Cellular Conditions Responsible for Methylmercury-Mediated Neurotoxicity

Differential Modulatory Effects of Methylmercury (MeHg) on Ahr-regulated Genes in Extrahepatic Tissues of C57BL/6 Mice

Methylmercury (MeHg) and 2,3,7,8-tetrachlorodibenzodioxin (TCDD) are potent environmental pollutants implicated in the modulation of xenobiotic-metabolizing enzymes, particularly the cytochrome P450 1 family (CYP1) which is regulated by the aryl hydrocarbon receptor (AHR). However, the co-exposure to MeHg and TCDD raises concerns about their potential combined effects, necessitating thorough investigation. The primary objective of this study was to investigate the individual and combined effects of MeHg and TCDD on AHR-regulated CYP1 enzymes in mouse extrahepatic tissues. Therefore, C57BL/6 mice were administrated with MeHg (2.5 mg/kg) in the absence and presence of TCDD (15 μg/kg) for 6 and 24 h. The AHR-regulated CYP1 mRNA and protein expression levels were measured in the heart, lung, and kidney, using RT real-time PCR and western blot, respectively. Interestingly, treatment with MeHg exhibited mainly inhibitory effect, particularly, it decreased the basal level of Cyp1a1 and Cyp1a2 mRNA and protein, and that was more evident at the 24 h time point in kidney followed by heart. Similarly, when mice were co-exposed, MeHg was able to reduce the TCDD-induced Cyp1a1 and Cyp1a2 expression, however, MeHg potentiated kidney Cyp1b1 mRNA expression, opposing the observed change on its protein level. Also, MeHg induced antioxidant NAD(P)H:quinone oxidoreductase (NQO1) mRNA and protein in kidney, while heme-oxygenase (HO-1) mRNA was up-regulated in heart and kidney. In conclusion, this study reveals intricate interplay between MeHg and TCDD on AHR-regulated CYP1 enzymes, with interesting inhibitory effects observed that might be significant for procarcinogen metabolism. Varied responses across tissues highlight the potential implications for environmental health.

Leek (Allium ampeloprasum var. kurrat) aqueous extract loaded on selenium nanoparticles protects against testis and brain injury induced by mercuric chloride in rats

BACKGROUND

Mercuric chloride (HgCl2) is poisonous to humans and animals and typically damages the nervous system and other organs. Mercuric chloride exposition disclosed to initiation of oxidative stress pathway can result in a defect in male fertility and testis tissue. Synthesized selenium nanoparticles (SeNPs) were characterized with a diameter range minimal than 100 nm, having the effective sets of the biological matter. The present study aimed to evaluate the effect of biosynthesized SeNPs, prepared by leek extract on Wistar rats' testicles and brain.

METHODS

Thirty-five Wistar male rats (120-150 g) were randomly split into five groups (n = 7), orally ingested with leek aqueous extract loaded on SeNPs, and then the animals were administered with mercury II chloride (HgCl2) to induce testis injury and damage the nervous system.

RESULTS

The used dose of mercuric chloride led to oxidative stress damage in the testis of the rats which was evidenced by a decrease in testosterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and proliferating cell nuclear antigen (PCNA) levels, and an increase in nuclear factor-kappa B (NF-κB) and caspase-3. Also, HgCl2 decreased the levels of dopamine (DA), serotonin (5-HT), norepinephrine (NE) and brain-derived neurotrophic factor (BDNF) in the brains of rats. In addition, A decrease was observed in the levels of antioxidant markers, B-cell lymphoma-2 (Bcl-2), as well as an increase in malondialdehyde (MDA), nitric oxide (NO), NF-κB, tumor necrosis factor (TNF)-α, interleukin (IL)-1β and Bax in both testes and brains. Pre-treatment with leek extract loaded on SeNPs significantly ameliorated testosterone, LH, FSH, PCNA and caspase-3 levels in the testis and DA, 5-HT, NE and BDNF in brains. Although the contents of MDA, NO, TNF-α, IL-1β, NF-κB and Bax decreased significantly in both. glutathione, glutathione peroxidase, glutathione reductase, catalase, superoxide dismutase and Bcl-2 levels were significantly improved in both organs.

CONCLUSION

Our findings suggest that treatment with aqueous leek extract loaded on SeNPs may offer promising prospects for the advancement of anti-inflammation activity against testis injury and also have a very key role in neurobehavioral alterations as a result of mercury toxicity. © 2024 Society of Chemical Industry.

Development of a sensor to detect methylmercury toxicity

Methylmercury (MeHg) is a well-known neurotoxicant that induces various cellular functions depending on cellular- and developmental-specific vulnerabilities. MeHg has a high affinity for selenol and thiol groups, thus impairing the antioxidant system. Such affinity characteristics of MeHg led us to develop sensor vectors to assess MeHg toxicity. In this study, MeHg-mediated defects in selenocysteine (Sec) incorporation were demonstrated using thioredoxin reductase 1 cDNA fused with the hemagglutinin tag sequence at the C-terminus. Taking advantage of such MeHg-mediated defects in Sec incorporation, a cDNA encoding luciferase with a Sec substituted for cysteine-491 was constructed. This construct showed MeHg-induced decreases in signaling in a dose-dependent manner. To directly detect truncated luciferase under MeHg exposure, we further constructed a new sensor vector fused with a target for proteasomal degradation. However, this construct was inadequate because of the low rate of Sec insertion, even in the absence of MeHg. Finally, a Krab transcriptional suppressor fused with Sec was constructed and assessed to demonstrate MeHg-dependent increases in signal intensity. We confirmed that the vector responded specifically and in a dose-dependent manner to MeHg in cultured cerebellar granule cells. This vector is expected to allow monitoring of MeHg-specific toxicity via spatial and temporal imaging.

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.

Biomarkers of methylmercury neurotoxicity and neurodevelopmental features: A systematic review

The issue of MeHg contamination is a significant concern due to its detrimental impact on the environment. This study aimed to thoroughly investigate the effects of MeHg on neurodevelopmental biomarkers, as there is a lack of systematic reviews in this area. We conducted a comprehensive search of three databases (PubMed, Scopus, and Web of Science) and found 522 records, which were then meticulously reviewed by two independent reviewers. A total of 66 studies were included, with biomarkers related to oxidative stress, neurotransmission, inflammation, epigenetics, and apoptosis being the most prominent. The results of both in vitro and in vivo models indicate that antioxidant enzymes and other oxidative stress-related markers are indeed, altered following MeHg exposure. Moreover, MeHg exposure causes significant disruptions to neurotransmitter levels, activities of neurotransmitter synthesis enzymes, receptor densities, and proteins involved in synaptic function. Proinflammatory biomarkers are consistently overexpressed in both MeHg-treated cells and the brains of exposed rats. Furthermore, studies on DNA methylation and biomarker activity suggest that MeHg exposure may lead to neurotoxicity and neurodevelopmental issues via perturbations to epigenetic markers and the apoptosis pathway.

Neurotoxicity of manganese via ferroptosis induced by redox imbalance and iron overload

Manganese (Mn) is an essential trace element for maintaining bodily functions. Excessive exposure to Mn can pose serious health risks to humans and animals, particularly to the nervous system. While Mn has been implicated as a neurotoxin, the exact mechanism of its toxicity remains unclear. Ferroptosis is a form of programmed cell death that results from iron-dependent lipid peroxidation. It plays a role in various physiological and pathological cellular processes and may be closely related to Mn-induced neurotoxicity. However, the mechanism of ferroptosis in Mn-induced neurotoxicity has not been thoroughly investigated. Therefore, this study aims to investigate the role and mechanism of ferroptosis in Mn-induced neurotoxicity. Using bioinformatics, we identified significant changes in genes associated with ferroptosis in Mn-exposed animal and cellular models. We then evaluated the role of ferroptosis in Mn-induced neurotoxicity at both the animal and cellular levels. Our findings suggest that Mn exposure causes weight loss and nervous system damage in mice. In vitro and in vivo experiments have shown that exposure to Mn increases malondialdehyde, reactive oxygen species, and ferrous iron, while decreasing glutathione and adenosine triphosphate. These findings suggest that Mn exposure leads to a significant increase in lipid peroxidation and disrupts iron metabolism, resulting in oxidative stress injury and ferroptosis. Furthermore, we assessed the expression levels of proteins and mRNAs related to ferroptosis, confirming its significant involvement in Mn-induced neurotoxicity.

Molecular crosstalk and putative mechanisms underlying mitochondrial quality control: The hidden link with methylmercury-induced cognitive impairment

Methylmercury (MeHg) is a neurotoxin associated with foetal neurodevelopmental and adult cognitive deficits. Neurons are highly dependent on the tricarboxylic acid cycle and oxidative phosphorylation to produce ATP and meet their high energy demands. Therefore, mitochondrial quality control (MQC) is critical for neuronal homeostasis. While existing studies have generated a wealth of data on the toxicity of MeHg, the complex cascades and molecular pathways governing the mitochondrial network remain to be elucidated. Here, 0.6, 1.2 and 2.4 mg/kg body weight of MeHg were administered intragastrically to pregnant Sprague Dawley rats to model maternal MeHg exposure. The results of the in vivo study revealed that MeHg-treated rats tended to perform more directionless repetitive strategies in the Morris Water Maze and fewer target-orientation strategies than control offspring. Moreover, pathological injury and synaptic toxicity were observed in the hippocampus. Transmission electron microscopy (TEM) demonstrated that the autophagosomes encapsulated damaged mitochondria, while showing a typical mitochondrial fission phenotype, which was supported by the activation of PINK1-dependent key regulators of mitophagy. Moreover, there was upregulation of DRP1 and FIS1. Additionally, MeHg compensation promoted mitochondrial biogenesis, as evidenced by the activation of the mitochondrial PGC1-α-NRF1-TFAM signalling pathway. Notably, SIRT3/AMPK was activated by MeHg, and the expression and activity of p-AMPK, p-LKB1 and SIRT3 were consistently coordinated. Collectively, these findings provide new insights into the potential molecular mechanisms regulating MeHg-induced cognitive deficits through SIRT3/AMPK MQC network coordination.

BDNF specifically expressed in hippocampal neurons is involved in methylmercury neurotoxicity resistance

Methylmercury (MeHg) causes selective neuronal damage to cerebrocortical neurons (CCNs) in the central nervous system, but not to hippocampal neurons (HiNs), which are highly vulnerable to neurodegenerative diseases. In our previous study using cultured rat neurons, we performed a comprehensive gene expression analysis and found that the brain-derived neurotrophic factor (BDNF), a neurotrophin (NT), was specifically expressed in HiNs. Therefore, to elucidate the causal factors of MeHg toxicity resistance in HiNs, we conducted a comparative study of the protein expression and function of several NTs, including BDNF, using CCNs showing vulnerability to MeHg toxicity and HiNs showing resistance. BDNF was specifically expressed in HiNs, whereas nerve growth factor was barely detectable in either neuron type. In addition, other NTs, NT3 and NT4/5, were expressed in small but nearly equal amounts in both neuron types. Furthermore, among the various pathways involved in MeHg neurotoxicity, the p44/42 MAPK pathway was specifically activated in HiNs, even without MeHg treatment. siRNAs were used to reduce NTs in both neuron types. Only a specific reduction in BDNF attenuated the resistance to MeHg toxicity and p44/42 MAPK activation in HiNs. In addition, the external addition of BDNF and NT4/5, which act on the same tyrosine receptor kinase (Trk), TrkB, suppressed MeHg neurotoxicity in both neuron types. These results suggest that BDNF, expressed specifically in HiNs, is involved in the resistance to MeHg neurotoxicity via TrkB. Additionally, the activation of the p44/42 MAPK pathway may contribute to the inhibitory effect of BDNF on MeHg neurotoxicity.

Cohort profile: China undergraduate cohort for environmental health study

The China Undergraduate Cohort (CUC) is an ambispective cohort study with its major purpose to better understand the effects of lifetime environmental exposures on health outcomes. We recruited 5322 college students with an average age of 18.3 ± 0.7 years in China from August 23, 2019 to October 28, 2019. Follow-up surveys were conducted annually. The dataset comprises individual demographic data (e.g. age, sex, height, weight, birth date, race, home address, annual family income, contact information), health-related behavior data (smoking status, smoking cessation, passive smoking exposure, drinking habit, physical activity, dietary status), lifestyle data (physical exercise, dietary habit, length of time spent outdoors), disease history (respiratory disease history, cardiovascular disease history, urinary system disease history, etc.), mental health status data (sleep quality, self-reported stress, anxiety and depression symptoms), lung function and blood samples data. Preliminary results from our cohort have found the association between air pollution, summer heat and mercury exposure and lung function among young adults in China.

Resveratrol alleviates perinatal methylmercury-induced neurobehavioral impairments by modulating the gut microbiota composition and neurotransmitter disturbances

Methylmercury (MeHg), a potent neurotoxic substance, causes adverse health outcomes by modulating metabolites through altered gut microbiota patterns. Among the many metabolites, neurotransmitters play a particularly important role in the nervous system and behavior. Resveratrol (RSV) has been investigated as an antiaging, antioxidant, anti-inflammatory, and neuroprotective agent. The current study evaluated that RSV is protective of neurodevelopmental toxicity induced by MeHg and further explored the underlying mechanisms. Sprague-Dawley rats were treated with 1.2 mg/kg/d of MeHg, and the effects were evaluated after supplementation with RSV (20 mg/kg/d). The results indicated that MeHg had adverse effects on early neurodevelopmental indicators in the experimental group offspring as compared to control pups. Interestingly, RSV significantly improved the MeHg-induced delays in the neurobehavioral reflexes and reduced the total mercury (THg) concentration in the colons of the offspring rats. In agreement, RSV administration improved the gut microbiota diversity and structure by increasing the abundance of probiotics and upregulating the expression of tight junction proteins. It also ameliorated the MeHg-induced abnormalities in the expression profiles of neurotransmitters. Furthermore, eight key bacteria that were strongly linked with the neurotransmitters and neuroreflex parameters were identified. Taken together, these results demonstrate that RSV treatment effectively reduces the occurrence of neurodevelopmental toxicity caused by perinatal MeHg exposure by modulating the intestinal flora and neurotransmitter metabolism. These findings provide a new therapeutic approach for treating MeHg-induced neurotoxicity. The cover image is based on the Research Article Resveratrol alleviates perinatal methylmercury-induced neurobehavioral impairments by modulating the gut microbiota composition and neurotransmitter disturbances by Fang Chen et al., https://doi.org/10.1002/tox.23973.

A Reaction-Based ESIPT Fluorescent Probe for the Detection of Hg2+ with Large Stokes Shift

A novel reaction-based fluorescent probe 1 for Hg2+ was designed and synthesized. 1 was almost nonfluoresent due to inhibition of the ESIPT process between hydroxy group and imid carbonyl oxygen by diphenylphosphinothioate group. After reacting with Hg2+, the fluorescence intensity of 1 exhibited significant enhancement owing to recovery of the ESIPT process via Hg2+-promoted desulfurization-hydrolysis of the diphenylphosphinothioate moiety and cleavage of the P-O bond. 1 not only showed rapid response, high sensitivity, excellent selectivity for Hg2+ over other metal ions, but also could detect Hg2+ with large Stokes shift (165 nm), which was attributed to the ESIPT process. Moreover, the reaction mechanism was fully validated by absorption spectra, fluorescence spectra, fluorescence color as well as ESI-MS analysis. 1 is the reaction-based ESIPT fluorescent probe for the detection of Hg2+ with large Stokes shift, rapid response, high sensitivity and selectivity.

A mitochondrion-targeted fluorescent probe based on ESIPT phthalimide for the detection of Hg2+ with large Stokes shift

A novel mitochondrion-targeted Hg2+-specific fluorescent probe 1 based on ESIPT phthalimide was designed and synthesized for the first time. Owing to the blockage of the ESIPT process between the hydroxy group and the carbonyl oxygen of the imide by the diphenylphosphinothioate group, 1 was almost nonfluorescent. After reacting with Hg2+, 1 exhibited a dramatic fluorescence enhancement due to the recovery of the ESIPT process through Hg2+-induced desulfurization-hydrolysis of the diphenylphosphinothioate moiety and the cleavage of the P-O bond. 1 showed a large Stokes shift, rapid response and high sensitivity and selectivity for Hg2+ over other metal ions. Moreover, 1 was successfully employed to image Hg2+ in the mitochondria of living cells.

Conversion of methylmercury into inorganic mercury via organomercurial lyase (MerB) activates autophagy and aggresome formation

Methylmercury (MeHg) is converted to inorganic mercury (iHg) in several organs; however, its impact on tissues and cells remains poorly understood. Previously, we established a bacterial organomercury lyase (MerB)-expressing mammalian cell line to overcome the low cell permeability of iHg and investigate its effects. Here, we elucidated the cytotoxic effects of the resultant iHg on autophagy and deciphered their relationship. Treatment of MerB-expressing cells with MeHg significantly increases the mRNA and protein levels of LC3B and p62, which are involved in autophagosome formation and substrate recognition, respectively. Autophagic flux assays using the autophagy inhibitor chloroquine (CQ) revealed that MeHg treatment activates autophagy in MerB-expressing cells but not in wild-type cells. Additionally, MeHg treatment induces the accumulation of ubiquitinated proteins and p62, specifically in MerB-expressing cells. Confocal microscopy revealed that large ubiquitinated protein aggregates (aggresomes) associated with p62 are formed transiently in the perinuclear region of MerB-expressing cells upon MeHg exposure. Meanwhile, inhibition of autophagic flux decreases the MeHg-induced cell viability of MerB-expressing cells. Overall, our results imply that cells regulate aggresome formation and autophagy activation by activating LC3B and p62 to prevent cytotoxicity caused by iHg. These findings provide insights into the role of autophagy against iHg-mediated toxicity.

Recent insights into autophagy and metals/nanoparticles exposure

Some anthropogenic pollutants, such as heavy metals and nanoparticles (NPs), are widely distributed and a major threat to environmental safety and public health. In particular, lead (Pb), cadmium (Cd), chromium (Cr), arsenic (As), and mercury (Hg) have systemic toxicity even at extremely low concentrations, so they are listed as priority metals in relation to their significant public health burden. Aluminum (Al) is also toxic to multiple organs and is linked to Alzheimer's disease. As the utilization of many metal nanoparticles (MNPs) gradually gain traction in industrial and medical applications, they are increasingly being investigated to address potential toxicity by impairing certain biological barriers. The dominant toxic mechanism of these metals and MNPs is the induction of oxidative stress, which subsequently triggers lipid peroxidation, protein modification, and DNA damage. Notably, a growing body of research has revealed the linkage between dysregulated autophagy and some diseases, including neurodegenerative diseases and cancers. Among them, some metals or metal mixtures can act as environmental stimuli and disturb basal autophagic activity, which has an underlying adverse health effect. Some studies also revealed that specific autophagy inhibitors or activators could modify the abnormal autophagic flux attributed to continuous exposure to metals. In this review, we have gathered recent data about the contribution of the autophagy/mitophagy mediated toxic effects and focused on the involvement of some key regulatory factors of autophagic signaling during exposure to selected metals, metal mixtures, as well as MNPs in the real world. Besides this, we summarized the potential significance of interactions between autophagy and excessive reactive oxygen species (ROS)-mediated oxidative damage in the regulation of cell survival response to metals/NPs. A critical view is given on the application of autophagy activators/inhibitors to modulate the systematic toxicity of various metals/MNPs.

Effects of Endocrine-Disrupting Heavy Metals on Human Health

Heavy metals play an important endocrine-disrupting role in the health consequences. However, the endocrine-disrupting mechanism of heavy metals is unclear. There are long-term and low-level metal/element exposure scenes for the human body in real life. Therefore, animal models exposed to high doses of heavy metals may not provide key information to elucidate the underlying pathogeny of human diseases. This review collects current knowledge regarding the endocrine-disrupting roles of heavy metals such as lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), nickel (Ni), copper (Cu), zinc (Zn), and manganese (Mn), summarizes the possible molecular mechanisms of these endocrine-disrupting chemicals (EDCs), and briefly evaluates their endocrine toxicity on animals and humans.