Mitochondrial metabolism in hematopoietic stem cells requires functional FOXO3

Hematopoietic stem cells (HSC) are primarily dormant but have the potential to become highly active on demand to reconstitute blood. This requires a swift metabolic switch from glycolysis to mitochondrial oxidative phosphorylation. Maintenance of low levels of reactive oxygen species (ROS), a by-product of mitochondrial metabolism, is also necessary for sustaining HSC dormancy. Little is known about mechanisms that integrate energy metabolism with hematopoietic stem cell homeostasis. Here, we identify the transcription factor FOXO3 as a new regulator of metabolic adaptation of HSC. ROS are elevated in Foxo3^-/- HSC that are defective in their activity. We show that Foxo3^-/- HSC are impaired in mitochondrial metabolism independent of ROS levels. These defects are associated with altered expression of mitochondrial/metabolic genes in Foxo3^-/- hematopoietic stem and progenitor cells (HSPC). We further show that defects of Foxo3^-/- HSC long-term repopulation activity are independent of ROS or mTOR signaling. Our results point to FOXO3 as a potential node that couples mitochondrial metabolism with HSC homeostasis. These findings have critical implications for mechanisms that promote malignant transformation and aging of blood stem and progenitor cells. Synopsis FOXO3 regulates oxidative stress in LT-HSC, which are highly sensitive to increased reactive oxygen species. However, while the impaired function of Foxo3^-/- LT-HSC is associated with defective mitochondrial metabolism, it is not mediated by oxidative stress or mTOR signaling. Mitochondrial metabolism is impaired in Foxo3^-/- LT-HSCs. Defects in Foxo3^-/- LT-HSC activity in vivo or Foxo3^-/- LT-HSC mitochondrial function are not mediated by oxidative stress. FOXO3 regulates oxidative stress in LT-HSCs, which are highly sensitive to increased reactive oxygen species. However, while the impaired function of Foxo3^-/- LT-HSCs is associated with defective mitochondrial metabolism, it is not mediated by oxidative stress or mTOR signaling.