| The capacity for self-maintenance is a unique property of stem cells that distinguishes them from all other cell types. In vertebrates, stem cell self-renewal is achieved through infrequent cellular division, in which some daughter cells undergo differentiation, while others remain in a quiescent undifferentiated state. Because embryonic and adult stem cells exhibit strikingly different anatomical and molecular features, it was thought that independent genetic pathways controlled their self-renewal. Work from our laboratory showed that the transcription factor Zfx is necessary for the self-renewal of embryonic and at least one adult stem cell type---hematopoietic stem cells.;Inactivation of Zfx in mouse embryonic stem cells (ESC) impaired their ability to self-renew and increased their rate of apoptosis, while their differentiation capacity was intact. Loss of Zfx in the hematopoietic system resulted in loss of phenotypic HSC, but did not have an effect on more differentiated erythro-megakaryocyte progenitors. Moreover, Zfxnull HSC could not reconstitute the blood system of irradiated animals, showing that loss of Zfx impaired their ability to self-renew in vivo. In contrast, Zfx deletion in brain of embryonic and adult animals did not show any overt phenotype. In order to complement the loss of function studies, we overexpressed Zfx in mouse ESC. Overexpression of Zfx enabled the maintenance of self-renewal under sub-optimal conditions, however impaired the differentiation capacity of ESC. We used the overexpression system to investigate the molecular mechanisms controlled by Zfx. Our preliminary results suggest that Zfx-overexpression expands the phenotypic HSC compartment, and may show functional enhancement as well. Overexpression of Zfx in human ESC led to increased expression of undifferentiated markers, and to apparent enhancement of self-renewal. Similarly to mouse ESC, overexpression of Zfx in human ESC impaired their differentiation. We are investigating the molecular mechanism responsible for these phenotypes. |
