Polychlorinated biphenyls exert selective effects on cellular composition of white matter in a manner inconsistent with thyroid hormone insufficiency

Developmental exposure to the brominated flame retardant DE-71 reduces serum thyroid hormones in rats without hypothalamic-pituitary-thyroid axis activation or neurobehavioral changes in offspring

Polybrominated diphenyl ethers (PBDEs) are legacy flame retardants for which human exposure remains ubiquitous. This is of concern since these chemicals can perturb development and cause adverse health effects. For instance, DE-71, a technical mixture of PBDEs, can induce liver toxicity as well as reproductive and developmental toxicity. DE-71 can also disrupt the thyroid hormone (TH) system which may induce developmental neurotoxicity indirectly. However, in developmental toxicity studies, it remains unclear how DE-71 exposure affects the offspring’s thyroid hormone system and if this dose-dependently relates to neurodevelopmental effects. To address this, we performed a rat toxicity study by exposing pregnant dams to DE-71 at 0, 40 or 60 mg/kg/day during perinatal development from gestational day 7 to postnatal day 16. We assessed the TH system in both dams and their offspring, as well as potential hearing and neurodevelopmental effects in prepubertal and adult offspring. DE-71 significantly reduced serum T4 and T3 levels in both dams and offspring without a concomitant upregulation of TSH, thus inducing a hypothyroxinemia-like effect. No discernible effects were observed on the offspring’s brain function when assessed in motor activity boxes and in the Morris water maze, or on offspring hearing function. Our results, together with a thorough review of the literature, suggest that DE-71 does not elicit a clear dose-dependent relationship between low serum thyroxine (T4) and effects on the rat brain in standard behavioral assays. However, low serum TH levels are in themselves believed to be detrimental to human brain development, thus we propose that we lack assays to identify developmental neurotoxicity caused by chemicals disrupting the TH system through various mechanisms.

Effects of endocrine disrupting chemicals on myelin development and diseases

In the central and peripheral nervous systems, myelin is essential for efficient conduction of action potentials. During development, oligodendrocytes and Schwann cells differentiate and ensure axon myelination, and disruption of these processes can contribute to neurodevelopmental disorders. In adults, demyelination can lead to important disabilities, and recovery capacities by remyelination often decrease with disease progression. Among environmental chemical pollutants, endocrine disrupting chemicals (EDCs) are of major concern for human health and are notably suspected to participate in neurodevelopmental and neurodegenerative diseases. In this review, we have combined the current knowledge on EDCs impacts on myelin including several persistent organic pollutants, bisphenol A, triclosan, heavy metals, pesticides, and nicotine. Besides, we presented several other endocrine modulators, including pharmaceuticals and the phytoestrogen genistein, some of which are candidates for treating demyelinating conditions but could also be deleterious as contaminants. The direct impacts of EDCs on myelinating cells were considered as well as their indirect consequences on myelin, particularly on immune mechanisms associated with demyelinating conditions. More studies are needed to describe the effects of these compounds and to further understand the underlying mechanisms in relation to the potential for endocrine disruption.

Evidence Implicating Non-Dioxin-Like Congeners as the Key Mediators of Polychlorinated Biphenyl (PCB) Developmental Neurotoxicity

Despite being banned from production for decades, polychlorinated biphenyls (PCBs) continue to pose a significant risk to human health. This is due to not only the continued release of legacy PCBs from PCB-containing equipment and materials manufactured prior to the ban on PCB production, but also the inadvertent production of PCBs as byproducts of contemporary pigment and dye production. Evidence from human and animal studies clearly identifies developmental neurotoxicity as a primary endpoint of concern associated with PCB exposures. However, the relative role(s) of specific PCB congeners in mediating the adverse effects of PCBs on the developing nervous system, and the mechanism(s) by which PCBs disrupt typical neurodevelopment remain outstanding questions. New questions are also emerging regarding the potential developmental neurotoxicity of lower chlorinated PCBs that were not present in the legacy commercial PCB mixtures, but constitute a significant proportion of contemporary human PCB exposures. Here, we review behavioral and mechanistic data obtained from experimental models as well as recent epidemiological studies that suggest the non-dioxin-like (NDL) PCBs are primarily responsible for the developmental neurotoxicity associated with PCBs. We also discuss emerging data demonstrating the potential for non-legacy, lower chlorinated PCBs to cause adverse neurodevelopmental outcomes. Molecular targets, the relevance of PCB interactions with these targets to neurodevelopmental disorders, and critical data gaps are addressed as well.

Thyroid hormone levels associate with exposure to polychlorinated biphenyls and polybrominated biphenyls in adults exposed as children

BACKGROUND

Michigan residents were directly exposed to endocrine-disrupting compounds, polybrominated biphenyl (PBB) and polychlorinated biphenyl (PCB). A growing body of evidence suggests that exposure to certain endocrine-disrupting compounds may affect thyroid function, especially in people exposed as children, but there are conflicting observations. In this study, we extend previous work by examining age of exposure's effect on the relationship between PBB exposure and thyroid function in a large group of individuals exposed to PBB.

METHODS

Linear regression models were used to test the association between serum measures of thyroid function (total thyroxine (T4), total triiodothyronine (T3), free T4, free T3, thyroid stimulating hormone (TSH), and free T3: free T4 ratio) and serum PBB and PCB levels in a cross-sectional analysis of 715 participants in the Michigan PBB Registry.

RESULTS

Higher PBB levels were associated with many thyroid hormones measures, including higher free T3 (p = 0.002), lower free T4 (p = 0.01), and higher free T3: free T4 ratio (p = 0.0001). Higher PCB levels were associated with higher free T4 (p = 0.0002), and higher free T3: free T4 ratio (p = 0.002). Importantly, the association between PBB and thyroid hormones was dependent on age at exposure. Among people exposed before age 16 (N = 446), higher PBB exposure was associated with higher total T3 (p = 0.01) and free T3 (p = 0.0003), lower free T4 (p = 0.04), and higher free T3: free T4 ratio (p = 0.0001). No significant associations were found among participants who were exposed after age 16. No significant associations were found between TSH and PBB or PCB in any of the analyses conducted.

CONCLUSIONS

This suggests that both PBB and PCB are associated with thyroid function, particularly among those who were exposed as children or prenatally.

Impact of endocrine-disrupting chemicals on thyroid function and brain development

Endocrine disrupting chemicals (EDCs) are synthetic or natural substances in the environment. EDCs have been shown to disrupt reproductive, developmental and other homeostatic systems by interfering with the synthesis, secretion, transport, metabolism and action of endogenous hormones including the thyroid hormone (TH) system. Since TH plays a critical role in brain development, the exposure to TH-system disrupting EDCs during development may have serious consequences. In this article, representative previous studies showing the effect of representative EDCs on the TH system are summarized. Then, the molecular mechanisms of action of polychlorinated biphenyls and polybrominated diphenyl ethers on the TH system are discussed further. Particularly, the effect of polychlorinated biphenyls and polybrominated diphenyl ethers on TH-mediated brain development is discussed. Our recent studies may provide a novel idea regarding the effect of EDCs on the TH system.

Alteration of rat fetal cerebral cortex development after prenatal exposure to polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are environmental contaminants that persist in environment and human tissues. Perinatal exposure to these endocrine disruptors causes cognitive deficits and learning disabilities in children. These effects may involve their ability to interfere with thyroid hormone (TH) action. We tested the hypothesis that developmental exposure to PCBs can concomitantly alter TH levels and TH-regulated events during cerebral cortex development: progenitor proliferation, cell cycle exit and neuron migration. Pregnant rats exposed to the commercial PCB mixture Aroclor 1254 ended gestation with reduced total and free serum thyroxine levels. Exposure to Aroclor 1254 increased cell cycle exit of the neuronal progenitors and delayed radial neuronal migration in the fetal cortex. Progenitor cell proliferation, cell death and differentiation rate were not altered by prenatal exposure to PCBs. Given that PCBs remain ubiquitous, though diminishing, contaminants in human systems, it is important that we further understand their deleterious effects in the brain.

Transient Maternal Hypothyroxinemia Potentiates the Transcriptional Response to Exogenous Thyroid Hormone in the Fetal Cerebral Cortex Before the Onset of Fetal Thyroid Function: A Messenger and MicroRNA Profiling Study

Thyroid hormone (TH) is essential for brain development both before and after birth. We have used gene expression microarrays to identify TH-regulated genes in the fetal cerebral cortex prior to the onset of fetal thyroid function to better understand the role of TH in early cortical development. TH levels were transiently manipulated in pregnant mice by treatment with goitrogens from gestational day (GD) 13-16 and/or by injection of TH 12 h before sacrifice on GD 16. The transcriptional response to exogenous TH in the GD 16 fetal cortex was potentiated by transient goitrogen treatment, suggesting that the hypothyroxinemic brain is a different substrate upon which TH can act, or that robust compensatory mechanisms are induced by transient hypothyroxinemia. Several known TH-responsive genes were identified including Klf9, and several novel TH-responsive genes such as Appbp2, Ppap2b, and Fgfr1op2 were identified in which TH response elements were confirmed. We also identified specific microRNAs whose expression in the fetal cortex was affected by TH treatment, and determined that Ppap2b and Klf9 are the target genes of miR-16 and miR-106, respectively. Thus, a complex redundant functional network appears to coordinate TH-mediated gene expression in the developing brain.

Prenatal transfer of polybrominated diphenyl ethers (PBDEs) results in developmental neurotoxicity in zebrafish larvae

Parental exposure to polybrominated diphenyl ethers (PBDEs) in animals has been found to be transferred to the offspring. The environmental health risk and toxicity to the offspring are still unclear. The objective of the present study was to identify environmentally relevant concentrations of PBDEs for parental exposure that would cause developmental neurotoxicity in the offspring. Adult zebrafish were exposed to environmentally relevant concentrations of DE-71 (0.16, 0.8, 4.0 μg/L) via water. The results showed that PBDE exposure did not affect larvae hatching, malformation, or survival. The residue of PBDEs was detected in F1 eggs upon parental exposure. Acetylcholinesterase (AChE) activity was significantly inhibited in F1 larvae. Genes of central nervous system development (e.g., myelin basic protein, synapsin IIa, α1-tubulin) were significantly downregulated in larvae. Protein levels of α1-tubulin and synapsin IIa were also reduced. Decreased locomotion activity was observed in the larvae. This study provides the first evidence that parental exposure to environmentally relevant concentrations of PBDEs could cause adverse effects on neurodevelopment in zebrafish offspring.

Magnetic resonance imaging and volumetric analysis: novel tools to study the effects of thyroid hormone disruption on white matter development

Humans and wildlife are exposed to environmental pollutants that have been shown to interfere with the thyroid hormone system and thus may affect brain development. Our goal was to expose pregnant rats to propylthiouracil (PTU) to measure the effects of a goitrogen on white matter development in offspring using magnetic resonance imaging (MRI) and volumetric analysis. We exposed pregnant Sprague Dawley (SD) rats to 3 or 10 ppm PTU from gestation day 7 (GD7) until postnatal day 25 (P25) to determine the effects on white matter (WM), gray matter (GM), and hippocampus volumes in offspring. We sacrificed offspring at P25 but continued the life of some offspring to P90 to measure persistent effects in adult animals. P25 offspring exposed to 10 ppm PTU displayed lowered levels of triiodothyronine (T3) and thyroxine (T4); cerebral WM, GM, and total brain volumes were significantly lower than the volumes in control animals. P90 adults exposed to 10 ppm PTU displayed normal T3 levels but lowered T4 levels; WM, GM, total brain, and hippocampal volumes were significantly lower than the volumes in control adults. Both P25 and P90 rats exposed to 10 ppm PTU displayed significant reductions in percent WM as well as heterotopias in the corpus callosum. Exposure to 3 ppm PTU did not produce any significant effects. These results suggest that MRI coupled with volumetric analysis is a powerful tool in assessing the effects of thyroid hormone disruption on white matter development and brain structure. This approach holds great promise in assessing neurotoxicity of xenobiotics in humans and wildlife.

Maternal hypothyroidism decreases progesterone receptor expression in the cortical subplate of foetal rat brain

Steroid hormones exert profound effects on the development of brain areas controlling complex cognitive function in adulthood. One class, progestins, may contribute by acting on the progestin receptor (PR), which is transiently expressed in a critical layer of developing cortex: the subplate. PR expression in the subplate coincides with the establishment of ongoing cortical connectivity and may play an important organisational role. Identification of the factor(s) that regulate the precise timing of PR expression within subplate may help elucidate the function of PR. Thyroid hormone may interact with hormone response elements within the PR gene. The present study examined the effects of maternal hypothyroidism on levels of PR immunoreactivity (PR-IR) within the foetal subplate. Pregnant rats were made hypothyroid by the administration of methimazole and potassium perchlorate in drinking water. Maternal hypothyroidism significantly decreased PR-IR within the foetal subplate. Using the incorporation of 5-bromo-2'-deoxyuridine (BrDU) during subplate cell neurogenesis (embryonic day 13.5) to determine subplate cell survival in hypothyroid animals, we found that decreases in PR-IR cannot be attributed to significant subplate cell loss but are more likely the result of altered PR expression. Gestational thyroxine replacement to hypothyroid dams prevented the decrease in PR-IR within the subplate. These results identify thyroid hormone as a potential factor in the regulation of PR expression in the developing brain. These results are consistent with the idea that endocrine cross-talk between progesterone and thyroid hormone may be one mechanism by which maternal hypothyroidism alters normal cortical development.

Thyroid effects of endocrine disrupting chemicals

In recent years, many studies of thyroid-disrupting effects of environmental chemicals have been published. Of special concern is the exposure of pregnant women and infants, as thyroid disruption of the developing organism may have deleterious effects on neurological outcome. Chemicals may exert thyroid effects through a variety of mechanisms of action, and some animal experiments and in vitro studies have focused on elucidating the mode of action of specific chemical compounds. Long-term human studies on effects of environmental chemicals on thyroid related outcomes such as growth and development are still lacking. The human exposure scenario with life long exposure to a vast mixture of chemicals in low doses and the large physiological variation in thyroid hormone levels between individuals render human studies very difficult. However, there is now reasonably firm evidence that PCBs have thyroid-disrupting effects, and there is emerging evidence that also phthalates, bisphenol A, brominated flame retardants and perfluorinated chemicals may have thyroid disrupting properties.

Developmental coexposure to polychlorinated biphenyls and polybrominated diphenyl ethers has additive effects on circulating thyroxine levels in rats

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are widespread environmental contaminants found in seafood and dairy products. PCBs and PBDEs are structurally similar chemicals and affect thyroid hormone function and behavior in children and laboratory rodents. Although coexposure frequently exists, the in vivo developmental effects of combined exposure to PCBs and PBDEs on thyroxine (T4) levels are unknown. We examined the effects of PCB and PBDE coexposure from gestational day 6 through postnatal day (p) 21, alone and in combination, on T4 levels in rat offspring. In males, exposure to PCBs and PBDEs at 1.7, 5, 10, 20, 40, and 60 μmol/kg/day induced equivalent and dose-dependent reductions in T4 from p 7 to p 21. Exposure to equimolar mixtures of PCBs and PBDEs at 3.4, 10, 20, 40, and 80 μmol/kg/day additively reduced T4 from p 7 to p 21 in males. In a second series of experiments, we determined sex effects on the mixture exposures and found that coexposure to PCBs and PBDEs had similar additive effects on T4 levels in male and female offspring. This study demonstrates that equimolar exposure to PCBs and PBDEs induces similar reductions in T4 levels and that coexposure to a mixture of PCBs and PBDEs has additive effects on T4 levels. These thyroid hormone effects of coexposure to PCBs and PBDEs are important when considering the cumulative effects of coexposure to multiple environmental thyroid hormone-disrupting agents in risk assessment for developmental disorders.

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.

Thyroxine for transient hypothyroxinemia and cerebral palsy in extremely preterm infants

BACKGROUND

The relationship of thyroxine supplementation for transient hypothyroxinemia of prematurity to the incidence of cerebral palsy (CP) in infants <28 weeks of gestation is unclear.

METHODS

The incidence of CP at a corrected age of 18 months was compared between infants born in a 3-year period in which routine measurement of free T4 (FT4) in the blood was not performed (first period, n= 54), and those born in a later 3-year period in which FT4 was measured (second period, n= 60; mainly at 7 days old), and in which l-thyroxine 5-10 µg/kg per day (mean, 9 µg/kg/day) was administered for FT4 levels <0.8 ng/dL. Incidence of CP at 3 years of age was also compared between the same groups.

RESULTS

Background clinical factors between the two groups were comparable except for prenatal steroid administration, which was reduced in the second period. Incidence of CP at a corrected age of 18 months was significantly lower in the second period (3.3%) than in the first period (16.6%). Incidence of CP at 3 years of age was also significantly lower in the second period. Multiple logistic regression analysis using factors except thyroxine supplementation, for the total of 114 infants from both groups, found no perinatal factors related to the development of CP at a corrected age of 18 months.

CONCLUSIONS

Thyroxine supplementation for transient hypothyroxinemia of prematurity may reduce the incidence of CP in extremely preterm infants. Large-scale randomized controlled trials are essential to determine the effects of thyroxine supplementation in reducing the incidence of CP among extremely preterm infants.

Individual polychlorinated biphenyl (PCB) congeners produce tissue- and gene-specific effects on thyroid hormone signaling during development

Polychlorinated biphenyls (PCB) are industrial chemicals linked to developmental deficits that may be caused in part by disrupting thyroid hormone (TH) action by either reducing serum TH or interacting directly with the TH receptor (TR). Individual PCB congeners can activate the TR in vitro when the metabolic enzyme cytochrome P4501A1 (CYP1A1) is induced, suggesting that specific PCB metabolites act as TR agonists. To test this hypothesis in vivo, we compared two combinations of PCB congeners that either activate the TR (PCB 105 and 118) or not (PCB 138 and 153) in the presence or absence of a PCB congener (PCB 126) that induces CYP1A1 in vitro. Aroclor 1254 was used as a positive control, and a group treated with propylthiouracil was included to characterize the effects of low serum TH. We monitored the effects on TH signaling in several peripheral tissues by measuring the mRNA expression of well-known TH-response genes in these tissues. Aroclor 1254 and its component PCB 105/118/126 reduced total T(4) to the same extent as that of propylthiouracil but increased the expression of some TH target genes in liver. This effect was strongly correlated with CYP1A1 expression supporting the hypothesis that metabolism is necessary. Effects were gene and tissue specific, indicating that tissue-specific metabolism is an important component of PCB disruption of TH action and that PCB metabolites interact in complex ways with the TR. These are essential mechanisms to consider when evaluating the health risks of contaminant exposures, for both PCB and other polycyclic compounds known to interact with nuclear hormone receptors.

Deciphering the oligodendrogenic program of neural progenitors: cell intrinsic and extrinsic regulators

In the developing and adult CNS, neural stem/progenitor cells (NSPCs) and oligodendroglial progenitor cells (OPCs) follow an oligodendrogenic process with the aim of myelinating axons. This process is to a high degree regulated by an oligodendrogenic program (OPr) composed of intrinsic and extrinsic factors that modulate the different steps required for NSPCs to differentiate into myelinating oligodendrocytes. Even though NSPCs and OPCs are present in the diseased CNS and have the capacity to generate oligodendrocytes, sparse remyelination of axons constitutes a major constraint in therapies toward multiple sclerosis (MS) and spinal cord injury (SCI). Lack of pro-oligodendrogenic factors and presence of anti-oligodendrogenic activities are thought to be the main reasons for this limitation. Thus, molecular and cellular strategies aiming at remyelination and at targeting such pro- and anti-oligodendrogenic mechanisms are currently under investigation. The present review summarizes the current knowledge on the OPr; it implements our own findings on mesenchymal stem cell-derived pro-oligodendroglial factors and on the role of p57/kip2 in oligodendroglial differentiation. Moreover, it describes molecular and cellular approaches for the development of future therapies toward remyelination.

Developmental PCB exposure induces hypothyroxinemia and sex-specific effects on cerebellum glial protein levels in rats

Polychlorinated biphenyls (PCBs) are persistent lipophilic environmental contaminants which are found in fatty tissues of humans and wild-life alike. Maternal transfer of PCBs to offspring is easily achieved across the placenta and via lactation. In male rats, perinatal PCB exposure induces behavioral abnormalities, in addition to hypothyroxinemia and white matter changes. There are sex differences in white matter volume synthesis and density in adult and aged rodents. Yet whether PCB exposure effects on white matter are sex-specific is unclear, because the previous studies were conducted in male offspring. Furthermore, although hypothyroxinemia induced by PCB exposure is thought to trigger white matter changes, PCBs also affect interleukin-6 (IL-6) expression, and IL-6 regulates white matter growth. We hypothesized that perinatal PCB exposure would have sex-specific effects on white matter development associated with altered IL-6 levels. We found that female offspring had higher levels of myelin basic protein (MBP) than males did, at postnatal day (PND) 7, 18 and 21. PCB exposure induced hypothyroxinemia in males and females at PND7, 14, 21, and 42. PCB exposure also increased MBP and reduced glial fibrillary acidic protein (GFAP) levels in males at PND21, but had the opposite effect in females. In addition, at PND14 and 21, PCB exposure elevated IL-6 levels in male offspring only. The induction of sex-specific changes in white matter proteins, in the absence of sex differences in thyroxine levels after PCB exposure, suggests that serum thyroxine levels do not directly contribute to the white matter alterations. Instead, IL-6 may contribute to increased MBP levels in males, whereas in females estromimetic and thyromimetic PCB metabolites may affect white matter development. This data adds to an increasing body of literature showing that perinatal insults induce sex-specific effects in offspring.

The nature of the compensatory response to low thyroid hormone in the developing brain

Thyroid hormone is essential for normal brain development, although the degree to which the developing brain is sensitive to small perturbations in serum thyroxin is not clear. An important concept related to this is that the developing brain possesses potent mechanisms to compensate for low serum thyroid hormone, and this concept is routinely employed in discussions concerning clinical treatments or public health. However, experimental studies have not directly tested whether (or the degree to which) putative compensatory mechanisms can ameliorate the consequences of small reductions in serum thyroxin (T(4)). To formally test this concept, we employed a model of graded T(4) reductions using doses of propylthiouracil (PTU) that were 200- to 67-fold lower than the dose traditionally used to produce hypothyroidism in rats. PTU produced a stepwise decrease in serum total T(4), and a stepwise increase in serum thyroid-stimulating hormone (TSH), in type 2 deiodinase mRNA expression and enzyme activity in the brain, and in the expression of the mRNA encoding the tri-iodothyronine (T(3)) transporter MCT8 in the postnatal day (P) 15 cortex. However, the mRNA encoding RC3/neurogranin, a direct target of T(3) action, exhibited a strong negative linear correlation with serum total T(4) despite these adaptive responses. In addition, single-cell analysis of RC3 mRNA levels in cortical neurones demonstrated that the co-expression of MCT8 did not alter the relationship between RC3 mRNA and serum T(4). These findings do not support the currently envisioned concept of the developing brain being capable of compensating for low T(4).

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Identification of thyroid hormone receptor binding sites and target genes using ChIP-on-chip in developing mouse cerebellum

Thyroid hormone (TH) is critical to normal brain development, but the mechanisms operating in this process are poorly understood. We used chromatin immunoprecipitation to enrich regions of DNA bound to thyroid receptor beta (TRβ) of mouse cerebellum sampled on post natal day 15. Enriched target was hybridized to promoter microarrays (ChIP-on-chip) spanning −8 kb to +2 kb of the transcription start site (TSS) of 5000 genes. We identified 91 genes with TR binding sites. Roughly half of the sites were located in introns, while 30% were located within 1 kb upstream (5′) of the TSS. Of these genes, 83 with known function included genes involved in apoptosis, neurodevelopment, metabolism and signal transduction. Two genes, MBP and CD44, are known to contain TREs, providing validation of the system. This is the first report of TR binding for 81 of these genes. ChIP-on-chip results were confirmed for 10 of the 13 binding fragments using ChIP-PCR. The expression of 4 novel TH target genes was found to be correlated with TH levels in hyper/hypothyroid animals providing further support for TR binding. A TRβ binding site upstream of the coding region of myelin associated glycoprotein was demonstrated to be TH-responsive using a luciferase expression system. Motif searches did not identify any classic binding elements, indicating that not all TR binding sites conform to variations of the classic form. These findings provide mechanistic insight into impaired neurodevelopment resulting from TH deficiency and a rich bioinformatics resource for developing a better understanding of TR binding.

Polychlorinated biphenyls (Aroclor 1254) do not uniformly produce agonist actions on thyroid hormone responses in the developing rat brain

Thyroid hormone (TH) is essential for normal brain development, and polychlorinated biphenyls (PCBs) are known to interfere with TH action in the developing brain. Thus, it is possible that the observed neurotoxic effects of PCB exposure in experimental animals and humans are mediated in part by their ability to interfere with TH signaling. PCBs may interfere with TH signaling by reducing circulating levels of TH, acting as TH receptor analogs, or both. If PCBs act primarily by reducing serum TH levels, then their effects should mimic those of low TH. In contrast, if PCBs act primarily as TH agonists in the developing brain, then they should mimic the effect of T(4) in hypothyroid animals. We used a two-factor design to test these predictions. Both hypothyroidism (Htx) and/or PCB treatment reduced serum free and total T(4) on postnatal d 15. However, only Htx increased pituitary TSHbeta expression. RC3/neurogranin expression was decreased by Htx and increased by PCB treatment. In contrast, Purkinje cell protein-2 expression was reduced in hypothyroid animals and restored by PCB treatment. Finally, PCB treatment partially ameliorated the effect of Htx on the thickness of the external granule layer of the cerebellum. These studies demonstrate clearly that PCB exposure does not mimic the effect of low TH on several important TH-sensitive measures in the developing brain. However, neither did PCBs mimic T(4) in hypothyroid animals on all end points measured. Thus, PCBs exert a complex action on TH signaling in the developing brain.

The balance between oligodendrocyte and astrocyte production in major white matter tracts is linearly related to serum total thyroxine

Thyroid hormone (TH) may control the ratio of oligodendrocytes to astrocytes in white matter by acting on a common precursor of these two cell types. If so, then TH should produce an equal but opposite effect on the density of these two cells types across all TH levels. To test this, we induced graded TH insufficiency by treating pregnant rats with increasing doses of propylthiouracil. Propylthiouracil induced a dose-dependent decrease in serum T(4) in postnatal d 15 pups, a dose-dependent decrease in the density of MAG-positive oligodendrocytes, and an equal increase in the density of glial fibrillary acidic protein-positive astrocytes in both the corpus callosum and anterior commissure. Linear regression analyses demonstrated a strong correlation between glial densities and serum T(4); this correlation was positive for astrocytes and negative for oligodendrocytes. Surprisingly, oligodendrocyte density in the corpus callosum was more sensitive to changes in TH than in the anterior commissure, as indicated by the slope of the regressions. Furthermore, we measured an overall reduction in the cellular density that was independent of changes in myelin-associated glycoprotein and glial fibrillary acidic protein-positive cells. These data strongly support the interpretation that TH controls the balance of production of oligodendrocytes and astrocytes in major white matter tracts of the developing brain by acting on a common precursor of these cell types. Moreover, these findings indicate that major white matter tracts may differ in their sensitivity to TH insufficiency.

Nescient helix-loop-helix 2 interacts with signal transducer and activator of transcription 3 to regulate transcription of prohormone convertase 1/3

Mechanisms controlling body weight involve gene regulation through the activation of signal transduction pathways. The Janus kinase/signal transducer and activator of transcription (STAT) signal transduction pathway is the mechanism primarily used by leptin in the hypothalamus. The transcription factor nescient helix-loop-helix 2 (Nhlh2) is a downstream target of leptin signaling and is expressed in proopiomelanocortin arcuate neurons. Proopiomelanocortin is cleaved by prohormone convertase 1/3 (PC1/3) to produce peptides that regulate the body's response to energy availability. Previous studies show that the PC1/3 promoter contains STAT3 sites mediating leptin-induced PC1/3 expression, and that Nhlh2 is required for hypothalamic PC1/3 expression because Nhlh2 knockout mice have reduced PC1/3 mRNA levels. Studies herein reveal that leptin-induced PC1/3 gene expression is abrogated in N2KO mice, and that in a hypothalamic cell line both STAT3 and Nhlh2 are required for the full transcriptional response of a PC1/3 reporter gene after leptin stimulation. Furthermore, it is shown that Nhlh2 binds to E-box motifs found adjacent to STAT3 sites in the PC1/3 promoter both in vitro and in chromatin immunoprecipitation assays. Finally, two different protein-protein interaction assays confirm the presence of a STAT3:Nhlh2 heterodimer on the PC1/3 promoter. The Nhlh2:STAT3 heterodimer may be an important transcriptional regulator of other hypothalamic genes in the leptin signaling pathway. These data confirm Nhlh2 as an integral element of the Janus kinase/STAT signaling pathway and are the first to demonstrate coordinated control of PC1/3 transcription by Nhlh2 and STAT3 after leptin stimulation.

Environmental chemicals impacting the thyroid: targets and consequences

Thyroid hormone (TH) is essential for normal brain development, but the specific actions of TH differ across developmental time and brain region. These actions of TH are mediated largely by a combination of thyroid hormone receptor (TR) isoforms that exhibit specific temporal and spatial patterns of expression during animal and human brain development. In addition, TR action is influenced by different cofactors, proteins that directly link the TR protein to functional changes in gene expression. Considering the importance of TH signaling in development, it is important to consider environmental chemicals that may interfere with this signaling. Recent research indicates that environmental chemicals can interfere with thyroid function and with TH signaling. The key issues are to understand the mechanism by which these chemicals act and the dose at which they act, and whether adaptive responses intrinsic to the thyroid system can ameliorate potential adverse consequences (i.e., compensate). In addition, several recent studies show that TRs may be unintended targets of chemicals manufactured for industrial purposes to which humans and wildlife are routinely exposed. Polychlorinated biphenyls, polybrominated diphenyl ethers, bisphenol-A, and specific halogenated derivatives and metabolites of these compounds have been shown to bind to TRs and perhaps have selective effects on TR functions. A number of common chemicals, including polybrominated biphenyls and phthalates, may also exert such effects. When we consider the importance of TH in brain development, it will be important to pursue the possibilities that these chemicals-or interactions among chemical classes-are affecting children's health by influencing TH signaling in the developing brain.

Transgenerational exposures: persistent chemical pollutants in the environment and breast milk

Persistent organic pollutants (POPs) are anthropogenic chemicals that are poorly biodegradable and have the potential for adverse human health effects. Although national regulations and an international treaty have resulted in the gradual decline of many POPs in human blood and breast milk, the levels of other POPs continue to rise. Children and developing fetuses are sensitive to health effects from these substances. This article reviews the health risks posed by the POPs that have been largely banned or regulated and the potential for health effects from a variety of other chemicals in widespread use today.

4-Hydroxy-PCB106 acts as a direct thyroid hormone receptor agonist in rat GH3 cells

Polychlorinated biphenyls (PCBs) may interfere with thyroid hormone (TH) action by interacting directly with the TH receptor (TR). We found that the hydroxylated PCB metabolite, 4-OH-CB106, bound to the human TRbeta1 and significantly elevated endogenous growth hormone (GH) expression in GH3 cells in a manner similar to that of T(3) itself. This effect was also observed using a consensus TH response element (TRE) in a luciferase expression system, and was blocked by a single base-pair substitution in this TRE. In addition, we found that 4-OH-CB106 did not alter the ability of TRbeta1 to physically interact with the TRE in the GH promoter, or with SRC1 or NCoR. These effects were directly parallel to effects of T(3), indicating that 4-OH-CB106 exerts a direct agonistic effect on the TRbeta1.

Collision of Basic and Applied Approaches to Risk Assessment of Thyroid Toxicants

Thyroid hormone (TH) is essential for normal brain development; therefore, any environmental chemical that interferes sufficiently with thyroid function, TH metabolism, or TH action may exert adverse effects on brain development. Important known differences in aspects of thyroid endocrinology between the fetus, infant, and adult allow us to identify age-dependent vulnerabilities to thyroid toxicants with some confidence. These differences include the size of the hormone pool stored in the thyroid gland at different ages as well as the age-dependent sensitivity to mild TH insufficiency. Several recent studies that describe risk assessments of the environmental contaminant, ammonium perchlorate, provide good examples of conclusions based on the selective consideration of these known aspects of the thyroid system. Specifically, authors who consider age-dependent differences in thyroid endocrinology suggest that safe levels of perchlorate should be set at relatively low levels (low parts per billion). In contrast, authors who do not consider these known age-dependent differences in thyroid endocrinology recommend safe levels of perchlorate at high (hundreds) parts per billion to parts per million. Emerging evidence indicates that a variety of high production volume chemicals can directly interact with the TH receptor. As testing paradigms are designed by regulatory agencies, these age-dependent differences in thyroid endocrinology must be considered.

Gestational 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure effects on sensory cortex function

Gestational exposure to environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) poses a significant threat to normal growth and differentiation of the developing brain. To characterize the impact of gestational TCDD exposure on subsequent cortical function, pregnant Long Evans rats were exposed to a single acute dose (100 or 700ng/kg b.w. via gavage) on gestational day 15. This dosing regimen had no significant effect on birth index. After the TCDD-exposed animals were born and reached maturity, neural activity was recorded under urethane anesthesia from neurons in primary somatic sensory cortex. Spontaneous activity was reduced by approximately 50% in barrel cortex compared to corn oil vehicle controls. The magnitude of neuronal response to sensory (whisker) stimuli also was significantly reduced, and responses did not achieve control levels at any stimulus intensity. The greatest deficit was in the short latency component of the cortical responses. These decrements in cortical responsiveness were present in young F1 generation TCDD-exposed animals and persisted for up to 180 days. Because glutamate receptors are crucial to the evoked responses and show developmental regulation, selected iontotropic glutamate receptor subunits (NMDA NR2A+NR2B and GluR1) were profiled for RNA levels in the cortex of F1 generation rats. The expression of NR2B (NMDA receptor) and GluR1 (AMPA receptor) subunits was significantly reduced in the TCDD-exposed F1 generation animals compared to vehicle controls. The results indicate that gestational TCDD exposure results in cortical deficits that are paralled by diminished expression of certain NMDA and AMPA receptor subunits at a time when synapses are being formed for the first time in cortex.