Membrane depolarization induces the undulating phosphorylation/dephosphorylation of glycogen synthase kinase 3beta, and this dephosphorylation involves protein phosphatases 2A and 2B in SH-SY5Y human neuroblastoma cells

Cyclosporine A (CsA) prevents synaptic impairment caused by truncated tau by caspase-3

During Alzheimer's (AD), tau protein suffers from abnormal post-translational modifications, including cleaving by caspase-3. These tau forms affect synaptic plasticity contributing to the cognitive decline observed in the early stages of AD. In addition, caspase-3 cleaved tau (TauC3) impairs mitochondrial dynamics and organelles transport, which are both relevant processes for synapse. We recently showed that the absence of tau expression reverts age-associated cognitive and mitochondrial failure by blocking the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial complex involved in calcium regulation and apoptosis. Therefore, we studied the effects of TauC3 against the dendritic spine and synaptic vesicle formation and the possible role of mPTP in these alterations. We used mature hippocampal mice neurons to express a reporter protein (GFP, mCherry), coupled to full-length human tau protein (GFP-T4, mCherry-T4), and coupled to human tau protein cleaved at D421 by caspase-3 (GFP-T4C3, mCherry-T4C3) and synaptic elements were evaluated. Treatment with cyclosporine A (CsA), an immunosuppressive drug with inhibitory activity on mPTP, prevented ROS increase and mitochondrial depolarization induced by TauC3 in hippocampal neurons. These results were corroborated with immortalized cortical neurons in which ROS increase and ATP loss induced by this tau form were prevented by CsA. Interestingly, TauC3 expression significantly reduced dendritic spine density (filopodia type) and synaptic vesicle number in hippocampal neurons. Also, neurons transfected with TauC3 showed a significant accumulation of synaptophysin protein in their soma. More importantly, all these synaptic alterations were prevented by CsA, suggesting an mPTP role in these negative changes derived from TauC3 expression.

Increased spine PIP3 is sequestered from dendritic shafts

Phosphatidylinositol 3,4,5-trisphosphate (PIP3) is a lipid second messenger that is crucial for the synaptic plasticity underlying learning and memory in pyramidal neurons in the brain. Our previous study uncovered PIP3 enrichment in the dendritic spines of hippocampal pyramidal neurons in the static state using a fluorescence lifetime-based PIP3 probe. However, the extent to which PIP3 enrichment is preserved in different states has not been fully investigated. Here, we revealed that PIP3 accumulation in dendritic spines is strictly controlled even in an active state in which PIP3 is increased by glutamate stimulation and high potassium-induced membrane depolarization. Time-course PIP3 analysis clarified the gradual PIP3 accumulation in dendritic spines over days during neuronal development. Collectively, these results deepen our understanding of PIP3 dynamics in dendritic spines, and the dysregulation of the PIP3 gradient between dendritic spines and shafts could cause neuronal diseases and mental disorders, such as autism spectrum disorder.

Quantum Tunneling-Induced Membrane Depolarization Can Explain the Cellular Effects Mediated by Lithium: Mathematical Modeling and Hypothesis

Lithium imposes several cellular effects allegedly through multiple physiological mechanisms. Membrane depolarization is a potential unifying concept of these mechanisms. Multiple inherent imperfections of classical electrophysiology limit its ability to fully explain the depolarizing effect of lithium ions; these include incapacity to explain the high resting permeability of lithium ions, the degree of depolarization with extracellular lithium concentration, depolarization at low therapeutic concentration, or the differences between the two lithium isotopes Li-6 and Li-7 in terms of depolarization. In this study, we implemented a mathematical model that explains the quantum tunneling of lithium ions through the closed gates of voltage-gated sodium channels as a conclusive approach that decodes the depolarizing action of lithium. Additionally, we compared our model to the classical model available and reported the differences. Our results showed that lithium can achieve high quantum membrane conductance at the resting state, which leads to significant depolarization. The quantum model infers that quantum membrane conductance of lithium ions emerges from quantum tunneling of lithium through the closed gates of sodium channels. It also differentiates between the two lithium isotopes (Li-6 and Li-7) in terms of depolarization compared with the previous classical model. Moreover, our study listed many examples of the cellular effects of lithium and membrane depolarization to show similarity and consistency with model predictions. In conclusion, the study suggests that lithium mediates its multiple cellular effects through membrane depolarization, and this can be comprehensively explained by the quantum tunneling model of lithium ions.

The Potential Role of Lithium as an Antiviral Agent against SARS-CoV-2 via Membrane Depolarization: Review and Hypothesis

Studies on potential treatments of Coronavirus Disease 2019 (COVID-19) are important to improve the global situation in the face of the pandemic. This review proposes lithium as a potential drug to treat COVID-19. Our hypothesis states that lithium can suppress NOD-like receptor family pyrin domain containing-3 (NLRP3) inflammasome activity, inhibit cell death, and exhibit immunomodulation via membrane depolarization. Our hypothesis was formulated after finding consistent correlations between these actions and membrane depolarization induced by lithium. Eventually, lithium could serve to mitigate the NLRP3-mediated cytokine storm, which is allegedly reported to be the inciting event of a series of retrogressive events associated with mortality from COVID-19. It could also inhibit cell death and modulate the immune system to attenuate its release, clear the virus from the body, and interrupt the cycle of immune-system dysregulation. Therefore, these effects are presumed to improve the morbidity and mortality of COVID-19 patients. As the numbers of COVID-19 cases and deaths continue to rise exponentially without a clear consensus on potential therapeutic agents, urgent conduction of preclinical and clinical studies to prove the efficacy and safety of lithium is reasonable.

Glycogen Synthase Kinase-3β Facilitates Cytokine Production in 12-O-Tetradecanoylphorbol-13-Acetate/Ionomycin-Activated Human CD4+ T Lymphocytes

Cytokines are the major immune regulators secreted from activated CD4⁺ T lymphocytes that activate adaptive immunity to eradicate nonself cells, including pathogens, tumors, and allografts. The regulation of glycogen synthase kinase (GSK)-3β, a serine/threonine kinase, controls cytokine production by regulating transcription factors. The artificial in vitro activation of CD4⁺ T lymphocytes by a combination of 12-O-tetradecanoylphorbol-13-acetate and ionomycin, the so-called T/I model, led to an inducible production of cytokines, such as interferon-γ, tumor necrosis factor-α, and interleukin-2. As demonstrated by the approaches of pharmacological targeting and genetic knockdown of GSK-3β, T/I treatment effectively caused GSK-3β activation followed by GSK-3β-regulated cytokine production. In contrast, pharmacological inhibition of the proline-rich tyrosine kinase 2 and calcineurin signaling pathways blocked cytokine production, probably by deactivating GSK-3β. The blockade of GSK-3β led to the inhibition of the nuclear translocation of T-bet, a vital transcription factor of T lymphocyte cytokines. In a mouse model, treatment with the GSK-3β inhibitor 6-bromoindirubin-3’-oxime significantly inhibited T/I-induced mortality and serum cytokine levels. In summary, targeting GSK-3β effectively inhibits CD4⁺ T lymphocyte activation and cytokine production.

Akt phosphorylation of neuronal nitric oxide synthase regulates gastrointestinal motility in mouse ileum

Nitric oxide (NO) is a major inhibitory neurotransmitter that mediates nonadrenergic noncholinergic (NANC) signaling. Neuronal NO synthase (nNOS) is activated by Ca²⁺/calmodulin to produce NO, which causes smooth muscle relaxation to regulate physiologic tone. nNOS serine1412 (S1412) phosphorylation may reduce the activating Ca²⁺ requirement and sustain NO production. We developed and characterized a nonphosphorylatable nNOS^S1412A knock-in mouse and evaluated its enteric neurotransmission and gastrointestinal (GI) motility to understand the physiologic significance of nNOS S1412 phosphorylation. Electrical field stimulation (EFS) of wild-type (WT) mouse ileum induced nNOS S1412 phosphorylation that was blocked by tetrodotoxin and by inhibitors of the protein kinase Akt but not by PKA inhibitors. Low-frequency depolarization increased nNOS S1412 phosphorylation and relaxed WT ileum but only partially relaxed nNOS^S1412A ileum. At higher frequencies, nNOS S1412 had no effect. nNOS^S1412A ileum expressed less phosphodiesterase-5 and was more sensitive to relaxation by exogenous NO. Under non-NANC conditions, peristalsis and segmentation were faster in the nNOS^S1412A ileum. Together these findings show that neuronal depolarization stimulates enteric nNOS phosphorylation by Akt to promote normal GI motility. Thus, phosphorylation of nNOS S1412 is a significant regulatory mechanism for nitrergic neurotransmission in the gut.

GSK-3β at the Intersection of Neuronal Plasticity and Neurodegeneration

In neurons, Glycogen Synthase Kinase-3β (GSK-3β) has been shown to regulate various critical processes underlying structural and functional synaptic plasticity. Mouse models with neuron-selective expression or deletion of GSK-3β present behavioral and cognitive abnormalities, positioning this protein kinase as a key signaling molecule in normal brain functioning. Furthermore, mouse models with defective GSK-3β activity display distinct structural and behavioral abnormalities, which model some aspects of different neurological and neuropsychiatric disorders. Equalizing GSK-3β activity in these mouse models by genetic or pharmacological interventions is able to rescue some of these abnormalities. Thus, GSK-3β is a relevant therapeutic target for the treatment of many brain disorders. Here, we provide an overview of how GSK-3β is regulated in physiological synaptic plasticity and how aberrant GSK-3β activity contributes to the development of dysfunctional synaptic plasticity in neuropsychiatric and neurodegenerative disorders.

Zinc ions regulate opening of tight junction favouring efflux of macromolecules via the GSK3β/snail-mediated pathway

Zinc is an essential trace element presenting in particularly high concentration in the brain. In some regions, e.g. lateral amygdala, subiculum and hippocampus, rapidly-exchangeable zinc may transiently reach even up to 600 μM. To explore the possible roles of high-concentration Zn²⁺ in regulating the blood-brain barrier (BBB), we investigated the effects of Zn²⁺ on the functions and structures of the tight junction (TJ) with an in vitro model of a Madin-Darby canine kidney (MDCK) cell monolayer. The experimental results indicated that high concentrations (>200 μM) of Zn²⁺ can affect the TJ integrity in a polarized manner. Basolateral addition of Zn²⁺ led to reversible TJ opening with pore paths of r ∼ 2 nm or more depending on Zn²⁺ concentration. The efflux/influx ratios of different sized probes were found to be ∼4.6 for FD4 (M_W 4000) and ∼1.8 for Eu-DTPA (M_W 560), suggesting that the Zn²⁺-induced paracelluar channels favour efflux especially for macromolecules. Further mechanistic studies revealed that the elevated intracellular Zn²⁺ taken from the basolateral side can increase phosphorylation of glycogen synthase kinase (GSK) 3β, primarily due to the inhibition of calcineurin (CaN), thus resulting in the elevation of the snail transcriptional repressors. Subsequently, Zn²⁺ can cause the down-regulation of claudin-1, breakage of occludin and ZO-1 rings, and collapse of basolateral F-actin structures. These overall factors result in the formation of a trumpet-like paracellular channel, which allows asymmetric solute permeation. The ERK1/2 and JNK1/2 pathways may also be involved in the Zn²⁺-induced TJ opening process, while the activation of matrix metalloproteinase was not observed. Our results may suggest a potential role of zinc in regulation of BBB permeability associated with brain clearance of metabolites through the glymphatic system.

Endogenous bioelectric currents promote differentiation of the mammalian lens

The functional roles of bioelectrical signals (ES) created by the flow of specific ions at the mammalian lens equator are poorly understood. We detected that mature, denucleated lens fibers expressed high levels of the α1 and β1 subunits of Na⁺/K⁺‐ATPase (ATP1A1 and ATP1B1 of the sodium pump) and had a hyperpolarized membrane potential difference (V_mem). In contrast, differentiating, nucleated lens fiber cells had little ATP1A1 and ATP1B1 and a depolarized V_mem. Mimicking the natural equatorial ES with an applied electrical field (EF) induced a striking reorientation of lens epithelial cells to lie perpendicular to the direction of the EF. An EF also promoted the expression of β‐crystallin, aquaporin‐0 (AQP0) and the Beaded Filament Structural Protein 2 (BFSP2) in lens epithelial cells (LECs), all of which are hallmarks of differentiation. In addition, applied EF activated the AKT and CDC2 and inhibition of AKT reduced the activation of CDC2. Our results indicate that the endogenous bioelectrical signal at the lens equator promotes differentiation of LECs into denucleated lens fiber cells via depolarization of V_mem. Development of methods and devices of EF application or amplification in vivo may supply a novel treatment for lens diseases and even promote regeneration of a complete new lens following cataract surgery.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Alpha-Synuclein Suppresses Retinoic Acid-Induced Neuronal Differentiation by Targeting the Glycogen Synthase Kinase-3β/β-Catenin Signaling Pathway

Alpha-synuclein (α-SYN) is expressed during neuronal development and is mainly involved in the modulation of synaptic transmission. Missense mutations and amplifications of this gene have been associated with the pathogenesis of Parkinson's disease. Here, we evaluate whether α-SYN plays a detrimental role in the phenotypic and morphological regulation of neurons. We also identify the underlying mechanisms of this process in all-trans-retinoic acid (RA)-induced differentiated SH-SY5Y cells, which represents dopaminergic (DAergic) phenotype. Our results indicate that overexpression of wild-type or mutant A53T α-SYN attenuated the RA-induced upregulation of tyrosine hydroxylase and dopamine transporter as well as neurite outgrowth in SH-SY5Y cells. In addition, GSK-3β inactivation and downstream β-catenin stabilization were associated with RA-induced differentiation, which was attenuated by α-SYN. Moreover, protein phosphatase 2A was positively regulated by α-SYN and was implicated in the α-SYN-mediated interference with RA signaling. The results obtained from SH-SY5Y cells were verified in primary cultures of mesencephalic DAergic neurons from A53T α-SYN transgenic mice, which represent high levels of α-SYN and protein phosphatase 2A in the midbrain. The number and length of neurites in tyrosine hydroxylase-positive as well as Tau-positive cells from A53T α-SYN transgenic mice were significantly lower than those in littermate controls. The current results provide novel insight into the role of α-SYN in the regulation of neuronal differentiation, including DAergic neurons. Identifying the signaling pathway involved in the α-SYN-mediated dysregulation of neuronal differentiation could lead to a better understanding of the developmental processes underlying α-SYN-related pathologies and facilitate the discovery of specifically targeted therapeutics.

Protein tyrosine phosphatase SHP-1 modulates osteoblast differentiation through direct association with and dephosphorylation of GSK3β

SHP-1, the Src homology-2 (SH2) domain-containing phosphatase 1, is a cytosolic protein-tyrosine phosphatase (PTP) predominantly expressed in hematopoietic-derived cells. Previous studies have focused on the involvement of SHP-1 in osteoclastogenesis. Using primary cultured mouse fetal calvaria-derived osteoblasts as a model, this study aims to investigate the effects of SHP-1 on differentiation and mineralization of osteoblasts and elucidate the signaling pathways responsible for these effects. We found that osteoblasts treated by osteogenic media showed significant increase in SHP-1 expression, which contributed to osteoblastic differentiation and mineralization. Using immunoprecipitation assay, we found that a direct association between SHP-1 and glycogen synthase kinase (GSK)-3β could be detected in differentiated osteoblasts and was significantly inhibited by SHP-1 inhibitor NSC87877. Inhibition of SHP-1 activated GSK3β, thereby leading to suppression of osteoblast differentiation and mineralization, which could be rescued by the inhibitor of GSK3β. In addition, we found that rosiglitazone (RSG) treatment led to significant decrease in SHP-1 expression. Overexpression of SHP-1 reversed RSG-induced GSK3β activation, thus rescuing the inhibitory effect of RSG on osteoblast differentiation and mineralization. These findings suggest that protein tyrosine phosphatase SHP-1 may act as a positive regulator of osteoblast differentiation through direct association with and dephosphorylation of GSK3β. Downregulation of SHP-1 may contribute to RSG-induced inhibition of mouse calvaria osteoblast differentiation by activating GSK3β-dependent pathway.

Novel Non-phosphorylated Serine 9/21 GSK3β/α Antibodies: Expanding the Tools for Studying GSK3 Regulation

Glycogen synthase kinase 3 (GSK3) β and α are serine/threonine kinases involved in many biological processes. A primary mechanism of GSK3 activity regulation is phosphorylation of N-terminal serine (S) residues (S9 in GSK3β, S21 in GSK3α). Phosphorylation is inhibitory to GSK3 kinase activity because the phosphorylated N-terminus acts as a competitive inhibitor for primed substrates. Despite widespread interest in GSK3 across most fields of biology, the research community does not have reagents that specifically react with nonphosphoS9/21 GSK3β/α (the so-called "active" form). Here, we describe two novel monoclonal antibodies that specifically react with nonphosphoS9/21 GSK3β/α in multiple species (human, mouse, and rat). One of the antibodies is specific for nonphospho-S9 GSK3β (clone 12B2) and one for nonphospho-S9/21 GSK3β/α (clone 15C2). These reagents were validated for specificity and reactivity in several biochemical and immunochemical assays, and they show linear detection of nonphosphoS GSK3. Finally, these reagents provide significant advantages in studying GSK3β regulation. We used both antibodies to study the regulation of S9 phosphorylation by Akt and protein phosphatases. We used 12B2 (due to its specificity for GSK3β) and to demonstrate that protein phosphatase inhibition reduces nonphospho-S9 GSK3β levels and lowers kinase activity within cells. The ability to use the same reagent across biochemical, immunohistological and kinase activity assays provides a powerful approach for studying serine-dependent regulation of GSK3β/α.

High glucose upregulates BACE1-mediated Aβ production through ROS-dependent HIF-1α and LXRα/ABCA1-regulated lipid raft reorganization in SK-N-MC cells

There is an accumulation of evidence indicating that the risk of Alzheimer’s disease is associated with diabetes mellitus, an indicator of high glucose concentrations in blood plasma. This study investigated the effect of high glucose on BACE1 expression and amyloidogenesis in vivo, and we present details of the mechanism associated with those effects. Our results, using ZLC and ZDF rat models, showed that ZDF rats have high levels of amyloid-beta (Aβ), phosphorylated tau, BACE1, and APP-C99. In vitro result with mouse hippocampal neuron and SK-N-MC, high glucose stimulated Aβ secretion and apoptosis in a dose-dependent manner. In addition, high glucose increased BACE1 and APP-C99 expressions, which were reversed by a reactive oxygen species (ROS) scavenger. Indeed, high glucose increased intracellular ROS levels and HIF-1α expression, associated with regulation of BACE1 and Liver X Receptor α (LXRα). In addition, high glucose induced ATP-binding cassette transporter A1 (ABCA1) down-regulation, was associated with LXR-induced lipid raft reorganization and BACE1 localization on the lipid raft. Furthermore, silencing of BACE1 expression was shown to regulate Aβ secretion and apoptosis of SK-N-MC. In conclusion, high glucose upregulates BACE1 expression and activity through HIF-1α and LXRα/ABCA1-regulated lipid raft reorganization, leading to Aβ production and apoptosis of SK-N-MC.

Tau accumulation induces synaptic impairment and memory deficit by calcineurin-mediated inactivation of nuclear CaMKIV/CREB signaling

Intracellular accumulation of wild-type tau is a hallmark of sporadic Alzheimer's disease (AD), but the molecular mechanisms underlying tau-induced synapse impairment and memory deficit are poorly understood. Here we found that overexpression of human wild-type full-length tau (termed hTau) induced memory deficits with impairments of synaptic plasticity. Both in vivo and in vitro data demonstrated that hTau accumulation caused remarkable dephosphorylation of cAMP response element binding protein (CREB) in the nuclear fraction. Simultaneously, the calcium-dependent protein phosphatase calcineurin (CaN) was up-regulated, whereas the calcium/calmodulin-dependent protein kinase IV (CaMKIV) was suppressed. Further studies revealed that CaN activation could dephosphorylate CREB and CaMKIV, and the effect of CaN on CREB dephosphorylation was independent of CaMKIV inhibition. Finally, inhibition of CaN attenuated the hTau-induced CREB dephosphorylation with improved synapse and memory functions. Together, these data indicate that the hTau accumulation impairs synapse and memory by CaN-mediated suppression of nuclear CaMKIV/CREB signaling. Our findings not only reveal new mechanisms underlying the hTau-induced synaptic toxicity, but also provide potential targets for rescuing tauopathies.

Dietary regulation of PI3K/AKT/GSK-3β pathway in Alzheimer's disease

Alzheimer’s disease (AD) is characterized by the formation of senile plaques and neurofibrillary tangles composed of phosphorylated Tau. Several findings suggest that correcting signal dysregulation for Tau phosphorylation in AD may offer a potential therapeutic approach. The PI3K/AKT/GSK-3β pathway has been shown to play a pivotal role in neuroprotection, enhancing cell survival by stimulating cell proliferation and inhibiting apoptosis. This pathway appears to be crucial in AD because it promotes protein hyper-phosphorylation in Tau. Understanding those regulations may provide a better efficacy of new therapeutic approaches. In this review, we summarize advances in the involvement of the PI3K/AKT/GSK-3β pathways in cell signaling of neuronal cells. We also review recent studies on the features of several diets and the signaling pathway involved in AD.

Light-controlled inhibition of malignant glioma by opsin gene transfer

Glioblastomas are aggressive cancers with low survival rates and poor prognosis because of their highly proliferative and invasive capacity. In the current study, we describe a new optogenetic strategy that selectively inhibits glioma cells through light-controlled membrane depolarization and cell death. Transfer of the engineered opsin ChETA (engineered Channelrhodopsin-2 variant) gene into primary human glioma cells or cell lines, but not normal astrocytes, unexpectedly decreased cell proliferation and increased mitochondria-dependent apoptosis, upon light stimulation. These optogenetic effects were mediated by membrane depolarization-induced reductions in cyclin expression and mitochondrial transmembrane potential. Importantly, the ChETA gene transfer and light illumination in mice significantly inhibited subcutaneous and intracranial glioma growth and increased the survival of the animals bearing the glioma. These results uncover an unexpected effect of opsin ion channels on glioma cells and offer the opportunity for the first time to treat glioma using a light-controllable optogenetic approach.

Amyloid-β oligomers induce tau-independent disruption of BDNF axonal transport via calcineurin activation in cultured hippocampal neurons

The role of tau in axonal transport disruption during early-stage Alzheimer disease is controversial. The amyloid-β oligomers markedly impair BDNF transport in primary wild-type and tau-knockout neurons. This occurs by nonexcitotoxic activation of calcineurin, and inhibition of calcineurin rescues transport defects independent of tau.

Disruption of fast axonal transport (FAT) is an early pathological event in Alzheimer's disease (AD). Soluble amyloid-β oligomers (AβOs), increasingly recognized as proximal neurotoxins in AD, impair organelle transport in cultured neurons and transgenic mouse models. AβOs also stimulate hyperphosphorylation of the axonal microtubule-associated protein, tau. However, the role of tau in FAT disruption is controversial. Here we show that AβOs reduce vesicular transport of brain-derived neurotrophic factor (BDNF) in hippocampal neurons from both wild-type and tau-knockout mice, indicating that tau is not required for transport disruption. FAT inhibition is not accompanied by microtubule destabilization or neuronal death. Significantly, inhibition of calcineurin (CaN), a calcium-dependent phosphatase implicated in AD pathogenesis, rescues BDNF transport. Moreover, inhibition of protein phosphatase 1 and glycogen synthase kinase 3β, downstream targets of CaN, prevents BDNF transport defects induced by AβOs. We further show that AβOs induce CaN activation through nonexcitotoxic calcium signaling. Results implicate CaN in FAT regulation and demonstrate that tau is not required for AβO-induced BDNF transport disruption.

Neuroprotective effects of donepezil against Aβ42-induced neuronal toxicity are mediated through not only enhancing PP2A activity but also regulating GSK-3β and nAChRs activity

The main purpose of this study was to evaluate whether donepezil, acetylcholinesterase inhibitor, shown to play a protective role through inhibiting glycogen synthesis kinase-3β (GSK-3β) activity, could also exert neuroprotective effects by stimulating protein phosphatase 2A (PP2A) activity in the amyloid-beta (Aβ)42-induced neuronal toxicity model of Alzheimer's disease. In Aβ42-induced toxic conditions, each PP2A and GSK-3β activity measured at different times showed time-dependent reverse pattern toward the direction of accelerating neuronal deaths with the passage of time. In addition, donepezil pre-treatment showed dose-dependent stepwise increase of neuronal viability and stimulation of PP2A activity. However, such effects on them were significantly reduced through the depletion of PP2A activity with either okadaic acid or PP2Ac siRNA. In spite of blocked PP2A activity in this Aβ42 insult, however, donepezil pretreatment showed additional significant recovering effect on neuronal viability when compared to the value without donepezil. Moreover, donepezil partially recovered its dephosphorylating effect on hyperphosphorylated tau induced by Aβ42. This observation led us to assume that additional mechanisms of donepezil, including its inhibitory effect on GSK-3β activity and/or the activation role of nicotinic acetylcholine receptors (nAChRs), might be involved. Taken together, our results suggest that the neuroprotective effects of donepezil against Aβ42-induced neurotoxicity are mediated through activation of PP2A, but its additional mechanisms including regulation of GSK-3β and nAChRs activity would partially contribute to its effects. We investigated neuroprotective mechanisms of donepezil against Aβ42 toxicity: Donepezil increased neuronal viability with reduced p-tau by enhancing PP2A activity. Despite of blocked PP2A activity, donepezil showed additional recovering effect on neuronal viability, which findings led us to assume that additional mechanisms of donepezil including its inhibitory effect on GSK-3β activity and activating role of nicotinic AChRs might be involved.

Regulation of c-Myc Protein Abundance by a Protein Phosphatase 2A-Glycogen Synthase Kinase 3β-Negative Feedback Pathway

Regulation of Myc protein abundance is critical for normal cell growth as evidenced by the fact that deregulated Myc expression is a hallmark of many cancers. One of several important mechanisms that control Myc levels involves its phosphorylation-dependent proteolysis. Previous studies have shown that phosphorylation of threonine 58 by glycogen synthase kinase 3β (GSK3β) within the conserved Myc Box I sequence results in binding by the ubiquitin ligase Fbw7-SCF complex, followed by ubiquitination and proteasome-mediated degradation of Myc. Here, we show that induction of Myc in several cell types correlates with loss of the inhibitory serine 9 phosphorylation of GSK3β and its increased kinase activity. The Myc-induced decrease in serine 9 phosphorylation is blocked by okadaic acid, an inhibitor of protein phosphatase 2A (PP2A). We therefore examined components of PP2A complexes and found that, among the regulatory B56 subunits, only the promoter of the ppp2r5d gene, encoding the B56δ isoform, is directly bound and transcriptionally activated by Myc in an E-box-dependent manner. Furthermore, we find that B56δ associates with both GSK3β and Myc, resulting in phosphorylation of Myc threonine 58, the well-established signal for ubiquitination and degradation. Furthermore, overexpression, or siRNA-mediated knockdown, of B56δ respectively results in accelerated, or retarded, rates of Myc degradation. Together, our data indicate that Myc limits its own abundance through a negative feedback pathway involving PP2A and GSK3β.

Synaptic released zinc promotes tau hyperphosphorylation by inhibition of protein phosphatase 2A (PP2A)

Hyperphosphorylated tau is the major component of neurofibrillary tangles in Alzheimer disease (AD), and the tangle distribution largely overlaps with zinc-containing glutamatergic neurons, suggesting that zinc released in synaptic terminals may play a role in tau phosphorylation. To explore this possibility, we treated cultured hippocampal slices or primary neurons with glutamate or Bic/4-AP to increase the synaptic activity with or without pretreatment of zinc chelators, and then detected the phosphorylation levels of tau. We found that glutamate or Bic/4-AP treatment caused tau hyperphosphorylation at multiple AD-related sites, including Ser-396, Ser-404, Thr-231, and Thr-205, while application of intracellular or extracellular zinc chelators, or blockade of zinc release by extracellular calcium omission almost abolished the synaptic activity-associated tau hyperphosphorylation. The zinc release and translocation of excitatory synapses in the hippocampus were detected, and zinc-induced tau hyperphosphorylation was also observed in cultured brain slices incubated with exogenously supplemented zinc. Tau hyperphosphorylation induced by synaptic activity was strongly associated with inactivation of protein phosphatase 2A (PP2A), and this inactivation can be reversed by pretreatment of zinc chelator. Together, these results suggest that synaptically released zinc promotes tau hyperphosphorylation through PP2A inhibition.

Different effects of CsA and FK506 on aquaporin-2 abundance in rat primary cultured collecting duct cells

Calcineurin (Cn) inhibitors (CnI) such as cyclosporine A (CsA) and FK506 are nephrotoxic immunosuppressant drugs, which decrease tubular function. Here, we examined the direct effect of CnI on aquaporin-2 (AQP2) expression in rat primary cultured inner medullary collecting duct cells. CsA (0.5-5 μM) but not FK 506 (0.01-1 μM) decreased expression of AQP2 protein and messenger RNA (mRNA) in a concentration and time dependent manner, without affecting mRNA stability. This effect was observed despite similar inhibition of Cn activity by both CnI, thereby suggesting that the CsA-dependent decrease in AQP2 expression was Cn independent. Another inhibitor of cyclophilin A, the primary intracellular target of CsA, had no effect on AQP2 expression. In order to investigate the mechanism of decreased AQP2 transcription, we studied activation status of two suggested transcriptional regulators of AQP2, cAMP-responsive element binding protein (CREB), and tonicity enhancer binding protein (TonEBP). Localization of TonEBP, as well as TonEBP-mediated gene transcription, was not affected by CsA. Phosphorylation of CREB at an activating phosphorylation site (S133) was decreased by CsA, but not by FK506. However, both CnI did not affect cellular cAMP levels. We show that CsA decreases transcription of AQP2, a process that is in part independent of Cn or cyclophilin A and suggests dependence on decreased activity of CREB.

Inactivation of PI3K/AKT signaling inhibits glioma cell growth through modulation of β-catenin-mediated transcription

Aberrant Wnt/β-catenin signaling contributes to the development of many cancers, including glial tumorigenesis. While cross talk between the Wnt/β-catenin and PI3K/AKT signaling pathways has been proposed, the impact of PI3K/AKT inhibition on β-catenin signaling in glioma remains unknown. In the present study, we report decreased cell proliferation and invasive ability upon the LY294002-induced inhibition of PI3K in both U251 and LN229 human glioblastoma cells in vitro. Pharmacologic inhibition of PI3K resulted in the downregulation of several members of the β-catenin pathway, including Fra-1, c-Myc, and cyclin D1. Downregulation impacted β-catenin-mediated transcription, as LY294002 decreased β-catenin/TCF transcriptional activity, determined by the reporter assay. Similar results were observed in vivo, as intratumoral injection of LY294002 downregulated the expression of the components of the β-catenin pathway and delayed tumor growth in nude mice harboring subcutaneous LN229 xenografts. These results suggest that the PI3K/AKT signaling pathway regulates glioma cell proliferation, in part via repression of the Wnt/β-catenin pathway.

Optimal Suppression of Protein Phosphatase 2A Activity Is Critical for Maintenance of Human Embryonic Stem Cell Self-Renewal

The self-renewal of embryonic stem cells involves a balance between processes governed by crosstalk between intrinsic and extrinsic factors. We hypothesized that protein serine/threonine phosphatase 2A (PP2A) may play a central role in the signaling pathways that regulate human embryonic stem cell (hESC) self-renewal. Biochemical analyses revealed that PP2A activity gradually increases over the course of hESC differentiation; PP2A/C and PP2A/A levels also increased. The overexpression of PP2A/C or the addition of PP2A activator C2-ceramide promoted hESC differentiation. Accordingly, the addition of PP2A inactivator okadaic acid (OA) maintained hESC self-renewal in the absence of basic fibroblast growth factor (bFGF). The hESCs maintained with OA expressed pluripotency markers and exhibited substantial telomerase activity with normal karyotypes. The hESCs were able to differentiate into derivatives of the three germ layers, both in vitro and in vivo. Furthermore, the addition of OA and bFGF enabled the maintenance of hESC self-renewal without feeder cells, even in chemically defined xeno-free media. These findings shed a light on the role of PP2A in hESC differentiation and provide a novel strategy for maintaining the self-renewal capability of hESC in bFGF-free, feeder cell-free, and xeno-free media through the optimal suppression of PP2A activity using OA.

A New alpha5beta1 integrin-dependent survival pathway through GSK3beta activation in leukemic cells

Background

Cell survival mediated by integrin engagement has been implicated in cell adhesion-mediated drug resistance. We have recently demonstrated that the activation of glycogen synthase kinase 3 β (GSK3β) is a new pathway supporting the chemoresistance of leukemic cells adhered to fibronectin.

Methodology and Principal Findings

We show here that in conditions of serum starvation, the fibronectin receptor α₅β₁ integrin, but not α₄β₁, induced activation of GSK3β through Ser-9 dephosphorylation in adherent U937 cells. The GSK3β-dependent survival pathway occurred in adherent leukemic cells from patients but not in the HL-60 and KG1 cell lines. In adhesion, activated GSK3β was found in the cytosol/plasma membrane compartment and was co-immunoprecipitated with α₅ integrin, the phosphatase PP2A and the scaffolding protein RACK1. PP2A and its regulatory subunit B' regulated the Ser-9 phosphorylation of GSK3β. In adherent leukemic cells, α₅β₁ integrin but not α₄β₁ upregulated the resistance to TNFα-induced apoptosis. Both extrinsic and intrinsic apoptotic pathways were under the control of α₅β₁ and GSK3β.

Conclusions and Significance

Our data show that, upon serum starvation, α₅β₁ integrin engagement could regulate specific pro-survival functions through the activation of GSK3β.

Divergent pathways mediate spine alterations and cell death induced by amyloid-beta, wild-type tau, and R406W tau

Alzheimer's disease is characterized by synaptic alterations and neurodegeneration. Histopathological hallmarks represent amyloid plaques composed of amyloid-beta (Abeta) and neurofibrillary tangles containing hyperphosphorylated tau. To determine whether synaptic changes and neurodegeneration share common pathways, we established an ex vivo model using organotypic hippocampal slice cultures from amyloid precursor protein transgenic mice combined with virus-mediated expression of EGFP-tagged tau constructs. Confocal high-resolution imaging, algorithm-based evaluation of spines, and live imaging were used to determine spine changes and neurodegeneration. We report that Abeta but not tau induces spine loss and shifts spine shape from mushroom to stubby through a mechanism involving NMDA receptor (NMDAR), calcineurin, and GSK-3beta activation. In contrast, Abeta alone does not cause neurodegeneration but induces toxicity through phosphorylation of wild-type (wt) tau in an NMDAR-dependent pathway. We show that GSK-3beta levels are elevated in APP transgenic cultures and that inhibiting GSK-3beta activity or use of phosphorylation-blocking tau mutations prevented Abeta-induced toxicity of tau. FTDP-17 tau mutants are differentially affected by Abeta. While R406W tau shows increased toxicity in the presence of Abeta, no change is observed with P301L tau. While blocking NMDAR activity abolishes toxicity of both wt and R406W tau, the inhibition of GSK-3beta only protects against toxicity of wt tau but not of R406W tau induced by Abeta. Tau aggregation does not correlate with toxicity. We propose that Abeta-induced spine pathology and tau-dependent neurodegeneration are mediated by divergent pathways downstream of NMDAR activation and suggest that Abeta affects wt and R406W tau toxicity by different pathways downstream of NMDAR activity.

Vitamin B12 deficiency reduces proliferation and promotes differentiation of neuroblastoma cells and up-regulates PP2A, proNGF, and TACE

Vitamin B12 (cobalamin, Cbl) is indispensable for proper brain development and functioning, suggesting that it has neurotrophic effects beside its well-known importance in metabolism. The molecular basis of these effects remains hypothetical, one of the reasons being that no efficient cell model has been made available for investigating the consequences of B12 cellular deficiency in neuronal cells. Here, we designed an approach by stable transfection of NIE115 neuroblastoma cells to impose the anchorage of a chimeric B12-binding protein, transcobalamin-oleosin (TO) to the intracellular membrane. This model produced an intracellular sequestration of B12 evidenced by decreased methyl-Cbl and S-adenosylmethionine and increased homocysteine and methylmalonic acid concentrations. B12 deficiency affected the proliferation of NIE115 cells through an overall increase in catalytic protein phosphatase 2A (PP2A), despite its demethylation. It promoted cellular differentiation by improving initial outgrowth of neurites and, at the molecular level, by augmenting the levels of proNGF and p75(NTR). The up-regulation of PP2A and pro-nerve growth factor (NGF) triggered changes in ERK1/2 and Akt, two signaling pathways that influence the balance between proliferation and neurite outgrowth. Compared with control cells, a 2-fold increase of p75(NTR)-regulated intramembraneous proteolysis (RIP) was observed in proliferating TO cells (P < 0.0001) that was associated with an increased expression of two tumor necrosis factor (TNF)-alpha converting enzyme (TACE) secretase enzymes, Adam 10 and Adam 17. In conclusion, our data show that B12 cellular deficiency produces a slower proliferation and a speedier differentiation of neuroblastoma cells through interacting signaling pathways that are related with increased expression of PP2A, proNGF, and TACE.

Hydrogen peroxide as a cell-survival signaling molecule

Reactive oxygen species (ROS) were seen as destructive molecules, but recently, they have been shown also to act as second messengers in varying intracellular signaling pathways. This review concentrates on hydrogen peroxide (H2O2), as it is a more stable ROS, and delineates its role as a survival molecule. In the first part, the production of H2O2 through the NADPH oxidase (Nox) family is investigated. Through careful examination of Nox proteins and their regulation, it is determined how they respond to stress and how this can be prosurvival rather than prodeath. The pathways on which H2O2 acts to enable its prosurvival function are then examined in greater detail. The main survival pathways are kinase driven, and oxidation of cysteines in the active sites of various phosphatases can thus regulate those survival pathways. Regulation of transcription factors such as p53, NF-kappaB, and AP-1 also are reviewed. Finally, prodeath proteins such as caspases could be directly inhibited through their cysteine residues. A better understanding of the prosurvival role of H2O2 in cells, from the why and how it is generated to the various molecules it can affect, will allow more precise targeting of therapeutics to this pathway.

Cyanidin-3-glucoside reverses ethanol-induced inhibition of neurite outgrowth: role of glycogen synthase kinase 3 Beta

Ethanol is a potent teratogen for the developing central nervous system (CNS), and fetal alcohol syndrome (FAS) is the most common nonhereditary cause of mental retardation. Ethanol disrupts neuronal differentiation and maturation. It is important to identify agents that provide neuroprotection against ethanol neurotoxicity. Using an in vitro neuronal model, mouse Neuro2a (N2a) neuroblastoma cells, we demonstrated that ethanol inhibited neurite outgrowth and the expression of neurofilament (NF) proteins. Glycogen synthase kinase 3beta (GSK3beta), a multifunctional serine/threonine kinase negatively regulated neurite outgrowth of N2a cells; inhibiting GSK3beta activity by retinoic acid (RA) and lithium induced neurite outgrowth, while over-expression of a constitutively active S9A GSK3beta mutant prevented neurite outgrowth. Ethanol inhibited neurite outgrowth by activating GSK3beta through the dephosphorylation of GSK3beta at serine 9. Cyanidin-3-glucoside (C3G), a member of the anthocyanin family rich in many edible berries and other pigmented fruits, enhanced neurite outgrowth by promoting p-GSK3beta(Ser9). More importantly, C3G reversed ethanol-mediated activation of GSK3beta and inhibition of neurite outgrowth as well as the expression of NF proteins. C3G also blocked ethanol-induced intracellular accumulation of reactive oxygen species (ROS). However, the antioxidant effect of C3G appeared minimally involved in its protection. Our study provides a potential avenue for preventing or ameliorating ethanol-induced damage to the developing CNS.

Selective induction of calcineurin activity and signaling by oligomeric amyloid beta

Alzheimer's disease (AD) is a terminal age-associated dementia characterized by early synaptic dysfunction and late neurodegeneration. Although the presence of plaques of fibrillar aggregates of the amyloid beta peptide (Abeta) is a signature of AD, evidence suggests that the preplaque small oligomeric Abeta promotes both synaptic dysfunction and neuronal death. We found that young Tg2576 transgenic mice, which accumulate Abeta and develop cognitive impairments prior to plaque deposition, have high central nervous system (CNS) activity of calcineurin (CaN), a phosphatase involved in negative regulation of memory function via inactivation of the transcription factor cAMP responsive element binding proteins (CREB), and display CaN-dependent memory deficits. These results thus suggested the involvement of prefibrillary forms of Abeta. To investigate this issue, we compared the effect of monomeric, oligomeric, and fibrillar Abeta on CaN activity, CaN-dependent pCREB and phosphorylated Bcl-2 Associated death Protein (pBAD) levels, and cell death in SY5Y cells and in rat brain slices, and determined the role of CaN on CREB phosphorylation in the CNS of Tg2576 mice. Our results show that oligomeric Abeta specifically induces CaN activity and promotes CaN-dependent CREB and Bcl-2 Asociated death Protein (BAD) dephosphorylation and cell death. Furthermore, Tg2576 mice display Abeta oligomers and reduced pCREB in the CNS, which is normalized by CaN inhibition. These findings suggest a role for CaN in mediating effects of oligomeric Abeta on neural cells. Because elevated CaN levels have been reported in the CNS of cognitively impaired aged rodents, our results further suggest that abnormal CaN hyperactivity may be a common event exacerbating the cognitive and neurodegenerative impact of oligomeric Abeta in the aging CNS.

Interaction between Neuronal Depolarization and MK-801 in SH-SY5Y Cells and the Rat Cortex

OBJECTIVE

The interaction between MK-801, a model of psychosis and KCl-induced depolarization or electroconvulsive shock (ECS), a therapeutic model of electroconvulsive therapy (ECT), was investigated in SH-SY5Y cells and the rat frontal cortex.

METHODS

SH-SY5Y cells were pretreated with 1 microM MK-801 for 15 min, followed by cotreatment with 100 mM KCl for 5 min. MK-801 was reintroduced after the KCl was washed out, and the samples were incubated before harvesting. For the experiments in rats, male Sprague-Dawley rats were treated with MK-801 followed by ECS. Immunoblot analyses of glycogen synthase kinase 3beta (GSK3beta) (Ser9), AKT (Ser473) and extracellular legulated kinase (ERK)1/2 in SH-SY5Y cells and the rat frontal cortex were performed.

RESULTS

KCl-induced neuronal depolarization resulted in the transient dephosphorylation of AKT (Ser473) and GSK3beta (Ser9), followed by increased phosphorylation of the enzymes in SH-SY5Y cells. Cotreatment with MK-801 and KCl inhibited the initial dephosphorylation of AKT and GSK3beta produced by KCl-induced neuronal depolarization. Similarly, ECS resulted in the transient dephosphorylation of AKT (Ser473) and GSK3beta (Ser9), whereas cotreatment with MK-801 inhibited the initial dephosphorylation of AKT (Ser473) and GSK3beta (Ser9) produced by ECS in the rat frontal cortex. No significant interaction was observed between MK-801 and KCl in the dephosphorylation of ERK1/2.

CONCLUSION

These results suggest that an antagonistic interplay between MK-801 and neuronal depolarization by KCl or ECS is involved the regulation of AKT (Ser473) and GSK3beta (Ser9) phosphorylation.

The role of GSK-3 in synaptic plasticity

Glycogen synthase kinase-3 (GSK-3), an important component of the glycogen metabolism pathway, is highly expressed in the CNS. It has been implicated in major neurological disorders including Alzheimer's disease, schizophrenia and bipolar disorders. Despite its central role in these conditions it was not known until recently whether GSK-3 has neuronal-specific functions under normal conditions. However recent work has shown that GSK-3 is involved in the regulation of, and cross-talk between, two major forms of synaptic plasticity, N-methyl-D-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP) and NMDAR-dependent long-term depression (LTD). The present article summarizes this recent work and discusses its potential relevance to the treatment of neurological disorders.

Mechanisms of ROS modulated cell survival during carcinogenesis

There is increasing evidence within the literature that the decreased susceptibility of tumour cells to stimuli that induce apoptosis can be linked to their inherently increased redox potential. The review primarily focuses on the PI3-kinase/Akt pathway, and the multiple points along this signalling pathway that may be redox regulated. The PI3-kinase/Akt pathway can influence a cells' sensitivity to death inducing signals, through direct manipulation of apoptosis regulating molecules or by regulating the activity of key transcription factors. Proteins involved in the control of apoptosis that are directly regulated by the PI3-kinase/Akt pathway include caspase-9, Bad and the transcription factor GSK-3beta. Lately, it is becoming increasingly obvious that phosphatases are a major counter balance to the PI3-kinase/Akt pathway. Phosphatases such as PP2A and PP1alpha can dephosphorylate signalling molecules within the PI3-kinase/Akt pathway, blocking their activity. It is the balance between the kinase activity and the phosphatase activity that determines the presence and strength of the PI3-kinase/Akt signal. This is why any protein modifications that hinder dephosphorylation can increase the tumours survival advantage. One such modification is the oxidation of the sulphydryl group in key cysteine residues present within the active site of the phosphatases. This highlights the link between the increased redox stress in tumours with the PI3-kinase/Akt pathway. This review will discuss the various sources of reactive oxygen species within a tumour and the effect of these radicals on the PI3-kinase/Akt pathway.

Regulated recycling and plasma membrane recruitment of the high-affinity choline transporter

The high-affinity choline transporter (CHT1) is responsible for uptake of choline from the synaptic cleft and supplying choline for acetylcholine synthesis. CHT1 internalization by clathrin-coated vesicles is proposed to represent a mechanism by which high-affinity choline uptake can be modulated. We show here that internalized CHT1 is rapidly recycled back to the cell surface in both human embryonic kidney cells (HEK 293 cells) and SH-SY5Y neuroblastoma cells. This rapidly recycling pool of CHT1 comprises about 10% of total CHT1 protein. In the SH-SY5Y neuroblastoma cell line K(+)-depolarization promotes Ca(2+)-dependent increase in the rate of CHT1 recycling to the plasma membrane without affecting the rate of CHT1 internalization. K(+)-depolarization also increases the size of the pool of CHT1 protein that can be mobilized to the plasma membrane. Thus, the activity-dependent increase in plasma membrane CHT1 localization appears to be regulated by two mechanisms: (i) an increase in the rate of externalization of the intracellular CHT1 pool; and (ii) the recruitment of additional intracellular transporters to the recycling pool.

Coupling of mammalian target of rapamycin with phosphoinositide 3-kinase signaling pathway regulates protein phosphatase 2A- and glycogen synthase kinase-3 -dependent phosphorylation of Tau

Tau is an important microtubule-stabilizing protein in neurons. In its hyperphosphorylated form, Tau protein loses its ability to bind to microtubules and then accumulates and is part of pathological lesions characterizing tauopathies, e.g. Alzheimer disease. Glycogen synthase kinase-3beta (GSK-3beta), antagonized by protein phosphatase 2A (PP2A), regulates Tau phosphorylation at many sites. Diabetes mellitus is linked to an increased risk of developing Alzheimer disease. This could be partially caused by dysregulated GSK-3beta. In a long term experiment (-16 h) using primary murine neuron cultures, we interfered in the insulin/phosphoinositide 3-kinase (PI3K) (LY294002 treatment and insulin boost) and mammalian target of rapamycin (mTor) (AICAR and rapamycin treatment) signaling pathways and examined consequent changes in the activities of PP2A, GSK-3beta, and Tau phosphorylation. We found that the coupling of PI3K with mTor signaling, in conjunction with a regulatory interaction between PP2A and GSK-3beta, changed activities of both enzymes always in the same direction. These balanced responses seem to ensure the steady Tau phosphorylation at GSK/PP2A-dependent sites observed over a long period of time (>/=6 h). This may help in preventing severe changes in Tau phosphorylation under conditions when neurons undergo transient fluctuations either in insulin or nutrient supply. On the other hand, the investigation of Tau protein at Ser-262 showed that interference in the insulin/PI3K and mTor signaling potentially influenced the Tau phosphorylation status at sites where only one of two enzymes (in this case PP2A) is involved in the regulation.

Glycogen synthase kinase-3beta: homologous regulation of cell surface insulin receptor level via controlling insulin receptor mRNA stability in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, 48 h-treatment with 20 mmol/L LiCl, 1 mmol/L valproic acid, 30 micromol/L SB216763, 30 micromol/L SB415286, or 100 nmol/L insulin, a condition that inhibits constitutive active glycogen synthase kinase-3 (GSK-3), decreased cell surface (125)I-insulin binding capacity by approximately 39%, without altering the K(d) value; LiCl, SB216763 or insulin decreased insulin receptor (IR) and IR precursor levels, attenuating insulin-induced Tyr-autophosphorylation of IR. LiCl increased inhibitory Ser9-phosphorylation of GSK-3beta at 6 h, decreasing (125)I-insulin binding at 24 h. SB216763-induced (125)I-insulin binding reduction (IC(50) = 3 micromol/L) was preceded by beta-catenin level increase by SB216763 (EC(50) = 11 micromol/L), a hallmark of GSK-3 inhibition. Insulin-induced rapid (> 1 min) Ser9-phosphorylation of GSK-3beta (Nemoto et al. 2006) was followed by approximately 48% decrease of IR level. LiCl did not stimulate endocytosis, nor proteolysis of IR. LiCl destabilized IR mRNA (t(1/2) = 9.3 vs. 6.5 h), decreasing IR mRNA level by approximately 47%, without altering IR gene transcription. Decreases of (125)I-insulin binding and IR level, as well as increased Ser9-phosphorylation of GSK-3beta were restored to the control levels by washing the test compound-treated cells. Thus, GSK-3beta regulates IR level via controlling IR mRNA stability.

A plant oxylipin, 12-oxo-phytodienoic acid, inhibits proliferation of human breast cancer cells by targeting cyclin D1

Cyclin D1 overexpression has been associated with poor prognosis and resistance to therapy in human breast cancer. Thus, the development of therapeutic agents that selectively target cyclin D1 activity is of clinical interest. This study demonstrates that 12-oxo-phytodienoic acid (OPDA), a phytohormone with critical functions in growth and development in plants, induces growth arrest in MDA-MB-231 and T47D breast cancer cells. In response to OPDA treatment, the human breast cancer cell lines exhibit a progressive decline in cyclin D1 expression, which is tightly associated with the accumulation of hypophosphorylated form of the retinoblastoma protein (Rb) and G1 arrest. The decrease in cyclin D1 protein expression accompanies a dramatic decline in nuclear but not membranous beta-catenin expression and activation of glycogen synthase kinase-3-beta (GSK3beta) caused by inhibition of its serine-9 phosphorylation. The proteasome inhibitor MG132 blocks OPDA-mediated decrease in cyclin D1. In addition, the overexpression of T286A, a cyclin D1 mutant which is refractory to phosphorylation by GSK3beta and proteosomal degradation, is resistant to OPDA-mediated Rb dephosphorylation as well as G(1) cell cycle arrest. Thus, our results demonstrate that degradation of cyclin D1 protein is a key event in OPDA induced growth inhibition in breast cancer cells. These data provide the basic foundation for future efforts to develop OPDA-based approaches in the prevention and treatment of breast cancer and other types of cancer.

LTP inhibits LTD in the hippocampus via regulation of GSK3beta

Glycogen synthase kinase-3 (GSK3) has been implicated in major neurological disorders, but its role in normal neuronal function is largely unknown. Here we show that GSK3beta mediates an interaction between two major forms of synaptic plasticity in the brain, N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) and NMDA receptor-dependent long-term depression (LTD). In rat hippocampal slices, GSK3beta inhibitors block the induction of LTD. Furthermore, the activity of GSK3beta is enhanced during LTD via activation of PP1. Conversely, following the induction of LTP, there is inhibition of GSK3beta activity. This regulation of GSK3beta during LTP involves activation of NMDA receptors and the PI3K-Akt pathway and disrupts the ability of synapses to undergo LTD for up to 1 hr. We conclude that the regulation of GSK3beta activity provides a powerful mechanism to preserve information encoded during LTP from erasure by subsequent LTD, perhaps thereby permitting the initial consolidation of learnt information.

The protein phosphatase-1/inhibitor-2 complex differentially regulates GSK3 dephosphorylation and increases sarcoplasmic/endoplasmic reticulum calcium ATPase 2 levels

The ubiquitously expressed protein glycogen synthase kinase-3 (GSK3) is constitutively active, however its activity is markedly diminished following phosphorylation of Ser21 of GSK3alpha and Ser9 of GSK3beta. Although several kinases are known to phosphorylate Ser21/9 of GSK3, for example Akt, relatively much less is known about the mechanisms that cause the dephosphorylation of GSK3 at Ser21/9. In the present study KCl-induced plasma membrane depolarization of SH-SY5Y cells, which increases intracellular calcium concentrations caused a transient decrease in the phosphorylation of Akt at Thr308 and Ser473, and GSK3 at Ser21/9. Overexpression of the selective protein phosphatase-1 inhibitor protein, inhibitor-2, increased basal GSK3 phosphorylation at Ser21/9 and significantly blocked the KCl-induced dephosphorylation of GSK3beta, but not GSK3alpha. The phosphorylation of Akt was not affected by the overexpression of inhibitor-2. GSK3 activity is known to affect sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) levels. Overexpression of inhibitor-2 or treatment of cells with the GSK3 inhibitors lithium and SB216763 increased the levels of SERCA2. These results indicate that the protein phosphatase-1/inhibitor-2 complex differentially regulates GSK3 dephosphorylation induced by KCl and that GSK3 activity regulates SERCA2 levels.

Constitutive activity of glycogen synthase kinase-3beta: positive regulation of steady-state levels of insulin receptor substrates-1 and -2 in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, 12-h treatment with 1-20 mM LiCl, an inhibitor of glycogen synthase kinase-3 (GSK-3), increased Ser(9) phosphorylation of GSK-3beta by approximately 44%, while decreasing insulin receptor substrate-1 (IRS-1) and IRS-2 protein levels by approximately 38 and approximately 62% in a concentration-dependent manner. Treatment with SB216763 (0.1-30 microM for 12 h), a selective inhibitor of GSK-3, lowered IRS-1 and IRS-2 levels by approximately 38 and approximately 48%, while increasing beta-catenin protein level by approximately 47%, due to the prevention of GSK-3-induced degradation of beta-catenin by SB216763. Insulin (100 nM for 24 h) increased Ser(9) phosphorylation of GSK-3beta by approximately 104%, while decreasing IRS-1 and IRS-2 levels by approximately 41 and approximately 72%; the insulin-induced Ser(9) phosphorylation of GSK-3beta, as well as down-regulations of IRS-1 and IRS-2 levels were restored to the control levels of nontreated cells at 24 h after the washout of the insulin (100 nM for 12 h)-treated cells. Either clasto-lactacystin beta-lactone or lactacystin (an inhibitor of proteasome) prevented LiCl- or SB216763-induced decreases of IRS-1 and IRS-2 levels by approximately 100 and approximately 69%, respectively. In contrast, calpastatin (an inhibitor of calpain) and leupeptin (an inhibitor of lysosome) failed to prevent the decreases of IRS-1 and IRS-2 levels caused by LiCl or SB216763. LiCl or SB216763 lowered IRS-2 mRNA level, with no effect on IRS-1 mRNA level. These results suggest that constitutive activity of GSK-3beta in quiescent cells positively maintains steady-state levels of IRS-1 and IRS-2 via regulating proteasomal degradation and/or synthesis of IRS-1 and IRS-2 proteins.

Identification of okadaic acid-induced phosphorylation events by a mass spectrometry approach

Okadaic acid (OA) is a widely used small-molecule phosphatase inhibitor that is thought to selectively inhibit protein phosphatase 2A (PP2A). Multiple studies have demonstrated that PP2A activity is compromised in the brains of Alzheimer's disease patients. Thus, we set out to determine changes in phosphorylation that occur upon OA treatment of neuronal cells. Utilizing isotope-coded affinity tags and mass spectrometry analysis, we determined the relative abundance of proteins in a phosphoprotein enriched fraction from control and OA-treated primary cortical neurons. We identified many proteins whose phosphorylation state is regulated by OA, including glycogen synthase kinase 3beta, collapsin-response mediator proteins (DRP-2, DPYSL-5, and CRMP-4), and the B subunit of PP2A itself. Most interestingly, we have found that complexin 2, an important regulator of neurotransmitter release and synaptic plasticity, is phosphorylated at serine 93 upon OA treatment of neurons. This is the first report of a phosphorylation site on complexin 2.

Transient activation of protein phosphatase 2A induced by electroconvulsive shock in the rat frontal cortex

We have attempted to determine the effects of electroconvulsive shock (ECS) on protein phosphatase 2A (PP2A) in the frontal cortices of rats. PP2A exhibited a 30% increase in activity immediately after ECS treatment. Immunoblot analysis revealed that phosphorylation signals, including protein kinase B (Akt/PKB), glycogen synthase kinase-3beta (GSK-3beta), and cyclic adenosine monophosphate response element binding protein (CREB) were reduced immediately after ECS treatment. When an additional ECS was administered after the activation of these kinases, the immediate reactivation of PP2A overrode the kinase activity. ECS induces transient PP2A activation prior to kinase activation, and this pattern of activity may induce the biphasic phosphorylation of substrate proteins.

The use of okadaic acid to elucidate the intracellular role(s) of protein phosphatase 2A: Lessons from the mast cell model system

In recent years a heightened appreciation has emerged for the role(s) that phosphatases play in regulating signal transduction pathways and other cellular processes. The tumor-promoting agent okadaic acid (OA) has been an invaluable tool in efforts aimed at delineating the contributions of the most abundant mammalian serine/threonine phosphatase, protein phosphatase 2A (PP2A), to intracellular signaling and cell function. PP2A, which is ubiquitous and vital in virtually every cell system studied, continues to be the focus of much research on phosphorylation control machinery. Mast cells represent an excellent in vitro model for the study of protein phosphorylation events because they possess a number of distinct signaling pathways that lead to the production and/or release of discreet mediators in response to different stimuli. The utility of OA in analyzing PP2A function has been demonstrated in mast cells across several species. Results of these studies have contributed to the current recognition that PP2A plays a crucial role in the biology of mast cells and other cell types.