N-n-Propylnorapomorphine Inhibits Huntingtin Aggregation and Lipid Membrane Interactions

Abstract Aggregation of mutant huntingtin (HTT) contributes to Huntington’s disease (HD) pathogenesis. Lipid membranes strongly influence this process and can also be damaged by membrane-associated HTT species. N–n-propylnorapomorphine (NPA), an aporphine-derived dopamine agonist, was identified in a polydiacetylene (PDA)-based screen for compounds that reduce HTT-exon1-induced perturbation of lipid vesicles. Here, thioflavin T fluorescence assays, ex situ and in situ atomic force microscopy (AFM), lipid-vesicle binding assays, and supported-lipid-bilayer imaging were used to determine how NPA affects HTT aggregation and membrane interactions. In bulk solution, NPA reduced β-sheet-rich fibril formation in a dose-dependent manner, with high concentrations nearly abolishing fibril growth. AFM imaging further showed that NPA suppresses fibril formation while altering the oligomer morphology and heterogeneity. NPA also destabilized preformed HTT fibrils and suppressed the surface-mediated aggregation of an HTT-derived peptide. Importantly, NPA retained inhibitory activity in the presence of lipid vesicles that either suppressed or accelerated HTT aggregation. NPA reduced HTT association with lipid vesicles in a dose-dependent manner and markedly decreased HTT-induced disruption of supported brain lipid bilayers. Collectively, these observations are consistent with NPA acting at multiple, potentially connected stages of the HTT aggregation pathway by altering early assemblies, destabilizing fibrils, and reducing the formation or persistence of membrane-active species. The present data do not distinguish whether these effects arise from a single shared binding mode or from interactions with multiple HTT conformational states. Although its dopamine agonist activity may limit direct therapeutic application, NPA provides a useful proof-of-concept scaffold for compounds targeting both HTT aggregation and membrane-associated toxicity.