Inhibitory effect of polycations on phosphorylation of glycogen synthase by glycogen synthase kinase 3

Several polycations were tested for their abilities to inhibit the activity of glycogen synthase kinase 3 (GSK-3). L-Polylysine was the most powerful inhibitor of GSK-3 with half-maximal inhibition of glycogen synthase phosphorylation occurring at approx. 100 nM. D-Polylysine and histone H1 were also inhibitory, but the concentration dependence was complex, and DL-polylysine was the least effective inhibitor. Spermine caused about 50% inhibition of GSK-3 at 0.7 mM and 70% inhibition at 4 mM. Inhibition of GSK-3 by L-polylysine could be blocked or reversed by heparin. A heat-stable polycation antagonist isolated from swine kidney cortex also blocked the inhibitory effect of L-polylysine on GSK-3 and blocked histone H1 stimulation of protein phosphatase 2A activity. Under the conditions tested, L-polylysine also inhibited GSK-3 catalyzed phosphorylation of type II regulatory subunit of cAMP-dependent protein kinase and a 63 kDa brain protein, but only slightly inhibited phosphorylation of inhibitor 2 or proteolytic fragments of glycogen synthase that contain site 3 (a + b + c). L-Polylysine at a concentration (200 nM) that caused nearly complete inhibition of GSK-3 stimulated casein kinase I and casein kinase II, but had virtually no effect on the catalytic subunit of cAMP-dependent protein kinase. These results suggest that polycations can be useful in controlling GSK-3 activity. Polycations have the potential to decrease the phosphorylation state of glycogen synthase at site 3, both by inhibiting GKS-3 as shown in this study and by stimulating the phosphatase reaction as shown previously (Pelech, S. and Cohen, P. (1985) Eur. J. Biochem. 148, 245-251).     

Mechanism of zinc-induced phosphorylation of p70 S6 kinase and glycogen synthase kinase 3beta in SH-SY5Y neuroblastoma cells

We have previously reported an aberrant accumulation of activated protein kinase B (PKB), glycogen synthase kinase (GSK)-3beta, extracellular signal-regulated kinase (ERK1/2), c-Jun N-terminal kinase (JNK), p38 and p70 S6 kinase (p70S6K) in neurons bearing neurofibrillary tangles (NFTs) in Alzheimer's disease (AD). However, the mechanism by which these tau candidate kinases are involved in the regulation of p70S6K and GSK-3beta phosphorylation is unknown. In the current study, 100 microM zinc sulfate was used, and influences of various components of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways on p70S6K and GSK-3beta phosphorylation have been investigated in serum-deprived SH-SY5Y neuroblastoma cells. We found that zinc could induce an increase of phosphorylated (p) p70S6K, p-PKB, p-GSK-3beta, p-ERK1/2, p-JNK and p-p38, especially in long-term treatment (4-8 h). Treatment with different inhibitors including rapamycin, wortmannin, LY294002, and U0126, and their combinations, indicated that phosphorylation of p70S6K and GSK-3beta is regulated by rapamycin-dependent, PI3K and MAPK pathways. Furthermore, phosphorylation of p70S6K and GSK-3beta affected levels of tau unphosphorylated at the Tau-1 site and phosphorylated at the PHF-1 site, and p70S6K phosphorylation affected the total tau level. Thus, 100 microM zinc might activate PKB, GSK-3beta, ERK1/2, JNK, p38 and p70S6K, that are consequently involved in tau changes in SH-SY5Y cells.     

The roles of cyclin-dependent kinase 5 and glycogen synthase kinase 3 in tau hyperphosphorylation

Hyperphosphorylation of the microtubule-associated protein tau is a characteristic feature of neurodegenerative tauopathies including Alzheimer disease. Over-activation of proline-directed kinases, such as cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase 3 (GSK3), has been implicated in the aberrant phosphorylation of tau at proline-directed sites. In this study we tested the roles of Cdk5 and GSK3 in tau hyperphosphorylation in vivo using transgenic mice with p25-induced Cdk5 over-activation. We found that over-activation of Cdk5 in young transgenic animals does not induce tau hyperphosphorylation at sites recognized by the antibodies AT8, AT100, PHF-1, and TG3. In fact, we observed that Cdk5 over-activation leads to inhibition of GSK3. However, in old transgenic animals the inhibition of GSK3 is lost and results in increased GSK3 activity, which coincides with tau hyperphosphorylation at the AT8 and PHF-1 sites. Pharmacological inhibition of GSK3 in old transgenic mice by chronic treatment with lithium leads to a reduction of the age-dependent increase in tau hyperphosphorylation. Furthermore, we found that Cdk5, GSK3, and PP2A co-immunoprecipitate, suggesting a functional association of these molecules. Together, these results reveal the role of GSK3 as a key mediator of tau hyperphosphorylation, whereas Cdk5 acts as a modulator of tau hyperphosphorylation via the inhibitory regulation of GSK3. Furthermore, these findings suggest that disruption of regulation of GSK3 activity underlies tau hyperphosphorylation in neurodegenerative tauopathies. Hence, GSK3 may be a prime target for therapeutic intervention in tauopathies including Alzheimer disease.     

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.     

Stimulation of glycogen synthase phosphorylation by calcium-dependent regulator protein.

Phosphorylation of skeletal muscle glycogen synthase catalyzed by a protein kinase is stimulated up to 10-fold by the calcium-dependent regulator (CDR) protein. Half-maximal stimulation requires about 1 microgram of CDR/ml. Phosphorylation by the CDR-dependent synthase kinase is more rapid at pH 8.6 than at pH 6.8 and is blocked by ethylene glycol bis(beta-aminoethyl-ether)N,N'-tetraacetic acid and trifuloperazine. Approximately 60 to 70% of the phosphate is incorporated into the trypsin-insensitive region of glycogen synthase resulting in conversion of the a form to the b form of the enzyme. The CDR-dependent synthase kinase is not myosin light chain kinase, as this enzyme does not phosphorylate glycogen synthase. Furthermore, synthase phosphorylation by the cAMP-dependent protein kinase catalytic subunit is not affected by CDR. The possibility that CDR-dependent synthase kinase may be phosphorylase kinase is being investigated.     

Mck1, a member of the glycogen synthase kinase 3 family of protein kinases, is a negative regulator of pyruvate kinase in the yeast Saccharomyces cerevisiae.

An interaction between the Saccharomyces cerevisiae protein kinase Mck1 and pyruvate kinase (Pyk1) was detected by using the two-hybrid method. Purified Mck1 was able to phosphorylate purified Pyk1 on Ser in vitro. Pyruvate kinase activity was elevated in mck1 delta cells. Several of the phenotypes of mck1 delta mutants are similar to those observed in cells overexpressing PYK1. Co-overexpression of MCK1 suppressed all of the phenotypes associated with PYK1 overexpression. These results indicate that Mck1 negatively regulates pyruvate kinase activity, possibly by direct phosphorylation.     

Kinetic properties of glycogen synthase from skeletal muscle after phosphorylation by glycogen synthase kinase 4

Glycogen synthase I (EC 2.4.1.11) from rat and from rabbit skeletal muscle was phosphorylated in vitro by glycogen synthase kinase 4 (EC 2.7.1.37) to the extent of 0.8 phosphates/subunit. For both phosphorylated enzymes, the activity ratio (activity without glucose 6-P divided by activity with 8 mM glucose 6-P) was 0.8 when determined with low concentrations of glycogen synthase and/or short incubation times. However, the activity ratio was 0.5 with high enzyme concentrations and longer incubation times. It was found that the lower activity ratios result largely from UDP inhibition of activity measured in the absence of glucose 6-P. Inhibition by UDP was much less pronounced for glycogen synthase I, indicating that a major consequence of phosphorylation by glycogen synthase kinase 4 is an increased sensitivity to UDP inhibition.     

p90-RSK and Akt may promote rapid phosphorylation/inactivation of glycogen synthase kinase 3 in chemoattractant-stimulated neutrophils

Stimulation of neutrophils with the chemoattractant fMet-Leu-Phe (fMLP) triggers phosphorylation/inactivation of the a- and beta-isoforms of glycogen synthase kinase 3 (GSK-3) with phosphorylation of the alpha-isoform predominating. These reactions were monitored with a phosphospecific antibody that only recognized the alpha- or beta-isoforms of GSK-3 when these proteins were phosphorylated on serine residues 21 and 9, respectively. Inhibitor studies indicated that phosphorylation of GSK-3alpha may be catalyzed by the combined action of p90-RSK and Akt and may represent a new strategy by which G protein-coupled receptors inactivate GSK-3. Inactivation of GSK-3 may be one of the mechanisms that delay apoptosis in fMLP-stimulated neutrophils.     

Cyclic AMP promotes neuronal survival by phosphorylation of glycogen synthase kinase 3beta

Agents that elevate intracellular cyclic AMP (cAMP) levels promote neuronal survival in a manner independent of neurotrophic factors. Inhibitors of phosphatidylinositol 3 kinase and dominant-inactive mutants of the protein kinase Akt do not block the survival effects of cAMP, suggesting that another signaling pathway is involved. In this report, we demonstrate that elevation of intracellular cAMP levels in rat cerebellar granule neurons leads to phosphorylation and inhibition of glycogen synthase kinase 3beta (GSK-3beta). The increased phosphorylation of GSK-3beta by protein kinase A (PKA) occurs at serine 9, the same site phosphorylated by Akt. Purified PKA is able to phosphorylate recombinant GSK-3beta in vitro. Inhibitors of GSK-3 block apoptosis in these neurons, and transfection of neurons with a GSK-3beta mutant that cannot be phosphorylated interferes with the prosurvival effects of cAMP. These data suggest that activated PKA directly phosphorylates GSK-3beta and inhibits its apoptotic activity in neurons.     

Functional studies of shaggy/glycogen synthase kinase 3 phosphorylation sites in Drosophila melanogaster

Early studies of glycogen synthase kinase 3 (GSK-3) in mammalian systems focused on its pivotal role in glycogen metabolism and insulin-mediated signaling. It is now recognized that GSK-3 is central to a number of diverse signaling systems. Here, we show that the major form of the kinase Shaggy (Sgg), the GSK-3 fly ortholog, is negatively regulated during insulin-like/phosphatidylinositol 3-kinase (PI3K) signaling in vivo. Since genetic studies of Drosophila melanogaster had previously shown that Wingless (Wg) signaling also acts to antagonize Sgg, we investigate how the kinase might integrate, or else discriminate, signaling inputs by Wg and insulin. Using Drosophila cell line assays, we found, in contrast to previous reports, that Wg induces accumulation of its transducer Armadillo (Arm)/beta-catenin without significant alteration of global Sgg-specific activity. In agreement with a previous study using human GSK-3beta, Wg did not cause phosphorylation changes of the Ser9 or Tyr214 regulatory phosphorylated sites of Sgg. Conversely, as shown in mammalian systems, insulin-induced inhibition of Sgg-specific activity by phosphorylation at the N-terminal pseudosubstrate site (Ser9) did not induce Arm/beta-catenin accumulation, showing selectivity in response to the different signaling pathways. Interestingly, a minigene bearing a Ser9-to-Ala change rescued mutant sgg without causing abnormal development, suggesting that the regulation of Sgg via the inhibitory pseudosubstrate domain is dispensable for many aspects of its function. Our studies of Drosophila show that Wg and insulin or PI3K pathways do not converge on Sgg but that they exhibit cross-regulatory interactions.     

Image-based screening identifies novel roles for IkappaB kinase and glycogen synthase kinase 3 in axonal degeneration

Axon degeneration is an active, evolutionarily conserved self-destruction program by which compromised axons fragment in response to varied insults. Unlike programmed cell death, axon degeneration is poorly understood. We have combined robotic liquid handling with automated microscopy and image analysis to create a robust screening platform to measure axon degeneration in mammalian primary neuronal cultures. Using this assay, we performed an unbiased screen of 480 bioactive compounds, identifying 11 that reproducibly delay fragmentation of severed axons in vitro, including two inhibitors of glycogen synthase kinase 3 and two inhibitors of IκB kinase. Knockdown of each of these targets by shRNA lentivirus also delays axon degeneration in vitro, further supporting their role in the axon degeneration program.     

Direct and novel regulation of cAMP-dependent protein kinase by Mck1p, a yeast glycogen synthase kinase-3

The MCK1 gene of Saccharomyces cerevisiae encodes a protein kinase homologous to metazoan glycogen synthase kinase-3. Previous studies implicated Mck1p in negative regulation of pyruvate kinase. In this study we find that purified Mck1p does not phosphorylate pyruvate kinase, suggesting that the link is indirect. We find that purified Tpk1p, a cAMP-dependent protein kinase catalytic subunit, phosphorylates purified pyruvate kinase in vitro, and that loss of the cAMP-dependent protein kinase regulatory subunit, Bcy1p, increases pyruvate kinase activity in vivo. We find that purified Mck1p inhibits purified Tpk1p in vitro, in the presence or absence of Bcy1p. Mck1p must be catalytically active to inhibit Tpk1p, but Mck1p does not phosphorylate this target. We find that abolition of Mck1p autophosphorylation on tyrosine prevents the kinase from efficiently phosphorylating exogenous substrates, but does not block its ability to inhibit Tpk1p in vitro. We find that this mutant form of Mck1p appears to retain the ability to negatively regulate cAMP-dependent protein kinase in vivo. We propose that Mck1p, in addition to phosphorylating some target proteins, also acts by a separate, novel mechanism: autophosphorylated Mck1p binds to and directly inhibits, but does not phosphorylate, the catalytic subunits of cAMP-dependent protein kinase.     

Priming-dependent phosphorylation and regulation of the tumor suppressor pVHL by glycogen synthase kinase 3

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene is linked to the development of tumors of the eyes, kidneys, and central nervous system. VHL encodes two gene products, pVHL30 and pVHL19, of which one, pVHL30, associates functionally with microtubules (MTs) to regulate their stability. Here we report that pVHL30 is a novel substrate of glycogen synthase kinase 3 (GSK3) in vitro and in vivo. Phosphorylation of pVHL on serine 68 (S68) by GSK3 requires a priming phosphorylation event at serine 72 (S72) mediated in vitro by casein kinase I. Functional analysis of pVHL species carrying nonphosphorylatable or phosphomimicking mutations at S68 and/or S72 reveals a central role for these phosphorylation events in the regulation of pVHL's MT stabilization (but not binding) activity. Taken together, our results identify pVHL as a novel priming-dependent substrate of GSK3 and suggest a dual-kinase mechanism in the control of pVHL's MT stabilization function. Since GSK3 is a component of multiple signaling pathways that are altered in human cancer, our results further imply that normal operation of the GSK3-pVHL axis may be a critical aspect of pVHL's tumor suppressor mechanism through the regulation of MT dynamics.