Pathways that govern hematopoietic stem cell fate

Abstract: Hematopoietic stem cells (HSCs) compose a rare population of undifferentiated cells, residing in the bone marrow of adult individuals, ensuring life-long maintenance and replenishment of the blood system. This fantastic achievement is possible owing to two special characteristics of the HSCs: their ability to make copies of themselves (self-renew), and their capacity to differentiate to all lineages of the blood system. The process of blood formation, hematopoiesis, is a dynamic and complicated process reliant on the strict balance between a large number of regulatory factors. Hematopoietic stem cell transplantation (HSCT) is currently used to treat hematological disorders such as leukemia. Cord blood is an easily accessible source of stem cells, however the number of HSCs extracted from one cord are not enough to successfully transplant adult patients. This limitation could be circumvented if we were capable of expanding stem cells outside the body. However, to reach this goal it is crucial to first understand how these cells are regulated in their natural environment. More knowledge is required to understand the interplay between different intrinsic and extrinsic factors participating in governing HSCs. Ex vivo HSC expansion would not only be beneficial for making HSCT accessible to a larger number of patients, but would also enable profound studies of HSC function and regulation. In this thesis we have identified and evaluated factors involved in the regulation of HSC fate decisions. Transforming growth factor-β (TGFβ) is one of the most potent inhibitors of hematopoietic stem and progenitor cell (HSPC) proliferation in vitro. However, the complete mechanism behind the growth inhibitory effect and the precise function of this signaling pathway in vivo, is still to be unraveled. Our results in Article I suggest that Smad4 is a limiting factor for TGFβ-mediated Smad signaling critical for long-term HSC function and demonstrate that the level of Smad4 can modulate the response to TGFβ in human cells. Furthermore, we describe a negative regulatory role of the Smad signaling pathway on human HSPCs during regeneration after transplantation - affecting self-renewal capacity but not lineage choice. In Article II, we identify a transcriptional network, consisting of important stem cell regulators, TGFβ(Smad4)/GATA2/p57, that is critical in controlling the proliferation of primitive hematopoietic cells. We further generate a database of genes that become deregulated following TGFβ stimulation, and demonstrate that GATA2 is involved in a large part of the TGFβ response. At last, in Article III, we have studied the role of Pigment epithelium-derived factor (PEDF) in murine hematopoiesis. Our findings demonstrate that PEDF is an important regulatory factor for HSC regeneration and that PEDF in vivo works in a cell-autonomous fashion. For the first time, we propose a role of PEDF in HSC biology. Taken together, the work in this thesis has contributed to the field by increased understanding of mechanisms and factors involved in the regulation of HSC fate decisions.