The Role of Sphingomyelin and Ganglioside GM1 in the Interaction of Polyglutamine Peptides with Lipid Membranes

Abstract

Huntington's disease is a dominant genetic neurodegenerative disorder associated with motor and cognitive decline, caused by a mutation in the poly-glutamine (polyQ) region near the N-terminus of the huntingtin (htt) protein. Expansion of the polyQ region above 35-40 repeats results in the disease that is characterized by inclusion body aggregates of mutated protein. The polyQ expansion in htt is flanked by a 17 amino acid N-terminal sequence (Nt17) and a proline-rich (polyP) region. Patients with Huntington's disease have reduced ganglioside GM1 levels and restoring these levels in mouse models improves motor function, suggesting a neuroprotective effect, but with an unknown mechanism. To explore the role of GM1 in the cell membrane environment without downstream metabolic effectors, total brain lipid extract (TBLE) model membranes doped with either ganglioside GM1 or sphingomyelin (SM) were exposed to either a model, synthetic polyQ peptide or a full-length htt-exon1 recombinant protein. The interactions between htt and lipid membranes were measured with a combination of Langmuir trough monolayer techniques, vesicle permeability and binding assays, and in situ atomic force microscopy. While addition of GM1 to a TBLE membrane resulted in reduced htt insertion and binding, ganglioside inclusion into vesicles had no measureable effect on htt-induced membrane permeability. Atomic force microscopy studies parallel these findings with similar htt-induced morphological changes albeit with an overall decreased interaction between the protein and the membrane at higher GM1 concentrations suggesting the neuroprotective effect of GM1 may be partially attributed to decreased protein-membrane interactions. Parallel studies performed with sphingomyelin indicate that elevated levels of SM cause increases in htt membrane binding, htt-induced membrane permeability, and morphological and mechanical changes in the bilayer associated with exposure to htt-exon1.