Endomucin marks quiescent long-term multi-lineage repopulating hematopoietic stem cells and is essential for their transendothelial migration.

Endomucin (EMCN) currently represents the only hematopoietic stem cell (HSC) marker expressed by both murine and human HSCs. Here, we report that EMCN long-term repopulating HSCs (LT-HSCs; CD150CD48LSK) have a higher long-term multi-lineage repopulating capacity compared to EMCN LT-HSCs. Cell cycle analyses and transcriptional profiling demonstrated that EMCN LT-HSCs were more quiescent compared to EMCN LT-HSCs. Emcn and Emcn mice displayed comparable steady-state hematopoiesis, as well as frequencies, transcriptional programs, and long-term multi-lineage repopulating capacity of their LT-HSCs. Complementary functional analyses further revealed increased cell cycle entry upon treatment with 5-fluorouracil and reduced granulocyte colony-stimulating factor (GCSF) mobilization of Emcn LT-HSCs, demonstrating that EMCN expression by LT-HSCs associates with quiescence in response to hematopoietic stress and is indispensable for effective LT-HSC mobilization. Transplantation of wild-type bone marrow cells into Emcn or Emcn recipients demonstrated that EMCN is essential for endothelial cell-dependent maintenance/self-renewal of the LT-HSC pool and sustained blood cell production post-transplant.