Inhibiting the Nucleation of Amyloid Structure in a Huntingtin Fragment by Targeting α-Helix-Rich Oligomeric Intermediates

Although oligomeric intermediates are transiently formed in almost all known amyloid assembly reactions, their mechanistic roles are poorly understood. Recently, we demonstrated a critical role for the 17-amino-acid N-terminus (httNT segment) of huntingtin (htt) in the oligomer-mediated amyloid assembly of htt N-terminal fragments. In this mechanism, the httNT segment forms the α-helix-rich core of the oligomers, leaving much of the polyglutamine (polyQ) segment disordered and solvent-exposed. Nucleation of amyloid structure occurs within this local high concentration of disordered polyQ. Here we demonstrate the kinetic importance of httNT self-assembly by describing inhibitory httNT-containing peptides that appear to work by targeting nucleation within the oligomer fraction. These molecules inhibit amyloid nucleation by forming mixed oligomers with the httNT domains of polyQ-containing htt N-terminal fragments. In one class of inhibitors, nucleation is passively suppressed due to the reduced local concentration of polyQ within the mixed oligomer. In the other class, nucleation is actively suppressed by a proline-rich polyQ segment covalently attached to httNT. Studies with d-amino acid and scrambled sequence versions of httNT suggest that inhibition activity is strongly linked to the propensity of inhibitory peptides to make amphipathic α-helices. HttNT derivatives with C-terminal cell-penetrating peptide segments also exhibit excellent inhibitory activity. The httNT-based peptides described here, especially those with protease-resistant d-amino acids and/or with cell-penetrating sequences, may prove useful as lead therapeutics for inhibiting the nucleation of amyloid formation in Huntingtonʹs disease.