Richet, E; (2012) The Modulation of Tau Aggregation in a Cell Model of Alzheimer’s Disease by the Proteasome Adaptor Protein NUB1. Doctoral thesis , UCL (University College London).

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Abstract

Neurofibrillary tangles (NFT) in Alzheimer’s disease (AD) are mainly composed of hyperphosphorylated and aggregated wild-type tau. NFTs are decorated by the ubiquitin-like modifier NEDD8, a protein targeted for proteasomal degradation by the NEDD8 Ultimate Buster 1 (NUB1). NUB1 has been shown to reduce synphilin-1 positive inclusions in a model of Parkinson’s disease. Therefore, this study examined the subcellular localisation of NUB1 as well as the effect of NUB1 on tau phosphorylation and aggregation. Furthermore, the effect of reducing NUB1 expression by RNA interference was investigated. Brain sections from AD patients showed that NUB1 and NEDD8 were expressed in the pyramidal neurons of the hippocampus, where the accumulation of NFTs is most abundant. In rat primary cortical neurons, NUB1 and tau co-localised in neurites and signalling structures such as varicosities, suggesting a functional interaction between them. The upregulation of the tau kinase GSK3β in AD leads to increased tau hyperphosphorylation and accumulation. In SK-N-SH neuroblastoma cells, which lack endogenous tau, ectopic wild-type tau formed inclusions when it was co-expressed with GSK3β, and this was enhanced by proteasome inhibition. NUB1 co-localised with both tau and GSK3β and significantly reduced tau inclusion formation. In neuroblastoma cells, NUB1 could interact with both tau and GSK3β, disrupt their interaction, and decrease the GSK3β-dependent phosphorylation of tau. NUB1 can directly bind synphilin-1 and induce its proteasomal degradation. Therefore, the ability of NUB1 to regulate GSK3β degradation was investigated in neuroblastoma cells. The upregulation of NUB1 accelerated the turnover of GSK3β, and the ubiquitin-associated (UBA) domains of NUB1 were necessary for NUB1 to exert its effect. Conversely, the downregulation of endogenous NUB1 by RNA interference increased the stability of endogenous GSK3β. Thus, NUB1 might have a role in tau inclusion formation by modulating GSK3β levels.

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