Methylmercury-Induced Toxicopathologic Findings in Salivary Glands of Offspring Rats After Gestational and Lactational Exposure

Atrophy and Inflammatory Changes in Salivary Glands Induced by Oxidative Stress after Exposure to Drugs and Other Chemical Substances: A Systematic Review

Background and Objectives: Oxidative stress is involved in the alterations at the level of salivary glands, being the cause of oral pathologies like xerostomia, periodontitis, gingivitis, leucoplakia, and cancer. It is known that antioxidants can reverse changes induced by drugs or other chemicals in some organs, but the question is whether these substances can reduce or revert the effects of oxidative stress at the salivary gland level. Our aim was to find histopathological data at the level of salivary glands supporting the hypothesis of the reversal of oxidative stress-induced changes after the treatment with substances with antioxidant effect. Materials and Methods: A systematic search was conducted in PubMed, Science Direct, and Springer databases, including research articles on oxidative stress histological aspects and oxidative stress biomarkers induced by drugs or other chemicals on salivary glands. Results: Out of 1756 articles, 25 articles were selected with data on tissue homogenate used for biochemical analysis of oxidative and antioxidative markers, along with routine hematoxylin eosin (HE) and immunohistochemical analysis used for histopathological and immunohistochemical diagnosis. Drugs (antineoplastic drugs, antibiotics, and analgesics), alcohol, heavy metals, and fluoride can cause oxidative stress, resulting in morphological changes in different tissues, including in salivary glands. There are many antioxidants but only a few were evaluated regarding the effects on salivary glands in animal studies, such as hesperidin and selenium, which can reverse the damage induced by cyclophosphamide; 10-dehydrogingerdione (10-DHGD), a compound extracted from ginger, which has a protective effect against the oxidative stress and apoptosis induced by tramadol; and glycyrrhizic acid, which may repair the injuries incurred after the administration of sodium nitrite. Conclusions: Substances such as hesperidin, selenium, 10-dehydrogingerdione, and glycyrrhizic acid are antioxidants with proven restorative effects on salivary glands for the damage induced by oxidative stress after exposure to drugs and other chemical substances; however, demonstrating their similar effects in human salivary glands is challenging.

Maternal methylmercury exposure during early-life periods adversely affects mature enamel structure of offspring rats at human exposure levels: a concern for oral health

Although there are many studies on the health effects of methylmercury (MeHg) toxicity during in utero and early development, little is known about its effects on mineralized tissues present in the oral cavity, such as enamel structure. Therefore, this study evaluated the effects of MeHg exposure on the physico-chemical, ultrastructural and functional properties of mature tooth enamel. Specifically, we studied offspring of mothers exposed to MeHg during the prenatal and postnatal periods which are the developmental stages associated with tooth enamel formation. Female rats were exposed to MeHg at a dose of 40 μg/kg/day for 42 days of pregnancy and lactation. The enamel of offspring was analyzed by (1) Fourier Transform Infrared Spectroscopy and Raman to assess physicochemical composition, (2) Scanning Electron Microscopy for ultrastructural evaluation, (3) Transmitted Polarizing Light Microscopy for analysis of the enamel extracellular matrix, and (4) resistance and hardness were evaluated by microhardness. The results showed that MeHg exposure during this sensitive enamel formation period induced changes in inorganic and organic content and enamel prisms ultrastructure alterations and disturbed the organic extracellular matrix due to a decreased enamel strength. These novel findings establish for the first time that maternal exposure to MeHg pre and postnatal promoted relevant changes in mature enamel of their offspring rats.

In utero and lactational exposure to methylmercury elicits physical-chemical and morphological damages in the alveolar bone of offspring rats: The first toxicological findings

Methylmercury (MeHg) is the most common organic form of mercury (Hg) that humans are exposed and is considered an environmental pollutant. Several populations that live in endemic regions of MeHg exposure are subject to the toxicant for long periods, including pregnant women and children, causing damage to several organs during early periods of development. Alveolar bone is an essential structure for the oral cavity, responsible for supporting teeth and masticatory forces. However, evidence on the effects of MeHg on alveolar bone and the intrauterine and lactation period is lacking. Thus, this study aimed to investigate the effects of MeHg exposure during gestation and lactation on the developing alveolar bone of offspring rats after maternal exposure. Dams were exposed during 41 days of pregnancy and lactation, and the mandibles of the offspring were collected. The alveolar bone was analyzed by Fourier Transform Infrared Spectroscopy to evaluate the physicochemical composition; by Scanning Electron Microscopy for ultrastructural evaluation; by histopathological, histochemical, and morphometric for tissue analyses. In addition, bone quality was assessed by X-ray microtomography. MeHg exposure altered the mineral composition and caused histological damage associated with a lower quantity and thickness of bone trabeculae, as well as reduced osteocyte density and collagen fiber content. A reduction in trabecular thickness and bone volume and an increase in trabecular spaces were observed and were associated with anatomical compromise of the vertical bone dimensions. Thus, the results suggest that the developing alveolar bone is susceptible to the toxic effects of MeHg when organisms are exposed during intrauterine and lactation periods. From a translational perspective, these changes in the alveolar bone can help us understand possible abnormalities induced by toxic metals and highlight the need for care for structures other than those already seen as targets for damage triggered by environmental MeHg exposure.

DNA Damage and Proteomic Profile Changes in Rat Salivary Glands After Chronic Exposure to Inorganic Mercury

Mercury (Hg) is a toxic metal that became a public health problem due to environmental contamination caused by anthropogenic activity. In this sense, oral homeostasis can undergo changes due to the toxic effects of metal on the salivary glands. Therefore, our objective was to investigate the proteomic and genotoxic changes in salivary glands after exposure to inorganic mercury (IHg). Forty Wistar rats that were divided into a control group, which received distilled water, and an exposed group, which received 0.375 mg/kg of mercury chloride for 45 days via orogastric gavage. After that, the animals were euthanized, and the parotid and submandibular glands were collected for analysis of the genotoxic effects, using the comet assay and proteome global profile assessment. The results showed that IHg promoted damage to cellular DNA associated with proteomic changes that showed events such as oxidative stress, mitochondrial dysfunction, changes in the cytoskeleton, and apoptosis. Therefore, these findings show a profile of molecular changes due to the interactions of IHg with several proteins and mechanisms inherent to the cell, which consequently may result in dysfunction of the salivary glands and impaired homeostasis of the oral cavity.

Maternal Fluoride Exposure Exerts Different Toxicity Patterns in Parotid and Submandibular Glands of Offspring Rats

There is currently a controversial and heated debate about the safety and ethical aspects of fluoride (F) used for human consumption. Thus, this study assessed the effects of prenatal and postnatal F exposure of rats on the salivary glands of their offspring. Pregnant rats were exposed to 0, 10, or 50 mg F/L from the drinking water, from the first day of gestation until offspring weaning (42 days). The offspring rats were euthanized for the collection of the parotid (PA) and submandibular (SM) glands, to assess the oxidative biochemistry and to perform morphometric and immunohistochemical analyses. F exposure was associated with a decrease in the antioxidant competence of PA in the 10 mg F/L group, contrasting with the increase observed in the 50 mg F/L group. On the other hand, the antioxidant competence of the SM glands was decreased at both concentrations. Moreover, both 10 and 50 mg F/L groups showed lower anti-α-smooth muscle actin immunostaining area in SM, while exposure to 50 mg F/L was associated with changes in gland morphometry by increasing the duct area in both glands. These findings demonstrate a greater susceptibility of the SM glands of the offspring to F at high concentration in comparison to PA, reinforcing the need to adhere to the optimum F levels recommended by the regulatory agencies. Such findings must be interpreted with caution, especially considering their translational meaning.

Methylmercury Causes Neurodegeneration and Downregulation of Myelin Basic Protein in the Spinal Cord of Offspring Rats after Maternal Exposure

Methylmercury (MeHg) is one of the most dangerous toxic pollutants spread throughout the earth. Chronic MeHg intoxication by contaminated food ingestion is the most common threat to human health, including impairment to the developing fetus. The present study aims at investigating the effects of maternal exposure to MeHg during gestation and lactation on the spinal cord of offspring. Pregnant rats received oral doses of MeHg (40 μg/kg/day) over a period of 42 days (21 gestation and 21 lactation). Control animals received the vehicle only. Total mercury concentration was measured in blood samples from offspring collected at the 41st postnatal day. Counting of motor neurons and immunoreactivity for myelin basic protein (MBP) were assessed in the spinal cords in both control and MeHg-intoxicated animals. Our results showed that MeHg promoted an increase in blood Hg levels. In addition, it caused a reduction in the number of spinal cord motor neurons as well as decreased MBP immunoreactivity in the cervical, thoracic and lumbar segments. Our present findings suggest that MeHg intoxication during rat pregnancy and lactation is associated with a pattern of motor neuron degeneration and downregulation of myelin basic protein in different segments of a developing spinal cord. Further studies are needed to establish the effect of MeHg intoxication in both young and adult rats.

Salivary Glands after Prolonged Aluminum Exposure: Proteomic Approach Underlying Biochemical and Morphological Impairments in Rats

Aluminum (Al) is one of the most abundant elements on Earth, and its high extraction rate and industrial use make human exposure very common. As Al may be a human toxicant, it is important to investigate the effects of Al exposure, mainly at low doses and for prolonged periods, by simulating human exposure. This work aimed to study the effects of low-dose exposure to chloride aluminum (AlCl₃) on the oxidative biochemistry, proteomic profile, and morphology of the major salivary glands. Wistar male rats were exposed to 8.3 mg/kg/day of AlCl₃ via intragastric gavage for 60 days. Then, the parotid and submandibular glands were subjected to biochemical assays, proteomic evaluation, and histological analysis. Al caused oxidative imbalance in both salivary glands. Dysregulation of protein expression, mainly of those related to cytoarchitecture, energy metabolism and glandular function, was detected in both salivary glands. Al also promoted histological alterations, such as acinar atrophy and an increase in parenchymal tissue. Prolonged exposure to Al, even at low doses, was able to modulate molecular alterations associated with morphological impairments in the salivary glands of rats. From this perspective, prolonged Al exposure may be a risk to exposed populations and their oral health.

Methylmercury exposure during prenatal and postnatal neurodevelopment promotes oxidative stress associated with motor and cognitive damages in rats: an environmental-experimental toxicology study

The environmental contamination by methylmercury (MeHg) is a major concern for public health. The effects of MeHg in the central nervous system (CNS) of adult animals have been extensively investigated; however, little is known about the effects of MeHg exposure during intrauterine and lactation periods on motor and cognitive functions of adolescent rats. Therefore, this study aimed to investigate the effect of MeHg exposure during intrauterine life and lactation on both motor and cognitive functions of offspring rats. Ten female Wistar rats were exposed to 40 μg/kg/day of MeHg through cookie treats from the first day of pregnancy until the last day of breastfeeding. Both motor and cognitive functions of offspring male rats were assessed by open field, rotarod, and step-down inhibitory avoidance tests. Forty-one days after birth, the hippocampus and cerebellum were collected to determine total Hg content, antioxidant capacity against peroxyl radicals (ACAP), reduced glutathione (GSH) levels, lipid peroxidation (LPO), and nitrite levels. MeHg exposure during CNS development increased Hg levels in both hippocampal and cerebellar parenchymas, triggered oxidative stress throughout ACAP and GSH decrease, increased LPO and nitrite levels. These alterations resulted in reduced spontaneous and stimulated locomotion and short- and long-term memory deficits. Therefore, damages triggered by MeHg exposure during intrauterine life and lactation had detrimental effects on oxidative biochemistry and motor and cognitive functions of offspring rats.

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The MeHg exposure during CNS development increased mercury levels in hippocampal and cerebellar parenchyma.

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The MeHg intoxication during pregnancy and lactation impairs the redox status of hippocampus and cerebellum of the offspring.

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MeHg exposure causes behavioral effects in motor ability and cognition of offspring rats.

From Molecules to Behavior in Long-Term Inorganic Mercury Intoxication: Unraveling Proteomic Features in Cerebellar Neurodegeneration of Rats

Mercury is a severe environmental pollutant with neurotoxic effects, especially when exposed for long periods. Although there are several evidences regarding mercury toxicity, little is known about inorganic mercury (IHg) species and cerebellum, one of the main targets of mercury associated with the neurological symptomatology of mercurial poisoning. Besides that, the global proteomic profile assessment is a valuable tool to screen possible biomarkers and elucidate molecular targets of mercury neurotoxicity; however, the literature is still scarce. Thus, this study aimed to investigate the effects of long-term exposure to IHg in adult rats’ cerebellum and explore the modulation of the cerebellar proteome associated with biochemical and functional outcomes, providing evidence, in a translational perspective, of new mercury toxicity targets and possible biomarkers. Fifty-four adult rats were exposed to 0.375 mg/kg of HgCl₂ or distilled water for 45 days using intragastric gavage. Then, the motor functions were evaluated by rotarod and inclined plane. The cerebellum was collected to quantify mercury levels, to assess the antioxidant activity against peroxyl radicals (ACAPs), the lipid peroxidation (LPO), the proteomic profile, the cell death nature by cytotoxicity and apoptosis, and the Purkinje cells density. The IHg exposure increased mercury levels in the cerebellum, reducing ACAP and increasing LPO. The proteomic approach revealed a total 419 proteins with different statuses of regulation, associated with different biological processes, such as synaptic signaling, energy metabolism and nervous system development, e.g., all these molecular changes are associated with increased cytotoxicity and apoptosis, with a neurodegenerative pattern on Purkinje cells layer and poor motor coordination and balance. In conclusion, all these findings feature a neurodegenerative process triggered by IHg in the cerebellum that culminated into motor functions deficits, which are associated with several molecular features and may be related to the clinical outcomes of people exposed to the toxicant.

R. Buzalaf M, Crespo-Lopez ME, Lima RR. Maternal methylmercury exposure changes the proteomic profile of the offspring's salivary glands: Prospects on translational toxicology

Background

Methylmercury (MeHg) remains a public health issue since developing organisms are particularly vulnerable to this environmental contaminant. This study investigated the effect of maternal MeHg exposure on the modulation of proteomic profile of parotid (PA), submandibular (SM), and sublingual (SL) glands of offspring rats.

Materials and methods

Pregnant Wistar rats were daily exposed to 40 μg/kg MeHg during both gestational and lactation periods. The proteomic profiles of the major salivary glands of the offspring rats were analyzed through mass spectrometry.

Results

The offspring rats exposed to MeHg showed significant alterations in the proteomic profiles of the PA, SM, and SL glands. Altered proteins were associated with cytoskeleton components, tissue morphogenesis, and response to stimulus and stress.

Conclusion

This original study showed that maternal MeHg exposure significantly modulates the expression of proteins and induces alterations in the proteomic profiles of developing salivary glands.

Non-Lethal Concentration of MeHg Causes Marked Responses in the DNA Repair, Integrity, and Replication Pathways in the Exposed Human Salivary Gland Cell Line

In Brazilian northern Amazon, communities are potentially exposed and vulnerable to methylmercury (MeHg) toxicity through the vast ingestion of fish. In vivo and in vitro studies demonstrated that the salivary glands as a susceptible organ to this potent environmental pollutant, reporting alterations on physiological, biochemical, and proteomic parameters. However, the alterations caused by MeHg on the gene expression of the exposed human salivary gland cells are still unknown. Therefore, the goal was to perform the transcriptome profile of the human salivary gland cell line after exposure to MeHg, using the microarray technique and posterior bioinformatics analysis. The cell exposure was performed using 2.5 µM MeHg. A previously published study demonstrated that this concentration belongs to a range of concentrations that caused biochemical and metabolic alterations in this linage. As a result, the MeHg exposure did not cause lethality in the human salivary gland cells line but was able to alter the expression of 155 genes. Downregulated genes (15) are entirety relating to the cell metabolism impairment, and according to KEGG analysis, they belong to the glycosphingolipid (GSL) biosynthesis pathway. On the other hand, most of the 140 upregulated genes were related to cell-cycle progression, DNA repair, and replication pathway, or cellular defenses through the GSH basal metabolism. These genomic changes revealed the effort to the cell to maintain physiological and genomic stability to avoid cell death, being in accordance with the nonlethality in the toxicity test. Last, the results support in-depth studies on nonlethal MeHg concentrations for biomarkers identification that interpret transcriptomics data in toxicological tests serving as an early alert of physiological changes in vitro biological models.