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Amyloid-β Secretion, Generation, and Lysosomal Sequestration in Response to Proteasome Inhibition: Involvement of Autophagy
Linköping University, Department of Clinical and Experimental Medicine, Geriatric. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Experimental Pathology. Linköping University, Faculty of Health Sciences. Östergötlands Läns Landsting, Centre for Diagnostics, Department of Clinical Pathology and Clinical Genetics.
Department of Neurobiology, Division of Neurodegeneration, Care Sciences and Society, Karolinska Institute, Stockholm, Sweden.
Linköping University, Department of Clinical and Experimental Medicine, Geriatric. Linköping University, Faculty of Health Sciences. Östergötlands Läns Landsting, Local Health Care Services in Central Östergötland, Department of Geriatric Medicine.
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2012 (English)In: Journal of Alzheimer's Disease, ISSN 1387-2877, Vol. 31, no 2, 343-358 p.Article in journal (Refereed) Published
Abstract [en]

The proteasome is important for degradation of worn out and misfolded proteins. Decreased proteasome activity has been implicated in Alzheimer's disease (AD). Proteasome inhibition induces autophagy, but it is still unknown whether autophagy is beneficial or deleterious to AD neurons, as the autophagosome has been suggested as a site of amyloid-β (Aβ) generation. In this study, we investigated the effect of proteasome inhibition on Aβ accumulation and secretion, as well as the processing of amyloid-β protein precursor (AβPP) in AβPPSwe transfected SH-SY5Y neuroblastoma cells. We show that proteasome inhibition resulted in autophagy-dependent accumulation of Aβ in lysosomes, and increased levels of intracellular and secreted Aβ. The enhanced levels of Aβ could not be explained by increased amounts of AβPP. Instead, reduced degradation of the C-terminal fragment of AβPP (C99) by the proteasome makes C99 available for γ-secretase cleavage, leading to Aβ generation. Inhibition of autophagy after proteasome inhibition led to reduced levels of intracellular, but not secreted Aβ, and tended to further increase the C99 to AβPP ratio, supporting involvement of the autophagosome in Aβ generation. Furthermore, proteasome inhibition caused a reduction in cellular viability, which was reverted by inhibition of autophagy. Dysfunction of the proteasome could cause lysosomal accumulation of Aβ, as well as increased generation and secretion of Aβ, which is partly facilitated by autophagy. As a decrease in cellular viability was also detected, it is possible that upregulation of autophagy is an unsuccessful rescue mechanism, which instead of being protective, contributes to AD pathogenesis.

Place, publisher, year, edition, pages
I O S Press , 2012. Vol. 31, no 2, 343-358 p.
Keyword [en]
AβPP processing, Alzheimer’s disease, amyloid- peptide, autophagy, cell death, LC-3, lysosome, p70S6K, proteasome
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-81340DOI: 10.3233/JAD-2012-120001ISI: 000307377300011PubMedID: 22555375OAI: oai:DiVA.org:liu-81340DiVA: diva2:551725
Note

funding agencies|foundations of Engqvist, Wiberg, Hedlund, Osterman, and Stohne||Gustav V and Queen Victorias Foundation||Swedish Alzheimers foundation||Ostergotland County Council||Swedish Research Council||

Available from: 2012-09-12 Created: 2012-09-12 Last updated: 2012-09-26Bibliographically approved
In thesis
1. The involvement of degradation pathways and neuron-to-neuron transmission in Alzheimer’s disease
Open this publication in new window or tab >>The involvement of degradation pathways and neuron-to-neuron transmission in Alzheimer’s disease
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Although the vast majority of Alzheimer’s disease (AD) cases are of the sporadic type, mutations causing the familial form have been the focus of AD research for decades. The disease is pathologically characterised by β-amyloid (Aβ) and tau protein aggregates in neuritic plaques and neurofibrillary tangles. Furthermore, it is known that AD pathology spreads throughout the brain, most often along the same anatomical pattern. However, so far no cause for the sporadic form of the disease has been found. Accumulation of protein aggregates as well as decreased activity of the protein degradation systems, lysosomes and proteasomes, is found in diseased brains. This indicates that defective degradation contributes to sporadic AD.

The aim of this thesis was to develop an improved neuronal model, and study the effects of decreased proteasome function on tau phosphorylation and axonal transport. In addition, the effects on Aβ accumulation and generation upon proteasome inhibition were investigated. Finally, the possibility that intracellularly accumulated Aβ oligomers could be transferred from one neuron to another was tested.

Differentiation of human SH-SY5Y neuroblastoma cells in an extracellular matrix gel, using a set of neurotrophic factors, resulted in cells with neuronal phenotype, expressing neuron specific markers and all six adult isoforms of tau. Within this neuronal model, we found that reduced proteasome activity inhibited neuritic transport, and caused tau phosphorylation in a c-Jun and ERK 1/2 dependent manner. Using proteasome inhibition in APP overexpressing cells, we found an autophagy dependent intralysosomal Aβ accumulation, together with elevation of intra- and extracellular concentrations of Aβ. Autophagy inhibition protected the cells from the toxicity induced by decreased proteasome activity. Finally, we could, as the first group, show that Aβ can be directly transferred from one neuron to another through connected neurites. Furthermore, accumulation of Aβ in the endo-lysosomal compartment of receiving cells caused toxicity and neurodegeneration.

We believe that cells not able to degrade accumulated Aβ, due to increased generation or reduced degradative capacity, instead tries to clear its content through transfer to connected neurons. If not properly degraded in the receiving cell, this can accelerate AD pathology and cause neuritic and neuronal degeneration spreading throughout the brain. Increasing the activity of the degradative systems, or inhibiting transmission of Aβ between neurons could therefore be novel treatments for AD.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 69 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1317
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-81341 (URN)978-91-7519-848-4 (ISBN)
Public defence
2012-10-05, Nils-Holger salen, Hälsouniversitetet, Campus US, Linköpings universitet, Linköping, 13:00 (Swedish)
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Available from: 2012-09-12 Created: 2012-09-12 Last updated: 2012-09-12Bibliographically approved

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