The mechanism of trichloroethylene neurotoxicity and its relation to Parkinsonism

Abstract

Parkinson’s Disease (PD) is a progressive neurodegenerative condition characterised by deterioration of the dopaminergic (DA) neurons of the substantia nigra pars compacta (SNpc). The exact mechanism by which SNpc cell death in PD occurs is poorly understood but several lines of evidence implicate both environmental and genetic factors. Trichloroethylene (TCE) is an industrial solvent used as a degreasing agent and in dry cleaning. TCE is a major environmental contaminant in the air, the water system and soil and is therefore exposed at low levels to various groups in the population. There have been links between chronic exposure to TCE and Parkinson-like symptoms reported and in vivo experiments support this link. It has also been discovered that TCE can be converted, via chloral, to a potentially DA neurotoxin TaClo in man. This project investigated this link between TCE and PD and to elucidate any causative mechanisms.

Cellular exposure paradigms were used to show neurotoxicity of TCE, chloral and TaClo in DA neurons. The mechanism of this cell death in TaClo was found to be necroptotic - and not apoptotic - in nature and involve induction of autophagy, DNA damage and an increase in reactive oxygen species (ROS) in exposed cells. A possible source of this ROS was suggested with the findings that TaClo significantly inhibits Complex I of the mitochondrial oxidative phosphorylation chain - an effect known to produce superoxide - and an increase in mitochondrial ROS seen in cells following TaClo treatment. The cell death induced by chloral was found to follow a different path, with neither apoptotic or necroptotic characteristics observed in exposed cells, and Complex I only inhibited at high doses. However, chloral was found to block the reduction of cytochrome c at lower doses, a property that may be involved in the neurotoxicity seen with chloral.

Animal models of TCE, chloral and TaClo exposure found no significant motor or cognitive abnormalities in behavioural testing of either wild-type mice or rats or human A30P mutated α-synuclein overexpressing mice. However, TCE and TaClo exposed wild-type and A30P mice did show a significant decrease in DA neuronal number and density in the SNpc, suggesting both compounds are neurotoxic to DA neurons in vivo. An LC-MS/MS assay was developed to assess neurotransmitter levels in the brains of toxin exposed animals, however the method was not found to be consistently accurate and showed extreme variability in results.

In conclusion, the main results of this thesis suggest that TCE does lead to DA neurodegeneration in the SNpc of exposed individuals, probably through metabolism to the neurotoxic compound TaClo. The neurotoxic properties of TaClo are DA specific and relatively potent. The mechanism of neurotoxicity is hypothesised to be through inhibition of mitochondrial Complex I, inducing increased ROS production and damage of intracellular organelles, DNA and proteins, which in turn leads to the activation of autophagy and PARP activation. This intracellular stress instigates RIP1 mediated necroptosis and death of the cells.