Mechanisms underlying loss of neural stem cell competence.

7. Project Summary/Abstract In both vertebrates and invertebrates, a vast diversity of neurons is generated from a relatively small pool of neural stem cells that undergo stereotyped temporal transitions to ensure that each cell type is made at the right time and in the right quantities. These transitions are highly regulated to ensure the development of a functional brain. Over time, neural stem cells lose competence to specify earlier-born fates; thus specific neuronal cell types can be generated only during a specific developmental time window. The mechanisms of how this occurs are totally unknown, but have wide implications in understanding the basic rules of brain development and how developmental brain disorders may arise. We have chosen to address this question in Drosophila, in which each of the ~30 neuroblasts (neural stem cells) gives rise to distinct lineages, but always in a stereotyped birth order. This order is specified by the neuroblasts' sequential expression of a series of temporal identity factors as they divide. The zinc-finger transcription factor, Hunchback (Hb) specifies first-born progeny, and consequently these neurons transcribe hb. First-born fate, characterized by hb transcription, can be specified for only a limited time during what is called the early competence window, best characterized in neuroblast 7-1. Afterwards, the neuroblast can no longer respond to ectopic Hb expression to produce progeny that transcribes hb, suggesting epigenetic mechanisms may underlie competence loss. I recently established DNA fluorescent in situ hybridization (DNA-FISH) on whole-mount Drosophila embryo neuroblasts to track the subnuclear position of the hb gene. I found a robust repositioning of the hb gene from the nuclear interior in young neuroblasts to the nuclear periphery in older neuroblasts. Strikingly, the timing of hb gene repositioning to the nuclear periphery, generally associated with silent genes, is coincident with the end of the neuroblast 7-1 early competence window. Furthermore, the nuclear factor Distal antenna (Dan), which we found can extend neuroblast 7-1 competence, inhibits this gene repositioning. Based on the above observations, I hypothesize that genome reorganization underlies loss of competence and Dan functions in neuroblasts to establish a early competence genome architecture. Recent evidence indicates that Ikaros, the Hb orthologue in mammals, establishes early competence in mouse retinal progenitor cells, which undergo transitions between competence states to generate distinct progeny in a stereotyped birth order. Work on Ikaros function in hematopoietic stem cells suggest that perhaps chromatin organization may underlie transitions between competence states in retinal progenitors. I further propose to translate my work in Drosophila to the mouse retina model system to investigate the mechanisms underlying loss of competence during mammalian development. Together, the above information will provide crucial insight into the origins of neural diversity and have wide implications in harnessing stem cells for tissue replacement therapies.