N-Terminal Fragments of Huntingtin Longer than Residue 170 form Visible Aggregates Independently to Polyglutamine Expansion

Interaction between huntingtin exon 1 and HEAT repeat structure probed by chimeric model proteins

Huntington disease (HD) is associated with aggregation of huntingtin (HTT) protein containing over 35 continuous Q residues within the N-terminal exon 1 encoded region. The C-terminal of the HTT protein consists mainly of HEAT repeat structure which serves as a scaffold for multiple cellular activities. Structural and biochemical analysis of the intact HTT protein has been hampered by its huge size (~300 kDa) and most in vitro studies to date have focused on the properties of the exon 1 region. To explore the interaction between HTT exon 1 and the HEAT repeat structure, we constructed chimeric proteins containing the N-terminal HTT exon 1 region and the HEAT repeat protein PR65/A. The results indicate that HTT exon 1 slightly destabilizes the downstream HEAT repeat structure and endows the HEAT repeat structure with more conformational flexibility. Wild-type and pathological lengths of polyQ did not show differences in the interaction between HTT exon 1 and the HEAT repeats. With the C-terminal fusion of PR65/A, HTT exon 1 containing pathological lengths of polyQ could still form amyloid fibrils, but the higher-order architecture of fibrils and kinetics of fibril formation were affected by the C-terminal fusion of HEAT repeats. This indicates that interaction between HTT exon 1 and HEAT repeat structure is compatible with both normal function of HTT protein and the pathogenesis of HD, and this study provides a potential model for further exploration.

Mutant Huntingtin Protein Interaction Map Implicates Dysregulation of Multiple Cellular Pathways in Neurodegeneration of Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a genetic neurodegenerative disease caused by trinucleotide repeat (CAG) expansions in the human HTT gene encoding the huntingtin protein (Htt) with an expanded polyglutamine tract.

OBJECTIVE

HD models from yeast to transgenic mice have investigated proteins interacting with mutant Htt that may initiate molecular pathways of cell death. There is a paucity of datasets of published Htt protein interactions that include the criteria of 1) defining fragments or full-length Htt forms, 2) indicating the number of poly-glutamines of the mutant and wild-type Htt forms, and 3) evaluating native Htt interaction complexes. This research evaluated such interactor data to gain understanding of Htt dysregulation of cellular pathways.

METHODS

Htt interacting proteins were compiled from the literature that meet our criteria and were subjected to network analysis via clustering, gene ontology, and KEGG pathways using rigorous statistical methods.

RESULTS

The compiled data of Htt interactors found that both mutant and wild-type Htt interact with more than 2,971 proteins. Application of a community detection algorithm to all known Htt interactors identified significant signal transduction, membrane trafficking, chromatin, and mitochondrial clusters, among others. Binomial analyses of a subset of reported protein interactor information determined that chromatin organization, signal transduction and endocytosis were diminished, while mitochondria, translation and membrane trafficking had enriched overall edge effects.

CONCLUSION

The data support the hypothesis that mutant Htt disrupts multiple cellular processes causing toxicity. This dataset is an open resource to aid researchers in formulating hypotheses of HD mechanisms of pathogenesis.

A New Chemoenzymatic Semisynthetic Approach Provides Insight into the Role of Phosphorylation beyond Exon1 of Huntingtin and Reveals N-Terminal Fragment Length-Dependent Distinct Mechanisms of Aggregation

Huntington's disease is a neurodegenerative disorder caused by the expansion of a polyglutamine repeat (>36Q) in the N-terminal domain of the huntingtin protein (Htt), which renders the protein or fragments thereof more prone to aggregate and form inclusions. Although several Htt N-terminal fragments of different lengths have been identified within Htt inclusions, most studies on the mechanisms, sequence, and structural determinants of Htt aggregation have focused on the Httexon1 (Httex1). Herein, we investigated the aggregation properties of mutant N-terminal Htt fragments of various lengths (Htt171, Htt140, and Htt104) in comparison to mutant Httex1 (mHttex1). We also present a new chemoenzymatic semisynthetic strategy that enables site-specific phosphorylation of Htt beyond Httex1. These advances yielded insights into how post-translational modifications (PTMs) and structured domains beyond Httex1 influence aggregation mechanisms, kinetics, and fibril morphology of longer N-terminal Htt fragments. We demonstrate that phosphorylation at T107 significantly slows the aggregation of mHtt171, whereas phosphorylation at T107 and S116 accelerates the aggregation, underscoring the importance of crosstalk between different PTMs. The mHtt171 proteins aggregate via a different mechanism and form oligomers and fibrillar aggregates with morphological properties that are distinct from that of mHttex1. These observations suggest that different N-terminal fragments could have distinct aggregation mechanisms and that a single polyQ-targeting antiaggregation strategy may not effectively inhibit the aggregation of all N-terminal Htt fragments. Finally, our results underscore the need for further studies to investigate the aggregation mechanisms of Htt fragments and how the various fragments interact with each other and influence Htt toxicity and disease progression.

Huntingtin protein: A new option for fixing the Huntington's disease countdown clock

Huntington's disease is a dreadful, incurable disorder. It springs from the autosomal dominant mutation in the first exon of the HTT gene, which encodes for the huntingtin protein (HTT) and results in progressive neurodegeneration. Thus far, all the attempted approaches to tackle the mutant HTT-induced toxicity causing this disease have failed. The mutant protein comes with the aberrantly expanded poly-glutamine tract. It is primarily to blame for the build-up of β-amyloid-like HTT aggregates, deleterious once broadened beyond the critical ∼35-37 repeats threshold. Recent experimental findings have provided valuable information on the molecular basis underlying this HTT-driven neurodegeneration. These findings indicate that the poly-glutamine siding regions and many post-translation modifications either abet or counter the poly-glutamine tract. This review provides an overall, up-to-date insight into HTT biophysics and structural biology, particularly discussing novel pharmacological options to specifically target the mutated protein and thus inhibit its functions and toxicity.