Atomic force microscopy analysis of the Huntington protein nanofibril formation

Abstract Background Huntington's disease is an autosomal dominant progressive neurodegenerative disease associated with dramatic expansion of a polyglutamine sequence in exon 1 of the huntingtin protein htt that leads to cytoplasmic, and even nuclear aggregation of fibrils. Methods We have studied the in vitro fibril formation of mutant exon 1, and the shorter wild-type exon 1, with use of atomic force microscopy (AFM). Results Large aggregates are formed spontaneously after cleavage of the glutathione-S-transferase fusion protein of the mutant exon 1 protein. The AFM data showed that, unlike fibrils assembled by such proteins as amyloid β-peptide and α-synuclein, htt forms fibrils with extensive branched morphologic features. Branching can be observed even at earlier stages of the htt self-assembly, but the effect is much more pronounced at late stages of aggregation. We also found that fusing of htt with green fluorescent protein does not change the branched-type morphologic features of the aggregates. Conclusions On the basis of the results obtained, we propose a model for htt fibrillization that explains branched morphologic features of the aggregates.