Antibodies to Protease-Activated Receptor 3 Inhibit Activation of Mouse Platelets by Thrombin

CLIC4 Regulates Endothelial Barrier Control by Mediating PAR1 Signaling via RhoA

BACKGROUND

Endothelial CLICs (chloride intracellular channel proteins) CLIC1 and CLIC4 are required for the GPCRs (G-protein-coupled receptors) S1PR1 (sphingosine-1-phosphate receptor 1) and S1PR3 to activate the small GTPases Rac1 (Ras-related C3 botulinum toxin substrate 1) and RhoA (Ras homolog family member A). To determine whether CLIC1 and CLIC4 function in additional endothelial GPCR pathways, we evaluated CLIC function in thrombin signaling via the thrombin-regulated PAR1 (protease-activated receptor 1) and downstream effector RhoA.

METHODS

We assessed the ability of CLIC1 and CLIC4 to relocalize to cell membranes in response to thrombin in human umbilical vein endothelial cells (HUVEC). We examined CLIC1 and CLIC4 function in HUVEC by knocking down expression of each CLIC protein and compared thrombin-mediated RhoA or Rac1 activation, ERM (ezrin/radixin/moesin) phosphorylation, and endothelial barrier modulation in control and CLIC knockdown HUVEC. We generated a conditional murine allele of Clic4 and examined PAR1-mediated lung microvascular permeability and retinal angiogenesis in mice with endothelial-specific loss of Clic4.

RESULTS

Thrombin promoted relocalization of CLIC4, but not CLIC1, to HUVEC membranes. Knockdown of CLIC4 in HUVEC reduced thrombin-mediated RhoA activation, ERM phosphorylation, and endothelial barrier disruption. Knockdown of CLIC1 did not reduce thrombin-mediated RhoA activity but prolonged the RhoA and endothelial barrier response to thrombin. Endothelial-specific deletion of Clic4 in mice reduced lung edema and microvascular permeability induced by PAR1 activating peptide.

CONCLUSIONS

CLIC4 is a critical effector of endothelial PAR1 signaling and is required to regulate RhoA-mediated endothelial barrier disruption in cultured endothelial cells and murine lung endothelium. CLIC1 was not critical for thrombin-mediated barrier disruption but contributed to the barrier recovery phase after thrombin treatment.

Platelets and Their Role in Hemostasis and Thrombosis-From Physiology to Pathophysiology and Therapeutic Implications

Hemostasis is a physiological process critical for survival. Meanwhile, thrombosis is amongst the leading causes of death worldwide, making antithrombotic therapy one of the most crucial aspects of modern medicine. Although antithrombotic therapy has progressed tremendously over the years, it remains far from ideal, and this is mainly due to the incomplete understanding of the exceptionally complex structural and functional properties of platelets. However, advances in biochemistry, molecular biology, and the advent of 'omics' continue to provide crucial information for our understanding of the complex structure and function of platelets, their interactions with the coagulation system, and their role in hemostasis and thrombosis. In this review, we provide a comprehensive view of the complex role that platelets play in hemostasis and thrombosis, and we discuss the major clinical implications of these fundamental blood components, with a focus on hemostatic platelet-related disorders and existing and emerging antithrombotic therapies. We also emphasize a number of questions that remain to be answered, and we identify hotspots for future research.

Biolabel-led research pattern reveals serum profile in rats after treatment with Herba Lysimachiae: Combined analysis of metabonomics and proteomics

In traditional Chinese medicine, Herba Lysimachiae (HL) is mainly used to treat rheumatic arthralgia. Current pharmacological studies also showed that HL has therapeutic potential for synovial diseases. HL is an oral drug, whose compounds need to enter the blood circulation before reaching the injured tissue, thus potentially causing activity or toxicity to the blood system. In this study, the biolabel-led research pattern was used to analyze the serum profile after HL intervention, based on which the safety and efficacy of HL were explored. Metabonomics and proteomics were combined to analyze the biolabels responsible for the interventions of HL on serum. Bioinformatics databases were used to screen for the material basis that may interfere with biolabels. Omics analysis showed that differentially expressed 19 proteins and 5 metabolites were identified and considered as the potential biolabels, which were involved in 8 biochemical processes (platelet activation and aggregation, blood glucose release, immune and inflammatory regulation, oxidative stress, endoplasmic reticulum stress, tumor progression, blood pressure regulation, and uric acid synthesis). Thirty-one compounds may be the material basis to interfere with eleven biolabels. The present research reveals that the potential activities and toxicities of HL can be explored based on the biolabel-led research pattern.

The role of protease-activated receptor 1 signaling in CD8 T cell effector functions

CD8 T cells are essential for adaptive immunity against viral infections. Protease activated receptor 1 (PAR1) is expressed by CD8 T cells; however, its role in T cell effector function is not well defined. Here we show that in human CD8 T cells, PAR1 stimulation accelerates calcium mobilization. Furthermore, PAR1 is involved in cytotoxic T cell function by facilitating granule trafficking via actin polymerization and repositioning of the microtubule organizing center (MTOC) toward the immunological synapse. In vivo, PAR1^−/− mice have reduced cytokine-producing T cells in response to a lymphocytic choriomeningitis virus (LCMV) infection and fail to efficiently control the virus. Specific deletion of PAR1 in LCMV GP33-specific CD8 T cells results in reduced expansion and diminished effector function. These data demonstrate that PAR1 plays a role in T cell activation and function, and this pathway could represent a new therapeutic strategy to modulate CD8 T cell effector function.

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PAR1 signaling in human CD8 T cells accelerates TCR-induced calcium mobilization

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PAR1 participates in the repositioning of the MTOC at the immunological synapse

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PAR1 facilitates polarized secretion of cytotoxic granules at the immunological synapse

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PAR1^−/− Gp33-specific CD8 T cells show reduced expansion and effector function

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.

Diallyl Trisulfide Inhibits Platelet Aggregation through the Modification of Sulfhydryl Groups

Diallyl trisulfide (DATS) is a secondary metabolite of allicin, a volatile organosulfur flavoring compound generated by the crushing of garlic. These compounds have various medicinal effects such as antiplatelet activity. In this study, we demonstrated for the first time the cellular mechanism involved in the inhibition of platelet aggregation by DATS and dipropyl trisulfide (DPTS), which is a saturated analogue of DATS. Washed murine platelets were incubated with these sulfides, and platelet aggregation was evaluated by light transmission aggregometry. The amount of reaction products produced by DATS, DPTS, and glutathione (GSH) was measured using liquid chromatography-mass spectrometry. Compared with DPTS, DATS potently inhibited platelet aggregation induced by thrombin, U46619, and collagen. N-Ethylmaleimide (NEM), which is commonly used to modify sulfhydryl groups, also suppressed platelet aggregation. The reactivity of DATS with GSH was higher than that of DPTS. These data suggested that DATS inhibited platelet aggregation through the reaction of sulfhydryl groups.

Pleiotropic effects of the hemostatic system

Atherothrombosis is characterized by the inflammatory process of atherosclerosis combined with a hypercoagulable state leading to superimposed thrombus formation. In atherosclerotic plaques, cell signaling can occur via protease-activated receptors (PARs), four of which have been identified so far (PAR1-PAR4). Proteases that are able to activate PARs can be produced systemically, but also at the sites of lesions, and they include thrombin and activated factor X. After PAR activation, downstream signaling can lead to both proinflammatory effects and a hypercoagulable state. Which specific effect occurs depends on the type of protease and activated PAR, and the site of activation. Hypercoagulable effects are mainly exerted through PAR1 and PAR4, whereas proinflammatory responses are mostly seen after PAR1 and PAR2 activation. PAR signaling pathways contribute to atherothrombosis, suggesting that inhibition of these pathways possibly prevents cardiovascular events based on this pathophysiological mechanism. In this review, we highlight the pathways by which PAR activation leads to proinflammatory responses and a hypercoagulable state. Furthermore, we give an overview of potential pharmacological treatment targets that promote vascular protection.

Protease-activated receptors in kidney disease progression

Protease-activated receptors (PARs) are members of a well-known family of transmembrane G protein-coupled receptors (GPCRs). Four PARs have been identified to date, of which PAR1 and PAR2 are the most abundant receptors, and have been shown to be expressed in the kidney vascular and tubular cells. PAR signaling is mediated by an N-terminus tethered ligand that can be unmasked by serine protease cleavage. The receptors are activated by endogenous serine proteases, such as thrombin (acts on PARs 1, 3, and 4) and trypsin (PAR2). PARs can be involved in glomerular, microvascular, and inflammatory regulation of renal function in both normal and pathological conditions. As an example, it was shown that human glomerular epithelial and mesangial cells express PARs, and these receptors are involved in the pathogenesis of crescentic glomerulonephritis, glomerular fibrin deposition, and macrophage infiltration. Activation of these receptors in the kidney also modulates renal hemodynamics and glomerular filtration rate. Clinical studies further demonstrated that the concentration of urinary thrombin is associated with glomerulonephritis and type 2 diabetic nephropathy; thus, molecular and functional mechanisms of PARs activation can be directly involved in renal disease progression. We briefly discuss here the recent literature related to activation of PAR signaling in glomeruli and the kidney in general and provide some examples of PAR1 signaling in glomeruli podocytes.

Systems biology insights into the meaning of the platelet's dual-receptor thrombin signaling

Essentials Roles of the two thrombin receptors in platelet signaling are poorly understood. Computational systems biology modeling was used together with continuous flow cytometry. Dual-receptor system has wide-range sensitivity to thrombin and optimal response dynamics. Procoagulant platelet formation is determined by donor-specific activities of the two receptors.

SUMMARY

Background Activation of human platelets with thrombin proceeds via two protease-activated receptors (PARs), PAR1 and PAR4, that have identical main intracellular signaling responses. Although there is evidence that they have different cleavage/inactivation kinetics (and some secondary variations in signaling), the reason for such redundancy is not clear. Methods We developed a multicompartmental stochastic computational systems biology model of dual-receptor thrombin signaling in platelets to gain insight into the mechanisms and roles of PAR1 and PAR4 functioning. Experiments employing continuous flow cytometry of washed human platelets were used to validate the model and test its predictions. Activity of PAR receptors in donors was evaluated by mRNA measurement and by polymorphism sequencing. Results Although PAR1 activation produced rapid and short-lived response, signaling via PAR4 developed slowly and propagated in time. Response of the dual-receptor system was both rapid and prolonged in time. Inclusion of PAR1/PAR4 heterodimer formation promoted PAR4 signaling in the medium range of thrombin concentration (about 10 nm), with little contribution at high and low thrombin. Different dynamics and dose-dependence of procoagulant platelet formation in healthy donors was associated with individual variations in PAR1 and PAR4 activities and particularly by the Ala120Thr polymorphism in the F2RL3 gene encoding PAR4. Conclusions The dual-receptor combination is critical to produce a response combining three critical advantages: sensitivity to thrombin concentration, rapid onset and steady propagation; specific features of the protease-activated receptors do not allow combination of all three in a single receptor.

Cathepsin K induces platelet dysfunction and affects cell signaling in breast cancer - molecularly distinct behavior of cathepsin K in breast cancer

Background

Breast cancer comprises clinically and molecularly distinct tumor subgroups that differ in cell histology and biology and show divergent clinical phenotypes that impede phase III trials, such as those utilizing cathepsin K inhibitors. Here we correlate the epithelial-mesenchymal-like transition breast cancer cells and cathepsin K secretion with activation and aggregation of platelets. Cathepsin K is up-regulated in cancer cells that proteolyze extracellular matrix and contributes to invasiveness. Although proteolytically activated receptors (PARs) are activated by proteases, the direct interaction of cysteine cathepsins with PARs is poorly understood. In human platelets, PAR-1 and −4 are highly expressed, but PAR-3 shows low expression and unclear functions.

Methods

Platelet aggregation was monitored by measuring changes in turbidity. Platelets were immunoblotted with anti-phospho and total p38, Src-Tyr-416, FAK-Tyr-397, and TGFβ monoclonal antibody. Activation was measured in a flow cytometer and calcium mobilization in a confocal microscope. Mammary epithelial cells were prepared from the primary breast cancer samples of 15 women with Luminal-B subtype to produce primary cells.

Results

We demonstrate that platelets are aggregated by cathepsin K in a dose-dependent manner, but not by other cysteine cathepsins. PARs-3 and −4 were confirmed as the cathepsin K target by immunodetection and specific antagonists using a fibroblast cell line derived from PARs deficient mice. Moreover, through co-culture experiments, we show that platelets activated by cathepsin K mediated the up-regulation of SHH, PTHrP, OPN, and TGFβ in epithelial-mesenchymal-like cells from patients with Luminal B breast cancer.

Conclusions

Cathepsin K induces platelet dysfunction and affects signaling in breast cancer cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2203-7) contains supplementary material, which is available to authorized users.

Platelets and Their Interactions with Other Immune Cells

Platelets are anucleate blood cells, long known to be critically involved in hemostasis and thrombosis. In addition to their role in blood clots, increasing evidence reveals significant roles for platelets in inflammation and immunity. However, the notion that platelets represent immune cells is not broadly recognized in the field of Physiology. This article reviews the role of platelets in inflammation and immune responses, and highlights their interactions with other immune cells, including examples of major functional consequences of these interactions.

Thrombin-induced platelet activation via PAR4: pivotal role for exosite II

Thrombin-induced platelet activation via PAR1 and PAR4 is an important event in haemostasis. Although the underlying mechanisms responsible for ensuring efficient PAR1 activation by thrombin have been extensively studied, the potential involvement of recognitions sites outside the active site of the protease in thrombin-induced PAR4 activation is largely unknown. In this study, we developed a new assay to assess the importance of exosite I and II for PAR4 activation with α - and γ-thrombin. Surprisingly, we found that exosite II is critical for activation of PAR4. We also show that this dependency on exosite II likely represents a new mechanism, as it is unaffected by blockage of the previously known interaction between thrombin and glycoprotein Ibα.

Differential signaling by protease-activated receptors: implications for therapeutic targeting

Protease-activated receptors (PARs) are a family of four G protein-coupled receptors that exhibit increasingly appreciated differences in signaling and regulation both within and between the receptor class. By nature of their proteolytic self-activation mechanism, PARs have unique processes of receptor activation, "ligand" binding, and desensitization/resensitization. These distinctive aspects have presented both challenges and opportunities in the targeting of PARs for therapeutic benefit-the most notable example of which is inhibition of PAR1 on platelets for the prevention of arterial thrombosis. However, more recent studies have uncovered further distinguishing features of PAR-mediated signaling, revealing mechanisms by which identical proteases elicit distinct effects in the same cell, as well as how distinct proteases produce different cellular consequences via the same receptor. Here we review this differential signaling by PARs, highlight how important distinctions between PAR1 and PAR4 are impacting on the progress of a new class of anti-thrombotic drugs, and discuss how these more recent insights into PAR signaling may present further opportunities for manipulating PAR activation and signaling in the development of novel therapies.

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.

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

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

Thrombin activates platelets through protease activated receptors (PARs). Mouse platelets express PAR3 and PAR4. PAR3 does not signal in platelets. However, PAR4 is a relatively poor thrombin substrate and requires PAR3 as a cofactor at low thrombin concentrations. In this study we show that PAR3 also regulates PAR4 signaling. In response to thrombin (30–100 nM) or PAR4 activating peptide (AYPGKF), platelets from PAR3^−/− mice had increased G_q signaling compared to wild type mice as demonstrated by a 1.6-fold increase in the maximum intracellular calcium (Ca²⁺) mobilization, an increase in phosphorylation level of protein kinase C (PKC) substrates, and a 2-fold increase of Ca²⁺ release from intracellular stores. Moreover, platelets from heterozygous mice (PAR3^+/−) had an intermediate increase in maximum Ca²⁺ mobilization. Treatment of PAR3^−/− mice platelets with P2Y₁₂ antagonist (2MeSAMP) did not affect Ca²⁺ mobilization from PAR4 in response to thrombin or AYPGKF. The activation of RhoA-GTP downstream G_12/13 signaling in response to thrombin was not significantly different between wild type and PAR3^−/− mice. Since PAR3 influenced PAR4 signaling independent of agonist, we examined the direct interaction between PAR3 and PAR4 with bioluminescence resonance energy transfer (BRET). PAR3 and PAR4 form constitutive homodimers and heterodimers. In summary, our results demonstrate that in addition to enhancing PAR4 activation at low thrombin concentrations, PAR3 negatively regulates PAR4-mediated maximum Ca²⁺ mobilization and PKC activation in mouse platelets by physical interaction.

PAR-1-dependent and PAR-independent pro-inflammatory signaling in human lung fibroblasts exposed to thrombin

Proteinase-activated receptors (PARs) are crucial in orchestrating cellular responses to coagulation proteinases, such as thrombin and FXa. Four PARs have been characterized and have been shown to be differentially expressed in mice and humans and between tissues. We have previously shown that in murine lung fibroblasts, PAR-1 is solely responsible for all cellular responses to thrombin and FXa. In contrast, we report here that in primary human lung fibroblasts (pHLFs), known PARs fail to account for all of the cellular responses to thrombin, in particular in the presence of high, but physiologically achievable concentrations of thrombin. We report that pHLFs secrete CCL2 in a PAR-1-dependent manner at low thrombin concentration (∼0.3 nM). At or above 10 nM thrombin, pharmacological antagonism (RWJ-58259) fails to block thrombin-induced CCL2 release; whereas PAR-1 cleavage-blocking monoclonal antibodies (ATAP2 and WEDE15) only partially inhibit thrombin-induced CCL2 secretion. In addition, activation of PAR-3, PAR-4, and transactivation of either PAR-2 or EGFR were ruled out as being responsible for thrombin-mediated CCL2 secretion at high yet standard concentrations of the proteinase. We further provide evidence that PAR-1-dependent and PAR-independent signaling involves the rapid phosphorylation of ERK, which in turn is absolutely required for thrombin-induced CCL2 secretion at both low and standard concentration of the proteinase. Our findings suggest the existence of a PAR-independent signaling mechanism in human lung fibroblasts and have important implications for the design of therapeutic strategies aimed at blocking pro-inflammatory signaling responses associated with excessive thrombin generation.

Next-generation antithrombotics in ischemic stroke: preclinical perspective on 'bleeding-free antithrombosis'

The present antithrombotic drugs used to treat or prevent ischemic stroke have significant limitations: either they show only moderate efficacy (platelet inhibitors), or they significantly increase the risk for hemorrhages (thrombolytics, anticoagulants). Although most strokes are caused by thrombotic or embolic vessel occlusions, the pathophysiological role of platelets and coagulation is largely unclear. The introduction of novel transgenic mouse models and specific coagulation inhibitors facilitated a detailed analysis of molecular pathways mediating thrombus formation in models of acute ischemic stroke. Prevention of early platelet adhesion to the damaged vessel wall by blocking platelet surface receptors glycoprotein Ib alpha (GPIbα) or glycoprotein VI (GPVI) protects from stroke without provoking bleeding complications. In addition, downstream signaling of GPIbα and GPVI has a key role in platelet calcium homeostasis and activation. Finally, the intrinsic coagulation cascade, activated by coagulation factor XII (FXII), has only recently been identified as another important mediator of thrombosis in cerebrovascular disease, thereby disproving established concepts. This review summarizes the latest insights into the pathophysiology of thrombus formation in the ischemic brain. Potential clinical merits of novel platelet inhibitors and anticoagulants as powerful and safe tools to combat ischemic stroke are discussed.

Pluripotent stem cells reveal the developmental biology of human megakaryocytes and provide a source of platelets for clinical application

Human pluripotent stem cells [PSCs; including human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs)] can infinitely proliferate in vitro and are easily accessible for gene manipulation. Megakaryocytes (MKs) and platelets can be created from human ESCs and iPSCs in vitro and represent a potential source of blood cells for transfusion and a promising tool for studying the human thrombopoiesis. Moreover, disease-specific iPSCs are a powerful tool for elucidating the pathogenesis of hematological diseases and for drug screening. In that context, we and other groups have developed in vitro MK and platelet differentiation systems from human pluripotent stem cells (PSCs). Combining this co-culture system with a drug-inducible gene expression system enabled us to clarify the novel role played by c-MYC during human thrombopoiesis. In the next decade, technical advances (e.g., high-throughput genomic sequencing) will likely enable the identification of numerous gene mutations associated with abnormal thrombopoiesis. Combined with such technology, an in vitro system for differentiating human PSCs into MKs and platelets could provide a novel platform for studying human gene function associated with thrombopoiesis.

Genome-wide RNA-seq analysis of human and mouse platelet transcriptomes

Inbred mice are a useful tool for studying the in vivo functions of platelets. Nonetheless, the mRNA signature of mouse platelets is not known. Here, we use paired-end next-generation RNA sequencing (RNA-seq) to characterize the polyadenylated transcriptomes of human and mouse platelets. We report that RNA-seq provides unprecedented resolution of mRNAs that are expressed across the entire human and mouse genomes. Transcript expression and abundance are often conserved between the 2 species. Several mRNAs, however, are differentially expressed in human and mouse platelets. Moreover, previously described functional disparities between mouse and human platelets are reflected in differences at the transcript level, including protease activated receptor-1, protease activated receptor-3, platelet activating factor receptor, and factor V. This suggests that RNA-seq is a useful tool for predicting differences in platelet function between mice and humans. Our next-generation sequencing analysis provides new insights into the human and murine platelet transcriptomes. The sequencing dataset will be useful in the design of mouse models of hemostasis and a catalyst for discovery of new functions of platelets. Access to the dataset is found in the "Introduction."

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Thrombin enhances the barrier function of rat microvascular endothelium in a PAR-1-dependent manner

Thrombin is a multifunctional coagulation protease with pro- and anti-inflammatory vascular effects. We questioned whether thrombin may have segmentally differentiated effects on pulmonary endothelium. In cultured rat endothelial cells, rat thrombin (10 U/ml) recapitulated the previously reported decrease in transmonolayer electrical resistance (TER), F-actin stress fiber formation, paracellular gap formation, and increased permeability. In contrast, in rat pulmonary microvascular endothelial cells (PMVEC), isolated on the basis of Griffonia simplicifolia lectin recognition, thrombin increased TER, induced fewer stress fibers, and decreased permeability. To assess for differential proteinase-activated receptor (PAR) expression as a basis for the different responses, PAR family expression was analyzed. Both pulmonary artery endothelial cells and PMVEC expressed PAR-1 and PAR-2; however, only PMVEC expressed PAR-3, as shown by both RT-PCR and Western analysis. PAR-1 activating peptides (PAR-APs: SFLLRN-NH(2) and TFLLRN-NH(2)) were used to confirm a role for the PAR-1 receptor. PAR-APs (25-250 muM) also increased TER, formed fewer stress fibers, and did not induce paracellular gaps in PMVEC in contrast to that shown in pulmonary artery endothelial cells. These results were confirmed in isolated perfused rat lung preparations. PAR-APs (100 mug/ml) induced a 60% increase in the filtration coefficient over baseline. However, by transmission electron microscopy, perivascular fluid cuffs were seen only along conduit veins and arteries without evidence of intra-alveolar edema. We conclude that thrombin exerts a segmentally differentiated effect on endothelial barrier function in vitro, which corresponds to a pattern of predominant perivascular fluid cuff formation in situ. This may indicate a distinct role for thrombin in the microcirculation.

Morphological and functional differences in haemostatic axis between kidney transplanted and end-stage renal disease patients

End-stage renal disease (ESRD) is characterized by several atherothrombotic abnormalities, and kidney transplant seems to improve most of them. However, because it is not clear which mechanism is responsible for such improvement, our purpose was to clarify that point.We conducted a cross sectional study involving 30 ESRD patients, 30 ESRD kidney-transplanted patients (Ktx) and 30 healthy controls (C) to evaluate platelet morphology and function, atherothrombotic profile, endothelial abnormalities and cytokine levels involved in the insulin resistance/endothelial dysfunction. (i) Platelet morphology: The ESRD group showed platelet size similar to the other two groups (ESRD=3518x10(3)+/-549x10(3) nm2, C=3075x10(3)+/-197x10(3) nm2, Ktx=2862x10(3)+/-205x10(3) nm2) with similar platelet granules and number. (ii) Platelet surface glycoprotein: The CD41 and P-Selectin were similar between groups. (iii) Platelet intracellular calcium: Resting intracellular calcium was statistically higher in ESRD compared to the C group (ESRD=182.1+/-34.5, Ktx=126.7+/-14.1, C=72.0+/-11.0 nM, P<0.01). (iv) Hypercoagulability markers and natural anticoagulants: The Ktx and ESRD groups showed higher levels of hypercoagulability markers compared to the C group. A reduction in antithrombin activity was evident in ESRD compared to the Ktx group (P=0.03). (v) Endothelial morphology: The ESRD group showed a thickened vessel basal membrane compared to the Ktx and C groups with more endothelial sufference. (vi) Insulin resistance and pro-inflammatory cytokine profile: The ESRD showed a higher homeostasis model assessment provided equations for estimating insulin resistance (HOMA-IR) compared to the Ktx and C groups (ESRD=2.6+/-0.3, Ktx=1.8+/-0.2, C=1.1+/-0.1, P=0.005) and increased soluble tumor neurosis factor alpha (sTNFalpha) (P<0.05) and soluble vascular cell adhesion molecule (sVCAM) levels (P<0.01). Positive correlations were evident among HOMA-IR and sTNFalpha (P<0.001) and sVCAM (P=0.01), respectively. In a small subgroup of ESRD who underwent Ktx (five pts), our findings were confirmed at 1 year of follow-up, suggesting an improvement of almost haemostatic abnormalities. Kidney transplant is associated with a better atherothrombotic profile in ESRD, platelet intracellular calcium and cytokines seem to be most influenced by the transplant, while most morphological abnormalities are retained.

Platelets in atherothrombosis: lessons from mouse models

Platelets play a central role in hemostasis and thrombosis but also in the initiation of atherosclerosis, making platelet receptors and their intracellular signaling pathways important molecular targets for antithrombotic and anti-inflammatory therapy. Historically, much of the knowledge about hemostasis and thrombosis has been derived from patients suffering from bleeding and thrombotic disorders and the identification of the underlying molecular defects. In recent years, the availability of genetically modified mouse strains with defined defects in platelet function and the development of in vivo models to assess platelet-related physiologic and pathophysiologic processes have opened new ways to identify the individual roles and the interplay of platelet proteins in adhesion, activation, aggregation, secretion, and procoagulant activity in vitro and in vivo. This review will summarize key findings made by these approaches and discuss them in the context of human disease.

Aprotinin Inhibits Protease-Dependent Platelet Aggregation and Thrombosis

BACKGROUND

Hemostatic effects of the protease inhibitor aprotinin in cardiac surgery are well described, and recent evidence suggests an antithrombotic mechanism of aprotinin through inhibition of thrombin-mediated platelet activation. We hypothesized that aprotinin provides hemostasis while reducing vascular thrombosis by attenuating protease-dependent platelet function.

METHODS

Rabbits (3 to 4 kg) underwent carotid artery thrombosis induced by electrical current. Treatment animals (n = 8) received aprotinin by a 100,000-KIU bolus followed by a continuous infusion (25,000 KIU/h). Control animals (n = 8) received crystalloid solution. Thrombus weight and time to thrombotic occlusion were determined. Platelet aggregation was examined in response to protease-dependent (thrombin) and protease-independent (adenosine diphosphate, ADP) platelet agonists. Platelet thrombin protease-activated receptor (PAR) expression was analyzed by Western blot. Ear bleeding time and abdominal incisional bleeding were measured at baseline and serially.

RESULTS

Thrombus weight was decreased by aprotinin (6.1 +/- 1.1 mg versus 10.8 +/- 1.5 mg, aprotinin versus control, p < 0.05). Time to thrombotic occlusion was prolonged in the aprotinin group (17.4 +/- 1.0 minutes versus 8.3 +/- 1.3 minutes, p < 0.001). Rabbit platelet expression of thrombin PARs was demonstrated by Western blot analysis, and was not altered by aprotinin therapy. Platelet aggregation due to thrombin was decreased by aprotinin therapy (59.2% +/- 3.0% versus 95.8% +/- 1.5%, p < 0.001), whereas protease-independent, ADP-induced platelet aggregation was unchanged with aprotinin. Incisional bleeding was not different between groups. In the aprotinin group, bleeding time was unchanged at baseline and then reduced for the duration of the experiment (35.0 +/- 4.7 seconds versus 76.8 +/- 6.4 seconds, p < 0.05).

CONCLUSIONS

While providing hemostatic effects, aprotinin attenuates vascular thrombosis in part by inhibition of PAR activation, resulting in the prevention of thrombin-induced platelet aggregation.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Expression of protease activated receptor 3 (PAR3) is upregulated by induction of megakaryocyte phenotype in human erythroleukemia (HEL) cells

OBJECTIVE

Two major protease-activated receptors (PARs), PAR1 and PAR4, are involved in the activation of human platelets by thrombin. A third, PAR3, is preferentially expressed by tissues of hematopoietic origin and megakaryocytes. Although PAR3 is also a thrombin substrate, its low-level expression on human platelets suggests a function distinct from that of PAR1, the major receptor involved in thrombin-mediated platelet activation. We studied the expression of PARs during megakaryocyte differentiation of human erythroleukemia (HEL) cells in order to determine the role of PAR3 in megakaryocytopoiesis.

METHODS

HEL cells exposed to phorbol 12-myristate 13-acetate (PMA) to induce megakaryocyte differentiation were examined by light microscopy and flow cytometry (DNA ploidy, surface expression of PAR1, PAR3, GPIIb-IIIa). Northern blot, RT-PCR, and quantitative RT-PCR were used to evaluate the expression of PARs 1, 3, and 4 mRNA. HEL cells were also exposed to thrombin and thrombopoietin (TPO).

RESULTS

In baseline studies, unstimulated HEL cells were found to express comparable levels of PAR1 and PAR3 by Northern blot. Minimal expression of PAR4 was detected by RT-PCR, but not by Northern analysis. Exposure to PMA, but not thrombin or TPO, resulted in megakaryocytic differentiation as evident by increased cell size and nuclear complexity, increased ploidy, and enhanced expression of GPIIb-IIIa, a specific marker of megakaryocytes/platelets. PMA-stimulated HEL cells showed enhanced PAR3 cell-surface expression (approximately threefold increase by day 2) by flow cytometry. In contrast, there was no change in cell-surface PAR1 expression. Northern blot analysis (approximately 10-fold) and quantitative RT-PCR (approximately threefold) confirmed the upregulation of PAR3 mRNA expression (by 24 hours) in cells exposed to PMA. This did not occur with exposure to TPO.

CONCLUSION

These data demonstrate increased expression of PAR3 mRNA and protein in HEL cells undergoing megakaryocytic maturation following PMA exposure, suggesting a developmental role for PAR3. Furthermore, regulation of PAR3 expression appears to be specifically coupled to the protein kinase C system, but independent of the Ras/Raf/MAP kinase pathway.

Normalization of multiple hemostatic abnormalities in uremic type 1 diabetic patients after kidney-pancreas transplantation

To evaluate the effects of kidney-pancreas transplantation on hemostatic abnormalities in uremic type 1 diabetic patients, we conducted a cross-sectional study involving 12 type 1 diabetic patients, 30 uremic type 1 diabetic patients, 27 uremic type 1 diabetic patients who had a kidney-pancreas transplant, 12 uremic type 1 diabetic patients who had a kidney-alone transplant, and 13 healthy control subjects. We evaluated platelet and clotting system. Platelets in the group of uremic type 1 diabetic patients were significantly larger than platelets in the other groups. Resting calcium levels were significantly higher in the uremic type 1 diabetic patients and uremic type 1 diabetic patients who had a kidney-alone transplant than in the type 1 diabetic patients who had a kidney-pancreas transplant and control subjects. CD41 expression was significantly reduced in platelets from the uremic type 1 diabetic patients compared with the other groups. Levels of hypercoagulability markers in the type 1 diabetic patients who had a kidney-pancreas transplant and, to a lesser extent, the uremic type 1 diabetic patients who had a kidney-alone transplant but not the uremic type 1 diabetic patients were similar to those of the control subjects. A reduction in natural anticoagulants was evident in the uremic type 1 diabetic patients, whereas near-normal values were observed in the type 1 diabetic patients who had a kidney-pancreas transplant and uremic type 1 diabetic patients who had a kidney-alone transplant. Hemostatic abnormalities were not observed in type 1 diabetic patients who had a kidney-pancreas transplant. This finding might explain the lower cardiovascular death rate observed in type 1 diabetic patients who had a kidney-pancreas transplant compared with uremic type 1 diabetic patients who had a kidney-alone transplant or uremic type 1 diabetic patients.

Impaired hemostasis and protection against thrombosis in protease-activated receptor 4-deficient mice is due to lack of thrombin signaling in platelets

Platelets from protease-activated receptor 4 (PAR4)-deficient mice are unresponsive to thrombin, and Par4-/- mice have prolonged bleeding times and are protected against thrombosis. However, in addition to its role in platelets, PAR4 contributes to thrombin signaling in cells in the blood vessel wall that might participate in hemostasis and thrombosis, such as endothelial cells. To determine whether the hemostatic and thrombotic phenotypes of Par4-/- mice were due to loss of PAR4 function in hematopoietic vs. other cell types, tail bleed times and thromboplastin-induced pulmonary embolism were examined in lethally irradiated mice reconstituted with Par4+/+ or Par4-/- bone marrow. In Par4+/+ and Par4-/- mice reconstituted with Par4+/+ marrow, the median tail bleed times were 2.0 and 1.7 min, respectively, vs. > 10 min for both Par4+/+ and Par4-/- mice reconstituted with Par4-/- marrow. In the pulmonary embolism model, Par4+/+ and Par4-/- mice reconstituted with Par4+/+ marrow survived a median of 3.7 and 2.8 min, respectively, after administration of thromboplastin, vs. > 20 min for both Par4+/+ and Par4-/- mice reconstituted with Par4-/- marrow. Further, the phenotype of mice reconstituted with Par4-/- marrow was almost as dramatic as that seen in Nf-e2-/- mice, which lack platelets. These data strongly suggest that increased tail bleed times and protection against thrombosis in Par4-/- mice are accounted for by lack of PAR4 function in platelets, emphasize the importance of thrombin signaling in platelets among the multiple pathways and cell types that govern hemostasis and thrombosis.

Platelets of female mice are intrinsically more sensitive to agonists than are platelets of males

OBJECTIVE

It has been reported that women fare worse after ischemic coronary events, but the mechanisms remain unclear. Because platelets play a central role in the formation of occlusive thrombi at sites of ruptured atherosclerotic plaques, we studied male/female paired mouse littermates for sex differences in platelet function.

METHODS AND RESULTS

We compared platelet reactivity in male/female mouse littermates by monitoring agonist-induced fibrinogen (FGN) binding and platelet aggregation. Compared with the platelets from males, platelets from females bound more FGN in response to low concentrations of thrombin and collagen-related peptide. Female platelets also demonstrated greater aggregation in response to adenosine diphosphate and collagen-related peptide. Platelet protein tyrosine phosphorylation on activation also showed small differences between sexes. These differences are independent of platelet size and surface expression of alphaIIbbeta3 and GPIb-IX-V, and they were not blocked by apyrase or aspirin. The sex differences we observed were intrinsic to platelets, because they were observed in washed platelets, but not when platelets were in plasma.

CONCLUSIONS

The platelets of female mice were more reactive than those of males in a manner independent of COX-1 and secreted ADP.

Differential expression and regulation of protease-activated receptors in human peripheral monocytes and monocyte-derived antigen-presenting cells

Protease-activated receptors (PARs) are stimulated by proteolytic cleavage of their extracellular domain, unmasking a new N-terminus acting as tethered ligand. Whereas the role of PARs in platelets is well known, their presence and function in human monocytes and other antigen-presenting cells has not been characterized. Here it is demonstrated that human peripheral monocytes and monocyte-derived macrophages and dendritic cells differentially express PARs. Human monocytes express mainly PAR1 and less PAR3. Differentiation of monocytes into macrophages by either macrophage colony-stimulating factor (M-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF) elicits enhanced expression of PAR1, PAR2, and PAR3. In contrast, dendritic cells differentiated from monocytes by GM-CSF and interleukin-4 (IL-4) strongly down-regulated PAR1, PAR2, and PAR3, both at the mRNA and the protein level. Down-regulation of the PAR expression was apparently due to IL-4, because treatment of macrophages with IL-4 caused down-regulation of PAR1, PAR2, and PAR3. PAR4 mRNA expression remained undetectable in any of the cell types investigated. Stimulation of PAR1, PAR2, and PAR3 with thrombin, trypsin, or established receptor-activating peptides (PAR-APs) triggered cytosolic Ca2+ responses, indicating functionally active PARs. Further, stimulation of monocytes or macrophages with thrombin or PAR1-AP, but not with PAR2-or PAR4-AP, triggers expression of monocyte chemoattractant protein-1 (MCP-1) both at the mRNA and the protein level. These data demonstrate that differentiation of human monocytes is associated with differential expression of functionally active PARs that mediate distinct regulatory functions in inflammation and atherogenesis.

Protease-activated receptor subtype expression in developing eye and adult retina of the rat after optic nerve crush

Protease-activated receptors (PARs), 7-transmembrane domain G protein-coupled receptors, are involved in tissue degeneration and repair upon injury. We demonstrate the expression of all four PAR subtypes in the postnatal eye and in retina of the adult rat by reverse transcription-polymerase chain reaction (RT-PCR). PAR-1 is regulated developmentally in the eye, with a decrease from P1, P9, to P16, whereas levels for PAR-2, PAR-3, and PAR-4 remain unchanged throughout. In the retina of the adult rat, PAR-1 is highly expressed, whereas PAR-2 and PAR-3 are moderately expressed, compared to low PAR-4 expression. To elucidate possible roles of PARs after trauma, we carried out semiquantitative RT-PCR analysis of expression of all 4 PAR subtypes, beginning 6 hr after partial optic nerve crush (ONC) in the adult rat until 3 weeks after the mild trauma. Levels of PAR mRNA for all four subtypes were upregulated as early as 6 hr after unilateral ONC, except PAR-3, which showed a delayed upregulation. PAR-1, PAR-3, and PAR-4 mRNA levels returned to almost basal levels at 3 weeks post-crush, whereas PAR-2 mRNA level was still high by the end of 3 weeks after crush. Although the lesion was unilateral, PAR mRNA expression in the contralateral, uninjured side was affected to levels almost comparable to those in the injured side. Previous studies have shown an increase in thrombin levels at the site of injury, retinal ganglion cell degeneration by necrosis and apoptosis, and PAR activation as consequences of nerve crush. PAR upregulation because of nerve crush in the mild trauma model could act as an effector of early cell death. Eventual return of receptor mRNA to basal levels is consistent with neuroprotection.

Extracellular mediators in atherosclerosis and thrombosis: lessons from thrombin receptor knockout mice

It is well appreciated that thrombin as well as other proteases can act as signaling molecules that specifically regulate cells by cleaving and activating members of a novel class of protease-activated receptors (PARs). The utility of gene knockout strategies to define and better comprehend the physiological role of specific proteins is perhaps best exemplified in the field of thrombin receptors. The development of PAR knockout mice has provided the unique opportunity to identify and characterize new members of this novel family of GPCRs, evaluate the interaction of PARs jointly expressed in common cells and tissues, and better understand the role of PARs in thrombosis, restenosis, vascular remodeling, angiogenesis, and inflammation. Presently, 4 members of the PAR family have been cloned and identified. In this review, we examine experimental evidence gleaned from PAR-/- mouse models as well as how the use of PAR-/- mice has provided insights toward understanding the physiological role of thrombin in cells of the vascular system and vascular pathology.

PAR-3 is a low-affinity substrate, high affinity effector of thrombin

A polypeptide corresponding to the extracellular domain of protease-activated receptor 3 (PAR-3) is hydrolyzed by thrombin slowly because of high K(M) (>100 microM). However, thrombin is found to bind two PAR-3, one without catalyzing hydrolysis or blocking the active site, while the other is hydrolyzed. In a solvent lacking Na(+), hydrolysis of a nitroanilide substrate is enhanced 1.6-fold by addition of PAR-3 polypeptide, with half-saturation at 2.5 microM. In contrast, the fibrinogen clotting activity of thrombin is inhibited completely by PAR-3, with a K(I) of 3 microM. None of the activities of thrombin are affected by addition of 50 microM PAR-4 polypeptide. Thus, PAR-3 in low concentrations binds thrombin in a configuration that blocks the anion-binding exosite but not the catalytic site, while hydrolysis of PAR-3, PAR-4, and other substrates that do not interact with exosite I persists. The allosteric effect of PAR-3 is characteristic of that of Na(+).

Synthesis of 1-benzyl-3-(5'-hydroxymethyl-2'-furyl)indazole analogues as novel antiplatelet agents

1-Benzyl-3-(5'-hydroxymethyl-2'-furyl)indazole (28, YC-1) was selected as the lead compound for systemic structural modification. After screening for antiplatelet activity, SARs of YC-1 analogues were established. Among these potent active derivatives, compounds 29, 30, 31, 44, and 45 functioned as potent activators of sGC and inhibitors of PDE5 with potency comparable to that of YC-1. In addition, compound 58 was found to be a selective and potent inhibitor of protease-activated receptor type 4 (PAR4)-dependent platelet activation.

Role of thrombin signalling in platelets in haemostasis and thrombosis

Platelets are critical in haemostasis and in arterial thrombosis, which causes heart attacks and other events triggered by abnormal clotting. The coagulation protease thrombin is a potent activator of platelets ex vivo. However, because thrombin also mediates fibrin deposition and because multiple agonists can trigger platelet activation, the relative importance of platelet activation by thrombin in haemostasis and thrombosis is unknown. Thrombin triggers cellular responses at least in part through protease-activated receptors (PARs). Mouse platelets express PAR3 and PAR4 (ref. 9). Here we show that platelets from PAR4-deficient mice failed to change shape, mobilize calcium, secrete ATP or aggregate in response to thrombin. This result demonstrates that PAR signalling is necessary for mouse platelet activation by thrombin and supports the model that mouse PAR3 (mPAR3) does not by itself mediate transmembrane signalling but instead acts as a cofactor for thrombin cleavage and activation of mPAR4 (ref. 10). Importantly, PAR4-deficient mice had markedly prolonged bleeding times and were protected in a model of arteriolar thrombosis. Thus platelet activation by thrombin is necessary for normal haemostasis and may be an important target in the treatment of thrombosis.

Enhanced contractile response to thrombin in the pregnant rat myometrium

Thrombin causes various cellular events by activating protease-activated receptors (PARs). Here, we showed, for the first time, that thrombin induced myometrial contraction. To determine the mechanism of thrombin-induced myometrial contraction, we simultaneously measured intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension of fura-PE3-loaded rat myometrium using front-surface fluorimetry. The expression of thrombin receptor mRNA in the rat myometrium were determined by reverse transcription-polymerase chain reaction analysis (RT - PCR analysis). Thrombin (0.01 - 3 u ml(-1)) caused dose-dependent increase in [Ca(2+)](i) and tension in the rat myometrium, and this effect was greatly enhanced in the pregnant myometrium. PAR1-activating peptide mimicked the effects of thrombin. In Ca(2+)-free PSS, thrombin induced no increase in [Ca(2+)](i) and tension in the pregnant myometrium. Both diltiazem (10 microM) and SK-F 96365 (10 microM) significantly inhibited the thrombin-induced elevations of [Ca(2+)](i) and tension, and their effects were additive. RT - PCR analysis revealed an approximately 10 fold increase in the level of thrombin receptor mRNA in the pregnant myometrium compared to that obtained in the non-pregnant myometrium. In conclusion, the contractile response to thrombin was greatly enhanced in the pregnant myometrium, mainly due to the up-regulation of thrombin receptor. We propose that initiation of a post-parturitional myometrial contraction is one of the most important physiological roles of thrombin receptor.

Protease-activated receptors 1 and 4 are shut off with distinct kinetics after activation by thrombin

Protease-activated receptors 1 and 4 (PAR1 and PAR4) mediate thrombin signaling in human platelets. Whether these receptors are redundant, interact, or serve only partially overlapping functions is unknown. We report that PAR1 and PAR4 signal with distinct tempos. In transfected fibroblasts, PAR4 triggered substantially more phosphoinositide hydrolysis per activated receptor than PAR1 and was shut off more slowly than PAR1. Shutoff and internalization of PAR1 depends upon phosphorylation of its carboxyl tail upon receptor activation. In contrast to PAR1, phosphorylation of PAR4 was undetectable, and activation-dependent internalization of PAR4 was much slower than that seen for PAR1. Mutation of potential phosphorylation sites in the carboxyl tail of PAR1 enhanced PAR1 signaling, whereas analogous mutations in PAR4 had no effect. Thus PAR4 signaling is shut off less rapidly than PAR1, probably due to differences in receptor phosphorylation. PAR1 and PAR4 also signaled with distinct tempos in platelets. PAR1 triggered a rapid and transient increase in intracellular calcium, whereas PAR4 triggered a more prolonged response. Together, the tempo of these responses accounted for that triggered by thrombin. Thus differences in the rates at which PAR1 and PAR4 are shut off allow thrombin to trigger intracellular signaling with distinct temporal characteristics.

YD-3, a novel inhibitor of protease-induced platelet activation

1. In the present study, the antiplatelet effects and mechanisms of a new synthetic compound YD-3 [1-benzyl-3(ethoxycarbonylphenyl)-indazole] were examined. 2. YD-3 inhibited the aggregation of washed rabbit platelets caused by thrombin (IC(50)=28.3 microM), but had no or little inhibitory effect on that induced by arachidonic acid, collagen, platelet-activating factor (PAF) or U46619. YD-3 also suppressed generation of inositol phosphates caused by thrombin. On the other hand, thrombin-induced fibrin formation was not affected by YD-3, indicating YD-3 does not inhibit the proteolytic activity of thrombin. 3. In washed human platelets, however, YD-3 had only mild inhibitory effect on the low concentration (0.05 u ml(-1)) of thrombin-induced human platelet aggregation, and did not affect that induced by higher concentrations (> or =0.1 u ml(-1)) of thrombin or SFLLRN, the protease-activated receptor 1 (PAR1) agonist peptide. By contrast, YD-3 inhibited both human and rabbit platelet aggregation elicited by trypsin with IC(50) values of 38.1 microM and 5.7 microM, respectively. 4. YD-3, at 100 microM, had no effect on ristocetin-induced glycoprotein Ib (GPIb)-dependent aggregation of human platelets. In addition, platelets treated with chymotrypsin, which cleaves GPIb, enhanced rather than attenuated the inhibition of YD-3 on thrombin-induced human platelet aggregation. These data indicate that GPIb plays no role in the antiplatelet effect of YD-3. 5. In SFLLRN-desensitized human platelets, high concentration of thrombin (1 u ml(-1)) could still elicit intracellular Ca(2+) mobilization, and the rise of [Ca(2+)](i) was prevented by either leupeptin or YD-3. 6. Our results suggest that YD-3 inhibits a non-PAR1 thrombin receptor which mediates the major effect of thrombin in rabbit platelets, but in human platelets, this receptor function becomes significant only when the function of PAR1 has been blocked or attenuated.

Structure-function analysis of protease-activated receptor 4 tethered ligand peptides. Determinants of specificity and utility in assays of receptor function

Thrombin activates protease-activated receptors (PARs) by specific cleavage of their amino-terminal exodomains to unmask a tethered ligand that binds intramolecularly to the body of the receptor to effect transmembrane signaling. Peptides that mimic such ligands are valuable as agonists for probing PAR function, but the tethered ligand peptide for PAR4, GYPGKF, lacks potency and is of limited utility. In a structure-activity analysis of PAR4 peptides, AYPGKF was approximately 10-fold more potent than GYPGKF and, unlike GYPGKF, elicited PAR4-mediated responses comparable in magnitude to those elicited by thrombin. AYPGKF was relatively specific for PAR4 in part due to the tyrosine at position 2; substitution of phenylalanine or p-fluorophenylalanine at this position produced peptides that activated both PAR1 and PAR4. Because human platelets express both PAR1 and PAR4, it might be desirable to inhibit both receptors. Identifying a single agonist for both receptors raises the possibility that a single antagonist for both receptors might be developed. The AYPGKF peptide is a useful new tool for probing PAR4 function. For example, AYPGKF activated and desensitized PAR4 in platelets and, like thrombin, triggered phosphoinositide hydrolysis but not inhibition of adenylyl cyclase in PAR4-expressing cells. The latter shows that, unlike PAR1, PAR4 couples to G(q) and not G(i).

PAR3 is a cofactor for PAR4 activation by thrombin

Identification of the mechanisms by which the coagulation protease thrombin activates platelets is critical for understanding haemostasis and thrombosis. Thrombin activates cells at least in part by cleaving protease-activated G-protein-coupled receptors (PARs). PAR3 and PAR4 are thrombin receptors expressed in mouse platelets. Inhibition of thrombin binding to mPAR3 (ref. 4) and knockout of the mPAR3 gene inhibited mouse platelet activation at low but not high concentrations of thrombin. Thus PAR3 is important for thrombin signalling in mouse platelets. Expression of human PAR3 in heterologous expression systems reliably resulted in responsiveness to thrombin. Curiously, despite its importance for the activation of mouse platelets by thrombin, mouse PAR3 (mPAR3) did not lead to thrombin signalling even when overexpressed. We now report that mPAR3 and mPAR4 interact in a novel way: mPAR3 does not itself mediate transmembrane signalling but instead functions as a cofactor for the cleavage and activation of mPAR4 by thrombin. This establishes a paradigm for cofactor-assisted PAR activation and for a G-protein-coupled receptor's acting as an accessory molecule to present ligand to another receptor.

Influence of vortex speed on fresh versus stored platelet aggregation in the absence and presence of extracellular ATP

Platelets are subjected to vastly differing shear forces under laminar and nonlaminar flow patterns throughout the tortuous cardiovascular system. Different activation pathways appear to be associated with platelet adhesion and aggregation under high shear rates vs. low shear rates. We found that platelets continue to aggregate at very low stirring rates (100 RPM) and low shear forces although significantly less than at high stirring rates (1000 RPM). These conditions may model vortices encountered in vivo, such as downstream of partially occluded blood vessels. The extent of agonist-induced platelet aggregation, at varying stir rates, remained essentially unchanged between 1200 and 600 RPM. This was true for both freshly prepared and stored platelets even though the extent of aggregation was significantly reduced with stored platelets. Agonists used were thrombin, thrombin receptor activating peptide (TRAP), SFLLRNP, the thromboxane A2 mimetic, U46619, plus epinephrine and ADP+epinephrine. At lower stir rates (100-400 RPM), little or no difference in aggregation levels was observed between fresh and stored platelets, depending upon agonist used. This may indicate that old and young platelets, in vivo, would be equally active at vessel walls exposed to blood flowing through a slow vortex at low shear rates. ATP, released from activated platelets, may act as a potent regulator of platelet aggregation within a vortex where the resident time of platelets and bioactive molecules is greater than in laminar flow regions. High levels of extracellular ATP (100 microM) inhibited agonist-induced aggregation of fresh platelets to a greater extent than stored platelets, except with ADP+epinephrine where the converse was observed. Inhibition, in general, appeared to be inversely related to stir rates. Low levels of extracellular ATP (10 nM, 1 microM) generally stimulated agonist-induced aggregations independent of stir rates and to a greater extent with stored platelets than fresh platelets. Unraveling how hemostasis functions within microenvironments may facilitate ways to further regulate this process.

Glycoprotein V-Deficient Platelets Have Undiminished Thrombin Responsiveness and Do Not Exhibit a Bernard-Soulier Phenotype

Adhesion of platelets to extracellular matrix via von Willebrand factor (vWF) and activation of platelets by thrombin are critical steps in hemostasis. Glycoprotein (GP) V is a component of the GPIb-V-IX complex, the platelet receptor for vWF. GPV is also cleaved by thrombin. Deficiency of GPIb or GPIX results in Bernard-Soulier syndrome (BSS), a bleeding disorder in which platelets are giant and have multiple functional defects. Whether GPV-deficiency might also cause BSS is unknown as are the roles of GPV in platelet-vWF interaction and thrombin signaling. We report that GPV-deficient mice developed normally, had no evidence of spontaneous bleeding, and had tail bleeding times that were not prolonged compared with wild-type mice. GPV-deficient platelets were normal in size and structure as assessed by flow cytometry and electron microscopy. GPV-deficient and wild-type platelets were indistinguishable in botrocetin-mediated platelet agglutination and in their ability to adhere to mouse vWF A1 domain. Platelet aggregation and ATP secretion in response to low and high concentrations of thrombin were not decreased in GPV-deficient platelets compared with wild-type. Our results show that (1) GPV is not necessary for GPIb expression and function in platelets and that GPV deficiency is not likely to be a cause of human BSS and (2) GPV is not necessary for robust thrombin signaling. Whether redundancy accounts for the lack of phenotype of GPV-deficiency or whether GPV serves subtle or as yet unprobed functions in platelets or other cells remains to be determined.

Glycoprotein V-Deficient Platelets Have Undiminished Thrombin Responsiveness and Do Not Exhibit a Bernard-Soulier Phenotype

Adhesion of platelets to extracellular matrix via von Willebrand factor (vWF) and activation of platelets by thrombin are critical steps in hemostasis. Glycoprotein (GP) V is a component of the GPIb-V-IX complex, the platelet receptor for vWF. GPV is also cleaved by thrombin. Deficiency of GPIb or GPIX results in Bernard-Soulier syndrome (BSS), a bleeding disorder in which platelets are giant and have multiple functional defects. Whether GPV-deficiency might also cause BSS is unknown as are the roles of GPV in platelet-vWF interaction and thrombin signaling. We report that GPV-deficient mice developed normally, had no evidence of spontaneous bleeding, and had tail bleeding times that were not prolonged compared with wild-type mice. GPV-deficient platelets were normal in size and structure as assessed by flow cytometry and electron microscopy. GPV-deficient and wild-type platelets were indistinguishable in botrocetin-mediated platelet agglutination and in their ability to adhere to mouse vWF A1 domain. Platelet aggregation and ATP secretion in response to low and high concentrations of thrombin were not decreased in GPV-deficient platelets compared with wild-type. Our results show that (1) GPV is not necessary for GPIb expression and function in platelets and that GPV deficiency is not likely to be a cause of human BSS and (2) GPV is not necessary for robust thrombin signaling. Whether redundancy accounts for the lack of phenotype of GPV-deficiency or whether GPV serves subtle or as yet unprobed functions in platelets or other cells remains to be determined.

Altered vascular injury responses in mice deficient in protease-activated receptor-1

Expression of protease-activated receptor-1 (PAR-1), a cell-surface receptor for thrombin, is increased in balloon-injured rat carotid artery and human atherosclerotic tissue. To examine the role of PAR-1 in vascular injury, we compared vascular injury responses in wild-type (WT) and PAR-1-deficient (PAR-1(-/-)) mice. Arterial injury was induced by inserting a flexible guidewire into the common carotid artery and withdrawing it 6 times with rotation. Bromodeoxyuridine, delivered subcutaneously by osmotic minipump, was used to measure cellular proliferation. Mice were perfusion-fixed at 1, 2, 5, 10, and 14 days after injury. Extensive endothelial damage, mural thrombosis, platelet adherence, and medial smooth muscle cell loss and necrosis were apparent at day 1 in both WT and PAR-1(-/-) mice. The incidence of thrombosis or platelet deposition in WT and PAR-1(-/-) mice declined from 100% at day 1 to 25% and 21%, respectively, at 14 days. Endothelial disruption, as assessed by Evan's blue uptake, was maximum at day 1 and declined by day 14. This apparent endothelial regrowth was similar in WT and PAR-1(-/-) mice. Significant medial thickening at 14 days after injury was similar in WT (from 22.8+/-1.7 to 30.7+/-1.9 microm) and PAR-1(-/-) (from 23.2+/-2.1 to 30.5+/-2.2 microm) mice. Medial area also increased in response to injury but to a lesser extent in PAR-1(-/-) mice (from 0.0250+/-0.0044 to 0.0312+/-0.0047 mm(2)) than in WT mice (from 0.0266+/-0.0040 to 0.0398+/-0.0050 mm(2)). Neointima was variable and occurred in 6 of 13 WT and 5 of 12 PAR-1(-/-) mice. However, intimal area tended to be less in PAR-1(-/-) mice (0. 0016+/-0.0007 mm(2)) compared with WT mice (0.0082+/-0.0032 mm(2)), although this difference did not achieve statistical significance (P=0.06). Cell density was significantly greater in normal carotids from PAR-1(-/-) (6.4+/-0.5 x 10(3)/mm(2)) compared with WT (4.3+/-0. 8 x 10(3)/mm(2)) mice and remained elevated after injury. Vessel and lumen diameters tended to increase in WT mice after injury, whereas vessel diameter was unchanged and lumen diameter actually decreased in PAR-1(-/-) mice. Cell proliferation in injured carotid arteries was similar in PAR-1(-/-) and WT mice. These data suggest that PAR-1(-/-) may play a role in vascular injury responses in this mouse model via possible effects on extracellular matrix regulation.

Integrin alpha(IIb)beta(3) signaling in platelet adhesion and aggregation

Intracellular signals are received and generated by the alpha(IIb)beta(3) integrin on platelets. Recent advances have been made in the areas of agonist receptors that initiate platelet activation, downstream signaling molecules (e.g. small G-proteins and kinases) and changes in ligand-occupied alpha(IIb)beta(3) that cause further signaling and clot retraction.