Post-translational modifications in MeHg-induced neurotoxicity

Quercetin prevents methylmercury-induced mitochondrial dysfunction in the cerebral cortex of mice

Methylmercury (MeHg) exposure can cause nerve damage and mitochondrial dysfunction. Mitochondrial dysfunction is mainly mediated by mitochondrial biogenesis and mitochondrial dynamics disorders. Quercetin (QE) plays an important role in activating silencing information regulator 2 related enzyme 1 (SIRT1), and SIRT1 activates peroxisome-proliferator-activated receptor-γ co-activator 1α (PGC-1α), which can regulate mitochondrial biogenesis and mitochondrial dynamics. The main purpose of this study was to explore the alleviating effects of QE on MeHg-induced nerve damage and mitochondrial dysfunction. The results showed that QE could reduce the excessive production of reactive oxygen species (ROS) and the loss of membrane potential induced by MeHg. Meanwhile, QE activated SIRT1 activity and SIRT1/PGC-1α signaling pathway, improved mitochondrial biogenesis and fusion and reduced mitochondrial fission. In summary, we hypothesized that QE prevents MeHg-induced mitochondrial dysfunction by activating SIRT1/PGC-1α signaling pathway.

N-acetylcysteine protects against neurodevelopmental injuries induced by methylmercury exposure during pregnancy and lactation

As an extremely dangerous environmental contaminant, methylmercury (MeHg) results in detrimental health effects in human brain nervous system, one of its main targets. However, as a developmental toxicant, the brain of offspring is vulnerable to MeHg during pregnancy and lactation exposure. Unfortunately, mechanisms of neurodevelopmental injuries induced by MeHg have not been fully elucidated. N-acetylcysteine (NAC) has been used for several decades as an antioxidant to antagonize oxidative stress. However, the molecular mechanisms of NAC alleviating MeHg-induced neurodevelopmental toxicity are not clear. Here, for evaluation of the dose-dependent effects of MeHg exposure on neurodevelopmental injuries of offspring, and the possible protective effects of NAC, the pregnant female mice were exposed to MeHg (4, 8, 12 mg/L, respectively) and NAC (50, 100, 150 mg/kg, respectively) from gestational day 1 (GD1) to postnatal day 21 (PND21). Our results indicated that administering MeHg caused behavioral impairment and neuronal injuries in the cerebral cortex of newborn mice. MeHg dose-dependently caused reactive oxygen species (ROS) overproduction and oxidative stress aggravation, together with expression of Nrf2, HO-1, Notch1, and p21 up-regulation, and CDK2 inhibition. NAC treatment dose-dependently antagonized MeHg-induced oxidative stress that may contribute to alleviating neurobehavioral and neurodevelopmental impairments. These results give insight into that NAC can protect against MeHg-induced neurodevelopmental toxicity by its antioxidation capacity.

Methylmercury effects on avian brains

Methylmercury (MeHg) is a concerning contaminant due to its ubiquity and harmful effects on organisms. Although birds are important models in the neurobiology of vocal learning and adult neuroplasticity, the neurotoxic effects of MeHg are less understood in birds than mammals. We surveyed the literature on MeHg effects on biochemical changes in the avian brain. Publication rates of papers related to neurology and/or birds and/or MeHg increased with time and can be linked with historical events, regulations, and increased understanding of MeHg cycling in the environment. However, publications on MeHg effects on the avian brain remain relatively low across time. The neural effects measured to evaluate MeHg neurotoxicity in birds changed with time and researcher interest. The measures most consistently affected by MeHg exposure in birds were markers of oxidative stress. NMDA, acetylcholinesterase, and Purkinje cells also seem sensitive to some extent. MeHg exposure has the potential to affect most neurotransmitter systems but more studies are needed for validation in birds. We also review the main mechanisms of MeHg-induced neurotoxicity in mammals and compare it to what is known in birds. The literature on MeHg effects on the avian brain is limited, preventing full construction of an adverse outcome pathway. We identify research gaps for taxonomic groups such as songbirds, and age- and life-stage groups such as immature fledgling stage and adult non-reproductive life stage. In addition, results are often inconsistent between experimental and field studies. We conclude that future neurotoxicological studies of MeHg impacts on birds need to better connect the numerous aspects of exposure from molecular physiological effects to behavioural outcomes that would be ecologically or biologically relevant for birds, especially under challenging conditions.

Developmental Methylmercury Exposure Induced and Age-Dependent Glutamatergic Neurotoxicity in Caenorhabditis elegans

Developmental methylmercury (MeHg) exposures cause latent neurotoxic effects in adults; however, the mechanisms underlying the latent neurotoxicity are not fully understood. In the current study, we used C. elegans as an animal model to investigate the latent neurotoxic effects of developmental MeHg exposures on glutamatergic neurons. The young larvae stage 1 worms were exposed to MeHg (0.05 ~ 5 µM) for 48 h. The morphological and behavioral endpoints of glutamatergic neurons were compared when worms reached to adult stages including the young adult stage (day 1 adult) and the old adult stage (day 10 adult). Here, we showed that C. elegans glutamatergic neurons were morphologically intact following low or medium MeHg exposures (0.05 ~ 0.5 µM). The morphological damage of glutamatergic neurons appeared to be pronounced in day 10 adults developmentally exposed to 5 µM MeHg. Behavioral assays also showed an age-dependent latent effect of MeHg. In the nose touch response assay, only day 10 adult worms exhibited a functional decline following prior 5 µM MeHg exposure. Moreover, the disruption of NaCl memory appeared only in day 1 adults following MeHg exposures but not in day 10 adults. The expression of C. elegans homologs of mammalian vesicular glutamate transporter (eat-4) was repressed in day 1 adults, while the glutamate receptor homolog (glr-1) was upregulated in day 10 adults with 5 µM MeHg. In the comparison of age-dependent changes in the insulin-like pathway (daf-2/age-1/daf-16) following MeHg exposures, we showed that the daf-2/age-1/daf-16 pathway was mobilized in day 1 adults but repressed in day 10 adults. Collectively, our data supports a conclusion that MeHg-induced glutamatergic neurotoxicity exhibits an age-dependent pattern, possibly related to the prominent changes in age-dependent modulation in the glutamatergic neurotransmission and metabolic pathways.

The Threat Posed by Environmental Contaminants on Neurodevelopment: What Can We Learn from Neural Stem Cells?

Exposure to chemicals may pose a greater risk to vulnerable groups, including pregnant women, fetuses, and children, that may lead to diseases linked to the toxicants' target organs. Among chemical contaminants, methylmercury (MeHg), present in aquatic food, is one of the most harmful to the developing nervous system depending on time and level of exposure. Moreover, certain man-made PFAS, such as PFOS and PFOA, used in commercial and industrial products including liquid repellants for paper, packaging, textile, leather, and carpets, are developmental neurotoxicants. There is vast knowledge about the detrimental neurotoxic effects induced by high levels of exposure to these chemicals. Less is known about the consequences that low-level exposures may have on neurodevelopment, although an increasing number of studies link neurotoxic chemical exposures to neurodevelopmental disorders. Still, the mechanisms of toxicity are not identified. Here we review in vitro mechanistic studies using neural stem cells (NSCs) from rodents and humans to dissect the cellular and molecular processes changed by exposure to environmentally relevant levels of MeHg or PFOS/PFOA. All studies show that even low concentrations dysregulate critical neurodevelopmental steps supporting the idea that neurotoxic chemicals may play a role in the onset of neurodevelopmental disorders.

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.

Association between Heavy Metal Exposure and Parkinson's Disease: A Review of the Mechanisms Related to Oxidative Stress

Parkinson's disease (PD) is a gradually progressing neurodegenerative condition that is marked by a loss of motor coordination along with non-motor features. Although the precise cause of PD has not been determined, the disease condition is mostly associated with the exposure to environmental toxins, such as metals, and their abnormal accumulation in the brain. Heavy metals, such as iron (Fe), mercury (Hg), manganese (Mn), copper (Cu), and lead (Pb), have been linked to PD and contribute to its progression. In addition, the interactions among the components of a metal mixture may result in synergistic toxicity. Numerous epidemiological studies have demonstrated a connection between PD and either single or mixed exposure to these heavy metals, which increase the prevalence of PD. Chronic exposure to heavy metals is related to the activation of proinflammatory cytokines resulting in neuronal loss through neuroinflammation. Similarly, metals disrupt redox homeostasis while inducing free radical production and decreasing antioxidant levels in the substantia nigra. Furthermore, these metals alter molecular processes and result in oxidative stress, DNA damage, mitochondrial dysfunction, and apoptosis, which can potentially trigger dopaminergic neurodegenerative disorders. This review focuses on the roles of Hg, Pb, Mn, Cu, and Fe in the development and progression of PD. Moreover, it explores the plausible roles of heavy metals in neurodegenerative mechanisms that facilitate the development of PD. A better understanding of the mechanisms underlying metal toxicities will enable the establishment of novel therapeutic approaches to prevent or cure PD.

Deciphering the Global Proteomic Profile Involved in Methylmercury-Induced Cerebellar Neurodegeneration and Motor Dysfunction in Adult Rats

Mercury is a ubiquitous pollutant in the environment with potential neurotoxic effects. Several populations are susceptible to mercurial exposure, especially methylmercury (MeHg) at low doses for long periods through food consumption. Given this, the present work aimed to assess the effects of long-term MeHg exposure on the cerebellum of rats from a translational perspective using a representative dose, assessing molecular, biochemical, morphological, and behavioral parameters. The model was produced by administering 40 µg/kg of MeHg for 60 days to adult male Wistar rats by oral gavage. As a result of this exposure, the animals presented motor deficits in open field and rotarod tests which were associated with an increase in total mercury content in cerebellar parenchyma, a reduction in antioxidant competence against peroxyl radicals, and increased nitrite and lipid peroxidation levels. The proteomic approach showed 317 modulated proteins. Such findings were associated with reductions in mature neuron and Purkinje cell densities and glial fibrillary acidic protein immunostained areas and increased microglial density. In addition, decreases in myelin basic protein and synaptophysin immunostaining were also observed. The results thus provided new evidence of the mechanisms underlying complex MeHg-induced neurodegeneration, especially the proteins underlying the biochemical and morphological features associated with motor dysfunction.

Mitochondrial fission mediated by Drp1-Fis1 pathway and neurodegenerative diseases

In recent years, the role of mitochondrial dynamics in neurodegenerative diseases has becoming increasingly important. More and more evidences have shown that in pathological conditions, abnormal mitochondrial divisions, especially Drp1-Fis1-mediated divisions, play an important role in the occurrence and development of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, glaucoma, and other neurodegenerative diseases. This review highlights several new mechanisms of physiological fission of mitochondria and the difference/connection of physiological/pathological mitochondrial fission. In addition, we described the relationship between abnormal mitochondrial dynamics and neurodegenerative diseases in detail and emphatically summarized its detection indicators in basic experiments, trying to provide references for further mechanism exploration and therapeutic targets.

Methylmercury exposure-induced reproductive effects are mediated by dopamine in Caenorhabditis elegans

Methylmercury (MeHg) is a neurotoxicant that exists in the natural environment, which level can be greatly increased because of human activity. MeHg exposures have the risk of being detrimental to the development of the nervous system. Studies on MeHg toxicity have largely focused on the mechanisms of its neurotoxicity following developmental exposures. Additionally, reproductive toxicity of developmental MeHg exposures has been noted in rodent models. The model organism Caenorhabditis elegans (C. elegans) is a self-fertilizing animal which has a short lifespan around 20 days. Most C. elegans are hermaphrodites that can generate both sperm and oocytes. To investigate the effects of developmental MeHg exposures on the reproduction in C. elegans, larvae stage 1 worms were exposed to MeHg (0, 0.01 or 0.05 μM) for 24 h. The laid eggs and oocytes were compared during each day at adult stages for 6 days. We showed that MeHg exposure significantly induced an increased number of eggs in day 1 adults without an effect on the timing of egg laying or the total number of eggs or oocytes over the 6-day period. The expression of dat-1 and cat-2 and dopamine levels were increased in worms exposed to MeHg. Supplementation with 100 μM dopamine recapitulated the effect of MeHg on the number of eggs present in day 1 adults. Furthermore, the effect of MeHg on the number of eggs was abrogated in the cat-2 mutant worms CB1112. The number of oocytes in the 6-day adult stages was decreased by MeHg in the dat-1 mutant RM2702. MeHg exposures did not change the mating rate or the number of offspring from mating. Combined, these novel findings show that developmental exposure to low levels of MeHg has limited effects on the reproduction in C. elegans. Furthermore, our data support a modulatory role of dopamine in MeHg-induced effects on reproduction in this model system.

The Human LRRK2 Modulates the Age-Dependent Effects of Developmental Methylmercury Exposure in Caenorhabditis elegans

Methylmercury (MeHg) neurotoxicity exhibits age-dependent effects with a latent and/or persistent neurotoxic effect on aged animals. Individual susceptibility to MeHg neurotoxicity is governed by both exposure duration and genetic factors that can magnify or mitigate the pathologic processes associated with this exposure. We previously showed the G2019S mutation of leucine-rich repeat kinase 2 (LRRK2) modulates the response of worms to high levels of MeHg, mitigating its effect on neuronal morphology in pre-vesicles in cephalic (CEP) dopaminergic neurons. Here we sought to better understand the long-term effects of MeHg exposure at low levels (100-fold lower than that in our previous report) and the modulatory role of the LRRK2 mutation. Worms exposed to MeHg (10 or 50 nM) at the larval stage (L1 stage) were compared at adult stages (young age: day 1 adult; middle age: day 5 adult; old age: day 10 adult) for the swimming speeds in M9 buffer, moving speeds during locomotion on an OP50-seeded plate, and the numbers of CEP dopaminergic pre-vesicles, vesicular structures originating from the dendrites of CEP for exportation of cellular content. In addition, the expression levels of Caenorhabditis elegans homologs of dopamine transporter (dat-1) and tyrosine hydroxylase (cat-2) were also analyzed at these adult stages. Our data showed that swimming speeds were reduced in wild-type worms at the day 10 adult stage at 50 nM MeHg level; yet, reduced swimming speeds were noted in the G2019S LRRK2 transgenic line upon MeHg exposures as low as 10 nM. Compared to locomotor speeds, swimming speeds appear to be more sensitive to the behavioral effects of developmental MeHg exposures, as the locomotor speeds were largely intact and indistinguishable from controls following MeHg exposures. Furthermore, we showed an age-dependent modulation of dat-1 and cat-2 expressions, which could also be modified by the LRRK2 mutation. Although MeHg exposures did not change the number of pre-vesicles, the LRRK2 mutation was associated with increased numbers of pre-vesicles in aged worms. Our data suggest that the latent behavioral effects of MeHg are sensitized by the G2019S LRRK2 mutation, and the underlying mechanism likely involves age-dependent changes in dopaminergic signaling.

Ghrelin attenuates methylmercury-induced oxidative stress in neuronal cells

Methylmercury (MeHg) is a global pollutant, which can cause damage to the central nervous system at both high-acute and chronic-low exposures, especially in vulnerable populations, such as children and pregnant women. Nowadays, acute-high poisoning is rare. However, chronic exposure to low MeHg concentrations via fish consumption remains a health concern. Current therapeutic strategies for MeHg poisoning are based on the use of chelators. However, these therapies have limited efficacy. Ghrelin is a gut hormone with an important role in regulating physiologic processes. It has been reported that ghrelin plays a protective role against the toxicity of several xenobiotics. Here, we explored the role of ghrelin as a putative protector against MeHg-induced oxidative stress. Our data show that ghrelin was able to ameliorate MeHg-induced reactive oxygen species (ROS) production in primary neuronal hypothalamic and hippocampal cultures. An analogous effect was observed in mouse hypothalamic neuronal GT 1-7 cells. Using this model, our novel findings show that antioxidant protection of ghrelin against MeHg is mediated by glutathione upregulation and induction of the NRF2/NQO1 pathway.

Methylmercury plus Ethanol Exposure: How Much Does This Combination Affect Emotionality?

Mercury is a heavy metal found in organic and inorganic forms that represents an important toxicant with impact on human health. Mercury can be released in the environment by natural phenoms (i.e., volcanic eruptions), industrial products, waste, or anthropogenic actions (i.e., mining activity). Evidence has pointed to mercury exposure inducing neurological damages related to emotional disturbance, such as anxiety, depression, and insomnia. The mechanisms that underlie these emotional disorders remain poorly understood, although an important role of glutamatergic pathways, alterations in HPA axis, and disturbance in activity of monoamines have been suggested. Ethanol (EtOH) is a psychoactive substance consumed worldwide that induces emotional alterations that have been strongly investigated, and shares common pathophysiological mechanisms with mercury. Concomitant mercury and EtOH intoxication occur in several regions of the world, specially by communities that consume seafood and fish as the principal product of nutrition (i.e., Amazon region). Such affront appears to be more deleterious in critical periods of life, such as the prenatal and adolescence period. Thus, this review aimed to discuss the cellular and behavioral changes displayed by the mercury plus EtOH exposure during adolescence, focused on emotional disorders, to answer the question of whether mercury plus EtOH exposure intensifies depression, anxiety, and insomnia observed by the toxicants in isolation.

Salivary parameters alterations after early exposure to environmental methylmercury: A preclinical study in offspring rats

BACKGROUND

Methylmercury (MeHg) is still considered a global pollutant of major concern; thus, it becomes relevant to investigate and validate alternative diagnostic methods to track early-life human exposure. This study aimed to evaluate the salivary parameters and to characterize potential mechanisms of oxidative damage on the salivary glands (SG) of offspring rats after pre- and postnatal environmental-experimental MeHg exposure.

METHODS

Pregnant Wistar rats were daily exposed to 40 μg/kg MeHg during both gestational and lactation periods. Then, the saliva of offspring rats was analyzed in terms of flow rate, amylase activity, and total protein concentration. The SG of the offspring rats were dissected to perform the oxidative biochemistry analyses of antioxidant capacity against peroxyl radicals (ACAP), lipid peroxidation (LPO), and nitrite levels.

RESULTS

Exposure to MeHg significantly decreased the ACAP, increased LPO and nitrite levels, decreased salivary flow rate, amylase activity, and total protein concentration.

CONCLUSION

Saliva analyses can predict damages induced by early-life MeHg exposure and may be used as an auxiliary diagnostic method.

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.

The antioxidant role of STAT3 in methylmercury-induced toxicity in mouse hypothalamic neuronal GT1-7 cell line

Oxidative stress, impairment of antioxidant defenses, and disruption of calcium homeostasis are associated with the toxicity of methylmercury (MeHg). Yet, the relative contribution and interdependence of these effects and other molecular mechanisms that mediate MeHg-induced neurotoxicity remain uncertain. The signal transducer and activator of transcription 3 (STAT3) is a transcription factor that regulates the expression of anti-apoptotic and cell cycle progression genes. In addition to its role in cell growth and survival, STAT3 regulates redox homeostasis and prevents oxidative stress by the modulation of nuclear genes that encode for electron transport complexes (ETC) and antioxidant enzymes. Here we tested the hypothesis that STAT3 contributes to the orchestration of the antioxidant defense response against MeHg injury. We show that MeHg (>1 μM) exposure induced STAT3 activation within 1 h and beyond in mouse hypothalamic neuronal GT1-7 cells in a concentration-and time-dependent manner. Pharmacological inhibition of STAT3 phosphorylation exacerbated MeHg-induced reactive oxygen species (ROS) production and antioxidant responses. Finally, treatment with the antioxidant Trolox demonstrated that MeHg-induced STAT3 activation is mediated, at least in part, by MeHg-induced ROS generation. Combined, our results demonstrated a role for the STAT3 signaling pathway as an early response to MeHg-induced oxidative stress.

Mercury Toxicity and Neurogenesis in the Mammalian Brain

The mammalian brain is formed from billions of cells that include a wide array of neuronal and glial subtypes. Neural progenitor cells give rise to the vast majority of these cells during embryonic, fetal, and early postnatal developmental periods. The process of embryonic neurogenesis includes proliferation, differentiation, migration, the programmed death of some newly formed cells, and the final integration of differentiated neurons into neural networks. Adult neurogenesis also occurs in the mammalian brain, but adult neurogenesis is beyond the scope of this review. Developing embryonic neurons are particularly susceptible to neurotoxicants and especially mercury toxicity. This review focused on observations concerning how mercury, and in particular, methylmercury, affects neurogenesis in the developing mammalian brain. We summarized information on models used to study developmental mercury toxicity, theories of pathogenesis, and treatments that could be used to reduce the toxic effects of mercury on developing neurons.

DHA ameliorates MeHg‑induced PC12 cell apoptosis by inhibiting the ROS/JNK signaling pathway

Recent studies have reported that methylmercury (MeHg) induces neuronal apoptosis, which is accompanied by abnormal neurological development. Despite the important role of docosahexaenoic acid (DHA) in maintaining the structure and function of the brain, as well as improving neuronal apoptosis induced by MeHg, the exact mechanism remains unknown. The present study hypothesized that the reactive oxygen species (ROS)-mediated JNK signaling pathway may be associated with the protective effect of DHA against MeHg-induced PC12 cell apoptosis. Cell Counting Kit-8, TUNEL staining, flow cytometry, ROS detection, PCR and western blot analysis were performed. The results demonstrated that MeHg inhibited the activity of PC12 cells, causing oxidative damage and promoting apoptosis; however, DHA significantly attenuated this effect. Mechanistic studies revealed that MeHg increased intracellular ROS levels and JNK protein phosphorylation, and decreased the expression levels of the anti-apoptotic protein Bcl-2, whereas DHA reduced ROS levels and JNK phosphorylation, and increased Bcl-2 expression. In addition, the ROS inhibitor N-acetyl-l-cysteine (NAC) was used to verify the experimental results. After pretreatment with NAC, expression levels of Bcl-2, Bax, phosphorylated-JNK and JNK were assessed. Bcl-2 protein expression was increased and the Bcl-2/Bax ratio was increased. Moreover, the high expression levels of phosphorylated-JNK induced by MeHg were significantly decreased. Based on the aforementioned results, the present study indicated that the effects of DHA against MeHg-induced PC12 cell apoptosis may be mediated via the ROS/JNK signaling pathway.

Mechanisms of oxidative stress in methylmercury-induced neurodevelopmental toxicity

Methylmercury (MeHg) is a long-lasting organic environmental pollutant that poses a great threat to human health. Ingestion of seafood containing MeHg is the most important way by which it comes into contact with human body, where the central nervous system (CNS) is the primary target of MeHg toxicity. During periods of pre-plus postnatal, in particular, the brain of offspring is vulnerable to specific developmental insults that result in abnormal neurobehavioral development, even without symptoms in mothers. While many studies on neurotoxic effects of MeHg on the developing brain have been conducted, the mechanisms of oxidative stress in MeHg-induced neurodevelopmental toxicity is less clear. Hitherto, no single process can explain the many effects observed in MeHg-induced neurodevelopmental toxicity. This review summarizes the possible mechanisms of oxidative stress in MeHg-induced neurodevelopmental toxicity, highlighting modulation of Nrf2/Keap1/Notch1, PI3K/AKT, and PKC/MAPK molecular pathways as well as some preventive drugs, and thus contributes to the discovery of endogenous and exogenous molecules that can counteract MeHg-induced neurodevelopmental toxicity.

New insights on mechanisms underlying methylmercury-induced and manganese-induced neurotoxicity

Toxic and essential elements are widely distributed in the Earth's crust and individuals may be exposed to several of them. Indeed, exposure to toxic elements such as mercury (Hg) can be a potential health risk factor of health, mainly by ingestion of fish containing methylmercury (MeHg). On the other hand, essential elements such as manganese (Mn) play an important role in physiological process in human body. However, Mn overexposure may cause toxic effects. In this respect, the neurotoxic effects of MeHg and Mn on the developing brain are well recognized. Therefore, in this critical review, we address the effects of MeHg and Mn on cell signaling pathways which may contribute to molecular mechanisms involved in MeHg- and Mn-induced neurotoxicity.

Personalized Prevention in Mercury-Induced Amyotrophic Lateral Sclerosis: A Case Report

Chronic exposure to low levels of mercury is involved in the development of motor neuron diseases (MND). Genetic alterations may have a crucial role in the onset and progression. We presented a case of a TANK-binding kinase 1 (TBK1)-mutated 54-year-old male worker who developed a MND due to chronic mercury exposure at work. He was employed in a chlor-alkali plant in Central Italy. After two years of employment he had acute mercury intoxication with suggestive neurological symptoms and a high urinary level of the metal. Through years, many episodes of intoxication occurred, but he continued to perform the same job and be exposed to mercury. After yet another episode of intoxication in 2013, he showed fasciculations of the upper limbs and trunk, and electromyographic activity patterns were consistent with amyotrophic lateral sclerosis (ALS). In 2016, a genetic test revealed a mutation of TBK1, an ALS-related gene. This case highlights the important role of genetics in personalized occupational medicine. Occupational physicians should use genetic tests to identify conditions of individual susceptibility in workers with documented frequent episodes of mercury intoxication recorded during health surveillance programs to customize prevention measures in the workplace and act before damage appears.

Redox toxicology of environmental chemicals causing oxidative stress

Living organisms are surrounded with heavy metals such as methylmercury, manganese, cobalt, cadmium, arsenic, as well as pesticides such as deltamethrin and paraquat, or atmospheric pollutants such as quinone. Extensive studies have demonstrated a strong link between environmental pollutants and human health. Redox toxicity is proposed as one of the main mechanisms of chemical-induced pathology in humans. Acting as both a sensor of oxidative stress and a positive regulator of antioxidants, the nuclear factor erythroid 2-related factor 2 (NRF2) has attracted recent attention. However, the role NRF2 plays in environmental pollutant-induced toxicity has not been systematically addressed. Here, we characterize NRF2 function in response to various pollutants, such as metals, pesticides and atmospheric quinones. NRF2 related signaling pathways and epigenetic regulations are also reviewed.

Therapeutic Efficacy of the N,N' Bis-(2-Mercaptoethyl) Isophthalamide Chelator for Methylmercury Intoxication in Caenorhabditis elegans

Methylmercury (MeHg) is a global pollutant and potent neurotoxin. In humans, MeHg damages the central nervous system (CNS), causing irreversible neuronal shrinkage, and neuronal loss. Most chelators for clinical mercury detoxification are thiol-containing agents. N,N 'bis-(2-mercaptoethyl) isophthalamide (NBMI) is a lipophilic thiol agent synthesized from natural chemicals. NBMI has high affinity for mercury, cadmium and lead, and can decrease their concentrations in polluted water. However, the efficacy of NBMI for MeHg toxicity has yet to be evaluated in intact animals. Here we used the nematode Caenorhabditis elegans (C. elegans) to test the efficacy of NBMI in attenuating MeHg toxicity in vivo in the whole organism. The results showed that NBMI reduced both the acute toxicity (125 μM MeHg, 1 h) and chronic (5 μM MeHg, 24 h) MeHg toxicity. Co-treatment with NBMI achieved maximal efficacy against MeHg toxicity, however delayed treatment 6 days after initiation of exposure was also effective at reducing neurotoxicity. Co-treatment of NBMI reduced the worms' death rate, structural damage in DAergic neurons, and restored antioxidant response levels. While this study provides proof of principle for the therapeutic value of NBMI in MeHg toxicity, future studies are needed to address the cellular and molecular mechanisms and translatability of these effects to humans and other animals.

Mercury-induced autoimmunity: Report of two adolescent siblings with Morvan syndrome "plus" and review of the literature

Heavy metal toxicity is a global health concern. Mercury intoxication has been implicated in the etiology and pathogenesis of autoimmune disease, including Morvan syndrome. We describe two siblings with overlapping features of distinct autoimmune syndromes following accidental exposure to elemental mercury. Morvan syndrome was the predominant clinical phenotype. In addition to the characteristic anti-leucine-rich glioma-inactivated protein 1 (LGI1) and anti-contactin-associated protein-like 2 (Caspr2) autoantibodies, glutamic acid decarboxylase 65-kilodalton isoform (GAD65), and N-type and P/Q-type voltage-gated calcium channel (VGCC) antibodies were detected. Treatment with chelation therapy, glucocorticoids, and intravenous immunoglobulin was unsuccessful, but complete resolution of symptoms was achieved following treatment with rituximab. Herein, we perform an extensive review of the literature with a focus on the emerging concepts of mercury-induced autoimmunity and the role of mercury in the etiopathogenesis of autoimmune diseases of the nervous system.

Bacteria affect Caenorhabditis elegans responses to MeHg toxicity

The organic form of mercurial complex, methylmercury (MeHg), is a neurotoxin that bioaccumulates in the food web. Studies in model organisms, such as Caenorhabditis elegans (C. elegans), provide powerful insights on the role of genetic factors in MeHg-induced toxicity. C. elegans is a free living worm that is commonly cultured in nematode growth medium (NGM) agar plates seeded with bacteria. The bacteria have broad impact on C. elegans biology, including development, reproduction and lifespan, as well as metabolism of experimental chemicals. We hypothesized that MeHg toxicity in C. elegans could be modified by the bacterial food. Using a C. elegans wild-type (WT) strain and transgenic reporter strains, we found that bacterial food reduced the chronic toxicity of MeHg in C. elegans in a dose- and live-status-dependent manner. The MeHg-induced death rate in C. elegans was highest in fasted worms, followed by dehydrated dead bacteria, dead bacteria and live bacteria fed worms. Among the different bacterial foods, dehydrated dead bacteria fed worms were most sensitive to the toxicity of MeHg. The distinct bacteria food (dehydrated dead bacteria food) attenuated oxidative stress and development delay in C. elegans exposed to MeHg. The FOXO transcriptional factor DAF16 was not changed by MeHg but modified by the distinct bacteria food. Independent of MeHg treatment, daf-16 expression in fed worms migrated from the intestine to muscle. We conclude that, in chronic exposure studies in C. elegans, the effects of bacteria on toxicological outcomes should be considered.

Comparative effects of mercury chloride and methylmercury exposure on early neurodevelopment in zebrafish larvae

Mercury (Hg) is a ubiquitous environmental toxicant with important public health implications. Hg causes neurotoxicity through astrocytes, Ca²⁺, neurotransmitters, mitochondrial damage, elevations of reactive oxygen species and post-translational modifications. However, the similarities and differences between the neurotoxic mechanisms caused by different chemical forms of Hg remain unclear. Zebrafish embryos were exposed to methylmercury (MeHgCl) or mercury chloride (HgCl₂) (0, 4, 40, 400 nM) up for 96 h. HgCl₂ exposure could significantly decrease survival rate, body length and eye size, delay the hatching period, induce tail bending and reduce the locomotor activity, and these effects were aggravated in the MeHgCl group. The compounds could increase the number of apoptotic cells in the brain and downregulate the expression of Shha, Ngn1 and Nrd, which contribute to early nervous development. The underlying mechanisms were investigated by metabolomics data. Galactose metabolism, tyrosine metabolism and starch and sucrose metabolism pathways were disturbed after HgCl₂ or MeHgCl exposure. In addition, the levels of three neurotransmitters including tyrosine, dopamine and tryptophan were reduced after HgCl₂ or MeHgCl exposure. Oxidative stress is related to metabolite changes, such as changes in the putrescine, niacinamide and uric acid contents in the HgCl₂ group, and squalene in the MeHgCl group. These data indicated that downregulation of these genes and abnormal metabolic profile and pathways contribute to the neurotoxicity of HgCl₂ and MeHgCl.

The metabolomics and neurodevelopmental endpoints were integrated to reveal that abnormal metabolic pathway and expression of Shha, Ngn1 and Nrd may contribute to neurotoxicity induced by MeHg, which was more toxic than HgCl₂ in zebrafish larvae.