Glycogen synthase kinase-3beta regulates presenilin 1 C-terminal fragment levels

Macrophage PTEN controls STING-induced inflammation and necroptosis through NICD/NRF2 signaling in APAP-induced liver injury

BACKGROUND

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling has been known to play a critical role in maintaining cellular and tissue homeostasis, which also has an essential role in the inflammatory response. However, it remains unidentified whether and how the macrophage PTEN may govern the innate immune signaling stimulator of interferon genes (STING) mediated inflammation and hepatocyte necroptosis in APAP-induced liver injury (AILI).

METHODS

Myeloid-specific PTEN knockout (PTEN^M-KO) and floxed PTEN (PTEN^FL/FL) mice were treated with APAP (400 mg/kg) or PBS. In a parallel in vitro study, bone marrow-derived macrophages (BMMs) were isolated from these conditional knockout mice and transfected with CRISPR/Cas9-mediated Notch1 knockout (KO) or CRISPR/Cas9-mediated STING activation vector followed by LPS (100 ng/ml) stimulation.

RESULTS

Here, we report that myeloid-specific PTEN knockout (PTEN^M-KO) mice were resistant to oxidative stress-induced hepatocellular injury with reduced macrophage/neutrophil accumulation and proinflammatory mediators in AILI. PTEN^M-KO increased the interaction of nuclear Notch intracellular domain (NICD) and nuclear factor (erythroid-derived 2)-like 2 (NRF2) in the macrophage nucleus, reducing reactive oxygen species (ROS) generation. Mechanistically, it is worth noting that macrophage NICD and NRF2 co-localize within the nucleus under inflammatory conditions. Additionally, Notch1 promotes the interaction of immunoglobulin kappa J region (RBPjκ) with NRF2. Disruption of the Notch1 signal in PTEN deletion macrophages, reduced RBPjκ and NRF2 binding, and activated STING signaling. Moreover, PTEN^M-KO macrophages with STING activated led to ROS generation and TNF-α release, resulting in hepatocyte necroptosis upon co-culture with primary hepatocytes.

CONCLUSIONS

Our findings demonstrate that the macrophage PTEN-NICD/NRF2-STING axis is critical to regulating oxidative stress-induced liver inflammation and necroptosis in AILI and implies the therapeutic potential for managing sterile liver inflammation. Video Abstract.

Protective role of anticancer drugs in neurodegenerative disorders: A drug repurposing approach

The disease heterogeneity and little therapeutic progress in neurodegenerative diseases justify the need for novel and effective drug discovery approaches. Drug repurposing is an emerging approach that reinvigorates the classical drug discovery method by divulging new therapeutic uses of existing drugs. The common biological background and inverse tuning between cancer and neurodegeneration give weight to the conceptualization of repurposing of anticancer drugs as novel therapeutics. Many studies are available in the literature, which highlights the success story of anticancer drugs as repurposed therapeutics. Among them, kinase inhibitors, developed for various oncology indications evinced notable neuroprotective effects in neurodegenerative diseases. In this review, we shed light on the salient role of multiple protein kinases in neurodegenerative disorders. We also proposed a feasible explanation of the action of kinase inhibitors in neurodegenerative disorders with more attention towards neurodegenerative disorders. The problem of neurotoxicity associated with some anticancer drugs is also highlighted. Our review encourages further research to better encode the hidden potential of anticancer drugs with the aim of developing prospective repurposed drugs with no toxicity for neurodegenerative disorders.

The cross-talk of NOTCH and GSK-3 signaling in colon and other cancers

The GSK-3 kinases, GSK-3α and GSK-3β, have a central role in regulating multiple cellular processes such as glycogen synthesis, insulin signaling, cell proliferation and apoptosis. GSK-3β is the most well studied, and was originally described for its role in regulating glycogen synthase. GSK-3β has been studied as a participant in the oncogenic process in a variety of cancers due to its intersection with the PTEN/PI3K/AKT and RAS/RAF/MEK/ERK pathways. Dysregulated signaling through the Notch family of receptors can also promote oncogenesis. Normal Notch receptor signaling regulates cell fate determination in stem cell pools. GSK-3β and Notch share similar targets such β-catenin and the WNT pathway. WNT and β-catenin are involved in several oncogenic processes including those of the colon. In addition, GSK-3β may directly regulate aspects of Notch signaling. This review describes how crosstalk between GSK-3β and Notch can promote oncogenesis, using colon cancer as the primary example.

Phosphorylation Signaling in APP Processing in Alzheimer's Disease

The abnormal accumulation of amyloid-β (Aβ) in the central nervous system is a hallmark of Alzheimer’s disease (AD). The regulation of the processing of the single- transmembrane amyloid precursor protein (APP) plays an important role in the generation of Aβ in the brain. The phosphorylation of APP and key enzymes involved in the proteolytic processing of APP has been demonstrated to be critical for modulating the generation of Aβ by either altering the subcellular localization of APP or changing the enzymatic activities of the secretases responsible for APP processing. In addition, the phosphorylation may also have an impact on the physiological function of these proteins. In this review, we summarize the kinases and signaling pathways that may participate in regulating the phosphorylation of APP and secretases and how this further affects the function and processing of APP and Aβ pathology. We also discuss the potential of approaches that modulate these phosphorylation-signaling pathways or kinases as interventions for AD pathology.

Presenilins and γ-Secretase in Membrane Proteostasis

The presenilin (PS) proteins exert a crucial role in the pathogenesis of Alzheimer disease (AD) by mediating the intramembranous cleavage of amyloid precursor protein (APP) and the generation of amyloid β-protein (Aβ). The two homologous proteins PS1 and PS2 represent the catalytic subunits of distinct γ-secretase complexes that mediate a variety of cellular processes, including membrane protein metabolism, signal transduction, and cell differentiation. While the intramembrane cleavage of select proteins by γ-secretase is critical in the regulation of intracellular signaling pathways, the plethora of identified protein substrates could also indicate an important role of these enzyme complexes in membrane protein homeostasis. In line with this notion, PS proteins and/or γ-secretase has also been implicated in autophagy, a fundamental process for the maintenance of cellular functions and homeostasis. Dysfunction in the clearance of proteins in the lysosome and during autophagy has been shown to contribute to neurodegeneration. This review summarizes the recent knowledge about the role of PS proteins and γ-secretase in membrane protein metabolism and trafficking, and the functional relation to lysosomal activity and autophagy.

Candesartan Restores the Amyloid Beta-Inhibited Proliferation of Neural Stem Cells by Activating the Phosphatidylinositol 3-Kinase Pathway

Background and Purpose

Neurogenesis in the adult brain is important for memory and learning, and the alterations in neural stem cells (NSCs) may be an important aspect of Alzheimer's disease (AD) pathogenesis. The phosphatidylinositol 3-kinase (PI3K) pathway has been suggested to have an important role in neuronal cell survival and is highly involved in adult neurogenesis. Candesartan is an angiotensin II receptor antagonist used for the treatment of hypertension and several studies have reported that it also has some neuroprotective effects. We investigated whether candesartan could restore the amyloid-β(25–35) (Aβ_25-35) oligomer-inhibited proliferation of NSCs by focusing on the PI3K pathway.

Methods

To evaluate the effects of candesartan on the Aβ_25-35 oligomer-inhibited proliferation of NSCs, the NSCs were treated with several concentrations of candesartan and/or Aβ_25-35 oligomers, and MTT assay and trypan blue staining were performed. To evaluate the effect of candesartan on the Aβ-inhibited proliferation of NSCs, we performed a bromodeoxyuridine (BrdU) labeling assay. The levels of p85α PI3K, phosphorylated Akt (pAkt) (Ser473), phosphorylated glycogen sinthase kinase-3β (pGSK-3β) (Ser9), and heat shock transcription factor-1 (HSTF-1) were analyzed by Western blotting.

Results

The BrdU assays demonstrated that NSC proliferation decreased with Aβ_25-35 oligomer treatment; however, a combined treatment with candesartan restored it. Western blotting displayed that candesartan treatment increased the expression levels of p85α PI3K, pAkt (Ser473), pGSK-3β (Ser9), and HSTF. The NSCs were pretreated with a PI3K inhibitor, LY294002; the effects of candesartan on the proliferation of NSCs inhibited by Aβ_25-35 oligomers were almost completely blocked.

Conclusions

Together, these results suggest that candesartan restores the Aβ_25-35 oligomer-inhibited proliferation of NSCs by activating the PI3K pathway.

STRAP positively regulates TLR3-triggered signaling pathway

Toll-like receptor (TLR) signaling drives the innate immune response by activating nuclear factor-κB (NF-κB) and interferon regulatory factor (IRF). We have previously shown that STRAP interacts with TAK1 and IKKα along with NF-κB subunit p65, leading to the activation of pro-inflammatory cytokines. However, the roles of STRAP in TRIF/TBK1-mediated TLR3 activation and the subsequent type I interferon (IFN) production are not fully elucidated. Here, we demonstrate that STRAP acts as a scaffold protein in TLR3-triggered signaling. STRAP strongly interacts with TBK1 and IRF3, which enhances IFN-β production. As a consequence, STRAP knockdown reduces the level of both pro-inflammatory cytokine and IFN in TLR3 agonist-stimulated macrophages, whereas its overexpression significantly enhances production of these cytokines. Furthermore, the C-terminus of STRAP is essential for its functional activity in TLR3-mediated IL-6 and IFN-β production. These data suggest that STRAP is a positive regulator of the TLR3-meditated NF-κB and IRF signaling pathway.

γ-Secretase Activity Is Required for Regulated Intramembrane Proteolysis of Tumor Necrosis Factor (TNF) Receptor 1 and TNF-mediated Pro-apoptotic Signaling

The γ-secretase protease and associated regulated intramembrane proteolysis play an important role in controlling receptor-mediated intracellular signaling events, which have a central role in Alzheimer disease, cancer progression, and immune surveillance. An increasing number of γ-secretase substrates have a role in cytokine signaling, including the IL-6 receptor, IL-1 receptor type I, and IL-1 receptor type II. In this study, we show that following TNF-converting enzyme-mediated ectodomain shedding of TNF type I receptor (TNFR1), the membrane-bound TNFR1 C-terminal fragment is subsequently cleaved by γ-secretase to generate a cytosolic TNFR1 intracellular domain. We also show that clathrin-mediated internalization of TNFR1 C-terminal fragment is a prerequisite for efficient γ-secretase cleavage of TNFR1. Furthermore, using in vitro and in vivo model systems, we show that in the absence of presenilin expression and γ-secretase activity, TNF-mediated JNK activation was prevented, assembly of the TNFR1 pro-apoptotic complex II was reduced, and TNF-induced apoptosis was inhibited. These observations demonstrate that TNFR1 is a γ-secretase substrate and suggest that γ-secretase cleavage of TNFR1 represents a new layer of regulation that links the presenilins and the γ-secretase protease to pro-inflammatory cytokine signaling.

Twenty Years of Presenilins--Important Proteins in Health and Disease

Alzheimer's disease (AD) is characterized by progressive decline in cognitive functions associated with depositions of aggregated proteins in the form of extracellular plaques and neurofibrillary tangles in the brain. Extracellular plaques contain characteristic fibrils of amyloid β peptides (Aβ); tangles consist of paired helical filaments of the microtubuli-associated protein tau. Although AD manifests predominantly at ages above 65 years, rare cases show a much earlier onset of disease symptoms with very similar neuropathological characteristics. In 1995, two homologous genes were identified, in which mutations are associated with dominantly inherited familial forms of early onset AD. The genes therefore were dubbed presenilins (PS) and encode polytopic transmembrane proteins. At this time the role of these proteins in the pathogenesis of AD and their biological function in general were completely unknown. However, individuals carrying PS mutations showed alterations in the composition of different length variants of Aβ peptides in blood and cerebrospinal fluid, which indicated the potential involvement of presenilins in the metabolism of Aβ. After 20 years of intense research, the roles of presenilins in Aβ generation as well as important functions in biological processes have been identified. Presenilins represent the catalytic components of protease complexes that directly cleave the amyloid precursor protein (APP) but also many other proteins with important physiological functions. Here, the progress in presenilin research from basic characterization of their cellular functions to the targeting in clinical trials for AD therapy, and potential future directions, will be discussed.

Beyond γ-secretase activity: The multifunctional nature of presenilins in cell signalling pathways

The presenilins are the catalytic subunit of the membrane-embedded tetrameric γ-secretase protease complexes. More that 90 transmembrane proteins have been reported to be γ-secretase substrates, including the widely studied amyloid precursor protein (APP) and the Notch receptor, which are precursors for the generation of amyloid-β peptides and biologically active APP intracellular domain (AICD) and Notch intracellular domain (NICD). The diversity of γ-secretase substrates highlights the importance of presenilin-dependent γ-secretase protease activities as a regulatory mechanism in a range of biological systems. However, there is also a growing body of evidence that supports the existence of γ-secretase-independent functions for the presenilins in the regulation and progression of an array of cell signalling pathways. In this review, we will present an overview of current literature that proposes evolutionarily conserved presenilin functions outside of the γ-secretase complex, with a focus on the suggested role of the presenilins in the regulation of Wnt/β-catenin signalling, protein trafficking and degradation, calcium homeostasis and apoptosis.

Loss of Presenilin 2 Function Is Associated with Defective LPS-Mediated Innate Immune Responsiveness

The importance of presenilin-dependent γ-secretase protease activities in the development, neurogenesis, and immune system is highlighted by the diversity of its substrates and characterization of Psen1- and Psen2-deficient transgenic animals. Functional differences between presenilin 1 (PS1) and presenilin 2 (PS2) are incompletely understood. In this study, we have identified a Psen2-specific function, not shared by Psen1 in Toll-like receptor signaling. We show that immortalized fibroblasts and bone marrow-derived macrophages from Psen2- but not Psen1-deficient mice display reduced responsiveness to lipopolysaccharide (LPS) with decreased nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) activity and diminished pro-inflammatory cytokine production. In whole animal in vivo responses, Psen2-deficient animals have abnormal systemic production of LPS-stimulated pro-inflammatory cytokines. Mechanistically, we demonstrate that Psen2 deficiency is paralleled by reduced transcription of tlr4 mRNA and loss of LPS-induced tlr4 mRNA transcription regulation. These observations illustrate a novel PS2-dependent means of modulating LPS-mediated immune responses and identify a functional distinction between PS1 and PS2 in innate immunity.

A ubiquitin-binding CUE domain in presenilin-1 enables interaction with K63-linked polyubiquitin chains

The presenilins (PS1 and PS2) are the catalytic component of the γ-secretase intramembrane protease complex, involved in the regulated intramembrane proteolysis of numerous type I transmembrane proteins, including amyloid precursor protein (APP) and Notch. Herein, we describe the identification and characterization of a CUE (coupling of ubiquitin conjugation to endoplasmic reticulum degradation) ubiquitin-binding domain (UBD) in PS1, and demonstrate that the CUE domain of PS1 mediates non-covalent binding to Lysine 63-linked polyubiquitin chains. Our results highlight a γ-secretase-independent function for non-covalent ubiquitin signaling in the regulation of PS1, and add new insights into the structure and function of the presenilin proteins.

Glycogen synthase kinase-3 as a therapeutic target for cognitive dysfunction in neuropsychiatric disorders

The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) is involved in a broad range of cellular processes including cell proliferation, apoptosis and inflammation. It is now also increasingly acknowledged as having a role to play in cognitive-related processes such as neurogenesis, synaptic plasticity and neural cell survival. Cognitive impairment represents a major debilitating feature of many neurodegenerative and psychiatric disorders, including Alzheimer's disease, mood disorders, schizophrenia and fragile X syndrome, as well as being a result of traumatic brain injury or cranial irradiation. Accordingly, GSK-3 has been identified as an important therapeutic target for cognitive impairment, and recent preclinical studies have yielded important evidence demonstrating that GSK-3 inhibitors may be useful therapeutic interventions for restoring cognitive function in some of these brain disorders. The current review summarises the role of GSK-3 as a regulator of cognitive-dependent functions, examines current preclinical and clinical evidence of the potential of GSK-3 inhibitors as therapeutic agents for cognitive impairments in neuropsychiatric disorders, and offers some insight into the current obstacles that are impeding the clinical use of selective GSK-3 inhibitors in the treatment of cognitive impairment.

Neuroprotective effects of amlodipine besylate and benidipine hydrochloride on oxidative stress-injured neural stem cells

Hypertension is associated with oxidative stress. Amlodipine besylate (AB) and benidipine hydrochloride (BH), which are Ca(2+) antagonists, have been reported to reduce oxidative stress. In this study, we examined the neuroprotective effects of AB and BH on oxidative stress-injured neural stem cells (NSCs), with a focus on the phosphatidylinositol 3-kinase (PI3K) pathway and the extracellular signal-regulated kinase (ERK) pathway. After treatment with H2O2, the viability of NSCs decreased in a concentration-dependent manner; however, co-treatment with AB or BH restored the viability of H2O2-injured NSCs. H2O2 increased free radical production and apoptosis in NSCs, whereas co-treatment with AB or BH attenuated these effects. To evaluate the effects of AB or BH on the H2O2-inhibited proliferation of NSCs, we performed BrdU labeling and colony formation assays and found that NSC proliferation decreased upon H2O2 treatment but that combined treatment with AB or BH restored this proliferation. Western blot analysis showed that AB and BH increased the expression of cell survival-related proteins that were linked with the PI3K and ERK pathways but decreased the expression of cell death-related proteins. To investigate whether the PI3K and ERK pathways were directly involved in the neuroprotective effects of AB and BH on H2O2-treated NSCs, NSCs were pretreated with the PI3K inhibitor, LY294002, or the ERK inhibitor, FR180204, which significantly blocked the effects of AB and BH. Together, our results suggest that AB and BH restore the H2O2-inhibited viability and proliferation of NSCs by inhibiting oxidative stress and by activating the PI3K and ERK pathways.

Presenilins are novel substrates for TRAF6-mediated ubiquitination

Mutations in presenilins (PS1 and PS2) have been linked to the pathogenesis of early onset familial Alzheimer's disease. Presenilins function as the catalytic component of the γ-secretase protease complexes responsible for the cleavage of the amyloid precursor protein (APP), subsequent generation of amyloid-β and associated amyloid plaques in Alzheimer's disease. Biochemical and genetic studies have revealed that through interactions with several proteins, the presenilins are functionally involved in a range of cellular processes, including the regulation of intracellular calcium homeostasis. Our group has previously reported an association between presenilins and members of the tumour necrosis factor receptor-associated factor (TRAF) family of proteins. In this study we further investigated the association between TRAF6, an E3 ubiquitin ligase, and the presenilins. Here we show that the presenilin full-length holoproteins are novel substrates of TRAF6-mediated Lysine-63-linked ubiquitination. Interestingly, co-expression of catalytically active TRAF6 with the presenilins leads to decreased turnover of PS1 full-length holoprotein accompanying elevated presenilin protein levels. Similarly, while overexpression of TRAF6 increases presenilin holoprotein levels and ubiquitination in HEK293 cells, expression of catalytically deficient TRAF6 or TRAF6-deficiency leads to a reduction in presenilin protein levels and reduced PS1 ubiquitination. We also demonstrate that TRAF6 induces PS1 gene transcription in a JNK-dependent manner. Notably, we reveal that TRAF6-mediated ubiquitination of presenilin does not affect γ-secretase enzyme activity, but may regulate presenilin function in calcium signalling. Taken together, we propose that presenilins are novel substrates for TRAF6-mediated K63-linked ubiquitination and that ubiquitination of presenilins by TRAF6 increases presenilin holoprotein levels and in conditions in which TRAF6 ubiquitination of presenilins is reduced results in reduction of calcium release from the endoplasmic reticulum.

Coenzyme Q10 restores amyloid beta-inhibited proliferation of neural stem cells by activating the PI3K pathway

Neurogenesis in the adult brain is important for memory and learning, and the alterations in neural stem cells (NSCs) may be an important part of Alzheimer's disease pathogenesis. The phosphatidylinositol 3-kinase (PI3K) pathway has been suggested to play an important role in neuronal cell survival and is highly involved in adult neurogenesis. Recently, coenzyme Q10 (CoQ10) was found to affect the PI3K pathway. We investigated whether CoQ10 could restore amyloid β (Aβ)25-35 oligomer-inhibited proliferation of NSCs by focusing on the PI3K pathway. To evaluate the effects of CoQ10 on Aβ25-35 oligomer-inhibited proliferation of NSCs, NSCs were treated with several concentrations of CoQ10 and/or Aβ25-35 oligomers. BrdU labeling, Colony Formation Assays, and immunoreactivity of Ki-67, a marker of proliferative activity, showed that NSC proliferation decreased with Aβ25-35 oligomer treatment, but combined treatment with CoQ10 restored it. Western blotting showed that CoQ10 treatment increased the expression levels of p85α PI3K, phosphorylated Akt (Ser473), phosphorylated glycogen synthase kinase-3β (Ser9), and heat shock transcription factor, which are proteins related to the PI3K pathway in Aβ25-35 oligomers-treated NSCs. To confirm a direct role for the PI3K pathway in CoQ10-induced restoration of proliferation of NSCs inhibited by Aβ25-35 oligomers, NSCs were pretreated with a PI3K inhibitor, LY294002; the effects of CoQ10 on the proliferation of NSCs inhibited by Aβ25-35 oligomers were almost completely blocked. Together, these results suggest that CoQ10 restores Aβ25-35 oligomer-inhibited proliferation of NSCs by activating the PI3K pathway.

Pro-inflammatory interleukin-18 increases Alzheimer's disease-associated amyloid-β production in human neuron-like cells

Background

Alzheimer’s disease (AD) involves increased accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles as well as neuronal loss in various regions of the neocortex. Neuroinflammation is also present, but its role in AD is not fully understood. We previously showed increased levels of pro-inflammatory cytokine interleukin-18 (IL-18) in different regions of AD brains, where it co-localized with Aβ-plaques, as well as the ability of IL-18 to increase expression of glycogen synthase kinase-3β (GSK-3β) and cyclin dependent kinase 5, involved in hyperphosphorylation of tau-protein. Elevated IL-18 has been detected in several risk conditions for AD, including obesity, type-II diabetes, and cardiovascular diseases as well as in stress.

Methods

We differentiated SH-SY5Y neuroblastoma cells as neuron-like and exposed them to IL-18 for various times. We examined the protein levels of amyloid-β precursor protein (APP) and its processing products, its cleaving enzymes, involved in amyloidogenic processing of APP, and markers of apoptosis.

Results

IL-18 increased protein levels of the β-site APP-cleaving enzyme BACE-1, the N-terminal fragment of presenilin-1 and slightly presenilin enhancer 2, both of which are members of the γ-secretase complex, as well as Fe65, which is a binding protein of the C-terminus of APP and one regulator for GSK-3β. IL-18 also increased APP expression and phosphorylation, which preceded increased BACE-1 levels. Further, IL-18 altered APP processing, increasing Aβ40 production in particular, which was inhibited by IL-18 binding protein. Increased levels of soluble APPβ were detected in culture medium after the IL-18 exposure. IL-18 also increased anti-apoptotic bcl-xL levels, which likely counteracted the minor increase of the pro-apoptotic caspase-3. Lactate dehydrogenase activity in culture medium was unaffected.

Conclusions

The IL-18 induction of BACE-1, APP processing, and Aβ is likely to be linked to stress-associated adaptations in neurons during the course of normal functioning and development. However, in the course of wider changes in the aging brain, and particularly in AD, the effects of heightened or prolonged levels of IL-18 may contribute to the process of AD, including via increased Aβ.

Abnormal cross-talk between mutant presenilin 1 (I143T, G384A) and glycosphingolipid biosynthesis

Mutations in the presenilin 1 (PS1) gene are associated with early onset familial Alzheimer's disease (FAD). In this study, we found that the expression of mutant-PS1 in stable transfectants of SH-SY5Y neuroblastoma cells results in a reduction of the biosynthesis and steady-state levels of glucosylceramide. As an in vivo corroboration of these data, there was a significant reduction of brain glucosylceramide and gangliosides in an animal model of FAD. In mutant-PS1-transfectants (I143T, G384A), immunocytochemistry disclosed a remarkable reduction of glucosylceramide synthase (GlcT-1)-like immunoreactivity in the cells when compared with those of mock- and wild-PS1 transfectants. Immunoprecipitation of GlcT-1 protein from mutant-PS1 transfectants demonstrated a marked reduction in GlcT-1 protein, but there was no reduction in the levels of GlcT-1 mRNA. Both coprecipitation and γ-secretase inhibition experiments suggest that mutant-PS1 seems to form a complex with GlcT-1 protein and to be involved in GlcT-1 degradation, which was never found in other cell types. Thus, mutations in the PS1 gene result in profound glycosphingolipids abnormalities by abnormal molecular interaction with GlcT-1.

Protease regulation: the Yin and Yang of neural development and disease

The formation, maintenance, and plasticity of neural circuits rely upon a complex interplay between progressive and regressive events. Increasingly, new functions are being identified for axon guidance molecules in the dynamic processes that occur within the embryonic and adult nervous system. The magnitude, duration, and spatial activity of axon guidance molecule signaling are precisely regulated by a variety of molecular mechanisms. Here we focus on recent progress in understanding the role of protease-mediated cleavage of guidance factors required for directional axon growth, with a particular emphasis on the role of metalloprotease and γ-secretase. Since axon guidance molecules have also been linked to neural degeneration and regeneration in adults, studies of guidance receptor proteolysis are beginning to define new relationships between neurodevelopment and neurodegeneration. These findings raise the possibility that the signaling checkpoints controlled by proteases could be useful targets to enhance regeneration.

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.

L-DOPA neurotoxicity is prevented by neuroprotective effects of erythropoietin

The neurotoxicity of L-3,4-dihydroxyphenylalanine (L-DOPA), one of the most important drugs for the treatment of Parkinson's disease, still remains controversial, although much more data on L-DOPA neurotoxicity have been presented. Considering the well known neuroprotective effects of erythropoietin (EPO), the inhibitory effects of EPO on L-DOPA neurotoxicity need to be evaluated. Neuronally differentiated PC12 (nPC12) cells were treated with different concentrations of L-DOPA and/or EPO for 24h. Cell viability was evaluated using trypan blue, 4',6-diamidino-2-phenylindole (DAPI) and TUNEL staining, and cell counting. Free radicals and intracellular signaling protein levels were measured with 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and Western blotting, respectively. L-DOPA reduced nPC12 cell viability at higher concentrations, but combined treatment with EPO and L-DOPA significantly restored cell viability. Free radicals and hydroxyl radical levels increased by L-DOPA were decreased after combined treatment of L-DOPA and EPO. Levels of survival-related intracellular signaling proteins decreased in nPC12 cells treated with 200 μM L-DOPA but increased significantly in cells treated with 200μM L-DOPA and 5 μM EPO. However, cleaved caspase-3, a death-related protein, increased in nPC12 cells treated with 200 μM L-DOPA but decreased significantly in cells treated with 200 μM L-DOPA and 5 μM EPO. Pretreatment with LY294002, a phosphatidylinositol 3-kinase inhibitor, prior to combined treatment with EPO and L-DOPA almost completely blocked the protective effects of EPO. These results indicate that EPO can prevent L-DOPA neurotoxicity by activating the PI3K pathway as well as reducing oxidative stress.

What Are the bona fide GSK3 Substrates?

Nearly 100 proteins are proposed to be substrates for GSK3, suggesting that this enzyme is a fundamental regulator of almost every process in the cell, in every tissue in the body. However, it is not certain how many of these proposed substrates are regulated by GSK3 in vivo. Clearly, the identification of the physiological functions of GSK3 will be greatly aided by the identification of its bona fide substrates, and the development of GSK3 as a therapeutic target will be highly influenced by this range of actions, hence the need to accurately establish true GSK3 substrates in cells. In this paper the evidence that proposed GSK3 substrates are likely to be physiological targets is assessed, highlighting the key cellular processes that could be modulated by GSK3 activity and inhibition.

Regulation of cell survival mechanisms in Alzheimer's disease by glycogen synthase kinase-3

A pivotal role has emerged for glycogen synthase kinase-3 (GSK3) as an important contributor to Alzheimer's disease pathology. Evidence for the involvement of GSK3 in Alzheimer's disease pathology and neuronal loss comes from studies of GSK3 overexpression, GSK3 localization studies, multiple relationships between GSK3 and amyloid β-peptide (Aβ), interactions between GSK3 and the microtubule-associated tau protein, and GSK3-mediated apoptotic cell death. Apoptotic signaling proceeds by either an intrinsic pathway or an extrinsic pathway. GSK3 is well established to promote intrinsic apoptotic signaling induced by many insults, several of which may contribute to neuronal loss in Alzheimer's disease. Particularly important is evidence that GSK3 promotes intrinsic apoptotic signaling induced by Aβ. GSK3 appears to promote intrinsic apoptotic signaling by modulating proteins in the apoptosis signaling pathway and by modulating transcription factors that regulate the expression of proteins involved in apoptosis. Thus, GSK3 appears to contribute to several neuropathological mechanisms in Alzheimer's disease, including apoptosis-mediated neuronal loss.

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.

Cdk5: multitasking between physiological and pathological conditions

Cyclin-dependent kinase 5 (Cdk5) is a peculiar proline-directed serine/threonine kinase. Unlike the other members of the Cdk family, Cdk5 is not directly involved in cell cycle regulation, being normally associated with neuronal processes such as migration, cortical layering and synaptic plasticity. This kinase is present mainly in post-mitotic neurons and its activity is tightly regulated by the interaction with the specific activators, p35 and p39. Despite its pivotal role in CNS development, Cdk5 dysregulation has been implicated in different pathologies, such as Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and, most recently, prion-related encephalopathies (PRE). In these neurodegenerative conditions, Cdk5 overactivation and relocalization occurs upon association with p25, a truncated form of the normal activator p35. This activator switching will cause a shift in the phosphorylative pattern of Cdk5, with an alteration both in targets and activity, ultimately leading to neuronal demise. In AD and PRE, two disorders that share clinical and neuropathological features, Cdk5 dysregulation is a linking event between the major neuropathological markers: amyloid plaques, tau hyperphosphorylation and synaptic and neuronal loss. Moreover, this kinase was shown to be involved in abortive cell cycle re-entry, a feature recently proposed as a possible step in the neuronal apoptosis mechanism of several neurological diseases. This review focuses on the role of Cdk5 in neurons, namely in the regulation of cytoskeletal dynamics, synaptic function and cell survival, both in physiological and in pathological conditions, highlighting the relevance of Cdk5 in the main mechanisms of neurodegeneration in Alzheimer's disease and other brain pathologies.

Brief review of the role of glycogen synthase kinase-3β in amyotrophic lateral sclerosis

Glycogen synthase kinase-3β (GSK-3β) is known to affect a diverse range of biological functions controlling gene expression, cellular architecture, and apoptosis. GSK-3β has recently been identified as one of the important pathogenic mechanisms in motor neuronal death related to amyotrophic lateral sclerosis (ALS). Therefore, the development of methods to control GSK-3β could be helpful in postponing the symptom progression of ALS. Here we discuss the known roles of GSK-3β in motor neuronal cell death in ALS and the possibility of employing GSK-3β modulators as a new therapeutic strategy.

Dyrk1A-mediated phosphorylation of Presenilin 1: a functional link between Down syndrome and Alzheimer's disease

The dual-specificity tyrosine(Y)-phosphorylation-regulated kinase 1A (Dyrk1A) gene is located on human chromosome 21 and encodes a proline-directed protein kinase that might be responsible for mental retardation and early onset of Alzheimer's disease (AD) in Down syndrome (DS) patients. Presenilin 1 (PS1) is a key component of the γ-secretase complex in the generation of β-amyloid (Aβ), an important trigger protein in the pathogenesis of AD. Increased Dyrk1A expression has been reported in human AD and DS brains. We previously showed that Dyrk1A increased Aβ production in mammalian cells and transgenic mice that over-express Dyrk1A. In this study, we describe a potential mechanism by which Aβ is increased in Dyrk1A-over-expressing DS and AD brains. First, we show that PS1 is phosphorylated by the Dyrk1A at Thr(354) and that this phosphorylation increases γ-secretase activity. Then, using transgenic mice that over-express human Dyrk1A, we demonstrate that phospho-Thr354-PS1 (pT354-PS1) expression is enhanced when Dyrk1A level is increased. We also show that pT354-PS1 is more stable than the unphosphorylated form of PS1. These results reveal a potential regulatory link between Dyrk1A and PS1 in the Aβ pathway of DS and AD brains, suggesting that up-regulated Dyrk1A may accelerate AD pathogenesis through PS1 phosphorylation.

Characterization of an atypical gamma-secretase complex from hematopoietic origin

Gamma-secretase is a widely expressed multisubunit enzyme complex which is involved in the pathogenesis of Alzheimer disease and hematopoietic malignancies through its aberrant processing of the amyloid precursor protein (APP) and Notch1, respectively. While gamma-secretase has been extensively studied, there is a dearth of information surrounding the activity, composition, and function of gamma-secretase expressed in distinct cellular populations. Here we show that endogenous gamma-secretase complexes of hematopoietic origin are distinct from epithelial derived gamma-secretase complexes. Hematopoietic gamma-secretase has reduced activity for APP and Notch1 processing compared to epithelial gamma-secretase. Characterization of the active complexes with small molecule affinity probes reveals that hematopoietic gamma-secretase has an atypical subunit composition with significantly altered subunit stoichiometry. Furthermore, we demonstrate that these discrete complexes exhibit cell-line specific substrate selectivity suggesting a possible mechanism of substrate regulation. These data underscore the need for studying endogenous gamma-secretase to fully understand of the biology of gamma-secretase and its complexity as a molecular target for the development of disease therapeutics.

Cell-type dependent modulation of Notch signaling by the amyloid precursor protein

The amyloid precursor protein is a ubiquitously expressed transmembrane protein that has been long implicated in the pathogenesis of Alzheimer's disease but its normal biological function has remained elusive despite extensive effort. We have previously reported the identification of Notch2 as an amyloid precursor protein interacting protein in E18 rat neurons. Here, we sought to reveal the physiologic consequences of this interaction. We report a functional relationship between amyloid precursor protein and Notch1, which does not affect Delta ligand binding. First, we observed interactions between the amyloid precursor protein and Notch in mouse embryonic stem cells lacking both presenilin 1 and presenilin 2, the active proteolytic components of the gamma-secretase complex, suggesting that these two transmembrane proteins can interact in the absence of presenilin. Next, we demonstrated that the amyloid precursor protein affects Notch signaling by using Notch-dependent luciferase assays in two cell lines, the human embryonic kidney 293 and the monkey kidney, COS7. We found that the amyloid precursor protein exerts opposing effects on Notch signaling in human embryonic kidney 293 vs. COS7 cells. Finally, we show that more Notch Intracellular Domain is found in the nucleus in the presence of exogenous amyloid precursor protein or its intracellular domain, suggesting the mechanism by which the amyloid precursor protein affects Notch signaling in certain cells. Our results provide evidence of potentially important communications between the amyloid precursor protein and Notch.

Regulation of protein stability by GSK3 mediated phosphorylation

Glycogen synthase kinase-3 (GSK3) plays important roles in numerous signaling pathways that regulate a variety of cellular processes including cell proliferation, differentiation, apoptosis and embryonic development. In the canonical Wnt signaling pathway, GSK3 phosphorylation mediates proteasomal targeting and degradation of beta-catenin via the destruction complex. We recently reported a biochemical screen that discovered multiple additional protein substrates whose stability is regulated by Wnt signaling and/or GSK3 and these have important implications for Wnt/GSK3 regulation of different cellular processes.(1) In this article, we also present a bio-informatics based screen for proteins whose stability may be controlled by GSK3 and beta-Trcp, the SCF E3 ubiquitin ligase that is responsible for beta-catenin degradation in the Wnt signaling pathway. Furthermore, we review various GSK3 regulated proteolysis substrates described in the literature. We propose that GSK3 phosphorylation dependent proteolysis is a widespread mechanism that the cell employs to regulate a variety of cell processes in response to signals.

PI3K inhibition causes the accumulation of ubiquitinated presenilin 1 without affecting the proteasome activity

gamma-Secretase is an enzymatic complex, composed of presenilin 1 (PS1), nicastrin, pen-2, and aph-1, and is responsible for the intramembranous cleavage of various type-I membrane proteins. The level of each component is tightly regulated in a cell via proteasomal degradation. On the other hand, it has previously been reported that PS1/gamma-secretase is involved in the activation of phosphatidylinositol-3 kinase/Akt (PI3K/Akt) pathway. PI3K is inhibited in Alzheimer's disease (AD) brain, whereas the effects of PI3K inhibition on the metabolism of PS1/gamma-secretase have not been elucidated. Here, we demonstrate that the treatment of neurons with PI3K inhibitors leads to increased levels of PS1/gamma-secretase components through an inhibitory effect on their degradation. Moreover, PI3K inhibition accelerated ubiquitination of PS1. We further show the evidence that the PS1 ubiquitination after PI3K inhibition is represented by the multiple mono-ubiquitination, instead of poly-ubiquitination. Accordingly, treatment of cells with PI3K inhibitor led to a differential intracellular redistribution of PS1 from the one observed after the proteasomal inhibition. These results suggest that PI3K inhibition may trigger the multiple mono-ubiquitination of PS1, which precludes the degradation of PS1/gamma-secretase through the proteasomal pathway. Since PS1/gamma-secretase is deeply involved in the production of Abeta protein, a deeper knowledge into its metabolism could contribute to a better elucidation of AD pathogenesis.

L-DOPA-induced neurotoxicity is reduced by the activation of the PI3K signaling pathway

L-3,4-Dihydroxyphenylalanine (L-DOPA) is one of the most important drugs for the treatment of Parkinson's disease (PD). Although neurotoxicity of L-DOPA remains controversial, there are many reports suggesting that L-DOPA causes neuronal death. We investigated whether the neurotoxic effect of L-DOPA could be inhibited by the activation of the phosphatidylinositol 3-kinase (PI3K) pathway. Cell counting kit-8, trypan blue staining, and DAPI staining all showed that L-DOPA decreased nPC12 cell viability at high concentrations. However, combined treatment with the PI3K activator and L-DOPA significantly increased the viability of nPC12 cells when compared with treatment with L-DOPA only. Phosphorylated Akt (Ser473), phosphorylated glycogen synthase kinase-3beta (GSK-3beta) (Ser9), and heat shock transcription factor-1, which are survival-related signaling proteins, were decreased in nPC12 cells treated with 200 microM L-DOPA, but were significantly increased with combined treatment with the PI3K activator in a concentration-dependent manner. However, treatment of L-DOPA significantly increased expressions of cytosolic cytochrome c and cleaved caspase-3, which are death-related signaling proteins, in nPC12 cells, but combined treatment with the PI3K activator reduced those expressions. To confirm whether the effect of the PI3K activator is associated with direct activation of PI3K, LY294002, a PI3K inhibitor, was used to pretreat the nPC12 cells prior to combined treatment with the PI3K activator and L-DOPA. The protective effect of the PI3K activator was almost completely blocked. Together, these results suggest that L-DOPA neurotoxicity can be prevented by PI3K activation.

Aberrant phosphorylation in the pathogenesis of Alzheimer's disease

The modification of proteins by reversible phosphorylation is a key mechanism in the regulation of various physiological functions. Abnormal protein kinase or phosphatase activity can cause disease by altering the phosphorylation of critical proteins in normal cellular and disease processes. Alzheimer's disease (AD), typically occurring in the elderly, is an irreversible, progressive brain disorder characterized by memory loss and cognitive decline. Accumulating evidence suggests that protein kinase and phosphatase activity are altered in the brain tissue of AD patients. Tau is a highly recognized phosphoprotein that undergoes hyperphosphorylation to form neurofibrillary tangles, a neuropathlogical hallmark with amyloid plaques in AD brains. This study is a brief overview of the altered protein phosphorylation pathways found in AD. Understanding the molecular mechanisms by which the activities of protein kinases and phosphatases are altered as well as the phosphorylation events in AD can potentially reveal novel insights into the role aberrant phosphorylation plays in the pathogenesis of AD, providing support for protein phosphorylation as a potential treatment strategy for AD.

Repression of tau hyperphosphorylation by chronic endurance exercise in aged transgenic mouse model of tauopathies

The present study was undertaken to investigate whether chronic endurance exercise affects tau phosphorylation levels in the brain with Alzheimer's disease (AD)-like pathology. To address this, the transgenic (Tg) mouse model of tauopathies, Tg-NSE/htau23, which expresses human tau23 in the brain, was chosen. Animals were subjected to chronic exercise for 3 months from 16 months of age. The exercised Tg mouse groups were treadmill run at speeds of 12 m/min (intermediate exercise group) or 19 m/min (high exercise group) for 1 hr/day, 5 days/week, during the 3-month period. Chronic endurance exercise in Tg mice increased the expression of Cu/Zn-superoxide dismutase (SOD) and catalase, and also their enzymatic activities in the brain. In parallel, chronic exercise in Tg mice up-regulated the expression of phospho-PKCalpha, phospho-AKT, and phospho-PI3K, and down-regulated the expressions of phospho-PKA, phosphor-p38, phospho-JNK, and phospho-ERK. Moreover, chronic exercise up-regulated both cytosolic and nuclear levels of beta-catenin, and the expression of T-cell factor-4 (Tcf-4) and cyclin D1 in the brain. As a consequence of such changes, the levels of phospho-tau in the brain of Tg mice were markedly decreased after exercise. Immunohistochemical analysis showed an exercised-induced decrease of the phospho-tau levels in the CA3 subregion of the hippocampus. These results suggest that chronic endurance exercise may provide a therapeutic potential to alleviate the tau pathology.

CD147, a gamma-secretase associated protein is upregulated in Alzheimer's disease brain and its cellular trafficking is affected by presenilin-2

Gamma-secretase activity has been extensively investigated due to its role in Alzheimer's disease. Here, we studied the association of CD147, a transmembrane glycoprotein belonging to the immunoglobulin family, with gamma-secretase and its expression in Alzheimer's disease and control tissues. Subcellular fractionation of postmitochondrial supernatant from rat brain on step iodixanol gradient in combination with co-immunoprecipitation using an anti-nicastrin antibody showed association of limited amount of CD147 to gamma-secretase. By immunoblotting of postnuclear pellets from Alzheimer's disease and control human brain tissues we showed that CD147 with molecular weight 75 kDa is upregulated in frontal cortex and thalamus of the Alzheimer's disease brains. Immunohistochemistry of brain tissues from Alzheimer's disease and control revealed specific upregulation of CD147 in neurons, axons and capillaries of Alzheimer's disease frontal cortex and thalamus. The effect of presenilin-1 and -2, which are the catalytic subunits of gamma-secretase, on CD147 expression and subcellular localization was analyzed by confocal microscopy in combination with flow cytometry and showed that PS2 affected the subcellular localization of CD147 in mouse embryonic fibroblast cells. We suggest that a small fraction of CD147 present in the brain is associated with the gamma-secretase, and can be involved in mechanisms dysregulated in Alzheimer's disease brain.

Review of lithium effects on brain and blood

Clinicians have long used lithium to treat manic depression. They have also observed that lithium causes granulocytosis and lymphopenia while it enhances immunological activities of monocytes and lymphocytes. In fact, clinicians have long used lithium to treat granulocytopenia resulting from radiation and chemotherapy, to boost immunoglobulins after vaccination, and to enhance natural killer activity. Recent studies revealed a mechanism that ties together these disparate effects of lithium. Lithium acts through multiple pathways to inhibit glycogen synthetase kinase-3beta (GSK3 beta). This enzyme phosphorylates and inhibits nuclear factors that turn on cell growth and protection programs, including the nuclear factor of activated T cells (NFAT) and WNT/beta-catenin. In animals, lithium upregulates neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor, neurotrophin-3 (NT3), as well as receptors to these growth factors in brain. Lithium also stimulates proliferation of stem cells, including bone marrow and neural stem cells in the subventricular zone, striatum, and forebrain. The stimulation of endogenous neural stem cells may explain why lithium increases brain cell density and volume in patients with bipolar disorders. Lithium also increases brain concentrations of the neuronal markers n-acetyl-aspartate and myoinositol. Lithium also remarkably protects neurons against glutamate, seizures, and apoptosis due to a wide variety of neurotoxins. The effective dose range for lithium is 0.6-1.0 mM in serum and >1.5 mM may be toxic. Serum lithium levels of 1.5-2.0 mM may have mild and reversible toxic effects on kidney, liver, heart, and glands. Serum levels of >2 mM may be associated with neurological symptoms, including cerebellar dysfunction. Prolonged lithium intoxication >2 mM can cause permanent brain damage. Lithium has low mutagenic and carcinogenic risk. Lithium is still the most effective therapy for depression. It "cures" a third of the patients with manic depression, improves the lives of about a third, and is ineffective in about a third. Recent studies suggest that some anticonvulsants (i.e., valproate, carbamapazine, and lamotrigene) may be useful in patients that do not respond to lithium. Lithium has been reported to be beneficial in animal models of brain injury, stroke, Alzheimer's, Huntington's, and Parkinson's diseases, amyotrophic lateral sclerosis (ALS), spinal cord injury, and other conditions. Clinical trials assessing the effects of lithium are under way. A recent clinical trial suggests that lithium stops the progression of ALS.

Association between Presenilin-1 and TRAF6 modulates regulated intramembrane proteolysis of the p75NTR neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) is a member of the tumour necrosis factor superfamily, which relies on the recruitment of cytosolic protein partners including the tumour necrosis factor receptor-associated factor 6 (TRAF6) E3 ubiquitin ligase to produce cellular responses. Recently, p75(NTR) was also shown to undergo presenilin-dependent, gamma-secretase-mediated regulated intramembrane proteolysis. In this study, we report the characterization of a highly conserved TRAF6-binding site (PxExxAr/Ac) in presenilin-1 (PS1) that mediates nerve growth factor (NGF)-induced association between PS1 and TRAF6. We demonstrate that disruption of this interaction between PS1 and TRAF6 inhibits TRAF6 autoubiquitination and gamma-secretase cleavage of p75(NTR). Additionally, we show that PS1-deficiency antagonizes NGF-induced I-kappaB degradation. Finally, we also show that p75(NTR) is a substrate for TRAF6-mediated ubiquitination and that TRAF6 E3 ligase activity is required for regulated intramembrane proteolysis of p75(NTR). In summary, our data suggest that an NGF-induced association between PS1 and TRAF6 influences regulated intramembrane proteolysis of p75(NTR).

Interleukin-1 receptor type 1 is a substrate for gamma-secretase-dependent regulated intramembrane proteolysis

Biochemical and genetic studies have revealed that the presenilins interact with several proteins and are involved in the regulated intramembrane proteolysis of numerous type 1 membrane proteins, thereby linking presenilins to a range of cellular processes. In this study, we report the characterization of a highly conserved tumor necrosis factor receptor-associated factor-6 (TRAF6) consensus-binding site within the hydrophilic loop domain of presenilin-1 (PS-1). In coimmunoprecipitation studies we indicate that presenilin-1 interacts with TRAF6 and interleukin-1 receptor-associated kinase 2. Substitution of presenilin-1 residues Pro-374 and Glu-376 by site-directed mutagenesis greatly reduces the ability of PS1 to associate with TRAF6. By studying these interactions, we also demonstrate that the interleukin-1 receptor type 1 (IL-1R1) undergoes intramembrane proteolytic processing, mediated by presenilin-dependent gamma-secretase activity. A metalloprotease-dependent proteolytic event liberates soluble IL-1R1 ectodomain and produces an approximately 32-kDa C-terminal domain. This IL-1R1 C-terminal domain is a substrate for subsequent gamma-secretase cleavage, which generates an approximately 26-kDa intracellular domain. Specific pharmacological gamma-secretase inhibitors, expression of dominant negative presenilin-1, or presenilin deficiency independently inhibit generation of the IL-1R1 intracellular domain. Attenuation of gamma-secretase activity also impairs responsiveness to IL-1beta-stimulated activation of the MAPKs and cytokine secretion. Thus, TRAF6 and interleukin receptor-associated kinase 2 are novel binding partners for PS1, and IL-1R1 is a new substrate for presenilin-dependent gamma-secretase cleavage. These findings also suggest that regulated intramembrane proteolysis may be a control mechanism for IL-1R1-mediated signaling.

Glucocorticoids trigger Alzheimer disease-like pathobiochemistry in rat neuronal cells expressing human tau

Amyloid precursor protein (APP) mis-processing and aberrant tau hyperphosphorylation are causally related to the pathogenesis and neurodegenerative processes that characterize Alzheimer's disease (AD). Abnormal APP metabolism leads to the generation of neurotoxic amyloid beta (Abeta), whereas tau hyperphosphorylation culminates in cytoskeletal disturbances, neuronal dysfunction and death. Many AD patients hypersecrete glucocorticoids (GC) while neuronal structure, function and survival are adversely influenced by elevated GC levels. We report here that a rat neuronal cell line (PC12) engineered to express the human ortholog of the tau protein (PC12-htau) becomes more vulnerable to the toxic effects of either Abeta or GC treatment. Importantly, APP metabolism in GC-treated PC12-htau cells is selectively shifted towards increased production of the pro-amyloidogenic peptide C99. Further, GC treatment results in hyperphosphorylation of human tau at AD-relevant sites, through the cyclin-dependent kinase 5 (E.C. 2.7.11.26) and GSK3 (E.C. 2.7.11.22) protein kinases. Pulse-chase experiments revealed that GC treatment increased the stability of tau protein rather than its de novo synthesis. GC treatment also induced accumulation of transiently expressed EGFP-tau in the neuronal perikarya. Together with previous evidence showing that Abeta can activate cyclin-dependent kinase 5 and GSK3, these results uncover a potential mechanism through which GC may contribute to AD neuropathology.

Tumor necrosis factor-alpha-elicited stimulation of gamma-secretase is mediated by c-Jun N-terminal kinase-dependent phosphorylation of presenilin and nicastrin

Gamma-secretase is a multiprotein complex composed of presenilin (PS), nicastrin (NCT), Aph-1, and Pen-2, and it catalyzes the final proteolytic step in the processing of amyloid precursor protein to generate amyloid-beta. Our previous results showed that tumor necrosis factor-alpha (TNF-alpha) can potently stimulate gamma-secretase activity through a c-Jun N-terminal kinase (JNK)-dependent pathway. Here, we demonstrate that TNF-alpha triggers JNK-dependent serine/threonine phosphorylation of PS1 and NCT to stimulate gamma-secretase activity. Blocking of JNK activity with a potent JNK inhibitor (SP600125) reduces TNF-alpha-triggered phosphorylation of PS1 and NCT. Consistent with this, we show that activated JNKs can be copurified with gamma-secretase complexes and that active recombinant JNK2 can promote the phosphorylation of PS1 and NCT in vitro. Using site-directed mutagenesis and a synthetic peptide, we clearly show that the Ser(319)Thr(320) motif in PS1 is an important JNK phosphorylation site that is critical for the TNF-alpha-elicited regulation of gamma-secretase. This JNK phosphorylation of PS1 at Ser(319)Thr(320) enhances the stability of the PS1 C-terminal fragment that is necessary for gamma-secretase activity. Together, our findings strongly suggest that JNK is a critical intracellular mediator of TNF-alpha-elicited regulation of gamma-secretase and governs the pivotal step in the assembly of functional gamma-secretase.

Development of anaplastic lymphoma kinase (ALK) small-molecule inhibitors for cancer therapy

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) involved in the genesis of several human cancers; indeed, ALK was initially identified in constitutively activated and oncogenic fusion forms--the most common being nucleophosmin (NPM)-ALK--in a non-Hodgkin's lymphoma (NHL) known as anaplastic large-cell lymphoma (ALCL) and subsequent studies identified ALK fusions in the human sarcomas called inflammatory myofibroblastic tumors (IMTs). In addition, two recent reports have suggested that the ALK fusion, TPM4-ALK, may be involved in the genesis of a subset of esophageal squamous cell carcinomas. While the cause-effect relationship between ALK fusions and malignancies such as ALCL and IMT is very well established, more circumstantial links implicate the involvement of the full-length, normal ALK receptor in the genesis of additional malignancies including glioblastoma, neuroblastoma, breast cancer, and others; in these instances, ALK is believed to foster tumorigenesis following activation by autocrine and/or paracrine growth loops involving the reported ALK ligands, pleiotrophin (PTN) and midkine (MK). There are no currently available ALK small-molecule inhibitors approved for clinical cancer therapy; however, recognition of the variety of malignancies in which ALK may play a causative role has recently begun to prompt developmental efforts in this area. This review provides a succinct summary of normal ALK biology, the confirmed and putative roles of ALK fusions and the full-length ALK receptor in the development of human cancers, and efforts to target ALK using small-molecule kinase inhibitors.

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.