PDI Cleavage of Disulfide Bonds within the TP Receptor Inhibits Signaling through Gα 13

G-protein coupled receptors (GPCRs) are the most abundant superfamily of cell surface receptors. Approximately 35% of approved drugs target GPCRs and class A GPCRs account for ~85% of this superfamily. Class A GPCRs are characterized in part by two highly conserved disulfides in their extracellular domains that are thought to stabilize protein structure. Whether these disulfides can be enzymatically modified to influence G-protein signaling is not known. We and others have previously shown that disulfide bond modification by thiol isomerases such as protein disulfide isomerase (PDI) represent a previously unrecognized level of control of thrombus formation. We therefore evaluated the ability of recombinant PDI (rPDI) to modulate signaling through modification of the canonical disulfides within platelet GPCRs. Exposure of platelets to rPDI had no effect on stimulation through PAR1, PAR4, or the α 2A-adrenergic receptor. In contrast, rPDI exposure dramatically decreased platelet activation induced by the TP receptor agonists U46619 or arachidonic acid, implicating rPDI-mediated modulation of TP receptor signaling. Consistent with this finding, rPDI blocked U46619-mediated activation of α IIbβ 3, α-granule release, and dense granule release. Conversely, inhibition of endogenous PDI using either inhibitory antibodies or PDI-targeted small molecules enhanced TP receptor-mediated platelet aggregation and granule release, indicating that endogenous platelet PDI influences TP receptor signaling. Inhibition of TP receptor-mediated signaling required PDI active site cysteines since rPDI mutants lacking these cysteines lost inhibitory activity. Evaluation of the inhibitory activity of different PDI fragments showed that the PDI substrate binding domain is also critical for inhibitory activity. The ability to inhibit signaling through the TP receptor was specific for rPDI since incubation with other recombinant thiol isomerases including ERp57, ERp5, and ERp72 had no effect. To determine the specific modifications to TP receptor canonical disulfides induced by PDI, HEK cells were transfected with TP receptor and exposed to rPDI. TP receptor was subsequently immunopreciptated and disulfide-linked peptide analysis was performed using mass spectrometry. Compared to untreated controls, TP receptor exposed to rPDI demonstrated cleavage of Cys11-Cys102 and Cys105-Cys183 bonds and the generation of a new Cys102-Cys183 bond. To determine how modification of the disulfide bonding pattern affected signaling through the TP receptor, we evaluated signaling through specific Gα subunits in platelets. The platelet TP receptor signals through Gα q(which couples to phospholipase and increases calcium flux) and Gα 13(which couples to RhoA and myosin light chain kinase). PDI-mediated cleavage of the platelet TP receptor resulted in biased signaling, with substantial inhibition of G α13-mediated RhoA-GTP activation and myosin light chain phosphorylation and little effect on Gα q-mediated calcium flux. These results show how PDI can modify platelet signaling and represent the first demonstration that a thiol isomerase can modulate the function of a GPCR via rearrangement of canonical disulfide bonds.