Mutant (R406W) Human Tau Is Hyperphosphorylated and Does Not Efficiently Bind Microtubules in a Neuronal Cortical Cell Model

Abstract

Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) is an autosomal dominant neurodegenerative disorder caused by mutations in the gene that encodes for tau, a microtubule-binding protein. Neuropathologically the disease is characterized by extensive neuronal loss in the frontal and temporal lobes and the filamentous accumulation of hyperphosphorylated tau. The R406W missense mutation was originally described in an American and a Dutch family. Although R406W tau is hyperphosphorylated in FTDP-17 cases, R406W tau expressed in cell model systems has not shown increased phosphorylation. The purpose of this study was to establish a neuronal model system in which the phosphorylation of R406W tau is increased and thus more representative of the in vivo situation. To accomplish this goal immortalized mouse cortical cells that express low levels of endogenous tau were stably transfected with human wild type or R406W tau. In this neuronal model R406W tau was more highly phosphorylated at numerous epitopes and showed decreased microtubule binding compared with wild type tau, an effect that could be reversed by dephosphorylation. In addition the expression of R406W tau in the cortical cells resulted in increased cell death as compared with wild type tau-expressing cells when the cells were exposed to an apoptotic stressor. These results indicate that in an appropriate cellular context R406W tau is hyperphosphorylated, which leads to decreased microtubule binding. Furthermore, expression of R406W tau sensitized cells to apoptotic stress, which may contribute to the neuronal cell loss that occurs in this FTDP-17 tauopathy.Frontotemporal dementia (FTD) 1 refers to a group of neurological disorders that are characterized clinically by progressive behavioral changes and neuropathologically by neuronal loss in the frontal and temporal lobes and the presence of filamentous deposits of abnormally hyperphosphorylated tau protein in neurons and/or glial cells (1-4). FTDs occur mainly as sporadic cases but also as familial forms with an autosomal dominant mode of inheritance and age-dependent penetrance (1). In 1998 it was demonstrated that several familial FTDs were due to mutations in the tau gene and have been referred to as frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17) (5-7). Since these initial discoveries, additional tau mutations have been described in frontotemporal dementia families, and presently over 25 mutations in the tau gene have been identified (8, 9). They include missense mutations in coding regions, amino acid deletions, and intronic mutations in the region following exon 10. The mutations may be divided into two main groups, those that affect alternative splicing of exon 10, leading to changes in the ratio of tau mRNAs with or without exon 10, and thus the proportion of tau with three microtubule binding repeats versus tau with four microtubule binding repeats, and missense mutations.Tau is a microtubule-associated protein that facilitates microtubule assembly an...

Promotion of Hyperphosphorylation by Frontotemporal Dementia Tau Mutations

“…In cell lines, 4L R406W seems not to be phosphorylated at Ser-396, although another study found it to be phosphorylated to a similar extent as the wild-type tau (49). Consistent with the present study, when 4L R406W was expressed in a neuronal cortical cell model (52), it was found to be phosphorylated at several sites, including Ser-396, and unable to bind to microtubules due to the phosphorylation state. In 4L R406W transgenic mice, the protein was found to be phosphorylated at several sites, including Ser-396, and to be aggregated into filaments (53).…”

Oxidative stress mediates tau-induced neurodegeneration in Drosophila

“…While there is no evidence of neuronal apoptosis in mouse models of tauopathy, in the Drosophila model, neuronal cell death is consistent with an apoptotic mechanism (28,34). Moreover, the tau mutations R406W, V337M, and N279K, associated with frontotemporal dementia, were shown to enhance the vulnerability of cultured neurons to apoptotic stressors (51,52). Interestingly, hallmarks of both apoptotic and necrotic cell death have been recently identified in neurons from transgenic mice expressing nonmutant forms of human tau (53).…”

Familial FTDP-17 Missense Mutations Inhibit Microtubule Assembly-promoting Activity of Tau by Increasing Phosphorylation at Ser202 in Vitro

“…When expressed in SH-SY5Y cells subsequently differentiated into neurons, the R406W, P301L, and V337M mutations reduce tau phosphorylation (23). In contrast, in hippocampal neurons, R406W increases tau phosphorylation (24). When phosphorylated by recombinant GSK3␤ in vitro, the P301L and V337M mutations do not have any effect, and the R406W mutation inhibits phosphorylation (25).…”