nacre encodes a zebrafish microphthalmia-related protein that regulates neural-crest-derived pigment cell fate

Curcumin-mediated NRF2 induction limits inflammatory damage in, preclinical models of cystic fibrosis

BACKGROUND

Overactive neutrophilic inflammation causes damage to the airways and death in people with cystic fibrosis (CF), a genetic disorder resulting from mutations in the CFTR gene. Reducing the impact of inflammation is therefore a major concern in CF. Evidence indicates that dysfunctional NRF2 signaling in CF individuals may impair their ability to regulate their oxidative and inflammatory responses, although the role of NRF2 in neutrophil-dominated inflammation and tissue damage associated with CF has not been determined. Therefore, we examined whether curcumin, an activator of NRF2, might provide a beneficial effect in the context of CF.

METHODS

Combining Cftr-depleted zebrafish as an innovative biomedical model with CF patient-derived airway organoids (AOs), we aimed to understand how NRF2 dysfunction leads to abnormal inflammatory status and tissue remodeling and determine the effects of curcumin in reducing inflammation and tissue damage in CF.

RESULTS

We demonstrate that NFR2 is instrumental in regulating neutrophilic inflammation and repair processes in vivo, thereby preventing inflammatory damage. Importantly, curcumin treatment restores NRF2 activity in both CF zebrafish and AOs. Curcumin reduces neutrophilic inflammation in CF context, by rebalancing the production of epithelial ROS and pro-inflammatory cytokines. Furthermore, curcumin improves tissue repair by reducing CF-associated fibrosis. Our findings demonstrate that curcumin prevents CF-mediated inflammation via activating the NRF2 pathway.

CONCLUSIONS

This work highlights the protective role of NRF2 in limiting inflammation and injury and show that therapeutic strategies to normalize NRF2 activity, using curcumin or others NRF2 activators, might simultaneously reduce airway inflammation and damage in CF.

Cutaneous Pigment Cell Distributions and Skin Structure of Xenopus

Pigment cells not only are intrinsic factors to determine animal patterns, but also play vital roles in numerous behavioral and physiological processes as well as health, such as melanomas originating from melanocytes. Model organisms are commonly used to study pigment cell development and the mechanisms underlying related diseases, with zebrafish and mice, and Xenopus being well-established examples. Xenopus tropicalis, a diploid amphibian model, offers advantages such as high fecundity and easily observable pigment cell development. Recent advancements in gene-editing techniques have increased its prominence in research on pigment cell biology and melanoma pathogenesis. Here, we compare the skin pigment cell distribution as well as the skin structure in X. tropicalis, zebrafish, mice, and humans and point out the potential value of using X. tropicalis to model human skin diseases, such as melanoma.

Neural-activity-regulated and glia-mediated control of brain lymphatic development

The nervous system regulates peripheral immune responses under physiological and pathological conditions, but the brain's impact on immune system development remains unknown. Meningeal mural lymphatic endothelial cells (muLECs), embedded in the leptomeninges, form an immune niche surrounding the brain that contributes to brain immunosurveillance. Here, we report that the brain controls the development of muLECs via a specialized glial subpopulation, slc6a11b+ radial astrocytes (RAs), a process modulated by neural activity in zebrafish. slc6a11b+ RAs, with processes extending to the meninges, govern muLEC formation by expressing vascular endothelial growth factor C (vegfc). Moreover, neural activity regulates muLEC development, and this regulation requires Vegfc in slc6a11b+ RAs. Intriguingly, slc6a11b+ RAs cooperate with calcium-binding EGF domain 1 (ccbe1)+ fibroblasts to restrict muLEC growth on the brain surface via controlling mature Vegfc distribution. Thus, our study uncovers a glia-mediated and neural-activity-regulated control of brain lymphatic development and highlights the importance of inter-tissue cellular cooperation in development.

Modeling ANKRD26 5'-UTR mutation-related thrombocytopenia

Mutations in the 5'-untranslated region (5'-UTR) of ankyrin repeat domain-containing protein 26 (ANKRD26) are associated with hereditary thrombocytopenia 2 (THC2). However, the causative role of these mutations and the mechanisms underlying THC2 are not fully established. Here, we report, for the first time, that zebrafish carrying a deletion of two nucleotides (Δ2) in the 5'-UTR of ankrd26 recapitulate some of the key laboratory features of THC2. ankrd26ku6 (homozygous for the Δ2 deletion in the 5'-UTR) fish larvae exhibited significantly increased expression of ankrd26 mRNA and protein. Adult ankrd26ku6 fish exhibited spontaneous thrombocytopenia. Furthermore, the thrombocytes from ankrd26ku6 fish showed enhanced ability to adhere and aggregate on a collagen surface under flow. Proteomic profiling demonstrated marked upregulation of Ninjurin 1 in young thrombocytes from ankrd26ku6 fish compared with those from wild-type controls. The ankrd26ku6 fish with a homozygous nacre allele developed myelodysplastic syndrome at old age. ANKRD26 protein levels were also significantly increased in platelets and plasma from patients with immune thrombotic thrombocytopenic purpura compared with those from unaffected controls. We conclude that ANKRD26 overexpression, resulting from either hereditary or acquired mechanisms, contributes to thrombocytopenia, thrombosis and hematologic malignancies.

The small molecule ML233 is a direct inhibitor of tyrosinase function

Melanogenesis is the biological process regulating the synthesis of melanin pigments in melanocytes. Defective melanogenesis is associated with numerous human skin diseases, including, but not limited to, albinism, vitiligo, melasma, and hypo- and hyperpigmentation disorders. Tyrosinase is the rate-limiting enzyme controlling melanogenesis, and hence tremendous efforts have been made to identify potent and safe inhibitors of tyrosinase function. However, despite decades of research, currently there is no effective treatment that inhibits melanogenesis or tyrosinase activity with no adverse side effects. In this study, we report the characterization of the ML233 chemical as a potent inhibitor of tyrosinase activity in vivo and in vitro. We demonstrate that ML233 reduces melanin production in the zebrafish model with no observable significant toxic side effects, and in murine melanoma cells. We also predict that these effects are mediated through direct tyrosinase-ML233 interaction, i.e., binding of the ML233 molecule to the active site of the protein to inhibit its function. Together, our results reveal that ML233 plays roles in both healthy and pathological skin cells via inhibition of melanin production. ML233-mediated tyrosinase inhibition is a potentially safe and effective approach to alleviate the symptoms of melanocyte-associated diseases and thereby substantially improve human health.

Regulatory and disruptive variants in the CLCN2 gene are associated with modified skin color pattern phenotypes in the corn snake

BACKGROUND

Snakes exhibit a broad variety of adaptive colors and color patterns, generated by the spatial arrangement of chromatophores, but little is known of the mechanisms responsible for these spectacular traits. Here, we investigate a mono-locus trait with two recessive alleles, motley and stripe, that both cause pattern aberrations in the corn snake.

RESULTS

We use mapping-by-sequencing to identify the genomic interval where the causal mutations reside. With our differential gene expression analyses, we find that CLCN2 (Chloride Voltage-Gated Channel 2), a gene within the genomic interval, is significantly downregulated in Motley embryonic skin. Furthermore, we identify the stripe allele as the insertion of an LTR-retrotransposon in CLCN2, resulting in a disruptive mutation of the protein. We confirm the involvement of CLCN2 in color pattern formation by producing knock-out snakes that present a phenotype similar to Stripe. In humans and mice, disruption of CLCN2 results in leukoencephalopathy, as well as retinal and testes degeneration. Our single-cell transcriptomic analyses in snakes reveal that CLCN2 is indeed expressed in chromatophores during embryogenesis and in the adult brain, but the behavior and fertility of Motley and Stripe corn snakes are not impacted.

CONCLUSIONS

Our genomic, transcriptomic, and functional analyses identify a plasma membrane anion channel to be involved in color pattern development in snakes and show that an active LTR-retrotransposon might be a key driver of trait diversification in corn snakes.

Protocol for generating a pericyte reporter zebrafish line Ki(pdgfrb-P2A-GAL4-VP16) using a CRISPR-Cas9-mediated knockin technique

Pericytes, the mural cells that envelop small blood vessels, play crucial roles in the formation of the blood-brain barrier (BBB). Here, we present a protocol for generating a pericyte reporter zebrafish line Ki(pdgfrb-P2A-GAL4-VP16) using a CRISPR-Cas9-mediated knockin technique. We describe steps for identifying efficient single guide RNA (sgRNA), constructing donor plasmid, and generating and maintaining the knockin line. We then detail procedures for in vivo imaging of brain pericytes. This protocol is adaptable for creating other knockin lines for specific cell labeling. For complete details on the use and execution of this protocol, please refer to Zi et al.1.

Dominant Negative Mitf Allele Impacts Melanophore and Xanthophore Development and Reveals Collaborative Interactions With Tfec in Zebrafish Chromatophore Lineages

Ectothermic vertebrates exhibit a diverse array of pigment cell types-chromatophores-that provide valuable opportunities to uncover mechanisms of fate specification and how they evolve. Like melanocytes of mammals, the melanophores of teleosts and other ectotherms depend on basic helix-loop-helix leucine zipper transcription factors encoded by orthologues of MITF. A different chromatophore, the iridescent iridophore, depends on the closely related transcription factor Tfec. Requirements for the specification of other chromatophore lineages remain largely uncertain. Here we identify a new allele of the zebrafish Mitf gene, mitfa, that results in a complete absence of not only melanophores but also yellow-orange xanthophores. Harboring a missense substitution in the DNA-binding domain identical to previously isolated alleles of mouse, we show that this new allele has defects in chromatophore precursor survival and xanthophore differentiation that extend beyond those of mitfa loss-of-function. Additional genetic analyses revealed interactions between Mitfa and Tfec as a likely basis for the observed phenotypes. Our findings point to collaborative roles for Mitfa and Tfec in promoting chromatophore development, particularly in xanthophore lineages, and provide new insights into evolutionary aspects of MITF functions across vertebrates.

Behavioural pharmacology predicts disrupted signalling pathways and candidate therapeutics from zebrafish mutants of Alzheimer's disease risk genes

By exposing genes associated with disease, genomic studies provide hundreds of starting points that should lead to druggable processes. However, our ability to systematically translate these genomic findings into biological pathways remains limited. Here, we combine rapid loss-of-function mutagenesis of Alzheimer's risk genes and behavioural pharmacology in zebrafish to predict disrupted processes and candidate therapeutics. FramebyFrame, our expanded package for the analysis of larval behaviours, revealed that decreased night-time sleep was common to F0 knockouts of all four late-onset Alzheimer's risk genes tested. We developed an online tool, ZOLTAR, which compares any behavioural fingerprint to a library of fingerprints from larvae treated with 3677 compounds. ZOLTAR successfully predicted that sorl1 mutants have disrupted serotonin signalling and identified betamethasone as a drug which normalises the excessive day-time sleep of presenilin-2 knockout larvae with minimal side effects. Predictive behavioural pharmacology offers a general framework to rapidly link disease-associated genes to druggable pathways.

Subsets of extraocular motoneurons produce kinematically distinct saccades during hunting and exploration

Animals construct diverse behavioral repertoires by moving a limited number of body parts with varied kinematics and patterns of coordination. There is evidence that distinct movements can be generated by changes in activity dynamics within a common pool of motoneurons or by selectively engaging specific subsets of motoneurons in a task-dependent manner. However, in most cases, we have an incomplete understanding of the patterns of motoneuron activity that generate distinct actions and of how upstream premotor circuits select and assemble such motor programs. In this study, we used two closely related but kinematically distinct types of saccadic eye movement in larval zebrafish as a model to examine circuit control of movement diversity. In contrast to the prevailing view of a final common pathway, we found that in the oculomotor nucleus, distinct subsets of motoneurons were engaged for each saccade type. This type-specific recruitment was topographically organized and aligned with ultrastructural differences in motoneuron morphology and afferent synaptic innervation. Medially located motoneurons were active for both saccade types, and circuit tracing revealed a type-agnostic premotor pathway that appears to control their recruitment. By contrast, a laterally located subset of motoneurons was specifically active for hunting-associated saccades and received premotor input from pretectal hunting command neurons. Our data support a model in which generalist and action-specific premotor pathways engage distinct subsets of motoneurons to elicit varied movements of the same body part that subserve distinct behavioral functions.

Super-resolution imaging of fast morphological dynamics of neurons in behaving animals

Neurons are best studied in their native states in which their functional and morphological dynamics support animals' natural behaviors. Super-resolution microscopy can potentially reveal these dynamics in higher details but has been challenging in behaving animals due to severe motion artifacts. Here we report multiplexed, line-scanning, structured illumination microscopy, which can tolerate motion of up to 50 μm s-1 while achieving 150-nm and 100-nm lateral resolutions in its linear and nonlinear forms, respectively. We continuously imaged the dynamics of spinules in dendritic spines and axonal boutons volumetrically over thousands of frames and tens of minutes in head-fixed mouse brains during sleep-wake cycles. Super-resolution imaging of axonal boutons revealed spinule dynamics on a scale of seconds. Simultaneous two-color imaging further enabled analyses of the spatial distributions of diverse PSD-95 clusters and opened up possibilities to study their correlations with the structural dynamics of dendrites in the brains of head-fixed awake mice.

The Ednrb-Aim2-AKT axis regulates neural crest-derived melanoblast proliferation during early development

Ednrb is specifically required to develop neural crest (NC) stem cell-derived lineages. However, it is still unknown why Ednrb signaling is only needed for the early development of melanoblasts in the skin and eye. We show that Ednrb is required for the proliferation of melanoblasts during early mouse development. To understand the mechanism of melanoblast proliferation, we found that the gene absent in melanoma 2 (Aim2) is upregulated in Ednrb-deficient NC cells by RNA-sequencing analysis. Consequently, the knockdown or knockout of Aim2 partially rescued the proliferation of Ednrb-deficient melanoblasts. Conversely, the overexpression of Aim2 in melanoblasts suppressed their proliferation. We further show that Ednrb signaling could act through the microRNA miR-196b to block the suppression of melanoblast proliferation by Aim2 in primary NC cell cultures. These results reveal the Ednrb-Aim2-AKT axis in regulating melanocyte development and suggest that Ednrb signaling functions as a negative regulator of Aim2, which inhibits the proliferation of melanoblasts in early development. These findings uncover a previously unreported role for Aim2 outside the inflammasome, showing that it is a significant regulator controlling NC stem cell-derived lineage development.

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

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

Kinematically distinct saccades are used in a context-dependent manner by larval zebrafish

Saccades are rapid eye movements that are used by all species with good vision. In this study, we set out to characterize the complete repertoire of larval zebrafish horizontal saccades to gain insight into their contributions to visually guided behavior and underlying neural control. We identified five saccade types, defined by systematic differences in kinematics and binocular coordination, which were differentially expressed across a variety of behavioral contexts. Conjugate saccades formed a large group that serves at least four functions. These include fast phases of the optokinetic nystagmus, visual scanning in stationary animals, and shifting gaze in coordination with body turns. In addition, we discovered a previously undescribed pattern of eye-body coordination in which small conjugate saccades partially oppose head rotation to maintain gaze during forward locomotion. Convergent saccades were coordinated with body movements to foveate prey targets during hunting. Detailed kinematic analysis showed that conjugate and convergent saccades differed in the millisecond coordination of the eyes and body and followed distinct velocity main sequence relationships. This challenges the prevailing view that all horizontal saccades are controlled by a common brainstem circuit and instead indicates saccade-type-specific neural control.

ABCA7-dependent induction of neuropeptide Y is required for synaptic resilience in Alzheimer's disease through BDNF/NGFR signaling

Genetic variants in ABCA7, an Alzheimer's disease (AD)-associated gene, elevate AD risk, yet its functional relevance to the etiology is unclear. We generated a CRISPR-Cas9-mediated abca7 knockout zebrafish to explore ABCA7's role in AD. Single-cell transcriptomics in heterozygous abca7+/- knockout combined with Aβ42 toxicity revealed that ABCA7 is crucial for neuropeptide Y (NPY), brain-derived neurotrophic factor (BDNF), and nerve growth factor receptor (NGFR) expressions, which are crucial for synaptic integrity, astroglial proliferation, and microglial prevalence. Impaired NPY induction decreased BDNF and synaptic density, which are rescuable with ectopic NPY. In induced pluripotent stem cell-derived human neurons exposed to Aβ42, ABCA7-/- suppresses NPY. Clinical data showed reduced NPY in AD correlated with elevated Braak stages, genetic variants in NPY associated with AD, and epigenetic changes in NPY, NGFR, and BDNF promoters linked to ABCA7 variants. Therefore, ABCA7-dependent NPY signaling via BDNF-NGFR maintains synaptic integrity, implicating its impairment in increased AD risk through reduced brain resilience.

PMEL is involved in snake colour pattern transition from blotches to stripes

Corn snakes are emerging models for animal colouration studies. Here, we focus on the Terrazzo morph, whose skin pattern is characterized by stripes rather than blotches. Using genome mapping, we discover a disruptive mutation in the coding region of the Premelanosome protein (PMEL) gene. Our transcriptomic analyses reveal that PMEL expression is significantly downregulated in Terrazzo embryonic tissues. We produce corn snake PMEL knockouts, which present a comparable colouration phenotype to Terrazzo and the subcellular structure of their melanosomes and xanthosomes is also similarly impacted. Our single-cell expression analyses of wild-type embryonic dorsal skin demonstrate that all chromatophore progenitors express PMEL at varying levels. Finally, we show that in wild-type embryos PMEL-expressing cells are initially uniformly spread before forming aggregates and eventually blotches, as seen in the adults. In Terrazzo embryos, the aggregates fail to form. Our results provide insights into the mechanisms governing colouration patterning in reptiles.

Myron Gordon Award Lecture 2023: Painting the neural crest: How studying pigment cells illuminates neural crest cell biology

It has been 30 (!!) years since I began working on zebrafish pigment cells, as a postdoc in the laboratory of Prof. Christiane Nüsslein-Volhard. There, I participated in the first large-scale mutagenesis screen in zebrafish, focusing on pigment cell mutant phenotypes. The isolation of colourless, shady, parade and choker mutants allowed us (as a postdoc in Prof. Judith Eisen's laboratory, and then in my own laboratory at the University of Bath since 1997) to pursue my ambition to address long-standing problems in the neural crest field. Thus, we have studied how neural crest cells choose individual fates, resulting in our recent proposal of a new, and potentially unifying, model which we call Cyclical Fate Restriction, as well as addressing how pigment cell patterns are generated. A key feature of our work in the last 10 years has been the use of mathematical modelling approaches to clarify our biological models and to refine our interpretations. None of this would have been possible without a hugely talented group of laboratory members and other collaborators from around the world-it has been, and I am sure will continue to be, a pleasure and privilege to work with you all!

Evolutionary Loss of Male-Specific Coloration Is Associated with the Loss of Androgen Receptor Expression in Skin of Sceloporus Lizards

AbstractHormones can induce trait development in one species yet have no effect on the same trait in a closely related species, but the mechanisms underlying these differences are unclear. Here, we compare two closely related lizard species to explore the cellular mechanisms associated with the evolutionary loss of hormonally mediated ventral coloration. The eastern fence lizard (Sceloporus undulatus) has sexually dimorphic blue and black ventral coloration that develops when maturational increases in androgens induce melanin synthesis in males. The closely related striped plateau lizard (Sceloporus virgatus) has sexually monomorphic white ventral skin that does not produce melanin in response to the same signal. We used immunohistochemistry to localize the androgen receptor (AR) in the skin of both species and to test whether the loss of ventral coloration in S. virgatus corresponds to the loss of AR in the skin. We found that the ventral skin of S. virgatus displays little or no AR staining in the pigment cell layer, potentially explaining the loss of androgen sensitivity in this tissue, relative to the robust AR staining in the same layer of S. undulatus. Based on the location of three markers for melanophores (microphthalmia-associated transcription factor, dopachrome tautomerase, and tyrosinase), AR appears to be present in melanophores in S. undulatus. However, we could not detect these melanophore markers in the skin of S. virgatus. Therefore, the evolutionary loss of ventral coloration may have occurred via the loss of the AR-producing melanophore in mature ventral skin, preventing the development of a male-typical trait and sexual dimorphism in this tissue.

Transdifferentiation is temporally uncoupled from progenitor pool expansion during hair cell regeneration in the zebrafish inner ear

Death of mechanosensory hair cells in the inner ear is a common cause of auditory and vestibular impairment in mammals, which have a limited ability to regrow these cells after damage. In contrast, non-mammalian vertebrates, including zebrafish, can robustly regenerate hair cells after severe organ damage. The zebrafish inner ear provides an understudied model system for understanding hair cell regeneration in organs that are highly conserved with their mammalian counterparts. Here, we quantitatively examine hair cell addition during growth and regeneration of the larval zebrafish inner ear. We used a genetically encoded ablation method to induce hair cell death and we observed gradual regeneration with correct spatial patterning over a 2-week period following ablation. Supporting cells, which surround and are a source of new hair cells, divide in response to hair cell ablation, expanding the possible progenitor pool. In parallel, nascent hair cells arise from direct transdifferentiation of progenitor pool cells temporally uncoupled from supporting cell division. These findings reveal a previously unrecognized mechanism of hair cell regeneration with implications for how hair cells may be encouraged to regenerate in the mammalian ear.

In vivo multiscale analyses of spring viremia of carp virus (SVCV) infection: From model organism to target species

Spring viremia of carp virus (SVCV) has a broad fish host spectrum and is responsible for a disease that generally affects juvenile fishes with a mortality rate of up to 90%. In the absence of treatments or vaccines against SVCV, the search for prophylactic or therapeutic solutions is thus relevant, particularly to identify solutions compatible with mass vaccination. In addition to being a threat to aquaculture and ecosystems, SVCV is a unique pathogen to study virus-host interactions in the zebrafish model. Establishing the first reverse genetics system for SVCV and the design of recombinant SVCV (rSVCV) expressing fluorescent or bioluminescent proteins adds a new dimension for the study of these interactions using innovative imaging techniques. The infection by bath immersion of zebrafish larvae with rSVCV expressing mCherry allows us to define the first SVCV replication sites and the host innate immune responses using different transgenic lines of zebrafish. The fins were found as the main initial sites of infection in both zebrafish and carp, its natural host. Hence, new insights into the physiopathology of SVCV infection have been described. We report that neutrophils are recruited at the sites of infection and persist up to the death of the animal leading to an uncontrolled inflammation correlated with the expression of the pro-inflammatory cytokine IL1β. Tissue damage was observed at the site of initial replication, a likely consequence of virus-induced injury or the pro-inflammatory response. Interestingly, SVCV infection by bath immersion triggers a persistent pro-inflammatory response rather than activation of the antiviral IFN signaling pathway as observed following intravenous injection, highlighting the importance of the route of infection on the progression of pathogenicity. Thus, this model of zebrafish larvae infection by rSVCV offers new perspectives to study in detail virus-host interactions and to discover new prophylactic or therapeutic solutions.

Highly sensitive response to the toxicity of environmental chemicals in transparent casper zebrafish

The response sensitivity to toxic substances is the most concerned performance of animal model in chemical risk assessment. Casper (mitfaw2/w2;mpv17a9/a9), a transparent zebrafish mutant, is a useful in vivo model for toxicological assessment. However, the ability of casper to respond to the toxicity of exogenous chemicals is unknown. In this study, zebrafish embryos were exposed to five environmental chemicals, chlorpyrifos, lindane, α-endosulfan, bisphenol A, tetrabromobisphenol A (TBBPA), and an antiepileptic drug valproic acid. The half-lethal concentration (LC50) values of these chemicals in casper embryos were 62-87 % of that in the wild-type. After TBBPA exposure, the occurrence of developmental defects in the posterior blood island of casper embryos was increased by 67-77 % in relative to the wild-type, and the half-maximal effective concentration (EC50) in casper was 73 % of that in the wild-type. Moreover, the casper genetic background significantly increased the hyperlocomotion caused by chlorpyrifos and lindane exposure compared with the wild-type. These results demonstrated that casper had greater susceptibility to toxicity than wild-type zebrafish in acute toxicity, developmental toxicity and neurobehavioral toxicity assessments. Our data will inform future toxicological studies in casper and accelerate the development of efficient approaches and strategies for toxicity assessment via the use of casper.

Behavioral control through the direct, focal silencing of neuronal activity

The ability to optically stimulate and inhibit neurons has revolutionized neuroscience research. Here, we present a direct, potent, user-friendly chemical approach for optically silencing neurons. We have rendered saxitoxin (STX), a naturally occurring paralytic agent, transiently inert through chemical protection with a previously undisclosed nitrobenzyl-derived photocleavable group. Exposing the caged toxin, STX-bpc, to a brief (5 ms) pulse of light effects rapid release of a potent STX derivative and transient, spatially precise blockade of voltage-gated sodium channels (NaVs). We demonstrate the efficacy of STX-bpc for parametrically manipulating action potentials in mammalian neurons and brain slice. Additionally, we show the effectiveness of this reagent for silencing neural activity by dissecting sensory-evoked swimming in larval zebrafish. Photo-uncaging of STX-bpc is a straightforward method for non-invasive, reversible, spatiotemporally precise neural silencing without the need for genetic access, thus removing barriers for comparative research.

The evolutionarily conserved choroid plexus contributes to the homeostasis of brain ventricles in zebrafish

The choroid plexus (ChP) produces cerebrospinal fluid (CSF). It also contributes to brain development and serves as the CSF-blood barrier. Prior studies have identified transporters on the epithelial cells that transport water and ions from the blood vasculature to the ventricles and tight junctions involved in the CSF-blood barrier. Yet, how the ChP epithelial cells control brain physiology remains unresolved. We use zebrafish to provide insights into the physiological roles of the ChP. Upon histological and transcriptomic analyses, we identify that the zebrafish ChP is conserved with mammals and expresses transporters involved in CSF secretion. Next, we show that the ChP epithelial cells secrete proteins into CSF. By ablating the ChP epithelial cells, we identify a reduction of the ventricular sizes without alterations of the CSF-blood barrier. Altogether, our findings reveal that the zebrafish ChP is conserved and contributes to the size and homeostasis of the brain ventricles.

A chronic signaling TGFb zebrafish reporter identifies immune response in melanoma

Developmental signaling pathways associated with growth factors such as TGFb are commonly dysregulated in melanoma. Here we identified a human TGFb enhancer specifically activated in melanoma cells treated with TGFB1 ligand. We generated stable transgenic zebrafish with this TGFb Induced Enhancer driving green fluorescent protein (TIE:EGFP). TIE:EGFP was not expressed in normal melanocytes or early melanomas but was expressed in spatially distinct regions of advanced melanomas. Single-cell RNA-sequencing revealed that TIE:EGFP+ melanoma cells down-regulated interferon response while up-regulating a novel set of chronic TGFb target genes. ChIP-sequencing demonstrated that AP-1 factor binding is required for activation of chronic TGFb response. Overexpression of SATB2, a chromatin remodeler associated with tumor spreading, showed activation of TGFb signaling in early melanomas. Confocal imaging and flow cytometric analysis showed that macrophages localize to TIE:EGFP+ regions and preferentially phagocytose TIE:EGFP+ melanoma cells compared to TIE:EGFP- melanoma cells. This work identifies a TGFb induced immune response and demonstrates the need for the development of chronic TGFb biomarkers to predict patient response to TGFb inhibitors.

Identification of pigmentation genes in skin, muscle and tail of a Thai-flag variety of Siamese fighting fish Betta splendens

Pigmentation genes expressed in skin, body muscle and tail of Thai-flag compared with Blue, White and Red varieties of Siamese fighting fish Betta splendens were identified. In total, 22,919 new unigenes were found. Pearson correlation and PCA analysis revealed that expression profiles of genes in muscle, skin and tail across solid color variety were similar. In contrast, those in skin and red tail part of Thai-flag were closely related but they showed different expression profiles with the white tail part. Moreover, 21,347-64,965 SNPs were identified in exonic regions of identified genes. In total, 28,899 genes were differentially expressed between paired comparisons of libraries where 13,907 genes (48.12 %) were upregulated and 14,992 genes (51.88 %) were downregulated. DEGs between paired libraries were 106-5775 genes relative to the compared libraries (56-2982 and 50-2782 for upregulated and downregulated DEGs). Interestingly, 432 pigmentation genes of B. splendens were found. Of these, 297 DEGs showed differential expression between varieties. Many DEGs in melanogenesis (Bsmcr1r, Bsmcr5r, and Bsslc2a15b), tyrosine metabolism (Bstyr, Bstyrp1b and Bsdct), stripe repressor (BsAsip1 and BsAsip2b), pteridine (Bsgch2) and carotenoid (BsBco2) biosynthesis were downregulated in the Thai-flag compared with solid color varieties. Expression of Bsbco1l, Bsfrem2b, Bskcnj13, Bszic2a and Bspah in skin, muscle and tail of Thai-flag, Blue, Red and White varieties was analyzed by qRT-PCR and revealed differential expression between fish varieties and showed anatomical tissue-preferred expression patterns in the same fish variety. The information could be applied to assist genetic-based development of new B. splendens varieties in the future.

Sleep pressure modulates single-neuron synapse number in zebrafish

Sleep is a nearly universal behaviour with unclear functions1. The synaptic homeostasis hypothesis proposes that sleep is required to renormalize the increases in synaptic number and strength that occur during wakefulness2. Some studies examining either large neuronal populations3 or small patches of dendrites4 have found evidence consistent with the synaptic homeostasis hypothesis, but whether sleep merely functions as a permissive state or actively promotes synaptic downregulation at the scale of whole neurons is unclear. Here, by repeatedly imaging all excitatory synapses on single neurons across sleep-wake states of zebrafish larvae, we show that synapses are gained during periods of wake (either spontaneous or forced) and lost during sleep in a neuron-subtype-dependent manner. However, synapse loss is greatest during sleep associated with high sleep pressure after prolonged wakefulness, and lowest in the latter half of an undisrupted night. Conversely, sleep induced pharmacologically during periods of low sleep pressure is insufficient to trigger synapse loss unless adenosine levels are boosted while noradrenergic tone is inhibited. We conclude that sleep-dependent synapse loss is regulated by sleep pressure at the level of the single neuron and that not all sleep periods are equally capable of fulfilling the functions of synaptic homeostasis.

The Calmodulin-interacting peptide Pcp4a regulates feeding state-dependent behavioral choice in zebrafish

Animals constantly need to judge the valence of an object in their environment: is it potential food or a threat? The brain makes fundamental decisions on the appropriate behavioral strategy by integrating external information from sensory organs and internal signals related to physiological needs. For example, a hungry animal may take more risks than a satiated one when deciding to approach or avoid an object. Using a proteomic profiling approach, we identified the Calmodulin-interacting peptide Pcp4a as a key regulator of foraging-related decisions. Food intake reduced abundance of protein and mRNA of pcp4a via dopamine D2-like receptor-mediated repression of adenylate cyclase. Accordingly, deleting the pcp4a gene made zebrafish larvae more risk averse in a binary decision assay. Strikingly, neurons in the tectum became less responsive to prey-like visual stimuli in pcp4a mutants, thus biasing the behavior toward avoidance. This study pinpoints a molecular mechanism modulating behavioral choice according to internal state.

Live Imaging of Cutaneous Wound Healing after Rotary Tool Injury in Zebrafish

Cutaneous wounds are common afflictions that follow a stereotypical healing process involving hemostasis, inflammation, proliferation, and remodeling phases. In the elderly and those suffering from vascular or metabolic diseases, poor healing after cutaneous injuries can lead to open chronic wounds susceptible to infection. The discovery of new therapeutic strategies to improve this defective wound healing requires a better understanding of the cellular behaviors and molecular mechanisms that drive the different phases of wound healing and how these are altered with age or disease. The zebrafish provides an ideal model for visualization and experimental manipulation of the cellular and molecular events during wound healing in the context of an intact, living vertebrate. To facilitate studies of cutaneous wound healing in zebrafish, we have developed an inexpensive, simple, and effective method for generating reproducible cutaneous injuries in adult zebrafish using a rotary tool. We demonstrate that our injury system can be used in combination with high-resolution live imaging to monitor skin re-epithelialization, immune cell recruitment and activation, and vessel regrowth in the same animal over time. This injury system provides a valuable experimental platform to study key cellular and molecular events during wound healing in vivo with unprecedented resolution.

Transcriptomic and Physiological Analysis Reveal Melanin Synthesis-Related Genes and Pathways in Pacific Oysters (Crassostrea gigas)

Shell color is one of the shell traits of molluscs, which has been regarded as an economic trait in some bivalves. Pacific oysters (Crassostrea gigas) are important aquaculture shellfish worldwide. In the past decade, several shell color strains of C. gigas were developed through selective breeding, which provides valuable materials for research on the inheritance pattern and regulation mechanisms of shell color. The inheritance patterns of different shell colors in C. gigas have been identified in certain research; however, the regulation mechanism of oyster pigmentation and shell color formation remains unclear. In this study, we performed transcriptomic and physiological analyses using black and white shell oysters to investigate the molecular mechanism of melanin synthesis in C. gigas. Several pigmentation-related pathways, such as cytochrome P450, melanogenesis, tyrosine metabolism, and the cAMP signaling pathway were found. The majority of differentially expressed genes and some signaling molecules from these pathways exhibited a higher level in the black shell oysters than in the white, especially after L-tyrosine feeding, suggesting that those differences may cause a variation of tyrosine metabolism and melanin synthesis. In addition, the in vitro assay using primary cells from mantle tissue showed that L-tyrosine incubation increased cAMP level, gene and protein expression, and melanin content. This study reveals the difference in tyrosine metabolism and melanin synthesis in black and white shell oysters and provides evidence for the potential regulatory mechanism of shell color in oysters.

Revealing mitf functions and visualizing allografted tumor metastasis in colorless and immunodeficient Xenopus tropicalis

Transparent immunodeficient animal models not only enhance in vivo imaging investigations of visceral organ development but also facilitate in vivo tracking of transplanted tumor cells. However, at present, transparent and immunodeficient animal models are confined to zebrafish, presenting substantial challenges for real-time, in vivo imaging studies addressing specific biological inquiries. Here, we employed a mitf-/-/prkdc-/-/il2rg-/- triple-knockout strategy to establish a colorless and immunodeficient amphibian model of Xenopus tropicalis. By disrupting the mitf gene, we observed the loss of melanophores, xanthophores, and granular glands in Xenopus tropicalis. Through the endogenous mitf promoter to drive BRAFV600E expression, we confirmed mitf expression in melanophores, xanthophores and granular glands. Moreover, the reconstruction of the disrupted site effectively reinstated melanophores, xanthophores, and granular glands, further highlighting the crucial role of mitf as a regulator in their development. By crossing mitf-/- frogs with prkdc-/-/il2rg-/- frogs, we generated a mitf-/-/prkdc-/-/il2rg-/- Xenopus tropicalis line, providing a colorless and immunodeficient amphibian model. Utilizing this model, we successfully observed intravital metastases of allotransplanted xanthophoromas and migrations of allotransplanted melanomas. Overall, colorless and immunodeficient Xenopus tropicalis holds great promise as a valuable platform for tumorous and developmental biology research.

Mitf involved in shell pigmentation by activating tyrosinase-mediated melanin synthesis in Pacific oyster (Crassostrea gigas)

Pigmentation is frequently observed in the molluscan shells, whereas the molecular regulation about these shell pigments formation is not clear. The microphthalmia-associated transcription factor (Mitf) is an important transactivator in melanin synthesis in vertebrates. Here, the Mitf containing a highly conserved basic helix-loop-helixleucine zipper (bHLH-LZ) domain was identified in an economically important marine bivalve Pacific oyster Crassostrea gigas. The Mitf was found to widespread tissue distribution and the expression was higher in the marginal mantle than in the central mantle. Particularly, the expression level of Mitf was high in black shell color oysters compared with white shell oysters. After injecting siRNA, the expression of Mitf decreased significantly, and the efficiency of RNA interference reached 53%. Besides, knockdown Mitf obviously decreased expression of tyrosinase family genes and tyrosinase activity of mantles, indicating a potential regulatory relationship between Mitf and Tyr or Typs. Simultaneously, there was a sharply reduce in the number of the melanosomes in the outer fold of mantle by silencing of Mitf. Luciferase assays in cell culture further verified that Mitf was involved in transcriptional regulation of Typ-2 and Typ-3 genes through binding to their specific promoter regions. These data argue that Mitf is involved in shell pigmentation through activating tyrosinase-mediated melanin synthesis in C. gigas.

Zebrafish as a model for podocyte research

Podocytes, specialized postmitotic cells, are central players in various kidney-related diseases. Zebrafish have become a valuable model system for studying podocyte biology because they are genetically easy to manipulate, transparent, and their glomerular structure is similar to that of mammals. This review provides an overview of the knowledge of podocyte biology in zebrafish larvae, with particular focus on their essential contribution to understanding the mechanisms that underlie kidney diseases as well as supporting drug development. In addition, special attention is given to advances in live-imaging techniques allowing the observation of dynamic processes, including podocyte motility, podocyte process behavior, and glomerulus maturation. The review further addresses the functional aspects of podocytes in zebrafish larvae. This includes topics such as glomerular filtration, ultrastructural analyses, and evaluation of podocyte response to nephrotoxic insults. Studies presented in this context have provided important insights into the maintenance and resistance of the glomerular filtration barrier in zebrafish larvae and explored the potential transferability of these findings to mammals such as mice, rats, and most importantly, humans. The recent ability to identify potential therapeutic targets represents a promising new way to identify drugs that could effectively treat podocyte-associated glomerulopathies in humans. In summary, this review gives an overview about the importance of zebrafish as a model for podocyte-related disease and targeted drug development. It also highlights the key role of advanced imaging techniques in transparent zebrafish larvae, improving our understanding of glomerular diseases and the significant potential for translation of these findings to humans.

Importin 13-dependent axon diameter growth regulates conduction speeds along myelinated CNS axons

Axon diameter influences the conduction properties of myelinated axons, both directly, and indirectly through effects on myelin. However, we have limited understanding of mechanisms controlling axon diameter growth in the central nervous system, preventing systematic dissection of how manipulating diameter affects myelination and conduction along individual axons. Here we establish zebrafish to study axon diameter. We find that importin 13b is required for axon diameter growth, but does not affect cell body size or axon length. Using neuron-specific ipo13b mutants, we assess how reduced axon diameter affects myelination and conduction, and find no changes to myelin thickness, precision of action potential propagation, or ability to sustain high frequency firing. However, increases in conduction speed that occur along single myelinated axons with development are tightly linked to their growth in diameter. This suggests that axon diameter growth is a major driver of increases in conduction speeds along myelinated axons over time.

Recording physiological history of cells with chemical labeling

Recordings of the physiological history of cells provide insights into biological processes, yet obtaining such recordings is a challenge. To address this, we introduce a method to record transient cellular events for later analysis. We designed proteins that become labeled in the presence of both a specific cellular activity and a fluorescent substrate. The recording period is set by the presence of the substrate, whereas the cellular activity controls the degree of the labeling. The use of distinguishable substrates enabled the recording of successive periods of activity. We recorded protein-protein interactions, G protein-coupled receptor activation, and increases in intracellular calcium. Recordings of elevated calcium levels allowed selections of cells from heterogeneous populations for transcriptomic analysis and tracking of neuronal activities in flies and zebrafish.

On the role of TFEC in reptilian coloration

Reptilian species, particularly snakes and lizards, are emerging models of animal coloration. Here, I focus on the role of the TFEC transcription factor in snake and lizard coloration based on a study on wild-type and piebald ball pythons. Genomic mapping previously identified a TFEC mutation linked to the piebald ball python phenotype. The association of TFEC with skin coloration was further supported by gene-editing experiments in the brown anole lizard. However, novel histological analyses presented here reveal discrepancies between the ball python and the anole TFEC mutants phenotype, cautioning against broad generalizations. Indeed, both wild-type and piebald ball pythons completely lack iridophores, whereas the TFEC anole lizard mutants lose their iridophores compared to the wild-type anole. Based on these findings, I discuss the potential role of the MiT/TFE family in skin pigmentation across vertebrate lineages and advocate the need for developmental analyses and additional gene-editing experiments to explore the reptilian coloration diversity.

Cancer Modeling by Transgene Electroporation in Adult Zebrafish (TEAZ)

Transgenic expression of genes is a mainstay of cancer modeling in zebrafish. Traditional transgenic techniques rely upon injection into one-cell embryos, but ideally these transgenes would be expressed only in adult somatic tissues. We provide a method to model cancer in adult zebrafish in which transgenes can be expressed via electroporation. Using melanoma as an example, we demonstrate the feasibility of expressing oncogenes such as BRAFV600E as well as CRISPR/Cas9 inactivation of tumor suppressors such as PTEN. These approaches can be performed in any genetic background such as existing fluorophore reporter lines or the casper line. These methods can readily be extended to other cell types allowing for rapid adult modeling of cancer in zebrafish.

The forkhead transcription factor Foxj1 controls vertebrate olfactory cilia biogenesis and sensory neuron differentiation

In vertebrates, olfactory receptors localize on multiple cilia elaborated on dendritic knobs of olfactory sensory neurons (OSNs). Although olfactory cilia dysfunction can cause anosmia, how their differentiation is programmed at the transcriptional level has remained largely unexplored. We discovered in zebrafish and mice that Foxj1, a forkhead domain-containing transcription factor traditionally linked with motile cilia biogenesis, is expressed in OSNs and required for olfactory epithelium (OE) formation. In keeping with the immotile nature of olfactory cilia, we observed that ciliary motility genes are repressed in zebrafish, mouse, and human OSNs. Strikingly, we also found that besides ciliogenesis, Foxj1 controls the differentiation of the OSNs themselves by regulating their cell type-specific gene expression, such as that of olfactory marker protein (omp) involved in odor-evoked signal transduction. In line with this, response to bile acids, odors detected by OMP-positive OSNs, was significantly diminished in foxj1 mutant zebrafish. Taken together, our findings establish how the canonical Foxj1-mediated motile ciliogenic transcriptional program has been repurposed for the biogenesis of immotile olfactory cilia, as well as for the development of the OSNs.

Radial astrocyte synchronization modulates the visual system during behavioral-state transitions

Glial cells support the function of neurons. Recent evidence shows that astrocytes are also involved in brain computations. To explore whether and how their excitable nature affects brain computations and motor behaviors, we used two-photon Ca2+ imaging of zebrafish larvae expressing GCaMP in both neurons and radial astrocytes (RAs). We found that in the optic tectum, RAs synchronize their Ca2+ transients immediately after the end of an escape behavior. Using optogenetics, ablations, and a genetically encoded norepinephrine sensor, we observed that RA synchronous Ca2+ events are mediated by the locus coeruleus (LC)-norepinephrine circuit. RA synchronization did not induce direct excitation or inhibition of tectal neurons. Nevertheless, it modulated the direction selectivity and the long-distance functional correlations among neurons. This mechanism supports freezing behavior following a switch to an alerted state. These results show that LC-mediated neuro-glial interactions modulate the visual system during transitions between behavioral states.

Comparative Transcriptome Analysis of the Skin and the Peritoneal Wall Layer of Triplophysa stenura Distributed in High Elevations

A total of 81,868 All-Unigenes were sequenced and assembled by the transcriptome in the dorsal skin, the lateral skin, and the peritoneal wall layer of Triplophysa stenura with a total assembly length of 123,827,585 bp, and 68,750 unigenes were annotated to seven functional databases. A total of 588 DEGs were screened between the dorsal and lateral skin, 17,097 DEGs were screened between the dorsal skin and the peritoneal wall layer, and 16,598 DEGs were screened between the lateral skin and the peritoneal wall layer. Most of DEGs in three tissues were annotated to GO terms related to cellular structures, binding, cellular processes, and catalytic activity. They were also annotated to KEGG pathways such as the MAPK signaling pathway, PI3K-Akt signaling pathway, Wnt signaling pathway, melanogenesis, tyrosine metabolism, and cell cycle. A total of twenty-three DEGs were found to be enriched in the melanin synthesis pathway by a local Blast comparison, of which nine DEGs were significantly upregulated in the peritoneal wall layer and six DEGs were significantly upregulated in the dorsal and lateral skin. The results suggest that these genes may be associated with the molecular mechanism of melanin synthesis in T. stenura, and the differential regulation of genes may be related to the differences in UVR intensity and tissue sites of melanin synthesis. Further investigation is needed on how these genes specifically regulate melanin synthesis.

A synaptic corollary discharge signal suppresses midbrain visual processing during saccade-like locomotion

In motor control, the brain not only sends motor commands to the periphery, but also generates concurrent internal signals known as corollary discharge (CD) that influence sensory information processing around the time of movement. CD signals are important for identifying sensory input arising from self-motion and to compensate for it, but the underlying mechanisms remain unclear. Using whole-cell patch clamp recordings from neurons in the zebrafish optic tectum, we discovered an inhibitory synaptic signal, temporally locked to spontaneous and visually driven locomotion. This motor-related inhibition was appropriately timed to counteract visually driven excitatory input arising from the fish's own motion, and transiently suppressed tectal spiking activity. High-resolution calcium imaging revealed localized motor-related signals in the tectal neuropil and the upstream torus longitudinalis, suggesting that CD enters the tectum via this pathway. Together, our results show how visual processing is suppressed during self-motion by motor-related phasic inhibition. This may help explain perceptual saccadic suppression observed in many species.

mitfa deficiency promotes immune vigor and potentiates antitumor effects in zebrafish

The mitfa gene is a well-known transcription factor associated with microphthalmia and is essential for early melanophore development. However, little is known about how mitfa affects the immune system. Here, we generated a novel mitfa knock-out zebrafish line using the CRISPR/Cas9 system. The mitfa-/- zebrafish exhibited reduced melanin levels compared to the nacre mutant. We investigated the impact on the immune system after exposure to Edwardsiella tarda and bifenazate in zebrafish larvae, and observed that the macrophage numbers were reduced in both treated groups. Remarkably, the expression levels of immune-related genes exhibited significant increases after bacterial challenge or bifenazate exposure in the mitfa-/- zebrafish, except for tlr4 and rela. Furthermore, we conducted xenograft experiments using mouse B16 melanoma cells. Notably, the cancer cells didn't show a high cell migration ratio, implying that the immune system was highly activated after the loss of mifta. Taken together, our findings suggest that mitfa-/- zebrafish serve as a valuable model for investigating the relationship between the immune system and melanocytes, providing new insights into the role of mitfa in immune responses.

Charge and lipophilicity are required for effective block of the hair-cell mechano-electrical transducer channel by FM1-43 and its derivatives

The styryl dye FM1-43 is widely used to study endocytosis but behaves as a permeant blocker of the mechano-electrical transducer (MET) channel in sensory hair cells, loading rapidly and specifically into the cytoplasm of hair cells in a MET channel-dependent manner. Patch clamp recordings of mouse outer hair cells (OHCs) were used to determine how a series of structural modifications of FM1-43 affect MET channel block. Fluorescence microscopy was used to assess how the modifications influence hair-cell loading in mouse cochlear cultures and zebrafish neuromasts. Cochlear cultures were also used to evaluate otoprotective potential of the modified FM1-43 derivatives. Structure-activity relationships reveal that the lipophilic tail and the cationic head group of FM1-43 are both required for MET channel block in mouse cochlear OHCs; neither moiety alone is sufficient. The extent of MET channel block is augmented by increasing the lipophilicity/bulkiness of the tail, by reducing the number of positive charges in the head group from two to one, or by increasing the distance between the two charged head groups. Loading assays with zebrafish neuromasts and mouse cochlear cultures are broadly in accordance with these observations but reveal a loss of hair-cell specific labelling with increasing lipophilicity. Although FM1-43 and many of its derivatives are generally cytotoxic when tested on cochlear cultures in the presence of an equimolar concentration of the ototoxic antibiotic gentamicin (5 µM), at a 10-fold lower concentration (0.5 µM), two of the derivatives protect OHCs from cell death caused by 48 h-exposure to 5 µM gentamicin.

A screen of pharmacologically active compounds to identify modulators of the Adgrg6/Gpr126 signalling pathway in zebrafish embryos

Adhesion G protein-coupled receptors (GPCRs) are an underrepresented class of GPCRs in drug discovery. We previously developed an in vivo drug screening pipeline to identify compounds with agonist activity for Adgrg6 (Gpr126), an adhesion GPCR required for myelination of the peripheral nervous system in vertebrates. The screening assay tests for rescue of an ear defect found in adgrg6tb233c-/- hypomorphic homozygous mutant zebrafish, using the expression of versican b (vcanb) mRNA as an easily identifiable phenotype. In the current study, we used the same assay to screen a commercially available library of 1280 diverse bioactive compounds (Sigma LOPAC). Comparison with published hits from two partially overlapping compound collections (Spectrum, Tocris) confirms that the screening assay is robust and reproducible. Using a modified counter screen for myelin basic protein (mbp) gene expression, we have identified 17 LOPAC compounds that can rescue both inner ear and myelination defects in adgrg6tb233c-/- hypomorphic mutants, three of which (ebastine, S-methylisothiourea hemisulfate, and thapsigargin) are new hits. A further 25 LOPAC hit compounds were effective at rescuing the otic vcanb expression but not mbp. Together, these and previously identified hits provide a wealth of starting material for the development of novel and specific pharmacological modulators of Adgrg6 receptor activity.

A gene regulatory network combining Pax3/7, Sox10 and Mitf generates diverse pigment cell types in medaka and zebrafish

Neural crest cells generate numerous derivatives, including pigment cells, and are a model for studying how fate specification from multipotent progenitors is controlled. In mammals, the core gene regulatory network for melanocytes (their only pigment cell type) contains three transcription factors, Sox10, Pax3 and Mitf, with the latter considered a master regulator of melanocyte development. In teleosts, which have three to four pigment cell types (melanophores, iridophores and xanthophores, plus leucophores e.g. in medaka), gene regulatory networks governing fate specification are poorly understood, although Mitf function is considered conserved. Here, we show that the regulatory relationships between Sox10, Pax3 and Mitf are conserved in zebrafish, but the role for Mitf is more complex than previously emphasized, affecting xanthophore development too. Similarly, medaka Mitf is necessary for melanophore, xanthophore and leucophore formation. Furthermore, expression patterns and mutant phenotypes of pax3 and pax7 suggest that Pax3 and Pax7 act sequentially, activating mitf expression. Pax7 modulates Mitf function, driving co-expressing cells to differentiate as xanthophores and leucophores rather than melanophores. We propose that pigment cell fate specification should be considered to result from the combinatorial activity of Mitf with other transcription factors.

Seizure-induced increase in microglial cell population in the developing zebrafish brain

Epilepsy is a chronic brain disorder characterized by unprovoked and recurrent seizures, of which 60% are of unknown etiology. Recent studies implicate microglia in the pathophysiology of epilepsy. However, their role in this process, in particular following early-life seizures, remains poorly understood due in part to the lack of suitable experimental models allowing the in vivo imaging of microglial activity. Given the advantage of zebrafish larvae for minimally-invasive imaging approaches, we sought for the first time to describe the microglial responses after acute seizures in two different zebrafish larval models: a chemically-induced epileptic model by the systemic injection of kainate at 3 days post-fertilization, and the didys552 genetic epilepsy model, which carries a mutation in scn1lab that leads to spontaneous epileptiform discharges. Kainate-treated larvae exhibited transient brain damage as shown by increased numbers of apoptotic nuclei as early as one day post-injection, which was followed by an increase in the number of microglia in the brain. A similar microglial phenotype was also observed in didys552-/- mutants, suggesting that microglia numbers change in response to seizure-like activity in the brain. Interestingly, kainate-treated larvae also displayed a decreased seizure threshold towards subsequent pentylenetetrazole-induced seizures, as shown by higher locomotor and encephalographic activity in comparison with vehicle-injected larvae. These results are comparable to kainate-induced rodent seizure models and suggest the suitability of these zebrafish seizure models for future studies, in particular to elucidate the links between epileptogenesis and microglial dynamic changes after seizure induction in the developing brain, and to understand how these modulate seizure susceptibility.

Optogenetic manipulation of Gq- and Gi/o-coupled receptor signaling in neurons and heart muscle cells

G-protein-coupled receptors (GPCRs) transmit signals into cells depending on the G protein type. To analyze the functions of GPCR signaling, we assessed the effectiveness of animal G-protein-coupled bistable rhodopsins that can be controlled into active and inactive states by light application using zebrafish. We expressed Gq- and Gi/o-coupled bistable rhodopsins in hindbrain reticulospinal V2a neurons, which are involved in locomotion, or in cardiomyocytes. Light stimulation of the reticulospinal V2a neurons expressing Gq-coupled spider Rh1 resulted in an increase in the intracellular Ca2+ level and evoked swimming behavior. Light stimulation of cardiomyocytes expressing the Gi/o-coupled mosquito Opn3, pufferfish TMT opsin, or lamprey parapinopsin induced cardiac arrest, and the effect was suppressed by treatment with pertussis toxin or barium, suggesting that Gi/o-dependent regulation of inward-rectifier K+ channels controls cardiac function. These data indicate that these rhodopsins are useful for optogenetic control of GPCR-mediated signaling in zebrafish neurons and cardiomyocytes.

Optogenetic manipulation of neuronal and cardiomyocyte functions in zebrafish using microbial rhodopsins and adenylyl cyclases

Even though microbial photosensitive proteins have been used for optogenetics, their use should be optimized to precisely control cell and tissue functions in vivo. We exploited GtCCR4 and KnChR, cation channelrhodopsins from algae, BeGC1, a guanylyl cyclase rhodopsin from a fungus, and photoactivated adenylyl cyclases (PACs) from cyanobacteria (OaPAC) or bacteria (bPAC), to control cell functions in zebrafish. Optical activation of GtCCR4 and KnChR in the hindbrain reticulospinal V2a neurons, which are involved in locomotion, induced swimming behavior at relatively short latencies, whereas activation of BeGC1 or PACs achieved it at long latencies. Activation of GtCCR4 and KnChR in cardiomyocytes induced cardiac arrest, whereas activation of bPAC gradually induced bradycardia. KnChR activation led to an increase in intracellular Ca2+ in the heart, suggesting that depolarization caused cardiac arrest. These data suggest that these optogenetic tools can be used to reveal the function and regulation of zebrafish neurons and cardiomyocytes.

kcnj13 regulates pigment cell shapes in zebrafish and has diverged by cis-regulatory evolution between Danio species

Teleost fish of the genus Danio are excellent models to study the genetic and cellular bases of pigment pattern variation in vertebrates. The two sister species Danio rerio and Danio aesculapii show divergent patterns of horizontal stripes and vertical bars that are partly caused by the divergence of the potassium channel gene kcnj13. Here, we show that kcnj13 is required only in melanophores for interactions with xanthophores and iridophores, which cause location-specific pigment cell shapes and thereby influence colour pattern and contrast in D. rerio. Cis-regulatory rather than protein coding changes underlie kcnj13 divergence between the two Danio species. Our results suggest that homotypic and heterotypic interactions between the pigment cells and their shapes diverged between species by quantitative changes in kcnj13 expression during pigment pattern diversification.

An inducible model of chronic hyperglycemia

Transgene driven expression of Escherichia coli nitroreductase (NTR1.0) renders animal cells susceptible to the antibiotic metronidazole (MTZ). Many NTR1.0/MTZ ablation tools have been reported in zebrafish, which have significantly impacted regeneration studies. However, NTR1.0-based tools are not appropriate for modeling chronic cell loss as prolonged application of the required MTZ dose (10 mM) is deleterious to zebrafish health. We established that this dose corresponds to the median lethal dose (LD50) of MTZ in larval and adult zebrafish and that it induced intestinal pathology. NTR2.0 is a more active nitroreductase engineered from Vibrio vulnificus NfsB that requires substantially less MTZ to induce cell ablation. Here, we report on the generation of two new NTR2.0-based zebrafish lines in which acute β-cell ablation can be achieved without MTZ-associated intestinal pathology. For the first time, we were able to sustain β-cell loss and maintain elevated glucose levels (chronic hyperglycemia) in larvae and adults. Adult fish showed significant weight loss, consistent with the induction of a diabetic state, indicating that this paradigm will allow the modeling of diabetes and associated pathologies.