Progressive neurogenesis defines lateralis somatotopy

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

A neuronal blueprint for directional mechanosensation in larval zebrafish

Animals have a remarkable ability to use local cues to orient in space in the absence of a panoramic fixed reference frame. Here we use the mechanosensory lateral line in larval zebrafish to understand rheotaxis, an innate oriented swimming evoked by water currents. We generated a comprehensive light-microscopy cell-resolution projectome of lateralis afferent neurons (LANs) and used clustering techniques for morphological classification. We find surprising structural constancy among LANs. Laser-mediated microlesions indicate that precise topographic mapping of lateral-line receptors is not essential for rheotaxis. Recording neuronal-activity during controlled mechanical stimulation of neuromasts reveals unequal representation of water-flow direction in the hindbrain. We explored potential circuit architectures constrained by anatomical and functional data to suggest a parsimonious model under which the integration of lateralized signals transmitted by direction-selective LANs underlies the encoding of water-flow direction in the brain. These data provide a new framework to understand how animals use local mechanical cues to orient in space.

Elmo1 function, linked to Rac1 activity, regulates peripheral neuronal numbers and myelination in zebrafish

Peripheral nervous system development involves a tight coordination of neuronal birth and death and a substantial remodelling of the myelinating glia cytoskeleton to achieve myelin wrapping of its projecting axons. However, how these processes are coordinated through time is still not understood. We have identified engulfment and cell motility 1, Elmo1, as a novel component that regulates (i) neuronal numbers within the Posterior Lateral Line ganglion and (ii) radial sorting of axons by Schwann cells (SC) and myelination in the PLL system in zebrafish. Our results show that neuronal and myelination defects observed in elmo1 mutant are rescued through small GTPase Rac1 activation. Inhibiting macrophage development leads to a decrease in neuronal numbers, while peripheral myelination is intact. However, elmo1 mutants do not show defective macrophage activity, suggesting a role for Elmo1 in PLLg neuronal development and SC myelination independent of macrophages. Forcing early Elmo1 and Rac1 expression specifically within SCs rescues elmo1-/- myelination defects, highlighting an autonomous role for Elmo1 and Rac1 in radial sorting of axons by SCs and myelination. This uncovers a previously unknown function of Elmo1 that regulates fundamental aspects of PNS development.

Retrograde Ret signaling controls sensory pioneer axon outgrowth

The trafficking mechanisms and transcriptional targets downstream of long-range neurotrophic factor ligand/receptor signaling that promote axon growth are incompletely understood. Zebrafish carrying a null mutation in a neurotrophic factor receptor, Ret, displayed defects in peripheral sensory axon growth cone morphology and dynamics. Ret receptor was highly enriched in sensory pioneer neurons and Ret51 isoform was required for pioneer axon outgrowth. Loss-of-function of a cargo adaptor, Jip3, partially phenocopied Ret axonal defects, led to accumulation of activated Ret in pioneer growth cones, and reduced retrograde Ret51 transport. Jip3 and Ret51 were also retrogradely co-transported, ultimately suggesting Jip3 is a retrograde adapter of active Ret51. Finally, loss of Ret reduced transcription and growth cone localization of Myosin-X, an initiator of filopodial formation. These results show a specific role for Ret51 in pioneer axon growth, and suggest a critical role for long-range retrograde Ret signaling in regulating growth cone dynamics through downstream transcriptional changes.

Studying Axonal Regeneration by Laser Microsurgery and High-Resolution Videomicroscopy

Heterogeneous and unpredictable environmental insult, disease, or trauma can affect the integrity and function of neuronal circuits, leading to irreversible neural dysfunction. The peripheral nervous system can robustly regenerate axons after damage to recover the capacity to transmit sensory information to the brain. The mechanisms that allow axonal repair remain incompletely understood. Here we present a preparation in zebrafish that combines laser microsurgery of sensory axons and videomicroscopy of neurons in multicolor transgenic specimens. This simple protocol allows controlled damage of axons and dynamic high-resolution visualization and quantification of repair.

The Order and Place of Neuronal Differentiation Establish the Topography of Sensory Projections and the Entry Points within the Hindbrain

Establishing topographical maps of the external world is an important but still poorly understood feature of the vertebrate sensory system. To study the selective innervation of hindbrain regions by sensory afferents in the zebrafish embryo, we mapped the fine-grained topographical representation of sensory projections at the central level by specific photoconversion of sensory neurons. Sensory ganglia located anteriorly project more medially than do ganglia located posteriorly, and this relates to the order of sensory ganglion differentiation. By single-plane illumination microscopy (SPIM) in vivo imaging, we show that (1) the sequence of arrival of cranial ganglion inputs predicts the topography of central projections, and (2) delaminated neuroblasts differentiate in close contact with the neural tube, and they never loose contact with the neural ectoderm. Afferent entrance points are established by plasma membrane interactions between primary differentiated peripheral sensory neurons and neural tube border cells with the cooperation of neural crest cells. These first contacts remain during ensuing morphological growth to establish pioneer axons. Neural crest cells and repulsive slit1/robo2 signals then guide axons from later-differentiating neurons toward the neural tube. Thus, this study proposes a new model by which the topographical representation of cranial sensory ganglia is established by entrance order, with the entry points determined by cell contact between the sensory ganglion cell bodies and the hindbrain.

There and back again: development and regeneration of the zebrafish lateral line system

The zebrafish lateral line is a sensory system used to detect changes in water flow. It is comprised of clusters of mechanosensory hair cells called neuromasts. The lateral line is initially established by a migratory group of cells, called a primordium, that deposits neuromasts at stereotyped locations along the surface of the fish. Wnt, FGF, and Notch signaling are all important regulators of various aspects of lateral line development, from primordium migration to hair cell specification. As zebrafish age, the organization of the lateral line becomes more complex in order to accommodate the fish's increased size. This expansion is regulated by many of the same factors involved in the initial development. Furthermore, unlike mammalian hair cells, lateral line hair cells have the capacity to regenerate after damage. New hair cells arise from the proliferation and differentiation of surrounding support cells, and the molecular and cellular pathways regulating this are beginning to be elucidated. All in all, the zebrafish lateral line has proven to be an excellent model in which to study a diverse array of processes, including collective cell migration, cell polarity, cell fate, and regeneration.

Axon-Schwann cell interactions during peripheral nerve regeneration in zebrafish larvae

Background

Peripheral nerve injuries can severely affect the way that animals perceive signals from the surrounding environment. While damage to peripheral axons generally has a better outcome than injuries to central nervous system axons, it is currently unknown how neurons re-establish their target innervations to recover function after injury, and how accessory cells contribute to this task. Here we use a simple technique to create reproducible and localized injury in the posterior lateral line (pLL) nerve of zebrafish and follow the fate of both neurons and Schwann cells.

Results

Using pLL single axon labeling by transient transgene expression, as well as transplantation of glial precursor cells in zebrafish larvae, we individualize different components in this system and characterize their cellular behaviors during the regenerative process. Neurectomy is followed by loss of Schwann cell differentiation markers that is reverted after nerve regrowth. We show that reinnervation of lateral line hair cells in neuromasts during pLL nerve regeneration is a highly dynamic process with promiscuous yet non-random target recognition. Furthermore, Schwann cells are required for directional extension and fasciculation of the regenerating nerve. We provide evidence that these cells and regrowing axons are mutually dependant during early stages of nerve regeneration in the pLL. The role of ErbB signaling in this context is also explored.

Conclusion

The accessibility of the pLL nerve and the availability of transgenic lines that label this structure and their synaptic targets provides an outstanding in vivo model to study the different events associated with axonal extension, target reinnervation, and the complex cellular interactions between glial cells and injured axons during nerve regeneration.

Optical stimulation of zebrafish hair cells expressing channelrhodopsin-2

Vertebrate hair cells are responsible for the high fidelity encoding of mechanical stimuli into trains of action potentials (spikes) in afferent neurons. Here, we generated a transgenic zebrafish line expressing Channelrhodopsin-2 (ChR2) under the control of the hair-cell specific myo6b promoter, in order to examine the role of the mechanoelectrical transduction (MET) channel in sensory encoding in afferent neurons. We performed in vivo recordings from afferent neurons of the zebrafish lateral line while activating hair cells with either mechanical stimuli from a waterjet or optical stimuli from flashes of ∼470-nm light. Comparison of the patterns of encoded spikes during 100-ms stimuli revealed no difference in mean first spike latency between the two modes of activation. However, there was a significant increase in the variability of first spike latency during optical stimulation as well as an increase in the mean number of spikes per stimulus. Next, we compared encoding of spikes during hair-cell stimulation at 10, 20, and 40-Hz. Consistent with the increased variability of first spike latency, we saw a significant decrease in the vector strength of phase-locked spiking during optical stimulation. These in vivo results support a physiological role for the MET channel in the high fidelity of first spike latency seen during encoding of mechanical sensory stimuli. Finally, we examined whether remote activation of hair cells via ChR2 activation was sufficient to elicit escape responses in free-swimming larvae. In transgenic larvae, 100-ms flashes of ∼470-nm light resulted in escape responses that occurred concomitantly with field recordings indicating Mauthner cell activity. Altogether, the myo6b:ChR2 transgenic line provides a platform to investigate hair-cell function and sensory encoding, hair-cell sensory input to the Mauthner cell, and the ability to remotely evoke behavior in free-swimming zebrafish.

Dynamic neuroanatomy at subcellular resolution in the zebrafish

Genetic means to visualize and manipulate neuronal circuits in the intact animal have revolutionized neurobiology. "Dynamic neuroanatomy" defines a range of approaches aimed at quantifying the architecture or subcellular organization of neurons over time during their development, regeneration, or degeneration. A general feature of these approaches is their reliance on the optical isolation of defined neurons in toto by genetically expressing markers in one or few cells. Here we use the afferent neurons of the lateral line as an example to describe a simple method for the dynamic neuroanatomical study of axon terminals in the zebrafish by laser-scanning confocal microscopy.

Developmental and architectural principles of the lateral-line neural map

The transmission and central representation of sensory cues through the accurate construction of neural maps is essential for animals to react to environmental stimuli. Structural diversity of sensorineural maps along a continuum between discrete- and continuous-map architectures can influence behavior. The mechanosensory lateral line of fishes and amphibians, for example, detects complex hydrodynamics occurring around the animal body. It triggers innate fast escape reactions but also modulates complex navigation behaviors that require constant knowledge about the environment. The aim of this article is to summarize recent work in the zebrafish that has shed light on the development and structure of the lateralis neural map, which is helping to understand how individual sensory modalities generate appropriate behavioral responses to the sensory context.

Innervation is required for sense organ development in the lateral line system of adult zebrafish

Superficial mechanosensory organs (neuromasts) distributed over the head and body of fishes and amphibians form the "lateral line" system. During zebrafish adulthood, each neuromast of the body (posterior lateral line system, or PLL) produces "accessory" neuromasts that remain tightly clustered, thereby increasing the total number of PLL neuromasts by a factor of more than 10. This expansion is achieved by a budding process and is accompanied by branches of the afferent nerve that innervates the founder neuromast. Here we show that innervation is essential for the budding process, in complete contrast with the development of the embryonic PLL, where innervation is entirely dispensable. To obtain insight into the molecular mechanisms that underlie the budding process, we focused on the terminal system that develops at the posterior tip of the body and on the caudal fin. In this subset of PLL neuromasts, bud neuromasts form in a reproducible sequence over a few days, much faster than for other PLL neuromasts. We show that wingless/int (Wnt) signaling takes place during, and is required for, the budding process. We also show that the Wnt activator R-spondin is expressed by the axons that innervate budding neuromasts. We propose that the axon triggers Wnt signaling, which itself is involved in the proliferative phase that leads to bud formation. Finally, we show that innervation is required not only for budding, but also for long-term maintenance of all PLL neuromasts.

Developmental origin of a major difference in sensory patterning between zebrafish and bluefin tuna

The posterior lateral line system (PLL) of teleost fish comprises a number of mechanosensory organs arranged in defined patterns on the body surface. Embryonic patterns are largely conserved among teleosts, yet adult patterns are highly diverse. Although changes in pattern modify the perceptual abilities of the system, their developmental origin remains unknown. Here we compare the processes that underlie the formation of the juvenile PLL pattern in Thunnus thynnus, the bluefin tuna, to the processes that were elucidated in Danio rerio, the zebrafish. In both cases, the embryonic PLL comprises five neuromasts regularly spaced along the horizontal myoseptum, but the juvenile PLL comprises four roughly parallel anteroposterior lines in zebrafish, whereas it is a simple dorsally arched line in tuna fish. We examined whether this difference involves evolutionary novelties, and show that the same mechanisms mediate the transition from embryonic to juvenile patterns in both species. We conclude that the marked difference in juveniles depends on a single change (dorsal vs. ventral migration of neuromasts) in the first days of larval life.

Zebrafish larvae exhibit rheotaxis and can escape a continuous suction source using their lateral line

Zebrafish larvae show a robust behavior called rheotaxis, whereby they use their lateral line system to orient upstream in the presence of a steady current. At 5 days post fertilization, rheotactic larvae can detect and initiate a swimming burst away from a continuous point-source of suction. Burst distance and velocity increase when fish initiate bursts closer to the suction source where flow velocity is higher. We suggest that either the magnitude of the burst reflects the initial flow stimulus, or fish may continually sense flow during the burst to determine where to stop. By removing specific neuromasts of the posterior lateral line along the body, we show how the location and number of flow sensors play a role in detecting a continuous suction source. We show that the burst response critically depends on the presence of neuromasts on the tail. Flow information relayed by neuromasts appears to be involved in the selection of appropriate behavioral responses. We hypothesize that caudally located neuromasts may be preferentially connected to fast swimming spinal motor networks while rostrally located neuromasts are connected to slow swimming motor networks at an early age.

Neuronal birth order identifies a dimorphic sensorineural map

Spatially distributed sensory information is topographically mapped in the brain by point-to-point correspondence of connections between peripheral receptors and central target neurons. In fishes, for example, the axonal projections from the mechanosensory lateral line organize a somatotopic neural map. The lateral line provides hydrodynamic information for intricate behaviors such as navigation and prey detection. It also mediates fast startle reactions triggered by the Mauthner cell. However, it is not known how the lateralis neural map is built to subserve these contrasting behaviors. Here we reveal that birth order diversifies lateralis afferent neurons in the zebrafish. We demonstrate that early- and late-born lateralis afferents diverge along the main axes of the hindbrain to synapse with hundreds of second-order targets. However, early-born afferents projecting from primary neuromasts also assemble a separate map by converging on the lateral dendrite of the Mauthner cell, whereas projections from secondary neuromasts never make physical contact with the Mauthner cell. We also show that neuronal diversity and map topology occur normally in animals permanently deprived of mechanosensory activity. We conclude that neuronal birth order correlates with the assembly of neural submaps, whose combination is likely to govern appropriate behavioral reactions to the sensory context.

Building the posterior lateral line system in zebrafish

The posterior lateral line (pLL) in zebrafish has emerged as an excellent system to study how a sensory organ system develops. Here we review recent studies that illustrate how interactions between multiple signaling pathways coordinate cell fate,morphogenesis, and collective migration of cells in the posterior lateral line primordium. These studies also illustrate how the pLL system is contributing much more broadly to our understanding of mechanisms operating during the growth, regeneration, and self-organization of other organ systems during development and disease.

Physiology of afferent neurons in larval zebrafish provides a functional framework for lateral line somatotopy

Fishes rely on the neuromasts of their lateral line system to detect water flow during behaviors such as predator avoidance and prey localization. Although the pattern of neuromast development has been a topic of detailed research, we still do not understand the functional consequences of its organization. Previous work has demonstrated somatotopy in the posterior lateral line, whereby afferent neurons that contact more caudal neuromasts project more dorsally in the hindbrain than those that contact more rostral neuromasts (Gompel N, Dambly-Chaudiere C, Ghysen A. Development 128: 387-393, 2001). We performed patch-clamp recordings of afferent neurons that contact neuromasts in the posterior lateral line of anesthetized, transgenic larval zebrafish (Danio rerio) to show that larger cells are born earlier, have a lower input resistance, a lower spontaneous firing rate, and tend to contact multiple neuromasts located closer to the tail than smaller neurons, which are born later, have a higher input resistance, a higher spontaneous firing rate, and tend to contact single neuromasts. We suggest that early-born neurons are poised to detect large stimuli during the initial stages of development. Later-born neurons are more easily driven to fire and thus likely to be more sensitive to local, weaker flows. Afferent projections onto identified glutamatergic regions in the hindbrain lead us to hypothesize a novel mechanism for lateral line somatotopy. We show that afferent fibers associated with tail neuromasts respond to stronger stimuli and are wired to dorsal hindbrain regions associated with Mauthner-mediated escape responses and fast, avoidance swimming. The ability to process flow stimuli by circumventing higher-order brain centers would ease the task of processing where speed is of critical importance. Our work lays the groundwork to understand how the lateral line translates flow stimuli into appropriate behaviors at the single cell level.

Delaying Gal4-driven gene expression in the zebrafish with morpholinos and Gal80

The modular Gal4/UAS gene expression system has become an indispensable tool in modern biology. Several large-scale gene- and enhancer-trap screens in the zebrafish have generated hundreds of transgenic lines expressing Gal4 in unique patterns. However, the early embryonic expression of the Gal4 severely limits their use for studies on regeneration or behavior because UAS-driven effectors could disrupt normal organogenesis. To overcome this limitation, we explored the use of the Gal4 repressor Gal80 in transient assays and with stable transgenes to temporally control Gal4 activity. We also validated a strategy to delay Gal4-driven gene expression using a morpholino targeted to Gal4. The first approach is limited to transgenes expressing the native Gal4. The morphant approach can also be applied to transgenic lines expressing the Gal4-VP16 fusion protein. It promises to become a standard approach to delay Gal4-driven transgene expression and enhance the genetic toolkit for the zebrafish.

Glial cell line-derived neurotrophic factor defines the path of developing and regenerating axons in the lateral line system of zebrafish

How the peripheral axons of sensory neurons are guided to distant target organs is not well understood. Here we examine this question in the case of the posterior lateral line (PLL) system of zebrafish, where sensory organs are deposited by a migrating primordium. Sensory neurites accompany this primordium during its migration and are thereby guided to their prospective target organs. We show that the inactivation of glial cell line-derived neurotrophic factor (GDNF) signaling leads to defects of innervation and that these defects are due to the inability of sensory axons to track the migrating primordium. GDNF signaling is also used as a guidance cue during axonal regeneration following nerve cut. We conclude that GDNF is a major determinant of directed neuritic growth and of target finding in this system, and we propose that GDNF acts by promoting local neurite outgrowth.

zTrap: zebrafish gene trap and enhancer trap database

BACKGROUND

We have developed genetic methods in zebrafish by using the Tol2 transposable element; namely, transgenesis, gene trapping, enhancer trapping and the Gal4FF-UAS system. Gene trap constructs contain a splice acceptor and the GFP or Gal4FF (a modified version of the yeast Gal4 transcription activator) gene, and enhancer trap constructs contain the zebrafish hsp70l promoter and the GFP or Gal4FF gene. By performing genetic screens using these constructs, we have generated transgenic zebrafish that express GFP and Gal4FF in specific cells, tissues and organs. Gal4FF expression is visualized by creating double transgenic fish carrying a Gal4FF transgene and the GFP reporter gene placed downstream of the Gal4-recognition sequence (UAS). Further, the Gal4FF-expressing cells can be manipulated by mating with UAS effector fish. For instance, when fish expressing Gal4FF in specific neurons are crossed with the UAS:TeTxLC fish carrying the tetanus neurotoxin gene downstream of UAS, the neuronal activities are inhibited in the double transgenic fish. Thus, these transgenic fish are useful to study developmental biology and neurobiology.

DESCRIPTION

To increase the usefulness of the transgenic fish resource, we developed a web-based database named zTrap http://kawakami.lab.nig.ac.jp/ztrap/. The zTrap database contains images of GFP and Gal4FF expression patterns, and genomic DNA sequences surrounding the integration sites of the gene trap and enhancer trap constructs. The integration sites are mapped onto the Ensembl zebrafish genome by in-house Blat analysis and can be viewed on the zTrap and Ensembl genome browsers. Furthermore, zTrap is equipped with the functionality to search these data for expression patterns and genomic loci of interest. zTrap contains the information about transgenic fish including UAS reporter and effector fish.

CONCLUSION

zTrap is a useful resource to find gene trap and enhancer trap fish lines that express GFP and Gal4FF in desired patterns, and to find insertions of the gene trap and enhancer trap constructs that are located within or near genes of interest. These transgenic fish can be utilized to observe specific cell types during embryogenesis, to manipulate their functions, and to discover novel genes and cis-regulatory elements. Therefore, zTrap should facilitate studies on genomics, developmental biology and neurobiology utilizing the transgenic zebrafish resource.