Nuclear targeting of mutant huntingtin increases toxicity

Huntington's disease is a neurodegenerative disorder resulting from expansi on of the polyglutamine region in huntingtin. Although huntingtin is normal ly cytoplasmic, in affected brain regions proteolytic fragments of mutant h untingtin containing the polyglutamine repeat form intranuclear inclusions. Here, we examine the contribution of nuclear localization to toxicity by t ransiently transfecting neuro-aa cells with an N-terminal huntingtin fragme nt similar in size to that believed to be present in patients. The huntingt in fragment, HD-N63, was targeted either to the cytoplasm with a nuclear ex port signal (NES) or to the nucleus with a nuclear localization signal (NLS ). The NES decreased the number of cells with aggregates in the nucleus whi le an NLS had the opposite effect. By cotransfecting HD-N63 with GFP as a m arker, we observed direct cell loss with constructs containing expanded pol yglutamine repeats. Compared to unmodified HD-N63-75Q, adding an NES reduce d cell loss by 57% while an NLS increased cell loss by 111%. These results indicate that nuclear localization of mutant huntingtin fragments plays an important role in cell toxicity.