Vagus Motor Neuron Topographic Map Determined by Parallel Mechanisms of hox5 Expression and Time of Axon Initiation

Abstract

Many networks throughout the nervous system are organized into topographic maps, where the positions of neuron cell bodies in the projecting field correspond with the positions of their axons in the target field. Previous studies of topographic map development show evidence for spatial patterning mechanisms, in which molecular determinants expressed across the projecting and target fields are matched directly in a point-to-point mapping process. Here, we describe a novel temporal mechanism of topographic map formation that depends on spatially regulated differences in the timing of axon outgrowth and functions in parallel with spatial point-to-point mapping mechanisms. We focus on the vagus motor neurons, which are topographically arranged in both mammals and fish. We show that cell position along the anterior-posterior axis of hindbrain rhombomere 8 determines expression of hox5 genes, which are expressed in posterior, but not anterior, vagus motor neurons. Using live imaging and transplantation in zebrafish embryos, we additionally reveal that axon initiation is delayed in posterior vagus motor neurons independent of neuron birth time. We show that hox5 expression directs topographic mapping without affecting time of axon outgrowth and that time of axon outgrowth directs topographic mapping without affecting hox5 expression. The vagus motor neuron topographic map is therefore determined by two mechanisms that act in parallel: a hox5-dependent spatial mechanism akin to classic mechanisms of topographic map formation and a novel axon outgrowth-dependent temporal mechanism in which time of axon formation is spatially regulated to direct axon targeting.

Retinoic acid organizes the vagus motor topographic map via spatiotemporal regulation of Hgf/Met signaling

“…We recently identified a key role for developmental timing in the topographic organization of the zebrafish vagus nerve, which functions independently of neuron birth timing (Barsh et al, 2017). The vagus nerve (cranial nerve X) carries sensory information of multiple modalities from the pharynx, larynx, heart and viscera, and returns appropriate motor and parasympathetic innervation for control of speech, swallowing, heart rate, and multiple aspects of digestion.…”

“…We previously identified a critical role for innervation timing in the formation of the vagus topographic map (Barsh et al, 2017). In order to elucidate the mechanisms that guide timing- Spatiotemporal regulation of vagus map development 6 dependent targeting, we first established a precise developmental timeline of the formation of pharyngeal arches 4-7 and their innervation by mX neurons.…”

“…The A-P position of a mX neuron within the vagus nucleus determines which branch it innervates, indicating that mX neurons exhibit molecular identity differences along the A-P axis that control axon targeting (Barsh et al, 2017). Furthermore, experimentally altering the A-P position of a post-mitotic mX neuron is sufficient to change target selection, indicating the presence of inductive signals along the A-P axis of the mX nucleus (Barsh et al, 2017). The signals that regulate A-P mX identity, and how these signals control axon targeting, are not known.…”

“…While retinal axon targeting relies purely on the conveyance of spatial information, in some cases the timing of developmental events is an important component of topographic map development (Eerdunfu et al, 2017;Kulkarni et al, 2016;Petrovic and Hummel, 2008;Takeuchi Isabella et al, 2019 Spatiotemporal regulation of vagus map development 5 neurons innervating more posterior branches ( Fig 1B) (Barsh et al, 2017;Bieger and Hopkins, 1987; Morita and Finger, 1985). We found that mX neurons at different A-P positions extend axons and innervate their targets at different developmental times, and that experimentally changing the timing of axon outgrowth in individual mX neurons is sufficient to override spatial cues and alter axon target selection (Barsh et al, 2017). Thus, mX neurons must integrate spatial and temporal cues to coordinate topographic axon targeting decisions.…”

“…It remains unclear whether and how a temporal mechanism might guide projections of groups of neurons in the brain that share a common birth time. Additionally, it is not clear how changes in molecular communication between neurons and their targets are coordinated over time to ensure precise axon-target matching over the period of axonal projection.We recently identified a key role for developmental timing in the topographic organization of the zebrafish vagus nerve, which functions independently of neuron birth timing (Barsh et al, 2017). The vagus nerve (cranial nerve X) carries sensory information of multiple modalities from the pharynx, larynx, heart and viscera, and returns appropriate motor and parasympathetic innervation for control of speech, swallowing, heart rate, and multiple aspects of digestion.…”