Murine Hematopoietic Stem Cells Require Akt1 and Akt2 for Long-Term Self-Renewal

Murine hematopoietic stem cells (HSCs) rely on components of the Akt signaling pathway, such as FOXO family members and PTEN, for efficient self-renewal and continued survival. However, it is unknown whether Akt is also required for murine HSC function. We hypothesized that Akt would be required for HSC self-renewal, and that the absence of Akt would lead to hematopoietic failure resulting in developmental defects in multiple lineages. To address the effect of Akt loss in HSCs we used competitive and noncompetitive murine fetal liver-bone marrow chimeras. In short-term assays, Akt1^−/−Akt2^−/− fetal liver cells reconstituted the LSK compartment of an irradiated host as well or better than wildtype cells, although failed to generate wildtype levels of more differentiated cells in multiple lineages. When placed in a competitive environment, Akt1^−/−Akt2^−/− HSCs were outcompeted by wildtype HSCs in serial bone marrow transplant assays, indicating a requirement for Akt1 and Akt2 in the maintainance of long-term hematopoietic stem cells. Akt1^−/−Akt2^−/− LSKs tend to remain in the G0 phase of the cell cycle compared to wildtype LSKs, suggesting the failure in serial transplant assays may be due to increased quiesence in the absence of Akt1 and Akt2. Additionally, the intracellular content of reactive oxygen species (ROS) in HSCs is dependent on Akt signaling because Akt1^−/−Akt2^−/− HSCs have decreased ROS levels. Furthermore, pharmacologic augmentation of ROS in the absence of Akt1 and Akt2 results in an exit from quiescence and rescue of differentiation both in vivo and in vitro. Together, these data implicate Akt1 and Akt2 as critical regulators of long-term HSC function and suggest that defective ROS homeostasis may contribute to failed hematopoiesis.