The comparative effects of methylmercuric chloride and mercuric chloride upon DNA synthesis in mouse fetal astrocytes in vitro

Single cell RNA sequencing detects persistent cell type- and methylmercury exposure paradigm-specific effects in a human cortical neurodevelopmental model

The developing human brain is uniquely vulnerable to methylmercury (MeHg) resulting in lasting effects especially in developing cortical structures. Here we assess by single-cell RNA sequencing (scRNAseq) persistent effects of developmental MeHg exposure in a differentiating cortical human-induced pluripotent stem cell (hiPSC) model which we exposed to in vivo relevant and non-cytotoxic MeHg (0.1 and 1.0 μM) concentrations. The cultures were exposed continuously for 6 days either once only during days 4-10, a stage representative of neural epithelial- and radial glia cells, or twice on days 4-10 and days 14-20, a somewhat later stage which includes intermediate precursors and early postmitotic neurons. After the completion of MeHg exposure the cultures were differentiated further until day 38 and then assessed for persistent MeHg-induced effects by scRNAseq. We report subtle, but significant changes in the population size of different cortical cell types/stages and cell cycle. We also observe MeHg-dependent differential gene expression and altered biological processes as determined by Gene Ontology analysis. Our data demonstrate that MeHg results in changes in gene expression in human developing cortical neurons that manifest well after cessation of exposure and that these changes are cell type-, developmental stage-, and exposure paradigm-specific.

Effects of gestational and lactational exposure to low dose mercury chloride (HgCl2) on behaviour, learning and hearing thresholds in WAG/Rij rats

We investigated the effects of inorganic mercury exposure during gestational/lactational periods on the behaviour, learning and hearing functions in a total of 32, 5-week-old and 5-month-old WAG/Rij rats (equally divided into 4 groups as 5-week and 5-month control mercury exposure groups). We evaluated the rats in terms of locomotor activity (LA), the Morris-water-maze (MWM) test and the passive avoidance (PA) test to quantify learning and memory performance; we used distortion product otoacoustic emission (DPOAE) tests to evaluate hearing ability. There were no significant differences between the 5-week-old rat groups in LA, and we detected a significant difference (p < 0.05) in the HgCl₂-treated group in PA, MWM and DPOAE tests compared with the control group. The HgCl₂-treated 5-week-old group exhibited worse emotional memory performance in PA, worse spatial learning and memory performances in MWM. There were no significant differences between the groups of 5-month-old rats in LA, MWM or PA. However, the DPOAE tests worsened in the mid- and high-frequency hearing thresholds. The HgCl₂-treated 5-month-old group exhibited the most hearing loss of all groups. Our results convey that mercury exposure in young rats may worsen learning and memory performances as well as hearing at high-frequency levels. While there was no statistically significant difference in the behavior and learning tests in adult rats, the DPOAE test produced poorer results. Early detection of effects of mercury exposure provides medicals team with an opportunity to determinate treatment regimens and mitigate ototoxicity. DPOAE test can be used in clinical and experimental research investigating heavy metal ototoxicity.

Evaluation of the effects of chronic intoxication with inorganic mercury on memory and motor control in rats

The aims of this study were to evaluate whether chronic intoxication with mercury chloride (HgCl₂), in a low concentration over a long time, can be deposited in the central nervous tissue and to determine if this exposure induces motor and cognitive impairments. Twenty animals were intoxicated for 45 days at a dose of 0.375 mg/kg/day. After this period, the animals underwent a battery of behavioral tests, in a sequence of open field, social recognition, elevated T maze and rotarod tests. They were then sacrificed, their brains collected and the motor cortex and hippocampus dissected for quantification of mercury deposited. This study demonstrates that long-term chronic HgCl₂ intoxication in rats promotes functional damage. Exposure to HgCl₂ induced anxiety-related responses, short- and long-term memory impairments and motor deficits. Additionally, HgCl₂ accumulated in both the hippocampus and cortex of the brain with a higher affinity for the cortex.

Comparison of cytopathological changes induced by mercury chloride exposure in renal cell lines (VERO and BGM)

The response to mercury chloride was assessed in two cell lines of renal origin, determining the range of toxic concentrations by Neutral Red assay after 24-h of exposure. Morphological changes in the Buffalo Green Monkey (BGM) and VERO cell lines after exposure to subcytotoxic doses (0.045 and 0.038mM, respectively) equivalent to EC10 (effective concentrations 10%) of mercury chloride were evaluated at the structural and ultrastructural level by optic, transmission and scanning microscopy. Using transmission electron microscopy, the most notable findings in treated cells were the presence of intracytoplasmic inclusion bodies and apoptotic bodies. Scanning microscopy pointed to a cell with a disrupted perinuclear region and a decreased number of surface microvilli. Similar alterations in both in vivo and in vitro experiments have been described by other authors. We conclude that BGM and VERO renal cell lines can be considered as useful tools for toxicological studies involving mercury chloride.

Glutamate: a potential mediator of inorganic mercury neurotoxicity

Exposure to mercury vapor (Hg0) produces neurotoxic effects which are for the most part subsequent to its biotransformation in brain to the mercuric cation (Hg2 +), which has an exceptionally strong affinity towards the SH groups in proteins. However, neurologic symptoms are often encountered in subjects in which Hg+ concentration in the brain remains in the submicromolar range, markedly below the anticipated threshold for direct inhibition of cerebral metabolism and function. In this report we review biochemical and morphological evidence obtained in this and other laboratories in tissue culture studies suggesting that in such instances mercury neurotoxicity may be mediated by excitotoxic activity of glutamate (GLU). Mercuric chloride (MC) at 1 microM concentration (or less) inhibits GLU uptake and stimulates GLU release in cultured astrocytes, which in vivo is likely to result in excessive GLU accumulation in the extracellular space of the CNS. Inhibition of GLU uptake and stimulation of GLU release by MC may be attenuated by addition to the cultures of a cell membrane-penetrating agent dithiothreitol (DTT) but not of glutathione (GSH), which is not transported to the inside of the cells. However, MC-stimulated release of GLU is suppressed when the intracellular GSH levels are increased by metabolic manipulation. The results indicate that the MC-vulnerable SH groups critical for GLU transport are located within the astrocytic membranes. Ultrastructural evidence for GLU-mediated MC neurotoxicity came from studies in an organotypic culture of rat cerebellum. We have shown that: 1) 1 microM MC lowers the threshold of GLU neurotoxicity, 2) the combined neurotoxic effect of GLU plus MC is attenuated by DTT but not by GSH, which is consistent with the involvement of impaired astrocytic GLU transport, and 3) neuronal damage induced by GLU plus MC becomes less accentuated in a medium with dizocilpine (MK-801), a noncompetitive NMDA receptor antagonist.

Genotoxicity of mercury compounds. A review

This article reviews literature data concerning the genotoxicity of 29 mercury-containing agents, including laboratory compounds as well as ingredients of preparations used as fungicides, dyes, disinfectants and drugs. A variety of genetic end-points were investigated in bacteria, yeasts, moulds, plants, insects, cultured cells from fishes, rodents or humans, aquatic organisms, amphibians, mammalia and exposed humans. The overall evaluation is quite complex. Mercury compounds failed to induce point mutations in bacteria but often exerted clastogenic effects in eukaryotes, especially by binding SH groups and acting as spindle inhibitors, thereby causing c-mitosis and consequently aneuploidy and/or polyploidy. Inorganic mercury compounds were also found to induce the generation of reactive oxygen species and glutathione depletion in cultured mammalian cells. Although different mercury compounds tended to produce qualitatively comparable genetic effects, which suggests the involvement of a common toxic entity, methylmercury derivatives and other ionizable organomercury compounds were more active in short-term tests than either non-ionizable mercury compounds (e.g., dimethylmercury) or inorganic mercury salts (e.g., mercuric chloride). The results of cytogenetic monitoring in peripheral blood lymphocytes of individuals exposed to elemental mercury or mercury compounds from accidental, occupational or alimentary sources were either negative or borderline or uncertain as to the actual role played by mercury in some positive findings. Both genotoxic and non-genotoxic mechanisms may contribute to the renal carcinogenicity of mercury, which so far has been convincingly demonstrated only in male rodents treated with methylmercury chloride.

Changes of the Na/K ATPase activity in the cerebral cortical microvessels of rat after single intraperitoneal administration of mercuric chloride: histochemical demonstration with light and electron microscopy

Since inorganic mercury salts only poorly penetrate the cerebral microvascular endothelial cells comprising the blood-brain barrier (BBB), their neurotoxicity may be predicted to result from interference with BBB transport enzymes. In the present study, we tested the effect of mercuric chloride (HgCl2) on Na+/K+ ATPase activity, a key enzyme involved in the ion transport in and out of the brain. Routine histochemical staining in conjunction with light and electron microscopy was used to evaluate the changes in the Na+/K+ ATPase activity in cerebral cortical microvessels of rats who received a single intraperitoneal injection of 6 mg/kg HgCl2. At 1 h after HgCl2 administration, light microscopy revealed uniform reduction of the Na+/K+ ATPase reaction in all cortical layers. Electron microscopy confirmed the enzyme reaction to be very weak to completely absent in both the luminal and abluminal endothelial cell membranes, and the luminal plasmalemma showed invaginations and pinocytic vesicles indicative of changes in its transport functions. The enzyme inhibition coincided with, and was likely to contribute to, profound perivascular swelling, involving mainly the astrocytic endfeet. The enzyme activity showed a partial recovery 18 h after HgCl2 treatment, mainly in cortical layers II and III. After 5 days, the recovery of the enzyme activity appeared complete as observed by light and electron microscopy. The recovery of the microvascular Na+/K+ ATPase coincided with the appearance of a strongly positive Na+/K+ ATPase reaction in the adjacent astrocytic processes and with the diminution of perivascular swelling.

Mercuric chloride affects protein secretion in rat primary hepatocyte cultures: a biochemical ultrastructural, and gold immunocytochemical study

The toxicity of mercury on hepatocytes was studied at the ultrastructural, biochemical, and immunocytochemical levels. Albumin metabolism was examined because it is a representative liver-specific function. A novel cytochemical method using the protein A-gold technique for the in situ localization of albumin in hepatocyte cultures was applied. Primary rat hepatocyte cultures were exposed to increasing HgCl2 concentrations. Cytotoxicity was assessed by measuring the release of lactic dehydrogenase from the cells. At the highest exposure concentration tested (50 microM), Hg was found to be significantly cytotoxic in contrast to what occurred at 5.0 and 0.5 microM. The level of albumin secreted, as measured by ELISA, was decreased by approximately 38% at 5.0 microM HgCl2 and was found not to be different from that of controls at lower concentrations. The ultrastructural analysis showed that hepatocytes treated with 5.0 microM HgCl2 undergo drastic morphological changes such as a decreased number of ribosomes associated with the rough endoplasmic reticulum, and the disappearance of the latter organelle, proliferation of the smooth endoplasmic reticulum, and dilatation of both the Golgi apparatus and the biliary canaliculus-like structures. Immunocytochemical detection of albumin-immunoreactive sites using protein A-gold labeling further revealed that these were less abundant in hepatocytes treated with 5.0 microM HgCl2 (-64%) as compared to control preparations. These results suggest that one of the effects of mercury on hepatocytes is to affect liver-specific functions such as albumin production, possibly through interference with ribosomal function. This study also demonstrates for the first time the applicability of the high-resolution protein A-gold technique for toxicological investigations on hepatocytes in vitro.

Mercuric Chloride Affects Morphology and Thymidine Incorporation into DNA in Astroglial Cells in Culture

Primary astroglial cell cultures from neonatal rat cerebral cortex revealed morphological changes when incubated in HgCl2 at various concentrations. After incubation for 30 minutes with 5 x 10-6 HgCl2, most cells changed their form from a flat to a more rounded shape and extended processes. After longer incubations (>60 minutes) or at higher concentrations (≥10-5M), vacuoles appeared in many cells. [3H]-Thymidine incorporation into DNA was slightly increased after acute incubation in 10-9M HgCl2, while decreases were obtained after incubation in the μM range. The results are important due to the functions of astroglia during development and in the mature nervous system.

Specificity and reversibility of the inhibition by HgCl2 of glutamate transport in astrocyte cultures

Inhibition of glutamate transport is a potential indirect cause of excitotoxic damage by glutamate in the CNS. The mercuric ion, the form in which metallic mercury vapor is believed to exert its neurotoxic action, is a known inhibitor of amino acid transport. This study examines the specificity with which HgCl2 inhibits glutamate transport in mouse cerebral astrocytes by means of comparative measurements of 2-deoxyglucose uptake. Uptake of 2-deoxyglucose is an index of glucose utilization that reflects the function of Na+,K+-ATPase and hexokinase, and is sensitive to Na+ entry. The kinetic parameters, ionic dependence, and substrate specificity of glutamate transport in these astrocyte cultures were consistent with the commonly occurring system designated X-AG. Acute exposure to 0.5 microM HgCl2 inhibited by 50% the initial rate of glutamate transport but did not affect 2-deoxyglucose uptake. Glutamate transport was not detectably inhibited by Al2+, Pb2+, Co2+, Sr2+, Cd2+, or Zn2+ (10 microM as chlorides). The inhibitory action of 0.5 microM HgCl2 on glutamate transport was rapidly reversible. The action of 1-2 microM HgCl2 was progressive when exposures were extended to 1-3 h, and was more slowly reversible. These results suggest that Hg2+ can impair glial glutamate transport reversibly at exposure levels that do not compromise some other vital cell functions.

A cell line with decreased sensitivity to the methyl mercury-induced stimulation of alpha-amanitin sensitive RNA synthesis in isolated nuclei

1. In nuclei isolated from cells of the B50 rat neuroblastoma line the stimulatory effect of methyl mercury on alpha-amanitin-sensitive RNA synthesis is very much reduced compared to the stimulatory effect in HeLa nuclei (see: Frenkel G. D. and Randles K. (1982) Specific stimulation of alpha-amanitin-sensitive RNA synthesis in isolated HeLa nuclei by methyl mercury. J. biol. Chem. 257, 6275-6279). 2. The stimulatory effect of another mercury compound, p-hydroxymercuribenzoate, was also much less pronounced in the B50 nuclei. 3. Similar results were obtained with nuclei isolated from B50 cells which had been induced to differentiate by exposure to dibutaryl cyclic AMP. 4. Nuclei isolated from cells of another rat neuroblastoma line (B35), and nuclei from cells of a human neuroblastoma line both exhibited levels of stimulation similar to that of HeLa nuclei. 5. The B50 and HeLa cells were also compared as to their sensitivity to other effects of methyl mercury.

The enhanced rate of transcription of methyl mercury-exposed DNA by RNA polymerase is not sufficient to explain the stimulatory effect of methyl mercury on RNA synthesis in isolated nuclei

Previous work demonstrated two stimulatory effects of methyl mercury on nucleic acid synthesis: (1) in isolated nuclei, methyl mercury stimulates RNA synthesis which is catalyzed by RNA polymerase II [Frenkel and Randles, J. Biol. Chem. 257, 6275-6279 (1982)]. (2) Brief exposure of purified DNA to methyl mercury increases the rate of its transcription by purified RNA polymerase II [Frenkel, Cain, and Chao, Biochem. Biophys. Res. Commun. 127, 849-856 (1985)]. The latter effect was considered as a possible mechanism of the former. Two lines of evidence are presented here which demonstrate that the latter effect is not a sufficient explanation for the former. (1) Mercuric perchlorate has been found to increase the rate of DNA transcription by purified polymerase and the template properties of the mercuric perchlorate-exposed DNA have been found to resemble those of methyl mercury-exposed DNA. Nevertheless, mercuric perchlorate has been shown not to stimulate RNA synthesis in isolated HeLa nuclei. (2) In isolated nuclei of the B50 rat neuroblastoma cell line, RNA synthesis has been found to be stimulated only minimally by methyl mercury. Nevertheless, RNA polymerase II purified from the B50 cells has been found to transcribe methyl mercury-exposed DNA at a higher rate than unexposed control DNA.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

Changes in the molecular structure of mouse fetal astrocyte nucleosomes produced in vitro by methylmercuric chloride

The fluorescent probe N-(3-pyrene)maleimide, which specifically labels the cysteine residues of histone H3 within the nucleosome, was used to monitor changes in the nucleosomal structure of mouse fetal astrocytes exposed to varying concentrations of methylmercuric chloride. Methylmercuric chloride treatment (10 microM) for 6 hr produced a significant decrease in the degree of fluorescence of the probe. The decrease was much smaller following a 4-hr incubation period. These results correlate with recent observations showing that significant changes in the thymidine labeling index occur following 4-6 hr of exposure to methylmercury (MeHg). It is hypothesized that MeHg enters the cells during the growth phase and attaches to the protein moiety of the nucleosome in or near the cysteine groups of histone H3, thus diminishing the binding capacity of the fluorescent probe. Addition of a detergent (sodium dodecyl sulfate) resulted in only a small increase in the degree of fluorescence of the treated nucleosomes as compared to controls, showing that the interaction of MeHg with the nuclear proteins was not dissociated by detergent. In view of the strong interaction between DNA and the histone dimer H3-H4 and the potential importance of the latter in gene regulation, these results suggest an additional means by which MeHg may exert its toxic effects.

Surface charge alterations in mouse fetal astrocytes due to methyl mercury: an ultrastructural study with cationized ferritin

A brief exposure of low dose methylmercuric chloride to monolayer cultures of mouse fetal astrocytes caused a marked shift in the distribution of anionic groups on the surface membrane as evidenced by irregular disruption of cationized ferritin. It is suggested that one of the earliest changes following methyl mercury exposure in embryonic astrocytes includes alterations in the surface charge which may in turn trigger cascading toxic actions of methyl mercury in the developing brain.