase C and trigger cytosolic Ca2+ increase and protein kinase C stimulation. Platelets also express several tyrosine kinases, including different members of the Src family (mainly Src and Lyn) and of the focal adhesion family (such as FAK and Pyk2), which are known to be activated upon stimulation with thrombin. Both Src family kinases (SFKs) and Pyk2 are required for thrombin-induced aggregation and thromboxane A2 production, and both of them have been shown to be regulated by intracellular Ca2+ (Xiang et al, J Biol Chem 2012; Canobbio et al, Blood 2013). However, the hierarchy of protein tyrosine kinases activation, as well as the molecular mechanism linking agonist-induced increase of cytosolic Ca2+ to protein tyrosine phosphorylation is poorly understood. In this study, we investigated the interplay between intracellular Ca2+, Pyk2 and SFKs in the regulation of protein tyrosine phosphorylation in thrombin-stimulated platelets. ResultsIn the presence of the ADP scavenger apyrase, thrombin caused the time-dependent tyrosine phosphorylation of several proteins in aspirin-treated human, as well as mouse platelets. This effect was severely reduced by the intracellular Ca2+ chelator BAPTA-AM and by the SFKs inhibitor PP2, and was totally suppressed only when BAPTA-AM and PP2 were used in combination, indicating that cytosolic Ca2+ and SFKs activity cooperate in the regulation of the whole process of protein tyrosine phosphorylation elicited by thrombin. Analysis of SFKs activity in human and mouse platelets with an anti-pY416Src antibody, that detects the autophosphorylation site in the catalytic domain, revealed that cytosolic Ca2+ is upstream SFKs activation upon thrombin stimulation. Accordingly, SFKs phosphorylation in the catalytic domain was also directly promoted by the Ca2+ ionophore A23187. Immunoblotting analysis on high resolution gels allowed to distinguish between Src and Lyn, and revealed that PP2 prevented phosphorylation of both kinases, but BAPTA-AM only inhibited Src , but not Lyn phosphorylation caused by thrombin. The Ca2+ ionophore A23187 also triggered the phosphorylation and activation of the Ca2+-dependent focal adhesion kinase Pyk2, and stimulation with thrombin promoted Pyk2 activation by a Ca2+-dependent mechanism. However, thrombin- or A23187-induced activation of Pyk2 was also reduced by the SFKs inhibitor PP2, and both PP2 and BAPTA-AM were required to completely suppress Pyk2 activation. In platelets from Pyk2 knockout mice, stimulation of SFKs by thrombin was strongly reduced, as it was the overall pattern of protein tyrosine phosphorylation. Importantly, also SFKs activation caused by Ca2+ionophore was significantly impaired in Pyk2-deficient platelets. Finally, in the absence of Pyk2, thrombin-induced aggregation was reduced to a level comparable to that of wild type platelets treated with PP2, although inhibition of SFKs by PP2 further reduced aggregation of Pyk2-deficient platelets. ConclusionsThese results indicate that in thrombin-stimulated platelets intracellular Ca2+ increase promotes Pyk2 stimulation, which initiates SFKs activation and establishes a positive loop which reinforces the Pyk2/SFKs axis to allow the subsequent massive tyrosine phosphorylation of multiple platelet substrates. Disclosures:No relevant conflicts of interest to declare.

Factor VIII Binds to an Acidic Fibrinogen Variant, a Critical Component of the High-Affinity Fibrin Binding Site, Resulting in Increased Cofactor Activity

Thrombin-stimulated platelets express binding sites for factor VIII (fVIII) despite exposing insufficient phosphatidylserine (PS) to constitute these binding sites. Our previous work implicated soluble fibrin bound to the αIIbβ3 integrin as an important component of these binding sites. Further, we demonstrated that fVIII has direct, high affinity binding to fibrin, with an associated increase in the activity of the purified Xase complex (Gilbert et al, Blood 2015). However, the stoichiometry between fVIII binding sites and fibrin monomers was at least 30:1, suggesting that fVIII binds a minor fibrin(ogen) variant. We undertook experiments to determine the nature of the fVIII-binding fibrin(ogen).fVIII binding to fibrin(ogen) was evaluated with three complementary assays: i) direct binding of fVIII-fluorescein (fVIII-fluor) to soluble fibrin linked to Superose beads, ii) a competition assay in which binding of fVIII-fluor to fibrinogen competed for binding to Superose-linked von Willebrand factor (VWF) and iii) a factor Xase assay in which fibrin(ogen) enhanced activity. Results were benchmarked against fVIII-fluor binding to thrombin-stimulated platelets.When fibrinogen (depleted of plasminogen, fibronectin, and VWF) was fractionated by anion exchange chromatography, the fibrinogen γA fraction did not bind fVIII. Rather, the fVIII-binding fractions (constituting 10-15% of total fibrinogen) eluted in association with fibrinogen γ'. However, commercially available γ' fibrinogen does not bind fVIII, suggesting that additional modification is necessary or that fVIII-binding fibrinogen (fbgn*) elutes near fibrinogen γ' on the basis of acidity rather than shared structure. Mass spectroscopy analysis indicated that only traces of VWF remained in the active fractions. Addition of mAb 418, which blocks the fVIII binding site on VWF, had no significant effect on binding of fVIII-fluor to fibrin-Superose showing that residual VWF is not the source of high affinity fVIII binding. Two rabbits inoculated with fbgn*-enriched fractions developed anti-sera that block binding of fVIII to fibrin. In contrast, 5 commercially available polyclonal antibodies against fibrin(ogen) had no effect on fVIII binding, verifying that fbgn* contains a distinct epitope relevant to fVIII binding.The fbgn*-enriched fraction competed directly with VWF for fVIII binding with at least a 3-fold increase in fVIII binding sites compared to total fibrinogen. Following treatment with thrombin, the fbgn*-enriched fraction had twice as many binding sites for fVIII compared to fibrin from the starting material and a similar affinity in the direct binding assay. FVIII-fluor bound immobilized fibrin with a KD of 2-4 nM while the competition binding experiment yielded an implied KD to fibrinogen of approx. 100 nM. This suggests that thrombin cleavage of fbgn* or assembly into fibrin protofibrils or fibrils increases fVIII affinity approx. 50-fold.Fbgn* increased activity of the factor Xase complex 2-3 fold. The enhancement was maintained when fbgn* was not exposed to thrombin, indicating that the enhancement was not restricted to the higher affinity binding sites on fibrin.Plasmin degradation of fibrinogen produces five major fragments that can be separated by ion-exchange chromatography. Enhanced Xase activity and binding of digested fbgn* to fVIII in the VWF competition assay were both similar to intact fbgn*. In order to identify the active fragment, plasmin-digested fbgn* was separated using anion exchange chromatography and tested for factor VIII binding and Xase activity. Only the latest eluting peak bound to fVIII-fluor. SDS-PAGE shows that this peak consists primarily of two bands. IEF gel analysis indicates that the isoelectric points of these two bands differ compared to bands from commercially available γ' digested in the same manner. We are currently determining the identities of the unique bands.These results show that fVIII binds to a minor, acidic fibrinogen variant (fbgn*) in a manner that competes with VWF binding. Binding to fbgn* enables fVIII binding to thrombin-stimulated platelets and enhances fVIII activity in the Xase complex. Ongoing experiments are expected to identify the molecular features that distinguish fbgn* from fibrinogens γA and γ' and produce selective antibodies that will enable testing of the physiologic contribution of the fVIII-fbgn* interaction to hemostasis. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Acetyl-CoA carboxylase inhibition alters tubulin acetylation and aggregation in thrombin-stimulated platelets

Acetyl-CoA carboxylase (ACC), the first enzyme regulating lipid synthesis, promotes thrombus formation by increasing platelet phospholipid content. Inhibition of its activity decreases lipogenesis and increases the content in acetyl-CoA which can serve as a substrate for protein acetylation. This posttranslational modification plays a key role in the regulation of platelet aggregation, via tubulin acetylation. To demonstrate that ACC inhibition may affect platelet functions via an alteration of lipid content and/or tubulin acetylation. Platelets were treated 2 hours with CP640.186, a pharmacological ACC inhibitor, prior to thrombin stimulation. Platelet functions were assessed by aggregometry and flow cytometry. Lipogenesis was measured via 14 C-acetate incorporation into lipids. Lipidomics analysis was carried out on the commercial Lipidyzer platform. Protein phosphorylation and acetylation were evaluated by western blot. Treatment with CP640.186 drastically decreased platelet lipogenesis. However, the quantitative lipidomics analyses showed that preincubation with the compound did not affect global platelet lipid content. Interestingly, this short-term ACC inhibition was sufficient to increase tubulin acetylation level, at basal state and after thrombin stimulation. It was associated with an impaired platelet aggregation, in response to low thrombin concentration, while granules secretion was not affected. Mechanistically, we highlighted a decrease in Rac1 activity, associated with a reduced phosphorylation of its downstream effector PAK2. Surprisingly, actin cytoskeleton was not impacted but we evidenced a significant decrease in ROS production, which could result from a decreased NOX2 activity. Pharmacological ACC inhibition decreases platelet aggregation upon thrombin stimulation. The mechanism depends on increased tubulin acetylation, with subsequent alteration of the Rac1/PAK2/NOX2 signaling pathway.

The Secreted Tyrosine Kinase Vlk Is Essential for Normal Platelet Activation and Thrombus Formation

Tyrosine phosphorylation of proteins secreted into the extracellular space has been observed in cell cultures and in vivo, yet little is known about the role that phosphorylation of extracellular proteins serves in modulating cell function. An important reason for the gap in our knowledge of the functional significance of extracellular protein phosphorylation has been the delay in identifying extracellular kinases. Within the last decade, however, bioinformatic strategies to identify kinases with signal peptides, coupled with biochemical approaches to characterize kinases in the secretory pathway, have described several kinases that phosphorylate secretory pathway and extracellular substrates. Of the known kinases containing signal sequences, Fam20B and VLK have been identified in platelets. VLK has been identified as a broadly expressed secretory pathway tyrosine kinase secreted from platelets in an activation dependent manner. Its role in platelet function, however, has not been previously studied. To understand the contribution of tyrosine phosphorylation of secreted factors and extracellular domains of transmembrane proteins in platelet function and thrombus formation, we generated mice whose platelets lacked VLK. Mice with megakaryocyte/platelet-specific VLK deficiency (Vlk-cKO) exhibited normal platelet abundance, volume and morphology, and tail clip bleeding times, but showed dramatic changes in platelet function in vitro and in vivo. In vivo, platelet accumulation was reduced by 90% in Vlk-cKO mice compared to control (Vlkf/f) littermates (P = 0.02) following laser-induced injury of cremaster arterioles (Figure). Likewise, fibrin generation was reduced in mice lacking platelet VLK by 62% (P = 0.009). In vitro, evaluation of resting and thrombin-stimulated VLK-deficient platelets demonstrated a significant decrease of several tyrosine phosphobands compared to control. Platelet function testing of VLK-deficient platelets (Figure) showed decreased platelet aggregation in response to stimulation with 100 µM AYPGKF, a PAR4 agonist, (Vlkf/f: 70+5.1%; Vlk-cKO: 23+8.0%) or 4 µg/mL collagen (Vlkf/f: 53+2.5%; Vlk-cKO: 27.5+2.9%). Dense and α-granule release in response to AYPGKF were also significantly decreased in platelets in which VLK had been silenced. In contrast, Vlk-cKO platelets aggregated normally in response to either 10 µM, 40 µM, or 100 µM ADP, and the aggregation defect in response to low doses of AYPGKF was reversed by subthreshold concentrations (2.5 µM) of ADP. Furthermore, stimulation with high-dose 150 µM AYPGFK or 5 U/ml thrombin resulted in comparable platelet function and ATP secretion in control and Vlk-cKO platelets respectively, ruling out a storage pool defect. Taken together, these results suggest that a dense granule secretion defect contributes to the decrease in platelet aggregation observed in platelets in which VLK is absent. In human platelets, tyrosines phosphorylated in secreted and extracellular domains of transmembrane proteins implicated in the regulation of platelet function were identified by mass spectroscopy analysis. Extracellular proteins or proteins with phosphosites that mapped to extracellular domains included ectonucleoside triphosphate diphosphodydrolase 6 [ENTPD6], platelet basic protein, integrin αIIß, and multimerin-1. These studies demonstrate that the secretory pathway tyrosine kinase VLK is critical for stimulus dependent platelet aggregation and thrombus formation, and provide the first evidence that secreted kinases contribute to platelet function. Disclosures De Ceunynck: Sanofi: Current Employment. Dilks:PlateletBiogenesis: Current Employment. Peters:PlateletBiogenesis: Current Employment. Noetzli:Anylam: Current Employment. Rosen:Keros Therapeutics: Membership on an entity's Board of Directors or advisory committees. Italiano:PlateletBioGenesis: Consultancy, Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding. Flaumenhaft:STRM.Bio: Membership on an entity's Board of Directors or advisory committees; PlateletDiagnostics: Consultancy, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties, Research Funding; QuercisPharma: Membership on an entity's Board of Directors or advisory committees.

Cryopreserved Platelets Retain Agonist Responsiveness and Support for Factor VIII Function

Platelet activation supports procoagulant activity through phosphatidylserine exposure, secretion of procoagulant factors, and receptor conformational change. For example, thrombin-stimulated platelets bind factor VIII (fVIII) via a macromolecular complex including oligomeric fibrin and the active αIIbβ3 receptor (Phillips et al, JTH 2004; Gilbert et al, Blood 2015). Thus, coagulation assays in which phospholipid vesicles are substituted for platelets do not fully emulate modulators of fVIII activity. Indeed, inhibition of platelet-supported fVIII activity by a panel of mAbs against the C2 domain was not correlated to inhibition of vesicle-dependent activity (Chatterjee et al, JTH 2020). An obstacle to adoption of a platelet-based assay for fVIII is the need for fresh platelets. Therefore we asked whether cryopreserved platelets might support fVIII activity similarly to fresh platelets. Apheresis platelets were mixed with cryopreservatives with or without calcium chelators, in various aliquot sizes, and frozen on various cooling media. Cryopreserved platelets were compared to non-preserved apheresis platelets with regard to agonist response and support of procoagulant activity. Cryopreservation resulted in an increase in subcellular debris and an unresponsive fraction of platelets with decreased forward scatter judged by flow cytometry. Optimized results were obtained when platelet rich plasma with 5% DMSO, in 1 mL aliquots was frozen on powdered dry ice, and stored at -150C. Purification of thawed platelets using a density gradient removed debris and decreased unresponsive platelets resulting in a forward and side scatter profile comparable to fresh platelets. We refer to these as cryopreserved platelets (CryoPlts). CryoPlts were compared to control and outdated apheresis platelets. As with fresh platelets, procoagulant activity of CryoPlts increased with thrombin receptor agonist peptides (TRAP) 1 & 4 and supported a log-linear relationship between time to initial fibrin strand formation and fVIII activity over a range of 0.0001 - 1 u/mL (Fig 1). Further, the degree of inhibition of fVIII activity by mAbs ESH4 and G99 against the fVIII C2 domain, was the same on control and CryoPlts, but markedly different from inhibition in an aPTT-based inhibitor assay. In contrast, outdated apheresis platelets had increased procoagulant activity, minimal agonist response and a shallow curve with varying fVIII concentration. Flow cytometry studies with lactadherin-FITC indicated that 33 ± 14% of CryoPlts had high PS exposure, and the size of this population was minimally affected by TRAP 1+4. In contrast, the main platelet population had a small, uniform, increment in PS exposure comparable to control platelets. Surprisingly, the PS-rich platelets did not significantly affect the time to fibrin formation, confirming that the viable platelets, with limited PS exposure, provide much of the support for fVIII-related procoagulant activity. Flow cytometry indicated αIIbβ3 activation (PAC1-FITC) and α-granule release (anti-P-selectin-PE) were qualitatively intact on CryoPlts, although staining was decreased 70% for PAC1 and 57% for Psel. We also tested whether CryoPlts may be utilized for evaluating response to anti-PF4-heparin antibodies, relevant to heparin-induced thrombocytopenia (HIT). We evaluated platelet response to platelet factor 4 (PF4) and a platelet-activating anti-PF4 antibody (KKO), a combination that induces activation similar to authentic autoimmune antibodies for HIT. The non-activating PF4 antibody RTO served as a negative control. Geometric mean response, corrected for background, was normalized to response to thrombin activation. Both fresh and CryoPlts responded with increases in PAC1 (Fig 2A) and anti-Psel (Fig 2B) binding in response to KKO/PF4 compared to RTO/PF4 . This data demonstrates that the qualitative αIIbβ3 and P-selectin response to HIT-like antibodies is intact. Our results demonstrate a refined cryopreservation protocol of apheresis platelets. These platelets maintain qualitative agonist responsiveness with near-normal support for factor VIII activity, suggesting that they could be used for other platelet-based laboratory or diagnostic assays. Further, our results suggest that major procoagulant activity is provided by platelets with very limited PS exposure, an area for further investigation. Disclosures No relevant conflicts of interest to declare.

AMPK inhibition protects against arterial thrombosis while sparing hemostasis through differential modulation of platelet responses

AMP-activated protein kinase (AMPK) is a metabolic master switch that has critical role in wide range of pathologies including cardiovascular disorders. As AMPK-α2 knockout mice exhibit impaired thrombus stability, we asked whether pharmacological inhibition of AMPK with a specific small-molecule inhibitor, compound C, could protect against arterial thrombosis without affecting hemostasis. Mice pre-administered with compound C exhibited decreased mesenteric arteriolar thrombosis but normal tail bleeding time compared to vehicle-treated animals. Compound C potently restricted platelet aggregation, clot retraction and integrin activation induced by thrombin and collagen. It impaired platelet spreading on both immobilized fibrinogen and collagen matrices; it, however, had no significant effect on thrombin-induced phosphatidylserine exposure that is characteristic of procoagulant platelets. In parallel, compound C brought about significant drop in thrombin-induced phosphorylation of myosin light chain (MLC) and MLC phosphatase (MYPT1) as well as abrogated rise in level of RhoA-GTP in thrombin-stimulated platelets. Thus, effects of compound C on agonist-induced platelet responses could be at least in part attributed to modulation of cytoskeletal changes mediated by RhoA-MYPT1-MLC signaling. An ideal antithrombotic drug would spare hemostatic responses that maintain vascular integrity while preferentially protecting against thrombosis. The present study suggests that AMPK could be one such potential therapeutic target.