Cholesterol Modulates the Binding and Subsequent Aggregation of Huntingtin on Lipid Bilayers

Abstract

Huntington disease (HD) is an autosomal dominant inherited neurodegenerative disease caused by abnormally long CAG-repeats in the huntingtin gene. This mutation encodes an expanded polyglutamine (polyQ) domain in the N-terminus of the huntingtin (htt) protein which directly leads to its disease-related aggregation. Htt is found highly associated with a variety of cellular and subcellular membranes, which are predominately comprised of lipids. The interaction of htt with lipid membranes is facilitated by its first 17 amino acids, whose secondary structure is an amphipathic α- helix. There are alterations in the relative amounts of specific membrane components in the brains of HD patients, and in particular, cholesterol homeostasis is altered. Here, we investigate how cholesterol modifies the interaction of htt with lipid bilayers. Using atomic force microscopy (AFM), we track aggregation of htt on supported lipid bilayer containing varying amounts of exogenously added cholesterol. As the amount of cholesterol in the bilayer increased, htt binding to the membrane, and subsequent aggregation, was reduced. This reduced lipid-membrane interaction was further validated using a colorimetric polydiacetylene (PDA) lipid binding assay. More interestingly, morphological changes on the bilayer induced by exposure to htt are significantly altered upon addition of cholesterol.