17beta-estradiol induced vitellogenesis is inhibited by cortisol at the post-transcriptional level in Arctic char (Salvelinus alpinus)

Hormonal dynamics of matrotrophy vs. lecithotrophy in live-bearing fish reproduction

We explored the relationship between gestational states, fecundity, and steroid hormone levels in three species of live-bearing fish with different maternal provisioning strategies. We studied two lecithotrophic species, Gambusia affinis and Xiphophorus couchianus, where embryos feed exclusively on yolk stored in the eggs, and one matrotrophic species, Heterandria formosa, which actively transfers nutrients to embryos through a follicular placenta. We measured water-borne cortisol, estradiol, and progesterone along with brood size (fecundity) and gestational stage(s). We examined the physiological costs of both maternal provisioning modes. Matrotrophy likely imposes energetic demands due to active nutrient transfer, while lecithotrophy may incur costs from carrying many large embryos. We hypothesized that fecundity, gestational stage, and hormones would covary differently in lecithotrophic vs. matrotrophic species. We found no relationships between hormones and fecundity or gestational stage in any species. However, in H. formosa, we found a positive relationship between estradiol levels and female mass, and a negative relationship between progesterone levels and female mass indicating a change in the circulating levels of both hormones as females grow. We observed differences in average hormone levels among species: the matrotrophic species had higher progesterone and lower estradiol compared to lecithotrophic species. Higher estradiol in lecithotrophic species may relate to egg yolk formation, while placental structures could play a role in progesterone production in matrotrophic species. Elevated cortisol in H. formosa suggests either higher energetic costs or a preparative role for reproduction. Our findings highlight progesterone's importance in maintaining gestation in matrotrophic species, like other placental species.

Assessment of environmental exposure to betamethasone on the reproductive function of female Japanese medaka (Oryzias latipes)

Betamethasone has been extensively used in medicine in recent years and poses potential hazards to aquatic organisms. This study investigated the reproductive toxic effects of betamethasone exposure in fish, employing female Japanese medaka (Oryzias latipes) as a model. Betamethasone exposure at environmentally relevant concentrations (0, 20, 200, and 2000 ng/L) for a period of 15 weeks resulted in its high accumulation in the ovary, leading to abnormal oogenesis in female Japanese medaka. The production of gonadotropins (LH and FSH) in the pituitary gland was inhibited, and sex steroid biosynthesis in the ovary was significantly influenced at the transcriptional level. The imbalance of androgens and estrogens resulted in a decrease in the E2/T ratio and hepatic VTG synthesis, and the suppression of estrogen receptor signaling was also induced. Furthermore, betamethasone exposure delayed spawning and reduced fertility in the F0 generation, and had detrimental effects on the fertilization rate and hatchability of the F1 generation. Our results showed that environmental betamethasone had the potential to adversely affect female fertility and steroid hormone dynamics in fish.

Corticosteroid plasma kinetics and gonadal receptor gene expression during the reproductive cycle in female Eurasian Perch: Investigation of the roles of corticosteroids in vitellogenesis

To improve the quality of reproduction in Eurasian perch, Perca fluviatilis L., which is a promising candidate for Eurasian freshwater aquaculture that is currently cultivated in recirculating aquaculture systems (RAS), investigating the hormones that mediate and affect reproduction in this species is indispensable. The literature defines a group of four major corticosteroids (11-deoxycorticosterone, 11-deoxycortisol, corticosterone and cortisol) that might mediate critical stages of reproduction in female perch. Unfortunately, neither the basic roles nor the kinetics of these four corticosteroids throughout the reproductive cycle of female perch have been well defined to date. In this study, we therefore elucidated the plasma kinetics of these four corticosteroids during the reproductive cycle of domesticated female perch while monitoring the expression of the different receptors and enzymes that mediate their production and possible functions. Additionally, we performed an in vitro experiment during late vitellogenesis to investigate the possible direct roles of these steroids during that stage. Our results revealed that these four corticosteroids were detectable throughout the reproductive cycle, and the levels of most of them (11-deoxycorticosterone, 11-deoxycortisol, and cortisol) fluctuated significantly depending on the stage of reproduction. 11-Deoxycorticosterone and 11-deoxycortisol exhibited their highest levels, 1.8 ng/ml and 58 ng/ml, respectively, at the beginning of the reproductive cycle. By the end of the reproductive cycle, 11-deoxycortisol and cortisol plasma levels exhibited a surge, reaching 58 ng/ml and 150 ng/ml, respectively. During the perch reproductive cycle, the corticosteroid receptor complex is not regulated only at the hormone level, as the expression levels of all corticosteroid receptor genes showed a progressive and similar decline. In vitro exposure of vitellogenic oocytes to some of these corticosteroids (11-deoxycorticosterone and 11-deoxycortisol) induced an increase in yolk globule diameter and a decrease in the density of yolk globules, which indicates the involvement of both of these hormones in yolk globule coalescence. Taken together, these results implicate corticosteroids in the reproductive cycle, although the related cellular mechanisms remain to be investigated.

Cortisol safeguards oogenesis by promoting follicular cell survival

The role of glucocorticoids in oogenesis remains to be elucidated. cyp11c1 encodes the key enzyme involved in the synthesis of cortisol, the major glucocorticoid in teleosts. In our previous study, we mutated cyp11c1 in tilapia and analyzed its role in spermatogenesis. In this study, we analyzed its role in oogenesis. cyp11c1^+/- XX tilapia showed normal ovarian morphology but poor egg quality, as indicated by the mortality of embryos before 3 d post fertilization, which could be partially rescued by the supplement of exogenous cortisol to the mother fish. Transcriptome analyses revealed reduced expression of maternal genes in the eggs of the cyp11c1^+/- XX fish. The cyp11c1^-/- females showed impaired vitellogenesis and arrested oogenesis due to significantly decreased serum cortisol. Further analyses revealed decreased serum E2 level and expression of amh, an important regulator of follicular cell development, and increased follicular cell apoptosis in the ovaries of cyp11c1^-/- XX fish, which could be rescued by supplement of either exogenous cortisol or E2. Luciferase assays revealed a direct regulation of cortisol and E2 on amh transcription via GRs or ESRs. Taken together, our results demonstrate that cortisol safeguards oogenesis by promoting follicular cell survival probably via Amh signaling.

Effects of long‐term exposure to TDCPP in zebrafish ( Danio rerio ) – Alternations of hormone balance and gene transcriptions along hypothalamus‐pituitary axes

Background

TDCPP is one of the major chemical of organophosphate flame retardants (OPFRs) that has been detected ubiquitously in both the environment and biota. Previously we observed that it influenced the concentrations of sex and thyroid hormones in a sex‐dependent pattern, leading to reproductive impairments after short‐term exposure in zebrafish. Here we investigate the consequences of longer‐term exposure to TDCPP on the hypothalamic‐pituitary‐gonad (HPG), hypothalamic‐pituitary‐interrenal (HPI), and hypothalamic‐pituitary‐thyroid (HPT) axes of zebrafish (Danio rerio).

Methods

A 120‐day exposure test to 0.005, 0.05 and 0.5 mg/L TDCPP was initiated with fertilized eggs. Sex steroid hormones in the treated fishes were measured and transcriptional changes were analyzed.

Results

In female fish, exposure to TDCPP resulted in increases in plasma cortisol, follicle stimulating hormone (FSH), luteinizing hormone (LH), 17β‐estradiol (E2), cortisol, thyroxine (T4), and triiodothyronine (T3). Transcription of most target genes along HPG, HPI and HPT axes were increased by the exposure. While in male fish the exposure led to decreases in cortisol, FSH, LH, T4, T3, testosterone (T), and 11‐ketotestosterone (11‐KT). Transcription of genes along HPG, HPI and HPT axes, especially steroidogenic genes, were inhibited in male zebrafish. While, E2/T or E2/11‐KT ratio was increased in both female and females. The sex‐dependent changes in hormones might be due to differential responses to TDCPP induced stresses. An increase in cortisol level coincided with increases in E2 and THs in female fish, while in males decreases in cortisol as well as T, 11‐KT and THs were observed. Long‐term exposure to TDCPP at very low (μg/L) concentrations could disrupt hormone balances in a sex dependent way.

Conclusion

This study revealed that TDCPP could affect endocrine axes – HPG, HPI and HPT – in zebrafish, and impair zebrafish development.

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Exposure to Boscalid Induces Reproductive Toxicity of Zebrafish by Gender-Specific Alterations in Steroidogenesis

Boscalid is a succinate dehydrogenase inhibitor fungicide and is frequently detected in surface water. Due to the frequent detection of boscalid, we evaluated its impact on the reproduction of adult zebrafish following a 21 d exposure to 0, 0.01, 0.1, and 1.0 mg/L. Following exposure to boscalid, the fertility of female zebrafish and fertilization rate of spawning eggs were reduced in a concentration-dependent manner up to a respective 87% and 20% in the highest concentration. A significant 16% reduction in the percentage of late vitellogenic oocytes was noted in ovaries, and a significant 74% reduction in the percentage of spermatids in testis was also observed after treatment with 1.0 mg/L. 17β-Estradiol (E2) concentrations decreased significantly in females (34% decrease) but significantly increased in males (15% increase) following 1.0 mg/L boscalid treatment. The expression of genes (such as era, er2b, cyp19a, and cyp19b) related to the hypothalamus-pituitary-gonad-liver (HPGL) axis was significantly altered and positively correlated with E2 concentrations in female and male zebrafish (p < 0.05). Molecular docking results revealed that the binding modes between boscalid and target proteins (ER and CYP19) of zebrafish were similar to that of the reference compounds and the target proteins. The binding energies indicate that boscalid may have a weak estrogen-like binding effect or CYP19 inhibition, potentially altering the HPGL axis, thereby reducing E2 concentrations and fecundity in females. In contrast, boscalid caused significant induction of E2 steroidogenesis and subsequent feminization of gonads in males, indicating gender-specific adverse outcome pathways.

Transcriptomic analysis of juvenile Chinese sea bass (Lateolabrax maculatus) anesthetized by MS-222 (tricaine methanesulfonate) and eugenol

MS-222 (tricaine methanesulfonate) and eugenol are the two frequently used fish anesthetics. This study intends to analyze the regulation of these anesthetics in Chinese sea bass, Lateolabrax maculatus, through transcriptomic analysis. L. maculatus were exposed to MS-222 or eugenol, and those without any treatments were regarded as controls. Gills and livers were extracted for transcriptomic analysis after recovery in fresh water for 6 h. Identified genes were assigned to NR, COG, SWISS, GO, and KEGG database for predicting gene functions. A FDR ≤ 0.05 and |log2(FC)| ≥ 1 were applied to determined differentially expressed gene (DEG). A total of 45,626 unigenes were annotated using at least one database. The eugenol-treated liver group presented less DEGs compared with that treated by MS-222. Both the MS-222- and eugenol-treated liver groups presented notable DEGs that participated in human disease and metabolism pathways. The eugenol group showed more pathways related to detoxification activity and xenobiotics biodegradation, and those from the MS-222 group were related to organismal system such as reproduction. By comparing gill and liver samples using the same drug, the enriched pathways were generally consistent among the three comparisons. In conclusion, eugenol and MS-222 could change the pathways related to metabolism and immunity in L. maculatus. MS-222 may trigger more damages on the fish liver and reproduction.

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Characterization of gonadal glucocorticoid-induced leucine zipper (GILZ) protein expression during sex change in the protogynous orange-spotted grouper, Epinephelus coioides

Steroid hormones have been proven as a key drive of sex change in sequentially hermaphroditic organisms. However, the upstream mechanism of sex steroid hormones regulation that affect sex change remain unknown. The main glucocorticoid in teleost fish is cortisol, which both regulates steroidogenesis and has antistress action. Thus, cortisol might be one of the prime factors in sex change. In this study, the glucocorticoid-induced leucine zipper (GILZ) gene, was proven to have a dramatic effect in orange-spotted groupers (Epinephelus coioides) during sex change at the early stage of gonadal transition. The specific action of the GILZ protein is at the pouch-shaped proliferative spermatogonia instead of the degenerative oocyte at the onset of sex change. Immunohistochemical (IHC) evidence revealed that GILZ performs intensively at undifferentiated spermatogonia in the early testis stage. These results imply that cortisol provokes a rise of GILZ through regulation caused by steroid hormones leading to sex change.

Acute air exposure modulates the microRNA abundance in stress responsive tissues and circulating extracellular vesicles in rainbow trout (Oncorhynchus mykiss)

The hypothalamic-pituitary-interrenal axis is an important regulator of stress and metabolism in teleosts. Cortisol is secreted by the head kidney where it increases gluconeogenesis in the liver to increase circulating glucose levels. MicroRNAs (miRNAs) are small, non-coding RNA molecules that bind to the 3' untranslated region of specific mRNA to regulate their expression. MicroRNAs can also be secreted into circulation by association with extracellular vesicles (EVs) where they can influence the phenotype of other tissues. In this study, adult rainbow trout were exposed to a 3-minute acute air stress and allowed to recover for 1-, 3-, or 24-h to determine how miRNAs were altered. MicroRNAs measured in this study were chosen based on their high relative abundance in tissues that drive the stress response (miR-21a-3p, let-7a-5p, miR-143-3p) or their role in regulating DNA methylation (miR-29a-3p). In general, miRNAs increased in circulating EVs during the recovery period while decreasing in head kidney and liver at the same timepoints. Predicted targets for these miRNAs were analyzed using KEGG and DAVID functional enrichment analysis. Pathways involved in metabolism and cell signaling were predicted to be upregulated. Future studies can use these results to investigate how pathways are regulated after stress. Overall, our results indicate that miRNAs are regulated during teleost stress responses and could be supporting the cortisol-mediated changes that occur.

Detection of biomarkers to differentiate endocrine disruption from hepatotoxicity in zebrafish (Danio rerio) using proteomics

Measurement of specific biomarkers identified by proteomics provides a potential alternative method for risk assessment, which is required to discriminate between hepatotoxicity and endocrine disruption. In this study, adult zebrafish (Danio rerio) were exposed to the hepatotoxic substance acetaminophen (APAP) for 21 days, in a fish short-term reproduction assay (FSTRA). The molecular changes induced by APAP exposure were studied in liver and gonads by applying a previously developed combined FSTRA and proteomics approach. We observed a significant decrease in egg numbers, an increase in plasma hyaluronic acid, and the presence of single cell necrosis in liver tissue. Furthermore, nine common biomarkers (atp5f1b, etfa, uqcrc2a, cahz, c3a.1, rab11ba, mettl7a, khdrbs1a and si:dkey-108k21.24) for assessing hepatotoxicity were detected in both male and female liver, indicating hepatic damage. In comparison with exposure to fadrozole, an endocrine disrupting chemical (EDC), three potential biomarkers for liver injury, i.e. cahz, c3a.1 and atp5f1b, were differentially expressed. The zebrafish proteome response to fadrozole exposure indicated a significant regulation in estrogen synthesis and perturbed binding of sperm to zona pellucida in the ovary. This study demonstrates that biomarkers identified and quantified by proteomics can serve as additional weight-of-evidence for the discrimination of hepatotoxicity and endocrine disruption, which is necessary for hazard identification in EU legislation and to decide upon the option for risk assessment.

A Preliminary Survey of Estrogenic Effects in Cultured Adult Male Nile Tilapia (Oreochromis niloticus) in the Seven Lakes of San Pablo City, Philippines

This study aimed to investigate the impact of estrogenic endocrine disruptors in adult male Nile Tilapia (Oreochromis niloticus) from the Seven Lakes (Bunot, Calibato, Mohicap, Palakpakin, Pandin, Sampaloc and Yambo) of San Pablo City in Laguna, Philippines. Fish were collected from aquaculture operations in each lake from July to September 2017 and examined for plasma vitellogenin (VTG) levels, hepatosomatic index (HSI) and gonadosomatic index (GSI). In comparison to the reference group (Pandin Lake), fish from Mohicap, Sampaloc and Yambo had higher levels of plasma VTG. HSI was increased in fish from Bunot Lake while it was markedly reduced in fish from Sampaloc Lake. Comparable GSI were obtained in all samples. While most examined fish are in active spermatogenesis, fish from Sampaloc Lake had immature gonads with no spermatozoa, and with minimal to moderate Leydig cell vacuolation and hyperplasia. The results suggest that cultured Nile tilapia from Sampaloc Lake is most adversely affected by endocrine-active compounds. The identification and quantification of these chemicals are recommended in future studies.

Interaction of nuclear ERs and GPER in vitellogenesis in zebrafish

Estrogens exert their biological functions through the estrogen receptors (ERs). In zebrafish, three nuclear estrogen receptors (nERs) named ERα, ERβ1 and ERβ2 and one membrane-bound G protein-coupled estrogen receptor (GPER) are identified. Vitellogenin (Vtg) is predominantly expressed in liver and strongly response to the stimulation of estrogen. It has been proposed that all three nERs are functionally involved in vitellogenesis and ERα may act as the major mediator in teleost. However, the role of GPER and its interaction with nERs in this process are not yet defined in teleost species. In the present study, we provide genetic evidence for the functional significance of ERα that the expression of Vtg genes (vtg1, vtg2, vtg3) and their response to estradiol stimulation were significantly decreased in esr1 mutant zebrafish. Activation of ERβ1 and ERβ2 induced Vtg expression through ERα. Moreover, the involvement of GPER in vitellogenesis and its interaction with nERs in zebrafish were firstly proposed in this work. Activation of GPER induced Vtg genes expression while inhibition of GPER significantly attenuated the estrogenic effect on Vtg. Both treatments altered the expression levels of nERs, suggesting GPER acts interactively with nERs. Collectively, the involvement of both nERs and GPER in regulation of vitellogenesis is demonstrated. ERα is the central factor, acting interactively with ERβ1, ERβ2 and GPER, and GPER regulates vitellogenesis directly and interactively with nERs.

Expression of estrogen receptors in female rainbow trout (Oncorhynchus mykiss) during first ovarian development and under dense rearing condition

To study the expression of four estrogen receptor genes (erα1, erα2, erβ1, erβ2) of female rainbow trout (Oncorhynchus mykiss) during first ovarian development, trouts were sampled from different ovarian stages. Serum E₂ (estradiol) was measured by ELISA and estrogen receptors mRNA expression were examined by qRT-PCR. Our results showed a close association between increased erα1 and vitellogenin mRNA expression during ovarian maturation and increased erα2 mRNA expression in mature ovarian stages. Correlation analysis revealed that a negative relationship between serum E₂ and ovarian erβ1 (or hepatic erβ2), but ovarian erβ2 mRNA expression was relatively unchanged during first ovarian development. Trout were also reared in different densities as stocking density 1, 2 and 3 (SD1, 4.6-31.1 kg/m³; SD2, 6.6-40.6 kg/m³; SD3, 8.6-49.3 kg/m³) to elucidate effects of high density on estrogen receptor expression. Histology observation showed ovarian development of trout in higher densities were retard with a relatively early stage and fewer vitellogenin accumulation. Trout in high densities showed significantly decreased serum E₂, erα mRNA expression and increasing trends of erβ mRNA expression. A noticeable increase of ovarian erβ2 mRNA expression was seen in trout when density is approaching to 50 kg/m³. In conclusion, we may hypothesize that increased erβ mRNA expression triggered by high density result in decreased erα mRNA expression and vitellogenesis. As a result, ovarian development in higher densities was retard.

Corticosteroids deeply depress the in vitro steroidogenic capacity of Eurasian perch ovary at the end of the reproductive cycle

Corticosteroids play positive or negative role in the reproductive mechanisms of many fish species but the physiological contexts relating to such biphasic actions are not well defined. In the present study we investigated to what extent corticosteroids (cortisol-Co, 11-deoxycorticosterone-DOC) hormones may interfere with the steroidogenic capacity of Eurasian perch ovarian tissues, and we tested whether the negative effects of corticosteroids may be mitigated by potential stimulating endocrine factors, namely insulin-like growth factor-1 (IGF), human chorionic gonadotropin (HCG) or thyroid hormones (Triidothyronine-T3, thyroxine-T4). Ovarian tissues from six maturing fish at late vitellogenesis developmental stage (LVO) or at the start of the final meiotic oocyte maturation (FMO) were incubated during 6h in Cortland medium containing various endocrine compounds. Both corticosteroids drastically suppressed aromatase activity (AA) and sex-steroid production, namely 17-β estradiol (E2), 17α-20β-dihydroxy-4-pregnen-3-one (DHP) and testosterone (T). HCG significantly prevented the suppression of both AA and sex-steroid production by low and high cortisol doses, but a lesser AA protection was observed in the case of DOC. The protection of DHP and T productions by HCG from the negative effects by the two corticosteroids was higher at FMO than at LVO stage. IGF or thyroid hormone treatments were lesser effective or ineffective in mitigating the suppression of AA or sex-steroid production by cortisol. The results suggest that an increase in cortisol or DOC such as after mild or high stress intensity may inhibit drastically the ovarian steroidogenic capacity whatever the final oocyte maturation stage in percid fish by hampering AA and sex-steroid production. That inhibition may be partly mitigated by gonadotropins but not IGF nor thyroid hormones, especially at final meiotic oocyte maturation stage.

Effects of a Novel Fish Transport System on the Health of Adult Fall Chinook Salmon

Movement past hydroelectric dams and related in-river structures has important implications for habitat connectivity and population persistence in migratory fish. A major problem is that many of these structures lack effective fish passage facilities, which can fragment spawning and rearing areas and negatively impact recruitment. While traditional fish passage facilities (e.g., ladders, trap and haul) can effectively enable fish to pass over barriers, their capital or operational costs can be significant. We evaluated the utility of a novel transport device that utilizes a flexible tube with differential internal air pressure to pass fish around in-river barriers. We apportioned a total of 147 adult fall Chinook salmon (Oncorhynchus tshawytscha) nearing maturation to three treatments and a control group. In two of the treatments, adult fall Chinook salmon were transported through the device via two lengths of tube (12 or 77 m) and we compared their injury, stress, and immune system responses and reproductive function to a third treatment where fish were moved by a standard trap-and-haul method and also to a control group. We observed no significant differences among the treatment or control groups in posttreatment adult survival, injury, or stress. Indicators of immune system response and reproductive readiness were also not significantly different among the four groups. Egg survival was significantly different among the groups, with the highest survival in the eggs from females transported 77 m and lowest in the control group; the differences were highly variable within groups and not consistent with the duration of treatment or degree of handling. Taken together, the results suggest the device did not injure or alter normal physiological functioning of adult fall Chinook salmon nearing maturation and may provide an effective method for transporting such fish around in-river barriers during their spawning migration.

Glucocorticoid synthesis inhibitor metyrapone blocks stress-induced suppression along luteinizing hormone secreting cells–ovary axis in the fish Oreochromis mossambicus

We showed previously that exposure to mild acute stressors leads to inhibition of follicular development and luteinizing hormone (LH) secretion in tilapia. In this study, we examined whether the hypothalamo–pituitary–interrenal axis was involved in such inhibition. Administration (i.p.) of corticotropin-releasing hormone (CRH) to stripped Oreochromis mossambicus (eggs manually removed from mouth brooder) during the ovarian cycle for 22 days resulted in a significant increase in the serum levels of cortisol, and significantly lower gonadosomatic and hepatosomatic indices concomitant with complete absence of stage V (vitellogenic) follicles in the ovary compared to controls. Furthermore, the LH secreting cells at the proximal pars distalis (PPD) in the pituitary gland showed weak immunostaining in contrast to the intensely stained immunoreactive cells in controls during prespawning phase. On the other hand, while exposure of fish to aquacultural stressors produced effects similar to that of CRH treatment, treatment of glucocorticoid synthesis inhibitor metyrapone to stressed fish during the ovarian cycle did not show significant serum cortisol response. The LH secreting cells in these fish showed intense immunostaining at the PPD in the pituitary gland, and the ovary contained stage V follicles similar to that of controls prior to spawning phase. These results suggest that the inhibitory effects of CRH treatment on LH secretion and recruitment of follicles for vitellogenic growth are mediated through the stress hormone cortisol in O. mossambicus.

Potential role of HSP90 in mediating the interactions between estrogen receptor (ER) and aryl hydrocarbon receptor (AhR) signaling pathways

The estrogen receptor (ER) and aryl hydrocarbon receptor (AhR) are ligand-activated transcription factors involved in estrogen or xenobiotic exposure, whereas the 90-kDa heat shock protein (HSP90), which is a ubiquitously expressed molecular chaperone, is involved in the signal transduction process. Although the interactions between these pathways have been under investigation, the mechanisms are unclear and the potential role of HSP90 in these interactions has not been reported. The results of goldfish primary hepatocytes showed that exposure to PCB77 and 17β-estradiol (E2) alone induced significant protein expression of cytochrome P450 1A (CYP1A) and vitellogenin (VTG), respectively. On the other hand, the combined exposure to PCB77 and E2 led to the reduction of CYP1A and VTG compared to the single treatments. Although the AhRs and ERs were naturally induced during the co-treatment, the total amount of HSP90 binding to the receptors was not changed. Furthermore, while the HSP90 chaperon activity was blocked by the specific inhibitor (geldanamycin), reciprocal inhibition between AhR and ER pathways was not observed. These findings indicate a potential role of HSP90 where competition between AhR and ER for binding to HSP90 can occur and cause reciprocal inhibition.

Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus

The tilapia, Oreochromis mossambicus, shows a short ovarian cycle of 24-26 days in nonmouthbrooding condition. In this study, the stripped female O. mossambicus were exposed to repeated mild acute stressors such as handling, chasing, frequent netting and low water levels daily for a period of 26 days. The follicular dynamics did not show significant difference during previtellogenic phase (day 12), whereas the mean number of stage IV (vitellogenic) follicles remained significantly lower compared with controls at the end of vitellogenic phase (day 18). The stage V (vitellogenic, preovulatory) follicles were completely absent in contrast to their presence in controls prior to spawning (day 23). The control fish spawned spontaneously after 24 days and entered mouthbrooding phase, whereas those exposed to stressors did not spawn. Furthermore, the serum levels of estradiol (E(2)) remained significantly lower concomitant with a significant increase in the serum cortisol concentration during vitellogenic and prespawning phase compared with those of the controls. The LH cells in the PPD of the pituitary gland showed weak immunoreactivity through vitellogenic and prespawning phase in fish exposed to stressors indicating the diminished secretory activity in contrast to the intensely stained ir-material in controls. The study reveals the disruptive effects of aquacultural stressors on the spawning cycle through suppression of LH and E(2) secretion along the pituitary-ovary axis. The results suggest that the ovarian stress response depends on the phase of the cycle and that the interruption of the spawning cycle is due to inhibition of recruitment of preovulatory follicles in O. mossambicus.

Influence of divalent metal ions on E2-induced ER pathway in goldfish (Carassius auratus) hepatocytes

Metal ions existing in the environment could influence the estrogen pathway in aquatic animal, but the detailed mechanism is still delusive. We here showed that in male Carassius auratus hepatocytes, copper (Cu) or cadmium (Cd), did not directly induce vitellogenin (VTG) expression. Interestingly, co-exposure with Cd²⁺ (or Cu²⁺) and 17-β-estradiol (E2) greatly increased the VTG level, comparing with single treatment of E2. Meanwhile, Cd²⁺ or Cu²⁺ (but not E2) triggers HSP70 expression. But, mixture of Cd²⁺ or Cu²⁺ with E2 did not obviously raise HSP70 level. E2 also had no obvious effect on reactive oxygen species. Co-treatment of Cd²⁺ and E2 showed no obvious increase compared to single treatment with Cd²⁺. We further assume that Cd²⁺-involved oxidative stress generates misfolded proteins, resulting in the competition of HSP70 proteins from a heterocomplex (with estrogen receptor). Thus, dissociation of the heterocomplex actives the receptor-ligand binding activity and promotes the E2-induced VTG expression.

Stressors, glucocorticoids and ovarian function in teleosts

The purpose of this overview is to re-examine the postulated direct and indirect actions of glucocorticoids on ovarian function in teleosts. The re-examination is undertaken in light of recent advances in the understanding of the stress response itself, the mode of action of the hypothalamus-pituitary gland-ovarian (HPO) axis, the mechanisms of control of oestrogen-dependent hepatic vitellogenin (VtG) secretion and the apparent roles of corticotrophin-releasing hormone (CRH) and CRH-related factors in the regulation of feeding activity. Many of the results of different studies, particularly whole-animal studies, are conflicting, and little is known as to whether the hormone acts directly on various components of the HPO axis or indirectly by virtue of redirection of energy resources away from ovarian growth to provide a source of metabolic resources for other organ systems involved in the physiological stress response. In vitro studies provide some new insights into the direct actions of glucocorticoid on hepatic VtG synthesis and ovarian follicle steroidogenesis, but even here, in some studies the cellular sites of action of these hormones is not altogether clear. The overview emphasizes the complexity of the stress response, the complexity of the regulation of glucocorticoid-dependent gene expression and the extensive interactive nature of the HPO with other hypothalamus-pituitary gland-peripheral endocrine gland axes, such as the thyroid (HPT), 'somatic' (GH-IGF) and interrenal tissue (HPI) axes.

Stress transcriptomics in fish: a role for genomic cortisol signaling

The physiological responses to stressors, including hormonal profiles and associated tissue responsiveness have been extensively studied in teleosts, but the molecular mechanisms associated with this adaptive response are not well understood. The advent of cDNA microarray technology has transformed the field of functional genomics by revealing global gene expression changes in response to stressor exposures even in non-mammalian vertebrates, including fish. A unifying response in studies related to stressor exposure is activation of the hypothalamus-pituitary-interrenal (HPI) axis in fish, leading to cortisol release into the circulation. Here we will discuss the implications of some of the gene expression changes observed in response to acute stress in fish, while highlighting a role for cortisol in this adaptive stress response. As liver is a key organ for metabolic adjustments to stressors and also is a major target for cortisol action, the genomic studies pertaining to stress and glucocorticoid regulation have focused mainly on this tissue. The studies have identified several genes that are altered transiently after an acute stressor exposure in fish. A number of these stress-responsive genes were also modulated by glucocorticoid receptor activation, suggesting that elevation in cortisol levels during stressor exposure may be a key signal for target tissue molecular programming, essential for stress adaptation. The identification of regulatory gene networks that are stress activated, and modulated by cortisol, both in hepatic and extra-hepatic tissues, including gonads, brain, immune cells and gills, will provide a mechanistic framework to characterize the multifaceted role of cortisol during stress adaptation.

Localisation of estrogen responsive genes in the liver and testis of Murray rainbowfish Melanotaenia fluviatilis exposed to 17beta-estradiol

The localisation of estrogen receptors (ERalpha and ERbeta) and vitellogenin (VTG) transcripts were examined in the liver and testis in male rainbowfish exposed to 17beta-estradiol (E2; 0, 50 and 500 ng/L) via the water for up to 7 days. The ER transcripts were localised within the perinuclear region of the hepatocytes and were up-regulated with E2 exposure. A parallel induction of liver VTG transcripts and protein was observed within 24h, followed by a time-dependent increase in VTG protein. In the testis, both ERs were up-regulated in the germ and epithelial cells, while VTG protein was detected in the cellular space surrounding the spermatids and in association with the connective tissue of the sperm tubules. These results indicate that the ERs are positively auto-regulated in the liver and testis of male rainbowfish. The cellular localisation of VTG within the testis may suggest implication in the mediation of adverse effects of endocrine disrupting chemicals such as testicular growth inhibition, testis-ova and sex reversal.

Molecular Characterization and Expression Pattern of Zona Pellucida Proteins in Gilthead Seabream (Sparus aurata)1

The developing oocyte is surrounded by an acellular envelope that is composed of 2-4 isoforms of zona pellucida (ZP) proteins. The ZP proteins comprise the ZP1, ZP2, ZP3, and ZPX isoforms. While ZP1 (ZPB) and ZP3 (ZPC) are present in all species, ZP2 (ZPA) is not found in teleost fish and ZPX is not found in mammals. In the present study, we identify and characterize the ZP1, ZP3 and ZPX isoforms of gilthead seabream. Furthermore, by analyzing the conserved domains, which include the external hydrophobic patch and the internal hydrophobic patch, we show that ZP2 and ZPX are closely related isoforms. ZP proteins are synthesized in either the liver or ovary of most teleosts. Only in rainbow trout has it been shown that zp3 has dual transcription sites. In gilthead seabream, all four mRNA isoforms are transcribed in both the liver and ovary, with zp1a, zp1b, and zp3 being highly expressed in the liver, and zpx being primarily expressed in the ovary. However, determination of the ZP proteins in plasma showed high levels of ZP1b, ZP3, and ZPX, with low or non-detectable levels of ZP1a. In similarity to other teleost ZPs, the hepatic transcription of all four ZP isoforms is under estrogenic control. Previously, we have shown that cortisol can potentiate estrogen-induced ZP synthesis in salmonids, and now we show that this is not the case in the gilthead seabream. The present study shows for the first time the endocrine regulation of a teleost ZPX isoform, and demonstrates the dual-organ transcriptional activities of all the ZP proteins in one species.

Development of estrogen-responsive transgenic medaka for environmental monitoring of endocrine disrupters

To develop a transgenic fish system to monitor environmental pollution, we generated a mvtg1:gfp transgenic medaka line, in which the gfp reporter gene was under the control of medaka vitellogenin1 (mvtg1) gene promoter. In this transgenic line, GFP was exclusively expressed in the liver of the mature adult female. Male and juvenile transgenic fish did not express GFP but could be induced to express GFP in the liver after exposure to 17-beta-estradiol (E2). Concurrent accumulation of mvtg1 and gfp mRNAs was observed during both development and estrogen treatment, indicating that the gfp transgene was faithfully expressed under the mvtg1 promoter. Dose- and time-dependent induction of GFP expression by E2 was investigated in male transgenic fish. The lowest-observed-effect concentration (LOEC) of E2 to induce GFP expression was 0.5 microg/L by observation of live fish and 0.05-0.1 microg/L by observation of dissection-exposed liver in a 30 day exposure experiment. GFP expression was observed within 36 h after treatment in high concentrations of E2 (5 microg/L), and it took longer to detect GFP expression under lower concentrations of E2. By removal and readdition of E2, we demonstrated that GFP expression was repeatedly induced. Finally, we also demonstrated that GFP expression could be induced by other estrogenic compounds, including 17-alpha-ethynylestradiol (EE2, 0.05 microg/L), diethylstibestrol (DES, 5 microg/L), estriol (10 microg/ L), and bisphenol A (BPA, 1 mg/L), but not by weak estrogenic chemicals such as nonylphenol (NP, up to 1 mg/ L) and methoxychlor (MXC, up to 20 microg/L). Our experiments indicated the broad application of the transgenic line to monitor a wide range of estrogenic chemicals.