Regulation and function of glycogen synthase kinase-3 isoforms in neuronal survival

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase consisting of two isoforms, alpha and beta. The activities of GSK-3 are regulated negatively by serine phosphorylation but positively by tyrosine phosphorylation. GSK-3 inactivation has been proposed as a mechanism to promote neuronal survival. We used GSK-3 isoform-specific small interfering RNAs, dominant-negative mutants, or pharmacological inhibitors to search for functions of the two GSK-3 isoforms in regulating neuronal survival in cultured cortical neurons in response to glutamate insult or during neuronal maturation/aging. Surprisingly, RNA interference-induced depletion of either isoform was sufficient to block glutamate-induced excitotoxicity, and the resulting neuroprotection was associated with enhanced N-terminal serine phosphorylation in both GSK-3 isoforms. However, GSK-3beta depletion was more effective than GSK-3alpha depletion in suppressing spontaneous neuronal death in extended culture. This phenomenon is likely due to selective and robust inhibition of GSK-3beta activation resulting from GSK-3beta Ser9 dephosphorylation during the course of spontaneous neuronal death. GSK-3alpha silencing resulted in reduced tyrosine phosphorylation of GSK-3beta, suggesting that tyrosine phosphorylation is also a critical autoregulatory event. Interestingly, GSK-3 inhibitors caused a rapid and long-lasting increase in GSK-3alpha Ser21 phosphorylation levels, followed by a delayed increase in GSK-3beta Ser9 phosphorylation and a decrease in GSK-3alpha Tyr279 and GSK-3beta Tyr216 phosphorylation, thus implying additional levels of GSK-3 autoregulation. Taken together, our results underscore important similarities and dissimilarities of GSK-3alpha and GSK-3beta in the roles of cell survival as well as their distinct modes of regulation. The development of GSK-3 isoform-specific inhibitors seems to be warranted for treating GSK-3-mediated pathology.     

Both the establishment and the maintenance of neuronal polarity require active mechanisms: critical roles of GSK-3beta and its upstream regulators

Axon-dendrite polarity is a cardinal feature of neuronal morphology essential for information flow. Here we report a differential distribution of GSK-3beta activity in the axon versus the dendrites. A constitutively active GSK-3beta mutant inhibited axon formation, whereas multiple axons formed from a single neuron when GSK-3beta activity was reduced by pharmacological inhibitors, a peptide inhibitor, or siRNAs. An active mechanism for maintaining neuronal polarity was revealed by the conversion of preexisting dendrites into axons upon GSK-3 inhibition. Biochemical and functional data show that the Akt kinase and the PTEN phosphatase are upstream of GSK-3beta in determining neuronal polarity. Our results demonstrate that there are active mechanisms for maintaining as well as establishing neuronal polarity, indicate that GSK-3beta relays signaling from Akt and PTEN to play critical roles in neuronal polarity, and suggest that application of GSK-3beta inhibitors can be a novel approach to promote generation of new axons after neural injuries.     

GSK-3beta inhibitors reduce protein degradation in muscles from septic rats and in dexamethasone-treated myotubes

Sepsis is associated with muscle wasting, mainly reflecting increased muscle proteolysis. Recent studies suggest that inhibition of GSK-3beta activity may counteract catabolic stimuli in skeletal muscle. We tested the hypothesis that treatment of muscles from septic rats with the GSK-3beta inhibitors LiCl and TDZD-8 would reduce sepsis-induced muscle proteolysis. Because muscle wasting during sepsis is, at least in part, mediated by glucocorticoids, we also tested the effects of GSK-3beta inhibitors on protein degradation in dexamethasone-treated cultured myotubes. Treatment of incubated extensor digitorum longus muscles with LiCl or TDZD-8 reduced basal and sepsis-induced protein breakdown rates. When cultured myotubes were treated with LiCl or one of the GSK-3beta inhibitors SB216763 or SB415286, protein degradation was reduced. Treatment of incubated muscles or cultured myotubes with LiCl, but not the other GSK-3beta inhibitors, resulted in increased phosphorylation of GSK-3beta at Ser9, consistent with inactivation of the kinase and suggesting that the other inhibitors used in the present experiments inhibit GSK-3beta by phosphorylation-independent mechanisms. The present results suggest that GSK-3beta inhibitors may be used to prevent or treat sepsis-induced, glucocorticoid-regulated muscle proteolysis.     

Structural characterization of the GSK-3beta active site using selective and non-selective ATP-mimetic inhibitors

GSK-3beta is a regulatory serine/threonine kinase with a plethora of cellular targets. Consequently, selective small molecule inhibitors of GSK-3beta may have a variety of therapeutic uses including the treatment of neurodegenerative diseases, type II diabetes and cancer. In order to characterize the active site of GSK-3beta, we determined crystal structures of unphosphorylated GSK-3beta in complex with selective and non-selective ATP-mimetic inhibitors. Analysis of the inhibitors' interactions with GSK-3beta in the structures reveals how the enzyme can accommodate a number of diverse molecular scaffolds. In addition, a conserved water molecule near Thr138 is identified that can serve a functional role in inhibitor binding. Finally, a comparison of the interactions made by selective and non-selective inhibitors highlights residues on the edge of the ATP binding-site that can be used to obtain inhibitor selectivity. Information gained from these structures provides a promising route for the design of second-generation GSK-3beta inhibitors.     

Indirubins inhibit glycogen synthase kinase-3 beta and CDK5/p25, two protein kinases involved in abnormal tau phosphorylation in Alzheimer's disease. A property common to most cyclin-dependent kinase inhibitors?

The bis-indole indirubin is an active ingredient of Danggui Longhui Wan, a traditional Chinese medicine recipe used in the treatment of chronic diseases such as leukemias. The antitumoral properties of indirubin appear to correlate with their antimitotic effects. Indirubins were recently described as potent (IC(50): 50-100 nm) inhibitors of cyclin-dependent kinases (CDKs). We report here that indirubins are also powerful inhibitors (IC(50): 5-50 nm) of an evolutionarily related kinase, glycogen synthase kinase-3beta (GSK-3 beta). Testing of a series of indoles and bis-indoles against GSK-3 beta, CDK1/cyclin B, and CDK5/p25 shows that only indirubins inhibit these kinases. The structure-activity relationship study also suggests that indirubins bind to GSK-3 beta's ATP binding pocket in a way similar to their binding to CDKs, the details of which were recently revealed by crystallographic analysis. GSK-3 beta, along with CDK5, is responsible for most of the abnormal hyperphosphorylation of the microtubule-binding protein tau observed in Alzheimer's disease. Indirubin-3'-monoxime inhibits tau phosphorylation in vitro and in vivo at Alzheimer's disease-specific sites. Indirubins may thus have important implications in the study and treatment of neurodegenerative disorders. Indirubin-3'-monoxime also inhibits the in vivo phosphorylation of DARPP-32 by CDK5 on Thr-75, thereby mimicking one of the effects of dopamine in the striatum. Finally, we show that many, but not all, reported CDK inhibitors are powerful inhibitors of GSK-3 beta. To which extent these GSK-3 beta effects of CDK inhibitors actually contribute to their antimitotic and antitumoral properties remains to be determined. Indirubins constitute the first family of low nanomolar inhibitors of GSK-3 beta to be described.     

Recruitment of active glycogen synthase kinase-3 into neuronal lipid rafts

Glycogen synthase kinase (GSK)-3beta has emerged as a key molecule that regulates neuronal apoptosis. To examine the molecular mechanism(s) through which GSK-3beta regulates this process, we studied the subcellular localization of GSK-3beta following exposure of the cells to well-characterized apoptotic stimuli. Here, we report that the induction of apoptosis by withdrawal of serum and potassium triggers dephosphorylation of GSK-3beta at serine 9 and subsequent translocation of these molecules into neuronal lipid raft microdomains. Inhibition of GSK-3beta by small molecule inhibitors blocks specific phosphorylation of lipid raft associated protein Tau. Consistent with the notion that the lipid raft domains may serve as a platform for the cellular signaling complexes, disruption of lipid rafts protected neurons from apoptosis induced by withdrawal of serum and potassium as well as by HIV-1 Tat. Our observations reveal novel interaction of GSK-3beta and raft domains, and suggest that such interaction could contribute to neuronal apoptosis.     

Flow cytometry-based method to analyze the change in Tau phosphorylation in a hGSK-3beta and hTau over-expressing EcR-293 cell line

Neurofibrillary tangles are composed of insoluble aggregates of microtubule-associated protein Tau. In the pathology of Alzheimer's disease (AD), accumulation of hyperphosphorylated Tau results in formation of paired helical filaments. One of the main candidate to hyperphosphorylate Tau in AD is glycogen synthase kinase 3beta (GSK-3beta). Here we introduce a non-neuronal cell line, stably co-expressing human Tau and GSK-3beta proteins, where the effect of potential kinase inhibitors on Tau phosphorylation can be monitored. The aim of our study was to establish a new flow-cytometry-based method to quantitatively analyze the changing of Tau phosphorylation, which is a suitable alternative to the well-accepted but non-quantitative Western blot technique. Our results demonstrate that the flow cytometry-based method is a convenient tool to analyze the effect of GSK-3beta inhibitors on Tau phosphorylation. This new approach provides appropriate throughput for screening purposes in preclinical research for characterization of GSK-3beta inhibitors, as potential drug candidate to cure Alzheimer's disease.     

Rational design of 4-amino-5,6-diaryl-furo[2,3-d]pyrimidines as potent glycogen synthase kinase-3 inhibitors

4-Amino-5,6-diaryl-furo[2,3-d]pyrimidines have been identified as inhibitors of glycogen synthase kinase-3beta (GSK-3beta). One representative derivative, 4-amino-3-(4-(benzenesulfonylamino)-phenyl)-2-(3-pyridyl)-furo[2,3-d]pyrimidine (12) exhibited potent GSK-3beta inhibitory activity in low nanomolar level of IC(50). The binding mode was proposed from a docking study.     

Glycogen synthase kinase-3 beta activity is critical for neuronal death caused by inhibiting phosphatidylinositol 3-kinase or Akt but not for death caused by nerve growth factor withdrawal

Numerous studies reveal that phosphatidylinositol (PI) 3-kinase and Akt protein kinase are important mediators of cell survival. However, the survival-promoting mechanisms downstream of these enzymes remain uncharacterized. Glycogen synthase kinase-3 beta (GSK-3 beta), which is inhibited upon phosphorylation by Akt, was recently shown to function during cell death induced by PI 3-kinase inhibitors. In this study, we tested whether GSK-3 beta is critical for the death of sympathetic neurons caused by the withdrawal of their physiological survival factor, the nerve growth factor (NGF). Stimulation with NGF resulted in PI 3-kinase-dependent phosphorylation of GSK-3 beta and inhibition of its protein kinase activity, indicating that GSK-3 beta is targeted by PI 3-kinase/Akt in these neurons. Expression of the GSK-3 beta inhibitor Frat1, but not a mutant Frat1 protein that does not bind GSK-3 beta, rescued neurons from death caused by inhibiting PI 3-kinase. Similarly, expression of Frat1 or kinase-deficient GSK-3 beta reduced death caused by inhibiting Akt. In NGF-maintained neurons, overexpression of GSK-3 beta caused a small but significant decrease in survival. However, expression of neither Frat1, kinase-deficient GSK-3 beta, nor GSK-3-binding protein inhibited NGF withdrawal-induced death. Thus, although GSK-3 beta function is required for death caused by inactivation of PI 3-kinase and Akt, neuronal death caused by NGF withdrawal can proceed through GSK-3 beta-independent pathways.     

Erythropoietin protects cardiac myocytes against anthracycline-induced apoptosis

The cardiotoxic adverse effects of anthracycline antibiotics limit their therapeutic utility as essential components of chemotherapy regimens for hematologic and solid malignancies. Here we show that the hematopoietic cytokine erythropoietin attenuates doxorubicin-induced apoptosis of primary neonatal rat ventricular cardiomyocytes in a dose-dependent manner. Erythropoietin treatment induced rapid, time-dependent phosphorylation of MAP kinases (MAPK) Erk1/2 and the phosphatidylinositol 3-kinase substrate Akt. Treatment of cardiomyocytes with inhibitors of phosphatidylinositol 3-kinase (LY294002) or Akt (Akti-1/2) abolished the protective effect of erythropoietin, whereas treatment with MAPK kinase (MEK1) inhibitor U0126 did not. Erythropoietin also induced the phosphorylation of GSK-3beta, a downstream target of PI3K-Akt. Because phosphorylation is known to inactivate GSK-3beta, we investigated whether GSK-3beta inhibition is cardioprotective. We found that GSK-3beta inhibitors SB216763 or lithium chloride blocked doxorubicin-induced cardiomyocyte apoptosis in a manner similar to erythropoietin, suggesting that GSK-3beta inhibition is involved in erythropoietin-mediated cardioprotection. Erythropoietin may serve as a novel cardioprotective agent against anthracycline-induced cardiotoxicity.     

Biosynthesis of new indigoid inhibitors of protein kinases using recombinant cytochrome P450 2A6

Glycogen synthase kinase-3 (GSK-3) is a potential drug target for a number of human diseases. Some indigoids have been found to be potent inhibitors of GSK-3, and individual compounds with better activity, specificity, and solubility are desired. In this work, a new disubstituted indigoid generation system was developed with a tryptophanase-deficient Escherichia coli strain as a host to express the human cytochrome P450 2A6 mutant L240C/N297Q, which catalyzes the oxidation of indole to isatin and indoxyl, which in turn react to generate indigoids. Forty-five substituted 1H-indoles from commercial sources were used as substrates in the system, and indigoid mixtures were tested as potential inhibitors of GSK-3. After preliminary screening, cell extracts with high inhibitory activity towards GSK-3alpha/beta were fractionated, and the IC50 values of twelve individual indigoids were measured for GSK-3alpha/beta as well as the protein kinases CDK1/cyclinB and CDK5/p25. Several indigoids, including an indigo, showed stronger inhibition than found in previous work. The most potent towards GSK-3alpha/beta, dimethyl indirubin 5,5'-dicarboxylate (IC50 of 51 nM), was modified by chemical reactions. One product, indirubin 5,5'-dicarboxylic acid 5-methyl ester, inhibited GSK-3alpha/beta with an IC50 of 14 nM and selectivity nearly 40-fold over CDK1 and CDK5. Indirubin-5-5'-dicarbonitrile was also modified to the corresponding 3'-oxime, which had low specificity but showed very high inhibition of all three kinases with IC50 values of 5, 13, and 10 nM towards GSK-3alpha/beta, CDK1, and CDK5, respectively. Thus, this system has the potential to generate new indigoids with therapeutic potential.     

Glycogen synthase kinase-3β positively regulates the proliferation of human ovarian cancer cells

Although glycogen synthase kinase-3 （GSK-3） might act as a tumor suppressor since its inhibition is expected to mimic the activation of Wnt-signaling pathway, GSK-3β may contribute to NF-κB activation in cancer cells leading to increased cancer cell proliferation and survival. Here we report that GSK-3β activity was involved in the proliferation of human ovarian cancer cell both in vitro and in vivo. Inhibition of GSK-3 activity by pharmacological inhibitors suppressed proliferation of the ovarian cancer cells. Overexpressing constitutively active form of GSK-3β induced entry into the S phase, increased cyclin D1 expression and facilitated the proliferation of ovarian cancer cells. Furthermore, GSK-3 inhibition prevented the formation of the tumor in nude mice generated by the inoculation of human ovarian cancer cells. Our findings thus suggest that GSK-3β activity is important for the proliferation of ovarian cancer cells, implicating this kinase as a potential therapeutic target in ovarian cancer.     

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.     

Inhibition of alanyl-aminopeptidase suppresses the activation-dependent induction of glycogen synthase kinase-3beta (GSK-3beta) in human T cells

Inhibition of alanyl-aminopeptidase (APN, CD13) gene expression or enzymatic activity compromises T cell proliferation and function. Molecular mechanisms mediating these effects are not known as yet. Recently, we found the expression of the proto-oncogen Wnt-5a to be strongly affected by APN-inhibition. Wnt-5a and other members of the Wnt family of secreted factors are implicated in cell growth and differentiation. Here, we analyzed by quantitative RT-PCR and immunoblotting the expression in mitogen-activated T cells of a major constituent of the Wnt-5a pathway, glycogen synthase kinase-3beta (GSK-3beta). T cell activation by phytohaemagglutinin or pokeweed mitogen results in a strong increase of GSK-3beta mRNA amounts. At the protein level, we observed an up-regulation of both GSK-3beta and phosphorylated GSK-3beta. This induction-dependent increase of GSK-3beta is markedly reduced in response to inhibitors of alanyl-aminopeptidase, actinonin, leuhistin, and RB3014. These findings may provide a rational for the growth inhibition resulting from a diminished expression or activity of alanyl aminopeptidase.     

Regulation of Glycogen Synthase Kinase 3β and Downstream Wnt Signaling by Axin

Axin is a recently identified protein encoded by the fused locus in mice that is required for normal vertebrate axis formation. We have defined a 25-amino-acid sequence in axin that comprises the glycogen synthase kinase 3beta (GSK-3beta) interaction domain (GID). In contrast to full-length axin, which has been shown to antagonize Wnt signaling, the GID inhibits GSK-3beta in vivo and activates Wnt signaling. Similarly, mutants of axin lacking key regulatory domains such as the RGS domain, which is required for interaction with the adenomatous polyposis coli protein, bind and inhibit GSK-3beta in vivo, suggesting that these domains are critical for proper regulation of GSK-3beta activity. We have identified a novel self-interaction domain in axin and have shown that formation of an axin regulatory complex in vivo is critical for axis formation and GSK-3beta activity. Based on these data, we propose that the axin complex may directly regulate GSK-3beta enzymatic activity in vivo. These observations also demonstrate that alternative inhibitors of GSK-3beta can mimic the effect of lithium in developing Xenopus embryos.     

Glycogen synthase kinase 3alpha-specific regulation of murine hepatic glycogen metabolism

Glycogen synthase kinase 3 comprises two isoforms (GSK-3alpha and GSK-3beta) that are implicated in type II diabetes, neurodegeneration, and cancer. GSK-3 activity is elevated in human and rodent models of diabetes, and various GSK-3 inhibitors improve glucose tolerance and insulin sensitivity in rodent models of obesity and diabetes. Here, we report the generation of mice lacking GSK-3alpha. Unlike GSK-3beta mutants, which die before birth, GSK-3alpha knockout (GSK-3alpha KO) animals are viable but display enhanced glucose and insulin sensitivity accompanied by reduced fat mass. Fasted and glucose-stimulated hepatic glycogen content was enhanced in GSK-3alpha KO mice, whereas muscle glycogen was unaltered. Insulin-stimulated protein kinase B (PKB/Akt) and GSK-3beta phosphorylation was higher in GSK-3alpha KO livers compared to wild-type littermates, and IRS-1 expression was markedly increased. We conclude that GSK-3 isoforms exhibit tissue-specific physiological functions and that GSK-3alpha KO mice are insulin sensitive, reinforcing the potential of GSK-3 as a therapeutic target for type II diabetes.     

Spatial learning deficit in transgenic mice that conditionally over-express GSK-3beta in the brain but do not form tau filaments

Deregulation of glycogen synthase kinase-3 (GSK-3) activity in neurones has been postulated as a key feature in Alzheimer's disease (AD) pathogenesis. This was further supported by our recent characterization of transgenic mice that conditionally over-express GSK-3beta in hippocampal and cortical neurones. These mice, designated Tet/GSK-3beta, showed many of the biochemical and cellular aspects of AD neuropathology such as tau hyperphosphorylation and somatodendritic localization, decreased nuclear beta-catenin, neuronal death and reactive gliosis. Tet/GSK-3beta mice, however, did not show tau filament formation up to the latest tested age of 3 months at least. Here we report spatial learning deficits of Tet/GSK-3beta mice in the Morris water maze. In parallel, we also measured the increase in GSK-3 activity while further exploring the possibility of tau filament formation in aged mice. We found a significant increase in GSK-3 activity in the hippocampus of Tet/GSK-3beta mice whereas no tau fibrils could be found even in very old mice. These data reinforce the hypothesis of GSK-3 deregulation in AD pathogenesis, and suggest that Tet/GSK-3beta mice can be used as an AD model and, most remarkably, can be used to test the therapeutic potential of the selective GSK-3 inhibitors that are currently under development. Additionally, these experiments suggest that destabilization of microtubules and alteration of intracellular metabolic pathways contribute to AD pathogenesis independent of toxicity triggered by the aberrant tau deposits.     

GSK-3beta activity is increased in skeletal muscle after burn injury in rats

Previous reports suggest that burn-induced muscle proteolysis can be inhibited by treatment with GSK-3beta inhibitors, suggesting that burn injury may be associated with increased GSK-3beta activity. The influence of burn injury on muscle GSK-3beta activity, however, is not known. We determined the effect of a 30% total body surface full-thickness burn injury in rats on muscle GSK-3beta activity by measuring GSK-3beta activity and tissue levels of serine 9 phosphorylated GSK-3beta, p(Ser9)-GSK-3beta, by Western blot analysis and immunohistochemistry. Because burn-induced muscle wasting is, at least in part, mediated by glucocorticoids, we used dexamethasone-treated cultured muscle cells in which GSK-3beta expression was reduced with small interfering RNA (siRNA) to further assess the role of GSK-3beta in muscle atrophy. Burn injury resulted in a seven-fold increase in GSK-3beta activity in skeletal muscle. This effect of burn was accompanied by reduced tissue levels of p(Ser9)-GSK-3beta, suggesting that burn injury stimulates GSK-3beta in skeletal muscle secondary to inhibited phosphorylation of the enzyme. In addition, burn injury resulted in inhibited phosphorylation and activation of Akt, an upstream regulatory mechanism of GSK-3beta activity. Reducing the expression of GSK-3beta in cultured muscle cells with siRNA inhibited dexamethasone-induced protein degradation by approximately 50%. The results suggest that burn injury stimulates GSK-3beta activity in skeletal muscle and that GSK-3beta may, at least in part, regulate glucocorticoid-mediated muscle wasting.