Food intake, body weight, and brain histopathology in mice following chronic methylmercury treatment

Association between Heavy Metals, Metalloids and Metabolic Syndrome: New Insights and Approaches

Metabolic syndrome (MetS) is an important public health issue that affects millions of people around the world and is growing to pandemic-like proportions. This syndrome is defined by the World Health Organization (WHO) as a pathologic condition characterized by abdominal obesity, insulin resistance, hypertension, and hyperlipidemia. Moreover, the etiology of MetS is multifactorial, involving many environmental factors, including toxicant exposures. Several studies have associated MetS with heavy metals exposure, which is the focus of this review. Environmental and/or occupational exposure to heavy metals are a major risk, contributing to the development of chronic diseases. Of particular note, toxic metals such as mercury, lead, and cadmium may contribute to the development of MetS by altering oxidative stress, IL-6 signaling, apoptosis, altered lipoprotein metabolism, fluid shear stress and atherosclerosis, and other mechanisms. In this review, we discuss the known and potential roles of heavy metals in MetS etiology as well as potential targeted pathways that are associated with MetS. Furthermore, we describe how new approaches involving proteomic and transcriptome analysis, as well as bioinformatic tools, may help bring about an understanding of the involvement of heavy metals and metalloids in MetS.

Methylmercury Induces Metabolic Alterations in Caenorhabditis elegans: Role for C/EBP Transcription Factor

Methylmercury (MeHg) is a well-known neurotoxicant; however, its role in metabolic diseases has been gaining wider attention. We have previously shown that MeHg causes metabolic alterations in Caenorhabditis elegans, leading to decreased nicotinamide adenine dinucleotide cofactor, mitochondrial dysfunction, and oxidative stress. We were, therefore, interested in whether MeHg also affects nutrient metabolism, particularly lipid homeostasis, which may contribute to the development of metabolic conditions such as obesity or metabolic syndrome (MS). RNA from wild-type worms exposed to MeHg was collected immediately after treatment and used for gene expression analysis by DNA microarray. MeHg differentially regulated 215 genes, 17 genes involved in lipid homeostasis, and 12 genes involved in carbohydrate homeostasis. Of particular interest was cebp-1, the worm ortholog to human C/EBP, a pro-adipogenic transcription factor implicated in MS. MeHg increased the expression of cebp-1 as well as pro-adipogenic transcription factors sbp-1 and nhr-49, triglyceride synthesis enzyme acl-6, and lipid transport proteins vit-2 and vit-6. Concurrent with the altered gene expression, MeHg increased triglyceride levels, lipid storage, and feeding behaviors. Worms expressing mutant cebp-1 were protected from MeHg-induced alterations in lipid content, feeding behaviors, and gene expression, highlighting the importance of this transcription factor in the worm's response to MeHg. Taken together, our data demonstrate that MeHg induces biochemical, metabolic, and behavioral changes in C. elegans that can lead to metabolic dysfunction.

Lipophilic compounds in garlic decrease the toxicity of methylmercury by forming sulfur adducts

Garlic (Allium sativum L.) contains numerous sulfur compounds. We have previously found that reactive sulfur species such as glutathione persulfide, glutathione polysulfide, protein-bound persulfides, and hydrogen sulfide can bind to methylmercury to give bismethylmercury sulfide, which is less toxic than methylmercury. It was not clear, however, whether such reactive sulfur species are present in garlic. The aim of the study presented here was to determine whether garlic contains reactive sulfur species that can bind to methylmercury. We extracted garlic with organic solvents and then performed silica gel column chromatography to separate constituents that could cause bismethylmercury sulfide to form. We found numerous garlic constituents could bind to methylmercury to form bismethylmercury sulfide. A hexane extract of garlic decreased methylmercury cytotoxicity in vitro and body weight loss in mice. The results suggest that ingesting garlic may decrease methylmercury toxicity by causing the formation of sulfur adducts that inhibit adverse reactions.

Chronic exposure to methylmercury enhances the anorexigenic effects of leptin in C57BL/6J male mice

Several studies have demonstrated that heavy metals disrupt energy homeostasis. Leptin inhibits food intake and decreases body weight through activation of its receptor in the hypothalamus. The impact of heavy metals on leptin signaling in the hypothalamus is unclear. Here, we show that the environmental pollutant, methylmercury (MeHg), favors an anorexigenic profile in wild-type males. C57BL/6J mice were exposed to MeHg via drinking water (5 ppm) up to 30 days. Our data shows that MeHg exposure was associated with changes in leptin induced activation of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway in the hypothalamus. In males, the activation of JAK2/STAT3 signaling pathway was sustained by an increase in SOCS3 protein levels. In females, MeHg-activated STAT3 was inhibited by a concomitant increase in PTP1B. Taken together, our data suggest that MeHg enhanced leptin effects in males, favoring an anorexigenic profile in males, which notably, have been shown to be more sensitive to the neurological effects of this organometal than females. A better understanding of MeHg-induced molecular mechanism alterations in the hypothalamus advances the understanding of its neurotoxicity and provides molecular sites for novel therapies.

Chronic exposure to methylmercury disrupts ghrelin actions in C57BL/6J mice

Methylmercury (MeHg) is a neurotoxic pollutant widely present in the environment. Initial symptoms of MeHg may include loss of body weight. However, the mechanisms by which MeHg induces body weight changes have yet to be fully elucidated. Body weight is regulated by multiple mechanisms. Whereas multiple peripheral peptides lead to food intake cessation, ghrelin is the only recognized peripheral hormone that stimulates food intake. It exerts its action on Neuropeptide Y/Agouti-related peptide neurons in the hypothalamus. To test if MeHg affects ghrelin signaling C57BL/6J mice (males and females) were exposed to 5 ppm MeHg via drinking water during a month. On days 15 and 30 of MeHg exposure ghrelin was administered intraperitoneally and changes in body weight and food intake were recorded. In addition, changes in ghrelin-induced signaling pathways in hypothalamus were also analyzed. Here, we show that in males, MeHg enhanced ghrelin-induced body weight gain by activating the AMP-activated Kinase (AMPK)/Uncoupled protein 2 (UCP2) signaling pathway. In contrast, in females, MeHg inhibited ghrelin-induced mTOR signaling activation and decreased Npy mRNA expression, thus mitigating the ghrelin-induced weight gain. Combined, our novel results demonstrate, for the first time, that MeHg disrupts the physiological functions of ghrelin differently in males and females.

Methylmercury Affects the Expression of Hypothalamic Neuropeptides That Control Body Weight in C57BL/6J Mice

Methylmercury (MeHg) is an environmental pollutant that affects primarily the central nervous system (CNS), causing neurological alterations. An early symptom of MeHg poisoning is the loss of body weight and appetite. Moreover, the CNS has an important role in controlling energy homeostasis. It is known that in the hypothalamus nutrient and hormonal signals converge to orchestrate control of body weight and food intake. In this study, we investigated if MeHg is able to induce changes in the expression of key hypothalamic neuropeptides that regulate energy homeostasis. Thus, hypothalamic neuronal mouse cell line GT 1-7 was treated with MeHg at different concentrations (0, 0.5, 1, and 5 µM). MeHg induced the expression of the anorexigenic neuropeptide pro-omiomelanocortin (Pomc) and the orexigenic peptide Agouti-related peptide (Agrp) in a concentration-dependent manner, suggesting deregulation of mechanisms that control body weight. To confirm these in vitro observations, 8-week-old C57BL/6J mice (males and females) were exposed to MeHg in drinking water, modeling the most prevalent exposure route to this metal. After 30-day exposure, no changes in body weight were detected. However, MeHg treated males showed a significant decrease in fat depots. Moreover, MeHg affected the expression of hypothalamic neuropeptides that control food intake and body weight in a gender- and dose-dependent manner. Thus, MeHg increases Pomc mRNA only in males in a dose-dependent way, and it does not have effects on the expression of Agrp mRNA. The present study shows, for first time, that MeHg is able to induce changes in hypothalamic neuropeptides that regulate energy homeostasis, favoring an anorexigenic/catabolic profile.

Methylmercuric chloride induces activation of neuronal stress circuitry and alters exploratory behavior in the mouse

Methylmercury (MeHg) is a well-known neurotoxicant, responsible for neurological and cognitive alterations. However, there is very little information available on the effects of MeHg administration on activation of murine neuronal pathways involved in the stress response, and whether this is altered as a function of repeated exposure to MeHg. Moreover, interactions between MeHg and other psychogenic and inflammatory stressors have yet to be fully determined. Acute i.p. exposure of male C57BL/6J mice to MeHg (2-8 mg/kg) dose-dependently attenuated exploratory behavior in the open field in the presence and absence of a novel object. In addition, increased numbers of c-Fos immunoreactive cells appeared in response to acute i.p. and i.c.v. MeHg within thalamic (anterior paraventricular nucleus of the thalamus (PVA)/posterior paraventricular nucleus of the thalamus (PV)), hypothalamic (paraventricular nucleus of the hypothalamus (PVN)), central amygdaloid nucleus (CeC), septal and hippocampal (dentate gyrus) nuclei, medial bed nucleus (BSTm) and the locus coeruleus (Lc). The increase in c-Fos positive cells in response to acute i.p. and i.c.v. MeHg did not appear to be influenced further by open field exposure. Repeated administration of MeHg led to an attenuation of most parameters of open field behavior altered by acute MeHg. However, increased c-Fos was significant in the CeC, Dg, supracapsular bed nucleus (BSTs), and Lc. Moreover, open field exposure after repeated treatments resulted in significant c-Fos responses in similar areas. Interestingly, 3 days after the final repeated MeHg dose (2 or 4 mg/kg) c-Fos increases to an immunogenic stressor (LPS) were not affected by MeHg pretreatment. These results demonstrate that systemic exposure to acute and repeated MeHg serves to activate the brain's stress circuitry, and furthermore appears to engage normal neuronal habituation processes.

Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

Widespread calcium deposits, as detected using the alizarin red S technique, in the nervous system of rats treated with dimethyl mercury

It has been reported that the alizarin red S technique may be used to visualize both intracellular and extracellular calcium deposits. Using this method histologic observations of the nervous system were made in rats that were given dimethyl mercury at 5 mg/kg per day for 12 consecutive days, and killed on days 1, 4, 7, 10, 12, 24, 32, 49, 100 and 140 (day 0 was the day that the final dose was administered). Neuronal degeneration with calcium deposition was found in the nervous system from day 4 onward. In the cerebellum alizarin red S-positive granules became gradually larger with time after dimethyl mercury administration, and large calcospherites were observed from day 32 onward. In contrast, the visualization of calcium deposits in the cerebral cortex was restricted to days 10-12. Calcium deposits were found in the ascending axons of the dorsal root ganglion neurons (dorsal fascicles of the spinal cord), but not in their perikarya. These findings suggest that widespread calcium deposition could occur in the nervous system following dimethyl mercury exposure, and that in the rat the mechanism of calcium deposition differs depending upon the brain region.

Distribution of bismuth in the brain after intraperitoneal dosing of bismuth subnitrate in mice: implications for routes of entry of xenobiotic metals into the brain

Bismuth (Bi) can produce neurotoxic effects in both humans and animals under certain dosing conditions, but little else is known about the effects of Bi in the brain. In the present study we determined the distribution of Bi in the brains of adult female Swiss-Webster mice 4, 7, 14, 21 and 28 days after a single 2500 mg/kg i.p. injection of Bi subnitrate (BSN), which establishes a depot of absorbable Bi and produces morphological signs of neurotoxicity. Sections of brains were processed by autometallographic (AMG) procedures that produced silver grains at the site of Bi localization (AMGBi). Ventricular dilation was observed in all BSN-dosed mice. Among treated mice there were marked interanimal differences in the absolute amount of AMGBi, but consistent regional and cellular patterns of AMGBi were observed. AMGBi was observed in many cell types in brain regions adjacent to fenestrated blood vessels of the circumventricular organs (CVOs) and olfactory epithelium. Prominent intrasomal AMGBi was observed in nuclei containing large cell bodies, including cranial motor neurons innervating somatic muscle, lateral vestibular and red nucleus and pontine/medullary reticular nuclei. In the hypothalamus, the supraoptic and paraventricular nuclei demonstrated the densest AMGBi. In the cerebellum, Purkinje and granule cell layers with the densest AMGBi were in folia adjacent to the fourth ventricle. In the hippocampus, AMGBi was densest in the fasciola cinerum, polymorph cells of the dentate gyrus, and pyramidal cell layer of the CA3 regions. Neuropil of subcortical auditory nuclei (cochlear nucleus, trapezoid body, lateral lemniscus and nucleus of lateral lemniscus, medial geniculate nucleus and inferior colliculus) had a high density of AMGBi. Among nonneuronal cells, ependyma and meninges lining the ventricular and subarachnoid spaces were labeled extensively. Glial labeling was prominent adjacent to CVOs, in subependymal regions, and in fiber tracts. Presumptive perivascular cells lining large blood vessels had extremely dense AMGBi as early as 4 days after dosing. Smaller blood vessels had moderate AMGBi. However, in regions (e.g. cerebral cortex, striatum) known to have low brain Bi levels after i.p. dosing, vascular deposits accounted for most of the AMGBi. Several animals had foci of AMGBi which suggested that vascular or perivascular aberrations may have contributed to the unusually dense accumulations. The results of the present studies indicate that Bi accumulates predictably in certain regions and cell types. The pattern of regions and cells with the highest AMGBi accumulations is very similar to pattern reported for other xenobiotic metals (i.e. mercury, silver, gold), and supports the hypothesis that these metals may share some mechanisms for entry, distribution and storage in the brain.

Cutaneous and auditory function in rats following methyl mercury poisoning

Rats were given a total dose of 50 mg/kg (Exp. 1), 13.3 or 40 mg/kg (Exp. 2), or 40 mg/kg (Exp. 3) of methyl mercury chloride subcutaneously over a course of 5 days. At varying times after the toxic exposure, up to 1 year, their sensory functioning was assessed by reflex modulation methods: stimuli of interest were presented just before an intense tone which elicited the startle reflex, and stimulus reception was measured by the inhibitory control of the stimuli over the amplitude of the reflex. In Experiment 1 cutaneous prestimuli (electric shock to the tail) and brief acoustic transients (silent periods in noise) were less effective inhibitors of reflex activity in poisoned animals, compared to controls, indicating that the poisoned animals had impairments in cutaneous sensitivity and audition. In Experiment 2 the time course of sensory loss and subsequent recovery was studied. Impaired auditory function was shown further by a deficit in the effectiveness of weak noise pulses, and, in addition, the cutaneous deficit for weak tail shocks was accompanied by an exaggerated or hyperpathic response to more intense tail shocks. Experiment 3 confirmed the finding that the loss of sensitivity to weak shock was accompanied by an enhancement of the response to more intense shock. These data were related to peripheral neuropathy and shown to be analogous to certain clinical symptoms of Minamata disease reported in humans.

Review of the health effects of methylmercury

The study critically reviews recent data relating to the health effects of methylmercury in man and the attendant dose-response relationships. New data obtained from animal studies, including pre-and postnatal exposure, are also examined. The consumption of fish and fish produce represents the major source of methylmercury exposure in the general population. Reported mercury concentrations in fish throughout the world are examined, particularly in the Mediterranean Sea. Here there is limited knowledge of methylmercury intake in critically exposed populations such as fishermen, employees of the fish industries and their families. The measurement of mercury in hair is now regarded as the most useful indicator of exposure but more experimental data are still required to increase the value of this index. The threshold levels of methylmercury in blood, hair and for dietary intake, as estimated by the World Health Organization, have been largely endorsed. However, new information from Japan and Canada suggests the existence of a latency period for some effects, so that the frequency or probability of their occurrence is inversely related to the duration of exposure. Incorporation of such findings would therefore lead to the designation of lower threshold values than are presently recognized. Pregnant women and the fetus have been identified as groups that are at special risk. The fetal blood mercury level is up to twice that of the mother and the sensitivity of both mother and fetus may be higher than in non-pregnant adults. This should be taken into account when assigning protective threshold concentrations.

Methyl mercury ototoxicity in mice determined by auditory brainstem responses

Mice of the C57 B1/6 strain were used to assess auditory deficits due to methyl mercury chloride intoxication. Auditory Brainstem Responses were obtained to pure-tone stimuli from 4 to 78 kHz. Physiologic thresholds were compared with behavioral methods for determining audibility in mice. Two dosage levels were studied; 4 and 8 mg/kg. Recordings were taken weekly, for each mouse, for 3 consecutive weeks after initial injection. Analysis of physiologic threshold and latency indicated that methyl mercury chloride causes auditory deficits at all frequencies tested with the greater effect in the higher frequencies. Additionally, latencies indicated nerve conduction hypersensitivity in the brainstem.

Assessment of the effects of acrylamide, methylmercury, and 2,5-hexanedione on motor functions in mice

Neurotoxic effects of acrylamide, methylmercury, and 2,5-hexanedione were studied in forth female BALB/c mice. The chemicals were dissolved in distilled water and administered via light-tight drinking bottles. Three control groups were used. The first received distilled water, the second received concentrated saccharin solution to assess the effects of reduced water intake, and the third was maintained on a reduced food diet. Motor functions were quantified by measuring landing foot-spread and rotarod performance. Baseline data were collected before dosing started. Mice were placed, twice weekly, on an accelerating rotarod, and their retention time was recorded. In the landing foot-spread test, the experimenter dropped mice from 15 cm onto a flat, smooth surface once a week. The hindlimb splay was then measured by the examiner. Both experimenter and examiner were unaware of the identity of each group (except of the food deprived group, in the case of the experimenter) during the first exposure. Decreased retention time and increased hindlimb splay were observed in mice after 12 d of exposure to acrylamide. Recovery followed treatment cessation. Increased hindlimb splay preceded an obvious decline of rotarod performance in the group receiving the 10 ppm of methylmercury solution. Mice receiving the 20 and 40 ppm of methylmercury solutions did not display any change in these tests before overt signs of toxicity. 2,6-Hexanedione produced a small decline in performance to a constant level after 85 d of exposure. After dosing termination, performance returned to baseline values. Control groups showed no change in performance on either the rotarod or the landing foot-spread test. Our data show that the rotarod and hindlimb splay tests in mice are about equal in sensitivity to the effects of the neurotoxic chemicals tested.

Distribution and biotransformation of methyl mercuric chloride in different tissues of mice

The distribution of 203Hg radioactivity has been studied in various organs of adult male and female mice from one hour to 21 days after treating with 203 Hg-labeled methyl mercuric chloride (MMC). The amount of methyl mercury (MeHg) and inorganic mercury (Hg) has also been determined by injecting single doses of non-radioactive MMC, and subsequently measuring total, organic and inorganic Hg content by atomic absorption technique. In addition, photoemulsion histochemical method (PEHM) was used to demonstrate localization of Hg grains in various cellular compartments of organs and tissues. The highest levels of radioactivity were attained at 7 hours post-treatment in all organs except for brain and testis. The testis showed the highest radioactivity at one day and the brain at two days post-treatment. MeHg persisted in brain over a longer period though the level was not as high. The content of MeHg and inorganic Hg was maximum in kidneys as compared to other organs. The brain and the reproductive organs contained the least amount of inorganic Hg. By PEHM, Hg grains were most prominently observed in the sinusoids, Kupfer cells, hepatic cells and bile duct epithelium of liver; in the lumen of blood vessels, convoluted and collecting tubules of kidneys; and in the gastrointestinal epithelium. The pattern of uptake and distribution of MeHg correlated well with the morphological demonstration of Hg grains in tissue sections.