mTORC1 mediates the expansion of hematopoietic stem and progenitor cells through ribosome biogenesis protein Urb2 in zebrafish.

Mammalian target of rapamycin (mTOR) serves as the key sensor to control protein synthesis, cell growth, and survival. Despite mTOR is reported to regulate hematopoietic stem and progenitor cell (HSPC) engraftment and multiple-lineage hematopoiesis in mice, the roles of unique mTOR complexes (mTORCs) in early HSPC development and HSPC pool formation have not been adequately elucidated. Here, we uncover that mTORC1 is essential for early HSPC expansion in zebrafish. mTORC1 signaling was highly activated in definitive HSPCs during the emerging and expanding stages. Pharmacological or genetic inactivation of mTORC1 would cause defective HSPC expansion and migration due to disrupted cell proliferation. Interestingly, mTORC2 is dispensable for early HSPC development. Ribosome biogenesis protein Urb2 was downregulated upon mTORC1 inhibition, and urb2 overexpression partially rescued the hematopoietic defects in mTORC1-deficient embryos. These data demonstrate that mTORC1 signaling regulates early HSPC expansion through Urb2, and this work will deepen our understanding of mTOR in different physiological processes.