Vagus Nerve Development
One specific focus in the lab is on the development of the vagus nerve which provides the major route of neuronal communication between the brain and the visceral organs (the “gut-brain axis”). Using the zebrafish, we previously discovered a novel “temporal matching” mechanism that guides the motor neurons of the vagus nerve to their target muscles in the head—muscles that in humans are used for speech and swallowing. Now, using neuronal tracing, single-cell RNA-Seq and CRISPR-mediated gene targeting, we are seeking to discover how the viscera-innervating vagus motor neurons find their appropriate visceral organs. This is an important goal, because the vagus nerve carries many different information modalities both to and from the brain, and its miswiring can cause serious disorders such as dysphagia (difficulty swallowing), gastropareisis (stomach paralysis), or tachycardia (low heart rate).
We have found that vagus neurons that innervate different targets can lie next to one another in the brain, suggesting that spatial cues are insufficient to distinguish different motor neuron target groups. We hypothesize that once neurons have innervated their organ targets, they undergo further refinement so that they interact with upstream neurons that innervate the same target, thereby completing a fine-tuned reflex circuit. We are using single-cell transplantation and live imaging of neuronal activity to understand the underlying mechainsm of activity-dependent circuit refinment by vagus neurons.
Long-distance axon guidance by local cues
Axon guidance is the process by which neurons find their targets in order to build the neural circuits that allow animals to respond to information they sense in their environment. Axon guidance involves a specialized structure at the end of the neuron called the growth cone, which probes the complex three-dimensional environment of the embryo and makes directional decisions in response to the cues it detects. In the vertebrate spinal cord, dorsal commissural axons extend towards and across the midline floorplate, and then turn longitudinally to ascend towards the brain. This axon trajectory is a prominent feature of vertebrate and invertebrate nervous systems, necessary for the left-right coordination of sensory and motor systems, locomotion, and posture. While the growth cone’s voyage to and across the floorplate has been intensively studied, its final decision—whether to ascend or descend after emerging from the midline—is less well understood. Genetic studies clearly implicate the Planar Cell Polarity (PCP) pathway in this decision, but our understanding of how PCP signaling guides the growth cone is incomplete. The PCP pathway is a cell-cell contact-mediated signaling pathway that transmits polarity information between cells to orient them for directed migration. Yet our mechanistic understanding of the role of PCP signaling in commissural axon guidance is largely informed by studies of isolated growth cones in vitro. Thus, a major gap in our understanding of commissural axon guidance is the role that cell contact-mediated cues play in longitudinal guidance. Using the transparent zebrafish embryo to visualize the axons and growth cones of single identified pioneer commissural interneurons in PCP mutants, we have found that core components of the PCP signaling pathway are required equally within the commissural neuron and in its environment for correct axon targeting. PCP proteins localize to the growth cone and to the cells on its trajectory. Using genetics, live imaging and CRISPR screening we are testing the hypothesis that the growth cone uses PCP signaling to polarize its growth in response to planar-polarized cues in its immediate neuroepithelial environment, and asking what the underlying mechanisms are that transmit planar polarity information to the growth cone cytoskeleton.