Pivotal Role for Gsk3 in Hematopoietic Stem Cell Homeostasis

Abstract 1292

Hematopoietic stem cells (HSCs) maintain the ability to self-renew and to differentiate into all lineages of the blood. The signaling pathways regulating hematopoietic stem cell (HSCs) self-renewal and differentiation are not well understood. We are very interested in understanding the roles of glycogen synthase kinase-3 (Gsk3) and the signaling pathways regulated by Gsk3 in HSCs. In our recent study (Journal of Clinical Investigation, December 2009) using loss of function approaches (inhibitors, RNAi, and knockout) in mice, we found that Gsk3 plays a pivotal role in controlling the decision between self-renewal and differentiation of HSCs. Disruption of Gsk3 in bone marrow transiently expands HSCs in a μ-catenin dependent manner, consistent with a role for Wnt signaling. However, in long-term repopulation assays, disruption of Gsk3 progressively depletes HSCs through activation of mTOR. This long-term HSC depletion is prevented by mTOR inhibition and exacerbated by μ-catenin knockout. Thus GSK3 regulates both Wnt and mTOR signaling in HSCs, with opposing effects on HSC self-renewal such that inhibition of Gsk3 in the presence of rapamycin expands the HSC pool in vivo. These findings identify unexpected functions for GSK3 in HSC homeostasis, suggest a therapeutic approach to expand HSCs in vivo using currently available medications that target GSK3 and mTOR, and provide a compelling explanation for the clinically prevalent hematopoietic effects of lithium, a widely prescribed GSK3 inhibitor. In the following study, we found that the combination of Gsk3 inhibitor and mTOR inhibitor can expand phenotypic HSCs in vivo and maintain functional HSC in ex vivo culture. This study will provide the basis for a new clinical approach to improve the efficiency of bone marrow transplantation.