Transmembrane thiol isomerase TMX1 inhibits fibrin formation by down-regulation of platelet phosphatidylserine exposure.

OBJECTIVE: Membrane exposure of phosphatidylserine (PS) on platelets is critical for the binding of coagulation factors leading to coagulation activation, however, the mechanism controlling PS exposure remains largely unknown. Using genetically modified mouse models, we previously reported that a transmembrane disulfide isomerase TMX1 inhibited integrin αIIbβ3 outside-in signaling which is important for PS exposure on platelets. In this study, we investigated the role of TMX1 in PS exposure and coagulation. APPROACH AND RESULTS: We found that the deficiency of TMX1 in platelets enhanced fibrin formation and PS exposure at the site of injury. In vitro, TMX1 inhibited thrombin generation mediated by activated platelets, and attenuated PS exposure on platelets, the effect of which was prevented when integrin αIIbβ3 outside-in signaling was blocked, suggesting that TMX1 inhibition of integrin αIIbβ3 outside-in signaling suppresses PS exposure. Moreover, TMX1 deficiency increased the free thiols of TMEM16 F in platelets including Cys338, Cys349 and Cys352. In HEK293 T cells overexpressing C338S-, C349S-mutated TMEM16 F, the PS exposure was increased, suggesting that TMX1 oxidizes these disulfide bonds of TMEM16 F, decreasing its activity to externalize PS on the membrane. CONCLUSION: Together, our observations for the first time demonstrate that TMX1 inhibits PS exposure in platelets downregulating the procoagulant activity, by which TMX1 plays a critical role in maintaining vascular quiescence.