Genetic dissection of neural circuits by Tol2 transposon-mediated Gal4 gene and enhancer trapping in zebrafish

Smuggling on the Nanoscale-Fusogenic Liposomes Enable Efficient RNA-Transfer with Negligible Immune Response In Vitro and In Vivo

The efficient and biocompatible transfer of nucleic acids into mammalian cells for research applications or medical purposes is a long-standing, challenging task. Viral transduction is the most efficient transfer system, but often entails high safety levels for research and potential health impairments for patients in medical applications. Lipo- or polyplexes are commonly used transfer systems but result in comparably low transfer efficiencies. Moreover, inflammatory responses caused by cytotoxic side effects were reported for these transfer methods. Often accountable for these effects are various recognition mechanisms for transferred nucleic acids. Using commercially available fusogenic liposomes (Fuse-It-mRNA), we established highly efficient and fully biocompatible transfer of RNA molecules for in vitro as well as in vivo applications. We demonstrated bypassing of endosomal uptake routes and, therefore, of pattern recognition receptors that recognize nucleic acids with high efficiency. This may underlie the observed almost complete abolishment of inflammatory cytokine responses. RNA transfer experiments into zebrafish embryos and adult animals fully confirmed the functional mechanism and the wide range of applications from single cells to organisms.

Identification of overlapping and distinct mural cell populations during early embryonic development

Mural cells are an essential perivascular cell population that associate with blood vessels and contribute to vascular stabilization and tone. In the embryonic zebrafish vasculature, pdgfrb and tagln are commonly used as markers for identifying pericytes and vascular smooth muscle cells (vSMCs). However, the expression patterns of these markers used in tandem have not been fully described. Here, we used the Tg(pdgfrb:Gal4FF; UAS:RFP) and Tg(tagln:NLS-EGFP) transgenic lines to identify single- and double-positive perivascular populations in the cranial, axial, and intersegmental vessels between 1 and 5 days post-fertilization. From this comparative analysis, we discovered two novel regions of tagln-positive cell populations that have the potential to function as mural cell precursors. Specifically, we found that the hypochord- a reportedly transient structure-contributes to tagln-positive cells along the dorsal aorta. We also identified a unique sclerotome-derived mural cell progenitor population that resides along the midline between the neural tube and notochord and contributes to intersegmental vessel mural cell coverage. Together, our findings highlight the variability and versatility of tracking pdgfrb and tagln expression in mural cells of the developing zebrafish embryo.

PRDM1 DNA-binding zinc finger domain is required for normal limb development and is disrupted in split hand/foot malformation

Split hand/foot malformation (SHFM) is a rare limb abnormality with clefting of the fingers and/or toes. For many individuals, the genetic etiology is unknown. Through whole-exome and targeted sequencing, we detected three novel variants in a gene encoding a transcription factor, PRDM1, that arose de novo in families with SHFM or segregated with the phenotype. PRDM1 is required for limb development; however, its role is not well understood and it is unclear how the PRDM1 variants affect protein function. Using transient and stable overexpression rescue experiments in zebrafish, we show that the variants disrupt the proline/serine-rich and DNA-binding zinc finger domains, resulting in a dominant-negative effect. Through gene expression assays, RNA sequencing, and CUT&RUN in isolated pectoral fin cells, we demonstrate that Prdm1a directly binds to and regulates genes required for fin induction, outgrowth and anterior/posterior patterning, such as fgfr1a, dlx5a, dlx6a and smo. Taken together, these results improve our understanding of the role of PRDM1 in the limb gene regulatory network and identified novel PRDM1 variants that link to SHFM in humans.

Endoderm-derived islet1-expressing cells differentiate into endothelial cells to function as the vascular HSPC niche in zebrafish

Endothelial cells (ECs) line blood vessels and serve as a niche for hematopoietic stem and progenitor cells (HSPCs). Recent data point to tissue-specific EC specialization as well as heterogeneity; however, it remains unclear how ECs acquire these properties. Here, by combining live-imaging-based lineage-tracing and single-cell transcriptomics in zebrafish embryos, we identify an unexpected origin for part of the vascular HSPC niche. We find that islet1 (isl1)-expressing cells are the progenitors of the venous ECs that constitute the majority of the HSPC niche. These isl1-expressing cells surprisingly originate from the endoderm and differentiate into ECs in a process dependent on Bmp-Smad signaling and subsequently requiring npas4l (cloche) function. Single-cell RNA sequencing analyses show that isl1-derived ECs express a set of genes that reflect their distinct origin. This study demonstrates that endothelial specialization in the HSPC niche is determined at least in part by the origin of the ECs.

TEMPO enables sequential genetic labeling and manipulation of vertebrate cell lineages

During development, regulatory factors appear in a precise order to determine cell fates over time. Consequently, to investigate complex tissue development, it is necessary to visualize and manipulate cell lineages with temporal control. Current strategies for tracing vertebrate cell lineages lack genetic access to sequentially produced cells. Here, we present TEMPO (Temporal Encoding and Manipulation in a Predefined Order), an imaging-readable genetic tool allowing differential labeling and manipulation of consecutive cell generations in vertebrates. TEMPO is based on CRISPR and powered by a cascade of gRNAs that drive orderly activation and inactivation of reporters and/or effectors. Using TEMPO to visualize zebrafish and mouse neurogenesis, we recapitulated birth-order-dependent neuronal fates. Temporally manipulating cell-cycle regulators in mouse cortex progenitors altered the proportion and distribution of neurons and glia, revealing the effects of temporal gene perturbation on serial cell fates. Thus, TEMPO enables sequential manipulation of molecular factors, crucial to study cell-type specification.

Novel analytical tools reveal that local synchronization of cilia coincides with tissue-scale metachronal waves in zebrafish multiciliated epithelia

Motile cilia are hair-like cell extensions that beat periodically to generate fluid flow along various epithelial tissues within the body. In dense multiciliated carpets, cilia were shown to exhibit a remarkable coordination of their beat in the form of traveling metachronal waves, a phenomenon which supposedly enhances fluid transport. Yet, how cilia coordinate their regular beat in multiciliated epithelia to move fluids remains insufficiently understood, particularly due to lack of rigorous quantification. We combine experiments, novel analysis tools, and theory to address this knowledge gap. To investigate collective dynamics of cilia, we studied zebrafish multiciliated epithelia in the nose and the brain. We focused mainly on the zebrafish nose, due to its conserved properties with other ciliated tissues and its superior accessibility for non-invasive imaging. We revealed that cilia are synchronized only locally and that the size of local synchronization domains increases with the viscosity of the surrounding medium. Even though synchronization is local only, we observed global patterns of traveling metachronal waves across the zebrafish multiciliated epithelium. Intriguingly, these global wave direction patterns are conserved across individual fish, but different for left and right noses, unveiling a chiral asymmetry of metachronal coordination. To understand the implications of synchronization for fluid pumping, we used a computational model of a regular array of cilia. We found that local metachronal synchronization prevents steric collisions, i.e., cilia colliding with each other, and improves fluid pumping in dense cilia carpets, but hardly affects the direction of fluid flow. In conclusion, we show that local synchronization together with tissue-scale cilia alignment coincide and generate metachronal wave patterns in multiciliated epithelia, which enhance their physiological function of fluid pumping.

A Critical Review of Zebrafish Neurological Disease Models-1. The Premise: Neuroanatomical, Cellular and Genetic Homology and Experimental Tractability

The last decade has seen a dramatic rise in the number of genes linked to neurological disorders, necessitating new models to explore underlying mechanisms and to test potential therapies. Over a similar period, many laboratories adopted zebrafish as a tractable model for studying brain development, defining neural circuits and performing chemical screens. Here we discuss strengths and limitations of using the zebrafish system to model neurological disorders. The underlying premise for many disease models is the high degree of homology between human and zebrafish genes, coupled with the conserved vertebrate Bauplan and repertoire of neurochemical signaling molecules. Yet, we caution that important evolutionary divergences often limit the extent to which human symptoms can be modeled meaningfully in zebrafish. We outline advances in genetic technologies that allow human mutations to be reproduced faithfully in zebrafish. Together with methods that visualize the development and function of neuronal pathways at the single cell level, there is now an unprecedented opportunity to understand how disease-associated genetic changes disrupt neural circuits, a level of analysis that is ideally suited to uncovering pathogenic changes in human brain disorders.

Skeletal Muscle Regeneration in Zebrafish

Muscle regeneration models have revealed mechanisms of inflammation, wound clearance, and stem cell-directed repair of damage, thereby informing therapy. Whereas studies of muscle repair are most advanced in rodents, the zebrafish is emerging as an additional model organism with genetic and optical advantages. Various muscle wounding protocols (both chemical and physical) have been published. Here we describe simple, cheap, precise, adaptable, and effective wounding protocols and analysis methods for two stages of a larval zebrafish skeletal muscle regeneration model. We show examples of how muscle damage, ingression of muscle stem cells, immune cells, and regeneration of fibers can be monitored over an extended timecourse in individual larvae. Such analyses have the potential to greatly enhance understanding, by reducing the need to average regeneration responses across individuals subjected to an unavoidably variable wound stimulus.

Tiltable objective microscope visualizes selectivity for head motion direction and dynamics in zebrafish vestibular system

Spatio-temporal information about head orientation and movement is fundamental to the sense of balance and motion. Hair cells (HCs) in otolith organs of the vestibular system transduce linear acceleration, including head tilt and vibration. Here, we build a tiltable objective microscope in which an objective lens and specimen tilt together. With in vivo Ca2+ imaging of all utricular HCs and ganglion neurons during 360° static tilt and vibration in pitch and roll axes, we reveal the direction- and static/dynamic stimulus-selective topographic responses in larval zebrafish. We find that head vibration is preferentially received by striolar HCs, whereas static tilt is preferentially transduced by extrastriolar HCs. Spatially ordered direction preference in HCs is consistent with hair-bundle polarity and is preserved in ganglion neurons through topographic innervation. Together, these results demonstrate topographically organized selectivity for direction and dynamics of head orientation/movement in the vestibular periphery.

Kiaa1024L/Minar2 is essential for hearing by regulating cholesterol distribution in hair bundles

Unbiased genetic screens implicated a number of uncharacterized genes in hearing loss, suggesting some biological processes required for auditory function remain unexplored. Loss of Kiaa1024L/Minar2, a previously understudied gene, caused deafness in mice, but how it functioned in the hearing was unclear. Here, we show that disruption of kiaa1024L/minar2 causes hearing loss in the zebrafish. Defects in mechanotransduction, longer and thinner hair bundles, and enlarged apical lysosomes in hair cells are observed in the kiaa1024L/minar2 mutant. In cultured cells, Kiaa1024L/Minar2 is mainly localized to lysosomes, and its overexpression recruits cholesterol and increases cholesterol labeling. Strikingly, cholesterol is highly enriched in the hair bundle membrane, and loss of kiaa1024L/minar2 reduces cholesterol localization to the hair bundles. Lowering cholesterol levels aggravates, while increasing cholesterol levels rescues the hair cell defects in the kiaa1024L/minar2 mutant. Therefore, cholesterol plays an essential role in hair bundles, and Kiaa1024L/Minar2 regulates cholesterol distribution and homeostasis to ensure normal hearing.

Using pERK immunostaining to quantify neuronal activity induced by stress in zebrafish larvae

The larval zebrafish has emerged as a very useful model organism to study the neuronal circuits controlling neuroendocrine and behavioral responses to stress. This protocol describes how to expose zebrafish larvae to hyperosmotic stress and test whether candidate populations of neurons are activated or inhibited by the stressor using a relatively rapid immunofluorescence staining approach. This approach takes advantage of the phosphorylation of the extracellular signal-regulated kinase (ERK) upon neuronal activation. For complete details on the use and execution of this protocol, please refer to Corradi et al. (2022).

Endothelial versus pronephron fate decision is modulated by the transcription factors Cloche/Npas4l, Tal1, and Lmo2

Endothelial specification is a key event during embryogenesis; however, when, and how, endothelial cells separate from other lineages is poorly understood. In zebrafish, Npas4l is indispensable for endothelial specification by inducing the expression of the transcription factor genes etsrp, tal1, and lmo2. We generated a knock-in reporter in zebrafish npas4l to visualize endothelial progenitors and their derivatives in wild-type and mutant embryos. Unexpectedly, we find that in npas4l mutants, npas4l reporter-expressing cells contribute to the pronephron tubules. Single-cell transcriptomics and live imaging of the early lateral plate mesoderm in wild-type embryos indeed reveals coexpression of endothelial and pronephron markers, a finding confirmed by creERT2-based lineage tracing. Increased contribution of npas4l reporter-expressing cells to pronephron tubules is also observed in tal1 and lmo2 mutants and is reversed in npas4l mutants injected with tal1 mRNA. Together, these data reveal that Npas4l/Tal1/Lmo2 regulate the fate decision between the endothelial and pronephron lineages.

Isolation of Reporter Cells That Respond to Vitamin A and/or D Using a piggyBac Transposon Promoter-Trapping Vector System

Previously, we established a highly sensitive promoter-trapping vector system using the piggyBac transposon for the efficient isolation of reporter cells. Herein, we examine whether this screening system can be applied to obtain vitamin-responsive cells. As a result, one and two reporter cells that responded to bexarotene (vitamin A) and calcitriol (vitamin D), respectively, were isolated from 4.7 × 106 seeded HeLaS3 cells. 5' RACE analyses identified the well-known CYP24A1 gene as a calcitriol-responsive gene, as well as two new bexarotene- or calcitriol-responsive genes, BDKRB2 and TSKU, respectively. TSKU, interestingly, also responded to bexarotene. Endogenous levels of the TSKU and BDKRB2 transcripts displayed only slight changes and were not detected in the comprehensive analyses performed to date. Dose-response analyses of BDKRB2 and TSKU reporter cells in parallel revealed a differential profile in response to each vitamin A agonist, suggesting a bioanalyzer. The present study demonstrates that producing multiple reporter cells by a type of random screening can efficiently identify novel genes with unusual characteristics and be used for the profiling of the properties of vitamin compounds. Similar approaches to the method shown here may be useful for identifying new markers and for the analysis or diagnosis of nutrients, toxins, metabolites, etc.

Habenula bibliometrics: Thematic development and research fronts of a resurgent field

The habenula (Hb) is a small structure of the posterior diencephalon that is highly conserved across vertebrates but nonetheless has attracted relatively little research attention until the past two decades. The resurgent interest is motivated by neurobehavioral studies demonstrating critical functions in a broad spectrum of motivational and cognitive processes, including functions relevant to psychiatric diseases. The Hb is widely conceived as an "anti-reward" center that acts by regulating brain monoaminergic systems. However, there is still no general conceptual framework for habenula research, and no study has focused on uncovering potentially significant but overlooked topics that may advance our understanding of physiological functions or suggest potential clinical applications of Hb-targeted interventions. Using science mapping tools, we quantitatively and qualitatively analyzed the relevant publications retrieved from the Web of Science Core Collection (WoSCC) database from 2002 to 2021. Herein we present an overview of habenula-related publications, reveal primary research trends, and prioritize some key research fronts by complementary bibliometric analysis. High-priority research fronts include Ventral Pallidum, Nucleus Accumbens, Nicotine and MHb, GLT-1, Zebrafish, and GCaMP, Ketamine, Deep Brain Stimulation, and GPR139. The high intrinsic heterogeneity of the Hb, extensive connectivity with both hindbrain and forebrain structures, and emerging associations with all three dimensions of mental disorders (internalizing, externalizing, and psychosis) suggest that the Hb may be the neuronal substrate for a common psychopathology factor shared by all mental illnesses termed the p factor. A future challenge is to explore the therapeutic potential of habenular modulation at circuit, cellular, and molecular levels.

In vivo dissection of Rhoa function in vascular development using zebrafish

The small monomeric GTPase RHOA acts as a master regulator of signal transduction cascades by activating effectors of cellular signaling, including the Rho-associated protein kinases ROCK1/2. Previous in vitro cell culture studies suggest that RHOA can regulate many critical aspects of vascular endothelial cell (EC) biology, including focal adhesion, stress fiber formation, and angiogenesis. However, the specific in vivo roles of RHOA during vascular development and homeostasis are still not well understood. In this study, we examine the in vivo functions of RHOA in regulating vascular development and integrity in zebrafish. We use zebrafish RHOA-ortholog (rhoaa) mutants, transgenic embryos expressing wild type, dominant negative, or constitutively active forms of rhoaa in ECs, pharmacological inhibitors of RHOA and ROCK1/2, and Rock1 and Rock2a/b dgRNP-injected zebrafish embryos to study the in vivo consequences of RHOA gain- and loss-of-function in the vascular endothelium. Our findings document roles for RHOA in vascular integrity, developmental angiogenesis, and vascular morphogenesis in vivo, showing that either too much or too little RHOA activity leads to vascular dysfunction.

A viral toolbox for conditional and transneuronal gene expression in zebrafish

The zebrafish is an important model in systems neuroscience but viral tools to dissect the structure and function of neuronal circuitry are not established. We developed methods for efficient gene transfer and retrograde tracing in adult and larval zebrafish by herpes simplex viruses (HSV1). HSV1 was combined with the Gal4/UAS system to target cell types with high spatial, temporal, and molecular specificity. We also established methods for efficient transneuronal tracing by modified rabies viruses in zebrafish. We demonstrate that HSV1 and rabies viruses can be used to visualize and manipulate genetically or anatomically identified neurons within and across different brain areas of adult and larval zebrafish. An expandable library of viruses is provided to express fluorescent proteins, calcium indicators, optogenetic probes, toxins and other molecular tools. This toolbox creates new opportunities to interrogate neuronal circuits in zebrafish through combinations of genetic and viral approaches.

Remodeling of the hyomandibular skeleton and facial nerve positioning during embryonic and postembryonic development of teleost fish

The vertebrate skeleton changes its shape during development through the activities of chondrocytes, osteoblasts and osteoclasts. Although much is known about the mechanisms for differentiation in these cells, it is less understood how they behave in a region-specific manner to acquire unique bone shapes. To address this question, we investigated the development of the hyomandibular (Hm) system in zebrafish. The Hm originates as cartilage carrying a single foramen (the Hm foramen), through which the facial (VII) nerve passes. We reveal that Schwann cells, which myelinate the VII nerve, regulate rearrangement of the chondrocytes to enlarge the Hm foramen. The Hm cartilage then becomes ossified in the perichondrium, where the marrow chondrocytes are replaced by adipocytes. Then, the bone matrix along the VII nerve is resorbed by osteoclasts, generating a gateway to the bone marrow. Subsequent movement of the VII nerve into the marrow, followed by deposition of new bone matrix, isolates the nerve from the jaw muscle insertion. Genetic ablation of osteoblasts and osteoclasts reveals specific roles of these cells during remodeling processes. Interestingly, the VII nerve relocation does not occur in medaka; instead, bone deposition distinct from those in zebrafish separates the VII nerve from the muscle insertion. Our results define novel mechanisms for skeletal remodeling, by which the bone shapes in a region- and species-specific manner.

Retinoic acid signaling regulates late stages of semicircular canal morphogenesis and otolith maintenance in the zebrafish inner ear

BACKGROUND

The vitamin A derivative all-trans retinoic acid (RA) regulates early stages of inner ear development. As the early disruption of the RA pathway results in profound mispatterning of the developing inner ear, this confounds analyses of specific roles in later stages. Therefore, we used the temporal-specific exposure of all-trans RA or diethylaminobenzaldehyde to evaluate RA functions in late otic development.

RESULTS

Perturbing late RA signaling causes behavioral defects analogous to those expected in larvae suffering from vestibular dysfunction. These larvae also demonstrate malformations of the semi-circular canals, as visualized through (a) use of the transgenic strain nkhspdmc12a, a fluorescent reporter expressed in otic epithelium; and (b) injection of the fluorescent lipophilic dye DiI. We also noted the altered expression of genes encoding ECM proteins or modifying enzymes. Other malformations of the inner ear observed in our work include the loss or reduced size of the utricular and saccular otoliths, suggesting a role for RA in otolith maintenance.

CONCLUSION

Our work has identified a previously undescribed late phase of RA activity in otic development, demonstrating that vestibular defects observed in human patients in relation to perturbed RA signaling are not solely due to its early disruption in otic development.

Divergent cis-regulatory evolution underlies the convergent loss of sodium channel expression in electric fish

South American and African weakly electric fish independently evolved electric organs from muscle. In both groups, a voltage-gated sodium channel gene independently lost expression from muscle and gained it in the electric organ, allowing the channel to become specialized for generating electric signals. It is unknown how this voltage-gated sodium channel gene is targeted to muscle in any vertebrate. We describe an enhancer that selectively targets sodium channel expression to muscle. Next, we demonstrate how the loss of this enhancer, but not trans-activating factors, drove the loss of sodium channel gene expression from muscle in South American electric fish. While this enhancer is also altered in African electric fish, key transcription factor binding sites and enhancer activity are retained, suggesting that the convergent loss of sodium channel expression from muscle in these two electric fish lineages occurred via different processes.

Integrated Behavioral, Genetic and Brain Circuit Visualization Methods to Unravel Functional Anatomy of Zebrafish Amygdala

The mammalian amygdala is a complex forebrain structure consisting of a heterogeneous group of nuclei derived from the pallial and subpallial telencephalon. It plays a critical role in a broad range of behaviors such as emotion, cognition, and social behavior; within the amygdala each nucleus has a distinct role in these behavioral processes. Topological, hodological, molecular, and functional studies suggest the presence of an amygdala-like structure in the zebrafish brain. It has been suggested that the pallial amygdala homolog corresponds to the medial zone of the dorsal telencephalon (Dm) and the subpallial amygdala homolog corresponds to the nuclei in the ventral telencephalon located close to and topographically basal to Dm. However, these brain regions are broad and understanding the functional anatomy of the zebrafish amygdala requires investigating the role of specific populations of neurons in brain function and behavior. In zebrafish, the highly efficient Tol2 transposon-mediated transgenesis method together with the targeted gene expression by the Gal4-UAS system has been a powerful tool in labeling, visualizing, and manipulating the function of specific cell types in the brain. The transgenic resource combined with neuronal activity imaging, optogenetics, pharmacology, and quantitative behavioral analyses enables functional analyses of neuronal circuits. Here, we review earlier studies focused on teleost amygdala anatomy and function and discuss how the transgenic resource and tools can help unravel the functional anatomy of the zebrafish amygdala.

KCNJ8/ABCC9-containing K-ATP channel modulates brain vascular smooth muscle development and neurovascular coupling

Loss- or gain-of-function mutations in ATP-sensitive potassium channel (K-ATP)-encoding genes, KCNJ8 and ABCC9, cause human central nervous system disorders with unknown pathogenesis. Here, using mice, zebrafish, and cell culture models, we investigated cellular and molecular causes of brain dysfunctions derived from altered K-ATP channel function. We show that genetic/chemical inhibition or activation of KCNJ8/ABCC9-containing K-ATP channel function leads to brain-selective suppression or promotion of arterial/arteriolar vascular smooth muscle cell (VSMC) differentiation, respectively. We further show that brain VSMCs develop from KCNJ8/ABCC9-containing K-ATP channel-expressing mural cell progenitor and that K-ATP channel cell autonomously regulates VSMC differentiation through modulation of intracellular Ca²⁺ oscillation via voltage-dependent calcium channels. Consistent with defective VSMC development, Kcnj8 knockout mice showed deficiency in vasoconstrictive capacity and neuronal-evoked vasodilation leading to local hyperemia. Our results demonstrate a role for KCNJ8/ABCC9-containing K-ATP channels in the differentiation of brain VSMC, which in turn is necessary for fine-tuning of cerebral blood flow.

Developmental independence of median fins from the larval fin fold revises their evolutionary origin

The median fins of modern fish that show discrete forms (dorsal, anal, and caudal fins) are derived from a continuous fold-like structure, both in ontogeny and phylogeny. The median fin fold (MFF) hypothesis assumes that the median fins evolved by reducing some positions in the continuous fin fold of basal chordates, based on the classical morphological observation of developmental reduction in the larval fin folds of living fish. However, the developmental processes of median fins are still unclear at the cellular and molecular levels. Here, we describe the transition from the larval fin fold into the median fins in zebrafish at the cellular and molecular developmental level. We demonstrate that reduction does not play a role in the emergence of the dorsal fin primordium. Instead, the reduction occurs along with body growth after primordium formation, rather than through actively scrapping the non-fin forming region by inducing cell death. We also report that the emergence of specific mesenchymal cells and their proliferation promote dorsal fin primordium formation. Based on these results, we propose a revised hypothesis for median fin evolution in which the acquisition of de novo developmental mechanisms is a crucial evolutionary component of the discrete forms of median fins.

Endocardial identity is established during early somitogenesis by Bmp signalling acting upstream of npas4l and etv2

The endocardium plays important roles in the development and function of the vertebrate heart; however, few molecular markers of this tissue have been identified and little is known about what regulates its differentiation. Here, we describe the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of endocardial development in zebrafish. Transcriptomic comparison between endocardium and pan-endothelium confirms molecular distinction between these populations and time-course analysis suggests differentiation as early as eight somites. To investigate what regulates endocardial identity, we employed npas4l, etv2 and scl loss-of-function models. Endocardial expression is lost in npas4l mutants, significantly reduced in etv2 mutants and only modestly affected upon scl loss-of-function. Bmp signalling was also examined: overactivation of Bmp signalling increased endocardial expression, whereas Bmp inhibition decreased expression. Finally, epistasis experiments showed that overactivation of Bmp signalling was incapable of restoring endocardial expression in etv2 mutants. By contrast, overexpression of either npas4l or etv2 was sufficient to rescue endocardial expression upon Bmp inhibition. Together, these results describe the differentiation of the endocardium, distinct from vasculature, and place npas4l and etv2 downstream of Bmp signalling in regulating its differentiation.

Summary: A zebrafish transgenic reporter of the endocardium is identified, permitting transcriptomic analysis and identification of new endocardial markers. Epistasis experiments demonstrate npas4l and etv2 act downstream of Bmp signalling to regulate endocardial differentiation.

Blood Flow Regulates Glomerular Capillary Formation in Zebrafish Pronephros

Background

The renal glomerulus is a tuft of capillaries in Bowman's capsule and functions as a blood-filtration unit in the kidney. The unique glomerular capillary tuft structure is relatively conserved through vertebrate species. However, the morphogenetic mechanism governing glomerular capillary tuft formation remains elusive.

Methods

To clarify how glomerular capillaries develop, we analyzed glomerular capillary formation in the zebrafish pronephros by exploiting fluorescence-based bio-imaging technology.

Results

During glomerular capillary formation in the zebrafish pronephros, endothelial cells initially sprouted from the dorsal aorta and formed the capillaries surrounding the bilateral glomerular primordia in response to podocyte progenitor-derived vascular endothelial growth factor-A. After formation, blood flow immediately occurred in the glomerular primordia-associated capillaries, while in the absence of blood flow, they were transformed into sheet-like structures enveloping the glomerular primordia. Subsequently, blood flow induced formation of Bowman's space at the lateral sides of the bilateral glomerular primordia. Concomitantly, podocyte progenitors enveloped their surrounding capillaries while moving toward and coalescing at the midline. These capillaries then underwent extensive expansion and remodeling to establish a functional glomerular capillary tuft. However, stopping blood flow inhibited the remodeling of bilateral glomerular primordia, which therefore remained unvascularized but covered by the vascular sheets.

Conclusions

We delineated the morphogenetic processes governing glomerular capillary tuft formation in the zebrafish pronephros and demonstrated crucial roles of blood flow in its formation. Blood flow maintains tubular structures of the capillaries surrounding the glomerular primordia and promotes glomerular incorporation of these vessels by inducing the remodeling of glomerular primordia.

Jagged 2b induces intercellular signaling within somites to establish hematopoietic stem cell fate in zebrafish

During development, the somites play a key role in the specification of hematopoietic stem cells (HSCs). In zebrafish, the somitic Notch ligands Delta-c (Dlc) and Dld, both of which are regulated by Wnt16, directly instruct HSC fate in a shared vascular precursor. However, it remains unclear how this signaling cascade is spatially and temporally regulated within somites. Here, we show in zebrafish that an additional somitic Notch ligand, Jagged 2b (Jag2b), induces intercellular signaling to drive wnt16 expression. Jag2b activated Notch signaling in segmented somites at the early stage of somitogenesis. Loss of jag2b led to a reduction in the expression of wnt16 in the somites and an HSC marker, runx1, in the dorsal aorta, whereas overexpression of jag2b increased both. However, Notch-activated cells were adjacent to, but did not overlap with, wnt16-expressing cells within the somites, suggesting that an additional signaling molecule mediates this intercellular signal transduction. We uncover that Jag2b-driven Notch signaling induces efna1b expression, which regulates wnt16 expression in neighboring somitic cells. Collectively, we provide evidence for previously unidentified spatiotemporal regulatory mechanisms of HSC specification by somites.

Integration of vascular progenitors into functional blood vessels represents a distinct mechanism of vascular growth

During embryogenesis, the initial vascular network forms by the process of vasculogenesis, or the specification of vascular progenitors de novo. In contrast, the majority of later-forming vessels arise by angiogenesis from the already established vasculature. Here, we show that new vascular progenitors in zebrafish embryos emerge from a distinct site along the yolk extension, or secondary vascular field (SVF), incorporate into the posterior cardinal vein, and contribute to subintestinal vasculature even after blood circulation has been initiated. We further demonstrate that SVF cells participate in vascular recovery after chemical ablation of vascular endothelial cells. Inducible inhibition of the function of vascular progenitor marker etv2/etsrp prevented SVF cell differentiation and resulted in the defective formation of subintestinal vasculature. Similar late-forming etv2+ progenitors were also observed in mouse embryos, suggesting that SVF cells are evolutionarily conserved. Our results characterize a distinct mechanism by which new vascular progenitors incorporate into established vasculature.

The crustacean model Parhyale hawaiensis

Arthropods are the most abundant and diverse animals on earth. Among them, pancrustaceans are an ancient and morphologically diverse group, comprising a wide range of aquatic and semi-aquatic crustaceans as well as the insects, which emerged from crustacean ancestors to colonize most terrestrial habitats. Within insects, Drosophila stands out as one of the most powerful animal models, making major contributions to our understanding of development, physiology and behavior. Given these attributes, crustaceans provide a fertile ground for exploring biological diversity through comparative studies. However, beyond insects, few crustaceans are developed sufficiently as experimental models to enable such studies. The marine amphipod Parhyale hawaiensis is currently the best established crustacean system, offering year-round accessibility to developmental stages, transgenic tools, genomic resources, and established genetics and imaging approaches. The Parhyale research community is small but diverse, investigating the evolution of development, regeneration, aspects of sensory biology, chronobiology, bioprocessing and ecotoxicology.

Proper migration of lymphatic endothelial cells requires survival and guidance cues from arterial mural cells

The migration of lymphatic endothelial cells (LECs) is key for the development of the complex and vast lymphatic vascular network that pervades most tissues in an organism. In zebrafish, arterial intersegmental vessels together with chemokines have been shown to promote lymphatic cell migration from the horizontal myoseptum (HM). We observed that emergence of mural cells around the intersegmental arteries coincides with lymphatic departure from HM which raised the possibility that arterial mural cells promote LEC migration. Our live imaging and cell ablation experiments revealed that LECs migrate slower and fail to establish the lymphatic vascular network in the absence of arterial mural cells. We determined that mural cells are a source for the C-X-C motif chemokine 12 (Cxcl12a and Cxcl12b), vascular endothelial growth factor C (Vegfc) and collagen and calcium-binding EGF domain-containing protein 1 (Ccbe1). We showed that chemokine and growth factor signalling function cooperatively to induce robust LEC migration. Specifically, Vegfc-Vegfr3 signalling, but not chemokines, induces extracellular signal-regulated kinase (ERK) activation in LECs, and has an additional pro-survival role in LECs during the migration. Together, the identification of mural cells as a source for signals that guide LEC migration and survival will be important in the future design for rebuilding lymphatic vessels in disease contexts.

Hypothalamic Galanin-producing neurons regulate stress in zebrafish through a peptidergic, self-inhibitory loop

Animals possess neuronal circuits inducing stress to avoid or cope with threats present in their surroundings, for instance, by promoting behaviors, such as avoidance and escape. However, mechanisms must exist to tightly control responses to stressors, since overactivation of stress circuits is deleterious for the wellbeing of an organism. The underlying neuronal dynamics responsible for controlling behavioral responses to stress have remained unclear. Here, we describe a neuronal circuit in the hypothalamus of zebrafish larvae that inhibits stress-related behaviors and prevents excessive activation of the neuroendocrine pathway hypothalamic-pituitary-interrenal axis. Central components of this circuit are neurons secreting the neuropeptide Galanin, as ablation of these neurons led to abnormally high levels of stress. Surprisingly, we found that Galanin has a self-inhibitory action on Galanin-producing neurons. Our results suggest that hypothalamic Galanin-producing neurons play an important role in fine-tuning stress responses by preventing potentially harmful overactivation of stress-regulating circuits.

An engineered abcb4 expression model reveals the central role of NF-κB in the regulation of drug resistance in zebrafish

Multi-drug resistance (MDR) is a phenomenon that tumor cells are exposed to a chemotherapeutic drug for a long time and then develop resistance to a variety of other anticancer drugs with different structures and different mechanisms. The in vitro studies of tumor cell lines cannot systematically reflect the role of MDR gene in vivo, and the cost of in vivo studies of transgenic mice as animal models is high. Given the myriad merits of zebrafish relative to other animal models, we aimed to establish a screening system using zebrafish stably expressing ATP-binding cassette (ATP-cassette) superfamily transporters and unveil the potential regulatory mechanism. We first used the Tol2-mediated approach to construct a Tg (abcb4:EGFP) transgenic zebrafish line with ATP-binding cassette (ABC) subfamily B member 4 (abcb4) gene promoter to drive EGFP expression. The expression levels of abcb4 and EGFP were significantly increased when Tg(abcb4:EGFP) transgenic zebrafish embryos were exposed to doxorubicin (DOX) or vincristine (VCR), and the increases were accompanied by a marked decreased accumulation of rhodamine B (RhB) in embryos, indicating a remarkable increase in DOX or VCR efflux. Mechanistically, Akt and Erk signalings were activated upon the treatment with DOX or VCR. With the application of Akt and Erk inhibitors, drug resistance was reversed with differing responsive effects. Notably, downstream NF-κB played a central role in the regulation of abcb4-mediated drug resistance. Taken together, the data indicate that the engineered Tg(abcb4:EGFP) transgenic zebrafish model is a new platform for screening drug resistance in vivo, which may facilitate and accelerate the process of drug development.

A novel gene trap line for visualization and manipulation of erbb3b+ neural crest and glial cells in zebrafish

Glia are a diverse and essential cell type in the vertebrate nervous system. Transgenic tools and fluorescent reporter lines are critical resources to investigate how glial subtypes develop and function. However, despite the many lines available in zebrafish, the community still lacks the ability to label all unique stages of glial development and specific subpopulations of cells. To address this issue, we screened zebrafish gene and enhancer trap lines to find a novel reporter for peripheral glial subtypes. From these, we generated the gSAIzGFFD37A transgenic line that expresses GFP in neural crest cells and central and peripheral glia. We found that the gene trap construct is located within an intron of erbb3b, a gene essential for glial development. Additionally, we confirmed that GFP+ ​cells express erbb3b along with sox10, a known glial marker. From our screen, we have identified the gSAIzGFFD37A line as a novel and powerful tool for studying glia in the developing zebrafish, as well as a new resource to manipulate erbb3b+ ​cells.

In Vivo Live Imaging of Axonal Transport in Developing Zebrafish Axons

Axonal transport is crucial for neuronal homeostasis, survival, and development. Indeed, axonal transport needs to be precisely regulated for developing axons to swiftly and accurately respond to their complex and evolving environment in space and time. A growing number of studies have started to unravel the diversity of regulatory and adaptor proteins required to orchestrate the axonal transport machinery. Despite some discrepancies between in vitro and in vivo axonal transport studies, most analyses aiming at deciphering these regulatory complexes, as well as their mode of action, were carried out in vitro in primary cultures of neurons, and mainly focused on their impact on axon specification and elongation, but rarely on axon navigation per se. Given the clear influence of the in vivo environment on axonal transport, including chemical and physical interactions with neighboring cells, it is essential to develop in vivo models to identify and characterize the molecular complexes involved in this key process. Here, we describe an experimental system to monitor axonal transport in vivo in developing axons of live zebrafish embryos with high spatial and temporal resolution. Due to its optical transparency and easy genetic manipulation, the zebrafish embryo is ideally suited to study such cellular dynamics at a single axon scale. Using this approach, we were able to unravel the key role of Fidgetin-like 1 in the regulation of bidirectional axonal transport required for motor axon targeting. Moreover, this protocol can be easily adapted to characterize a wide range of axonal transport regulators and components in physiological conditions and may additionally be used to screen new therapeutic compounds based on their ability to recue axonal transport defects in pathological conditions.

IQ-Switch is a QF-based innocuous, silencing-free, and inducible gene switch system in zebrafish

Though various transgene expression switches have been adopted in a wide variety of organisms for basic and biomedical research, intrinsic obstacles of those existing systems, including toxicity and silencing, have been limiting their use in vertebrate transgenesis. Here we demonstrate a novel QF-based binary transgene switch (IQ-Switch) that is relatively free of driver toxicity and transgene silencing, and exhibits potent and highly tunable transgene activation by the chemical inducer tebufenozide, a non-toxic lipophilic molecule to developing zebrafish with negligible background. The interchangeable IQ-Switch makes it possible to elicit ubiquitous and tissue specific transgene expression in a spatiotemporal manner. We generated a RASopathy disease model using IQ-Switch and demonstrated that the RASopathy symptoms were ameliorated by the specific BRAF^(V600E) inhibitor vemurafenib, validating the therapeutic use of the gene switch. The orthogonal IQ-Switch provides a state-of-the-art platform for flexible regulation of transgene expression in zebrafish, potentially applicable in cell-based systems and other model organisms.

Integrated molecular analysis identifies a conserved pericyte gene signature in zebrafish

Pericytes reside in capillary beds where they share a basement membrane with endothelial cells and regulate their function. However, little is known about embryonic pericyte development, in part, due to lack of specific molecular markers and genetic tools. Here, we applied single cell RNA-sequencing (scRNA-seq) of platelet derived growth factor beta (pdgfrb)-positive cells to molecularly characterize pericytes in zebrafish larvae. scRNA-seq revealed zebrafish cells expressing mouse pericyte gene orthologs, and comparison with bulk RNA-seq from wild-type and pdgfrb mutant larvae further refined a pericyte gene set. Subsequent integration with mouse pericyte scRNA-seq profiles revealed a core set of conserved pericyte genes. Using transgenic reporter lines, we validated pericyte expression of two genes identified in our analysis: NDUFA4 mitochondrial complex associated like 2a (ndufa4l2a), and potassium voltage-gated channel, Isk-related family, member 4 (kcne4). Both reporter lines exhibited pericyte expression in multiple anatomical locations, and kcne4 was also detected in a subset of vascular smooth muscle cells. Thus, our integrated molecular analysis revealed a molecular profile for zebrafish pericytes and allowed us to develop new tools to observe these cells in vivo.

Selective autophagy: the rise of the zebrafish model

Selective autophagy is a specific elimination of certain intracellular substrates by autophagic pathways. The most studied macroautophagy pathway involves tagging and recognition of a specific cargo by the autophagic membrane (phagophore) followed by the complete sequestration of targeted cargo from the cytosol by the double-membrane vesicle, autophagosome. Until recently, the knowledge about selective macroautophagy was minimal, but now there is a panoply of links elucidating how phagophores engulf their substrates selectively. The studies of selective autophagy processes have further stressed the importance of using the in vivo models to validate new in vitro findings and discover the physiologically relevant mechanisms. However, dissecting how the selective autophagy occurs yet remains difficult in living organisms, because most of the organelles are relatively inaccessible to observation and experimental manipulation in mammals. In recent years, zebrafish (Danio rerio) is widely recognized as an excellent model for studying autophagic processes in vivo because of its optical accessibility, genetic manipulability and translational potential. Several selective autophagy pathways, such as mitophagy, xenophagy, lipophagy and aggrephagy, have been investigated using zebrafish and still need to be studied further, while other selective autophagy pathways, such as pexophagy or reticulophagy, could also benefit from the use of the zebrafish model. In this review, we shed light on how zebrafish contributed to our understanding of these selective autophagy processes by providing the in vivo platform to study them at the organismal level and highlighted the versatility of zebrafish model in the selective autophagy field.Abbreviations: AD: Alzheimer disease; ALS: amyotrophic lateral sclerosis; Atg: autophagy-related; CMA: chaperone-mediated autophagy; CQ: chloroquine; HsAMBRA1: human AMBRA1; KD: knockdown; KO: knockout; LD: lipid droplet; MMA: methylmalonic acidemia; PD: Parkinson disease; Tg: transgenic.

Bioelectric signaling and the control of cardiac cell identity in response to mechanical forces

[Figure: see text].

Mechanosensitive Notch-Dll4 and Klf2-Wnt9 signaling pathways intersect in guiding valvulogenesis in zebrafish

In the zebrafish embryo, the onset of blood flow generates fluid shear stress on endocardial cells, which are specialized endothelial cells that line the interior of the heart. High levels of fluid shear stress activate both Notch and Klf2 signaling, which play crucial roles in atrioventricular valvulogenesis. However, it remains unclear why only individual endocardial cells ingress into the cardiac jelly and initiate valvulogenesis. Here, we show that lateral inhibition between endocardial cells, mediated by Notch, singles out Delta-like-4-positive endocardial cells. These cells ingress into the cardiac jelly, where they form an abluminal cell population. Delta-like-4-positive cells ingress in response to Wnt9a, which is produced in parallel through an Erk5-Klf2-Wnt9a signaling cascade also activated by blood flow. Hence, mechanical stimulation activates parallel mechanosensitive signaling pathways that produce binary effects by driving endocardial cells toward either luminal or abluminal fates. Ultimately, these cell fate decisions sculpt cardiac valve leaflets.

Similarities of developmental gene expression changes in the brain between human and experimental animals: rhesus monkey, mouse, Zebrafish, and Drosophila

AIM

Experimental animals, such as non-human primates (NHPs), mice, Zebrafish, and Drosophila, are frequently employed as models to gain insights into human physiology and pathology. In developmental neuroscience and related research fields, information about the similarities of developmental gene expression patterns between animal models and humans is vital to choose what animal models to employ. Here, we aimed to statistically compare the similarities of developmental changes of gene expression patterns in the brains of humans with those of animal models frequently used in the neuroscience field.

METHODS

The developmental gene expression datasets that we analyzed consist of the fold-changes and P values of gene expression in the brains of animals of various ages compared with those of the youngest postnatal animals available in the dataset. By employing the running Fisher algorithm in a bioinformatics platform, BaseSpace, we assessed similarities between the developmental changes of gene expression patterns in the human (Homo sapiens) hippocampus with those in the dentate gyrus (DG) of the rhesus monkey (Macaca mulatta), the DG of the mouse (Mus musculus), the whole brain of Zebrafish (Danio rerio), and the whole brain of Drosophila (D. melanogaster).

RESULTS

Among all possible comparisons of different ages and animals in developmental changes in gene expression patterns within the datasets, those between rhesus monkeys and mice were highly similar to those of humans with significant overlap P-value as assessed by the running Fisher algorithm. There was the highest degree of gene expression similarity between 40-59-year-old humans and 6-12-year-old rhesus monkeys (overlap P-value = 2.1 × 10^- 72). The gene expression similarity between 20-39-year-old humans and 29-day-old mice was also significant (overlap P = 1.1 × 10^- 44). Moreover, there was a similarity in developmental changes of gene expression patterns between 1-2-year-old Zebrafish and 40-59-year-old humans (Overlap P-value = 1.4 × 10^- 6). The overlap P-value of developmental gene expression patterns between Drosophila and humans failed to reach significance (30 days Drosophila and 6-11-year-old humans; overlap P-value = 0.0614).

CONCLUSIONS

These results indicate that the developmental gene expression changes in the brains of the rhesus monkey, mouse, and Zebrafish recapitulate, to a certain degree, those in humans. Our findings support the idea that these animal models are a valid tool for investigating the development of the brain in neurophysiological and neuropsychiatric studies.

In vivo wide-field voltage imaging in zebrafish with voltage-sensitive dye and genetically encoded voltage indicator

The brain consists of neural circuits, which are assemblies of various neuron types. For understanding how the brain works, it is essential to identify the functions of each type of neuron and neuronal circuits. Recent advances in our understanding of brain function and its development have been achieved using light to detect neuronal activity. Optical measurement of membrane potentials through voltage imaging is a desirable approach, enabling fast, direct, and simultaneous detection of membrane potentials in a population of neurons. Its high speed and directness can help detect synaptic and action potentials and hyperpolarization, which encode critical information for brain function. Here, we describe in vivo voltage imaging procedures that we have recently established using zebrafish, a powerful animal model in developmental biology and neuroscience. By applying two types of voltage sensors, voltage-sensitive dyes (VSDs, Di-4-ANEPPS) and genetically encoded voltage indicators (GEVIs, ASAP1), spatiotemporal dynamics of voltage signals can be detected in the whole cerebellum and spinal cord in awake fish at single-cell and neuronal population levels. Combining this method with other approaches, such as optogenetics, behavioral analysis, and electrophysiology would facilitate a deeper understanding of the network dynamics of the brain circuitry and its development.

Insulin-producing β-cells regenerate ectopically from a mesodermal origin under the perturbation of hemato-endothelial specification

To investigate the role of the vasculature in pancreatic β-cell regeneration, we crossed a zebrafish β-cell ablation model into the avascular npas4l mutant (i.e. cloche). Surprisingly, β-cell regeneration increased markedly in npas4l mutants owing to the ectopic differentiation of β-cells in the mesenchyme, a phenotype not previously reported in any models. The ectopic β-cells expressed endocrine markers of pancreatic β-cells, and also responded to glucose with increased calcium influx. Through lineage tracing, we determined that the vast majority of these ectopic β-cells has a mesodermal origin. Notably, ectopic β-cells were found in npas4l mutants as well as following knockdown of the endothelial/myeloid determinant Etsrp. Together, these data indicate that under the perturbation of endothelial/myeloid specification, mesodermal cells possess a remarkable plasticity enabling them to form β-cells, which are normally endodermal in origin. Understanding the restriction of this differentiation plasticity will help exploit an alternative source for β-cell regeneration.

Chemical screening reveals Ronidazole is a superior prodrug to Metronidazole for nitroreductase-induced cell ablation system in zebrafish larvae

The Metronidazole (MTZ)/nitroreductase (NTR)-mediated cell ablation system is the most commonly used chemical-genetic cell ablation method in zebrafish. This system can specifically ablate target cells under spatial and temporal control. The MTZ/NTR system has become a widely used cell ablation system in biological, developmental, and functional studies. However, the inadequate cell-ablation ability of some cell types and the side effects of high concentration MTZ impede extensive applications of the MTZ/NTR system. In the present study, the US drug collection library was searched to extend the NTR system. Six MTZ analogs were found, and the cell-ablation ability of these analogs was tested in zebrafish larvae. The results revealed that two of the NTR substrates, Furazolidone and Ronidazole, ablated target cells more efficiently than MTZ at lower concentrations. Furthermore, the working concentration of Ronidazole, but not Furazolidone and MTZ, did not affect axonal bridge formation during spinal cord regeneration. Our results, taken together, indicate that Ronidazole is a superior prodrug to MTZ for the NTR system, especially for the study of neuron regeneration in zebrafish larvae.

Twisting of the zebrafish heart tube during cardiac looping is a tbx5-dependent and tissue-intrinsic process

Organ laterality refers to the left-right asymmetry in disposition and conformation of internal organs and is established during embryogenesis. The heart is the first organ to display visible left-right asymmetries through its left-sided positioning and rightward looping. Here, we present a new zebrafish loss-of-function allele for tbx5a, which displays defective rightward cardiac looping morphogenesis. By mapping individual cardiomyocyte behavior during cardiac looping, we establish that ventricular and atrial cardiomyocytes rearrange in distinct directions. As a consequence, the cardiac chambers twist around the atrioventricular canal resulting in torsion of the heart tube, which is compromised in tbx5a mutants. Pharmacological treatment and ex vivo culture establishes that the cardiac twisting depends on intrinsic mechanisms and is independent from cardiac growth. Furthermore, genetic experiments indicate that looping requires proper tissue patterning. We conclude that cardiac looping involves twisting of the chambers around the atrioventricular canal, which requires correct tissue patterning by Tbx5a.

Two-Photon Laser Ablation and In Vivo Wide-Field Imaging of Inferior Olive Neurons Revealed the Recovery of Olivocerebellar Circuits in Zebrafish

The cerebellum, a brain region with a high degree of plasticity, is pivotal in motor control, learning, and cognition. The cerebellar reserve is the capacity of the cerebellum to respond and adapt to various disorders via resilience and reversibility. Although structural and functional recovery has been reported in mammals and has attracted attention regarding treatments for cerebellar dysfunction, such as spinocerebellar degeneration, the regulatory mechanisms of the cerebellar reserve are largely unidentified, particularly at the circuit level. Herein, we established an optical approach using zebrafish, an ideal vertebrate model in optical techniques, neuroscience, and developmental biology. By combining two-photon laser ablation of the inferior olive (IO) and long-term non-invasive imaging of "the whole brain" at a single-cell resolution, we succeeded in visualization of the morphological changes occurring in the IO neuron population and showed at a single-cell level that structural remodeling of the olivocerebellar circuit occurred in a relatively short period. This system, in combination with various functional analyses, represents a novel and powerful approach for uncovering the mechanisms of the cerebellar reserve, and highlights the potential of the zebrafish model to elucidate the organizing principles of neuronal circuits and their homeostasis in health and disease.

ubtor Mutation Causes Motor Hyperactivity by Activating mTOR Signaling in Zebrafish

Mechanistic target of rapamycin (mTOR) signaling governs important physiological and pathological processes key to cellular life. Loss of mTOR negative regulators and subsequent over-activation of mTOR signaling are major causes underlying epileptic encephalopathy. Our previous studies showed that UBTOR/KIAA1024/MINAR1 acts as a negative regulator of mTOR signaling, but whether UBTOR plays a role in neurological diseases remains largely unknown. We therefore examined a zebrafish model and found that ubtor disruption caused increased spontaneous embryonic movement and neuronal activity in spinal interneurons, as well as the expected hyperactivation of mTOR signaling in early zebrafish embryos. In addition, mutant ubtor larvae showed increased sensitivity to the convulsant pentylenetetrazol, and both the motor activity and the neuronal activity were up-regulated. These phenotypic abnormalities in zebrafish embryos and larvae were rescued by treatment with the mTORC1 inhibitor rapamycin. Taken together, our findings show that ubtor regulates motor hyperactivity and epilepsy-like behaviors by elevating neuronal activity and activating mTOR signaling.

Establishment of reporter cells that respond to glucocorticoids by a transposon-mediated promoter-trapping system

Previously, we had established a highly sensitive trap vector system for the efficient isolation of reporter cells for a certain condition of interest. In this study, we used this system to screen reporter cells that express the luciferase and enhanced green fluorescent protein genes in response to dexamethasone, a glucocorticoid receptor agonist to facilitate glucocorticoid signaling research. In total, 10 clones were isolated. The insertion sites of the trap vector were analyzed using 5' rapid amplification of cDNA ends (5' RACE), whereupon LPIN1, PKP2, and FKBP5 were identified as genes that were upregulated by the dexamethasone treatment. Specifically, PKP2 has not previously been focused as a gene that responds to glucocorticoids. The PKP2 mRNA was analyzed and induction of the endogenous gene was confirmed by real-time polymerase chain reaction. Given that PKP2 does not appear to have a consensus glucocorticoid response element (GRE) sequence, this reporter clone could supplement the current GRE-based reporter systems that are prevalently used. Because different clones showed different responses to glucocorticoids, these clones should provide more information than analysis with a single reporter clone. This paper demonstrates that the previously developed trap vector technology can contribute to the rapid construction of drug evaluation systems.

FAM19A5l Affects Mustard Oil-Induced Peripheral Nociception in Zebrafish

Family with sequence similarity 19 (chemokine (C-C motif)-like) member A5 (FAM19A5) is a chemokine-like secretory protein recently identified as involved in the regulation of osteoclast formation, post-injury neointima formation, and depression. Although roles for FAM19A5 have been described in nervous system development and psychiatric disorders, its role in the nervous system remains poorly understood. Here, we analyzed the evolutionary history of FAM19A genes in vertebrates and identified FAM19A5l, a paralogous zebrafish gene originating from a common ancestral FAM19A5 gene. Further, zebrafish FAM19A5l is expressed in trigeminal and dorsal root ganglion neurons as well as distinct neuronal subsets of the central nervous system. Interestingly, FAM19A5l⁺ trigeminal neurons are nociceptive neurons that localized with TRPA1b and TRPV1 and respond to mustard oil treatment. Behavioral analysis further revealed that the nociceptive response to mustard oil decreases in FAM19A5l-knockout zebrafish larvae. In addition, TRPA1b and NGFa mRNA levels are down- and upregulated in FAM19A5l-knockout and -overexpressing transgenic zebrafish, respectively. Together, our data suggest that FAM19A5l plays a role in nociceptive responses to mustard oil by regulating TRPA1b and NGFa expression in zebrafish.

Involvement of Cerebellar Neural Circuits in Active Avoidance Conditioning in Zebrafish

When animals repeatedly receive a combination of neutral conditional stimulus (CS) and aversive unconditional stimulus (US), they learn the relationship between CS and US, and show conditioned fear responses after CS. They show passive responses such as freezing or panic movements (classical or Pavlovian fear conditioning), or active behavioral responses to avoid aversive stimuli (active avoidance). Previous studies suggested the roles of the cerebellum in classical fear conditioning but it remains elusive whether the cerebellum is involved in active avoidance conditioning. In this study, we analyzed the roles of cerebellar neural circuits during active avoidance in adult zebrafish. When pairs of CS (light) and US (electric shock) were administered to wild-type zebrafish, about half of them displayed active avoidance. The expression of botulinum toxin, which inhibits the release of neurotransmitters, in cerebellar granule cells (GCs) or Purkinje cells (PCs) did not affect conditioning-independent swimming behaviors, but did inhibit active avoidance conditioning. Nitroreductase (NTR)-mediated ablation of PCs in adult zebrafish also impaired active avoidance. Furthermore, the inhibited transmission of GCs or PCs resulted in reduced fear-conditioned Pavlovian fear responses. Our findings suggest that the zebrafish cerebellum plays an active role in active avoidance conditioning.

Regenerated interneurons integrate into locomotor circuitry following spinal cord injury

Whereas humans and other adult mammals lack the ability to regain locomotor function after spinal cord injury, zebrafish are able to recover swimming behavior even after complete spinal cord transection. We have previously shown that zebrafish larvae regenerate lost spinal cord neurons within 9 days post-injury (dpi), but it is unknown whether these neurons are physiologically active or integrate into functional circuitry. Here we show that genetically defined premotor interneurons are regenerated in injured spinal cord segments as functional recovery begins. Further, we show that these newly-generated interneurons receive excitatory input and fire synchronously with motor output by 9 dpi. Taken together, our data indicate that regenerative neurogenesis in the zebrafish spinal cord produces interneurons with the ability to integrate into existing locomotor circuitry.

Zebrafish as the toxicant screening model: Transgenic and omics approaches

The production of large amounts of synthetic industrial and biomedical compounds, together with environmental pollutants, poses a risk to our ecosystem and induces negative effects on the health of wildlife and human beings. With the emergence of the global problem of chemical contamination, the adverse biological effects of these chemicals are gaining attention among the scientific communities, industry, governments, and the public. Among these chemicals, endocrine disrupting chemicals (EDCs) are regarded as one of the major global issues that potentially affecting our health. There is an urgent need of understanding the potential hazards of such chemicals. Zebrafish have been widely used in the aquatic toxicology. In this review, we first discuss the strategy of transgenic lines that used in the toxicological studies, followed by summarizing the current omics approaches (transcriptomics, proteomics, metabolomics, and epigenomics) on toxicities of EDCs in this model. We will also discuss the possible transgenerational effects in zebrafish and future prospective of the integrated omics approaches with customized transgenic organism. To conclude, we summarize the current findings in the field, and provide our opinions on future environmental toxicity research in the zebrafish model.