Succinate independently stimulates full platelet activation via cAMP and phosphoinositide 3‐kinase‐β signaling

Summary. Background: The citric cycle intermediate succinate has recently been identified as a ligand for the G‐protein‐coupled receptor (GPCR) SUCNR1. We have previously found that this receptor is one of the most highly expressed GPCRs in human platelets. Objective: The aim of this study was to investigate the role of SUCNR1 in platelet aggregation and to explore the signaling pathways of this receptor in platelets. Methods and Results: Using real‐time‐PCR, we demonstrated that SUCNR1 is expressed in human platelets at a level corresponding to that of the P2Y₁ receptor. Light transmission aggregation experiments showed dose‐dependent aggregation induced by succinate, reaching a maximum response at 0.5 mm . The effect of succinate on platelet aggregation was confirmed with flow cytometry, showing increased surface expression of activated glycoprotein IIb–IIIa and P‐selectin. Intracellular SUCNR1 signaling was found to result in decreased cAMP levels, Akt phosphorylation mediated by phosphoinositide 3‐kinase‐β activation, and receptor desensitization. Furthermore, succinate‐induced platelet aggregation was demonstrated to depend on Src, generation of thromboxane A₂, and ATP release. Platelet SUCNR1 is subject to desensitization through both homologous and heterologous mechanisms. In addition, the P2Y₁₂ receptor inhibitor ticagrelor completely prevented platelet aggregation induced by succinate. Conclusions: Our experiments show that succinate induces full aggregation of human platelets via SUCNR1. Succinate‐induced platelet aggregation depends on thromboxane A₂ generation, ATP release, and P2Y₁₂ activation.     

Human target validation of phosphoinositide 3‐kinase (PI3K)β: effects on platelets and insulin sensitivity,using AZD6482 a novel PI3Kβ inhibitor

Nylander S, Kull B, Björkman JA, Ulvinge JC, Oakes N, Emanuelsson BM, Andersson M, Skärby T, Inghardt T, Fjellström O, Gustafsson D. Human target validation of phosphoinositide 3‐kinase (PI3K)β:effects on platelets and insulin sensitivity, using AZD6482 a novel PI3Kβ inhibitor. J Thromb Haemost 2012; 10: 2127–36. See also Jackson SP, Schoenwaelder SM. Antithrombotic phosphoinositide 3‐kinase β inhibitors in humans – a ‘shear’ delight! This issue, pp 2123–6. Summary. Background: Based on in vitro and animal data, PI3Kβ is given an important role in platelet adhesion and aggregation but its role in insulin signaling is unclear. Objective: To strengthen the PI3Kβ target validation using the novel, short‐acting inhibitor AZD6482. Methods and results: AZD6482 is a potent, selective and ATP competitive PI3Kβ inhibitor (IC₅₀ 0.01 μm ). A maximal anti‐platelet effect was achieved at 1 μm in the in vitro and ex vivo tests both in dog and in man. In dog, in vivo AZD6482 produced a complete anti‐thrombotic effect without an increased bleeding time or blood loss. AZD6482 was well tolerated in healthy volunteers during a 3‐h infusion. The ex vivo anti‐platelet effect and minimal bleeding time prolongation in the dog model translated well to data obtained in healthy volunteers. AZD6482 inhibited insulin‐induced human adipocyte glucose uptake in vitro (IC₅₀ of 4.4 μm ). In the euglycemic hyperinsulinemic clamp model, in rats, glucose infusion rate was not affected at 2.3 μm but reduced by about 60% at a plasma exposure of 27 μm . In man, the homeostasis model analysis (HOMA) index increased by about 10–20% at the highest plasma concentration of 5.3 μm . Conclusions: This is the first human target validation for PI3Kβ inhibition as anti‐platelet therapy showing a mild and generalized antiplatelet effect attenuating but not completely inhibiting multiple signaling pathways with an impressive separation towards primary hemostasis. AZD6482 at ‘supratherapeutic’ plasma concentrations may attenuate insulin signaling, most likely through PI3Kα inhibition.     

Invasive Streptococcus pneumoniae trigger platelet activation via Toll‐like receptor 2

Summary. Background: Sepsis is the most common manifestation of invasive pneumococcal disease and is characterized by a severe systemic inflammatory state that leads to circulatory compromise or end organ malperfusion or dysfunction. Patients suffering from sepsis often display low platelet counts characterized by thrombocytopenia as a result of platelet activation. Objective: To investigate the mechanism through which platelets become activated in sepsis upon binding to Streptococcus pneumoniae. Patients and methods: We determined S. pneumoniae inducible platelet reactivity using light transmission aggregometry. Dense granule secretion was measured by luminometry using a luciferin/luciferase assay . Results : Streptococcus pneumoniae induced platelet aggregation in a strain‐dependent manner. Induction of aggregation was not attributable to capsule serotype, as unencapsulated strains also induced platelet aggregation. Platelet aggregation was not associated with pneumolysin toxin, as a pneumolysin‐deficient mutant of S. pneumoniae induced aggregation equally as well as the parent strain. Platelet aggregation also occurred in the absence of plasma proteins or antibody, and was GPIIbIIIa dependent but aspirin independent. Toll‐like receptor 2 (TLR2) is present on platelets and acts as a receptor for gram‐positive bacterial lipoteichoic acid and peptidoglycan. Inhibition of TLR2 but not TLR4 (also present on platelets) completely abolished platelet aggregation. S. pneumoniae‐induced platelet aggregation resulted in activation of the PI3kinase/RAP1 pathway, leading to integrin GPIIbIIIa activation and dense granule release. Conclusions : Our results demonstrate a novel interaction between S. pneumoniae and TLR2, which results in platelet activation that is likely to contribute to the thrombotic complications of sepsis.     

A role for adhesion and degranulation‐promoting adapter protein in collagen‐induced platelet activation mediated via integrin α2β1

Summary. Background: Collagen‐induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α₂β₁. Adhesion and degranulation‐promoting adapter protein (ADAP) regulates α_IIbβ₃ in platelets and α_Lβ₂ in T cells, and is phosphorylated in GPVI‐deficient platelets activated by collagen. Objectives: To determine whether ADAP plays a role in collagen‐induced platelet activation and in the regulation and function of α₂β₁. Methods: Using ADAP^?/? mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation. Results and Conclusions: Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP^?/? platelets. However, aggregation and signaling induced by collagen‐related peptide (CRP), a GPVI‐selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α₂β₁‐selective ligand GFOGER and to a peptide (III‐04), which supports adhesion that is dependent on both GPVI and α₂β₁, was reduced in ADAP^?/? platelets. An impedance‐based label‐free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non‐fluorescent differential‐interference contrast microscopy, which revealed reduced filpodia formation in ADAP^?/? platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen‐binding integrin α₂β₁. In addition, we found that ADAP^?/? mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild‐type animals. This may reflect increased removal of platelets from the circulation.     

Diverse functions of protein kinase C isoforms in platelet activation and thrombus formation

Summary. Platelet activation is a complex balance of positive and negative signaling pathways. The protein kinase C (PKC) family is a major regulator of platelet granule secretion, integrin activation, aggregation, spreading and procoagulant activity. As broad‐spectrum PKC inhibitors reduce secretion and aggregation, the PKC family is generally considered to be a positive regulator of platelet activation. However, the individual members of the PKC family that are expressed in platelets are regulated in different ways, and an increasing body of evidence indicates that they have distinct, and often opposing, roles. Many of the recent advances in understanding the contributions of individual PKC isoforms have come from mouse gene knockout studies. PKCα, a classic isoform, is an essential positive regulator of granule secretion and thrombus formation, both in vitro and in vivo. Mice lacking PKCα show much reduced thrombus formation in vivo but do not have a bleeding defect, suggesting that PKCα could be an attractive antithrombotic target. Important, apparently non‐redundant, roles, both positive and negative, for the novel PKC isoforms δ, θ and ε in granule secretion have also been proposed, indicating highly complex regulation of this essential process. Similarly, PKCβ, PKCδ and PKCθ have non‐redundant roles in platelet spreading, as absence of either PKCβ or PKCθ reduces spreading, whereas PKCδ negatively regulates filopodial formation. This negative signaling by PKCδ may reduce platelet aggregation and so restrict thrombus formation. In this review, we discuss the current understanding of the regulation and functions of individual PKC isoforms in platelet activation and thrombus formation.     

Phosphoinositide 3‐kinase mediates CD40 ligand‐induced oxidative stress and endothelial dysfunction via Rac1 and NADPH oxidase 2

Summary. Objectives: CD40 ligand (CD40L) has been implicated as an inducer of reactive oxygen species (ROS) generation in endothelial cells, but definitive evidence for this and the in vivo relevance haves not been demonstrated fully. We thus investigated whether phosphoinositide 3‐kinase (PI3K) was linked to ROS generation and endothelial reactivity in response to CD40L. Methods and Results: CD40L treatment activated PI3K activity by regulating the association between PI3K p85 and the CD40 receptor. CD40L exposure also stimulated the GTPase Rac1, which is known to activate NADPH oxidases, and enhanced ROS formation, whereas PI3K inhibition or depletion by small interfering RNA (siRNA) prevented these responses. Subsequently, PI3K overexpression activated Rac1 and increased ROS generation. These responses were not observed in the presence of inactive Rac1 or siRNA against the NADPH oxidase subunit NOX2. Protein kinase Cζ mediates PI3K‐regulated NADPH oxidase activation by promoting cellular p47phox translocation. Importantly, PI3K inhibition prevented CD40L‐mediated ROS generation and endothelial dysfunction in a mouse model. In summary, PI3K mediates CD40L‐induced ROS production and subsequent endothelial dysfunction. Conclusions: Targeting PI3K may provide a new therapeutic approach in diseases associated with oxidative stress and endothelial dysfunction.     

依布硒啉对大鼠心肌缺血再灌注损伤后磷酸肌醇-3激酶/蛋白激酶B信号通路的影响

目的探讨依布硒啉预处理对大鼠心肌缺血再灌注后磷酸肌醇-3激酶/蛋白激酶B（P13K/Akt）信号及其内质网应激的影响。 方法将30只大鼠分成对照组,缺血再灌注组及依布硒啉组,每组10只。对照组大鼠冠状动脉左室支左心耳下缘约0.5 cm处只穿线,不结扎,常规腹腔注射生理盐水2 ml;缺血再灌注组大鼠缺血再灌注前30 min腹腔注射生理盐水2 ml;依布硒啉组大鼠缺血再灌注前30 min腹腔注射依布硒啉溶液5 mg/kg。每组各取8只大鼠,采用酶联免疫吸附试验（ELISA）检测各组血清炎症因子高迁移率族蛋白1（HMGB1）、丙二醛、超氧化物歧化酶（SOD）、天冬氨酸转氨酶（AST）及乳酸脱氢酶（LDH）的表达,Tunel法检测缺血心肌细胞凋亡指数,Western-blotting检测各组心肌心肌葡萄糖调节蛋白78（GRP78）蛋白表达及P13K/Akt通路磷酸化水平。 结果三组大鼠间HMGB1、丙二醛、SOD、AST及LDH水平比较,差异均有统计学意义（F = 63.755、73.023、99.220、110.439、120.557,P均< 0.05）,进一步比较发现,缺血再灌注组及依布硒啉组的HMGB1[（9.2 ± 2.7）、（5.5 ± 1.1）、（2.2 ± 0.3）U/L]、丙二醛[（7.2 ± 0.4）、（5.6 ± 0.7）、（4.1 ± 0.9）μmol/L]、AST [（1 011 ± 226）、（813 ± 82）、（671 ± 60）U/L]及LDH [（2 783 ± 674）、（2 043 ± 489）、（1 528 ± 524）U/L]水平较对照组均明显升高,SOD水平[（249 ± 28）、（149 ± 10）、（172 ± 17）kU/L]较对照组明显降低（P均< 0.05）。三组大鼠间的凋亡指数（F = 139.942,P < 0.001）、GRP78蛋白表达（F = 177.846,P < 0.001）及P13K/Akt磷酸化水平（F = 86.286,P < 0.001）比较差异均存在统计学意义,进一步比较发现,缺血再灌注组和依布硒啉组凋亡指数[（38.1 ± 4.6）、（25.4 ± 3.9）、（8.2 ± 1.5）%]明显高于对照组,且I/R组更高（P均< 0.05）。 结论依布硒啉预处理可以抑制大鼠内质网应激,可能与调节P13K/Akt信号通路磷酸化水平有关。     

视网膜神经节细胞中AKAP1的丢失与青光眼小鼠病情发展的相关性及机制研究

目的 探究视网膜神经节细胞中A激酶锚定蛋白1(AKAP1)的丢失与青光眼小鼠病情发展的相关性及机制.方法 选择无特定病原体(SPF)级C57BL/6小鼠,通过硬膜外静脉阻塞并烧灼建立具有慢性高眼压的小鼠青光眼模型,然后按照眼压高低分为青光眼早期组(相对低眼压+造模后7 d)、青光眼中期组(相对中眼压+造模后15 d)和...     

Relative contributions of stromal interaction molecule 1 and CalDAG‐GEFI to calcium‐dependent platelet activation and thrombosis

Summary. Background: Stromal interaction molecule 1 (STIM1) was recently identified as a critical component of store‐operated calcium entry (SOCE) in platelets. We previously reported the Ca²⁺‐sensing guanine nucleotide exchange factor CalDAG‐GEFI as a critical molecule in Ca²⁺ signaling in platelets. Objective: To evaluate the contribution of STIM1/SOCE to Ca²⁺‐dependent platelet activation and thrombosis, we here compared the activation responses of platelets lacking STIM1 and platelets lacking CalDAG‐GEFI. Methods: The murine Stim1 gene was conditionally deleted in the megakaryocyte/platelet lineage. CalDAG‐GEFI^–/– and Stim1^fl/flPF4‐Cre mice, along with littermate control mice, were used for in vitro and in vivo experiments under flow as well as static conditions. Results: Integrin α_IIbβ₃‐mediated aggregation was markedly impaired in CalDAG‐GEFI‐deficient but not STIM1‐deficient platelets, under both static and flow conditions. In contrast, deficiency in either STIM1 or CalDAG‐GEFI significantly impaired the ability of platelets to express phosphatidylserine on the cell surface. When subjected to a laser injury thrombosis model, mice lacking STIM1 in platelets were characterized by the formation of unstable platelet‐rich thrombi and delayed and reduced fibrin generation in injured arterioles. In CalDAG‐GEFI^–/– mice, fibrin generation was also delayed and reduced, but platelet accumulation was almost abolished. Conclusions: Our studies suggest that: (i) STIM1/SOCE is critical for the procoagulant activity but not the proadhesive function of platelets; and (ii) at the site of vascular injury, STIM1 and CalDAG‐GEFI are critical for the first wave of thrombin generation mediated by procoagulant platelets.     

TC21/RRas2 regulates glycoprotein VI–FcRγ‐mediated platelet activation and thrombus stability

Essentials

• RAS proteins are expressed in platelets but their functions are largely uncharacterized.
• TC21/RRas2 is required for glycoprotein VI‐induced platelet responses and for thrombus stability in vivo.
• TC21 regulates platelet aggregation by control of α_IIbβ₃ integrin activation, via crosstalk with Rap1b.
• This is the first indication of functional importance of a proto‐oncogenic RAS protein in platelets.

Summary

Background

Many RAS family small GTPases are expressed in platelets, including RAC, RHOA, RAP, and HRAS/NRAS/RRAS1, but most of their signaling and cellular functions remain poorly understood. Like RRAS1, TC21/RRAS2 reverses HRAS‐induced suppression of integrin activation in CHO cells. However, a role for TC21 in platelets has not been explored.

Objectives

To determine TC21 expression in platelets, TC21 activation in response to platelet agonists, and roles of TC21 in platelet function in in vitro and in vivo thrombosis.

Results

We demonstrate that TC21 is expressed in human and murine platelets, and is activated in response to agonists for the glycoprotein (GP) VI–FcRγ immunoreceptor tyrosine‐based activation motif (ITAM)‐containing collagen receptor, in an Src‐dependent manner. GPVI‐induced platelet aggregation, integrin α_IIbβ₃ activation, and α‐granule and dense granule secretion, as well as phosphorylation of Syk, phospholipase Cγ2, AKT, and extracellular signal‐regulated kinase, were inhibited in TC21‐deficient platelets ex vivo. In contrast, these responses were normal in TC21‐deficient platelets following stimulation with P2Y, protease‐activated receptor 4 and C‐type lectin receptor 2 receptor agonists, indicating that the function of TC21 in platelets is GPVI–FcRγ‐ITAM‐specific. TC21 was required for GPVI‐induced activation of Rap1b. TC21‐deficient mice did not show a significant delay in injury‐induced thrombosis as compared with wild‐type controls; however, thrombi were unstable. Hemostatic responses showed similar effects.

Conclusions

TC21 is essential for GPVI–FcRγ‐mediated platelet activation and for thrombus stability in vivo via control of Rap1b and integrins.

     

Circulating matrix γ‐carboxyglutamate protein (MGP) species are refractory to vitamin K treatment in a new case of Keutel syndrome

Summary. Background and objectives: Matrix γ‐carboxyglutamate protein (MGP), a vitamin K‐dependent protein, is recognized as a potent local inhibitor of vascular calcification. Studying patients with Keutel syndrome (KS), a rare autosomal recessive disorder resulting from MGP mutations, provides an opportunity to investigate the functions of MGP. The purpose of this study was (i) to investigate the phenotype and the underlying MGP mutation of a newly identified KS patient, and (ii) to investigate MGP species and the effect of vitamin K supplements in KS patients. Methods: The phenotype of a newly identified KS patient was characterized with specific attention to signs of vascular calcification. Genetic analysis of the MGP gene was performed. Circulating MGP species were quantified and the effect of vitamin K supplements on MGP carboxylation was studied. Finally, we performed immunohistochemical staining of tissues of the first KS patient originally described focusing on MGP species. Results: We describe a novel homozygous MGP mutation (c.61+1G>A) in a newly identified KS patient. No signs of arterial calcification were found, in contrast to findings in MGP knockout mice. This patient is the first in whom circulating MGP species have been characterized, showing a high level of phosphorylated MGP and a low level of carboxylated MGP. Contrary to expectations, vitamin K supplements did not improve the circulating carboxylated MGP levels. Phosphorylated MGP was also found to be present in the first KS patient originally described. Conclusions: Investigation of the phenotype and MGP species in the circulation and tissues of KS patients contributes to our understanding of MGP functions and to further elucidation of the difference in arterial phenotype between MGP‐deficient mice and humans.