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Keywords:

  • ADP;
  • P2Y12;
  • procoagulant activity;
  • thrombin

Abstract

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

During thrombus formation, thrombin, which is abundantly present at sites of vascular injury, activates platelets in part via autocrine-produced ADP. We investigated the signaling pathways by which thrombin and ADP in synergy induced platelet Ca2+ elevation and procoagulant activity, and we monitored the consequences for the coagulation process. Even at high thrombin concentration, autocrine and added ADP enhanced and prolonged Ca2+ depletion from internal stores via stimulation of the P2Y12 receptors. This P2Y12-dependent effect was mediated via two distinct signaling pathways. The first is enhanced Ca2+ mobilization by the inositol 1,4,5-trisphosphate receptors due to inhibition of protein kinase A. The second pathway concerns prolonged activation of phosphoinositide 3-kinase (PI3-K) and phospholipase C. Experiments with phosphoinositide 3-kinase isoform-selective inhibitors and p110γ deficient platelets demonstrated that the phosphoinositide 3-kinase β and not the phosphoinositide 3-kinase γ isoform is responsible for the prolonged Ca2+ response and for the subsequent increases in procoagulant activity and coagulation. Taken together, these results demonstrate a dual P2Y12-dependent signaling mechanism, which increases the platelet-activating effect of thrombin by prolongation of Ca2+ elevation, thereby facilitating the coagulation process.

Abbreviations
AM

acetoxymethyl ester

FITC

fluorescein isothiocyanate

InsP3

inositol 1,4,5-trisphosphate

OG

Oregon green

PI3-K

phosphoinositide 3-kinase, SERCA, sarco- and endoplasmic reticulum Ca2+-ATPase

PKA

protein kinase A

PRP

platelet-rich plasma

Platelets are activated at sites of vascular injury, and then clump together to form a vaso-occlusive thrombus. Platelet activation is usually triggered by the exposure of a thrombogenic surface such as collagen, and continues by the availability of soluble agonists that are derived from the injured vessel wall or the activated platelets themselves. One of the most potent, soluble platelet-activating agents is thrombin. Intravital imaging studies of thrombus formation in damaged mouse arteries indicate that thrombin is rapidly formed at thrombotic sites via the tissue factor/factor VIIa pathway of coagulation [1]. This is confirmed by inhibitory studies in various experimental models, showing that thrombin generation plays a key, driving role in the thrombotic process [2–4]. Once thrombin is formed, it will inevitably interact with and activate all nearby platelets [5].

Thrombin stimulates platelets mostly or only via the protease-activated receptors, PAR1/4 on human and PAR3/4 on mouse platelets, which all signal through the G proteins, Gq and G12/13 [6,7]. Thrombin evokes multiple responses, such as shape change, Ca2+ mobilization, secretion, integrin αIIbβ3 activation, and assembly of a platelet aggregate [8]. Furthermore, thrombin enhances the development of platelet procoagulant activity [9]. Particularly, in combination with collagen, it causes a prolonged elevation in cytosolic [Ca2+]i, which leads to the exposure of phosphatidylserine at the platelet outer membrane. This provides a procoagulant surface, upon which coagulation factors assemble to produce factor Xa and thrombin. Hence, the generation of initial traces of thrombin via the tissue factor/factor VII pathway leads to a strong positive feedback loop, where thrombin activates platelets, platelets become procoagulant, and more thrombin is formed at the surface of these platelets [9,10].

Although ADP is considered to be a weak agonist, studies with human and mouse platelets have indicated that it does play an important role in thrombus formation [11,12]. Being secreted from platelets in large amounts, ADP functions as an autocrine agonist sustaining many activation processes. Secreted ADP binds to the P2Y1 and P2Y12 purinergic receptors, and triggers shape change, Ca2+ mobilization and platelet aggregation [13–15]. The P2Y1 receptors are linked to Gq, but they evoke much weaker responses than thrombin receptor activation [16]. The result is limited activation of phospholipase C, leading to formation of inositol 1,4,5-trisphosphate (InsP3) and InsP3 receptor-mediated elevation in [Ca2+]i. The P2Y12 receptors are coupled to Gi and signal in a different way. In both human and mouse platelets, P2Y12 induces Gi-dependent inhibition of adenylyl cyclase and consequent down-regulation of cAMP [13,17]. In this way, autocrine-produced ADP can relieve the platelet-inactivating effect of cAMP and its effector, protein kinase A (PKA) [14]. Also downstream of Gi, P2Y12 receptors stimulate the less well understood phosphoinositide 3-kinase (PI3-K) pathway, which leads to αIIbβ3 integrin activation and platelet aggregation [18]. We and others have shown that both the PI3-Kβ and PI3-Kγ isoforms contribute to the P2Y12-mediated stabilization of platelet aggregates under static and shear conditions [19–21].

Recently, it was established that P2Y12 signaling is implicated in the stimulating effect of thrombin on phosphatidylserine exposure and procoagulant activity of platelets [5,22,23]. Hence, we hypothesized that the thrombin and P2Y12 receptors signal in a synergistic way towards this platelet response. Since elevation in [Ca2+]i is a key feature in phosphatidylserine exposure, we started to investigate how thrombin and ADP receptor stimulation co-operate to induce Ca2+ mobilization and to provoke platelet procoagulant activity. We found that autocrine-released ADP via P2Y12 causes a marked prolongation of the [Ca2+]i elevation, even with high doses of thrombin. Subsequently, using platelets that were co-stimulated with fixed concentrations of thrombin and ADP, we unraveled the signaling mechanism underlying this P2Y12 effect. The results point to a dual regulatory pathway evoked by P2Y12. It involves increased InsP3 receptor function due to inactivation of the cAMP/PKA route. This effect is accompanied by prolongation of thrombin/ADP-evoked phospholipase C activity and Ca2+ mobilization in a way controlled by PI3-Kβ.

Results

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

Autocrine and added ADP increases and prolongs thrombin receptor-induced Ca2+ responses via P2Y12 receptor stimulation

Previously, the selective P2Y12 antagonist, cangrelor (AR-C69931MX, AR-C), was used to demonstrate that autocrine-produced ADP stimulates the procoagulant activity of thrombin-stimulated platelets exclusively via P2Y12 receptors; the selective P2Y1 antagonist, MRS2179, was found to be without influence [5]. This procoagulant effect of P2Y12 was proposed to result from synergy with signaling via the platelet thrombin receptors. To investigate how autocrine ADP contributes to the Ca2+ response induced by thrombin, Fura-2-loaded human platelets were stimulated with low or high thrombin concentrations (0.5–20 nm), and the effects of pre- or post-addition of AR-C or MRS2179 were examined. As shown in Fig. 1, AR-C pretreatment lowered the Ca2+ signal at all thrombin doses. The effect of AR-C was marked in showing a persistent reduction of 30–50% (P < 0.05) of the later phase of the Ca2+ response, in contrast to MRS2179. Interestingly, late addition of AR-C (i.e. when given after the initial Ca2+ peak) resulted in an almost immediate abolition of the remaining part of the Ca2+ signal, which then reached the level as in platelets preincubated with AR-C. In the experiments, a concentration of 2–5 μm AR-C was sufficient for maximal reduction of the Ca2+ response, whereas higher concentrations of 10–30 μm did not give additional effects (not shown). In marked contrast, post-addition of MRS2179 did not affect the thrombin-induced Ca2+ response (Fig. 1).

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Figure 1.  Autocrine ADP and P2Y12 prolong thrombin-induced Ca2+ responses. Fura-2-loaded platelets were activated with thrombin (0.5, 4 or 20 nm) in the presence of 1 mm CaCl2. Vehicle solution (black lines) or AR-C (10 μm, grey lines) was added 10 min before thrombin or shortly after thrombin (arrows). Dotted lines indicate the effect of the addition of MRS2179 (100 μm). Traces are representative of three or more experiments.

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To determine whether the P2Y12 contribution to the thrombin-evoked Ca2+ response was limited by incomplete or impaired secretion, we determined how it was influenced by pre- or post-addition of AR-C using platelets that were co-stimulated with thrombin (4 nm) and ADP (20 μm). Again, when given before or after the agonists, AR-C greatly suppressed the late phase of the [Ca2+]i increase (Fig. 2A). Here, an AR-C concentration of 10–30 μm was needed for an optimal effect (data not shown). As a comparison, the general PI3-K inhibitor wortmannin was given after thrombin + ADP; wortmannin had a slower, but similar type of effect as AR-C (Fig. 2B).

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Figure 2.  P2Y12 prolongs thrombin-induced Ca2+ responses partly via PI3-K signaling. Fura-2-loaded platelets were activated with 4 nm thrombin + 20 μm ADP in the presence of 1 mm CaCl2. (A) Effect of pre- or post-addition of vehicle (black lines) or AR-C (30 μm, grey lines) on the Ca2+ response. (B) Effect of pre- or post-addition of wortmannin (WT, 200 nm, dotted lines) on the Ca2+ response. Traces are representative of three or more experiments.

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To quantify the contribution of P2Y12 to the Ca2+ response in platelets stimulated with 4 nm thrombin with/without ADP, we measured not only [Ca2+]i peaks, but also changes in [Ca2+]i-time integrals, which reflect both the extent and duration of the [Ca2+]i increase [9]. Platelet pretreatment with AR-C reduced the Ca2+ peak (–9%) and the Ca2+ integral (–34%) after thrombin stimulation (Table 1). Further addition of MRS2179 was without effect, thus excluding a contribution of P2Y1 receptors. Co-stimulation with thrombin and ADP increased both the Ca2+ peak (+19%) and the Ca2+ integral (+35%) in comparison to thrombin alone. Importantly, pretreatment with AR-C reversed both parameters to the same level as that seen with thrombin alone.

Table 1.   Contribution of P2Y12 signaling to thrombin- and ADP-induced Ca2+ responses. Fura-2-loaded platelets (1 × 108·mL−1) were preincubated with vehicle, AR-C (30 μm) and/or MRS2179 (100 μm) for 10 min. Changes in [Ca2+]i were measured after activation with 4 nm thrombin ± 20 μm ADP in the presence of 1 mm CaCl2. Data show [Ca2+]i peak levels and [Ca2+]i-time integrals over 5 min. *P < 0.05, **P < 0.1 (n = 3–5).
AgonistAntagonistPeak (nm) (% versus thrombin)Integral (nm × s) (% versus thrombin)
ThrombinVehicle646 ± 38 (100%)40173 ± 3032 (100%)
AR-C591 ± 69 (91%)*26513 ± 2660 (66%)*
AR-C + MRS2179648 ± 142 (100%)29064 ± 4098 (72%)**
Thrombin + ADPVehicle767 ± 131 (119%)54149 ± 7418 (135%)*
AR-C603 ± 103 (93%)*25762 ± 2784 (64%)*
AR-C + MRS2179565 ± 131 (87%)*23744 ± 3421 (59%)*

The thrombin receptors PAR1 and PAR4 have been implicated in early and late stages of thrombin-induced human platelet activation, respectively. To investigate whether PAR1 alone or in combination with PAR4 co-signals with P2Y12, platelets were stimulated with the PAR1 agonist SFFLRN (15 μm) ± the PAR4 agonist AYPGKF (200 μm). In either case, AR-C (but not MRS2179) suppressed the Ca2+ integral to a similar degree; 38 ± 1% and 37 ± 1% (n = 3). Thus, the P2Y12-dependent part of the Ca2+ signal with thrombin does not rely on PAR4 activation.

Together, these results demonstrate that both autocrine-released and externally-added ADP reinforce the thrombin receptor-induced Ca2+ responses by a moderate increase of the first Ca2+ peak and a more marked increase of the later Ca2+ signal. Furthermore, the strong inhibitory effect of post-added AR-C indicates that long-term signaling via P2Y12 receptors is needed for the prolonged thrombin-induced Ca2+ signal.

P2Y12 stimulation increases thrombin-induced Ca2+ mobilization from internal stores

To prevent response variation due to incomplete or impaired ADP secretion, subsequent experiments were carried out by co-stimulation of platelets with fixed concentrations of thrombin and ADP. Since ADP was proposed to trigger unspecified Ca2+ entry channels [24], we measured its contribution to thrombin-induced Ca2+ signals in the presence or absence of external CaCl2. Typically, ADP increased and prolonged the Ca2+ response in either case (Fig. 3). In comparison to the condition where P2Y12 activity was fully blocked (+AR-C), ADP increased the thrombin-induced Ca2+ integral by 93 ± 16% or 76 ± 10% in the presence of EGTA or CaCl2, respectively (Fig. 3). This suggested that P2Y12 primarily stimulated mobilization of Ca2+ from internal stores, and it only secondarily enhanced store-regulated Ca2+ entry in the presence of CaCl2.

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Figure 3.  P2Y12 enhances thrombin-induced Ca2+ responses independent of Ca2+ entry. Fura-2-loaded platelets were preincubated with vehicle or AR-C (30 μm), and stimulated with thrombin (4 nm) ± ADP (20 μm) in the presence of either 1 mm CaCl2 or 1 mm EGTA. Data are presented as normalized Ca2+-time integrals (5 min) relative to the condition of AR-C + thrombin; *P < 0.05 (n = 4–6).

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P2Y12 stimulation increases InsP3 receptor function via PKA

By linking to Gi, P2Y12 inhibits adenylyl cyclase and causes inactivation of cAMP-dependent PKA [14,25]. Knowing that PKA-induced phosphorylation of platelet InsP3 receptors inhibits their Ca2+ channel function [26], we determined how P2Y12 signaling affects InsP3-induced mobilization of Ca2+ from intracellular stores. Using saponin-permeabilized platelets, the Ca2+ release was measured in response to a sub-optimal dose of InsP3 [27]. Platelet activation with ADP had a clear stimulating effect on InsP3-induced Ca2+ release, whereas AR-C completely antagonized this effect (Fig. 4A). In marked contrast, preincubation with the PI3-K inhibitor wortmannin was ineffective.

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Figure 4.  P2Y12 enhances InsP3-induced Ca2+ mobilization in saponin-permeabilized platelets. Washed platelets in ATP-regenerating medium were stimulated with ADP (20 μm), as indicated, and permeabilized with saponin in the presence of Fluo-3. After adjustment of the free Ca2+ level to 300 nm, InsP3 (100 nm) was added, and Ca2+ mobilization was measured. (A) Platelets were pretreated with vehicle, AR-C (30 μm) or wortmannin (WT, 200 nm) for 5 min, and then activated with ADP. (B) Platelets were pretreated with KT5720 (2.5 μm), prostaglandin E1 (10 μm) or heparin (20 μg·mL−1). Representative traces of InsP3-induced increases in [Ca2+]i from three or more experiments are shown. Values are percentages of maximal InsP3-induced Ca2+ mobilization compared to control condition; *P < 0.05 compared to control (n = 3–8).

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Further experiments confirmed the sensitivity of InsP3-induced Ca2+ mobilization for modulation of PKA activity. Platelets were therefore pretreated with the PKA inhibitors, KT5720 and H89 [28]. Following saponin permeabilization, this resulted in increased InsP3-induced Ca2+ release with either inhibitor, with an EC50 of 1 μm KT5720 and 4 μm H89. Pretreatment with an optimal dose of 2.5 μm KT5720 doubled the Ca2+ release with ADP (Fig. 4B). Conversely, pretreatment with the PKA-stimulating agent prostaglandin E1 (IC50 0.5 μm) more than halved this Ca2+ release. In control experiments, saponin-permeabilized platelets were treated with heparin, which was used as an established InsP3 receptor [29]. Heparin completely inhibited all InsP3-induced Ca2+ mobilization (Fig. 4B). Note that no thrombin was used in this experimental set. As an alternative method of reducing cAMP, platelets were preincubated with the Gz-coupled agonist, adrenaline [27]. Similar to the P2Y12/Gi-mediated inhibition of adenylate cyclase, this treatment resulted in a 61 ± 10% increase of InsP3-induced Ca2+ mobilization. Together, these results show that P2Y12 receptor activation, by lowering cAMP and PKA activity, can enhance the Ca2+-mobilizing function of InsP3 receptors.

P2Y12 stimulation increases Ca2+ mobilization via both PKA and PI3-K pathways

The effects of PKA inhibition were also measured with respect to the Ca2+ responses of non-permeabilized, Fura-2-loaded platelets. Pretreatment of platelets with an optimal dose of 10 μm H89 resulted in an overall increase in Ca2+ integral with thrombin alone, but not with thrombin + ADP (Fig. 5). Accordingly, with H89 present, the contribution of ADP/P2Y12 to the thrombin-induced Ca2+ integral was reduced by 47%. Essentially similar results were obtained with KT5720, but these were difficult to quantify because this compound strongly interfered with Fura-2 fluorescence (data not shown). The ADP/P2Y12 effect on the thrombin-induced Ca2+ response was independent of integrin signaling because, in platelets treated with the αIIbβ3 antagonist, tirofiban, it changed insignificantly from 176% to 167–170%.

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Figure 5.  P2Y12 enhances thrombin-induced Ca2+ responses via both PKA and PI3-K. Fura-2-loaded platelets in 1 mm EGTA were preincubated with vehicle, AR-C (30 μm), H89 (10 μm) and/or wortmannin (WT, 200 nm), as indicated. Platelets were activated with 4 nm thrombin in combination with either 30 μm AR-C or 20 μm ADP, as described in Fig. 3. Data are presented as normalized [Ca2+]i-time integrals relative to the condition of AR-C + thrombin. *P < 0.05 compared to respective control (n = 5–6).

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The contribution of PI3-K was further examined using two structurally distinct inhibitors, wortmannin and LY294002 [21]. In the presence of ADP/P2Y12 activity, wortmannin or LY294002 suppressed the thrombin-induced Ca2+ integral with an IC50 of approximately 10 nm and 1 μm, respectively, which is in accordance with the known affinity of these compounds for the PI3-K catalytic subunits. At these concentrations (required for notable inhibition of Akt phosphorylation; see below), wortmannin and LY294002 reduced the Ca2+ integral by 24.4 ± 4.1% and 24.0 ± 1.7% (n = 3), respectively. In contrast, when AR-C was present and P2Y12 was not active, these compounds influenced the thrombin-induced Ca2+ mobilization insignificantly by < 6% (P = 0.34). At a maximally effective dose of 200 nm, wortmannin suppressed the thrombin + ADP response by 35 ± 3.4% (Fig. 5). Notably, when combined with H89 to block PKA, wortmannin treatment almost completely abolished the stimulating effect of ADP (Fig. 5). In other words, the combined antagonism of PKA and PI3-K was sufficient to almost completely block the effect of ADP/P2Y12 on thrombin-induced Ca2+ mobilization.

P2Y12 stimulation increases Ca2+ mobilization via prolonged phospholipase C activity

The PI3-K pathway might enhance Ca2+ mobilization by reducing Ca2+ removal via sarco- and endoplasmic reticulum Ca2+-ATPase (SERCA) inhibition, in a similar way to that proposed for pancreatic acinar cells [30]. In platelets, the SERCA inhibitor thapsigargin prolonged the thrombin-induced Ca2+ response, and abolished the effects of ADP, AR-C and wortmannin on this response (Fig. 6A,B). Wortmannin pretreatment did not change the decay rate of the Ca2+ signal with thrombin + ADP. Direct measurement of SERCA activity in saponin-permeabilized platelets showed that neither AR-C nor wortmannin decreased this activity by < 3%. Together, these results indicate that ADP/P2Y12 activity prolongs Ca2+ mobilization in a way that requires normal SERCA activity. However, the data provide no evidence for a direct effect of P2Y12/PI3-K on SERCA activity in platelets.

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Figure 6.  Contribution of SERCA and phospholipase C to P2Y12-dependent prolongation of Ca2+ responses. (A, B) Fura-2-loaded platelets were preincubated with vehicle, AR-C (30 μm) or wortmannin (WT, 200 nm) for 10 min, as indicated. Platelets then were stimulated with thrombin (4 nm) ± ADP (20 μm) in the presence or absence of thapsigargin (TG, 2 μm). Bars show the quantitative effect of wortmannin relative to thrombin + ADP. (C, D) Fura-2-loaded platelets were stimulated with thrombin and ADP as above. At 60 s after activation (arrow), the following substances were added: vehicle (control), U73343 (2 μm), U73122 (2 μm), ET-18-OCH3 (40 μm) or manoalide (10 μm). Bars indicate Ca2+ levels, relative to thrombin + ADP, measured 60 s after the addition of the indicated substance. Representative Ca2+ traces are shown (n = 3–5).

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If PI3-K does not affect Ca2+ reuptake, it may enhance or prolong the activation of phospholipase C, in particular its γ isoforms which partly rely on PIP3 formation [31]. To explore this possibility, platelets were stimulated with thrombin + ADP, after which phospholipase C-inhibiting agents were added. Post-addition of the phospholipase C inhibitor U73122 completely abrogated the prolonged phase of the Ca2+ response, whereas the control substance U73343 was ineffective (Fig. 6C,D). As U73122 can have non-specific effects, control experiments were performed with other phospholipase C inhibitors: ET-18-OCH3 and manoalide. Similarly, post-addition of these compounds blocked the prolonged phase of the Ca2+ response (Fig. 6D). As mentioned above, a similar, but slower effect was obtained by post-addition of wortmannin (Fig. 2B). To confirm that PI3-K contributes to late phospholipase C activation, levels of InsP3 were measured in platelets stimulated for 5 min with thrombin + ADP. This stimulation resulted in a 1.73 ± 0.16-fold increase in InsP3, which was significantly reduced to 1.43 ± 0.20-fold in the presence of wortmannin (P = 0.02, n = 6). Together, these results indicate that the ADP/P2Y12-dependent prolongation of the Ca2+ response relies on both phospholipase C and PI3-K activity.

PI3-Kβ and not PI3-Kγ mediates the P2Y12 effect on thrombin-evoked Ca2+ responses

In man and mouse, the PI3-Kβ (p110β) and PI3-Kγ (p110γ) isoforms are involved in P2Y12-dependent platelet aggregation [19,21,32]. To examine how these isoforms contribute to the Ca2+ signal, the PI3-Kβ selective inhibitor, TGX221 [21], and platelets from p110γ−/− mice, lacking active PI-3Kγ, were used. It was established that, in murine platelets, TGX221 dose-dependently inhibited PI3-K-dependent phosphorylation of Akt; full inhibition was achieved at a concentration of 0.5 μm (data not shown). Typically, platelets from wild-type p110γ+/+ and knockout p110γ−/− mice showed a similar enhancement with ADP of the thrombin-induced Ca2+ response, which was always inhibited by AR-C (Fig. 7A). In either genotype, this enhancement was also antagonized by the general PI3-K inhibitor LY294002 (Fig. 7B), and by the PI3-Kß specific inhibitor TGX221 (Fig. 7C).

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Figure 7.  Unchanged contribution of P2Y12 to Ca2+ responses in PI3-Kγ deficient platelets. Washed platelets, obtained from p110γ+/+ and p110γ−/− mice, were loaded with Ca2+ indicator dyes. Changes in [Ca2+]i were monitored after preincubation of the platelets with inhibitor (10 min), and stimulation with thrombin alone (4 nm) or in combination with ADP (20 μm). (A) Effect of AR-C (10 μm) preincubation on Ca2+ response. (B) Effect of general PI3-K inhibitor LY294002 (LY, 25 μm) on Ca2+ response. (C) Effect of PI3-Kβ inhibitor TGX221 (TGX, 0.5 μm) on Ca2+ response. Graphs are representative and show the fold increases in [Ca2+]i after agonist stimulation. Bars indicate [Ca2+]i-time integrals, expressed relative to values with thrombin + ADP (n = 4, duplicate experiments).

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Similar results were obtained with human platelets, showing that pretreatment with TGX221 was almost as active as wortmannin in suppressing the thrombin + ADP-induced Ca2+ response (Fig. 8A). On the other hand, pretreatment with the PI3-Kγ-specific inhibitor, AS252424 [21], was without effect. As protein kinase B/Akt is a downstream mediator of PI3-K in platelets [33,34], the effects of the isoform-specific inhibitors were examined on thrombin + ADP-induced Akt activation. In platelets that were stimulated with thrombin alone or in combination with ADP, Akt was phosphorylated at its activation site of Ser473, peaking after 5–10 min. This phosphorylation was completely absent in the presence of the P2Y12 antagonist AR-C, regardless of whether ADP was added (Fig. 8B,C). Furthermore, pretreatment with LY294002 or TGX221 caused complete inhibition of the thrombin + ADP-evoked Ser473 phosphorylation of Akt (Fig. 8D). Apparently, in thrombin-stimulated platelets, Akt phosphorylation and activation is completely dependent on autocrine-produced or externally-added ADP via stimulation of the P2Y12 and PI3-Kß pathway. These results not only show that the regulatory role for PI3-K in P2Y12 signaling is conserved in mouse and human platelets, but also highlight the importance of the PI3-Kβ isoform.

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Figure 8.  PI3-K ß-isoform mediates P2Y12-dependent enhancement of platelet activation by thrombin. (A) Human, Fura-2-loaded platelets were preincubated with vehicle, wortmannin (WT, 200 nm), TGX221 (TGX, 0.5 μm) or AS252424 (AS, 1 μm). Cells were then stimulated by thrombin (4 nm) and ADP (20 μm). The effects of preincubation on Ca2+-time integrals are shown (n = 4–6, relative to thrombin + ADP). (B–D) Washed platelets were preincubated with AR-C (10 μm), LY294002 (LY, 25 μm) or TGX221 (TGX, 0.5 μm) for 10 min. Platelets then remained unstimulated (rest), or were stimulated with thrombin ± ADP (as above), and were then boiled in the presence of reducing buffer. Equal volumes of platelet samples were analyzed for Akt activation by western blot. Representative images are shown from four independent experiments. Bars indicate the density of Akt phosphorylation on Ser473 (n = 4).

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PI3-Kβ and not PI3-Kγ mediates P2Y12-dependent procoagulant activity of mouse and human platelets

Prolonged elevation in [Ca2+]i can signal for surface exposure of phosphatidylserine, thus facilitating platelet-dependent thrombin generation [9]. It was studied whether the P2Y12/PI3-Kß pathway contributed to thrombin generation. In platelet-rich plasma (PRP) from wild-type and p110γ−/− mice, lacking PI3-Kγ, thrombin generation was induced by triggering with tissue factor/CaCl2. In either genotype, activation with ADP via P2Y12 resulted in a quite similar increase in thrombin generation (Fig. 9A,B). In PRP from all mice, TGX221 partly antagonized the stimulating effect of ADP, reducing the rate of thrombin generation by approximately 25%.

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Figure 9.  PI3-K ß-isoform mediates P2Y12-dependent stimulation of coagulation in wild-type and PI3-Kγ deficient mice. PRP from (A) p110γ+/+ or (B) p110γ−/− mice was pretreated with vehicle, AR-C (30 μm) or TGX221 (TGX, 0.5 μm) and activated with ADP. Coagulant activity was measured by the thrombin generation assay, after triggering with tissue factor/CaCl2. Representative thrombin generation curves are given for wild-type and p110γ−/− PRP.

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Similar experiments were conducted with human PRP. In the human system, ADP enhanced the thrombin generation triggered by tissue factor/CaCl2 in a way that was inhibitable by AR-C (Fig. 10A). Pretreatment of PRP with wortmannin or TGX221 reduced the initial rate of thrombin generation half as effective as AR-C (Fig. 10B,C). In contrast, pretreatment with the PI3-Kγ inhibitor AS252424 was without any effect. Controls showed that neither wortmannin nor TGX221 affected thrombin generation in the presence of AR-C (not shown). With only wortmannin present, the PKA inhibitor H89 further reduced the rate of thrombin generation by another 25%, thus indicating the additional involvement of PKA. To assess more directly the role of the P2Y12/PI3-Kβ pathway in procoagulant activity, effects of ADP on phosphatidylserine exposure were examined in PRP that was triggered with tissue factor/CaCl2. Plasma was depleted from fibrinogen to prevent formation of clots. Flow cytometric analysis using fluorescein isothiocyanate (FITC)-labeled annexin A5 (detecting exposed phosphatidylserine) showed that ADP increased the fraction of phosphatidylserine-exposing platelets by 70% (Fig. 10D). Wortmannin pretreatment almost fully antagonized this increase, whereas TGX221 pretreatment was somewhat less inhibitory, and AS252424 was ineffective. Taken together, these results suggest that, in both mouse and human platelets, the PI3-Kβ but not the PI3-Kγ isoform contributes to platelet procoagulant activity following P2Y12 stimulation.

image

Figure 10.  PI3-K ß-isoform mediates P2Y12-dependent stimulation of coagulation and phosphatidylserine exposure. Human PRP was preincubated with vehicle, wortmannin (WT, 200 nm), AR-C (30 μm), TGX221 (TGX, 0.5 μm) or AS252424 (AS, 1 μm), and then activated with ADP (20 μm). Thrombin generation was measured by triggering with tissue factor/CaCl2. (A) Traces are representative thrombin generation curves, showing the treatment effects of AR-C, WT and TGX221. (B) Initial part of the same thrombin generation curves. (C) Bars indicate the effects of preincubation on initial rates (5 min) of thrombin generation (n = 3–5). (D) Human platelets in fibrin-depleted human plasma were preincubated with inhibitors and activated with tissue factor/CaCl2. After 10 min, FITC-labeled annexin A5 was added, and fractions of phosphatidylserine-exposing platelets were determined by flow cytometry (n = 3–5).

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Discussion

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

The results of the present study highlight the importance of the ‘weak’ agonist, ADP, as a key platelet activator that is effective at low and high thrombin concentrations, as well as under coagulant conditions, where thrombin is generated in situ. We find that (autocrine) ADP, acting via P2Y12, enhances and extends the thrombin-induced platelet activation by increasing Ca2+ mobilization from internal stores, without directly affecting a specific Ca2+ entry channel, as was previously suggested. This potentiation by P2Y12 signaling is conserved between platelets from man and mouse, despite the different thrombin receptor types employed by these species. The data are compatible with the earlier findings indicating that P2Y12 activates platelets mostly or exclusively via Gi [14,25], whereas thrombin and P2Y1 stimulate the Gq pathway, which is directly coupled to Ca2+ mobilization [35]. The present results are also in agreement with a previous study demonstrating that P2Y12 activation enhances the Ca2+ response induced by thrombin receptor-activating peptide [36]. Thus, in the presence of thrombin, the Gi signaling pathway via P2Y12 provides platelets with a mechanism to extend their activation.

Platelet and mature megakaryocytic InsP3 receptors are sensitive to small changes in cAMP levels and ensuing PKA activation [27,28]. This sensitivity is likely regulated by PKA phosphorylation sites, present in the type-I InsP3 receptor Ca2+ channels, which control the Ca2+-mobilizing properties of platelets [26]. The current data indicate that ADP, acting via P2Y12 and Gi, can down-regulate adenylyl cyclase and hence PKA with consequently increased Ca2+ mobilization. This pathway still operates in the presence of thrombin (e.g. the PKA inhibitor H89 reinforces the thrombin-induced Ca2+ response when P2Y12 is active).

In addition, the present study demonstrates an important role for PI3-K in the P2Y12-dependent enhancement of thrombin receptor signaling, which is most prominent in the late stage of the Ca2+ response and is quite substantial in longer-term Ca2+ integrals. This long Ca2+ signal is shortened by PI3-K inhibition with wortmannin or LY294002. It apparently does not implicate modulation of InsP3 receptor Ca2+ channels because InsP3-induced Ca2+ mobilization is not affected by PI3-K inhibition. As wortmannin and the prototype PI-3K inhibitor, LY294002 [21], had similar shortening effects on the Ca2+ response evoked by thrombin + ADP, there is no evidence that LY294002 may affect this response in an aspecific way, as was proposed for smooth muscle cells [37].

Platelets from PI3-Kγ deficient mice exhibited an unchanged Ca2+ response and procoagulant activity, whereas the PI3-Kß inhibitor TGX221 suppressed this response in both wild-type and deficient platelets. Similarly, in human platelets, TGX221 but not the PI3-Kγ specific inhibitor, AS252424, antagonized the P2Y12-dependent part of the Ca2+ response, indicating that PI3-Kß is the main isoform in Ca2+ signal modulation via P2Y12. Examination of targets downstream of P2Y12 revealed a clear role for PI3-Kß in the regulation of Akt activation, in platelets stimulated with thrombin + ADP. This agrees well with the earlier finding in mouse platelets that Akt is activated downstream of Gi and G12/13 [33,38]. However, under the present conditions of thrombin + ADP receptor stimulation, we could not confirm that also PI3-Kγ contributes to Akt activation [19] or to Ca2+ mobilization [32].

The mechanism whereby PI3-Kß enhances Ca2+ signaling is not entirely clear. Its effect relies on SERCA activity because it disappears in the presence of thapsigargin. However, in contrast to a report on pancreatic acinar cells, where PI3-K inhibition increased Ca2+ mobilization via SERCA activation [30], PI-3K inhibition did not alter SERCA activity in platelets. This suggests that it is not Ca2+ reuptake itself that is controlled by PI3-K, but a different process that is still dependent on normal Ca2+-ATPase function.

A remarkable finding is that the persistent effect of P2Y12 on Ca2+ mobilization relies on prolonged phospholipase C activation. Similar to AR-C, late application of each of the three phospholipase C inhibitors, U73122, ET-18-OCH3 or manoalide, rapidly abolished the remaining Ca2+ response. Late application of wortmannin to block PI3-K had a similar, although slower effect. That PI3-K contributes to phospholipase C activity was further confirmed by the finding that treatment with wortmannin suppressed the cytosolic InsP3 level in thrombin + ADP stimulated platelets. Knowing that the PI3-Kß isoform is responsible for a considerable amount of the PIP3 formed in platelets [21], this may suggest that the PIP3 produced by this isoform leads to plasma membrane binding and, hence, activation of PH domain-containing phospholipase Cγ. Indeed, thrombin (e.g., via ADP) provokes activation of phospholipase Cγ along with phospholipase Cβ isozymes [39]. A similar mechanism of prolonged Ca2+ signaling by PIP3 and membrane translocation of phospholipase Cγ has been proposed for other cell types [31].

In experiments where thrombin is induced in situ by activation of PRP with tissue factor and CaCl2, the P2Y12/PI3-K pathway significantly enhances the activation state and, hence, the procoagulant activity of platelets. Experiments with inhibitors and PI3-Kγ deficient mice indicated that especially the PI3-Kβ isoform is involved. Flow cytometry further indicated that the PI3-K pathway increased the fraction of platelets with phosphatidylserine exposure under these conditions. Since phosphatidylserine exposure is a strongly Ca2+-dependent response, which in turn mediates thrombin generation, this indicates that P2Y12 signaling via PI3-Kβ plays a regulating role in the positive feedback loop of thrombin-induced platelet activation, platelet procoagulant activity, and new thrombin formation. These data thereby support the earlier finding that ADP enhances the procoagulant activity of platelets via the P2Y12 receptors [5,22,23].

The present results significantly extend the earlier work investigating the interaction of P2Y12 with P2Y1 signaling [25,40]. We find that P2Y12 enhances the thrombin-induced Ca2+ response in a way not only involving adenylyl cyclase/PKA inhibition, but also by PI3-K stimulation. P2Y12 appears to increase InsP3 receptor function via PKA inhibition. This effect is further enlarged by a PI3-Kβ-dependent prolongation of phospholipase C activation and InsP3 production (see scheme in supplementary Fig. S1). The data thereby reveal a novel function for the β-isoform of PI3-K. Previously, this isoform had been linked to shear-dependent activation of platelets, regulating the stability of platelet adhesion and aggregation [20,21]. We now advocate that PI3-Kβ also plays a role in the prolongation of thrombin-induced Ca2+ signaling via P2Y12.

Experimental procedures

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

Animals

Animal experiments were approved by the local animal experimental committees. Wild-type control (p110γ+/+) and PI3-Kγ deficient (p110γ−/−) mice, with identical genetic backgrounds, were generated as described previously [21,41].

Materials

Human α-thrombin (3270 NIH units/mg, 12 750 units·mL−1, 1.12 μm) was obtained from Enzyme Research Laboratories (Swansea, UK). Fura-2, Fura red and Oregon green (OG)-BAPTA acetoxymethyl esters (AM) as well as non-esterified Fluo-3 were obtained from Molecular Probes (Leiden, the Netherlands); ultra-pure calcium-free water was from Baker Analytical (Phillipsburg, NJ, USA). Cangrelor (AR-C69931MX, AR-C) was kindly provided by The Medicines Company (Parsippany, NJ, USA). Fluorescent thrombin substrate, Z-Gly-Gly-Arg aminomethyl coumarin (Z-GGR-AMC) was from Bachem (Bubendorf, Switzerland); recombinant human tissue factor from Dade Behring (Marburg, Germany); and human thrombin calibrator from Synapse (Maastricht, the Netherlands). InsP3 was from Alexis Biochemicals (Lausen, Switzerland); tirofiban from Merck, Sharp and Dohme (Haarlem, the Netherlands); Akt inhibitor and LY294002 from Calbiochem (La Jolla, CA, USA). ET-18-OCH3, manoalide, U73122 and U73343 came from Biomol (Plymouth Meeting, PA, USA). PI3-K isoform-specific inhibitors were kind gifts of Baker Heart Institute (Melbourne, Australia), namely TGX221, selective for the PI3-Kβ isoform (p110ß), and AS252424, selective for PI3-Kγ (p110γ), both synthesized as described [21,42]. Fluorescent-labeled annexin A5 was obtained from Nexins Research (Hoeven, the Netherlands). Other materials were from Sigma (St Louis, MO, USA).

Platelet preparation and isolation

Blood was taken from healthy volunteers, who provided full informed consent; subjects were free from medication for at least 2 weeks. Blood was collected into a 1 : 6 volume of acid-citrate glucose solution (80 mm trisodium citrate, 52 mm citric acid and 180 mm glucose). Human PRP was obtained by 15 min of centrifuging at 240 g. Blood from mice was collected and handled as described previously [3]. Murine PRP was prepared by centrifuging blood at 280 g for 3 min, and centrifuging the upper phase once more at 625 g for 10 s. Platelet counts were determined with a thrombocounter (Coulter Electronics, Luton, UK).

Platelet shape change was measured in PRP or washed platelets by turbidometry in the presence of tirofiban to prevent aggregation. Shape change evoked by up to 20 μm ADP was not significantly (< 10%) influenced by AR-C within the range 1–30 μm, indicating that AR-C did not interfere with the P2Y1-dependent response.

Measurements of cytosolic Ca2+ in intact and permeabilized platelets

To measure [Ca2+]i in intact human platelets, PRP was incubated with the fluorescent probe Fura-2 AM (2.5 μm) at 37 °C for 45 min. After addition of a 1 : 15 volume of acid-citrate glucose solution and apyrase (0.1 U·mL−1 ADPase), platelets were centrifuged from plasma, washed and resuspended in Hepes buffer pH 7.45 (10 mm Hepes, 136 mm NaCl, 2.7 mm KCl, 2 mm MgCl2, 0.1% glucose and 0.1% BSA), as described previously [43]. Washed Fura-2-loaded platelets were diluted in Hepes buffer pH 7.45 to 1 × 108 platelets·mL−1. Changes in cytosolic [Ca2+]i were measured by ratio fluorometry under stirring; [Ca2+]i-time integrals were measured to quantify prolonged Ca2+ responses [44].

Changes in [Ca2+]i in mouse platelets were measured as previously described [45]. Briefly, the washed platelets (5.0 × 108 mL−1) were incubated with OG-BAPTA AM (1 μm) and Fura red AM (1.25 μm) for 30 min at 37 °C, and subsequently resuspended at 2.5 × 108 mL−1 in Hepes buffer pH 7.45, containing BSA (5 mg·mL−1), CaCl2 (1 mm), apyrase (0.02 U·mL−1) and probenecid (1.75 mm). Platelets were stimulated by addition of thrombin (4 nm) and/or ADP (20 μm), and ratiometric changes in [Ca2+]i were then determined [45].

In permeabilized human platelets, InsP3-induced Ca2+ mobilization from internal stores was measured using a previously established procedure [27]. Washed human platelets were resuspended in Ca2+-free Hepes buffer pH 7.45 containing 0.1 mm EGTA (1.5 × 109 platelets·mL−1). The cells were diluted in ATP-regenerating medium, preincubated for 15 min with (ant)agonist, stimulated with ADP, and then permeabilized with 15 μg·mL−1 saponin in the presence of 1 μm free Fluo-3. After 10 min of stirring, free [Ca2+] was adjusted to 300 nm with a CaCl2 stock solution, after which InsP3 was added. Fluorescence intensities were continuously recorded at 488 nm excitation and 526 nm emission wavelengths using an SLM-Aminco DMX-1100 spectrofluorometer (Rochester, NY, USA). Calibrations were performed by adding excess amounts of CaCl2 or EGTA/Tris; levels of [Ca2+] were calculated from the binding equation of Fluo-3 for Ca2+ [27]. Ultra-pure calcium-free water was used for preparation of all buffers and (ant)agonists.

Measurement of cytosolic cAMP and InsP3

Intracellular levels of cAMP and InsP3 in resting and activated platelets were measured as described previously [27]. Basal concentrations were 2.59 ± 0.17 and 0.96 ± 0.08 pmol/108 platelets (n = 6), respectively.

Measurement of SERCA activity

Decay constants of [Ca2+]i decreases following peak values were determined by mono-exponential fitting of 2-s data points. SERCA activity in saponin-permeabilized platelets was determined by measuring fluorescence accumulation due to cleavage of the Ca2+-ATPase substrate, 3-O-methylfluorescein phosphate [46]. SERCA activity represented the ATP- and thapsigargin-sensitive phosphatase activity.

Measurement of Akt activation

Akt activation was measured by western blot analysis, using a phosphoserine-473 Akt polyclonal antibody (Biosource International, Camarillo, CA, USA) to detect active Akt kinase, as well as a separate Akt polyclonal antibody (Cell Signaling Technology, Danvers, MA, USA) to determine total Akt, as described previously [19].

Thrombin generation

For thrombin generation measurements, human or mouse blood was collected in a 1 : 10 volume of 129 mm trisodium citrate. Isolated PRP was centrifuged twice at 2700 g for 10 min to prepare platelet-poor plasma. The PRP was diluted to the desired platelet count with autologous platelet-poor plasma.

Normalized PRP (1.5 × 108 platelets·mL−1) was preincubated with inhibitors (15 min), and platelets were activated as required. Thrombin generation was initiated in PRP with tissue factor (1 pm final concentration) at 37 °C and measured according to the thrombogram method [47]. Briefly, PRP samples (4 volumes) were pipetted into a polystyrene 96-well plate (Immulon 2HB; Dynex Technologies, Chantilly, VA, USA), already containing 1 volume of buffer A (20 mm Hepes, 140 mm NaCl, 5 mg·mL−1 BSA and 6 pm tissue factor). Coagulation was started by adding 1 volume of buffer B (2.5 mm Z-GGR-AMC, 20 mm Hepes, 140 mm NaCl, 100 mm CaCl2 and 60 mg·mL−1 BSA). Fluorescence accumulation was measured with a Fluoroskan Ascent well-plate reader (Thermolab Systems, Helsinki, Finland), equipped with thrombinoscope software (Synapse).

Flow cytometry

Washed platelets (1.0 × 108·mL−1) were resuspended in citrate-anticoagulated, fibrin-depleted plasma [5]. Coagulation was triggered with CaCl2 (16.6 mm) and tissue factor (0.5 pm, f.c.). After 10 min of activation, exposure of phosphatidylserine was determined by flow cytometry using FITC-labeled annexin A5.

Statistical analysis

Statistical analysis was performed with Student’s t-test, using the Statistical Package for Social Sciences, version 11.0 (SPSS Inc., Chicago, IL, USA). Data are presented as mean ± SEM, unless indicated otherwise.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

This work was supported by the Netherlands Organization for Scientific Research (902-16-276) and the Netherlands Heart Foundation (2002B014).

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  4. Discussion
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  6. Acknowledgements
  7. References
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Results
  4. Discussion
  5. Experimental procedures
  6. Acknowledgements
  7. References
  8. Supporting Information

Fig. S1.  Proposed role of the P2Y12 receptor in regulating Ca2+ signaling and platelet procoagulant activity.

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