Toxicity of methylmercury conjugated with L-cysteine on rat thymocytes and human leukemia K562 cells in comparison with that of methylmercury chloride

Carnosine prevents necrotic and apoptotic death of rat thymocytes via ouabain-sensitive Na/K-ATPase

It is known that ouabain, a selective inhibitor of Na/K-ATPase, not only can cause the activation of signal cascades, which regulate the cell viability, but also can cause the accumulation of free radicals, which can evoke the oxidative stress. We have shown that the nanomolar concentrations of ouabain result in the temporary increase in the level of intracellular free radicals, but the millimolar concentration of ouabain induces a stable intracellular accumulation of free radicals in rat thymocytes. The increasing level of free radicals resulting from both low and high concentrations of ouabain can be attenuated by the antioxidant, carnosine. Moreover, the long-term incubation with ouabain leads to the cell death by necrosis and apoptosis. Ouabain-mediated apoptosis and necrosis were also abolished by carnosine.

Dietary mercury exposure resulted in behavioral differences in mice contaminated with fish-associated methylmercury compared to methylmercury chloride added to diet

Methylmercury (MeHg) is a potent neurotoxin, and humans are mainly exposed to this pollutant through fish consumption. However, in classical toxicological studies, pure methylmercury chloride (MeHgCl) is injected, given to drink or incorporated within feed assuming that its effects are identical to those of MeHg naturally associated to fish. In the present study, we wanted to address the question whether a diet containing MeHg associated to fish could result in observable adverse effects in mice as compared to a diet containing the same concentration of MeHg added pure to the diet and whether beneficial nutriments from fish were able to counterbalance the deleterious effects of fish-associated mercury, if any. After two months of feeding, the fish-containing diet resulted in significant observable effects as compared to the control and MeHg-containing diets, encompassing altered behavioral performances as monitored in a Y-shaped maze and an open field, and an increased dopamine metabolic turnover in hippocampus, despite the fact that the fish-containing diet was enriched in polyunsaturated fatty acids and selenium compared to the fish-devoid diets.

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.

Does selenium modify neurobehavioural impacts of developmental methylmercury exposure in mice?

There is controversy as to whether low-level chronic exposure to methylmercury (MeHg) through maternal fish consumption may cause subtle effects in the developing child, owing in part to the potential ameliorating effects of beneficial seafood nutrients. The aim of the present investigation was to assess the ameliorating potential of selenium (Se; as the naturally occurring methionine complex) on the neurobehavioural toxicity of foodborne MeHg (as the naturally occurring cysteinate) in prenatally exposed mice. Pups from dams exposed to a diet containing 3mg/kg of MeHg fed throughout gestation showed delayed fur development and impaired performance in a motor function assessment. These effects were not apparent in pups born to dams concurrently exposed to Se (at 1.3mg/kg). These results, using natural dietary forms of the elements administered through the relevant exposure pathway, suggest only minor impacts of MeHgCys on neurobehaviour, and possible amelioration of these effects by Se.

Flow-cytometric analysis on cytotoxic effect of thimerosal, a preservative in vaccines, on lymphocytes dissociated from rat thymic glands

There is a concern on the part of public health community that adverse health consequence by thimerosal, a preservative in vaccines for infants, may occur among infants during immunization schedule. Therefore, the cytotoxic action of thimerosal was examined on lymphocytes dissociated from thymic glands of young rats using a flow cytometer and respective fluorescent probes for monitoring changes in intracellular Ca2+ concentration ([Ca2+]i) and membrane potential, and for discriminating intact living cells, apoptotic living cells and dead cells. Incubation with thimerosal at 3 microM or more (up to 30 microM) for 60 min depolarized the membranes, associated with increasing the [Ca2+]i. Thimerosal at 30 microM induced an apoptotic change in membranes of almost all living cells. Furthermore, the prolonged incubation with 30 microM thimerosal induced a loss of membrane integrity, leading to cell death. Since the blood concentration of thimerosal after receiving vaccines is theoretically submicromolar, it may be unlikely that thimerosal affects lymphocytes of infants.

Higher faecal excretion and lower tissue accumulation of mercury in Wistar rats from contaminated fish than from methylmercury chloride added to fish

A short-term low level exposure experiment was conducted on rats in order to determine urinary and faecal excretion, accumulation, and biological responses to methylmercury from fish products. Male Wistar rats were fed fish-meal diets containing methylmercury contaminated fish (1.45 or 2.61 mgHg/kg as methylmercury), uncontaminated fish supplemented with methylmercury chloride (CH3HgCl) at similar levels (1.24 and 2.49 mgHg/kg, respectively) or uncontaminated fish as a control (0.052 mgHg/kg) for 4 weeks (n=6 rats per treatment). After 2 and 4 weeks of exposure, rats were placed in metabolic chambers for 48 h to assess overall faecal and urinary excretion of mercury. The overall faecal excretion in rats fed fish supplemented with CH3HgCl (12%) was significantly lower (P <0.05) than rats fed methylmercury in fish muscle (19%) or rats fed control diet (76%). Urinary excretion did not differ among the experimental groups. Rats fed the highest level of CH3HgCl had a significantly higher (P <0.05) blood, liver, kidney and brain mercury contamination compared to rats fed methylmercury contaminated fish or rats fed control diet. Metallothionein levels in kidney were significantly higher in CH3HgCl-fed rats compared to rats fed contaminated fish. The results indicate a higher faecal excretion and lower tissue accumulation, and metallothionein induction in rats following exposure to methylmercury naturally incorporated in fish compared to methylmercury chloride added to the same matrix.

Effect of thimerosal, a preservative in vaccines, on intracellular Ca2+ concentration of rat cerebellar neurons

The effect of thimerosal, an organomercurial preservative in vaccines, on cerebellar neurons dissociated from 2-week-old rats was compared with those of methylmercury using a flow cytometer with appropriate fluorescent dyes. Thimerosal and methylmercury at concentrations ranging from 0.3 to 10 microM increased the intracellular concentration of Ca2+ ([Ca2+]i) in a concentration-dependent manner. The potency of 10 microM thimerosal to increase the [Ca2+]i was less than that of 10 microM methylmercury. Their effects on the [Ca2+]i were greatly attenuated, but not completely suppressed, under external Ca(2+)-free condition, suggesting a possibility that both agents increase membrane Ca2+ permeability and release Ca2+ from intracellular calcium stores. The effect of 10 microM thimerosal was not affected by simultaneous application of 30 microM L-cysteine whereas that of 10 microM methylmercury was significantly suppressed. The potency of thimerosal was similar to that of methylmercury in the presence of L-cysteine. Both agents at 1 microM or more similarly decreased the cellular content of glutathione in a concentration-dependent manner, suggesting an increase in oxidative stress. Results indicate that thimerosal exerts some cytotoxic actions on cerebellar granule neurons dissociated from 2-week-old rats and its potency is almost similar to that of methylmercury.