UCLA

Parkinson’s disease (PD) is an increasingly prevalent neurodegenerative disorder that presents as an emerging public health concern with the prospective increase of the aging population in the United States. PD is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, pathological aggregation of α-synuclein (α-syn) protein, and chronic neuroinflammation. The disease etiology of PD is not well understood though both genetic and environmental factors appear to play significant roles in modifying disease risk. Since only a minority of cases can be attributed to genetic (inherited) forms of PD, there is particular interest in elucidating the role of the environment in modifying PD risk. Epidemiological evidence has pointed to pesticide exposure as a strong risk factor for PD with numerous studies documenting this positive association. In this thesis, I explore the mechanisms of neurotoxicity of pesticides used in the California Central Valley. These mechanisms include altered autophagy, increased α-syn transmission and neuroinflammation. To this end, we developed a multi-pronged screen of sixty-two pesticides to determine which pesticides promoted α-syn pre-formed fibril (PFF) transmission in primary murine neuron cultures, altered autophagic components in human neuroblastoma SK-N-MC cells, and exhibited an increased odds ratio for PD in a case-control study of PD. Each independent screen yielded results, and the pesticides that were triple-hits (matched the inclusion criteria of all three screens) were identified for further study; the triple-hits include carbaryl, carbofuran, copper sulfate, chlorthal dimethyl, copper hydroxide, diuron. To support biological plausibility, we tested whether these triple-hits induced aminergic neuron loss in VMAT2:eGFP zebrafish (ZF) at 7 days post fertilization (dpf). We found that carbaryl, carbofuran, copper sulfate, chlorthal dimethyl, and copper hydroxide all induce the loss of aminergic neurons in the ZF brain compared to vehicle following a six-day exposure. To further investigate the mechanisms of toxicity that pesticides may act through to increase PD risk, we identified four fungicides (mancozeb, triflumizole, zineb, ziram) with distinct autophagy component profiles for in-depth study. Fungicides mancozeb, triflumizole, and ziram were found to increase autophagic cargo protein p62 mean fluorescence intensity (MFI) levels, and only ziram increased lysosomal marker LAMP2a MFI levels compared to vehicle in SK-N-MC cells. To further support biological plausibility, we conducted studies in 3-7 dpf ZF to assess behavior, autophagic flux, aminergic neuron loss and neuroinflammation following exposure. Light-stimulated locomotive behavior in 7 dpf ZF was significantly decreased following mancozeb and triflumizole exposure and significantly increased following ziram pesticide exposure. Autophagic flux was impaired with ziram and zineb exposure in 3 dpf ZF. This was supported by the observed increase in p62 protein levels in the brain with ziram exposure, though we did not observe an increase in p62 with zineb exposure. Further, ziram, zineb, and triflumizole exposure induced selective aminergic neuron loss in 7 dpf ZF. Of the fungicides tested, only mancozeb and ziram exposure resulted in reduced microglial structural ramification, which is indicative of a neuroinflammatory response in 5 dpf ZF. Taken together, this work explores the neurotoxicity of individual pesticides and the mechanisms by which they act to alter the risk of PD.