In the developing brain, neural progenitor cells (NPCs) initially divide symmetrically, make more of themselves, and expand the size of the NPC pool. Later, their behavior irreversibly changes as they asymmetrically divide to produce a neuron with each division (the process of neurogenesis). Once neurogenesis begins, NPCs are committed and they are not able to return to symmetric cell divisions. It is important to the development of a brain that the change from symmetric to asymmetric cell division does not occur too early, which limits the number of neurons generated, or too late, which improperly expands the brain. Pockets of NPCs persist in the brain and continue to divide in adulthood. Limiting neurogenesis in the adult, where the newborn neurons contribute to learning and memory, may accelerate cognitive decline and inhibit the ability of the brain to recover from injury. Consequently, mutations, disease or traumas that alter NPC proliferation contribute to the etiology of many brain disorders. Intrinsic signals like gene expression regulate NPC fate but equally important are the extrinsic inputs such as brain activity or diffusible signaling molecules. We do not understand the interplay between these intrinsic and extrinsic inputs and how they come together to control NPCs. Increasing evidence suggests mitochondria may be central to this process. NPCs rely on aerobic glycolysis, a non-mitochondrial ATP generating process. Yet, NPCs contain abundant mitochondria with morphological features that change as the NPC fate changes. The role of mitochondria in NPCs is unclear, but interfering with mitochondrial structure and function limits the ability of NPC to proliferate. Assessments of cell proliferation have only been conducted thus far in vitro, where NPCs are divorced from the extrinsic signals of the environmental niche. This is a concern that my students and I circumvent by studying the NPCs in transparent Xenopus laevis, using in vivo timelapse confocal microscopy where individual fluorescently-labeled NPCs, their cellular progeny and their labeled mitochondria can be tracked over days. Will test our hypothesis that mitochondrial biogenesis, mitochondrial dynamics and the metabolic activity of mitochondria within NPCs are regulated in order to support changes in the fate of the NPCs. We will analyze and quantify the diversity of mitochondria, their motility and metabolic potential in the NPCs to determine the features that forecast cell fate (Aim 1). By manipulating mitochondrial biogenesis in NPCs, we predict that we can force them to leave the symmetrically-dividing population and commit to neurogenic cell divisions (Aim 2). Finally, sensory experience is a potent regulator cell proliferation, acting through signaling pathways that also affect mitochondrial function.
In Aim 3 we will test whether increasing or suppressing mitochondrial biogenesis mediates experience-dependent changes in NPC proliferation and neurogenesis. In this work, students gain expertise in molecular biology techniques and 4D imaging and analysis. Our results will give insights into the control of cell proliferation in the brain, an important source of plasticity that may one day be harnessed to treat disease or injury.
Understanding how neural progenitor cells (NPCs) respond to intrinsic (e.g., transcription factor expression) and extrinsic (e.g., brain activity) factors in order to regulate whether they divide and expand the progenitor pool or irreversibly begin the process of producing neurons with each cell division (neurogenesis) is critical to developing therapeutic interventions that will prevent developmental brain malformations and neurological disorders. Although they are metabolically silent, the morphologically dynamic and motile mitochondria in NPCs are necessary for normal NPC proliferation and neurogenesis, but this has yet only been tested after the NPCs were isolated from the brain and therefore divorced from their external inputs that play a crucial role in these developmental processes. Using in vivo timelapse microscopy to track the NPCs, their mitochondria, and their cellular progeny in the living animal where we can control levels of sensory experience, we will investigate the mitochondria?s role in controlling cell proliferation and neurogenesis, thus providing insights into how one day NPCs could be harnessed to treat disease or injury.
