Mutant Huntingtin exon-1 proteins form trans-dimers/ oligomers as detected by bimolecular fluorescence complementation

Toxic oligomers of mutant Huntingtin (mHtt) proteins have been implicated in the pathogenesis of Huntington's disease (HD). To examine how oligomerization of mHtt is initiated within cells, we have developed a novel set of bimolecular fluorescence complementation (BiFC) probes to evaluate dimerization/oligomerization of Huntingtin protein by live cell imaging. Expression of either the wild-type Huntingtin exon 1 GFP (wtHttPolyQ25-GFP) or the mutant Huntingtin exon 1 GFP (mHttPolyQ97-GFP) was revealed by the GFP signal. By splitting mCherry, we were able to further differentiate between the diffuse signals associated with monomeric Htt proteins and aggregated Htt proteins that may implicate the early developments of inclusion bodies or insoluble oligomers associated with HD pathogenesis. As expected, we detected dimerization between mHttPoly97 proteins. To our surprise, we also found that wtHttPolyQ25 formed dimers. Moreover, we discovered that dimerization of wtHttPolyQ25 or mHttPoly97 proteins occurred in a trans- but not in a cis-manner. Our findings suggest that mHtt could potentially form hetero- dimers and sequester wtHtt, thereby disrupting its normal functio