Extracellular thiol isomerase ERp5 regulates integrin αIIbβ3 activation by inhibition of fibrinogen binding

Recent studies have shown that anti-ERp5 antibodies inhibit platelet activation and thrombus formation; Moreover, ERp5-deficient platelets exhibit enhanced platelet reactivity via regulation of endoplasmic reticulum (ER) stress. In this study, we used a new ERp5-knockout mouse model as well as recombinant ERp5 (rERp5) protein, to examine the role of ERp5 in platelet function and thrombosis. Although platelet-specific ERp5-deficient mice had decreased platelet count, the mice had shortened tail-bleeding times and enhanced platelet accumulation in FeCl3-induced mesenteric artery injury, compared with wild-type mice. Using platelet-specific ERp5-deficient mice, we found that ERp5 deficiency increased platelet aggregation, granule secretion, and integrin alpha IIb beta 3 activation. Wild-type recombinant ERp5 protein (rERp5-wt) and inactive mutant ERp5 protein (rERp5-mut) both inhibited human platelet aggregation and the binding of fibrinogen to human platelets, indicating that ERp5 protein interferes with the interaction between integrin alpha IIb beta 3 and its ligand fibrinogen, and its enzymatic activity is not required for this process. Consistently, wild-type mice injected with rERp5-wt or rERp5-mut protein had prolonged tail-bleeding times. Our results provide important evidence that platelet ERp5 negatively regulates platelet activation and thrombus formation, via steric hindrance interfering with integrin alpha IIb beta 3 ligation.