Hsp104 targets multiple intermediates on the amyloid pathway and suppresses the seeding capacity of Abeta fibrils and protofibrils

The heat shock protein Hsp104 has been reported to possess the ability to modulate protein aggregation and toxicity and to “catalyze” the disaggregation and recovery of protein aggregates, including amyloid fibrils, in yeast, Escherichia coli, mammalian cell cultures, and animal models of Huntington's disease and Parkinson's disease. To provide mechanistic insight into the molecular mechanisms by which Hsp104 modulates aggregation and fibrillogenesis, the effect of Hsp104 on the fibrillogenesis of amyloid beta (Aβ) was investigated by characterizing its ability to interfere with oligomerization and fibrillogenesis of different species along the amyloid-formation pathway of Aβ. To probe the disaggregation activity of Hsp104, its ability to dissociate preformed protofibrillar and fibrillar aggregates of Aβ was assessed in the presence and in the absence of ATP. Our results show that Hsp104 inhibits the fibrillization of monomeric and protofibrillar forms of Aβ in a concentration-dependent but ATP-independent manner. Inhibition of Aβ fibrillization by Hsp104 is observable up to Hsp104/Aβ stoichiometric ratios of 1:1000, suggesting a preferential interaction of Hsp104 with aggregation intermediates (e.g., oligomers, protofibrils, small fibrils) on the pathway of Aβ amyloid formation. This hypothesis is consistent with our observations that Hsp104 (i) interacts with Aβ protofibrils, (ii) inhibits conversion of protofibrils into amyloid fibrils, (iii) arrests fibril elongation and reassembly, and (iv) abolishes the capacity of protofibrils and sonicated fibrils to seed the fibrillization of monomeric Aβ. Together, these findings suggest that the strong inhibition of Aβ fibrillization by Hsp104 is mediated by its ability to act at different stages and target multiple intermediates on the pathway to amyloid formation.     

Can size alone explain some of the differences in toxicity between β‐amyloid oligomers and fibrils?

β‐Amyloid (Aβ) peptide is believed to play a key role in the mechanism of Alzheimer's disease (AD). Aβ tends to aggregate to form amyloid fibrils. A variety of evidence indicates that Aβ aggregates are toxic in vitro and in vivo. An early “Aβ hypothesis” postulated that AD was the consequence of neuron death induced by insoluble deposits of large Aβ fibrils. Newer findings indicate that small soluble Aβ oligomers are the neurotoxic species, yet their structure is still unknown. Many researchers have tried to probe the differences in molecular structure between Aβ oligomers, protofibrils, and fibrils that give rise to their unique toxicities, but with limited success. In this report, we examine the hypothesis that differences in the toxicity of different aggregated Aβ species are the result of differences in species concentration and diffusivity. Using a simple mathematical analysis based on the assumption of a diffusion‐limited reaction, we demonstrate that near 10‐fold differences in toxicity between spherical oligomers and fibrils can be explained from size and concentration arguments. While this work does not suggest that Aβ oligomers and fibrils have identical molecular structures, it highlights the possibility that simple physical phenomena may contribute to the biological processes induced by Aβ. Biotechnol. Bioeng. 2010;106: 333–337. © 2010 Wiley Periodicals, Inc.