Methylmercury efflux from brain capillary endothelial cells is modulated by intracellular glutathione but not ATP

Methylmercury displays pro-adipogenic properties in rainbow trout preadipocytes

Methylmercury (MeHg) is a ubiquitous contaminant largely found in aquatic environments, especially in species at high trophic level such as salmonids. The aim of this study was to evaluate the effects of MeHg on adipocyte differentiation and lipid metabolism in rainbow trout. Primary cultured preadipocytes were exposed to increasing concentrations of MeHg during six days with or without a hormonal cocktail. Main results showed a dose-dependent intracellular accumulation of neutral lipids with a preferential uptake of n-3 polyunsaturated fatty acids. Interestingly, this accumulation occurred after a fairly low uptake of MeHg by preadipocytes and was maintained after the cellular exposure to MeHg. In membrane phospholipids, arachidonic acid (20:4 n-6) was released in a dose-dependent manner. At the transcriptional level, the expression of several adipocyte-specific genes (perilipin 2 and apolipoprotein Eb) as well as lipid-related genes (fatty acid synthase and fatty acid binding protein 11a) was up-regulated in preadipocytes exposed to MeHg. These results highlight for the first time the disrupting effect of MeHg in trout adipocyte metabolism, providing new insights regarding the role of environmental pollutants in adipose tissue dysfunction and related pathologies.

Drosophotoxicology: Elucidating Kinetic and Dynamic Pathways of Methylmercury Toxicity in a Drosophila Model

The risks of methylmercury (MeHg) toxicity are greatest during early life where it has long been appreciated that the developing nervous system is an especially sensitive target. Yet, understanding the discrete mechanisms of MeHg toxicity have been obscured by the wide variation in the nature and severity of developmental outcomes that are typically seen across individuals in MeHg exposed populations. Some insight has come from studies aimed at identifying a role for genetic background as a modifier of MeHg toxicity, which have predominantly focused on factors influencing MeHg toxicokinetics, notably, polymorphisms in genes related to glutathione (GSH) metabolism. For example, variants in genes encoding the catalytic and modifier subunits of glutamyl-cysteine ligase (GCLc and GCLm), the rate limiting enzyme for GSH synthesis, have been reported to associate with Hg body burden (Hg levels in blood or hair) in humans. However, GSH can facilitate both toxicokinetics and toxicodynamics of MeHg by forming MeHg-GSH conjugates, which are readily transported and excreted, and by acting indirectly as an anti-oxidant. In this study, we refine a model to distinguish kinetic and dynamic traits of MeHg toxicity using a paradigm of Drosophotoxicolgy. First, we identify that the pupal stage is selectively sensitive to MeHg toxicity. Using a protocol of larval feeding, measurements of Hg body burden, and assays of development to adulthood (pupal eclosion), we identify strain-dependent variation in MeHg elimination as a potential kinetic determinant of differential tolerance to MeHg. We also find that global upregulation of GSH levels, with GCLc trans-gene expression, can induce MeHg tolerance and reduce Hg body burden. However, we demonstrate that MeHg tolerance can also be achieved independently of reducing Hg body burden, in both wild-derived strains and with targeted expression of GCLc in developing neuronal and muscle tissue, pointing to a robust toxicodynamic mechanism. Our findings have important implications for understanding variation in MeHg toxic potential on an individual basis and for informing the process of relating a measurement of Hg body burden to the potential for adverse developmental outcome.

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

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

A new page on the road book of inorganic mercury in fish body – tissue distribution and elimination following waterborne exposure and post-exposure periods

Time-related accumulation/distribution of inorganic Hg upon exposure and post-exposure periods. Body compartments selected: gills, eye wall, lens, blood, liver, brain and bile.

Menopause and blood mercury levels: the Korea National Health and Nutrition Examination Survey (KNHANES) 2008-2011

This study aimed to evaluate the association between menopause and blood mercury concentrations in South Korean women. Women aged ≥20 years who participated in the Korean National Health and Nutrition Examination Survey 2008-2011 were included in this study. Primary and secondary analyses included women aged ≥20 years (n = 1,642) and 45-55 years (i.e., perimenopausal; n = 325), respectively. For all analyses, the mercury levels were log-transformed. The linear regression model for mercury levels was adjusted for age, body mass index, household income, menopausal status, hormone replacement therapy, use of oral contraceptives, smoking history, alcohol intake, physical activity, number of pregnancies, serum ferritin levels, and fish consumption. After adjusting for covariates, log-transformed blood mercury levels were significantly lower in women who were menopausal [β-coefficient -0.1488; 95 % confidence interval -0.2586, -0.0389; P = 0.01) than in those who were premenopausal. A similar relationship was identified in perimenopausal women (β-coefficient -0.1753; 95 % confidence interval -0.3357, -0.015; P = 0.03). The blood mercury concentration was lower in postmenopausal women than in premenopausal women. There was a significant positive correlation between blood mercury concentrations and both the frequency of alcohol intake and serum ferritin levels.

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.

Mechanisms and Modifiers of Methylmercury-Induced Neurotoxicity

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 many mechanistic insights, particularly 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 genetic, environmental and nutritional factors capable of modifying MeHg toxicity.

Methylated Metal(loid) Species in Humans

While the metal(loid)s arsenic, bismuth, and selenium (probably also tellurium) have been shown to be enzymatically methylated in the human body, this has not yet been demonstrated for antimony, cadmium, germanium, indium, lead, mercury, thallium, and tin, although the latter elements can be biomethylated in the environment. Methylated metal(loid)s exhibit increased mobility, thus leading to a more efficient metal(loid) transport within the body and, in particular, opening chances for passing membrane barriers (blood-brain barrier, placental barrier). As a consequence human health may be affected. In this review, relevant data from the literature are compiled, and are discussed with respect to the evaluation of assumed and proven health effects caused by alkylated metal(loid) species.

Methylmercury disruption of embryonic neural development in Drosophila

Methylmercury (MeHg) is a potent environmental neurotoxin that preferentially targets the developing embryonic nervous system. While a number of cytotoxic mechanisms of MeHg have been characterized in differentiated cells its mode of action in the developing nervous system in vivo is less clear. Studies in primate and rodent models demonstrate aberrant cell migration and disorganized patterning of cortical layers in the brain following MeHg exposure. However, defining the molecular and cellular pathways targeted by MeHg will require more genetically accessible animal models. In this study, we instigate a method of in vitro MeHg exposure using Drosophila embryos. We demonstrate dose-dependent inhibition of embryonic development with MeHg revealed by a failure of embryos to hatch to the larval stage. In addition, we document definitive phenotypes in neural development showing abnormalities in neuronal and glial cell patterning consistent with disrupted migration. We observe pronounced defects in neurite outgrowth in both central and peripheral neurons. Ectopic expression of the Nrf2 transcription factor in embryos, a core factor in the antioxidant response element (ARE) pathway, enhances embryonic development and hatching in the presence of MeHg, illustrating the power of this model for investigation of candidate MeHg tolerance genes. Our data establish a utility for the Drosophila embryo model as a platform for elucidating MeHg sensitive pathways in neural development.

Mechanisms of mercury disposition in the body

Today the most widespread human exposures to mercury are to mercury vapor emitted from amalgam tooth fillings, to ethylmercury as a preservative in vaccines, and to methylmercury in edible tissues of fish. This review will focus on the mechanisms of transport of these three species of mercury. All three species are freely moveable throughout the body. Inhaled vapor in view of its physical properties as an uncharged atomic gas is believed to be transported by passive diffusion. Methylmercury and ethylmercury also move freely in the body. Methylmercury, and presumably its closely related chemical cousin ethylmercury, cross cell membranes as complexes with small molecular weight thiol compounds, entering the cell in part as a cysteine complex on the large neutral amino acid carriers and exiting the cell in part as a complex with reduced glutathione on endogenous carriers. The implications of these mechanisms with regard to biological monitoring are discussed.

Induction of glutamate-cysteine ligase (gamma-glutamylcysteine synthetase) in the brains of adult female mice subchronically exposed to methylmercury

Methylmercury (MeHg) is widely known for its potent neurotoxic properties. One proposed mechanism of action of MeHg relates to its high affinity for sulfhydryl groups, especially those found on glutathione (GSH) and proteins. Previous studies have shown that acute MeHg exposure results in an increase in the mRNA for the rate-limiting enzyme in GSH synthesis, glutamate-cysteine ligase (GLCL) (also known as gamma-glutamylcysteine synthetase). In this study, we evaluated the effects of subchronic (12-week) MeHg exposure at 0, 3 or 10 ppm in the drinking water on GSH levels, GLCL catalytic (GLCLC) and regulatory subunit mRNA and protein levels, and GLCL activity in brain, liver and kidney tissue of C57B1/6 female mice. Contrary to previous findings in rats, there were no changes in GSH concentration in any of the tissues examined. However, there was an increase in GLCLC protein in the brain, which was accompanied by a 30% increase in GLCL activity. We conclude that up-regulation of GSH synthetic capacity in the brains of mice is a sensitive biomarker of subchronic MeHg exposure.

Uptake of HgCl2 and MeHgCl in an insect cell line (Aedes albopictus C6/36)

We studied the uptake mechanism of mercuric chloride (Hg) and methylmercuric chloride (MeHg) in Aedes albopictus C6/36 cells. The uptake kinetics, together with the effect of temperature and a metabolic inhibitor (2, 4-dinitrophenol) on the mercury accumulation, were examined. Both amounts of internalized Hg and MeHg increased linearly with the extracellular concentration. Initially, the influx rate was high for both metal species but MeHg was found to accumulate seven times faster than Hg. At longer exposure times it leveled off for Hg, while for MeHg, the intracellular concentration decreased. Hg toxicity was not significantly influenced by elevated temperatures; in contrast there was a marked decrease of the LC50/24h value for MeHg. On the other hand, Hg accumulation was temperature dependent but MeHg was not. The different toxicity and uptake rate of both mercury compounds can be explained in terms of membrane permeability and target site. For Hg the main target seems to be the plasma membrane, while MeHg readily crosses this barrier and reacts with intracellular targets. 2, 4-Dinitrophenol had no effect on the accumulation of Hg but that of MeHg was doubled. This increased MeHg accumulation might be the result of the inhibition of an active MeHg efflux mechanism; this is in agreement with the MeHg influx kinetics. Despite these differences between Hg and MeHg, which probably result from their physicochemical properties, our experiments indicate that, for both mercury species, simple diffusion is probably the main way to entrance in Aedes cells.