A platelet secretion pathway mediated by cGMP-dependent protein kinase

Role of the NO-GC/cGMP signaling pathway in platelet biomechanics

Cyclic guanosine monophosphate (cGMP) is a second messenger produced by the NO-sensitive guanylyl cyclase (NO-GC). The NO-GC/cGMP pathway in platelets has been extensively studied. However, its role in regulating the biomechanical properties of platelets has not yet been addressed and remains unknown. We therefore investigated the stiffness of living platelets after treatment with the NO-GC stimulator riociguat or the NO-GC activator cinaciguat using scanning ion conductance microscopy (SICM). Stimulation of human and murine platelets with cGMP-modulating drugs decreased cellular stiffness and downregulated P-selectin, a marker for platelet activation. We also quantified changes in platelet shape using deep learning-based platelet morphometry, finding that platelets become more circular upon treatment with cGMP-modulating drugs. To test for clinical applicability of NO-GC stimulators in the context of increased thrombogenicity risk, we investigated the effect of riociguat on platelets from human immunodeficiency virus (HIV)-positive patients taking abacavir sulfate (ABC)-containing regimens. Our results corroborate a functional role of the NO-GC/cGMP pathway in platelet biomechanics, indicating that biomechanical properties such as stiffness or shape could be used as novel biomarkers in clinical research.

Integrin β3 directly inhibits the Gα13-p115RhoGEF interaction to regulate G protein signaling and platelet exocytosis

The integrins and G protein-coupled receptors are both fundamental in cell biology. The cross talk between these two, however, is unclear. Here we show that β3 integrins negatively regulate G protein-coupled signaling by directly inhibiting the Gα13-p115RhoGEF interaction. Furthermore, whereas β3 deficiency or integrin antagonists inhibit integrin-dependent platelet aggregation and exocytosis (granule secretion), they enhance G protein-coupled RhoA activation and integrin-independent secretion. In contrast, a β3-derived Gα13-binding peptide or Gα13 knockout inhibits G protein-coupled RhoA activation and both integrin-independent and dependent platelet secretion without affecting primary platelet aggregation. In a mouse model of myocardial ischemia/reperfusion injury in vivo, the β3-derived Gα13-binding peptide inhibits platelet secretion of granule constituents, which exacerbates inflammation and ischemia/reperfusion injury. These data establish crucial integrin-G protein crosstalk, providing a rationale for therapeutic approaches that inhibit exocytosis in platelets and possibly other cells without adverse effects associated with loss of cell adhesion.

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

Signaling mechanisms of the platelet glycoprotein Ib-IX complex

The glycoprotein Ib-IX (GPIb-IX) complex mediates initial platelet adhesion to von Willebrand factor (VWF) immobilized on subendothelial matrix and endothelial surfaces, and transmits VWF binding-induced signals to stimulate platelet activation. GPIb-IX also functions as part of a mechanosensor to convert mechanical force received via VWF binding into intracellular signals, thereby greatly enhancing platelet activation. Thrombin binding to GPIb-IX initiates GPIb-IX signaling cooperatively with protease-activated receptors to synergistically stimulate the platelet response to low-dose thrombin. GPIb-IX signaling may also occur following the binding of other GPIb-IX ligands such as leukocyte integrin αMβ2 and red cell-derived semaphorin 7A, contributing to thrombo-inflammation. GPIb-IX signaling requires the interaction between the cytoplasmic domains of GPIb-IX and 14-3-3 protein and is mediated through Src family kinases, the Rho family of small GTPases, phosphoinositide 3-kinase-Akt-cGMP-mitogen-activated protein kinase, and LIM kinase 1 signaling pathways, leading to calcium mobilization, integrin activation, and granule secretion. This review summarizes the current understanding of GPIb-IX signaling.

The Manifold Cellular Functions of von Willebrand Factor

The plasma glycoprotein von Willebrand factor (VWF) is exclusively synthesized in endothelial cells (ECs) and megakaryocytes, the precursor cells of platelets. Its primary function lies in hemostasis. However, VWF is much more than just a "fishing hook" for platelets and a transporter for coagulation factor VIII. VWF is a true multitasker when it comes to its many roles in cellular processes. In ECs, VWF coordinates the formation of Weibel-Palade bodies and guides several cargo proteins to these storage organelles, which control the release of hemostatic, inflammatory and angiogenic factors. Leukocytes employ VWF to assist their rolling on, adhesion to and passage through the endothelium. Vascular smooth muscle cell proliferation is supported by VWF, and it regulates angiogenesis. The life cycle of platelets is accompanied by VWF from their budding from megakaryocytes to adhesion, activation and aggregation until the end in apoptosis. Some tumor cells acquire the ability to produce VWF to promote metastasis and hide in a shell of VWF and platelets, and even the maturation of osteoclasts is regulated by VWF. This review summarizes the current knowledge on VWF's versatile cellular functions and the resulting pathophysiological consequences of their dysregulation.

Shear and Integrin Outside-In Signaling Activate NADPH-Oxidase 2 to Promote Platelet Activation

[Figure: see text].

Upregulation of cGMP-dependent Protein Kinase (PRKG1) in the Development of Adolescent Idiopathic Scoliosis

Objective

To explore the molecular regulatory mechanisms underlying fibroblast differentiation and dysfunction in the development of adolescent idiopathic scoliosis (AIS) in an effort to identify candidate therapeutic targets for AIS.

Methods

The GSE110359 dataset, obtained from the bone marrow stromal cells of 12 AIS patients and five healthy controls, was retrieved from the GEO database. The data were preprocessed and differentially expressed genes (DEGs) were identified. KEGG pathway and Gene Ontology (GO)‐Biological Process (BP) enrichment analyses were performed to identify the function of the DEGs. A protein–protein interaction (PPI) and a microRNA‐transcription factor (TF)‐target co‐regulatory network were constructed to identify hub genes in the development of AIS. In addition, hub DEGs were evaluated by quantitative PCR (qPCR) and immunohistochemical staining.

Results

A total of 188 DEGs including 100 up‐regulated and 88 down‐regulated genes were obtained. The up‐regulated DEGs were related to “p53 signaling pathway”, “FoxO signaling pathway”, and “cGMP‐PKG signaling pathway” terms, while the down‐regulated DEGs were significantly enriched in seven terms including “protein processing in endoplasmic reticulum”. The key up‐regulated genes, PRKG1, CCNG2, and KAT2B, and the key down‐regulated genes, MAP2K1 and DUSP6, were identified by the PPI and miRNA‐TF‐Target regulatory network analyses. mRNA expression patterns for PRKG1, DUSP6, and KAT2B were successfully verified by qPCR. In addition, PRKG1 protein levels were found to be elevated during the immunohistochemical analysis.

Conclusion

Increased expression of PRKG1 in AIS patients might be an attractive therapeutic target for AIS. However, further gain or loss‐of‐function studies should be conducted.

Platelet activity is negatively modulated by tumor necrosis factor alpha through reductions of cytosolic calcium levels and integrin alphaIIbbeta3 phosphorylation

INTRODUCTION

Tumor necrosis factor-alpha (TNF-α) exerts a critical role in inflammatory events through two distinct receptors, TNFR1 and TNFR2. Platelets have been recognized as important inflammatory cells, but little is known about the effects of TNF-α on the platelet activity.

OBJECTIVES

In the present study we have studied the role of TNF-α on ADP-induced platelet aggregation and its downstream signaling (c-Src and fibrinogen receptor phosphorylation, cytosolic Ca²⁺ mobilization, cAMP and cGMP levels and cell viability).

METHODS AND RESULTS

Washed rat platelets were incubated with TNF-α (1-3000 pg/ml) for different time-periods (5-60 min) before the addition of ADP (5 μM) to induce platelet aggregation. TNF-α concentration- and time-dependently inhibits ADP-induced aggregation, which was significantly prevented by incubation with the non-selective TNF-α receptor antagonist R7050. TNF-α (300 pg/ml, 30 min) decreases thrombin-induced elevation of cytosolic Ca⁺⁺ levels by 2.2- fold compared to untreated platelets. TNF-α decreases the cAMP levels, while significantly increases the intracellular cyclic cGMP levels. However, the pre-incubation of platelets with the guanylyl cyclase inhibitor ODQ, despite decreasing the cGMP levels, does not modify the inhibitory effect of TNF-α on ADP-induced platelet aggregation. Additionally, western blotting analysis showed that TNF-α significantly reduced (Tyr 416)-c-Src and (Tyr773)-β3 subunit of αIIbβ3 integrin phosphorylation. TNF-α does not affect the platelet viability in any condition tested.

CONCLUSION

Therefore, our results show that TNF-α negatively modulates ADP-induced aggregation via TNFR1/TNFR2 receptors by reducing cytosolic Ca⁺⁺ levels and by inhibiting c-Src and fibrinogen receptor activation, which take place through cAMP- and cGMP-independent mechanisms.

Toll-Like Receptor 4 Signalling and Its Impact on Platelet Function, Thrombosis, and Haemostasis

Platelets are anucleated blood cells that participate in a wide range of physiological and pathological functions. Their major role is mediating haemostasis and thrombosis. In addition to these classic functions, platelets have emerged as important players in the innate immune system. In particular, they interact with leukocytes, secrete pro- and anti-inflammatory factors, and express a wide range of inflammatory receptors including Toll-like receptors (TLRs), for example, Toll-like receptor 4 (TLR4). TLR4, which is the most extensively studied TLR in nucleated cells, recognises lipopolysaccharides (LPS) that are compounds of the outer surface of Gram-negative bacteria. Unlike other TLRs, TLR4 is able to signal through both the MyD88-dependent and MyD88-independent signalling pathways. Notably, despite both pathways culminating in the activation of transcription factors, TLR4 has a prominent functional impact on platelet activity, haemostasis, and thrombosis. In this review, we summarise the current knowledge on TLR4 signalling in platelets, critically discuss its impact on platelet function, and highlight the open questions in this area.

Ambient fine particles (PM2.5 ) attenuate collagen-induced platelet activation through interference of the PLCγ2/Akt/GSK3β signaling pathway

AIMS

It has been proven that carbon nanoparticles or diesel exhaust particles stimulate platelet activation. However, the effect of fine particle matter (PM_2.5 ) on platelet activation remains unknown, which motivates this study.

METHODS

PM_2.5 samples were collected in an urban area of Zhengzhou, China. To study the morphological characteristics and the mass concentrations of trace elements of PM_2.5 samples, a filed-emission scanning electron microscope, the Image-J software, and an inductively coupled plasma mass spectrometry were used. Washed human platelets or platelet-rich-plasma were used to study the effect of PM_2.5 on platelet aggregation, P-selectin expression, or platelet signaling pathways. The cytotoxicity in platelets exposed to PM_2.5 was evaluated by a lactate dehydrogenase assay kit. In addition, platelet adhesion and spreading were studied on collagen-coated surfaces in stable conditions.

RESULTS

The filed-emission scanning electron microscope scanning showed that PM_2.5 samples varied in shape and size distributions. The mean equivalent spherical diameter of these particles was 1.97 ± 0.04 μm, of which 82.40% were particles with equivalent spherical diameters of less than 2.5 μm. The mass concentration of Ca was higher than that of other elements. The other elements followed the trend of Al>Fe>Zn>Mg>Pb>K>Mn>Cu>Ti>Ba>As>Sr>Sn>Sb>Cd>B>Se>Mo>Ag>Ni>TI>V>Co. Furthermore, pretreatment of PM_2.5 significantly inhibited rather than potentiated collagen-induced platelet aggregation and P-selectin expression, whereas it had no significant effect on ADP-induced platelet aggregation and P-selectin expression. The lactate dehydrogenase analysis showed trivial cytotoxic effect of PM_2.5 exposure on platelets. Pretreatment of PM_2.5 inhibited platelet adhesion on immobilized collagen-coated surfaces; however, it almost did not impact the platelet spreading. Immunoblotting analysis indicated that PM_2.5 reduced collagen-induced phosphorylation of phospholipase C gamma-2 (PLCγ2) at Tyr759, Akt at Ser473, and glycogen synthase kinase 3β (GSK3β) at Ser9.

CONCLUSIONS

PM_2.5 attenuated collagen-induced platelet aggregation, α-granule secretion and adhesion, with the potential mechanism of impairing PLCγ2, Akt, and GSK3β signaling. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 530-540, 2017.

Epigenetic and genetic variations at the TNNT1 gene locus are associated with HDL-C levels and coronary artery disease

AIM

To assess whether epigenetic and genetic variations at the TNNT1 gene locus are associated with high-density lipoprotein cholesterol (HDL-C) and coronary artery disease (CAD). Patients, materials & methods: TNNT1 DNA methylation and c.-20G>A polymorphism were genotyped in subjects with and without familial hypercholesterolemia (FH).

RESULTS

Lower TNNT1 DNA methylation levels were independently associated with lower HDL-C levels and with the TNNT1 c.-20G>A polymorphism. In FH men, carriers of the TNNT1 c.-20G>A polymorphism had lower HDL-C levels and an increased risk of CAD compared with noncarriers. In non-FH men, a higher TNNT1 DNA methylation level was associated with CAD.

CONCLUSION

These results suggest that TNNT1 genetic and epigenetic variations are associated with HDL-C levels and CAD.

Platelet-derived HMGB1 is a critical mediator of thrombosis

Thrombosis and inflammation are intricately linked in several major clinical disorders, including disseminated intravascular coagulation and acute ischemic events. The damage-associated molecular pattern molecule high-mobility group box 1 (HMGB1) is upregulated by activated platelets in multiple inflammatory diseases; however, the contribution of platelet-derived HMGB1 in thrombosis remains unexplored. Here, we generated transgenic mice with platelet-specific ablation of HMGB1 and determined that platelet-derived HMGB1 is a critical mediator of thrombosis. Mice lacking HMGB1 in platelets exhibited increased bleeding times as well as reduced thrombus formation, platelet aggregation, inflammation, and organ damage during experimental trauma/hemorrhagic shock. Platelets were the major source of HMGB1 within thrombi. In trauma patients, HMGB1 expression on the surface of circulating platelets was markedly upregulated. Moreover, evaluation of isolated platelets revealed that HMGB1 is critical for regulating platelet activation, granule secretion, adhesion, and spreading. These effects were mediated via TLR4- and MyD88-dependent recruitment of platelet guanylyl cyclase (GC) toward the plasma membrane, followed by MyD88/GC complex formation and activation of the cGMP-dependent protein kinase I (cGKI). Thus, we establish platelet-derived HMGB1 as an important mediator of thrombosis and identify a HMGB1-driven link between MyD88 and GC/cGKI in platelets. Additionally, these findings suggest a potential therapeutic target for patients sustaining trauma and other inflammatory disorders associated with abnormal coagulation.

The Inflammatory Role of Platelets via Their TLRs and Siglec Receptors

Platelets are non-nucleated cells that play central roles in the processes of hemostasis, innate immunity, and inflammation; however, several reports show that these distinct functions are more closely linked than initially thought. Platelets express numerous receptors and contain hundreds of secretory products. These receptors and secretory products are instrumental to the platelet functional responses. The capacity of platelets to secrete copious amounts of cytokines, chemokines, and related molecules appears intimately related to the role of the platelet in inflammation. Platelets exhibit non-self-infectious danger detection molecules on their surfaces, including those belonging to the “toll-like receptor” family, as well as pathogen sensors of other natures (Ig- or complement receptors, etc.). These receptors permit platelets to both bind infectious agents and deliver differential signals leading to the secretion of cytokines/chemokines, under the control of specific intracellular regulatory pathways. In contrast, dysfunctional receptors or dysregulation of the intracellular pathway may increase the susceptibility to pathological inflammation. Physiological vs. pathological inflammation is tightly controlled by the sensors of danger expressed in resting, as well as in activated, platelets. These sensors, referred to as pathogen recognition receptors, primarily sense danger signals termed pathogen associated molecular patterns. As platelets are found in inflamed tissues and are involved in auto-immune disorders, it is possible that they can also be stimulated by internal pathogens. In such cases, platelets can also sense danger signals using damage associated molecular patterns (DAMPs). Some of the most significant DAMP family members are the alarmins, to which the Siglec family of molecules belongs. This review examines the role of platelets in anti-infection immunity via their TLRs and Siglec receptors.

BDNF-induced nitric oxide signals in cultured rat hippocampal neurons: time course, mechanism of generation, and effect on neurotrophin secretion

BDNF and nitric oxide signaling both contribute to plasticity at glutamatergic synapses. However, the role of combined signaling of both pathways at the same synapse is largely unknown. Using NO imaging with diaminofluoresceine in cultured hippocampal neurons we analyzed the time course of neurotrophin-induced NO signals. Application of exogenous BDNF, NT-4, and NT-3 (but not NGF) induced NO signals in the soma and in proximal dendrites of hippocampal neurons that were sensitive to NO synthase activity, TrkB signaling, and intracellular calcium elevation. The effect of NO signaling on neurotrophin secretion was analyzed in BDNF-GFP, and NT-3-GFP transfected hippocampal neurons. Exogenous application of the NO donor sodium-nitroprusside markedly inhibited neurotrophin secretion. However, endogenously generated NO in response to depolarization and neurotrophin stimulation, both did not result in a negative feedback on neurotrophin secretion. These results suggest that a negative feedback of NO signaling on synaptic secretion of neurotrophins operates only at high intracellular levels of nitric oxide that are under physiological conditions not reached by depolarization or BDNF signaling.

Time-dependent inhibitory effects of cGMP-analogues on thrombin-induced platelet-derived microparticles formation, platelet aggregation, and P-selectin expression

In platelets, nitric oxide (NO) activates cGMP/PKG signalling, whereas prostaglandins and adenosine signal through cAMP/PKA. Cyclic nucleotide signalling has been considered to play an inhibitory role in platelets. However, an early stimulatory effect of NO and cGMP-PKG signalling in low dose agonist-induced platelet activation have recently been suggested. Here, we investigated whether different experimental conditions could explain some of the discrepancy reported for platelet cGMP-PKG-signalling. We treated gel-filtered human platelets with cGMP and cAMP analogues, and used flow cytometric assays to detect low dose thrombin-induced formation of small platelet aggregates, single platelet disappearance (SPD), platelet-derived microparticles (PMP) and thrombin receptor agonist peptide (TRAP)-induced P-selectin expression. All four agonist-induced platelet activation phases were blocked when platelets were costimulated with the PKG activators 8-Br-PET-cGMP or 8-pCPT-cGMP and low-doses of thrombin or TRAP. However, extended incubation with 8-Br-PET-cGMP decreased its inhibition of TRAP-induced P-selectin expression in a time-dependent manner. This effect did not involve desensitisation of PKG or PKA activity, measured as site-specific VASP phosphorylation. Moreover, PKG activators in combination with the PKA activator Sp-5,6-DCL-cBIMPS revealed additive inhibitory effect on TRAP-induced P-selectin expression. Taken together, we found no evidence for a stimulatory role of cGMP/PKG in platelets activation and conclude rather that cGMP/PKG signalling has an important inhibitory function in human platelet activation.

Extracellular ATP signaling via P2X(4) receptor and cAMP/PKA signaling mediate ATP oscillations essential for prechondrogenic condensation

Prechondrogenic condensation is the most critical process in skeletal patterning. A previous study demonstrated that ATP oscillations driven by Ca(2+) oscillations play a critical role in prechondrogenic condensation by inducing oscillatory secretion. However, it remains unknown what mechanisms initiate the Ca(2+)-driven ATP oscillations, mediate the link between Ca(2+) and ATP oscillations, and then result in oscillatory secretion in chondrogenesis. This study has shown that extracellular ATP signaling was required for both ATP oscillations and prechondrogenic condensation. Among P2 receptors, the P2X(4) receptor revealed the strongest expression level and mediated ATP oscillations in chondrogenesis. Moreover, blockage of P2X(4) activity abrogated not only chondrogenic differentiation but also prechondrogenic condensation. In addition, both ATP oscillations and secretion activity depended on cAMP/PKA signaling but not on K(ATP) channel activity and PKC or PKG signaling. This study proposes that Ca(2+)-driven ATP oscillations essential for prechondrogenic condensation is initiated by extracellular ATP signaling via P2X(4) receptor and is mediated by cAMP/PKA signaling and that cAMP/PKA signaling induces oscillatory secretion to underlie prechondrogenic condensation, in cooperation with Ca(2+) and ATP oscillations.

Mechanism of anti-platelet activity of Oligoporus tephroleucus oligoporin A: involvement of extracellular signal-regulated kinase phosphorylation and cyclic nucleotide elevation

This study investigated the inhibitory effects of oligoporin A on platelet aggregation and the mechanism of its action on downstream signaling molecules. Oligoporin A was isolated from the fruiting bodies of Oligoporus tephroleucus (Polyporaceae). The anti-platelet activities of oligoporin A were studied using rat platelets. The effects of oligoporin A on intracellular Ca(2+) mobilization, ATP release, production of the cyclic nucleotides cAMP and cGMP, extracellular signal-regulated kinase (ERK) 2 phosphorylation, and fibrinogen binding to active integrin α(II)(b)β(3) were assessed. Oligoporin A, but not oligoporins B and C, inhibited collagen-induced platelet aggregation in a concentration-dependent manner. Interestingly, oligoporin A did not affect ADP- and thrombin-induced platelet aggregations, which act on different types of membrane receptors. Granule secretion analysis demonstrated that oligoporin A significantly and dose-dependently reduced collagen-induced ATP release and intracellular Ca(2+) mobilization. Additionally, oligoporin A induced the dynamic increase in cAMP and cGMP. Increased cGMP production was further confirmed by the simultaneous production of nitric oxide. Pretreatment with oligoporin A significantly blocked collagen-induced ERK2 phosphorylation. Finally, oligoporin A vaguely diminished the binding of fibrinogen to its cognate receptor, integrin α(II)(b)β(3). The results indicate that oligoporin A inhibits only collagen-induced platelet aggregation mediated through the modulation of downstream signaling molecules. Oligoporin A may be beneficial against cardiovascular disease provoked by aberrant platelet activation.

Current concepts of platelet activation: possibilities for therapeutic modulation of heterotypic vs. homotypic aggregation

Thrombogenic and inflammatory activity are two distinct aspects of platelet biology, which are sustained by the ability of activated platelets to interact with each other (homotypic aggregation) and to adhere to circulating leucocytes (heterotypic aggregation). These two events are regulated by distinct biomolecular mechanisms that are selectively activated in different pathophysiological settings. They can occur simultaneously, for example, as part of a pro-thrombotic/pro-inflammatory response induced by vascular damage, or independently, as in certain clinical conditions in which abnormal heterotypic aggregation has been observed in the absence of intravascular thrombosis. Current antiplatelet drugs have been developed to target specific molecular signalling pathways mainly implicated in thrombus formation, and their ever increasing clinical use has resulted in clear benefits in the treatment and prevention of arterial thrombotic events. However, the efficacy of currently available antiplatelet drugs remains suboptimal, most likely because their therapeutic action is limited to only few of the signalling pathways involved in platelet homotypic aggregation. In this context, modulation of heterotypic aggregation, which is believed to contribute importantly to acute thrombotic events, as well to the pathophysiology of atherosclerosis itself, may offer benefits over and above the classical antiplatelet approach. This review will focus on the distinct biomolecular pathways that, following platelet activation, underlie homotypic and heterotypic aggregation, aiming potentially to identify novel therapeutic targets.

An important role for Akt3 in platelet activation and thrombosis

The Akt family of serine/threonine kinases includes Akt1, Akt2, and Akt3 isoforms. Prior studies have reported that Akt1 and Akt2, but not Akt3, are expressed in platelets. Here, we show that Akt3 is expressed in substantial amounts in platelets. Akt3(-/-) mouse platelets selectively exhibit impaired platelet aggregation and secretion in response to low concentrations of thrombin receptor agonists and thromboxane A₂ (TXA₂), but not collagen or VWF. In contrast, platelets from Akt1(-/-) or Akt2(-/-) mice are defective in platelet activation induced by thrombin, TXA₂, and VWF, but only Akt1(-/-) platelets show significant defects in response to collagen, indicating differences among Akt isoforms. Akt3(-/-) platelets exhibit a significant reduction in thrombin-induced phosphorylation of glycogen synthase kinase 3β (GSK-3β) at Ser9, which is known to inhibit GSK-3β function. Thus, Akt3 is important in inhibiting GSK-3β. Accordingly, treatment of Akt3(-/-) platelets with a GSK-3β inhibitor rescued the defect of Akt3(-/-) platelets in thrombin-induced aggregation, suggesting that negatively regulating GSK-3β may be a mechanism by which Akt3 promotes platelet activation. Importantly, Akt3(-/-) mice showed retardation in FeCl₃-induced carotid artery thrombosis in vivo. Thus, Akt3 plays an important and distinct role in platelet activation and in thrombosis.

Collagen stimulation of platelets induces a rapid spatial response of cAMP and cGMP signaling scaffolds

Intracellular communication is tightly regulated in both space and time. Spatiotemporal control is important to achieve a high level of specificity in both dimensions. For instance, cAMP-dependent kinase (PKA) attains spatial resolution by interacting with distinct members of the family of A-kinase anchoring proteins (AKAPs) that position PKA at specific loci within the cell. To control the cAMP induced signal in time, distinct signal terminators such as phosphodiesterases and phosphatases are often co-localized at the AKAP scaffold. In platelets, high levels of cAMP/cGMP maintain the resting state to allow free circulation. Exposure to collagen, for instance when the vessel is damaged, triggers platelet activation through initiation of the GPVI (glycoprotein VI)/FcRγ-chain forming the onset of a plethora of signaling pathways. Consequently overall intra-platelet cAMP and cGMP levels drop, however detail on how PKA, but also cGMP-dependent protein kinase (PKG) respond in relation to their localized signaling scaffolds is currently missing. To investigate this, we employed a quantitative chemical proteomics approach in activated human platelets enabling the specific enrichment of cAMP/cGMP signaling nodes. Our data reveal that within a few minutes several specific PKA and PKG signaling nodes respond significantly to the activating signal, whereas others do not, suggesting a rapid adaption of specific localized cAMP and cGMP pools to the stimulus. Using protein phosphorylation data gathered we touch upon the potential cross-talk between protein phosphorylation and signaling scaffold function as a general theme in platelet spatiotemporal control.

Extracellular-purine metabolism in blood vessels (part I). Extracellular-purine level in blood of patients with abdominal aortic aneurysm

Adenosine and adenosine derivatives are the main regulators of purinoceptors (P1 and P2) mediated hemostasis and blood pressure. Since impaired hemostasis and high blood pressure lead to atherosclerosis and to the development of aneurysm, in this study we tested and compared the concentration of extracellular purines (e-purines) in the blood in of patients having abdominal aortic aneurysm with that from healthy volunteers. Whereas adenine nucleosides and nucleotides level in human blood plasma was analysed using reverse phase high performance liquid chromatography (HPLC), cholesterol concentration was estimated by an enzymatic assay. We did not find any correlation between e-purines concentration and the age of healthy volunteers. Furthermore, the sum level of e-purines (ATP, ADP, AMP, adenosine, and inosine) in the control group did not exceed 70 microM, while it was nearly two-fold higher in the blood of patients having abdominal aortic aneurysm, (123 microM). In a special case of people with Leriche Syndrome, a disease characterized by deep atherosclerotic changes, the e-purines level had further increased. Additionally, we also report typical atherosclerotic changes in the aorta using histological assays as well as total cholesterol rise. The significant rise in cholesterol concentration in the blood of the patients with abdominal aortas aneurysm, compared with the control groups, was not unique since 23% of the healthy people also exceeded the normal level of cholesterol. Therefore, our results strongly indicate that the estimation of e-purines concentration in the blood may serve as another indicator of atherosclerosis and warrant further consideration as a futuristic diagnostic tool.

Signaling during platelet adhesion and activation

Upon vascular injury, platelets are activated by adhesion to adhesive proteins, such as von Willebrand factor and collagen, or by soluble platelet agonists, such as ADP, thrombin, and thromboxane A(2). These adhesive proteins and soluble agonists induce signal transduction via their respective receptors. The various receptor-specific platelet activation signaling pathways converge into common signaling events that stimulate platelet shape change and granule secretion and ultimately induce the "inside-out" signaling process leading to activation of the ligand-binding function of integrin α(IIb)β(3). Ligand binding to integrin α(IIb)β(3) mediates platelet adhesion and aggregation and triggers "outside-in" signaling, resulting in platelet spreading, additional granule secretion, stabilization of platelet adhesion and aggregation, and clot retraction. It has become increasingly evident that agonist-induced platelet activation signals also cross talk with integrin outside-in signals to regulate platelet responses. Platelet activation involves a series of rapid positive feedback loops that greatly amplify initial activation signals and enable robust platelet recruitment and thrombus stabilization. Recent studies have provided novel insight into the molecular mechanisms of these processes.

Biphasic effects of nitric oxide on calcium influx in human platelets

In this study the effects of nitric oxide (NO) donors on intracellular free calcium ([Ca(2+)](i)) in human platelets was examined. Inhibition of guanylyl cyclase (GC) with either methylene blue or ODQ slightly inhibited the ability of submaximal concentrations of thrombin to increase [Ca(2+)](i) which suggests that a small portion of the thrombin mediated increase in [Ca(2+)](i) was due to an increase in NO and subsequent increase in cGMP and activation of cGMP dependent protein kinase (cGPK). Thrombin predominantly increases [Ca(2+)](i) by stimulating store-operated Ca(2+) entry (SOCE). The NO donor GEA3162 was previously shown to stimulate SOCE in some cells. In platelets GEA3162 had no effect to increase [Ca(2+)](i) however it inhibited the ability of thrombin to increase [Ca(2+)](i) and this effect was reversed by ODQ. The addition of low concentrations (2.0 - 20 nM) of the NO donor sodium nitroprusside (SNP) slightly potentiated the ability of thrombin to increase [Ca(2+)](i) whereas higher concentrations (>200 nM) of SNP inhibited thrombin induced increases in [Ca(2+)](i). Both of these effects of SNP were reversed by ODQ which implies that they were both mediated by cGPK. Ba(2+) influx was stimulated by low concentrations (2.0 nM) of SNP and inhibited by high concentrations (>200 nM) of SNP and both effects were inhibited by ODQ. Previous studies showed that Ba(2+) influx was blocked by the SOCE inhibitors 2-aminoethoxydipheny borate and diethylstilbestrol. It was concluded that low levels of SNP can stimulate SOCE in platelets and this effect may account for the increased aggregation and secretion previously observed with low concentrations of NO donors. Of the proteins known to be involved in SOCE (e.g. stromal interaction molecule 1 (Stim1), Stim2 and Orai1) only Stim2 has cGPK phosphorylation sites. The possibility that Stim2 phosphorylation regulates SOCE in platelets is discussed.

A G(i) -independent mechanism mediating Akt phosphorylation in platelets

BACKGROUND

The serine-threonine kinase Akt plays an important role in regulating platelet activation. Stimulation of platelets with various agonists results in Akt activation as indicated by Akt phosphorylation. However, the mechanisms of Akt phosphorylation in platelets are not completely understood.

OBJECTIVES AND METHODS

We used P2Y₁ knockout mice to address the role of P2Y₁₂ in Akt phosphorylation in response to thrombin receptors in platelets.

RESULTS

Thrombin or the PAR4 thrombin receptor peptide AYPGKF at high concentrations stimulated substantial phosphorylation of Akt residues Thr³⁰⁸ and Ser⁴⁷³ in P2Y₁₂-deficient platelets. AYPGKF-induced Akt phosphorylation is enhanced by expression of recombinant human PAR4 cDNA in Chinese hamster ovary (CHO) cells. P2Y₁₂ -independent Akt phosphorylation was not inhibited by integrin inhibitor peptide RGDS or integrin β₃ deficiency. Akt phosphorylation induced by thrombin or AYPGKF in P2Y₁₂-deficient platelets was inhibited by the calcium chelator dimethyl-BAPTA, the Src family kinase inhibitor PP2, and PI3K inhibitors, respectively.

CONCLUSIONS

Our results reveal a novel P2Y₁₂-independent signaling pathway mediating Akt phosphorylation in response to thrombin receptors.

Novel thienylacylhydrazone derivatives inhibit platelet aggregation through cyclic nucleotides modulation and thromboxane A2 synthesis inhibition

The aim of this study has been to investigate the antiplatelet activity of a new series of thienylacylhydrazone derivatives analogous to the lead compound LASSBio-294 ((2-thienylidene) 3,4-methylenedioxybenzoylhydrazine). The antiplatelet effect was investigated in rabbit and human platelet rich plasma stimulated by arachidonic acid, collagen, ADP and in washed platelet stimulated by thrombin. The effects on the production of cyclic nucleotides and thromboxane A(2) (TXA(2)) in human platelets were also investigated. Compounds LASSBio-785 (N-Methyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-786 (N-Benzyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-787 ((5-Methyl-2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-788 (N-Allyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine) and LASSBio-789 ((5-Bromo-2-thienylidene) 3,4-methylenedioxybezoylhydrazine) inhibited platelet aggregation induced by arachidonic acid, collagen and ADP. LASSBio-785, LASSBio-788 and LASSBio-789 presented the higher potency in platelet aggregation induced by arachidonic acid (IC(50) values of 0.3, 0.2 and 3.1 microM, respectively) and collagen (IC(50) values of 0.9, 1.5 and 3.4 microM, respectively), with a 20 to 70-fold increase in potency compared to LASSBio-294. They inhibited the ATP release reaction by 95%, the whole blood aggregation by 35-45% and the TXB(2) production was totally abolished. In addition, they presented a significant effect on bleeding time. Qualitative studies in thrombin-induced washed platelet aggregation in the presence of sodium nitroprusside (SNP) suggested a phosphodiesterase-2 (PDE2) like effect for LASSBio-785, LASSBio-788 and LASSBio-789. They were able to increase the cGMP levels in non-stimulated platelets, in SNP-stimulated platelets and in the presence of 1-H- [1, 2, 4] oxadiazolo [4, 3- a] quinoxalin- 1- one (ODQ). The antiplatelet aggregation activity exerted by thienylacylhydrazone derivatives seems to be related to cyclic nucleotides regulation and TXA(2) synthesis inhibition. The structural modification of compound LASSBio-294 led to the optimization of its pharmacological properties and to the discovery of new potent antiplatelet prototypes with an antithrombotic potential.

S-nitrosylation in cardiovascular signaling

Well over 2 decades have passed since the endothelium-derived relaxation factor was reported to be the gaseous molecule nitric oxide (NO). Although soluble guanylyl cyclase (which generates cyclic guanosine monophosphate, cGMP) was the first identified receptor for NO, it has become increasingly clear that NO exerts a ubiquitous influence in a cGMP-independent manner. In particular, many, if not most, effects of NO are mediated by S-nitrosylation, the covalent modification of a protein cysteine thiol by an NO group to generate an S-nitrosothiol (SNO). Moreover, within the current framework of NO biology, endothelium-derived relaxation factor activity (ie, G protein-coupled receptor-mediated, or shear-induced endothelium-derived NO bioactivity) is understood to involve a central role for SNOs, acting both as second messengers and signal effectors. Furthermore, essential roles for S-nitrosylation have been implicated in virtually all major functions of NO in the cardiovascular system. Here, we review the basic biochemistry of S-nitrosylation (and denitrosylation), discuss the role of S-nitrosylation in the vascular and cardiac functions of NO, and identify current and potential clinical applications.

An important role of the SRC family kinase Lyn in stimulating platelet granule secretion

The Src family kinases (SFKs) have been proposed to play stimulatory and inhibitory roles in platelet activation. The mechanisms for these apparently contradictory roles are unclear. Here we show that SFK, mainly Lyn, is important in stimulating a common signaling pathway leading to secretion of platelet granules. Lyn knock-out or an isoform-nonselective SFK inhibitor, PP2, inhibited platelet secretion of both dense and alpha granules and the secretion-dependent platelet aggregation induced by thrombin, collagen, and thromboxane A(2). The inhibitory effect of Lyn knock-out on platelet aggregation was reversed by supplementing granule content ADP, indicating that the primary role of Lyn is to stimulate granule secretion. Inhibitory effect of PP2 on platelet aggregation induced by thrombin and thromboxane A(2) were also reversed by supplementing ADP. Furthermore, PP2 treatment or Lyn knock-out diminished agonist-induced Akt activation and cyclic GMP production. The inhibitory effect of PP2 or Lyn knock-out on platelet response can be corrected by supplementing cyclic GMP. These data indicate that Lyn stimulates platelet secretion by activating the phosphoinositide 3-kinase-Akt-nitric oxide (NO)-cyclic GMP pathway and also provide an explanation why Lyn can both stimulate and inhibit platelet activation.

The catalytic subunit of protein phosphatase 1 gamma regulates thrombin-induced murine platelet alpha(IIb)beta(3) function

Background

Hemostasis and thrombosis are regulated by agonist-induced activation of platelet integrin α_IIbβ₃. Integrin activation, in turn is mediated by cellular signaling via protein kinases and protein phosphatases. Although the catalytic subunit of protein phosphatase 1 (PP1c) interacts with α_IIbβ₃, the role of PP1c in platelet reactivity is unclear.

Methodology/Principal Findings

Using γ isoform of PP1c deficient mice (PP1cγ^−/−), we show that the platelets have moderately decreased soluble fibrinogen binding and aggregation to low concentrations of thrombin or protease-activated receptor 4 (PAR4)-activating peptide but not to adenosine diphosphate (ADP), collagen or collagen-related peptide (CRP). Thrombin-stimulated PP1cγ^−/− platelets showed decreased α_IIbβ₃ activation despite comparable levels of α_IIbβ₃, PAR3, PAR4 expression and normal granule secretion. Functions regulated by outside-in integrin α_IIbβ₃ signaling like adhesion to immobilized fibrinogen and clot retraction were not altered in PP1cγ^−/− platelets. Thrombus formation induced by a light/dye injury in the cremaster muscle venules was significantly delayed in PP1cγ^−/− mice. Phosphorylation of glycogen synthase kinase (GSK3)β-serine 9 that promotes platelet function, was reduced in thrombin-stimulated PP1cγ^−/− platelets by an AKT independent mechanism. Inhibition of GSK3β partially abolished the difference in fibrinogen binding between thrombin-stimulated wild type and PP1cγ^−/− platelets.

Conclusions/Significance

These studies illustrate a role for PP1cγ in maintaining GSK3β-serine9 phosphorylation downstream of thrombin signaling and promoting thrombus formation via fibrinogen binding and platelet aggregation.

Lipopolysaccharide stimulates platelet secretion and potentiates platelet aggregation via TLR4/MyD88 and the cGMP-dependent protein kinase pathway

Bacterial LPS induces rapid thrombocytopenia, hypotension, and sepsis. Although growing evidence suggests that platelet activation plays a critical role in LPS-induced thrombocytopenia and tissue damage, the mechanism of LPS-mediated platelet activation is unclear. In this study, we show that LPS stimulates platelet secretion of dense and alpha granules as indicated by ATP release and P-selectin expression, and thus enhances platelet activation induced by low concentrations of platelet agonists. Platelets express components of the LPS receptor-signaling complex, including TLR (TLR4), CD14, MD2, and MyD88, and the effect of LPS on platelet activation was abolished by an anti-TLR4-blocking Ab or TLR4 knockout, suggesting that the effect of LPS on platelet aggregation requires the TLR4 pathway. Furthermore, LPS-potentiated thrombin- and collagen-induced platelet aggregation and FeCl(3)-induced thrombus formation were abolished in MyD88 knockout mice. LPS also induced cGMP elevation and the stimulatory effect of LPS on platelet aggregation was abolished by inhibitors of NO synthase and the cGMP-dependent protein kinase (PKG). LPS-induced cGMP elevation was inhibited by an anti-TLR4 Ab or by TLR4 deficiency, suggesting that activation of the cGMP/protein kinase G pathway by LPS involves the TLR4 pathway. Taken together, our data indicate that LPS stimulates platelet secretion and potentiates platelet aggregation through a TLR4/MyD88- and cGMP/PKG-dependent pathway.

Hyperbaric oxygen treatment induces platelet aggregation and protein release, without altering expression of activation molecules

OBJECTIVES

To investigate the effect of hyperbaric oxygen (HBO) on platelet physiology.

DESIGN AND METHODS

Human platelets were exposed to HBO (97.7% O(2), balance CO(2) at 2.2 ata) or control (CON; 5% CO(2), balance air at 1 ata) for 90 min, and analyzed for aggregation, protein release, ()NO production, and activation.

RESULTS

HBO induced 29.8+/-3.0% of platelets to aggregate compared with CON (5.5+/-0.9%). Proteins observed to be released in greater abundance from HBO- compared with CON-treated platelets included 14-3-3 zeta and alpha-2-macroglobulin. Release of ()NO by platelets was unaffected following exposure to HBO, as was platelet activation as measured by surface expression of PECAM-1, CD62P and the activated form of alpha(IIB)beta(IIIa).

CONCLUSIONS

Exposure to HBO induces both platelet aggregation and protein release. Further study will better define the precise mechanisms and effects of HBO on platelet activation.

Two distinct roles of mitogen-activated protein kinases in platelets and a novel Rac1-MAPK-dependent integrin outside-in retractile signaling pathway

Mitogen-activated protein kinases (MAPK), p38, and extracellular stimuli-responsive kinase (ERK), are acutely but transiently activated in platelets by platelet agonists, and the agonist-induced platelet MAPK activation is inhibited by ligand binding to the integrin alpha(IIb)beta(3). Here we show that, although the activation of MAPK, as indicated by MAPK phosphorylation, is initially inhibited after ligand binding to integrin alpha(IIb)beta(3), integrin outside-insignaling results in a late but sustained activation of MAPKs in platelets. Furthermore, we show that the early agonist-induced MAPK activation and the late integrin-mediated MAPK activation play distinct roles in different stages of platelet activation. Agonist-induced MAPK activation primarily plays an important role in stimulating secretion of platelet granules, while integrin-mediated MAPK activation is important in facilitating clot retraction. The stimulatory role of MAPK in clot retraction is mediated by stimulating myosin light chain (MLC) phosphorylation. Importantly, integrin-dependent MAPK activation, MAPK-dependent MLC phosphorylation, and clot retraction are inhibited by a Rac1 inhibitor and in Rac1 knockout platelets, indicating that integrin-induced activation of MAPK and MLC and subsequent clot retraction is Rac1-dependent. Thus, our results reveal 2 different activation mechanisms of MAPKs that are involved in distinct aspects of platelet function and a novel Rac1-MAPK-dependent cell retractile signaling pathway.

Signaling-mediated functional activation of inducible nitric-oxide synthase and its role in stimulating platelet activation

Nitric oxide (NO) is a short lived secondary messenger, synthesized by nitric-oxide synthases (NOS). It is believed that the activity of inducible NOS (iNOS) is regulated primarily at the transcription level by inducing expression of iNOS mRNA and protein, which then continuously produces NO, until its degradation. Platelets do not have the nuclear transcriptional regulatory mechanisms of the iNOS gene and are believed to generate NO in response to agonist stimulation via endothelial NOS (eNOS). However, here we show that agonist-induced NO production is only partially eNOS-dependent and is also mediated by iNOS. Platelet agonist-induced NO production is significantly reduced in iNOS-knockout platelets. Platelet NO production occurs within seconds after agonist addition and is not accompanied by changes in iNOS protein levels, indicating a signaling-mediated functional activation mechanism of iNOS. Importantly, iNOS knock-out and iNOS inhibitors reduce agonist-induced platelet secretion and aggregation and cGMP levels, indicating that iNOS activation is important in stimulating platelets via the newly identified NO-cGMP-dependent platelet secretion pathway. Furthermore, iNOS knock-out mice have prolonged bleeding time, suggesting that this novel mode of regulation of iNOS activity plays a physiologically relevant role in hemostasis.

RGT, a synthetic peptide corresponding to the integrin beta 3 cytoplasmic C-terminal sequence, selectively inhibits outside-in signaling in human platelets by disrupting the interaction of integrin alpha IIb beta 3 with Src kinase

Mutational analysis has established that the cytoplasmic tail of the integrin beta 3 subunit binds c-Src (termed as Src in this study) and is critical for bidirectional integrin signaling. Here we show in washed human platelets that a cell-permeable, myristoylated RGT peptide (myr-RGT) corresponding to the integrin beta 3 C-terminal sequence dose-dependently inhibited stable platelet adhesion and spreading on immobilized fibrinogen, and fibrin clot retraction as well. Myr-RGT also inhibited the aggregation-dependent platelet secretion and secretion-dependent second wave of platelet aggregation induced by adenosine diphosphate, ristocetin, or thrombin. Thus, myr-RGT inhibited integrin outside-in signaling. In contrast, myr-RGT had no inhibitory effect on adenosine diphosphate-induced soluble fibrinogen binding to platelets that is dependent on integrin inside-out signaling. Furthermore, the RGT peptide induced dissociation of Src from integrin beta 3 and dose-dependently inhibited the purified recombinant beta 3 cytoplasmic domain binding to Src-SH3. In addition, phosphorylation of the beta 3 cytoplasmic tyrosines, Y(747) and Y(759), was inhibited by myr-RGT. These data indicate an important role for beta 3-Src interaction in outside-in signaling. Thus, in intact human platelets, disruption of the association of Src with beta 3 and selective blockade of integrin alpha IIb beta 3 outside-in signaling by myr-RGT suggest a potential new antithrombotic strategy.

Unresolved roles of platelet nitric oxide synthase

Endothelial-derived nitric oxide (NO) is a key regulator of platelet function, inhibiting both adhesion to the extracellular matrix and aggregation at sites of vascular injury. Platelets also have the capacity to synthesize and release bioactive NO, which is thought to make a significant contribution to the vascular pool of NO. The regulation of platelet NO production is poorly understood and studies examining the physiological role of platelet-derived NO have produced contradictory and controversial findings. In the present article, we discuss the current understanding of the biochemical and molecular regulation of platelet NO synthesis and outline the potential physiological and clinical significance of this molecule.

The role of Akt in the signaling pathway of the glycoprotein Ib-IX induced platelet activation

The platelet von Willebrand factor (vWF) receptor, glycoprotein Ib-IX (GPIb-IX), mediates platelet adhesion and induces signaling leading to integrin activation. Phosphoinositol 3-kinase (PI3K) is important in GPIb-IX-mediated signaling. PI3K-dependent signaling mechanisms, however, are unclear. We show that GPIb-IX-induced platelet aggregation and stable adhesion under flow were impaired in mouse platelets deficient in PI3K effectors, Akt1 and Akt2, and in human platelets treated with an Akt inhibitor, SH-6. Akt1 and Akt2 play important roles in early GPIb-IX signaling independent of Syk, adenosine diphosphate (ADP), or thromboxane A2 (TXA2), in addition to their recognized roles in ADP- and TXA2-dependent secondary amplification pathways. Knockout of Akt1 or Akt2 diminished platelet spreading on vWF but not on immobilized fibrinogen. Thus, Akt1 and Akt2 are both required only in the GPIb-IX-mediated integrin activation (inside-out signaling). In contrast, PI3K inhibitors abolished platelet spreading on both vWF and fibrinogen, indicating a role for PI3K in integrin outside-in signaling distinct from that in GPIb-IX-mediated inside-out signaling. Furthermore, Akt1- or Akt2-deficiency diminished vWF-induced cGMP elevation, and their inhibitory effects on GPIb-IX-dependent platelet adhesion were reversed by exogenous cGMP. Thus, Akt1 and Akt2 mediate GPIb-IX signaling via the cGMP-dependent signaling pathway.

Platelet-Derived Nitric Oxide Signaling and Regulation

Nitric oxide (NO) exerts important vasodilatory, antiplatelet, antioxidant, antiadhesive, and antiproliferative effects. Although endothelium derived NO has been shown to be of prime importance in cardio- and vasculoprotection, until recently little was known about the role of platelet-derived NO. New evidence suggests that NO synthesized by platelets regulates platelet functions, in particular suppressing platelet activation and intravascular thrombosis. Moreover, platelet NO biosynthesis may be decreased in patients with cardiovascular risk factors or with coronary heart disease, and this may contribute to arterial thrombotic disease in these patients. Here, we review the current state of knowledge as regards the role of platelet-derived NO, both in normal physiology and in cardiovascular disease states, and compare platelet NO signaling and regulation with that in endothelial cells.

Genetic and pharmacologic evidence that Rac1 GTPase is involved in regulation of platelet secretion and aggregation

BACKGROUND

Rac1 GTPase, a member of the Ras-related Rho GTPase family, is the major Rac isoform present in platelets and has been shown to be involved in cell actin cytoskeleton reorganization and adhesion. Agonists that induce platelet secretion and aggregation also activate Rac1 GTPase, raising the possibility that Rac1 GTPase may be involved in regulation of platelet function.

OBJECTIVES

To rigorously define the role of Rac1 in platelet regulation.

METHODS

We have used a dual approach of gene targeting in mice and pharmacologic inhibition of Rac1 by NSC23766, a rationally designed specific small molecule inhibitor, to study the role of Rac1 in platelet function.

RESULTS

Platelets from mice as well as human platelets treated with NSC23766 exhibited a significant decrease in: (i) active Rac1 species and phosphorylation of the Rac effector, p21-activated kinase; (ii) expression of P-selectin and secretion of adenosine triphosphate induced by thrombin or U46619; and (iii) aggregation induced by adenosine 5'-diphosphate, collagen, thrombin and U46619, a stable analog of thromboxane A(2). NSC23766 did not alter the cAMP or cGMP levels in platelets. Consistent with the requirement of Rac1 for normal platelet function, the bleeding times in Rac1(-/-) mice or mice given NSC23766 were significantly prolonged.

CONCLUSIONS

Our data show that deficiency or inhibition of Rac1 GTPase blocks platelet secretion. The inhibition of secretion, at least in part, is responsible for diminished platelet aggregation and prolonged bleeding times observed in Rac1 knockout or Rac1 inhibitor-treated mice.

Regulation of protease-activated receptor (PAR) 1 and PAR4 signaling in human platelets by compartmentalized cyclic nucleotide actions

Thrombin potently regulates human platelets by the G protein-coupled receptors protease-activated receptor (PAR) 1 and PAR4. Platelet activation by thrombin and other agonists is broadly inhibited by prostacyclin and nitric oxide acting through adenylyl and guanylyl cyclases to elevate cAMP and cGMP levels, respectively. Using forskolin and YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole] to selectively activate the adenylyl and guanylyl cyclases, respectively, and the membrane-permeable analogs N(6),2'-O-dibutyryladenosine-3'-5'-cAMP (dibutyryl-cAMP) and 8-(4-parachlorophenylthoi)-cGMP (8-pCPT-cGMP), we sought to identify key antiplatelet steps for cyclic nucleotide actions in blocking platelet activation by PAR1 versus PAR4. Platelet aggregation by PAR1 or PAR4 was inhibited with similar EC(50) of 1.2 to 2.1 microM forskolin, 31 to 33 microM YC-1, 57 to 150 microM dibutyryl-cAMP, and 220 to 410 microM 8-pCPT-cGMP. There was a marked left shift in the inhibitory potencies of forskolin and YC-1 for alpha-granule release and glycoprotein IIbIIIa/integrin alphaIIbbeta3 activation (i.e., EC(50) of 1-60 and 40-1300 nM, respectively) that was not observed for dibutyryl-cAMP and 8-pCPT-cGMP (i.e., EC(50) of 200-600 and 40-140 microM, respectively). This inhibition was essentially instantaneous, and measurements of cyclic nucleotide levels and kinase activities support a model of compartmentation involving the cyclic nucleotide effectors and regulators and the key molecular targets for this platelet inhibition. The different sensitivities of PAR1 and PAR4 to inhibition of calcium mobilization and dense granule release identify key antiplatelet steps for cyclic nucleotide actions and are consistent with the signaling models for these receptors. Specifically, PAR4 inhibition depends on the regulation of both calcium mobilization and dense granule release, and PAR1 inhibition depends predominantly on the regulation of dense granule release.

Signaling and regulation of the platelet glycoprotein Ib-IX-V complex

PURPOSE OF REVIEW

The platelet adhesion receptor, the glycoprotein Ib-IX-V complex, not only mediates platelet adhesion but also transmits signals leading to platelet activation, aggregation and secretion. Significant progress has been made recently on the signaling pathways and regulatory mechanisms involving glycoprotein Ib-IX-V function.

RECENT FINDINGS

The interaction of glycoprotein Ib-IX-V with its ligand, von Willebrand factor, is dually controlled by von Willebrand factor conformation and intracellular signal-mediated regulation of glycoprotein Ib-IX-V receptor function that requires the zeta isoform of the 14-3-3 protein family (14-3-3zeta). Glycoprotein Ib-IX-V signaling is mediated by the Src family of protein kinases, phospholipase C, calcium elevation, phosphoinositol 3-kinase, and multiple amplification mechanisms including the nitric oxide-cGMP pathway, the mitogen-activated protein kinase pathway, the immunoreceptor tyrosine-based activation motif pathway, and ADP and thromboxane A2 pathways.

SUMMARY

Progress in understanding the mechanism(s) regulating glycoprotein Ib-IX-V should help develop inhibitors and modifiers that interfere or augment its von Willebrand factor binding function and thus be useful for treating thrombosis and bleeding disorders. Characterization of intracellular molecules and pathways in glycoprotein Ib-IX-V signaling has implications in the development of new agents and for the use of existing drugs that affect glycoprotein Ib-IX-V signaling.

A guide to murine platelet structure, function, assays, and genetic alterations

Platelets play an important role in hemostasis, thrombosis and several other biological processes. The adaptability of mice to genetic manipulation and their genetic similarity to humans has resulted in a plethora of murine models to study platelet function. Although murine platelets differ from human platelets with regard to size, number and structure, functionally they are very similar. Thus, studies which employed these model systems have greatly improved our current understanding of the contribution of platelets to hemostasis and thrombosis. This review presents general recommendations with respect to collection, isolation and processing of murine platelets. It also describes the assays currently available to study platelet function and critically assesses their utility. The extensive literature on the effects of genetic alterations on murine platelet function is considered in detail. This review is intended to provide a convenient source of reference for platelet investigators.

Regulation of leukocyte degranulation by cGMP-dependent protein kinase and phosphoinositide 3-kinase: potential roles in phosphorylation of target membrane SNARE complex proteins in rat mast cells

We examined the roles of cGMP-dependent protein kinase (PKG) and PI3K in degranulation induced by fMLF and by FcepsilonRI cross-linking. In rat basophilic leukemia-2H3 cells expressing formyl peptide receptor, the PKG inhibitors KT5823 and Rp-8-Br-PET-cGMP, as well as the PI3K inhibitor LY294002, reduced agonist-stimulated beta-hexosaminidase release in a dose-dependent manner. These inhibitors also abolished vesicular fusion with the plasma membrane, as evidenced by diminished annexin V staining. Agonist-induced degranulation was completely blocked when LY294002 was applied together with one of the PKG inhibitors, suggesting an additive and possibly synergistic effect. In contrast, the PKG inhibitors did not affect fMLF-induced intracellular calcium mobilization and Akt phosphorylation. Likewise, LY294002 did not alter fMLF-induced elevation of intracellular cGMP concentration, and the inhibitory effect of LY294002 was not reversed by a cell-permeable analog of cGMP. Treatment with fMLF induced phosphorylation of soluble N-ethylmaleimide-sensitive factor-attachment protein (SNAP)-23, syntaxins 2, 4, and 6, and Monc18-3. The induced phosphorylation of SNAP-23 and syntaxins 2 and 4 was blocked by Rp-8-Br-PET-cGMP and LY294002. However, LY294002 was less effective in inhibiting Munc18-3 phosphorylation. The induced phosphorylation of syntaxin 6 was not effectively blocked by either Rp-8-Br-PET-cGMP or LY294002. Treatment of human neutrophils with the PKG inhibitors and LY294002 reduced enzyme release from primary, secondary, and tertiary granules. These results suggest that PKG and PI3K are involved in degranulation, possibly through phosphorylation of target membrane SNAP receptor proteins and their binding proteins.

Thrombin stimulation of p38 MAP kinase in human platelets is mediated by ADP and thromboxane A2 and inhibited by cGMP/cGMP-dependent protein kinase

p38 MAP kinase in human platelets is activated by platelet agonists including thrombin, thromboxane A2 (TxA2), ADP, and others. However, both upstream mechanisms of p38 MAP kinase activation, and their downstream sequelae, are presently controversial and essentially unclear. Certain studies report sequential activation of cGMP-dependent protein kinase (PKG) and p38/ERK pathways by platelet agonists, leading to integrin activation and secretion, whereas others establish an essential role of Src/ERK-mediated TxA2 generation for fibrinogen receptor activation in human platelets. Here, we show that ADP secreted from platelet-dense granules, and subsequent activation of P2Y12 receptors, as well as TxA2 release are important upstream mediators of p38 MAP kinase activation by thrombin. However, p38 MAP kinase activation did not significantly contribute to calcium mobilization, P-selectin expression, alphaIIbbeta3 integrin activation, and aggregation of human platelets in response to thrombin. Finally, PKG activation did not stimulate, but rather inhibited, p38 MAP kinase in human platelets.

IRAG mediates NO/cGMP-dependent inhibition of platelet aggregation and thrombus formation

Defective regulation of platelet activation/aggregation is a predominant cause for arterial thrombosis, the major complication of atherosclerosis triggering myocardial infarction and stroke. A central regulatory pathway conveying inhibition of platelet activation/aggregation is nitric oxide (NO)/cyclic GMP (cGMP) signaling by cGMP-dependent protein kinase I (cGKI). However, the regulatory cascade downstream of cGKI mediating platelet inhibition is still unclear. Here, we show that the inositol-1,4,5-trisphosphate receptor-associated cGMP kinase substrate (IRAG) is abundantly expressed in platelets and assembled in a macrocomplex together with cGKIbeta and the inositol-1,4,5-trisphosphate receptor type I (InsP3RI). cGKI phosphorylates IRAG at Ser664 and Ser677 in intact platelets. Targeted deletion of the IRAG-InsP3RI interaction in IRAGDelta12/Delta12 mutant mice leads to a loss of NO/cGMP-dependent inhibition of fibrinogen-receptor activation and platelet aggregation. Intracellular calcium transients were not affected by DEA/NO or cGMP in mutant platelets. Furthermore, intravital microscopy shows that NO fails to prevent arterial thrombosis of the injured carotid artery in IRAGDelta12/Delta12 mutants. These findings reveal that interaction between IRAG and InsP3RI has a central role in NO/cGMP-dependent inhibition of platelet aggregation and in vivo thrombosis.

A phosphoinositide 3-kinase-AKT-nitric oxide-cGMP signaling pathway in stimulating platelet secretion and aggregation

Phosphoinositide 3-kinase (PI3K) and Akt play important roles in platelet activation. However, the downstream mechanisms mediating their functions are unclear. We have recently shown that nitric-oxide (NO) synthase 3 and cGMP-dependent protein kinase stimulate platelet secretion and aggregation. Here we show that PI3K-mediated Akt activation plays an important role in agonist-stimulated platelet NO synthesis and cGMP elevation. Agonist-induced elevation of NO and cGMP was inhibited by Akt inhibitors and reduced in Akt-1 knock-out platelets. Akt-1 knock-out or Akt inhibitor-treated platelets showed reduced platelet secretion and aggregation in response to low concentrations of agonists, which can be reversed by low concentrations of 8-bromo-cGMP or sodium nitroprusside (an NO donor). Similarly, PI3K inhibitors diminished elevation of cGMP and inhibited platelet secretion and the second wave platelet aggregation, which was also partially reversed by 8-bromo-cGMP. These results indicate that the NO-cGMP pathway is an important downstream mechanism mediating PI3K and Akt signals leading to platelet secretion and aggregation. Conversely, the PI3K-Akt pathway is the major upstream mechanism responsible for activating the NO-cGMP pathway in platelets. Thus, this study delineates a novel platelet activation pathway involving sequential activation of PI3K, Akt, nitric-oxide synthase 3, sGC, and cGMP-dependent protein kinase.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Stimulatory Roles of Nitric-oxide Synthase 3 and Guanylyl Cyclase in Platelet Activation

Nitric oxide (NO) stimulates soluble guanylyl cyclase and, thus, enhances cyclic guanosine monophosphate (cGMP) levels. It is a currently prevailing concept that NO inhibits platelet activation. This concept, however, does not fully explain why platelet agonists stimulate NO production. Here we show that a major platelet NO synthase (NOS) isoform, NOS3, plays a stimulatory role in platelet secretion and aggregation induced by low doses of platelet agonists. Furthermore, we show that NOS3 promotes thrombosis in vivo. The stimulatory role of NOS is mediated by soluble guanylyl cyclase and results from a cGMP-dependent stimulation of platelet granule secretion. These findings delineate a novel signaling pathway in which agonists sequentially activate NOS3, elevate cGMP, and induce platelet secretion and aggregation. Our data also suggest that NO plays a biphasic role in platelet activation, a stimulatory role at low NO concentrations and an inhibitory role at high NO concentrations.