David W. Essex

The disulfide isomerase ERp57 mediates platelet aggregation, hemostasis, and thrombosis

A close homologue to protein disulfide isomerase (PDI) called ERp57 forms disulfide bonds in glycoproteins in the endoplasmic reticulum and is expressed on the platelet surface. We generated 2 rabbit Abs to ERp57. One Ab strongly inhibited ERp57 in a functional assay and strongly inhibited platelet aggregation. There was minimal cross-reactivity of this Ab with PDI by Western blot or in the functional assay. This Ab substantially inhibited activation of the αIIbβ3 fibrinogen receptor and P-selectin expression. Furthermore, adding ERp57 to platelets potentiated aggregation. In contrast, adding a catalytically inactive ERp57 inhibited platelet aggregation. When infused into mice the inactive ERp57 prolonged the tail bleeding times. We generated 2 IgG2a mAbs that reacted with ERp57 by immunoblot. One of these Abs inhibited both ERp57 activity and platelet aggregation. The other Ab did not inhibit ERp57 activity or platelet aggregation. The inhibitory Ab inhibited activation of αIIbβ3 and P-selectin expression, prolonged tail bleeding times, and inhibited FeCl(3)-induced thrombosis in mice. Finally, we found that a commonly used mAb to PDI also inhibited ERp57 activity. We conclude that a glycoprotein-specific member of the PDI family, ERp57, is required for platelet aggregation, hemostasis, and thrombosis.

Platelet-derived ERp57 mediates platelet incorporation into a growing thrombus by regulation of the αIIbβ3 integrin

The platelet protein disulfide isomerase called ERp57 mediates platelet aggregation, but its role in thrombus formation is unknown. To determine the specific role of platelet-derived ERp57 in hemostasis and thrombosis, we generated a megakaryocyte/platelet-specific knockout. Despite normal platelet counts and platelet glycoprotein expression, mice with ERp57-deficient platelets had prolonged tail-bleeding times and thrombus occlusion times with FeCl3-induced carotid artery injury. Using a mesenteric artery thrombosis model, we found decreased incorporation of ERp57-deficient platelets into a growing thrombus. Platelets lacking ERp57 have defective activation of the αIIbβ3 integrin and platelet aggregation. The defect in aggregation was corrected by the addition of exogenous ERp57, implicating surface ERp57 in platelet aggregation. Using mutants of ERp57, we demonstrate the second active site targets a platelet surface substrate to potentiate platelet aggregation. Binding of Alexa 488-labeled ERp57 to thrombin-activated and Mn(2+)-treated platelets lacking β3 was decreased substantially, suggesting a direct interaction of ERp57 with αIIbβ3. Surface expression of ERp57 protein and activity in human platelets increased with platelet activation, with protein expression occurring in a physiologically relevant time frame. In conclusion, platelet-derived ERp57 directly interacts with αIIbβ3 during activation of this receptor and is required for incorporation of platelets into a growing thrombus.

Complementary DNA cloning of the alternatively expressed endothelial cell glycoprotein Ib beta (GPIb beta) and localization of the GPIb beta gene to chromosome 22.

Glycoprotein Ib beta (GPIb beta) exists in platelets disulfide-linked to glycoprotein Ib alpha (GPIb alpha), a major receptor for von Willebrand factor. Both GPIb alpha and GPIb beta are expressed in endothelial cells (EC). While the GPIb alpha mRNA and protein appear similar in platelets and EC, EC GPIb beta mRNA is larger than platelet GPIb beta and encodes a larger protein. We have cloned and sequenced EC GPIb beta cDNA and report a 2793-nucleotide sequence which contains a 411-amino acid open reading frame. The EC sequence contains all of the platelet cDNA sequence and all but three amino acids of the primary translation product. Like the genes encoding GPIb alpha, GPIX, and GPV, the GPIb beta gene appears simple in structure. Using human hamster hybrids, we have localized the GPIb beta gene to chromosome 22pter-->22q11.2. When we examined poly (A)+ RNA from several human tissues for GPIb beta mRNA expression, we found that GPIb beta mRNA was expressed in a variety of tissues but was most abundant in heart and brain, while GPIb alpha and GPIX mRNA expression was found only in lung and placenta at very low levels. The broad distribution of GPIb beta mRNA suggests that it may be playing a role different than or additional to its function in platelets.

The C-terminal CGHC motif of protein disulfide isomerase supports thrombosis

Protein disulfide isomerase (PDI) has two distinct CGHC redox-active sites; however, the contribution of these sites during different physiologic reactions, including thrombosis, is unknown. Here, we evaluated the role of PDI and redox-active sites of PDI in thrombosis by generating mice with blood cells and vessel wall cells lacking PDI (Mx1-Cre Pdifl/fl mice) and transgenic mice harboring PDI that lacks a functional C-terminal CGHC motif [PDI(ss-oo) mice]. Both mouse models showed decreased fibrin deposition and platelet accumulation in laser-induced cremaster arteriole injury, and PDI(ss-oo) mice had attenuated platelet accumulation in FeCl3-induced mesenteric arterial injury. These defects were rescued by infusion of recombinant PDI containing only a functional C-terminal CGHC motif [PDI(oo-ss)]. PDI infusion restored fibrin formation, but not platelet accumulation, in eptifibatide-treated wild-type mice, suggesting a direct role of PDI in coagulation. In vitro aggregation of platelets from PDI(ss-oo) mice and PDI-null platelets was reduced; however, this defect was rescued by recombinant PDI(oo-ss). In human platelets, recombinant PDI(ss-oo) inhibited aggregation, while recombinant PDI(oo-ss) potentiated aggregation. Platelet secretion assays demonstrated that the C-terminal CGHC motif of PDI is important for P-selectin expression and ATP secretion through a non-αIIbβ3 substrate. In summary, our results indicate that the C-terminal CGHC motif of PDI is important for platelet function and coagulation.

Vicinal thiols are required for activation of the αIIbβ3 platelet integrin.

Summary.  Background: Closely spaced thiols in proteins that interconvert between the dithiol form and disulfide bonds are called vicinal thiols. These thiols provide a mechanism to regulate protein function. We previously found that thiols in both αIIb and β3 of the αIIbβ3 fibrinogen receptor were required for platelet aggregation. Methods and Results: Using p‐chloromercuribenzene sulfonate (pCMBS) we provide evidence that surface thiols in αIIbβ3 are exposed during platelet activation. Phenylarsine oxide (PAO), a reagent that binds vicinal thiols, inhibits platelet aggregation and labeling of sulfhydryls in both αIIb and β3. For the aggregation and labeling studies, binding of PAO to vicinal thiols was confirmed by reversal of PAO binding with the dithiol reagent 2,3‐Dimercapto‐1‐propanesulfonic acid (DMPS). In contrast, the monothiol β‐mercaptoethanol did not reverse the effects of PAO. Additionally, PAO did not inhibit sulfhydryl labeling of the monothiol protein albumin, confirming the specificity of PAO for vicinal thiols in αIIbβ3. As vicinal thiols represent redox sensitive sites that can be regulated by reducing equivalents from the extracellular or cytoplasmic environment, they are likely to be important in regulating activation of αIIbβ3. Additionally, when the labeled integrin was passed though a lectin column containing wheat germ agglutinin and lentil lectin a substantial amount of non‐labeled αIIbβ3 eluted separately from the labeled receptor. This suggests that two populations of integrin exist on platelets that can be distinguished by thiol labeling. Conclusion: A vicinal thiol‐containing population of αIIbβ3 provides redox sensitive sites for regulation of αIIbβ3.

The disulfide isomerase ERp57 is required for fibrin deposition in vivo

ERp57 is required for platelet function; however, whether ERp57 contributes to fibrin generation is unknown.

A new antithrombotic strategy: inhibition of the C-terminal active site of protein disulfide isomerase

Protein disulfide isomerase (PDI) was discovered over fifty years ago as an enzyme that forms disulfide bonds in newly synthesized proteins in the endoplasmic reticulum [1]. Twenty five years ago PDI was shown to be released from activated platelets [2] opening up a new field of research on extracellular functions of this enzyme. PDI was subsequently localized to the platelet surface [3] and shown to mediate platelet aggregation [4, 5]. This article is protected by copyright. All rights reserved.