Hagit Eldar-Finkelman

Glycogen synthase kinase 3: an emerging therapeutic target

Glycogen synthase kinase 3 (GSK-3) is a serine/threonine protein kinase that has recently emerged as a key target in drug discovery. It has been implicated in multiple cellular processes and linked with the pathogenesis of several diseases. GSK-3 inhibitors might prove useful as therapeutic compounds in the treatment of conditions associated with elevated levels of enzyme activity, such as type 2 diabetes and Alzheimers disease. The pro-apoptotic feature of GSK-3 activity suggests a potential role for its inhibitors in protection against neuronal cell death, and in the treatment of traumatic head injury and stroke. Finally, selective inhibitors of GSK-3 could mimic the action of mood stabilizers such as lithium and valproic acid and be used in the treatment of bipolar mood disorders.

Rapid antidepressive-like activity of specific glycogen synthase kinase-3 inhibitor and its effect on β-catenin in mouse hippocampus

BACKGROUND Inhibition of glycogen synthase kinase-3 (GSK-3) is thought to be a key feature in the therapeutic mechanism of several mood stabilizers; however, the role of GSK-3 in depressive behavior has not been determined. In these studies, we evaluated the antidepressive effect of L803-mts, a novel GSK-3 peptide inhibitor, in an animal model of depression, the mouse forced swimming test (FST). METHODS Animals were intracerebroventricularly injected with L803-mts or with respective control peptide (cp) 1 hour, 3 hours, or 12 hours before their subjection to FST. RESULTS Animals administered L803-mts showed reduced duration of immobility at all three time points tested, as compared with cp-treated animals. Expression levels of beta-catenin, the endogenous substrate of GSK-3, increased in the hippocampus of L803-mts-treated animals by 20%-50%, as compared with cp-treated animals. CONCLUSIONS Our studies show, for the first time, that in-vivo inhibition of GSK-3 produces antidepressive-like behavior and suggest the potential of GSK-3 inhibitors as antidepressants.

GSK-3 Inhibitors: Preclinical and Clinical Focus on CNS

Inhibiting glycogen synthase kinase-3 (GSK-3) activity via pharmacological intervention has become an important strategy for treating neurodegenerative and psychiatric disorders. The known GSK-3 inhibitors are of diverse chemotypes and mechanisms of action and include compounds isolated from natural sources, cations, synthetic small-molecule ATP-competitive inhibitors, non-ATP-competitive inhibitors, and substrate–competitive inhibitors. Here we describe the variety of GSK-3 inhibitors with a specific emphasis on their biological activities in neurons and neurological disorders. We further highlight our current progress in the development of non-ATP-competitive inhibitors of GSK-3. The available data raise the hope that one or more of these drug design approaches will prove successful at stabilizing or even reversing the aberrant neuropathology and cognitive deficits of certain central nervous system disorders.

Inhibition of glycogen synthase kinase-3β by bivalent zinc ions: insight into the insulin-mimetic action of zinc

Zinc is an important trace element found in most body tissues as bivalent cations and has essential roles in human health. The insulin-like effect of zinc cations raises the possibility that they inhibit glycogen synthase kinase-3beta (GSK-3beta), a serine/threonine protein kinase linked with insulin resistance and type 2 diabetes. Here we show that physiological concentrations of zinc ions directly inhibit GSK-3beta in vitro in an uncompetitive manner. Treatment of HEK-293 cells with zinc enhanced glycogen synthase activity and increased the intracellular levels of beta-catenin, providing evidence for inhibition of endogenous GSK-3beta by zinc. Moreover, zinc ions enhanced glucose uptake 3-fold in isolated mouse adipocytes, an increase similar to activation with saturated concentrations of insulin. We propose that the in vivo insulin-mimetic actions of zinc are mediated via direct inhibition of endogenous GSK-3beta.

Inhibition of Glycogen Synthase Kinase-3 Ameliorates β-Amyloid Pathology and Restores Lysosomal Acidification and Mammalian Target of Rapamycin Activity in the Alzheimer Disease Mouse Model: IN VIVO AND IN VITRO STUDIES*

Background: The mechanisms behind the contribution of GSK-3 to Alzheimer disease pathogenesis remain elusive. Results: A GSK-3 inhibitor reduced Aβ pathology and ameliorated cognitive decline in an Alzheimer disease mouse model. GSK-3 impairs lysosomal acidification and impacts mTOR activity. Conclusion: Inhibition of GSK-3 reverses Alzheimer disease pathogenesis via restoration of lysosomal acidification and reactivation of mTOR. Significance: We identified novel mechanisms linking GSK-3 with Aβ pathology. Accumulation of β-amyloid (Aβ) deposits is a primary pathological feature of Alzheimer disease that is correlated with neurotoxicity and cognitive decline. The role of glycogen synthase kinase-3 (GSK-3) in Alzheimer disease pathogenesis has been debated. To study the role of GSK-3 in Aβ pathology, we used 5XFAD mice co-expressing mutated amyloid precursor protein and presenilin-1 that develop massive cerebral Aβ loads. Both GSK-3 isozymes (α/β) were hyperactive in this model. Nasal treatment of 5XFAD mice with a novel substrate competitive GSK-3 inhibitor, L803-mts, reduced Aβ deposits and ameliorated cognitive deficits. Analyses of 5XFAD hemi-brain samples indicated that L803-mts restored the activity of mammalian target of rapamycin (mTOR) and inhibited autophagy. Lysosomal acidification was impaired in the 5XFAD brains as indicated by reduced cathepsin D activity and decreased N-glycoyslation of the vacuolar ATPase subunit V0a1, a modification required for lysosomal acidification. Treatment with L803-mts restored lysosomal acidification in 5XFAD brains. Studies in SH-SY5Y cells confirmed that GSK-3α and GSK-3β impair lysosomal acidification and that treatment with L803-mts enhanced the acidic lysosomal pool as demonstrated in LysoTracker Red-stained cells. Furthermore, L803-mts restored impaired lysosomal acidification caused by dysfunctional presenilin-1. We provide evidence that mTOR is a target activated by GSK-3 but inhibited by impaired lysosomal acidification and elevation in amyloid precursor protein/Aβ loads. Taken together, our data indicate that GSK-3 is a player in Aβ pathology. Inhibition of GSK-3 restores lysosomal acidification that in turn enables clearance of Aβ burdens and reactivation of mTOR. These changes facilitate amelioration in cognitive function.

The role of glycogen synthase kinase-3 in insulin resistance and type 2 diabetes.

Glycogen synthase kinase-3 (GSK-3) is a ubiquitous cytosolic serine/threonine protein kinase that has been implicated in multiple receptor-mediated intracellular processes. Its unique feature, which distinguishes it from other protein kinases, is that it is constitutively active in resting conditions and acts as a suppressor of signalling pathways. The fact that the function of two key targets of insulin action, glycogen synthase and insulin receptor substrate-1, are suppressed by GSK-3, as well as the fact that GSK-3 activity is higher in diabetic tissues, makes it a promising drug discovery target for insulin resistance and Type 2 diabetes. Thus, the development of GSK-3 inhibitors has received attention as an attempt to control both the spread of the disease and its severity.

Role of glycogen synthase kinase-3β in early depressive behavior induced by mild traumatic brain injury

Traumatic brain injury (TBI) is a triggering event for a set of pathophysiological changes and concomitant depressive behavior. Glycogen synthase kinase-3 (GSK-3) is a potent in vivo regulator of cell apoptosis and, in addition, is implicated in depressive behavior. In this study, we investigated the role of GSK-3 in the physiological model of mild TBI (mTBI) at both the cellular and behavior levels. mTBI resulted in increased phosphorylation of inhibitory site serine(9) of GSK-3beta, which coincided with increased serine(473) phosphorylation of its upstream kinase PKB and accumulation of its downstream target beta-catenin in the hippocampus. mTBI induced a depressive behavior which was evident as early as 24 h post-injury. Pretreatment with GSK-3 inhibitors, lithium, or L803-mts prevented mTBI-induced depression. We suggest that mTBI elicits a pro-survival cascade of PKB/GSK-3beta/beta-catenin as part of a rehabilitation program. Furthermore, the use of selective GSK-3 inhibitors may have therapeutic benefits in treatment conditions associated with brain injury.

Lithium-mediated phosphorylation of glycogen synthase kinase-3β involves PI3 kinase-dependent activation of protein kinase C-α

Lithium, a known mood-stabilizer frequently used in treatment of bipolar disorders, is an effective glycogen synthase kinase-3β (GSK-3β) inhibitor. This led to the idea that GSK-3β is an in vivo target directly inhibited by lithium. As lithium is a weak in vitro inhibitor of GSK-3β (IC50=2 mM), however, we speculated that it inhibits GSK-3β via an indirect, yet unknown, mechanism. The present studies show that lithium increased the phosphorylation of a key inhibitory site of GSK-3β, serine-9 (Ser-9), in HEK293 cells and in PC12 cells. This phosphorylation was significantly reduced by protein kinase C (PKC) inhibitors GF109203X and Ro31-8425, as well as GÖ6976, and effective inhibitor toward conventional PKC isoforms (cPKC). Consistent with these results, lithium increased PKC-α activity approximately twofold in both cell lines. Because PI3 kinase is a potential upstream regulator of cPKC, its inhibition by wortmannin or LY294002 also abolished the lithium-induced serine phosphorylation of GSK-3β in HEK293 and PC12 cells. Moreover, lithium did not activate PKB, and in addition, its activity was not dependent on the presence of medium inositol nor did it affect the autophosphorylation activity of GSK-3β. Finally, intracerebroventricular injection of lithium increased GSK-3β Ser-9 phosphorylation and enhanced PKC-α activity 1.8-fold in mouse hippocampus, confirming this lithium response in vivo. Our studies propose a new mechanism by which lithium indirectly inhibits GSK-3β via phosphatidylinositol 3 kinase-dependent activation of PKC-α.

Challenges and opportunities with glycogen synthase kinase-3 inhibitors for insulin resistance and Type 2 diabetes treatment

The role of the serine/threonine protein kinase, glycogen synthase kinase-3 (GSK-3), in attenuating the insulin signalling pathway has led to the concept that inhibition of GSK-3 may have therapeutic benefits in the treatment of insulin resistance and Type 2 diabetes. Indeed, various selective GSK-3 inhibitors have been developed recently and have proven to promote insulin-like effects and to act as insulin sensitisers in both in vitro and in vivo systems. GSK-3 inhibition may thus present a new, effective approach for the treatment of insulin resistance and Type 2 diabetes. This review describes the qualifications of GSK-3 as a novel drug-discovery target for Type 2 diabetes and discusses the strategies and challenges in developing small-molecule inhibitors for this important protein kinase.

Substrate competitive GSK-3 inhibitors - strategy and implications.

Glycogen synthase kinase-3 (GSK-3) is a highly conserved protein serine/threonine kinase ubiquitously distributed in eukaryotes as a constitutively active enzyme. Abnormally high GSK-3 activity has been implicated in several pathological disorders, including diabetes and neuron degenerative and affective disorders. This led to the hypothesis that inhibition of GSK-3 may have therapeutic benefit. Most GSK-3 inhibitors developed so far compete with ATP and often show limited specificity. Our goal is to develop inhibitors that compete with GSK-3 substrates, as this type of inhibitor is more specific and may be useful for clinical applications. We have employed computational, biochemical, and molecular analyses to gain in-depth understanding of GSK-3s substrate recognition. Here we argue that GSK-3 is a promising drug discovery target and describe the strategy and practice for developing specific substrate-competitive inhibitors of GSK-3.