TULA-2 (T-Cell Ubiquitin Ligand-2) Inhibits the Platelet Fc Receptor for IgG IIA (FcγRIIA) Signaling Pathway and Heparin-Induced Thrombocytopenia in Mice

Objective-The objective of this study is to investigate the role of T-cell ubiquitin ligand-2 (TULA-2) in the platelet Fc receptor for IgG IIA (Fc gamma RIIA) pathway and in the pathogenesis of heparin-induced thrombocytopenia (HIT). Approach and Results-HIT is a life-threatening thrombotic disease in which IgG antibodies against the heparin-platelet factor 4 complex activate platelets via Fc gamma RIIA. We reported previously differential expression of TULA-2 in human population was linked to Fc gamma RIIA responsiveness. In this study, we investigated the role of TULA-2, a protein phosphatase, in the Fc gamma RIIA pathway and HIT pathogenesis by crossing TULA-2(-/-) mice with transgenic Fc gamma RIIA(+/+) mice. Ablation of TULA-2 resulted in hyperphosphorylation of spleen tyrosine kinase, linker for the activation of T cells, and phospholipase C gamma 2 in platelets via Fc gamma RIIA activation. Platelet integrin activation, granule secretion, phosphatidylserine exposure, and aggregation were also enhanced in TULA-2(-/-) murine platelets. Compared with wild-type mice, TULA-2(-/-) mice showed aggravated antibody-mediated thrombocytopenia, augmented thrombin generation, and shortened tail bleeding time. In contrast, there was no significant difference between TULA-2(-/-) and TULA-2(+/+) platelets in platelet spreading and clot retraction. Of note, heterozygous TULA-2(+/-) mice, whose platelets contained 50% as much protein as the TULA-2(+/+) platelets, showed significantly increased platelet reactivity and more severe thrombocytopenia in vivo compared with TULA-2(+/+) mice. Conclusions-Together, the data demonstrate that not only the absence of TULA-2 but also the relative level of TULA-2 expression modulates Fc gamma RIIA-mediated platelet reactivity and HIT in vivo. TULA-2 expression could be a valuable marker for HIT and inhibiting TULA-2 may serve as a potential therapy to reverse the bleeding adverse effect of anticoagulants.