Curvature Enhances Binding and Aggregation of Huntingtin at Lipid Membranes

Abstract

Huntington disease (HD) is a genetic neurodegenerative disease caused by an expanded polyglutamine (polyQ) domain in the first exon of the huntingtin (Htt) protein, facilitating its aggregation. Htt interacts with a variety of membraneous structures within the cell, and the first 17 amino acids (Nt17) of Htt directly flanking the polyQ domain comprise an amphiphathic α-helix (AH) lipid-binding domain. AHs are also known to detect membrane curvature. To determine if Htt exon 1 preferentially binds curved membranes, in situ atomic force microscopy (AFM) studies were performed. Supported lipid bilayers are commonly used as model membranes for AFM studies of protein aggregation. However, these supported bilayers usually lack curvature. By forming a bilayer on top of silica nanobeads (50 ± 10 nm) deposited on a silicon substrate, model supported lipid bilayers with flat and curved regions were developed for AFM studies. The presence of the bilayer over the beads was validated by continual imaging of the formation of the bilayer, height measurements, and spatially resolved mechanical measurements of the resulting bilayer using scanning probe acceleration microscopy. Interpretation of this data was facilitated by numerical simulations of the entire imaging process. The curved supported bilayers associated with the beads were found to be more compliant than flat supported bilayers, consistent with the altered packing density of lipids caused by the induced curvature. This model bilayer system was exposed to a synthetic truncated Htt exon 1 peptide (Nt17Q35P10KK), and this peptide preferentially accumulated on curved membranes, consistent with the ability of AHs to sense membrane curvature.

Preparation Protocols of Aβ(1–40) Promote the Formation of Polymorphic Aggregates and Altered Interactions with Lipid Bilayers

“…The observed decreases in the magnitude of F min associated with disrupted regions of the bilayer are primarily due to a weakened adhesive interaction between the disrupted bilayer and the AFM tip. 28,47,48 The observed decrease in the magnitude of F max associated with disrupted regions of the bilayer indicates a decreased membrane rigidity or compression modulus. 28,47,48 Collectively, these results indicate that interaction between lipid bilayers and Aβ(1−40), independent of the preparatory history, results in regions of altered membrane morphology (increased roughness) and mechanical properties (decreased compression modulus of the bilayer and a weakened adhesive interaction between the tip and surface).…”

“…37 Specific features of these time-resolved tip/sample forces, the maximal tapping force (F max ) and minimal tapping force (F min ) per oscillation cycle, are sensitive to the mechanical properties of the surface and can be used to map relative changes in such properties along a surface with a high spatial resolution. 28,47,48 F max refers to the peak or largest repulsive (positive) force between the tip and surface associated with an individual tapping event, and F min is the largest magnitude attractive (negative) force experienced during a tapping event. Several numerical simulations and experimental studies have demonstrated that F max can be directly related to the compression modulus of the surface and that F min is sensitive to the adhesive interaction between the tip and surface.…”

The emerging role of the first 17 amino acids of huntingtin in Huntington’s disease

“…Due to their ability to weakly bind membranes, their interaction with membranes is easily regulated. Nt17 has been shown to sequester truncated htt exon 1 peptides to regions of curvature on supported lipid bilayers (81). The structure of an AH is compatible with the formation of a variety of helix bundles in aqueous environment that is driven by maximizing the interaction of the hydrophobic faces of adjacent helices (82, 83).…”

“…Nt17 facilitates the trafficking of htt exon 1 to membranes associated with the ER, autophagic vacuoles, mitochondria, and Golgi (14, 142). Membrane curvature sensing, facilitated by AH structural characteristics of Nt17, may play a mechanistic role in these functions (81). Additional evidence that Nt17 directly interacts with lipid membranes was provided by the observation that the binding of htt exon 1 to large unilamellar vesicles composed of phosphatidylcholines (PC) or a mixture of PC and phosphatidylserines (PS) enhanced the helical content of Nt17 (14).…”

Identification of novel polyglutamine-expanded aggregation species in spinal and bulbar muscular atrophy

“…The novel observation of slow-migrating AR species may reflect intrinsic features of specific AR functional domains. It may be that slow-migrating species have lipophilic properties, resulting from the presence of lipophilic hormone in the ligand-binding pocket, or from interactions with lipid membranes, as has been shown with other polyglutamine-expanded peptides (Burke et al, 2013b; Chaibva et al, 2014). Alternatively, the low density of slow-migrating species might arise from aberrant conformation of AR structural domains.…”