Defined Tau Phosphospecies Differentially Inhibit Fast Axonal Transport Through Activation of Two Independent Signaling Pathways.

Svenja König,Gerardo Morfini,Karin Bechberger,Ming-Ying Tsai,Sarah L Morris,Sarah Aloe,Scott T Brady

doi:10.3389/fnmol.2020.610037

Abstract

Tau protein is subject to phosphorylation by multiple kinases at more than 80 different sites. Some of these sites are associated with tau pathology and neurodegeneration, but other sites are modified in normal tau as well as in pathological tau. Although phosphorylation of tau at residues in the microtubule-binding repeats is thought to reduce tau association with microtubules, the functional consequences of other sites are poorly understood. The AT8 antibody recognizes a complex phosphoepitope site on tau that is detectable in a healthy brain but significantly increased in Alzheimer’s disease (AD) and other tauopathies. Previous studies showed that phosphorylation of tau at the AT8 site leads to exposure of an N-terminal sequence that promotes activation of a protein phosphatase 1 (PP1)/glycogen synthase 3 (GSK3) signaling pathway, which inhibits kinesin-1-based anterograde fast axonal transport (FAT). This finding suggests that phosphorylation may control tau conformation and function. However, the AT8 includes three distinct phosphorylated amino acids that may be differentially phosphorylated in normal and disease conditions. To evaluate the effects of specific phosphorylation sites in the AT8 epitope, recombinant, pseudophosphorylated tau proteins were perfused into the isolated squid axoplasm preparation to determine their effects on axonal signaling pathways and FAT. Results from these studies suggest a mechanism where specific phosphorylation events differentially impact tau conformation, promoting activation of independent signaling pathways that differentially affect FAT. Implications of findings here to our understanding of tau function in health and disease conditions are discussed.

Highlights

Tau is a neuronal microtubule-associated protein enriched in axons, which becomes abnormally phosphorylated and aggregated in a group of neurodegenerative diseases including Alzheimer’s disease (AD), collectively known as tauopathies (Wang and Mandelkow, 2016)
Phosphorylated sites within the AT8 antibody epitope are of particular interest, as these sites are detected in normal brain tissues and widespread AT8 immunoreactivity is frequently used for the diagnosis of tauopathies (Goedert et al, 1993; Watanabe et al, 1993; Maurage et al, 2003)
Co-perfusion with TNT1 failed to prevent the inhibitory effect of hTau40-S199E in either anterograde (Figures 3B,E) or retrograde (Figures 3B,F) directions. These results indicated that exposure of the phosphatase activating domain (PAD) is necessary for hTau40-T205E to inhibit anterograde fast axonal transport (FAT) and that hTau40-S199E inhibits both directions of FAT through a PAD-independent mechanism

Summary

Introduction

Tau is a neuronal microtubule-associated protein enriched in axons, which becomes abnormally phosphorylated and aggregated in a group of neurodegenerative diseases including Alzheimer’s disease (AD), collectively known as tauopathies (Wang and Mandelkow, 2016). A wide variety of observations support additional roles for this protein, including regulation of intracellular organelle trafficking through modulation of phosphotransferases (Kanaan et al, 2013; Kneynsberg et al, 2017). Such roles appear consistent with findings of tau interactions with various protein kinases and phosphatases (Lee et al, 1998; Liao et al, 1998; Sun et al, 2002; Sontag et al, 2012). The question of how these conformational changes may affect tau function other than microtubule-binding has not been addressed

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