Impaired activation of murine platelets lacking Gα_i2

The intracellular signaling pathways by which G protein–coupled receptors on the platelet surface initiate aggregation, a critical process for hemostasis and thrombosis, are not well understood. In particular, the contribution of the G_i pathway has not been directly addressed. We have investigated the activation of platelets from mice in which the gene for the predominant platelet Gα_i subtype, Gα_i2, has been disrupted. In intact platelets from Gα_i2-deficient mice, the inhibition of adenylyl cyclase by ADP was found to be partially impaired compared with wild-type platelets. Moreover, both ADP-dependent platelet aggregation and the activation of the integrin αIIbβ3 (GPIIb-IIIa) were strongly reduced in platelets from Gα_i2-deficient mice. In addition, Gα_i2-deficient platelets displayed impaired activation at low thrombin concentrations. This defect was mimicked by blocking the adenylyl cyclase–coupled platelet ADP receptor (P2Y₁₂) on wild-type platelets with a selective antagonist. These observations suggest that Gα_i2 is involved in the inhibition of platelet adenylyl cyclase in vivo and is a critical component of the signaling pathway for integrin activation by ADP, resulting in platelet aggregation. In addition, thrombin-dependent activation of mouse platelets is mediated, at least in part, by secreted ADP acting on the Gα_i2–linked ADP receptor.

Platelets play a critical role in hemostasis and thrombosis, and drugs inhibiting platelet aggregation are effective antithrombotics. Platelet aggregation requires induction of an active conformation of the integrin αIIbβ3 (GPIIb-IIIa), capable of binding soluble fibrinogen and thus cross-linking the platelets. The signaling pathways leading to integrin activation (inside-out signaling) are not well understood (1). Most platelet agonists (e.g., thrombin, ADP, thromboxane A₂, epinephrine, and serotonin) bind to G protein–coupled receptors on the platelet surface. This results in the activation of heterotrimeric G proteins consisting of various α and βγ subunits that initiate multiple signaling pathways by interacting with downstream effectors. Platelets express Gα_s, Gα_q, Gα_12/13, Gα_z, Gα_i2, and Gα_i3 (2–5). Whereas Gα_s appears to mediate prostacyclin-dependent platelet inhibition by stimulating adenylyl cyclase, Gα_q is known to activate phospholipase C, resulting in release of intracellular calcium and activation of protein kinase C. Gα_12/13 might be involved in events leading to platelet shape change, and Gα_z is capable of inhibition of adenylyl cyclase. Similarly, it has been shown in other cells that Gα_i2 and Gα_i3 can mediate repression of cAMP formation whereas the released βγ subunits may activate phospholipase Cβ or other signaling molecules (4, 5). The requirement of intracellular calcium mobilization and the G_q pathway for platelet aggregation has been well established. Platelets from mice in which the gene for Gα_q has been disrupted are defective in calcium mobilization and aggregation by all agonists (6). Of the inhibitory G proteins in platelets, only Gα_z has been investigated in vivo. Platelets from Gα_z-knockout mice appear abnormal only for epinephrine-mediated inhibition of adenylyl cyclase and the potentiating effect of epinephrine on aggregation induced by other agonists (7).

Recently, however, the potential relevance of the G_i pathway for platelet activation has been implied from pharmacological studies. For example, ADP-dependent aggregation requires not only activation of a receptor mediating calcium mobilization (P2Y₁), but also concomitant stimulation of an ADP receptor coupled to the inhibition of adenylyl cyclase (P2Y₁₂, previously called P2T_AC, P2Y_ADP, P2Y_AC, P2Y_cyc) (8–14). Furthermore, after blockade of the P2Y₁₂ receptor, ADP-dependent aggregation could be rescued by epinephrine, which lowers cAMP levels (10, 15). Therefore, in addition to the G_q pathway, it appears that full platelet aggregation requires activation of receptors that are capable of inhibiting adenylyl cyclase and are likely to couple to a G protein of the G_i family. Indeed, in isolated human platelet membranes, stimulation of ADP receptors or the PAR1 thrombin receptor resulted in activation of Gα_i2 (3). However, this does not mean that adenylyl cyclase inhibition or a G_i protein is part of the signaling cascade leading to integrin activation and platelet aggregation. In fact, lowering cAMP levels using an adenylyl cyclase inhibitor did not induce platelet aggregation, even with concomitant activation of the G_q pathway (16–18). Thus, it was assumed previously that repression of the cAMP pathway does not lead to aggregation, although low cAMP levels are required to permit aggregation.

It remains to be clarified which of the inhibitory G proteins (G_i2, G_i3, G_z) mediate inhibition of adenylyl cyclase in intact platelets, and more importantly, whether a G_i protein is involved in the pathway leading to integrin activation and aggregation. In the present study we have investigated the role of the predominant platelet Gα_i protein, Gα_i2, in thrombin, epinephrine, and ADP-dependent platelet activation by using platelets from mice in which the gene for the α subunit Gα_i2 has been disrupted. Gα_i2^–/– mouse strains have been established, but their platelet function has not been reported (19, 20). Because released ADP appears to be a crucial cofactor of other platelet agonists as well (21), we have used the selective ADP receptor antagonist 2-methylthioadenosine 5′-monophosphate (2MeSAMP) to determine the contribution of released ADP activating a Gα_i2-coupled ADP receptor in thrombin-induced platelet signaling.

Effect of Gαi2 gene disruption on platelet expression of Gα subtypes. Mice with a disruption of the Gα_i2 gene did not display any obvious abnormalities of the hemostatic system, but the effects on platelet function have not been investigated (19, 20). We have found no significant difference in the platelet counts between wild-type and Gα_i2^–/– mice (599 ± 73 × 10³ /μl, n = 5, versus 621 ± 67 × 10³/μl, n = 5). The expression of Gα_i2 and other Gα subtypes was analyzed in crude platelet membranes using Western blot analysis. As expected, no Gα_i2 protein could be detected in membranes from Gαi2^–/– mice, and the expression of Gα_i2 was reduced in platelet membranes from heterozygous mice (Figure 1). By comparison, no difference was seen in the abundance of Gα_q. To determine whether the absence of Gα_i2 resulted in upregulation of other G proteins capable of inhibiting adenylyl cyclase, as reported for Gα_i3 in SV40-transformed embryonic fibroblasts from Gαi2^–/– mice (19), the expression levels for Gα_i1 , Gα_i3 , and Gα_z were compared in platelet membranes prepared from wild-type and Gαi2^–/– mice. No significant difference in the abundance of Gα_i3 and Gα_z proteins could be detected (Figure 1). There was also no change in the level of Gβ protein that could reflect different expression of βγ subunits in Gαi2^–/– platelets (data not shown). In confirmation of a previous report on human platelets (3), we were unable to detect expression of Gα_i1 in wild-type mouse platelet membranes (data not shown). It thus appears that the lack of Gα_i2 does not lead to an increased expression in platelets of other G protein subunits capable of mediating the inhibition of adenylyl cyclase.

Figure 1

Expression of Gα subunits in platelet membranes from Gα_i2^–/– mice. Western blots of platelet membranes (30 μg protein) from wild-type (+/+), Gα_i2 knockout (–/–), and heterozygote (–/+) mice were probed with Gα_i2, Gα_q, Gα_i3, and Gα_z Ab’s as indicated. Blots shown are representative of at least three independent experiments controlled for load and detectable protein.

Effect of Gαi2 gene disruption on the inhibition of adenylyl cyclase in platelets. The G protein subunit Gα_i2 is known to be capable of mediating inhibition of stimulated adenylyl cyclase in many cell types and is also the most abundant Gαi form in platelets. To determine the role of Gα_i2 in the regulation of platelet adenylyl cyclase in intact mouse platelets, we have compared the ability of ADP, thrombin, and epinephrine to inhibit forskolin-stimulated cAMP accumulation in washed platelets from wild-type and Gαi2^–/– mice. ADP inhibited cAMP formation in platelets from wild-type mice resulting in 60% and 76% inhibition at 1 and 10 μM ADP, respectively (Figure 2a). In platelets from Gαi2^–/– mice, however, ADP-dependent repression of cAMP levels was impaired with only 30% and 52% inhibition observed at 1 and 10 μM ADP, respectively. This suggests that Gα_i2 is important for ADP-dependent inhibition of mouse platelet adenylyl cyclase in vivo. Interestingly, this defect can be detected even in the presence of other inhibitory G proteins, such as Gα_i3 and Gα_z , which may partially substitute for Gα_i2 in Gαi2^–/– platelets.

Figure 2

Inhibition of platelet cAMP accumulation by ADP and thrombin in Gα_i2^–/– mice. (a, b and d) In WPs from wild-type mice (filled bars) and Gα_i2 knockout mice (open bars), cAMP formation was stimulated by forskolin (Control, 100%) in the presence of increasing concentrations of ADP (a), thrombin (b), and epinephrine (d). (c) Blockade of thrombin-mediated inhibition of cAMP formation by the P2Y₁₂ ADP receptor antagonist 2MeSAMP. Platelets from wild-type mice treated with forskolin and thrombin (filled bars) were incubated with 2MeSAMP (hatched bars). Data shown are the mean ± SEM of one representative experiment out of at least three performed in triplicate, normalized to cAMP levels in platelets treated with forskolin only (Control, 100%).

Thrombin repressed forskolin-stimulated cAMP accumulation in wild-type mouse platelets, reaching 65% and 62% inhibition at 10 and 100 nM thrombin, respectively. This inhibition was impaired in platelets prepared from Gαi2^–/– mice with 38% and 46% inhibition at 10 and 100 nM thrombin (Figure 2b). To determine whether this defect is based on a direct coupling of Gα_i2 to mouse thrombin receptors or to thrombin-dependent release of ADP, which then activates the Gα_i2 -coupled platelet ADP receptor P2Y₁₂ , the cAMP experiment with wild-type platelets was performed in the presence of 2MeSAMP. It has been reported previously that 2MeSAMP is a selective antagonist of the human platelet ADP receptor (P2Y₁₂), mediating the inhibition of adenylyl cyclase (13, 14). We have confirmed the properties of this antagonist on mouse ADP receptors (data not shown). Interestingly, addition of 5 μM 2MeSAMP prevented the inhibition of cAMP accumulation even at the relatively high thrombin concentration of 100 nM (Figure 2c). These results indicate that thrombin-dependent inhibition of platelet adenylyl cyclase is not mediated by thrombin receptors, but by ADP released upon stimulation of platelets activating the Gα_i2 -coupled ADP receptor (P2Y₁₂) in wild-type mice. Therefore, thrombin-dependent inhibition of adenylyl cyclase is defective in Gα_i2^–/– mice.

By contrast, no significant impairment of epinephrine-mediated inhibition of adenylyl cyclase could be detected (Figure 2d). Epinephrine inhibited forskolin-stimulated cAMP formation in wild-type platelets by 21%, 53%, and 80% and in Gαi2^–/– platelets by 9%, 55%, and 86% at 0.01, 0.1, and 1 μM epinephrine, respectively. This inhibition suggests either that Gα_i2 does not mediate epinephrine-dependent inhibition of platelet adenylyl cyclase or that other inhibitory G proteins, e.g., Gα_i3 or Gα_z , can functionally replace Gα_i2 in Gαi2^–/– platelets. The former interpretation is consistent with the suggestion based on studies with Gα_z^–/– platelets that the effects of epinephrine appear to be predominantly mediated by Gα_z (7).

Effect of Gαi2 gene disruption on platelet aggregation and integrin activation. To determine the role of Gα_i2 in platelet aggregation, we compared ADP- and thrombin-induced aggregation of washed platelets from wild-type and Gαi2^–/– mice. ADP induced half-maximal aggregation of wild-type platelets at about 0.3 μM and full aggregation at 10 μM (Figure 3a). In Gαi2^–/– platelets, the final extent of aggregation was reduced by 35–67% and the slopes of aggregation tracings were diminished as well. Even at the very high ADP concentration of 100 μM, aggregation was still impaired by 21–38% (data not shown). These results support the conclusion that Gα_i2 is not only important for the inhibition of platelet adenylyl cyclase but also for platelet aggregation. ADP-dependent aggregation was also investigated in platelets from heterozygous mice. No defect was detected, which is consistent with the assumption that the amount of Gα_i2 in platelets from heterozygous mice is sufficient to saturate the small number of ADP receptors (data not shown).

Figure 3

Platelet aggregation in Gα_i2^–/– mice. (a) ADP-induced aggregation of WPs of wild-type mice (+/+) and Gα_i2 knockout mice (–/–) was determined as an increase in light transmission in an aggregometer. (b) Aggregation responses to threshold (0.2 nM) and higher (1 nM) concentrations of thrombin were obtained with wild-type (+/+) and Gα_i2 knockout platelets (–/–), and with wild-type (+/+) platelets in the presence and absence of the P2Y₁₂ ADP receptor antagonist 2MeSAMP. The aggregometer tracings shown are representative of at least three independent experiments.

Thrombin-induced aggregation was also impaired in platelets from Gαi2^–/– mice, but only at threshold concentrations of thrombin that induce slightly reversible aggregation (Figure 3b). With platelets from Gαi2^–/– mice, maximum aggregation was reduced to about 50% when compared with wild-type platelets, and aggregation was almost completely reversible within 5 minutes. At a fivefold higher concentration of thrombin, which induces irreversible aggregation in wild-type platelets, no effect of Gα_i2 gene disruption was observed. Since aggregation induced by low concentrations of thrombin has been shown previously to be dependent on released ADP (21), the P2Y₁₂ ADP receptor antagonist 2MeSAMP was included in the experiment. Blockade of the P2Y₁₂ receptor on wild-type platelets by 10 μM 2MeSAMP mimicked essentially the observations with thrombin-induced platelet aggregation of platelets from Gαi2^–/– mice (Figure 3c). Apparently, Gα_i2 does not couple to mouse platelet thrombin receptors in the pathway leading to aggregation, or Gα_i3 together with Gq, and possibly other G proteins can functionally replace Gα_i2.

Platelet aggregation requires activation of the platelet integrin αIIbβ3 (GPIIb-IIIa) enabling the binding of soluble fibrinogen with high affinity and avidity. Integrin activation was determined as an ADP or thrombin-induced increase in binding of FITC-conjugated fibrinogen to platelets using flow cytometry. FITC-fibrinogen binding to wild-type platelets was induced 4.1-fold, 13-fold, and 18-fold by 1, 10, and 100 μM ADP, respectively (Figure 4a). By comparison, in Gαi2^–/– platelets, induction of FITC-fibrinogen binding was strongly reduced even at high ADP concentrations, resulting in an induction factor of only 1.4, 1.8, and 2.5. Apparently, Gα_i2 is required for ADP-mediated inside-out integrin signaling with little substitution by other G proteins still present in Gαi2^–/– platelets. Thrombin-induced FITC-fibrinogen binding is also reduced in Gαi2^–/– platelets, however, to a lesser degree (Figure 4b). FITC-fibrinogen binding was induced 15- and 23-fold by 10 and 100 nM thrombin in wild-type platelets and 7- and 20-fold in Gαi2^–/– platelets, respectively. In wild-type mouse platelets, blockade of the P2Y₁₂ ADP receptor linked to inhibition of adenylyl cyclase with 100 μM 2MeSAMP also resulted in a reduction of thrombin-mediated FITC-fibrinogen binding (Figure 4c). These data support the hypothesis that Gα_i2 is involved in inside-out integrin signaling by thrombin predominantly through released ADP.

Figure 4

Stimulation of FITC-fibrinogen binding to platelets from Gα_i2^–/– mice. (a and b) Diluted PRP from wild-type (filled bars) and Gα_i2 knockout mice (open bars) was incubated with FITC-fibrinogen in the absence (Control) and presence of increasing concentrations of ADP and thrombin. The mean fluorescence intensity of platelet-bound FITC-fibrinogen was determined by flow cytometry. (c) Blockade of thrombin-mediated stimulation of FITC-fibrinogen binding by the P2Y₁₂ ADP receptor antagonist 2MeSAMP. Platelets from wild-type mice treated with thrombin were incubated in the absence (filled bars) and presence of the P2Y₁₂ ADP receptor antagonist 2MeSAMP (gray bars). Data shown are the mean ± SEM of one representative experiment out of at least three performed in duplicate.

Effect of Gαi2 gene disruption on platelet shape change and calcium mobilization. ADP mediates platelet shape change through the P2Y₁ receptor, which is linked to intracellular calcium mobilization. Antagonists of the P2Y₁₂ ADP receptor coupled to inhibition of adenylyl cyclase do not block shape change (8, 13, 21). Furthermore, no ADP-dependent shape change has been detected in platelets from mice in which the Gα_q gene has been disrupted (6, 23). However, signaling pathways independent of Gα_q and calcium have been proposed to be involved in ADP-mediated shape change (5, 24). As shown in Figure 5, platelet shape change upon addition of ADP was not affected in Gαi2^–/– mice. This is consistent with Gα_q being at least the predominant mediator of this response, although some involvement of Gα_i2 , substituted by Gα_i3 and Gα_z in Gαi2^–/– platelets, cannot be excluded.

Figure 5

Platelet shape change in Gα_i2^–/– mice. Shape change in response to ADP and thrombin was determined in WPs from wild-type (+/+) and Gα_i2 knockout mice (–/–) as a decrease of light transmission in an aggregometer. Arrows indicate addition of agonists. Aggregation was prevented by addition of EDTA, and the amplitude of the tracings was amplified for clarity. The aggregometer tracings shown are representative of at least three independent experiments.

The Gα_q pathway and intracellular calcium mobilization do not appear to be required for thrombin-dependent platelet shape change, but Gα₁₂ and/or Gα13 might be involved (5, 6). The contribution of G_I proteins has not been investigated. Here we show that the shape change response in platelets from Gαi2^–/– mice was not impaired, even at low concentrations of thrombin (0.1 nM; Figure 5), suggesting that Gα_i2 is not a major component in the signaling pathway for thrombin-dependent platelet shape change.

Intracellular calcium mobilization is a critical event for platelet activation and has been attributed to the Gq pathway. Consequently, in platelets from Gα_q^–/– mice no intracellular calcium mobilization was detected in response to ADP, thrombin, or other platelet agonists (6). However, the contribution of other G proteins, e.g., βγ subunits released from Gα_i2 and capable of activation of phospholipase Cβ, cannot be excluded from those data. Wild-type and Gαi2^–/– platelets were analyzed for intracellular calcium mobilization using the fura-2 ratio method. In three independent experiments no impairment of responses to 1 and 10 μM ADP or 1 nM thrombin was found in Gαi2^–/– platelets (data not shown). These findings are consistent with the hypothesis that Gα_i2 does not play any role in this signaling pathway.

This study provides direct evidence for the importance of Gα_i2 for platelet activation resulting in aggregation. Platelets from Gαi2^–/– mice have been found to be defective for ADP-dependent inhibition of adenylyl cyclase, integrin activation, and aggregation. Intracellular calcium mobilization and shape change were not affected, which is consistent with previous results with Gα_q^–/– platelets and selective ADP receptor antagonists, which suggested a predominant role of the G_q pathway for these responses (6, 23). Lack of expression of Gα_i2 resulted in only partial signaling defects. It is conceivable that other inhibitory G proteins expressed in the Gαi2^–/– platelets, such as Gα_i3 and maybe Gα_z , can be activated by ADP receptors even though they might not couple efficiently to these receptors in wild-type platelets when Gα_i2 is present. This would be consistent with the finding that Gα_z^–/– platelets display normal cAMP signaling in response to ADP and thrombin-receptor stimulation (7). The fact that any defects could be detected in the presence of a likely excess of other inhibitory G proteins supports the crucial role of Gα_i2 in this signaling pathway.

The finding that integrin activation was more affected in the Gαi2^–/– platelets than inhibition of adenylyl cyclase points to a pathway between Gα_i2 and the integrin that is independent of cAMP and strongly suggests a pivotal role for Gα_i2 in inside-out signaling. However, the almost complete loss of ADP-dependent integrin activation did not result in a similar dramatic effect on platelet aggregation, although integrin activation is thought to be a prerequisite of aggregation. There are at least two possible explanations for this apparent discrepancy. First, platelets from β₃ integrin heterozygous -/+mice that express only 50% of wild-type level of GPIIb-IIIa are still capable of full aggregation (25). Second, in contrast to measurements of FITC-fibrinogen binding, aggregation assays are performed with an excess of fibrinogen and stirring, facilitating integrin outside-in signaling that enhances aggregation (1). A contribution of released thromboxane A₂ to the observed aggregation can be excluded, because addition of a thromboxane A₂ receptor antagonist did not affect aggregation induced by ADP or thrombin (H.-M. Jantzen, unpublished observations).

The data in the present study support a role of Gα_i2 in thrombin-dependent inhibition of adenylyl cyclase, integrin activation, and aggregation, although the effect was less pronounced than with ADP. Interestingly, blockade of the ADP receptor mediating inhibition of adenylyl cyclase (P2Y₁₂) by 2MeSAMP in wild-type platelets had effects on thrombin signaling that were similar to those of the Gα_i2 knockout. This supports the hypothesis that Gα_i2 does not directly couple to mouse platelet thrombin receptors, but instead ADP released upon thrombin activation of platelets activates Gα_i2 -coupled ADP receptors. These results are also consistent with other reports emphasizing the role of released ADP and a G_i-linked ADP receptor in platelet activation by other agonists (21, 26–29). The observation that the P2Y₁₂ antagonist 2MeSAMP interfered with thrombin effects on adenylyl cyclase and fibrinogen binding to a larger extent than did the Gα_i2 gene disruption, substantiates the conclusion above that the P2Y₁₂ receptor couples to other proteins of the G_i family in addition to Gα_i2. The findings presented here suggesting that mouse thrombin receptors do not directly mediate inhibition of adenylyl cyclase in intact platelets support the observation that the cloned thrombin receptors PAR3 and PAR4 do not couple to adenylate cyclase (30). Unlike mouse platelets, human platelets express PAR1 instead of PAR3, and PAR1 is capable of mediating inhibition of adenylyl cyclase, indicating a different role for Gα_i2 in thrombin signaling in human platelets (31).

The signaling cascade between G_i2 and activation of the platelet integrin α_IIbβ₃ (GPIIb-IIIa) still remains to be elucidated. Gα_i2 mediates inhibition of adenylyl cyclase, but direct inhibition of the enzyme, even with concomitant activation of the G_q pathway, is not sufficient for platelet aggregation (16–18). Since high cAMP levels prevent platelet activation, the phosphorylation-dephosphorylation equilibrium of substrates of the cAMP-dependent protein kinase A might be important for inside-out signaling. Alternatively, cAMP and Gα_i2 could directly interact with Epac and Rap1GAPII, respectively, which are regulators of Rap1, a small guanine nucleotide-binding protein activated by ADP and thrombin (32–34).

This study demonstrates a critical role for Gα_i2 in platelet activation, especially by ADP. However, it is possible that the absence of the βγ subunits normally released from activated Gα_i2 and not the lack of Gα_i2 per se, is responsible for some of the observed defects. For example, βγ subunits can activate isoforms of phosphoinositide-3-OH kinase (PI 3-kinase). PI 3-kinases and released ADP acting on the adenylyl cyclase–coupled ADP receptor (P2Y₁₂) appear to be important for the stabilization of platelet aggregates (1, 21, 27, 29). Alternatively, released βγ subunits may activate nonreceptor tyrosine kinases and other signaling proteins (1, 35). The challenge for future research will be the identification of the missing links of the signaling cascade between Gα_i2 and platelet integrin α_IIbβ₃ (GPIIb-IIIa). The analysis of platelet function from mice lacking Gα_i2 , other signaling proteins, or the Gα_i2 -coupled ADP receptor P2Y₁₂ in combination with selective inhibitors will facilitate progress toward this goal.

We thank F. Deguzman and the COR Therapeutics Inc. animal facility, G. Li and E. Thompson for technical assistance. We thank E. Reynolds and C. Homcy for help with initiating the project; D. Oksenberg, D. Law, D. Phillips, C. Homcy, V. Ramakrishnan, and R. Scarborough for advice or critical reading of the manuscript; and R. Wong, D. DeGuzman and D. Stockett for help with the figures. D.S. Milstone was supported by grants from the NIH (HL-5409 and HL-30628). R.M. Mortensen was supported by NIH grant R01 HL-58606.

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