Interaction of methylmercury with microtubules in cultured cells and in vitro

Characteristics of Abnormalities in Somatosensory Submodalities Observed in Residents Exposed to Methylmercury

Hundreds of thousands of people living along the Yatsushiro Sea coast have been exposed to methylmercury from the contaminated water of the Chisso factory in Minamata. The most common neurological disorder caused by methylmercury is somatosensory disturbance, but very few studies have been conducted in the world to determine its pathophysiology and origin, including the Japanese cases, which have produced numerous intoxicated individuals. We have already shown in previous studies the body part where the disorder occurs and that its cause is not peripheral nerve damage but damage to the parietal lobes of the cerebrum. We reanalyzed the results of subjective symptoms, neurological findings, and quantitative sensory measurements in 197 residents (63.2 ± 10.7 years old) from contaminated areas exposed to methylmercury from seafood and 130 residents (63.7 ± 9.3 years old) from control areas, the same subjects as in previous studies, to determine the characteristics of somatosensory disturbance in detail. The most commonly affected sensory modalities were superficial peripheral touch and pain in the extremities, followed by two-point discrimination and deep senses, and in the most severe cases, full-body sensory dysfunction and impairment of all sensory submodalities. The severity of sensory submodalities correlated with each other but not with peripheral nerve conduction test indices, further confirming the correctness of our assertion about the responsible foci of sensory disturbance. The health effects of chronic methylmercury toxicosis can be elucidated by a detailed examination of sensory deficits.

Astrocyte-Like Cells Transcriptome Changes After Exposure to a Low and Non-cytotoxic MeHg Concentration

The central nervous system is the main target of MeHg toxicity and glial cells are the first line of defense; however, their true role remains unclear. This study aimed to identify the global map of human glial-like (U87) cells transcriptome after exposure to a non-toxic and non-lethal MeHg concentration and to investigate the related molecular changes. U87 cells were exposed upon 0.1, 0.5, and 1 µM MeHg for 4 and 24 h. Although no changes were observed in the percentage of viable cells, the metabolic viability was significantly decreased after exposure to 1 µM MeHg for 24 h; thus, the non-toxic concentration of 0.1 µM MeHg was chosen to perform microarray analysis. Significant changes in U87 cells transcriptome were observed only after 24 h. The expression of 392 genes was down regulated while 431 genes were up-regulated. Gene ontology showed alterations in biological processes (75%), cellular components (21%), and molecular functions (4%). The main pathways showed by KEGG and Reactome were cell cycle regulation and Rho GTPase signaling. The complex mechanism of U87 cells response against MeHg exposure indicates that even a low and non-toxic concentration is able to alter the gene expression profile.

Oxygen as an important factor modulating in vitro MeHgCl toxicity associated with mitochondrial genes in hiPSCs

Mitochondria are energy factories of cells and important targets for methylmercury chloride (MgHgCl). Methylmercury (MeHg) is a well-known environmental toxicant that bioaccumulates in fish and shellfish. It readily crosses the placental barrier, making it a threat to correct fetal development. Despite being comprehensively investigated for years, this compound has not been assessed for its in vitro mitochondrial toxicity under different oxygen conditions. In this study, human induced pluripotent stem cells (hiPSCs) were used to evaluate the dependence of the expression of genes associated with pluripotency and mitochondria on atmospheric (21% O₂) and low (5% O₂) oxygen concentrations upon MeHgCl treatment. We showed that the toxicity of MeHgCl was strongly related to an increased mtDNA copy number and downregulation of the expression of an mtDNA replication and damage repair-associated gene POLG1 (Mitochondrial Polymerase Gamma Catalytic Subunit) in both tested oxygen conditions. In addition, the viability and mitochondrial membrane potential of hiPSCs were significantly lowered by MeHgCl regardless of the oxygen concentration. However, reactive oxygen species accumulation significantly increased only under atmospheric oxygen conditions; what was associated with increased expression of TFAM (Transcription Factor A, Mitochondrial) and NRF1 (Nuclear Respiratory Factor 1) and downregulation of PARK2 (Parkin RBR E3 Ubiquitin Protein Ligase). Taken together, our results demonstrated that MeHgCl could induce in vitro toxicity in hiPSCs through altering mitochondria-associated genes in an oxygen level-dependent manner. Thus, our work suggests that oxygen should be considered a factor was modulating the in vitro toxicity of environmental pollutants. Typical atmospheric conditions of in vitro culture significantly lower the predictive value of studies of such toxicity.

Mercury interactions with selenium and sulfur and the relevance of the Se:Hg molar ratio to fish consumption advice

Eating fish is often recommended as part of a healthful diet. However, fish, particularly large predatory fish, can contain significant levels of the highly toxic methylmercury (MeHg). Ocean fish in general also contain high levels of selenium (Se), which is reported to confer protection against toxicity of various metals including mercury (Hg). Se and Hg have a high mutual binding affinity, and each can reduce the toxicity of the other. This is an evolving area of extensive research and controversy with variable results in the animal and epidemiologic literature. MeHg is toxic to many organ systems through high affinity for -SH (thiol) ligands on enzymes and microtubules. Hg toxicity also causes oxidative damage particularly to neurons in the brain. Hg is a potent and apparently irreversible inhibitor of the selenoenzymes, glutathione peroxidases (GPX), and thioredoxin reductases (TXNRD) that are important antioxidants, each with a selenocysteine (SeCys) at the active site. Hg binding to the SeCys inhibits these enzymes, accounting in part for the oxidative damage that is an important manifestation of Hg toxicity, particularly if there is not a pool of excess Se to synthesize new enzymes. A molar excess of Se reflected in an Se:Hg molar ratio > 1 is often invoked as evidence that the Hg content can be discounted. Some recent papers now suggest that if the Se:Hg molar ratio exceeds 1:1, the fish is safe and the mercury concentration can be ignored. Such papers suggested that the molar ratio rather than the Hg concentration should be emphasized in fish advisories. This paper examines some of the limitations of current understanding of the Se:Hg molar ratio in guiding fish consumption advice; Se is certainly an important part of the Hg toxicity story, but it is not the whole story. We examine how Hg toxicity relates also to thiol binding. We suggest that a 1:1 molar ratio cannot be relied on because not all of the Se in fish or in the fish eater is available to interact with Hg. Moreover, in some fish, Se levels are sufficiently high to warrant concern about Se toxicity.

Methylmercury cytotoxicity and possible mechanisms in human trophoblastic HTR-8/SVneo cells

Methylmercury (MeHg) exposure during pregnancy can lead to adverse outcomes, including miscarriage and intrauterine growth retardation. In this study, MeHg cytotoxicity and its mechanisms in HTR-8/SVneo cells were investigated. MeHg inhibited HTR-8/SVneo cell viability and severely disrupted the cellular submicrostructure, showing a time-dose effect relationship. After MeHg treatment, the reactive oxygen species levels, malondialdehyde content, and superoxide dismutase (SOD) and catalase activities in the HTR-8/SVneo cells increased significantly with increased MeHg concentration (P<0.05). Similarly, MeHg also induced HTR-8/SVneo cell apoptosis in a dose-dependent manner. The proportion of cells in G1 phase decreased with increasing MeHg concentration, while that in the S and G2/M phases gradually increased. Moreover, cell migration and invasion capacities gradually decreased with increasing MeHg concentration, showing a significant difference between the MeHg-treated and control groups. Genes related to oxidative stress (HSPA6, HSPA1A, Nrf2, SOD1, HO-1, NQO1, OSGIN1, and gPX1), cell cycle (P21 and CDC25A), apoptosis (CYCS and AIFM2), and migration and invasion (CXCL8, CXCL3, CLU, IL24, COL3A1, MAPT, and ITGA7) were differentially expressed in the MeHg-treated group, indicating MeHg toxicity and mechanism of action. This study will provide insights into the prevention and treatment of pregnancy-related diseases caused by MeHg.

Neutral Red Uptake, Cellular Growth and Lysosomal Function: In Vitro Effects of 24 Metals

Specific toxic interference in lysosomal activity was evaluated by the relative uptake of neutral red by lysosomes. In this adaptation of the standard neutral red cytotoxicity assay, the results are expressed relative to cell culture protein content to avoid misinterpretation due to the influence on cell proliferation of the chemicals tested. Neuro-2a mouse neuroblastoma cells were exposed in vitro to 24 metal compounds and the lysosomal activity was quantified. Five different patterns of alterations were identified. Group A (ZnCl2, NaAsO2, Na2HAsO4, CdCl2, SnCl2, HgCl2, HgCH3Cl and TlCOOCH3) produced an inhibition of the relative uptake of the dye, in marked contrast to the greater inhibition shown by the neutral red uptake assay. Group B (MgCl2, A1C13, KMnO4) NiCl2, BaCl2 and Pb[NO3]2) showed less inhibition with strong parallelism with the neutral red assay. In Group C (CrCl3, Na2Cr2O7, FeCl3, CoCl2, CuCl2 and Sn[C2H5]4), low-dose stimulation, and inhibition at higher concentrations were found. Group D (LiCl and CrCl2) stimulated uptake, and Group E (MnCl2 and Sr[NO3]2) produced no significant modifications. The relative uptake of neutral red can be a convenient tool for the study of specific toxic alterations of lysosomes.

An autophagy deficiency promotes methylmercury-induced multinuclear cell formation

Methylmercury (MeHg) is a highly toxic pollutant, and is considered hazardous to human health. In our previous study, we found that MeHg induces autophagy and that Atg5-dependent autophagy plays a protective role against MeHg toxicity. To further characterize the role of autophagy in MeHg-induced toxicity, we examined the impact of autophagy on microtubules and nuclei under MeHg exposure using Atg5KO mouse embryonic fibroblasts (MEFs). Low concentrations of MeHg induced a decrease in α-tubulin and acetylated-tubulin in both wild-type and Atg5KO cells. While α-tubulin acetylation was promoted by treatment with tubacin, a selective inhibitor of histone deacetylase 6, MeHg treatment inhibits the increase of tubacin-induced acetylated-tubulin. However, similar effects were observed for treatment with either tubacin or tubacin + MeHg in wild-type and Atg5KO cells. We also found a significant increase in the number of multinuclear cells upon MeHg exposure in Atg5KO MEFs compared to wild-type MEFs. In addition, DNA double strand breaks (DSBs), measured by phosphorylation of the core histone H2A variant (H2AX) on serine 139 (γH2AX), markedly increased in Atg5KO MEFs compared to wild-type MEFs. Our results therefore suggest that autophagy is not a simple elimination pathway of MeHg-induced damaged proteins, but that it also plays a protective role in the context of MeHg-associated DSBs.

Methylmercury's chemistry: From the environment to the mammalian brain

Methylmercury is a neurotoxicant that is found in fish and rice. MeHg's toxicity is mediated by blockage of -SH and -SeH groups of proteins. However, the identification of MeHg's targets is elusive. Here we focus on the chemistry of MeHg in the abiotic and biotic environment. The toxicological chemistry of MeHg is complex in metazoans, but at the atomic level it can be explained by exchange reactions of MeHg bound to -S(e)H with another free -S(e)H group (R¹S(e)-HgMe + R²-S(e)H ↔ R¹S(e)H + R²-S(e)-HgMe). This reaction was first studied by professor Rabenstein and here it is referred as the "Rabenstein's Reaction". The absorption, distribution, and excretion of MeHg in the environment and in the body of animals will be dictated by Rabenstein's reactions. The affinity of MeHg by thiol and selenol groups and the exchange of MeHg by Rabenstein's Reaction (which is a diffusion controlled reaction) dictates MeHg's neurotoxicity. However, it is important to emphasize that the MeHg exchange reaction velocity with different types of thiol- and selenol-containing proteins will also depend on protein-specific structural and thermodynamical factors. New experimental approaches and detailed studies about the Rabenstein's reaction between MeHg with low molecular mass thiol (LMM-SH) molecules (cysteine, GSH, acetyl-CoA, lipoate, homocysteine) with abundant high molecular mass thiol (HMM-SH) molecules (albumin, hemoglobin) and HMM-SeH (GPxs, Selenoprotein P, TrxR1-3) are needed. The study of MeHg migration from -S(e)-Hg- bonds to free -S(e)H groups (Rabenstein's Reaction) in pure chemical systems and neural cells (with special emphasis to the LMM-SH and HMM-S(e)H molecules cited above) will be critical to developing realistic constants to be used in silico models that will predict the distribution of MeHg in humans.

Comparison of neurons derived from mouse P19, rat PC12 and human SH-SY5Y cells in the assessment of chemical- and toxin-induced neurotoxicity

Background

Exposure to chemicals might be toxic to the developing brain. There is a need for simple and robust in vitro cellular models for evaluation of chemical-induced neurotoxicity as a complement to traditional studies on animals. In this study, neuronally differentiated mouse embryonal carcinoma P19 cells (P19 neurons) were compared with human neuroblastoma SH-SY5Y cells and rat adrenal pheochromocytoma PC12 cells for their ability to detect toxicity of methylmercury (MeHg), okadaic acid and acrylamide.

Methods

Retinoic acid-treated P19 and SH-SY5Y cells and nerve growth factor-stimulated PC12 cells, allowed to differentiate for 6 days, were exposed to MeHg, okadaic acid and acrylamide for 48 h. Cell survival and neurite outgrowth were assessed with the calcein-AM assay and fluorescence detection of antibodies against the cytoskeletal neuron-specific protein βIII-tubulin, respectively. The effects of glutathione (GSH) and the potent inhibitor of GSH synthesis buthionine sulfoximine (BSO) on the MeHg induced-toxicity were assessed using the PrestoBlue™ cell viability assay and the TMRE mitochondrial membrane potential assay.

Results

Differentiated P19 cells developed the most extensive neuronal network among the three cell models and were the most sensitive neuronal model to detect neurotoxic effects of the test compounds. MeHg produced a concentration-dependent toxicity in differentiated P19 cells and SH-SY5Y cells, with statistically significant effects at concentrations from 0.1 μM in the P19 neurons and 1 μM in the SH-SY5Y cells. MeHg induced a decrease in the cellular metabolic activity and mitochondrial membrane potential (ΔΨm) in the differentiated P19 cells and SH-SY5Y cells, that were attenuated by GSH. Okadaic acid and acrylamide also showed statistically significant toxicity in the P19 neurons, but not in the SH-SY5Y cells or the P12 cells.

Conclusions

P19 neurons are more sensitive to detect cytotoxicity of MeHg, okadaic acid and acrylamide than retinoic acid-differentiated SH-SY5Y cells and nerve growth factor-treated PC12 cells. P19 neurons are at least as sensitive as differentiated SH-SY5Y cells to detect the loss of mitochondrial membrane potential produced by MeHg and the protective effects of extracellular GSH on MeHg toxicity. P19 neurons may be a useful model to study neurotoxic effects of chemicals.

Methylmercury and brain development: A review of recent literature

Methylmercury (MeHg) is a potent environmental pollutant, which elicits significant toxicity in humans. The central nervous system (CNS) is the primary target of toxicity, and is particularly vulnerable during development. Maternal exposure to MeHg via consumption of fish and seafood can have irreversible effects on the neurobehavioral development of children, even in the absence of symptoms in the mother. It is well documented that developmental MeHg exposure may lead to neurological alterations, including cognitive and motor dysfunction. The neurotoxic effects of MeHg on the developing brain have been extensively studied. The mechanism of toxicity, however, is not fully understood. No single process can explain the multitude of effects observed in MeHg-induced neurotoxicity. This review summarizes the most current knowledge on the effects of MeHg during nervous system development considering both, in vitro and in vivo experimental models. Considerable attention was directed towards the role of glutamate and calcium dyshomeostasis, mitochondrial dysfunction, as well as the effects of MeHg on cytoskeletal components/regulators.

Protective effect of prolactin against methylmercury-induced mutagenicity and cytotoxicity on human lymphocytes

Mercury exhibits cytotoxic and mutagenic properties as a result of its effect on tubulin. This toxicity mechanism is related to the production of free radicals that can cause DNA damage. Methylmercury (MeHg) is one of the most toxic of the mercury compounds. It accumulates in the aquatic food chain, eventually reaching the human diet. Several studies have demonstrated that prolactin (PRL) may be differently affected by inorganic and organic mercury based on interference with various neurotransmitters involved in the regulation of PRL secretion. This study evaluated the cytoprotective effect of PRL on human lymphocytes exposed to MeHg in vitro, including observation of the kinetics of HL-60 cells (an acute myeloid leukemia lineage) treated with MeHg and PRL at different concentrations, with both treatments with the individual compounds and combined treatments. All treatments with MeHg produced a significant increase in the frequency of chromatid gaps, however, no significant difference was observed in the chromosomal breaks with any treatment. A dose-dependent increase in the mitotic index was observed for treatments with PRL, which also acts as a co-mitogenic factor, regulating proliferation by modulating the expression of genes that are essential for cell cycle progression and cytoskeleton organization. These properties contribute to the protective action of PRL against the cytotoxic and mutagenic effects of MeHg.

Alterations in the Atlantic cod (Gadus morhua) hepatic thiol-proteome after methylmercury exposure

Proteomic studies in general have demonstrated that the most effective and thorough analysis of biological samples requires subfractionation and/or enrichment prior to downstream processing. In the present study, Atlantic cod (Gadus morhua) liver samples were fractionated using activated thiol sepharose to isolate hepatic proteins containing free/reactive cysteines. This subset of proteins is of special interest when studying the physiological effects attributed to methylmercury (MeHg) exposure. Methylmercury is a persistent environmental contaminant that has a potent affinity toward thiol groups, and can directly bind proteins via available cysteine residues. Further, alterations in the cod thiol-proteome following MeHg exposure (2 mg/kg body weight) were explored with two-dimensional gel electrophoresis combined with downstream mass spectrometry analyses for protein identifications. Thirty-five protein spots were found to respond to MeHg exposure, and 13 of these were identified when searching cod-specific databases with acquired mass spectrometry data. Among the identified thiol-containing proteins, some are known to respond to MeHg treatment, including constituents of the cytoskeleton, and proteins involved in oxidative stress responses, protein synthesis, protein folding, and energy metabolism. Methylmercury also appeared to affect cod heme metabolism/turnover, producing significantly altered levels of hemoglobin and hemopexin in liver following metal exposure. The latter finding suggests that MeHg may also affect the hematological system in Atlantic cod.

Developmental stage dependent neural stem cells sensitivity to methylmercury chloride on different biofunctional surfaces

Sensitivity of neural stem cells viability, proliferation and differentiation upon exposure to methylmercury chloride (MeHgCl) was investigated on different types of biofunctional surfaces. Patterns of biodomains created by microprinting/microspotting of poly-l-lysine or extracellular matrix proteins (fibronectin and vitronectin) allowed for non-specific electrostatic or specific, receptor mediated interactions, respectively, between stem cells and the surface. The neural stem cell line HUCB-NSC has been previously shown to be susceptible to MeHgCl in developmentally dependent manner. Here we demonstrated that developmental sensitivity of HUCB-NSC to MeHgCl depends upon the type of adhesive biomolecules and the geometry of biodomains. Proliferation of HUCB-NSC was diminished in time and MeHgCl concentration dependent manner. In addition, the response to MeHgCl was found to be cell-type dependent. Undifferentiated cells were the most sensitive independently of the type of bioactive domain. Significant decrease of GFAP+ cells was detected among cells growing on poly-l-lysine, while on fibronectin and vitronectin, this effect was observed only in the highest (1μM) concentration of MeHgCl. β-Tubulin III expressing cells were most sensitive on fibronectin domains. In addition, limited bioactive domains to μm in size, as compared to non-patterned larger area of the same adhesive substrate, exerted protective role. Thus, the surface area and type of cell/biofunctional surface interaction exerted significant influence on developmental stage and cell-type specific response of HUCB-NSC to MeHgCl.

Disruption of cytoskeleton by methylmercury in cultured CHO cells

The effect of methylmercury (MM) on three main cytoskeletal components [i.e. microtubules (MT), microfilaments (MF) and intermediate filaments (IF)] and on specific biochemical parameters (i.e. glutathione transferase (GST), glutathione reductase (RED), glutathione peroxidase (GSH-Px), glyoxalase 1 (GLY 1) and total -SH groups (TSH) of the cytosolic fraction) was studied in cultured Chinese hamster ovary (CHO) cells. The experiments were conducted with increasing doses of MM (i.e. 1, 4 and 8 mum), using an exposure time of 16 hr; and with a fixed dose of MM (2 mum), using increasing exposure periods (i.e. 0-24 hr). The morphological changes observed by immunofluorescence seemed to indicate that MF were damaged as much as (if not more than) MT after 16 hr of exposure to 4 mum-MM. At a concentration of 1 mum, MM only affected MF. The time-course experiments revealed that IF as well as MF and MT were severely disorganized after 3 and 6 hr of incubation in the presence of 2 mum-MM. However, an obvious reorganization was observed after 24 hr of exposure. In experiments using increasing MM doses, changes in the enzymatic activities were less noticeable than those observed in the morphology; only a modest decrease in TSH and RED activities (<30%) was recorded at the highest dose of MM used (i.e. 8 mum). In contrast, increasing the time of exposure to MM induced changes in both the cytoskeletal structures and the biochemical parameters: the lowest RED activity and TSH were observed after 3-6 hr exposure; control values were obtained after an exposure period of 24 hr. Ultrastructural observations on cells treated with increasing doses of MM showed changes in plasmamembrane profile, cytoskeleton organization and mitochondrion structure. The results confirm that MM causes non-specific damage to CHO cells and suggest that a functional interaction may exist between GSH-dependent enzymes and cytoskeletal structures.

The toxicology of mercury and its compounds

A concentrated review on the toxicology of inorganic mercury together with an extensive review on the neurotoxicology of methylmercury is presented. The challenges of using inorganic mercury in dental amalgam are reviewed both regarding the occupational exposure and the possible health problems for the dental patients. The two remaining "mysteries" of methylmercury neurotoxicology are also being reviewed; the cellular selectivity and the delayed onset of symptoms. The relevant literature on these aspects has been discussed and some suggestions towards explaining these observations have been presented.

Degeneration of peripheral nervous system in rats experimentally induced by methylmercury intoxication

The objective of this study is to elucidate the primary action of methylmercury chloride (MMC) intoxication on peripheral nervous system. We chronologically observed the pathological changes of sciatic nerve, dorsal root ganglion (DRG) neurons, ventral and dorsal roots in rats given 4 mg/kg/day of MMC on consecutive days and killed on days 11, 15, 18 and 21. On day 11, an initial axonal degeneration of type B neuron occurred, predominantly in the distal portions of sciatic nerve. The DRG type A neuron was infiltrated by MRF-1-positive macrophages on day 11. Electron microscopy also demonstrated degenerated mitochondria in type A neuron. On day 21, most of type A neurons seemed to have disappeared. However, type B neurons were well preserved. Immunoblotting with monoclonal antibodies, P0 and neurofilament, demonstrated that both of proteins significantly decreases from day 15. In conclusion, these results indicate that the primary action on type A neuron is the neuron body that consequently results in an anterograde degeneration of nerve fibers, while the type B neuron degeneration occurs in a dying-back process in this subacute model. These findings suggest that the mechanisms involved in the degeneration induced by MMC vary and may depend on certain intrinsic factors peculiar to these neurons.

Methylmercury (MeHg) elicits mitochondrial-dependent apoptosis in developing hippocampus and acts at low exposures

The developing brain is particularly sensitive to environmental teratogens, such as methylmercury (MeHg), which may induce cell death. Although several mechanisms of MeHg-induced apoptosis have been defined in culture models, pathways mediating caspase-3 activation in vivo remain unclear, especially in the developing hippocampus. To explore apoptotic mechanisms, Sprague-Dawley rats were exposed to 5 μg/g MeHg or PBS vehicle on postnatal day 7 (P7) and the hippocampus was assessed at various times for levels of apoptotic proteins. MeHg induced a 38% increase in Bax protein and an increase in cytosolic cytochrome c at 4h, followed by later increases in caspase-9 (40% at 12h; 33% at 24h) and caspase-8 (33% at 24h), compared to controls. MeHg also induced an increase in executioner caspase-3, a protease activated by both mitochondrial-dependent caspase-9 and mitochondrial-independent caspase-8. To further define pathways, we used a forebrain culture model and found that the MeHg-induced increases in caspase-3 and caspase-8 were completely blocked by a caspase-9-specific inhibitor, while caspase-9 induction was unperturbed by the caspase-8 inhibitor. These observations suggest that MeHg acts primarily through the mitochondrial-dependent cascade to activate caspase-3 in forebrain precursors, a pathway that may contribute to previously documented neurotoxicity in developing hippocampus. In turn, using the endpoint protein, caspase-3, as a sensitive marker for neural injury, we were able to detect hippocampal cell death in vivo at ten-fold lower levels of MeHg exposure (0.6 μg/g) than previously reported. Thus mitochondrial-dependent cell death in the hippocampus may serve as a sensitive index for teratogenic insults to the developing brain.

Neurobehavioural and molecular changes induced by methylmercury exposure during development

There is an increasing body of evidence on the possible environmental influence on neurodevelopmental and neurodegenerative disorders. Both experimental and epidemiological studies have demonstrated the distinctive susceptibility of the developing brain to environmental factors such as lead, mercury and polychlorinated biphenyls at levels of exposure that have no detectable effects in adults. Methylmercury (MeHg) has long been known to affect neurodevelopment in both humans and experimental animals. Neurobehavioural effects reported include altered motoric function and memory and learning disabilities. In addition, there is evidence from recent experimental neurodevelopmental studies that MeHg can induce depression-like behaviour. Several mechanisms have been suggested from in vivo- and in vitro-studies, such as effects on neurotransmitter systems, induction of oxidative stress and disruption of microtubules and intracellular calcium homeostasis. Recent in vitro data show that very low levels of MeHg can inhibit neuronal differentiation of neural stem cells. This review summarises what is currently known about the neurodevelopmental effects of MeHg and consider the strength of different experimental approaches to study the effects of environmentally relevant exposure in vivo and in vitro.

Chemically diverse toxicants converge on Fyn and c-Cbl to disrupt precursor cell function

Identification of common mechanistic principles that shed light on the action of the many chemically diverse toxicants to which we are exposed is of central importance in understanding how toxicants disrupt normal cellular function and in developing more effective means of protecting against such effects. Of particular importance is identifying mechanisms operative at environmentally relevant toxicant exposure levels. Chemically diverse toxicants exhibit striking convergence, at environmentally relevant exposure levels, on pathway-specific disruption of receptor tyrosine kinase (RTK) signaling required for cell division in central nervous system (CNS) progenitor cells. Relatively small toxicant-induced increases in oxidative status are associated with Fyn kinase activation, leading to secondary activation of the c-Cbl ubiquitin ligase. Fyn/c-Cbl pathway activation by these pro-oxidative changes causes specific reductions, in vitro and in vivo, in levels of the c-Cbl target platelet-derived growth factor receptor-α and other c-Cbl targets, but not of the TrkC RTK (which is not a c-Cbl target). Sequential Fyn and c-Cbl activation, with consequent pathway-specific suppression of RTK signaling, is induced by levels of methylmercury and lead that affect large segments of the population, as well as by paraquat, an organic herbicide. Our results identify a novel regulatory pathway of oxidant-mediated Fyn/c-Cbl activation as a shared mechanism of action of chemically diverse toxicants at environmentally relevant levels, and as a means by which increased oxidative status may disrupt mitogenic signaling. These results provide one of a small number of general mechanistic principles in toxicology, and the only such principle integrating toxicology, precursor cell biology, redox biology, and signaling pathway analysis in a predictive framework of broad potential relevance to the understanding of pro-oxidant–mediated disruption of normal development.

Chemically different toxins (lead, methylmercury, and paraquat) each cause the intracellular environment to become more oxidized, and thereby activate a common pathway that suppresses signaling from growth factor receptors that may be associated with developmental impairments.

Discovering general principles underlying the effects of toxicant exposure on biological systems is one of the central challenges of toxicological research. We have discovered a previously unrecognized regulatory pathway on which chemically diverse toxicants converge, at environmentally relevant exposure levels, to disrupt the function of progenitor cells of the developing central nervous system. We found that the ability of low levels of methylmercury, lead, and paraquat to make progenitor cells more oxidized causes activation of an enzyme called Fyn kinase. Activated Fyn then activates another enzyme (c-Cbl) that modifies specific proteins—receptors that are required for cell division and survival—to initiate the proteins' degradation. By enhancing degradation of these receptors, their downstream signaling functions are repressed. Analysis of developmental exposure to methylmercury provided evidence that this same pathway is activated in vivo by environmentally relevant toxicant levels. The remarkable sensitivity of progenitor cells to low levels of toxicant exposure, and the discovery of the redox/Fyn/c-Cbl pathway as a mechanism by which small increases in oxidative status can markedly alter cell function, provide a novel and specific means by which exposure to chemically diverse toxicants might perturb normal development. In addition, the principles revealed in our studies appear likely to have broad applicability in understanding the regulation of cell function by alterations in redox balance, regardless of how they might be generated.

Salicylic acid triggers genotoxic adaptation to methyl mercuric chloride and ethyl methane sulfonate, but not to maleic hydrazide in root meristem cells of Allium cepa L

Salicylic acid (SA), 0.01 mM, a signalling phytohormone, was tested for induction of adaptive response against genotoxicity of methyl mercuric chloride (MMCl), 0.013 mM; ethylmethane sulfonate (EMS), 2.5 mM, or maleic hydrazide (MH), 5 mM, in root meristem cells of Allium cepa. Induction of adaptive response to EMS by hydrogen peroxide (H2O2), 1 mM, and yet another secondary signal molecule was tested for comparison. Assessed by the incidence of mitoses with spindle and/or chromosome aberration and micronucleus, the findings provided evidence that SA-conditioning triggered adaptive response against the genotoxic-challenges of MMCl and EMS, but failed to do so against MH. H2O2, which is known to induce adaptive response to MMCl and MH, failed to induce the same against EMS in the present study. The findings pointed to the possible role of signal transduction in the SA-induced adaptive response to genotoxic stress that perhaps ruled out an involvement of H2O2.

Persistence and prevention of aluminium- and paraquat-induced adaptive response to methyl mercuric chloride in plant cells in vivo

Induction and persistence of adaptive response by aluminium (Al), 1 or 10 microM, and paraquat (PQ), 5 or 10 microM, against genotoxicity of methyl mercuric chloride (MMCl), 1.26 microM, a standard environmental genotoxin, was investigated in root meristem cells of Allium cepa. Subsequently, three metabolic inhibitors, namely, 3-aminobezamide (3-AB, 10 or 100 microM), an inhibitor of poly(ADP-ribose) polymerase (PARP) implicated in DNA repair and/or apoptosis, cycloheximide (CH, 0.1 or 1 microM), an inhibitor of protein synthesis, and buthionine sulfoximine (BSO, 100 microM or 1mM), an inhibitor of glutathione synthesis were tested for their ability to prevent the adaptive response induced by conditioning doses of Al, 10 or 100 microM; and PQ, 5 or 100 microM, against MMCl-challenge, 1.26 or 100 microM, in root meristems of A. cepa or embryonic shoots of Hordeum vulgare, respectively. The findings demonstrated that once triggered, the Al- or PQ-adaptive response to MMCl could persist for at least 48h in root meristems of A. cepa. Furthermore, the adaptive response could effectively be prevented by 3-AB, to a lesser degree by CH, and the least by BSO, suggesting primarily the involvement of PARP and implicating DNA repair in the underlying mechanisms of adaptive response in plant cells in vivo.

Methylmercury, but not inorganic mercury, causes abnormality of centrosome integrity (multiple foci of gamma-tubulin), multipolar spindles and multinucleated cells without microtubule disruption in cultured Chinese hamster V79 cells

Abnormalities of centrosome integrity and spindle organization in the cultured Chinese hamster fibroblast cell line V79 exposed to methylmercury (MeHgCl) and inorganic mercury (Hg(2+)) were investigated in conjunction with inductions of mitotic arrest and multinucleated cells. The centrosome integrity and spindle organization were investigated by immunofluorescence of centrosome proteins, gamma-tubulin, and beta-tubulin, respectively. MeHgCl at subtoxic concentrations caused an increase in mitotic index 6 h after exposure. Ameboid cells with multiple pseudopodia were also induced and chromosomes were distributed even in the pseudopodia. After the increase in mitotic index caused by MeHgCl, multinucleated cells with multiple micronuclei appeared. MeHgCl caused abnormality of centrosome integrity (multiple foci of gamma-tubulin) colocalized with aberrant spindles in a concentration-dependent manner, while it did not cause disruption of centrosome integrity and microtubule organization in interphase cells. In addition, MeHgCl led to the appearance of monoastral cells with a one-dot signal of gamma-tubulin. By contrast, Hg(2+) did not cause any of the changes induced by MeHgCl. Thus, MeHgCl caused centrosome abnormality and the related changes without microtubule disruption, suggesting that the mitotic centrosome is a critical target for the cytotoxic effects of MeHgCl.

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.

Neurotoxicity and molecular effects of methylmercury

The neurotoxicity of high levels of methylmercury (MeHg) and the high susceptibility of the developing brain are well established both in humans and experimental animals. Prenatally poisoned children display a range of effects varying from severe cerebral palsy to subtle developmental delays. Still unknown is the lowest dose that impairs neurodevelopment. The primary source of human exposure is the fish. The data obtained so far from epidemiological studies on fish-eating populations are not consistent. A reference dose of 0.1 microg MeHg/kg per day has been established by the U.S. Environmental Protection Agency based on a study on Iraqi children exposed to MeHg in utero. However, these exposures occurred at high level for a limited period of time, and consequently were not typical of lower chronic exposure levels associated with fish consumption. Major obstacles for estimation of a threshold dose for MeHg include the delayed appearance of the neurodevelopmental effects following prenatal exposure and limited knowledge of cellular and molecular processes underlying these neurological changes. In this respect, a strategy which aims at identifying sensitive molecular targets of MeHg at environmentally relevant levels may prove particularly useful to risk assessment. Here some examples of MeHg molecular effects occurring at low doses/concentrations are presented.

Cytogenetic damage related to low levels of methyl mercury contamination in the Brazilian Amazon

The mercury rejected in the water system, from mining operations and lixiviation of soils after deforestation, is considered to be the main contributors to the contamination of the ecosystem in the Amazon Basin. The objectives of the present study were to examine cytogenetic functions in peripheral lymphocytes within a population living on the banks of the Tapajós River with respect to methylmercury (MeHg) contamination, using hair mercury as a biological indicator of exposure. Our investigation shows a clear relation between methylmercury contamination and cytogenetic damage in lymphocytes at levels well below 50 micrograms/gram, the level at which initial clinical signs and symptoms of mercury poisoning occur. The first apparent biological effect with increasing MeHg hair level was the impairment of lymphocyte proliferation measured as mitotic index (MI). The relation between mercury concentration in hair and MI suggests that this parameter, an indicator of changes in lymphocytes and their ability to respond to culture conditions, may be an early marker of cytotoxicity and genotoxicity in humans and should be taken into account in the preliminary evaluation of the risks to populations exposed in vivo. This is the first report showing clear cytotoxic effects of long-term exposure to MeHg. Although the results strongly suggest that, under the conditions examined here, MeHg is both a spindle poison and a clastogen, the biological significance of these observations are as yet unknown. A long-term follow-up of these subjects should be undertaken.

Effects of methylmercury and inorganic mercury on the growth of nerve fibers in cultured chick dorsal root ganglia

Inhibition of the growth of nerve fibers by mercurials was quantitatively estimated by measuring the length of fibers in the cultured chick dorsal root ganglion. Morphological changes in nonneuronal cells were also evaluated. The growth rates of nerve fibers were constant for 2 to 6 days after the start of incubation. Methylmercury depressed nerve fiber growth dose- and time-dependently by 50% and completely at 3 x 10(-6) M and 7 x 10(-6) M, respectively. About 10-fold higher concentrations of inorganic mercury were required for the same extent of inhibition. The nerve fibers exposed to inorganic mercury shrank at an early stage of exposure and thereafter grew again within 24 hours. Electron microscopic examination revealed that methylmercury decreased microtubule mass extensively in nerve fibers, while inorganic mercury markedly altered surface membrane structure. These results suggested that microtubule disruption is involved in methylmercury-induced depression of nerve fibers but not in that induced by inorganic mercury. Characteristic effects on the growth of nerve fibers and the proliferation of nonneuronal cells were observed on the treatment with other metals such as cadmium, silver and chromium. Thus, dorsal root ganglion culture seems to be useful for the evaluation of toxic effects of metals in vitro.

Determination of a site-specific reference dose for methylmercury for fish-eating populations

Environmental risk-management decisions in the U.S. involving potential exposures to methylmercury currently use a reference dose (RfD) developed by the U.S. Environmental Protection Agency (USEPA). This RfD is based on retrospective studies of an acute poisoning incident in Iraq in which grain contaminated with a methylmercury fungicide was inadvertently used in the baking of bread. The exposures, which were relatively high but lasted only a few months, were associated with neurological effects in both adults (primarily paresthesia) and infants (late walking, late talking, etc.). It is generally believed that the developing fetus represents a particularly sensitive subpopulation for the neurological effects of methylmercury. The USEPA derived an RfD of 0.1 microg/kg/day based on benchmark dose (BMD) modeling of the combined neurological endpoints reported for children exposed in utero. This RfD included an uncertainty factor of 10 to consider human pharmacokinetic variability and database limitations (lack of data on multigeneration effects or possible long-term sequelae of perinatal exposure). Alcoa signed an Administrative Order of Consent for the conduct of a remedial investigation/feasibility study (RI/FS) at their Point Comfort Operations and the adjacent Lavaca Bay in Texas to address the effects of historical discharges of mercury-containing wastewater. In cooperation with the Texas Natural Resource Conservation Commission and USEPA Region VI, Alcoa conducted a baseline risk assessment to assess potential risk to human health and the environment. As a part of this assessment. Alcoa pursued the development of a site-specific RfD for methylmercury to specifically address the potential human health effects associated with the ingestion of contaminated finfish and shellfish from Lavaca Bay. Application of the published USEPA RfD to this site is problematic; while the study underlying the RfD represented acute exposure to relatively high concentrations of methylmercury, the exposures of concern for the Point Comfort site are from the chronic consumption of relatively low concentrations of methylmercury in fish. Since the publication of the USEPA RfD, several analyses of chronic exposure to methylmercury in fish-eating populations have been reported. The purpose of the analysis reported here was to evaluate the possibility of deriving an RfD for methylmercury, specifically for the case of fish ingestion, on the basis of these new studies. In order to better support the risk-management decisions associated with developing a remediation approach for the site in question, the analysis was designed to provide information on the distribution of acceptable ingestion rates across a population, which could reasonably be expected to be consistent with the results of the epidemiological studies of other fish-eating populations. Based on a review of the available literature on the effects of methylmercury, a study conducted with a population in the Seychelles Islands was selected as the critical study for this analysis. The exposures to methylmercury in this population result from chronic, multigenerational ingestion of contaminated fish. This prospective study was carefully conducted and analyzed, included a large cohort of mother-infant pairs, and was relatively free of confounding factors. The results of this study are essentially negative, and a no-observed-adverse-effect level (NOAEL) derived from the estimated exposures has recently been used by the Agency for Toxic Substances and Disease Registry (ATSDR) as the basis for a chronic oral minimal risk level (MRL) for methylmercury. In spite of the fact that no statistically significant effects were observed in this study, the data as reported are suitable for dose-response analysis using the BMD method. Evaluation of the BMD method used in this analysis, as well as in the current USEPA RfD, has demonstrated that the resulting 95% lower bound on the 10% benchmark dose (BMDL) represents a conservative estimate of the traditional NOAEL, and that it is superior to the use of "average" or "grouped" exposure estimates when dose-response information is available, as is the case for the Seychelles study. A more recent study in the Faroe Islands, which did report statistically significant associations between methylmercury exposure and neurological effects, could not be used for dose-response modeling due to inadequate reporting of the data and confounding from co-exposure to polychlorinated biphenyls (PCBs). BMD modeling over the wide range of neurological endpoints reported in the Seychelles study yielded a lowest BMDL for methylmercury in maternal hair of 21 ppm. This BMDL was then converted to an expected distribution of daily ingestion rates across a population using Monte Carlo analysis with a physiologically based pharmacokinetic (PBPK) model to evaluate the impact of interindividual variability. The resulting distribution of ingestion rates at the BMDL had a geometric mean of 1.60 microg/kg/day with a geometric standard deviation of 1.33; the 1st, 5th, and 10th percentiles of the distribution were 0.86, 1.04, and 1.15 microg/kg/day. In place of the use of an uncertainty factor of 3 for pharmacokinetic variability, as is done in the current RfD, one of these lower percentiles of the daily ingestion rate distribution provides a scientifically based, conservative basis for taking into consideration the impact of pharmacokinetic variability across the population. On the other hand, it was felt that an uncertainty factor of 3 for database limitations should be used in the current analysis. Although there can be high confidence in the benchmark-estimated NOAEL of 21 ppm in the Seychelles study, some results in the New Zealand and Faroe Islands studies could be construed to suggest the possibility of effects at maternal hair concentrations below 10 ppm. In addition, while concerns regarding the possibility of chronic sequelae are not supported by the available data, neither can they be absolutely ruled out. The use of an uncertainty factor of 3 is equivalent to using a NOAEL of 7 ppm in maternal hair, which provides additional protection against the possibility that effects could occur at lower concentrations in some populations. Based on the analysis described above, the distribution of acceptable daily ingestion rates (RfDs) recommended to serve as the basis for site-specific risk-management decisions at Alcoa's Point Comfort Operations ranges from approximately 0.3 to 1.1 microg/kg/day, with a population median (50th percentile) of 0.5 microg/kg/day. By analogy with USEPA guidelines for the use of percentiles in applications of distributions in exposure assessments, the 10th percentile provides a reasonably conservative measure. On this basis, a site-specific RfD of 0.4 microg/kg/day is recommended.

Early acute necrosis, delayed apoptosis and cytoskeletal breakdown in cultured cerebellar granule neurons exposed to methylmercury

Cerebellar granule cells (CGCs) are a sensitive target for methylmercury (MeHg) neurotoxicity. In vitro exposure of primary cultures of rat CGCs to MeHg resulted in a time- and concentration-dependent cell death. Within 1 hr exposure, MeHg at 5-10 microM caused impairment of mitochondrial activity, de-energization of mitochondria and plasma membrane lysis, resulting in necrotic cell death. Lower MeHg concentrations (0.5-1 microM) did not compromise cell viability, mitochondrial membrane potential and function at early time points. Later, however, the cells progressively underwent apoptosis and 100% cell death was reached by 18 hr treatment. Neuronal network fragmentation and microtubule depolymerization were detected as early as within 1.5 hr of MeHg (1 microM) exposure, long before the occurrence of nuclear condensation (6-9 hr). Neurite damage worsened with longer exposure time and proceeded to the complete dissolution of microtubules and neuronal processes (18 hr). Microtubule stabilization by taxol did not prevent MeHg-induced delayed apoptosis. Similarly ineffective were the caspase inhibitors z-VAD-fluoromethylketone and z-DEVD-chloromethylketone, the L-type calcium channel inhibitor nifedipine, the calcium chelator EGTA and BAPTA, and the NMDA receptor antagonist MK-801. On the other hand, insulin-like growth factor-I partially rescued CGCs from MeHg-triggered apoptosis. Altogether these results provide evidence that the intensity of MeHg insult is decisive in the time of onset and the mode of neuronal death that follows, i.e., necrosis vs. apoptosis, and suggest that cytoskeletal breakdown and deprivation of neurotrophic support play a role in MeHg delayed toxicity.

The involvement of microtubular disruption in methylmercury-induced apoptosis in neuronal and nonneuronal cell lines

Methylmercury (MeHg) is known to interfere with cell cycle progression by disruption of microtubules. The relationship between the changes in cell cycle and the induction of apoptosis caused by MeHg was investigated in cultured mammalian cells. MeHg caused nuclear fragmentation and DNA ladder formation in rat pheochromocytoma (PC12) and mouse neuroblastoma cells exposed to MeHg. Flow cytometric analysis revealed that the occurrence of apoptosis was preceded by the accumulation of cells in G2/M after MeHg treatment. Exposure to colchicine, a well-characterized mitotic inhibitor, also caused G2/M-phase arrest followed by the appearance of apoptotic cells. These results suggest that G2/M-phase arrest through the disruption of microtubules is an important event in the development of apoptosis by MeHg. MeHg treatment led to G2/M-phase arrest followed by apoptosis in nonneuronal HeLa cells also. Bcl-2 was phosphorylated by MeHg treatment in HeLa cells but not in PC12 cells; however, p53 expression was not changed in either cell line. Thus, MeHg induces apoptosis via a p53-independent pathway in both cell lines, however, different pathways may be activated after the disruption of microtubules in PC12 and HeLa cells.

Breast-feeding exposure of infants to cadmium, lead, and mercury: a public health viewpoint

The purpose of this report is to provide an overview of the public health implications of exposure via breast milk to cadmium, lead, and mercury for nursing infants and to provide health-based guidance. Daily intakes were calculated and compared with guidance values used for public health assessments at hazardous waste sites. Cadmium, lead, and mercury under normal conditions are found in breast milk at concentration ranges of < 1 microgram/L, 2-5 micrograms/L, and 1.4-1.7 micrograms/L, respectively. Women exposed environmentally or occupationally can have higher levels in their breast milk. Concentrations of about 5 micrograms/L (cadmium), 20 micrograms/L (lead), and 3.5 micrograms/L (mercury) appear to be adequate screening levels. Many factors affect both the distribution of cadmium, lead, and mercury in breast milk and the health consequences to an infant. It is not clear what additional impact low-level exposure via breast milk may have on an infant born with a body burden to one of these metals. There is sufficient evidence to make the case that contaminated breast milk is a source of potential risk to infants in certain populations. Prevention strategies that include behavior modification and proper nutrition should be communicated to women at risk. Identification and elimination of exposure pathways and a critical analysis of the benefits of breast feeding versus heavy metal exposure are needed on a site-specific or individual basis. Research is required to better understand the impact of low-level exposure to heavy metals via breast milk. Breastfeeding should be encouraged under most circumstances.

Detection of micronuclei in peripheral erythrocytes of Cyprinus carpio exposed to metallic mercury

Cyprinus carpio fish (carp), exposed to elemental or metallic mercury (Hg0) at concentrations of 2.0, 20.0, and 200.0 mg per liter of water, were kept in concrete tanks for 159 days. Ten fish were used for each concentration level. Thirteen samples of peripheral blood were collected from each animal through gill puncture, 12 during the first 90 days of the experiment, and the last one at the end of the experiment. The micronucleus test (MNT) was designed to study dose and time yield effects of mercury after indirect exposure in vivo. The results indicated that for a concentration of 2.0 mg Hg0/l, there was no significant increase in frequency of micronuclei (MN), but at higher concentrations (20.0 and 200.0 mg Hg0/l) there was a significant increase in MN frequencies. This effect was higher after 31 days of exposure, followed by slight stabilization and gradual decrease.

Cycloheximide and buthionine sulfoximine prevent induction of genotoxic adaptation by cadmium salt against methyl mercuric chloride in embryonic shoot cells of Hordum vulgare L

Presoaked seeds of barley Hordeum vulgare L. pretreated with cycloheximide (CH), 10(-6) M or bythionine sulfoximine (BSO), 10(-4) M, were exposed to methyl mercuric chloride (MMCl), 10(-4) M, with or without prior conditioning with cadmium sulfate (CdSO4), 10(-4) M. Subsequently as the seeds germinated the endpoints measured were mitotic index, cells with mitotic aberrations and micronuclei (MNC) in embryonic shoot cells fixed at 40, 44, 48 and 52 h of recovery. Indicated by the significant reduction (p < or = 0.05) of the yield of cells with aberrations or MNC, the results confirmed that CdSO4-conditioning triggered an adaptive response to MMCl-challenge. Pretreatments of CH and BSO, whereas they potentiated the genotoxicity of MMCl, significantly prevented (p < or = 0.05) the Cd-induced genotoxic adaptation. That underscores a possible involvement of proteins in addition to phytochelatins in the underlaying mechanisms.

Prophylaxis of antioxidants against the genotoxicity of methyl mercuric chloride and maleic hydrazide in Allium micronucleus assay

Antioxidants, namely cysteine, 2.46 x 10(-5) M; glutathione 9.75 x 10(-6), 9.75 x 10(-4) M; vitamin C, 10(-2) M; mannitol, 5 x 10(-2) M; potassium iodide, 5 x 10(-2) M and sodium selenite, 1.73 x 10(-6) M; were tested for their prophylactic activity against the genotoxicity of methylmercuric chloride, 1.26 x 10(-6) M and maleic hydrazide, 3 x 10(-4) M in Allium micronucleus assay. Antioxidants doses were administered simultaneously or prior to the genotoxic exposures. The results from the present experiments indicated that antioxidants conferred protection against the genotoxicity of both methyl mercuric chloride and maleic hydrazide. Furthermore, the protection of GSH against methyl mercuric chloride depending on the concentration persisted for 12 h.

Metal-induced genotoxic adaptation in barley (Hordeum vulgare L.) to maleic hydrazide and methyl mercuric chloride

Presoaked seeds of barley, Hordeum vulgare L., were exposed for 2 h to maleic hydrazide (MH), 5 x 10(-2) M or methyl mercuric chloride (MMCl), 10(-4) M with or without a prior conditioning with MH, 5 x 10(-3) M; MMCl, 10(-5) M; cadmium sulfate (CdSO4), 10(-4) M or zinc sulfate (ZnSO4), 10(-1) M; the interexposure time was 2 h. Subsequently as the seeds germinated a number of endpoints were measured that included mitotic index, mitotic chromosome aberrations and micronuclei (MNC) in embryonic shoot cells fixed at 32, 36, 40, 44, 48 and 52 h of recovery, and seedling height on day 7. The results demonstrated that prior conditioning exposure to MH or metals induced genotoxic adaptation to the subsequent challenge exposure to MH and MMCl. Cadmium-induced genotoxic adaptation against either MH or MMCl challenge exposure was, however, significantly prevented when the presoaked seeds were pre-exposed to buthionine sulfoximine, 10(-3) M for 2 h, thereby providing evidence that the underlying mechanism of genotoxic adaptation possibly involved phytochelatins.

A scanning electron-microscopic study of the effects of methylmercury on the neuronal cytoskeleton

Cultured mouse neuroblastoma cells, Neuro-2a (c1300), were exposed to 2.5 and 5.0 microM methylmercury (MeHg) with or without the concomitant administration of 10 mM glutathione for 24 h. Treated cells viewed by scanning electron microscopy (SEM) appeared sponge-like and were surrounded by fragments of cytoplasmic processes. SEM cytoskeletal preparations of treated cells showed a collapsed matrix containing globular bodies. Microtubules were not seen in treated cells, but intermediate and microfilaments were observed. SDS-PAGE analysis of cytoskeletal extracts revealed bands ranging in size from 90 to 27 kDa in all treatment groups except in the 5.0 microM-MeHg-treated group. This group showed a single band co-migrating with actin. Cells exposed to glutathione alone or concomitantly with MeHg appeared similar to control cells under all experimental conditions. These observations suggest that MeHg may predominantly affect microtubules to form a condensation product.

Effects of the tumor inhibitor IKP-104, a 4(1H)-pyridinone derivative, on cytoskeletal microtubules of cultured tumor cells

The effects of IKP-104, a 4(1H)-pyridinone derivative, on the mitotic profile and cytoskeletal microtubule dynamics of cultured B16 melanoma cells were examined in order to investigate the mechanism of its antitumor activity. The exposure to IKP-104 caused accumulation of cells in abnormal metaphase with chromosomes scattered within the cytoplasm and induced polyploid and multinucleate cells as detected by differential staining microscopy with brilliant blue R and safranin O. An immunofluorescence study with monoclonal anti-alpha-tubulin antibody revealed that IKP-104 diminished cytoskeletal microtubules of both interphase and mitotic cells, resulting in induction of a few fragments resembling "microtubular bundles" induced by vinblastine (VLB). These results indicated that IKP-104 arrests cells in the mitotic phase by inhibition of polymerization and induction of depolymerization of cytoskeletal microtubules, similarly to VLB.

The protective effects of glutathione against methylmercury cytotoxicity

Mouse neuroblastoma cells exposed to 2.5 and 5.0 microM methylmercury for 24 h appeared rounded with the loss of processes. Immunohistochemical staining directed against beta-tubulin revealed severe alterations in microtubular architecture. Non-membrane-bound condensation product was visualized ultrastructurally in the treated cells and appeared similar to what was seen histochemically. Reduced and oxidized glutathione levels suggest that methylmercury may manifest its deleterious effects via oxidation of tubulin sulfhydryls, and by alterations due to peroxidative injury. Cells exposed to methylmercury showed a decrease in glutathione peroxidase activity. Simultaneous administration of 10 mM glutathione with 2.5 and 5.0 microM methylmercury dramatically prevented cell injury.

Inhibition of cellular activities by triethyllead. Role of glutathione and accumulation of triethyllead in vitro

We investigated the interaction of triethyllead with ATP-coupled cellular enzymatic activities and the role of GSH to reverse the observed inhibition of these enzymes. Triethyllead inhibited the membrane bound Na+-K+-ATPase from HeLa cells (IC50 12 microM) and the ATP-hydrolysing activity of the mitochondrial F0-F1-ATPase complex (IC50 17 microM). Addition of 1 mM GSH reversed both enzyme activities totally, whereas lower GSH concentrations showed a less pronounced effect. Surprisingly, in freshly isolated rat liver mitochondria the ATP-synthesizing activity was also inhibited by triethyllead (IC50 16 microM), in spite of a measured high intramitochondrial GSH concentration (up to 10 mM). Further experiments in isolated submitochondrial particles revealed that ATP-synthesis and ATP-hydrolysis were inhibited by triethyllead with similar IC50 values, and both activities could be protected in vitro from the organolead compound in the presence of 1 mM GSH. Thus in all activities tested in vitro a high excess of GSH over triethyllead (greater than or equal to 25-fold) is necessary to restore the inhibited enzymes. The intramitochondrial triethyllead concentration was further determined after incubation of intact mitochondria with 10 microM of the organolead compound. The organolead concentration measured was as high as 600 microM. This means that in intact mitochondria there exists only a ca. 16-fold excess of GSH, which has been shown to be insufficient to protect ATP-synthesizing and ATP-hydrolyzing activities of the F0-F1-ATPase from triethyllead in vitro. We concluded that in intact mitochondria the F0-F1-ATPase complex is inhibited by triethyllead due to its accumulation in the matrix.

Mechanism of cytotoxicity of methylmercury. With special reference to microtubule disruption

Mechanism of methylmercury cytotoxicity was investigated with special reference to its preferential action on microtubules and protein biosynthesis in cultured cells. The tubulin synthesis analyzed by autoradiography of two-dimensional electropherogram using 35S-methionine was inhibited by 50-70% in mouse glioma cells exposed to 5 x 10(-6) M methylmercury for 3 h, which almost completely depolymerized microtubules. Total protein synthesis monitored by incorporation of labeled methionine into acid insoluble fraction was decreased slightly but significantly and the protein bands other than tubulin on gradient urea-PAGE gel appeared to remain unchanged under the experimental condition used. These results suggest that the inhibition of protein synthesis observed on exposure to methylmercury can be ascribed, at least partly, to a possible autoregulatory depression in tubulin synthesis owing to the increase in the pool of tubulin subunits resulted from microtubule depolymerization by methylmercury.

Analysis of methyl mercury binding sites on tubulin subunits and microtubules

We have studied the localization and affinity of methyl mercury hydroxide (MeHg) binding sites on microtubules. There is one class of binding sites for MeHg on tubulin, a high affinity class with fifteen sites. MeHg binds to tubulin stoichiometrically within microtubules, and does not induce microtubule disassembly at this low binding ratio. MeHg binds in microtubules either in the presence or absence of free tubulin subunits but free subunits act as uncompetitive inhibitors for MeHg binding to the polymer. These stoichiometric polymer surface binding sites for MeHg apparently do not interfere with subsequent polymerization, in contrast to the multiple sites in the free dimer whose occupation blocks subsequent assembly. In assembly cycles that follow MeHg binding to polymers, we continue to find MeHg bound to microtubules at substoichiometric ratios. Dimers with higher levels of MeHg binding are rendered assembly incompetent. These results show MeHg to have one class of binding site on tubulin, and the MeHg binding site, both to the polymer surface and to the free dimer, to be the same.

Selectivity of methyl mercury effects on cytoskeleton and mitotic progression in cultured cells

Methyl mercury (MeHg) disrupts microtubules, but effects on other cytoskeleton components are not well studied. Dose-effect relationships were used to determine the selectivity of MeHg effects on vimentin intermediate filaments, actin microfilaments, and microtubules. These were examined in PtK2 cells by immunofluorescence. At 0.5 microM MeHg the number of microtubules was reduced. After 1.0 or 2.0 microM MeHg, microtubules in most cells were disassembled except for a few "stable" microtubules. No effects on vimentin or actin filaments were observed except as secondary effects at concentrations of MeHg that caused extensive microtubule disassembly. Antimitotic effects of MeHg are well known. An assay based on fluid pinocytosis was used here to study kinetics of mitotic progression. HeLa cells were arrested at prometaphase with a lengthening of subsequent stages of mitosis. Progression from prophase to prometaphase was not affected. MeHg treatment also increased the number of micronucleated and multinucleated cells. Drugs specific for actin cause similar effects by blocking cytokinesis but the selective action of MeHg on microtubules argues against this mechanism. Data from both interphase and mitotic cultured cells indicate that MeHg acts selectively on microtubules. It further supports the hypothesis that the mechanism of damage of MeHg is related to its antimicrotubule activity.