Structural Variations in the Aggregation Pathways of Normal and Pathological Huntingtin-Like Peptides

Abstract

The current challenge in the amyloid field is to obtain structural information on the progression of associated aggregation pathways, relate them to the cause of disease, and specifically point to the species truly responsible for neurotoxicity. Huntington's disease (HD) is linked to the aggregation of mutant huntingtin (htt) peptide, which contains an abnormally long polyglutamine (polyGln) repeat that leads to the formation of fibrils with stable, β-sheet rich structures. Analogous to other neurodegenerative diseases like Alzheimer's, mature htt fibrils are thought to be overall protective while the earlier smaller intermediates are proposed to be toxic to neuronal cells. We performed time-resolved small-angle neutron scattering (SANS) experiments to monitor the structural kinetics for htt peptides having polyGln repeats in the normal (22 Glns) and pathological (42 Glns) range. In addition to observing the expected faster aggregation rate for longer polyGln repeats, we find distinct conformational differences between normal and pathological htt. We also present, for the first time, three-dimensional structures of intermediates formed at the earliest stages of htt aggregation as detected by SANS and obtained using ab initio shape reconstruction methods. Finally, we are able to investigate the internal structure of the mature fibrils, where the mass-per-length of pathological htt fibrils is considerably less than normal htt fibrils. These findings provide the first steps toward characterizing the oligomers formed by htt peptides and illustrate the utility of SANS for identifying different aggregate intermediates formed in the development of neurodegenerative diseases.