Methylmercury-induced changes in mitochondrial function in striatal synaptosomes are calcium-dependent and ROS-independent

Abstract

The brain is the main target organ for methylmercury (MeHg), a highly toxic compound that bioaccumulates in aquatic systems, leading to high exposure in humans who consume large amounts of fish. The mechanisms responsible for MeHg-induced changes in neuronal function are, however, not yet fully understood. In the present study we investigated whether MeHg-induced elevations in reactive oxygen species (ROS) or intracellular calcium are responsible for altering mitochondrial metabolic function in rat striatal synaptosomes. MeHg decreased mitochondrial function (measured by the conversion of MTT to formazan) and increased ROS levels in striatal synaptosomes after 30 min exposure. Although co-incubation with the antioxidant Trolox significantly reduced MeHg-induced ROS levels, it failed to restore mitochondrial function. MeHg also increased cytosolic and mitochondrial calcium levels in striatal synaptosomes. These elevations were largely independent of extrasynaptosomal calcium, given that nominal calcium-free buffer with 20 microM EGTA did not prevent MeHg-induced increases in cytosolic calcium. In conclusion, we suggest that ROS are not the cause of mitochondrial dysfunction in striatal synaptosomes after MeHg exposure; rather, we propose that ROS formation is a downstream event that reflects MeHg-induced mitochondrial dysfunction due to increased mitochondrial calcium levels.

<i>S</i>-Mercuration of cellular proteins by methylmercury and its toxicological implications

“…MeHg increases the production of reactive oxygen species (ROS) and the degree to which lipids are peroxidized Dreiem and Seegal, 2007;Garg and Chang, 2006;LeBel et al, 1990LeBel et al, , 1992Lee et al, 2009;Mori et al, 2007;Yin et al, 2007;Yonaha et al, 1983). Antioxidant enzymes such as superoxide dismutase (SOD), cat- Atchison and Hare, 1994;Clarkson, 1987;Miura and Imura, 1987 Aminoacyl-tRNA synthetase (?)…”

“…Ca 2+ concentrations are also much higher in some intracellular organelles, such as the endoplasmic reticulum (ER), than in cytoplasm. It has been found in a number of studies that MeHg causes the intracellular Ca 2+ concentration to increase, and this effect is possibly caused by an influx of Ca 2+ from outside the cells and by the release of Ca 2+ from intracellular stores, such as the ER and the mitochondria (Dreiem and Seegal, 2007;Hare et al, 1993;Limke and Atchison, 2002;Limke et al, 2003;Minnema et al, 1989;Sarafian, 1993;Tan et al, 1993). The disruption of Ca 2+ handling could ultimately lead to cell death.…”

Methylmercury-Dependent Increases in Fluo4 Fluorescence in Neonatal Rat Cerebellar Slices Depend on Granule Cell Migrational Stage and GABAA Receptor Modulation

“…Increases in [Ca 21 ] i are a consistent effect of MeHg, being seen in single neurons in culture Marty and Atchison, 1997;Bemis and Seegal, 2000;Limke and Atchison, 2002;Edwards et al, 2005), acutely exposed cerebellar slices (Yuan and Atchison, 2007), synaptosomes (Denny et al, 1993;Dreiem and Seegal, 2007), and even in mice treated chronically with MeHg (Johnson et al, 2011). Moreover, this effect occurs early in exposure, requiring neither extended exposure nor high MeHg concentrations, as pharmacokinetic barriers to MeHg absorption and distribution are negated in vitro.…”

Early Exposure to Mercuric Chloride or Methylmercury Alters Zebrafish Embryo (Danio rerio) Development

“…However, many studies have examined the effects of exposure to only relatively high levels of methyl mercury [37][38][39][40][41]. On the other hand, most human exposure takes place at low to moderate methyl mercury exposure levels.…”