Improved Long-Term Imaging of Embryos with Genetically Encoded α-Bungarotoxin

In toto live imaging of Erk signaling dynamics in developing zebrafish hepatocytes

Regional and tissue-wide regulation of signaling pathways orchestrates cellular proliferation and differentiation during organ development. In this study, we established an imaging platform for longitudinal analysis of liver development in live developing zebrafish. We generated hepatocyte-specific transgenic lines for kinase translocation reporters of extracellular signal-regulated kinase (Erk) and c-Jun N-terminal kinase (Jnk) signaling, and with these we captured signaling dynamics that govern rapid expansion of hepatocytes toward creation of the functioning liver at single-cell resolution. Our findings reveal Erk signaling fluctuations as the liver develops and introduce methodology for investigating cell-type specific signaling dynamics during organ morphogenesis.

Cell expansion for notochord mechanics and endochondral bone lengthening in zebrafish depends on the 5'-inositol phosphatase Inppl1a

Cell size is a key contributor to tissue morphogenesis. As a notable example, growth plate hypertrophic chondrocytes use cellular biogenesis and disproportionate fluid uptake to expand 10 to 20 times in size to drive lengthening of endochondral bone. Similarly, notochord vacuolated cells expand to one of the largest cell types in the developing embryo to drive axial extension. In zebrafish, the notochord vacuolated cells undergo vacuole fusion to form a single large, fluid-filled vacuole that fills the cytoplasmic space and contributes to vacuolated cell expansion. When this process goes awry, the notochord lacks sufficient hydrostatic pressure to support vertebral bone deposition, resulting in adult spines with misshapen vertebral bones and scoliosis. However, it remains unclear whether endochondral bone and the notochord share common genetic and cellular mechanisms for regulating cell and tissue expansion. Here, we demonstrate that the 5'-inositol phosphatase gene, inppl1a, regulates notochord expansion independent of vacuole fusion, thereby genetically decoupling these processes. We demonstrate that inppl1a-dependent vacuolated cell expansion is essential to establish normal mechanical properties of the notochord and to facilitate the development of a straight spine. Finally, we find that inppl1a is also important for hypertrophic chondrocyte differentiation and endochondral bone lengthening in fish, as has been shown in the human INPPL1-related endochondral bone disorder, opsismodysplasia. Overall, this work reveals a shared mechanism of cell size regulation that influences disparate tissues critical for skeletal development and short-stature disorders.

Far-red fluorescent genetically encoded calcium ion indicators

Genetically encoded calcium ion (Ca2+) indicators (GECIs) are widely-used molecular tools for functional imaging of Ca2+ dynamics and neuronal activities with single-cell resolution. Here we report the design and development of two far-red fluorescent GECIs, FR-GECO1a and FR-GECO1c, based on the monomeric far-red fluorescent proteins mKelly1 and mKelly2. FR-GECOs have excitation and emission maxima at ~596 nm and ~644 nm, respectively, display large responses to Ca2+ in vitro (ΔF/F0 = 6 for FR-GECO1a, 18 for FR-GECO1c), are bright under both one-photon and two-photon illumination, and have high affinities (apparent Kd = 29 nM for FR-GECO1a, 83 nM for FR-GECO1c) for Ca2+. FR-GECOs offer sensitive and fast detection of single action potentials in neurons, and enable in vivo all-optical manipulation and measurement of cellular activities in combination with optogenetic actuators.

Increasingly efficient chromatin binding of cohesin and CTCF supports chromatin architecture formation during zebrafish embryogenesis

The three-dimensional folding of chromosomes is essential for nuclear functions such as DNA replication and gene regulation. The emergence of chromatin architecture is thus an important process during embryogenesis. To shed light on the molecular and kinetic underpinnings of chromatin architecture formation, we characterized biophysical properties of cohesin and CTCF binding to chromatin and their changes upon cofactor depletion using single-molecule imaging in live developing zebrafish embryos. We found that chromatin-bound fractions of both cohesin and CTCF increased significantly between the 1000-cell and shield stages, which we could explain through changes in both their association and dissociation rates. Moreover, increasing binding of cohesin restricted chromatin motion, potentially via loop extrusion, and showed distinct stage-dependent nuclear distribution. Polymer simulations with experimentally derived parameters recapitulated the experimentally observed gradual emergence of chromatin architecture. Our findings reveal molecular kinetics underlying chromatin architecture formation during zebrafish embryogenesis.

Exploring the role of mechanical forces on tendon development using in vivo model: A scoping review

Tendons transmit the muscle contraction forces to bones and drive joint movement throughout life. While extensive research have indicated the essentiality of mechanical forces on tendon development, a comprehensive understanding of the fundamental role of mechanical forces still needs to be impaerted. This scoping review aimed to summarize the current knowledge about the role of mechanical forces during the tendon developmental phase. The electronic database search using PubMed, performed in May 2023, yielded 651 articles, of which 16 met the prespecified inclusion criteria. We summarized and divided the methods to reduce the mechanical force into three groups: loss of muscle, muscle dysfunction, and weight-bearing regulation. In contrast, there were few studies to analyze the increased mechanical force model. Most studies suggested that mechanical force has some roles in tendon development in the embryo to postnatal phase. However, we identified species variability and methodological heterogeneity to modulate mechanical force. To establish a comprehensive understanding, methodological commonality to modulate the mechanical force is needed in this field. Additionally, summarizing chronological changes in developmental processes across animal species helps to understand the essence of developmental tendon mechanobiology. We expect that the findings summarized in the current review serve as a groundwork for future study in the fields of tendon developmantal biology and mechanobiology.

A bioelectrical phase transition patterns the first vertebrate heartbeats

A regular heartbeat is essential to vertebrate life. In the mature heart, this function is driven by an anatomically localized pacemaker. By contrast, pacemaking capability is broadly distributed in the early embryonic heart1-3, raising the question of how tissue-scale activity is first established and then maintained during embryonic development. The initial transition of the heart from silent to beating has never been characterized at the timescale of individual electrical events, and the structure in space and time of the early heartbeats remains poorly understood. Using all-optical electrophysiology, we captured the very first heartbeat of a zebrafish and analysed the development of cardiac excitability and conduction around this singular event. The first few beats appeared suddenly, had irregular interbeat intervals, propagated coherently across the primordial heart and emanated from loci that varied between animals and over time. The bioelectrical dynamics were well described by a noisy saddle-node on invariant circle bifurcation with action potential upstroke driven by CaV1.2. Our work shows how gradual and largely asynchronous development of single-cell bioelectrical properties produces a stereotyped and robust tissue-scale transition from quiescence to coordinated beating.

Mechanical force regulates Sox9 expression at the developing enthesis

Entheses transmit force from tendons and ligaments to the skeleton. Regional organization of enthesis extracellular matrix (ECM) generates differences in stiffness required for force transmission. Two key transcription factors co-expressed in entheseal tenocytes, scleraxis (Scx) and Sox9, directly control production of enthesis ECM components. Formation of embryonic craniofacial entheses in zebrafish coincides with onset of jaw movements, possibly in response to the force of muscle contraction. We show dynamic changes in scxa and sox9a mRNA levels in subsets of entheseal tenocytes that correlate with their roles in force transmission. We also show that transcription of a direct target of Scxa, Col1a, in enthesis ECM is regulated by the ratio of scxa to sox9a expression. Eliminating muscle contraction by paralyzing embryos during early stages of musculoskeletal differentiation alters relative levels of scxa and sox9a in entheses, primarily owing to increased sox9a expression. Force-dependent TGF-β (TGFβ) signaling is required to maintain this balance of scxa and sox9a expression. Thus, force from muscle contraction helps establish a balance of transcription factor expression that controls specialized ECM organization at the tendon enthesis and its ability to transmit force.

Visualization of Actin Cytoskeleton in Cellular Protrusions in Medaka Embryos

Cellular protrusions are fundamental structures for a wide variety of cellular behaviors, such as cell migration, cell-cell interaction, and signal reception. Visualization of cellular protrusions in living cells can be achieved by labeling of cytoskeletal actin with genetically encoded fluorescent probes. Here, we describe a detailed experimental procedure to visualize cellular protrusions in medaka embryos, which consists of the following steps: preparation of Actin-Chromobody-GFP and α-bungarotoxin mRNAs for actin labeling and immobilization of the embryo, respectively; microinjection of the mRNAs into embryos in a mosaic fashion to sparsely label individual cells; removal of the hard chorion, which hampers observation; and visualization of cellular protrusions in the embryo with a confocal microscope. Overall, our protocol provides a simple method to reveal cellular protrusions in vivo by confocal microscopy.

Measurement of ultrasound velocity in yolk and blastula of fish embryo in vivo

An acoustic microscopy method for measuring the velocity of ultrasound in the yolk and blastula of bony fish embryos at early stages of development was proposed. The yolk and blastula were approximated as a sphere and a spherical dome, respectively, consisting of a homogeneous liquid. A theoretical model of ultrasonic wave propagation through a spherical liquid drop located on a solid substrate was developed in the ray approximation. The dependence of the wave propagation time on the speed of sound in the drop, its diameter, and the position of the focus of the ultrasonic transducer has been determined. It was shown that the velocity in the drop can be found by solving the inverse problem by minimizing the discrepancy between the experimental and model spatial distributions of the propagation time, assuming that the velocity in the immersion liquid and the radius of the drop are known. The velocities in the yolk and blastula of the loach (Misgurnus fossilis) embryo at the stage of development of the middle blastula were measured in vivo using a pulsed scanning acoustic microscope operating at a central frequency of 50 MHz. The yolk and blastula radii were determined from ultrasound images of the embryo. Acoustic microscopy measurements conducted with four embryos provide velocities of the acoustic longitudinal wave in the yolk and blastula. They were measured to be 1581 ± 5 m/s and 1525 ± 4 m/s when the temperature of the liquid in the water tank was kept at 22 ± 2 °C.

Zebrafish - The Neurobehavioural Model in Trend

Zebrafish (Danio rerio) is currently in vogue as a prevalently used experimental model for studies concerning neurobehavioural disorders and associated fields. Since the 1960s, this model has succeeded in breaking most barriers faced in the hunt for an experimental model. From its appearance to its high parity with human beings genetically, this model renders itself as an advantageous experimental lab animal. Neurobehavioural disorders have always posed an arduous task in terms of their detection as well as in determining their exact etiology. They are still, in most cases, diseases of interest for inventing or discovering novel pharmacological interventions. Thus, the need for a harbinger experimental model for studying neurobehaviours is escalating. Ensuring the same model is used for studying several neuro-studies conserves the results from inter-species variations. For this, we need a model that satisfies all the pre-requisite conditions to be made the final choice of model for neurobehavioural studies. This review recapitulates the progress of zebrafish as an experimental model with its most up-to-the-minute advances in the area. Various tests, assays, and responses employed using zebrafish in screening neuroactive drugs have been tabulated effectively. The tools, techniques, protocols, and apparatuses that bolster zebrafish studies are discussed. The probable research that can be done using zebrafish has also been briefly outlined. The various breeding and maintenance methods employed, along with the information on various strains available and most commonly used, are also elaborated upon, supplementing Zebrafish's use in neuroscience.

Wnt/β-Catenin Signaling Pathway Is Strongly Implicated in Cadmium-Induced Developmental Neurotoxicity and Neuroinflammation: Clues from Zebrafish Neurobehavior and In Vivo Neuroimaging

Cadmium (Cd) is a toxic heavy metal and worldwide environmental pollutant which seriously threatens human health and ecosystems. It is easy to be adsorbed and deposited in organisms, exerting adverse effects on various organs including the brain. In a very recent study, making full use of a zebrafish model in both high-throughput behavioral tracking and live neuroimaging, we explored the potential developmental neurotoxicity of Cd²⁺ at environmentally relevant levels and identified multiple connections between Cd²⁺ exposure and neurodevelopmental disorders as well as microglia-mediated neuroinflammation, whereas the underlying neurotoxic mechanisms remained unclear. The canonical Wnt/β-catenin signaling pathway plays crucial roles in many biological processes including neurodevelopment, cell survival, and cell cycle regulation, as well as microglial activation, thereby potentially presenting one of the key targets of Cd²⁺ neurotoxicity. Therefore, in this follow-up study, we investigated the implication of the Wnt/β-catenin signaling pathway in Cd²⁺-induced developmental disorders and neuroinflammation and revealed that environmental Cd²⁺ exposure significantly affected the expression of key factors in the zebrafish Wnt/β-catenin signaling pathway. In addition, pharmacological intervention of this pathway via TWS119, which can increase the protein level of β-catenin and act as a classical activator of the Wnt signaling pathway, could significantly repress the Cd²⁺-induced cell cycle arrest and apoptosis, thereby attenuating the inhibitory effects of Cd²⁺ on the early development, behavior, and activity, as well as neurodevelopment of zebrafish larvae to a certain degree. Furthermore, activation and proliferation of microglia, as well as the altered expression profiles of genes associated with neuroimmune homeostasis triggered by Cd²⁺ exposure could also be significantly alleviated by the activation of the Wnt/β-catenin signaling pathway. Thus, this study provided novel insights into the cellular and molecular mechanisms of Cd²⁺ toxicity on the vertebrate central nervous system (CNS), which might be helpful in developing pharmacotherapies to mitigate the neurological disorders resulting from exposure to Cd²⁺ and many other environmental heavy metals.

Defining the ultrastructure of the hematopoietic stem cell niche by correlative light and electron microscopy

The blood system is supported by hematopoietic stem and progenitor cells (HSPCs) found in a specialized microenvironment called the niche. Many different niche cell types support HSPCs, however how they interact and their ultrastructure has been difficult to define. Here, we show that single endogenous HSPCs can be tracked by light microscopy, then identified by serial block-face scanning electron microscopy (SBEM) at multiscale levels. Using the zebrafish larval kidney marrow (KM) niche as a model, we followed single fluorescently labeled HSPCs by light sheet microscopy, then confirmed their exact location in a 3D SBEM dataset. We found a variety of different configurations of HSPCs and surrounding niche cells, suggesting there could be functional heterogeneity in sites of HSPC lodgement. Our approach also allowed us to identify dopamine beta-hydroxylase (dbh) positive ganglion cells as a previously uncharacterized functional cell type in the HSPC niche. By integrating multiple imaging modalities, we could resolve the ultrastructure of single rare cells deep in live tissue and define all contacts between an HSPC and its surrounding niche cell types.

Calcium sparks enhance the tissue fluidity within epithelial layers and promote apical extrusion of transformed cells

In vertebrates, newly emerging transformed cells are often apically extruded from epithelial layers through cell competition with surrounding normal epithelial cells. However, the underlying molecular mechanism remains elusive. Here, using phospho-SILAC screening, we show that phosphorylation of AHNAK2 is elevated in normal cells neighboring RasV12 cells soon after the induction of RasV12 expression, which is mediated by calcium-dependent protein kinase C. In addition, transient upsurges of intracellular calcium, which we call calcium sparks, frequently occur in normal cells neighboring RasV12 cells, which are mediated by mechanosensitive calcium channel TRPC1 upon membrane stretching. Calcium sparks then enhance cell movements of both normal and RasV12 cells through phosphorylation of AHNAK2 and promote apical extrusion. Moreover, comparable calcium sparks positively regulate apical extrusion of RasV12-transformed cells in zebrafish larvae as well. Hence, calcium sparks play a crucial role in the elimination of transformed cells at the early phase of cell competition.

Wnt11 acts on dermomyotome cells to guide epaxial myotome morphogenesis

The dorsal axial muscles, or epaxial muscles, are a fundamental structure covering the spinal cord and vertebrae, as well as mobilizing the vertebrate trunk. To date, mechanisms underlying the morphogenetic process shaping the epaxial myotome are largely unknown. To address this, we used the medaka zic1/zic4-enhancer mutant Double anal fin (Da), which exhibits ventralized dorsal trunk structures resulting in impaired epaxial myotome morphology and incomplete coverage over the neural tube. In wild type, dorsal dermomyotome (DM) cells reduce their proliferative activity after somitogenesis. Subsequently, a subset of DM cells, which does not differentiate into the myotome population, begins to form unique large protrusions extending dorsally to guide the epaxial myotome dorsally. In Da, by contrast, DM cells maintain the high proliferative activity and mainly form small protrusions. By combining RNA- and ChIP-sequencing analyses, we revealed direct targets of Zic1, which are specifically expressed in dorsal somites and involved in various aspects of development, such as cell migration, extracellular matrix organization, and cell-cell communication. Among these, we identified wnt11 as a crucial factor regulating both cell proliferation and protrusive activity of DM cells. We propose that dorsal extension of the epaxial myotome is guided by a non-myogenic subpopulation of DM cells and that wnt11 empowers the DM cells to drive the coverage of the neural tube by the epaxial myotome.

Hand2 delineates mesothelium progenitors and is reactivated in mesothelioma

The mesothelium lines body cavities and surrounds internal organs, widely contributing to homeostasis and regeneration. Mesothelium disruptions cause visceral anomalies and mesothelioma tumors. Nonetheless, the embryonic emergence of mesothelia remains incompletely understood. Here, we track mesothelial origins in the lateral plate mesoderm (LPM) using zebrafish. Single-cell transcriptomics uncovers a post-gastrulation gene expression signature centered on hand2 in distinct LPM progenitor cells. We map mesothelial progenitors to lateral-most, hand2-expressing LPM and confirm conservation in mouse. Time-lapse imaging of zebrafish hand2 reporter embryos captures mesothelium formation including pericardium, visceral, and parietal peritoneum. We find primordial germ cells migrate with the forming mesothelium as ventral migration boundary. Functionally, hand2 loss disrupts mesothelium formation with reduced progenitor cells and perturbed migration. In mouse and human mesothelioma, we document expression of LPM-associated transcription factors including Hand2, suggesting re-initiation of a developmental program. Our data connects mesothelium development to Hand2, expanding our understanding of mesothelial pathologies.

Zebrafish imaging reveals TP53 mutation switching oncogene-induced senescence from suppressor to driver in primary tumorigenesis

Most tumours are thought to arise through oncogenic cell generation followed by additional mutations. How a new oncogenic cell primes tumorigenesis by acquiring additional mutations remains unclear. We show that an additional TP53 mutation stimulates primary tumorigenesis by switching oncogene-induced senescence from a tumour suppressor to a driver. Zebrafish imaging reveals that a newly emerged oncogenic cell with the Ras^G12V mutation becomes senescent and is eliminated from the epithelia, which is prevented by adding a TP53 gain-of-function mutation (TP53^R175H) into Ras^G12V cells. Surviving Ras^G12V-TP53^R175H double-mutant cells senesce and secrete senescence-associated secretory phenotype (SASP)-related inflammatory molecules that convert neighbouring normal cells into SASP factor-secreting senescent cells, generating a heterogeneous tumour-like cell mass. We identify oncogenic cell behaviours that may control the initial human tumorigenesis step. Ras and TP53 mutations and cellular senescence are frequently detected in human tumours; similar switching may occur during the initial step of human tumorigenesis.

It is unclear how a single oncogenic cell primes tumorigenesis. Here the authors visualised this behaviour using a zebrafish larval skin as a model and show that RasG12V oncogenic cell is eliminated through oncogene-senescence while a gain of function mutation in p53 alters this behaviour from tumour suppressive to tumour promoting.

Svep1 stabilises developmental vascular anastomosis in reduced flow conditions

Molecular mechanisms controlling the formation, stabilisation and maintenance of blood vessel connections remain poorly defined. Here, we identify blood flow and the large extracellular protein Svep1 as co-modulators of vessel anastomosis during developmental angiogenesis in zebrafish embryos. Both loss of Svep1 and blood flow reduction contribute to defective anastomosis of intersegmental vessels. The reduced formation and lumenisation of the dorsal longitudinal anastomotic vessel (DLAV) is associated with a compensatory increase in Vegfa/Vegfr pERK signalling, concomittant expansion of apelin-positive tip cells, but reduced expression of klf2a. Experimentally, further increasing Vegfa/Vegfr signalling can rescue the DLAV formation and lumenisation defects, whereas its inhibition dramatically exacerbates the loss of connectivity. Mechanistically, our results suggest that flow and Svep1 co-regulate the stabilisation of vascular connections, in part by modulating the Vegfa/Vegfr signalling pathway.

Summary: Blood flow and the large extracellular matrix protein Svep1 jointly regulate vessel anastomosis during developmental angiogenesis in zebrafish embryos partly by modulating the Vegfa/Vegfr signalling pathway.

Developmental exposure to environmental levels of cadmium induces neurotoxicity and activates microglia in zebrafish larvae: From the perspectives of neurobehavior and neuroimaging

Cadmium (Cd) is a worldwide environmental pollutant that postures serious threats to humans and ecosystems. Over the years, its adverse effects on the central nervous system (CNS) have been concerned, whereas the underlying cellular/molecular mechanisms remain unclear. In this study, taking advantages of zebrafish model in high-throughput imaging and behavioral tests, we have explored the potential developmental neurotoxicity of Cd at environmentally relevant levels, from the perspectives of neurobehavior and neuroimaging. Briefly, Cd²⁺ exposure resulted in a general impairment of zebrafish early development. Zebrafish neurobehavioral patterns including locomotion and reactivity to environmental signals were significantly perturbed upon Cd²⁺ exposure. Importantly, a combination of in vivo two-photon neuroimaging, flow cytometry and gene expression analyses revealed notable neurodevelopmental disorders as well as neuroimmune responses induced by Cd²⁺ exposure. Both cell-cycle arrest and apoptosis contributed jointly to a significant decrease of neuronal density in zebrafish larvae exposed to Cd²⁺. The dramatic morphological alterations of microglia from multi-branched to amoeboid, the microgliosis, as well as the modulation of gene expression profiles demonstrated a strong activation of microglia and neuroinflammation triggered by environmental levels of Cd²⁺. Together, our study points to the developmental toxicity of Cd in inducing CNS impairment and neuroinflammation thereby providing visualized etiological evidence of this heavy metal induced neurodevelopmental disorders. It's tempting to speculate that this research model might represent a promising tool not only for understanding the molecular mechanisms of Cd-induced neurotoxicity, but also for developing pharmacotherapies to mitigate the neurological damage resulting from exposure to Cd, and other neurotoxicants.

Computational simulations of the 4D micro-circulatory network in zebrafish tail amputation and regeneration

Wall shear stress (WSS) contributes to the mechanotransduction underlying microvascular development and regeneration. Using computational fluid dynamics, we elucidated the interplay between WSS and vascular remodelling in a zebrafish model of tail amputation and regeneration. The transgenic Tg (fli1:eGFP; Gata1:ds-red) zebrafish line was used to track the three-dimensional fluorescently labelled vascular endothelium for post-image segmentation and reconstruction of the fluid domain. Particle image velocimetry was used to validate the blood flow. Following amputation to the dorsal aorta and posterior cardinal vein (PCV), vasoconstriction developed in the dorsal longitudinal anastomotic vessel (DLAV) along with increased WSS in the proximal segmental vessels (SVs) from amputation. Angiogenesis ensued at the tips of the amputated DLAV and PCV where WSS was minimal. At 2 days post amputation (dpa), vasodilation occurred in a pair of SVs proximal to amputation, followed by increased blood flow and WSS; however, in the SVs distal to amputation, WSS normalized to the baseline. At 3 dpa, the blood flow increased in the arterial SV proximal to amputation and through anastomosis with DLAV formed a loop with PCV. Thus, our in silico modelling revealed the interplay between WSS and microvascular adaptation to changes in WSS and blood flow to restore microcirculation following tail amputation.

Computational simulations of the 4D micro-circulatory network in zebrafish tail amputation and regeneration

Wall shear stress (WSS) contributes to the mechanotransduction underlying microvascular development and regeneration. Using computational fluid dynamics, we elucidated the interplay between WSS and vascular remodelling in a zebrafish model of tail amputation and regeneration. The transgenic Tg (fli1:eGFP; Gata1:ds-red) zebrafish line was used to track the three-dimensional fluorescently labelled vascular endothelium for post-image segmentation and reconstruction of the fluid domain. Particle image velocimetry was used to validate the blood flow. Following amputation to the dorsal aorta and posterior cardinal vein (PCV), vasoconstriction developed in the dorsal longitudinal anastomotic vessel (DLAV) along with increased WSS in the proximal segmental vessels (SVs) from amputation. Angiogenesis ensued at the tips of the amputated DLAV and PCV where WSS was minimal. At 2 days post amputation (dpa), vasodilation occurred in a pair of SVs proximal to amputation, followed by increased blood flow and WSS; however, in the SVs distal to amputation, WSS normalized to the baseline. At 3 dpa, the blood flow increased in the arterial SV proximal to amputation and through anastomosis with DLAV formed a loop with PCV. Thus, our in silico modelling revealed the interplay between WSS and microvascular adaptation to changes in WSS and blood flow to restore microcirculation following tail amputation.

Chemical Synthesis of a Functional Fluorescent-Tagged α-Bungarotoxin

α-bungarotoxin is a large, 74 amino acid toxin containing five disulphide bridges, initially identified in the venom of Bungarus multicinctus snake. Like most large toxins, chemical synthesis of α-bungarotoxin is challenging, explaining why all previous reports use purified or recombinant α-bungarotoxin. However, only chemical synthesis allows easy insertion of non-natural amino acids or new chemical functionalities. Herein, we describe a procedure for the chemical synthesis of a fluorescent-tagged α-bungarotoxin. The full-length peptide was designed to include an alkyne function at the amino-terminus through the addition of a pentynoic acid linker. Chemical synthesis of α-bungarotoxin requires hydrazide-based coupling of three peptide fragments in successive steps. After completion of the oxidative folding, an azide-modified Cy5 fluorophore was coupled by click chemistry onto the toxin. Next, we determined the efficacy of the fluorescent-tagged α-bungarotoxin to block acetylcholine (ACh)-mediated currents in response to muscle nicotinic receptor activation in TE671 cells. Using automated patch-clamp recordings, we demonstrate that fluorescent synthetic α-bungarotoxin has the expected nanomolar affinity for the nicotinic receptor. The blocking effect of fluorescent α-bungarotoxin could be displaced by incubation with a 20-mer peptide mimicking the α-bungarotoxin binding site. In addition, TE671 cells could be labelled with fluorescent toxin, as witnessed by confocal microscopy, and this labelling was partially displaced by the 20-mer competitive peptide. We thus demonstrate that synthetic fluorescent-tagged α-bungarotoxin preserves excellent properties for binding onto muscle nicotinic receptors.

Extracellular hyaluronate pressure shaped by cellular tethers drives tissue morphogenesis

How tissues acquire complex shapes is a fundamental question in biology and regenerative medicine. Zebrafish semicircular canals form from invaginations in the otic epithelium (buds) that extend and fuse to form the hubs of each canal. We find that conventional actomyosin-driven behaviors are not required. Instead, local secretion of hyaluronan, made by the enzymes uridine 5'-diphosphate dehydrogenase (ugdh) and hyaluronan synthase 3 (has3), drives canal morphogenesis. Charged hyaluronate polymers osmotically swell with water and generate isotropic extracellular pressure to deform the overlying epithelium into buds. The mechanical anisotropy needed to shape buds into tubes is conferred by a polarized distribution of actomyosin and E-cadherin-rich membrane tethers, which we term cytocinches. Most work on tissue morphogenesis ascribes actomyosin contractility as the driving force, while the extracellular matrix shapes tissues through differential stiffness. Our work inverts this expectation. Hyaluronate pressure shaped by anisotropic tissue stiffness may be a widespread mechanism for powering morphological change in organogenesis and tissue engineering.

Insights Into Central Nervous System Glial Cell Formation and Function From Zebrafish

The term glia describes a heterogenous collection of distinct cell types that make up a large proportion of our nervous system. Although once considered the glue of the nervous system, the study of glial cells has evolved significantly in recent years, with a large body of literature now highlighting their complex and diverse roles in development and throughout life. This progress is due, in part, to advances in animal models in which the molecular and cellular mechanisms of glial cell development and function as well as neuron-glial cell interactions can be directly studied in vivo in real time, in intact neural circuits. In this review we highlight the instrumental role that zebrafish have played as a vertebrate model system for the study of glial cells, and discuss how the experimental advantages of the zebrafish lend themselves to investigate glial cell interactions and diversity. We focus in particular on recent studies that have provided insight into the formation and function of the major glial cell types in the central nervous system in zebrafish.

Malformations and mortality in zebrafish early stages associated with elevated caspase activity after 24 h exposure to MS-222

Tricaine methanesulfonate (MS-222) is a commonly used anaesthetic agent for immobilization of aquatic species. However, delayed development and malformations have been observed in 24 hpf (hours post-fertilization) zebrafish embryos after long-term immobilization. Still, no comprehensive study has been described regarding zebrafish exposure to MS-222 during the first hours of development, which are one of the most sensitive life stages to toxicants. Therefore, this research aimed to assess the toxicity of a 24 h exposure to MS-222 on zebrafish embryonic development. Based on the MS-222 LC₅₀, early blastula stage embryos (~2 hpf) were exposed to 0, 12.5, 25 and 50 mg L^-1 for 24 h and then allowed to develop up to 144 hpf. The chromatographic analysis showed that this anaesthetic agent bioaccumulates in 26 hpf zebrafish larvae in a concentration-dependent manner. In addition, increased mortalities and skeletal abnormalities were observed at 144 hpf, namely in the highest tested concentration. Yet, no craniofacial anomalies were observed either by alcian blue or calcein staining methods. Independently of the tested concentration, decreased speed and distance travelled were perceived in 144 hpf larvae. At the biochemical level, decreased in vivo reactive oxygen species (ROS) generation and apoptosis was observed. Additionally, catalase activity was increased at 26 hpf while results of mRNA expression showed a decreased gclc transcript content at the same time-point. Overall, data obtained highlight the toxicological risk of MS-222 and support ROS-mediated cell death signalling changes through the elevation of catalase activity as an adaptative or protective response.

Osmolarity-independent electrical cues guide rapid response to injury in zebrafish epidermis

The ability of epithelial tissues to heal after injury is essential for animal life, yet the mechanisms by which epithelial cells sense tissue damage are incompletely understood. In aquatic organisms such as zebrafish, osmotic shock following injury is believed to be an early and potent activator of a wound response. We find that, in addition to sensing osmolarity, basal skin cells in zebrafish larvae are also sensitive to changes in the particular ionic composition of their surroundings after wounding, specifically the concentration of sodium chloride in the immediate vicinity of the wound. This sodium chloride-specific wound detection mechanism is independent of cell swelling, and instead is suggestive of a mechanism by which cells sense changes in the transepithelial electrical potential generated by the transport of sodium and chloride ions across the skin. Consistent with this hypothesis, we show that electric fields directly applied within the skin are sufficient to initiate actin polarization and migration of basal cells in their native epithelial context in vivo, even overriding endogenous wound signaling. This suggests that, in order to mount a robust wound response, skin cells respond to both osmotic and electrical perturbations arising from tissue injury.

Development and regeneration dynamics of the Medaka notochord

The notochord is an embryonic tissue that acts as a hydrostatic skeleton until ossification begins in vertebrates. It is composed of outer sheath cells and inner vacuolated cells, which are generated from a common pool of disc-shaped precursors. Notochord extension during early embryogenesis is driven by the growth of vacuolated cells, reflecting in turn the expansion of their inner vacuole. Here we use desmogon, a novel desmosomal cadherin, to follow notochord development and regeneration in medaka (Oryzias latipes). We trace desmogon ​+ disc-shaped precursors at the single cell level to demonstrate that they operate as unipotent progenitors, giving rise to either sheath or vacuolated cells. We reveal that once specified, vacuolated cells grow asynchronously and drive notochord expansion bi-directionally. Additionally, we uncover distinct regenerative responses in the notochord, which depend on the nature of the injury sustained. By generating a desmogon CRISPR mutant we demonstrate that this cadherin is essential for proper vacuolated cell shape and therefore correct notochord and spine morphology. Our work expands the repertoire of model systems to study dynamic aspects of the notochord in vivo, and provides new insights in its development and regeneration properties.

A Layered Mounting Method for Extended Time-Lapse Confocal Microscopy of Whole Zebrafish Embryos

Dynamics of development can be followed by confocal time-lapse microscopy of live transgenic zebrafish embryos expressing fluorescence in specific tissues or cells. A difficulty with imaging whole embryo development is that zebrafish embryos grow substantially in length. When mounted as regularly done in 0.3-1% low melt agarose, the agarose imposes growth restriction, leading to distortions in the soft embryo body. Yet, to perform confocal time-lapse microscopy, the embryo must be immobilized. This article describes a layered mounting method for zebrafish embryos that restrict the motility of the embryos while allowing for the unrestricted growth. The mounting is performed in layers of agarose at different concentrations. To demonstrate the usability of this method, whole embryo vascular, neuronal and muscle development was imaged in transgenic fish for 55 consecutive hours. This mounting method can be used for easy, low-cost imaging of whole zebrafish embryos using inverted microscopes without requirements of molds or special equipment.

Standardized mounting method of (zebrafish) embryos using a 3D-printed stamp for high-content, semi-automated confocal imaging

Background

Developmental biology relies to a large extent on the observation and comparison of phenotypic traits through time using high resolution microscopes. In this context, transparent model organisms such as the zebrafish Danio rerio in which developing tissues and organs can be easily observed and imaged using fluorescent proteins have become very popular. One limiting factor however is the acquisition of a sufficient amount of data, in standardized and reproducible conditions, to allow robust quantitative analysis. One way to improve this is by developing mounting methods to increase the number of embryos that can be imaged simultaneously in near-to-identical orientation.

Results

Here we present an improved mounting method allowing semi-automated and high-content imaging of zebrafish embryos. It is based on a 3D-printed stamp which is used to create a 2D coordinate system of multiple μ-wells in an agarose cast. Each μ-well models a negative of the average zebrafish embryo morphology between 22 and 96 h-post-fertilization. Due to this standardized and reproducible arrangement, it is possible to define a custom well plate in the respective imaging software that allows for a semi-automated imaging process. Furthermore, the improvement in Z-orientation significantly reduces post-processing and improves comparability of volumetric data while reducing light exposure and thus photo-bleaching and photo-toxicity, and improving signal-to-noise ratio (SNR).

Conclusions

We present here a new method that allows to standardize and improve mounting and imaging of embryos. The 3D-printed stamp creates a 2D coordinate system of μ-wells in an agarose cast thus standardizing specimen mounting and allowing high-content imaging of up to 44 live or mounted zebrafish embryos simultaneously in a semi-automated, well-plate like manner on inverted confocal microscopes. In summary, image data quality and acquisition efficiency (amount of data per time) are significantly improved. The latter might also be crucial when using the services of a microscopy facility.

Size control of the inner ear via hydraulic feedback

Animals make organs of precise size, shape, and symmetry but how developing embryos do this is largely unknown. Here, we combine quantitative imaging, physical theory, and physiological measurement of hydrostatic pressure and fluid transport in zebrafish to study size control of the developing inner ear. We find that fluid accumulation creates hydrostatic pressure in the lumen leading to stress in the epithelium and expansion of the otic vesicle. Pressure, in turn, inhibits fluid transport into the lumen. This negative feedback loop between pressure and transport allows the otic vesicle to change growth rate to control natural or experimentally-induced size variation. Spatiotemporal patterning of contractility modulates pressure-driven strain for regional tissue thinning. Our work connects molecular-driven mechanisms, such as osmotic pressure driven strain and actomyosin tension, to the regulation of tissue morphogenesis via hydraulic feedback to ensure robust control of organ size.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Multi-sample SPIM image acquisition, processing and analysis of vascular growth in zebrafish

To quantitatively understand biological processes that occur over many hours or days, it is desirable to image multiple samples simultaneously, and automatically process and analyse the resulting datasets. Here, we present a complete multi-sample preparation, imaging, processing and analysis workflow to determine the development of the vascular volume in zebrafish. Up to five live embryos were mounted and imaged simultaneously over several days using selective plane illumination microscopy (SPIM). The resulting large imagery dataset of several terabytes was processed in an automated manner on a high-performance computer cluster and segmented using a novel segmentation approach that uses images of red blood cells as training data. This analysis yielded a precise quantification of growth characteristics of the whole vascular network, head vasculature and tail vasculature over development. Our multi-sample platform demonstrates effective upgrades to conventional single-sample imaging platforms and paves the way for diverse quantitative long-term imaging studies.

Loss of the Polycomb group protein Rnf2 results in derepression of tbx-transcription factors and defects in embryonic and cardiac development

The Polycomb group (PcG) protein family is a well-known group of epigenetic modifiers. We used zebrafish to investigate the role of Rnf2, the enzymatic subunit of PRC1. We found a positive correlation between loss of Rnf2 and upregulation of genes, especially of those whose promoter is normally bound by Rnf2. The heart of rnf2 mutants shows a tubular shaped morphology and to further understand the underlying mechanism, we studied gene expression of single wildtype and rnf2 mutant hearts. We detected the most pronounced differences at 3 dpf, including upregulation of heart transcription factors, such as tbx2a, tbx2b, and tbx3a. These tbx genes were decorated by broad PcG domains in wildtype whole embryo lysates. Chamber specific genes such as vmhc, myh6, and nppa showed downregulation in rnf2 mutant hearts. The marker of the working myocard, nppa, is negatively regulated by Tbx2 and Tbx3. Based on our findings and literature we postulate that loss of Rnf2-mediated repression results in upregulation and ectopic expression of tbx2/3, whose expression is normally restricted to the cardiac conductive system. This could lead to repression of chamber specific gene expression, a misbalance in cardiac cell types, and thereby to cardiac defects observed in rnf2 mutants.

Enhanced in vivo-imaging in medaka by optimized anaesthesia, fluorescent protein selection and removal of pigmentation

Fish are ideally suited for in vivo-imaging due to their transparency at early stages combined with a large genetic toolbox. Key challenges to further advance imaging are fluorophore selection, immobilization of the specimen and approaches to eliminate pigmentation. We addressed all three and identified the fluorophores and anaesthesia of choice by high throughput time-lapse imaging. Our results indicate that eGFP and mCherry are the best conservative choices for in vivo-fluorescence experiments, when availability of well-established antibodies and nanobodies matters. Still, mVenusNB and mGFPmut2 delivered highest absolute fluorescence intensities in vivo. Immobilization is of key importance during extended in vivo imaging. Here, traditional approaches are outperformed by mRNA injection of α-Bungarotoxin which allows a complete and reversible, transient immobilization. In combination with fully transparent juvenile and adult fish established by the targeted inactivation of both, oca2 and pnp4a via CRISPR/Cas9-mediated gene editing in medaka we could dramatically improve the state-of-the art imaging conditions in post-embryonic fish, now enabling light-sheet microscopy of the growing retina, brain, gills and inner organs in the absence of side effects caused by anaesthetic drugs or pigmentation.

Mechanical force regulates tendon extracellular matrix organization and tenocyte morphogenesis through TGFbeta signaling

Mechanical forces between cells and extracellular matrix (ECM) influence cell shape and function. Tendons are ECM-rich tissues connecting muscles with bones that bear extreme tensional force. Analysis of transgenic zebrafish expressing mCherry driven by the tendon determinant scleraxis reveals that tendon fibroblasts (tenocytes) extend arrays of microtubule-rich projections at the onset of muscle contraction. In the trunk, these form a dense curtain along the myotendinous junctions at somite boundaries, perpendicular to myofibers, suggesting a role as force sensors to control ECM production and tendon strength. Paralysis or destabilization of microtubules reduces projection length and surrounding ECM, both of which are rescued by muscle stimulation. Paralysis also reduces SMAD3 phosphorylation in tenocytes and chemical inhibition of TGFβ signaling shortens tenocyte projections. These results suggest that TGFβ, released in response to force, acts on tenocytes to alter their morphology and ECM production, revealing a feedback mechanism by which tendons adapt to tension.

Lamellar projections in the endolymphatic sac act as a relief valve to regulate inner ear pressure

The inner ear is a fluid-filled closed-epithelial structure whose function requires maintenance of an internal hydrostatic pressure and fluid composition. The endolymphatic sac (ES) is a dead-end epithelial tube connected to the inner ear whose function is unclear. ES defects can cause distended ear tissue, a pathology often seen in hearing and balance disorders. Using live imaging of zebrafish larvae, we reveal that the ES undergoes cycles of slow pressure-driven inflation followed by rapid deflation. Absence of these cycles in lmx1bb mutants leads to distended ear tissue. Using serial-section electron microscopy and adaptive optics lattice light-sheet microscopy, we find a pressure relief valve in the ES comprised of partially separated apical junctions and dynamic overlapping basal lamellae that separate under pressure to release fluid. We propose that this lmx1-dependent pressure relief valve is required to maintain fluid homeostasis in the inner ear and other fluid-filled cavities.

Feedback between tissue packing and neurogenesis in the zebrafish neural tube

Balancing the rate of differentiation and proliferation in developing tissues is essential to produce organs of robust size and composition. Although many molecular regulators have been established, how these connect to physical and geometrical aspects of tissue architecture is poorly understood. Here, using high-resolution timelapse imaging, we find that changes to cell geometry associated with dense tissue packing play a significant role in regulating differentiation rate in the zebrafish neural tube. Specifically, progenitors that are displaced away from the apical surface due to crowding, tend to differentiate in a Notch-dependent manner. Using simulations we show that interplay between progenitor density, cell shape and changes in differentiation rate could naturally result in negative-feedback control on progenitor cell number. Given these results, we suggest a model whereby differentiation rate is regulated by density dependent effects on cell geometry to: (1) correct variability in cell number; and (2) balance the rates of proliferation and differentiation over development to 'fill' the available space.

Observing the cell in its native state: Imaging subcellular dynamics in multicellular organisms

True physiological imaging of subcellular dynamics requires studying cells within their parent organisms, where all the environmental cues that drive gene expression, and hence the phenotypes that we actually observe, are present. A complete understanding also requires volumetric imaging of the cell and its surroundings at high spatiotemporal resolution, without inducing undue stress on either. We combined lattice light-sheet microscopy with adaptive optics to achieve, across large multicellular volumes, noninvasive aberration-free imaging of subcellular processes, including endocytosis, organelle remodeling during mitosis, and the migration of axons, immune cells, and metastatic cancer cells in vivo. The technology reveals the phenotypic diversity within cells across different organisms and developmental stages and may offer insights into how cells harness their intrinsic variability to adapt to different physiological environments.

Following Endocardial Tissue Movements via Cell Photoconversion in the Zebrafish Embryo

During embryogenesis, cells undergo dynamic changes in cell behavior, and deciphering the cellular logic behind these changes is a fundamental goal in the field of developmental biology. The discovery and development of photoconvertible proteins have greatly aided our understanding of these dynamic changes by providing a method to optically highlight cells and tissues. However, while photoconversion, time-lapse microscopy, and subsequent image analysis have proven to be very successful in uncovering cellular dynamics in organs such as the brain or the eye, this approach is generally not used in the developing heart due to challenges posed by the rapid movement of the heart during the cardiac cycle. This protocol consists of two parts. The first part describes a method for photoconverting and subsequently tracking endocardial cells (EdCs) during zebrafish atrioventricular canal (AVC) and atrioventricular heart valve development. The method involves temporally stopping the heart with a drug in order for accurate photoconversion to take place. Hearts are allowed to resume beating upon removal of the drug and embryonic development continues normally until the heart is stopped again for high-resolution imaging of photoconverted EdCs at a later developmental time point. The second part of the protocol describes an image analysis method to quantify the length of a photoconverted or non-photoconverted region in the AVC in young embryos by mapping the fluorescent signal from the three-dimensional structure onto a two-dimensional map. Together, the two parts of the protocol allows one to examine the origin and behavior of cells that make up the zebrafish AVC and atrioventricular heart valve, and can potentially be applied for studying mutants, morphants, or embryos that have been treated with reagents that disrupt AVC and/or valve development.

Sheath Cell Invasion and Trans-differentiation Repair Mechanical Damage Caused by Loss of Caveolae in the Zebrafish Notochord

The notochord, a conserved axial structure required for embryonic axis elongation and spine development, consists of giant vacuolated cells surrounded by an epithelial sheath [1-3]. During morphogenesis, vacuolated cells maintain their structural integrity despite being under constant mechanical stress [4]. We hypothesized that the high density of caveolae present in vacuolated cells [5, 6] could buffer mechanical tension. Caveolae are 50- to 80-nm membrane invaginations lined by cage-like polygonal structures [7, 8] formed by caveolin 1 (Cav1) or Cav3 and one of the cavin proteins [6, 9-11]. Recent in vitro work has shown that plasma membrane caveolae constitute a membrane reservoir that can buffer mechanical stresses such as stretching or osmotic swelling [12]. Moreover, mechanical integrity of vascular and muscle cells is partly dependent on caveolae [13-15]. However, the in vivo mechano-protective roles of caveolae have only begun to be explored. Using zebrafish mutants for cav1, cav3, and cavin1b, we show that caveolae are essential for notochord integrity. Upon loss of caveola function, vacuolated cells collapse at discrete positions under the mechanical strain of locomotion. Then, sheath cells invade the inner notochord and differentiate into vacuolated cells, thereby restoring notochord function and allowing normal spine development. Our data further indicate that nucleotides released by dying vacuolated cells promote sheath cell vacuolization and trans-differentiation. This work reveals a novel structural role for caveolae in vertebrates and provides unique insights into the mechanisms that safeguard notochord and spine development.

Recovery of shape and size in a developing organ pair

BACKGROUND

Paired organs in animals are largely bilaterally symmetric despite inherent noise in most biological processes. How is precise organ shape and size achieved during development despite this noise? Examining paired organ development is a challenge because it requires repeated quantification of two structures in parallel within living embryos. Here we combine bilateral quantification of morphology through time with asymmetric perturbations to study regulation of organ shape, size, and symmetry in developing organ pairs.

RESULTS

We present quantitative live imaging tools to measure the shape and size of the developing inner ears on both the left and right side simultaneously over time. By quantifying variation between the left and right inner ear (intrinsic noise) and between different individuals (extrinsic noise), we find that initial variability decreases over time in normal development to achieve symmetry. Early asymmetry is increased by environmental stress, but symmetry is still recovered over subsequent developmental time. Using multiple unilateral perturbations including Fgf signaling and ultraviolet light, we find that growth can be adjusted to compensate for a range of initial size and shape differences.

CONCLUSIONS

We propose that symmetry in developmental systems does not emerge through precise deterministic bilateral development, but rather through feedback mechanisms that adjust morphogenesis rates to account for variation. Developmental Dynamics 246:451-465, 2016. © 2017 Wiley Periodicals, Inc.

Imaging Myelination In Vivo Using Transparent Animal Models

Myelination by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system is essential for nervous system function and health. Despite its importance, we have a relatively poor understanding of the molecular and cellular mechanisms that regulate myelination in the living animal, particularly in the CNS. This is partly due to the fact that myelination commences around birth in mammals, by which time the CNS is complex and largely inaccessible, and thus very difficult to image live in its intact form. As a consequence, in recent years much effort has been invested in the use of smaller, simpler, transparent model organisms to investigate mechanisms of myelination in vivo. Although the majority of such studies have employed zebrafish, the Xenopus tadpole also represents an important complementary system with advantages for investigating myelin biology in vivo. Here we review how the natural features of zebrafish embryos and larvae and Xenopus tadpoles make them ideal systems for experimentally interrogating myelination by live imaging. We outline common transgenic technologies used to generate zebrafish and Xenopus that express fluorescent reporters, which can be used to image myelination. We also provide an extensive overview of the imaging modalities most commonly employed to date to image the nervous system in these transparent systems, and also emerging technologies that we anticipate will become widely used in studies of zebrafish and Xenopus myelination in the near future.

Membrane dynamics of dividing cells imaged by lattice light-sheet microscopy

Lattice light-sheet microscopy is used to examine two problems in membrane dynamics—molecular events in clathrin-coated pit formation and changes in cell shape during cell division. This methodology sets a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

Membrane remodeling is an essential part of transferring components to and from the cell surface and membrane-bound organelles and for changes in cell shape, which are particularly critical during cell division. Earlier analyses, based on classical optical live-cell imaging and mostly restricted by technical necessity to the attached bottom surface, showed persistent formation of endocytic clathrin pits and vesicles during mitosis. Taking advantage of the resolution, speed, and noninvasive illumination of the newly developed lattice light-sheet fluorescence microscope, we reexamined their assembly dynamics over the entire cell surface and found that clathrin pits form at a lower rate during late mitosis. Full-cell imaging measurements of cell surface area and volume throughout the cell cycle of single cells in culture and in zebrafish embryos showed that the total surface increased rapidly during the transition from telophase to cytokinesis, whereas cell volume increased slightly in metaphase and was relatively constant during cytokinesis. These applications demonstrate the advantage of lattice light-sheet microscopy and enable a new standard for imaging membrane dynamics in single cells and multicellular assemblies.

A Dynamic Anesthesia System for Long-Term Imaging in Adult Zebrafish

Long-term in vivo imaging in adult zebrafish (i.e., 1-24 h) has been limited by the fact that regimens for long-term anesthesia in embryos and larvae are ineffective in adults. Here, we examined the potential for dynamic administration of benzocaine to enable long-term anesthesia in adult zebrafish. We developed a computer-controlled perfusion system comprised of programmable peristaltic pumps that enabled automatic exchange between anesthetic and system water. Continuous administration of benzocaine in adult zebrafish resulted in a mean time to respiratory arrest of 5.0 h and 8-h survival of 14.3%. We measured characteristic sedation and recovery times in response to benzocaine, and used them to devise an intermittent dosing regimen consisting of 14.5 min of benzocaine followed by 5.5 min of system water. Intermittent benzocaine administration in adult zebrafish resulted in a mean time to respiratory arrest of 7.6 h and 8-h survival of 71.4%. Finally, we performed a single 24-h trial and found that intermittent dosing maintained anesthesia in an adult zebrafish over the entire 24-h period. In summary, our studies demonstrate the potential for dynamic administration of benzocaine to enable prolonged anesthesia in adult zebrafish, expanding the potential for imaging in adult physiologies that unfold over 1-24 h.

Methods to study the development, anatomy, and function of the zebrafish inner ear across the life course

The inner ear is a remarkably intricate structure able to detect sound, motion, and gravity. During development of the zebrafish embryo, the ear undergoes dynamic morphogenesis from a simple epithelial vesicle into a complex labyrinth, consisting of three semicircular canals and three otolithic sensory organs, each with an array of differentiated cell types. This microcosm of biology has led to advances in understanding molecular and cellular changes in epithelial patterning and morphogenesis, through to mechanisms of mechanosensory transduction and the origins of reflexive behavior. In this chapter, we describe different methods to study the zebrafish ear, including high-speed imaging of otic cilia, confocal microscopy, and light-sheet fluorescent microscopy. Many dyes, antibodies, and transgenic lines for labeling the ear are available, and we provide a comprehensive review of these resources. The developing ear is amenable to genetic, chemical, and physical manipulations, including injection and transplantation. Chemical modulation of developmental signaling pathways has paved the way for zebrafish to be widely used in drug discovery. We describe two chemical screens with relevance to the ear: a fluorescent-based screen for compounds that protect against ototoxicity, and an in situ-based screen for modulators of a signaling pathway involved in semicircular canal development. We also describe methods for dissection and imaging of the adult otic epithelia. We review both manual and automated methods to test the function of the inner ear and lateral line, defects in which can lead to altered locomotor behavior. Finally, we review a collection of zebrafish models that are generating new insights into human deafness and vestibular disorders.