Molecular regulation of hematopoietic stem cells
Abstract: Hematopoietic stem cells (HSCs) are ultimately responsible for the all mature blood cell production. They can primarily be found in the bone marrow (BM) at a low frequency (~0.02%). Unlike other blood cells, HSCs have a high degree of self-renewal capacity, the process where upon cell division, at least one daughter cell has the same properties as the stem cell it was generated from. In addition, HSCs need to be able to form mature blood cell components, a process called differentiation. In this thesis, we investigated how in vitro regulation of HSCs would affect their capacity to self-renew and/or differentiate. Whereas we found some growth factors, including kit ligand and Thrombopoietin, to be crucial for induction of self-renewal, we failed to find any direct role of flt3 ligand in these processes. In fact, we developed an efficient strategy to isolate HSCs based on their lack of flt3. Although we, using flt3 ligand deficient mice, failed to find any role of flt3 in HSC regulation, we found flt3 to be crucial in early lymphoid development, as flt3 ligand deficient mice had severely reduced numbers of early lymphoid, but not myeloid, precursors. Since we were capable of efficiently inducing self-renewal in vitro, we were able to characterize the roles of TNF-alpha and fas on HSC self-renewal. These two molecules had previously been suggested to negatively regulate early hematopoiesis. Whereas HSCs in steady state do not seem to be regulated by fas, we found that HSCs can be efficiently targeted by fas following exposure to TNF-alpha. In all, we here demonstrate critical and distinct cytokine requirements for the HSC regulatory processes self-renewal, differentiation and apoptosis.
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