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STUDIES OF FACTORS AFFECTING INTRACELLULAR TOXICITY OF THE SCA7 DISEASE PROTEIN ATAXIN - 7: FOCUS ON ATAXIN-7 DEGRADATION AND OXIDATIVE STRESS
Stockholm University, Faculty of Science, Department of Neurochemistry.ORCID iD: 0000-0001-9834-4554
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expansion of CAG/polyglutamine repeats. Proteins carrying expanded polyglutamine (polyQ) domains are suggested to be resistant to degradation and aggregate. Furthermore, a negative correlation between aggregation and toxicity has been shown. So far, little is known about the turn-over rate and degradation of the SCA7 disease protein ataxin-7 (ATXN7) and how this protein induces cellular toxicity. For the studies in this thesis work, we constructed stable inducible PC12 cell lines expressing GFP-tagged ATXN7 with 10 or 65 glutamines (Qs). Using these cell lines, we studied the turn-over of ATXN7 and the relationship between mutant ATXN7 and oxidative stress.

We showed that ATXN7 with a normal glutamine repeat (ATXN7Q10-GFP) has a short half-life and is mainly degraded by the UPS. In cells expressing expanded ATXN7 (ATXN7Q65-GFP), aggregation and reduced viability was observed. The aggregation increased the half-life of mutant ATXN7. For expanded full-length ATXN7, UPS was still the main degradation pathway; however autophagy also played a role in clearance of soluble ATXN7 fragments and possibly in aggregated ATXN7 material. Moreover, activation of autophagy reduced the level of aggregation and ameliorated the toxicity in cells expressing mutant ATXN7. From this study, we could get the conclusion that although expansion of the polyQ repeat increases the stability of expanded ATXN7, the protein can still be degraded via both UPS and autophagy. Furthermore, stimulation of autophagy could ameliorate the expanded ATXN7 toxicity and could therefore be a potential therapeutic approach for SCA7.

Regarding the role of oxidative stress we showed that expression of mutant ATXN7 leads to increased ROS levels and oxidative stress. Treatment with an antioxidant or blockage of NADPH oxidase complexes (NOX) decreased ATXN7 aggregation, the levels of ROS and ameliorated ATXN7 induced toxicity. Based on these results, we suggest that mutant ATXN7 cause increased ROS production from NOX and antioxidants treatment and or inhibition of NADPH-oxidase might potentially be used as a therapeutic strategy in SCA7.

Place, publisher, year, edition, pages
Stockholm: Department of Neurochemistry, Stockholm University , 2011. , 37 p.
Keyword [en]
Polyglutamine, SCA7, UPS, Autophagy
National Category
Biological Sciences Chemical Sciences
Research subject
Neurochemistry with Molecular Neurobiology
Identifiers
URN: urn:nbn:se:su:diva-64167ISBN: 978-91-7447-410-7 (print)OAI: oai:DiVA.org:su-64167DiVA: diva2:456058
Presentation
2011-12-09, Heilbronnsalen, Svante Arrhenius väg 21 A, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of defence the following paper was unpublished and had a status as follows: Paper 2: Manuscript

Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2015-03-16Bibliographically approved
List of papers
1. Expanded ataxin-7 cause toxicity by inducing ROS production from NADPH oxidase complexes in a stable inducible Spinocerebellar ataxia type 7 (SCA7) model
Open this publication in new window or tab >>Expanded ataxin-7 cause toxicity by inducing ROS production from NADPH oxidase complexes in a stable inducible Spinocerebellar ataxia type 7 (SCA7) model
Show others...
2012 (English)In: BMC neuroscience (Online), ISSN 1471-2202, E-ISSN 1471-2202, Vol. 13, 86Article in journal (Refereed) Published
Abstract [en]

Background: Spinocerebellar ataxia type 7 (SCA7) is one of nine inherited neurodegenerative disorders caused by polyglutamine (polyQ) expansions. Common mechanisms of disease pathogenesis suggested for polyQ disorders include aggregation of the polyQ protein and induction of oxidative stress. However, the exact mechanism(s) of toxicity is still unclear. Results: In this study we show that expression of polyQ expanded ATXN7 in a novel stable inducible cell model first results in a concomitant increase in ROS levels and aggregation of the disease protein and later cellular toxicity. The increase in ROS could be completely prevented by inhibition of NADPH oxidase (NOX) complexes suggesting that ATXN7 directly or indirectly causes oxidative stress by increasing superoxide anion production from these complexes. Moreover, we could observe that induction of mutant ATXN7 leads to a decrease in the levels of catalase, a key enzyme in detoxifying hydrogen peroxide produced from dismutation of superoxide anions. This could also contribute to the generation of oxidative stress. Most importantly, we found that treatment with a general anti-oxidant or inhibitors of NOX complexes reduced both the aggregation and toxicity of mutant ATXN7. In contrast, ATXN7 aggregation was aggravated by treatments promoting oxidative stress. Conclusion: Our results demonstrates that oxidative stress contributes to ATXN7 aggregation as well as toxicity and show that anti-oxidants or NOX inhibition can ameliorate mutant ATXN7 toxicity.

Keyword
Ataxin-7, NADPH oxidase complex, Neurodegeneration, Oxidative stress, Polyglutamine, SCA7
National Category
Chemical Sciences Biological Sciences
Research subject
Neurochemistry with Molecular Neurobiology
Identifiers
urn:nbn:se:su:diva-64162 (URN)10.1186/1471-2202-13-86 (DOI)000307239200001 ()
Funder
Swedish Research Council, K2010-68X-21449-01-3
Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08Bibliographically approved
2. Differential degradation of full-length and cleaved ataxin-7 fragments in a novel stable inducible SCA7 model
Open this publication in new window or tab >>Differential degradation of full-length and cleaved ataxin-7 fragments in a novel stable inducible SCA7 model
2012 (English)In: Journal of Molecular Neuroscience, ISSN 0895-8696, E-ISSN 1559-1166, Vol. 47, no 2, 219-233 p.Article in journal (Refereed) Published
Abstract [en]

Spinocerebellar ataxia type 7 (SCA7) is one of nine neurodegenerative disorders caused by expanded polyglutamine repeats, and a common toxic gain-of-function mechanism has been proposed. Proteolytic cleavage of several polyglutamine proteins has been identified and suggested to modulate the polyglutamine toxicity. In this study, we show that full-length and cleaved fragments of the SCA7 disease protein ataxin-7 (ATXN7) are differentially degraded. We found that the ubiquitin-proteosome system (UPS) was essential for the degradation of full-length endogenous ATXN7 or transgenic full-length ATXN7 with a normal or expanded glutamine repeat in both HEK 293T and stable PC12 cells. However, a similar contribution by UPS and autophagy was found for the degradation of proteolytically cleaved ATXN7 fragments. Furthermore, in our novel stable inducible PC12 model, induction of mutant ATXN7 expression resulted in toxicity and this toxicity was worsened by inhibition of either UPS or autophagy. In contrast, pharmacological activation of autophagy could ameliorate the ATXN7-induced toxicity. Based on our findings, we propose that both UPS and autophagy are important for the reduction of mutant ataxin-7-induced toxicity, and enhancing ATXN7 clearance through autophagy could be used as a potential therapeutic strategy in SCA7.

Keyword
Aggregation, Ataxin-7, Autophagy, Polyglutamine, Proteasome, SCA7
National Category
Biological Sciences Chemical Sciences
Research subject
Neurochemistry with Molecular Neurobiology
Identifiers
urn:nbn:se:su:diva-64160 (URN)10.1007/s12031-012-9722-8 (DOI)000304567700002 ()
Funder
Swedish Research Council, K2010-68X-21449-01-3
Note

AuthorCount: 4;

Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08Bibliographically approved

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