Joint PCB-Methylmercury Exposures and Neurobehavioral Outcomes

The level of intake at which methylmercury might result in neurobehavioral or neuro psychological deficits in children has been extensively debated in recent years. The human data base for methylmercury toxicity is very robust, particularly when compared with other environmental substances for which health guidance values have been developed by Federal organizations. However, despite the depth of the data base for methylmercury, some of the key studies have been clouded by the concurrent exposure of the study population to polychlorinated biphenys (PCBs) and other persistent organic pollutants (POPs). In this paper, the authors examine the key points comprising the methylmercury-PCB issue and contrast these with recent reports of concurrent methylmercury/PCB human exposures and laboratory studies. The authors conclude that research published to date concerning the Seychelles and Faroes Island populations suggests that both methylmercury and PCBs are jointly responsible for the effects reported in the Faroes. Further neurobehavioral testing in populations exposed to PCBs alone, as well as with methylmercury alone, is necessary to fully resolve the ongoing debate.     

Actions of aluminum on voltage-activated calcium channel currents

Summary 1. Extracellular and intracellular effects of aluminum (Al) on voltage-activated calcium channel currents (VACCCs) of cultured rat dorsal root ganglion (DRG) neurons were investigated. Al (0.54 to 5.4 µg/ml=20–200 µM) applied extracellularly reduces VACCCs in a concentration-dependent manner. The IC₅₀ was calculated to be 2.3 µg/ml (83 µM). All types of VACCCs were similarly reduced by Al treatment. A slight shift of the current-voltage relation to depolarized potentials was observed for higher Al concentrations (>2 µg/ml). The action of Al was found to be use dependent, with little recovery (max. 20%) after wash.2. The effect of Al was highly pH dependent in the investigated range (pH 6.4 to 7.8). We observed a rightward shift of the concentration-response curve at pH 7.7 (IC₅₀:3.1 µg/ml) and a leftward shift at pH 6.4 (IC₅₀:0.56 µg/ml) compared to the concentration-response curve at pH 7.3.3. The VACCC declined when 2.7 µg/ml Al was added to the internal solution. A steady state was reached within a few minutes. Additional extracellular application of the same concentration lead to an additional decrease of the current. These observations strongly suggest the existence of both intracellular and extracellular accessible binding sites for Al on voltage-activated calcium channels (VACCs).4. The special characteristics of the action of Al on VACCCs, i.e., the irreversibility, use dependence, and pH dependence, as well as the additional internal binding site may contribute to its neurotoxicity.     

LRRK2 enhances oxidative stress-induced neurotoxicity via its kinase activity

LRRK2 is an autosomal dominant gene whose mutations cause familial Parkinson's disease (PD). The LRRK2 protein contains a functional kinase and a GTPase domain. PD phenotypes caused by LRRK2 mutations are similar to those of idiopathic PD, implying that LRRK2 is an important participant in PD pathogenesis. Of LRRK2's PD-specific mutations, the G2019S is the most frequently observed one. Its over-expression is known to increase kinase activity and neurotoxicity compared to wild type (WT) LRRK2. Here, using a simple colorimetric cell viability assay, we analyzed LRRK2's neurotoxicity in dopaminergic SN4741 cells following treatment with hydrogen peroxide. When WT, G2019S, or empty vector was expressed in SN4741 cells, cell death was modestly and significantly increased in the order of G2019S > WT > vector. When these transfected cells were treated with hydrogen peroxide to mimic oxidative stress, cellular neurotoxicity was enhanced in the same order (i.e. G2019S > WT > vector). Moreover, incubation of SN4741 cells with conditioned medium from cells expressing G2019S and subjected to hydrogen peroxide treatment exhibited 10-15% more cell death than conditioned medium from cells transfected with vector or WT, suggesting that G2019S-expressing cells secrete a factor(s) affecting viability of neighboring cells. The kinase domain was mapped to be responsible for oxidative stress-induced neurotoxicity. In addition, over-expression of WT and G2019S LRRK2 lead to a weak, but significant, increase in intracellular reactive oxygen species (ROS) in the order of G2019S > WT as measured by DCFH-DA assay in both the presence and absence of H₂O₂ treatment. Furthermore, in G2019S-expressing cells, co-expression of the anti-oxidant protein DJ-1 or ERK inhibitor treatment restored survival rate to a level similar to that of cells transfected with control vector under H₂O₂ treatment. Taken together, our data suggest that the LRRK2 kinase domain increases the generation of ROS and causes enhanced neurotoxicity under H₂O₂ treatment, which can be at least partially rescued by DJ-1 or the ERK inhibitor.     

Tricyclic Antidepressants Amitriptyline and Desipramine Induced Neurotoxicity Associated with Parkinson’s Disease

Recent studies report that a history of antidepressant use is strongly correlated with the occurrence of Parkinson’s disease (PD). However, it remains unclear whether antidepressant use can be a causative factor for PD. In the present study, we examined whether tricyclic antidepressants amitriptyline and desipramine can induce dopaminergic cell damage, both in vitro and in vivo. We found that amitriptyline and desipramine induced mitochondria-mediated neurotoxicity and oxidative stress in SH-SY5Y cells. When injected into mice on a subchronic schedule, amitriptyline induced movement deficits in the pole test, which is known to detect nigrostriatal dysfunction. In addition, the number of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta was reduced in amitriptyline-injected mice. Our results suggest that amitriptyline and desipramine may induce PD-associated neurotoxicity.     

Patient‐derived olfactory mucosa for study of the non‐neuronal contribution to amyotrophic lateral sclerosis pathology

Amyotrophic lateral sclerosis (ALS) is a degenerative motor neuron disease which currently has no cure. Research using rodent ALS models transgenic for mutant superoxide dismutase 1 (SOD1) has implicated that glial–neuronal interactions play a major role in the destruction of motor neurons, but the generality of this mechanism is not clear as SOD1 mutations only account for less than 2% of all ALS cases. Recently, this hypothesis was backed up by observation of similar effects using astrocytes derived from post‐mortem spinal cord tissue of ALS patients which did not carry SOD1 mutations. However, such necropsy samples may not be easy to obtain and may not always yield viable cell cultures. Here, we have analysed olfactory mucosa (OM) cells, which can be easily isolated from living ALS patients. Disease‐specific changes observed when ALS OM cells were co‐cultured with human spinal cord neurons included decreased neuronal viability, aberrant neuronal morphology and altered glial inflammatory responses. Our results show the potential of OM cells as new cell models for ALS.     

Blocking the GABA transporter GAT‐1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life‐saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel‐induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT‐1 but not GAT‐3. In the spinal dorsal horn, GAT‐1 was expressed at presynaptic terminals and astrocytes while GAT‐3 was only expressed in astrocytes. In rats with paclitaxel‐induced neuropathic pain, the protein expression of GAT‐1 was increased while GAT‐3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel‐induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT‐1 but not GAT‐3 transporters. Paclitaxel‐induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT‐1 inhibitor. These findings suggest that targeting GAT‐1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel‐induced neuropathic pain.

     

Macrophages treated with particulate matter PM2.5 induce selective neurotoxicity through glutaminase‐mediated glutamate generation

Exposure to atmospheric particulate matter PM_2.5 (aerodynamic diameter ≤ 2.5 μm) has been epidemiologically associated with respiratory illnesses. However, recent data have suggested that PM_2.5 is able to infiltrate into circulation and elicit a systemic inflammatory response. Potential adverse effects of air pollutants to the central nervous system (CNS) have raised concerns, but whether PM_2.5 causes neurotoxicity remains unclear. In this study, we have demonstrated that PM_2.5 impairs the tight junction of endothelial cells and increases permeability and monocyte transmigration across endothelial monolayer in vitro, indicating that PM_2.5 is able to disrupt blood–brain barrier integrity and gain access to the CNS. Exposure of primary neuronal cultures to PM_2.5 resulted in decrease in cell viability and loss of neuronal antigens. Furthermore, supernatants collected from PM_2.5‐treated macrophages and microglia were also neurotoxic. These macrophages and microglia significantly increased extracellular levels of glutamate following PM_2.5 exposure, which were negatively correlated with neuronal viability. Pre‐treatment with NMDA receptor antagonist MK801 alleviated neuron loss, suggesting that PM_2.5 neurotoxicity is mediated by glutamate. To determine the potential source of excess glutamate production, we investigated glutaminase, the main enzyme for glutamate generation. Glutaminase was reduced in PM_2.5‐treated macrophages and increased in extracellular vesicles, suggesting that PM_2.5 induces glutaminase release through extracellular vesicles. In conclusion, these findings indicate PM_2.5 as a potential neurotoxic factor, crucial to understanding the effects of air pollution on the CNS.