1,2,5,6,9,10-αHexabromocyclododecane (HBCD) Impairs Thyroid Hormone-Induced Dendrite Arborization of Purkinje Cells and Suppresses Thyroid Hormone Receptor-Mediated Transcription

Effects of Perfluorooctane Sulfonate on Cerebellar Cells via Inhibition of Type 2 Iodothyronine Deiodinase Activity

Perfluorooctane sulfonate (PFOS) has been used in a wide variety of industrial and commercial products. The adverse effects of PFOS on the developing brain are becoming of a great concern. However, the molecular mechanisms of PFOS on brain development have not yet been clarified. We investigated the effect of early-life exposure to PFOS on brain development and the mechanism involved. We investigated the change in thyroid hormone (TH)-induced dendrite arborization of Purkinje cells in the primary culture of newborn rat cerebellum. We further examined the mechanism of PFOS on TH signaling by reporter gene assay, quantitative RT-PCR, and type 2 iodothyronine deiodinase (D2) assay. As low as 10-7 M PFOS suppressed thyroxine (T4)-, but not triiodothyronine (T3)-induced dendrite arborization of Purkinje cells. Reporter gene assay showed that PFOS did not affect TRα1- and TRβ1-mediated transcription in CV-1 cells. RT-PCR showed that PFOS suppressed D2 mRNA expression in the absence of T4 in primary cerebellar cells. D2 activity was also suppressed by PFOS in C6 glioma-derived cells. These results indicate that early-life exposure of PFOS disrupts TH-mediated cerebellar development possibly through the disruption of D2 activity and/or mRNA expression, which may cause cerebellar dysfunction.

Thyroid-disrupting effects caused by exposure to alternative flame retardants from groundwater contamination in rural central China

Accumulating evidence reveals that exposure to alternative flame retardants (AFRs) results in defective thyroid functions. AFRs are detectable in various environmental media in developed cities in China. However, few studies have reported the contamination levels of AFR in groundwater in rural areas, indicating an urgent need to investigate exposure of AFRs and perform health risk assessment for populations that use groundwater as the main source of drinking water. This study investigated the concentrations of AFRs in groundwater in rural areas of central China. Moreover, Nthy-ori-3-1 cells were used to determine the thyroid cytotoxicities and thyroid-interfering effects of a single AFR as well as the mixtures of AFRs based on the AFR contamination levels in real-world. The results revealed that all classes of AFRs were detectable in rural areas in central China. Dechlorane plus, hexabromocyclododecane, bromophenols (BPs), novel brominated flame retardants (NBFRs) and organophosphate flame retardants (OPFRs) exhibited spatial contamination patterns, with an average concentrations (median) of 157.89 ± 88.61 (185.47) pg/L, 0.09 ± 0.29 (not detectable) ng/L, 5.20 ± 5.92 (3.43) ng/L, 3338.11 ± 3758.78 (2836.72) pg/L, and 79.35 ± 97.19 (53.62) ng/L, respectively. The half maximal effective concentrations (EC₅₀) of BPs, OPFRs, and NBFRs ranged 98.4-4012 μM, 42.0-2506 μM, and 10.1-203.7 μM, respectively. Several AFRs exhibited more cytotoxic effects than did traditional brominated flame retardants. It is intriguing that several single AFRs and mixtures at environmentally-relevant exposure levels promoted the viability of Nthy-ori-3-1 cells. Taken together, our study demonstrates that AFRs are present in the groundwater in rural areas in central China and AFRs exhibit thyroid disrupting effects.

A PBPK model describing the pharmacokinetics of γ-HBCD exposure in mice

The brominated flame retardant, hexabromocyclododecane (HBCD), is added-but not bound-to consumer products and is eventually found in the environment and human tissues. Commercial-grade HBCD mixtures contain three major stereoisomers, alpha (α), beta (β), and gamma (γ), that are typically at a ratio of 12%:6%:82%, respectively. Although HBCD is widely used, the toxicological effects from its exposure in humans are not clearly understood. Using a physiologically based pharmacokinetic (PBPK) model could help improve our understanding of the toxicity of HBCD. The aim of this work was to develop a PBPK model, consisting of five permeability limited compartments (i.e., brain, liver, adipose tissue, blood, and rest of the body), to evaluate the pharmacokinetics of γ-HBCD in C57BL/6 mice. Physiological parameters related to body size, organ weights, and blood flow were taken from the literature. All partition coefficients were calculated based on the log Kow. The elimination in urine and feces was optimized to reflect the percent dose eliminated, as published in the literature. Compared with data from the literature for brain, liver, blood, and adipose tissue, the model simulations accurately described the mouse data set within 1.5-fold of the data points. Also, two examples showing the utility of the PBPK model supplement the information regarding the internal dose that caused the health effects observed during these studies. Although this version of the PBPK model expressly describes γ-HBCD, more efforts are needed to clarify and improve the model to discriminate between the α, β, and γ stereoisomers.

Health toxicity effects of brominated flame retardants: From environmental to human exposure

Hexabromocyclododecane (HBCD) and Tetrabromobisphenol A (TBBP-A) are brominated flame retardants widely used in variety of industrial and consumer products (e.g., automobiles, electronics, furniture, textiles and plastics) to reduce flammability. HBCD and TBBPA can also contaminate the environment, mainly water, dust, air and soil, from which human exposure occurs. This constant exposure has raised some concerns against human health. These compounds can act as endocrine disruptors, a property that gives them the ability to interfere with hormonal function and quantity, when HBCD and TBBPA bind target tissues in the body. Studies in human and animals suggest a correlation between HBCD and TBBPA exposure and adverse health outcomes, namely thyroid disorders, neurobehavior and development disorders, reproductive health, immunological, oncological and cardiovascular diseases. However, in humans these effects are still poorly understood, once only a few data evaluated the human health effects. Thus, the purpose of this review is to present the toxicity effects of HBCD and TBBPA and how these compounds affect the environment and health, resorting to data and knowledge of 255 published papers from 1979 to 2020.

Update of the risk assessment of hexabromocyclododecanes (HBCDDs) in food

The European Commission asked EFSA to update its 2011 risk assessment on hexabromocyclododecanes (HBCDDs) in food. HBCDDs, predominantly mixtures of the stereoisomers α-, β- and γ-HBCDD, were widely used additive flame retardants. Concern has been raised because of the occurrence of HBCDDs in the environment, food and in humans. Main targets for toxicity are neurodevelopment, the liver, thyroid hormone homeostasis and the reproductive and immune systems. The CONTAM Panel concluded that the neurodevelopmental effects on behaviour in mice can be considered the critical effects. Based on effects on spontaneous behaviour in mice, the Panel identified a lowest observed adverse effect level (LOAEL) of 0.9 mg/kg body weight (bw) as the Reference Point, corresponding to a body burden of 0.75 mg/kg bw. The chronic intake that would lead to the same body burden in humans was calculated to be 2.35 μg/kg bw per day. The derivation of a health-based guidance value (HBGV) was not considered appropriate. Instead, the margin of exposure (MOE) approach was applied to assess possible health concerns. Over 6,000 analytical results for HBCDDs in food were used to estimate the exposure across dietary surveys and age groups of the European population. The most important contributors to the chronic dietary LB exposure to HBCDDs were fish meat, eggs, livestock meat and poultry. The CONTAM Panel concluded that the resulting MOE values support the conclusion that current dietary exposure to HBCDDs across European countries does not raise a health concern. An exception is breastfed infants with high milk consumption, for which the lowest MOE values may raise a health concern.

Distribution of polybrominated diphenyl ethers and HBCD in sediments of the Hunhe River in Northeast China

Forty surface sediment samples from the Hunhe River in Northeast China were evaluated for contamination by polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD). The results showed that decabromodiphenyl ether (BDE-209) was the predominant congener, accounting for >98 % of PBDEs in all sediment. The concentrations of BDE-209 and HBCD ranged from 3.96 to 327 ng/g dry weight and 0.05 to 25.8 ng/g dry weight, respectively, suggesting that BDE-209 was more widely applied in the study area. The mean concentrations of BDE-209 and HBCD in the downstream portion of the Hunhe River (BDE-209 148 ng/g dry weight and HBCD 3.74 ng/g dry weight) were found to be relatively higher than those in the upstream portion of the Hunhe River and the Dahuofang Reservoir, revealing an association with municipal sewage and industrial effluent received from the cities of Fushun and Shenyang. γ-HBCD was the most abundant diastereoisomer of all three analyzed HBCD isomers; however, marked elevations of α-HBCD were also found in most sediment samples. Surprisingly, the relative abundance (mean 38%) of α-HBCD in sediment from the upstream portion of the Hunhe River was significantly higher (p < 0.006, t test) than those in Dahuofang Reservoir (mean 24%). Moreover, the severe heavy metal contamination associated with the frequent mining activities in this region was tentatively suggested as being responsible for the increased levels of α-HBCD.

Selective damage to dopaminergic transporters following exposure to the brominated flame retardant, HBCDD

Over the last several decades, the use of halogenated organic compounds has become the cause of environmental and human health concerns. Of particular notoriety has been the establishment of the neurotoxicity of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). The subsequent banning of PBDEs has led to greatly increased use of 1,2,5,6,9,10-hexabromocyclododecane (HBCDD, also known as HBCD) as a flame retardant in consumer products. The physiochemical similarities between HBCDD and PBDEs suggest that HBCDD may also be neurotoxic to the dopamine system, which is specifically damaged in Parkinson disease (PD). The purpose of this study was to assess the neurotoxicity of HBCDD on the nigrostriatal dopamine system using an in vitro and in vivo approach. We demonstrate that exposure to HBCDD (0-25 μM) for 24 h causes significant cell death in the SK-N-SH catecholaminergic cell line, as well as reductions in the growth and viability of TH+ primary cultured neurons at lower concentrations (0-10 μM) after 72 h of treatment. Assessment of the in vivo neurotoxicity of HBCDD (25 mg/kg for 30 days) resulted in significant reductions in the expression of the striatal dopamine transporter and vesicular monoamine transporter 2, both of which are integral in mediating dopamine homeostasis and neurotransmission in the dopamine circuit. However, no changes were seen in the expression of tyrosine hydroxylase in the dopamine terminal, or striatal levels of dopamine. To date, these are the first data to demonstrate that exposure to HBCDD disrupts the nigrostriatal dopamine system. Given these results and the ubiquitous nature of HBCDD in the environment, its possible role as an environmental risk factor for PD should be further investigated.

Halogenated flame-retardant concentrations in settled dust, respirable and inhalable particulates and polyurethane foam at gymnastic training facilities and residences

Halogenated flame-retardants (FRs) are used in a wide array of polymer-containing products. Animal studies and structure-activity modeling exercises suggest that FR exposure may result in detrimental toxicological effects. Workers with extended contact with such polymers (e.g., electronic dismantlers, carpet installers and aircraft personnel) have previously been observed to exhibit elevated body burdens of FRs, e.g., polybrominated diphenyl ethers (PBDEs). Recently, elevated PBDE blood levels were also reported in a non-occupational exposure group, gymnasts. These levels were hypothesized to be related to the large volumes of FR-treated polyurethane foam in gymnastics facilities. To further our understanding of workers' potential exposure, we analyzed FR concentrations in indoor dust and size-fractionated air particulates (respirable (<4 μm) and inhalable (>4 μm)) from gymnastic studios. Values were compared to samples from the homes of coaches employed at these facilities. Polyurethane foam blocks (i.e., pit foam) were also analyzed to characterize potential FR sources. FRs examined included those used to flame-retard polyurethane foam: 8 PBDE congeners, two brominated components of Firemaster 550 (2-ethylhexyl 2, 3, 4, 5-tetrabromobenzoate (TBB) and bis(2-ethylhexyl) 3, 4, 5, 6-tetrabromophthalate (TBPH)) and three chlorinated organophosphates (tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2-propyl) phosphate (TCPP) and tris(1,3-dichloro-2-propyl) phosphate (TDCPP)). Several additional FRs not used in polyurethane were also evaluated. These have also been detected in indoor dust and air and may also lead to adverse health effects. These include: BDE-183 and its replacement product (1, 2-bis(2, 4, 6-tribromophenoxy) ethane (BTBPE), two congeners of the deca-BDE formulation (BDE-206, -209) and their replacement decabromodiphenyl ethane (DBDPE) and hexabromocyclododecane (α-, β-, γ-HBCD), and tetrabromobisphenol-A (TBBPA)). Pit foam contained multiple FRs at cumulative concentrations of 12,100 to 25,800 μg g(-1), or 1.2% to 2.6% by weight. TBB and TBPH were the most abundant FRs detected, followed by TDCPP and several PBDEs. The mean total FR burden detected at the gyms was 8.6-fold higher (574 μg g(-1)) than that observed in the house dust samples (66.8 μg g(-1)). However, the polyurethane additives TBB and TDCPP were the only FRs that exhibited significantly greater levels (P<0.05) in gym than house dust. Mean levels of five FRs (BDE-99, -100, -153, -209 and TDCPP) were also higher in respirable particulates from the gyms than the homes and four FRs (BDE-47, TBB, TBPH and DBDPE) were higher at the homes than the gyms; these differences were not significant (P>0.05). Several additional FRs were detected in inhalable particulates; mean levels of BDE-66, -206 and TCPP were higher in the homes and BDE-47, -85, -99, -100, -153, -209, TBB, TBPH, and TDCPP were higher at the gyms. But, only the polyurethane additives i.e., BDE-100, TBB and TDCPP were significantly greater (P<0.05) in inhalable particulates from the gyms than at the homes. In conclusion, polyurethane foam collected from gymnastic studios exhibited a variety of FR compositional signatures; likely reflective of changes in FR usage over time and by different manufacturers. FR concentrations and compositional signatures also differed between settled dust, respirable and inhalable particulates between the gyms and homes. Concentrations of FRs used in polyurethane foam were higher in gym air and dust compared to homes, particularly TBB and TDCPP - which were also the primary FRs detected in the pit foam samples. Although these results should be interpreted with caution, as the sample size was small, these findings do suggest that FR concentrations observed in dust and air particulates from the gymnastic studios are further evidence that individuals frequenting these environments are at greater risk for exposure to these polymer additives.

Review of hexabromocyclododecane (HBCD) with a focus on legislation and recent publications concerning toxicokinetics and -dynamics

In this paper, we review recent publications regarding the toxicokinetics and -dynamics of the flame retardant Hexabromocyclododecane (HBCD). HBCD has recently been listed as a persistent organic pollutant, which therefore influenced the legislation concerning its manufacturing and formulation. However, under specific circumstances it may still be used until 2024. Early toxicity studies have only focussed on HBCD itself, which is a mixture of different isomers with different physical and toxicological characteristics. Here we take a more differentiated look at the three diastereomers α-, β- and γ-HBCD. We also address the different enantiomers to give an overview of the toxicity of HBCD to identify present gaps in our knowledge about this chemical, especially with respect to its possible formulation until 2024.

Human health risk associated with brominated flame-retardants (BFRs)

The purposes of this review are to assess the human exposure and human and experimental evidence for adverse effects of brominated flame-retardants (BFRs) with specific focus on intake from seafood. The leakage of BFRs from consumer products leads to exposure of humans from fetal life to adulthood. Fish and fish products contain the highest levels of BFRs and dominate the dietary intake of frequent fish eaters in Europe, while meat, followed by seafood and dairy products accounted for the highest US dietary intake. House dust is also reported as an important source of exposure for children as well as adults. The levels of BFRs in the general North American populations are higher than those in Europe and Japan and the highest levels are detected in infants and toddlers. The daily intake via breast milk exceeds the RfD in 10% of US infants. BFRs including PBDEs, HBCDs and TBBP-A have induced endocrine-, reproductive- and behavior effects in laboratory animals. Furthermore, recent human epidemiological data demonstrated association between exposure to BFRs and similar adverse effects as observed in animal studies. Fish including farmed fish and crude fish oil for human consumption may contain substantial levels of BFRs and infants and toddlers consuming these products on a daily basis may exceed the tolerable daily intake suggesting that fish and fish oil alone represent a risk to human health. This intake comes in addition to exposure from other sources (breast milk, other food, house dust). Because potential harmful concentrations of BFRs and other toxicants occur in fish and fish products, research on a wider range of products is warranted, to assess health hazard related to the contamination of fish and fish products for human consumption.

Prenatal exposure to the brominated flame retardant hexabromocyclododecane (HBCD) impairs measures of sustained attention and increases age-related morbidity in the Long-Evans rat

Hexabromocyclododecane (HBCD) is a brominated flame retardant that is widely-used in foam building materials and to a lesser extent, furniture and electronic equipment. After decades of use, HBCD and its metabolites have become globally-distributed environmental contaminants that can be measured in the atmosphere, water bodies, wildlife, food staples and human breastmilk. Emerging evidence suggests that HBCD can affect early brain development and produce behavioral consequences for exposed organisms. The current study examined some of the developmental and lifelong neurobehavioral effects of prenatal HBCD exposure in a rat model. Pregnant rats were gavaged with 0, 3, 10, or 30mg/kg HBCD from gestation day 1 to parturition. A functional observation battery was used to assess sensorimotor behaviors in neonates. Locomotor and operant responding under random ratio and Go/no-go schedules of food reinforcement were examined in cohorts of young adult and aged rats. HBCD exposure was associated with increased reactivity to a tailpinch in neonates, decreased forelimb grip strength in juveniles, and impaired sustained attention indicated by Go/no-go responding in aged rats. In addition, HBCD exposure was associated with a significant increase in morbidity in the aged cohort. One health complication, a progressive loss of hindleg function, was observed only in the aged, 3mg/kg HBCD animals. These effects suggest that HBCD is a developmental neurotoxicant that can produce long-term behavioral impairments that emerge at different points in the lifespan following prenatal exposure.

Mechanisms of hexabromocyclododecanes induced developmental toxicity in marine medaka (Oryzias melastigma) embryos

Hexabromocyclododecanes (HBCDs) are widely used as additive brominated flame retardants, and are now ubiquitous contaminants in the environmental media and biota, including the marine environment and marine organisms. However, the impacts of HBCDs on marine fish are not well known. In this study the embryos of marine medaka (Oryzias melastigma) were used to assess the developmental toxicity of HBCDs. Freshly fertilized marine medaka embryos were exposed to various concentrations of technical HBCD (tHBCD, 0, 5, 20 and 50μg/L) until the first fry stage, and hatch success, morphology and cardiac function were examined. In all the exposure groups (5, 20 and 50μg/L) tHBCD significantly increased the embryo heart beats. The measurement of sinus venosus-bulbus arteriosus (SV-BA) distance indicated that tHBCD significantly enlarged the SV-BA distance at exposure concentrations of 20 and 50μg/L. The malformation rate at the first fry stage was also induced by tHBCD in a dose dependent manner, with the formation of pericardial edema and yolk sac edema as the most frequently observed malformation. In addition, the concentrations of total HBCD isomers (ΣHBCDs) in embryos in the current study were comparative with environmental levels and increased with increasing exposure duration. Furthermore, exposure to tHBCD also induced the level of 8-oxodG, a representative oxidative DNA damage. The mechanisms of HBCD-induced developmental toxicity were further explored by TUNEL assay, gel-based quantitative proteomic approach and measurement of the expression of several stress responsive genes, such as p53, TNF-α, IL-1β, CYP1A, COX-1 and COX-2, together with the activities of caspases. The results suggested that HBCDs exposure at environmentally realistic concentrations induced oxidative stress and apoptosis, and suppressed nucleotide and protein synthesis, which all together resulted in developmental toxicity, particularly in the cardiovascular system, in the embryos of O. melastigma.

Actions of endocrine-disrupting chemicals on stem/progenitor cells during development and disease

Development and fate of the stem cell are regulated by extrinsic signals from the environment. Endocrine-disrupting chemicals which perturb hormonal signaling in utero and during early childhood may cause deregulation of multiple developmental processes, ranging from breakdown of stem cell niche architecture, developmental reprograming and altered stem cell fate to impaired organ and gonad development and sexual differentiation. Therefore, study of the environmental effects on stem cell integrity and normal development is a new and emerging focus for developmental biologists and cell toxicologists. When combined with new human and mouse stem cell-based models, stem cell differentiation dynamics can be studied in more biologically relevant ways. In this study, we review the current status of our understanding of the molecular mechanisms by which endocrine disruptors alter embryonic stem cell and adult stem/progenitor cell fate, organ development, cancer stem cell activity, and tumorigenesis.

Invited commentary: Maternal plasma polybrominated diphenyl ethers and thyroid hormones--challenges and opportunities

Thyroid hormones play a fundamental role in fetal and child development. While iodine deficiency-related maternal and child hypothyroidism may cause severe mental retardation, recent evidence suggests that milder forms of maternal hypothyroidism and hypothyroxinemia during pregnancy are also associated with altered neurodevelopment. On the other hand, hyperthyroidism during pregnancy has been associated with adverse fetal outcomes. Findings published by Abdelouahab et al. in the American Journal of Epidemiology (Am J Epidemiol. 2013;178(5):701-713) suggest that plasma concentrations of maternal polybrominated diphenyl ethers (PBDEs), which were used as flame retardants until recently and are detected in the tissues of virtually every North American, are associated with umbilical cord and maternal thyroid hormone levels during pregnancy. Although PBDEs have been consistently shown to reduce levels of free and total thyroxine in experimental animal studies, the direction of associations in human studies has been inconsistent. In this commentary, I discuss challenges beyond the factors often cited in the epidemiologic literature to explain inconsistent findings which more specifically apply to the study of PBDEs and thyroid hormones. These include the determination of iodine intake status, the method used to adjust for blood lipid concentrations, the measurement of free thyroid hormone levels, the possible effect of PBDE metabolites, and the potential for reverse causality.

The fate of β-hexabromocyclododecane in female C57BL/6 mice

1,2,5,6,9,10-Hexabromocyclododecane (HBCD) is a high production volume cycloaliphatic used as an additive flame retardant primarily in polystyrene foam building materials. HBCD mixtures contain three major stereoisomers, alpha (α), beta (β), and gamma (γ), at a typical ratio of 1.2:0.6:8.2. The toxicokinetic properties of the α and γ isomers differ. For instance, α-HBCD has greater bioavailability and potential for accumulation in mice than γ-HBCD. The present study reports comparative kinetics data for β-HBCD needed to support toxicological evaluations of HBCD mixtures. Results indicated that a single oral dose of 3mg/kg of [(14)C]-labeled β-HBCD was absorbed rapidly (≥ 85% total dose) in the female C57BL/6 mouse. The C max for β-HBCD-derived radioactivity in tissues, except adipose, was observed 3h following gavage. Approximately 90% of the administered dose was excreted in urine and feces within 24h, primarily as β-HBCD-derived metabolites. A portion of the dose (circa 9%) was excreted in feces as γ-HBCD. Oral administration of 30 or 100mg/kg of β-HBCD resulted initially in slower rates of [(14)C] elimination; however, cumulative excretion data were similar across the dosing range 4 days postdosing. Residual concentrations of [(14)C] in tissues were highest in adipose and liver. β-HBCD-derived radioactivity accumulated in most tissues following four consecutive daily oral doses of 3mg/kg. The extent of metabolism and excretion of β-HBCD in female C57BL/6 mice was similar to that for γ-HBCD. The potential for accumulation of β-HBCD-derived material in most tissues appeared to be less than for α-HBCD.

The cytological effects of HBCDs on human hepatocyte L02 and the potential molecular mechanism

The concentration of hexabromocyclododecanes (HBCDs) in the environment media and organism samples are gradually rising with the increase of HBCDs usage. This study is designed to investigate the cytological effects of HBCDs on human hepatocyte L02 and explore the potential molecular mechanism. The results of CCK-8 assay showed that high concentration of HBCDs (>20 μM) significantly suppressed cell survival, while comparatively lower dose of HBCDs (10(-13)-10(-7)M) slightly stimulated cell proliferation (P < 0.05). In the mean time, high concentration HBCDs markedly induced cell apoptosis and DNA damage, accompanying with increase of intracellular Ca(2+)level and decrease of mitochondrial membrane potential (P < 0.05). ROS level induced by low concentration of HBCDs was comparatively lower than that by high concentration of HBCDs. In addition, low concentration HBCDs exposure (10(-13)-10(-7)M) resulted in up-regulation of PCNA protein expression level in a time-dependent manner. However, high concentration HBCDs exposure led to increase of Apaf-1 expression level. In conclusion, loss of mitochondrial membrane potential and activation of Apaf-1 mediated pathway involve the L02 cell apoptosis induced by high concentration HBCDs exposure. However, low concentration HBCDs exposure could stimulate cell proliferation of L02 cells, which might be associated with enhancement of PCNA expression.

Profound morphological and functional changes of rodent Purkinje cells between the first and the second postnatal weeks: a metamorphosis?

Between the first and the second postnatal week, the development of rodent Purkinje cells is characterized by several profound transitions. Purkinje cells acquire their typical dendritic "espalier" tree morphology and form distal spines. During the first postnatal week, they are multi-innervated by climbing fibers and numerous collateral branches sprout from their axons, whereas from the second postnatal week, the regression of climbing fiber multi-innervation begins, and Purkinje cells become innervated by parallel fibers and inhibitory molecular layer interneurons. Furthermore, their periods of developmental cell death and ability to regenerate their axon stop and their axons become myelinated. Thus a Purkinje cell during the first postnatal week looks and functions differently from a Purkinje cell during the second postnatal week. These fundamental changes occur in parallel with a peak of circulating thyroid hormone in the mouse. All these features suggest to some extent an interesting analogy with amphibian metamorphosis.

Brain-derived neurotrophic factor (BDNF) ameliorates the suppression of thyroid hormone-induced granule cell neurite extension by hexabromocyclododecane (HBCD)

Thyroid hormone (TH) plays an essential role in growth and differentiation of the central nervous system. Deficiency of TH during perinatal period results in abnormal brain development known as cretinism in human. We recently reported that an environmental chemical 1,2,5,6,9,10-α-hexabromocyclododecane (HBCD) suppressed TH receptor (TR)-mediated transcription. To examine the effect of HBCD on cerebellar granule cells, we used purified rat cerebellar granule cells in reaggregate culture. Low dose HBCD (10(-10)M) significantly suppressed TH-induced neurite extension of granule cell aggregate. To clarify further the mechanisms of such suppression, we added brain-derived neurotrophic factor (BDNF) into culture medium, since BDNF plays a critical role in promoting granule cell development and is regulated by TH. BDNF completely rescued HBCD-induced suppression of granule cell neurite extension in the presence of T3. These results indicate that HBCD may disrupt TH-mediated brain development at least in part due to a disruption of the T3 stimulated increase in BDNF and BDNF may possess ability to ameliorate the effect of HBCD in granule cells.