Length of polyglutamine tract affects secondary and tertiary structures of huntingtin protein

Stabilization of elongated polyglutamine tracts by a helical peptide derived from N-terminal huntingtin

In Huntington's disease, the length of the polyglutamine tract in the mutant protein correlates positively with the formation of aggregates and disease symptoms and severity of the disease. Some disease-modifying factors exist. However, no organized study has been carried out to investigate the effect of polyglutamine length in the mutant protein on the efficacy of a therapeutic strategy. We had shown earlier that the helical peptide arising out of the N-terminal stretch of normal huntingtin is able to inhibit aggregation of a number of proteins, including luciferase, α-synuclein, p53, and Rnq1. In this work, we show that polyglutamine stretches of differing lengths, namely 51Q, 72Q, and 103Q, form a mixture of aggregates at different rates, with the rate increasing in a polyQ length-dependent manner. The helical peptide is able to inhibit the rate of aggregation. The extent of inhibition was different when measuring either total aggregation or only fibrillar aggregates, suggesting that the helical peptide with benign polyQ stretch alters the aggregation landscape of different elongated polyQ lengths differently. Our results suggest that designing a therapeutic approach to inhibit protein aggregation must take note of polyQ length of the protein.

Selection pressure on human STR loci and its relevance in repeat expansion disease

Short Tandem Repeats (STRs) comprise repeats of one to several base pairs. Because of the high mutability due to strand slippage during DNA synthesis, rapid evolutionary change in the number of repeating units directly shapes the range of repeat-number variation according to selection pressure. However, the remaining questions include: Why are STRs causing repeat expansion diseases maintained in the human population; and why are these limited to neurodegenerative diseases? By evaluating the genome-wide selection pressure on STRs using the database we constructed, we identified two different patterns of relationship in repeat-number polymorphisms between DNA and amino-acid sequences, although both patterns are evolutionary consequences of avoiding the formation of harmful long STRs. First, a mixture of degenerate codons is represented in poly-proline (poly-P) repeats. Second, long poly-glutamine (poly-Q) repeats are favored at the protein level; however, at the DNA level, STRs encoding long poly-Qs are frequently divided by synonymous SNPs. Furthermore, significant enrichments of apoptosis and neurodevelopment were biological processes found specifically in genes encoding poly-Qs with repeat polymorphism. This suggests the existence of a specific molecular function for polymorphic and/or long poly-Q stretches. Given that the poly-Qs causing expansion diseases were longer than other poly-Qs, even in healthy subjects, our results indicate that the evolutionary benefits of long and/or polymorphic poly-Q stretches outweigh the risks of long CAG repeats predisposing to pathological hyper-expansions. Molecular pathways in neurodevelopment requiring long and polymorphic poly-Q stretches may provide a clue to understanding why poly-Q expansion diseases are limited to neurodegenerative diseases.

Inhibition of Aggregation of Mutant Huntingtin by Nucleic Acid Aptamers In Vitro and in a Yeast Model of Huntington's Disease

Elongated polyglutamine stretch in mutant huntingtin (mhtt) correlates well with the pathology of Huntington's disease (HD). Inhibition of aggregation of mhtt is a promising strategy to arrest disease progression. In this work, specific, high-affinity RNA aptamers were selected against monomeric mhtt (51Q-htt). Some of them inhibited its aggregation in vitro by stabilizing the monomer. They also recognized 103Q-htt but not 20Q-htt (nonpathogenic length). Inhibition of aggregation corresponded with reduced leakage of a fluorescent probe from liposomes and diminished oxidative stress in RBCs. The presence of aptamers was able to rescue the sequestration of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by aggregated mhtt. Some of the aptamers were able to enhance the partitioning of mhtt in the soluble fraction in a yeast model of HD. They were also able to rescue endocytotic defect due to aggregation of mhtt. The beneficial effect of a combination of aptamers was enhanced with improvement in cell survival. Since HD is a monogenic autosomal dominant disorder, aptamers may be developed as a viable strategy to slow down the progress of the disease. Since they are nonimmunogenic and nontoxic, aptamers may emerge as strong candidates to reduce protein-protein interaction and hence protein aggregation in protein misfolding disorders in general.

Aggregation of polyglutamine-expanded ataxin-3 sequesters its specific interacting partners into inclusions: implication in a loss-of-function pathology

Expansion of polyglutamine (polyQ) tract may cause protein misfolding and aggregation that lead to cytotoxicity and neurodegeneration, but the underlying mechanism remains to be elucidated. We applied ataxin-3 (Atx3), a polyQ tract-containing protein, as a model to study sequestration of normal cellular proteins. We found that the aggregates formed by polyQ-expanded Atx3 sequester its interacting partners, such as P97/VCP and ubiquitin conjugates, into the protein inclusions through specific interactions both in vitro and in cells. Moreover, this specific sequestration impairs the normal cellular function of P97 in down-regulating neddylation. However, expansion of polyQ tract in Atx3 does not alter the conformation of its surrounding regions and the interaction affinities with the interacting partners, although it indeed facilitates misfolding and aggregation of the Atx3 protein. Thus, we propose a loss-of-function pathology for polyQ diseases that sequestration of the cellular essential proteins via specific interactions into inclusions by the polyQ aggregates causes dysfunction of the corresponding proteins, and consequently leads to neurodegeneration.

Effect of osmolytes on the fibrillation of HypF-N

We have studied the effect of a series of stabilizing and destabilizing osmolytes on the fibrillation pattern of a model amyloidogenic protein, HypF-N. Under mildly denaturing conditions, HypF-N forms cross β-sheet structures, characteristic of amyloid fibrils. In the presence of all stabilizing osmolytes except proline, fibrillation of HypF-N is inhibited. Notably, fibrillation kinetics is retarded at subdenaturing concentrations of chaotropes. In case of proline, fibrillation of HypF-N is accelerated. Thus, the changes during exposure of a protein to denaturing conditions in the presence of osmolyes cannot be extrapolated from their role as anti-fibrillation agents.

Chaperone-like activity of high-mobility group box 1 protein and its role in reducing the formation of polyglutamine aggregates

High-mobility group box 1 protein (HMGB1), which mainly exists in the nucleus, has recently been shown to function as a sentinel molecule for viral nucleic acid sensing and an autophagy regulator in the cytoplasm. In this study, we studied the chaperone-like activity of HMGB1 and found that HMGB1 inhibited the chemically induced aggregation of insulin and lysozyme, as well as the heat-induced aggregation of citrate synthase. HMGB1 also restored the heat-induced suppression of cytoplasmic luciferase activity as a reporter protein in hamster lung fibroblast O23 cells with expression of HMGB1. Next, we demonstrated that HMGB1 inhibited the formation of aggregates and toxicity caused by expanded polyglutamine (polyQ), one of the main causes of Huntington disease. HMGB1 directly interacted with polyQ on immunofluorescence and coimmunoprecipitation assay, whereas the overexpression of HMGB1 or exogenous administration of recombinant HMGB1 protein remarkably reduced polyQ aggregates in SHSY5Y cells and hmgb1(-/-) mouse embryonic fibroblasts upon filter trap and immunofluorescence assay. Finally, overexpressed HMGB1 proteins in mouse embryonic primary striatal neurons also bound to polyQ and decreased the formation of polyQ aggregates. To this end, we have demonstrated that HMGB1 exhibits chaperone-like activity and a possible therapeutic candidate in polyQ disease.