Structural and Cholesterol Binding Properties of the Amyloid Precursor Protein C-Terminal Fragment C99 and the Etiology of Alzheimer's Disease
Barrett, Paul John
Biochemistry Structural and Cholesterol Binding Properties of the Amyloid Precursor Protein C-Terminal Fragment C99 and the Etiology of Alzheimer’s Disease Paul John Barrett Dissertation under the direction of Professor Charles Sanders Alzheimer’s Disease (AD) is a severe form of dementia that currently affects nearly 40 million people worldwide, a number that is estimated to increase by the year 2050 to nearly 120 million. The production and oligomerization of the amyloid beta polypeptides (Aβ) is widely thought to play a central role in AD onset and progression. Aβ oligomers are neurotoxic, such that factors that increase Aβ production and propensity to oligomerize or that reduce its degradation and transport out of the brain are viewed as pro-AD risk factors. Recently, it has been established that cholesterol may be a pro-AD risk factor, but this mechanism is still highly controversial. Aβ polypeptides are derived from two step proteolytic processing of the amyloid precursor protein (APP) by the enzymes β- and γ-secretase. This thesis will investigate both how the structure of C99 and the ability of C99 to bind cholesterol regulate and promote these cleavage events. First, this thesis will focus on the three dimensional structure determination of C99 using NMR spectroscopy. Prior to this work, much had been inferred about how the structure of C99 regulated the cleavage events of the amyloidogenic pathway, but without an actual structure, this work was incomplete. I demonstrate that C99 contains a membrane embedded helical turn N-terminal of the transmembrane helix, and that the transmembrane helix is both highly curved and flexible in nature. We hypothesize from this work that the curved structure of the transmembrane helix allows for proper insertion in the γ-secretase complex, and may be a potential target for modulating Aβ production. Secondly, this thesis will focus on the mechanism that makes cholesterol a pro-AD risk factor. We have shown that not only can C99 specifically bind cholesterol, but that this binding event partitions C99 to cholesterol rich membrane domains. It is known that both β- and γ-secretase reside in these cholesterol rich membranes. These findings show that cholesterol binding by APP may be the first step in promoting Aβ formation during AD.