In Vivo Screening Using Transgenic Zebrafish Embryos Reveals New Effects of HDAC Inhibitors Trichostatin A and Valproic Acid on Organogenesis

Comparative toxicological assessment of 2 bisphenols using a systems approach: evaluation of the behavioral and transcriptomic responses of Danio rerio to bisphenol A and tetrabromobisphenol A

The zebrafish (Danio rerio) is becoming a critical component of new approach methods (NAMs) in chemical risk assessment. As a whole organism in vitro NAM, the zebrafish model offers significant advantages over individual cell-line testing, including toxicokinetic and toxicodynamic competencies. A transcriptomic approach not only allows for insight into mechanism of action for both apical endpoints and unobservable adverse outcomes, but also changes in gene expression induced by lower, environmentally relevant concentrations. In this study, we used a larval zebrafish model to assess the behavioral and transcriptomic alterations caused by subphenotypic concentrations of 2 chemicals with the same structural backbone, the endocrine-disrupting chemicals bisphenol A and tetrabromobisphenol A. Following assessment of behavioral toxicity, we used a transcriptomic approach to identify molecular pathways associated with previously described phenotypes. We also determined the transcriptomic point of departure for each chemical by modeling gene expression changes as continuous systems which allows for the identification of a single concentration at which toxic effects can be predicted. This can then be investigated with confirmatory cell-based testing in an integrated approach to testing and assessment to determine risk to human health and the environment with greater confidence. This paper demonstrates the impact of using a multi-faceted approach for evaluating the physiological and neurotoxic effects of exposure to structurally related chemicals. By comparing phenotypic effects with transcriptomic outcomes, we were able to differentiate, characterize, and rank the toxicities of related bisphenols, which demonstrates methodological advantages unique to the larval zebrafish NAM.

Epigenetics and the evolution of form: Experimental manipulation of a chromatin modification causes species-specific changes to the craniofacial skeleton

A central question in biology is the molecular origins of phenotypic diversity. While genetic changes are key to the genotype-phenotype relationship, alterations to chromatin structure and the physical packaging of histone proteins may also be important drivers of vertebrate divergence. We investigate the impact of such an epigenetic mechanism, histone acetylation, within a textbook example of an adaptive radiation. Cichlids of Lake Malawi have adapted diverse craniofacial structures, and here we investigate how histone acetylation influences morphological variation in these fishes. Specifically, we assessed the effect of inhibiting histone deacetylation using the drug trichostatin A (TSA) on developing facial structures. We examined this during three critical developmental windows in two cichlid species with alternate adult morphologies. Exposure to TSA during neural crest cell (NCC) migration and as postmigratory NCCs proliferate in the pharyngeal arches resulted in significant changes in lateral and ventral shape in Maylandia, but not in Tropheops. This included an overall shortening of the head, widening of the lower jaw, and steeper craniofacial profile, all of which are paedomorphic morphologies. In contrast, treatment with TSA during early chondrogenesis did not result in significant morphological changes in either species. Together, these data suggest a sensitivity to epigenetic alterations that are both time- and species-dependent. We find that morphologies are due to nonautonomous or potentially indirect effects on NCC development, including in part a global developmental delay. Our research bolsters the understanding that proper histone acetylation is essential for early craniofacial development and identifies a species-specific robustness to developmental change. Overall, this study demonstrates how epigenetic regulation may play an important role in both generating and buffering morphological variation.

Sodium valproate exposure influences the expression of pparg in the zebrafish model

Valproic acid (VPA) is an anti-epileptic drug used alone or in combination with other medications to treat seizures, mania, and bipolar disorder. VPA recognized as a teratogenic chemical can cause severe birth defects mainly affecting the brain and spinal cord when administered during pregnancy. However, the potential mechanisms of developmental toxicity are still less studied, and in the present study, the influence of VPA exposure was evaluated on zebrafish early-life stages. Zebrafish were exposed to two sublethal concentrations of sodium valproate (SV) (0.06 mM and 0.15 mM) from 24 hours post-fertilization (hpf) to 96 hpf and the SV teratogenic potential was investigated through morphometric analysis of zebrafish larvae combined with the evaluation of cartilage profile. Moreover, the effect of SV on the transcription level of pparg was also performed. The results of the study showed the teratogenic potential of SV, which disrupts the morphometric signature of the head and body. The marked distortion of cartilage structures was paralleled to a malformation of telencephalon and optic tectum in both concentrations suggesting a high teratogen effect of SV on the brain. These data were further confirmed by the increased expression of pparg in the zebrafish head. Overall, the present study confirms the teratogenic activity of SV in the zebrafish model and, for the first time, points out the potential protective role of pparg in the SV dose-dependent toxicity.

Potential of the zebrafish (Danio rerio) embryo test to discriminate between chemicals of similar molecular structure-a study with valproic acid and 14 of its analogues

Valproic acid is a frequently used antiepileptic drug and known pediatric hepatotoxic agent. In search of pharmaceuticals with increased effectiveness and reduced toxicity, analogue chemicals came into focus. So far, toxicity and teratogenicity data of drugs and metabolites have usually been collected from mammalian model systems such as mice and rats. However, in an attempt to reduce mammalian testing while maintaining the reliability of toxicity testing of new industrial chemicals and drugs, alternative test methods are being developed. To this end, the potential of the zebrafish (Danio rerio) embryo to discriminate between valproic acid and 14 analogues was investigated by exposing zebrafish embryos for 120 h post fertilization in the extended version of the fish embryo acute toxicity test (FET; OECD TG 236), and analyzing liver histology to evaluate the correlation of liver effects and the molecular structure of each compound. Although histological evaluation of zebrafish liver did not identify steatosis as the prominent adverse effect typical in human and mice, the structure–activity relationship (SAR) derived was comparable not only to human HepG2 cells, but also to available in vivo mouse and rat data. Thus, there is evidence that zebrafish embryos might serve as a tool to bridge the gap between subcellular, cell-based systems and vertebrate models.

Neurotoxic effects in zebrafish embryos by valproic acid and nine of its analogues: the fish-mouse connection?

Since teratogenicity testing in mammals is a particular challenge from an animal welfare perspective, there is a great need for the development of alternative test systems. In this context, the zebrafish (Danio rerio) embryo has received increasing attention as a non-protected embryonic vertebrate in vivo model. The predictive power of zebrafish embryos for general vertebrate teratogenicity strongly depends on the correlation between fish and mammals with respect to both overall general toxicity and more specific endpoints indicative of certain modes-of-action. The present study was designed to analyze the correlation between (1) effects of valproic acid and nine of its analogues in zebrafish embryos and (2) their known neurodevelopmental effects in mice. To this end, zebrafish embryos exposed for 120 h in an extended version of the acute fish embryo toxicity test (FET; OECD TG 236) were analyzed with respect to an extended list of sublethal endpoints. Particular care was given to endpoints putatively related to neurodevelopmental toxicity, namely jitter/tremor, deformation of sensory organs (eyes) and craniofacial deformation, which might correlate to neural tube defects caused by valproic acid in mammals. A standard evaluation of lethal (LC according to OECD TG 236) and sublethal toxicity (EC) merely indicated that four out of ten compounds tested in zebrafish correlate with positive results in mouse in vivo studies. A detailed assessment of more specific effects, however, namely, jitter/tremor, small eyes and craniofacial deformation, resulted in a correspondence of 75% with in vivo mouse data. A refinement of endpoint analysis from an integration of all observations into one LC_x or EC_x data (as foreseen by current ecotoxicology-driven OECD guidelines) to a differential evaluation of endpoints specific of selected modes-of-action thus increases significantly the predictive power of the zebrafish embryo model for mammalian teratogenicity. However, for some of the endpoints observed, e.g., scoliosis, lordosis, pectoral fin deformation and lack of movement, further experiments are required for the identification of underlying modes-of-action and an unambiguous interpretation of their predictive power for mammalian toxicity.

Micromolar Valproic Acid Doses Preserve Survival and Induce Molecular Alterations in Neurodevelopmental Genes in Two Strains of Zebrafish Larvae

Autism spectrum disorders (ASDs) comprise a genetically heterogeneous group of conditions characterized by a multifaceted range of impairments and multifactorial etiology. Epidemiological studies have identified valproic acid (VPA), an anticonvulsant used to treat epilepsy, as an environmental factor for ASDs. Based on these observations, studies using embryonic exposure to VPA have been conducted in many vertebrate species to model ASD. The zebrafish is emerging as a popular model in biomedical research to study the molecular pathways involved in nervous system disorders. VPA exposure in zebrafish larvae has been shown to produce a plethora of effects on social, motor and anxiety behavior, and several genetic pathways altered by VPA have been described. However, the doses and regimen of administration reported in the literature are very heterogenous, creating contradictory results and posing serious limits to the interpretation of VPA action on neurodevelopment. To shed light on the toxic effect of VPA, we tested micromolar concentrations of VPA, using exposure for 24 and 48 h in two different zebrafish strains. Our results show that micromolar doses of VPA mildly affect embryo survival but are sufficient to induce molecular alterations in neurodevelopmental genes previously shown to be influenced by VPA, with substantial differences between strains.

Redundant Trojan horse and endothelial-circulatory mechanisms for host-mediated spread of Candida albicans yeast

The host innate immune system has developed elegant processes for the detection and clearance of invasive fungal pathogens. These strategies may also aid in the spread of pathogens in vivo, although technical limitations have previously hindered our ability to view the host innate immune and endothelial cells to probe their roles in spreading disease. Here, we have leveraged zebrafish larvae as a model to view the interactions of these host processes with the fungal pathogen Candida albicans in vivo. We examined three potential host-mediated mechanisms of fungal spread: movement inside phagocytes in a "Trojan Horse" mechanism, inflammation-assisted spread, and endothelial barrier passage. Utilizing both chemical and genetic tools, we systematically tested the loss of neutrophils and macrophages and the loss of blood flow on yeast cell spread. Both neutrophils and macrophages respond to yeast-locked and wild type C. albicans in our model and time-lapse imaging revealed that macrophages can support yeast spread in a "Trojan Horse" mechanism. Surprisingly, loss of immune cells or inflammation does not alter dissemination dynamics. On the other hand, when blood flow is blocked, yeast can cross into blood vessels but they are limited in how far they travel. Blockade of both phagocytes and circulation reduces rates of dissemination and significantly limits the distance of fungal spread from the infection site. Together, this data suggests a redundant two-step process whereby (1) yeast cross the endothelium inside phagocytes or via direct uptake, and then (2) they utilize blood flow or phagocytes to travel to distant sites.

A whole organism small molecule screen identifies novel regulators of pancreatic endocrine development

An early step in pancreas development is marked by the expression of the transcription factor Pdx1 within the pancreatic endoderm, where it is required for the specification of all endocrine cell types. Subsequently, Pdx1 expression becomes restricted to the β-cell lineage, where it plays a central role in β-cell function. This pivotal role of Pdx1 at various stages of pancreas development makes it an attractive target to enhance pancreatic β-cell differentiation and increase β-cell function. In this study, we used a newly generated zebrafish reporter to screen over 8000 small molecules for modulators of pdx1 expression. We found four hit compounds and validated their efficacy at different stages of pancreas development. Notably, valproic acid treatment increased pancreatic endoderm formation, while inhibition of TGFβ signaling led to α-cell to β-cell transdifferentiation. HC toxin, another HDAC inhibitor, enhances β-cell function in primary mouse and human islets. Thus, using a whole organism screening strategy, this study identified new pdx1 expression modulators that can be used to influence different steps in pancreas and β-cell development.

Histone deacetylase activity mediates thermal plasticity in zebrafish (Danio rerio)

Regulatory mechanisms underlying thermal plasticity determine its evolution and potential to confer resilience to climate change. Here we show that class I and II histone deacetylases (HDAC) mediated thermal plasticity globally by shifting metabolomic profiles of cold acclimated zebrafish (Danio rerio) away from warm acclimated animals. HDAC activity promoted swimming performance, but reduced slow and fast myosin heavy chain content in cardiac and skeletal muscle. HDAC increased sarco-endoplasmic reticulum ATPase activity in cold-acclimated fish but not in warm-acclimated animals, and it promoted cardiac function (heart rate and relative stroke volume) in cold but not in warm-acclimated animals. HDAC are an evolutionarily ancient group of proteins, and our data show that they mediate the capacity for thermal plasticity, although the actual manifestation of plasticity is likely to be determined by interactions with other regulators such as AMP-activated protein kinase and thyroid hormone.

Folic acid-nanoscale gadolinium-porphyrin metal-organic frameworks: fluorescence and magnetic resonance dual-modality imaging and photodynamic therapy in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is the most common primary liver cancer and severely threatens human health. Since the prognosis of advanced HCC remains poor, there is an urgent need to develop new therapeutic approaches. Porphyrin metal-organic frameworks are a class of porous organic-inorganic hybrid functional materials with good biocompatibility.

Methods

Gadolinium-porphyrin metal-organic frameworks were used as a skeleton for folic acid (FA) conjugation to synthesize a novel type of nanoparticle, denoted as folic acid-nanoscale gadolinium-porphyrin metal-organic frameworks (FA-NPMOFs). The FA-NPMOFs were characterized using transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric-differential thermal analysis. The biotoxicity and imaging capability of the FA-NPMOFs were determined using HepG2 cells and embryonic and larval zebrafish. The delivery and photodynamic therapeutic effect of FA-NPMOFs were explored in transgenic zebrafish with doxycycline-induced HCC.

Results

FA-NPMOFs were spherical in structure with good dispersion and water solubility. They showed low biotoxicity, emitted bright red fluorescence, and exhibited an excellent magnetic resonance imaging capability, both in vitro and in vivo. Meanwhile, the FA-NPMOFs exhibited a strong affinity for folate receptor (FR)-expressing cells and were delivered to the tumor site in a targeted manner. Moreover, HCC tumor cells were eliminated following laser irradiation.

Conclusion

FA-NPMOFs can be used for dual-modality imaging and photodynamic therapy in HCC and show promise for use as a carrier in new therapies for HCC and other FR-positive tumors.

Evaluation of the rewarding properties of nicotine and caffeine by implementation of a five-choice conditioned place preference task in zebrafish

The rewarding properties of drugs in zebrafish can be studied using the conditioned place preference (CPP) paradigm. Most devices that have been used for CPP consist of two-half tanks with or without a central chamber. Here we evaluated the rewarding effects of nicotine and caffeine using a tank with five arms distributed radially from a central chamber that we have denoted Fish Tank Radial Maze (FTRM). Zebrafish were trained to associate nicotine or caffeine with a coloured arm. In testing sessions to assess CPP induction, between two and five different arms were available to explore. We found that when offering the two arms, one of them associated to the drug mediating conditioning for 14 days, zebrafish showed nicotine-induced CPP but not caffeine-induced CPP. When zebrafish had the option to explore drug-paired arms together with new coloured arms as putative distractors, the nicotine-CPP strength was maintained for at least three days. The presence of novel environments induced caffeine-CPP, which was still positive after three days of testing sessions. Complementary behavioural data supported these findings. Nicotine-CPP was prevented by the histone deacetylase inhibitor phenylbutyrate administered during conditioning; however, there were no effects on caffeine-CPP. The specific acetylation of lysine 9 in histone 3 (H3-K9) was increased in nicotine-conditioned zebrafish brains. This study suggests that novel environmental cues facilitate drug-environment associations, and hence, the use of drugs of abuse.

Epigenetics in teleost fish: From molecular mechanisms to physiological phenotypes

While the field of epigenetics is increasingly recognized to contribute to the emergence of phenotypes in mammalian research models across different developmental and generational timescales, the comparative biology of epigenetics in the large and physiologically diverse vertebrate infraclass of teleost fish remains comparatively understudied. The cypriniform zebrafish and the salmoniform rainbow trout and Atlantic salmon represent two especially important teleost orders, because they offer the unique possibility to comparatively investigate the role of epigenetic regulation in 3R and 4R duplicated genomes. In addition to their sequenced genomes, these teleost species are well-characterized model species for development and physiology, and therefore allow for an investigation of the role of epigenetic modifications in the emergence of physiological phenotypes during an organism's lifespan and in subsequent generations. This review aims firstly to describe the evolution of the repertoire of genes involved in key molecular epigenetic pathways including histone modifications, DNA methylation and microRNAs in zebrafish, rainbow trout, and Atlantic salmon, and secondly, to discuss recent advances in research highlighting a role for molecular epigenetics in shaping physiological phenotypes in these and other teleost models. Finally, by discussing themes and current limitations of the emerging field of teleost epigenetics from both theoretical and technical points of view, we will highlight future research needs and discuss how epigenetics will not only help address basic research questions in comparative teleost physiology, but also inform translational research including aquaculture, aquatic toxicology, and human disease.

Histone deacetylases and their roles in mineralized tissue regeneration

Histone acetylation is an important epigenetic mechanism that controls expression of certain genes. It includes non-sequence-based changes of chromosomal regional structure that can alter the expression of genes. Acetylation of histones is controlled by the activity of two groups of enzymes: the histone acetyltransferases (HATs) and histone deacetylases (HDACs). HDACs remove acetyl groups from the histone tail, which alters its charge and thus promotes compaction of DNA in the nucleosome. HDACs render the chromatin structure into a more compact form of heterochromatin, which makes the genes inaccessible for transcription. By altering the transcriptional activity of bone-associated genes, HDACs control both osteogenesis and osteoclastogenesis. This review presents an overview of the function of HDACs in the modulation of bone formation. Special attention is paid to the use of HDAC inhibitors in mineralized tissue regeneration from cells of dental origin.

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HDACs regulate the transcription activity of bone related genes.

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Inhibition of HDAC promotes osteogenic/odontogenic differentiation.

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HDAC inhibitors are applicable for mineral tissue regeneration therapy.

Molecular Mechanisms of Developmental Toxicity Induced by Graphene Oxide at Predicted Environmental Concentrations

Developmental toxicity is a critical issue in nanotoxicity. However, very little is known about the effects of graphene oxide (GO, a widely used carbon material) at predicted environmental concentrations on biological development or the specific molecular mechanisms. The present study established that the development of zebrafish embryos exposed to trace concentrations (1-100 μg/L) of GO was impaired because of DNA modification, protein carbonylation and excessive generation of reactive oxygen species (ROS), especially the superoxide radical. Noticeably, there was a nonmonotonic response of zebrafish developmental toxicity to GO at μg/L to mg/L levels. Transcriptomics analysis revealed that disturbing collagen- and matrix metalloproteinase (MMP)-related genes affected the skeletal and cardiac development of zebrafish. Moreover, metabolomics analysis showed that the inhibition of amino acid metabolism and the ratios of unsaturated fatty acids (UFAs) to saturated fatty acids (SFAs) contributed to the above developmental toxicity. The present work verifies the developmental toxicity of GO at trace concentrations and illustrates for the first time the specific molecular mechanisms thereof. Because of the potential developmental toxicity of GO at trace concentrations, government administrators and nanomaterial producers should consider its potential risks prior to the widespread environmental exposure to GO.

Comparative study of herbal plants on the phenolic and flavonoid content, antioxidant activities and toxicity on cells and zebrafish embryo

Natural antioxidants derived from plants have shown a tremendous inhibitory effect on free radicals in actively metabolizing cells. Overproduction of free radicals increases the risk factor of chronic diseases associated with diabetes, cancer, arthritis and cardiovascular disease. Andrographis paniculata, Cinnamon zeylanicum, Curcuma xanthorrhiza, Eugenia polyantha and Orthosiphon stamineus are ethnomedicinal plants used in the Asian region to treat various illnesses from a common fever to metabolic disease. In this study, we have quantified the total phenolic (TPC) and flavonoid content (TFC) in these plants and its inhibitory effect on 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals as well as the cytotoxicity effect on cell lines proliferation and zebrafish embryogenesis. Results showed that Cinnamon zeylanicum and E. polyantha have the highest phenolic and flavonoid content. Furthermore, both herbs significantly inhibited the formation of DPPH and ABTS free radicals. Meanwhile, O. stamineus exhibited minimum cytotoxicity and embryotoxicity on tested models. Good correlation between IC50 of 3T3-L1 cells and LC50 embyrotoxicity was also found. This study revealed the potent activity of antioxidant against free radical and the toxicology levels of the tested herbal plants.

The transcription factor, Nuclear factor, erythroid 2 (Nfe2), is a regulator of the oxidative stress response during Danio rerio development

Development is a complex and well-defined process characterized by rapid cell proliferation and apoptosis. At this stage in life, a developmentally young organism is more sensitive to toxicants as compared to an adult. In response to pro-oxidant exposure, members of the Cap'n'Collar (CNC) basic leucine zipper (b-ZIP) transcription factor family (including Nfe2 and Nfe2-related factors, Nrfs) activate the expression of genes whose protein products contribute to reduced toxicity. Here, we studied the role of the CNC protein, Nfe2, in the developmental response to pro-oxidant exposure in the zebrafish (Danio rerio). Following acute waterborne exposures to diquat or tert-buytlhydroperoxide (tBOOH) at one of three developmental stages, wildtype (WT) and nfe2 knockout (KO) embryos and larvae were morphologically scored and their transcriptomes sequenced. Early in development, KO animals suffered from hypochromia that was made more severe through exposure to pro-oxidants; this phenotype in the KO may be linked to decreased expression of alas2, a gene involved in heme synthesis. WT and KO eleutheroembryos and larvae were phenotypically equally affected by exposure to pro-oxidants, where tBOOH caused more pronounced phenotypes as compared to diquat. Comparing diquat and tBOOH exposed embryos relative to the WT untreated control, a greater number of genes were up-regulated in the tBOOH condition as compared to diquat (tBOOH: 304 vs diquat: 148), including those commonly found to be differentially regulated in the vertebrate oxidative stress response (OSR) (e.g. hsp70.2, txn1, and gsr). When comparing WT and KO across all treatments and times, there were 1170 genes that were differentially expressed, of which 33 are known targets of the Nrf proteins Nrf1 and Nrf2. More specifically, in animals exposed to pro-oxidants a total of 968 genes were differentially expressed between WT and KO across developmental time, representing pathways involved in coagulation, embryonic organ development, body fluid level regulation, erythrocyte differentiation, and oxidation-reduction, amongst others. The greatest number of genes that changed in expression between WT and KO occurred in animals exposed to diquat at 2h post fertilization (hpf). Across time and treatment, there were six genes (dhx40, cfap70, dnajb9b, slc35f4, spi-c, and gpr19) that were significantly up-regulated in KO compared to WT and four genes (fhad1, cyp4v7, nlrp12, and slc16a6a) that were significantly down-regulated. None of these genes have been previously identified as targets of Nfe2 or the Nrf family. These results demonstrate that the zebrafish Nfe2 may be a regulator of both primitive erythropoiesis and the OSR during development.