Quantitative NMR analysis of the kinetics of prenucleation oligomerization and aggregation of pathogenic huntingtin exon-1 protein

The N-terminal region of the huntingtin protein, encoded by exon-1 (htt(ex1)) and containing an expanded polyglutamine tract, forms fibrils that accumulate in neuronal inclusion bodies, resulting in Huntington’s disease. We previously showed that reversible formation of a sparsely populated tetramer of the N-terminal amphiphilic domain, comprising a dimer of dimers in a four-helix bundle configuration, occurs on the microsecond timescale and is an essential prerequisite for subsequent nucleation and fibril formation that takes place orders of magnitude slower on a timescale of hours. For pathogenic htt(ex1), such as htt(ex1)Q(35) with 35 glutamines, NMR signals decay too rapidly to permit measurement of time-intensive exchange-based experiments. Here, we show that quantitative analysis of both the kinetics and mechanism of prenucleation tetramerization and aggregation can be obtained simultaneously from a series of (1)H-(15)N band–selective optimized flip-angle short-transient heteronuclear multiple quantum coherence (SOFAST-HMQC) correlation spectra. The equilibria and kinetics of tetramerization are derived from the time dependence of the (15)N chemical shifts and (1)H-(15)N cross-peak volume/intensity ratios, while the kinetics of irreversible fibril formation are afforded by the decay curves of (1)H-(15)N cross-peak intensities and volumes. Analysis of data on htt(ex1)Q(35) over a series of concentrations ranging from 200 to 750 μM and containing variable (7 to 20%) amounts of the Met(7)O sulfoxide species, which does not tetramerize, shows that aggregation of native htt(ex1)Q(35) proceeds via fourth-order primary nucleation, consistent with the critical role of prenucleation tetramerization, coupled with first-order secondary nucleation. The Met(7)O sulfoxide species does not nucleate but is still incorporated into fibrils by elongation.