Maintenance of hematopoietic stem cells through regulation of Wnt and mTOR pathways

Hematopoietic stem cells are difficult to maintain in vitro , hampering their clinical use. Jian Huang et al . show that mouse and human hematopoietic stem cells can be maintained in culture in the absence of exogenous cytokines by combined treatment with agents that activate Wnt and inhibit mTOR signaling, two major signaling pathways implicated in stem cell homeostasis. Moreover, treatment with two such compounds used clinically increases the number of hematopoietic stem cells in mice in vivo . Hematopoietic stem cell (HSC) self renewal and lineage commitment depend on complex interactions with the microenvironment. The ability to maintain or expand HSCs for clinical applications or basic research has been substantially limited because these interactions are not well defined. Recent evidence suggests that HSCs reside in a low-perfusion, reduced-nutrient niche and that nutrient-sensing pathways contribute to HSC homeostasis. Here we report that suppression of the mTOR pathway, an established nutrient sensor, combined with activation of canonical Wnt–β-catenin signaling, allows for the ex vivo maintenance of human and mouse long-term HSCs under cytokine-free conditions. We also show that the combination of two clinically approved medications that together activate Wnt–β-catenin and inhibit mTOR signaling increases the number (but not the proportion) of long-term HSCs in vivo .