Fulvic acid accelerates hatching and stimulates antioxidative protection and the innate immune response in zebrafish larvae

Aquaculture plays a pivotal role in covering dietary animal protein demands and restocking endangered fish populations. However, high mortality takes place at the earliest life stages: prior and immediately after hatching. Improving growth and health parameters by immunostimulants is widely used in older fish, but rarely studied in larvae. Fulvic acids (FAs) are natural substances found in soil and water. Using zebrafish as a model organism, we evaluated the effects of exposure to a FA at concentrations ranging from 1 to 500 mg C/L (mg dissolved organic carbon per liter) on embryonic development. Furthermore, the concentration of reactive oxygen species (ROS) inside the larvae as well as the molecular mechanisms involved in growth, immune response, and antioxidative protection were determined at 5, 50, and 500 mg C/L. 20 to 200 mg C/L accelerated the hatching, which was mediated by increased expression of ifg-1, gh, and he1-α. Furthermore, lyz and mpx were significantly increased at 5 and 50 mg C/L. A concentration of 500 mg C/L induced genes involved in the protection against ROS (nrf-2, keap-1, cat, sod-1), increased the concentration of ROS inside the larvae and caused tissue damage and mortality. Interestingly, 50 mg C/L activated ROS protection as well (nrf-2, sod-2), while no increase of ROS was found in the larvae. Our results show, that FA at low to medium concentrations can increase the health of larvae, but becomes detrimental at higher concentrations.

Environmental glucocorticoids corticosterone, betamethasone and flumethasone induce more potent physiological than transcriptional effects in zebrafish embryos

Many glucocorticoids occur in the aquatic environments but their adverse effects to fish are poorly known. Here we investigate effects of the natural glucocorticoid corticosterone and the synthetic glucocorticoids betamethasone and flumethasone in zebrafish embryos. Besides studying the effects of each steroid, we compared effects of natural with synthetic glucocorticoids, used as drugs. Exposure at concentrations of 1 μg/L and higher led to concentration-related decrease in spontaneous muscle contractions at 24 h post fertilization (hpf) and increase in heart rate at 48 hpf. Betamethasone showed a significant increase at 0.11 μg/L in heart rate. Corticosterone also accelerated hatching at 60 hpf at 0.085 μg/L. Transcription of up to 24 genes associated with different pathways showed alterations at 96 and 120 hpf for all glucocorticoids, although with low potency. Corticosterone caused transcriptional induction of interleukin-17, while betamethasone caused transcriptional down-regulation of the androgen receptor, aromatase and hsd11b2, indicating an effect on the sex hormone system. Furthermore, transcripts encoding proteins related to immune system regulation (irg1l, gilz) and fkbp5 were differentially expressed by corticosterone and betamethasone, while flumethasone caused only little effects, mainly alteration of the irg1l transcript. Our study shows that these glucocorticoids caused more potent physiological effects in early embryos than transcriptional alterations in hatched embryos, likely due to increased metabolism in later developmental stages. Thus, these glucocorticoids may be of concern for early stages of fish embryos in contaminated aquatic environments.

Endogenous regulation of 11-deoxycorticosterone (DOC) and corticosteroid receptors (CRs) during rainbow trout early development and the effects of corticosteroids on hatching

Clear evidence for a physiological role of the mineralocorticoid-like hormone 11-deoxycorticosterone (DOC) and the mineralocorticoid receptor (MR) in fish is still lacking. Efforts to demonstrate an osmoregulatory role for this hormone has so far not been conclusive, while a few scattered studies have indicated a role for DOC in development and reproduction. In this study, we investigate the onset of de novo DOC synthesis in parallel with endogenous corticosteroid receptor mRNA production from fertilization to the swim-up stage in rainbow trout. Whole egg DOC content decreased from fertilization until hatching followed by an increase to pre-fertilization levels just after hatching. Onset of de novo transcription of corticosteroid receptor mRNA's was observed shortly after the midblastula transition; initially glucocorticoid receptor 2 (GR2) followed by MR and then GR1. Non-invasive introduction of DOC or cortisol at fertilization resulted in altered corticosteroid receptor regulation and accelerated hatching date, suggesting a regulatory role in trout ontogenesis of both hormones through MR signaling pathway. The results presented in this study suggest a possible physiological role of the DOC-MR signaling pathway during fish ontogenesis, at fertilization and just after hatching.

Active Glucocorticoids Have a Range of Important Adverse Developmental and Physiological Effects on Developing Zebrafish Embryos

Glucocorticoids in aquatic systems originating from natural excretion and medical use may pose a risk to fish. Here, we analyzed physiological and transcriptional effects of clobetasol propionate (CLO), cortisol and cortisone in zebrafish embryos as single compounds and binary mixtures. CLO and cortisol, but not cortisone showed a concentration-dependent decrease in muscle contraction, increase in heart rate, and accelerated hatching. CLO also induced immobilization and edema at high concentrations. Transcription analysis covering up to 26 genes showed that mostly genes related to glucose metabolism, immune system and development were differentially expressed at 91 ng/L and higher. CLO showed stronger effects on immune system genes than cortisol, which was characterized by upregulation of fkbp5, irg1l, gilz, and socs3, and development genes, matrix metalloproteinases mmp-9 and mmp-13, while cortisol led to stronger upregulation of the gluconeogenesis genes g6pca and pepck1. CLO also induced genes regulating the circadian rhythm, nr1d1 and per1a. In contrast, cortisone led to down-regulation of vitellogenin. Binary mixtures of cortisol and CLO mostly showed a similar activity as CLO alone on physiological and transcriptional end points but additive effects in heart rate and pepck1 upregulation, which indicates that mixtures of glucocorticoids may be of concern for developing fish.

Physiological and Behavioral Effects of Exposure to Environmentally Relevant Concentrations of Prednisolone During Zebrafish (Danio rerio) Embryogenesis

The presence of synthetic glucocorticoids within the aquatic environment has been highlighted as a potential environmental concern as they may mimic the role of endogenous glucocorticoids during vertebrate ontogeny. Prednisolone is a commonly prescribed synthetic glucocorticoid which has been repeatedly detected in the environment. This study investigated the impact of environmentally relevant concentrations of prednisolone (0.1, 1, and 10 μg/L) during zebrafish embryogenesis using physiological and behavioral end points which are known to be mediated by endogenous glucocorticoids. The frequency of spontaneous muscle contractions (24 hpf) was significantly reduced by prednisolone and 0.1 μg/L increased the distance embryos swam in response to a mechanosensory stimulus (48 hpf). The percentage of embryos hatched significantly increased following prednisolone treatment (1 and 10 μg/L), while growth and mortality were unaffected. The onset of heart contraction was differentially affected by prednisolone while heart rate and oxygen consumption both increased significantly throughout embryogenesis. No substantial effect on the axial musculature was observed. Morphological changes to the lower jaw were detected at 96 hpf in response to 1 μg/L of prednisolone. Several parameters of swim behavior were also significantly affected. Environmentally relevant concentrations of prednisolone therefore alter early zebrafish ontogeny and significantly affect embryo behavior.

PFOS induced precocious hatching of Oryzias melastigma--from molecular level to individual level

Perfluorooctane sulfonate (PFOS) showed a risk of causing adverse effects including hatching of animals. However, there are no systematical studies about the effects of PFOS on fish embryo hatching. In this study, a precocious hatching was detected in Oryzias melastigma embryos upon PFOS exposure. The average hatching time was shortened and the hatching rate increased after PFOS exposure. The subsequent survival rate decreased significantly in the larvae hatching from the PFOS-treated embryos compared to the control. The influence occurred in a PFOS concentration-dependent manner. To further elucidate the inner molecular change, full length cDNAs of hatching enzymes HCE and LCE were first cloned in O. melastigma by degenerate RT-PCR and RACE. PFOS elicited a transcriptional response of both HCE and LCE. The expressions of both genes were significantly up-regulated earlier in the PFOS-treated group than in the control group. The activity of hatching enzyme also significantly increased upon exposure. In all, our results showed that exposure to PFOS could induce the hatching enzyme both at transcriptional and enzymatic activity level and further lead to the precocious hatching of O. melastigma embryos, which induced the decrease of the subsequent larvae survival.

The mislabelling of deoxycorticosterone: making sense of corticosteroid structure and function

Over the 70 or so years since their discovery, there has been continuous interest and activity in the field of corticosteroid functions. However, despite major advances in the characterisation of receptors and coregulators, in some ways we still lack clear insight into the mechanism of receptor activation, and, in particular, the relationship between steroid hormone structure and function remains obscure. Thus, why should deoxycorticosterone (DOC) reportedly be a weak mineralocorticoid, while the addition of an 11β-hydroxyl group produces glucocorticoid activity, yet further hydroxylation at C18 leads to the most potent mineralocorticoid, aldosterone? This review aims to show that the field has been confused by the misreading of the earlier literature and that DOC, far from being relatively inactive, in fact has a wide range of activities not shared by the other corticoids. In contrast to the accepted view, the presence of an 11β-hydroxyl group yields, in corticosterone or cortisol, hormones with more limited functions, and also more readily regulated, by 11β-hydroxysteroid dehydrogenase. This interpretation leads to a more systematic understanding of structure-function relationships in the corticosteroids and may assist more rational drug design.

Corticosterone stimulates hatching of late-term tree lizard embryos

The regulation of hatching in oviparous animals is important for successful reproduction and survival, but is poorly understood. We unexpectedly found that RU-486, a progesterone and glucocorticoid antagonist, interferes with hatching of viable tree lizard (Urosaurus ornatus) embryos in a dose-dependent manner and hypothesized that embryonic glucocorticoids regulate hatching. To test this hypothesis, we treated eggs with corticosterone (CORT) or vehicle on Day 30 (85%) of incubation, left other eggs untreated, and observed relative hatch order and hatch time. In one study, the CORT egg hatched first in 9 of 11 clutches. In a second study, the CORT egg hatched first in 9 of 12 clutches, before vehicle-treated eggs in 10 of 12 clutches, and before untreated eggs in 7 of 9 clutches. On average, CORT eggs hatched 18.2 h before vehicle-treated eggs and 11.6 h before untreated eggs. Thus, CORT accelerates hatching of near-term embryos and RU-486 appears to block this effect. CORT may mobilize energy substrates that fuel hatching and/or accelerate lung development, and may provide a mechanism by which stressed embryos escape environmental stressors.

Maternal transfer of complement components C3-1, C3-3, C3-4, C4, C5, C7, Bf, and Df to offspring in rainbow trout (Oncorhynchus mykiss)

Defense mechanisms in developing fish are poorly known but before maturation of lymphoid organs and immunocompetence, innate mechanisms are essential. The complement system represents a major part of innate immunity. Our main objective was to map the presence of complement components early in fish development. Rainbow trout eggs, embryos, and hatchlings were assayed for the onset and duration of C3-1, C3-3, C3-4, C4, C5, C7, factor B, and factor D transcription using real-time reverse transcription-polymerase chain reaction. In general, complement transcript levels increased steadily from day 28 postfertilization to hatch, followed by a decrease during yolk-sac resorption. All the complement proteins studied were found in unfertilized eggs. There was no correlation between the transcript and protein levels throughout the study period. Complement proteins appeared in the liver, kidney, and intestine between day 7 and 35 but not until day 77 in the heart. This study is the first to address the ontogeny of several complement components and represents the first evidence that maternal transfer of complement components, other than C3, occurs in teleost fish.

Catecholaminergic and cholinergic regulation of swimming motility development in free embryos of Cichlasoma Nigrofasciatum

The divergence of progeny from the same spawners of Cichlasoma nigrofasciatum into two groups by duration of embryogenesis and the level of motor activity was studied close to the end of the embryonic period. Free embryos were also studied. During the study, eggs were treated with agents, modifying the activity of catecholaminergic and cholinergic systems. 3-Hydroxytyramine and L-Tyrosine were found to exert a weak influence on embryonic motility. After hatching, these substances modify swimming performance of free embryos, approximating movements of fish at later stages. 6-Hydroxydopamine and, still more, alpha-Bungarotoxin, decrease embryonic motility and postpone the hatching. The influence of these substances on the development of embryo motility increases during early ontogenesis, as indicated by decreased concentration of the substance, necessary for adequate reaction. Neither L-Tyrosine nor 6-Hydroxydopamine influenced the divergence of the progeny into two groups. Injection of the perivitelline fluid with high concentration of hatching enzyme from pre-hatching embryos into the perivitelline space of earlier embryos was found to induce the appearance of rotation movements, typical for more advanced embryos. Changes of correlation between the miogenic and neurogenic motor activity during early development of fish are discussed.

Effects of bis(2-ethylhexyl) phthalate and benzo[a]pyrene on the embryos of Japanese medaka (Oryzias latipes)

Effects of two widely found chemical pollutants, bis(2-ethylhexyl) phthalate (DEHP) and benzo[a]pyrene (BaP), on the embryos of Japanese medaka were investigated. The embryos were exposed to different concentrations (0.01, 0.1, 1, and 10μg/l) of DEHP and BaP. The following were investigated: (1) hatching time and hatching rate in embryos, (2) mortality, sex ratio, body weight and gonadosomatic index (GSI) in adulthood. These two chemicals delayed the hatching time without dose-dependence, but these chemicals had no effect on hatching rate. Mortality was raised and body weight was reduced by DEHP and BaP-treatment; distortion of sex ratio appeared at the lowest concentration of DEHP tested. GSI was decreased because of the BaP-treatment. DEHP and BaP negatively affected Japanease medaka embryos, and the influences of the effects continued into adult stage. Moreover, the effects did not appear to be necessarily dose-dependent.

Posttranscriptional silencing of cytochrome P4501A1 (CYP1A1) during zebrafish (Danio rerio) development

Induction patterns of cytochrome P4501A1 (CYP1A1), an early biochemical marker of exposure to the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, or dioxin) were investigated during zebrafish (Danio rerio) development. A zebrafish CYP1A1 cDNA fragment was cloned and used to detect CYP1A1 mRNA in embryos exposed to TCDD (1 or 10 nM). Induction of CYP1A1 activity was dependent on age and state of hatch. CYP1A1 mRNA was observed by 15 hr postfertilization. CYP1A1 protein and monooxygenase activity were not detected until 3 days postfertilization and after hatch, as determined by Western immunoblot analysis and ethoxyresorufin O-deethylase (EROD) activity, respectively. In contrast to embryos, concomitant induction of mRNA and activity was detected in juvenile zebrafish (3 days posthatch) after 6 hr of TCDD exposure. Asynchronous induction of CYP1A1 mRNA and activity during development may be a general regulatory mechanism, as similar ontogenetic expression of this gene was demonstrated in mouse embryos. To our knowledge, this is the first report of CYP1A1 posttranscriptional silencing during embryogenesis. Our data suggest that TCDD-mediated induction of CYP1A1 activity is regulated differentially in developing and mature systems.

Estrogen and xenoestrogens upregulate the brain aromatase isoform (P450aromB) and perturb markers of early development in zebrafish (Danio rerio)

Estrogen synthesized in the brain itself by the action of cytochrome P450 aromatase (P450arom) is known to have permanent organizing effects on the developing CNS. In fish, estrogen upregulates the predominant brain isoform (P450aromB), implying that xenoestrogens (XE) could act as neurodevelopmental toxicants by altering P450aromB. To test this hypothesis, zebrafish embryos were exposed to 17beta-estradiol (E(2)), diethylstilbestrol (DES, a potent agonist), and bisphenol A (BPA, a weak agonist). RT-PCR/Southern transfer analysis showed that E(2) (0.01-10 microM) upregulated P450aromB in a dose-response manner. The effect of DES (0.01 microM) was similar to 1 microM E(2) (three- to four-fold higher than control), but BPA was less effective (<threefold increase at 10 microM). mRNA levels of the predominant ovarian isoform (P450aromA) were unchanged by estrogen. Treatment with E(2) (0.1-10 microM) between 2 and 72 hpf had dose-response effects on mortality and hatching and induced a 'curved tail down' phenotype characteristic of mutants with defects of early CNS development. The critical period of estrogen sensitivity for effects on mortality and curved tails was 2-24 hpf, whereas hatching effects were both stage- and duration-dependent. Developmental effects of DES and BPA were similar to E(2) but testosterone, and 5alpha-dihydrotestosterone were ineffective. 17alpha-Estradiol showed a small but significant effect on curved tails. We conclude that P450aromB mRNA is a sensitive marker of XE effect during embryogenesis, but further studies are required to determine whether changes in neural aromatase expression and estrogen biosynthesis have consequences for CNS development.

Effect of thyroid hormones on hatching in the tilapia, Oreochromis mossambicus

Incubation of fertilized eggs of Oreochromis mossambicus in media containing varying doses of T3, T4, and phenylthiocarbamide (PTC) indicated that the thyroid hormones delayed hatching, while the antithyroid drug accelerated it. Denuded eggs were also incubated in the above media and the media were assayed at various time intervals for hatching enzyme. The thyroid hormones delayed hatching enzyme release, while PTC enhanced it. The possible role of thyroid hormones in the control of the hatching process in fish eggs is discussed.

Dopaminergic regulation of hatching in fish embryos

Enveloped medaka embryos and denuded zebrafish embryos were exposed to agents that are known to modify the activity of dopaminergic systems. Precocious emergence of medaka embryos was found in the presence of pimozide, salsolinol, and alpha-methyl-rho-tyrosine, whereas delayed hatching occurred with bromocriptine and apomorphine. Moreover, the hatching rate in the light period of medaka eggs, exposed to a 12-hr light/12-hr dark cycle, is significantly higher than in the dark period. Precocious hatching enzyme secretion from denuded zebrafish embryos is caused by salsolinol, whereas dopamine has an opposite effect. At the same time it turned out that in controls hatching enzyme release from denuded zebrafish embryos is well correlated with hatching of enveloped zebrafish embryos. These results do not support the hypothesis proposed by several authors that hatching enzyme is solely mechanically released, but suggest a controlling influence of dopamine receptors, probably located in the developing central nervous system. Assuming a stimulating effect of prolactin on teleostean hatching enzyme secretion, the present data indicate that hypothalamic-hypophyseal tracts are functional at the time of hatching.