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Global changes to the ubiquitin system in Huntington's disease

Abstract

Huntington’s disease (HD) is a dominantly inherited neurodegenerative disorder caused by expansion of CAG triplet repeats in the huntingtin (HTT) gene (also called HD) and characterized by accumulation of aggregated fragments of polyglutamine-expanded HTT protein in affected neurons1,2. Abnormal enrichment of HD inclusion bodies with ubiquitin, a diagnostic characteristic of HD and many other neurodegenerative disorders including Alzheimer’s and Parkinson’s diseases3,4, has suggested that dysfunction in ubiquitin metabolism may contribute to the pathogenesis of these diseases5,6. Because modification of proteins with polyubiquitin chains regulates many essential cellular processes including protein degradation, cell cycle, transcription, DNA repair and membrane trafficking7, disrupted ubiquitin signalling is likely to have broad consequences for neuronal function and survival. Although ubiquitin-dependent protein degradation is impaired in cell-culture models of HD8,9,10,11 and of other neurodegenerative diseases12,13, it has not been possible to evaluate the function of the ubiquitin–proteasome system (UPS) in HD patients or in animal models of the disease, and a functional role for UPS impairment in neurodegenerative disease pathogenesis remains controversial14,15,16. Here we exploit a mass-spectrometry-based method to quantify polyubiquitin chains17 and demonstrate that the abundance of these chains is a faithful endogenous biomarker of UPS function. Lys 48-linked polyubiquitin chains accumulate early in pathogenesis in brains from the R6/2 transgenic mouse model of HD, from a knock-in model of HD and from human HD patients, establishing that UPS dysfunction is a consistent feature of HD pathology. Lys 63- and Lys 11-linked polyubiquitin chains, which are not typically associated with proteasomal targeting, also accumulate in the R6/2 mouse brain. Thus, HD is linked to global changes in the ubiquitin system to a much greater extent than previously recognized.

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Figure 1: UbK48 ion intensity correlates with a loss of UPS function.
Figure 2: HTTQ150 aggregation in an inducible cell model of HD correlates with elevated UbK48 levels.
Figure 3: UbK48 isopeptide ion intensity is elevated in mouse models of HD and in the human HD brain.
Figure 4: Elevated levels of polyubiquitin chain linkages not typically associated with proteasomal targeting in cellular and mouse models of HD.

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Acknowledgements

We are grateful to P. Howley, R. Baker and N. Nukina for reagents. We thank D. Kirkpatrick and N. Hathaway for suggestions, and J.-P. Vonsattel and the New York Brain Bank for the human brain tissue. This work was supported by a predoctoral training grant from NIGMS (E.J.B.), a small business innovation research grant from NINDS (H.S.), grants from the Huntington’s Disease Society of America Coalition for the Cure, Hereditary Disease Foundation and High Q Foundation (G.P.B. and R.R.K.), and a grant from the Wellcome Trust (G.P.B.).

Author Contributions E.J.B., T.A.S., C.H.B., H.S. and R.R.K. devised the overall proteomic approach. E.J.B. performed all of the biochemical analyses, the pull-down assays and, together with T.A.S., obtained and analysed all the mass spectrometry data. All of the mouse breeding and dissection was performed by B.W. and G.P.B. T.S.Z. performed all experiments in Supplementary Fig. 2 and the analysis of the HdhQ150/Q150 knock-in mice. K.-Y.R. contributed the real-time RT–PCR data in Supplementary Fig. 3 and performed the ubiquitin ELISA on R6/2 and control mice. E.J.B. and R.R.K. wrote the manuscript. All authors discussed the results and contributed to the manuscript.

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Correspondence to Ron R. Kopito.

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H.S., T.A.S. and C.H.B. are employees of PPD Biomarker Discovery Inc.

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This file contains Supplementary Discussion, Supplementary Figures S1-S7 with Legends and Supplementary Table S1. (PDF 377 kb)

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Bennett, E., Shaler, T., Woodman, B. et al. Global changes to the ubiquitin system in Huntington's disease. Nature 448, 704–708 (2007). https://doi.org/10.1038/nature06022

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