Huntingtin N17 domain is a reactive oxygen species sensor regulating huntingtin phosphorylation and localization

Abstract

The N17 domain of the huntingtin protein is post-translationallymodified and is themaster regulator of huntingtin intracellular localization. In Huntington's disease (HD),mutant huntingtin is hypo-phosphorylated at serines 13 and 16 within N17, and increasing N17 phosphorylation has been shown to be protective in HDmousemodels. Thus, N17 phosphorylation is defined as a sub-target of huntingtin for potential therapeutic intervention.We have previously shown that cellular stress can affect huntingtin nuclear entry and phosphorylation. Here, we demonstrate that huntingtin localization can be specifically affected by reactive oxygen species (ROS) stress.We have located the sensor of this stress to the N17 domain, specifically to a highly conserved methionine at position 8. In vitro, we show by circular dichroismspectroscopy structural studies that the alpha-helical structure of N17 changes in response to redox conditions and show that the consequence of this change is enhanced N17 phosphorylation and nuclear targeting of endogenous huntingtin. Using N17 substitution pointmutants, we demonstrate that N17 sulphoxidation enhances N17 dissociation fromthe endoplasmic reticulum(ER)membrane. This enhanced solubilitymakes N17 a better substrate for phosphorylation and subsequent nuclear retention. This ability of huntingtin to sense ROS levels at the ER, with phosphorylation and nuclear localization as a response, suggests that ROS stress due to aging could be a criticalmolecular trigger of huntingtin functions and dysfunctions in HD andmay explain the age-onset nature of the disorder.