Psi Domain of b3 Integrin Has Endogenous Thiol Isomerase Function and Is a Potential New Target for Anti-Thrombotic Therapy

Platelets are critical for maintaining hemostasis, but inappropriate platelet activation can lead to pathogenic thrombosis. Integrin aIIbb3, the most abundant protein on the platelet surface, is a key molecule for platelet aggregation and thrombus formation. The PSI domain of b3 integrin is highly conserved among different species but the function of the PSI domain in the integrin family has not been well defined and the role of this domain in hemostasis and thrombosis is poorly understood. It has been reported that b3 integrin possess PDI activity, which may play a role in integrin activation and platelet aggregation. However, whether the PSI domain of b3 integrin has PDI function is currently unknown.

We generated recombinant protein of PSI domain of mouse b3 integrin. Mouse anti-mouse PSI domain mAbs were generated utilising b3 gene deficient mice (b3−/−) immunized with the recombinant protein. Antibody specificity was determined by flow cytometry and western blot. PDI activity assay of mouse PSI domain and native human b3 integrin was performed using reduced and denatured RNase (rdRNase). The effects of the mAbs on platelet function were measured in vitro using aggregometry and in vivo using intravital microscopy thrombosis model.

Analysis of the PSI domain of b3 integrin reveals that it contains two CXXC amino acid sequences (the active site motif of PDI), which are highly conserved in different species. Refolding of rdRNase assay showed that the PSI recombinant protein has endogenous PDI activity. Bacitracin, a well-known PDI inhibitor, inhibited PSI domain PDI function in a dose dependent manner. Four anti-PSI domain monoclonal antibodies (mAbs) were generated and showed different inhibitory effects on PDI function of the recombinant PSI domain and purified human platelet b3 integrin. In vitro and ex-vivo studies showed that anti-PSI antibodies inhibited mouse and human platelet aggregation. Using intravital microscopy we demonstrated that anti-PSI mAbs inhibited mouse platelet aggregation and thrombus formation in laser injury thrombosis model.

To the best of our knowledge, this is the first time in which it has been demonstrated that the PSI domain of b3 integrin has endogenous PDI activity, which may play important roles in cell biology of platelets and other cells. Our data suggest that the PSI domain of b3 may be a new target in controlling platelet function and our mAbs may have potential in anti-thrombotic therapy.

No relevant conflicts of interest to declare.