Phagocytosis of full-length Tau oligomers by Actin-remodeling of activated microglia

Rashmi Das,Subashchandrabose Chinnathambi,Abhishek Ankur Balmik

doi:10.1186/s12974-019-1694-y

Rashmi Das, Subashchandrabose Chinnathambi + Show 1 more

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https://doi.org/10.1186/s12974-019-1694-y

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Abstract

BackgroundAlzheimer’s disease is associated with the accumulation of intracellular Tau tangles within neurons and extracellular amyloid-β plaques in the brain parenchyma, which altogether results in synaptic loss and neurodegeneration. Extracellular concentrations of oligomers and aggregated proteins initiate microglial activation and convert their state of synaptic surveillance into a destructive inflammatory state. Although Tau oligomers have fleeting nature, they were shown to mediate neurotoxicity and microglial pro-inflammation. Due to the instability of oligomers, in vitro experiments become challenging, and hence, the stability of the full-length Tau oligomers is a major concern.MethodsIn this study, we have prepared and stabilized hTau40WT oligomers, which were purified by size-exclusion chromatography. The formation of the oligomers was confirmed by western blot, thioflavin-S, 8-anilinonaphthaalene-1-sulfonic acid fluorescence, and circular dichroism spectroscopy, which determine the intermolecular cross-β sheet structure and hydrophobicity. The efficiency of N9 microglial cells to phagocytose hTau40WT oligomer and subsequent microglial activation was studied by immunofluorescence microscopy with apotome. The one-way ANOVA was performed for the statistical analysis of fluorometric assay and microscopic analysis.ResultsFull-length Tau oligomers were detected in heterogeneous globular structures ranging from 5 to 50 nm as observed by high-resolution transmission electron microscopy, which was further characterized by oligomer-specific A11 antibody. Immunocytochemistry studies for oligomer treatment were evidenced with A11+ Iba1high microglia, suggesting that the phagocytosis of extracellular Tau oligomers leads to microglial activation. Also, the microglia were observed with remodeled filopodia-like actin structures upon the exposure of oligomers and aggregated Tau.ConclusionThe peri-membrane polymerization of actin filament and co-localization of Iba1 relate to the microglial movements for phagocytosis. Here, these findings suggest that microglia modified actin cytoskeleton for phagocytosis and rapid clearance of Tau oligomers in Alzheimer’s disease condition.

Highlights

Alzheimer’s disease is associated with the accumulation of intracellular Tau tangles within neurons and extracellular amyloid-β plaques in the brain parenchyma, which altogether results in synaptic loss and neurodegeneration
Tau oligomers have found to be increased in age-dependent Alzheimer’s disease (AD) brain, which resulted in NFT formation and subsequent neurodegeneration [13, 48]
The Tau oligomers which are isolated from AD brain have induced cytotoxicity in neuroblastoma cell line at a lower concentration, which has been reversed by oligomer-specific antibody TOMA [50]

Summary

Introduction

Alzheimer’s disease is associated with the accumulation of intracellular Tau tangles within neurons and extracellular amyloid-β plaques in the brain parenchyma, which altogether results in synaptic loss and neurodegeneration. Tauopathies are characterized by abnormal accumulation of Tau protein in various locations of the brain, leading to progressive neuronal loss, inflammation, and dementia [2]. Other groups have shown that the propagated Tau oligomers lead to reduced long-term potentiation and increased short-term depression effect on cortical neurons, which has been partially blocked by the administration of oligomer-specific antibody [13, 14]. It was shown that the exposure of extracellular oligomers can induce the aggregation of intracellular Tau in HEK293T in vitro cell model [18]. The most effectively endocytosed Tau species were globular in structure and having a high molecular weight (HMW) of > 670 kDa, as observed by size-exclusion chromatography and immunoreactive to oligomer-specific antibody [19]. Engulfed oligomers get degraded by cellular proteostasis machinery [20] while endocytosed Tau oligomers were found to be located more in lysosomal compartment than Golgi bodies [21]

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