Inhibition of amino acid transport and protein synthesis by HgCl2 and methylmercury in astrocytes: selectivity and reversibility

Association of blood mercury levels during pregnancy with infant birth size by blood selenium levels in the Japan Environment and Children's Study: A prospective birth cohort

BACKGROUND

It is necessary to determine whether there are adverse health effects of prenatal exposure to long-term, low levels of mercury and selenium. However, there are limited that reports on the association between mercury levels by selenium levels and birth size. Therefore, we examined whether maternal mercury levels during pregnancy had any effect on infant birth size, and size, and whether selenium levels influenced this relationship.

OBJECTIVES

To examine the association between mercury and selenium levels during pregnancy with infant birth size.

METHODS

The Japan Environment and Children's Study is a prospective birth cohort conducted between 2011 and 2014. Total mercury levels and total selenium levels in maternal blood during the second and third trimesters were measured using Inductively Coupled Plasma-Mass Spectrometry. Birth weight and small-for-gestational-age were confirmed by medical records. Small-for-gestational-age was defined as birth weight below the 10th percentile according to standard percentile for gender, parity, and gestational age. Multiple linear and logistic regression analyses were used to examine the association between maternal mercury exposure and birth weight or small-for-gestational-age adjusted for confounders (including maternal age and body mass index pregnancy).

RESULTS

Overall, 15,444 pregnant women were included in this study. Median (inter-quartile range) of blood mercury and selenium levels were 3.66 (2.59-5.18) ng/g and 170.0 (158.0-183.0) ng/g, respectively. Compared to infants of mothers with the highest blood selenium level, those of mothers with the lowest blood selenium level had neither a significant birth weight increase (9 g, 95% confidence interval: -6, 25) nor a significant odds ratio for small-for-gestational-age (0.903, 95% confidence interval: 0.748, 1.089). Compared to infants of mothers with the lowest blood mercury level, those of mothers with the highest blood mercury level had neither a significant birth weight reduction (-12 g, 95% confidence interval: -27, 4) nor a significant odds ratio for small-for-gestational-age (0.951, 95% confidence interval: 0.786, 1.150). Compared to infants of mothers with the lowest quartile of maternal blood mercury level, all infants of mothers with the highest quartile of maternal blood mercury level had a reduced birth head circumference of 0.073 cm (95% confidence interval: -0.134, -0.011).

CONCLUSIONS

There was no association between maternal blood mercury levels and small-for-gestational-age and birth weight among 15,444 pregnant women. In a Japanese population, which has a relatively higher blood mercury level than reported in Western population, reduced birth size was not found to be associated with blood mercury levels, with the exception of birth head circumference.

Human-induced pluripotent stems cells as a model to dissect the selective neurotoxicity of methylmercury

Methylmercury (MeHg) is a potent neurotoxicant affecting both the developing and mature central nervous system (CNS) with apparent indiscriminate disruption of multiple homeostatic pathways. However, genetic and environmental modifiers contribute significant variability to neurotoxicity associated with human exposures. MeHg displays developmental stage and neural lineage selective neurotoxicity. To identify mechanistic-based neuroprotective strategies to mitigate human MeHg exposure risk, it will be critical to improve our understanding of the basis of MeHg neurotoxicity and of this selective neurotoxicity. Here, we propose that human-based pluripotent stem cell cellular approaches may enable mechanistic insight into genetic pathways that modify sensitivity of specific neural lineages to MeHg-induced neurotoxicity. Such studies are crucial for the development of novel disease modifying strategies impinging on MeHg exposure vulnerability.

Brain morphometric profiles and their seasonal modulation in fish (Liza aurata) inhabiting a mercury contaminated estuary

Mercury (Hg) is a potent neurotoxicant known to induce important adverse effects on fish, but a deeper understanding is lacking regarding how environmental exposure affects the brain morphology and neural plasticity of specific brain regions in wild specimens. In this work, it was evaluated the relative volume and cell density of the lateral pallium, hypothalamus, optic tectum and molecular layer of the cerebellum on wild Liza aurata captured in Hg-contaminated (LAR) and non-contaminated (SJ) sites of a coastal system (Ria de Aveiro, Portugal). Given the season-related variations in the environment that fish are naturally exposed, this assessment was performed in the winter and summer. Hg triggered a deficit in cell density of hypothalamus during the winter that could lead to hormonal dysfunctions, while in the summer Hg promoted larger volumes of the optic tectum and cerebellum, indicating the warm period as the most critical for the manifestation of putative changes in visual acuity and motor-dependent tasks. Moreover, in fish from the SJ site, the lateral pallium relative volume and the cell density of the hypothalamus and optic tectum were higher in the winter than in summer. Thus, season-related stimuli strongly influence the size and/or cell density of specific brain regions in the non-contaminated area, pointing out the ability of fish to adapt to environmental and physiological demands. Conversely, fish from the Hg-contaminated site showed a distinct seasonal profile of brain morphology, presenting a larger optic tectum in the summer, as well as a larger molecular layer of the cerebellum with higher cell density. Moreover, Hg exposure impaired the winter-summer variation of the lateral pallium relative size (as observed at SJ). Altogether, seasonal variations in fish neural morphology and physiology should be considered when performing ecotoxicological studies in order to better discriminate the Hg neurotoxicity.

Possible Mechanisms of Mercury Toxicity and Cancer Promotion: Involvement of Gap Junction Intercellular Communications and Inflammatory Cytokines

A number of observations indicate that heavy metals are able to alter cellular metabolic pathways through induction of a prooxidative state. Nevertheless, the outcome of heavy metal-mediated effects in the development of human diseases is debated and needs further insights. Cancer is a well-established DNA mutation-linked disease; however, epigenetic events are perhaps more important and harmful than genetic alterations. Unfortunately, we do not have reliable screening methods to assess/validate the epigenetic (promoter) effects of a physical or a chemical agent. We propose a mechanism of action whereby mercury acts as a possible promoter carcinogen. In the present contribution, we resume our previous studies on mercury tested at concentrations comparable with its occurrence as environmental pollutant. It is shown that Hg(II) elicits a prooxidative state in keratinocytes linked to inhibition of gap junction-mediated intercellular communication and proinflammatory cytokine production. These combined effects may on one hand isolate cells from tissue-specific homeostasis promoting their proliferation and on the other hand tamper the immune system defense/surveillance checkmating the whole organism. Since Hg(II) is not a mutagenic/genotoxic compound directly affecting gene expression, in a broader sense, mercury might be an example of an epigenetic tumor promoter or, further expanding this concept, a “metagenetic” effector.

Heavy metals (Pb, Cd, As and MeHg) as risk factors for cognitive dysfunction: A general review of metal mixture mechanism in brain

Human exposure to toxic heavy metals is a global challenge. Concurrent exposure of heavy metals, such as lead (Pb), cadmium (Cd), arsenic (As) and methylmercury (MeHg) are particularly important due to their long lasting effects on the brain. The exact toxicological mechanisms invoked by exposure to mixtures of the metals Pb, Cd, As and MeHg are still unclear, however they share many common pathways for causing cognitive dysfunction. The combination of metals may produce additive/synergetic effects due to their common binding affinity with NMDA receptor (Pb, As, MeHg), Na⁺ - K⁺ ATP-ase pump (Cd, MeHg), biological Ca⁺² (Pb, Cd, MeHg), Glu neurotransmitter (Pb, MeHg), which can lead to imbalance between the pro-oxidant elements (ROS) and the antioxidants (reducing elements). In this process, ROS dominates the antioxidants factors such as GPx, GS, GSH, MT-III, Catalase, SOD, BDNF, and CERB, and finally leads to cognitive dysfunction. The present review illustrates an account of the current knowledge about the individual metal induced cognitive dysfunction mechanisms and analyse common Mode of Actions (MOAs) of quaternary metal mixture (Pb, Cd, As, MeHg). This review aims to help advancement in mixture toxicology and development of next generation predictive model (such as PBPK/PD) combining both kinetic and dynamic interactions of metals.

Unveiling the neurotoxicity of methylmercury in fish (Diplodus sargus) through a regional morphometric analysis of brain and swimming behavior assessment

The current study aims to shed light on the neurotoxicity of MeHg in fish (white seabream - Diplodus sargus) by the combined assessment of: (i) MeHg toxicokinetics in the brain, (ii) brain morphometry (volume and number of neurons plus glial cells in specific brain regions) and (iii) fish swimming behavior (endpoints associated with the motor performance and the fear/anxiety-like status). Fish were surveyed for all the components after 7 (E7) and 14 (E14) days of dietary exposure to MeHg (8.7μgg^-1), as well as after a post-exposure period of 28days (PE28). MeHg was accumulated in the brain of D. sargus after a short time (E7) and reached a maximum at the end of the exposure period (E14), suggesting an efficient transport of this toxicant into fish brain. Divalent inorganic Hg was also detected in fish brain along the experiment (indicating demethylation reactions), although levels were 100-200 times lower than MeHg, which pinpoints the organic counterpart as the great liable for the recorded effects. In this regard, a decreased number of cells in medial pallium and optic tectum, as well as an increased hypothalamic volume, occurred at E7. Such morphometric alterations were followed by an impairment of fish motor condition as evidenced by a decrease in the total swimming time, while the fear/anxiety-like status was not altered. Moreover, at E14 fish swam a greater distance, although no morphometric alterations were found in any of the brain areas, probably due to compensatory mechanisms. Additionally, although MeHg decreased almost two-fold in the brain during post-exposure, the levels were still high and led to a loss of cells in the optic tectum at PE28. This is an interesting result that highlights the optic tectum as particularly vulnerable to MeHg exposure in fish. Despite the morphometric alterations reported in the optic tectum at PE28, no significant changes were found in fish behavior. Globally, the effects of MeHg followed a multiphasic profile, where homeostatic mechanisms prevented circumstantially morphometric alterations in the brain and behavioral shifts. Although it has become clear the complexity of matching brain morphometric changes and behavioral shifts, motor-related alterations induced by MeHg seem to depend on a combination of disruptions in different brain regions.

Effects of gestational and lactational exposure to low dose mercury chloride (HgCl2) on behaviour, learning and hearing thresholds in WAG/Rij rats

We investigated the effects of inorganic mercury exposure during gestational/lactational periods on the behaviour, learning and hearing functions in a total of 32, 5-week-old and 5-month-old WAG/Rij rats (equally divided into 4 groups as 5-week and 5-month control mercury exposure groups). We evaluated the rats in terms of locomotor activity (LA), the Morris-water-maze (MWM) test and the passive avoidance (PA) test to quantify learning and memory performance; we used distortion product otoacoustic emission (DPOAE) tests to evaluate hearing ability. There were no significant differences between the 5-week-old rat groups in LA, and we detected a significant difference (p < 0.05) in the HgCl₂-treated group in PA, MWM and DPOAE tests compared with the control group. The HgCl₂-treated 5-week-old group exhibited worse emotional memory performance in PA, worse spatial learning and memory performances in MWM. There were no significant differences between the groups of 5-month-old rats in LA, MWM or PA. However, the DPOAE tests worsened in the mid- and high-frequency hearing thresholds. The HgCl₂-treated 5-month-old group exhibited the most hearing loss of all groups. Our results convey that mercury exposure in young rats may worsen learning and memory performances as well as hearing at high-frequency levels. While there was no statistically significant difference in the behavior and learning tests in adult rats, the DPOAE test produced poorer results. Early detection of effects of mercury exposure provides medicals team with an opportunity to determinate treatment regimens and mitigate ototoxicity. DPOAE test can be used in clinical and experimental research investigating heavy metal ototoxicity.

Dysregulation of Glutamate Cycling Mediates Methylmercury-Induced Neurotoxicity

To examine the toxicological implications of glutamate, this chapter will focus specifically on its impact in the brain. More explicitly, it will illustrate the role glutamate plays in mediating methylmercury (MeHg)-induced neurotoxicity. In this chapter, one intends to highlight the processes that occur prior to glutamate-stimulated excitotoxicity and subsequent neurodegeneration. As such, it will emphasize three main routes by which MeHg alters glutamate homeostasis. It is essential to recognize that these effects are not mutually exclusive, and that they synergistically influence glutamate dysregulation. Furthermore, the consequences of MeHg exposure will be presented here as a direct pathway; however, it must be noted these effects occur simultaneously. First, glutamate uptake will be reviewed emphasizing the function of astrocytes. Next, the induction of oxidative stress by MeHg exposure will be discussed. This process has a two-fold effect on glutamate homeostasis by (1) inhibiting extracellular glutamate uptake and (2) altering transcription of genes vital to glutamate cycling. Finally, the impact glutamate dysregulation has on glutathione synthesis will be examined. Although this chapter centers on the link between glutamate and MeHg toxicity, it is imperative that the reader acknowledges the processes discussed here can be extended to any pro-oxidant.

Protective actions of 17β-estradiol and progesterone on oxidative neuronal injury induced by organometallic compounds

Steroid hormones synthesized in and secreted from peripheral endocrine glands pass through the blood-brain barrier and play a role in the central nervous system. In addition, the brain possesses an inherent endocrine system and synthesizes steroid hormones known as neurosteroids. Increasing evidence shows that neuroactive steroids protect the central nervous system from various harmful stimuli. Reports show that the neuroprotective actions of steroid hormones attenuate oxidative stress. In this review, we summarize the antioxidative effects of neuroactive steroids, especially 17β-estradiol and progesterone, on neuronal injury in the central nervous system under various pathological conditions, and then describe our recent findings concerning the neuroprotective actions of 17β-estradiol and progesterone on oxidative neuronal injury induced by organometallic compounds, tributyltin, and methylmercury.

Quantitative proteomic analysis reveals proteins involved in the neurotoxicity of marine medaka Oryzias melastigma chronically exposed to inorganic mercury

Mercury is a ubiquitous environmental contaminant which exerts neurotoxicity upon animals. Nevertheless, the molecular mechanisms involved in inorganic mercury neurotoxicity are unknown. We investigated protein profiles of marine medaka, chronically exposed to mercuric chloride using two-dimensional difference gel electrophoresis (2D-DIGE) and matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF-TOF MS) analysis. The mercury accumulation and ultrastructure were also examined in the brain. The results showed that mercury was significantly accumulated in the treated brain, and subsequently caused a noticeable damage. The comparison of 2D-DIGE protein profiles between the control and treatment revealed that 16 protein spots were remarkably altered in abundance, which were further submitted for MALDI-TOF-TOF MS analysis. The identified proteins indicated that inorganic mercury may cause neurotoxicity through the induction of oxidative stress, cytoskeletal assembly dysfunction and metabolic disorders. Thus, this study provided a basis for a better understanding of the molecular mechanisms involved in mercury neurotoxicity.

Evaluation of the effects of chronic intoxication with inorganic mercury on memory and motor control in rats

The aims of this study were to evaluate whether chronic intoxication with mercury chloride (HgCl₂), in a low concentration over a long time, can be deposited in the central nervous tissue and to determine if this exposure induces motor and cognitive impairments. Twenty animals were intoxicated for 45 days at a dose of 0.375 mg/kg/day. After this period, the animals underwent a battery of behavioral tests, in a sequence of open field, social recognition, elevated T maze and rotarod tests. They were then sacrificed, their brains collected and the motor cortex and hippocampus dissected for quantification of mercury deposited. This study demonstrates that long-term chronic HgCl₂ intoxication in rats promotes functional damage. Exposure to HgCl₂ induced anxiety-related responses, short- and long-term memory impairments and motor deficits. Additionally, HgCl₂ accumulated in both the hippocampus and cortex of the brain with a higher affinity for the cortex.

Methylmercury alters the activities of Hsp90 client proteins, prostaglandin E synthase/p23 (PGES/23) and nNOS

Methylmercury (MeHg) is a persistent pollutant with known neurotoxic effects. We have previously shown that astrocytes accumulate MeHg and play a prominent role in mediating MeHg toxicity in the central nervous system (CNS) by altering glutamate signaling, generating oxidative stress, depleting glutathione (GSH) and initiating lipid peroxidation. Interestingly, all of these pathways can be regulated by the constitutively expressed, 90-kDa heat shock protein, Hsp90. As Hsp90 function is regulated by oxidative stress, we hypothesized that MeHg disrupts Hsp90-client protein functions. Astrocytes were treated with MeHg and expression of Hsp90, as well as the abundance of complexes of Hsp90-neuronal nitric oxide synthase (nNOS) and Hsp90-prostaglandin E synthase/p23 (PGES/p23) were assessed. MeHg exposure decreased Hsp90 protein expression following 12 h of treatment while shorter exposures had no effect on Hsp90 protein expression. Interestingly, following 1 or 6 h of MeHg exposure, Hsp90 binding to PGES/p23 or nNOS was significantly increased, resulting in increased prostaglandin E₂ (PGE₂) synthesis from MeHg-treated astrocytes. These effects were attenuated by the Hsp90 antagonist, geldanmycin. NOS activity was increased following MeHg treatment while cGMP formation was decreased. This was accompanied by an increase in •O₂⁻ and H₂O₂ levels, suggesting that MeHg uncouples NO formation from NO-dependent signaling and increases oxidative stress. Altogether, our data demonstrates that Hsp90 interactions with client proteins are increased following MeHg exposure, but over time Hsp90 levels decline, contributing to oxidative stress and MeHg-dependent excitotoxicity.

Metals, oxidative stress and neurodegeneration: a focus on iron, manganese and mercury

Essential metals are crucial for the maintenance of cell homeostasis. Among the 23 elements that have known physiological functions in humans, 12 are metals, including iron (Fe) and manganese (Mn). Nevertheless, excessive exposure to these metals may lead to pathological conditions, including neurodegeneration. Similarly, exposure to metals that do not have known biological functions, such as mercury (Hg), also present great health concerns. This review focuses on the neurodegenerative mechanisms and effects of Fe, Mn and Hg. Oxidative stress (OS), particularly in mitochondria, is a common feature of Fe, Mn and Hg toxicity. However, the primary molecular targets triggering OS are distinct. Free cationic iron is a potent pro-oxidant and can initiate a set of reactions that form extremely reactive products, such as OH. Mn can oxidize dopamine (DA), generating reactive species and also affect mitochondrial function, leading to accumulation of metabolites and culminating with OS. Cationic Hg forms have strong affinity for nucleophiles, such as -SH and -SeH. Therefore, they target critical thiol- and selenol-molecules with antioxidant properties. Finally, we address the main sources of exposure to these metals, their transport mechanisms into the brain, and therapeutic modalities to mitigate their neurotoxic effects.

Riluzole-triggered GSH synthesis via activation of glutamate transporters to antagonize methylmercury-induced oxidative stress in rat cerebral cortex

OBJECTIVE

This study was to evaluate the effect of riluzole on methylmercury- (MeHg-) induced oxidative stress, through promotion of glutathione (GSH) synthesis by activating of glutamate transporters (GluTs) in rat cerebral cortex.

METHODS

Eighty rats were randomly assigned to four groups, control group, riluzole alone group, MeHg alone group, and riluzole + MeHg group. The neurotoxicity of MeHg was observed by measuring mercury (Hg) absorption, pathological changes, and cell apoptosis of cortex. Oxidative stress was evaluated via determining reactive oxygen species (ROS), 8-hydroxy-2-deoxyguanosine (8-OHdG), malondialdehyde (MDAs), carbonyl, sulfydryl, and GSH in cortex. Glutamate (Glu) transport was studied by measuring Glu, glutamine (Gln), mRNA, and protein of glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1).

RESULT

(1) MeHg induced Hg accumulation, pathological injury, and apoptosis of cortex; (2) MeHg increased ROS, 8-OHdG, MDA, and carbonyl, and inhibited sulfydryl and GSH; (3) MeHg elevated Glu, decreased Gln, and downregulated GLAST and GLT-1 mRNA expression and protein levels; (4) riluzole antagonized MeHg-induced downregulation of GLAST and GLT-1 function and expression, GSH depletion, oxidative stress, pathological injury, and apoptosis obviously.

CONCLUSION

Data indicate that MeHg administration induced oxidative stress in cortex and that riluzole could antagonize this situation through elevation of GSH synthesis by activating of GluTs.

Administration of thimerosal to infant rats increases overflow of glutamate and aspartate in the prefrontal cortex: protective role of dehydroepiandrosterone sulfate

Thimerosal, a mercury-containing vaccine preservative, is a suspected factor in the etiology of neurodevelopmental disorders. We previously showed that its administration to infant rats causes behavioral, neurochemical and neuropathological abnormalities similar to those present in autism. Here we examined, using microdialysis, the effect of thimerosal on extracellular levels of neuroactive amino acids in the rat prefrontal cortex (PFC). Thimerosal administration (4 injections, i.m., 240 μg Hg/kg on postnatal days 7, 9, 11, 15) induced lasting changes in amino acid overflow: an increase of glutamate and aspartate accompanied by a decrease of glycine and alanine; measured 10-14 weeks after the injections. Four injections of thimerosal at a dose of 12.5 μg Hg/kg did not alter glutamate and aspartate concentrations at microdialysis time (but based on thimerosal pharmacokinetics, could have been effective soon after its injection). Application of thimerosal to the PFC in perfusion fluid evoked a rapid increase of glutamate overflow. Coadministration of the neurosteroid, dehydroepiandrosterone sulfate (DHEAS; 80 mg/kg; i.p.) prevented the thimerosal effect on glutamate and aspartate; the steroid alone had no influence on these amino acids. Coapplication of DHEAS with thimerosal in perfusion fluid also blocked the acute action of thimerosal on glutamate. In contrast, DHEAS alone reduced overflow of glycine and alanine, somewhat potentiating the thimerosal effect on these amino acids. Since excessive accumulation of extracellular glutamate is linked with excitotoxicity, our data imply that neonatal exposure to thimerosal-containing vaccines might induce excitotoxic brain injuries, leading to neurodevelopmental disorders. DHEAS may partially protect against mercurials-induced neurotoxicity.

Oxidative stress in MeHg-induced neurotoxicity

Methylmercury (MeHg) is an environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. Although the molecular mechanisms mediating MeHg-induced neurotoxicity are not completely understood, several lines of evidence indicate that oxidative stress represents a critical event related to the neurotoxic effects elicited by this toxicant. The objective of this review is to summarize and discuss data from experimental and epidemiological studies that have been important in clarifying the molecular events which mediate MeHg-induced oxidative damage and, consequently, toxicity. Although unanswered questions remain, the electrophilic properties of MeHg and its ability to oxidize thiols have been reported to play decisive roles to the oxidative consequences observed after MeHg exposure. However, a close examination of the relationship between low levels of MeHg necessary to induce oxidative stress and the high amounts of sulfhydryl-containing antioxidants in mammalian cells (e.g., glutathione) have led to the hypothesis that nucleophilic groups with extremely high affinities for MeHg (e.g., selenols) might represent primary targets in MeHg-induced oxidative stress. Indeed, the inhibition of antioxidant selenoproteins during MeHg poisoning in experimental animals has corroborated this hypothesis. The levels of different reactive species (superoxide anion, hydrogen peroxide and nitric oxide) have been reported to be increased in MeHg-exposed systems, and the mechanisms concerning these increments seem to involve a complex sequence of cascading molecular events, such as mitochondrial dysfunction, excitotoxicity, intracellular calcium dyshomeostasis and decreased antioxidant capacity. This review also discusses potential therapeutic strategies to counteract MeHg-induced toxicity and oxidative stress, emphasizing the use of organic selenocompounds, which generally present higher affinity for MeHg when compared to the classically studied agents.

Mechanisms of methylmercury-induced neurotoxicity: evidence from experimental studies

Neurological disorders are common, costly, and can cause enduring disability. Although mostly unknown, a few environmental toxicants are recognized causes of neurological disorders and subclinical brain dysfunction. One of the best known neurotoxins is methylmercury (MeHg), a ubiquitous environmental toxicant that leads to long-lasting neurological and developmental deficits in animals and humans. In the aquatic environment, MeHg is accumulated in fish, which represent a major source of human exposure. Although several episodes of MeHg poisoning have contributed to the understanding of the clinical symptoms and histological changes elicited by this neurotoxicant in humans, experimental studies have been pivotal in elucidating the molecular mechanisms that mediate MeHg-induced neurotoxicity. The objective of this mini-review is to summarize data from experimental studies on molecular mechanisms of MeHg-induced neurotoxicity. While the full picture has yet to be unmasked, in vitro approaches based on cultured cells, isolated mitochondria and tissue slices, as well as in vivo studies based mainly on the use of rodents, point to impairment in intracellular calcium homeostasis, alteration of glutamate homeostasis and oxidative stress as important events in MeHg-induced neurotoxicity. The potential relationship among these events is discussed, with particular emphasis on the neurotoxic cycle triggered by MeHg-induced excitotoxicity and oxidative stress. The particular sensitivity of the developing brain to MeHg toxicity, the critical role of selenoproteins and the potential protective role of selenocompounds are also discussed. These concepts provide the biochemical bases to the understanding of MeHg neurotoxicity, contributing to the discovery of endogenous and exogenous molecules that counteract such toxicity and provide efficacious means for ablating this vicious cycle.

Effects of methyl mercury in combination with polychlorinated biphenyls and brominated flame retardants on the uptake of glutamate in rat brain synaptosomes: a mathematical approach for the study of mixtures

Regulatory limit values for toxicants are in general determined by the toxicology of the single compounds. However, little is known about their combined effects. Methyl mercury (MeHg), polychlorinated biphenyls (PCBs), and brominated flame retardants (BFRs) are dominant contaminants in the environment and food. MeHg is a well known neurotoxicant, especially affecting the developing brain. There is increasing evidence that PCB and BFRs also have neurotoxic effects. An enhanced effect of these toxicants, due to either synergistic or additive effects, would be considered as a risk for the fetal development. Here we studied the combinatorial effects of MeHg in combination with PCB or BFR on the reuptake of glutamate in synaptosomes. To provide the optimal conclusion regarding type of interaction, we have analyzed the data using two mathematical models, the Löewe model of additivity and Bliss' model of independent action. Binary and ternary mixtures in different proportions were made. The toxicants had primarily additive effects, as shown with both models, although tendencies towards synergism were observed. MeHg was by far the most potent inhibitor of uptake with an EC(50) value of 0.33 microM. A reconstituted mixture from a relevant fish sample was made in order to elucidate which chemical was responsible for the observed effect. Some interaction was experienced between PCB and MeHg, but in general MeHg seemed to explain the observed effect. We also show that mixture effects should not be assessed by effect addition.

Comparison of alterations in amino acids content in cultured astrocytes or neurons exposed to methylmercury separately or in co-culture

Methylmercury (MeHg) is an environmental toxicant that induces enduring neuropsychological deficits in humans. Although the mechanisms associated with MeHg-induced neurotoxicity have not yet been fully elucidated, some lines of evidence point out to excitatory amino acids dyshomeostasis as an important outcome of MeHg exposure. The present study was designed to characterize the effects of MeHg on amino acid content in co-cultured astrocytes and neurons or in each cell type under solitary conditions. The results showed that glutamate concentrations significantly decreased in neurons, but not in astrocyte cultures exposed to 10 microM MeHg. The decrease in neurons was fully reversed when these cells were co-cultured with astrocytes. The content of other amino acids (aspartate, alanine, glycine and serine) decreased upon exposure to 10 microM MeHg in both neurons and astrocytes cultured in solitary conditions, although the effect was generally smaller in astrocytes than in neurons. However, the content of these amino acids in each of the cell types was indistinguishable from controls when co-cultures were treated with MeHg. Overall, the results indicate that astrocytes, which are more resistant to amino acid modulation by MeHg, can (i) mitigate the effects of MeHg that occur in neurons cultured in solitary conditions and (ii) become themselves more MeHg resistant in the presence of neurons. Delineating the mechanisms underlying the mutual neuroprotective effects of astrocytes and neurons in co-culture to MeHg-induced amino acid imbalance requires further investigation.

Relationships among mercury, selenium, and neurochemical parameters in common loons (Gavia immer) and bald eagles (Haliaeetus leucocephalus)

Fish-eating birds can be exposed to levels of dietary methylmercury (MeHg) known or suspected to adversely affect normal behavior and reproduction, but little is known regarding Hg's subtle effects on the avian brain. In the current study, we explored relationships among Hg, Se, and neurochemical receptors and enzymes in two fish-eating birds--common loons (Gavia immer) and bald eagles (Haliaeetus leucocephalus). In liver, both species demonstrated a wide range of total Hg (THg) concentrations, substantial demethylation of MeHg, and a co-accumulation of Hg and Se. In liver, there were molar excesses of Se over Hg up to about 50-60 microg/g THg, above which there was an approximate 1:1 molar ratio of Hg:Se in both species. However, in brain, bald eagles displayed a greater apparent ability to demethylate MeHg than common loons. There were molar excesses of Se over Hg in brains of bald eagles across the full range of THg concentrations, whereas common loons often had extreme molar excesses of Hg in their brains, with a higher proportion of THg remaining as MeHg compared with eagles. There were significant positive correlations between brain THg and muscarinic cholinergic receptor concentrations in both species studied; whereas significant negative correlations were observed between N-methyl-D-aspartic acid (NMDA) receptor levels and brain Hg concentration. There were no significant correlations between brain Se and neurochemical receptors or enzymes (cholinesterase and monoamine oxidase) in either species. Our findings suggest that there are significant differences between common loons and bald eagles with respect to cerebral metabolism and toxicodynamics of MeHg and Se. These interspecies differences may influence relative susceptibility to MeHg toxicity; however, neurochemical responses to Hg in both species were similar.

Metallothionein in the central nervous system: Roles in protection, regeneration and cognition

Metallothionein (MT) is an enigmatic protein, and its physiological role remains a matter of intense study and debate 50 years after its discovery. This is particularly true of its function in the central nervous system (CNS), where the challenge remains to link its known biochemical properties of metal binding and free radical scavenging to the intricate workings of brain. In this compilation of four reports, first delivered at the 11th International Neurotoxicology Association (INA-11) Meeting, June 2007, the authors present the work of their laboratories, each of which gives an important insight into the actions of MT in the brain. What emerges is that MT has the potential to contribute to a variety of processes, including neuroprotection, regeneration, and even cognitive functions. In this article, the properties and CNS expression of MT are briefly reviewed before Dr Hidalgo describes his pioneering work using transgenic models of MT expression to demonstrate how this protein plays a major role in the defence of the CNS against neurodegenerative disorders and other CNS injuries. His group's work leads to two further questions, what are the mechanisms at the cellular level by which MT acts, and does this protein influence higher order issues of architecture and cognition? These topics are addressed in the second and third sections of this review by Dr West, and Dr Levin and Dr Eddins, respectively. Finally, Dr Aschner examines the ability of MT to protect against a specific toxicant, methylmercury, in the CNS.

Methylmercury Neurotoxicity: Exploring Potential Novel Targets

Mechanistic studies on the effects of MeHg in the central nervous system (CNS) have been limited to morphology, substrate uptake and macromolecular synthesis, differentiation, and changes in gene expression during development and adulthood, but its primary site of action has yet to be identified. Proper functioning of the nitric oxide synthase (NOS)-cyclic GMP and the cyclooxygenase (COX)-prostaglandin (PG) signaling pathways in the CNS depend on post-translational modifications of key enzymes by chaperone proteins. The ability of MeHg to alter or inhibit chaperone-client protein interactions is hitherto unexplored, and potentially offers an upstream unifying mechanism for the plethora of MeHg effects, ranging from reactive species generation (ROS) generation, mitochondrial dysfunction, changes in redox potential, macromolecule synthesis, and cell swelling. In view of the prominent function of astrocytes in the maintenance of the extracellular milieu and their critical role in mediating MeHg neurotoxicity, they afford a relevant and well-established experimental model. The present review is predicated on (a) the remarkable affinity of mercurials for the anionic form of sulfhydryl (-SH) groups, (b) the essential role of thiols in protein biochemistry, and (c) the role of molecular chaperone proteins, such as heat shock protein 90 (Hsp90) in the regulation of protein redox status by facilitating the formation and breakage of disulfide bridges. We offer potential sites where MeHg may interfere with cellular homeostasis and advance a novel mechanistic model for MeHg-induced neurotoxicity.

Decreased N-methyl-d-aspartic acid (NMDA) receptor levels are associated with mercury exposure in wild and captive mink

Mercury (Hg) impairs glutamate homeostasis but little is known about its effects on the N-methyl-d-aspartic acid (NMDA) receptor. Here, we investigated NMDA receptor levels, as determined by [(3)H]-MK801 binding, in both wild and captive mink (Mustela vison) that experienced different levels of methylmercury (MeHg) exposure. Competitive in vitro binding experiments showed that inorganic Hg (HgCl(2); IC(50)=1.5-20.7 microM), but not MeHg (MeHgCl; IC(50)>320 microM), inhibited binding to the NMDA receptor in several brain regions of mink. In a survey of trapped wild mink, NMDA receptor levels in the brain were negatively correlated (p<0.005) with concentrations of total Hg (R=-0.618) and MeHg (R=-0.714). These findings were supported by a laboratory feeding study in which captive mink were exposed to dietary MeHg (0-2 ppm) for 89 days. Concentration-dependent decreases in NMDA receptor levels were found in the basal ganglia, cerebellum, brain stem and occipital cortex. These findings are of physiological and ecological concern because they demonstrate that Hg, at dietary concentrations as low as 0.1 ppm, can significantly reduce NMDA receptor levels.

Involvement of glutamate and reactive oxygen species in methylmercury neurotoxicity

This review addresses the mechanisms of methylmercury (MeHg)-induced neurotoxicity, specifically examining the role of oxidative stress in mediating neuronal damage. A number of critical findings point to a central role for astrocytes in mediating MeHg-induced neurotoxicity as evidenced by the following observations: a) MeHg preferentially accumulates in astrocytes; b) MeHg specifically inhibits glutamate uptake in astrocytes; c) neuronal dysfunction is secondary to disturbances in astrocytes. The generation of reactive oxygen species (ROS) by MeHg has been observed in various experimental paradigms. For example, MeHg enhances ROS formation both in vivo (rodent cerebellum) and in vitro (isolated rat brain synaptosomes), as well as in neuronal and mixed reaggregating cell cultures. Antioxidants, including selenocompounds, can rescue astrocytes from MeHg-induced cytotoxicity by reducing ROS formation. We emphasize that oxidative stress plays a significant role in mediating MeHg-induced neurotoxic damage with active involvement of the mitochondria in this process. Furthermore, we provide a mechanistic overview on oxidative stress induced by MeHg that is triggered by a series of molecular events such as activation of various kinases, stress proteins and other immediate early genes culminating in cell damage.

Methylmercury induces oxidative injury, alterations in permeability and glutamine transport in cultured astrocytes

The neurotoxicity of high levels of methylmercury (MeHg) is well established both in humans and experimental animals. Astrocytes accumulate MeHg and play a prominent role in mediating MeHg toxicity in the central nervous system (CNS). Although the precise mechanisms of MeHg neurotoxicity are ill-defined, oxidative stress and altered mitochondrial and cell membrane permeability appear to be critical factors in its pathogenesis. The present study examined the effects of MeHg treatment on oxidative injury, mitochondrial inner membrane potential, glutamine uptake and expression of glutamine transporters in primary astrocyte cultures. MeHg caused a significant increase in F(2)-isoprostanes (F(2)-IsoPs), lipid peroxidation biomarkers of oxidative damage, in astrocyte cultures treated with 5 or 10 microM MeHg for 1 or 6 h. Consistent with this observation, MeHg induced a concentration-dependant reduction in the inner mitochondrial membrane potential (DeltaPsi(m)), as assessed by the potentiometric dye, tetramethylrhodamine ethyl ester (TMRE). Our results demonstrate that DeltaPsi(m) is a very sensitive endpoint for MeHg toxicity, since significant reductions were observed after only 1 h exposure to concentrations of MeHg as low as 1 microM. MeHg pretreatment (1, 5 and 10 microM) for 30 min also inhibited the net uptake of glutamine ((3)H-glutamine) measured at 1 min and 5 min. Expression of the mRNA coding the glutamine transporters, SNAT3/SN1 and ASCT2, was inhibited only at the highest (10 microM) MeHg concentration, suggesting that the reduction in glutamine uptake observed after 30 min treatment with lower concentrations of MeHg (1 and 5 microM) was not due to inhibition of transcription. Taken together, these studies demonstrate that MeHg exposure is associated with increased mitochondrial membrane permeability, alterations in glutamine/glutamate cycling, increased ROS formation and consequent oxidative injury. Ultimately, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to cellular dysfunction and cell death.

Region-dependent differences and alterations of protective thiol compound levels in cultured astrocytes and brain tissues

We examined region-dependent differences and alterations in the levels of protective thiol compounds, glutathione (GSH) and metallothionein (MT)-I and -II, in cultured rat astrocytes under several culture conditions and in brain tissues of rats at postnatal and weaning periods. Regardless of culture conditions, both protein concentrations and mRNA expressions of MT-I and -II were much higher in the cerebral hemisphere than in cerebellar astrocytes, whereas no difference was observed in GSH concentration. In both astrocytes, the GSH concentrations did not change within 12 h but significantly increased 24 h after being maintained in a serum-free defined medium. At 24 h, protein concentrations and mRNA expressions of MT-I and -II also increased in the respective astrocytes, and were further enhanced when maintained in the presence of 50 microM Zn(2+). In the brain tissues, the MT-I/-II protein concentrations were significantly higher in the cerebral cortex (a part of the cerebral hemisphere) than in the cerebellum, whereas the GSH concentration was similar at both postnatal day (P)1 and P35. In addition, the concentrations in the respective regions were significantly higher at P35 than at P1. These results suggest that region-dependent differences in the cellular levels of GSH and MTs in cultured astrocytes might reflect the in vivo differences, and that the levels of the respective thiol compounds in cultured astrocytes increase after serum elimination along with the region-dependent differences.

Site-directed mutagenesis of cysteine residues of large neutral amino acid transporter LAT1

The large neutral amino acid transporter type 1, LAT1, is the principal neutral amino acid transporter expressed at the blood-brain barrier (BBB). Owing to the high affinity (low Km) of the LAT1 isoform, BBB amino acid transport in vivo is very sensitive to transport competition effects induced by hyperaminoacidemias, such as phenylketonuria. The low Km of LAT1 is a function of specific amino acid residues, and the transporter is comprised of 12 phylogenetically conserved cysteine (Cys) residues. LAT1 is highly sensitive to inhibition by inorganic mercury, but the specific cysteine residue(s) of LAT1 that account for the mercury sensitivity is not known. LAT1 forms a heterodimer with the 4F2hc heavy chain, which are joined by a disulfide bond between Cys160 of LAT1 and Cys110 of 4F2hc. The present studies use site-directed mutagenesis to convert each of the 12 cysteines of LAT1 and each of the 2 cysteines of 4F2hc into serine residues. Mutation of the cysteine residues of the 4F2hc heavy chain of the hetero-dimeric transporter did not affect transporter activity. The wild type LAT1 was inhibited by HgCl2 with a Ki of 0.56+/-0.11 microM. The inhibitory effect of HgCl2 for all 12 LAT1 Cys mutants was examined. However, except for the C439S mutant, the inhibition by HgCl2 for 11 of the 12 Cys mutants was comparable to the wild type transporter. Mutation of only 2 of the 12 cysteine residues of the LAT1 light chain, Cys88 and Cys439, altered amino acid transport. The Vmax was decreased 50% for the C88S mutant. A kinetic analysis of the C439S mutant could not be performed because transporter activity was not significantly above background. Confocal microscopy showed the C439S LAT1 mutant was not effectively transferred to the oocyte plasma membrane. These studies show that the Cys439 residue of LAT1 plays a significant role in either folding or insertion of the transporter protein in the plasma membrane.

Effect of methylmercury on glutamate-cysteine ligase expression in the placenta and yolk sac during mouse development

The placenta and the yolk sac play critical roles in fetal development, including protection from oxidative stress through the presence of detoxifying enzymes. Glutathione (GSH; gamma-glutamylcysteinylglycine), a crucial molecule in the maintenance of cellular redox status, plays a critical role in development, and it is also protective against methylmercury toxicity. Glutamate-cysteine ligase (GCL), the enzyme that catalyzes the rate-limiting step in GSH synthesis, is widely expressed in the mouse embryo and extraembryonic membranes throughout development. The aim of this study was to investigate the effect of low-level subchronic methylmercury exposure on GCL expression in the mouse placenta and yolk sac, after describing the basal developmental expression of the enzyme in these tissues. We found that basal mRNA expression levels increased dramatically in the placenta and the yolk sac at gd 18, whereas protein levels did not increase in parallel with the mRNA. We also found that methylmercury induced GCLc mRNA expression in the placenta at gd 18 in a dose-dependent manner, suggesting an important role for this enzyme in the response of the placenta to toxicants. These changes in expression may be useful as a biomarker of MeHg exposure during development.

Modulatory effect of glutathione status and antioxidants on methylmercury-induced free radical formation in primary cultures of cerebral astrocytes

Excessive free radical formation has been implicated as one of the causative factors in neurotoxic damage associated with variety of metals, including methylmercury (MeHg). Although the mechanism(s) associated with MeHg-dependent neurotoxicity remains far from clear, overwhelming data give credence to a mediatory role for astrocytes, a major cell type that preferentially accumulates MeHg. To extend our recent findings of MeHg-induced increase in ROS formation (G. Shanker, J.L. Aschner, T. Syversen et al., Free radical formation in cerebral cortical astrocytes in culture induced by methylmercury, Mol. Brain Res. 128 (2004) 48-57), the present studies were designed to assess the effect of modulating intracellular glutathione (GSH) content, on ROS generation, in the absence and presence of MeHg. Intracellular GSH was reduced by treatment with 100 microM buthionine-L-sulfoxane (BSO) for 24 h, and increased by treatment with 1 mM l-2-oxothiazolidine-4-carboxylic acid (OTC) for 24 h. Additionally, the effects of the selective antioxidants, catalase (1000 U/ml for 1 h), an H2O2 scavenger, and n-propyl gallate (100 microM for 1 h), a superoxide radical (*O2-) and possibly hydroxyl radical (*OH) scavenger on MeHg-induced ROS formation were examined. After these treatments, astrocytes were exposed to +/-10 microM MeHg for 30 min, following which the fluorescent probes, CM-H2DCFA and CM-H2XRos were added; 20 min later, laser scanning confocal microscopy (LSCM) images were obtained. Exposure of astrocytes for 24 h to 100 microM BSO, a GSH synthesis inhibitor, led to a significant increase in mitochondrial ROS (i.e., *O2-, *NO, and ONOO-) formation, as assessed with CM-H2XRos mitotracker red dye. Similarly, BSO increased ROS formation in various intracellular organelles, as assessed with CM-H2DCFDA. BSO in combination with MeHg increased fluorescence levels in astrocytes to levels above those noted with BSO or MeHg alone, but this effect was statistically indistinguishable from either of these groups (BSO or MeHg). Pretreatment of astrocytes for 24 h with 1 mM OTC abolished the MeHg-induced increase in ROS. Results similar to those obtained with OTC were observed with the free radical scavenger, n-propyl gallate (n-PG). The latter had no significant effects on astrocytic fluorescence when administered alone. This *O2- and possibly *OH radical scavenger significantly attenuated MeHg-induced ROS formation. Catalase, an H2O2 scavenger, was less effective in reducing MeHg-induced ROS formation. Taken together, these studies point to the important protective effect of adequate intracellular GSH content as well as antioxidants against MeHg-triggered oxidative stress in primary astrocyte cultures.

Free radical formation in cerebral cortical astrocytes in culture induced by methylmercury

Oxidative stress has been implicated in neurotoxic damage associated with various metals, including methylmercury (MeHg). Although the mechanism(s) of MeHg-induced neurotoxicity remains unclear, evidence supports a mediatory role for astrocytes, a cell type that preferentially accumulates MeHg. Using scanning confocal microscopy (LSCM), the present study was undertaken to examine the role of astrocytes as the site of reactive oxygen species (ROS). Three redox-sensitive fluorescent probes were used for ROS analysis, (a) CM-H2DCFDA (chloromethyl derivative of dichlorodihydrofluorescein diacetate), a probe for intracellular hydrogen peroxide (H2O2); (b) hydroethidine (HETH), a probe for superoxide anion (*O2-), and (c) CM-H2XRos (chloromethyl derivative of dihydro X-rosamine), and a probe that is selective for mitochondrial reactive oxygen intermediates. Astrocytes were treated with 10 microM MeHg for 30 min, following which the various fluorescent probes were added; 20 min later LSCM images were collected. Astrocytes loaded with CM-H2DCFDA and HE demonstrated a significant MeHg-induced increase in fluorescence intensity indicative of increased intracellular H2O2 and *O2-, respectively. Similar results were obtained with the mitotracker dye, CM-H2XRos. Additionally, exposure of astrocytes for 24 h to 100 microM buthionine-L-sulfoxane (BSO), a glutathione (GSH) synthesis inhibitor, caused a significant increase in ROS formation. Furthermore, BSO pretreatment significantly enhanced the MeHg-induced formation of *O2-, indicating an important role for GSH in the maintenance of optimal cellular redox status. Time-course experiments performed in the simultaneous presence of CM-H2XRos and CM-H2DCFDA demonstrated that the MeHg-induced CM-H2XRos fluorescence changes preceded those of CM-H2DCFDA, suggesting that the mitochondria represent an early primary site for ROS formation. Taken together, these studies illustrate that MeHg induces the generation of astrocyte-derived ROS and support a role for astrocytic ROS in MeHg-associated neurotoxic damage.

Acute Cytotoxic Effects of Mercuric Compounds in Cultured Astrocytes Prepared from Cerebral Hemisphere and Cerebellum of Newborn Rats

We investigated acute cytotoxic effects and Hg accumulation after exposure to methylmercury (MeHg) or Hg(2+) in the presence or absence of serum in cultured astrocytes prepared from the cerebral hemisphere or cerebellum of newborn rats. Dose-related changes in viable cell numbers after exposure to mercuric compounds were not different between astrocytes from both regions under the specified conditions. Accumulation of each compound for 3 h was similar in both astrocytes but that for 24 h became different, especially that of Hg(2+). In both astrocytes, susceptibility to the respective compounds was higher in the order of those exposed immediately after, without, and 24 h after changing the serum-containing medium to a serum-free defined medium (SFDM). Accumulation for 3 h was higher in the respective astrocytes exposed to MeHg or Hg(2+) immediately after being maintained in SFDM than in those exposed 24 h after. These results suggest that accumulation of mercuric compounds up to 3 h strongly correlates with susceptibility, at least when maintained in SFDM. Astrocytic morphology changed to a satellite shape after the medium change to SFDM particularly in cerebellar astrocytes but only a few in cerebral hemisphere astrocytes, and it was reverted to a polygonal shape by MeHg but not Hg(2+) at 3 microM. The present results suggest that although some properties such as morphological changes and Hg accumulation are different between cerebral hemisphere and cerebellar astrocytes, these differences are not simply reflected by susceptibility to the acute cytotoxicity of mercuric compounds.

Mitochondrial viability and apoptosis induced by aluminum, mercuric mercury and methylmercury in cell lines of neural origin

Mercury and aluminum are considered to be neurotoxic metals, and they are often connected with the onset of neurodegenerative diseases. In this study, mercuric mercury, methylmercury and aluminum were studied in three different cell lines of neural origin. To evaluate the effects, mitochondrial cytotoxicity and apoptosis induced by the metals were measured after various incubation times. SH-SY5Y neuroblastoma, U 373MG glioblastoma, and RPE D407 retinal pigment epithelial cells were subcultured to appropriate cell culture plates and 0.01-1,000 microM concentrations of methylmercury, mercuric and aluminum chloride were added into the growth medium. In the assay measuring the mitochondrial dehydrogenase activity, WST-1, the cultures were exposed for 15 min, 24 or 48 h before measurement. Cells were allowed to recover from the exposure in part of the study. Apoptosis induced by the metals was measured after 6-, 24- and 48-h exposure times with the determination of activated caspase 3 enzyme. Mitochondrial assays showed a clear dose-response and exposure time-response to the metals. The most toxic was methylmercury (EC50 ~0.8 microM, 48 h), and the most sensitive cell line was the neuroblastoma cell line SH-SY5Y. Furthermore, there was marked mitochondrial activation, especially in connection with aluminum and methylmercury at low concentrations. This activation may be important during the initiation of cellular processes. All the metals tested induced apoptosis, but with a different time-course and cell-line specificity. In microscopic photographs, glioblastoma cells formed fibrillary tangles, and neuroblastoma cells settled along the fibrilles in cocultures of glial and neuronal cell lines during aluminum exposure. The study emphasized the toxicity of methylmercury to neural cells and showed that aluminum alters various cellular activities.

Effect of methylmercury on glutamate metabolism in cerebellar astrocytes in culture

The effect of methylmercury (MeHg) on [U-13C]glutamate metabolism was studied in cerebellar astrocytes using 13C nuclear magnetic resonance spectroscopy. The cells were preincubated in medium containing 25 or 50 microM MeHg and 10% fetal calf serum for 4h and then in medium with [U-13C]glutamate (0.5mM) for 2h. Labeled glutamate, glutamine and aspartate were observed both in the cell extracts and media, labeled glutathione in the cell extracts and labeled lactate and alanine in the media. The amount of glutamate removed from the media was decreased in the 50 microM MeHg group, furthermore, the levels of both labeled and unlabeled glutamine were decreased. This might indicate a decreased synthesis and/or increased degradation. An increase was observed for glutathione in the 25 microM group, which might be due to an upregulated synthesis of glutathione in response to the toxic effects of MeHg. The percentage of [U-13C]glutamate used for the synthesis of metabolites via the tricarboxylic acid cycle was increased in the presence of 50 microM MeHg. However, the percentage used for energy production was decreased in both groups, indicating selective mitochondrial vulnerability due to the inhibitory effect of MeHg.

Activation of mu-calpain in developing cortical neurons following methylmercury treatment

In order to examine the possible involvement of mu-calpain in methylmercury (MeHg)-induced neurotoxicity in developing cortical neurons, we performed biochemical and immunohistochemical studies utilizing two antibodies which specifically recognize the 150-kDa mu-calpain-specific alpha-spectrin breakdown product (SBDP) and the active form of mu-calpain in rats on postnatal day 16. Soluble fractions of the cerebral cortex from control rats exhibited slight immunoreactivity for SBDP. Although the amount of SBDP in the cerebral cortex was only slightly increased the day after the final treatment of MeHg (10 mg/kg) for 3 or 7 consecutive days, there was a prominent accumulation of SBDP 3 days after the final treatment of MeHg for 7 consecutive days. On the other hand, the 76-kDa isoform of mu-calpain gradually increased after chronic treatment of MeHg, but markedly decreased 3 days after the final treatment of MeHg for 7 consecutive days. At this stage, many cortical neurons were densely stained with anti-SBDP antibody. The delayed increase in SBDP corresponded well with the delayed nature of the MeHg-induced neurotoxicity. When MK-801 (0.1 mg/kg), a non-competitive antagonist of N-methyl-D-aspartate (NMDA), was administered intraperitoneally with MeHg for 7 consecutive days, both neuronal damage and accumulation of SBDP were markedly depressed in the cerebral cortex 3 days after the final treatment. Our results indicate that mu-calpain activation and mu-calpain-mediated proteolysis of alpha-spectrin preceded neuronal damage in the developing cerebral cortex induced by chronic treatment of MeHg.

Methylmercury-induced reactive oxygen species formation in neonatal cerebral astrocytic cultures is attenuated by antioxidants

Excessive generation of reactive oxygen species (ROS) has been suggested as a causal factor in various neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease [Brain Res. 830 (1999) 10-15; Biochem. J. 310 (1995) 83-90; Free Radic. Biol. Med. 27 (1999) 612-616]. The present work examined the role of ROS in the neurotoxicity of methylmercury (MeHg). ROS formation in primary astrocytic cultures of neonatal rat cerebral cortex was monitored by 2',7'-dichlorodihydrofluorescein diacetate (H(2)DCF-DA) fluorescence. MeHg, at 10 and 20 microM caused a significant increase in ROS formation (10 microM, P<0.01; 20 microM, P<0.001). Additional studies established the effectiveness of antioxidants/free radical scavengers in attenuating the MeHg-stimulated ROS formation in the following rank-order: (1) Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), a non-thiol containing antioxidant, (2) n-propyl gallate (PG), a free radical scavenger, (3) superoxide dismutase (SOD), an antioxidant enzyme that dismutates superoxide anion radical, (4) alpha-phenyl-tert-butyl nitrone (PBN), a lipophilic hydroxyl radical spin trapping agent. A significant inhibition of MeHg-induced ROS generation was also noted in astrocytes preincubated (3 h) with arachidonyl trifluoromethyl ketone (AACOCF(3,) 20 microM, P<0.05), a specific inhibitor of cytosolic phospholipase A(2) (cPLA(2)). Conversely, pretreatment (24 h) with 100 microM buthionine-L-sulfoxamine [BSO, a glutathione (GSH) synthesis inhibitor], significantly increased (P<0.05) ROS formation in MeHg treated astrocytes compared to controls. Combined, these studies invoke ROS as potent mediators of MeHg cytotoxicity and support the hypothesis that excessive ROS generation, at least in part, plays an important role in MeHg-induced neurotoxicity.

Neurotoxic effects of mercury on auditory cortex networks growing on microelectrode arrays: a preliminary analysis

Mercury is known to cause sensorineural hearing loss and impaired speech perception. However, there is still a lack of a quantitative description of mercury toxicity on central auditory structures. This is a preliminary study using the novel technique of microelectrode array (MEA) recordings to evaluate acute and chronic neurotoxic effects of mercury on auditory cortex networks (ACNs) in vitro. Morphological and electrophysiological effects of mercuric chloride (HgCl(2)) were studied. Neurons dissociated from auditory cortices of 14-day-old mouse embryos were grown on photoetched MEAs containing 64 transparent indium-tin oxide (ITO) electrodes. For acute electrophysiological experiments, the spontaneous spiking and bursting activity from ACNs were compared before and after application of HgCl(2). For chronic electrophysiological experiments, auditory cortex cultures were treated with various concentrations of HgCl(2) from the day of seeding, and were tested 4 weeks later for the presence of spontaneous activity. Morphological analysis was conducted on 8-day-old ACNs treated with HgCl(2) for 3 days. Results of acute experiments indicated that <75 mM of HgCl(2) had an excitatory effect of variable magnitude on the spontaneous activity of ACNs; however, concentrations above 100 microM completely and irreversibly inhibited spike and burst activity. Chronic exposure of ACNs to 10 microM HgCl(2) completely blocked the spontaneous activity. Morphological analysis indicated that 10 microM HgCl(2) caused neuronal cell death in 3 days. It is concluded that HgCl(2) has a more toxic effect on auditory networks when exposed chronically, and the levels of mercury showing toxic effects on ACNs are within the dose range shown to cause neurologic symptoms in humans.

Methylmercury increases glutamate extracellular levels in frontal cortex of awake rats

A current hypothesis about methylmercury (MeHg) neurotoxicity proposes that neuronal damage is due to excitotoxicity following glutamate uptake alterations in the astrocyte. By sampling from a microdialysis probe implanted in the frontal cortex of adult Wistar rats, we measured the effects of acute exposure to either 10 or 100 microM MeHg through the microdialysis probe, on glutamate extracellular levels in 15 awake animals. After baseline measurements, the perfusion of MeHg during 90 min induced immediate and significant elevations in extracellular glutamate at 10 microM (9.8-fold, P<.001) and at 100 microM (2.4-fold, P=.001). This in vivo demonstration of increments of extracellular glutamate supports the hypothesis that dysfunction of glutamate neurotransmission plays a key role in MeHg-induced neural damage.

Methylmercury enhances arachidonic acid release and cytosolic phospholipase A2 expression in primary cultures of neonatal astrocytes

Cytosolic phospholipase A(2) (cPLA(2)) stimulates the hydrolysis of sn-2 ester bond in membrane phospholipids releasing arachidonic acid (AA) and lysophospholipids. The present study examined the effect of methylmercury (MeHg) on cPLA(2) activation and AA release in primary cultures of neonatal rat cerebral astrocytes. Astrocytes were preloaded overnight at 37 degrees C with 3H-AA to metabolically label phospholipids. The effect of MeHg on the activation of cPLA(2) was measured by the release of 3H-AA from astrocytes over 120 min. MeHg (5 microM) caused a significant increase in AA release at 10, 30, 60, and 120 min, whereas 2.5 microM MeHg significantly increased AA release only at 120 min. MeHg-induced increase in 3H-AA release was due to cPLA(2) activation, since arachidonyl trifluoromethyl ketone (AACOCF(3)), a selective inhibitor of cPLA(2), completely abolished MeHg's effect. Consistent with these observations, MeHg (5.0 and 10.0 microM) increased cPLA(2) mRNA (6 h) and cPLA(2) protein expression (5.0 and 10.0 microM; 24 h). The time-course of these effects suggests an immediate direct or indirect effect of MeHg on cPLA(2) activation and 3H-AA release as well as a long-term effect involving the induction of cPLA(2). Thin layer chromatographic analysis of 3H-AA-labeled phospholipids showed that MeHg-stimulated astrocyte 3H-AA release was not due to increased incorporation of 3H-AA into the putative substrates of cPLA(2). These results invoke cPLA(2) as a putative target for MeHg toxicity, and support the notion that cPLA(2)-stimulated hydrolysis and release of AA play a critical role in MeHg-induced neurotoxicity.

XANES spectroscopy: a promising tool for toxicology: a tutorial

X-ray absorption near edge structure (XANES) spectroscopy can provide information on the oxidation state of metal ions within a biological sample and also the complexes in which it is found. This type of information could be of great use to toxicologists in understanding the mechanism of action of many toxic agents. The prospect of using a sophisticated physical technique such as XANES may be somewhat intimidating for those without a strong physical background. Here, we explain the concepts necessary to understand XANES spectroscopy at a level that can be easily understood by biological scientists without a strong physics background and describe useful sample preparation and data analysis techniques which can be adapted for a variety of applications. Examples are taken from an ongoing study of manganese in brain mitochondria and neuron-like cells.

Protection of methylmercury effects on the in vivo dopamine release by NMDA receptor antagonists and nitric oxide synthase inhibitors

The possible protective effects of NMDA receptor antagonists dizocilpine (MK-801) and D(-)-2-amino-5-phosphonopentanoic acid (AP5), and nitric oxide synthase (NOS) inhibitors L-nitro-arginine methyl ester (L-NAME) and 7-nitro-indazol (7-NI) on the methylmercury (MeHg)-induced dopamine (DA) release from rat striatum were investigated using in vivo microdialysis. Intrastriatal infusion of 400 microM or 4 mM MeHg increased the extracellular DA levels to 1941+/-199 and 7971+/-534% with respect to basal levels. Infusion of 400 microM or 4 mM MeHg in 400 microM MK-801 pretreated animals, increased striatal DA levels to 677+/-126 and 2926+/-254%, with respect to basal levels, these increases being 65 and 63% smaller than those induced by MeHg in non-pretreated animals. Infusion of 400 microM or 4 mM MeHg in 400 microM AP5 pretreated animals, increased striatal DA levels to 950+/-234 and 2251+/-254% with respect to basal levels, these increases being 51 and 72% smaller than those induced by MeHg in non-pretreated animals. Infusion of 400 microM MeHg in 100 microM L-NAME or 7-NI pretreated animals, increased the extracellular DA levels to 1159+/-90 and 981+/-292%, with respect to basal levels, these increases being 40 and 50% smaller than those induced by MeHg in non-pretreated animals. In summary, MeHg acts, at last in part, through an overstimulation of NMDA receptors with possible NO production to induce DA release, and administration of NMDA receptor antagonists and NOS inhibitors protects against MeHg-induced DA release from rat striatum.

Identification and characterization of uptake systems for cystine and cysteine in cultured astrocytes and neurons: evidence for methylmercury-targeted disruption of astrocyte transport

Maintenance of appropriate intracellular glutathione (GSH) levels is crucial for cellular defense against oxidative damage. A suggested mechanism of methylmercury (MeHg) neurotoxicity implicates the involvement of oxygen radical formation and a decrease in cellular levels of GSH. Astrocytes play an important role in providing GSH precursors to neurons, and as will be discussed in this review, altered GSH homeostasis likely leads to impairment of astrocytic handling of glutamate, and neuronal energy metabolism. The review summarizes recent observations on transport systems for cysteine and cystine, precursors of GSH, in primary cultures of astrocytes and neurons, and their sensitivity to MeHg treatment.

Methylmercury inhibits cysteine uptake in cultured primary astrocytes, but not in neurons

The maintenance of adequate intracellular glutathione (GSH) concentrations is dependent on the availability and transport of the rate-limiting substrate, cysteine. A suggested mechanism of methylmercury (MeHg) neurotoxicity in brain involves the formation of oxygen radicals, and a decrease in intracellular levels of GSH. Recently, we have characterized various cysteine transport systems (both Na(+)-dependent and -independent) in cerebrocortical astrocytes and hippocampal neurons. The present study was carried out to investigate the effect of MeHg on cysteine uptake in both astrocytes and neurons, and to determine whether cysteine transport is differentially affected in the two cell types by MeHg treatment. Sixty-minute pretreatment with MeHg caused significant concentration-dependent inhibition in cysteine uptake in astrocytes, but not in neurons. As most of the cysteine transport is Na(+)-dependent (80-90% of total), additional studies focused on MeHg's effect on the Na(+)-dependent cysteine transporters X(AG(-)) and ASC. An additive inhibitory effect on cysteine uptake was observed in astrocytes treated with MeHg (5 microM) plus sub-maximal inhibitory concentrations (0.1 and 0.5 mM) of threo-beta-hydroxy-aspartate (THA), a specific inhibitor of the Na(+)-dependent transporter, X(AG(-)), compared to astrocytes treated with MeHg (P<0.001) or THA alone (P<0.05). There was no additive effect of MeHg and maximal inhibitory concentrations of THA (1.0 and 5.0 mM) on astrocytic cysteine uptake inhibition. Additional studies examined the sensitivity of the Na(+)-dependent ASC transport system to MeHg treatment. Maximal inhibitory concentration of L-serine (10 mM) alone had a rather modest inhibitory effect on cysteine uptake, and when applied in the presence of MeHg there was no additive effect. These results suggest that the inhibition of cysteine uptake by MeHg in astrocytes occurs through specific inhibition of both the X(AG(-)) as well as the ASC transport system.

Methylmercury-mediated inhibition of 3H-d-aspartate transport in cultured astrocytes is reversed by the antioxidant catalase

Astrocytes are essential for removal of glutamate from the extracellular space in the central nervous system. The neurotoxic heavy metal methylmercury potently and specifically inhibits the transport of glutamate in cultured astrocytes by an unknown mechanism. Glutamate transport in astrocytes is also inhibited by reactive oxygen species. A glutamate-induced transporter current is inhibited both by reactive oxygen species and thiol oxidizing agents. These observations suggest that oxidation of the transporter might mediate methylmercury-induced inhibition of glutamate transport. In the present study, we examined the ability of thiol reducing or oxidizing agents to inhibit transport of 3H-D-aspartate, a glutamate analog, in primary cultures of neonatal rat astrocytes. To assess if methylmercury-mediated inhibition of 3H-aspartate transport was due to overproduction of reactive oxygen species, we tested the ability of Trolox, alpha-phenyl-tert-butyl nitrone (PBN), or catalase to attenuate the methylmercury-induced inhibition of aspartate uptake. Neither the thiol reducing agent dithiothreitol (DTT), nor the thiol oxidizing agent 5,5'-dithio-bis(2-nitrobenzoic) acid (DTNB) had any effect on 3H-aspartate transport suggesting that the thiol redox state does not alter transporter function. In contrast, the antioxidant catalase (1000 U/ml) significantly attenuated methylmercury-induced inhibition of 3H-aspartate uptake, suggesting that excess reactive oxygen species, specifically H2O2, inhibit the function of an astrocytic excitatory amino acid transporter (EAAT1). Prolonged exposure (6 h) to inhibitors of glutamate transport significantly decreased EAAT1 mRNA levels suggesting that transporter expression is related to function. This study suggests that methylmercury-induced overproduction of H2O2 is a mechanism for inhibition of glutamate transport and transporter expression in cultured astrocytes.

Methylmercury inhibits the in vitro uptake of the glutathione precursor, cystine, in astrocytes, but not in neurons

Maintenance of adequate intracellular glutathione (GSH) levels is vital for intracellular defense against oxidative damage. The toxic effects of methylmercury (MeHg) are attributable, at least in part, to elevated levels of reactive oxygen species, and thus decreases in GSH synthesis may increase methylmercury toxicity. Astrocytes have recently been proposed to play an essential role in providing GSH precursors to neurons. Therefore, cystine transport, a prerequisite to GSH production, was characterized in cultured astrocytes and neurons, and the effects of methylmercury on this transport were assessed. Astrocytes and neurons both possessed temperature dependent transport systems for cystine. Astrocytes accumulated cystine by Na+-independent (X(C)-) and -dependent (X(AG)-) systems while neurons used exclusively Na+-independent systems. Inhibition of the X(AG)- transport system decreased cystine transport in astrocytes to levels equivalent to those in sodium-depleted conditions, suggesting that cystine is carried by a glutamate/aspartate transporter in astrocytes. Inhibition of the multifunction ectoenzyme/amino acid transporter gamma-glutamyltranspeptidase (GGT) decreased cystine transport in both neurons and astrocytes. Inhibition of System X(C)- with quisqualate also decreased cystine uptake in both astrocytes and neurons. These data demonstrate that cultured astrocytes accumulate cystine via three independent mechanisms, System X(AG)-, System X(C)-, and GGT, while cultured hippocampal neurons use System X(C)- and GGT exclusively. Inhibition of cystine uptake in astrocytes by methylmercury appears to be due to actions on the System X(AG)- transporter.

In vivo effects of inorganic mercury (HgCl(2)) on striatal dopaminergic system

In the present study, the effects of intrastriatal administration of different concentrations (40 microM, 400 microM, and 4 mM) of inorganic mercury (HgCl(2)) on the dopaminergic system of rat striatum were evaluated, using a microdialysis technique coupled to liquid chromatography-electrochemical detection. In previous studies, we discussed the effects of organic mercury (MeHg) administration on the striatal dopaminergic system on the basis of changes in the release and metabolism of striatal dopamine (DA). In the present study it is demonstrated that intrastriatal administration of all concentrations of HgCl(2) produced significant increases in the output of DA (1240, 2500, and 2658% for the concentrations of 40 microM, 400 microM, and 4 mM HgCl(2), respectively) from rat striatal tissue, associated with significant decreases in striatal levels of its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with the concentrations of 400 microM and 4 mM HgCl(2) (74.4 and 3.4% for DOPAC and 71.0 and 50.6% for HVA, respectively), whereas no changes in metabolite levels were observed with the concentration of 40 microM HgCl(2). These effects are explained as a result of stimulated DA release and/or changed DA metabolism. The effects of intrastriatal administration of HgCl(2) were compared with those of MeHg on DA extracellular levels.