αIIbβ3 Activation Research Articles

Non-activating ITGB3 mutation in β3 cytoplasmic region causes macrothrombocytopenia with impaired αIIbβ3/RhoA pathway

Almost all mutations of ITGA2B or ITGB3 identified in congenital macrothrombocytopenia induce constitutive activation of αIIbβ3. However, whether concomitant αIIbβ3 activation is essential for macrothrombocytopenia development remains unknown. Recently, we identified the β3(R760C) mutation that does not induce αIIbβ3 activation in a subject with macrothrombocytopenia. The family study showed that macrothrombocytopenia with reduced expression of αIIbβ3 and GPVI appeared to be associated with subjects heterozygous for β3(R760C). We generated β3(R760C) knock-in (KI) mice and investigated the effects of the mutation on platelet/Mgk biology. Macrothrombocytopenia was decreased to 76% and 40% of platelet counts in heterozygous (Hetero) and homozygous (Homo) KI mice, respectively, compared to the wild-type mice. Platelet αIIbβ3 and GPVI expression were decreased in KI mice, and αIIbβ3 activation was not detected on non-stimulated KI platelets. Thus, the Hetero KI mice reproduced the phenotype of the human subject, indicating that the β3(R760C) mutation is responsible for the macrothrombocytopenia. Platelet aggregation, agonist-induced JON/A binding and P-selectin expression were impaired in KI mice. Platelet spreading on fibrinogen was also impaired in Homo mice with ADP or thrombin stimulation. Filopodia and lamellipodia formation was impaired in fibrinogen-adhered megakaryocytes of Homo mice with significantly impaired RhoA activation. Proplatelet formation in Homo mice was impaired with abnormal morphology. In addition, platelet lifespan was shortened in Homo mice. These data indicate that β3(R760C) mutation impairs the inside-out and outside-in signaling of αIIbβ3, and abnormal actin rearrangement and impaired RhoA activation may play major roles in leading to macrothrombocytopenia.

Loss of endogenous Nox2-NADPH oxidase does not prevent age-induced platelet activation and arterial thrombosis in mice

BackgroundReactive oxygen species are known to contribute to platelet hyperactivation and thrombosis during aging; however, the mechanistic contribution of the specific oxidative pathway remains elusive. ObjectivesWe hypothesized that during aging, endogenous Nox2-NADPH oxidase contributes to platelet reactive oxygen species accumulation and that loss of Nox2 will protect from platelet activation and thrombosis. MethodsWe studied littermates of Nox2 knockout (Nox2-KO) and -wild-type (Nox2-WT) mice at young (3-4 months) and old (18-20 months) age. Within platelets, we examined the expression of subunits of NADPH oxidase and enzyme activity, oxidant levels, activation markers, aggregation, and secretion. We also assessed susceptibility to in vivo thrombosis in 2 experimental models. ResultsWhile aged Nox2-WT mice displayed increased mRNA levels for Nox2, aged Nox2-KO mice showed an increase in Nox4 mRNA. However, neither the protein levels of several subunits nor the activity of NADPH oxidase were found to be altered by age or genotype. Both aged Nox2-WT and aged Nox2-KO mice exhibited similar enhancement in levels of platelet oxidants, granule release, αIIbβ3 activation, annexin V binding, aggregation and secretion, and a greater susceptibility to platelet-induced pulmonary thrombosis compared with young mice. In a photochemical injury model, adoptive transfer of platelets from aged Nox2-WT or Nox2-KO mice to the aged host mice resulted in a similar time to develop occlusive thrombus in the carotid artery. These findings suggest that loss of endogenous Nox2 does not protect against age-related platelet activation and arterial thrombosis in mice. ConclusionWe conclude that Nox2 is not an essential mediator of prothrombotic effects associated with aging.

Regulation of Reactive Oxygen Species on Platelet Activation and Apoptosis

To investigate how reactive oxygen species (ROS) regulates the signal transduction of platelet activation and apoptosis, and to explore the relationship between platelet activation and apoptosis. Platelets were directly stimulated with thrombin or pretreated with ROS inhibitor N-acetylcysteine (NAC) before being stimulated with thrombin, and then flow cytometry was used to detect the effects of thrombin and NAC on P-selectin expression, αⅡbβ3 activation, mitochondrial membrane potential depolarization, phosphatidylserine (PS) externalization, ROS expression and platelet aggregation. Thrombin could induce the production of ROS in platelets in a concentration- and time-dependent manner. 0.01 U thrombin induced ROS-dependent high degree of integrin αⅡbβ3 activation, P-selectin expression, and platelet aggregation. The platelets induced by different concentration gradients of thrombin exhibited ROS-dependent mitochondrial membrane potential depolarization and PS externalization in platelets. After induction with thrombin for 30 min, the activation of integrin αⅡbβ3 in platelets reached its maximum level, and after 60 minutes, the depolarization of mitochondrial membrane potential in platelets reached its maximum level. However, the expression of P-selectin, depolarization of mitochondrial membrane potential, and platelet aggregation function were all inhibited to a certain extent when the platelets were pretreated with ROS inhibitor NAC and then induced with thrombin. When platelets are induced by thrombin, ROS first regulates the activation of platelets, and then regulates the apoptosis of platelets. Both platelet activation and apoptosis depend on the production of ROS in platelets, and the signals of activation and apoptosis occur orderly. Inhibiting the ROS signal in platelets can effectively inhibit the activation and apoptosis of platelets.

USP15-Mediated Deubiquitination of FKBP 5 and Activation of the αIIbβ3 Signaling Pathway Regulate Thrombosis in Mice.

Platelets have the hemostatic function, and their aberrant activation is associated with occlusive thrombus formation. Plasma exosomes are rich in platelets containing ubiquitin-specific peptidase 15 (USP15). Herein, we aim to explore the effect of USP15 on thrombosis, as well as expounding whether USP15 acts as an upstream target of FK506 binding protein 5 (FKBP5) to regulate occlusive thrombus formation. Washed human platelets were treated with thrombin for measurement of USP15 and FKBP5 expressions. USP15 loss/gain-of-function variant in HEK293 cells was performed by cell transfection, and the interaction between USP15 and FKBP5 was examined using immunoprecipitation and ubiquitination assays. Mice with USP15-knockout platelets (Plt USP15-/-) were modeled, and subjected to calculation of bleeding time, artery thrombosis imaging and clot retraction assay. FKBP5 expression and the inhibitor of nuclear factor kappa B kinase subunit epsilon (IKBKE)/phosphatidylinositol 3-kinase (PI3K)/Rap1 pathway in wild-type and Plt USP15-/- mice-derived platelets were detected using Western blot. The activation of αIIbβ3 in washed platelets was analyzed using flow cytometry. USP15 and FKBP5 expressions were upregulated in platelets after thrombin treatment. Following transfection of USP15 knockdown and USP15 overexpression plasmids into HEK293 cells, FKBP5 protein expression was downregulated by USP15 knockdown while being upregulated by USP15 overexpression. USP15 bound to FKBP5 and protected FKBP5 against ubiquitination. Knockdown of platelet USP15 prolonged bleeding time, inhibited arterial thrombosis and delayed clot retraction in mice. Knockdown of platelet USP15 also decreased protein expressions of FKBP5, IKBKE and Rap1, p-PI3K/PI3K ratio, and activation of αIIbβ3 in mice. USP15 knockdown in platelets affects thrombosis in mice by promoting the instability of FKBP5 to repress the activation of IKBKE/PI3K/Rap1 pathway-mediated αIIbβ3.

NADPH oxidase 1/4 dual inhibitor setanaxib suppresses platelet activation and thrombus formation

AimsThe production of reactive oxygen species (ROS) by NADPH oxidase (NOX) is able to induce platelet activation, making NOX a promising target for antiplatelet therapy. In this study, we examined the effects of setanaxib, a dual NOX1/4 inhibitor, on human platelet function and ROS-related signaling pathways. Materials and methodsIn collagen-stimulated human platelets, aggregometry, assessment of ROS and Ca2+, immunoblotting, ELISA, flow cytometry, platelet adhesion assay, and assessment of mouse arterial thrombosis were performed in this study. Key findingsSetanaxib inhibited both intracellular and extracellular ROS production in collagen-activated platelets. Additionally, setanaxib significantly inhibited collagen-induced platelet aggregation, P-selectin exposure from α-granule release, and ATP release from dense granules. Setanaxib blocked the specific tyrosine phosphorylation-mediated activation of Syk, LAT, Vav1, and Btk within collagen receptor signaling pathways, leading to reduced activation of PLCγ2, PKC, and Ca2+ mobilization. Setanaxib also inhibited collagen-induced activation of integrin αIIbβ3, which is linked to increased cGMP levels and VASP phosphorylation. Furthermore, setanaxib suppressed collagen-induced p38 MAPK activation, resulting in decreased phosphorylation of cytosolic PLA2 and reduced TXA2 generation. Setanaxib also inhibited ERK5 activation, affecting the exposure of procoagulant phosphatidylserine. Setanaxib reduced thrombus formation under shear conditions by preventing platelet adhesion to collagen. Finally, in vivo administration of setanaxib in animal models led to the inhibition of arterial thrombosis. SignificanceThis study is the first to show that setanaxib suppresses ROS generation, platelet activation, and collagen-induced thrombus formation, suggesting its potential use in treating thrombotic or cardiovascular diseases.

A novel role for thioredoxin-related transmembrane protein TMX4 in platelet activation and thrombus formation

BackgroundThe functions of critical platelet proteins are controlled by thiol-disulfide exchanges, which are mediated by the protein disulfide isomerase (PDI) family. It has been shown that some PDI family members are important in platelet activation and thrombosis with distinct functions. TMX4, a membrane-type PDI family member, is expressed in platelets, but whether it has a role in platelet activation remains unknown. ObjectivesTo determine the role of TMX4 in platelet activation and thrombosis. MethodsThe phenotypes of TMX4-deficient mice were evaluated in tail bleeding time assay and laser-induced and FeCl3-induced arterial injury models. The functions of TMX4 in platelets were assessed in vitro using TMX4-null platelets, recombinant TMX4 protein, and anti-TMX4 antibody. ResultsCompared with the control mice, Tie2-Cre/TMX4fl/fl mice deficient of hematopoietic and endothelial TMX4 exhibited prolonged tail bleeding times and reduced platelet thrombus formation. Pf4-Cre/TMX4fl/fl mice deficient of platelet TMX4 also had prolonged tail bleeding times and decreased thrombus formation, which was rescued by injection of recombinant TMX4 protein. Consistently, TMX4 deficiency inhibited platelet aggregation, integrin αIIbβ3 activation, P-selectin expression, phosphatidylserine exposure, and thrombin generation, without affecting tyrosine phosphorylation of intracellular signaling molecules Syk, LAT, PLCγ2 and calcium mobilization. Recombinant TMX4 protein enhanced platelet aggregation and reduced integrin αIIbβ3 disulfide bonds, and TMX4 deficiency decreased free thiols of integrin αIIbβ3, consistent with a potent reductase activity of TMX4. In contrast, an inactive TMX4 protein and a specific anti-TMX4 antibody inhibited platelet aggregation. ConclusionTMX4 is a novel PDI family member that enhances platelet activation and thrombosis.

Platelet-specific Rictor knockout inhibits platelet production and activation and reduces thrombosis in mice

To investigate the effects of platelet-specific Rictor knockout on platelet activation and thrombus formation in mice. PF4-Cre and Rictorfl/fl transgenic mice were crossed to obtain platelet-specific Rictor knockout (Rictor-KO) mice and wild-type mice (n=65), whose expression levels of Rictor, protein kinase B (AKT) and p-AKT were detected using Western blotting. Platelet counts of the mice were determined using routine blood tests, and hemostatic function was assessed by tail vein hemorrhage test. Venous thrombosis models were established in the mice to evaluate the effect of Rictor knockout on thrombosis. Platelet aggregation induced by ADP and thrombin was observed in Rictor-KO and wild-type mice, and flow cytometry was used to analyze the expression levels of integrin αIIbβ3 and CD62P in resting and activated platelets. Plasma PF4 levels were determined with ELISA. Megakaryocytes from Rictor-KO and wild-type mice were incubated by vWF immunohistochemical antibody and APC-CD41 antibody to detect the number and ploidy of megakaryocytes, respectively. Platelet elongation on collagen surface was observed with scanning electron microscopy. Compared with the wild-type mice, Rictor-KO mice showed significantly decreased AKT phosphorylation, decreased platelet production, reduced thrombosis, and decreased platelet activation in response to ADP and thrombin stimulation. The Rictor-KO mice also showed lowered expression level of P-selectin protein and activation of integrin αIIbβ3 with suppression of platelet extension, reduced plasma PF4 level and decreased number of megakaryocytes in the bone marrow. The ploidy of megakaryocytes and the mean area of proplatelets were both significantly decreased in Rictor-KO mice. Platelet-specific Rictor knockout inhibits platelet generation and activation to result in decreased thrombus formation in mice, suggesting the potential of mTORC2 activity inhibition as an efficient antithrombotic strategy.

Pharmacological inhibition of Septins with Forchlorfenuron attenuates thrombus formation in experimental thrombotic mice models with modulating multiple signaling pathways in platelets

IntroductionThe Septin family of cytoskeletal proteins is abundant in platelets. When these proteins are functionally blocked using the compound forchlorfenuron (FCF), it hampers the normal activation processes of purified human platelets. ObjectivesTo evaluate the in vivo effects of FCF on physiological haemostasis and pathological thrombosis in mice and to investigate possible molecular mechanisms. MethodsThe impact of FCF on haemorrhage risk in the brain, liver, and tail of mice was investigated. Using several experimental models, thrombus development in the lung, mesenteric arteries, and postcava was studied. Functional assays were performed on mice and human platelets, both with and without FCF pretreatment. These tests included aggregation, granule release, ROS production, integrin αIIbβ3 activation, cytoskeletal remodeling imaging, and clot retraction. ResultsNeither oral nor intravenous administration of FCF showed any apparent impairment of haemostasis in the tissues studied, but only later administration resulted in a significant reduction in thrombus formation in different mice vessel types. FCF generally inhibited agonist-induced platelet aggregation, degranulation, ROS burst, morphological expansion on the fibrinogen matrix with completely disordered dynamic organizations of the cytoskeleton for septin, tubulin and actin. In addition, FCF was found to antagonise agonist-induced dephosphorylation of VASP (Ser239) and PI3K/AKT and ERK1/2 phosphorylation. ConclusionFCF showed preferences in attenuating pathological thrombus formation, apart from physiological haemostasis, with possible mechanisms to prevent cytoskeletal remodelling and signal transduction of AKT, ERK1/2 and VASP signalling pathways, suggesting that Septin may serve as a promising target for the prevention and treatment of thrombotic diseases.

Western Diet Modifies Platelet Activation Profiles in Male Mice.

The correlation between obesity and cardiovascular disease has long been understood, yet scant investigations endeavored to determine the impact of an obesogenic diet on platelet activation or function. As platelets drive clot formation, the terminus of cardiovascular events, we aimed to elucidate the longitudinal effect of an obesogenic diet on platelet phenotype by assessing markers of platelet activation using flow cytometry. Male, weanling mice were fed either a Western diet (30% kcal sucrose, 40% kcal fat, 8.0% sodium) or Control diet (7% kcal sucrose, 10% kcal fat, 0.24% sodium). At 12, 16 and 20 weeks on diets, platelets were collected and stained to visualize glycoprotein Ibα (GPIbα), P-selectin and the conformationally active state of αIIbβ3 (a platelet specific integrin) after collagen stimulation. At all time points, a Western diet reduced GPIbα and αIIbβ3 expression in platelets broadly while P-selectin levels were unaffected. However, P-selectin was diminished by a Western diet in the GPIbα- subpopulation. Thus, a Western diet persistently primed platelets towards a blunted activation response as indicated by reduced active αIIbβ3 and P-selectin surface expression. This study provides a first look at the influence of diet on platelet activation and revealed that platelet activation is susceptible to dietary intervention.

An αIIbβ3 monoclonal antibody traps a semiextended conformation and allosterically inhibits large ligand binding

AbstractMonoclonal antibodies (mAbs) have provided valuable information regarding the structure and function of platelet αIIbβ3. Protein disulfide isomerase (PDI) has been implicated in αIIbβ3 activation and binds to thrombin-activated αIIbβ3. Using human platelets as the immunogen, we identified a new mAb (R21D10) that inhibits the binding of PDI to platelets activated with thrombin receptor–activating peptide (T6). R21D10 also partially inhibited T6-induced fibrinogen and PAC-1 binding to platelets, as well as T6- and adenosine 5'-diphosphate–induced platelet aggregation. Mutual competition experiments showed that R21D10 does not inhibit the binding of mAbs 10E5 (anti-αIIb cap domain) or 7E3 (anti-β3 β-I domain), and immunoblot studies indicated that R21D10 binds to β3. The dissociation of αIIbβ3 by EDTA had a minimal effect on R21D10 binding. Cryogenic electron microscopy of the αIIbβ3-R21D10 Fab complex revealed that R21D10 binds to the β3 integrin-epidermal growth factor 1 (I-EGF1) domain and traps an intermediate conformation of αIIbβ3 with semiextended leg domains. The binding of R21D10 produces a major structural change in the β3 I-EGF2 domain associated with a new interaction between the β3 I-EGF2 and αIIb thigh domains, which may prevent the swing-out motion of the β3 hybrid domain required for high-affinity ligand binding and protect αIIbβ3 from EDTA-induced dissociation. R21D10 partially reversed the ligand binding priming effect of eptifibatide, suggesting that it could convert the swung-out conformation into a semiextended conformation. We concluded that R21D10 inhibits ligand binding to αIIbβ3 via a unique allosteric mechanism, which may or may not be related to its inhibition of PDI binding.

Utility of thromboelastography with platelet mapping for monitoring platelet transfusion in qualitative platelet disorders

BackgroundPatients with pathogenic variants in RASGRP2 (inherited platelet disorder (IPD)-18) exhibit normal platelet counts but impaired platelet aggregation and αIIbβ3 activation. Moderate-to-severe bleeding episodes require patients to be transfused with platelets and/or pro-hemostatic agents. We recently demonstrated that hemostatic efficacy of transfused platelets is limited by dysfunctional endogenous platelets in a mouse model of IPD-18 (Rasgrp2−/− mice), as dysfunctional platelets were recruited to the forming hemostatic plug but did not participate in clot contraction. Thus, higher amounts of transfused platelets were required to outcompete these dysfunctional cells and to reverse bleeding. ObjectiveWe here studied the usefulness of thromboelastography with platelet mapping (TEG-PM) for ex vivo monitoring of the hemostatic potential in Rasgrp2−/− mice transfused with various amounts of wild-type (WT) platelets. MethodsWhole blood (WB) samples from WT and Rasgrp2−/− mice were tested in TEG-PM and parameters for clot formation and contraction (K time, α-angle, maximum amplitude [MA]) were measured. ResultsRasgrp2−/− WB samples did not contract in TEG-PM, consistent with a critical role of this protein in αIIbβ3 activation. Addition of WT platelets improved TEG parameters in a ratio-dependent manner, consistent with our recent in vivo studies showing impaired hemostasis at a 5:1, but not at a 2:1 ratio of mutant to WT platelets. K and α values were identified as better predictors of transfusion efficacy than MA, the most platelet-dependent TEG parameter. ConclusionThis proof-of-concept study supports the use of TEG-PM to monitor platelet transfusion ratios and hemostatic potential in IPD-18 and potentially other platelet disorders.

Transmembrane thiol isomerase TMX1 counterbalances the effect of ERp46 to inhibit platelet activation and integrin αIIbβ3 function

BackgroundPrevious studies have shown that thiol isomerases such as ERp46 positively regulate platelet function by reducing integrin αIIbβ3 disulfides, and the transmembrane thiol isomerase TMX1 negatively regulates integrin αIIbβ3 activation. However, whether and how the positive and negative thiol isomerases interact with each other and their interactions participate in platelet activation remain unknown. ObjectivesTo investigate whether and how TMX1 regulates the effect of ERp46 on platelet function. MethodsUsing ERp46- and TMX1-deficient platelets, anti-TMX1 antibody, and wild-type TMX1 (TMX1-CPAC, TMX1-SS) and inactive TMX1 (TMX1-SPAS, TMX1-OO) proteins, we studied the antagonistic effect of TMX1 on ERp46 in platelet aggregation, clot retraction, and integrin αIIbβ3 signaling. The underlying mechanisms were further determined using thiol labeling, reductase activity, and other assays. ResultsAnti-TMX1 antibody and TMX1-OO reversed the decreased aggregation of ERp46-deficient platelets induced by thrombin, convulxin, and U46619. Anti-TMX1 antibody reversed the attenuated integrin αIIbβ3 function of ERp46-deficient platelets. TMX1 inhibited ERp46 reductase activity in a concentration-dependent manner. TMX1 oxidized thiols of ERp46 and those of integrin αIIbβ3 generated by ERp46. Moreover, TMX1 deficiency increased free thiols of ERp46 in platelets, which was reversed by the addition of wild-type TMX1 protein. Besides, anti-TMX1 antibody increased free thiols of ERp46 in wild-type activated platelets. ConclusionTMX1 not only oxidizes integrin αIIbβ3 disulfides that are reduced by ERp46 but also directly oxidizes ERp46 to suppress its reduction of integrin αIIbβ3. Thus, TMX1 is critical for maintaining platelets in a quiescent state and counterbalancing the effect of ERp46 to prevent platelet overactivation.

Iron deficiency anemia and platelet dysfunction: A comprehensive analysis of the underlying mechanisms

AimsThis research aimed to study the changes in platelet function and their underlying mechanisms in iron deficiency anemia. Main methodsInitially, we evaluated platelet function in an IDA mice model. Due to the inability to accurately reduce intracellular Fe2+ concentrations, we investigated the impact of Fe2+ on platelet function by introducing varying concentrations of Fe2+. To probe the underlying mechanism, we simultaneously examined the dynamics of calcium in the cytosol, and integrin αIIbβ3 activation in Fe2+-treated platelets. Ferroptosis inhibitors Lip-1 and Fer-1 were applied to determine whether ferroptosis was involved in this process. Key findingsOur study revealed that platelet function was suppressed in IDA mice. Fe2+ concentration-dependently facilitated platelet activation and function in vitro. Mechanistically, Fe2+ promoted calcium mobilization, integrin αIIbβ3 activation, and its downstream outside-in signaling. Additionally, we also demonstrated that ferroptosis might play a role in this process. SignificanceOur data suggest an association between iron and platelet activation, with iron deficiency resulting in impaired platelet function, while high concentrations of Fe2+ contribute to platelet activation and function by promoting calcium mobilization, αIIbβ3 activation, and ferroptosis.

Lysophosphatidylcholine induces oxidative stress and calcium-mediated cell death in human blood platelets.

Platelets are essential component of circulation that plays a major role in hemostasis and thrombosis. During activation and its demise, platelets release platelet-derived microvesicles, with lysophosphatidylcholine (LPC) being a prominent component in their lipid composition. LPC, an oxidized low-density lipoprotein, is involved in cellular metabolism, but its higher level is implicated in pathologies like atherosclerosis, diabetes, and inflammatory disorders. Despite this, its impact on platelet function remains relatively unexplored. To address this, we studied LPC's effects on washed human platelets. A multimode plate reader was employed to measure reactive oxygen species and intracellular calcium using H2DCF-DA and Fluo-4-AM, respectively. Flow cytometry was utilized to measure phosphatidylserine expression, mitochondrial membrane potential (ΔΨm), and mitochondrial permeability transition pore (mPTP) formation using FITC-Annexin V, JC-1, and CoCl2/calcein-AM, respectively. Additionally, platelet morphology and its ultrastructure were observed via phase contrast and electron microscopy. Sonoclot and light transmission aggregometry were employed to examine fibrin formation and platelet aggregation, respectively. The findings demonstrate that LPC induced oxidative stress and increased intracellular calcium in platelets, resulting in increased phosphatidylserine expression and reduced ΔΨm. LPC triggered caspase-independent platelet death and mPTP opening via cytosolic and mitochondrial calcium, along with microvesiculation and reduced platelet counts. LPC increased the platelet's size, adopting a balloon-shaped morphology, causing membrane fragmentation and releasing its cellular contents, while inducing a pro-coagulant phenotype with increased fibrin formation and reduced integrin αIIbβ3 activation. Conclusively, this study reveals LPC-induced oxidative stress and calcium-mediated platelet death, necrotic in nature with pro-coagulant properties, potentially impacting inflammation and repair mechanisms during vascular injury.

Essential role of glycoprotein Ibα in platelet activation

AbstractGlycoprotein Ibα (GPIbα), the ligand-binding subunit of platelet GPIb-IX complex, interacts with von Willebrand factor (VWF) exposed at the injured vessel wall, initiating platelet adhesion, activation, hemostasis, and thrombus formation. The cytoplasmic tail of GPIbα interacts with 14-3-3ζ, regulating the VWF-GPIbα–elicited signal transduction and VWF binding function of GPIbα. However, we unexpectedly found that the GPIbα–14-3-3ζ association, beyond VWF-dependent function, is essential for general platelet activation. We found that the myristoylated peptide of GPIbα C-terminus MPαC, a potential GPIbα inhibitor, by itself induced platelet aggregation, integrin αIIbβ3 activation, granule secretion, and phosphatidylserine (PS) exposure. Conversely, the deletion of the cytoplasmic tail of GPIbα in mouse platelets (10aa−/−) decreased platelet aggregation, integrin αIIbβ3 activation, granule secretion, and PS exposure induced by various physiological agonists. Phosphoproteome-based kinase activity profiling revealed significantly upregulated protein kinase C (PKC) activity in MPαC-treated platelets. MPαC-induced platelet activation was abolished by the pan-PKC inhibitor and PKCα deletion. Decreased PKC activity was observed in both resting and agonist-stimulated 10aa−/− platelets. GPIbα regulates PKCα activity by sequestering 14-3-3ζ from PKCα. In vivo, the deletion of the GPIbα cytoplasmic tail impaired mouse hemostasis and thrombus formation and protected against platelet-dependent pulmonary thromboembolism. Therefore, our findings demonstrate an essential role for the GPIbα cytoplasmic tail in regulating platelet general activation and thrombus formation beyond the VWF–GPIbα axis.

Platelet Activation Pathways Controlling Reversible Integrin αIIbβ3 Activation.

Background Agonist-induced platelet activation, with the integrin αIIbβ3 conformational change, is required for fibrinogen binding. This is considered reversible under specific conditions, allowing a second phase of platelet aggregation. The signaling pathways that differentiate between a permanent or transient activation state of platelets are poorly elucidated. Objective To explore platelet signaling mechanisms induced by the collagen receptor glycoprotein VI (GPVI) or by protease-activated receptors (PAR) for thrombin that regulate time-dependent αIIbβ3 activation. Methods Platelets were activated with collagen-related peptide (CRP, stimulating GPVI), thrombin receptor-activating peptides, or thrombin (stimulating PAR1 and/or 4). Integrin αIIbβ3 activation and P-selectin expression was assessed by two-color flow cytometry. Signaling pathway inhibitors were applied before or after agonist addition. Reversibility of platelet spreading was studied by microscopy. Results Platelet pretreatment with pharmacological inhibitors decreased GPVI- and PAR-induced integrin αIIbβ3 activation and P-selectin expression in the target order of protein kinase C (PKC) > glycogen synthase kinase 3 > β-arrestin > phosphatidylinositol-3-kinase. Posttreatment revealed secondary αIIbβ3 inactivation (not P-selectin expression), in the same order, but this reversibility was confined to CRP and PAR1 agonist. Combined inhibition of conventional and novel PKC isoforms was most effective for integrin closure. Pre- and posttreatment with ticagrelor, blocking the P2Y 12 adenosine diphosphate (ADP) receptor, enhanced αIIbβ3 inactivation. Spreading assays showed that PKC or P2Y 12 inhibition provoked a partial conversion from filopodia to a more discoid platelet shape. Conclusion PKC and autocrine ADP signaling contribute to persistent integrin αIIbβ3 activation in the order of PAR1/GPVI > PAR4 stimulation and hence to stabilized platelet aggregation. These findings are relevant for optimization of effective antiplatelet treatment.

A mouse model of the protease-activated receptor 4 Pro310Leu variant has reduced platelet reactivity

BackgroundProtease-activated receptor 4 (PAR4) mediates thrombin signaling on platelets and other cells. Our recent structural studies demonstrated that a single nucleotide polymorphism in extracellular loop 3 and PAR4-P310L (rs2227376) leads to a hyporeactive receptor. ObjectivesThe goal of this study was to determine how the hyporeactive PAR4 variant in extracellular loop 3 impacts platelet function in vivo using a novel knock-in mouse model (PAR4-322L). MethodsA point mutation was introduced into the PAR4 gene F2rl3 via CRISPR/Cas9 to create PAR4-P322L, the mouse homolog to human PAR4-P310L. Platelet response to PAR4 activation peptide (AYPGKF), thrombin, ADP, and convulxin was monitored by αIIbβ3 integrin activation and P-selectin translocation using flow cytometry or platelet aggregation. In vivo responses were determined by the tail bleeding assay and the ferric chloride–induced carotid artery injury model. ResultsPAR4-P/L and PAR4-L/L platelets had a reduced response to AYPGKF and thrombin measured by P-selectin translocation or αIIbβ3 activation. The response to ADP and convulxin was unchanged among genotypes. In addition, both PAR4-P/L and PAR4-L/L platelets showed a reduced response to thrombin in aggregation studies. There was an increase in the tail bleeding time for PAR4-L/L mice. The PAR4-P/L and PAR4-L/L mice both showed an extended time to arterial thrombosis. ConclusionPAR4-322L significantly reduced platelet responsiveness to AYPGKF and thrombin, which is in agreement with our previous structural and cell signaling studies. In addition, PAR4-322L had prolonged arterial thrombosis time. Our mouse model provides a foundation to further evaluate the role of PAR4 in other pathophysiological contexts.

Constitutive hypercoagulability in pediatric sickle cell disease patients with hemoglobin SS genotype

BackgroundConstitutive inflammation and hemostatic activation have been identified as key contributors to the pathophysiology of sickle cell disease (SCD), leading to clinical consequences such as vaso-occlusive crises and stroke. Patients with hemoglobin SS (HbSS) and hemoglobin SC (HbSC) genotypes are reported to have different symptoms, as do patients in steady-state and crisis situations. Differences among these groups remain unclear in pediatric patients. ObjectivesTo compare hemostatic activity in HbSS and HbSC pediatric patients during steady state, in crisis, and in clinical follow-up and compare HbSS and HbSC patients with normal healthy children. MethodsWhole-blood coagulation assay thromboelastography (TEG) was used to assess hemostatic activity. In parallel, flow cytometry was used to assess procoagulant surface expression of platelets and red blood cells. ResultsTEG results indicated no significant differences in clotting onset (R time), clot maximum amplitude, or maximum rate of thrombus generation among steady-state, crisis, and follow-up subgroups of HbSS and HbSC patients. TEG parameters did not differ significantly between HbSC patients and healthy children, while HbSS patients showed significantly shorter R time and greater maximum amplitude and maximum rate of thrombus generation, all indicative of a constitutive hypercoagulable state. Flow cytometry results did not detect increased platelet integrin αIIbβ3 activation or red blood cell procoagulant surface expression in SCD patients compared with unaffected children. ConclusionOur results indicate that pediatric SCD patients with the HbSS genotype have constitutively activated hemostasis relative to HbSC patients and healthy children. It remains to be determined how treatments that improve clinical outcomes in SCD patients affect this constitutively hypercoagulable state.

Icaritin Sensitizes Thrombin- and TxA2-Induced Platelet Activation and Promotes Hemostasis via Enhancing PLCγ2-PKC Signaling Pathways.

Vascular injury results in uncontrollable hemorrhage in hemorrhagic diseases and excessive antithrombotic therapy. Safe and efficient hemostatic agents which can be orally administered are urgently needed. Platelets play indispensable roles in hemostasis, but there is no drug exerting hemostatic effects through enhancing platelet function. The regulatory effects of icaritin, a natural compound isolated from Herba Epimedii, on the dense granule release, thromboxane A2 (TxA2) synthesis, α-granule release, activation of integrin αIIbβ3, and aggregation of platelets induced by multiple agonists were investigated. The effects of icaritin on tail vein bleeding times of warfarin-treated mice were also evaluated. Furthermore, we investigated the underlying mechanisms by which icaritin exerted its pharmacological effects. Icaritin alone did not activate platelets, but significantly potentiated the dense granule release, α-granule release, activation of integrin αIIbβ3, and aggregation of platelets induced by thrombin and U46619. Icaritin also shortened tail vein bleeding times of mice treated with warfarin. In addition, phosphorylated proteome analysis, immunoblotting analysis, and pharmacological research revealed that icaritin sensitized the activation of phospholipase Cγ2 (PLCγ2)-protein kinase C (PKC) signaling pathways, which play important roles in platelet activation. Icaritin can sensitize platelet activation induced by thrombin and TxA2 through enhancing the activation of PLCγ2-PKC signaling pathways and promote hemostasis, and has potential to be developed into a novel orally deliverable therapeutic agent for hemorrhages.

Inhibition of mitochondrial calcium transporters alters adp-induced platelet responses.

ADP-stimulated elevation of cytosolic Ca2+ is an important effector mechanism for platelet activation. The rapidly elevating cytosolic Ca2+ is also transported to mitochondrial matrix via Mitochondrial Ca2+ Uniporter (MCU) and extruded via Na+/Ca2+/Li+ Exchanger (NCLX). However, the exact contribution of MCU and NCLX in ADP-mediated platelet responses remains incompletely understood. The present study aimed to elucidate the role of mitochondrial Ca2+ transport in ADP-stimulated platelet responses by inhibition of MCU and NCLX with mitoxantrone (MTX) and CGP37157 (CGP), respectively. As these inhibitory strategies are reported to cause distinct effects on matrix Ca2+ concentration, we hypothesized to observe opposite impact of MTX and CGP on ADP-induced platelet responses. Platelet aggregation profiling was performed by microplate-based spectrophotometery while p-selectin externalization and integrin αIIbβ3 activation were analyzed by fluorescent immunolabeling using flow cytometery. Our results confirmed the expression of both MCU and NCLX mRNAs with relatively low abundance of NCLX in human platelets. In line with our hypothesis, MTX caused a dose-dependent inhibition of ADP-induced platelet aggregation without displaying any cytotoxicity. Likewise, ADP-induced p-selectin externalization and integrin αIIbβ3 activation was also significantly attenuated in MTX-treated platelets. Concordantly, inhibition of NCLX with CGP yielded an accelerated ADP-stimulated platelet aggregation which was associated with an elevation of p-selectin surface expression and αIIbβ3 activation. Together, these findings uncover a vital and hitherto poorly characterized role of mitochondrial Ca2+ transporters in ADP-induced platelet activation.