Multisite phosphorylation of glycogen synthase from rabbit skeletal muscle. Phosphorylation of site 5 by glycogen synthase kinase-5 (casein kinase-II) is a prerequisite for phosphorylation of sites 3 by glycogen synthase kinase-3

Highly efficient conversion of mouse fibroblasts into functional hepatic cells under chemical induction

Previous studies have shown that hepatocyte-like cells can be generated from fibroblasts using either lineage-specific transcription factors or chemical induction methods. However, these methods have their own deficiencies that restrict the therapeutic applications of such induced hepatocytes. In this study, we present a transgene-free, highly efficient chemical-induced direct reprogramming approach to generate hepatocyte-like cells from mouse embryonic fibroblasts (MEFs). Using a small molecule cocktail (SMC) as an inducer, MEFs can be directly reprogrammed into hepatocyte-like cells, bypassing the intermediate stages of pluripotent and immature hepatoblasts. These chemical-induced hepatocyte-like cells (ciHeps) closely resemble mature primary hepatocytes in terms of morphology, biological behavior, gene expression patterns, marker expression levels, and hepatic functions. Furthermore, transplanted ciHeps can integrate into the liver, promote liver regeneration, and improve survival rates in mice with acute liver damage. ciHeps can also ameliorate liver fibrosis caused by chronic injuries and enhance liver function. Notably, ciHeps exhibit no tumorigenic potential either in vitro or in vivo. Mechanistically, SMC-induced mesenchymal-to-epithelial transition and suppression of SNAI1 contribute to the fate conversion of fibroblasts into ciHeps. These results indicate that this transgene-free, chemical-induced direct reprogramming technique has the potential to serve as a valuable means of producing alternative hepatocytes for both research and therapeutic purposes. Additionally, this method also sheds light on the direct reprogramming of other cell types under chemical induction.

Inhibitory protein-protein interactions of the SIRT1 deacetylase are choreographed by post-translational modification

Regulation of SIRT1 activity is vital to energy homeostasis and plays important roles in many diseases. We previously showed that insulin triggers the epigenetic regulator DBC1 to prime SIRT1 for repression by the multifunctional trafficking protein PACS-2. Here, we show that liver DBC1/PACS-2 regulates the diurnal inhibition of SIRT1, which is critically important for insulin-dependent switch in fuel metabolism from fat to glucose oxidation. We present the x-ray structure of the DBC1 S1-like domain that binds SIRT1 and an NMR characterization of how the SIRT1 N-terminal region engages DBC1. This interaction is inhibited by acetylation of K112 of DBC1 and stimulated by the insulin-dependent phosphorylation of human SIRT1 at S162 and S172, catalyzed sequentially by CK2 and GSK3, resulting in the PACS-2-dependent inhibition of nuclear SIRT1 enzymatic activity and translocation of the deacetylase in the cytoplasm. Finally, we discuss how defects in the DBC1/PACS-2-controlled SIRT1 inhibitory pathway are associated with disease, including obesity and non-alcoholic fatty liver disease.

Interfering with the Ubiquitin-Mediated Regulation of Akt as a Strategy for Cancer Treatment

The serine/threonine kinase Akt modulates the functions of numerous substrates, many of them being involved in cell proliferation and growth, metabolism, angiogenesis, resistance to hypoxia and migration. Akt is frequently deregulated in many types of human cancers, its overexpression or abnormal activation being associated with the increased proliferation and survival of cancer cells. A promising avenue for turning off the functionality of Akt is to either interfere with the K63-linked ubiquitination that is necessary for Akt membrane recruitment and activation or increase the K48-linked polyubiquitination that aims to target Akt to the proteasome for its degradation. Recent evidence indicates that targeting the ubiquitin proteasome system is effective for certain cancer treatments. In this review, the functions and roles of Akt in human cancer will be discussed, with a main focus on molecules and compounds that target various elements of the ubiquitination processes that regulate the activation and inactivation of Akt. Moreover, their possible and attractive implications for cancer therapy will be discussed.

Mechanism of glycogen synthase inactivation and interaction with glycogenin

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress against GS regulatory helices. This keeps GS in an inactive conformation mediated by phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided mutagenesis perturbing interactions with phosphorylated tails led to increased basal/unstimulated GS activity. We propose that multivalent phosphorylation supports GS autoinhibition through interactions from a dynamic “spike” region, allowing a tuneable rheostat for regulating GS activity. This work therefore provides insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.

Glycogen is a major energy reserve in eukaryotes and is synthesised in part by glycogenin (GN) and glycogen synthase (GS). Here, authors describe the structural basis of GS regulation, specifically the mechanism of inactivation by phosphorylation.

Non-redundant activity of GSK-3α and GSK-3β in T cell-mediated tumor rejection

Glycogen synthase kinase-3 (GSK-3) is a positive regulator of PD-1 expression in CD8+ T cells and GSK-3 inhibition enhances T cell function and is effective in the control of tumor growth. GSK-3 has two co-expressed isoforms, GSK-3α and GSK-3β. Using conditional gene targeting, we demonstrate that both isoforms contribute to T cell function to different degrees. Gsk3b-/- mice suppressed tumor growth to the same degree as Gsk3a/b-/- mice, whereas Gsk3a-/- mice behaved similarly to wild-type, revealing an important role for GSK-3β in regulating T cell-mediated anti-tumor immunity. The individual GSK-3α and β isoforms have differential effects on PD-1, IFNγ, and granzyme B expression and operate in synergy to control PD-1 expression and the infiltration of tumors with CD4 and CD8 T cells. Our data reveal a complex interplay of the GSK-3 isoforms in the control of tumor immunity and highlight non-redundant activity of GSK-3 isoforms in T cells, with implications for immunotherapy.

The role of GSK3 in metabolic pathway perturbations in cancer

Malignant transformation and tumor progression are accompanied by significant perturbations in metabolic programs. As such, cancer cells support high ATP turnover to construct the building blocks needed to fuel neoplastic growth. The coordination of metabolic networks in malignant cells is dependent on the collaboration with cellular signaling pathways. Glycogen synthase kinase 3 (GSK3) lies at the convergence of several signaling axes, including the PI3K/AKT/mTOR, AMPK, and Wnt pathways, which influence cancer initiation, progression and therapeutic responses. Accordingly, GSK3 modulates metabolic processes, including protein and lipid synthesis, glucose and mitochondrial metabolism, as well as autophagy. In this review, we highlight current knowledge of the role of GSK3 in metabolic perturbations in cancer.

Crosstalks of GSK3 signaling with the mTOR network and effects on targeted therapy of cancer

The introduction of therapeutics targeting specific tumor-promoting oncogenic or non-oncogenic signaling pathways has revolutionized cancer treatment. Mechanistic (previously mammalian) target of rapamycin (mTOR), a highly conserved Ser/Thr kinase, is a central hub of the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR network, one of the most frequently deregulated signaling pathways in cancer, that makes it an attractive target for therapy. Numerous mTOR inhibitors have progressed to clinical trials and two of them have been officially approved as anticancer therapeutics. However, mTOR-targeting drugs have met with a very limited success in cancer patients. Frequently, the primary impediment to a successful targeted therapy in cancer is drug-resistance, either from the very beginning of the therapy (innate resistance) or after an initial response and upon repeated drug treatment (evasive or acquired resistance). Drug-resistance leads to treatment failure and relapse/progression of the disease. Resistance to mTOR inhibitors depends, among other reasons, on activation/deactivation of several signaling pathways, included those regulated by glycogen synthase kinase-3 (GSK3), a protein that targets a vast number of substrates in its repertoire, thereby orchestrating many processes that include cell proliferation and survival, metabolism, differentiation, and stemness. A detailed knowledge of the rewiring of signaling pathways triggered by exposure to mTOR inhibitors is critical to our understanding of the consequences such perturbations cause in tumors, including the emergence of drug-resistant cells. Here, we provide the reader with an updated overview of intricate circuitries that connect mTOR and GSK3 and we relate them to the efficacy (or lack of efficacy) of mTOR inhibitors in cancer cells.

Glycogen synthase kinase-3 and alternative splicing

Glycogen synthase kinase-3 (GSK-3) is a highly conserved negative regulator of receptor tyrosine kinase, cytokine, and Wnt signaling pathways. Stimulation of these pathways inhibits GSK-3 to modulate diverse downstream effectors that include transcription factors, nutrient sensors, glycogen synthesis, mitochondrial function, circadian rhythm, and cell fate. GSK-3 also regulates alternative splicing in response to T-cell receptor activation, and recent phosphoproteomic studies have revealed that multiple splicing factors and regulators of RNA biosynthesis are phosphorylated in a GSK-3-dependent manner. Furthermore, inhibition of GSK-3 alters the splicing of hundreds of mRNAs, indicating a broad role for GSK-3 in the regulation of RNA processing. GSK-3-regulated phosphoproteins include SF3B1, SRSF2, PSF, RBM8A, nucleophosmin 1 (NPM1), and PHF6, many of which are mutated in leukemia and myelodysplasia. As GSK-3 is inhibited by pathways that are pathologically activated in leukemia and loss of Gsk3 in hematopoietic cells causes a severe myelodysplastic neoplasm in mice, these findings strongly implicate GSK-3 as a critical regulator of mRNA processing in normal and malignant hematopoiesis. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Phosphoproteomics reveals that glycogen synthase kinase-3 phosphorylates multiple splicing factors and is associated with alternative splicing

Glycogen synthase kinase-3 (GSK-3) is a constitutively active, ubiquitously expressed protein kinase that regulates multiple signaling pathways. In vitro kinase assays and genetic and pharmacological manipulations of GSK-3 have identified more than 100 putative GSK-3 substrates in diverse cell types. Many more have been predicted on the basis of a recurrent GSK-3 consensus motif ((pS/pT)XXX(S/T)), but this prediction has not been tested by analyzing the GSK-3 phosphoproteome. Using stable isotope labeling of amino acids in culture (SILAC) and MS techniques to analyze the repertoire of GSK-3-dependent phosphorylation in mouse embryonic stem cells (ESCs), we found that ∼2.4% of (pS/pT)XXX(S/T) sites are phosphorylated in a GSK-3-dependent manner. A comparison of WT and Gsk3a;Gsk3b knock-out (Gsk3 DKO) ESCs revealed prominent GSK-3-dependent phosphorylation of multiple splicing factors and regulators of RNA biosynthesis as well as proteins that regulate transcription, translation, and cell division. Gsk3 DKO reduced phosphorylation of the splicing factors RBM8A, SRSF9, and PSF as well as the nucleolar proteins NPM1 and PHF6, and recombinant GSK-3β phosphorylated these proteins in vitro RNA-Seq of WT and Gsk3 DKO ESCs identified ∼190 genes that are alternatively spliced in a GSK-3-dependent manner, supporting a broad role for GSK-3 in regulating alternative splicing. The MS data also identified posttranscriptional regulation of protein abundance by GSK-3, with ∼47 proteins (1.4%) whose levels increased and ∼78 (2.4%) whose levels decreased in the absence of GSK-3. This study provides the first unbiased analysis of the GSK-3 phosphoproteome and strong evidence that GSK-3 broadly regulates alternative splicing.

Ablation of Protein Kinase CK2β in Skeletal Muscle Fibers Interferes with Their Oxidative Capacity

The tetrameric protein kinase CK2 was identified playing a role at neuromuscular junctions by studying CK2β-deficient muscle fibers in mice, and in cultured immortalized C2C12 muscle cells after individual knockdown of CK2α and CK2β subunits. In muscle cells, CK2 activity appeared to be at least required for regular aggregation of nicotinic acetylcholine receptors, which serves as a hallmark for the presence of a postsynaptic apparatus. Here, we set out to determine whether any other feature accompanies CK2β-deficient muscle fibers. Hind limb muscles gastrocnemius, plantaris, and soleus of adult wildtype and CK2β-deficient mice were dissected, cross-sectioned, and stained histochemically by Gomori trichrome and for nicotinamide adenine dinucleotide (NADH) dehydrogenase and succinate dehydrogenase (SDH) enzymatic activities. A reduction of oxidative enzymatic activity was determined for CK2β-deficient muscle fibers in comparison with wildtype controls. Importantly, the CK2β-deficient fibers, muscle fibers that typically exhibit high NADH dehydrogenase and SDH activities, like slow-type fibers, showed a marked reduction in these activities. Altogether, our data indicate additional impairments in the absence of CK2β in skeletal muscle fibers, pointing to an eventual mitochondrial myopathy.

Expression and purification of functional human glycogen synthase-1:glycogenin-1 complex in insect cells

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GYS1:GN1 complex expressed using bicistronic pFastBac-Dual vector in insect cells.

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A large quantity of highly-pure stoichiometric GYS1:GN1 complex obtained.

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Purified GYS1 is functional and heavily phosphorylated at several Ser/Thr residues.

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GYS1:GN1 complex will be useful to reveal its structural and biochemical properties.

We report the successful expression and purification of functional human muscle glycogen synthase (GYS1) in complex with human glycogenin-1 (GN1). Stoichiometric GYS1:GN1 complex was produced by co-expression of GYS1 and GN1 using a bicistronic pFastBac™-Dual expression vector, followed by affinity purification and subsequent size-exclusion chromatography. Mass spectrometry analysis identified that GYS1 is phosphorylated at several well-characterised and uncharacterised Ser/Thr residues. Biochemical analysis, including activity ratio (in the absence relative to that in the presence of glucose-6-phosphate) measurement, covalently attached phosphate estimation as well as phosphatase treatment, revealed that recombinant GYS1 is substantially more heavily phosphorylated than would be observed in intact human or rodent muscle tissues. A large quantity of highly-pure stoichiometric GYS1:GN1 complex will be useful to study its structural and biochemical properties in the future, which would reveal mechanistic insights into its functional role in glycogen biosynthesis.

A model integration approach linking signalling and gene-regulatory logic with kinetic metabolic models

Systems biology has to increasingly cope with large- and multi-scale biological systems. Many successful in silico representations and simulations of various cellular modules proved mathematical modelling to be an important tool in gaining a solid understanding of biological phenomena. However, models spanning different functional layers (e.g. metabolism, signalling and gene regulation) are still scarce. Consequently, model integration methods capable of fusing different types of biological networks and various model formalisms become a key methodology to increase the scope of cellular processes covered by mathematical models. Here we propose a new integration approach to couple logical models of signalling or/and gene-regulatory networks with kinetic models of metabolic processes. The procedure ends up with an integrated dynamic model of both layers relying on differential equations. The feasibility of the approach is shown in an illustrative case study integrating a kinetic model of central metabolic pathways in hepatocytes with a Boolean logical network depicting the hormonally induced signal transduction and gene regulation events involved. In silico simulations demonstrate the integrated model to qualitatively describe the physiological switch-like behaviour of hepatocytes in response to nutritionally regulated changes in extracellular glucagon and insulin levels. A simulated failure mode scenario addressing insulin resistance furthermore illustrates the pharmacological potential of a model covering interactions between signalling, gene regulation and metabolism.

Inhibitory phosphorylation of GSK-3β by AKT, PKA, and PI3K contributes to high NaCl-induced activation of the transcription factor NFAT5 (TonEBP/OREBP)

High NaCl activates the transcription factor nuclear factor of activated T cells 5 (NFAT5), leading to increased transcription of osmoprotective target genes. Kinases PKA, PI3K, AKT1, and p38α were known to contribute to the high NaCl-induced increase of NFAT5 activity. We now identify another kinase, GSK-3β. siRNA-mediated knock-down of GSK-3β increases NFAT5 transcriptional and transactivating activities without affecting high NaCl-induced nuclear localization of NFAT5 or NFAT5 protein expression. High NaCl increases phosphorylation of GSK-3β-S9, which inhibits GSK-3β. In GSK-3β-null mouse embryonic fibroblasts transfection of GSK-3β, in which serine 9 is mutated to alanine, so that it cannot be inhibited by phosphorylation at that site, inhibits high NaCl-induced NFAT5 transcriptional activity more than transfection of wild-type GSK-3β. High NaCl-induced phosphorylation of GSK-3β-S9 depends on PKA, PI3K, and AKT, but not p38α. Overexpression of PKA catalytic subunit α or of catalytically active AKT1 reduces inhibition of NFAT5 by GSK-3β, but overexpression of p38α together with its catalytically active upstream kinase, MKK6, does not. Thus, GSK-3β normally inhibits NFAT5 by suppressing its transactivating activity. When activated by high NaCl, PKA, PI3K, and AKT1, but not p38α, increase phosphorylation of GSK-3β-S9, which reduces the inhibitory effect of GSK-3β on NFAT5, and thus contributes to activation of NFAT5.

Regulation of glycogen synthase from mammalian skeletal muscle--a unifying view of allosteric and covalent regulation

It is widely accepted that insufficient insulin-stimulated activation of muscle glycogen synthesis is one of the major components of non-insulin-dependent (type 2) diabetes mellitus. Glycogen synthase, a key enzyme in muscle glycogen synthesis, is extensively regulated, both allosterically (by glucose-6-phosphate, ATP, and others) and covalently (by phosphorylation). Although glycogen synthase has been a topic of intense study for more than 50 years, its kinetic characterization has been confounded by its large number of phosphorylation states. Questions remain regarding the function of glycogen synthase regulation and the relative importance of allosteric and covalent modification in fulfilling this function. In this review, we consider both earlier kinetic studies and more recent site-directed mutagenesis and crystal structure studies in a detailed qualitative discussion of the effects of regulation on the kinetics of glycogen synthase. We propose that both allosteric and covalent modification of glycogen synthase may be described by a Monod-Wyman-Changeux model in terms of apparent changes to L, the equilibrium constant for transition between the T and R conformers. As, with the exception of L, all parameters of this model are independent of the glycogen synthase phosphorylation state, the need to determine kinetic parameters for all possible states is eliminated; only the relationship between a particular state and L must be established. We conclude by suggesting that renewed efforts to characterize the relationship between phosphorylation and the kinetics of glycogen synthase are essential in order to obtain a better quantitative understanding of the function of glycogen synthesis regulation. The model we propose may prove useful in this regard.

GSK-3: Functional Insights from Cell Biology and Animal Models

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded in mammals by two genes that generate two related proteins: GSK-3α and GSK-3β. GSK-3 is active in cells under resting conditions and is primarily regulated through inhibition or diversion of its activity. While GSK-3 is one of the few protein kinases that can be inactivated by phosphorylation, the mechanisms of GSK-3 regulation are more varied and not fully understood. Precise control appears to be achieved by a combination of phosphorylation, localization, and sequestration by a number of GSK-3-binding proteins. GSK-3 lies downstream of several major signaling pathways including the phosphatidylinositol 3′ kinase pathway, the Wnt pathway, Hedgehog signaling and Notch. Specific pools of GSK-3, which differ in intracellular localization, binding partner affinity, and relative amount are differentially sensitized to several distinct signaling pathways and these sequestration mechanisms contribute to pathway insulation and signal specificity. Dysregulation of signaling pathways involving GSK-3 is associated with the pathogenesis of numerous neurological and psychiatric disorders and there are data suggesting GSK-3 isoform-selective roles in several of these. Here, we review the current knowledge of GSK-3 regulation and targets and discuss the various animal models that have been employed to dissect the functions of GSK-3 in brain development and function through the use of conventional or conditional knockout mice as well as transgenic mice. These studies have revealed fundamental roles for these protein kinases in memory, behavior, and neuronal fate determination and provide insights into possible therapeutic interventions.

Dual regulation of muscle glycogen synthase during exercise by activation and compartmentalization

Glycogen synthase (GS) is considered the rate-limiting enzyme in glycogenesis but still today there is a lack of understanding on its regulation. We have previously shown phosphorylation-dependent GS intracellular redistribution at the start of glycogen re-synthesis in rabbit skeletal muscle (Prats, C., Cadefau, J. A., Cussó, R., Qvortrup, K., Nielsen, J. N., Wojtaszewki, J. F., Wojtaszewki, J. F., Hardie, D. G., Stewart, G., Hansen, B. F., and Ploug, T. (2005) J. Biol. Chem. 280, 23165-23172). In the present study we investigate the regulation of human muscle GS activity by glycogen, exercise, and insulin. Using immunocytochemistry we investigate the existence and relevance of GS intracellular compartmentalization during exercise and during glycogen re-synthesis. The results show that GS intrinsic activity is strongly dependent on glycogen levels and that such regulation involves associated dephosphorylation at sites 2+2a, 3a, and 3a + 3b. Furthermore, we report the existence of several glycogen metabolism regulatory mechanisms based on GS intracellular compartmentalization. After exhausting exercise, epinephrine-induced protein kinase A activation leads to GS site 1b phosphorylation targeting the enzyme to intramyofibrillar glycogen particles, which are preferentially used during muscle contraction. On the other hand, when phosphorylated at sites 2+2a, GS is preferentially associated with subsarcolemmal and intermyofibrillar glycogen particles. Finally, we verify the existence in human vastus lateralis muscle of the previously reported mechanism of glycogen metabolism regulation in rabbit tibialis anterior muscle. After overnight low muscle glycogen level and/or in response to exhausting exercise-induced glycogenolysis, GS is associated with spherical structures at the I-band of sarcomeres.

Glycogen synthase kinase 3 (GSK3) in the heart: a point of integration in hypertrophic signalling and a therapeutic target? A critical analysis

Glycogen synthase kinase 3 (GSK3, of which there are two isoforms, GSK3alpha and GSK3beta) was originally characterized in the context of regulation of glycogen metabolism, though it is now known to regulate many other cellular processes. Phosphorylation of GSK3alpha(Ser21) and GSK3beta(Ser9) inhibits their activity. In the heart, emphasis has been placed particularly on GSK3beta, rather than GSK3alpha. Importantly, catalytically-active GSK3 generally restrains gene expression and, in the heart, catalytically-active GSK3 has been implicated in anti-hypertrophic signalling. Inhibition of GSK3 results in changes in the activities of transcription and translation factors in the heart and promotes hypertrophic responses, and it is generally assumed that signal transduction from hypertrophic stimuli to GSK3 passes primarily through protein kinase B/Akt (PKB/Akt). However, recent data suggest that the situation is far more complex. We review evidence pertaining to the role of GSK3 in the myocardium and discuss effects of genetic manipulation of GSK3 activity in vivo. We also discuss the signalling pathways potentially regulating GSK3 activity and propose that, depending on the stimulus, phosphorylation of GSK3 is independent of PKB/Akt. Potential GSK3 substrates studied in relation to myocardial hypertrophy include nuclear factors of activated T cells, beta-catenin, GATA4, myocardin, CREB, and eukaryotic initiation factor 2Bvarepsilon. These and other transcription factor substrates putatively important in the heart are considered. We discuss whether cardiac pathologies could be treated by therapeutic intervention at the GSK3 level but conclude that any intervention would be premature without greater understanding of the precise role of GSK3 in cardiac processes.

GSK-3beta inhibition enhances sorafenib-induced apoptosis in melanoma cell lines

Glycogen synthase kinase-3beta (GSK-3beta) can participate in the induction of apoptosis or, alternatively, provide a survival signal that minimizes cellular injury. We previously demonstrated that the multikinase inhibitor sorafenib induces apoptosis in melanoma cell lines. In this report, we show that sorafenib activates GSK-3beta in multiple subcellular compartments and that this activation undermines the lethality of the drug. Pharmacologic inhibition and/or down-modulation of the kinase enhances sorafenib-induced apoptosis as determined by propidium iodide staining and by assessing the mitochondrial release of apoptosis-inducing factor and Smac/DIABLO. Conversely, the forced expression of a constitutively active form of the enzyme (GSK-3beta(S9A)) protects the cells from the apoptotic effects of the drug. This protective effect is associated with a marked increase in basal levels of Bcl-2, Bcl-x(L), and survivin and a diminution in the degree to which these anti-apoptotic proteins are down-modulated by sorafenib exposure. Sorafenib down-modulates the pro-apoptotic Bcl-2 family member Noxa in cells with high constitutive GSK-3beta activity. Pharmacologic inhibition of GSK-3beta prevents the disappearance of Noxa induced by sorafenib and enhances the down-modulation of Mcl-1. Down-modulation of Noxa largely eliminates the enhancing effect of GSK-3 inhibition on sorafenib-induced apoptosis. These data provide a strong rationale for the use of GSK-3beta inhibitors as adjuncts to sorafenib treatment and suggest that preservation of Noxa may contribute to their efficacy.

Glycogen synthase kinase-3 phosphorylates CdGAP at a consensus ERK 1 regulatory site

Rho GTPases regulate a multitude of cellular processes from cytoskeletal reorganization to gene transcription and are negatively regulated by GTPase-activating proteins (GAPs). Cdc42 GTPase-activating protein (CdGAP) is a ubiquitously expressed GAP for Rac1 and Cdc42. In this study, we set out to identify CdGAP-binding partners and, using a yeast two-hybrid approach, glycogen synthase kinase 3alpha (GSK-3alpha) was identified as a partner for CdGAP. GSK-3 exists in two isoforms, alpha and beta, and is involved in regulating many cellular functions from insulin response to tumorigenesis. We show that GSK-3alpha and -beta interact with CdGAP in mammalian cells. We also demonstrate that GSK-3 phosphorylates CdGAP both in vitro and in vivo on Thr-776, which we have previously shown to be an ERK 1/2 phosphorylation site involved in CdGAP regulation. We report that the mRNA and protein levels of CdGAP are increased upon serum stimulation and that GSK-3 activity is necessary for the up-regulation of the protein levels of CdGAP but not for the increase in mRNA. We conclude that GSK-3 is an important regulator of CdGAP and that regulation of CdGAP protein levels by serum presents a novel mechanism for cells to control Cdc42/Rac1 GTPase signaling pathways.

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Casein kinase 2, circadian clocks, and the flight from mutagenic light

Circadian clocks play a fundamental role in biology and disease. Much has been learned about the molecular underpinnings of these biological clocks from genetic studies in model organisms, such as the fruit fly, Drosophila melanogaster. Here we review the literature from our lab and others that establish a role for the protein kinase CK2 in Drosophila clock timing. Among the clock genes described thus far, CK2 is unique in its involvement in plant, fungal, as well as animal circadian clocks. We propose that this reflects an ancient, conserved function for CK2 in circadian clocks. CK2 and other clock genes have been implicated in cellular responses to DNA damage, particularly those induced by ultraviolet (UV) light. The finding of a dual function of CK2 in clocks and in UV responses supports the notion that clocks evolved to assist organisms in avoiding the mutagenic effects of daily sunlight.

In vivo circadian function of casein kinase 2 phosphorylation sites in Drosophila PERIOD

Phosphorylation plays a key role in the precise timing of circadian clocks. Daily rhythms of phosphorylation of the Drosophila circadian clock component PERIOD (PER) were first described more than a decade ago, yet little is known about their phosphorylation sites and their function in circadian behavior. Here we show that serines 151 and 153 in PER are required for robust in vitro phosphorylation by the casein kinase 2 (CK2) holoenzyme, a cytoplasmic kinase shown to be involved in circadian rhythms. Mutation of these sites in transgenic flies results in significant period lengthening of behavioral rhythms, altered PER rhythms, and delayed PER nuclear localization in circadian pacemaker neurons. In many respects, mutation of these phosphorylation sites phenocopies mutation of the catalytic subunit of CK2. We propose that CK2 phosphorylation at these sites triggers PER nuclear localization.

Control of mammalian glycogen synthase by PAS kinase

The regulation of glycogen metabolism is critical for the maintenance of glucose and energy homeostasis in mammals. Glycogen synthase, the enzyme responsible for glycogen production, is regulated by multisite phosphorylation in yeast and mammals. We have previously identified PAS kinase as a physiological regulator of glycogen synthase in Saccharomyces cerevisiae. We provide evidence here that PAS kinase is an important regulator of mammalian glycogen synthase. Glycogen synthase is efficiently phosphorylated by PAS kinase in vitro at Ser-640, a known regulatory phosphosite. Efficient phosphorylation requires a region of PAS kinase outside the catalytic domain. This region appears to mediate a direct interaction between glycogen synthase and PAS kinase, thereby targeting kinase activity to this substrate specifically. This interaction is regulated by the PAS kinase PAS domain, raising the possibility that this interaction (and phosphorylation event) is modulated by the cellular metabolic state. This mode of regulation provides a mechanism for metabolic status to impinge directly on the cellular decision of whether to store or use available energy.

Physiological roles of glycogen synthase kinase-3: potential as a therapeutic target for diabetes and other disorders

Glycogen synthase kinase-3 (GSK-3) has perplexed signal transduction researchers since its detection in skeletal muscle 25 years ago. The enzyme confounds most of the rules normally associated with protein kinases in that it exhibits significant activity, even in resting, unstimulated cells. However, the protein is highly regulated and potently inactivated in response to signals such as insulin and polypeptide growth factors. The enzyme also displays a distinct and unusual preference for substrates that have been previously phosphorylated by other protein kinases which provides obvious opportunities for cross-talk. Its substrates are diverse and are predominantly regulatory molecules. The molecular cloning of the kinase revealed it to be encoded by two related but distinct genes. Moreover, the mammalian proteins showed remarkable similarity to a fruitfly protein isolated on the basis of its role in cell fate determination. From these humble beginnings, study of the enzyme has accrued further surprises such as its inhibition by lithium, its regulation by serine and tyrosine phosphorylation and its implication in several human disorders including Alzheimers disease, bipolar disorder, cancer and diabetes. Most recently, small molecule inhibitors of GSK-3 have been developed and assessed for therapeutic potential in several of models of pathophysiology. The question is whether modulation of such an "involved" enzyme could lead to selective restoration of defects without multiple unwanted side effects. This review summarizes current knowledge of GSK-3 with respect to its known functions, together with an assessment of its real-life potential as a drug target for chronic conditions such as type 2 diabetes.

Calmodulin phosphorylation and modulation of endothelial nitric oxide synthase catalysis

The endothelial NO synthase (eNOS) is regulated by diverse protein kinase pathways, yet eNOS activity ultimately depends on the ubiquitous calcium regulatory protein calmodulin (CaM). In these studies, we establish that CaM itself undergoes phosphorylation in endothelial cells and that CaM phosphorylation attenuates eNOS activation. Using [(32)P]orthophosphoric acid biosynthetic labeling, we found that CaM is a phosphoprotein in bovine aortic endothelial cells (BAEC) and that the kinase CK2 promotes CaM phosphorylation in BAEC. Phosphorylation of CaM by purified CK2 in vitro reduces the V(max) of immunopurified eNOS by a factor of 2 but has no effect on the K(A) for CaM or calcium. Additionally, [(32)P]orthophosphoric acid biosynthetic labeling of mutant CaM-transfected BAEC revealed that phosphorylation of Ser-81 to alanine mutant CaM ("phosphonull" S81A mutant) is dramatically reduced relative to WT, whereas phosphorylation of the "phosphomimetic" Ser-81 to aspartate (S81D) mutant is unchanged. Further studies using Escherichia coli-expressed and phenyl-Sepharose-purified CaM mutants revealed that the S81A mutation abrogates in vitro CK2-mediated phosphorylation of CaM, whereas phosphorylation of the S81D CaM mutant by CK2 is preserved. Additionally, we found that the phosphomimetic S101D CaM mutant is impaired in its ability to activate eNOS. Taken together, these results suggest that phosphorylation of CaM inhibits eNOS catalysis and proceeds in a hierarchical manner, initially requiring phosphorylation of the CaM Ser-81 residue. We conclude that CaM phosphorylation may represent a unique pathway in the regulation of eNOS signaling and thereby may play a role in modulating NO-dependent vascular responses.

Regulation of glycogen synthase in skeletal muscle during exercise

Glycogen synthase (GS) catalyses the incorporation of uridine diphosphate-glucose into glycogen in skeletal muscle. In concert with the glucose transport step, GS activity is thought to be rate-limiting in the disposal of glucose as muscle glycogen. Glycogen synthase is regulated by both allosteric factors (primarily glucose 6-phosphate) and covalent modification by reversible phosphorylation and dephosphorylation leading to inactivation and activation of GS, respectively. Exercise activates both stimulatory and inhibitory regulators of GS and it is thought that the resultant activity of GS during exercise depends on the relative strength of opposing signals. However, the mechanisms by which exercise regulates GS activity are not fully understood. Glycogen breakdown, the GM-protein phosphatase 1 complex and possibly cellular relocalization of GS may be considered important factors involved in the stimulation of GS activity during exercise, while adenosine monophosphate-activated protein kinase and plasma adrenaline (via protein kinase A) can be considered as essential for the exercise-induced inhibitory signals to GS.

A reinvestigation of the multisite phosphorylation of the transcription factor c-Jun

We have used phospho-specific antibodies to re-examine the multisite phosphorylation of c-Jun in murine RAW macrophages and embryonic fibroblasts. Our results indicate that JNK isoforms are required and sufficient for the phosphorylation of Thr91 and Thr93, as well as the phosphorylation of Ser63 and Ser73, in response to LPS or anisomycin in macrophages and TNFalpha or anisomycin in fibroblasts. However, the phorbol ester (TPA) and EGF-induced phosphorylation of Ser63 and Ser73 is mediated by ERK1/ERK2, as well as JNK1/JNK2, in fibroblasts from wild-type mice and by ERK1/ERK2 alone in fibroblasts from JNK-deficient mice. The phosphorylation of Thr239 is catalysed by GSK3 and the phosphorylation of Ser243 by an as yet unidentified protein kinase. The inhibition of GSK3 is not required for the dephosphorylation of Thr239 in response to LPS, and nor is the phosphorylation of Thr91 and Thr93 required for the TPA- or EGF-induced dephosphorylation of Thr239 in fibroblasts. The agonist-induced dephosphorylation of Thr239 may involve a conformational change that exposes Thr239 to dephosphorylation and/or the activation of a Thr239 phosphatase.

Characterisation of the phosphorylation of beta-catenin at the GSK-3 priming site Ser45

Activation of the canonical Wnt signalling pathway results in stabilisation and nuclear translocation of beta-catenin. In the absence of a Wnt signal, beta-catenin is phosphorylated at four conserved serine and threonine residues at the N-terminus of the protein, which results in beta-catenin ubiquitination and proteasome-dependent degradation. The phosphorylation of three of these residues, Thr41, Ser37, and Ser33, is mediated by glycogen synthase kinase-3 (GSK-3) in a sequential manner, beginning from the C-terminal Thr41. It has recently been shown that the GSK-3 dependent phosphorylation of beta-catenin requires prior priming through phosphorylation of Ser45. However, it is not known whether phosphorylation of Ser45 is carried out by GSK-3 itself or by an alternative kinase. In this study, the phosphorylation of beta-catenin at Ser45 was characterised using a phospho-specific antibody. GSK-3beta was found to be unable to phosphorylate beta-catenin at Ser45 in vitro and in intact cells. However, inhibition of GSK-3 in intact cells reduced Ser45 phosphorylation, suggesting that GSK-3 kinase activity is required for the phosphorylation event. In vitro, CK1, but not CK2, phosphorylates Ser45. Ser45 phosphorylation in intact cells is not mediated by CK1varepsilon, a known positive regulator of Wnt signalling, as overexpression of this kinase leads to decreased phosphorylation levels. In conclusion, phosphorylation of beta-catenin at the GSK-3 priming site Ser45 is not mediated by GSK-3 itself, but by an alternative kinase, indicating that beta-catenin is not an unprimed substrate for GSK-3 in vivo. Priming of GSK-3 dependent phosphorylation of beta-catenin by a different kinase could have important implications for the regulation of Wnt signalling.

Casein kinase I phosphorylates the Armadillo protein and induces its degradation in Drosophila

Casein kinase I (CKI) was recently reported as a positive regulator of Wnt signaling in vertebrates and Caenorhabditis elegans. To elucidate the function of Drosophila CKI in the wingless (Wg) pathway, we have disrupted its function by double-stranded RNA-mediated interference (RNAi). While previous findings were mainly based on CKI overexpression, this is the first convincing loss-of-function analysis of CKI. Surprisingly, CKIalpha- or CKIepsilon-RNAi markedly elevated the Armadillo (Arm) protein levels in Drosophila Schneider S2R+ cells, without affecting its mRNA levels. Pulse-chase analysis showed that CKI-RNAi stabilizes Arm protein. Moreover, Drosophila embryos injected with CKIalpha double-stranded RNA showed a naked cuticle phenotype, which is associated with activation of Wg signaling. These results indicate that CKI functions as a negative regulator of Wg/Arm signaling. Overexpression of CKIalpha induced hyper-phosphorylation of both Arm and Dishevelled in S2R+ cells and, conversely, CKIalpha-RNAi reduced the amount of hyper-modified forms. His-tagged Arm was phosphorylated by CKIalpha in vitro on a set of serine and threonine residues that are also phosphorylated by Zeste-white 3. Thus, we propose that CKI phosphorylates Arm and stimulates its degradation.

Effects of diabetes, vanadium, and insulin on glycogen synthase activation in Wistar rats

In vivo effects of insulin and vanadium treatment on glycogen synthase (GS), glycogen synthase kinase-3 (GSK-3) and protein phosphatase-1 (PP1) activity were determined in Wistar rats with streptozotocin (STZ)-induced diabetes. The skeletal muscle was freeze-clamped before or following an insulin injection (5 U/kg i.v.). Diabetes, vanadium, and insulin in vivo treatment did not affect muscle GSK-3beta activity as compared to controls. Following insulin stimulation in 4-week STZ-diabetic rats muscle GS fractional activity (GSFA) was increased 3 fold (p < 0.05), while in 7-week diabetic rats it remained unchanged, suggesting development of insulin resistance in longer term diabetes. Muscle PP1 activity was increased in diabetic rats and returned to normal after vanadium treatment, while muscle GSFA remained unchanged. Therefore, it is possible that PP1 is involved in the regulation of some other cellular events of vanadium (other than regulation of glycogen synthesis). The lack of effect of vanadium treatment in stimulating glycogen synthesis in skeletal muscle suggests the involvement of other metabolic pathways in the observed glucoregulatory effect of vanadium.

Structural features underlying the multisite phosphorylation of the A domain of the NF-AT4 transcription factor by protein kinase CK1

The phosphorylation and dephosphorylation of the NF-AT family of transcription factors play a key role in the activation of T lymphocytes and in the control of the immune response. The mechanistic aspects of NF-AT4 phosphorylation by protein kinase CK1 have been studied in this work with the aid of a series of 27 peptides, reproducing with suitable modifications the regions of NF-AT4 that have been reported to be phosphorylated by this protein kinase. The largest parent peptide, representing the three regions A, Z, and L spanning amino acids 173-218, is readily phosphorylated by CK1 at seryl residues belonging to the A2 segment, none of which fulfill the canonical consensus sequence for CK1. An acidic cluster of amino acids in the linker region between domains A and Z is essential for high-efficiency phosphorylation of the A2 domain, as shown by the increase in K(m) caused by a deletion of the linker region or a substitution of the acidic residues with glycines. Individual substitutions with alanine of each of the five serines in the A2 domain (S-177, S-180, S-181, S-184, and S-186) reduce the phosphorylation rate, the most detrimental effect being caused by Ser177 substitution which results in a 10-fold drop in V(max). On the contrary, the replacement of Ser177 with phosphoserine triggers a hierarchical effect with a dramatic improvement in phosphorylation efficiency, which no longer depends on the linker region for optimal efficiency. These data are consistent with a two-phase phosphorylation mechanism of NF-AT4 by CK1, initiated by the linker region which provides a functional docking site for CK1 and allows the unorthodox phosphorylation of Ser177; once achieved, this phosphoserine residue primes the phosphorylation of other downstream seryl residues, according to a hierarchical mechanism typically exploited by CK1.

Judging a protein by more than its name: GSK-3

As knowledge of cellular signal transduction has accumulated, general truisms have emerged, including the notion that signaling proteins are usually activated by stimuli and that they, in turn, mediate the actions of specific agonists. Glycogen synthase kinase-3 (GSK-3) is an unusual protein-serine kinase that bucks these conventions. This evolutionarily conserved protein kinase is active in resting cells and is inhibited in response to activation of several distinct pathways, including those acting by elevation of 3' phosphorylated phosphatidylinositol lipids and adenosine 3'-5'-monophosphate (cAMP). In addition, GSK-3 is distinctly regulated by, and is a core component of, the Wnt pathway. This review describes the unique characteristics of this decidedly oddball protein kinase in terms of its diverse biological functions, plethora of targets, role in several human diseases, and consequential potential as a therapeutic target.

A common phosphate binding site explains the unique substrate specificity of GSK3 and its inactivation by phosphorylation

The inhibition of GSK3 is required for the stimulation of glycogen and protein synthesis by insulin and the specification of cell fate during development. Here, we demonstrate that the insulin-induced inhibition of GSK3 and its unique substrate specificity are explained by the existence of a phosphate binding site in which Arg-96 is critical. Thus, mutation of Arg-96 abolishes the phosphorylation of "primed" glycogen synthase as well as inhibition by PKB-mediated phosphorylation of Ser-9. Hence, the phosphorylated N terminus acts as a pseudosubstrate, occupying the same phosphate binding site used by primed substrates. Significantly, this mutation does not affect phosphorylation of "nonprimed" substrates in the Wnt-signaling pathway (Axin and beta-catenin), suggesting new approaches to design more selective GSK3 inhibitors for the treatment of diabetes.

Specificity and mechanism of action of some commonly used protein kinase inhibitors

The specificities of 28 commercially available compounds reported to be relatively selective inhibitors of particular serine/threonine-specific protein kinases have been examined against a large panel of protein kinases. The compounds KT 5720, Rottlerin and quercetin were found to inhibit many protein kinases, sometimes much more potently than their presumed targets, and conclusions drawn from their use in cell-based experiments are likely to be erroneous. Ro 318220 and related bisindoylmaleimides, as well as H89, HA1077 and Y 27632, were more selective inhibitors, but still inhibited two or more protein kinases with similar potency. LY 294002 was found to inhibit casein kinase-2 with similar potency to phosphoinositide (phosphatidylinositol) 3-kinase. The compounds with the most impressive selectivity profiles were KN62, PD 98059, U0126, PD 184352, rapamycin, wortmannin, SB 203580 and SB 202190. U0126 and PD 184352, like PD 98059, were found to block the mitogen-activated protein kinase (MAPK) cascade in cell-based assays by preventing the activation of MAPK kinase (MKK1), and not by inhibiting MKK1 activity directly. Apart from rapamycin and PD 184352, even the most selective inhibitors affected at least one additional protein kinase. Our results demonstrate that the specificities of protein kinase inhibitors cannot be assessed simply by studying their effect on kinases that are closely related in primary structure. We propose guidelines for the use of protein kinase inhibitors in cell-based assays.

Specificity and mechanism of action of some commonly used protein kinase inhibitors

The specificities of 28 commercially available compounds reported to be relatively selective inhibitors of particular serine/threonine-specific protein kinases have been examined against a large panel of protein kinases. The compounds KT 5720, Rottlerin and quercetin were found to inhibit many protein kinases, sometimes much more potently than their presumed targets, and conclusions drawn from their use in cell-based experiments are likely to be erroneous. Ro 318220 and related bisindoylmaleimides, as well as H89, HA1077 and Y 27632, were more selective inhibitors, but still inhibited two or more protein kinases with similar potency. LY 294002 was found to inhibit casein kinase-2 with similar potency to phosphoinositide (phosphatidylinositol) 3-kinase. The compounds with the most impressive selectivity profiles were KN62, PD 98059, U0126, PD 184352, rapamycin, wortmannin, SB 203580 and SB 202190. U0126 and PD 184352, like PD 98059, were found to block the mitogen-activated protein kinase (MAPK) cascade in cell-based assays by preventing the activation of MAPK kinase (MKK1), and not by inhibiting MKK1 activity directly. Apart from rapamycin and PD 184352, even the most selective inhibitors affected at least one additional protein kinase. Our results demonstrate that the specificities of protein kinase inhibitors cannot be assessed simply by studying their effect on kinases that are closely related in primary structure. We propose guidelines for the use of protein kinase inhibitors in cell-based assays.

Glycogen synthase association with the striated muscle glycogen-targeting subunit of protein phosphatase-1. Synthase activation involves scaffolding regulated by beta-adrenergic signaling

Glycogen-binding subunits for protein phosphatase-1 (PP1) target the PP1 catalytic subunit (PP1C) to glycogen particles, where the enzymes glycogen synthase and glycogen phosphorylase are concentrated. Here we identify sites within the striated muscle glycogen-binding subunit (G(M)) that mediate direct binding to glycogen synthase. Both PP1C and glycogen synthase were coimmunoprecipitated with a full-length FLAG-tagged G(M) transiently expressed in COS7 cells or C2C12 myotubes. Deletion and mutational analysis of a glutathione S-transferase (GST) fusion of the N-terminal domain of G(M) (residues 1-240) identified two putative sites for binding to glycogen synthase, one of which is the WXNXGXNYX(I/L) motif that is conserved among the family of PP1 glycogen-binding subunits. Either deletion of this motif or Ala substitution of Asn-228 in this motif disrupted the binding of glycogen synthase. Expression of full-length FLAG-G(M) in cells increased the activity of endogenous glycogen synthase, but protein disabled in either PP1 binding or glycogen synthase binding did not produce synthase activation. The results show that efficient activation of glycogen synthase requires a scaffold function of G(M) that involves simultaneous binding of both PP1C and glycogen synthase. Isoproterenol and forskolin treatment of cells decreased glycogen synthase binding to FLAG-G(M), thereby limiting synthase activation by PP1. This response was insensitive to inhibition by H-89, therefore probably not involving cAMP-dependent protein kinase, but did require inclusion of microcystin-LR during cell lysis, implying that phosphorylation was modulating binding of glycogen synthase. Phosphorylation control of binding to a scaffold site on the G(M) subunit of PP1 offers a new mechanism for regulation of muscle glycogen synthase in response to beta-adrenergic signals.

A GSK3-binding peptide from FRAT1 selectively inhibits the GSK3-catalysed phosphorylation of axin and beta-catenin

The Axin-dependent phosphorylation of beta-catenin catalysed by glycogen synthase kinase-3 (GSK3) is inhibited during embryogenesis. This protects beta-catenin against ubiquitin-dependent proteolysis, leading to its accumulation in the nucleus, where it controls the expression of genes important for development. Frequently rearranged in advanced T-cell lymphomas 1 (FRAT1) is a mammalian homologue of a GSK3-binding protein (GBP), which appears to play a key role in the correct establishment of the dorsal-ventral axis in Xenopus laevis. Here, we demonstrate that FRATtide (a peptide corresponding to residues 188-226 of FRAT1) binds to GSK3 and prevents GSK3 from interacting with Axin. FRATtide also blocks the GSK3-catalysed phosphorylation of Axin and beta-catenin, suggesting a potential mechanism by which GBP could trigger axis formation. In contrast, FRATtide does not suppress GSK3 activity towards other substrates, such as glycogen synthase and eIF2B, whose phosphorylation is independent of Axin but dependent on a 'priming' phosphorylation. This may explain how the essential cellular functions of GSK3 can continue, despite the suppression of beta-catenin phosphorylation.

Phosphorylation of a vesicular monoamine transporter by casein kinase II

The vesicular monoamine transporters (VMATs) package monoamine neurotransmitters into secretory vesicles for regulated exocytotic release. One isoform occurs in the adrenal gland (VMAT1) and another in the brain (VMAT2). To assess their potential for regulation, we have investigated the phosphorylation of the VMATs. Using heterologous expression in Chinese hamster ovary, PC12, and COS cells, we find that rat VMAT2, but not VMAT1, is constitutively phosphorylated. Phosphoamino acid analysis indicates that this phosphorylation occurs on serine residues, and the analysis of VMAT1-VMAT2 chimeras and site-directed mutagenesis localize the phosphorylation sites to serines 512 and 514 at the carboxyl terminus of VMAT2. Since these residues occur in an acidic region, we tested the ability of the acidotropic kinases casein kinase I (CKI) and casein kinase II (CKII) to phosphorylate bacterial fusion proteins containing the carboxyl terminus of VMAT2. Purified CKI and CKII phosphorylate the wild-type carboxyl terminus of VMAT2, but not a double mutant with both serines 512 and 514 replaced by alanine. The protein kinase inhibitor CKI-7 and unlabeled GTP both block in vitro phosphorylation by cell homogenates, indicating a role for CKII and possibly CKI in vivo. Both kinases phosphorylate the VMAT2 fusion protein to a much greater extent than a similar fusion protein containing the carboxyl terminus of VMAT1, consistent with differential phosphorylation of the two transporters observed in intact cells. These results provide the first demonstration of phosphorylation of a vesicular neurotransmitter transporter and a potential mechanism for differential regulation of the two VMATs.

The Lx1 gene maps to mouse chromosome 17 and codes for a protein that is homologous to glucose and polyspecific transmembrane transporters

A novel mouse gene, provisionally named Lx1, has been cloned and sequenced. Lx1 most likely represents the mouse homolog of the rat gene OCT1, which encodes a polyspecific transmembrane transporter that is possibly involved in drug elimination. The LX1 predicted protein is highly hydrophobic, possesses twelve putative transmembrane domains, and also shares significant homology with members of the sugar transporter family, particularly the novel liver-specific transporter NLT. Lx1 mRNA is expressed at high levels in mouse liver, kidney, and intestine, and at low levels in the adrenals and in lactating mammary glands. The Lx1 gene maps very close to the imprinted Igf2r/Mpr300 gene on mouse Chromosome (Chr) 17, in a region that is syntenic to human Chr 6q. Chr 6q has been previously associated with transient neonatal diabetes mellitus and breast cancer.

Multiple mechanisms for the phosphorylation of C-terminal regulatory sites in rabbit muscle glycogen synthase expressed in COS cells

Glycogen synthase can be inactivated by sequential phosphorylation at the C-terminal residues Ser652 (site 4), Ser648 (site 3c), Ser644 (site 3b) and Ser640 (site 3a) catalysed by glycogen synthase kinase-3. In vitro, glycogen synthase kinase-3 action requires that glycogen synthase has first been phosphorylated at Ser656 (site 5) by casein kinase II. Recently we demonstrated that inactivation is linked only to phosphorylation at site 3a and site 3b, and that, in COS cells, modification of these sites can occur by alternative mechanisms independent of any C-terminal phosphorylations [Skurat and Roach (1995) J. Biol. Chem. 270, 12491-12497]. To address these mechanisms multiple Ser-->Ala mutations were introduced in glycogen synthase such that only site 3a or site 3b remained intact. Additional mutation of Arg637-->Gln eliminated phosphorylation of site 3a, indicating that Arg637 may be important for recognition of site 3a by its corresponding protein kinase(s). Similarly, additional mutation of Pro645-->Ala eliminated phosphorylation of site 3b, indicating a possible involvement of 'proline-directed' protein kinase(s). Mutation of Arg637 alone did not activate glycogen synthase as expected from the loss of phosphorylation at site 3a. Rather, mutation of both Arg637 and the Ser-->Ala substitution at site 3b was required for substantial activation. The results suggest that sites 3a and 3b can be phosphorylated independently of one another by distinct protein kinases. However, phosphorylation of site 3b can potentiate phosphorylation of site 3a, by an enzyme such as glycogen synthase kinase-3.

Glycogen synthase kinase-3 beta phosphorylates tau protein at multiple sites in intact cells

Hyperphosphorylated tau protein is the major constituent of the paired helical filament (PHF), the major fibrous component of the neurofibrillary lesions of Alzheimer's disease (AD). Hyperphosphorylation of tau is believed to be the critical event that leads to filament assembly. Identification of the responsible protein kinases is therefore a key step towards an understanding of the pathogenesis of AD. Mitogen-activated protein kinase, glycogen synthase kinase-3 (GSK3) and neuronal cdc2-like kinase have been shown to phosphorylate tau protein in vitro at a number of sites that are phosphorylated in PHFs. In this study, we report that transient transfection of human GSK3 beta into Chinese hamster ovary cells stably transfected with individual human tau isoforms leads to hyperphosphorylation of tau at all the sites investigated with phosphorylation-dependent anti-tau antibodies. Thus, GSK3 beta is a protein kinase that phosphorylates tau protein in intact cells.

Phosphorylation of sites 3a and 3b (Ser640 and Ser644) in the control of rabbit muscle glycogen synthase

Glycogen synthase kinase-3 inactivates rabbit muscle glycogen synthase by sequential phosphorylation of four COOH-terminal residues Ser652 (site 4), Ser648 (site 3c), Ser644 (site 3b), and Ser640 (site 3a). Effective recognition of glycogen synthase by glycogen synthase kinase-3 occurs only after the phosphorylation of Ser656 (site 5) catalyzed by casein kinase II. The present study addresses specifically the role of sites 3a and 3b in the regulation of glycogen synthase expressed in COS cells. Simultaneous Ser-->Ala substitutions at sites 3 a, b and c, 4, and 5 in the same protein molecule eliminated 32P labeling in the proteolytic fragment Arg634-Lys682, which contains these sites. This mutant enzyme (which also had a Ser-->Ala substitution at site 2 in the NH2 terminus) had a -/+ glucose-6-P activity ratio of approximately 0.8, similar to that of totally dephosphorylated enzyme. Reinstating serine residues at either site 3a or site 3b restored labeling in the Arg634-Lys682 peptide and caused a decrease in the activity ratio to 0.4-0.6. When both sites 3a and 3b were reintroduced, there was complete inactivation of the enzyme. Thus, sites 3a and 3b are sufficient for the inactivation of glycogen synthase and act synergistically to control activity. This investigation demonstrates the existence of an alternate mechanism for the phosphorylation of sites 3a and 3b that does not depend on prior phosphorylation of site 5.

Modulation of GSK-3-catalyzed phosphorylation of microtubule-associated protein tau by non-proline-dependent protein kinases

The phosphorylation of bovine tau, either by GSK-3 alone or by a combination of GSK-3 and several non-proline-dependent protein kinases (non-PDPKs), was studied. GSK-3 alone catalyzed the incorporation of approximately 3 mol 32P/mol tau at a relatively slow rate. Prephosphorylation of tau by A-kinase, C-kinase, or CK-2 (but not by CK-1, CaM kinase II or Gr kinase) increased both the rate and extent of a subsequent phosphorylation catalyzed by GSK-3 by several-fold. These results suggest that the phosphorylation of tau by PDPKs such as GSK-3 (and possibly MAP kinase, cdk5) may be positively modulated at the substrate level by non-PDPK-catalyzed phosphorylations.