Atomic force microscopy analysis of the Huntington protein nanofibril formation.

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 beta-peptide and alpha-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.