Neurons expressing the aryl hydrocarbon receptor in the locus coeruleus and island of Calleja major are novel targets of dioxin in the mouse brain

The aryl hydrocarbon receptor (AhR) acts as a receptor that responds to ligands, including dioxin. The AhR-ligand complex translocates from the cytoplasm into the nucleus to induce gene expression. Because dioxin exposure impairs cognitive and neurobehavioral functions, AhR-expressing neurons need to be identified for elucidation of the dioxin neurotoxicity mechanism. Immunohistochemistry was performed to detect AhR-expressing neurons in the mouse brain and confirm the specificity of the anti-AhR antibody using Ahr^-/- mice. Intracellular distribution of AhR and expression level of AhR-target genes, Cyp1a1, Cyp1b1, and Ahr repressor (Ahrr), were analyzed by immunohistochemistry and quantitative RT-PCR, respectively, using mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The mouse brains were shown to harbor AhR in neurons of the locus coeruleus (LC) and island of Calleja major (ICjM) during developmental period in Ahr^+/+ mice but not in Ahr^-/- mice. A significant increase in nuclear AhR of ICjM neurons but not LC neurons was found in 14-day-old mice compared to 5- and 7-day-old mice. AhR was significantly translocated into the nucleus in LC and ICjM neurons of TCDD-exposed adult mice. Additionally, the expression levels of Cyp1a1, Cyp1b1, and Ahrr genes in the brain, a surrogate of TCDD in the tissue, were significantly increased by dioxin exposure, suggesting that dioxin-activated AhR induces gene expression in LC and ICjM neurons. This histochemical study shows the ligand-induced nuclear translocation of AhR at the single-neuron level in vivo. Thus, the neurotoxicological significance of the dioxin-activated AhR in the LC and ICjM warrants further studies.

Cardiotoxicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure through lactation in mice

Dioxins are a group of structurally related chemicals that persist in the environment. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener, is a suspected risk factor for cardiac diseases in humans. TCDD induces signs of cardiotoxicity in various animals. Mouse models of TCDD exposure suggest cardiotoxicity phenotypes develop differently depending on the timing and time-course of exposure. In order to clarify and characterize the TCDD-induced cardiotoxicity in the developing period, we utilized mouse pups exposed to TCDD. One day after delivery, groups of nursing C57BL/6J dams were orally administered TCDD at a dose of 0 (Control), 20 (TCDD-20), or 80 μg/kg (TCDD-80) body weight (BW). On postnatal days (PNDs) 7 and 21, pups' hearts were examined by histological and gene expression analyses. The TCDD-80 group was found to have a left ventricular remodeling on PND 7, and to develop heart hypertrophy on PND 21. It was accompanied by fibrosis and increased expression of associated genes, such as those for atrial natriuretic peptide (ANP), β-myosin heavy chain (β-MHC), and endothelin-1 (ET-1). These results revealed that TCDD directly induces cardiotoxicity in the postnatal period represented by progressive hypertrophy in which ANP, β-MHC, and ET-1 have potentials to mediate the cardiac hypertrophy and heart failure.

The role of prostaglandin E2 receptor EP1 in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced neonatal hydronephrosis in mice

Prostaglandin E₂ (PGE₂) is a critical factor in the pathogenesis of dioxin-induced neonatal hydronephrosis. Since the PGE₂ receptor has four subtypes, EP1 - EP4, this study was aimed to challenge the hypothesis that at least one of the four subtypes is responsible for the pathogenesis of dioxin-induced hydronephrosis. To this end, we used mouse pups, with a C57BL/6 J background, genetically lacking EP1, EP2, or EP3, and wild-type pups in whom EP4 was suppressed by administering ONO-AE3-208 (ONO), an EP4 antagonist, from postnatal day 1 (PND 1) to PND 13. To expose the pups to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via lactation, the dams were administered TCDD at an oral dose of 20 μg/kg on PND 1. The pups' urine and kidneys were collected on PND 14 for urinalysis and histological examination, respectively. We found that the incidence of hydronephrosis was 80% in the EP1^+/+ group, but was markedly reduced to 28.6% in the EP1^-/- group despite the fact that PGE₂ concentration in the urine was similarly increased in the both groups. In contrast, the incidence of hydronephrosis was 80% and 100% in the EP2^+/+ and EP2^-/-groups, respectively, and 88.9% and 100% in the EP3^+/+ and EP3^-/- groups, respectively. With regard to EP4, the incidence of hydronephrosis in vehicle (saline)-treated groups and ONO-treated was 88.9% and 100%, respectively. Therefore, we concluded that among PGE₂ receptor subtypes, EP1 plays a predominant role in the onset of TCDD-induced neonatal hydronephrosis in mouse pups.

Significance of AHR nuclear translocation sequence in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced cPLA2α activation and hydronephrosis

The aryl hydrocarbon receptor (AHR) plays a major role in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicity phenotypes. TCDD bound to AHR elicits both genomic action in which target genes are transcriptionally upregulated and nongenomic action in which cytosolic phospholipase A₂α (cPLA₂α) is rapidly activated. However, how either of these actions, separately or in combination, induces toxicity phenotypes is largely unknown. In this study, we used AHR^nls/nls mice as a model in which AHR was mutated to lack nuclear translocation sequence (NLS), and AHR^d/- mice as the corresponding control. Using this model, we studied TCDD-induced alterations in cPLA₂α activation and related factors because of the pivotal roles of cPLA₂α both in AHR's nongenomic action and in regulation of causative genes of TCDD-induced hydronephrosis. Dams were orally administered TCDD at a dose of 300 µg/kg body weight on postnatal day 1, and pups subsequently exposed to TCDD via milk were examined for gene expression on PND 7 and for histological changes on PND 14. The activation of the AHR genomic action and hydronephrosis onset were observed in the control group but not in the AHR^nls/nls group. An ex vivo experiment using peritoneal macrophages exposed to 100 nM TCDD resulted in rapid activation of cPLA₂α, an indicator of the nongenomic action, only in the control group but not in the AHR^nls/nls group. These results indicated that an NLS is required for the AHR's genomic and nongenomic actions.

Mechanisms of Developmental Toxicity of Dioxins and Related Compounds

Dioxins and related compounds induce morphological abnormalities in developing animals in an aryl hydrocarbon receptor (AhR)-dependent manner. Here we review the studies in which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is used as a prototypical compound to elucidate the pathogenesis of morphological abnormalities. TCDD-induced cleft palate in fetal mice involves a delay in palatogenesis and dissociation of fused palate shelves. TCDD-induced hydronephrosis, once considered to be caused by the anatomical obstruction of the ureter, is now separated into TCDD-induced obstructive and non-obstructive hydronephrosis, which develops during fetal and neonatal periods, respectively. In the latter, a prostaglandin E₂ synthesis pathway and urine concentration system are involved. TCDD-induced abnormal development of prostate involves agenesis of the ventral lobe. A suggested mechanism is that AhR activation in the urogenital sinus mesenchyme by TCDD modulates the wingless-type MMTV integration site family (WNT)/β-catenin signaling cascade to interfere with budding from urogenital sinus epithelium. TCDD exposure to zebrafish embryos induces loss of epicardium progenitor cells and heart malformation. AHR2-dependent downregulation of Sox9b expression in cardiomyocytes is a suggested underlying mechanism. TCDD-induced craniofacial malformation in zebrafish is considered to result from the AHR2-dependent reduction in SRY-box 9b (SOX9b), probably partly via the noncoding RNA slincR, resulting in the underdevelopment of chondrocytes and cartilage.

Exposures of children to neonicotinoids in pine wilt disease control areas

Neonicotinoid insecticides that have been on the market since 1992 have been used globally including in Japan. Because they are sprayed over forests and agricultural areas, inadvertent toxicity in nontarget insects (especially honey bees) and humans is a matter of public concern. However, information on exposure levels and potential health impacts of neonicotinoids in children living around sprayed areas is scarce. Thus, we determined neonicotinoid exposure levels in children living in communities where thiacloprid was used to control pine wilt disease. A total of 46 children (23 males and 23 females) were recruited for the present study, and informed written consent was obtained from their guardians. Urine specimens were collected before, during, and after insecticide spraying events; and atmospheric particulate matter was also collected. Concentrations of thiacloprid and 6 other neonicotinoid compounds were determined in urine samples and in atmospheric particulate matter specimens using liquid chromatography-electrospray ionization-tandem mass spectrometry. In urine specimens, thiacloprid concentrations were <0.13 μg/L and were detectable in approximately 30% of all samples. Concentrations of the other neonicotinoids, N-dm-acetamiprid, thiamethoxam, dinotefuran, and clothianidin, were 18.7, 1.92, 72.3, and 6.02 µg/L, respectively. Estimated daily intakes of these neonicotinoids were then calculated from urinary levels; although the estimated daily intakes of the neonicotinoids were lower than current acceptable daily intake values, the children were found to be exposed to multiple neonicotinoids on a daily basis. Environ Toxicol Chem 2019;38:71-79. © 2018 SETAC.

Multiple animal positioning system shows that socially-reared mice influence the social proximity of isolation-reared cagemates

Social relationships are a key determinant of social behaviour, and disruption of social behaviour is a major symptom of several psychiatric disorders. However, few studies have analysed social relationships among multiple individuals in a group or how social relationships within a group influence the behaviour of members with impaired socialisation. Here, we developed a video-analysis-based system, the Multiple-Animal Positioning System (MAPS), to automatically and separately analyse the social behaviour of multiple individuals in group housing. Using MAPS, we show that social isolation of male mice during adolescence leads to impaired social proximity in adulthood. The phenotype of these socially isolated mice was partially rescued by cohabitation with group-housed (socially-reared) mice, indicating that both individual behavioural traits and those of cagemates influence social proximity. Furthermore, we demonstrate that low reactive behaviour of other cagemates also influence individual social proximity in male mice.

Nozomi Endo, Waka Ujita et al. present a video-based system for analysing social behaviour of groups of mice. They find that male mice reared in isolation have impaired social proximity in adulthood, but this is partially rescued by cohabitation with socially-reared mice.

Glutathione has a more important role than metallothionein-I/II against inorganic mercury-induced acute renal toxicity

Inorganic mercury is a harmful heavy metal that causes severe kidney damage. Glutathione (GSH), a tripeptide comprising L-glutamic acid, glycine and L-cysteine, and metallothionein (MT), a cysteine-rich and metal-binding protein, are biologically important protective factors for renal toxicity by inorganic mercury. However, the relationship between GSH and MT for the prevention of renal toxicity by inorganic mercury is unknown. We examined the sensitivity of the mice depleted in GSH by treatment with L-Buthionine-SR-sulfoximine (L-BSO), and MT-I/II null mice genetically deleted for MT-I and MT-II, to inorganic mercury (HgCl₂). Kidney damage was not induced in the wild-type mice treated with HgCl₂ (30 µmol/kg). In the MT-I/II null mice, renal toxicity was induced by HgCl₂ at a dose of 30 µmol/kg but not 1.0 µmol/kg. All GSH-depleted mice of both strains were dead following the injection of HgCl₂ (30 µmol/kg). GSH-depleted wild-type mice treated with HgCl₂ (1.0 µmol/kg) developed kidney damage similar to MT-I/II null mice treated with HgCl₂ (30 µmol/kg). Moreover, renal toxicity induced by HgCl₂ (1.0 µmol/kg) was more severe in GSH-depleted MT-I/II null mice compared with GSH-depleted wild-type mice. The present study found that GSH and MT-I/II play cooperatively an important role in the detoxification of severe kidney damage caused by inorganic mercury. In addition, GSH may act as a primary protective factor against inorganic mercury-induced acute renal toxicity, because GSH-depleted mice were more sensitive to inorganic mercury than MT-I/II null mice.

Vocalization as a novel endpoint of atypical attachment behavior in 2,3,7,8-tetrachlorodibenzo-p-dioxin-exposed infant mice

Mammalian attachment behaviors, such as crying, are essential for infant survival by receiving food, protection, and warmth from caregivers. Ultrasonic vocalization (USV) of infant rodents functions to promote maternal proximity. Impaired USV emission has been reported in mouse models of autism spectrum disorder, suggesting that USV is associated with higher brain function. In utero and lactational dioxin exposure is known to induce higher brain function abnormalities in adulthood; however, whether perinatal dioxin exposure affects behavior during infancy is unclear. Therefore, we studied the impact of dioxin exposure on USV emission in infant mice born to dams treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 0.6 or 3.0 µg/kg) on gestational day 12.5. On postnatal days 3-9, USVs of the offspring were recorded for 1 min using a microphone in a sound-attenuated chamber. The total USV and mean call durations in infant mice exposed to 3.0 µg/kg, but not 0.6 µg/kg, were shorter than those in the control mice. In addition, the percentages of complicated call types (i.e., chevron and wave) in mice exposed to 3.0 µg/kg were decreased. Dioxin-induced gene expression changes occurred in the brains of mice exposed to 3.0 µg/kg; however, body weight, motor activity, and vocal fold structure were not significantly affected. These results suggest that infant USV is a useful behavioral endpoint in developmental neurotoxicity assessment that may be used to evaluate effects of chemical exposure on the infant-caregiver interaction.

Roles of cytosolic phospholipase A2α in reproductive and systemic toxicities in 2,3,7,8-tetrachlorodibenzo-p-dioxin-exposed mice

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces a variety of toxicities upon binding of TCDD to aryl hydrocarbon receptor. Although this binding upregulates the synthesis of prostaglandins and their related lipid mediators via cytosolic phospholipase A₂α (cPLA₂α), toxicological significance of this signaling pathway remains elusive. Herein, we investigated the roles of cPLA₂α in TCDD toxicities using cPLA₂α-null mice. In a first set of experiments, pregnant mice were orally administered TCDD at a dose of 40 μg/kg on gestation day (GD) 12.5, and fetuses were collected on GD 18 for subsequent analyses. The number of live male fetuses of cPLA₂α-null type was significantly less than that of wild-type in TCDD-exposed litters. TCDD-induced hydronephrosis was more severe in wild-type fetuses than in cPLA₂α-null fetuses regardless of sex, and kidney expression levels of the inflammatory cytokines interleukin-1β and tumor necrosis factor-α were increased in a cPLA₂α-dependent manner in TCDD-exposed fetuses. In a second set of experiments, following intraperitoneal administration of TCDD at 50 μg/kg, body weight of the male adult mice was decreased within 2 days in wild-type mice but was not changed in cPLA₂α-null mice. In addition, TCDD-induced lipid accumulation in the livers of cPLA₂α-null mice was at an intermediate level compared with TCDD-exposed wild-type and vehicle-control mice. In conclusion, the present results show that cPLA₂α is involved in TCDD-induced body weight loss, lipid accumulation in the liver, fetal hydronephrosis, and cytokine gene expression, and that the molecular basis of TCDD toxicity differs considerably between target tissues and life stages.

Impaired dendritic growth and positioning of cortical pyramidal neurons by activation of aryl hydrocarbon receptor signaling in the developing mouse

The basic helix-loop-helix (bHLH) transcription factors exert multiple functions in mammalian cerebral cortex development. The aryl hydrocarbon receptor (AhR), a member of the bHLH-Per-Arnt-Sim subfamily, is a ligand-activated transcription factor reported to regulate nervous system development in both invertebrates and vertebrates, but the functions that AhR signaling pathway may have for mammalian cerebral cortex development remains elusive. Although the endogenous ligand involved in brain developmental process has not been identified, the environmental pollutant dioxin potently binds AhR and induces abnormalities in higher brain function of laboratory animals. Thus, we studied how activation of AhR signaling influences cortical development in mice. To this end, we produced mice expressing either constitutively active-AhR (CA-AhR), which has the capacity for ligand-independent activation of downstream genes, or AhR, which requires its ligands for activation. In brief, CA-AhR-expressing plasmid and AhR-expressing plasmid were each transfected into neural stems cells in the developing cerebrum by in utero electroporation on embryonic day 14.5. On postnatal day 14, mice transfected in utero with CA-AhR, but not those transfected with AhR, exhibited drastically reduced dendritic arborization of layer II/III pyramidal neurons and impaired neuronal positioning in the developing somatosensory cortex. The effects of CA-AhR were observed for dendrite development but not for the commissural fiber projection, suggesting a preferential influence on dendrites. The present results indicate that over-activation of AhR perturbs neuronal migration and morphological development in mammalian cortex, supporting previous observations of impaired dendritic structure, cortical dysgenesis, and behavioral abnormalities following perinatal dioxin exposure.

Developmental neurotoxicity test guidelines: problems and perspectives

Epidemiologic evidence has demonstrated associations between early life exposure to industrial chemicals and the occurrence of disease states, including cognitive and behavioral abnormalities, in children. The developing brain in the fetal and infantile periods is extremely vulnerable to chemicals because the blood-brain barrier is not completely formed during these periods. The Organisation for Economic Co-operation and Development (OECD) developmental neurotoxicity (DNT) test guideline, TG426, updated in 2007, comprises in vivo behavioral observational tests and other tests intended to assess DNT induced by exposure to industrial chemicals. These chemicals may enter the market without having been subjected to DNT testing, as DNT test data is not mandated by law at the time of chemical registration. In addition, proprietary rights have led to problems concerning the non-disclosure of industrial chemical toxicity test data, including DNT test data. To overcome the disadvantages of high-cost and low time efficiency of in vivo DNT tests, in vitro or in silico tests are the proposed alternatives, but it is unlikely that the results of such tests would reflect changes in higher brain functions. Accordingly, the current DNT test guidelines need to be revised to avoid overlooking or neglecting the occurrence of DNT induced by exposure to low doses of chemicals. This review also proposes the introduction of novel in vivo DNT testing methods in light of a cost-performance analysis.

Excessive activation of AhR signaling disrupts neuronal migration in the hippocampal CA1 region in the developing mouse

The aryl hydrocarbon receptor (AhR) avidly binds dioxin, a ubiquitous environmental contaminant. Disruption of downstream AhR signaling has been reported to alter neuronal development, and rodent offspring exposed to dioxin during gestation and lactation showed abnormalities in learning and memory, emotion, and social behavior. However, the mechanism behind the disrupted AhR signaling and developmental neurotoxicity induced by xenobiotic ligands remains elusive. Therefore, we studied how excessive AhR activation affects neuronal migration in the hippocampal CA1 region of the developing mouse brain. We transfected constitutively active (CA)-AhR, AhR, or control vector plasmids into neurons via in utero electroporation on gestational day 14 and analyzed neuronal positioning in the hippocampal CA1 region of offspring on postnatal day 14. CA-AhR transfection affected neuronal positioning, whereas no change was observed in AhR-transfected or control hippocampus. These results suggest that constitutively activated AhR signaling disrupts neuronal migration during hippocampal development. Further studies are needed to investigate whether such developmental disruption in the hippocampus leads to the abnormal cognition and behavior of rodent offspring upon maternal exposure to AhR xenobiotic ligands.

Association of impaired neuronal migration with cognitive deficits in extremely preterm infants

Many extremely preterm infants (born before 28 gestational weeks [GWs]) develop cognitive impairment in later life, although the underlying pathogenesis is not yet completely understood. Our examinations of the developing human neocortex confirmed that neuronal migration continues beyond 23 GWs, the gestational week at which extremely preterm infants have live births. We observed larger numbers of ectopic neurons in the white matter of the neocortex in human extremely preterm infants with brain injury and hypothesized that altered neuronal migration may be associated with cognitive impairment in later life. To confirm whether preterm brain injury affects neuronal migration, we produced brain damage in mouse embryos by occluding the maternal uterine arteries. The mice showed delayed neuronal migration, ectopic neurons in the white matter, altered neuronal alignment, and abnormal corticocortical axonal wiring. Similar to human extremely preterm infants with brain injury, the surviving mice exhibited cognitive deficits. Activation of the affected medial prefrontal cortices of the surviving mice improved working memory deficits, indicating that decreased neuronal activity caused the cognitive deficits. These findings suggest that altered neuronal migration altered by brain injury might contribute to the subsequent development of cognitive impairment in extremely preterm infants.

Methylated site display (MSD)-AFLP, a sensitive and affordable method for analysis of CpG methylation profiles

Background

It has been pointed out that environmental factors or chemicals can cause diseases that are developmental in origin. To detect abnormal epigenetic alterations in DNA methylation, convenient and cost-effective methods are required for such research, in which multiple samples are processed simultaneously. We here present methylated site display (MSD), a unique technique for the preparation of DNA libraries. By combining it with amplified fragment length polymorphism (AFLP) analysis, we developed a new method, MSD-AFLP.

Results

Methylated site display libraries consist of only DNAs derived from DNA fragments that are CpG methylated at the 5′ end in the original genomic DNA sample. To test the effectiveness of this method, CpG methylation levels in liver, kidney, and hippocampal tissues of mice were compared to examine if MSD-AFLP can detect subtle differences in the levels of tissue-specific differentially methylated CpGs. As a result, many CpG sites suspected to be tissue-specific differentially methylated were detected. Nucleotide sequences adjacent to these methyl-CpG sites were identified and we determined the methylation level by methylation-sensitive restriction endonuclease (MSRE)-PCR analysis to confirm the accuracy of AFLP analysis. The differences of the methylation level among tissues were almost identical among these methods. By MSD-AFLP analysis, we detected many CpGs showing less than 5% statistically significant tissue-specific difference and less than 10% degree of variability. Additionally, MSD-AFLP analysis could be used to identify CpG methylation sites in other organisms including humans.

Conclusion

MSD-AFLP analysis can potentially be used to measure slight changes in CpG methylation level. Regarding the remarkable precision, sensitivity, and throughput of MSD-AFLP analysis studies, this method will be advantageous in a variety of epigenetics-based research.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-017-0083-2) contains supplementary material, which is available to authorized users.

Embryonic and Postnatal Expression of Aryl Hydrocarbon Receptor mRNA in Mouse Brain

Aryl hydrocarbon receptor (AhR), a member of the basic helix-loop-helix-Per-Arnt-Sim transcription factor family, plays a critical role in the developing nervous system of invertebrates and vertebrates. Dioxin, a ubiquitous environmental pollutant, avidly binds to this receptor, and maternal exposure to dioxin has been shown to impair higher brain functions and dendritic morphogenesis, possibly via an AhR-dependent mechanism. However, there is little information on AhR expression in the developing mammalian brain. To address this issue, the present study analyzed AhR mRNA expression in the brains of embryonic, juvenile, and adult mice by reverse transcription (RT)-PCR and in situ hybridization. In early brain development (embryonic day 12.5), AhR transcript was detected in the innermost cortical layer. The mRNA was also expressed in the hippocampus, cerebral cortex, cerebellum, olfactory bulb, and rostral migratory stream on embryonic day 18.5, postnatal days 3, 7, and 14, and in 12-week-old (adult) mice. Hippocampal expression was abundant in the CA1 and CA3 pyramidal and dentate gyrus granule cell layers, where expression level of AhR mRNA in 12-week old is higher than that in 7-day old. These results reveal temporal and spatial patterns of AhR mRNA expression in the mouse brain, providing the information that may contribute to the elucidation of the physiologic and toxicologic significance of AhR in the developing brain.

Dioxin induces Ahr-dependent robust DNA demethylation of the Cyp1a1 promoter via Tdg in the mouse liver

The aryl hydrocarbon receptor (Ahr) is a highly conserved nuclear receptor that plays an important role in the manifestation of toxicity induced by polycyclic aromatic hydrocarbons. As a xenobiotic sensor, Ahr is involved in chemical biotransformation through activation of drug metabolizing enzymes. The activated Ahr cooperates with coactivator complexes to induce epigenetic modifications at target genes. Thus, it is conceivable that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent Ahr ligand, may elicit robust epigenetic changes in vivo at the Ahr target gene cytochrome P450 1a1 (Cyp1a1). A single dose of TCDD administered to adult mice induced Ahr-dependent CpG hypomethylation, changes in histone modifications, and thymine DNA glycosylase (Tdg) recruitment at the Cyp1a1 promoter in the liver within 24 hrs. These epigenetic changes persisted until 40 days post-TCDD treatment and there was Cyp1a1 mRNA hyperinduction upon repeat administration of TCDD at this time-point. Our demethylation assay using siRNA knockdown and an in vitro methylated plasmid showed that Ahr, Tdg, and the ten-eleven translocation methyldioxygenases Tet2 and Tet3 are required for the TCDD-induced DNA demethylation. These results provide novel evidence of Ahr-driven active DNA demethylation and epigenetic memory. The epigenetic alterations influence response to subsequent chemical exposure and imply an adaptive mechanism to xenobiotic stress.

Polyuria-associated hydronephrosis induced by xenobiotic chemical exposure in mice

Hydronephrosis is a commonly found disease state characterized by the dilation of renal calices and pelvis, resulting in the loss of kidney function in the severest cases. A generally accepted etiology of hydronephrosis involves the obstruction of urine flow along the urinary tract. In the recent years, we have developed a mouse model of hydronephrosis induced by lactational exposure to dioxin and demonstrated a lack of anatomical obstruction in this model. We also showed that prostaglandin E2 synthesis system plays a critical role in the onset of hydronephrosis. In the present study, we found that neonatal hydronephrosis was not likely to be associated with functional obstruction (impaired peristalsis) but was found to be associated with polyuria and low urine osmolality with the downregulation of proteins involved in the urine concentrating process. The administration of an antidiuretic hormone analog to the dioxin-exposed pups not only suppressed the increased urine output but also decreased the incidence and severity of hydronephrosis. In contrast to the case in pups, administration of dioxin to adult mice failed to induce polyuria and upregulation of prostaglandin E2 synthesis system, and the adult mice were resistant to develop hydronephrosis. These findings suggest the possibility that polyuria could induce hydronephrosis in the absence of anatomical or functional obstruction of the ureter. It is concluded that the present animal model provides a unique example of polyuria-associated type of hydronephrosis, suggesting a need to redefine this disease state.

Ultrastructural Demonstration of Mercury Granules in the Placenta of Metallothionein-Null Pregnant Mice after Exposure to Mercury Vapor

The placenta plays an important role in the regulation of maternal to fetal transfer of toxic substances, including nonessential metals. Metallothioneins (MTs), which are known to have protective effects against heavy metal toxicity, exist in the placenta, but the exact localization of placental MTs (both MT-I and MT-III) and their physiological role in the placenta exposed to mercury are unclear. The present study was performed to examine the localization of MTs and mercury granules in the placenta of mice exposed to mercury vapor. On gestational day 16, MT-I & II-null and wild-type mice were exposed to mercury vapor at 4.9 to 5.9 mg/m3 for 2 hours. At 24 and 48 hours after exposure, the placentas were examined for mercury distribution (autometallography), MT immunoreactivity, and MT mRNA expression (in situ hybridization). No histological changes were observed in the placentas of either MT-null or wild-type mice. Mercury deposition was demonstrated along the boundary between the junctional zone and the labyrinth zone, as well as in the yolk sac, maternal decidual cells, and labyrinth trophoblasts of both MT-null and wild-type mice. MT-I & -II immunoreactivity, which was confined to wild-type mice, was demonstrated in the yolk sac and decidual cells; mercury was also shown in both structures, suggesting that mercury granules were bound to MTs. MT-III mRNA expression was observed in the yolk sac, decidual cells, and spongiotrophoblasts in both MT-null and wild-type mice. There was, however, no evidence of MT at the boundary between the junctional and labyrinth zones, where substantial mercury deposits were demonstrated. These results suggest that placental MTs and the other unknown molecules may be related to the barrier to the placental transfer of mercury.

In utero and Lactational Exposure to Acetamiprid Induces Abnormalities in Socio-Sexual and Anxiety-Related Behaviors of Male Mice

Neonicotinoids, a widely used group of pesticides designed to selectively bind to insect nicotinic acetylcholine receptors, were considered relatively safe for mammalian species. However, they have been found to activate vertebrate nicotinic acetylcholine receptors and could be toxic to the mammalian brain. In the present study, we evaluated the developmental neurotoxicity of acetamiprid (ACE), one of the most widely used neonicotinoids, in C57BL/6J mice whose mothers were administered ACE via gavage at doses of either 0 mg/kg (control group), 1.0 mg/kg (low-dose group), or 10.0 mg/kg (high-dose group) from gestational day 6 to lactation day 21. The results of a battery of behavior tests for socio-sexual and anxiety-related behaviors, the numbers of vasopressin-immunoreactive cells in the paraventricular nucleus of the hypothalamus, and testosterone levels were used as endpoints. In addition, behavioral flexibility in mice was assessed in a group-housed environment using the IntelliCage, a fully automated mouse behavioral analysis system. In adult male mice exposed to ACE at both low and high doses, a significant reduction of anxiety level was found in the light-dark transition test. Males in the low-dose group also showed a significant increase in sexual and aggressive behaviors. In contrast, neither the anxiety levels nor the sexual behaviors of females were altered. No reductions in the testosterone level, the number of vasopressin-immunoreactive cells, or behavioral flexibility were detected in either sex. These results suggest the possibility that in utero and lactational ACE exposure interferes with the development of the neural circuits required for executing socio-sexual and anxiety-related behaviors in male mice specifically.

In utero and lactational dioxin exposure induces Sema3b and Sema3g gene expression in the developing mouse brain

In the developing mammalian brain, neural network formation is regulated by complex signaling cascades. In utero and lactational dioxin exposure is known to induce higher brain function abnormalities and dendritic growth disruption in rodents. However, it is unclear whether perinatal dioxin exposure affects the expression of genes involved in neural network formation. Therefore, we investigated changes in gene expression in the brain regions of developing mice born to dams administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dose: 0, 0.6, or 3.0 μg/kg) on gestational day 12.5. Quantitative RT-PCR showed that TCDD exposure induced Ahrr expression in the cerebral cortex, hippocampus, and olfactory bulb of 3-day-old mice. Gene microarray analysis indicated that the mRNA expression levels of Sema3b and Sema3g, which encode proteins that are known to control axonal projections, were elevated in the olfactory bulb of TCDD-exposed mice, and the induction of these genes was observed during a 2-week postnatal period. Increased Sema3g expression was also observed in the brain but not in the kidney, liver, lung, and spleen of TCDD-exposed neonatal mice. These results indicate that the Sema3b and Sema3g genes are sensitive to brain-specific induction by dioxin exposure, which may disrupt neural network formation in the mammalian nervous system, thereby leading to abnormal higher brain function in adulthood.

Prenatal Exposure to Arsenic Impairs Behavioral Flexibility and Cortical Structure in Mice

Exposure to arsenic from well water in developing countries is suspected to cause developmental neurotoxicity. Although, it has been demonstrated that exposure to sodium arsenite (NaAsO₂) suppresses neurite outgrowth of cortical neurons in vitro, it is largely unknown how developmental exposure to NaAsO₂ impairs higher brain function and affects cortical histology. Here, we investigated the effect of prenatal NaAsO₂ exposure on the behavior of mice in adulthood, and evaluated histological changes in the prelimbic cortex (PrL), which is a part of the medial prefrontal cortex that is critically involved in cognition. Drinking water with or without NaAsO₂ (85 ppm) was provided to pregnant C3H mice from gestational days 8 to 18, and offspring of both sexes were subjected to cognitive behavioral analyses at 60 weeks of age. The brains of female offspring were subsequently harvested and used for morphometrical analyses. We found that both male and female mice prenatally exposed to NaAsO₂ displayed an impaired adaptation to repetitive reversal tasks. In morphometrical analyses of Nissl- or Golgi-stained tissue sections, we found that NaAsO₂ exposure was associated with a significant increase in the number of pyramidal neurons in layers V and VI of the PrL, but not other layers of the PrL. More strikingly, prenatal NaAsO₂ exposure was associated with a significant decrease in neurite length but not dendrite spine density in all layers of the PrL. Taken together, our results indicate that prenatal exposure to NaAsO₂ leads to behavioral inflexibility in adulthood and cortical disarrangement in the PrL might contribute to this behavioral impairment.

In Utero Bisphenol A Exposure Induces Abnormal Neuronal Migration in the Cerebral Cortex of Mice

Bisphenol A (BPA) has been known to have endocrine-disrupting activity to induce reproductive and behavioral abnormalities in offspring of laboratory animal species. However, morphological basis of this abnormality during brain development is largely unknown. Cerebral cortex plays a crucial role in higher brain function, and its precisely laminated structure is formed by neuronal migration. In the present study, transfecting a plasmid (pCAG-mCherry) by in utero electroporation (IUE), we visualized developing neurons and investigated the possible effects of in utero BPA exposure on neuronal migration. Pregnant mice were exposed to BPA by osmotic pump at estimated daily doses of 0, 40 (BPA-40), or 400 (BPA-400) μg/kg from embryonic day 14.5 (E14.5) to E18.5. IUE was performed at E14.5 and neuronal migration was analyzed at E18.5. Compared with the control group, neuronal migration in the cortical plate was significantly decreased in the BPA-40 group; however, there was no significant difference in the BPA-400 group. Among several neuronal migration-related genes and cortical layer-specific genes, TrkB in the BPA-400 group was found significantly upregulated. In conclusion, in utero exposure to low BPA dose was found to disrupt neuronal migration in the cerebral cortex in a dose-specific manner.

Application of NeuroTrace staining in the fresh frozen brain samples to laser microdissection combined with quantitative RT-PCR analysis

Background

The heterogeneity of the brain requires appropriate molecular biological approaches to account for its morphological complexity. Laser-assisted microdissection followed by transcript profiling by quantitative determination has been reported to be an optimal methodology. Nevertheless, not all brain regions can be identified easily without staining, restricting the accuracy and efficiency in sampling. The aim of the present study was to validate whether fixation and staining treatments are suitable for quantitative transcript expression analysis in laser microdissection (LMD) samples. Quantitative RT-PCR was used to determine the absolute transcript expression levels and profiles of samples obtained from the hippocampal dentate gyrus from fresh frozen mice brain sections that had been fixed with ethanol and stained with NeuroTrace. The results were compared with those obtained from unfixed and unstained samples.

Results

We found that the quantitative relationship of transcript expression levels between various housekeeping genes and immediate early genes was preserved, although the preparation compromised the yield of the transcripts. In addition, histological and molecular integrities of the fixed and stained specimens were preserved for at least a week at room temperature. Based on the lobe specific profiles of transcripts in the anterior and posterior lobes of the pituitary, we confirmed that no cross-contamination on transcription expressions occurred as a result of the fixation and staining.

Conclusions

We have provided detailed information of the procedures on ethanol fixation followed by NeuroTrace staining on the absolute quantitative RT-PCR analysis using microdissected fresh frozen mouse brain tissues. The present study demonstrated that quantitative transcript expression analysis can be conducted reliably on stained tissues. This method is suitable for applications in basic and clinical studies on particular transcript expressions in various regions of the brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-015-1222-9) contains supplementary material, which is available to authorized users.

Environmental insults in early life and submissiveness later in life in mouse models

Dominant and subordinate dispositions are not only determined genetically but also nurtured by environmental stimuli during neuroendocrine development. However, the relationship between early life environment and dominance behavior remains elusive. Using the IntelliCage-based competition task for group-housed mice, we have previously described two cases in which environmental insults during the developmental period altered the outcome of dominance behavior later in life. First, mice that were repeatedly isolated from their mother and their littermates (early deprivation; ED), and second, mice perinatally exposed to an environmental pollutant, dioxin, both exhibited subordinate phenotypes, defined by decreased occupancy of limited resource sites under highly competitive circumstances. Similar alterations found in the cortex and limbic area of these two models are suggestive of the presence of neural systems shared across generalized dominance behavior.

[Clarification of molecular targets of dioxin toxicity]

Health effects associated with exposure to various chemicals have been extensively studied. However, in most cases, the molecular basis of the underlying mechanism has been elusive. Dioxin toxicity, which has raised a significant concern in society, was discovered to be mediated by a high-affinity receptor, aryl hydrocarbon receptor (AhR), more than three decades ago. This receptor has been established to be essential for the manifestation of various toxicities, such as carcinogenicity, reproductive toxicity, developmental neurotoxicity, and immunotoxicity. However, it has not been clarified how AhR mediates such a wide variety of dioxin toxicities through AhR-dependent mechanisms. In recent years, several lines of experimental evidence have provided clues for opening the "black box" that contains the molecular mechanisms of dioxin action. In this review, I focus on dioxin toxicity phenotypes for which downstream molecular targets have begun to be elucidated. The toxicity phenotypes include impaired prostate development and hydronephrosis in mouse fetuses and pups, respectively, as well as abnormality in organogenesis in zebrafish embryos and adults. As the molecular basis of the tissue-specific endpoints of dioxin toxicity, dysregulation of AhR downstream pathways, such as signaling of prostanoid synthesis, Wnt/β-catenin signaling, and signaling by receptors for inflammatory cytokines, are discussed. The new findings on the molecular targets of dioxin may provide clues to the prospective discovery of new molecular events associated with the growth and development of organs and pathogenesis of diseases.

A mouse strain less responsive to dioxin-induced prostaglandin E2 synthesis is resistant to the onset of neonatal hydronephrosis

Dioxin is a ubiquitous environmental pollutant that induces toxicity when bound to the aryl hydrocarbon receptor (AhR). Significant differences in susceptibility of mouse strains to dioxin toxicity are largely accounted for by the dissociation constant of binding to dioxins of AhR subtypes encoded by different alleles. We showed that cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1), components of a prostanoid synthesis pathway, play essential roles in the onset of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced hydronephrosis of neonatal mice. Although C57BL/6J and BALB/cA mice harbor AhR receptors highly responsive to TCDD, they were found by chance to differ significantly in the incidence of TCDD-induced hydronephrosis. Therefore, the goal of the present study was to determine the molecular basis of this difference in susceptibility to TCDD toxicity. For this purpose, we administered C57BL/6J and BALB/cA dams' TCDD at an oral dose of 15 or 80 μg/kg on postnatal day (PND) 1 to expose pups to TCDD via lactation, and the pups' kidneys were collected on PND 7. The incidence of hydronephrosis in C57BL/6J pups (64%) was greater than in BALB/cA pups (0%, p < 0.05), despite similarly increased levels of COX-2 mRNA. The incidence of hydronephrosis in these mouse strains paralleled the levels of renal mPGES-1 mRNA and early growth response 1 (Egr-1) that modulates mPGES-1 gene expression, as well as PGE2 concentrations in urine. Although these mouse strains possess AhR alleles tightly bound to TCDD, their difference in incidence and severity of hydronephrosis can be explained, in part, by differences in the expression of mPGES-1 and Egr-1.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin modulates dysregulation of the lipid metabolism in mouse offspring fed a high-calorie diet

Exposure to environmental chemicals, including dioxins, is a risk factor for type 2 diabetes mellitus in humans. This study explored the hypothesis that in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener among dioxins, aggravates this disease state later in adulthood. Pregnant C57Bl/6 J mice were administered either a single oral dose of TCDD (3.0 µg kg(-1) body weight) or corn oil on gestational day 12.5. The male pups born to these two groups of dams were given either a regular diet or a high-calorie diet, after postnatal day (PND) 28. The four groups of investigated offspring were thus termed T-R (TCDD regular diet), T-H (TCDD high-calorie diet), V-R (vehicle regular diet), and V-H (vehicle high-calorie diet). The mice were regularly monitored for body weight, blood pressure and glucose, until they reached 26 weeks of age. Mice in the V-H group were significantly obese at weeks 15 and 26, but they exhibited no diabetes-associated signs of insulin resistance or hypertension. However, metabolic syndrome-related alterations with marginal signs of liver damage were found at week 26. Pronounced signs of dysregulated lipid metabolism with altered gene expression and liver inflammation were already present at week 15, whereas such alterations were suppressed in the T-H group. Although the mechanism is unclear, this study showed that in utero and lactational exposure to low-dose TCDD does not aggravate obesity-induced disease states, such as adult-onset diabetes, but instead attenuates the dysregulation of lipid metabolism brought on by a high-calorie diet.

Polybrominated dibenzo-p-dioxins, dibenzofurans, and biphenyls: inclusion in the toxicity equivalency factor concept for dioxin-like compounds

In 2011, a joint World Health Organization (WHO) and United Nations Environment Programme (UNEP) expert consultation took place, during which the possible inclusion of brominated analogues of the dioxin-like compounds in the WHO Toxicity Equivalency Factor (TEF) scheme was evaluated. The expert panel concluded that polybrominated dibenzo-p-dioxins (PBDDs), dibenzofurans (PBDFs), and some dioxin-like biphenyls (dl-PBBs) may contribute significantly in daily human background exposure to the total dioxin toxic equivalencies (TEQs). These compounds are also commonly found in the aquatic environment. Available data for fish toxicity were evaluated for possible inclusion in the WHO-UNEP TEF scheme (van den Berg et al., 1998). Because of the limited database, it was decided not to derive specific WHO-UNEP TEFs for fish, but for ecotoxicological risk assessment, the use of specific relative effect potencies (REPs) from fish embryo assays is recommended. Based on the limited mammalian REP database for these brominated compounds, it was concluded that sufficient differentiation from the present TEF values of the chlorinated analogues (van den Berg et al., 2006) was not possible. However, the REPs for PBDDs, PBDFs, and non-ortho dl-PBBs in mammals closely follow those of the chlorinated analogues, at least within one order of magnitude. Therefore, the use of similar interim TEF values for brominated and chlorinated congeners for human risk assessment is recommended, pending more detailed information in the future.

Prenatal zinc deficiency-dependent epigenetic alterations of mouse metallothionein-2 gene

Zinc (Zn) deficiency in utero has been shown to cause a variety of disease states in children in developing countries, which prompted us to formulate the hypothesis that fetal epigenetic alterations are induced by zinc deficiency in utero. Focusing on metallothionein (MT), a protein that contributes to Zn transport and homeostasis, we studied whether and how the prenatal Zn status affects gene expression. Pregnant mice were fed low-Zn (IU-LZ, 5.0 μg Zn/g) or control (IU-CZ, 35 μg Zn/g) diet ad libitum from gestation day 8 until delivery, with a regular diet thereafter. Bisulfite genomic sequencing for DNA methylation and chromatin immunoprecipitation assay for histone modifications were performed on the MT2 promoter region. We found that 5-week-old IU-LZ mice administered cadmium (Cd) (5.0 mg/kg b.w.) have an elevated abundance of MT2 mRNA compared with IU-CZ mice. Alteration of histone modifications in the MT2 promoter region having metal responsive elements (MREs) was observed in 1-day-old and 5-week-old IU-LZ mice compared with IU-CZ mice. In addition, prolongation of MTF1 binding to the MT2 promoter region in 5-week-old IU-LZ mice upon Cd exposure is considered to contribute to the enhanced MT2 induction. In conclusion, we found for the first time that Zn deficiency in utero induces fetal epigenetic alterations and that these changes are being stored as an epigenetic memory until adulthood.

Executive function deficits and social-behavioral abnormality in mice exposed to a low dose of dioxin in utero and via lactation

An increasing prevalence of mental health problems has been partly ascribed to abnormal brain development that is induced upon exposure to environmental chemicals. However, it has been extremely difficult to detect and assess such causality particularly at low exposure levels. To address this question, we here investigated higher brain function in mice exposed to dioxin in utero and via lactation by using our recently developed automated behavioral flexibility test and immunohistochemistry of neuronal activation markers Arc, at the 14 brain areas. Pregnant C57BL/6 mice were given orally a low dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at a dose of either 0, 0.6 or 3.0 µg/kg on gestation day 12.5. When the pups reached adulthood, they were group-housed in IntelliCage to assess their behavior. As a result, the offspring born to dams exposed to 0.6 µg TCDD/kg were shown to have behavioral inflexibility, compulsive repetitive behavior, and dramatically lowered competitive dominance. In these mice, immunohistochemistry of Arc exhibited the signs of hypoactivation of the medial prefrontal cortex (mPFC) and hyperactivation of the amygdala. Intriguingly, mice exposed to 3.0 µg/kg were hardly affected in both the behavioral and neuronal activation indices, indicating that the robust, non-monotonic dose-response relationship. In conclusion, this study showed for the first time that perinatal exposure to a low dose of TCDD in mice develops executive function deficits and social behavioral abnormality accompanied with the signs of imbalanced mPFC-amygdala activation.

Fluorescence laser microdissection reveals a distinct pattern of gene activation in the mouse hippocampal region

A histoanatomical context is imperative in an analysis of gene expression in a cell in a tissue to elucidate physiological function of the cell. In this study, we made technical advances in fluorescence laser microdissection (LMD) in combination with the absolute quantification of small amounts of mRNAs from a region of interest (ROI) in fluorescence-labeled tissue sections. We demonstrate that our fluorescence LMD-RTqPCR method has three orders of dynamic range, with the lower limit of ROI-size corresponding to a single cell. The absolute quantification of the expression levels of the immediate early genes in an ROI equivalent to a few hundred neurons in the hippocampus revealed that mice transferred from their home cage to a novel environment have distinct activation profiles in the hippocampal regions (CA1, CA3, and DG) and that the gene expression pattern in CA1, but not in the other regions, follows a power law distribution.

Maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin and the body burden in offspring of long-evans rats

OBJECTIVES

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) results in a wide variety of developmental effects in pups at doses much lower than those causing overt toxicity in adult animals. We investigated the relationship between tissue concentrations of TCDD in dams and fetuses and developmental effects on pups.

MATERIALS AND METHODS

Pregnant Long-Evans rats were given TCDD at a single oral dose of 12.5, 50, 200, or 800 ng of TCDD or [(3)H]-TCDD/kg bw on gestation day (GD) 15. Dams were sacrificed on GD16 and GD21, and the tissue concentrations of TCDD were measured in dams and fetuses. Pups were sacrificed on postnatal day (PND) 49 and PND63 for males and PND70 for females, and the reproductive effects and tissue concentrations of TCDD were determined.

RESULTS

The sex ratio (male/female) on GD21 was significantly reduced at 50 ng TCDD/kg and at 12.5 and 50 ng TCDD/kg at birth, but not at other doses. Delayed puberty was observed in males at 200 ng TCDD/kg and in males and females at 800 ng TCDD/kg. Anogenital distance, testis weight, epididymal sperm count, sperm motility, and ejaculated sperm count were not affected. Estrous cyclicity was not different from that of the control in any treatment group. A dose-dependent decrease in weight of seminal vesicle and prostate on PND49 was observed. Prostate weight was significantly decreased at 800 ng TCDD/kg. At this dose, maternal body burden and TCDD concentration in fetuses were 290 pg TCDD/g and 52 pg TCDD/g on GD16, respectively. Reduced prostate weight is a sensitive and commonly observed endpoint so that the body burdens of dams and fetuses at the LOAEL of this endpoint could be served as the basis for establishing TDI for dioxins.

Organic Chemicals Adsorbed onto Diesel Exhaust Particles Directly Alter the Differentiation of Fetal Thymocytes Through Arylhydrocarbon Receptor but Not Oxidative Stress Responses

Diesel exhaust particles (DEP) were reported to have adverse effects on the immune system of laboratory animals and to induce thymic involution, particularly when exposure occurred during the fetal or lactational period. DEP consist of a carbon core to which many organic compounds are adsorbed, including polyaromatic hydrocarbons (PAHs) and their derivatives (e.g., dioxins and quinones). Although it has been suggested that these organic compounds were responsible for mediating the effects of DEP through their regulation of gene expression, the molecular mechanism of action of DEP has not been fully elucidated. In this study, we examined the direct effect of DEP extracts and their constituents on gene expression and phenotype in the fetal thymus. Fetal thymuses from C57BL/6 mice were exposed to DEP extracts for 24 hrs, after which their gene expression was analyzed using an Affymetrix GeneChip system. DEP extracts up-regulated several genes known as arylhydrocarbon receptor (AhR)-target genes, including cytochrome P450 1a1 (Cyp1a1), 1b1 (Cyp1b1), TCDD-inducible poly(ADP-ribose) polymerase (Tiparp), and scinderin (Scin). Similarly, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene (B[a]P), which are AhR ligands, induced remarkably similar changes in gene expression compared to DEP extracts. In addition, our data showed little contribution of quinones to DEP extracts-induced changes in gene expression in fetal thymus through oxidative stress responses. These changes in gene expression were also confirmed by semi-quantitative RT-PCR. Furthermore, DEP extracts skewed thymic T-cell differentiation in favor of the production of CD8 T-cells, which was also observed when exposed to AhR ligands. Our results suggest that organic compounds adsorbed onto DEP alter thymic gene expression and directly affect thymocyte development by activating the AhR.

Molecular targets that link dioxin exposure to toxicity phenotypes

Many toxicology studies have elucidated health effects associated with exposure to various chemicals, but few have identified the molecular targets that cause specific endpoints of toxicity. Our understanding of the toxicity of dioxins, a group of chemicals capable of causing toxicity at environmentally relevant levels of exposure, is no exception. Dioxins are unique compared to most chemicals that we are exposed to in the environment because they activate a high affinity receptor, aryl hydrocarbon receptor (AhR), that was identified more than three decades ago. In recent years, several lines of experimental evidence have provided clues for opening the "black box" that contains the molecular mechanisms of dioxin action. These clues have emerged by toxicologists beginning to identify the molecular targets that link AhR signaling to tissue-specific toxicity phenotypes. Endpoints of dioxin toxicity for which downstream molecular targets have begun to be elucidated are observed in developmental or tissue regeneration processes, and include impaired prostate development and hydronephrosis in mouse fetuses and pups, reduced midbrain blood flow and jaw malformation in zebrafish embryos, and impaired fin regeneration in larval and adult zebrafish. Significant progress in identifying molecular targets for dioxin-induced hepatotoxicity in adult mice also has occurred. Misregulation of AhR downstream pathways, such as conversion of arachidonic acid to prostanoids via cyclooxygenase-2, and altered Wnt/β-catenin signaling downregulating Sox9, and signaling by receptors for inflammatory cytokines have been implicated in tissue-specific endpoints of dioxin toxicity. These findings may not only begin to clarify the molecular targets of dioxin action but shed light on new molecular events associated with development and disease.

Schema-dependent gene activation and memory encoding in neocortex

When new learning occurs against the background of established prior knowledge, relevant new information can be assimilated into a schema and thereby expand the knowledge base. An animal model of this important component of memory consolidation reveals that systems memory consolidation can be very fast. In experiments with rats, we found that the hippocampal-dependent learning of new paired associates is associated with a striking up-regulation of immediate early genes in the prelimbic region of the medial prefrontal cortex, and that pharmacological interventions targeted at that area can prevent both new learning and the recall of remotely and even recently consolidated information. These findings challenge the concept of distinct fast (hippocampal) and slow (cortical) learning systems, and shed new light on the neural mechanisms of memory assimilation into schemas.

Involvement of microRNAs in dioxin-induced liver damage in the mouse

MicroRNA (miRNA) is a class of small RNA that functions as a negative regulator of gene expression. Human and mouse genomes encode over 1400 and 700 miRNAs, respectively, and most of the cellular pathways are considered to be modulated by miRNAs. However, the pathophysiological role of miRNAs is still largely unknown. Thus, we investigated the possible involvement of miRNAs in the toxic responses to xenobiotic chemicals. Here, we searched for miRNAs responsible for inducing liver damage in mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and found that miR-101a and miR-122 are differentially downregulated by TCDD in a time-dependent manner. Because miRNA exerts multiple actions by repressing its target genes, we quantified the target genes of miR-101a, such as cyclooxygenase-2 (COX-2), enhancer of zeste homolog 2, and cFos, and found the upregulation of these genes, which suggests that miR-101a downregulates the expressions of these genes in the mouse liver. A COX-2 selective inhibitor, NS-398, suppressed the onset of TCDD-induced liver damage. In conclusion, this study demonstrated that TCDD dysregulates the expression of miR101a and miR122 and that COX-2, a target gene of miR101a, plays a significant role in liver damage in mice exposed to TCDD.

Polychlorinated dioxins, furans, and biphenyls in blood of children and adults living in a dioxin-contaminated area in Tokyo

The soil of a residential area in Tokyo was found to contain dioxins, namely polychlorinated dioxins, furans, and dioxin-like biphenyls, the levels of which exceeded the environmental guideline [1,000 pg toxic equivalent (TEQ)/g] by up to 6.8 times. To assess the exposure levels of people living in this area and to study the possible relationship of blood dioxin concentrations of children with breast milk and/or formula feeding, a health survey was carried out in 2006, involving a total of 138 people, including 66 children aged 3-15 years, and blood dioxin concentrations and the characteristics and lifestyles of these people were analyzed. Mean ± standard error of the mean (SEM) of blood dioxin concentrations (pg/g-lipid) of group 1 (3-6 years old), group 2 (7-15 years old), and group 3 (≥16 years old) were 13 ± 1.9, 6.6 ± 0.65, and 10 ± 0.54, respectively. The congener/isomer profile of dioxins in blood samples differed markedly from that of the contaminated soil samples. According to the feeding mode of children, blood dioxin concentrations (pg/g-lipid) were 17 ± 2.9 for breast milk only, 7.4 ± 0.82 for both breast milk and formula, and 4.7 ± 1.1 for formula only, with a significant difference from one another. We conclude that people living in the dioxin-contaminated area are less likely to be exposed to excessive amounts of dioxins, and that blood dioxin concentrations of children aged 3-15 years seem to be strongly affected by breast feeding duration.

In utero and lactational exposure to low doses of chlorinated and brominated dioxins induces deficits in the fear memory of male mice

Environmental-level in utero and lactational exposures to dioxins have been considered to affect brain functions of offspring. Here, we determined whether in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrabromodibenzo-p-dioxin (TBDD), at the dose that does not harm the dams, affects the acquisition and retention of fear memory in mouse offspring. Pregnant C57BL/6J mice were administered by gavages TCDD or TBDD at a dose of 0 or 3.0 microg/kg body weight on gestation day 12.5, and their male offspring were examined for their behavior in adulthood. In the fear conditioning, a paired presentation of tone and foot shock was repeated three times, and retention tests for contextual and auditory fear memory were carried out 1 and 24h after the fear conditioning. Groups of mice that were exposed to TCDD and TBDD in utero and via lactation showed deficits in the contextual and auditory retention tests at 1 and 24h retention intervals. The present results suggest that maternal exposure to a low dose of TCDD or TBDD disrupts the functions of memory and emotion in male mouse offspring, and that the developmental toxicities of these chemicals are similar to each other.

Aryl hydrocarbon receptor-mediated effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on glucose-stimulated insulin secretion in mice

Epidemiological and laboratory studies suggested that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects glucose homeostasis and increases the incidence of type 2 diabetes mellitus. To evaluate the effects of TCDD on insulin secretion from islets of Langerhans (islets), we designed in vivo, ex vivo and in vitro experiments. For the in vivo experiment, male C57BL/6J and aryl hydrocarbon receptor (AhR)-null mice were injected intraperitoneally with TCDD (10 microg kg(-1) b.w.), fasted for 12 h and administered glucose 24 h post-administration. TCDD exposure significantly decreased the plasma insulin concentration at 60 and 120 min after a glucose challenge in C57BL/6J mice but not in AhR-null mice. In contrast, the plasma glucose concentration was not changed by TCDD exposure in both C57BL/6J and AhR-null mice. For the ex vivo experiment, we isolated islets 24 h after TCDD administration and determined the glucose-stimulated insulin secretion from the islets. The insulin secretion level was found to be significantly decreased by TCDD exposure at 60 min after glucose treatment. For the in vitro experiment, islets harvested from untreated C57BL/6J mice were exposed to 0.1, 1, 10 or 100 nM TCDD for 24 h and analyzed for glucose-stimulated insulin secretion. Insulin secretion from the islets remained unchanged regardless of TCDD dose. In conclusion, TCDD exposure impaired the second phase of glucose-stimulated secretion of insulin from the islets via the AhR signaling pathway.

Induction of spermatogenic cell apoptosis in prepubertal rat testes irrespective of testicular steroidogenesis: a possible estrogenic effect of di(n-butyl) phthalate

Although di(n-butyl) phthalate (DBP), a suspected endocrine disruptor, induces testicular atrophy in prepubertal male rats, whether it exerts estrogenic activity in vivo remains a matter of debate. In the present study, we explored the estrogenic potency of DBP using 3-week-old male rats, and then examined the relationship between estrogen-induced spermatogenic cell apoptosis and testicular steroidogenesis. Daily exposure to DBP for 7 days caused testicular atrophy due to loss of spermatogenic cells, whereas testicular steroidogenesis was almost the same with the control values. A single exposure of DBP decreased testicular steroidogenesis in addition to decreasing the level of serum LH at 3 h after DBP treatment, with an extremely high incidence of apoptotic spermatogenic cells at 6 h after administration. To elucidate the estrogenic activity of DBP, we carried out an inhibition study using pure antiestrogen ICI 182,780 (ICI) in a model of spermatogenic cell apoptosis induced by DBP or estradial-3-benzoate (EB). Although both the DBP- and EB-treated groups showed a significant increase in spermatogenic cell apoptosis, ICI pretreatment significantly decreased the number of apoptotic spermatogenic cells in these two groups. In contrast, testicular steroidogenesis and serum FSH were significantly reduced in all the treated groups, even in the DBP+ICI and EB+ICI groups. Taken together, these findings led us to conclude that estrogenic compounds such as DBP and EB induce spermatogenic cell apoptosis in prepubertal rats, probably by activating estrogen receptors in testis, and that reduction in testicular steroidogenic function induced by estrogenic compounds is not associated with spermatogenic cell apoptosis.

Severe toxicity and cyclooxygenase (COX)-2 mRNA increase by lithium in the neonatal mouse kidney

Functions of the kidney of mammals are immature during the neonatal period, and the neonatal kidney could be susceptible to chemicals, including drugs and environmental toxicants. Among these chemicals, cyclooxygenase (COX)-inducing chemicals should be given attentions as the potential kidney toxicants during the period, and we hypothesized that lithium chloride (LiCl) has such toxicity. Neonatal mice of C57BL/J strain were intraperitoneally injected with LiCl (2 mmol/kg body weight) daily until 21 days of age, and examined on 7 days and 21 days of age. Neonatal treatment of LiCl caused a significant increase in COX-2 mRNA and a decrease in mRNAs of aquaporins on day 7 of age. Osmolarity of urine from LiCl-treated neonates was significantly lower than that of control neonate. Most of the LiCl-treated neonates died during the second week of age. Histological examination revealed renal cysts on day 7 and hydronephrosis on day 21. in the surviving neonates. The present results showed that the kidney of mouse neonates is vulnerable to lithium, and suggested the possibility that COX-2 upregulation is responsible for the severe renal toxicity including hydronephrosis.