Category: Huntington Disease proteostasis group

Abstract

Neurodegenerative disorders such as Huntington’s disease are hallmarked by neuronal intracellular inclusion body formation. Whether proteasomes are irreversibly recruited into inclusion bodies in these protein misfolding disorders is a controversial subject. In addition, it has been proposed that the proteasomes may become clogged by the aggregated protein fragments, leading to impairment of the ubiquitin-proteasome system. Here, we show by fluorescence pulse-chase experiments in living cells that proteasomes are dynamically and reversibly recruited into inclusion bodies. As these recruited proteasomes remain catalytically active and accessible to substrates, our results challenge the concept of proteasome sequestration and impairment in Huntington’s disease, and support the reported absence of proteasome impairment in mouse models of Huntington’s disease.

Keywords: ABP; Aggregate; C4; FRAP; HD; Huntington; IB; Polyglutamine; Proteasome; UPS; Ub; Ubiquitin; activity-based probe; fluorescence recovery after photobleaching; huntington’s disease; inclusion body; mHtt; mutant huntingtin; polyQ; polyglutamine; tetracysteine; ubiquitin; ubiquitin–proteasome system.

Copyright © 2013 Federation of European Biochemical Societies. All rights reserved.

Authors

Sabine Schipper-Krom, Katrin Juenemann, Anne H Jansen, Anne Wiemhoefer, Rianne van den Nieuwendijk, Donna L Smith, Mark A Hink, Gillian P Bates, Hermen Overkleeft, Huib Ovaa, Eric Reits

Link

https://doi.org/10.1016/j.febslet.2013.11.023

Abstract

Huntington’s disease (HD) is an autosomal dominant inherited neurodegenerative disorder that is caused by a CAG expansion in the Huntingtin (HTT) gene, leading to HTT inclusion formation in the brain. The mutant huntingtin protein (mHTT) is ubiquitously expressed and therefore nuclear inclusions could be present in all brain cells. The effects of nuclear inclusion formation have been mainly studied in neurons, while the effect on glia has been comparatively disregarded. Astrocytes, microglia, and oligodendrocytes are glial cells that are essential for normal brain function and are implicated in several neurological diseases. Here we examined the number of nuclear mHTT inclusions in both neurons and various types of glia in the two brain areas that are the most affected in HD, frontal cortex, and striatum. We compared nuclear mHTT inclusion body formation in three HD mouse models that express either full-length HTT or an N-terminal exon1 fragment of mHTT, and we observed nuclear inclusions in neurons, astrocytes, oligodendrocytes, and microglia. When studying the frequency of cells with nuclear inclusions in mice, we found that half of the population of neurons contained nuclear inclusions at the disease end stage, whereas the proportion of GFAP-positive astrocytes and oligodendrocytes having a nuclear inclusion was much lower, while microglia hardly showed any nuclear inclusions. Nuclear inclusions were also present in neurons and all studied glial cell types in human patient material. This is the first report to compare nuclear mHTT inclusions in glia and neurons in different HD mouse models and HD patient brains. GLIA 2016;65:50–61

Authors

Anne H P Jansen, Maurik van Hal, Ilse C Op den Kelder, Romy T Meier, Anna-Aster de Ruiter, Menno H Schut, Donna L Smith, Corien Grit, Nieske Brouwer, Willem Kamphuis, H W G M Boddeke, Wilfred F A den Dunnen, Willeke M C van Roon, Gillian P Bates, Elly M Hol, Eric A Reits

Link

https://doi.org/10.1002/glia.23050

Abstract

Huntington’s disease is caused by a polyglutamine repeat expansion in the huntingtin protein which affects the function and folding of the protein, and results in intracellular protein aggregates. Here, we examined whether this mutation leads to altered ubiquitination of huntingtin and other proteins in both soluble and insoluble fractions of brain lysates of the Q175 knock-in Huntington’s disease mouse model and the Q20 wild-type mouse model. Ubiquitination sites are detected by identification of Gly-Gly (diGly) remnant motifs that remain on modified lysine residues after digestion. We identified K6, K9, K132, K804, and K837 as endogenous ubiquitination sites of soluble huntingtin, with wild-type huntingtin being mainly ubiquitinated at K132, K804, and K837. Mutant huntingtin protein levels were strongly reduced in the soluble fraction whereas K6 and K9 were mainly ubiquitinated. In the insoluble fraction increased levels of huntingtin K6 and K9 diGly sites were observed for mutant huntingtin as compared with wild type. Besides huntingtin, proteins with various roles, including membrane organization, transport, mRNA processing, gene transcription, translation, catabolic processes and oxidative phosphorylation, were differently expressed or ubiquitinated in wild-type and mutant huntingtin brain tissues. Correlating protein and diGly site fold changes in the soluble fraction revealed that diGly site abundances of most of the proteins were not related to protein fold changes, indicating that these proteins were differentially ubiquitinated in the Q175 mice. In contrast, both the fold change of the protein level and diGly site level were increased for several proteins in the insoluble fraction, including ubiquitin, ubiquilin-2, sequestosome-1/p62 and myo5a. Our data sheds light on putative novel proteins involved in different cellular processes as well as their ubiquitination status in Huntington’s disease, which forms the basis for further mechanistic studies to understand the role of differential ubiquitination of huntingtin and ubiquitin-regulated processes in Huntington’s disease.

Authors

Karen A Sap, Arzu Tugce Guler, Karel Bezstarosti, Aleksandra E Bury, Katrin Juenemann, JeroenA A Demmers, Eric A Reits

Link

https://doi.org/10.1074/mcp.RA119.001486