Molecular characterization of an extended binding site for coagulation factor Va in the positive exosite of activated protein C

Computational analysis of the functional and structural impact of the most deleterious missense mutations in the human Protein C

Protein C (PC) is a vitamin K-dependent factor that plays a crucial role in controlling anticoagulant processes and acts as a cytoprotective agent to promote cell survival. Several mutations in human PC are associated with decreased protein production or altered protein structure, resulting in PC deficiency. In this study, we conducted a comprehensive analysis of nonsynonymous single nucleotide polymorphisms in human PC to prioritize and confirm the most high-risk mutations predicted to cause disease. Of the 340 missense mutations obtained from the NCBI database, only 26 were classified as high-risk mutations using various bioinformatic tools. Among these, we identified that 12 mutations reduced the stability of protein, and thereby had the greatest potential to disturb protein structure and function. Molecular dynamics simulations revealed moderate alterations in the structural stability, flexibility, and secondary structural organization of the serine protease domain of human PC for five missense mutations (L305R, W342C, G403R, V420E, and W444C) when compared to the native structure that could maybe influence its interaction with other molecules. Protein-protein interaction analyses demonstrated that the occurrence of these five mutations can affect the regular interaction between PC and activated factor V. Therefore, our findings assume that these mutants can be used in the identification and development of therapeutics for diseases associated with PC dysfunction, although assessment the effect of these mutations need to be proofed in in-vitro.

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Targeted inhibition of activated protein C by a non-active-site inhibitory antibody to treat hemophilia

Activated protein C (APC) is a plasma serine protease with antithrombotic and cytoprotective functions. Based on the hypothesis that specific inhibition of APC’s anticoagulant but not its cytoprotective activity can be beneficial for hemophilia therapy, 2 types of inhibitory monoclonal antibodies (mAbs) are tested: A type I active-site binding mAb and a type II mAb binding to an exosite on APC (required for anticoagulant activity) as shown by X-ray crystallography. Both mAbs increase thrombin generation and promote plasma clotting. Type I blocks all APC activities, whereas type II preserves APC’s cytoprotective function. In normal monkeys, type I causes many adverse effects including animal death. In contrast, type II is well-tolerated in normal monkeys and shows both acute and prophylactic dose-dependent efficacy in hemophilic monkeys. Our data show that the type II mAb can specifically inhibit APC’s anticoagulant function without compromising its cytoprotective function and offers superior therapeutic opportunities for hemophilia.

Activated protein C (APC) is a plasma serine protease with antithrombotic and cytoprotective functions. Here, the authors develop a monoclonal antibody that specifically inhibits APC’s anticoagulant function without compromising its cytoprotective function, and shows efficacy in animal models.

Protein C deficiency (a novel mutation: ala291Thr) with systemic lupus erythematosus leads to the deep vein thrombosis

: The current study aims to explore the phenotype and genotype of a mutation Ala291Thr, which responsible for type I protein C (PC) deficiency in a Chinese woman. The PROC antigen was tested with chromogenic substrate method. PROC gene were amplified by PCR with direct sequencing. Bioinformatics and model analysis were used to study the harm of the mutation. PC activity (PC: A) levels of three members were reduced to 39, 57 and 56%, respectively, PC: antigen was decreased parallelly same as PC: A. Sequencing analysis showed proband with a novel heterozygous c.997G>A point mutation in exon 9 of PROC gene resulting in Ala291Thr. The Ala291Thr mutation is responsible for the decrease of PC: A, which is cross-reacting material negative deficiency and the first reported in the world. This mutation alone may not have significant clinical symptoms, whereas it will cause deep vein thrombosis when combined with systemic lupus erythematosus.

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.

Regulation of immune cell signaling by activated protein C

Innate immune cells are an essential part of the host defense response, promoting inflammation through release of proinflammatory cytokines or formation of neutrophil extracellular traps. While these processes are important for defense against infectious agents or injury, aberrant activation potentiates pathologic inflammatory disease. Thus, understanding regulatory mechanisms that limit neutrophil extracellular traps formation and cytokine release is of therapeutic interest for targeting pathologic diseases. Activated protein C is an endogenous serine protease with anticoagulant activity as well as anti-inflammatory and cytoprotective functions, the latter of which are mediated through binding cell surface receptors and inducing intracellular signaling. In this review, we discuss certain leukocyte functions, namely neutrophil extracellular traps formation and cytokine release, and the inhibition of these processes by activated protein C.

PAR1 agonists stimulate APC-like endothelial cytoprotection and confer resistance to thromboinflammatory injury

Stimulation of protease-activated receptor 1 (PAR1) on endothelium by activated protein C (APC) is protective in several animal models of disease, and APC has been used clinically in severe sepsis and wound healing. Clinical use of APC, however, is limited by its immunogenicity and its anticoagulant activity. We show that a class of small molecules termed "parmodulins" that act at the cytosolic face of PAR1 stimulates APC-like cytoprotective signaling in endothelium. Parmodulins block thrombin generation in response to inflammatory mediators and inhibit platelet accumulation on endothelium cultured under flow. Evaluation of the antithrombotic mechanism showed that parmodulins induce cytoprotective signaling through Gβγ, activating a PI3K/Akt pathway and eliciting a genetic program that includes suppression of NF-κB-mediated transcriptional activation and up-regulation of select cytoprotective transcripts. STC1 is among the up-regulated transcripts, and knockdown of stanniocalin-1 blocks the protective effects of both parmodulins and APC. Induction of this signaling pathway in vivo protects against thromboinflammatory injury in blood vessels. Small-molecule activation of endothelial cytoprotection through PAR1 represents an approach for treatment of thromboinflammatory disease and provides proof-of-principle for the strategy of targeting the cytoplasmic surface of GPCRs to achieve pathway selective signaling.

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

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.

Activation-resistant homozygous protein C R229W mutation causing familial perinatal intracranial hemorrhage and delayed onset of thrombosis

INTRODUCTION

We describe a family with two first-degree cousins who presented with similar phenotypes characterized by neonatal intracranial hemorrhage and subsequent onset of thrombosis.

PATIENTS/METHODS

We enrolled the two affected patients, five unaffected family members and fifty-five normal controls. Clinical, laboratory, and radiological characteristics of patients were obtained. Exome sequencing was performed for the older affected child. PROC c.811 C>T was genotyped by PCR in patients, family members, and controls. Protein C amidolytic activity and antigen were measured using the STACHROM® protein C kit and ELISAs. To define functional abnormalities caused by the patients' mutation, recombinant wildtype protein C and its mutants R229W, R229Q and R229A were studied.

RESULTS

For the two cousins, protein C amidolytic activity was 61% and 59% and antigen was 57% and 73% (nl 70-140%), respectively. Exome sequencing revealed a homozygous variant in exon 9 of the protein C (PROC) gene c.811 C>T (R229W). The R229W mutation is located in the calcium binding loop of protein C's protease domain that mediates thrombomodulin interactions. Recombinant R229W-protein C mutant was strikingly defective in rate of activation by thrombin: thrombomodulin, suggesting an in vivo deficit in these children for generation of activated protein C.

CONCLUSIONS

These cases emphasize that protein C and activated protein C are important in maintaining the integrity of the brain vascular endothelium in humans. Moreover, routine protein C assays utilizing snake venom protease fail to detect protein C mutants that are resistant to thrombin:thrombomodulin activation.

Protective effects of non-anticoagulant activated protein C variant (D36A/L38D/A39V) in a murine model of ischaemic stroke

Ischaemic stroke is caused by occlusive thrombi in the cerebral vasculature. Although tissue-plasminogen activator (tPA) can be administered as thrombolytic therapy, it has major limitations, which include disruption of the blood-brain barrier and an increased risk of bleeding. Treatments that prevent or limit such deleterious effects could be of major clinical importance. Activated protein C (APC) is a natural anticoagulant that regulates thrombin generation, but also confers endothelial cytoprotective effects and improved endothelial barrier function mediated through its cell signalling properties. In murine models of stroke, although APC can limit the deleterious effects of tPA due to its cell signalling function, its anticoagulant actions can further elevate the risk of bleeding. Thus, APC variants such as APC(5A), APC(Ca-ins) and APC(36-39) with reduced anticoagulant, but normal signalling function may have therapeutic benefit. Human and murine protein C (5A), (Ca-ins) and (36-39) variants were expressed and characterised. All protein C variants were secreted normally, but 5-20% of the protein C (Ca-ins) variants were secreted as disulphide-linked dimers. Thrombin generation assays suggested reductions in anticoagulant function of 50- to 57-fold for APC(36-39), 22- to 27-fold for APC(Ca-ins) and 14- to 17-fold for APC(5A). Interestingly, whereas human wt APC, APC(36-39) and APC(Ca-ins) were inhibited similarly by protein C inhibitor (t_½ - 33 to 39 mins), APC(5A) was inactivated ~9-fold faster (t_½ - 4 mins). Using the murine middle cerebral artery occlusion ischaemia/repurfusion injury model, in combination with tPA, APC(36-39), which cannot be enhanced by its cofactor protein S, significantly improved neurological scores, reduced cerebral infarct area by ~50% and reduced oedema ratio. APC(36-39) also significantly reduced bleeding in the brain induced by administration of tPA, whereas wt APC did not. If our data can be extrapolated to clinical settings, then APC(36-39) could represent a feasible adjunctive therapy for ischaemic stroke.

Activated protein C analog protects from ischemic stroke and extends the therapeutic window of tissue-type plasminogen activator in aged female mice and hypertensive rats

BACKGROUND AND PURPOSE

3K3A-activated protein C (APC) protects young, healthy male rodents after ischemic stroke. 3K3A-APC is currently under development as a neuroprotectant for acute ischemic stroke in humans. Stroke Therapy Academic Industry Roundtable recommends that after initial studies in young, healthy male animals, further studies should be performed in females, aged animals, and animals with comorbid conditions. Here, we studied the effects of delayed 3KA-APC therapy alone and with tissue-type plasminogen activator (tPA) in aged female mice and spontaneously hypertensive rats.

METHODS

We used Stroke Therapy Academic Industry Roundtable recommendations for ensuring good scientific inquiry. Murine recombinant 3K3A-APC (0.2 mg/kg) alone or with recombinant tPA (10 mg/kg) was given intravenously 4 hours after transient middle cerebral artery occlusion in aged female mice and rats and after embolic stroke in spontaneously hypertensive rat. 3K3A-APC was additionally administered within 3 to 7 days after stroke. The neuropathological analysis and neurological scores, foot-fault, forelimb asymmetry, and adhesive removal tests were performed within 7 and 28 days of stroke.

RESULTS

In all models, tPA alone had no effects on the infarct volume or behavior. 3K3A-APC alone or with tPA reduced the infarct volume 7 days after the middle cerebral artery occlusion in aged female mice and embolic stroke in spontaneously hypertensive rat by 62% to 66% and 50% to 53%, respectively, significantly improved (P<0.05) behavior, and eliminated tPA-induced intracerebral microhemorrhages. In aged female mice, 3K3A-APC was protective within 4 weeks of stroke.

CONCLUSIONS

3K3A-APC protects from ischemic stroke and extends the therapeutic window of tPA in aged female mice and in spontaneously hypertensive rat with a comorbid condition.

An activated protein C analog stimulates neuronal production by human neural progenitor cells via a PAR1-PAR3-S1PR1-Akt pathway

Activated protein C (APC) is a protease with anticoagulant and cell-signaling activities. In the CNS, APC and its analogs with reduced anticoagulant activity but preserved cell signaling activities, such as 3K3A-APC, exert neuroprotective, vasculoprotective, and anti-inflammatory effects. Murine APC promotes subependymal neurogenesis in rodents in vivo after ischemic and traumatic brain injury. Whether human APC can influence neuronal production from resident progenitor cells in humans is unknown. Here we show that 3K3A-APC, but not S360A-APC (an enzymatically inactive analog of APC), stimulates neuronal mitogenesis and differentiation from fetal human neural stem and progenitor cells (NPCs). The effects of 3K3A-APC on proliferation and differentiation were comparable to those obtained with fibroblast growth factor and brain-derived growth factor, respectively. Its promoting effect on neuronal differentiation was accompanied by inhibition of astroglial differentiation. In addition, 3K3A-APC exerted modest anti-apoptotic effects during neuronal production. These effects appeared to be mediated through specific protease activated receptors (PARs) and sphingosine-1-phosphate receptors (S1PRs), in that siRNA-mediated inhibition of PARs 1-4 and S1PRs 1-5 revealed that PAR1, PAR3, and S1PR1 are required for the neurogenic effects of 3K3A-APC. 3K3A-APC activated Akt, a downstream target of S1PR1, which was inhibited by S1PR1, PAR1, and PAR3 silencing. Adenoviral transduction of NPCs with a kinase-defective Akt mutant abolished the effects of 3K3A-APC on NPCs, confirming a key role of Akt activation in 3K3A-APC-mediated neurogenesis. Therefore, APC and its pharmacological analogs, by influencing PAR and S1PR signals in resident neural progenitor cells, may be potent modulators of both development and repair in the human CNS.

Neurotoxicity of the anticoagulant-selective E149A-activated protein C variant after focal ischemic stroke in mice

Wild type (WT) activated protein C (APC) and cytoprotective-selective APC variants such as 3K3A-APC (<10% anticoagulant but normal cytoprotective activity) are neuroprotective in murine focal ischemic stroke models. Here we compared the neuroprotective effects of the anticoagulant-selective E149A-APC variant (>3-fold increased anticoagulant activity but defective cytoprotective activities) to those of the cytoprotective-selective 5A-APC variant (<10% anticoagulant activity). After transient distal middle cerebral artery occlusion, mice received a vehicle, E149A-APC or 5A-APC at 0.2mg/kg at 4h after stroke. Treatment with 5A-APC was neuroprotective, as it improved performance on forelimb use asymmetry test and foot fault test (P<0.05), reduced by 48% and 50% the infarct and edema volumes, respectively (P<0.05), and was not associated with an increased risk of bleeding as indicated by normal hemoglobin levels in the ischemic brain at day 7. In contrast, E149A-APC treatment worsened neurological outcome determined by foot fault tests and forelimb use asymmetry tests, and increased significantly by 44% and 60% infarct and edema volume, respectively (P<0.05). At 7days after treatment, E149A-APC compared to vehicle or 5A-APC notably increased by ~5-fold the hemoglobin level in the ischemic hemisphere suggesting it provoked significant intracerebral bleeding. Thus, the enhanced anticoagulant activity of E149A-APC increased post-ischemic accumulation of neurotoxic erythrocyte-derived hemoglobin which likely worsened the neurological and neuropathological outcomes after stroke. Our data emphasize that APC's cytoprotective activities, but not its anticoagulant activity, are key for APC neuroprotection after transient ischemic stroke.

Preclinical safety and pharmacokinetic profile of 3K3A-APC, a novel, modified activated protein C for ischemic stroke

Activated protein C (APC), a protease with anticoagulant and cytoprotective activities, protects neurons and cerebrovascular endothelium from ischemic injury. A recombinant APC, drotrecogin alfa (activated) (DrotAA) (Xigris®), was approved by the Food and Drug Administration for the treatment of sepsis; however, serious bleeding was a dose-limiting side effect. A modified APC, containing 405 amino acid residues, 3K3A-APC, was designed to possess significantly reduced anticoagulant activity ( < 10 %) while maintaining full cytoprotective properties. The preclinical safety assessment of 3K3A-APC was conducted to support initiation of ischemic stroke clinical trials.The safety and toxicokinetics of 3K3A-APC were studied in CD-1 mice and cynomolgus monkeys. Multiple-dose (14-day), intravenous GLP toxicology assessed toxicity, histopathology, immunogenicity, and toxicokinetics.Dose-related increases in plasma total 3K3A-APC were observed in mice and monkeys with no evidence of accumulation over 14 days. The elimination T(1/2) in monkeys was 1 hour. 3K3A-APC was well tolerated in mice and monkeys, and no signs of 3K3A-APC toxicity were identified in mice or monkeys at any time. Additionally,wild-type APC (DrotAA) was studied to obtain comparative anticoagulant data using clotting assays. Anticoagulant activity of 3K3A-APC was observed in monkeys at doses of 1 and 5 mg/kg/day .In contrast, DrotAA showed prolongation of clotting assays in monkeys at doses 1/10(th) of those showing effects with 3K3A-APC. Based upon the anticoagulant profiles, the risk for APC-induced bleeding in clinical trials of 3K3A-APC is greatly reduced relative to wild type APC which makes this new drug a feasible therapy for ischemic stroke patients.

Variable contributions of basic residues forming an APC exosite in the binding and inactivation of factor VIIIa

Basic residues contained in the 39-, 60-, and 70-80-loops of activated protein C (APC) comprise an exosite that contributes to the binding and subsequent proteolytic inactivation of factor (F) VIIIa. Surface plasmon resonance (SPR) showed that WT APC bound to FVIII light chain (LC) and the FVIIIa A1/A3C1C2 dimer with equivalent affinity (Kd = 525 and 546 nM, respectively). These affinity values may reflect binding interactions to the acidic residue-rich a1 and a3 segments adjacent to A1 domain in the A1/A3C1C2 and A3 domain in LC, respectively. Results from SPR, using a panel of APC exosite variants where basic residues were mutated, in binding to immobilized FVIIIa A1/A3C1C2 or LC indicated ~4-10-fold increases in the Kd values relative to WT for several of the variants including Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, and Arg67Ala. On the other hand, a number of APC variants including Lys38Ala, Lys62Ala, and Lys78Ala showed little if any change in binding affinity to the FVIII substrates. FXa generation assays and Western blotting, used to monitor rates of FVIIIa inactivation and proteolysis at the primary cleavage site in the cofactor (Arg(336)), respectively, showed marked rate reductions relative to WT for the Lys39Ala, Lys37-Lys38-Lys39/Pro-Gln-Glu, Arg67Ala, and Arg74Ala variants. Furthermore, kinetic analysis monitoring FVIIIa inactivation by APC variants at varying FVIIIa substrate concentration showed ~2.6-4.4-fold increases in Km values relative to WT. These results show a variable contribution of basic residues comprising the APC exosite, with significant contributions from Lys39, Arg67, and Arg74 to forming a FVIIIa-interactive site.

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.

PROC c.574_576del polymorphism: a common genetic risk factor for venous thrombosis in the Chinese population

BACKGROUND

There are ethnic differences in the genetic risk factors for venous thrombosis (VT). The genetic causes of VT in the Chinese population are not fully understood.

OBJECTIVES

To identify possible common abnormal factors that could contribute to thrombosis susceptibility.

METHODS/RESULTS

We measured the levels of nine types of plasma coagulation factor, three types of anticoagulation factor and two types of fibrinolytic factor in 310 VT patients. Factor V activity was higher in 32 cases. Eleven of the 32 cases also had low protein C (PC) or protein S (PS) activities, indicating PC or PS deficiency. No other abnormalities were observed in the other 21 cases. All of the samples were sensitive to activated PC inactivation. Therefore, the abnormal factor involved may be FV inactivator or its cofactor rather than FV itself. Resequencing identified a common PROC c.574_576del variant in 10 of the 32 subjects. In a case-control study, this variant was detected in 68 of the 1003 patients and in 25 of the 1031 controls. It had an adjusted odds ratio of 2.71 (95% confidence interval [CI] 1.68-4.36). PC amidolytic activities of most variant carriers were similar to those of non-carriers, but the mean anticoagulant activity was only 72.7 U dL(-1). Expression studies in vitro showed that the anticoagulant activity of the mutant PC was 43.6% of that of the wild-type PC.

CONCLUSIONS

We identified what is, so far, the most common genetic risk factor for VT in the Chinese population, with its prevalence being approximately 2.36%.

An activated protein C analog with reduced anticoagulant activity extends the therapeutic window of tissue plasminogen activator for ischemic stroke in rodents

BACKGROUND AND PURPOSE

Tissue plasminogen activator (tPA) is the only approved therapy for acute ischemic stroke. However, tPA has a brief therapeutic window. Its side effects include intracerebral bleeding and neurotoxicity. Therefore, a combination therapy with tPA and agents that can extend the therapeutic window of tPA and/or counteract its side effects are warranted. Here, we studied whether 3K3A-APC, a neuroprotective analog of activated protein C with reduced anticoagulant activity, can enhance the therapeutic effects of tPA in models of ischemic stroke in rodents.

METHODS

Human recombinant tPA (10 mg/kg), alone or in combination with human recombinant 3K3A-APC (2 mg/kg), was administered intravenously 4 hours after proximal or distal transient middle cerebral artery occlusion in mice and embolic stroke in rats. The 3K3A-APC was additionally administered for 3 to 4 consecutive days after stroke. The neuropathological and neurological analyses were performed at 1 to 7 days after stroke.

RESULTS

In all models, tPA alone had no effects on the infarct volume or behavior (ie, neurological score, foot-fault, forelimb asymmetry, adhesive removal) compared with vehicle. The tPA and 3K3A-APC combination therapy reduced the infarct volume 24 hours and 7 days after proximal or distal transient middle cerebral artery occlusion in mice and 7 days after embolic stroke in rats by 65%, 63%, and 52%, respectively, significantly (P<0.05) improved behavior and eliminated tPA-induced intracerebral microhemorrhages.

CONCLUSIONS

The 3K3A-APC extends the therapeutic window of tPA for ischemic stroke in rodents. Therefore, this combination therapy also should be considered for treating stroke in humans.

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.

Function of the activated protein C (APC) autolysis loop in activated FVIII inactivation

Activated protein C (APC) binds to its substrates activated factor V (FVa) and activated factor VIII (FVIIIa) with a basic exosite that consists of loops 37, 60, 70 and the autolysis loop. These loops have a high density of basic residues, resulting in a positive charge on the surface of APC. Many of these residues are important in the interaction of APC with FVa and FVIIIa. The current study focused on the function of the autolysis loop in the interaction with FVIIIa. This loop was previously shown to interact with FVa, and it inhibits APC inactivation by plasma serpins. Charged residues of the autolysis loop were individually mutated to alanine and the activity of these mutants was assessed in functional FVIIIa inactivation assays. The autolysis loop was functionally important for FVIIIa inactivation. Mutation of R306, K311 and R314 each resulted in significantly reduced FVIIIa inactivation. The inactivating cleavages of FVIIIa at R336 and R562 were affected equally by the mutations. Protein S and FV stimulated cleavage at R562 more than cleavage at R336, independent of mutations in the autolysis loop. Together, these results confirmed that the autolysis loop plays a significant role as part of the basic exosite on APC in the interaction with FVIIIa.

Cytoprotective protein C pathways and implications for stroke and neurological disorders

Recent studies indicate that single-action-single-target agents are unlikely to cure CNS disorders sharing a pathogenic triad consisting of vascular damage, neuronal injury/neurodegeneration and neuroinflammation. Here we focus on a recent example of a multiple-action-multiple-target approach for CNS disorders based on newly discovered biological properties of activated protein C (APC), an endogenous plasma protease with antithrombotic, cytoprotective and anti-inflammatory activities in the CNS. We propose that APC-mediated signaling through the protease activated receptor-1 (PAR1) can favorably regulate multiple pathways within the neurovascular unit in non-neuronal cells and neurons during acute or chronic CNS insults, leading to stabilization of the blood-brain barrier (BBB), neuroprotection and control of neuroinflammation. Although much remains to be understood regarding the biology of APC, preclinical studies suggest that APC has promising applications as disease-modifying therapy for ischemic stroke and other neuropathologies whose underlying pathology involves deficits in the vasculo-neuronal-inflammatory triad.

Regulation of the protein C anticoagulant and antiinflammatory pathways

Protein C is a vitamin K-dependent anticoagulant serine protease zymogen in plasma which upon activation by the thrombin-thrombomodulin complex down-regulates the coagulation cascade by degrading cofactors Va and VIIIa by limited proteolysis. In addition to its anticoagulant function, activated protein C (APC) also binds to endothelial protein C receptor (EPCR) in lipid-rafts/caveolar compartments to activate protease- activated receptor 1 (PAR-1) thereby eliciting antiinflammatory and cytoprotective signaling responses in endothelial cells. These properties have led to FDA approval of recombinant APC as a therapeutic drug for severe sepsis. The mechanism by which APC selects its substrates in the anticoagulant and antiinflammatory pathways is not well understood. Recent structural and mutagenesis data have indicated that basic residues of three exposed surface loops known as 39-loop (Lys-37, Lys-38, and Lys-39), 60-loop (Lys-62, Lys- 63, and Arg-67), and 70-80-loop (Arg-74, Arg-75, and Lys-78) (chymotrypsin numbering) constitute an anion binding exosite in APC that interacts with the procoagulant cofactors Va and VIIIa in the anticoagulant pathway. Furthermore, two negatively charged residues on the opposite side of the active-site of APC on a helical structure have been demonstrated to determine the specificity of the PAR-1 recognition in the cytoprotective pathway. This article will review the mechanism by which APC exerts its proteolytic function in two physiologically inter-related pathways and how the structure- function insights into determinants of the specificity of APC interaction with its substrates in two pathways can be utilized to tinker with the structure of the molecule to obtain APC derivatives with potentially improved therapeutic profiles.

Activated protein C analog with reduced anticoagulant activity improves functional recovery and reduces bleeding risk following controlled cortical impact

The anticoagulant activated protein C (APC) protects neurons and vascular cells from injury through its direct cytoprotective effects that are independent of its anticoagulant action. Wild-type recombinant murine APC (wt-APC) exerts significant neuroprotection in mice if administered early after traumatic brain injury (TBI). Here, we compared efficacy and safety of a late therapy for TBI with wt-APC and 3K3A-APC, an APC analog with approximately 80% reduced anticoagulant activity but normal cytoprotective activity, using a controlled cortical impact model of TBI. Mice received 0.8 mg/kg intraperitoneally of recombinant murine 3K3A-APC, wt-APC or saline at 6, 12, 24 and 48 h after injury. 3K3A-APC (n=15) relative to wt-APC (n=15) improved motor and sensorimotor recovery within the first three days post-trauma as demonstrated by rotarod (p<0.05) and beam balance test (p<0.05), respectively. Both, wt-APC and 3K3A-APC reduced the lesion volume seven days after injury by 36% (n=8; p<0.01) and 56% (n=8; p<0.01), respectively, compared to saline (n=8). Three days post-TBI, the hemoglobin levels in the injured brain were increased by approximately 3-fold after wt-APC treatment compared to saline indicating an increased risk for intracerebral bleeding. In contrast, comparable levels of brain hemoglobin in 3K3A-APC-treated and saline-treated mice suggested that 3K3A-APC treatment did not increase risk for bleeding after TBI. Thus, compared to wt-APC, 3K3A-APC is more efficacious and safer therapy for TBI with no risk for intracerebral hemorrhage.

Inhibition of thrombin formation by active site mutated (S360A) activated protein C

Activated protein C (APC) down-regulates thrombin formation through proteolytic inactivation of factor Va (FVa) by cleavage at Arg(506) and Arg(306) and of factor VIIIa (FVIIIa) by cleavage at Arg(336) and Arg(562). To study substrate recognition by APC, active site-mutated APC (APC(S360A)) was used, which lacks proteolytic activity but exhibits anticoagulant activity. Experiments in model systems and in plasma show that APC(S360A), and not its zymogen protein C(S360A), expresses anticoagulant activities by competing with activated coagulation factors X and IX for binding to FVa and FVIIIa, respectively. APC(S360A) bound to FVa with a K(D) of 0.11 +/- 0.05 nm and competed with active site-labeled Oregon Green activated coagulation factor X for binding to FVa. The binding of APC(S360A) to FVa was not affected by protein S but was inhibited by prothrombin. APC(S360A) binding to FVa was critically dependent upon the presence of Arg(506) and not Arg(306) and additionally required an active site accessible to substrates. Inhibition of FVIIIa activity by APC(S360A) was >100-fold less efficient than inhibition of FVa. Our results show that despite exosite interactions near the Arg(506) cleavage site, binding of APC(S360A) to FVa is almost completely dependent on Arg(506) interacting with APC(S360A) to form a nonproductive Michaelis complex. Because docking of APC to FVa and FVIIIa constitutes the first step in the inactivation of the cofactors, we hypothesize that the observed anticoagulant activity may be important for in vivo regulation of thrombin formation.

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.

Novel APC-cleavage sites in FVa provide insights into mechanisms of action of APC and its cofactor protein S

BACKGROUND

Activated protein C (APC) inhibits factor Va (FVa) by cleaving at Arg306, Arg506 and Arg679. Protein S serves as cofactor, in particular for the Arg306 site, and a protein S-mediated relocation of the active site of APC closer to the membrane has been proposed as a mechanism. Recently, it was demonstrated that FVa, which was mutated at all three APC-cleavage sites (FVa-306Q/506Q/679Q), could still be cleaved by APC. These sites were close to Arg306 and Arg506 but not further defined.

OBJECTIVE

To identify and characterize the additional APC-cleavage sites in FVa.

METHODS

The cDNA for FV-306Q/506Q/679Q was used as a template to create FV variants with one or more possible cleavage sites being mutated. The FV variants were expressed and their sensitivity for APC characterized functionally and with Western blotting.

RESULTS

The additional APC-cleavage sites were located at Lys309, Arg313, Arg316, Arg317 and Arg505. FVa-306Q/309Q/313Q/316Q/317Q/505Q/506Q/679Q (denoted 8M-FVa) was APC resistant. To investigate individual sites, they were mutated back using 8M-FV as a template. The kinetics of APC-degradation of these variants demonstrated that protein S was equally efficient in enhancing the APC effect for all the novel sites.

CONCLUSIONS

Multiple APC-cleavage sites close to Arg306 and a single site close to Arg506 were identified. Protein S was equally efficient as APC cofactor for all novel sites. The stimulation by protein S of the Arg505 cleavage argues against a specific protein S-mediated stimulation of cleavage at Arg306 due to relocation of the APC active site closer to the membrane.

Activated protein C therapy slows ALS-like disease in mice by transcriptionally inhibiting SOD1 in motor neurons and microglia cells

Activated protein C (APC) is a signaling protease with anticoagulant activity. Here, we have used mice expressing a mutation in superoxide dismutase-1 (SOD1) that is linked to amyotrophic lateral sclerosis (ALS) to show that administration of APC or APC analogs with reduced anticoagulant activity after disease onset slows disease progression and extends survival. A proteolytically inactive form of APC with reduced anticoagulant activity provided no benefit. APC crossed the blood-spinal cord barrier in mice via endothelial protein C receptor. When administered after disease onset, APC eliminated leakage of hemoglobin-derived products across the blood-spinal cord barrier and delayed microglial activation. In microvessels, motor neurons, and microglial cells from SOD1-mutant mice and in cultured neuronal cells, APC transcriptionally downregulated SOD1. Inhibition of SOD1 synthesis in neuronal cells by APC required protease-activated receptor-1 (PAR1) and PAR3, which inhibited nuclear transport of the Sp1 transcription factor. Diminished mutant SOD1 synthesis by selective gene excision within endothelial cells did not alter disease progression, which suggests that diminished mutant SOD1 synthesis in other cells, including motor neurons and microglia, caused the APC-mediated slowing of disease. The delayed disease progression in mice after APC administration suggests that this approach may be of benefit to patients with familial, and possibly sporadic, ALS.

Growing insights into the potential benefits and risks of activated protein C administration in sepsis: a review of preclinical and clinical studies

Recombinant human activated protein C (rhAPC) was developed to reduce excessive coagulant and inflammatory activity during sepsis. Basic and clinical research has suggested these pathways contribute to the pathogenesis of this lethal syndrome and are inhibited by rhAPC. Based in large part on the results of a single multicenter randomized controlled trial, rhAPC was first approved in 2001 by the US Food and Drug Administration (FDA) as adjunctive therapy in septic patients with a high risk of death. This was followed closely by approval in Europe, Australia, and New Zealand. At the original FDA review of rhAPC, concerns were raised as to whether a confirmatory trial should be done before final regulatory approval because of concerns that rhAPCs bleeding risk might outweigh its potential benefit during clinical use. Since 2001, continuing basic and clinical research has further elucidated the complex role activated protein C may have in both adaptive and maladaptive responses during sepsis. Moreover, subsequent controlled trials in other types of septic patients and observational studies appear to support earlier concerns that the benefit-to-risk ratio of rhAPC may not support its clinical use. This experience has prompted additional trials presently underway, to define whether treatment with rhAPC as it was originally indicated in septic patients with persistent shock, is safe and effective. Until such trials are complete, physicians employing this agent must carefully consider which patients may be appropriate candidates for rhAPC administration.

An exosite-specific ssDNA aptamer inhibits the anticoagulant functions of activated protein C and enhances inhibition by protein C inhibitor

Activated protein C (APC) is a serine protease with anticoagulant, anti-inflammatory, and cytoprotective properties. Using recombinant APC, we identified a class of single-stranded DNA aptamers (HS02) that selectively bind to APC with high affinity. Interaction of HS02 with APC modulates the protease activity in a way such that the anticoagulant functions of APC are inhibited and its reactivity toward the protein C inhibitor is augmented in a glysoaminoglycan-like fashion, whereas APC's antiapoptotic and cytoprotective functions remain unaffected. Based on these data, the binding site of HS02 was localized to the basic exosite of APC. These characteristics render the exosite-specific aptamers a promising tool for the development of APC assays and a potential therapeutic agent applicable for the selective control of APC's anticoagulant activity.

Neuroprotective activities of activated protein C mutant with reduced anticoagulant activity

The anticoagulant activated protein C (APC) protects neurons and endothelium via protease activated receptor (PAR)1, PAR3 and endothelial protein C receptor. APC is neuroprotective in stroke models. Bleeding complications may limit the pharmacologic utility of APC. Here, we compared the 3K3A-APC mutant with 80% reduced anticoagulant activity and wild-type (wt)-APC. Murine 3K3A-APC compared with wt-APC protected mouse cortical neurons from N-methyl-D-aspartate-induced apoptosis with twofold greater efficacy and more potently reduced N-methyl-D-aspartate excitotoxic lesions in vivo. Human 3K3A-APC protected human brain endothelial cells (BECs) from oxygen/glucose deprivation with 1.7-fold greater efficacy than wt-APC. 3K3A-APC neuronal protection required PAR1 and PAR3, as shown by using PAR-specific blocking antibodies and PAR1- and PAR3-deficient cells and mice. BEC protection required endothelial protein C receptor and PAR1. In neurons and BECs, 3K3A-APC blocked caspase-9 and -3 activation and induction of p53, and decreased the Bax/Bcl-2 pro-apoptotic ratio. After distal middle cerebral artery occlusion (dMCAO) in mice, murine 3K3A-APC compared with vehicle given 4:00 h after dMCAO improved the functional outcome and reduced the infarction volume by 50% within 3 days. 3K3A-APC compared with wt-APC multi-dosing therapy at 12:00 h, 1, 3, 5 and 7 days after dMCAO significantly improved functional recovery and reduced the infarction volume by 75% and 38%, respectively, within 7 days. The wt-APC, but not 3K3A-APC, significantly increased the risk of intracerebral bleeding as indicated by a 50% increase in hemoglobin levels in the ischemic hemisphere. Thus, 3K3A-APC offers a new approach for safer and more efficacious treatments of neurodegenerative disorders and stroke with APC.

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.

Species-dependent neuroprotection by activated protein C mutants with reduced anticoagulant activity

Activated protein C (APC) is a protease with anticoagulant and cytoprotective activities. APC is neuroprotective in rodent models of stroke. But, an APC variant with reduced anticoagulant activity, 3K3A-APC, compared to wild-type APC shows greater neuroprotection with no risk for bleeding in stroke models. To determine whether 3K3A-APC exhibits species-dependent neuroprotection similar to that as seen with wild-type APC, we studied murine and human recombinant 3K3A-APC mutants which show approximately 80% reduced anticoagulant activity. Murine 3K3A-APC (0.2 mg/kg i.v.) administered at 4 h after embolic stroke improved substantially functional outcome and reduced by 80% the infract volume 7 days after stroke. Human 3K3A-APC was neuroprotective after embolic stroke in mice, but at significantly higher concentrations (i.e. 2 mg/kg i.v.). Species-dependent neuroprotection, i.e. murine > human 3K3A-APC, was confirmed in a mouse model of permanent middle cerebral artery occlusion. Human 3K3A-APC had by fivefold greater cytoprotective activity than murine 3K3A-APC in oxygen-glucose deprivation model in human brain endothelial cells, whereas murine 3K3A-APC was by 2.5-fold more potent than human 3K3A-APC in a mouse model of NMDA-induced neuronal apoptosis. Thus, 3K3A-APC exhibits species-dependent neuroprotection which should be taken into account when designing human trials for ischemic stroke with APC mutants.

Differential neuroprotection and risk for bleeding from activated protein C with varying degrees of anticoagulant activity

BACKGROUND AND PURPOSE

Activated protein C (APC), a protease with anticoagulant and cytoprotective activities, protects neurons and endothelium from ischemic injury. Drotrecogin-alfa activated, a hyperanticoagulant form of human recombinant APC, is currently being studied in patients with ischemic stroke. How changes in APC anticoagulant activity influence APC's neuroprotection and risk for bleeding is not clear.

METHODS

We used neuronal and brain endothelial cell injury models and middle cerebral artery occlusion in mice to compare efficacy and safety of drotrecogin-alfa activated and human 3K3A-APC, an APC nonanticoagulant mutant.

RESULTS

Drotrecogin-alfa activated and 3K3A-APC exhibited 148% and 10% of plasma-derived APC's anticoagulant activity and differ in the carbohydrate content. 3K3A-APC protected mouse neurons from N-methyl-d-aspartate-induced apoptosis and human brain endothelial cell from oxygen-glucose deprivation with 1.8- and 3.1-fold greater efficacy than drotrecogin-alfa activated. Given 5 minutes before transient middle cerebral artery occlusion, 3K3A-APC and drotrecogin-alfa activated (0.5 and 2 mg/kg intravenously) reduced comparably and dose-dependently the infarction lesion up to 85%. 3K3A-APC, but not drotrecogin-alfa activated, improved neurological score dose-dependently (P<0.05). 3K3A-APC did not cause bleeding. In contrast, drotrecogin-alfa activated dose-dependently increased hemoglobin content in postischemic brain. After permanent middle cerebral artery occlusion, 3K3A-APC multidose therapy (1 mg/kg intravenously at 12 hours and 1, 3, 5, and 7 days) improved functional recovery and reduced infarction by 60% with no risk for bleeding, whereas drotrecogin-alfa activated increased hemoglobin deposition in the postischemic brain and showed relatively modest neuroprotection.

CONCLUSIONS

Nonanticoagulant 3K3A-APC exhibits greater neuroprotective efficacy with no risk for bleeding compared with drotrecogin-alfa activated, a hyperanticoagulant form of APC.

Identification of surface epitopes of human coagulation factor Va that are important for interaction with activated protein C and heparin

Inactivation of factor Va (FVa) by activated protein C (APC) is a key reaction in the down-regulation of thrombin formation. FVa inactivation by APC is correlated with a loss of FXa cofactor activity as a result of three proteolytic cleavages in the FVa heavy chain at Arg306, Arg506, and Arg679. Recently, we have shown that heparin specifically inhibits the APC-mediated cleavage at Arg506 and stimulates cleavage at Arg306. Three-dimensional molecular models of APC docked at the Arg306 and Arg506 cleavage sites in FVa have identified several FVa amino acids that may be important for FVa inactivation by APC in the absence and presence of heparin. Mutagenesis of Lys320, Arg321, and Arg400 to Ala resulted in an increased inactivation rate by APC at Arg306, which indicates the importance of these residues in the FVa-APC interaction. No heparin-mediated stimulation of Arg306 cleavage was observed for these mutants, and stimulation by protein S was similar to that of wild type FVa. With this, we have now demonstrated that a cluster of basic residues in FVa comprising Lys320, Arg321, and Arg400 is required for the heparin-mediated stimulation of cleavage at Arg306 by APC. Furthermore, mutations that were introduced near the Arg506 cleavage site had a significant but modest effect on the rate of APC-catalyzed FVa inactivation, suggesting an extended interaction surface between the FVa Arg506 site and APC.

Activated protein C mutant with minimal anticoagulant activity, normal cytoprotective activity, and preservation of thrombin activable fibrinolysis inhibitor-dependent cytoprotective functions

Activated protein C (APC) reduces mortality in severe sepsis patients and exhibits beneficial effects in multiple animal injury models. APC anticoagulant activity involves inactivation of factors Va and VIIIa, whereas APC cytoprotective activities involve the endothelial protein C receptor and protease-activated receptor-1 (PAR-1). The relative importance of the anticoagulant activity of APC versus the direct cytoprotective effects of APC on cells for the in vivo benefits is unclear. To distinguish cytoprotective from the anticoagulant activities of APC, a protease domain mutant, 5A-APC (RR229/230AA and KKK191-193AAA), was made and compared with recombinant wild-type (rwt)-APC. This mutant had minimal anticoagulant activity but normal cytoprotective activities that were dependent on endothelial protein C receptor and protease-activated receptor-1. Whereas anticoagulantly active rwt-APC inhibited secondary-extended thrombin generation and concomitant thrombin-dependent activation of thrombin activable fibrinolysis inhibitor (TAFI) in plasma, secondary-extended thrombin generation and the activation of TAFI were essentially unopposed by 5A-APC due to its low anticoagulant activity. Compared with rwt-APC, 5A-APC had minimal profibrinolytic activity and preserved TAFI-mediated anti-inflammatory carboxypeptidase activities toward bradykinin and presumably toward the anaphlatoxins, C3a and C5a, which are well known pathological mediators in sepsis. Thus, genetic engineering can selectively alter the multiple activities of APC and provide APC mutants that retain the beneficial cytoprotective effects of APC while diminishing bleeding risk due to reduction in APC's anticoagulant and APC-dependent profibrinolytic activities.

Factor Va residues 311-325 represent an activated protein C binding region

Activated protein C (APC) inactivates factor Va (fVa) by proteolytically cleaving fVa heavy chain at Arg(506), Arg(306), and Arg(679). Factor Xa (fXa) protects fVa from inactivation by APC. To test the hypothesis that fXa and APC share overlapping fVa binding sites, 15 amino acid-overlapping peptides representing the heavy chain (residues 1-709) of fVa were screened for inhibition of fVa inactivation by APC. As reported, VP311-325, a peptide comprising residues 311-325 in fVa, dose-dependently and potently inhibited fVa-dependent prothrombin activation by fXa in the absence of APC. This peptide also inhibited the inactivation of fVa by APC, suggesting that this region of fVa interacts with APC. The peptide inhibited the APC-dependent cleavage of both Arg(506) and Arg(306) because inhibition was observed with plasma-derived fVa and recombinant R506Q and RR306/679QQ fVa. VP311-325 altered the fluorescence emission of dansyl-active site-labeled APC(i) but not a dansyl-active site-labeled thrombin control, showing that the peptide binds to APC(i). This peptide also inhibited the resonance energy transfer between membrane-bound fluorescein-labeled fVa (donor) and rhodamine-active site-labeled S360C-APC (acceptor). These data suggest that peptide VP311-325 represents both an APC and fXa binding region in fVa.

Exosites in the substrate specificity of blood coagulation reactions

The specificity of blood coagulation proteinases for substrate, inhibitor, and effector recognition is mediated by exosites on the surfaces of the catalytic domains, physically separated from the catalytic site. Some thrombin ligands bind specifically to either exosite I or II, while others engage both exosites. The involvement of different, overlapping constellations of exosite residues enables binding of structurally diverse ligands. The flexibility of the thrombin structure is central to the mechanism of complex formation and the specificity of exosite interactions. Encounter complex formation is driven by electrostatic ligand-exosite interactions, followed by conformational rearrangement to a stable complex. Exosites on some zymogens are in low affinity proexosite states and are expressed concomitant with catalytic site activation. The requirement for exosite expression controls the specificity of assembly of catalytic complexes on the coagulation pathway, such as the membrane-bound factor Xa*factor Va (prothrombinase) complex, and prevents premature assembly. Substrate recognition by prothrombinase involves a two-step mechanism with initial docking of prothrombin to exosites, followed by a conformational change to engage the FXa catalytic site. Prothrombin and its activation intermediates bind prothrombinase in two alternative conformations determined by the zymogen to proteinase transition that are hypothesized to involve prothrombin (pro)exosite I interactions with FVa, which underpin the sequential activation pathway. The role of exosites as the major source of substrate specificity has stimulated development of exosite-targeted anticoagulants for treatment of thrombosis.

Activated protein C

Protein C is a vitamin K-dependent plasma protein zymogen whose genetic mild or severe deficiencies are linked with risk for venous thrombosis or neonatal purpura fulminans, respectively. Studies over past decades showed that activated protein C (APC) inactivates factors (F) Va and VIIIa to down-regulate thrombin generation. More recent basic and preclinical research on APC has characterized the direct cytoprotective effects of APC that involve gene expression profile alterations, anti-inflammatory and anti-apoptotic activities and endothelial barrier stabilization. These actions generally require endothelial cell protein C receptor (EPCR) and protease activated receptor-1. Because of these direct cytoprotective actions, APC reduces mortality in murine endotoxemia and severe sepsis models and provides neuroprotective benefits in murine ischemic stroke models. Furthermore, APC reduces mortality in patients with severe sepsis (PROWESS clinical trial). Although much remains to be clarified about mechanisms for APC's direct effects on various cell types, it is clear that APC's molecular features that determine its antithrombotic action are partially distinct from those providing cytoprotective actions because we have engineered recombinant APC variants with selective reduction or retention of either anticoagulant or cytoprotective activities. Such APC variants can provide relatively enhanced levels of either cytoprotective or anticoagulant activities for various therapeutic applications. We speculate that APC variants with reduced anticoagulant action but normal cytoprotective actions hold the promise of reducing bleeding risk because of attenuated anticoagulant activity while reducing mortality based on direct cytoprotective effects on cells.

Identification of a specific exosite on activated protein C for interaction with protease-activated receptor 1

Activated protein C (APC) is a vitamin K-dependent plasma serine protease which down-regulates the clotting cascade by inactivating procoagulant factors Va and VIIIa by limited proteolysis. In addition to its anticoagulant effect, APC also exhibits cytoprotective and antiinflammatory activity through the endothelial protein C receptor-dependent cleavage of protease activated receptor 1 (PAR-1) on endothelial cells. Recent mutagenesis data have indicated that the basic residues of two surface loops including those on 39 and the Ca2+-binding 70-80 loops constitute interactive sites for both factors Va and VIIIa, thereby mediating the interaction of APC specifically with these procoagulant cofactors. The basic residues of both loops have been discovered to be dispensable for the interaction of APC with PAR-1. It is not known if a similar exosite-dependent interaction contributes to the specificity of APC recognition of PAR-1 on endothelial cells. In this study, we have identified two acidic residues on helix-162 (Glu-167 and Glu-170) on the protease domain of APC which are required for the protease interaction with PAR-1, but not for its interaction with the procoagulant cofactors. Thus, the substitution of either Glu-167 or Glu-170 with Ala eliminated the cytoprotective signaling properties of APC without affecting its anticoagulant activity. These mutants provide useful tools for initiating in vivo studies to understand the extent to which the anticoagulant versus antiinflammatory activity of APC contributes to its beneficial effect in treating severe sepsis.

Engineering a disulfide bond to stabilize the calcium-binding loop of activated protein C eliminates its anticoagulant but not its protective signaling properties

In addition to an anticoagulant activity, activated protein C (APC) also exhibits anti-inflammatory and cytoprotective properties. These properties may contribute to the beneficial effect of APC in treating severe sepsis patients. A higher incidence of bleeding because of its anticoagulant function has been found to be a major drawback of APC as an effective anti-inflammatory drug. In this study, we have prepared a protein C variant in which an engineered disulfide bond between two beta-sheets stabilized the functionally critical Ca2+-binding 70-80 loop of the molecule. The 70-80 loop of this mutant no longer bound Ca2+, and the activation of the mutant by thrombin was enhanced 60-80-fold independently of thrombomodulin. The anticoagulant activity of the activated protein C mutant was nearly eliminated as determined by a plasma-based clotting assay. However, the endothelial protein C receptor- and protease-activated receptor-1-dependent protective signaling properties of the mutant were minimally altered as determined by staurosporine-induced endothelial cell apoptosis, thrombin-induced endothelial cell permeability, and tumor necrosis-alpha-mediated neutrophil adhesion and migration assays. These results suggest that the mutant lost its ability to interact with the procoagulant cofactors but not with the protective signaling molecules; thus this mutant provides an important tool for in vivo studies to examine the role of anticoagulant versus anti-inflammatory function of activated protein C.

The cytoprotective protein C pathway

Protein C is best known for its mild deficiency associated with venous thrombosis risk and severe deficiency associated with neonatal purpura fulminans. Activated protein C (APC) anticoagulant activity involves proteolytic inactivation of factors Va and VIIIa, and APC resistance is often caused by factor V Leiden. Less known is the clinical success of APC in reducing mortality in severe sepsis patients (PROWESS trial) that gave impetus to new directions for basic and preclinical research on APC. This review summarizes insights gleaned from recent in vitro and in vivo studies of the direct cytoprotective effects of APC that include beneficial alterations in gene expression profiles, anti-inflammatory actions, antiapoptotic activities, and stabilization of endothelial barriers. APC's cytoprotection requires its receptor, endothelial cell protein C receptor, and protease-activated receptor-1. Because of its pleiotropic activities, APC has potential roles in the treatment of complex disorders, including sepsis, thrombosis, and ischemic stroke. Although much about molecular mechanisms for APC's effects on cells remains unclear, it is clear that APC's structural features mediating anticoagulant actions and related bleeding risks are distinct from those mediating cytoprotective actions, suggesting the possibility of developing APC variants with an improved profile for the ratio of cytoprotective to anticoagulant actions.

Role of P1 residues Arg336 and Arg562 in the activated-Protein-C-catalysed inactivation of Factor VIIIa

APC (activated Protein C) inactivates human Factor VIIIa following cleavage at residues Arg336 and Arg562 within the A1 and A2 subunits respectively. The role of the P1 arginine in APC-catalysed inactivation of Factor VIIIa was examined by employing recombinant Factor VIIIa molecules where residues 336 and 562 were replaced with alanine and/or glutamine. Stably expressed Factor VIII proteins were activated by thrombin and resultant Factor VIIIa was reacted at high concentration with APC to minimize cofactor inactivation due to A2 subunit dissociation. APC cleaved wild-type Factor VIIIa at the A1 site with a rate approximately 25-fold greater than that for the A2 site. A1 mutants R336A and R336Q were inactivated approximately 9-fold slower than wild-type Factor VIIIa, whereas the A2 mutant R562A was inactivated approximately 2-fold slower. No cleavage at the mutated sites was observed. Taken together, these results suggested that cleavage at the A1 site was the dominant mechanism for Factor VIIIa inactivation catalysed by the proteinase. On the basis of cleavage at Arg336, a K(m) value for wild-type Factor VIIIa of 102 nM was determined, and this value was significantly greater than K(i) values (approximately 9-18 nM) obtained for an R336Q/R562Q Factor VIIIa. Furthermore, evaluation of a series of cluster mutants in the C-terminal region of the A1 subunit revealed a role for acidic residues in segment 341-345 in the APC-catalysed proteolysis of Arg336. Thus, while P1 residues contribute to catalytic efficiency, residues removed from these sites make a primary contribution to the overall binding of APC to Factor VIIIa.

Activation of protein C by the thrombin-thrombomodulin complex: cooperative roles of Arg-35 of thrombin and Arg-67 of protein C

The binding of Ca(2+) to the 70-80 loop of protein C inhibits protein C activation by thrombin in the absence of thrombomodulin (TM), but the metal ion is required for activation in the presence of TM. Structural data suggests that the 70-80 loop is located between two antiparallel beta strands comprised of residues 64-69 and 81-91 on the protease domain of protein C. To test the hypothesis that a salt-bridge/hydrogen bond interaction between Arg-67 of the former strand and Asp-82 of the latter strand modulates the unique Ca(2+)-binding properties of protein C, we engineered a disulfide bond between the two strands by substituting both Arg-67 and Asp-82 with Cys residues. The activation of this mutant was enhanced 40- to 50-fold independent of TM and Ca(2+). Furthermore, the Arg-67 to Ala mutant of protein C was activated in the absence of TM by the Arg-35 to Glu mutant of thrombin with the same efficiency as wild-type protein C by wild-type thrombin-TM complex. These results suggest that TM functions by alleviating the Ca(2+)-dependent inhibitory interactions of Arg-67 of protein C and Arg-35 of thrombin.

Regulation of Blood Coagulation by the Protein C Anticoagulant Pathway

The protein C system provides important control of blood coagulation by regulating the activities of factor VIIIa (FVIIIa) and factor Va (FVa), cofactors in the activation of factor X and prothrombin, respectively. The system comprises membrane-bound and circulating proteins that assemble into multi-molecular complexes on cell surfaces. Vitamin K-dependent protein C, the key component of the system, circulates in blood as zymogen to an anticoagulant serine protease. It is efficiently activated on the surface of endothelial cells by thrombin bound to the membrane protein thrombomodulin. The endothelial protein C receptor (EPCR) further stimulates the protein C activation. Activated protein C (APC) together with its cofactor protein S inhibits coagulation by degrading FVIIIa and FVa on the surface of negatively charged phospholipid membranes. Efficient FVIIIa degradation by APC requires not only protein S but also intact FV, which like thrombin is a Janus-faced protein with both procoagulant and anticoagulant potential. In addition to its anticoagulant properties, APC has antiinflammatory and antiapoptotic functions, which are exerted when APC binds to EPCR and proteolytic cleaves protease-activated receptor 1 (PAR-1). The protein C system is physiologically important, and genetic defects affecting the system are the most common risk factors of venous thrombosis. The proteins of the protein C system are composed of multiple domains and the 3-dimensional structures of several of the proteins are known. The molecular recognition of the protein C system is progressively being unraveled, giving us new insights into this fascinating and intricate molecular scenario at the atomic level.

Exosite-mediated substrate recognition of factor IX by factor XIa. The factor XIa heavy chain is required for initial recognition of factor IX

Studies of the mechanisms of blood coagulation zymogen activation demonstrate that exosites (sites on the activating complex distinct from the protease active site) play key roles in macromolecular substrate recognition. We investigated the importance of exosite interactions in recognition of factor IX by the protease factor XIa. Factor XIa cleavage of the tripeptide substrate S2366 was inhibited by the active site inhibitors p-aminobenzamidine (Ki 28 +/- 2 microM) and aprotinin (Ki 1.13 +/- 0.07 microM) in a classical competitive manner, indicating that substrate and inhibitor binding to the active site was mutually exclusive. In contrast, inhibition of factor XIa cleavage of S2366 by factor IX (Ki 224 +/- 32 nM) was characterized by hyperbolic mixed-type inhibition, indicating that factor IX binds to free and S2366-bound factor XIa at exosites. Consistent with this premise, inhibition of factor XIa activation of factor IX by aprotinin (Ki 0.89 +/- 0.52 microM) was non-competitive, whereas inhibition by active site-inhibited factor IXa beta was competitive (Ki 0.33 +/- 0.05 microM). S2366 cleavage by isolated factor XIa catalytic domain was competitively inhibited by p-aminobenzamidine (Ki 38 +/- 14 microM) but was not inhibited by factor IX, consistent with loss of factor IX-binding exosites on the non-catalytic factor XI heavy chain. The results support a model in which factor IX binds initially to exosites on the factor XIa heavy chain, followed by interaction at the active site with subsequent bond cleavage, and support a growing body of evidence that exosite interactions are critical determinants of substrate affinity and specificity in blood coagulation reactions.

Exosite-driven substrate specificity and function in coagulation

Macromolecular substrate recognition and serine proteinase specificity lie at the heart of the tightly regulated hemostatic response. Mechanisms established for the less specific serine proteinases of digestion have played a dominant role in guiding investigations of the basis for the narrow specificities exhibited by the coagulation enzymes. These concepts have also dominated the development of specific inhibitors of coagulation for therapeutic purposes. Studies of the enzymology and physical biochemistry of prothrombinase challenge these prevailing ideas by establishing a principal role for exosites within the enzyme in determining substrate recognition and directing the action of the enzyme on its biological substrate. Mechanisms by which narrow protein substrate specificity is achieved by prothrombinase also apply to several other reactions of coagulation. These strategies are increasingly evident in the action of other families of enzymes that act with high specificity on protein substrates. Exosite-driven enzymic function probably represents a widely employed biological strategy for the achievement of high macromolecular substrate specificity.

Zymogenic and enzymatic properties of the 70-80 loop mutants of factor X/Xa

The Ca(2+) binding 70-80 loop of factor X (fX) contains one basic (Arg(71)) and three acidic (Glu(74), Glu(76), and Glu(77)) residues whose contributions to the zymogenic and enzymatic properties of the protein have not been evaluated. We prepared four Ala substitution mutants of fX (R71A, E74A, E76A, and E77A) and characterized their activation kinetics by the factor VIIa and factor IXa in both the absence and presence of cofactors. Factor VIIa exhibited normal activity toward E74A and E76A and less than a twofold impaired activity toward R71A and E77A in both the absence and presence of tissue factor. Similarly, factor IXa in the absence of factor VIIIa exhibited normal activity toward both E74A and E76A; however, its activity toward R71A and E77A was impaired approximately two- to threefold. In the presence of factor VIIIa, factor IX activated all mutants with approximately two- to fivefold impaired catalytic efficiency. In contrast to changes in their zymogenic properties, all mutant enzymes exhibited normal affinities for factor Va, and catalyzed the conversion of prothrombin to thrombin with normal catalytic efficiencies. However, further studies revealed that the affinity of mutant enzymes for interaction with metal ions Na(+) and Ca(2+) was impaired. These results suggest that although charged residues of the 70-80 loop play an insignificant role in fX recognition by the factor VIIa-tissue factor complex, they are critical for the substrate recognition by factor IXa in the intrinsic Xase complex. The results further suggest that mutant residues do not play a specific role in the catalytic function of fXa in the prothrombinase complex.