Delayed impacts of developmental exposure to 17-α-ethinylestradiol in the self-fertilizing fish Kryptolebias marmoratus

Early-life exposure to permethrin affects phenotypic traits in both larval and adult mangrove rivulus Kryptolebias marmoratus

In fishes, the impacts of environmental constraints undergone during development on the behavioural response of individuals are not well understood. Obtaining more information is important since the aquatic environment is widely exposed to pollution involving neurotoxic compounds likely to cause phenotypic changes that can affect animal fitness. We explored how early exposure to the pyrethroid insecticide permethrin (PM), a compound known for its neurotoxicity, influences the phenotypic traits in both larvae and adults of the self-fertilizing fish mangrove rivulus, Kryptolebias marmoratus. First, we investigated immediate effects of PM on larvae after one-week exposure (0-7 days post-hatching): larvae exposed to high concentration (200 µg.L^-1) grew less, were less active, had negative thigmotaxis and were less likely to capture prey than control individuals and those exposed to low concentration (5 µg.L^-1). No difference was found between treatments when considering oxygen consumption rate and cortisol levels. Persistent effects of early exposure to PM on adults (147-149 days post-hatching) showed that fish previously exposed to high concentration of PM overcompensated growth, leading them to finally be longer and heavier than fish from other treatments. Moreover, we evidenced that levels of cortisol interacted with early PM exposure to affect behaviours during dyadic contests. Fish were more likely to initiate fighting behaviours and were more likely to be aggressive when they have low pre-contest levels of cortisol, but these effects were less pronounced when individuals were exposed to PM. This study shows that PM can have both immediate and persistent effects on phenotypic traits in a self-fertilizing vertebrate and suggests that a pyrethroid can interact with hormones action to affect animal behaviour.

Early-life exposure to methylmercury induces reversible behavioral impairments and gene expression modifications in one isogenic lineage of mangrove rivulus fish Kryptolebias marmoratus

Methylmercury (MeHg) is a ubiquitous bioaccumulative neurotoxicant present in aquatic ecosystems. It is known to alter behaviors, sensory functions and learning abilities in fish and other vertebrates. Developmental and early-life stages exposure to MeHg can lead to brain damage with immediate consequences on larvae behavior, but may also induce long term effects in adults after a detoxification period. However, very little is known about developmental origin of behavioral impairment in adults due to early exposure to MeHg. The aim of this study is to assess whether early-life MeHg exposure induces immediate and/or delayed effects on behaviors, related genes expression and DNA methylation (one of epigenetic mechanisms). To reach this goal, newly hatched larvae of mangrove rivulus fish, Kryptolebias marmoratus, were exposed to two sub-lethal concentrations of MeHg (90 μg/L and 135 µg/L) for 7 days, and immediate and delayed effects were assessed respectively in 7 dph (days post-hatching) and 90 dph fish. This species naturally produces isogenic lineages due to its self-fertilizing reproduction system, which is unique among vertebrates. It allows to study how environment stressors can influence organism's phenotype while minimizing genetic variability. As results, both MeHg exposures are associated with a decreased foraging efficiency and thigmotaxis, and a dose-dependent reduction in larvae locomotor activity. Regarding molecular analysis in larvae whole bodies, both MeHg exposures induced significant decreased expression of DNMT3a, MAOA, MeCP2 and NIPBL, and significant increase of GSS, but none of those genes underwent methylation changes in targeted CpGs. None of significant behavioral and molecular impairments observed in 7-dph larvae were found in 90-dph adults, which highlight a distinction between immediate and delayed effects of developmental MeHg exposure. Our results suggest implications of aminergic system and its neurotransmitters, redox/methylation trade-off and possibly other epigenetic mechanisms in MeHg neurotoxicity underlying behavioral alterations in rivulus.

Effects of 17α-ethinylestradiol on the neuroendocrine gonadotropic system and behavior of European sea bass larvae (Dicentrarchus labrax)

The widespread use of 17α-ethinylestradiol (EE2), and other estrogenic endocrine disruptors, results in a continuous release of estrogenic compounds into aquatic environments. Xenoestrogens may interfere with the neuroendocrine system of aquatic organisms and may produce various adverse effects. The aim of the present study was to expose European sea bass larvae (Dicentrarchus labrax) to EE2 (0.5 and 50 nM) for 8 d and determine the expression levels of brain aromatase (cyp19a1b), gonadotropin-releasing hormones (gnrh1, gnrh2, gnrh3), kisspeptins (kiss1, kiss2) and estrogen receptors (esr1, esr2a, esr2b, gpera, gperb). Growth and behavior of larvae as evidenced by locomotor activity and anxiety-like behaviors were measured 8 d after EE2 treatment and a depuration period of 20 d. Exposure to 0.5 nM EE2 induced a significant increase in cyp19a1b expression levels, while upregulation of gnrh2, kiss1, and cyp19a1b expression was noted after 8 d at 50 nM EE2. Standard length at the end of the exposure phase was significantly lower in larvae exposed to 50 nM EE2 than in control; however, this effect was no longer observed after the depuration phase. The upregulation of gnrh2, kiss1, and cyp19a1b expression levels was found in conjunction with elevation in locomotor activity and anxiety-like behaviors in larvae. Behavioral alterations were still detected at the end of the depuration phase. Evidence indicates that the long-lasting effects of EE2 on behavior might impact normal development and subsequent fitness of exposed fish.

Genome-wide DNA methylation of the liver reveals delayed effects of early-life exposure to 17-α-ethinylestradiol in the self-fertilizing mangrove rivulus

Organisms exposed to endocrine disruptors in early life can show altered phenotype later in adulthood. Although the mechanisms underlying these long-term effects remain poorly understood, an increasing body of evidence points towards the potential role of epigenetic processes. In the present study, we exposed hatchlings of an isogenic lineage of the self-fertilizing fish mangrove rivulus for 28 days to 4 and 120 ng/L of 17-α-ethinylestradiol. After a recovery period of 140 days, reduced representation bisulphite sequencing (RRBS) was performed on the liver in order to assess the hepatic genome-wide methylation landscape. Across all treatment comparisons, a total of 146 differentially methylated fragments (DMFs) were reported, mostly for the group exposed to 4 ng/L, suggesting a non-monotonic effect of EE2 exposure. Gene ontology analysis revealed networks involved in lipid metabolism, cellular processes, connective tissue function, molecular transport and inflammation. The highest effect was reported for nipped-B-like protein B (NIPBL) promoter region after exposure to 4 ng/L EE2 (+ 21.9%), suggesting that NIPBL could be an important regulator for long-term effects of EE2. Our results also suggest a significant role of DNA methylation in intergenic regions and potentially in transposable elements. These results support the ability of early exposure to endocrine disruptors of inducing epigenetic alterations during adulthood, providing plausible mechanistic explanations for long-term phenotypic alteration. Additionally, this work demonstrates the usefulness of isogenic lineages of the self-fertilizing mangrove rivulus to better understand the biological significance of long-term alterations of DNA methylation by diminishing the confounding factor of genetic variability.

Early-life exposure to the endocrine disruptor 17-α-ethinylestradiol induces delayed effects in adult brain, liver and ovotestis proteomes of a self-fertilizing fish

Early-life represents a critically sensitive window to endocrine disrupting chemicals, potentially leading to long-term repercussions on the phenotype later in life. The mechanisms underlying this phenomenon, referred to as the Developmental Origins of Health and Disease (DOHaD), are still poorly understood. To gain molecular understanding of these effects, we exposed mangrove rivulus (Kryptolebias marmoratus) for 28 days post hatching (dph) to 4 and 120 ng/L 17-α-ethinylestradiol, a model xenoestrogen. After 28 days, fish were raised for 140 days in clean water and we performed quantitative label-free proteomics on brain, liver and ovotestis of 168 dph adults. A total of 820, 888 and 420 proteins were robustly identified in the brain, liver and ovotestis, respectively. Effects of 17-α-ethinylestradiol were tissue and dose-dependent: a total of 31, 51 and 18 proteins were differentially abundant at 4 ng/L in the brain, liver and ovotestis, respectively, compared to 20, 25 and 39 proteins at 120 ng/L. Our results suggest that estrogen-responsive pathways, such as lipid metabolism, inflammation, and the innate immune system were affected months after the exposure. In addition, the potential perturbation of S-adenosylmethionine metabolism encourages future studies to investigate the role of DNA methylation in mediating the long-term effects of early-life exposures. SIGNIFICANCE: The Developmental Origins of Health and Disease (DOHaD) states that early life stages of humans and animals are sensitive to environmental stressors and can develop health issues later in life, even if the stress has ceased. Molecular mechanisms supporting DOHaD are still unclear. The mangrove rivulus is a new fish model species naturally reproducing by self-fertilization, making it possible to use isogenic lineages in which all individuals are highly homozygous. This species therefore permits to strongly reduce the confounding factor of genetic variability in order to investigate the effects of environmental stress on the phenotype. After characterizing the molecular phenotype of brain, liver and ovotestis, we obtained true proteomic reaction norms of these three organs in adults after early life stages have been exposed to the common endocrine disruptor 17-α-ethinylestradiol (EE2). Our study demonstrates long-term effects of early-life endocrine disruption at the proteomic level in diverse estrogen-responsive pathways 5 months after the exposure. The lowest tested and environmentally relevant concentration of 4 ng/L had the highest impact on the proteome in brain and liver, highlighting the potency of endocrine disruptors at low concentrations.

Behavioral effects of the neurotoxin -N-methylamino- L-alanine on the mangrove rivulus (Kryptolebias marmoratus) larvae

Mangrove rivulus, Kryptolebias marmoratus, is a hermaphrodite fish capable of self-fertilization. This particularity allows to naturally produce highly homozygous and isogenic individuals. Despite the low genetic diversity, rivulus can live in extremely variable environments and adjust its phenotype accordingly. This species represents a unique opportunity to clearly distinguish the genetic and non-genetic factors implicated in adaptation and evolution, such as epigenetic mechanisms. It is thus a great model in aquatic ecotoxicology to investigate the effects of xenobiotics on the epigenome, and their potential long-term impacts. In the present study, we used the mangrove rivulus to investigate the effects of the neurotoxin b-N-methylamino-L-alanine (BMAA) on larvae behaviors after 7 days exposure to two sub-lethal concentrations. Results show that BMAA can affect the maximal speed and prey capture (trials and failures), suggesting potential impacts on the organism’s fitness.

Embryonic exposure to benzo(a)pyrene inhibits reproductive capability in adult female zebrafish and correlation with DNA methylation

This study was conducted to investigate the effects of embryonic short-term exposure to benzo(a)pyrene (BaP), a model polycyclic aromatic hydrocarbon, on ovarian development and reproductive capability in adult female zebrafish. In 1-year-old fish after embryonic exposure to BaP for 96 h, the gonadosomatic indices and the percentage of mature oocytes were significantly decreased in the 0.5, 5 and 50 nmol/L treatments. The spawned egg number, the fertilization rate and the hatching success were significantly reduced, while the malformation rate of the F1 unexposed larvae were increased. The mRNA levels of follicle-stimulating hormone, luteinizing hormone, ovarian cytochrome P450 aromatase cyp19a1a and cyp19b, estrogen receptor esr1 and esr2, and hepatic vitellogenin vtg1 and vtg2 genes, were down-regulated in adult female zebrafish that were exposed to BaP during embryonic stage. Both 17β-estradiol and testosterone levels were reduced in the ovary of adult females. The methylation levels of the gonadotropin releasing hormone (GnRH) gene gnrh3 were significantly increased in the adult zebrafish brain, and those of the GnRH receptor gene gnrhr3 were elevated both in the larvae exposed to BaP and in the adult brain, which might cause the down-regulation of the mRNA levels of gnrh3 and gnrhr3. This epigenetic change caused by embryonic exposure to BaP might be a reason for physiological changes along the brain-pituitary-gonad axis. These results suggest that short-term exposure in early life should be included and evaluated in any risk assessment of pollutant exposure to the reproductive health of fish.

DNA methylation in adults and during development of the self-fertilizing mangrove rivulus, Kryptolebias marmoratus

In addition to genetic variation, epigenetic mechanisms such as DNA methylation might make important contributions to heritable phenotypic diversity in populations. However, it is often difficult to disentangle the contributions of genetic and epigenetic variation to phenotypic diversity. Here, we investigated global DNA methylation and mRNA expression of the methylation-associated enzymes during embryonic development and in adult tissues of one natural isogenic lineage of mangrove rivulus fish, Kryptolebias marmoratus. Being the best-known self-fertilizing hermaphroditic vertebrate affords the opportunity to work with genetically identical individuals to examine, explicitly, the phenotypic effects of epigenetic variance. Using the LUminometric Methylation Assay (LUMA), we described variable global DNA methylation at CpG sites in adult tissues, which differed significantly between hermaphrodite ovotestes and male testes (79.6% and 87.2%, respectively). After fertilization, an immediate decrease in DNA methylation occurred to 15.8% in gastrula followed by re-establishment to 70.0% by stage 26 (liver formation). Compared to zebrafish, at the same embryonic stages, this reprogramming event seems later, deeper, and longer. Furthermore, genes putatively encoding DNA methyltransferases (DNMTs), Ten-Eleven Translocation (TET), and MeCP2 proteins showed specific regulation in adult gonad and brain, and also during early embryogenesis. Their conserved domains and expression profiles suggest that these proteins play important roles during reproduction and development. This study raises questions about mangrove rivulus' peculiar reprogramming period in terms of epigenetic transmission and physiological adaptation of individuals to highly variable environments. In accordance with the general-purpose genotype model, epigenetic mechanisms might allow for the expression of diverse phenotypes among genetically identical individuals. Such phenotypes might help to overcome environmental challenges, making the mangrove rivulus a valuable vertebrate model for ecological epigenetic studies. The mangrove rivulus, Kryptolebias marmoratus, is the best-known self-fertilizing hermaphroditic vertebrate that allows to work with genetically identical individuals to examine, explicitly, the phenotypic effects of epigenetic variance. The reprogramming event is later, more dramatic and longer than in other described vertebrates. High evolutionary conservation and expression patterns of DNMT, TET, and MeCP2 proteins in K. marmoratus suggest biological roles for each member in gametogenesis and development.

The Role of Epigenomics in Aquatic Toxicology

Over the past decade, the field of molecular biology has rapidly incorporated epigenetic studies to evaluate organism-environment interactions that can result in chronic effects. Such responses arise from early life stage stress, the utilization of genetic information over an individual's life time, and transgenerational inheritance. Knowledge of epigenetic mechanisms provides the potential for a comprehensive evaluation of multigenerational and heritable effects from environmental stressors, such as contaminants. Focused studies have provided a greater understanding of how many responses to environmental stressors are driven by epigenetic modifiers. We discuss the promise of epigenetics and suggest future research directions within the field of aquatic toxicology, with a particular focus on the potential for identifying key heritable marks with consequential impacts at the organism and population levels. Environ Toxicol Chem 2017;36:2565-2573. © 2017 SETAC.