The role of branched-chain amino acids and BCAT1 in the maintenance of stem cell identity of mouse embryonic stem cells

Abstract

Embryonic stem cells (ESCs) derived from the inner cell mass (ICM) of preimplantation embryos are among the best-studied stem cell types because they can be cultured under defined culture conditions and can be propagated indefinitely. Indeed, ESCs retain their pluripotent stem cell character, i.e., the capacity for continuous self-renewal and the potential for differentiation into any cell type of the organism. Therefore, they have promising potential for many medical applications. The metabolism of branched-chain amino acids (BCAAs) and its role in cellular proliferation and differentiation of stem cells has recently received considerable attention. Here we show that ground-state mouse embryonic stem cells (mESCs) rely on high intracellular BCAA levels to maintain their stem cell identity. In contrast, heterogeneous naive mESCs, representing a more advanced stage of maturation, enter a quiescent state comparable to embryonic diapause when cultured with reduced leucine. BCAA metabolism is regulated by two branched-chain amino acid transaminases (BCAT1/2), that reversibly catalyze the transamination of BCAAs with alpha-ketoglutarate (α-KG), leading to the production of the corresponding branched-chain keto acids (BCKAs) and glutamate. We have recently shown that BCAT1 is overexpressed in acute myeloid leukemia (AML) stem cells, leading to a reduction in α-KG and subsequent DNA hypermethylation. An increased α-KG/succinate ratio has been suggested to be essential for the maintenance of pluripotency of mESCs. Therefore, we aimed to investigate the role of BCAT1 in controlling α-KG homeostasis and stem cell potential. In this work, I showed that ground-state mESCs display a high expression of BCAT1, whereas mESCs in a heterogeneous naive state downregulate the BCAT1 protein. Studies conducted with Bcat1 knockdown and knockout mESC lines showed that this enzyme positively affects cell growth and pluripotency of mESCs. In addition, BCAT1 impacts mTORC1 signaling and autophagy through controlling intracellular BCAA concentrations. I hypothesize that BCAT1 plays an important role in the BCAA-sensing pathway in ground-state mESCs. Modulation of BCAT1 alters intracellular BCAA concentrations and thereby senses the current nutrient status via mTORC1 signaling. My data shed light on the complexity of the influence of BCAA metabolism and BCAT1 on the stem cell function of pluripotent mESCs.