Polyglutamine Toxicity Is Controlled by Prion Composition and Gene Dosage in Yeast

School of Biology, Georgia Institute of Technology, Atlanta, Georgia, United States of America.
PLoS Genetics (Impact Factor: 7.53). 04/2012; 8(4):e1002634. DOI: 10.1371/journal.pgen.1002634
Source: PubMed


Polyglutamine expansion causes diseases in humans and other mammals. One example is Huntington's disease. Fragments of human huntingtin protein having an expanded polyglutamine stretch form aggregates and cause cytotoxicity in yeast cells bearing endogenous QN-rich proteins in the aggregated (prion) form. Attachment of the proline(P)-rich region targets polyglutamines to the large perinuclear deposit (aggresome). Aggresome formation ameliorates polyglutamine cytotoxicity in cells containing only the prion form of Rnq1 protein. Here we show that expanded polyglutamines both with (poly-QP) or without (poly-Q) a P-rich stretch remain toxic in the presence of the prion form of translation termination (release) factor Sup35 (eRF3). A Sup35 derivative that lacks the QN-rich domain and is unable to be incorporated into aggregates counteracts cytotoxicity, suggesting that toxicity is due to Sup35 sequestration. Increase in the levels of another release factor, Sup45 (eRF1), due to either disomy by chromosome II containing the SUP45 gene or to introduction of the SUP45-bearing plasmid counteracts poly-Q or poly-QP toxicity in the presence of the Sup35 prion. Protein analysis confirms that polyglutamines alter aggregation patterns of Sup35 and promote aggregation of Sup45, while excess Sup45 counteracts these effects. Our data show that one and the same mode of polyglutamine aggregation could be cytoprotective or cytotoxic, depending on the composition of other aggregates in a eukaryotic cell, and demonstrate that other aggregates expand the range of proteins that are susceptible to sequestration by polyglutamines.

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    • "Prion variants that do not cause toxicity on their own may become toxic in combination with other factors. For example, endogenous prions, such as [PIN + ] and/or [PSI + ] (Gokhale et al., 2005; Gong et al., 2012; Kochneva-Pervukhova et al., 2012; Zhao et al., 2012), promote aggregation and toxicity of expanded polyQ constructs in the yeast huntingtin model. Overproduction of Sup35 (Chernoff et al., 1992; Derkatch et al., 1997; Vishveshwara et al., 2009) or Rnq1 (Douglas et al., 2008; Stein & True, 2011; Treusch & Lindquist, 2012) is toxic to prion-containing cells. "
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    • "Although the Q25 htt variant (corresponding to a normal polyQ length) did not aggregate, insoluble inclusion formation increased with an increase in the polyQ length (Krobitsch and Lindquist 2000), recapitulating results obtained in cultured mammalian cells and animal models (Apostol et al. 2003, 2006; Woodman et al. 2007). The correlation between aggregation and toxicity of htt fragments in yeast was found to be dependent on the sequences flanking the polyglutamine stretches as well as on the existence of specific interacting proteins of the yeast strain expressing it, in particular the prion composition of the cell (Meriin et al. 2002; Duennwald et al. 2006a, b; Gong et al. 2012). Specifically, the htt exon 1 with expanded polyQ tracts was shown to impair protein homeostasis of the ER (Duennwald and Lindquist 2008) and endocytosis (Meriin et al. 2003, 2007), cause transcriptional deregulation (Hughes et al. 2001), increase ROS production by affecting mitochondrial function and morphology (Giorgini et al. 2005; Sokolov et al. 2006; Solans et al. 2006), induce activation of apoptotic pathways (Sokolov et al. 2006), and cause cell cycle dysfunction (Sokolov et al. 2006; Bocharova et al. 2008). "
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    • "Perhaps, there is a certain threshold concentration for a misfolded protein where it can no longer be efficiently recruited to a single site and neutralized; proteins with efficient targeting would only cause cytotoxic effects upon accumulation above this threshold. Alternatively, 'aggresome' targeting does not ameliorate cytotoxicity in yeast in the presence of specific prions (Gong et al., 2012). Cellular proteins involved in managing misfolded protein species can be anticipated to mitigate cytotoxicity. "
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