Sphingomyelin and GM1 Influence Huntingtin Binding to, Disruption of, and Aggregation on Lipid Membranes

Abstract

Huntington disease (HD) is an inherited neurodegenerative disease caused by the expansion beyond a critical threshold of a polyglutamine (polyQ) tract near the N-terminus of the huntingtin (htt) protein. Expanded polyQ promotes the formation of a variety of oligomeric and fibrillar aggregates of htt that accumulate into the hallmark proteinaceous inclusion bodies associated with HD. htt is also highly associated with numerous cellular and subcellular membranes that contain a variety of lipids. As lipid homeostasis and metabolism abnormalities are observed in HD patients, we investigated how varying both the sphingomyelin (SM) and ganglioside (GM1) contents modifies the interactions between htt and lipid membranes. SM composition is altered in HD, and GM1 has been shown to have protective effects in animal models of HD. A combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy (AFM) were used to directly monitor the interaction of a model, synthetic htt peptide and a full-length htt-exon1 recombinant protein with model membranes comprised of total brain lipid extract (TBLE) and varying amounts of exogenously added SM or GM1. The addition of either SM or GM1 decreased htt insertion into the lipid monolayers. However, TBLE vesicles with an increased SM content were more susceptible to htt-induced permeabilization, whereas GM1 had no effect on permeablization. Pure TBLE bilayers and TBLE bilayers enriched with GM1 developed regions of roughened, granular morphologies upon exposure to htt-exon1, but plateau-like domains with a smoother appearance formed in bilayers enriched with SM. Oligomeric aggregates were observed on all bilayer systems regardless of induced morphology. Collectively, these observations suggest that the lipid composition and its subsequent effects on membrane material properties strongly influence htt binding and aggregation on lipid membranes.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications

“…The underlying mechanisms remain to be determined, although it was found that GM1 treatment increases phosphorylation of mHTT at Ser13 and Ser16, a crucial post-translational modification associated with a reduction in mHTT aggregation and toxicity ( Gu et al, 2009 ). Furthermore, a recent study showed that the presence of GM1 in artificial membranes made with total brain lipid extract dramatically decreases membrane insertion of exon 1 mHTT, and slows down the appearance of mHTT oligomers formed on membranes ( Chaibva et al, 2018 ). Whether administration of GM1 can affect the kinetics of mHTT aggregation or the aggregate species that are formed in vivo remains unknown.…”

Investigating the interactions of the first 17 amino acid residues of Huntingtin with lipid vesicles using mass spectrometry and molecular dynamics

“…The subcellular location of htt, necessitates its interaction with the lipid membrane bilayer and is mediated by posttranslational modifications of Nt17 residues . Additionally, the interaction of Nt17 with the lipid membrane bilayer has been shown to be an important factor in protein aggregation in HD where Nt17 can participate in lipid‐binding as well as protein‐protein interactions.…”

Membrane interactions accelerate the self-aggregation of huntingtin exon 1 fragments in a polyglutamine length-dependent manner

“…Such lipid interactions have been shown to be modulators of aggregation and fibril formation also for α -synuclein (57,62,64), islet amyloid polypeptide (9) and βamyloid (58,84). Within these studies the detailed membrane composition including the resulting physico-chemical properties such as negative charge density, fluidity, saturation, curvature or interactions with specific lipids all play important roles in the aggregation process (9,39,56,76,85). Thereby, the membrane interactions of polyQ flanking regions and their modulation by posttranslational modifications provides a possible therapeutic intervention site which has to our knowledge not been explored in greater detail.…”

“…Another huntingtin domain mediating membrane interactions has been identified within residues 171-371, a region bearing an overall high cationic character (10). Furthermore, biochemical and cell biological assays demonstrate the potential role of these anchoring domains in disease development (10,21,41,42,55,56).…”

“…glutamines the slower aggregation kinetics allow for a more controlled, quantitative evaluation of the processes involved in aggregation and fibril formation similar to the use of htt17-polyQ constructs used in previous biophysical studies (e.g. (41,42,48,56,65)). The polyglutamine was then successively extended in small steps to quantitatively measure the effect of polyQs on the aggregation dynamics.…”