Regulation of Hematopoietic Stem Cell Self-Renewal during Homeostasis and Regeneration

The hematopoietic system is maintained throughout the lifetime of an organism by hematopoietic stem cells (HSCs). HSCs are uniquely able to balance both self-renewal, to maintain a population of stem cells, and differentiation, to produce all of the immune cells required for hematopoietic function. Further, in response to a severe injury, such as chemotherapy or radiation, HSCs are able to respond and regenerate the hematopoietic compartment. To better understand the intrinsic and extrinsic mechanisms that endow this functionality, I focused on two main facets of hematopoietic stem cell biology: the bone marrow microenvironment and metabolism. To compare the role of distinct bone marrow niches in hematopoiesis we developed two tools. First, a live calvarial bone marrow imaging system that allowed dynamic real-time tracking of individual cells. Secondly, a transgenic mouse that enabled the visualization of hematopoietic stem/progenitor cells (HSPCs) in vivo. To track HSPCs in real-time via live imaging, we developed a new Msi2-GFP knock-in reporter mouse. Using these tools, I determined that HSPCs display a preference for contacting the vasculature niche, as well as making long-term interactions there. Further application of these tools will continue to deepen the understanding of how HSPCs interact with and are influenced by their bone marrow microenvironment.To understand the role of metabolism in HSC function, I focused on uncovering the function of the monocarboxylate transporters 1 and 2 (MCT1/2). MCT1/2 were identified from a transcriptional analysis of genes impacted by the treatment of mice with G-CSF, which induces HSC proliferation and mobilization. Inhibition of MCT1/2 via AR-C155858 (AR-C) improved HSC function both in vitro and in vivo. AR-C improved bone marrow recovery after injury and the engraftment of mouse and human HSCs. This data supports the use of AR-C as a clinical tool to both aid hematopoietic regeneration and hematopoietic stem cell transplant efficacy. Collectively, this dissertation advances the understanding of mechanisms regulating homeostatic and regenerating HSCs. A thorough understanding of these mechanisms allows them to be manipulated for therapeutic uses to improve patient outcomes after hematopoietic injury and engraftment after transplantation