Stephanie J. B.
Fretham
,
Samuel
Caito
,
Ebany J.
Martinez-Finley
and
Michael
Aschner
*
Department of Pediatrics and Department of Pharmacology, and the Kennedy Center for Research on Human Development, Vanderbilt University Medical Center, Nashville, TN, USA. E-mail: michael.aschner@vanderbilt.edu; Fax: +1 615-936-4080; Tel: +1 615-322-8024
First published on 2nd May 2012
The neurotoxic consequences of methylmercury (MeHg) exposure have long been known, however a complete understanding of the mechanisms underlying this toxicity is elusive. Recent epidemiological and experimental studies have provided mechanistic insights into the contribution of genetic and environmental factors that interact with MeHg to modify toxicity. This review will outline cellular processes directly and indirectly affected by MeHg, including oxidative stress, cellular signaling and gene expression, and discuss epigenetic modifications, genetic polymorphisms and gene–environment interactions capable of modifying MeHg neurotoxicity.
As early as the mid 1800s the neurotoxic effects of MeHg were evident from poisonings and deaths caused by industrial exposure. In Minamata, Japan, industrial wastewater containing MeHg was released into the environment over a number of years and bioaccumulated in fish consumed by the local population. In the 1950s many adults developed Minamata disease, characterized by paresthesia, sensory deficits, slurred speech, unsteady gait, muscle weakness, irritability, memory loss, depression, and sleeping disturbance.2 Similar symptoms were also observed in Iraq in 1956 and 1960 after consumption of bread made from grain treated with ethylmercury and again in 1971 from grain treated with methylmercury.3
The first recorded case of prenatal MeHg neurotoxicity in humans occurred in the 1950s, a few years later children in Minamata, Japan, presented with severe neurodevelopmental deficits that were later related to maternal consumption of fish contaminated with MeHg. Studies from the Iraq exposures provided further evidence that the developing brain is particularly sensitive to MeHg.3,4 Since these incidences, several large-scale epidemiological studies have examined prenatal MeHg toxicity in humans from Minamata and fish-eating regions in the Seychelles, Faroe Islands and New Zealand.
This review will outline cellular processes directly and indirectly affected by MeHg and discuss genetic, environmental and nutritional factors capable of modifying MeHg toxicity.
Several important regulators of the antioxidant response are rich in thiol and selenol groups. Glutathione (γ-glutamyl-cysteinyl-glycine; GSH) is a highly abundant tripeptide, which serves as a reducing agent for reactive oxygen species (ROS) and other unstable molecules, a reaction catalyzed by the selenoprotein glutathione peroxidase (GPx). GSH can also be conjugated to toxins to facilitate excretion during detoxification. The majority of GSH is present in the reduced form (GSH) while approximately 10% is present as the oxidized form glutathione disulfide (GSSG), the GSSG:GSH ratio reflects the oxidative state of the cell.18 MeHg interacts with GSH to form an excretable GS–HgCH3 complex,14 increasing the GSSG:GSH ratio and reducing the antioxidant capacity both in astrocytes and microglia.19,20 Furthermore, thioredoxins (Trxs), a family of proteins with abundant thiol groups, are essential in regulating cellular thiol/disulfide redox status. Trxs contain an invariant active center, -Cys-Gly-Pro-Cys- where the two Cys residues undergo reversible oxidation/reduction in the presence of the selenoprotein Trx reductase (TrxR) and NADPH.21 The cytosolic Trx (Trx 1) is also implicated in growth stimulation, transcriptional regulation and apoptosis.22 The mitochondrial Trx (Trx 2) is a critical component of the mitochondrial antioxidant system and protects against oxidative stress-induced cell death.23In vitro, MeHg inhibits GPx and TrxR activity.24 Prior to inducing cell death, MeHg inhibits GPx activity in cultured cerebellar granule cells, and overexpression of GPx prevents cell death.25 These effects also occur in vivo in mice and fish.26,27
In addition to impairing the antioxidant response of the cell, MeHg increases ROS production. Hydrogen peroxide (H2O2) levels are increased concomitant with altered motor reflexes in mouse pups exposed to MeHg through lactation.28 H2O2 levels are also increased by MeHg in cultured astrocytes29 and in mitochondria isolated from mouse and rat brain.30,31 Decreased H2O2 detoxification resulting from reduced GSH and GPx activity contributes to these increases, although Mori and colleagues observed increased H2O2 production from MeHg-induced changes in the mitochondrial electron transport chain (ETC) complexes II and III in mitochondria isolated from cerebellum, but not liver or cerebrum.31,32 Superoxide anion (O2˙−) levels also increased due to changes in ETC complexes I and III respiration.33 Nitric oxide (NO) production rises as well following MeHg exposure,34–36 likely through stimulation of nitric oxide synthase (NOS) by increased Ca2+.
The resulting pro-oxidative shift has wide-reaching consequences including lipid peroxidation, protein oxidation, DNA oxidation, altered Ca2+ homeostasis, impaired mitochondria and apoptosis. Owing to the high content of polyunsaturated fatty acids (PUFAs) in the central nervous system, lipid peroxidation is a major consequence of free radical-mediated injury. H2O2 undergoes Fenton chemistry, producing the hydroxyl radical (˙OH), a primary cause of lipid oxidation.33 The oxidative shift caused by MeHg also affects DNA. A study of Chinese mine workers and local residents chronically exposed to various forms of MeHg from Hg mining plants revealed increased urinary 8-oxo-2′-deoxyguanosine (8-OHdG) levels, a metabolite of oxidized DNA, as well as increased serum markers of oxidation in exposed individuals compared to age and gender matched control individuals.37
Mitochondrial function is also affected by oxidation of thiol rich proteins, such as mitochondrial creatine kinase and respiratory chain complexes.38,39 Mori et al. demonstrated that MeHg disrupts ETC function in cerebellar mitochondria from young adult rats exposed to MeHg for five days.32 Consistent with mitochondrial dysfunction, Hg2+ increased mitochondrial permeability and swelling, promoted de-coupling, and caused depolarization in isolated rat liver mitochondria.40 Furthermore, MeHg inhibits uptake of Ca2+ and stimulates Ca2+ release in neuronal mitochondria.41
The pro-oxidative chemical properties and biological effects of MeHg have been shown to induce neuronal apoptosis in a variety of in vitro and in vivo models and neuronal cell types (for comprehensive review see ref. 42). MeHg-induced apoptosis is reduced in cultured primary cortical neurons and rat cerebral cortical neurons by the cysteine derivative N-acetylcysteine,43 suggesting that the high affinity of MeHg for thiol groups (including cysteine) may contribute to neuronal apoptosis. Dysregulation of Ca2+ homeostasis is another important factor mediating MeHg-induced apoptosis. In addition to mitochondrial Ca2+, MeHg increases intracellular Ca2+ through N-methyl-D-aspartate (NMDA) receptors and inositol trisphosphate (IP3) sensitive endoplasmic reticulum stores (discussed below). In an in vivo rat model, inhibition of NMDA receptors reduces apoptosis in the frontal cortex.49 Altogether, MeHg reduces ETC capacity, increases ROS production and alters Ca2+ causing opening of the mitochondrial permeability transition pore (mPTP).44,45 The mPTP allows release of cytochrome c oxidase from the inner mitochondrial membrane and triggers apoptotic cell death (for review see ref. 46).
Along with glutamate stimulated Ca2+ entry through NMDA receptors, MeHg causes release of IP3 sensitive endoplasmic reticulum Ca2+ stores through direct inhibition of the thiol-rich endoplasmic Ca2+-ATPase.52,53 Disrupted Ca2+ homeostasis contributes to toxicity by increasing neurotransmitter release, altering mitochondrial membrane potential, and promoting Ca2+-mediated intracellular signaling.52,53
In contrast to increased excitatory glutamate signaling, MeHg decreases GABA signaling, the most abundant inhibitory neurotransmitter. In primary cerebellar granule cells, MeHg interacts with thiol resides on GABAA receptors, which modifies receptor conformation.54 Chronic low-level MeHg exposure in captive minks reduces the activity of the synthetic enzyme glutamic acid decarboxylase, and significantly decreases of GABAA receptors and transport activity in the brain stem and basal ganglia.55 Glutamate and GABA signaling are crucial for establishing equilibrium between excitation and inhibition. Shifts in this equilibrium are associated with regulation of normative critical periods in neurodevelopment56 as well as with neurologic disorders such as epilepsy.57 In combination, these direct effects of MeHg on the regulation of neuronal signaling likely contribute to MeHg-induced neurotoxicity.
In pancreatic β-cells, MeHg increases PI3K/Akt activity,61 in part due to increased ROS production and in part through inactivation of PTEN, a thiol-rich PI3K inhibitor.62 Low concentrations of MeHg trigger ROS production and increase PI3K activity and its downstream effector phospho-Akt,63 pharmacological inhibition of PI3K attenuates these changes in astrocytes.19 PI3K is activated by a G-protein-coupled receptor or receptor tyrosine kinase such as the insulin receptor.64 Once activated, PI3K phosphorylates phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to form phosphatidylinositol (3,4,5)-triphosphate [PtdIns(3,4,5)P3]. Akt functions downstream of PI3K and is activated by phosphorylation.64 Akt activation has many consequences including regulation of the protective transcription factors forkhead box protein O (FOXO) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and additional signaling cascades regulating cellular growth and morphology. These observations are consistent with MeHg's widespread damage in fetal and neonatal brain, characterized by hypoplastic and symmetrical brain atrophy, and reflective of aberrant cell division, migration, differentiation and synaptogenesis.2,65–67
Nrf2 induces the expression of several cytoprotective proteins including NAD(P)H quinine oxidoreductase (NQO1), glutamate-cysteine ligase catalytic subunit and modifier subunit (GCLC and GCLM), and heme oxygenase-1 (HO-1).68 MeHg has been shown to activate Nrf2 in both cell lines and primary cells, allowing for an up-regulation of NQO1 and HO-1.19 In addition to oxidative stress derived from MeHg exposure, Toyama et al. have shown MeHg's ability to bind to recombinant Keap1 and to activate Nrf2.74 Nrf2 has been shown to be protective in a Caenorhabditis elegans model of dopaminergic neurodegeneration induced by MeHg, where reduction of SKN-1, the Nrf2 homologue, increases the vulnerability for neurodegeneration.75 Activation of Nrf2 by MeHg is a protective response; however MeHg often alters gene expression resulting in neurotoxic effects.
Recently, several toxicogenomics studies have been performed to characterize changes in gene expression upon MeHg exposure.60,76,77 In the cerebellum of eight-week-old mice exposed to 10 mg kg−1 per day for 7 days, several genes involved in inflammation (such as the chemokines, CCL2 and CCL4), metabolism, anti-apoptosis (Bcl2a1b) and signal transduction were increased above control, while genes involved in cell proliferation, oxidative stress (selenoprotein H) and apoptosis (Ifi27) were decreased.60 Increased oxidative stress can cause inflammation through the activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) transcription factor, and inflammation contributes to several neurologic diseases, such as Alzheimer's disease and Parkinson's disease.78 MeHg has been shown to increase IL-6 release from glial cells79 and activation of NF-kB has been observed in rat cerebrum and cerebellum.80
Changes in gene expression can be more severe if the exposure occurs during neurodevelopment. The adult brain has a complex structure formed by several cell types and numerous connections. Disruption of normal neurodevelopment or death of cells caused by exposure to a xenobiotic can have consequences on the functioning of the adult brain. There are several reports of genome wide analyses of genes that are up-regulated and down-regulated in adults or in various stages of development. In a comparative study, pooling transcriptomics data sets from studies involving mouse embryos exposed to MeHg in utero, young mice exposed both in utero and postnatally, adult rats chronically exposed to MeHg, whole embryo cultures exposed to MeHg, embryonic stem cells induced into cardiomyocytes or neurons and mouse embryonic fibroblasts exposed to MeHg, Robinson et al. found that although different models were used, as well as experimental variables of dose and time of exposure, there is a set of genes that are consistently altered by MeHg that effect development of brain as well as heart and kidney.76 Using neural cells differentiated from murine embryonic stem cells exposed to 25 nM MeHg for 8 consecutive days, Theunissen et al. found MeHg to increase late differentiation gene and neuroectodermal gene sets, and to decrease early differentiation gene, mesodermal and endodermal derived tissue gene sets, resulting in cultures with retarded development that deviated from their normal differentiation track.77 Large studies like these are important in identifying pathways affected by MeHg exposure although exact mechanisms of how an individual gene is up or down regulated may not be clear.
The BDNF promoter in hippocampus of mice exposed to MeHg showed decreased histone H3 acetylation on K9 and K14, a marker of active chromatin state, and increased trimethylation on histone H3 K27, a marker of repressed gene expression.81,82 Additionally, DNA methylation was enriched in four separate CpG sites in the BDNF promoter after exposure to MeHg.81 Hypomethylated DNA correlates with increased DNA binding to transcription factors whereas hypermethylated DNA is associated with reduced transcription factor binding.83 All of these markers cumulatively suggest that the reduction in BDNF levels in the hippocampus after MeHg exposure is a result of epigenetic modifications, suggesting that MeHg may affect other genes through alteration of epigenetic markers. The extent to which epigenetic modifications contribute to MeHg-induced gene expression and the mechanisms through which MeHg alters epigenetic markers are not known.
A study in Austria by Gundacker and colleagues, examined in medical students the potential associations between Hg exposure and the glutathione S-transferases (GSTs), GST theta 1, GSTT1; GST mu 1, GSTM1; GSTA1, GST pi 1, GSTP1; glutamate-cysteine ligase catalytic subunit, GCLC; and metallothionine (MT) polymorphisms. The levels of Hg exposure of students were low as determined by analysis of blood, urine, and hair. GSTP1-114 and MT4 allele carriers as well as GSTT1−/− and GSTM1−/− jointly deleted phenotypes were associated with higher hair mercury levels than homozygous wild types. GSTP1-114/GSTT1 and GSTP1-105/GCLC combinations showed synergistic effects on hair Hg levels compared to single-gene variants and the authors suggest that the GSTP1 variants are more important in mercury toxicokinetics than the other GST polymorphisms.84
In a study of Michigan dental professionals, Goodrich and colleagues evaluated polymorphisms in key glutathione synthesizing enzymes, glutathione S-transferase, and selenoprotein genes underlying inter-individual differences in Hg body burden. They measured urine and hair Hg levels. Five polymorphisms were significantly associated with urine Hg levels (GSTT1 deletion), hair Hg levels (GSTP1-105, GSTP1-114, GSS 5′), or both (SEPP1 3′UTR). The authors suggest that polymorphisms in these genes may influence elimination of Hg in the urine and hair or retention following exposures to elemental Hg through dental amalgams and MeHg through fish consumption.85 Similar data was presented by Schlawicke Engstrom and colleagues, demonstrating that carriers of GSTP1-105G and GSTP1-114T accumulated less Hg in erythrocytes after controlling for levels of polyunsaturated fatty acids.86 Custodio and colleagues reported that the T allele of the GSTP1-114 polymorphism was associated with increased blood mercury levels.87
Autism is a neurodevelopmental disorder with genetic and environmental components, and genetic susceptibility to high Hg has been suggested as a potential risk factor. Owens and colleagues tested the hypothesis that Hg could be implicated in the etiology of autism if genetic susceptibility altered Hg's metabolism or intracellular compartmentalization. Genetic sequences of four genes implicated in the transport [LAT1 and divalent metal transporter1 (DMT1)] and response to Hg [metal regulatory transcription factor1 (MTF1) and metallothionein1a (MT1a)] were screened for variation and association with autism. The group identified and characterized 74 variants in MT1a, DMT1, LAT1 and MTF1. Polymorphisms identified were evaluated for differences in allele frequencies. They reported no association of any variant evaluated with autism suggesting that variations in these genes may not play a significant role in the etiology of autism.88 Another group using the C. elegans model system to study synapse formation and function as related to autism showed that neuroligin mutants (postsynaptic cell adhesion proteins that bind specifically to presynaptic membrane proteins-neurexins) are hypersensitive to Hg compounds and are defective in sensory behaviors and processing. They suggest that the behavioral deficits are similar to traits frequently associated with autism spectrum disorders and report a possible link between genetic defects in neuroligin and sensitivity to Hg in the development of autism.89
Hg exposure is associated with cardiovascular problems. A common polymorphism of matrix metalloproteinase (MMP)-2 gene is the C(−1306)T which disrupts a promoter site and is associated with lower promoter activity when the T allele is present affecting the expression and activity of the enzyme. A study by Jacob-Ferreira and colleagues examined how this polymorphism affects the circulating MMP-2 levels and tissue inhibitor of metalloproteinase-2 (TIMP-2) following environmental exposure to Hg. They reported a positive association between plasma Hg concentrations and the ratio of MMP-2/TIMP-2. The C(−1306)T polymorphism modified MMP-2 concentrations and MMP-2/TIMP-2 ratio in subjects exposed to Hg, with higher MMP-2 levels found in subjects carrying the C allele. Their findings suggest a significant interaction between the C(−1306)T polymorphism and Hg exposure, with possible ramifications for those carrying the C allele.90
Another study examined the role polymorphisms of the endothelial nitric oxide (eNOS) gene (T-786C and Glu298Asp) on nitrite concentrations following Hg exposure in humans. The hypothesis tested was that Hg exposure might decrease circulating nitrite concentrations and these polymorphisms might enhance the effects of Hg resulting in increased risk of cardiovascular disease. Decreased nitric oxide (NO) production was found to be predominantly related to Hg, age and gender rather than polymorphisms. These findings suggest that there is not an association between increased risk for cardiovascular disease in MeHg-exposed subjects and eNOS gene polymorphisms (T-786C and Glu298Asp).91 The same group investigated the contribution of the 27 nt tandem repeat of intron 4 of the eNOS gene to NO production, which could enhance susceptibility to cardiovascular disease in the MeHg-exposed study population. They found no significant differences in age, arterial blood pressure, body mass index or cardiac frequency between genotype groups, but observed different nitrite concentrations, with lower nitrite levels for the 4a4a genotype carriers. Age, gender and the presence of intron 4 polymorphism contributed to nitrite reduction as a result of blood Hg concentration. Their results suggest increased susceptibility to cardiovascular diseases after MeHg exposure in carriers of the 27 nt repeat polymorphism of intron 4 in the eNOS gene.92
In a Swedish study, Engström and colleagues studied whether genetic polymorphisms in glutathione-related genes modify the association between the PUFAs eicosapentaenoic and docosahexaenoic acid (DHA) or MeHg and risk of first ever myocardial infarction. Polymorphisms in glutathione-synthesizing (glutamyl-cysteine ligase catalytic subunit, GCLC and glutamyl-cysteine ligase modifier subunit, GCLM) or glutathione-conjugating (glutathione S-transferase P, GSTP1) genes were assessed. The authors evaluated the impact of these polymorphisms on the association between erythrocyte-Hg and risk of myocardial infarction, as well as between plasma eicosapentaenoic and DHA and risk of myocardial infarction. They reported no statistically significant genetic modifying effects for the association between plasma eicosapentaenoic and DHA or erythrocyte-Hg and risk of myocardial infarction but do report a relatively rare GCLM-588 TT genotype that may have an impact.93
An association between the Hg resistance merA gene and antibiotic resistance has been described in the literature and has been shown to be widely distributed among bacteria (both Escherichia coli and Staphylococcus aureus) isolated from healthy adults and children.94–99 To study this further, Skurnik and colleagues analyzed Hg resistance in collections of strains from two populations with different levels of Hg exposure and various levels of antibiotic resistance. Hg-resistant E. coli was found significantly more frequently in the population that had the highest antibiotic-resistant E. coli and antibiotic resistance was higher in the population living in an environment with a high exposure to Hg.100
Optimal neurodevelopment requires large amounts of energy, and as a result mitochondrial activity and subsequent ROS production is higher in the developing nervous system than in the adult nervous system.104 Accordingly, in mice the GSH system develops rapidly, with significant increases in GSH, glutathione reductase (GR) and GPx occurring in the first three postnatal weeks.105 Prenatal MeHg exposure not only prevents these increases in antioxidant capacity, but also continues to suppress the system at postnatal day 21, when MeHg levels were no longer elevated, GSH, GR and GPx remained decreased relative to controls.105 This suggests that the developing nervous system is vulnerable to the acute pro-oxidative effects of MeHg, which may affect the antioxidant response throughout life.
In addition to promoting plasticity and survival throughout life, growth factors such as BDNF and neuronal growth factor influence cell migration, differentiation and survival, axonal and dendrite structure and targeting, and synapse formation during development.106 Most neurotrophic factors signal through intracellular cascades susceptible to the oxidative effects of MeHg as described above. Together these early changes not only cause immediate developmental abnormalities such as observed in congenital Minamata disease, but may also contribute to latent effects. For example, neurological deficits have emerged months and years following MeHg exposure in Minamata as well as in other instances of human exposure and experimental primate studies.107,108
Experimental studies have shown that Se supplementation attenuates growth and motor deficits in rats chronically exposed to MeHg following weaning, the severity of deficits was not directly related to MeHg levels in the brain but rather by the Hg:Se ratio.110 In 15-day-old mice born to dams fed MeHg with or without Se supplementation through gestation and lactation, combined Se and MeHg regulated expression of 63 genes in whole genome cerebral microarray compared to 8 and 5 genes affected by Se or MeHg alone.111 This is consistent with functional Se deficiency due to MeHg inhibition of selenoproteins, effects that can be lessened by Se supplementation.112 In a similar study, Jayashankar and colleagues exposed mice to MeHg and/or DHA through maternal diet. DHA decreased accumulation of MeHg in the brain and eliminated reflexive deficits observed in non-DHA supplemented MeHg pups, as well as altering gene expression.113
On the other hand, the contribution of concomitant exposure to additional environmental toxins such as arsenic, pesticides, and polychorinated biphenyls (PCB) to MeHg toxicity has also been investigated. There is limited support for interactions between MeHg and these additional toxins; however many studies have failed to demonstrate consistent interactive toxicity and more research is needed.114
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