Exposure to PCB 77 induces tissue-dependent changes in iodothyronine deiodinase activity patterns in the embryonic chicken

Halogen Bonding Interactions of Polychlorinated Biphenyls and the Potential for Thyroid Disruption

Polychlorinated biphenyl (PCB) flame retardants are persistent pollutants and inhibit neurodevelopment, particularly in the early stages of life. Halogen bonding (XB) to the iodothyronine deiodinases (Dio) that modulate thyroid hormones (THs) is a potential mechanism for endocrine disruption. Cl⋅⋅⋅Se XB interactions of PCBs with SeMe- , a small model of the Dio active site selenocysteine, are compared with previous results on polybrominated diphenylethers (PBDEs) and THs using density functional theory. PCBs generally display weaker XB interactions compared to PBDEs and THs, consistent with the dependence of XB strength on the size of the halogen (I>Br>Cl). PCBs also do not meet a proposed energy threshold for substrates to undergo dehalogenation, suggesting they may behave as competitive inhibitors of Dio in addition to other mechanisms of endocrine disruption. XB interactions in PCBs are position-dependent, with ortho interactions slightly more favorable than meta and para interactions, suggesting that PCBs may have a greater effect on certain classes of Dio. Flexibility of PCBs around the biphenyl C-C bond is limited by ortho substitutions relative to the biphenyl linkage, which may contribute to the ability to inhibit Dio and other TH-related proteins.

Dissecting the role of regulators of thyroid hormone availability in early brain development: Merits and potential of the chicken embryo model

Thyroid hormones (THs) are important mediators of vertebrate central nervous system (CNS) development, thereby regulating the expression of a wide variety of genes by binding to nuclear TH receptors. TH transporters and deiodinases are both needed to ensure appropriate intracellular TH availability, but the precise function of each of these regulators and their coaction during brain development is only partially understood. Rodent knockout models already provided some crucial insights, but their in utero development severely hampers research regarding the role of TH regulators during early embryonic stages. The establishment of novel gain- and loss-of-function techniques has boosted the position of externally developing non-mammalian vertebrates as research models in developmental endocrinology. Here, we elaborate on the chicken as a model organism to elucidate the function of TH regulators during embryonic CNS development. The fast-developing, relatively big and accessible embryo allows easy experimental manipulation, especially at early stages of brain development. Recent data on the characterisation and spatiotemporal expression pattern of different TH regulators in embryonic chicken CNS have provided the necessary background to dissect the function of each of them in more detail. We highlight some recent advances and important strategies to investigate the role of TH transporters and deiodinases in various CNS structures like the brain barriers, the cerebellum, the retina and the hypothalamus. Exploiting the advantages of this non-classical model can greatly contribute to complete our understanding of the regulation of TH bioavailability throughout embryonic CNS development.

Association between exposure to organochlorine compounds and maternal thyroid status: Role of the iodothyronine deiodinase 1 gene

INTRODUCTION

Exposure to organochlorine compounds (OCs) may interfere with thyroid hormone (TH) homeostasis. The disruption of the deiodinase (DIO) enzymes has been proposed as a mechanism of action.

AIM

To evaluate the association between exposure to OCs and TH status in pregnant women, as well as to explore the role of genetic variations in the DIO1 and DIO2 genes.

METHODS

The study population (n=1128) was composed of pregnant women who participated in the INMA Project (Spain, 2003-2006). Hexachlorobenzene (HCB), 1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene (4,4´-DDE), b-hexachlorocyclohexane (b-HCH), polychlorobiphenyl (PCB) congeners 138, 153 and 180, thyroid stimulating hormone (TSH), total triiodothyronine (TT3) and free thyroxine (FT4) were measured in serum samples taken during the first trimester of pregnancy (mean [standard deviation (SD)]: 13.5 [2] weeks of gestation). Polymorphisms in DIO1 (rs2235544) and DIO2 (rs12885300) were genotyped in maternal DNA. Sociodemographic and dietary characteristics were obtained by questionnaire.

RESULTS

A 2-fold increase in HCB was associated with lower TT3 (% change=-1.48; 95%CI: -2.36, -0.60). Women in the third tertile for b-HCH had lower TT3 (% change=-3.19; 95%CI: -5.64, -0.67). The interactions between DIO1 rs2235544 and PCB153 and b-HCH were statistically significant. The inverse association between PCB153 and TT3 was the strongest among women with AA genotype. Women with CC genotype presented the strongest inverse association between b-HCH and FT4.

CONCLUSION

Exposure to HCB and b-HCH was associated to a disruption in maternal TT3. The DIO1 rs2235544 SNP modified the association between exposure to some of the OCs (specifically b-HCH and PCB153) and maternal thyroid hormone levels. These results strengthen the hypothesis that DIO enzymes play a role in explaining the disruption of thyroid hormones in relation to exposure to OCs.

Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming

The hypothalamus-pituitary-thyroid feedback control is a dynamic, adaptive system. In situations of illness and deprivation of energy representing type 1 allostasis, the stress response operates to alter both its set point and peripheral transfer parameters. In contrast, type 2 allostatic load, typically effective in psychosocial stress, pregnancy, metabolic syndrome, and adaptation to cold, produces a nearly opposite phenotype of predictive plasticity. The non-thyroidal illness syndrome (NTIS) or thyroid allostasis in critical illness, tumors, uremia, and starvation (TACITUS), commonly observed in hospitalized patients, displays a historically well-studied pattern of allostatic thyroid response. This is characterized by decreased total and free thyroid hormone concentrations and varying levels of thyroid-stimulating hormone (TSH) ranging from decreased (in severe cases) to normal or even elevated (mainly in the recovery phase) TSH concentrations. An acute versus chronic stage (wasting syndrome) of TACITUS can be discerned. The two types differ in molecular mechanisms and prognosis. The acute adaptation of thyroid hormone metabolism to critical illness may prove beneficial to the organism, whereas the far more complex molecular alterations associated with chronic illness frequently lead to allostatic overload. The latter is associated with poor outcome, independently of the underlying disease. Adaptive responses of thyroid homeostasis extend to alterations in thyroid hormone concentrations during fetal life, periods of weight gain or loss, thermoregulation, physical exercise, and psychiatric diseases. The various forms of thyroid allostasis pose serious problems in differential diagnosis of thyroid disease. This review article provides an overview of physiological mechanisms as well as major diagnostic and therapeutic implications of thyroid allostasis under a variety of developmental and straining conditions.

The effect of maternal exposure to endocrine disrupting chemicals on fetal and neonatal development: A review on the major concerns

There is a widespread exposure of general population, including pregnant women and developing fetuses, to the endocrine disrupting chemicals (EDCs). These chemicals have been reported to be present in urine, blood serum, breast milk, and amniotic fluid. Endocrine disruptions induced by environmental toxicants have placed a heavy burden on society, since environmental exposures during critical periods of development can permanently reprogram normal physiological responses, thereby increasing susceptibility to disease later in life-a process known as developmental reprogramming. During development, organogenesis and tissue differentiation occur through a continuous series of tightly-regulated and precisely-timed molecular, biochemical, and cellular events. Humans may encounter EDCs daily and during all stages of life, from conception and fetal development through adulthood and senescence. Nevertheless, prenatal and early postnatal windows are the most critical for proper development, due to rapid changes in system growth. Although there are still gaps in our knowledge, currently available data support the urgent need for health and environmental policies aimed at protecting the public and, in particular, the developing fetus and women of reproductive age. Birth Defects Research (Part C) 108:224-242, 2016. © 2016 Wiley Periodicals, Inc.

Bisphenol S alters embryonic viability, development, gallbladder size, and messenger RNA expression in chicken embryos exposed via egg injection

Amid concerns about the toxicological effects and environmental prevalence of bisphenol A (BPA), efforts to find suitable, safer replacement alternatives are essential. Bisphenol S (BPS) is a potential chemical substitute for BPA; however, few studies are available confirming that it has a more desirable ecotoxicological profile. In the present study, BPS was injected into the air cell of unincubated, fertilized chicken embryos at 6 concentrations ranging from 0 μg/g to 207 μg/g egg to determine effects on pipping success, development, hepatic messenger ribonucleic acid (mRNA) expression, thyroid hormone levels, and circulating bile acid concentrations. Concentrations of BPS increased in a dose-dependent manner in whole-embryo homogenates, and exposure to the highest dose, 207 μg/g, resulted in decreased pipping success (estimated median lethal dose  = 279 μg/g; 95% confidence interval = 161-486 μg/g). Exposure to BPS also reduced growth metrics including embryo mass and tarsus length, whereas the most pronounced phenotypic effect was the concentration-dependent, significant increase in gallbladder size at concentrations ≥52.8 μg/g. These adverse phenotypic outcomes were associated with the modulation of gene targets from a chicken ToxChip polymerase chain reaction array, which are involved with xenobiotic metabolism, lipid homeostasis, bile acid synthesis, and the thyroid hormone pathway. Expression levels of 2 estrogen-responsive genes, apolipoprotein II and vitellogenin, were too low at the sampling time point assessed (i.e., pipping embryos) to quantify changes, and no effects were observed on circulating free thyroxine or bile acid concentrations. The present study provides novel, whole-animal toxicological data for a BPA replacement alternative that is not well characterized. Environ Toxicol Chem 2016;35:1541-1549. © 2015 SETAC.

Associations between organohalogen concentrations and transcription of thyroid-related genes in a highly contaminated gull population

A number of studies have reported altered circulating thyroid hormone levels in birds exposed either in controlled settings or in their natural habitat to ubiquitous organohalogen compounds including organochlorines (OCs) and polybrominated diphenyl ether (PBDE) flame retardants. However, limited attention has been paid to underlying homeostatic mechanisms in wild birds such as changes in the expression of genes in the hypothalamic-pituitary-thyroid (HPT) axis. The objective of the present study was to investigate the relationships between hepatic concentrations of major organohalogens (PBDEs and OCs), and circulating thyroid hormone (free and total thyroxine (T4) and triiodothyronine (T3)) levels and transcription of 14 thyroid-related genes in three tissues (thyroid, brain, and liver) of an urban-adapted bird exposed to high organohalogen concentrations in the Montreal area (QC, Canada), the ring-billed gull (Larus delawarensis). Positive correlations were found between liver concentrations of several polychlorinated biphenyls (PCBs), PBDEs as well as chlordanes and total plasma T4 levels. Hepatic concentrations of several PBDEs were negatively correlated with mRNA levels of deiodinase type 3, thyroid peroxidase, and thyroid hormone receptor β (TRβ) in the thyroid gland. Liver PCB (deca-CB) correlated positively with mRNA levels of sodium-iodide symporter and TRα. In brain, concentrations of most PBDEs were positively correlated with mRNA levels of organic anion transporter protein 1C1 and transthyretin, while PCBs positively correlated with expression of TRα and TRβ as well as deiodinase type 2. These multiple correlative linkages suggest that organohalogens operate through several mechanisms (direct or compensatory) involving gene transcription, thus potentially perturbing the HPT axis of this highly organohalogen-contaminated ring-billed gull population.

The PI3K/Akt and ERK pathways elevate thyroid hormone receptor β1 and TRH receptor to decrease thyroid hormones after exposure to PCB153 and p,p'-DDE

PCBs and DDT cause the disturbance of thyroid hormone (TH) homeostasis in humans and animals. To test the hypothesis that the PI3K/Akt and MAPK pathways would play significant roles in TH imbalance caused by PCBs and DDT, Sprague-Dawley rats were dosed with PCB153 and p,p'-DDE intraperitoneally for 5 consecutive days, and human thyroid follicular epithelial (Nthy-ori 3-1 cell line) were treated with PCB153 and p,p'-DDE for different time. Results showed that serum total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3) and thyroid stimulating hormone (TSH) were decreased, whereas serum free triiodothyronine (FT3) and thyrotropin releasing hormone (TRH) were not changed. The PI3K/Akt and ERK pathways were activated in vivo and in vitro after the treatment with PCB153 and p,p'-DDE. Moreover, TH receptor β1 (TRβ1) was elevated after the activation of the PI3K/Akt pathway and was depressed after the inhibition of the PI3K/Akt pathway; TRH receptor (TRHr) was increased after the activation of the ERK pathway and was decreased after the inhibition of the ERK pathway. Though TH receptor α1 (TRα1) level was increased in the hypothalamus, TRα1 and TSHr were not influenced by the status of signaling pathways in in vitro study. Taken together, after exposure to PCB153 and p,p'-DDE, activated PI3K/Akt and ERK pathways disrupt the hypothalamic-pituitary-thyroid (HPT) axis via TRβ1 and TRHr and then decrease TH levels, and that would be a potential mechanism by which PCBs and DDT disturb TH homeostasis.

Thyroid hormones and deiodinase activity in plasma and tissues in relation to high levels of organohalogen contaminants in East Greenland polar bears (Ursus maritimus)

Previous studies have shown relationships between organohalogen contaminants (OHCs) and circulating levels of thyroid hormones (THs) in arctic wildlife. However, there is a lack of knowledge concerning the possible functional effects of OHCs on TH status in target tissues for TH-dependent activity. The relationships between circulating (plasma) levels of OHCs and various TH variables in plasma as well as in liver, muscle and kidney tissues from East Greenland sub-adult polar bears (Ursus maritimus) sampled in 2011 (n=7) were therefore investigated. The TH variables included 3.3',5.5'-tetraiodothyronine or thyroxine (T4), 3.3',5-triiodothyronine (T3) and type 1 (D1) and type 2 (D2) deiodinase activities. Principal component analysis (PCA) combined with correlation analyses demonstrated negative relationships between individual polychlorinated biphenyls (PCBs) and their hydroxylated (OH-) metabolites and T4 in both plasma and muscle. There were both positive and negative relationships between individual OHCs and D1 and D2 activities in muscle, liver and kidney tissues. In general, PCBs, OH-PCBs and polybrominated dipehenyl ethers (PBDEs) were positively correlated to D1 and D2 activities, whereas organochlorine pesticides and byproducts (OCPs) were negatively associated with D1 and D2 activities. These results support the hypothesis that OHCs can affect TH status and action in the target tissues of polar bears. TH levels and deiodinase activities in target tissues can be sensitive endpoints for exposure of TH-disrupting compounds in arctic wildlife, and thus, tissue-specific responses in target organs should be further considered when assessing TH disruption in wildlife studies.

Behavioral changes following PCB 153 exposure in the spontaneously hypertensive rat - an animal model of Attention-Deficit/Hyperactivity Disorder

Background

Attention-Deficit/Hyperactivity Disorder (ADHD) is a behavioral disorder affecting 3-5% of children. Although ADHD is highly heritable, environmental factors like exposure during early development to various toxic substances like polychlorinated biphenyls (PCBs) may contribute to the prevalence. PCBs are a group of chemical industrial compounds with adverse effects on neurobiological and cognitive functioning, and may produce behavioral impairments that share significant similarities with ADHD. The present study examined the relation between exposure to PCB 153 and changes in ADHD-like behavior in an animal model of ADHD, the spontaneously hypertensive rats (SHR/NCrl), and in Wistar Kyoto (WKY/NHsd) controls.

Methods

SHR/NCrl and WKY/NHsd, males and females, were orally given PCB 153 dissolved in corn oil at around postnatal day (PND) 8, 14, and 20 at a dosage of 1, 3 or 6 mg/kg bodyweight at each exposure. The control groups were orally administered corn oil only. The animals were behaviorally tested for exposure effects from PND 37 to 64 using an operant procedure.

Results

Exposure to PCB 153 was associated with pronounced and long-lasting behavioral changes in SHR/NCrl. Exposure effects in the SHR/NCrl depended on dose, where 1 mg/kg tended to reduce ADHD-like behaviors and produce opposite behavioral effects compared to 3 mg/kg and 6 mg/kg, especially in the females. In the WKY/NHsd controls and for the three doses tested, PCB 153 exposure produced a few specific behavioral changes only in males. The data suggest that PCB 153 exposure interacts with strain and sex, and also indicate a non-linear dose–response relation for the behaviors observed.

Conclusions

Exposure to PCB 153 seems to interact with several variables including strain, sex, dose, and time of testing. To the extent that the present findings can be generalized to humans, exposure effects of PCB 153 on ADHD behavior depends on amount of exposure, where high doses may aggravate ADHD symptoms in genetically vulnerable individuals. In normal controls, exposure may not constitute an environmental risk factor for developing the full range of ADHD symptoms, but can produce specific behavioral changes.

The effect of chitosan on the concentration of 17β-estradiol and free triiodothyronine in mice exposed to polychlorinated biphenyls (PCBs)

The aim of this work was to examine (i) how the applied PCB mixture influences the level of 17β-estradiol (E2) and free triiodothyronine (FT3) in the blood plasma of mice (C57/BL/6J) and (ii) whether supplementation with chitosan would protect against the observed changes in the examined plasma hormone concentrations. In the study we used a mixture of indicator PCBs (CB no. 28, 52, 101, 118, 138, 153, 180) and our results showed their anti-estrogenic effects. Exposure to the mixture resulted in a significant decrease (P < 0.05) in plasma concentration of E2 relative to the control, and chitosan administration did not prevent the decrease. To the contrary, E2 concentration in the blood plasma of the mice which received both the PCB mixture and chitosan was lower compared to those which did not receive chitosan. Exposure to the PCBs also resulted in a decrease in FT3 concentration in the treatedgroup, although it was not as pronounced as for E2 and was prevented with dietary supplementation with chitosan, with the observed FT3 level in the chitosan-treated group similar to the control. In summary, supplementation with chitosan can only to a certain extent minimize the negative effects of exposure to PCBs.

Contrasting retinoid and thyroid hormone status in differentially-contaminated northern fulmar colonies from the Canadian Arctic, Svalbard and the Faroe Islands

The northern fulmar (Fulmarus glacialis) has previously been shown to accumulate a wide range, and occasionally high concentrations of organochlorines (OCs) (e.g., PCBs, chlorobenzenes, DDT- and chlordane-related compounds, dioxins and furans). The present study aimed to investigate, using a meta-analysis approach, the variations in cytochrome P450 (CYP) 1A-like enzyme induction based on ethoxyresorufin O-deethylase activity (EROD) and selected physiological variables (retinoids and thyroid hormones) in northern fulmar breeding in three differentially OC-exposed populations: Nunavut (Canadian Arctic), Svalbard (Norwegian Arctic) and the Faroe Islands. Substantially higher (roughly two-fold) OC levels were uncovered in the liver of this long-lived fulmarine petrel breeding in the Faroe Islands relative to Svalbard and Nunavut. Liver levels of PCDDs, PCDFs and non-ortho PCBs in Faroe Islands fulmars were amongst the highest reported thus far in any seabirds from the northern regions. Positive correlations were depicted in combined fulmars (all three populations) between hepatic EROD activity and concentrations of OCs, in which strongest associations were found for dioxin-like compound (PCDFs and PCDDs) and TEQ concentrations. Moreover, moderate to strong positive correlations were found between liver OC concentrations and plasma total thyroxin (TT(4)) levels and TT(4)/total triiodothyronine (TT(3)) level ratios, as well as strong negative correlations between the same suite of OCs and plasma TT(3) levels. Hepatic OC concentrations (PCBs, PCDDs, PCDFs, HCB, p,p'-DDE and oxychlordane) also were positively correlated with hepatic retinyl palmitate levels which, in turn, were associated with a significant decrease in plasma retinol levels and somewhat unchanged liver retinol levels. The present meta-analysis investigation on northern fulmar breeding in three geographically-distant sites illustrated that OC exposure (mainly PCBs and dioxins/furans) may be associated with modulation of the thyroid and retinoid homeostasis. However, the impact of confounding environmental factors (e.g., temperature and nutritional status) on current physiological variable variations could not be ruled out, and thus any cause-effect linkages between thyroid and retinoid system perturbation and OC exposure cannot be ascertained.

Cord blood thyroid tests in boys born with and without cryptorchidism: correlations with birth parameters and in utero xenobiotics exposure

BACKGROUND

In utero exposure to environmental chemicals can result in reproductive toxicity via endocrine disruption mechanisms. Whether some of those contaminants also have an impact on fetal thyroid function or pathways, and, thus, potentially on neuropsychological development, is still debated.

METHODS

We used samples from a cord blood (CB) and milk bank, established for a research on cryptorchidism and xenobiotic exposure to compounds known for their anti-androgenic and/or estrogenic activity, to study CB thyroid tests and their correlation with CB and milk xenobiotics concentrations in boys born in Nice area.

RESULTS

No difference was found in thyroid tests between 60 cryptorchid boys and 76 matched controls (median thyroid stimulating hormone 5.97 vs. 6.55 mUI/L, free thyroxine [fT4] 13.1 vs. 12.9 pmol/L, free triiodothyronine [fT3] 1.9 vs. 2.1 pmol/L), with no influence of season of birth, gestational age, maternal smoking, or mode of delivery (except for higher fT4 in control boys born vaginally). FT4 was correlated with fetal growth only in cryptorchid boys. Since we had previously shown differences between cryptorchid and controls exposure, we studied correlations of thyroid tests with xenobiotics in control boys only. All tested CB or maternal milk was contaminated by one or more selected xenobiotics, mainly polychlorinated biphenyls (PCBs), dichloro diphenyl dichloroethylène (DDE), dibutylphthalate, hexachlorobenzene, and bisphenol A. We found a significant negative correlation between fT4 and concentrations of PCB118, PC180, and DDE in milk (respectively r = -0.342, p < 0.03, r = -0.296, p = 0.031, r = -0.315, p = 0.016), persisting after adjustment for mode of delivery. There was a significant positive correlation of fT3 with milk concentrations of PCB138, PCB153, ΣPCB, and dibutylphthalate (respectively r = 0.31, p = 0.016, r = 0.28, p = 0.029; r = 0.34, p = 0.0079 and r = 0.272, p = 0.0295), with a trend for PCB180 (r = 0.259, p = 0.061). There was no correlation of thyroid stimulating hormone with any of the measured xenobiotics, except for a weak negative trend with CB bisphenol A (r = -0.25, p = 0.077).

CONCLUSIONS

CB thyroid tests are within normal range in cryptorchid boys, similar to controls. Our data in controls suggest a possible weak correlation between in utero exposure to some xenobiotics (PCBs, DDE) and fT3 and fT4 CB concentrations, with usually negative correlations with fT4 and positive with fT3 concentrations, which we speculate could suggest an impact on deiodinases.

Polyhalogenated aromatic hydrocarbons and metabolites: Relation to circulating thyroid hormone and retinol in nestling bald eagles (Haliaeetus leucocephalus)

Polyhalogenated aromatic hydrocarbons are global contaminants that are often considered to be endocrine disruptors and include 1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene (p,p'-DDE), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs). The present study examined these compounds and their hydroxylated metabolites or analogues and relationships with circulating thyroid hormones and retinols in plasma from nestling and adult bald eagles in British Columbia, Canada, and California, USA. We also compared our results with published data. Thyroxine (T4) decreased with summation operatorPCB and CB153 in nestling bald eagles, which was congruent with results from nine of 14 other published avian laboratory and field studies. Free thyroid hormone levels also decreased with CB-153 and hydroxylated PCBs (OH-PCBs). Retinol increased with CB118 and CB180 in nestling eagles, decreased with OH-PCBs in a subset of nestlings, and decreased in 7 of 12 PCB published studies. Thyroxine decreased with p,p'-DDE for nestlings and with data reported in one of five other published studies. In our samples, plasma retinol, triiodothyronine (T3), and T4 were independent of summation operatorPBDEs, whereas summation operatorOH-PBDEs were weakly but significantly correlated with increases in T3 and retinol. Adult bald eagles showed no relationship between contaminants and thyroid hormones, which is consistent with other studies of long-lived birds, perhaps because adult birds have time to adjust to contaminant levels. Measurement of circulating thyroid hormones appears to be a more useful biomarker than retinols, given the more consistent response of T4 to PCBs here and reported in the literature. We conclude that current environmental exposures to PCBs in British Columbia and in southern California are associated with significant decreases in T4, suggesting a potential negative effect on the endocrine system of nestling bald eagles.

Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain

Persistent polyhalogenated organic pollutants are present worldwide and accumulate along the food chain. They interfere with human and animal health and are particularly harmful for pre- and perinatal neurodevelopment. The mechanisms behind the observed effects vary depending on the specific compound investigated. Co-planar polychlorinated biphenyls (PCBs) can act via the arylhydrocarbon receptor while many ortho-substituted PCBs disrupt intracellular Ca(2+) homeostasis. A common mechanism for a wide variety of PCBs is interference with thyroid hormone (TH) signalling in developing brain, by changing intracellular TH availability or by interacting directly at the level of the TH receptors. Studies on gene expression in cortex and cerebellum revealed both hypothyroid- and hyperthyroid-like effects. However, since THdependent gene expression plays a crucial role in the coordination of neuronal proliferation, migration, synaptogenesis, myelination, etc., both reduced/delayed and increased/premature expression may result in permanent structural changes in neuronal communication networks, leading to lifelong deficits in cognitive performance, motor functions, and psychobehavior. In a similar way, PCBs are able to interfere with estrogen- and androgen-dependent brain development and in some studies neurobehavioral outcome was shown to be gender-specific. Other persistent organohalogens like polychlorinated dibenzo-p-dioxins (PCDDs) and polybrominated diphenyl ethers (PBDEs) also act as endocrine disrupters in the developing brain. Several of the mechanisms involved are similar to those of PCBs, but each group also works via own specific pathways. The fact that persistent organohalogens can amplify the neurotoxic effects of other environmental pollutants, such as heavy metals, further increases their risk for human and animal neurodevelopment.

Changes of thyroid hormone levels and related gene expression in Chinese rare minnow (Gobiocypris rarus) during 3-amino-1,2,4-triazole exposure and recovery

Thyroid hormones (THs) play an important role in the development and metabolism of fish through their influences on genetic transcription and are targets for endocrine disruptive agents in the aquatic environment. Amitrole is a pesticide potentially interfering with thyroid hormone regulation. In this study, the rare minnow (Gobiocypris rarus) was exposed to different levels of 3-amino-1,2,4-triazole (amitrole) and allowed to recover in clean water. Plasma TH levels and the expression of TH-related genes, including transthyretin (ttr), deiodinases (d1 and d2), and the thyroid hormone receptor (tralpha) from the livers and brains were evaluated. After exposure, the plasma TH levels did not change. Histopathological observations showed that livers were degenerated at 10,000 ng/l and these damages could be recovered by the withdrawal of amitrole. However, the ttr, d1, and d2 mRNA levels in the livers of males were significantly up-regulated in all exposure groups (p<0.05). The ttr and d2 mRNA levels were significantly up-regulated at 10,000 ng/l and 10, 100, and 1000 ng/l in the livers of females, respectively (p<0.05). In the brains of males, a twofold increase of d2 mRNA levels at > or = 100 ng/l and a fivefold decrease of tralpha mRNA levels at > or = 10 ng/l were observed (p<0.05), whereas no significant differences were observed in the expression of d2 and tralpha in the brains of females. After a recovery period, the ttr, d1, and d2 mRNA levels in the livers of males returned to control levels, but the tralpha mRNA levels were irreversibly decreased at all treatments (p<0.05). In addition, the d2 mRNA levels in the livers of females were significantly induced at > or = 100 ng/l. Moreover, the d2 mRNA levels in the brains of males and females were up-regulated at 10,000 ng/l. These results indicated that amitrole exposure could result in alternations of ttr, d1, d2, and tralpha gene expression in different tissues of the rare minnow. The expression of these TH-related genes in males was more sensitive to amitrole than those of females. Recovery in clean water was associated with the selective regulation of TH-related gene transcription in the rare minnow. Therefore, these TH-related genes can serve as biomarkers to screen the effects of thyroid disruption chemicals in rare minnow.

Effects of maternal exposure to ammonium perchlorate on thyroid function and the expression of thyroid-responsive genes in Japanese quail embryos

Perchlorate, a known thyroid disruptor, is deposited in eggs of exposed female birds, raising concerns that the embryos from these eggs may become hypothyroid, which may in turn affect the development and function of thyroid-dependent organs. We hypothesized that exposure to ammonium perchlorate (AP) would decrease hen and embryonic thyroid function and affect the expression of thyroid-responsive genes in embryonic brain and liver. Laying Japanese quail hens were treated with 2000 mg/l or 4000 mg/l AP in drinking water. Thyroid status and expression of thyroid-responsive genes were examined in the embryos from eggs of exposed hens. Perchlorate exposure led to hypothyroidism in hens from both treatment groups; egg production was decreased in the high dosage group only. Embryos from eggs of perchlorate-exposed hens had hypertrophied thyroid glands and significantly lower thyroidal hormone storage, indicating hypothyroidism in these embryos. The embryonic hypothyroidism was associated with decreased embryonic growth, delayed hatching and greater mortality during hatching. The mRNA level of type 2 deiodinase (D2) in the liver of embryos from eggs of perchlorate-exposed hens was increased compared to the control embryos, a compensatory response that increases the production of metabolically active T(3). However, the mRNA levels of D2 and RC3 in the brain were not affected. These results suggest that the embryonic brain is protected from hypothyroidism by other mechanisms known to influence hormone entry into and exit from the brain. Our study shows that maternal perchlorate exposure led to embryonic hypothyroidism and may have interfered with embryonic development.

The effect of 3,5,3'-triiodothyronine supplementation on zebrafish (Danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors

The importance of thyroid hormones (TH) for embryonic development has long been shown in many vertebrates, but is not yet established in pre-hatch teleost models despite the presence of TH, TH receptors and iodothyronine deiodinases. Lack of data about the dynamics of TH metabolism in embryonic stages of fish does not allow to speculate about the involvement and/or role of TH in fish embryonic development. We therefore set up an experiment to examine the effect of 3,5,3'-triiodothyronine (T(3)) on zebrafish (Danio rerio) embryonic development and on the expression of some thyroid hormone-regulated genes as measured by real-time PCR. Maternally derived thyroxine (T(4)) and T(3) were detected throughout embryonic development and total levels remained stable. Thyroid hormone receptor (TR) alpha and beta mRNA were found to be present in early embryos. After an initial fall, TRalpha mRNA levels in the control group increased gradually from 12h post fertilization (HPF) onwards. TRbeta mRNA levels of control embryos were relatively stable during embryonic development, but increased around the hatching period. We also quantified type I (D1) and type II (D2) deiodinase mRNA expression in zebrafish embryos. D1 mRNA levels in the control group gradually increased during development while D2 levels were kept at a low and stable level until hatching. At 75 HPF, a fivefold increase of D2 expression was observed compared to embryonic levels. Exogenous T(3) added to the water (5nM) was taken up by the embryos, causing increased pigmentation and accelerated hatching. T(3) treatment significantly up regulated TRalpha mRNA levels at 48 HPF, while D2 mRNA was significantly down regulated at 75 HPF. Neither TRbeta nor D1 mRNA levels seemed responsive to the treatment. Taken together, these data show that during embryonic development zebrafish already have the necessary regulatory machinery for TH activation and signaling, and that T(3) treatment at that stage indeed influence embryonic development.