Inhibition of follicular development, vitellogenesis, and serum 17beta-estradiol concentrations in zebrafish following chronic, sublethal dietary exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Early life developmental effects induced by dioxins and PCBs in novel bioassays with C. elegans

This study assessed the effects of TCDD, two PCB mixtures (Clophen A50 and Aroclor 1254), and field extracts from marine sediments and swimming crab tissues on early-life development in Caenorhabditis elegans. Gravid nematodes were exposed on agar, and isolated eggs and larvae were tested in solution. Larval development was evaluated after 72 hours. Reporter gene assays (DR-CALUX) were also used to quantify dioxin-equivalent toxicity (TEQ). Exposure to 10 pM Clophen A50 and TCDD on agar inhibited L3-L4 transition by 60 % and 50 %, respectively. Liquid exposure to 5 µM Aroclor 1254 or TCDD (10 nM and 10 µM) delayed development by 20-40 %. Field extracts contained TEQ values of 0.67-4.91 ng/kg (0.2-1.47 pM TCDD), reducing L3-L4 development by 40-60 %. Both bioassays effectively assessed the toxicity of persistent organic pollutants in environmental samples. Agar exposure mimics realistic uptake, while liquid assays offer faster, high-throughput screening.

Defining the cellular complexity of the zebrafish bipotential gonad

Zebrafish are routinely used to model reproductive development, function, and disease, yet we still lack a clear understanding of the fundamental steps that occur during early bipotential gonad development, including when endothelial cells, pericytes, and macrophage arrive at the bipotential gonad to support gonad growth and differentiation. Here, we use a combination of transgenic reporters and single-cell sequencing analyses to define the arrival of different critical cell types to the larval zebrafish gonad. We determined that blood initially reaches the gonad via a vessel formed from the swim bladder artery, which we have termed the gonadal artery. We find that vascular and lymphatic development occurs concurrently in the bipotential zebrafish gonad and our data suggest that similar to what has been observed in developing zebrafish embryos, lymphatic endothelial cells in the gonad may be derived from vascular endothelial cells. We mined preexisting sequencing datasets to determine whether ovarian pericytes had unique gene expression signatures. We identified 215 genes that were uniquely expressed in ovarian pericytes, but not expressed in larval pericytes. Similar to what has been shown in the mouse ovary, our data suggest that pdgfrb+ pericytes may support the migration of endothelial tip cells during ovarian angiogenesis. Using a macrophage-driven photoconvertible protein, we found that macrophage established a nascent resident population as early as 12 dpf and can be observed removing cellular material during gonadal differentiation. This foundational information demonstrates that the early bipotential gonad contains complex cellular interactions, which likely shape the health and function of the mature gonad.

Defining the cellular complexity of the zebrafish bipotential gonad

Zebrafish are routinely used to model reproductive development, function, and disease, yet we still lack a clear understanding of the fundamental steps that occur during early bipotential gonad development, including when endothelial cells, pericytes, and macrophage cells arrive at the bipotential gonad to support gonad growth and differentiation. Here, we use a combination of transgenic reporters and single-cell sequencing analyses to define the arrival of different critical cell types to the larval zebrafish gonad. We determined that blood initially reaches the gonad via a vessel formed from the swim bladder artery, which we have termed the gonadal artery. We find that vascular and lymphatic development occurs concurrently in the bipotential zebrafish gonad and our data suggest that similar to what has been observed in developing zebrafish embryos, lymphatic endothelial cells in the gonad may be derived from vascular endothelial cells. We mined preexisting sequencing data sets to determine whether ovarian pericytes had unique gene expression signatures. We identified 215 genes that were uniquely expressed in ovarian pericytes that were not expressed in larval pericytes. Similar to what has been shown in the mouse ovary, our data suggest that pdgfrb+ pericytes may support the migration of endothelial tip cells during ovarian angiogenesis. Using a macrophage-driven photoconvertible protein, we found that macrophage established a nascent resident population as early as 12 dpf and can be observed removing cellular material during gonadal differentiation. This foundational information demonstrates that the early bipotential gonad contains complex cellular interactions, which likely shape the health and function of the mature, differentiated gonad.

Current Evidence on Bisphenol A Exposure and the Molecular Mechanism Involved in Related Pathological Conditions

Bisphenol A (BPA) is one of the so-called endocrine disrupting chemicals (EDCs) and is thought to be involved in the pathogenesis of different morbid conditions: immune-mediated disorders, type-2 diabetes mellitus, cardiovascular diseases, and cancer. The purpose of this review is to analyze the mechanism of action of bisphenol A, with a special focus on mesenchymal stromal/stem cells (MSCs) and adipogenesis. Its uses will be assessed in various fields: dental, orthopedic, and industrial. The different pathological or physiological conditions altered by BPA and the related molecular pathways will be taken into consideration.

Anthropogenic impact on the reproductive health of two wild Patagonian fish species with differing reproductive strategies

A particularly concerning outcome of environmental pollution is the disturbance of reproductive processes. However, studies on the impacts of pollution on the reproductive health of fish inhabiting South American environments are limited. We studied the impact of anthropogenic pollution on the reproductive health of two sympatric Patagonian marine fish species with different reproductive strategies: the live-bearing rockfish Sebastes oculatus and the egg-laying Brazilian sandperch Pinguipes brasilianus. Our findings reveal that both species presented some degree of reproductive disturbance when inhabiting an affected site, but the specific alterations differed depending on the species, sex, and season. During the reproductive season, 17β-estradiol levels were elevated in females of both species living in polluted areas, while no differences in androgen levels were observed in either species or season. The gonadosomatic index (GSI) was affected in both sexes of S. oculatus during the non-reproductive season, while the gonadal stages were mainly affected in both sexes of P. brasilianus. No signs of intersex condition were observed. Our results highlight the importance of including diverse reproductive parameters to better understand anthropogenic effects on wild animals. Long-term studies including other fish species and including offspring (to evaluate possible transgenerational effects) will be necessary to determine the consequences of the documented reproductive alterations, particularly whether fish species inhabiting Patagonian marine reef areas will be able to reproductively adapt to increasing marine anthropogenic disturbances.

The antiestrogen-like activity and reproductive toxicity of 2,6-DCBQ on female zebrafish upon sub-chronic exposure

2,6-Dichloro-1,4-benzoquinone (2,6-DCBQ), an emerging water disinfection by-product, is widely detected in water resources. However, its potential effects on the reproductive system are largely unknown. Here, we investigated the long-term effects of 2,6-DCBQ on gonadal development by exposing zebrafish from 15 to 180 days postfertilization (dpf). Following exposure to 2,6-DCBQ (20 and 100 µg/L), female-specific effects including delayed puberty onset, retarded ovarian growth and breakdown of the zona radiata were observed, resulting in subfertility in adult females. Adverse effects in folliculogenesis disappeared two months after cessation of 2,6-DCBQ administration. In contrast, no adverse impacts were noted in male testes. The effects on females were associated with significant reduction in 17β-estradiol (E2) level, suggesting a role for 2,6-DCBQ in anti-estrogenic activity. E2 level change in blood was further supported by dysregulated expression of genes (cyp19a1a, fshb, kiss3, esr2b, vtg1, and vtg3) related to the hypothalamic-pituitary-gonad-liver (HPGL) axis. The present study demonstrates for the first time that 2,6-DCBQ induces reproductive impairments in female zebrafish through disrupting 17β-estradiol level.

Atresia of ovarian follicles in fishes, and implications and uses in aquaculture and fisheries

Atresia of ovarian follicles, that is the degenerative process of germ cells and their associated somatic cells, is a complex process involving apoptosis, autophagy and heterophagy. Follicular atresia is a normal component of fish oogenesis and it is observed throughout the ovarian cycle, although it is more frequent in regressing ovaries during the postspawning period. An increased occurrence of follicular atresia above physiological rates reduces fish fecundity and even causes reproductive failure in both wild and captive-reared fish stocks, and hence, this phenomenon has a wide range of implications in applied sciences such as fisheries and aquaculture. The present article reviews the available literature on both basic and applied traits of oocyte loss by atresia, including its morpho-physiological aspects and factors that cause a supraphysiological increase of follicular atresia. Finally, the review presents the use of early follicular atresia identification in the selection process of induced spawning in aquaculture and the implications of follicular atresia in fisheries management.

Transcriptome responses in polar cod (Boreogadus saida) liver slice culture exposed to benzo[a]pyrene and ethynylestradiol: insights into anti-estrogenic effects

Polar cod (Boreogadus saida) is a key species in the arctic marine ecosystem vulnerable to effects of pollution, particularly from petroleum related activities. To facilitate studying the effects of those pollutants, we adapted a precision-cut liver slice culture protocol for this species. Using this system on board a research vessel, we studied gene expression in liver slice after exposure to the polycyclic aromatic hydrocarbon (PAH) benzo[a]pyrene (BaP), ethynylestradiol (EE2), and their mixtures, to map their molecular targets and examine possible anti-estrogenic effects of BaP. The exposure experiments were performed with BaP alone (0.1, 1, and 10 μM) or in combination with low concentrations of EE2 (5 nM) to mimic physiological estradiol levels in early vitellogenic female fish. Transcriptome analysis (RNA-seq) was performed after 72 h exposure in culture to map the genes and cellular pathways affected. The results provide a view of global transcriptome responses to BaP and EE2, which resulted in enrichment of many pathways such as the aryl hydrocarbon (Ahr) and estrogen receptor pathways. In the mixture exposure, BaP resulted in anti-estrogenic effects, shown by attenuation of EE2 activated transcription of many estrogen target genes. The results from this ex vivo experiment suggest that pollutants that activate the Ahr pathway such as the PAH compound BaP can result in anti-estrogenic effects that may lead to endocrine disruption in polar cod.

Endocrine-Disrupting Air Pollutants and Their Effects on the Hypothalamus-Pituitary-Gonadal Axis

Anthropogenic endocrine-disrupting chemicals (EDCs) can contaminate air, soil, and water. Human exposures to EDCs occur through inhalation, absorption, and ingestion. EDCs act by disrupting various pathways in the endocrine system. When the hypothalamic–pituitary–gonadal (HPG) axis is disrupted by EDCs, there can be effects on fertility in both men and women. Not only can fertility be indirectly affected by EDC disruptions of the HPG axis, but EDCs can also directly affect the menstrual cycle and sperm morphology. In this review, we will discuss the current findings on EDCs that can be inhaled. This review examines effects of exposure to prominent EDCs: brominated and organophosphate flame retardants, diesel exhaust, polycyclic aromatic hydrocarbons, cadmium and lead, TCDD, and polychlorinated biphenyls on fertility through alterations that disrupt the HPG axis and fertility through inhalation. Although the studies included herein include multiple exposure routes, all the studies indicate receptor interactions that can occur from inhalation and the associated effects of all compounds on the HPG axis and subsequent fertility.

Continuous nationwide atmospheric PCDD/F monitoring network in Taiwan (2006-2016): Variation in concentrations and apportionment of emission sources

Atmospheric polychlorinated-dibenzo-dioxins/dibenzo-furans (PCDD/Fs) remains an important environmental health concern. Although the total emission inventories of PCDD/Fs in Taiwan decreased from 320 to 52.1 g-I-TEQ/year during 2002-2016, the resulting concentrations of atmospheric PCDD/F and distributions in Taiwan are unknown. We, therefore, conducted a comprehensive investigation of spatial and seasonal variations and apportioned potential sources of ambient PCDD/F concentrations in Taiwan-based on 11-year observation data. A total of 1,008 atmospheric PCDD/F samples were collected from 25 air monitoring stations (from seven areas) and 1 background station for 2006-2016. Linear regression was used to model changes in PCDD/F concentrations. Principal component analysis (PCA) and positive matrix factorization (PMF) were used to identify potential contributors. PCDD/F concentrations in the ambient air gradually decreased during the study period, with a median concentration of 28.2 fg I-TEQ/m3 over 11 years. The highest median PCDD/F concentrations were found in the highly industrialized regions of western Taiwan (38.0-43.4 fg I-TEQ/m3). Lower concentrations were found in eastern Taiwan (∼10 fg I-TEQ/m3). Background stations reported the lowest concentrations of PCDD/Fs, with a median concentration of 1.47 fg I-TEQ/m3. Overall, the concentrations of atmospheric PCDD/Fs in Taiwan were higher in winter (13.4-86.7 fg I-TEQ/m3) than in summer (9.65-27.2 fg I-TEQ/m3). The PCA results indicated that PCDD/F profiles varied by both region (industrialized, urbanized, and background areas) and season. The PMF model for the overall data revealed that the major sources of PCDD/Fs were industrial activities (71.2%). However, in less industrialized areas, traffic activities, long-range transport, and open burning were dominant.

Triphenyl Phosphate at Environmental Levels Retarded Ovary Development and Reduced Egg Production in Japanese Medaka (Oryzias latipes)

Since triphenyl phosphate (TPhP) elicits both antiestrogenic activities via blocking the estrogen receptor (ER) and estrogenic activity by elevating 17β-estradiol (17β-E2) synthesis, its adverse effect on female reproduction is uncertain. In this study, we exposed Japanese medaka to TPhP at 131, 363, and 1773 ng/L for 100 days following hatching. TPhP significantly induced ovary retardation in all exposure groups (incidence: from 11.9 to 37.8%) and reduced egg production by 38.9 and 50.9% in the 363 and 1773 ng/L exposure groups, respectively. Vitellogenin (vtg) transcription was significantly downregulated by 35.4-57.4% after TPhP exposure, explaining the ovary retardation. Considering that 17β-E2 was only significantly decreased in the 1773 ng/L exposure group, ER antagonism could be the dominant contributor to the inhibition of vtg transcription and female reproductive toxicity of TPhP. As 4-hydroxyphenyl diphenyl phosphate, a metabolite of TPhP, was detected in livers with similar concentration [68.4-1237 ng/g lipid weight (lw)] to that of TPhP (485-1594 ng/g lw) and elicited medaka ER antagonistic activity (50% inhibitory concentration = 78.1 μM), TPhP and its metabolite should both contribute to the reproductive inhibition. We demonstrate that TPhP at environmentally relevant concentrations is toxic to female reproduction, which poses an ecological risk to wild fish at the population level.

The effects of fluorene-9-bisphenol on female zebrafish (Danio rerio) reproductive and exploratory behaviors

Endocrine disruptor chemicals induce adverse effects to animals' development, reproduction and behavior in environment. We investigated the effects of fluorene-9-bisphenol (BHPF), one substitute of bisphenol A, on courtship behavior and exploratory behavior of adult zebrafish. Customized apparatus was used to evaluate courtship behavior. The result showed that the male spent less time with BHPF and anti-oestrogenic fulvestrant (FULV) treated female in region of approaching (ROA). Courtship index between BHPF-exposed female and male decreased. The body orientation of BHPF- and FULV-exposed female to male decreased. Furthermore, BHPF exposure downregulated the expression of genes related to estrogen receptor, steroidogenesis and upregulated oxidative stress related genes. It indicated that BHPF exposure interfered the preference of male and female in courtship, and induced detrimental effects on reproduction. BHPF treatment decreased locomotor activity and time spent in top, increased freezing bouts, and induced anxiety/depression-like behavior. The tyrosine hydroxylase in brain decreased under BHPF exposure. Here we showed the potential adverse effects of BHPF on reproduction and exploratory behaviors.

Imatinib mesylate effects on zebrafish reproductive success: Gonadal development, gamete quality, fertility, embryo-larvae viability and development, and related genes

Imatinib (IM) is a tyrosine kinase (TK) inhibitor (TKI) used to treat chronic myeloid leukemia. Clinical case reports and a few laboratory mammal studies provide inconclusive evidence about its deleterious effects on reproduction. The aim of the current study was to evaluate the potential of zebrafish to characterize IM-induced effects on reproduction and clarify IM effects on reproductive success. To this end, we exposed adult zebrafish to four concentrations of IM for 30 days followed by a 30-day depuration period. IM exposure caused a concentration-dependent, irreversible, suppression of folliculogenesis, reversible decrease in sperm density and motility, decreased fecundity and fertility, but no significant change in atretic follicle abundance. We also observed IM-induced premature hatching, but no significant change in embryo-larvae survivability. However, we found significant IM-induced morphometric malformations. IM decreased expression of vegfaa and igf2a (two reproductive-, angiogenic-, and growth-related genes) in testes and ovaries. The results demonstrate IM can induce significant changes in critical reproductive endpoints and zebrafish as a suitable model organism to show effects of IM on reproduction. The findings suggest that TKI effects on reproductive success should be considered.

Temporal changes in the transcriptomic profile of granulosa cells of pigs treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) compound is an environmental chemical adversely affecting reproductive processes. Intracellular TCDD effects are mediated via aryl hydrocarbon receptor (AhR). The aim of the current study was to identify genes linking the AhR pathway with phenotypic consequences of TCDD action in granulosa cells of pigs. By applying multifactorial analysis, with TCDD and incubation time as factors, it was possible to determine temporal changes induced by TCDD in the cell transcriptome. Among the identified 144 differentially expressed genes (DEGs; P_adjusted<0.05, log2 fold change (FC)≥1), 111 DEGs were classified as sustained genes (FC values changing between 3 and 24 h). Eighty six DEGs were classified as early genes and only nine as late genes (FC changes observed between 3 and 12 h or 12 and 24 h, respectively). The sustained gene category included genes related to TCDD mechanism of action (AHR, ARNTL, CYP1A1), cell proliferation (TGFβ3), follicular development and ovulation (PTGS2) as well as stress response (NR3C1). The early gene category contained DEGs associated with cell proliferation (DUSP4, TAB1) and cellular response to stress (DHX34). The CYP1A1 gene was the only DEG classified as an early, late and sustained gene. The multifactorial approach allowed for statistically analyzing TCDD-induced changes over time in the gene expression in granulosa cells of pigs. Changes over time in the granulosal transcriptome profile indicated the involvement of stress related molecules in the cellular response to TCDD and TCDD effects on ovulation. The TCDD effects were particularly evident during the early stage of action by this compound.

RNA-Seq analysis of transcriptome responses in Atlantic cod (Gadus morhua) precision-cut liver slices exposed to benzo[a]pyrene and 17α-ethynylestradiol

Polycyclic aromatic hydrocarbons such as benzo[a]pyrene (BaP) that activate the aryl hydrocarbon receptor (Ahr) pathway, and endocrine disruptors acting through the estrogen receptor pathway are among environmental pollutants of major concern. In this work, we exposed Atlantic cod (Gadus morhua) precision-cut liver slices (PCLS) to BaP (10 nM and 1000 nM), ethynylestradiol (EE2) (10 nM and 1000 nM), and equimolar mixtures of BaP and EE2 (10 nM and 1000 nM) for 48 h, and performed RNA-Seq based transcriptome mapping followed by systematic bioinformatics analyses. Our gene expression analysis showed that several genes were differentially expressed in response to BaP and EE2 treatments in PCLS. Strong up-regulation of genes coding for the cytochrome P450 1a (Cyp1a) enzyme and the Ahr repressor (Ahrrb) was observed in BaP treated PCLS. EE2 treatment of liver slices strongly up-regulated genes coding for precursors of vitellogenin (Vtg) and eggshell zona pellucida (Zp) proteins. As expected, pathway enrichment and network analysis showed that the Ahr and estrogen receptor pathways are among the top affected by BaP and EE2 treatments, respectively. Interestingly, two genes coding for fibroblast growth factor 3 (Fgf3) and fibroblast growth factor 4 (Fgf4) were up-regulated by EE2 in this study. To our knowledge, the fgf3 and fgf4 genes have not previously been described in relation to estrogen signaling in fish liver, and these results suggest the modulation of the FGF signaling pathway by estrogens in fish. The signature expression profiles of top differentially expressed genes in response to the single compound (BaP or EE2) treatment were generally maintained in the expression responses to the equimolar binary mixtures. However, in the mixture-treated groups, BaP appeared to have anti-estrogenic effects as observed by lower number of differentially expressed putative EE2 responsive genes. Our in-depth quantitative analysis of changes in liver transcriptome in response to BaP and EE2, using PCLS tissue culture provides further mechanistic insights into effects of the compounds. Moreover, the analyses demonstrate the usefulness of PCLS in cod for omics experiments.

AHR2 required for normal behavioral responses and proper development of the skeletal and reproductive systems in zebrafish

The aryl hydrocarbon receptor (AHR) is a conserved ligand-activated transcription factor required for proper vertebrate development and homeostasis. The inappropriate activation of AHR by ubiquitous pollutants can lead to adverse effects on wildlife and human health. The zebrafish is a powerful model system that provides a vertebrate data stream that anchors hypothesis at the genetic and cellular levels to observations at the morphological and behavioral level, in a high-throughput format. In order to investigate the endogenous functions of AHR, we generated an AHR2 (homolog of human AHR)-null zebrafish line (ahr2osu1) using the clustered, regulatory interspaced, short palindromic repeats (CRISPR)-Cas9 precision genome editing method. In zebrafish, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mediated toxicity requires AHR2. The AHR2-null line was resistant to TCDD-induced toxicity, indicating the line can be used to investigate the biological and toxicological functions of AHR2. The AHR2-null zebrafish exhibited decreased survival and fecundity compared to the wild type line. At 36 weeks, histological evaluations of the AHR2-null ovaries revealed a reduction of mature follicles when compared to wild type ovaries, suggesting AHR2 regulates follicle growth in zebrafish. AHR2-null adults had malformed cranial skeletal bones and severely damaged fins. Our data suggests AHR2 regulates some aspect(s) of neuromuscular and/or sensory system development, with impaired behavioral responses observed in larval and adult AHR2-null zebrafish. This study increases our understanding of the endogenous functions of AHR, which may help foster a better understanding of the target organs and molecular mechanisms involved in AHR-mediated toxicities.

Effects of the bioconcentration and parental transfer of environmentally relevant concentrations of difenoconazole on endocrine disruption in zebrafish (Danio rerio)

Difenoconazole, a typical triazole fungicide, inhibits lanosterol-14R-demethylase (CYP51) to prevent fungal sterol synthesis and its residues are frequently detected in the environment due to its wide application. Previous studies have demonstrated that difenoconazole altered the triglyceride levels, and gene expression relevant to cholesterol biosynthesis in zebrafish. However, endocrine-disruption in the hypothalamus-pituitary-gonadal-liver (HPGL) axis, the effects of transferring to offspring, and the underlying mechanisms of difenoconazole in aquatic organisms are still unknown. In this study, we defined the effects of difenoconazole at environmental concentrations on endocrine disturbance using zebrafish as an experimental model. The results indicated that difenoconazole induced a significant change in the somatic index, and pathological variations in tissues, and steroid hormone levels. RT-PCR experiments further confirmed that difenoconazole significantly induced expression alteration of lhr, hsd3β, hsd11β, cyp19a in the ovary and star, cyp19a, cyp3c1 in the testis, and erα genes in livers. In addition, difenoconazole exposure in parental zebrafish affected the hatchability and length of its offspring. Moreover, the burdens of difenoconazole and difenoconazole alcohol in females were higher than in males. These findings highlighted that difenoconazole exposure at environmentally relevant concentrations elicited estrogenic endocrine-disruption effects via altering homeostasis of sex steroid hormones in the HPGL axis and the adverse effects can be transferred to the offspring.

Fertility Enhancement but Premature Ovarian Failure in esr1-Deficient Female Zebrafish

It is well established that estrogens regulate female reproduction through estrogen receptors (ERs) in the ovary. However, the precise physiological role of estrogen/ER signaling in reproduction processes remains poorly defined in zebrafish. In this study, we successfully generated an ERα (esr1) mutant line in zebrafish via transcription activator-like effectors nucleases (TALENs). It was found in the mutant females that the fertility was enhanced and the ovarian histology was normal at 90 days post-fertilization (dpf). However, the number of fertile females decreased with age. By 180 dpf, esr1 mutant females were infertile with degenerated ovaries, while the age-matched wild-type females were still fertile. Additionally, few large vitellogenic granules can be found in full grown (FG) follicles at 90 dpf and the expression of vtg genes were down-regulated at both 90 and 180 dpf in esr1 mutant zebrafish. Moreover, steroidogenesis pathway and mTOR signaling pathway were over-activated at 90 dpf, but declined prematurely in esr1 mutant zebrafish by 180 dpf. Collectively, the present study provides evidence that esr1 is fundamental for ovarian maintenance in zebrafish.

Characterizing combined effects of antiestrogenic chemicals on vitellogenin production in rainbow trout (Oncorhynchus mykiss) hepatocytes

Fish are exposed to a complex mixture of endocrine disrupting compounds (EDC), some of which display antiestrogenic activity leading to suppression of estrogen receptor (ER)- mediated reproductive processes. Although the main mode of action (MoA) of these antiestrogens is to directly interfere with natural ligand binding of the ER, several other MoA have been proposed. The aim of the present study was to characterize single and combined antiestrogenic effects of the aryl hydrocarbon receptor (AhR)-agonist β-naphthoflavone (BNF) and ER-antagonist 4-hydroxytamoxifen (OHT) on vitellogenin (Vtg) protein using primary rainbow trout (Oncorhynchus mykiss) hepatocytes. Supporting transcriptional analysis of ER-responsive genes (estrogen receptor-α (er-α), vitellogenin-1 (vtg-1), eggshell zona radiata protein (zrp)) and AhR-mediated genes (aryl hydrocarbon receptor-2β, cytochrome p450-1a (cyp1a)) was performed by qPCR to characterize the antiestrogenic influence on ER- and AhR-mediated responses. Data demonstrated that both BNF and OHT significantly reduced 17β-estradiol (E2)-induced Vtg protein expression in a concentration responsive manner, whereas exposure to a mixture of these produced an additive antiestrogenic effect. The results observed at the protein level were further supported by transcriptional analysis of ER-responsive genes (er-α, vtg-1, zrp), where only E2-induced vtg-1 gene expression was significantly decreased by OHT and the mixture of OHT and BNF. E2-induced er-α and zrp gene expression was not markedly altered. The significant reduction of E2-induced vtg-1 gene expression by OHT suggested that the antiestrogenic effect of this compound may be associated with ER signaling pathway. Specific genes involved in putative AhR-ER cross-talk were also investigated, however none were directly associated with the compound anti-estrogenic MoA. Although the MoA of the single compounds and mixture were not completely characterized, the present study enhanced our knowledge of the combined toxicity mediated by antiestrogens acting through different MoA.

Does 2,4-dichlorophenoxyacetic acid (2,4-D) induce genotoxic effects in tissue cultured Allium roots?

2,4-Dichlorophenoxyacetic acid (2,4-D) is a synthetic plant growth regulator that is highly toxic to most broad leaved plants and relatively nontoxic to monocotyledonous plants; is frequently used as weed killer. The study aimed to investigate cytogenetic effects of different concentrations of 2,4-D (0.67, 1.34, 2.01, 2.68, 3.35 and 4.02 mg/L) on Allium cepa bulblets' root tips treated for 24 and 48 h. The results showed six types of structural aberrations: C-mitosis, stickiness, laggards, bridges, fragments and multipolarity that varied numerically compared to control. It significantly affected mitotic index (MI) at 24 and 48 h treatment. In the Allium test, MI increased significantly at three lower concentrations (0.67, 1.34, 2.01 mg/L) after treatment with 2,4-D for 24 h and decreased significantly at higher concentration. Whereas, 2,4-D treatment for 48 h increased MI at all concentrations with significantly decreased MI at the highest concentration. The experiment was extended using comet test that did not reveal significant difference among treatments except for application of 4.02 mg/L 2,4-D for 48 h; where cell damages were verified by comet test. Rest of the concentrations for any duration of time were not damaging and toxic to cells. The results showed, visible mitodepressive action of 4.02 mg/L 2,4-D when treated for 48 h that had tendency to become toxic if the roots had been in touch with 2,4-D for a longer time.

Zebrafish: A Versatile Animal Model for Fertility Research

The utilization of zebrafish in biomedical research is very common in the research world nowadays. Today, it has emerged as a favored vertebrate organism for the research in science of reproduction. There is a significant growth in amount numbers of scientific literature pertaining to research discoveries in reproductive sciences in zebrafish. It has implied the importance of zebrafish in this particular field of research. In essence, the current available literature has covered from the very specific brain region or neurons of zebrafish, which are responsible for reproductive regulation, until the gonadal level of the animal. The discoveries and findings have proven that this small animal is sharing a very close/similar reproductive system with mammals. More interestingly, the behavioral characteristics and along with the establishment of animal courtship behavior categorization in zebrafish have laid an even stronger foundation and firmer reason on the suitability of zebrafish utilization in research of reproductive sciences. In view of the immense importance of this small animal for the development of reproductive sciences, this review aimed at compiling and describing the proximate close similarity of reproductive regulation on zebrafish and human along with factors contributing to the infertility, showing its versatility and its potential usage for fertility research.

Reproductive toxicity of chlorpyrifos tested in zebrafish (Danio rerio)

This study was carried out to find out whether exposure to environmentally relevant concentration of chlorpyrifos (CP) modulates reproductive competence. To understand this, eight adult male and eight adult female zebrafish ( Danio rerio) were exposed to 200 µg/L of CP for 24, 48, 72, and 96 h. Vitellogenin levels did not show much change in female fish, whereas in male the levels increased with increasing exposure time thereby indicating estrogenecity of the toxicant. Attenuation of serum 11-ketotestosterone in male and serum 17β-estradiol in female was noticed in the exposed fish and thus signified interference of CP in the reproductive endocrine system. Structural damage common to both the gonads was vacuolization. Elongation of seminiferous tubules in testes and atretic follicles in ovary was also observed.

Interpretation of fish biomarker data for identification, classification, risk assessment and testing of endocrine disrupting chemicals

Chemical induced changes in fish biomarkers vitellogenin (VTG), secondary sex characteristics (SSC), and sex ratio indicate modes/mechanisms of action (MOAs) of EAS (estrogen, androgen and steroidogenesis) pathways. These biomarkers could be used for defining MOAs and the causal link between MOAs and adverse effects in fish for the identification of endocrine disrupting chemicals (EDCs). This paper compiled data sets of 150 chemicals for VTG, 57 chemicals for SSC and 38 chemicals for sex ratio in fathead minnow, medaka and zebrafish. It showed 1) changes in fish biomarkers can indicate the MOAs as anticipated; 2) in addition to EAS pathways, chemicals with non-EAS pathways induced changes in fish biomarkers; 3) responses of fish biomarkers did not always follow the anticipated patterns of EAS pathways. These responses may result from the interaction of chemical-induced multiple MOAs and confounding factors like fish diet, infection, culture conditions, general toxicity and stress response. The complex response of fish biomarkers to a chemical of interest requires EDC testing at multiple biological levels. Interpretation of fish biomarker data should be combined with relevant information at different biological levels, which is critical for defining chemical specific MOAs. The utility of fish biomarker data for identification, classification, PBT assessment, risk assessment, and testing of EDCs in the regulatory context was discussed. This paper emphasizes the importance of fish biomarker data in the regulatory context, a weight of evidence approach for the interpretation of fish biomarker data and the need for defining levels of evidence for the identification of EDCs.

Regulation of vitellogenin (vtg1) and estrogen receptor (er) gene expression in zebrafish (Danio rerio) following the administration of Cd²⁺ and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

We evaluated the individual and joint estrogenic effects of cadmium (Cd(2+)) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the zebrafish liver (ZFL) cell line, zebrafish embryo, larvae and the liver of adult zebrafish. The mRNA expression of vtg1 was inhibited by Cd(2+), but unaffected by TCDD in ZFL cells. Similar changes in the mRNA levels of ERα, ERβ1, ERβ2 and GPER (G protein coupled estrogen receptor) in ZFL cells were also observed. Deletion mutants of vtg1 gene promoters were constructed to investigate transcriptional regulation, and we found that all of the constructs failed to respond to TCDD or Cd(2+). However, after co-transfection with a vtg1 promoter-luciferase construct to the ERα, ERβ1, ERβ2 and GPER expression vectors, decreased luciferase activity was observed in the ERα co-transfection group after treatment with Cd(2+), suggesting that ERα participates in vtg1 transcriptional regulation and is affected by Cd(2+). Differences in the regulation of the mRNA levels of these genes were also observed between different developmental stages and between the livers of male and female zebrafish.

[Application of Zebrafish Model to Environmental Toxicology]

Recently, a tropical freshwater fish, the zebrafish, has been generally used as a useful model organism in various fields of life science worldwide. The zebrafish model has also been applied to environmental toxicology; however, in Japan, it has not yet become widely used. In this review, we will introduce the biological and historical backgrounds of zebrafish as an animal model and their breeding. We then present the current status of toxicological experiments using zebrafish that were treated with some important environmental contaminants, including cadmium, organic mercury, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and tributyltin. Finally, the future possible application of genetically modified zebrafish to the study of environmental toxicology is discussed.

Very low-dose (femtomolar) 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) disrupts steroidogenic enzyme mRNAs and steroid secretion by human luteinizing granulosa cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic congener of the polyhalogenated aromatic hydrocarbons (PAH), which causes anatomical abnormalities and developmental defects, impairs ovulation and reduces fertility. TCDD's endocrine-disrupting effects are, in part, caused by a direct action at the ovary. Herein we investigated the in-vitro effects of environmentally relevant doses of TCDD on estradiol-17β (E2) production by human luteinizing granulosa cells (hLGC) obtained from women stimulated for in-vitro fertilization (IVF). TCDD at all concentrations tested (3.1fM, 3.1pM and 3.1nM) significantly decreased E2 secretion when assayed for by radioimmunoassay (RIA). Herein we confirm that TCDD alters E2 secretion by hLGC in a time-, not dose-dependent fashion and are the first to show decreases in E2 secretion with fM concentrations of TCDD. Using real-time quantitative PCR (RT-qPCR), the decreased E2 secretion correlates with a decrease in the mRNA expression levels two enzymes in the estrogen biosynthesis pathway: CYP11A1 and CYP19A1.

Dioxin induction of transgenerational inheritance of disease in zebrafish

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD) is an aryl hydrocarbon receptor (AHR) agonist, an endocrine disruptor, and a potent global pollutant. TCDD exposure is associated with diseases of almost every organ system, and its toxicity is highly conserved across vertebrates. While the acute developmental effects of dioxin exposure have been extensively studied, the ability of early sublethal exposure to produce toxicity in adulthood or subsequent generations is poorly understood. This type of question is difficult to study because of the time frame of the effects. With human subjects, such a study could span more than a lifetime. We have chosen zebrafish (Danio rerio) as a model because they are vertebrates with short generation times and consistent genetic backgrounds. Zebrafish have very modest housing needs, facilitating single and multigenerational studies with minimal time and expense. We have used this model to identify transgenerational effects of TCDD on skeletal development, sex ratio, and male-mediated decreases in reproductive capacity. Here we compare these findings with transgenerational effects described in laboratory rodent species. We propose that the zebrafish is a cost-effective model system for evaluating the transgenerational effects of toxic chemicals and their role in the fetal basis of adult disease.

Zebrafish: A marvel of high-throughput biology for 21st century toxicology

The evolutionary conservation of genomic, biochemical and developmental features between zebrafish and humans is gradually coming into focus with the end result that the zebrafish embryo model has emerged as a powerful tool for uncovering the effects of environmental exposures on a multitude of biological processes with direct relevance to human health. In this review, we highlight advances in automation, high-throughput (HT) screening, and analysis that leverage the power of the zebrafish embryo model for unparalleled advances in our understanding of how chemicals in our environment affect our health and wellbeing.

Histopathologic alterations associated with global gene expression due to chronic dietary TCDD exposure in juvenile zebrafish

The goal of this project was to investigate the effects and possible developmental disease implication of chronic dietary TCDD exposure on global gene expression anchored to histopathologic analysis in juvenile zebrafish by functional genomic, histopathologic and analytic chemistry methods. Specifically, juvenile zebrafish were fed Biodiet starter with TCDD added at 0, 0.1, 1, 10 and 100 ppb, and fish were sampled following 0, 7, 14, 28 and 42 d after initiation of the exposure. TCDD accumulated in a dose- and time-dependent manner and 100 ppb TCDD caused TCDD accumulation in female (15.49 ppb) and male (18.04 ppb) fish at 28 d post exposure. Dietary TCDD caused multiple lesions in liver, kidney, intestine and ovary of zebrafish and functional dysregulation such as depletion of glycogen in liver, retrobulbar edema, degeneration of nasal neurosensory epithelium, underdevelopment of intestine, and diminution in the fraction of ovarian follicles containing vitellogenic oocytes. Importantly, lesions in nasal epithelium and evidence of endocrine disruption based on alternatively spliced vasa transcripts are two novel and significant results of this study. Microarray gene expression analysis comparing vehicle control to dietary TCDD revealed dysregulated genes involved in pathways associated with cardiac necrosis/cell death, cardiac fibrosis, renal necrosis/cell death and liver necrosis/cell death. These baseline toxicological effects provide evidence for the potential mechanisms of developmental dysfunctions induced by TCDD and vasa as a biomarker for ovarian developmental disruption.

Fish biomarkers for regulatory identification of endocrine disrupting chemicals

Demonstrating chemical-induced adverse effects, endocrine mechanisms/modes of action (MOAs) and their causal link is needed for regulatory identification of endocrine disrupting chemicals (EDCs). This paper addresses critical issues over MOAs and their causal link to changes in endpoints. Vitellogenin (VTG), secondary sex characteristics (SSC), and sex ratio (also an apical endpoint) are indicative of chemicals interfering with EAS (estrogen, androgen and steroidogenesis) pathways. These biomarkers, however, can be changed by non-EAS chemicals, systemic toxicity and the stress response. Examples are shown that proving causal link between MOAs and changes in endpoints may be difficult for regulatory identification of EDCs. The paper concludes that both in vitro and in vivo data are needed to define MOAs for regulatory identification of EDCs. Further development of guidance documents for data interpretation and for defining the level of evidence is needed for regulatory identification of EDCs.

AhR signalling and dioxin toxicity

Dioxins are a family of molecules associated to several industrial accidents such as Ludwigshafen in 1953 or Seveso in 1976, to the Agent Orange used during the war of Vietnam, and more recently to the poisoning of the former president of Ukraine, Victor Yushchenko. These persistent organic pollutants are by-products of industrial activity and bind to an intracellular receptor, AhR, with a high potency. In humans, exposure to dioxins, in particular 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces a cutaneous syndrome known as chloracne, consisting in the development of many small skin lesions (hamartoma), lasting for 2-5 years. Although TCDD has been classified by the WHO as a human carcinogen, its carcinogenic potential to humans is not clearly demonstrated. It was first believed that AhR activation accounted for most, if not all, biological properties of dioxins. However, certain AhR agonists found in vegetables do not induce chloracne, and other chemicals, in particular certain therapeutic agents, may induce a chloracne-like syndrome without activating AhR. It is time to rethink the mechanism of dioxin toxicity and analyse in more details the biological events following exposure to these compounds and other AhR agonists, some of which have a very different chemical structure than TCDD. In particular various food-containing AhR agonists are non-toxic and may on the contrary have beneficial properties to human health.

Inhibition of vitellogenin gene induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin is mediated by aryl hydrocarbon receptor 2 (AHR2) in zebrafish (Danio rerio)

Vitellogenins are hepatically derived yolk-protein precursors required for oogenesis in all oviparous teleosts. Altered gene-regulation of vitellogenesis by environmental contaminants can have profound effects on reproductive success, and ultimately population sustainability. To better understand chemical effects on vitellogenin gene regulation, we tested the hypothesis that activation of the aryl hydrocarbon receptor 2 (AHR2) by dioxin inhibits the estrogen receptor pathway regulation of 3 vitellogenin genes (vtg1-3) in vivo, using zebrafish (Danio rerio) as a model teleost. Using an embryo-larval bioassay, embryos were either treated with 1000 pptr (parts-per-trillion, pg/mL) 17α-ethynylestradiol (EE2) alone from 6h post fertilization (hpf) to 4 days post fertilization (dpf), or pre-treated with dioxin (4-5 hpf) prior to EE2. Pre-treatment with 400 pptr 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) or 1,2,3,7,8-pentachlorodibenzo-p-dioxin inhibited the EE2 induction of vtg1, vtg2 and vtg3 by >95% (p≤0.05). In comparison, a splice-blocking AHR2 morpholino used to down-regulate ahr2 expression significantly reduced the inhibition of vtg1, vtg2 and vtg3 by 400 pptr 2,3,7,8-TCDD (20.7-27.4% rescue). These studies demonstrate that 2,3,7,8-TCDD directly inhibits the vitellogenin pathway in vivo through activation of the AHR2. This work provides evidence for AHR2 dependent cross-talk inhibition of vitellogenin genes and offers insight into anti-estrogenic reproductive effects observed in oviparous species exposed to AHR agonist contaminants.

Neurokinin Bs and neurokinin B receptors in zebrafish-potential role in controlling fish reproduction

The endocrine regulation of vertebrate reproduction is achieved by the coordinated actions of several peptide neurohormones, tachykinin among them. To study the evolutionary conservation and physiological functions of neurokinin B (NKB), we identified tachykinin (tac) and tac receptor (NKBR) genes from many fish species, and cloned two cDNA forms from zebrafish. Phylogenetic analysis showed that piscine Tac3s and mammalian neurokinin genes arise from one lineage. High identity was found among different fish species in the region encoding the NKB; all shared the common C-terminal sequence. Although the piscine Tac3 gene encodes for two putative tachykinin peptides, the mammalian ortholog encodes for only one. The second fish putative peptide, referred to as neurokinin F (NKF), is unique and found to be conserved among the fish species when tested in silico. tac3a was expressed asymmetrically in the habenula of embryos, whereas in adults zebrafish tac3a-expressing neurons were localized in specific brain nuclei that are known to be involved in reproduction. Zebrafish tac3a mRNA levels gradually increased during the first few weeks of life and peaked at pubescence. Estrogen treatment of prepubertal fish elicited increases in tac3a, kiss1, kiss2, and kiss1ra expression. The synthetic zebrafish peptides (NKBa, NKBb, and NKF) activated Tac3 receptors via both PKC/Ca(2+) and PKA/cAMP signal-transduction pathways in vitro. Moreover, a single intraperitoneal injection of NKBa and NKF significantly increased leuteinizing hormone levels in mature female zebrafish. These results suggest that the NKB/NKBR system may participate in neuroendocrine control of fish reproduction.

Reproductive and developmental toxicity of dioxin in fish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is a global environmental contaminant and the prototypical ligand for investigating aryl hydrocarbon receptor (AHR)-mediated toxicity. Environmental exposure to TCDD results in developmental and reproductive toxicity in fish, birds and mammals. To resolve the ecotoxicological relevance and human health risks posed by exposure to dioxin-like AHR agonists, a vertebrate model is needed that allows for toxicity studies at various levels of biological organization, assesses adverse reproductive and developmental effects and establishes appropriate integrative correlations between different levels of effects. Here we describe the reproductive and developmental toxicity of TCDD in feral fish species and summarize how using the zebrafish model to investigate TCDD toxicity has enabled us to characterize the AHR signaling in fish and to better understand how dioxin-like chemicals induce toxicity. We propose that such studies can be used to predict the risks that AHR ligands pose to feral fish populations and provide a platform for integrating risk assessments for both ecologically relevant organisms and humans.

Impact of environmental exposures on ovarian function and role of xenobiotic metabolism during ovotoxicity

The mammalian ovary is a heterogeneous organ and contains oocyte-containing follicles at varying stages of development. The most immature follicular stage, the primordial follicle, comprises the ovarian reserve and is a finite number, defined at the time of birth. Depletion of all follicles within the ovary leads to reproductive senescence, known as menopause. A number of chemical classes can destroy follicles, thus hastening entry into the menopausal state. The ovarian response to chemical exposure can determine the extent of ovotoxicity that occurs. Enzymes capable of bioactivating as well as detoxifying xenobiotics are expressed in the ovary and their impact on ovotoxicity has been partially characterized for trichloroethylene, 7,12-dimethylbenz[a]anthracene, and 4-vinylcyclohexene. This review will discuss those studies, as well as illustrate where knowledge gaps remain for chemicals that have also been established as ovotoxicants.

Retention and maternal transfer of environmentally relevant polybrominated dibenzo-p-dioxins and dibenzofurans, polychlorinated dibenzo-p-dioxins and dibenzofurans, and polychlorinated biphenyls in zebrafish (Danio rerio) after dietary exposure

High levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), mono- and non-ortho-polychlorinated biphenyls (PCBs), and polybrominated dibenzo-p-dioxins (PBDDs) are found in fish from coastal areas in the Baltic Sea, which may cause ecotoxicological effects. To increase our understanding of the persistency of the emerging pollutants polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), fish feed was spiked with 21 PBDD/Fs, 17 PCDD/Fs, and 30 PCBs and fed to zebrafish (Danio rerio). Concentrations in fish and eggs were examined during a 6- or 12-week uptake period, and a 6-week elimination period. Steady-state was reached for 2-, 3-, 7-, and/or 8-substituted tri- and tetra-BDD/Fs; 2,3,7,8-tetra-BDD (2,3,7,8-TeBDD) was the most strongly retained. Steady-state was not reached for tetra- to hexa-CDDs. Non-2,3,7,8 congeners showed little or no retention. Most PCBs had high retention and did not reach steady state. Half-lives decreased in the order PCBs > PCDD/Fs > PBDD/Fs. Concentrations of 2,3,7,8-substituted penta- to octa-CDD/Fs decreased with their degree of chlorination, suggesting that the rate-limiting factor for uptake is low bioavailability. Maternal transfer was observed for all retained compounds, with most transfer factors <1, indicating that transfer rates are affected by the poor water solubility of the compounds. The limited retention of the major PBDD congeners found in Baltic Sea fish suggests that they are exposed to high or very high concentrations via either food or water.

Retention and maternal transfer of brominated dioxins in zebrafish (Danio rerio) and effects on reproduction, aryl hydrocarbon receptor-regulated genes, and ethoxyresorufin-O-deethylase (EROD) activity

Brominated dioxins have recently been detected in Baltic Sea biota. Due to their similarities to the highly toxic chlorinated dioxins, concern has been raised about their potential biological effects. The present study investigated retention and effects of brominated dioxins in adult zebrafish, as well as maternal transfer and effects on offspring. We exposed adult zebrafish for nine weeks via feed to 2,3,7,8-tetrabromodibenzo-p-dioxin (TBDD) or to a mixture of brominated dioxins (Baltic Sea mixture), which was designed to reflect relative concentrations found in Baltic Sea biota. We studied spawning success, gonad morphology, hepatic vitellogenin gene expression, and offspring early life-stage development to investigate effects on zebrafish reproduction. Hepatic ethoxyresorufin-O-deethylase (EROD) activity and hepatic expression of a number of aryl hydrocarbon receptor (AHR)-regulated genes were studied to investigate if the brominated dioxins can activate gene transcription through the AHR pathway in zebrafish. In addition, glutathione reductase activity and expression of genes involved in adaptive responses to intracellular stress were studied to investigate potential stress effects of brominated dioxins. After nine weeks of exposure, all brominated dioxins spiked to the feed were detected in female fish and transferred to eggs. Exposure to the Baltic Sea mixture and TBDD clearly induced AHR-regulated genes and EROD activity. Exposure to TBDD reduced spawning success, altered ovarian morphology and reduced hepatic vitellogenin gene expression, which implies that TBDD has a similar effect pattern as the chlorinated analogue. Overall, our results show that dietary exposure to sublethal concentrations of brominated dioxins may impair reproductive physiology in fish and induce AHR-regulated genes.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Endocrine disruptor, dioxin (TCDD)-induced mitochondrial dysfunction and apoptosis in human trophoblast-like JAR cells

The endocrine disruptor 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been demonstrated to disrupt hormone signalling, reduce fertility, interfere with embryo development and cause spontaneous miscarriage in humans. The precise mechanisms of its effects on early implantation in humans are still unclear. In this study, we examined the relationship between mitochondrial function and dioxin-induced toxicity in JAR cells, a human trophoblast-like cell line. Several experiments were performed to address the effects of TCDD on cell viability, reactive oxygen species (ROS) generation, oxidative damage (indicated by the presence of lipoperoxides and oxidized DNA bases), mitochondrial DNA (mtDNA) copy number, ATP content, mtDNA mutations and the protein levels of p53, Bax, Bcl2, cytochrome c and caspase 3. Increased oxidative damage and mitochondrial dysfunction in TCDD-treated trophoblast-like cells was demonstrated. A 2.58-fold increase in lipid peroxides was detected in cells treated with 2 nM TCDD for 4 h. The oxidative DNA damage marker 8-hydroxy-2'-deoxyguanosine was significantly increased by TCDD treatment in a time-dependent manner. Meanwhile, reductions in mtDNA copy number and ATP content and an increase in mtDNA deletions were found. Furthermore, we observed increased apoptosis, p53 accumulation, Bax overexpression, cytochrome c release and sequential caspase 3 activation after TCDD exposure. These results indicate that oxidative damage and mitochondrial dysfunction may be responsible for the apoptotic effects of TCDD.

Revealing genes associated with vitellogenesis in the liver of the zebrafish (Danio rerio) by transcriptome profiling

Background

In oviparous vertebrates, including fish, vitellogenesis consists of highly regulated pathways involving 17β-estradiol (E2). Previous studies focused on a relatively small number of hepatic expressed genes during vitellogenesis. This study aims to identify hepatic genes involved in vitellogenesis and regulated by E2, by using zebrafish microarray gene expression profiling, and to provide information on functional distinctive genes expressed in the liver of a vitellogenic female, using zebrafish as a model fish.

Results

Genes associated with vitellogenesis were revealed by the following paired t-tests (SAM) comparisons: a) two-month old vitellogenic (Vit2) females were compared with non-vitellogenic (NV) females, showing 825 differentially expressed transcripts during early stages of vitellogenesis, b) four-month old vitellogenic (Vit4) females were compared with NV females, showing 1,046 differentially expressed transcripts during vitellogenesis and c) E2-treated males were compared with control males, showing 1,828 differentially expressed transcripts regulated by E2. A Venn diagram revealed 822 common transcripts in the three groups, indicating that these transcripts were involved in vitellogenesis and putatively regulated by E2. In addition, 431 transcripts were differentially expressed in Vit2 and Vit4 females but not in E2-treated males, indicating that they were putatively not up-regulated by E2. Correspondence analysis showed high similarity in expression profiles of Vit2 with Vit4 and of NV females with control males. The E2-treated males differed from the other groups. The repertoire of genes putatively regulated by E2 in vitellogenic females included genes associated with protein synthesis and reproduction. Genes associated with the immune system processes and biological adhesion, were among the genes that were putatively not regulated by E2. E2-treated males expressed a large array of transcripts that were not associated with vitellogenesis.

The study revealed several genes that were not reported before as being regulated by E2. Also, the hepatic expression of several genes was reported here for the first time.

Conclusion

Gene expression profiling of liver samples revealed 1,046 differentially expressed transcripts during vitellogenesis of which at least ~64% were regulated by E2. The results raise the question on the regulation pattern and temporal pleiotropic expression of hepatic genes in vitellogenic females.

Persistent adverse effects on health and reproduction caused by exposure of zebrafish to 2,3,7,8-tetrachlorodibenzo-p-dioxin during early development and gonad differentiation

Little is understood regarding the impacts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure during early development on the health, survival, and reproductive capability of adults. Here we use zebrafish to determine whether early life stage exposure to TCDD induces toxicity in adult zebrafish and their offspring. Zebrafish were exposed to graded concentrations of TCDD (0-400 pg/ml) via waterborne exposure for 1 h/week from 0 to 7 weeks of age. The heart and swim bladder were identified as being most sensitive to TCDD exposure during early development. Subtle developmental toxic responses collectively impaired survival, and only zebrafish in the 0, 25, and 50 pg TCDD/ml groups survived to adulthood. Surviving fish exhibited TCDD toxicity in craniofacial structures (i.e., operculum and jaw), heart, swim bladder, and ovary. Exposure to 25 pg TCDD/ml impaired egg production (40% of control), fertility (90% of control), and gamete quality. TCDD-treated males contributed more than females to impaired reproductive capacity. Transgenerational effects were also discovered in that offspring from parents exposed to TCDD during early life stages showed a 25% increase in mortality compared with the F1 of dimethyl sulfoxide fish, reduced egg production (30-50% of control) and fertility (96% of control). Thus, adverse effects resulting from TCDD exposure during early life stages for one generation of zebrafish were sufficient to cause adverse health and reproductive effects on a second generation of zebrafish. In the environment, transgenerational effects such as these may contribute to population declines for the most TCDD sensitive fish species.

Proteomic analysis of the rat ovary following chronic low-dose exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitously distributed endocrine-disrupting chemical and reproductive toxicant. In order to elucidate low-dose TCDD-mediated effects on reproductive or endocrine functions, female Sprague-Dawley rats were orally administered various concentrations (20, 50, or 125 ng/kg once weekly) TCDD for 29 wk. A proteomic analysis of the ovaries by two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization (MALDI) tandem mass spectrometry showed distinct changes in the levels of several proteins that are relevant markers of TCDD toxicity. Serum estradiol (E2) levels of TCDD-treated animals were markedly lower than control. There were no significant differences in bone mineral density (BMD) of femurs. The body weight of the 125-ng/kg TCDD group was significantly decreased relative to control and there was also a significant reduction in absolute and relative ovarian weights. Expressions of selenium binding protein 2, glutathione S-transferase mu type 3, Lrpap1 protein, NADPH, and peptidylprolyl isomerase D were upregulated, while prohibitin and N-ethylmaleimide-sensitive factor expression levels were downregulated. Data provide further insight into the mechanisms by which TCDD disrupts ovarian function by indicating which differential protein expressions following low-dose TCDD exposure.