Oxidative stress increases BACE1 protein levels through activation of the PKR-eIF2α pathway

Beta-site APP cleaving enzyme 1 (BACE1) is the rate limiting enzyme for accumulation of amyloid β (Aβ)-peptide in the brain in Alzheimer's disease (AD). Oxidative stress (OS) that leads to metabolic dysfunction and apoptosis of neurons in AD enhances BACE1 expression and activity. The activation of c-jun N-terminal kinase (JNK) pathway was proposed to explain the BACE1 mRNA increase under OS. However, little is known about the translational control of BACE1 in OS. Recently, a post-transcriptional increase of BACE1 level controlled by phosphorylation of eIF2α (eukaryotic translation initiation factor-2α) have been described after energy deprivation. PKR (double-stranded RNA dependant protein kinase) is a pro-apoptotic kinase that phosphorylates eIF2α and modulates JNK activation in various cellular stresses. We investigated the relations between PKR, eIF2α and BACE1 in AD brains in APP/PS1 knock-in mice and in hydrogen peroxide-induced OS in human neuroblastoma (SH-SY5Y) cell cultures. Immunoblotting results showed that activated PKR (pPKR) and activated eIF2α (peIF2α) and BACE1 levels are increased in AD cortices and BACE1 correlate with phosphorylated eIF2α levels. BACE1 protein levels are increased in response to OS in SH-SY5Y cells and specific inhibitions of PKR-eIF2α attenuate BACE1 protein levels in this model. Our findings provide a new translational regulation of BACE1, under the control of PKR in OS, where eIF2α phosphorylation regulates BACE1 protein expression.

The PKR activator PACT is induced by Aβ: involvement in Alzheimer's disease

The neuropathological hallmarks of Alzheimer's disease (AD) include senile plaques made of Aβ peptide, neurofibrillary tangles containing hyperphosphorylated tau protein and neuronal loss. The pro-apoptotic kinase PKR can be activated by Aβ and can phosphorylate tau protein via GSK3β kinase activation. The activated form of PKR (pPKR) accumulates in affected neurons and could participate in neuronal degeneration in AD. The mechanism of abnormal PKR activation in AD is not elucidated but could be linked to the PKR activator PACT. PACT stainings, and levels were assessed in the brains of AD patients and in APP/PS1 knock-in transgenic mice and in cell cultures exposed to stresses. We showed that PACT and pPKR colocalizations are enhanced in AD brains. Their levels are increased and correlated in AD and APP/PS1 knock-in mice brains. In human neuroblastoma cells exposed to Aβ, tunicamycin or H2O2, PACT and pPKR concentrations are increased. PACT then PKR inhibitions indicate that PACT is upstream of PKR activation. Our findings demonstrate that PACT levels are enhanced in AD brains and could partly be caused by the action of Aβ. In addition, PACT participates in PKR activation. The PACT-PKR pathway represents a potential link between Aβ accumulation, PKR activation and tau phosphorylation.

No replication of genetic association between candidate polymorphisms and Alzheimer's disease

Alzheimer's disease is a genetically complex disorder, for which new putative susceptibility genes are constantly proposed in the literature. We selected 16 candidate genes involved in biological pathways closely related to the pathology, and for which a genetic association with Alzheimer's disease was previously detected: ACE, BACE1, BDNF, ECE1, HSPG2, IDE, IL1a, IL6, IL10, MAPT, PLAU, PrnP, PSEN1, SORL1, TFCP2 and TGFb1. The variants originally associated with the disease were genotyped in a French Caucasian sample including 428 cases and 475 controls and tested for association in order to replicate the initial results. Despite a careful replication study design, we failed to validate the initial findings for any of these variants, with the possible exception of MAPT, SORL1 and TFCP2 for which some nominal but inconsistent evidence of association was observed.

The FunGenES database: a genomics resource for mouse embryonic stem cell differentiation

Embryonic stem (ES) cells have high self-renewal capacity and the potential to differentiate into a large variety of cell types. To investigate gene networks operating in pluripotent ES cells and their derivatives, the “Functional Genomics in Embryonic Stem Cells” consortium (FunGenES) has analyzed the transcriptome of mouse ES cells in eleven diverse settings representing sixty-seven experimental conditions. To better illustrate gene expression profiles in mouse ES cells, we have organized the results in an interactive database with a number of features and tools. Specifically, we have generated clusters of transcripts that behave the same way under the entire spectrum of the sixty-seven experimental conditions; we have assembled genes in groups according to their time of expression during successive days of ES cell differentiation; we have included expression profiles of specific gene classes such as transcription regulatory factors and Expressed Sequence Tags; transcripts have been arranged in “Expression Waves” and juxtaposed to genes with opposite or complementary expression patterns; we have designed search engines to display the expression profile of any transcript during ES cell differentiation; gene expression data have been organized in animated graphs of KEGG signaling and metabolic pathways; and finally, we have incorporated advanced functional annotations for individual genes or gene clusters of interest and links to microarray and genomic resources. The FunGenES database provides a comprehensive resource for studies into the biology of ES cells.

Bcl2, a transcriptional target of p38alpha, is critical for neuronal commitment of mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain pluripotent in vitro when grown in the presence of leukemia inhibitory factor (LIF) cytokine. LIF starvation leads to cell commitment, and part of the ES-derived differentiated cells die by apoptosis together with caspase3-cleavage and p38alpha activation. Inhibition of p38 activity by chemical compounds (PD169316 and SB203580), along with LIF withdrawal, leads to different outcomes on cell apoptosis, giving the opportunity to study the influence of apoptosis on cell differentiation. By gene profiling studies on ES-derived differentiated cells treated or not with these inhibitors, we have characterized the common and specific set of genes modulated by each inhibitor. We have also identified key genes that might account for their different survival effects. In addition, we have demonstrated that some genes, similarly regulated by both inhibitors (upregulated as Bcl2, Id2, Cd24a or downregulated as Nodal), are bona fide p38alpha targets involved in neurogenesis and found a correlation with their expression profiles and the onset of neuronal differentiation triggered upon retinoic acid treatment. We also showed, in an embryoid body differentiation protocol, that overexpression of EGFP (enhanced green fluorescent protein)-BCL2 fusion protein and repression of p38alpha are essential to increase formation of TUJ1-positive neuronal cell networks along with an increase in Map2-expressing cells.

Intraneuronal APP/A beta trafficking and plaque formation in beta-amyloid precursor protein and presenilin-1 transgenic mice

Neuropil deposition of beta-amyloid peptides A beta40 and A beta42 is believed to be the key event in the neurodegenerative processes of Alzheimer's disease (AD). Since A beta seems to carry a transport signal that is required for axonal sorting of its precursor beta-amyloid precursor protein (APP), we studied the intraneuronal staining profile of A beta peptides in a transgenic mouse model expressing human mutant APP751 (KM670/671NL and V7171) and human mutant presenilin-1 (PS-1 M146L) in neurons. Using surface plasmon resonance we analyzed the A beta antibodies and defined their binding profile to APP, A beta40 and A beta42. Immunohistochemical staining revealed that intraneuronal A beta40 and A beta42 staining preceded plaque deposition, which started at 3 months of age. A beta was observed in the somatodendritic and axonal compartments of many neurons. Interestingly, the striatum, which lacks transgenic APP expression harbored many plaques at 10 months of age. This is most likely due to an APP/A beta transport problem and may be a model region to study APP/A beta trafficking as an early pathological event.

Unraveling substantia nigra sequential gene expression in a progressive MPTP-lesioned macaque model of Parkinson's disease

Taking advantage of a progressive nonhuman primate model mimicking Parkinson's disease (PD) evolution, we monitored transcriptional fluctuations in the substantia nigra using Affymetrix microarrays in control (normal), saline-treated (normal), 6 days-treated (asymptomatic with 20% cell loss), 12 days-treated (asymptomatic with 40% cell loss) and 25 days-treated animals (fully parkinsonian with 85% cell loss). Two statistical methods were used to ascertain the regulation and real-time quantitative PCR was used to confirm their regulation. Surprisingly, the number of deregulated transcripts is limited at all time points and five clusters exhibiting different profiles were defined using a hierarchical clustering algorithm. Such profiles are likely to represent activation/deactivation of mechanisms of different nature. We briefly speculate about (i) the existence of yet unknown compensatory mechanisms is unraveled, (ii) the putative triggering of a developmental program in the mature brain in reaction to progressing degeneration and finally, (iii) the activation of mechanisms leading eventually to death in final stage. These data should help development of new therapeutic approaches either aimed at enhancing existing compensatory mechanisms or at protecting dopamine neurons.

mTOR/p70S6k signalling alteration by Abeta exposure as well as in APP-PS1 transgenic models and in patients with Alzheimer's disease

In Alzheimer's disease, neuropathological hallmarks include the accumulation of beta-amyloid peptides (Abeta) in senile plaques, phosphorylated tau in neurofibrillary tangles and neuronal death. Abeta is the major aetiological agent according to the amyloid cascade hypothesis. Translational control includes phosphorylation of the kinases mammalian target of rapamycin (mTOR) and p70S6k which modulate cell growth, proliferation and autophagy. It is mainly part of an anti-apoptotic cellular signalling. In this study, we analysed modifications of mTOR/p70S6k signalling in cellular and transgenic models of Alzheimer's disease, as well as in lymphocytes of patients and control individuals. Abeta 1-42 produced a rapid and persistent down-regulation of mTOR/p70S6k phosphorylation in murine neuroblastoma cells associated with caspase 3 activation. Using western blottings, we found that phosphorylated forms of mTOR and p70S6k are decreased in the cortex but not in the cerebellum (devoid of plaques) of double APP/PS1 transgenic mice compared with control mice. These results were confirmed by immunohistochemical methods. Finally, the expression of phosphorylated p70S6k was significantly reduced in lymphocytes of Alzheimer's patients, and levels of phosphorylated p70S6k were statistically correlated with Mini Mental Status Examination (MMSE) scores. Taken together, these findings demonstrate that the mainly anti-apoptotic mTOR/p70S6k signalling is altered in cellular and transgenic models of Alzheimer's disease and in peripheral cells of patients, and could contribute to the pathogenesis of the disease.

Antibody-bound β-amyloid precursor protein stimulates the production of tumor necrosis factor-α and monocyte chemoattractant protein-1 by cortical neurons

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by the accumulation of extracellular depositions of fibrillar beta-amyloid (A beta), which is derived from the alternative processing of beta-amyloid precursor protein (APP). Although APP is thought to function as a cell surface receptor, its mode of action still remains elusive. In this study, we found that the culture medium derived from cortical neurons treated with an anti-APP antibody triggers the death of naive neurons. Biochemical and immunocytochemical analyses revealed the presence, both in the conditioned medium and in neurons, of increased levels of tumor necrosis factor-alpha and monocyte chemoattractant protein-1. Furthermore, the expression of these proinflammatory mediators occurred through a c-Jun N-terminal protein kinase/c-Jun-dependent mechanism. Taken together, our findings provide evidence for a novel mechanism whereby neuronal APP in its full-length configuration induces neuronal death. Such a mechanism might be relevant to neuroinflammatory processes as those observed in AD.

Expression of human FE65 in amyloid precursor protein transgenic mice is associated with a reduction in beta-amyloid load

FE65 is an adaptor protein that interacts with the cytoplasmic tail of the amyloid precursor protein (APP). In cultured non-neuronal cells, the formation of the FE65-APP complex is a key element for the modulation of APP processing, signalling and beta-amyloid (Abeta) production. The functions of FE65 in vivo, including its role in the metabolism of neuronal APP, remain to be investigated. In this study, transgenic mice expressing human FE65 were generated and crossbred with APP transgenic mice, known to develop Abeta deposits at 6 months of age. Compared with APP mice, APP/FE65 double transgenic mice exhibited a lower Abeta accumulation in the cerebral cortex as demonstrated by immunohistochemistry and immunoassay, and a lower level of APP-CTFs. The reduced accumulation of Abeta in APP/FE65 double transgenics, compared with APP mice, could be linked to the low Abeta42 level observed at 4 months of age and to the lower APP-CTFs levels. The present work provides evidence that FE65 plays a role in the regulation of APP processing in an in vivo model.

ABCA2 is a strong genetic risk factor for early-onset Alzheimer's disease

Recent epidemiological, biological and genetic data indicate a relationship between cholesterol and Alzheimer's disease (AD) including the association of polymorphisms of ABCA1 (a gene that is known to participate in cholesterol and phospholipid transport) with AD prevalence. Based on these data, we postulated that genetic variation in the related and brain-specific ABCA2 gene leads to increase risk of AD. A large case-control study was conducted where the sample was randomly divided into a hypothesis-testing sample (230 cases/286 controls) and a validation sample (210 cases/233 controls). Among the 45 SNPs we tested, one synonymous SNP (rs908832) was found significantly associated with AD in both samples. Additional analyses performed on the whole sample showed a very strong association between this marker and early-onset AD (OR = 3.82, 95% C.I. = [2.00 - 7.30], P = 5 x 10(-5)). Further research is needed to understand the functional role of this polymorphism. However, together with the reported associations of AD with APOE, CYP46A1 and ABCA1, the present result adds a very significant support for the role of cholesterol and phospholipid homeostasis in AD and a rationale for testing novel cholesterol homeostasis-related therapeutic strategies in AD.

Impaired Cu/Zn-SOD activity contributes to increased oxidative damage in APP transgenic mice

Oxidative stress plays an important role in the pathogenesis of Alzheimer's disease. To determine which mechanisms cause the origin of oxidative damage, we analyzed enzymatic antioxidant defense (Cu/Zn-superoxide dismutase Cu/Zn-SOD, glutathione peroxidase GPx and glutathione reductase GR) and lipid peroxidation products malondialdehyde MDA and 4-hydroxynonenal HNE in two different APP transgenic mouse models at 3-4 and 12-15 months of age. No changes in any parameter were observed in brains from PDGF-APP695(SDL) mice, which have low levels of Abeta and no plaque load. In contrast, Thy1-APP751(SL) mice show high Abeta accumulation with aging and plaques from an age of 6 months. In brains of these mice, HNE levels were increased at 3 months (female transgenic mice) and at 12 months (both gender), that is, before and after plaque deposition, and the activity of Cu/Zn-SOD was reduced. Interestingly, beta-amyloidogenic cleavage of APP was increased in female Thy1-APP751(SL) mice, which also showed increased HNE levels with simultaneously reduced Cu/Zn-SOD activity earlier than male Thy1-APP751(SL) mice. Our results demonstrate that impaired Cu/Zn-SOD activity contributes to oxidative damage in Thy1-APP751(SL) transgenic mice, and these findings are closely linked to increased beta-amyloidogenic cleavage of APP.

Astrocytic calcium/zinc binding protein S100A6 over expression in Alzheimer's disease and in PS1/APP transgenic mice models

Astrocytes recruitment and activation are a hallmark of many neurodegenerative diseases including Alzheimer's disease (AD). We have previously observed an overexpression for S100A6 protein, a Ca(2+)/Zn(2+) binding protein presenting more affinity for zinc than for calcium, in amyotrophic lateral sclerosis (ALS). Here we demonstrated in AD patients but also in two different AD mouse models, that astrocytic S100A6 protein was homogeneously up-regulated within the white matter. However, within the grey matter, almost all S100A6 immunoreactivity was concentrated in astrocytes surrounding the Abeta amyloid deposits of senile plaques. These S100A6 neocortex labelled astrocytes were also positive for the glial fibrillary acidic protein (GFAP) and S100B protein. Contrasting with S100A6, the distribution for S100B and GFA astrocytic labelled cells was not restricted to the Abeta amyloid deposit in grey matter, but widely distributed throughout the neocortex. Coupling the knowledge that biometals such as zinc are highly concentrated in the amyloid deposits in AD and S100A6 having a high affinity for Zn(2+) may suggest that S100A6 plays a role in AD neuropathology.

Subcellular topography of neuronal Abeta peptide in APPxPS1 transgenic mice

In transgenic mice expressing human mutant beta-amyloid precursor protein (APP) and mutant presenilin-1 (PS1), Abeta antibodies labeled granules, about 1 microm in diameter, in the perikaryon of neurons clustered in the isocortex, hippocampus, amygdala, thalamus, and brainstem. The granules were present before the onset of Abeta deposits; their number increased up to 9 months and decreased in 15-month-old animals. They were immunostained by antibodies against Abeta 40, Abeta 42, and APP C-terminal region. In double immunofluorescence experiments, the intracellular Abeta co-localized with lysosome markers and less frequently with MG160, a Golgi marker. Abeta accumulation correlated with an increased volume of lysosomes and Golgi apparatus, while the volume of endoplasmic reticulum and early endosomes did not change. Some granules were immunolabeled with an antibody against flotillin-1, a raft marker. At electron microscopy, Abeta, APP-C terminal, cathepsin D, and flotillin-1 epitopes were found in the lumen of multivesicular bodies. This study shows that Abeta peptide and APP C-terminal region accumulate in multivesicular bodies containing lysosomal enzymes, while APP N-terminus is excluded from them. Multivesicular bodies could secondarily liberate their content in the extracellular space as suggested by the association of cathepsin D with Abeta peptide in the extracellular space.

Massive CA1/2 neuronal loss with intraneuronal and N-terminal truncated Abeta42 accumulation in a novel Alzheimer transgenic model

Alzheimer's disease (AD) is characterized by a substantial degeneration of pyramidal neurons and the appearance of neuritic plaques and neurofibrillary tangles. Here we present a novel transgenic mouse model, APP(SL)PS1KI that closely mimics the development of AD-related neuropathological features including a significant hippocampal neuronal loss. This transgenic mouse model carries M233T/L235P knocked-in mutations in presenilin-1 and overexpresses mutated human beta-amyloid (Abeta) precursor protein. Abeta(x-42) is the major form of Abeta species present in this model with progressive development of a complex pattern of N-truncated variants and dimers, similar to those observed in AD brain. At 10 months of age, an extensive neuronal loss (>50%) is present in the CA1/2 hippocampal pyramidal cell layer that correlates with strong accumulation of intraneuronal Abeta and thioflavine-S-positive intracellular material but not with extracellular Abeta deposits. A strong reactive astrogliosis develops together with the neuronal loss. This loss is already detectable at 6 months of age and is PS1KI gene dosage-dependent. Thus, APP(SL)PS1KI mice further confirm the critical role of intraneuronal Abeta(42) in neuronal loss and provide an excellent tool to investigate therapeutic strategies designed to prevent AD neurodegeneration.

TC-1734: an orally active neuronal nicotinic acetylcholine receptor modulator with antidepressant, neuroprotective and long-lasting cognitive effects

The development of selective ligands targeting neuronal nicotinic acetylcholine receptors to alleviate symptoms associated with neurodegenerative diseases presents the advantage of affecting multiple deficits that are the hallmarks of these pathologies. TC-1734 is an orally active novel neuronal nicotinic agonist with high selectivity for neuronal nicotinic receptors. Microdialysis studies indicate that TC-1734 enhances the release of acetylcholine from the cortex. TC-1734, by either acute or repeated administration, exhibits memory enhancing properties in rats and mice and is neuroprotective following excitotoxic insult in fetal rat brain in cultures and against alterations of synaptic transmission induced by deprivation of glucose and oxygen in hippocampal slices. At submaximal doses, TC-1734 produced additive cognitive effects when used in combination with tacrine or donepezil. Unlike (-)-nicotine, behavioral sensitization does not develop following repeated administration of TC-1734. Its pharmacokinetic (PK) profile (half-life of 2 h) contrasts with the long lasting improvement in working memory (18 h) demonstrating that cognitive improvement extends beyond the lifetime of the compound. The very low acute toxicity of TC-1734 and its receptor activity profile provides additional mechanistic basis for its suggested potential as a clinical candidate. TC-1734 was very well tolerated in acute and chronic oral toxicity studies in mice, rats and dogs. Phase I clinical trials demonstrated TC-1734's favorable pharmacokinetic and safety profile by acute oral administration at doses ranging from 2 to 320 mg. The bioavailability, pharmacological, pharmacokinetic, and safety profile of TC-1734 provides an example of a safe, potent and efficacious neuronal nicotinic modulator that holds promise for the management of the hallmark symptomatologies observed in dementia.

Amyloid beta-induced changes in nitric oxide production and mitochondrial activity lead to apoptosis

Increasing evidence suggests an important role of mitochondrial dysfunction in the pathogenesis of Alzheimer's disease. Thus, we investigated the effects of acute and chronic exposure to increasing concentrations of amyloid beta (Abeta) on mitochondrial function and nitric oxide (NO) production in vitro and in vivo. Our data demonstrate that PC12 cells and human embryonic kidney cells bearing the Swedish double mutation in the amyloid precursor protein gene (APPsw), exhibiting substantial Abeta levels, have increased NO levels and reduced ATP levels. The inhibition of intracellular Abeta production by a functional gamma-secretase inhibitor normalizes NO and ATP levels, indicating a direct involvement of Abeta in these processes. Extracellular treatment of PC12 cells with comparable Abeta concentrations only leads to weak changes, demonstrating the important role of intracellular Abeta. In 3-month-old APP transgenic (tg) mice, which exhibit no plaques but already detectable Abeta levels in the brain, reduced ATP levels can also be observed showing the in vivo relevance of our findings. Moreover, we could demonstrate that APP is present in the mitochondria of APPsw PC12 cells. This presence might be directly involved in the impairment of cytochrome c oxidase activity and depletion of ATP levels in APPsw PC12 cells. In addition, APPsw human embryonic kidney cells, which produce 20-fold increased Abeta levels compared with APPsw PC12 cells, and APP tg mice already show a significantly decreased mitochondrial membrane potential under basal conditions. We suggest a hypothetical sequence of pathogenic steps linking mutant APP expression and amyloid production with enhanced NO production and mitochondrial dysfunction finally leading to cell death.

Heterologous expression of human {alpha}6{beta}4{beta}3{alpha}5 nicotinic acetylcholine receptors: binding properties consistent with their natural expression require quaternary subunit assembly including the {alpha}5 subunit

Heterologous expression and lesioning studies were conducted to identify possible subunit assembly partners in nicotinic acetylcholine receptors (nAChR) containing alpha6 subunits (alpha6(*) nAChR). SH-EP1 human epithelial cells were transfected with the requisite subunits to achieve stable expression of human alpha6beta2, alpha6beta4, alpha6beta2beta3, alpha6beta4beta3, or alpha6beta4beta3alpha5 nAChR. Cells expressing subunits needed to form alpha6beta4beta3alpha5 nAChR exhibited saturable [(3)H]epibatidine binding (K(d) = 95.9 +/- 8.3 pM and B(max) = 84.5 +/- 1.6 fmol/mg of protein). The rank order of binding competition potency (K(i)) for prototypical nicotinic compounds was alpha-conotoxin MII (6 nM) > nicotine (156 nM) approximately methyllycaconitine (200 nM) > alpha-bungarotoxin (>10 microM), similar to that for nAChR in dopamine neurons displaying a distinctive pharmacology. 6-Hydroxydopamine lesioning studies indicated that beta3 and alpha5 subunits are likely partners of the alpha6 subunits in nAChR expressed in dopaminergic cell bodies. Similar to findings in rodents, quantitative real-time reverse transcription-polymerase chain reactions of human brain indicated that alpha6 subunit mRNA expression was 13-fold higher in the substantia nigra than in the cortex or the rest of the brain. Thus, heterologous expression studies suggest that the human alpha5 subunit makes a critical contribution to alpha6beta4beta3alpha5 nAChR assembly into a ligand-binding form with native alpha6(*)-nAChR-like pharmacology and of potential physiological and pathophysiological relevance.

Alzheimer pathology disorganizes cortico-cortical circuitry: direct evidence from a transgenic animal model

It has been proposed that Alzheimer disease (AD) is associated with a "disconnection syndrome" due to the gradual loss of morphological and functional integrity of cortico-cortical pathways. This hypothesis derives from indirect neuropathological observations, but definitive evidence that AD primarily targets cortico-cortical networks is still lacking. By means of neuroanatomical anterograde tracing methods, we have investigated, in a murine transgenic model of AD, the impact of the amyloid burden on axonal terminals in different neural systems. Axonal tracings revealed, in accordance with the "disconnection syndrome" hypothesis, that cortico-cortical fibers are significantly disorganized. Terminal fields in local and distant cortical areas contained numerous swollen dystrophic neurites often grouped in grape-like clusters at the plaque periphery. In contrary to fibers of cortical origin, those originating from subcortical brain structures only showed limited signs of degeneration upon reaching their cortical targets. These observations suggest a selective disruption of cortico-cortical connections induced by AD brain pathology.