Date of Award

January 2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Sciences

First Advisor

Othman Ghribi

Abstract

Synucleinopathies is the overarching term used to describe a group of neurodegenerative disorders characterized by aggregates of α-synuclein (α-syn) protein in the cytoplasm of neurons, nerve fibers, and glial cells. The cause(s) for synucleinopathies are likely multi-factorial with genetic predisposition and environmental factors participating in the pathogenesis of the diseases. Dietary factors including dyslipidemia of cholesterol, its metabolites, and fatty acids have shown conflicting results as risk factors in recent years. Our overarching hypothesis is that dietary factors including 27-Hydroxycholesterol (27-OHC) and Palmitic Acid (PA) can affect key proteins involved in synucleinopathies.

Findings regarding risk related specifically to dietary cholesterol have indicated either an increased risk, decreased risk, or no association. We believe the reason for the conflicting association between cholesterol and synucleinopathies lies in the metabolites of cholesterol and not cholesterol per se. Supporting our hypothesis many studies have shown increased levels of many different oxysterols, including 27-OHC, within the brains of synucleinopathy patients. However, the extent to which increased 27-OHC levels in the brain causes α-syn deposition and promotes synucleinopathies is yet to be determined. Therefore, in this dissertation we explore the effects of 27-OHC on the accumulation of α-syn and investigate the mechanisms of such involvement.

We demonstrate that 27-OHC induces an increase in α-syn levels in human dopaminergic neurons. The mechanism involved in the α-syn increase does not appear to involve LXRs as we did not observe any significant changes in α-syn mRNA with 27-OHC or LXR agonist and antagonistic treatments. To the best of our knowledge, our results are the first to show that 27-OHC increases α-syn in dopaminergic neurons and that this increase may emanate from inhibition of the proteasomal function. Also, 27-OHC decreases levels of HSP70 protein which is involved in protein folding, and protein degradation through the Ubiquitin-Proteasomal System (UPS). The extent to which a decrease in HSP70 protein levels leads to decreased protein folding and degradation through specific pathways needs to be further elucidated. All-together, our results potentially suggest that restoring proteasomal function and HSP70 protein levels may attenuate the 27-OHC-induced increase in α-syn protein levels in vitro and reduce α-syn accumulation that can increase the risk for synucleinopathies.

Additionally, to date, studies focused on the contributions of dietary fat intake to the risk of PD type synucleinopathy have yielded inconsistent results. Epidemiological studies of dietary fat intake and PD have found positive associations, no association, and even protective effects. As implied by all the conflicting studies, the jury is still out on the role(s) of FAs in PD-type synucleinopathy risk.

Palmitic acid (PA) (16:0) is the most abundant saturated fatty acid in the body and the most abundant fatty acid in our diet. It has been shown to increase ER stress proinflammatory cytokine expression in astrocytes and microglia, and

activation of TLRs via NFKβ but its role in pathological hallmark formation of PD-type synucleinopathy remains unknown. Throughout this dissertation we aim to examine the role(s) of PA on various hallmarks of PD-type synucleinopathy pathology in various animal and cellular models. We demonstrate that a PA-enriched diet induces an increase in α-syn and TH protein and mRNA expression in both B6D2 and m-Thy1 mice. We also show that the PA-enriched diet does not affect biogenic amine content in control B6D2 mice but significantly changes dopamine and serotonin levels in m-Thy1 mice relative to control-fed mice. Our results demonstrate that a diet enriched in PA increases the levels of Tyrosine Hydroxylase (TH), and serotonin, an effect that can provide beneficial effects in a variety of conditions.

Additionally, we demonstrate that PA treatment in mouse dopaminergic neurons decreases α-syn protein and mRNA expression as well as it decreases TH protein content. Our study is the first to show that within MPTP-injected C57BL/6 mice a PA-enriched diet preserves motor function, decreases α-syn accumulation, increases TH protein, and increases dopaminergic neuronal survival. Altogether, our results suggest that a diet enriched in PA is protective against MPTP-induced Parkinsonism. Future studies are needed to elucidate the mechanisms by which a PA-enriched diet modulates these proteins. Establishing the effects of a smaller percentage of PA in the diet may reveal beneficial effects of this saturated free fatty acid in neurodegenerative conditions including PD and other synucleinopathies.

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