Insulin transiently increases tau phosphorylation: involvement of glycogen synthase kinase-3beta and Fyn tyrosine kinase

Insulin Resistance as a Link between Amyloid-Beta and Tau Pathologies in Alzheimer's Disease

Current hypotheses and theories regarding the pathogenesis of Alzheimer’s disease (AD) heavily implicate brain insulin resistance (IR) as a key factor. Despite the many well-validated metrics for systemic IR, the absence of biomarkers for brain-specific IR represents a translational gap that has hindered its study in living humans. In our lab, we have been working to develop biomarkers that reflect the common mechanisms of brain IR and AD that may be used to follow their engagement by experimental treatments. We present two promising biomarkers for brain IR in AD: insulin cascade mediators probed in extracellular vesicles (EVs) enriched for neuronal origin, and two-dimensional magnetic resonance spectroscopy (MRS) measures of brain glucose. As further evidence for a fundamental link between brain IR and AD, we provide a novel analysis demonstrating the close spatial correlation between brain expression of genes implicated in IR (using Allen Human Brain Atlas data) and tau and beta-amyloid pathologies. We proceed to propose the bold hypotheses that baseline differences in the metabolic reliance on glycolysis, and the expression of glucose transporters (GLUT) and insulin signaling genes determine the vulnerability of different brain regions to Tau and/or Amyloid beta (Aβ) pathology, and that IR is a critical link between these two pathologies that define AD. Lastly, we provide an overview of ongoing clinical trials that target IR as an angle to treat AD, and suggest how biomarkers may be used to evaluate treatment efficacy and target engagement.

Insulin resistance in Alzheimer's disease

The links between systemic insulin resistance (IR), brain-specific IR, and Alzheimer's disease (AD) have been an extremely productive area of current research. This review will cover the fundamentals and pathways leading to IR, its connection to AD via cellular mechanisms, the most prominent methods and models used to examine it, an introduction to the role of extracellular vesicles (EVs) as a source of biomarkers for IR and AD, and an overview of modern clinical studies on the subject. To provide additional context, we also present a novel analysis of the spatial correlation of gene expression in the brain with the aid of Allen Human Brain Atlas data. Ultimately, examining the relation between IR and AD can be seen as a means of advancing the understanding of both disease states, with IR being a promising target for therapeutic strategies in AD treatment. In conclusion, we highlight the therapeutic potential of targeting brain IR in AD and the main strategies to pursue this goal.

Glycogen Synthase Kinase 3: A Kinase for All Pathways?

Glycogen synthase kinase-3 (GSK-3) is an unusual protein-serine kinase in that it is primarily regulated by inhibition and lies downstream of multiple cell signaling pathways. This raises a variety of questions in terms of its physiological role(s), how signaling specificity is maintained and why so many eggs have been placed into one basket. There are actually two baskets, as there are two isoforms, GSK-3α and β, that are highly related and largely redundant. Their many substrates range from regulators of cellular metabolism to molecules that control growth and differentiation. In this chapter, we review the characteristics of GSK-3, update progress in understanding the kinase, and try to answer some of the questions raised by its unusual properties. Indeed, the kinase may trigger transformation in our thinking of how cellular signals are organized and controlled.

Effects of mutations in Wnt/β-catenin, hedgehog, Notch and PI3K pathways on GSK-3 activity-Diverse effects on cell growth, metabolism and cancer

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase that participates in an array of critical cellular processes. GSK-3 was first characterized as an enzyme that phosphorylated and inactivated glycogen synthase. However, subsequent studies have revealed that this moon-lighting protein is involved in numerous signaling pathways that regulate not only metabolism but also have roles in: apoptosis, cell cycle progression, cell renewal, differentiation, embryogenesis, migration, regulation of gene transcription, stem cell biology and survival. In this review, we will discuss the roles that GSK-3 plays in various diseases as well as how this pivotal kinase interacts with multiple signaling pathways such as: PI3K/PTEN/Akt/mTOR, Ras/Raf/MEK/ERK, Wnt/beta-catenin, hedgehog, Notch and TP53. Mutations that occur in these and other pathways can alter the effects that natural GSK-3 activity has on regulating these signaling circuits that can lead to cancer as well as other diseases. The novel roles that microRNAs play in regulation of the effects of GSK-3 will also be evaluated. Targeting GSK-3 and these other pathways may improve therapy and overcome therapeutic resistance.

Retroviral induction of GSK-3β expression blocks the stimulatory action of physical exercise on the maturation of newborn neurons

Adult hippocampal neurogenesis (AHN) is a key process for certain types of hippocampal-dependent learning. Alzheimer's disease (AD) is accompanied by memory deficits related to alterations in AHN. Given that the increased activity of GSK-3β has been related to alterations in the population of hippocampal granule neurons in AD patients, we designed a novel methodology by which to induce selective GSK-3β overexpression exclusively in newborn granule neurons. To this end, we injected an rtTA-IRES-EGFP-expressing retrovirus into the hippocampus of tTO-GSK-3β mice. Using this novel retroviral strategy, we found that GSK-3β caused a cell-autonomous impairment of the morphological and synaptic maturation of newborn neurons. In addition, we examined whether GSK-3β overexpression in newborn neurons limits the effects of physical activity. While physical exercise increased the number of dendritic spines, the percentage of mushroom spines, and the head diameter of the same in tet-OFF cells, these effects were not triggered in tet-ON cells. This observation suggests that GSK-3β blocks the stimulatory actions of exercise. Given that the activity of GSK-3β is increased in the brains of individuals with AD, these data may be relevant for non-pharmacological therapies for AD.

The rise and fall of insulin signaling in Alzheimer's disease

The prevalence of both diabetes and Alzheimer's disease (AD) are reaching epidemic proportions worldwide. Alarmingly, diabetes is also a risk factor for Alzheimer's disease. The AD brain is characterised by the accumulation of peptides called Aβ as plaques in the neuropil and hyperphosphorylated tau protein in the form of neurofibrillary tangles within neurons. How diabetes confers risk is unknown but a simple linear relationship has been proposed whereby the hyperinsulinemia associated with type 2 diabetes leads to decreased insulin signaling in the brain, with downregulation of the PI3K/AKT signalling pathway and its inhibition of the major tau kinase, glycogen synthase kinase 3β. The earliest studies of post mortem AD brain tissue largely confirmed this cascade of events but subsequent studies have generally found either an upregulation of AKT activity, or that the relationship between insulin signaling and AD is independent of glycogen synthase kinase 3β altogether. Given the lack of success of beta-amyloid-reducing therapies in clinical trials, there is intense interest in finding alternative or adjunctive therapeutic targets for AD. Insulin signaling is a neuroprotective pathway and represents an attractive therapeutic option. However, this incredibly complex signaling pathway is not fully understood in the human brain and particularly in the context of AD. Here, we review the ups and downs of the research efforts aimed at understanding how diabetes modifies AD risk.

Metformin promotes tau aggregation and exacerbates abnormal behavior in a mouse model of tauopathy

Background

Alzheimer disease (AD) and other tauopathies develop cerebral intracellular inclusions of hyperphosphorylated tau. Epidemiological and experimental evidence suggests a clear link between type 2 diabetes mellitus and AD. In AD animal models, tau pathology is exacerbated by metabolic comorbidities, such as insulin resistance and diabetes. Within this context, anitidiabetic drugs, including the widely-prescribed insulin-sensitizing drug metformin, are currently being investigated for AD therapy. However, their efficacy for tauopathy in vivo has not been tested.

Results

Here, we report that in the P301S mutant human tau (P301S) transgenic mouse model of tauopathy, chronic administration of metformin exerts paradoxical effects on tau pathology. Despite reducing tau phosphorylation in the cortex and hippocampus via AMPK/mTOR and PP2A, metformin increases insoluble tau species (including tau oligomers) and the number of inclusions with β-sheet aggregates in the brain of P301S mice. In addition, metformin exacerbates hindlimb atrophy, increases P301S hyperactive behavior, induces tau cleavage by caspase 3 and disrupts synaptic structures.

Conclusions

These findings indicate that metformin pro-aggregation effects mitigate the potential benefits arising from its dephosphorylating action, possibly leading to an overall increase of the risk of tauopathy in elderly diabetic patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0082-7) contains supplementary material, which is available to authorized users.

Tau Hyperphosphorylation and Oxidative Stress, a Critical Vicious Circle in Neurodegenerative Tauopathies?

Hyperphosphorylation and aggregation of the microtubule-associated protein tau in brain, are pathological hallmarks of a large family of neurodegenerative disorders, named tauopathies, which include Alzheimer's disease. It has been shown that increased phosphorylation of tau destabilizes tau-microtubule interactions, leading to microtubule instability, transport defects along microtubules, and ultimately neuronal death. However, although mutations of the MAPT gene have been detected in familial early-onset tauopathies, causative events in the more frequent sporadic late-onset forms and relationships between tau hyperphosphorylation and neurodegeneration remain largely elusive. Oxidative stress is a further pathological hallmark of tauopathies, but its precise role in the disease process is poorly understood. Another open question is the source of reactive oxygen species, which induce oxidative stress in brain neurons. Mitochondria have been classically viewed as a major source for oxidative stress, but microglial cells were recently identified as reactive oxygen species producers in tauopathies. Here we review the complex relationships between tau pathology and oxidative stress, placing emphasis on (i) tau protein function, (ii) origin and consequences of reactive oxygen species production, and (iii) links between tau phosphorylation and oxidative stress. Further, we go on to discuss the hypothesis that tau hyperphosphorylation and oxidative stress are two key components of a vicious circle, crucial in neurodegenerative tauopathies.

Glycogen synthase kinase-3: a potential preventive target for prostate cancer management

OBJECTIVES

Prostate cancers are the frequently diagnosed cancers in men, and patients with metastatic disease only have 28% chance for 5-year survival. Patients with low-risk tumors are subjected to active surveillance, whereas high-risk cases are actively treated. Unfortunately, there is no cure for patients with late-stage disease. Glycogen synthase kinase-3 (GSK-3, α and β) is a protein serine/threonine kinase and has diverse cellular functions and numerous substrates. We sought to summarize all the studies done with GSK-3 in prostate cancers and to provide a prospective direction for future work.

METHODS AND MATERIALS

A comprehensive search of the literature on the electronic databases PubMed was conducted for the subject terms of GSK-3 and prostate cancer. Gene mutation and expression information was extracted from Oncomine and COSMIC databases. Case reports were not included.

RESULTS

Accumulating evidence indicates that GSK-3α is mainly expressed in low-risk prostate cancers and is related to hormone-dependent androgen receptor (AR)-mediated gene expression, whereas GSK-3β is mainly expressed in high-risk prostate cancers and is related to hormone-independent AR-mediated gene expression. GSK-3 has been demonstrated as a positive regulator in AR transactivation and prostate cancer growth independent of the Wnt/β-catenin pathway. Different types of GSK-3inhibitors including lithium show promising results in suppressing tumor growth in different animal models of prostate cancer. Importantly, clinical use of lithium is associated with reduced cancer incidence in psychiatric patients.

CONCLUSIONS

Taken together, GSK-3 inhibition might be implicated in prostate cancer management as a preventive treatment.

Expression of β-site APP-cleaving enzyme 1 in the hippocampal tissue of an insulin-resistant rat model of Alzheimer's disease

The aim of this study was to investigate the expression of β-site APP-cleaving enzyme 1 (BACE1) in the hippocampal tissue of an insulin-resistant rat model, and thereby explore the roles of BACE1 and insulin resistance (IR) in the pathogenesis of Alzheimer's disease (AD). A total of 36 male Sprague-Dawley rats, aged 2 months, were randomly divided into three groups. These were an insulin-resistant (experimental) group, a high fat control group and a blank control group. The cognitive function and behavioral changes of the rats were tested by a Morris water maze experiment. Amyloid β (Aβ) deposition was detected by an immunohistochemical method. The expression levels of BACE1 in the rat hippocampal tissues were detected by enzyme-linked immunosorbent assay, western blotting and reverse transcription-quantitative polymerase chain reaction technology. The rats in the experimental group had evident learning and memory impairment, with significantly decreased learning memory. The modeling was successful; in the experimental group, the rats exhibited IR and their glucose metabolism was significantly abnormal. However, there was no characteristic pathology of AD. The expression of BACE1 in the brain tissue of rats in the experimental group was significantly higher than that in high fat control and blank control groups (P<0.01). In conclusion, the expression of BACE1 in the brain tissue of insulin-resistant rats increased, and IR was indicated to participate in the pathogenesis of AD.

GSK-3 as potential target for therapeutic intervention in cancer

The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) was initially identified and studied in the regulation of glycogen synthesis. GSK-3 functions in a wide range of cellular processes. Aberrant activity of GSK-3 has been implicated in many human pathologies including: bipolar depression, Alzheimer's disease, Parkinson's disease, cancer, non-insulin-dependent diabetes mellitus (NIDDM) and others. In some cases, suppression of GSK-3 activity by phosphorylation by Akt and other kinases has been associated with cancer progression. In these cases, GSK-3 has tumor suppressor functions. In other cases, GSK-3 has been associated with tumor progression by stabilizing components of the beta-catenin complex. In these situations, GSK-3 has oncogenic properties. While many inhibitors to GSK-3 have been developed, their use remains controversial because of the ambiguous role of GSK-3 in cancer development. In this review, we will focus on the diverse roles that GSK-3 plays in various human cancers, in particular in solid tumors. Recently, GSK-3 has also been implicated in the generation of cancer stem cells in various cell types. We will also discuss how this pivotal kinase interacts with multiple signaling pathways such as: PI3K/PTEN/Akt/mTORC1, Ras/Raf/MEK/ERK, Wnt/beta-catenin, Hedgehog, Notch and others.

SIL1 Rescued Bip Elevation-Related Tau Hyperphosphorylation in ER Stress

Endoplasmic reticulum (ER) stress has been indicated in the early stage of Alzheimer's disease (AD), in which tau hyperphosphorylation is one major pathological alteration. The elevation of binding immunoglobulin protein (Bip), an important ER chaperon, was reported in AD brain. It is important to study the roles of ER-related chaperons in tau hyperphosphorylation. In this research, increased Bip was found in the brains of the AD model mice (Tg2576) compared to the age-matched control mice. Meanwhile, deficiency of SIL1, an important co-chaperon of Bip, was observed in brains of Tg2576 mice and in ER stress both in vivo and in vitro. Then, we transfected Bip-EGFP plasmid into HEK293 cells stably expressing the longest human tau (HEK293/tau) or N2a cells and found that increased Bip induced tau hyperphosphorylation via activating glycogen synthase kinase-3β (GSK-3β), an important tau kinase, and increased the association with tau and GSK-3β. When we overexpressed SIL1 in Bip-transfected HEK293/tau cells and thapsigargin-treated HEK293/tau cells, significantly reduced tau hyperphosphorylation and GSK-3β activation were observed. These results suggested the important roles of ER-related chaperons, Bip and SIL1, in AD-like tau hyperphosphorylation.

The association of single nucleotide polymorphism of the Fyn gene with sporadic Alzheimer's disease in the Chinese Han population

Recent studies suggested genetic factors contribute to the pathogenesis of sporadic Alzheimer's disease (sAD). Fibroblast Yes related novel (Fyn), a tau kinase, has been reported to be associated with aberrant phosphorylated tau and neurofibrillary tangles formation. Fyn gene may be a potential candidate gene for AD. To investigate the association of the polymorphisms in Fyn gene with the susceptibility to sAD, we conducted a case-control study in a Chinese Han cohort including 200 sAD patients and 243 control participants. Four single nucleotide polymorphisms (SNPs) (rs111787668, rs1057979, rs6916861 and rs12910) within the promoter region of Fyn gene and one (rs7768046) in intron were selected and genotyped with a polymerase chain reaction-ligase detection reaction (PCR-LDR) method. Logistic regression under four genetic models was used to analyze the association between target SNPs and the risk of sAD. After adjusting for age, sex and APOE ɛ4 status, no association was revealed between these SNPs or the haplotypes containing four SNPs and the risk of sAD (P>0.05). The SNPs in the selected regions of the Fyn gene are unlikely to confer the susceptibility of sAD in the Chinese Han population. Further studies with a larger sample size and different ethnic populations are needed to reveal the role of Fyn gene in the pathogenesis of sAD.

Elevated risk of type 2 diabetes for development of Alzheimer disease: a key role for oxidative stress in brain

Alzheimer disease (AD) is the most common form of dementia among the elderly and is characterized by progressive loss of memory and cognition. Epidemiological data show that the incidence of AD increases with age and doubles every 5 years after 65 years of age. From a neuropathological point of view, amyloid-β-peptide (Aβ) leads to senile plaques, which, together with hyperphosphorylated tau-based neurofibrillary tangles and synapse loss, are the principal pathological hallmarks of AD. Aβ is associated with the formation of reactive oxygen (ROS) and nitrogen (RNS) species, and induces calcium-dependent excitotoxicity, impairment of cellular respiration, and alteration of synaptic functions associated with learning and memory. Oxidative stress was found to be associated with type 2 diabetes mellitus (T2DM), which (i) represents another prevalent disease associated with obesity and often aging, and (ii) is considered to be a risk factor for AD development. T2DM is characterized by high blood glucose levels resulting from increased hepatic glucose production, impaired insulin production and peripheral insulin resistance, which close resemble to the brain insulin resistance observed in AD patients. Furthermore, growing evidence suggests that oxidative stress plays a pivotal role in the development of insulin resistance and vice versa. This review article provides molecular aspects and the pharmacological approaches from both preclinical and clinical data interpreted from the point of view of oxidative stress with the aim of highlighting progresses in this field.

Neuronal deletion of GSK3β increases microtubule speed in the growth cone and enhances axon regeneration via CRMP-2 and independently of MAP1B and CLASP2

Background

In the adult central nervous system, axonal regeneration is abortive. Regulators of microtubule dynamics have emerged as attractive targets to promote axonal growth following injury as microtubule organization is pivotal for growth cone formation. In this study, we used conditioned neurons with high regenerative capacity to further dissect cytoskeletal mechanisms that might be involved in the gain of intrinsic axon growth capacity.

Results

Following a phospho-site broad signaling pathway screen, we found that in conditioned neurons with high regenerative capacity, decreased glycogen synthase kinase 3β (GSK3β) activity and increased microtubule growth speed in the growth cone were present. To investigate the importance of GSK3β regulation during axonal regeneration in vivo, we used three genetic mouse models with high, intermediate or no GSK3β activity in neurons. Following spinal cord injury, reduced GSK3β levels or complete neuronal deletion of GSK3β led to increased growth cone microtubule growth speed and promoted axon regeneration. While several microtubule-interacting proteins are GSK3β substrates, phospho-mimetic collapsin response mediator protein 2 (T/D-CRMP-2) was sufficient to decrease microtubule growth speed and neurite outgrowth of conditioned neurons and of GSK3β-depleted neurons, prevailing over the effect of decreased levels of phosphorylated microtubule-associated protein 1B (MAP1B) and through a mechanism unrelated to decreased levels of phosphorylated cytoplasmic linker associated protein 2 (CLASP2). In addition, phospho-resistant T/A-CRMP-2 counteracted the inhibitory myelin effect on neurite growth, further supporting the GSK3β-CRMP-2 relevance during axon regeneration.

Conclusions

Our work shows that increased microtubule growth speed in the growth cone is present in conditions of increased axonal growth, and is achieved following inactivation of the GSK3β-CRMP-2 pathway, enhancing axon regeneration through the glial scar. In this context, our results support that a precise control of microtubule dynamics, specifically in the growth cone, is required to optimize axon regrowth.

Deregulation of brain insulin signaling in Alzheimer's disease

Contrary to the previous belief that insulin does not act in the brain, studies in the last three decades have demonstrated important roles of insulin and insulin signal transduction in various functions of the central nervous system. Deregulated brain insulin signaling and its role in molecular pathogenesis have recently been reported in Alzheimer's disease (AD). In this article, we review the roles of brain insulin signaling in memory and cognition, the metabolism of amyloid β precursor protein, and tau phosphorylation. We further discuss deficiencies of brain insulin signaling and glucose metabolism, their roles in the development of AD, and recent studies that target the brain insulin signaling pathway for the treatment of AD. It is clear now that deregulation of brain insulin signaling plays an important role in the development of sporadic AD. The brain insulin signaling pathway also offers a promising therapeutic target for treating AD and probably other neurodegenerative disorders.

Insulin, IGF-1 and GLP-1 signaling in neurodegenerative disorders: targets for disease modification?

Insulin and Insulin Growth Factor-1 (IGF-1) play a major role in body homeostasis and glucose regulation. They also have paracrine/autocrine functions in the brain. The Insulin/IGF-1 signaling pathway contributes to the control of neuronal excitability, nerve cell metabolism and cell survival. Glucagon like peptide-1 (GLP-1), known as an insulinotropic hormone has similar functions and growth like properties as insulin/IGF-1. Growing evidence suggests that dysfunction of these pathways contribute to the progressive loss of neurons in Alzheimer's disease (AD) and Parkinson's disease (PD), the two most frequent neurodegenerative disorders. These findings have led to numerous studies in preclinical models of neurodegenerative disorders targeting insulin/IGF-1 and GLP-1 signaling with currently available anti-diabetics. These studies have shown that administration of insulin, IGF-1 and GLP-1 agonists reverses signaling abnormalities and has positive effects on surrogate markers of neurodegeneration and behavioral outcomes. Several proof-of-concept studies are underway that attempt to translate the encouraging preclinical results to patients suffering from AD and PD. In the first part of this review, we discuss physiological functions of insulin/IGF-1 and GLP-1 signaling pathways including downstream targets and receptors distribution within the brain. In the second part, we undertake a comprehensive overview of preclinical studies targeting insulin/IGF-1 or GLP-1 signaling for treating AD and PD. We then detail the design of clinical trials that have used anti-diabetics for treating AD and PD patients. We close with future considerations that treat relevant issues for successful translation of these encouraging preclinical results into treatments for patients with AD and PD.

Insulin dysfunction and Tau pathology

The neuropathological hallmarks of Alzheimer's disease (AD) include senile plaques of β-amyloid (Aβ) peptides (a cleavage product of the Amyloid Precursor Protein, or APP) and neurofibrillary tangles (NFT) of hyperphosphorylated Tau protein assembled in paired helical filaments (PHF). NFT pathology is important since it correlates with the degree of cognitive impairment in AD. Only a small proportion of AD is due to genetic variants, whereas the large majority of cases (~99%) is late onset and sporadic in origin. The cause of sporadic AD is likely to be multifactorial, with external factors interacting with biological or genetic susceptibilities to accelerate the manifestation of the disease. Insulin dysfunction, manifested by diabetes mellitus (DM) might be such factor, as there is extensive data from epidemiological studies suggesting that DM is associated with an increased relative risk for AD. Type 1 diabetes (T1DM) and type 2 diabetes (T2DM) are known to affect multiple cognitive functions in patients. In this context, understanding the effects of diabetes on Tau pathogenesis is important since Tau pathology show a strong relationship to dementia in AD, and to memory loss in normal aging and mild cognitive impairment. Here, we reviewed preclinical studies that link insulin dysfunction to Tau protein pathogenesis, one of the major pathological hallmarks of AD. We found more than 30 studies reporting Tau phosphorylation in a mouse or rat model of insulin dysfunction. We also payed attention to potential sources of artifacts, such as hypothermia and anesthesia, that were demonstrated to results in Tau hyperphosphorylation and could major confounding experimental factors. We found that very few studies reported the temperature of the animals, and only a handful did not use anesthesia. Overall, most published studies showed that insulin dysfunction can promote Tau hyperphosphorylation and pathology, both directly and indirectly, through hypothermia.

Arsenic trioxide suppressed mantle cell lymphoma by downregulation of cyclin D1

Mantle cell lymphoma (MCL) is aggressive with poor prognosis. Due to t(11;14)(q13;q32), cyclin D1 is overexpressed. The in vitro activities of arsenic trioxide (As2O3) in MCL were investigated. In MCL lines Jeko-1 and Granta-519, As2O3 induced dose-dependent and time-dependent increases in apoptosis accompanied by cyclin D1 suppression. Downregulation of cyclin D1 resulted in decreased retinoblastoma protein phosphorylation, which led to repressed G1 progression to S/G2 phases. As2O3 did not affect cyclin D1 gene transcription. Instead, As2O3 activated glycogen synthase kinase-3beta (by tyrosine-216 phosphorylation) and IkappaB kinase alpha/beta (by serine-176/180 phosphorylation), both of which phosphorylated cyclin D1 at threonine-286, leading to its poly-ubiquitination and degradation in the proteasome. These observations were recapitulated partly in primary MCL samples obtained from patients refractory to conventional treatment. Our findings suggested that As2O3 might be clinically useful in MCL.

Serine 9 and tyrosine 216 phosphorylation of GSK-3β differentially regulates autophagy in acquired cadmium resistance

Glycogen synthase kinase-3β (GSK-3β) plays an important role in the regulation of apoptosis. To investigate its involvement in acquired cadmium (Cd) resistance, Cd-resistant cells (RH460) were established from H460 lung carcinoma cells. Cd resistance led to interruption of apoptosis and autophagy, as determined by an apoptotic sub-G1 population, procaspase-3 clevage, and LC3-II induction. Cd-induced autophagy preceded apoptosis as determined by 3-methyladenine or zVAD and time-course experiments after Cd treatment. Despite β-catenin accumulation, phospho(p)-Ser/Tyr GSK-3α/β increased in the nucleus until 12h after treatment and then p-Ser partly translocated to the cytoplasm. The GSK-3 inhibitor lithium augmented Cd-induced p-Ser GSK-3α/β, which accumulated in the nucleus and cytoplasm, and increased autophagy. SB216763 inhibited p-Ser/p-Tyr GSK-3α/β and subsequent autophagy. GSK-3β knockdown decreased Cd-induced autophagy. Cd exposure to RH460 cells overexpressed with pcDNA-GSK-3β-HA strongly phosphorylated Ser(9)/Tyr(216) residues and decreased LC3-II. Constitutively active pcDNA-GSK-3β(S9A)-HA overexpression phosphorylated Tyr(216) and decreased LC3-II, suggesting that p-Tyr inhibits autophagy. PI3K inhibitors decreased Cd-induced p-Ser GSK-3αβ and LC3-II, whereas a Ser/Thr phosphatase inhibitor, okadaic acid, hyperphosphorylated Ser residues, which accumulated in the nucleus and cytosol, and enhanced LC3-II. The general tyrosine kinase inhibitor genistein suppressed Cd-induced p-Tyr/p-Ser GSK-3α/β and LC3-II. Mouse lung tissues respond to long-term Cd exposure increased p-Tyr, downregulated LC3-II, and accumulated full-length Bax and procaspase-3. Taken together, this study shows that acquired Cd resistance is regulated by GSK-3β phosphorylation state, but not activation state, and intracellular localization of p-Ser GSK-3 regulates Cd-induced autophagy and apoptosis.

Multifaceted roles of GSK-3 and Wnt/β-catenin in hematopoiesis and leukemogenesis: opportunities for therapeutic intervention

Glycogen synthase kinase-3 (GSK-3) is well documented to participate in a complex array of critical cellular processes. It was initially identified in rat skeletal muscle as a serine/threonine kinase that phosphorylated and inactivated glycogen synthase. This versatile protein is involved in numerous signaling pathways that influence metabolism, embryogenesis, differentiation, migration, cell cycle progression and survival. Recently, GSK-3 has been implicated in leukemia stem cell pathophysiology and may be an appropriate target for its eradication. In this review, we will discuss the roles that GSK-3 plays in hematopoiesis and leukemogenesis as how this pivotal kinase can interact with multiple signaling pathways such as: Wnt/β-catenin, phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homolog (PTEN)/Akt/mammalian target of rapamycin (mTOR), Ras/Raf/MEK/extracellular signal-regulated kinase (ERK), Notch and others. Moreover, we will discuss how targeting GSK-3 and these other pathways can improve leukemia therapy and may overcome therapeutic resistance. In summary, GSK-3 is a crucial regulatory kinase interacting with multiple pathways to control various physiological processes, as well as leukemia stem cells, leukemia progression and therapeutic resistance. GSK-3 and Wnt are clearly intriguing therapeutic targets.

Glycogen synthase kinase-3β (GSK-3β) and its dysregulation in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most frequently occurring and devastating human brain malignancy, retaining almost universal mortality and a median survival of only 14 months, even with recent advances in multimodal treatments. Gliomas are characterised as being both highly resistant to chemo- and radiotherapy and highly invasive, rendering conventional interventions palliative. The continual dismal prognosis for GBM patients identifies an urgent need for the evolutionary development of new treatment modalities. This includes molecular targeted therapies as many signaling molecules and associated pathways have been implicated in the development and survival of malignant gliomas including the protein kinase, glycogen synthase kinase 3 beta (GSK-3β). Here we review the activity and function of GSK-3β in a number of signaling pathways and its role in gliomagenesis.

Glucose dependence of glycogen synthase activity regulation by GSK3 and MEK/ERK inhibitors and angiotensin-(1-7) action on these pathways in cultured human myotubes

Glycogen synthase (GS) is activated by glucose/glycogen depletion in skeletal muscle cells, but the contributing signaling pathways, including the chief GS regulator GSK3, have not been fully defined. The MEK/ERK pathway is known to regulate GSK3 and respond to glucose. The aim of this study was to elucidate the GSK3 and MEK/ERK pathway contribution to GS activation by glucose deprivation in cultured human myotubes. Moreover, we tested the glucose-dependence of GSK3 and MEK/ERK effects on GS and angiotensin (1-7) actions on these pathways. We show that glucose deprivation activated GS, but did not change phospho-GS (Ser640/1), GSK3β activity or activity-activating phosphorylation of ERK1/2. We then treated glucose-replete and -depleted cells with SB415286, U0126, LY294 and rapamycin to inhibit GSK3, MEK1/2, PI3K and mTOR, respectively. SB415286 activated GS and decreased the relative phospho-GS (Ser640/1) level, more in glucose-depleted than -replete cells. U0126 activated GS and reduced the phospho-GS (Ser640/1) content significantly in glucose-depleted cells, while GSK3β activity tended to increase. LY294 inactivated GS in glucose-depleted cells only, without affecting relative phospho-GS (Ser640/1) level. Rapamycin had no effect on GS activation. Angiotensin-(1-7) raised phospho-ERK1/2 but not phospho-GSK3β (Ser9) content, while it inactivated GS and increased GS phosphorylation on Ser640/1, in glucose-replete cells. In glucose-depleted cells, angiotensin-(1-7) effects on ERK1/2 and GS were reverted, while relative phospho-GSK3β (Ser9) content decreased. In conclusion, activation of GS by glucose deprivation is not due to GS Ser640/1 dephosphorylation, GSK3β or ERK1/2 regulation in cultured myotubes. However, glucose depletion enhances GS activation/Ser640/1 dephosphorylation due to both GSK3 and MEK/ERK inhibition. Angiotensin-(1-7) inactivates GS in glucose-replete cells in association with ERK1/2 activation, not with GSK3 regulation, and glucose deprivation reverts both hormone effects. Thus, the ERK1/2 pathway negatively regulates GS activity in myotubes, without involving GSK3 regulation, and as a function of the presence of glucose.

Unfolded protein response activates glycogen synthase kinase-3 via selective lysosomal degradation

The unfolded protein response (UPR) is a stress response that is activated upon disturbed homeostasis in the endoplasmic reticulum. In Alzheimer's disease, as well as in other tauopathies, the UPR is activated in neurons that contain early tau pathology. A recent genome-wide association study identified genetic variation in a UPR transducer as a risk factor for tauopathy, supporting a functional connection between UPR activation and tau pathology. Here we show that UPR activation increases the activity of the major tau kinase glycogen synthase kinase (GSK)-3 in vitro via a selective removal of inactive GSK-3 phosphorylated at Ser(21/9). We demonstrate that this is mediated by the autophagy/lysosomal pathway. In brain tissue from patients with different tauopathies, lysosomal accumulations of pSer(21/9) GSK-3 are found in neurons with markers for UPR activation. Our data indicate that UPR activation increases the activity of GSK-3 by a novel mechanism, the lysosomal degradation of the inactive pSer(21/9) GSK-3. This may provide a functional explanation for the close association between UPR activation and early tau pathology in neurodegenerative diseases.

Deregulation of protein phosphatase 2A and hyperphosphorylation of τ protein following onset of diabetes in NOD mice

The histopathological hallmarks of Alzheimer disease (AD) include intraneuronal neurofibrillary tangles composed of abnormally hyperphosphorylated τ protein. Insulin dysfunction might influence AD pathology, as population-based and cohort studies have detected higher AD incidence rates in diabetic patients. But how diabetes affects τ pathology is not fully understood. In this study, we investigated the impact of insulin dysfunction on τ phosphorylation in a genetic model of spontaneous type 1 diabetes: the nonobese diabetic (NOD) mouse. Brains of young and adult female NOD mice were examined, but young NOD mice did not display τ hyperphosphorylation. τ phosphorylation at τ-1 and pS422 epitopes was slightly increased in nondiabetic adult NOD mice. At the onset of diabetes, τ was hyperphosphorylated at the τ-1, AT8, CP13, pS262, and pS422. A subpopulation of diabetic NOD mice became hypothermic, and τ hyperphosphorylation further extended to paired helical filament-1 and TG3 epitopes. Furthermore, elevated τ phosphorylation correlated with an inhibition of protein phosphatase 2A (PP2A) activity. Our data indicate that insulin dysfunction in NOD mice leads to AD-like τ hyperphosphorylation in the brain, with molecular mechanisms likely involving a deregulation of PP2A. This model may be a useful tool to address further mechanistic association between insulin dysfunction and AD pathology.

Glutamate system, amyloid ß peptides and tau protein: functional interrelationships and relevance to Alzheimer disease pathology

Alzheimer disease is the most prevalent form of dementia globally and is characterized premortem by a gradual memory loss and deterioration of higher cognitive functions and postmortem by neuritic plaques containing amyloid ß peptide and neurofibrillary tangles containing phospho-tau protein. Glutamate is the most abundant neurotransmitter in the brain and is essential to memory formation through processes such as long-term potentiation and so might be pivotal to Alzheimer disease progression. This review discusses how the glutamatergic system is impaired in Alzheimer disease and how interactions of amyloid ß and glutamate influence synaptic function, tau phosphorylation and neurodegeneration. Interestingly, glutamate not only influences amyloid ß production, but also amyloid ß can alter the levels of glutamate at the synapse, indicating that small changes in the concentrations of both molecules could influence Alzheimer disease progression. Finally, we describe how the glutamate receptor antagonist, memantine, has been used in the treatment of individuals with Alzheimer disease and discuss its effectiveness.

Tau phosphorylation and μ-calpain activation mediate the dexamethasone-induced inhibition on the insulin-stimulated Akt phosphorylation

Evidence has suggested that insulin resistance (IR) or high levels of glucocorticoids (GCs) may be linked with the pathogenesis and/or progression of Alzheimer's disease (AD). Although studies have shown that a high level of GCs results in IR, little is known about the molecular details that link GCs and IR in the context of AD. Abnormal phosphorylation of tau and activation of μ-calpain are two key events in the pathology of AD. Importantly, these two events are also related with GCs and IR. We therefore speculate that tau phosphorylation and μ-calpain activation may mediate the GCs-induced IR. Akt phosphorylation at Ser-473 (pAkt) is commonly used as a marker for assessing IR. We employed two cell lines, wild-type HEK293 cells and HEK293 cells stably expressing the longest human tau isoform (tau-441; HEK293/tau441 cells). We examined whether DEX, a synthetic GCs, induces tau phosphorylation and μ-calpain activation. If so, we examined whether the DEX-induced tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. The results showed that DEX increased tau phosphorylation and induced tau-mediated μ-calpain activation. Furthermore, pre-treatment with LiCl prevented the effects of DEX on tau phosphorylation and μ-calpain activation. Finally, both LiCl pre-treatment and calpain inhibition prevented the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. In conclusion, our study suggests that the tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation.

Signal transducer and activator of transcription 3 (STAT3) protein suppresses adenoma-to-carcinoma transition in Apcmin/+ mice via regulation of Snail-1 (SNAI) protein stability

STAT3 was recently reported to suppress tumor invasion in Apc(min)(/+) mice. We investigated the mechanisms by which STAT3 inhibits intestinal epithelial tumors using Apc(min)(/+)/Stat3(IEC-KO) mice (intestinal epithelial cell (IEC)-specific deletion of STAT3 in the Apc(min)(/+) background) to determine the role of STAT3 in carcinogenesis in vivo as well as colorectal cancer cell lines in vitro. To inhibit invasion of IEC tumors, STAT3 functions as a molecular adaptor rather than a transcription factor. Accordingly, the tumors in Apc(min)(/+)/Stat3(IEC-KO) mice undergo adenoma-to-carcinoma transition and acquire an invasive phenotype. Similarly, STAT3 knockdown in a colorectal cell line enhances IEC invasion. We demonstrate that STAT3 down-regulates SNAI (Snail-1) expression levels and hence suppresses epithelial-mesenchymal transition of colorectal cancer cells. Mechanistically, STAT3 facilitates glycogen synthase kinase (GSK) 3β-mediated degradation of SNAI by regulating phosphorylation of GSK3β. Our data identified a new role for STAT3 in the adenoma-to-carcinoma sequence of intestinal tumors.

The role of GSK3beta in the development of the central nervous system

Glycogen synthase kinase 3β (GSK3β) is a multifunctional serine/threonine kinase. It is particularly abundant in the developing central nervous system (CNS). Since GSK3β has diverse substrates ranging from metabolic/signaling proteins and structural proteins to transcription factors, it is involved in many developmental events in the immature brain, such as neurogenesis, neuronal migration, differentiation and survival. The activity of GSK3β is developmentally regulated and is affected by various environmental/cellular insults, such as deprivation of nutrients/trophic factors, oxidative stress and endoplasmic reticulum stress. Abnormalities in GSK3β activity may disrupt CNS development. Therefore, GSK3β is a critical signaling protein that regulates brain development. It may also determine neuronal susceptibility to damages caused by various environmental insults.

Insulin in central nervous system: more than just a peripheral hormone

Insulin signaling in central nervous system (CNS) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning, memory, and age-related neurodegenerative diseases. Thus, besides its well-known role in longevity, insulin may constitute a promising therapy against diabetes- and age-related neurodegenerative disorders. More interestingly, insulin has been also faced as the potential missing link between diabetes and aging in CNS, with Alzheimer's disease (AD) considered as the "brain-type diabetes." In fact, brain insulin has been shown to regulate both peripheral and central glucose metabolism, neurotransmission, learning, and memory and to be neuroprotective. And a future challenge will be to unravel the complex interactions between aging and diabetes, which, we believe, will allow the development of efficient preventive and therapeutic strategies to overcome age-related diseases and to prolong human "healthy" longevity. Herewith, we aim to integrate the metabolic, neuromodulatory, and neuroprotective roles of insulin in two age-related pathologies: diabetes and AD, both in terms of intracellular signaling and potential therapeutic approach.

A pivotal role of GSK-3 in synaptic plasticity

Glycogen synthase kinase-3 (GSK-3) has many cellular functions. Recent evidence suggests that it plays a key role in certain types of synaptic plasticity, in particular a form of long-term depression (LTD) that is induced by the synaptic activation of N-methyl-D-aspartate receptors (NMDARs). In the present article we summarize what is currently known concerning the roles of GSK-3 in synaptic plasticity at both glutamatergic and GABAergic synapses. We summarize its role in cognition and speculate on how alterations in the synaptic functioning of GSK-3 may be a major factor in certain neurodegenerative disorders.

Targeting of GSK3β promotes imatinib-mediated apoptosis in quiescent CD34+ chronic myeloid leukemia progenitors, preserving normal stem cells

The targeting of BCR-ABL, a hybrid oncogenic tyrosine (Y) kinase, does not eradicate chronic myeloid leukemia (CML)-initiating cells. Activation of β-catenin was linked to CML leukemogenesis and drug resistance through its BCR-ABL-dependent Y phosphorylation and impaired binding to GSK3β (glycogen synthase kinase 3β). Herein, we show that GSK3β is constitutively Y(216) phospho-activated and predominantly relocated to the cytoplasm in primary CML stem/progenitor cells compared with its balanced active/inactive levels and cytosolic/nuclear distribution in normal cells. Under cytokine support, persistent GSK3β activity and its altered subcellular localization were correlated with BCR-ABL-dependent and -independent activation of MAPK and p60-SRC/GSK3β complex formation. Specifically, GSK3β activity and nuclear import were increased by imatinib mesylate (IM), a selective ABL inhibitor, but prevented by dasatinib that targets both BCR-ABL- and cytokine-dependent MAPK/p60-SRC activity. SB216763, a specific GSK3 inhibitor, promoted an almost complete suppression of primary CML stem/progenitor cells when combined with IM, but not dasatinib, while sparing bcr-abl-negative cells. Our data indicate that GSK3 inhibition acts to prime a pro-differentiative/apoptotic transcription program in the nucleus of IM-treated CML cells by affecting the β-catenin, cyclinD1, C-EBPα, ATF5, mTOR, and p27 levels. In conclusion, our data gain new insight in CML biology, indicating that GSK3 inhibitors may be of therapeutic value in selectively targeting leukemia-initiating cells in combination with IM but not dasatinib.

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.

Modulation of GSK-3 as a Therapeutic Strategy on Tau Pathologies

Glycogen synthase kinase-3 (GSK-3) is ubiquitously expressed and unusually active in resting, non-stimulated cells. In mammals, at least three proteins (α, β1, and β2), generated from two different genes, gsk-3α and gsk-3β, are widely expressed at both the RNA and protein levels although some tissues show preferential expression of some of the three proteins. Control of GSK-3 activity occurs by complex mechanisms that depend on specific signaling pathways, often controlling the inhibition of the kinase activity. GSK-3 appears to integrate different signaling pathways from a wide selection of cellular stimuli. The unique position of GSK-3 in modulating the function of a diverse series of proteins and its association with a wide variety of human disorders has attracted significant attention as a therapeutic target and as a means to understand the molecular basis of brain disorders. Different neurodegenerative diseases including frontotemporal dementia, progressive supranuclear palsy, and Alzheimer's disease, present prominent tau pathology such as tau hyperphosphorylation and aggregation and are collectively referred to as tauopathies. GSK-3 has also been associated to different neuropsychiatric disorders, like schizophrenia and bipolar disorder. GSK-3β is the major kinase to phosphorylate tau both in vitro and in vivo and has been proposed as a target for therapeutic intervention. The first therapeutic strategy to modulate GSK-3 activity was the direct inhibition of its kinase activity. This review will focus on the signaling pathways involved in the control of GSK-3 activity and its pathological deregulation. We will highlight different alternatives of GSK-3 modulation including the direct pharmacological inhibition as compared to the modulation by upstream regulators.

Possible implications of insulin resistance and glucose metabolism in Alzheimer's disease pathogenesis

Type 2 diabetes mellitus (DM) appears to be a significant risk factor for Alzheimer disease (AD). Insulin and insulin-like growth factor-1 (IGF-1) also have intense effects in the central nervous system (CNS), regulating key processes such as neuronal survival and longevity, as well as learning and memory. Hyperglycaemia induces increased peripheral utilization of insulin, resulting in reduced insulin transport into the brain. Whereas the density of brain insulin receptor decreases during age, IGF-1 receptor increases, suggesting that specific insulin-mediated signals is involved in aging and possibly in cognitive decline. Molecular mechanisms that protect CNS neurons against β-amyloid-derived-diffusible ligands (ADDL), responsible for synaptic deterioration underlying AD memory failure, have been identified. The protection mechanism does not involve simple competition between ADDLs and insulin, but rather it is signalling dependent down-regulation of ADDL-binding sites. Defective insulin signalling make neurons energy deficient and vulnerable to oxidizing or other metabolic insults and impairs synaptic plasticity. In fact, destruction of mitochondria, by oxidation of a dynamic-like transporter protein, may cause synapse loss in AD. Moreover, interaction between Aβ and τ proteins could be cause of neuronal loss. Hyperinsulinaemia as well as complete lack of insulin result in increased τ phosphorylation, leading to an imbalance of insulin-regulated τ kinases and phosphatates. However, amyloid peptides accumulation is currently seen as a key step in the pathogenesis of AD. Inflammation interacts with processing and deposit of β-amyloid. Chronic hyperinsulinemia may exacerbate inflammatory responses and increase markers of oxidative stress. In addition, insulin appears to act as 'neuromodulator', influencing release and reuptake of neurotransmitters, and improving learning and memory. Thus, experimental and clinical evidence show that insulin action influences cerebral functions. In this paper, we reviewed several mechanisms by which insulin may affect pathophysiology in AD.

Mechanisms of brain aging regulation by insulin: implications for neurodegeneration in late-onset Alzheimer's disease

Insulin and IGF seem to be important players in modulating brain aging. Neurons share more similarities with islet cells than any other human cell type. Insulin and insulin receptors are diffusely found in the brain, especially so in the hippocampus. Caloric restriction decreases insulin resistance, and it is the only proven mechanism to expand lifespan. Conversely, insulin resistance increases with age, obesity, and sedentarism, all of which have been shown to be risk factors for late-onset Alzheimer's disease (AD). Hyperphagia and obesity potentiate the production of oxidative reactive species (ROS), and chronic hyperglycemia accelerates the formation of advanced glucose end products (AGEs) in (pre)diabetes—both mechanisms favoring a neurodegenerative milieu. Prolonged high cerebral insulin concentrations cause microvascular endothelium proliferation, chronic hypoperfusion, and energy deficit, triggering β-amyloid oligomerization and tau hyperphosphorylation. Insulin-degrading enzyme (IDE) seems to be the main mechanism in clearing β-amyloid from the brain. Hyperinsulinemic states may deviate IDE utilization towards insulin processing, decreasing β-amyloid degradation.

Accelerated human mutant tau aggregation by knocking out murine tau in a transgenic mouse model

Many models of human tauopathies have been generated in mice by expression of a human mutant tau with maintained expression of mouse endogenous tau. Because murine tau might interfere with the toxic effects of human mutant tau, we generated a model in which a pathogenic human tau protein is expressed in the absence of wild-type tau protein, with the aim of facilitating the study of the pathogenic role of the mutant tau and to reproduce more faithfully a human tauopathy. The Tg30 line is a tau transgenic mouse model overexpressing human 1N4R double-mutant tau (P301S and G272V) that develops Alzheimer's disease-like neurofibrillary tangles in an age-dependent manner. By crossing Tg30 mice with mice invalidated for their endogenous tau gene, we obtained Tg30xtau(-/-) mice that express only exogenous human double-mutant 1N4R tau. Although Tg30xtau(-/-) mice express less tau protein compared with Tg30, they exhibit signs of decreased survival, increased proportion of sarkosyl-insoluble tau in the brain and in the spinal cord, increased number of Gallyas-positive neurofibrillary tangles in the hippocampus, increased number of inclusions in the spinal cord, and a more severe motor phenotype. Deletion of murine tau accelerated tau aggregation during aging of this mutant tau transgenic model, suggesting that murine tau could interfere with the development of tau pathology in transgenic models of human tauopathies.

GSK-3 in Neurodegenerative Diseases

Glycogen synthase kinase-3 (GSK-3) regulates multiple cellular processes, and its dysregulation is implicated in the pathogenesis of diverse diseases. In this paper we will focus on the dysfunction of GSK-3 in Alzheimer's disease and Parkinson's disease. Specifically, GSK-3 is known to interact with tau, β-amyloid (Aβ), and α-synuclein, and as such may be crucially involved in both diseases. Aβ production, for example, is regulated by GSK-3, and its toxicity is mediated by GSK-induced tau phosphorylation and degeneration. α-synuclein is a substrate for GSK-3 and GSK-3 inhibition protects against Parkinsonian toxins. Lithium, a GSK-3 inhibitor, has also been shown to affect tau, Aβ, and α-synuclein in cell culture, and transgenic animal models. Thus, understanding the role of GSK-3 in neurodegenerative diseases will enhance our understanding of the basic mechanisms underlying the pathogenesis of these disorders and also facilitate the identification of new therapeutic avenues.

Deconstructing GSK-3: The Fine Regulation of Its Activity

Glycogen synthase kinase-3 (GSK-3) unique position in modulating the function of a diverse series of proteins in combination with its association with a wide variety of human disorders has attracted significant attention to the protein both as a therapeutic target and as a means to understand the molecular basis of these disorders. GSK-3 is ubiquitously expressed and, unusually, constitutively active in resting, unstimulated cells. In mammals, GSK-3α and β are each expressed widely at both the RNA and protein levels although some tissues show preferential levels of some of the two proteins. Neither gene appears to be acutely regulated at the transcriptional level, whereas the proteins are controlled posttranslationally, largely through protein-protein interactions or by posttranslational regulation. Control of GSK-3 activity thus occurs by complex mechanisms that are each dependent upon specific signalling pathways. Furthermore, GSK-3 appears to be a cellular nexus, integrating several signalling systems, including several second messengers and a wide selection of cellular stimulants. This paper will focus on the different ways to control GSK-3 activity (phosphorylation, protein complex formation, truncation, subcellular localization, etc.), the main signalling pathways involved in its control, and its pathological deregulation.

Tyrosine phosphorylation-mediated signaling pathways in dictyostelium

While studies on metazoan cell proliferation, cell differentiation, and cytokine signaling laid the foundation of the current paradigms of tyrosine kinase signaling, similar studies using lower eukaryotes have provided invaluable insight for the understanding of mammalian pathways, such as Wnt and STAT pathways. Dictyostelium is one of the leading lower eukaryotic model systems where stress-induced cellular responses, Wnt-like pathways, and STAT-mediated pathways are well investigated. These Dictyostelium pathways will be reviewed together with their mammalian counterparts to facilitate the comparative understanding of these variant and noncanonical pathways.

Is glycogen synthase kinase-3 a central modulator in mood regulation?

Little is known regarding the mechanisms underlying the complex etiology of mood disorders, represented mainly by major depressive disorder and bipolar disorder. The 1996 discovery that lithium inhibits glycogen synthase kinase-3 (GSK3) raised the possibility that impaired inhibition of GSK3 is associated with mood disorders. This is now supported by evidence from animal biochemical, pharmacological, molecular, and behavioral studies and from human post-mortem brain, peripheral tissue, and genetic studies that are reviewed here. Mood disorders may result in part from impairments in mechanisms controlling the activity of GSK3 or GSK3-regulated functions, and disruptions of these regulating systems at different signaling sites may contribute to the heterogeneity of mood disorders. This substantial evidence supports the conclusion that bolstering the inhibitory control of GSK3 is an important component of the therapeutic actions of drugs used to treat mood disorders and that GSK3 is a valid target for developing new therapeutic interventions.