Huntington’s disease is a neurological disease that is caused by mutations in the gene that encodes a protein called Htt. Individuals with this mutation gradually lose neurons as they age, resulting in declines in muscle coordination and mental abilities. The mutant Htt proteins tend to form clumps inside cells, but it is not clear if these clumps are the cause of the disease symptoms or whether they have a protective effect.

Yang et al. used yeast as a model to investigate whether the mutant Htt proteins need other molecules to allow them to form clumps. The experiments identified several new molecules that are required for mutated Htt to form clumps. Some of these are components of a system called the Ribosome Quality Control (RQC) complex, which monitors newly made proteins and labels abnormal ones for destruction. However, Yang et al.’s findings suggest that the RQC complex regulates the formation of Htt clumps through a different pathway involving a protein called heat shock factor 1. In this case, cells would need to fine-tune heat shock factor 1 activity to make mutant Htt proteins clump together to protect cells from damage.

Future experiments should expand Yang et al.’s findings to animal models of Huntington’s disease and identify which other molecules contribute to the formation of Htt clumps. One challenge will be to find out why older neurons fail to form clumps of Htt proteins, and whether this can be overcome by drugs that boost the activity of the molecules that Yang et al. identified.

The Huntington disease (HD) is predominantly inherited, with a single gene, HTT, encoding the Huntingtin protein, at its origin (MacDonald, 1993). Mutated and aggregation-prone poly-glutamine-expanded (Poly (Q)) Huntingtins (mHtt) are causing HD by toxic gain-of-functions and, possibly, dominant-negative mechanisms, which are typically manifested in aged individuals (Ross and Tabrizi, 2011). While the formation of mHtt inclusion bodies (IBs) correlates with toxicity and disease, such formation might, in effect, be a protective response to limit proteotoxicity (Ross and Tabrizi, 2011; Arrasate et al., 2004): For example, IB formation predicts improved survival in neurons (Arrasate et al., 2004) and the IB-forming mHtt103QP protein (Figure 1a; exon-1 with 97Q repeats) are not, or only mildly, cytotoxic even when produced at high levels in young yeast cells (Dehay and Bertolotti, 2006; Duennwald et al., 2006). In contrast, when the innate proline-rich region adjacent the poly (Q) stretch of exon-1 is removed, the protein, mHtt103Q, forms multiple small, highly cytotoxic aggregates/oligomers (Figure 1a) (Dehay and Bertolotti, 2006; Duennwald et al., 2006; Meriin et al., 2002). These aggregates are associated with the actin cytoskeleton (Song et al., 2014) and interfere with the cytosolic ubiquitin-proteasome-system (UPS) by sequestering the Hsp40 chaperone Sis1 (Park et al., 2013). Chaperones, peptides, and prion-like proteins that either prevent/modify oligomer production (Behrends et al., 2006; Dehay and Bertolotti, 2006; Krobitsch and Lindquist, 2000; Muchowski et al., 2000; Gokhale et al., 2005) or convert small aggregates/oligomers into IBs (Kayatekin et al., 2014; Wolfe et al., 2014) can suppress the toxicity of the proline-less exon-1, suggesting that small aggregates and oligomers are likely culprits in mHtt103Q-derived toxicity (Arrasate et al., 2004; Miller et al., 2011).

Figure 1

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Ubiquitination is another process suggested to prevent mHtt toxicity in both mammals (Steffan, 2004) and yeast (Willingham et al., 2003). IBs of mHtt contain ubiquitin in mice (Davies et al., 1997) and the human ubiquitin-conjugating enzyme, hE2-25K, interacts with mHtt, which has been shown to be ubiquitinated in both humans and flies (Kalchman et al., 1996; Steffan, 2004). However, an E3 ubiquitin ligase directly responsible for mHtt ubiquitin-tagging, IB formation, and detoxification has not been identified.

We approached mHtt toxicity by a different route than recent mHtt103Q toxicity-suppression screens (Kayatekin et al., 2014; Mason et al., 2013; Wolfe et al., 2014) by asking if the non-toxic, IB-forming mHtt103QP carrying the innate proline-rich stretch of exon-1, requires trans-acting factors to form IBs and if such factors convert mHtt103QP into non-toxic conformers. This approach was prompted also by our finding that the ability to form large and single mHtt103QP IBs was lost upon mother cell aging and the mHtt proteins accumulated instead in multiple, three or more smaller aggregates per cell, referred to as Class 3 cells (Figure 1b; Class 1 cells contain one aggregate and Class 2 cells contain two aggregates). To identify trans-acting factors required for IB formation in an unbiased genome-wide manner, we used high content microscopy (HCM) and a galactose-regulated version of mHtt103QP, which we introduced into the ordered yeast deletion library (SGA-V2) (Tong, 2001) of S. cerevisiae (Figure 1c). Upon galactose-induction, mHtt103QP formed aggregates in about 50% of the cells within 180 min (Figure 1d) and 70% of these cells contain one large IB. HCM was used to identify mutants that formed multiple aggregates/oligomers rather than a big IB (Class 3 mutants; Figure 1e), which revealed that IB formation requires proteasome/chaperone and ubiquitination functions, Golgi-vesicle trafficking, mRNA transport/metabolism, and cell cycle control (Figure 1f&g, see Supplementary file 1 for a list of confirmed mutants). Among these factors, Ltn1 and Rqc1 are especially interesting as they are both partners of the ribosome quality control complex (RQC) (Brandman et al., 2012) and Ltn1 is the yeast homologue of the E3 RING ubiquitin ligase Listerin of mammalian cells (Bengtson and Joazeiro, 2010), which reduced activity causes premature neurodegeneration in mice (Chu et al., 2009).

Complementation analysis revealed that the ubiquitin E3 ligase activity of Ltn1 was required for both mHtt103QP IB formation (Figure 2a) and ubiquitination (Figure 2b). It’s been reported that the absence of Ltn1, but not Rqc1, results in the failure to tag non-stop protein with ubiquitin (Brandman et al., 2012). Contrasting such data on non-stop proteins, both Ltn1 and Rqc1-deficieny resulted in a failure of cells to tag also full-length mHtt103QP properly with ubiquitin (Figure 2b, Figure 2—figure supplement 1) and to form IBs, even though the effect of rqc1∆ was markedly smaller than ltn1∆ on IB formation (Figure 2a). Moreover, both soluble and aggregated mHtt103QP was stable in the absence and presence of Ltn1 (Figure 2c, Figure 2—figure supplement 2), and the levels of soluble and aggregated mHtt103QP was somewhat lower in ltn1∆ cells (Figure 2—figure supplements 2 & 4). These data suggest that Ltn1 is involved in mHTT103QP sequestration into IBs rather than its decay.

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Ltn1-, and to a lesser extent, Rqc1-deficieny results in hyper-activation of the heat shock transcription factor Hsf1 through the RQC component Tae2 and such activation can thus be suppressed by removing the TAE2 gene (Brandman et al., 2012). We found that deleting TAE2 in ltn1∆ or rqc1∆ cells restored IB formation (Figure 2a, Figure 2—figure supplements 2 & 5) but did not restore ubiquitination (Figure 2b), demonstrating that ubiquitination is not an absolute requirement for the formation of mHtt103QP IBs. Moreover, overproducing a hyperactive Hsf1 (HSF1-R206S [Hou et al., 2013]) alone was sufficient to reduce IB formation, as was reducing Hsf1 activity using the hsf1-848(ts) allele (Figure 2d). demonstrating that maintaining a proper, intermediate, range of Hsf1 activity is required to efficiently sequester mHtt103QP into IBs. In support of this notion, a deletion in the C-terminal trans-activation domain of Hsf1 resulted in defects in IB formation that could not be further abrogated by an ltn1 deletion (Figure 2d).

The mHtt103QP protein displays no obvious toxicity in yeast (Dehay and Bertolotti, 2006; Duennwald et al., 2006) but we found that it became detrimental in the absence of Ltn1, and to a somewhat lesser extent, Rqc1 (Figure 2e), supporting the idea that IB formation protects the cell against Huntingtin toxicity. Consistently, a tae2∆ mutation completely suppressed the toxicity of mHtt103QP in the ltn1∆ cells (Figure 2e). Since the TAE2 deletion did not restore mHtt103QP ubiquitination, we conclude that IB formation is more important than ubiquitination for the detoxification of mHtt103QP, at least in the yeast model system. Contrasting the LTN1 data, the absence of TAE2 failed to fully suppress toxicity in rqc1∆ cells indicating that the roles of Ltn1 and Rqc1 in RQC are overlapping (Brandman et al., 2012) but not identical. Consistent with small mHtt103QP aggregates/conformers being toxic, both overactive and diminished Hsf1 activity rendered mHtt103QP toxic (Figure 2f&g). Since the proline-less, intrinsically noxious, mHtt103Q protein requires the presence of the prion-forming protein Rnq1 to display cytotoxicity in yeast (Meriin et al., 2002), we tested whether the toxicity of mHtt103QP in Ltn1-deficient also required the presence of Rnq1 and found that this was not the case (Figure 2e).

The small cytotoxic mHtt103Q aggregates have been shown to associate with the actin cytoskeleton (Song et al., 2014), and we, therefore, investigated if mHtt103QP in wild type and ltn1∆ cells likewise interacted with and affected actin cytoskeletal structures. First, using co-staining with the misfolded protein Ubc9^ts-mCherry, we confirmed that the mHtt103QP proteins of wild type cells were deposited in IBs adjacent to the Ubc9^ts-associated insoluble-protein-deposit, IPOD (Kaganovich et al., 2008) (Figure 3a). Super resolution, three-dimensional structured illumination microscopy (SIM) revealed that these mHtt103QP IBs were associated with dense actin cytoskeletal structures (Figure 3b, Video 1). Moreover, the actin cytoskeleton appears to harness latent mHtt103QP toxicity as a screen for conditional ts mutations causing synthetic sickness/lethality with mHtt103QP (Figure 3d&e) revealed that cells carrying ts mutations in genes encoding actin itself (act1), profiling (pfy1) involved in actin polymerization, cofilin (cof1) regulating assembly/disassembly of actin filaments, Arp3 of the actin-nucleation center, Las17, an activator of Arp2/3 and actin assembly factors, and Mss4, a phosphatidylinositol-4-phosphate 5-kinase involved in actin cytoskeleton organization, were drastically sensitized to mHtt103QP (Figure 3d&e, also see Supplementary file 2 for a list of alleles). The multiple mHtt103QP aggregates formed in ltn1∆ cells also co-localized with actin cytoskeletal structures (Figure 3c, Video 2), akin to those of the toxic mHtt103Q aggregates reported previously (Song et al., 2014). Actin-mHtt103QP-associated structures were more abundant in Ltn1-deficient cells than in wild type cells whereas the number of aggregate-free forms of actin structures, including actin patches, was reduced (Figure 3f). Because the actin cytoskeleton is required for proper endocytosis, we tested the effect of mHtt103QP and an ltn1 deletion on the rate of endocytic internalization of the dye FM4-64, and found that Htt103QP retarded endocytosis and that such retardation was more pronounced in cells lacking Ltn1 (Figure 3g; Figure 3—figure supplement 1). In contrast, Ltn1 deficiency did not by itself cause actin cytoskeleton defects or endocytosis retardation (Figure 3g, Figure 3—figure supplement 2).

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The conserved Listerin (Ltn1) E3 ligase is a key factor involved in targeting protein products derived from defective mRNA or aborted translation for degradation by the 26S proteasome (Bengtson and Joazeiro, 2010; Brandman et al., 2012). Upon translation stalling, ribosome recycling factors dissociate 80S ribosome-nascent chain complexes to 60S ribosome-nascent chain-tRNA complexes, which are recognized by Ltn1 and Tae2 (Shen et al., 2015; Shao et al., 2015; Shao et al., 2013). Both nascent chains and, for example, K12- and R12-arrested polypeptides are substrates for Ltn1-dependent ubiquitin tagging, which signal their destruction by the 26S proteasome (Bengtson and Joazeiro, 2010; Brandman et al., 2012; Preissler et al., 2015). Herein, we report on another pivotal role of Ltn1 in protein quality control – detoxification of mutant Huntingtin through a Tae2/Hsf1-dependent sequestration of mHtt103QP into actin-associated inclusions (Figure 3h). As depicted in Figure 3h, the effect of Ltn1 on mHtt103QP aggregation appears to act through Tae2, which in turn is known to negatively control Hfs1 activity (Brandman et al., 2012). Thus, the presence of Tae2 is known to cause hyperactivation of Hsf1 when LTN1 is deleted (Brandman et al., 2012), which could be enough to inhibit IB formation. On the other hand, mutations reducing Hsf1 activity also inhibited IB formation suggesting that maintaining a proper, intermediate, range of Hsf1 activity is required to efficiently sequester mHtt103QP into IBs (Figure 3h). In worms, elevated production of small heat shock proteins through Hsf1 activity has been shown to delay the onset of polyglutamine-expansion protein aggregation (Hsu, 2003) and reducing hsf-1 activity accelerates aging (Hsu, 2003; Morley and Morimoto, 2004). Reciprocally, hsf-1 overexpression extends worm lifespan (Hsu, 2003). (Baird et al., 2014). The data presented here, however, demonstrate that both Hsf1 elevation and Hsf1 deficiency in cells expressing the Huntingtin disease protein is detrimental (Figure 3h), suggesting, again, that a fine balance of Hsf1 activity has to be maintained to assuage proteotoxicity. This notion might explain why alterations in Hsf1 levels in mammalian cells have been shown to either inhibit mHtt IB formation (Fujimoto et al., 2005) or lower the concentration threshold at which HTT forms IB (Bersuker et al., 2013). These results raise the question of whether age-dependent penetrance of HD could be due to a reduced Hsf1 activity in aging tissues or a malignant hyperactivation of Hsf1. The latter scenario could be the result of an age-dependent increase in translational processivity errors, which could titrate the RQC complex eliciting a Tae2-dependent activation of Hsf1 (Figure 3h), possibly through Tae2-directed tagging of incomplete translation products with carboxyl-terminal Ala and Thr extensions. (Shen et al., 2015).

The exact mechanism behind Hsf1-dependent modulation of mHtt IB formation might be complex in that Hsf1 targets other genes than heat shock genes. It has been shown in worms that over-expression of hsf-1, with or without the C-terminal trans-activation domain, elevates the levels of pat-10, a troponin-like protein, that increase actin cytoskeleton integrity leading to lifespan extension and resistance to proteotoxic stress (Baird et al., 2014). Thus, it is possible that Hsf1 may regulate mHtt IB formation/toxicity in the yeast model system through the regulation of actin cytoskeleton dynamics since we found that mHtt103QP is associated with dense actin structures and that genes involved in actin dynamics are required to harness the latent toxicity of mHtt103QP. In addition, our data cannot rule out the possibility that the expression of mHtt in general raises proteostasis stress in the cell leading to Hsf1 activation and that such activation is epistatically affecting the effect of Ltn1-deficieny.

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Author details

1. Junsheng Yang

   Department of Chemistry and Molecular Biology, University of Gothenburg, Göteborg, Sweden

   Contribution

   JY, designed the study and experiments, performed old cell isolations, the HMC screen, IP, FACS and endocytosis assay, performed microscopy, toxicity assays and statistics, assisted to write the manuscript, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. Xinxin Hao

   Department of Chemistry and Molecular Biology, University of Gothenburg, Göteborg, Sweden

   Contribution

   XH, performed microscopy, toxicity assays and statistics, performed the SGA assay, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

3. Xiuling Cao

   Department of Chemistry and Molecular Biology, University of Gothenburg, Göteborg, Sweden

   Contribution

   XC, performed the His-Ub pull-down assay, Acquisition of data

   Competing interests

   The authors declare that no competing interests exist.

4. Beidong Liu

   Department of Chemistry and Molecular Biology, University of Gothenburg, Göteborg, Sweden

   Contribution

   BL, designed the study and experiments, designed the HMC screen and analyzed the HMC data, performed the SGA assay, performed 3D-SIM imaging, assisted to write the manuscript, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   beidong.liu@cmb.gu.se

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0001-6052-8411

5. Thomas Nyström

   Institute of Biomedicine, Sahlgrenska Academy, University of Gothenburg, Göteborg, Sweden

   Contribution

   TN, designed the study and experiments, wrote the manuscript, Conception and design, Drafting or revising the article

   For correspondence

   thomas.nystrom@cmb.gu.se

   Competing interests

   The authors declare that no competing interests exist.

   "This ORCID iD identifies the author of this article:" 0000-0001-5489-2903

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