The occupancy of endothelial protein C receptor by its ligand modulates the par-1 dependent signaling specificity of coagulation proteases

Targeting biased signaling by PAR1: function and molecular mechanism of parmodulins

The G protein-coupled receptor (GPCR) protease-activated receptor 1 (PAR1) is a therapeutic target that was originally pursued with the aim of restricting platelet activation and the burden of cardiovascular diseases. In clinical studies, the use of orthosteric PAR1 inhibitors was associated with an increased risk of hemorrhage, including intracranial hemorrhage. Because (1) PAR1 is expressed by various cell types, including endothelial cells, (2) conveys in mice a physiological indispensable function for vascular development during embryogenesis, and (3) is subject to biased signaling dependent on the activating proteases, orthosteric PAR1 inhibition may be associated with unwanted side effects. Alternatively, the protease-activated protein C (aPC) and its variants can promote valuable anti-inflammatory signaling via PAR1. Most recently, small molecule allosteric modulators of PAR1 signaling, called parmodulins, have been developed. Parmodulins inhibit coagulation and platelet activation yet maintain cytoprotective effects typically provoked by PAR1 signaling upon the activation by aPC. In this study, we review the discovery of parmodulins and their preclinical data, summarize the current knowledge about their mode of action, and compare the structural interaction of parmodulin and PAR1 with that of other intracellularly binding allosteric GPCR modulators. Thus, we highlight the pharmaceutical potential and challenges associated with the future development of parmodulins.

Protease-activated receptors in kidney diseases: A comprehensive review of pathological roles, therapeutic outcomes and challenges

Studies have demonstrated that protease-activated receptors (PARs) with four subtypes (PAR1-4) are mainly expressed in the renal epithelial, endothelial, and podocyte cells. Some endogenous and urinary proteases, namely thrombin, trypsin, urokinase, and kallikrein released during diseased conditions, are responsible for activating different subtypes of PARs. Each PAR receptor subtype is involved in kidney disease of distinct aetiology. PAR1 and PAR2 have shown differential therapeutic outcomes in rodent models of type-1 and type-2 diabetic kidney diseases due to the distinct etiological basis of each disease type, however such findings need to be confirmed in other diabetic renal injury models. PAR1 and PAR2 blockers have been observed to abolish drug-induced nephrotoxicity in rodents by suppressing tubular inflammation and fibrosis and preventing mitochondrial dysfunction. Notably, PAR2 inhibition improved autophagy and prevented fibrosis, inflammation, and remodeling in the urethral obstruction model. Only the PAR1/4 subtypes have emerged as a therapeutic target for treating experimentally induced nephrotic syndrome, where their respective antibodies attenuated the podocyte apoptosis induced upon thrombin activation. Strikingly PAR2 and PAR4 subtypes involvement has been tested in sepsis-induced acute kidney injury (AKI) and renal ischemia-reperfusion injury models. Thus, more studies are required to delineate the role of other subtypes in the sepsis-AKI model. Evidence suggests that PARs regulate oxidative, inflammatory stress, immune cell activation, fibrosis, autophagic flux, and apoptosis during kidney diseases.

Selective inhibition of activated protein C anticoagulant activity protects against hemophilic arthropathy in mice

Recurrent spontaneous or trauma-related bleeding into joints in hemophilia leads to hemophilic arthropathy (HA), a debilitating joint disease. Treatment of HA consists of preventing joint bleeding by clotting factor replacement, and in extreme cases, orthopedic surgery. We recently showed that administration of endothelial cell protein C receptor (EPCR) blocking monoclonal antibodies (mAb) markedly reduced the severity of HA in factor VIII (FVIII)-/- mice. EPCR blocking inhibits activated protein C (APC) generation and EPCR-dependent APC signaling. The present study was aimed to define the role of inhibition of APC anticoagulant activity, APC signaling, or both in suppressing HA. FVIII-/- mice were treated with a single dose of isotype control mAb, MPC1609 mAb, that inhibits anticoagulant, and signaling properties of APC, or MAPC1591 mAb that only blocks the anticoagulant activity of APC. Joint bleeding was induced by needle puncture injury. HA was evaluated by monitoring joint bleeding, change in joint diameter, and histopathological analysis of joint tissue sections for synovial hypertrophy, macrophage infiltration, neoangiogenesis, cartilage degeneration, and chondrocyte apoptosis. No significant differences were observed between MPC1609 and MAPC1591 in inhibiting APC anticoagulant activity in vitro and equally effective in correcting acute bleeding induced by the saphenous vein incision in FVIII-/- mice. Administration of MAPC1591, and not MPC1609, markedly reduced the severity of HA. MAPC1591 inhibited joint bleed-induced inflammatory cytokine interleukin-6 expression and vascular leakage in joints, whereas MPC1609 had no significant effect. Our data show that an mAb that selectively inhibits APC's anticoagulant activity without compromising its cytoprotective signaling offers a therapeutic potential alternative to treat HA.

Factor VIIa suppresses inflammation and barrier disruption through the release of EEVs and transfer of microRNA 10a

Coagulation protease, factor VIIa (FVIIa), binds to endothelial cell protein C receptor (EPCR) and induces anti-inflammatory and endothelial barrier protective responses via protease-activated receptor-1 (PAR1)-mediated, biased signaling. Our recent studies had shown that the FVIIa-EPCR-PAR1 axis induces the release of extracellular vesicles (EVs) from endothelial cells. In the present study, we investigated the mechanism of FVIIa release of endothelial EVs (EEVs) and the contribution of FVIIa-released EEVs to anti-inflammatory and vascular barrier protective effects, in both in vitro and in vivo models. Multiple signaling pathways regulated FVIIa release of EVs from endothelial cells, but the ROCK-dependent pathway appeared to be a major mechanism. FVIIa-released EEVs were enriched with anti-inflammatory microRNAs (miRs), mostly miR10a. FVIIa-released EEVs were taken up readily by monocytes/macrophages and endothelial cells. The uptake of FVIIa-released EEVs by monocytes conferred anti-inflammatory phenotype to monocytes, whereas EEV uptake by endothelial cells resulted in barrier protection. In additional experiments, EEV-mediated delivery of miR10a to monocytes downregulated the expression of TAK1 and activation of the NF-κB-mediated inflammatory pathway. In in vivo experiments, administration of FVIIa-released EEVs to wild-type mice attenuated LPS-induced increased inflammatory cytokines in plasma and vascular leakage into vital tissues. The incorporation of anti-miR10a into FVIIa-released EEVs diminished the ability of FVIIa-released EEVs to confer cytoprotective effects. Administration of the ROCK inhibitor Y27632, which significantly inhibits FVIIa release of EEVs into the circulation, to mice attenuated the cytoprotective effects of FVIIa. Overall, our study revealed novel insights into how FVIIa induces cytoprotective effects and communicates with various cell types.

Coagulation Dysfunction in Acute Respiratory Distress Syndrome and Its Potential Impact in Inflammatory Subphenotypes

The Acute Respiratory Distress Syndrome (ARDS) has caused innumerable deaths worldwide since its initial description over five decades ago. Population-based estimates of ARDS vary from 1 to 86 cases per 100,000, with the highest rates reported in Australia and the United States. This syndrome is characterised by a breakdown of the pulmonary alveolo-epithelial barrier with subsequent severe hypoxaemia and disturbances in pulmonary mechanics. The underlying pathophysiology of this syndrome is a severe inflammatory reaction and associated local and systemic coagulation dysfunction that leads to pulmonary and systemic damage, ultimately causing death in up to 40% of patients. Since inflammation and coagulation are inextricably linked throughout evolution, it is biological folly to assess the two systems in isolation when investigating the underlying molecular mechanisms of coagulation dysfunction in ARDS. Although the body possesses potent endogenous systems to regulate coagulation, these become dysregulated and no longer optimally functional during the acute phase of ARDS, further perpetuating coagulation, inflammation and cell damage. The inflammatory ARDS subphenotypes address inflammatory differences but neglect the equally important coagulation pathway. A holistic understanding of this syndrome and its subphenotypes will improve our understanding of underlying mechanisms that then drive translation into diagnostic testing, treatments, and improve patient outcomes.

Factor VIIa induces extracellular vesicles from the endothelium: a potential mechanism for its hemostatic effect

Recombinant factor FVIIa (rFVIIa) is used as a hemostatic agent to treat bleeding disorders in hemophilia patients with inhibitors and other groups of patients. Our recent studies showed that FVIIa binds endothelial cell protein C receptor (EPCR) and induces protease-activated receptor 1 (PAR1)-mediated biased signaling. The importance of FVIIa-EPCR-PAR1-mediated signaling in hemostasis is unknown. In the present study, we show that FVIIa induces the release of extracellular vesicles (EVs) from endothelial cells both in vitro and in vivo. Silencing of EPCR or PAR1 in endothelial cells blocked the FVIIa-induced generation of EVs. Consistent with these data, FVIIa treatment enhanced the release of EVs from murine brain endothelial cells isolated from wild-type (WT), EPCR-overexpressing, and PAR1-R46Q-mutant mice, but not EPCR-deficient or PAR1-R41Q-mutant mice. In vivo studies revealed that administration of FVIIa to WT, EPCR-overexpressing, and PAR1-R46Q-mutant mice, but not EPCR-deficient or PAR1-R41Q-mutant mice, increased the number of circulating EVs. EVs released in response to FVIIa treatment exhibit enhanced procoagulant activity. Infusion of FVIIa-generated EVs and not control EVs to platelet-depleted mice increased thrombin generation at the site of injury and reduced blood loss. Administration of FVIIa-generated EVs or generation of EVs endogenously by administering FVIIa augmented the hemostatic effect of FVIIa. Overall, our data reveal that FVIIa treatment, through FVIIa-EPCR-PAR1 signaling, releases EVs from the endothelium into the circulation, and these EVs contribute to the hemostatic effect of FVIIa.

Role of Thrombin in Central Nervous System Injury and Disease

Thrombin is a Na+-activated allosteric serine protease of the chymotrypsin family involved in coagulation, inflammation, cell protection, and apoptosis. Increasingly, the role of thrombin in the brain has been explored. Low concentrations of thrombin are neuroprotective, while high concentrations exert pathological effects. However, greater attention regarding the involvement of thrombin in normal and pathological processes in the central nervous system is warranted. In this review, we explore the mechanisms of thrombin action, localization, and functions in the central nervous system and describe the involvement of thrombin in stroke and intracerebral hemorrhage, neurodegenerative diseases, epilepsy, traumatic brain injury, and primary central nervous system tumors. We aim to comprehensively characterize the role of thrombin in neurological disease and injury.

Endothelial Cell Protein C Receptor Deficiency Attenuates Streptococcus pneumoniae-induced Pleural Fibrosis

Streptococcus pneumoniae is the leading cause of hospital community-acquired pneumonia. Patients with pneumococcal pneumonia may develop complicated parapneumonic effusions or empyema that can lead to pleural organization and subsequent fibrosis. The pathogenesis of pleural organization and scarification involves complex interactions between the components of the immune system, coagulation, and fibrinolysis. EPCR (endothelial protein C receptor) is a critical component of the protein C anticoagulant pathway. The present study was performed to evaluate the role of EPCR in the pathogenesis of S. pneumoniae infection-induced pleural thickening and fibrosis. Our studies show that the pleural mesothelium expresses EPCR. Intrapleural instillation of S. pneumoniae impairs lung compliance and lung volume in wild-type and EPCR-overexpressing mice but not in EPCR-deficient mice. Intrapleural S. pneumoniae infection induces pleural thickening in wild-type mice. Pleural thickening is more pronounced in EPCR-overexpressing mice, whereas it is reduced in EPCR-deficient mice. Markers of mesomesenchymal transition are increased in the visceral pleura of S. pneumoniae-infected wild-type and EPCR-overexpressing mice but not in EPCR-deficient mice. The lungs of wild-type and EPCR-overexpressing mice administered intrapleural S. pneumoniae showed increased infiltration of macrophages and neutrophils, which was significantly reduced in EPCR-deficient mice. An analysis of bacterial burden in the pleural lavage, the lungs, and blood revealed a significantly lower bacterial burden in EPCR-deficient mice compared with wild-type and EPCR-overexpressing mice. Overall, our data provide strong evidence that EPCR deficiency protects against S. pneumoniae infection-induced impairment of lung function and pleural remodeling.

Platelet-derived calpain cleaves the endothelial protease-activated receptor 1 to induce vascular inflammation in diabetes

Diabetes mellitus is a major risk factor for cardiovascular disease. Platelets from diabetic patients are hyperreactive and release microparticles that carry activated cysteine proteases or calpains. Whether platelet-derived calpains contribute to the development of vascular complications in diabetes is unknown. Here we report that platelet-derived calpain1 (CAPN1) cleaves the protease-activated receptor 1 (PAR-1) on the surface of endothelial cells, which then initiates a signaling cascade that includes the activation of the tumor necrosis factor (TNF)-α converting enzyme (TACE). The latter elicits the shedding of the endothelial protein C receptor and the generation of TNF-α, which in turn, induces intracellular adhesion molecule (ICAM)-1 expression to promote monocyte adhesion. All of the effects of CAPN1 were mimicked by platelet-derived microparticles from diabetic patients or from wild-type mice but not from CAPN1^−/− mice, and were not observed in PAR-1-deficient endothelial cells. Importantly, aortae from diabetic mice expressed less PAR-1 but more ICAM-1 than non-diabetic mice, effects that were prevented by treating diabetic mice with a calpain inhibitor as well as by the platelet specific deletion of CAPN1. Thus, platelet-derived CAPN1 contributes to the initiation of the sterile vascular inflammation associated with diabetes via the cleavage of PAR-1 and the release of TNF-α from the endothelial cell surface.

The endothelial protein C receptor plays an essential role in the maintenance of pregnancy

Placenta-mediated pregnancy complications are a major challenge in the management of maternal-fetal health. Maternal thrombophilia is a suspected risk factor, but the role of thrombotic processes in these complications has remained unclear. Endothelial protein C receptor (EPCR) is an anticoagulant protein highly expressed in the placenta. EPCR autoantibodies and gene variants are associated with poor pregnancy outcomes. In mice, fetal EPCR deficiency results in placental failure and in utero death. We show that inhibition of molecules involved in thrombin generation or in the activation of maternal platelets allows placental development and embryonic survival. Nonetheless, placentae exhibit venous thrombosis in uteroplacental circulation associated with neonatal death. In contrast, maternal EPCR deficiency results in clinical and histological features of placental abruption and is ameliorated with concomitant Par4 deficiency. Our findings unveil a causal link between maternal thrombophilia, uterine hemorrhage, and placental abruption and identify Par4 as a potential target of therapeutic intervention.

Podocyte Integrin-β 3 and Activated Protein C Coordinately Restrict RhoA Signaling and Ameliorate Diabetic Nephropathy

BACKGROUND

Diabetic nephropathy (dNP), now the leading cause of ESKD, lacks efficient therapies. Coagulation protease-dependent signaling modulates dNP, in part via the G protein-coupled, protease-activated receptors (PARs). Specifically, the cytoprotective protease-activated protein C (aPC) protects from dNP, but the mechanisms are not clear.

METHODS

A combination of in vitro approaches and mouse models evaluated the role of aPC-integrin interaction and related signaling in dNP.

RESULTS

The zymogen protein C and aPC bind to podocyte integrin-β ₃, a subunit of integrin-α _v β ₃. Deficiency of this integrin impairs thrombin-mediated generation of aPC on podocytes. The interaction of aPC with integrin-α _v β ₃ induces transient binding of integrin-β ₃ with G _α13 and controls PAR-dependent RhoA signaling in podocytes. Binding of aPC to integrin-β ₃ via its RGD sequence is required for the temporal restriction of RhoA signaling in podocytes. In podocytes lacking integrin-β ₃, aPC induces sustained RhoA activation, mimicking the effect of thrombin. In vivo, overexpression of wild-type aPC suppresses pathologic renal RhoA activation and protects against dNP. Disrupting the aPC-integrin-β ₃ interaction by specifically deleting podocyte integrin-β ₃ or by abolishing aPC's integrin-binding RGD sequence enhances RhoA signaling in mice with high aPC levels and abolishes aPC's nephroprotective effect. Pharmacologic inhibition of PAR1, the pivotal thrombin receptor, restricts RhoA activation and nephroprotects RGE-aPC^high and wild-type mice.Conclusions aPC-integrin-α _v β ₃ acts as a rheostat, controlling PAR1-dependent RhoA activation in podocytes in diabetic nephropathy. These results identify integrin-α _v β ₃ as an essential coreceptor for aPC that is required for nephroprotective aPC-PAR signaling in dNP.

Activated Protein C Attenuates Experimental Autoimmune Encephalomyelitis Progression by Enhancing Vascular Integrity and Suppressing Microglial Activation

Background

Activated protein C (APC), a serine protease with antithrombotic effects, protects in animal models of ischemic stroke by suppressing inflammation and enhancing vascular integrity, angiogenesis, neurogenesis and neuroprotection. A small number of animal studies suggest it might also have therapeutic potential in multiple sclerosis (MS), though results have been mixed. Based on these conflicting data, the goals of this study were to clarify the therapeutic potential of APC in the experimental autoimmune encephalomyelitis (EAE) model of MS and to determine mechanistically how APC mediates this protective effect.

Methods

The protective potential of APC was examined in a chronic progressive model of EAE. Vascular breakdown, tight junction protein expression and vascular expression of fibronectin and α5β1 integrin as well as vascularity and glial activation were analyzed using immunofluorescence (IF) of spinal cord sections taken from mice with established EAE. The direct influence of APC on microglial activation was evaluated in vitro by a combination of morphology and MMP-9 expression.

Results

APC attenuated the progression of EAE, and this was strongly associated at the histopathological level with reduced levels of leukocyte infiltration and concomitant demyelination. Further analysis revealed that APC reduced vascular breakdown which was associated with maintained endothelial expression of the tight junction protein zonula occludens-1 (ZO-1). In addition, APC suppressed microglial activation in this EAE model and in vitro studies revealed that APC strongly inhibited microglial activation at both the morphological level and by the expression of the pro-inflammatory protease MMP-9.

Conclusion

These findings build on the work of others in demonstrating strong therapeutic potential for APC in the treatment of inflammatory demyelinating disease and suggest that enhancement of vascular integrity and suppression of microglial activation may be important mediators of this protection.

Host's Endogenous Caveolin-1 Expression is Downregulated in the Lung During Sepsis to Promote Cytoprotection

The present study focuses on the profile of "endogeneous" caveolin-1 protein in septic lung (CLP model).Caveolin-1, CD25, pP38, pAkt, and 14-3-3b protein expression profiles were studied using flow cytometry and immunohistochemistry 6, 12, 24, 36, and 48 h after sepsis induction. Cell viability was determined by 7-AAD staining and fibrosis by Masson trichrome stain. The effect of protein C zymogen concentrate (PC) on caveolin-1 expression was also investigated given that PC, once dissociated from caveolin-1, elicits a PAR-1-mediated protective signaling by forming a complex with endothelial protein C receptor (EPCR).CLP treatment increased lung inflammation and cell apoptosis. Fibrosis was apparent in vessels and alveoli. Caveolin-1+ cells presented reduced protein expression, especially 12 h post-CLP (P = 0.002). Immunohistochemistry revealed caveolin-1 positive expression mainly in regions with strong inflammatory reaction. Early induction of pP38+ cell population (P = 0.014) and gradual increase of CD25+ cells were also observed. Alternations in 14-3-3b expression related to apoptosis were apparent and accompanied by increased AKT phosphorylation activity late during sepsis progression.After PC administration, cell apoptosis was reduced (P = 0.004) and both the percentile and expression intensity of caveolin-1 positive cells were compromised (P = 0.009 and P = 0.027, respectively). 14-3-3b, CD25, and pP38 protein expression were decreased (P = 0.014, P = 0.004, and P = 0.007, respectively), whereas pAkt expression was induced (P = 0.032).The observed decline of endogenous caveolin-1 protein expression during sepsis implies its involvement in host's cytoprotective reaction either directly, by controlling caveolae population to decrease bacterial burden, or indirectly via regulating 14-3-3b-dependent apoptosis and EPCR-PAR-1-dependent protective signaling.

Activated protein C in neuroprotection and malaria

PURPOSE OF REVIEW

Activated protein C (APC) is a homeostatic coagulation protease with anticoagulant and cytoprotective activities. Focusing on APC's effects in the brain, this review discusses three different scenarios that illustrate how APC functions are intimately affecting the physiology and pathophysiology of the brain.

RECENT FINDINGS

Cytoprotective APC therapy holds promise for the treatment of ischemic stroke, and a recently completed trial suggested that cytoprotective-selective 3K3A-APC reduced bleeding in ischemic stroke patients. In contrast, APC's anticoagulant activity contributes to brain bleeding as shown by the disproportional upregulation of APC generation in cerebral cavernous malformations lesions in mice. However, too little APC generation also contributes to maladies of the brain, such as in case of cerebral malaria where the binding of infected erythrocytes to the endothelial protein C receptor (EPCR) may interfere with the EPCR-dependent functions of the protein C pathway. Furthermore, discoveries of new activities of APC such as the inhibition of the NLRP3-mediated inflammasome and of new applications of APC therapy such as in Alzheimer's disease and graft-versus-host disease continue to advance our knowledge of this important proteolytic regulatory system.

SUMMARY

APC's many activities or lack thereof are intimately involved in multiple neuropathologies, providing abundant opportunities for translational research.

Endothelial Protein C Receptor (EPCR), Protease Activated Receptor-1 (PAR-1) and Their Interplay in Cancer Growth and Metastatic Dissemination

Endothelial protein C receptor (EPCR) and protease activated receptor 1 (PAR-1) by themselves play important role in cancer growth and dissemination. Moreover, interactions between the two receptors are essential for tumor progression. EPCR is a cell surface transmembrane glycoprotein localized predominantly on endothelial cells (ECs). It is a vital component of the activated protein C (APC)—mediated anticoagulant and cytoprotective signaling cascade. PAR-1, which belongs to a family of G protein–coupled cell surface receptors, is also widely distributed on endothelial and blood cells, where it plays a critical role in hemostasis. Both EPCR and PAR-1, generally considered coagulation-related receptors, are implicated in carcinogenesis and dissemination of diverse tumor types, and their expression correlates with clinical outcome of cancer patients. Existing data explain some mechanisms by which EPCR/PAR-1 affects cancer growth and metastasis; however, the exact molecular basis of cancer invasion associated with the signaling is still obscure. Here, we discuss the role of EPCR and PAR-1 reciprocal interactions in cancer progression as well as potential therapeutic options targeted specifically to interact with EPCR/PAR-1-induced signaling in cancer patients.

Endothelial cell protein C receptor-dependent signaling

PURPOSE OF REVIEW

Endothelial cell protein C receptor (EPCR), a transmembrane glycoprotein present on the surface of endothelial cells and other cell types, is an essential component of the protein C (PC) anticoagulant system. EPCR is also shown to play a critical role in mediating activated protein C (APC)-induced cytoprotective signaling. The purpose of this review is to outline the mechanisms of EPCR-dependent cell signaling and discuss recent findings made in this area.

RECENT FINDINGS

Recent studies showed that the cleavage of protease-activated receptor (PAR)1 at a noncanonical site by APC-EPCR or the canonical site by thrombin when PC occupies EPCR induces β-arrestin-2-mediated biased cytoprotective signaling. Factor VIIa binding to EPCR is also shown to induce the cytoprotective signaling. EPCR is found to be a reliable surface marker for identifying human hematopoietic stem cells in culture. EPCR, binding to diverse ligands, is thought to play a role in the pathogenesis of severe malaria, immune functions, and cancer by either blocking the APC-mediated signaling or by mechanisms that are yet to be elucidated.

SUMMARY

Recent studies provide a mechanistic basis to how EPCR contributes to PAR1-mediated biased signaling. EPCR may play a role in influencing a wide array of biological functions by binding to diverse ligands.

Activated protein C, protease activated receptor 1, and neuroprotection

Protein C is a plasma serine protease zymogen whose active form, activated protein C (APC), exerts potent anticoagulant activity. In addition to its antithrombotic role as a plasma protease, pharmacologic APC is a pleiotropic protease that activates diverse homeostatic cell signaling pathways via multiple receptors on many cells. Engineering of APC by site-directed mutagenesis provided a signaling selective APC mutant with 3 Lys residues replaced by 3 Ala residues, 3K3A-APC, that lacks >90% anticoagulant activity but retains normal cell signaling activities. This 3K3A-APC mutant exerts multiple potent neuroprotective activities, which require the G-protein-coupled receptor, protease activated receptor 1. Potent neuroprotection in murine ischemic stroke models is linked to 3K3A-APC-induced signaling that arises due to APC's cleavage in protease activated receptor 1 at a noncanonical Arg46 site. This cleavage causes biased signaling that provides a major explanation for APC's in vivo mechanism of action for neuroprotective activities. 3K3A-APC appeared to be safe in ischemic stroke patients and reduced bleeding in the brain after tissue plasminogen activator therapy in a recent phase 2 clinical trial. Hence, it merits further clinical testing for its efficacy in ischemic stroke patients. Recent studies using human fetal neural stem and progenitor cells show that 3K3A-APC promotes neurogenesis in vitro as well as in vivo in the murine middle cerebral artery occlusion stroke model. These recent advances should encourage translational research centered on signaling selective APC's for both single-agent therapies and multiagent combination therapies for ischemic stroke and other neuropathologies.

Factor VIIa induces anti-inflammatory signaling via EPCR and PAR1

Recent studies show that endothelial cell protein C receptor (EPCR) interacts with diverse ligands, in addition to its known ligands protein C and activated protein C (APC). We showed in earlier studies that procoagulant clotting factor VIIa (FVIIa) binds EPCR and downregulates EPCR-mediated anticoagulation and induces an endothelial barrier protective effect. Here, we investigated the effect of FVIIa's interaction with EPCR on endothelial cell inflammation and lipopolysaccharide (LPS)-induced inflammatory responses in vivo. Treatment of endothelial cells with FVIIa suppressed tumor necrosis factor α (TNF-α)- and LPS-induced expression of cellular adhesion molecules and adherence of monocytes to endothelial cells. Inhibition of EPCR or protease-activated receptor 1 (PAR1) by either specific antibodies or small interfering RNA abolished the FVIIa-induced suppression of TNF-α- and LPS-induced expression of cellular adhesion molecules and interleukin-6. β-Arrestin-1 silencing blocked the FVIIa-induced anti-inflammatory effect in endothelial cells. In vivo studies showed that FVIIa treatment markedly suppressed LPS-induced inflammatory cytokines and infiltration of innate immune cells into the lung in wild-type and EPCR-overexpressing mice, but not in EPCR-deficient mice. Mechanistic studies revealed that FVIIa treatment inhibited TNF-α-induced ERK1/2, p38 MAPK, JNK, NF-κB, and C-Jun activation indicating that FVIIa-mediated signaling blocks an upstream signaling event in TNFα-induced signaling cascade. FVIIa treatment impaired the recruitment of TNF-receptor-associated factor 2 into the TNF receptor 1 signaling complex. Overall, our present data provide convincing evidence that FVIIa binding to EPCR elicits anti-inflammatory signaling via a PAR1- and β-arrestin-1 dependent pathway. The present study suggests new therapeutic potentials for FVIIa, which is currently in clinical use for treating bleeding disorders.

Pharmacological modulation of procoagulant microparticles improves haemodynamic dysfunction during septic shock in rats

Circulating microparticles play a pro-inflammatory and procoagulant detrimental role in the vascular dysfunction of septic shock. It was the objective of this study to investigate mechanisms by which a pharmacological modulation of microparticles could affect vascular dysfunction in a rat model of septic shock. Septic or sham rats were treated by activated protein C (aPC) and resuscitated during 4 hours. Their microparticles were harvested and inoculated to another set of healthy recipient rats. Haemodynamic parameters were monitored, circulating total procoagulant microparticles assessed by prothrombinase assay, and their cell origin characterised. Mesenteric resistance arteries, aorta and heart were harvested for western blotting analysis. We found that a) the amount and phenotype of circulating microparticles were altered in septic rats with an enhanced endothelial, leucocyte and platelet contribution; b) aPC treatment significantly reduced the generation of leucocyte microparticles and norepinephrine requirements to reach the mean arterial pressure target in septic rats; c) Microparticles from untreated septic rats, but not from aPC-treated ones, significantly reduced the healthy recipients’ mean arterial pressure; d) Microparticle thromboxane content and aPC activity were significantly increased in aPC-treated septic rats. In inoculated naïve recipients, microparticles from aPC-treated septic rats prompted reduced NF-κB and cyclooxygenase-2 arterial activation, blunted the generation of pro-inflammatory iNOS and secondarily increased platelet and endothelial microparticles. In conclusion, in this septic shock model, increased circulating levels of procoagulant microparticles led to negative haemodynamic outcomes. Pharmacological treatment by aPC modified the cell origin and levels of circulating microparticles, thereby limiting vascular inflammation and favouring haemodynamic improvement.

2016 Scientific Sessions Sol Sherry Distinguished Lecturer in Thrombosis: Thrombotic Stroke: Neuroprotective Therapy by Recombinant-Activated Protein C

APC (activated protein C), derived from the plasma protease zymogen, is antithrombotic and anti-inflammatory. In preclinical injury models, recombinant APC provides neuroprotection for multiple injuries, including ischemic stroke. APC acts directly on brain endothelial cells and neurons by initiating cell signaling that requires multiple receptors. Two or more major APC receptors mediate APC's neuroprotective cell signaling. When bound to endothelial cell protein C receptor, APC can cleave protease-activated receptor 1, causing biased cytoprotective signaling that reduces ischemia-induced injury. Pharmacological APC alleviates bleeding induced by tissue-type plasminogen activator in murine ischemic stroke studies. Remarkably, APC's signaling promotes neurogenesis. The signaling-selective recombinant variant of APC, 3K3A-APC, was engineered to lack most of the APC's anticoagulant activity but retain APC's cell signaling actions. Recombinant 3K3A-APC is in ongoing National Institutes of Health (NIH)-funded clinical trials for ischemic stroke.

Coagulation and non-coagulation effects of thrombin

Thrombin is a multifunctional serine protease produced from prothrombin, and is a key regulator in hemostatic and non-hemostatic processes. It is the main effector protease in primary hemostasis by activating platelets, and plays a key role in secondary hemostasis. Besides its well-known functions in hemostasis, thrombin also plays a role in various non-hemostatic biological and pathophysiologic processes, predominantly mediated through activation of protease-activated receptors (PARs). Depending on several factors, such as the concentration of thrombin, the duration of activation, the location of PARs, the presence of coreceptors, and the formation of PAR heterodimers, activation of the receptor by thrombin can induce different cellular responses. Moreover, thrombin can have opposing effects in the same cell; it can induce both inflammatory and anti-inflammatory signals. Owing to the complexity of thrombin's signal transduction pathways, the exact mechanism behind the dichotomy of thrombin is yet still unknown. In this review, we highlight the hemostatic and non-hemostatic functions of thrombin, and specifically focus on the non-hemostatic dual role of thrombin under various conditions and in relation to cardiovascular disease.

Protease-activated receptors (PARs)--biology and role in cancer invasion and metastasis

Although many studies have demonstrated that components of the hemostatic system may be involved in signaling leading to cancer progression, the potential mechanisms by which they contribute to cancer dissemination are not yet precisely understood. Among known coagulant factors, tissue factor (TF) and thrombin play a pivotal role in cancer invasion. They may be generated in the tumor microenvironment independently of blood coagulation and can induce cell signaling through activation of protease-activated receptors (PARs). PARs are transmembrane G-protein-coupled receptors (GPCRs) that are activated by a unique proteolytic mechanism. They play important roles in vascular physiology, neural tube closure, hemostasis, and inflammation. All of these agents (TF, thrombin, PARs—mainly PAR-1 and PAR-2) are thought to promote cancer invasion and metastasis at least in part by facilitating tumor cell migration, angiogenesis, and interactions with host vascular cells, including platelets, fibroblasts, and endothelial cells lining blood vessels. Here, we discuss the role of PARs and their activators in cancer progression, focusing on TF- and thrombin-mediated actions. Therapeutic options tailored specifically to inhibit PAR-induced signaling in cancer patients are presented as well.

Physiological cerebrovascular remodeling in response to chronic mild hypoxia: A role for activated protein C

Activated protein C (APC) is a serine protease that promotes favorable changes in vascular barrier integrity and post-ischemic angiogenic remodeling in animal models of ischemic stroke, and its efficacy is currently being investigated in clinical ischemic stroke trials. Interestingly, application of sub-clinical chronic mild hypoxia (CMH) (8% O2) also promotes angiogenic remodeling and increased tight junction protein expression, suggestive of enhanced blood-brain barrier (BBB) integrity, though the role of APC in mediating the influence of CMH has not been investigated. To examine this potential link, we studied CMH-induced cerebrovascular remodeling after treating mice with two different reagents: (i) a function-blocking antibody that neutralizes APC activity, and (ii) exogenous recombinant murine APC. While CMH promoted endothelial proliferation, increased vascular density, and upregulated the angiogenic endothelial integrins α5β1 and αvβ3, these events were almost completely abolished by functional blockade of APC. Consistent with these findings, addition of exogenous recombinant APC enhanced CMH-induced endothelial proliferation, expansion of total vascular area and further enhanced the CMH-induced right-shift in vessel size distribution. Taken together, our findings support a key role for APC in mediating physiological remodeling of cerebral blood vessels in response to CMH.

Cell painting with an engineered EPCR to augment the protein C system

The protein C (PC) system conveys beneficial anticoagulant and cytoprotective effects in numerous in vivo disease models. The endothelial protein C receptor (EPCR) plays a central role in these pathways as cofactor for PC activation and by enhancing activated protein C (APC)-mediated protease-activated receptor (PAR) activation. During inflammatory disease, expression of EPCR on cell membranes is often diminished thereby limiting PC activation and APC's effects on cells. Here a caveolae-targeting glycosylphosphatidylinositol (GPI)-anchored EPCR (EPCR-GPI) was engineered to restore EPCR's bioavailability via "cell painting." The painting efficiency of EPCR-GPI on EPCR-depleted endothelial cells was time- and dose-dependent. The EPCR-GPI bioavailability after painting was long lasting since EPCR surface levels reached 400 % of wild-type cells after 2 hours and remained > 200 % for 24 hours. EPCR-GPI painting conveyed APC binding to EPCR-depleted endothelial cells where EPCR was lost due to shedding or shRNA. EPCR painting normalised PC activation on EPCR-depleted cells indicating that EPCR-GPI is functional active on painted cells. Caveolin-1 lipid rafts were enriched in EPCR after painting due to the GPI-anchor targeting caveolae. Accordingly, EPCR painting supported PAR1 and PAR3 cleavage by APC and augmented PAR1-dependent Akt phosphorylation by APC. Thus, EPCR-GPI painting achieved physiological relevant surface levels on endothelial cells, restored APC binding to EPCR-depleted cells, supported PC activation, and enhanced APC-mediated PAR cleavage and cytoprotective signalling. Therefore, EPCR-GPI provides a novel tool to restore the bioavailability and functionality of EPCR on EPCR- depleted and -deficient cells.

Activated protein C modulates the proinflammatory activity of dendritic cells

Background

Previous studies have demonstrated the beneficial activity of activated protein C in allergic diseases including bronchial asthma and rhinitis. However, the exact mechanism of action of activated protein C in allergies is unclear. In this study, we hypothesized that pharmacological doses of activated protein C can modulate allergic inflammation by inhibiting dendritic cells.

Materials and methods

Dendritic cells were prepared using murine bone marrow progenitor cells and human peripheral monocytes. Bronchial asthma was induced in mice that received intratracheal instillation of ovalbumin-pulsed dendritic cells.

Results

Activated protein C significantly increased the differentiation of tolerogenic plasmacytoid dendritic cells and the secretion of type I interferons, but it significantly reduced lipopolysaccharide-mediated maturation and the secretion of inflammatory cytokines in myeloid dendritic cells. Activated protein C also inhibited maturation and the secretion of inflammatory cytokines in monocyte-derived dendritic cells. Activated protein C-treated dendritic cells were less effective when differentiating naïve CD4 T-cells from Th1 or Th2 cells, and the cellular effect of activated protein C was mediated by its receptors. Mice that received adoptive transfer of activated protein C-treated ovalbumin-pulsed dendritic cells had significantly less airway hyperresponsiveness, significantly decreased lung concentrations of Th1 and Th2 cytokines, and less plasma concentration of immunoglobulin E when compared to control mice.

Conclusion

These results suggest that dendritic cells mediate the immunosuppressive effect of activated protein C during allergic inflammation.

Activated protein C: biased for translation

The homeostatic blood protease, activated protein C (APC), can function as (1) an antithrombotic on the basis of inactivation of clotting factors Va and VIIIa; (2) a cytoprotective on the basis of endothelial barrier stabilization and anti-inflammatory and antiapoptotic actions; and (3) a regenerative on the basis of stimulation of neurogenesis, angiogenesis, and wound healing. Pharmacologic therapies using recombinant human and murine APCs indicate that APC provides effective acute or chronic therapies for a strikingly diverse range of preclinical injury models. APC reduces the damage caused by the following: ischemia/reperfusion in brain, heart, and kidney; pulmonary, kidney, and gastrointestinal inflammation; sepsis; Ebola virus; diabetes; and total lethal body radiation. For these beneficial effects, APC alters cell signaling networks and gene expression profiles by activating protease-activated receptors 1 and 3. APC's activation of these G protein-coupled receptors differs completely from thrombin's activation mechanism due to biased signaling via either G proteins or β-arrestin-2. To reduce APC-associated bleeding risk, APC variants were engineered to lack >90% anticoagulant activity but retain normal cell signaling. Such a neuroprotective variant, 3K3A-APC (Lys191-193Ala), has advanced to clinical trials for ischemic stroke. A rich data set of preclinical knowledge provides a solid foundation for potential translation of APC variants to future novel therapies.

Cytoprotective-selective activated protein C therapy for ischaemic stroke

Despite years of research and efforts to translate stroke research to clinical therapy, ischaemic stroke remains a major cause of death, disability, and diminished quality of life. Primary and secondary preventive measures combined with improved quality of care have made significant progress. However, no novel drug for ischaemic stroke therapy has been approved in the past decade. Numerous studies have shown beneficial effects of activated protein C (APC) in rodent stroke models. In addition to its natural anticoagulant functions, APC conveys multiple direct cytoprotective effects on many different cell types that involve multiple receptors including protease activated receptor (PAR) 1, PAR3, and the endothelial protein C receptor (EPCR). Application of molecular engineered APC variants with altered selectivity profiles to rodent stroke models demonstrated that the beneficial effects of APC primarily require its cytoprotective activities but not its anticoagulant activities. Extensive basic, preclinical, and clinical research provided a compelling rationale based on strong evidence for translation of APC therapy that has led to the clinical development of the cytoprotective-selective APC variant, 3K3A-APC, for ischaemic stroke. Recent identification of non-canonical PAR1 and PAR3 activation by APC that give rise to novel tethered-ligands capable of inducing biased cytoprotective signalling as opposed to the canonical signalling provides a mechanistic explanation for how APC-mediated PAR activation can selectively induce cytoprotective signalling pathways. Collectively, these paradigm-shifting discoveries provide detailed insights into the receptor targets and the molecular mechanisms for neuroprotection by cytoprotective-selective 3K3A-APC, which is currently a biologic drug in clinical trials for ischaemic stroke.

Endothelial cell protein C receptor: a multiliganded and multifunctional receptor

Endothelial cell protein C receptor (EPCR) was first identified and isolated as a cellular receptor for protein C on endothelial cells. EPCR plays a crucial role in the protein C anticoagulant pathway by promoting protein C activation. In the last decade, EPCR has received wide attention after it was discovered to play a key role in mediating activated protein C (APC)-induced cytoprotective effects, including antiapoptotic, anti-inflammatory, and barrier stabilization. APC elicits cytoprotective signaling through activation of protease activated receptor-1 (PAR1). Understanding how EPCR-APC induces cytoprotective effects through activation of PAR1, whose activation by thrombin is known to induce a proinflammatory response, has become a major research focus in the field. Recent studies also discovered additional ligands for EPCR, which include factor VIIa, Plasmodium falciparum erythrocyte membrane protein, and a specific variant of the T-cell receptor. These observations open unsuspected new roles for EPCR in hemostasis, malaria pathogenesis, innate immunity, and cancer. Future research on these new discoveries will undoubtedly expand our understanding of the role of EPCR in normal physiology and disease, as well as provide novel insights into mechanisms for EPCR multifunctionality. Comprehensive understanding of EPCR may lead to development of novel therapeutic modalities in treating hemophilia, inflammation, cerebral malaria, and cancer.

Protease-activated receptor signalling by coagulation proteases in endothelial cells

Endothelial cells express several types of integral membrane protein receptors, which upon interaction and activation by their specific ligands, initiate a signalling network that links extracellular cues in circulation to various biological processes within a plethora of cells in the vascular system. A small family of G-protein coupled receptors, termed protease-activated receptors (PAR1-4), can be specifically activated by coagulation proteases, thereby modulating a diverse array of cellular activities under various pathophysiological conditions. Thrombin and all vitamin K-dependent coagulation proteases, with the exception of factor IXa for which no PAR signalling has been attributed, can selectively activate cell surface PARs on the vasculature. Thrombin can activate PAR1, PAR3 and PAR4, but not PAR2 which can be specifically activated by factors VIIa and Xa. The mechanistic details of the specificity of PAR signalling by coagulation proteases are the subject of extensive investigation by many research groups worldwide. However, analysis of PAR signalling data in the literature has proved to be challenging since a single coagulation protease can elicit different signalling responses through activation of the same PAR receptor in endothelial cells. This article is focused on briefly reviewing the literature with respect to determinants of the specificity of PAR signalling by coagulation proteases with special emphasis on the mechanism of PAR1 signalling by thrombin and activated protein C in endothelial cells.

The Northwick Park Heart Study: evidence from the laboratory

The Northwick Park Heart Study (NPHS) has shown associations of high plasma fibrinogen and factor VII (FVIIc) levels with the risk of death from coronary heart disease (CHD). The finding for fibrinogen has been confirmed in many other studies. Whereas one further study has found a similar prospective association for FVIIc, several have not. Experimental studies have demonstrated the impact that the coagulation activity of fibrinogen and FVIIc have on the progression and phenotype of atherosclerotic lesions. FVIIc-driven thrombin generation and fibrin formation within the vessel wall are important determinants of both plaque (in)stability and atherothrombosis. In blood, local concentrations of FVIIc and thrombin may be sufficient to allow interactions between these serine proteases and protease-activated receptors, to drive cellular inflammatory reactions that further promote these processes. Local fibrinogen concentrations dictate fibrin clot structure and resistance to fibrinolysis. Within the atherosclerotic plaque, coagulation reactions driven by proinflammatory stimuli may initially support lesion stability (as part of wound healing), but, with advanced inflammation, thrombin and fibrin generation diminish because of proteolytic activity contributing to plaque instability. The NPHS findings have proved controversial, but, in the light of current knowledge, a reappraisal of the importance of FVIIc and fibrinogen in atherosclerosis, atherothrombosis and CHD is justified. Hypercoagulability, reflected in turn by thrombin generation capacity, and local concentrations of coagulation proteins, including FVIIc and fibrinogen, is linked to plaque phenotype, and even minute local concentrations of fibrinogen and proteases such as FVIIc may affect thrombin generation capacity.

The anticoagulant activated protein C (aPC) promotes metaplasticity in the hippocampus through an EPCR-PAR1-S1P1 receptors dependent mechanism

Thrombin and other clotting factors regulate long-term potentiation (LTP) in the hippocampus through the activation of the protease activated receptor 1 (PAR1) and consequent potentiation of N-methyl-d-aspartate receptor (NMDAR) functions. We have recently shown that the activation of PAR1 either by thrombin or the anticoagulant factor activated protein C (aPC) has differential effects on LTP. While thrombin activation of PAR1 induces an NMDAR-mediated slow onset LTP, which saturates the ability to induce further LTP in the exposed network, aPC stimulation of PAR1 enhances tetanus induced LTP through a voltage-gated calcium channels mediated mechanism. In this study, we addressed the mechanisms by which aPC enhances LTP in hippocampal slices. Using extracellular recordings, we show that a short tetanic stimulation, which does not induce LTP, is able to enhance plasticity in the presence of aPC through a mechanism that requires the activation of sphingosine-1 phosphate receptor 1 and intracellular Ca(2+) stores. These data identify aPC as a "metaplastic molecule", capable of shifting the threshold of LTP towards further potentiation. Our findings propose novel strategies to enhance plasticity in neurological diseases associated with the breakdown of the blood brain barrier and alterations in synaptic plasticity.

New insights into modulation of thrombin formation

Thrombin is a pleiotropic enzyme that regulates hemostasis and nonhemostatic functions, including an array of actions within and on the vasculature. Physiologically, thrombin generation serves mainly to protect against thrombosis, but also to maintain vascular endothelial integrity. This protective effect is mediated in part through generation of anticoagulant enzymes, including activated protein C, formed on the action of thrombin on the endothelial receptor thrombomodulin. Partly, thrombin's vascular effects are effectuated through interaction with protease-activated receptors on various cell types. Pathophysiologically, downregulation and shedding of anticoagulant-acting receptors such as thrombomodulin and endothelial protein C receptor may contribute to a shift in activities of thrombin towards thrombogenic and proinflammatory actions. This shift may typically occur in the process of atherosclerosis, leading to a proatherogenic direction of the effects of thrombin. Therapeutically, the long-term inhibition of thrombin may create new ways of reducing atherosclerosis burden, altering the plaque phenotype.

RAGE controls activation and anti-inflammatory signalling of protein C

AIMS

The receptor for advanced glycation endproducts, RAGE, is a multiligand receptor and NF-κB activator leading to perpetuation of inflammation. We investigated whether and how RAGE is involved in mediation of anti-inflammatory properties of protein C.

METHODS AND RESULTS

We analyzed the effect of protein C on leukocyte adhesion and transmigration in WT- and RAGE-deficient mice using intravital microscopy of cremaster muscle venules during trauma- and TNFα-induced inflammation. Both, protein C (PC, Ceprotin, 100 U/kg) and activated protein C (aPC, 24 µg/kg/h) treatment significantly inhibited leukocyte adhesion in WT mice in these inflammation models. The impaired leukocyte adhesion after trauma-induced inflammation in RAGE knockout mice could not be further reduced by PC and aPC. After TNFα-stimulation, however, aPC but not PC treatment effectively blocked leukocyte adhesion in these mice. Consequently, we asked whether RAGE is involved in PC activation. Since RAGE-deficient mice and endothelial cells showed insufficient PC activation, and since thrombomodulin (TM) and endothelial protein C receptor (EPCR) are reduced on the mRNA and protein level in RAGE deficient endothelial cells, an involvement of RAGE in TM-EPCR-dependent PC activation is likely. Moreover, TNFα-induced activation of MAPK and upregulation of ICAM-1 and VCAM-1 are reduced both in response to aPC treatment and in the absence of RAGE. Thus, there seems to be interplay of the RAGE and the PC pathway in inflammation.

CONCLUSION

RAGE controls anti-inflammatory properties and activation of PC, which might involve EPCR and TM.

Biased signaling of protease-activated receptors

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

Novel mechanisms for activated protein C cytoprotective activities involving noncanonical activation of protease-activated receptor 3

The direct cytoprotective activities of activated protein C (APC) on cells convey therapeutic, relevant, beneficial effects in injury and disease models in vivo and require the endothelial protein C receptor (EPCR) and protease activated receptor 1 (PAR1). Thrombin also activates PAR1, but its effects on cells contrast APC's cytoprotective effects. To gain insights into mechanisms for these contrasting cellular effects, protease activated receptor 3 (PAR3) activation by APC and thrombin was studied. APC cleaved PAR3 on transfected and endothelial cells in the presence of EPCR. Remarkably, APC cleaved a synthetic PAR3 N-terminal peptide at Arg41, whereas thrombin cleaved at Lys38. On cells, APC failed to cleave R41Q-PAR3, whereas K38Q-PAR3 was still cleaved by APC but not by thrombin. PAR3 tethered-ligand peptides beginning at amino acid 42, but not those beginning at amino acid 39, conveyed endothelial barrier-protective effects. In vivo, the APC-derived PAR3 tethered-ligand peptide, but not the thrombin-derived PAR3 peptide, blunted vascular endothelial growth factor (VEGF)-induced vascular permeability. These data indicate that PAR3 cleavage by APC at Arg41 can initiate distinctive APC-like cytoprotective effects. These novel insights help explain the differentiation of APC's cytoprotective versus thrombin's proinflammatory effects on cells and suggest a unique contributory role for PAR3 in the complex mechanisms underlying APC cytoprotective effects.

Thrombomodulin and the vascular endothelium: insights into functional, regulatory, and therapeutic aspects

Thrombomodulin (TM) is a 557-amino acid protein with a broad cell and tissue distribution consistent with its wide-ranging physiological roles. When expressed on the lumenal surface of vascular endothelial cells in both large vessels and capillaries, its primary function is to mediate endothelial thromboresistance. The complete integral membrane-bound protein form displays five distinct functional domains, although shorter soluble (functional) variants comprising the extracellular domains have also been reported in fluids such as serum and urine. TM-mediated binding of thrombin is known to enhance the specificity of the latter serine protease toward both protein C and thrombin activatable fibrinolysis inhibitor (TAFI), increasing their proteolytic activation rate by almost three orders of magnitude with concomitant anticoagulant, antifibrinolytic, and anti-inflammatory benefits to the vascular wall. Recent years have seen an abundance of research into the cellular mechanisms governing endothelial TM production, processing, and regulation (including flow-mediated mechanoregulation)--from transcriptional and posttranscriptional (miRNA) regulation of TM gene expression, to posttranslational processing and release of the expressed protein--facilitating greater exploitation of its therapeutic potential. The goal of the present paper is to comprehensively review the endothelial/TM system from these regulatory perspectives and draw some fresh conclusions. This paper will conclude with a timely examination of the current status of TM's growing therapeutic appeal, from novel strategies to improve the clinical efficacy of recombinant TM analogs for resolution of vascular disorders such as disseminated intravascular coagulation (DIC), to an examination of the complex pleiotropic relationship between statin treatment and TM expression.

Mechanisms of anticoagulant and cytoprotective actions of the protein C pathway

The protein C pathway provides multiple important functions to maintain a regulated balance between hemostasis and host defense systems in response to vascular and inflammatory injury. The anticoagulant protein C pathway is designed to regulate coagulation, maintain the fluidity of blood within the vasculature, and prevent thrombosis, whereas the cytoprotective protein C pathway prevents vascular damage and stress. The cytoprotective activities of activated protein C (APC) include anti-apoptotic activity, anti-inflammatory activity, beneficial alterations of gene expression profiles, and endothelial barrier stabilization. These cytoprotective activities of APC, which require the endothelial protein C receptor (EPCR) and protease-activated receptor-1 (PAR1), have been a major research focus. Recent insights, such as non-canonical activation of PAR1 at Arg46 by APC and biased PAR1 signaling, provided better understanding of the molecular mechanisms by which APC elicits cytoprotective signaling through cleavage of PAR1. The discovery and development of anticoagulant-selective and cytoprotective-selective APC mutants provided unique opportunities for preclinical research that has been and may continue to be translated to clinical research. New mechanisms for the regulation of EPCR functionality, such as modulation of EPCR-bound lipids that affect APC's cytoprotective activities, may provide new research directions to improve the efficacy of APC to convey its cytoprotective activities to cells. Moreover, emerging novel functions for EPCR expand the roles of EPCR beyond mediating protein C activation and APC-induced PAR1 cleavage. These discoveries increasingly develop our understanding of the protein C pathway, which will conceivably expand its physiological implications to many areas in the future.

Thrombin regulation of synaptic plasticity: implications for physiology and pathology

Thrombin, a serine protease involved in the coagulation cascade has been recently shown to affect neuronal function following blood-brain barrier breakdown. Several lines of evidence have shown that thrombin may exist in the brain parenchyma under normal physiological conditions, yet its role in normal brain functions and synaptic transmission has not been established. In an attempt to shed light on the physiological functions of thrombin and Protease Activated Receptor 1 (PAR1) in the brain, we studied the effects of thrombin and a PAR1 agonist on long term potentiation (LTP) in mice hippocampal slices. Surprisingly, different concentrations of thrombin affect LTP through different molecular routes converging on PAR1. High thrombin concentrations induced an NMDA dependent, slow onset LTP, whereas low concentrations of thrombin promoted a VGCCs, mGluR-5 dependent LTP through activated Protein C (aPC). Remarkably, aPC facilitated LTP by activating PAR1 through an Endothelial Protein C Receptor (EPCR)-mediated mechanism which involves intracellular calcium stores. These findings reveal a novel mechanism by which PAR1 may regulate the threshold for synaptic plasticity in the hippocampus and provide additional insights into the role of this receptor in normal and pathological conditions.

Organ-specific protection against lipopolysaccharide-induced vascular leak is dependent on the endothelial protein C receptor

OBJECTIVE

To study the role of the endothelial protein C receptor (EPCR) in the modulation of susceptibility to inflammation-induced vascular leak in vivo.

APPROACH AND RESULTS

Genetically modified mice with low, <10% EPCR expression (EPCR(low)) and control mice were challenged with lipopolysaccharides in a mouse model of endotoxemia. Infrared fluorescence and quantification of albumin-bound Evans Blue in tissues and intravascular plasma volumes were used to assess plasma extravasation. Pair-wise analysis of EPCR(low) and control mice matched for sex, age, and weight allowed determination of EPCR-dependent vascular leak. Kidney, lung, and brain were the organs with highest discriminative increased Evans Blue accumulation in EPCR(low) versus control mice in response to lipopolysaccharides. Histology of kidney and lung confirmed the EPCR-specific pathology. In addition to severe kidney injury in response to lipopolysaccharides, EPCR(low) and anti-EPCR-treated wild-type mice suffered from enhanced albuminuria and profound renal hemorrhage versus controls. Intravascular volume loss at the same extent of weight loss in EPCR(low) mice compared with control mice provided proof that plasma leak was the predominant cause of Evans Blue tissue accumulation.

CONCLUSIONS

This study demonstrates an important protective role for EPCR in vivo against vascular leakage during inflammation and suggests that EPCR-dependent vascular protection is organ-specific.

Down-regulation of the clotting cascade by the protein C pathway

The protein C pathway provides important biological activities to maintain the fluidity of the circulation, prevent thrombosis, and protect the integrity of the vasculature in response to injury. Activated protein C (APC), in concert with its cofactors and cell receptors, assembles in specific macromolecular complexes to provide efficient proteolysis of multiple substrates that result in anticoagulant and cytoprotective activities. Numerous studies on APC's structure-function relation with its cofactors, cell receptors, and substrates provide valuable insights into the molecular mechanisms and presumed assembly of the macromolecular complexes that are responsible for APC's activities. These insights allow for molecular engineering approaches specifically targeting the interaction of APC with one of its substrates or cofactors. Thus far, these approaches resulted in several anticoagulant-selective and cytoprotective-selective APC mutants, which provide unique insights into the relative contributions of APC's anticoagulant or cytoprotective activities to the beneficial effects of APC in various murine injury and disease models. Because of its multiple physiological and pharmacological activities, the anticoagulant and cytoprotective protein C pathway have important implications for the (patho)physiology of vascular disease and for translational research exploring novel therapeutic strategies to combat complex medical disorders such as thrombosis, inflammation, ischemic stroke and neurodegenerative disease.

Neutrophil proteinase 3 acts on protease-activated receptor-2 to enhance vascular endothelial cell barrier function

OBJECTIVE

The principle role of the vascular endothelium is to present a semi-impermeable barrier to soluble factors and circulating cells, while still permitting the passage of leukocytes from the bloodstream into the tissue. The process of diapedesis involves the selective disruption of endothelial cell junctions, which could compromise vascular integrity. It is therefore somewhat surprising that neutrophil transmigration does not significantly impair endothelial barrier function. We examined whether neutrophils might secrete factors that promote vascular integrity during the latter stages of neutrophil transmigration, in particular, the role of neutrophil serine proteinase 3 (PR3).

METHODS AND RESULTS

Endothelial cells were treated with PR3 either in its soluble form or in a complex form with cell surface NB1. We observed that PR3 mediated the enhancement of endothelial cell junctional integrity and that this required its proteolytic activity, as well as endothelial cell expression of the protease-activated receptor-2. Importantly, PR3 suppressed the vascular permeability changes and disruption of junctional proteins induced by the action of protease-activated receptor-1 agonists.

CONCLUSIONS

These findings establish the potential for neutrophil-derived PR3 to play a role in reestablishing vascular integrity after leukocyte transmigration and in protecting endothelial cells from protease-activated receptor-1-induced permeability changes that occur during thrombotic and inflammatory events.

Biased agonism of protease-activated receptor 1 by activated protein C caused by noncanonical cleavage at Arg46

Activated protein C (APC) exerts endothelial cytoprotective actions that require protease-activated receptor 1 (PAR1), whereas thrombin acting via PAR1 causes endothelial disruptive, proinflammatory actions. APC's activities, but not thrombin's, require PAR1 located in caveolae. PAR1 is a biased 7-transmembrane receptor because G proteins mediate thrombin's signaling, whereas β-arrestin 2 mediates APC's signaling. Here we elucidate novel mechanisms for APC's initiation of signaling. Biochemical studies of APC's protease specificity showed that APC cleaved PAR1 sequences at both Arg41 and Arg46. That PAR1 cleavage at Arg46 can occur on cells was supported by APC's cleavage of N-terminal-SEAP-tagged R41Q-PAR1 but not R41Q/R46Q-PAR1 mutants transfected into cells and by anti-PAR1 epitope mapping of APC-treated endothelial cells. A synthetic peptide composing PAR1 residues 47-66, TR47, stimulated protective signaling in endothelial cells as reflected in Akt and glycogen synthase kinase 3β phosphorylation, Ras-related C3 botulinum toxin substrate 1 activation, and barrier stabilization effects. In mice, the TR47 peptide reduced VEGF-induced vascular leakage. These in vitro and in vivo data imply that the novel PAR1 N-terminus beginning at residue Asn47, which is generated by APC cleavage at Arg46, mediates APC's cytoprotective signaling and that this unique APC-generated N-terminal peptide tail is a novel biased agonist for PAR1.

The protein C pathway and sepsis

After the discovery of the key components of the protein C (PC) pathway a beneficial effect on survival of the infusion of activated protein C (APC) in animal models of sepsis was demonstrated, leading to the development of recombinant human activated protein C (rh-APC) as a therapeutic agent. It soon became clear that rather than the anticoagulant and profibrinolytic activities of APC, its anti-inflammatory and cytoprotective properties played a major role in the treatment of patients with severe sepsis. Such properties affect the response to inflammation of endothelial cells and leukocytes and are exerted through binding of APC to at least five receptors with intracellular signaling. The main APC protective mechanism involves binding of the Gla-domain to the endothelial protein C receptor (EPCR) and cleavage of protease activated receptor 1 (PAR-1), eliciting suppression of proinflammatory cytokines synthesis and of intracellular proapoptotic pathways and activation of endothelial barrier properties. However, thrombin cleaves PAR-1 with much higher catalytic efficiency, followed by pro-inflammatory, pro-apoptotic and barrier disruptive intracellular signaling, and it is unclear how APC can exert a protective activity through the cleavage of PAR-1 when thrombin is also present in the same environment. Interestingly, in endothelial cell cultures, PAR-1 cleavage by thrombin results in anti-inflammatory and barrier protective signaling provided occupation of EPCR by the PC gla-domain, raising the possibility that the beneficial effects of rh-APC might be recapitulated in vivo by administration of h-PC zymogen to patients with severe sepsis. Recent reports of h-PC infusion in animal models of sepsis support this hypothesis.