Novel Genetic Pathways in Functional Regulation of Hematopoietic Stem Cells

Abstract

Hematopoietic stem cells (HSCs) are a rare population of bone marrow cells that have self-renewal and differentiating capabilities enabling them to produce all blood lineages during normal hematopoiesis. Many molecular pathways are involved in the regulation of HSCs, and the survival, maintenance and proliferation of these cells must be tightly controlled to avoid aberrant activities that can cause blood diseases, such as hematopoietic malignancies. Therefore, additional factors involved in the functional regulation of HSCs must be discovered to provide new therapeutic treatments for hematopoietic diseases. In chapter I, I briefly introduce the hematopoietic system and hierarchy through which HSCs can produce all mature blood cells in the lifespan. I also describe various methods to identify different hematopoietic cells, with a focus on using cell surface antigen markers. I additionally discuss an important method to study HSCs in mice by using bone marrow transplantation in which the donor cells are manipulated to examine the role of a gene of interest. I also briefly describe several signaling pathways important in HSCs, such as the Bmp, Wnt, Hedgehog and Notch signaling pathways. I provide known relevant information to my thesis work on c-Kit and Sca-1 receptors, as well as on LSK and LSK- cells. In chapter II, I describe my findings regarding a novel mechanism in which Ikzf3 plays a suppressive role in regulating HSC survival and maintenance. Ikzf3 suppresses the population of LSK (lineage-Sca-1+c-Kit+) cells that contains HSCs, and increases the LSK- (lineage-Sca-1+c-Kit-) population, which is highly apoptotic and derived from the LSK population. The DNA binding domain of Ikzf3 is required for its inhibition of HSCs, and Ikzf3 downregulates the expression of Bcl-2, Bcl-xL and c-Myc. Ikzf3 expression in HSCs is maintained at low levels by the c-Kit pathway. In chapter III, on the basis of data from microarray analysis previously performed to compare gene expression profiles between Hif1a knockout HSCs and wild type HSCs, the effects of both Hif1a and Notch1 deletion on HSC regulation are examined. Loss of both Hif1a and Notch1 induces the development of myelodysplastic/myeloproliferative diseases, which are clonal hematopoietic stem cell neoplasms characterized by abnormal regulation of the myeloid pathways for cellular proliferation, maturation and survival. Loss of both Hif1a and Notch1 also leads to a loss of HSC function. These findings indicate a mechanism through which the hypoxia pathway acts in coordination with the Notch pathway in HSCs. In chapter IV, I summarize the findings from chapter II and III and discuss the importance of these results in the field. I also provide the future directions that can answer more questions to expand our knowledge on these pathways. Together, the findings reveal two novel pathways involved in functional regulation of HSCs: (i) the Ikzf3 pathway, which involves c-Kit, Icsbp, Bcl-2, Bcl-xL and c-Myc, and suppresses normal HSCs to maintain homeostasis and (ii) the synergy of Hif1a and Notch1 in regulating HSCs. The loss of both genes can cause myelodysplastic/myeloproliferative-like diseases.