Amyloid β-protein oligomers promote the uptake of tau fibril seeds potentiating intracellular tau aggregation

Shin et al. Alzheimer's Research & Therapy https://doi.org/10.1186/s13195-019-0541-9 (2019) 11:86 RESEARCH Open Access Amyloid β-protein oligomers promote the uptake of tau fibril seeds potentiating intracellular tau aggregation Woo Shik Shin1, Jing Di1, Qin Cao2, Binsen Li1, Paul M. Seidler2, Kevin A. Murray2, Gal Bitan1,3 and Lin Jiang1,3* Abstract Background: Repeated failure of drug candidates targeting Alzheimer’s disease (AD) in clinical trials likely stems from a lack of understanding of the molecular mechanisms underlying AD pathogenesis. Recent research has highlighted synergistic interactions between aggregated amyloid-β (Aβ) and tau proteins in AD, but the molecular details of how these interactions drive AD pathology remain elusive and speculative. Methods: Here, we test the hypothesis that Aβ potentiates intracellular tau aggregation, and show that oligomeric Aβ specifically exacerbates proteopathic seeding by tau. Using tau-biosensor cells, we show that treatment with sub-toxic concentrations of Aβ oligomers, but not monomers or fibrils, “primes” cells, making them more susceptible to tau seeding. The treatment with Aβ oligomers enhances intracellular tau aggregation in a dosedependent manner when the cells are seeded with either recombinant or brain-derived tau fibrils, whereas little or no aggregation is observed in the absence of Aβ-oligomer priming. Results: Priming by Aβ oligomers appears to be specific to tau, as α-synuclein seeding is unaffected by this treatment. Aβ oligomer-enhanced tau seeding also occurs in primary mouse neurons and human neuroblastoma cells. Using fluorescently labeled tau seeds, we find that treatment with Aβ oligomers significantly enhances the cellular uptake of tau seeds, whereas a known tau-uptake inhibitor blocks the effect of Aβ on tau uptake. Conclusion: The ability of Aβ to promote tau seeding suggests a specific and plausible mechanism by which extracellular Aβ initiates a deleterious cascade that is unique to AD. These data suggest that the Aβ-mediated potentiation of tau uptake into cells should also be taken into account when designing Aβ-targeted therapeutics. Keywords: Amyloid beta, Tau, Biosensor cell, Oligomer, Alzheimer’s disease Background Amyloid plaques and neurofibrillary tangles (NFTs), comprising Aβ and hyperphosphorylated tau, respectively, are the two major hallmarks of Alzheimer’s disease (AD) pathology [1–3]. The relationships among Aβ, tau, and neurodegeneration in AD are not fully understood [4, 5]. Although the amyloid cascade hypothesis has posited that Aβ aggregation is the initiating pathologic event in AD [6, 7], biomarker and pathology studies have shown a strong correlation between NFT accumulation, neurodegeneration, and clinical * Correspondence: 1 Department of Neurology, David Geffen School of Medicine, UCLA, 635 Charles E Young Drive South, Los Angeles, CA 90095, USA 3 Brain Research Institute, and Molecular Biology Institute, UCLA, Los Angeles, CA 90095, USA Full list of author information is available at the end of the article decline, whereas plaque pathology correlated poorly with AD progression [8–12]. Moreover, despite promising results in pre-clinical models, therapies that reduce plaque load have not yielded significant benefits in clinical trials for AD. The limited understanding of the link between Aβ accumulation and tau deposition in AD is a key piece that is missing from our knowledge of the disease mechanism, and may factor into failures of existing Aβ therapies in clinical trials. The interplay between Aβ and tau is exemplified in animal models and biomarker studies of AD patients [13–16]. Studies in animal models show synergistic enhancement of tau accumulation in the presence of Aβ in the cortex of young mice overexpressing the frontotemporal-dementiaassociated variant P301L-tau. In wild-type mice, Aβ plaques enhance tau seeding and pathology [17, 18]. Biomarker © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Shin et al. Alzheimer's Research & Therapy (2019) 11:86 Page 2 of 13 studies in patients concluded that the progression of AD dementia is driven by the synergistic interaction between Aβ and tau [19–22]. This body of work provides the basis for our hypothesis that Aβ and tau synergize to create a defining pathology in AD, the details of which are the focus of our current study. Aβ and tau aggregate to form small soluble oligomers, and large insoluble fibrils that are seen in AD and related diseases [14, 23]. Several studies have shown that oligomeric and fibrillar species contribute differently to disease progression. For example, soluble Aβ oligomers are thought to be a major toxic agent in AD [24]. Tau oligomers and fibrils have been proposed to template the conversion of monomers into aggregates in recipient cells, leading to pathological spread in the brain [25–28]. Cell culture assays are useful tools for dissecting the contribution of specific assemblies in each pathological process. Experiments using primary neurons or neuronal cell lines have shown that application of Aβ oligomers increased tau phosphorylation, demonstrating a link between Aβ toxicity and tau pathology [29]. However, it is unclear how a direct interaction might occur between Aβ and tau in pathological tau seeding. Here, we sought to explore how different Aβ assemblies might contribute to the process of tau seeding. We investigated the relationship between different assemblies of Aβ—freshly prepared, oligomeric, and fibrillar— and tau seeding using several cell-culture models, including FRET-based tau biosensor cells [30, 31], human neuroblastoma cells, and primary hippocampal neurons from transgenic mice expressing human P301S tau. We also explored the mechanism by which Aβ oligomers affect cellular uptake of tau seeds. Our findings connect toxic Aβ oligomers to tau seeding, a currently missing link in our understanding of AD pathology. Examining the preparation by electron microscopy (EM) showed occasional spherical structures, but mostly the species in freshly prepared Aβ were too small to be observed by EM (Additional file 1: Figure S1c). The freshly prepared Aβ was then quiescently incubated at 37 °C at 10 μM in PBS to generate different self-assembly states. We monitored the self-assembly of Aβ by thioflavin T (ThT) fluorescence (Fig. 1c). After incubation for 18 h, native PAGE showed a smeary band consistent with mole (...truncated)