Amino Acid Transporter X Is Required for Hematopoietic Stem Cell Maintenance through Regulating Specific Amino Acids Level

Hematopoietic stem cells (HSCs) maintenance is required to preserve stem cell pool and compensate the dynamic loss of blood cells. Previous studies of HSCs maintenance mainly focus on the quiescent versus active state of HSCs and accumulated evidence indicates that metabolism plays a critical role in coordinating divergent stem cell states. While recent reports largely emphasized the role of catabolic glycolysis on long-term (LT) HSC maintenance, we found that free amino acids are enriched in primitive stem cell by ~1.5 fold. Given that amino acid metabolism in HSCs is largely unknown, we first cultured bone marrow (BM) cells with individual amino acid deprived medium to study the function of individual amino acids on HSCs in vitro. Surprisingly, we found that specific amino acids, including valine, methionine and threonine (VMT), are essential for maintaining primitive HSCs, as removing them (VMT) individually from media dramatically reduced primitive HSC number by over 95%. Thus, we hypothesize that specific amino acids are critical for preserving the stem cell pool and maintaining their function.  To test it, we transplanted equal number of cells cultured with complete or individual VMT deprived media into lethally irradiated recipient mice and found VMT deprivation in vitro impaired stem cell repopulation ability. We also identified the amino acid transporter X (AATX) that is specifically expressed in HSCs and maintain VMT levels within the cell. Furthermore, inhibition of AATX reduced LT-HSC (LSK CD34^- Flk2^-) number in vivo. BM transplantation indicated that AATX inhibition impaired stem cell long-term reconstitution ability by over 2 fold. Our studies uncovered a role of amino acid metabolism in HSC maintenance and discovered the underlying  molecular mechanism related to the amino acid transport. This finding may impact clinical treatment of blood disorders including leukemia.