Studies have demonstrated that hematopoiesis requires coordinated expression of many genes that may directly or indirectly govern HSC and progenitor cell maintenance, lineage commitment, differentiation and mature blood cell function. Although it is evident that correct regulation of proliferation, survival and differentiation is critical for normal homeostasis within the bone marrow, the precise molecular mechanisms involved in the regulation of these processes are still incompletely understood. The aim of the work described in the thesis was to characterize the cytokine-mediated intracellular signal transduction pathways regulating HSC and myeloid progenitor function. Furthermore, no studies have been performed to elucidate the molecular mechanism underlying the differentiation defects in MDS patients. In order to investigate the mechanisms underlying myeloid differentiation in human CD34+ cells, an ex-vivo hematopoiesis culture system as well as an in vivo mouse transplantation model was utilized. Retroviral experiments allowed to ectopically express genes of interest and to study their role in regulation of hematopoiesis. In Chapter 3, we show a role for MKK3-p38MAPK signalling in the regulation of lineage choices during myelopoiesis through modulation of C/EBP? activity. In Chapter 4, we further studied MAPK function in the regulation of myeloid differentiation and identified a novel role for MEK-ERK signalling in regulating the balance between proliferation and apoptosis of neutrophil progenitors. In Chapter 5, we investigated the role of mTOR in regulation of myelopoiesis, and demonstrate that mTOR activity is essential for expansion of CD34+ hematopoietic progenitor cells during myelopoiesis. In Chapter 6, we performed a candidate-directed screen for molecules improving defective hematopoiesis in MDS CD34+ progenitor cells and show that ectopic expression of C/EBP? resulted in improved neutrophil maturation of MDS progenitors. This suggests that targeting C/EBP? may be of benefit in the design of novel therapies for low risk MDS. Aberrant regulation of hematopoiesis can lead to bone marrow failure or development of hematological malignancies. By identifying the molecular mechanisms underlying the function of protein kinases in hematopoietic progenitor cells it will be possible to design novel therapies targeting these kinase activities for treatment of bone marrow disorders
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