SUMMARY Adult hippocampal neurogenesis has garnered significant interest over the past two decades as a robust and unique form of plasticity in a region critical for learning and memory. It has also proven to be fertile ground for understanding fundamental principles of stem cell biology, neuronal development, as well as illustrating the capacity of the mature brain to integrate immature neurons, which has important implications for regeneration and transplantation efforts for neural repair following injury or diseases. Despite considerable progress in understanding the molecular and cellular mechanisms underlying adult neurogenesis, there are still critical outstanding questions in the field that have not been addressed due to the technical limitations of traditional experimental approaches. In the proposed series of studies, we will use several cutting-edge techniques that we have developed or adapted to investigate the developmental origin of adult neurogenesis, its functional impact in the adult brain, and the fidelity of rodent models to human neuronal development. First, we will characterize the origin and properties of embryonic neural precursor cells that give rise to the largely quiescent pool of neural stem cells that maintain neurogenesis throughout life in a rodent model. Building on our recent findings that Hopx-expressing neural progenitors in the embryonic dentate gyrus can generate the constitutive populations in the dentate gyrus before adopting a quiescent state indicative of adult neural stem cells, we will identify the molecular mechanisms regulate this precursor population and its transition into quiescence. These studies will provide novel insight into the intrinsic and extrinsic signaling cues that establish a long-term pool of stem cells in the developing and adult brain. Second, we have developed a 3D organoid model of dentate gyrus development using human induced pluripotent stem cells to investigate the properties of neural progenitors, neurogenesis and fate specification. These studies could lead to the potential identification of human-specific markers of neural stem cells and new granule neurons in the dentate gyrus and mechanistic differences and similarities with rodent models, which would inform the current debate over the extent of postnatal neurogenesis in the human dentate gyrus. Third, we will investigate the functional properties of adult neurogenesis in adult behaving mice using an optogenetic strategy to identify and record electrophysiological activity of single newborn granule cells at different stages of maturation. We will also investigate the circuit- level impact of silencing these cells at the population level. These data would provide novel information to evaluate the hypothesis that adult-born granule cells make a unique contribution to information processing in the hippocampus using techniques with high temporal resolution. Together, these studies combine an array of approaches to answer fundament...