Rapid expansion of neural stem cells is a normal component of central nervous system (CNS) development. However, at mid-gestation of mouse development, neural progenitors exit from active cycling and begin differentiation, a process that is initially directed towards the neuronal and later the astroglial lineages. Unrestrained proliferation and differentiation block are hallmarks of childhood and adult brain cancer, for which neural stem cells and/or immature progenitors have been proposed as cell of origin. Indeed, brain tumors appear to hijack the homeostasis of neural stem cells in that only a small fraction of cells within brain tumors displays stem-like features and retains tumor-initiating capacity (brain tumor stem cells, BTSC). Here we have identified the E3 ubiquitin ligase Ureb1 as a positive regulator of neural differentiation and inhibitor of the stem cell state. The preliminary data for this proposal indicate that Ureb1 exerts these effects primarily through destabilization of the oncoprotein N-Myc, a factor required for the ability of neural stem cells to expand and populate the brain and frequently activated in neural tumors. The model that we will pursue in this proposal predicts that, through the control of N-Myc protein turnover, Ureb1 acts as a restraining factor for the self- renewal and tumor initiating capacity of BTSC derived from glioblastoma multiforme (GBM), the most aggressive form of brain tumor in humans. In GBM, N-Myc is the hub of a molecular network that controls the stem cell state. This proposal will employ powerful biochemical and genetic tools available in the mouse to develop a comprehensive picture of the Ureb1 ubiquitin ligase for the regulation of the homeostasis of neural stem cells and BTSC derived from GBM. First, we will use a series of biochemical approaches to functionally characterize the major levels of post-translational regulation (phosphorylation) of the Ureb1 protein. We will also take advantage of a state-of-the-art mass spectrometry approach to identify the relevant Ureb1-containing protein complexes in neural cells. Second, we will address the function of Ureb1 in normal and tumor cells from the nervous system in mice carrying a conditionally mutant allele of Ureb1 in the neural stem cell compartment. Finally, to directly test the hypothesis that Ureb1 acts as a brake for the stem cell and tumor initiating activity of GBM through N-Myc we will manipulate the activity of Ureb1 and N-My in BTSC derived from human GBM and determine whether the Ureb1 gene is mutated in human high grade gliomas.
Unrestrained proliferation combined with a differentiation block are hallmarks of childhood and adult brain cancer, for which neural stem cells and/or immature progenitors have been proposed as cell of origin. N-Myc, is a very unstable protein involved in neural stem cell state and frequently deregulated in neural cancer. We have identified a new ubiquitin ligase for N-Myc in neural cells and will test whether defects in N-myc turnover affects neural stem cells and initiates cancer in the brain.
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