Cell viability and proteomic analysis in cultured neurons exposed to methylmercury

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

Astrocyte-Like Cells Transcriptome Changes After Exposure to a Low and Non-cytotoxic MeHg Concentration

The central nervous system is the main target of MeHg toxicity and glial cells are the first line of defense; however, their true role remains unclear. This study aimed to identify the global map of human glial-like (U87) cells transcriptome after exposure to a non-toxic and non-lethal MeHg concentration and to investigate the related molecular changes. U87 cells were exposed upon 0.1, 0.5, and 1 µM MeHg for 4 and 24 h. Although no changes were observed in the percentage of viable cells, the metabolic viability was significantly decreased after exposure to 1 µM MeHg for 24 h; thus, the non-toxic concentration of 0.1 µM MeHg was chosen to perform microarray analysis. Significant changes in U87 cells transcriptome were observed only after 24 h. The expression of 392 genes was down regulated while 431 genes were up-regulated. Gene ontology showed alterations in biological processes (75%), cellular components (21%), and molecular functions (4%). The main pathways showed by KEGG and Reactome were cell cycle regulation and Rho GTPase signaling. The complex mechanism of U87 cells response against MeHg exposure indicates that even a low and non-toxic concentration is able to alter the gene expression profile.

Metal Profiles in Autism Spectrum Disorders: A Crosstalk between Toxic and Essential Metals

Since hundreds of years ago, metals have been recognized as impacting our body's physiology. As a result, they have been studied as a potential cure for many ailments as well as a cause of acute or chronic poisoning. However, the link between aberrant metal levels and neuropsychiatric illnesses such as schizophrenia and neurodevelopmental disorders, such as autism spectrum disorders (ASDs), is a relatively new finding, despite some evident ASD-related consequences of shortage or excess of specific metals. In this review, we will summarize past and current results explaining the pathomechanisms of toxic metals at the cellular and molecular levels that are still not fully understood. While toxic metals may interfere with dozens of physiological processes concurrently, we will focus on ASD-relevant activity such as inflammation/immune activation, mitochondrial malfunction, increased oxidative stress, impairment of axonal myelination, and synapse formation and function. In particular, we will highlight the competition with essential metals that may explain why both the presence of certain toxic metals and the absence of certain essential metals have emerged as risk factors for ASD. Although often investigated separately, through the agonistic and antagonistic effects of metals, a common metal imbalance may result in relation to ASD.

Mechanisms of Metal-Induced Mitochondrial Dysfunction in Neurological Disorders

Metals are actively involved in multiple catalytic physiological activities. However, metal overload may result in neurotoxicity as it increases formation of reactive oxygen species (ROS) and elevates oxidative stress in the nervous system. Mitochondria are a key target of metal-induced toxicity, given their role in energy production. As the brain consumes a large amount of energy, mitochondrial dysfunction and the subsequent decrease in levels of ATP may significantly disrupt brain function, resulting in neuronal cell death and ensuing neurological disorders. Here, we address contemporary studies on metal-induced mitochondrial dysfunction and its impact on the nervous system.

Environmentally relevant developmental methylmercury exposures alter neuronal differentiation in a human-induced pluripotent stem cell model

Developmental methylmercury (MeHg) exposure selectively targets the cerebral and cerebellar cortices, as seen by disruption of cytoarchitecture and glutamatergic (GLUergic) neuron hypoplasia. To begin to understand the mechanisms of this loss of GLUergic neurons, we aimed to develop a model of developmental MeHg neurotoxicity in human-induced pluripotent stem cells differentiating into cortical GLUergic neurons. Three dosing paradigms at 0.1 μM and 1.0 μM MeHg, which span different stages of neurodevelopment and reflect toxicologically relevant accumulation levels seen in human studies and mammalian models, were established. With these exposure paradigms, no changes were seen in commonly studied endpoints of MeHg toxicity, including viability, proliferation, and glutathione levels. However, MeHg exposure induced changes in mitochondrial respiration and glycolysis and in markers of neuronal differentiation. Our novel data suggests that GLUergic neuron hypoplasia seen with MeHg toxicity may be due to the partial inhibition of neuronal differentiation, given the increased expression of the early dorsal forebrain marker FOXG1 and corresponding decrease in expression on neuronal markers MAP2 and DCX and the deep layer cortical neuronal marker TBR1. Future studies should examine the persistent and latent functional effects of this MeHg-induced disruption of neuronal differentiation as well as transcriptomic and metabolomic alterations that may mediate MeHg toxicity.

Probucol Protects Neuronal Cells Against Peroxide-Induced Damage and Directly Activates Glutathione Peroxidase-1

Experimental evidence has shown that probucol, a hypocholesterolemic agent, is also able to increase glutathione peroxidase (GPx) activity. However, there is a lack of knowledge about the mechanism(s) involved in this event. In this study, in vitro experiments with purified GPx1 from bovine erythrocytes and cultured SH-SY5Y neuroblastoma cells, as well as in silico studies with GPx1, were performed in order to elucidate mechanisms mediating the stimulatory effect of probucol on GPx activity and to investigate the relevance of this event in terms of susceptibility against peroxide-induced cytotoxicity. In vitro experiments with purified GPx1 showed a direct stimulatory effect of probucol on the activity of GPx1, which was related to an increase in Vmax with no changes in KM. Probucol also increased GPx activity in cultured SH-SY5Y neuroblastoma cells, while the levels of GPx1 expression were not changed, corroborating the results found with the purified enzyme. In addition, probucol rendered SH-SY5Y cells more resistant to hydroperoxide-induced cytotoxicity, and this event was abolished in GPx1 knocked-down cells. In silico studies with GPx1 pointed to a potential binding site for probucol at the close vicinity of the GSH pocket. Collectively, the results presented herein indicate that GPx1 plays a central role in the probucol-induced protective effects against peroxide toxicity. This highlights a novel target (GPx1) and a new mechanism of action (direct activation) for an "old drug." The relevance of such results for in vivo conditions deserves further investigation.

Toxicity of mercury: Molecular evidence

Minamata disease in Japan and the large-scale poisoning by methylmercury (MeHg) in Iraq caused wide public concerns about the risk emanating from mercury for human health. Nowadays, it is widely known that all forms of mercury induce toxic effects in mammals, and increasing evidence supports the concern that environmentally relevant levels of MeHg could impact normal biological functions in wildlife. The information of mechanism involved in mercurial toxicity is growing but knowledge gaps still exist between the adverse effects and mechanisms of action, especially at the molecular level. A body of data obtained from experimental studies on mechanisms of mercurial toxicity in vivo and in vitro points to that disruption of the antioxidant system may play an important role in the mercurial toxic effects. Moreover, the accumulating evidence indicates that signaling transduction, protein or/and enzyme activity, and gene regulation are involving in mediating toxic and adaptive response to mercury exposure. We conducted here a comprehensive review of mercurial toxic effects on wildlife and human, in particular synthesized key findings of molecular pathways involved in mercurial toxicity from the cells to human. We discuss the molecular evidence related mercurial toxicity to the adverse effects, with particular emphasis on the gene regulation. The further studies relying on Omic analysis connected to adverse effects and modes of action of mercury will aid in the evaluation and validation of causative relationship between health outcomes and gene expression.

Proteome changes in methylmercury-exposed mouse primary cerebellar granule neurons and astrocytes

Methylmercury (MeHg) is a neurotoxicant, with the cerebellum as the main target of toxicity; however, the toxic effects of MeHg on specific cell types remain unclear. Here, primary cerebellar granule neurons (CGNs) and cerebellar astrocytes were isolated and analyzed for total mercury accumulation, cellular reactive oxygen species (ROS) production, and whole-cell proteome expression after exposure to 0-10 μM MeHg for 24 h. Intracellular mercury and ROS levels showed dose-dependent increases. Mercury accumulation was greater in CGNs than astrocytes. The proteomic analysis identified a total of 1966 and 3214 proteins in CGNs and astrocytes, among which 183 and 262 proteins were differentially expressed after mercury exposure, respectively. Enrichment analysis revealed mitochondrial-associated organelles as the main targets of MeHg in both cell types. Whereas multiple functions/pathways were affected in CGNs, the oxidation-reduction process was the most significantly changed function/pathway in astrocytes. CGNs were more sensitive to MeHg-mediated neurotoxicity than astrocytes. The two cell types showed distinct mechanistic responses to MeHg. In astrocytes, the mitochondrion was the primary target of toxicity, resulting in increases in oxidation-reduction process responses. In CGNs, the neurotrophin signaling pathway, cytoskeleton, cAMP signaling pathway, and thyroid hormone signaling pathway were affected.

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.

Biosafety assessment of conducting nanostructured materials by using co-cultures of neurons and astrocytes

Neural electrode implants are made mostly of noble materials. We have synthesized a nanostructured material combining the good electrochemical properties of iridium oxide (IrOx) and carbon-nanotubes (CNT) and the properties of poly(3,4-ethylenedioxythiophene) (PEDOT). IrOx-CNT-PEDOT charge storage capacity was lower than that of IrOx and IrOx-CNT, but higher than that of other PEDOT-containing hybrids and Pt. Cyclic voltammetry, SEM, XPS and micro-Raman spectroscopy suggest that PEDOT encapsulates IrOx and CNT. In our search for a cell culture platform that could optimize modelling the in vivo environment, we determined cell viability, neuron and astrocyte functionality and the response of astrocytes to an inflammatory insult by using primary cultures of neurons, of astrocytes and co-cultures of both. The materials tested (based on IrOx, CNT and PEDOT, as well as Pt as a reference) allowed adhesion and proliferation of astrocytes and full compatibility for neurons grown in co-cultures. Functionality assays show that uptake of glutamate in neuron-astrocyte co-culture was significantly higher than the sum of the uptake in astrocytes and neurons. In co-cultures on IrOx, IrOx-CNT and IrOx-CNT-PEDOT, glutamate was released by a depolarizing stimulus and induced a significant increase in intracellular calcium, supporting the expression of functional NMDA/glutamate receptors. LPS-induced inflammatory response in astrocytes showed a decreased response in NOS2 and COX2 mRNA expression for IrOx-CNT-PEDOT. Results indicate that neuron-astrocyte co-cultures are a reliable model for assessing the biocompatibility and safety of nanostructured materials, evidencing also that hybrid IrOx-CNT-PEDOT nanocomposite materials may offer larger resistance to inflammatory insults.

Methylmercury exposure alters RNA splicing in human neuroblastoma SK-N-SH cells: Implications from proteomic and post-transcriptional responses

The neurotoxic effects of methylmercury (MeHg) have been intensively studied. However, the molecular mechanisms responsible for the neurotoxicity of MeHg are not fully understood. To decipher these mechanisms, proteomic and high-throughput mRNA sequencing (RNA-seq) technique were utilized, comprehensively evaluating the cellular responses of human neuroblastoma SK-N-SH cells to MeHg exposure. Proteomic results revealed that MeHg exposure interfered with RNA splicing via splicesome, along with the known molecular mechanisms of mercury-related neurotoxicity (e.g. oxidative stress, protein folding, immune system processes, and cytoskeletal organization). The effects of MeHg on RNA splicing were further verified using RNA-seq. Compared to control, a total of 658 aberrant RNA alternative splicing (AS) events were observed after MeHg exposure. Proteomics and RNA-seq results also demonstrated that mercury chloride (HgCl₂) influenced the expression levels of several RNA splicing related proteins and 676 AS events compared to control. These results suggested that RNA splicing could be a new molecular mechanism involved in MeHg and HgCl₂ neurotoxicity.

Chronic intoxication by methylmercury leads to oxidative damage and cell death in salivary glands of rats

Methylmercury (MeHg) is one of the most toxic species of mercury, causing several systemic damages; however, its effect on the salivary glands has rarely been explored to date. This study was aimed at analyzing the mercury deposit, oxidative stress markers, and cell viability in parotid and submandibular rat salivary glands after chronic methylmercury intoxication. Herein, forty male Wistar rats (40 days old) were used in the experiment. The animals of the experimental group were intoxicated by intragastric gavage with MeHg at a dose of 0.04 mg per kg body weight per day for 35 days, whereas the control group received only corn oil, a diluent. After the period of intoxication, the glands were obtained for evaluation of total mercury deposit, cell viability, and the malondialdehyde (MDA) and the nitrite levels. Our results indicated mercury deposits in salivary glands, with a decrease in cell viability, higher levels of MDA in both glands of intoxicated animals, and a higher concentration of nitrite only in the submandibular gland of the mercury group. Thus, the intoxication by MeHg was able to generate deposits and oxidative stress in salivary glands that resulted in a decrease in cell viability in both types of glands.

Biomarkers of mercury toxicity: Past, present, and future trends

Mercury (Hg) toxicity continues to represent a global health concern. Given that human populations are mostly exposed to low chronic levels of mercurial compounds (methylmercury through fish, mercury vapor from dental amalgams, and ethylmercury from vaccines), the need for more sensitive and refined tools to assess the effects and/or susceptibility to adverse metal-mediated health risks remains. Traditional biomarkers, such as hair or blood Hg levels, are practical and provide a reliable measure of exposure, but given intra-population variability, it is difficult to establish accurate cause-effect relationships. It is therefore important to identify and validate biomarkers that are predictive of early adverse effects prior to adverse health outcomes becoming irreversible. This review describes the predominant biomarkers used by toxicologists and epidemiologists to evaluate exposure, effect and susceptibility to Hg compounds, weighing on their advantages and disadvantages. Most importantly, and in light of recent findings on the molecular mechanisms underlying Hg-mediated toxicity, potential novel biomarkers that might be predictive of toxic effect are presented, and the applicability of these parameters in risk assessment is examined.

Methylmercury-induced developmental toxicity is associated with oxidative stress and cofilin phosphorylation. Cellular and human studies

Environmental exposure to methylmercury (MeHg) during development is of concern because it is easily incorporated in children's body both pre- and post-natal, it acts at several levels of neural pathways (mitochondria, cytoskeleton, neurotransmission) and it causes behavioral impairment in child. We evaluated the effects of prolonged exposure to 10-600nM MeHg on primary cultures of mouse cortical (CCN) and of cerebellar granule cells (CGC) during their differentiation period. In addition, it was studied if prenatal MeHg exposure correlated with altered antioxidant defenses and cofilin phosphorylation in human placentas (n=12) from the INMA cohort (Spain). Exposure to MeHg for 9days in vitro (DIV) resulted in protein carbonylation and in cell death at concentrations ≥200nM and ≥300nM, respectively. Exposure of CCN and CGC to non-cytotoxic MeHg concentrations for 5 DIV induced an early concentration-dependent decrease in cofilin phosphorylation. Furthermore, in both cell types actin was translocated from the cytosol to the mitochondria whereas cofilin translocation was found only in CGC. Translocation of cofilin and actin to mitochondria in CGC occurred from 30nM MeHg onwards. We also found an increased expression of cortactin and LIMK1 mRNA in CGC but not in CCN. All these effects were prevented by the antioxidant probucol. Cofilin phosphorylation was significantly decreased and a trend for decreased activity of glutathione reductase and glutathione peroxidase was found in the fetal side of human placental samples from the highest (20-40μg/L) MeHg-exposed group when compared with the low (<7μg/L) MeHg-exposed group. In summary, cofilin dephosphorylation and oxidative stress are hallmarks of MeHg exposure in both experimental and human systems.

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.

Influence of prenatal exposure to environmental pollutants on human cord blood levels of glutamate

Some chemicals released into the environment, including mercury and some organochlorine compounds (OCs), are suspected to have a key role on subclinical brain dysfunction in childhood. Alteration of the glutamatergic system may be one mechanistic pathway. We aimed to determine whether mercury and seven OCs, including PCBs 138, 153, and 180, DDT and DDE, hexachlorobenzene (HCB), and beta-hexachlorocyclohexane (β-HCH) influence the cord levels of two excitatory amino acids, glutamate and aspartate. Second, we evaluated if this association was mediated by glutamate uptake measured in human placental membranes. The study sample included 40 newborns from a Spanish cohort selected according to cord mercury levels. We determined the content of both amino acids in cord blood samples by means of HPLC and assessed their associations with the contaminants using linear regression analyses, and the effect of the contaminants on glutamate uptake by means of [(3)H]-aspartate binding in human placenta samples. PCB138, β-HCH, and the sum of the three PCBs and seven OCs showed a significant negative association with glutamate levels (decrease of 51, 24, 56 and 54%, respectively, in glutamate levels for each 10-fold increase in the contaminant concentration). Mercury did not show a significant correlation neither with glutamate nor aspartate levels in cord blood, however a compensatory effect between T-Hg and both PCB138, and 4,4'-DDE was observed. The organo-metallic derivative methylmercury completely inhibited glutamate uptake in placenta while PCB138 and β-HCH partially inhibited it (IC50 values: 4.9±0.8 μM, 14.2±1.2 nM and 6.9±2.9 nM, respectively). We conclude that some environmental toxicants may alter the glutamate content in the umbilical cord blood, which might underlie alterations in human development.

Identification of Potential Serum Biomarkers in Mercury-Treated Mice Using a Glycoproteomic Approach

Mercury is a well-recognized health hazard and a deleterious environmental contaminant. Exposure to mercury can cause neurotoxic manifestations, nephrotoxicity, and immune function alterations; however, the mechanisms and related proteins responsible for these effects are still unclear. Our goal is to understand the relationship between the toxicity of mercury and the proteins affected by this toxic heavy metal and to define biomarkers for mercury intoxication. Two different forms of mercury, organic methylmercury or inorganic mercury sulfide, were orally administered to the mice for 4 weeks. To reduce complexity of the serum proteome, we enriched glycoproteins from mice serum with lectin concanavalin A resin, and the tryptic peptides of the purified glycoproteins were subjected to nanoultra performance liquid chromatography-Quadrupole time-of-flight for identification and label-free quantification. In this study, we characterized approximately 209 proteins from mice serum, and, among them, 21 proteins were differentially expressed in organic methylmercury-treated mice serum compared with the control group. Two proteins, serum amyloid P component (SAP) and inter α-trypsin inhibitor heavy chain 4 (ITI-H4), were upregulated in organic methylmercury-treated mice and confirmed with different doses of both types of mercury by Western blot analysis. Results of immunohistochemistry also confirmed the validity of SAP and ITI-H4 as biomarker candidates for organic methylmercury exposure. Findings of this study may assist in understanding the relationship between toxicity of mercury and upregulated proteins in mouse serum. Furthermore, the proteins identified here might be used as biomarker candidates in mercury intoxication.

Chronic exposure of adult rats to low doses of methylmercury induced a state of metabolic deficit in the somatosensory cortex

Because of the ever-increasing bioaccumulation of methylmercury (MeHg) in the marine food chain, human consumers are exposed to low doses of MeHg continually through seafood consumption. Epidemiological studies strongly suggest that chronic prenatal exposure to nanomolar of MeHg has immense negative impacts on neurological development in neonates. However, effects of chronic exposure to low doses (CELDs) of MeHg in adult brains on a molecular level are unknown. The current study aims to investigate the molecular effects of CELD of MeHg on adult somatosensory cortex in a rat model using proteomic techniques. Young adult rats were fed with a low dose of MeHg (40 μg/kg body weight/day) for a maximum of 12 weeks. Whole proteome expression of the somatosensory cortex (S1 area) of normal rats and those with CELD to MeHg were compared. Levels of MeHg, total calcium, adenosine triphosphate (ATP), and pyruvate were also measured. Comparative proteomic studies of the somatosensory cortexes revealed that 94 proteins involved in the various metabolic processes (including carbohydrate metabolism, generation of precursors for essential metabolites, energy, proteins, cellular components for morphogenesis, and neurotransmission) were down-regulated. Consequently, levels of important end products of active metabolism including ATP, pyruvate, and total calcium were also found to be significantly reduced concomitantly. Our results showed that CELD of MeHg induced a state of metabolic deficit in the somatosensory cortex of adult rats.

Comparative study on methyl- and ethylmercury-induced toxicity in C6 glioma cells and the potential role of LAT-1 in mediating mercurial-thiol complexes uptake

Various forms of mercury possess different rates of absorption, metabolism and excretion, and consequently, toxicity. Methylmercury (MeHg) is a highly neurotoxic organic mercurial. Human exposure is mostly due to ingestion of contaminated fish. Ethylmercury (EtHg), another organic mercury compound, has received significant toxicological attention due to its presence in thimerosal-containing vaccines. This study was designed to compare the toxicities induced by MeHg and EtHg, as well as by their complexes with cysteine (MeHg-S-Cys and EtHg-S-Cys) in the C6 rat glioma cell line. MeHg and EtHg caused significant (p<0.0001) decreases in cellular viability when cells were treated during 30min with each mercurial following by a washing period of 24h (EC50 values of 4.83 and 5.05μM, respectively). Significant cytotoxicity (p<0.0001) was also observed when cells were treated under the same conditions with MeHg-S-Cys and EtHg-S-Cys, but the respective EC50 values were significantly increased (11.2 and 9.37μM). l-Methionine, a substrate for the l-type neutral amino acid carrier transport (LAT) system, significantly protected against the toxicities induced by both complexes (MeHg-S-Cys and EtHg-S-Cys). However, no protective effects of l-methionine were observed against MeHg and EtHg toxicities. Corroborating these findings, l-methionine significantly decreased mercurial uptake when cells were exposed to MeHg-S-Cys (p=0.028) and EtHg-S-Cys (p=0.023), but not to MeHg and EtHg. These results indicate that the uptake of MeHg-S-Cys and EtHg-S-Cys into C6 cells is mediated, at least in part, through the LAT system, but MeHg and EtHg enter C6 cells by mechanisms other than LAT system.

Toxicogenomic approaches for understanding molecular mechanisms of heavy metal mutagenicity and carcinogenicity

Heavy metals that are harmful to humans include arsenic, cadmium, chromium, lead, mercury, and nickel. Some metals or their related compounds may even cause cancer. However, the mechanism underlying heavy metal-induced cancer remains unclear. Increasing data show a link between heavy metal exposure and aberrant changes in both genetic and epigenetic factors via non-targeted multiple toxicogenomic technologies of the transcriptome, proteome, metabolome, and epigenome. These modifications due to heavy metal exposure might provide a better understanding of environmental disorders. Such informative changes following heavy metal exposure might also be useful for screening of biomarker-monitored exposure to environmental pollutants and/or predicting the risk of disease. We summarize advances in high-throughput toxicogenomic-based technologies and studies related to exposure to individual heavy metal and/or mixtures and propose the underlying mechanism of action and toxicant signatures. Integrative multi-level expression analysis of the toxicity of heavy metals via system toxicology-based methodologies combined with statistical and computational tools might clarify the biological pathways involved in carcinogenic processes. Although standard in vitro and in vivo endpoint testing of mutagenicity and carcinogenicity are considered a complementary approach linked to disease, we also suggest that further evaluation of prominent biomarkers reflecting effects, responses, and disease susceptibility might be diagnostic. Furthermore, we discuss challenges in toxicogenomic applications for toxicological studies of metal mixtures and epidemiological research. Taken together, this review presents toxicogenomic data that will be useful for improvement of the knowledge of carcinogenesis and the development of better strategies for health risk assessment.

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 lead and mercury on the blood proteome of children

Heavy metal exposure in children has been associated with a variety of physiological and neurological problems. The goal of this study was to utilize proteomics to enhance the understanding of biochemical interactions responsible for the health problems related to lead and mercury exposure at concentrations well below CDC guidelines. Blood plasma and serum samples from 34 children were depleted of their most abundant proteins using antibody-based affinity columns and analyzed using two different methods, LC-MS/MS and 2-D electrophoresis coupled with MALDI-TOF/MS and tandem mass spectrometry. Apolipoprotein E demonstrated an inverse significant association with lead concentrations (average being one microgram/deciliter) as deduced from LC-MS/MS and 2-D electrophoresis and confirmed by Western blot analysis. This coincides with prior findings that Apolipoprotein E genotype moderates neurobehavioral effects in individuals exposed to lead. Fifteen other proteins were identified by LC-MS/MS as proteins of interest exhibiting expressional differences in the presence of environmental lead and mercury.

Methylmercury-induced neurotoxicity and apoptosis

Methylmercury is a widely distributed environmental toxicant with detrimental effects on the developing and adult nervous system. Due to its accumulation in the food chain, chronic exposure to methylmercury via consumption of fish and sea mammals is still a major concern for human health, especially developmental exposure that may lead to neurological alterations, including cognitive and motor dysfunctions. Mercury-induced neurotoxicity and the identification of the underlying mechanisms has been a main focus of research in the neurotoxicology field. Three major mechanisms have been identified as critical in methylmercury-induced cell damage including (i) disruption of calcium homeostasis, (ii) induction of oxidative stress via overproduction of reactive oxygen species or reduction of antioxidative defenses and (iii) interactions with sulfhydryl groups. In vivo and in vitro studies have provided solid evidence for the occurrence of neural cell death, as well as cytoarchitectural alterations in the nervous system after exposure to methylmercury. Signaling cascades leading to cell death induced by methylmercury involve the release of mitochondrial factors, such as cytochrome c and AIF with subsequent caspase-dependent or -independent apoptosis, respectively; induction of calcium-dependent proteases calpains; interaction with lysosomes leading to release of cathepsins. Interestingly, several pathways can be activated in parallel, depending on the cell type. In this paper, we provide an overview of recent findings on methylmercury-induced neurotoxicity and cell death pathways that have been described in neural and endocrine cell systems.

Probucol increases glutathione peroxidase-1 activity and displays long-lasting protection against methylmercury toxicity in cerebellar granule cells

Methylmercury (MeHg) is an environmental neurotoxicant whose molecular mechanisms underlying toxicity remain elusive. Here, we investigated molecular events involved in MeHg-induced neurotoxicity in cultured cerebellar granule cells (CGCs) as well as potential protective strategies for such toxicity. Glutathione peroxidase, isozyme 1 (GPx-1) activity was significantly (p = 0.0017) decreased at 24 h before MeHg-induced neuronal death (day in vitro 4). This event was related to enhanced susceptibilities to hydrogen peroxide or tert-butyl peroxide and increased lipid peroxidation. However, intracellular calcium levels, glutamate uptake, and glutathione levels, as well as glutathione reductase and catalase activities, were not changed by MeHg exposure at this time point. Probucol (PB), a lipid-lowering drug, displayed a long-lasting protective effect against MeHg-induced neurotoxicity. The beneficial effects of PB were correlated with increased GPx-1 activity and decreased lipid peroxidation. The protection afforded by PB was significantly higher when compared to the antioxidants, ascorbic acid and trolox. In vitro studies with the purified GPx-1 proved that MeHg inhibits and PB activates the enzyme activity. Overexpression of GPx-1 prevented MeHg-induced neuronal death. These data indicate that (1) GPx-1 is an important molecular target involved in MeHg-induced neurotoxicity and (2) PB, which increases GPx-1 activity in CGCs, induces enduring protection against such toxicity. The results bring out new insights on the potential therapeutic strategies for poisonings to MeHg and other pathological conditions related to increased production and/or decreased detoxification of peroxides.

Mercury immune toxicity in harbour seals: links to in vitro toxicity

BACKGROUND

Mercury is known to bioaccumulate and to magnify in marine mammals, which is a cause of great concern in terms of their general health. In particular, the immune system is known to be susceptible to long-term mercury exposure. The aims of the present study were (1) to determine the mercury level in the blood of free-ranging harbour seals from the North Sea and (2) to examine the link between methylmercury in vitro exposure and immune functions using seal and human mitogen-stimulated peripheral blood mononuclear cells (T-lymphocytes).

METHODS

Total mercury was analysed in the blood of 22 harbour seals. Peripheral blood mononuclear cells were isolated from seals (n = 11) and from humans (n = 9). Stimulated lymphocytes of both species were exposed to functional tests (proliferation, metabolic activity, radioactive precursor incorporation) under increasing doses of methylmercury (0.1 to 10 microM). The expression of cytokines (IL-2, IL-4 and TGF-beta) was investigated in seal lymphocytes by RT-PCR and by real time quantitative PCR (n = 5) at methylmercury concentrations of 0.2 and 1 microM. Finally, proteomics analysis was attempted on human lymphocytes (cytoplasmic fraction) in order to identify biochemical pathways of toxicity at concentration of 1 microM (n = 3).

RESULTS

The results showed that the number of seal lymphocytes, viability, metabolic activity, DNA and RNA synthesis were reduced in vitro, suggesting deleterious effects of methylmercury concentrations naturally encountered in free-ranging seals. Similar results were found for human lymphocytes. Functional tests showed that a 1 microM concentration was the critical concentration above which lymphocyte activity, proliferation and survival were compromised. The expression of IL-2 and TGF-beta mRNA was weaker in exposed seal lymphocytes compared to control cells (0.2 and 1 microM). Proteomics showed some variation in the protein expression profile (e.g. vimentin).

CONCLUSION

Our results suggest that seal and human PBMCs react in a comparable way to MeHg in vitro exposure with, however, larger inter-individual variations. MeHg could be an additional cofactor in the immunosuppressive pollutant cocktail generally described in the blood of seals and this therefore raises the possibility of additional additive effects in the marine mammal immune system.

Studies with neuronal cells: From basic studies of mechanisms of neurotoxicity to the prediction of chemical toxicity

Neurotoxicology considers that chemicals perturb neurological functions by interfering with the structure or function of neural pathways, circuits and systems. Using in vitro methods for neurotoxicity studies should include evaluation of specific targets for the functionalism of the nervous system and general cellular targets. In this review we present the neuronal characteristics of primary cultures of cortical neurons and of cerebellar granule cells and their use in neurotoxicity studies. Primary cultures of cortical neurons are constituted by around 40% of GABAergic neurons, whereas primary cultures of cerebellar granule cells are mainly constituted by glutamatergic neurons. Both cultures express functional GABAA and ionotropic glutamate receptors. We present neurotoxicity studies performed in these cell cultures, where specific neural targets related to GABA and glutamate neurotransmission are evaluated. The effects of convulsant polychlorocycloalkane pesticides on the GABAA, glycine and NMDA receptors points to the GABAA receptor as the neural target that accounts for their in vivo acute toxicity, whereas NMDA disturbance might be relevant for long-term toxicity. Several compounds from a list of reference compounds, whose severe human poisoning result in convulsions, inhibited the GABAA receptor. We also present cell proteomic studies showing that the neurotoxic contaminant methylmercury affect mitochondrial proteins. We conclude that the in vitro assays that have been developed can be useful for their inclusion in an in vitro test battery to predict human toxicity.

Prenatal methylmercury exposure hampers glutathione antioxidant system ontogenesis and causes long-lasting oxidative stress in the mouse brain

During the perinatal period, the central nervous system (CNS) is extremely sensitive to metals, including methylmercury (MeHg). Although the mechanism(s) associated with MeHg-induced developmental neurotoxicity remains obscure, several studies point to the glutathione (GSH) antioxidant system as an important molecular target for this toxicant. To extend our recent findings of MeHg-induced GSH dyshomeostasis, the present study was designed to assess the developmental profile of the GSH antioxidant system in the mouse brain during the early postnatal period after in utero exposure to MeHg. Pregnant mice were exposed to different doses of MeHg (1, 3 and 10 mg/l, diluted in drinking water, ad libitum) during the gestational period. After delivery, pups were killed at different time points - postnatal days (PND) 1, 11 and 21 - and the whole brain was used for determining biochemical parameters related to the antioxidant GSH system, as well as mercury content and the levels of F(2)-isoprostane. In control animals, cerebral GSH levels significantly increased over time during the early postnatal period; gestational exposure to MeHg caused a dose-dependent inhibition of this developmental event. Cerebral glutathione peroxidase (GPx) and glutathione reductase (GR) activities significantly increased over time during the early postnatal period in control animals; gestational MeHg exposure induced a dose-dependent inhibitory effect on both developmental phenomena. These adverse effects of prenatal MeHg exposure were corroborated by marked increases in cerebral F(2)-isoprostanes levels at all time points. Significant negative correlations were found between F(2)-isoprostanes and GSH, as well as between F(2)-isoprostanes and GPx activity, suggesting that MeHg-induced disruption of the GSH system maturation is related to MeHg-induced increased lipid peroxidation in the pup brain. In utero MeHg exposure also caused a dose-dependent increase in the cerebral levels of mercury at birth. Even though the cerebral mercury concentration decreased to nearly basal levels at postnatal day 21, GSH levels, GPx and GR activities remained decreased in MeHg-exposed mice, indicating that prenatal exposure to MeHg affects the cerebral GSH antioxidant systems by inducing biochemical alterations that endure even when mercury tissue levels decrease and become indistinguishable from those noted in pups born to control dams. This study is the first to show that prenatal exposure to MeHg disrupts the postnatal development of the glutathione antioxidant system in the mouse brain, pointing to an additional molecular mechanism by which MeHg induces pro-oxidative damage in the developing CNS. Moreover, our experimental observation corroborates previous reports on the permanent functional deficits observed after prenatal MeHg exposure.