Concomitant activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin αIIbβ3 activation and irreversible aggregation in Gαq-deficient platelets.
The study demonstrates that Gq-mediated signaling is not strictly required for platelet integrin αIIbβ3 activation, as synergistic activation of G12/G13 and Gi pathways is sufficient to induce platelet aggregation.
Platelet activation is a complex process induced by a variety of stimuli, which act in concert to ensure the rapid formation of a platelet plug at places of vascular injury. We show here that fibrillar collagen, which initiates platelet activation at the damaged vessel wall, activates only a small fraction of platelets in suspension directly, whereas the majority of platelets becomes activated by mediators released from collagen-activated platelets. In Gαq-deficient platelets that do not respond with activation of integrin αIIbβ3 to a variety of mediators like thromboxane A2 (TXA2), thrombin, or ADP, collagen at high concentrations was able to induce aggregation, an effect that could be blocked by antagonists of the TXA2 or P2Y12 receptors. The activation of TXA2 or P2Y12 receptors alone, which in Gαq-deficient platelets couple to G12/G13 and Gi, respectively, did not induce platelet integrin activation or aggregation. However, concomitant activation of both receptors resulted in irreversible integrin αIIbβ3-mediated aggregation of Gαq-deficient platelets. Thus, the activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin αIIbβ3 activation. Although Gq-mediated signaling plays an important role in platelet activation, it is not strictly required for the activation of integrin αIIbβ3. This indicates that the efficient induction of platelet aggregation through G-protein-coupled receptors is an integrated response mediated by various converging G-protein-mediated signaling pathways involving Gq and Gi as well as G12/G13. Platelet activation is a complex process induced by a variety of stimuli, which act in concert to ensure the rapid formation of a platelet plug at places of vascular injury. We show here that fibrillar collagen, which initiates platelet activation at the damaged vessel wall, activates only a small fraction of platelets in suspension directly, whereas the majority of platelets becomes activated by mediators released from collagen-activated platelets. In Gαq-deficient platelets that do not respond with activation of integrin αIIbβ3 to a variety of mediators like thromboxane A2 (TXA2), thrombin, or ADP, collagen at high concentrations was able to induce aggregation, an effect that could be blocked by antagonists of the TXA2 or P2Y12 receptors. The activation of TXA2 or P2Y12 receptors alone, which in Gαq-deficient platelets couple to G12/G13 and Gi, respectively, did not induce platelet integrin activation or aggregation. However, concomitant activation of both receptors resulted in irreversible integrin αIIbβ3-mediated aggregation of Gαq-deficient platelets. Thus, the activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin αIIbβ3 activation. Although Gq-mediated signaling plays an important role in platelet activation, it is not strictly required for the activation of integrin αIIbβ3. This indicates that the efficient induction of platelet aggregation through G-protein-coupled receptors is an integrated response mediated by various converging G-protein-mediated signaling pathways involving Gq and Gi as well as G12/G13. thromboxane A2 thromboxane B2 forward/side scatter Under normal conditions, platelets circulate freely in the blood and do not adhere to each other. At sites of vascular damage, however, platelets adhere to subendothelial surfaces and become activated. Platelet activation involves a rapid change of shape followed by degranulation and integrin αIIbβ3-mediated aggregation. The activation of platelets is induced by various extracellular stimuli and involves positive feedback loops, which in a complex process ensure the rapid formation of a platelet plug. Collagen, exposed at subendothelial surfaces at sites of vascular injury, induces platelet activation initially by binding to glycoprotein VI, which through the associated FcRγ chain signals in a G-protein-independent manner (1Watson S.P. Gibbins J. Immunol. Today. 1998; 19: 260-264Abstract Full Text Full Text PDF PubMed Scopus (171) Google Scholar). Full platelet aggregation by collagen and subsequent recruitment of platelets into a growing platelet plug, however, requires the formation and release of mediators like thromboxane A2(TXA2)1 and ADP. The transmembrane receptors of TXA2 and ADP couple to heterotrimeric G proteins, which represent central mediators of platelet activation. TXA2 receptors couple to Gq and to G12/G13 (2Offermanns S. Laugwitz K.L. Spicher K. Schultz G. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 504-508Crossref PubMed Scopus (398) Google Scholar, 3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar), whereas ADP acts through two G-protein-coupled receptors, P2Y1, which couples to Gq, and P2Y12, which couples to Gi (4Gachet C. Thromb. Haemostasis. 2001; 86: 222-232Crossref PubMed Scopus (375) Google Scholar, 5Hollopeter G. Jantzen H.-M. Vincent D., Li, G. England L. Ramakrishnan V. Yang R.-B. Nurden P. Nurden A. Julius D. Conley P.B. Nature. 2001; 409: 202-207Crossref PubMed Scopus (1305) Google Scholar). Some of the roles of individual G-protein-mediated signaling pathways in platelet activation have been described during recent years. It is generally believed that Gq-mediated phospholipase Cβ activation plays an essential role in platelet activation as demonstrated by the phenotype of Gαq-deficient platelets, which fail to aggregate and secrete in response to thrombin, ADP, and the TXA2 mimetic U46619 because of a lack of agonist-induced phospholipase C activation (6Offermanns S. Toombs C.F., Hu, Y.H. Simon M.I. Nature. 1997; 389: 183-186Crossref PubMed Scopus (501) Google Scholar). Evidence for a role of Gq-independent processes in the regulation of platelet activation emerged from studies describing the mechanism of ADP-induced platelet activation. Platelets deficient in P2Y1 or in which P2Y1 was blocked pharmacologically do not aggregate in response to low and intermediate concentrations of ADP (7Hechler B. Leon C. Vial C. Vigne P. Frelin C. Cazenave J.P. Gachet C. Blood. 1998; 92: 152-159Crossref PubMed Google Scholar, 8Jin J. Kunapuli S.P. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 8070-8074Crossref PubMed Scopus (481) Google Scholar, 9Savi P. Beauverger P. Labouret C. Delfaud M. Salel V. Kaghad M. Herbert J.M. FEBS Lett. 1998; 422: 291-295Crossref PubMed Scopus (180) Google Scholar, 10Fabre J.E. Nguyen M. Latour A. Keifer J.A. Audoly L.P. Coffman T.M. Koller B.H. Nat. Med. 1999; 5: 1199-1202Crossref PubMed Scopus (396) Google Scholar, 11Leon C. Hechler B. Freund M. Eckly A. Vial C. Ohlmann P. Dierich A. LeMeur M. Cazenave J.P. Gachet C. J. Clin. Invest. 1999; 104: 1731-1737Crossref PubMed Scopus (403) Google Scholar). Aggregation could be restored by serotonin, which induces Gq-mediated phospholipase C activation but alone is not able to induce platelet aggregation (8Jin J. Kunapuli S.P. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 8070-8074Crossref PubMed Scopus (481) Google Scholar, 9Savi P. Beauverger P. Labouret C. Delfaud M. Salel V. Kaghad M. Herbert J.M. FEBS Lett. 1998; 422: 291-295Crossref PubMed Scopus (180) Google Scholar, 11Leon C. Hechler B. Freund M. Eckly A. Vial C. Ohlmann P. Dierich A. LeMeur M. Cazenave J.P. Gachet C. J. Clin. Invest. 1999; 104: 1731-1737Crossref PubMed Scopus (403) Google Scholar). Platelet activation by ADP obviously requires an additional signal, which appears to be mediated by the Gi-coupled P2Y12 receptor (5Hollopeter G. Jantzen H.-M. Vincent D., Li, G. England L. Ramakrishnan V. Yang R.-B. Nurden P. Nurden A. Julius D. Conley P.B. Nature. 2001; 409: 202-207Crossref PubMed Scopus (1305) Google Scholar). Platelets deficient in P2Y12 or in which P2Y12was blocked did not aggregate in response to ADP unless the Gi-mediated pathway was activated by adrenaline, which itself is unable to induce platelet aggregation (8Jin J. Kunapuli S.P. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 8070-8074Crossref PubMed Scopus (481) Google Scholar, 12Foster C.J. Prosser D.M. Agans J.M. Zha I.Y. Smith M.D. Lachowicz J.E. Zhang F.L. Gustafson E. Monsma Jr., F.J. Wiekowski M.T. Abbondanzo S.J. Cook D.N. Bayne M.L. Lira S.A. Chintala M.S. J. Clin. Invest. 2001; 107: 1591-1598Crossref PubMed Scopus (386) Google Scholar). Thus, Gq and Gi can synergize to induce platelet aggregation, and full platelet activation in response to ADP at intermediate concentrations requires concomitant activation of both G-proteins. G-proteins of the G12 family, G12 and G13, which are activated through TXA2 and thrombin receptors (2Offermanns S. Laugwitz K.L. Spicher K. Schultz G. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 504-508Crossref PubMed Scopus (398) Google Scholar, 3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar), have been involved in the induction of the platelet shape change. This is mainly based on the finding that in Gαq-deficient platelets in which TXA2receptors only couple to G12 and G13, U46619still induces a rapid shape change. This effect appears to be mediated by the Rho/Rho kinase pathway (3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar). Here we demonstrate a role for G12/G13-mediated signaling in platelet aggregation and show that in the absence of Gαq, TXA2, and ADP or adrenaline can induce integrin αIIbβ3-dependent platelet aggregation by concomitant activation of G12/G13 through TXA2 receptors and of Gi via P2Y12 or α2-adrenergic receptors, whereas each of the stimuli given alone was without effect. Thus, G12/G13-mediated as well as Gi-mediated signaling processes can synergize to activate the platelet fibrinogen receptor. Mutant mice deficient in Gαq were produced as described previously (6Offermanns S. Toombs C.F., Hu, Y.H. Simon M.I. Nature. 1997; 389: 183-186Crossref PubMed Scopus (501) Google Scholar). Gαq-deficient and wild-type littermates, which were of 129/Sv × C57BL/6 genetic background were used for experiments. The ATP analog AR-C69931MX was a generous gift from ASTRA Charnwood (Loughborough Leics, England). ADP, adrenaline, essentially fatty acid-free bovine serum albumin, and human fibrinogen were from Sigma. The thromboxane analog U46619 was purchased from Alexis Biochemicals (Grünberg, Germany), and fibrillar type I collagen (Horm) from equine tendon was from Nycomed (Munich, Germany). The thromboxane A2 receptor antagonist SQ29548 was fromBiomol (Hamburg, Germany), and the Rho kinase inhibitor Y-27632 was kindly provided by Yoshitomi Pharmaceutical Industries, Ltd. (Saitama, Japan). JON/APE, which selectively binds to activated mouse αIIbβ3 integrin (13Bergmeier W. Schulte V. Brockhoff G. Bier U. Zirngibl H. Nieswandt B. Cytometry. 2002; 48: 80-86Crossref PubMed Scopus (123) Google Scholar), was produced and modified in our laboratory. Fluorescein isothiocyanate-conjugated anti-P-selectin and control IgG antibodies were purchased from BD Pharmingen. Unlabeled JON/A was used to block αIIbβ3 in aggregometry as described previously (13Bergmeier W. Schulte V. Brockhoff G. Bier U. Zirngibl H. Nieswandt B. Cytometry. 2002; 48: 80-86Crossref PubMed Scopus (123) Google Scholar). Mice were bled under ether anesthesia from the retroorbital plexus. Blood was collected in a tube containing 10% (v/v) 7.5 units/ml heparin, and platelet-rich plasma was obtained by centrifugation at 300 × g for 10 min at room temperature. Platelet-rich plasma was removed and centrifuged at 1570 × g for 10 min. The platelet pellet was washed twice in Tyrode's buffer (137 mmol/liter NaCl, 2 mmol/liter KCl, 12 mmol/liter NaHCO3, 0.3 mmol/liter NaH2PO4, 5.5 mmol/liter glucose, 5 mmol/liter Hepes, pH 7.3) containing 0.35% bovine serum albumin and finally resuspended at a density of 5 × 105 platelet/μl in the same buffer in the presence of 0.02 units/ml ADP scavenger apyrase, a concentration sufficient to prevent desensitization of platelet ADP receptors during storage. Platelets were kept at 37 °C throughout all experiments. To block thromboxane A2 or P2Y12 receptors, platelets were incubated with 10 μm SQ29548 or 10 μm AR-C69931MX, respectively, for 10 min at 37 °C before the start of the experiment. To inhibit Rho kinase activity, platelets were incubated with Y-27632 (10 μm) for 30 min before the start of the experiment. To determine platelet aggregation, light transmission was measured using washed platelets adjusted to a platelet concentration of 3 × 105 platelets/μl with Tyrode's buffer containing CaCl2 (1 mm) and human fibrinogen (0.2 mg/ml). Transmission was recorded on a Fibrintimer 4-channel aggregometer (APACT Laborgeräte und Analysensysteme, Hamburg, Germany) and is expressed as arbitrary units with 100% transmission adjusted with plasma. Agonists were added as 20–100-fold concentrates. For determination of platelet degranulation, platelets were loaded with serotonin by incubation of platelet-rich plasma for 1 h at 37 °C with 2 μCi/ml 3Hserotonin (80–130 Ci/mmol). Thereafter, platelets were washed once in Tyrode's buffer, and platelets were adjusted at 2 × 105/ml. Platelets were exposed for 3 min to the indicated stimuli and chilled. After centrifugation, the secreted serotonin was determined by scintillation counting of the supernatants. TXB2 formation was determined using an enzyme immunoassay (Cayman, Ann Arbor, MI). 2.5 × 106 washed platelets were incubated for 3 min in the presence of the indicated stimuli. After centrifugation, TXB2 levels in supernatants were determined according to manufacturer's instructions. Washed platelets were adjusted to 2 × 104 platelets/μl with Tyrode's buffer, and 50 μl of this dilution were stimulated with the indicated agonists for 5 min at 37 °C. Samples were then stained with fluorophore-labeled monoclonal antibodies for 10 min at room temperature and directly analyzed on a FACScalibur (BD Pharmingen). Platelets were gated by forward/side scatter (FSC/SSC) characteristics. We have previously shown that Gαq-deficient platelets display defective activation by collagen (6Offermanns S. Toombs C.F., Hu, Y.H. Simon M.I. Nature. 1997; 389: 183-186Crossref PubMed Scopus (501) Google Scholar) although primary signaling through the activating platelet collagen receptor, GPVI/FcRγ, is not affected by the absence of Gαq (14Nieswandt B. Bergmeier W. Eckly A. Schulte V. Ohlmann P. Cazenave J.P. Zirngibl H. Offermanns S. Gachet C. Blood. 2001; 97: 3829-3835Crossref PubMed Scopus (89) Google Scholar). This may be because of the fact that fibrillar collagen is an insoluble macromolecule and, therefore, only a small fraction of platelets comes into direct contact with collagen (15Nieswandt B. Brakebusch C. Bergmeier W. Schulte V. Bouvard D. Mokhtari-Nejad R. Lindhout T. Heemskerk J.W. Zirngibl H. Fassler R. EMBO J. 2001; 20: 2120-2130Crossref PubMed Scopus (457) Google Scholar), whereas the majority of platelets exposed to collagen may be activated secondarily by mediators like ADP and TXA2, which are released from the collagen-responsive fraction of platelets. To test this possibility, we performed flow cytometric analysis of single platelets in diluted suspensions under experimental conditions that largely exclude the accumulation of released mediators. Only 11% of the wild-type platelets were directly activated by collagen at high concentrations (50 μg/ml) as shown by the activation of αIIbβ3 using the JON/APEantibody (13Bergmeier W. Schulte V. Brockhoff G. Bier U. Zirngibl H. Nieswandt B. Cytometry. 2002; 48: 80-86Crossref PubMed Scopus (123) Google Scholar) and by of In of platelets are activated to stimuli like ADP and TXA2 under the same conditions not The primary activation of a of platelets by fibrillar collagen was not in Gαq-deficient platelets of which to Thus, a small fraction of platelets is activated by collagen directly and agonists like ADP, TXA2, and which in activate the majority of platelets, in the full aggregation This mechanism not only activation of platelets in suspension as here but the activation of platelets by subendothelial collagen at the damaged vessel in The of platelet activation by mediators like ADP and TXA2 is to be largely because both ADP and TXA2 fail to induce platelet aggregation in the absence of Gαq (6Offermanns S. Toombs C.F., Hu, Y.H. Simon M.I. Nature. 1997; 389: 183-186Crossref PubMed Scopus (501) Google Scholar). To test this we aggregation of Gαq-deficient platelets in response to concentrations of fibrillar Although low concentrations of collagen did not induce aggregation of platelets in the absence of Gαq, collagen at concentrations of induced aggregation of Gαq-deficient platelets, which with a shape change shown by the in light transmission before the start of collagen was in Gαq-deficient platelets with wild-type platelets, the to be only affected by the absence of Gq-mediated The aggregation of Gαq-deficient platelets in response to high collagen concentrations was by serotonin release and the formation of TXA2 whereas collagen at low concentrations was unable to induce release or formation of mediators in the absence of that agonists released from collagen-activated platelets can induce shape change and aggregation in the absence of In Gαq-deficient platelets, the receptors for ADP and TXA2 couple to Gi and we the role of signaling pathways in the process of platelet To test the of activation of G12/G13 for the shape change and aggregation response in Gαq-deficient platelets, we the platelet TXA2 receptor by the antagonist SQ29548 A. M.L. D.N. J. Google Scholar). by light this the shape change of Gαq-deficient platelets 2 Thus, the activation of G12/G13 through TXA2receptors plays a role in this the of the TXA2 receptor the aggregation that is involved in the activation of the platelet integrin αIIbβ3. the the of the Gi-coupled P2Y12 receptor by the antagonist AR-C69931MX (10 μm) on the shape change but the aggregation response The finding that both TXA2 and ADP are required for platelet aggregation in the absence of Gαq that G12/G13- and Gi-mediated signaling pathways act in concert to activate integrin αIIbβ3. To the role of G12/G13 in platelet integrin activation, we used the TXA2 mimetic which is to Gq and G12/G13 through the TXA2 receptor. shown U46619 induced a rapid shape change but aggregation in Gαq-deficient platelets at concentrations to 30 whereas aggregation was at 0.3 μm in wild-type platelets the role of Gq in this aggregation of wild-type platelets been shown to largely on released ADP on the Gi-coupled P2Y12 receptor J. Kunapuli S.P. J. Biol. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). This finding that the activation of Gq and G12/G13 is not sufficient to full integrin activation and, degranulation in platelets unless Gi is activated. This was we wild-type platelets by flow for activated αIIbβ3 integrin and U46619 at concentrations of to 30 only low levels of and in wild-type platelets Thus, the activation of Gq and G12/G13 is not sufficient for efficient integrin activation and release in platelets. However, the of low concentrations of ADP μm) resulted in αIIbβ3 activation and At this ADP alone is to induce only low levels of αIIbβ3 activation and in wild-type platelets, in platelet aggregation 3 (4Gachet C. Thromb. Haemostasis. 2001; 86: 222-232Crossref PubMed Scopus (375) Google Scholar). The effect of ADP was on of the Gi-coupled P2Y12 receptor, because it was in the presence of the P2Y12 antagonist AR-C69931MX (10 This finding that the of Gαq-deficient platelets to aggregate in response to U46619 is not only because of the absence of the Gq-mediated pathway to activation of integrin αIIbβ3 but is based on defective release of ADP and subsequent To test this we analyzed αIIbβ3 activation on Gαq-deficient platelets in the absence or presence of added ADP. In platelets, which do not in response to TXA2 receptor activation, U46619 alone did not induce αIIbβ3 activation at concentrations to 30 μm 3 however, but activation of the of 5 μm ADP, which by itself effect because of the lack of P. Eckly A. Freund M. Cazenave Offermanns S. Gachet C Blood. PubMed Google Scholar). to wild-type platelets, the effect of ADP was by the AR-C69931MX (10 μm) 3 The low levels of induced by U46619 and ADP in Gαq-deficient platelets were sufficient to and irreversible aggregation that was by a αIIbβ3 integrin 3 aggregation was blocked in the presence of AR-C69931MX, the role of the Gi activation in this process were obtained G12/G13 activation was induced by thrombin of Although thrombin (0.2 alone induced shape change but aggregation of Gαq-deficient platelets, αIIbβ3-dependent aggregation in the presence of added ADP This effect of ADP was by AR-C69931MX (10 μm) not It is that Gαq-deficient platelets do not aggregate in response to U46619 or ADP because of the lack of (6Offermanns S. Toombs C.F., Hu, Y.H. Simon M.I. Nature. 1997; 389: 183-186Crossref PubMed Scopus (501) Google Scholar). To test the of both agonists be able to this by we measured levels in Gαq-deficient platelets in response to concentrations of U46619 and ADP. However, although wild-type platelets with to a of 1 and 5 μm ADP, response was in Gαq-deficient platelets, at 10 50 μm ADP not that concomitant of G12/G13 and Gi in integrin activation in platelets and that Gq is not required to induce platelet activation through G-protein-coupled receptors. To test this we stimulated wild-type and Gαq-deficient platelets with U46619 in the absence or presence of the alone is unable to platelet shape change or aggregation (14Nieswandt B. Bergmeier W. Eckly A. Schulte V. Ohlmann P. Cazenave J.P. Zirngibl H. Offermanns S. Gachet C. Blood. 2001; 97: 3829-3835Crossref PubMed Scopus (89) Google Scholar, Cazenave Thromb. Haemostasis. PubMed Scopus Google Scholar) but can various platelet through Gi and the Gi J. A. D. M. J.A. Proc. Natl. Acad. Sci. U. S. A. 97: PubMed Scopus (171) Google Scholar). wild-type Gαq-deficient platelets to adrenaline (10 μm) with αIIbβ3 activation and or C and not and aggregation and not the effect of U46619 in both mouse as shown by αIIbβ3 activation and in wild-type and αIIbβ3 activation in Gαq-deficient platelets The αIIbβ3 activation in Gαq-deficient platelets in response to U46619 and adrenaline was sufficient to irreversible platelet aggregation that was by a the integrin This finding the of both G12/G13 and Gi in full integrin activation and release in response to the TXA2 mimetic U46619 and indicates that activation of G12/G13- and Gi-mediated pathways is sufficient for platelet activation. that the G12/G13-mediated platelet shape change response is mediated by the Rho/Rho kinase pathway (3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar, Kunapuli S.P. J. Biol. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, M. M. P. M. M. S.P. W. Blood. 1999; PubMed Google Scholar). In with this the shape change of Gαq-deficient platelets in response to high collagen concentrations was in the presence of the Rho kinase inhibitor Y-27632 (10 μm) T. M. J. K. M. S. Google Scholar). however, the of Rho kinase on the aggregation that to integrin activation by a Rho mechanism To test this we activated Gαq-deficient platelets with U46619 alone or in with ADP or adrenaline in the absence or presence of Y-27632 (10 whereas the of Rho kinase the shape change induced by it effect on αIIbβ3 activation 5 and platelet aggregation 5 of like which have been shown to be activated through G12/G13 in platelets (3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar), effect on the activation of Gαq-deficient platelets in response to U46619 and ADP not The the G12/G13-mediated signaling pathway to integrin αIIbβ3 activation in platelets To test the of the G12/G13- and Gi-mediated signaling in normal platelets, we stimulated wild-type platelets with in the presence or absence of At this U46619 alone is to induce a rapid shape change through G12/G13 activation but aggregation to Gq signaling (3Klages B. Brandt U. Simon M.I. Schultz G. Offermanns S. J. Cell Biol. 1999; 144: 745-754Crossref PubMed Scopus (317) Google Scholar, FEBS Lett. PubMed Scopus Google Scholar, S. J. PubMed Scopus Google Scholar) 5 However, the concomitant of Gi by the of adrenaline (10 μm) induced irreversible integrin αIIbβ3-dependent aggregation 5 demonstrate that the of G12/G13 signaling to platelet activation plays a role in wild-type platelets, at at low This may the in a variety of stimuli, which are at induce platelet activation during our show that the activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin αIIbβ3 activation. Gq-mediated signaling processes are essential for and an important role in platelet activation, are not required for the activation of integrin αIIbβ3. that the activation of platelets through G-protein-coupled receptors is a integrated which in to with high requires the activation of at G-protein-mediated pathways involving Gq, Gi, and We for
Nieswandt et al. (Tue,) conducted a other in Platelet activation. Concomitant activation of G12/G13 and Gi pathways vs. Activation of either pathway alone was evaluated on Integrin αIIbβ3 activation and platelet aggregation. Concomitant activation of G12/G13- and Gi-mediated signaling pathways is sufficient to induce integrin αIIbβ3 activation and irreversible aggregation in Gαq-deficient platelets.