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Overexposure to manganese (Mn) is an important environmental risk factor for Parkinsonian-like symptoms referred to as manganism. Alpha-synuclein (α-Syn) oligomerization is a major cause in Mn-induced neurotoxicity. Autophagy, as an adjust response to control intracellular protein homeostasis, is involved in the degradation of α-Syn monomers or oligomers. Furthermore, autophagy dysregulation is also related to development of neurodegenerative disorders. Hence, we speculated that there was an interaction effect between α-Syn oligomerization and autophagy upon Mn exposure. In this study, we applied α-Syn gene knockout mice (α-Syn−/−) and wild-type mice (α-Syn+/+) treated with three different concentrations of MnCl2 (50, 100, and 200 μmol/kg) to elucidate the physiological role of α-Syn in Mn-induced autophagy dysregulation and neurocytes injury. We found that activation of chaperone-mediated autophagy (CMA) pathway by Mn was independent of α-Syn. Additionally, α-Syn could ameliorate excessive autophagy induced by high dose Mn (200 μmol/kg). Next, we used 5 mg/kg Rapamycin (Rap) or 3-methyladenine (3-MA) to regulate autophagy. The study revealed that autophagy is involved in Mn-induced α-Syn oligomerization and neurocytes injury. Taken together, these findings indicated that α-Syn oligomerization might be the major responsible for the Mn-induced autophagy dysregulation and neurocytes injury. [ABSTRACT FROM AUTHOR]

Over-exposure to manganese (Mn) has been known to induce endoplasmic reticulum (ER) stress involving protein misfolding. The proper maturation and folding of native proteins rely on the activity of protein disulfide isomerase (PDI). However, the exact mechanism of Mn-induced alpha-synuclein oligomerization is unclear. To explore whether alpha-synuclein oligomerization was associated with S-nitrosylation of PDI, we made the rat brain slice model of manganism and pretreated slices with l-Canavanine, a selective iNOS inhibitor. After slices were treated with Mn (0, 25, 100, and 400 μM) for 24 h, there were dose-dependent increases in apoptotic percentage of cells, lactate dehydrogenase (LDH) releases, production of NO, inducible nitric oxide synthase (iNOS) activity, the mRNA and protein expressions of iNOS, and PDI. Moreover, S-nitrosylated PDI and alpha-synuclein oligomerization also increased. However, there was a significant increase in the PDI activity of 25-μM Mn-treated slices. Then, PDI activity and the affinity between PDI and alpha-synuclein decreased significantly in response to Mn (100 and 400 μM), which was associated with S-nitrosylation of PDI. The results indicated that S-nitrosylated PDI could affect its activity. We use the l-Canavanine pretreatment brain slices to inhibit S-nitrosylation of PDI. The results showed that l-Canavanine pretreatment could reduce Mn-induced nerve cell injury and alpha-synuclein oligomerization. Additionally, there was a significant recovery in PDI activity in l-Canavanine-pretreated slices. The findings revealed that Mn induced nitrosative stress via the activation of iNOS and subsequent S-nitrosylation of PDI in cultured slices. Moreover, S-nitrosylation of PDI is an important signaling event in the Mn-induced alpha-synuclein oligomerization in brain slices. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) is a well-known environmental pollutant leading to neurotoxicant associated with aberrant central nervous system (CNS) functions, but its toxic mechanisms have not yet been fully recognized. In the present study, we tested the hypothesis that MeHg induces neuronal injury via glutamate (Glu) dyshomeostasis and oxidative damage mechanisms and that these effects are attenuated by dextromethorphan (DM), a low-affinity and noncompetitive N-methyl- d-aspartate receptor (NMDAR) antagonist. Seventy-two rats were randomly divided into four groups of 18 animals in each group: control group, MeHg-treated group (4 and 12 μmol/kg), and DM-pretreated group. After the 4-week treatment, we observed that the administration of MeHg at a dose of 12 μmol/kg significantly increased in total mercury (Hg) levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload in the cerebral cortex. We also found that MeHg reduced nonenzymatic and enzymatic antioxidants, enhanced neurocyte apoptosis, induced reactive oxygen species (ROS), and caused lipid, protein, and DNA peroxidative damage in the cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appeared to be inhibited by MeHg exposure. These alterations were significantly prevented by the pretreatment with DM at a dose of 13.5 μmol/kg. In conclusion, these findings strongly implicate that DM has potential to protect the brain from Glu dyshomeostasis and oxidative damage resulting from MeHg-induced neurotoxicity in rat. [ABSTRACT FROM AUTHOR]

Overexposure to manganese (Mn) has been known to induce neuronal damage involving endoplasmic reticulum (ER) stress. However, the exact mechanism of Mn-induced ER stress is unclear. Increasing evidence suggested that the overexpression of alpha-synuclein played a critical role in Mn-induced neurotoxicity. To explore whether the occurrence of ER stress was associated with alpha-synuclein overexpression, we made the rat brain slices model of silencing alpha-synuclein using short-interference RNA. After non-silencing alpha-synuclein slices were treated with Mn (0-400 μM) for 24 h, there was a dose-dependent increase in apoptotic rates of cells and levels of lactate dehydrogenase in the culture medium. Moreover, there was a dose-dependent increase in the protein expression of 78, 94-kDa glucose-regulated protein (GRP78/94), C/EBP homologous protein (CHOP), and caspase-12. Moreover, PKR-like ER kinase (PERK) phosphorylation, PERK-mediated phosphorylation of eIF2a, and ATF4 expression also increased. Inositol-requiring enzyme 1 (IRE1) activation and X-box-binding protein-1 (Xbp1) mRNA splicing increased. Activating transcription factor 6 p90 levels did not change. However, after silencing alpha-synuclein slices were treated with 400 μM Mn for 24 h, there was a significant decrease in the expression of GRP78/94, CHOP, and caspase-12 compared with 400 μM Mn-treated non-silencing alpha-synuclein slices. Furthermore, PERK phosphorylation, PERK-mediated phosphorylation of eIF2a, and ATF4 mRNA expression also decreased. However, IRE1 phosphorylation and Xbp1 mRNA splicing did not change. The findings revealed that Mn induced ER stress via activation of PERK and IRE1 signaling pathways and subsequent apoptosis in cultured slices. Moreover, alpha-synuclein protein was associated with Mn-induced activation of PERK signaling pathway. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) is one of the ubiquitous environmental toxicant that leads to long-lasting neurological deficits in animals and humans. The identification of the underlying mechanisms has been a main focus of research in the neurotoxicology field. Glutamate (Glu) dyshomeostasis and oxidative stress have been identified as two critical mechanisms mediating MeHg-induced neurotoxicity. However, little has been known of the interaction between these two mechanisms that play in MeHg poisoning in vivo. We, therefore, developed a rat model of MeHg subchronic poisoning to evaluate its neurotoxic effects and investigated the neuroprotective role of memantine, a low-affinity, noncompetitive N-methyl- d-aspartate receptors (NMDARs) antagonist, against MeHg-induced neurotoxicity. Ninety rats were randomly divided into five groups: control, memantine control, MeHg-treated (4 and 12 μmol/kg), and memantine pretreated. Administration of 12 μmol/kg MeHg for 4 weeks significantly elevated total Hg levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload, which might be critical to excessive reactive oxygen species (ROS) formation in cerebral cortex. Meanwhile, MeHg administration reduced non-enzymatic (non-protein sulfhydryl, NPSH) and enzymatic (superoxide dismutase, SOD and glutathione peroxidase, GSH-Px) antioxidants; caused lipid, protein, and DNA oxidative damage; and enhanced neurocyte apoptosis in cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appear to be inhibited by MeHg exposure. Pretreatment with memantine at a dose of 5 μmol/kg significantly prevented MeHg-induced alterations of Glu metabolism and oxidative stress, alleviated neurocyte apoptosis, and pathological injury. In conclusion, the results suggested that Glu dyshomeostasis and oxidative stress resulting from MeHg exposure contributed to neuronal injury. Memantine possesses the ability to attenuate MeHg-induced neurotoxicity through mechanisms involving its NMDARs-binding properties and indirect antioxidation. [ABSTRACT FROM AUTHOR]

Overexposure to methylmercury (MeHg) has been known to induce neurotoxicity. The objective of this study is to explore mechanisms that contribute to MeHg-induced nerve cell apoptosis focusing on the alteration of intracellular Ca homeostasis and expression of N-methyl- d-aspartate receptors (NMDARs) subunits in rat cerebral cortex and whether MK801, a non-competitive NMDAR antagonist, could attenuate MeHg-induced neurotoxicity. Fifty rats were randomly divided into five groups of 10 animals in each group: control group, MK801 control group, MeHg-treated group (4 and 12 μmol/kg) and MK801 pre-treated group. Administration of MeHg at a dose of 12 μmol/kg for 4 weeks significantly increased in intracellular [Ca] and total Hg levels and that enhanced neurocyte apoptosis rate in cerebral cortex. In addition, the inhibitory effect of MeHg on Na-K-ATPase and Ca-ATPases might be one of the reasons that cause a significant increase of [Ca] in neurocyte. Over activated by increased cytosolic Ca loading, calpains degraded NMDAR subunits leading ultimately to nerve cell damage. However, pretreatment with MK801 at a dose of 0.3 μmol/kg could prevent Ca homeostasis dysregulation and alleviate the neurocyte apoptosis. In conclusion, the neuroprotective effects of MK801 appeared to be mediated not only via its NMDA receptor binding properties but also by maintaining intracellular calcium homeostasis. [ABSTRACT FROM AUTHOR]

As a highly toxic environmental pollutant, methylmercury (MeHg) can cause neurotoxicity in animals and humans. Considering the antioxidant property of grape seed proanthocyanidin extracts (GSPE), this study was aimed to evaluate the effect of GSPE on MeHg-induced neurotoxicity in rats. Rats were exposed to MeHg by intraperitoneal injection (4, 12 μmol/kg, respectively) and GSPE was administered by gavage (250 mg/kg) 2 h later. After a 4-week treatment, phosphate-activated glutaminase, glutamine synthetase, glutathione peroxidase and superoxide dismutase activities, glutamate, glutamine, malondialdehyde and glutathione contents in cerebral cortex were measured. Reactive oxygen species (ROS) and apoptosis were also estimated in cells. The results showed that the MeHg-induced neurotoxicity was significantly attenuated. GSPE significantly decreased the production of ROS, counteracted oxidative damage and increased the antioxidants and antioxidant enzymes activities in rats prior to MeHg exposure. Moreover, the effects on the rate of apoptotic cells and the disturbance of glutamate homeostasis were correspondingly modulated. These observations highlighted the potential of GSPE in offering protection against MeHg-induced neurotoxicity. [ABSTRACT FROM AUTHOR]