Targeting a Proteinase-Activated Receptor 4 (PAR4) Carboxyl Terminal Motif to Regulate Platelet Function

PAR4 Inhibition Reduces Coronary Artery Atherosclerosis and Myocardial Fibrosis in SR-B1/LDLR Double Knockout Mice

BACKGROUND

SR-B1 (scavenger receptor class B type 1)/LDLR (low-density lipoprotein receptor) double knockout mice fed a high-fat, high-cholesterol diet containing cholate exhibit coronary artery disease characterized by occlusive coronary artery atherosclerosis, platelet accumulation in coronary arteries, and myocardial fibrosis. Platelets are involved in atherosclerosis development, and PAR (protease-activated receptor) 4 has a prominent role in platelet function in mice. However, the role of PAR4 on coronary artery disease in mice has not been tested.

METHODS

We tested the effects of a PAR4 inhibitory pepducin (RAG8) on diet-induced aortic sinus and coronary artery atherosclerosis, platelet accumulation in atherosclerotic coronary arteries, and myocardial fibrosis in SR-B1/LDLR double knockout mice. SR-B1/LDLR double knockout mice were fed a high-fat, high-cholesterol diet containing cholate and injected daily with 20 mg/kg of either the RAG8 pepducin or a control reverse-sequence pepducin (SRQ8) for 20 days.

RESULTS

Platelets from the RAG8-treated mice exhibited reduced thrombin and PAR4 agonist peptide-mediated activation compared with those from control SRQ8-treated mice when tested ex vivo. Although aortic sinus atherosclerosis levels did not differ, RAG8-treated mice exhibited reduced coronary artery atherosclerosis, reduced platelet accumulation in atherosclerotic coronary arteries, and reduced myocardial fibrosis. These protective effects were not accompanied by changes in circulating lipids, inflammatory cytokines, or immune cells. However, RAG8-treated mice exhibited reduced VCAM-1 (vascular cell adhesion molecule 1) protein levels in nonatherosclerotic coronary artery cross sections and reduced leukocyte accumulation in atherosclerotic coronary artery cross sections compared with those from SRQ8-treated mice.

CONCLUSIONS

The PAR4 inhibitory RAG8 pepducin reduced coronary artery atherosclerosis and myocardial fibrosis in SR-B1/LDLR double knockout mice fed a high-fat, high-cholesterol diet containing cholate. Furthermore, RAG8 reduced VCAM-1 in nonatherosclerotic coronary arteries and reduced leukocyte and platelet accumulation in atherosclerotic coronary arteries. These findings identify PAR4 as an attractive target in reducing coronary artery disease development, and the use of RAG8 may potentially be beneficial in cardiovascular disease.

Discovery of Ghrelin(1-8) Analogues with Improved Stability and Functional Activity for PET Imaging

The highest affinity ghrelin-based analogue for fluorine-18 positron emission tomography, [Inp¹,Dpr³(6-FN),1Nal⁴,Thr⁸]ghrelin(1-8) amide (1), has remarkable subnanomolar receptor affinity (IC₅₀ = 0.11 nM) toward the growth hormone secretagogue receptor 1a (GHSR). However, initial in vivo PET imaging and biodistribution of [¹⁸F]1 in mice demonstrated an unfavorable pharmacokinetic profile with rapid clearance and accumulation in liver and intestinal tissue, prompting concerns about the metabolic stability of this probe. The aims of the present study were to examine the proteolytic stability of ghrelin analogue 1 in the presence of blood and liver enzymes, structurally modify the peptide to improve stability without impeding the strong binding affinity, and measure the presently unknown functional activity of ghrelin(1-8) analogues. The in vitro stability and metabolite formation of 1 in human serum and liver S9 fraction revealed a metabolic soft spot between amino acids Leu⁵ and Ser⁶ in the peptide sequence. A focused library of ghrelin(1-8) analogues was synthesized and evaluated in a structure-activity-stability relationship study to further understand the structural importance of the residues at these positions in the context of stability and receptor affinity. The critical nature of l-stereochemistry at position 5 was identified and substitution of Ser⁶ with l-2,3-diaminopropionic acid led to a novel ligand with substantially improved in vitro stability while maintaining subnanomolar GHSR affinity. Despite the highly modified nature of these analogues compared to human ghrelin, ghrelin(1-8) analogues were found to recruit all G protein subtypes (Gα_{q/11/13/i1/oB}) known to associate with GHSR as well as β-arrestins with low micromolar to nanomolar potencies. The study of these analogues demonstrates the ability to balance desirable ligand properties, including affinity, stability, and potency to produce well-rounded candidate molecules for further in vivo evaluation.

Human osteoarthritis knee joint synovial fluids cleave and activate Proteinase-Activated Receptor (PAR) mediated signaling

Osteoarthritis (OA) is the most prevalent joint disorder with increasing worldwide incidence. Mechanistic insights into OA pathophysiology are evolving and there are currently no disease-modifying OA drugs. An increase in protease activity is linked to progressive degradation of the cartilage in OA. Proteases also trigger inflammation through a family of G protein-coupled receptors (GPCRs) called the Proteinase-Activated Receptors (PARs). PAR signaling can trigger pro-inflammatory responses and targeting PARs is proposed as a therapeutic approach in OA. Several enzymes can cleave the PAR N-terminus, but the endogenous protease activators of PARs in OA remain unclear. Here we characterized PAR activating enzymes in knee joint synovial fluids from OA patients and healthy donors using genetically encoded PAR biosensor expressing cells. Calcium signaling assays were performed to examine receptor activation. The class and type of enzymes cleaving the PARs was further characterized using protease inhibitors and fluorogenic substrates. We find that PAR1, PAR2 and PAR4 activating enzymes are present in knee joint synovial fluids from healthy controls and OA patients. Compared to healthy controls, PAR1 activating enzymes are elevated in OA synovial fluids while PAR4 activating enzyme levels are decreased. Using enzyme class and type selective inhibitors and fluorogenic substrates we find that multiple PAR activating enzymes are present in OA joint fluids and identify serine proteinases (thrombin and trypsin-like) and matrix metalloproteinases as the major classes of PAR activating enzymes in the OA synovial fluids. Synovial fluid driven increase in calcium signaling was significantly reduced in cells treated with PAR1 and PAR2 antagonists, but not in PAR4 antagonist treated cells. OA associated elevation of PAR1 cleavage suggests that targeting this receptor may be beneficial in the treatment of OA.

PH-Binding Motif in PAR4 Oncogene: From Molecular Mechanism to Drug Design

While the role of G-protein-coupled receptors (GPCR) in cancer is acknowledged, their underlying signaling pathways are understudied. Protease-activated receptors (PAR), a subgroup of GPCRs, form a family of four members (PAR1-4) centrally involved in epithelial malignancies. PAR4 emerges as a potent oncogene, capable of inducing tumor generation. Here, we demonstrate identification of a pleckstrin-homology (PH)-binding motif within PAR4, critical for colon cancer growth. In addition to PH-Akt/PKB association, other PH-containing signal proteins such as Gab1 and Sos1 also associate with PAR4. Point mutations are in the C-tail of PAR4 PH-binding domain; F347 L and D349A, but not E346A, abrogate these associations. Pc(4-4), a lead backbone cyclic peptide, was selected out of a mini-library, directed toward PAR2&4 PH-binding motifs. It effectively attenuates PAR2&4-Akt/PKB associations; PAR4 instigated Matrigel invasion and migration in vitro and tumor development in vivo. EGFR/erbB is among the most prominent cancer targets. AYPGKF peptide ligand activation of PAR4 induces EGF receptor (EGFR) Tyr-phosphorylation, effectively inhibited by Pc(4-4). The presence of PAR2 and PAR4 in biopsies of aggressive breast and colon cancer tissue specimens is demonstrated. We propose that Pc(4-4) may serve as a powerful drug not only toward PAR-expressing tumors but also for treating EGFR/erbB-expressing tumors in cases of resistance to traditional therapies. Overall, our studies are expected to allocate new targets for cancer therapy. Pc(4-4) may become a promising candidate for future therapeutic cancer treatment.

Human and mouse PAR4 are functionally distinct receptors: Studies in novel humanized mice

BACKGROUND

Human and mouse platelets both express protease-activated receptor (PAR) 4 but sequence alignment reveals differences in several functional domains. These differences may result in functional disparities between the receptors which make it difficult to translate PAR4 studies using mice to human platelet physiology.

OBJECTIVES

To generate transgenic mice that express human, but not mouse, PAR4 and directly compare human and mouse PAR4 function in the same platelet environment.

METHODS

Transgenic mice were made using a genomic clone of the F2RL3 gene (encoding PAR4) and backcrossed with Par4 KO mice. For certain experiments, mice were bred with GRK6 KO mice. Tail bleeding time and platelet function in response to PAR4-activating peptide were assessed.

RESULTS

Human F2RL3 was successfully integrated into the mouse genome, transgenic mice were crossed to the mPar4 KO background (PAR4 tg/KO), and PAR4 was functionally expressed on platelets. Compared to WT, PAR4 tg/KO mice exhibited shortened tail bleeding time and their platelets were more responsive to PAR4-AP as assessed by α-granule release and integrin activation. The opposite was observed with thrombin. Knocking out GRK6 had no effect on human PAR4-expressing platelets, unlike mouse Par4-expressing platelets. PAR4 tg/KO platelets exhibited greater Ca2+ area under the curve and more robust extracellular vesicle release than WT stimulated with PAR4-AP.

CONCLUSION

These data suggest that (1) human PAR4- and mouse Par4-mediated signaling are different and (2) the feedback regulation mechanisms of human and mouse PAR4 are different. These functional differences are important to consider when interpreting PAR4 studies done with mice.

Inhibitors of protease activated receptor 4 (PAR4): a review of recent patents (2013-2021)

INTRODUCTION

Protease-activated receptor 4 (PAR4), belonging to a subfamily of G-protein-coupled receptors (GPCR), is expressed on the surface of Human platelets, and the activation of it can lead to platelets aggregation. Studies demonstrated that PAR4 inhibition protect mice from arterial/arteriolar thrombosis, pulmonary embolism and cerebral infarct, while do not affect the haemostatic responses integrity. Therefore, PAR4 has been a promising target for the development of anti-thrombotic agents.

AREAS COVERED

This review covers recent patents and literature on PAR4 and their application published between 2013 and 2021.

EXPERT OPINION

PAR4 is a promising anti-thrombotic target and PAR4 inhibitors are important biologically active compounds for the treatment of thrombosis. Most the recent patents and literature focus on PAR4 selective inhibitors, and BMS-986120 and BMS-986141, which were developed by BMS, have entered clinical trials. With the deep understanding of the crystal structures and biological functions of PAR4, we believe that many other novel types of molecules targeting PAR4 would enter the clinical studies or the market.

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.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Novel antiplatelet targets in the treatment of acute coronary syndromes

Acute coronary syndromes (ACS) are a global cause of mortality and morbidity that affect millions of lives worldwide. Following atherosclerotic plaque rupture, platelet activation and aggregation are the two major elements that initiate thrombus formation inside a coronary artery, which can obstruct blood flow and cause myocardial ischemia; ergo, antiplatelet therapy forms a major part of the treatment strategy for ACS. Patients with ACS routinely receive dual antiplatelet therapy (DAPT), which consists of aspirin and a platelet P2Y₁₂ inhibitor to both treat and prevent atherothrombosis. Use of platelet glycoprotein (GP) IIb/IIIa inhibitors is now limited due to the risk of severe bleeding and thrombocytopenia. Thus, administration of GPIIb/IIIa inhibitors is generally restricted to bail out thrombotic events associated with PCI. Furthermore, current antiplatelet medications mainly rely on thromboxane A₂ and P2Y₁₂ inhibition, which have broad-acting effects on platelets and are known to cause bleeding, which especially limits the long-term use of these agents. In addition, not all ACS patients treated with current antiplatelet treatments are protected from recurrence of arterial thrombosis, since many platelet mechanisms and activation pathways remain uninhibited by current antiplatelet therapy. Pharmacological antagonism of novel targets involved in platelet function could shape future antiplatelet therapies that could ultimately lead to more effective or safer therapeutic approaches. In this article, we focus on inhibitors of promising targets that have not yet been introduced into clinical practice, including inhibitors of GPVI, protease-activated receptor (PAR)-4, GPIb, 5-hydroxytryptamine receptor subtype 2A (5-HT_2A), protein disulfide isomerase, P-selectin and phosphoinositide 3-kinase β.

Proteinase-Activated Receptor 4 Activation Triggers Cell Membrane Blebbing through RhoA and β-Arrestin

Proteinase-activated receptors (PARs) are a four-member family of G-protein-coupled receptors that are activated via proteolysis. PAR4 is a member of this family that is cleaved and activated by serine proteinases such as thrombin, trypsin, and cathepsin-G. PAR4 is expressed in a variety of tissues and cell types, including platelets, vascular smooth muscle cells, and neuronal cells. In studying PAR4 signaling and trafficking, we observed dynamic changes in the cell membrane, with spherical membrane protrusions that resemble plasma membrane blebbing. Since nonapoptotic membrane blebbing is now recognized as an important regulator of cell migration, cancer cell invasion, and vesicular content release, we sought to elucidate the signaling pathway downstream of PAR4 activation that leads to such events. Using a combination of pharmacological inhibition and CRISPR/CRISPR-associated protein 9 (Cas9)-mediated gene editing approaches, we establish that PAR4-dependent membrane blebbing occurs independently of the Gα _q/11- and Gα _i-signaling pathways and is dependent on signaling via the β-arrestin-1/2 and Ras homolog family member A (RhoA) signaling pathways. Together these studies provide further mechanistic insight into PAR4 regulation of cellular function. SIGNIFICANCE STATEMENT: We find that the thrombin receptor PAR4 triggers cell membrane blebbing in a RhoA-and β-arrestin-dependent manner. In addition to identifying novel cellular responses mediated by PAR4, these data provide further evidence for biased signaling in PAR4 since membrane blebbing was dependent on some, but not all, signaling pathways activated by PAR4.

Molecular basis for activation and biased signaling at the thrombin-activated GPCR proteinase activated receptor-4 (PAR4)

Proteinase-activated receptor (PAR)-4 is a member of the proteolytically-activated PAR family of G-protein-coupled receptors (GPCR) that represents an important target in the development of anti-platelet therapeutics. PARs are activated by proteolytic cleavage of their receptor N terminus by enzymes such as thrombin, trypsin, and cathepsin-G. This reveals the receptor-activating motif, termed the tethered ligand that binds intramolecularly to the receptor and triggers signaling. However, PARs are also activated by exogenous application of synthetic peptides derived from the tethered-ligand sequence. To better understand the molecular basis for PAR4-dependent signaling, we examined PAR4-signaling responses to a peptide library derived from the canonical PAR4-agonist peptide, AYPGKF-NH2, and we monitored activation of the Gαq/11-coupled calcium-signaling pathway, β-arrestin recruitment, and mitogen-activated protein kinase (MAPK) pathway activation. We identified peptides that are poor activators of PAR4-dependent calcium signaling but were fully competent in recruiting β-arrestin-1 and -2. Peptides that were unable to stimulate PAR4-dependent calcium signaling could not trigger MAPK activation. Using in silico docking and site-directed mutagenesis, we identified Asp230 in the extracellular loop-2 as being critical for PAR4 activation by both agonist peptide and the tethered ligand. Probing the consequence of biased signaling on platelet activation, we found that a peptide that cannot activate calcium signaling fails to cause platelet aggregation, whereas a peptide that is able to stimulate calcium signaling and is more potent for β-arrestin recruitment triggered greater levels of platelet aggregation compared with the canonical PAR4 agonist peptide. These findings uncover molecular determinants critical for agonist binding and biased signaling through PAR4.

Single Amino Acid Replacement in G-7039 Leads to a 70-fold Increase in Binding toward GHS-R1a

The growth hormone secretagogue receptor type 1a (GHS-R1a) is a class A rhodopsin-like G protein coupled receptor (GPCR) that is expressed in a variety of human tissues and is differentially expressed in benign and malignant prostate cancer. Previously, the peptidomimetic [1-Nal⁴ ,Lys⁵ (4-fluorobenzoyl)]G-7039 was designed as a molecular imaging tool for positron emission tomography (PET). However, this candidate was a poor binder (IC₅₀ =69 nm), required a lengthy four-step radiosynthesis, and had a cLogP above 8. To address these challenges, we now report on changes targeted at the 4^th position of G-7039. A 2-fluoropropionic acid (2-FPA) group was added on to Lys⁵ to determine the potential binding affinity of the [¹⁸ F]-2-FP radiolabeled analogue, which could be prepared by simplified radiochemistry. Lead candidate [Tyr⁴ ,Lys⁵ (2-fluoropropionyl)]G-7039 exhibited an IC₅₀ of 0.28 nm and low picomolar activity toward GHS-R1a. Molecular docking revealed a molecular basis for this picomolar affinity.

Acetylcholine Inhibits Platelet Activation

Platelets are key mediators of thrombosis. Many agonists of platelet activation are known, but fewer endogenous inhibitors of platelets, such as prostacyclin and nitric oxide (NO), have been identified. Acetylcholinesterase inhibitors, such as donepezil, can cause bleeding in patients, but the underlying mechanisms are not well understood. We hypothesized that acetylcholine is an endogenous inhibitor of platelets. We measured the effect of acetylcholine or analogs of acetylcholine on human platelet activation ex vivo. Acetylcholine and analogs of acetylcholine inhibited platelet activation, as measured by P-selectin translocation and glycoprotein IIb IIIa conformational changes. Conversely, we found that antagonists of the acetylcholine receptor, such as pancuronium, enhance platelet activation. Furthermore, drugs inhibiting acetylcholinesterase, such as donepezil, also inhibit platelet activation, suggesting that platelets release acetylcholine. We found that NO mediates acetylcholine inhibition of platelets. Our data suggest that acetylcholine is an endogenous inhibitor of platelet activation. The cholinergic system may be a novel target for antithrombotic therapies.

Protease-Activated Receptor 4 (PAR4): A Promising Target for Antiplatelet Therapy

Cardiovascular diseases (CVDs) are currently among the leading causes of death worldwide. Platelet aggregation is a key cellular component of arterial thrombi and major cause of CVDs. Protease-activated receptors (PARs), including PAR1, PAR2, PAR3 and PAR4, fall within a subfamily of seven-transmembrane G-protein-coupled receptors (GPCR). Human platelets express PAR1 and PAR4, which contribute to the signaling transduction processes. In association with CVDs, PAR4 not only contributes to platelet activation but also is a modulator of cellular responses that serve as hallmarks of inflammation. Although several antiplatelet drugs are available on the market, they have many side effects that limit their use. Emerging evidence shows that PAR4 targeting is a safer strategy for preventing thrombosis and consequently may improve the overall cardiac safety profile. Our present review summarizes the PAR4 structural characteristics, activation mechanism, role in the pathophysiology of diseases and understanding the association of PAR4 targeting for improved cardiac protection. Conclusively, this review highlights the importance of PAR4 antagonists and its potential utility in different CVDs.