TCDD modulation of gut microbiome correlated with liver and immune toxicity in streptozotocin (STZ)-induced hyperglycemic mice

Exposure to Environmental Toxins: Potential Implications for Stroke Risk via the Gut- and Lung-Brain Axis

Recent evidence indicates that exposure to environmental toxins, both short-term and long-term, can increase the risk of developing neurological disorders, including neurodegenerative diseases (i.e., Alzheimer's disease and other dementias) and acute brain injury (i.e., stroke). For stroke, the latest systematic analysis revealed that exposure to ambient particulate matter is the second most frequent stroke risk after high blood pressure. However, preclinical and clinical stroke investigations on the deleterious consequences of environmental pollutants are scarce. This review examines recent evidence of how environmental toxins, absorbed along the digestive tract or inhaled through the lungs, affect the host cellular response. We particularly address the consequences of environmental toxins on the immune response and the microbiome at the gut and lung barrier sites. Additionally, this review highlights findings showing the potential contribution of environmental toxins to an increased risk of stroke. A better understanding of the biological mechanisms underlying exposure to environmental toxins has the potential to mitigate stroke risk and other neurological disorders.

Dioxin(-like)-Related Biological Effects through Integrated Chemical-wide and Metabolome-wide Analyses

Dioxin(-like) exposures are linked to adverse health effects, including cancer. However, metabolic alterations induced by these chemicals remain largely unknown. Beyond known dioxin(-like) compounds, we leveraged a chemical-wide approach to assess chlorinated co-exposures and parent compound products [termed dioxin(-like)-related compounds] among 137 occupational workers. Endogenous metabolites were profiled by untargeted metabolomics, namely, reversed-phase chromatography with negative electrospray ionization (C18-negative) and hydrophilic interaction liquid chromatography with positive electrospray ionization (HILIC-positive). We performed a metabolome-wide association study to select dioxin(-like) associated metabolic features using a 20% false discovery rate threshold. Metabolic features were then characterized by pathway enrichment analyses. There are no significant features associated with polychlorinated dibenzo-p-dioxins (PCDDs), a subgroup of known dioxin(-like) compounds. However, 3,110 C18-negative and 2,894 HILIC-positive features were associated with at least one of the PCDD-related compounds. Abundant metabolic changes were also observed for polychlorinated dibenzofuran-related and polychlorinated biphenyl-related compounds. These metabolic features were primarily enriched in pathways of amino acids, lipid and fatty acids, carbohydrates, cofactors, and nucleotides. Our study highlights the potential of chemical-wide analysis for comprehensive exposure assessment beyond targeted chemicals. Coupled with advanced endogenous metabolomics, this approach allows for an in-depth exploration of metabolic alterations induced by environmental chemicals.

Effects of tris (2-chloroethyl) phosphate exposure on gut microbiome using the simulator of the human intestinal microbial ecosystem (SHIME)

Tris (2-chloroethyl) phosphate (TCEP) has been widely used, and its health risk has received increasing attention. However, the rare research has been conducted on the effects of TCEP exposure on changes in the structure of the human gut microbiome and metabolic functions. In this experiment, Simulator of the human intestinal microbial ecosystem (SHIME) was applied to explore the influences of TCEP on the human gut bacteria community and structure. The results obtained from high-throughput sequencing of 16S rRNA gene have clearly revealed differences among control and exposure groups. High-dose TCEP exposure increased the Shannon and Simpson indexes in the results of α-diversity of the gut microbiome. At phylum level, Firmicutes occupied a higher proportion of gut microbiota, while the proportion of Bacteroidetes decreased. In the genus-level analysis, the relative abundance of Bacteroides descended with the TCEP exposure dose increased in the ascending colon, while the abundances of Roseburia, Lachnospira, Coprococcus and Lachnoclostridium were obviously correlated with exposure dose in each colon. The results of short chain fatty acids (SCFAs) showed a remarkable effect on the distribution after TCEP exposure. In the ascending colon, the control group had the highest acetate concentration (1.666 ± 0.085 mg⋅mL-1), while acetate concentrations in lose-dose medium-dose and high-doseTCEP exposure groups were 1.119 ± 0.084 mg⋅mL-1, 0.437 ± 0.053 mg⋅mL-1 and 0.548 ± 0.106 mg⋅mL-1, respectively. TCEP exposure resulted in a decrease in acetate and propionate concentrations, while increasing butyrate concentrations in each colon. Dorea, Fusicatenibacter, Kineothrix, Lachnospira, and Roseburia showed an increasing tendency in abundance under TCEP exposure, while they had a negatively correlation with acetate and propionate concentrations and positively related with butyrate concentrations. Overall, this study confirms that TCEP exposure alters both the composition and metabolic function of intestinal microbial communities, to arouse public concern about its negative health effects.

Beyond the amyloid hypothesis: how current research implicates autoimmunity in Alzheimer's disease pathogenesis

The amyloid hypothesis has so far been at the forefront of explaining the pathogenesis of Alzheimer's Disease (AD), a progressive neurodegenerative disorder that leads to cognitive decline and eventual death. Recent evidence, however, points to additional factors that contribute to the pathogenesis of this disease. These include the neurovascular hypothesis, the mitochondrial cascade hypothesis, the inflammatory hypothesis, the prion hypothesis, the mutational accumulation hypothesis, and the autoimmunity hypothesis. The purpose of this review was to briefly discuss the factors that are associated with autoimmunity in humans, including sex, the gut and lung microbiomes, age, genetics, and environmental factors. Subsequently, it was to examine the rise of autoimmune phenomena in AD, which can be instigated by a blood-brain barrier breakdown, pathogen infections, and dysfunction of the glymphatic system. Lastly, it was to discuss the various ways by which immune system dysregulation leads to AD, immunomodulating therapies, and future directions in the field of autoimmunity and neurodegeneration. A comprehensive account of the recent research done in the field was extracted from PubMed on 31 January 2022, with the keywords "Alzheimer's disease" and "autoantibodies" for the first search input, and "Alzheimer's disease" with "IgG" for the second. From the first search, 19 papers were selected, because they contained recent research on the autoantibodies found in the biofluids of patients with AD. From the second search, four papers were selected. The analysis of the literature has led to support the autoimmune hypothesis in AD. Autoantibodies were found in biofluids (serum/plasma, cerebrospinal fluid) of patients with AD with multiple methods, including ELISA, Mass Spectrometry, and microarray analysis. Through continuous research, the understanding of the synergistic effects of the various components that lead to AD will pave the way for better therapeutic methods and a deeper understanding of the disease.

Maternal polychlorinated biphenyl 126 (PCB 126) exposure modulates offspring gut microbiota irrespective of diet and exercise

The gut microbiota plays an important role throughout the lifespan in maintaining host health, and several factors can modulate microbiota composition including diet, exercise, and environmental exposures. Maternal microbiota is transferred to offspring during early life; thus, environmental exposures before gestation may also modulate offspring microbiota. Here we aimed to investigate the effects of maternal exposure to dioxin-like polychlorinated biphenyls (PCBs) on the microbiota of aged offspring and to determine if lifestyle factors, including maternal exercise or offspring high-fat feeding alter these associations. To test this, dams were exposed to PCB 126 (0.5 μmole/kg body weight) or vehicle oil by oral gavage during preconception, gestation, and during lactation. Half of each group was allowed access to running wheels for ≥ 7 days before and during pregnancy and up through day 14 of lactation. Female offspring born from the 4 maternal groups (PCB exposure or not, with/without exercise) were subsequently placed either on regular diet or switched to a high-fat diet during adulthood. Microbiota composition was quantified in female offspring at 49 weeks of age by 16 S rRNA sequencing. Maternal exposure to PCB 126 resulted in significantly reduced richness and diversity in offspring microbiota regardless of diet or exercise. Overall compositional differences were largely driven by offspring diet, but alterations in specific taxa due to maternal PCB 126 exposure, included the depletion of Verrucomicrobiaceae and Akkermansia muciniphila, and an increase in Anaeroplasma. Perturbation of microbiota due to PCB 126 may predispose offspring to a variety of chronic diseases later in adulthood.

Nicotinamide Riboside Improves Enteric Neuropathy in Streptozocin-Induced Diabetic Rats Through Myenteric Plexus Neuroprotection

BACKGROUND

Diabetes Mellitus causes a systemic oxidative stress due in part to the hyperglycemia and the reactive oxygen species generated. Up to 75% of diabetic patients present with an autonomic neuropathy affecting the Enteric Nervous System. Deficits in the human population are chronic dysmotilities with either increased (i.e., constipation) or decreased (i.e., diarrhea) total gastrointestinal transit times. These are recapitulated in the streptozocin-induced diabetic rat, which is a model of Type I Diabetes Mellitus.

AIMS

Examine the effects that a precursor of nicotinamide adenosine dinucleotide (NAD), nicotinamide riboside (NR), had on the development of dysmotility in induced diabetic rats and if fecal microbiota transplant (FMT) could produce the same results.

MATERIALS AND METHODS

Utilizing a 6-week treatment paradigm, NR was administered intraperitoneally every 48 h. Total gastrointestinal transit time was assessed weekly utilizing the carmine red method. Three weeks following hyperglycemic induction, FMT was performed between NR-treated animals and untreated animals.

SIGNIFICANT RESULTS

There is improvement in overall gastrointestinal transit time with the use of NR. 16S microbiome sequencing demonstrated decreased alpha and beta diversity in induced diabetic rats without change in animals receiving FMT. Improvements in myenteric plexus ganglia density in small and large intestines in diabetic animals treated with NR were seen.

CONCLUSIONS

NR treatment led to functional improvement in total gastrointestinal transit time in induced diabetic animals. This was associated with neuroprotection in the myenteric plexuses of both small and large intestines of induced diabetic rats. This represents an important first step in showing NR's benefit as a treatment for diabetic enteric neuropathy. Streptozocin-induced diabetic rats have improved transit times and increased myenteric plexus ganglia density when treated with intraperitoneal nicotinamide riboside.

Associated long-term effects of decabromodiphenyl ethane on the gut microbial profiles and metabolic homeostasis in Sprague-Dawley rat offspring

Decabromodiphenyl ethane (DBDPE) as a widely used brominated flame retardant is harmful to human health due to its toxicity, including cardiovascular toxicity, reproductive toxicity, and hepatotoxicity. However, the knowledge of the long-term effects and structural and metabolic function influence on gut microbiota from DBDPE exposure remains limited. This study was mainly aimed at the gut microbiome and fecal metabolome of female rats and their offspring exposed to DBDPE in early life. 16S rRNA gene sequencing demonstrated that maternal DBDPE exposure could increase the α-diversity of gut microbiota in immature offspring while decreasing the abundance of Bifidobacterium, Clostridium, Muribaculum, Escherichia, and Lactobacillus in adult offspring. The nonmetric multidimensional scaling showed a consistency in the alternation of β-diversity between pregnant rats and their adult offspring. Furthermore, the short-chain fatty acids produced by gut microbiota dramatically increased in adult offspring after maternal DBDPE exposure, revealing that DBDPE treatment disrupted the gut microbial compositions and altered the gut community's metabolic functions. Untargeted metabolomics identified 41 differential metabolites and seven metabolic pathways between adult offspring from various groups. Targeted metabolomic showed that maternal high dose DBDPE exposure obviously decreased the level of glutathione, taurine, and l-carnitine in their adult offspring, which verified the correlation between weight loss and amino acid metabolites. An interesting link between some gut bacteria (especially the Firmicutes) and fecal metabolites demonstrated the shifts in gut microbiota may drive the metabolic process of fecal metabolites. The current findings provide new insight into long-term effects on human health.

The disruption on gut microbiome of Decabromodiphenyl ethane exposure in the simulator of the human intestinal microbial ecosystem (SHIME)

The health risks of Decabromodiphenyl ethane (DBDPE) with its cardiovascular toxicity, liver toxicity and cytotoxicity had been generally acknowledged. However, the influence on gut microbiome and short-chain fatty acids (SCFAs) metabolism caused by DBDPE exposure remained unknown. In this study, three exposure groups (5, 50, 500 mg/L) and control group were used to investigate the effect of DBDPE by using simulator of the human intestinal microbial ecosystem (SHIME). 16S rRNA gene high-throughput sequencing illustrated that high dose DBDPE exposure increased the α-diversity of gut microbiota, while reduced the abundance of Firmicutes and Proteobacteria. In addition, the low dose (5 mg/L) DBDPE inhibited the increasing of SCFAs, but the medium and high dose (50 and 500 mg/L) DBDPE promoted the advancement, especially in ascending colon. Notably, DBDPE exposure lead a similar changing of acetic acid and butyric acid contents in different sections of the colon. This study confirmed the alternation of composition and metabolic function in gut microbial community due to DBDPE exposure, indicating an intestinal damage and appealing for more attention concentrated on the health effects of DBDPE exposure.

Persistent organic pollutants in foods, their interplay with gut microbiota and resultant toxicity

Persistent Organic Pollutants (POPs) have become immensely prevalent in the environment as a result of their unique chemical properties (persistent, semi-volatile and bioaccumulative nature). Their occurrence in the soil, water and subsequently in food has become a matter of concern. With food being one of the major sources of exposure, the detrimental impact of these chemicals on the gut microbiome is inevitable. The gut microbiome is considered as an important integrant for human health. It participates in various physiological, biochemical and immunological activities; thus, affects the metabolism and physiology of the host. A myriad of studies have corroborated an association between POP-induced gut microbial dysbiosis and prevalence of disorders. For instance, ingestion of polychlorinated biphenyls, polybrominated diphenyl ethers or organochlorine pesticides influenced bile acid metabolism via alteration of bile salt hydrolase activity of Lactobacillus, Clostridium or Bacteroides genus. At the same time, some chemicals such as DDE have the potential to elevate Proteobacteria and Firmicutes/Bacteriodetes ratio influencing their metabolic activity leading to enhanced short-chain fatty acid synthesis, ensuing obesity or a pre-diabetic state. This review highlights the impact of POPs exposure on the gut microbiota composition and metabolic activity, along with an account of its corresponding consequences on the host physiology. The critical role of gut microbiota in impeding the POPs excretion out of the body resulting in their prolonged exposure and consequently, enhanced degree of toxicity is also emphasized.

Environmental Toxicants and NAFLD: A Neglected yet Significant Relationship

The liver is an organ of vital importance in the body; it is the center of metabolic activities and acts as the primary line of defense against toxic compounds. Exposure to environmental toxicants is an unavoidable fallout from rapid industrialization across the world and is even higher in developing countries. Technological development and industrialization have led to the release of toxicants such as pollutant toxic gases, chemical discharge, industrial effluents, pesticides and solvents, into the environment. In the last few years, a growing body of evidence has shed light on the potential impact of environmental toxicants on liver health, in particular, on non-alcoholic fatty liver disease (NAFLD) incidence and progression. NAFLD is a multifactorial disease linked to metabolic derangement including diabetes and other complications. Environmental toxicants including xenobiotics and pollutants may have a direct or indirect steatogenic/fibrogenic impact on the liver and should be considered as risk factors associated with NAFLD. This review discusses the contribution of environmental toxicants toward the increasing disease burden of NAFLD.

Subacute effects of the chlorinated flame retardant dechlorane 602 on intestinal microenvironment in mice

BACKGROUND

Chlorinated flame retardant Dechlorane 602 (Dec 602) has been detected in daily food, indicating that it may pose a risk to intestinal health. The intestinal microenvironment plays an important role in intestinal health. Intestinal microbiota and metabolites are two important factors for maintaining the microenvironment. However, little is known about the effects of Dec 602 on intestinal microbiota and metabolites.

OBJECTIVES

We aimed to probe the effects of Dec 602 on the intestine by revealing the changes that Dec 602 caused to the intestinal microbiota and metabolites.

METHODS

Adult female C57BL/6 mice were exposed to Dec 602 (low/high doses: 1.0/10.0 μg/kg body weight per day) orally for 7 consecutive days, and sacrificed after 7 days of recovery. The composition of colonic microbiota was measured by 16S rRNA gene sequencing, and the colonic metabolites were determined by LC-ESI-MS/MS. Finally, the effects of Dec 602 on the colon were validated by histopathological analysis.

RESULTS

The intestinal microbiota composition was altered toward a pro-inflammatory status after exposure to Dec 602. Dec 602 exposure also up-regulated oxidative metabolites (glutathione disulfide, taurine and retinoic acid) and pro-inflammatory metabolites (prostaglandin E2). On the other hand, antioxidative metabolites (s-adenosylmethionine and 11-cis-retinol) and anti-inflammatory metabolites (alpha-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid) were down-regulated after exposure to Dec 602. Infiltration of lymphocytes in the colonic lamina propria was observed in the mice treated with Dec 602 for 7 days, and it was not recovered after another 7 days without further treatment.

CONCLUSION

Dec 602 interfered with the colonic microbiota and metabolome, and exhibited inflammatory features. Histopathological studies confirmed that Dec 602 exposure did induce colonic inflammation.

Effects of perinatal TCDD exposure on colonic microbiota and metabolism in offspring and mother mice

Emerging evidence supports that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) impacts the gut microbiota and metabolic pathways. TCDD can be transmitted from mother to child; thus, we hypothesize that maternal exposure to TCDD may affect the gut microbiota in mothers and offspring. To acquire in vivo evidence supporting this hypothesis, female C57BL/6 mice were administered with TCDD (0.1 and 10 μg/kg body weight (bw)) during pregnancy and lactation periods, and then changes of colonic microbiota in offspring and mothers were evaluated. High-throughput sequencing of the V4 regions of the 16S rRNA gene was performed. The composition and structure of the colonic microbiota in offspring and mothers were significantly influenced by 10 μg/kg bw TCDD as demonstrated by upregulation of harmful bacteria and downregulation of beneficial bacteria. Paradoxically, pathogenic bacteria and opportunistic pathogens were conversely decreased in the offspring of the low-dose TCDD treatment group. Tryptophan (Trp) metabolism exhibited a noticeable change caused by the alteration of colonic microbiota in offspring after maternal exposure to 10 μg/kg bw TCDD, which showed a linear dependence, demonstrating that pathogens or opportunistic pathogens may accelerate the dysbiosis of Trp metabolism. Trp metabolism dysregulation caused by the changed colonic microbiota may subsequently impact other intestinal segments or even living organisms. Our study provides new evidence indicating a potential influence of early TCDD exposure on the colonic microbiota and metabolism.

16S rRNA metabarcoding unearths responses of rare gut microbiome of fathead minnows exposed to benzo[a]pyrene

Activities of gut microbiomes are often overlooked in assessments of ecotoxicological effects of environmental contaminants. Effects of the polycyclic aromatic hydrocarbon, benzo[a]pyrene (BaP) on active gut microbiomes of juvenile fathead minnows (Pimephales promelas) were investigated. Fish were exposed for two weeks, to concentrations of 0, 1, 10, 100, or 1000 μg BaP g^-1 in the diet. The active gut microbiome was characterized using 16S rRNA metabarcoding to determine its response to dietary exposure of BaP. BaP reduced alpha-diversity at the greatest exposure concentrations. Additionally, exposure to BaP altered community composition of active microbiome and resulted in differential proportion of taxa associated with hydrocarbon degradation and fish health. Neighborhood selection networks of active microbiomes were not reduced with greater concentrations of BaP, which suggests ecological resistance and/or resilience of gut microbiota. The active gut microbiome had a similar overall biodiversity as that of the genomic gut microbiota, but had a distinct composition from that of the 16S rDNA profile. Responses of alpha- and beta-diversities of the active microbiome to BaP exposure were consistent with that of genomic microbiomes. Normalized activity of microbiome via the ratio of rRNA to rDNA abundance revealed rare taxa that became active or dormant due to exposure to BaP. These differences highlight the need to assess both 16S rDNA and rRNA metabarcoding to fully derive bacterial compositional changes resulting from exposure to contaminants.

Studies of xenobiotic-induced gut microbiota dysbiosis: from correlation to mechanisms

Environmental chemicals can alter gut microbial community composition, known as dysbiosis. However, the gut microbiota is a highly dynamic system and its functions are still largely underexplored. Likewise, it is unclear whether xenobiotic exposure affects host health through impairing host-microbiota interactions. Answers to this question not only can lead to a more precise understanding of the toxic effects of xenobiotics but also can provide new targets for the development of new therapeutic strategies. Here, we aim to identify the major challenges in the field of microbiota-exposure research and highlight the need to exam the health effects of xenobiotic-induced gut microbiota dysbiosis in host bodies. Although the changes of gut microbiota frequently co-occur with the xenobiotic exposure, the causal relationship of xenobiotic-induced microbiota dysbiosis and diseases is rarely established. The high dynamics of the gut microbiota and the complex interactions among exposure, microbiota, and host, are the major challenges to decipher the specific health effects of microbiota dysbiosis. The next stage of study needs to combine various technologies to precisely assess the xenobiotic-induced gut microbiota perturbation and the subsequent health effects in host bodies. The exposure, gut microbiota dysbiosis, and disease outcomes have to be causally linked. Many microbiota-host interactions are established by previous studies, including signaling metabolites and response pathways in the host, which may use as start points for future research to examine the mechanistic interactions of exposure, gut microbiota, and host health. In conclusion, to precisely understand the toxicity of xenobiotics and develop microbiota-based therapies, the causal and mechanistic links of exposure and microbiota dysbiosis have to be established in the next stage study.

Impacts of benzo(a)pyrene exposure on scallop (Chlamys farreri) gut health and gut microbiota composition

The gut tissue interacts with nutrients and pollutants which can impact gut health. Gut microbiota is essential to the host health, but is also easily affected by external environment. However, little is known about the toxicological assessment of environmental contaminants on gut health and microbiota, especially in marine invertebrates. In this study, we first explored the effect of benzo(a)pyrene (BaP) on the gut health and gut microbiota of scallops (Chlamys farreri). The scallops were exposed to different concentrations (0, 0.4, 2 and 10 μg/L) of BaP for 21 days. The histological morphology, immune- and oxidative enzyme-related gene expression, and lipid peroxidation of the scallops were analyzed at 7, 14 and 21 days. The results revealed that BaP could impair intestinal barrier function, increasing the intestinal permeability of scallops. Moreover, immune and antioxidant responses were induced in the gut tissue. After a 21-day exposure to different concentrations of BaP, the intestinal microbial community was analyzed based on 16S rRNA sequencing. Our results suggested that BaP exposure altered the gut microbial diversity and composition in scallops. Many beneficial genera declined after BaP treatment, while the potential pathogens were increased, such as Mycoplasma and Tenacibaculum. A series of hydrocarbon-degrading bacteria were recognized in BaP-treated groups, such as Pseudomonas, Polaribacter, Amphritea and Kordiimonas. Interestingly, the degrading bacteria present varied after exposure to different concentrations of BaP. Overall, this study provides new insights into gut health and gut microbiota in marine invertebrates following exposure to persistent organic pollutants.

Aryl Hydrocarbon Receptor (AhR) Activation by 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) Dose-Dependently Shifts the Gut Microbiome Consistent with the Progression of Non-Alcoholic Fatty Liver Disease

Gut dysbiosis with disrupted enterohepatic bile acid metabolism is commonly associated with non-alcoholic fatty liver disease (NAFLD) and recapitulated in a NAFLD-phenotype elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice. TCDD induces hepatic fat accumulation and increases levels of secondary bile acids, including taurolithocholic acid and deoxycholic acid (microbial modified bile acids involved in host bile acid regulation signaling pathways). To investigate the effects of TCDD on the gut microbiota, the cecum contents of male C57BL/6 mice orally gavaged with sesame oil vehicle or 0.3, 3, or 30 µg/kg TCDD were examined using shotgun metagenomic sequencing. Taxonomic analysis identified dose-dependent increases in Lactobacillus species (i.e., Lactobacillus reuteri). Increased species were also associated with dose-dependent increases in bile salt hydrolase sequences, responsible for deconjugation reactions in secondary bile acid metabolism. Increased L. reuteri levels were further associated with mevalonate-dependent isopentenyl diphosphate (IPP) biosynthesis and o-succinylbenzoate synthase, a menaquinone biosynthesis associated gene. Analysis of the gut microbiomes from cirrhosis patients identified an increased abundance of genes from the mevalonate-dependent IPP biosynthesis as well as several other menaquinone biosynthesis genes, including o-succinylbenzoate synthase. These results extend the association of lactobacilli with the AhR/intestinal axis in NAFLD progression and highlight the similarities between TCDD-elicited phenotypes in mice to human NAFLD.

Effects of Acute 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Exposure on the Circulating and Cecal Metabolome Profile

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a polyhalogenated planar hydrocarbon belonging to a group of highly toxic and persistent environmental contaminants known as "dioxins". TCDD is an animal teratogen and carcinogen that is well characterized for causing immunosuppression through activation of aryl hydrocarbon receptor (AHR). In this study, we investigated the effect of exposure of mice to an acute dose of TCDD on the metabolic profile within the serum and cecal contents to better define the effects of TCDD on host physiology. Our findings demonstrated that within the circulating metabolome following acute TCDD exposure, there was significant dysregulation in the metabolism of bioactive lipids, amino acids, and carbohydrates when compared with the vehicle (VEH)-treated mice. These widespread changes in metabolite abundance were identified to regulate host immunity via modulating nuclear factor-kappa B (NF-κB) and extracellular signal-regulated protein kinase (ERK1/2) activity and work as biomarkers for a variety of organ injuries and dysfunctions that follow TCDD exposure. Within the cecal content of mice exposed to TCDD, we were able to detect changes in inflammatory markers that regulate NF-κB, markers of injury-related inflammation, and changes in lysine degradation, nicotinamide metabolism, and butanoate metabolism, which collectively suggested an immediate suppression of broad-scale metabolic processes in the gastrointestinal tract. Collectively, these results demonstrate that acute TCDD exposure results in immediate irregularities in the circulating and intestinal metabolome, which likely contribute to TCDD toxicity and can be used as biomarkers for the early detection of individual exposure.

Developmental toxicity of bisphenol S in Caenorhabditis elegans and NODEF mice

The growing concern surrounding bisphenol A (BPA) has led to increased industrial production and application of its analog bisphenol S (BPS). The goals of this study were: (1) To examine the generational effects in the nematode C. elegans for up to three generations following developmental exposure to BPS (0.1, 1.0, 5.0 and 10.0 μM), and (2) To examine the neurotoxicity and metabolic toxicity in NODEF mouse offspring exposed to BPS (3 μg/kg BW) in utero throughout gestation once/day via oral pipette. First, worms were exposed to BPS developmentally for a single period of 48 hours and then propagated for 2 additional generations. Exposure to 0.1 and 1.0 μM BPS decreased lifespan and the number of progeny with an ability to recover in subsequent generations. In contrast, worms exposed to 5.0 or 10.0 μM BPS exhibited a continuous effect in the second generation, e.g., decreased lifespan and reduced number of progeny. Only worms exposed to 10.0 μM BPS continued to have a significant long-term effect (e.g., decreased lifespan) through the third generation. In addition, worms developmentally exposed to BPS at 5.0 μM and 10.0 μM also showed decreases in body bends. In contrast, worms exposed to 0.1 μM BPS exhibited a significant increase in head thrashes. When the multigenerational effects were examined by exposing worms to BPS for 48 hours developmentally at each generation for three generations, an accumulative effect was observed in worms treated with 0.1 or 1.0 μM BPS for two generations, but not for three generations, suggesting a threshold existed. Worms exposed to either 5.0 or 10.0 μM BPS demonstrated accumulative effects through two and three generations. When the developmental effects of BPS were studied in NODEF mice, offspring exposed gestationally exhibited behavioral deficits at 12, but not at 3, weeks of age. Specifically, female offspring had decreases in working and short-term memories while male offspring showed increases in hyperactivity and anxiety-like behaviors. In summary, this study demonstrates the sex-related effects of BPS in NODEF mouse offspring exposed in utero, along with the generational effects observed in C. elegans.

Cutaneous Effects of In Utero and Lactational Exposure of C57BL/6J Mice to 2,3,7,8-Tetrachlorodibenzo-p-dioxin

To determine the cutaneous effects of in utero and lactational exposure to the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), pregnant C57BL/6J mice were exposed by gavage to a vehicle or 5 μg TCDD/kg body weight at embryonic day 12 and epidermal barrier formation and function were studied in their offspring from postnatal day 1 (P1) through adulthood. TCDD-exposed pups were born with acanthosis. This effect was AHR-dependent and subsided by P6 with no evidence of subsequent inflammatory dermatitis. The challenge of adult mice with MC903 showed similar inflammatory responses in control and treated animals, indicating no long-term immunosuppression to this chemical. Chloracne-like sebaceous gland hypoplasia and cyst formation were observed in TCDD-exposed P21 mice, with concomitant microbiome dysbiosis. These effects were reversed by P35. CYP1A1 and CYP1B1 expression in the skin was increased in the exposed mice until P21, then declined. Both CYP proteins co-localized with LRIG1-expressing progenitor cells at the infundibulum. CYP1B1 protein also co-localized with a second stem cell niche in the isthmus. These results indicate that this exposure to TCDD causes a chloracne-like effect without inflammation. Transient activation of the AhR, due to the shorter half-life of TCDD in mice, likely contributes to the reversibility of these effects.

The Impact of Environmental Chemicals on the Gut Microbiome

Since the surge of microbiome research in the last decade, many studies have provided insight into the causes and consequences of changes in the gut microbiota. Among the multiple factors involved in regulating the microbiome, exogenous factors such as diet and environmental chemicals have been shown to alter the gut microbiome significantly. Although diet substantially contributes to changes in the gut microbiome, environmental chemicals are major contaminants in our food and are often overlooked. Herein, we summarize the current knowledge on major classes of environmental chemicals (bisphenols, phthalates, persistent organic pollutants, heavy metals, and pesticides) and their impact on the gut microbiome, which includes alterations in microbial composition, gene expression, function, and health effects in the host. We then discuss health-related implications of gut microbial changes, which include changes in metabolism, immunity, and neurological function.

Responses of juvenile fathead minnow (Pimephales promelas) gut microbiome to a chronic dietary exposure of benzo[a]pyrene

The microbiome has been described as an additional host "organ" with well-established beneficial roles. However, the effects of exposures to chemicals on both structure and function of the gut microbiome of fishes are understudied. To determine effects of benzo[a]pyrene (BaP), a model persistent organic pollutant, on structural shifts of gut microbiome in juvenile fathead minnows (Pimephales promelas), fish were exposed ad libitum in the diet to concentrations of 1, 10, 100, or 1000 μg BaP g^-1 food, in addition to a vehicle control, for two weeks. To determine the link between exposure to BaP and changes in the microbial community, concentrations of metabolites of BaP were measured in fish bile and 16S rRNA amplicon sequencing was used to evaluate the microbiome. Exposure to BaP only reduced alpha-diversity at the greatest exposure concentrations. However, it did alter community composition assessed as differential abundance of taxa and reduced network complexity of the microbial community in all exposure groups. Results presented here illustrate that environmentally-relevant concentrations of BaP can alter the diversity of the gut microbiome and community network connectivity.

Preliminary evidence for an influence of exposure to polycyclic aromatic hydrocarbons on the composition of the gut microbiota and neurodevelopment in three-year-old healthy children

Background

During the second and third year after birth the gut microbiota (GM) is subjected to important development. The polycyclic aromatic hydrocarbon (PAH) exposure could influence the GM in animal and early postnatal exposure is associated with neurodevelopment disorder in children. This study was designed to explore the possible influence of the polycyclic aromatic hydrocarbons (PAHs) on the composition of the gut microbiota (GM) and neurodevelopment in a sample of 38 healthy children at the age of 3 years.

Methods

A brief development (Gesell Development Inventory, GDI) and behavior test (Child Behavior Checklist, CBCL) were completed on 3-yr-olds and stool samples were collected for 16S rRNA V4-V5 sequencing. The PAH-DNA adduct in the umbilical cord blood and the urinary hydroxyl PAHs (OH-PAHs) at the age of 12 months were measured as pre- and postnatal PAH exposure, respectively.

Results

The most abundant two phyla were Bacteroidetes (68.6%) and Firmicutes (24.2%). The phyla Firmicutes, Actinobacteria, Proteobacteria, Tenericutes, and Lentisphaerae were positively correlated with most domain behaviors of the GDI, whereas the Bacteroidetes, Cyanobacteria, and Fusobacteria were negatively correlated. Correspondingly, the phyla Bacteroidetes, Actinobacteria, and Fusobacteria showed positive correlations with most CBCL core and broadband syndromes, whereas the Firmicutes, Verrucomicrobia, Synergistetes, Proteobacteria and Tenericules were negatively correlated. The OH-PAH levels were not significantly associated with the Firmicutes phylum whereas the Bacteroidetes, Bacteroidia, and Bacteroidales all showed significant negative association with the OH-PAH levels.

Conclusion

The current findings suggest that composition of the GM is associated with neurodevelopment of the child. PAHs seem to change the relative abundance of some taxa (some deleted and some recruited) to counteract the negative effects of the PAHs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-021-02539-w.

Tryptamine Attenuates Experimental Multiple Sclerosis Through Activation of Aryl Hydrocarbon Receptor

Tryptamine is a naturally occurring monoamine alkaloid which has been shown to act as an aryl hydrocarbon receptor (AHR) agonist. It is produced in large quantities from the catabolism of the essential amino acid tryptophan by commensal microorganisms within the gastrointestinal (GI) tract of homeothermic organisms. Previous studies have established microbiota derived AHR ligands as potent regulators of neuroinflammation, further defining the role the gut-brain axis plays in the complex etiology in multiple sclerosis (MS) progression. In the current study, we tested the ability of tryptamine to ameliorate symptoms of experimental autoimmune encephalomyelitis (EAE), a murine model of MS. We found that tryptamine administration attenuated clinical signs of paralysis in EAE mice, decreased the number of infiltrating CD4⁺ T cells in the CNS, Th17 cells, and RORγ T cells while increasing FoxP3+Tregs. To test if tryptamine acts through AHR, myelin oligodendrocyte glycoprotein (MOG)-sensitized T cells from wild-type or Lck-Cre AHR^flox/flox mice that lacked AHR expression in T cells, and cultured with tryptamine, were transferred into wild-type mice to induce passive EAE. It was noted that in these experiments, while cells from wild-type mice treated with tryptamine caused marked decrease in paralysis and attenuated neuroinflammation in passive EAE, similar cells from Lck-Cre AHR^flox/flox mice treated with tryptamine, induced significant paralysis symptoms and heightened neuroinflammation. Tryptamine treatment also caused alterations in the gut microbiota and promoted butyrate production. Together, the current study demonstrates for the first time that tryptamine administration attenuates EAE by activating AHR and suppressing neuroinflammation.

AhR Activation Leads to Alterations in the Gut Microbiome with Consequent Effect on Induction of Myeloid Derived Suppressor Cells in a CXCR2-Dependent Manner

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent ligand for AhR and a known carcinogen. While AhR activation by TCDD leads to significant immunosuppression, how this translates into carcinogenic signal is unclear. Recently, we demonstrated that activation of AhR by TCDD in naïve C57BL6 mice leads to massive induction of myeloid derived-suppressor cells (MDSCs). In the current study, we investigated the role of the gut microbiota in TCDD-mediated MDSC induction. TCDD caused significant alterations in the gut microbiome, such as increases in Prevotella and Lactobacillus, while decreasing Sutterella and Bacteroides. Fecal transplants from TCDD-treated donor mice into antibiotic-treated mice induced MDSCs and increased regulatory T-cells (Tregs). Injecting TCDD directly into antibiotic-treated mice also induced MDSCs, although to a lesser extent. These data suggested that TCDD-induced dysbiosis plays a critical role in MDSC induction. Interestingly, treatment with TCDD led to induction of MDSCs in the colon and undetectable levels of cysteine. MDSCs suppressed T cell proliferation while reconstitution with cysteine restored this response. Lastly, blocking CXC chemokine receptor 2 (CXCR2) impeded TCDD-mediated MDSC induction. Our data demonstrate that AhR activation by TCDD triggers dysbiosis which, in turn, regulates, at least in part, induction of MDSCs.

Effects of the husky oil spill on gut microbiota of native fishes in the North Saskatchewan River, Canada

In July 2016, a Husky Energy pipeline spilled 225,000 L of diluted heavy crude oil, with a portion of the oil entering the North Saskatchewan River near Maidstone, SK, Canada. This event provided a unique opportunity to assess potential effects of a crude oil constituent (namely polycyclic aromatic hydrocarbons, PAHs) on a possible sensitive indicator of freshwater ecosystem health, the gut microbiota of native fishes. In summer 2017, goldeye (Hiodon alosoides), walleye (Sander vitreus), northern pike (Esox lucius), and shorthead redhorse (Moxostoma macrolepidotum) were collected at six locations upstream and downstream of the spill. Muscle and bile were collected from individual fish for quantification of PAHs and intestinal contents were collected for characterization of the microbial community of the gut. Results suggested that host species is a significant determinant of gut microbiota, with significant differences among the species across sites. Concentrations of PAHs in dorsal muscle were significantly correlated with gut community compositions of walleye, but not of the other fishes. Concentrations of PAHs in muscle were also correlated with abundances of several families of bacteria among fishes. This study represents one of the first to investigate the response of the gut microbiome of wild fishes to chemical stressors.

Crohn's disease and environmental contamination: Current challenges and perspectives in exposure evaluation

Although the incidence of Crohn's disease has increased worldwide over the past 30 years, the disorder's exact causes and physiological mechanisms have yet to be determined. Given that genetic determinants alone do not explain the development of Crohn's disease, there is growing interest in "environmental" determinants. In medical science, the term "environment" refers to both the ecological and social surroundings; however, most published studies have focused on the latter. In environmental and exposure sciences, the term "environment" mostly relates to contamination of the biotope. There are many unanswered questions on how environmental hazards might contribute to the pathogenesis of Crohn's disease. Which pollutants should be considered? Which mechanisms are involved? And how should environmental contamination and exposure be evaluated? The objective was to perform a systematic review of the literature on Crohn's disease and environmental contamination. We searched the PubMed, Google Scholar, Scopus, ISI Web of Science and Prospero databases. We considered all field studies previous to April 2019 conducted on human health indicators, and evaluating exposure to all type of physical, biological and chemical contamination of the environment. The lack of clear answers to date can be ascribed to the small total number of field studies (n = 16 of 39 publications, most of which were conducted by pioneering medical scientists), methodological differences, and the small number of contaminants evaluated. This make it impossible to conduct a coherent and efficient meta-analysis. Based on individual analysis of available studies, we formulated five recommendations on improving future research: (i) follow up the currently identified leads - especially metals and endocrine disruptors; (ii) explore soil contamination; (iii) gain a better knowledge of exposure mechanisms by developing transdisciplinary studies; (iv) identify the most plausible contaminants by developing approaches based on the source-to-target distance; and (v) develop registries and cohort-based analyses.

The Impact of Environmental Chemicals on the Gut Microbiome

Since the surge of microbiome research in the last decade, many studies have provided insight into the causes and consequences of changes in the gut microbiota. Among the multiple factors involved in regulating the microbiome, exogenous factors such as diet and environmental chemicals have been shown to alter the gut microbiome significantly. Although diet substantially contributes to changes in the gut microbiome, environmental chemicals are major contaminants in our food and are often overlooked. Herein, we summarize the current knowledge on major classes of environmental chemicals (bisphenols, phthalates, persistent organic pollutants, heavy metals, and pesticides) and their impact on the gut microbiome, which includes alterations in microbial composition, gene expression, function, and health effects in the host. We then discuss health-related implications of gut microbial changes, which include changes in metabolism, immunity, and neurological function.

Endocrine Disrupting Chemicals and Type 1 Diabetes

Type 1 diabetes (T1D) is the most common chronic metabolic disease in children and adolescents. The etiology of T1D is not fully understood but it seems multifactorial. The genetic background determines the predisposition to develop T1D, while the autoimmune process against β-cells seems to be also determined by environmental triggers, such as endocrine disrupting chemicals (EDCs). Environmental EDCs may act throughout different temporal windows as single chemical agent or as chemical mixtures. They could affect the development and the function of the immune system or of the β-cells function, promoting autoimmunity and increasing the susceptibility to autoimmune attack. Human studies evaluating the potential role of exposure to EDCs on the pathogenesis of T1D are few and demonstrated contradictory results. The aim of this narrative review is to summarize experimental and epidemiological studies on the potential role of exposure to EDCs in the development of T1D. We highlight what we know by animals about EDCs’ effects on mechanisms leading to T1D development and progression. Studies evaluating the EDC levels in patients with T1D were also reported. Moreover, we discussed why further studies are needed and how they should be designed to better understand the causal mechanisms and the next prevention interventions.

Acute oral methylmercury exposure perturbs the gut microbiome and alters gut-brain axis related metabolites in rats

Environmental pollutants like methylmercury (MeHg) can bring devastating neurotoxicity to animals and human beings. Gut microbiota has been found to demethylate MeHg and promote the excretion of Hg through feces. However, the impacts of MeHg on gut microbiota and metabolites related to gut-brain interactions were less studied in mammals. The object of this study was to investigate the impacts of acute MeHg exposure on gut microbiome and metabolites together with its impact on gut integrity and related biological responses in rats. Rats were exposed to MeHg through oral administration and were sacrificed after 24 h 16 S rRNA gene sequencing was used to study the perturbance to gut microbiome and liquid chromatography mass spectrometry (LC-MS) was used for metabolomics profiling. It was found that gut was one of the target tissues of MeHg. MeHg induce the changes of intestinal microbial community structure and induce the regulating neuron activity change of intestinal neurotransmitters and metabolites on intestinal neurotransmitters and metabolites regulating the neuron activity. This was supported by the increased BDNF level. These findings may suggest a potential new mechanism regarding the neurotoxicity of MeHg. The protocols used in this study may also be applied to understand the neurotoxicity of other environmental neurotoxins like Pb, Mn, polychlorinated biphenyls, and pesticides, etc and to screen the neurotoxicity of emerging environmental contaminants.

Chronic oral exposure to glycated whey proteins increases survival of aged male NOD mice with autoimmune prostatitis by regulating the gut microbiome and anti-inflammatory responses

Glycated whey proteins have been shown to be protective against type 1 diabetes in our previous studies, suggesting their potential application as medical food. To determine if the protection could be extended to other autoimmune diseases, aged male non-obese diabetic (NOD) mice that develop a wide spectrum of autoimmune pathologies, including spontaneous autoimmune prostatitis, were used. After a 6-month oral exposure to whey protein-derived early glycation products (EGPs), EGP-treated NOD mice had an increased survival rate, decreased macrophage infiltration in the anterior lobe and decreased inflammation in the prostate when compared to the mice that received non-reacted controls. The systemic immunity was regulated towards anti-inflammation, evidenced by an increase in serum IL-10 level and decreases in total splenocytes, splenic M1 macrophages, CD4+ T cells, CD8+ T cells and B cells. Consistent with an overall anti-inflammatory status, the gut microbiome was altered in abundance but not diversity, with increased Allobaculum, Anaerostipes, Bacteroides, Parabacteroides and Prevotella and decreased Adlercreutzia and Roseburia at the genus level. Moreover, increased Bacteroides acidifaciens correlated with most of the immune parameters measured. Collectively, chronic oral exposure to EGPs produced an anti-inflammatory effect in aged male NOD mice, which might contribute to the protective effects against spontaneous autoimmune prostatitis and/or other organ specific autoimmune diseases.

Metatranscriptomic Analysis of the Mouse Gut Microbiome Response to the Persistent Organic Pollutant 2,3,7,8-Tetrachlorodibenzofuran

Persistent organic pollutants (POPs) are important environmental chemicals and continued study of their mechanism of action remains a high priority. POPs, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and polychlorinated biphenyls (PCBs), are widespread environmental contaminants that are agonists for the aryl hydrocarbon receptor (AHR). Activation of the AHR modulates the gut microbiome community structure and function, host immunity, and the host metabolome. In the current study, male C57BL6/J mice were exposed, via the diet, to 5 µg/kg body weight (BW) TCDF or 24 µg/kg BW of TCDF every day for 5 days. The functional and structural changes imparted by TCDF exposure to the gut microbiome and host metabolome were explored via 16S rRNA gene amplicon sequencing, metabolomics, and bacterial metatranscriptomics. Significant changes included increases in lipopolysaccharide (LPS) biosynthesis gene expression after exposure to 24 µg/kg BW of TCDF. Increases in LPS biosynthesis were confirmed with metabolomics and LPS assays using serum obtained from TCDF-treated mice. Significant increases in gene expression within aspartate and glutamate metabolism were noted after exposure to 24 µg/kg BW of TCDF. Together, these results suggest that after exposure to 24 µg/kg BW of TCDF, the gut microbiome increases the production of LPS and glutamate to promote localized gut inflammation, potentially using glutamate as a stress response.

Effects of cyanidin-3-O-glucoside on 3-chloro-1,2-propanediol induced intestinal microbiota dysbiosis in rats

Gastrointestinal studies suggested that balanced gut microbial community contribute to a healthy gut. Our previous studies have suggested that cyanidin-3-O-glucoside (C3G) can alleviate food contaminant 3-Chloro-1,2-propanediol (3-MCPD) induced testis injury and improve the spermatogenesis in rats. To the best of our knowledge, the effects of 3-MCPD exposure and C3G intervention on intestinal microbiota have not been studied. In the present study, male Wistar rats were used to investigate the effects of C3G and 3-MCPD on microbiota composition. After 3-MCPD treatment, the small intestinal showed histopathological alterations, including villus atrophy, necrosis, decreased number of epithelial cells and cellular infiltration. Supplementation of C3G brings the small intestine closer to normal histology. Meanwhile, 3-MCPD exposure significantly changed the diversity and composition of gut microbiota. At the phylum level, Cyanobacteria and Firmicutes were enriched in 3-MCPD groups, while Actinobacteria and Proteobacteria were decreased. Supplementation of C3G significantly increased the relative abundance of Lachnospiraceae_NK4A136_group and Actinobacteria, indicating that C3G may regulate the communities of gut microbiota towards a beneficial orientation. Our results indicate that C3G may protect the intestinal mucosa damage caused by 3-MCPD, and appropriate dose of C3G restrains gut microbial dysbiosis caused by 3-MCPD, which is a potential way to promote gut healthy.

Exacerbation of Type 1 Diabetes in Perinatally Genistein Exposed Female Non-Obese Diabetic (NOD) Mouse Is Associated With Alterations of Gut Microbiota and Immune Homeostasis

Despite various hypothesized benefits of dietary isoflavone genistein (GEN) from soy-based products, many questions surrounding GEN's immunotoxic effects, especially during perinatal exposure, have yet to be answered. The objective of the study was to determine if there existed a sex-specific effect of GEN on type 1 diabetes (T1D) following perinatal exposure. We exposed offspring of non-obese diabetic (NOD) mice to GEN per oral at a physiological dose (20 mg/kg body weight) from embryonic day 7 to postnatal day (PND) 21. In female offspring, perinatal GEN dosing significantly increased the incidence of T1D at early time points, and the exacerbation was associated with decreased serum levels of interleukin (IL)-10, IgG2a, and IgM. In male offspring dosed with GEN, a decrease in serum IgG1 was also observed. Flow cytometric analysis in females suggested an increased pro-inflammatory splenic CD5+CD24- and CD4-CD8+ cell counts, while both %T cells and %CD4+ T cells were significantly decreased in males, suggesting an anti-inflammatory effect. Gut microbiota (GMB) analysis indicated that fecal microbiota from PND 90 female offspring exhibited an increased level of Enterobacteriales (suggesting a pro-inflammatory response), while the similar changes were not found in PND 30 females. Moreover, RNA sequencing showed that intestinal α-defensin expression was down-regulated in GEN-treated females, supporting a pro-inflammatory response. However, perinatal GEN administration perturbed GMB toward an anti-inflammatory response in PND 90 males. Taken together, a strong sex-specific effect was found in the perinatal GEN exposure window, and the T1D exacerbation in NOD females was associated with GMB-related immunomodulatory mechanisms.

Cross-regulatory Circuit Between AHR and Microbiota

BACKGROUND

The gut microbes have a close symbiotic relationship with their host. Interactions between host and the microbiota affect the nutritional, immunological, and physiological status of the host. The Aryl Hydrocarbon Receptor (AHR) is a ligand activated transcription factor that mediates the toxicity of xenobiotics. Recently, the relationship between the gut microbiota and AHR has attracted the attention of many researchers.

METHODS

We undertook a structured search of bibliographic databases for peer-reviewed research literature.

RESULTS

We found and reviewed 49 peer-reviewed papers dealing with the major aspects related to the crosstalk between AHR and microbiota. The AHR influences the intestinal microbiota population and mediates host-microbe homeostasis. Interestingly, the gut microbiota also produces ligands of AHR from bacterial metabolism and thereby activates the AHR signaling pathway. Concusion: This review presents current knowledge of the cross-regulatory circuit between the AHR and intestinal microbiota. The findings of this review confirm the importance of AHR-microbiota interactions in health and disease.

Effects of single and combined toxic exposures on the gut microbiome: Current knowledge and future directions

Human populations are chronically exposed to mixtures of toxic chemicals. Predicting the health effects of these mixtures require a large amount of information on the mode of action of their components. Xenobiotic metabolism by bacteria inhabiting the gastrointestinal tract has a major influence on human health. Our review aims to explore the literature for studies looking to characterize the different modes of action and outcomes of major chemical pollutants, and some components of cosmetics and food additives, on gut microbial communities in order to facilitate an estimation of their potential mixture effects. We identified good evidence that exposure to heavy metals, pesticides, nanoparticles, polycyclic aromatic hydrocarbons, dioxins, furans, polychlorinated biphenyls, and non-caloric artificial sweeteners affect the gut microbiome and which is associated with the development of metabolic, malignant, inflammatory, or immune diseases. Answering the question 'Who is there?' is not sufficient to define the mode of action of a toxicant in predictive modeling of mixture effects. Therefore, we recommend that new studies focus to simulate real-life exposure to diverse chemicals (toxicants, cosmetic/food additives), including as mixtures, and which combine metagenomics, metatranscriptomics and metabolomic analytical methods achieving in that way a comprehensive evaluation of effects on human health.

Sex-dependent effects of bisphenol A on type 1 diabetes development in non-obese diabetic (NOD) mice

Type 1 diabetes (T1D) is an autoimmune disease caused by immune-mediated pancreatic β-cell destruction. The endocrine disrupting chemical bisphenol A (BPA) has widespread human exposure and can modulate immune function and the gut microbiome (GMB), which may contribute to the increasing T1D incidence worldwide. It was hypothesized that BPA had sex-dependent effects on T1D by modulating immune homeostasis and GMB. Adult female and male non-obese diabetic (NOD) mice were orally administered BPA at environmentally relevant doses (30 or 300 µg/kg). Antibiotic-treated adult NOD females were exposed to 0 or 30 µg/kg BPA. BPA accelerated T1D development in females, but delayed males from T1D. Consistently, females had a shift towards pro-inflammation (e.g., increased macrophages and Bacteroidetes), while males had increases in anti-inflammatory immune factors and a decrease in both anti- and pro-inflammatory GMB. Although bacteria altered during sub-acute BPA exposure differed from bacteria altered from chronic BPA exposure in both sexes, the GMB profile was consistently pro-inflammatory in females, while males had a general decrease of both anti- and pro-inflammatory gut microbes. However, treatment of females with the antibiotic vancomycin failed to prevent BPA-induced glucose intolerance, suggesting changes in Gram-positive bacteria were not a primary mechanism. In conclusion, BPA exposure was found to have sex dimorphic effects on T1D with detrimental effects in females, and immunomodulation was identified as the primary mechanism.

Dioxin-like PCB 126 increases intestinal inflammation and disrupts gut microbiota and metabolic homeostasis

The gut microbiome is sensitive to diet and environmental exposures and is involved in the regulation of host metabolism. Additionally, gut inflammation is an independent risk factor for the development of metabolic diseases, specifically atherosclerosis and diabetes. Exposures to dioxin-like pollutants occur primarily via ingestion of contaminated foods and are linked to increased risk of developing cardiometabolic diseases. We aimed to elucidate the detrimental impacts of dioxin-like pollutant exposure on gut microbiota and host gut health and metabolism in a mouse model of cardiometabolic disease. We utilized 16S rRNA sequencing, metabolomics, and regression modeling to examine the impact of PCB 126 on the microbiome and host metabolism and gut health. 16S rRNA sequencing showed that gut microbiota populations shifted at the phylum and genus levels in ways that mimic observations seen in chronic inflammatory diseases. PCB 126 reduced cecum alpha diversity (0.60 fold change; p = 0.001) and significantly increased the Firmicutes to Bacteroidetes ratio (1.63 fold change; p = 0.044). Toxicant exposed mice exhibited quantifiable concentrations of PCB 126 in the colon, upregulation of Cyp1a1 gene expression, and increased markers of intestinal inflammation. Also, a significant correlation between circulating Glucagon-like peptide-1 (GLP-1) and Bifidobacterium was evident and dependent on toxicant exposure. PCB 126 exposure disrupted the gut microbiota and host metabolism and increased intestinal and systemic inflammation. These data imply that the deleterious effects of dioxin-like pollutants may be initiated in the gut, and the modulation of gut microbiota may be a sensitive marker of pollutant exposures.

The gut microbiome and aquatic toxicology: An emerging concept for environmental health

The microbiome plays an essential role in the health and onset of diseases in all animals, including humans. The microbiome has emerged as a central theme in environmental toxicology because microbes interact with the host immune system in addition to its role in chemical detoxification. Pathophysiological changes in the gastrointestinal tissue caused by ingested chemicals and metabolites generated from microbial biodegradation can lead to systemic adverse effects. The present critical review dissects what we know about the impacts of environmental contaminants on the microbiome of aquatic species, with special emphasis on the gut microbiome. We highlight some of the known major gut epithelium proteins in vertebrate hosts that are targets for chemical perturbation, proteins that also directly cross-talk with the microbiome. These proteins may act as molecular initiators for altered gut function, and we propose a general framework for an adverse outcome pathway that considers gut dysbiosis as a major contributing factor to adverse apical endpoints. We present 2 case studies, nanomaterials and hydrocarbons, with special emphasis on the Deepwater Horizon oil spill, to illustrate how investigations into the microbiome can improve understanding of adverse outcomes. Lastly, we present strategies to functionally relate chemical-induced gut dysbiosis with adverse outcomes because this is required to demonstrate cause-effect relationships. Further investigations into the toxicant-microbiome relationship may prove to be a major breakthrough for improving animal and human health. Environ Toxicol Chem 2018;37:2758-2775. © 2018 SETAC.

Food Chemicals Disrupt Human Gut Microbiota Activity And Impact Intestinal Homeostasis As Revealed By In Vitro Systems

Growing evidence indicates that the human gut microbiota interacts with xenobiotics, including persistent organic pollutants and foodborne chemicals. The toxicological relevance of the gut microbiota-pollutant interplay is of great concern since chemicals may disrupt gut microbiota functions, with a potential impairment of host homeostasis. Herein we report within batch fermentation systems the impact of food contaminants (polycyclic aromatic hydrocarbons, polychlorobiphenyls, brominated flame retardants, dioxins, pesticides and heterocyclic amines) on the human gut microbiota by metatranscriptome and volatolome i.e. “volatile organic compounds” analyses. Inflammatory host cell response caused by microbial metabolites following the pollutants-gut microbiota interaction, was evaluated on intestinal epithelial TC7 cells. Changes in the volatolome pattern analyzed via solid-phase microextraction coupled to gas chromatography-mass spectrometry mainly resulted in an imbalance in sulfur, phenolic and ester compounds. An increase in microbial gene expression related to lipid metabolism processes as well as the plasma membrane, periplasmic space, protein kinase activity and receptor activity was observed following dioxin, brominated flame retardant and heterocyclic amine exposure. Conversely, all food contaminants tested induced a decreased in microbial transcript levels related to ribosome, translation and nucleic acid binding. Finally, we demonstrated that gut microbiota metabolites resulting from pollutant disturbances may promote the establishment of a pro-inflammatory state in the gut, as stated with the release of cytokine IL-8 by intestinal epithelial cells.

Flux, Impact, and Fate of Halogenated Xenobiotic Compounds in the Gut

Humans and their associated microbiomes are exposed to numerous xenobiotics through drugs, dietary components, personal care products as well as environmental chemicals. Most of the reciprocal interactions between the microbiota and xenobiotics, such as halogenated compounds, occur within the human gut harboring diverse and dense microbial communities. Here, we provide an overview of the flux of halogenated compounds in the environment, and diverse exposure routes of human microbiota to these compounds. Subsequently, we review the impact of halogenated compounds in perturbing the structure and function of gut microbiota and host cells. In turn, cultivation-dependent and metagenomic surveys of dehalogenating genes revealed the potential of the gut microbiota to chemically alter halogenated xenobiotics and impact their fate. Finally, we provide an outlook for future research to draw attention and attract interest to study the bidirectional impact of halogenated and other xenobiotic compounds and the gut microbiota.

Aryl hydrocarbon receptor (AhR) mediated short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid homeostasis in mice

The effects of the most potent aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid (BA) homeostasis was examined in male and female wild-type and AhR-null mice shortly after 4-day exposure, rather than at a later time when secondary non-AhR dependent effects are more likely to occur. TCDD had similar effects on BA homeostasis in male and female mice. TCDD decreased the concentration of total-(Σ) BAs in liver by approximately 50% (all major BA categories except for the non-6,12-OH BAs), without decreasing the expression of the rate limiting BA synthetic enzyme (Cyp7a1) or altering the major BA regulatory pathways (FXR) in liver and intestine. Even though the Σ-BAs in liver were markedly decreased, the Σ-BAs excreted into bile were not altered. TCDD decreased the relative amount of 12-OH BAs (TCA, TDCA, CA, DCA) in bile and increased the biliary excretion of TCDCA and its metabolites (TαMCA, TUDCA); this was likely due to the decreased Cyp8b1 (12α-hydroxylase) in liver. The concentration of Σ-BAs in serum was not altered by TCDD, indicating that serum BAs do not reflect BA status in liver. However, proportions of individual BAs in serum reflected the decreased expression of Cyp8b1. All these TCDD-induced changes in BA homeostasis were absent in AhR-null mice. In summary, through the AhR, TCDD markedly decreases BA concentrations in liver and reduces the 12α-hydroxylation of BAs without altering Cyp7a1 and FXR signaling. The TCDD-induced decrease in Σ-BAs in liver did not result in a decrease in biliary excretion or serum concentrations of Σ-BAs.

Persistent Endocrine-Disrupting Chemicals and Fatty Liver Disease

PURPOSE OF REVIEW

Non-alcoholic fatty liver disease (NAFLD) is the most prominent chronic liver disease in Western countries, affecting approximately 25% of the population worldwide. Sex-specific differences in the development of NAFLD are apparent. While obesity and insulin resistance are major contributors to the increasing prevalence of NAFLD, a growing body of literature suggests that exposure to persistent endocrine-disrupting chemicals (pEDCs) may also play a role. This review summarizes recent (2011 and later) scientific literature investigating exposures to pEDCs, specifically persistent organic pollutants (POPs), and NAFLD, with a focus on sex-specific associations.

RECENT FINDINGS

The overwhelming majority of studies were conducted in single-sex animal models and provide biological evidence that exposures to 2,3,7,8-tetrachlorodibenzo-p-dioxin polychlorinated biphenyls, and other POPs or POP mixtures are negatively associated with liver health. There were four cross-sectional epidemiological studies in humans that reported associations for several POPs, including polychlorinated biphenyls and perfluorinated chemicals, with elevated liver enzymes. Only one of these studies, using a sample of gastric bypass surgery patients, examined sex-specific associations of POPs and liver enzymes, finding adverse associations among women only. The noticeable lack of studies investigating how differences (i.e., biochemical, physiological, and behavioral) between men and women may influence associations of pEDCs and NAFLD represents a large research gap in environmental health. Sexual dimorphism in metabolic processes throughout the body, including the liver, is established but often overlooked in the designs and analyses of studies. Other factors identified in this review that may also act to modulate associations of environmental chemicals and NAFLD are reproductive status and dietary nutrient intakes, which also remain understudied in the literature. Despite knowledge of sexual dimorphism in the actions of pEDCs, as well as in metabolic processes related to NAFLD development, few experimental or epidemiological studies have investigated sex-dependent associations. Future studies, especially those in humans, should be designed to address this research need. Consideration of other factors, such as reproductive status, dietary intakes, and mixtures of chemicals with varying endocrine-disrupting capabilities, should be explored.

Aryl hydrocarbon receptor (AhR) mediated short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid homeostasis in mice

The effects of the most potent aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid (BA) homeostasis was examined in male and female wild-type and AhR-null mice shortly after 4-day exposure, rather than at a later time when secondary non-AhR dependent effects are more likely to occur. TCDD had similar effects on BA homeostasis in male and female mice. TCDD decreased the concentration of total-(Σ) BAs in liver by approximately 50% (all major BA categories except for the non-6,12-OH BAs), without decreasing the expression of the rate limiting BA synthetic enzyme (Cyp7a1) or altering the major BA regulatory pathways (FXR) in liver and intestine. Even though the Σ-BAs in liver were markedly decreased, the Σ-BAs excreted into bile were not altered. TCDD decreased the relative amount of 12-OH BAs (TCA, TDCA, CA, DCA) in bile and increased the biliary excretion of TCDCA and its metabolites (TαMCA, TUDCA); this was likely due to the decreased Cyp8b1 (12α-hydroxylase) in liver. The concentration of Σ-BAs in serum was not altered by TCDD, indicating that serum BAs do not reflect BA status in liver. However, proportions of individual BAs in serum reflected the decreased expression of Cyp8b1. All these TCDD-induced changes in BA homeostasis were absent in AhR-null mice. In summary, through the AhR, TCDD markedly decreases BA concentrations in liver and reduces the 12α-hydroxylation of BAs without altering Cyp7a1 and FXR signaling. The TCDD-induced decrease in Σ-BAs in liver did not result in a decrease in biliary excretion or serum concentrations of Σ-BAs.

Nicotine Alters the Gut Microbiome and Metabolites of Gut-Brain Interactions in a Sex-Specific Manner

As the primary active substance in tobacco, nicotine affects the activity of the central nervous system, and its effects are sex-dependent. There are complex interactions between the gut and brain, and the gut microbiome can influence neuronal activity and host behavior, with diverse chemical signaling being involved. However, it is unclear whether nicotine can affect the normal gut microbiome and associated chemical signaling of the gut-brain axis. Sex is an important factor that shapes the gut microbiome, but the role of sex in the interaction among nicotine, gut bacteria, and related metabolites remains unknown. In this study, we applied high-throughput sequencing and gas chromatography-mass spectrometry (GC-MS) to explore how nicotine exposure affects the gut microbiome and its metabolism in female and male C57BL/6J mice, with a focus on the chemical signaling involved in gut-brain interactions. 16S sequencing results indicated that the community composition of the gut microbiome was differentially perturbed by nicotine in females and males. Differential alterations of bacterial carbohydrate metabolic pathways are consistent with lower body weight gain in nicotine-treated males. Oxidative stress response and DNA repair genes were also specifically enriched in the nicotine-treated male gut microbiome. The fecal metabolome indicated that multiple neurotransmitters, such as glutamate, gamma-aminobutyric acid (GABA), and glycine, were differentially altered in female and male mice. Some neuroactive metabolites, including leucine and uric acid, were also changed. This study demonstrates a sex-dependent effect of nicotine on gut microbiome community composition, functional bacterial genes, and the fecal metabolome.

TCDD influences reservoir of antibiotic resistance genes in murine gut microbiome

Dysbiosis of the gut microbiome via antibiotics, changes in diet and infection can select for bacterial groups that more frequently harbor antimicrobial resistance genes (ARGs) and mobile genetic elements (MGEs). However, the impact of environmental toxicants on the reservoir of ARGs in the gut microbiome has received less attention. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist with multiple toxic health effects including immune dysfunction. The selective pressure of TCDD on the abundance of ARG and MGE-harboring gut populations was examined using C57BL/6 mice exposed to 0-30 μg/kg TCDD for 28 and 92 days with the latter having a 30-day recovery period. DNA extracted from temporally collected fecal pellets was characterized using a qPCR array with 384 assays targeting ARGs and MGEs. Fourteen genes, typically observed in Enterobacteriaceae, increased significantly within 8 days of initial dosing, persisted throughout the treatment period, and remained induced 30 days post dosing. A qPCR primer set targeting Enterobacteriaceae also showed 10- to 100-fold higher abundance in TCDD-treated groups, which was further verified using metagenomics. Results show a bloom of ARG-harboring bacterial groups in the gut due to a xenobiotic compound that is not a metal, biocide or antimicrobial.

Genistein prevention of hyperglycemia and improvement of glucose tolerance in adult non-obese diabetic mice are associated with alterations of gut microbiome and immune homeostasis

Although studies have linked soy phytoestrogen 4,7,4-trihydroxyisoflavone genistein (GEN) to reduced type 1 diabetes (T1D) risk, the mechanism of dietary GEN on T1D remains unknown. In our studies, adult non-obese diabetic (NOD) mouse model was employed to investigate the effects of GEN exposure on blood glucose level (BGL), glucose tolerance, gut microbiome, and immune responses. Adult male and female NOD mice were fed with either soy-based or casein-based diet, and received GEN at 20mg/kg body weight by gavage daily. The BGL and immune responses (represented by serum antibodies, cytokines and chemokines, and histopathology) were monitored, while the fecal gut microbiome was sequenced for 16S ribosomal RNA to reveal any alterations in gut microbial communities. A significantly reduced BGL was found in NOD males fed with soy-based diet on day 98 after initial dosing, and an improved glucose tolerance was observed on both diets. In addition, an anti-inflammatory response (suggested by reduced IgG2b and cytokine/chemokine levels, and alterations in the microbial taxonomy) was accompanied by an altered β-diversity in gut microbial species. Among the NOD females exposed to GEN, a later onset of T1D was observed. However, the profiles of gut microbiome, antibodies and cytokines/chemokines were all indicative of pro-inflammation. This study demonstrated an association among GEN exposure, gut microbiome alteration, and immune homeostasis in NOD males. Although the mechanisms underlying the protective effects of GEN in NOD mice need to be explored further, the current study suggested a GEN-induced sex-specific effect in inflammatory status and gut microbiome.

Modulatory Influence of Segmented Filamentous Bacteria on Transcriptomic Response of Gnotobiotic Mice Exposed to TCDD

Environmental toxicants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR), are known to induce host toxicity and structural shifts in the gut microbiota. Key bacterial populations with similar or opposing functional responses to AhR ligand exposure may potentially help regulate expression of genes associated with immune dysfunction. To examine this question and the mechanisms for AhR ligand-induced bacterial shifts, C57BL/6 gnotobiotic mice were colonized with and without segmented filamentous bacteria (SFB) – an immune activator. Mice were also colonized with polysaccharide A producing Bacteroides fragilis – an immune suppressor to serve as a commensal background. Following colonization, mice were administered TCDD (30 μg/kg) every 4 days for 28 days by oral gavage. Quantified with the nCounter^® mouse immunology panel, opposing responses in ileal gene expression (e.g., genes associated with T-cell differentiation via the class II major histocompatibility complex) as a result of TCDD dosing and SFB colonization were observed. Genes that responded to TCDD in the presence of SFB did not show a significant response in the absence of SFB, and vice versa. Regulatory T-cells examined in the mesenteric lymph-nodes, spleen, and blood were also less impacted by TCDD in mice colonized with SFB. TCDD-induced shifts in abundance of SFB and B. fragilis compared with previous studies in mice with a traditional gut microbiome. With regard to the mouse model colonized with individual populations, results indicate that TCDD-induced host response was significantly modulated by the presence of SFB in the gut microbiome, providing insight into therapeutic potential between AhR ligands and key commensals.

TCDD administered on activated carbon eliminates bioavailability and subsequent shifts to a key murine gut commensal

Activated carbon (AC) is an increasingly attractive remediation alternative for the sequestration of dioxins at contaminated sites globally. However, the potential for AC to reduce the bioavailability of dioxins in mammals and the residing gut microbiota has received less attention. This question was partially answered in a recent study examining 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced hallmark toxic responses in mice administered with TCDD sequestered by AC or freely available in corn oil by oral gavage. Results from that study support the use of AC to significantly reduce the bioavailability of TCDD to the host. Herein, we examined the bioavailability of TCDD sequestered to AC on a key murine gut commensal and the influence of AC on the community structure of the gut microbiota. The analysis included qPCR to quantify the expression of segmented filamentous bacteria (SFB) in the mouse ileum, which has responded to TCDD-induced host toxicity in previous studies and community structure via sequencing the 16S ribosomal RNA (rRNA) gene. The expression of SFB 16S rRNA gene and functional genes significantly increased with TCDD administered with corn oil vehicle. Such a response was absent when TCDD was sequestered by AC. In addition, AC appeared to have a minimal influence on murine gut community structure and diversity, affecting only the relative abundance of Lactobacillaceae and two other groups. Results of this study further support the remedial use of AC for eliminating bioavailability of TCDD to host and subsequent influence on the gut microbiome.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-elicited effects on bile acid homeostasis: Alterations in biosynthesis, enterohepatic circulation, and microbial metabolism

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant which elicits hepatotoxicity through activation of the aryl hydrocarbon receptor (AhR). Male C57BL/6 mice orally gavaged with TCDD (0.01–30 µg/kg) every 4 days for 28 days exhibited bile duct proliferation and pericholangitis. Mass spectrometry analysis detected a 4.6-fold increase in total hepatic bile acid levels, despite the coordinated repression of genes involved in cholesterol and primary bile acid biosynthesis including Cyp7a1. Specifically, TCDD elicited a >200-fold increase in taurolithocholic acid (TLCA), a potent G protein-coupled bile acid receptor 1 (GPBAR1) agonist associated with bile duct proliferation. Increased levels of microbial bile acid metabolism loci (bsh, baiCD) are consistent with accumulation of TLCA and other secondary bile acids. Fecal bile acids decreased 2.8-fold, suggesting enhanced intestinal reabsorption due to induction of ileal transporters (Slc10a2, Slc51a) and increases in whole gut transit time and intestinal permeability. Moreover, serum bile acids were increased 45.4-fold, consistent with blood-to-hepatocyte transporter repression (Slco1a1, Slc10a1, Slco2b1, Slco1b2, Slco1a4) and hepatocyte-to-blood transporter induction (Abcc4, Abcc3). These results suggest that systemic alterations in enterohepatic circulation, as well as host and microbiota bile acid metabolism, favor bile acid accumulation that contributes to AhR-mediated hepatotoxicity.