Understanding the hepatoxicity of inorganic mercury through guts: Perturbance to gut microbiota, alteration of gut-liver axis related metabolites and damage to gut integrity

Toxic and essential metals: metabolic interactions with the gut microbiota and health implications

Human exposure to heavy metals, which encompasses both essential and toxic varieties, is widespread. The intestine functions as a critical organ for absorption and metabolism of heavy metals. Gut microbiota plays a crucial role in heavy metal absorption, metabolism, and related processes. Toxic heavy metals (THMs), such as arsenic (As), mercury (Hg), lead (Pb), and cadmium (Cd), can cause damage to multiple organs even at low levels of exposure, and it is crucial to emphasize their potential high toxicity. Nevertheless, certain essential trace elements, including iron (Fe), copper (Cu), and manganese (Mn), play vital roles in the biochemical and physiological functions of organisms at low concentrations but can exert toxic effects on the gut microbiota at higher levels. Some potentially essential micronutrients, such as chromium (Cr), silicon (Si), and nickel (Ni), which were considered to be intermediate in terms of their essentiality and toxicity, had different effects on the gut microbiota and their metabolites. Bidirectional relationships between heavy metals and gut microbiota have been found. Heavy metal exposure disrupts gut microbiota and influences its metabolism and physiological functions, potentially contributing to metabolic and other disorders. Furthermore, gut microbiota influences the absorption and metabolism of heavy metals by serving as a physical barrier against heavy metal absorption and modulating the pH, oxidative balance, and concentrations of detoxification enzymes or proteins involved in heavy metal metabolism. The interactions between heavy metals and gut microbiota might be positive or negative according to different valence states, concentrations, and forms of the same heavy metal. This paper reviews the metabolic interactions of 10 common heavy metals with the gut microbiota and their health implications. This collated information could provide novel insights into the disruption of the intestinal microbiota caused by heavy metals as a potential contributing factor to human diseases.

Protective effects of oral administration of lactic acid bacteria strains against methylmercury-induced intestinal toxicity in a murine model

The utilization of lactic acid bacteria has been proposed to mitigate the burden of heavy metal exposure through processes probably involving chelation and reduced metal bioaccessibility. We evaluated the effects of daily intake of two strains of lactobacilli (Lactobacillus intestinalis LE1 or Lactobacillus johnsonii LE2) on intestinal toxicity during methylmercury (MeHg) exposure through drinking water (5 mg/L) for two months in mice. MeHg exposure resulted in inflammation and oxidative stress at the colon, as well as an increase in intestinal permeability accompanied by decreased fecal short-chain fatty acids (SCFA). The administration of the strains resulted in a differential protective effect that, based on their chelation capacity, supported the existence of additional mechanisms of action besides chelation. Both strains reduced IL-1β levels and oxidative stress, while LE1 lowered TNF-α, diminished MeHg-induced mucus over-secretion triggered by the IL-4/IL-13/STAT6 pathway, reduced intestinal permeability, and ameliorated inflammation and oxidative stress, probably by acting on the Keap1/Nrf2/ARE pathway. Administration of LE1 partially restored SCFA contents, which could be partly responsible for the positive effects of this strain in alleviating MeHg toxicity. These results demonstrate that lactobacilli strains can be useful tools in reducing the intestinal toxicity of MeHg, the main mercurial form conveyed by food.

Could the gut microbiota be capable of making individuals more or less susceptible to environmental toxicants?

Environmental toxicants are chemical substances capable to impair environmental quality and exert adverse effects on humans and other animals. The main routes of exposure to these pollutants are through the respiratory tract, skin, and oral ingestion. When ingested orally, they will encounter trillions of microorganisms that live in a community - the gut microbiota (GM). While pollutants can disrupt the GM balance, GM plays an essential role in the metabolism and bioavailability of these chemical compounds. Under physiological conditions, strategies used by the GM for metabolism and/or excretion of xenobiotics include reductive and hydrolytic transformations, lyase and functional group transfer reactions, and enzyme-mediated functional transformations. Simultaneously, the host performs metabolic processes based mainly on conjugation, oxidation, and hydrolysis reactions. Thus, due to the broad variety of bacterial enzymes present in GM, the repertoire of microbial transformations of chemicals is considered a key component of the machinery involved in the metabolism of pollutants in humans and other mammals. Among pollutants, metals deserve special attention once contamination by metals is a worldwide problem, and their adverse effects can be observed even at very low concentrations due to their toxic properties. In this review, bidirectional interaction between lead, arsenic, cadmium, and mercury and the host organism and its GM will be discussed given the most recent literature, presenting an analysis of the ability of GM to alter the host organism's susceptibility to the toxic effects of heavy metals, as well as evaluating the extent to which interventions targeting the microbiota could be potential initiatives to mitigate the adverse effects resulting from poisoning by heavy metals. This study is the first to highlight the overlap between some of the bacteria found to be altered by metal exposure and the bacteria that also aid the host organism in the metabolism of these metals. This could be a key factor to determine the beneficial species able to minimize the toxicity of metals in future therapeutic approaches.

Association between gut microbiota and primary ovarian insufficiency: a bidirectional two-sample Mendelian randomization study

Background

Recent studies have indicated a potential correlation between intestinal bacteria and primary ovarian insufficiency (POI). However, the causal relationship between the gut microbiota (GM) and POI remains unclear.

Methods

A bidirectional two-sample Mendelian randomization (MR) study was conducted to investigate the relationship between the GM and POI. Data on the GM were based on the MiBioGen consortium's summary statistics from the most comprehensive genome-wide association study meta-analysis to date (n=13,266), and POI data were obtained from the R8 release of the FinnGen consortium, containing a total of 424 cases and 181,796 controls. A variety of analytical methods, including inverse variance weighting, maximum likelihood, MR-Egger, weighted median, and constrained maximum likelihood and model averaging and Bayesian information criterion, were utilized to explore the connection between the GM and POI. The Cochran's Q statistics were used to evaluate the heterogeneity of instrumental variables. The MR-Egger and MR-pleiotropy residual sum and outlier (PRESSO) methods were used to identify the horizontal pleiotropy of instrumental variables. The MR Steiger test was used to evaluate the strength of causal relationships. A reverse MR study was performed to investigate the causal relationship between POI and the targeted GMs which were indicated to have a causal relationship with POI in the forward MR evaluation.

Results

The inverse variance weighted analysis indicated that Eubacterium (hallii group) (odds ratio [OR]=0.49, 95% confidence interval [CI]: 0.26-0.9, P=0.022) and Eubacterium (ventriosum group) (OR=0.51, 95% CI: 0.27-0.97, P=0.04) had protective effects on POI, and Intestinibacter (OR=1.82, 95% CI: 1.04-3.2, P=0.037) and Terrisporobacter (OR=2.47, 95% CI: 1.14-5.36, P=0.022) had detrimental effects on POI. Results of the reverse MR analysis indicated that POI had no significant influence on the four GMs. No significant heterogeneity or horizontal pleiotropy was observed in the performance of the instrumental variables.

Conclusion

This bidirectional two-sample MR study revealed a causal link between Eubacterium (hallii group), Eubacterium (ventriosum group), Intestinibacter, and Terrisporobacter and POI. Additional clinical trials are needed to gain a clearer understanding of the beneficial or detrimental effects of the GMs on POI and their mechanisms of action.

Challenges and strategies for preventing intestinal damage associated to mercury dietary exposure

Food represents the major risk factor for exposure to mercury in most human populations. Therefore, passage through the gastrointestinal tract plays a fundamental role in its entry into the organism. Despite the intense research carried out on the toxicity of Hg, the effects at the intestinal level have received increased attention only recently. In this review we first provide a critical appraisal of the recent advances on the toxic effects of Hg at the intestinal epithelium. Next, dietary strategies aimed to diminish Hg bioavailability or modulate the epithelial and microbiota responses will be revised. Food components and additives, including probiotics, will be considered. Finally, limitations of current approaches to tackle this problem and future lines of research will be discussed.

Oral exposure to inorganic mercury or methylmercury elicits distinct pro-inflammatory and pro-oxidant intestinal responses in a mouse model system

Humans are mainly exposed to mercury (Hg) through contaminated foodstuffs. However, the effects of Hg on the intestinal tract have received little attention. We performed a subchronic exposure to inorganic mercury or methylmercury in mice through drinking water (1, 5 or 10 mg/L for four months) to evaluate their intestinal impact. Histological, biochemical and gene expression analyses showed that both Hg species induced oxidative stress in small intestine and colon, while inflammation was mainly detected in the colon. Increased fecal albumin content indicated a compromised epithelial barrier. Mucus production was possibly also affected, as an increase in Muc2 expression was detected. However, differential effects were detected between both Hg species. Activation of p38 MAPK and increased crypt depth were detected in colon only with MeHg. Minor differences in microbiota composition were detected between unexposed and exposed mice. Although significant differences were detected between both Hg species at 10 mg/L, only the relative abundances of low abundance taxa were affected. Concentrations of microbial-derived short-chain fatty acids were decreased, suggesting an effect on microbial metabolism or increased demand by the intestinal epithelium. Results obtained confirm previous in vitro studies and highlights the intestinal mucosa as an initial target of Hg.

Changed cecal microbiota involved in growth depression of broiler chickens induced by immune stress

A previous study identified genes and metabolites associated with amino acid metabolism, glycerophospholipid metabolism, and inflammatory response in the liver of broilers with immune stress. The present research was designed to investigate the effect of immune stress on the cecal microbiome in broilers. In addition, the correlation between altered microbiota and liver gene expression, the correlation between altered microbiota and serum metabolites were compared using the Spearman correlation coefficients. Eighty broiler chicks were randomly assigned to 2 groups with 4 replicate pens per group and 10 birds per pen. The model broilers were intraperitoneally injected of 250 µg/kg LPS at 12, 14, 33, and 35 d of age to induce immunological stress. Cecal contents were taken after the experiment and kept at -80°C for 16S rDNA gene sequencing. Then the Pearson's correlation between gut microbiome and liver transcriptome, between gut microbiome and serum metabolites were calculated using R software. The results showed that immune stress significantly changed microbiota composition at different taxonomic levels. KEGG pathways analysis suggested that these gut microbiota were mainly involved in biosynthesis of ansamycins, glycan degradation, D-glutamine and D-glutamate metabolism, valine, leucine, and isoleucine biosynthesis and biosynthesis of vancomycin group antibiotics. Moreover, immune stress increased the activities of metabolism of cofactors and vitamins, as well as decreased the ability of energy metabolism and digestive system. Pearson's correlation analysis identified several bacteria were positively correlated with the gene expression while a few of bacteria were negatively correlated with the gene expression. The results identified potential microbiota involvement in growth depression mediated by immune stress and provided strategies such as supplement of probiotic for alleviating immune stress in broiler chickens.

Nano mercury selenide as a source of mercury for rice

Mercury (Hg) is a persistent and toxic metal while mercury selenide (HgSe) is generally considered as the environmental sink of Hg in its biogeochemical cycle. Recent studies found nano-sized HgSe (nano-HgSe) could be transformed by certain bacteria. This raises safety concerns about the application of selenium (Se) to curb Hg contamination in farmlands. Therefore, hydroponic experiments were performed in which rice plants were cultured with different concentrations of nano-HgSe and micro-sized HgSe (micro-HgSe) to explore their bioavailability and toxicity. It was found that both nano-HgSe and micro-HgSe did not affect the germination of rice seeds but affected the growth of rice seedlings. However, nano-HgSe could be more readily absorbed by roots and transferred to the aboveground parts compared to micro-HgSe. The highest Hg and Se levels were found to be 5255.67 ± 2496.14 μg/g and 1743.75 ± 61.87 μg/g, respectively in roots when exposed to 5000 mg/L nano-HgSe. Besides, small portion (1.2%) of methylmercury (MeHg) to total Hg was found accumulated in rice stem when exposed to 100 mg/L nano-HgSe, suggesting that nano-HgSe could be decomposed. Furthermore, nano-HgSe exposure brought oxidative damage to rice with decreased chlorophyll content and GSH-Px activity. In all, nano-HgSe was found to be more absorbable, transportable and methylated in rice plant compared to micro-HgSe. This suggests that although Se application in Hg contaminated farmland is an effective way to reduce the bioavailability of Hg, the risk of the possible remobilization of HgSe should not be neglected. Besides, the finding that nano-HgSe can act as an environmental source of Hg for plants deepens the understanding of biogeochemical cycle of Hg. More works are required to study the factors affecting the formation of nano-HgSe in the environment and the mechanisms of Hg methylation in rice plants after exposure to nano-HgSe.

Gut as the target tissue of mercury and the extraintestinal effects

Mercury (Hg) is harmful to the environment and human health. The gut plays important roles as the biological, chemical, mechanical, and immune barriers in animals and human beings. It has been known that Hg can be absorbed and methylated/demethylated in the gut, on the other hand, the impacts of Hg to the gut (especially the gut microbiota) is less studied. This review paper summarizes the impacts of inorganic Hg (IHg) and methyl Hg (MeHg) on gut barriers and the extraintestinal effects (damage to other organs such as the liver and brain). Both IHg and MeHg were found to cause intestinal microbial disorders, abnormal metabolites production, tight junction damage, and immune responses in the gut. The damage to the gut also contributed to the extraintestinal effects like the hepatotoxicity by IHg and the neurotoxicity by MeHg. In all, it is proposed that the gut should be considered as an important target tissue of Hg exposure, and the regulation of gut microbiota may have the potential for the prevention and control of the toxicity of Hg.

Radix paeoniae alba polysaccharide attenuates lipopolysaccharide-induced intestinal injury by regulating gut microbiota

Accumulating evidence indicated that oxidative stress is closely related to inflammation and the progression of multiple chronic diseases, which seriously threaten the host health. Currently, multiple plant-derived polysaccharides have been demonstrated to ameliorate the negative effects of oxidative stress on the host, but the potential protective effect of radix paeoniae alba polysaccharide (RPAP) on host have not been well characterized. Here, we investigated whether different doses of RPAP administration could alleviate lipopolysaccharide (LPS)-induced intestinal injury and gut microbial dysbiosis in mice. Results indicated that RPAP administration effectively alleviated LPS-induced intestinal damage in dose dependent. Additionally, amplicon sequencing showed that RPAP administration reversed the significant decrease in gut microbial diversity caused by LPS exposure and restored the alpha-diversity indices to normal levels. Microbial taxonomic investigation also indicated that LPS exposure resulted in significant changes in the gut microbial composition, characterized by a decrease in the abundances of beneficial bacteria (Lactobacillus, Alistipes, Bacillus, Rikenellaceae_RC9_gut_group, etc.) and an increase in the contents of pathogenic bacteria (Klebsiella, Helicobacter, Enterococcus, etc.). However, RPAP administration, especially in high doses, could improve the composition of the gut microbiota by altering the abundance of some bacteria. Taken together, this study demonstrated that RPAP administration could ameliorate LPS-induced intestinal injury by regulating gut microbiota. Meanwhile, this also provides the basis for the popularization and application of RPAP and alleviating oxidative stress from the perspective of gut microbiota.

Gut-on-a-chip for exploring the transport mechanism of Hg(II)

Animal models and static cultures of intestinal epithelial cells are commonly used platforms for exploring mercury ion (Hg(II)) transport. However, they cannot reliably simulate the human intestinal microenvironment and monitor cellular physiology in situ; thus, the mechanism of Hg(II) transport in the human intestine is still unclear. Here, a gut-on-a-chip integrated with transepithelial electrical resistance (TEER) sensors and electrochemical sensors is proposed for dynamically simulating the formation of the physical intestinal barrier and monitoring the transport and absorption of Hg(II) in situ. The cellular microenvironment was recreated by applying fluid shear stress (0.02 dyne/cm²) and cyclic mechanical strain (1%, 0.15 Hz). Hg(II) absorption and physical damage to cells were simultaneously monitored by electrochemical and TEER sensors when intestinal epithelial cells were exposed to different concentrations of Hg(II) mixed in culture medium. Hg(II) absorption increased by 23.59% when tensile strain increased from 1% to 5%, and the corresponding expression of Piezo1 and DMT1 on the cell surface was upregulated.

Microbiome analysis reveals the effects of black soldier fly oil on gut microbiota in pigeon

The gut microbiota plays a vital roles in poultry physiology, immunity and metabolism. Black soldier fly oil is known to have a positive effect on the gut microbiota. However, the specific effect of black soldier fly oil on the composition and structure of the gut microbiota of the pigeon is unknown. In this experiment, 16S rDNA high-throughput sequencing was performed to study the effect of different doses of black soldier fly oil on the changes of pigeon intestinal microbes. Results indicated that the different doses of black soldier fly oil had no effect on the gut microbial diversity of the pigeon. Although the dominant phyla (Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria) and genus (uncultured_bacterium_f_Lachnospiraceae and Desulfovibrio) in control group and experimental group with different doses were the same, the abundances of some beneficial bacteria (Megasphaera, Intestinimonas, Prevotella_9, Lachnospiraceae_UCG-001, Faecalibacterium, Coprococcus_2, Parabacteroides, Megasphaera, Leuconostoc, Prevotellaceae_UCG-001, Lactococcus, Ruminococcaceae_UCG-014, and Coprococcus_2) increased significantly as the concentration of black soldier fly oil increased. Taken together, this study indicated that black soldier fly oil supplementation could improve gut microbial composition and structure by increasing the proportions of beneficial bacteria. Notably, this is the first report on the effects of black soldier fly oil on the gut microbiota of pigeon, which contribute to understanding the positive effects of black soldier fly oil from the gut microbial perspective.

Comprehensive Review Regarding Mercury Poisoning and Its Complex Involvement in Alzheimer's Disease

Mercury (Hg) is considered one of the most widespread toxic environmental pollutants, which seems to have multiple effects on organisms even at low concentrations. It has a critical role in many health problems with harmful consequences, with Hg primarily targeting the brain and its components, such as the central nervous system (CNS). Hg exposure was associated with numerous CNS disorders that frequently trigger Alzheimer's disease (AD). Patients with AD have higher concentrations of Hg in blood and brain tissue. This paper aims to emphasize a correlation between Hg and AD based on the known literature in the occupational field. The outcome shows that all these concerning elements could get attributed to Hg. However, recent studies did not investigate the molecular level of Hg exposure in AD. The present review highlights the interactions between Hg and AD in neuronal degenerations, apoptosis, autophagy, oxidative stress (OS), mitochondrial malfunctions, gastrointestinal (GI) microflora, infertility and altering gene expression.

Potential protective mechanism of Tibetan kefir underlying gut-derived liver injury induced by ochratoxin A

Tibetan kefir against Ochratoxin A-induced liver injury by maintaining the intestinal barrier and modulating the gut microbiota and metabolites.