An allosteric disulfide bond is involved in enhanced activation of factor XI by protein disulfide isomerase

A novel role for thioredoxin-related transmembrane protein TMX4 in platelet activation and thrombus formation

BACKGROUND

The functions of critical platelet proteins are controlled by thiol-disulfide exchanges, which are mediated by the protein disulfide isomerase (PDI) family. It has been shown that some PDI family members are important in platelet activation and thrombosis with distinct functions. TMX4, a membrane-type PDI family member, is expressed in platelets, but whether it has a role in platelet activation remains unknown.

OBJECTIVES

To determine the role of TMX4 in platelet activation and thrombosis.

METHODS

The phenotypes of TMX4-deficient mice were evaluated in tail bleeding time assay and laser-induced and FeCl3-induced arterial injury models. The functions of TMX4 in platelets were assessed in vitro using TMX4-null platelets, recombinant TMX4 protein, and anti-TMX4 antibody.

RESULTS

Compared with the control mice, Tie2-Cre/TMX4fl/fl mice deficient of hematopoietic and endothelial TMX4 exhibited prolonged tail bleeding times and reduced platelet thrombus formation. Pf4-Cre/TMX4fl/fl mice deficient of platelet TMX4 also had prolonged tail bleeding times and decreased thrombus formation, which was rescued by injection of recombinant TMX4 protein. Consistently, TMX4 deficiency inhibited platelet aggregation, integrin αIIbβ3 activation, P-selectin expression, phosphatidylserine exposure, and thrombin generation, without affecting tyrosine phosphorylation of intracellular signaling molecules Syk, LAT, PLCγ2 and calcium mobilization. Recombinant TMX4 protein enhanced platelet aggregation and reduced integrin αIIbβ3 disulfide bonds, and TMX4 deficiency decreased free thiols of integrin αIIbβ3, consistent with a potent reductase activity of TMX4. In contrast, an inactive TMX4 protein and a specific anti-TMX4 antibody inhibited platelet aggregation.

CONCLUSION

TMX4 is a novel PDI family member that enhances platelet activation and thrombosis.

Protein disulfide isomerase uses thrombin-antithrombin complex as a template to bind its target protein and alter the blood coagulation rates

During inflammation and situations of cellular stress protein disulfide isomerase (PDI) is released in the blood plasma from the platelet and endothelial cells to influence thrombosis. The addition of exogenous PDI makes the environment pro-thrombotic by inducing disulfide bond formation in specific plasma protein targets like vitronectin, factor V, and factor XI. However, the mechanistic details of PDI interaction with its target remain largely unknown. A decrease in the coagulation time was detected in activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT) on addition of the purified recombinant PDI (175 nM). The coagulation time can be controlled using an activator (quercetin penta sulfate, QPS) or an inhibitor (quercetin 3-rutinoside, Q3R) of PDI activity. Likewise, the PDI variants that increase the PDI activity (H399R) decrease, and the variant with low activity (C53A) increases the blood coagulation time. An SDS-PAGE and Western blot analysis showed that the PDI does not form a stable complex with either thrombin or antithrombin (ATIII) but it uses the ATIII-thrombin complex as a template to bind and maintain its activity. A complete inhibition of thrombin activity on the formation of ATIII-thrombin-PDI complex, and the complex-bound PDI-catalyzed disulfide bond formation of the target proteins may control the pro- and anti-thrombotic role of PDI.

Mudanpioside C Discovered from Paeonia suffruticosa Andr. Acts as a Protein Disulfide Isomerase Inhibitor with Antithrombotic Activities

Paeonia suffruticosa Andr. is a well-known landscape plant worldwide and also holds significant importance in China due to its medicinal and dietary properties. Previous studies have found that Cortex Moutan (CM), the dried root bark of P. suffruticosa, showed antiplatelet and cardioprotective effects, although the underlying mechanism and active compounds remain to be revealed. In this study, protein disulfide isomerase (PDI) inhibitors in CM were identified using a ligand-fishing method combined with the UHPLC-Q-TOF-MS assay. Further, their binding sites and inhibitory activities toward PDI were validated. The antiplatelet aggregation and antithrombotic activity were investigated. The results showed that two structurally similar compounds in CM were identified as the inhibitor for PDI with IC50 at 3.22 μM and 16.73 μM; among them Mudanpioside C (MC) is the most effective PDI inhibitor. Molecular docking, site-directed mutagenesis, and MST assay unequivocally demonstrated the specific binding of MC to the b'-x domain of PDI (Kd = 3.9 μM), acting as a potent PDI inhibitor by interacting with key amino acids K263, D292, and N298 within the b'-x domain. Meanwhile, MC could dose-dependently suppress collagen-induced platelet aggregation and interfere with platelet activation, adhesion, and spreading. Administration of MC can significantly inhibit thrombosis formation without disturbing hemostasis in mice. These findings present a promising perspective on the antithrombotic properties of CM and highlight the potential application of MC as lead compounds for targeting PDI in thrombosis therapy.

Naringin binds to protein disulfide isomerase to inhibit its activity and modulate the blood coagulation rates: Implications in controlling thrombosis

Currently used antithrombotic drugs are beset with several drawbacks which necessitates the need for new and cheaper alternatives. Protein disulfide isomerase (PDI) is secreted in the blood plasma in cellular stress conditions and initiates the thrombus formation. A screening of library of natural compounds revealed that naringin had a high binding affinity for the PDI (-8.2 kcal/mol). Recombinant PDI was purified using the affinity chromatography. Incubation of purified PDI (3 μM) with naringin (0-100 μM, pH 7.4, 25 °C) partially modulated its conformation. Consequently, the fluorescence emission spectra of the PDI binding to naringin were assessed using the Stern-Volmer equation, which indicated an association constant of 2.78 × 104 M-1 suggesting an appreciable affinity for the naringin, with a unique binding site. An insulin turbidity assay showed that PDI activity is decreased in the presence of naringin indicating inhibition. Molecular dynamic simulation studies showed the changes in the PDI structure on binding to the naringin. Incubation of naringin (80 μM) in fresh human plasma along with exogenous PDI (175 nM) showed a significant delay in the intrinsic and extrinsic coagulation pathways. We show that naringin is able to modulate the PDI conformation and activity resulting in altered blood coagulation rates.

Juvenile patients with the homozygous MTHFR C677T genotype develop ischemic stroke 5 years earlier than wild type

To compare age at 1st ischaemic stroke (IS) in a cohort of juvenile (< 46 years of age) IS patients evaluated for the rs1801133 polymorphism (C → T677) of the methylene tetrahydrofolate reductase (MTHFR) gene; to identify predictors of age at IS and of type of cerebral vessel involvement, small vessel disease (SVD) vs large vessel disease (LVD) responsible for the IS; to evaluate possible associations between other clinical and laboratory variables. Retrospective cohort study on 82 MTHFR TT, 54 MTHFR TC and 34 MTHFR CC participants; data regarding age, sex, age at IS, history of dyslipidaemia, hypertension, smoking, migraine and homocysteine (HC) as well as neuroimaging were collected. Age at IS was lower in MTHFR TT than MTHFR TC and CC (35 ± 4 vs 38 ± 0 vs 40 ± 3 years, respectively, p = 0.002); plasma HC (median, interquartile range) was higher in MTHFR TT than in the other groups [16.7 (11.8, 28.6) vs 11.4 (8.2, 16.1) vs 9.8 (7.9, 1.3) respectively, p < 0.0001)] and was higher in SVD than LVD [17.4 (12.4, 32.5) vs  11.4 (8.8, 16.4) p < 0.0001]. MTHFR TT independently predicted age at IS (p = 0.0008) alongside smoking both as a categorical (p = 0.003) or continuous variable (p = 0.02), whereas HC independently predicted SVD as categorical (p = 0.01) and continuous variable (p < 0.0001). Smoking positively predicted plasma HC (p = 0.005) and negatively the activated partial thromboplastin ratio (aPTTr) (p = 0.02). Juvenile IS carriers of the MTHFR TT genotype develop their 1st occlusion on average 5 years earlier compared to the CC genotype; smoking contributes to this prematurity adversely affecting plasma HC and coagulation whereas plasma HC predicts IS secondary to SVD. Public health campaigns against smoking should highlight the prematurity of IS in the juvenile population.

Association of FXI activity with thrombo-inflammation, extracellular matrix, lipid metabolism and apoptosis in venous thrombosis

Animal experiments and early phase human trials suggest that inhibition of factor XIa (FXIa) safely prevents venous thromboembolism (VTE), and specific murine models of sepsis have shown potential efficacy in alleviating cytokine storm. These latter findings support the role of FXI beyond coagulation. Here, we combine targeted proteomics, machine learning and bioinformatics, to discover associations between FXI activity (FXI:C) and the plasma protein profile of patients with VTE. FXI:C was measured with a modified activated partial prothrombin time (APTT) clotting time assay. Proximity extension assay-based protein profiling was performed on plasma collected from subjects from the Genotyping and Molecular Phenotyping of Venous Thromboembolism (GMP-VTE) Project, collected during an acute VTE event (n = 549) and 12-months after (n = 187). Among 444 proteins investigated, N = 21 and N = 66 were associated with FXI:C during the acute VTE event and at 12 months follow-up, respectively. Seven proteins were identified as FXI:C-associated at both time points. These FXI-related proteins were enriched in immune pathways related to causes of thrombo-inflammation, extracellular matrix interaction, lipid metabolism, and apoptosis. The results of this study offer important new avenues for future research into the multiple properties of FXI, which are of high clinical interest given the current development of FXI inhibitors.

Earlier onset of peripheral arterial thrombosis in homozygous MTHFR C677T carriers than in other MTHFR genotypes: a cohort study

To investigate whether age at first presentation of pure peripheral arterial thrombosis (PAT) in lower and upper limbs and in the splanchnic circulation occurs earlier in carriers of the methylenetetrahydrofolate reductase (MTHFR) T677T genotype compared to the heterozygous and wild type and to identify predictors of a possible earlier onset. Retrospective cohort study on 27 MTHFR TT, 29 MTHFR TC and 29 MTHFR CC participants; data regarding age, sex, age at PAT, clinical history (dyslipidaemia, hypertension, smoking, obesity) and homocysteine (HC) measured by immunoassay were collected. Age at PAT was lower in MTHFR TT than MTHFR TC and CC (43 ± 9 vs 47 ± 9 vs 51 ± 4 years, respectively, p = 0.02); plasma HC was higher in MTHFR TT than in the other groups (25 ± 19 vs 12.7 ± 6.7 vs 11.3 ± 3.3 μmol/l, respectively, p < 0.001) while the activated partial thromboplastin ratio (aPTTr) was lower in MTHFR TT than in other genotypes (0.90 ± 0.10 vs 0.97 ± 0.12 vs 0.97 ± 0.08 μmol/L p < 0.001). Among categorical variables, MTHFR TT and dyslipidaemia independently predicted age at AT (p = 0.01 & p = 0.03, respectively) whereas among the continuous variables HC independently predicted age at PAT (p = 0.02) as well as the aPTTr (p = 0.001); smoking predicted lower limb PAT (p = 0.005). MTHFR TT carriers develop their first PAT an average of 4 and 8 years earlier than MTHF CT and CC genotypes; MTHFR TT, dyslipidaemia and plasma HC contribute to the prematurity of the PAT while the interplay between elevated HC and smoking may affect type of arterial district occlusion.

A proximity-based in silico approach to identify redox-labile disulfide bonds: The example of FVIII

Allosteric disulfide bonds permit highly responsive, transient 'switch-like' properties that are ideal for processes like coagulation and inflammation that require rapid and localised responses to damage or injury. Haemophilia A (HA) is a rare bleeding disorder managed with exogenous coagulation factor(F) VIII products. FVIII has eight disulfide bonds and is known to be redox labile, but it is not known how reduction/oxidation affects the structure-function relationship, or its immunogenicity-a serious complication for 30% severe HA patients. Understanding how redox-mediated changes influence FVIII can inform molecular engineering strategies aimed at improving activity and stability, and reducing immunogenicity. FVIII is a challenging molecule to work with owing to its poor expression and instability so, in a proof-of-concept study, we used molecular dynamics (MD) to identify which disulfide bonds were most likely to be reduced and how this would affect structure/function; results were then experimentally verified. MD identified Cys1899-Cys1903 disulfide as the most likely to undergo reduction based on energy and proximity criteria. Further MD suggested this reduction led to a more open conformation. Here we present our findings and highlight the value of MD approaches.

Plasma Homocysteine in Behcet's Disease: A Systematic Review and Meta-Analysis

AIM

 To evaluate the relevance of plasma homocysteine (HC) in Behcet's disease (BD) and its clinical manifestations.

METHODS

 Systematic review of EMBASE and PubMed databases according to PRISMA guidelines from inception to July 2021; random-effects meta-analyses for continuous outcomes.

RESULTS

 The search strategy retrieved 48 case-control (2,669 BD and 2,245 control participants) and 5 cohort studies (708 BD participants). Plasma HC was higher in BD than in controls (p < 0.0001) with wide heterogeneity (I2  = 89.7%) that remained unchanged after sensitivity analysis according to year of article publication, age of BD participants, study size, study quality, method of HC determination, and male/female ratio >1.5; some pooled ethnicities explained a small part of the heterogeneity (I2  = 16.3%). Active BD participants had higher HC than inactive ones (p < 0.0001), with moderate heterogeneity (I2  = 49.2%) that disappeared after removal of an outlier study with very high disease activity. BD participants with any vascular involvement had higher HC than those without (p < 0.0001) with wide heterogeneity (I2  = 89.7%); subgroup analysis on venous thrombosis only changed neither effect size (p < 0.0001) nor heterogeneity (I2  = 72.7%). BD participants with ocular involvement had higher HC than those without (p < 0.0001) with moderate heterogeneity (I2  = 40.3%).

CONCLUSION

 Although causality cannot be inferred, the consistency of the elevation of plasma HC in BD, particularly in patients with active disease, with vascular and ocular involvement suggests an intrinsic involvement of HC in these clinical manifestations.

Protein disulfide isomerase-A1 regulates intraplatelet reactive oxygen species-thromboxane A2 -dependent pathway in human platelets

BACKGROUND

Platelet-derived protein disulfide isomerase 1 (PDIA1) regulates thrombus formation, but its role in the regulation of platelet function is not fully understood.

AIMS

The aim of this study was to characterize the role of PDIA1 in human platelets.

METHODS

Proteomic analysis of PDI isoforms in platelets was performed using liquid chromatography tandem mass spectometry, and the expression of PDIs on platelets in response to collagen, TRAP-14, or ADP was measured with flow cytometry. The effects of bepristat, a selective PDIA1 inhibitor, on platelet aggregation, expression of platelet surface activation markers, thromboxane A2 (TxA2 ), and reactive oxygen species (ROS) generation were evaluated by optical aggregometry, flow cytometry, ELISA, and dihydrodichlorofluorescein diacetate-based fluorescent assay, respectively.

RESULTS

PDIA1 was less abundant compared with PDIA3 in resting platelets and platelets stimulated with TRAP-14, collagen, or ADP. Collagen, but not ADP, induced a significant increase in PDIA1 expression. Bepristat potently inhibited the aggregation of washed platelets induced by collagen or convulxin, but only weakly inhibited platelet aggregation induced by TRAP-14 or thrombin, and had the negligible effect on platelet aggregation induced by arachidonic acid. Inhibition of PDIA1 by bepristat resulted in the reduction of TxA2 and ROS production in collagen- or thrombin-stimulated platelets. Furthermore, bepristat reduced the activation of αIIbβ3 integrin and expression of P-selectin.

CONCLUSIONS

PDIA1 acts as an intraplatelet regulator of the ROS-TxA2 pathway in collagen-GP VI receptor-mediated platelet activation that is a mechanistically distinct pathway from extracellular regulation of αIIbβ3 integrin by PDIA3.

Vascular thiol isomerases: Structures, regulatory mechanisms, and inhibitor development

Vascular thiol isomerases (VTIs), including PDI, ERp5, ERp57, ERp72, and thioredoxin-related transmembrane protein 1 (TMX1), have important roles in platelet aggregation and thrombosis. Research on VTIs, their substrates in thrombosis, their regulatory mechanisms, and inhibitor development is an emerging and rapidly evolving area in vascular biology. Here, we describe the structures and functions of VTIs, summarize the relationship between the vascular TIs and thrombosis, and focus on the development of VTI inhibitors for antithrombotic applications.

Homozygous methylentetrahydrofolate reductase C667T genotype anticipates age at venous thromboembolism by one decade

The aim of the study was to compare age at first venous thromboembolism (VTE), plasma homocysteine and activated partial thromboplastin time ratio (aPTTr) amongst unprovoked VTE patients with the methylentetrahydrofolate reductase (MTHFR) C667T genotypes, and to identify predictors of age at first VTE, of plasma homocysteine and of the aPTTr; to evaluate whether heterozygous or homozygous prothrombin (PT) G20210A mutation lowered the age at first VTE when associated with MTHFR TT. Retrospective cohort study on 259 MTHFR TT, 76 MTHFR TC and 64 MTHFR CC participants with unprovoked VTE; each participant contributed age, sex, age at VTE, history of dyslipidaemia, hypertension, smoking, homocysteine (measured by enzyme immunoassay) and aPTTr (measured by standard coagulation assay). Age at first VTE was lower in MTHFR TT than MTHFR TC and CC (41 ± 14 vs. 50 ± 16 vs. 51 ± 12 years, respectively, P < 0.0001); plasma homocysteine was higher in MTHFR TT than in the other groups (22 ± 21 vs. 12 ± 11.6 vs. 10 ± 3.3 μmol/l, respectively, P = 0.0005) whilst aPTTr was not different. MTHFR TT independently predicted age at first VTE (P = 0.001), plasma homocysteine (P < 0.0001) alongside sex (P = 0.0007), age and smoking (P = 0.03 for both). Compound MTHFR TT with PT GA or AA had no lowering effect on age at first VTE compared with MTHFR TT alone (41 ± 13 vs. 41 ± 14 years). Plasma homocysteine inversely related to aPTTr in the MTHFR TT group (P = 0.003). MTHFR TT anticipates age at first VTE by an average of 10 years compared with MTHFR TC and CC genotypes.

Vascular thiol isomerases in thrombosis: The yin and yang

There has recently been considerable progress of the field of extracellular protein disulfide isomerases with vascular thiol isomerases in the forefront. Four members of protein disulfide isomerase (PDI) family of enzymes, PDI, ERp57, ERp72, and ERp5, have been shown to be secreted from activated platelets and endothelial cells at the site of vascular injury. Each isomerase individually supports platelet accumulation and coagulation, as indicated by multiple levels of evidence, including inhibitory antibodies, targeted knockout mice, and mutant isomerases. The transmembrane PDI family member TMX1 was recently shown to inhibit platelet function and thrombosis, demonstrating that the PDIs can have opposing functions in thrombosis. These observations provide a new concept that thiol isomerases can both positively and negatively regulate hemostasis, constituting off-on redox switches controlling activation of hemostatic factors. This redox network serves to maintain vascular homeostasis. Integrins such as the αIIbβ3 fibrinogen receptor on platelets appear to be major substrates, with the platelet receptor for von Willebrand factor, glycoprotein Ibα, as another substrate. S-nitrosylation of the prothrombotic PDIs may additionally negatively regulate platelets and thrombosis. Thiol isomerases also regulate coagulation in mouse models, and a clinical trial with the oral PDI inhibitor isoquercetin substantially decreased markers of coagulation in patients at risk for thrombosis. This review updates recent findings in the field and addresses emerging evidence that thiol/disulfide-based reactions mediated by the prothrombotic secreted PDIs are balanced by the transmembrane member of this family, TMX1.

Peri/Epicellular Thiol Oxidoreductases as Mediators of Extracellular Redox Signaling

Significance: Supracellular redox networks regulating cell-extracellular matrix (ECM) and organ system architecture merge with structural and functional (catalytic or allosteric) properties of disulfide bonds. This review addresses emerging evidence that exported thiol oxidoreductases (TORs), such as thioredoxin, protein disulfide isomerases (PDIs), quiescin sulfhydryl oxidases (QSOX)1, and peroxiredoxins, composing a peri/epicellular (pec)TOR pool, mediate relevant signaling. pecTOR functions depend mainly on kinetic and spatial regulation of thiol-disulfide exchange reactions governed by redox potentials, which are modulated by exported intracellular low-molecular-weight thiols, together conferring signal specificity. Recent Advances: pecTOR redox-modulates several targets including integrins, ECM proteins, surface molecules, and plasma components, although clear-cut documentation of direct effects is lacking in many cases. TOR catalytic pathways, displaying common patterns, culminate in substrate thiol reduction, oxidation, or isomerization. Peroxiredoxins act as redox/peroxide sensors, contrary to PDIs, which are likely substrate-targeted redox modulators. Emerging evidence suggests important pecTOR roles in patho(physio)logical processes, including blood coagulation, vascular remodeling, mechanosensing, endothelial function, immune responses, and inflammation. Critical Issues: Effects of pecPDIs supporting thrombosis/platelet activation have been well documented and reached the clinical arena. Roles of pecPDIA1 in vascular remodeling/mechanosensing are also emerging. Extracellular thioredoxin and pecPDIs redox-regulate immunoinflammation. Routes of TOR externalization remain elusive and appear to involve Golgi-independent routes. pecTORs are particularly accessible drug targets. Future Directions: Further understanding mechanisms of thiol redox reactions and developing assays for assessing pecTOR redox activities remain important research avenues. Also, addressing pecTORs as disease markers and achieving more efficient/specific drugs for pecTOR modulation are major perspectives for diagnostic/therapeutic improvements.

Multiple protein disulfide isomerases support thrombosis

PURPOSE OF REVIEW

The present review provides an overview of recent findings on new members of the protein disulfide isomerase (PDI) family required for thrombosis.

RECENT FINDINGS

Twenty years ago PDI was shown to mediate platelet aggregation, and 10 years ago PDI was shown to support thrombosis in vivo. Subsequently, other members of this endoplasmic reticulum family of enzymes, ERp57 and ERp5, were demonstrated to support thrombosis. A fourth member, ERp72, was recently shown to be required for platelet accumulation and fibrin deposition in vivo. None of these enzymes can individually support these processes. Moreover, aggregation of platelets deficient in a specific PDI is only recovered by the PDI that is missing. This implies that each PDI has a distinct role in activation of the αIIbβ3 fibrinogen receptor and platelet aggregation. Free thiols can be labeled in both subunits of αIIbβ3, suggesting cysteine-based reactions are involved in relaying conformational changes from the cytoplasmic tails to the integrin headpiece of this integrin.

SUMMARY

Multiple members of the PDI family support platelet function, and hemostasis and thrombosis with distinct roles in these processes. The individual cysteine targets of each enzyme and how these enzymes are integrated into a network that supports hemostasis and thrombosis remain to be elucidated.

Targeting protein disulfide isomerase with the flavonoid isoquercetin to improve hypercoagulability in advanced cancer

BACKGROUND

Protein disulfide isomerase (PDI) is a thiol isomerase secreted by vascular cells that is required for thrombus formation. Quercetin flavonoids inhibit PDI activity and block platelet accumulation and fibrin generation at the site of a vascular injury in mouse models, but the clinical effect of targeting extracellular PDI in humans has not been studied.

METHODS

We conducted a multicenter phase II trial of sequential dosing cohorts to evaluate the efficacy of targeting PDI with isoquercetin to reduce hypercoagulability in cancer patients at high risk for thrombosis. Patients received isoquercetin at 500 mg (cohort A, n = 28) or 1000 mg (cohort B, n = 29) daily for 56 days, with laboratory assays performed at baseline and the end of the study, along with bilateral lower extremity compression ultrasound. The primary efficacy endpoint was a reduction in D-dimer, and the primary clinical endpoint included pulmonary embolism or proximal deep vein thrombosis.

RESULTS

The administration of 1000 mg isoquercetin decreased D-dimer plasma concentrations by a median of -21.9% (P = 0.0002). There were no primary VTE events or major hemorrhages observed in either cohort. Isoquercetin increased PDI inhibitory activity in plasma (37.0% in cohort A, n = 25, P < 0.001; 73.3% in cohort B, n = 22, P < 0.001, respectively). Corroborating the antithrombotic efficacy, we also observed a significant decrease in platelet-dependent thrombin generation (cohort A median decrease -31.1%, P = 0.007; cohort B median decrease -57.2%, P = 0.004) and circulating soluble P selectin at the 1000 mg isoquercetin dose (median decrease -57.9%, P < 0.0001).

CONCLUSIONS

Isoquercetin targets extracellular PDI and improves markers of coagulation in advanced cancer patients.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02195232.

FUNDING

Quercegen Pharmaceuticals; National Heart, Lung, and Blood Institute (NHLBI; U54HL112302, R35HL135775, and T32HL007917); and NHLBI Consortium Linking Oncology and Thrombosis (U01HL143365).

The reduced form of coagulation factor XI is associated with illness severity and coagulopathy in critically-ill septic patients

Coagulation Factor XI (FXI) contributes to the pathobiology of sepsis-associated thrombosis and is a target for new therapeutics. Through cleavage of disulfide bonds, FXI becomes reduced (rFXI), accelerating intrinsic coagulation cascade activation. The role of rFXI in human sepsis has never been studied. We determined levels of total FXI and rFXI in critically-ill septic patients with and without overt disseminated intravascular coagulation (DIC, a dysregulated pro-thrombotic condition). Total FXI and rFXI plasma levels were measured on ICU admission in prospectively enrolled septic patients (n = 32) from three academic medical centers and matched, healthy controls (n = 15). In septic patients, hematologic and physiologic parameters and pathological thrombosis (presence or absence of overt DIC) were determined. rFXI was higher in septic patients than controls (p < 0.05). In septic patients, rFXI was significantly associated with platelet count (r = 0.3511, p < 0.05) and APACHE II score (r = - 0.359, p < 0.05), indices of illness severity. rFXI was lower in patients with overt DIC (p = 0.088), suggesting a consumptive coagulopathy. In contrast, while total FXI levels were reduced in sepsis, they failed to correlate with illness severity, thrombosis, or hematologic parameters. We establish, for the first time, that rFXI is increased in patients with sepsis and correlates with illness severity (APACHE II score and platelet count) and pathological coagulopathy (overt DIC). Total FXI levels, in contrast, are decreased in sepsis but fail to associate with any indices. These findings suggest that rFXI has unique activity in human sepsis.

The intersection of protein disulfide isomerase and cancer associated thrombosis

The mechanisms underlying the hypercoagulability of cancer are complex and include the upregulation coagulation factors or procoagulant proteins, shedding of microparticles, and direct activation of vascular cells. Protein disulfide isomerase (PDI) is a thiol isomerase secreted from activated platelets and endothelial cells and plays a critical role in both platelet aggregation and fibrin generation. A number of potential intravascular targets of PDI have been identified including cell surface receptors (e.g. β-integrins and glycoprotein Ib), receptor ligands (e.g. fibrinogen and von Willebrand factor), serine proteases (e.g. cathepsin G and kallekrein-14), and coagulation factors (e.g. factor XI and factor V). Recent clinical studies demonstrated that a small molecule inhibitor of PDI, isoquercetin, decreases platelet-dependent thrombin generation and PDI activity in plasma following oral administration. This review explores the mechanistic overlap between the molecular drivers of cancer associated thrombosis and the potential roles PDI plays in mediating thrombosis. These molecular insights provide rationale for clinical trials targeting PDI to prevent thrombosis in cancer patients.

Mechano-redox control of integrin de-adhesion

How proteins harness mechanical force to control function is a significant biological question. Here we describe a human cell surface receptor that couples ligand binding and force to trigger a chemical event which controls the adhesive properties of the receptor. Our studies of the secreted platelet oxidoreductase, ERp5, have revealed that it mediates release of fibrinogen from activated platelet αIIbβ3 integrin. Protein chemical studies show that ligand binding to extended αIIbβ3 integrin renders the βI-domain Cys177-Cys184 disulfide bond cleavable by ERp5. Fluid shear and force spectroscopy assays indicate that disulfide cleavage is enhanced by mechanical force. Cell adhesion assays and molecular dynamics simulations demonstrate that cleavage of the disulfide induces long-range allosteric effects within the βI-domain, mainly affecting the metal-binding sites, that results in release of fibrinogen. This coupling of ligand binding, force and redox events to control cell adhesion may be employed to regulate other protein-protein interactions.

Regulatory role of thiol isomerases in thrombus formation

INTRODUCTION

The protein disulfide isomerase (PDI) family of thiol isomerases are intracellular enzymes known to catalyze the oxidation, reduction and isomerization of disulfide bonds during protein synthesis in the endoplasmic reticulum. PDI and related members of the thiol isomerase family are known to localize extracellularly where they possess various functions. Among these, the role of PDI in the initiation of thrombus formation is best characterized. PDI is secreted within seconds from activated platelets and endothelial cells at the site of vascular injury and accumulates in the developing platelet-fibrin thrombus. Inhibition of PDI by antibodies or small molecule inhibitors blocks thrombus formation. Efforts are underway to identify extracellular substrates of PDI that participate in the network pathways linking thiol isomerases to thrombus formation. ERp57, ERp5 and ERp72 also play a role in initiation of thrombus formation but their specific extracellular substrates are unknown. Areas covered: The following review gives an overview of biochemistry of vascular thiol isomerases followed by a detailed description of their role in thrombosis and its clinical implications. Expert commentary: The thiol isomerase system, by controlling the initiation of thrombus formation, provides the regulatory switch by which the normal vasculature is protected under physiologic conditions from thrombi generation.

The disulfide isomerase ERp72 supports arterial thrombosis in mice

Several CGHC motif-containing disulfide isomerases support thrombosis. We here report that endoplasmic reticulum protein 72 (ERp72), with 3 CGHC redox-active sites (a^o, a, and a'), supports thrombosis. We generated a new conditional knockout mouse model and found that Tie2-Cre/ERp72^fl/fl mice with blood and endothelial cells lacking ERp72 had prolonged tail bleeding times and decreased platelet accumulation in laser-induced cremaster arteriole injury and FeCl₃-induced mesenteric arterial injury. Fibrin deposition was decreased in the laser injury model. Both platelet and fibrin accumulation defects were fully rescued by infusion of recombinant ERp72 containing functional a and a' CGHC motifs (ERp72(oo-ss-ss)). Infusion of ERp72 containing inactivated a and a' CGHC motifs (ERp72(ss-oo-oo)) inhibited platelet accumulation and fibrin deposition in wild-type mice. Infusion of ERp72(oo-ss-ss) into β3-null mice increased fibrin deposition in the absence of platelets. ERp72-null platelets had defective aggregation, JON/A binding, P-selectin expression, and adenosine triphosphate (ATP) secretion. The aggregation and ATP secretion defects were fully rescued by ERp72(oo-ss-ss) but partially rescued by ERp72(ss-oo-ss) and ERp72(ss-ss-oo). Aggregation and ATP secretion of human platelets was potentiated by ERp72(oo-ss-ss) but inhibited by ERp72(ss-oo-ss) and ERp72(ss-ss-oo). These data suggest that both the a and a' active sites are required for platelet function. ERp72 bound poorly to β3-null mouse platelets, and the addition of ERp72(oo-ss-ss) to human platelets generated thiols in αIIbβ3, suggesting a direct interaction of ERp72 with αIIbβ3. Defective aggregation of ERp72-null platelets was recovered by ERp72, but not other thiol isomerases. In summary, ERp72 plays a critical role in platelet function and coagulation through the a and a' CGHC motifs.

Thioredoxin-1 Selectively Activates Transglutaminase 2 in the Extracellular Matrix of the Small Intestine: IMPLICATIONS FOR CELIAC DISEASE

Transglutaminase 2 (TG2) catalyzes transamidation or deamidation of its substrates and is ordinarily maintained in a catalytically inactive state in the intestine and other organs. Aberrant TG2 activity is thought to play a role in celiac disease, suggesting that a better understanding of TG2 regulation could help to elucidate the mechanistic basis of this malady. Structural and biochemical analysis has led to the hypothesis that extracellular TG2 activation involves reduction of an allosteric disulfide bond by thioredoxin-1 (TRX), but cellular and in vivo evidence for this proposal is lacking. To test the physiological relevance of this hypothesis, we first showed that macrophages exposed to pro-inflammatory stimuli released TRX in sufficient quantities to activate their extracellular pools of TG2. By using the C35S mutant of TRX, which formed a metastable mixed disulfide bond with TG2, we demonstrated that these proteins specifically recognized each other in the extracellular matrix of fibroblasts. When injected into mice and visualized with antibodies, we observed the C35S TRX mutant bound to endogenous TG2 as its principal protein partner in the small intestine. Control experiments showed no labeling of TG2 knock-out mice. Intravenous administration of recombinant TRX in wild-type mice, but not TG2 knock-out mice, led to a rapid rise in intestinal transglutaminase activity in a manner that could be inhibited by small molecules targeting TG2 or TRX. Our findings support the potential pathophysiological relevance of TRX in celiac disease and establish the Cys³⁷⁰-Cys³⁷¹ disulfide bond of TG2 as one of clearest examples of an allosteric disulfide bond in mammals.