Glycogen synthase kinase 3beta phosphorylates Alzheimer's disease-specific Ser396 of microtubule-associated protein tau by a sequential mechanism

Phosphorylation of tau on S(396) was suggested to be a key step in the development of neurofibrillary pathology in Alzheimer's disease brain [Bramblett, G. T., Goedert, M., Jacks, R., Merrick, S. E., Trojanowski, J. Q., and Lee, V. M.-Y. (1993) Neuron 10, 1089-1099]. GSK3beta phosphorylates Ser(396) of tau in the brain by a mechanism which is not clear. In this study, when HEK-293 cells were cotransfected with tau and GSK3beta, GSK3beta co-immunoprecipitated with tau and phosphorylated tau on S(202), T(231), S(396), and S(400) but not on S(262), S(235), and S(404). Blocking phosphorylation on T(231), S(235), S(396), S(400), or S(404) did not prevent the subsequent phosphorylation on S(202) by GSK3beta. These data suggest that GSK3beta directly phosphorylates tau on S(202) (without requiring prephosphorylation). However, preventing phosphorylation on S(235), S(400), and S(404) prevented GSK3beta-dependent phosphorylation of T(231), S(396), and S(400), respectively. This indicates that phosphorylation of T(231), S(396), and S(400) by GSK3beta depends on a previous phosphorylation of S(235), S(400), and S(404), respectively. To examine S(396) phosphorylation, we analyzed phosphorylation of S(396), S(400), and S(404). Blocking phosphorylation of S(404) prevented the subsequent GSK3beta-dependent phosphorylation of both S(400) and S(396). When phosphorylation of S(404) was allowed but S(400) blocked, GSK3beta failed to phosphorylate S(396). Thus, GSK3beta phosphorylates S(396) by a two-step mechanism. In the first step, GSK3beta phosphorylates S(400) of previously S(404)-phosphorylated tau. This event primes tau for second-step phosphorylation of S(396) by GSK3beta. We conclude that GSK3beta phosphorylates tau directly at S(202) but requires the previous phosphorylation on S(235) to phosphorylate T(231). Phosphorylation of S(396), on the other hand, occurs sequentially. Once a priming kinase phosphorylates S(404), GSK3beta sequentially phosphorylates S(400) and then S(396).     

Cyclin-dependent protein kinase 5 primes microtubule-associated protein tau site-specifically for glycogen synthase kinase 3beta

In the preceding paper, we showed that GSK3beta phosphorylates tau at S(202), T(231), S(396), and S(400) in vivo. Phosphorylation of S(202) occurs without priming. Phosphorylation of T(231), on the other hand, requires priming phosphorylation of S(235). Similarly, priming phosphorylation of S(404) is essential for the sequential phosphorylation of S(400) and S(396) by GSK3beta. The priming kinase that phosphorylates tau at S(235) and S(404) in the brain is not known. In this study, we find that in HEK-293 cells cotransfected with tau, GSK3beta, and Cdk5, Cdk5 phosphorylates tau at S(202), S(235), and S(404). S(235) phosphorylation enhances GSK3beta-catalyzed T(231) phosphorylation. Similarly, Cdk5 by phosphorylating S(404) stimulates phosphorylation of S(400) and S(396) by GSK3beta. These data indicate that Cdk5 primes tau for GSK3beta in intact cells. To evaluate if Cdk5 primes tau for GSK3beta in mammalian brain, we examined localizations of Cdk5, tau, and GSK3beta in rat brain. We also analyzed the interaction of Cdk5 with tau and GSK3beta in brain microtubules. We found that Cdk5, GSK3beta, and tau are virtually colocalized in rat brain cortex. When bovine brain microtubules are analyzed by FPLC gel filtration, Cdk5, GSK3beta, and tau coelute within an approximately 450 kDa complex. From the fractions containing the approximately 450 kDa complex, tau, Cdk5, and GSK3beta co-immunoprecipitate with each other. In HEK-293 cells transfected with tau, Cdk5, and GSK3beta in different combinations, tau binds to Cdk5 in a manner independent of GSK3beta and to GSK3beta in a manner independent of Cdk5. However, Cdk5 and GSK3beta bind to each other only in the presence of tau, suggesting that tau connects Cdk5 and GSK3beta. Our results suggest that in the brain, tau, Cdk5, and GSK3beta are components of an approximately 450 kDa complex. Within the complex, Cdk5 phosphorylates tau at S(235) and primes it for phosphorylation of T(231) by GSK3beta. Similarly, Cdk5 by phosphorylating tau at S(404) primes tau for a sequential phosphorylation of S(400) and S(396) by GSK3beta.     

Site-specific phosphorylation and caspase cleavage differentially impact tau-microtubule interactions and tau aggregation

The microtubule-associated protein tau is hyperphosphorylated and forms neurofibrillary tangles in Alzheimer disease. Additionally caspase-cleaved tau is present in Alzheimer disease brains co-localized with fibrillar tau pathologies. To further understand the role of site-specific phosphorylation and caspase cleavage of tau in regulating its function, constructs of full-length tau (T4) or tau truncated at Asp421 (T4C3) to mimic caspase-3 cleavage with and without site-directed mutations that mimic phosphorylation at Thr231/Ser235, Ser396/Ser404, or at all four sites (Thr231/Ser235/Ser396/Ser404) were made and expressed in cells. Pseudophosphorylation of T4, but not T4C3, at either Thr231/Ser235 or Ser396/Ser404 increased its phosphorylation at Ser262 and Ser199. Pseudophosphorylation at Thr231/Ser235 impaired the microtubule binding of both T4 and T4C3. In contrast, pseudophosphorylation at Ser396/Ser404 only affected microtubule binding of T4C3 but did make T4 less soluble and more aggregated, which is consistent with the previous finding (Abraha, A., Ghoshal, N., Gamblin, T. C., Cryns, V., Berry, R. W., Kuret, J., and Binder, L. I. (2000) J. Cell Sci. 113, 3737-3745) that pseudophosphorylation at Ser396/Ser404 enhances tau polymerization in vitro. In situ T4C3 was more prevalent in the cytoskeletal and microtubule-associated fractions compared with T4, whereas purified recombinant T4 bound microtubules with higher affinity than did T4C3 in an in vitro assay. These data indicate the importance of cellular factors in regulating tau-microtubule interactions and that, in the cells, phosphorylation of T4 might impair its microtubule binding ability more than caspase cleavage. Treatment of cells with nocodazole revealed that pseudophosphorylation of T4 at both Thr231/Ser235 and Ser396/Ser404 diminished the ability of tau to protect against microtubule depolymerization, whereas with T4C3 only pseudophosphorylation at Ser396/Ser404 attenuated the ability of tau to stabilize the microtubules. These results show that site-specific phosphorylation and caspase cleavage of tau differentially affect the ability of tau to bind and stabilize microtubules and facilitate tau self-association.     

Physiological regulation of tau phosphorylation during hibernation

The microtubule-associated protein tau is abnormally hyperphosphorylated in the brains of individuals with Alzheimer disease and other tauopathies, and is believed to play a critical role in the pathogenesis of these diseases. While the mechanisms leading to abnormal tau phosphorylation remain elusive, the recent demonstration of reversible tau phosphorylation during hibernation provides an ideal physiological model to study this critical process in vivo . In this study, arctic ground squirrels (AGS) during hibernation were used to study mechanisms related to tau hyperphosphorylation. Our data demonstrate that tau is hyperphosphorylated at all six sites (S199, T205, S214, S262, S396, and S404) examined in hibernating AGS. Interestingly, only three of these sites (S199, S262, and S404) are dephosphorylated in aroused animals, suggesting a reversible phosphorylation at selective sites. Summer-active AGS demonstrated the lowest tau phosphorylation at all these sites. To explore the mechanisms underlying increased tau phosphorylation during hibernation, the expression level and enzyme activity of various potential tau kinases and protein phosphatases were examined. The kinetic analysis of enzyme activity at different temperatures revealed differential changes in enzyme activity with temperature decline. Specifically, increased protein kinase A activity, decreased protein phosphatase 2A activity, as well as substantial contribution from glycogen synthase kinase-3β, likely play a key role in increased tau phosphorylation during hibernation in AGS.     

Primed phosphorylation of tau at Thr231 by glycogen synthase kinase 3beta (GSK3beta) plays a critical role in regulating tau's ability to bind and stabilize microtubules

Site-specific phosphorylation of tau negatively regulates its ability to bind and stabilize microtubule structure. Although tau is a substrate of glycogen synthase kinase 3beta (GSK3beta), the exact sites on tau that are phosphorylated by this kinase in situ have not yet been established, and the effect of these phosphorylation events on tau-microtubule interactions have not been fully elucidated. GSK3beta phosphorylates both primed and unprimed sites on tau, but only primed phosphorylation events significantly decrease the ability of tau to bind microtubules. The focus of the present study is on determining the importance of the GSK3beta-mediated phosphorylation of a specific primed site, Thr231, in regulating tau's function. Pre-phosphorylation of Ser235 primes tau for phosphorylation by GSK3beta at Thr231. Phosphorylation by GSK3beta of wild-type tau or tau with Ser235 mutated to Ala decreases tau-microtubule interactions. However, when Thr231 alone or Thr231 and Ser235 in tau were mutated to Ala, phosphorylation by GSK3beta did not decrease the association of tau with the cytoskeleton. Further, T231A tau was still able to efficiently bind microtubules after phosphorylation by GSK3beta. Expression of each tau construct alone increased tubulin acetylation, a marker of microtubule stability. However, when cells were cotransfected with wild-type tau and GSK3beta, the level of tubulin acetylation was decreased to vector-transfected levels. In contrast, coexpression of GSK3beta with mutated tau (T231A/S235A) did not significantly decrease the levels of acetylated tubulin. These results strongly indicate that phosphorylation of Thr231 in tau by GSK3beta plays a critical role in regulating tau's ability to bind and stabilize microtubules.     

Effects of FTDP-17 mutations on the in vitro phosphorylation of tau by glycogen synthase kinase 3beta identified by mass spectrometry demonstrate certain mutations exert long-range conformational changes

In vitro phosphorylation of recombinant wild-type 2N4R tau and FTDP-17 exonic mutant forms P301L, V337M and R406W by glycogen synthase kinase 3beta (GSK3beta) was examined by two dimensional phosphopeptide mapping analysis on thin layer cellulose plates. Comparison of these peptide maps with those generated from wild-type 1N4R tau isoform from which the phosphopeptide constituents and sites of phosphorylation had been determined previously, enabled us to monitor directly changes in phosphorylation of the individual tau proteins. No differences were found in the phosphorylation of wild-type, P301L or V337M tau by GSK3beta but the R406W mutant showed at least two clear differences from the other three tau proteins. The peptides, identified by mass spectrometry corresponding to phosphorylation at both threonine 231 and serine 235 (spot 3), serines 396, 400 and 404 (spot 6a) and serines 195 and 199 (spot 6b) were absent from the R406W peptide map. The findings imply that the R406W mutation in tau exerts long-range conformational effects on the structure of tau.     

New application of beta-galactosidase complementation to monitor tau self-association

Neurofibrillary tangles composed of hyperphosphorylated and aberrantly cleaved microtubule-associated protein tau are a major neuropathological hallmark of Alzheimer's disease. Recent studies suggest that the predominant neurotoxic effect of pathologically processed tau is mediated by intermediate tau multimers rather than the mature tau tangles, thus underscoring the importance of studying tau self-association preceding tangle formation. However, experimental approaches for such studies are limited. Here, we describe a modification of the β-galactosidase (β-gal) complementation assay, which provides a simple, sensitive and quantitative system to monitor pre-tangle tau–tau interactions in a cell model. Full-length tau (T4) and tau truncated at D421 (C3, to mimic caspase-cleaved tau) were fused to one of a pair of weakly complementing β-gal mutants (Δα and Δω) and expressed in human embryonic kidney cells. The tau–tau interactions and the subsequent juxtapositioning of Δα and Δω led to β-gal complementation and an increase in β-gal activity which was detected by histochemical staining and quantified by chemiluminescent assays. After cross-linking with disuccinimidyl suberate, tau formed high molecular weight complexes which were detected on denaturing acrylamide gels, further confirming the close proximity among self-associated tau molecules. The self-association of C3 appeared to be less efficient than that of T4. Furthermore, treatment with lithium decreased β-gal complementation of both T4 and C3 indicating that the interaction of these proteins was attenuated. Overall, this study suggests that β-gal complementation assay can be a useful tool to monitor tau self-association.     

Cleavage of GSK‐3β by calpain counteracts the inhibitory effect of Ser9 phosphorylation on GSK‐3β activity induced by H2O2

Glycogen synthase kinase‐3 beta (GSK‐3β) dysfunction may play an essential role in the pathogenesis of psychiatric, metabolic, neurodegenerative diseases, in which oxidative stress exists concurrently. Some studies have shown that GSK‐3β activity is up‐regulated under oxidative stress. This study evaluated how oxidative stress regulates GSK‐3β activity in human embryonic kidney 293 (HEK293)/Tau cells treated with hydrogen peroxide (H₂O₂). Here, we show that H₂O₂ induced an obvious increase of GSK‐3β activity. Surprisingly, H₂O₂ dramatically increased phosphorylation of GSK‐3β at Ser9, an inactive form of GSK‐3β,while there were no changes of phosphorylation of GSK‐3β at Tyr216. Moreover, H₂O₂ led to a transient [Ca²⁺]_i elevation, and simultaneously increased the truncation of GSK‐3β into two fragments of 40 kDa and 30 kDa, whereas inhibition of calpain decreased the truncation and recovered the activity of GSK‐3β. Furthermore, tau was hyperphosphorylated at Ser396, Ser404, and Thr231, three most common GSK‐3β targeted sites after 100 μM H₂O₂ administration in HEK293/Tau cells, whereas inhibition of calpain blocked the tau phosphorylation. In addition, we found that there were no obvious changes of Cyclin‐dependent kinase 5 (CDK5) expression (responsible for tau phosphorylation) and of p35 cleavage, the regulatory subunit of CDK5 in H₂O₂‐treated HEK293/Tau cells. In conclusion, Ca²⁺‐dependent calpain activation leads to GSK‐3β truncation, which counteracts the inhibitory effect of Ser9 phosphorylation, up‐regulates GSK‐3β activity, and phosphorylates tau in H₂O₂‐treated HEK293/Tau cells.     

Involvement of aberrant glycosylation in phosphorylation of tau by cdk5 and GSK-3beta

Microtubule-associated protein tau is abnormally hyperphosphorylated, glycosylated, and aggregated in affected neurons in the brains of individuals with Alzheimer’s disease (AD). We recently found that the glycosylation might precede hyperphosphorylation of tau in AD. In this study, we investigated the effect of glycosylation on phosphorylation of tau catalyzed by cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase-3β (GSK-3β). The phosphorylation of the longest isoform of recombinant human brain tau, tau₄₄₁, at various sites was detected by Western blots and by radioimmuno-dot-blot assay with phosphorylation-dependent and site-specific tau antibodies. We found that cdk5 phosphorylated tau₄₄₁ at Thr-181, Ser-199, Ser-202, Thr-205, Thr-212, Ser-214, Thr-217, Thr-231, Ser-235, Ser-396, and Ser-404, but not at Ser-262, Ser-400, Thr-403, Ser-409, Ser-413, or Ser-422. GSK-3β phosphorylated all the cdk5-catalyzed sites above except Ser-235. Deglycosylation by glycosidases depressed the subsequent phosphorylation of AD-tau (i) with cdk5 at Thr-181, Ser-199, Ser-202, Thr-205, and Ser-404, but not at Thr-212; and (ii) with GSK-3β at Thr-181, Ser-202, Thr-205, Ser-217, and Ser-404, but not at Ser-199, Thr-212, Thr-231, or Ser-396. These data suggest that aberrant glycosylation of tau in AD might be involved in neurofibrillary degeneration by promoting abnormal hyperphosphorylation by cdk5 and GSK-3β.     

The retinoic acid and brain-derived neurotrophic factor differentiated SH-SY5Y cell line as a model for Alzheimer's disease-like tau phosphorylation

The paired helical filaments of highly phosphorylated tau protein are the main components of neurofibrillary tangles (NFT) in Alzheimer's disease (AD). Protein kinases including glycogen synthase kinase 3 beta (GSK3beta), cyclin-dependent kinase 5 (Cdk5), and c-Jun N-terminal kinase (JNK) have been implicated in NFT formation making the use of selective kinase inhibitors an attractive treatment possibility in AD. When sequentially treated with retinoic acid (RA) and brain-derived neurotrophic factor (BDNF), the human neuroblastoma SH-SY5Y differentiates to neuron-like cells. We found that coincident with morphologically evident neurite outgrowth, both the content and phosphorylation state of tau increased in RA-BDNF differentiated SH-SY5Y cells. Tau phosphorylation increased at all the examined sites ser-199, ser-202, thr-205, ser-396, and ser-404, all of which are hyperphosphorylated in AD brain. We also investigated whether GSK3beta, Cdk5 or JNK was involved in tau phosphorylation in the differentiated SH-SY5Y cells. We found that GSK3beta contributed most and that Cdk5 made a minor contribution. JNK was not involved in tau phosphorylation in this system. The GSK3beta-inhibitor, lithium, inhibited tau phosphorylation in a concentration-dependent manner and with good reproducibility, which enables ranking of substances in this cell model. RA-BDNF differentiated SH-SY5Y cells could serve as a suitable model for studying the mechanisms of tau phosphorylation and for screening potential GSK3beta inhibitors.     

PKA modulates GSK-3beta- and cdk5-catalyzed phosphorylation of tau in site- and kinase-specific manners

Phosphorylation of tau protein is regulated by several kinases, especially glycogen synthase kinase 3beta (GSK-3beta), cyclin-dependent protein kinase 5 (cdk5) and cAMP-dependent protein kinase (PKA). Phosphorylation of tau by PKA primes it for phosphorylation by GSK-3beta, but the site-specific modulation of GSK-3beta-catalyzed tau phosphorylation by the prephosphorylation has not been well investigated. Here, we found that prephosphorylation by PKA promotes GSK-3beta-catalyzed tau phosphorylation at Thr181, Ser199, Ser202, Thr205, Thr217, Thr231, Ser396 and Ser422, but inhibits its phosphorylation at Thr212 and Ser404. In contrast, the prephosphorylation had no significant effect on its subsequent phosphorylation by cdk5 at Thr181, Ser199, Thr205, Thr231 and Ser422; inhibited it at Ser202, Thr212, Thr217 and Ser404; and slightly promoted it at Ser396. These studies reveal the nature of the inter-regulation of tau phosphorylation by the three major tau kinases.     

Casein kinase 1 delta phosphorylates tau and disrupts its binding to microtubules

Tau hyperphosphorylation precedes neuritic lesion formation in Alzheimer's disease, suggesting it participates in the tau fibrillization reaction pathway. Candidate tau protein kinases include members of the casein kinase 1 (CK1) family of phosphotransferases, which are highly overexpressed in Alzheimer's disease brain and colocalize with neuritic and granulovacuolar lesions. Here we characterized the contribution of one CK1 isoform, Ckidelta, to the phosphorylation of tau at residues Ser202/Thr205 and Ser396/Ser404 in human embryonic kidney 293 cells using immunodetection and fluorescence microscopy. Treatment of cells with membrane permeable CK1 inhibitor 3-[(2,3,6-trimethoxyphenyl)methylidenyl]-indolin-2-one (IC261) lowered occupancy of Ser396/Ser404 phosphorylation sites by >70% at saturation, suggesting that endogenous CK1 was the major source of basal phosphorylation activity at these sites. Overexpression of Ckidelta increased CK1 enzyme activity and further raised tau phosphorylation at residues Ser202/Thr205 and Ser396/Ser404 in situ. Inhibitor IC261 reversed tau hyperphosphorylation induced by Ckidelta overexpression. Co-immunoprecipitation assays showed direct association of tau and Ckidelta in situ, consistent with tau being a Ckidelta substrate. Ckidelta overexpression also produced a decrease in the fraction of bulk tau bound to detergent-insoluble microtubules. These results suggest that Ckidelta phosphorylates tau at sites that modulate tau/microtubule binding, and that the expression pattern of Ckidelta in Alzheimer's disease is consistent with it playing an important role in tau aggregation.     

An Unbiased Approach to Identifying Tau Kinases That Phosphorylate Tau at Sites Associated with Alzheimer Disease

Neurofibrillary tangles, one of the hallmarks of Alzheimer disease (AD), are composed of paired helical filaments of abnormally hyperphosphorylated tau. The accumulation of these proteinaceous aggregates in AD correlates with synaptic loss and severity of dementia. Identifying the kinases involved in the pathological phosphorylation of tau may identify novel targets for AD. We used an unbiased approach to study the effect of 352 human kinases on their ability to phosphorylate tau at epitopes associated with AD. The kinases were overexpressed together with the longest form of human tau in human neuroblastoma cells. Levels of total and phosphorylated tau (epitopes Ser(P)-202, Thr(P)-231, Ser(P)-235, and Ser(P)-396/404) were measured in cell lysates using AlphaScreen assays. GSK3α, GSK3β, and MAPK13 were found to be the most active tau kinases, phosphorylating tau at all four epitopes. We further dissected the effects of GSK3α and GSK3β using pharmacological and genetic tools in hTau primary cortical neurons. Pathway analysis of the kinases identified in the screen suggested mechanisms for regulation of total tau levels and tau phosphorylation; for example, kinases that affect total tau levels do so by inhibition or activation of translation. A network fishing approach with the kinase hits identified other key molecules putatively involved in tau phosphorylation pathways, including the G-protein signaling through the Ras family of GTPases (MAPK family) pathway. The findings identify novel tau kinases and novel pathways that may be relevant for AD and other tauopathies.     

Activation of glycogen synthase kinase 3beta promotes the intermolecular association of tau. The use of fluorescence resonance energy transfer microscopy

Tau is hyperphosphorylated and undergoes proteolysis in Alzheimer disease brain. Caspase-cleaved tau efficiently forms fibrillary structures in vitro and in situ. Glycogen synthase kinase 3beta (GSK3beta) phosphorylates tau and induces the aggregation of caspase-cleaved tau in situ. Given the hypothesis that increased association of tau precedes the formation of fibrillar structures, we generated a cell model to quantitate the extent of tau association in situ using fluorescence resonance energy transfer (FRET) microscopy. The cyan and yellow fluorescent proteins were attached to full-length (T4) and caspase-cleaved (T4C3) tau at either the N or C termini, and a pair of cyan and yellow fluorescent protein-tagged tau were co-transfected into human embryonic kidney cells. The FRET efficiency was examined in the presence of a constitutively active or a kinase-dead GSK3beta. Active GSK3beta significantly increased FRET efficiency with both T4 and T4C3, indicating that GSK3beta activation resulted in an increase in the self-association of both T4 and T4C3, but interestingly only T4 is efficiently phosphorylated by GSK3beta. There was no significant difference in FRET efficiency between T4 and T4C3, although only T4C3 in the presence of active GSK3beta leads to the formation of Sarkosyl-insoluble inclusions. These FRET studies demonstrate that GSK3beta facilitates the association of T4 and T4C3, and the presence of caspase-cleaved tau is necessary for the evolution of tau oligomers into Sarkosyl-insoluble inclusions even though it is not extensively phosphorylated. These data imply that increased association of tau should not be regarded as a direct indicator of the formation of insoluble tau aggregates.     

Phosphorylation of PPP(S/T)P motif of the free LRP6 intracellular domain is not required to activate the Wnt/β‐catenin pathway and attenuate GSK3β activity

The canonical Wnt/β‐catenin signaling pathway plays a critical role in numerous physiological and pathological processes. LRP6 is an essential co‐receptor for Wnt/β‐catenin signaling; as transduction of the Wnt signal is strongly dependent upon GSK3β‐mediated phosphorylation of multiple PPP(S/T)P motifs within the membrane‐anchored LRP6 intracellular domain. Previously, we showed that the free LRP6 intracellular domain (LRP6‐ICD) can activate the Wnt/β‐catenin pathway in a β‐catenin and TCF/LEF‐1 dependent manner, as well as interact with and attenuate GSK3β activity. However, it is unknown if the ability of LRP6‐ICD to attenuate GSK3β activity and modulate activation of the Wnt/β‐catenin pathway requires phosphorylation of the LRP6‐ICD PPP(S/T)P motifs, in a manner similar to the membrane‐anchored LRP6 intracellular domain. Here we provide evidence that the LRP6‐ICD does not have to be phosphorylated at its PPP(S/T)P motif by GSK3β to stabilize endogenous cytosolic β‐catenin resulting in activation of TCF/LEF‐1 and the Wnt/β‐catenin pathway. LRP6‐ICD and a mutant in which all 5 PPP(S/T)P motifs were changed to PPP(A)P motifs equivalently interacted with and attenuated GSK3β activity in vitro, and both constructs inhibited the in situ GSK3β‐mediated phosphorylation of β‐catenin and tau to the same extent. These data indicate that the LRP6‐ICD attenuates GSK3β activity similar to other GSK3β binding proteins, and is not a result of it being a GSK3β substrate. Our findings suggest the functional and regulatory mechanisms governing the free LRP6‐ICD may be distinct from membrane‐anchored LRP6, and that release of the LRP6‐ICD may provide a complimentary signaling cascade capable of modulating Wnt‐dependent gene expression. J. Cell. Biochem. 108: 886–895, 2009. © 2009 Wiley‐Liss, Inc.     

Caspase-Cleaved Tau Impairs Mitochondrial Dynamics in Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by the presence of aggregates of tau protein. Tau truncated by caspase-3 (D421) or tau hyperphosphorylated at Ser396/S404 might play a role in the pathogenesis of AD. Mitochondria are dynamic organelles that modify their size and function through mitochondrial dynamics. Recent studies have shown that alterations of mitochondrial dynamics affect synaptic communication. Therefore, we studied the effects of pathological forms of tau on the regulation of mitochondrial dynamics. We used primary cortical neurons from tau(?/?) knockout mice and immortalized cortical neurons (CN1.4) that were transfected with plasmids containing green fluorescent protein (GFP) or GFP with different tau forms: full-length (GFP-T4), truncated (GFP-T4C3), pseudophosphorylated (GFP-T42EC), or both truncated and pseudophosphorylated modifications of tau (GFP-T4C3-2EC). Cells expressing truncated tau showed fragmented mitochondria compared to cells that expressed full-length tau. These findings were corroborated using primary neurons from tau(?/?) knockout mice that expressed the truncated and both truncated and pseudophosphorylated forms of tau. Interestingly, mitochondrial fragmentation was accompanied by a significant reduction in levels of optic atrophy protein 1 (Opa1) in cells expressing the truncated form of tau. In addition, treatment with low concentrations of amyloid-beta (Aβ) significantly reduced mitochondrial membrane potential, cell viability, and mitochondrial length in cortical cells and primary neurons from tau(?/?) mice that express truncated tau. These results indicate that the presence of tau pathology impairs mitochondrial dynamics by reducing Opa1 levels, an event that could lead to mitochondrial impairment observed in AD.     

14-3-3 binds to and mediates phosphorylation of microtubule-associated tau protein by Ser9-phosphorylated glycogen synthase kinase 3beta in the brain

In mammalian brain, tau, glycogen synthase kinase 3beta (GSK3beta), and 14-3-3, a phosphoserine-binding protein, are parts of a multiprotein tau phosphorylation complex. Within the complex, 14-3-3 simultaneously binds to tau and GSK3beta (Agarwal-Mawal, A., Qureshi, H. Y., Cafferty, P. W., Yuan, Z., Han, D., Lin, R., and Paudel, H. K. (2003) J. Biol. Chem. 278, 12722-12728). The molecular mechanism by which 14-3-3 connects GSK3beta to tau within the complex is not clear. In this study, we find that GSK3beta within the tau phosphorylation complex is phosphorylated on Ser(9). From extracts of rat brain and rat primary cultured neurons, Ser(9)-phosphorylated GSK3beta precipitates with glutathione-agarose beads coated with glutathione S-transferase-14-3-3. Similarly, from rat brain extract, Ser(9)-phosphorylated GSK3beta co-immunoprecipitates with tau. In vitro, 14-3-3 binds to GSK3beta only when the kinase is phosphorylated on Ser(9). In transfected HEK-293 cells, 14-3-3 binds to Ser(9)-phosphorylated GSK3beta and does not bind to GSK3beta (S9A). Tau, on the other hand, binds to both GSK3beta (WT) and GSK3beta (S9A). Moreover, 14-3-3 enhances the binding of tau with Ser(9)-phosphorylated GSK3beta by approximately 3-fold but not with GSK3beta (S9A). Similarly, 14-3-3 stimulates phosphorylation of tau by Ser(9)-phosphorylated GSK3beta but not by GSK3beta (S9A). In transfected HEK-293 cells, Ser(9) phosphorylation suppresses GSK3beta-catalyzed tau phosphorylation in the absence of 14-3-3. In the presence of 14-3-3, however, Ser(9)-phosphorylated GSK3beta remains active and phosphorylates tau. Our data indicate that within the tau phosphorylation complex, 14-3-3 connects Ser(9)-phosphorylated GSK3beta to tau and Ser(9)-phosphorylated GSK3beta phosphorylates tau.     

Axin negatively affects tau phosphorylation by glycogen synthase kinase 3beta

Glycogen synthase kinase 3beta (GSK3beta) is an essential protein kinase that regulates numerous functions within the cell. One critically important substrate of GSK3beta is the microtubule-associated protein tau. Phosphorylation of tau by GSK3beta decreases tau-microtubule interactions. In addition to phosphorylating tau, GSK3beta is a downstream regulator of the wnt signaling pathway, which maintains the levels of beta-catenin. Axin plays a central role in regulating beta-catenin levels by bringing together GSK3beta and beta-catenin and facilitating the phosphorylation of beta-catenin, targeting it for ubiquitination and degradation by the proteasome. Although axin clearly facilitates the phosphorylation of beta-catenin, its effects on the phosphorylation of other GSK3beta substrates are unclear. Therefore in this study the effects of axin on GSK3beta-mediated tau phosphorylation were examined. The results clearly demonstrate that axin is a negative regulator of tau phosphorylation by GSK3beta. This negative regulation of GSK3beta-mediated tau phosphorylation is due to the fact that axin efficiently binds GSK3beta but not tau and thus sequesters GSK3beta away from tau, as an axin mutant that does not bind GSK3beta did not inhibit tau phosphorylation by GSK3beta. This is the first demonstration that axin negatively affects the phosphorylation of a GSK3beta substrate, and provides a novel mechanism by which tau phosphorylation and function can be regulated within the cell.     

Glycogen synthase kinase 3 beta induces caspase-cleaved tau aggregation in situ

Tau is a substrate of caspases, and caspase-cleaved tau has been detected in Alzheimer's disease brain but not in control brain. Furthermore, in vitro studies have revealed that caspase-cleaved tau is more fibrillogenic than full-length tau. Considering these previous findings, the purpose of this study was to determine how the caspase cleavage of tau affected tau function and aggregation in a cell model system. The effects of glycogen synthase kinase 3 beta (GSK3 beta), a well established tau kinase, on these processes also were examined. Tau or tau that had been truncated at Asp-421 to mimic caspase cleavage (Tau-D421) was transfected into cells with or without GSK3 beta, and phosphorylation, microtubule binding, and tau aggregation were examined. Tau-D421 was not as efficiently phosphorylated by GSK3 beta as full-length tau. Tau-D421 efficiently bound microtubules, and in contrast to the full-length tau, co-expression with GSK3 beta did not result in a reduction in the ability of Tau-D421 to bind microtubules. In the absence of GSK3 beta, neither Tau-D421 nor full-length tau formed Sarkosyl-insoluble inclusions. However, in the presence of GSK3 beta, Tau-D421, but not full-length tau, was present in the Sarkosyl-insoluble fraction and formed thioflavin-S-positive inclusions in the cell. Nonetheless, co-expression of GSK3 beta and Tau-D421 did not result in an enhancement of cell death. These data suggest that a combination of phosphorylation events and caspase activation contribute to the tau oligomerization process in Alzheimer's disease, with GSK3 beta-mediated tau phosphorylation preceding caspase cleavage.     

Role of GSK-3beta activation and alpha7 nAChRs in Abeta(1-42)-induced tau phosphorylation in PC12 cells

β-amyloid peptide 1–42 (Aβ_1–42) and hyperphosphorylated tau are associated with neurodegeneration in Alzheimer's disease. Emerging evidence indicates that Aβ_1–42 can potentiate hyperphosphorylation of tau in cell lines and in transgenic mice, but the underlying mechanism(s) remains unclear. In this study, Aβ_1–42-induced tau phosphorylation was investigated in differentiated PC12 cells. Treatment of cells with Aβ_1–42 increased phosphorylation of tau at serine-202 as detected by AT8 antibody. This Aβ_1–42-induced tau phosphorylation paralleled phosphorylation of glycogen synthase kinase-3β (GSK-3β) at tyrosine-216 (GSK-3β-pY216), which was partially inhibited by the GSK-3β inhibitor, CHIR98023. Aβ_1–42-induced tau phosphorylation and increase in GSK-3β-pY216 phosphorylation were also partially attenuated by α7 nicotinic acetylcholine receptor (α7 nAChR) selective ligands including agonist A-582941 and antagonists methyllycaconitine and α-bungarotoxin. The α7 nAChR agonist and the GSK-3β inhibitor had no additive effect. These observations suggest that α7 nAChR modulation can influence Aβ_1–42-induced tau phosphorylation, possibly involving GSK-3β. This study provides evidence of nAChR mechanisms underlying Aβ_1–42 toxicity and tau phosphorylation, which, if translated in vivo , could provide additional basis for the utility of α7 nAChR ligands in the treatment of Alzheimer's disease.