Thiol Isomerase Research Articles

Endoplasmic reticulum–resident protein 57 (ERp57) oxidatively inactivates human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed, intracellular as well as extracellular protein with multiple modes of post-translational regulation, including an allosteric disulfide bond between Cys-370-Cys-371 that renders the enzyme inactive in the extracellular matrix. Although recent studies have established that extracellular TG2 is switched "on" by the redox cofactor protein thioredoxin-1 (TRX), it is unclear how TG2 is switched "off." Here, we demonstrate that TG2 oxidation by small-molecule biological oxidants, including glutathione, cystine, and hydrogen peroxide, is unlikely to be the inactivation mechanism. Instead, endoplasmic reticulum (ER)-resident protein 57 (ERp57), a protein in the ER that promotes folding of nascent proteins and is also present in the extracellular environment, has the cellular and biochemical characteristics for inactivating TG2. We found that ERp57 colocalizes with extracellular TG2 in cultured human umbilical vein endothelial cells (HUVECs). ERp57 oxidized TG2 with a rate constant that was 400-2000-fold higher than those of the aforementioned small molecule oxidants. Moreover, its specificity for TG2 was also markedly higher than those of other secreted redox proteins, including protein disulfide isomerase (PDI), ERp72, TRX, and quiescin sulfhydryl oxidase 1 (QSOX1). Lastly, siRNA-mediated ERp57 knockdown in HUVECs increased TG2-catalyzed transamidation in the extracellular environment. We conclude that, to the best of our knowledge, the disulfide bond switch in human TG2 represents the first example of a post-translational redox regulatory mechanism that is reversibly and allosterically modulated by two distinct proteins (ERp57 and TRX).

Advances in vascular thiol isomerase function.

The present review will provide an overview of several recent advances in the field of vascular thiol isomerase function. The initial observation that protein disulfide isomerase (PDI) functions in thrombus formation occurred approximately a decade ago. At the time, there was little understanding regarding how PDI or other vascular thiol isomerases contribute to thrombosis. Although this problem is far from solved, the past few years have seen substantial progress in several areas that will be reviewed in this article. The relationship between PDI structure and its function has been investigated and applied to identify domains of PDI that are critical for thrombus formation. The mechanisms that direct thiol isomerase storage and release from platelets and endothelium have been studied. New techniques including kinetic-based trapping have identified substrates that vascular thiol isomerases modify during thrombus formation. Novel inhibitors of thiol isomerases have been developed that are useful both as tools to interrogate PDI function and as potential therapeutics. Human studies have been conducted to measure circulating PDI in disease states and evaluate the effect of oral administration of a PDI inhibitor on ex-vivo thrombin generation. Current findings indicate that thiol isomerase-mediated disulfide bond modification in receptors and plasma proteins is an important layer of control of thrombosis and vascular function more generally.

The disulfide isomerase ERp72 supports arterial thrombosis in mice

AbstractSeveral CGHC motif–containing disulfide isomerases support thrombosis. We here report that endoplasmic reticulum protein 72 (ERp72), with 3 CGHC redox-active sites (ao, a, and a′), supports thrombosis. We generated a new conditional knockout mouse model and found that Tie2-Cre/ERp72fl/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.

Kinetic-based trapping by intervening sequence variants of the active sites of protein-disulfide isomerase identifies platelet protein substrates

Thiol isomerases such as protein-disulfide isomerase (PDI) direct disulfide rearrangements required for proper folding of nascent proteins synthesized in the endoplasmic reticulum. Identifying PDI substrates is challenging because PDI catalyzes conformational changes that cannot be easily monitored (e.g. compared with proteolytic cleavage or amino acid phosphorylation); PDI has multiple substrates; and it can catalyze either oxidation, reduction, or isomerization of substrates. Kinetic-based substrate trapping wherein the active site motif CGHC is modified to CGHA to stabilize a PDI-substrate intermediate is effective in identifying some substrates. A limitation of this approach, however, is that it captures only substrates that are reduced by PDI, whereas many substrates are oxidized by PDI. By manipulating the highly conserved -GH- residues in the CGHC active site of PDI, we created PDI variants with a slowed reaction rate toward substrates. The prolonged intermediate state allowed us to identify protein substrates that have biased affinities for either oxidation or reduction by PDI. Because extracellular PDI is critical for thrombus formation but its extracellular substrates are not known, we evaluated the ability of these bidirectional trapping PDI variants to trap proteins released from platelets and on the platelet surface. Trapped proteins were identified by mass spectroscopy. Of the trapped substrate proteins identified by mass spectroscopy, five proteins, cathepsin G, glutaredoxin-1, thioredoxin, GP1b, and fibrinogen, showed a bias for oxidation, whereas annexin V, heparanase, ERp57, kallekrein-14, serpin B6, tetranectin, and collagen VI showed a bias for reduction. These bidirectional trapping variants will enable more comprehensive identification of thiol isomerase substrates and better elucidation of their cellular functions.

Thiol isomerase ERp72 enhances the development of antibody-induced arthritis

Abstract Thiol isomerases control labile disulfide bonds that are important in cell activation, cytokine signaling and in a variety of diseases. However, their role in autoimmune diseases such as arthritis remains unknown. ERp72, a thiol isomerases which contains three highly conserved CGHC active motifs, is capable of oxidizing, reducing and isomerizing disulfide bonds. In this study, we generated a new knockout mouse strain deficient of ERp72 (ERp72−/−). Using K/BxN serum-transfer-induced arthritis model (STIA), we determined the role of ERp72 in the pathogenesis of arthritis. To induce STIA, ERp72+/+and ERp72−/− mice received intraperitoneal injection of 200μl K/BxN serum twice on day 0 and day 2. Severity of arthritis was evaluated by daily measurements of joint diameter and clinical scores. Compared with ERp72+/+ mice, ERp72−/− mice developed much more severe arthritis, their joint swelling diameter was significantly increased. Clinical scoring of hind paw edema and hyperemia showed similar results. As indicated by H&E and Safranin O staining, the joint tissue from ERp72−/− mice exhibits more severe synovial hyperplasia, inflammatory cell infiltration and bone erosion on day 12. To determine the role of ERp72 in regulation of cytokine production, we measured the profiles of proinflammatory cytokines. In the joint tissue of ERp72−/− mice bearing STIA, both IL-1β and IL-6 levels were markedly increased by more than 50% than that of ERp72+/+ mice. Interestingly, the basal level of proinflammatory cytokine TNF-α in ERp72−/− mice was significantly higher in that of ERp72−/− mice. Taken together, our observation demonstrates that ERp72 is a negative regulator of innate immunity and plays an important role in the development of arthritis.

The integrin PSI domain has an endogenous thiol isomerase function and is a novel target for antiplatelet therapy

AbstractIntegrins are a large family of heterodimeric transmembrane receptors differentially expressed on almost all metazoan cells. Integrin β subunits contain a highly conserved plexin-semaphorin-integrin (PSI) domain. The CXXC motif, the active site of the protein-disulfide-isomerase (PDI) family, is expressed twice in this domain of all integrins across species. However, the role of the PSI domain in integrins and whether it contains thiol-isomerase activity have not been explored. Here, recombinant PSI domains of murine β3, and human β1 and β2 integrins were generated and their PDI-like activity was demonstrated by refolding of reduced/denatured RNase. We identified that both CXXC motifs of β3 integrin PSI domain are required to maintain its optimal PDI-like activity. Cysteine substitutions (C13A and C26A) of the CXXC motifs also significantly decreased the PDI-like activity of full-length human recombinant β3 subunit. We further developed mouse anti-mouse β3 PSI domain monoclonal antibodies (mAbs) that cross-react with human and other species. These mAbs inhibited αIIbβ3 PDI-like activity and its fibrinogen binding. Using single-molecular Biomembrane-Force-Probe assays, we demonstrated that inhibition of αIIbβ3 endogenous PDI-like activity reduced αIIbβ3-fibrinogen interaction, and these anti-PSI mAbs inhibited fibrinogen binding via different levels of both PDI-like activity-dependent and -independent mechanisms. Importantly, these mAbs inhibited murine/human platelet aggregation in vitro and ex vivo, and murine thrombus formation in vivo, without significantly affecting bleeding time or platelet count. Thus, the PSI domain is a potential regulator of integrin activation and a novel target for antithrombotic therapies. These findings may have broad implications for all integrin functions, and cell-cell and cell-matrix interactions.

Extracellular Protein Disulfide Isomerase Regulates Vitronectin during the Initiation of Thrombus Formation

Protein disulfide isomerase (PDI), secreted by platelets and endothelial cells upon vascular injury, is required for thrombus formation. However, the precise mechanism by which PDI regulates thrombosis remains elusive. Using PDI variants that form stable mixed disulfide complexes with their substrates, we performed kinetic trapping experiment in platelet rich plasma and identified multiple substrate proteins for PDI, including vitronectin. Importantly, when using variants of endoplasmic reticulum protein 57 (ERp57), a thiol isomerase that has a similar domain structure as PDI and is also important for thrombus formation, the trapping mutants of ERp57 do not interact with vitronectin. This result has demonstrated the substrate specificity of PDI during our kinetic trapping experiment. Further study using polyethylene glycol (PEG)-based gel mobility shift assay combined with mass spectrometry has identified the redox reaction between PDI and vitronectin occurs on two disulfide bonds Cys 137-161 and Cys 274-453 in the hemopexin-like domains of plasma vitronectin.Vitronectin, as a substrate of extracellular PDI, has been shown to be important for thrombus formation. Vitronectin null mice have reduced platelet accumulation and fibrin deposition in the cremaster arterioles following laser injury. Vitronectin null mice also have significantly prolonged large-vessel thrombosis in the carotid artery using the ferric chloride thrombosis model. Using intravital microscopy we showed that vitronectin rapidly accumulates in a growing thrombus following vessel injury. When mice are treated with eptifibatide to eliminate platelet accumulation, we still observe significant amount of vitronectin accumulation on the vessel wall in the absence of platelet thrombus. This observation was further confirmed using confocal intravital microscopy. After 3D reconstruction of a growing thrombus in mouse cremaster arteriole, vitronectin was identified to locate primarily on the CD31 stained vessel wall. These combined studies suggest that plasma-derived vitronectin and not platelet-derived vitronectin is the primary substrate of PDI.Our study further showed that the indispensable role of vitronectin to a growing thrombus depends on extracellular PDI. Native plasma vitronectin does not bind to αvβ3 or αIIbβ3-integrins on endothelial cells and platelets. On solid phase binding assay, plasma sample pre-treated with wild-type PDI showed significantly increased binding of vitronectin to its ligand αvβ3 or αIIbβ3-integrins. However, this increase was not observed in plasma pre-treated with dead-mutant PDI or ERp57. In addition, using immunofluorescent staining, PDI treated plasma sample also showed significantly increased binding of vitronectin to activated human umbilical vein endothelial cells (HUVECs) and this binding was abrogated by RGD peptides or an αvβ3 blocking antibody. The critical role of extracellular PDI for the regulation of vitronectin in a growing thrombus was further confirmed in our in vivo studies. When mice were treated with quecetin-3-rutinoside or two different inhibitory antibodies that selectively block PDI activity, the accumulation of vitronectin and platelets was significantly reduced. These combined results demonstrate that extracellular PDI regulates vitronectin in a growing thrombus to promote platelet accumulation and fibrin generation.In summary, our studies have revealed a novel regulatory mechanism during the initiation of thrombus formation. Under normal physiologic conditions in the absence of secreted PDI, thrombus formation is suppressed and maintains a quiescent, patent vasculature. The release of PDI during vascular injury serves as a novel regulatory switch that allows activation of proteins, including vitronectin, which are critical for the following platelet accumulation and fibrin generation. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Characterizing the morbid genome of ciliopathies.

BackgroundCiliopathies are clinically diverse disorders of the primary cilium. Remarkable progress has been made in understanding the molecular basis of these genetically heterogeneous conditions; however, our knowledge of their morbid genome, pleiotropy, and variable expressivity remains incomplete.ResultsWe applied genomic approaches on a large patient cohort of 371 affected individuals from 265 families, with phenotypes that span the entire ciliopathy spectrum. Likely causal mutations in previously described ciliopathy genes were identified in 85% (225/265) of the families, adding 32 novel alleles. Consistent with a fully penetrant model for these genes, we found no significant difference in their “mutation load” beyond the causal variants between our ciliopathy cohort and a control non-ciliopathy cohort. Genomic analysis of our cohort further identified mutations in a novel morbid gene TXNDC15, encoding a thiol isomerase, based on independent loss of function mutations in individuals with a consistent ciliopathy phenotype (Meckel-Gruber syndrome) and a functional effect of its deficiency on ciliary signaling. Our study also highlighted seven novel candidate genes (TRAPPC3, EXOC3L2, FAM98C, C17orf61, LRRCC1, NEK4, and CELSR2) some of which have established links to ciliogenesis. Finally, we show that the morbid genome of ciliopathies encompasses many founder mutations, the combined carrier frequency of which accounts for a high disease burden in the study population.ConclusionsOur study increases our understanding of the morbid genome of ciliopathies. We also provide the strongest evidence, to date, in support of the classical Mendelian inheritance of Bardet-Biedl syndrome and other ciliopathies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-016-1099-5) contains supplementary material, which is available to authorized users.

A substrate-driven allosteric switch that enhances PDI catalytic activity.

Protein disulfide isomerase (PDI) is an oxidoreductase essential for folding proteins in the endoplasmic reticulum. The domain structure of PDI is a–b–b′–x–a′, wherein the thioredoxin-like a and a′ domains mediate disulfide bond shuffling and b and b′ domains are substrate binding. The b′ and a′ domains are connected via the x-linker, a 19-amino-acid flexible peptide. Here we identify a class of compounds, termed bepristats, that target the substrate-binding pocket of b′. Bepristats reversibly block substrate binding and inhibit platelet aggregation and thrombus formation in vivo. Ligation of the substrate-binding pocket by bepristats paradoxically enhances catalytic activity of a and a′ by displacing the x-linker, which acts as an allosteric switch to augment reductase activity in the catalytic domains. This substrate-driven allosteric switch is also activated by peptides and proteins and is present in other thiol isomerases. Our results demonstrate a mechanism whereby binding of a substrate to thiol isomerases enhances catalytic activity of remote domains.

Platelet‐targeting thiol reduction sensor detects thiol isomerase activity on activated platelets in mouse and human blood under flow

Essentials Protein disulfide isomerases may have an essential role in thrombus formation. A platelet-binding sensor (PDI-sAb) was developed to detect thiol reductase activity under flow. Primary human platelet adhesion to collagen at 200 s(-1) was correlated with the PDI-sAb signal. Detected thiol reductase activity was localized in the core of growing thrombi at the site of injury in mice. Background Protein disulfide isomerases (PDIs) may regulate thrombus formation in vivo, although the sources and targets of PDIs are not fully understood. Methods and results Using click chemistry to link anti-CD61 and a C-terminal azido disulfide-linked peptide construct with a quenched reporter, we developed a fluorogenic platelet-targeting antibody (PDI-sAb) for thiol reductase activity detection in whole blood under flow conditions. PDI-sAb was highly responsive to various exogenous reducing agents (dithiothreitol, glutathione and recombinant PDI) and detected thiol reductase activity on P-selectin/phosphatidylserine-positive platelets activated with convulxin/PAR1 agonist peptide, a signal partially blocked by PDI inhibitors and antibody. In a microfluidic thrombosis model using 4 μg mL(-1) corn trypsin inhibitor-treated human blood perfused over collagen (wall shear rate = 100 s(-1) ), the PDI-sAb signal increased mostly over the first 200 s, whereas platelets continually accumulated for over 500 s, indicating that primary adhesion to collagen, but not secondary aggregation, was correlated with the PDI-sAb signal. Rutin and the PDI blocking antibody RL90 reduced platelet adhesion and the PDI-sAb signal only when thrombin production was inhibited with PPACK, suggesting limited effects of platelet thiol isomerase activity on platelet aggregation on collagen in the presence of thrombin. With anti-mouse CD41 PDI-sAb used in an arteriolar laser injury model, thiol reductase activity was localized in the core of growing thrombi where platelets displayed P-selectin and were in close proximity to disrupted endothelium. Conclusion PDI-sAb is a sensitive and real-time reporter of platelet- and vascular-derived disulfide reduction that targets clots as they form under flow to reveal spatial gradients.

Probing for thiol isomerase activity in thrombi

Probing for thiol isomerase activity in thrombi

Intracellular Trafficking, Localization, and Mobilization of Platelet-Borne Thiol Isomerases.

Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including protein disulfide isomerase and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation, and thrombus formation. In this study, we examined the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets, and identified the cellular events responsible for their movement to the platelet surface on activation. Immunofluorescence microscopy imaging was used to localize protein disulfide isomerase and ERp57 in murine and human megakaryocytes at various developmental stages. Immunofluorescence microscopy and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell-surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and sarco/endoplasmic reticulum calcium ATPase 3. Immunofluorescence microscopy and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin) and in the presence or absence of membrane fusion mediated by Munc13-4 (absent in platelets from Unc13d(Jinx) mice). Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not soluble N-ethylmaleimide-sensitive fusion protein attachment receptor/Munc13-4-dependent vesicular-plasma membrane fusion.

Inhibition of Protein Disulfide Isomerase in Thrombosis.

This MiniReview addresses our current understanding of the mechanisms by which protein disulfide isomerase (PDI) mediates thrombus formation and discusses the potential of blocking thrombosis by targeting PDI. Thiol isomerases are ubiquitous oxidoreductases primarily localized to the endoplasmic reticulum (ER) where they serve a critical role in protein folding. PDI is the founding member of the thiol isomerase family. Although PDI is an essential intracellular enzyme, it can participate in pathological processes once released from cells. In particular, PDI serves a critical role in thrombus formation, the underlying cause of myocardial infarction and stroke. Both platelets and endothelial cells secrete PDI upon vascular injury. Secreted PDI appears to activate multiple extracellular substrates in the vasculature, enabling the initiation of thrombus formation. As an essential component of thrombus formation, extracellular PDI represents a new target for pharmacological inhibition of clinical thrombosis. Quercetin-3-rutinoside, a flavonol highly abundant in common foods, inhibits PDI and blocks thrombus formation both in vitro and in vivo. Such observations have prompted clinical trials targeting PDI in thrombotic diseases.

Platelet Surface PDI Controls the Ligand-Binding Function of Glycoprotein Ibalpha and Platelet-Neutrophil Interactions Under Thromboinflammatory Conditions

We previously showed that the isomerase activity of platelet surface protein disulfide isomerase (PDI), a prototypic thiol isomerase, regulates the full activation of αIIbβ3 integrin and is important for platelet accumulation, but not hemostasis, following vascular injury (Kim et al. Blood 2013). However, it remains unclear whether platelet PDI regulates the function of other platelet surface receptors. Since it was reported that platelet PDI and glycoprotein Ibα (GPIbα) were physically close on the platelet surface (Burgess et al. JBC 2000), we investigated whether platelet PDI regulates GPIbα function using megakaryocyte-specific PDI conditional knockout (CKO) mice developed by us. We found that PDI-null platelets showed a significant defect in GPIbα-mediated agglutination and von Willebrand factor (vWF) binding. Those defects were completely rescued by exogenously-added recombinant wild-type PDI (wtPDI), but not mutant PDI (dmPDI) lacking the isomerase activity. Consistently, inhibition of platelet PDI with blocking antibodies impaired GPIbα-mediated agglutination and vWF binding in human and mouse platelets, suggesting that the isomerase activity of platelet surface PDI controls the ligand-binding function of GPIbα. Studies using surface plasmon resonance revealed that wtPDI and dmPDI bound to immobilized GPIbα with a dissociation constant of 0.9 and 1.2 µM, respectively. Mass spectrometric analysis indicated that the Cys4-Cys17 disulfide bond in GPIbα, which has an allosteric -RHStaple configuration, was preferentially reduced by wtPDI, but not dmPDI. Consistent with previous studies showing that platelet GPIbα is important for platelet-neutrophil interactions, we observed that deletion of platelet PDI significantly reduced platelet-neutrophil interactions under shear conditions. Importantly, using real-time fluorescence intravital microscopy with megakaryocyte-specific PDI CKO mice, we demonstrated that platelet-specific PDI deletion did not influence neutrophil adhesion but markedly reduced platelet adhesion and accumulation on the adherent neutrophils on TNF-α-inflamed cremaster muscle venules. Infusion of wtPDI, but not dmPDI, restored reduced platelet-neutrophil interactions in the PDI CKO mice, implying the importance of platelet surface PDI activity in regulating GPIbα-mediated platelet-neutrophil interactions. Using a thromboinflammation model of hepatic ischemia/reperfusion injury, we found that platelet-specific PDI CKO mice exhibited a significant reduction in the infarct size and the serum levels of aspartate aminotransferase, a marker of liver damage. Our results suggest that the isomerase activity of platelet surface PDI regulates the redox state of the Cys4-Cys17 disulfide bond in GPIbα and is required for the ligand-binding function of GPIbα, thereby playing a critical role during platelet-neutrophil interactions under thromboinflammatory conditions. DisclosuresNo relevant conflicts of interest to declare.

Extracellular Thiol Isomerases and Their Role in Thrombus Formation.

The mammalian endoplasmic reticulum (ER) houses a large family of twenty thioredoxin-like proteins of which protein disulfide isomerase (PDI) is the archetypal member. Although the PDI family is best known for its role in oxidative protein folding of secretory proteins in the ER, these thioredoxin-like proteins fulfill ever-expanding roles, both within the secretory pathway and beyond. Secreted PDI family proteins have now been shown to serve a critical role in platelet thrombus formation and fibrin generation. Utilizing intravital microscopy to visualize thrombus formation in mice, we have demonstrated the presence of extracellular PDI antigen during thrombus formation following injury of the vascular wall. Inhibition of PDI abrogates thrombus formation in vivo (16, 26, 46, 55). These observations have been extended to other PDI family members, including ERp57 (39, 116, 118, 123) and ERp5 (77). The vascular thiol isomerases are those PDI family members secreted from platelets and/or endothelium (40): PDI, ERp57, ERp5, ERp72, ERp44, ERp29, and TMX3. We focus here on PDI (16, 46, 55), ERp57 (39, 116, 118, 123), and ERp5 (77), which have been implicated in thrombus formation in vivo. It would appear that a system of thiol isomerase redox catalysts has been hijacked from the ER to regulate thrombus formation in the vasculature. How this redox system is trafficked to and regulated at the cell surface, the identity of extracellular substrates, why so many thiol isomerases are required, and which thiol isomerase functions are necessary are critical unanswered questions in understanding the role of thiol isomerases in thrombus formation.

Revisiting the mechanistic basis of the French Paradox: Red wine inhibits the activity of protein disulfide isomerase in vitro

IntroductionAlthough epidemiologic evidence points to cardioprotective activity of red wine, the mechanistic basis for antithrombotic activity has not been established. Quercetin and related flavonoids are present in high concentrations in red but not white wine. Quercetin-glycosides were recently shown to prevent thrombosis in animal models through the inhibition of extracellular protein disulfide isomerase (PDI). We evaluated whether red or white wine inhibited PDI activity in vitro. MethodsQuercetin levels in red and white wines were measured by HPLC analysis. Inhibition of PDI activity by red and white wines was assessed by an insulin reduction turbidity assay at various concentrations of wine. PDI inhibition was confirmed using a reduced peptide that contained a disulfide containing peptide as a substrate. The inhibition of PDI related thiol isomerases ERp5 and ERp57 was also assessed. ResultsWe observed a dose-dependent decrease of PDI activity for a variety of red but not white wines. Red wine diluted to 3% final concentration resulted in over 80% inhibition of PDI activity by insulin reductase assay for all varieties tested. This inhibition was also observed in the peptide based assay. Red grape juice yielded similar results but ethanol alone did not affect PDI activity. Interestingly, red wine also inhibited the PDI related thiol isomerases ERp5 and ERp57, albeit to a lesser degree than PDI. ConclusionsPDI activity is inhibited by red wine and grape juice, identifying a potentially novel mechanism underlying the cardiovascular benefits attributed to wine consumption.

Both platelet- and endothelial cell–derived ERp5 support thrombus formation in a laser-induced mouse model of thrombosis

AbstractProtein disulfide isomerase (PDI) and endoplasmic reticulum protein 57 (ERp57) are emerging as important regulators of thrombus formation. Another thiol isomerase, endoplasmic reticulum protein 5 (ERp5), is involved in platelet activation. We show here the involvement of ERp5 in thrombus formation using the mouse laser-injury model of thrombosis and a specific antibody raised against recombinant ERp5. Anti-ERp5 antibody inhibited ERp5-dependent platelet and endothelial cell disulfide reductase activity in vitro. ERp5 release at the thrombus site was detected after infusion of Alexa Fluor 488–labeled anti-ERp5 antibody at 0.05 μg/g body weight, a dose that does not inhibit thrombus formation. Anti-ERp5 at 3 μg/g body weight inhibited laser-induced thrombus formation in vivo by causing a 70% decrease in the deposition of platelets and a 62% decrease in fibrin accumulation compared to infusion of control antibody (P < .01). ERp5 binds to β3 integrin with an equilibrium dissociation constant (KD) of 21 µM, measured by surface plasmon resonance. The cysteine residues in the ERp5 active sites are not required for binding to β3 integrin. These results provide evidence for a novel role of ERp5 in thrombus formation, a function that may be mediated through its association with αIIbβ3.

Inhibition of thiol isomerase activity diminishes endothelial activation of plasminogen, but not of protein C

IntroductionCell surface thiol isomerase enzymes, principally protein disulphide isomerase (PDI), have emerged as important regulators of platelet function and tissue factor activation via their action on allosteric disulphide bonds. Allosteric disulphides are present in other haemostasis-related proteins, and we have therefore investigated whether thiol isomerase inhibition has any influence on two endothelial activities relevant to haemostatic regulation, namely activation of protein C and activation of plasminogen, with subsequent fibrinolysis. Materials and MethodsThe study was performed using the human microvascular endothelial cell line HMEC-1. Thiol isomerase gene expression was measured by RT-PCR and activation of protein C and plasminogen by cell-based assays using chromogenic substrates S2366 and S2251, respectively. Cell mediated fibrinolysis was measured by monitoring absorbance at 405nm following fibrin clot formation on the surface of HMEC-1 monolayers. Results and ConclusionsA variety of thiol isomerase enzymes, including PDI, were expressed by HMEC-1 cells and thiol reductase activity detectable on the cell surface was inhibited by both RL90 anti-PDI antibody and by the PDI inhibitor quercetin-3-rutinoside (rutin). In cell-based assays, activation of plasminogen, but not of protein C, was inhibited by RL90 antibody and, to a lesser extent, by rutin. Fibrin clot lysis occurring on a HMEC-1 monolayer was also significantly slowed by RL90 antibody and by rutin, but RL90-mediated inhibition was abolished in the presence of exogenous tissue plasminogen activator (tPA). We conclude that thiol isomerases, including PDI, are involved in fibrinolytic regulation at the endothelial surface, although not via a direct action on tPA. These findings broaden understanding of haemostatic regulation by PDI, and may aid in development of novel anti-thrombotic therapeutic strategies targeted via the fibrinolysis system.

Animal Models of Arterial and Venous Thrombosis

The intersection of the availability of genetically altered mice, new technologies for intravital microscopy and high speed computing systems has led to the use of animal models to build on the concepts that have emerged from in vitro studies of the molecular and cellular biology of hemostasis and thrombosis. In an effort to improve the understanding of the etiology and pathogenesis of thrombosis, thrombus formation has been initiated in experimental systems via mechanical disruption, laser-induced, photochemical-induced and ferric chloride-induced injury to the vessel wall, among others. None of these methods are physiologic, and as such, remain models that require extrapolation from a living animal – a mouse – to human biology. We have focused on laser-induced injury of the arteriole vessel wall in the cremasteric muscle of the mouse. Using high speed digital imaging of fluorescently labeled components and real time intravital microscopy, our group has been able to demonstrate that platelet accumulation and fibrin generation during thrombus formation occur simultaneously, that tissue factor and collagen are independent initiators of platelet activation, and that monocyte-derived microparticles deliver tissue factor to the site of thrombus development. Perhaps the most important and unanticipated observation has been that thiol isomerases, thought only to be involved in protein biosynthesis via the formation of disulfide bonds in the endoplasmic reticulum, play a critical role in thrombus formation. Protein disulfide isomerase (PDI), ERp5 and ERp57 are among the vascular thiol isomerases that are known to be important for the initiation of thrombus formation. Laser-induced thrombosis in mice is associated with PDI, ERp5 and ERp57 secretion by platelets and endothelium. Inhibition of these thiol isomerases blocks platelet thrombus formation and fibrin generation. The integrins αIIbβ3 and αVβ3 play a key role in this process, binding directly with the secreted thiol isomerases and capturing them in the vicinity of vessel wall injury. These enzymes are required for the initiation of platelet thrombus formation and fibrin generation, but the mechanism by which they function remains to be elucidated. At present, it would appear that there is an electron transfer pathway involving these enzymes that regulates the initiation of thrombus formation. The mechanism by which PDI participates in thrombus generation is being evaluated by using trapping mutant forms to identify substrates of PDI that participate in the network pathways linking thiol isomerases, platelet receptor activation and fibrin generation. Several proteins, including vitronectin, thrombospondin and Factor V, have been identified as forming covalent disulfide intermediates with PDI. We are currently exploring PDI as an antithrombotic target using isoquercetin and quercetin 3-rutinoside, inhibitors of PDI identified by high throughput screening. Tail bleeding times are equivalent for mice treated with quercetin-3-rutinoside and isoquercetin compared to untreated mice. In an in vivo mouse pulmonary embolism model, PDI inhibitors rescue a high percentage of mice from death. The b-domain of PDI binds to quercetin-3-rutinoside, and infusion of the isolated b’ domain into a mouse treated with quercetin-3-rutinoside restores thrombus formation. This suggests a method for reversal of bleeding if these PDI inhibitors are found to be complicated by bleeding. The antithrombotic properties of quercetin and isoquercetin in humans have been tested. A pharmacokinetic study with quercetin and isoquercetin determined optimal oral delivery with isoquercetin. The effectiveness of these PDI inhibitors in human studies is being evaluated in a clinical trial evaluating prophylaxis of thromboembolic events in patients with cancer-associated thrombosis. PDI is a novel target for antithrombotic therapy and is unique in that its inhibition simultaneously blocks platelet thrombus formation and fibrin generation. Disclosures Zwicker: Quercegen Pharma: Research Funding.

ML359, a Small Molecule Inhibitor of Protein Disulfide Isomerase That Prevents Thrombus Formation and Inhibits Oxidoreductase but Not Transnitrosylase Activity

Vascular thiol isomerases comprise a family of enzymes including protein disulfide isomerase (PDI), ERp5, and ERp57 that are important in the process of thrombus formation. PDI is secreted at sites of vascular injury, and antibody-mediated PDI inhibition prevents thrombus formation in a mouse laser injury model. Our group has previously reported on the discovery of the small molecule PDI inhibitors quercetin-3-rutinoside and ML359. Identified as part of a high-throughput screen, ML359 is a second-generation PDI inhibitor that selectively blocks PDI oxidoreductase activity with approximately ten-fold the potency of quercetin-3-rutinoside. To better understand the mechanism of allosteric modulation of PDI by small molecules, we evaluated the association of ML359 with isolated domains of PDI, determined the effects of ML359 on a variety of PDI functions, and compared the activity of ML359 to that of quercetin-3-rutinoside. PDI is composed of four thioredoxin-like domains and an x-linker region in the sequence a-b-b’-x-a’. Major substrate binding is thought to occur in the b-b’ region while the a and a’ domains contain catalytically active cysteine motifs (CGHC) that mediate the oxidoreducase, nitrosylase, and thiol isomerase functions of PDI. In order to identify potential binding sites of ML359 on PDI, we constructed and expressed the domain fragments a, ab, abb’, b’xa’, and a’. These fragments were tested in the presence of 10 µM ML359 using an insulin turbidometric assay that measures the oxidoreductase activity of PDI. ML359 demonstrated full inhibition of oxidoreductase activity when full-length PDI and the b’xa’ fragment were used whereas no inhibition was observed with the other fragments assayed. These results are consistent with docking studies showing that ML359 likely binds in a pocket formed at the b’x interface. In contrast, when the same experiment was performed in the presence of 30 µM of quercetin-3-rutinoside, inhibition was only noted with full-length PDI and the abb’ and b’xa’ fragments, suggesting that binding was dependent on the b’ and not the x-linker region. To determine if ML359 has differential effects on the oxidoreductase and nitrosylase functions of PDI, we utilized a platelet-based assay in which fluorescence intensity stemming from the NO-sensitive intracellular dye DAF-FM was measured as an indicator of PDI-mediated translocation of NO from the extracellular surface into the cytosol (transnitrosylation). While quercetin-3-rutinoside potently inhibited PDI-mediated transnitrosylation activity, ML359 had no effect. These results are consistent with the idea that the transnitrosylase and oxidoreducase functions of PDI are separable and inhibition of either is specific to the small molecule used. We evaluated the ability of ML359 to inhibit thrombosis in a mouse laser injury model. Intravital microscopy was used to follow thrombus formation in mouse cremaster arterioles after laser-induced vascular injury. Infusion of ML359 resulted in inhibition of thrombus formation, in contrast to thrombosis seen after infusion of vehicle alone. In summary, ML359 is a second generation small molecule inhibitor of PDI that likely binds at the b’x interface of the enzyme. Furthermore, ML359 is able to selectively inhibit PDI oxidoreductase activity without affecting transnitrosylase activity. ML359 may prove a useful molecular probe to better understand the different functions of PDI relative to thrombus formation in vivo. DisclosuresNo relevant conflicts of interest to declare.