Concentration-dependent structural transition of huntingtin protein in Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by the abnormal expansion of the polyglutamine (polyQ) tract (> 35Q) in the first exon of the huntingtin (Htt), HttEx1. This N-terminal fragment tends to form fibrillar inclusions, which constitute a key pathological hallmark of HD. Although polyQ expansion is commonly understood to be a primary cause of HttEx1 pathology, the molecular mechanism of aggregations of non-pathogenic polyQ tract with the N-terminally flanking region of N17 in HttEx1 (HttEx1-17Q) remains largely unknown. In this study, we exclusively investigated the effect of the protein concentration on the structural transition of HttEx1-17Q and its relation to the amyloid fibril formation by employing biophysical techniques including nuclear magnetic resonance (NMR) and circular dichroism (CD) spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM), and thioflavin T (ThT) fluorescence. Complementary analyses showed that monomeric HttEx1-17Q undergoes a multiple structural transition from largely unfolded structures to β structures via helical structures in a concentration-dependent manner in the early stages of aggregation. This structural rearrangement accelerates kinetically the formation of short amyloid fibrils of HttEx1-17Q by facilitating nucleation. Our findings provide new insights into the amyloid formation of HttEx1 by highlighting the critical role of a structural conversion into an amyloidogenic structure, of which mechanism is helpful to understand amyloidogenesis of other amyloid-forming molecules.