Recombinant murine-activated protein C is neuroprotective in a murine ischemic stroke model

An orthosteric/allosteric bivalent peptide agonist comprising covalently linked protease-activated receptor-derived peptides mimics in vitro and in vivo activities of activated protein C

BACKGROUND

Activated protein C (APC) has anticoagulant and cytoprotective cell-signaling activities, which often require protease-activated receptor (PAR) 1 and PAR3 and PAR cleavages at noncanonical sites (R46-N47 and R41-G42, respectively). Some PAR1-derived (P1) peptides and PAR3-derived (P3) peptides, eg, P1-47-66 and P3-42-65, mimic APC's cell signaling. In anti-inflammatory assays, these 2 peptides at low concentrations synergistically attenuate cellular inflammation.

OBJECTIVES

To determine whether a P1 peptide covalently linked to a P3 peptide mimics APC's anti-inflammatory and endothelial barrier stabilization activities.

METHODS

Anti-inflammatory assays employed stimulated THP-1 cells and caspase-1 measurements. Cultured human EA.hy926 or murine aortic endothelial cells (ECs) exposed to thrombin were monitored for transendothelial electrical resistance. Bivalent covalently linked P1:P3 peptides were studied for APC-like activities.

RESULTS

In anti-inflammatory assays, P1-47-55 was as active as P1-47-66 and some P3 peptides (eg, P3-44-54 and P3-51-65) were as active as P3-42-65. The bivalent P1:P3 peptide comprising P1-47-55-(Gly[10 residues])-P3-51-65 (designated "G10 peptide") was more potently anti-inflammatory than the P1 or P3 peptide alone. In transendothelial electrical resistance studies of thrombin-challenged ECs, P1-47-55 and the G10 peptide mimicked APC's protective actions. In dose-response studies, the G10 peptide was more potent than the P1-47-55 peptide. In murine EC studies, the murine PAR-sequence-derived G10 peptide mimicked murine APC's activity. Anti-PAR1 and anti-PAR3 antibodies, but not anti-endothelial protein C receptor antibodies, abated G10's cytoprotection, showing that G10's actions involve PAR1:PAR3. G10 significantly increased survival in murine endotoxemia.

CONCLUSION

The PAR-sequence-derived G10 peptide is a bivalent agonist that mimics APC's cytoprotective, anti-inflammatory, and endothelial barrier-stabilizing actions and APC's protection against endotoxemic mortality.

Differential Vulnerability and Response to Injury among Brain Cell Types Comprising the Neurovascular Unit

The neurovascular unit (NVU) includes multiple different cell types, including neurons, astrocytes, endothelial cells, and pericytes, which respond to insults on very different time or dose scales. We defined differential vulnerability among these cell types, using response to two different insults: oxygen-glucose deprivation (OGD) and thrombin-mediated cytotoxicity. We found that neurons are most vulnerable, followed by endothelial cells and astrocytes. After temporary focal cerebral ischemia in male rats, we found significantly more injured neurons, compared with astrocytes in the ischemic area, consistent with differential vulnerability in vivo. We sought to illustrate different and shared mechanisms across all cell types during response to insult. We found that gene expression profiles in response to OGD differed among the cell types, with a paucity of gene responses shared by all types. All cell types activated genes relating to autophagy, apoptosis, and necroptosis, but the specific genes differed. Astrocytes and endothelial cells also activated pathways connected to DNA repair and antiapoptosis. Taken together, the data support the concept of differential vulnerability in the NVU and suggest that different elements of the unit will evolve from salvageable to irretrievable on different time scales while residing in the same brain region and receiving the same (ischemic) blood flow. Future work will focus on the mechanisms of these differences. These data suggest future stroke therapy development should target different elements of the NVU differently.

Met343Val mutation disrupts the shuttling of Trp380 leading to a low-activity conformer of activated protein C and causes thrombosis

BACKGROUND

Protein C (PC) pathway serves as a major defense mechanism against thrombosis by the activation of PC through the thrombin-thrombomodulin complex and subsequent inactivation of the activated factor (F)V (FVa) and FVIII (FVIIIa) with the assistance of protein S, thereby contributing to hemostatic balance. We identified 2 unrelated patients who suffered from recurrent thrombosis and carried the same heterozygous mutation c.1153A>G, p.Met343Val (M343V), in PROC gene. This mutation had not been previously reported.

OBJECTIVES

To explore the molecular basis underlying the anticoagulant defect in patients carrying the M343V mutation in PROC.

METHODS

We expressed PC-M343V variant in mammalian cells and characterized its properties through coagulation assays.

RESULTS

Our findings demonstrated that while activation of mutant zymogen by thrombin-thrombomodulin complex was slightly affected, cleavage of chromogenic substrate by APC-M343V was significantly impaired. However, Ca2+ increased the cleavage efficiency by approximately 50%. Additionally, there was a severe reduction in affinity between APC-M343V and Na+. Furthermore, the inhibitory ability of APC-M343V toward FVa was markedly impaired. Structural and simulation analyses suggested that Val343 might disrupt the potential hydrogen bonds with Trp380 and cause Trp380 to orient closer to His211, potentially interfering with substrate binding and destabilizing the catalytic triad of APC.

CONCLUSION

The M343V mutation in patients adversely affects the reactivity and/or folding of the active site as well as the binding of the physiological substrate to the protease, resulting in impaired protein C anticoagulant activity and ultimately leading to thrombosis.

Neutrophil extracellular traps: A novel target for the treatment of stroke

Stroke is a threatening cerebrovascular disease caused by thrombus with high morbidity and mortality rates. Neutrophils are the first to be recruited in the brain after stroke, which aggravate brain injury through multiple mechanisms. Neutrophil extracellular traps (NETs), as a novel regulatory mechanism of neutrophils, can trap bacteria and secret antimicrobial molecules, thereby degrading pathogenic factors and killing bacteria. However, NETs also exacerbate certain non-infectious diseases by activating autoimmune or inflammatory responses. NETs have been found to play important roles in the pathological process of stroke in recent years. In this review, the mechanisms of NETs formation, the physiological roles of NETs, and the dynamic changes of NETs after stroke are summarized. NETs participate in stroke through various mechanisms. NETs promote the coagulation cascade and interact with platelets to induce thrombosis. tPA induces the degranulation of neutrophils to form NETs, leading to hemorrhagic transformation and thrombolytic resistance. NETs aggravate stroke by mediating inflammation, atherosclerosis and vascular injury. In addition, the regulation of NETs in stroke, the potential of NETs as biomarker and the treatment of stroke targeting NETs are discussed. The increasing evidences suggest that NETs may be a potential target for stroke treatment. Inhibition of NETs formation or promotion of NETs degradation plays protective effects in stroke. However, how to avoid the adverse effects of NETs-targeted therapy deserves further study. In summary, this review provides a reference for the pathogenesis, drug targets, biomarkers and drug development of NETs in stroke.

3K3A-Activated Protein C Prevents Microglia Activation, Inhibits NLRP3 Inflammasome and Limits Ocular Inflammation

3K3A-Activated Protein C (APC) is a recombinant variant of the physiological anticoagulant APC with pleiotropic cytoprotective properties albeit without the bleeding risks. The anti-inflammatory activities of 3K3A-APC were demonstrated in multiple preclinical injury models, including various neurological disorders. We determined the ability of 3K3A-APC to inhibit ocular inflammation in a murine model of lipopolysaccharide (LPS)-induced uveitis. Leukocyte recruitment, microglia activation, NLRP3 inflammasome and IL-1β levels were assessed using flow cytometry, retinal cryosection histology, retinal flatmount immunohistochemistry and vascular imaging, with and without 3K3A-APC treatment. LPS triggered robust inflammatory cell recruitment in the posterior chamber. The 3K3A-APC treatment significantly decreased leukocyte numbers and inhibited leukocyte extravasation from blood vessels into the retinal parenchyma to a level similar to controls. Resident microglia, which underwent an inflammatory transition following LPS injection, remained quiescent in eyes treated with 3K3A-APC. An inflammation-associated increase in retinal thickness, observed in LPS-injected eyes, was diminished by 3K3A-APC treatment, suggesting its clinical relevancy. Finally, 3K3A-APC treatment inhibited inflammasome activation, determined by lower levels of NLRP3 and its downstream effector IL-1β. Our results highlight the anti-inflammatory properties of 3K3A-APC in ocular inflammation and suggest its potential use as a novel treatment for retinal diseases associated with inflammation.

Blocking human protein C anticoagulant activity improves clotting defects of hemophilia mice expressing human protein C

We generated novel hemophilia A or B mice expressing human protein C.

Selectively blocking the anticoagulant activity of human activated protein C improves the clotting defects in hemophilia mice.

Hemophilia A and B are hereditary coagulation defects resulting in unstable blood clotting and recurrent bleeding. Current factor replacement therapies have major limitations such as the short half-life of the factors and development of inhibitors. Alternative approaches to rebalance the hemostasis by inhibiting the anticoagulant pathways have recently gained considerable interest. In this study, we tested the therapeutic potential of a monoclonal antibody, HAPC1573, that selectively blocks the anticoagulant activity of human activated protein C (APC). We generated F8^−/− or F9^−/− hemophilia mice expressing human protein C by genetically replacing the murine Proc gene with the human PROC. The resulting PROC^+/+;F8^−/− or PROC^+/+;F9^−/− mice had bleeding characteristics similar to their corresponding F8^−/− or F9^−/− mice. Pretreating the PROC^+/+;F8^−/− mice with HAPC1573 shortened the tail bleeding time. HAPC1573 pretreatment significantly reduced mortality and alleviated joint swelling, similar to those treated with either FVIII or FIX, of either PROC^+/+;F8^−/− or PROC^+/+;F9^−/− mice in a needle puncture–induced knee-joint bleeding model. Additionally, we found that HAPC1573 significantly improved the thrombin generation of PROC^+/+;F8^−/− mice but not F8^−/− mice, indicating that HAPC1573 enhanced the coagulant activity of hemophilia mice by modulating human APC in vivo. We further documented that HAPC1573 inhibited the APC anticoagulant activity to improve the clotting time of human plasma deficient of FVIII, FIX, FXI, FVII, VWF, FV, or FX. These results demonstrate that selectively blocking the anticoagulant activity of human APC may be an effective therapeutic and/or prophylactic approach for bleeding disorders lacking FVIII, FIX, or other clotting factors.

Collateral Circulation Augmentation and Neuroprotection as Adjuvant to Mechanical Thrombectomy in Acute Ischemic Stroke

PURPOSE OF THE REVIEW

Mechanical thrombectomy (MT)-mediated endovascular recanalization has dramatically transformed treatment and outcomes after acute ischemic stroke caused by a large vessel occlusion (LVO). Current guidelines recommend MT up to 24 hours from stroke onset in carefully selected patients based on favorable clinical and imaging parameters. Despite optimal patient selection and low complication rates with current recanalization technology, approximately 1 in 2 patients with LVO stroke do not achieve functional independence at 3 months. This ceiling effect of MT efficacy may be explained by ischemic core expansion into the ischemic penumbra before recanalization and neuronal loss occurring after recanalization. Factors affecting the efficacy of MT, or the degree of irreversible injury, include time from symptom onset to recanalization, collateral circulation status, and differences in neuronal vulnerability. The purpose of this brief review is to discuss potential targets for neuroprotection, present and future potential pharmacologic and nonpharmacologic agents, and the data available in the literature.

RECENT FINDINGS

In experimental ischemia models, several authors reported that pharmacologic and nonpharmacologic agents are able to slow the progression of ischemic core expansion. However, in the era of unsuccessful recanalization of the occluded artery, several neuroprotective agents that were promising in the preclinical stage failed phase II/III clinical trials.

SUMMARY

Providing neuroprotection before and after recanalization of an LVO may play an important role in improving outcomes in the era of MT. Neuroprotection is classically defined as a process that results in the salvage, recovery, or regeneration of neuronal (and other supporting CNS cell) structure or function. The advent of successful recanalization of acute LVO by MT in the majority of patients may spur the growth of effective neuroprotection.

L-ARGININE AND L-GLUTAMIC ACID INCREASE THE CONTENT OF PROTEIN C IN THE EARLY STAGES OF ISOLATION FROM DONOR PLASMA

Current large-scale production of blood-derived pharmacological preparations is aimed at expanding the list of products and deeper extraction of target proteins especially at the pre-purification stage. In particular, this problem becomes critical for the isolation of proteins like protein C (PC), which is present in plasma in trace amounts. Aim. We aimed to improve the buffer composition to minimize the interaction of PC with other proteins and lipids that are inevitably present in the stock material. Methods. The content of protein C in plasma and its derivatives was assessed by the amidolytic activity to the chromogenic substrate S2366. A decrease in homologous impurities and plasma enrichment with protein C was provided by selective bulk adsorption on DEAE-cellulose. Results. Here we describe that an equimolar mixture of two amino acids (L-arginine and L-glutamic acid) essentially increased the content of protein C at the stage of cryo-depleted plasma pre-purification, including initial dilution and subsequent enrichment of plasma with protein C due to selective bulk adsorption on DEAE- cellulose. Additionally, it was revealed that solutions of these amino acids, when combined, inhibit the induced amidolytic activity of protein C and increase its solubility (in contrast to other plasma proteases). Conclusion. Pre-adding of a mixture of amino acids L-arginine and L-glutamic acid to cryo-depleted plasma significantly optimizes the pre-purification stage of protein C, providing a 5-fold increase in its yield after elution from DEAE-cellulose.

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.

A Novel Highly Sensitive Method for Measuring Inflammatory Neural-Derived APC Activity in Glial Cell Lines, Mouse Brain and Human CSF

Background: Neural inflammation is linked to coagulation. Low levels of thrombin have a neuroprotective effect, mediated by activated protein C (APC). We describe a sensitive novel method for the measurement of APC activity at the low concentrations found in neural tissue. Methods: APC activity was measured using a fluorogenic substrate, Pyr-Pro-Arg-AMC, cleaved preferentially by APC. Selectivity was assessed using specific inhibitors and activators. APC levels were measured in human plasma, in glia cell lines, in mice brain slices following mild traumatic brain injury (mTBI) and systemic lipopolysaccharide (LPS) injection, and in cerebrospinal fluid (CSF) taken from viral meningoencephalitis patients and controls. Results: Selectivity required apixaban and alpha-naphthylsulphonylglycyl-4-amidinophenylalanine piperidine (NAPAP). APC levels were easily measurable in plasma and were significantly increased by Protac and CaCl₂. APC activity was significantly higher in the microglial compared to astrocytic cell line and specifically lowered by LPS. Brain APC levels were higher in posterior regions and increased by mTBI and LPS. Highly elevated APC activity was measured in viral meningoencephalitis patients CSF. Conclusions: This method is selective and sensitive for the measurement of APC activity that significantly changes during inflammation in cell lines, animal models and human CSF.

Acute effects of human protein S administration after traumatic brain injury in mice

Despite years of effort, no effective acute phase treatment has been discovered for traumatic brain injury. One impediment to successful drug development is entangled secondary injury pathways. Here we show that protein S, a natural multifunctional protein that regulates coagulation, inflammation, and apoptosis, is able to reduce the extent of multiple secondary injuries in traumatic brain injury, and therefore improve prognosis. Mice subjected to controlled cortical impact were treated acutely (10–15 minutes post-injury) with a single dose of either protein S (1 mg/kg) or vehicle phosphate buffered saline via intravenous injection. At 24 hours post-injury, compared to the non-treated group, the protein S treated group showed substantial improvement of edema and fine motor coordination, as well as mitigation of progressive tissue loss. Immunohistochemistry and western blot targeting caspase-3, B-cell lymphoma 2 (Bcl-2) along with terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay revealed that apoptosis was suppressed in treated animals. Immunohistochemistry targeting CD11b showed limited leukocyte infiltration in the protein S-treated group. Moreover, protein S treatment increased the ipsilesional expression of aquaporin-4, which may be the underlying mechanism of its function in reducing edema. These results indicate that immediate intravenous protein S treatment after controlled cortical impact is beneficial to traumatic brain injury prognosis. Animal Use Protocols (AUPs) were approved by the University Committee on Animal Resources (UCAR) of University of Rochester Medical Center (approval No. UCAR-2008-102R) on November 12, 2013.

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.

The structure-function relationship of activated protein C

Protein C is the central enzyme of the natural anticoagulant pathway and its activated form APC (activated protein C) is able to proteolyse non-active as well as active coagulation factors V and VIII. Proteolysis renders these cofactors inactive, resulting in an attenuation of thrombin formation and overall down-regulation of coagulation. Presences of the APC cofactor, protein S, thrombomodulin, endothelial protein C receptor and a phospholipid surface are important for the expression of anticoagulant APC activity. Notably, APC also has direct cytoprotective effects on cells: APC is able to protect the endothelial barrier function and expresses anti-inflammatory and anti-apoptotic activities. Exact molecular mechanisms have thus far not been completely described but it has been shown that both the protease activated receptor 1 and EPCR are essential for the cytoprotective activity of APC. Recently it was shown that also other receptors like sphingosine 1 phosphate receptor 1, Cd11b/CD18 and tyrosine kinase with immunoglobulin-like and EGFlike domains 2 are likewise important for APC signalling. Mutagenesis studies are being performed to map the various APC functions and interactions onto its 3D structure and to dissect anticoagulant and cytoprotective properties. The results of these studies have provided a wealth of structure-function information. With this review we describe the state-of-the-art of the intricate structure-function relationships of APC, a protein that harbours several important functions for the maintenance of both humoral and tissue homeostasis.

Lessons from natural and engineered mutations

Can adjunctive therapies augment the efficacy of endovascular thrombolysis? A potential role for activated protein C

In the management of acute ischemic stroke, vessel recanalization correlates with functional status, mortality, cost, and other outcome measures. Thrombolysis with intravenous tissue plasminogen activator has many limitations that restrict its applicability, but recent advances in the development of mechanical thrombectomy devices as well as improved systems of stroke care have resulted in greater likelihood of vessel revascularization. Nonetheless, there remains substantial discrepancy between rates of recanalization and rates of favorable outcome. The poor neurological recovery among some stroke patients despite successful recanalization confirms the need for adjuvant pharmacological therapy for neuroprotection and/or neurorestoration. Prior clinical trials of such drugs may have failed due to the inability of the agent to access the ischemic tissue beyond the occluded artery. A protocol that couples revascularization with concurrent delivery of a neuroprotectant drug offers the potential to enhance the benefit of thrombolysis. Analogs of activated protein C (APC) exert pleiotropic anti-inflammatory, anti-apoptotic, antithrombotic, cytoprotective, and neuroregenerative effects in ischemic stroke and thus appear to be promising candidates for this novel approach. A multicenter, prospective, double-blinded, dose-escalation Phase 2 randomized clinical trial has enrolled 110 patients to assess the safety, pharmacokinetics, and efficacy of human recombinant 3K3A-APC following endovascular thrombolysis. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Organ-specific effects on inflammation and apoptosis of recombinant human activated protein C in a murine model of sepsis

There is legitimate interest in the effects of recombinant human activated protein C (rhAPC) on various organs and individual patients, but the specific effects on organ tissues during early sepsis remain unknown. Differences in the levels of organ damage may influence responses to drug therapy. We aimed to investigate whether rhAPC induces organ-specific effects on inflammation and apoptosis using randomized, experimental trials with male NMRI mice. Animals underwent caecal ligation and puncture, and after 12 h, sepsis inflammation and apoptosis were assessed by plasma cytokines, gene expression ratios and immunohistochemistry (IHC). RhAPC-treated animals exhibited increased physical activity and decreased cytokine release compared to untreated animals (interleukin-6 reduction 58%, P < 0.001). CD14 expression was higher in the heart and liver and decreased upon rhAPC application in the heart (−35%), liver and kidney (both −60%). Macrophage inflammatory protein 2 (MIP2) expression decreased in the heart (−58%) but not in the liver or kidney. IHC revealed decreased cleaved caspase-3 in the heart and kidney due to rhAPC intervention. Preservation of the endothelial PC receptor was significant only in the heart during sepsis ( P = 0.007). In early polymicrobial sepsis, inflammation was more pronounced in the heart and liver compared to the kidney. RhAPC exhibited protective effects, especially in the heart tissue, and led to reduced plasma levels of pro-inflammatory cytokines and improved physical activity.

The role of extracellular histone in organ injury

Histones are intra-nuclear cationic proteins that are present in all eukaryotic cells and are highly conserved across species. Within the nucleus, they provide structural stability to chromatin and regulate gene expression. Histone may be released into the extracellular space in three forms: freely, as a DNA-bound nucleosome or as part of neutrophil extracellular traps, and all three can be detected in serum after significant cellular death such as sepsis, trauma, ischaemia/reperfusion injury and autoimmune disease. Once in the extracellular space, histones act as damage-associated molecular pattern proteins, activating the immune system and causing further cytotoxicity. They interact with Toll-like receptors (TLRs), complement and the phospholipids of cell membranes inducing endothelial and epithelial cytotoxicity, TLR2/TLR4/TLR9 activation and pro-inflammatory cytokine/chemokine release via MyD88, NFκB and NLRP3 inflammasome-dependent pathways. Drugs that block the release of histone, neutralise circulating histone or block histone signal transduction provide significant protection from mortality in animal models of acute organ injury but warrant further research to inform future clinical applications.

Activated protein C promotes neuroprotection: mechanisms and translation to the clinic

Activated protein C (APC) is a plasma serine protease that is capable of antithrombotic, anti-inflammatory, anti-apoptotic, and cell-signaling activities. Animal injury studies show that recombinant APC and some of its mutants are remarkably therapeutic for a wide range of injuries. In particular, for neurologic injuries, APC reduces damage caused by ischemia/reperfusion in the brain, by acute brain trauma, and by chronic neurodegenerative conditions. For these neuroprotective effects, APC requires endothelial cell protein C receptor. APC activates cell signaling networks with alterations in gene expression profiles by activating protease activated receptors 1 and 3. To minimize APC-induced bleeding risk, APC variants were engineered to lack > 90% anticoagulant activity but retain normal cell signaling. The neuroprotective APC mutant, 3K3A-APC which has Lys191-193 mutated to Ala191-193, is very neuroprotective and it is currently in clinical trials for ischemic stroke.

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.

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.

Hereditary protein S deficiency leads to ischemic stroke

Hereditary protein S (PS) deficiency is an independent risk factor for venous thromboembolism. However, the correlation between PS and arterial thrombotic disease, such as cerebral thrombosis, is not clear. The present study focused on the molecular mechanisms underlying ischemic stroke caused by a PS gene mutation in one family. The activity of antithrombin, protein C and PS in the plasma of the proband was measured, and the genes encoding PS were amplified and sequenced. The cellular localization and expression of PS were analyzed in HEK‑293 cells. The proband was a 50‑year‑old male. Plasma PS activity of the proband was 38.9%, which was significantly decreased compared with normal levels. Sequencing analysis revealed a PROS1 c.1486_1490delGATTA mutation on exon 12. This frameshift mutation converts Asp496 in the precursor PS into the termination codon. In addition, the PROS1 mutation was correlated with low PS activity in the family. Functional tests revealed that the mutant protein aggregated in the cytoplasm and its secretion and expression decreased. In conclusion, protein S mutation appeared to be the primary cause of thrombosis in the family of the present study. However, the correlation between PS deficiency and ischemic stroke requires further investigation.

Effects of exogenous recombinant APC in mouse models of ischemia reperfusion injury and of atherosclerosis

Activated protein C (APC) is a serine protease that has both anticoagulant and cytoprotective properties. The cytoprotective effects are protease activated receptor 1 (PAR-1) and endothelial protein C receptor (EPCR) dependent and likely underlie protective effects of APC in animal models of sepsis, myocardial infarction and ischemic stroke. S360A-(A)PC, a variant (A)PC that has no catalytic activity, binds EPCR and shifts pro-inflammatory signaling of the thrombin-PAR-1 complex to anti-inflammatory signaling. In this study we investigated effects of human (h)wt-PC, hS360A-PC, hwt-APC and hS360A-APC in acute (mouse model of acute myocardial ischemia/reperfusion (I/R) injury) and chronic inflammation (apoE-/- mouse model of atherosclerosis). All h(A)PC variants significantly reduced myocardial infarct area (p<0.05) following I/R injury. IL-6 levels in heart homogenates did not differ significantly between sham, placebo and treatment groups in I/R injury. None of the h(A)PC variants decreased number and size of atherosclerotic plaques in apoE-/- mice. Only hS360A-APC slightly affected phenotype of plaques. IL-6 levels in plasma were significantly (p<0.001) decreased in hwt-APC and hS360A-PC treated mice. In the last group levels of monocyte chemotactic protein 1 (MCP-1) were significantly increased (p<0.05). In this study we show that both hwt and hS360A-(A)PC protect against acute myocardial I/R injury, which implies that protection from I/R injury is independent of the proteolytic activity of APC. However, in the chronic atherosclerosis model hwt and hS360-(A)PC had only minor effects. When the dose, species and mode of (A)PC administration will be adjusted, we believe that (A)PC will have potential to influence development of chronic inflammation as occurring during atherosclerosis as well.

Activated protein C analog promotes neurogenesis and improves neurological outcome after focal ischemic stroke in mice via protease activated receptor 1

3K3A-APC is a recombinant analog of activated protein C (APC) which is an endogenous protease with multiple functions in the body. Compared to APC, 3K3A-APC has reduced anticoagulant activity but preserved cell signaling activities. In the brain, 3K3A-APC exerts neuroprotective effects after an acute or chronic injury. 3K3A-APC is currently under clinical assessment as a neuroprotective agent following acute ischemic stroke. Whether 3K3A-APC can influence post-ischemic neurogenesis and improve neurological outcome by promoting brain repair remains unknown. Here we show that murine 3K3A-APC 0.8mg/kg intraperitoneally given at 12h, 1, 3, 5 and 7 days after permanent distal middle cerebral artery occlusion (dMCAO) in mice compared to vehicle improves significantly sensorimotor and locomotor activity 7 and 14 days after stroke, reduces infarct and edema volumes 7 days after stroke by 43% (P<0.05) and 50% (P<0.05), respectively, increases the number of newly formed neuroblasts in the subventricular zone, corpus callosum and the peri-infarct area 7 days after stroke by 2.2-fold, 2.3-fold and 2.2-fold (P<0.05), respectively, and increases the cortical width index 14 days after stroke by 28% (P<0.05). Functional outcome in 3K3A-APC-treated group, but not in vehicle-treated group, correlated inversely with the reductions in the infarct volume, and positively with the number of neuroblasts migrating in the peri-infarct area and the cortical width index. The effects of 3K3A-APC on neuroprotection, neurogenesis and brain repair were lost in protease activated receptor 1 (PAR1) deficient mice. Thus, late therapy with 3K3A-APC is neuroprotective and promotes stroke-induced neurogenesis and repair through PAR1 in mice.

Antibody SPC-54 provides acute in vivo blockage of the murine protein C system

Multiple protective effects of pharmacological activated protein C (APC) are reported in several organ pathologies. To help evaluate the endogenous murine PC system, we characterized a rat monoclonal anti-mouse PC antibody, SPC-54, which inhibited the amidolytic and anticoagulant activities of murine APC by>95%. SPC-54 blocked active site titration of purified APC using the active site titrant, biotinylated FPR-chloromethylketone, showing that SPC-54 blocks access to APC's active site to inhibit all enzymatic activity. A single injection of SPC-54 (10mg/kg) neutralized circulating PC in mice for at least 7days, and immunoblotting and immuno-precipitation with protein G-agarose confirmed that SPC-54 in vivo was bound to PC in plasma. Pre-infusion of SPC-54 in tissue factor-induced murine acute thromboembolism experiments caused a major decrease in mean survival time compared to controls (7min vs. 42.5min, P=0.0016). SPC-54 decreased lung perfusion in this model by 54% when monitored by vascular perfusion methodologies using infrared fluorescence of Evans blue dye. In LD50 endotoxemia murine models, SPC-54 infused at 7hr after endotoxin administration increased mortality from 42% to 100% (P<0.001). In summary, monoclonal antibody SPC-54 ablates in vitro and in vivo APC protective functions and enzymatic activity. The ability of SPC-54 to block the endogenous PC/APC system provides a powerful tool to understand better the role of the endogenous PC system in murine injury models and in cell bioassays and also to neutralize the enzymatic activities of murine APC in any assay system.

Novel association of a PROC variant with ischemic stroke in a Chinese Han population

Protein C (PC) is a well-characterized anticoagulant enzyme. However, the association between PC and ischemic stroke (IS) remains controversial. The aim of the present study was to investigate whether any genetic variant in the human protein C gene (PROC) was associated with susceptibility to IS in the Chinese Han population. All exons and the 5'- and 3'-untranslated regions of PROC were initially sequenced to identify informative variants. Potential abnormal variants were analyzed in a population of 788 IS patients and 1,200 healthy controls. The analysis was stratified by stroke etiology, and the results were replicated in 262 IS patients and 288 healthy controls. Finally, functional studies were performed to evaluate the effects of the variant. A three-nucleotide duplication/deletion variant (c.574_576del) was identified and found to be significantly associated with IS (OR 2.56, 95 % CI 1.45-4.52, P = 0.001). Stratification by stroke etiology after adjustment for IS risk factors showed that this association persisted in the lacunar and cardioembolic subtypes (P < 0.001 and P = 0.008, respectively) but not in the atherothrombotic and undetermined subtypes (P = 0.070 and P = 0.998, respectively). The functional studies showed a significant difference in the anticoagulant activity of PC in c.574_576del carriers and non-carriers (P < 0.001). Our results suggested that the novel PROC c.574_576del variant is a possible genetic determinant of an increased risk of IS and diminished anticoagulant activity of PC.

Extracellular chromatin is an important mediator of ischemic stroke in mice

OBJECTIVE

Recently, a growing number of studies have revealed a prothrombotic and cytotoxic role for extracellular chromatin. Cerebral ischemia/reperfusion injury is characterized by a significant amount of cell death and neutrophil activation, both of which may result in the release of chromatin. The goal of this study was to assess the effect of extracellular chromatin in ischemic stroke using a mouse model of transient middle cerebral artery occlusion.

METHODS AND RESULTS

Similar to reports in stroke patients, we observed increased levels of circulating nucleosomes and DNA after ischemic stroke in mice. In addition, we observed that general hypoxia also augmented extracellular chromatin. We hypothesized that targeting extracellular chromatin components would be protective in ischemic stroke. Indeed, treatment with recombinant human DNase 1 significantly improved stroke outcome. Neutralization of histones using an antihistone antibody was also protective as evidenced by smaller infarct volumes, whereas increasing levels of extracellular histones via histone infusion exacerbated stroke outcome by increasing infarct size and worsening functional outcome.

CONCLUSIONS

Our results indicate that extracellular chromatin is generated and is detrimental during cerebral ischemia/reperfusion in mice. Targeting DNA and histones may be a new therapeutic strategy to limit injury resulting from ischemic stroke.

Protein C anticoagulant and cytoprotective pathways

Plasma protein C is a serine protease zymogen that is transformed into the active, trypsin-like protease, activated protein C (APC), which can exert multiple activities. For its anticoagulant action, APC causes inactivation of the procoagulant cofactors, factors Va and VIIIa, by limited proteolysis, and APC's anticoagulant activity is promoted by protein S, various lipids, high-density lipoprotein, and factor V. Hereditary heterozygous deficiency of protein C or protein S is linked to moderately increased risk for venous thrombosis, while a severe or total deficiency of either protein is linked to neonatal purpura fulminans. In recent years, the beneficial direct effects of APC on cells which are mediated by several specific receptors have become the focus of much attention. APC-induced signaling can promote multiple cytoprotective actions which can minimize injuries in various preclinical animal injury models. Remarkably, pharmacologic therapy using APC demonstrates substantial neuroprotective effects in various murine injury models, including ischemic stroke. This review summarizes the molecules that are central to the protein C pathways, the relationship of pathway deficiencies to venous thrombosis risk, and mechanisms for the beneficial effects of APC.

Human thrombomodulin knock-in mice reveal differential effects of human thrombomodulin on thrombosis and atherosclerosis

OBJECTIVE

We sought to develop a murine model to examine the antithrombotic and antiinflammatory functions of human thrombomodulin in vivo.

METHODS AND RESULTS

Knock-in mice that express human thrombomodulin from the murine thrombomodulin gene locus were generated. Compared with wild-type mice, human thrombomodulin knock-in mice exhibited decreased protein C activation in the aorta (P<0.01) and lung (P<0.001). Activation of endogenous protein C following infusion of thrombin was decreased by 90% in knock-in mice compared with wild-type mice (P<0.05). Carotid artery thrombosis induced by photochemical injury occurred more rapidly in knock-in mice (12±3 minutes) than in wild-type mice (31±6 minutes; P<0.05). No differences in serum cytokine levels were detected between knock-in and wild-type mice after injection of endotoxin. When crossed with apolipoprotein E-deficient mice and fed a Western diet, knock-in mice had a further decrease in protein C activation but did not exhibit increased atherosclerosis.

CONCLUSION

Expression of human thrombomodulin in place of murine thrombomodulin produces viable mice with a prothrombotic phenotype but unaltered responses to systemic inflammatory or atherogenic stimuli. This humanized animal model will be useful for investigating the function of human thrombomodulin under pathophysiological conditions in vivo.

The development of inflammatory joint disease is attenuated in mice expressing the anticoagulant prothrombin mutant W215A/E217A

Thrombin is a positive mediator of thrombus formation through the proteolytic activation of protease-activated receptors (PARs), fibrinogen, factor XI (fXI), and other substrates, and a negative regulator through activation of protein C, a natural anticoagulant with anti-inflammatory/cytoprotective properties. Protease-engineering studies have established that 2 active-site substitutions, W215A and E217A (fII(WE)), result in dramatically reduced catalytic efficiency with procoagulant substrates while largely preserving thrombomodulin (TM)-dependent protein C activation. To explore the hypothesis that a prothrombin variant favoring antithrombotic pathways would be compatible with development but limit inflammatory processes in vivo, we generated mice carrying the fII(WE) mutations within the endogenous prothrombin gene. Unlike fII-null embryos, fII(WE/WE) mice uniformly developed to term. Nevertheless, these mice ultimately succumbed to spontaneous bleeding events shortly after birth. Heterozygous fII(WT/WE) mice were viable and fertile despite a shift toward an antithrombotic phenotype exemplified by prolonged tail-bleeding times and times-to-occlusion after FeCl₃ vessel injury. More interestingly, prothrombin(WE) expression significantly ameliorated the development of inflammatory joint disease in mice challenged with collagen-induced arthritis (CIA). The administration of active recombinant thrombin(WE) also suppressed the development of CIA in wild-type mice. These studies provide a proof-of-principle that pro/thrombin variants engineered with altered substrate specificity may offer therapeutic opportunities for limiting inflammatory disease processes.

Thrombin-induced activation of astrocytes in mixed rat hippocampal cultures is inhibited by soluble thrombomodulin

Thrombin, a serine protease known for its role in coagulation, also induces a variety of protease activated receptor (PAR)-mediated responses in the central nervous system that contribute to many brain pathologies. Since the proteolytic specificity of thrombin is uniquely controlled by thrombomodulin (TM), we investigated the mechanisms by which thrombin and a recombinant soluble form of human TM (Solulin, INN: sothrombomodulin alpha; rhsTM) could influence rat hippocampal cultures. Treatment of hippocampal cultures with thrombin for up to 48h resulted in a significant morphological rearrangement with the formation of expansive cell-free areas (CFAs) and a reduction in cell viability; both effects were blocked by rhsTM. Treatment with the selective PAR-1 agonist, TRAP (SFLLRN) caused the formation of CFAs, suggesting that CFA formation involved PAR-1 signaling. Astrocytes prepared from PAR-1(-/-) mice also had an attenuated CFA response to thrombin. Thrombin-induced CFA formation was a consequence of cell movement and substantial changes in cell morphology, rather than due to cell detachment. Immunocytochemical and functional analyses revealed that the thrombin-sensitive cells within these hippocampal cultures were astrocytes. The effects of thrombin on CFA development were mediated by astrocyte-specific release of intracellular calcium and signalling through ERK1/2. rhsTM was able to attenuate thrombin-induced ERK1/2 phosphorylation. Finally, astrocytes were shown to maintain thrombin-sensitivity following neuronal depletion with NMDA, a result which was confirmed with pure astrocyte cultures. Hence thrombin mediates PAR-1-induced activation of hippocampal astrocytes, but not neurons, in a process that can be modulated by rhsTM.

Recombinant human activated protein C attenuates cardiovascular and microcirculatory dysfunction in acute lung injury and septic shock

Introduction

This prospective, randomized, controlled, experimental animal study looks at the effects of recombinant human activated protein C (rhAPC) on global hemodynamics and microcirculation in ovine acute lung injury (ALI) and septic shock, resulting from smoke inhalation injury.

Methods

Twenty-one sheep (37 ± 2 kg) were operatively prepared for chronic study and randomly allocated to either the sham, control, or rhAPC group (n = 7 each). The control and rhAPC groups were subjected to insufflation of four sets of 12 breaths of cotton smoke followed by instillation of live Pseudomonas aeruginosa into both lung lobes, according to an established protocol. Healthy sham animals were not subjected to the injury and received only four sets of 12 breaths of room air and instillation of the vehicle (normal saline). rhAPC (24 μg/kg/hour) was intravenously administered from 1 hour post injury until the end of the 24-hour experiment. Regional microvascular blood flow was analyzed using colored microspheres. All sheep were mechanically ventilated with 100% oxygen, and fluid resuscitated with lactated Ringer's solution to maintain hematocrit at baseline levels.

Results

The rhAPC-associated reduction in heart malondialdehyde (MDA) and heart 3-nitrotyrosine (a reliable indicator of tissue injury) levels occurred parallel to a significant increase in mean arterial pressure and to a significant reduction in heart rate and cardiac output compared with untreated controls that showed a typical hypotensive, hyperdynamic response to the injury (P < 0.05). In addition, rhAPC significantly attenuated the changes in microvascular blood flow to the trachea, kidney, and spleen compared with untreated controls (P < 0.05 each). Blood flow to the ileum and pancreas, however, remained similar between groups. The cerebral blood flow as measured in cerebral cortex, cerebellum, thalamus, pons, and hypothalamus, was significantly increased in untreated controls, due to a loss of cerebral autoregulation in septic shock. rhAPC stabilized cerebral blood flow at baseline levels, as in the sham group.

Conclusions

We conclude that rhAPC stabilized cardiovascular functions and attenuated the changes in visceral and cerebral microcirculation in sheep suffering from ALI and septic shock by reduction of cardiac MDA and 3-nitrotyrosine.

Endothelial protein C receptor-expressing hematopoietic stem cells reside in the perisinusoidal niche in fetal liver

Hematopoietic stem cells (HSCs) are maintained in specialized niches in adult bone marrow. However, niche and HSC maintenance mechanism in fetal liver (FL) still remains unclear. Here, we investigated the niche and the molecular mechanism of HSC maintenance in mouse FL using HSCs expressing endothelial protein C receptor (EPCR). The antiapoptotic effect of activated protein C (APC) on EPCR(+) HSCs and the expression of protease-activated receptor 1 (Par-1) mRNA in these cells suggested the involvement of the cytoprotective APC/EPCR/Par-1 pathway in HSC maintenance. Immunohistochemistry revealed that EPCR(+) cells were localized adjacent to, or integrated in, the Lyve-1(+) sinusoidal network, where APC and extracellular matrix (ECM) are abundant, suggesting that HSCs in FL were maintained in the APC- and ECM-rich perisinusoidal niche. EPCR(+) HSCs were in a relatively slow cycling state, consistent with their high expression levels of p57 and p18. Furthermore, the long-term reconstitution activity of EPCR(+) HSCs decreased significantly after short culture but not when cocultured with feeder layer of FL-derived Lyve-1(+) cells, which suggests that the maintenance of the self-renewal activity of FL HSCs largely depended on the interaction with the perisinusoidal niche. In conclusion, EPCR(+) HSCs resided in the perisinusoidal niche in mouse FL.

Activated protein C action in inflammation

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

Roles of inflammation and the activated protein C pathway in the brain edema associated with cerebral venous sinus thrombosis

BACKGROUND AND PURPOSE

Increased blood-brain barrier (BBB) permeability, brain edema, and hemorrhage are important consequences of cerebral venous sinus thrombosis (CVST). The objective of this study was to define the role of the protein C pathway in the BBB permeability and edema elicited by experimental CVST. The role of neutrophil recruitment was also evaluated.

METHODS

Edema, BBB permeability, leukocyte-endothelial cell adhesion (LECA) and inflammatory cytokine levels were monitored in a murine model of CVST. The role of activated protein C (APC) was assessed in wild type mice (WT) receiving APC neutralizing antibody and in endothelial protein C receptor overexpressing mice (EPCR-tg). Neutrophil involvement was evaluated using an anti-CD18 antibody (Ab) and antineutrophil serum.

RESULTS

Brain edema and increases in BBB permeability and LECA were noted 48 hours after CVST. APC immunoblockade exacerbated these responses, while EPCR-tg exhibited blunted responses, as did WT treated with either antineutrophil serum or the CD18 Ab.

CONCLUSIONS

The protein C pathway protects the brain against the deleterious microvascular responses to CVST, a response that appears to be linked to the recruitment of inflammatory cells.

Species-specific anticoagulant and mitogenic activities of murine protein S

BACKGROUND

The protein C pathway down-regulates thrombin generation and promotes cytoprotection during inflammation and stress. In preclinical studies using models of murine injury (e.g., sepsis and ischemic stroke), murine protein S may be required because of restrictive species specificity.

DESIGN AND METHODS

We prepared and characterized recombinant murine protein S using novel coagulation assays, immunoassays, and cell proliferation assays.

RESULTS

Purified murine protein S had good anticoagulant co-factor activity for murine activated protein C, but not for human activated protein C, in mouse or rat plasma. In human plasma, murine protein S was a poor co-factor for murine activated protein C and had no anticoagulant effect with human activated protein C, suggesting protein S species specificity for factor V in addition to activated protein C. We estimated that mouse plasma contains 22+/-1 microg/mL protein S and developed assays to measure activated protein C co-factor activity of the protein S in murine plasma. Activated protein C-independent anticoagulant activity of murine protein S was demonstrable and quantifiable in mouse plasma, and this activity was enhanced by exogenous murine protein S. Murine protein S promoted the proliferation of mouse and human smooth muscle cells. The potency of murine protein S was higher for mouse cells than for human cells and similarly, human protein S was more potent for human cells than for mouse cells.

CONCLUSIONS

The spectrum of bioactivities of recombinant murine protein S with mouse plasma and smooth muscle cells is similar to that of human protein S. However, in vitro and in vivo studies of the protein C pathway in murine disease models are more appropriately performed using murine protein S. This study extends previous observations regarding the remarkable species specificity of protein S to the mouse.

Mouse recombinant protein C variants with enhanced membrane affinity and hyper-anticoagulant activity in mouse plasma

Mouse anticoagulant protein C (461 residues) shares 69% sequence identity with its human ortholog. Interspecies experiments suggest that there is an incompatibility between mouse and human protein C, such that human protein C does not function efficiently in mouse plasma, nor does mouse protein C function efficiently in human plasma. Previously, we described a series of human activated protein C (APC) Gla domain mutants (e.g. QGNSEDY-APC), with enhanced membrane affinity that also served as superior anticoagulants. To characterize these Gla mutants further in mouse models of diseases, the analogous mutations were now made in mouse protein C. In total, seven mutants (mutated at one or more of positions P(10)S(12)D(23)Q(32)N(33)) and wild-type protein C were expressed and purified to homogeneity. In a surface plasmon resonance-based membrane-binding assay, several high affinity protein C mutants were identified. In Ca(2+) titration experiments, the high affinity variants had a significantly reduced (four-fold) Ca(2+) requirement for half-maximum binding. In a tissue factor-initiated thrombin generation assay using mouse plasma, all mouse APC variants, including wild-type, could completely inhibit thrombin generation; however, one of the variants denoted mutant III (P10Q/S12N/D23S/Q32E/N33D) was found to be a 30- to 50-fold better anticoagulant compared to the wild-type protein. This mouse APC variant will be attractive to use in mouse models aiming to elucidate the in vivo effects of APC variants with enhanced anticoagulant activity.

Endothelial protein C receptor is expressed in rat cortical and hippocampal neurons and is necessary for protective effect of activated protein C at glutamate excitotoxicity

Activated protein C (APC) is an anticoagulant and anti-inflammatory factor that acts via endothelial protein C receptor (EPCR). Interestingly, APC also exhibits neuroprotective activities. In the present study, we demonstrate for the first time expression of EPCR, the receptor for APC, in rat cortical and hippocampal neurons. Moreover, exposing the neurons to glutamate excitotoxicity we studied the functional consequence of the expression of EPCR. By cytotoxicity assay we showed that EPCR was necessary for the APC-mediated protective effect in both neuronal cell types in culture. The effect of APC was abrogated in the presence of blocking EPCR antibodies. Analysis of neuronal death by cell labelling with dyes which allow distinguishing living and dead cells confirmed that the anti-apoptotic effect of APC was dependent on both EPCR and protease-activated receptor-1. Thus, we suggest that binding of APC to EPCR on neurons and subsequent activation of protease-activated receptor-1 by the complex of APC-EPCR promotes survival mechanisms after exposure of neurons to damaging factors.

Generation and phenotypic analysis of protein S-deficient mice

Protein S (PS) is an important natural anticoagulant with potentially multiple biologic functions. To investigate further the role of PS in vivo, we generated Pros(+/-) heterozygous mice. In the null (-) allele, the Pros exons 3 to 7 have been excised through conditional gene targeting. Pros(+/-) mice did not present any signs of spontaneous thrombosis and had reduced PS plasma levels and activated protein C cofactor activity in plasma coagulation and thrombin generation assays. Tissue factor pathway inhibitor cofactor activity of PS could not be demonstrated. Heterozygous Pros(+/-) mice exhibited a notable thrombotic phenotype in vivo when challenged in a tissue factor-induced thromboembolism model. No viable Pros(-/-) mice were obtained through mating of Pros(+/-) parents. Most E17.5 Pros(-/-) embryos were found dead with severe intracranial hemorrhages and most likely presented consumptive coagulopathy, as demonstrated by intravascular and interstitial fibrin deposition and an increased number of megakaryocytes in the liver, suggesting peripheral thrombocytopenia. A few E17.5 Pros(-/-) embryos had less severe phenotype, indicating that life-threatening manifestations might occur between E17.5 and the full term. Thus, similar to human phenotypes, mild heterozygous PS deficiency in mice was associated with a thrombotic phenotype, whereas total homozygous deficiency in PS was incompatible with life.

Hyperantithrombotic, noncytoprotective Glu149Ala-activated protein C mutant

Activated protein C (APC) reduces mortality in severe sepsis patients. APC exerts anticoagulant activities via inactivation of factors Va and VIIIa and cytoprotective activities via endothelial protein C receptor and protease-activated receptor-1. APC mutants with selectively altered and opposite activity profiles, that is, greatly reduced anticoagulant activity or greatly reduced cytoprotective activities, are compared here. Glu149Ala-APC exhibited enhanced in vitro anticoagulant and in vivo antithrombotic activity, but greatly diminished in vitro cytoprotective effects and in vivo reduction of endotoxin-induced murine mortality. Thus, residue Glu149 and the C-terminal region of APC's light chain are identified as functionally important for expression of multiple APC activities. In contrast to Glu149Ala-APC, 5A-APC (Lys191-193Ala + Arg229/230Ala) with protease domain mutations lacked in vivo antithrombotic activity, although it was potent in reducing endotoxin-induced mortality, as previously shown. These data imply that APC molecular species with potent antithrombotic activity, but without robust cytoprotective activity, are not sufficient to reduce mortality in endotoxemia, emphasizing the need for APC's cytoprotective actions, but not anticoagulant actions, to reduce endotoxin-induced mortality. Protein engineering can provide APC mutants that permit definitive mechanism of action studies for APC's multiple activities, and may also provide safer and more effective second-generation APC mutants with reduced bleeding risk.

Role of activated protein C and its receptor in inhibition of tumor metastasis

Engagement of endothelial protein C receptor (EPCR) by activated protein C (aPC) decreases expression of endothelial adhesion molecules implicated in tumor-endothelium interactions. We examined the role of the aPC/EPCR pathway on tumor migration and metastasis. In vitro, B16-F10 melanoma cells showed decreased adhesion to and transmigration through endothelium treated with recombinant human aPC (rhaPC). In murine B16-F10 metastasis models, transgenic EPCR overexpressing (Tie2-EPCR) mice exhibited marked reductions in liver (50%) and lung (92%) metastases compared with wild-type (WT) animals. Intravital imaging showed reduced B16-F10 entrapment within livers of Tie2-EPCR compared with WT mice. A similar reduction was observed in WT mice treated with rhaPC. Strikingly, rhaPC treatment resulted in a 44% reduction in lung metastases. This was associated with decreased lung P-selectin and TNF-alpha mRNA levels. These findings support an important role for the aPC/EPCR pathway in reducing metastasis via inhibition of tumor cell adhesion and transmigration.

Protection of vascular barrier integrity by activated protein C in murine models depends on protease-activated receptor-1

Protease activated receptor-1 (PAR1) mediates barrier protective signalling of activated protein C (APC) in human endothelial cells in vitro and may contribute to APC's beneficial effects in patients with severe sepsis. Mouse models are of key importance for translational research but species differences may limit conclusions for the human system. We analysed whether mouse APC can cleave, activate and induce signalling through murine PAR1 and tested in newly established mouse models if long-term infusion of APC prevents from vascular leakage. Cell surface immunoassays demonstrated efficient cleavage of endogenous murine endothelial PAR1 by either murine or human APC. Pharmacological concentrations of APC of either species had powerful barrier protective effects on cultured murine endothelial cells that required PAR1 cleavage. Vascular endothelial growth factor-mediated hyperpermeability in the skin was reduced by either endogenously generated as well as directly infused recombinant mouse APC in wild-type mice. However APC did not significantly alter the vascular barrier function in PAR1-deficient mice. In endotoxin-challenged mice, infused APC significantly prevented from pulmonary fluid accumulation in the wild-type mice but not in mice lacking PAR1. Our results directly show that murine APC cleaves and signals through PAR1 in mouse endothelial cells. APC reduces vascular permeability in mouse models and PAR1 plays a major role in mediating these effects. Our data in vitro and in vivo support the paradigm that PAR1 contributes to protective effects of APC on vascular barrier integrity in sepsis.

Activated protein C promotes neovascularization and neurogenesis in postischemic brain via protease-activated receptor 1

Activated protein C (APC) is a serine protease with anticoagulant and direct cytoprotective activities. Early postischemic APC application activates the cellular protein C pathway in brain endothelium and neurons, which is neuroprotective. Whether late APC administration after a transient ischemic attack is neuroprotective and whether APC influences brain repair is not known. Here, we determined safety and efficacy of late APC and tissue-plasminogen activator (tPA) administrations in a mouse model of transient brain ischemia. tPA given at 6 h after onset of ischemia killed all mice within 2 d, whereas APC given at 6 or 24 h after ischemia onset improved significantly functional outcome and reduced spread of the ischemic lesion. At 7 d postischemia, APC multiple dosing (0.8 mg/kg, i.p.) at 6-72 or 72-144 h enhanced comparably cerebral perfusion in the ischemic border by approximately 40% as shown by in vivo lectin-FITC angiography, blocked blood-brain barrier leakage of serum proteins, and increased the number of endothelial replicating cells by 4.5- to 4.7-fold. APC multidosing at 6-72 h or 72-144 h increased proliferation of neuronal progenitor cells in the subventricular zone (SVZ) by 40-50% and migration of newly formed neuroblasts from the SVZ toward the ischemic border by approximately twofold. The effects of APC on neovascularization and neurogenesis were mediated by protease-activated receptor 1 and were independent of the reduction by APC of infarction volume. Our data show that delayed APC administration is neuroprotective and mediates brain repair (i.e., neovascularization and neurogenesis), suggesting a significant extension of the therapeutic window for APC intervention in postischemic brain.

New therapeutic applications for the anticoagulant, activated protein C

BACKGROUND

Activated protein C (APC) is derived from its precursor, protein C (PC). Originally thought to be synthesised exclusively by the liver, recent reports have shown that PC is also produced by endothelial cells, smooth muscle cells, keratinocytes and some leukocytes.

OBJECTIVE

To provide an update on the emerging therapeutic effects of APC.

RESULTS/CONCLUSION

APC functions as an anticoagulant with cytoprotective, anti-inflammatory and antiapoptotic properties. In vitro and preclinical data have revealed that APC exerts its protective effects via an intriguing mechanism requiring endothelial protein C receptor and protease activated receptor-1. Approved as a therapeutic agent for severe sepsis, APC is emerging as a potential treatment for a number of autoimmune and inflammatory diseases including spinal cord injury, asthma, chronic wounds and possibly rheumatoid arthritis. The future therapeutic uses of APC look very promising.

The protein C pathway in inflammatory bowel disease: the missing link between inflammation and coagulation

Traditionally described as a major anti-coagulant system, the protein C (PC) pathway, consisting of thrombomodulin, the endothelial cell protein C receptor and activated PC (APC), is gaining increasing attention as an important regulator of microvascular inflammation. Although they possess several anti-inflammatory and cytoprotective functions, the expression and function of the components of the PC pathway is downregulated during inflammation. Recent evidence suggests that the PC pathway is defective in patients with inflammatory bowel disease (IBD) and that restoring its function has anti-inflammatory effects on cultured intestinal microvascular endothelial cells and in animal models of colitis. Here, we propose that the PC pathway has an important role in governing intestinal microvascular inflammation and might provide a novel therapeutic target in the management of IBD.

Recombinant human activated protein C for the postexposure treatment of Ebola hemorrhagic fever

BACKGROUND

Infection of primates with Zaire ebolavirus (ZEBOV) leads to hypotension, coagulation disorders, and an impaired immune response and, in many ways, resembles severe sepsis. Rapid decreases in plasma levels of protein C are a prominent feature of severe sepsis and ZEBOV hemorrhagic fever (ZHF). Currently, recombinant human activated protein C (rhAPC [Xigris; Eli Lilly]) is licensed for treating human patients with severe sepsis who are at high risk of death. The aim of this study was to test the efficacy of rhAPC as a potential treatment for ZHF.

METHODS

Fourteen rhesus macaques were challenged with a uniformly lethal dose of ZEBOV; 11 of these monkeys were treated by intravenous infusion with rhAPC beginning 30-60 min after challenge and continuing for 7 days. Three control monkeys received sterile saline in parallel.

RESULTS

All 3 control monkeys died on day 8, whereas 2 of the 11 rhAPC-treated monkeys survived. The mean time to death for the rhAPC-treated monkeys that did not survive ZEBOV challenge was 12.6 days. The difference in survival was significant when the rhAPC-treated monkeys were compared with historical controls.

CONCLUSIONS

The experimental findings provide evidence that ZHF and severe sepsis share underlying mechanisms and may respond to the same therapies.

Activated protein C--an anticoagulant that does more than stop clots

Activated protein C (APC) is a glycoprotein derived from its precursor, protein C and formed by the cleavage of an activation peptide by thrombin bound to thrombomodulin. Originally thought to be synthesized exclusively by the liver, recent reports have shown that protein C is synthesized by endothelial cells, keratinocytes and some hematopoietic cells. APC functions as a physiological anticoagulant with cytoprotective, anti-inflammatory and anti-apoptotic properties. In vitro and preclinical data have revealed that APC exerts its protective effects via an intriguing mechanism requiring endothelial protein C receptor and the thrombin receptor, protease-activated receptor-1. Remarkably, even though APC cleaves this receptor in an identical fashion to thrombin, it exerts opposing effects. Recently approved as a therapeutic agent for severe sepsis, APC is now emerging as a potential treatment for a number of autoimmune and inflammatory diseases including lung disorders, spinal cord injury and chronic wounds. The future pharmacologic use of APC holds remarkable promise.

Activated protein C-cleaved protease activated receptor-1 is retained on the endothelial cell surface even in the presence of thrombin

Activated protein C (APC) signals in endothelial cells ex vivo through protease activated receptor-1 (PAR1). However, it is controversial whether PAR1 can mediate APC's protective effects in sepsis because the inflammatory response results in thrombin generation and thrombin proteolytically activates PAR1 much more efficiently than APC. Here we show that APC can induce powerful barrier protective responses in an endothelial cell monolayer in the presence of thrombin. Using cell surface immunoassays with conformation sensitive monoclonal anti-PAR1 antibodies we analyzed cleavage of endogenous PAR1 on the endothelial cell surface by APC in the absence and presence of thrombin. Incubation with APC caused efficient PAR1 cleavage and upon coincubation with thrombin APC supported additional PAR1 cleavage. Thrombin-cleaved PAR1 rapidly disappeared from the cell surface whereas, unexpectedly, the APC-cleaved PAR1 remained and could be detected on the cell surface, even when thrombin at concentrations of up to 1 nM was also present. Our findings demonstrate for the first time directly that APC can generate a distinct PAR1 population on endothelial cells in the presence of thrombin. The data suggest that different trafficking of activated PAR1 might explain how PAR1 signaling by APC can be relevant when thrombin is present.

A guide to murine coagulation factor structure, function, assays, and genetic alterations

Murine blood coagulation factors and function are quite similar to those of humans. Because of this similarity and the adaptability of mice to genetic manipulation, murine coagulation factors and inhibitors have been extensively studied. These studies have provided significant insights into human hemostasis. They have also provided useful experimental models for evaluation of the pathophysiology and treatment of thrombosis. This review contains recommendations for obtaining, processing and assaying mouse blood hemostatic components, and it summarizes the extensive literature on murine coagulation factor structure and function, assays and genetic alteration. It is intended to be a convenient reference source for investigators of hemostasis and thrombosis.