Distinct localization and function of1,4,5IP3 receptor subtypes and the1,3,4,5IP4 receptor GAP1IP4BP in highly purified human platelet membranes

Protein Kinase A Regulates Platelet Phosphodiesterase 3A through an A-Kinase Anchoring Protein Dependent Manner

Platelet activation is critical for haemostasis, but if unregulated can lead to pathological thrombosis. Endogenous platelet inhibitory mechanisms are mediated by prostacyclin (PGI2)-stimulated cAMP signalling, which is regulated by phosphodiesterase 3A (PDE3A). However, spatiotemporal regulation of PDE3A activity in platelets is unknown. Here, we report that platelets possess multiple PDE3A isoforms with seemingly identical molecular weights (100 kDa). One isoform contained a unique N-terminal sequence that corresponded to PDE3A1 in nucleated cells but with negligible contribution to overall PDE3A activity. The predominant cytosolic PDE3A isoform did not possess the unique N-terminal sequence and accounted for >99% of basal PDE3A activity. PGI2 treatment induced a dose and time-dependent increase in PDE3A phosphorylation which was PKA-dependent and associated with an increase in phosphodiesterase enzymatic activity. The effects of PGI2 on PDE3A were modulated by A-kinase anchoring protein (AKAP) disruptor peptides, suggesting an AKAP-mediated PDE3A signalosome. We identified AKAP7, AKAP9, AKAP12, AKAP13, and moesin expressed in platelets but focussed on AKAP7 as a potential PDE3A binding partner. Using a combination of immunoprecipitation, proximity ligation techniques, and activity assays, we identified a novel PDE3A/PKA RII/AKAP7 signalosome in platelets that integrates propagation and termination of cAMP signalling through coupling of PKA and PDE3A.

Evidence that inositol 1,4,5-trisphosphate 3-kinase and inositol 1,3,4,5-tetrakisphosphate are negative regulators of platelet function

Background

Inositol 1,3,4,5-tetrakisphosphate (IP4) is formed from inositol 1,4,5-trisphosphate (IP3) by IP3 3-kinase (ITPK) in most cells. Its function is unknown but has been suggested to be involved in Ca2+ entry, IP3 regulation, and phosphoinositide 3-kinase antagonism.

Objectives

To better elucidate a function for IP4, we tested a specific inhibitor of ITPK (GNF362) on platelets, the effects of IP4 directly in permeabilized platelets and its effect on phosphatidylinositol 3,4,5-trisphosphate (PIP3) binding to pleckstrin-homology (PH) domain-containing proteins in platelets.

Methods

Human platelets were utilized in whole blood for thrombus formation, in platelet-rich plasma and washed suspensions for aggregation, and for Ca2+ studies, or resuspended in high K+ and low Na+ buffers for permeabilization experiments. Phosphorylation of AKT-Ser473 and Rap1-GTP formation were measured by Western blotting and PIP3 binding using PIP3 beads.

Results

GNF362-enhanced platelet aggregation stimulated by low concentrations of ADP, collagen, thrombin, U46619, and thrombus formation in collagen-coated capillaries. GNF362 induced a transient elevation of Ca2+ concentration, elevated basal levels of IP3, and enhanced the peak height of Ca2+ elevated by agonists. In permeabilized platelets, IP4 inhibited GTPγS induced formation of AKT-Ser473 phosphorylation and platelet aggregation. IP4 reduced GTPγS-stimulated Rap1-GTP levels and potently reduced extraction of RASA3 and BTK by PIP3 beads.

Conclusion

ITPK and IP4 are negative regulators of platelet function. IP4 regulation of PH domain-containing proteins may represent a pathway by which platelet activation may be controlled during thrombosis.

Impaired Platelet Function and Thrombus Formation in PDE5A-Deficient Mice

Intracellular cyclic GMP (cGMP) inhibits platelet function. Platelet cGMP levels are controlled by phosphodiesterase 5A (PDE5A)-mediated degradation. However, the exact role of PDE5A in platelet function and thrombus formation remains poorly understood. In this study, we characterized the role of PDE5A in platelet activation and function. Platelets were isolated from wild type or PDE5A^-/- mice to measure platelet aggregation, activation, phosphatidylserine exposure (annexin-V binding), reactive oxygen species (ROS) generation, platelet spreading as well as clot retraction. Cytosolic calcium mobilization was measured using Fluo-4 AM by a microplate reader. Western blot was used to measure the phosphorylation of VASP, ERK1/2, p38, JNK, and AKT. FeCl₃-induced arterial thrombosis and venous thrombosis were assessed to evaluate the in vivo hemostatic function and thrombus formation. Additionally, in vitro thrombus formation was assessed in a microfluidic whole-blood perfusion assay. PDE5A-deficient mice presented significantly prolonged tail bleeding time and delayed arterial and venous thrombus formation. PDE5A deficiency significantly inhibited platelet aggregation, ATP release, P-selectin expression, and integrin aIIbb3 activation. In addition, an impaired spreading on collagen or fibrinogen and clot retraction was observed in PDE5A-deficient platelets. Moreover, PDE5A deficiency reduced phosphatidylserine exposure, calcium mobilization, ROS production, and increased intracellular cGMP level along with elevated VASP phosphorylation and reduced phosphorylation of ERK1/2, p38, JNK, and AKT. In conclusion, PDE5A modulates platelet activation and function and thrombus formation, indicating that therapeutically targeting it might be beneficial for the treatment of thrombotic diseases.

Medical Gas Plasma—A Potent ROS-Generating Technology for Managing Intraoperative Bleeding Complications

Cold medical gas plasmas are under pre-clinical investigation concerning their hemostatic activity and could be applied for intra-operative bleeding control in the future. The technological leap innovation was their generation at body temperature, thereby causing no thermal harm to the tissue and ensuring tissue integrity. This directly contrasts with current techniques such as electrocautery, which induces hemostasis by carbonizing the tissue using a heated electrode. However, the necrotized tissue is prone to fall, raising the risk of post-operative complications such as secondary bleedings or infection. In recent years, various studies have reported on the ability of medical gas plasmas to induce blood coagulation, including several suggestions concerning their mode of action. As non-invasive and gentle hemostatic agents, medical gas plasmas could be particularly eligible for vulnerable tissues, e.g., colorectal surgery and neurosurgery. Further, their usage could be beneficial regarding the prevention of post-operative bleedings due to the absence or sloughing of eschar. However, no clinical trials or individual healing attempts for medical gas plasmas have been reported to pave the way for clinical approvement until now, despite promising results in experimental animal models. In this light, the present mini-review aims to emphasize the potential of medical gas plasmas to serve as a hemostatic agent in clinical procedures. Providing a detailed overview of the current state of knowledge, feasible application fields are discussed, and possible obstacles are addressed.

CRACking the Molecular Regulatory Mechanism of SOCE during Platelet Activation in Thrombo-Occlusive Diseases

Thrombo-occlusive diseases such as myocardial infarction, ischemic stroke and deep vein thrombosis with subsequent pulmonary embolism still represent a major health burden worldwide. Besides the cells of the vasculature or other hematopoietic cells, platelets are primarily responsible for the development and progression of an occluding thrombus. The activation and function of platelets crucially depend on free cytosolic calcium (Ca2+) as second messenger, which modulates platelet secretion, aggregation and thrombus formation. Ca2+ is elevated upon platelet activation by release of Ca2+ from intracellular stores thus triggering of the subsequent store-operated Ca2+ entry (SOCE), which is facilitated by Ca2+ release-activated channels (CRACs). In general, CRACs are assembled by the pore-forming unit Orai in the plasma membrane and the Ca2+-sensing stromal interaction molecule (STIM) in the endoplasmic reticulum after the depletion of internal Ca2+ stores. In the last few years, there is a growing body of the literature demonstrating the importance of STIM and Orai-mediated mechanism in thrombo-occlusive disorders. Thus, this review provides an overview of the recent understanding of STIM and Orai signaling in platelet function and its implication in the development and progression of ischemic thrombo-occlusive disorders. Moreover, potential pharmacological implications of STIM and Orai signaling in platelets are anticipated and discussed in the end.

Proteomics: A Tool to Study Platelet Function

Platelets are components of the blood that are highly reactive, and they quickly respond to multiple physiological and pathophysiological processes. In the last decade, it became clear that platelets are the key components of circulation, linking hemostasis, innate, and acquired immunity. Protein composition, localization, and activity are crucial for platelet function and regulation. The current state of mass spectrometry-based proteomics has tremendous potential to identify and quantify thousands of proteins from a minimal amount of material, unravel multiple post-translational modifications, and monitor platelet activity during drug treatments. This review focuses on the role of proteomics in understanding the molecular basics of the classical and newly emerging functions of platelets. including the recently described role of platelets in immunology and the development of COVID-19.The state-of-the-art proteomic technologies and their application in studying platelet biogenesis, signaling, and storage are described, and the potential of newly appeared trapped ion mobility spectrometry (TIMS) is highlighted. Additionally, implementing proteomic methods in platelet transfusion medicine, and as a diagnostic and prognostic tool, is discussed.

Physiological testosterone attenuates profibrotic activities of rat cardiac fibroblasts through modulation of nitric oxide and calcium homeostasis

Testosterone deficiency is associated with poor prognosis among patients with chronic heart failure (HF). Physiological testosterone improves the exercise capacity of patients with HF. In this study, we evaluated whether treatment with physiological testosterone contributes to anti-fibrogenesis by modifying calcium homeostasis in cardiac fibroblasts and we studied the underlying mechanisms. Nitric oxide (NO) analyses, calcium (Ca²⁺) fluorescence, and Western blotting were performed in primary isolated rat cardiac fibroblasts with or without (control cells) testosterone (10, 100, 1,000 nmol/L) treatment for 48 hours. Physiological testosterone (10 nmol/L) increased NO production and phosphorylation at the inhibitory site of the inositol trisphosphate (IP3) receptor, thereby reducing Ca²⁺ entry, phosphorylated Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) expression, type I and type III pro-collagen production. Non-physiological testosterone-treated fibroblasts exhibited similar NO and collagen production capabilities as compared to control (testosterone deficient) fibroblasts. These effects were blocked by co-treatment with NO inhibitor (L-NG-nitro arginine methyl ester [L-NAME], 100 μmol/L). In the presence of the IP3 receptor inhibitor (2-aminoethyl diphenylborinate [2-APB], 50 μmol/L), testosterone-deficient and physiological testosterone-treated fibroblasts exhibited similar phosphorylated CaMKII expression. When treated with 2-APB or CaMKII inhibitor (KN93, 10 μmol/L), testosterone-deficient and physiological testosterone-treated fibroblasts exhibited similar type I, and type III collagen production. In conclusion, physiological testosterone activates NO production, and attenuates the IP3 receptor/Ca²⁺ entry/CaMKII signaling pathway, thereby inhibiting the collagen production capability of cardiac fibroblasts.

IP3 Receptor Plasticity Underlying Diverse Functions

In the body, extracellular stimuli produce inositol 1,4,5-trisphosphate (IP₃), an intracellular chemical signal that binds to the IP₃ receptor (IP₃R) to release calcium ions (Ca²⁺) from the endoplasmic reticulum. In the past 40 years, the wide-ranging functions mediated by IP₃R and its genetic defects causing a variety of disorders have been unveiled. Recent cryo-electron microscopy and X-ray crystallography have resolved IP₃R structures and begun to integrate with concurrent functional studies, which can explicate IP₃-dependent opening of Ca²⁺-conducting gates placed ∼90 Å away from IP₃-binding sites and its regulation by Ca²⁺. This review highlights recent research progress on the IP₃R structure and function. We also propose how protein plasticity within IP₃R, which involves allosteric gating and assembly transformations accompanied by rapid and chronic structural changes, would enable it to regulate diverse functions at cellular microdomains in pathophysiological states.

Cyclic nucleotide-dependent inhibitory signaling interweaves with activating pathways to determine platelet responses

Platelets are regulated by extracellular cues that impact on intracellular signaling. The endothelium releases prostacyclin and nitric oxide which stimulate the synthesis of cyclic nucleotides cAMP and cGMP leading to platelet inhibition. Other inhibitory mechanisms involve immunoreceptor tyrosine-based inhibition motif-containing receptors, intracellular receptors and receptor desensitization. Inhibitory cyclic nucleotide pathways are traditionally thought to represent a passive background system keeping platelets in a quiescent state. In contrast, cyclic nucleotides are increasingly seen to be dynamically involved in most aspects of platelet regulation. This review focuses on crosstalk between activating and cyclic nucleotide-mediated inhibitory pathways highlighting emerging new hub structures and signaling mechanisms. In particular, interactions of plasma membrane receptors like P2Y12 and GPIb/IX/V with the cyclic nucleotide system are described. Furthermore, differential regulation of the RGS18 complex, second messengers, protein kinases, and phosphatases are presented, and control over small G-proteins by guanine-nucleotide exchange factors and GTPase-activating proteins are outlined. Possible clinical implications of signaling crosstalk are discussed.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

CK2β regulates thrombopoiesis and Ca2+-triggered platelet activation in arterial thrombosis

CK2β is critically required for thrombopoiesis by regulating tubulin polymerization, MK fragmentation, and proplatelet formation. CK2β facilitates inositol triphosphate–mediated increase of cytosolic Ca2+ and is essential for platelet activation in arterial thrombosis in vivo.

Transcriptomic analysis of the ion channelome of human platelets and megakaryocytic cell lines

Ion channels have crucial roles in all cell types and represent important therapeutic targets. Approximately 20 ion channels have been reported in human platelets; however, no systematic study has been undertaken to define the platelet channelome. These membrane proteins need only be expressed at low copy number to influence function and may not be detected using proteomic or transcriptomic microarray approaches. In our recent work, quantitative real-time PCR (qPCR) provided key evidence that Kv1.3 is responsible for the voltage-dependent K+ conductance of platelets and megakaryocytes. The present study has expanded this approach to assess relative expression of 402 ion channels and channel regulatory genes in human platelets and three megakaryoblastic/erythroleukaemic cell lines. mRNA levels in platelets are low compared to other blood cells, therefore an improved method of isolating platelets was developed. This used a cocktail of inhibitors to prevent formation of leukocyte-platelet aggregates, and a combination of positive and negative immunomagnetic cell separation, followed by rapid extraction of mRNA. Expression of 34 channel-related transcripts was quantified in platelets, including 24 with unknown roles in platelet function, but that were detected at levels comparable to ion channels with established roles in haemostasis or thrombosis. Trace expression of a further 50 ion channel genes was also detected. More extensive channelomes were detected in MEG-01, CHRF-288-11 and HEL cells (195, 185 and 197 transcripts, respectively), but lacked several channels observed in the platelet. These "channelome" datasets provide an important resource for further studies of ion channel function in the platelet and megakaryocyte.

MRP4 (ABCC4) as a potential pharmacologic target for cardiovascular disease

This review focuses on multidrug resistance protein 4 (MRP4 or ABCC4) that has recently been shown to play a role in cAMP homeostasis, a key-pathway in vascular biology and in platelet functions. In vascular system, recent data provide evidence that inhibition of MRP4 prevents human coronary artery smooth muscle cell proliferation in vitro and in vivo, as well as human pulmonary artery smooth muscle cell proliferation in vitro and pulmonary hypertension in mice in vivo. In the heart, MRP4 silencing in adult rat ventricular myocytes results in an increase in intracellular cAMP levels leading to enhanced cardiomyocyte contractility. However, a prolonged inhibition of MRP4 can promote cardiac hypertrophy. In addition, secreted cAMP, through its metabolite adenosine, prevents adrenergically induced cardiac hypertrophy and fibrosis. Finally, MRP4 inhibition in platelets induces a moderate thrombopathy. The localization of MRP4 underlines the emerging concept of cAMP compartmentalization in platelets, which is a major regulatory mechanism in other cells. cAMP storage in platelet dense granules might limit the cAMP cytosolic concentration upon adenylate cyclase activation, a necessary step to induce platelet activation. In this review, we discuss the therapeutic potential of direct pharmacological inhibition of MRP4 in atherothrombotic disease, via its vasodilating and antiplatelet effects.

Regulation of Platelet Function by Orai, STIM and TRP

Agonist-induced changes in cytosolic Ca(2+) concentration ([Ca(2+)]c) are central events in platelet physiology. A major mechanism supporting agonist-induced Ca(2+) signals is store-operated Ca(2+) entry (SOCE), where the Ca(2+) sensor STIM1 and the channels of the Orai family, as well as TRPC members are the key elements. STIM1-dependent SOCE plays a major role in collagen-stimulated Ca(2+) signaling, phosphatidylserine exposure and thrombin generation. Furthermore, studies involving Orai1 gain-of-function mutants and platelets from Orai1-deficient mice have revealed the importance of this channel in thrombosis and hemostasis to those found in STIM1-deficient mice indicating that SOCE might play a prominent role in thrombus formation. Moreover, increase in TRPC6 expression might lead to thrombosis in humans. The role of STIM1, Orai1 and TRPCs, and thus SOCE, in thrombus formation, suggests that therapies directed against SOCE and targeting these molecules during cardiovascular and cerebrovascular events could significantly improve traditional anti-thrombotic treatments.

The Spatiotemporal Regulation of cAMP Signaling in Blood Platelets-Old Friends and New Players

Atherothrombosis, the pathology underlying numerous cardiovascular diseases, is a major cause of death globally. Hyperactive blood platelets play a key role in the atherothrombotic process through the release of inflammatory mediators and formation of thrombi. In healthy blood vessels, excessive platelet activation is restricted by endothelial-derived prostacyclin (PGI₂) through cyclic adenosine-5′-monophosphate (cAMP) and protein kinase A (PKA)-dependent mechanisms. Elevation in intracellular cAMP is associated with the control of a number of distinct platelet functions including actin polymerisation, granule secretion, calcium mobilization and integrin activation. Unfortunately, in atherosclerotic disease the protective effects of cAMP are compromised, which may contribute to pathological thrombosis. The cAMP signaling network in platelets is highly complex with the presence of multiple isoforms of adenylyl cyclase (AC), PKA, and phosphodiesterases (PDEs). However, a precise understanding of the relationship between specific AC, PKA, and PDE isoforms, and how individual signaling substrates are targeted to control distinct platelet functions is still lacking. In other cells types, compartmentalisation of cAMP signaling has emerged as a key mechanism to allow precise control of specific cell functions. A-kinase anchoring proteins (AKAPs) play an important role in this spatiotemporal regulation of cAMP signaling networks. Evidence of AKAP-mediated compartmentalisation of cAMP signaling in blood platelets has begun to emerge and is providing new insights into the regulation of platelet function. Dissecting the mechanisms that allow cAMP to control excessive platelet activity without preventing effective haemostasis may unleash the possibility of therapeutic targeting of the pathway to control unwanted platelet activity.

RASA3 is a critical inhibitor of RAP1-dependent platelet activation

The small GTPase RAP1 is critical for platelet activation and thrombus formation. RAP1 activity in platelets is controlled by the GEF CalDAG-GEFI and an unknown regulator that operates downstream of the adenosine diphosphate (ADP) receptor, P2Y12, a target of antithrombotic therapy. Here, we provide evidence that the GAP, RASA3, inhibits platelet activation and provides a link between P2Y12 and activation of the RAP1 signaling pathway. In mice, reduced expression of RASA3 led to premature platelet activation and markedly reduced the life span of circulating platelets. The increased platelet turnover and the resulting thrombocytopenia were reversed by concomitant deletion of the gene encoding CalDAG-GEFI. Rasa3 mutant platelets were hyperresponsive to agonist stimulation, both in vitro and in vivo. Moreover, activation of Rasa3 mutant platelets occurred independently of ADP feedback signaling and was insensitive to inhibitors of P2Y12 or PI3 kinase. Together, our results indicate that RASA3 ensures that circulating platelets remain quiescent by restraining CalDAG-GEFI/RAP1 signaling and suggest that P2Y12 signaling is required to inhibit RASA3 and enable sustained RAP1-dependent platelet activation and thrombus formation at sites of vascular injury. These findings provide insight into the antithrombotic effect of P2Y12 inhibitors and may lead to improved diagnosis and treatment of platelet-related disorders.

Headpiece domain of dematin regulates calcium mobilization and signaling in platelets

Dematin is a broadly expressed membrane cytoskeletal protein that has been well characterized in erythrocytes and to a lesser extent in non-erythroid cells. However, dematin's function in platelets is not known. Here, we show that dematin is abundantly expressed in both human and mouse platelets. Platelets harvested from the dematin headpiece knock-out (HPKO) mouse model exhibit a striking defect in the mobilization of calcium in response to multiple agonists of platelet activation. The reduced calcium mobilization in HPKO platelets is associated with concomitant inhibition of platelet aggregation and granule secretion. Integrin α(IIb)β(3) activation in response to agonists is attenuated in the HPKO platelets. The mutant platelets show nearly normal spreading on fibrinogen and an unaltered basal cAMP level; however, the clot retraction was compromised in the mutant mice. Immunofluorescence analysis indicated that dematin is present both at the dense tubular system and plasma membrane fractions of platelets. Proteomic analysis of dematin-associated proteins in human platelets identified inositol 1,4,5-trisphosphate 3-kinase isoform B (IP3KB) as a binding partner, which was confirmed by immunoprecipitation analysis. IP3KB, a dense tubular system protein, is a major regulator of calcium homeostasis. Loss of the dematin headpiece resulted in a decrease of IP3KB at the membrane and increased levels of IP3KB in the cytosol. Collectively, these findings unveil dematin as a novel regulator of internal calcium mobilization in platelets affecting multiple signaling and cytoskeletal functions. Implications of a conserved role of dematin in the regulation of calcium homeostasis in other cell types will be discussed.

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.

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.

A live cell micro-imaging technique to examine platelet calcium signaling dynamics under blood flow

The platelet is a specialized adhesive cell that plays a key role in thrombus formation under both physiological and pathological blood flow conditions. Platelet adhesion and activation are dynamic processes associated with rapid morphological and functional changes, with the earliest signaling events occurring over a subsecond time-scale. The relatively small size of platelets combined with the dynamic nature of platelet adhesion under blood flow means that the investigation of platelet signaling events requires techniques with both high spatial discrimination and rapid temporal resolution. Unraveling the complex signaling processes governing platelet adhesive function under conditions of hemodynamic shear stress has been a longstanding goal in platelet research and has been greatly influenced by the development and application of microimaging-based techniques. Advances in the area of epi-fluorescence and confocal-based platelet calcium (Ca(2+)) imaging have facilitated the in vitro and in vivo elucidation of the early signaling events regulating platelet adhesion and activation. These studies have identified distinct Ca(2+) signaling mechanisms that serve to dynamically regulate activation of the major platelet integrin α(IIb)β(3) and associated adhesion and aggregation processes under flow. This chapter describes in detail a ratiometric calcium imaging protocol and associated troubleshooting procedures developed in our laboratory to examine live platelet Ca(2+) signaling dynamics. This technique provides a method for high-resolution imaging of the Ca(2+) dynamics underpinning platelet adhesion and thrombus formation under conditions of pathophysiological shear stress.

Ca(2+) channels on the move

The versatility of Ca(2+) as an intracellular messenger derives largely from the spatial organization of cytosolic Ca(2+) signals, most of which are generated by regulated openings of Ca(2+)-permeable channels. Most Ca(2+) channels are expressed in the plasma membrane (PM). Others, including the almost ubiquitous inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, the ryanodine receptors (RyR), are predominantly expressed in membranes of the sarcoplasmic or endoplasmic reticulum (ER). Targeting of these channels to appropriate destinations underpins their ability to generate spatially organized Ca(2+) signals. All Ca(2+) channels begin life in the cytosol, and the vast majority are then functionally assembled in the ER, where they may either remain or be dispatched to other membranes. Here, by means of selective examples, we review two issues related to this trafficking of Ca(2+) channels via the ER. How do cells avoid wayward activity of Ca(2+) channels in transit as they pass from the ER via other membranes to their final destination? How and why do some cells express small numbers of the archetypal intracellular Ca(2+) channels, IP(3)R and RyR, in the PM?

Calcium signaling in platelets

Agonist-induced elevation in cytosolic Ca2+ concentrations is essential for platelet activation in hemostasis and thrombosis. It occurs through Ca2+ release from intracellular stores and Ca2+ entry through the plasma membrane (PM). Ca2+ store release is a well-established process involving phospholipase (PL)C-mediated production of inositol-1,4,5-trisphosphate (IP3), which in turn releases Ca2+ from the intracellular stores through IP3 receptor channels. In contrast, the mechanisms controlling Ca2+ entry and the significance of this process for platelet activation have been elucidated only very recently. In platelets, as in other non-excitable cells, the major way of Ca2+ entry involves the agonist-induced release of cytosolic sequestered Ca2+ followed by Ca2+ influx through the PM, a process referred to as store-operated calcium entry (SOCE). It is now clear that stromal interaction molecule 1 (STIM1), a Ca2+ sensor molecule in intracellular stores, and the four transmembrane channel protein Orai1 are the key players in platelet SOCE. The other major Ca2+ entry mechanism is mediated by the direct receptor-operated calcium (ROC) channel, P2X1. Besides these, canonical transient receptor potential channel (TRPC) 6 mediates Ca2+ entry through the PM. This review summarizes the current knowledge of platelet Ca2+ homeostasis with a focus on the newly identified Ca2+ entry mechanisms.

cGMP and cGMP-dependent protein kinase in platelets and blood cells

Platelets are specialized adhesive cells that play a key role in normal and pathological hemostasis through their ability to rapidly adhere to subendothelial matrix proteins (platelet adhesion) and to other activated platelets (platelet aggregation). NO plays a crucial role in preventing platelet adhesion and aggregation. In platelets, cGMP synthesis is catalyzed by sGC, whereas PDE2, PDE3 and PDE5 are responsible for cGMP degradation. Stimulation of cGK by cGMP leads to phosphorylation of multiple target substrates. These substrates inhibit elevation of intracellular calcium, integrin activation, cytoskeletal reorganization, and platelet granule secretion, events normally associated with platelet activation. The NO/cGMP pathway also plays a significant role in many other blood cell types in addition to platelets. In leukocytes, depending on the specific cell type, cGMP signaling regulates gene expression, differentiation, migration, cytokine production, and apoptosis.

An EF hand mutation in Stim1 causes premature platelet activation and bleeding in mice

Changes in cytoplasmic Ca2+ levels regulate a variety of fundamental cellular functions in virtually all cells. In nonexcitable cells, a major pathway of Ca2+ entry involves receptor-mediated depletion of intracellular Ca2+ stores followed by the activation of store-operated calcium channels in the plasma membrane. We have established a mouse line expressing an activating EF hand motif mutant of stromal interaction molecule 1 (Stim1), an ER receptor recently identified as the Ca2+ sensor responsible for activation of Ca2+ release-activated (CRAC) channels in T cells, whose function in mammalian physiology is not well understood. Mice expressing mutant Stim1 had macrothrombocytopenia and an associated bleeding disorder. Basal intracellular Ca2+ levels were increased in platelets, which resulted in a preactivation state, a selective unresponsiveness to immunoreceptor tyrosine activation motif-coupled agonists, and increased platelet consumption. In contrast, basal Ca2+ levels, but not receptor-mediated responses, were affected in mutant T cells. These findings identify Stim1 as a central regulator of platelet function and suggest a cell type-specific activation or composition of the CRAC complex.

Follicle-stimulating hormone-induced Galphah/phospholipase C-delta1 signaling mediating a noncapacitative Ca2+ influx through T-type Ca2+ channels in rat sertoli cells

Our previous study demonstrated that FSH-induced immediate Ca(2+) influx in rat Sertoli cells (SCs) is mediated by the Galphah/phospholipase C-delta1 (PLC-delta1) signaling pathway. As to which Ca(2+) channel is responsible for such Ca(2+) influx was not understood. In this study, thapsigargin triggered an in-store calcium release and evoked a 1.5-fold elevation of intracellular Ca(2+) in Ca(2+)-free media, whereas FSH exhibited no effect. The readdition of CaCl(2) (2.5 mm) to FSH-pretreated or thapsigargin-sensitized SCs in Ca(2+)-free media immediately elicited a rapid Ca(2+) influx or a 2-fold increase of second intracellular Ca(2+) elevation, respectively. The addition of Ca(2+) chelator EGTA (0.2 mm) reduced the FSH-induced elevation of intracellular Ca(2+) in SCs incubated with CaCl(2). However, pretreatment with dantrolene (25 microM), which inhibits in-store calcium release, did not affect the FSH-induced elevation of intracellular Ca(2+). NiCl(2) (10 microM), a T-type calcium channel blocker, abolished the FSH-induced SC Ca(2+) influx. Furthermore, mibefradil (10 and 100 microm), another specific blocker for T-type Ca(2+) channels, dose-dependently suppressed the FSH-induced Ca(2+) influx. In contrast, nifedipine (10 and 50 microm) or omega-conotoxin GVIA (100 and 500 nm), blocker of L- or N-type Ca(2+) channels, respectively, did not affect the FSH-induced SC Ca(2+) influx. On the other hand, FSH-induced Ca(2+) influx was significantly reduced by pretreatment of SCs with myristoylated synthetic peptide (0.1 and 1 microm) of PLC-delta1 fragment TIPWNSLKQGYRHVHLL but not affected by 2',5'-dideoxyadenosine (3 and 15 microm), a selective inhibitor of adenylate cyclase. In conclusion, the FSH-induced Galphah/PLC-delta1 pathway-dependent Ca(2+) influx of rat SCs is mediated by T-type Ca(2+) channels and independent of in-store calcium release.

Plasma membrane IP3 receptors

IP3Rs (inositol 1,4,5-trisphosphate receptors) are expressed in the membranes of non-mitochondrial organelles in most animal cells, but their presence and role within the plasma membrane are unclear. Whole-cell patch-clamp recording from DT40 cells expressing native or mutated IP3Rs has established that each cell expresses just two or three functional IP3Rs in its plasma membrane. Only approx. 50% of the Ca2+ entry evoked by stimulation of the B-cell receptor is mediated by store-operated Ca2+ entry, the remainder appears to be carried by the IP3Rs expressed in the plasma membrane. Ca2+ entering the cell via just two large-conductance IP3Rs is likely to have very different functional consequences from the comparable amount of Ca2+ that enters through the several thousand low-conductance store-operated channels.

TRP channels in platelet function

Ca2+ entry forms an essential component of platelet activation; however, the mechanisms associated with this process are not understood. Ca2+ entry upon receptor activation occurs as a consequence of intracellular store depletion (referred to as store-operated Ca2+ entry or SOCE), a direct action of second messengers on cation entry channels or the direct occupancy of a ligand-gated P2(Xi) receptor. The molecular identity of the SOCE channel has yet to be established. Transient receptor potential (TRP) proteins are candidate cation entry channels and are classified into a number of closely related subfamilies including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin) and TRPML (mucolipins). From the TRPC family, platelets have been shown to express TRPC6 and TRPC1, and are likely to express other TRPC and other TRP members. TRPC6 is suggested to be involved with receptor-activated, diacyl-glycerol-mediated cation entry. TRPC1 has been suggested to be involved with SOCE, though many of the suggested mechanisms remain controversial. As no single TRP channel has the properties described for SOCE in platelets, it is likely that it is composed of a heteromeric association of TRP and related subunits, some of which may be present in intracellular compartments in the resting cell.

Transient Receptor Potential Channels and Intracellular Signaling

The transient receptor potential (TRP) family of ion channels is composed of more than 50 functionally versatile cation-permeant ion channels expressed in most mammalian cell types. Considerable research has been brought to bear on the members of this family, especially with regard to their possible role as store-operated calcium channels, although studies have provided evidence that TRP channels exhibit a number of regulatory and functional aspects. Endogenous and transiently expressed TRP channels can be activated by different mechanisms grouped into four main categories: receptor-operated activation, store depletion-mediated activation, ligand-induced activation, and direct activation. This article reviews the biochemical characteristics of the different members of the TRP family and summarizes their involvement in a number of physiological events ranging from sensory transduction to development, which might help in understanding the relationship between TRP channel dysfunction and the development of several diseases.

Mobility of proteins associated with the plasma membrane by interaction with inositol lipids

Translocation of a protein to the plasma membrane in response to the generation of polyphosphoinositol lipids is believed to be an important component of cellular regulation, in part because it increases the effective concentration of that protein relative to other proteins in the same membrane by restricting it to a two-dimensional space. However, such a concept assumes that, once translocated, a protein retains the free mobility it had in the cytoplasm, and also that the possible existence of partitioned pools of inositol lipids does not restrict its sphere of influence. We have explored by fluorescence recovery after photobleaching (FRAP) the mobility of four green-fluorescent-protein-tagged proteins, GAP1(IP4BP) and GAP1(m), when they are either cytoplasmic or attached to the plasma membrane, and the PH domain of PI-PLCdelta(1) and ICAM as representative of, respectively, another inositol-lipid-anchored protein and a single-transmembrane-span-domain protein. The data from GAP1(m) and the PI-PLCdelta(1) PH domain show that, when proteins associate with inositol lipids in the plasma membrane, they retain a mobility similar to that in the cytoplasm, and probably also similar to the inositol lipid to which they are attached, suggesting a free diffusion within the plane of the membrane. Moreover, this free diffusion is similar whether they are bound to PtdIns(3,4,5)P(3) or to PtdIns(4,5)P(2), and no evidence was found by these criteria for restricted pools of PtdIns(4,5)P(2). The mobility of GAP1(IP4BP), which has been reported to associate with PtdIns(4,5)P(2) in the plasma membrane, is much lower, suggesting that it might interact with other cellular components. Moreover, the mobility of GAP1(IP4BP) is not detectably altered by the generation of either of its two potential regulators, Ins(1,3,4,5)P(4) or PtdIns(3,4,5)P(3).

Protease-activated receptor-1-induced calcium signaling in gingival fibroblasts is mediated by sarcoplasmic reticulum calcium release and extracellular calcium influx

Thrombin is a serine protease activated during injury and inflammation. Thrombin and other proteases generated by periodontal pathogens affect the behavior of periodontal cells via activation of protease-activated receptors (PARs). We noted that thrombin and PAR-1 agonist peptide stimulated intracellular calcium levels ([Ca2+]i) of gingival fibroblasts (GF). This increase of [Ca2+]i was inhibited by EGTA and verapamil. U73122 and neomycin inhibited thrombin- and PAR-1-induced [Ca2+]i. Furthermore, 2-APB (75-100 microM, inositol triphosphate [IP3] receptor antagonist), thapsigargin (1 microM), SKF-96365 (200 microM) and W7 (50 and 100 microM) also suppressed the PAR-1- and thrombin-induced [Ca2+]i. However, H7 (100, 200 microM) and ryanodine showed little effects. Blocking Ca2+ efflux from mitochondria by CGP37157 (50, 100 microM) inhibited both thrombin- and PAR-1-induced [Ca2+]i. Thrombin induced the IP3 production of GF within 30-seconds of exposure, which was inhibited by U73122. These results indicate that mitochondrial calcium efflux and calcium-calmodulin pathways are related to thrombin and PAR-1 induced [Ca2+]i in GF. Thrombin-induced [Ca2+]i of GF is mainly due to PAR-1 activation, extracellular calcium influx via L-type calcium channel, PLC activation, then IP3 binding to IP3 receptor in sarcoplasmic reticulum, which leads to intracellular calcium release and subsequently alters cell membrane capacitative calcium entry.

The relative role of PLCbeta and PI3Kgamma in platelet activation

Stimulation of platelet G protein-coupled receptors results in the cleavage of phosphatidylinositol 4,5-trisphosphate (PIP(2)) into inositol 1,4,5-trisphosphate and 1,2-diacylglycerol by phospholipase C (PLCbeta). It also results in the phosphorylation of PIP2 by the gamma isoform of phosphatidylinositol 3-kinase (PI3Kgamma) to synthesize phosphatidylinositol 3,4,5-trisphosphate. To understand the role of PIP2 in platelet signaling, we evaluated knock-out mice lacking 2 isoforms of PLCbeta (PLCbeta2 and PLCbeta3) or lacking the G(betagamma)-activated isoform of PI3K (PI3Kgamma). Both knock-out mice were unable to form stable thrombi in a carotid injury model. To provide a functional explanation, knock-out platelets were studied ex vivo. PLCbeta2/beta3-/- platelets failed to assemble filamentous actin, had defects in both secretion and mobilization of intracellular calcium, and were unable to form stable aggregates following low doses of agonists. Platelets lacking PI3Kgamma disaggregated following low-dose adenosine diphosphate (ADP) and had a mildly impaired ability to mobilize intracellular calcium. Yet, they exhibited essentially normal actin assembly and secretion. Remarkably, both PLCbeta2/beta3-/- and PI3Kgamma-/- platelets spread more slowly upon fibrinogen. These results suggest substantial redundancy in platelet signaling pathways. Nonetheless, the diminished ability of knock-out platelets to normally spread after adhesion and to form stable thrombi in vivo suggests that both PLCbeta2/beta3 and PI3Kgamma play vital roles in platelet cytoskeletal dynamics.

A role for hTRPC1 and lipid raft domains in store-mediated calcium entry in human platelets

We have previously suggested that store-mediated Ca2+ entry (SMCE) in human platelets may be activated by a secretion-like coupling model, involving de novo coupling of the type II inositol 1,4,5-trisphosphate receptor (IP(3)RII) to the putative Ca2+ entry channel, hTRPC1. In other cells, hTRPC1 has been reported to be associated with cholesterol-rich lipid raft domains (LRDs) in the plasma membrane. Here we have shown that hTRPC1 is largely associated with detergent-resistant platelet membranes, from which it is partially released when the cells are depleted of cholesterol by treatment with methyl-beta-cyclodextrin (MBCD). MBCD treatment inhibited thapsigargin (TG)-evoked SMCE in a concentration-dependent manner, reducing it to 38.1+/-4.1% at a concentration of 10mM. Similarly, the Ca2+ entry evoked by thrombin (1unit/ml) was reduced to 48.2+/-4.5% of control following MBCD (10mM) treatment. Thrombin- and TG-evoked coupling between IP(3)RII and hTRPC1 was also reduced following cholesterol depletion. These results suggest that hTRPC1 is associated with LRDs in human platelets and that these domains are important for its participation in SMCE.

The role of endothelial cell Ca2+store release in the regulation of microvascular permeabilityin vivo

Microvascular permeability is regulated by changes in intracellular calcium concentration. The mechanism by which this increase in calcium determines permeability under normal conditions and during stimulation with agonists remains to be elucidated. In order to determine whether calcium release from intracellular stores could contribute towards the regulation of vascular permeability, hydraulic conductivity (Lp) was measured in frog mesenteric microvessels during stimulation of the endothelial cells of these vessels with agonists that release calcium from the intracellular stores. ATP (which acts through activation of inositol 1,4,5-trisphosphate (IP3) receptors) increased Lp in the absence of calcium influx across the plasma membrane 2.3 +/- 0.3 fold (mean +/- s.e.m., P < 0.01, n = 8), which was less than the increase in the presence of calcium influx (3.1 +/- 1.1 fold). Caffeine (which acts through activation of ryanodine receptors) also increased Lp in the absence of calcium influx across the plasma membrane 3.8 +/- 1.0 fold (P < 0.01, n = 9), but by at least as much as it does in the presence of calcium influx (2.8 +/- 0.5 fold). It is surprising that there was a strong positive correlation between the size of the response during store release and the baseline permeability (r = 0.91 for ATP, r = 0.75 for caffeine). This suggests that the filling state of the stores may regulate the baseline permeability of the microvessels.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.

Targeting of Inositol 1,4,5-Trisphosphate Receptors to the Endoplasmic Reticulum by Multiple Signals within Their Transmembrane Domains

Most inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in the endoplasmic reticulum (ER), where their precise distribution underlies the spatially complex Ca2+ signals evoked by extracellular stimuli. The signals that target IP3R to the ER or, less commonly, to other membranes are unknown. We expressed yellow fluorescent protein-tagged fragments of type 1 IP3R alone or fused with a plasma membrane protein to establish the determinants of ER targeting in COS-7 cells. By using a combination of confocal imaging and glycoprotein analyses, we demonstrated that any pair of the six transmembrane domains (TMD) linked by a luminal loop retains the protein within the ER, and when attached to a plasma membrane protein (ICAM-1), prevents it from reaching the medial Golgi. TMD1 or TMD2 alone were accumulated in mitochondria, whereas TMD5 and TMD6 were retained in ER, but were unable to prevent ICAM from reaching the plasma membrane. We conclude that IP3R are targeted to the ER membrane only after synthesis of TMDs 1 and 2, and that after co-translational insertion of the remaining TMDs, redundant retention signals present in any pair of TMD retain IP3R in the ER.

Initial accumulation of platelets during arterial thrombus formation in vivo is inhibited by elevation of basal cAMP levels

Platelet accumulation at sites of vascular injury is the primary event in arterial thrombosis. Initial platelet accrual into thrombi is mediated by interactions of platelet adhesion receptors with ligands on the injured endothelium or in the sub-endothelial matrix. The role of intracellular signals in initial platelet accumulation at sites of endothelial injury, however, is the subject of debate. We have used a newly discovered inhibitor of phosphodiesterase 3A (PDE3A) and the well-characterized PDE3A inhibitor, cilostazol, to modulate 3',5'-cyclic adenosine monophosphate (cAMP) levels in an in vivo model that enables the kinetic analysis of platelet accumulation. These studies demonstrate that elevation of basal cAMP levels results in an overall decline in platelet accumulation at the site of vascular injury. In particular, the initial rate of accumulation of platelets is inhibited by elevation of cAMP. Analysis of the kinetics of individual platelets at injury sites using intravital microscopy demonstrates that cAMP directs the rate at which platelets attach to and detach from thrombi. These studies demonstrate that cAMP in circulating platelets controls attachment to and detachment from sites of arteriolar injury. Thus, the status of the intracellular signaling machinery prior to engagement of platelet receptors influences the rate of platelet accumulation during thrombus formation.

Cyclic GMP-Dependent Protein Kinases and the Cardiovascular System

Signaling cascades initiated by nitric oxide (NO) and natriuretic peptides (NPs) play an important role in the maintenance of cardiovascular homeostasis. It is currently accepted that many effects of these endogenous signaling molecules are mediated via stimulation of guanylyl cyclases and intracellular production of the second messenger cGMP. Indeed, cGMP-elevating drugs like glyceryl trinitrate have been used for more than 100 years to treat cardiovascular diseases. However, the molecular mechanisms of NO/NP signaling downstream of cGMP are not completely understood. Recent in vitro and in vivo evidence identifies cGMP-dependent protein kinases (cGKs) as major mediators of cGMP signaling in the cardiovascular system. In particular, the analysis of conventional and conditional knockout mice indicates that cGKs are critically involved in regulating vascular remodeling and thrombosis. Thus, cGKs may represent novel drug targets for the treatment of human cardiovascular disorders.

Inositol 1,3,4,5-tetrakisphosphate is essential for T lymphocyte development

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) is phosphorylated by Ins(1,4,5)P(3) 3-kinase, generating inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). The physiological function of Ins(1,3,4,5)P(4) is still unclear, but it has been reported to be a potential modulator of calcium mobilization. Disruption of the gene encoding the ubiquitously expressed Ins(1,4,5)P(3) 3-kinase isoform B (Itpkb) in mice caused a severe T cell deficiency due to major alterations in thymocyte responsiveness and selection. However, we were unable to detect substantial defects in Ins(1,4,5)P(3) amounts or calcium mobilization in Itpkb(-/-) thymocytes. These data indicate that Itpkb and Ins(1,3,4,5)P(4) define an essential signaling pathway for T cell precursor responsiveness and development.

Actin binding of human LIM and SH3 protein is regulated by cGMP- and cAMP-dependent protein kinase phosphorylation on serine 146

Various drugs that elevate cGMP levels and activate cGMP-dependent protein kinase (cGK) inhibit agonist-induced platelet activation. In the present study we identified the LIM and SH3 domain protein (LASP) that was recently cloned from human breast cancer cells (Tomasetto, C., Regnier, C., Moog-Lutz, C., Mattei, M. G., Chenard, M. P., Liderau, R., Basset, P., and Rio, M. C. (1995) Genomics 28, 367-376) as a novel substrate of cGK in human platelets. Recombinant human LASP was phosphorylated by cGMP- and cAMP-dependent protein kinase (cAK) in vitro. Cotransfection of PtK-2 cells with LASP and cGK confirmed phosphorylation of LASP in vivo. Studies with human LASP mutants identified serine 146 as a specific phosphorylation site for cGK and cAK in vivo. LASP is an actin-binding protein, and the phospho-LASP-mimicking mutant S146D showed reduced binding affinity for F-actin in cosedimentation experiments. Immunofluorescence of transfected PtK2 cells demonstrated the localization of LASP in the tips of cell membrane extensions and at cell-cell contacts. Expression of the human LASP mutant S146D resulted in nearly complete relocalization to the cytosol and reduced migration of the cells. Taken together, these data suggest that phosphorylation of LASP by cGK and cAK may be involved in cytoskeletal organization and cell motility.

Expression and role of TRPC proteins in human platelets: evidence that TRPC6 forms the store-independent calcium entry channel

Store-operated Ca(++) entry (SOCE) is thought to comprise the major pathway for Ca(++) entry in platelets. Recently, a number of transient receptor potential (TRP) proteins, which have been divided into 3 groups (TRPC, TRPM, and TRPV), have been suggested as SOCE channels. We report the expression and function of TRPC proteins in human platelets. TRPC6 is found at high levels and TRPC1 at low levels. Using purified plasma (PM) and intracellular membranes (IM), TRPC6 is found in the PM, but TRPC1 is localized to the IM. Using Fura-2-loaded platelets, we report that, in line with TRPC6 expression, 1-oleoyl-2-acetyl-sn-glycerol (OAG) stimulated the entry of Ca(++) and Ba(2+) independently of protein kinase C. Thrombin also induced the entry of Ca(++) and Ba(2+), but thapsigargin, which depletes the stores, induced the entry of only Ca(++). Thus, thrombin activated TRPC6 via a SOCE-independent mechanism. In phosphorylation studies, we report that neither TRPC6 nor TRPC1 was a substrate for tyrosine kinases. TRPC6 was phosphorylated by cAMP-dependent protein kinase (cAMP-PK) and associated with other cAMP-PK substrates. TRPC1 was not phosphorylated by cAMP-PK but also associated with other substrates. Activation of cAMP-PK inhibited Ca(++) but not Ba(2+) entry induced by thrombin and neither Ca(++) nor Ba(2+) entry stimulated by OAG. These results suggest that TRPC6 is a SOCE-independent, nonselective cation entry channel stimulated by thrombin and OAG. TRPC6 is a substrate for cAMP-PK, although phosphorylation appears to not affect cation permeation. TRPC1 is located in IM, suggesting a role at the level of the stores.

Development of platelet inhibition by cAMP during megakaryocytopoiesis

Prostacyclin is a potent inhibitor of agonist-induced Ca2+ increases in platelets, but in the megakaryocytic cell line MEG-01 this inhibition is absent. Using human megakaryocytic cell lines representing different stages in megakaryocyte (Mk) maturation as well as stem cells and immature and mature megakaryocytes, we show that the inhibition by prostacyclin develops at a late maturation stage shortly before platelets are formed. This late appearance is not caused by insufficient cAMP formation or absent protein kinase A (PKA) activity in immature cells. Instead, the appearance of Ca2+ inhibition by prostacyclin is accompanied by a sharp increase in the expression of the catalytic subunit of PKA (PKA-C) but not by changes in the expression of the PKA-regulatory subunits Ialpha/beta, IIalpha, and IIbeta. Overexpression of PKA-C in the megakaryocytic cell line CHRF-288-11 potentiates the Ca2+ inhibition by prostacyclin. Thus, up-regulation of PKA-C appears to be a key step in the development of Ca2+ inhibition by prostacyclin in platelets.

Fast biphasic regulation of type 3 inositol trisphosphate receptors by cytosolic calcium

In cytosol-like medium (CLM) with a free [Ca(2+)] of 200 nm, a supramaximal concentration of inositol 1,4,5-trisphosphate (IP(3)) (30 microm) evoked (45)Ca(2+) release from type 3 IP(3) receptors only after a latency of 48 +/- 6 ms; this latency could not be reduced by increasing the IP(3) concentration. In CLM containing a low free [Ca(2+)] ( approximately 4 nm), 300 microm IP(3) evoked (45)Ca(2+) release after a latency of 66 +/- 11 ms; this was reduced to 14 +/- 3 ms when the [Ca(2+)] was 1 mm. Preincubation with CLM containing 100 microm Ca(2+) caused a rapid (half-time = 33 +/- 9 ms), complete, and fully reversible inhibition that could not be overcome by a high concentration of IP(3) (300 microm). Hepatic (type 2) IP(3) receptors were not inhibited by Ca(2+) once they had bound IP(3), but 100 microm Ca(2+) rapidly inhibited type 3 IP(3) receptors whether it was delivered before addition of IP(3) or at any stage during a response to IP(3). Ca(2+) increases the affinity of IP(3) for hepatic receptors by slowing IP(3) dissociation, but Ca(2+) had no effect on IP(3) binding to type 3 receptors. The rate of inhibition of type 3 IP(3) receptors by Ca(2+) was faster than the rate of IP(3) dissociation, and occurred at similar rates whether receptors had bound a high (adenophostin) or low affinity (3-deoxy-3-fluoro-IP(3)) agonist. Dissociation of agonist is not therefore required for Ca(2+) to inhibit type 3 IP(3) receptors. We conclude that type 2 and 3 IP(3) receptors are each biphasically regulated by Ca(2+), but by different mechanisms. For both, IP(3) binding causes a stimulatory Ca(2+)-binding site to be exposed allowing Ca(2+) to bind and open the channel. IP(3) binding protects type 2 receptors from Ca(2+) inhibition, but type 3 receptors are inhibited by Ca(2+) whether or not they have IP(3) bound. Increases in cytosolic [Ca(2+)] will immediately inhibit type 3 receptors, but inhibit type 2 receptors only after IP(3) has dissociated.

Irregular spiking in free calcium concentration in single, human platelets. Regulation by modulation of the inositol trisphosphate receptors

Fluorescence ratio imaging indicates that immobilized, aspirin-treated platelets, loaded with Fura-2, respond to inositol 1,4,5-trisphosphate- (InsP3)-generating agonists such as thrombin by high-frequency, irregular rises in cytosolic [Ca2+]i with spikes that vary in peak level and peak-to-peak interval. This differs from the regular [Ca2+]i oscillations observed in other, larger cells. We found that the thiol-reactive compounds thimerosal (10 microm) and U73122 (10 microm) evoked similar irregular Ca2+ responses in platelets, but in this case in the absence of InsP3 generation. Thrombin-induced spiking was acutely abolished by inhibiting phospholipase C or elevating intracellular cAMP levels, while spiking with sulfhydryl reagents was only partially blocked by cAMP elevation. Confocal laser scanning microscopy using fluo-3-loaded platelets indicated that, with all agonists or conditions, the irregular spikes were almost instantaneously raised in various regions within a single platelet. When using saponin-permeabilized platelets, we found that InsP3-induced Ca2+ release from stores was stimulated by modest Ca2+ concentrations, pointing to a mechanism of InsP3-dependent Ca2+-induced Ca2+ release (CICR). This process was completely inhibitable by heparin. The Ca2+ release by InsP3, but not the CICR sensor, was negatively regulated by cAMP elevation. Thimerosal treatment did not release Ca2+ from intracellular stores, but markedly potentiated the stimulatory effect of InsP3. In contrast, U73122 caused a heparin/cAMP-insensitive Ca2+ leak from stores that differed from those used by InsP3. Taken together, these results demonstrate that InsP3 receptor channels play a crucial role in the irregular, spiking Ca2+ signal of intact platelets, even when induced by agents such as thimerosal or U73122 which do not stimulate InsP3 formation. The irregular Ca2+ release events appear to be subjected to extensive regulation by: (a) InsP3 level, (b) the potentiating effect of elevated Ca2+ on InsP3 action via CICR, (c) InsP3 channel sensitization by sulfhydryl (thimerosal) modification, (d) InsP3 channel-independent Ca2+ leak with U73122, and (e) down-regulation via cAMP elevation. The observation that individual Ca2+ peaks were generated in various parts of a platelet at similar intervals and amplitudes points to effective cooperation of the various stores in the Ca2+-release process.

Taming platelets with cyclic nucleotides

Cardiovascular diseases are often accompanied and aggravated by pathologic platelet activation. Tight regulation of platelet function is an essential prerequisite for intact vessel physiology or effective cardiovascular therapy. Physiological platelet antagonists as well as various pharmacological vasodilators inhibit platelet function by activating adenylyl and guanylyl cyclases and increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP) levels, respectively. Elevation of platelet cyclic nucleotides interferes with basically all known platelet activatory signaling pathways, and effectively blocks complex intracellular signaling networks, cytoskeletal rearrangements, fibrinogen receptor activation, degranulation, and expression of pro-inflammatory signaling molecules. The major target molecules of cyclic nucleotides in platelets are cyclic nucleotide-dependent protein kinases that mediate their effects through phosphorylation of specific substrates. They directly affect receptor/G-protein activation and interfere with a variety of signal transduction pathways, including the phospholipase C, protein kinase C, and mitogen-activated protein kinase pathways. Regulation of these pathways blocks several steps of cytosolic Ca(2+) elevation and controls a multitude of cytoskeleton-associated proteins that are directly involved in organization of the platelet cytoskeleton. Due to their multiple sites of action and strong inhibitory potencies, cyclic nucleotides and their regulatory pathways are of particular interest for developing new approaches for the treatment of thrombotic and cardiovascular disorders.

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.