Projects

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.

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.

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 sspatial 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

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 sspatial 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.

Spinal commissural axon guidance. top: a single spinal commissural neuron (magenta) has an axon that passes under the floorplate (some cells of which are labeled green) and then turns towards the brain. Bottom: higher magnification image of the growth cone of one neuron at the moment of deciding whether to go towards the brain or the tail. The proteins of the Planar Polarity Pathway bias this decision because in mutants the decision is 50-50 compared to 100% towards the brain in wildtype. Both images are lateral views of the spinal cord with anterior to the left.

Moens Lab