beta-Amyloid-induced neuronal apoptosis requires c-Jun N-terminal kinase activation

Neuroprotective effect of honokiol and magnolol, compounds from Magnolia officinalis, on beta-amyloid-induced toxicity in PC12 cells

Amyloid β peptide (Aβ) induced toxicity is a well-established pathway of neuronal cell death which might play a role in Alzheimer's disease. In this regard, the toxic effect of Aβ on a cultured Aβ-sensitive neuronal cell line was used as a primary screening tool for potential anti-Alzheimer's therapeutic agents. The effects of nine pure compounds (vitamin E, α-asarone, salidroside, baicolin, magnolol, gastrodin, bilobalide, honokiol and β-asarone) from selected Chinese herbs on neuronal cell death induced by Aβ in NGF-differentiated PC12 cells were examined. Only two of the studied compounds, honokiol and magnolol, significantly decreased Aβ-induced cell death. Further experiments indicated that their neuroprotective effects are possibly mediated through reduced ROS production as well as suppression of intracellular calcium elevation and inhibition of caspase-3 activity. The results provide for the first time a scientific rationale for the clinical use of honokiol and magnolol in the treatment of Alzheimer's disease.

Linking Alzheimer's disease to insulin resistance: the FoxO response to oxidative stress

Oxidative stress is an important determinant not only in the pathogenesis of Alzheimer's disease (AD), but also in insulin resistance (InsRes) and diabetic complications. Forkhead box class O (FoxO) transcription factors are involved in both insulin action and the cellular response to oxidative stress, thereby providing a potential integrative link between AD and InsRes. For example, the expression of intra- and extracellular antioxidant enzymes, such as manganese-superoxide dismutase and selenoprotein P, is regulated by FoxO proteins, as is the expression of important hepatic enzymes of gluconeogenesis. Here, we review the molecular mechanisms involved in the pathogenesis of AD and InsRes and discuss the function of FoxO proteins in these processes. Both InsRes and oxidative stress may promote the transcriptional activity of FoxO proteins, resulting in hyperglycaemia and a further increased production of reactive oxygen species (ROS). The consecutive activation of c-Jun N-terminal kinases and inhibition of Wingless (Wnt) signalling may result in the formation of β-amyloid plaques and τ protein phosphorylation. Wnt inhibition may also result in a sustained activation of FoxO proteins with induction of apoptosis and neuronal loss, thereby completing a vicious circle from oxidative stress, InsRes and hyperglycaemia back to the formation of ROS and consecutive neurodegeneration. In view of their central function in this model, FoxO proteins may provide a potential molecular target for the treatment of both InsRes and AD.

αB-Crystallin inhibits the cell toxicity associated with amyloid fibril formation by κ-casein and the amyloid-β peptide

Amyloid fibril formation is associated with diseases such as Alzheimer's, Parkinson's, and prion diseases. Inhibition of amyloid fibril formation by molecular chaperone proteins, such as the small heat-shock protein αB-crystallin, may play a protective role in preventing the toxicity associated with this form of protein misfolding. Reduced and carboxymethylated κ-casein (RCMκ-CN), a protein derived from milk, readily and reproducibly forms fibrils at physiological temperature and pH. We investigated the toxicity of fibril formation by RCMκ-CN using neuronal model PC12 cells and determined whether the inhibition of fibril formation altered its cell toxicity. To resolve ambiguities in the literature, we also investigated whether fibril formation by amyloid-β1-40 (Aβ(1-40)), the peptide associated with Alzheimer's disease, was inhibited by αB-crystallin and if this affected the toxicity of Aβ. To this end, either RCMκ-CN or Aβ(1-40) was incubated at neutral pH to induce fibril formation before treating PC12 cells and assessing cell viability. Incubated (fibrillar) RCMκ-CN was more toxic to PC12 cells than native RCMκ-CN with the highest level of toxicity being associated with mature fibrils and protofibrils. Furthermore, the toxicity of RCMκ-CN was attenuated when its fibril formation was inhibited, either through the chaperone action of αB-crystallin or when it interacted with its natural binding partners in milk, α(S)- and β-casein. Likewise, incubating Aβ(1-40) with αB-crystallin inhibited both Aβ(1-40) fibril formation and the associated cell toxicity. Importantly, by inhibiting fibril formation, αB-crystallin prevents the cell toxicity associated with protein misfolding.

Down-expression of PGC-1alpha partially mediated by JNK/c-Jun through binding to CRE site during apoptotic procedure in cerebellar granule neurons

In eukaryotes, mitochondria are critical for cellular bioenergetics and mediating apoptosis. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is an important regulator of mitochondrial biogenesis and function. However, the role of PGC-1alpha in neuronal apoptosis and its regulation by apoptotic pathway are still unknown. We demonstrated that PGC-1alpha expression was down-regulated in cerebellar granule neurons(CGNs) after activation of the JNK/c-Jun pathway by potassium deprivation. Overexpression of PGC-1alpha partially protected CGNs from potassium deprivation-induced apoptosis. JNK-specific inhibitors, SP600125 and CEP11004, partially blocked the inhibitory effects of JNK on PGC-1alpha expression and its promoter activity. Furthermore, ChIP assays revealed that c-Jun was able to bind to the CRE site (-188 to -180) in the PGC-1alpha promoter. In conclusion, these results suggest that down-expression of PGC-1alpha partially mediated by activation of JNK/c-Jun may be through the binding of c-Jun to the CRE site in the PGC-1alpha promoter, and it might be involved in potassium deprivation-induced apoptosis in CGNs.

Microglial receptor for advanced glycation end product-dependent signal pathway drives beta-amyloid-induced synaptic depression and long-term depression impairment in entorhinal cortex

Overproduction of beta-amyloid (Abeta) is a pathologic feature of Alzheimer's disease, leading to cognitive impairment. Here, we investigated the impact of cell-specific receptor for advanced glycation end products (RAGE) on Abeta-induced entorhinal cortex (EC) synaptic dysfunction. We found both a transient depression of basal synaptic transmission and inhibition of long-term depression (LTD) after the application of Abeta in EC slices. Synaptic depression and LTD impairment induced by Abeta were rescued by functional suppression of RAGE. Remarkably, the rescue was only observed in slices from mice expressing a defective form of RAGE targeted to microglia, but not in slices from mice expressing defective RAGE targeted to neurons. Moreover, we found that the inflammatory cytokine IL-1beta (interleukin-1beta) and stress-activated kinases [p38 MAPK (p38 mitogen-activated protein kinase) and JNK (c-Jun N-terminal kinase)] were significantly altered and involved in RAGE signaling pathways depending on RAGE expression in neuron or microglia. These findings suggest a prominent role of microglial RAGE signaling in Abeta-induced EC synaptic dysfunction.

AP-1 inhibitory peptides attenuate in vitro cortical neuronal cell death induced by kainic acid

This study has assessed the neuroprotective efficacy of five AP-1 inhibitory peptides in an in vitro excitotoxicity model. The five AP-1 inhibitory peptides and controls of the JNK inhibitor peptide (JNKI-1D-TAT) and TAT cell-penetrating-peptide were administered to primary cortical neuronal cultures prior to kainic acid exposure. All five AP-1 inhibitory peptides and JNKI-1D-TAT provided significant neuroprotection from kainic acid induced neuronal cell death. Kainic acid exposure induced caspase and calpain activation in neuronal cultures, with caspase-induced cleavage of α-fodrin reduced by administration of the AP-1 inhibitory peptides. Sequence analysis of the AP-1 inhibitory peptides did not reveal the presence of any secondary structures; however two peptides shared 66% amino-acid sequence homology. As a result, truncated sequences were designed and synthesised to identify the active region of the peptides. All truncated peptides were significantly neuroprotective following kainic acid and glutamate exposure. We have shown for the first time the neuroprotective efficacy of full-length and truncated AP-1 inhibitory peptides in kainic acid and glutamate neuronal excitotoxicity models. The identification of therapeutic targets, such as the AP-1 complex, is an important step for the development of pharmaceuticals to reduce neuronal loss in disorders with a prevalence of excitotoxic cell death such as epilepsy, cerebral ischaemia, and traumatic brain injury.

Neuronal cell death in Alzheimer's disease and a neuroprotective factor, humanin

Brain atrophy caused by neuronal loss is a prominent pathological feature of Alzheimer's disease (AD). Amyloid beta (Abeta), the major component of senile plaques, is considered to play a central role in neuronal cell death. In addition to removal of the toxic Abeta, direct suppression of neuronal loss is an essential part of AD treatment; however, no such neuroprotective therapies have been developed. Excess amount of Abeta evokes multiple cytotoxic mechanisms, involving increase of the intracellular Ca(2+) level, oxidative stress, and receptor-mediated activation of cell-death cascades. Such diversity in cytotoxic mechanisms induced by Abeta clearly indicates a complex nature of the AD-related neuronal cell death. We have identified a 24-residue peptide, Humanin (HN), which suppresses in vitro neuronal cell death caused by all AD-related insults, including Abeta, so far tested. The anti-AD effect of HN has been further confirmed in vivo using mice with Abeta-induced amnesia. Altogether, such potent neuroprotective activity of HN against AD-relevant cytotoxicity both in vitro and in vivo suggests the potential clinical applications of HN in novel AD therapies aimed at controlling neuronal death.

Abeta(31-35)-induced neuronal apoptosis is mediated by JNK-dependent extrinsic apoptosis pathway

OBJECTIVE

To investigate whether JNK-caspase-dependent apoptotic pathway is involved in Abeta(31-35)-induced apoptosis of cultured cortical neurons.

METHODS

Cultured cortical neurons were treated with Abeta(31-35) (25 micromol/L) for 4 h, 8 h, 16 h and 24 h, respectively. Caspase activities were measured using a spectrophotometer. Levels of c-Jun phosphorylation (p-c-Jun) and Fas ligand (FasL) expression were assessed by immunocytochemistry method and quantified using Image-pro plus11.0 image processing and analysis software.

RESULTS

Treatment with Abeta(31-35) (25 micromol/L) for 24 h induced significant increases in the activities of caspase-3 and caspase-8 in the cortical neurons. Besides, Abeta(31-35) could time-dependently enhance the expression of p-c-Jun protein. Moreover, SP600125 application (100 nmol/L) could completely abolish Abeta(31-35) neurotoxicity. The increase in FasL expression was detected at 8 h, 16 h and 24 h after Abeta(31-35) treatment, and SP600125 (100 nmol/L) significantly inhibited FasL expression.

CONCLUSION

JNK-c-Jun-FasL-caspase-dependent extrinsic apoptotic pathway plays a critical role in mediating Abeta(31-35)-induced apoptosis of cultured neurons.

The consequences of mitochondrial amyloid beta-peptide in Alzheimer's disease

The Abeta (amyloid-beta peptide) has long been associated with Alzheimer's disease, originally in the form of extracellular plaques. However, in the present paper we review the growing evidence for the role of soluble intracellular Abeta in the disease progression, with particular reference to Abeta found within the mitochondria. Once inside the cell, Abeta is able to interact with a number of targets, including the mitochondrial proteins ABAD (amyloid-binding alcohol dehydrogenase) and CypD (cyclophilin D), which is a component of the mitochondrial permeability transition pore. Interference with the normal functions of these proteins results in disruption of cell homoeostasis and ultimately cell death. The present review explores the possible mechanisms by which cell death occurs, considering the evidence presented on a molecular, cellular and in vivo level.

MAPK, beta-amyloid and synaptic dysfunction: the role of RAGE

Genetic and biological studies provide strong support for the hypothesis that accumulation of beta amyloid peptide (Abeta) contributes to the etiology of Alzheimer's disease (AD). Growing evidence indicates that oligomeric soluble Abeta plays an important role in the development of synaptic dysfunction and the impairment of cognitive function in AD. The receptor for advanced glycation end products (RAGE), a multiligand receptor in the immunoglobulin superfamily, acts as a cell surface binding site for Abeta and mediates alternations in the phosphorylation state of mitogen-activated protein kinase (MAPKs). Recent results have shown that MAPKs are involved in neurodegenerative processes. In particular, changes in the phosphorylation state of various MAPKs by Abeta lead to synaptic dysfunction and cognitive decline, as well as development of inflammatory responses in AD. The present review summarizes the evidence justifying a novel therapeutic approach focused on inhibition of RAGE signaling in order to arrest or halt the development of neuronal dysfunction in AD.

Adenosine A2A receptor blockade prevents synaptotoxicity and memory dysfunction caused by beta-amyloid peptides via p38 mitogen-activated protein kinase pathway

Alzheimer's disease (AD) is characterized by memory impairment, neurochemically by accumulation of beta-amyloid peptide (namely Abeta(1-42)) and morphologically by an initial loss of nerve terminals. Caffeine consumption prevents memory dysfunction in different models, which is mimicked by antagonists of adenosine A(2A) receptors (A(2A)Rs), which are located in synapses. Thus, we now tested whether A(2A)R blockade prevents the early Abeta(1-42)-induced synaptotoxicity and memory dysfunction and what are the underlying signaling pathways. The intracerebral administration of soluble Abeta(1-42) (2 nmol) in rats or mice caused, 2 weeks later, memory impairment (decreased performance in the Y-maze and object recognition tests) and a loss of nerve terminal markers (synaptophysin, SNAP-25) without overt neuronal loss, astrogliosis, or microgliosis. These were prevented by pharmacological blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261); 0.05 mg . kg(-1) . d(-1), i.p.; for 15 d] in rats, and genetic inactivation of A(2A)Rs in mice. Moreover, these were synaptic events since purified nerve terminals acutely exposed to Abeta(1-42) (500 nm) displayed mitochondrial dysfunction, which was prevented by A(2A)R blockade. SCH58261 (50 nm) also prevented the initial synaptotoxicity (loss of MAP-2, synaptophysin, and SNAP-25 immunoreactivity) and subsequent loss of viability of cultured hippocampal neurons exposed to Abeta(1-42) (500 nm). This A(2A)R-mediated control of neurotoxicity involved the control of Abeta(1-42)-induced p38 phosphorylation and was independent from cAMP/PKA (protein kinase A) pathway. Together, these results show that A(2A)Rs play a crucial role in the development of Abeta-induced synaptotoxicity leading to memory dysfunction through a p38 MAPK (mitogen-activated protein kinase)-dependent pathway and provide a molecular basis for the benefits of caffeine consumption in AD.

Beta-amyloid oligomers induce phosphorylation of tau and inactivation of insulin receptor substrate via c-Jun N-terminal kinase signaling: suppression by omega-3 fatty acids and curcumin

Both insulin resistance (type II diabetes) and beta-amyloid (Abeta) oligomers are implicated in Alzheimer's disease (AD). Here, we investigate the role of Abeta oligomer-induced c-Jun N-terminal kinase (JNK) activation leading to phosphorylation and degradation of the adaptor protein insulin receptor substrate-1 (IRS-1). IRS-1 couples insulin and other trophic factor receptors to downstream kinases and neuroprotective signaling. Increased phospho-IRS-1 is found in AD brain and insulin-resistant tissues from diabetics. Here, we report Abeta oligomers significantly increased active JNK and phosphorylation of IRS-1 (Ser616) and tau (Ser422) in cultured hippocampal neurons, whereas JNK inhibition blocked these responses. The omega-3 fatty acid docosahexaenoic acid (DHA) similarly inhibited JNK and the phosphorylation of IRS-1 and tau in cultured hippocampal neurons. Feeding 3xTg-AD transgenic mice a diet high in saturated and omega-6 fat increased active JNK and phosphorylated IRS-1 and tau. Treatment of the 3xTg-AD mice on high-fat diet with fish oil or curcumin or a combination of both for 4 months reduced phosphorylated JNK, IRS-1, and tau and prevented the degradation of total IRS-1. This was accompanied by improvement in Y-maze performance. Mice fed with fish oil and curcumin for 1 month had more significant effects on Y-maze, and the combination showed more significant inhibition of JNK, IRS-1, and tau phosphorylation. These data indicate JNK mediates Abeta oligomer inactivation of IRS-1 and phospho-tau pathology and that dietary treatment with fish oil/DHA, curcumin, or a combination of both has the potential to improve insulin/trophic signaling and cognitive deficits in AD.

p75 reduces beta-amyloid-induced sympathetic innervation deficits in an Alzheimer's disease mouse model

Beta-amyloid (Abeta) has adverse effects on brain cells, but little is known about its effects on the peripheral nervous system in Alzheimer's disease (AD). Several lines of in vitro evidence suggest that the neurotrophin receptor p75 mediates or exacerbates Abeta-induced neurotoxicity. Here, we show that p75-deficient sympathetic neurons are more sensitive to Abeta-induced neurite growth inhibition. To investigate the role of p75 in the sympathetic nervous system of AD, p75 mutant mice were crossed with a mouse line of AD model. The majority of p75-deficient AD mice died by 3 weeks of age. The lethality is associated with severe defects in sympathetic innervation to multiple organs. When 1 copy of the BACE1 gene encoding a protein essential in Abeta production was deleted in p75-deficient AD mice, sympathetic innervation was significantly restored. These results suggest that p75 is neuroprotective for the sympathetic nervous system in a mouse model of AD.

Amyloid-beta-induced occludin down-regulation and increased permeability in human brain endothelial cells is mediated by MAPK activation

Vascular dysfunction is emerging as a key pathological hallmark in Alzheimer’s disease (AD). A leaky blood–brain barrier (BBB) has been described in AD patient tissue and in vivo AD mouse models. Brain endothelial cells (BECs) are linked together by tight junctional (TJ) proteins, which are a key determinant in restricting the permeability of the BBB. The amyloid β (Aβ) peptides of 1–40 and 1–42 amino acids are believed to be pivotal in AD pathogenesis. We therefore decided to investigate the effect of Aβ 1–40, the Aβ variant found at the highest concentration in human plasma, on the permeability of an immortalized human BEC line, hCMEC/D3. Aβ 1–40 induced a marked increase in hCMEC/D3 cell permeability to the paracellular tracer 70 kD FITC-dextran when compared with cells incubated with the scrambled Aβ 1–40 peptide. Increased permeability was associated with a specific decrease, both at the protein and mRNA level, in the TJ protein occludin, whereas claudin-5 and ZO-1 were unaffected. JNK and p38MAPK inhibition prevented both Aβ 1–40-mediated down-regulation of occludin and the increase in paracellular permeability in hCMEC/D3 cells. Our findings suggest that the JNK and p38MAPK pathways might represent attractive therapeutic targets for preventing BBB dysfunction in AD.

Expression of inflammatory genes induced by beta-amyloid peptides in human brain endothelial cells and in Alzheimer's brain is mediated by the JNK-AP1 signaling pathway

Alzheimer's disease (AD) is characterized by accumulation and deposition of Abeta peptides in the brain. Abeta deposition in cerebral vessels occurs in many AD patients and results in cerebral amyloid angiopathy (AD/CAA). Abeta deposits evoke neuro- and neurovascular inflammation contributing to neurodegeneration. In this study, we found that exposure of cultured human brain endothelial cells (HBEC) to Abeta(1-40) elicited expression of inflammatory genes MCP-1, GRO, IL-1beta and IL-6. Up-regulation of these genes was confirmed in AD and AD/CAA brains by qRT-PCR. Profiling of 54 transcription factors indicated that AP-1 was strongly activated not only in Abeta-treated HBEC but also in AD and AD/CAA brains. AP-1 complex in nuclear extracts from Abeta-treated HBEC bound to AP-1 DNA-binding sequence and activated the reporter gene of a luciferase vector carrying AP-1-binding site from human MCP-1 gene. AP-1 is a dimeric protein complex and supershift assay identified c-Jun as a component of the activated AP-1 complex. Western blot analyses showed that c-Jun was activated via JNK-mediated phosphorylation, suggesting that as a result of c-Jun phosphorylation, AP-1 was activated and thus up-regulated MCP-1 expression. A JNK inhibitor SP600125 strongly inhibited Abeta-induced c-Jun phosphorylation, AP-1 activation, AP-1 reporter gene activity and MCP-1 expression in cells stimulated with Abeta peptides. The results suggested that JNK-AP1 signaling pathway is responsible for Abeta-induced neuroinflammation in HBEC and Alzheimer's brain and that this signaling pathway may serve as a therapeutic target for relieving Abeta-induced inflammation.

Different protection of K252a and N-acetyl-L-cysteine against amyloid-beta peptide-induced cortical neuron apoptosis involving inhibition of MLK3-MKK7-JNK3 signal cascades

Amyloid-beta peptide (Abeta) has been implicated in the etiopathogenesis of Alzheimer's disease (AD). However, the molecular mechanisms underlying Abeta neurotoxicity remain to be elucidated. This study showed that Abeta treatment resulted in the increased phosphorylation (activation) of MLK3, MKK7, and JNK3 in cultured cortical neurons, which characterized as biphasic activation (first peaked at 1 hr and second peaked at 12 hr after Abeta treatment). K252a blocked Abeta-induced neuronal apoptosis, both early and late phases of MLK3-MKK7-JNK3 activation, as well as downstream signal events involving p-JNKs nuclear translocation, c-Jun phosphorylation, and Bad translocation to the mitochondria. The neuroprotective effect of K252a on Abeta-induced apoptosis was partially dependent on Akt activation. In contrast, antioxidant N-acetyl-L-cysteine (NAC) reduced early, but not late, MLK3-MKK7-JNK3 activation by Abeta treatment and provided a weak neuroprotective ability in Abeta-induced apoptosis. Taken together, Abeta neurotoxicity is mainly due to MLK3-MKK7-JNK3 signal cascades. The late signal events of MLK3 activation after Abeta treatment may play an important role in AD neuronal loss and will be a promising pharmacological target for AD therapeutic intervention.

The forkhead transcription factor FOXO3a controls microglial inflammatory activation and eventual apoptotic injury through caspase 3

Memory loss and cognitive failure are increasingly being identified as potential risks with the recognized increase in life expectancy of the general population. As a result, the development of novel therapeutic strategies for disorders such as Alzheimer's disease have garnered increased attention. The etiologies that can lead to Alzheimer's disease are extremely varied, but a number of therapeutic options are directed against amyloid-beta peptide and inflammatory cell regulation to prevent or halt progressive cognitive loss. In particular, inflammatory microglial cells may have disparate functions that in some scenarios lead to disability through the removal of functional neurovascular cells and in other circumstances foster tissue repair. Given the significance microglial cells hold for neurodegenerative disorders, we therefore examined the function that amyloid (Abeta(1-42)) has upon the microglial cell line EOC 2 and identified a novel role for the forkhead transcription factor FoxO3a and caspase 3. Here we show that Abeta(1-42) leads to progressive injury and apoptotic cell loss in microglial cells that involves both early phosphatidylserine (PS) externalization and late genomic DNA fragmentation over a 24 hour course. Prior to these injury programs, Abeta(1-42) results in the activation and proliferation of microglia as demonstrated by increased proliferating cell nuclear antigen (PCNA) expression and bromodeoxyuridine (BrdU) uptake. Both apoptotic injury as well as the prior activation and proliferation of microglial cells relies upon the presence of FoxO3a, since specific gene silencing of FoxO3a promotes microglial cell protection and prevents the early activation and proliferation of these cells. Furthermore, Abeta(1-42) exposure maintained FoxO3a in an unphosphorylated "active" state and facilitated the cellular trafficking of FoxO3a from the cytoplasm to the cell nucleus to potentially lead to "pro-apoptotic" programs by this transcription factor. One apoptotic program in particular appears to involve the activation of caspase 3, since loss of FoxO3a through gene silencing prevents the induction of caspase 3 activity by Abeta(1-42).

Reduction of cerebral oxidative stress following environmental enrichment in mice with Alzheimer-like pathology

Oxidative stress is a key feature during progression of chronic neurodegenerative conditions such as Alzheimer's disease. In aging humans and animals, voluntary exercise lowers oxidative stress reactions. Additionally, recent work in our lab demonstrated that cognitive and physical stimulation (termed environmental enrichment) counteracts amyloid beta pathology, neurovascular dysfunction and behavioral symptoms in mice with Alzheimer-like disease. Based on these facts, we hypothesized that cognitive and physical activity can also protect against oxidative stress in Alzheimer-diseased brain. We, therefore, kept female TgCRND8 mice under standard and enriched housing from day 30 until 5 months of age. Environmental stimulation attenuated pro-oxidative processes and triggered anti-oxidative defense mechanisms as indicated by diminished biomarkers for reactive oxygen and nitrogen species, downregulation of pro-inflammatory and pro-oxidative mediators, decreased expression of pro-apoptotic caspases, and upregulation of SOD1 and SOD2. This study identifies a thus far undescribed antagonizing effect of environmental stimulation on Alzheimer's disease-related oxidative damage.

Oxidative stress signaling in Alzheimer's disease

Multiple lines of evidence demonstrate that oxidative stress is an early event in Alzheimer's disease (AD), occurring prior to cytopathology, and therefore may play a key pathogenic role in AD. Oxidative stress not only temporally precedes the pathological lesions of the disease but also activates cell signaling pathways, which, in turn, contribute to lesion formation and, at the same time, provoke cellular responses such as compensatory upregulation of antioxidant enzymes found in vulnerable neurons in AD. In this review, we provide an overview of the evidence of oxidative stress and compensatory responses that occur in AD, particularly focused on potential sources of oxidative stress and the roles and mechanism of activation of stress-activated protein kinase pathways.

Toll-like receptor-4 mediates neuronal apoptosis induced by amyloid beta-peptide and the membrane lipid peroxidation product 4-hydroxynonenal

The innate immune system senses the invasion of pathogenic microorganisms and tissue injury through Toll-like receptors (TLR), a mechanism thought to be limited to immune cells. We recently found that neurons express several TLRs, and that the levels of TLR2 and TLR4 are increased in neurons in response to energy deprivation. Here we report that TLR4 expression increases in neurons when exposed to amyloid beta-peptide (Abeta1-42) or the lipid peroxidation product 4-hydroxynonenal (HNE). Neuronal apoptosis triggered by Abeta and HNE was mediated by jun N-terminal kinase (JNK); neurons from TLR4 mutant mice exhibited reduced JNK and caspase-3 activation and were protected against apoptosis induced by Abeta and HNE. Levels of TLR4 were decreased in inferior parietal cortex tissue specimens from end-stage AD patients compared to aged-matched control subjects, possibly as the result of loss of neurons expressing TLR4. Our findings suggest that TLR4 signaling increases the vulnerability of neurons to Abeta and oxidative stress in AD, and identify TLR4 as a potential therapeutic target for AD.

Erythropoietin: elucidating new cellular targets that broaden therapeutic strategies

Given that erythropoietin (EPO) is no longer believed to have exclusive biological activity in the hematopoietic system, EPO is now considered to have applicability in a variety of nervous system disorders that can overlap with vascular disease, metabolic impairments, and immune system function. As a result, EPO may offer efficacy for a broad number of disorders that involve Alzheimer's disease, cardiac insufficiency, stroke, trauma, and diabetic complications. During a number of clinical conditions, EPO is robust and can prevent metabolic compromise, neuronal and vascular degeneration, and inflammatory cell activation. Yet, use of EPO is not without its considerations especially in light of frequent concerns that may compromise clinical care. Recent work has elucidated a number of novel cellular pathways governed by EPO that can open new avenues to avert deleterious effects of this agent and offer previously unrecognized perspectives for therapeutic strategies. Obtaining greater insight into the role of EPO in the nervous system and elucidating its unique cellular pathways may provide greater cellular viability not only in the nervous system but also throughout the body.

Calphostin C-induced apoptosis is mediated by a tissue transglutaminase-dependent mechanism involving the DLK/JNK signaling pathway

A role for tissue transglutaminase (TG2) and its substrate dual leucine zipper-bearing kinase (DLK), an upstream component of the c-Jun N-terminal kinase (JNK) signaling pathway, has been previously suggested in the apoptotic response induced by calphostin C. In the current study, we directly tested this hypothesis by examining via pharmacological and RNA-interference approaches whether inhibition of expression or activity of TG2, DLK and JNK in mouse NIH 3T3 fibroblasts and human MDA-MB-231 breast cancer epithelial cells affects calphostin C-induced apoptosis. Our experiments with the selective JNK inhibitor SP600125 reveal that calphostin C is capable of causing JNK activation and JNK-dependent apoptosis in both cell lines. Small interfering RNA-mediated depletion of TG2 alone strongly reduces calphostin C action on JNK activity and apoptosis. Consistent with an active role for DLK in this cascade of event, cells deficient in DLK demonstrate a substantial delay of JNK activation and poly-ADP-ribose polymerase (PARP) cleavage in response to calphostin C, whereas overexpression of a recombinant DLK resistant to silencing, but sensitive to TG2-mediated oligomerization, reverses this effect. Importantly, combined depletion of TG2 and DLK further alters calphostin C effects on JNK activity, Bax translocation, caspase-3 activation, PARP cleavage and cell viability, demonstrating an obligatory role for TG2 and DLK in calphostin C-induced apoptosis.

Glutamate-induced c-Jun expression in neuronal PC12 cells: the effects of ketamine and propofol

Transcription factor c-Jun affects neuronal cell death and survival in mammalian brain. As general anesthetics, such as ketamine and propofol, are thought to provide some degree of neuroprotection, this study was intended to test whether the protection of injured neuronal PC12 cells by ketamine and propofol is related to the inhibition of phospho-c-Jun. Using neuronal PC12 cells from rat pheochromocytoma cells differentiated with nerve growth factor, we found that 24 hours of exposure to glutamate (1 to 100 mM) induced concentration-dependent cell death as determined by an ability to reduce the tetrazolium derivative, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) into a blue formazan salt. Neuronal PC12 cells were exposed to ketamine (0.1, 1.0 mM) or propofol (0.5, 5.0 microM) and glutamate (0, 20 mM) for 24 hours. Cell injury was assessed using MTT, in situ terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling, and c-Jun activity assay. Glutamate, 20 mM, induced about 70% of cell death as determined by MTT and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling staining. Glutamate-induced cell death was related to an increase in expression of phospho-c-Jun. Glutamate-induced cell death was reduced by ketamine (0.1, 1.0 mM) in a dose-dependent manner and also by propofol (0.5, 5.0 microM). In addition, the expression of phospho-c-Jun was substantially reduced by ketamine (0.1, 1.0 mM) and propofol (0.5, 5.0 microM), respectively, as determined by Western blot assay. These results suggest that inhibition of c-Jun activity is involved in the neuroprotective effects of ketamine and propofol on glutamate-induced injury in neuronal PC12 cells.

Erythropoietin and oxidative stress

Unmitigated oxidative stress can lead to diminished cellular longevity, accelerated aging, and accumulated toxic effects for an organism. Current investigations further suggest the significant disadvantages that can occur with cellular oxidative stress that can lead to clinical disability in a number of disorders, such as myocardial infarction, dementia, stroke, and diabetes. New therapeutic strategies are therefore sought that can be directed toward ameliorating the toxic effects of oxidative stress. Here we discuss the exciting potential of the growth factor and cytokine erythropoietin for the treatment of diseases such as cardiac ischemia, vascular injury, neurodegeneration, and diabetes through the modulation of cellular oxidative stress. Erythropoietin controls a variety of signal transduction pathways during oxidative stress that can involve Janus-tyrosine kinase 2, protein kinase B, signal transducer and activator of transcription pathways, Wnt proteins, mammalian forkhead transcription factors, caspases, and nuclear factor kappaB. Yet, the biological effects of erythropoietin may not always be beneficial and may be poor tolerated in a number of clinical scenarios, necessitating further basic and clinical investigations that emphasize the elucidation of the signal transduction pathways controlled by erythropoietin to direct both successful and safe clinical care.

Receptor for advanced glycation end product-dependent activation of p38 mitogen-activated protein kinase contributes to amyloid-beta-mediated cortical synaptic dysfunction

Soluble amyloid-beta (Abeta) peptide is likely to play a key role during early stages of Alzheimer's disease (AD) by perturbing synaptic function and cognitive processes. Receptor for advanced glycation end products (RAGE) has been identified as a receptor involved in Abeta-induced neuronal dysfunction. We investigated the role of neuronal RAGE in Abeta-induced synaptic dysfunction in the entorhinal cortex, an area of the brain important in memory processes that is affected early in AD. We found that soluble oligomeric Abeta peptide (Abeta42) blocked long-term potentiation (LTP), but did not affect long-term depression, paired-pulse facilitation, or basal synaptic transmission. In contrast, Abeta did not inhibit LTP in slices from RAGE-null mutant mice or in slices from wild-type mice treated with anti-RAGE IgG. Similarly, transgenic mice expressing a dominant-negative form of RAGE targeted to neurons showed normal LTP in the presence of Abeta, suggesting that neuronal RAGE functions as a signal transducer for Abeta-mediated LTP impairment. To investigate intracellular pathway transducing RAGE activation by Abeta, we used inhibitors of stress activated kinases. We found that inhibiting p38 mitogen-activated protein kinase (p38 MAPK), but not blocking c-Jun N-terminal kinase activation, was capable of maintaining LTP in Abeta-treated slices. Moreover, Abeta-mediated enhancement of p38 MAPK phosphorylation in cortical neurons was reduced by blocking antibodies to RAGE. Together, our results indicate that Abeta impairs LTP in the entorhinal cortex through neuronal RAGE-mediated activation of p38 MAPK.

Differential activation of mitogen-activated protein kinase signalling pathways in the hippocampus of CRND8 transgenic mouse, a model of Alzheimer's disease

Transgenic Centre for Research in Neurodegenerative Diseases 8 (TgCRND8) mice expressing a double mutant form of human amyloid precursor protein represent a good model of Alzheimer's disease, and can be useful to clarify the involvement of mitogen-activated protein kinases (MAPK) dysregulation in the pathophysiology of this neurodegenerative disorder. Activation of extracellular regulated kinase (ERK) 1/2, jun kinase (JNK) and p38MAPK was studied in the hippocampus of 7-month-old TgCRND8 mice by immunohistochemistry and Western blot analysis using antibodies selective for the phosphorylated, and thus active, forms of the enzymes. We demonstrated that the three main MAPK pathways were differentially activated in cells of the hippocampus of TgCRND8 mice in comparison to wild type (Wt) littermates, p38MAPK and JNK being more activated, while ERK less activated. p38MAPK was significantly activated in microglia, astrocytes and neurons, around and distant from the plaques. JNK was highly activated in cells closely surrounding the plaques. No difference was observed in the activation of the two major bands of JNK, at a molecular weight of 46 kDa and 54 kDa. These data indicate the possible involvement of p38MAPK and JNK pathways dysregulation in the pathogenesis of Alzheimer's disease. The ERK2 isoform of the ERK pathway was less activated in the hippocampal dentate gyrus of Tg mice in basal conditions. Furthermore activation of the ERK pathway by ex vivo cholinergic stimulation with carbachol caused significantly higher activation of ERK in the hippocampus of Wt mice than in Tg mice. These findings may pose a molecular basis for the memory disruption of Alzheimer's disease, since proper functioning of the basal forebrain cholinergic neurons and of ERK2 is critical for memory formation.

Endophilin I expression is increased in the brains of Alzheimer disease patients

Alzheimer patients have increased levels of both the 42 amyloid-beta-peptide (Abeta) and the amyloid binding alcohol dehydrogenase (ABAD), which is an intracellular binding site for Abeta. The overexpression of Abeta and ABAD in transgenic mice has shown that the binding of Abeta to ABAD results in amplified neuronal stress and impairment of learning and memory. From a proteomic analysis of the brains from these animals, we have identified for the first time that the protein endophilin I increases in Alzheimer diseased brain. The increase in endophilin I levels in neurons is linked to an increase in the activation of the stress kinase c-Jun N-terminal kinase with the subsequent death of the neurons. We also demonstrate in living animals that the expression level of endophilin I is an indicator for the interaction of ABAD and Abeta as its expression levels return to normal if this interaction is perturbed. Therefore this identifies endophilin I as a new indicator of the progression of Alzheimer disease.

Complement component C1q inhibits beta-amyloid- and serum amyloid P-induced neurotoxicity via caspase- and calpain-independent mechanisms

Alzheimer's disease is a neurodegenerative disorder characterized by neuronal loss, beta-amyloid (Abeta) plaques, and neurofibrillary tangles. Complement protein C1q has been found associated with fibrillar Abeta deposits, however the exact contributions of C1q to Alzheimer's disease is still unknown. There is evidence that C1q, as an initiator of the inflammatory complement cascade, may accelerate disease progression. However, neuronal C1q synthesis is induced after injury/infection suggesting that it may be a beneficial response to injury. In this study, we report that C1q enhances the viability of neurons in culture and protects neurons against Abeta- and serum amyloid P (SAP)-induced neurotoxicity. Investigation of potential signaling pathways indicates that caspase and calpain are activated by Abeta, but C1q had no effect on either of these pathways. Interestingly, SAP did not induce caspase and calpain activation, suggesting that C1q neuroprotection is in distinct from caspase and calpain pathways. In contrast to Abeta- and SAP-induced neurotoxicity, neurotoxicity induced by etoposide or FCCP was unaffected by the addition of C1q, indicating pathway selectivity for C1q neuroprotection. These data support a neuroprotective role for C1q which should be further investigated to uncover mechanisms which may be therapeutically targeted to slow neurodegeneration via direct inhibition of neuronal loss.

Pharmacogenomics in Alzheimer's disease

Pharmacological treatment in Alzheimer's disease (AD) accounts for 10-20% of direct costs, and fewer than 20% of AD patients are moderate responders to conventional drugs (donepezil, rivastigmine, galantamine, memantine), with doubtful cost-effectiveness. Both AD pathogenesis and drug metabolism are genetically regulated complex traits in which hundreds of genes cooperatively participate. Structural genomics studies demonstrated that more than 200 genes might be involved in AD pathogenesis regulating dysfunctional genetic networks leading to premature neuronal death. The AD population exhibits a higher genetic variation rate than the control population, with absolute and relative genetic variations of 40-60% and 0.85-1.89%, respectively. AD patients also differ in their genomic architecture from patients with other forms of dementia. Functional genomics studies in AD revealed that age of onset, brain atrophy, cerebrovascular hemodynamics, brain bioelectrical activity, cognitive decline, apoptosis, immune function, lipid metabolism dyshomeostasis, and amyloid deposition are associated with AD-related genes. Pioneering pharmacogenomics studies also demonstrated that the therapeutic response in AD is genotype-specific, with apolipoprotein E (APOE) 4/4 carriers the worst responders to conventional treatments. About 10-20% of Caucasians are carriers of defective cytochrome P450 (CYP) 2D6 polymorphic variants that alter the metabolism and effects of AD drugs and many psychotropic agents currently administered to patients with dementia. There is a moderate accumulation of AD-related genetic variants of risk in CYP2D6 poor metabolizers (PMs) and ultrarapid metabolizers (UMs), who are the worst responders to conventional drugs. The association of the APOE-4 allele with specific genetic variants of other genes (e.g., CYP2D6, angiotensin-converting enzyme [ACE]) negatively modulates the therapeutic response to multifactorial treatments affecting cognition, mood, and behavior. Pharmacogenetic and pharmacogenomic factors may account for 60-90% of drug variability in drug disposition and pharmacodynamics. The incorporation of pharmacogenetic/pharmacogenomic protocols to AD research and clinical practice can foster therapeutics optimization by helping to develop cost-effective pharmaceuticals and improving drug efficacy and safety.

Activating transcription factor 3 up-regulated by c-Jun NH(2)-terminal kinase/c-Jun contributes to apoptosis induced by potassium deprivation in cerebellar granule neurons

Cerebellar granule neurons (CGNs) depend on potassium depolarization for survival and undergo apoptosis when deprived of depolarizing concentration of potassium. Activating transcription factor 3 (ATF3), a stress-inducible protein, belongs to the ATF/CREB family of transcription factors family and is involved in cell growth and apoptosis. However, the role of ATF3 in neuronal apoptosis remains unknown. Here, we showed that ATF3 was up-regulated under potassium deprivation in CGNs, and this induction was preceded by a rapid and sustained activation of c-Jun NH(2)-terminal kinase/c-Jun signaling pathway, which plays a fundamental role in neuronal apoptosis. Furthermore, ATF3 up-regulation was abolished by inhibition of JNK or knockdown of c-Jun. Finally, knockdown of ATF3 by RNA interference protected CGNs from potassium deprivation-induced apoptosis. Taken together, our results indicate that ATF3 is a downstream target of JNK/c-Jun pathway and contributes to apoptosis induced by potassium deprivation in rat CGNs.

Antagonizing beta-amyloid peptide neurotoxicity of the anti-aging fungus Ganoderma lucidum

Ganoderma lucidum (Leyss. ex Fr.) Karst. (Lingzhi) is a medicinal fungus used clinically in many Asian countries to promote health and longevity. Synaptic degeneration is another key mode of neurodegeneration in Alzheimer's disease (AD). Recent studies have shown the loss of synaptic density proteins in each individual neuron during the progression of AD. It was recently reported that beta-amyloid (Abeta) could cause synaptic dysfunction and contribute to AD pathology. In this study, we reported that aqueous extract of G. lucidum significantly attenuated Abeta-induced synaptotoxicity by preserving the synaptic density protein, synaptophysin. In addition, G. lucidum aqueous extract antagonized Abeta-triggered DEVD cleavage activities in a dose-dependent manner. Further studies elucidated that phosphorylation of c-Jun N-terminal kinase, c-Jun, and p38 MAP kinase was attenuated by G. lucidum in Abeta-stressed neurons. Taken together, the results prove a hypothesis that anti-aging G. lucidum can prevent harmful effects of the exterminating toxin Abeta in AD.

c-Jun phosphorylation in Alzheimer disease

The c-Jun N-terminal kinase (JNK) pathway is known to be activated by oxidative stress and can lead to either defensive-protective adaptations in the cell or apoptosis. The JNK pathway is activated in Alzheimer disease (AD), as demonstrated in studies showing higher levels of phospho-JNK in affected neurons in AD brains than in controls. c-Jun, a transcription factor, is the downstream effector of JNK, whose activation requires phosphorylation of Ser63/Ser73. In this study, we characterized and compared the localization of c-Jun phosphorylated at either Ser63 or Ser73 in the hippocampi of AD cases with that in age-matched controls. Phospho-c-Jun (Ser73) was found to be strongly associated with neurofibrillary tangles and granulovacuolar degeneration (GVD) in addition to the nuclei in neurons in the hippocampal regions of the AD brain, but was virtually absent in most controls. Phospho-c-Jun (Ser63) was also found to be associated with GVD in AD brains. Indeed, phospho-c-Jun (Ser73) immunostaining was much more extensive than that of phospho-c-Jun (Ser63), with all the phospho-c-Jun (Ser63)-positive neurons also being phospho-c-Jun (Ser73) positive. Significant overlap between phospho-c-Jun and phospho-JNK suggested a mechanistic link. In addition, the neurons showing increased levels of phospho-c-Jun (Ser73) in the cytoplasmic GVD were negative for TUNEL, suggesting a mechanism protecting the cells from death. Overall, this study demonstrated specific alterations in c-Jun phosphorylation and distribution in AD which is not necessarily linked to apoptosis but rather may represent an adaptation process in the face of oxidative stress.

Glycogen synthase kinase-3beta induces neuronal cell death via direct phosphorylation of mixed lineage kinase 3

Mixed lineage kinase 3 (MLK3) is a mitogen-activated protein kinase kinase kinase member that activates the c-Jun N-terminal kinase (JNK) pathway. Aberrant activation of MLK3 has been implicated in neurodegenerative diseases. Similarly, glycogen synthase kinase (GSK)-3beta has also been shown to activate JNK and contribute to neuronal apoptosis. Here, we show a functional interaction between MLK3 and GSK-3beta during nerve growth factor (NGF) withdrawal-induced cell death in PC-12 cells. The protein kinase activities of GSK-3beta, MLK3, and JNK were increased upon NGF withdrawal, which paralleled increased cell death in NGF-deprived PC-12 cells. NGF withdrawal-induced cell death and MLK3 activation were blocked by a GSK-3beta-selective inhibitor, kenpaullone. However, the MLK family inhibitor, CEP-11004, although preventing PC-12 cell death, failed to inhibit GSK-3beta activation, indicating that induction of GSK-3beta lies upstream of MLK3. In GSK-3beta-deficient murine embryonic fibroblasts, ultraviolet light was unable to activate MLK3 kinase activity, a defect that was restored upon ectopic expression of GSK-3beta. The activation of MLK3 by GSK-3beta occurred via phosphorylation of MLK3 on two amino acid residues, Ser(789) and Ser(793), that are located within the C-terminal regulatory domain of MLK3. Furthermore, the cell death induced by GSK-3beta was mediated by MLK3 in a manner dependent on its phosphorylation of the specific residues within the C-terminal domain by GSK-3beta. Taken together, our data provide a direct link between GSK-3beta and MLK3 activation in a neuronal cell death pathway and identify MLK3 as a direct downstream target of GSK-3beta. Inhibition of GSK-3 is thus a potential therapeutic strategy for neurodegenerative diseases caused by trophic factor deprivation.

β-Amyloid(1–42) induces a reduction in the parallel fiber responses of Purkinje cells: Possible involvement of pro-inflammatory processes

In Alzheimer's disease there is an increased production of the toxic beta-amyloid peptides (Abeta), especially the longer forms such as Abeta(1-42). Using the patch-clamp technique we have studied the contribution of early pro-inflammatory processes to the acute effects of 1 microM Abeta(1-42) on the parallel fiber EPSC (PF-EPSC) of Purkinje cells in cerebellar slices. Abeta(1-42) induces a decrease in the PF-EPSC amplitude. This decrease is accompanied by a decrease in the frequency and amplitude of the miniature EPSCs, suggesting that Abeta acts at both pre- and post-synaptic sites. In the presence of L-NAME, a nitric oxide synthase inhibitor, the effects of Abeta were partially blocked. The frequency of mEPSCs was unchanged while Abeta still reduced the mEPSCs amplitude. The anti-inflammatory agent flurbiprofen blocked the depressant action of Abeta on the mEPSCs amplitude but not its effect on mEPSCs frequency. Both a p38 inhibitor (SB203580) and a JNK inhibitor (SP600125) reverse the effects of Abeta as an increase in the mEPSCs frequency and amplitude was observed. This study provides evidence that the Abeta-induced depression of the PF-EPSCs was mediated via an activation of JNK and p38 and by the action of NO and raises the possibility of the involvement of an early pro-inflammatory process.

Glutathione S-transferase P1 and T1 gene polymorphisms predict longitudinal course and age at onset of Alzheimer disease

OBJECTIVE

Oxidative stress has been suggested as a contributor of Alzheimer disease (AD) neurodegeneration, particularly in those patients with late-onset AD (LOAD). Therefore, the authors studied the effect of glutathione S-transferase (GST) P1-M1-T1 gene polymorphisms and their interactions with the apolipoprotein E (ApoE) epsilon4 allelic variant on the three-year longitudinal course of AD.

METHODS

Global cognitive level as measured by the Mini-Mental State Exam, basic activities of daily living (BADLs) as measured by the Physical Self-Maintenance Scale, and behavior as measured by the Neuropsychiatric Inventory, were assessed at baseline and after 1, 2, and 3 years in a sample of 99 LOAD patients. These subjects were drug naive and had undergone the first clinical examination for the diagnosis of AD.

RESULTS

A multiple regression analysis indicated that the presence of ApoE epsilon4 allelic variant or GSTT1 null phenotype predicted the faster age at onset of the illness (F = 5.76, df = 2, 96, p = 0.0043). Carriers of GSTP1 *C allelic variant had a faster decline in cognitive functions (repeated measures analysis of variance [ANOVA]: F = 4.00, df = 3, 285, p = 0.008) and in BADLs (repeated measures ANOVA: F = 5.27, df = 3, 285, p = 0.001). This faster decline was independent from ApoE epsilon4 allele possession. No effect of GST P1-M1-T1 polymorphisms was found on behavioral symptom severity.

CONCLUSION

These data are in line with the hypotheses that oxidative damage is a prominent feature in the clinical progression and the age at onset of LOAD.

Pro-apoptotic Bim induction in response to nerve growth factor deprivation requires simultaneous activation of three different death signaling pathways

Bim is a pro-apoptotic member of the Bcl-2 family that is induced and contributes to neuron death in response to nerve growth factor (NGF) deprivation. Past work has revealed that Bim is downstream of multiple independent transcriptional pathways in neurons, including those culminating in activation of the c-Jun, FoxO, and Myb transcription factors. This study addresses the issue of whether the three signaling pathways are redundant with respect to Bim induction or whether they act cooperatively. Examination of the proximal Bim promoter reveals binding sites for FoxO, Mybs, and, as shown here, c-Jun. We find that mutation of any one of these types of sites abolishes induction of a Bim promoter-driven reporter in response to NGF deprivation. Moreover, down-regulation of either c-Jun, FoxOs, or Mybs by short hairpin RNAs blocks induction of Bim promoter-reporter activity triggered by withdrawal of NGF. This was the case for reporters driven by either the proximal promoter or a promoter that also includes additional regulatory elements in the first intron of the Bim gene. Such short hairpin RNAs also suppressed the induction of endogenous Bim protein. These findings thus indicate that the Bim promoter acts as a coincidence detector that optimally responds to the simultaneous activation of three different pro-apoptotic transcriptional pathways. Such a mechanism provides a "fail-safe" that prevents neurons from dying by accidental activation of any single pathway. It also permits neurons to utilize individual pathways such as JNK signaling for other purposes without risk of demise.

Insulin rescues amyloid beta-induced impairment of hippocampal long-term potentiation

Cerebral accumulation of amyloid beta-protein (Abeta) is generally believed to play a critical role in the pathogenesis of Alzheimer's disease (AD). Recent evidence suggests that Abeta-induced synaptic dysfunction is one of earliest pathogenic events observed in AD. Here we report that synthetic Abeta(1-42) strongly inhibited the induction of long-term potentiation (LTP) in the CA1 region of rat hippocampal slices. To ascertain which Abeta(1-42) sequences contribute to the impairment of LTP, we compared actions of several Abeta fragments and found that the sequence within 25-35 region of Abeta mainly contributes to the expression of LTP impairment. Importantly, we show that insulin and insulin-like growth factor-1 significantly inhibit Abeta oligomer formation, particularly dimers and trimers, and ameliorate the synthetic Abeta-induced suppression of LTP. Furthermore, dithiothreitol was found to be capable of significantly preventing the inhibitory effect of insulin on Abeta oligomer formation. In contrast, hemoglobin promotes Abeta oligomer formation and enhances Abeta-mediated inhibition of LTP induction. These results suggest that insulin may have utility in treating the earliest stages of Abeta-induced synaptic dysfunction in AD patients.

Protective effects of Aβ-derived tripeptide, Aβ32–34, on Aβ1–42-induced phosphatidylinositol 4-kinase inhibition and neurotoxicity

We previously reported that the neurotoxicity of pathophysiological concentrations of amyloid beta proteins (Abetas, 0.1-10nM) as assessed by the inhibition of type II phosphatidylinositol 4-kinase (PI4KII) activity and the enhancement of glutamate toxicity was blocked by a short fragment of Abeta, Abeta(31-35). Such protective effects of shorter fragments derived from Abeta(31-35) were examined in this study to reach the shortest effective peptide, using recombinant human PI4KII and primary cultured rat hippocampal neurons. Among the peptides tested (Abeta(31-34), Abeta(31-33), Abeta(31-32), Abeta(32-35), Abeta(33-35), Abeta(34-35), Abeta(32-34), Abeta(33-34) and Abeta(32-33)), Abeta(31-34), Abeta(32-35) and Abeta(32-34) blocked both the Abeta(1-42)-induced inhibition of PI4KII activity and enhancement of glutamate toxicity on cell viability. The shortest peptide among them, Abeta(32-34), showed a dose-dependent protective effect with 50% effective concentration near 1nM, while Abeta(34-32), with a reverse amino acid sequence for Abeta(32-34), showed no protective effects. Thus, a tripeptide, Abeta(32-34) i.e. Ile-Gly-Leu, may be available as a lead compound for designing effective Abeta antagonists.

Connecting TNF-alpha signaling pathways to iNOS expression in a mouse model of Alzheimer's disease: relevance for the behavioral and synaptic deficits induced by amyloid beta protein

Increased brain deposition of amyloid beta protein (Abeta) and cognitive deficits are classical signals of Alzheimer's disease (AD) that have been highly associated with inflammatory alterations. The present work was designed to determine the correlation between tumor necrosis factor-alpha (TNF-alpha)-related signaling pathways and inducible nitric oxide synthase (iNOS) expression in a mouse model of AD, by means of both in vivo and in vitro approaches. The intracerebroventricular injection of Abeta(1-40) in mice resulted in marked deficits of learning and memory, according to assessment in the water maze paradigm. This cognition impairment seems to be related to synapse dysfunction and glial cell activation. The pharmacological blockage of either TNF-alpha or iNOS reduced the cognitive deficit evoked by Abeta(1-40) in mice. Similar results were obtained in TNF-alpha receptor 1 and iNOS knock-out mice. Abeta(1-40) administration induced an increase in TNF-alpha expression and oxidative alterations in prefrontal cortex and hippocampus. Likewise, Abeta(1-40) led to activation of both JNK (c-Jun-NH2-terminal kinase)/c-Jun and nuclear factor-kappaB, resulting in iNOS upregulation in both brain structures. The anti-TNF-alpha antibody reduced all of the molecular and biochemical alterations promoted by Abeta(1-40). These results provide new insights in mouse models of AD, revealing TNF-alpha and iNOS as central mediators of Abeta action. These pathways might be targeted for AD drug development.

JNK pathway: diseases and therapeutic potential

c-Jun N-terminal protein kinases (JNK), also known as stress-activated protein kinases, were originally identified by their ability to phosphorylate the N-terminal of the transcription factor c-Jun and by their activation in response to a variety of stresses. JNK are multifunctional kinases involved in many physiological processes. The JNK pathway has been shown to play a major role in apoptosis in many cell death paradigms and its association with a variety of pathological processes is gradually been recognized. This review will concentrate on describing the involvement of the JNK pathway in the context of different diseases and the potential to adopt the JNK pathway components as therapeutic targets.

Trypanosome trans-sialidase mediates neuroprotection against oxidative stress, serum/glucose deprivation, and hypoxia-induced neurite retraction in Trk-expressing PC12 cells

Trypanosome trans-sialidase (TS) is a sialic acid-transferring enzyme and a novel ligand of tyrosine kinase (TrkA) receptors but not of neurotrophin receptor p75NTR. Here, we show that TS targets TrkB receptors on TrkB-expressing pheochromocytoma PC12 cells and colocalizes with TrkB receptor internalization and phosphorylation (pTrkB). Wild-type TS but not the catalytically inactive mutant TSDeltaAsp98-Glu induces pTrkB and mediates cell survival responses against death caused by oxidative stress in TrkA- and TrkB-expressing cells like those seen with nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). These same effects are not observed in Trk deficient PC12(nnr5) cells, but are re-established in PC12(nnr5) cells stably transfected with TrkA or TrkB, are partially blocked by inhibitors of tyrosine kinase (K-252a), mitogen-activated protein/mitogen-activated kinase (PD98059) and completely blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K). Both TrkA- and TrkB-expressing cells pretreated with TS or their natural ligands are protected against cell death caused by serum/glucose deprivation or from hypoxia-induced neurite retraction. The cell survival effects of NGF and BDNF against oxidative stress are significantly inhibited by the neuraminidase inhibitor, Tamiflu. Together, these observations suggest that trypanosome TS mimics neurotrophic factors in cell survival responses against oxidative stress, hypoxia-induced neurite retraction and serum/glucose deprivation.

Estrogen regulates Bcl-w and Bim expression: role in protection against beta-amyloid peptide-induced neuronal death

Estrogen is neuroprotective against a variety of insults, including beta-amyloid peptide (Abeta); however, the underlying mechanism(s) is not fully understood. Here, we report that 17beta-estradiol (E2) selectively regulates neuronal expression of the Bcl-2 family (bcl-2, bcl-x, bcl-w, bax, bak, bad, bik, bnip3, bid, and bim). In primary cerebrocortical neuron cultures under basal conditions, we observe that E2 upregulates expression of antiapoptotic Bcl-w and downregulates expression of proapoptotic Bim in an estrogen receptor (ER)-dependent manner. In the presence of toxic levels of Abeta, we observe that E2 attenuates indices of neuronal apoptosis: c-Jun N-terminal kinase (JNK)-dependent downregulation of Bcl-w and upregulation of Bim, mitochondrial release of cytochrome c and Smac, and cell death. These neuroprotective effects of E2 against Abeta-induced apoptosis are mimicked by the JNK inhibitor SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one). In addition, E2 attenuates Abeta-induced JNK phosphorylation in an ER-dependent manner, but does not affect basal levels of JNK phosphorylation. These results suggest that E2 may reduce Abeta-induced neuronal apoptosis at least in part by two complementary pathways: (1) ER-dependent, JNK-independent upregulation of Bcl-w and downregulation of Bim under basal conditions, and (2) ER-dependent inhibition of Abeta-induced JNK activation and subsequent JNK-dependent downregulation of Bcl-w and upregulation of Bim, resulting in mitochondrial release of cytochrome c and Smac and eventual cell death. These data provide new understanding into the mechanisms contributing to estrogen neuroprotection, a neural function with potential therapeutic relevance to Alzheimer's disease.

Bim is elevated in Alzheimer's disease neurons and is required for beta-amyloid-induced neuronal apoptosis

The molecules that mediate neuron death in Alzheimer's disease (AD) are largely unknown. We report that beta-amyloid (Abeta), a death-promoting peptide implicated in the pathophysiology of AD, induces the proapoptotic protein Bcl-2 interacting mediator of cell death (Bim) in cultured hippocampal and cortical neurons. We further find that Bim is an essential mediator of Abeta-induced neurotoxicity. Our examination of postmortem AD human brains additionally reveals upregulation of Bim in vulnerable entorhinal cortical neurons, but not in cerebellum, a region usually unaffected by AD. Accumulating evidence links inappropriate induction/activation of cell cycle-related proteins to AD, but their roles in the disease have been unclear. We find that the cell cycle molecule cyclin-dependent kinase 4 (cdk4) and its downstream effector B-myb, are required for Abeta-dependent Bim induction and death in cultured neurons. Moreover, neurons that overexpress Bim in AD brains also show elevated levels of the cell cycle-related proteins cdk4 and phospho-Rb. Our observations indicate that Bim is a proapoptotic effector of Abeta and of dysregulated cell cycle proteins in AD and identify both Bim and cell cycle elements as potential therapeutic targets.

Cell cycle molecules define a pathway required for neuron death in development and disease

We review here evidence defining a molecular pathway that includes cell cycle-related molecules and that appears to play a required role in neuron death during normal development as well as in disease and trauma. The pathway starts with inappropriate activation of cyclin dependent kinase 4 (Cdk4) in neurons which leads to hyper-phosphorylation of the pRb family member p130. This in turn results in dissociation of p130 and its associated chromatin modifiers Suv39H1 and HDAC1 from the transcription factor E2F4. Dissociation of this complex results in de-repression of genes with E2F binding sites including those encoding the transcription factors B- and C-Myb. Once elevated in neurons, B- and C-Myb proteins bind to the promoter for the pro-apoptotic BH3-only protein Bim and promote its induction. Bim then interacts with the core cellular apoptotic machinery, leading to caspase activation and apoptotic death. This pathway is supported by a variety of observations and experimental findings that implicate it as a required element for neuron loss in development and in many nervous system traumas and disorders. The components of this pathway appear to represent potential therapeutic targets for prevention of disease-associated neuron death.