Investigation of intracellular factors involved in methylmercury toxicity

Comprehensive review on toxic heavy metals in the aquatic system: sources, identification, treatment strategies, and health risk assessment

Heavy metal pollution in water sources has become a major worldwide environmental issue, posing a threat to aquatic ecosystems and human health. The pollution of the aquatic environment is increasing as a result of industrialization, climate change, and urban development. The sources of heavy metal pollution in water include mining waste, leachates from landfills, municipal and industrial wastewater, urban runoff, and natural events such as volcanism, weathering, and rock abrasion. Heavy metal ions are toxic and potentially carcinogenic. They can also buildup in biological systems and cause bioaccumulation even at low levels of exposure, heavy metals can cause harm to organs such as the nervous system, liver and lungs, kidneys and stomach, skin, and reproductive systems. There were various approaches tried to purify water and maintain water quality. The main purpose of this article was to investigate the occurrence and fate of the dangerous contaminants (Heavy metal and metalloids) found in domestic and industrial effluents. This effluent mixes with other water streams and is used for agricultural activities and other domestic activities further complicating the issue. It also discussed conventional and non-conventional treatment methods for heavy metals from aquatic environments. Conclusively, a pollution assessment of heavy metals and a human health risk assessment of heavy metals in water resources have been explained. In addition, there have been efforts to focus on heavy metal sequestration from industrial waste streams and to create a scientific framework for reducing heavy metal discharges into the aquatic environment.

Are mercury levels in fishery products appropriate to ensure low risk to high fish-consumption populations?

The main goal of the present study is to determine the sources of methylmercury (MeHg) for high fish-consumption populations with the Portuguese population as showcase, as Portugal is the EU country with the highest fish consumption per capita (2019: 59.91 kg year^-1). Since limited information is available on the effective levels of mercury after culinary treatments, cooked and raw codfish (Gadus morhua), hake (Merluccius merluccius), octopus (Octopus vulgaris), horse mackerel (Trachurus trachurus) and sardine (Sardina pilchardus) were considered. The mercury concentration ranking Hake > Horse mackerel > Codfish > Octopus > Sardine was observed in all situations (cooked and raw samples) for both MeHg and total mercury (T-Hg). The gathered results reinforce the general assumption that the loss of moisture during cooking increases MeHg and T-Hg concentrations in fish, but the idea that MeHg in fish muscle tissue represents the bulk of T-Hg cannot be generalised, as our study determined a MeHg/T-Hg ratio of 0.43 for grilled sardines.

Identification of metal binding motifs in protein frameworks to develop novel remediation strategies for Hg2+ and Cr(VI)

Amino acid sequences in metal-binding proteins with chelating properties offer exciting applications in biotechnology and medical research. To enhance their application in bioremediation studies, we explicitly aimed to identify specific metal-binding chelating motifs in protein structures for two significant pollutants, such as mercury (Hg²⁺) and chromium Cr(V1). For this purpose, we have performed an extensive coordination chemistry approach by retrieving Hg²⁺ and Cr(V1) binding protein structures from the protein database and validated using the B-factor, a term defining uncertainty of the atoms and with occupancy to obtain the best binding motifs. Our analysis revealed that acidic amino acids like aspartic acid, glutamic acid, and basic amino acids such as cysteine and histidine are predominant in coordinating with these metals. The order of preference in Hg²⁺-bound structures is predicted to be Cys > His > Asp > Glu, and for Cr(V1) is His > Asp > Glu. Examination of the atomic coordinates and their distance from each metal revealed that the sulfur atoms of cysteine showing more preference towards Hg²⁺coordination with an atomic distance ranging from 1.5 to 2.9 Å. Likewise, oxygen atoms of aspartic acid, glutamic acid and nitrogen atoms of histidine are within 2 Å of Cr(V1) coordination. Based on these observations, we obtained C-C-C, C-X(2)-C-C-(X)2-C, H-C-H motifs for Hg²⁺, and D-X(1)-D, H-X(3)-E motif for Cr(V1) to be shared within the coordination space of 3 Å. As a future scope, we propose that the identified metal-binding chelating motifs are oligopeptides and can display on the surface of microorganisms such as Escherichia coli and Saccharomyces cerevisiae for effective removal of natural Hg²⁺ and Cr(V1) through biosorption. Hence, our results will provide the basis for futuristic bioremediation.

Macrobrachium amazonicum (Crustacea, Decapoda) Used to Biomonitor Mercury Contamination in Rivers

Open-air landfill's may be are considered as a potential source of human environmental exposure to chemical substances such as, polycyclic aromatic hydrocarbons and toxic metals. Due to possible availability of mercury in the environment caused by open landfill emissions, this study evaluates the spatiality and seasonality of macroinvertebrates, in particular shrimps (Macrobrachium amazonicum), exposure to mercury (Hg). Information regarding Hg accumulation in this crustacean may be important for the development of public policies aiming conservation and preservation of ecosystems surrounding landfills in Amazon, and around the world. Sampling occurred quarterly in the following months: November/2015; February/2016; May/2016 and; August/2016. In each of these months, three points were selected: P1, P2 and P3. The samples were processed via acid digestion and the quantification of metal was performed by inductively coupled plasma mass spectrometry. The average concentration of total mercury (T-Hg) was 24.565 ± 6.610 µg kg^-1 wet weight, with minimum and maximum limits of 12.742 ± 11.367 (P3) and 35.509 ± 14.761 µg kg^-1 wet weight (P1) in November/2015 and August/2016, respectively. The concentration of total mercury (T-Hg) in shrimps was different between points (p = 0.004) and months (p = 0.000). The T-Hg concentrations were significantly higher in May and August 2016, which corresponds to the dry season. The presence of landfills promotes large accumulation of T-Hg in the aquatic biota and represents a risk to human health. However, seasonal changes in T-Hg levels were observed. In the wettest period, bioconcentration factor levels decrease in aquatic organisms.

Nitrate and Phosphate Transporters Rescue Fluoride Toxicity in Yeast

Organisms are exposed to fluoride in the air, water, and soil. Yeast and other microbes utilize fluoride channels as a method to prevent intracellular fluoride accumulation and mediate fluoride toxicity. Consequently, deletion of fluoride exporter genes (FEX) in S. cerevisiae resulted in over 1000-fold increased fluoride sensitivity. We used this FEX knockout strain to identify genes, that when overexpressed, are able to partially relieve the toxicity of fluoride exposure. Overexpression of five genes, SSU1, YHB1, IPP1, PHO87, and PHO90, increase fluoride tolerance by 2- to 10-fold. Overexpression of these genes did not provide improved fluoride resistance in wild-type yeast, suggesting that the mechanism is specific to low fluoride toxicity in yeast. Ssu1p and Yhb1p both function in nitrosative stress response, which is induced upon fluoride exposure along with metal influx. Ipp1p, Pho87p, and Pho90p increase intracellular orthophosphate. Consistent with this observation, fluoride toxicity is also partially mitigated by the addition of high levels of phosphate to the growth media. Fluoride inhibits phosphate import upon stress induction and causes nutrient starvation and organelle disruption, as supported by gene induction monitored through RNA-Seq. The combination of observations suggests that transmembrane nutrient transporters are among the most sensitized proteins during fluoride-instigated stress.

The Clinical Importance of the Mercury Problem in Artisanal Small-Scale Gold Mining

Artisanal small-scale mining is widely operated in various countries serving as a livelihood to many rural communities. However, it is a significant source of environmental mercury contamination which affects human health. Amalgamation and amalgam smelting, two significant steps in the artisanal small-scale mining operations generate lots of mercury vapors, leading to chronic exposure among miners. Thus, this article seeks to provide a topical review of recent findings on organ damage and metabolic disorders among mercury-exposed artisanal small-scale miners with emphasis on the contributing factors such as personal protective equipment usage and artisanal small-scale gold mining-specific occupational activities. Also, insights into the effect of mercury intoxication and mechanisms of action on organ and metabolic systems among exposed individuals are provided.

Environmental mercury and its toxic effects

Mercury exists naturally and as a man-made contaminant. The release of processed mercury can lead to a progressive increase in the amount of atmospheric mercury, which enters the atmospheric-soil-water distribution cycles where it can remain in circulation for years. Mercury poisoning is the result of exposure to mercury or mercury compounds resulting in various toxic effects depend on its chemical form and route of exposure. The major route of human exposure to methylmercury (MeHg) is largely through eating contaminated fish, seafood, and wildlife which have been exposed to mercury through ingestion of contaminated lower organisms. MeHg toxicity is associated with nervous system damage in adults and impaired neurological development in infants and children. Ingested mercury may undergo bioaccumulation leading to progressive increases in body burdens. This review addresses the systemic pathophysiology of individual organ systems associated with mercury poisoning. Mercury has profound cellular, cardiovascular, hematological, pulmonary, renal, immunological, neurological, endocrine, reproductive, and embryonic toxicological effects.

Comparative toxicogenomic responses of mercuric and methyl-mercury

Background

Mercury is a ubiquitous environmental toxicant that exists in multiple chemical forms. A paucity of information exists regarding the differences or similarities by which different mercurials act at the molecular level.

Results

Transcriptomes of mixed-stage C. elegans following equitoxic sub-, low- and high-toxicity exposures to inorganic mercuric chloride (HgCl₂) and organic methylmercury chloride (MeHgCl) were analyzed. In C. elegans, the mercurials had highly different effects on transcription, with MeHgCl affecting the expression of significantly more genes than HgCl₂. Bioinformatics analysis indicated that inorganic and organic mercurials affected different biological processes. RNAi identified 18 genes that were important in C. elegans response to mercurial exposure, although only two of these genes responded to both mercurials. To determine if the responses observed in C. elegans were evolutionarily conserved, the two mercurials were investigated in human neuroblastoma (SK-N-SH), hepatocellular carcinoma (HepG2) and embryonic kidney (HEK293) cells. The human homologs of the affected C. elegans genes were then used to test the effects on gene expression and cell viability after using siRNA during HgCl₂ and MeHgCl exposure. As was observed with C. elegans, exposure to the HgCl₂ and MeHgCl had different effects on gene expression, and different genes were important in the cellular response to the two mercurials.

Conclusions

These results suggest that, contrary to previous reports, inorganic and organic mercurials have different mechanisms of toxicity. The two mercurials induced disparate effects on gene expression, and different genes were important in protecting the organism from mercurial toxicity.

A system-based comparison of gene expression reveals alterations in oxidative stress, disruption of ubiquitin-proteasome system and altered cell cycle regulation after exposure to cadmium and methylmercury in mouse embryonic fibroblast

Environmental and occupational exposures to heavy metals such as methylmercury (MeHg) and cadmium (Cd) pose significant health risks to humans, including neurotoxicity. The underlying mechanisms of their toxicity, however, remain to be fully characterized. Our previous studies with Cd and MeHg have demonstrated that the perturbation of the ubiquitin-proteasome system (UPS) was associated with metal-induced cytotoxicity and apoptosis. We conducted a microarray-based gene expression analysis to compare metal-altered gene expression patterns with a classical proteasome inhibitor, MG132 (0.5 microM), to determine whether the disruption of the UPS is a critical mechanism of metal-induced toxicity. We treated mouse embryonic fibroblast cells at doses of MeHg (2.5 microM) and Cd (5.0 microM) for 24 h. The doses selected were based on the neutral red-based cell viability assay where initial statistically significant decreases in variability were detected. Following normalization of the array data, we employed multilevel analysis tools to explore the data, including group comparisons, cluster analysis, gene annotations analysis (gene ontology analysis), and pathway analysis using GenMAPP and Ingenuity Pathway Analysis (IPA). Using these integrated approaches, we identified significant gene expression changes across treatments within the UPS (Uchl1 and Ube2c), antioxidant and phase II enzymes (Gsta2, Gsta4, and Noq1), and genes involved in cell cycle regulation pathways (ccnb1, cdc2a, and cdc25c). Furthermore, pathway analysis revealed significant alterations in genes implicated in Parkinson's disease pathogenesis following metal exposure. This study suggests that these pathways play a critical role in the development of adverse effects associated with metal exposures.

Characterization of antioxidant protection of cultured neural progenitor cells (NPC) against methylmercury (MeHg) toxicity

Methylmercury (MeHg) is an environmental pollutant known to cause neurobehavioral defects and is especially toxic to the developing brain. With recent studies showing that fetal exposure to low-dose MeHg causes developmental abnormalities, it is therefore important to find ways to combat its effects as well as to clarify the mechanism(s) underlying MeHg toxicity. In the present study, the effects of MeHg on cultured neural progenitor cells (NPC) derived from mouse embryonic brain were investigated. We first confirmed the vulnerability of embryonic NPC to MeHg toxicity, NPC from the telencephalon were more sensitive to MeHg compared to those from the diencephalon. Buthionine sulfoximine (BSO) which is known to inhibit glutathione synthesis accelerated MeHg toxicity. Furthermore, antioxidants such as N-acetyl cysteine and alpha-tocopherol dramatically rescued the NPC from MeHg's toxic effects. Interestingly, a 12 hr delay in the addition of either antioxidant was still able to prevent the cells from undergoing cell death. Although it is now difficult to avoid MeHg exposure from our environment and contaminated foods, taking anti-oxidants from foods or supplements may prevent or diminish the toxicological effects of MeHg.

Neurotoxic effects in workers of the clinical thermometer manufacture plant

OBJECTIVES

Occupational exposure to mercury can induce adverse health effects, and the central nervous system is the major target of its toxic action. This problem especially arises in plants involved in the manufacture of mercury-containing products, where an appropriate protection against mercury exposure is not ensured. The aim of this study was to assess health effects of mercury, especially neurotoxic effects and oral disorders, in workers employed in a clinical thermometer manufacture plant and to determine mercury concentrations in the workplace ambient air.

MATERIALS AND METHODS

The study population comprised 143 workers, including 51 (35.7%) men and 92 (64.3%) women employed in the plant. Mean age in the whole group was 29 years (range, 18-55 years). It was divided into three groups: control, mercury absorption and mercury poisoning. A questionnaire-based interview was used to collect data on medical history, occupational exposure and employment. For clinical diagnosis, all subjects underwent physical, neurological and oral examinations. Mercury concentrations in the air were recorded by Hg monitoring instrument and mercury levels in collected urine samples were determined by cold vapor atomic absorption spectrophotometry.

RESULTS

Neurasthenic symptoms were found in 51.75% of the subjects, emotional changes in 27.27%, tremors in 11.19%, and inflammations in 21.68%. The case percentage of neurological symptoms in the control and mercury absorption groups did not show significant difference, but it was significantly higher in the mercury poisoning group.

CONCLUSIONS

The high occupational exposure to mercury, found in the plant and evidenced by urinary Hg concentration > or = 0.05 mg/l, can result in chronic quantitative neurotoxic effects and qualitative health changes. Therefore, constant monitoring of the work environment and checking of workers' health status should be ensured. In addition, appropriate steps should be taken to improve work conditions and promote health among the employees.

Micromolar HgCl2 concentrations transitorily duplicate the ATP level in Saccharomyces cerevisiae cells

Low concentrations of HgCl2 elicited, in Saccharomyces cerevisiae, a transitory increase in the ATP level followed by a decrease of its concentration, until almost disappearance. At 1 microM HgCl2, the increase in ATP lasted for about 30 min, while at 10 microM the increase was only observed in the first 5 min of treatment. The initial burst of ATP was accompanied by a decrease in the level of hexose phosphates, whereas during the decrease of ATP an increase in the inosine and hexose phosphates levels took place. The treatment with HgCl2 inhibited the plasma membrane proton ATPase but not the activities of hexokinase or 6-phosphofructokinase.

Overexpression of Rad23 confers resistance to methylmercury in saccharomyces cerevisiae via inhibition of the degradation of ubiquitinated proteins

We report here that overexpression of Rad23, a protein related to the ubiquitin-proteasome system, renders yeast cells resistant to methylmercury. Rad23 has three domains: two ubiquitin-associated (UBA) domains that bind to the multiubiquitin chain of ubiquitinated proteins and a single ubiquitin-like (UbL) domain that binds to proteasomes. To examine the mechanism of acquisition of methylmercury resistance that is induced by overexpression of Rad23, we expressed variants of Rad23 in which one or the other of the two types of domain was defective in yeast cells. In cells that overexpressed full-length intact Rad23, we detected elevated levels of intracellular ubiquitinated proteins, and the cells were resistant to methylmercury. In contrast, cells that overexpressed Rad23 with a defective UBA domain were not resistant to methylmercury and contained control levels of ubiquitinated proteins. Yeast cells that overexpressed Rad23 with a defective UbL domain exhibited enhanced resistance to methylmercury and contained even higher levels of ubiquitinated proteins than cells that overexpressed intact full-length Rad23. Rad23 is known to have two mutually contradictory functions. It suppresses the degradation of ubiquitinated proteins by proteasomes via a mechanism mediated by the UBA domains, and it enhances the degradation of ubiquitinated proteins via a mechanism that is mediated by the UbL domain. Therefore, our findings suggest that Rad23 might induce resistance to methylmercury in yeast cells by suppressing the degradation of proteins that reduce the toxicity of methylmercury via a UBA domain-mediated mechanism.

Overexpression of Bop3 confers resistance to methylmercury in Saccharomyces cerevisiae through interaction with other proteins such as Fkh1, Rts1, and Msn2

We found that overexpression of Bop3, a protein of unknown function, confers resistance to methylmercury in Saccharomyces cerevisiae. Bmh2, Fkh1, and Rts1 are proteins that have been previously shown to bind Bop3 by the two-hybrid method. Overexpression of Bmh2 and the homologous protein Bmh1 confers resistance to methylmercury in yeast, but overexpression of either Fkh1 or Rts1 has a minimal effect. However, the increased level of resistance to methylmercury produced by overexpression of Bop3 was smaller in Fhk1-deleted yeast as compared with that of the wild-type strain. In contrast, the degree of resistance was significantly elevated in Rts1-deleted yeast. Msn2 and Msn4 were previously reported as proteins that bind to Bmh1 and Bmh2. Overexpression of Msn2 conferred a much greater sensitivity to methylmercury in yeast, while deletion of the corresponding gene lowered the degree of resistance to methylmercury induced by overexpression of Bop3. These results suggest that multiple proteins are involved in minimizing the toxicity of methylmercury induced by overexpression of Bop3.