Emerging role of the mitochondrial branched-chain aminotransferase (BCATm) as an immunosuppressive enzyme during CD4+ T cell activation

Abstract

Abstract The mitochondrial branched-chain aminotransferase (BCATm) catalyzes the transamination of leucine, an amino acid that is essential for the upregulation of complex 1 of the mammalian target of rapamycin (mTORC1) during T cell activation. Because BCATm is responsible for the mitochondrial degradation of leucine, we hypothesized that BCATm limits leucine availability for mTORC1 signaling and subsequently suppresses T cell activation. To test the hypothesis, we isolated CD4+T cells from the global BCATmKO mouse and measured the rates of leucine transamination/oxidation, the activity of the mTORC1 downstream targets S6 and 4EBP-1, and the release of IFNγ after 24–72h of T cell activation. To elucidate the role of BCATm in T cells in vivo, we used the Cre-LoxP strategy to create a T cell specific deletion of BCATm in C57BL/6 mice. We completed the initial characterization of the mice, which included a validation of absence of BCATm in T cells by using qRT-PCR and Western blotting. Results demonstrated that BCATm-deficient T cells experienced significant reductions in the transamination and oxidation of leucine, which led to increased intracellular leucine concentrations. This correlated with an increased phosphorylation of S6, but not 4EBP-1, suggesting that BCATm regulates mTORC1 signaling in S6 dependent manner. The T cells released more IFNγ in the absence of BCATm, which was highest after 72h of T cell activation. Thus, BCATm appears to play an immunosuppressive role by reducing CD4+ T cell activation. The newly designed T cell conditional BCATmKO mice, which do not express BCATm in CD4+ and CD8+ T cells will be engaged in preclinical studies aiming at deciphering the therapeutic potential of BCATm in T cell-driven immunological responses.