• Platelet PDI regulates a IIb b 3 integrin activation without affecting platelet activation and inside-out integrin signaling.• Platelet PDI is essential for platelet accumulation but not for fibrin generation and hemostasis in mice.Protein disulfide isomerase (PDI) derived from intravascular cells is required for thrombus formation. However, it remains unclear whether platelet PDI contributes to the process. Using platelet-specific PDI-deficient mice, we demonstrate that PDI-null platelets have defects in aggregation and adenosine triphosphate secretion induced by thrombin, collagen, and adenosine diphosphate. Such defects were rescued by wildtype but not mutant PDI, indicating that the isomerase activity of platelet surface PDI is critical for the regulatory effect. PDI-deficient platelets expressed increased levels of intracellular ER protein 57 (ERp57) and ERp72. Platelet PDI regulated a IIb b 3 integrin activation but not P-selectin exposure, Ca 21 mobilization, b 3 -talin1 interaction, or platelet spreading on immobilized fibrinogen. Inhibition of ERp57 further diminished a IIb b 3 integrin activation and aggregation of activated PDI-deficient platelets, suggesting distinct roles of PDI and ERp57 in platelet functions. We found that platelet PDI is important for thrombus formation on collagen-coated surfaces under shear. Intravital microscopy demonstrates that platelet PDI is important for platelet accumulation but not initial adhesion and fibrin generation following laser-induced arteriolar injury. Tail bleeding time in platelet-specific PDI-deficient mice were not significantly increased. Our results provide important evidence that platelet PDI is essential for thrombus formation but not for hemostasis in mice. (Blood. 2013;122(6):1052-1061 IntroductionPlatelets play a central role in hemostasis and atherothrombosis. Following vascular injury, platelets rapidly adhere to activated endothelial cells and/or subendothelial matrix proteins such as collagen and von Willebrand factor through receptor-ligand interactions.1 Subsequently, activated platelets expose P-selectin from a-granules to the plasma membrane and release other granular molecules such as adenosine diphosphate (ADP), which activates other platelets and facilitates a IIb b 3 integrin-mediated platelet accumulation at the site of vascular injury. Although it is not fully understood how integrin function is regulated, it has been postulated that thiol rearrangement in integrins could be one of the regulatory mechanisms. [2][3][4] Previous studies showed that a IIb b 3 integrin has an endogenous isomerase activity and exposes free sulfhydryl groups during platelet activation. [4][5][6] Consistently, reducing agents such as reduced glutathione and cysteine affect platelet aggregation. 2,7,8 Using a IIb b 3 integrin with mutations on Cys residues, Mor-Cohen et al 9 reported that different disulfide bonds in the b 3 subunit change the structure and function of a IIb b 3 integrin. Moreover, disruption of the disulfide bonds of Cys5-Cys435 or Cys663...

Background: Thiol isomerases are a family of endoplasmic reticulum enzymes which orchestrate redox-based modifications of protein disulphide bonds. Previous studies have identified important roles for the thiol isomerases PDI and ERp5 in the regulation of normal platelet function. Aim: Recently, we demonstrated the presence of a further five thiol isomerases at the platelet surface. In this report we aim to report the role of one of these enzymes – ERp57 in the regulation of platelet function. Methods/Results: Using enzyme activity function blocking antibodies, we demonstrate a role for ERp57 in platelet aggregation, dense granule secretion, fibrinogen binding, calcium mobilisation and thrombus formation under arterial conditions. In addition to the effects of ERp57 on isolated platelets, we observe the presence of ERp57 in the developing thrombus in vivo. Furthermore the inhibition of ERp57 function was found to reduce laser-injury induced arterial thrombus formation in a murine model of thrombosis. Conclusions: These data suggest that ERp57 is important for normal platelet function and opens up the possibility that the regulation of platelet function by a range of cell surface thiol isomerases may represent a broad paradigm for the regulation of haemostasis and thrombosis.

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Platelets have long been recognized to be of central importance in haemostasis, but their participation in pathological conditions such as thrombosis, atherosclerosis and inflammation is now also well established. The platelet has therefore become a key target in therapies to combat cardiovascular disease. Anti‐platelet therapies are used widely, but current approaches lack efficacy in a proportion of patients, and are associated with side effects including problem bleeding. In the last decade, substantial progress has been made in understanding the regulation of platelet function, including the characterization of new ligands, platelet‐specific receptors and cell signalling pathways. It is anticipated this progress will impact positively on the future innovations towards more effective and safer anti‐platelet agents. In this review, the mechanisms of platelet regulation and current anti‐platelet therapies are introduced, and strong, and some more speculative, potential candidate target molecules for future anti‐platelet drug development are discussed. British Journal of Pharmacology (2008) 154, 918–939; doi:; published online 21 April 2008

In affluent societies the prevalences of so-called 'Western' diseases such as atherosclerosis, allergies and autoimmune disorders appear to have increased, while many diseases caused by communicable infections are now relatively less common. To test whether there may be a causal relationship we examined the effects of Schistosoma mansoni infections in mice that develop cardiovascular pathology as a result of a genetic deficiency in apolipoprotein E (apoE-/-). The development of atherosclerotic lesions in the aortic arch and brachiocephalic artery of the apoE-/- mice was reduced by approximately 50% in mice with the parasitic infection, when comparison was made with uninfected control mice fed the same diet. Observations on S. mansoni-infected conventional laboratory mice indicate that patent schistosome infections could be counteracting the effects of an atherogenic diet by modulating host lipid metabolism and inducing a reduction in blood total cholesterol concentrations.