Hyperphosphorylated tau proteins accumulate in the paired helical filaments of neurofibrillary tangles seen in such tauopathies as Alzheimer's disease. In the present paper we show that tau turnover is dependent on degradation by the proteasome (inhibited by MG132) in HT22 neuronal cells. Recombinant human tau was rapidly degraded by the 20 S proteasome in vitro, but tau phosphorylation by GSK3β (glycogen synthase kinase 3β) significantly inhibited proteolysis. Tau phosphorylation was increased in HT22 cells by OA [okadaic acid; which inhibits PP (protein phosphatase) 1 and PP2A] or CsA [cyclosporin A; which inhibits PP2B (calcineurin)], and in PC12 cells by induction of a tet-off dependent RCAN1 transgene (which also inhibits PP2B). Inhibition of PP1/PP2A by OA was the most effective of these treatments, and tau hyperphosphorylation induced by OA almost completely blocked tau degradation in HT22 cells (and in cell lysates to which purified proteasome was added) even though proteasome activity actually increased. Many tauopathies involve both tau hyperphosphorylation and the oxidative stress of chronic inflammation. We tested the effects of both cellular oxidative stress, and direct tau oxidative modification in vitro, on tau proteolysis. In HT22 cells, oxidative stress alone caused no increase in tau phosphorylation, but did subtly change the pattern of tau phosphorylation. Tau was actually less susceptible to direct oxidative modification than most cell proteins, and oxidized tau was degraded no better than untreated tau. The combination of oxidative stress plus OA treatment caused extensive tau phosphorylation and significant inhibition of tau degradation. HT22 cells transfected with tau–CFP (cyan fluorescent protein)/tau–GFP (green fluorescent protein) constructs exhibited significant toxicity following tau hyperphosphorylation and oxidative stress, with loss of fibrillar tau structure throughout the cytoplasm. We suggest that the combination of tau phosphorylation and tau oxidation, which also occurs in tauopathies, may be directly responsible for the accumulation of tau aggregates.
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Research Article|
November 28 2006
Phosphorylation inhibits turnover of the tau protein by the proteasome: influence of RCAN1 and oxidative stress
Diana Poppek;
Diana Poppek
1
*Research Institute of Environmental Medicine, Heinrich Heine University, Düsseldorf, Germany
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Susi Keck;
Susi Keck
1
†Neuroscience Research Center, Humboldt University, Berlin, Germany
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Gennady Ermak;
Gennady Ermak
‡Ethel Percy Andrus Gerontology Center, and Division of Molecular and Computational Biology, The University of Southern California, Los Angeles, CA 90089-0191, U.S.A.
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Tobias Jung;
Tobias Jung
*Research Institute of Environmental Medicine, Heinrich Heine University, Düsseldorf, Germany
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Alexandra Stolzing;
Alexandra Stolzing
†Neuroscience Research Center, Humboldt University, Berlin, Germany
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Oliver Ullrich;
Oliver Ullrich
§Institute of Immunology, University of Magdeburg, Germany
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Kelvin J. A. Davies;
Kelvin J. A. Davies
‡Ethel Percy Andrus Gerontology Center, and Division of Molecular and Computational Biology, The University of Southern California, Los Angeles, CA 90089-0191, U.S.A.
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Tilman Grune
Tilman Grune
2
*Research Institute of Environmental Medicine, Heinrich Heine University, Düsseldorf, Germany
†Neuroscience Research Center, Humboldt University, Berlin, Germany
2To whom correspondence should be addressed (email grune@uni.hohenheim.de).
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Publisher: Portland Press Ltd
Received:
March 29 2006
Revision Received:
August 24 2006
Accepted:
August 29 2006
Accepted Manuscript online:
August 29 2006
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London
2006
Biochem J (2006) 400 (3): 511–520.
Article history
Received:
March 29 2006
Revision Received:
August 24 2006
Accepted:
August 29 2006
Accepted Manuscript online:
August 29 2006
Citation
Diana Poppek, Susi Keck, Gennady Ermak, Tobias Jung, Alexandra Stolzing, Oliver Ullrich, Kelvin J. A. Davies, Tilman Grune; Phosphorylation inhibits turnover of the tau protein by the proteasome: influence of RCAN1 and oxidative stress. Biochem J 15 December 2006; 400 (3): 511–520. doi: https://doi.org/10.1042/BJ20060463
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