Developmental exposure to methylmercury and resultant muscle mercury accumulation and adult motor deficits in mice

Natural mercury exposure of European insectivorous bats may exceed a recognized toxicity threshold

Heavy metals are an important group of toxic substances harmful for many organisms. Of these, mercury is one of the most monitored in the environment. Several matrices are used for the monitoring of environmental load, including a range of organisms; bats, however, have only been examined rarely. Insectivorous bats are apex predators threatened by several human interventions in their natural environment, including heavy metal pollution. The aim of this study was to analyze the content of total mercury in the fur, flight membrane, and pectoral muscle of greater mouse-eared bats (Myotis myotis). Total mercury concentrations were also measured in carabid beetles from the catch locality Zastávka u Brna. Samples were obtained from 43 bat carcasses at two different localities in the Czech Republic (Zastávka u Brna, Malá Morávka). Total mercury content varied between 1.76-72.20 µg/g in fur, 0.04-0.14 µg/g in skin, and 0.05-0.20 µg/g in muscle. Total mercury values in the fur of some individuals from Malá Morávka exceeded the recognized toxicity limit. Furthermore, there was a significant difference (p < 0.001) in content of total mercury in fur between localities, and there was a clear effect of age on concentrations in fur, skin, and muscle, the concentrations being significantly correlated (fur and skin rs = 0.783; fur and muscle rs = 0.716; skin and muscle rs = 0.884). These findings confirm the usefulness of fur samples from living bats for biomonitoring mercury burden in the environment.

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.

Review: myogenic and muscle toxicity targets of environmental methylmercury exposure

A number of environmental toxicants are noted for their activity that leads to declined motor function. However, the role of muscle as a proximal toxicity target organ for environmental agents has received considerably less attention than the toxicity targets in the nervous system. Nonetheless, the effects of conventional neurotoxicants on processes of myogenesis and muscle maintenance are beginning to resolve a concerted role of muscle as a susceptible toxicity target. A large body of evidence from epidemiological, animal, and in vitro studies has established that methylmercury (MeHg) is a potent developmental toxicant, with the nervous system being a preferred target. Despite its well-recognized status as a neurotoxicant, there is accumulating evidence that MeHg also targets muscle and neuromuscular development as well as contributes to the etiology of motor defects with prenatal MeHg exposure. Here, we summarize evidence for targets of MeHg in the morphogenesis and maintenance of skeletal muscle that reveal effects on MeHg distribution, myogenesis, myotube formation, myotendinous junction formation, neuromuscular junction formation, and satellite cell-mediated muscle repair. We briefly recapitulate the molecular and cellular mechanisms of skeletal muscle development and highlight the pragmatic role of alternative model organisms, Drosophila and zebrafish, in delineating the molecular underpinnings of muscle development and MeHg-mediated myotoxicity. Finally, we discuss how toxicity targets in muscle development may inform the developmental origins of health and disease theory to explain the etiology of environmentally induced adult motor deficits and accelerated decline in muscle fitness with aging.

Maternal DHA intake in mice increased DHA metabolites in the pup brain and ameliorated MeHg-induced behavioral disorder

Although pregnant women's fish consumption is beneficial for the brain development of the fetus due to the DHA in fish, seafood also contains methylmercury (MeHg), which adversely affects fetal brain development. Epidemiological studies suggest that high DHA levels in pregnant women's sera may protect the fetal brain from MeHg-induced neurotoxicity, but the underlying mechanism is unknown. Our earlier study revealed that DHA and its metabolite 19,20-dihydroxydocosapentaenoic acid (19,20-DHDP) produced by cytochrome P450s (P450s) and soluble epoxide hydrolase (sEH) can suppress MeHg-induced cytotoxicity in mouse primary neuronal cells. In the present study, DHA supplementation to pregnant mice suppressed MeHg-induced impairments of pups' body weight, grip strength, motor function, and short-term memory. DHA supplementation also suppressed MeHg-induced oxidative stress and the decrease in the number of subplate neurons in the cerebral cortex of the pups. DHA supplementation to dams significantly increased the DHA metabolites 19,20-epoxydocosapentaenoic acid (19,20-EDP) and 19,20-DHDP as well as DHA itself in the fetal and infant brains, although the expression levels of P450s and sEH were low in the fetal brain and liver. DHA metabolites were detected in the mouse breast milk and in human umbilical cord blood, indicating the active transfer of DHA metabolites from dams to pups. These results demonstrate that DHA supplementation increased DHA and its metabolites in the mouse pup brain and alleviated the effects of MeHg on fetal brain development. Pregnant women's intake of fish containing high levels of DHA (or DHA supplementation) may help prevent MeHg-induced neurotoxicity in the fetus.

Species and reproductive status influence element concentrations in bat fur

To assess the impact of increasing pollutant levels on wildlife, we measured chemical contaminant loads in bats with different habitat and dietary preferences. Samples were taken from the fur of bats (Eptesicus nilssonii, Myotis brandtii, Myotis mystacinus and Plecotus auritus) to measure concentrations of 55 elements by inductively coupled plasma mass spectrometry (ICP-MS). Variations in element concentrations between different bat groups (species, sex, reproductive status) were analysed with a focus on arsenic (As), mercury (Hg) and lead (Pb) as these are known to cause specific health concerns in wildlife. For M. brandtii we found the highest As concentrations, especially in lactating bats, with a maximum exceeding those from other studies where bats had compromised health. Whereas for M. mystacinus there was a negative correlation between body condition index (BCI) and As concentration, indicating a potential danger for bats in the study area. In M. mystacinus and M. brandtii Hg concentrations were higher for sixteen individuals than in other studies where bats suffered genotoxic effects, although median levels were still below this threshold. Lactating bats from P. auritus and M. brandtii had higher Hg concentrations than bats of other reproductive status, which could endanger offspring as Hg can be transferred through lactation. In females from M. mystacinus Pb concentrations were more than three times higher compared to males. There was also a negative correlation between Pb concentration and BCI, which could mean that Pb has an adverse effect on health. Although many other biotic and abiotic factors should be considered, some of the variations in element concentrations could be due to different behaviours (foraging, roosting, etc.) in the studied species. The high levels of chemical contamination in some of the bats in our study, particularly reproductive individuals, is of conservation concern as bats are important agents for insect control.

Methylmercury-induced reactive oxygen species-dependent and independent dysregulation of MAP kinase-related signaling pathway in cultured normal rat cerebellar astrocytes

Methylmercury (MeHg), a global environmental pollutant, could seriously damage the central nervous system (CNS) and cause neurological disorders such as cerebellar symptoms. Although numerous studies have revealed detailed toxicity mechanisms of MeHg in neurons, toxicity in astrocytes is barely known. Here, we tried to shed light on the toxicity mechanisms of MeHg exposure in cultured normal rat cerebellar astrocytes (NRA), focusing on the involvement of reactive oxygen species (ROS) in MeHg toxicity by assessing the effects of major antioxidants Trolox, a free-radical scavenger, N-acetyl-L-cysteine (NAC), a potent thiol-containing antioxidant, and glutathione (GSH), an endogenous thiol-containing antioxidant. Exposure to MeHg at just approximately 2 µM for 96 h increased cell viability, which was accompanied by the increase in intracellular ROS level and at ≥ 5 µM induced significant cell death and lowered ROS level. Trolox and NAC suppressed 2 µM MeHg-induced increases in cell viability and ROS level corresponding to control, although GSH with 2 µM MeHg induced significant cell death and ROS increase. On the contrary, against 4 µM MeHg-induced cell loss and ROS decrease, NAC inhibited both cell loss and ROS decrease, Trolox inhibited cell loss and further enhanced ROS decrease, and GSH moderately inhibited cell loss and increased ROS level above the control level. MeHg-induced oxidative stress was suggested by increases in the protein expression levels of heme oxygenase-1 (HO-1), Hsp70, and Nrf2, except for the decrease in SOD-1 and no change in catalase. Furthermore, MeHg exposure dose-dependently induced increases in the phosphorylation of MAP kinases (ERK1/2, p38MAPK, and SAPK/JNK) and phosphorylation and/or expression levels of transcription factors (CREB, c-Jun, and c-Fos) in NRA. NAC successfully suppressed 2 µM MeHg-induced alterations in all of the above-mentioned MeHg-responsive factors, whereas Trolox suppressed some MeHg-responsive factors but failed to suppress MeHg-induced increases in the protein expression levels of HO-1 and Hsp70 and increase in p38MAPK phosphorylation. Protein expression analyses in NRA exposed to 2 µM MeHg and GSH were excluded because of devastating cell death. These results suggested that MeHg could induce aberrant NRA activation, and ROS must be substantially involved in the toxicity mechanism of MeHg in NRA; however, other factors should be assumed.

Adolescence as a sensitive period for neurotoxicity: Lifespan developmental effects of methylmercury

Neurotoxicity resulting from the environmental contaminant, methylmercury (MeHg), is a source of concern for many human populations that rely heavily on the consumption of fish and rice as stable ingredients in the diet. The developmental period of exposure is important both to the qualitative effects of MeHg and to the dose required to produce those effects. MeHg exposure during the sensitive prenatal period causes deleterious and long-lasting changes in neurodevelopment at particularly low doses. The effects include a wide host of cognitive and behavioral outcomes expressed in adulthood and sometimes not until aging. However, neurotoxic outcomes of methylmercury when exposure occurs during adolescence are only recently revealing impacts on human populations and animal models. This review examines the current body of work and showcases the sensitivity of adolescence, a period that straddles early development and adulthood, to methylmercury neurotoxicity and the implications such toxicity has in our understanding of methylmercury's effects in human populations and animal models.

Methylmercury Causes Neurodegeneration and Downregulation of Myelin Basic Protein in the Spinal Cord of Offspring Rats after Maternal Exposure

Methylmercury (MeHg) is one of the most dangerous toxic pollutants spread throughout the earth. Chronic MeHg intoxication by contaminated food ingestion is the most common threat to human health, including impairment to the developing fetus. The present study aims at investigating the effects of maternal exposure to MeHg during gestation and lactation on the spinal cord of offspring. Pregnant rats received oral doses of MeHg (40 μg/kg/day) over a period of 42 days (21 gestation and 21 lactation). Control animals received the vehicle only. Total mercury concentration was measured in blood samples from offspring collected at the 41st postnatal day. Counting of motor neurons and immunoreactivity for myelin basic protein (MBP) were assessed in the spinal cords in both control and MeHg-intoxicated animals. Our results showed that MeHg promoted an increase in blood Hg levels. In addition, it caused a reduction in the number of spinal cord motor neurons as well as decreased MBP immunoreactivity in the cervical, thoracic and lumbar segments. Our present findings suggest that MeHg intoxication during rat pregnancy and lactation is associated with a pattern of motor neuron degeneration and downregulation of myelin basic protein in different segments of a developing spinal cord. Further studies are needed to establish the effect of MeHg intoxication in both young and adult rats.

In utero exposure to methylmercury impairs cognitive function in adult offspring: Insights from proteomic modulation

Methylmercury (MeHg) is a hazardous substance that has unique neurodevelopmental toxic effects. However, its molecular alteration profile, sensitive response biomarkers, and mechanism of neuronal injury remain largely unknown. Here, the effects of intrauterine methylmercury chloride (low-, medium- and high-dose groups: 0.6 mg/kg/d, 1.2 mg/kg/d, 2.4 mg/ kg /d, respectively) exposure on learning and memory were assessed in offspring rats by behavioral tests, pathological analysis and hippocampal proteomic analysis. The results suggested that intrauterine MeHg exposure impairs spatial learning and memory and leads a significant reduction in the number and dispersion scattered arrangement in the hippocampus of offspring. Furthermore, in the tandem mass tag-based proteomics analysis, compared with the control group, a total of 74 differentially expressed proteins (DEPs) were found in the MeHg exposure groups; specifically, 32 down-regulated and 42 up-regulated proteins were identified. In addition, the pathways enrichment analysis indicated that these DEPs are implicated in several biological processes, such as synaptic plasticity and energy metabolism, as well as various molecular functional categories. Simultaneously, MeHg reduced the postsynaptic density, diminished the active zone, amplified the synaptic cleft and changed the synaptic interface of pyramidal cells. Western blot analysis further revealed that MeHg significantly reduced the levels of Forkhead box protein (FOXP2), Synaptophysin (SYP) and Postsynaptic density protein 95 (PSD-95), and down-regulated the N-methyl-D-aspartate receptor 1 (NMDAR1), N-methyl-D-aspartate receptor 2 A (NR2A) and N-methyl-D-aspartate receptor 2B (NR2B). In general, from a functional perspective, most overlapping proteins were related to NMDA receptor-mediated glutamatergic signaling, which is an excitotoxicity mechanism known to influence learning and memory. These discoveries contribute to our understanding of the relationship between MeHg and cognitive deficits and provide insight into the protein mediators of this relationship and possible prospective early biomarkers.

Latent effects of early-life methylmercury exposure on motor function in Drosophila

The developmental toxicant, methylmercury (MeHg), can elicit motor deficits that last well into adulthood. Recent studies using Drosophila showed that the developing musculature is sensitive to high doses of MeHg, where a larval feeding paradigm resulted in compromised myotendinous junction (MTJ) formation during development, by a mechanism involving the NG2 homologue, kon-tiki (kon). Low-dose exposures to MeHg that do not produce muscle pathology during development, nevertheless result in impaired flight behavior later in adult life. The present study evaluated the potential for relatively low-dose exposure to produce latent adult muscle pathology and motor impairments, as assayed by climbing and flight, as well as to evaluate molecular mechanisms that may contribute to motor deficits. Wildtype larvae were fed 0, 2, 2.5, or 5 μM MeHg laden food until eclosion. The effect of 5 μM MeHg on MTJ-related gene expression during pupal development was assessed via quantitative RT-qPCR analysis. Upon eclosion, adults were transferred to standard food bottles for 4, 11, or 30 days prior to motor testing. Survivorship (%) was determined from a subset of 200 flies per treatment. Average climbing speed (cm/s) was quantified 4-days post-eclosion (PE). Flight ability was assayed 11- or 30-days PE by measuring landing height (cm) of flies dropped into an adhesive-lined vertical column. In parallel, total body mercury was measured to estimate the influence of residual MeHg at the time of motor testing. Muscle morphology was assessed using immuno-fluorescence microscopy. Exposure to 5uM MeHg significantly reduced climbing speed, and flight ability 4 and 11 - days PE, respectively. While age-related flight deficits were seen in each sex, flight deficits due to MeHg persisted to 30-day PE timepoints exclusively in males. Expression of kon was upregulated across the window of pupal development essential to establishing adult MTJ. However, experimentally restricting the induction of comparable levels of kon to muscle during the same periods did not recapitulate the flight deficits, indicating that muscle-specific induction of kon alone is not sufficient to contribute to latent flight impairments. Adult flight muscle morphology of 11-day PE flies treated with 5 μM MeHg was indistinct from controls, implying muscle structure is not grossly perturbed to impair flight. Collectively, the current data suggest that developmental exposure to 5 μM MeHg reduces flight ability in each sex at 11 day-PE and that latent deficits at 30-day PE are male-specific. It remains to be determined whether the developing MTJ of Drosophila is a sensitive target of MeHg, and whether or not kon acts in conjunction with additional MTJ factors to constitute a MeHg target.

Nrf2/HO-1 Signaling Activator Acetyl-11-keto-beta Boswellic Acid (AKBA)-Mediated Neuroprotection in Methyl Mercury-Induced Experimental Model of ALS

Methylmercury (MeHg) is a potent neurotoxin that causes neurotoxicity and neuronal cell death. MeHg exposure also leads to oligodendrocyte destruction, glial cell overactivation, and demyelination of motor neurons in the motor cortex and spinal cord. As a result, MeHg plays an important role in the progression of amyotrophic lateral sclerosis (ALS)-like neurocomplications. ALS is a fatal neurodegenerative disorder in which neuroinflammation is the leading cause of further CNS demyelination. Nuclear factor erythroid-2-related factor-2 (Nrf2)/Heme oxygenase-1 (HO-1) signaling pathway was thought to be a potential target for neuroprotection in ALS. Acetyl-11-keto-beta-boswellic acid (AKBA) is a multi-component pentacyclic triterpenoid mixture derived from Boswellia serrata with anti-inflammatory and antioxidant properties. The research aimed to investigate whether AKBA, as a Nrf2 / HO-1 activator, can provide protection against ALS. Thus, we explored the role of AKBA on the Nrf2/HO-1 signaling pathway in a MeHg-induced experimental ALS model. In this study, ALS was induced in Wistar rats by oral gavage of MeHg 5 mg/kg for 21 days. An open field test, force swim test, and grip strength were performed to observe experimental rats' motor coordination behaviors. In contrast, a morris water maze was performed for learning and memory. Administration of AKBA 50 mg/kg and AKBA 100 mg/kg continued from day 22 to 42. Neurochemical parameters were evaluated in the rat's brain homogenate. In the meantime, post-treatment with AKBA significantly improved behavioral, neurochemical, and gross pathological characteristics in the brain of rats by increasing the amount of Nrf2/HO-1 in brain tissue. Collectively, our findings indicated that AKBA could potentially avoid demyelination and encourage remyelination.

Variation in Methylmercury Metabolism and Elimination in Humans: Physiological Pharmacokinetic Modeling Highlights the Role of Gut Biotransformation, Skeletal Muscle, and Hair

The biological half-life (t1/2) of methylmercury (MeHg) shows considerable individual variability (t1/2 < 30 to > 120 days), highlighting the importance of mechanisms controlling MeHg metabolism and elimination. Building on a prior physiologically based pharmacokinetic (PBPK) model, we elucidate parameters that have the greatest influence on variability of MeHg t1/2 in the human body. Employing a dataset of parameters for mean organ volumes and blood flow rates appropriate for man and woman (25-35 years) and child (4 - 6 years), we demonstrate model fitness by simulating data from our prior controlled study of MeHg elimination in people. Model predictions give MeHg t1/2 of 46.9, 38.9, and 31.5 days and steady-state blood MeHg of 2.6, 2.6, and 2.3 µg/l in man, woman, and child, respectively, subsequent to a weekly dose of 0.7 µg/kg body weight. The major routes of elimination are biotransformation to inorganic Hg in the gut lumen (73% in adults, 61% in child) and loss of MeHg via excretion within growing hair (13% in adults, 24% in child). Local and global sensitivity analyses of model parameters reveal that variation in biotransformation rate in the gut lumen, and rates of transport between gut lumen and gut tissue, have the greatest influence on MeHg t1/2. Volume and partition coefficients for skeletal muscle (SM) and gut tissue also show significant sensitivity affecting model output of MeHg t1/2. Our results emphasize the role of gut microbiota in MeHg biotransformation, transport kinetics at the level of the gut, and SM mass as moderators of MeHg kinetics in the human body.