Differential Hyperphosphorylation of Tau-S199, -T231 and -S396 in Organotypic Brain Slices of Alzheimer Mice. A Model to Study Early Tau Hyperphosphorylation Using Okadaic Acid.

Bettina M Foidl,Christian Humpel

doi:10.3389/fnagi.2018.00113

Abstract

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder of the brain, characterized by extracellular aggregation of beta-amyloid (Aβ) and hyperphosphorylation of tau causing intraneuronal neurofibrillary tangles (NFTs). There is urgent need to study the interactions between Aβ and tau, especially to solve the question of the pathological cascade. In the present study, we aim to develop a model of organotypic brain slices in which both plaque and tau pathology can be examined. Organotypic brain slices (150 μm thick, coronal, at the hippocampal level) from adult (9 month) wildtype (WT, C57BL/6N) and transgenic AD mice (TG, APP_SweDI) were cultured for 2 weeks. To induce tau hyperphosphorylation 100 nM okadaic acid (OA), 10 μM wortmannin (WM) or both were added to the slices. Hyperphosphorylation of tau was tested at tau-S199, tau-T231 and tau-S396 using Western blot. Our data show that in TG mice with plaques a 50 kDa fragment of tau-S396 was hyperphosphorylated and that OA induced hyperphosphorylation of tau-S199. In WT mice (without plaques) OA caused hyperphosphorylation of a 50 kDa and a 38 kDa tau-T231 form and a 25 kDa sdftau-S396 fragment. The N-methyl-D-aspartate (NMDA) antagonist MK801 (1 μM) did not block these effects. Immunohistochemistry showed diffuse increased tau-S396 and tau-T231-like immunoreactivities at the hippocampal level but no formation of NFTs. Confocal microscopy indicated, that pTau-T231 was preferentially located in cytoplasma surrounding nuclei whereas pTau-S396 was found mainly in nerve fibers and strongly associated with plaques. In conclusion we provide a novel in vitro model to study both plaque and tau hyperphosphorylation but not NFTs, which could be useful to study pathological processes in AD and to screen for drugs.

Highlights

Alzheimer’s disease (AD) is the most common form of dementia worldwide but the underlying mechanisms leading to AD are still not fully understood
We show that okadaic acid (OA) induces hyperphosphorylation of p-tau S396, tau-T231 and tau-S199 in organotypic brain slices of adult WT and TG Alzheimer mice
There is some discrepancy on the role of tau: while it is well known that tau regulates the stability of microtubuli via phosphorylation and subsequent axonal transport (Hanger et al, 2009b; Ward et al, 2012; Luna-Munoz et al, 2013; Wang and Mandelkow, 2016), others suggest that the deletion or overexpression of tau did not affect the axonal transport rates both in vivo (Yuan et al, 2008, 2013) or in cultured sympathetic neurons (Tint et al, 1998)

Summary

Introduction

Alzheimer’s disease (AD) is the most common form of dementia worldwide but the underlying mechanisms leading to AD are still not fully understood. Two major hallmarks appear to be characteristic for AD: the accumulation of extracellular beta-amyloid (Aβ) plaques and the hyperphosphorylation of tau leading to the formation of intraneuronal neurofibrillary tangles (NFTs; Loeffler et al, 2015; Ahn et al, 2016). The hypothesis has limitations as the interplay between plaques, the hyperphosphorylation of tau with the formation of NFTs and the exact initiation of the cascade is still not fully understood (Haass and Mandelkow, 2010; Reitz, 2012). Studies about the formation of mutated tau show in contrast that e.g., in frontotemporal dementia formation of tangles is existent without the deposition of Aβ (Jin et al, 2011; Selkoe and Hardy, 2016)

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