Motor Neuron Migration and Planar Polarity

The non-canonical Wnt/Planar Cell Polarity (PCP) signaling pathway is required for neuronal migration and axon guidance events that follow the anterior-posterior axis of the central nervous system. In static epithelial cells, asymmetrically localized PCP protein complexes signal across anterior-posterior cell boundaries to inform cells of their orientation in the plane of the epithelium. The function of PCP signaling in migrating neurons or neuronal processes (axons) is poorly understood. Using timelapse imaging of single neurons in developing embryos, we have found that PCP proteins are required both in the migrating neurons and their surrounding neuroepithelial environment for optimal migration, in part by controlling neuronal filopodial activity. We are currently using a CRISPR-based strategy to identify novel PCP effectors involved in migration and to discover how they control neuronal cytoskeletal dynamics. We are also exploring how the PCP proteins themselves become asymmetrically localized within neuroepithelial cells and how this localization influences polarized axon growth in vivo. Mutations in components of the PCP pathway cause human birth defects including spina bifida, and PCP signaling has been found to promote the motility and invasiveness of breast cancer cells in response to stromal signals. Our findings will elucidate the basis of PCP-mediated cell motility in development and disease.

motor neuron migration and planar polarity
A dorsal view of the zebrafish hindbrain, with anterior to the top. Cranial motor neurons of the facial nerve differentiate in rhombomere 4 and migrate posteriorly adjacent to the floorplate to rhombomere 6, leaving a characteristic axon tract (the genu) that exits the hindbrain in rhombomere 4. Red: nuclear isl1 protein; green: isl1:GFP; blue: DAPI.

Musculotopic Map Formation in the Head

The vertebrate brain contains neuronal representations of the outside world, known as topographic maps. Most of these maps form during development through the use of spatial cues that guide axons in a point-to-point mapping process. We have discovered a novel spatio-temporal mechanism of topographic mapping that guides cranial motor neurons of the vagus nerve to their target muscles in the pharyngeal arches. Using neuronal tracing and single-cell transplantation, we have found that the timing with which a motor neuron elaborates its axon determines which pharyngeal arch it will innervate. Our current work addresses several outstanding questions: for example, how is the timing of axon initiation spatially regulated, and how is the timing of pharyngeal arch development coordinated with axon initiation? We are exploring the roles of retinoic acid (RA) and hepatocyte growth factor (HGF) in vagus motor nucleus patterning and axon guidance, and we are using RNA-Seq of isolated motor neurons to identify genes whose expression correlates with time of axogenesis and asking how their motor neuron-specific gain- and loss of function affects axon targeting. Longer term, we hope to discover the activity-dependent mechanisms that refine the topographic map to provide accurate sensory-motor connectivity.

Musculotopic map formation in the head
A live 36-hour zebrafish embryo viewed from the side, showing cranial motor neurons (green) in the process of innervating the segmentally organized muscle progenitors of the pharyngeal arches (magenta).