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The transmembrane protein disulfide isomerase TMX1 negatively regulates platelet responses

Zhenzhen Zhao, Yi Wu, Junsong Zhou, Fengwu Chen, Aizhen Yang and David W. Essex

Key points

  • Extracellular TMX1 negatively regulates activation of the αIIbβ3 integrin and platelet aggregation.

Abstract

Secreted platelet protein disulfide isomerases, PDI, ERp57, ERp5 and ERp72, have important roles as positive regulators of platelet function and thrombosis. Thioredoxin-related transmembrane protein 1 (TMX1) was the first described transmembrane member of the protein disulfide isomerase family of enzymes. Using a specific antibody, recombinant TMX1 protein (rTMX1), a knockout mouse model, and thiol-labeling approach, we examined the role of TMX1 in platelet function and thrombosis. Expression of TMX1 on the platelet surface increased with thrombin stimulation. The anti-TMX1 antibody increased platelet aggregation induced by convulxin and thrombin, and potentiated platelet ATP release. In contrast, rTMX1 inhibited platelet aggregation and ATP release. TMX1-deficient platelets had increased aggregation, ATP release, αIIbβ3 activation, and P-selectin expression, which were reversed by addition of rTMX1. TMX1-knockout mice had increased incorporation of platelets into a growing thrombus in a FeCl3-induced mesenteric arterial injury, and shortened tail-bleeding times. rTMX1 oxidized thiols in the αIIbβ3 integrin and TMX1-deficient platelets had increased thiols in the β3 subunit of αIIbβ3, consistent with oxidase activity of rTMX1 against αIIbβ3. Thus, TMX1 is the first identified extracellular inhibitor of platelet function and the first disulfide isomerase that negatively regulates platelet function.

  • Submitted April 12, 2018.
  • Accepted November 5, 2018.