Methylmercury increases S100B content in rat cerebrospinal fluid

Mercury-induced toxicity: Mechanisms, molecular pathways, and gene regulation

Mercury is a well-known neurotoxicant for humans and wildlife. The epidemic of mercury poisoning in Japan has clearly demonstrated that chronic exposure to methylmercury (MeHg) results in serious neurological damage to the cerebral and cerebellar cortex, leading to the dysfunction of the central nervous system (CNS), especially in infants exposed to MeHg in utero. The occurrences of poisoning have caused a wide public concern regarding the health risk emanating from MeHg exposure; particularly those eating large amounts of fish may experience the low-level and long-term exposure. There is growing evidence that MeHg at environmentally relevant concentrations can affect the health of biota in the ecosystem. Although extensive in vivo and in vitro studies have demonstrated that the disruption of redox homeostasis and microtube assembly is mainly responsible for mercurial toxicity leading to adverse health outcomes, it is still unclear whether we could quantitively determine the occurrence of interaction between mercurial and thiols and/or selenols groups of proteins linked directly to outcomes, especially at very low levels of exposure. Furthermore, intracellular calcium homeostasis, cytoskeleton, mitochondrial function, oxidative stress, neurotransmitter release, and DNA methylation may be the targets of mercury compounds; however, the primary targets associated with the adverse outcomes remain to be elucidated. Considering these knowledge gaps, in this article, we conducted a comprehensive review of mercurial toxicity, focusing mainly on the mechanism, and genes/proteins expression. We speculated that comprehensive analyses of transcriptomics, proteomics, and metabolomics could enhance interpretation of "omics" profiles, which may reveal specific biomarkers obviously correlated with specific pathways that mediate selective neurotoxicity.

Potential Association between Methylmercury Neurotoxicity and Inflammation

Methylmercury (MeHg) is the causal substrate of Minamata disease and a major environmental toxicant. MeHg is widely distributed, mainly in the ocean, meaning its bioaccumulation in seafood is a considerable problem for human health. MeHg has been intensively investigated and is known to induce inflammatory responses and neurodegeneration. However, the relationship between MeHg-induced inflammatory responses and neurodegeneration is not understood. In the present review, we first describe recent findings showing an association between inflammatory responses and certain MeHg-unrelated neurological diseases caused by neurodegeneration. In addition, cell-specific MeHg-induced inflammatory responses are summarized for the central nervous system including those of microglia, astrocytes, and neurons. We also describe MeHg-induced inflammatory responses in peripheral cells and tissue, such as macrophages and blood. These findings provide a concept of the relationship between MeHg-induced inflammatory responses and neurodegeneration, as well as direction for future research of MeHg-induced neurotoxicity.

Revisiting Astrocytic Roles in Methylmercury Intoxication

Intoxication by heavy metals such as methylmercury (MeHg) is recognized as a global health problem, with strong implications in central nervous system pathologies. Most of these neuropathological conditions involve vascular, neurotransmitter recycling, and oxidative balance disruption leading to accelerated decline in fine balance, and learning, memory, and visual processes as main outcomes. Besides neurons, astrocytes are involved in virtually all the brain processes and perform important roles in neurological response following injuries. Due to astrocytes' strategic functions in brain homeostasis, these cells became the subject of several studies on MeHg intoxication. The most heterogenous glial cells, astrocytes, are composed of plenty of receptors and transporters to dialogue with neurons and other cells and to monitor extracellular environment responding tightly through fluctuation of cytosolic ions. The overall toxicity of MeHg might be determined on the basis of the balance between MeHg-mediated injury to neurons and protective responses from astrocytes. Although the role of neurons in MeHg intoxication is relatively well-established, the role of the astrocytes is only beginning to be understood. In this review, we update the information on astroglial modulation of the MeHg-induced neurotoxicity, providing remarks on their protective and deleterious roles and insights for future studies.

Cellular and Molecular Mechanisms Mediating Methylmercury Neurotoxicity and Neuroinflammation

Methylmercury (MeHg) toxicity is a major environmental concern. In the aquatic reservoir, MeHg bioaccumulates along the food chain until it is consumed by riverine populations. There has been much interest in the neurotoxicity of MeHg due to recent environmental disasters. Studies have also addressed the implications of long-term MeHg exposure for humans. The central nervous system is particularly susceptible to the deleterious effects of MeHg, as evidenced by clinical symptoms and histopathological changes in poisoned humans. In vitro and in vivo studies have been crucial in deciphering the molecular mechanisms underlying MeHg-induced neurotoxicity. A collection of cellular and molecular alterations including cytokine release, oxidative stress, mitochondrial dysfunction, Ca²⁺ and glutamate dyshomeostasis, and cell death mechanisms are important consequences of brain cells exposure to MeHg. The purpose of this review is to organize an overview of the mercury cycle and MeHg poisoning events and to summarize data from cellular, animal, and human studies focusing on MeHg effects in neurons and glial cells. This review proposes an up-to-date compendium that will serve as a starting point for further studies and a consultation reference of published studies.

Mercury Accumulation and Effects in the Brain of the Atlantic Sharpnose Shark (Rhizoprionodon terraenovae)

Few published studies have examined whether the elevated concentrations of the nonessential toxic metal mercury (Hg) often observed in shark muscle also occur in the shark brain or whether Hg accumulation affects shark neurophysiology. Therefore, this study examined accumulation and distribution of Hg in the shark brain, as well as effects of Hg on oxidative stress in the shark central nervous system, with particular focus on the Atlantic sharpnose shark (Rhizoprionodon terraenovae). Sharks were collected along the southeastern U.S. coast throughout most of this species' U.S. geographical range. Total Hg (THg) concentrations were measured in and compared between shark muscle and brain, whereas known biomarkers of Hg-induced neurological effects, including glutathione depletion, lipid peroxidation, and concentrations of a protein marker of glial cell damage (S100b), were measured in shark cerebrospinal fluid. Brain THg concentrations were correlated with muscle THg levels but were significantly lower and did not exceed most published thresholds for neurological effects, suggesting limited potential for detrimental responses. Biomarker concentrations supported this premise, because these data were not correlated with brain THg levels. Hg speciation also was examined. Unlike muscle, methylmercury (MeHg) did not comprise a high percentage of THg in the brain, suggesting that differential uptake or loss of organic and inorganic Hg and/or demethylation of MeHg may occur in this organ. Although Hg accumulation in the shark brain generally fell below toxicity thresholds, higher THg levels were measured in the shark forebrain compared with the midbrain and hindbrain. Therefore, there is potential for selective effects on certain aspects of shark neurophysiology if brain Hg accumulation is increased.

Assessing mercury intoxication in isolated/remote populations: Increased S100B mRNA in blood in exposed riverine inhabitants of the Amazon

Mercury is a heavy metal responsible for human intoxication worldwide and especially in the Amazon, where both natural and anthropogenic sources are responsible for exposure in riverine populations. Methylmercury is the most toxic specie of mercury with recognized neurotoxicity due to its affinity for the central nervous system. S100B protein is a well-established biomarker of brain damage and it was recently associated with mercury-related neurotoxicity. Accurate measurement is especially challenging in isolated/remote populations due to the difficulty of adequate sample conservation, therefore here we use S100B mRNA levels in blood as a way to assay mercury neurotoxicity. We hypothesized that individuals from chronically exposed populations showing mercury levels above the limit of 10 μg/g in hair would present increased levels of S100B mRNA, likely due to early brain damage. A total of 224 riverine individuals were evaluated for anthropometric data (age, body mass index), self-reported symptoms of mercury intoxication, c-reactive protein in blood, and mercury speciation in hair. Approximately 20% of participants showed mercury levels above the limit, and prevalence for most symptoms was not different between individuals exposed to high or low mercury levels. Rigorous exclusion criteria were applied to avoid confounding factors and S100B mRNA in blood was tested by RT-qPCR. Participants with ≥10 μg/g of mercury had S100B mRNA levels over two times higher than that of individuals with lower exposure. A significant correlation was also detected between mercury content in hair and S100B mRNA levels in blood, supporting the use of the latter as a possible candidate to predict mercury-induced neurotoxicity. This is the first report of an association between S100B mRNA and mercury exposure in humans. The combination of both exposure and intoxication biomarkers could provide additional support for the screening and early identification of high-risk individuals in isolated populations and subsequent referral to specialized centers.

Subchronic oral administration of Benzo[a]pyrene impairs motor and cognitive behavior and modulates S100B levels and MAPKs in rats

Benzo[a]pyrene (BaP) is an environmental contaminant produced during incomplete combustion of organic material that is well known as a mutagenic and carcinogenic toxin. There are few studies addressing the molecular and cellular basis of behavioural alterations related to BaP exposure. The aim of this study was to evaluate the effect of subchronic oral administration of BaP on behavioral and neurochemical parameters. Wistar male rats received BaP (2 mg/kg) or corn oil (control), once a day for 28 days (n = 12/group). Spontaneous locomotor activity and short- and long-term memories were evaluated. Glial fibrillary acid protein and S100B content in the hippocampus, serum and CSF were measured using ELISA and total and phosphorylated forms of mitogen activated protein kinases (MAPKs) named extracellular signal-regulated kinases 1 and 2, p38(MAPK) and c-Jun amino-terminal kinases 1 and 2, in the hippocampus, were evaluated by western blotting. BaP induced a significant increase on locomotor activity and a decrease in short-term memory. S100B content was increased significantly in cerebrospinal fluid. BaP induced a decrease on ERK2 phosphorylation in the hippocampus. Thus, BaP subchronic treatment induces an astroglial response and impairs both motor and cognitive behavior, with parallel inhibition of ERK2, a signaling enzyme involved in the hippocampal neuroplasticity. All these effects suggest that BaP neurotoxicity is a concern for environmental pollution.

Methionine stimulates motor impairment and cerebellar mercury deposition in methylmercury-exposed mice

Methylmercury (MeHg) is a highly toxic environmental contaminant that produces neurological and developmental impairments in animals and humans. Although its neurotoxic properties have been widely reported, the molecular mechanisms by which MeHg enters the cells and exerts toxicity are not yet completely understood. Taking into account that MeHg is found mostly bound to sulfhydryl-containing molecules such as cysteine in the environment and based on the fact that the MeHg-cysteine complex (MeHg-S-Cys) can be transported via the L-type neutral amino acid carrier transport (LAT) system, the potential beneficial effects of L-methionine (L-Met, a well known LAT substrate) against MeHg (administrated as MeHg-S-Cys)-induced neurotoxicity in mice were investigated. Mice were exposed to MeHg (daily subcutaneous injections of MeHg-S-Cys, 10 mg Hg/kg) and/or L-Met (daily intraperitoneal injections, 250 mg/kg) for 10 consecutive days. After treatments, the measured hallmarks of toxicity were mostly based on behavioral parameters related to motor performance, as well as biochemical parameters related to the cerebellar antioxidant glutathione (GSH) system. MeHg significantly decreased motor activity (open-field test) and impaired motor performance (rota-rod task) compared with controls, as well as producing disturbances in the cerebellar antioxidant GSH system. Interestingly, L-Met administration did not protect against MeHg-induced behavioral and cerebellar changes, but rather increased motor impairments in animals exposed to MeHg. In agreement with this observation, cerebellar levels of mercury (Hg) were higher in animals exposed to MeHg plus L-Met compared to those only exposed to MeHg. However, this event was not observed in kidney and liver. These results are the first to demonstrate that L-Met enhances cerebellar deposition of Hg in mice exposed to MeHg and that this higher deposition may be responsible for the greater motor impairment observed in mice simultaneously exposed to MeHg and L-Met.

Protective effects of Polygala paniculata extract against methylmercury-induced neurotoxicity in mice

We have examined the possible protective effects of Polygala paniculata extract against methylmercury (MeHg)-induced neurotoxicity in adult mice. MeHg was diluted in drinking water (40 mg L−1, freely available) and the hydroalcoholic Polygala extract was diluted in a 150 mm NaCl solution and administered by gavage (100 mg kg−1 b.w., twice a day). After a two-week treatment, MeHg exposure significantly inhibited glutathione peroxidase and increased glutathione reductase activity, while the levels of thiobarbituric acid reactive substances were increased in the cerebral cortex and cerebellum. These alterations were prevented by administration of Polygala extract, except for glutathione reductase activity, which remained elevated in the cerebral cortex. Behavioural interference in the MeHg-exposed animals was evident through a marked deficit in the motor performance in the rotarod task, which was completely recovered to control levels by Polygala extract co-administration. This study has shown, for the first time, the in-vivo protective effects of Polygala extract against MeHg-induced neurotoxicity. In addition, our findings encourage studies concerning the beneficial effects of P. paniculata on neurological conditions related to excitotoxicity and oxidative stress.

Synergistic neurotoxicity induced by methylmercury and quercetin in mice

Methylmercury (MeHg) is a highly neurotoxic pollutant, whose mechanisms of toxicity are related to its pro-oxidative properties. A previous report showed under in vivo conditions the neuroprotective effects of plants of the genus Polygala against MeHg-induced neurotoxicity. Moreover, the flavonoid quercetin, isolated from Polygala sabulosa, displayed beneficial effects against MeHg-induced oxidative damage under in vitro conditions. In this study, we sought for potential beneficial effects of quercetin against the neurotoxicity induced by MeHg in Swiss female mice. Animals were divided into six experimental groups: control, quercetin low dose (5 mg/kg), quercetin high dose (50 mg/kg), MeHg (40 mg/L, in tap water), MeHg+quercetin low dose, and MeHg+quercetin high dose. After the treatment (21 days), a significant motor deficit was observed in MeHg+quercetin groups. Biochemical parameters related to oxidative stress showed that the simultaneous treatment with quercetin and MeHg caused a higher cerebellar oxidative damage when compared to the individual exposures. MeHg plus quercetin elicited a higher cerebellar lipid peroxidation than MeHg or quercetin alone. The present results indicate that under in vivo conditions quercetin and MeHg cause additive pro-oxidative effects toward the mice cerebellum and that such phenomenon is associated with the observed motor deficit.

Cipura paludosa extract prevents methyl mercury-induced neurotoxicity in mice

Cipura paludosa (Iridaceae), a native plant widely distributed in the north of Brazil, is used in traditional medicine as an anti-inflammatory and analgesic agent, against tuberculosis and gonorrhoea and for regulation of menstrual flow. However, scientific studies on the pharmacological properties of C. paludosa are scarce. We have examined the potential protective effects of the ethanolic extract of C. paludosa against methyl mercury (MeHg)-induced neurotoxicity in adult mice. MeHg was diluted in drinking water (40 mg/l, freely available) and the ethanolic C. paludosa extract (CE) was diluted in a 150 mM NaCl solution and administered by gavage (10 and 100 mg/kg body weight, respectively, twice a day). Because treatment lasted for 14 days and each animal weighed around 40 g, the total dosage of plant extract given to each mouse was 5.6 and 56 g, respectively. After the treatment period, MeHg exposure induced a significant deficit in the motor coordination, which was evident by a reduction (90%) in the falling latency in the rotarod apparatus. Interestingly, this phenomenon was completely recovered to control levels by CE co-administration, independent of dosages. MeHg exposure inhibited cerebellar glutathione peroxidase (mean percentage inhibition of 42%) - an important enzyme involved in the detoxification of endogenous peroxides - and this effect was prevented by co-administration of CE. Conversely, MeHg exposure increased cerebellar glutathione reductase activity (mean percentage inhibition of 70%), and this phenomenon was not affected by C. paludosa co-administration. Neither MeHg nor CE changed the cerebellar glutathione levels. This study has shown for the first time, the in vivo protective effects of CE against MeHg-induced neurotoxicity. In addition, our findings encourage studies concerning the beneficial effects of C. paludosa on neurological conditions related to excitotoxicity and oxidative stress.

Behavioral alterations induced by HgCl2 depend on the postnatal period of exposure

This paper shows the toxicity of mercury (HgCl(2) 5mg/kg/day for 5 days, sc) applied at specific stages of development (1-5, 8-12 or 17-21 days old, 1st, 2nd and 3rd phases, respectively) on the performance of rats in three behavioral tasks and on cerebral mercury levels. The mercury exposure at the 1st and 2nd phases affected the performances of rats in the rim escape. Spontaneous alternation behavior was not altered by mercury exposure. In the open field task, habituation was absent when the rats were treated at the 1st phase, and the crossing response number was lower in rats exposed to mercury at the last period. In general, the brain accumulated large quantities of mercury. In short, the first days of postnatal life (1st phase) appeared to be more sensitive to mercury exposure than the other phases studied, since they presented behavioral deficits even at a time period somewhat after the exposure.

Lactational exposure to inorganic mercury: evidence of neurotoxic effects

This study examined the effects of inorganic mercury (mercuric chloride - HgCl2) exposure exclusively through maternal milk on biochemical parameters related to oxidative stress (glutathione and thiobarbituric acid reactive substances levels, glutathione peroxidase and glutathione reductase activities) in the cerebellum of weanling mice. These parameters were also evaluated in the cerebellum of mothers, which were subjected to intraperitoneal injections of HgCl2 (0, 0.5 and 1.5 mg/kg, once a day) during the lactational period. Considering the relationship between cerebellar function and motor activity, the presence of motor impairment was also evaluated in the offspring exposed to HgCl2 during lactation. After treatments (at weaning), pups lactationally exposed to inorganic mercury showed high levels of mercury in the cerebellar tissue, as well as significant impairment in motor performance in the rotarod task and decreased locomotor activity in the open field. Offspring and dams did not show changes in cerebellar glutathione levels or glutathione peroxidase activity. In pups, lactational exposure to inorganic mercury significantly increased cerebellar lipoperoxidation, as well as the activity of cerebellar glutathione reductase. However, these phenomena were not observed in dams. These results indicate that inorganic mercury exposure through maternal milk is capable of inducing biochemical changes in the cerebellum of weanling mice, as well as motor deficit and these phenomena appear to be related to the pro-oxidative properties of inorganic mercury.

Cerebellar thiol status and motor deficit after lactational exposure to methylmercury

This study examined the exclusive contribution of methylmercury (MeHg) exposure through maternal milk on biochemical parameters related to the thiol status (glutathione (GSH) levels, glutathione peroxidase (GPx) and glutathione reductase (GR) activities) in the cerebellums of suckling mice. The same biochemical parameters were also evaluated in the cerebellums of mothers, which were submitted to a direct oral exposure to MeHg (10 mg/L in drinking water). With regard to the relationship between cerebellar function and motor activity, the presence of signs of motor impairment was also evaluated in the offspring exposed to MeHg during lactation. After the treatment (at weaning period), the pups lactationally exposed to MeHg showed increased levels of mercury in the cerebellum compared to pups in the control group and a significant impairment in the motor performance in the rotarod apparatus. In addition, these pups showed decreased levels of GSH in the cerebellum compared to pups in the control group. In dams, MeHg significantly increased the levels of cerebellar GSH and the activities of cerebellar GR. However, this was not observed in pups. This study indicates that (1) the exposure of lactating mice to MeHg causes significant impairments in motor performance in the offspring which may be related to a decrease in the cerebellar thiol status and (2) the increased GSH levels and GR activity, observed only in the cerebellums of MeHg-exposed dams, could represent compensatory pathophysiologic responses to the oxidative effects of MeHg toward endogenous GSH.

Postnatal methylmercury exposure induces hyperlocomotor activity and cerebellar oxidative stress in mice: dependence on the neurodevelopmental period

During the early postnatal period the central nervous system (CNS) is extremely sensitive to external agents. The present study aims at the investigation of critical phases where methylmercury (MeHg) induces cerebellar toxicity during the suckling period in mice. Animals were treated with daily subcutaneous injections of MeHg (7 mg/kg of body weight) during four different periods (5 days each) at the early postnatal period: postnatal day (PND) 1-5, PND 6-10, PND 11-15, or PND 16-20. A control group was treated with daily subcutaneous injections of a 150 mM NaCl solution (10 ml/kg of body weight). Subjects exposed to MeHg at different postnatal periods were littermate. At PND 35, behavioral tests were performed to evaluate spontaneous locomotor activity in the open field and motor performance in the rotarod task. Biochemical parameters related to oxidative stress (levels of glutathione and thiobarbituric acid reactive substances, as well as glutathione peroxidase and glutathione reductase activity) were evaluated in cerebellum. Hyperlocomotor activity and high levels of cerebellar thiobarbituric acid reactive substances were observed in animals exposed to MeHg during the PND 11-15 or PND 16-20 periods. Cerebellar glutathione reductase activity decreased in MeHg-exposed animals. Cerebellar glutathione peroxidase activity was also decreased after MeHg exposure and the lowest enzymatic activity was found in animals exposed to MeHg during the later days of the suckling period. In addition, low levels of cerebellar glutathione were found in animals exposed to MeHg during the PND 16-20 period. The present results show that the postnatal exposure to MeHg during the second half of the suckling period causes hyperlocomotor activity in mice and point to this phase as a critical developmental stage where mouse cerebellum is a vulnerable target for the neurotoxic and pro-oxidative effects of MeHg.