Ild type (black) and PAR32/2 (gray) platelets were incubated at 37uC for 5 min in the absence or the presence of 100 mM 2MeSAMP. After treatment, platelets were activated 100 nM thrombin (A,) or 2 mM AYPGKF (B) for 10 min at 37uC in the presence of 2 mM of CaCl2. The I-BRD9 site difference between the maximum increase and the basal intracellular Ca2+ mobilization was measured. The results are the mean (6 SD) of three independent experiments (* p,0.05). doi:10.1371/journal.pone.0055740.gfrom the intracellular stores for PAR32/2 platelets compared to wild type platelets.G12/13 and Gi-mediated signaling are not affected in PAR32/2 mouse plateletsPAR4 couples to Gq and G12/13 in human platelets [7,8]. We next determined if G12/13-mediated signaling was also negatively regulated by PAR3 in 
response to thrombin in mouse platelets. The G12/13 pathway was tested by measuring the activation of the small GTPase RhoA by a G-LISA in response to thrombin. As expected, the level of RhoA activation is decreased in PAR32/2 compared to wild type mouse platelets at low thrombin concentrations (#10 nM) because PAR4 was unable to mediate the signaling in the absence of PAR3 (Figure 6). However, there was no significant difference in the level of RhoA activation in response to thrombin concentrations ( 30 nM) in PAR32/2 platelets compared to wild type mouse platelets. We next examined the activation of Gi pathway in response to thrombin by measuring the phosphorylation level of Akt. The activation of Akt plays an important role in platelet aggregation and secretion by negatively regulating glycogen synthase kinase 3b (GSK-3b) [24,25]. Our data show that in response to increasing concentrations of thrombin, there was no significant difference in Akt activation between PAR32/2 and wild type 18055761 mouse platelets (Figure 7A and B). These data indicate that PAR3 negatively regulates PAR4 induced Gq signaling pathways without affecting G12/13 and Gi pathways in mouse platelets.PAR4 and form constitutive homodimers and heterodimers. Finally, we verified the expression level of PAR3 and PAR4 in HEK293 cells by flow cytometry using HA or V5 tag antibodies conjugated to Alexa Fluor 647. The mean fluorescence intensity from each antibody was converted to antibody binding sites using quantitative flow cytometry (Figure 8 F, G and H).DiscussionThe accepted physiological role of PAR3 in mouse platelets is to serve as a cofactor for cleavage and activation of PAR4 at low thrombin concentrations [6]. The results from the current study provide the first evidence that PAR3 plays an additional role in mouse platelets by negative regulation of PAR4 mediated Ca2+ mobilization and protein kinase C (PKC) activation without affecting the downstream signaling of the G12/13 pathways. Throughout our study we have used thrombin concentrations of 30 and 100 nM. It is common to use low thrombin concentrations to examine signaling pathways in platelets so that one can detect subtle differences that would otherwise be missed. It is important to consider that the thrombin concentration generated at the platelet surface at the site of injury likely reaches .100 nM locally [19]. In human platelets, the elevation in intracellular Ca2+ concentration regulates various platelet functions, such as integrin activation, granule secretion, and rapid procoagulant phosphatidylserine (PS) exposure [26,27]. One important initiator of Ca2+ signaling is the activation of Gq pathways, which induce the Pentagastrin generation of diacylg.Ild type (black) and PAR32/2 (gray) platelets were incubated at 37uC for 5 min in the absence or the presence of 100 mM 2MeSAMP. After treatment, platelets were activated 100 nM thrombin (A,) or 2 mM AYPGKF (B) for 10 min at 37uC in the presence of 2 mM of CaCl2. The difference between the maximum increase and the basal intracellular Ca2+ mobilization was measured. The results are the mean (6 SD) of three independent experiments (* p,0.05). doi:10.1371/journal.pone.0055740.gfrom the intracellular stores for PAR32/2 platelets compared to wild type platelets.G12/13 and Gi-mediated signaling are not affected in PAR32/2 mouse plateletsPAR4 couples to Gq and G12/13 in human platelets [7,8]. We next determined if G12/13-mediated signaling was also negatively regulated by PAR3 in response to thrombin in mouse platelets. The G12/13 pathway was tested by measuring the activation of the small GTPase RhoA by a G-LISA in response to thrombin. As expected, the level of RhoA activation is decreased in PAR32/2 compared to wild type mouse platelets at low thrombin concentrations (#10 nM) because PAR4 was unable to mediate the signaling in the absence of PAR3 (Figure 6). However, there was no significant difference in the level of RhoA activation in response to thrombin concentrations ( 30 nM) in PAR32/2 platelets compared to wild type mouse platelets. We next examined the activation of Gi pathway in response to thrombin by measuring the phosphorylation level of Akt. The activation of Akt plays an important role in platelet aggregation and secretion by negatively regulating glycogen synthase kinase 3b (GSK-3b) [24,25]. Our data show that in response to increasing concentrations of thrombin, there was no significant difference in Akt activation between PAR32/2 and wild type 18055761 mouse platelets (Figure 7A and B). These data indicate that PAR3 negatively regulates PAR4 induced Gq signaling pathways without affecting G12/13 and Gi pathways in mouse platelets.PAR4 and form constitutive homodimers and heterodimers. Finally, we verified the expression level of PAR3 and PAR4 in HEK293 cells by flow cytometry using HA or V5 tag antibodies conjugated to Alexa Fluor 647. The mean fluorescence intensity from each antibody was converted to antibody binding sites using quantitative flow cytometry (Figure 8 F, G and H).DiscussionThe accepted physiological role of PAR3 in mouse platelets is to serve as a cofactor for cleavage and activation of PAR4 at low thrombin concentrations [6]. The results from the current study provide the first evidence that PAR3 plays an additional role in mouse platelets by negative regulation of PAR4 mediated Ca2+ mobilization and protein kinase C (PKC) activation without affecting the downstream signaling of the G12/13 pathways. Throughout our study we have used thrombin concentrations of 30 and 100 nM. It is common to use low thrombin concentrations to examine signaling pathways in platelets so that one can detect subtle differences that would otherwise be missed. It is important to consider that the thrombin concentration generated at the platelet surface at the site of injury likely reaches .100 nM locally [19]. In human platelets, the elevation in intracellular Ca2+ concentration regulates various platelet functions, such as integrin activation, granule secretion, and rapid procoagulant phosphatidylserine (PS) exposure [26,27]. One important initiator of Ca2+ signaling is the activation of Gq pathways, which induce the generation of diacylg.