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

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Impaired anterior swim bladder inflation following exposure to the thyroid peroxidase inhibitor 2-mercaptobenzothiazole part I: Fathead minnow

In the present study, a hypothesized adverse outcome pathway linking inhibition of thyroid peroxidase (TPO) activity to impaired swim bladder inflation was investigated in two experiments in which fathead minnows (Pimephales promelas) were exposed to 2-mercaptobenzothiazole (MBT). Continuous exposure to 1mg MBT/L for up to 22 days had no effect on inflation of the posterior chamber of the swim bladder, which typically inflates around 6 days post fertilization (dpf), a period during which maternally-derived thyroid hormone is presumed to be present. In contrast, inflation of the anterior swim bladder, which occurs around 14dpf, was impacted. Specifically, at 14dpf, approximately 50% of fish exposed to 1mg MBT/L did not have an inflated anterior swim bladder. In fish exposed to MBT through 21 or 22dpf, the anterior swim bladder was able to inflate, but the ratio of the anterior/posterior chamber length was significantly reduced compared to controls. Both abundance of thyroid peroxidase mRNA and thyroid follicle histology suggest that fathead minnows mounted a compensatory response to the presumed inhibition of TPO activity by MBT. Time-course characterization showed that fish exposed to MBT for at least 4 days prior to normal anterior swim bladder inflation had significant reductions in anterior swim bladder size, relative to the posterior chamber, compared to controls. These results, along with similar results observed in zebrafish (see part II, this issue) are consistent with the hypothesis that thyroid hormone signaling plays a significant role in mediating anterior swim bladder inflation and development in cyprinids, and that role can be disrupted by exposure to thyroid hormone synthesis inhibitors. Nonetheless, possible thyroid-independent actions of MBT on anterior swim bladder inflation cannot be ruled out based on the present results. Overall, although anterior swim bladder inflation has not been directly linked to survival as posterior swim bladder inflation has, potential links to adverse ecological outcomes are plausible given involvement of the anterior chamber in sound production and detection.

Deiodinase knockdown affects zebrafish eye development at the level of gene expression, morphology and function

Retinal development in vertebrates relies extensively on thyroid hormones. Their local availability is tightly controlled by several regulators, including deiodinases (Ds). Here we used morpholino technology to explore the roles of Ds during eye development in zebrafish. Transcriptome analysis at 3 days post fertilization (dpf) revealed a pronounced effect of knockdown of both T4-activating Ds (D1D2MO) or knockdown of T3-inactivating D3 (D3bMO) on phototransduction and retinoid recycling. This was accompanied by morphological defects (studied from 1 to 7 dpf) including reduced eye size, disturbed retinal lamination and strong reduction in rods and all four cone types. Defects were more prominent and persistent in D3-deficient fish. Finally, D3-deficient zebrafish larvae had disrupted visual function at 4 dpf and were less sensitive to a light stimulus at 5 dpf. These data demonstrate the importance of TH-activating and -inactivating Ds for correct zebrafish eye development, and point to D3b as a central player.

Deiodinase knockdown during early zebrafish development affects growth, development, energy metabolism, motility and phototransduction

Thyroid hormone (TH) balance is essential for vertebrate development. Deiodinase type 1 (D1) and type 2 (D2) increase and deiodinase type 3 (D3) decreases local intracellular levels of T3, the most important active TH. The role of deiodinase-mediated TH effects in early vertebrate development is only partially understood. Therefore, we investigated the role of deiodinases during early development of zebrafish until 96 hours post fertilization at the level of the transcriptome (microarray), biochemistry, morphology and physiology using morpholino (MO) knockdown. Knockdown of D1+D2 (D1D2MO) and knockdown of D3 (D3MO) both resulted in transcriptional regulation of energy metabolism and (muscle) development in abdomen and tail, together with reduced growth, impaired swim bladder inflation, reduced protein content and reduced motility. The reduced growth and impaired swim bladder inflation in D1D2MO could be due to lower levels of T3 which is known to drive growth and development. The pronounced upregulation of a large number of transcripts coding for key proteins in ATP-producing pathways in D1D2MO could reflect a compensatory response to a decreased metabolic rate, also typically linked to hypothyroidism. Compared to D1D2MO, the effects were more pronounced or more frequent in D3MO, in which hyperthyroidism is expected. More specifically, increased heart rate, delayed hatching and increased carbohydrate content were observed only in D3MO. An increase of the metabolic rate, a decrease of the metabolic efficiency and a stimulation of gluconeogenesis using amino acids as substrates may have been involved in the observed reduced protein content, growth and motility in D3MO larvae. Furthermore, expression of transcripts involved in purine metabolism coupled to vision was decreased in both knockdown conditions, suggesting that both may impair vision. This study provides new insights, not only into the role of deiodinases, but also into the importance of a correct TH balance during vertebrate embryonic development.

Knockdown of type 3 iodothyronine deiodinase severely perturbs both embryonic and early larval development in zebrafish

Exposure to appropriate levels of thyroid hormones (THs) at the right time is of key importance for normal development in all vertebrates. Type 3 iodothyronine deiodinase (D3) is the prime TH-inactivating enzyme, and its expression is highest in the early stages of vertebrate development, implying that it may be necessary to shield developing tissues from overexposure to THs. We used antisense morpholino knockdown to examine the role of D3 during early development in zebrafish. Zebrafish possess 2 D3 genes, dio3a and dio3b. Here, we show that both genes are expressed during development and both contribute to in vivo D3 activity. However, dio3b mRNA levels in embryos are higher, and the effects of dio3b knockdown on D3 activity and on the resulting phenotype are more severe. D3 knockdown induced an overall delay in development, as determined by measurements of otic vesicle length, eye and ear size, and body length. The time of hatching was also severely delayed in D3-knockdown embryos. Importantly, we also observed a severe disturbance of several aspects of development. Swim bladder development and inflation was aberrant as was the development of liver and intestine. Furthermore, D3-knockdown larvae spent significantly less time moving, and both embryos and larvae exhibited perturbed escape responses, suggesting that D3 knockdown affects muscle development and/or functioning. These data indicate that D3 is essential for normal zebrafish embryonic and early larval development and show the value of morpholino knockdown in this model to further elucidate the specific role of D3 in some aspects of vertebrate development.

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

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

Corticosterone stimulates hatching of late-term tree lizard embryos

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

Toxicity of a phytosterol mixture to grayling (Thymallus thymallus) during early developmental stages

The study concerns the toxicity of a phytosterol mixture, ultrasitosterol, consisting mainly of beta-sitosterol 75.7% and beta-sitostanol 13.0%, to grayling (Thymallus thymallus) embryos. Eyed eggs were exposed to three concentrations (1 microg/l, 10 microg/l, and 50 microg/l) of ultrasitosterol for 4 weeks. Embryos and later on hatched fry were taken for triiodothyronine (T3), thyroxine (T4), and histopathological analyses after 7, 14, 21, and 28 days of exposure. Most of the eggs (>95%) hatched during the first week of exposure, and ultrasitosterol treatment shortened hatching time significantly (p < or = 0.0001) at all exposure concentrations in comparison to the control. Ultrasitosterol did not have any significant effect on T3 or T4 levels in the embryo extracts. However, an interesting observation was that T3 levels increased in all treatments and in the control near the time of hatching. In conclusion, ultrasitosterol showed potential to affect the development of grayling embryos and fry, but further long-term exposure experiments are needed to verify these changes in more detail.

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

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