Aryl hydrocarbon receptor activation inhibits regenerative growth

Radix Rehmanniae Praeparata promoted zebrafish fin regeneration through aryl hydrocarbon receptor-dependent autophagy

HEADINGS ETHNOPHARMACOLOGICAL RELEVANCE

Rehmanniae Radix Praeparata (RRP), a staple in traditional Chinese medicine, is derived from Rehmannia glutinosa Libosch and is renowned for its wound-healing properties. Despite its clinical prevalence, the molecular mechanisms underlying RRP's wound-healing effects have not been fully elucidated.

AIM OF THE STUDY

This research endeavored to delineate the molecular and cellular mechanisms underlying the beneficial effects of RRP on wound healing, utilizing a zebrafish model.

MATERIALS AND METHODS

Zebrafish larvae at 3 days post-fertilization were amputated at the fin and subsequently treated with RRP. The pro-wound healing and regenerative effects of RRP were evaluated through morphological analysis, assessment of cell proliferation and apoptosis, Additionally, mechanistic insights were gained through a comprehensive approach encompassing network pharmacology analysis, cell tracing, RNA-sequencing, CRISPR/Cas9 gene editing, and pharmacological inhibition.

RESULTS

Our findings demonstrate that RRP significantly accelerates caudal fin regeneration in zebrafish following injury by suppressing cell apoptosis, promoting cell proliferation, and upregulating the expression of regenerative-related genes. Furthermore, RRP triggers autophagy signals during the regenerative process, which is attenuated by the autophagy inhibitor chloroquine (CQ). Notably, the administration of RRP enhances the expression of ahr1 and ahr2 in the regenerating fin. Genetic knockout of ahr1a, ahr1b, or ahr2 using CRISPR/Cas9, or pharmacological blockade of AHR signals with the antagonist CH-223191, diminishes the regenerative potential of RRP. Remarkably, zebrafish lacking ahr2 completely lose their fin regeneration ability. Additionally, inhibition of AHR signaling suppresses autophagy signaling during fin regeneration.

CONCLUSIONS

This study uncovers that RRP stimulates fin regeneration in zebrafish by inducing AHR signals and, at least partially, activating the autophagy process. These findings provide novel insights into the molecular mechanisms underlying the wound-healing effects of RRP and may pave the way for the development of novel therapeutic strategies.

Lingering Effects of Legacy Industrial Pollution on Yellow Perch of the Detroit River

We used yellow perch (Perca flavescens) captured at four sites differing in legacy industrial pollution in the Lake St. Clair-Detroit River system to evaluate the lingering sublethal effects of industrial pollution. We emphasized bioindicators of direct (toxicity) and indirect (chronic stress, impoverished food web) effects on somatic and organ-specific growth (brain, gut, liver, heart ventricle, gonad). Our results show that higher sediment levels of industrial contaminants at the most downstream Detroit River site (Trenton Channel) are associated with increased perch liver detoxification activity and liver size, reduced brain size, and reduced scale cortisol content. Trenton Channel also displayed food web disruption, where adult perch occupied lower trophic positions than forage fish. Somatic growth and relative gut size were lower in perch sampled at the reference site in Lake St. Clair (Mitchell's Bay), possibly because of increased competition for resources. Models used to determine the factors contributing to site differences in organ growth suggest that the lingering effects of industrial pollution are best explained by trophic disruption. Thus, bioindicators of fish trophic ecology may prove advantageous to assess the health of aquatic ecosystems. Environ Toxicol Chem 2023;42:2158-2170. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Developmental Exposure to Kynurenine Affects Zebrafish and Rat Behavior

Proper nutrition and supplementation during pregnancy and breastfeeding are crucial for the development of offspring. Kynurenine (KYN) is the central metabolite of the kynurenine pathway and a direct precursor of other metabolites that possess immunoprotective or neuroactive properties, with the ultimate effect on fetal neurodevelopment. To date, no studies have evaluated the effects of KYN on early embryonic development. Thus, the aim of our study was to determine the effect of incubation of larvae with KYN in different developmental periods on the behavior of 5-day-old zebrafish. Additionally, the effects exerted by KYN administered on embryonic days 1-7 (ED 1-7) on the behavior of adult offspring of rats were elucidated. Our study revealed that the incubation with KYN induced changes in zebrafish behavior, especially when zebrafish embryos or larvae were incubated with KYN from 1 to 72 h post-fertilization (hpf) and from 49 to 72 hpf. KYN administered early during pregnancy induced subtle differences in the neurobehavioral development of adult offspring. Further research is required to understand the mechanism of these changes. The larval zebrafish model can be useful for studying disturbances in early brain development processes and their late behavioral consequences. The zebrafish-medium system may be applicable in monitoring drug metabolism in zebrafish.

Inhibitory effects of estrogenic endocrine disrupting chemicals on fin regeneration in zebrafish are dependent on estrogen receptors

For fish and other aquatic organisms, disrupting their capacity for repair and regeneration will reduce their quality of life and survivorship in the wild. Studies have shown that 17α-ethinylestradiol (EE2), a synthetic estrogenic endocrine disrupting chemical (EEDC), can inhibit caudal fin regeneration in larval zebrafish following fin amputation. However, whether the inhibitory effects of EE2 are dependent on estrogen receptor (ER) remains unknown. Therefore, in this study, amputated zebrafish larvae were exposed to the ER agonist EE2 alone and in combination with the ER antagonist ICI 182,780 (ICI), and the change in regenerative capacity was determined. The inhibition of fin regeneration caused by EE2 alone (100 ng/L) was ameliorated after combination with ICI (30-300 μg/L), and these changes in regeneration-related signaling and the immune system corresponded with morphological observations, implying that the effects of EE2 on regeneration were possibly initiated by the activation of ER. Furthermore, the role of ER was confirmed with a natural ligand of ER, namely, 17β-estradiol (E2), and as expected, the effects of E2 (10, 100 and 1000 ng/L) paralleled those of EE2. In conclusion, EEDCs can disrupt the regenerative capacity in zebrafish, possibly due to the binding and activation of ERs and the consequent alteration of signaling pathways that regulate fin regeneration and immune competence. Given that EEDCs appear to be ubiquitous in the aquatic environment, the risk of these chemicals might be readdressed regarding their potential effects on tissue repair and regeneration.

Aryl Hydrocarbon Receptor: From Homeostasis to Tumor Progression

Transcription factor aryl hydrocarbon receptor (AHR) has emerged as one of the main regulators involved both in different homeostatic cell functions and tumor progression. Being a member of the family of basic-helix-loop-helix (bHLH) transcriptional regulators, this intracellular receptor has become a key member in differentiation, pluripotency, chromatin dynamics and cell reprogramming processes, with plenty of new targets identified in the last decade. Besides this role in tissue homeostasis, one enthralling feature of AHR is its capacity of acting as an oncogene or tumor suppressor depending on the specific organ, tissue and cell type. Together with its well-known modulation of cell adhesion and migration in a cell-type specific manner in epithelial-mesenchymal transition (EMT), this duality has also contributed to the arise of its clinical interest, highlighting a new potential as therapeutic tool, diagnosis and prognosis marker. Therefore, a deregulation of AHR-controlled pathways may have a causal role in contributing to physiological and homeostatic failures, tumor progression and dissemination. With that firmly in mind, this review will address the remarkable capability of AHR to exert a different function influenced by the phenotype of the target cell and its potential consequences.

New insights into benzo[⍺]pyrene osteotoxicity in zebrafish

Persistent and ubiquitous organic pollutants, such as the polycyclic aromatic hydrocarbon benzo[⍺]pyrene (BaP), represent a major threat to aquatic organisms and human health. Beside some well-documented adverse effects on the development and reproduction of aquatic organisms, BaP was recently shown to affect fish bone formation and skeletal development through mechanisms that remain poorly understood. In this work, zebrafish bone-related in vivo assays were used to evaluate the osteotoxic effects of BaP during bone development and regeneration. Acute exposure of zebrafish larvae to BaP from 3 to 6 days post-fertilization (dpf) induced a dose-dependent reduction of the opercular bone size and a depletion of osteocalcin-positive cells, indicating an effect on osteoblast maturation. Chronic exposure of zebrafish larvae to BaP from 3 to 30 dpf affected the development of the axial skeleton and increased the incidence and severity of skeletal deformities. In young adults, BaP affected the mineralization of newly formed fin rays and scales, and impaired fin ray patterning and scale shape, through mechanisms that involve an imbalanced bone remodeling. Gene expression analyses indicated that BaP induced the activation of xenobiotic and metabolic pathways, while negatively impacting extracellular matrix formation and organization. Interestingly, BaP exposure positively regulated inflammation markers in larvae and increased the recruitment of neutrophils. A direct interaction between neutrophils and bone extracellular matrix or bone forming cells was observed in vivo, suggesting a role for neutrophils in the mechanisms underlying BaP osteotoxicity. Our work provides novel data on the cellular and molecular players involved in BaP osteotoxicity and brings new insights into a possible role for neutrophils in inflammatory bone reduction.

Spirulina maxima derived marine pectin promotes the in vitro and in vivo regeneration and wound healing in zebrafish

Purified bioactive components of marine algae have shown great pharmaceutical and biomedical potential, including wound healing activity. However, the activity of Spirulina maxima is the least documented with regard to wound healing potential. In the present study, we investigated the regenerative and wound healing activities of a Spirulina (Arthrospira) maxima based pectin (SmP) using in vitro human dermal fibroblasts (HDFs) and in vivo zebrafish model. SmP treated (12.5-50 μg/mL) HDFs showed increased cell proliferation by 20-40% compared to the untreated HDFs. Moreover, in vitro wound healing results in HDFs demonstrated that SmP decreased the open wound area % in concentration-dependent manner at 12.5 (32%) and 25 μg/mL (12%) compared to the control (44%). Further, zebrafish larvae displayed a greater fin regenerated area in the SmP exposed group at 25 (0.48 mm²) and 50 μg/mL (0.51 mm²), whereas the untreated group had the lowest regenerated area (0.40 mm²) at 3 days post amputation. However, fin regeneration was significantly (P < 0.001) higher only in the SmP treated group at 50 μg/mL. Furthermore, the open skin wound healing % in adult zebrafish was significantly higher (P < 0.05) after topical application (600 μg/fish) of SmP (46%) compared to the control (38%). Upregulation of genes such as tgfβ1, timp2b, mmp9, tnf-α, and il-1β, and chemokines such as cxcl18b, ccl34a.4, and ccl34b.4, in the muscle and kidney tissues of SmP treated fish compared to the respective control group was demonstrated using qRT-PCR. Histological analysis results further supported the rapid epidermal growth and tissue remodeling in SmP treated fish, suggesting that SmP exerts positive effects associated with wound healing. Therefore, SmP can be considered a potential regenerative and wound healing agent.

A Review of the Functional Roles of the Zebrafish Aryl Hydrocarbon Receptors

Over the last 2 decades, the zebrafish (Danio rerio) has emerged as a stellar model for unraveling molecular signaling events mediated by the aryl hydrocarbon receptor (AHR), an important ligand-activated receptor found in all eumetazoan animals. Zebrafish have 3 AHRs-AHR1a, AHR1b, and AHR2, and studies have demonstrated the diversity of both the endogenous and toxicological functions of the zebrafish AHRs. In this contemporary review, we first highlight the evolution of the zebrafish ahr genes, and the characteristics of the receptors including developmental and adult expression, their endogenous and inducible roles, and the predicted ligands from homology modeling studies. We then review the toxicity of a broad spectrum of AHR ligands across multiple life stages (early stage, and adult), discuss their transcriptomic and epigenetic mechanisms of action, and report on any known interactions between the AHRs and other signaling pathways. Through this article, we summarize the promising research that furthers our understanding of the complex AHR pathway through the extensive use of zebrafish as a model, coupled with a large array of molecular techniques. As much of the research has focused on the functions of AHR2 during development and the mechanism of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) toxicity, we illustrate the need to address the considerable knowledge gap in our understanding of both the mechanistic roles of AHR1a and AHR1b, and the diverse modes of toxicity of the various AHR ligands.

Inhibitory effects of polystyrene microplastics on caudal fin regeneration in zebrafish larvae

Microplastic pollution is pervasive in aquatic environments, but the potential effects of microplastics on aquatic organisms are still under debate. Given that tissue damage is unavoidable in fish and the available data mostly concentrate on healthy fish, there is a large chance that the ecotoxicological risk of microplastic pollution is underrated. Therefore, in this study, the effects of microplastics on the regenerative capacity of injured fish were investigated using a zebrafish caudal fin regeneration model. After fin amputation at 72 h post fertilization, the larvae were exposed to polystyrene microplastics (0.1-10 mg/L) with diameters of 50 or 500 nm. Microplastic exposure significantly inhibited fin regeneration, both morphologically and functionally. Furthermore, the signaling networks that regulate fin regeneration, as well as reactive oxygen species signaling and the immune response, both of which are essential for tissue repair and regeneration, were altered. Transcriptomic analyses of the regenerating fin confirmed that genes related to fin regeneration were transcriptionally modulated in response to microplastic exposure and that metabolic pathways were also extensively involved. In conclusion, this study demonstrated for the first time that microplastic exposure could disrupt the regenerative capacity of fish and might eventually impair their fitness in the wild.

A nontoxic fungal natural product modulates fin regeneration in zebrafish larvae upstream of FGF-WNT developmental signaling

BACKGROUND

The regeneration of larvae zebrafish fin emerged as a new model of regeneration in the last decade. In contrast to genetic tools to study fin regeneration, chemical probes to modulate and interrogate regeneration processes are not well developed.

RESULTS

We set up a zebrafish larvae fin regeneration assay system and tested activities of natural product compounds and extracts, prepared from various microbes. Colomitide C, a recently isolated product from a fungus obtained from Antarctica, inhibited larvae fin regeneration. Using fluorescent reporter transgenic lines, we show that colomitide C inhibited fibroblast growth factor (FGF) signaling and WNT/β-catenin signaling, which were activated after larvae fin amputation. By using the endothelial cell reporter line and immunofluorescence, we showed that colomitide C did not affect migration of the blood vessel and nerve into the injured larvae fin. Colomitide C did not show any cytotoxic activities when tested against FGF receptor-amplified human cancer cell lines.

CONCLUSION

Colomitide C, a natural product, modulated larvae fin regeneration likely acting upstream of FGF and WNT signaling. Colomitide C may serve as a template for developing new chemical probes to study regeneration and other biological processes.

Aryl Hydrocarbon Receptor Mediates Larval Zebrafish Fin Duplication Following Exposure to Benzofluoranthenes

The aryl hydrocarbon receptor (AHR) mediates developmental toxicity of several xenobiotic classes including polycyclic aromatic hydrocarbons. Using embryonic zebrafish, we previously identified 4 polycyclic aromatic hydrocarbons that caused a novel phenotype among AHR ligands-growth of a lateral, duplicate caudal fin fold. The window of sensitivity to the most potent inducer of this phenotype, benzo[k]fluoranthene (BkF), was prior to 36 h postfertilization (hpf), although the phenotype was not manifest until 60 hpf. AHR dependency via Ahr2 was demonstrated using morpholino knockdown. Hepatocyte ablation demonstrated that hepatic metabolism of BkF was not required for the phenotype, nor was it responsible for the window of sensitivity. RNA sequencing performed on caudal trunk tissue from BkF-exposed animals collected at 48, 60, 72, and 96 hpf showed upregulation of genes associated with AHR activation, appendage development, and tissue patterning. Genes encoding fibroblast growth factor and bone morphogenic protein ligands, along with retinaldehyde dehydrogenase, were prominently upregulated. Gene Ontology term analysis revealed that upregulated genes were enriched for mesoderm development and fin regeneration, whereas downregulated genes were enriched for Wnt signaling and neuronal development. MetaCore (Clarivate Analytics) systems analysis of orthologous human genes predicted that R-SMADs, AP-1, and LEF1 regulated the expression of an enriched number of gene targets across all time points. Our results demonstrate a novel aspect of AHR activity with implications for developmental processes conserved across vertebrate species.

A comprehensive analysis of AHRR gene as a candidate for cleft lip with or without cleft palate

Cleft lip and palate (CL/P) is among the most common congenital malformations and affects 1 in 700 newborns. CL/P is caused by genetic and environmental factors (maternal smoking, alcohol or drug use and others). Many genes and loci were associated with cleft lip/palate but the amount of heterogeneity justifies identifying new causal genes and variants. AHRR (Aryl-Hydrocarbon Receptor Repressor) gene has recently been related to CL/P however, few functional studies analyze the genotypephenotype interaction of AHRR with CL/P. Several studies associate the molecular pathway of AHRR to CL/P which indicates this gene as a functional candidate in CL/P etiology.

METHODS

Systematic Literature Review was performed using PUBMED database with the keywords cleft lip, cleft palate, orofacial cleft, AHRR and synonyms. SLR resulted in 37 included articles.

RESULTS

AHRR is a positional and functional candidate gene for CL/P. In silico analysis detected interactions with other genes previously associated to CL/P like ARNT and CYP1A1. AHRR protein regulates cellular toxicity through TCDD mediated AHR pathway. Exposure to TCDD in animal embryos is AHR mediated and lead to cleft palate due to palate fusion failure and post fusion rupture. AHRR regulates cellular growth and differentiation, fundamental to lip and palatogenesis. AHRR decreases carcinogenesis and recently a higher tumor risk has been described in CL/P patients and families. AHRR is also a smoking biomarker due to changed methylation sites found in smokers DNA although folate intake may partially revert these methylation alterations. This corroborates the role of maternal smoking and lack of folate supplementation as risk factors for CL/P.

CONCLUSION

This research identified the importance of AHRR in dioxin response and demonstrated an example of genetic and environmental interaction, indispensable in the development of many complex diseases.

Structural, functional, and molecular impact on the cardiovascular system in ApoE-/- mice exposed to aerosol from candidate modified risk tobacco products, Carbon Heated Tobacco Product 1.2 and Tobacco Heating System 2.2, compared with cigarette smoke

AIM

To investigate the molecular, structural, and functional impact of aerosols from candidate modified risk tobacco products (cMRTP), the Carbon Heated Tobacco Product (CHTP) 1.2 and Tobacco Heating System (THS) 2.2, compared with that of mainstream cigarette smoke (CS) on the cardiovascular system of ApoE-/- mice.

METHODS

Female ApoE-/- mice were exposed to aerosols from THS 2.2 and CHTP 1.2 or to CS from the 3R4F reference cigarette for up to 6 months at matching nicotine concentrations. A Cessation and a Switching group (3 months exposure to 3R4F CS followed by filtered air or CHTP 1.2 for 3 months) were included. Cardiovascular effects were investigated by echocardiographic, histopathological, immunohistochemical, and transcriptomics analyses.

RESULTS

Continuous exposure to cMRTP aerosols did not affect atherosclerosis progression, heart function, left ventricular (LV) structure, or the cardiovascular transcriptome. Exposure to 3R4F CS triggered atherosclerosis progression, reduced systolic ejection fraction and fractional shortening, caused heart LV hypertrophy, and initiated significant dysregulation in the transcriptomes of the heart ventricle and thoracic aorta. Importantly, the structural, functional, and molecular changes caused by 3R4F CS were improved in the smoking cessation and switching groups.

CONCLUSION

Exposure to cMRTP aerosols lacked most of the CS exposure-related functional, structural, and molecular effects. Smoking cessation or switching to CHTP 1.2 aerosol caused similar recovery from the 3R4F CS effects in the ApoE-/- model, with no further acceleration of plaque progression beyond the aging-related rate.

The Henna pigment Lawsone activates the Aryl Hydrocarbon Receptor and impacts skin homeostasis

As a first host barrier, the skin is constantly exposed to environmental insults that perturb its integrity. Tight regulation of skin homeostasis is largely controlled by the aryl hydrocarbon receptor (AhR). Here, we demonstrate that Henna and its major pigment, the naphthoquinone Lawsone activate AhR, both in vitro and in vivo. In human keratinocytes and epidermis equivalents, Lawsone exposure enhances the production of late epidermal proteins, impacts keratinocyte differentiation and proliferation, and regulates skin inflammation. To determine the potential use of Lawsone for therapeutic application, we harnessed human, murine and zebrafish models. In skin regeneration models, Lawsone interferes with physiological tissue regeneration and inhibits wound healing. Conversely, in a human acute dermatitis model, topical application of a Lawsone-containing cream ameliorates skin irritation. Altogether, our study reveals how a widely used natural plant pigment is sensed by the host receptor AhR, and how the physiopathological context determines beneficial and detrimental outcomes.

Glucocorticoid receptor-dependent induction of cripto-1 (one-eyed pinhead) inhibits zebrafish caudal fin regeneration

We previously used a chemical genetics approach with the larval zebrafish to identify small molecule inhibitors of tissue regeneration. This led to the discovery that glucocorticoids (GC) block early stages of tissue regeneration by the inappropriate activation of the glucocorticoid receptor (GR). We performed a microarray analysis to identify the changes in gene expression associated with beclomethasone dipropionate (BDP) exposure during epimorphic fin regeneration. Oncofetal cripto-1 showed > eight-fold increased expression in BDP-treated regenerates. We hypothesized that the mis-expression of cripto-1 was essential for BDP to block regeneration. Expression of cripto-1 was not elevated in GR morphants in the presence of BDP indicating that cripto-1 induction was GR-dependent. Partial translational suppression of Cripto-1 in the presence of BDP restored tissue regeneration. Retinoic acid exposure prevented increased cripto-1 expression and permitted regeneration in the presence of BDP. We demonstrated that BDP exposure increased cripto-1 expression in mouse embryonic stem cells and that regulation of cripto-1 by GCs is conserved in mammals.

Coupling Genome-wide Transcriptomics and Developmental Toxicity Profiles in Zebrafish to Characterize Polycyclic Aromatic Hydrocarbon (PAH) Hazard

Polycyclic Aromatic Hydrocarbons (PAHs) are diverse environmental pollutants associated with adverse human health effects. Many studies focus on the carcinogenic effects of a limited number of PAHs and there is an increasing need to understand mechanisms of developmental toxicity of more varied yet environmentally relevant PAHs. A previous study characterized the developmental toxicity of 123 PAHs in zebrafish. Based on phenotypic responses ranging from complete inactivity to acute mortality, we classified these PAHs into eight bins, selected 16 representative PAHs, and exposed developing zebrafish to the concentration of each PAH that induced 80% phenotypic effect. We conducted RNA sequencing at 48 h post fertilization to identify gene expression changes as a result of PAH exposure. Using the Context Likelihood of Relatedness algorithm, we inferred a network that links the PAHs based on coordinated gene responses to PAH exposure. The 16 PAHs formed two broad clusters: Cluster A was transcriptionally more similar to the controls, while Cluster B consisted of PAHs that were generally more developmentally toxic, significantly elevated cyp1a transcript levels, and induced Ahr2-dependent Cyp1a protein expression in the skin confirmed by gene-silencing studies. We found that cyp1a transcript levels were associated with transcriptomic response, but not with PAH developmental toxicity. While all cluster B PAHs predominantly activated Ahr2, they also each enriched unique pathways like ion transport signaling, which likely points to differing molecular events between the PAHs downstream of Ahr2. Thus, using a systems biology approach, we have begun to evaluate, classify, and define mechanisms of PAH toxicity.

Effects of 17α‑ethinylestradiol on caudal fin regeneration in zebrafish larvae

The ability to restore tissue function and morphology after injury is a key advantage of many fish for a greater chance of survival. The tissue regeneration process is regulated by multiple pathways, and it can therefore be hypothesized that environmental contaminants targeting components of these signaling pathways, may disrupt the fish's capability to repair or regenerate. This could lead to higher mortality and eventually even to a decline in populations. In this study, the effects of 17α‑ethinylestradiol (EE2), a synthetic estrogen, were assessed on the regenerative capacity of larval zebrafish. Zebrafish aged 2 hour post fertilization (hpf) were exposed to 1, 10, or 100 ng/L EE2, and the caudal fins were amputated at 72 hpf. It was found that EE2 exposure significantly inhibited fin regeneration and changed locomotor behavior. The transcription levels for most of the genes involved in the signaling networks regulating the fin regeneration, such as axin2, fgfr1, bmp2b and igf2b, were down-regulated in the amputated fish in response to EE2 exposure, which was in contrast to their increased patterns in the vehicle-exposed control fish. Additionally, the mRNA levels of several immune-related genes, such as il-1β, il-6, il-10 and nf-κb2, were significantly decreased after EE2 exposure, accompanied by a lower density of neutrophils migrated into the wound site. In conclusion, the present study indicated for the first time that estrogenic endocrine disrupting chemicals (EEDCs) could inhibit the regenerative capacity of zebrafish, and this effect was speculated to be mediated through the alteration in regeneration-related signaling pathways and immune competence. This work expands our knowledge of the potential effects of EEDCs on injured aquatic organisms, and highlights the ecotoxicological significance of relationships between regenerative process and endocrine system. This study also implies the potential application of fin regeneration assay for assessing immunotoxicity in ecotoxicological risk assessment.

Developmental Regulation of Nuclear Factor Erythroid-2 Related Factors (nrfs) by AHR1b in Zebrafish (Danio rerio)

Interactions between regulatory pathways allow organisms to adapt to their environment and respond to stress. One interaction that has been recently identified occurs between the aryl hydrocarbon receptor (AHR) and the nuclear factor erythroid-2 related factor (NRF) family. Each transcription factor regulates numerous downstream genes involved in the cellular response to toxicants and oxidative stress; they are also implicated in normal developmental pathways. The zebrafish model was used to explore the role of AHR regulation of nrf genes during development and in response to toxicant exposure. To determine if AHR1b is responsible for transcriptional regulation of 6 nrf genes during development, a loss-of-function experiment using morpholino-modified oligonucleotides was conducted followed by a chromatin immunoprecipitation study at the beginning of the pharyngula period (24 h postfertilization). The expression of nrf1a was AHR1b dependent and its expression was directly regulated through specific XREs in its cis-promoter. However, nrf1a expression was not altered by exposure to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), a toxicant and prototypic AHR agonist. The expression of nrf1b, nrf2a, and nfe2 was induced by TCDD, and AHR1b directly regulated their expression by binding to cis-XRE promoter elements. Last, nrf2b and nrf3 were neither induced by TCDD nor regulated by AHR1b. These results show that AHR1b transcriptionally regulates nrf genes under toxicant modulation via binding to specific XREs. These data provide a better understanding of how combinatorial molecular signaling potentially protects embryos from embryotoxic events following toxicant exposure.

sox9b is required in cardiomyocytes for cardiac morphogenesis and function

The high mobility group transcription factor SOX9 is expressed in stem cells, progenitor cells, and differentiated cell-types in developing and mature organs. Exposure to a variety of toxicants including dioxin, di(2-ethylhexyl) phthalate, 6:2 chlorinated polyfluorinated ether sulfonate, and chlorpyrifos results in the downregulation of tetrapod Sox9 and/or zebrafish sox9b. Disruption of Sox9/sox9b function through environmental exposures or genetic mutations produce a wide range of phenotypes and adversely affect organ development and health. We generated a dominant-negative sox9b (dnsox9b) to inhibit sox9b target gene expression and used the Gal4/UAS system to drive dnsox9b specifically in cardiomyocytes. Cardiomyocyte-specific inhibition of sox9b function resulted in a decrease in ventricular cardiomyocytes, an increase in atrial cardiomyocytes, hypoplastic endothelial cushions, and impaired epicardial development, ultimately culminating in heart failure. Cardiomyocyte-specific dnsox9b expression significantly reduced end diastolic volume, which corresponded with a decrease in stroke volume, ejection fraction, and cardiac output. Further analysis of isolated cardiac tissue by RT-qPCR revealed cardiomyocyte-specific inhibition of sox9b function significantly decreased the expression of the critical cardiac development genes nkx2.5, nkx2.7, and myl7, as well as c-fos, an immediate early gene necessary for cardiomyocyte progenitor differentiation. Together our studies indicate sox9b transcriptional regulation is necessary for cardiomyocyte development and function.

Systematic developmental neurotoxicity assessment of a representative PAH Superfund mixture using zebrafish

Superfund sites often consist of complex mixtures of polycyclic aromatic hydrocarbons (PAHs). It is widely recognized that PAHs pose risks to human and environmental health, but the risks posed by exposure to PAH mixtures are unclear. We constructed an environmentally relevant PAH mixture with the top 10 most prevalent PAHs (SM10) from a Superfund site derived from environmental passive sampling data. Using the zebrafish model, we measured body burden at 48 hours post fertilization (hpf) and evaluated the developmental and neurotoxicity of SM10 and the 10 individual constituents at 24 hours post fertilization (hpf) and 5 days post fertilization (dpf). Zebrafish embryos were exposed from 6 to 120 hpf to (1) the SM10 mixture, (2) a variety of individual PAHs: pyrene, fluoranthene, retene, benzo[a]anthracene, chrysene, naphthalene, acenaphthene, phenanthrene, fluorene, and 2-methylnaphthalene. We demonstrated that SM10 and only 3 of the individual PAHs were developmentally toxic. Subsequently, we constructed and exposed developing zebrafish to two sub-mixtures: SM3 (comprised of 3 of the developmentally toxicity PAHs) and SM7 (7 non-developmentally toxic PAHs). We found that the SM3 toxicity profile was similar to SM10, and SM7 unexpectedly elicited developmental toxicity unlike that seen with its individual components. The results demonstrated that the overall developmental toxicity in the mixtures could be explained using the general concentration addition model. To determine if exposures activated the AHR pathway, spatial expression of CYP1A was evaluated in the 10 individual PAHs and the 3 mixtures at 5 dpf. Results showed activation of AHR in the liver and vasculature for the mixtures and some individual PAHs. Embryos exposed to SM10 during development and raised in chemical-free water into adulthood exhibited decreased learning and responses to startle stimulus indicating that developmental SM10 exposures affect neurobehavior. Collectively, these results exemplify the utility of zebrafish to investigate the developmental and neurotoxicity of complex mixtures.

Role of DNA methylation in altered gene expression patterns in adult zebrafish (Danio rerio) exposed to 3, 3', 4, 4', 5-pentachlorobiphenyl (PCB 126)

There is growing evidence that environmental toxicants can affect various physiological processes by altering DNA methylation patterns. However, very little is known about the impact of toxicant-induced DNA methylation changes on gene expression patterns. The objective of this study was to determine the genome-wide changes in DNA methylation concomitant with altered gene expression patterns in response to 3, 3', 4, 4', 5-pentachlorobiphenyl (PCB126) exposure. We used PCB126 as a model environmental chemical because the mechanism of action is well-characterized, involving activation of aryl hydrocarbon receptor, a ligand-activated transcription factor. Adult zebrafish were exposed to 10 nM PCB126 for 24 h (water-borne exposure) and brain and liver tissues were sampled at 7 days post-exposure in order to capture both primary and secondary changes in DNA methylation and gene expression. We used enhanced Reduced Representation Bisulfite Sequencing and RNAseq to quantify DNA methylation and gene expression, respectively. Enhanced reduced representation bisulfite sequencing analysis revealed 573 and 481 differentially methylated regions in the liver and brain, respectively. Most of the differentially methylated regions are located more than 10 kilobases upstream of transcriptional start sites of the nearest neighboring genes. Gene Ontology analysis of these genes showed that they belong to diverse physiological pathways including development, metabolic processes and regeneration. RNAseq results revealed differential expression of genes related to xenobiotic metabolism, oxidative stress and energy metabolism in response to polychlorinated biphenyl exposure. There was very little correlation between differentially methylated regions and differentially expressed genes suggesting that the relationship between methylation and gene expression is dynamic and complex, involving multiple layers of regulation.

Comparative developmental toxicity of a comprehensive suite of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants that occur in complex mixtures. Several PAHs are known or suspected mutagens and/or carcinogens, but developmental toxicity data is lacking for PAHs, particularly their oxygenated and nitrated derivatives. Such data are necessary to understand and predict the toxicity of environmental mixtures. 123 PAHs were assessed for morphological and neurobehavioral effects for a range of concentrations between 0.1 and 50 µM, using a high throughput early-life stage zebrafish assay, including 33 parent, 22 nitrated, 17 oxygenated, 19 hydroxylated, 14 methylated, 16 heterocyclic, and 2 aminated PAHs. Additionally, each PAH was evaluated for AHR activation, by assessing CYP1A protein expression using whole animal immunohistochemistry (IHC). Responses to PAHs varied in a structurally dependent manner. High-molecular weight PAHs were significantly more developmentally toxic than the low-molecular weight PAHs, and CYP1A expression was detected in five distinct tissues, including vasculature, liver, skin, neuromasts and yolk.

Fishing anti(lymph)angiogenic drugs with zebrafish

Zebrafish, an amenable small teleost fish with a complex mammal-like circulatory system, is being increasingly used for drug screening and toxicity studies. It combines the biological complexity of in vivo models with a higher-throughput screening capability compared with other available animal models. Externally growing, transparent embryos, displaying well-defined blood and lymphatic vessels, allow the inexpensive, rapid, and automatable evaluation of drug candidates that are able to inhibit neovascularisation. Here, we briefly review zebrafish as a model for the screening of anti(lymph)angiogenic drugs, with emphasis on the advantages and limitations of the different zebrafish-based in vivo assays.

Lung regeneration after toxic injury is improved in absence of dioxin receptor

Recent experimental evidences from cellular systems and from mammalian and non-mammalian animal models highlight novel functions for the aryl hydrocarbon/dioxin receptor (AhR) in maintaining cell differentiation and tissue homeostasis. Notably, AhR depletion stimulates an undifferentiated and pluripotent phenotype likely associated to a mesenchymal transition in epithelial cells and to increased primary tumorigenesis and metastasis in melanoma. In this work, we have used a lung model of epithelial regeneration to investigate whether AhR regulates proper tissue repair by adjusting the expansion of undifferentiated stem-like cells. AhR-null mice developed a faster and more efficient repair of the lung bronchiolar epithelium upon naphthalene injury that required increased cell proliferation and the earlier activation of stem-like Clara, Basal and neuroepithelial cells precursors. Increased basal content in multipotent Sca1⁺/CD31^-/CD4^- cells and in cells expressing pluripotency factors NANOG and OCT4 could also improve re-epithelialization in AhR-null lungs. The reduced response of AhR-deficient lungs to Sonic Hedgehog (Shh) repression shortly after injury may also help their improved bronchiolar epithelium repair. These results support a role for AhR in the regenerative response against toxins, and open the possibility of modulating its activation level to favor recovery from lesions caused by environmental contaminants.

Dioxin Receptor Adjusts Liver Regeneration After Acute Toxic Injury and Protects Against Liver Carcinogenesis

The aryl hydrocarbon receptor (AhR) has roles in cell proliferation, differentiation and organ homeostasis, including the liver. AhR depletion induces undifferentiation and pluripotency in normal and transformed cells. Here, AhR-null mice (AhR-/-) were used to explore whether AhR controls liver regeneration and carcinogenesis by restricting the expansion of stem-like cells and the expression of pluripotency genes. Short-term CCl₄ liver damage was earlier and more efficiently repaired in AhR-/- than in AhR+/+ mice. Stem-like CK14 + and TBX3 + and pluripotency-expressing OCT4 + and NANOG + cells expanded sooner in AhR-/- than in AhR+/+ regenerating livers. Stem-like side population cells (SP) isolated from AhR-/- livers had increased β-catenin (β-Cat) signaling with overexpression of Axin2, Dkk1 and Cyclin D1. Interestingly, β-Cat, Axin2 and Dkk1 also increased during regeneration but more notably in AhR-null livers. Liver carcinogenesis induced by diethylnitrosamine (DEN) produced large carcinomas in all AhR-/- mice but mostly premalignant adenomas in less than half of AhR+/+ mice. AhR-null tumoral tissue, but not their surrounding non-tumoral parenchyma, had nuclear β-Cat and Axin2 overexpression. OCT4 and NANOG were nevertheless similarly expressed in AhR+/+ and AhR-/- lesions. We suggest that AhR may serve to adjust liver repair and to block tumorigenesis by modulating stem-like cells and β-Cat signaling.

Hyaluronic acid synthesis is required for zebrafish tail fin regeneration

Using genome-wide transcriptional profiling and whole-mount expression analyses of zebrafish larvae, we have identified hyaluronan synthase 3 (has3) as an upregulated gene during caudal fin regeneration. has3 expression is induced in the wound epithelium within hours after tail amputation, and its onset and maintenance requires fibroblast growth factor, phosphoinositide 3-kinase, and transforming growth factor-ß signaling. Inhibition of hyaluronic acid (HA) synthesis by the small molecule 4-methylumbelliferone (4-MU) impairs tail regeneration in zebrafish larvae by preventing injury-induced cell proliferation. In addition, 4-MU reduces the expression of genes associated with wound epithelium and blastema function. Treatment with glycogen synthase kinase 3 inhibitors rescues 4-MU-induced defects in cell proliferation and tail regeneration, while restoring a subset of wound epithelium and blastema markers. Our findings demonstrate a role for HA biosynthesis in zebrafish tail regeneration and delineate its epistatic relationships with other regenerative processes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) increases necroinflammation and hepatic stellate cell activation but does not exacerbate experimental liver fibrosis in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant and high-affinity ligand for the aryl hydrocarbon receptor (AhR). Increasing evidence indicates that AhR signaling contributes to wound healing, which involves the coordinated deposition and remodeling of the extracellular matrix. In the liver, wound healing is attributed to the activation of hepatic stellate cells (HSCs), which mediate fibrogenesis through the production of soluble mediators and collagen type I. We recently reported that TCDD treatment increases the activation of human HSCs in vitro. The goal of this study was to determine how TCDD impacts HSC activation in vivo using a mouse model of experimental liver fibrosis. To elicit fibrosis, C57BL6/male mice were treated twice weekly for 8weeks with 0.5ml/kg carbon tetrachloride (CCl₄). TCDD (20μg/kg) or peanut oil (vehicle) was administered once a week during the last 2weeks. Results indicate that TCDD increased liver-body-weight ratios, serum alanine aminotransferase activity, and hepatic necroinflammation in CCl₄-treated mice. Likewise, TCDD treatment increased mRNA expression of HSC activation and fibrogenesis genes, namely α-smooth muscle actin, desmin, delta-like homolog-1, TGF-β1, and collagen type I. However, TCDD treatment did not exacerbate fibrosis, nor did it increase the collagen content of the liver. Instead, TCDD increased hepatic collagenase activity and increased expression of matrix metalloproteinase (MMP)-13 and the matrix regulatory proteins, TIMP-1 and PAI-1. These results support the conclusion that TCDD increases CCl₄-induced liver damage and exacerbates HSC activation, yet collagen deposition and the development of fibrosis may be limited by TCDD-mediated changes in extracellular matrix remodeling.

Keratinocyte Growth Factor Regulation of Aryl Hydrocarbon Receptor Activation in Colorectal Cancer Cells

BACKGROUND

Keratinocyte growth factor (KGF) stimulates normal growth, development and intestinal epithelial cell proliferation. Cyclin D1 promotes the cell cycle by inhibiting retinoblastoma protein (RB1). The activated aryl hydrocarbon receptor (AhR) has an important influence on the development of tumors through its interactions with the cell cycle.

AIM

The aim of the present study was to explore a new role for AhR in KGF-induced colon cancer cell growth.

MATERIALS AND METHODS

Real-time PCR, western blot or immunofluorescence analysis were used to detect the expression of KGF, AhR, cyclin D1 and CYP1A1. Immunohistochemistry was used to observe the localization of AhR. MTT assay and flow cytometric analyses were performed to measure cell viability and the cell cycle.

RESULTS

Real-time PCR analysis revealed that KGF, AhR, and CYP1A1 mRNAs were overexpressed in colorectal cancer tissues. Meanwhile, overexpression of AhR was primarily observed in epithelial cells. In in vitro assay, KGF promoted colon cancer cell growth, as well as up-regulated and activated AhR. At the same time, AhR-knockdown colon cancer cells were less responsive to KGF. Western blot analysis, real-time PCR, or immunofluorescence data indicated that cyclin D1 expression was up-regulated by KGF but this up-regulation was compromised when AhR was silenced, and the cell cycle was arrested in the G0/G1 stage in these cells.

CONCLUSIONS

Our study suggests that KGF, AhR, and CYP1A1 are overexpressed in colorectal cancer tissues. Moreover, we reveal a new mechanism by which KGF promotes cell proliferation through the AhR-cyclin D1 pathway in colon cancer cells.

Nrf2 and Nrf2-related proteins in development and developmental toxicity: Insights from studies in zebrafish (Danio rerio)

Oxidative stress is an important mechanism of chemical toxicity, contributing to developmental toxicity and teratogenesis as well as to cardiovascular and neurodegenerative diseases and diabetic embryopathy. Developing animals are especially sensitive to effects of chemicals that disrupt the balance of processes generating reactive species and oxidative stress, and those anti-oxidant defenses that protect against oxidative stress. The expression and inducibility of anti-oxidant defenses through activation of NFE2-related factor 2 (Nrf2) and related proteins is an essential process affecting the susceptibility to oxidants, but the complex interactions of Nrf2 in determining embryonic response to oxidants and oxidative stress are only beginning to be understood. The zebrafish (Danio rerio) is an established model in developmental biology and now also in developmental toxicology and redox signaling. Here we review the regulation of genes involved in protection against oxidative stress in developing vertebrates, with a focus on Nrf2 and related cap'n'collar (CNC)-basic-leucine zipper (bZIP) transcription factors. Vertebrate animals including zebrafish share Nfe2, Nrf1, Nrf2, and Nrf3 as well as a core set of genes that respond to oxidative stress, contributing to the value of zebrafish as a model system with which to investigate the mechanisms involved in regulation of redox signaling and the response to oxidative stress during embryolarval development. Moreover, studies in zebrafish have revealed nrf and keap1 gene duplications that provide an opportunity to dissect multiple functions of vertebrate NRF genes, including multiple sensing mechanisms involved in chemical-specific effects.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

Chemical genetics and its potential in cardiac stem cell therapy

Over the last decade or so, intensive research in cardiac stem cell biology has led to significant discoveries towards a potential therapy for cardiovascular disease; the main cause of morbidity and mortality in humans. The major goal within the field of cardiovascular regenerative medicine is to replace lost or damaged cardiac muscle and coronaries following ischaemic disease. At present, de novo cardiomyocytes can be generated either in vitro, for cell transplantation or disease modelling using directed differentiation of embryonic stem cells or induced pluripotent stem cells, or in vivo via direct reprogramming of resident adult cardiac fibroblast or ectopic stimulation of resident cardiac stem or progenitor cells. A major bottleneck with all of these approaches is the low efficiency of cardiomyocyte differentiation alongside their relative functional immaturity. Chemical genetics, and the application of phenotypic screening with small molecule libraries, represent a means to enhance understanding of the molecular pathways controlling cardiovascular cell differentiation and, moreover, offer the potential for discovery of new drugs to invoke heart repair and regeneration. Here, we review the potential of chemical genetics in cardiac stem cell therapy, highlighting not only the major contributions to the field so far, but also the future challenges.

Comparative developmental toxicity of environmentally relevant oxygenated PAHs

Oxygenated polycyclic aromatic hydrocarbons (OPAHs) are byproducts of combustion and photo-oxidation of parent PAHs. OPAHs are widely present in the environment and pose an unknown hazard to human health. The developing zebrafish was used to evaluate a structurally diverse set of 38 OPAHs for malformation induction, gene expression changes and mitochondrial function. Zebrafish embryos were exposed from 6 to 120h post fertilization (hpf) to a dilution series of 38 different OPAHs and evaluated for 22 developmental endpoints. AHR activation was determined via CYP1A immunohistochemistry. Phenanthrenequinone (9,10-PHEQ), 1,9-benz-10-anthrone (BEZO), xanthone (XAN), benz(a)anthracene-7,12-dione (7,12-B[a]AQ), and 9,10-anthraquinone (9,10-ANTQ) were evaluated for transcriptional responses at 48hpf, prior to the onset of malformations. qRT-PCR was conducted for a number of oxidative stress genes, including the glutathione transferase(gst), glutathione peroxidase(gpx), and superoxide dismutase(sod) families. Bioenergetics was assayed to measure in vivo oxidative stress and mitochondrial function in 26hpf embryos exposed to OPAHs. Hierarchical clustering of the structure-activity outcomes indicated that the most toxic of the OPAHs contained adjacent diones on 6-carbon moieties or terminal, para-diones on multi-ring structures. 5-carbon moieties with adjacent diones were among the least toxic OPAHs while the toxicity of multi-ring structures with more centralized para-diones varied considerably. 9,10-PHEQ, BEZO, 7,12-B[a]AQ, and XAN exposures increased expression of several oxidative stress related genes and decreased oxygen consumption rate (OCR), a measurement of mitochondrial respiration. Comprehensive in vivo characterization of 38 structurally diverse OPAHs indicated differential AHR dependency and a prominent role for oxidative stress in the toxicity mechanisms.

Exposure to diesel exhaust particulates induces cardiac dysfunction and remodeling

Chronic exposure to diesel exhaust particulates (DEP) increases the risk of cardiovascular disease in urban residents, predisposing them to the development of several cardiovascular stresses, including myocardial infarctions, arrhythmias, thrombosis, and heart failure. DEP contain a high level of polycyclic aromatic hydrocarbons, which activate the aryl hydrocarbon receptor (AHR). We hypothesize that exposure to DEP elicits ventricular remodeling through the activation of the AHR pathway, leading to ventricular dilation and dysfunction. Male Sprague-Dawley rats were exposed by nose-only nebulization to DEP (SRM 2975, 0.2 mg/ml) or vehicle for 20 min/day × 5 wk. DEP exposure resulted in eccentric left ventricular dilation (8% increased left ventricular internal diameter at diastole and 23% decreased left ventricular posterior wall thickness at diastole vs. vehicle), as shown by echocardiograph assessment. Histological analysis using Picrosirius red staining revealed that DEP reduced cardiac interstitial collagen (23% decrease vs. vehicle). Further assessment of cardiac function using a pressure-volume catheter indicated impaired diastolic function (85% increased end-diastolic pressure and 19% decreased Tau vs. vehicle) and contractility (57 and 48% decreased end-systolic pressure-volume relationship and maximum change in pressure over time vs. end-diastolic volume compared with vehicle, respectively) in the DEP-exposed animals. Exposure to DEP significantly increased cardiac expression of AHR (19% increase vs. vehicle). In addition, DEP significantly decreased the cardiac expression of hypoxia inducible factor-1α, the competitive pathway to the AHR, and vascular endothelial growth factor, a downstream mediator of hypoxia inducible factor-1α (26 and 47% decrease vs. vehicle, respectively). These findings indicate that exposure to DEP induced left ventricular dilation by loss of collagen through an AHR-dependent mechanism.

Global gene expression analysis reveals pathway differences between teratogenic and non-teratogenic exposure concentrations of bisphenol A and 17β-estradiol in embryonic zebrafish

Transient developmental exposure to 0.1μM bisphenol A (BPA) results in larval zebrafish hyperactivity and learning impairments in the adult, while exposure to 80μM BPA results in teratogenic responses, including craniofacial abnormalities and edema. The mode of action underlying these effects is unclear. We used global gene expression analysis to identify candidate genes and signaling pathways that mediate BPA's developmental toxicity in zebrafish. Exposure concentrations were selected and anchored to the positive control, 17β-estradiol (E2), based on previously determined behavioral or teratogenic phenotypes. Functional analysis of differentially expressed genes revealed distinct expression profiles at 24h post fertilization for 0.1μM versus 80μM BPA and 0.1μM versus 15μM E2 exposure, identification of prothrombin activation as a top canonical pathway impacted by both 0.1μM BPA and 0.1μM E2 exposure, and suppressed expression of several genes involved in nervous system development and function following 0.1μM BPA exposure.

TCDD inhibits heart regeneration in adult zebrafish

Normal adult zebrafish can completely regenerate lost myocardium following partial amputation of the ventricle apex. We report that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) significantly impairs this regeneration. Adult male zebrafish were injected with vehicle (control) or TCDD (70ng/g, ip) 1 day prior to partial amputation of the ventricle apex. Gross observation and histological analysis of the amputated heart at 21 days postamputation revealed that TCDD-exposed fish had not progressed beyond the initial clot formation stage, whereas the vehicle control fish showed substantial recovery and almost complete resolution of the formed clot. In contrast, hearts that were not surgically wounded showed no signs of TCDD toxicity. Striking features in the TCDD-exposed hearts were the absence of the normal sheath of new tissue enveloping the wound and the absence of intense cell proliferation at the site of the wound. In addition, the patterns of collagen deposition at the wound site were different between the TCDD and vehicle groups. Because the receptor for TCDD is the aryl hydrocarbon receptor ligand-activated transcriptional regulator, we examined the effects of TCDD exposure on gene expression in the ventricle using DNA microarrays. Samples were collected just prior to amputation and at 6h and 7 days postamputation. TCDD-pretreated hearts had dysregulated expression of genes involved in heart function, tissue regeneration, cell growth, and extracellular matrix. Because embryonic, but not adult, hearts are major targets for TCDD-induced cardiotoxicity, we speculate that the need for embryonic-like cells in regeneration is connected with the effects of TCDD in inhibiting the response to wounding.

Alternate glucocorticoid receptor ligand binding structures influence outcomes in an in vivo tissue regeneration model

Since their characterization, glucocorticoids (GCs), the most commonly prescribed immunomodulatory drugs, have undergone numerous structural modifications designed to enhance their activity. In vivo assessment of these corticosteroid analogs is essential to understand the difference in molecular signaling of the ligands that share the corticosteroid backbone. Our research identified a novel function of GCs as modulators of tissue regeneration and demonstrated that GCs activate the glucocorticoid receptor (GR) to inhibit early stages of tissue regeneration in zebrafish (Danio rerio). We utilized this phenomenon to assess the effect of different GC analogs on tissue regeneration and identified that some GCs such as beclomethasone dipropionate (BDP) possess inhibitory properties, while others, such as dexamethasone and hydrocortisone have no effect on regeneration. We performed in silico molecular docking and dynamic studies and demonstrated that type and size of substitution at the C17 position of the cortisol backbone confer a unique stable conformation to GR on ligand binding that is critical for inhibitory activity. In the field of tissue regeneration, our study is one of the first Structure Activity Relationship (SAR) investigations performed in vertebrates demonstrating that the in vivo tissue regeneration model is a powerful tool to probe structure function relationships, to understand regenerative biology, and to assist in rational drug design.

Potential protective mechanisms of aryl hydrocarbon receptor (AHR) signaling in benign prostatic hyperplasia

The aryl hydrocarbon receptor (AHR) is an evolutionarily conserved ligand activated transcription factor best known for its role in mediating toxic responses to dioxin-like environmental contaminants. However, AHR signaling has also emerged as an active participant in processes of normal development and disease progression. Here, we review the role of AHR signaling in prostate development and disease processes, with a particular emphasis on benign prostatic hyperplasia (BPH). Inappropriate AHR activation has recently been associated with a decreased risk of symptomatic BPH in humans and has been shown to impair prostate development and disrupt endocrine signaling in rodents. We highlight known physiological responses to AHR activation in prostate and other tissues and discuss potential mechanisms by which it may act in adult human prostate to protect against symptomatic BPH.

AHR2 mutant reveals functional diversity of aryl hydrocarbon receptors in zebrafish

The aryl hydrocarbon receptor (AHR) is well known for mediating the toxic effects of TCDD and has been a subject of intense research for over 30 years. Current investigations continue to uncover its endogenous and regulatory roles in a wide variety of cellular and molecular signaling processes. A zebrafish line with a mutation in ahr2 (ahr2(hu3335)), encoding the AHR paralogue responsible for mediating TCDD toxicity in zebrafish, was developed via Targeting Induced Local Lesions IN Genomes (TILLING) and predicted to express a non-functional AHR2 protein. We characterized AHR activity in the mutant line using TCDD and leflunomide as toxicological probes to investigate function, ligand binding and CYP1A induction patterns of paralogues AHR2, AHR1A and AHR1B. By evaluating TCDD-induced developmental toxicity, mRNA expression changes and CYP1A protein in the AHR2 mutant line, we determined that ahr2(hu3335) zebrafish are functionally null. In silico modeling predicted differential binding of TCDD and leflunomide to the AHR paralogues. AHR1A is considered a non-functional pseudogene as it does not bind TCCD or mediate in vivo TCDD toxicity. Homology modeling, however, predicted a ligand binding conformation of AHR1A with leflunomide. AHR1A-dependent CYP1A immunohistochemical expression in the liver provided in vivo confirmation of the in silico docking studies. The ahr2(hu3335) functional knockout line expands the experimental power of zebrafish to unravel the role of the AHR during development, as well as highlights potential activity of the other AHR paralogues in ligand-specific toxicological responses.

TCDD disrupts hypural skeletogenesis during medaka embryonic development

Defective bone and cartilage development account for a large number of human birth defects annually. Normal skeletogenesis involves cartilage development in early morphogenesis through a highly coordinated and orchestrated series of events involving commitment and differentiation of mesenchymal cells to chondrocytes followed by a highly programmed process of structural maturation. Recent developmental studies with laboratory model fish demonstrate that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) results in cartilage and skeletal abnormalities. In this study, we exposed embryonic medaka to TCDD to induce developmental modification(s) of both cartilage and bone formation. Emphasis is placed on cell-rich hyaline cartilage of the hypural plate where both chondrogenesis and osteogenesis are impaired by TCDD exposure. In this model, TCDD exposure results in a concentration-dependent impairment of mesenchymal cell recruitment, chondrocyte cell proliferation, differentiation, and progression to hypertrophy. Gene expression of ColA2, a marker of chondrocyte terminal differentiation in hypural structures, is markedly attenuated consistent with hypural dysmorphogenesis. Assessment of hypural structure using a transgenic medaka expressing mCherry under control of the osterix promoter illustrated significant attenuation in expression of the osteoblast gene marker and lack of formation of a calcified perichondral sheath surrounding hypural anlage. Overall, these studies illustrate that TCDD impacts terminal differentiation and growth of cartilage and bone in axial structures not likely derived from neural crest progenitors in medaka hypurals.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

The anti-inflammatory drug leflunomide is an agonist of the aryl hydrocarbon receptor

BACKGROUND

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxicity and biological activity of dioxins and related chemicals. The AhR influences a variety of processes involved in cellular growth and differentiation, and recent studies have suggested that the AhR is a potential target for immune-mediated diseases.

METHODOLOGY/PRINCIPAL FINDINGS

During a screen for molecules that activate the AhR, leflunomide, an immunomodulatory drug presently used in the clinic for the treatment of rheumatoid arthritis, was identified as an AhR agonist. We aimed to determine whether any biological activity of leflunomide could be attributed to a previously unappreciated interaction with the AhR. The currently established mechanism of action of leflunomide involves its metabolism to A771726, possibly by cytochrome P450 enzymes, followed by inhibition of de novo pyrimidine biosynthesis by A771726. Our results demonstrate that leflunomide, but not its metabolite A771726, caused nuclear translocation of AhR into the nucleus and increased expression of AhR-responsive reporter genes and endogenous AhR target genes in an AhR-dependent manner. In silico Molecular Docking studies employing AhR ligand binding domain revealed favorable binding energy for leflunomide, but not for A771726. Further, leflunomide, but not A771726, inhibited in vivo epimorphic regeneration in a zebrafish model of tissue regeneration in an AhR-dependent manner. However, suppression of lymphocyte proliferation by leflunomide or A771726 was not dependent on AhR.

CONCLUSIONS

These data reveal that leflunomide, an anti-inflammatory drug, is an agonist of the AhR. Our findings link AhR activation by leflunomide to inhibition of fin regeneration in zebrafish. Identification of alternative AhR agonists is a critical step in evaluating the AhR as a therapeutic target for the treatment of immune disorders.

Effects of 3,4-dichloroaniline on expression of ahr2 and cyp1a1 in zebrafish adults and embryos

Arylhydrocarbon receptor (Ahr) and cytochrome P4501a1 (Cyp1a1) are members of the Ahr/Cyp1a1 pathway that oxygenates various toxic chemicals including aryl hydrocarbons. To elucidate Ahr/Cyp1a1 pathway responses in teleost fish tissues, we examined the effects of 3,4-dichloroaniline (3,4-DCA), a reference toxic compound known to activate the Ahr/Cyp1a1 pathway, on the expression of arh and cyp1a1 in zebrafish tissues and embryos by means of in situ hybridization (ISH). Our ISH analysis showed that cyp1a1 expression was markedly activated by 3,4-DCA in the gill and intestinal epithelia, skin epidermis, and liver parenchymal cells of adult zebrafish. Before differentiation of the gill, intestine, and liver, skin was the site of cyp1a1 activation in embryos. Unlike the cyp1a1 response, 3,4-DCA-mediated ahr activation was not marked in either adults or embryos, indicating a possibility that stable ahr transcripts persist in the cytoplasm of these cells to induce cyp1a1. Young oocytes (previtellogenic to early vitellogic stage) express ahr; however activation of cyp1a1 by 3,4-DCA was negligible in these oocytes, suggesting that ahr expression in oocytes is not directly linked to cyp1a1 activation. Based on our finding that skin epidermis up-regulates cyp1a1 in response to 3,4-DCA, we demonstrated that fin explants, which can be harvested without sacrificing fish, can be used as a standard for assaying cyp1a1 activation in addition to embryos that are now used.

2,3,7,8-Tetrachlorodibenzo-p-dioxin upregulates FoxQ1b in zebrafish jaw primordium

Vertebrate jaw development can be disrupted by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-a potent activator of the aryl hydrocarbon receptor (AHR) transcription factor required for transducing the toxic effects of TCDD. We used zebrafish (Danio rerio) embryos to investigate transcriptional responses to TCDD with the goal of discovering novel, jaw-specific genes affected by TCDD exposure. Our results uncovered a novel target of TCDD-activated Ahr belonging to the evolutionarily conserved family of forkhead box transcription factors. Quantitative real-time polymerase chain reaction analysis demonstrated that FoxQ1b was upregulated by TCDD 7- and 10-fold at 24 and 48 h postfertilization (hpf), respectively. The rate of TCDD-induced FoxQ1b expression was more rapid than that of Cyp1a, a known direct target of TCDD-activated Ahr. TCDD-mediated induction of FoxQ1b was suppressed in the presence of an Ahr antagonist, alpha-naphthoflavone, as well as following knockdown of Ahr2 expression using an Ahr2-specific morpholino antisense oligonucleotide. In situ hybridization analysis of FoxQ1b expression at 48 hpf demonstrated that FoxQ1b is specifically expressed in the jaw primordium where it discretely outlines a developing jaw structure known as Meckel's cartilage--a conserved structure in all jawed vertebrates that develops abnormally in the presence of TCDD. These results identify a novel target of TCDD-activated Ahr and suggest that FoxQ1b may play a role in craniofacial abnormalities induced by developmental exposure to TCDD.

Time course Analysis of Gene expression patterns in ZebrafIsh Eye during Optic Nerve Regeneration

It is well-established that neurons in the adult mammalian central nervous system (CNS) are terminally differentiated and, if injured, will be unable to regenerate their connections. In contrast to mammals, zebrafish and other teleosts display a robust neuroregenerative response. Following optic nerve crush (ONX), retinal ganglion cells (RGC) regrow their axons to synapse with topographically correct targets in the optic tectum, such that vision is restored in ~21 days. What accounts for these differences between teleostean and mammalian responses to neural injury is not fully understood. A time course analysis of global gene expression patterns in the zebrafish eye after ONX can help to elucidate cellular and molecular mechanisms that contribute to a successful neuroregeneration. To define different phases of regeneration after ONX, alpha tubulin 1 ( tuba1) and growth-associated protein 43 ( gap43), markers previously shown to correspond to morphophological events, were measured by real time quantitative PCR (qPCR). Microarray analysis was then performed at defined intervals (6 hours, 1, 4, 12, and 21 days) post-ONX and compared to SHAM. Results show that optic nerve damage induces multiple, phase-related transcriptional programs, with the maximum number of genes changed and highest fold-change occurring at 4 days. Several functional groups affected by optic nerve regeneration, including cell adhesion, apoptosis, cell cycle, energy metabolism, ion channel activity, and calcium signaling, were identified. Utilizing the whole eye allowed us to identify signaling contributions from the vitreous, immune and glial cells as well as the neural cells of the retina. Comparisons between our dataset and transcriptional profiles from other models of regeneration in zebrafish retina, heart and fin revealed a subset of commonly regulated transcripts, indicating shared mechanisms in different regenerating tissues. Knowledge of gene expression patterns in all components of the eye in a model of successful regeneration provides an entry point for functional analyses, and will help in devising hypotheses for testing normal and toxic regulatory factors.

Molecular signaling networks that choreograph epimorphic fin regeneration in zebrafish - a mini-review

This short review provides a current synopsis of caudal fin regeneration in zebrafish with an emphasis on the molecular signaling networks that dictate epimorphic regeneration. At the outset, the fundamentals of caudal fin architecture and the stages of epimorphic regeneration are described. This is followed by a detailed look at the main networks implicated in fin regeneration, namely the Wnt, fibroblast growth factor, activin-betaA, retinoic acid and hedgehog signaling pathways. Throughout this mini-review, these molecular networks are examined through the lens of wound healing, blastema formation or regenerative outgrowth, three of the main stages of epimorphic regeneration. Next, the emerging role of noncoding RNAs as regulators of regeneration and mechanisms of regenerative termination are discussed. Finally, the implications for future research and the broader field of regenerative medicine are examined.

Comparative expression profiling reveals an essential role for raldh2 in epimorphic regeneration

Zebrafish have the remarkable ability to regenerate body parts including the heart and fins by a process referred to as epimorphic regeneration. Recent studies have illustrated that similar to adult zebrafish, early life stage larvae also possess the ability to regenerate the caudal fin. A comparative microarray analysis was used to determine the degree of conservation in gene expression among the regenerating adult caudal fin, adult heart, and larval fin. Results indicate that these tissues respond to amputation/injury with strikingly similar genomic responses. Comparative analysis revealed raldh2, a rate-limiting enzyme for the synthesis of retinoic acid, as one of the most highly induced genes across the three regeneration platforms. In situ localization and functional studies indicate that raldh2 expression is critical for the formation of wound epithelium and blastema. Patterning during regenerative outgrowth was considered to be the primary function of retinoic acid signaling; however, our results suggest that it is also required for early stages of tissue regeneration. Expression of raldh2 is regulated by Wnt and fibroblast growth factor/ERK signaling.

The zebrafish gill model: induction of CYP1A, EROD and PAH adduct formation

Zebrafish (Danio rerio) is a common model species in fish toxicology, and the zebrafish gill is potentially useful in screening waterborne pollutants. We have previously developed a gill-based ethoxyresorufin-O-deethylase (EROD) assay, and proposed gill EROD activity as a biomarker for exposure to waterborne aryl hydrocarbon receptor (AHR) agonists. In this study we modified the gill EROD assay for use in zebrafish. We used immunohistochemistry to localize CYP1A induction, and microautoradiography to localize irreversible binding of the prototypic polycyclic aromatic hydrocarbon, 7,12-dimethylbenz[a]anthracene (DMBA) in zebrafish gills. Gill filament and liver microsomal EROD activities were measured after waterborne exposure of zebrafish and rainbow trout to benzo[a]pyrene (BaP) or beta-naphthoflavone (betaNF). The results showed considerably lower relative EROD induction by betaNF (1microM) in zebrafish than in rainbow trout, both in gills (13-fold versus 230-fold compared to control) and in liver (5-fold versus 320-fold compared to control). The induced hepatic EROD activity was similar in the two species, whereas the basal activity was considerably higher in zebrafish than in rainbow trout. In zebrafish gills, betaNF enhanced DMBA adduct formation and CYP1A immunostaining. Ellipticine blocked DMBA adduct formation and EROD activity following betaNF exposure but had no effect on CYP1A immunostaining. A notable finding was that the localization of DMBA adducts differed from that of CYP1A protein in betaNF-induced fish; CYP1A immunoreactivity was evenly distributed in the gills whereas DMBA adduction was confined to the leading edges of the filaments and the gill rakers, i.e., structures being highly exposed to DMBA-containing inhaled water. The results show that the modified method is suitable for determination of gill EROD activity in zebrafish, although rainbow trout seems more sensitive. They also imply that the sites of DMBA adduct formation in zebrafish gills are markedly influenced by kinetic factors.