ADP receptor P2Y12 is the capstone of the cross-talk between Ca2+ mobilization pathways dependent on Ca2+ ATPases sarcoplasmic/endoplasmic reticulum type 3 and type 2b in platelets.

Background: Blood platelet Ca2+ stores are regulated by 2 Ca2+-ATPases (SERCA2b and SERCA3). On thrombin stimulation, nicotinic acid adenosine dinucleotide phosphate mobilizes SERCA3-dependent stores, inducing early adenosine 5'-diphosphate (ADP) secretion, potentiating later SERCA2b-dependent secretion. Objectives: The aim of this study was to identify which ADP P2 purinergic receptor (P2Y1 and/or P2Y12) is(are) involved in the amplification of platelet secretion dependent on the SERCA3-dependent Ca2+ mobilization pathway (SERCA3 stores mobilization) as triggered by low concentration of thrombin. Methods: The study used the pharmacologic antagonists MRS2719 and AR-C69931MX, of the P2Y1 and P2Y12, respectively, as well as Serca3 -/- mice and mice exhibiting platelet lineage-specific inactivation of the P2Y1 or P2Y12 genes. Results: We found that in mouse platelets, pharmacological blockade or gene inactivation of P2Y12 but not of P2Y1 led to a marked inhibition of ADP secretion after platelet stimulation with low concentration of thrombin. Likewise, in human platelets, pharmacological inhibition of P2Y12 but not of P2Y1 alters amplification of thrombin-elicited secretion through SERCA2b stores mobilization. Finally, we show that early SERCA3 stores secretion of ADP is a dense granule secretion, based on parallel adenosine triphosphate and serotonin early secretion. Furthermore, early secretion involves a single granule, based on the amount of adenosine triphosphate released. Conclusion: Altogether, these results show that at low concentrations of thrombin, SERCA3- and SERCA2b-dependent Ca2+ mobilization pathways cross-talk via ADP and activation of the P2Y12, and not the P2Y1 ADP receptor. The relevance in hemostasis of the coupling of the SERCA3 and the SERCA2b pathways is reviewed.

NAADP/SERCA3-Dependent Ca2+ Stores Pathway Specifically Controls Early Autocrine ADP Secretion Potentiating Platelet Activation.

RATIONALE: Ca2+ signaling is a key and ubiquitous actor of cell organization and its modulation controls many cellular responses. SERCAs (sarco-endoplasmic reticulum Ca2+-ATPases) pump Ca2+ into internal stores that play a major role in the cytosolic Ca2+ concentration rise upon cell activation. Platelets exhibit 2 types of SERCAs, SERCA2b and SERCA3 (SERCA3 deficient mice), which may exert specific roles, yet ill-defined. We have recently shown that Ca2+ mobilization from SERCA3-dependent stores was required for full platelet activation in weak stimulation conditions. OBJECTIVE: To uncover the signaling mechanisms associated with Ca2+ mobilization from SERCA3-dependent stores leading to ADP secretion. METHODS AND RESULTS: Using platelets from wild-type or Serca3-deficient mice, we demonstrated that an early (within 5-10 s following stimulation) secretion of ADP specifically dependent on SERCA3 stored Ca2+ is exclusively mobilized by nicotinic acid adenosine dinucleotide-phosphate (NAADP): both Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion are blocked by the NAADP receptor antagonist Ned-19, and reciprocally both are stimulated by permeant NAADP. In contrast, Ca2+ mobilization from SERCA3-dependent stores and primary ADP secretion were unaffected by inhibition of the production of IP3 (inositol-1,4,5-trisphosphate) by phospholipase-C and accordingly were not stimulated by permeant IP3. CONCLUSIONS: Upon activation, an NAADP/SERCA3 Ca2+ mobilization pathway initiates an early ADP secretion, potentiating platelet activation, and a secondary wave of ADP secretion driven by both an IP3/SERCA2b-dependent Ca2+ stores pathway and the NAADP/SERCA3 pathway. This does not exclude that Ca2+ mobilized from SERCA3 stores may also enhance platelet global reactivity to agonists. Because of its modulating effect on platelet activation, this NAADP-SERCA3 pathway may be a relevant target for anti-thrombotic therapy. Graphic Abstract: A graphic abstract is available for this article.

Platelet Functions are Decreased in Obesity and Restored after Weight Loss: Evidence for a Role of the SERCA3-Dependent ADP Secretion Pathway.

In obesity, platelets are described as hyperactive, mainly based on increased platelet size and presence of pro-thrombotic plasmatic molecules. We explored platelet functions, including calcium signalling in obesity, and the effect of weight loss. We included 40 obese patients (women with body mass index [BMI] of ≥ 35 kg/m2) who were to undergo gastric bypass surgery and 40 healthy lean subjects (women with BMI of < 25 kg/m2) as a control group. Approximately 1 year after surgery, the obese patients lost weight (75% had a BMI < 35 kg/m2). They were explored a second time with the same healthy control for the same platelet experiments. Compared with controls, obese patients' platelets displayed reduced sensitivity to thrombin (aggregation EC50 increased by 1.9 ± 0.3-fold, p = 0.005) and a lower Ca2+ response (70 ± 7% decrease, p < 10-4). In 17 pairs of patients, we performed additional experiments: in obese patients' platelets, thrombin-induced αIIbß3 activation was significantly lower (p = 0.003) and sarco-endoplasmic reticulum Ca2+ATPase (SERCA3) expression was decreased (48 ± 6% decrease, p < 10-4). These differences were abolished after weight loss. Interestingly, pharmacological inhibition of SERCA3 activity in control group's platelets mimicked similar alterations than in obese patients' platelets and was associated with defective adenosine diphosphate (ADP) secretion. Addition of ADP to agonist restored platelet functions in obese patients and in SERCA3-inhibited control platelets (five experiments) confirming the direct involvement of the SERCA3-dependent ADP secretion pathway. This is the first study demonstrating that platelets from obese patients are hypo-reactive, due to a deficiency of SERCA3-dependent ADP secretion. Weight loss restores SERCA3 activity and subsequent calcium signalling, αIIbß3 activation, platelet aggregation and ADP secretion.

A mutation of the human EPHB2 gene leads to a major platelet functional defect.

The ephrin transmembrane receptor family of tyrosine kinases is involved in platelet function. We report the first EPHB2 variant affecting platelets in 2 siblings (P1 and P2) from a consanguineous family with recurrent bleeding and normal platelet counts. Whole-exome sequencing identified a c.2233C>T variant (missense p.R745C) of the EPHB2 gene. P1 and P2 were homozygous for this variant, while their asymptomatic parents were heterozygous. The p.R745C variant within the tyrosine kinase domain was associated with defects in platelet aggregation, αIIbß3 activation, and granule secretion induced by G-protein-coupled receptor (GPCR) agonists and convulxin, as well as in thrombus formation on collagen under flow. In contrast, clot retraction, flow-dependent platelet adhesion, and spreading on fibrinogen were only mildly affected, indicating limited effects on αIIbß3 outside-in signaling. Most importantly, Lyn, Syk, and FcRγ phosphorylation, the initial steps in glycoprotein VI (GPVI) platelet signaling were drastically impaired in the absence of platelet-platelet contact, indicating a positive role for EPHB2 in GPVI activation. Likewise platelet activation by PAR4-AP showed defective Src activation, as opposed to normal protein kinase C activity and Ca2+ mobilization. Overexpression of wild-type and R745C EPHB2 variant in RBL-2H3 (rat basophilic leukemia) cells stably expressing human GPVI confirmed that EPHB2 R745C mutation impaired EPHB2 autophosphorylation but had no effect on ephrin ligand-induced EPHB2 clustering, suggesting it did not interfere with EPHB2-ephrin-mediated cell-to-cell contact. In conclusion, this novel inherited platelet disorder affecting EPHB2 demonstrates this tyrosine kinase receptor plays an important role in platelet function through crosstalk with GPVI and GPCR signaling.

Full activation of mouse platelets requires ADP secretion regulated by SERCA3 ATPase-dependent calcium stores.

The role of the sarco-endoplasmic reticulum calcium (Ca(2+)) adenosine triphosphatase (ATPase) 3 (SERCA3) in platelet physiology remains poorly understood. Here, we show that SERCA3 knockout (SERCA3(-/-)) mice exhibit prolonged tail bleeding time and rebleeding. Thrombus formation was delayed both in arteries and venules in an in vivo ferric chloride-induced thrombosis model. Defective platelet adhesion and thrombus growth over collagen was confirmed in vitro. Adenosine 5'-diphosphate (ADP) removal by apyrase diminished adhesion and thrombus growth of control platelets to the level of SERCA3(-/-) platelets. Aggregation, dense granule secretion, and Ca(2+) mobilization of SERCA3(-/-) platelets induced by low collagen or low thrombin concentration were weaker than controls. Accordingly, SERCA3(-/-) platelets exhibited a partial defect in total stored Ca(2+) and in Ca(2+) store reuptake following thrombin stimulation. Importantly ADP, but not serotonin, rescued aggregation, secretion, and Ca(2+) mobilization in SERCA3(-/-) platelets, suggesting specificity. Dense granules appeared normal upon electron microscopy, mepacrine staining, and total serotonin content, ruling out a dense granule defect. ADP induced normal platelet aggregation, excluding a defect in ADP activation pathways. The SERCA3-specific inhibitor 2,5-di-(tert-butyl)-1,4-benzohydroquinone diminished both Ca(2+) mobilization and secretion of control platelets, as opposed to the SERCA2b inhibitor thapsigargin. This confirmed the specific role of catalytically active SERCA3 in ADP secretion. Accordingly, SERCA3-dependent Ca(2+) stores appeared depleted in SERCA3(-/-) platelets. Finally, αIIbß3 integrin blockade did not affect SERCA3-dependent secretion, therefore proving independent of αIIbß3 engagement. Altogether, these results show that SERCA3-dependent Ca(2+) stores control a specific ADP secretion pathway required for full platelet secretion induced by agonists at low concentration and independent of αIIbß3.

Comparison between electronic cigarette refill liquid and nicotine on metabolic parameters in rats.

AIMS: Nicotine is known to promote body weight loss and to disturb glucose homeostasis and lipoprotein metabolism. Electronic cigarettes, as a substitute to nicotine, are becoming increasingly popular, although there is no evidence regarding their safety. Considering the dearth of information about e-cigarette toxicity, the present study was designed to compare nicotine alone to e-liquid with or without nicotine on metabolic parameters in Wistar rats. MAIN METHODS: For this purpose, e-liquid with or without nicotine and nicotine alone (0.5mg/kg of body weight) were administered intra-peritoneally during 28 days. KEY FINDINGS: Our results show a significant decrease in food and energy intake after nicotine or e-liquid with nicotine exposure, when compared to control or e-liquid without nicotine. Analysis of lipid status identified a significant decrease in cholesterol and LDL levels in e-cigarette groups, suggesting an improvement in lipid profile. Interestingly, e-liquid without nicotine induced hyperglycemia which is negatively correlated to hepatic glycogen level, acting like nicotine alone. Furthermore, an increase in liver biomarkers was observed in all treated groups. qRT-PCR analysis showed GSK3ß up-regulation in e-liquid with nicotine as well as, surprisingly, in e-liquid without nicotine exposure. In contrast, PEPCK genes were only up-regulated in e-liquid with nicotine. SIGNIFICANCE: While some features observed in rats may not be observed in human smokers, most of our data are consistent with, e-liquid per se i.e. without nicotine, not being neutral from a metabolic stand point since disrupting glucose homeostasis in rats.

A new form of macrothrombocytopenia induced by a germ-line mutation in the PRKACG gene.

Macrothrombocytopenias are the most important subgroup of inherited thrombocytopenias. This subgroup is particularly heterogeneous because the affected genes are involved in various functions such as cell signaling, cytoskeleton organization, and gene expression. Herein we describe the clinical and hematological features of a consanguineous family with a severe autosomal recessive macrothrombocytopenia associated with a thrombocytopathy inducing a bleeding tendency in the homozygous mutated patients. Platelet activation and cytoskeleton reorganization were impaired in these homozygous patients. Exome sequencing identified a c.222C>G mutation (missense p.74Ile>Met) in PRKACG, a gene encoding the γ-catalytic subunit of the cyclic adenosine monophosphate-dependent protein kinase, the mutated allele cosegregating with the macrothrombocytopenia. We demonstrate that the p.74Ile>Met PRKACG mutation is associated with a marked defect in proplatelet formation and a low level in filamin A in megakaryocytes (MKs). The defect in proplatelet formation was rescued in vitro by lentiviral vector-mediated overexpression of wild-type PRKACG in patient MKs. We thus conclude that PRKACG is a new central actor in platelet biogenesis and a new gene involved in inherited thrombocytopenia with giant platelets associated with a thrombocytopathy.

von Willebrand factor mutation promotes thrombocytopathy by inhibiting integrin αIIbß3.

von Willebrand disease type 2B (vWD-type 2B) is characterized by gain-of-function mutations in von Willebrand factor (vWF) that enhance its binding to the glycoprotein Ib-IX-V complex on platelets. Patients with vWD-type 2B have a bleeding tendency that is linked to loss of vWF multimers and/or thrombocytopenia. In this study, we uncovered evidence that platelet dysfunction is a third possible mechanism for bleeding tendency. We found that platelet aggregation, secretion, and spreading were diminished due to inhibition of integrin αIIbß3 in platelets from mice expressing a vWD-type 2B-associated vWF (vWF/p.V1316M), platelets from a patient with the same mutation, and control platelets pretreated with recombinant vWF/p.V1316M. Impaired platelet function coincided with reduced thrombus growth. Further, αIIbß3 activation and activation of the small GTPase Rap1 were impaired by vWF/p.V1316M following exposure to platelet agonists (thrombin, ADP, or convulxin). Conversely, thrombin- or ADP-induced Ca2+ store release, which is required for αIIbß3 activation, was normal, indicating that vWF/p.V1316M acts downstream of Ca2+ release and upstream of Rap1. We found normal Syk phosphorylation and PLCγ2 activation following collagen receptor signaling, further implying that vWF/p.V1316M acts directly on or downstream of Ca2+ release. These data indicate that the vWD-type 2B mutation p.V1316M is associated with severe thrombocytopathy, which likely contributes to the bleeding tendency in vWD-type 2B.

Evidence for a granule targeting sequence within platelet factor 4.

Platelets achieve bleeding arrest at sites of vascular injury via secretion of secretory proteins from their storage granules, termed alpha-granules. We have recently analyzed granule targeting of platelet factor 4 (PF4), a secretory alpha-granule chemokine, and demonstrated that PF4 alpha-granule storage relied upon determinants within PF4 mature sequence. To define these determinants, PF4 mutants fused to the fluorescent reporter protein green fluorescent protein were generated by progressive deletions and site-directed mutagenesis. They were then transfected in AtT20 cells and assessed for granule targeting by colocalization with ACTH-containing granules, using laser scanning confocal microscopy. This strategy identified the amino acid 41-50 (LIATLKNGRK) sequence as most critical for PF4 granule targeting and/or storage; its deletion from PF4 induced a marked decrease in granule storage (from 81 +/- 2% to 17 +/- 3%, p < or = 0.0001). Ala-scanning mutagenesis of LIATLKNGRK narrowed down the targeting motif to LKNG. A direct role for LKNG in alpha-granule targeting was confirmed in the thrombopoietin-induced human megakaryocytic Dami cells, in which the LKNG-green fluorescent protein chimera exhibited an 82.5 +/- 1.8% colocalization with the alpha-granule proteins von Willebrand factor and P-selectin. LKNG is poorly conserved within the chemokine family. However three-dimensional alignments of the human alpha-granule chemokines Nap-2 (neutrophil-activating peptide) and RANTES (Regulated upon Activation Normal T Cell Expressed and Secreted) with PF4 revealed that LKNG, a surface-exposed hydrophilic turn/loop, matched Nap-2 (LKDG) and RANTES (TRKN) peptides with similar features. Moreover Nap-2 and RANTES peptides exhibited the same alpha-granule targeting efficiency than LKNG. We therefore postulate that the three-dimensional and physicochemical characteristics of PF4 LKNG are of general relevance to alpha-granule targeting of chemokines and possibly of other alpha-granule proteins.

Thrombopoietin-induced Dami cells as a model for alpha-granule biogenesis.

Megakaryocytic alpha-granules contain secretory proteins relevant to megakaryocyte and platelet functions. Understanding alpha-granule biogenesis is hampered because human primary megakaryocytes are difficult to manipulate. Existing promegakaryocytic cell lines do not spontaneously exhibit mature alpha-granules. Dami cells, transfected with the megakaryocytic platelet factor 4, fused to GFP (PF4-GFP), were induced in the presence of thrombopoietin (TPO), a megakaryocyte cytokine and PMA. Using confocal microscopy, PF4-GFP colocalized with von Willebrand Factor (vWF) in newly formed storage granules. Immunoelectron microscopy demonstrated alpha-granule-like features, including a dense core or parallel tubules and colocalization of PF4-GFP and vWF. Hence, TPO-treated Dami cells are a suitable model to study alpha-granules and their biogenesis.

Costimulation of the Gi-coupled ADP receptor and the Gq-coupled TXA2 receptor is required for ERK2 activation in collagen-induced platelet aggregation.

The stimulation of platelets by low doses of collagen induces extracellular signal-regulated kinase 2 (ERK2) activation. In this report, we demonstrate that collagen-induced ERK2 activation depends on thromboxane A(2) (TXA(2)) formation and ADP release. The collagen-induced ERK2 activation was inhibited by indomethacin (88%) and by AR-C69931MX (70%), a specific antagonist of P2Y12, a Gi-coupled ADP receptor. AR-C69931MX (10 microM) inhibition was overcome by epinephrine (1 microM), an agonist of the Gi-coupled alpha(2A)-adrenergic receptor, suggesting that the Gi-coupled receptor was necessary for ERK2 activation by collagen. By contrast, MRS 2179 (10 microM), a specific antagonist of P2Y1, a Gq-coupled ADP receptor, did not affect collagen-induced ERK2 activation. Little or no ERK2 activation was observed with ADP alone (10 microM). By contrast, U46619 (10 microM), a stable analog of TXA(2), induced ERK2 activation in an ADP-dependent manner, via the P2Y12 receptor. These results suggest that the Gi-dependent signaling pathway, stimulated by ADP or epinephrine, was not the only pathway required for ERK2 activation by collagen. Costimulation of the specific G(12/13)-coupled TXA(2) receptor with a low dose of U46619 (10 nM) and of Gi- and Gq-coupled ADP receptor (10 microM) induced very low levels of ERK2 activation, similar to those observed with ADP alone, suggesting that G(12/13) is not involved or not sufficient to induce the additional pathway necessary for ERK2 activation. The Gq-coupled TXA(2) receptor was required for ERK2 activation by U46619 (10 microM) and low doses of collagen, clearly showing that a coordinated pathway through both Gq from TXA(2) and Gi from ADP was necessary for ERK2 activation. Finally, we demonstrate that ERK2 activation is involved in collagen-induced aggregation and secretion.

Platelet ERK2 activation by thrombin is dependent on calcium and conventional protein kinases C but not Raf-1 or B-Raf.

Extracellular signal-regulated kinase (ERK) activation pathways have been well characterized in a number of cell types but very few data are available for platelets. The thrombin-induced signaling pathway leading to ERK2 activation in platelets is largely uncharacterized. In this study, we investigated the kinases involved in thrombin-induced ERK2 activation in conditions of maximal ERK2 activation. We found that thrombin-induced mitogen-activated protein kinase/ERK kinase (MEK)1/2 activation was necessary for ERK2 phosphorylation. We obtained strong evidence that conventional protein kinase Cs (PKCs) and calcium are involved in thrombin-induced ERK2 activation. First, ERK2 and MEK1/2 phosphorylation was totally inhibited by low concentrations (1 microM) of RO318425, a specific inhibitor of conventional PKCs. Second, Ca(2+), from either intracellular pools or the extracellular medium, was necessary for ERK2 activation and conventional PKC activation, excluding the involvement of a new class of calcium-insensitive PKCs. Third, LY294002 and wortmannin had no significant effect on ERK2 activation, even at concentrations that inhibit phosphatidylinositol (PI)3-kinase (5 microM to 25 microM and 50 nM, respectively). This suggests that PI3-kinase was not necessary for ERK2 activation and therefore, that PI3-kinase-dependent atypical PKCs were not involved. Surprisingly, in contrast to proliferative cells, we found that the serine/threonine kinases Raf-1 and B-Raf were not an intermediate kinase between conventional PKCs and MEK1/2. After immunoprecipitation of Raf-1 and B-Raf, the basal glutathione S-transferase-MEK1 phosphorylation observed in resting platelets was not upregulated by thrombin and was still observed in the absence of anti-Raf-1 or anti-B-Raf antibodies. In these conditions, the in vitro cascade kinase assay did not detect any MEK activity. Thus in platelets, thrombin-induced ERK2 activation is activated by conventional PKCs independently of Raf-1 and B-Raf activation.