Structural flexibility of huntingtin oligomers plays a key role in directly binding lipid membranes.

Abstract

Huntingtin disease (HD) is a neurodegenerative disease caused by expansion of a polyglutamine (polyQ) tract within the huntingtin (htt) protein, leading to aggregation into a variety of species from oligomers to fibrils. The ability of htt to directly bind and damage cellular membranes represents a fundamental step in a variety of toxic mechanisms. For example, membrane damage associated with the ER, mitochondria, and nuclear membrane occurs in several HD models. However, how distinct htt aggregates interact with lipid membranes remains unclear. Directly preceding the polyQ domain, the first seventeen amino acids of htt (Nt17) functions as an amphipathic α-helix lipid-binding domain. Thus, Nt17 plays a prominent role in the direct interaction of htt with membranes. Nt17 also facilitates the formation of α-helical oligomers that serve as intermediates in fibril formation. Here, the membrane activity of different aggregate species was systematically determined using lipid-binding assays. While htt monomers readily bound membranes, oligomer formation enhanced the htt/lipid interaction several fold. Htt fibrils displayed minimal membrane activity. To elucidate mechanisms by which oligomers bind membranes, two strategies were implemented: 1) stabilizing oligomers with small peptides based on Nt17 and 2) chemically cross-linking oligomers via lysine residues within Nt17. Some of the small Nt17 peptides contained point mutations that alter the total charge of the oligomer. While changing charge slightly reduced the apparent affinity of oligomers for membranes, structural flexibility within the oligomer appeared to play a larger role in the oligomer/membrane interaction. In particular, chemical cross-linking effectively eliminated the ability of oligomers to bind lipid membranes. Additionally, cross-link stabilized oligomers lost their toxic gain of function when exogenously added to cell culture. Collectively, this suggests that targeting structural flexibility of htt oligomers may be an effective therapeutic strategy.