Slow Amyloid Nucleation via α-Helix-Rich Oligomeric Intermediates in Short Polyglutamine-Containing Huntingtin Fragments

The 17-amino-acid N-terminal segment (htt(NT)) that leads into the polyglutamine (polyQ) segment in the Huntington's disease protein huntingtin (htt) dramatically increases aggregation rates and changes the aggregation mechanism, compared to a simple polyQ peptide of similar length. With polyQ segments near or above the pathological repeat length threshold of about 37, aggregation of htt N-terminal fragments is so rapid that it is difficult to tease out mechanistic details. We describe here the use of very short polyQ repeat lengths in htt N-terminal fragments to slow this disease-associated aggregation. Although all of these peptides, in addition to htt(NT) itself, form alpha-helix-rich oligomeric intermediates, only peptides with Q(N) of eight or longer mature into amyloid-like aggregates, doing so by a slow increase in beta-structure. Concentration-dependent circular dichroism and analytical ultracentrifugation suggest that the htt(NT) sequence, with or without added glutamine residues, exists in solution as an equilibrium between disordered monomer and alpha-helical tetramer. Higher order, alpha-helix rich oligomers appear to be built up via these tetramers. However, only htt(NT)Q(N) peptides with N=8 or more undergo conversion into polyQ beta-sheet aggregates. These final amyloid-like aggregates not only feature the expected high beta-sheet content but also retain an element of solvent-exposed a-helix. The alpha-helix-rich oligomeric intermediates appear to be both on- and off-pathway, with some oligomers serving as the pool from within which nuclei emerge, while those that fail to undergo amyloid nucleation serve as a reservoir for release of monomers to support fibril elongation. Based on a regular pattern of multimers observed in analytical ultracentrifugation, and a concentration dependence of alpha-helix formation in CD spectroscopy, it is likely that these oligomers assemble via a four-helix assembly unit. PolyQ expansion in these peptides appears to enhance the rates of both oligomer formation and nucleation from within the oligomer population, by structural mechanisms that remain unclear. (C) 2011 Elsevier Ltd. All rights reserved.