Genetic control of neural stem cell homeostasis - Neural stem cell (NSC) homeostasis represents a state of delicate equilibrium between self-renewal, differentiation, and survival. It is fundamental to the development, growth, and regeneration of the nervous system. Defects in NSC homeostasis underlie broad neurodevelopmental, psychiatric, and neurodegenerative disorders. The mechanisms underlying the control of NSC homeostasis remain incompletely understood. Drosophila has been instrumental in discovering signaling molecules such as Notch and Numb and cellular mechanisms such as asymmetric cell division that are centrally involved in NSC homeostasis. The Drosophila larval brain type II neuroblasts (NBs) have served as an excellent model for studying NSC homeostasis. Similar to mammalian NSCs in lineage hierarchy, the type II NB lineages in the Drosophila larval brain contain transit-amplifying intermediate progenitors (IPs), which can generate a vast number of differentiated progenies. Notch signaling is critical for maintaining the homeostasis of type II NB lineages. Inhibition of Notch signaling results in NB not being properly maintained, whereas Notch hyperactivation causes ectopic NB formation and brain tumorigenesis. Notch signaling also regulates the homeostasis of mammalian NSCs, with deregulated Notch signaling having been linked to brain cancer. The molecular mechanisms by which Notch signaling regulates NSC homeostasis, however, are not well delineated. Previous studies have focused heavily on canonical Notch signaling mediated by transcription factors acting in the nucleus. We have found that a non- canonical Notch signaling (NNS) pathway operating in the cytosol and involving mitochondria critically mediates Notch function in NSC homeostasis. The clinical significance of this NNS pathway is underscored by our observation that tumor-initiating cancer stem cells (CSCs) are particularly sensitive to perturbation of this pathway. Moreover, we found that this NNS pathway exerts co-translational quality control over a master regulator of cell growth. The goal of this proposal is to move away from the status quo of transcriptional control of NSC behavior by Notch and focus on the newly discovered co-translational quality control mechanism by NNS signaling. Our central hypothesis is that non- canonical N signaling regulates NSC homeostasis through a signaling cascade emanating from mitochondria and impinging on the ribosome-associated quality control (RQC) of a master regulator of cell growth. To test this hypothesis, we propose three integrated Specific Aims. Aim 1 will characterize features of the translation of the growth regulator that necessitates RQC. Aim 2 will examine the functional relationship between a key RQC factor and this growth regulator in NSCs and CSCs. Aim 3 will dissect the molecular mechanism by which mitochondrial activity signals to the RQC machinery to regulate the translation of the growth regulator. Upon successful completion of these Aims, we will have generated new mechanistic insights into the control of NSC homeostasis by Notch. We anticipate that this will open entirely new directions for studying the fundamental roles of Notch in NSC and cancer biology.
