Methylmercury-Induced Increase of Intracellular Ca2+ Increases Spontaneous Synaptic Current Frequency in Rat Cerebellar Slices

Oleanolic acid-3-glucoside, a synthetic oleanane-type saponin, ameliorates methylmercury-induced dysfunction of synaptic transmission in mice

Methylmercury (MeHg) is widely distributed in nature and is known to cause neurotoxic effects. This study aimed to examine the anti-MeHg activity of oleanolic acid-3-glucoside (OA3Glu), a synthetic oleanane-type saponin derivative, by evaluating its effects on motor function, pathology, and electrophysiological properties in a mouse model of MeHg poisoning. Mice were orally administered 2 or 4 mg·kg-1·d-1 MeHg with or without 100 µg·kg-1·d-1 OA3Glu 5x/week for four weeks. Motor function was evaluated using beam-walking and dynamic weight-bearing (DWB) tests. High-dose MeHg exposure significantly increased the frequency of stepping off the hind leg while crossing the beam in the beam-walking test, and increased weight on forelegs when moving freely in the DWB test. OA3Glu treatment alleviated motor abnormality caused by high-dose MeHg exposure in both motor function tests. Additionally, OA3Glu treatment reduced the number of contracted Purkinje cells frequently observed in the cerebellum of MeHg-treated groups, although cerebrum histology was similar in all experimental groups. The synaptic potential amplitude in the cerebellum decreased as MeHg exposure increased, which was restored by OA3Glu treatment. Even in the cerebrum, where the effects of MeHg were not observed, the amplitude of the field potential was suppressed with increasing MeHg exposure but was restored with OA3Glu treatment. Taken together, the study findings suggest that OA3Glu improves neurotransmission and movement disorders associated with MeHg exposure via protection of Purkinje cells in the cerebellum while ameliorating pre/post-synaptic deficits in the cerebral cortex in which no changes were observed at the tissue level, potentially providing a treatment to mitigate MeHg toxicity.

Preferential potentiation of AMPA-mediated currents in brainstem hypoglossal motoneurons by subchronic exposure of mice expressing the human superoxide dismutase 1 G93A gene mutation to neurotoxicant methylmercury in vivo

The exact causes of Amyotrophic lateral sclerosis (ALS), a progressive and fatal neurological disorder due to loss of upper and/or lower motoneurons, remain elusive. Gene-environment interactions are believed to be an important factor in the development of ALS. We previously showed that in vivo exposure of mice overexpressing the human superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; G93A), a mouse model for ALS, to environmental neurotoxicant methylmercury (MeHg) accelerated the onset of ALS-like phenotype. Here we examined the time-course of effects of MeHg on AMPA receptor (AMPAR)-mediated currents in hypoglossal motoneurons in brainstem slices prepared from G93A, hSOD1wild-type (hWT) and non-carrier WT mice following in vivo exposure to MeHg. Mice were exposed daily to 3 ppm (approximately 0.7 mg/kg/day) MeHg via drinking water beginning at postnatal day 28 (P28) and continued until P47, 64 or 84, then acute brainstem slices were prepared, and spontaneous excitatory postsynaptic currents (sEPSCs) or AMPA-evoked currents were examined using whole cell patch-clamp recording technique. Brainstem slices of untreated littermates were prepared at the same time points to serve as control. MeHg exposure had no significant effect on either sEPSCs or AMPA-evoked currents in slices from hWT or WT mice during any of those exposure time periods under our experimental conditions. MeHg also did not cause any significant effect on sEPSCs or AMPA-currents in G93A hypoglossal motoneurons at P47 and P64. However, at P84, MeHg significantly increased amplitudes of both sEPSCs and AMPA-evoked currents in hypoglossal motineurons from G93A mice (p < 0.05), but not the sEPSC frequency, suggesting a postsynaptic action on AMPARs. MeHg exposure did not cause any significant effect on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). Therefore, MeHg exposure in vivo caused differential effects on AMPARs in hypoglossal motoneurons from mice with different genetic backgrounds. MeHg appears to preferentially stimulate the AMPAR-mediated currents in G93A hypoglossal motoneurons in an exposure time-dependent manner, which may contribute to the AMPAR-mediated motoneuron excitotoxicity, thereby facilitating development of ALS-like phenotype.

Effect of methylmercury on fetal neurobehavioral development: an overview of the possible mechanisms of toxicity and the neuroprotective effect of phytochemicals

Methylmercury (MeHg) is a global environmental pollutant with neurotoxic effects. Exposure to MeHg via consumption of seafood and fish can severely impact fetal neurobehavioral development even when MeHg levels in maternal blood are as low as about 5 μg/L, which the mother tolerates well. Persistent motor dysfunctions and cognitive deficits may result from trans-placental exposure. The present review summarizes current knowledge on the mechanisms of MeHg toxicity during the period of nervous system development. Although cerebellar Purkinje cells are MeHg targets, the actions of MeHg on thiol components in the neuronal cytoskeleton as well as on mitochondrial enzymes and induction of disturbances of glutamate signaling can impair extra-cerebellar functions, also at levels well tolerated by adult individuals. Numerous herbal substances possess neuroprotective effects, predominantly represented by natural polyphenolic molecules that might be utilized to develop natural drugs to alleviate neurotoxicity symptoms caused by MeHg or other Hg compounds.

MICE WITH GNAO1-ASSOCIATED MOVEMENT DISORDER EXHIBIT REDUCED INHIBITORY SYNAPTIC INPUT TO CEREBELLAR PURKINJE CELLS

GNAO1 encodes Gα_o, a heterotrimeric G protein alpha subunit in the G_i/o family. In this report, we used a Gnao1 mouse model "G203R" previously described as a "gain-of-function" Gnao1 mutant with movement abnormalities and enhanced seizure susceptibility. Here, we report an unexpected second mutation resulting in a loss-of-function Gα_o protein and describe alterations in central synaptic transmission. Whole cell patch clamp recordings from Purkinje cells (PCs) in acute cerebellar slices from Gnao1 mutant mice showed significantly lower frequencies of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) compared to WT mice. There was no significant change in sEPSCs or mEPSCs. Whereas mIPSC frequency was reduced, mIPSC amplitudes were not affected, suggesting a presynaptic mechanism of action. A modest decrease in the number of molecular layer interneurons was insufficient to explain the magnitude of IPSC suppression. Paradoxically, G_i/o inhibitors (pertussis toxin), enhanced the mutant-suppressed mIPSC frequency and eliminated the difference between WT and Gnao1 mice. While GABA_B receptor regulates mIPSCs, neither agonists nor antagonists of this receptor altered function in the mutant mouse PCs. This study is the first electrophysiological investigation of the role of G_i/o proteinin cerebellar synaptic transmission using an animal model with a loss-of-function G_i/o protein.

Developmental exposure to methylmercury and ADHD, a literature review of epigenetic studies

Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects the competence of academic performance and social wellness in children and adults. The causes of ADHD are unclear. Both genetic and environmental factors contribute to the development of ADHD. The behavioral impairments in ADHD are associated with epigenetic changes in genes that are important for neurodevelopment. Among environmental causes of ADHD, the neurotoxin methylmercury (MeHg) is associated with an increased risk for ADHD. Developing children are susceptible to neurotoxic effects of prenatal MeHg exposure. Human epidemiology studies have shown that prenatal MeHg exposure could invoke epigenetic changes in genes that are involved in ADHD. In addition, the pathogenesis of ADHD involves dopaminergic system, which is a target of developmental MeHg exposure. MeHg-induced alterations in the dopaminergic system have a profound impact on behavioral functions in adults. As a trace level of MeHg (around nM) can induce long-lasting behavioral alterations, potential mechanisms of MeHg-induced functional changes in the dopaminergic system may involve epigenetic mechanisms. Here, we review the relevant evidence on developmental MeHg exposures and the risk for ADHD. We also point out research gaps in understanding environmental causes of ADHD.

Transcriptional Analyses of Acute Exposure to Methylmercury on Erythrocytes of Loggerhead Sea Turtle

To understand changes in enzyme activity and gene expression as biomarkers of exposure to methylmercury, we exposed loggerhead turtle erythrocytes (RBCs) to concentrations of 0, 1, and 5 mg L-1 of MeHg and de novo transcriptome were assembled using RNA-seq. The analysis of differentially expressed genes (DEGs) indicated that 79 unique genes were dysregulated (39 upregulated and 44 downregulated genes). The results showed that MeHg altered gene expression patterns as a response to the cellular stress produced, reflected in cell cycle regulation, lysosomal activity, autophagy, calcium regulation, mitochondrial regulation, apoptosis, and regulation of transcription and translation. The analysis of DEGs showed a low response of the antioxidant machinery to MeHg, evidenced by the fact that genes of early response to oxidative stress were not dysregulated. The RBCs maintained a constitutive expression of proteins that represented a good part of the defense against reactive oxygen species (ROS) induced by MeHg.

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.

Acute neurotoxicant exposure induces hyperexcitability in mouse lumbar spinal motor neurons

Spinal motor neurons (MNs) are susceptible to glutamatergic excitotoxicity, an effect associated with lumbar MN degeneration in amyotrophic lateral sclerosis (ALS). MN susceptibility to environmental toxicant exposure, one prospective contributor to sporadic ALS, has not been systematically studied. The goal of this study was to test the ability of a well-known environmental neurotoxicant to induce hyperexcitability in mouse lumbar MNs. Methylmercury (MeHg) causes neurotoxicity through mechanisms involving elevated intracellular Ca²⁺ concentration ([Ca²⁺]_i), a hallmark of excitotoxicity. We tested whether acute exposure to MeHg induces hyperexcitability in MNs by altering synaptic transmission, using whole cell patch-clamp recordings of lumbar spinal MNs in vitro. Acute MeHg exposure (20 μM) led to an increase in the frequency of both spontaneous excitatory postsynaptic currents (EPSCs) and miniature EPSCs. The frequency of inhibitory postsynaptic currents (IPSCs) was also increased by MeHg. Action potential firing rates, both spontaneous and evoked, were increased by MeHg, despite increases in both EPSCs and IPSCs, indicating a shift toward hyperexcitability. Also consistent with hyperexcitability, fluo 4-AM microfluorimetry indicated that MeHg exposure induced an increase in [Ca²⁺]_i. Spinal cord hyperexcitability is partially mediated by Ca²⁺-permeable AMPA receptors, as MeHg-dependent increases in EPSCs were blocked by 1-napthyl spermine. Therefore, spinal MNs appear highly susceptible to MeHg exposure, leading to significant increases in spontaneous network excitability and disruption of normal function. Prolonged hyperexcitability could lead to eventual neurodegeneration and loss of motor function as observed in spinal cord after MeHg exposure in vivo and may contribute to MeHg-induced acceleration of ALS symptoms.NEW & NOTEWORTHY Spinal motor neurons (MN) are susceptible to glutamatergic excitotoxicity, an effect associated with lumbar MN degeneration in amyotrophic lateral sclerosis (ALS). This study investigated MN susceptibility to environmental toxicant exposure, one prospective contributor to sporadic ALS. Spinal MNs appear highly susceptible to methylmercury exposure, leading to significant increases in spontaneous network excitability and disruption of normal function. Prolonged hyperexcitability could lead to neurodegeneration and loss of motor function as observed in ALS spinal cord symptoms.

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.

Human iPSC-derived neuronal models for in vitro neurotoxicity assessment

Neurotoxicity testing still relies on ethically debated, expensive and time consuming in vivo experiments, which are unsuitable for high-throughput toxicity screening. There is thus a clear need for a rapid in vitro screening strategy that is preferably based on human-derived neurons to circumvent interspecies translation. Recent availability of commercially obtainable human induced pluripotent stem cell (hiPSC)-derived neurons and astrocytes holds great promise in assisting the transition from the current standard of rat primary cortical cultures to an animal-free alternative. We therefore composed several hiPSC-derived neuronal models with different ratios of excitatory and inhibitory neurons in the presence or absence of astrocytes. Using immunofluorescent stainings and multi-well micro-electrode array (mwMEA) recordings we demonstrate that these models form functional neuronal networks that become spontaneously active. The differences in development of spontaneous neuronal activity and bursting behavior as well as spiking patterns between our models confirm the importance of the presence of astrocytes. Preliminary neurotoxicity assessment demonstrates that these cultures can be modulated with known seizurogenic compounds, such as picrotoxin (PTX) and endosulfan, and the neurotoxicant methylmercury (MeHg). However, the chemical-induced effects on different parameters for neuronal activity, such as mean spike rate (MSR) and mean burst rate (MBR), may depend on the ratio of inhibitory and excitatory neurons. Our results thus indicate that hiPSC-derived neuronal models must be carefully designed and characterized prior to large-scale use in neurotoxicity screening.

Evaluating a Gene-Environment Interaction in Amyotrophic Lateral Sclerosis: Methylmercury Exposure and Mutated SOD1

PURPOSE OF REVIEW

Gene-environment (GxE) interactions likely contribute to numerous diseases, but are often difficult to model in the laboratory. Such interactions have been widely hypothesized for amyotrophic lateral sclerosis (ALS); recent controlled laboratory studies are discussed here and hypotheses related to possible mechanisms of action are offered. Using methylmercury exposure and mutated SOD1 to model the impacts of such an interaction, we interpret evidence about their respective mechanisms of toxicity to interrogate the possibility of additive (or synergistic) effects when combined.

RECENT FINDINGS

Recent work has converged on mechanisms of calcium-mediated glutamate excitotoxicity as a likely contributor in one model of a gene-environment interaction affecting the onset and progression of ALS-like phenotype. The current experimental literature on mechanisms of metal-induced neuronal injury and their relevant interactions with genetic contributions in ALS is sparse, but we describe those studies here and offer several integrative hypotheses about the likely mechanisms involved.

Effects of methylmercury on spinal cord afferents and efferents-A review

Methylmercury (MeHg) is an environmental neurotoxicant of public health concern. It readily accumulates in exposed humans, primarily in neuronal tissue. Exposure to MeHg, either acutely or chronically, causes severe neuronal dysfunction in the central nervous system and spinal neurons; dysfunction of susceptible neuronal populations results in neurodegeneration, at least in part through Ca2+-mediated pathways. Biochemical and morphologic changes in peripheral neurons precede those in central brain regions, despite the fact that MeHg readily crosses the blood-brain barrier. Consequently, it is suggested that unique characteristics of spinal cord afferents and efferents could heighten their susceptibility to MeHg toxicity. Transient receptor potential (TRP) ion channels are a class of Ca2+-permeable cation channels that are highly expressed in spinal afferents, among other sensory and visceral organs. These channels can be activated in numerous ways, including directly via chemical irritants or indirectly via Ca2+ release from intracellular storage organelles. Early studies demonstrated that MeHg interacts with heterologous TRP channels, though definitive mechanisms of MeHg toxicity on sensory neurons may involve more complex interaction with, and among, differentially-expressed TRP populations. In spinal efferents, glutamate receptors of the N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and possibly kainic acid (KA) classes are thought to play a major role in MeHg-induced neurotoxicity. Specifically, the Ca2+-permeable AMPA receptors, which are abundant in motor neurons, have been identified as being involved in MeHg-induced neurotoxicity. In this review, we will describe the mechanisms that could contribute to MeHg-induced spinal cord afferent and efferent neuronal degeneration, including the possible mediators, such as uniquely expressed Ca2+-permeable ion channels.

Methylmercury induces an initial increase in GABA-evoked currents in Xenopus oocytes expressing α1 and α6 subunit-containing GABAA receptors

Early onset effects of methylmercury (MeHg) on recombinant α₁β₂γ_2S or α₆β₂γ_2S subunit-containing GABA_A receptors were examined. These are two of the most prevalent receptor types found in cerebellum-a consistent target of MeHg-induced neurotoxicity. Heterologously expressed receptors were used in order to: (1) isolate receptor-mediated events from extraneous effects of MeHg due to stimulation of the receptor secondary to increased release of GABA seen with MeHg in neurons in situ and (2) limit the phenotypes of GABA_A receptors present at one time. Initial changes in I_GABA in Xenopus laevis oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors were compared during continuous bath application of MeHg. A time-dependent increase in I_GABA mediated by both receptor subtypes occurred following the first 25-30min of MeHg (5μM) exposure. In α₆β₂γ_2S containing receptors, the MeHg-induced increase in I_GABA was less pronounced compared to that mediated by α₁β₂γ_2S containing receptors, although the pattern of effects was generally similar. Washing with MeHg-free solution reversed the increase in current amplitude. Application of bicuculline at the time of peak potentiation of I_GABA rapidly and completely reversed the MeHg-induced currents. Therefore these MeHg-increased inward currents are mediated specifically by the two subtypes of GABA_A receptors and appear to entail direct actions of MeHg on the receptor. However bicuculline did not affect stimulation by MeHg of oocyte endogenous Cl^- -mediated current, which presumably results from increased [Ca²⁺]_i. Thus, MeHg initially potentiates I_GABA in oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors prior to its more defined later effects, suggesting that MeHg may initially interact directly with GABA_A receptors in a reversible manner to cause this potentiation.

Heavy metal chelator TPEN attenuates fura-2 fluorescence changes induced by cadmium, mercury and methylmercury

Stimulation with heavy metals is known to induce calcium (Ca²⁺) mobilization in many cell types. Interference with the measurement of intracellular Ca²⁺ concentration by the heavy metals in cells loaded with Ca²⁺ indicator fura-2 is an ongoing problem. In this study, we analyzed the effect of heavy metals on the fura-2 fluorescence ratio in human SH-SY5Y neuroblastoma cells by using TPEN, a specific cell-permeable heavy metal chelator. Manganese chloride (30–300 µM) did not cause significant changes in the fura-2 fluorescence ratio. A high concentration (300 µM) of lead acetate induced a slight elevation in the fura-2 fluorescence ratio. In contrast, stimulation with cadmium chloride, mercury chloride or MeHg (3–30 µM) elicited an apparent elevation of the fura-2 fluorescence ratio in a dose-dependent manner. In cells stimulated with 10 or 30 µM cadmium chloride, the addition of TPEN decreased the elevated fura-2 fluorescence ratio to basal levels. In cells stimulated with mercury or MeHg, the addition of TPEN significantly decreased the elevation of the fura-2 fluorescence ratio induced by lower concentrations (10 µM) of mercury or MeHg, but not by higher concentrations (30 µM). Pretreatment with Ca²⁺ channel blockers, such as verapamil, 2-APB or lanthanum chloride, resulted in different effects on the fura-2 fluorescence ratio. Our study provides a characterization of the effects of several heavy metals on the mobilization of divalent cations and the toxicity of heavy metals to neuronal cells.

Multiple Sources of Ca2+ Contribute to Methylmercury-Induced Increased Frequency of Spontaneous Inhibitory Synaptic Responses in Cerebellar Slices of Rat

We previously showed that elevated intracellular Ca(2+) ([Ca(2+)]i) in the molecular layer and granule cells in cerebellar slices is responsible for the initial increases in frequency of spontaneous or miniature inhibitory postsynaptic currents (sIPSCs or mIPSCs) of Purkinje cells following methylmercury (MeHg) treatment. To identify the contribution of different Ca(2+) sources to MeHg-induced stimulation of spontaneous GABA release, we examined sIPSC or mIPSC frequency of Purkinje cells in acutely prepared cerebellar slices using whole-cell patch-clamp recording techniques under conditions of lowered [Ca(2+)]o, pretreatment with caffeine, cyclopiazonic acid (CPA), thapsigargin or ruthenium red (RR) to deplete ryanodine-sensitive and insensitive intracellular Ca(2+) stores or mitochondria, or a combination of lowering [Ca(2+)]o and increased BAPTA buffering. Lowering [Ca(2+)]o significantly reduced sIPSC or mIPSC frequency and amplitudes, but failed to completely prevent MeHg-induced increase in these events frequency. Caffeine, CPA, or thapisgargin also minimized MeHg-induced increase in sIPSC frequency, yet none of them completely blocked MeHg-induced increase in sIPSC frequency. Similarly, the mitochondrial Ca(2+) transport inhibitor RR, or a combination of lowering [Ca(2+)]o and BAPTA buffering reduced but did not prevent MeHg-induced changes in mIPSC frequency. Consistently, confocal Ca(2+) imaging under low [Ca(2+)]o conditions or in the presence of caffeine or CPA exhibited a marked reduction of MeHg-induced increases in [Ca(2+)]i in both molecular and granule layers. Thus, these results verify that a combination of extracellular Ca(2+) influx and Ca(2+) release from different intracellular Ca(2+) pools all contribute to MeHg-induced increase in [Ca(2+)]i and spontaneous GABA release, although extracellular Ca(2+) appears to be the primary contributor.

Methylmercury-Dependent Increases in Fluo4 Fluorescence in Neonatal Rat Cerebellar Slices Depend on Granule Cell Migrational Stage and GABAA Receptor Modulation

Methylmercury (MeHg) disrupts cerebellar function, especially during development. Cerebellar granule cells (CGC), which are particularly susceptible to MeHg by unknown mechanisms, migrate during this process. Transient changes in intracellular Ca(2+) (Ca(2+) i) are crucial to proper migration, and MeHg is well known to disrupt CGC Ca(2+) i regulation. Acutely prepared slices of neonatal rat cerebellum in conjunction with confocal microscopy and fluo4 epifluorescence were used to track changes induced by MeHg in CGC Ca(2+) i regulation in the external (EGL) and internal granule cell layers (IGL) as well as the molecular layer (ML). MeHg caused no cytotoxicity but did cause a time-dependent increase in fluo4 fluorescence that depended on the stage of CGC development. CGCs in the EGL were most susceptible to MeHg-induced increases in fluo4 fluorescence. MeHg increased fluorescence in CGC processes but only diffusely; Purkinje cells rarely fluoresced in these slices. Neither muscimol nor bicuculline alone altered baseline fluo4 fluorescence in any CGC layer, but each delayed the onset and reduced the magnitude of effect of MeHg on fluo4 fluorescence in the EGL and ML. In the IGL, both muscimol and bicuculline delayed the onset of MeHg-induced increases in fluo4 fluorescence but did not affect fluorescence magnitude. Thus, acute exposure to MeHg causes developmental stage-dependent increases in Ca(2+) i in CGCs. Effects are most prominent in CGCs during development or early stages of migration. GABAA receptors participate in an as yet unclear manner to MeHg-induced Ca(2+) i dysregulation of CGCs.

Alteration in MARCKS phosphorylation and expression by methylmercury in SH-SY5Y cells and rat brain

The molecular mechanisms mediating methylmercury (MeHg)-induced neurotoxicity are not completely understood. Because myristoylated alanine-rich C kinase substrate (MARCKS) plays an essential role in the differentiation and development of neuronal cells, we studied the alteration of MARCKS expression and phosphorylation in MeHg-induced neurotoxicity of neuroblastoma SH-SY5Y cells and in the rat brain. Exposure to MeHg induced a decrease in cell viability of SH-SY5Y cells, which was accompanied by a significant increase in phosphorylation and a reduction in MARCKS expression. Pretreatment of cells with a protein kinase C inhibitor or an extracellular Ca(2+) chelator suppressed MeHg-induced MARCKS phosphorylation. In MARCKS knock-down cells, MeHg-induced cell death was significantly augmented in comparison to control siRNA. In brain tissue from MeHg-treated rats, MARCKS phosphorylation was enhanced in the olfactory bulb in comparison to control rats. The present study may indicate that alteration in MARCKS expression or phosphorylation has consequences for MeHg-induced neurotoxicity.

Methylmercury decreases cellular excitability by a direct blockade of sodium and calcium channels in bovine chromaffin cells: an integrative study

Methylmercury, a potent environmental pollutant responsible for fatal food poisoning, blocked calcium channels of bovine chromaffin cells in a time- and concentration-dependent manner with an IC50 of 0.93 μM. This blockade was not reversed upon wash-out and was greater at more depolarising holding potentials (i.e. 21 % at -110 mV and 60 % at -50 mV, after 3 min perfusion with methylmercury). In ω-toxins-sensitive calcium channels, methylmercury caused a higher blockade of I Ba than in ω-toxins-resistant ones, in which a lower blockade was detected. The sodium current was also blocked by acute application of methylmercury in a time- and concentration-dependent manner with an IC50 of 1.05 μM. The blockade was not reversed upon wash-out of the drug. The drug inhibited sodium current at all test potentials and shows a shift of the I-V curve to the left of about 10 mV. Intracellular dialysis with methylmercury caused no blockade of calcium or sodium channels. Voltage-dependent potassium current was not affected by methylmercury. Calcium- and voltage-dependent potassium current was also drastically depressed. This blockade was related to the prevention of Ca(2+) influx through voltage-dependent calcium channels coupled to BK channels. Under current-clamp conditions, the blockade of ionic current present during the generation and termination of action potentials led to a drastic alteration of cellular excitability. The application of methylmercury greatly reduced the shape and the number of electrically evoked action potentials. Taken together, these results point out that the neurotoxic action evoked by methylmercury may be associated to alteration of cellular excitability by blocking ionic currents responsible for the generation and termination of action potentials.

Dietary nimodipine delays the onset of methylmercury neurotoxicity in mice

Adult-onset methylmercury (MeHg) exposure is thought to result primarily in sensory and motor deficits but effects on learning are poorly understood. One mechanism by which chronic MeHg may exert its neurotoxicity is via sustained disruption of intracellular calcium homeostasis, with a consequent increase of intracellular Ca(2+) ions in vulnerable neurons. A biochemically heterogeneous group of compounds, calcium channel blockers, have been shown in vitro to attenuate MeHg's toxicity. To evaluate the role of calcium antagonism in MeHg toxicity in vivo, adult BALB/c mice were exposed chronically to 0 or 15 ppm of Hg (as MeHg) via drinking water and to nimodipine, a dihydropryidine, L-type Ca(2+) channel blocker with action in the CNS. Nimodipine was administered orally in diets (0, 20, or 200 ppm, producing approximately 0, 2, or 20 mg/kg/day of nimodipine). An incremental repeated acquisition (IRA) of response chains procedure was used to detect MeHg-induced deficits in learning or motoric function and to evaluate possible neuroprotection by nimodipine. MeHg impaired performance on the IRA task, and this was partially or completely blocked by dietary nimodipine, depending on dose. Measures of learning co-varied with measures of motoric function as indicated by overall response rate. Nimodipine delayed or prevented the behavioral toxicity of MeHg exposure as evidenced by IRA performance; effects on learning seemed secondary to response rate decreases.

Role of calcium and mitochondria in MeHg-mediated cytotoxicity

Methylmercury (MeHg) mediated cytotoxicity is associated with loss of intracellular calcium (Ca²⁺) homeostasis. The imbalance in Ca²⁺ physiology is believed to be associated with dysregulation of Ca²⁺ intracellular stores and/or increased permeability of the biomembranes to this ion. In this paper we summarize the contribution of glutamate dyshomeostasis in intracellular Ca²⁺ overload and highlight the mitochondrial dysfunctions induced by MeHg via Ca²⁺ overload. Mitochondrial disturbances elicited by Ca²⁺ may involve several molecular events (i.e., alterations in the activity of the mitochondrial electron transport chain complexes, mitochondrial proton gradient dissipation, mitochondrial permeability transition pore (MPTP) opening, thiol depletion, failure of energy metabolism, reactive oxygen species overproduction) that could culminate in cell death. Here we will focus on the role of oxidative stress in these phenomena. Additionally, possible antioxidant therapies that could be effective in the treatment of MeHg intoxication are briefly discussed.

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.

Probing the bioinorganic chemistry of toxic metals in the mammalian bloodstream to advance human health

The etiology of numerous grievous human diseases, including Alzheimer's and Parkinson's Disease is not well understood. Conversely, the concentration toxic metals and metalloids, such as As, Cd, Hg and Pb in human blood of the average population is well established, yet we know strikingly little about the role that they might play in the etiology of disease processes. Establishing functional connections between the chronic exposure of humans to these and other inorganic pollutants and the etiology of certain human diseases is therefore viewed by many as one of the greatest challenges in the post-genomic era. Conceptually, this task requires us to uncover hitherto unknown biomolecular mechanisms which must explain how small doses of a toxic metal/metalloid compound (low μg per day) - or mixtures thereof - may eventually result in a particular human disease. The biological complexity that is inherently associated with mammals, however, makes the discovery of these mechanisms a truly monumental task. Recent findings suggest that a better understanding of the bioinorganic chemistry of inorganic pollutants in the mammalian bloodstream represents a fruitful strategy to unravel relevant biomolecular mechanisms. The adverse effect(s) that toxic metals/metalloid compounds exert on the transport of essential ultratrace elements to internal organs appear particularly pertinent. A brief overview of the effect that arsenite and Hg(2+) exert on the mammalian metabolism of selenium is presented.

Ca2+ entry pathways in mouse spinal motor neurons in culture following in vitro exposure to methylmercury

Methylmercury (MeHg) is a widespread environmental toxicant with major actions on the central nervous system. Among the neurons reportedly affected in cases of Hg poisoning are motor neurons; however, the direct cellular effects of MeHg on motor neurons have not been reported. Ratiometric fluorescence imaging, using the Ca(2+)-sensitive fluorophore fura-2, was used to examine the effect of MeHg on Ca(2+) homeostasis in primary cultures of mouse spinal motor neurons. In vitro MeHg exposure at concentrations (0.1-2 μM) known to affect other neurons in culture differentially, induced a biphasic rise in fura-2 fluorescence ratio indicating an increase in [Ca(2+)](i). The time-to-onset of these fura-2 fluorescence ratio changes was inversely correlated with MeHg concentration. TPEN (20 μM), a non-Ca(2+), divalent cation chelator, reduced the amplitude of the increase in fura-2 fluorescence induced by MeHg in the first phase, indicating that both Ca(2+) and non-Ca(2+) divalent cations contribute to the MeHg-induced effect. When examining various Ca(2+) entry pathways as possible targets contributing to Ca(2+) influx, we found that excitatory amino acid receptor blockers MK-801 (15 μM), and AP-5 (100 μM)-both NMDA receptor-operated ion channel blockers, CNQX (20 μM), a non-NMDA receptor blocker, and the voltage-dependent Ca(2+) channel blockers nifedipine (1 μM) and ω-conotoxin-GVIA (1 μM) all significantly delayed the development of increased Ca(2+) caused by MeHg. The voltage-dependent Na(+) channel blocker tetrodotoxin (TTX, 1 μM) did not alter the MeHg-induced increases in fura-2 fluorescence ratio. Thus, MeHg alters Ca(2+) homeostasis in mouse spinal motor neurons through excitatory amino acid receptor-mediated pathways, and nifedipine and ω-conotoxin-GVIA-sensitive pathways. Spinal motor neurons are highly sensitive to this effect of acute exposure to MeHg.

Exposure to an environmental neurotoxicant hastens the onset of amyotrophic lateral sclerosis-like phenotype in human Cu2+/Zn2+ superoxide dismutase 1 G93A mice: glutamate-mediated excitotoxicity

Mice expressing the human Cu(2+)/Zn(2+) superoxide dismutase 1 (hSOD1) gene mutation (hSOD1(G93A); G93A) were exposed to methylmercury (MeHg) at concentrations that did not cause overt motor dysfunction. We hypothesized that low concentrations of MeHg could hasten development of the amyotrophic lateral sclerosis (ALS)-like phenotype in G93A mice. MeHg (1 or 3 ppm/day in drinking water) concentration-dependently accelerated the onset of rotarod failure in G93A, but not wild-type, mice. At the time of rotarod failure, MeHg increased Fluo-4 fluorescence (free intracellular calcium concentration [Ca(2+)](i)) in soma of brainstem-hypoglossal nucleus. These motor neurons control intrinsic and some extrinsic tongue function and exhibit vulnerability in bulbar-onset ALS. The α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)/kainic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione reduced [Ca(2+)](i) in all G93A mice, irrespective of MeHg treatment. N-acetyl spermine, which antagonizes Ca(2+)-permeable AMPA receptors, further reduced [Ca(2+)](i) more effectively in MeHg-treated than untreated G93A mice, suggesting that MeHg-treated mice have a greater Ca(2+)-permeable AMPA receptor contribution. The non-Ca(2+) divalent cation chelator N,N,N',N'-tetrakis(pyridylmethyl)ethylenediamine reduced Fluo-4 fluorescence in all G93A mice; FluoZin-(Zn(2+) indicator) fluorescence was increased in all MeHg-treated mice. Thus in G93A mice Zn(2+) apparently contributed measurably to the MeHg-induced effect. This is the initial demonstration of accelerated onset of ALS-like phenotype in a genetically susceptible organism by exposure to low concentrations of an environmental neurotoxicant. Increased [Ca(2+)](i) induced by the G93A-MeHg interaction apparently was associated with Ca(2+)-permeable AMPA receptors and may contribute to the hastened development of ALS-like phenotypes by subjecting motor neurons to excessive elevation of [Ca(2+)](i), leading to excitotoxic cell death.

Application of single-cell microfluorimetry to neurotoxicology assays

Intracellular signaling events play fundamental roles in regulating physiological function. In neurons, these include inducing growth and differentiation, secretion, gene expression, and controlling processes associated with learning and memory. All of these processes have in common the vital dependence on changes in intracellular Ca²(+) [Ca²(+)](i). Numerous toxicants, including metals, polychlorinated biphenyls, and biological neurotoxins, can disrupt [Ca²(+)](i). Understanding how toxicants disrupt Ca²(+)-dependent neuronal signaling, and thus induce neuronal death or dysfunction, requires the ability to monitor [Ca²(+)](i) at the level of individual cells. A series of fluorophores that can report on changes in [Ca²(+)](i) has been pivotal in this process. This section describes how to use these fluorophores to study effects of neurotoxicants on two types of processes: changes in [Ca²(+)](i) in individual cells and changes in mitochondrial membrane potential. Similar techniques using distinct fluorophores can be applied to other physiological processes.

Comparative study on the response of rat primary astrocytes and microglia to methylmercury toxicity

As the two major glial cell types in the brain, astrocytes and microglia play pivotal but different roles in maintaining optimal brain function. Although both cell types have been implicated as major targets of methylmercury (MeHg), their sensitivities and adaptive responses to this metal can vary given their distinctive properties and physiological functions. This study was carried out to compare the responses of astrocytes and microglia following MeHg treatment, specifically addressing the effects of MeHg on cell viability, reactive oxygen species (ROS) generation and glutathione (GSH) levels, as well as mercury (Hg) uptake and the expression of NF-E2-related factor 2 (Nrf2). Results showed that microglia are more sensitive to MeHg than astrocytes, a finding that is consistent with their higher Hg uptake and lower basal GSH levels. Microglia also demonstrated higher ROS generation compared with astrocytes. Nrf2 and its downstream genes were upregulated in both cell types, but with different kinetics (much faster in microglia). In summary, microglia and astrocytes each exhibit a distinct sensitivity to MeHg, resulting in their differential temporal adaptive responses. These unique sensitivities appear to be dependent on the cellular thiol status of the particular cell type.

Probing bioinorganic chemistry processes in the bloodstream to gain new insights into the origin of human diseases

In the context of elucidating the origin of human diseases, past poisoning epidemics have revealed that exceedingly small doses of inorganic environmental pollutants can result in dramatic effects on human health. Today, numerous organic and inorganic pollutants have been quantified in human blood, but the interpretation of these concentrations remains--from a public health point of view--problematic. Conversely, the biomolecular origin for several grievous human diseases is essentially unknown. Taken together and viewed in the context of recent bioinorganic research findings, the established human blood concentrations of toxic metals and metalloids may be functionally connected with the etiology of specific human diseases. To unravel the underlying biomolecular mechanisms, and taking into account the basic flow of dietary matter through mammalian organisms, a better understanding of the bioinorganic chemistry of toxic metals and metalloid compounds in the bloodstream is emerging as a promising avenue for future research. To this end, the concerted application of modern proteomic methodologies, synchrotron-based X-ray absorption spectroscopy and established spectroscopic techniques will contribute to better define the role that blood-based bioinorganic chemistry-related processes play in the origin of human diseases. The application of this and other modern proteomic methodologies could contribute to a better understanding of the role that blood-based bioinorganic chemistry-related processes play in the origin and etiology of human diseases.

Low level postnatal methylmercury exposure in vivo alters developmental forms of short-term synaptic plasticity in the visual cortex of rat

Methylmercury (MeHg) has been previously shown to affect neurotransmitter release. Short-term synaptic plasticity (STP) is primarily related to changes in the probability of neurotransmitter release. To determine if MeHg affects STP development, we examined STP forms in the visual cortex of rat following in vivo MeHg exposure. Neonatal rats received 0 (0.9% NaCl), 0.75 or 1.5 mg/kg/day MeHg subcutaneously for 15 or 30 days beginning on postnatal day 5, after which visual cortical slices were prepared for field potential recordings. In slices prepared from rats treated with vehicle, field excitatory postsynaptic potentials (fEPSPs) evoked by paired-pulse stimulation at 20-200 ms inter-stimulus intervals showed a depression (PPD) of the second fEPSP (fEPSP2). PPD was also seen in slices prepared from rats after 15 day treatment with 0.75 or 1.5 mg/kg/day MeHg. However, longer duration treatment (30 days) with either dose of MeHg resulted in paired-pulse facilitation (PPF) of fEPSP2 in the majority of slices examined. PPF remained observable in slices prepared from animals in which MeHg exposure had been terminated for 30 days after completion of the initial 30 day MeHg treatment, whereas slices from control animals still showed PPD. MeHg did not cause any frequency- or region-preferential effect on STP. Manipulations of [Ca2+](e) or application of the GABA(A) receptor antagonist bicuculline could alter the strength and polarity of MeHg-induced changes in STP. Thus, these data suggest that low level postnatal MeHg exposure interferes with the developmental transformation of STP in the visual cortex, which is a long-lasting effect.

Cerebellum cholinergic muscarinic receptor (subtype-2 and -3) and cytoarchitecture after developmental exposure to methylmercury: an immunohistochemical study in rat

The developing central nervous system (CNS) is a target of the environmental toxicant methylmercury (MeHg), and the cerebellum seems the most susceptible tissue in response to this neurotoxicant. The cholinergic system is essential for brain development, acting as a modulator of neuronal proliferation, migration and differentiation processes; its muscarinic receptors (MRs) play pivotal roles in regulating important basic physiologic functions. By immunohistochemistry, we investigated the effects of perinatal (GD7-PD21) MeHg (0.5 mg/kg bw/day in drinking water) administration on cerebellum of mature (PD36) and immature (PD21) rats, evaluating the: (i) M2- and M3-MR expression; (ii) presence of gliosis; (iii) cytoarchitecture alterations. Regarding to M2-MRs, we showed that: at PD21, MeHg-treated animals did not display any differences compared to controls, while, at PD36 there was a significant increase of M2-immunopositive Bergmann cells in the molecular layer (ML), suggesting a MeHg-related cytotoxic effect. Similarly to M2-MRs, at PD21 the M3-MRs were not affected by MeHg, while, at PD36 a lacking immunoreactivity of the granular layer (IGL) was observed after MeHg treatment. In MeHg-treated rats, at both developmental points, we showed reactive gliosis, e.g. a significant increase in Bergmann glia of the ML and astrocytes of the IGL, identified by their expression of glial fibrillar acidic protein. No MeHg-related effects on Purkinje cells were detected neither at weaning nor at puberty. These findings suggest: (i) a delayed MeHg exposure-related effect on M2- and M3-MRs, (ii) an overt MeHg-related cytotoxic effect on cerebellar oligodendroglia, e.g. reactive gliosis, (iii) a selective vulnerability of granule cells and Purkinje neurons to MeHg, with the latter that remain unharmed.

Perinatal co-exposure to methylmercury and PCB153 or PCB126 in rats alters the cerebral cholinergic muscarinic receptors at weaning and puberty

In the last few decades, combined exposure to methylmercury (MeHg) and polychlorinated biphenyls (PCBs) from fish and seafood, and their potentially interactive effects on neurodevelopment, have been giving increasing cause for concern. We examined the combined effects of MeHg and either a non-dioxin PCB (PCB153) or a dioxin-like PCB (PCB126) congener on the developing brain cholinergic muscarinic receptors (MRs). These receptors are known to play a major role in many central functions including higher cognitive processes and the modulation of extrapyramidal motor activity. MRs in pup rat brains diminished following prenatal and lactational exposure, from gestational day [GD]7 to postnatal day [PND]21, to MeHg (0.5mg/kgbodyweight[bw]/day), PCB153 (5mg/kgbw/day), and PCB126 (100ng/kg/day), alone or in combination. Total MR density, as well as M1, M2, and M3 receptor subtypes of the weanling and pubertal rats, were affected in a brain-area-, gender-, time- and compound-dependent fashion. MeHg decreased (by 15-20%) the total MR density in a delayed (PND36) manner in the cerebral cortex of both genders, and early (at weaning) in the cerebellum of both genders, with the effect lasting until puberty (in males only). MeHg decreased the ACh M1- and M3-immunopositive neurons in the cerebral cortex and also increased the M2-immunopositive Bergmann glia in the cerebellum. PCB153 also induced a delayed (PND36) decrease (of 20%) in total MR number in the cerebellum of the male offspring and in the cerebral cortex of both genders. The latter effect was coupled with a decrease in ACh M1- and ACh M3-immunopositive neuron populations. PCB126 decreased (by 30-40%) total MR density in a gender-dependent manner, males being more sensitive than females. The effect was evident early (at PND21) and lasted until puberty in the cerebellum, while it was observed later (at PND36) in the cerebral cortex. The M1 and M3 receptors were similarly affected by PCB126. Co-exposure to MeHg and either PCB153 or PCB126 had the same effect on the cerebral MRs as exposure to each compound alone. The results rule out additive or synergistic interactions between MeHg and PCB153 or PCB126 on MRs in the brain areas examined. Some early-onset changes persisted until puberty, while other modifications became manifest only at the advanced time point (PND36), when the brain levels of total Hg, PCB153, and PCB126 had declined. These data support the ability of MeHg and PCBs to induce delayed neurotoxicity after developmental exposure.