The role of Pumilio genes in maintenance and self-renewal of hematopoietic stem cells and progenitors
Date of Award
Doctor of Philosophy (Ph.D.)
Microbiology and Immunology
First Committee Member
Roland Jurecic, Committee Chair
Common properties of stem cells are their capacity to self-renew and to generate progeny that is destined to differentiate into mature cells. However, the mechanisms that regulate self-renewal of stem cells remain unknown. Genetic studies have revealed an evolutionarily conserved family of Pumilio proteins that are essential for stem cell maintenance and self-renewal in Drosophila and C. elegans. To study the function of mammalian Pumilio proteins we have cloned and characterized two mouse (Pum1 and Pum2) and human orthologs (PUM1 and PUM2) of Drosophila Pumilio gene. We have also found that Pum1 and Pum2 are expressed preferentially in hematopoietic stem cells (HSC), but are down-regulated in hematopoietic progenitors. To start elucidating the function of Pum proteins in HSC, Pum2 was over-expressed in a SCF-dependent multipotent hematopoietic progenitor cell line EML, which has the capacity for multilineage (erythroid, myeloid, B and T lymphoid) differentiation.Pum2 over-expression in EML cells leads to (a) SCF-independent growth and maintenance, (b) up-regulation of Sca-1 and c-kit markers, and (c) complete block of differentiation into multiple lineages in the absence of SCF. The SCF-independent maintenance of EML cells over-expressing Pum2 (Pum2-EML cells) is caused by up-regulation and constitutive activation of the SCF receptor c-kit, accompanied by constitutive activation of MAPK, PI3K and PLCgamma signaling pathways. SCF is indispensable for differentiation of Pum2-EML cells, and activation of c-kit receptor and its downstream signaling through ligand binding may be necessary for differentiation of multipotent hematopoietic progenitors. More importantly, we have found that EML cells and HSC, but not progenitors, express truncated forms of c-kit receptor, which contain only the intracellular tyrosine kinase domain. These truncated c-kit forms are up-regulated and constitutively active in Pum2-EML cells, and could play a critical role in their SCF-independent maintenance and inhibition of their multilineage differentiation in the absence of SCF. These data suggest a model in which self-renewal and maintenance of stem cells are mediated through SCF-independent c-kit signaling, whereas their differentiation depends on the canonical SCF-induced c-kit signaling. In summary, by regulating the function and activation of c-kit Pumilio genes could play a critical role in supporting HSC maintenance and self-renewal.
Spassov, Danislav Stefanov, "The role of Pumilio genes in maintenance and self-renewal of hematopoietic stem cells and progenitors" (2004). Dissertations from ProQuest. 2089.