Date of Award
Doctor of Philosophy (PhD)
This study was directed at examining the neurotoxic mechanisms of several classes of environmental toxicants implicated in neurodegenerative disease. Primary cortical cultures were exposed to organophosphorus pesticides, heavy metals and the cyanobacterial toxin, beta-N-methylamino-L-alanine (BMAA). Several components relating to neuronal injury were assessed in each study and novel aspects are described.
The main action of organophosphorous insecticides is generally believed to be the inhibition of acetylcholinesterase. However, these compounds are now recognized to inhibit many other enzymes and cause neuronal death through a variety of mechanisms. I found that exposure to chlorpyrifos or diazinon caused concentration-dependent neurotoxicity that could not be attributed to acetylcholinesterase inhibition. Chlorpyrifos exposure increased extracellular glutamate and induced a diffuse nuclear staining characteristic of necrosis; the toxicity was sensitive to ionotropic glutamate receptor antagonists. Diazinon toxicity was blocked by caspase inhibitors. Additionally, diazinon induced punctuate chromatin staining characteristic of apoptosis. These results represent two distinct, novel mechanisms of organophosphorous neurotoxicity.
Heavy metals are ubiquitous in the environment and are of significant health concern worldwide. Exposure to lead, iron, mercurials (inorganic mercury, methylmercury, or thimerosal, i.e. ethylmercury) or other heavy metals is implicated as a risk factor for neurodegenerative disease. I found that the toxicity of these metals may be enhanced when interacting with chelators used to treat metal intoxication. As well, my studies describe a new role for mercury-induced oxidative stress as a cytoprotective signal to enhance glutathione levels. My data also suggests an obligate role for MRP1 in the detoxification of methylmercury.
Neurodegenerative diseases likely involve complex interactions between genetic predisposition and multiple environmental factors. My final study tested the interaction of the methylmercury and BMAA. Importantly, concentrations of BMAA that caused no toxicity by themselves potentiated methyl mercury toxicity. BMAA plus methylmercury, at concentrations that had no effect by themselves, depleted cellular glutathione. The combined toxicity was attenuated by glutathione monoethyl ester, and the free radical scavenger, trolox, but not by the NMDA receptor antagonist, MK-801. The results indicate a synergistic neurotoxic interaction targeting the cellular redox state. This finding may have implications for neurodegenerative disease caused by environmental toxicant exposure.