Methylmercury induces oxidative injury, alterations in permeability and glutamine transport in cultured astrocytes

Abstract

The neurotoxicity of high levels of methylmercury (MeHg) is well established both in humans and experimental animals. Astrocytes accumulate MeHg and play a prominent role in mediating MeHg toxicity in the central nervous system (CNS). Although the precise mechanisms of MeHg neurotoxicity are ill-defined, oxidative stress and altered mitochondrial and cell membrane permeability appear to be critical factors in its pathogenesis. The present study examined the effects of MeHg treatment on oxidative injury, mitochondrial inner membrane potential, glutamine uptake and expression of glutamine transporters in primary astrocyte cultures. MeHg caused a significant increase in F 2 -isoprostanes (F 2 -IsoPs), lipid peroxidation biomarkers of oxidative damage, in astrocyte cultures treated with 5 or 10 μ M MeHg for 1 or 6 hours. Consistent with this observation, MeHg induced a concentration-dependant reduction in the inner mitochondrial membrane potential (ΔΨm), as assessed by the potentiometric dye, tetramethylrhodamine ethyl ester (TMRE). Our results demonstrate that ΔΨ m is a very sensitive endpoint for MeHg toxicity, since significant reductions were observed after only 1 h exposure to concentrations of MeHg as low as 1 μ M. MeHg pretreatment (1, 5 and 10 μ M) for 30 min also inhibited the net uptake of glutamine ( 3 H-glutamine) measured at 1 min and 5 min. Expression of the mRNA coding the glutamine transporters, SNAT3/SN1 and ASCT2, was inhibited only at the highest (10 μ M) MeHg concentration, suggesting that the reduction in glutamine uptake observed after 30 min treatment with lower concentrations of MeHg (1 and 5 μ M) was not due to inhibition of transcription. Taken together, these studies demonstrate that MeHg exposure is associated with increased mitochondrial membrane permeability, alterations in glutamine/glutamate cycling, increased ROS formation and consequent oxidative injury. Ultimately, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to cellular dysfunction and cell death.Please send request for reprints to Michael Aschner, Ph.D.,

Inactivation by Hg2+ and methylmercury of the glutamine/amino acid transporter (ASCT2) reconstituted in liposomes: Prediction of the involvement of a CXXC motif by homology modelling

“…These effects are explained by the direct inhibition of the transporter, the activity of which is suppressed by 10 µM methylmercury (see Fig. 3), i.e., the concentration used in the previous study [26]. The effects of cysteine and N-acetylcysteine in reversing the inhibition by mercurials correlates well with the use of these antioxidant compounds in mercury detoxification [4].…”

“…It was previously reported that methylmercury inhibits the expression of the amino acid transporters SNAT1 and ASCT2 in astrocytes. However, this mechanism could not explain the effects caused at prolonged times of exposure to methylmercury [26]. These effects are explained by the direct inhibition of the transporter, the activity of which is suppressed by 10 µM methylmercury (see Fig.…”

“…The study of interaction of mercuric reagents with transport systems for glutamine is still at an initial stage, in spite of the essential role of these transporters in cell homeostasis [25]. It was reported that methylmercury modify the permeability to glutamine of cultured astrocytes by an indirect mechanism based on inhibition of expression of the amino acid transporters ASCT2 and SNAT3; however, this mechanism only partially explained the permeability alteration [26]. The rat glutamine/amino acid transporter ASCT2 which is expressed in kidney, intestine and nervous system besides other tissues [25], was reconstituted in liposomes [27][28].…”

Flavonoid constituents of Dobera glabra leaves: amelioration impact against CCl 4 -induced changes in the genetic materials in male rats

“…Within the excitatory amino acid transporters (EAATs), the glutamate transporters, GLAST, GLT-1 and SNAT3 are the main players in provoking glutamate uptake into astrocytes in the CNS. Therefore, it was hypothesized that GLAST, GLT-1 and SNAT3 played an important role in maintaining GSH in the CNS (Yin et al 2007). In the current work, we found that the mRNA expression of the glutamate transporters was down-regulated obviously in CCl 4 alone.…”

Heavy metals, islet function and diabetes development

“…98 The study of Nakagawa has shown that the total mercury concentrations in the hair of ordinary diseased people, including diabetes, were from 2.08 to 36.5 ppm; those values were considerably higher than that of healthy people of the same age-groups. 99 Organic or inorganic mercury compounds are well known to induce cellular damage in various cell types, such as renal cells, 92 astrocytes, 100 lymphoma cells, 101 human gingival fibroblast cells, 102 alveolar epithelial cells 103 and pancreatic islet β-cells. 104 Except for organic and inorganic forms of mercury toxicity, many studies focus on the vaccine preservative-thimerosal.…”