Key Features of Structural and Functional Organization of Zebrafish Facial Motor Neurons Are Resilient to Disruption of Neuronal Migration

Cranial motor neuron input specificity refined by activity

A recent study by Kaneko and colleagues provides evidence that developing cranial motor neurons in larval zebrafish refine their input specificity over time, using an activity-dependent mechanism that may depend, in part, on adaptive dendrite extension. These findings illuminate the mechanism by which spatially overlapping motor pools are recruited into distinct motor circuits.

Structure and Topography of Facial Branchiomotor Neuron Dendrites in Larval Zebrafish (Danio rerio)

Motor circuits in the vertebrate hindbrain need to become functional early in development. What are the fundamental mechanisms that establish early synaptic inputs to motor neurons? Previous evidence is consistent with the hypothesis that motor neuron dendrite positioning serves a causal role in early spinal motor circuit development, with initial connectivity determined by the overlap between premotor axons and motor neuron dendrites (perhaps without the need for molecular recognition). Does motor neuron dendrite topography serve a similar role in the hindbrain? In the current study, we provide the first quantitative analysis of the dendrites of facial branchiomotor neurons (FBMNs) in larval zebrafish. We previously demonstrated that FBMNs exhibit functional topography along the dorsoventral axis, with the most ventral cell bodies most likely to exhibit early rhythmic activity-suggesting that FBMNs with ventral cell bodies are most likely to receive inputs from premotor neurons carrying rhythmic respiratory signals. We hypothesized that this functional topography can be explained by differences in dendrite positioning, giving ventral FBMNs preferential access to premotor axons carrying rhythmic signals. If this hypothesis is true, we predicted that FBMN cell body position would be correlated with dendrite position along the dorsoventral axis. To test this prediction, we used single-cell labeling to trace the dendritic arbors of FBMNs in larval zebrafish at 5-days post-fertilization (dpf). FBMN dendrites varied in complexity, and this variation could not be attributed to differences in the relative age of neurons. Most dendrites grew caudally, laterally, and ventrally from the cell body-though FBMN dendrites could extend their dendrites dorsally. Across our sample, FBMN cell body position correlated with dendrite position along the dorsoventral axis, consistent with our hypothesis that differences in dendrite positioning serve as the substrate for differences in activity patterns across neurons. Future studies will build on this foundational data, testing additional predictions of the central hypothesis-to further investigate the mechanisms of early motor circuit development.

Position-independent functional refinement within the vagus motor topographic map

Motor neurons in the central nervous system often lie in a continuous topographic map, where neurons that innervate different body parts are spatially intermingled. This is the case for the efferent neurons of the vagus nerve, which innervate diverse muscle and organ targets in the head and viscera for brain-body communication. It remains elusive how neighboring motor neurons with different fixed peripheral axon targets develop the separate somatodendritic (input) connectivity they need to generate spatially precise body control. Here, we show that vagus motor neurons in the zebrafish indeed generate spatially appropriate peripheral responses to focal sensory stimulation even when they are transplanted into ectopic positions within the topographic map, indicating that circuit refinement occurs after the establishment of coarse topography. Refinement depends on motor neuron synaptic transmission, suggesting that an experience-dependent periphery-to-brain feedback mechanism establishes specific input connectivity among intermingled motor populations.

Developing zebrafish utilize taste-signaling pathways for oxygen chemoreception

A fundamental requirement for all animals is to sense and respond to changes in environmental O2 availability. Low O2 (hypoxia) typically stimulates breathing, a universal and critical response termed the hypoxic ventilatory response (HVR). In this study, we test the hypothesis that taste-signaling pathways are used for O2 sensing and activation of the HVR. We show that Merkel-like cells (MLCs), which are part of the taste-bud complex, function as O2 chemoreceptor cells in larval zebrafish and that transduction of the O2 signal uses taste-signaling pathways. Specifically, MLCs responded to hypoxia in vivo with an increase in Ca2+ activity that can drive the HVR. In addition, MLCs transmit O2 signals to afferent cranial nerves IX and X (nIX/X), which project into the area postrema within the hindbrain and synapse with interneurons that are in contact with vagal motor neurons. Hypoxia or chemo-activation of nIX/X caused Ca2+ activity to increase within the area postrema and elicited hyperventilation. The results provide the first demonstration of an O2 signaling pathway that commences with the activation of taste receptors (MLCs) to yield a critical physiological reflex, the HVR.

Position-independent functional refinement within the vagus motor topographic map

Motor neurons in the central nervous system often lie in a continuous topographic map, where neurons that innervate different body parts are spatially intermingled. This is the case for the efferent neurons of the vagus nerve, which innervate diverse muscle and organ targets in the head and viscera for brain-body communication. It remains elusive how neighboring motor neurons with different fixed peripheral axon targets develop the separate somatodendritic (input) connectivity they need to generate spatially precise body control. Here we show that vagus motor neurons in the zebrafish indeed generate spatially appropriate peripheral responses to focal sensory stimulation even when they are transplanted into ectopic positions within the topographic map, indicating that circuit refinement occurs after the establishment of coarse topography. Refinement depends on motor neuron synaptic transmission, suggesting that an experience-dependent periphery-to-brain feedback mechanism establishes specific input connectivity amongst intermingled motor populations.

Early development of respiratory motor circuits in larval zebrafish (Danio rerio)

Rhythm-generating circuits in the vertebrate hindbrain form synaptic connections with cranial and spinal motor neurons, to generate coordinated, patterned respiratory behaviors. Zebrafish provide a uniquely tractable model system to investigate the earliest stages in respiratory motor circuit development in vivo. In larval zebrafish, respiratory behaviors are carried out by muscles innervated by cranial motor neurons-including the facial branchiomotor neurons (FBMNs), which innervate muscles that move the jaw, buccal cavity, and operculum. However, it is unclear when FBMNs first receive functional synaptic input from respiratory pattern-generating neurons, and how the functional output of the respiratory motor circuit changes across larval development. In the current study, we used behavior and calcium imaging to determine how early FBMNs receive functional synaptic inputs from respiratory pattern-generating networks in larval zebrafish. Zebrafish exhibited patterned operculum movements by 3 days postfertilization (dpf), though this behavior became more consistent at 4 and 5 dpf. Also by 3dpf, FBMNs fell into two distinct categories ("rhythmic" and "nonrhythmic"), based on patterns of neural activity. These two neuron categories were arranged differently along the dorsoventral axis, demonstrating that FBMNs have already established dorsoventral topography by 3 dpf. Finally, operculum movements were coordinated with pectoral fin movements at 3 dpf, indicating that the operculum behavioral pattern was driven by synaptic input. Taken together, this evidence suggests that FBMNs begin to receive initial synaptic input from a functional respiratory central pattern generator at or prior to 3 dpf. Future studies will use this model to study mechanisms of normal and abnormal respiratory circuit development.

Coordinated cadherin functions sculpt respiratory motor circuit connectivity

Breathing, and the motor circuits that control it, is essential for life. At the core of respiratory circuits are Dbx1-derived interneurons, which generate the rhythm and pattern of breathing, and phrenic motor neurons (MNs), which provide the final motor output that drives diaphragm muscle contractions during inspiration. Despite their critical function, the principles that dictate how respiratory circuits assemble are unknown. Here, we show that coordinated activity of a type I cadherin (N-cadherin) and type II cadherins (Cadherin-6, -9, and -10) is required in both MNs and Dbx1-derived neurons to generate robust respiratory motor output. Both MN- and Dbx1-specific cadherin inactivation in mice during a critical developmental window results in perinatal lethality due to respiratory failure and a striking reduction in phrenic MN bursting activity. This combinatorial cadherin code is required to establish phrenic MN cell body and dendritic topography; surprisingly, however, cell body position appears to be dispensable for the targeting of phrenic MNs by descending respiratory inputs. Our findings demonstrate that type I and II cadherins function cooperatively throughout the respiratory circuit to generate a robust breathing output and reveal novel strategies that drive the assembly of motor circuits.

From Cell States to Cell Fates: How Cell Proliferation and Neuronal Differentiation Are Coordinated During Embryonic Development

The central nervous system (CNS) exhibits an extraordinary diversity of neurons, with the right cell types and proportions at the appropriate sites. Thus, to produce brains with specific size and cell composition, the rates of proliferation and differentiation must be tightly coordinated and balanced during development. Early on, proliferation dominates; later on, the growth rate almost ceases as more cells differentiate and exit the cell cycle. Generation of cell diversity and morphogenesis takes place concomitantly. In the vertebrate brain, this results in dramatic changes in the position of progenitor cells and their neuronal derivatives, whereas in the spinal cord morphogenetic changes are not so important because the structure mainly grows by increasing its volume. Morphogenesis is under control of specific genetic programs that coordinately unfold over time; however, little is known about how they operate and impact in the pools of progenitor cells in the CNS. Thus, the spatiotemporal coordination of these processes is fundamental for generating functional neuronal networks. Some key aims in developmental neurobiology are to determine how cell diversity arises from pluripotent progenitor cells, and how the progenitor potential changes upon time. In this review, we will share our view on how the advance of new technologies provides novel data that challenge some of the current hypothesis. We will cover some of the latest studies on cell lineage tracing and clonal analyses addressing the role of distinct progenitor cell division modes in balancing the rate of proliferation and differentiation during brain morphogenesis. We will discuss different hypothesis proposed to explain how progenitor cell diversity is generated and how they challenged prevailing concepts and raised new questions.

Defective Neuronal Positioning Correlates With Aberrant Motor Circuit Function in Zebrafish

Precise positioning of neurons resulting from cell division and migration during development is critical for normal brain function. Disruption of neuronal migration can cause a myriad of neurological disorders. To investigate the functional consequences of defective neuronal positioning on circuit function, we studied a zebrafish frizzled3a (fzd3a) loss-of-function mutant off-limits (olt) where the facial branchiomotor (FBM) neurons fail to migrate out of their birthplace. A jaw movement assay, which measures the opening of the zebrafish jaw (gape), showed that the frequency of gape events, but not their amplitude, was decreased in olt mutants. Consistent with this, a larval feeding assay revealed decreased food intake in olt mutants, indicating that the FBM circuit in mutants generates defective functional outputs. We tested various mechanisms that could generate defective functional outputs in mutants. While fzd3a is ubiquitously expressed in neural and non-neural tissues, jaw cartilage and muscle developed normally in olt mutants, and muscle function also appeared to be unaffected. Although FBM neurons were mispositioned in olt mutants, axon pathfinding to jaw muscles was unaffected. Moreover, neuromuscular junctions established by FBM neurons on jaw muscles were similar between wildtype siblings and olt mutants. Interestingly, motor axons innervating the interhyoideus jaw muscle were frequently defasciculated in olt mutants. Furthermore, GCaMP imaging revealed that mutant FBM neurons were less active than their wildtype counterparts. These data show that aberrant positioning of FBM neurons in olt mutants is correlated with subtle defects in fasciculation and neuronal activity, potentially generating defective functional outputs.

Zebrafish Minichromosome Maintenance Protein 5 Gene Regulates the Development and Migration of Facial Motor Neurons via Fibroblast Growth Factor Signaling

Minichromosome maintenance protein 5 (MCM5), a member of the microchromosomal maintenance protein family, plays an important role in the initiation and extension of DNA replication. However, its role in neural development in zebrafish remains unclear. Here, we used morpholino (MO) and CRISPR/Cas9 to knock down mcm5 and investigated the developmental features of facial motor neurons (FMNs) in the hindbrain of zebrafish. We found that knockdown of mcm5 using mcm5 MO resulted in a small head, small eyes, and a blurred midbrain-hindbrain boundary, while MO injection of mcm5 led to decrease in FMNs and their migration disorder. However, the mutant of mcm5 only resulted in the migration defect of FMNs rather than quantity change. We further investigated the underlying mechanism of mcm5 in the development of hindbrain using in situ hybridization (ISH) and fgfr1a mRNA co-injected with mcm5 MO. Results from ISH showed that the fibroblast growth factor (FGF) signaling pathway was changed when the MCM5 function was lost, with the decrease in fgfr1a and the increase in fgf8, while that of pea3 had opposite trend. FMN development defects were rescued by fgfr1a mRNA co-injected with mcm5 MO. Our results demonstrated that FGF signaling pathway is required for FMN development in zebrafish. Specifically, mcm5 regulates FMN development during zebrafish growing.

Cadherins regulate nuclear topography and function of developing ocular motor circuitry

In the vertebrate central nervous system, groups of functionally related neurons, including cranial motor neurons of the brainstem, are frequently organised as nuclei. The molecular mechanisms governing the emergence of nuclear topography and circuit function are poorly understood. Here we investigate the role of cadherin-mediated adhesion in the development of zebrafish ocular motor (sub)nuclei. We find that developing ocular motor (sub)nuclei differentially express classical cadherins. Perturbing cadherin function in these neurons results in distinct defects in neuronal positioning, including scattering of dorsal cells and defective contralateral migration of ventral subnuclei. In addition, we show that cadherin-mediated interactions between adjacent subnuclei are critical for subnucleus position. We also find that disrupting cadherin adhesivity in dorsal oculomotor neurons impairs the larval optokinetic reflex, suggesting that neuronal clustering is important for co-ordinating circuit function. Our findings reveal that cadherins regulate distinct aspects of cranial motor neuron positioning and establish subnuclear topography and motor function.

Etv1 Controls the Establishment of Non-overlapping Motor Innervation of Neighboring Facial Muscles during Development

The somatotopic motor-neuron projections onto their cognate target muscles are essential for coordinated movement, but how that occurs for facial motor circuits, which have critical roles in respiratory and interactive behaviors, is poorly understood. We report extensive molecular heterogeneity in developing facial motor neurons in the mouse and identify markers of subnuclei and the motor pools innervating specific facial muscles. Facial subnuclei differentiate during migration to the ventral hindbrain, where neurons with progressively later birth dates—and evolutionarily more recent functions—settle in more-lateral positions. One subpopulation marker, ETV1, determines both positional and target muscle identity for neurons of the dorsolateral (DL) subnucleus. In Etv1 mutants, many markers of DL differentiation are lost, and individual motor pools project indifferently to their own and neighboring muscle targets. The resulting aberrant activation patterns are reminiscent of the facial synkinesis observed in humans after facial nerve injury.

Tenney et al. demonstrate that embryonic facial motor neurons are transcriptionally diverse as they establish somatotopic innervation of the facial muscles, a process that requires the transcription factor ETV1. Facial-motor axon-targeting errors in Etv1 mutants cause coordination of whisking and eyeblink evocative of human blepharospasm.

Hindbrain neurovascular anatomy of adult goldfish (Carassius auratus)

The goldfish hindbrain develops from a segmented (rhombomeric) neuroepithelial scaffold, similar to other vertebrates. Motor, reticular and other neuronal groups develop in specific segmental locations within this rhombomeric framework. Teleosts are unique in possessing a segmental series of unpaired, midline central arteries that extend from the basilar artery and penetrate the pial midline of each hindbrain rhombomere (r). This study demonstrates that the rhombencephalic arterial supply of the brainstem forms in relation to the neural segments they supply. Midline central arteries penetrate the pial floor plate and branch within the neuroepithelium near the ventricular surface to form vascular trees that extend back towards the pial surface. This intramural branching pattern has not been described in any other vertebrate, with blood flow in a ventriculo-pial direction, vastly different than the pial-ventricular blood flow observed in most other vertebrates. Each central arterial stem penetrates the pial midline and ascends through the floor plate, giving off short transverse paramedian branches that extend a short distance into the adjoining basal plate to supply ventromedial areas of the brainstem, including direct supply of reticulospinal neurons. Robust r3 and r8 central arteries are significantly larger and form a more interconnected network than any of the remaining hindbrain vascular stems. The r3 arterial stem has extensive vascular branching, including specific vessels that supply the cerebellum, trigeminal motor nucleus located in r2/3 and facial motoneurons found in r6/7. Results suggest that some blood vessels may be predetermined to supply specific neuronal populations, even traveling outside of their original neurovascular territories in order to supply migrated neurons.

Longitudinal monitoring of cancer cell subpopulations in monolayers, 3D spheroids, and xenografts using the photoconvertible dye DiR

A central challenge in cancer biology is the identification, longitudinal tracking, and -omics analysis of specific cells in vivo. To this aim, photoconvertible fluorescent dyes are reporters that are characterized by a set of excitation and emission spectra that can be predictably altered, resulting in a distinct optical signature following irradiation with a specific light source. One such dye, DiR, is an infrared fluorescent membrane probe that can irreversibly undergo such a switch. Here, we demonstrate a method using DiR for the spatiotemporal labeling of specific cells in the context of cancer cell monolayer cultures, 3D tumor spheroids, and in vivo melanoma xenograft models to monitor the proliferation of cellular subpopulations of interest over time. Importantly, the photoconversion process is performed in situ, supporting the pursuit of novel avenues of research in molecular pathology.

Molecular specification of facial branchial motor neurons in vertebrates

Orofacial muscles are critical for life-sustaining behaviors, such as feeding and breathing. Centuries of work by neuroanatomists and surgeons resulted in the mapping of bulbar motor neurons in the brainstem and the course of the cranial nerves that carry their axons. Despite the sophisticated understanding of the anatomy of the region, the molecular mechanisms that dictate the development and maturation of facial motor neurons remain poorly understood. This fundamental problem has been recently revisited by physiologists with novel techniques of studying the rhythmic contraction of orofacial muscles in relationship to breathing. The molecular understanding of facial motor neuron development will not only lead to the comprehension of the neural basis of facial expression but may also unlock new avenues to generate stem cell-derived replacements. This review summarizes the current understanding of molecular programs involved in facial motor neuron generation, migration, and maturation, including neural circuit assembly.

Role of branchiomotor neurons in controlling food intake of zebrafish larvae

The physical act of eating or feeding involves the coordinated action of several organs like eyes and jaws, and associated neural networks. Moreover, the activity of the neural networks controlling jaw movements (branchiomotor circuits) is regulated by the visual, olfactory, gustatory and hypothalamic systems, which are largely well characterized at the physiological level. By contrast, the behavioral output of the branchiomotor circuits and the functional consequences of disruption of these circuits by abnormal neural development are poorly understood. To begin to address these questions, we sought to evaluate the feeding ability of zebrafish larvae, a direct output of the branchiomotor circuits, and developed a qualitative assay for measuring food intake in zebrafish larvae at 7 days post-fertilization. We validated the assay by examining the effects of ablating the branchiomotor neurons. Metronidazole-mediated ablation of nitroreductase-expressing branchiomotor neurons resulted in a predictable reduction in food intake without significantly affecting swimming ability, indicating that the assay is robust. Laser-mediated ablation of trigeminal motor neurons resulted in a significant decrease in food intake, indicating that the assay is sensitive. Importantly, in larvae of a genetic mutant with severe loss of branchiomotor neurons, food intake was abolished. These studies establish a foundation for dissecting the neural circuits driving a motor behavior essential for survival.