Calcium mobilization and protein kinase C activation downstream of protease activated receptor 4 (PAR4) is negatively regulated by PAR3 in mouse platelets

The C-Type Lectin Receptor CD93 Regulates Platelet Activation and Surface Expression of the Protease Activated Receptor 4

BACKGROUND

 The C-type lectin receptor CD93 is a single pass type I transmembrane glycoprotein involved in inflammation, immunity, and angiogenesis. This study investigates the role of CD93 in platelet function. CD93 knockout (KO) mice and wild-type (WT) controls were compared in this study.

METHODS

 Platelet activation and aggregation were investigated by flow cytometry and light transmission aggregometry, respectively. Protein expression and phosphorylation were analyzed by immunoblotting. Subcellular localization of membrane receptors was investigated by wide-field and confocal microscopy.

RESULTS

 The lack of CD93 in mice was not associated to any evident bleeding defect and no alterations of platelet activation were observed upon stimulation with thromboxane A2 analogue and convulxin. Conversely, platelet aggregation induced by stimulation of the thrombin receptor PAR4 was significantly reduced in the absence of CD93. This defect was associated with a significant reduction of α-granule secretion, integrin αIIbβ3 activation, and protein kinase C (PKC) stimulation. Resting WT and CD93-deficient platelets expressed comparable amounts of PAR4. However, upon stimulation with a PAR4 activating peptide, a more pronounced clearance of PAR4 from the platelet surface was observed in CD93-deficient platelets compared with WT controls. Confocal microscopy analysis revealed a massive movement of PAR4 in cytosolic compartments of activated platelets lacking CD93. Accordingly, platelet desensitization following PAR4 stimulation was more pronounced in CD93 KO platelets compared with WT controls.

CONCLUSION

 These results demonstrate that CD93 supports platelet activation triggered by PAR4 stimulation and is required to stabilize the expression of the thrombin receptor on the cell surface.

A Mouse Model of the Protease Activated Receptor 4 (PAR4) Pro310Leu Variant has Reduced Platelet Reactivity

Background

Protease activated receptor 4 (PAR4) mediates thrombin signaling on platelets and other cells. Our recent structural studies demonstrated a single nucleotide polymorphism in extracellular loop 3 (ECL3), PAR4-P310L (rs2227376) leads to a hypo-reactive receptor.

Objectives

The goal of this study was to determine how the hypo-reactive PAR4 variant in ECL3 impacts platelet function in vivo using a novel knock-in mouse model (PAR4-322L).

Methods

A point mutation was introduced into the PAR4 gene, F2rl3, via CRISPR/Cas9 to create PAR4-P322L, the mouse homolog to human PAR4-P310L. Platelet response to PAR4 activation peptide (AYPGKF), thrombin, ADP, and convulxin was monitored by αIIbβ3 integrin activation and P-selectin translocation using flow cytometry or platelet aggregation. In vivo responses were determined by the tail bleeding assay and the ferric chloride-induced carotid artery injury model.

Results

PAR4-P/L and PAR4-L/L platelets had a reduced response to AYPGKF and thrombin measured by P-selectin translocation or αIIbβ3 activation. The response to ADP and convulxin was unchanged among genotypes. In addition, both PAR4-P/L and PAR4-L/L platelets showed a reduced response to thrombin in aggregation studies. There was an increase in the tail bleeding time for PAR4-L/L mice. The PAR4-P/L and PAR4-L/L mice both showed an extended time to arterial thrombosis.

Conclusions

PAR4-322L significantly reduced platelet responsiveness to AYPGKF and thrombin, which is in agreement with our previous structural and cell signaling studies. In addition, PAR4-322L had prolonged arterial thrombosis time. Our mouse model provides a foundation to further evaluate the role of PAR4 in other pathophysiological contexts.

Mice lacking PECAM-1 and Ceacam1 have an aberrant platelet and thrombus phenotype

The immunoglobulin (Ig)-immunoreceptor tyrosine-based inhibitory motif (ITIM) bearing receptors, PECAM-1 and CEACAM1 have been shown net negative regulators of platelet-collagen interactions and hemi-ITAM signalling pathways. In this study, a double knockout (DKO) mouse was developed with deleted PECAM-1 and CEACAM1 to study their combined contribution in platelet activation by glycoprotein VI, C-type lectin-like receptor 2 (CLEC-2), protease activated receptor PAR-4, ADP purinergic receptors and thromboxane receptor TP A2 pathways. Additionally, their collective contribution was examined in thrombus formation under high shear and microvascular thrombosis using in vivo models. DKO platelets responded normally to ADP purinergic receptors and TP A2 pathway. However, DKO platelets released significantly higher amounts of P-selectin compared to hyper-responsive Pecam-1-/- or Ceacam1-/- versus wild-type (WT) upon stimulation with collagen related peptide or rhodocytin. Contrastingly, DKO platelets released increased amounts of P-selectin upon stimulation with PAR-4 agonist peptide or thrombin but not Pecam-1-/-, Ceacam1-/- or WT platelets. Blockade of phospholipase C (PLC) or Rho A kinase revealed that DKO platelets enhanced alpha granule release via PAR-4/Gαq/PLC signalling without crosstalk with Src/Syk or G12/13 signalling pathways. This DKO model showed a significant increase in thrombus formation compared to the hyper-responsive Ceacam1-/- or Pecam-1-/- versus WT phenotype. DKO platelets have similar glycoprotein surface expression compared to Pecam-1-/-, Ceacam1-/- and WT platelets. PECAM-1 and CEACAM1 work in concert to negatively regulate hemiITAM signalling, platelet-collagen interactions and PAR-4 Gαq protein coupled signalling pathways. Both PECAM-1 and CEACAM1 are required for negative regulation of platelet activation and microvascular thrombosis in vivo.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

A Role for Protease Activated Receptor Type 3 (PAR3) in Nociception Demonstrated Through Development of a Novel Peptide Agonist

The protease activated receptor (PAR) family is a group of G-protein coupled receptors (GPCRs) activated by proteolytic cleavage of the extracellular domain. PARs are expressed in a variety of cell types with crucial roles in homeostasis, immune responses, inflammation, and pain. PAR3 is the least researched of the four PARs, with little known about its expression and function. We sought to better understand its potential function in the peripheral sensory nervous system. Mouse single-cell RNA sequencing data demonstrates that PAR3 is widely expressed in dorsal root ganglion (DRG) neurons. Co-expression of PAR3 mRNA with other PARs was identified in various DRG neuron subpopulations, consistent with its proposed role as a coreceptor of other PARs. We developed a lipid tethered PAR3 agonist, C660, that selectively activates PAR3 by eliciting a Ca2+ response in DRG and trigeminal neurons. In vivo, C660 induces mechanical hypersensitivity and facial grimacing in WT but not PAR3-/- mice. We characterized other nociceptive phenotypes in PAR3-/- mice and found a loss of hyperalgesic priming in response to IL-6, carrageenan, and a PAR2 agonist, suggesting that PAR3 contributes to long-lasting nociceptor plasticity in some contexts. To examine the potential role of PAR3 in regulating the activity of other PARs in sensory neurons, we administered PAR1, PAR2, and PAR4 agonists and assessed mechanical and affective pain behaviors in WT and PAR3-/- mice. We observed that the nociceptive effects of PAR1 agonists were potentiated in the absence of PAR3. Our findings suggest a complex role of PAR3 in the physiology and plasticity of nociceptors. PERSPECTIVE: We evaluated the role of PAR3, a G-protein coupled receptor, in nociception by developing a selective peptide agonist. Our findings suggest that PAR3 contributes to nociception in various contexts and plays a role in modulating the activity of other PARs.

Murine cadherin-6 mediates thrombosis in vivo in a platelet-independent manner

BACKGROUND

Platelet adhesion is the critical process mediating stable thrombus formation. Previous reports of cadherin-6 on human platelets have demonstrated its role in platelet aggregation and thrombus formation.

OBJECTIVES

We aimed to further characterize the importance of cadherin-6 in thrombosis in vivo.

METHODS

Cadherin-6 platelet expression was evaluated by western blotting, flow cytometry, and immunoprecipitation. Thrombosis was evaluated using the FeCl3 and Rose Bengal carotid artery models in C57Bl6 mice treated with anti-cadherin-6 or IgG and wild-type or Cdh6-/- mice. Platelet function was compared in wild-type and Cdh6-/- mice using tail-clip assays, aggregometry, and flow cytometry.

RESULTS

Human platelet expression of cadherin-6 was confirmed at ~3000 copies per platelet. Cdh6-/- mice or those treated with anti-cadherin-6 antibody showed an increased time to occlusion in both thrombosis models. Cadherin-6 was not expressed on mouse platelets, and there were no differences in tail bleeding times, platelet aggregation, or platelet activation in wild-type versus Cdh6-/- mice.

CONCLUSIONS

Cadherin-6 plays an essential role in thrombosis in vivo. However, cadherin-6 is not expressed on murine platelets. These data are in contrast to human platelets, which express a functional cadherin-6/catenin complex. The essential, platelet-independent role for cadherin-6 in hemostasis may allow it to be an effective and safe therapeutic target.

PAR4 activation involves extracellular loop 3 and transmembrane residue Thr153

Protease-activated receptor 4 (PAR4) mediates sustained thrombin signaling in platelets and is required for a stable thrombus. PAR4 is activated by proteolysis of the N terminus to expose a tethered ligand. The structural basis for PAR4 activation and the location of its ligand binding site (LBS) are unknown. Using hydrogen/deuterium exchange (H/D exchange), computational modeling, and signaling studies, we determined the molecular mechanism for tethered ligand-mediated PAR4 activation. H/D exchange identified that the LBS is composed of transmembrane 3 (TM3) domain and TM7. Unbiased computational modeling further predicted an interaction between Gly48 from the tethered ligand and Thr153 from the LBS. Mutating Thr153 significantly decreased PAR4 signaling. H/D exchange and modeling also showed that extracellular loop 3 (ECL3) serves as a gatekeeper for the interaction between the tethered ligand and LBS. A naturally occurring sequence variant (P310L, rs2227376) and 2 experimental mutations (S311A and P312L) determined that the rigidity conferred by prolines in ECL3 are essential for PAR4 activation. Finally, we examined the role of the polymorphism at position 310 in venous thromboembolism (VTE) using the International Network Against Venous Thrombosis (INVENT) consortium multi-ancestry genome-wide association study (GWAS) meta-analysis. Individuals with the PAR4 Leu310 allele had a 15% reduction in relative risk for VTE (odds ratio, 0.85; 95% confidence interval, 0.77-0.94) compared with the Pro310 allele. These data are consistent with our H/D exchange, molecular modeling, and signaling studies. In conclusion, we have uncovered the structural basis for PAR4 activation and identified a previously unrecognized role for PAR4 in VTE.

Loss of Tet2 affects platelet function but not coagulation in mice

Ten-eleven translocation 2 (TET2) functions as a methylcytosine dioxygenase that catalyzes the iterative oxidation of 5-methylcytosine to 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine. TET2 has been shown to be crucial for the maintenance and differentiation of hematopoietic stem cells, and its deletion and/or mutations results in the expansion of HSPCs, and leads to hematological malignancies. TET2 mutations were found in a variety of hematological disorders such as CMML (60%), MDS (30%), MPN (13%) and AML (20%). Interestingly, it was shown that CMML patients with TET2 mutation exhibited fewer platelets than CMML patients without TET2 mutation. However, the role and function of TET2 in platelet hemostasis and thrombogenesis is not well defined. Here in this study, using a genetically engineered Tet2 deletion mouse model, we found that the absence of Tet2 caused a decrease in the proportion of MEP cells and hyperploid megakaryocytes. Additionally, Tet2-deficient mice displayed impaired platelet activation and aggregation under stimulation of ADP and low concentrations of thrombin, although the modestly compromised platelet function and MEP differentiation in Tet2-deficient mice could be compensated without affecting blood coagulation function. Our study indicate that Tet2 deficiency leads to mild impairment of platelet function and thrombopoiesis in mice.

The endothelial barrier and cancer metastasis: Does the protective facet of platelet function matter?

Overwhelming evidence suggests that platelets have a detrimental role in promoting cancer spread via platelet-cancer cell interactions linked to thrombotic mechanisms. On the other hand, a beneficial role of platelets in the preservation of the endothelial barrier in inflammatory conditions has been recently described, a phenomenon that could also operate in cancer-related inflammation. It is tempting to speculate that some antiplatelet strategies to combat cancer metastasis may impair the endogenous platelet-dependent mechanisms preserving endothelial barrier function. If the protective function of platelets is impaired, it may lead to increased endothelial permeability and more efficient cancer cell intravasation in the primary tumor and cancer cell extravasation at metastatic sites. In this commentary, we discuss current evidence that could support this hypothesis.

Expression and Purification of Protease-Activated Receptor 4 (PAR4) and Analysis with Histidine Hydrogen-Deuterium Exchange

Protease-activated receptors (PARs) are G-protein-coupled receptors that are activated by proteolysis of the N-terminus, which exposes a tethered ligand that interacts with the receptor. Numerous studies have focused on the signaling pathways mediated by PARs. However, the structural basis for initiation of these pathways is unknown. Here, we describe a strategy for the expression and purification of PAR4. This is the first PAR family member to be isolated without stabilizing modifications for biophysical studies. We monitored PAR4 activation with histidine hydrogen-deuterium exchange. PAR4 has nine histidines that are spaced throughout the protein, allowing a global view of solvent accessible and nonaccessible regions. Peptides containing each of the nine His residues were used to determine the t_1/2 for each His residue in apo or thrombin-activated PAR4. The thrombin-cleaved PAR4 exhibited a 2-fold increase (p > 0.01) in t_1/2 values observed for four histidine residues (His¹⁸⁰, His²²⁹, His²⁴⁰, and His³⁸⁰), demonstrating that these regions have decreased solvent accessibility upon thrombin treatment. In agreement, thrombin-cleaved PAR4 also was resistant to thermolysin digestion. In contrast, the rate of thermolysin proteolysis following stimulation with the PAR4 activation peptide was the same as that of unstimulated PAR4. Further analysis showed the C-terminus is protected in thrombin-activated PAR4 compared to uncleaved or agonist peptide-treated PAR4. The studies described here are the first to examine the tethered ligand activation mechanism for a PAR family member biophysically and shed light on the overall conformational changes that follow activation of PARs by a protease.

The protease-activated receptor 4 Ala120Thr variant alters platelet responsiveness to low-dose thrombin and protease-activated receptor 4 desensitization, and is blocked by non-competitive P2Y12 inhibition

Essentials The rs773902 SNP results in differences in platelet protease-activated receptor (PAR4) function. The functional consequences of rs773902 were analyzed in human platelets and stroke patients. rs773902 affects thrombin-induced platelet function, PAR4 desensitization, stroke association. Enhanced PAR4 Thr120 effects on platelet function are blocked by ticagrelor. SUMMARY: Background F2RL3 encodes protease-activated receptor (PAR) 4 and harbors an A/G single-nucleotide polymorphism (SNP) (rs773902) with racially dimorphic allelic frequencies. This SNP mediates an alanine to threonine substitution at residue 120 that alters platelet PAR4 activation by the artificial PAR4-activation peptide (PAR4-AP) AYPGKF. Objectives To determine the functional effects of rs773902 on stimulation by a physiological agonist, thrombin, and on antiplatelet antagonist activity. Methods Healthy human donors were screened and genotyped for rs773902. Platelet function in response to thrombin was assessed without and with antiplatelet antagonists. The association of rs773902 alleles with stroke was assessed in the Stroke Genetics Network study. Results As compared with rs773902 GG donors, platelets from rs773902 AA donors had increased aggregation in response to subnanomolar concentrations of thrombin, increased granule secretion, and decreased sensitivity to PAR4 desensitization. In the presence of PAR1 blockade, this genotype effect was abolished by higher concentrations of or longer exposure to thrombin. We were unable to detect a genotype effect on thrombin-induced PAR4 cleavage, dimerization, and lipid raft localization; however, rs773902 AA platelets required a three-fold higher level of PAR4-AP for receptor desensitization. Ticagrelor, but not vorapaxar, abolished the PAR4 variant effect on thrombin-induced platelet aggregation. A significant association of modest effect was detected between the rs773902 A allele and stroke. Conclusion The F2RL3 rs773902 SNP alters platelet reactivity to thrombin; the allelic effect requires P2Y12 , and is not affected by gender. Ticagrelor blocks the enhanced reactivity of rs773902 A platelets. PAR4 encoded by the rs773902 A allele is relatively resistant to desensitization and may contribute to stroke risk.

Targeting PAR1: Now What?

Protease-activated receptors (PARs) are a ubiquitously expressed class of G-protein-coupled receptors (GPCRs) that enable cells to respond to proteases in the extracellular environment in a nuanced and dynamic manner. PAR1 is the archetypal family member and has been the object of large-scale drug development programs since the 1990s. Vorapaxar and drotrecogin-alfa are approved PAR1-targeted therapeutics, but safety concerns have limited the clinical use of vorapaxar and questions regarding the efficacy of drotrecogin-alfa led to its withdrawal from the market. New understanding of mechanisms of PAR1 function, discovery of improved strategies for modifying PAR1 function, and identification of novel indications for PAR1 modulators have provided new opportunities for therapies targeting PAR1. In this review, we critically evaluate prospects for the next generation of PAR1-targeted therapeutics.

A novel mechanism regulating human platelet activation by MMP-2-mediated PAR1 biased signaling

Platelets contain and release several matrix metalloproteinases (MMPs). Among these, active MMP-2 enhances platelet aggregation by favoring the activation of phosphatidylinositol 3- kinase (PI3K) and contributes to arterial thrombosis. The platelet surface target of MMP-2 and the mechanism through which it primes platelets to respond to subsequent stimuli are still unknown. We show that active MMP-2 enhances platelet activation induced by weak stimuli by cleaving PAR1 at a noncanonical extracellular site different from the thrombin-cleavage site and thus initiates biased receptor signaling, triggering only some of the signaling pathways normally activated by full PAR1 agonism. The novel PAR1-tethered ligand exposed by MMP-2 stimulates PAR1-dependent G_q and G_12/13 pathway activation, triggering p38-MAPK phosphorylation, Ca⁺² fluxes, and PI3K activation, but not G_i signaling; this is insufficient to cause platelet aggregation, but it is enough to predispose platelets to fully respond to G_i-activating stimuli. Integrin α_IIbβ₃ is a necessary cofactor for PAR1 cleavage by MMP-2 by binding the MMP-2 hemopexin domain, thus favoring the interaction of the enzyme with PAR1. Our studies unravel a novel mechanism regulating platelet activation that involves the binding of MMP-2 to integrin α_IIbβ₃ and the subsequent cleavage of PAR1 by active MMP-2 at a noncanonical site, exposing a previously undescribed tethered ligand that triggers biased G-protein agonism and thus predisposes platelets to full activation by other stimuli. These results identify the MMP-2-α_IIbβ₃-PAR1 interaction as a potential target for the prevention of arterial thrombosis.

Protease-activated receptors in hemostasis

Protease signaling in cells elicits multiple physiologically important responses via protease-activated receptors (PARs). There are 4 members of this family of G-protein-coupled receptors (PAR1-4). PARs are activated by proteolysis of the N terminus to reveal a tethered ligand. The rate-limiting step of PAR signaling is determined by the efficiency of proteolysis of the N terminus, which is regulated by allosteric binding sites, cofactors, membrane localization, and receptor dimerization. This ultimately controls the initiation of PAR signaling. In addition, these factors also control the cellular response by directing signaling toward G-protein or β-arrestin pathways. PAR1 signaling on endothelial cells is controlled by the activating protease and heterodimerization with PAR2 or PAR3. As a consequence, the genetic and epigenetic control of PARs and their cofactors in physiologic and pathophysiologic conditions have the potential to influence cellular behavior. Recent studies have uncovered polymorphisms that result in PAR4 sequence variants with altered reactivity that interact to influence platelet response. This further demonstrates how interactions within the plasma membrane can control the physiological output. Understanding the structural rearrangement following PAR activation and how PARs are allosterically controlled within the plasma membrane will determine how best to target this family of receptors therapeutically. The purpose of this article is to review how signaling from PARs is influenced by alternative cleavage sites and the physical interactions within the membrane. Going forward, it will be important to relate the altered signaling to the molecular arrangement of PARs in the cell membrane and to determine how these may be influenced genetically.

Protease-activated receptor 4: a critical participator in inflammatory response

Protease-activated receptors (PARs) are G protein-coupled receptors of which four members PAR1, PAR2, PAR3, and PAR4 have been identified, characterized by a typical mechanism of activation involving various related proteases. The amino-terminal sequence of PARs is cleaved by a broad array of proteases, leading to specific proteolytic cleavage which forms endogenous tethered ligands to induce agonist-biased PAR activation. The biological effect of PARs activated by coagulation proteases to regulate hemostasis and thrombosis plays an enormous role in the cardiovascular system, while PAR4 can also be activated by trypsin, cathepsin G, the activated factor X of the coagulation cascade, and trypsin IV. Irrespective of its role in thrombin-induced platelet aggregation, PAR4 activation is believed to be involved in inflammatory lesions, as show by investigations that have unmasked the effects of PAR4 on neutrophil recruitment, the regulation of edema, and plasma extravasation. This review summarizes the roles of PAR4 in coagulation and other extracellular protease pathways, which activate PAR4 to participate in normal regulation and disease.

PKCs in thrombus formation

The protein kinase C (PKC) family has been implicated in several physiological processes regulating platelet activation. Each isoform of PKC expressed on platelets, may have a positive and/or negative role depending on the nature and concentration of the agonist. Mice lacking PKCα show much reduced thrombus formation in vivo, while PKCθ(-/-) showed inhibition of aggregation in response to PAR4. On the other hand, PKCδ by associating with Fyn, inhibits platelet aggregation. In addition, PKCβ by interacting with its receptor RACK1 has been implicated in the primary phases of signaling via the αIIbβ3 and finally PKCɛ appears to be involved in platelet function downstream GPVI. The present review discusses the latest observations relevant to the role of individual PKC isoforms in platelet activation and thrombus formation.

Platelet specific promoters are insufficient to express protease activated receptor 1 (PAR1) transgene in mouse platelets

The in vivo study of protease activated receptors (PARs) in platelets is complicated due to species specific expression profiles. Human platelets express PAR1 and PAR4 whereas mouse platelets express PAR3 and PAR4. Further, PAR subtypes interact with one another to influence activation and signaling. The goal of the current study was to generate mice expressing PAR1 on their platelets using transgenic approaches to mimic PAR expression found in human platelets. This system would allow us to examine specific signaling from PAR1 and the PAR1-PAR4 heterodimer in vivo. Our first approach used the mouse GPIbα promoter to drive expression of mouse PAR1 in platelets (GPIbα-Tg-mPAR1). We obtained the expected frequency of founders carrying the transgene and had the expected Mendelian distribution of the transgene in multiple founders. However, we did not observe expression or a functional response of PAR1. As a second approach, we targeted human PAR1 with the same promoter (GPIbα-Tg-hPAR1). Once again we observed the expected frequency and distributing of the transgene. Human PAR1 expression was detected in platelets from the GPIbα-Tg-hPAR1 mice by flow cytometry, however, at a lower level than for human platelets. Despite a low level of PAR1 expression, platelets from the GPIbα-Tg-hPAR1 mice did not respond to the PAR1 agonist peptide (SFLLRN). In addition, they did not respond to thrombin when crossed to the PAR4^−/− mice. Finally, we used an alternative platelet specific promoter, human α_IIb, to express human PAR1 (α_IIb-Tg-hPAR1). Similar to our previous attempts, we obtained the expected number of founders but did not detect PAR1 expression or response in platelets from α_IIb-Tg-hPAR1 mice. Although unsuccessful, the experiments described in this report provide a resource for future efforts in generating mice expressing PAR1 on their platelets. We provide an experimental framework and offer considerations that will save time and research funds.

Biased signaling of protease-activated receptors

In addition to their role in protein degradation and digestion, proteases can also function as hormone-like signaling molecules that regulate vital patho-physiological processes, including inflammation, hemostasis, pain, and repair mechanisms. Certain proteases can signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. PARs are expressed by almost all cell types, control important physiological and disease-relevant processes, and are an emerging therapeutic target for major diseases. Most information about PAR activation and function derives from studies of a few proteases, for example thrombin in the case of PAR1, PAR3, and PAR4, and trypsin in the case of PAR2 and PAR4. These proteases cleave PARs at established sites with the extracellular N-terminal domains, and expose tethered ligands that stabilize conformations of the cleaved receptors that activate the canonical pathways of G protein- and/or β-arrestin-dependent signaling. However, a growing number of proteases have been identified that cleave PARs at divergent sites to activate distinct patterns of receptor signaling and trafficking. The capacity of these proteases to trigger distinct signaling pathways is referred to as biased signaling, and can lead to unique patho-physiological outcomes. Given that a different repertoire of proteases are activated in various patho-physiological conditions that may activate PARs by different mechanisms, signaling bias may account for the divergent actions of proteases and PARs. Moreover, therapies that target disease-relevant biased signaling pathways may be more effective and selective approaches for the treatment of protease- and PAR-driven diseases. Thus, rather than mediating the actions of a few proteases, PARs may integrate the biological actions of a wide spectrum of proteases in different patho-physiological conditions.

Protease-activated receptor 1 (PAR1) and PAR4 heterodimers are required for PAR1-enhanced cleavage of PAR4 by α-thrombin

Thrombin is a potent platelet agonist that activates platelets and other cells of the cardiovascular system by cleaving its G-protein-coupled receptors, protease-activated receptor 1 (PAR1), PAR4, or both. We now show that cleaving PAR1 and PAR4 with α-thrombin induces heterodimer formation. PAR1-PAR4 heterodimers were not detected when unstimulated; however, when the cells were stimulated with 10 nm α-thrombin, we were able to detect a strong interaction between PAR1 and PAR4 by bioluminescence resonance energy transfer. In contrast, activating the receptors without cleavage using PAR1 and PAR4 agonist peptides (TFLLRN and AYPGKF, respectively) did not enhance heterodimer formation. Preventing PAR1 or PAR4 cleavage with point mutations or hirugen also prevented the induction of heterodimers. To further characterize the PAR1-PAR4 interactions, we mapped the heterodimer interface by introducing point mutations in transmembrane helix 4 of PAR1 or PAR4 that prevented heterodimer formation. Finally, we show that mutations in PAR1 or PAR4 at the heterodimer interface prevented PAR1-assisted cleavage of PAR4. These data demonstrate that PAR1 and PAR4 require allosteric changes induced via receptor cleavage by α-thrombin to mediate heterodimer formation, and we have determined the PAR1-PAR4 heterodimer interface. Our findings show that PAR1 and PAR4 have dynamic interactions on the cell surface that should be taken into account when developing and characterizing PAR antagonists.

Cofactoring and dimerization of proteinase-activated receptors

Proteinase-activated receptors (PARs) are G protein-coupled receptors that transmit cellular responses to extracellular proteases and have important functions in vascular physiology, development, inflammation, and cancer progression. The established paradigm for PAR activation involves proteolytic cleavage of the extracellular N terminus, which reveals a new N terminus that functions as a tethered ligand by binding intramolecularly to the receptor to trigger transmembrane signaling. Most cells express more than one PAR, which can influence the mode of PAR activation and signaling. Clear examples include murine PAR3 cofactoring of PAR4 and transactivation of PAR2 by PAR1. Thrombin binds to and cleaves murine PAR3, which facilitates PAR4 cleavage and activation. This process is essential for thrombin signaling and platelet activation, since murine PAR3 cannot signal alone. Although PAR1 and PAR4 are both competent to signal, PAR1 is able to act as a cofactor for PAR4, facilitating more rapid cleavage and activation by thrombin. PAR1 can also facilitate PAR2 activation through a different mechanism. Cleavage of the PAR1 N terminus by thrombin generates a tethered ligand domain that can bind intermolecularly to PAR2 to activate signaling. Thus, PARs can regulate each other's activity by localizing thrombin when in complex with PAR3 and PAR4 or by cleaved PAR1, providing its tethered ligand domain for PAR2 activation. The ability of PARs to cofactor or transactivate other PARs would necessitate that the two receptors be in close proximity, likely in the form of a heterodimer. Here, we discuss the cofactoring and dimerization of PARs and the functional consequences on signaling.