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.     

Glycogen synthase kinase 3beta interacts with and phosphorylates the spindle-associated protein astrin

Emerging evidence shows that glycogen synthase kinase 3beta (GSK3beta) is involved in mitotic division and that inhibiting of GSK3beta kinase activity causes defects in spindle microtubule length and chromosome alignment. However, the purpose of GSK3beta involvement in spindle microtubule assembly and accurate chromosome segregation remains obscure. Here, we report that GSK3beta interacts with the spindle-associated protein Astrin both in vitro and in vivo. Additionally, Astrin acts as a substrate for GSK3beta and is phosphorylated at Thr-111, Thr-937 ((S/T)P motif) and Ser-974/Thr-978 ((S/T)XXX(S/T)-p motif; p is a phosphorylatable residue). Inhibition of GSK3beta impairs spindle and kinetochore accumulation of Astrin and spindle formation at mitosis, suggesting that Astrin association with the spindle microtubule and kinetochore may be dependent on phosphorylation by GSK3beta. Conversely, depletion of Astrin by small interfering RNA has no detectable influence on the localization of GSK3beta. Interestingly, in vitro assays demonstrated that Astrin enhances GSK3beta-mediated phosphorylation of other substrates. Moreover, we showed that coexpression of Astrin and GSK3beta differentially increases GSK3beta-mediated Tau phosphorylation on an unprimed site. Collectively, these data indicate that GSK3beta interacts with and phosphorylates the spindle-associated protein Astrin, resulting in targeting Astrin to the spindle microtubules and kinetochores. In turn, the GSK3beta-Astrin complex may also facilitate further physiological and pathological phosphorylation.     

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.     

Glycogen synthase kinase-3 and the Alzheimer-like state of microtubule-associated protein tau.

The Alzheimer-like state of tau protein includes phosphorylation by a proline-directed Ser/Thr kinase present in normal or pathological human brain. Extending earlier results on MAP kinase, we show here that the proline-directed kinase, GSK3, can induce an Alzheimer-like immune response involving several distinct and phosphorylatable epitopes at Ser-Pro motifs, as well as a gel mobility shift, similar to MAP kinase. Both kinases behave like microtubule-associated proteins in that they co-purify through cycles of assembly and disassembly, and both kinases are directly associated with paired helical filaments.     

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.     

Glycogen synthase kinase 3beta phosphorylates tau at both primed and unprimed sites. Differential impact on microtubule binding

Glycogen synthase kinase 3beta (GSK3beta) phosphorylates substrates, including the microtubule-associated protein tau, at both primed and unprimed epitopes. GSK3beta phosphorylation of tau negatively regulates tau-microtubule interactions; however the differential effects of phosphorylation at primed and unprimed epitopes on tau is unknown. To examine the phosphorylation of tau at primed and unprimed epitopes and how this impacts tau function, the R96A mutant of GSK3beta was used, a mutation that prevents phosphorylation of substrates at primed sites. Both GSK3beta and GSK3beta-R96A phosphorylated tau efficiently in situ. However, expression of GSK3beta-R96A resulted in significantly less phosphorylation of tau at primed sites compared with GSK3beta. Conversely, GSK3beta-R96A phosphorylated unprimed tau sites to a significantly greater extent than GSK3beta. Prephosphorylating tau with cdk5/p25 impaired the ability of GSK3beta-R96A to phosphorylate tau, whereas GSK3beta-R96A phosphorylated recombinant tau to a significantly greater extent than GSK3beta. Moreover, the amount of tau associated with microtubules was reduced by overexpression of GSK3beta but only when tau was phosphorylated at primed sites, as phosphorylation of tau by GSK3beta-R96A did not negatively regulate the association of tau with microtubules. These results demonstrate that GSK3beta-mediated phosphorylation of tau at primed sites plays a more significant role in regulating the interaction of tau with microtubules than phosphorylation at unprimed epitopes.     

Glycogen synthase kinase-3beta phosphorylates protein tau and rescues the axonopathy in the central nervous system of human four-repeat tau transgenic mice

Protein tau filaments in brain of patients suffering from Alzheimer's disease, frontotemporal dementia, and other tauopathies consist of protein tau that is hyperphosphorylated. The responsible kinases operating in vivo in neurons still need to be identified. Here we demonstrate that glycogen synthase kinase-3beta (GSK-3beta) is an effective kinase for protein tau in cerebral neurons in vivo in adult GSK-3beta and GSK-3beta x human tau40 transgenic mice. Phosphorylated protein tau migrates slower during electrophoretic separation and is revealed by phosphorylation-dependent anti-tau antibodies in Western blot analysis. In addition, its capacity to bind to re-assembled paclitaxel (Taxol((R)))-stabilized microtubules is reduced, compared with protein tau isolated from mice not overexpressing GSK-3beta. Co-expression of GSK-3beta reduces the number of axonal dilations and alleviates the motoric impairment that was typical for single htau40 transgenic animals (Spittaels, K., Van den Haute, C., Van Dorpe, J., Bruynseels, K., Vandezande, K., Laenen, I., Geerts, H., Mercken, M., Sciot, R., Van Lommel, A., Loos, R., and Van Leuven, F. (1999) Am. J. Pathol. 155, 2153-2165). Although more hyperphosphorylated protein tau is available, neither an increase in insoluble protein tau aggregates nor the presence of paired helical filaments or tangles was observed. These findings could have therapeutic implications in the field of neurodegeneration, as discussed.     

Alzheimer's disease-like phosphorylation of the microtubule-associated protein tau by glycogen synthase kinase-3 in transfected mammalian cells

Background: Paired helical filaments (PHFs) are a characteristic pathological feature of Alzheimer's disease; their principal component is the microtubule-associated protein tau. The tau in PHFs (PHF-tau) is hyperphosphorylated, but the cellular mechanisms responsible for this hyperphosphorylation have yet to be elucidated. A number of kinases, including mitogen-activated protein (MAP) kinase, glycogen synthase kinase (GSK)-3α, GSK-3β and cyclin-dependent kinase-5, phosphorylate recombinant tau in vitro so that it resembles PHF-tau as judged by its reactivity with a panel of antibodies capable of discriminating between normal tau and PHF-tau, and by a reduced electrophoretic mobility that is characteristic of PHF-tau. To determine whether MAP kinase, GSK-3α and GSK-3β can also induce Alzheimer's disease-like phosphorylation of tau in mammalian cells, we studied the phosphorylation status of tau in primary neuronal cultures and transfected COS cells following changes in the activities of MAP kinase and GSK-3.Results Activating MAP kinase in cultures of primary neurons or transfected COS cells expressing tau isoforms did not increase the level of phosphorylation for any PHF-tau epitope investigated. But elevating GSK-3 activity in the COS cells by co-transfection with GSK-3α or GSK-3β decreased the electrophoretic mobility of tau so that it resembled that of PHF-tau, and induced reactivity with eight PHF-tau-selective monoclonal antibodies.Conclusion Our data indicate that GSK-3α and/or GSK-3β, but not MAP kinase, are good candidates for generating PHF-type phosphorylation of tau in Alzheimer's disease. The involvement of other kinases in the generation of PHFs cannot, however, be eliminated. Our results suggest that aberrant regulation of GSK-3 may be a pathogenic mechanism in Alzheimer's disease.     

Glycogen synthase kinase-3beta is complexed with tau protein in brain microtubules.

In Alzheimer's disease, microtubule-associated protein tau is hyperphosphorylated by an unknown mechanism and is aggregated into paired helical filaments. Hyperphosphorylation causes loss of tau function, microtubule instability, and neurodegeneration. Glycogen synthase kinase-3beta (GSK3beta) has been implicated in the phosphorylation of tau in normal and Alzheimer's disease brain. The molecular mechanism of GSK3beta-tau interaction has not been clarified. In this study, we find that when microtubules are disassembled, microtubule-associated GSK3beta dissociates from microtubules. From a gel filtration column, the dissociated GSK3beta elutes as an approximately 400-kDa complex. When fractions containing the approximately 400-kDa complex are chromatographed through an anti-GSK3beta immunoaffinity column, tau co-elutes with GSK3beta. From fractions containing the approximately 400-kDa complex, both tau and GSK3beta co-immunoprecipitate with each other. GSK3beta binds to nonphosphorylated tau, and the GSK3beta-binding region is located within the N-terminal projection domain of tau. In vitro, GSK3beta associates with microtubules only in the presence of tau. From brain extract, approximately 6-fold more GSK3beta co-immunoprecipitates with tau than GSK3alpha. These data indicate that, in brain, GSK3beta is bound to tau within a approximately 400-kDa microtubule-associated complex, and GSK3beta associates with microtubules via tau.     

Glycogen synthase kinase 3 phosphorylates RBL2/p130 during quiescence

Phosphorylation of the retinoblastoma-related or pocket proteins RB1/pRb, RBL1/p107, and RBL2/p130 regulates cell cycle progression and exit. While all pocket proteins are phosphorylated by cyclin-dependent kinases (CDKs) during the G1/S-phase transition, p130 is also specifically phosphorylated in G0-arrested cells. We have previously identified several phosphorylated residues that match the consensus site for glycogen synthase kinase 3 (GSK3) in the G0 form of p130. Using small-molecule inhibitors of GSK3, site-specific mutants of p130, and phospho-specific antibodies, we demonstrate here that GSK3 phosphorylates p130 during G0. Phosphorylation of p130 by GSK3 contributes to the stability of p130 but does not affect its ability to interact with E2F4 or cyclins. Regulation of p130 by GSK3 provides a novel link between growth factor signaling and regulation of the cell cycle progression and exit.     

Glycogen synthase kinase 3beta is tyrosine phosphorylated by PYK2

Glycogen synthase kinase 3beta (GSK3beta) is a Ser/Thr kinase that is involved in numerous cellular activities. GSK3beta is activated by tyrosine phosphorylation. However, very little is known about the tyrosine kinases that are responsible for phosphorylating GSK3beta. In this report, we investigated the ability of the calcium-dependent tyrosine kinase, proline-rich tyrosine kinase 2 (PYK2) to tyrosine phosphorylate GSK3beta. In transfected CHO cells, it was demonstrated that PYK2 tyrosine phosphorylates GSK3beta in situ. The two kinases also coimmunoprecipitated. Furthermore, GSK3beta was tyrosine phosphorylated in vitro by an active, wild type PYK2, but not by the inactive, kinase dead form of PYK2. Therefore, this study is the first to demonstrate that GSK3beta is a substrate of PYK2 both in vitro and in situ.     

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.     

14-3-3 connects glycogen synthase kinase-3 beta to tau within a brain microtubule-associated tau phosphorylation complex

In a recent study, we reported that in bovine brain extract, glycogen synthase kinase-3beta and tau are parts of an approximately 400-500 kDa microtubule-associated tau phosphorylation complex (Sun, W., Qureshi, H. Y., Cafferty, P. W., Sobue, K., Agarwal-Mawal, A., Neufield, K. D., and Paudel, H. K. (2002) J. Biol. Chem. 277, 11933-11940). In this study, we find that when purified brain microtubules are subjected to Superose 12 gel filtration column chromatography, the dimeric scaffold protein 14-3-3 zeta co-elutes with the tau phosphorylation complex components tau and GSK3 beta. From gel filtration fractions containing the tau phosphorylation complex, 14-3-3 zeta, GSK3 beta, and tau co-immunoprecipitate with each other. From extracts of bovine brain, COS-7 cells, and HEK-293 cells transfected with GSK3 beta, 14-3-3 zeta co-precipitates with GSK3 beta, indicating that GSK3 beta binds to 14-3-3 zeta. From HEK-293 cells transfected with tau, GSK3 beta, and 14-3-3 zeta in different combinations, tau co-immunoprecipitates with GSK3 beta only in the presence of 14-3-3 zeta. In vitro, approximately 10-fold more tau binds to GSK3 beta in the presence of than in the absence of 14-3-3 zeta. In transfected HEK-293 cells, 14-3-3 zeta stimulates GSK3 beta-catalyzed tau phosphorylation in a dose-dependent manner. These data indicate that in brain, the 14-3-3 zeta dimer simultaneously binds and bridges tau and GSK3 beta and stimulates GSK3 beta-catalyzed tau phosphorylation.     

Glycogen synthase kinase 3 phosphorylates kinesin light chains and negatively regulates kinesin-based motility

Membrane-bounded organelles (MBOs) are delivered to different domains in neurons by fast axonal transport. The importance of kinesin for fast antero grade transport is well established, but mechanisms for regulating kinesin-based motility are largely unknown. In this report, we provide biochemical and in vivo evidence that kinesin light chains (KLCs) interact with and are in vivo substrates for glycogen synthase kinase 3 (GSK3). Active GSK3 inhibited anterograde, but not retrograde, transport in squid axoplasm and reduced the amount of kinesin bound to MBOs. Kinesin microtubule binding and microtubule-stimulated ATPase activities were unaffected by GSK3 phosphorylation of KLCs. Active GSK3 was also localized preferentially to regions known to be sites of membrane delivery. These data suggest that GSK3 can regulate fast anterograde axonal transport and targeting of cargos to specific subcellular domains in neurons.     

Akt/PKB kinase phosphorylates separately Thr212 and Ser214 of tau protein in vitro

Microtubule-associated protein tau contains a consensus motif for protein kinase B/Akt (Akt), which plays an essential role in anti-apoptotic signaling. The motif encompasses the AT100 double phospho-epitope (Thr212/Ser214), a specific marker for Alzheimer's disease (AD) and other neurodegenerations, raising the possibility that it could be generated by Akt. We studied Akt-dependent phosphorylation of tau protein in vitro. We found that Akt phosphorylated both Thr212 and Ser214 in the longest and shortest tau isoforms as determined using phospho site-specific antibodies against tau. Akt did not phosphorylate other tau epitopes, including Tau-1, AT8, AT180, 12E8 and PHF-1. The Akt-phosphorylated tau retained its initial electrophoretic mobility. Immunoprecipitation studies with phospho-specific Thr212 and Ser214 antibodies revealed that only one of the two sites is phosphorylated per single tau molecule, resulting in tau immunonegative for AT100. Mixed kinase studies showed that prior Ser214 phosphorylation by Akt blocked protein kinase A but not GSK3beta activity. On the other hand, GSK3beta selectively blocked Ser214 phosphorylation, which was prevented by lithium. The results suggest that Akt may be involved in AD-specific phosphorylation of tau at the AT100 epitope in conjunction with other kinases. Our data suggest that phosphorylation of tau by Akt may play specific role(s) in Akt-mediated anti-apoptotic signaling, particularly relevant to AD and other neurodegenerations.     

Glycogen synthase kinase 3beta phosphorylates p21WAF1/CIP1 for proteasomal degradation after UV irradiation

UV irradiation has been reported to induce p21(WAF1/CIP1) protein degradation through a ubiquitin-proteasome pathway, but the underlying biochemical mechanism remains to be elucidated. Here, we show that ser-114 phosphorylation of p21 protein by glycogen synthase kinase 3beta (GSK-3beta) is required for its degradation in response to UV irradiation and that GSK-3beta activation is a downstream event in the ATR signaling pathway triggered by UV. UV transiently increased GSK-3beta activity, and this increase could be blocked by caffeine or by ATR small interfering RNA, indicating ATR-dependent activation of GSK-3beta. ser-114, located within the putative GSK-3beta target sequence, was phosphorylated by GSK-3beta upon UV exposure. The nonphosphorylatable S114A mutant of p21 was protected from UV-induced destabilization. Degradation of p21 protein by UV irradiation was independent of p53 status and prevented by proteasome inhibitors. In contrast to the previous report, the proteasomal degradation of p21 appeared to be ubiquitination independent. These data show that GSK-3beta is activated by UV irradiation through the ATR signaling pathway and phosphorylates p21 at ser-114 for its degradation by the proteasome. To our knowledge, this is the first demonstration of GSK-3beta as the missing link between UV-induced ATR activation and p21 degradation.     

The function of the microtubule-associated protein tau is variably modulated by graded changes in glycogen synthase kinase-3beta activity

The microtubule-associated protein tau favors microtubule nucleation and stabilization and plays a role in the elongation of axons. We have investigated the ability of glycogen synthase kinase-3beta (GSK-3beta) to control tau-induced processes outgrowth. Tau-transfected Chinese hamster ovary (CHO) cells developed processes containing microtubule bundles after cytochalasin treatment, but a significant reduction in the number of cells harboring processes was observed in tau/GSK-3beta-co-transfected cells. Lithium, an inhibitor of GSK-3beta, counteracted in a dose-dependent manner this inhibitory effect of GSK-3beta. These findings suggest that GSK-3beta modulates in a graded manner the ability of tau to control the microtubule-dependent induction of cell processes.