Involvement of Na+/Ca2+ Exchanger in Inside-Out Signaling Through the Platelet Integrin IIbβ3

Study on the Mechanism of the Adrenaline-Evoked Procoagulant Response in Human Platelets

Adrenaline has recently been found to trigger phosphatidylserine (PS) exposure on blood platelets, resulting in amplification of the coagulation process, but the mechanism is only fragmentarily established. Using a panel of platelet receptors' antagonists and modulators of signaling pathways, we evaluated the importance of these in adrenaline-evoked PS exposure by flow cytometry. Calcium and sodium ion influx into platelet cytosol, after adrenaline treatment, was examined by fluorimetric measurements. We found a strong reduction in PS exposure after blocking of sodium and calcium ion influx via Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger (NCX), respectively. ADP receptor antagonists produced a moderate inhibitory effect. Substantial limitation of PS exposure was observed in the presence of GPIIb/IIIa antagonist, phosphoinositide-3 kinase (PI3-K) inhibitors, or prostaglandin E1, a cyclic adenosine monophosphate (cAMP)-elevating agent. We demonstrated that adrenaline may develop a procoagulant response in human platelets with the substantial role of ion exchangers (NHE and NCX), secreted ADP, GPIIb/IIIa-dependent outside-in signaling, and PI3-K. Inhibition of the above mechanisms and increasing cytosolic cAMP seem to be the most efficient procedures to control adrenaline-evoked PS exposure in human platelets.

Role of Na+/Ca2+ exchanger (NCX) in modulating postovulatory aging of mouse and rat oocytes

We studied the role of the Na+/Ca2+ exchanger (NCX) in modulating oocyte postovulatory aging by observing changes in NCX contents and activities in aging mouse and rat oocytes. Whereas the NCX activity was measured by observing oocyte activation following culture with NCX inhibitor or activator, the NCX contents were determined by immunohistochemical quantification. Although NCX was active in freshly-ovulated rat oocytes recovered 13 h post hCG injection and in aged oocytes recovered 19 h post hCG in both species, it was not active in freshly-ovulated mouse oocytes. However, NCX became active when the freshly-ovulated mouse oocytes were activated with ethanol before culture. Measurement of cytoplasmic Ca2+ revealed Ca2+ increases always before NCX activation. Whereas levels of the reactive oxygen species (ROS) and the activation susceptibility increased, the density of NCX member 1 (NCX1) decreased significantly with oocyte aging in both species. While culture with H2O2 decreased the density of NCX1 significantly, culture with NaCl supplementation sustained the NCX1 density in mouse oocytes. It was concluded that (a) the NCX activity was involved in the modulation of oocyte aging and spontaneous activation; (b) ROS and Na+ regulated the NCX activity in aging oocytes by altering its density as well as functioning; and (c) cytoplasmic Ca2+ elevation was essential for NCX activation in the oocyte.

Epithelial sodium channel modulates platelet collagen activation

Activated platelets adhere to the exposed subendothelial extracellular matrix and undergo a rapid cytoskeletal rearrangement resulting in shape change and release of their intracellular dense and alpha granule contents to avoid hemorrhage. A central step in this process is the elevation of the intracellular Ca(2+) concentration through its release from intracellular stores and on throughout its influx from the extracellular space. The Epithelial sodium channel (ENaC) is a highly selective Na(+) channel involved in mechanosensation, nociception, fluid volume homeostasis, and control of arterial blood pressure. The present study describes the expression, distribution, and participation of ENaC in platelet migration and granule secretion using pharmacological inhibition with amiloride. Our biochemical and confocal analysis in suspended and adhered platelets suggests that ENaC is associated with Intermediate filaments (IF) and with Dystrophin-associated proteins (DAP) via α-syntrophin and β-dystroglycan. Migration assays, quantification of soluble P-selectin, and serotonin release suggest that ENaC is dispensable for migration and alpha and dense granule secretion, whereas Na(+) influx through this channel is fundamental for platelet collagen activation.

A functional interaction between TRPC/NCKX induced by DAG plays a role in determining calcium influx independently from PKC activation

Ca(2+)influx might occur through K(+)-dependent Na(+)/Ca(2+) exchanger operating in reverse mode (rNCKX). In a cellular model different from platelets, an interaction between canonical transient receptor potential cation (TRPC) channels and NCX has been found. The aim of this study was to verify whether the TRPC/NCKX interaction operates in human platelets. Our results showed that the diacylglycerol (DAG) analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG) induced rNCKX-mediated Ca(2+) influx through TRPC-mediated Na(+) influx. DAG-induced activation of TRPC/NCKX occurs independently of protein kinase C (PKC) activation, as PKC inhibitor did not modify OAG-mediated Ca(2+) influx. Moreover, as both rNCKX and TRPC inhibitors reduced OAG-induced platelet aggregation which, conversely, was increased by flufenamic acid, known to develop TRPC activity, it could be suggested that the TRPC/NCKX interaction has a role in OAG-dependent platelet aggregation.

Microparticle formation after co-culture of human whole blood and umbilical artery in a novel in vitro model of flow

Cardiovascular disease (CVD) is now the largest killer in western society, and the importance of interactions between vascular endothelium and circulating blood components in disease pathogenesis is well established. Microparticles are a heterogeneous population of <1 μm blood borne particles that arise from blebbing or shedding of cell membranes. The microparticle population includes several classes of apoptotic bodies; however, increased numbers of procoagulant microparticles have been described in plasma from people with CVD. We have previously demonstrated that interactions of monocytes and platelets with isolated inflamed endothelial cells lead to production of pro-coagulant tissue factor bearing microparticles under laminar flow conditions. Here we have investigated microparticle production after perfusion of human whole blood through intact inflamed human umbilical artery. When blood was perfused through umbilical arteries which had been pre-stimulated with tumour necrosis factor (TNFα) for 18 h under flow conditions, there was significantly increased production of microparticles from both platelet and non-platelet sources, in particular from erythrocytes. To determine whether microparticles generated during interactions with inflamed endothelium could induce a pro-inflammatory response in trans, we isolated microparticles by centrifugation after co-culture and incubated with isolated quiescent endothelial cells followed by measurement of reactive oxygen species formation. Microparticles derived from co-culture with inflamed endothelium induced significantly enhanced levels of reactive oxygen species (ROS). These data suggest that presence of an inflamed endothelium causes release of pro-inflammatory microparticles from circulating blood cells, which could contribute to prolonged endothelial activation and subsequent atherosclerotic changes in blood vessels subjected to inflammatory insult.

Effect on ex vivo platelet aggregation and in vivo cyclic flow with Na+/H+ exchange inhibition: Gumina, NHE-1 inhibition and platelet aggregation

BIIB 513 and EMD 85131 are selective inhibitors of the Na+/H+ exchanger-1 (NHE-1) that are benzoylguanidine derivatives of the clinically employed diuretic amiloride. Prior studies have suggested a role for NHE-1 activity in platelet activation and aggregation using amiloride or its non- benzoylguanidines derivatives. However, the concentrations employed in these prior studies were at levels known to exert effects on other ion transport systems besides the NHE-1. Therefore, the purpose of this study was to examine the effects of more selective NHE-1 inhibitors, BIIB 513 and EMD 85131, on platelet aggregation and in vivo cyclic flow following arterial injury. BIIB 513 and EMD 85131 effects on ex vivo canine and human platelet aggregation in response to various agents was monitored via platelet aggregation. For analysis of in vivo thrombus formation, a femoral artery crush injury model was employed and a flow meter was used to monitor the effect of BIIB 513 on cyclic blood flow. Treatment of either canine or human platelets with up to 1 mM of BIIB 513 had no effect on aggregation induced by platelet activating factor (PAF), thrombin receptor activator peptide (TRAP), or adenosine diphosphate (ADP). Additionally, the structurally related compound EMD 85131 at up to 1 mM failed to inhibit TRAP induced platelet aggregation. In vivo administration of up to 9 mg/kg of BIIB 513 intravenously failed to affect cyclic flow in a canine model of femoral artery injury. These data demonstrate that the specific and selective NHE-1 inhibitors BIIB 513 or EMD 85131 have no effect on ex vivo platelet aggregation or in vivo cyclic flow following arterial injury.

A key role for dense granule secretion in potentiation of the Ca2+ signal arising from store-operated calcium entry in human platelets

Recent work has demonstrated a role for Na(+)/Ca(2+) exchange in potentiation of the Ca(2+) entry elicited through the human platelet store-operated channel by controlling a Mn(2+)-impermeable Ca(2+) entry pathway. Here we demonstrate that this involves control over the secretion of dense granules by a Na(+)/Ca(2+) exchanger (NCX) and so autocrine signalling between platelets. NCX inhibition reduced dense granule secretion. The reduction in SOCE elicited by NCX inhibition could be reversed by the addition of uninhibited donor cells, their releasate alone, or exogenous ADP and 5-HT. The use of specific receptor antagonists indicated that ATP, ADP and 5-HT all played a role in NCX-dependent autocrine signalling between platelets following thapsigargin stimulation, by activating Mn(2+)-impermeable Ca(2+) entry pathways. These data provide further insight into the mechanisms underlying the known interrelationship between platelet Ca(2+) signalling and dense granule secretion, and suggest an important role for the NCX in potentiation of platelet activation via dense granule secretion and so autocrine signalling. Our results caution the interpretation of platelet Ca(2+) signalling studies involving pharmacological or other manipulations that do not assess possible effects on NCX activity and dense granule secretion.

Decreased platelet expression of myosin regulatory light chain polypeptide (MYL9) and other genes with platelet dysfunction and CBFA2/RUNX1 mutation: insights from platelet expression profiling

We have reported on a patient with thrombocytopenia, impaired platelet aggregation, secretion, phosphorylation of pleckstrin and myosin light chain (MLC), and GPIIb-IIIa activation, associated with a heterozygous mutation in transcription factor CBFA2 (core binding factor A2, RUNX1 or AML1). To obtain insights into the abnormal platelet mechanisms and CBFA2-regulated genes, we performed platelet expression profiling in four control subjects and the patient using the Affymetrix U133 GeneChips. In the patient, 298 probe sets were significantly downregulated at least 2-fold. MLC regulatory polypeptide (MYL9 gene) was decreased approximately 77-fold; this is an important finding because agonist-stimulated MLC phosphorylation is decreased in patient platelets. Genes downregulated > or = 5-fold include those involving calcium binding proteins (CABP5), ion transport (sodium/potassium/Ca exchanger, SLC24A3), cytoskeletal/microtubule proteins (erythrocyte membrane protein band 4.1-like 3, EPB41L3; tropomyosin 1, TPM1; tubulin, alpha 1, TUBA1), signaling proteins (RAB GTPase activating protein 1-like, RABGAP1L; beta3-endonexin, ITGB3 BP) and chemokines (platelet factor 4 variant 1, PF4V1; chemokine CXCL5, CXCL5). These and other downregulated genes are relevant to the patient's platelet defects in function and production. These studies provide the first proof of concept that platelet expression profiling can be applied to obtain insights into the molecular basis of inherited platelet defects.

Critical role of ADP interaction with P2Y12 receptor in the maintenance of alpha(IIb)beta3 activation: association with Rap1B activation

OBJECTIVE

Platelet integrin alpha(IIb)beta3 plays a crucial role in platelet aggregation, and the affinity of alpha(IIb)beta3 for fibrinogen is dynamically regulated. Employing modified ligand-binding assays, we analyzed the mechanism by which alpha(IIb)beta3 maintains its high-affinity state.

METHODS AND RESULTS

Washed platelets adjusted to 50 x 10(3) microL(-1) were stimulated with 0.2 U mL(-1) thrombin or 5 microm U46619 under static conditions. After the completion of alpha(IIb)beta3 activation and granule secretion, different kinds of antagonists were added to the activated platelets. The activated alpha(IIb)beta3 was then detected by fluorescein isothiocyanate (FITC)-labeled PAC1. The addition of 1 mum AR-C69931MX (a P2Y12 antagonist) or 1 mm A3P5P (a P2Y1 antagonist) disrupted the sustained alpha(IIb)beta3 activation by approximately 92% and approximately 38%, respectively, without inhibiting CD62P or CD63 expression. Dilution of the platelet preparation to 500 microL(-1) also disrupted the sustained alpha(IIb)beta3 activation, and the disruption by such dilution was abrogated by the addition of exogenous adenosine 5'-diphosphate (ADP) in a dose-dependent fashion. The amounts of ADP released from activated platelets determined by high-performance liquid chromatography were compatible with the amounts of exogenous ADP required for the restoration. We next examined the effects of antagonists on protein kinase C (PKC) and Rap1B activation induced by 0.2 U mL(-1) thrombin. Thrombin induced long-lasting PKC and Rap1B activation. AR-C69931MX markedly inhibited Rap1B activation without inhibiting PKC activation.

CONCLUSIONS

Our data indicate that the continuous interaction between released ADP and P2Y12 is critical for the maintenance of alpha(IIb)beta3 activation.

Impaired platelet function in a patient with P2Y12 deficiency caused by a mutation in the translation initiation codon

In this study, we have identified a patient (OSP-1) with a congenital P2Y12 deficiency showing a mild bleeding tendency from her childhood and examined the role of P2Y12 in platelet function. At low concentrations of agonists OSP-1 platelets showed an impaired aggregation to several kinds of stimuli, whereas at high concentrations they showed a specifically impaired platelet aggregation to adenosine diphosphate (ADP). ADP normally induced platelet shape change and failed to inhibit PGE1-stimulated cAMP accumulation in OSP-1 platelets. Molecular genetic analysis revealed that OSP-1 was a homozygous for a mutation in the translation initiation codon (ATG to AGG) in the P2Y12 gene. Heterologous cell expression of wild-type or mutant P2Y12 confirmed that the mutation was responsible for the deficiency in P2Y12. OSP-1 platelets showed a markedly impaired platelet spreading onto immobilized fibrinogen. Real-time observations of thrombogenesis under a high shear rate (2000 s(-1)) revealed that thrombi over collagen were small and loosely packed and most of the aggregates were unable to resist against high shear stress in OSP-1. Our data suggest that secretion of endogenous ADP and subsequent P2Y12-mediated signaling are critical for platelet aggregation, platelet spreading, and as a consequence, for stabilization of thrombus.

Negative regulation of platelet function by a secreted cell repulsive protein, semaphorin 3A

Semaphorin 3A (Sema3A) is a secreted disulfide-bound homodimeric molecule that induces growth cone collapse and repulsion of axon growth in the nervous system. Recently, it has been demonstrated that Sema3A is produced by endothelial cells and inhibits integrin function in an autocrine fashion. In this study, we investigated the effects of Sema3A on platelet function by using 2 distinct human Sema3A chimera proteins. We detected expression of functional Sema3A receptors in platelets and dose-dependent and saturable binding of Sema3A to platelets. Sema3A dose-dependently inhibited activation of integrin alphaIIbbeta3 by all agonists examined including adenosine diphosphate (ADP), thrombin, convulxin, phorbol 12-myristate 13-acetate, and A23187. Sema3A inhibited not only platelet aggregation induced by thrombin or collagen but also platelet adhesion and spreading on immobilized fibrinogen. Moreover, Sema3A impaired alphaIIbbeta3-independent spreading on glass coverslips and aggregation-independent granular secretion. Sema3A inhibited agonist-induced elevation of filamentous action (F-actin) contents, phosphorylation of cofilin, and Rac1 activation. In contrast, Sema3A did not affect the levels of cyclic nucleotides or agonist-induced increase of intracellular Ca2+ concentrations. Thus, the extensive inhibition of platelet function by Sema3A appears to be mediated, at least in part, through impairment of agonist-induced Rac1-dependent actin rearrangement.

SHPS-1 negatively regulates integrin alphaIIbbeta3 function through CD47 without disturbing FAK phosphorylation

CD47 (integrin-associated protein) serves as a receptor for thrombospondin-1 (TSP-1) and Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1), and the TSP-1/CD47 interaction has been believed to augment integrin-mediated platelet function. Here, employing SHPS-1-immunoglobulin (Ig) as a ligand, we have newly demonstrated that CD47 acts as an inhibitory receptor for platelet function. The binding of SHPS-1-Ig was solely mediated by CD47, because CD47-deficient platelets failed to bind murine SHPS-1-Ig. The human SHPS-1/CD47 interaction inhibited the platelet aggregation induced by several kinds of agonists at a low concentration. Moreover, human SHPS-1 expressed on the cell surface as well as soluble SHPS-1-Ig markedly inhibited the platelet spreading on, but not initial adhesion to, immobilized fibrinogen. Again, neither murine SHPS-1 expressed on the cell surface nor murine SHPS-1-Ig inhibited the spreading of CD47-deficient platelets. We further investigated the tyrosine phosphorylation of signaling proteins during platelet spreading on immobilized fibrinogen. Unexpectedly, SHPS-1 inhibited alpha(IIb)beta(3)-mediated platelet spreading without disturbing focal adhesion kinase (FAK) tyrosine phosphorylation. Further examination revealed that SHPS-1 inhibited the tyrosine phosphorylation of alpha-actinin, a downstream effector of FAK, but not of cortactin. Thus, it is likely that the SHPS-1/CD47 interaction inhibits alpha(IIb)beta(3)-mediated outside-in signaling by interfering with the downstream pathway of FAK. Taken together, our data suggest that SHPS-1 negatively regulates platelet function via CD47, especially alpha(IIb)beta(3)-mediated outside-in signaling.

Expression and subcellular localization of WAVE isoforms in the megakaryocyte/platelet lineage

WAVE isoforms, which consist of WAVE-1, WAVE-2 and WAVE-3, are members of the Wiskott-Aldrich syndrome protein (WASP) family. They are implicated in the regulation of actin-cytoskeletal reorganization downsteam of the small GTPase, Rac. Since platelet attachment to extracellular matrices leads to filopodial and lamellipodial extension, we examined the expression and subcellular localization of WAVEs in platelets. Employing primary megakaryocytic cells derived from cord blood as well as megakaryocytic cell lines, we also examined their expression during megakaryocytic differentiation. Immunoblotting and immunohistochemical analysis revealed that platelets expressed WAVE-1 and WAVE-2, whereas WAVE-3 expression was hardly to be detected. WAVE-1 expression was associated with megakaryocytic differentiation, whereas WAVE-2 and WAVE-3 expression was not changed by the differentiation. In adhered platelets both WAVE-1 and WAVE-2 were localized at the edge of the lamellipodia and at the tips of filopodia. In WASP-deficient platelets we found normal lamellipodial formation and localization of WAVE-1 and WAVE-2 at the edges of lamellipodia. Furthermore, we demonstrated that WAVE-1 and WAVE-2 moved from a detergent-soluble cytosolic fraction to insoluble cytoskeleton fraction after platelet aggregation. These results suggest that WAVE-1 and WAVE-2 regulate actin reorganization during platelet spreading and aggregate formation.

Effects of the NHE-1 inhibitor cariporide alone or together with the P2Y12 antagonist AR-C 69331 MX on CD62p expression and formation of platelet-leukocyte aggregates

The sodium-hydrogen exchanger isoform 1 (NHE-1) contributes to platelet activation at elevated pH. Effects of NHE-1 inhibitors on platelet degranulation and formation of proinflammatory and procoagulatory platelet-leukocyte aggregates (PLA) and possible interactions with P2Y(12) inhibitors--which also affect platelet degranulation--have not been investigated. Whole blood from healthy human subjects was incubated with the NHE-1 inhibitor cariporide and the P2Y(12) inhibitor AR-C 69331 MX at clinically reasonable concentrations, in the presence of normal pH or in a propionate model to activate the NHE-1 (approximately pH 7.0). The degranulation marker CD62p, the expression of the activated GPIIb/IIIa receptor (PAC-1), and formation of platelet-leukocyte (monocyte) aggregates (PLA) was assessed by flow cytometry. Cariporide at concentrations up to 20 microg/ml had no effects on ADP- (5 microM) or TRAP- (2 microM) induced CD62p expression or PLA formation at normal pH. At pH 7.0 and stimulation with ADP, PLA decreased from 64+/-24% (control) to 47+/-23% under cariporide at 2 microg/ml (p<0.05), and the MFI of PLA (i.e. the platelet mass attached at monocytes) decreased from 547+/-203 to 360+/-96 units (p<0.05). PAC-1 MFI decreased from 66+/-23 to 34+/-18 units (p<0.05) after ADP and from 74+/-29 to 42+/-17 units (p<0.05) after TRAP, respectively. AR-C 69331 MX (10 nM) had inhibitory effects on all parameters irrespectively of the pH, and the combination of both agents at pH 7.0 shows additive effects. In conclusion, our investigation points to-perhaps clinically relevant-effects of NHE-1 inhibition on the degranulation of platelets and formation of platelet-leukocyte aggregates.

A novel action of the antianginal drug bepredil: induction of internal Ca(2+) release and external Ca(2+) influx in Madin-Darby canine kidney (MDCK) epithelial cells

The effect of the antianginal drug bepridil on Ca(2+) signaling in Madin-Darby canine kidney (MDCK) cells was investigated by using fura-2 as a Ca(2+) probe. Bepridil at 10-50 microM evoked a significant rise in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in a dose-dependent manner. The [Ca(2+)](i) rise consisted of an immediate initial rise and a slow decay. Removal of external Ca(2+) partly inhibited the Ca(2+) signals by reducing both the initial rise and the decay phase, suggesting that bepridil activated both external Ca(2+) influx and internal Ca(2+) release. In Ca(2+)-free medium, pretreatment with 50 microM bepridil nearly abolished the Ca(2+) release induced by thapsigargin (1 microM), an endoplasmic reticulum Ca(2+) pump inhibitor, and vice versa, pretreatment with thapsigargin inhibited most of the bepridil-induced Ca(2+) release, suggesting that the thapsigargin-sensitive Ca(2+) store was the main source of bepridil-induced Ca(2+) release. Bepridil (50 microM) induced considerable Mn(2+) quench of fura-2 fluorescence at an excitation wavelength of 360 nm, which was partly inhibited by La(3+) (0.1 mM). Consistently, La(3+) (0.1 mM) pretreatment significantly inhibited the bepridil-induced [Ca(2+)](i) rise. Addition of 3 mM Ca(2+) induced a significant [Ca(2+)](i) rise after prior incubation with 10-50 microM bepridil in Ca(2+)-free medium, suggesting that bepridil induced dose-dependent capacitative Ca(2+) entry. However, 50 microM bepridil inhibited 1 microM thapsigargin-induced capacitative Ca(2+) entry by 38%. Pretreatment with aristolochic acid (40 microM) so as to inhibit phospholipase A(2) inhibited 50 microM bepridil-induced internal Ca(2+) release by 42%, but inhibition of phospholipase C with U73122 (2 microM) or inhibition of phospholipase D with propranolol (0.1 mM) had little effect, suggesting that bepridil induced internal Ca(2+) release in an inositol 1,4,5-trisphosphate-independent manner that could be modulated by phospholipase A(2)-coupled events. This is the first report providing evidence that bepridil, currently used as an antianginal drug, induced a rise in [Ca(2+)](i) in a non-excitable cell line.