Species-specific anticoagulant and mitogenic activities of murine protein S

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.

Platelet protein S limits venous but not arterial thrombosis propensity by controlling coagulation in the thrombus

Anticoagulant protein S (PS) in platelets (PSplt) resembles plasma PS and is released on platelet activation, but its role in thrombosis has not been elucidated. Here we report that inactivation of PSplt expression using the Platelet factor 4 (Pf4)-Cre transgene (Pros1lox/loxPf4-Cre+) in mice promotes thrombus propensity in the vena cava, where shear rates are low, but not in the carotid artery, where shear rates are high. At a low shear rate, PSplt functions as a cofactor for both activated protein C and tissue factor pathway inhibitor, thereby limiting factor X activation and thrombin generation within the growing thrombus and ensuring that highly activated platelets and fibrin remain localized at the injury site. In the presence of high thrombin concentrations, clots from Pros1lox/loxPf4-Cre- mice contract, but not clots from Pros1lox/loxPf4-Cre+ mice, because of highly dense fibrin networks. Thus, PSplt controls platelet activation as well as coagulation in thrombi in large veins, but not in large arteries.

Roles of Factor XII in Innate Immunity

Factor XII (FXII) is the zymogen of serine protease, factor XIIa (FXIIa). FXIIa enzymatic activities have been extensively studied and FXIIa inhibition is emerging as a promising target to treat or prevent thrombosis without creating a hemostatic defect. FXII and plasma prekallikrein reciprocally activate each other and result in liberation of bradykinin. Due to its unique structure among coagulation factors, FXII exerts mitogenic activity in endothelial and smooth muscle cells, indicating that zymogen FXII has activities independent of its protease function. A growing body of evidence has revealed that both FXII and FXIIa upregulate neutrophil functions, contribute to macrophage polarization and induce T-cell differentiation. In vivo, these signaling activities contribute to host defense against pathogens, mediate the development of neuroinflammation, influence wound repair and may facilitate cancer maintenance and progression. Here, we review the roles of FXII in innate immunity as they relate to non-sterile and sterile immune responses.

Protein S K196E mutation reduces its cofactor activity for APC but not for TFPI

BACKGROUND

Protein S (PS) is an anticoagulant molecule that functions as a cofactor for activated protein C (APC) in the inactivation of activated coagulation factors Va (FVa) and VIIIa. It also serves as a cofactor for tissue factor pathway inhibitor (TFPI) in the efficient inhibition of factor Xa (FXa). The Lys196-to-Glu (K196E, Tokushima) mutation in the EGF-2 domain of PS is a genetic risk factor for venous thromboembolism (VTE) in the Japanese population.

OBJECTIVES

To investigate the molecular basis of the thrombophilic phenotype of Japanese patients carrying the PS K196E mutation.

METHODS

We expressed recombinant human PS wild-type (PS-K) and K196E-mutant (PS-E) in CHO cells, and purified them by Ni2+-affinity and anion exchange column chromatography. We investigated the anticoagulant functions of PS-K and PS-E by measuring APC cofactor activity, TFPI cofactor activity, affinity for the β chain of complement component C4b-binding protein (C4BP), and cleavage by thrombin.

RESULTS

PS-E had approximately 40% APC cofactor activity compared with PS-K in a clotting-based assay and a FVa inactivation assay. The TFPI cofactor activity of PS-E in the FXa inactivation assay was equivalent to that of PS-K in the absence and presence of coagulation factor V. The strengths of PS-E and PS-K binding to the β chain of C4BP were comparable, and both were equally cleaved by thrombin.

CONCLUSIONS

The PS K196E mutation increases the risk of VTE because of reduced APC cofactor activity but does not alter various other properties, including the TFPI cofactor activity.

A case control study on the structural equation model of the mechanism of coagulation and fibrinolysis imbalance in chronic schistosomiasis

A structural equation model was used for verification with chronic schistosomiasis to investigate the coagulation-anticoagulation system imbalance and to deduce the mechanism of D-dimer (D-D) level elevation in patients with advanced schistosome hepatic disease. We detected the plasma levels of tissue-type fiber plasminogen activator (tPA), urokinase type plasminogen activator (uPA), plasmin-antiplasmin complex (PAP), plasminogen (PLG), antithrombin (AT), plasminogen activator inhibitor 1 (PAI1), D-D, factor VIII: C (FVIII:C), antithrombin-III (AT-III), PLG, protein S (PS), and protein C (PC) in the healthy people as control (69), patients with chronic schistosomiasis (150) or advanced chronic schistosomiasis (90). FVIII, PAP, D-D, tPA, and uPA plasma levels were significantly higher in the chronic group than in the control group and were also significantly higher in the advanced group. However, AT-III, PC, PS, AT, PLG, and PAI1 plasma levels in the advanced and chronic groups were significantly lower than those in the control group. With progression of disease in patients with schistosomiasis japonica, a hypercoagulable state is induced by the coagulation-anticoagulation imbalance, eventually leading to patients with high levels of D-D. Furthermore, we established a structural equation model path of a "chronic schistosomiasis disease stage-(coagulation-anticoagulation-fibrinolysis)-D-D." By using analysis of moment structures (AMOS), it was shown that the chronic schistosomiasis stage was positively related to factor VIII and had negative correlation with AT-III; a good positive correlation with PAP, tPA, and uPA; and a good negative correlation with PLG and PAI1. In addition, our results show that the path coefficient of anticoagulation-fibrinolysis system to the chronic stage of schistosomiasis or D-D levels was significantly higher than that of the coagulation system. In conclusion, the coagulation and fibrinolysis imbalance in patients with chronic schistosomiasis, especially with advanced schistosomiasis, is due to the progression of disease stages.

Role of protein S in castration-resistant prostate cancer-like cells

Understanding how castration-resistant prostate cancer (CRPC) cells survive the androgen-deprivation condition is crucial for treatment of this advanced prostate cancer (PCa). Here, we reported for the first time the up-regulation of protein S (PROS), an anticoagulant plasma glycoprotein with multiple biological functions, in androgen-insensitive PCa cells and in experimentally induced castration-resistant PCa cells. Overexpression of exogenous PROS in LNCaP cells reduced androgen deprivation-induced apoptosis and enhanced anchorage-dependent clonogenic ability under androgen deprivation condition. Reciprocally, PROS1 knockdown inhibited cell invasiveness and migration, caused the growth inhibition of castration-resistant tumor xenograft under androgen-depleted conditions, and potentiated Taxol (a widely prescribed anti-neoplastic agent)-mediated cell death in PC3 cells. Furthermore, PROS overexpression significantly stimulated AKT activation but failed to evoke oxidative stress in LNCaP cells under normal condition, suggesting that the malignance-promoting effects of the above-mentioned pathway may occur in the order of oxidative stress/PROS/AKT. The potential mechanism may be due to control of oxidative stress-elicited activation of PI3K-AKT-mTOR pathway. Taken together, our gain-of-function, loss-of-function analyses suggest that PROS may facilitate cell proliferation and promote castration resistance in human castration-resistant PCa-like cells via its apoptosis-regulating property. Future study emphasizing on delineating how PROS regulate cellular processes controlling transformation during the development of castration resistance should open new doors for the development of novel therapeutic targets for CRPC.

Protein S is protective in pulmonary fibrosis

Essentials Epithelial cell apoptosis is critical in the pathogenesis of idiopathic pulmonary fibrosis. Protein S, a circulating anticoagulant, inhibited apoptosis of lung epithelial cells. Overexpression of protein S in lung cells reduced bleomycin-induced pulmonary fibrosis. Intranasal therapy with exogenous protein S ameliorated bleomycin-induced pulmonary fibrosis.

SUMMARY

Background Pulmonary fibrosis is the terminal stage of interstitial lung diseases, some of them being incurable and of unknown etiology. Apoptosis plays a critical role in lung fibrogenesis. Protein S is a plasma anticoagulant with potent antiapoptotic activity. The role of protein S in pulmonary fibrosis is unknown. Objectives To evaluate the clinical relevance of protein S and its protective role in pulmonary fibrosis. Methods and Results The circulating level of protein S was measured in patients with pulmonary fibrosis and controls by the use of enzyme immunoassays. Pulmonary fibrosis was induced with bleomycin in transgenic mice overexpressing human protein S and wild-type mice, and exogenous protein S or vehicle was administered to wild-type mice; fibrosis was then compared in both models. Patients with pulmonary fibrosis had reduced circulating levels of protein S as compared with controls. Inflammatory changes, the levels of profibrotic cytokines, fibrosis score, hydroxyproline content in the lungs and oxygen desaturation were significantly reduced in protein S-transgenic mice as compared with wild-type mice. Wild-type mice treated with exogenous protein S showed significant decreases in the levels of inflammatory and profibrotic markers and fibrosis in the lungs as compared with untreated control mice. After bleomycin infusion, mice overexpressing human protein S showed significantly low caspase-3 activity, enhanced expression of antiapoptotic molecules and enhanced Akt and Axl kinase phosphorylation as compared with wild-type counterparts. Protein S also inhibited apoptosis of alveolar epithelial cells in vitro. Conclusions These observations suggest clinical relevance and a protective role of protein S in pulmonary fibrosis.

Exacerbated venous thromboembolism in mice carrying a protein S K196E mutation

Protein S (PS) acts as an anticoagulant cofactor for activated protein C in regulation of blood coagulation. The K196E mutation in PS is a race-specific genetic risk factor for venous thromboembolism with a prevalence of ∼2% within the Japanese population. To evaluate the thrombosis risk of the PS-K196E mutation, we generated PS-K196E knockin mice and heterozygous PS-deficient mice. We analyzed their thrombotic states, comparing with mice carrying the factor V Leiden mutation (FV-R504Q), a race-specific genetic risk for venous thrombosis in whites. PS-K196E mice grew normally but had decreased activated protein C cofactor activity in plasma. Purified recombinant murine PS-K196E showed the same decreased activated protein C cofactor activity. A deep vein thrombosis model of electrolytic inferior vena cava injury and pulmonary embolism models induced by infusion of tissue factor or polyphosphates revealed that PS-K196E mice, heterozygous PS-deficient mice, and FV-R504Q mice were much more susceptible to venous thrombosis compared with wild-type mice. Transient middle cerebral artery ischemia-reperfusion injury model studies demonstrated that both PS-K196E mice and heterozygous PS-deficient mice had cerebral infarction similar to wild-type mice, consistent with human observations. Our in vitro and in vivo results support a causal relationship between the PS-K196E mutation and venous thrombosis and indicate that PS-K196E mice can provide an in vivo evaluation system to help uncovering racial differences in thrombotic diseases.

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.

TAM receptor signaling in immune homeostasis

The TAM receptor tyrosine kinases (RTKs)-TYRO3, AXL, and MERTK-together with their cognate agonists GAS6 and PROS1 play an essential role in the resolution of inflammation. Deficiencies in TAM signaling have been associated with chronic inflammatory and autoimmune diseases. Three processes regulated by TAM signaling may contribute, either independently or collectively, to immune homeostasis: the negative regulation of the innate immune response, the phagocytosis of apoptotic cells, and the restoration of vascular integrity. Recent studies have also revealed the function of TAMs in infectious diseases and cancer. Here, we review the important milestones in the discovery of these RTKs and their ligands and the studies that underscore the functional importance of this signaling pathway in physiological immune settings and disease.

Protein S exacerbates alcoholic hepatitis by stimulating liver natural killer T cells

BACKGROUND

Alcohol consumption is a major cause of liver injury but the mechanisms are not completely understood. Protein S (PS) is an anticoagulant glycoprotein with multiple functions. The role of PS in liver injury is unknown.

OBJECTIVES

This study investigated the role of PS in acute alcoholic hepatitis.

METHODS

A mouse overexpressing human PS (hPS-TG) was generated in which acute hepatitis was induced by intraperitoneal injection of ethanol.

RESULTS

The levels of serum liver enzymes and liver tissue inflammatory cytokines and the degree of hepatic steatosis were significantly increased in hPS-TG mice treated with ethanol compared with ethanol-treated wild type (WT) mice. Cell expansion, activation and inhibition of apoptosis were significantly augmented in natural killer T (NKT) cells from hPS-TG mice compared with WT mice. Liver mononuclear cells from hPS-TG mice express higher levels of inflammatory cytokines than those from WT mice after stimulation with a specific stimulant of NKT cells in vitro. In a co-culture system of hepatocytes and NKT cells, the effects of PS on ethanol-mediated cell injury were suppressed by a CD1d neutralizing antibody. Alcoholic liver injury was significantly improved in mice pre-treated with PS siRNA and anti-protein S antibody compared with control mice. Patients with alcoholic hepatitis showed significantly increased plasma PS levels and enhanced liver expression of PS and CD1d compared with controls.

CONCLUSIONS

The results of this study suggest that PS exacerbates acute alcoholic hepatitis by inhibiting apoptosis of activated NKT cells.

An engineered factor Va prevents bleeding induced by anticoagulant wt activated protein C

OBJECTIVE

An increased risk of bleeding is observed in patients receiving activated protein C (APC), which may be a limiting factor for the application of novel APC therapies. Since APC's therapeutic effects often require its cytoprotective activities on cells but not APC's anticoagulant activities, an agent that specifically antagonizes APC's anticoagulant effects but not its cytoprotective effects could provide an effective means to control concerns for risk of bleeding. We hypothesized that superFVa, an engineered activated FVa-variant that restores hemostasis in hemophilia could reduce APC-induced bleeding.

APPROACH AND RESULTS

SuperFVa was engineered with mutations of the APC cleavage sites (Arg506/306/679Gln) and a disulfide bond (Cys609-Cys1691) between the A2 and A3 domains, which augment its biological activity and cause high resistance to APC. SuperFVa normalized APC-prolonged clotting times and restored APC-suppressed thrombin generation in human and murine plasma at concentrations where wild-type (wt) FVa did not show effects. Following intravenous injection of APC into BALB/c mice, addition to whole blood ex vivo of superFVa but not wt-FVa significantly normalized whole blood clotting. Blood loss following tail clip or liver laceration was significantly reduced when superFVa was administered intravenously to BALB/c mice prior to intravenous APC-treatment. Furthermore, superFVa abolished mortality (∼50%) associated with excessive bleeding following liver laceration in mice treated with APC.

CONCLUSIONS

Our results provide proof of concept that superFVa is effective in preventing APC-induced bleeding and may provide therapeutic benefits as a prohemostatic agent in various situations where bleeding is a serious risk.

Plasma protein S residues 37-50 mediate its binding to factor Va and inhibition of blood coagulation

Protein S (PS) is an anticoagulant plasma protein whose deficiency is associated with increased risk of venous thrombosis. PS directly inhibits thrombin generation by the blood coagulation pathways by several mechanisms, including by binding coagulation factors (F) Va and Xa. To identify PS sequences that mediate inhibition of FVa activity, antibodies and synthetic peptides based on PS sequence were prepared and employed in plasma coagulation assays, purified component prothrombinase assays, binding assays, and immunoblots. In the absence of activated protein C, monoclonal antibody (Mab) S4 shortened FXa-induced clotting in normal plasma but not in PS-depleted plasma. Mab S4 also blocked PS inhibition of FVa-dependent prothrombinase activity in purified component assays in the absence or presence of phospholipids and inhibited binding of PS to immobilised FVa. Epitope mapping identified N-terminal region residues 37-67 of PS as this antibody's epitope. A peptide representing PS residues 37-50 inhibited FVa-dependent prothrombinase activity in a non-competitive manner, with 50% inhibition observed at 11 µM peptide, whereas a peptide with a D-amino acid sequence of 37-50 was ineffective. FVa, but not FXa, bound specifically to the immobilised peptide representing residues 37-50, and the peptide inhibited binding of FVa to immobilised PS. These data implicate PS residues 37-50 as a binding site for FVa that mediates, at least in part, the direct inhibition of FVa-dependent procoagulant activity by PS.

Antithrombotic activity of protein S infused without activated protein C in a baboon thrombosis model

Protein S (ProS) is an essential plasma protein that enhances the anticoagulant activity of activated protein C (APC). In vitro , purified native human Zn2+-containing ProS also exerts direct anticoagulant activity by inhibiting prothrombinase and extrinsic FXase activities independently of APC. We investigated antithrombotic effects of ProS infused without APC in a baboon shunt model of thrombogenesis that employs a device consisting of arterial and venous shear flow segments. In in vitro experiments, the Zn2+-containing human ProS used for the studies displayed >10-fold higher prothrombinase inhibitory activity and anticoagulant activity in tissue factor-stimulated plasma, and four-fold higher inhibition of the intrinsic pathway than the Zn2+-deficient ProS used. In the thrombosis model, ProS (33 μg/minute for 1 hour) or saline was infused locally; platelet and fibrin deposition in the shunt were measured over 2 hours. During experiments performed at 50 ml/minute blood flow, Zn2+-containing ProS inhibited platelet deposition 73-96% in arterial-type flow segments and 90-99% in venous-type flow segments; Zn2+-deficient ProS inhibited platelet deposition 52% in arterial-type flow segments and 65-73% in venous-type flow segments. At 100 ml/min blood flow rate, Zn2+-containing ProS inhibited platelet deposition by 39% and 73% in the respective segments; Zn2+-deficient ProS inhibited platelet deposition by 5% and 0% in the respective segments. Zn2+-containing ProS suppressed fibrin deposition by 67-90%. Systemic APC-independent ProS activity was significantly increased and thrombin-antithrombin complex levels were significantly decreased after infusion of ProS. Thus, infused human Zn2+-containing ProS is antithrombotic in primates, and may have therapeutic potential even in protein C-deficient human patients.

Protein S blocks the extrinsic apoptotic cascade in tissue plasminogen activator/N-methyl D-aspartate-treated neurons via Tyro3-Akt-FKHRL1 signaling pathway

Background

Thrombolytic therapy with tissue plasminogen activator (tPA) benefits patients with acute ischemic stroke. However, tPA increases the risk for intracerebral bleeding and enhances post-ischemic neuronal injury if administered 3-4 hours after stroke. Therefore, combination therapies with tPA and neuroprotective agents have been considered to increase tPA's therapeutic window and reduce toxicity. The anticoagulant factor protein S (PS) protects neurons from hypoxic/ischemic injury. PS also inhibits N-methyl-D-aspartate (NMDA) excitotoxicity by phosphorylating Bad and Mdm2 which blocks the downstream steps in the intrinsic apoptotic cascade. To test whether PS can protect neurons from tPA toxicity we studied its effects on tPA/NMDA combined injury which in contrast to NMDA alone kills neurons by activating the extrinsic apoptotic pathway. Neither Bad nor Mdm2 which are PS's targets and control the intrinsic apoptotic pathway can influence the extrinsic cascade. Thus, based on published data one cannot predict whether PS can protect neurons from tPA/NMDA injury by blocking the extrinsic pathway. Neurons express all three TAM (Tyro3, Axl, Mer) receptors that can potentially interact with PS. Therefore, we studied whether PS can activate TAM receptors during a tPA/NMDA insult.

Results

We show that PS protects neurons from tPA/NMDA-induced apoptosis by suppressing Fas-ligand (FasL) production and FasL-dependent caspase-8 activation within the extrinsic apoptotic pathway. By transducing neurons with adenoviral vectors expressing the kinase-deficient Akt mutant Akt^K179A and a triple FKHRL1 Akt phosphorylation site mutant (FKHRL1-TM), we show that Akt activation and Akt-mediated phosphorylation of FKHRL1, a member of the Forkhead family of transcription factors, are critical for FasL down-regulation and caspase-8 inhibition. Using cultured neurons from Tyro3, Axl and Mer mutants, we show that Tyro3, but not Axl and Mer, mediates phosphorylation of FHKRL1 that is required for PS-mediated neuronal protection after tPA/NMDA-induced injury.

Conclusions

PS blocks the extrinsic apoptotic cascade through a novel mechanism mediated by Tyro3-dependent FKHRL1 phosphorylation which inhibits FasL-dependent caspase-8 activation and can control tPA-induced neurotoxicity associated with pathologic activation of NMDA receptors. The present findings should encourage future studies in animal stroke models to determine whether PS can increase the therapeutic window of tPA by reducing its post-ischemic neuronal toxicity.

Protein S protects neurons from excitotoxic injury by activating the TAM receptor Tyro3-phosphatidylinositol 3-kinase-Akt pathway through its sex hormone-binding globulin-like region

The anticoagulant factor protein S (PS) protects neurons from hypoxic/ischemic injury. However, molecular mechanisms mediating PS protection in injured neurons remain unknown. Here, we show mouse recombinant PS protects dose-dependently mouse cortical neurons from excitotoxic NMDA-mediated neuritic bead formation and apoptosis by activating the phosphatidylinositol 3-kinase (PI3K)-Akt pathway (EC(50) = 26 ± 4 nm). PS stimulated phosphorylation of Bad and Mdm2, two downstream targets of Akt, which in neurons subjected to pathological overstimulation of NMDA receptors (NMDARs) increased the antiapoptotic Bcl-2 and Bcl-X(L) levels and reduced the proapoptotic p53 and Bax levels. Adenoviral transduction with a kinase-deficient Akt mutant (Ad.Akt(K179A)) resulted in loss of PS-mediated neuronal protection, Akt activation, and Bad and Mdm2 phosphorylation. Using the TAM receptors tyrosine kinases Tyro3-, Axl-, and Mer-deficient neurons, we showed that PS protected neurons lacking Axl and Mer, but not Tyro3, suggesting a requirement of Tyro3 for PS-mediated protection. Consistent with these results, PS dose-dependently phosphorylated Tyro3 on neurons (EC(50) = 25 ± 3 nm). In an in vivo model of NMDA-induced excitotoxic lesions in the striatum, PS dose-dependently reduced the lesion volume in control mice (EC(50) = 22 ± 2 nm) and protected Axl(-/-) and Mer(-/-) transgenic mice, but not Tyro3(-/-) transgenic mice. Using different structural PS analogs, we demonstrated that the C terminus sex hormone-binding globulin-like (SHBG) domain of PS is critical for neuronal protection in vitro and in vivo. Thus, our data show that PS protects neurons by activating the Tyro3-PI3K-Akt pathway via its SHGB domain, suggesting potentially a novel neuroprotective approach for acute brain injury and chronic neurodegenerative disorders associated with excessive activation of NMDARs.

Protein S controls hypoxic/ischemic blood-brain barrier disruption through the TAM receptor Tyro3 and sphingosine 1-phosphate receptor

The anticoagulant factor protein S (PS) has direct cellular activities. Lack of PS in mice causes lethal coagulopathy, ischemic/thrombotic injuries, vascular dysgenesis, and blood-brain barrier (BBB) disruption with intracerebral hemorrhages. Thus, we hypothesized that PS maintains and/or enhances the BBB integrity. Using a BBB model with human brain endothelial cells, we show PS inhibits time- and dose-dependently (half maximal effective concentration [EC(50)] = 27 +/- 3 nM) oxygen/glucose deprivation-induced BBB breakdown, as demonstrated by measurements of the transmonolayer electrical resistance, permeability of endothelial monolayers to dextran (40 kDa), and rearrangement of F-actin toward the cortical cytoskeletal ring. Using Tyro-3, Axl, and Mer (TAM) receptor, tyrosine kinase silencing through RNA interference, specific N-terminus-blocking antibodies, Tyro3 phosphorylation, and Tyro3-, Axl- and Mer-deficient mouse brain endothelial cells, we show that Tyro3 mediates PS vasculoprotection. After Tyro3 ligation, PS activated sphingosine 1-phosphate receptor (S1P(1)), resulting in Rac1-dependent BBB protection. Using 2-photon in vivo imaging, we show that PS blocks postischemic BBB disruption in Tyro3(+/+), Axl(-/-), and Mer(-/-) mice, but not in Tyro3(-/-) mice or Tyro3(+/+) mice receiving low-dose W146, a S1P(1)-specific antagonist. Our findings indicate that PS protects the BBB integrity via Tyro3 and S1P(1), suggesting potentially novel treatments for neurovascular dysfunction resulting from hypoxic/ischemic BBB damage.

Lack of protein S in mice causes embryonic lethal coagulopathy and vascular dysgenesis

Protein S (ProS) is a blood anticoagulant encoded by the Pros1 gene, and ProS deficiencies are associated with venous thrombosis, stroke, and autoimmunity. These associations notwithstanding, the relative risk that reduced ProS expression confers in different disease settings has been difficult to assess without an animal model. We have now described a mouse model of ProS deficiency and shown that all Pros1-/- mice die in utero,from a fulminant coagulopathy and associated hemorrhages. Although ProS is known to act as a cofactor for activated Protein C (aPC), plasma from Pros1+/- heterozygous mice exhibited accelerated thrombin generation independent of aPC, and Pros1 mutants displayed defects in vessel development and function not seen in mice lacking protein C. Similar vascular defects appeared in mice in which Pros1 was conditionally deleted in vascular smooth muscle cells. Mutants in which Pros1 was deleted specifically in hepatocytes, which are thought to be the major source of ProS in the blood, were viable as adults and displayed less-severe coagulopathy without vascular dysgenesis. Finally, analysis of mutants in which Pros1 was deleted in endothelial cells indicated that these cells make a substantial contribution to circulating ProS. These results demonstrate that ProS is a pleiotropic anticoagulant with aPC-independent activities and highlight new roles for ProS in vascular development and homeostasis.