Interactions of methylmercury with rat primary astrocyte cultures: inhibition of rubidium and glutamate uptake and induction of swelling

Methylmercury neurotoxicity: Beyond the neurocentric view

Mercury is a highly toxic metal widely used in human activities worldwide, therefore considered a global public health problem. Many cases of mercury intoxication have occurred in history and represent a huge challenge nowadays. Of particular importance is its methylated form, methylmercury (MeHg). This mercurial species induces damage to several organs in the human body, especially to the central nervous system. Neurological impairments such as executive, memory, motor and visual deficits are associated with MeHg neurotoxicity. Molecular mechanisms involved in MeHg-induced neurotoxicity include excitotoxicity due to glutamatergic imbalance, disturbance in calcium homeostasis and oxidative balance, failure in synaptic support, and inflammatory response. Although neurons are largely affected by MeHg intoxication, they only represent half of the brain cells. Glial cells represent roughly 50 % of the brain cells and are key elements in the functioning of the central nervous system. Particularly, astrocytes and microglia are deeply involved in MeHg-induced neurotoxicity, resulting in distinct neurological outcomes depending on the context. In this review, we discuss the main findings on astroglial and microglial involvement as mediators of neuroprotective and neurotoxic responses to MeHg intoxication. The literature shows that these responses depend on chemical and morphophysiological features, thus, we present some insights for future investigations, considering the particularities of the context, including time and dose of exposure, brain region, and species of study.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Neurotoxicity and the Global Worst Pollutants: Astroglial Involvement in Arsenic, Lead, and Mercury Intoxication

Environmental pollution is a global threat and represents a strong risk factor for human health. It is estimated that pollution causes about 9 million premature deaths every year. Pollutants that can cross the blood-brain barrier and reach the central nervous system are of special concern, because of their potential to cause neurological and development disorders. Arsenic, lead and mercury are usually ranked as the top three in priority lists of regulatory agencies. Against xenobiotics, astrocytes are recognised as the first line of defence in the CNS, being involved in virtually all brain functions, contributing to homeostasis maintenance. Here, we discuss the current knowledge on the astroglial involvement in the neurotoxicity induced by these pollutants. Beginning by the main toxicokinetic characteristics, this review also highlights the several astrocytic mechanisms affected by these pollutants, involving redox system, neurotransmitter and glucose metabolism, and cytokine production/release, among others. Understanding how these alterations lead to neurological disturbances (including impaired memory, deficits in executive functions, and motor and visual disfunctions), by revisiting the current knowledge is essential for future research and development of therapies and prevention strategies.

Long-Term Exposure to Inorganic Mercury Leads to Oxidative Stress in Peripheral Blood of Adult Rats

Mercury chloride (HgCl₂) is a compound found in the environment that presents low risk due to low liposolubility. Considering the importance of blood as access rout to the systemic distribution of this toxicant to the organism as well as functions performed by it, this study aimed to investigate the effects of HgCl₂ on the peripheral blood of rats, evaluating the oxidative biochemistry, blood count, and morphology of cell populations. For this, 20 adult Wistar male rats were divided into control (n = 10) and exposed (n = 10) groups and received distilled water or HgCl₂ at a dose of 0.375 mg/kg for 45 days, respectively, through intragastric gavage. Then, the animals were euthanized and the blood was collected for total mercury (Hg) levels determination, complete blood and reticulocyte count, oxidative biochemistry by Trolox Equivalent Antioxidant Capacity (TEAC), reduced glutathione (GSH) levels, superoxide dismutase activity (SOD), thiobarbituric acid reactive substances (TBARS), and nitric oxide (NO), in blood cells and plasma. Long-term exposure increased total Hg in plasma and blood cells. In blood cells, only TEAC has decreased; in plasma, the HgCl₂ increased TBARS and NO levels, followed by a decrease in TEAC and GSH levels. There were no quantitative changes in reticulocytes, erythrocytes, and hemoglobin; however, the number of leukocytes have increased and platelets have decreased. Our results suggest that even in the face of low toxicity when compared with other mercury species, HgCl₂ at low doses is able to modulate the systemic redox balance and affect some blood cell populations.

Revisiting Astrocytic Roles in Methylmercury Intoxication

Intoxication by heavy metals such as methylmercury (MeHg) is recognized as a global health problem, with strong implications in central nervous system pathologies. Most of these neuropathological conditions involve vascular, neurotransmitter recycling, and oxidative balance disruption leading to accelerated decline in fine balance, and learning, memory, and visual processes as main outcomes. Besides neurons, astrocytes are involved in virtually all the brain processes and perform important roles in neurological response following injuries. Due to astrocytes' strategic functions in brain homeostasis, these cells became the subject of several studies on MeHg intoxication. The most heterogenous glial cells, astrocytes, are composed of plenty of receptors and transporters to dialogue with neurons and other cells and to monitor extracellular environment responding tightly through fluctuation of cytosolic ions. The overall toxicity of MeHg might be determined on the basis of the balance between MeHg-mediated injury to neurons and protective responses from astrocytes. Although the role of neurons in MeHg intoxication is relatively well-established, the role of the astrocytes is only beginning to be understood. In this review, we update the information on astroglial modulation of the MeHg-induced neurotoxicity, providing remarks on their protective and deleterious roles and insights for future studies.

Neurotoxicity of mercury: an old issue with contemporary significance

Mercury exerts a variety of toxic effects, depending on the specific compound and route of exposure. However, neurotoxicity in virtue of its consequence to health causes the greatest concern for toxicologists. This is particularly true regarding fetal development, where neurotoxic effects are much more severe than in adults, and the toxicity threshold is lower. Here, we review the major concepts regarding the neurotoxicity of mercury compounds (mercury vapor; methylmercury and ethylmercury), from exposure routes to toxicokinetic particularities leading to brain deposition and the development of neurotoxic effects. Albeit research on the neurotoxicity of mercury compounds has significantly advanced from the second half of the twentieth century onwards, several grey areas regarding the mechanism of toxicity still exist. Thus, we emphasize research advances during the last two decades concerning the molecular interactions of mercury which cause neurotoxic effects. Highlights include the disruption of glutamate signaling and excitotoxicity resulting from exposure to mercury and the interaction with redox active residues such as cysteines and selenocysteines which are the premise accounting for the disruption of redox homeostasis caused by mercurials. We also address how immunotoxic effects at the CNS, namely microglia and astrocyte activation modulate developmental neurotoxicity, a major topic in contemporary research.

Brain morphometric profiles and their seasonal modulation in fish (Liza aurata) inhabiting a mercury contaminated estuary

Mercury (Hg) is a potent neurotoxicant known to induce important adverse effects on fish, but a deeper understanding is lacking regarding how environmental exposure affects the brain morphology and neural plasticity of specific brain regions in wild specimens. In this work, it was evaluated the relative volume and cell density of the lateral pallium, hypothalamus, optic tectum and molecular layer of the cerebellum on wild Liza aurata captured in Hg-contaminated (LAR) and non-contaminated (SJ) sites of a coastal system (Ria de Aveiro, Portugal). Given the season-related variations in the environment that fish are naturally exposed, this assessment was performed in the winter and summer. Hg triggered a deficit in cell density of hypothalamus during the winter that could lead to hormonal dysfunctions, while in the summer Hg promoted larger volumes of the optic tectum and cerebellum, indicating the warm period as the most critical for the manifestation of putative changes in visual acuity and motor-dependent tasks. Moreover, in fish from the SJ site, the lateral pallium relative volume and the cell density of the hypothalamus and optic tectum were higher in the winter than in summer. Thus, season-related stimuli strongly influence the size and/or cell density of specific brain regions in the non-contaminated area, pointing out the ability of fish to adapt to environmental and physiological demands. Conversely, fish from the Hg-contaminated site showed a distinct seasonal profile of brain morphology, presenting a larger optic tectum in the summer, as well as a larger molecular layer of the cerebellum with higher cell density. Moreover, Hg exposure impaired the winter-summer variation of the lateral pallium relative size (as observed at SJ). Altogether, seasonal variations in fish neural morphology and physiology should be considered when performing ecotoxicological studies in order to better discriminate the Hg neurotoxicity.

Unveiling the neurotoxicity of methylmercury in fish (Diplodus sargus) through a regional morphometric analysis of brain and swimming behavior assessment

The current study aims to shed light on the neurotoxicity of MeHg in fish (white seabream - Diplodus sargus) by the combined assessment of: (i) MeHg toxicokinetics in the brain, (ii) brain morphometry (volume and number of neurons plus glial cells in specific brain regions) and (iii) fish swimming behavior (endpoints associated with the motor performance and the fear/anxiety-like status). Fish were surveyed for all the components after 7 (E7) and 14 (E14) days of dietary exposure to MeHg (8.7μgg^-1), as well as after a post-exposure period of 28days (PE28). MeHg was accumulated in the brain of D. sargus after a short time (E7) and reached a maximum at the end of the exposure period (E14), suggesting an efficient transport of this toxicant into fish brain. Divalent inorganic Hg was also detected in fish brain along the experiment (indicating demethylation reactions), although levels were 100-200 times lower than MeHg, which pinpoints the organic counterpart as the great liable for the recorded effects. In this regard, a decreased number of cells in medial pallium and optic tectum, as well as an increased hypothalamic volume, occurred at E7. Such morphometric alterations were followed by an impairment of fish motor condition as evidenced by a decrease in the total swimming time, while the fear/anxiety-like status was not altered. Moreover, at E14 fish swam a greater distance, although no morphometric alterations were found in any of the brain areas, probably due to compensatory mechanisms. Additionally, although MeHg decreased almost two-fold in the brain during post-exposure, the levels were still high and led to a loss of cells in the optic tectum at PE28. This is an interesting result that highlights the optic tectum as particularly vulnerable to MeHg exposure in fish. Despite the morphometric alterations reported in the optic tectum at PE28, no significant changes were found in fish behavior. Globally, the effects of MeHg followed a multiphasic profile, where homeostatic mechanisms prevented circumstantially morphometric alterations in the brain and behavioral shifts. Although it has become clear the complexity of matching brain morphometric changes and behavioral shifts, motor-related alterations induced by MeHg seem to depend on a combination of disruptions in different brain regions.

Dysregulation of Glutamate Cycling Mediates Methylmercury-Induced Neurotoxicity

To examine the toxicological implications of glutamate, this chapter will focus specifically on its impact in the brain. More explicitly, it will illustrate the role glutamate plays in mediating methylmercury (MeHg)-induced neurotoxicity. In this chapter, one intends to highlight the processes that occur prior to glutamate-stimulated excitotoxicity and subsequent neurodegeneration. As such, it will emphasize three main routes by which MeHg alters glutamate homeostasis. It is essential to recognize that these effects are not mutually exclusive, and that they synergistically influence glutamate dysregulation. Furthermore, the consequences of MeHg exposure will be presented here as a direct pathway; however, it must be noted these effects occur simultaneously. First, glutamate uptake will be reviewed emphasizing the function of astrocytes. Next, the induction of oxidative stress by MeHg exposure will be discussed. This process has a two-fold effect on glutamate homeostasis by (1) inhibiting extracellular glutamate uptake and (2) altering transcription of genes vital to glutamate cycling. Finally, the impact glutamate dysregulation has on glutathione synthesis will be examined. Although this chapter centers on the link between glutamate and MeHg toxicity, it is imperative that the reader acknowledges the processes discussed here can be extended to any pro-oxidant.

The use of glial data in human health assessments of environmental contaminants

Central nervous system (CNS) glia (i.e., astrocytes, microglia, and oligodendrocytes) are essential for maintaining neuronal homeostasis, and they orchestrate an organized cellular response to CNS injury. In addition to their beneficial roles, studies have demonstrated that disrupted glial function can have disastrous consequences on neuronal health. While effects on neuron-supportive glia are important to consider when evaluating neurotoxicity risk, interpreting glial changes is not always straightforward, particularly when attempting to discern pro-neurotoxic phenotypes from homeostatic processes or adaptive responses. To better understand how glia have been characterized and used in human health assessments of environmental contaminants (e.g., chemicals), an evaluation of all finalized assessments conducted by the U.S. Environmental Protection Agency's influential Integrated Risk Information System (IRIS) program between 1987 and 2013 was performed. Human health assessments to date have placed a clear emphasis on the neuronal cell response to potential toxicants, although more recent assessments increasingly include descriptions of glial changes. However, these descriptions are generally brief and non-specific, and they primarily consist of documenting gliosis following overt neuronal injury. As research interest in this topic continues to increase, methods for evaluating changes in glia continue to be expanded and refined, and assessors' confidence in the reliability of these data is likely to rise. Thus, glial data are anticipated to have an increasingly influential impact on the interpretation of neurotoxicity risk and underlying mechanisms. As our understanding of the complex roles these cells play grows, this knowledge is expected to support the inclusion of more extensive and specific descriptions of glial changes, including informed interpretations of the potential impact on CNS health, in future human health assessments.

Methylmercury alters the activities of Hsp90 client proteins, prostaglandin E synthase/p23 (PGES/23) and nNOS

Methylmercury (MeHg) is a persistent pollutant with known neurotoxic effects. We have previously shown that astrocytes accumulate MeHg and play a prominent role in mediating MeHg toxicity in the central nervous system (CNS) by altering glutamate signaling, generating oxidative stress, depleting glutathione (GSH) and initiating lipid peroxidation. Interestingly, all of these pathways can be regulated by the constitutively expressed, 90-kDa heat shock protein, Hsp90. As Hsp90 function is regulated by oxidative stress, we hypothesized that MeHg disrupts Hsp90-client protein functions. Astrocytes were treated with MeHg and expression of Hsp90, as well as the abundance of complexes of Hsp90-neuronal nitric oxide synthase (nNOS) and Hsp90-prostaglandin E synthase/p23 (PGES/p23) were assessed. MeHg exposure decreased Hsp90 protein expression following 12 h of treatment while shorter exposures had no effect on Hsp90 protein expression. Interestingly, following 1 or 6 h of MeHg exposure, Hsp90 binding to PGES/p23 or nNOS was significantly increased, resulting in increased prostaglandin E₂ (PGE₂) synthesis from MeHg-treated astrocytes. These effects were attenuated by the Hsp90 antagonist, geldanmycin. NOS activity was increased following MeHg treatment while cGMP formation was decreased. This was accompanied by an increase in •O₂⁻ and H₂O₂ levels, suggesting that MeHg uncouples NO formation from NO-dependent signaling and increases oxidative stress. Altogether, our data demonstrates that Hsp90 interactions with client proteins are increased following MeHg exposure, but over time Hsp90 levels decline, contributing to oxidative stress and MeHg-dependent excitotoxicity.

Methylmercury-induced alterations in astrocyte functions are attenuated by ebselen

Methylmercury (MeHg) preferentially accumulates in glia of the central nervous system (CNS), but its toxic mechanisms have yet to be fully recognized. In the present study, we tested the hypothesis that MeHg induces neurotoxicity via oxidative stress mechanisms, and that these effects are attenuated by the antioxidant, ebselen. Rat neonatal primary cortical astrocytes were pretreated with or without 10 μM ebselen for 2h followed by MeHg (0, 1, 5, and 10 μM) treatments. MeHg-induced changes in astrocytic [(3)H]-glutamine uptake were assessed along with changes in mitochondrial membrane potential (ΔΨ(m)), using the potentiometric dye tetramethylrhodamine ethyl ester (TMRE). Western blot analysis was used to detect MeHg-induced ERK (extracellular-signal related kinase) phosphorylation and caspase-3 activation. MeHg treatment significantly decreased (p<0.05) astrocytic [(3)H]-glutamine uptake at all time points and concentrations. Ebselen fully reversed MeHg's (1 μM) effect on [(3)H]-glutamine uptake at 1 min. At higher MeHg concentrations, ebselen partially reversed the MeHg-induced astrocytic inhibition of [(3)H]-glutamine uptake [at 1 min (5 and 10 μM) (p<0.05); 5 min (1, 5 and 10 μM) (p<0.05)]. MeHg treatment (1h) significantly (p<0.05) dissipated the ΔΨ(m) in astrocytes as evidenced by a decrease in mitochondrial TMRE fluorescence. Ebselen fully reversed the effect of 1 μM MeHg treatment for 1h on astrocytic ΔΨ(m) and partially reversed the effect of 5 and 10 μM MeHg treatments for 1h on ΔΨ(m). In addition, ebselen inhibited MeHg-induced phosphorylation of ERK (p<0.05) and blocked MeHg-induced activation of caspase-3 (p<0.05-0.01). These results are consistent with the hypothesis that MeHg exerts its toxic effects via oxidative stress and that the phosphorylation of ERK and the dissipation of the astrocytic mitochondrial membrane potential are involved in MeHg toxicity. In addition, the protective effects elicited by ebselen reinforce the idea that organic selenocompounds represent promising strategies to counteract MeHg-induced neurotoxicity.

Comparison of alterations in amino acids content in cultured astrocytes or neurons exposed to methylmercury separately or in co-culture

Methylmercury (MeHg) is an environmental toxicant that induces enduring neuropsychological deficits in humans. Although the mechanisms associated with MeHg-induced neurotoxicity have not yet been fully elucidated, some lines of evidence point out to excitatory amino acids dyshomeostasis as an important outcome of MeHg exposure. The present study was designed to characterize the effects of MeHg on amino acid content in co-cultured astrocytes and neurons or in each cell type under solitary conditions. The results showed that glutamate concentrations significantly decreased in neurons, but not in astrocyte cultures exposed to 10 microM MeHg. The decrease in neurons was fully reversed when these cells were co-cultured with astrocytes. The content of other amino acids (aspartate, alanine, glycine and serine) decreased upon exposure to 10 microM MeHg in both neurons and astrocytes cultured in solitary conditions, although the effect was generally smaller in astrocytes than in neurons. However, the content of these amino acids in each of the cell types was indistinguishable from controls when co-cultures were treated with MeHg. Overall, the results indicate that astrocytes, which are more resistant to amino acid modulation by MeHg, can (i) mitigate the effects of MeHg that occur in neurons cultured in solitary conditions and (ii) become themselves more MeHg resistant in the presence of neurons. Delineating the mechanisms underlying the mutual neuroprotective effects of astrocytes and neurons in co-culture to MeHg-induced amino acid imbalance requires further investigation.

Hydrogen sulfide protects astrocytes against H(2)O(2)-induced neural injury via enhancing glutamate uptake

Excess extracellular glutamate, the main excitatory neurotransmitter, may result in excitotoxicity and neural injury. The present study was designed to study the effect of hydrogen sulfide (H(2)S), a novel neuromodulator, on hydrogen peroxide (H(2)O(2)) -induced glutamate uptake impairment and cellular injuries in primary cultured rat cortical astrocytes. We found that NaHS (an H(2)S donor, 0.1-1000 microM) reversed H(2)O(2)-induced cellular injury in a concentration-dependent manner. This effect was attenuated by L-trans-pyrrolidine-2,4-dicarboxylic (PDC), a specific glutamate uptake inhibitor. Moreover, NaHS significantly increased [(3)H]glutamate transport in astrocytes treated with H(2)O(2), suggesting that H(2)S may protect astrocytes via enhancing glutamate uptake function. NaHS also reversed H(2)O(2)-impaired glutathione (GSH) production. Blockade of glutamate uptake with PDC attenuated this effect, indicating that the effect of H(2)S on GSH production is secondary to the stimulation of glutamate uptake. In addition, it was also found that H(2)S may promote glutamate uptake activity via decreasing ROS generation, enhancing ATP production and suppressing ERK1/2 activation. In conclusion, our findings provide direct evidence that H(2)S has potential therapeutic value for oxidative stress-induced brain damage via a mechanism involving enhancing glutamate uptake function.

Dynamic effects of Hg2+-induced changes in cell volume

Using a microfluidic volume sensor, we studied the dynamic effects of Hg2+ on hypotonic stress-induced volume changes in CHO cells. A hypotonic challenge to control cells caused them to swell but did not evoke a significant regulatory volume decrease (RVD). Treatment with 100 muM HgCl2 caused a substantial increase in the steady-state volume following osmotic stress. Continuous hypotonic challenge following a single 10-min exposure to HgCl2 produced a biphasic volume increase with a steady-state volume 100% larger than control cells. Repeated hypotonic challenges to cells exposed once to Hg2+ resulted in a sequential approach to the same steady-state volume. Stimulation after reaching steady state caused a reduction in peak cell volume. Repeated stimulation was different than continuous stimulation resulting in a more rapid approach to steady state. Substituting extracellular Na+ with impermeant NMDG+ in the hypotonic solution produced a rapid RVD-like volume decrease and eliminated the Hg2+-induced excess swelling. The volume decrease in the presence of Hg2+ was inhibited by tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium, blockers of K+ and Cl(-) channels, respectively, suggesting that part of the Hg2+ effect was increasing NaCl influx over KCl efflux. The presence of multiple phases of steady-state volume and their sensitivity to the stimulation history suggests that factors beyond solute fluxes, such as modification of mechanical stress within the cytoskeleton also plays a role in the response to hypotonic stress.

Methylmercury Neurotoxicity: Exploring Potential Novel Targets

Mechanistic studies on the effects of MeHg in the central nervous system (CNS) have been limited to morphology, substrate uptake and macromolecular synthesis, differentiation, and changes in gene expression during development and adulthood, but its primary site of action has yet to be identified. Proper functioning of the nitric oxide synthase (NOS)-cyclic GMP and the cyclooxygenase (COX)-prostaglandin (PG) signaling pathways in the CNS depend on post-translational modifications of key enzymes by chaperone proteins. The ability of MeHg to alter or inhibit chaperone-client protein interactions is hitherto unexplored, and potentially offers an upstream unifying mechanism for the plethora of MeHg effects, ranging from reactive species generation (ROS) generation, mitochondrial dysfunction, changes in redox potential, macromolecule synthesis, and cell swelling. In view of the prominent function of astrocytes in the maintenance of the extracellular milieu and their critical role in mediating MeHg neurotoxicity, they afford a relevant and well-established experimental model. The present review is predicated on (a) the remarkable affinity of mercurials for the anionic form of sulfhydryl (-SH) groups, (b) the essential role of thiols in protein biochemistry, and (c) the role of molecular chaperone proteins, such as heat shock protein 90 (Hsp90) in the regulation of protein redox status by facilitating the formation and breakage of disulfide bridges. We offer potential sites where MeHg may interfere with cellular homeostasis and advance a novel mechanistic model for MeHg-induced neurotoxicity.

Region-dependent differences and alterations of protective thiol compound levels in cultured astrocytes and brain tissues

We examined region-dependent differences and alterations in the levels of protective thiol compounds, glutathione (GSH) and metallothionein (MT)-I and -II, in cultured rat astrocytes under several culture conditions and in brain tissues of rats at postnatal and weaning periods. Regardless of culture conditions, both protein concentrations and mRNA expressions of MT-I and -II were much higher in the cerebral hemisphere than in cerebellar astrocytes, whereas no difference was observed in GSH concentration. In both astrocytes, the GSH concentrations did not change within 12 h but significantly increased 24 h after being maintained in a serum-free defined medium. At 24 h, protein concentrations and mRNA expressions of MT-I and -II also increased in the respective astrocytes, and were further enhanced when maintained in the presence of 50 microM Zn(2+). In the brain tissues, the MT-I/-II protein concentrations were significantly higher in the cerebral cortex (a part of the cerebral hemisphere) than in the cerebellum, whereas the GSH concentration was similar at both postnatal day (P)1 and P35. In addition, the concentrations in the respective regions were significantly higher at P35 than at P1. These results suggest that region-dependent differences in the cellular levels of GSH and MTs in cultured astrocytes might reflect the in vivo differences, and that the levels of the respective thiol compounds in cultured astrocytes increase after serum elimination along with the region-dependent differences.

Ebselen protects glutamate uptake inhibition caused by methyl mercury but does not by Hg2+

Alterations of the neurotransmitter release systems in CNS have been reported in a variety of neuropathological processes associated with heavy metal toxicity. Neurotoxic effects of mercurials were investigated in vitro in cerebral cortex slices from young rats. The present study indicates that: (i) the environmental contaminants methylmercury (MeHg) and mercuric chloride (Hg2+) (50 microM) inhibited the glutamate net uptake from the cerebral cortex of 17-day-old rats; (ii) ebselen (10 microM) reverted the MeHg-induced inhibition of glutamate net uptake but did not protect the inhibition caused by Hg2+. At same time, we investigated another diorganochalcogenide, diphenyl diselenide (PhSe)2 and it was observed that this compound did not revert the action of MeHg or Hg2+; (iii) in addition, we observed that exposure of slices to 50 microM MeHg and Hg2+ for 30 min followed by Trypan blue exclusion assay resulted in 58.5 and 67.5% of staining cells, respectively, indicating a decrease in cell viability. Ebselen protected slices from the deleterious effects of MeHg, but not of Hg2+ on cell viability. Conversely, ebselen did not modify the reduction of MTT caused by MeHg and Hg2+; (iv) the protective effect of ebselen on MeHg-induced inhibition of glutamate net uptake seems to be related to its ability in maintaining cell viability.

Acute Cytotoxic Effects of Mercuric Compounds in Cultured Astrocytes Prepared from Cerebral Hemisphere and Cerebellum of Newborn Rats

We investigated acute cytotoxic effects and Hg accumulation after exposure to methylmercury (MeHg) or Hg(2+) in the presence or absence of serum in cultured astrocytes prepared from the cerebral hemisphere or cerebellum of newborn rats. Dose-related changes in viable cell numbers after exposure to mercuric compounds were not different between astrocytes from both regions under the specified conditions. Accumulation of each compound for 3 h was similar in both astrocytes but that for 24 h became different, especially that of Hg(2+). In both astrocytes, susceptibility to the respective compounds was higher in the order of those exposed immediately after, without, and 24 h after changing the serum-containing medium to a serum-free defined medium (SFDM). Accumulation for 3 h was higher in the respective astrocytes exposed to MeHg or Hg(2+) immediately after being maintained in SFDM than in those exposed 24 h after. These results suggest that accumulation of mercuric compounds up to 3 h strongly correlates with susceptibility, at least when maintained in SFDM. Astrocytic morphology changed to a satellite shape after the medium change to SFDM particularly in cerebellar astrocytes but only a few in cerebral hemisphere astrocytes, and it was reverted to a polygonal shape by MeHg but not Hg(2+) at 3 microM. The present results suggest that although some properties such as morphological changes and Hg accumulation are different between cerebral hemisphere and cerebellar astrocytes, these differences are not simply reflected by susceptibility to the acute cytotoxicity of mercuric compounds.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Ethanol-induced swelling in neonatal rat primary astrocyte cultures

We tested the hypothesis that astrocytes swell in response to ethanol (EtOH) exposure. The experimental approach consisted of an electrical impedance method designed to measure cell volume. In chronic experiments, EtOH (100 mM) was added to the culture media for 1, 3, or 7 days. The cells were subsequently exposed for 15 min to isotonic buffer (122 mM NaCl) also containing 100 mM EtOH. Subsequently, the cells were washed and exposed to hypotonic buffer (112 mM NaCl) containing 100 mM mannitol. Chronic exposure to EtOH led to a marked increase in cell volume compared with control cells. Specific anion cotransport blockers, such as SITS, DIDS, furosemide, or bumetanide, when simultaneously added with EtOH to hyponatremic buffer, failed to reverse the EtOH-induced effect on swelling. In acute experiments, confluent neonatal rat primary astrocyte cultures were exposed to isotonic media (122 mM NaCl) for 15 min, followed by 45-min exposure to hypotonic media (112 mM NaCl, mimicking in vivo hyponatremic conditions associated with EtOH withdrawal) in the presence of 0-100 mM EtOH. This exposure led to a concentration-dependent increase in cell volume. Combined, these studies suggest that astrocytes exposed to EtOH accumulate compensatory organic solutes to maintain cell volume, and that in response to hyponatremia and EtOH withdrawal their volume increases to a greater extent than in cells exposed to hyponatremia alone. Furthermore, the changes associated with EtOH are osmotic in nature, and they are not reversed by anion cotransport blockers.

Mercuric chloride, but not methylmercury, inhibits glutamine synthetase activity in primary cultures of cortical astrocytes

Methylmercury (MeHg) is highly neurotoxic with an apparent dose-related latency period between time of exposure and the appearance of symptoms. Astrocytes are known targets for MeHg toxicity and a site of mercury localization within the central nervous system (CNS). Glutamine synthetase (GS) is an enzyme localized predominately within astrocytes. GS converts two potentially toxic molecules, glutamate and ammonia, to the relatively non-toxic amino acid, glutamine. During prolonged exposure to MeHg, inorganic mercury (I-Hg) accumulates within the brain, suggesting in situ demethylation of MeHg to I-Hg. To determine if speciation of mercurials would differentially alter GS activity and expression, neonatal rat primary astrocyte cultures were exposed to MeHg or mercuric chloride (HgCl2) for 1 or 6 h. MeHg produced no changes in GS activity, protein, or mRNA at any time or dose tested. In contrast, HgCl2 produced a dose dependent decrease in astrocytic GS activity at both 1 and 6 h. There were no changes in GS protein or mRNA levels following HgCl2 exposure. Additional studies were carried out to determine GS activity in cell lysates incubated with HgCl2 or MeHg. In cell lysates, HgCl2 was three-times more potent than MeHg in inhibiting GS activity. The inhibition of GS activity in cell lysates by HgCl2 was reversed by the addition of dithiothreitol (DTT), while DTT did not restore GS activity following MeHg. These data suggest that astrocytic GS activity is not inhibited by physiologically relevant concentrations of MeHg, but is inhibited by I-Hg, which is present in CNS following chronic MeHg exposure.

Developmental exposure to methylmercury alters behavioral sensitivity to D-amphetamine and pentobarbital in adult rats

Female rats were exposed to 0, 0.5, or 6.4 ppm methylmercury in their drinking water before mating, and throughout gestation and lactation. When the female offspring were 4-6 months old, they were trained to respond under a multiple differential reinforcement of high rate (DRH) 9:4-- Extinction schedule of reinforcement. No differences among exposure groups were apparent in steady-state behavior. Drug challenges were conducted with multiple doses of D-amphetamine, scopolamine, pentobarbital, haloperidol, and dizocilpine, drugs selected for their different pharmacological effects. The ED(50) values for amphetamine's reinforcement rate-reducing effects for the control, 0.5-, and 6.4-ppm groups were 3.1, 1.9, and 0.9 mg amphetamine/kg body weight, respectively, demonstrating an increased sensitivity to D-amphetamine in methylmercury-exposed rats. Rats in the 6.4-ppm group also demonstrated a relative insensitivity to pentobarbital. Further, these exposed rats exhibited an inverted U-shaped dose-effect curve under the pentobarbital dose-effect determination, while controls showed only a declining curve. Exposed rats did not respond differentially to haloperidol, scopolamine, or dizocilpine, suggesting specificity. The present data suggest an involvement of catecholaminergic and GABAergic activity in methylmercury's neurotoxicity.

Glial cells in neurotoxicity development

Neuroglial cells of the central nervous system include the astrocytes, oligodendrocytes, and microglia. Their counterparts in the peripheral nervous system are the Schwann cells. The term neuroglia comes from an erroneous concept originally coined by Virchow (1850), in which he envisioned the neurons to be embedded in a layer of connective tissue. The term, or its shortened form--glia, has persisted as the preferred generic term for these cells. A reciprocal relationship exists between neurons and glia, and this association is vital for mutual differentiation, development, and functioning of these cell types. Therefore, perturbations in glial cell function, as well as glial metabolism of chemicals to active intermediates, can lead to neuronal dysfunction. The purpose of this review is to explore neuroglial sites of neurotoxicant actions, discuss potential mechanisms of glial-induced or glial-mediated central nervous system and peripheral nervous system damage, and review the role of glial cells in neurotoxicity development.

Induction of astrocyte metallothioneins (MTs) by zinc confers resistance against the acute cytotoxic effects of methylmercury on cell swelling, Na+ uptake, and K+ release

Metallothionein (MT) proteins play an important role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, we investigated the ability of the astrocyte-specific MT isoform, MT-I, to attenuate MeHg-induced cytotoxicity. Increased astrocytic MT expression was achieved by 24-h pretreatment of neonatal rat primary astrocyte cultures with 100 microM zinc (ZnSO4). Subsequently, the astrocytes were treated with MeHg (10 microM), and its toxic effects on cell volume, Na+ uptake, and K+ release were investigated and compared to cells treated with or without MeHg, but in the absence of Zn pretreatment. Pretreatment of astrocytes with Zn was associated with a 2.9-fold increase in MT protein levels (P<0.02), and a 5.6-fold increase in MT mRNA levels (p<0.002) compared to control astrocytes. Astrocytes expressing increased MT protein levels were resistant to MeHg-induced swelling. In isotonic buffer the effect of MeHg on swelling was abolished (p<0.01) by 24-h Zn pretreatment, in such a way that volume profiles in these cells did not differ from controls. Zn-induced increased expression of MTs was also associated with significant attenuation of astrocytic Na+ uptake (p<0.01) and Rb+ (a marker for K+) release (p<0.001) in response to treatment with MeHg. These results demonstrate (1) that astrocytes can be induced to express high levels of MT proteins by pretreatment with Zn, and (2) that Zn confers resistance against the acute effect of MeHg on astrocytic swelling and the associated changes in ion (Na+ and K+) transport. Taken together, the data suggest that astrocytic MT induction offers effective cellular adaptation to MeHg cytotoxicity.

Methylmercury-induced inhibition of regulatory volume decrease in astrocytes: characterization of osmoregulator efflux and its reversal by amiloride

Swelling of neonatal rat primary astrocyte cultures by hypotonic media leads to regulatory volume decrease (RVD) and the resumption of resting cell volume. RVD is associated with activation of conductive K+ and Cl- channels, allowing for the escape of KCl, as well as the release of osmoregulators, such as taurine and myoinositol. As we have previously shown [D. Vitarella, H.K. Kimelberg, M. Aschner, Inhibition of RVD in swollen rat primary astrocyte cultures by methylmercury (MeHg) is due to increase amiloride-sensitive Na+ uptake, Brain Res. 732 (1996) 169-178.], MeHg, when added to hypotonic buffer inhibits RVD, primarily due to increased cellular permeability to Na+ via the Na+/H+ antiporter. The present study was, therefore, undertaken to assess the ability of cation-anion cotransport blockers to reverse the inhibitory effect of MeHg on RVD in swollen astrocytes, and to further characterize MeHg-induced changes in astrocytic osmoregulatory release processes. The studies demonstrate the following: (1) MeHg-induced inhibition of RVD is partially inhibited by the Na+/H+ antiporter blocker, amiloride, but not SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid), DIDS (4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid), furosemide or bumetanide; (2) exposure of swollen astrocytes to MeHg is associated with specific effects on osmoregulatory release, leading to significant inhibition of taurine release and a significant increase in potassium and myoinositol release compared with release in hypotonic conditions.

Methylmercury-induced astrocytic swelling is associated with activation of the Na+/H+ antiporter, and is fully reversed by amiloride

Astrocytes are a known 'sink' for brain methylmercury (MeHg) deposition. Yet, the significance of the preferential accumulation of MeHg within these cells is imprecisely defined. To determine whether MeHg in isotonic buffer has the potential to interfere with homeostatic functions, we measured its effect on astrocytic volume using an electrical impedance method [E.R. O'Connor, H.K. Kimelberg, C.R. Keese, I. Giaever, Electrical impedance method for measuring volume changes in astrocytes, Am. J. Physiol. 264 (1993) C471-C478.]. In addition, we have characterized the alterations in astrocytic ion permeability associated with exposure to this organometal. The results show that MeHg rapidly induces astrocytic swelling, and that this effect is secondary to increased astrocytic Na+ uptake. Furthermore, the effect of MeHg on astrocytic swelling is completely inhibited by amiloride, but not by SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid), furosemide, or bumetanide. Accordingly, increased cellular permeability to Na+ via the Na+/H+ antiporter is invoked as the primary mechanism of MeHg-induced astrocytic swelling.

Manganese decreases glutamate uptake in cultured astrocytes

Recent data have shown an accumulation of manganese in the basal ganglia in patients with chronic hepatic encephalopathy (HE). Astrocytes and ammonia are critically involved in the pathogenesis of HE, and we have recently demonstrated that ammonia decreases glutamate uptake in cultured astrocytes. Since failure by astrocytes to take up glutamate may represent an important pathogenetic mechanism in HE, we, therefore, examined the effect of manganese on glutamate transport in these cells. Treatment of cultured astrocytes with 100 microM manganese for 2 days resulted in a 54% decrease in the uptake of D-aspartate, a nonmetabolizable analogue of glutamate. Kinetic analysis revealed a 28% decline in Vmax, with no change in the K(m). Treatment of cultures with 5 mM NH4 Cl inhibited D-aspartate uptake by 21%, and a combination of 5 mM NH4Cl with 100 microM manganese produced an additive effect on uptake inhibition. These results suggest a pathogenetic role for manganese in HE, possibly involving glutamate transport.

Astrocyte metallothioneins (MTs) and their neuroprotective role

I have briefly detailed in this review the role of astrocytes in MeHg neurotoxicity, emphasizing the mechanisms and significance of astrocytic swelling in neuropathological conditions. I have also described the functions of brain MTs and have reported recent observations on their propensity to attenuate cytotoxicity. While it is unclear why three different MT genes are expressed in the brain, this redundancy should allow for greater accumulation of MTs under stressful conditions compared to its accumulation if only a single gene was present. Another explanation may be that genes encoding functionally identical MTs might be regulated independently, thus permitting cell-specific MT expression. Finally, each of the three MT isoforms may have distinct functions. As discussed herein, astrocytic MTs afford protection from the acute cytotoxic effects of MeHg, reversing the effect of this organometal on RVD and inhibition of taurine release. Whether other vital cellular functions are protected by MTs will have to await future studies, as will the mechanisms associated with MT-induced cellular protection. That the resistance to heavy metal toxicity is closely related to the cellular ability to synthesize MTs, raises interesting questions regarding the potential involvement of heavy metals in neurodegenerating (amyotrophic lateral sclerosis, Parkinson's disease, Alzheimer's disease) under conditions of compromised MT synthesis. Future studies on the expression and regulation of MT genes are likely to culminate in novel strategies for manipulating intracellular MT levels, providing insight to their role in both health and disease.

Metallothionein induction protects swollen rat primary astrocyte cultures from methylmercury-induced inhibition of regulatory volume decrease

Metallothionein (MT) proteins have been postulated to play a role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, and MT-1 and MT-2 are astrocyte-specific MT isoforms, we investigated the ability of MTs to attenuate MeHg-induced cytotoxicity. The toxic effects of MeHg on astrocytes were investigated in a model of regulatory volume decrease (RVD) in which the cells are swollen by exposure to a hypotonic buffer. Preexposure to CdCl2 (1 microM) for 72, 96 or 120 h, prior to acute exposure to hypotonic buffer and MeHg (10 microM) led to a time-dependent increase in the intracellular levels of astrocyte MT proteins. The acute MeHg-induced inhibition of RVD was significantly, and almost fully reversed by preexposure to CdCl2. This reversal was time-dependent, 120-h preexposure to CdCl2 producing the greatest reversibility. Furthermore, the ability of astrocytes to efficiently volume regulate in the presence of MeHg-containing hypotonic buffer was highly correlated (r = 0.99) with the intracellular levels of MT proteins. The release of [3H]taurine, an osmolyte involved in the RVD process was also measured. The inhibitory effect of MeHg on [3H]taurine in swollen cells was significantly, and fully reversed by CdCl2 preexposure. The study suggests that astrocytes induced to express high levels of MT proteins are resistant to the acute inhibitory effect of MeHg on RVD.

Inhibition of regulatory volume decrease in swollen rat primary astrocyte cultures by methylmercury is due to increased amiloride-sensitive Na+ uptake

Primary astrocyte cultures from neonatal rats were swollen by exposure to hypotonic buffer with and without 10 microM methylmercury (MeHg). We investigated the effects of MeHg on K+ (using 86Rb), taurine, D-aspartate (a non metabolizable analogue of glutamate) and Na+ fluxes during regulatory volume decrease (RVD), with an electrical impedance method for determination of cell volume, coupled with on-line measurements of efflux of radioactive ions and amino acids. Addition of 10 microM MeHg completely inhibited RVD in swollen astrocytes, increased the uptake of 22Na+, increased 86Rb release, and decreased 3H-taurine release. There was no effect on the rate of release of 3H-D-aspartate from swollen astrocytes. 0.5 mM amiloride completely inhibited MeHg-induced increased Na+ influx during RVD, while 1 mM furosemide had no effect. When Na+ in the hypotonic buffer was replaced with N-methyl-D-glucamine (NMDG), RVD in the presence of MeHg was indistinguishable from controls. These results indicate that MeHg increases cellular permeability to ions such as Na+ and K+, and that an increase in Na+ permeability via Na+/H+ exchange, offsetting K+ loss, is the primary mechanism in its inhibition of RVD in swollen astrocytes.

Adenosine modulates methylmercuric chloride (MeHgCl)-induced D-aspartate release from neonatal rat primary astrocyte cultures

The effects of adenosine, and selective adenosine receptor agonists and antagonists on methylmercury (MeHg)-induced aspartate release were studied in neonatal rat primary astrocyte cultures. Whereas basal levels of D-[3H]aspartate release were unchanged upon treatment with adenosine or the selective A1 receptor agonists, N6-cyclopentyladenosine (CPA), cyclohexyladenosine (CHA), and R-phenylisopropyladenosine (R-PIA), all partially reversed the MeHg-induced release of D-aspartate. Treatment of astrocytes with the xanthine derivative, theophylline, an adenosine antagonist, reversed the inhibitory effect of adenosine on MeHg-induced D-[3H]aspartate release. Since the effect of MeHg on D-[3H]aspartate release is known to be associated with sulfhydryl (-SH) groups which are controlled by intracellular glutathione concentrations [GSH]i, we also evaluated the effects of adenosine, the A1 agonists CPA and CHP, and the adenosine antagonist, theophylline, on astrocytic [GSH]i. Attenuation of the stimulatory effect of MeHg on D-[3H]aspartate release by adenosine and its agonists occurred in the presence of reduced astrocytic [GSH]i, suggesting that other mechanisms must be invoked for this protective effect. Whilst the mechanism of MeHg-induced D-[3H]aspartate release is not known, the data suggest a role for adenosine in its regulation.

Effects of lead and mercury on histamine uptake by glial and endothelial cells

The effects of lead and mercury on [3H]-histamine uptake by cultured astroglial and endothelial cells of rat brain were studied. Experimental data showed that both metal ions inhibited the uptake in both cell types of concentrations as low as 1-10 microM. The effects were consistent with non/competitive inhibitions. With either lead or mercury exposure, the inhibition of the uptake was greater in astroglial than in cerebral endothelial cells. Contrary to the above findings, 100 microM of mercuric chloride produced stimulation of histamine uptake and this stimulation was much more pronounced in cultured cerebral endothelial cells than in astroglial cells. Inhibition of [3H]-histamine uptake by lead acetate and mercuric chloride was considered to be association with a loss of the transmembrane Na+ and/or K+ gradient while stimulation of the uptake by high concentration of mercury might be related to a direct effect on histamine transporter. It is noteworthy, that cultured astroglial cells, derived from neonatal rat brain, are much more sensitive to the toxic effects of these heavy metal ions than cultured endothelial cells derived from the brain capillaries of the same species of animals.

Astrocytes as potential modulators of mercuric chloride neurotoxicity

1. MC has been shown to inhibit the uptake of L-glutamate and increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. 2. Two sulfhydryl (SH-)-protecting agents; reduced glutathione (GSH), a cell membrane-nonpenetrating compound, and the membrane permeable dithiothreitol (DTT), have been shown consistently to reverse the above effects. MC-induced D-aspartate release is completely inhibited by the addition of 1 mM DTT or GSH during the actual 5-min perfusion period with MC (5 microM); when added after MC treatment, DTT fully inhibits the MC-induced D-aspartate release, while GSH does not. 3. Neither DTT nor GSH, in the absence of MC, have any effect on the rate of astrocytic D-aspartate release. Other studies demonstrate that although MC treatment (5 microM) does not induce astrocytic swelling, its addition to astrocytes swollen by exposure to hypotonic medium leads to their failure to volume regulate. 4. Omission of calcium from the medium greatly potentiates the effect of MC on astrocytic D-aspartate release, an effect which can be reversed by cotreatment of astrocytes with the dihydropyridine Ca(2+)-channel antagonist nimodipine (10 microM), indicating that one possible route of MC entry into the cells is through voltage-gated L-type channels.

Intracellular glutathione (GSH) levels modulate mercuric chloride (MC)- and methylmercuric chloride (MeHgCl)-induced amino acid release from neonatal rat primary astrocytes cultures

Mercuric chloride (MC) and methylmercury (MeHg) were found to increase amino acid release from astrocytes. This suggests interaction with sulfhydryl (-SH) groups which are controlled by glutathione [GSH] levels. In the present study, we evaluated the effects of alterations in intracellular glutathione concentrations [GSH]i on the outcome of MC and MeHg treatment. [GSH]i were increased in a time-dependent fashion by incubating the astrocytes with 1 mM L-2-oxothiazolidine-4-carboxylic acid (OTC), a cysteine precursor. OTC attenuated the release of [2,3-3H]D-aspartic acid from astrocytes exposed to MC- (5 microM) and MeHg-(10 microM). MeHg-induced [3H]D-taurine release was also reduced by pretreatment of astrocytes with OTC. Treatment with BSO (50 microM) decreased [GSH]i in astrocytes, and increased [2,3-3H]D-aspartate release from MC- and MeHg-treated astrocytes, and [3H]D-taurine release from MeHg-treated cells. Neither OTC nor BSO when added to cultures in the absence of MC or MeHg had an effect on amino acid release by astrocytes. The current study underscores both the sensitivity of astrocytes to mercurials in terms of amino acid release and the relationship of these effects of astrocytic [GSH]i.

The role of sulfhydryl groups in D-aspartate and rubidium release from neonatal rat primary astrocyte cultures

We have recently demonstrated that both methylmercury (MeHg) and mercuric chloride (MC) induce D-aspartate release from neonatal rat primary astrocyte cultures maintained in isotonic conditions. In the present study, we compare several other sulfhydryl-(-SH) selective alkylating reagents [methyl methanethiosulfonate (MMTS), N-ethylmaleimide (NEM), and iodoacetamide (IA)] in isotonic, as well as hypotonic conditions to discern the functional importance of -SH groups in [3H]D-aspartate and 86rubidium (86Rb) release from astrocytes. Treatment of astrocytes (5 min) in isotonic buffer with the hydrophobic reagent NEM (10 microM) caused a marked increase in 86Rb release but had no effect on [3H]D-aspartate release. Neither IA-, nor MMTS-treatment (both at 10 microM) induced increase in [3H]D-aspartate or 86Rb release in isotonic buffer. In hypotonic condition (-50 mM Na+), astrocytes were most sensitive to MC exposure (5 microM), exhibiting an increase in both [3H]D-aspartate and 86Rb efflux. The hydrophobic compounds MMTS and NEM, and the hydrophilic -SH modifying reagent, IA, attenuated the hypotonic-induced efflux of [3H]D-aspartate, in the absence of an effect on 86Rb release. These observations are consistent with a critical role for -SH groups both in basal (i.e. isotonic) and hypotonic-induced release of D-aspartate and Rb from astrocytes. Lack of uniformity of these effects may be attributed to site-specificity, related to the physicochemical properties of these -SH alkylating reagents.

The role of -SH groups in methylmercuric chloride-induced D-aspartate and rubidium release from rat primary astrocyte cultures

Methylmercuric chloride (MeHgCl) was shown to increase D-aspartate and rubidium (Rb; a marker for potassium) release from preloaded astrocytes in a dose- and time-dependent fashion. Two sulfhydryl (-SH) protecting agents: a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MeHgCl on the efflux of radiolabeled D-aspartate as well as Rb. MeHgCl-induced D-aspartate and Rb release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MeHgCl (10 microM). However, when added after MeHgCl treatment, this inhibition could not be fully sustained by GSH, while DTT fully inhibited the MeHgCl-induced release of D-aspartate. Neither DTT or GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MeHgCl on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MeHgCl-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier cannot be invoked to explain MeHgCl-induced D-aspartate release. Omission of Ca2+ from the perfusion solution increased the time-dependent MeHgCl-induced D-aspartate release.

Stimulation of D-aspartate efflux by mercuric chloride from rat primary astrocyte cultures

Mercuric chloride (HgCl2; MC) was shown to increase D-aspartate release from preloaded astrocytes in a dose-dependent fashion. Two sulfhydryl (-SH) protecting agents, a cell membrane non-penetrating compound, reduced glutathione (GSH), and the membrane-permeable dithiothreitol (DTT), were found to inhibit the stimulatory action of MC on the efflux of radiolabeled D-aspartate. MC-induced D-aspartate release was completely inhibited by the addition of 1 mM DTT or GSH during the actual 5 min perfusion period with MC (5 microM). However, when added after MC treatment, this inhibition could not be sustained by GSH, while DTT fully inhibited the MC-induced release of D-aspartate. Neither DTT nor GSH alone had any effect on the rate of astrocytic D-aspartate release. Accordingly, it is postulated that the stimulatory effect exerted by MC on astrocytic D-aspartate release is associated with vulnerable -SH groups located within, but not on the surface of the cell membrane. Omission of Na+ from the perfusion solution did not accelerate MC-induced D-aspartate release, suggesting that reversal of the D-aspartate carrier can not be invoked to explain MC-induced D-aspartate release. Furthermore, MC did not appear to be associated with astrocytic swelling.

Distribution of metallothionein in the human central nervous system

The distribution of metallothionein (MT), a metal-binding protein, was examined immunohistochemically in the normal human brain and spinal cord. Paraffin-embedded brain tissue from three patients who had died from a non-neurological disease and were free of histopathological central nervous system alterations were processed. The results of the present study demonstrate that MT is readily detectable in a subgroup of astrocytes in the normal human brain. MT staining is most intense on grey matter astrocytes that bear short stout processes and which probably represent protoplasmic astrocytes. Using anti-MT and anti-glial fibrillary acidic protein immunostaining, we could demonstrate two subpopulations of astrocytes that were mutually exclusive. The functional significance of MT-expression in protoplasmic astrocytes is not entirely clear. Metal detoxification is only one of the many postulated functions of MT. The finding that staining for MT permits subtyping of astrocytes may be of great importance in glia research and surgical pathology of the human brain.

The role of sulfhydryl groups and calcium in the mercuric chloride-induced inhibition of glutamate uptake in rat primary astrocyte cultures

Inhibition by mercuric chloride (MC) of the astrocytic uptake of the excitotoxic neurotransmitter L-glutamate (L-GLU) has been postulated to contribute to MC neurotoxicity. In the present study, we analyzed the ability of two sulfhydryl (SH)-protecting agents: a cell membrane non-penetrating compound-reduced glutathione (GSH), and the membrane permeable dithiothreitol (DTT), to reverse the inhibitory action of MC on the initial rate of uptake of radiolabelled GLU (100 microM) in primary cultures of rat astrocytes. MC at 5 microM concentration reduced the uptake to 46% of control when present in the incubation medium during the 5 min of actual uptake, and to 27% of control when astrocytes were preincubated for 30 min in HEPES buffer containing MC prior to GLU uptake measurements. GLU uptake inhibition caused by 30 min preincubation with MC was partly relieved by the addition of 1 mM DTT during the actual 5 min uptake period. However, this inhibition could not be reversed by 1 mM GSH. Accordingly, it is postulated that the inhibitory effect exerted by MC on GLU uptake is associated with vulnerable SH groups located within, but not on the surface of the cell membrane. Neither 5 microM N-ethylmaleimide (NEM) nor 5 microM or 25 microM iodoacetate (IA) affected GLU uptake, indicating steric hindrance of the access of these two sulfhydryl reagents to the SH groups critical for the uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Astrocytes: targets and mediators of chemical-induced CNS injury

It is now well established that a reciprocal relationship exists between neurons and astrocytes, and that this association is vital for mutual differentiation, development, and functioning of both cell types. It had also become apparent that perturbations in astrocytic function may lead to deleterious consequences in juxtaposed neurons. It is therefore possible that neuronal damage induced by chemicals or neuropathic disease involves dissociation of astrocytic-neuronal interactions. The purpose of this review is to explore astrocytic-neuronal interactions, focusing on potential sites of neurotoxicant actions. In developing this thesis, we briefly examine the functional interactions between astrocytes and neurons, followed by specific examples of astrocyte-mediated neurotoxicity.

Methylmercury-induced alterations in excitatory amino acid transport in rat primary astrocyte cultures

To determine whether methylmercury (MeHg) has the potential to interfere with homeostatic functions in neonatal rat cortical primary astrocyte cultures, the effects of MeHg on the uptake and efflux of both L-glutamate and D-aspartate were examined. Uptake of both of these excitatory amino acids (EAAs) was significantly (P < 0.05) reduced in the presence of MeHg concentrations as low as 10(-5) M. Efflux of both glutamate and aspartate from preloaded astrocytes was also increased by MeHg in a dose- and time-dependent fashion. Since in our earlier studies we had found that MeHg causes dose-dependent astrocytic swelling, which could have been the mechanism of the increased efflux, we examined whether blockage of conductive ion fluxes, which have been implicated in astrocytic swelling, could reverse the MeHg-induced increase in L-glutamate and D-aspartate efflux. Three compounds which inhibit the hypotonic-media-induced efflux of EAA, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), L-644,711 (a derivative of ethacrynic acid) and furosemide were tested at their maximal concentrations for their ability to reverse MeHg-induced EAA efflux. Only furosemide (5 mM) could sustain the reversal for the entire 120 min duration of the efflux measurement. Since hypotonic-media swelling-induced release of EAAs is inhibited by these anion inhibitors (in the following rank order: L-644,711 > SITS > furosemide), we conclude that different mechanisms account for EAA release from primary astrocyte cultures during MeHg exposure as compared to hypotonic media-induced efflux.

Metallothionein immunoreactivity is increased in the spinal cord of patients with amyotrophic lateral sclerosis

Sections of the spinal cord from 10 patients with classic amyotrophic lateral sclerosis and from 10 control cases were examined by immunocytochemical methods to localize metallothionein. Metallothionein immunoreactivity was seen in the nucleus and cytoplasm of a subset of astrocytes, largely confined to the gray matter. Also, diffuse gray matter staining was observed, probably representing small glial fibers. Astrocytic metallothionein immunoreactivity (P less than 0.01) and strong gray matter matrix staining (P less than 0.03) was increased in the spinal cords from patients with amyotrophic lateral sclerosis. Although compatible with induction by metals, increased metallothionein expression in the spinal cords from patients with amyotrophic lateral sclerosis may also have resulted from inflammation or gliosis.

Effect of thiol group modification on ion flux and ligand binding properties of the GABAA-benzodiazepine receptor chloride channel complex

Agents that modify thiol groups have been shown to alter ligand binding at a variety of receptor sites. In addition, alkylation of sulfhydryls has been shown to block ion channel conductance. We studied the effects of thiol reagents on gamma-aminobutyric acid (GABA)-activated chloride flux (36Cl-) and [3H]-diazepam binding in mouse brain membrane preparation (microsacs). Incubation of microsacs in the presence of: mercuric chloride (HgCl2), p-chloromercuriphenylsulfonic acid (pCMBS), hydroxymercuribenzoate (HMB), n-ethylmaleimide (NEM), or iodoacetic acid (IAA) attenuated GABA-stimulated Cl- uptake. The thiol reagents reduced both maximal stimulation and the potency of GABA to induce Cl- uptake. Thiol reagent treatment decreased the affinity of high-affinity [3H]-muscimol equilibrium binding. Supernatant prepared from microsacs treated with pCMBS stimulated Cl- uptake in the absence of GABA agonist in microsacs unexposed to thiol reagents. The supernatant taken from pCMBS-treated microsacs also stimulated [3H]-diazepam binding. This effect was blocked by the addition of the GABA receptor antagonist bicuculline. The concentration of endogenous GABA in supernatant from pCMBS-treated microsacs was sixfold greater than that in supernatant from control microsacs. This increase in levels of endogenous GABA by thiol reagents was due to both an increase in GABA release and a decrease in high-affinity GABA uptake.

TCP enhances the survival of human fetal spinal cord cells in culture

Dissociated cell cultures were prepared from human spinal cords of 7-10-week-old fetuses. After 10 weeks progressive neuronal necrosis was observed in controls whereas N/1-(2-thienyl)cyclohexyl/piperidine (TCP) enhanced the survival time of the cells. After 21 weeks the number of gamma-aminobutyric acid (GABA)ergic and neuron specific enolase (NSE)-stained neurons was higher in the TCP-treated cultures than in controls. TCP appears to be a promising drug for long term survival of neurons.