Date of Award

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Medical Education

First Advisor

Omar El -Agnaf

Second Advisor

Md Emdad ul Haque

Third Advisor

Murat Oz

Abstract

α-Synuclein aggregation is the key pathogenic event in several important neurological disorders including Parkinson’s Disease, dementia with Lewy bodies and multiple system atrophy, giving rise to a distinct group of neurodegenerative diseases known as synucleinopathies. Although the molecular basis of α – syn toxicity is mediated by a nucleation-dependent aggregation process. To elucidate the structural basis of α-synuclein mediated toxicity, we developed various methods to prepare different α-synuclein species of a defined size and morphology distribution, and we investigated their toxicity in different human dopaminergic cell lines. We observed that crude α-synuclein oligomers preparations, containing both monomeric and heterogeneous mixtures of α-synuclein oligomers, were the most toxic species. The toxicity of α-synuclein aggregates was directly linked to the presence of the monomeric α-synuclein, and strongly dependent on its ability in seeded nucleation-dependent aggregation converting into amyloid fibrils. Therefore any effort to identify compounds that could inhibit or even reverse the aggregation process should assess the effect of the potential inhibitors on the seeded aggregation of α-synuclein, among others. We screened thirty Chinese herbal medicinal compounds for their effect on α-synuclein aggregation, seeded polymerization and toxicity by employing biophysical, biochemical and cell-culture-based techniques. Among the screened compounds, only ginsenoside Rbl, salvianolic acid B, dihydromyricetin and gallic acid were shown to be strong inhibitors of α-syn fibrillation, seeded aggregation ant toxicity. Our results showed that gallic acid, ginsenoside Rb1 and salvianolic acid B inhibit α-synuclein fibrillation by binding and stabilizing the structure of the soluble, non-toxic oligomers, which are devoid of β-sheet content. In contrast, dihydromyricetin was found to be able to bind both oligomeric and monomeric species of α-synuclein. In the case of gallic acid, the inhibition of α-synuclein fibrillation is related to the compound’s hydroxyl moieties whose number and position on the phenyl ring were proven to be significant for the process of inhibition, as indicated by the structure activity relationship data obtained from fourteen structurally similar benzoic acid derivatives. Overall, the compounds identified herein may represent the starting point for designing new molecules that could be utilized as drugs for the treatment of Parkinson’s Disease and related disorders.

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