Concentration and regulation of cyclic nucleotides, cyclic-nucleotide-dependent protein kinases and one of their major substrates in human platelets. Estimating the rate of cAMP-regulated and cGMP-regulated protein phosphorylation in intact cells

Surface Engineering for Endothelium-Mimicking Functions to Combat Infection and Thrombosis in Extracorporeal Life Support Technologies

Blood-contacting medical devices routinely fail from the cascading effects of biofouling toward infection and thrombosis. Nitric oxide (NO) is an integral part of endothelial homeostasis, maintaining platelet quiescence and facilitating oxidative/nitrosative stress against pathogens. Recently, it is shown that the surface evolution of NO can mediate cell-surface interactions. However, this technique alone cannot prevent the biofouling inherent in device failure with dynamic blood-contacting applications. This work proposes an endothelium-mimicking surface design pairing controlled NO release with an inherently antifouling polyethylene glycol interface (NO+PEG). This simple, robust, and scalable platform develops surface-localized NO availability with surface hydration, leading to a significant reduction in protein adsorption as well as bacteria/platelet adhesion. Further in vivo thrombogenicity studies show a decrease in thrombus formation on NO+PEG interfaces, with preservation of circulating platelet and white blood cell counts, maintenance of activated clotting time, and reduced coagulation cascade activation. It is anticipated that this bio-inspired surface design will enable a facile alternative to existing surface technologies to address clinical manifestations of infection and thrombosis in dynamic blood-contacting environments.

Cucurbitacin C suppresses the progression of pancreatic ductal adenocarcinoma via inhibition of the cGMP-PKG-VASP axis

Pancreatic ductal adenocarcinoma (PDAC) remains one of the most devastating diseases; it has a considerably poor prognosis and may become the second most lethal malignancy in the next 10 years. Chemotherapeutic resistance is common in PDAC; thus, it is necessary to exploit effective alternative drugs. In recent years, traditional folk medicines and their extracts have shown great potential in cancer treatment. The seed of Lagenaria siceraria (Molina) Standl. is a traditional medicine in Asia. Because of its analgesic effects and ability to reduce swelling, it is often used as an adjuvant treatment for abdominal tumors. Cucurbitacin compounds are extracts abundant in Lagenaria siceraria (Molina) Standl. Here, we found that cucurbitacin C (CuC), a member of the cucurbitacin family, has apparent anti-PDAC therapeutic properties. CuC decreased the viability and suppressed the proliferation of PDAC cells in a time- and dose-dependent manner. Further studies revealed that CuC inhibited cell migration and invasion by inhibiting epithelial-mesenchymal transition (EMT). In addition, G2/M arrest was induced, and the apoptotic pathway was activated. Transcriptomic and bioinformatic analyses showed that CuC inhibited the cGMP-PKG-VASP axis, increasing the content of cGMP to restore tumor characteristics. The antitumor activity of CuC in vivo was verified through animal experiments, and no obvious side effects were observed. Overall, our study indicates a candidate therapeutic compound for PDAC that is worthy of further development.

Complementary Sets of Autoantibodies Induced by SARS-CoV-2, Adenovirus and Bacterial Antigens Cross-React with Human Blood Protein Antigens in COVID-19 Coagulopathies

COVID-19 patients often develop coagulopathies including microclotting, thrombotic strokes or thrombocytopenia. Autoantibodies are present against blood-related proteins including cardiolipin (CL), serum albumin (SA), platelet factor 4 (PF4), beta 2 glycoprotein 1 (β2GPI), phosphodiesterases (PDE), and coagulation factors such as Factor II, IX, X and von Willebrand factor (vWF). Different combinations of autoantibodies associate with different coagulopathies. Previous research revealed similarities between proteins with blood clotting functions and SARS-CoV-2 proteins, adenovirus, and bacterial proteins associated with moderate-to-severe COVID-19 infections. This study investigated whether polyclonal antibodies (mainly goat and rabbit) against these viruses and bacteria recognize human blood-related proteins. Antibodies against SARS-CoV-2 and adenovirus recognized vWF, PDE and PF4 and SARS-CoV-2 antibodies also recognized additional antigens. Most bacterial antibodies tested (group A streptococci [GAS], staphylococci, Escherichia coli [E. coli], Klebsiella pneumoniae, Clostridia, and Mycobacterium tuberculosis) cross-reacted with CL and PF4. while GAS antibodies also bound to F2, Factor VIII, Factor IX, and vWF, and E. coli antibodies to PDE. All cross-reactive interactions involved antibody-antigen binding constants smaller than 100 nM. Since most COVID-19 coagulopathy patients display autoantibodies against vWF, PDE and PF4 along with CL, combinations of viral and bacterial infections appear to be necessary to initiate their autoimmune coagulopathies.

Mesenchymal stromal cells mitigate liver damage after extended resection in the pig by modulating thrombospondin-1/TGF-β

Post-surgery liver failure is a serious complication for patients after extended partial hepatectomies (ePHx). Previously, we demonstrated in the pig model that transplantation of mesenchymal stromal cells (MSC) improved circulatory maintenance and supported multi-organ functions after 70% liver resection. Mechanisms behind the beneficial MSC effects remained unknown. Here we performed 70% liver resection in pigs with and without MSC treatment, and animals were monitored for 24 h post surgery. Gene expression profiles were determined in the lung and liver. Bioinformatics analysis predicted organ-independent MSC targets, importantly a role for thrombospondin-1 linked to transforming growth factor-β (TGF-β) and downstream signaling towards providing epithelial plasticity and epithelial-mesenchymal transition (EMT). This prediction was supported histologically and mechanistically, the latter with primary hepatocyte cell cultures. MSC attenuated the surgery-induced increase of tissue damage, of thrombospondin-1 and TGF-β, as well as of epithelial plasticity in both the liver and lung. This suggests that MSC ameliorated surgery-induced hepatocellular stress and EMT, thus supporting epithelial integrity and facilitating regeneration. MSC-derived soluble factor(s) did not directly interfere with intracellular TGF-β signaling, but inhibited thrombospondin-1 secretion from thrombocytes and non-parenchymal liver cells, therewith obviously reducing the availability of active TGF-β.

Potential and limitations of PKA/ PKG inhibitors for platelet studies

Cyclic nucleotides (cAMP and cGMP) and corresponding protein kinases, protein kinase A (PKA) and protein kinase G (PKG), are the main intracellular mediators of endothelium-derived platelet inhibitors. Pharmacological PKA/PKG inhibitors are often used to discriminate between these two kinase activities and to analyze their underlying mechanisms. Previously we showed that all widely used PKG inhibitors (KT5823, DT3, RP isomers) either did not inhibit PKG or inhibited and even activated platelets independently from PKG. In this study, we examined several PKA inhibitors as well as inhibitors of adenylate and guanylate cyclases to reveal their effects on platelets and establish whether they are mediated by PKA/PKG. The commonly used PKA inhibitor H89 inhibited both PKA and PKG but PKA-independently inhibited thrombin-induced platelet activation. In our experiments, KT5720 did not inhibit PKA and had no effect on platelet activation. PKI inhibited PKA activity in platelets but also strongly PKA-independently activated platelets. Inhibition of adenylate and guanylate cyclases may be an alternative approach to analyze PKA/PKG function. Based on our previous and presented data, we conclude that all results where the mentioned PKA inhibitors were used for the analysis of PKA activity in intact platelets should be considered with caution.

Platelet inhibition by P2Y12 antagonists is potentiated by adenosine signalling activators

BACKGROUND AND PURPOSE

P2Y₁₂ receptor antagonists reduce platelet aggregation and the incidence of arterial thrombosis. Adenosine signalling in platelets directly affects cyclic nucleotide tone which we have previously shown to have a synergistic relationship with P2Y₁₂ inhibition. Several studies suggest that ticagrelor inhibits erythrocyte uptake of adenosine and that this could also contribute to its antiplatelet effects. We therefore examined the effects on platelet activation of adenosine signalling activators in combination with the P2Y₁₂ receptor antagonists ticagrelor and prasugrel.

EXPERIMENTAL APPROACH

Human washed platelets, platelet-rich plasma, and whole blood were used to test the interactions between ticagrelor or prasugrel, and adenosine or 5'-N-ethylcarboxamidoadenosine (NECA). Platelet reactivity to thrombin, PAR-1 activation or collagen was assessed by a combination of 96-well plate aggregometry, light transmission aggregometry, whole blood aggregometry, ATP release assay, and levels of cAMP.

KEY RESULTS

The inhibitory effects of ticagrelor and prasugrel on platelet aggregation and ATP release were enhanced in the presence of adenosine or NECA. Isobolographic analysis indicated a powerful synergy between P2Y₁₂ receptor inhibition and adenosine signalling activators. Increased levels of cAMP in platelets were also observed. In all cases, ticagrelor showed similar synergistic effects on platelet inhibition as prasugrel in the presence of adenosine or NECA.

CONCLUSION AND IMPLICATIONS

These results indicate that P2Y₁₂ antagonists have a synergistic relationship with adenosine signalling and that their efficacy may depend partly upon the presence of endogenous adenosine. This effect was common to both prasugrel and ticagrelor despite reports of differences in their effects upon adenosine reuptake.

The effects of ginsenosides on platelet aggregation and vascular intima in the treatment of cardiovascular diseases: From molecular mechanisms to clinical applications

Thrombosis initiated by abnormal platelet aggregation is a pivotal pathological event that precedes most cases of cardiovascular diseases (CVD). Recently, growing evidence indicates that platelet could be a potential target for CVD prevention. However, as the conventional antithrombotic management strategy, applications of current antiplatelet agents are somewhat limited by their various side effects, such as bleeding risk and drug resistance. Hence, efforts have been made to search for agents as complementary therapies. Ginsenoside, the principal active component extracted from Panax ginseng, has gained much attention for its regulations on multiple crucial events of platelet aggregation. From structural characteristics to clinical applications, this review anatomized the intrinsic structure-function relationship of antiplatelet potency of ginsenosides, and the involved signal pathways were specifically summarized. Additionally, the emphasis was placed on clinical studies that investigate the antithrombotic efficacy of ginsenosides in the treatment of CVD. Further, a broad overview of approaches for improving the bioavailability of ginsenosides was concluded. Limitations and prospects of current studies were also discussed. This study may provide some new insights into the systematic understanding of ginsenosides in CVD treatment and lay a foundation for future research.

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Interaction of Nucleotides with a Trinuclear Terbium(III)-Dizinc(II) Complex: Efficient Sensitization of Terbium Luminescence by Guanosine Monophosphate and Application to Real-Time Monitoring of Phosphodiesterase Activity

An in-depth study of the interaction of a trinuclear terbium(III)-dizinc(II) complex with an array of nucleotides differing in the type of nucleobase and number of phosphate groups, as well as cyclic versus acyclic variants, is presented. The study examined the nature of the interaction and the efficiency at which guanine was able to sensitize terbium(III) luminescence. Competitive binding and titration studies were performed to help establish the nature/mode of the interactions. These established that (1) interaction occurs by the coordination of phosphate groups to zinc(II) (in addition to uridine in the case of uridine monophosphate), (2) acyclic nucleotides bind more strongly than cyclic counterparts because of their higher negative charge, (3) guanine-containing nucleotides are able to sensitize terbium(III) luminescence with the efficiency of sensitization following the order guanosine monophosphate (GMP) > guanosine diphosphate > guanosine triphosphate because of the mode of binding, and (4) nucleoside monophosphates bind to a single zinc(II) ion, whereas di- and triphosphates appear to bind in a bridging mode between two host molecules. Furthermore, it has been shown that guanine is a sensitizer of terbium(III) luminescence. On the basis of the ability of GMP to effectively sensitize terbium(III)-based luminescence while cyclic GMP (cGMP) does not, the complex has been utilized to monitor the catalytic conversion of cGMP to GMP by a phosphodiesterase enzyme in real time using time-gated luminescence on a benchtop fluorimeter. The complex has the potential to find broad application in monitoring the activity of enzymes that process nucleotides (co)substrates, including high-throughput drug-screening programs.

Structure-based design, synthesis, and biological evaluation of novel pyrimidinone derivatives as PDE9 inhibitors

The pathological processes of Alzheimer's disease and type 2 diabetes mellitus have been demonstrated to be linked together. Both PDE9 inhibitors and PPARγ agonists such as rosiglitazone exhibited remarkable preclinical and clinical treatment effects for these two diseases. In this study, a series of PDE9 inhibitors combining the pharmacophore of rosiglitazone were discovered. All the compounds possessed remarkable affinities towards PDE9 and four of them have the IC₅₀ values <5 nmol/L. In addition, these four compounds showed low cell toxicity in human SH-SY5Y neuroblastoma cells. Compound 11a, the most effective one, gave the IC₅₀ of 1.1 nmol/L towards PDE9, which is significantly better than the reference compounds PF-04447943 and BAY 73-6691. The analysis of putative binding patterns and binding free energy of the designed compounds with PDE9 may explain the structure—activity relationships and provide evidence for further structural modifications.

TRPC5-eNOS Axis Negatively Regulates ATP-Induced Cardiomyocyte Hypertrophy

Cardiac hypertrophy, induced by neurohumoral factors, including angiotensin II and endothelin-1, is a major predisposing factor for heart failure. These ligands can induce hypertrophic growth of neonatal rat cardiomyocytes (NRCMs) mainly through Ca2+-dependent calcineurin/nuclear factor of activated T cell (NFAT) signaling pathways activated by diacylglycerol-activated transient receptor potential canonical 3 and 6 (TRPC3/6) heteromultimer channels. Although extracellular nucleotide, adenosine 5'-triphosphate (ATP), is also known as most potent Ca2+-mobilizing ligand that acts on purinergic receptors, ATP never induces cardiomyocyte hypertrophy. Here we show that ATP-induced production of nitric oxide (NO) negatively regulates hypertrophic signaling mediated by TRPC3/6 channels in NRCMs. Pharmacological inhibition of NO synthase (NOS) potentiated ATP-induced increases in NFAT activity, protein synthesis, and transcriptional activity of brain natriuretic peptide. ATP significantly increased NO production and protein kinase G (PKG) activity compared to angiotensin II and endothelin-1. We found that ATP-induced Ca2+ signaling requires inositol 1,4,5-trisphosphate (IP3) receptor activation. Interestingly, inhibition of TRPC5, but not TRPC6 attenuated ATP-induced activation of Ca2+/NFAT-dependent signaling. As inhibition of TRPC5 attenuates ATP-stimulated NOS activation, these results suggest that NO-cGMP-PKG axis activated by IP3-mediated TRPC5 channels underlies negative regulation of TRPC3/6-dependent hypertrophic signaling induced by ATP stimulation.

Label-Free Quantitative Phosphoproteomics Reveals Regulation of Vasodilator-Stimulated Phosphoprotein upon Stathmin-1 Silencing in a Pair of Isogenic Colorectal Cancer Cell Lines

In this communication, we present the phosphoproteome changes in an isogenic pair of colorectal cancer cell lines, viz., the poorly metastatic HCT-116 and the highly metastatic derivative E1, upon stathmin-1 (STMN1) knockdown. The aim was to better understand how the alterations of the phosphoproteins in these cells are involved in cancer metastasis. After the phosphopeptides were enriched using the TiO₂ HAMMOC approach, comparative proteomics analysis was carried out using sequential window acquisition of all theoretical mass spectra-MS. Following bioinformatics analysis using MarkerView and OneOmics platforms, we identified a list of regulated phosphoproteins that may play a potential role in signaling, maintenance of cytoskeletal structure, and focal adhesion. Among these phosphoproteins, was the actin cytoskeleton regulator protein, vasodilator-stimulated phosphoprotein (VASP), where its change in phosphorylation status was found to be concomitant with STMN1-associated roles in metastasis. We further showed that silencing of stathmin-1 altered the expression, subcellular localization and phosphorylation status of VASP, which suggested that it might be associated with remodeling of the cell cytoskeleton in colorectal cancer metastasis.

Mathematical Modelling of Nitric Oxide/Cyclic GMP/Cyclic AMP Signalling in Platelets

Platelet activation contributes to normal haemostasis but also to pathologic conditions like stroke and cardiac infarction. Signalling by cGMP and cAMP inhibit platelet activation and are therefore attractive targets for thrombosis prevention. However, extensive cross-talk between the cGMP and cAMP signalling pathways in multiple tissues complicates the selective targeting of their activities. We have used mathematical modelling based on experimental data from the literature to quantify the steady state behaviour of nitric oxide (NO)/cGMP/cAMP signalling in platelets. The analysis provides an assessment of NO-induced cGMP synthesis and PKG activation as well as cGMP-mediated cAMP and PKA activation though modulation of phosphodiesterase (PDE2 and 3) activities. Both one- and two-compartment models of platelet cyclic nucleotide signalling are presented. The models provide new insight for understanding how NO signalling to cGMP and indirectly cAMP, can inhibit platelet shape-change, the initial step of platelet activation. Only the two-compartment models could account for the experimental observation that NO-mediated PKA activation can occur when the bulk platelet cAMP level is unchanged. The models revealed also a potential for hierarchical interplay between the different platelet phosphodiesterases. Specifically, the models predict, unexpectedly, a strong effect of pharmacological inhibitors of cGMP-specific PDE5 on the cGMP/cAMP cross-talk. This may explain the successful use of weak PDE5-inhibitors, such as dipyridamole, in anti-platelet therapy. In conclusion, increased NO signalling or PDE5 inhibition are attractive ways of increasing cGMP-cAMP cross-talk selectively in platelets.

Mathematical Techniques for Understanding Platelet Regulation and the Development of New Pharmacological Approaches

Mathematical and computational modeling is currently in the process of becoming an accepted tool in the arsenal of methods utilized for the investigation of complex biological systems. For some problems in the field, like cellular metabolic regulation, neural impulse propagation, or cell cycle, progress is already unthinkable without use of such methods. Mathematical models of platelet signaling, function, and metabolism during the last years have not only been steadily increasing in their number, but have also been providing more in-depth insights, generating hypotheses, and allowing predictions to be made leading to new experimental designs and data. Here we describe the basic approaches to platelet mathematical model development and validation, highlighting the challenges involved. We then review the current theoretical models in the literature and how these are being utilized to increase our understanding of these complex cells.

RXR Ligands Negatively Regulate Thrombosis and Hemostasis

Supplemental Digital Content is available in the text.

Objective—

Platelets have been found to express intracellular nuclear receptors including the retinoid X receptors (RXRα and RXRβ). Treatment of platelets with ligands of RXR has been shown to inhibit platelet responses to ADP and thromboxane A2; however, the effects on responses to other platelet agonists and the underlying mechanism have not been fully characterized.

Approach and Results—

The effect of 9-cis-retinoic acid, docosahexaenoic acid and methoprene acid on collagen receptor (glycoprotein VI [GPVI]) agonists and thrombin-stimulated platelet function; including aggregation, granule secretion, integrin activation, calcium mobilization, integrin αIIbβ3 outside-in signaling and thrombus formation in vitro and in vivo were determined. Treatment of platelets with RXR ligands resulted in attenuation of platelet functional responses after stimulation by GPVI agonists or thrombin and inhibition of integrin αIIbβ3 outside-in signaling. Treatment with 9-cis-retinoic acid caused inhibition of thrombus formation in vitro and an impairment of thrombosis and hemostasis in vivo. Both RXR ligands stimulated protein kinase A activation, measured by VASP S157 phosphorylation, that was found to be dependent on both cAMP and nuclear factor κ-light-chain-enhancer of activated B cell activity.

Conclusions—

This study identifies a widespread, negative regulatory role for RXR in the regulation of platelet functional responses and thrombus formation and describes novel events that lead to the upregulation of protein kinase A, a known negative regulator of many aspects of platelet function. This mechanism may offer a possible explanation for the cardioprotective effects described in vivo after treatment with RXR ligands.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

Huntingtin-associated protein 1 (HAP1) is a cGMP-dependent kinase anchoring protein (GKAP) specific for the cGMP-dependent protein kinase Iβ isoform

Protein-protein interactions are important in providing compartmentalization and specificity in cellular signal transduction. Many studies have hallmarked the well designed compartmentalization of the cAMP-dependent protein kinase (PKA) through its anchoring proteins. Much less data are available on the compartmentalization of its closest homolog, cGMP-dependent protein kinase (PKG), via its own PKG anchoring proteins (GKAPs). For the enrichment, screening, and discovery of (novel) PKA anchoring proteins, a plethora of methodologies is available, including our previously described chemical proteomics approach based on immobilized cAMP or cGMP. Although this method was demonstrated to be effective, each immobilized cyclic nucleotide did not discriminate in the enrichment for either PKA or PKG and their secondary interactors. Hence, with PKG signaling components being less abundant in most tissues, it turned out to be challenging to enrich and identify GKAPs. Here we extend this cAMP-based chemical proteomics approach using competitive concentrations of free cyclic nucleotides to isolate each kinase and its secondary interactors. Using this approach, we identified Huntingtin-associated protein 1 (HAP1) as a putative novel GKAP. Through sequence alignment with known GKAPs and secondary structure prediction analysis, we defined a small sequence domain mediating the interaction with PKG Iβ but not PKG Iα. In vitro binding studies and site-directed mutagenesis further confirmed the specificity and affinity of HAP1 binding to the PKG Iβ N terminus. These data fully support that HAP1 is a GKAP, anchoring specifically to the cGMP-dependent protein kinase isoform Iβ, and provide further evidence that also PKG spatiotemporal signaling is largely controlled by anchoring proteins.

Effect of Cordycepin-Enriched WIB801C from Cordyceps militaris Suppressing Fibrinogen Binding to Glycoprotein IIb/IIIa

In this study, we investigated the effects of cordycepin-enriched (CE)-WIB801C, a n-butanol extract of Cordyceps militaris-hypha on collagen-stimulated platelet aggregation. CE-WIB801C dose dependently inhibited collagen-induced platelet aggregation, and had a synergistic effect together with cordycepin (W-cordycepin) from CE-WIB801C on the inhibition of collagen-induced platelet aggregation. CE-WIB801C and cordycepin stimulated the phosphorylation of VASP (Ser(157)) and the dephosphorylation of PI3K and Akt, and inhibited the binding of fibrinogen to glycoprotein IIb/IIIa (αIIb/β3) and the release of ATP and serotonin in collagen-induced platelet aggregation. A-kinase inhibitor Rp-8-Br-cAMPS reduced CE-WIB801C-, and cordycepin-increased VASP (Ser(157)) phosphorylation, and increased CE-WIB801C-, and cordycepin-inhibited the fibrinogen binding to αIIb/β3. Therefore, we demonstrate that CE-WIB801C-, and cordycepin-inhibited fibrinogen binding to αIIb/β3 are due to stimulation of cAMP-dependent phosphorylation of VASP (Ser(157)), and inhibition of PI3K/Akt phosphorylation. These results strongly indicate that CE-WIB801C and cordycepin may have preventive or therapeutic potential for platelet aggregation-mediated diseases, such as thrombosis, myocardial infarction, atherosclerosis, and ischemic cerebrovascular disease.

Specific inhibitory effects of the NO donor MAHMA/NONOate on human platelets

Nitric oxide (NO) is a physiological inhibitor of platelet function and has vaso-dilating effects. Therefore, synthesized NO releasing agents are used e.g. in cardiovascular medicine. The aim of this study was to characterise specific effects of the short living agent MAHMA/NONOate, a NO donor of the diazeniumdiolate class, on human platelets. Whole blood was obtained from healthy volunteers. In washed human platelets, the MAHMA/NONOate induced phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) and cyclic nucleotide production were studied by Western Blot and by enzyme immunoassay kits. Agonist induced aggregation was measured in platelet rich plasma. Paired Student׳s t-test was used for statistical analysis. MAHMA/NONOate significantly stimulated platelet VASP phosphorylation in a concentration dependent manner and increased intracellular cGMP, but not cAMP levels, transiently. ODQ, a specific inhibitor of the soluble guanylyl cyclase, completely prevented VASP phosphorylation induced by low MAHMA/NONOate concentrations (5nM-15nM). The effects of higher concentrations (30-200nM) were only partially inhibited by ODQ. MAHMA/NONOate reduced platelet aggregation induced by low doses of agonists (2µM ADP, 0.5µg/mL collagen, 5µM TRAP-6) in a concentration dependent manner. MAHMA/NONOate leads to a rapid and transient activation of platelet inhibitory systems, accompanied by decreased platelet aggregation induced by low dose agonists. At low MAHMA/NONOate concentrations, the effects are cGMP dependent and at higher concentrations additionally cGMP independent. The substance could be of interest for clinical situations requiring transient and subtotal inhibition of platelet function.

Antiplatelet effects of caffeic acid due to Ca(2＋) mobilizationinhibition via cAMP-dependent inositol-1, 4, 5-trisphosphate receptor phosphorylation

AIM

In this study, we investigated the effects of caffeic acid (CAFA), a phenolic acid, on Ca(2＋)-antagonistic cyclic nucleotides associated with the phosphorylation of inositol 1,4,5-trisphosphate receptor (IP3R) and vasodilator-stimulated phosphoprotein (VASP) and the thromboxane A2 (TXA2)-associated microsomal cyclooxygenase-1 (COX-1) activity in collagen (10 μg/mL)-stimulated platelet aggregation.

METHODS

Washed platelets (10(8)/mL) obtained from Sprague-Dawley rats (6-7 weeks old, male) were preincubated for 3 minutes at 37℃ in the presence of 2 mM exogenous CaCl2 with or without CAFA or other materials, stimulated with collagen (10 μg/mL) for 5 minutes, then used to determine the levels of intracellular cytosolic Ca(2＋) ([Ca(2＋)]i), TXA2, cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), COX-1 activity, VASP and IP3R phosphorylation.

RESULTS

CAFA dose-dependently inhibited collagen-induced platelet aggregation and suppressed the production of TXA2, an aggregation-inducing autacoid associated with the strong inhibition of COX-1 in platelet microsomes exhibiting cytochrome C reductase activity. CAFA dose-dependently inhibited collagen-elevated [Ca(2＋)]i mobilization, which was increased by a cAMP-dependent protein kinase (A-kinase) inhibitor, Rp-8-Br-cAMPS, but not a cGMP-dependent protein kinase (G-kinase) inhibitor, Rp-8-Br-cGMPS. In addition, CAFA significantly increased the formation of cAMP and cGMP, intracellular Ca(2＋)-antagonists that function as aggregation-inhibiting molecules. CAFA increased IP3R (320 kDa) phosphorylation, indicating the inhibition of IP3-mediated Ca(2＋) release from internal stores (i.e. the dense tubular system) via the IP3R on collagen-activated platelets. Furthermore, CAFA-induced IP3R phosphorylation was strongly inhibited by an A-kinase inhibitor, Rp-8-Br-cAMPS, but only mildly inhibited by a G-kinase inhibitor, Rp-8-Br-cGMPS. These results suggest that the inhibition of [Ca(2＋)]i mobilization by CAFA is resulted from the cAMP/A-kinase-dependent phosphorylation of IP3R. CAFA elevated the phosphorylation of VASP-Ser(157), an A-kinase substrate, but not the phosphorylation of VASP-Ser(239), a G-kinase substrate. We demonstrate that CAFA increases cAMP and subsequently phosphorylates both IP3R and VASP-Ser(157) through A-kinase activation to inhibit [Ca(2＋)]i mobilization and TXA2 production via the inhibition of the COX-1 activity.

CONCLUSIONS

These results strongly indicate that CAFA is a potent beneficial compound that elevates the level of cAMP-dependent protein phosphorylation in collagen-platelet interactions, which may result in the prevention of platelet aggregation-mediated thrombotic diseases.

The mediation of platelet quiescence by NO-releasing polymers via cGMP-induced serine 239 phosphorylation of vasodilator-stimulated phosphoprotein

Nitric oxide (NO) releasing (NORel) materials have been shown to create localized increases in NO concentration by the release of NO from a diazeniumdiolate-containing or S-nitrosothiol-containing polymer coating and the improvement of extracorporeal circulation (ECC) hemocompatibility. However, the mechanism and, in particular, the platelet upregulation of the NO/cGMP signaling protein, vasodilator-stimulated phosphoprotein phosphorylated at serine 239 (P-VASP (ser 239)), for the improved ECC hemocompatibility via NO release still needs elucidation. In this work, two NORel polymeric coatings were evaluated in a 4 h rabbit thrombogenicity model and the anti-thrombotic mechanism investigated for rabbit platelet P-VASP upregulation. Polymer films containing 25 wt% diazeniumdiolated dibutylhexanediamine (DBHD) or 5 wt% S-nitroso-N-acetylpenicillamine (SNAP) coated on the inner walls of ECC circuits yielded significantly reduced ECC thrombus formation and maintained normal platelet aggregation compared to polymer controls after 4 h of blood exposure. Platelet P-VASP (ser 239), a useful tool to monitor NO/cGMP signaling, was upregulated after 4 h on ECC and markedly increased after ex vivo sodium nitroprusside (SNP) stimulation. Interestingly, in the rabbit platelet, NO did not upregulate the cAMP P-VASP phosphoprotein P-VASP (ser 157) as previously shown in human platelets. These results suggest that NORel polymers preserve rabbit platelet quiescence by sustaining a level of cGMP signaling as monitored by P-VASP (ser 239) upregulation. The upregulation of this NO-mediated platelet signaling mechanism in this rabbit thrombogenicity model indicates the potential for improved thromboresistance of any NORel-coated medical device.

ATP antagonizes thrombin-induced signal transduction through 12(S)-HETE and cAMP

In this study we have investigated the role of extracellular ATP on thrombin induced-platelet aggregation (TIPA) in washed human platelets. ATP inhibited TIPA in a dose-dependent manner and this inhibition was abolished by apyrase but not by adenosine deaminase (ADA) and it was reversed by extracellular magnesium. Antagonists of P2Y1 and P2Y12 receptors had no effect on this inhibition suggesting that a P2X receptor controlled ATP-mediated TIPA inhibition. ATP also blocked inositol phosphates (IP1, IP2, IP3) generation and [Ca(2+)]i mobilization induced by thrombin. Thrombin reduced cAMP levels which were restored in the presence of ATP. SQ-22536, an adenylate cyclase (AC) inhibitor, partially reduced the inhibition exerted by ATP on TIPA. 12-lipoxygenase (12-LO) inhibitors, nordihidroguaretic acid (NDGA) and 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid (15(S)-HETE), strongly prevented ATP-mediated TIPA inhibition. Additionally, ATP inhibited the increase of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) induced by thrombin. Pretreatment with both SQ-22536 and NDGA almost completely abolished ATP-mediated TIPA inhibition. Our results describe for the first time that ATP implicates both AC and 12-LO pathways in the inhibition of human platelets aggregation in response to agonists.

Inhibitory effects of total saponin from Korean red ginseng via vasodilator-stimulated phosphoprotein-Ser(157) phosphorylation on thrombin-induced platelet aggregation

In this study, we have investigated the effects of total saponin from Korean red ginseng (TSKRG) on thrombin-induced platelet aggregation. TSKRG dose-dependently inhibited thrombin-induced platelet aggregation with IC₅₀ value of about 81.1 μg/mL. In addition, TSKRG dose-dependently decreased thrombin-elevated the level of cytosolic-free Ca²⁺ ([Ca²⁺]_i), one of aggregation-inducing molecules. Of two Ca²⁺-antagonistic cyclic nucleotides as aggregation-inhibiting molecules, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), TSKRG significantly dose-dependently elevated intracellular level of cAMP, but not cGMP. In addition, TSKRG dose-dependently inhibited thrombin-elevated adenosine triphosphate (ATP) release from platelets. These results suggest that the suppression of [Ca²⁺]_i elevation, and of ATP release by TSKRG are associated with upregulation of cAMP. TSKRG elevated the phosphorylation of vasodilator-stimulated phosphoprotein (VASP)-Ser¹⁵⁷, a cAMP-dependent protein kinase (A-kinase) substrate, but not the phosphorylation of VASP-Ser²³⁹, a cGMPdependent protein kinase substrate, in thrombin-activated platelets. We demonstrate that TSKRG involves in increase of cAMP level and subsequent elevation of VASP-Ser¹⁵⁷ phosphorylation through A-kinase activation to inhibit [Ca²⁺]_i mobilization and ATP release in thrombin-induced platelet aggregation. These results strongly indicate that TSKRG is a beneficial herbal substance elevating cAMP level in thrombin-platelet interaction, which may result in preventing of platelet aggregation-mediated thrombotic diseases.

The oligopeptide DT-2 is a specific PKG I inhibitor only in vitro, not in living cells

BACKGROUND AND PURPOSE

cGMP is involved in the regulation of many cellular processes including cardiac and smooth muscle contractility, aldosterone synthesis and inhibition of platelet activation. Intracellular effects cGMP are mediated by cGMP-dependent PKs, cGMP-regulated PDEs and cGMP-gated ion channels. PKG inhibitors are widely used to discriminate PKG-specific effects. They can be divided into cyclic nucleotide-binding site inhibitors such as Rp-phosphorothioate analogues (Rp-cGMPS), ATP-binding site inhibitors such as KT5823, and substrate binding site inhibitors represented by the recently described DT-oligopeptides. As it has been shown that Rp-cGMPS and KT5823 have numerous non-specific effects, we analysed the pharmacological properties of the oligopeptide (D)-DT-2 described as a highly specific, membrane-permeable, PKG inhibitor.

EXPERIMENTAL APPROACH

Specificity and potency of (D)-DT-2 to inhibit PKG activity was evaluated using biochemical assays in vitro and by substrate phosphorylation analysis in various cell types including human platelets, rat mesangial cells and rat neonatal cardiomyocytes.

KEY RESULTS

Despite potent inhibition of PKGI in vitro, (D)-DT-2 lost specificity for PKG in cell homogenates and particularly in living cells, as demonstrated by phosphorylation of different substrates. Instead, (D)-DT-2 modulated activity of other kinases including ERK, p38, PKB and PKC, thereby inducing unpredicted and often opposing functional effects.

CONCLUSIONS AND IMPLICATIONS

We conclude that DT-oligopeptides, as other inhibitors, cannot be used to specifically inhibit PKG in intact cells. Therefore, no specific pharmacological PKG inhibitors are available, and reliable studies of PKG signalling can only be made by using RNA knockdown or genetic deletion methods.

The P2Y(12) antagonists, 2MeSAMP and cangrelor, inhibit platelet activation through P2Y(12)/G(i)-dependent mechanism

Background

ADP is an important physiological agonist that induces integrin activation and platelet aggregation through its receptors P2Y₁ (Gα_q-coupled) and P2Y₁₂ (Gα_i-coupled). P2Y₁₂ plays a critical role in platelet activation and thrombosis. Adenosine-based P2Y₁₂ antagonists, 2-methylthioadenosine 5′-monophosphate triethylammonium salt hydrate (2MeSAMP) and Cangrelor (AR-C69931MX) have been widely used to demonstrate the role of P2Y₁₂ in platelet function. Cangrelor is being evaluated in clinical trials of thrombotic diseases. However, a recent study reported that both 2MeSAMP and Cangrelor raise intra-platelet cAMP levels and inhibit platelet aggregation through a P2Y₁₂-independent mechanism.

Methodology/Principal Findings

The present work, using P2Y₁₂ deficient mice, sought to clarify previous conflicting reports and to elucidate the mechanisms by which 2MeSAMP and Cangrelor inhibit platelet activation and thrombosis. 2MeSAMP and Cangrelor inhibited aggregation and ATP release of wild-type but not P2Y₁₂ deficient platelets. 2MeSAMP and Cangrelor neither raised intracellular cAMP concentrations nor induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP) in washed human or mouse platelets. Furthermore, unlike the activators (PGI₂ and forskolin) of the cAMP pathway, 2MeSAMP and Cangrelor failed to inhibit Ca²⁺ mobilization, Akt phosphorylation, and Rap1b activation in P2Y₁₂ deficient platelets. Importantly, while injection of Cangrelor inhibited thrombus formation in a FeCl₃-induced thrombosis model in wild-type mice, it failed to affect thrombus formation in P2Y₁₂ deficient mice.

Conclusions

These data together demonstrate that 2MeSAMP and Cangrelor inhibit platelet function through the P2Y₁₂-dependent mechanism both in vitro and in vivo.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

The first comprehensive and quantitative analysis of human platelet protein composition allows the comparative analysis of structural and functional pathways

Antiplatelet treatment is of fundamental importance in combatting functions/dysfunction of platelets in the pathogenesis of cardiovascular and inflammatory diseases. Dysfunction of anucleate platelets is likely to be completely attributable to alterations in posttranslational modifications and protein expression. We therefore examined the proteome of platelets highly purified from fresh blood donations, using elaborate protocols to ensure negligible contamination by leukocytes, erythrocytes, and plasma. Using quantitative mass spectrometry, we created the first comprehensive and quantitative human platelet proteome, comprising almost 4000 unique proteins, estimated copy numbers for ∼ 3700 of those, and assessed intersubject (4 donors) as well as intrasubject (3 different blood samples from 1 donor) variations of the proteome. For the first time, our data allow for a systematic and weighted appraisal of protein networks and pathways in human platelets, and indicate the feasibility of differential and comprehensive proteome analyses from small blood donations. Because 85% of the platelet proteome shows no variation between healthy donors, this study represents the starting point for disease-oriented platelet proteomics. In the near future, comprehensive and quantitative comparisons between normal and well-defined dysfunctional platelets, or between platelets obtained from donors at various stages of chronic cardiovascular and inflammatory diseases will be feasible.

Epigallocatechin-3-gallate has an anti-platelet effect in a cyclic AMP-dependent manner

AIM

In this study, we investigated the effect of (-)-epigallocatechin-3-gallate (EGCG) on cyclic nucleotide production and vasodilator-stimulated phosphoprotein (VASP) phosphorylation in collagen (10 µg/mL)-stimulated platelet aggregation.

METHODS

Washed platelets (10(8)/mL) from Sprague-Dawley rats (6-7 weeks old, male) were preincubated for 3 min at 37°C in the presence of 2 mM exogenous CaCl(2) with or without EGCG or other materials, stimulated with collagen (10 µg/mL) for 5 min, and then used for the determination of intracellular cytosolic Ca(2+) ([Ca(2+)](i)), thromboxane A(2) (TXA(2)), adenosine 3',5'-cyclic monophosphate (cAMP), guanosine 3',5'-cyclic monophosphate (cGMP), and VASP phosphorylation.

RESULTS

EGCG dose-dependently inhibited collagen-induced platelet aggregation by inhibiting both [Ca(2+)](i) mobilization and TXA(2) production. Of two aggregation-inhibiting molecules, cAMP and cGMP, EGCG significantly increased intracellular levels of cAMP, but not cGMP. EGCG-elevated cAMP level was decreased by SQ22536, an adenylate cyclase inhibitor, but not by etazolate, a cAMPspecific phosphodiesterase inhibitor. In addition, EGCG elevated the phosphorylation of VASP-Ser(157), a cAMP-dependent protein kinase (A-kinase) substrate, but not the phosphorylation of VASP-Ser(239), a cGMP-dependent protein kinase substrate, in intact platelets and collagen-induced platelets, and VASP-Ser(157) phosphorylation by EGCG was inhibited by both an adenylate cyclase inhibitor SQ22536 and an A-kinase inhibitor Rp-8-Br-cAMPS. We have demonstrated that EGCG increases cAMP via adenylate cyclase activation and subsequently phosphorylates VASP-Ser(157) through A-kinase activation to inhibit [Ca(2+)](i) mobilization and TXA(2) production on collagen-induced platelet aggregation.

CONCLUSIONS

These results strongly indicate that EGCG is a beneficial compound elevating cAMP level in collagen-platelet interaction, which may result in the prevention of platelet aggregation-mediated thrombotic diseases.

Novel roles of cAMP/cGMP-dependent signaling in platelets

Endothelial prostacyclin and nitric oxide potently inhibit platelet functions. Prostacyclin and nitric oxide actions are mediated by platelet adenylyl and guanylyl cyclases, which synthesize cyclic AMP (cAMP) and cyclic GMP (cGMP), respectively. Cyclic nucleotides stimulate cAMP-dependent protein kinase (protein kinase A [PKA]I and PKAII) and cGMP-dependent protein kinase (protein kinase G [PKG]I) to phosphorylate a broad panel of substrate proteins. Substrate phosphorylation results in the inactivation of small G-proteins of the Ras and Rho families, inhibition of the release of Ca(2+) from intracellular stores, and modulation of actin cytoskeleton dynamics. Thus, PKA/PKG substrates translate prostacyclin and nitric oxide signals into a block of platelet adhesion, granule release, and aggregation. cAMP and cGMP are degraded by phosphodiesterases, which might restrict signaling to specific subcellular compartments. An emerging principle of cyclic nucleotide signaling in platelets is the high degree of interconnection between activating and cAMP/cGMP-dependent inhibitory signaling pathways at all levels, including cAMP/cGMP synthesis and breakdown, and PKA/PKG-mediated substrate phosphorylation. Furthermore, defects in cAMP/cGMP pathways might contribute to platelet hyperreactivity in cardiovascular disease. This article focuses on recent insights into the regulation of the cAMP/cGMP signaling network and on new targets of PKA and PKG in platelets.

Neisseria meningitidis induces platelet inhibition and increases vascular endothelial permeability via nitric oxide regulated pathways

Despite antibiotic therapy, infections with Neisseria meningitidis still demonstrate a high rate of morbidity and mortality even in developed countries. The fulminant septicaemic course, named Waterhouse-Friderichsen syndrome, with massive haemorrhage into the adrenal glands and widespread petechial bleeding suggest pathophysiological inhibition of platelet function. Our data show that N. meningitidis produces the important physiological platelet inhibitor and cardiovascular signalling molecule nitric oxide (NO), also known as endothelium-derived relaxing factor (EDRF). N. meningitidis -derived NO inhibited ADP-induced platelet aggregation through the activation of soluble guanylyl cyclase (sGC) followed by an increase in platelet cyclic nucleotide levels and subsequent activation of platelet cGMP- and cAMP- dependent protein kinases (PKG and PKA). Furthermore, direct measurement of horseradish peroxidase (HRP) passage through a vascular endothelial cell monolayer revealed that N. meningitidis significantly increased endothelial monolayer permeability. Immunfluorescence analysis demonstrated NO dependent disturbances in the structure of endothelial adherens junctions after co-incubation with N. meningitidis . In contrast to platelet inhibition, the NO effects on HBMEC were not mediated by cyclic nucleotides. Our study provides evidence that NO plays an essential role in the pathophysiology of septicaemic meningococcal infection.

Time-resolved in silico modeling of fine-tuned cAMP signaling in platelets: feedback loops, titrated phosphorylations and pharmacological modulation

Background

Hemostasis is a critical and active function of the blood mediated by platelets. Therefore, the prevention of pathological platelet aggregation is of great importance as well as of pharmaceutical and medical interest. Endogenous platelet inhibition is predominantly based on cyclic nucleotides (cAMP, cGMP) elevation and subsequent cyclic nucleotide-dependent protein kinase (PKA, PKG) activation. In turn, platelet phosphodiesterases (PDEs) and protein phosphatases counterbalance their activity. This main inhibitory pathway in human platelets is crucial for countervailing unwanted platelet activation. Consequently, the regulators of cyclic nucleotide signaling are of particular interest to pharmacology and therapeutics of atherothrombosis. Modeling of pharmacodynamics allows understanding this intricate signaling and supports the precise description of these pivotal targets for pharmacological modulation.

Results

We modeled dynamically concentration-dependent responses of pathway effectors (inhibitors, activators, drug combinations) to cyclic nucleotide signaling as well as to downstream signaling events and verified resulting model predictions by experimental data. Experiments with various cAMP affecting compounds including anti-platelet drugs and their combinations revealed a high fidelity, fine-tuned cAMP signaling in platelets without cross-talk to the cGMP pathway. The model and the data provide evidence for two independent feedback loops: PKA, which is activated by elevated cAMP levels in the platelet, subsequently inhibits adenylyl cyclase (AC) but as well activates PDE3. By multi-experiment fitting, we established a comprehensive dynamic model with one predictive, optimized and validated set of parameters. Different pharmacological conditions (inhibition, activation, drug combinations, permanent and transient perturbations) are successfully tested and simulated, including statistical validation and sensitivity analysis. Downstream cyclic nucleotide signaling events target different phosphorylation sites for cAMP- and cGMP-dependent protein kinases (PKA, PKG) in the vasodilator-stimulated phosphoprotein (VASP). VASP phosphorylation as well as cAMP levels resulting from different drug strengths and combined stimulants were quantitatively modeled. These predictions were again experimentally validated. High sensitivity of the signaling pathway at low concentrations is involved in a fine-tuned balance as well as stable activation of this inhibitory cyclic nucleotide pathway.

Conclusions

On the basis of experimental data, literature mining and database screening we established a dynamic in silico model of cyclic nucleotide signaling and probed its signaling sensitivity. Thoroughly validated, it successfully predicts drug combination effects on platelet function, including synergism, antagonism and regulatory loops.

Quantification of cAMP and cGMP analogs in intact cells: pitfalls in enzyme immunoassays for cyclic nucleotides

Immunoassays are routinely used as research tools to measure intracellular cAMP and cGMP concentrations. Ideally, this application requires antibodies with high sensitivity and specificity. The present work evaluates the cross-reactivity of commercially available cyclic nucleotide analogs with two non-radioactive and one radioactive cAMP and cGMP immunoassay. Most of the tested cyclic nucleotide analogs showed low degree competition with the antibodies; however, with Rp-cAMPS, 8-Br-cGMP and 8-pCPT-cGMP, a strong cross-reactivity with the corresponding cAMP and cGMP, respectively, immunoassays was observed. The determined EIA-binding constants enabled the measurement of the intracellular cyclic nucleotide concentrations and revealed a time- and lipophilicity-dependent cell membrane permeability of the compounds in the range of 10–30% of the extracellular applied concentration, thus allowing a more accurate prediction of the intracellular analog levels in a given experiment.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Dipyridamole synergizes with nitric oxide to prolong inhibition of thrombin-induced platelet shape change

We and others have previously demonstrated that nitric oxide (NO)-induced inhibition of platelet shape change is important in regulating platelet adhesion and aggregation, and therapeutic intervention of this pathway is clinically relevant for secondary prevention of stroke with dipyridamole. In the present study, we investigated whether dipyridamole affected the shape change of aspirinated platelets. Platelet shape change was inhibited using both authentic NO and sodium nitroprusside, as monitored by light scattering and mean platelet volume measurements. Dipyridamole synergized with NO, even at supra-therapeutic levels, to inhibit thrombin-induced shape change and further potentiated cAMP dependent protein kinase (PKA) mediated phosphorylation of vasodilator stimulated phosphoprotein (VASP) Ser157, even without altered levels of platelet cAMP. The effect of dipyridamole on NO-inhibited shape change depended on cGMP synthesis as evaluated by inhibition of soluble guanylyl cyclase. Measured increases in cGMP levels by dipyridamole and NO was assessed by mathematical modeling and found to be consistent with inhibition of phosphodiesterase 5 (PDE5). The model could explain the unexpected efficiency of dipyridamole in inhibiting PDE5 at the measured cGMP levels, by the majority of cGMP being bound to cGMP-dependent protein kinase (PKG). Still, selective activators of PKG failed to extend NO-mediated inhibition of the thrombin-induced platelet shape change, suggesting that PKG was not responsible for the inhibitory effect of NO and dipyridamole on shape change. The effects of dipyridamole were independent of the prostanoid and ADP pathways. Thus, the effect of dipyridamole on NO-mediated inhibition of platelet shape change may be an important and additional beneficial therapeutic effect of dipyridamole, which we suggest, is acting though localized amplification of the NO/cGMP/Phosphodiesterase3/cAMP/PKA-pathway. Probably, the efficiency of dipyridamole could be amplified clinically with NO donors.

Roles of cAMP and cAMP-dependent protein kinase in the progression of prostate cancer: cross-talk with the androgen receptor

Prostate carcinomas are among the most frequently diagnosed and death causing cancers affecting males in the developed world. It has become clear that the molecular mechanisms that drive the differentiation of normal prostate cells towards neoplasia involve multiple signal transduction cascades that often overlap and interact. A critical mediator of cellular proliferation and differentiation in various cells (and cancers) is the cAMP-dependent protein kinase, also known as protein kinase A (PKA), and its activating secondary messenger, cAMP. PKA and cAMP have been shown to play critical roles in prostate carcinogenesis and are the subject of this review. In particular we will focus on the cross-talk between PKA/cAMP signaling and that of the androgen receptor.

A protein phosphorylation-based assay for screening and monitoring of drugs modulating cyclic nucleotide pathways

Cyclic nucleotide regulation is an important target for drug development, particularly for treatment and prophylaxis of cardiovascular diseases. Determination of cyclic nucleotide levels for screening and monitoring of cyclic nucleotide modulating drug action is necessary, yet the techniques available are cumbersome and not sufficiently accurate. Here we present an approach based on the detection of cyclic nucleotide-dependent protein phosphorylation. By use of a common substrate of cyclic nucleotide-dependent protein kinases, the protein vasodilator-stimulated phosphoprotein (VASP) featuring two phosphorylation sites specifically phosphorylated by these kinases, an assay was developed for the monitoring of intracellular cyclic nucleotide levels. The assay was tested with human platelets ex vivo treated with stimulants of nucleotide cyclases, kinases, and phosphodiesterase inhibitors. Phosphorylation of the protein VASP correlates with intracellular cyclic nucleotide concentration (R(2)>0.90 for cGMP and cAMP); however, VASP phosphorylation is more sensitive to elevated cyclic nucleotide levels and significantly more stable over time. Quantification of VASP phosphorylation offers a reliable and robust tool for fast and easy monitoring of cyclic nucleotide levels and is also applicable to unprocessed biological matrices. Owing to these properties, VASP is a promising biomarker for screening and monitoring of cyclic nucleotide modulating drugs.

Decreased renal corin expression contributes to sodium retention in proteinuric kidney diseases

Patients with proteinuric kidney diseases often have symptoms of salt and water retention. It has been hypothesized that dysregulated sodium absorption is due to increased proteolytic cleavage of epithelial sodium channels (ENaCs) and increased Na,K-ATPase expression. Microarray analysis identified a reduction in kidney corin mRNA expression in rat models of puromycin aminonucleoside-induced nephrotic syndrome and acute anti-Thy1 glomerulonephritis (GN). As atrial natriuretic peptide (ANP) resistance is a mechanism accounting for volume retention, we analyzed the renal expression and function of corin; a type II transmembrane serine protease that converts pro-ANP to active ANP. Immunohistochemical analysis found that corin colocalized with ANP. The nephrotic and glomerulonephritic models exhibited concomitant increased pro-ANP and decreased ANP protein levels in the kidney consistent with low amounts of corin. Importantly, kidneys from corin knockout mice had increased amounts of renal β-ENaC and its activators, phosphodiesterase (PDE) 5 and protein kinase G II, when compared to wild-type mice. A similar expression profile was also found in cell culture suggesting the increase in PDE5 and kinase G II could account for the increase in β-ENaC seen in nephrotic syndrome and GN. Thus, we suggest that corin might be involved in the salt retention seen in glomerular diseases.

Thrombin and collagen induce a feedback inhibitory signaling pathway in platelets involving dissociation of the catalytic subunit of protein kinase A from an NFkappaB-IkappaB complex

Protein kinase A (PKA) activation by cAMP phosphorylates multiple target proteins in numerous platelet inhibitory pathways that have a very important role in maintaining circulating platelets in a resting state. Here we show that in thrombin- and collagen-stimulated platelets, PKA is activated by cAMP-independent mechanisms involving dissociation of the catalytic subunit of PKA (PKAc) from an NFkappaB-IkappaBalpha-PKAc complex. We demonstrate mRNA and protein expression for most of the NFkappaB family members in platelets. From resting platelets, PKAc was co-immunoprecipitated with IkappaBalpha, and conversely, IkappaBalpha was also co-immunoprecipitated with PKAc. This interaction was significantly reduced in thrombin- and collagen-stimulated platelets. Stimulation of platelets with thrombin- or collagen-activated IKK, at least partly by PI3 kinase-dependent pathways, leading to phosphorylation of IkappaBalpha, disruption of an IkappaBalpha-PKAc complex, and release of free, active PKAc, which phosphorylated VASP and other PKA substrates. IKK inhibitor inhibited thrombin-stimulated IkBalpha phosphorylation, PKA-IkBalpha dissociation, and VASP phosphorylation, and potentiated integrin alphaIIbbeta3 activation and the early phase of platelet aggregation. We conclude that thrombin and collagen not only cause platelet activation but also appear to fine-tune this response by initiating downstream NFkappaB-dependent PKAc activation, as a novel feedback inhibitory signaling mechanism for preventing undesired platelet activation.

Allosteric communication between cAMP binding sites in the RI subunit of protein kinase A revealed by NMR

The activation of protein kinase A involves the synergistic binding of cAMP to two cAMP binding sites on the inhibitory R subunit, causing release of the C subunit, which subsequently can carry out catalysis. We used NMR to structurally characterize in solution the RIalpha-(98-381) subunit, a construct comprising both cyclic nucleotide binding (CNB) domains, in the presence and absence of cAMP, and map the effects of cAMP binding at single residue resolution. Several conformationally disordered regions in free RIalpha become structured upon cAMP binding, including the interdomain alphaC:A and alphaC':A helices that connect CNB domains A and B and are primary recognition sites for the C subunit. NMR titration experiments with cAMP, B site-selective 2-Cl-8-hexylamino-cAMP, and A site-selective N(6)-monobutyryl-cAMP revealed that cyclic nucleotide binding to either the B or A site affected the interdomain helices. The NMR resonances of this interdomain region exhibited chemical shift changes upon ligand binding to a single site, either site B or A, with additional changes occurring upon binding to both sites. Such distinct, stepwise conformational changes in this region reflect the synergistic interplay between the two sites and may underlie the positive cooperativity of cAMP activation of the kinase. Furthermore, nucleotide binding to the A site also affected residues within the B domain. The present NMR study provides the first structural evidence of unidirectional allosteric communication between the sites. Trp(262), which lines the CNB A site but resides in the sequence of domain B, is an important structural determinant for intersite communication.

Mechanisms of activity-dependent plasticity in cellular nitric oxide-cGMP signaling

Cellular responsiveness to nitric oxide (NO) is shaped by past history of NO exposure. The mechanisms behind this plasticity were explored using rat platelets in vitro, specifically to determine the relative contributions made by desensitization of NO receptors, which couple to cGMP formation, and by phosphodiesterase-5 (PDE5), which is activated by cGMP and also hydrolyzes it. Repeated delivery of brief NO pulses (50 nm peak) at 1-min intervals resulted in a progressive loss of the associated cGMP responses, which was the combined consequence of receptor desensitization and PDE5 activation, with the former dominating. Delivery of pulses of differing amplitude showed that NO stimulated and desensitized receptors with similar potency (EC₅₀ = 10–20 nm). PDE5 activation was highly sensitive to NO, with a single pulse peaking at 2 nm being sufficient to evoke a 50% loss of response to a subsequent near-maximal NO pulse. However, the activated state of the PDE subsided quickly after removal of NO, the half-time for recovery being 25 s. In contrast, receptor desensitization reverted much more slowly, the half-time being 16 min. Accordingly, with long (20-min) exposures, NO concentrations as low as 600 pm provoked significant desensitization. The results indicate that PDE5 activation and receptor desensitization subserve distinct short term and longer term roles as mediators of plasticity in NO-cGMP signaling. A kinetic model explicitly describing the complex interplay between NO concentration, cGMP synthesis, PDE5 activation, and the resulting cGMP accumulation successfully simulated the present and previous data.

Platelet membrane proteomics: a novel repository for functional research

Being central players in thrombosis and hemostasis, platelets react in manifold and complex ways to extracellular stimuli. Cell-matrix and cell-cell interactions are mandatory for initial adhesion as well as for final development of stable plugs. Primary interfaces for interactions are plasma membrane proteins, of which many have been identified over the past decades in individual studies. However, due to their enucleate structure, platelets are not accessible to large-scale genomic screens and thus a comprehensive inventory of membrane proteins is still missing. For this reason, we here present an advanced proteomic setup for the detailed analysis of enriched platelet plasma membranes and the so far most complete collection of platelet membrane proteins. In summary, 1282 proteins were identified, of which more than half are termed to be of membrane origin. This study provides a brief overview of gene ontology subcellular and functional classification, as well as interaction network analysis. In addition, the mass spectrometric data were used to assemble a first tentative relative quantification of large-scale data on the protein level. We therefore estimate the presented data to be of major interest to the platelet research field and to support rational design of functional studies.

VASP is a CXCR2-interacting protein that regulates CXCR2-mediated polarization and chemotaxis

Chemotaxis regulates the recruitment of leukocytes, which is integral for a number of biological processes and is mediated through the interaction of chemokines with seven transmembrane G-protein-coupled receptors. Several studies have indicated that chemotactic signaling pathways might be activated via G-protein-independent mechanisms, perhaps through novel receptor-interacting proteins. CXCR2 is a major chemokine receptor expressed on neutrophils. We used a proteomics approach to identify unique ligand-dependent CXCR2-interacting proteins in differentiated neutrophil-like HL-60 cells. Using this approach, vasodilator-stimulated phosphoprotein (VASP) was identified as a CXCR2-interacting protein. The interaction between CXCR2 and VASP is direct and enhanced by CXCL8 stimulation, which triggers VASP phosphorylation via PKA- and PKCdelta-mediated pathways. The interaction between CXCR2 and VASP requires free F-actin barbed ends to recruit VASP to the leading edge. Finally, knockdown of VASP in HL-60 cells results in severely impaired CXCR2-mediated chemotaxis and polarization. These data provide the first demonstration that direct interaction of VASP with CXCR2 is essential for proper CXCR2 function and demonstrate a crucial role for VASP in mediating chemotaxis in leukocytes.

Protein kinase C-mediated phosphorylation and activation of PDE3A regulate cAMP levels in human platelets

The elevation of [cAMP](i) is an important mechanism of platelet inhibition and is regulated by the opposing activity of adenylyl cyclase and phosphodiesterase (PDE). In this study, we demonstrate that a variety of platelet agonists, including thrombin, significantly enhance the activity of PDE3A in a phosphorylation-dependent manner. Stimulation of platelets with the PAR-1 agonist SFLLRN resulted in rapid and transient phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492), in parallel with the PKC (protein kinase C) substrate, pleckstrin. Furthermore, phosphorylation and activation of PDE3A required the activation of PKC, but not of PI3K/PKB, mTOR/p70S6K, or ERK/RSK. Activation of PKC by phorbol esters also resulted in phosphorylation of the same PDE3A sites in a PKC-dependent, PKB-independent manner. This was further supported by the finding that IGF-1, which strongly activates PI3K/PKB, but not PKC, did not regulate PDE3A. Platelet activation also led to a PKC-dependent association between PDE3A and 14-3-3 proteins. In contrast, cAMP-elevating agents such as PGE(1) and forskolin-induced phosphorylation of Ser(312) and increased PDE3A activity, but did not stimulate 14-3-3 binding. Finally, complete antagonism of PGE(1)-evoked cAMP accumulation by thrombin required both G(i) and PKC activation. Together, these results demonstrate that platelet activation stimulates PKC-dependent phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492) leading to a subsequent increase in cAMP hydrolysis and 14-3-3 binding.

Inhibition of platelet activation in rats with severe congestive heart failure by a novel endothelial nitric oxide synthase transcription enhancer

AIMS

Increased risk of thrombo-embolic events in congestive heart failure (CHF) has been attributed to a hypercoagulable state including vascular endothelial dysfunction and reduced bioavailability of nitric oxide (NO) as well as platelet activation. We investigated whether treatment with a novel endothelial NO synthase (eNOS)-transcription enhancer positively modulates systemic NO bioavailability and reduces platelet activation in rats with CHF.

METHODS AND RESULTS

After experimental myocardial infarction, male Wistar rats were treated with either placebo or the eNOS-transcription enhancer, AVE9488 (25 ppm/day) for 10 weeks. In rats with severe CHF (left ventricular end-diastolic pressure >15 mmHg), platelet vasodilator-stimulated phosphoprotein (VASP)-phosphorylation reflecting the integrity of the NO/cGMP pathway was significantly reduced (mean immunofluorescence at Ser(157): Sham, 61.4 +/- 9.1; CHF-Placebo, 37.4 +/- 4.9; P < 0.05; Ser(239): Sham, 18.1 +/- 2.5; CHF-Placebo, 13.2 +/- 0.6; P < 0.05). Platelet surface expression of P-selectin and glycoprotein 53 were increased in CHF rats compared with sham-operated animals. Chronic treatment with AVE9488 significantly enhanced platelet VASP-phosphorylation in CHF rats (Ser(157): 70.4 +/- 16.2; Ser(239): 19.3 +/- 1.8). In parallel, platelet surface expression of P-selectin and glycoprotein 53 was reduced in the treatment group.

CONCLUSION

Platelet activation was evident in CHF rats. Therapy with the eNOS-transcription enhancer, AVE9488, reduced platelet activation in parallel to normalization of platelet NO bioavailability.

cGMP-dependent protein kinase modulators

The first cGMP-dependent protein kinase (PKG) modulators were described nearly 30 years ago and since then more than 200 compounds have been synthesized and tested, but only a small subset of these compounds has found widespread application. The aim of this review is to suggest a framework for evaluating and using PKG activators and inhibitors and to explore and interpret PKG signal transduction in cell culture-based model systems. Therefore, cross-reactivity of cGMP-analogs with other classes of cyclic nucleotide binding proteins, as well as the advantages and problems of newly designed PKG inhibitors, are discussed. Additional information and a search option are available at www.cyclic-nucleotides.org