Ather activation of the Gi pathway is mediated by secondary release of ADP, which acts on the Gi-coupled ADP receptor, P2Y12 [8,11,12]. A common feature of PAR4 across species is that, on its own, PAR4 is not an efficient thrombin substrate [13?5]. As a result, PAR1 in human Epigenetic Reader Domain platelets or PAR3 in mouse platelets serves as acofactor for PAR4 activation at low thrombin concentrations (,10 nM). However, at high concentrations of thrombin ( 30 nM), PAR4 is sufficient to induce platelet activation [6]. Two independent studies show that PAR3 can affect PAR4 signaling, Nakanishi-Matsui et al, reported that the amount of accumulated inositol phosphate (IP) in response to thrombin (10?100 nM) was 1.7-fold increased in COS7 cells expressing mouse PAR4 alone compared to COS7 cells expressing mouse PAR4 and PAR3 [6]. In addition, Mao et al. showed an increase in intracellular Ca2+ mobilization and platelet aggregation in response to plasmin, in PAR3 knockout (PAR32/2) mouse platelets compared to wild type [16]. These studies show that PAR3 can influence PAR4 inhibitor signaling in addition to enhancing PAR4 activation. There are also examples of PAR3 regulating signaling from other PAR family members in endothelial cells and podocytes [17,18]. In the present study we aimed to determine if the activation of PAR4 with thrombin concentrations that occur at the site of the growing thrombus [19] is affected by the presence of PAR3 in mouse platelets. We report here that PAR3 negatively regulates PAR4-mediated Gq signaling by down regulation of Ca2+ mobilization and PKC activation without affecting the G12/13 pathway as measured by RhoA activation. The negative regulationPAR3 Regulates PAR4 Signaling in Mouse Plateletsof PAR3 on PAR4 signaling was independent of the PAR4 agonist. Therefore, we examined the physical interaction between PAR3 and PAR4 with bioluminescence resonance energy transfer (BRET). We also show for the first time that PAR3 forms a constitutive heterodimer with PAR4, and this interaction may affect PAR4 signaling when 
PAR3 is absent. The results from this study demonstrate that PAR4 signaling can be modulated by other PAR subtypes at thrombin concentrations that are found in vivo at the site of the thrombus. This may have important implications for PAR4 signaling in human platelets where it is co-expressed with PAR1. More generally, the physical interaction between platelet GPCRs may provide unique signaling and may have broad implications for the design of antiplatelet agents.Measurement of the concentration of free intracellular Ca2+ ([Ca2+]i)Washed mouse platelets adjusted to a final concentration of 26108 platelets/mL were loaded with 10 mM Fura-2 for 45 minutes at room temperature. Platelets were washed once and resuspended to their original concentration in HEPES-Tyrode buffer (pH 7.4) containing 2 mM CaCl2 or 0.1 mM EGTA. In some experiments, Fura-2 loaded platelets were treated with 100 mM 2-MeSAMP for 5 min in the dark at 37uC prior to measuring intracellular Ca2+ mobilization. Ca2+ release from internal stores was determined by stimulating platelets with 3 mM thapsigargin. Eighty microliters of Fura-2 loaded platelets were placed 
in 96-well plates, stimulated with agonist, and read in a NOVOstar plate reader (BMG Labtech, Durham, NC) at 37uC. Intracellular Ca2+ variations were monitored by measuring the Fura-2 fluorescence ratio at 340/380 nm with emission at 510 nm. Fluorescence measurement was converted to the concentration of intrac.Ather activation of the Gi pathway is mediated by secondary release of ADP, which acts on the Gi-coupled ADP receptor, P2Y12 [8,11,12]. A common feature of PAR4 across species is that, on its own, PAR4 is not an efficient thrombin substrate [13?5]. As a result, PAR1 in human platelets or PAR3 in mouse platelets serves as acofactor for PAR4 activation at low thrombin concentrations (,10 nM). However, at high concentrations of thrombin ( 30 nM), PAR4 is sufficient to induce platelet activation [6]. Two independent studies show that PAR3 can affect PAR4 signaling, Nakanishi-Matsui et al, reported that the amount of accumulated inositol phosphate (IP) in response to thrombin (10?100 nM) was 1.7-fold increased in COS7 cells expressing mouse PAR4 alone compared to COS7 cells expressing mouse PAR4 and PAR3 [6]. In addition, Mao et al. showed an increase in intracellular Ca2+ mobilization and platelet aggregation in response to plasmin, in PAR3 knockout (PAR32/2) mouse platelets compared to wild type [16]. These studies show that PAR3 can influence PAR4 signaling in addition to enhancing PAR4 activation. There are also examples of PAR3 regulating signaling from other PAR family members in endothelial cells and podocytes [17,18]. In the present study we aimed to determine if the activation of PAR4 with thrombin concentrations that occur at the site of the growing thrombus [19] is affected by the presence of PAR3 in mouse platelets. We report here that PAR3 negatively regulates PAR4-mediated Gq signaling by down regulation of Ca2+ mobilization and PKC activation without affecting the G12/13 pathway as measured by RhoA activation. The negative regulationPAR3 Regulates PAR4 Signaling in Mouse Plateletsof PAR3 on PAR4 signaling was independent of the PAR4 agonist. Therefore, we examined the physical interaction between PAR3 and PAR4 with bioluminescence resonance energy transfer (BRET). We also show for the first time that PAR3 forms a constitutive heterodimer with PAR4, and this interaction may affect PAR4 signaling when PAR3 is absent. The results from this study demonstrate that PAR4 signaling can be modulated by other PAR subtypes at thrombin concentrations that are found in vivo at the site of the thrombus. This may have important implications for PAR4 signaling in human platelets where it is co-expressed with PAR1. More generally, the physical interaction between platelet GPCRs may provide unique signaling and may have broad implications for the design of antiplatelet agents.Measurement of the concentration of free intracellular Ca2+ ([Ca2+]i)Washed mouse platelets adjusted to a final concentration of 26108 platelets/mL were loaded with 10 mM Fura-2 for 45 minutes at room temperature. Platelets were washed once and resuspended to their original concentration in HEPES-Tyrode buffer (pH 7.4) containing 2 mM CaCl2 or 0.1 mM EGTA. In some experiments, Fura-2 loaded platelets were treated with 100 mM 2-MeSAMP for 5 min in the dark at 37uC prior to measuring intracellular Ca2+ mobilization. Ca2+ release from internal stores was determined by stimulating platelets with 3 mM thapsigargin. Eighty microliters of Fura-2 loaded platelets were placed in 96-well plates, stimulated with agonist, and read in a NOVOstar plate reader (BMG Labtech, Durham, NC) at 37uC. Intracellular Ca2+ variations were monitored by measuring the Fura-2 fluorescence ratio at 340/380 nm with emission at 510 nm. Fluorescence measurement was converted to the concentration of intrac.
