| Summary: | Mitochondrial impairment, glutathione depletion and oxidative stress have been implicated in the pathogenesis of Parkinson's disease, linked recently to proteasomal dysfunction. This study analyses how these factors influence the various activities of the proteasome in SH-SY5Y human neuroblastoma cells treated with the PD mimetics MPP+ (a complex I inhibitor) or dopamine. Treatment with these toxins led to dose and time dependent reductions in ATP and glutathione levels and also chymotrypsin-like and postacidic-like activities; however, trypsin-like activity was unaffected. Antioxidants blocked the effects of dopamine but not MPP+, suggesting that oxidative stress was more important in the dopamine-mediated effects. With MPP+, ATP depletion was a pre-requisite for loss of proteasomal function. This study also shows that addition of MPP+ or dopamine to purified samples of the human 20S proteasome also reduced proteasomal activities; with dopamine being most damaging. As was the case with toxin-treated cells chymotrypsin-like activity was the most sensitive and trypsin-like activity, the least sensitive. The direct effect of both compounds on proteasomal activity was, at least, partly due to oxidative damage to the proteasome, since the antioxidant vitamin C could partially alleviate the proteasomal impairment. Indeed, Western blot analyses showed that some of the ?- and ?-subunits of the proteasome were modified by dopamine treatment. One of the hallmarks of Parkinson's disease is the appearance of Lewy bodies, which are protein inclusions containing ?-synuclein, neurofilament proteins and ubiquitinated proteins. A growing body of evidence suggests that the UPS might be involved in the formation of these aggregates. This thesis, reports that neurofilaments can undergo proteasomal degradation and that MPP+ and dopamine alter the expression/phosphorylation and distribution of these cytoskeletal proteins in SH-SY5Y cells. Therefore aberrant changes in both neurofilament profiles and proteasomal degradation may influence inclusion formation in dopaminergic neurons.
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