The Expression and Significance of Metallothioneins in Murine Organs and Tissues Following Mercury Vapour Exposure

Ultrastructural Demonstration of Mercury Granules in the Placenta of Metallothionein-Null Pregnant Mice after Exposure to Mercury Vapor

The placenta plays an important role in the regulation of maternal to fetal transfer of toxic substances, including nonessential metals. Metallothioneins (MTs), which are known to have protective effects against heavy metal toxicity, exist in the placenta, but the exact localization of placental MTs (both MT-I and MT-III) and their physiological role in the placenta exposed to mercury are unclear. The present study was performed to examine the localization of MTs and mercury granules in the placenta of mice exposed to mercury vapor. On gestational day 16, MT-I & II-null and wild-type mice were exposed to mercury vapor at 4.9 to 5.9 mg/m3 for 2 hours. At 24 and 48 hours after exposure, the placentas were examined for mercury distribution (autometallography), MT immunoreactivity, and MT mRNA expression (in situ hybridization). No histological changes were observed in the placentas of either MT-null or wild-type mice. Mercury deposition was demonstrated along the boundary between the junctional zone and the labyrinth zone, as well as in the yolk sac, maternal decidual cells, and labyrinth trophoblasts of both MT-null and wild-type mice. MT-I & -II immunoreactivity, which was confined to wild-type mice, was demonstrated in the yolk sac and decidual cells; mercury was also shown in both structures, suggesting that mercury granules were bound to MTs. MT-III mRNA expression was observed in the yolk sac, decidual cells, and spongiotrophoblasts in both MT-null and wild-type mice. There was, however, no evidence of MT at the boundary between the junctional and labyrinth zones, where substantial mercury deposits were demonstrated. These results suggest that placental MTs and the other unknown molecules may be related to the barrier to the placental transfer of mercury.

Metallothionein-1 and nitric oxide expression are inversely correlated in a murine model of Chagas disease

Chagas disease, caused by Trypanosoma cruzi, represents an endemic among Latin America countries. The participation of free radicals, especially nitric oxide (NO), has been demonstrated in the pathophysiology of seropositive individuals with T. cruzi. In Chagas disease, increased NO contributes to the development of cardiomyopathy and megacolon. Metallothioneins (MTs) are efficient free radicals scavengers of NO in vitro and in vivo. Here, we developed a murine model of the chronic phase of Chagas disease using endemic T. cruzi RyCH1 in BALB/c mice, which were divided into four groups: infected non-treated (Inf), infected N-monomethyl-L-arginine treated (Inf L-NAME), non-infected L-NAME treated and non-infected vehicle-treated. We determined blood parasitaemia and NO levels, the extent of parasite nests in tissues and liver MT-I expression levels. It was observed that NO levels were increasing in Inf mice in a time-dependent manner. Inf L-NAME mice had fewer T. cruzi nests in cardiac and skeletal muscle with decreased blood NO levels at day 135 post infection. This affect was negatively correlated with an increase of MT-I expression (r = -0.8462, p < 0.0001). In conclusion, we determined that in Chagas disease, an unknown inhibitory mechanism reduces MT-I expression, allowing augmented NO levels.

Mercury in the spinal cord after inhalation of mercury

Amyotrophic lateral sclerosis (ALS) affects anterior horn cells of the spinal cord causing an indolent slow and steady deterioration of muscle strength leading inevitably to death in respiratory failure. ALS is a model condition for neurodegenerative disorders. Exposure to different agents dispersed in the environment has been suggested to cause neurodegeneration but no convincing evidence for such a link has yet been presented. Respiratory exposure to metallic mercury (Hg(0)) from different sources may be suspected. Body distribution of metallic mercury is fast and depends on solubility properties. Routes of transport, metabolism, excretion and biological half-life determine the overall toxic effects. Inhalation experiments were performed in 1984 where small marmoset monkeys (Callithrix jacchus) were exposed to (203) Hg(0 vapour) mixed into the breathing air (4-5 μg/l). After 1 hr of exposure, they were killed and whole body autoradiograms prepared to study the distribution of mercury within organs. Autoradiograms showed that Hg was deposited inside the spinal cord. Areas of enhanced accumulation anatomically corresponding to motor nuclei could be observed. This study describes a reinvestigation, with new emphasis on the spinal cord, of these classical metal exposure data in a primate, focusing on their relevance for the causation of neurodegenerative disorders. A comparison with more recent rodent experiments with similar findings is included. The hypothesis that long-time low-dose respiratory exposure to metals, for example, Hg, contributes to neurodegenerative disorders is forwarded and discussed.

Atrophy of Large Myelinated Motor Axons and Declining Muscle Grip Strength Following Mercury Vapor Inhalation in Mice

The effects of acute mercury vapor (Hg(0)) exposure on the peripheral motor system have not been previously addressed in the literature. Early case studies report that acute exposure in humans can cause symptoms resembling motor neuron disease (MND). Mercury granules can be histochemically demonstrated in the cytoplasm of murine motor neurons following Hg(0) exposure, suggesting it is transported from the neuromuscular junction (NMJ) to the cell body by retrograde axonal transport mechanisms. We considered the hypothesis that morphological damage to the peripheral motor axonal cytoskeleton possibly involving neurofilaments (NFs) follows Hg(0) exposure. Eight-week-old wild type (Wt) 129S/v mice were exposed to 500 microg/m(3) of Hg(0) for 4 h in an experimental vapor exposure chamber. Forelimb grip strength (FGS) was measured over 4-wk intervals prior to removal of the murine phrenic nerves (MPN) 7 mo postexposure. Autometallography of 7-microm-thick spinal-cord sections from Hg(0)-exposed mice confirmed the presence of mercury deposits in ventral horn motor neurons. The morphology of the myelinated motor axons was assessed by computer-assisted image analysis of 1-microm-thick resin cross sections of the MPN. The group exposed to Hg(0) showed a significant reduction in the mean axon caliber, p < .0001. Gaussian spectral analysis of axon diameter distribution showed atrophy principally to large myelinated fibers, a subpopulation of axons that is also affected in MND. This atrophic change was also accompanied by an increased irregularity in axon shape. FGS initially increased with age until 20 wk and then progressively decreased after 22 wk to 36 wk. In conclusion, Hg(0) exposure appears to reduce axon diameter, suggesting axon caliber-determining cytoskeletal components such as neurofilaments may be damaged by heavy metal-induced oxidative stress mechanisms, resulting in functional changes to motor units.

Induction of reactive oxygen species and apoptosis in BEAS-2B cells by mercuric chloride

Mercury is a widespread environmental and industrial pollutant that induces serious adverse effects in both humans and the environment. However, the toxicities and its mechanisms have not been fully elucidated. Among the proposed mechanisms of biological toxicities, the intracellular level of thiol group (-SH) and oxidative stress have been widely studied. In this study, production of reactive oxygen species (ROS) by mercuric chloride (2, 4, 6, and 8 ppm as of mercury) was investigated in cultured human bronchial epithelial cells (BEAS-2B cell line). Exposure of cultured cells to mercuric chloride led to cell death, ROS increase, and cytosolic caspase-3 activation. The ROS increase was related to the decreased level of GSH. Chromatin condensation evaluated by 4',6-diamidino-2-phenylindole (DAPI) staining were also shown in mercury-treated cells and this suggest the apoptotic process of cells by mercuric chloride.

Metallothioneins I and II: Neuroprotective significance during CNS pathology

Metallothioneins (MTs) constitutes a superfamily of highly conserved, low molecular weight polypeptides, which are characterized by high contents of cysteine (sulphur) and metals. As intracellular metal-binding proteins they play a significant role in the regulation of essential metals. The major isoforms of the protein (MT-I and MT-II) are induced by numerous stimuli and pathogens but most importantly their induction by metals is closely linked to the physiological metabolism of zinc and protection from the toxic affects following heavy metal exposure. Although the preservation of their genetic expression across animal phyla suggests that MTs may play an important physiological role, MT-I, II knock out (KO) mice survive to adulthood. In both central and peripheral nervous tissues, MT-I, II have neuroprotective roles, which are also induced by exogenous MT-I and/or MT-II treatment. Hence, MT-I, II may provide neurotherapeutic targets offering protection against neuronal injury and degeneration.

Decreased neurofilament density in large myelinated axons of metallothionein-I, II knockout mice

Metallothioneins (MTs) are small proteins, two isoforms (I, II) of which bind metals. Their physiological role has been difficult to establish, but recent reports suggested that they serve an important function in nerve repair and in the protection against oxidative stress in the peripheral nervous system. We previously reported a decreased axon calibre in the large myelinated fibres of the phrenic nerve in the MT-I, II double knock out (MT-I, II KO) mouse model. We propose that this could be due to the effects of oxidative stress on neurofilaments (NFs). In this study, we examined the same subset of large myelinated axons using transmission electron microscopy (TEM). There was a decreased NF density in the axons of MT-I, II KO phrenic nerve (P<0.005). This observation may have novel therapeutic implications in the treatment of amyotrophic lateral sclerosis (ALS), particularly as the terminal phases of the disease involve respiratory insufficiency.

The experimental toxicology of metallic mercury on the murine peripheral motor system: a novel method of assessing axon calibre spectra using the phrenic nerve

The toxicology of metallic mercury on motor neurons and their processes requires further work to resolve controversial implications in the aetiology of human motor neuron disease (MND). The assessment of experimental neurotoxicity in the peripheral motor system is, however, technically problematic and difficult to interpret. The mean number of axons in a nerve can vary considerably due to a high degree of biological variation. Atrophy of large axons can appear as loss when, in fact, their numbers appear in smaller diameter axonal categories. We addressed these quantitative problems using the murine phrenic nerve (MPN), a mono-fascicular, predominantly motor nerve as a model system. One micrometer transverse sections of gluteraldehyde/osmium tetroxide fixed MPNs were stained for myelin using a silver technique. Axon areas were measured from digital images of the nerve in cross-section (ImagePro Plus software) and transformed to circular diameter equivalents, then displayed as frequency distributions. We found a high biological variation in the mean axon number between paired nerves within experimental groups. Therefore, axon diameter data within individuals group was pooled. Theoretical simulation of axonal degeneration, atrophy and hypertrophy of larger myelinated axons (also affected in MND) were modelled by manipulating the original data set. With this model, by comparing normal distributions, it is possible to distinguish axonal atrophy, degenerative loss, and hypertrophy as distinct pathological processes in the large calibre axon subgroup that are selectively vulnerable to metallic toxins such as mercury.

Atrophy of Large Myelinated Axons in Metallothionein-I, II Knockout Mice

1. Metallothioneins (MTs) are small metal-binding proteins. The genes that encode MT isoforms I and II are also induced by metal at transcription level. Commonly expressed in the central nervous system (CNS) their putative function is protection against reactive oxygen species (ROS), however their role may not be restricted to this sole purpose. The physiological function of MTs in the peripheral nervous system (PNS) requires further investigation. 2. Examination of phrenic nerve cross-sections from MT-I and MT-II double knockout mice (MT-I, II KO) showed a significant reduction (P=0.0032) in the mean myelinated axons calibre compared to 129/Sv wild type (Wt) counterparts. 3. Analysis of the Gaussian spectra specifically attributes this atrophy to the large myelinated class (>or=4 microm) of axon considered selectively vulnerable in motor neuron disease (MND). Supporting the results, these axons also showed increased irregularity in shape. 4. In conclusion, MTs directly or indirectly influence the radial equilibrium of large myelinated motor axons.

Lespeflan, a bioflavonoid, and amidinotransferase interaction in mercury chloride intoxication

Amidinotransferase (transamidinase, L-arginine: glycine amidinotransferase, EC 2.1.4.1) is an enzyme that catalyses the first step in creatine synthesis primarily in the kidney and pancreas. The kidney is also the primary target organ for the toxic effect of mercury. Therefore, we studied the effect of acute uremic syndrome on enzyme activity induced by mercury chloride. Because of the potential beneficial effect of bioflavonoids, we have investigated the effects of the bioflavonoid lespeflan on acute uremic syndrome and amidinotransferase activity. Male Spraque Dawley rats weighing about 200 g were used in this study. Acute renal failure was induced by intraperitoneally (i.p.) administration of mercury chloride in a dose of 3 mg/kg. One group of animals was given lespeflan (1.0 mL/kg) 1 hr before mercury chloride administration. Urea and creatinine levels in blood plasma were significantly elevated 48 hr after the induction of acute uremic syndrome (p< 0.001). Kidney transamidinase activity was decreased compared to the control group (p<0.001). Pretreatment by lespeflan potentiates the inhibitory effect of mercury chloride on enzyme activity. We discussed mechanisms of transamidinase inhibition and point thiol group of cysteine forming thiol-conjugates on enzyme inhibition both by mercury and lespeflan.