Modulation of Prion-dependent Polyglutamine Aggregation and Toxicity by Chaperone Proteins in the Yeast Model

Abstract

In yeast, aggregation and toxicity of the expanded polyglutamine fragment of human huntingtin strictly depend on the presence of the endogenous self-perpetuating aggregated proteins (prions), which contain glutamine/asparagine-rich domains. Some chaperones of the Hsp100/70/40 complex, modulating propagation of yeast prions, were also reported to influence polyglutamine aggregation in yeast, but it was not clear whether they do it directly or via affecting prions. Our data show that although some chaperone alterations indeed act on polyglutamines via curing endogenous prions, other alterations decrease size and ameliorate toxicity of polyglutamine aggregates without affecting prion propagation. Therefore, the role of yeast chaperones in polyglutamine aggregation and toxicity is not restricted only to their effects on the endogenous prions. Moreover, chaperone interactions with prion and polyglutamine aggregates appear to be of a highly specific nature. One and the same chaperone alteration, substitution A503V in the middle region of the chaperone Hsp104, exhibited opposite effects on one of the endogenous prions ([PSI ؉ ], the prion form of Sup35) and on polyglutamines, increasing aggregate size and toxicity in the former case and decreasing them in the latter case. On the other hand, different members of a single chaperone family exhibited opposite effects on one and the same type of aggregates: excess of the Hsp40 chaperone Ydj1 increased polyglutamine aggregate size and toxicity, whereas excess of the other Hsp40 chaperone, Sis1, decreased them. As many stress-defense proteins are conserved between yeast and mammals, these data shed light on possible mechanisms modulating polyglutamine aggregation and toxicity in mammalian cells. Expansion of glutamine repeats (poly-Q)1 in certain proteins is responsible for neurodegenerative disorders. The hallmark of poly-Q diseases is the formation of insoluble cytosolic and nuclear inclusions (1). Huntington disease is one of the best known poly-Q disorders (2). It is caused by an expansion of the poly-Q stretch in the essential protein called huntingtin (Htt) to more than 37 amino acids. The length of the poly-Q stretch inversely correlates with the time of onset of the disease and the time of formation of Htt aggregates (3). The role of various types of aggregates in cell toxicity remains a matter of debate (4). Recent models propose that toxicity of poly-Q Htt arises from sequestration of certain essential proteins by Htt aggregates (5-7). The poly-Q stretch is located within the N-terminal (exon 1) region of Htt, which is involved in numerous proteinprotein interactions (8). The N-terminal fragment of Htt with poly-Q extensions, when expressed in mice, aggregated and was sufficient to cause Huntington disease-like neurodegeneration (9, 10). This indicates that at least some parameters of poly-Q associated aggregation and toxicity could be reproduced in the experimental assays using only N-terminal poly-Q expanded fragments of Htt.Several models for studying poly-Q aggr...

The yeast prion protein Ure2: Structure, function and folding

“…Involvement of the Hsp70 chaperone system in yeast prion maintenance and curing is consistent with the finding that overexpression of Hsp70 suppresses polyglutamine-associated pathogenicity and reduces the formation of amyloid fibrils [97,98], and suggests that the Hsp70 family may have a universal role in suppressing the formation of pathogenic aggregates, including prions. However, to date, direct demonstration of an interaction between Hsp70 and yeast prions by in vitro biochemical analysis has not yet been reported.…”

Ordered Assembly of Heat Shock Proteins, Hsp26, Hsp70, Hsp90, and Hsp104, on Expanded Polyglutamine Fragments Revealed by Chemical Probes

“…As a first test of this idea, we found that 115-7c (100 M) did not alter the localization of Rnq1p-YFP, suggesting that prion status is unchanged by this treatment (supplemental ϩ status. Perhaps consistent with this general idea, the Chernoff Laboratory has observed that some chaperones impact polyQ toxicity through effects on prions, while others do not (48).…”

Effects of Ubiquitin System Alterations on the Formation and Loss of a Yeast Prion

“…Another chaperone of the Hsp70 family, Ssb, consistently manifests itself as a [PSI ϩ ] antagonist (26). Hsp40 chaperones that act as co-chaperones of Hsp70s were shown to control propagation of [PIN ϩ ] (27) and modulate aggregation of heterologous poly(Q) proteins in yeast (28,29). Mechanisms of the chaperone effects on prions are not yet completely understood.…”