Subacute exposure to a mixture of tributyltin plus mercury impairs reproductive axis function, exacerbating premature ovarian insufficiency features and reducing fertility in female rats

Tributyltin (TBT) and mercury (Hg) are endocrine-disrupting chemicals that individually cause reproductive complications. However, the reproductive consequences of exposure to a mixture of TBT plus Hg are not well known. We hypothesized that exposure to a mixture of TBT plus Hg would alter hypothalamic-pituitary-gonadal (HPG) axis function. Female rats were exposed to this mixture daily for 15 days, after which chemical accumulation in the tissues, morphology, hormone levels, inflammation, fibrosis, and protein expression in the reproductive organs were assessed. Increases in tin (Sn) and Hg levels were detected in the serum, HPG axis, and uterus of TBT-Hg rats. TBT-Hg rats exhibited irregular estrous cycles. TBT-Hg rats showed an increase in gonadotropin-releasing hormone (GnRH) protein expression and follicle-stimulating hormone (FSH) levels and a reduction in luteinizing hormone (LH) levels. Reduced ovarian reserve, antral follicles, corpora lutea (CL) number, and estrogen levels and increased atretic and cystic follicles were found, suggesting that TBT-Hg exposure exacerbated premature ovarian insufficiency (POI) features. Furthermore, TBT-Hg rats exhibited increased ovarian mast cell numbers, expression of the inflammatory markers IL-6 and collagen deposition. Apoptosis and reduced gland number were observed in the uteri of TBT-Hg rats. A reduction in the number of pups/litter for 90 days was found in TBT-Hg rats, suggesting impaired fertility. Strong negative correlations were found between serum and ovarian Sn levels and ovarian Hg levels and ovarian reserve and CL number. Collectively, these data suggest that TBT plus Hg exposure leads to abnormalities in the HPG axis, exacerbating POI features and reducing fertility in female rats.

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

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

Transgenerational inheritance of methylmercury and vitamin A-induced toxicological effects in a Wistar rats environmental-based model

Mercury (Hg) and vitamin A (VitA) are two environmental factors with potential health impacts, especially during pregnancy and early childhood. Fish and seafood may present elevated levels of methylmercury (MeHg), the major Hg derivative, and VitA. This study aimed to evaluate the transgenerational effects of exposure to MeHg and/or VitA on epigenetic and toxicological parameters in a Wistar rat model. Our findings revealed persistent toxicological effects in generations F1 and F2 following low/mild doses of MeHg and/or VitA exposure during dams' (F0) gestation and breastfeeding. Toxicological effects observed in F2 included chronic DNA damage, bone marrow toxicity, altered microglial content, reduced neuronal signal, and diminished male longevity. Sex-specific patterns were also observed. Co-exposure to MeHg and VitA showed both synergistic and antagonistic effects. Additionally, the study demonstrated that MeHg and VitA affected histone methylation and caused consistent effects in F2. While MeHg exposure has been associated with transgenerational inheritance effects in other organisms, this study provides the first evidence of transgenerational inheritance of MeHg and VitA-induced toxicological effects in rodents. Although the exact mechanism is not yet fully understood, these findings suggest that MeHg and VitA may perpetuate their impacts across generations. The study highlights the need for remedial policies and interventions to mitigate the potential health problems faced by future generations exposed to MeHg or VitA. Further research is warranted to investigate the transgenerational effects beyond F2 and determine the matrilineal or patrilineal inheritance patterns.

Occupational and environmental mercury exposure and human reproductive health - a review

Mercury is a toxic heavy metal. Humans are exposed to mercury through several sources including environmental, occupational, contaminated food and water and from mercury-containing dental amalgam. Mercury exposure is known to harm the nervous system profoundly, and have a negative impact on digestive and immune systems, and other organs. To review and discuss the effect of mercury exposure through environmental or occupational routes on human reproduction, pregnancy, and its outcome. Published information about the potential toxic effects of mercury on human reproduction were collected and summarized. Literature was identified by systematic search using relevant keywords. Literature review revealed a number of negative impacts of mercury on human reproduction. These included effects on semen quality, including reduced sperm count, motility, and changes in morphology that may reduce fertility potential. There may also be an effect in changing reproductive hormone levels. Mercury exposure might also affect pregnancy but the data concerning mercury effects on female reproduction are limited except for some data about mercury exposure and poor pregnancy outcomes. Available data indicate that mercury exposure may have a toxicity effect on reproductive potential, especially in males. Prenatal mercury exposure may affect pregnancy or its outcome and this appears to be dependent upon dose, duration, and timing of exposure. Nutritional status of exposed individual might also influence the impact of mercury.

Effects of methylmercury and retinol palmitate co-administration in rats during pregnancy and breastfeeding: Metabolic and redox parameters in dams and their offspring

Ubiquitous low-dose methylmercury (MeHg) exposure through an increased fish consumption represents a global public health problem, especially among pregnant women. A plethora of micronutrients presented in fish affects MeHg uptake/distribution, but limited data is available. Vitamin A (VitA), another fish micronutrient is used in nutritional supplementation, especially during pregnancy. However, there is no information about the health effects arising from their combined exposure. Therefore, the present study aimed to examine the effects of both MeHg and retinyl palmitate administered on pregnant and lactating rats in metabolic and redox parameters from dams and their offspring. Thirty Wistar female rats were orally supplemented with MeHg (0,5 mg/kg/day) and retinyl palmitate (7500 µg RAE/kg/day) via gavage, either individually or in combination from the gestational day 0 to weaning. For dams (150 days old) and their offspring (31 days old), glycogen accumulation (hepatic and cardiac) and retinoid contents (plasma and liver) were analyzed. Hg deposition in liver tissue was quantified. Redox parameters (liver, kidney, and heart) were evaluated for both animals. Cytogenetic damage was analyzed with micronucleus test. Our results showed no general toxic or metabolic alterations in dams and their offspring by MeHg-VitA co-administration during pregnancy and lactation. However, increased lipoperoxidation in maternal liver and a disrupted pro-oxidant response in the heart of male pups was encountered, with apparently no particular effects in the antioxidant response in female offspring. GST activity in dam kidney was altered leading to possible redox disruption of this tissue with no alterations in offspring. Finally, the genomic damage was exacerbated in both male and female pups. In conclusion, low-dose MeHg exposure and retinyl palmitate supplementation during gestation and lactation produced a potentiated pro-oxidant effect, which was tissue-specific. Although this is a pre-clinical approach, we recommend precaution for pregnant women regarding food consumption, and we encourage more epidemiological studies to assess possible modulations effects of MeHg-VitA co-administration at safe or inadvertently used doses in humans, which may be related to specific pathologies in mothers and their children.

Delayed neurochemical effects of prenatal exposure to MeHg in the cerebellum of developing rats

Human fetuses and neonates are particularly vulnerable to methylmercury (MeHg)-induced brain damage and are sensitive even to low exposure levels. Previous work of our group evidence that prenatal exposure to MeHg causes cognitive and behavioral alterations and disrupt hippocampus signaling. The current study aimed to investigate the effect of gestational exposure of rats to MeHg at low doses (1 or 2 mg/kg) on parameters of redox imbalance and key signaling pathways in the cerebellum of their offspring. Pregnant females received MeHg (treated group) or 0.9% saline water (control group) by gavage in alternated days from gestational day 5 (GD5) until parturition and analyzes were proceed in the cerebellum of 30-day-old pups. We found increased lipid peroxidation and protein carbonylation levels as well as decreased SH content in pups prenatally exposed to 2 mg/kg MeHg. In addition, misregulated SOD/catalase activities supported imbalanced redox equilibrium. We found decreased GSK3β(Ser9) phosphorylation, suggesting activation of this enzyme and dephosphorylation/inhibition of ERK1/2 and JNK pathways. Increased PKAα catalytic subunit could be upstream of hyperphosphorylated c-Raf(Ser259) and downregulated MAPK pathway. In addition, we found raised levels of the Ca²⁺-dependent protein phosphatase 2 B (PP2B). We also found preserved immunohistochemical staining for both glial fibrillary acidic protein (GFAP) and NeuN in MeHg-exposed pups. Western blot analysis showed unaltered levels of BAX/BCL-XL, BAD/BCL-2 and active caspase 3. Together, these findings support absence of reactive astrocytes, neuronal damage and apoptotic cell death in the cerebellum of MeHg treated pups. The present study provides evidence that prenatal exposure to MeHg leads to later redox imbalance and disrupted signaling mechanisms in the cerebellum of 30-day-old pups potentially predisposing them to long-lasting neurological impairments in CNS.

Developmental neurotoxicity of the hippocampus following in utero exposure to methylmercury: impairment in cell signaling

In this study, we assessed some hippocampal signaling cascades and behavioral impairments in 30-day-old rat pups prenatally exposed to methylmercury (MeHg). Pregnant rats were exposed to 1.0 or 2.0 mg/kg MeHg by gavage in alternated days from gestational day 5 until parturition. We found increased anxiety-like and decreased exploration behavior evaluated by open field test and deficit of both short- and long-term memories by novel object recognition task, respectively, in MeHg-treated pups. Downregulated PI3K/Akt/mTOR pathway and activated/hypophosphorylated (Ser9) GSK3β in MeHg-treated pups could be upstream of hyperphosphorylated Tau (Ser396) destabilizing microtubules and contributing to neural dysfunction in the hippocampus of these rats. Hyperphosphorylated/activated p38MAPK and downregulated phosphoErk1/2 support a role for mitogen-activated protein kinase (MAPK) cascade on MeHg neurotoxicity. Decreased receptor of advanced glycation end products (RAGE) immunocontent supports the assumption that downregulated RAGE/Erk1/2 pathway could be involved in hypophosphorylated lysine/serine/proline (KSP) repeats on neurofilament subunits and disturbed axonal transport. Downregulated myelin basic protein (MBP), the major myelin protein, is compatible with dysmyelination and neurofilament hypophosphorylation. Increased glial fibrillary acidic protein (GFAP) levels suggest reactive astrocytes, and active apoptotic pathways BAD/BCL-2, BAX/BCL-XL, and caspase 3 suggest cell death. Taken together, our findings get light on important signaling mechanisms that could underlie the behavioral deficits in 30-day-old pups prenatally exposed to MeHg.

Developmental toxicity from exposure to various forms of mercury compounds in medaka fish (Oryzias latipes) embryos

This study examined developmental toxicity of different mercury compounds, including some used in traditional medicines. Medaka (Oryzias latipes) embryos were exposed to 0.001–10 µM concentrations of MeHg, HgCl2, α-HgS (Zhu Sha), and β-HgS (Zuotai) from stage 10 (6–7 hpf) to 10 days post fertilization (dpf). Of the forms of mercury in this study, the organic form (MeHg) proved the most toxic followed by inorganic mercury (HgCl₂), both producing embryo developmental toxicity. Altered phenotypes included pericardial edema with elongated or tube heart, reduction of eye pigmentation, and failure of swim bladder inflation. Both α-HgS and β-HgS were less toxic than MeHg and HgCl₂. Total RNA was extracted from survivors three days after exposure to MeHg (0.1 µM), HgCl₂ (1 µM), α-HgS (10 µM), or β-HgS (10 µM) to examine toxicity-related gene expression. MeHg and HgCl₂ markedly induced metallothionein (MT) and heme oxygenase-1 (Ho-1), while α-HgS and β-HgS failed to induce either gene. Chemical forms of mercury compounds proved to be a major determinant in their developmental toxicity.

Preliminary study of blood methylmercury effects on reproductive hormones and relevant factors among infertile and pregnant women in Taiwan

Methylmercury (MeHg) is the most poisonous mercury species and an endocrine-disrupting chemical that could cause reproductive and developmental harm effects in animals. In this study, we recruited 310 infertile women and 57 pregnant women and investigated their blood MeHg levels. The distribution of blood reproductive hormone, selenium and zinc levels, and the difference of relevant factors by the reference level of blood MeHg (5.8 μg/L) of infertile women were further examined. Results showed that greater percentages of sashimi consumption, frequencies of Chinese herbal medicine use, alcohol consumption, and lack of physical activity were observed in infertile women than those for pregnant women. Blood MeHg concentration was significantly greater in infertile than that in pregnant women. Significant concentration differences for FSH and LH by the dichotomized reference level of blood MeHg (5.8 μg/L) in infertile women were not observed, which may stem from that these reproductive hormones in participated infertile women were mostly in the normal reference range. Consumption of fish and sashimi represented the major source of MeHg exposure in infertile women. MeHg levels were elevated in infertile women, and consistent with fish consumption frequency. Compared to the referent level of blood MeHg levels <5.8 μg/L, the elevated blood MeHg levels (⩾5.8 μg/L) in infertile women were 3.35 and 4.42 folds risk in categorized frequencies of fish consumption 1-2 meals per week and more than 3 meals per week, respectively. The obtained results provide evidences and help updating the advisory of fish consumption and improving women's reproductive health.