Ontogeny of the thyroid hormones and cortisol in the gilthead sea bream, Sparus aurata

The transcriptional landscape underlying larval development and metamorphosis in the Malabar grouper (Epinephelus malabaricus)

Most teleost fishes exhibit a biphasic life history with a larval oceanic phase that is transformed into morphologically and physiologically different demersal, benthic, or pelagic juveniles. This process of transformation is characterized by a myriad of hormone-induced changes, during the often abrupt transition between larval and juvenile phases called metamorphosis. Thyroid hormones (TH) are known to be instrumental in triggering and coordinating this transformation but other hormonal systems such as corticoids, might be also involved as it is the case in amphibians. In order to investigate the potential involvement of these two hormonal pathways in marine fish post-embryonic development, we used the Malabar grouper (Epinephelus malabaricus) as a model system. We assembled a chromosome-scale genome sequence and conducted a transcriptomic analysis of nine larval developmental stages. We studied the expression patterns of genes involved in TH and corticoid pathways, as well as four biological processes known to be regulated by TH in other teleost species: ossification, pigmentation, visual perception, and metabolism. Surprisingly, we observed an activation of many of the same pathways involved in metamorphosis also at an early stage of the larval development, suggesting an additional implication of these pathways in the formation of early larval features. Overall, our data brings new evidence to the controversial interplay between corticoids and thyroid hormones during metamorphosis as well as, surprisingly, during the early larval development. Further experiments will be needed to investigate the precise role of both pathways during these two distinct periods and whether an early activation of both corticoid and TH pathways occurs in other teleost species.

Thyroid axis participates in high-temperature-induced male sex reversal through its activation by the stress response

Environmental changes alter the sex fate in about 15% of vertebrate orders, mainly in ectotherms such as fish and reptiles. However, the effects of temperature changes on the endocrine and molecular processes controlling gonadal sex determination are not fully understood. Here, we provide evidence that thyroid hormones (THs) act as co-players in heat-induced masculinization through interactions with the stress axis to promote testicular development. We first demonstrated that the thyroid axis (through thyroid-related genes and T3 levels) is highly active in males during the gonadal development in medaka (Oryzias latipes). Similarly, T3 treatments promoted female-to-male sex reversal in XX embryos. Subsequently, embryonic exposure to temperature-induced stress up-regulated the genes related to the thyroid and stress axes with a final increase in T3 levels. In this context, we show that blocking the stress axis response by the loss of function of the corticotropin-releasing hormone receptors suppresses thyroid-stimulating hormone expression, therefore, heat-induced activation of the thyroid axis. Thus, our data showed that early activation of the stress axis and, in consequence, the TH axis, too, leaves us with that both being important endocrine players in inducing female-to-male reversal, which can help predict possible upcoming physiological impacts of global warming on fish populations.

Egg-mediated maternal effects in a cooperatively breeding cichlid fish

Mothers can influence offspring phenotype through egg-mediated maternal effects, which can be influenced by cues mothers obtain from their environment during offspring production. Developing embryos use these components but have mechanisms to alter maternal signals. Here we aimed to understand the role of mothers and embryos in how maternal effects might shape offspring social phenotype. In the cooperatively breeding fish Neolamprologus pulcher different social phenotypes develop in large and small social groups differing in predation risk and social complexity. We manipulated the maternal social environment of N. pulcher females during egg laying by allocating them either to a small or a large social group. We compared egg mass and clutch size and the concentration of corticosteroid metabolites between social environments, and between fertilized and unfertilized eggs to investigate how embryos deal with maternal signalling. Mothers in small groups produced larger clutches but neither laid smaller eggs nor bestowed eggs differently with corticosteroids. Fertilized eggs scored lower on a principal component representing three corticosteroid metabolites, namely 11-deoxycortisol, cortisone, and 11-deoxycorticosterone. We did not detect egg-mediated maternal effects induced by the maternal social environment. We discuss that divergent social phenotypes induced by different group sizes may be triggered by own offspring experience.

The intersection of stress, sex and immunity in fishes

While sexual dimorphism in immune responses has been documented in other vertebrates, evidence for a similar phenomenon in fish is lacking. Here, we review the relationship between immunity, stress, spawning, and sex hormones in fish to gain a better understanding of sex-based differences in fish immune responses and its consequences for aquaculture. It is well known that there is a strong link between the stress response and immune function in fish. In addition, research to date has demonstrated that sexual dimorphism in the stress response exists in many species; yet, the relationship between the sexual dimorphic stress responses and immune function has rarely been explored together. Aside from stress, spawning is also known to trigger changes in fish immune responses. Estrogens and androgens have been shown to modulate the immune system which could account for differences between the two sexes of fish when spawning; however, evidence regarding the sexual dimorphism of these changes varies between fishes and is likely related to the spawning strategy employed by a given species. Sex hormones are also used in aquaculture practices to produce monosex populations, and exposure to these hormones early in development has been shown to impact the development of immune organs in several fishes. While female fish are generally thought to be more robust than males, aquaculture practices should also consider the role that maternal stress has on the immune function of the offspring and what role this plays in compromising the immune response of farmed fish.

Hypoxia affects the ontogeny of the hypothalamus-pituitary-interrenal axis functioning in the lake whitefish (Coregonus clupeaformis)

Early life stages are sensitive to environmental insults and changes during critical developmental periods; this can often result in altered adult behaviour and physiology. Examining the development of the hypothalamus-pituitary-interrenal (HPI) axis and its responsiveness, or lack thereof, during development are important for understanding the short- and long-term impacts of stressors on embryonic and larval fish. We examined the ontogeny of the HPI axis in embryonic (21, 38, 63, 83 and 103 days post-fertilisation (dpf)) and larval (1, 2, 3 and 4 weeks post-hatch (wph)) lake whitefish (Coregonus clupeaformis) by quantifying changes in mRNA levels of several genes associated with HPI axis functioning and whole animal cortisol levels throughout development and in response to a severe or mild hypoxic stress. Cortisol, and crh, crhbp1, pomc and star transcripts were detected from the earliest embryonic age studied. Cortisol levels in control embryos decreased between 21 and 63 dpf, suggesting the utilisation of maternal cortisol deposits. However, by 83 dpf (70% developed) endogenous de novo synthesis had generated a 4.5-fold increase in whole embryo cortisol. Importantly, we provide novel data showing that the HPI axis can be activated even earlier. Whole body cortisol increased in eyed lake whitefish embryos (38 dpf; ~32% developed) in response to hypoxia stress. Coincident with this hypoxia-induced increase in cortisol in 38 dpf embryos were corresponding increases in crh, crhbp1, pomc and star transcript levels. Beyond 38 dpf, the HPI axis in lake whitefish embryos was hyporesponsive to hypoxia stress at all embryonic ages examined (63, 83 and 103 dpf; 54, 72 and 85% developed, respectively). Post-hatch, larvae responded to hypoxia with an increase in cortisol levels and HPI axis genes at 1 wph, but this response was lost and larvae appeared hyporesponsive at subsequent ages (2, 3 and 4 wph). Collectively our work demonstrates that during fish embryogenesis and the larval stage there are windows where the HPI axis is responsive and windows where it is truly hyporesponsive; both could be beneficial in ensuring undisrupted development particularly in the face of increasing environmental changes.

Growth and survival of Takifugu rubripes larvae cultured under different light conditions

We assessed the effects of light intensity and spectrum on the growth and survival of Takifugu rubripes larvae from 30 to 69 days after hatching. Five lighting regimes were applied using 0.5, 1.5, and 3.0 W m^-2 full spectrum white (W0.5, W1.5, W3.0), 0.5 W m^-2 yellow (Y0.5), and 0.5 W m^-2 blue light (B0.5). At the end of the experiment, body length, wet weight, and specific growth rate from day 0 to day 39 were significantly greater in larvae reared under W3.0 than under B0.5 (P ˂ 0.05). No significant differences were observed among W0.5, W1.5, and W3.0, or among W0.5, Y0.5, and B0.5 (P > 0.05). Survival rate was significantly higher in larvae reared under W1.5 than W0.5 (P ˂ 0.05), but no significant differences were observed among W0.5, Y0.5, and B0.5 (P > 0.05). Additionally, light conditioning did not affect the total thickness of the retina. Although the ratio of the thickness of the retinal pigment epithelium layer/total thickness (TT) was significantly higher in larvae exposed to W3.0 compared with those exposed to other light conditions, and the thickness of the outer nuclear layer/TT was significantly lower in larvae exposed to W3.0 compared with those exposed to W0.5 (P < 0.05), no relationship was confirmed between the structure of the retina and the growth performance of the T. rubripes larvae. Expression patterns of two stress-related and seven growth-related genes were also compared with the biometric parameters investigated in the experimental groups. No significant differences in the aanat1a, crh, ss1, igf1, or igf2 expression were observed among the five treatments. Pomc expression was significantly lower in larvae exposed to W1.5 than the larvae exposed to W0.5, and it was significantly lower in larvae exposed to Y0.5 than in larvae exposed to W0.5 or B0.5 (P < 0.05). Significant differences were also found in the expression of gh, with the highest levels being observed under W3.0, while the lowest levels were observed in B0.5 (P < 0.05). Ghrh expression was significantly higher in W3.0 (P < 0.05). These results should be considered when designing rearing protocols for fugu larvae in aquaculture systems.

Characterization and Expression Dynamics of Key Genes Involved in the Gilthead Sea Bream (Sparus aurata) Cortisol Stress Response during Early Ontogeny

The present study identified and characterized six key genes involved in the hypothalamic-pituitary-interrenal (HPI) axis of gilthead sea bream (Sparus aurata), a commercially important European aquaculture species. The key genes involved in the HPI axis for which gene structure and synteny analysis was carried out, comprised of two functional forms of glucocorticoid receptors (GR), as well as three forms of pro-opiomelanocortin (POMC) genes and one form of mineralocorticoid receptor (MR) gene. To explore their functional roles during development but also in the stress response, the expression profiles of gr1, gr2, mr, pomc_aI, pomc_aII, and pomc_β were examined during early ontogeny and after an acute stress challenge. The acute stress challenge was applied at the stage of full formation of all fins, where whole body cortisol was also measured. Both the cortisol and the molecular data implied that sea bream larvae at the stage of the full formation of all fins at 45 dph are capable of a response to stress of a similar profile as observed in adult fish.

Lamprey metamorphosis: Thyroid hormone signaling in a basal vertebrate

As one of the most basal living vertebrates, lampreys represent an excellent model system to study the evolution of thyroid hormone (TH) signaling. The lamprey hypothalamic-pituitary-thyroid and reproductive axes overlap functionally. Lampreys have 3 gonadotropin-releasing hormones and a single glycoprotein hormone from the hypothalamus and pituitary, respectively, that regulate both the reproductive and thyroid axes. TH synthesis in larval lampreys takes place in an endostyle that transforms into typical vertebrate thyroid tissue during metamorphosis; both the endostyle and follicular tissue have all the typical TH synthetic components found in other vertebrates. Furthermore, lampreys also have the vertebrate suite of peripheral regulators including TH distributor proteins (THDPs), deiodinases and TH receptors (TRs). Although at the molecular level the components of the lamprey thyroid system are ancestral to other vertebrates, their functions have been largely conserved. TH signaling as it relates to lamprey metamorphosis represents a particularly interesting phenomenon. Unlike other metamorphosing vertebrates, lamprey THs increase throughout the larval period, peak prior to metamorphosis and decline rapidly at the onset of metamorphosis; patterns of deiodinase activity are consistent with these increases and declines. Moreover, goitrogens (which suppress TH levels) initiate precocious metamorphosis, and exogenous TH treatment blocks goitrogen-induced metamorphosis and disrupts natural metamorphosis. Despite this clear physiological difference, TH action via TRs is consistent with higher vertebrates. Based on observations that TRs are upregulated in a tissue-specific fashion during morphogenesis and the finding that lamprey TRs upregulate genes via THs in a fashion similar to higher vertebrates, we propose the following hypothesis for further testing. THs have a dual role in lampreys where high TH levels promote larval feeding and growth and then at the onset of metamorphosis TH levels decrease rapidly; at this time the relatively low TH levels function via TRs in a fashion similar to that of other metamorphosing vertebrates.

Pituitary-thyroid axis development during the larval-juvenile transition in the pejerrey Odontesthes bonariensis

The morphological development of the thyroid gland of pejerrey Odontesthes bonariensis during larval-juvenile transition was studied and related to whole-body concentrations of thyroxine (T₄ ) and tri-iodothyronine (T₃ ). A complementary (c)DNA fragment of the thyroid-stimulating hormone β-subunit (tshb) was sequenced and transcript levels quantified during this period. Follicles with eosinophilic and T₄ -immunoreactive colloids were detected at hatching together with tshb transcript levels and whole-body concentrations of T₄ and T₃ hormones. Thyroid follicles were located in the subpharyngeal region associated with the ventral aorta below the hyoid bone. Follicle structure switched from the rounded form at hatching to oval in juveniles. Significant increase of follicle number per larva, mean colloidal area and total colloidal area was observed throughout development with maximum values at the end of the larval-juvenile transition. A significant decrease of tshb expression together with a significant increase in T₄ and T₃ whole-body concentrations was observed prior to achieving the juvenile phenotype. These results are in accordance with a negative feedback regulation of tshb expression by thyroid hormones and a possible association between thyroid hormone levels and the acceleration of metabolic processes necessary to complete metamorphosis.

Determination of cortisol in lake sturgeon (Acipenser fulvescens) eggs by liquid chromatography tandem mass spectrometry

Quantifying cortisol concentrations in fish eggs is important to understand the effects of environmental conditions on maternal physiological condition and on egg provisioning and quality. Data are particularly relevant to studies of the ecology of threatened species such as lake sturgeon (Aciperser fulvescens) as well as assessments of larval physical and behavioral phenotypes, fish health and caviar quality in sturgeon aquaculture. This study focuses on development of bioanalytical methods for high sensitivity and robust determination of cortisol in sturgeon eggs. Sample preparation was optimized after investigating protein precipitation and liquid-liquid extraction techniques. Ethyl acetate was found to be the most efficient solvent (recovery parameter) and also provided the best sample clean up (matrix effect parameter). The method was determined to be linear for cortisol concentrations between 0.025 and 100ng/mL. The limits of detection and quantification were 0.025 and 0.1ng/mL respectively. Intra- and inter-day performances of the method were validated at three concentrations (0.25; 10 and 100ng/mL). The method was applied to field-collected samples for the determination of endogenous cortisol in lake sturgeon eggs. Cortisol was detected in all egg samples and statistical analysis showed significant differences between fertilized and non-fertilized eggs.

Triclosan exposure results in alterations of thyroid hormone status and retarded early development and metamorphosis in Cyprinodon variegatus

Thyroid hormones are critically involved in somatic growth, development and metamorphosis of vertebrates. The structural similarity between thyroid hormones and triclosan, an antimicrobial compound widely employed in consumer personal care products, suggests triclosan can have adverse effects on the thyroid system. The sheepshead minnow, Cyprinodon variegatus, is now used in ecotoxicological studies that have recently begun to focus on potential disruption of the thyroid axis by endocrine disrupting compounds. Here, we investigate the in vivo effects of exposure to triclosan (20, 50, and 100μgL^-1) on the thyroid system and the embryonic and larval development of C. variegatus. Triclosan exposure did not affect hatching success, but delayed hatching time by 6-13h compared to control embryos. Triclosan exposure affected the ontogenetic variations of whole body thyroid hormone concentrations during the larval phase. The T3 peak around 12-15 dph, described to be indicative for the metamorphosis climax in C. variegatus, was absent in triclosan-exposed larvae. Triclosan exposure did not produce any deformity or allometric repatterning, but a delayed development of 18-32h was observed. We conclude that the triclosan-induced disruption of the thyroid system delays in vivo the start of metamorphosis in our experimental model. We observed a global developmental delay of 24-45h, equivalent to 4-7% prolongation of the developmental time in C. variegatus. The costs of delayed metamorphosis can lead to reduction of juvenile fitness and could be a determining factor in the outcome of competitive interactions.

The thyroid gland and thyroid hormones in sheepshead minnow (Cyprinodon variegatus) during early development and metamorphosis

The sheepshead minnow is widely used in ecotoxicological studies that only recently have begun to focus on disruption of the thyroid axis by xenobiotics and endocrine disrupting compounds. However, reference levels of the thyroid prohormone thyroxine (T4) and biologically active hormone 3,5,3'-triiodothyronine (T3) and their developmental patterns are unknown. This study set out to describe the ontogeny and morphology of the thyroid gland in sheepshead minnow, and to correlate these with whole-body concentrations of thyroid hormones during early development and metamorphosis. Eggs were collected by natural spawning in our laboratory. T4 and T3 were extracted from embryos, larvae and juveniles and an enzyme-linked immunoassay was used to measure whole-body hormone levels. Length and body mass, hatching success, gross morphology, thyroid hormone levels and histology were measured. The onset of metamorphosis at 12-day post-hatching coincided with surges in whole-body T4 and T3 concentrations. Thyroid follicles were first observed in pre-metamorphic larvae at hatching and were detected exclusively in the subpharyngeal region, surrounding the ventral aorta. Follicle size and thyrocyte epithelial cell heights varied during development, indicating fluctuations in thyroid hormone synthesis activity. The increase in the whole-body T3/T4 ratio was indicative of an increase in outer ring deiodination activity. This study establishes a baseline for thyroid hormones in sheepshead minnows, which will be useful for the understanding of thyroid hormone functions and in future studies of thyroid toxicants in this species.

The onset of stress response in rainbow trout Oncorhynchus mykiss embryos subjected to density and handling

The present study made an attempt to measure the cortisol content, as an indicator of stress response, in rainbow trout embryos which were exposed to different densities and handling stress (air exposure) during incubation. The three densities of experimental embryos at early development stages were considered as 2.55 embryos/cm(2) (low density), 5.10 embryos/cm(2) (normal density) and 7.65 embryos/cm(2) (high density). The cortisol content of eggs (5.09 ± 0.12 ng/g) decreased to 3.68 ± 0.14 ng/g in newly fertilized eggs. Resting level of cortisol dropped at three densities by day 18 of post fertilization. Then, cortisol increased at hatching stage to 1.16 ± 0.11, 1.20 ± 0.12 and 1.21 ± 0.14 ng/g at low, normal and high densities, respectively. There were no statistically significant differences between cortisol concentrations in three densities. The acute handling stress test (5-min out-of-water), conducted on embryos (48 h post fertilization, organogenesis and eyed stage) in three densities, revealed no differences in whole-body cortisol levels between stressed and unstressed experimental groups. At hatching stage in low-density group, level of cortisol increased but the difference with the pre-stress levels was not statistically significant. Furthermore, significant differences in cortisol levels of stressed and unstressed embryos were detected on hatching in normal and high density groups [1.20 ± 0.12 at time 0-1.49 ± 0.11 ng/g at 1 hps (hours post stress) and from 1.21 ± 0.14 at time 0 to 1.53 ± 0.10 ng/g at 3 hps, respectively]. The results showed no difference in profile of cortisol in different densities, but acute stress conducted on embryos, incubated in different densities, revealed differences in cortisol stress response at hatching between normal and high density, which lead to cortisol increase at hatching time. It indicates that the lag time in the cortisol response to stressors immediately after hatching does not occur when the siblings were stressed during the embryo stage. Results, finally, indicated that hypothalamus-pituitary-interrenal axis was active and responded to an acute stressor under normal and high density, but it is unresponsive to a stressor around hatching under low density.

Ontogenesis of the HPI axis and molecular regulation of the cortisol stress response during early development in Dicentrarchus labrax

The cortisol stress response and the molecular programming of the corticoid axis were characterized for the first time during early ontogeny in a Mediterranean marine teleost, the European sea bass (Dicentrarchus labrax). Sea bass embryos, pre-larvae and larvae at specific points of development were exposed to acute stressors and the temporal patterns of cortisol whole body concentrations and the expression of genes involved in corticosteroid biosynthesis, degradation and signaling were determined. Expression of genes (gr1, gr2, mr, crf) involved into the corticoid response regulation combined with histological data indicated that, although a cortisol stress response is evident for the first time around first feeding, a pattern becomes established in larvae at flexion until the formation of all fins. Moreover, mRNA transcript levels of 11β-hydroxylase and 11β-hsd2 showed a strong correlation with the whole body cortisol concentrations. Concluding, our data reveal the presence of an adaptive mechanism in European sea bass at early ontogeny enabling to cope with external stressful stimuli and provide a better insight into the onset and regulation of the stress response in this species.

Thyroid hormone signaling during early neurogenesis and its significance as a vulnerable window for endocrine disruption

The essential roles of thyroid hormone (TH) in perinatal brain development have been known for decades. More recently, many of the molecular mechanisms underlying the multiple effects of TH on proliferation, differentiation, migration, synaptogenesis and myelination in the developing nervous system have been elucidated. At the same time data from both epidemiological studies and animal models have revealed that the influence of thyroid signaling on development of the nervous system, extends to all periods of life, from early embryogenesis to neurogenesis in the adult brain. This review focuses on recent insights into the actions of TH during early neurogenesis. A key concept is that, in contrast to the previous ideas that only the unliganded receptor was implicated in these early phases, a critical role of the ligand, T3, is increasingly recognized. These findings are considered in the light of increasing knowledge of cell specific control of T3 availability as a function of deiodinase activity and transporter expression. These requirements for TH in the early stages of neurogenesis take on new relevance given the increasing epidemiological data on adverse effects of TH lack in early pregnancy on children's neurodevelopmental outcome. These ideas lead logically into a discussion on how the actions of TH during the first phases of neurogenesis can be potentially disrupted by gestational iodine lack and/or chemical pollution. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Metamorphosis in teleosts

Teleosts are the largest and most diverse group of vertebrates, and many species undergo morphological, physiological, and behavioral transitions, "metamorphoses," as they progress between morphologically divergent life stages. The larval metamorphosis that generally occurs as teleosts mature from larva to juvenile involves the loss of embryo-specific features, the development of new adult features, major remodeling of different organ systems, and changes in physical proportions and overall phenotype. Yet, in contrast to anuran amphibians, for example, teleost metamorphosis can entail morphological change that is either sudden and profound, or relatively gradual and subtle. Here, we review the definition of metamorphosis in teleosts, the diversity of teleost metamorphic strategies and the transitions they involve, and what is known of their underlying endocrine and genetic bases. We suggest that teleost metamorphosis offers an outstanding opportunity for integrating our understanding of endocrine mechanisms, cellular processes of morphogenesis and differentiation, and the evolution of diverse morphologies and life histories.

Thyroid hormone signaling in the Xenopus laevis embryo is functional and susceptible to endocrine disruption

Thyroid hormone (TH) is essential for vertebrate brain development. Most research on TH and neuronal development focuses on late development, mainly the perinatal period in mammals. However, in human infants neuromotor development correlates best with maternal TH levels in the first trimester of pregnancy, suggesting that TH signaling could affect early brain development. Studying TH signaling in early embryogenesis in mammals is experimentally challenging. In contrast, free-living embryos, such as Xenopus laevis, permit physiological experimentation independent of maternal factors. We detailed key elements of TH signaling: ligands, receptors (TR), and deiodinases during early X. laevis development, before embryonic thyroid gland formation. Dynamic profiles for all components were found. Between developmental stages 37 and 41 (~48 h after hatching, coincident with a phase of continuing neurogenesis) significant increases in T(3) levels as well as in mRNA encoding deiodinases and TR occurred. Exposure of embryos at this developmental stage for 24 h to either a TH antagonist, NH-3, or to tetrabromobisphenol A, a flame retardant and known TH disruptor, differentially modulated the expression of a number of TH target genes implicated in neural stem cell function or neural differentiation. Moreover, 24-h exposure to either NH-3 or tetrabromobisphenol A diminished cell proliferation in the brain. Thus, these data show first, that TH signaling exerts regulatory roles in early X. laevis neurogenesis and second, that this period represents a potential window for endocrine disruption.

Stressing zebrafish for behavioral genetics

The stress response is a normal reaction to a real or perceived threat. However, stress response systems that are overwhelmed or out of balance can increase both the incidence and severity of diseases including addiction and mood and anxiety disorders. Using an animal model with both genetic diversity and large family size can help discover the specific genetic and environmental contributions to these behavioral diseases. The stress response has been studied extensively in teleosts because of their importance in food production. The zebrafish (Danio rerio) is a major model organism with a strong record for use in developmental biology, genetic screening, and genomic studies. More recently, the stress response of larval and adult zebrafish has been documented. High-throughput automated tracking systems make possible behavioral readouts of the stress response in zebrafish. This non-invasive measure of the stress response can be combined with mutagenesis methods to dissect the genes involved in complex stress response behaviors in vertebrates. Understanding the genetic and epigenetic basis for the stress response in vertebrates will help to develop advanced screening and therapies for stress-aggravated diseases such as addiction and mood and anxiety disorders.

Identification of a β1 integrin isoform with restricted tissue expression in a teleost fish

The composition and organisation of extracellular matrix (ECM)-related molecules change during development. These components interact with different cell surface receptors to modulate the transduction of signals for cell growth, differentiation, migration, proliferation and apoptosis. Previous findings in the teleost fish gilthead seabream (Sparus aurata L., Teleostei), a marine protandrous hermaphrodite fish, showed that endocrine and immune stimuli are able to modulate the expression of ECM-related molecules, as well as specific correlations between them. In the present study, quantitative reverse transcription–polymerase chain reaction was used to examine the gene expression profile of β1 integrin isoform b (ITGB1b) and its possible role in reproductive physiology, especially in relation to spermatogenesis. Expression profiles were analysed in the context of the reproductive cycle (RC) and in relation with other ECM-related molecules, including matrix metalloproteinase (MMP)-2, MMP-9, MMP-13, tissue-specific inhibitor of metalloproteinase (TIMP)-2a, TIMP-2b, collagen (COL1A1) and ITGB1a. Expression of ITGB1b was found in the testis and brain and, to some extent, in endothelial cells. In contrast, ITGB1a was expressed ubiquitously. In the testis, the ITGB1b expression peaked during spermatogenesis, whereas the expression of the other ECM-related molecules is induced mainly during the post-spawning stage, both stages of marked tissue remodelling during the first and second RC in males. In addition, in fish exposed to the endocrine disruptor 17α-ethynyloestradiol (at 5 and 50 μg g–1 food during 7, 14 and 21 days), ITGB1b expression in the testis was inhibited in a dose- and time-dependent manner and was related to reduced serum levels of testosterone. Together, these results suggest a different functionality for the two ITGB1 isoforms in the gilthead seabream, where ITGB1b is more specifically involved in reproduction. This is the first report of an ITGB1 gene isoform whose expression is restricted to endocrine-related tissues in vertebrates.

The onset of cortisol synthesis and the stress response is independent of changes in CYP11B or CYP21 mRNA levels in larval red drum (Sciaenops ocellatus)

Although cortisol plays an important role in teleost development, the onset of cortisol production and the cortisol stress response in teleosts remain poorly understood. Here we have reported basal cortisol levels and the development of the cortisol stress response in larval red drum (Sciaenops ocellatus). We isolated partial nucleic acid sequences encoding two key corticosteroidogenic enzymes, CYP11B and CYP21 and assessed ontogenetic patterns of their mRNA levels relative to basal and stress-induced cortisol production. Basal cortisol was first detected 3 days post-hatch (DPH) and reached a maximum at 9 DPH. Cortisol did not increase in response to an acute stressor prior to 6 DPH. From 6 DPH forward, stress caused significant increases in larval cortisol content. Stress-induced cortisol levels in 6-9 DPH larvae were highest 1h post-stress. In larvae 11 DPH and older, the highest cortisol measurements occurred 0.5h post-stress. Elevated cortisol was still evident after 3h in 6 DPH larvae. From 11 DPH onward, basal cortisol levels were reestablished in larvae by 1h post-stress. CYP11B and CYP21 transcripts were detected in red drum 12h prior to hatching and in all post-hatch larvae examined. Changes in CYP11B and CYP21 mRNA levels did not occur in association with the ontogenetic appearance of cortisol, or the onset of the stress response. As larvae developed, the dynamics of the cortisol stress response matured from a low magnitude, slow recovery response, to a response similar to that observed in juvenile and adult fish.

Gender-related expression of TRalpha and TRbeta in the protandrous black porgy, Acanthopagrus schlegeli, during sex change processes

We cloned the thyroid hormone receptor alpha (TRalpha) and beta (TRbeta) cDNAs from the ovaries of the protandrous black porgy and compared the expression levels of TRalpha and TRbeta mRNA during the sex change in black porgy. We observed that the TRalpha mRNA by quantitative real-time PCR and protein levels by Western blot were highest in the mature ovaries. Additionally, TRbeta mRNA levels were only expressed highly in the mature ovaries when compared to any other gonadal stages. Then, we injected gonadotropin-releasing hormone analogue (GnRHa) to know the effects on TRs mRNA in immature black porgy. Injection with GnRHa resulted in a significant increase in TRalpha level while significantly reducing TRbeta level after 12h. We concluded that TRalpha was related in testicular development as well as ovarian development and TRbeta was only affect to ovarian development in black porgy. These results will provide a framework for better understanding of the role of TRs during sex change processes in this fish.

Ontogeny of the corticotropin-releasing factor system in zebrafish

The corticotropin-releasing factor (CRF) system in fish functions to maintain homeostasis during stress in part by regulating cortisol production via the hypothalamus-pituitary-interrenal (HPI) axis. Towards understanding the role of the CRF system in vertebrate development, we describe the ontogeny of the CRF system, cortisol, and the stress response in the zebrafish, Danio rerio. Early embryonic expression of mRNA encoding CRF, urotensin I (UI), CRF-binding protein (CRF-BP), and two CRF receptors (CRF-R1 and CRF-R2) suggest a function in the early organization of the developing embryo. The expression patterns of CRF, UI, and CRF-BP in the larval brain are consistent with the adult distribution patterns for these genes and support HPI-axis independent functions. The relative amounts of CRF and UI mRNA in the heads and tails of developing and adult zebrafish suggest that CRF functions primarily in the brain while UI also plays an important role in the caudal neurosecretory system. The amount of cortisol in developing zebrafish is low and relatively constant through the first 6 days of development. The commencement of feeding after 4 dpf, however, significantly increases basal cortisol production. Finally, we show that zebrafish larvae are able to respond to an osmotic stressor as early as 3 dpf. Overall, results from this study establish the zebrafish as a model species for research on stress during ontogeny and offer new insights into an HPI-axis independent function for the CRF system during embryogenesis.

Melatonin concentrations during larval and postlarval development of gilthead sea bream Sparus auratus: more than a time-keeping molecule?

In this study, melatonin (MEL) and thyroxine (T(4)) concentrations were measured during larval and postlarval development of gilthead sea bream Sparus auratus Hormones were measured in whole bodies of larvae or the head and trunk of postlarvae after 67 days of exposure to constant light, 24L:0D, constant darkness, 0L:24D or 12L:12D and in the plasma of 6 month juveniles kept under the 12L:12D, 0L:24D and 24L:0D regimes. High MEL concentrations in larvae suggested a distinct role of MEL in early organogenesis and development of S. auratus. In larvae, the gastro-intestinal tract seemed to be an important extrapineal and extraretinal source of MEL. No endogenous rhythm of MEL synthesis was demonstrated in 67 day larvae; however, in 6 month juveniles, it was evident. At early ontogenesis of S. auratus, the role of MEL is probably related mostly to the control of development and protection against free radicals, whereas its action as a time-keeping molecule develops later. The increase in T(4) concentration during the S. auratus larva-juvenile transition, i.e. between 50 and 70 days post-hatch, which was observed concurrently with the decrease of MEL concentration, may suggest an inverse relationship between T(4) and MEL.

Hormonally mediated maternal effects shape offspring survival potential in stressful environments

In most egg-laying vertebrates, maternal responses to stressful conditions are translated into the release of glucocorticoid hormones such as cortisol, which are then transmitted to their developing embryos. Although such maternally transmitted hormonal resources have been shown to influence or even interfere with the optimal developmental trajectories of offspring in many taxa, their influence on the dynamics of wild fish populations remains largely unexplored. Here, we examined the extent to which simulated hormonally mediated maternal effects influence the development and early survival of the coral reef damselfish, Pomacentrus amboinensis. Concentrations of cortisol in the eggs were manipulated within naturally occurring limits by immersion. We found that the proportion of embryos that delayed hatching when exposed to high levels of cortisol was considerably lower than in the other two treatments (low cortisol dose and control). High cortisol levels in P. amboinensis eggs resulted in increased egg mortality and greater asymmetry in hatchlings. For embryos that successfully hatched, individuals from the elevated cortisol treatments (especially low dose) survived longer after hatching. Although individuals that originated from eggs with elevated cortisol levels survived longer after hatching, they may not gain an overall survival advantage. Our results suggest that subtle increases in the allocation of maternally derived hormones, such as cortisol, to offspring are a direct way for stressed mothers to endow their young with an immediate survival advantage. We propose that this immediate benefit outweighs the developmental costs which may be expressed as reduced fitness at later life stages.

Gene expression patterns during the larval development of European sea bass (dicentrarchus labrax) by microarray analysis

During the larval period, marine teleosts undergo very fast growth and dramatic changes in morphology, metabolism, and behavior to accomplish their metamorphosis into juvenile fish. Regulation of gene expression is widely thought to be a key mechanism underlying the management of the biological processes required for harmonious development over this phase of life. To provide an overall analysis of gene expression in the whole body during sea bass larval development, we monitored the expression of 6,626 distinct genes at 10 different points in time between 7 and 43 days post-hatching (dph) by using heterologous hybridization of a rainbow trout cDNA microarray. The differentially expressed genes (n = 485) could be grouped into two categories: genes that were generally up-expressed early, between 7 and 23 dph, and genes up-expressed between 25 and 43 dph. Interestingly, among the genes regulated during the larval period, those related to organogenesis, energy pathways, biosynthesis, and digestion were over-represented compared with total set of analyzed genes. We discuss the quantitative regulation of whole-body contents of these specific transcripts with regard to the ontogenesis and maturation of essential functions that take place over larval development. Our study is the first utilization of a transcriptomic approach in sea bass and reveals dynamic changes in gene expression patterns in relation to marine finfish larval development.

Urea cycle enzymes through the development of pacu (Piaractus mesopotamicus): the role of ornithine carbamoyl transferase

The present work reports the activities of urea cycle enzymes during the ontogenic development of the teleost pacu (Piaractus mesopotamicus). Urea cycle enzymes from the kidney and liver of adult fish were compared with those from the fish's embryonic phases. Samples were evaluated over all phases of embryonic development, the larval period and alevin. Ammonia and urea concentrations were determined during embryogenesis and in the plasma of adult fish. Except for carbamoyl phosphate synthetase-III (CPS-III), all enzymes of the urea cycle were expressed in the larvae and alevins as well as in the liver and kidney of adult fish. In spite of the low level of activity of the ornithine urea cycle (OUC) enzymes compared to those in mammals, and the low levels of tissue urea concentration compared to ammonia, the ureogenesis was evaluated in pacu. Ammonia seems to be the main nitrogenous waste during embryonic development. In this phase glutamine synthetase (GS) may play a role in ammonia detoxification, and the OUC enzymes can be individually involved in functions other than urea production. The presence of ornithine carbamoyl transferase (OCT) in all developmental phases of pacu and in the adult liver and kidney suggests that this enzyme is performing different metabolic pathways. OCT in the kidney, wherein the activity is less than in the liver, should work in the biosynthesis of polyamines and control the arginine plasma concentration given that renal arginase and argininosuccinate synthetase-argininosuccinate lyase are more active than from the liver. We suppose that OCT during the embryogenesis is a control step regulating the cellular concentration of ornithine for polyamines synthesis.

Glucocorticoid receptor (DlGR1) is expressed in pre-larval and larval stages of the teleost fish Dicentrarchus labrax

Glucocorticoid hormone receptors (GR), members of the nuclear hormone receptor superfamily, are ligand-dependent transcription factors expressed in various tissues by binding to specific DNA sequences. Since glucocorticoids have a role in maintaining the homeostatic status in fish, we previously cloned and sequenced a GR (DlGR1) of adult Dicentrarchus labrax; we also showed mRNA expression (in situ hybridization) and tissue immunohistochemical localization of DlGR1 in several organs. This work has now been extended to the examination of the expression, tissue distribution, and cytolocalization of DlGR1 in larval developmental stages by similar methods to those used for the adult organs. The riboprobe included the DlGR1 cDNA transcriptional activation domain (1.0–1,300 nucleotide sequence) showing no significant similarity with a known second GR cDNA sequence of sea bass. The antibody was specific for an opportunely selected peptide sequence of the DlGR1 transcriptional domain. In histological sections of brain, head kidney, gills, liver, anterior intestine, and spleen cells, the riboprobe was mainly located in the cell nucleus. The antibody identified DlGR1 in the head kidney, gills, liver, and anterior intestine, mainly located in the cytosol. These results are in agreement with the receptor location in adult tissues. The greater presence of both the transcript and protein of DlGR1 in the late developmental stages suggests an increasing expression of this receptor. The cytolocalization (nuclear-cytosolic) and presumptive roles of DlGR1-containing tissues are discussed.

Comparative thyroidology: thyroid gland location and iodothyronine dynamics in Mozambique tilapia (Oreochromis mossambicus Peters) and common carp (Cyprinus carpio L.)

In teleosts, the thyroid gland is mostly found in the subpharyngeal region. However, in several species thyroid follicles are found in, for example, heart, head kidney and kidney. Such heterotopic thyroid follicles are active, and considered to work in concert with the subpharyngeal thyroid. In Mozambique tilapia (Oreochromis mossambicus) thyroid activity is, indeed, restricted to the subpharyngeal region; in common carp (Cyprinus carpio) the functional endocrine thyroid is associated with renal tissues. The subpharyngeal follicles of carp comprise only 10% of the total thyroid tissue, and these follicles neither accumulate iodide nor synthesize or secrete thyroid hormones to a significant degree. Although the shape and size of carp subpharyngeal and renal follicles vary, the epithelial cell height of the thyrocytes and thyroxine immunoreactivity do not differ, which suggests that the activity of the carp subpharyngeal thyroid follicles is dormant. Differences in thyroid physiology between the two fish species were further assessed at the level of peripheral thyroid hormone metabolism. Carp clears plasma of thyroid hormones faster than tilapia does. Furthermore, a significant amount of conjugated thyroid hormones was observed in the plasma of tilapia, which was preceded by the occurrence of thyroid hormone conjugates in the subpharyngeal region and coincides with the appearance of conjugates in the surrounding water. Apparently, plasma thyroid hormone conjugates in tilapia originate from the thyroid gland and function in the excretion of thyroid hormones. Our data illustrate the variability in teleostean thyroidology, an important notion for those studying thyroid physiology.

The thyroid gland and thyroid hormones in Senegalese sole (Solea senegalensis) during early development and metamorphosis

We here describe the ontogeny and morphology of the thyroid gland in Senegalese sole (Solea senegalensis), and correlate these with whole body concentrations of thyroid hormones during early development and metamorphosis. Under our rearing conditions at 19.5 degrees C, most larvae entered metamorphosis in stage 1 at 15 days post-hatching (dph), and completed metamorphosis in stage 4 at 25dph. The onset of metamorphosis coincided with surges in whole body T4 and T3 concentrations. Crossmon's trichrome stain colored the lumen of follicular structures brightly red, and this co-localized with a T4-immunoreactivity. Thyroid follicles were first observed in stage 0 pre-metamorphic larvae at 5dph of age, and were detected exclusively in the subpharyngeal region, surrounding the ventral aorta. Increases in whole body thyroid hormone levels coincided with a 2(1/2)-fold increase in the total thyroidal colloid area in stage 1 larvae (aged 15dph) compared to stage 0 larvae (12dph). This was preceded by an approximately 40%-increase in the follicles' epithelial cell height in stage 0 larvae at 12dph compared to larvae at 5dph, and by an increase in the whole body T3/T4 ratio, indicative of an increase in outer ring deiodination. We conclude that in S. senegalensis there is a clear chronology in the activation of the thyroid gland that starts in early pre-metamorphic larvae.

Thyroid hormones down-regulate thyrotropin beta subunit and thyroglobulin during metamorphosis in the flatfish Senegalese sole (Solea senegalensis Kaup)

Thyroid hormones (TH) play a critical role in flatfish metamorphosis. Their levels are regulated by the pituitary-thyroid axis. The expression profile of thyroid stimulating hormone (TSH) beta subunit and thyroglobulin (Tg) was investigated using a real-time PCR approach. Both genes exhibited different expression patterns during larval development in Senegalese sole. TSH beta mRNAs reduced progressively at the commencement of metamorphosis. On the contrary, Tg transcripts increased sharply at the onset of metamorphosis and dropped after the metamorphosis climax. T4 levels, as determined by radioimmunoassay, clearly resembled the Tg expression profile with a peak at the metamorphosis climax. To investigate if such expression profiles were regulated by TH, premetamorphic larvae were exposed to the goitrogen thiourea (TU). TU-treated larvae were not able to complete metamorphosis. However, the addition of exogenous T4 enabled to revert this effect. Expression analysis showed higher mRNA levels of both TSH beta and Tg in TU-treated larvae in comparison to control larvae. Moreover, the TU+T4 treated larvae exhibited similar or lower mRNA levels than in the control. Present results demonstrate that TH mediate metamorphosis and down-regulate TSH beta and Tg at transcriptional level in Senegalese sole.

Development of the corticosteroid stress axis and receptor expression in zebrafish

Using zebrafish embryos and larvae, we examined the temporal patterns of cortisol and expression of genes involved in corticosteroid synthesis and signaling. Embryonic cortisol levels decreased approximately 70% from 1.5 h postfertilization (hpf) to hatch (approximately 42 hpf) and then increased 27-fold by 146 hpf. The mRNA abundances of steroidogenic acute regulatory protein, 11beta-hydroxylase and 11beta-hydroxysteroid dehydrogenase type 2, increased severalfold after hatch and preceded the rise of cortisol levels. In contrast to other teleosts that possess two glucocorticoid receptors (GRs) and one mineralocorticoid receptor (MR), only one GR and MR were identified in zebrafish, which were cloned and sequenced. GR mRNA abundance decreased from 1.5 to 25 hpf, rebounded, and then was stable from 49 to 146 hpf. MR transcripts increased continuously from 1.5 hpf and were 52-fold higher by 97 hpf. An acute cortisol response to a stressor was not detected until 97 hpf, whereas melanocortin type 2 receptor mRNA increased between 25 and 49 hpf. Collectively, the patterns of cortisol and the expression of cortisol biosynthetic genes and melanocortin type 2 receptor suggest that the corticoid stress axis in zebrafish is fully developed only after hatch. The temporal differences in GR, MR, and 11beta-hydroxysteroid dehydrogenase type 2 gene expression lead us to propose a key role for MR signaling by maternal cortisol during embryogenesis, whereas cortisol secretion after hatch may be regulating GR expression and signaling in zebrafish.

Regulation of troponin T expression during muscle development in sea bream Sparus auratus Linnaeus: the potential role of thyroid hormones

In the sea bream Sparus auratus three stage-specific fast troponin T (fTnT) isoforms have been cloned and correspond to embryonic-, larval- and adult-specific isoforms. Characterisation, using database searches, of the putative genomic organisation of Fugu rubripes and Tetraodon nigroviridis fTnT indicates that alternative exon splicing in the 5 region of the gene generates the different isoforms. Moreover, comparison of teleost fTnTs suggests that alternative splicing of fTnT appears to be common in teleosts. A different temporal expression pattern for each fTnT splice varotnt is found during sea bream development and probably relates to differing functional demands, as a highly acidic embryonic form (pI 5.16) is substituted by a basic larval form (pI 9.57). Thyroid hormones (THs), which play an important regulatory role in muscle development in flatfish and tetrapods, appear also to influence TnT gene expression in the sea bream. However, THs have a divergent action on different sea bream TnT genes and although the slow isoform (sTnT1) is TH-responsive, fTnT, sTnT2 and the itronless isoform (iTnT) are unaffected. The present results taken together with those published for flatfish seem to suggest differences may exist in the regulation of larval muscle development in teleosts.

Hepatic and muscle expression of thyroid hormone receptors in association with body and muscle growth in large yellow croaker, Pseudosciaena crocea (Richardson)

The role of thyroid hormone (TH) and its receptors (TRs) in the regulation of body growth and muscle accretion is well established in mammals and birds, whereas the involvement of THs and TRs in fish growth, especially during the muscle accretion period of juvenile-adult transition, is unknown. This study describes the cloning of the partial cDNA sequences of TRalpha and TRbeta in large yellow croaker, Pseudosciaena crocea (Richardson) and the patterns of TRalpha and TRbeta mRNA expression in liver and muscle of 1- and 2-year-old large yellow croaker, associated with changes in body mass and muscle characteristics. Two TRalpha isoforms (TRalpha1, TRalpha2) and TRbeta were identified in large yellow croaker. The deduced amino acid sequences showed high homology to the TRs of human and other teleosts. Hepatic TRbeta mRNA expression was markedly lower in 2-year-old large yellow croaker compared with the 1-year-old, while no significant age difference was observed for hepatic TRalpha mRNA expression. Muscle expression of TRalpha mRNA was significantly higher in 2-year-old large yellow croaker, whereas TRbeta exhibited no significant age difference. Meanwhile, serum concentration of T(4) was significantly decreased in 2-year-old large yellow croaker, but no change was observed for T(3). The body mass, fork length and body height of 2-year-old large yellow croaker were 4.7, 1.6 and 1.7 times greater, respectively compared with that of 1-year-old. Average diameters of skeletal muscle in 2-year-old large yellow croaker were remarkably larger than that in 1-year-old with no significant difference in muscle crude fat content. The down-regulation of hepatic TRbeta expression was associated with the decrease in general growth rate and the increase in muscle expression of TRalpha was accompanied with muscle accretion and myofiber hypertrophy, implicating the different roles of TRs in the regulation of growth in large yellow croaker.

The Effects of Methimazole on Development of the Fathead Minnow, Pimephales promelas, from Embryo to Adult

The importance of thyroid hormones in regulating early developmental processes of many amphibian and fish species is well known, but the impacts of exposure to disrupters of thyroid homeostasis during the embryo-larval-juvenile transitions are unclear. To investigate these impacts, fathead minnows, Pimephales promelas, were exposed to a model thyroid axis disrupter, methimazole, an inhibitor of thyroid hormone synthesis, at control (0), 32, 100, and 320 mug/l, starting at <24-h postfertilization, for 28, 56, and 83/84 days postfertilization (dpf). Thyroid disruption was evident at 28 dpf, when survival was significantly reduced by 32 or 100 mug/l methimazole concomitant with a reduced thyroxine (T(4)) content. However, the T(3) content of these fish was similar to that of control fish, and body mass was unaffected (as in all groups), suggesting compensatory mechanisms overcame reduced T(4) synthesis. At the highest concentration of methimazole (320 mug/l), activation of feedback mechanisms on the hypothalamic-pituitary-thyroid axis was suggested by the normal T(4) content after 28 dpf exposure to methimazole, although triiodothyronine (T(3)) content of these fish was significantly reduced. The generally less pronounced disruption of thyroid hormone homeostasis after 56 days exposure to methimazole also suggests compensatory mechanisms in juvenile/adult fish that may regulate T(4) content, despite exposure to methimazole at 32 or 100 mug/l (in fish held in 320 mug/l methimazole, the T(4) content was significantly higher than in controls). Whole body T(3) content at 56 dpf was significantly depressed only in fish held in 100 mug/l methimazole. By 83/84 dpf, length, body mass, and thyroid hormone concentrations were similar in all experimental groups and controls, indicating that adult fish may achieve regulation of their thyroid axis despite prolonged exposures to thyroid disruptors throughout early development.