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

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 F₂-isoprostanes (F₂-IsoPs), lipid peroxidation biomarkers of oxidative damage, in astrocyte cultures treated with 5 or 10 μM MeHg for 1 or 6 h. Consistent with this observation, MeHg induced a concentration-dependant reduction in the inner mitochondrial membrane potential (ΔΨ_m), 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 μM. MeHg pretreatment (1, 5 and 10 μM) for 30 min also inhibited the net uptake of glutamine (³H-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 μM) MeHg concentration, suggesting that the reduction in glutamine uptake observed after 30 min treatment with lower concentrations of MeHg (1 and 5 μM) 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.