Glycogen Synthase Kinase 3β Phosphorylates Tau at Both Primed and Unprimed Sites

128

Tauopathies, including Alzheimer's disease, are neurodegenerative disorders in which tau protein accumulates as a consequence of alterations in its metabolism. At least three different types of alterations have been described; in some cases, an aberrant mRNA splicing of tau exon 10 occurs; in other cases, the disorder is a consequence of missense mutations and, in most cases, aberrant tau hyperphosphorylation takes place. Glycogen synthase kinase-3 (GSK-3) has emerged as a key kinase that is able to interact with several proteins involved in the ethiology of Alzheimer's disease and other tauopathies. Here, we have evaluated whether GSK-3 is also able to modulate tau-mRNA splicing. Our data demonstrate that GSK-3 inhibition in cultured neurons affects tau splicing resulting in an increase in tau mRNA containing exon 10. Pre-mRNA splicing is catalyzed by a multimolecular complex including members of the serine/arginine-rich (SR) family of splicing factors. Immunofluorescence studies showed that after GSK-3 inhibition, SC35, a member of the SR family, is redistributed and enriched in nuclear speckles and colocalizes with the kinase. Furthermore, immunoprecipitated SC35 is phosphorylated by recombinant GSK-3␤. Phosphorylation of a peptide from the SR domain by GSK-3 revealed that the peptide needs to be prephosphorylated, suggesting the involvement of a priming kinase. Our results demonstrate that GSK-3 plays a crucial role in tau exon 10 splicing, raising the possibility that GSK3 could contribute to tauopathies via aberrant tau splicing.

Section: Gsk-3 Tau Splicing and Sc35 Phosphorylationmentioning

confidence: 69%

Glycogen Synthase Kinase-3 Plays a Crucial Role in Tau Exon 10 Splicing and Intranuclear Distribution of SC35

Hernández

Pérez

Lucas

et al. 2004

128

“…In fact, these mutants impaired MAPK signaling relative to rTau4, suggesting that these mutants have a dominant-negative effect. As T231A Tau is able to bind microtubules well (41,42), this finding also underscored the dissociation between microtubule binding activity and MAPK potentiation activity. Most likely, phosphorylation of Tau at Thr-231 is required for the interaction with the MAPK pathway.…”

Section: Effects Of Tau Phosphorylation On Its Ability To Potentiatementioning

confidence: 91%

Tau Potentiates Nerve Growth Factor-induced Mitogen-activated Protein Kinase Signaling and Neurite Initiation without a Requirement for Microtubule Binding

Leugers¹,

Lee

2010

Microtubule-associated protein Tau is known to bind to and stabilize microtubules, thereby regulating microtubule dynamics. However, recent evidence has indicated that Tau can also interact with various components of intracellular signaling pathways, leading to the possibility that Tau might have a role in signal transduction. Here we provide evidence that during growth factor stimulation of neuronal cells, Tau has functions in advance of the neurite elongation stage. Using Tau-depleted neuronal cell lines, we demonstrate that Tau is required for neurite initiation in a manner that does not involve its microtubule binding function. In addition, we demonstrate that Tau potentiates AP-1 transcription factor activation in response to nerve growth factor (NGF). The effect of Tau on AP-1 activation is mediated through its ability to potentiate the activation of mitogen-activated protein kinase (MAPK), which occurs in response to both NGF and epidermal growth factor. Phosphorylation of Tau at Thr-231 also occurs in response to NGF and is required for Tau to impact on MAPK signaling, whereas the ability of Tau to bind to microtubules is not required. Together, these findings indicate a new functional role for Tau in early neuronal development independent of its established role in microtubule stabilization.Microtubule-associated proteins are a class of proteins that includes Tau, MAP2, and MAP4. These proteins are so named for their ability to bind to and stabilize microtubules, thereby contributing to the regulation of microtubule dynamics. Tau in particular has been extensively studied due to its prominent role in the pathogenesis of neurodegenerative diseases such as Alzheimer disease and the fronto-temporal dementias (reviewed in Refs. 1-4). In general, research into the physiological functions of Tau has focused on its interactions with microtubules. However, additional roles for Tau beyond those associated with microtubules have been suggested by numerous studies. We have previously demonstrated that the amino terminus of Tau, a domain not involved in microtubule binding, is associated with the plasma membrane and can affect nerve growth factor (NGF) 2 -induced neurite outgrowth (5). Also, neurite outgrowth was shown to require phosphorylation of Tau at Ser-262, a modification that reduces the ability of Tau to bind to microtubules (6). In addition, interactions between Tau and various signaling proteins such as Src, Fyn, Abl, the p85 subunit of phosphatidylinositol 3-kinase, phospholipase C␥, 14-3-3, and Grb2 have been described (7-12). Such findings suggest that non-microtubule-associated Tau species may be associated with signaling components at the plasma membrane during early neurogenesis and differentiation. Recent reports have also indicated that Tau can be linked to the increased expression of cell cycle proteins. Mice expressing human Tau in the absence of mouse Tau and mice expressing FTDP-17 mutant Tau were found to have abnormal expression of cell cycle proteins (13-15) and a cell culture model expres...

“…Because glycogen synthase kinase 3␤ (GSK3␤) is a predominant tau kinase (58), we examined the interaction of R406W mutant tau and GSK3␤. But co-immunoprecipitation studies did not reveal any differences in the interaction of wild type or R406W mutant tau with GSK3␤ (data not shown), although increased phosphorylation at Thr-231, a primed GSK3␤ site that negatively impacts microtubule binding (48,59), was observed for R406W tau. Goedert et al (60) reported that FTDP-17 mutant tau, including the R406W mutant, showed a decreased ability to bind protein phosphatase 2A (PP2A), a major tau phosphatase.…”

Section: Cellular Localization Of Wild Type and Mutant R406wmentioning

confidence: 92%

“…In Vitro Microtubule Binding Assay-The microtubule binding assay was carried out as previously described (48). Unbound (supernatant) and microtubule-bound (pellet) fractions were electrophoresed on a 10% SDS-polyacrylamide gel, transferred to nitrocellulose membrane, and probed with the total tau antibodies Tau5/5A6.…”

Section: Methodsmentioning

confidence: 99%

“…3), indicating that mutant R406W tau is more phosphorylated and less associated with the neuronal cytoskeleton. It is interesting to note that AT180 immunoreactivity was only observed in the soluble fraction from the R406W mutant tau-expressing cells given that phosphorylation of Thr-231 likely plays a key role in decreasing the association of tau with microtubules (48).…”

Section: Cell Morphology Of Wild Type and R406w Mutant Tau-transfectementioning

confidence: 99%

See 1 more Smart Citation

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

Krishnamurthy

Johnson

2004

Self Cite

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...