THE HYPOXIC HEMATOPOIETIC STEM CELL NICHE Consequences of Hypoxia-induced Transcription on Stem Cell Fate

Abstract: Hematopoiesis is the process of blood formation that originates with the hematopoietic stem cell (HSC), a cell type that is responsible for the life-long supply of mature blood cells. HSCs are defined by their ability to self-renew as well as giving rise to differentiated cells of all blood lineages. Because of these features, HSCs are also the basis for bone marrow transplantation regimens for treatment of leukemia and various other hematopoietic disorders. HSCs reside in the bone marrow of adult mammals, in the so called niche. The HSC niche refers to the specific anatomical region where the stem cells reside, but also to the regulatory microenvironment consisting of adjacent cells and factors produced by these. An important feature of the HSC niche seems to be a relatively low level of oxygen: hypoxia. Hypoxia leads to activation of Hypoxia-Inducible Factors (HIF). We have investigated the role of hypoxia and HIFs in HSC biology. By in vitro hypoxic treatment, we could show that hypoxia leads to less proliferation of HSCs, with up-regulation of cell-cycle inhibitory genes, while full reconstitution potential of irradiated recipient mice is preserved. Ectopic activation of HIF, mediated by retroviral overexpression in hematopoietic stem and progenitor cells (HSPC), leads to even less proliferation and a disability to sustain hematopoiesis in vivo. The role of the HIF target gene Vegfa was studied in a mouse model where hypoxia-induced transcription of Vegfa is abrogated (Vegfaδ/δ). HSCs could be maintained under steady-state conditions without hypoxic induction of Vegfa. In transplantation assays however, we show that when Vegfa upregulation upon hypoxia is lacking, adult HSC function is clearly impaired. On the contrary, HSCs isolated from the fetal liver of developing Vegfaδ/δ mice had a normal function. Furthermore, we show that erythropoiesis during development and to a lesser extent in the adult involves hypoxic Vegfa expression. In summary, this thesis contributes new findings to the role of the hypoxic HSC niche. We show that hypoxia and HIFs are involved in regulating HSC proliferation. Furthermore, hypoxia-induced Vegfa is identified as an important player in HSCs.