Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Stephen J Valentine

Committee Co-Chair

Suzanne Bell

Committee Member

Lisa A Holland

Committee Member

Justin Legleiter

Committee Member

David M Smith

Abstract

Huntington's disease is neurodegenerative disease caused by an expanded polyglutamine-coding CAG repeat in exon 1 of the huntingtin gene. Huntingtin exon 1 forms the primary toxic amyloid structure in Huntington's disease; disease severity is directly correlated with polyglutamine length. Recent works have shown that fully formed amyloid plaques may not represent the most toxic species in Huntington's disease; the most neurotoxic species may be small, diffuse oligomer (4 - 20 monomer units) that are precursors to amyloid plaques. While the polyglutamine region is undisputed as the primary constituent of amyloid structure, aggregation kinetics and morphology are regulated by the presence of flanking sequences that are N- and C-terminal to theamyloid forming tract. The first seventeen residues of huntingtin exon 1 (Nt17) can form an amphipathic &agr;-helix depending upon solution conditions and the presence of a binding partner, and in most cases, mediates oligomer formation. C-terminal to the polyglutamine tract is a proline-rich region, or in the case of a model peptide a polyproline region (polyP), that can form a polyproline-type II (PPII) helix, which may regulate Nt17 in huntingtin protein with short polyglutamine regions. Much is unknown regarding residue-specific Nt17-Nt17 and Nt17-polyP interactions. The work described here utilized state-of-the-art deuterium exchange mass spectrometry techniques to identify critical hydrophilic residues in early stages of oligomer formation. Monomeric and multimeric conformations of Nt17, idependent og the polyglutamine domain, were then studied using ion mobility-mass spectrometry and molecular dynamics to gain insight into the earliest stages of Nt17-Nt17 association, and thus, aggregation. Monomeric and multimeric Nt17 could form extended helices in the gas phase. Key hydrophilic residues were chemically modified, which resulted in a sharp decline in multimer formation. Finally, Nt17-polyP interactions were probed using gas-phase deuterium exchange mass spectrometry, supplemented with molecular dynamics and an exchange kinetics model. The obtained gas-phase structures showed a reduction in Nt17 extended &agr;-helix, when compared to a monomeric and extended homodimeric conformation. Thus, it is hypothesized that polyP regulates Nt17 by not allowing transition to the amphipathic &agr;-helix. The results of this study examine the structural heterogeneity of a sequence thought to drive a potentially toxic aggregate morphology, pinpoint key residues in early oligomer formation, and provide strategies for regulation of oligomer formation.

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