A systems level analysis of transcriptional changes in Alzheimer's disease and normal aging

Alzheimer's disease (AD) is a debilitating neurodegenerative disorder affecting millions of elderly individuals worldwide. Advances in the genetics of AD have led to new levels of understanding and treatment opportunities. Here, we used a systems biology approach based on weighted gene coexpression network analysis to determine transcriptional networks in AD. This method permits a higher order depiction of gene expression relationships and identifies modules of coexpressed genes that are functionally related, rather than producing massive gene lists. Using this framework, we characterized the transcriptional network in AD, identifying 12 distinct modules related to synaptic and metabolic processes, immune response, and white matter, nine of which were related to disease progression. We further examined the association of gene expression changes with progression of AD and normal aging, and were able to compare functional modules of genes defined in both conditions. Two biologically relevant modules were conserved between AD and aging, one related to mitochondrial processes such as energy metabolism, and the other related to synaptic plasticity. We also identified several genes that were central, or hub, genes in both aging and AD, including the highly abundant signaling molecule 14.3.3 zeta (YWHAZ), whose role in AD and aging is uncharacterized. Finally, we found that presenilin 1 (PSEN1) is highly coexpressed with canonical myelin proteins, suggesting a role for PSEN1 in aspects of glial-neuronal interactions related to neurodegenerative processes.

The genetic architecture of Alzheimer's disease: beyond APP, PSENs and APOE

Alzheimer's disease (AD) is a complex disorder with a clear genetic component. Three genes have been identified as the cause of early onset familial AD (EOAD). The most common form of the disease, late onset Alzheimer's disease (LOAD), is, however, a sporadic one presenting itself in later stages of life. The genetic component of this late onset form of AD has been the target of a large number of studies, because only one genetic risk factor (APOE4) has been consistently associated with the disease. However, technological advances allow new approaches in the study of complex disorders. In this review, we discuss the new results produced by genome wide association studies, in light of the current knowledge of the complexity of AD genetics.

Bacopa monniera extract reduces amyloid levels in PSAPP mice

PSAPP mice expressing the "Swedish" amyloid precursor protein and M146L presenilin-1 mutations are a well-characterized model for spontaneous amyloid plaque formation. Bacopa monniera has a long history of use in India as an anti-aging and memory-enhancing ethnobotanical therapy. To evaluate the effect of Bacopa monniera extract (BME) on amyloid (Abeta) pathology in PSAPP mice, two doses of BME (40 or 160 mg/kg/day) were administered starting at 2 months of age for either 2 or 8 months. Our present data suggests that BME lowers Abeta 1-40 and 1-42 levels in cortex by as much as 60%, and reverses Y-maze performance and open field hyperlocomotion behavioral changes present in PSAPP mice. The areas encompassed by Congo Red-positive fibrillar amyloid deposits, however, were not altered by BME treatment. The data suggest that BME has potential application in Alzheimer's disease therapeutics.

The expression of presenilin 1 enhances carcinogenesis and metastasis in gastric cancer

Presenilin 1 (PS-1, encoded by PSEN1) is a part of the gamma- (γ-) secretase complex. Mutations in PSEN1 cause the majority of cases of familial Alzheimer's disease (FAD). Although in recent years PS-1 has been implicated as a tumor enhancer in various cancers, nothing is known regarding its role in gastric cancer (GC). In the present study, we investigate the role and clinical significance of PS-1 in GC. We observed that PS-1 was significantly upregulated and amplified in GC tissues and cell lines, and its aberrant expression was positively correlated with lymph node metastasis and with poor overall survival. Furthermore, PS-1 promoted tumor invasion and metastasis of GC both in vitro and vivo without affecting the proliferation of GC cells (MGC-803 and MKN-45). The results of treatment with the γ-secretase inhibitor DAPT were consistent with the outcomes of PS-1 silencing. PS-1/γ-secretase cleaves E-cadherin and releases its bound protein partner, β-catenin, from the actin cytoskeleton, thereby allowing it to translocate into the nucleus and to activate the TCF/LEF-1 transcriptional activator, which may promote GC invasion and metastasis.In conclusion, PS-1 promotes invasion and metastasis in GC and may represent a novel prognostic biomarker and potential therapeutic target for GC treatment.

Dietary deficiency increases presenilin expression, gamma-secretase activity, and Abeta levels: potentiation by ApoE genotype and alleviation by S-adenosyl methionine

Apolipoprotein E4 (ApoE4) is a risk factor for Alzheimer's disease (AD). Whether this risk arises from a deficient function of E4 or the lack of protection provided by E2 or E3 is unclear. Previous studies demonstrate that deprivation of folate and vitamin E, coupled with dietary iron as a pro-oxidant, for 1 month displayed increased presenilin 1 (PS-1) expression, gamma-secretase, and Abeta generation in mice lacking ApoE (ApoE-/- mice). While ApoE-/- mice are a model for ApoE deficiency, they may not reflect the entire range of consequences of E4 expression. We therefore compared herein the impact of the above deficient diet on mice expressing human E2, E3, or E4. As folate deficiency is accompanied by a decrease in the major methyl donor, S-adenosyl methionine (SAM), additional mice received the deficient diet plus SAM. E2 was more protective than murine ApoE or E3 and E4. Surprisingly, PS-1 and gamma-secretase were over-expressed in E3 to the same extent as in E4 even under a complete diet, and were not alleviated by SAM supplementation. Abeta increased only in E4 mice maintained under the complete diet, and was alleviated by SAM supplementation. These findings suggest dietary compromise can potentiate latent risk factors for AD.

Molecular characterization and temporal expression profiling of presenilins in the developing porcine brain

Background

The transmembrane presenilin (PSEN) proteins, PSEN1 and PSEN2, have been proposed to be the catalytic components of the γ-secretase protein complex, which is an intramembranous multimeric protease involved in development, cell regulatory processes, and neurodegeneration in Alzheimer's disease. Here we describe the sequencing, chromosomal mapping, and polymorphism analysis of PSEN1 and PSEN2 in the domestic pig (Sus scrofa domesticus).

Conclusion

The data provide evidence for structural and functional conservation of PSENs in mammalian lineages, and may suggest that the high sequence similarity and colocalization of PSEN1 and PSEN2 in brain tissue reflect a certain degree of functional redundancy. The data show that pigs may provide a new animal model for detailed analysis of the developmental functions of the PSENs.

Oral Treponema denticola Infection Induces Aβ1-40 and Aβ1-42 Accumulation in the Hippocampus of C57BL/6 Mice

Accumulation of amyloid-β (Aβ) in the brain is a central component of pathology in Alzheimer's disease. A growing volume of evidence demonstrates close associations between periodontal pathogens including Porphyromonas gingivalis (P. gingivalis) and Treponema denticola (T. denticola) and AD. However, the effect and mechanisms of T. denticola on accumulation of Aβ remain to be unclear. In this study, we demonstrated that T. denticola was able to enter the brain and act directly on nerve cells resulting in intra- and extracellular Aβ_1-40 and Aβ_1-42 accumulation in the hippocampus of C57BL/6 mice by selectively activating both β-secretase and γ-secretase. Furthermore, both KMI1303, an inhibitor of β-secretase, as well as DAPT, an inhibitor of γ- secretase, were found to be able to inhibit the effect of T. denticola on Aβ accumulation in N2a neuronal cells. Overall, it is concluded that T. denticola increases the expression of Aβ_1-42 and Aβ_1-40 by its regulation on beta-site amyloid precursor protein cleaving enzyme-1 and presenilin 1.

YY-1224, a terpene trilactone-strengthened Ginkgo biloba, attenuates neurodegenerative changes induced by β-amyloid (1-42) or double transgenic overexpression of APP and PS1 via inhibition of cyclooxygenase-2

BACKGROUND

Ginkgo biloba has been reported to possess free radical-scavenging antioxidant activity and anti-inflammatory properties. In our pilot study, YY-1224, a terpene trilactone-strengthened extract of G. biloba, showed anti-inflammatory, neurotrophic, and antioxidant effects.

RESULTS

We investigated the pharmacological potential of YY-1224 in β-amyloid (Aβ) (1-42)-induced memory impairment using cyclooxygenase-2 (COX-2) knockout (-/-) and APPswe/PS1dE9 transgenic (APP/PS1 Tg) mice. Repeated treatment with YY-1224 significantly attenuated Aβ (1-42)-induced memory impairment in COX-2 (+/+) mice, but not in COX-2 (-/-) mice. YY-1224 significantly attenuated Aβ (1-42)-induced upregulation of platelet-activating factor (PAF) receptor gene expression, reactive oxygen species, and pro-inflammatory factors. In addition, YY-1224 significantly inhibited Aβ (1-42)-induced downregulation of PAF-acetylhydrolase-1 (PAF-AH-1) and peroxisome proliferator-activated receptor γ (PPARγ) gene expression. These changes were more pronounced in COX-2 (+/+) mice than in COX-2 (-/-) mice. YY-1224 significantly attenuated learning impairment, Aβ deposition, and pro-inflammatory microglial activation in APP/PS1 Tg mice, whereas it significantly enhanced PAF-AH and PPARγ expression. A preferential COX-2 inhibitor, meloxicam, did not affect the pharmacological activity by YY-1224, suggesting that the COX-2 gene is a critical mediator of the neuroprotective effects of YY-1224. The protective activity of YY-1224 appeared to be more efficacious than a standard G. biloba extract (Gb) against Aβ insult.

CONCLUSIONS

Our results suggest that the protective effects of YY-1224 against Aβ toxicity may be associated with its PAF antagonistic- and PPARγ agonistic-potential as well as inhibition of the Aβ-mediated pro-inflammatory switch of microglia phenotypes through suppression of COX-2 expression.

Intraperitoneal injection of JNK-specific inhibitor SP600125 inhibits the expression of presenilin-1 and Notch signaling in mouse brain without induction of apoptosis

Presenilin-1 (PS1) is a multifunctional protein involved in many cellular functions including the processing of type 1 membrane proteins such as β-amyloid precursor protein (APP) and Notch 1 receptor. PS1 acts as the catalytic subunit of the γ-secretase complex, and participates in Notch 1 processing to release Notch intracellular domain (NICD) in the cytoplasm. NICD subsequently migrates to the nucleus and causes Notch signaling by increasing the expression of the Hes1 gene. We have previously shown that inhibition of basal activity of c-jun-NH2-terminal kinase (JNK) with JNK-specific inhibitor SP600125 represses the expression of PS1 and γ-secretase activity by increasing p53 level in SK-N-SH cell line in vitro (Lee and Das, 2008, 2010). However, it is largely unknown whether PS1 can be effectively suppressed in vivo in adult mouse brains. In this report we showed that intraperitoneal (i.p) injection of JNK-specific inhibitor SP600125 decreased p-JNK level, and reduced PS1 expression by increasing p53 level in adult mouse brains. We also showed that suppression of PS1 expression by SP600125 reduced γ-secretase activity which decreased Notch 1 processing to reduce NICD in mouse brains. Furthermore, inhibition of Notch 1 processing by SP600125 decreased Notch 1 signaling by reducing the expression of the NICD target Hes1 gene in mouse brains without induction of apoptosis. These results provide insights for further study on PS1-mediated reduction of Notch 1 and APP processing for the treatment of Alzheimer's disease.

An ancestral non-proteolytic role for presenilin proteins in multicellular development of the social amoeba Dictyostelium discoideum

Mutations in either of two presenilin genes can cause familial Alzheimer's disease. Presenilins have both proteolysis-dependent functions, as components of the γ-secretase complex, and proteolysis-independent functions in signalling. In this study, we investigate a conserved function of human presenilins in the development of the simple model organism Dictyostelium discoideum. We show that the block in Dictyostelium development caused by the ablation of both Dictyostelium presenilins is rescued by the expression of human presenilin 1, restoring the terminal differentiation of multiple cell types. This developmental role is independent of proteolytic activity, because the mutation of both catalytic aspartates does not affect presenilin ability to rescue development, and the ablation of nicastrin, a γ-secretase component that is crucial for proteolytic activity, does not block development. The role of presenilins during Dictyostelium development is therefore independent of their proteolytic activity. However, presenilin loss in Dictyostelium results in elevated cyclic AMP (cAMP) levels and enhanced stimulation-induced calcium release, suggesting that presenilins regulate these intracellular signalling pathways. Our data suggest that presenilin proteins perform an ancient non-proteolytic role in regulating intracellular signalling and development, and that Dictyostelium is a useful model for analysing human presenilin function.

Up-regulation of Bcl-2 in APP transgenic mice is associated with neuroprotection

Abeta-induced neurodegeneration is limited in APP and APP+PS1 transgenic mice. In middle-aged APP + PS1 transgenic mice, we found significantly increased Bcl-2 expression. The increase in Bcl-2 is restricted to amyloid-containing brain regions and is not found at young ages, suggesting that Abeta deposition is the stimulus for increased Bcl-2. Western blot results were confirmed with immunohistochemistry and qRT-PCR. In addition, we found that APP transgenic mice were protected from neurotoxicity caused by an injection of bak BH3 fusion peptides, known to induce apoptosis by antagonizing bcl protein activity. Nissl and fluorojade-stained slides showed that the active bak BH3 peptide caused substantial neuronal loss in the dentate gyrus and CA3 regions of nontransgenic, but not APP mice. The inactive mutant bak BH3 peptide did not cause degeneration in any mice. These data demonstrate that the increased Bcl-2 expression in brain regions containing Abeta deposits is associated with neuroprotection.

Antagonist of neuronal store-operated calcium entry exerts beneficial effects in neurons expressing PSEN1ΔE9 mutant linked to familial Alzheimer disease

Alzheimer's disease (AD) is the neurodegenerative disorder with no cure. Recent studies suggest that dysregulated postsynaptic store-operated calcium entry (nSOCE) may underlie mushroom spine loss that is related to AD pathology. In the present study we observed that PSEN1ΔE9 familial AD (FAD) mutation causes mushroom spine loss in hippocampal neuronal cultures. We also demonstrated that amplitude of TRPC6-mediated nSOCE is increased in PSEN1ΔE9-expressing neurons and we suggested that inhibition of nSOCE may help to rescue synaptic defects in this model. We further established that nSOCE antagonist EVP4593 decreases PSEN1ΔE9-mediated nSOCE upregulation and rescues mushroom spines in PSEN1ΔE9-expressing neurons. Obtained results further highlight the connection between dysregulation of endoplasmic reticulum calcium signaling and synaptic loss in AD and suggest that calcium signaling modulators may have a therapeutic value for treatment of memory loss in AD.

PS1 expression is downregulated by gonadal steroids in adult mouse brain

Mutations in presenilin (PS) 1 and PS2 genes are associated with early onset (< or =65 years) of Alzheimer's disease (AD). PS1 is involved in gamma-secretase mediated cleavage of beta-amyloid precursor protein (APP), but its regulation is poorly understood. Sex steroids influence APP cleavage pathways resulting in reduced burden of both intra- and extra-cellular nonamyloidogenic products. As gonadal hormones are implicated in AD and their levels change with age, we have analyzed the effect of 17beta-estradiol and testosterone on PS1 expression in the cerebral cortex of adult and old AKR mice of both sexes. Northern and Western-blot analysis revealed that PS1 mRNA and protein expression followed similar pattern of regulation. PS1 expression was downregulated by 17 beta-estradiol and testosterone in the cerebral cortex of females and adult male, but upregulated in old male mice. Such sex-dependent regulation of PS1 expression during aging by gonadal steroids might account for the PS-related brain functions.

Upregulation of presenilin 1 in brains of sporadic, late-onset Alzheimer's disease

The activity of the β-secretase involved in the cleavage of amyloid-β (Aβ) is increased in sporadic late-onset Alzheimer's disease (AD). Whether the corresponding γ-secretase activity is altered is still uncertain. We evaluated mRNA expression and protein levels of presenilin 1 (PS1) and γ-secretase activity in the frontal cortex of 32 cases with late-onset sporadic AD and those of 29 control subjects. We found a significant increase in PS1 mRNA, protein levels and γ-secretase activity in AD cases. These findings suggest that upregulation of PS1 leads to Aβ overproduction and accumulation in sporadic AD.

Amyloid precursor protein and Presenilin 1 interaction studied by FRET in human H4 cells

The mayor pathologic hallmarks of Alzheimer's disease (AD) are senile plaque and neurofibrillary tangles. Senile plaque are primarily made up of deposits of amyloid-beta protein, a proteolytic product derived from the amyloid precursor protein (APP). APP is a transmembrane protein detected into the endoplasmic reticulum, in the Golgi apparatus, at the cell surface, recycled by endocytosis to endosomes, whose physiological function is unclear. Presenilins (PS), are a component of gamma-secretase complex that cleave alpha-CTFs (carboxy-terminal fragment), or beta-CTFs, leaving 40 or 42 amino acids amyloid-beta peptides and 58 or 56 amino acids intracellular domains (AICD). Where the amyloid-beta peptides is generated is not clear. The study of APP-PS interaction in specific cell compartments provides a good opportunity to light upon the molecular mechanisms regulating the activity of the "gamma-secretase complex," and where beta-amyloid is generated. In our study we used a biophysical assay of protein proximity: fluorescence resonance energy transfer (FRET), that can provide information about molecular interactions when two proteins are in the close proximity (<10 nm), to examine the subcellular localization and interaction between APP and PS1 in human neuroglioma cells (H4). Confocal microscopic analysis reveals extensive colocalization in different cells' compartment, and centrosomal or microtubule organizing center (MTOC) localization of APP and PS1, but not necessarily a close molecular interaction. We used FRET to determine if APP and PS1 interact at the cell centrosome. FRET data suggest a close interaction between APP and PS1 in subcellular compartments and at the centrosome of H4 cells. Using this approach we show that APP and PS1 are closely associated in the centrosomes of the H4 cell.

Novel presenilin 1 variant (P117A) causing Alzheimer's disease in the fourth decade of life

Over 160 rare genetic variants in presenilin 1 (PSEN1) are known to cause Alzheimer's disease (AD). In this study we screened a family with early-onset AD for mutations in PSEN1 using direct DNA sequencing. We identified a novel PSEN1 genetic variant which results in the substitution of a Proline with an Alanine at codon 117 (P117A). The P117A variant was present in all demented individuals and fifty percent of at risk individuals. This variant occurs at a site where three other disease-causing variants have been previously observed. In vitro functional studies demonstrate that the P117A variant results in an altered Abeta42/total Abeta ratio consistent with an AD causing mutation. The P117A variant is a novel mutation in PSEN1, which causes early-onset AD in an autosomal dominant manner.

Morphological change by overexpression of D385A dominant negative presenilin 1 in human neuroblastoma SH-SY5Y cells

Presenilin 1 (PS1) is a multifunctional protein, and its mutations are highly related to familial Alzheimer's disease (AD). In this study, we examined the effects of PS1 overexpression on neuronal morphology using SH-SY5Y cells. Overexpression of dominant-negative D385A PS1 induced morphological change and impairment of neurite formation, while those of wild-type and pathogenic P117L mutant PS1 did not change cellular morphology compared with native cells. Moreover, filopodium-formation-related proteins were decreased only in cells overexpressing D385A PS1. Therefore, PS1 may be involved in neuritogenesis and morphological change in SH-SY5Y cells, and P117L mutation may linked to AD by different mechanisms.

[Expression of presenilin 1 on the cell surface in motile polarized cells]

Most cases of familial early-onset Alzheimer's disease are caused by mutations in the presenilin 1 (PS1) gene. However, the cellular functions of PS1 are not yet completely understood. We showed that endogenous PS1 and the adhesion protein CD44 are redistributed on the surface of cell projections (lamellipodia) in polarized T- lymphocytes (Jurkat cells) after the adhesion to a collagen matrix. This effect was not observed for another surface protein of T lymphocytes, which is not involved in cell adhesion processes, the T cell receptor. In primary cultures of mouse cortical neurons, PS1 was concentrated at the surface of extended growth cones and at the sites of neurite contacts. The concentration of PS1 at the surface of cellular structures that promote cell motility and cell contacts suggests an important role of PSI in cell adhesion in motile polarized cells.