Disruption of Intraneuronal Divalent Cation Regulation by Methylmercury: Are Specific Targets Involved in Altered Neuronal Development and Cytotoxicity in Methylmercury Poisoning?

Methylmercury is an environmental contaminant which causes relatively specific degeneration of the granular layer of the cerebellum, despite its ability to bind thiol groups in proteins of all cell types. The mechanisms underlying the specific targeting of cells during MeHg poisoning may depend on specific receptors and other targets related to divalent cation homeostasis, particularly intracellular calcium (Ca(2+)(i) signaling. MeHg disrupts Ca(2+)(i) homeostasis in a number of neuronal models, including cerebellar granule cells in primary culture, and contributes to MeHg-induced cell death, impaired synaptic function and disruption of neuronal development. Interestingly, the disruption of [Ca(2+)](i) regulation occurs through specific pathways which affect Ca(2+) regulation by organelles, particularly mitochondria and the smooth endoplasmic reticulum (SER). Cholinergic pathways which affect [Ca(2+)](i) signaling also appear to be critical targets, particularly muscarinic acetylcholine (ACh) receptors which are linked to Ca(2+) release through inositol-1,4,5-triphosphate (IP(3)) receptors. [Ca(2+)](i) dysregulation may also underlie observed alterations in cerebellar neuron development through interaction with specific target(s) in the developing axon. In this review, we examine the hypothesis that MeHg affects specific targets to cause disruption of neuronal development and cell death.

A new ultrasound protocol for extrusion of coelomocyte cells from the earthworm Eisenia fetida

There is mounting evidence that earthworms could be used as a sentinel species for soil ecotoxicity evaluation. In this aspect, phagocytosis by coelomocytes was shown to be a sensitive biomarker of exposure to xenobiotics. In this paper, we introduce a simple method for ultrasound extrusion of earthworm coelomocytes that generates a high cell yield, does not interfere with phagocytic competence, and requires a minimum of manipulations. Coelomocytes were extruded from the earthworm Eisenia fetida using this new ultrasound method and compared with ethanol and electrical extrusion. The ultrasonic extrusion showed the highest cell recovery with 3.17 +/ -0.8 x 10(6) cells per earthworm compared with 2.22 +/- 0.8 x 10(6) cells per earthworm for electrical extrusion and 1.57 +/- 0.07 x 10(6) cells per earthworm for ethanol extrusion. No significant differences in the cell viability were observed using propidium iodide and flow cytometry with viability for extrusion with ethanol of 63.8 +/- 12.7%, electrical 76.8 +/- 7.5%, and ultrasound 68.2 +/- 7.8%. To compare the potential effect of extrusion on cell quality, the cells extruded using the three methods were subjected to an 18-h in vitro exposure to methylmercury chloride (MeHgCl; CH3HgCl) with concentrations ranging from 10(-9) to 10(-4)M. The half-maximal effective concentration (EC50) for inhibition of phagocytosis occurred between 10(-7) and 10(-6)M. We found no significant differences among the extrusion methods for the phagocytic potential of the coelomocytes. This method does not harm the worms and can certainly improve collection of coelomocytes from earthworms and therefore contribute to the development of bioassays using invertebrates.

Chemical foundation of the attenuation of methylmercury(II) cytotoxicity by metallothioneins

To elucidate the chemical interactions underlying the role of metallothioneins (MTs) in reducing the cytotoxicity caused by MeHg(II), we monitored in parallel by electronic absorption and CD spectroscopies the stepwise addition of MeHgCl stock solution to mammalian Zn(7)-MT1 and the isolated Zn(4)-alphaMT1 and Zn(3)-betaMT1 fragments. The incorporation of MeHg(+) into Zn(7)-MT and Zn(3)-betaMT entails total displacement of Zn(II) and unfolding of the protein. However, both features are only partial for Zn(4)-alphaMT. The different behavior observed for this fragment, whether isolated or constituting one of the two domains of Zn(7)-MT, indicates interdomain interactions in the whole protein. Overall, the binding properties of Zn(7)-MT, Zn(4)-alphaMT and Zn(3)-betaMT toward MeHg(+) are unprecedented. In addition, the sequestration of MeHg(+) by Zn(7)-MT and the concomitant release of Zn(II) are probably two of the main contributions in the detoxifying role of mammalian MT.

Methylmercury stimulates arachidonic acid release and cytosolic phospholipase A2 expression in primary neuronal cultures

Cytosolic phospholipase A2 (cPLA2) plays an important role in the stimulus-dependent hydrolysis of sn-2 ester bond from membrane phospholipids, releasing arachidonic acid (AA), which along with its metabolites is involved in a number of regulatory functions. The present study examined the effect of methylmercury (MeHg; 0, 2.5, 5.0 microM) on cPLA2 activation in primary hippocampal neurons by assessing the release of 3H-AA. A significant increase in AA release was observed in cultures treated with 5 microM MeHg (10, 30, 60 and 120 min). This effect was due to neuronal cPLA2 activation, since it was completely abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a specific inhibitor of cPLA2. Additional studies confirmed, by means of western blot analysis, that MeHg (5.0 and 10 microM; 16h) potently increases neuronal cPLA2 protein expression. These results suggest that cPLA2-stimulated hydrolysis and release of AA are potential mediators of MeHg-induced neurotoxicity.

Methylmercury inhibits TrkA signaling through the ERK1/2 cascade after NGF stimulation of PC12 cells

Using PC12 cells as a model of neuronal differentiation, we have shown that acute exposure to methylmercury (CH3Hg) inhibits nerve growth factor (NGF)-induced activation of TrkA. In the present study, we examined the effects of CH3Hg on pathways activated by NGF. NGF-induced phosphorylation of ERK1/2 in PC12 cells was time-dependent. Concurrent exposure to CH3Hg and NGF for 2.5 min resulted in a concentration-dependent inhibition of ERK1/2 phosphorylation (EC50 = 0.018 microM). However, NGF-stimulated ERK1/2 phosphorylation was not altered after 5 min of exposure to CH3Hg. In vitro studies revealed that CH3Hg did not directly inhibit the ERK kinase MEK. As reported in other neuronal tissue, CH3Hg can inhibit PKC activity in vitro. Incubation of PC12 cell lysates with CH3Hg produced a concentration-dependent inhibition of PKC activity that was significant at 0.3-10 microM. Further studies using recombinant enzymes examined the effect of CH3Hg on PKC isoforms expressed in PC12 cells. CH3Hg inhibited PKCdelta, and zeta activity in a concentration-dependent manner at higher concentrations (3-10 microM), while a significant increase in PKCalpha activity was observed at lower concentrations (0.1 microM). However, CH3Hg had no affect on NGF-induced PKC activity in intact cells. These results show that CH3Hg inhibition of NGF-stimulated TrkA activation in PC12 cells decreases downstream signaling through the Raf/MEK/ERK cascade. In intact cells PKC does not appear to be a primary target for CH3Hg.

The Tohoku Study of Child Development: A Cohort Study of Effects of Perinatal Exposures to Methylmercury and Environmentally Persistent Organic Pollutants on Neurobehavioral Development in Japanese Children

Several birth cohort studies have shown adverse effects of perinatal exposures to methylmercury (MeHg) and environmentally persistent organic pollutants (POPs). These chemicals are ingested mainly through fish consumption, but little is known about the hazardous effects in Japanese, whose fish consumption is high. The present study, the Tohoku Study of Child Development, was designed to examine the effects of perinatal exposures to MeHg, polychlorinated biphenyls (PCB), dioxins, pesticides, and other chemicals in Japanese children. Six hundred eighty-seven pregnant women were participated in this study with their written informed consent. Maternal peripheral blood, cord blood, cord tissue, placenta, and breast milk samples were collected for chemical analysis. Maternal hair was also taken for MeHg analysis. Infants born at full term were assessed by neurobehavioral tests: the Brazelton Neonatal Behavioral Assessment Scale at three days old, the Kyoto Scale of Psychological Development and the Bayley Scales of Infant Development at 7 and 18 months old, and the Fagan Test of Infant Intelligence at 7 months old. The children will be continuously followed up to ages 6-7 years. Maternal food intake frequency, maternal IQ, socioeconomic status, and home environment were assessed as covariates. The results of this cohort study will allow us to evaluate associations between the neurobehavioral development of children and perinatal exposures to MeHg and environmentally POPs in Japan.

[Effect of mercuric chloride and methylmercury on photosynthetic activity of the diatom Thalassiosira weissflogii by a fluorescent method]

It was shown by the pulse-amplitude modulation fluorescent method that, at a weak illumination (6 microE m-2.s-1), methylmercury at a concentration of 10(-6)-10(-7) M decreases the photochemical activity of the reaction centers of photosystem II in cells of microalgae Thalassiosira weissflogii after a prolonged lag phase. Cells resistant to methylmercury at these low concentrations were detected by the microfluorimetric method. Chloride mercury decreased the activity of photosystem II of the algae only when at higher concentrations. Both toxicants at a concentration of 10(-6) M decreased the rate of recovery of photoinduced damage of centers of photosystem II and led to an increase in the energization component of nonphotochemical fluorescence quenching. These results indicate that the complex of fluorescent methods can be used to monitor early changes in the photosynthetic apparatus of algae in response to the toxic action of heavy metals.

Both mercury and cadmium directly influence calcium homeostasis resulting from the suppression of scale bone cells: the scale is a good model for the evaluation of heavy metals in bone metabolism

To examine the effects of heavy metals such as cadmium and mercury on calcium homeostasis, plasma calcium and calcitonin were measured in goldfish. Cadmium induced hypocalcemia both at 4 and at 8 days. In methylmercury-treated goldfish, the plasma calcium level increased at 2 days and then decreased at 8 days. The plasma calcitonin level increased in correspondence with the increased plasma calcium by methylmercury treatment, although cadmium did not cause a significant change. To elucidate the mechanism in detail, fish scales, which have both osteoclasts and osteoblasts and are similar to mammalian membrane bone, were used in the present study. We measured tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) activity as respective indicators of activity in both types of cells. TRAP activity in the scales decreased by treatment of cadmium and methylmercury at 6 h incubation. Particularly, cadmium (even at 10(-13) M) significantly suppressed TRAP activity, suggesting that this system is utilized as an acute biosensor for cadmium. ALP activity decreased after exposures of 64 and 96 h, although the activity did not change after 6, 18, and 36 h. In addition, mRNA expression of the estrogen receptor and insulin-like growth factor 1, which participate in osteoblastic growth and differentiation, was less than the control values by treatment with both metals. This study demonstrates that mercury directly acts on the bone cells and influences calcium homeostasis and indicates that, in a short-term exposure, mercury has a different action from that of cadmium and induces hypercalcemia.

Comparative effects of methylmercury and Hg(2+) on human neuronal N- and R-type high-voltage activated calcium channels transiently expressed in human embryonic kidney 293 cells

Expression cDNA clones of alpha1B-1 or alpha1E-3 subunits coding for human neuronal N-(Cav2.2) or R-subtype (Cav2.3) Ca2+ channels, respectively, was combined with alpha2-bdelta and beta3-a Ca2+ channel subunits, and transfected into human embryonic kidney cells for transient expression to determine whether specific types of neuronal voltage-sensitive Ca2+ channels are affected differentially by methylmercury (MeHg) and Hg2+. For both Ca2+ channel subtypes, MeHg (0.125-5.0 microM) or Hg2+ (0.1-5 microM) caused a time- and concentration-dependent reduction of current. MeHg caused an initial, rapid component and a subsequent more gradual component of inhibition. The rapid component of block was completed between 100 and 150 s after beginning treatment. At 0.125 to 1.25 microM, MeHg caused a more gradual decline in current. Apparent IC50 values were 1.3 and 1.1 microM for MeHg, and 2.2 and 0.7 microM for Hg2+ on N- and R-types, respectively. For N-type current, effects of Hg2+ were initially greater on the peak current than on the sustained current remaining at the end of a test pulse; subsequently, Hg2+ blocked both components of current. For R-type current, Hg2+ affected peak and sustained current approximately equally. Kinetics of inactivation also seemed to be affected by Hg2+ in cells expressing N-type but not R-type current. Washing with MeHg-free solution could not reverse effects of MeHg on either type of current. The effect of Hg2+ on N- but not R-type current was partially reversed by Hg2+-free wash solution. Therefore, different types of Ca2+ channels have differential susceptibility to neurotoxic mercurials even when expressed in the same cell type.

[Effect of methylmercury on primary photosynthesis processes in green microalgae Chlamydomonas reinhardtii]

The sensitivity of green microalgae Chlamydomonas reinhardtii to methylmercury chloride (MeHg) and chloride mercury (HgCl2) was evaluated by measuring chlorophyll fluorescence parameters by the pulse-amplitude-modulation (PAM) fluorometry. It was shown that MeHg at concentrations above 1 microM decreased the Fv/Fm ratio, which characterizes the maximal efficiency of energy utilization in photosystem II. The degree of inhibition depended on the time of treatment and was always higher under illumination conditions (50 microE.m-2.s-1) than under dark conditions. A similar regularity was observed for the delta F/Fm' ratio, which characterizes the real efficiency of energy storage at the given intensity of the photosynthesis-exciting light. Incubation with 5 microM HgCl2 for 5 h did not affect both ratios. The decrease in Fm at constant F0 as well as changes in the fast fluorescence kinetics after MeHg treatment of algae cells indicated the damage on the donor side of photosystem II and the damage of the electron transfer from QA to QB. The reduction of photochemical fluorescence quenching (qN) under MeHg treatment is also evidence of the increase in the fraction of closed reaction centers (QA-). At the same time, increase in the steady-state level of P700 photooxidation indicated a disturbance of electron transfer between photosystems. The present study demonstrates that methylmercury treatment damaged the photosynthetic electron transfer chain at several sites. The inhibitory effect of methylmercury is much stronger than the effect of mercury chloride on photosynthetic processes.

Methylmercury differentially affects GABA(A) receptor-mediated spontaneous IPSCs in Purkinje and granule cells of rat cerebellar slices

Using whole-cell recording techniques we compared effects of the environmental cerebellar neurotoxicant methylmercury (MeHg) on spontaneous IPSCs (sIPSCs) of both Purkinje and granule cells in cerebellar slices of the rat. In Purkinje cells, bath application of 10, 20 or 100 microM MeHg initially increased then suppressed the frequency of sIPSCs to zero. In granule cells, the initial increase in frequency was not observed in approximately 50% of cells examined, but suppression of sIPSCs by MeHg occurred in every cell tested. For both cells, time to onset of effects of MeHg was inversely related to the concentration; moreover, the pattern of changes in mIPSCs induced by MeHg in the presence of tetrodotoxin was similar to that in sIPSCs. For ea ch concentration of MeHg, it took 2-3 times longer to block sIPSCs in Purkinje cells than it did in granule cells. MeHg also initially increased then decreased amplitudes of sIPSCs to block in both cells; again the response was more variable in granule cells. In most Purkinje and some granule cells, MeHg induced a giant, slow inward current during the late stages of exposure. Appearance of this current appeared to be MeHg concentration dependent, and the direction of current flow was reversed by changing the holding potentials. Reduction of the [Cl-] in the internal solution caused inwardly directed, but not outwardly directed giant currents to disappear, suggesting that this current is a Cl(-)-mediated response. However, bicuculline and picrotoxin failed to block it. MeHg apparently acts at both presynaptic and postsynaptic sites to alter GABA(A) receptor-mediated inhibitory synaptic transmission. GABA(A) receptors in granule cells appear to be more sensitive to block by MeHg than are those in Purkinje cells, although the general patterns of effects on the two cells are similar.

Methylmercury induces apoptosis in cultured rat dorsal root ganglion neurons

Methylmercury is known to have devastating effects on the mammalian nervous system. In order to characterize the dose dependence of methylmercury-induced neurotoxicity, we first studied neurite outgrowth from rat dorsal root ganglia explants. In this model, methylmercury inhibited neurite outgrowth with a TD(50) of approximately 0.5 microM. We then used this relationship to optimize dosing for subsequent transcriptional profiling analyses in two independent neuronal model systems: dissociated sensory neurons and PC12 cells. As seen in previous studies, the expression of a number of genes associated with oxidative stress was altered following a 6h challenge with 1 microM methylmercury. When PC12 cells were subjected to a longer exposure (24h), a relative increase was noted in the representation of genes associated with cell cycling and apoptosis. To confirm the presence of apoptosis in cultured neurons, we then applied TUNEL staining and bis-benzimide staining techniques to primary cultures of dissociated sensory neurons. After 24h, 1 microM methylmercury increased both DNA end-labeling (P<0.01) and nuclear fragmentation (P<0.02). The latter effect appeared to be dose-dependent.

Cytotoxicity of ingredients of various dental materials and related compounds in L2- and A549 cells

Various ingredients of dental materials and related compounds were tested for cytotoxicity in two alveolar epithelial cell lines (L2 and A549 cells). Release of lactate dehydrogenase (LDH) from cells was measured after incubation with the test substances for time intervals up to 48 h and expressed as percentage of total LDH content of lysed cells. Furthermore, the glutathione content of cells was determined in the nonmalignant L2 cells. Additionally, cell viability was assessed by microscopic examination. The highest cytotoxicity was observed with mercury compounds (methylmercuric chloride and mercury dichloride) in the range of 5-20 micromol/l. The composite components 2-hydroxyethylmethacrylate (HEMA) and triethleneglycoldimethacrylate (TEGDMA) showed time- and concentration-dependent effects of cytotoxicity at high concentrations (about 1-5 mmol/l). A time dependence for GSH decrease was mainly found for the composite components up to 12 h of cellular exposure. L2 cells were more sensitive to both mercury and composite compounds than A549 cells. Gold compounds (sodiumaurothiomalate and gold particles < 1.5 microm) did not produce any sign of toxic reactions. A time-dependent increased toxicity in pulmonary cell lines was found for the composite components HEMA and TEGDMA, but not for mercury and gold compounds.

Methylmercury inhibits glutamate uptake by synaptic vesicles from rat brain

Methylmercury (MeHg) is an environmental contaminant that continues to cause risk to human health. The toxic effects of MeHg on the CNS implicate the involvement of glutamatergic system. In this study, we evaluated the effects of MeHg on [3H]glutamate uptake by synaptic vesicles. MeHg inhibited [3H]glutamate uptake in a concentration dependent manner. Since glutamate uptake by synaptic vesicles is driven by an electrochemical gradient, formed across the vesicle membrane by a bafilomycin A(1)-sensitive H+-ATPase, we further investigated the effect of MeHg on activity of this enzyme. MeHg inhibited the H+-ATPase activity and also dissipated the proton gradient (DeltapH), indicating that MeHg decreased [3H]glutamate uptake involving the H+-ATPase activity. Until now, the toxic effects of MeHg on CNS were attributed mainly to an impairment of glial glutamate transporters. These findings contribute for the understanding of the neurotoxicity by MeHg, pointing to the involvement of vesicular glutamate.

Sensitivity to methylmercury-induced autoimmune disease in mice correlates with resistance to apoptosis of activated CD4+ lymphocytes

The sensitivity of splenic lymphoid cells to apoptosis induced by low concentrations of methylmercury (MeHgCl) has been examined in C57BL/6 and SJL mice, which are, respectively, resistant and sensitive to a genetically determined autoimmune disease induced by subtoxic doses of MeHgCl. To determine the implications of subtoxic doses of MeHgCl in the susceptibility of SJL mice to autoimmune disease, Concanavalin A (ConA) stimulated spleen cells from both mouse strains were treated in vitro with MeHgCl concentrations varying between 0.001 and 1.0 microM for 48h. Results have shown that ConA-activated splenic lymphoid cells from SJL mice increased in the presence of low concentrations of MeHgCl while the number of lymphoid cells from C57BL/6 mice rather decreased. Flow cytometric analysis of the cells showing a typical lymphoid forward scatter (FSC)/side scatter (SSC) pattern (region R1), and those characterized by a lower FSC and a higher SSC parameters (region R2), morphologically corresponding to apoptotic cells, revealed that lymphoid cells from C57BL/6 mice suffered a dose-dependent shift from region R1 toward region R2 when treated with concentrations ranging between 0.01 and 1 microM of MeHgCl. However, SJL splenic lymphoid cells cultured in the presence of low concentrations of MeHgCl proved more resistant to apoptosis. The level of apoptosis induced by MeHgCl in both regions was verified by AnnexinV-propidium iodide (PI) and TdT-mediated dUTP nick end labeling (TUNEL) immunolabelings. Phenotyping of lymphoid cells from both mouse strains cultured in the presence of low concentrations of MeHgCl and stimulated with ConA, indicated that CD4+ T cells from SJL mice increased while the corresponding cell subset from C57BL/6 mice became apoptotic. The resistance to apoptosis of ConA-activated lymphoid cells from SJL mice seemed related to an increase of CD4+ cells induced by the lower concentrations of MeHgCl (0.001 and 0.01 microM). However, these SJL cells were sensitive to anti-Fas-mediated apoptosis while residual anti-Fas-resistant cells from C57BL/6 mice were, themselves, sensitive to MeHgCl-induced apoptosis. The in vivo significance of these results has been confirmed by an observed increase in splenic cellularity and in the percentage of activated CD4+ cells from SJL mice. These increases were not observed in C57BL/6 mice.

Effect of arsenic and mercury speciation on inhibition of respiration rate in activated sludge systems

The inhibition effect of arsenite, As(III), arsenate, As(V), inorganic mercury, Hg(II) and methylmercury, MeHg, on the respiration rate of activated heterotrophic sludge microorganisms was evaluated. As(III) and MeHg were much more toxic to activated sludge than As(V) and Hg(II) respectively. The effect of various experimental parameters on the toxicity, such as sludge age, concentration of suspended solids and exposure time, was investigated. An increase of sludge age or the concentration of suspended solids reduces the observed inhibition. Longer exposure seems to dramatically increase the inhibition of As(III), MeHg and Hg(II) during the first hours of exposure, while the later inhibition increases at a slower rate. On the contrary, in the presence of As(V), 24 hours after exposure, the respiration rate was similar to that of the control biomass.

Methylmercury enhances arachidonic acid release and cytosolic phospholipase A2 expression in primary cultures of neonatal astrocytes

Cytosolic phospholipase A(2) (cPLA(2)) stimulates the hydrolysis of sn-2 ester bond in membrane phospholipids releasing arachidonic acid (AA) and lysophospholipids. The present study examined the effect of methylmercury (MeHg) on cPLA(2) activation and AA release in primary cultures of neonatal rat cerebral astrocytes. Astrocytes were preloaded overnight at 37 degrees C with 3H-AA to metabolically label phospholipids. The effect of MeHg on the activation of cPLA(2) was measured by the release of 3H-AA from astrocytes over 120 min. MeHg (5 microM) caused a significant increase in AA release at 10, 30, 60, and 120 min, whereas 2.5 microM MeHg significantly increased AA release only at 120 min. MeHg-induced increase in 3H-AA release was due to cPLA(2) activation, since arachidonyl trifluoromethyl ketone (AACOCF(3)), a selective inhibitor of cPLA(2), completely abolished MeHg's effect. Consistent with these observations, MeHg (5.0 and 10.0 microM) increased cPLA(2) mRNA (6 h) and cPLA(2) protein expression (5.0 and 10.0 microM; 24 h). The time-course of these effects suggests an immediate direct or indirect effect of MeHg on cPLA(2) activation and 3H-AA release as well as a long-term effect involving the induction of cPLA(2). Thin layer chromatographic analysis of 3H-AA-labeled phospholipids showed that MeHg-stimulated astrocyte 3H-AA release was not due to increased incorporation of 3H-AA into the putative substrates of cPLA(2). These results invoke cPLA(2) as a putative target for MeHg toxicity, and support the notion that cPLA(2)-stimulated hydrolysis and release of AA play a critical role in MeHg-induced neurotoxicity.

[Promoter effect of mercury chloride and methyl-mercury on human keratinocytes in culture]

Mercury has received considerable media focus because it is present in dental amalgams and seafood. There is potential exposure in gas meters, thermometers and fluorescent lamps workers. To evaluate its possible epigenetic carcinogen effect, cultures of human keratinocytes were treated with increasing doses of HgCl2 for 30 min, 24 h and of CH3HgCl for 24 h, respectively. The red neutral method was used to evaluate the doses of HgCl2 and CH3HgCl which had no cytotoxic effect. Then, the dye transfer method was used to investigate the gap junctions-mediated intercellular communication (GJIC). Cells were microinjected with Lucifer Yellow CH by using the Eppendorf Apparatus and the Leica inverted microscope. After 30 min incubation at the concentration of 10 microM, HgCl2 did not exert inhibition of GJIC. Conversely, after 24 h at the concentration of 10 nM, HgCl2 inhibited GJIC. Incubation with CH3HgCl at the concentration of 250 nM for 24 h reduced the number of fluorescent cells, thus denoting a inhibition of GJIC. Taken together our data demonstrated that: i) HgCl2 and CH3HgCl exerted an inhibitory effect upon GJIC; ii) HgCl2 resulted to inhibit GJIC at concentrations 25 folds lower than CH3HgCl. Further studies will be addressed to evaluate whether the reversal of GJIC inhibition could be obtained by withdrawal of toxic substance, or by the addition of a GJIC activator like the retinoic acid. Finally to shed light on the possible effect of mercury derivates at the transcriptional or translational levels, the expression profile of the connexin 43 gene after HgCl2 and CH3HgCl exposure of cultured human keratinocytes will be investigated.

Transport of a neurotoxicant by molecular mimicry: the methylmercury-L-cysteine complex is a substrate for human L-type large neutral amino acid transporter (LAT) 1 and LAT2

Methylmercury (MeHg) readily crosses cell membrane barriers to reach its target tissue, the brain. Although it is generally assumed that this rapid transport is due to simple diffusion, recent studies have demonstrated that MeHg is transported as a hydrophilic complex, and possibly as an L-cysteine complex on the ubiquitous L-type large neutral amino acid transporters (LATs). To test this hypothesis, studies were carried out in Xenopus laevis oocytes expressing two of the major L-type carriers in humans, LAT1-4F2 heavy chain (4F2hc) and LAT2-4F2hc. Oocytes expressing LAT1-4F2hc or LAT2-4F2hc demonstrated enhanced uptake of [(14)C]MeHg when administered as the L-cysteine or D,L-homocysteine complexes, but not when administered as the D-cysteine, N -acetyl-L-cysteine, penicillamine or GSH complexes. Kinetic analysis of transport indicated that the apparent affinities ( K (m)) of MeHg-L-cysteine uptake by LAT1 and LAT2 (98+/-8 and 64+/-8 microM respectively) were comparable with those for methionine (99+/-9 and 161+/-11 microM), whereas the V (max) values were higher for MeHg-L-cysteine, indicating that it may be a better substrate than the endogenous amino acid. Uptake and efflux of [(3)H]methionine and [(14)C]MeHg-L-cysteine were trans -stimulated by leucine and phenylalanine, but not by glutamate, indicating that MeHg-L-cysteine is both a cis - and trans -substrate. In addition, [(3)H]methionine efflux was trans -stimulated by leucine and phenylalanine even in the presence of an inwardly directed methionine gradient, demonstrating concentrative transport by both LAT1 and LAT2. The present results describe a major molecular mechanism by which MeHg is transported across cell membranes and indicate that metal complexes may form a novel class of substrates for amino acid carriers. These transport proteins may therefore participate in metal ion homoeostasis and toxicity.

Effects of methylmercury on human neuronal L-type calcium channels transiently expressed in human embryonic kidney cells (HEK-293)

Methylmercury (MeHg) disrupts the function of native, high voltage-activated neuronal Ca(2+) channels in several types of cells. However, the effects of MeHg on isolated Ca(2+) channel phenotypes have not been examined. The aim of the present study was to examine the action of MeHg on recombinant, neuronal L-type voltage-sensitive Ca(2+) channels. Human embryonic kidney cells (HEK-293) were transfected with human neuronal cDNA clones of the alpha(1C-1) subunit in combination with alpha(2b) and beta(3a) Ca(2+) channel subunits and the reporter jellyfish green fluorescent protein for transient expression. Current from expressed channels (I(Ba)) and their response to MeHg applied acutely were measured using whole-cell voltage-clamp recording techniques and Ba(2+) (5 mM) as charge carrier. Amplitude of I(Ba) in these cells was reduced by the dihydropyridine (DHP), nimodipine, and enhanced by Bay K8644 [S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3 pyridine carboxylic acid methyl ester]. MeHg (0.125-5.0 microM) caused a time- and concentration-dependent reduction in amplitude of the peak and sustained current through these channels. However, even at the highest concentration of MeHg tested, reduction of current amplitude by MeHg was incomplete. Washing with MeHg-free solution could not reverse its effects. The steady-state inactivation curve was unaltered by MeHg. Increasing the stimulation frequency or the extracellular Ba(2+) concentration each attenuated slightly the reduction in amplitude of I(Ba) by MeHg. In the presence of MeHg (5.0 microM), Bay K8644 still increased the remaining current, and nimodipine (10 microM) reduced residual current that was resistant to MeHg. Thus, although MeHg reduces the amplitude of recombinant, heterologously expressed L-type channel current, a portion of current is resistant to reduction by MeHg. Furthermore, DHP agonists and antagonists retain their ability to affect L-type Ca(2+) channel current even in the presence of MeHg.

Mercury-induced apoptosis in human lymphocytes: caspase activation is linked to redox status

There is growing evidence that heavy metals, in general, and mercurial compounds, in particular, are toxic to the human immune system. We have previously shown that methyl mercuric chloride (MeHgCl) is a potent human T-cell apoptogen; moreover, mitochondria appear to be a target organelle for the induction of cell death. The objective of this study was to determine the impact of MeHgCl on mitochondrial function in lymphocytes in terms of modulating reactive oxygen species (ROS) generation, thiol status, and caspase activation. Using the fluorescent probe, 3,3'-dihexyloxacarbocyanine, we demonstrated that exposure to MeHgCl for 1 h resulted in a profound decrease in the mitochondrial transmembrane potential. We next observed the release of cytochrome c from mitochondria into the cytosol; significant translocation was noted between 4 and 8 h following treatment with mercury. ROS generation was monitored by following the conversion of dihydroethidium to the fluorescent product, ethidium. Kinetic analysis indicated that ROS generation was maximal after 16 h of exposure to MeHgCl. The toxicant also depleted the thiol reserves of the cell; glutathione levels were depleted in a dose-dependent fashion reaching minimal levels at 16 h. Real-time RT-PCR analysis demonstrated a significant reduction in both glutathione S-transferase and glutathione peroxidase gene expression in mercury-treated cells. Finally, after 16 h of treatment with MeHgCl, we observed activation of caspase-8, -9, and -3 along with increased expression of caspase-8 and -9. We propose that the target organelle for MeHgCl is the mitochondrion and that induction of oxidative stress is critical to activation of death-signaling pathways. Additonally, mercury acts as a genotoxin significantly altering the expression of genes that affect cell survival and apoptosis.

Methylmercury induces neurite degeneration in primary culture of mouse dopaminergic mesencephalic cells

Methylmercury cation (MeHg) is an hazardous environmental pollutant with neurotoxic action. Little is known about the effects of MeHg on catecholaminergic neurons. In the present study we have used epifluorescence microscopy and confocal microscopy to investigate the alterations induced by MeHg in primary DA (dopaminergic) cells isolated from the ventral mesencephalon of CD-1 embryonic mice and cultured for six days in vitro. DA cells were identified in the multi-culture by immunocytochemistry using a tyrosine-hydroxylase antibody. The morphometric analysis of DA neurons exposed to 1 microM MeHg demonstrated a striking decrease in the number of neurites, indicative of cytoskeletal alteration. In addition, DA neurons displayed cell shrinkage and a significant increase of nuclei with chromatin condensation. Based on these results it is concluded that MeHg is highly toxic to primary DA neurons.