Effects of Macromolecular Cosolutes on the Kinetics of Huntingtin Aggregation Monitored by NMR Spectroscopy

The effects of two macromolecular cosolutes, specifically the polysaccharide dextran-20 and the protein lysozyme, on the aggregation kinetics of a pathogenic huntingtin exon-1 protein (hht^ex1) with a 35 polyglutamine repeat, htt^ex1Q₃₅, are described. A unified kinetic model that establishes a direct connection between reversible tetramerization occurring on the microsecond time scale and irreversible fibril formation on a time scale of hours/days forms the basis for quantitative analysis of htt^ex1Q₃₅ aggregation, monitored by measuring cross-peak intensities in a series of 2D ¹H–¹⁵N NMR correlation spectra acquired during the course of aggregation. The primary effects of the two cosolutes are associated with shifts in the prenucleation tetramerization equilibrium resulting in substantial changes in concentration of “preformed” htt^ex1Q₃₅ tetramers. Similar effects of the two cosolutes on the tetramerization equilibrium observed for a shorter, nonaggregating huntingtin variant with a 7-glutamine repeat, htt^ex1Q₇, lend confidence to the conclusions drawn from the fits to the htt^ex1Q₃₅ aggregation kinetics.