Translational regulation of self-renewing factor mRNAs during intermediate neural progenitor development

Self-renewing factors that are required for maintaining the self-renewal of neural stem cells (NSCs) often also function similarly in transient amplifying intermediate neural progenitor cells (INPs). Dysregulation of the expression of self-renewing factors in NSCs or INPs could affect the maintenance and/or specification of NSCs and INPs, resulting in neural developmental disorders or brain tumor formation. However, whether the expression of the same self-renewing factor in NSCs and INPs are activated independently or co-regulated is not well known. In this proposed project, we will use Drosophila type II neuroblast (NB, the Drosophila NSC) lineages as a model system to investigate how the expression of self-renewing factors is dynamically regulated during INP development with primary focus on the Hes family protein Deadpan (Dpn). Like NSCs in developing human and other gyrencephalic brains, Type II NBs generate a large number of neurons and glia by producing transient amplifying INPs, which undergo a differentiation process from an immature status to a fate-committed mature status before dividing to generate neurons. Like mammalian Hes family proteins that are expressed in both NSCs and INPs, Dpn is also expressed in type II NBs and mature INPs (mINPs) to maintain their self-renewal. However, in the immature INPs (imINPs), Dpn must be inhibited to allow imINPs to differentiate into mINPs and prevent tumorigenic dedifferentiation of imINPs. In this proposed project, we will test our hypothesis that that dpn mRNAs inherited from type II NBs are transiently translationally repressed in imINPs by Brat and subsequently de-repressed in mINPs, enabling quick reactivation of Dpn protein expression without requiring de novo transcriptional activation. We will test this hypothesis by pursuing three specific aims. Aim 1. Determine if Dpn mRNAs transcribed in type II NBs are inherited by both imINPs and mINPs. Aim 2. Investigate if dpn mRNAs are transiently translationally repressed by Brat in imINPs. Aim 3. Examine if dpn transcription is only activated in the NBs but not in mINPs. We expect that findings from this project will reveal a novel translational mechanism that regulates Dpn expression and the bone fide function of Brat during of INP development. Given that the mammalian homologs of Dpn, Hes1, is also expressed in human neural stem cells/progenitors to regulate neural stem cell proliferation and differentiation, and TRIM3, the mammalian homolog of Brat, has been shown to function as a tumor suppressor of brain tumors (such as glioblastoma), our work will provide important information for understanding how the expression of mammalian homolog of Dpn, Hes1, is regulated in mammalian NSCs and INPs and how TRIM3 functions in normal mammalian brain development and brain tumor suppression.