Huntington's disease is a progressive neurodegenerative disorder caused by a polyglutamine [poly(Q)] repeat expansion in the first exon of the huntingtin protein.Previously, we showed that N-terminal huntingtin peptides with poly(Q) tracts in the pathological range (51-122 glutamines), but not with poly(Q) tracts in the normal range (20 and 30 glutamines), form high molecular weight protein aggregates with a fibrillar or ribbon-like morphology, reminiscent of scrapie prion rods and ␤-amyloid fibrils in Alzheimer's disease. Here we report that the formation of amyloidlike huntingtin aggregates in vitro not only depends on poly(Q) repeat length but also critically depends on protein concentration and time. Furthermore, the in vitro aggregation of huntingtin can be seeded by preformed fibrils. Together, these results suggest that amyloid fibrillogenesis in Huntington's disease, like in Alzheimer's disease, is a nucleation-dependent polymerization.

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The accumulation of insoluble protein aggregates in intra and perinuclear inclusions is a hallmark of Huntington's disease (HD) and related glutamine-repeat disorders. A central question is whether protein aggregation plays a direct role in the pathogenesis of these neurodegenerative diseases. Here we show by using a filter retardation assay that the mAb 1C2, which specifically recognizes the elongated polyglutamine (polyQ) stretch in huntingtin, and the chemical compounds Congo red, thioflavine S, chrysamine G, and Direct fast yellow inhibit HD exon 1 protein aggregation in a dose-dependent manner. On the other hand, potential inhibitors of amyloid-␤ formation such as thioflavine T, gossypol, melatonin, and rifampicin had little or no inhibitory effect on huntingtin aggregation in vitro. The results obtained by the filtration assay were confirmed by electron microscopy, SDS͞ PAGE, and MS. Furthermore, cell culture studies revealed that the Congo red dye at micromolar concentrations reduced the extent of HD exon 1 aggregation in transiently transfected COS cells. Together, these findings contribute to a better understanding of the mechanism of huntingtin fibrillogenesis in vitro and provide the basis for the development of new huntingtin aggregation inhibitors that may be effective in treating HD.glutamine repeat ͉ inhibition