Neurotoxicity and molecular effects of methylmercury

Elevated mercury exposure and neurochemical alterations in little brown bats (Myotis lucifugus) from a site with historical mercury contamination

Despite evidence of persistent methylmercury (MeHg) contamination in the South River (Virginia, USA) ecosystem, there is little information concerning MeHg-associated neurological impacts in resident wildlife. Here we determined mercury (Hg) concentrations in tissues of insectivorous little brown bats (Myotis lucifugus) collected from a reference site and a MeHg-contaminated site in the South River ecosystem. We also explored whether neurochemical biomarkers (monoamine oxidase, MAO; acetylcholinesterase, ChE; muscarinic acetylcholine receptor, mAChR; N-methyl-D-aspartate receptor, NMDAR) previously shown to be altered by MeHg in other wildlife were associated with brain Hg levels in these bats. Concentrations of Hg (total and MeHg) in tissues were significantly higher (10-40 fold difference) in South River bats when compared to reference sites. Mean tissue mercury levels (71.9 ppm dw in liver, 7.14 ppm dw in brain, 132 ppm fw in fur) in the South River bats exceed (sub)-clinical thresholds in mammals. When compared to the South River bats, animals from the reference site showed a greater ability to demethylate MeHg in brain (33.1% of total Hg was MeHg vs. 65.5%) and liver (8.9% of total Hg was MeHg vs. 50.8%) thus suggesting differences in their ability to detoxify and eliminate Hg. In terms of Hg-associated neurochemical biomarker responses, interesting biphasic responses were observed with an inflection point between 1 and 5 ppm dw in the brain. In the reference bats Hg-associated decreases in MAO (r = -0.61; p < 0.05) and ChE (r = -0.79; p < 0.01) were found in a manner expected but these were not found in the bats from the contaminated site. Owing to high Hg exposures, differences in Hg metabolism, and the importance of the aforementioned neurochemicals in multiple facets of animal health, altered or perhaps even a lack of expected neurochemical responses in Hg-contaminated bats raise questions about the ecological and physiological impacts of Hg on the bat population as well as the broader ecosystem in the South River.

Degeneration of peripheral nervous system in rats experimentally induced by methylmercury intoxication

The objective of this study is to elucidate the primary action of methylmercury chloride (MMC) intoxication on peripheral nervous system. We chronologically observed the pathological changes of sciatic nerve, dorsal root ganglion (DRG) neurons, ventral and dorsal roots in rats given 4 mg/kg/day of MMC on consecutive days and killed on days 11, 15, 18 and 21. On day 11, an initial axonal degeneration of type B neuron occurred, predominantly in the distal portions of sciatic nerve. The DRG type A neuron was infiltrated by MRF-1-positive macrophages on day 11. Electron microscopy also demonstrated degenerated mitochondria in type A neuron. On day 21, most of type A neurons seemed to have disappeared. However, type B neurons were well preserved. Immunoblotting with monoclonal antibodies, P0 and neurofilament, demonstrated that both of proteins significantly decreases from day 15. In conclusion, these results indicate that the primary action on type A neuron is the neuron body that consequently results in an anterograde degeneration of nerve fibers, while the type B neuron degeneration occurs in a dying-back process in this subacute model. These findings suggest that the mechanisms involved in the degeneration induced by MMC vary and may depend on certain intrinsic factors peculiar to these neurons.

Evidence for Improved Neuropharmacological Efficacy and Decreased Neurotoxicity in Mice with Traditional Processing of Rhizoma Arisaematis

Rhizoma Arisaematis (RA, the rhizome of Pinellia pedatisecta Schott) is a traditional Chinese medicine commonly used in the treatment of convulsions, inflammation, and cancer. Despite the fact that it has been used for more than 2000 years, the pharmacological and toxic effects of traditionally processed products of RA are still unclear. In this study, we attempted to investigate the effects exerted by untreated crude RA and different preparations of RA treated with alumen in combination with ginger juice (Zhinanxing) or bile juice (Dannanxing) in ICR mice. The results showed that both the Zhinanxing and Dannanxing water extracts exerted significantly increased sedative effects, as indicated by the inhibitory effects on ambulatory distances, jumps, vertical-plane entries, and prolonged pentobarbital-induced sleeping time. The extracts also exerted significantly increased analgesic effects (increase of tail flick latency in nociceptive testing) in mice than did the unprocessed crude RA after oral administration for one to three days, and effects persisted 18 days after the cessation of treatment. By contrast, the toxic effects, such as an increase in stereotype-1 episodes of locomotor activities and reduction of the retention time on a rotating rod (motor equilibrium dysfunction), were observed only in mice treated with the unprocessed crude RA for three consecutive days, and effects persisted for 18 days after the cessation of treatment. These neurotoxic effects were accompanied by an increase in plasma lipid peroxidation (LPO), decrease in whole blood nitric oxide (NOx) levels, and inhibition of Na +/ K +-ATPase activities in membrane fractions of erythrocytes and in the cerebral cortex. In conclusion, these findings provide scientific evidence that the processed RA indeed possesses not only enhanced neuropharmacological efficacy but also reduced neurotoxic effects as compared to the unprocessed crude RA. The signaling of NO x/oxidative stress/ Na +- K +- ATPase activities played a role, at least in part, in the underlying mechanisms of neurotoxic effects induced by the crude RA.

Hormetic effects of acute methylmercury exposure on grp78 expression in rat brain cortex

This study aims to explore the expression of GRP78, a marker of endoplasmic reticulum (ER) stress, in the cortex of rat brains acutely exposed to methylmercury (MeHg). Thirty Sprague-Dawley (SD) rats were randomly divided into six groups, and decapitated 6 hours (h) after intraperitoneal (i.p.) injection of MeHg (2, 4, 6, 8 or 10 mg/kg body weight) or normal saline. Protein and mRNA expression of Grp78 were detected by western blotting and real-time PCR, respectively. The results showed that a gradual increase in GRP78 protein expression was observed in the cortex of rats acutely exposed to MeHg (2, 4 or 6 mg/kg). Protein levels peaked in the 6 mg/kg group (p < 0.05 vs. controls), decreased in the 8 mg/kg group, and bottomed below the control level in the 10 mg/kg group. Parallel changes were noted for Grp78 mRNA expression. It may be implied that acute exposure to MeHg induced hormetic dose-dependent changes in Grp78 mRNA and protein expression, suggesting that activation of ER stress is involved in MeHg-induced neurotoxicity. Low level MeHg exposure may induce GRP78 protein expression to stimulate endogenous cytoprotective mechanisms.

Application of Neurochemical Markers for Assessing Health Effects after Developmental Methylmercury and PCB Coexposure

Cholinergic muscarinic receptors (MRs) and monoamine oxidase activity (MAO-B), expressed both in brain and blood cells, were investigated in animals and exposed subjects to assess (i) MeHg (0.5–1 mg/kg/day GD7-PD7) and/or PCB153 (20 mg/kg/day GD10–GD16) effects on cerebellar MAO-B and MRs, and lymphocyte MRs, in dams and offspring 21 days postpartum; (ii) MAO-B in platelets and MRs in lymphocytes of a Faroese 7-year-old children cohort, prenatally exposed to MeHg/PCBs. Animal Data. MAO-B was altered in male cerebellum by MeHg, PCB153, and their combination (35%, 45%, and 25% decrease, resp.). Cerebellar MRs were enhanced by MeHg alone in dams (87%) and male pups (27%). PCB153 alone and in mixture did not modify cerebellar MRs. Similarly to brain, lymphocyte MRs were enhanced in both dams and offspring by MeHg alone. All changes were caused by 1 MeHg mg/kg/day, the lower dose was ineffective. Human Data. Both biomarkers showed homogeneous distributions within the cohort (MRs, range 0.1–36.78 fmol/million cells; MAO-B, 0.95–14.95 nmol/mg protein/h). No correlation was found between the two biomarkers and neurotoxicant concentrations in blood (pre- and postnatally).

Red fox Vulpes vulpes (L., 1758) as a bioindicator of mercury contamination in terrestrial ecosystems of north-western Poland

In this study, we determined the concentrations of total mercury (Hg) in samples of liver, kidney and skeletal muscle of 27 red foxes Vulpes vulpes (L., 1758) from north-western Poland, and examined the morphometric characteristics of the collected specimens. The analysis also included the relationship between Hg concentration and the fox size, and the suitability of individual organs as bioindicators in indirect evaluation of environmental mercury contamination. Determination of Hg concentration was performed by atomic absorption spectroscopy. In the analysed samples, the Hg concentration was low and the maximum value did not exceed 0.85 mgHg/kg dry weight (dw). There were no significant differences in Hg concentrations in the analysed material between males and females or between immature and adult groups. The median concentrations of Hg in the liver, kidney and skeletal muscle were 0.22, 0.11 and 0.05 mgHg/kg dw, respectively. The correlation coefficients were significant between the concentrations of mercury in the liver, kidney and skeletal muscle (positive) and between the kidney Hg concentration and kidney mass (negative). Taking into account our results and findings of other authors, it may be argued that the red fox exhibits a measurable response to mercury environmental pollution and meets the requirements of a bioindicator.

Mangiferin, a dietary xanthone protects against mercury-induced toxicity in HepG2 cells

Mercury is one of the noxious heavy metal environmental toxicants and is a cause of concern for human exposure. Mangiferin (MGN), a glucosylxanthone found in Mangifera indica, reported to have a wide range of pharmacological properties. The objective of this study was to evaluate the cytoprotective potential of MGN, against mercury chloride (HgCl(2) ) induced toxicity in HepG2 cell line. The cytoprotective effect of MGN on HgCl(2) induced toxicity was assessed by colony formation assay, while antiapoptotic effect by fluorescence microscopy, flow cytometric DNA analysis, and DNA fragmentation pattern assays. Further, the cytoprotective effect of MGN against HgCl(2) toxicity was assessed by using biochemical parameters like reduced glutathione (GSH), glutathione-S-transferase (GST), superoxide dismutase (SOD), catalase (CAT) by spectrophotometrically, mitochondrial membrane potential by flowcytometry and the changes in reactive oxygen species levels by DCFH-DA spectrofluoremetric analysis. A significant increase in the surviving fraction was observed with 50 μM of MGN administered two hours prior to various concentrations of HgCl(2) . Further, pretreatment of MGN significantly decreased the percentage of HgCl(2) induced apoptotic cells. Similarly, the levels of ROS generated by the HgCl(2) treatment were inhibited significantly (P < 0.01) by MGN. MGN also significantly (P < 0.01) inhibited the HgCl(2) induced decrease in GSH, GST, SOD, and CAT levels at all the post incubation intervals. Our study demonstrated the cytoprotective potential of MGN, which may be attributed to quenching of the ROS generated in the cells due to oxidative stress induced by HgCl(2) , restoration of mitochondrial membrane potential and normalization of cellular antioxidant levels.

Methylmercury: a potential environmental risk factor contributing to epileptogenesis

Epilepsy or seizure disorder is one of the most common neurological diseases in humans. Although genetic mutations in ion channels and receptors and some other risk factors such as brain injury are linked to epileptogenesis, the underlying cause for the majority of epilepsy cases remains unknown. Gene-environment interactions are thought to play a critical role in the etiology of epilepsy. Exposure to environmental chemicals is an important risk factor. Methylmercury (MeHg) is a prominent environmental neurotoxicant, which targets primarily the central nervous system (CNS). Patients or animals with acute or chronic MeHg poisoning often display epileptic seizures or show increased susceptibility to seizures, suggesting that MeHg exposure may be associated with epileptogenesis. This mini-review highlights the effects of MeHg exposure, especially developmental exposure, on the susceptibility of humans and animals to seizures, and discusses the potential role of low level MeHg exposure in epileptogenesis. This review also proposes that a preferential effect of MeHg on the inhibitory GABAergic system, leading to disinhibition of excitatory glutamatergic function, may be one of the potential mechanisms underlying MeHg-induced changes in seizure susceptibility.

Spatial and temporal variations of total and methylmercury concentrations in plankton from a mercury-contaminated and eutrophic reservoir in Guizhou Province, China

Total mercury (THg) and methylmercury (MeHg) concentrations in four size fractions of plankton from three sampling stations in the Hg-contaminated and eutrophic Baihua Reservoir, Guizhou, China, were investigated for biomagnification and trophic transfer of Hg at different sites with various proximity to the major point sources of nutrients and metals. Total Hg concentrations in plankton of the various size fractions varied from 49 to 5,504 ng g(-1) and MeHg concentrations ranged from 3 to 101 ng g(-1). The percentage of Hg as MeHg varied from 0.16 to 70%. Total Hg and MeHg concentrations in plankton samples differed among the three sampling stations with different proximities from the major point sources. The plankton from the site closest to the dam contained the highest concentrations of MeHg. The successive increase of the ratios of MeHg to Hg from seston to macroplankton at all sites indicated that biomagnification is occurring along the plankton food web. However, biomagnification factors (BMF) for MeHg were low (1.5-2.0) between trophic levels. Concentrations of THg in seston decreased with an increase of chlorophyll concentrations, suggesting a significant dilution effect by the algae bloom for Hg. Eutrophication dilution may be a reason for lower MeHg accumulation by the four size classes of plankton in this Hg-contaminated reservoir.

Embryotoxicity assessment of developmental neurotoxicants using a neuronal endpoint in the embryonic stem cell test

The embryonic stem cell test (EST) is a validated in vitro embryotoxicity test; however, as the inhibition of cardiac differentiation alone is used as a differentiation endpoint in the EST, it may not be a useful test to screen embryotoxic chemicals that affect the differentiation of noncardiac tissues. Previously, methylmercury (MeHg), cadmium and arsenic compounds, which are heavy metals that induce developmental neurotoxicity in vivo, were misclassified as nonembryotoxic with the EST. The aim of this study was to improve the EST to correctly screen such developmental neurotoxicants. We developed a neuronal endpoint (Tuj-1 ID₅₀) using flow cytometry analysis of Tuj-1-positive cells to screen developmental neurotoxicants (MeHg, valproic acid, sodium arsenate and sodium arsenite) correctly using an adherent monoculture differentiation method. Using Tuj-1 ID₅₀ in the EST instead of cardiac ID₅₀, all of the tested chemicals were classified as embryotoxic, while the negative controls were correctly classified as nonembryotoxic. To support the validity of Tuj-1 ID₅₀) , we compared the results from two experimenters who independently tested MeHg using our modified EST. An additional neuronal endpoint (MAP2 ID₅₀), obtained by analyzing the relative quantity of MAP2 mRNA, was used to classify the same chemicals. There were no significant differences in the three endpoint values of the two experimenters or in the classification results, except for isoniazid. In conclusion, our results indicate that Tuj-1 ID₅₀ can be used as a surrogate endpoint of the traditional EST to screen developmental neurotoxicants correctly and it can also be applied to other chemicals.

Changes in metabolic enzymes, cortisol and glucose concentrations of Beluga (Huso huso) exposed to dietary methylmercury

In this paper, effects of dietary methylmercury (MeHg) on several blood biochemical parameters including GLU (glucose), LDH (lactate dehydrogenase), AST (aspartate aminotransferase), ALT (alanine aminotransferase), ALP (alkaline phosphatase) and cortisol were investigated in the Beluga sturgeon (Huso huso). Beluga juveniles were fed for 32 days on four diets containing MeHg (control: 0.04 mg kg⁻¹; low: 0.76 mg kg⁻¹; medium: 7.88 mg kg⁻¹; and high 16.22 mg kg⁻¹ treatment). Significant increases (P < 0.05) were observed in all biochemical parameters, except ALP levels, which decreased significantly (P < 0.05) compared to the control group with either dose- or time-dependent effects. These results suggest that long-term dietary MeHg exposure may affect metabolic enzyme activity and glucose levels in Belugas. These findings provide useful information for environmental and fishery officials to apply in future decisions for managing fish resources in Caspian Sea.

Mangiferin attenuates methylmercury induced cytotoxicity against IMR-32, human neuroblastoma cells by the inhibition of oxidative stress and free radical scavenging potential

Mangiferin (MGN), a C-glucosylxanthone was investigated for its ability to protect against methylmercury (MeHg) induced neurotoxicity by employing IMR-32 (human neuroblastoma) cell line. MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] and clonogenic cell survival assays confirmed the efficacy of MGN supplementation in attenuating MeHg-induced cytotoxicity. Pre-treatment with MGN significantly (p<0.01) inhibited MeHg-induced DNA damage (micronuclei, olive tail moment and % tail DNA) thereby demonstrating MGN's antigenotoxic potential. Also, pre-treatment with MGN significantly reduced MeHg-induced oxidative stress, intra-cellular Ca(2+) influx and inhibited depolarization of mitochondrial membrane. MGN pre-treated cells demonstrated a significant (p<0.05) increase in the GSH and GST levels followed by a significant (p<0.05) decrease in malondialdehyde (MDA) formation. In addition, inhibition of MeHg induced apoptotic cell death by MGN was demonstrated by microscopic, Annexin-V FITC and DNA fragmentation assays and further confirmed by western blot analysis. The present findings indicated the protective effect of MGN against MeHg induced toxicity, which may be attributed to its anti-genotoxic, anti-apoptotic and anti-lipid peroxidative potential plausibly because of its free radical scavenging ability, which reduced the oxidative stress and in turn facilitated the down-regulation of mitochondrial apoptotic signalling pathways.

Proteome profiles in medaka (Oryzias melastigma) liver and brain experimentally exposed to acute inorganic mercury

Mercury is a widespread and persistent pollutant occurring in a variety of forms in freshwater and marine ecosystems. Using the proteomic approach, this study examined the protein profiles of the medaka (Oryzias melastigma) liver and brain exposed to an acute mercuric chloride (HgCl(2)) concentration (1000μg/L) for 8h. The results showed that acute exposure of medaka to inorganic mercury enhanced metal accumulation in both the liver and brain, and a higher content of mercury was detected in the latter. Comparison of the two-dimensional electrophoresis protein profiles of HgCl(2)-exposed and non-exposed group revealed that altered protein expression was quantitatively detected in 20 spots in the brain and 27 in the liver. The altered protein spots were subjected to matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis, with the resultant identification of 46 proteins. The proteins identified were involved in oxidative stress, cytoskeletonal assembly, signal transduction, protein modification, metabolism and other related functions (e.g. immune response, ionoregulation and transporting), highlighting the fact that inorganic mercury toxicity in fish seems to be complex and diverse. This study provided basic information to aid our understanding of the possible molecular mechanisms of acute inorganic mercury toxicity in aquatic organisms, as well as potential protein biomarker candidates for aquatic environmental monitoring.

The plausibility of a role for mercury in the etiology of autism: a cellular perspective

Autism is defined by a behavioral set of stereotypic and repetitious behavioral patterns in combination with social and communication deficits. There is emerging evidence supporting the hypothesis that autism may result from a combination of genetic susceptibility and exposure to environmental toxins at critical moments in development. Mercury (Hg) is recognized as a ubiquitous environmental neurotoxin and there is mounting evidence linking it to neurodevelopmental disorders, including autism. Of course, the evidence is not derived from experimental trials with humans but rather from methods focusing on biomarkers of Hg damage, measurements of Hg exposure, epidemiological data, and animal studies. For ethical reasons, controlled Hg exposure in humans will never be conducted. Therefore, to properly evaluate the Hg-autism etiological hypothesis, it is essential to first establish the biological plausibility of the hypothesis. This review examines the plausibility of Hg as the primary etiological agent driving the cellular mechanisms by which Hg-induced neurotoxicity may result in the physiological attributes of autism. Key areas of focus include: (1) route and cellular mechanisms of Hg exposure in autism; (2) current research and examples of possible genetic variables that are linked to both Hg sensitivity and autism; (3) the role Hg may play as an environmental toxin fueling the oxidative stress found in autism; (4) role of mitochondrial dysfunction; and (5) possible role of Hg in abnormal neuroexcitory and excitotoxity that may play a role in the immune dysregulation found in autism. Future research directions that would assist in addressing the gaps in our knowledge are proposed.

Burdens of mercury in residents of Temirtau, Kazakhstan I: hair mercury concentrations and factors of elevated hair mercury levels

Mercury (Hg) is released either naturally in the environment or by anthropogenic activities. During its global circulation, Hg presents in a diversity of chemical forms and transforms between each other. Among Hg species, methylmercury (MeHg) is readily absorbed by humans via the aquatic food chain and thus it is very neurotoxic to exposed populations including fetuses due to perinatal exposure. In 2005, a survey was carried out in Temirtau, an Hg-contaminated site in North Central Kazakhstan, to investigate Hg concentrations in the hair samples of the residents and the relationship between Hg exposure levels and the related factors. Among the 289 hair samples, Hg concentrations ranged from 0.009 to 5.184µg/g with a mean of 0.577µg/g. Nearly 17% of the population exceeded 1µg/g for hair Hg, which corresponds to the reference of dose (RfD) 0.1µg/kg body weight/day developed by the United States Environmental Protection Agency (USEPA). Subgroups of males, people aged over 45 and fishermen or anglers were found to have elevated Hg exposure levels in their hair. A positive correlation was found between Hg concentrations in hair and frequencies of river fish consumption. As a result, the finding that people were exposed to high levels of Hg was expected due to the frequent consumption of fish caught from the polluted River Nura or the neighbouring lakes. A regression model showed that approximately 41% of variance of Hg concentrations in the study population's hair was attributed to the variables of gender, residential location, age and fishery occupation. The model implied that demographic characteristics together with dietary behaviour should be taken into account in studies associated with Hg exposure risk, in order to clearly define the group potentially sensitive to Hg exposure.

Gene expression changes in female zebrafish (Danio rerio) brain in response to acute exposure to methylmercury

Methylmercury (MeHg) is a potent neurotoxicant and endocrine disruptor that accumulates in aquatic systems. Previous studies have shown suppression of hormone levels in both male and female fish, suggesting effects on gonadotropin regulation in the brain. The gene expression profile in adult female zebrafish whole brain induced by acute (96 h) MeHg exposure was investigated. Fish were exposed by injection to 0 or 0.5 µg MeHg/g. Gene expression changes in the brain were examined using a 22,000-feature zebrafish microarray. At a significance level of p < 0.01, 79 genes were up-regulated and 76 genes were down-regulated in response to MeHg exposure. Individual genes exhibiting altered expression in response to MeHg exposure implicate effects on glutathione metabolism in the mechanism of MeHg neurotoxicity. Gene ontology (GO) terms significantly enriched among altered genes included protein folding, cell redox homeostasis, and steroid biosynthetic process. The most affected biological functions were related to nervous system development and function, as well as lipid metabolism and molecular transport. These results support the involvement of oxidative stress and effects on protein structure in the mechanism of action of MeHg in the female brain. Future studies will compare the gene expression profile induced in response to MeHg with that induced by other toxicants and will investigate responsive genes as potential biomarkers of MeHg exposure.

Effects of methylmercury on male reproductive functions in Wistar rats

In this study we investigated the effects of subacute exposure to methylmercury (MeHg) on male reproductive functions in rats by means of determination of alterations in structural and functional parameters. Adult male Wistar rats received 0, 0.5, 1.0 or 3.0 mg/kg/body weight/day orally, daily MeHg for 14 days. Sperm motility, the relative sperm count and transit time in the caput/corpus epididymis, were all reduced at all doses. The lowest dose increased the number of sperm head abnormalities; daily sperm production was elevated at the intermediate dose; while at the highest dose there was a decrease in serum testosterone levels and a rise in mercury (Hg) content in reproductive organs, liver and kidneys. In conclusion, MeHg exposure produced damages on male reproductive functions which may be attributed, at least in part, to the reduction in serum testosterone levels. These consequences could potentially result in infertility in rats.

Rapid methods to detect organic mercury and total selenium in biological samples

BACKGROUND

Organic mercury (Hg) is a global pollutant of concern and selenium is believed to afford protection against mercury risk though few approaches exist to rapidly assess both chemicals in biological samples. Here, micro-scale and rapid methods to detect organic mercury (< 1.5 ml total sample volume, < 1.5 hour) and total selenium (Se; < 3.0 ml total volume, < 3 hour) from a range of biological samples (10-50 mg) are described.

RESULTS

For organic Hg, samples are digested using Tris-HCl buffer (with sequential additions of protease, NaOH, cysteine, CuSO4, acidic NaBr) followed by extraction with toluene and Na2S2O3. The final product is analyzed via commercially available direct/total mercury analyzers. For Se, a fluorometric assay has been developed for microplate readers that involves digestion (HNO3-HClO4 and HCl), conjugation (2,3-diaminonaphthalene), and cyclohexane extraction. Recovery of organic Hg (86-107%) and Se (85-121%) were determined through use of Standard Reference Materials and lemon shark kidney tissues.

CONCLUSIONS

The approaches outlined provide an easy, rapid, reproducible, and cost-effective platform for monitoring organic Hg and total Se in biological samples. Owing to the importance of organic Hg and Se in the pathophysiology of Hg, integration of such methods into established research monitoring efforts (that largely focus on screening total Hg only) will help increase understanding of Hg's true risks.

Protective effects of Syzygium cumini seed extract against methylmercury-induced sistemic toxicity in neonatal rats

Syzygium cumini (L.) Skeels (Sc) belongs to the medicinal plants with an important source of phenolic compounds. Sc has been shown to possess antioxidant and anti-inflammatory properties. Methylmercury (MeHg), a highly toxic environmental pollutant, induces oxidative stress and dysfunction in many cell types. This study was aimed to evaluate the effect of aqueous seed extract of Sc (ASc) on MeHg-induced toxicity in rats. Two-day-old rats (P2) received a single dose of MeHg (10 mg/kg) and two doses of ASc (0.9 mg/kg) per os. After two days, the effects of the treatment were investigated in the cerebral cortex, hippocampus, kidney, liver and urine samples. Our results demonstrated that N-acetyl-β-D: -glucosaminidase (NAG) activity in the kidney and urine, the lipid peroxidation levels in the liver and kidney samples, as well as the adenosine deaminase (ADA) activity in the hippocampus, kidney and liver were higher in MeHg-group when compared to the control group. The administration of ASc reverted the toxic effects of MeHg. It is noteworthy to observe that the main compounds present in the ASc, as gallic acid (the major component), chlorogenic acid and rutin, might be the responsible for such benefit, since they were found to display antioxidant properties.

Neuroendocrine disruption: more than hormones are upset

Only a small proportion of the published research on endocrine-disrupting chemicals (EDC) directly examined effects on neuroendocrine processes. There is an expanding body of evidence that anthropogenic chemicals exert effects on neuroendocrine systems and that these changes might impact peripheral organ systems and physiological processes. Neuroendocrine disruption extends the concept of endocrine disruption to include the full breadth of integrative physiology (i.e., more than hormones are upset). Pollutants may also disrupt numerous other neurochemical pathways to affect an animal's capacity to reproduce, develop and grow, or deal with stress and other challenges. Several examples are presented in this review, from both vertebrates and invertebrates, illustrating that diverse environmental pollutants including pharmaceuticals, organochlorine pesticides, and industrial contaminants have the potential to disrupt neuroendocrine control mechanisms. While most investigations on EDC are carried out with vertebrate models, an attempt is also made to highlight the importance of research on invertebrate neuroendocrine disruption. The neurophysiology of many invertebrates is well described and many of their neurotransmitters are similar or identical to those in vertebrates; therefore, lessons learned from one group of organisms may help us understand potential adverse effects in others. This review argues for the adoption of systems biology and integrative physiology to address the effects of EDC. Effects of pulp and paper mill effluents on fish reproduction are a good example of where relatively narrow hypothesis testing strategies (e.g., whether or not pollutants are sex steroid mimics) have only partially solved a major problem in environmental biology. It is clear that a global, integrative physiological approach, including improved understanding of neuroendocrine control mechanisms, is warranted to fully understand the impacts of pulp and paper mill effluents. Neuroendocrine disruptors are defined as pollutants in the environment that are capable of acting as agonists/antagonists or modulators of the synthesis and/or metabolism of neuropeptides, neurotransmitters, or neurohormones, which subsequently alter diverse physiological, behavioral, or hormonal processes to affect an animal's capacity to reproduce, develop and grow, or deal with stress and other challenges. By adopting a definition of neuroendocrine disruption that encompasses both direct physiological targets and their indirect downstream effects, from the level of the individual to the ecosystem, a more comprehensive picture of the consequences of environmentally relevant EDC exposure may emerge.

Integrative environmental genomics of Cod (Gadus morhua): the proteomics approach

Atlantic cod (Gadus morhua) is an essential species in North Atlantic fisheries and increasingly relevant as an aquaculture species. However, potential conflicts with both coastal industry and petroleum industry expanding into northern waters make it important to understand how effluents (produced water, pharmaceuticals, food contaminants, and feed contaminants) affect the growth, reproduction, and health of this species in order to maintain a sustainable cod population and a healthy human food source, and to discover biomarkers for environmental monitoring and risk assessment. The ongoing genome sequencing effort of Atlantic cod has opened the possibility for a systems biology approach to elucidate molecular mechanisms of toxicity. Our study aims to be a first step toward such a systems toxicology understanding of genomic responses to environmental insults. A toxicogenomic approach was initiated that is combining data generated from proteomics analyses and transcriptomics analyses, and the concurrent development of searchable expressed sequence tags (EST) databases and genomic databases. This interdisciplinary study may also open new possibilities of gene annotation and pathway analyses.

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Responses in the brain proteome of Atlantic cod (Gadus morhua) exposed to methylmercury

The molecular mechanisms underlying the neurotoxicity of methylmercury (MeHg), a ubiquitous environmental contaminant, are not yet fully understood. Furthermore, there is a lack of biomarkers of MeHg neurotoxicity for use in environmental monitoring. We have undertaken a proteomic analysis of brains from Atlantic cod (Gadus morhua) exposed to 0, 0.5 and 2 mg/kg MeHg administered by intraperitoneal injection. The doses were given in two injections, half of the dose on the first day and the second half after 1 week, and the total exposure period lasted 2 weeks. Using 2-DE coupled with MALDI-TOF MS and MS/MS, we observed the level of 71 protein spots to be 20% or more significantly altered following MeHg exposure, and successfully identified 40 of these protein spots. Many of these proteins are associated with main known molecular targets and mechanisms of MeHg-induced neurotoxicity in mammals, such as mitochondrial dysfunction, oxidative stress, altered calcium homeostasis and tubulin/disruption of microtubules. More interestingly, several of the affected proteins, with well-established or recently demonstrated critical functions in nervous system-specific processes, have not previously been associated with MeHg exposure in any species. These proteins include the strongest up-regulated protein, pyridoxal kinase (essential for synthesis of several neurotransmitters), G protein (coupled to neurotransmitter receptors), nicotinamide phosphoribosyl-transferase (protection against axonal degeneration), dihydropyrimidinase-like 5 (or collapsin response mediator protein 5, CRMP-5) (axon guidance and regeneration), septin (dendrite development), phosphatidylethanolamine binding protein (precursor for hippocampal cholinergic neurostimulating peptide) and protein phosphatase 1 (control of brain recovery by synaptic plasticity). The results of the present study aid our understanding of molecular mechanisms underlying MeHg neurotoxicity and defense responses, and provide a large panel of protein biomarker candidates for aquatic environmental monitoring.

Human cell-based micro electrode array platform for studying neurotoxicity

At present, most of the neurotoxicological analyses are based on in vitro and in vivo models utilizing animal cells or animal models. In addition, the used in vitro models are mostly based on molecular biological end-point analyses. Thus, for neurotoxicological screening, human cell-based analysis platforms in which the functional neuronal networks responses for various neurotoxicants can be also detected real-time are highly needed. Microelectrode array (MEA) is a method which enables the measurement of functional activity of neuronal cell networks in vitro for long periods of time. Here, we utilize MEA to study the neurotoxicity of methyl mercury chloride (MeHgCl, concentrations 0.5–500 nM) to human embryonic stem cell (hESC)-derived neuronal cell networks exhibiting spontaneous electrical activity. The neuronal cell cultures were matured on MEAs into networks expressing spontaneous spike train-like activity before exposing the cells to MeHgCl for 72 h. MEA measurements were performed acutely and 24, 48, and 72 h after the onset of the exposure. Finally, exposed cells were analyzed with traditional molecular biological methods for cell proliferation, cell survival, and gene and protein expression. Our results show that 500 nM MeHgCl decreases the electrical signaling and alters the pharmacologic response of hESC-derived neuronal networks in delayed manner whereas effects can not be detected with qRT-PCR, immunostainings, or proliferation measurements. Thus, we conclude that human cell-based MEA platform is a sensitive online method for neurotoxicological screening.

Hair methylmercury: a new indication for therapeutic monitoring

Maternal exposure to methylmercury can adversely affect fetal neurodevelopment. Long-term mercury exposure is best estimated by hair measurement of the metal. The authors analyzed the appropriateness of therapeutic monitoring of hair mercury in women of reproductive age using widely accepted criteria for therapeutic drug monitoring. The analysis reveals that such monitoring can help protect babies from long-term adverse effects, while ensuring appropriate maternal fish consumption.

Cipura paludosa attenuates long-term behavioral deficits in rats exposed to methylmercury during early development

In the present study, we evaluated the effects of the ethanolic extract (EE) of Cipura paludosa on locomotor, and anxiety- and depression-like behaviors of adult rats exposed to MeHg during early development. Additionally, the antioxidant enzymes catalase (CAT) and selenium-glutathione peroxidase (Se-GPx) were measured in cortical, hippocampal, and cerebellar tissues. Pregnant Wistar rats were treated by gavage with a single dose of MeHg (8 mg/kg) on gestational day 15, the developmental stage critical for cortical neuron proliferation. Moreover, prenatal MeHg exposure inhibited CAT and Se-GPx in the cortex and cerebellum. Chronic treatment with the EE of C. paludosa attenuated these emotional and antioxidant deficits induced by prenatal MeHg toxic exposure. This study provides novel evidence that developmental exposure to MeHg can affect not only cognitive functions but also locomotor, and anxiety- and depression-like behaviors.

Molecular cloning of cDNA of mammalian and chicken II gonadotropin-releasing hormones (mGnRHs and cGnRH-II) in the beluga (Huso huso) and the disruptive effect of methylmercury on gene expression

Two gonadotropin-releasing hormone (GnRH) isoforms were identified in the beluga (Huso huso) brain by cDNA sequencing: prepro-mammalian GnRH (mGnRH) and prepro-chicken GnRH-II (cGnRH-II). The nucleotide sequences of the beluga mGnRH and cGnRH-II precursors are 273 and 258 base pairs (bp) long, encoding peptides of 91 and 86 amino acids, respectively. To investigate the effect of methylmercury (MeHg) on GnRH gene expression, animals were fed with four diets containing increasing levels of MeHg (0 mg kg(-1) [control]; 0.76 mg kg(-1) [low]; 7.8 mg kg(-1) [medium]; 16.22 mg kg(-1) [high]) for 32 days. The effects of MeHg on brain GnRH mRNA levels were evaluated by real-time PCR. A significant decrease in brain mGnRH and cGnRH-II mRNA levels were detected in fish receiving high dietary MeHg dose compared to controls on day 11 (P < 0.05). On day 18 and 32, all treatment groups had significantly lower brain mGnRH and cGnRH-II mRNA levels compared to the control group (P < 0.05). These findings demonstrate a disruptive role of MeHg on the level of brain mGnRH and cGnRH-II mRNAs in immature beluga.

The relevance of the individual genetic background for the toxicokinetics of two significant neurodevelopmental toxicants: mercury and lead

The heavy metals mercury and lead are well-known and significant developmental neurotoxicants. This review summarizes the genetic factors that modify their toxicokinetics. Understanding toxicokinetics (uptake, biotransformation, distribution, and elimination processes) is a key precondition to understanding the individual health risks associated with exposure. We selected candidate susceptibility genes when evidence was available for (1) genes/proteins playing a significant role in mercury and lead toxicokinetics, (2) gene expression/protein activity being induced by these metals, and (3) mercury and lead toxicokinetics being affected by gene knockout/knockdown or (4) by functional gene polymorphisms. The genetic background is far better known for mercury than for lead toxicokinetics. Involved are genes encoding L-type amino acid transporters, organic anion transporters, glutathione (GSH)-related enzymes, metallothioneins, and transporters of the ABC family. Certain gene variants can influence mercury toxicokinetics, potentially explaining part of the variable susceptibility to mercury toxicity. Delta-aminolevulinic acid dehydratase (ALAD), vitamin D receptor (VDR) and hemochromatosis (HFE) gene variants are the only well-established susceptibility markers of lead toxicity in humans. Many gaps remain in our knowledge about the functional genomics of this issue. This calls for studies to detect functional gene polymorphisms related to mercury- and lead-associated disease phenotypes, to demonstrate the impact of functional polymorphisms and gene knockout/knockdown in relation to toxicity, to confirm the in vivo relevance of genetic variation, and to examine gene-gene interactions on the respective toxicokinetics. Another crucial aspect is knowledge on the maternal-fetal genetic background, which modulates fetal exposure to these neurotoxicants. To completely define the genetically susceptible risk groups, research is also needed on the genes/proteins involved in the toxicodynamics, i.e., in the mechanisms causing adverse effects in the brain. Studies relating the toxicogenetics to neurodevelopmental disorders are lacking (mercury) or very scarce (lead). Thus, the extent of variability in susceptibility to heavy metal-associated neurological outcomes is poorly characterized.

Scopoli's work in the field of mercurialism in light of today's knowledge: past and present perspectives

The Idrija Mercury Mine (1490-1994) appointed its first physician, Joannes Antonius Scopoli, in 1754. Most of his descriptions of mercurialism are still relevant today. This study highlights Scopoli's observations on the interaction between elemental mercury (Hg degrees ) and alcohol, on the appearance of lung impairment, insomnia, and depressive mood in mercurialism. This presentation is based on Scopoli's experiences presented in his book, De Hydrargyro Idriensi Tentamina (1761), current knowledge, and our own experience acquired through health monitoring of occupational Hg degrees exposure. Some studies have confirmed Scopoli's observation that alcohol enhances mercurialism and his hypothesis that exposure to high Hg degrees concentrations causes serious lung impairment. Neurobiological studies have highlighted the influence of Hg degrees on sleep disorder and depressive mood observed by Scopoli. Although today's knowledge provides new perspectives of Scopoli's work on mercurialism, his work is still very important and can be considered a part of occupational medicine heritage.

Mercury-induced hepatotoxicity in zebrafish: in vivo mechanistic insights from transcriptome analysis, phenotype anchoring and targeted gene expression validation

Background

Mercury is a prominent environmental contaminant that causes detrimental effects to human health. Although the liver has been known to be a main target organ, there is limited information on in vivo molecular mechanism of mercury-induced toxicity in the liver. By using transcriptome analysis, phenotypic anchoring and validation of targeted gene expression in zebrafish, mercury-induced hepatotoxicity was investigated and a number of perturbed cellular processes were identified and compared with those captured in the in vitro human cell line studies.

Results

Hepato-transcriptome analysis of mercury-exposed zebrafish revealed that the earliest deregulated genes were associated with electron transport chain, mitochondrial fatty acid beta-oxidation, nuclear receptor signaling and apoptotic pathway, followed by complement system and proteasome pathway, and thereafter DNA damage, hypoxia, Wnt signaling, fatty acid synthesis, gluconeogenesis, cell cycle and motility. Comparative meta-analysis of microarray data between zebrafish liver and human HepG2 cells exposed to mercury identified some common toxicological effects of mercury-induced hepatotoxicity in both models. Histological analyses of liver from mercury-exposed fish revealed morphological changes of liver parenchyma, decreased nucleated cell count, increased lipid vesicles, glycogen and apoptotic bodies, thus providing phenotypic evidence for anchoring of the transcriptome analysis. Validation of targeted gene expression confirmed deregulated gene-pathways from enrichment analysis. Some of these genes responding to low concentrations of mercury may serve as toxicogenomic-based markers for detection and health risk assessment of environmental mercury contaminations.

Conclusion

Mercury-induced hepatotoxicity was triggered by oxidative stresses, intrinsic apoptotic pathway, deregulation of nuclear receptor and kinase activities including Gsk3 that deregulates Wnt signaling pathway, gluconeogenesis, and adipogenesis, leading to mitochondrial dysfunction, endocrine disruption and metabolic disorders. This study provides important mechanistic insights into mercury-induced liver toxicity in a whole-animal physiology context, which will help in understanding the syndromes caused by mercury poisoning. The molecular conservation of mercury-induced hepatotoxicity between zebrafish and human cell line reveals the feasibility of using zebrafish to model molecular toxicity in human for toxicant risk assessments.

Interaction between GSTM1/GSTT1 polymorphism and blood mercury on birth weight

BACKGROUND

Mercury (Hg) is toxic to both the reproductive and nervous systems. In addition, glutathione S-transferases (GSTs), which conjugate glutathione to a variety of electrophilic compounds, are involved in the detoxification of Hg.

OBJECTIVE

In this study we examined the association between prenatal exposure to Hg and birth weight as well as the influence of GST polymorphisms.

METHODS

The total Hg concentration in maternal and cord blood was measured from 417 Korean women and newborns in the Mothers and Children's Environmental Health study from 2006 to 2008. Information on birth weight was collected from the patients' medical records. The genotyping of glutathione S-transferase M1 (GSTM1) and glutathione S-transferase T1 (GSTT1) polymorphisms was carried out using polymerase chain reaction. Regression analysis was performed to determine the association between the blood Hg concentration and birth weight in mothers with GSTM1 and GSTT1 polymorphisms.

RESULTS

The geometric mean levels of Hg in the maternal blood during late pregnancy and in cord blood were 3.30 microg/L and 5.53 microg/L, respectively. For mothers with the GSTT1 null genotype, elevated Hg levels in maternal blood during late pregnancy were associated with an increased risk of lower birth weight. For mothers with both GSTM1 and GSTT1 null genotype, both maternal and cord blood Hg levels were associated with lower birth weight.

CONCLUSIONS

This study suggests that the interactions of Hg with GSTM1 and GSTT1 polymorphisms play a role in reducing birth weight.

The impact of methylmercury on 1,25-dihydroxyvitamin D3-induced transcriptomic responses in dolphin skin cells

The Atlantic bottlenose dolphin has been the focus of much attention owing to the considerable impact of environmental stress on its health and the associated implications for human health. Here, we used skin cells from the dolphin to investigate the protective role of the vitamin D pathway against environmental stressors. We previously reported that dolphin skin cells respond to 1,25-dihydroxyvitamin D3 (1,25D3), the bioactive metabolite of vitamin D3, by upregulation of the vitamin D receptor (VDR) and expression of several genes. Methylmercury is a highly bioaccumulative environmental stressor of relevance to the dolphin. We currently report that in dolphin cells sublethal concentrations of methylmercury compromise the ability of 1,25D3 to upregulate VDR, to transactivate a vitamin D-sensitive promoter, and to express specific target genes. These results help elucidate the effects of vitamin D and methylmercury on innate immunity in dolphin skin and potentially in human skin as well, considering similarities in the vitamin D pathway between the two species.

Toxicology of Alkylmercury Compounds

Methylmercury is a global pollutant and potent neurotoxin whose abundance in the food chain mandates additional studies on the consequences and mechanisms of its toxicity to the central nervous system. Formulation of our new hypotheses was predicated on our appreciation for (a) the remarkable affinity of mercurials for the anionic form of sulfhydryl (-SH) groups, and (b) the essential role of thiols in protein biochemistry. The present chapter addresses pathways to human exposure of various mercury compounds, highlighting their neurotoxicity and potential involvement in neurotoxic injury and neurodegenerative changes, both in the developing and senescent brain. Mechanisms that trigger these effects are discussed in detail.

Balancing the risks and the benefits of local fish consumption in Bermuda

Fish consumption today is widely recognized as highly beneficial since it constitutes a good source of several essential nutrients, such as selenium and polyunsaturated fatty acids (n-3 PUFA). However, fish can also contain contaminants such as mercury, which make the consumer, especially pregnant women, confused about the risk-benefit balance associated with fish consumption. This is particularly true for tropical fish species for which little information is available. We have previously reported that some Bermudian neonates had elevated mercury in their umbilical blood compared with international guidelines. The objective of this study was to give precise and balanced information on the content of mercury, selenium and PUFA in the most consumed fish species in Bermuda. In 2003 and 2006, a total of 307 fish were collected from 43 fish species and 351 samples were analysed (305 flesh samples, 44 liver samples, one roe and one fat sample) by inductively coupled plasma-mass spectrometry (ICP-MS) (metals) and high-resolution gas chromatography (HRGC) (fatty acids). Results show that mercury varies among species from 0.03 to 3.3 microg g(-1) and that it is possible for at-risk groups such as pregnant women to make informed choices concerning fish consumption, e.g. maximizing fish species rich in nutrients and low in mercury.

Assessment of toxic elements' content in swine kidneys: Pathomorphological analysis

Background: In order to ensure the safety of consumers in Serbia, the toxic elements' (Cd, Hg, As, Pb) content in swine kidneys collected from three different sites in Serbia (n=90), were determined by atomic absorption spectrometry. Also, in order to find information on the effects of accumulation of toxic elements in swine kidneys, histopathological examination of kidneys was carried out. Methods: Determination of toxic elements (Cd, Hg, As, Pb) in swine kidneys was performed by atomic absorption spectrometry. For microscopic examination, kidney samples were fixed in 10% neutral buffered formalin and absolute alcohol for 5 to 7 days, processed by routine methods, sectioned at 5-8 ?m, and stained with hematoxylin and eosin (HE) for light microscopy. Results: The presence of mercury was found in 33.3% of kidney samples in the range of 0.005-0.055 mg/kg, while presence of cadmium was found in less degree (27.7%) but in higher content (0.05-1,23 mg/kg). The presence of arsenic was found only in one sample, while presence of lead was not found. The metal-to-metal correlation analysis supported the theory that there were different sources of contamination. Histopathological examination of kidneys confirms tubulopathies with edema and cell vacuolization. In addition, hemorrhages and necrosis of proximal kidney tubules' cells were found. Conclusion: This study shows the presence of toxic elements in pigs butchered in Serbia at levels comparable to those reported in other countries., and consequently do not represent any concern from a consumer safety point of view. The lack of strong correlation between histopathological changes and incidence of toxic elements found in our trial may explain the possible synergism among toxic elements and other nephrotoxic compounds, which enhances the toxicity, especially in cases of low contamination.

Toxicological assessment of toxic element residues in swine kidney and its role in public health risk assessment

In order to ensure the safety of consumers in Serbia the prevalence of toxic elements (As, Cd, Hg, Pb) in swine kidney collected from three different areas in Serbia (n = 90) was determined by atomic absorption spectrometry. Also, in order to find information on the effects of accumulation of toxic elements on swine kidney, pathohistological examination of the kidneys was performed. The presence of mercury was found in 33.3% of kidney samples in the range of 0.005-0.055 mg/kg, while the presence of cadmium was detected less often (27.7%) but in larger amounts (0.05-1.23 mg/kg). The presence of arsenic was found only in one sample, while no lead was found. The results of the metal-to-metal correlation analysis supported there were the result of different sources of contamination. Pathohistological examination of kidneys confirms tubulopathies with oedema and cell vacuolization. In addition, haemorrhages and necrosis of proximal kidney tubule cells were found. This study demonstrates that toxic elements in Serbian slaughtered pigs are found at levels comparable to those reported in other countries, and consequently the levels reported in this study do not represent a concern from a consumer safety point of view. The lack of a strong correlation between histopathological changes and the incidence of toxic elements found in this study might be explained as the result of synergism among toxic elements and other nephrotoxic compounds which enhance the toxicity of the individual toxins even at the relatively low mean concentrations observed in this study.

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.

Comparative evaluation of adenosine deaminase activity in cerebral cortex and hippocampus of young and adult rats: effect of garlic extract (Allium sativum L.) on their susceptibility to heavy metal exposure

Adenosine plays an important neuromodulatory role in the central nervous system, and adenosine deaminase is an important enzyme in the degradation of adenine nucleotides. Methylmercury is the most prevalent form of mercury found in the environment. Methylmercury neurotoxicity has been correlated to the production of reactive oxygen species. In this study, its potential pathogenic effects were investigated in vitro in cerebral cortex and hippocampus of rats. We first observed that adenosine deaminase activity was higher in young rat brains when compared to the 60-day-old rats and was higher in hippocampus when compared to the cortex. Methylmercury (0.1, 1.0, 20 microM) inhibited adenosine deaminase activity in 7- and 60-day-old rats in a concentration-dependent manner. We have demonstrated that methylmercury-induced inhibition was antagonized by garlic alcoholic extract, but sodium selenate did not alter enzyme activity. In addition, glutathione and dithiothreitol restored the methylmercury-induced decrease of adenosine deaminase activity. These results demonstrated that there are age-related changes in adenosine deaminase activity and that thiol agents may contribute to the maintenance of adenosine deaminase activity and may be important in the neuromodulation of adenosine. Garlic alcoholic extract may be effective in reducing the effect of methylmercury-induced adenosine deaminase, which may be due to its sulphur-containing compounds.

Modifications in rat testicular morphology and increases in IFN-gamma serum levels by the oral administration of subtoxic doses of mercuric chloride

Mercury induces structural and functional damage in several organs, however the effects of subtoxic doses of the metal on the male reproductive system are not well defined. In order to analyze testicular and epididymal morphological alterations and changes in IL-4 or IFN-gamma serum levels, adult male Sprague-Dawley rats received 0.01, 0.05 or 0.1 microg/ml of mercuric chloride (HgCl(2)) in deionized water for 1 to 7 months by oral route. Controls received deionized water alone. Twenty rats, separated in four groups of five animals each, were used per time of exposure. Progressive degenerative lesions consisting of lack of germ cell cohesion and desquamation, arrest at spermatocyte stage and hypospermatogenesis were observed in seminiferous epithelium by light and electron microscopy. Leydig cells showed cytoplasmic vacuolation and nuclear signs of cell death. Loss of peritubular cell aggregation was evidenced in the epididymis. Mercury accumulation was detected in both organs by mass spectroscopy. Rats showed enhanced IFN-gamma serum levels as compared to controls but only reached significance after 7 months of mercury administration. Subtoxic doses of inorganic mercury could lead to reproductive and immunological alterations. The results demonstrate that sublethal concentrations of mercuric chloride are enough to induce morphological and ultrastructural modifications in male reproductive organs. These contribute to functional alterations of spermatogenesis with arrest at spermatocyte stage, hypospermatogenesis and possibly impaired steroidogenesis which together could affect male fertility.

Human neurospheres as three-dimensional cellular systems for developmental neurotoxicity testing

BACKGROUND

Developmental neurotoxicity (DNT) of environmental chemicals is a serious threat to human health. Current DNT testing guidelines propose investigations in rodents, which require large numbers of animals. With regard to the "3 Rs" (reduction, replacement, and refinement) of animal testing and the European regulation of chemicals [Registration, Evaluation, and Authorisation of Chemicals (REACH)], alternative testing strategies are needed in order to refine and reduce animal experiments and allow faster and less expensive screening.

OBJECTIVES

The goal of this study was to establish a three-dimensional test system for DNT screening based on human fetal brain cells.

METHODS

We established assays suitable for detecting disturbances in basic processes of brain development by employing human neural progenitor cells (hNPCs), which grow as neurospheres. Furthermore, we assessed effects of mercury and oxidative stress on these cells.

RESULTS

We found that human neurospheres imitate proliferation, differentiation, and migration in vitro. Exposure to the proapoptotic agent staurosporine further suggests that human neurospheres possess functioning apoptosis machinery. The developmental neurotoxicants methylmercury chloride and mercury chloride decreased migration distance and number of neuronal-like cells in differentiated hNPCs. Furthermore, hNPCs undergo caspase-independent apoptosis when exposed toward high amounts of oxidative stress.

CONCLUSIONS

Human neurospheres are likely to imitate basic processes of brain development, and these processes can be modulated by developmental neurotoxicants. Thus, this three-dimensional cell system is a promising approach for DNT testing.

Characterization of the effects of methylmercury on Caenorhabditis elegans

The rising prevalence of methylmercury (MeHg) in seafood and in the global environment provides an impetus for delineating the mechanism of the toxicity of MeHg. Deleterious effects of MeHg have been widely observed in humans and in other mammals, the most striking of which occur in the nervous system. Here we test the model organism, Caenorhabditis elegans (C. elegans), for MeHg toxicity. The simple, well-defined anatomy of the C. elegans nervous system and its ready visualization with green fluorescent protein (GFP) markers facilitated our study of the effects of methylmercuric chloride (MeHgCl) on neural development. Although MeHgCl was lethal to C. elegans, induced a developmental delay, and decreased pharyngeal pumping, other traits including lifespan, brood size, swimming rate, and nervous system morphology were not obviously perturbed in animals that survived MeHgCl exposure. Despite the limited effects of MeHgCl on C. elegans development and behavior, intracellular mercury (Hg) concentrations (<or=3 ng Hg/mg protein) in MeHgCl-treated nematodes approached levels that are highly toxic to mammals. If MeHgCl reaches these concentrations throughout the animal, this finding indicates that C. elegans cells, particularly neurons, may be less sensitive to MeHgCl toxicity than mammalian cells. We propose, therefore, that C. elegans should be a useful model for discovering intrinsic mechanisms that confer resistance to MeHgCl exposure.

Guanosine and synthetic organoselenium compounds modulate methylmercury-induced oxidative stress in rat brain cortical slices: involvement of oxidative stress and glutamatergic system

Excessive formation of reactive oxygen species (ROS) and disruption of glutamate uptake have been pointed as two key mechanisms in methylmercury-toxicity. Thus, here we investigate the involvement of glutamatergic system in methylmercury (MeHg) neurotoxicity and whether diphenyl diselenide, ebselen and guanosine could protect cortical rat brain slices from MeHg-induced ROS generation. MeHg (100 and 200 microM) increased 2',7'-dichlorodihydrofluorescin (DCFH) oxidation after 2h of exposure. At 50 microM, MeHg increased DCFH oxidation only after 5h of exposure. Guanosine (1 and 5 microM) did not caused any effect per se; however, it blocked the increase in DCFH caused by 200 or 50 microM MeHg. Ebselen (5 and 10 microM) decreased significantly the DCFH oxidation after 2 and 5h of exposure to MeHg. Diphenyl diselenide (5 microM) did not change the basal DCFH oxidation, but abolished the pro-oxidant effect of MeHg. MK-801 also abolished the pro-oxidant effect of MeHg. These results demonstrate for the first time the potential antioxidant properties of organoseleniun compounds and guanosine against MeHg-induced ROS generation after short-term exposure in a simple in vitro model. In conclusion, endogenous purine (guanosine) and two synthetic organoselenium compounds can modulate the pro-oxidant effect of MeHg in cortical brain slices.