Methylmercury Induces Mitochondria- and Endoplasmic Reticulum Stress-Dependent Pancreatic β-Cell Apoptosis via an Oxidative Stress-Mediated JNK Signaling Pathway

Methylmercury (MeHg), a long-lasting organic pollutant, is known to induce cytotoxic effects in mammalian cells. Epidemiological studies have suggested that environmental exposure to MeHg is linked to the development of diabetes mellitus (DM). The exact molecular mechanism of MeHg-induced pancreatic β-cell cytotoxicity is still unclear. Here, we found that MeHg (1-4 μM) significantly decreased insulin secretion and cell viability in pancreatic β-cell-derived RIN-m5F cells. A concomitant elevation of mitochondrial-dependent apoptotic events was observed, including decreased mitochondrial membrane potential and increased proapoptotic (Bax, Bak, p53)/antiapoptotic (Bcl-2) mRNA ratio, cytochrome c release, annexin V-Cy3 binding, caspase-3 activity, and caspase-3/-7/-9 activation. Exposure of RIN-m5F cells to MeHg (2 μM) also induced protein expression of endoplasmic reticulum (ER) stress-related signaling molecules, including C/EBP homologous protein (CHOP), X-box binding protein (XBP-1), and caspase-12. Pretreatment with 4-phenylbutyric acid (4-PBA; an ER stress inhibitor) and specific siRNAs for CHOP and XBP-1 significantly inhibited their expression and caspase-3/-12 activation in MeHg-exposed RIN-mF cells. MeHg could also evoke c-Jun N-terminal kinase (JNK) activation and reactive oxygen species (ROS) generation. Antioxidant N-acetylcysteine (NAC; 1mM) or 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox; 100 μM) markedly prevented MeH-induced ROS generation and decreased cell viability in RIN-m5F cells. Furthermore, pretreatment of cells with SP600125 (JNK inhibitor; 10 μM) or NAC (1 mM) or transfection with JNK-specific siRNA obviously attenuated the MeHg-induced JNK phosphorylation, CHOP and XBP-1 protein expression, apoptotic events, and insulin secretion dysfunction. NAC significantly inhibited MeHg-activated JNK signaling, but SP600125 could not effectively reduce MeHg-induced ROS generation. Collectively, these findings demonstrate that the induction of ROS-activated JNK signaling is a crucial mechanism underlying MeHg-induced mitochondria- and ER stress-dependent apoptosis, ultimately leading to β-cell death.

Selective dopaminergic effects on attention and memory in male mice exposed to Methylmercury during adolescence

Gestational exposure to methylmercury disrupts dopamine-mediated behavior and produces heightened sensitivity to monoamine agonists later in life. This has been reported and replicated following both pre- and post-natal exposure. Impacts of methylmercury when exposure occurs during the sensitive period of adolescence, a key period of dopaminergic development, remain underexplored. There have been variable results thus far in studies investigating links between adolescent exposure to methylmercury and alterations in executive function and altered sensitivity to monoamine agonists. The current study was designed to investigate adolescent exposure by exposing male mice to 0, 0.3, or 3 ppm methylmercury during adolescence and training them in a hybrid task to assess two executive functions, attention and memory, in adulthood. Behavior in these animals was probed with a range of doses of the dopamine agonist, d-amphetamine, and the norepinephrine agonist, desipramine. Attention and memory in these mice were sensitive to disruption by d-amphetamine and interacted with methylmercury exposure. Choice latencies were also longer in the MeHg-exposed mice. Desipramine did not affect behavior in these animals nor did it interact with methylmercury. It is concluded that methylmercury-related inhibition of behavior observed in this study were differentially sensitive to acute disruption in dopamine, but not norepinephrine, neurotransmission.

RNA sequencing and proteomic profiling reveal different alterations by dietary methylmercury in the hippocampal transcriptome and proteome in BALB/c mice

Methylmercury (MeHg) is a highly neurotoxic form of mercury (Hg) present in seafood. Here, we recorded and compared proteomic and transcriptomic changes in hippocampus of male BALB/c mice exposed to two doses of MeHg. Mice were fed diets spiked with 0.28 mg MeHg kg-1, 5 mg MeHg kg-1, or an unspiked control diet for 77 days. Total mercury content was significantly (P < 0.05) increased in brain tissue of both MeHg-exposed groups (18 ± 2 mg Hg kg-1 and 0.56 ± 0.06 mg Hg kg-1). Hippocampal protein and ribonucleic acid (RNA) expression levels were significantly altered both in tissues from mice receiving a low dose MeHg (20 proteins/294 RNA transcripts) and a high dose MeHg (61 proteins/876 RNA transcripts). The majority but not all the differentially expressed features in hippocampus were dose dependent. The combined use of transcriptomic and proteomic profiling data provided insight on the influence of MeHg on neurotoxicity, energy metabolism, and oxidative stress through several regulated features and pathways, including RXR function and superoxide radical degradation.

Amino Acid Transporter LAT1 (SLC7A5) Mediates MeHg-Induced Oxidative Stress Defense in the Human Placental Cell Line HTR-8/SVneo

The placental barrier can protect the fetus from contact with harmful substances. The potent neurotoxin methylmercury (MeHg), however, is very efficiently transported across the placenta. Our previous data suggested that L-type amino acid transporter (LAT)1 is involved in placental MeHg uptake, accepting MeHg-L-cysteine conjugates as substrate due to structural similarity to methionine. The aim of the present study was to investigate the antioxidant defense of placental cells to MeHg exposure and the role of LAT1 in this response. When trophoblast-derived HTR-8/SVneo cells were LAT1 depleted by siRNA-mediated knockdown, they accumulated less MeHg. However, they were more susceptible to MeHg-induced toxicity. This was evidenced in decreased cell viability at a usually noncytotoxic concentration of 0.03 µM MeHg (~6 µg/L). Treatment with ≥0.3 µM MeHg increased cytotoxicity, apoptosis rate, and oxidative stress of HTR-8/SVneo cells. These effects were enhanced under LAT1 knockdown. Reduced cell number was seen when MeHg-exposed cells were cultured in medium low in cysteine, a constituent of the tripeptide glutathione (GSH). Because LAT1-deficient HTR-8/SVneo cells have lower GSH levels than control cells (independent of MeHg treatment), we conclude that LAT1 is essential for de novo synthesis of GSH, required to counteract oxidative stress. Genetic predisposition to decreased LAT1 function combined with MeHg exposure could increase the risk of placental damage.

Low Doses of Methylmercury Induce the Proliferation of Thyroid Cells In Vitro Through Modulation of ERK Pathway

Exposure to environmental endocrine disruptors has been associated with an increased frequency of thyroid pathology. In this study, we evaluated the effects of various concentrations of methylmercury (MeHg) on immortalized, non-tumorigenic thyroid cells (Nthy-ori-3-1). Exposure to MeHg at 2.5 and 5 µM for 24 h caused a reduction in cell viability with a decrease of the cell population in sub-G0 phase, as detected by MTT and flow cytometry. Conversely, MeHg at the lower concentration of 0.1 µM increased the cell viability with a rise of G2/M phase. An immunoblot analysis showed higher expression levels of phospho-ERK and not of phospho-Akt. Further enhancement of the cell growth rate was observed after a prolonged exposure of the cells up to 18 days to MeHg 0.1 µM. The present findings demonstrate the toxicity of high concentrations of MeHg on thyroid cells, while showing that treatment with lower doses of Hg, as may occur after prolonged exposure to this environmental contaminant, exerts a promoting effect on thyroid cell proliferation, by acting on the ERK-mediated pro-oncogenic signal transduction pathway.

Methyl and Ethylmercury elicit oxidative stress and unbalance the antioxidant system in Saccharomyces cerevisiae

Methylmercury (MeHg) and Ethylmercury (EtHg) are toxic to the central nervous system. Human exposure to MeHg and EtHg results mainly from the consumption of contaminated fish and thimerosal-containing vaccines, respectively. The mechanisms underlying the toxicity of MeHg and EtHg are still elusive. Here, we compared the toxic effects of MeHg and EtHg in Saccharomyces cerevisiae (S. cerevisiae) emphasizing the involvement of oxidative stress and the identification of molecular targets from antioxidant pathways. Wild type and mutant strains with deleted genes for antioxidant defenses, namely: γ-glutamylcysteine synthetase, glutathione peroxidase, catalase, superoxide dismutase, mitochondrial peroxiredoxin, cytoplasmic thioredoxin, and redox transcription factor Yap1 were used to identify potential pathways and proteins from cell redox system targeted by MeHg and EtHg. MeHg and EtHg inhibited cell growth, decreased membrane integrity, and increased the granularity and production of reactive species (RS) in wild type yeast. The mutants were predominantly less tolerant of mercurial than wild type yeast. But, as the wild strain, mutants exhibited higher tolerance to MeHg than EtHg. Our results indicate the involvement of oxidative stress in the cytotoxicity of MeHg and EtHg and reinforce S. cerevisiae as a suitable model to explore the mechanisms of action of electrophilic toxicants.

Evidences of possible influences of methylmercury concentrations on condition factor and maturation of Lophius vomerinus (Cape monkfish)

Muscle and liver tissues of Lophius vomerinus off the coast of Namibia were analysed to investigated the influence of MeHg on the biological parameters of L.vomerinus by (i) determining if the variability in total MeHg concentrations is influenced by length, maturity status and sex, and (ii) assessing if there is a relationship between biological indices (Condition factor (K), Gonadosomatic Index (GSI) Hepatosomatic Index (HSI)) and MeHg concentrations. Correlations between total MeHg concentrations and fish length, K and HSI were observed. A weak positive correlation was observed between total MeHg and GSI for combined sex. Total MeHg concentration in tissues of L. vomerinus is significantly dependent on the maturity stages (p < 0.05). K was significantly inversely correlated with total MeHg in tissues of L. vomerinus. The evidence presented in this study suggests that MeHg in L. vomerinus tissues could be detrimental to both its physiology and population dynamics.

Low doses of methylmercury exposure during adulthood in rats display oxidative stress, neurodegeneration in the motor cortex and lead to impairment of motor skills

Despite the vast distribution among tissues, the central nervous system (CNS) represents the main target of methylmercury (MeHg) toxicity. However, few studies have evaluated the effects of MeHg exposure on the CNS at equivalent doses to human environmental exposure. In our study, we evaluated the motor cortex, an important area of motor control, in adult rats chronically exposed to MeHg in a concentration equivalent to those found in fish-eating populations exposed to mercury (Hg). The parameters evaluated were total Hg accumulation, oxidative stress, tissue damage, and behavioral assessment in functional actions that involved this cortical region. Our results show in exposed animals a significantly greater level of Hg in the motor cortex; increase of nitrite levels and lipid peroxidation, associated with decreased antioxidant capacity against peroxyl radicals; reduction of neuronal and astrocyte density; and poor coordination and motor learning impairment. Our data showed that chronic exposure at low doses to MeHg is capable of promoting damages to the motor cortex of adult animals, with changes in oxidative biochemistry misbalance, neurodegeneration, and motor function impairment.

Brain methylmercury uptake in fetal, neonate, weanling, and adult rats

Fetuses and neonates are known to be highly susceptible to methylmercury (MeHg) toxicity, but little is known about the relative uptake of MeHg from blood to the developing brain. We measured time-course changes in mercury (Hg) concentrations in the brain of fetal, neonate, weanling, and adult rats after an injection of 0.08 μg (0.4 nmol) Hg/g MeHg. In the prenatal experiment, MeHg was subcutaneously injected to pregnant dams on embryonic days 17, 18, 18.5, 19, 19.5, or 20, and Hg concentrations in tissues were measured in both mothers and fetuses on embryonic day 21 (1 day before parturition). Brain Hg levels in fetuses peaked 2 days after injection and were approximately 1.5 times higher than in mothers. In the postnatal experiment, the same MeHg dose was injected subcutaneously to male rats on postnatal days 1 (neonates), 35 (weanlings), or 56 (adults). Mercury concentrations in tissues were measured 1, 2, 3, 4, 5, or 6 days after the injection. Brain Hg levels peaked most rapidly in neonates, and were approximately 1.5 times higher than levels in weanlings or adults. Throughout the examined period, peak Hg levels in the brain and the Hg brain/blood ratio 24 h after injection were highest in fetuses, followed by the levels in neonates, and decreased with life stage. These findings suggest that relatively higher brain MeHg uptake is an important factor in the vulnerability of fetuses and neonates to MeHg exposure.

Female reproductive impacts of dietary methylmercury in yellow perch (Perca flavescens) and zebrafish (Danio rerio)

The purpose of this study was to evaluate the effects of environmentally relevant dietary MeHg exposures on adult female yellow perch (Perca flavescens) and female zebrafish (Danio rerio) ovarian development and reproduction. Yellow perch were used in the study for their socioeconomic and ecological importance within the Great Lakes basin, and the use of zebrafish allowed for a detailed analysis of the molecular effects of MeHg following a whole life-cycle exposure. Chronic whole life dietary exposure of F₁ zebrafish to MeHg mimics realistic wildlife exposure scenarios, and the twenty-week adult yellow perch exposure (where whole life-cycle exposures are difficult) captures early seasonal ovarian development. For both species, target dietary accumulation values were achieved prior to analyses. In zebrafish, several genes involved in reproductive processes were shown to be dysregulated by RNA-sequencing and quantitative real-time polymerase chain reaction (QPCR), but no significant phenotypic changes were observed regarding ovarian staging, fecundity, or embryo mortality. Yellow perch were exposed to dietary MeHg for 12, 16, or 20 weeks. In this species, a set of eight genes were assessed by QPCR in the pituitary, liver, and ovary, and no exposure-related changes were observed. The lack of genomic resources in yellow perch hinders the characterization of subtle molecular impacts. The ovarian somatic index, circulating estradiol and testosterone, and ovarian staging were not significantly altered by MeHg exposure in yellow perch. These results suggest that environmentally relevant MeHg exposures do not drastically reduce the reproductively important endpoints in these fish, but to capture realistic exposure scenarios, whole life-cycle yellow perch exposures are needed.

Methylmercury-l-Cysteine targeting L-type amino acid transporter conjugate cytotoxicity on C6 glioma cells

Glioma is the most common primary tumor in the brain, accounting for about 40~50% of intracranial primary tumors. Most chemotherapeutic drugs have difficulty in penetrating the blood-brain barrier, and their clinical applications are greatly limited. We evaluated the effects of methylmercury-L-cysteine (MeHg-L-cys) and methylmercury chloride (MMC) on apoptosis of C6 glioma cells. L-type amino acid transporter (LAT1) was used to investigate the targeted transport function and cytotoxicity of MeHg- L-cys in glioma. MeHg-L-cys enhanced the ability of targeting glioma cells and reduced the adverse reactions to normal brain tissues. Therefore, it is significantly important to develop new anti-glioma drugs targeting the blood-brain barrier.

Gestational Age and Sex Influence the Susceptibility of Human Neural Progenitor Cells to Low Levels of MeHg

The developing nervous system is highly susceptible to methylmercury (MeHg), a widespread environmental neurotoxic contaminant. A wide range of morphological and functional outcomes have been described; however, there are still open questions regarding the mechanisms behind the developmental neurotoxic effects induced by low-level exposure. In the present study, we have examined the effects of nanomolar concentrations of MeHg on primary fetal human progenitor cells (hNPCs) with special focus on the role played by developmental stage and sex on the neurotoxic outcome. We found that neurospheres derived from earlier gestational time points exhibit higher susceptibility to MeHg, as they undergo apoptosis at a much lower dose (25 nM) as compared to neurospheres established from older fetuses (100 nM). At subapoptotic concentrations (10 nM), MeHg inhibited neuronal differentiation and maturation of hNPCs, as shown by a reduced number of Tuj1-positive cells and a visible reduction in neurite extension and cell migration, associated with a misregulation of Notch1 and BDNF signaling pathways. Interestingly, cells derived from male fetuses showed more severe alterations of neuronal morphology as compared to cells from females, indicating that the MeHg-induced impairment of neurite extension and cell migration is sex-dependent. Accordingly, the expression of the CDKL5 gene, a major factor regulating neurite outgrowth, was significantly more downregulated in male-derived cells. Altogether, gestational age and sex appear to be critical factors influencing in vitro hNPC sensitivity to low levels of MeHg.

Methylmercury augments Nrf2 activity by downregulation of the Src family kinase Fyn

Methylmercury (MeHg) is a potent developmental neurotoxicant that induces an oxidative stress response in the brain. It has been demonstrated that MeHg exposure increases nuclear factor erythroid 2-related factor 2 (Nrf2) activity. Nrf2 is a transcription factor that translocates to the nucleus in response to oxidative stress, and upregulates phase II detoxifying enzymes. Although, Nrf2 activity is augmented subsequent to MeHg exposure, it has yet to be established whether Nrf2 moves into the nucleus as a result. Furthermore, the potential effect MeHg might have on the non-receptor tyrosine kinase, Fyn, has not been addressed. Fyn phosphorylates Nrf2 in the nucleus, resulting in its inactivation, and consequent downregulation of the oxidative stress response. Here, we observe Nrf2 translocates to the nucleus subsequent to MeHg-induced oxidative stress. This response is concomitant with reduced Fyn expression and nuclear localization. Moreover, we detected an increase in phosphorylated Akt and glycogen synthase kinase 3 beta (GSK-3β) at activating and inhibitory sites, respectively. Akt phosphorylates and inhibits GSK-3β, which subsequently prevents Fyn phosphorylation to signal nuclear import. Our results demonstrate MeHg downregulates Fyn to maintain Nrf2 activity, and further illuminate a potential mechanism by which MeHg elicits neurotoxicity.

Mercury-induced epigenetic transgenerational inheritance of abnormal neurobehavior is correlated with sperm epimutations in zebrafish

Methylmercury (MeHg) is a ubiquitous environmental neurotoxicant, with human exposures predominantly resulting from fish consumption. Developmental exposure of zebrafish to MeHg is known to alter their neurobehavior. The current study investigated the direct exposure and transgenerational effects of MeHg, at tissue doses similar to those detected in exposed human populations, on sperm epimutations (i.e., differential DNA methylation regions [DMRs]) and neurobehavior (i.e., visual startle and spontaneous locomotion) in zebrafish, an established human health model. F0 generation embryos were exposed to MeHg (0, 1, 3, 10, 30, and 100 nM) for 24 hours ex vivo. F0 generation control and MeHg-exposed lineages were reared to adults and bred to yield the F1 generation, which was subsequently bred to the F2 generation. Direct exposure (F0 generation) and transgenerational actions (F2 generation) were then evaluated. Hyperactivity and visual deficit were observed in the unexposed descendants (F2 generation) of the MeHg-exposed lineage compared to control. An increase in F2 generation sperm epimutations was observed relative to the F0 generation. Investigation of the DMRs in the F2 generation MeHg-exposed lineage sperm revealed associated genes in the neuroactive ligand-receptor interaction and actin-cytoskeleton pathways being effected, which correlate to the observed neurobehavioral phenotypes. Developmental MeHg-induced epigenetic transgenerational inheritance of abnormal neurobehavior is correlated with sperm epimutations in F2 generation adult zebrafish. Therefore, mercury can promote the epigenetic transgenerational inheritance of disease in zebrafish, which significantly impacts its environmental health considerations in all species including humans.

Effects of adolescent exposure to methylmercury and d-amphetamine on reversal learning and an extradimensional shift in male mice

Adolescence is associated with the continued maturation of dopamine neurotransmission and is implicated in the etiology of many psychiatric illnesses. Adolescent exposure to neurotoxicants that distort dopamine neurotransmission, such as methylmercury (MeHg), may modify the effects of chronic d-amphetamine (d-AMP) administration on reversal learning and attentional-set shifting. Male C57Bl/6n mice were randomly assigned to two MeHg-exposure groups (0 ppm and 3 ppm) and two d-AMP-exposure groups (saline and 1 mg/kg/day), producing four treatment groups (n = 10-12/group): control, MeHg, d-AMP, and MeHg + d-AMP. MeHg exposure (via drinking water) spanned postnatal days 21-59 (the murine adolescent period), and once daily intraperitoneal injections of d-AMP or saline spanned postnatal days 28-42. As adults, mice were trained on a spatial-discrimination-reversal (SDR) task in which the spatial location of a lever press predicted reinforcement. Following 2 SDRs, a visual-discrimination task (extradimensional shift) was instated in which the presence of a stimulus light above a lever predicted reinforcement. Responding was modeled using a logistic function, which estimated the rate (slope) of a behavioral transition and trials required to complete half a transition (half-max). MeHg, d-AMP, and MeHg + d-AMP exposure increased estimates of half-max on the second reversal. MeHg exposure increased half-max and decreased the slope term following the extradimensional shift, but these effects did not occur following MeHg + d-AMP exposure. MeHg + d-AMP exposure produced more perseverative errors and omissions following a reversal. Adolescent exposure to MeHg can modify the behavioral effects of chronic d-AMP administration. (PsycINFO Database Record

Methylmercury Causes Blood-Brain Barrier Damage in Rats via Upregulation of Vascular Endothelial Growth Factor Expression

Clinical manifestations of methylmercury (MeHg) intoxication include cerebellar ataxia, concentric constriction of visual fields, and sensory and auditory disturbances. The symptoms depend on the site of MeHg damage, such as the cerebellum and occipital lobes. However, the underlying mechanism of MeHg-induced tissue vulnerability remains to be elucidated. In the present study, we used a rat model of subacute MeHg intoxication to investigate possible MeHg-induced blood-brain barrier (BBB) damage. The model was established by exposing the rats to 20-ppm MeHg for up to 4 weeks; the rats exhibited severe cerebellar pathological changes, although there were no significant differences in mercury content among the different brain regions. BBB damage in the cerebellum after MeHg exposure was confirmed based on extravasation of endogenous immunoglobulin G (IgG) and decreased expression of rat endothelial cell antigen-1. Furthermore, expression of vascular endothelial growth factor (VEGF), a potent angiogenic growth factor, increased markedly in the cerebellum and mildly in the occipital lobe following MeHg exposure. VEGF expression was detected mainly in astrocytes of the BBB. Intravenous administration of anti-VEGF neutralizing antibody mildly reduced the rate of hind-limb crossing signs observed in MeHg-exposed rats. In conclusion, we demonstrated for the first time that MeHg induces BBB damage via upregulation of VEGF expression at the BBB in vivo. Further studies are required in order to determine whether treatment targeted at VEGF can ameliorate MeHg-induced toxicity.

Comparison of mercury sulfides with mercury chloride and methylmercury on hepatic P450, phase-2 and transporter gene expression in mice

Zuotai (mainly β-HgS) and Zhusha (also called as cinnabar, mainly α-HgS) are used in traditional medicines in combination with herbs or even drugs in the treatment of various disorders, while mercury chloride (HgCl2) and methylmercury (MeHg) do not have known medical values but are highly toxic. This study aimed to compare the effects of mercury sulfides with HgCl2 and MeHg on hepatic drug processing gene expression. Mice were orally administrated with Zuotai (β-HgS, 30mg/kg), α-HgS (HgS, 30mg/kg), HgCl2 (33.6mg/kg), or MeHg (3.1mg/kg) for 7days, and the expression of genes related to phase-1 drug metabolism (P450), phase-2 conjugation, and phase-3 (transporters) genes were examined. The mercurials at the dose and duration used in the study did not have significant effects on the expression of cytochrome P450 1-4 family genes and the corresponding nuclear receptors, except for a slight increase in PPARα and Cyp4a10 by HgCl2. The expressions of UDP-glucuronosyltransferase and sulfotransferase were increased by HgCl2 and MeHg, but not by Zuotai and HgS. HgCl2 decreased the expression of organic anion transporter (Oatp1a1), but increased Oatp1a4. Both HgCl2 and MeHg increased the expression of multidrug resistance-associated protein genes (Mrp1, Mrp2, Mrp3, and Mrp4). Zuotai and HgS had little effects on these transporter genes. In conclusion, Zuotai and HgS are different from HgCl2 and MeHg in hepatic drug processing gene expression; suggesting that chemical forms of mercury not only affect their disposition and toxicity, but also affect their effects on the expression of hepatic drug processing genes.

MeHg Suppressed Neuronal Potency of Hippocampal NSCs Contributing to the Puberal Spatial Memory Deficits

Hippocampal neurogenesis-related structural damage, particularly that leading to defective adult cognitive function, is considered an important risk factor for neurodegenerative and psychiatric diseases. Normal differentiation of neurons and glial cells during development is crucial in neurogenesis, which is particularly sensitive to the environmental toxicant methylmercury (MeHg). However, the exact effects of MeHg on hippocampal neural stem cell (hNSC) differentiation during puberty remain unknown. This study investigates whether MeHg exposure induces changes in hippocampal neurogenesis and whether these changes underlie cognitive defects in puberty. A rat model of methylmercury chloride (MeHgCl) exposure (0.4 mg/kg/day, PND 5-PND 33, 28 days) was established, and the Morris water maze was used to assess cognitive function. Primary hNSCs from hippocampal tissues of E16-day Sprague-Dawley rats were purified, identified, and cloned. hNSC proliferation and differentiation and the growth and morphology of newly generated neurons were observed by MTT and immunofluorescence assays. MeHg exposure induced defects in spatial learning and memory accompanied by a decrease in number of doublecortin (DCX)-positive cells in the dentate gyrus (DG). DCX is a surrogate marker for newly generated neurons. Proliferation and differentiation of hNSCs significantly decreased in the MeHg-treated groups. MeHg attenuated microtubule-associated protein-2 (MAP-2) expression in neurons and enhanced the glial fibrillary acidic protein (GFAP)-positive cell differentiation of hNSCs, thereby inducing degenerative changes in a dose-dependent manner. Moreover, MeHg induced deficits in hippocampus-dependent spatial learning and memory during adolescence as a consequence of decreased generation of DG neurons. Our findings suggested that MeHg exposure could be a potential risk factor for psychiatric and neurodegenerative diseases.

Mechanisms involved in methylmercuric chloride (MeHgCl)-induced suppression of human neutrophil apoptosis

We have previously demonstrated that concentrations of 1-10 μM of methylmercuric chloride (MeHgCl) that are cytotoxic to monocytes-macrophages can curiously inhibit neutrophil apoptosis by a yet unknown mechanism. In the present study, we demonstrate that, as with the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF), a classical inhibitor of neutrophil apoptosis, treatment of cells with 5 M MeHgCl inducesde novo protein synthesis and prevents the loss of expression of the antiapoptotic Mcl-1 protein. The expression of the cytoskeletal proteins gelsolin, paxillin and vinculin was similar in MeHgCl or GM-CSF-induced suppression of apoptosis. However, MeHgCl prevents the degradation of vimentin differently than GM-CSF. Apoptosis was further confirmed by flow cytometry (FITC annexin-V), and by monitoring CD16 cell surface expression. Curiously, unlike GM-CSF, MeHgCl did not prevent CD16 shedding. We conclude that, like GM-CSF, MeHgCl can delay neutrophil apoptosis by inducing de novoprotein synthesis and by preventing the loss of the antiapoptotic Mcl-1 protein. However, unlike GM-CSF, MeHgCl induces an atypical degradation of vimentin without preventing CD16 shedding.

Neuroprotective Effect of Portulaca oleraceae Ethanolic Extract Ameliorates Methylmercury Induced Cognitive Dysfunction and Oxidative Stress in Cerebellum and Cortex of Rat Brain

Methylmercury (MeHg) is highly toxic, and its principal target tissue in human is the nervous system, which has made MeHg intoxication a public health concern for many decades. Portulaca oleraceae (purslane), a member of the Portulacaceae family, is widespread as a weed and has been ranked the eighth most common plant in the world. In this study, we sought for potential beneficial effects of Portulaca oleracea ethanolic extract (POEE) against the neurotoxicity induced by MeHg in cerebellum and cortex of rats. Male Wistar rats were administered with MeHg orally at a dose of 5 mg/kg b.w. for 21 days. Experimental rats were given MeHg and also administered with POEE (4 mg/kg, orally) 1 h prior to the administration of MeHg for 21 days. After MeHg exposure, we determine the mercury concentration by atomic absorption spectroscopy (AAS); mercury content was observed high in MeHg-induced group. POEE reduced the mercury content. We also observed that the activities of catalase, superoxide dismutase, glutathione peroxidase, and the level of glutathione were reduced. The levels of glutathione reductase and thiobarbituric acid reactive substance were found to be increased. The above biochemical changes were found to be reversed with POEE. Behavioral changes like decrease tail flick response, longer immobility time, and decreased motor activity were noted down during MeHg exposure. POEE pretreatment offered protection from these behavioral changes. MeHg intoxication also caused histopathological changes in cerebellum and cortex, which was found to be normalized by treatment with POEE. The present results indicate that POEE has protective effect against MeHg-induced neurotoxicity.

Parental Whole Life Cycle Exposure to Dietary Methylmercury in Zebrafish (Danio rerio) Affects the Behavior of Offspring

Methylmercury (MeHg) is an established neurotoxicant of concern to fish-eating organisms. While most studies have focused on the fish consumers, much less is known about the effects of MeHg on the fish themselves, especially following exposures to chronic and environmentally relevant scenarios. Here we evaluated the behavioral effects of developmental MeHg insult by exposing parental generations of zebrafish to an environmentally realistic MeHg dietary concentration (1 ppm) and two higher concentrations (3 and 10 ppm) throughout their whole life span. Upon reaching adulthood, their offspring were analyzed through a series of behavioral tests, including the visual-motor response (VMR) assay, analysis of spontaneous swimming and evaluation of foraging efficiency. The VMR assay identified decreased locomotor output in the 6 day postfertilization (dpf) offspring of fish exposed to 3 and 10 ppm MeHg. However, in a second test 7 dpf fish revealed an increase in locomotor activity in all MeHg exposures tested. Increases in locomotion continued to be observed until 16 dpf, which coincided with increased foraging efficiency. These results suggest an association between MeHg and hyperactivity, and imply that fish chronically exposed to MeHg in the wild may be vulnerable to predation.

Effects of Methyl Mercury Chloride on Rat Hippocampus Structure

The objective of this study is to investigate the impacts of Methyl Mercury Chloride (MMC) on cognitive functions and ultrastructural changes of hippocampus in Sprague Dawley (SD) rats. Thirty healthy 20-day-old male SD rats weighing 30-40 g were randomly divided into three groups to receive daily injections. Two different dose levels were used: 4 mg/kg as high dose (H-MMC) and 2 mg/kg as low dose (L-MMC).The control group received 4 mg/kg saline solution (N-NaCl). After daily subcutaneous injection for 50 days, 6-day Morris water maze tests were used to assess the learning and memory functions of the rats. After a 5-day continuous training, spatial probe tests were conducted of times and paths crossing to the target quadrant on the 6th day. After the rats were euthanized, their hippocampus sections were stained with hematoxylin and eosin and analyzed under bothoptical microscope and electron microscope. The time H-MMC group spent in finding platform was significantly longer as compared toN-NaCl group on day 2 to day 5 and L-MMC group on day 4 to day 5. The number of crossing times of H-MMC group to the target quadrant was 0.63 ± 0.74, which is much lower than C-NaCl group (3.13 ± 1.56) with P value <0.05. No statistically significant difference in crossing times was found between L-MMC and C-NaCl groups. For H-MMC group, decreasing number of neurons and disorganized nerve cells were examined under light microscope. Swelling and dissolution of Golgi complex were examined under electron microscope, along with endoplasmic reticulum expansion and cytoplasmic edema. Mild cytoplasmic edema was found in L-MMC group. MMC can cause cognitive impairment in terms of learning and memory in SD rats. Additionally, it can also cause changes in the ultrastructure of neurons and morphological changes in the hippocampus, causing significant damage.

Toward Bioremediation of Methylmercury Using Silica Encapsulated Escherichia coli Harboring the mer Operon

Mercury is a highly toxic heavy metal and the ability of the neurotoxin methylmercury to biomagnify in the food chain is a serious concern for both public and environmental health globally. Because thousands of tons of mercury are released into the environment each year, remediation strategies are urgently needed and prompted this study. To facilitate remediation of both organic and inorganic forms of mercury, Escherichia coli was engineered to harbor a subset of genes (merRTPAB) from the mercury resistance operon. Protein products of the mer operon enable transport of mercury into the cell, cleavage of organic C-Hg bonds, and subsequent reduction of ionic mercury to the less toxic elemental form, Hg(0). E. coli containing merRTPAB was then encapsulated in silica beads resulting in a biological-based filtration material. Performing encapsulation in aerated mineral oil yielded silica beads that were smooth, spherical, and similar in diameter. Following encapsulation, E. coli containing merRTPAB retained the ability to degrade methylmercury and performed similarly to non-encapsulated cells. Due to the versatility of both the engineered mercury resistant strain and silica bead technology, this study provides a strong foundation for use of the resulting biological-based filtration material for methylmercury remediation.

[Effect of Methylmercury on the Light Dependence Fluorescence Parameters in a Green Alga Chlamydomonas moewusii]

The effect of a dangerous toxic substance, methylmercury, on light dependence curves of chlorophyll fluorescence in Chlamydomonas moewusii was studied. We found low concentration of methylmercury (10(-7) M) to cause a decrease in the relative rate of the non-cyclic electron transport activity of PS 2, a decline in the maximum utilization of light energy (α), and a decline in the saturation light intensity (E(s)). Non-photochemical fluorescence quenching increased after short-term exposure and decreased in the course of prolonged incubation. These parameters were more sensitive to the action of the toxic substance than the widely used parameter F(V)/F(M), which reflects the maximum quantum yield of PS 2. We propose the use of the method of fast measurement of light dependence curves of fluorescence to detect the changes in algal cells at the early stages of exposure to mercury salts.

Methylmercury-induced oxidative stress in rainbow trout (Oncorhynchus mykiss) liver: ameliorating effect of vitamin C

The present study was undertaken to evaluate methylmercury-induced alterations in hepatic enzymes and oxidative stress markers in liver tissue of rainbow trout (Oncorhynchus mykiss) by using a perfusion method, and to explore possible protective effect of vitamin C against these alterations. Forty-eight fish were divided into six groups containing control, test, and amelioration groups. The liver of fish in the test groups were exposed to different doses of methylmercury, i.e., 0.6, 1.2, and 2.4 μg L(-1), respectively, for 120 min. In the amelioration group, liver was treated with vitamin C (17.2 μg L(-1)) along with high dose (2.4 μg L(-1)) of methylmercury. The results of the present study showed that exposure with 0.6, 1.2, and 2.4 μg L(-1) of methylmercury significantly increased (p < 0.05) hepatic enzyme activities (alanine transaminase (ALT), aspartate transaminase (AST), and Lactate dehydrogenase (LDH)) and malondialdehyde (MDA) level, as a marker of lipid peroxidation. On the other hand, the concentration of reduced glutathione (GSH) and total antioxidant capacity of the liver decreased (p < 0.05) in the methylmercury-exposed groups when compared to the control group. Pearson's correlation analysis revealed a positive correlation between MDA concentration and ALT, AST, and LDH activities in the methylmercury groups, suggesting that the enhanced lipid peroxidation may be linked to hepatic damage caused by methylmercury. Treatment with vitamin C in methylmercury-exposed group led to a significant decrease (p < 0.05) in MDA concentration and hepatic enzyme activities and significant increase (p < 0.05) in levels of GSH and total antioxidant capacity. The values of measured parameters in the methylmercury + vitamin C group were comparable to those of the control group. The results of the present study demonstrated that methylmercury exposure induces oxidative stress in the liver of rainbow trout and treatment with vitamin C can protect fish liver against this oxidative insult.

Selenium prevents downregulation of antioxidant selenoprotein genes by methylmercury

Selenium (Se) is an essential nutrient required by Se-dependent proteins, termed selenoproteins. The selenoprotein family is small but diverse and includes key proteins in antioxidant, redox signaling, thyroid hormone metabolism, and protein folding pathways. Methylmercury (MeHg) is a toxic environmental contaminant that affects seafood safety. Selenium can reduce MeHg toxicity, but it is unclear how selenoproteins are affected in this interaction. In this study we explored how Se and MeHg interact to affect the mRNA expression of selenoprotein genes in whole zebrafish (Danio rerio) embryos. Embryos were obtained from adult zebrafish fed MeHg with or without elevated Se in a 2×2 factorial design. The embryo mRNA levels of 30 selenoprotein genes were then measured. These genes cover most of the selenoprotein families, including members of the glutathione peroxidase (GPX), thioredoxin reductase, iodothyronine deiodinase, and methionine sulfoxide reductase families, along with selenophosphate synthetase 2 and selenoproteins H, J-P, T, W, sep15, fep15, and fam213aa. GPX enzyme activity and larval locomotor activity were also measured. We found that around one-quarter of the selenoprotein genes were downregulated by elevated MeHg. These downregulated genes were dominated by selenoproteins from antioxidant pathways that are also susceptible to Se-deficiency-induced downregulation. MeHg also decreased GPX activity and induced larval hypoactivity. Elevated Se partially prevented MeHg-induced disruption of selenoprotein gene mRNA levels, GPX activity, and larval locomotor activity. Overall, the MeHg-induced downregulation and subsequent rescue by elevated Se levels of selenogenes regulated by Se status suggest that Se deficiency is a contributing factor to MeHg toxicity.

Isolation and characterization of mercury-resistant bacteria from sediments of Tagus Estuary (Portugal): implications for environmental and human health risk assessment

Mercury (Hg) contamination of aquatic systems has been recognized as a global and serious problem affecting both human and environmental health. In the aquatic ecosystems, mercurial compounds are microbiologically transformed with methylation responsible for generation of methylmercury (MeHg) and subsequent biomagnification in food chain, consequently increasing the risk of poisoning for humans and wildlife. High levels of Hg, especially MeHg, are known to exist in Tagus Estuary as a result of past industrial activities. The aim of this study was to isolate and characterize Hg-resistant bacteria from Tagus Estuary. Mercury-resistant (Hg-R) bacteria were isolated from sediments of two hotspots (Barreiro and North Channel) and one reserve area (Alcochete). Mercury contamination in these areas was examined and bacterial susceptibility to Hg compounds evaluated by determination of minimal inhibitory concentrations (MIC). The isolates characterization was based on morphological observation and biochemical testing. Bacteria characteristics, distribution, and Hg resistance levels were compared with metal levels. Barreiro and North Channel were highly contaminated with Hg, containing 126 and 18 μg/g total Hg, respectively, and in Alcochete, contamination was lower at 0.87 μg/g total Hg. Among the isolates there were aerobic and anaerobic bacteria, namely, sulfate-reducing bacteria, and Hg resistance levels ranged from 0.16 to 140 μg/ml for Hg(2+) and from 0.02 to 50.1 μg/ml for MeHg. The distribution of these bacteria and the resistance levels were consistent with Hg contamination along the depth of the sediments. Overall, results show the importance of the characterization of Tagus Estuary bacteria for ecological and human health risk assessment.

Mercury methylation in mesocosms with and without the aquatic macrophyte Eichhornia crassipes (mart.) Solms

Mercury is a toxic pollutant and spreads to several compartments in the environment. Previous in-vitro studies showed that roots of aquatic macrophytes are sites of methylmercury formation, performed mainly by sulfate-reducing bacteria (SRB). The objective of this study was to observe MMHg formation and distribution among filtered water (0.2µm), suspended and settled particles and macrophyte roots during seventeen days, in (203)Hg- spiked mesocosms with and without live Eichhornia crassipes whole plants and a SRB inhibitor. Root samples were also incubated in-vitro for comparison of MM(203)Hg formation under in-vitro and in-vivo conditions. To evaluate the effect of SRB inhibition by sodium molybdate on total heterotrophic activity, the latter was measured by (3)H-leucine uptake. Inhibition of Hg methylation by sodium molybdate decreased with time in mesocosms. MMHg averaged 10, 12.4 and 0.23 percent of total (203)Hg present in filtered water, suspended particles and roots respectively. In vitro MMHg formation in roots averaged 5.54 percent of total added (203)Hg, with a clearer SRB inhibition effect than in mesocosms. Though significant, MMHg formation in roots from in-vivo mesocosms was one order of magnitude lower than previously found in in-vitro incubations of roots alone.

Cerebral gene expression and neurobehavioural responses in mice pups exposed to methylmercury and docosahexaenoic acid through the maternal diet

Methylmercury (MeHg) is an environmental neurotoxicant with adverse effects particularly noted in the developing brain. The main source of MeHg exposure is seafood. However, fish is also an important source of n-3 fatty acids such as docosahexaenoic acid (DHA) which has neuroprotective effects, and which plays an important role during the prenatal development of the central nervous system. The aim of the present study was to examine the effects of DHA and MeHg individually, and in combination, on development using accumulation, behavioural and transcriptomic endpoints in a mammalian model. Analyses were performed on 15 day old mice which had been exposed to varying levels of DHA (8 or 24 mg/kg) and/or MeHg (4 mg/kg) throughout development via the maternal diet. Supplementation of the maternal diet with DHA reduced MeHg accumulation in the brain. An accelerated development of grasping reflex was seen in mice offspring in the 'MeHg+high DHA' group when compared to 'MeHg' and 'control'. Exposure to MeHg and DHA had an impact on cerebral gene expression as assessed by microarray and qPCR analysis. The results from the present study show the potential of DHA for alleviating toxicity caused by MeHg. This information may contribute towards refining risk/benefit assessment of seafood consumption and may enhance understanding of discrepancies between epidemiological studies of MeHg neurodevelopmental toxicity.

Effects of dietary methylmercury on the zebrafish brain: histological, mitochondrial, and gene transcription analyses

The neurotoxic compound methylmercury (MeHg) is a commonly encountered pollutant in the environment, and constitutes a hazard for wildlife and human health through fish consumption. To study the neurotoxic impact of MeHg on piscivorous fish, we contaminated the model fish species Danio rerio for 25 and 50 days with food containing 13.5 μg/g dry weight (dw) of MeHg (0.6 μg MeHg/fish/day), an environmentally relevant dose leading to brain mercury concentrations of 30 ± 4 μg of Hg g(-1) (dw) after 25 days of exposure and 46 ± 7 μg of Hg g(-1) (dw) after 50 days. Brain mitochondrial respiration was not modified by exposure to MeHg, contrary to what happens in skeletal muscles. A 6-fold increase in the expression of the sdh gene encoding the succinate dehydrogenase Fe/S protein subunit was detected in the contaminated brain after 50 days of exposure. An up regulation of 3 genes, atp2b3a, atp2b3b, and slc8a2b, encoding for calcium transporters was noticed after 25 days of exposure but the atp2b3a and atp2b3b were repressed and the slc8a2b gene expression returned to its basal level after 50 days, suggesting a perturbation of calcium homeostasis. After 50 days, we detected the up regulation of glial fibrillary acidic protein and glutathione S-transferase genes (gfap and gst), along with a repression of the glutathione peroxidase gene gpx1. These results match well with a MeHg-induced onset of oxidative stress and inflammation. A transmission electron microscopic observation confirmed an impairment of the optical tectum integrity, with a decrease of the nucleal area in contaminated granular cells compared to control cells, and a lower density of cells in the contaminated tissue. A potential functional significance of such changes observed in optical tectum when considering wild fish contaminated in their natural habitat might be an impaired vision and therefore a lowered adaptability to their environment.

Dietary mercury has no observable effects on thyroid-mediated processes and fitness-related traits in wood frogs

Mercury (Hg) is a neurotoxicant known to cause developmental and behavioral abnormalities in vertebrates. Increasing evidence suggests that Hg can also disrupt endocrine functions and endocrine-dependent processes. For example, dietary Hg has been shown to delay tail resorption during metamorphic climax in amphibians, a process mediated by thyroid hormones. However, a direct link between Hg, hormone disruption, and developmental delays in amphibians has not been explored. Therefore, we examined the effects of dietary Hg (0.01, 2.5, and 10 μg/g total Hg, dry wt) on thyroid hormone concentrations, development, growth, performance, and survival of wood frogs (Rana sylvatica). Tadpoles accumulated Hg in a concentration-dependent manner; total Hg concentrations in tadpoles at the beginning of metamorphic climax (Gosner stage 42) were 0.03, 1.06, 3.54 μg/g, dry wt, for control, low, and high Hg diets, respectively. During metamorphic climax, tadpoles eliminated 35% of the inorganic Hg from their tissues but retained most of their accumulated methylmercury. Contrary to our predictions, we found no effect of Hg on the duration of tadpole development, size at metamorphosis, tail resorption time, or hopping performance. Consistent with the lack of effects on development, we also detected no differences in whole-body thyroid hormone concentrations among our dietary treatments. Our results, when compared with the effects of Hg on other amphibians, suggest that amphibian species may differ substantially in their sensitivity to dietary Hg, emphasizing the need for data on multiple species when establishing toxicity benchmarks.

Involvement of oxidative stress-mediated ERK1/2 and p38 activation regulated mitochondria-dependent apoptotic signals in methylmercury-induced neuronal cell injury

Methylmercury (MeHg) is well-known for causing irreversible damage in the central nervous system as well as a risk factor for inducing neuronal degeneration. However, the molecular mechanisms of MeHg-induced neurotoxicity remain unclear. Here, we investigated the effects and possible mechanisms of MeHg in the mouse cerebrum (in vivo) and in cultured Neuro-2a cells (in vitro). In vivo study showed that the levels of LPO in the plasma and cerebral cortex significantly increased after administration of MeHg (50μg/kg/day) for 7 consecutive weeks. MeHg could also decrease glutathione level and increase the expressions of caspase-3, -7, and -9, accompanied by Bcl-2 down-regulation and up-regulation of Bax, Bak, and p53. Moreover, treatment of Neuro-2a cells with MeHg significantly reduced cell viability, increased oxidative stress damage, and induced several features of mitochondria-dependent apoptotic signals, including increased sub-G1 hypodiploids, mitochondrial dysfunctions, and the activation of PARP, and caspase cascades. These MeHg-induced apoptotic-related signals could be remarkably reversed by antioxidant NAC. MeHg also increased the phosphorylation of ERK1/2 and p38, but not JNK. Pharmacological inhibitors NAC, PD98059, and SB203580 attenuated MeHg-induced cytotoxicity, ERK1/2 and p38 activation, MMP loss, and caspase-3 activation in Neuro-2a cells. Taken together, these results suggest that the signals of ROS-mediated ERK1/2 and p38 activation regulated mitochondria-dependent apoptotic pathways that are involved in MeHg-induced neurotoxicity.

Serial analysis of gene expression in the skeletal muscles of zebrafish fed with a methylmercury-contaminated diet

Mercury (Hg) is a widespread environmental contaminant and its organic form, methylmercury (MeHg), has been known as a potent neurotoxic since the Minamata tragedy. In the Amazonian basin, gold mining leads to MeHg biomagnification all along the food web, culminating in piscivorous fish, ultimately responsible for contamination of human beings through fish consumption. In order to assess the biological impact of dietary MeHg on fish at the genome scale, we contaminated zebrafish with MeHg-contaminated food for 25 days (13.5 microg of Hg/g of food). A serial analysis of gene expression (SAGE) was conducted on the skeletal muscle because this tissue does not perform MeHg demethylation, and 19171 SAGE tags were sequenced from the control library versus 22 261 from the MeHg-contaminated library, corresponding to 5280 different transcripts. Among those identified, 60 genes appeared up-regulated and 15 down-regulated by more than 2 times. A net impact of MeHg was noticed on 14 ribosomal protein genes, indicating a perturbation of protein synthesis. Several genes involved in mitochondrial metabolism, the electron transport chain, endoplasmic reticulum (ER) function, detoxification, and general stress responses were differentially regulated, suggesting an onset of oxidative stress and ER stress. Several other genes for which expression varied with MeHg contamination could be clustered in various compartments of the cell's life, such as lipid metabolism, calcium homeostasis, iron metabolism, muscle contraction, and cell cycle regulation. This study reveals the effectiveness of the SAGE approach to acquire a better understanding of the MeHg global effects. Furthermore, this is the first time that the SAGE was used to characterize the effect of a toxicant at the genome scale in an aquatic organism.

Overexpression of the novel F-box protein Ymr258c confers resistance to methylmercury in Saccharomyces cerevisiae

We searched a yeast genomic library for genes that conferred methylmercury resistance to yeast. Ymr258c was identified as a factor that conferred strong methylmercury resistance when overexpressed in budding yeast. Ymr258c is regarded as one of the F-box proteins, i.e., component factors of the Skp1/Cullin/F-box protein (SCF) complex that functions as a ubiquitin ligase in the ubiquitin-proteasome system. In this study, Ymr258c was demonstrated to function as an F-box protein to reduce methylmercury toxicity. The overexpression of Ymr258c might promote the ubiquitination of proteins that are involved in the enhancement of methylmercury toxicity, and thereby promote their degradation by proteasomes to reduce methylmercury toxicity.

Influence of heavy metals upon the retention and mobilization of polonium‐210 in rats

PURPOSE

To provide information about the tissue retention and mobilization of the alpha-emitting radionuclide, polonium-210 (210Po), in rats under combined exposure to heavy metal ions and the chelating agent, 2, 3-dimercaptopropane-1-sulfonate (DMPS).

MATERIALS AND METHODS

Rats were pre-exposed intraperitoneally to either CdCl2 or Pb(CH3COO)2. 9 or 15 h later they received 210Po nitrate intravenously. The retention and excretion of 210Po via the urine and faeces of pre-exposed rats, as well as in pre-exposed rats treated with DMPS, were followed. The radioactivity due to 210Po in a broad spectrum of body tissues and excreta was measured by the liquid scintillation counting after sample digestion in a mixture of perchloric acid and hydrogen peroxide. The immunohistochemical localization of metallothioneins (MT) was studied using a mixture of murine monoclonal antibodies directed against MT I+II.

RESULTS

The present study revealed different tissue distributions of polonium-210 in the rats pre-exposed to lead or cadmium ions when compared with that in 210Po only controls. Under combined exposure to Pb or Cd, the spontaneous excretion of 210Po was enhanced and could be further enhanced by treatment with DMPS. Treatment with this chelator was efficient even when its start was postponed until 24h after internal contamination of the body with 210Po.

CONCLUSIONS

Polonium-210 is bound in vivo to binding sites on various biomolecules, among them erythrocytic enzymes and MT. This phenomenon explains the different affinity and overall distribution of 210Po in control body tissues. When the appropriate binding sites are occupied by lead or cadmium, enhanced natural excretion of polonium-210 occurs.

[Study of the effect of methylmercury and copper ions on primary photosynthesis processes in the green algae Chlamydomonas moevusii using the parameters of kinetic curves of the variable of chlorophyll fluorescence]

The effect of methylmercury and copper ions on the kinetics of light induction and dark relaxation of the variable of chlorophyll a fluorescence has been studied on cultures of the microalgae Chlamydomonas moevusii. It was shown that the toxicants added at concentrations that induce no decrease in photochemical activity of PS II (F(v)/F(M)) affect the electron transport on the acceptor side of PS II, the nonphotochemical quenching of excitation in the antenna, and the reoxidation of the quinone pool. At low concentrations, methylmercury produced a more toxic effect. The results obtained indicate that this approach can be used for detecting the changes in plant and algae cells at the early stages of the action of toxicants.

Gestational methylmercury exposure selectively increases the sensitivity of operant behavior to cocaine

Developmental methylmercury (MeHg) exposure alters dopamine neurotransmitter systems, but the selectivity of this and the effects of low, environmentally relevant MeHg exposure regimens are poorly understood. In previous reports, some including littermates of animals studied here, chronic, low-level exposures affected performance on reversal tasks and enhanced reinforcer efficacy. Using high- and low-rate operant behavior under a fixed interval (FI) schedule, sensitivity was examined to drugs that target noradrenergic and dopaminergic neurotransmitter systems. Female rats were exposed in utero to 0, 0.5, or 5 ppm of mercury, as MeHg, via maternal drinking water. Selenium (Se) is thought to attenuate MeHg's neurotoxicity, so animals consumed a diet containing 0.06 or 0.6 ppm of Se. At 11 months, they lever-pressed under a FI 120" schedule of sucrose reinforcement. Acute dose-effect curves were generated with cocaine, desipramine, SKF-38393, quinpirole, SCH-23390, and sulpiride. As compared with unexposed animals, those exposed to 5 ppm mercury, regardless of Se exposure, were 2 to 3 times more sensitive to the rate-reducing effects of high doses of cocaine and did not show increased responding earlier in the interval following moderate cocaine doses. Cocaine's effects in the 0.5 ppm Hg groups depended on dietary Se: low Se diet resulted in a rightward shift in the DEC compared to controls, whereas a high Se diet did not. No differential effects of MeHg were seen with the other drugs. Gestational MeHg exposure produces irreversible sensitivity to dopamine, but not norepinephrine, reuptake inhibitors and not to drugs that target D1 or D2 receptors.

Methyl mercury inhibits short-circuit current and Cl- influx across isolated epipodite of European lobster (Homarus gammarus)

The effect of methyl mercuric chloride (MeHg) on short-circuit current (I(SC)) was studied in the isolated perfused epipodite preparation from the branchial chamber of European lobster (Homarus gammarus) acclimated to dilute seawater. When applied at the apical surface, 0.2, 1.0 and 3.0 microM MeHg depressed I(SC) by a 26%, 81% and 98%, respectively. The half-maximal inhibitory concentration (IC(50)) of apically applied MeHg was 0.6 microM. Basolaterally added MeHg (3.0 microM) had no effect on I(SC), whereas addition of the specific Na(+),K(+)-ATPase inhibitor ouabain (1.5 mM) reduced I(SC) by approximately 90%. Ouabain effects were reversible, and I(SC) fully recovered upon removal of ouabain. The MeHg-induced block of I(SC) was partially reversed by the reducing agent, 1,4-dithiothreitol, suggesting that the formation of S-Hg-S bridges is important in the inhibitory mechanism. A significant reduction of I(SC) and conductance occurred when low Na(+) and Cl(-) salines were substituted. Furthermore, in the low Na(+) saline, J(Cl)(A-->B) fluxes were reduced by about 50%. In the highly conductive epipodite epithelium, coupling of Na(+) and Cl(-) fluxes was suggested. The effects of MeHg on I(SC) in the lobster epipodite are attributed to inhibition of an apical Cl(-) influx.

[A study on apoptosis and apoptotic mechanisms of HL-7702 cell line induced by methylmercury]

OBJECTIVE

To study the apoptotic effect and mechanisms of methylmercury (MeHg) on HL-7702 cell line in vitro.

METHODS

In this study, the cell apoptosis was observed by AO/EB method and FCM method; the mitochondrial membrane potential was detected by FCM; and the expression of proteins related to apoptosis was measured by immunocytochemical method.

RESULTS

After exposure to MeHg for 24 h in different doses, apoptotic rate ascended with the increasing of MeHg concentration. By AO/EB method, cell apoptotic ratio of negative control group was (2.62 +/- 0.19)%, cell apoptotic ratio of 10-50 micromol/L exposure groups were (7.97 +/- 0.64)%, (12.66 +/- 0.76)%, (19.16 +/- 0.87)%, (18.42 +/- 0.88)%, and (11.52 +/- 0.63)%, there were significant differences between the exposure and negative control groups (q values were 17.057, 32.009, 52.732, 50.373, 28.375; P<0.05). Mitochondrial membrane potential descended with the increase of MeHg, mitochondrial membrane potential of negative control group was (10.23 +/- 3.43) mV, mitochondrial membrane potential of 10-50 micromol/L exposure groups were (3.25 +/- 0.66), (3.03 +/- 0.35), (1.68 +/- 1.26), (1.69 +/- 1.13) and (1.77 +/- 0.88) mV, and there was significant differences between exposure and negative control groups (q values were 9.569, 9.871, 11.722, 11.708, 11.598; P<0.05). The expression of Bax, Bcl-2, CytC, Caspase-3 and AIF enhanced with the increase of MeHg, Bax/Bcl-2 ratio also appeared a trend of increase. Bax expression integral optical density (IOD) of negative control group was (21295.86 +/- 1969.81), Bax expression IOD of 10, 20, 30 micromol/L groups were 42807.87 +/- 4416.64, 55651.65 +/- 4662.72, and 72708.56 +/- 910.10, there were significant differences in Bax expression between 10, 20, 30 micromol/L groups and negative control group (q values were 14.191, 14.320, 33.917; P<0.05); Bcl-2 expression IOD of negative control group was (12588.33 +/- 4091.02), Bcl-2 expression IOD of 10, 20, 30 micromol/L groups were 20539.16 +/- 4906.09, 23689.97 +/- 2281.42, and 28692.80 +/- 4655.86, there were significant differences in Bcl-2 expression between 10, 20, 30 micromol/L groups and negative control group (q values were 4.322, 6.035, 8.754; P<0.05); and AIF expression IOD of negative control group was (12942.72 +/- 457.94), AIF expression IOD of 10, 20, 30, 40 micromol/L groups were 16973.57 +/- 1922.87, 29998.91 +/- 6803.58, 52467.16 +/- 1916.25 and 106342.53 +/- 1273.19, there were significant differences in AIF expression between 20, 30 and 40 micromol/L groups and negative control group (q values were 11.449, 26.530, 62.692; P<0.05).

CONCLUSION

MeHg could induce apoptosis on HL-7702 cell line in vitro. The mechanisms could be related to mitochondrial pathway in apoptosis.

Methylmercury toxicity and Nrf2-dependent detoxification in astrocytes

Methylmercury (MeHg) is a potent neurotoxicant and preferentially induces oxidative injury in astrocytes. In neuronal tissues, nuclear factor erythroid 2-related factor 2 (Nrf2) is a key factor determining the protective antioxidant response against various environmental toxicants. Nrf2 is subjected to regulation by many other signaling pathways. The purpose of this study is to characterize its interaction with the phosphatidylinositol 3 (PI3) kinase in cultured rat neonatal primary astrocytes. The results showed that at pathologically relevant concentrations, exposure of primary astrocytes to MeHg led to Nrf2 activation and upregulation of its downstream antioxidant genes. Inhibition of the PI3 kinase resulted in decreased Nrf2 activity, decreased cellular glutathione, and increased cell death to high-dose MeHg. The functional interaction between the two signaling pathways underlined an important mechanism for astrocyte protection against MeHg toxicity. Modulation of Nrf2 by pharmacological modalities should afford a treatment to attenuate MeHg-induced neurotoxicity.

Mercury toxicity and the mitigating role of selenium

Mercury is a well-known environmental toxicant, particularly in its most common organic form, methylmercury. Consumption of fish and shellfish that contain methylmercury is a dominant source of mercury exposure in humans and piscivorous wildlife. Considerable efforts have focused on assessment of mercury and its attendant risks in the environment and food sources, including the studies reported in this issue. However, studies of mercury intoxication have frequently failed to consider the protective effects of the essential trace element, selenium. Mercury binds to selenium with extraordinarily high affinity, and high maternal exposures inhibit selenium-dependent enzyme activities in fetal brains. However, increased maternal dietary selenium intakes preserve these enzyme activities, thereby preventing the pathological effects that would otherwise arise in their absence. Recent evidence indicates that assessments of mercury exposure and tissue levels need to consider selenium intakes and tissue distributions in order to provide meaningful risk evaluations.

Effects of methylmercury and spatial complexity on foraging behavior and foraging efficiency in juvenile white ibises (Eudocimus albus)

Methylmercury is a globally distributed neurotoxin, endocrine disruptor, and teratogen, the effects of which on wildlife at environmentally relevant levels are largely unknown. In birds, foraging efficiency and learning may be sensitive endpoints for sublethal methylmercury toxicity, and these endpoints also may be biologically relevant at the population level. In the present study, groups of wild-caught, prefledgling white ibises (Eudocimus albus) were raised in a free-flight, open-air aviary on diets that approximated the measured range of methylmercury exposure in the Everglades ecosystem (0, 0.05, 0.1, and 0.3 mg/kg/d). The effect of methylmercury exposure on group foraging efficiency was examined by allowing birds to forage on 200 fathead minnows (Pimephales promelas) in artificial ponds for 15 min by straining the arenas' contents through a seine net and counting all remaining prey. Additionally, we varied the difficulty of foraging by these tactile feeding birds by adding multiple levels of structural complexity (e.g., increased vegetation and prey refugia) to the pond. Structural complexity affected both foraging efficiency and the rate of increase in efficiency over time (improvement). Methylmercury exposure affected foraging efficiency (p = 0.03). It did not affect foraging improvement in the face of increasingly challenging environments, however, and the dose-response relationship was nonlinear (e.g., the control and high-exposure groups were the least efficient foragers). Evidence for an effect of methylmercury on foraging efficiency therefore was inconclusive because of unpredicted results and no interaction with time or habitat complexity. These data suggest a nonlinear dose-response relationship at low levels of methylmercury exposure; future research is needed to verify this hypothesis. This appears to be the first experimental demonstration of the effects of habitat complexity on foraging efficiency in long-legged wading birds.

Transcriptomics analysis of interactive effects of benzene, trichloroethylene and methyl mercury within binary and ternary mixtures on the liver and kidney following subchronic exposure in the rat

The present research aimed to study the interaction of three chemicals, methyl mercury, benzene and trichloroethylene, on mRNA expression alterations in rat liver and kidney measured by microarray analysis. These compounds were selected based on presumed different modes of action. The chemicals were administered daily for 14 days at the Lowest-Observed-Adverse-Effect-Level (LOAEL) or at a two- or threefold lower concentration individually or in binary or ternary mixtures. The compounds had strong antagonistic effects on each other's gene expression changes, which included several genes encoding Phase I and II metabolizing enzymes. On the other hand, the mixtures affected the expression of "novel" genes that were not or little affected by the individual compounds. The three compounds exhibited a synergistic interaction on gene expression changes at the LOAEL in the liver and both at the sub-LOAEL and LOAEL in the kidney. Many of the genes induced by mixtures but not by single compounds, such as Id2, Nr2f6, Tnfrsf1a, Ccng1, Mdm2 and Nfkb1 in the liver, are known to affect cellular proliferation, apoptosis and tissue-specific function. This indicates a shift from compound specific response on exposure to individual compounds to a more generic stress response to mixtures. Most of the effects on cell viability as concluded from transcriptomics were not detected by classical toxicological endpoints illustrating the benefit of increased sensitivity of assessing gene expression profiling. These results emphasize the benefit of applying toxicogenomics in mixture interaction studies, which yields biomarkers for joint toxicity and eventually can result in an interaction model for most known toxicants.

Methylmercuric chloride induces activation of neuronal stress circuitry and alters exploratory behavior in the mouse

Methylmercury (MeHg) is a well-known neurotoxicant, responsible for neurological and cognitive alterations. However, there is very little information available on the effects of MeHg administration on activation of murine neuronal pathways involved in the stress response, and whether this is altered as a function of repeated exposure to MeHg. Moreover, interactions between MeHg and other psychogenic and inflammatory stressors have yet to be fully determined. Acute i.p. exposure of male C57BL/6J mice to MeHg (2-8 mg/kg) dose-dependently attenuated exploratory behavior in the open field in the presence and absence of a novel object. In addition, increased numbers of c-Fos immunoreactive cells appeared in response to acute i.p. and i.c.v. MeHg within thalamic (anterior paraventricular nucleus of the thalamus (PVA)/posterior paraventricular nucleus of the thalamus (PV)), hypothalamic (paraventricular nucleus of the hypothalamus (PVN)), central amygdaloid nucleus (CeC), septal and hippocampal (dentate gyrus) nuclei, medial bed nucleus (BSTm) and the locus coeruleus (Lc). The increase in c-Fos positive cells in response to acute i.p. and i.c.v. MeHg did not appear to be influenced further by open field exposure. Repeated administration of MeHg led to an attenuation of most parameters of open field behavior altered by acute MeHg. However, increased c-Fos was significant in the CeC, Dg, supracapsular bed nucleus (BSTs), and Lc. Moreover, open field exposure after repeated treatments resulted in significant c-Fos responses in similar areas. Interestingly, 3 days after the final repeated MeHg dose (2 or 4 mg/kg) c-Fos increases to an immunogenic stressor (LPS) were not affected by MeHg pretreatment. These results demonstrate that systemic exposure to acute and repeated MeHg serves to activate the brain's stress circuitry, and furthermore appears to engage normal neuronal habituation processes.

Comparative effects of organic and inorganic mercury on in vivo dopamine release in freely moving rats

The present study was carried out in order to compare the effects of administration of organic (methylmercury, MeHg) and inorganic (mercury chloride, HgCl 2 ) forms of mercury on in vivo dopamine (DA) release from rat striatum. Experiments were performed in conscious and freely moving female adult Sprague-Dawley (230-280 g) rats using brain microdialysis coupled to HPLC with electrochemical detection. Perfusion of different concentrations of MeHg or HgCl 2 (2 microL/min for 1 h, N = 5-7/group) into the striatum produced significant increases in the levels of DA. Infusion of 40 microM, 400 microM, or 4 mM MeHg increased DA levels to 907 +/- 31, 2324 +/- 156, and 9032 +/- 70% of basal levels, respectively. The same concentrations of HgCl 2 increased DA levels to 1240 +/- 66, 2500 +/- 424, and 2658 +/- 337% of basal levels, respectively. These increases were associated with significant decreases in levels of dihydroxyphenylacetic acid and homovallinic acid. Intrastriatal administration of MeHg induced a sharp concentration-dependent increase in DA levels with a peak 30 min after injection, whereas HgCl 2 induced a gradual, lower (for 4 mM) and delayed increase in DA levels (75 min after the beginning of perfusion). Comparing the neurochemical profile of the two mercury derivatives to induce increases in DA levels, we observed that the time-course of these increases induced by both mercurials was different and the effect produced by HgCl 2 was not concentration-dependent (the effect was the same for the concentrations of 400 microM and 4 mM HgCl 2 ). These results indicate that HgCl 2 produces increases in extracellular DA levels by a mechanism differing from that of MeHg.

ERK-dependent and -independent pathways trigger human neural progenitor cell migration

Besides differentiation and apoptosis, cell migration is a basic process in brain development in which neural cells migrate several centimeters within the developing brain before reaching their proper positions and forming the right connections. For identifying signaling events that control neural migration and are therefore potential targets of chemicals to disturb normal brain development, we developed a human neurosphere-based migration assay based on normal human neural progenitor (NHNP) cells, in which the distance is measured that cells wander over time. Applying this assay, we investigated the role of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) in the regulation of NHNP cell migration. Exposure to model substances like ethanol or phorbol 12-myristate 13-acetate (PMA) revealed a correlation between ERK1/2 activation and cell migration. The participation of phospho-(P-) ERK1/2 was confirmed by exposure of the cells to the MEK inhibitor PD98059, which directly prohibits ERK1/2 phosphorylation and inhibited cell migration. We identified protein kinase C (PKC) and epidermal growth factor receptor (EGFR) as upstream signaling kinases governing ERK1/2 activation, thereby controlling NHNP cell migration. Additionally, treatments with src kinase inhibitors led to a diminished cell migration without affecting ERK1/2 phosphorylation. Based on these results, we postulate that migration of NHNP cells is controlled via ERK1/2-dependent and -independent pathways.