Beta-amyloid deposition in the brains of rats chronically infused with thiorphan or lipopolysaccharide: the role of ascorbic acid in the vehicle

The gut microbiome and Alzheimer's disease: Complex and bidirectional interactions

Structural and functional alterations to the gut microbiome, referred to as gut dysbiosis, have emerged as potential key mediators of neurodegeneration and Alzheimer disease (AD) pathogenesis through the "gut -brain" axis. Emerging data from animal and clinical studies support an important role for gut dysbiosis in mediating neuroinflammation, central and peripheral immune dysregulation, abnormal brain protein aggregation, and impaired intestinal and brain barrier permeability, leading to neuronal loss and cognitive impairment. Gut dysbiosis has also been shown to directly influence various mechanisms involved in neuronal growth and repair, synaptic plasticity, and memory and learning functions. Aging and lifestyle factors including diet, exercise, sleep, and stress influence AD risk through gut dysbiosis. Furthermore, AD is associated with characteristic gut microbial signatures which offer value as potential markers of disease severity and progression. Together, these findings suggest the presence of a complex bidirectional relationship between AD and the gut microbiome and highlight the utility of gut modulation strategies as potential preventative or therapeutic strategies in AD. We here review the current literature regarding the role of the gut-brain axis in AD pathogenesis and its potential role as a future therapeutic target in AD treatment and/or prevention.

MicroRNA-146a switches microglial phenotypes to resist the pathological processes and cognitive degradation of Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease and currently has no effective treatment. Mainstream research on the mechanisms and therapeutic targets of AD is focused on the two most important hallmarks, Aβ and Tau, but the results from clinical studies are not encouraging. Abnormal microglial polarization is a clear typical pathological feature in the progression of AD. Microglia can be neuroprotective by degrading and removing Aβ and Tau. However, under AD conditions, microglia transform into a pro-inflammatory phenotype that decreases the phagocytic activity of microglia, damages neurons and promotes the pathology of AD. We previously reported that a miR-146a polymorphism is associated with sporadic AD risk, and the nasal administration of miR-146a mimics reduced cognitive impairment and the main pathological features of AD. However, it is not clear by what mechanism miR-146a resists the pathological process of AD. In this study, we discovered that microglia-specific miR-146a overexpression reduced cognitive deficits in learning and memory, attenuated neuroinflammation, reduced Aβ levels, ameliorated plaque-associated neuritic pathology, and prevented neuronal loss in APP/PS1 transgenic mice. In addition, we found that miR-146a switched the microglial phenotype, reduced pro-inflammatory cytokines and enhanced phagocytic function to protect neurons in vitro and in vivo. Moreover, transcriptional analysis confirmed that miR-146a opposed the pathological process of AD mainly through neuroinflammation-related pathways. In summary, our results provide sufficient evidence for the mechanism by which miR-146a opposes AD and strengthen the conclusion that miR-146a is a promising target for AD and other microglia-related diseases.

Dysbiosis and Alzheimer's Disease: Cause or Treatment Opportunity?

Recent investigations have increased the interest on the connection between the microorganisms inhabiting the gut (gut microbiota) and human health. An imbalance of the intestinal bacteria representation (dysbiosis) could lead to different diseases, ranging from obesity and diabetes, to neurological disorders including Alzheimer's disease (AD). The term "gut-brain axis" refers to a crosstalk between the brain and the gut involving multiple overlapping pathways, including the autonomic, neuroendocrine, and immune systems as well as bacterial metabolites and neuromodulatory molecules. Through this pathway, microbiota can influence the onset and progression of neuropathologies such as AD. This review discusses the possible interaction between the gut microbiome and AD, focusing on the role of gut microbiota in neuroinflammation, cerebrovascular degeneration and Aβ clearance.

Complex Interaction between Resident Microbiota and Misfolded Proteins: Role in Neuroinflammation and Neurodegeneration

Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and Creutzfeldt-Jakob disease (CJD) are brain conditions affecting millions of people worldwide. These diseases are associated with the presence of amyloid-β (Aβ), alpha synuclein (α-Syn) and prion protein (PrP) depositions in the brain, respectively, which lead to synaptic disconnection and subsequent progressive neuronal death. Although considerable progress has been made in elucidating the pathogenesis of these diseases, the specific mechanisms of their origins remain largely unknown. A body of research suggests a potential association between host microbiota, neuroinflammation and dementia, either directly due to bacterial brain invasion because of barrier leakage and production of toxins and inflammation, or indirectly by modulating the immune response. In the present review, we focus on the emerging topics of neuroinflammation and the association between components of the human microbiota and the deposition of Aβ, α-Syn and PrP in the brain. Special focus is given to gut and oral bacteria and biofilms and to the potential mechanisms associating microbiome dysbiosis and toxin production with neurodegeneration. The roles of neuroinflammation, protein misfolding and cellular mediators in membrane damage and increased permeability are also discussed.

Low-Dose Ionizing Radiation Modulates Microglia Phenotypes in the Models of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common type of dementia. AD involves major pathologies such as amyloid-β (Aβ) plaques and neurofibrillary tangles in the brain. During the progression of AD, microglia can be polarized from anti-inflammatory M2 to pro-inflammatory M1 phenotype. The activation of triggering receptor expressed on myeloid cells 2 (TREM2) may result in microglia phenotype switching from M1 to M2, which finally attenuated Aβ deposition and memory loss in AD. Low-dose ionizing radiation (LDIR) is known to ameliorate Aβ pathology and cognitive deficits in AD; however, the therapeutic mechanisms of LDIR against AD-related pathology have been little studied. First, we reconfirm that LDIR (two Gy per fraction for five times)-treated six-month 5XFAD mice exhibited (1) the reduction of Aβ deposition, as reflected by thioflavins S staining, and (2) the improvement of cognitive deficits, as revealed by Morris water maze test, compared to sham-exposed 5XFAD mice. To elucidate the mechanisms of LDIR-induced inhibition of Aβ accumulation and memory loss in AD, we examined whether LDIR regulates the microglial phenotype through the examination of levels of M1 and M2 cytokines in 5XFAD mice. In addition, we investigated the direct effects of LDIR on lipopolysaccharide (LPS)-induced production and secretion of M1/M2 cytokines in the BV-2 microglial cells. In the LPS- and LDIR-treated BV-2 cells, the M2 phenotypic marker CD206 was significantly increased, compared with LPS- and sham-treated BV-2 cells. Finally, the effect of LDIR on M2 polarization was confirmed by detection of increased expression of TREM2 in LPS-induced BV2 cells. These results suggest that LDIR directly induced phenotype switching from M1 to M2 in the brain with AD. Taken together, our results indicated that LDIR modulates LPS- and Aβ-induced neuroinflammation by promoting M2 polarization via TREM2 expression, and has beneficial effects in the AD-related pathology such as Aβ deposition and memory loss.

The Links Between the Gut Microbiome, Aging, Modern Lifestyle and Alzheimer's Disease

Gut microbiome is a community of microorganisms in the gastrointestinal tract. These bacteria have a tremendous impact on the human physiology in healthy individuals and during an illness. Intestinal microbiome can influence one's health either directly by secreting biologically active substances such as vitamins, essential amino acids, lipids et cetera or indirectly by modulating metabolic processes and the immune system. In recent years considerable information has been accumulated on the relationship between gut microbiome and brain functions. Moreover, significant quantitative and qualitative changes of gut microbiome have been reported in patients with Alzheimer's disease. On the other hand, gut microbiome is highly sensitive to negative external lifestyle aspects, such as diet, sleep deprivation, circadian rhythm disturbance, chronic noise, and sedentary behavior, which are also considered as important risk factors for the development of sporadic Alzheimer's disease. In this regard, this review is focused on analyzing the links between gut microbiome, modern lifestyle, aging, and Alzheimer's disease.

Association between Alzheimer's Disease and Oral and Gut Microbiota: Are Pore Forming Proteins the Missing Link?

Alzheimer's disease (AD) is a neurodegenerative condition affecting millions of people worldwide. It is associated with cerebral amyloid-β (Aβ) plaque deposition in the brain, synaptic disconnection, and subsequent progressive neuronal death. Although considerable progress has been made to elucidate the pathogenesis of AD, the specific causes of the disease remain highly unknown. Recent research has suggested a potential association between certain infectious diseases and dementia, either directly due to bacterial brain invasion and toxin production, or indirectly by modulating the immune response. Therefore, in the present review we focus on the emerging issues of bacterial infection and AD, including the existence of antimicrobial peptides having pore-forming properties that act in a similar way to pores formed by Aβ in a variety of cell membranes. Special focus is placed on oral bacteria and biofilms, and on the potential mechanisms associating bacterial infection and toxin production in AD. The role of bacterial outer membrane vesicles on the transport and delivery of toxins as well as porins to the brain is also discussed. Aβ has shown to possess antimicrobial activity against several bacteria, and therefore could be upregulated as a response to bacteria and bacterial toxins in the brain. Although further research is needed, we believe that the control of biofilm-mediated diseases could be an important potential prevention mechanism for AD development.

Astaxanthin Ameliorates Lipopolysaccharide-Induced Neuroinflammation, Oxidative Stress and Memory Dysfunction through Inactivation of the Signal Transducer and Activator of Transcription 3 Pathway

Astaxanthin (AXT), a xanthophyll carotenoid compound, has potent antioxidant, anti-inflammatory and neuroprotective properties. Neuroinflammation and oxidative stress are significant in the pathogenesis and development of Alzheimer's disease (AD). Here, we studied whether AXT could alleviate neuroinflammation, oxidative stress and memory loss in lipopolysaccharide (LPS) administered mice model. Additionally, we investigated the anti-oxidant activity and the anti-neuroinflammatory response of AXT in LPS-treated BV-2 microglial cells. The AXT administration ameliorated LPS-induced memory loss. This effect was associated with the reduction of LPS-induced expression of inflammatory proteins, as well as the production of reactive oxygen species (ROS), nitric oxide (NO), cytokines and chemokines both in vivo and in vitro. AXT also reduced LPS-induced β-secretase and Aβ_1⁻42 generation through the down-regulation of amyloidogenic proteins both in vivo and in vitro. Furthermore, AXT suppressed the DNA binding activities of the signal transducer and activator of transcription 3 (STAT3). We found that AXT directly bound to the DNA- binding domain (DBD) and linker domain (LD) domains of STAT3 using docking studies. The oxidative stress and inflammatory responses were not downregulated in BV-2 cells transfected with DBD-null STAT3 and LD-null STAT3. These results indicated AXT inhibits LPS-induced oxidant activity, neuroinflammatory response and amyloidogenesis via the blocking of STAT3 activity through direct binding.

A Bacterial Component to Alzheimer's-Type Dementia Seen via a Systems Biology Approach that Links Iron Dysregulation and Inflammagen Shedding to Disease

The progression of Alzheimer's disease (AD) is accompanied by a great many observable changes, both molecular and physiological. These include oxidative stress, neuroinflammation, and (more proximal to cognitive decline) the death of neuronal and other cells. A systems biology approach seeks to organize these observed variables into pathways that discriminate those that are highly involved (i.e., causative) from those that are more usefully recognized as bystander effects. We review the evidence that iron dysregulation is one of the central causative pathway elements here, as this can cause each of the above effects. In addition, we review the evidence that dormant, non-growing bacteria are a crucial feature of AD, that their growth in vivo is normally limited by a lack of free iron, and that it is this iron dysregulation that is an important factor in their resuscitation. Indeed, bacterial cells can be observed by ultrastructural microscopy in the blood of AD patients. A consequence of this is that the growing cells can shed highly inflammatory components such as lipopolysaccharides (LPS). These too are known to be able to induce (apoptotic and pyroptotic) neuronal cell death. There is also evidence that these systems interact with elements of vitamin D metabolism. This integrative systems approach has strong predictive power, indicating (as has indeed been shown) that both natural and pharmaceutical iron chelators might have useful protective roles in arresting cognitive decline, and that a further assessment of the role of microbes in AD development is more than highly warranted.

Vitamin C, Aging and Alzheimer's Disease

Accumulating evidence in mice models of accelerated senescence indicates a rescuing role of ascorbic acid in premature aging. Supplementation of ascorbic acid appeared to halt cell growth, oxidative stress, telomere attrition, disorganization of chromatin, and excessive secretion of inflammatory factors, and extend lifespan. Interestingly, ascorbic acid (AA) was also found to positively modulate inflamm-aging and immunosenescence, two hallmarks of biological aging. Moreover, ascorbic acid has been shown to epigenetically regulate genome integrity and stability, indicating a key role of targeted nutrition in healthy aging. Growing in vivo evidence supports the role of ascorbic acid in ameliorating factors linked to Alzheimer’s disease (AD) pathogenesis, although evidence in humans yielded equivocal results. The neuroprotective role of ascorbic acid not only relies on the general free radical trapping, but also on the suppression of pro-inflammatory genes, mitigating neuroinflammation, on the chelation of iron, copper, and zinc, and on the suppression of amyloid-beta peptide (Aβ) fibrillogenesis. Epidemiological evidence linking diet, one of the most important modifiable lifestyle factors, and risk of Alzheimer's disease is rapidly increasing. Thus, dietary interventions, as a way to epigenetically modulate the human genome, may play a role in the prevention of AD. The present review is aimed at providing an up to date overview of the main biological mechanisms that are associated with ascorbic acid supplementation/bioavailability in the process of aging and Alzheimer’s disease. In addition, we will address new fields of research and future directions.

Possible role of resveratrol targeting estradiol and neprilysin pathways in lipopolysaccharide model of Alzheimer disease

Alzheimer's disease (AD) is an irreversible, progressive neurodegenerative brain disease that slowly destroys memory and thinking skills. It is the most common cause of dementia among older people. One of the most important hallmarks of AD is the presence of amyloid beta (Aβ) peptide in the brain that suggests that it is the primary trigger for neuronal loss. Herbal extracts have been studied over the years for their potential therapeutic effect in AD. Resveratrol (RSV), one of the most important phytoestrogens, is considered to be useful as estrogen plays an important role in AD. One of the most important amyloid degrading enzymes is neprilysin (NEP), which plays a major role in degrading Aβ, and mainly affected by estrogen. So, the aim of the present study is investigating the possible role of resveratrol in lipopolysaccharide model of AD and the implication of its possible role in regulating the estradiol and neprilysin pathways. Mice were divided into four groups: Control group (0.9 % saline), LPS group (0.8 mg/kg i.p once), Treatment group with RSV (mice were once injected with LPS then after 30 min given a dose of {4 mg/kg} RSV for 7 days), and RSV group only (mice received 4 mg/kg i.p for 7 days only). After 7 days mice were subjected to different behavioral tests using Y-maze, object recognition test, and open field tests. Estradiol and NEP level were measured using ELISA kit. Results showed RSV was able to reverse the decline in different types of memory (working, nonspatial, and locomotor functions) caused by LPS induction in mice. Moreover RSV was able to significantly increase both the estradiol level and NEP level and that may have a great role to decrease Aβ deposition as it has been confirmed that there is a link between NEP and estradiol level; by upregulation of estradiol level this consequently leads to increase in the level of NEP level, and by increasing the NEP level in brain, this lead to decrease in Aβ deposition and enhancing its degradation by NEP.

Valproic acid potentiates curcumin-mediated neuroprotection in lipopolysaccharide induced rats

The etiology of neuroinflammation is complex and comprises multifactorial, involving both genetic and environmental factors during which diverse genetic and epigenetic modulations are implicated. Curcumin (Cur) and valproic acid (VPA), histone deacetylase 1 inhibitor, have neuroprotective effects. The present study was designed with an aim to investigate the ability of co-treatment of both compounds (Cur or VPA, 200 mg/kg) for 4 weeks to augment neuroprotection and enhance brain recovery from intra-peritoneal injection of (250 μg/kg) lipopolysaccharide-stimulated neuroinflammatory condition on rat brain cortex. Cortex activation and the effects of combined treatment and production of proinflammatory mediators, cyclooxygenase-2 (COX-2), APE1, and nitric oxide/inducible nitric oxide synthase (iNOS) were investigated. Neuroinflammation development was assessed by histological analyses and by investigating associated indices [β-secretase (BACE1), amyloid protein precursor (APP), presenilin (PSEN-1), and PSEN-2)]. Furthermore we measured the expression profile of lethal-7 (let-7) miRNAs members a, b, c, e, and f in all groups, a highly abundant regulator of gene expression in the CNS. Protein and mRNA levels of neuroinflammation markers COX-2, BACE1, APP, and iNOS were also attenuated by combined therapy. On the other hand, assessment of the indicated five let-7 members, showed distinct expression profile pattern in the different groups. Let-7 a, b, and c disappeared in the induced group, an effect that was partially suppressed by co-addition of either Cur or VPA. These data suggest that the combined treatment induced significantly the expression of the five members when compared to rats treated with Cur or VPA only as well as to self-recovery group, which indicates a possible benefit from the synergistic effect of Cur-VPA combination as therapeutic agents for neuroinflammation and its associated disorders. The mechanism elucidated here highlights the particular drug-induced expression profile of let-7 family as new targets for future pharmacological development.

Glucagon-like peptide-1 protects synaptic and learning functions from neuroinflammation in rodents

Glucagon-like peptide-1 (GLP-1) is derived from the processing of proglucagon in intestinal L-cells and releases insulin from pancreatic β-cells as an incretin. The GLP-1 receptor has been proposed as a possible therapeutic target for the treatment of Alzheimer's disease, in which neuroinflammation is critical in the pathogenesis. The present study investigates whether GLP-1 (7-36) amide, an active fragment of GLP-1, protected against synaptic impairments induced by inflammation-related injurious agents (lipopolysaccharide [LPS], interleukin-1β [IL-1β], and H2 O2). In the Y-maze test, LPS (10 μg/mouse, i.c.v) significantly decreased the percentage alternation. Pretreatment with GLP-1 (7-36) amide (0.09-0.9 nmol/mouse, i.c.v.) prevented an impairment in spontaneous alternation performance. Pretreatment with LPS (10 μg/ml, 2 hr) impaired LTP induction but not paired-pulse facilitation in the CA1 region of rat hippocampal slices. This impairment was prevented by cotreatment with GLP-1 (7-36) amide (50 nM). IL-1β (0.57 nM) or H2 O2 (50 μM) also impaired LTP induction. This impairment was prevented by GLP-1 (7-36) amide (50 nM). These results suggest that GLP-1 (7-36) amide improves the synaptic impairments induced by inflammation-related injurious agents in the CA1 region of the hippocampus.

Dose-dependent changes in neuroinflammatory and arachidonic acid cascade markers with synaptic marker loss in rat lipopolysaccharide infusion model of neuroinflammation

Background

Neuroinflammation, caused by six days of intracerebroventricular infusion of bacterial lipopolysaccharide (LPS), stimulates rat brain arachidonic acid (AA) metabolism. The molecular changes associated with increased AA metabolism are not clear. We examined effects of a six-day infusion of a low-dose (0.5 ng/h) and a high-dose (250 ng/h) of LPS on neuroinflammatory, AA cascade, and pre- and post-synaptic markers in rat brain. We used artificial cerebrospinal fluid-infused brains as controls.

Results

Infusion of low- or high-dose LPS increased brain protein levels of TNFα, and iNOS, without significantly changing GFAP. High-dose LPS infusion upregulated brain protein and mRNA levels of AA cascade markers (cytosolic cPLA₂-IVA, secretory sPLA₂-V, cyclooxygenase-2 and 5-lipoxygenase), and of transcription factor NF-κB p50 DNA binding activity. Both LPS doses increased cPLA₂ and p38 mitogen-activated protein kinase levels, while reducing protein levels of the pre-synaptic marker, synaptophysin. Post-synaptic markers drebrin and PSD95 protein levels were decreased with high- but not low-dose LPS.

Conclusions

Chronic LPS infusion has differential effects, depending on dose, on inflammatory, AA and synaptic markers in rat brain. Neuroinflammation associated with upregulated brain AA metabolism can lead to synaptic dysfunction.

Emerging roles of pathogens in Alzheimer disease

Chronic spirochetal infection can cause slowly progressive dementia, cortical atrophy and amyloid deposition in the atrophic form of general paresis. There is a significant association between Alzheimer disease (AD) and various types of spirochete (including the periodontal pathogen Treponemas and Borrelia burgdorferi), and other pathogens such as Chlamydophyla pneumoniae and herpes simplex virus type-1 (HSV-1). Exposure of mammalian neuronal and glial cells and organotypic cultures to spirochetes reproduces the biological and pathological hallmarks of AD. Senile-plaque-like beta amyloid (Aβ) deposits are also observed in mice following inhalation of C. pneumoniae in vivo, and Aβ accumulation and phosphorylation of tau is induced in neurons by HSV-1 in vitro and in vivo. Specific bacterial ligands, and bacterial and viral DNA and RNA all increase the expression of proinflammatory molecules, which activates the innate and adaptive immune systems. Evasion of pathogens from destruction by the host immune reactions leads to persistent infection, chronic inflammation, neuronal destruction and Aβ deposition. Aβ has been shown to be a pore-forming antimicrobial peptide, indicating that Aβ accumulation might be a response to infection. Global attention and action is needed to support this emerging field of research because dementia might be prevented by combined antibiotic, antiviral and anti-inflammatory therapy.

Androgens regulate neprilysin expression: role in reducing beta-amyloid levels

Age-related testosterone depletion in men is a risk factor for Alzheimer's disease. Prior studies suggest that androgens affect Alzheimer's disease risk by regulating accumulation of beta-amyloid protein (Abeta) by an undefined mechanism. In this study, we investigated the role of the Abeta-catabolizing enzyme neprilysin (NEP) in this process. First, we observed that androgens positively regulate neural expression of NEP in adult male rats. Next, we investigated androgen regulatory effects on both NEP expression and Abeta levels using cultured hippocampal neurons and neuronally differentiated rat pheochromocytoma cell 12 with or without androgen receptor (AR). Dihydrotestosterone (DHT) induced a time-dependent increase in NEP expression. DHT also significantly decreased levels of Abeta in AR-expressing cells transfected with amyloid precursor protein, but did not affect levels of either full-length or non-amyloidogenic, soluble amyloid precursor protein. Importantly, the DHT induced decrease of Abeta was blocked by pharmacological inhibition of NEP. The DHT-mediated increase in NEP expression and decrease in Abeta levels were (i) not observed in rat pheochromocytoma cell 12 lacking AR and (ii) blocked in AR-expressing cells by the antagonists, cyproterone acetate and flutamide. Together, these findings suggest that androgen regulation of Abeta involves an AR-dependent mechanism requiring up-regulation of the Abeta catabolizing enzyme NEP.

(-)-Epigallocatechin-3-gallate prevents lipopolysaccharide-induced elevation of beta-amyloid generation and memory deficiency

Neuroinflammation has been known to play a role in the pathogenesis of AD. Our previous study showed that lipopolysaccharide (LPS) induced memory impairment through the accumulation of Abeta via the increase of beta- and gamma-secretase. In this study, we investigated the possible preventive effect of (-)-epigallocatechin-3-gallate (EGCG) on memory deficiency caused by LPS through the inhibition of Abeta(1-42) generation. Oral treatment with EGCG (1.5 and 3 mg/kg, for 3 weeks) into drinking water ameliorated LPS (1 microg/mouse, i.c.v.)-induced memory deficiency in a dose dependent manner. In addition, EGCG also dose-dependently inhibited LPS-induced elevation of Abeta level through attenuation of LPS-induced beta- and gamma-secretase activities and expression of its metabolic products; C99 and Abeta. Moreover, EGCG prevented LPS-induced neuronal cell death as well as the expression of inflammatory proteins, inducible nitric oxide synthetase and cyclooxygenase-2. This study therefore suggests that EGCG prevents LPS-mediated apoptotic cell death through the inhibition of the elevation of Abeta via the inhibition of beta- and gamma-secretases, and thus EGCG can be a useful agent against neuroinflammation-associated development or progression of AD.

Neuro-inflammation induced by lipopolysaccharide causes cognitive impairment through enhancement of beta-amyloid generation

Background

Alzheimer's disease (AD) is characterized by extensive loss of neurons in the brain of AD patients. Intracellular accumulation of beta-amyloid peptide (Aβ) has also shown to occur in AD. Neuro-inflammation has been known to play a role in the pathogenesis of AD.

Methods

In this study, we investigated neuro-inflammation and amyloidogenesis and memory impairment following the systemic inflammation generated by lipopolysaccharide (LPS) using immunohistochemistry, ELISA, behavioral tests and Western blotting.

Results

Intraperitoneal injection of LPS, (250 μg/kg) induced memory impairment determined by passive avoidance and water maze tests in mice. Repeated injection of LPS (250 μg/kg, 3 or 7 times) resulted in an accumulation of Aβ_1–42 in the hippocampus and cerebralcortex of mice brains through increased β- and γ-secretase activities accompanied with the increased expression of amyloid precursor protein (APP), 99-residue carboxy-terminal fragment of APP (C99) and generation of Aβ_1–42 as well as activation of astrocytes in vivo. 3 weeks of pretreatment of sulindac sulfide (3.75 and 7.5 mg/kg, orally), an anti-inflammatory agent, suppressed the LPS-induced amyloidogenesis, memory dysfunction as well as neuronal cell death in vivo. Sulindac sulfide (12.5–50 μM) also suppressed LPS (1 μg/ml)-induced amyloidogenesis in cultured neurons and astrocytes in vitro.

Conclusion

This study suggests that neuro-inflammatory reaction could contribute to AD pathology, and anti-inflammatory agent could be useful for the prevention of AD.

Neprilysin activity and expression are controlled by nicastrin

We recently demonstrated that the presenilin-dependent gamma-secretase complex regulates the expression and activity of neprilysin, one of the main enzymes that degrade the amyloid beta-peptide (Abeta) which accumulates in Alzheimer's disease. Here, we examined the influence of endogenous nicastrin (NCT), a member of the gamma-secretase complex, on neprilysin physiology. We show that nicastrin deficiency drastically lowers neprilysin expression, membrane-bound activity and mRNA levels, but it did not modulate the expression of two other putative Abeta-cleaving enzymes, endothelin-converting enzyme and insulin-degrading enzyme. Furthermore, we show that nicastrin restores neprilysin activity and expression in nicastrin-deficient, but not presenilin-deficient fibroblasts, indicating that the control of neprilysin necessitates the complete gamma-secretase complex harbouring its four reported components. Finally, we show that NCT expression peaked 24 h after NCT cDNA transfection of wild-type and NCT-/- fibroblasts, while neprilysin expression drastically increased only after 36 h and was maximal at 48 h. This delayed effect on neprilysin expression correlates well with our demonstration of an indirect gamma-secretase-dependent modulation of neprilysin at its transcriptional level.

Ascorbic acid decreases neutral endopeptidase activity in cultured osteoblastic cells

Neprilysin (NEP) is a plasma membrane-bound peptidase with wide expression in kidney, lung, brain and bone. Decreased NEP activity has been linked to increased growth of some cancer cells, but it is unknown whether its activity is related to growth of cells belonging to the osteoblast lineage. We assessed NEP activity in an osteoblastic cell line, PyMS, by cleavage of N-Dansyl-d-Ala-Gly-p-nitro-Phe-Gly to Dansyl-d-Ala-Gly. NEP activity was completely blocked by 1 muM thiorphan. Most agents affecting growth of these cells (e.g. calcium, insulin-like growth factor I and dexamethasone) did not regulate NEP activity. Ascorbic acid (ASA) increased thymidine incorporation into DNA and potentiated the stimulatory effect of IGF I on DNA synthesis, an effect which was attenuated by echistatin. ASA decreased NEP activity in a dose-dependent manner, and decreased Western-detectable NEP protein in plasma membranes. ASA affects both integrin receptor-mediated signalling and the processing of regulatory peptides.

Chronic intracerebroventricular infusion of lipopolysaccharide: effects of ibuprofen treatment and behavioural and histopathological correlates

Twenty male Wistar rats were trained under an alternating-lever cyclic-ratio (ALCR) schedule of food reinforcement. When responding showed no trends, each subject was subcutaneously implanted with an Alzet osmotic mini-pump, connected to a chronic indwelling cannula extending into the lateral ventricle of the brain. The mini-pumps were primed to infuse 0.25 microl lipopolysaccharide (LPS) (1.0 microg/0.25 ml) or 0.25 microl artificial cerebrospinal fluid (aCSF) per hour and were implanted for 28 days. LPS infusion produced behavioural deficits which chronic ibuprofen treatment (40 mg/kg every 12 h) alleviated. Infusion of LPS induced R 1282-positive amyloid deposits, and activation of microglia and astrocytes. Ibuprofen treatment reduced the numbers of activated microglia, and withdrawal of ibuprofen resulted in an increase in activated microglia; however, ibuprofen treatment had no effect on numbers of activated astrocytes in the LPS-infused subjects.

Lipopolysaccharide stimulates the secretion of the amyloid precursor protein via a protein kinase C-mediated pathway

The processing of the amyloid precursor protein (APP) by the secretase family of protease enzymes can be influenced by a variety of diverse factors, including elements of the immune response. In this study, we have investigated the effect of the pro-inflammatory lipopolysaccharide (LPS) on APP processing in rat glial cell cultures derived from both cortex and cerebellum. LPS activation of the cells, as monitored by the induction of the pro-inflammatory nitric oxide synthase (iNOS) enzyme, elicited no change in the overall cellular expression levels of APP, although there was a marked concentration-related increase in the secretion of the soluble APPs following both short- (4 h) and long-term (18 h) drug treatment times. The stimulation of APPs secretion was blocked by the protein kinase C (PKC) inhibitor GF109203x, suggesting that LPS may act via a PKC-mediated pathway to increase APPs secretion.

Presenilin-dependent transcriptional control of the Abeta-degrading enzyme neprilysin by intracellular domains of betaAPP and APLP

Amyloid beta-peptide (Abeta), which plays a central role in Alzheimer's disease, is generated by presenilin-dependent gamma-secretase cleavage of beta-amyloid precursor protein (betaAPP). We report that the presenilins (PS1 and PS2) also regulate Abeta degradation. Presenilin-deficient cells fail to degrade Abeta and have drastic reductions in the transcription, expression, and activity of neprilysin, a key Abeta-degrading enzyme. Neprilysin activity and expression are also lowered by gamma-secretase inhibitors and by PS1/PS2 deficiency in mouse brain. Neprilysin activity is restored by transient expression of PS1 or PS2 and by expression of the amyloid intracellular domain (AICD), which is cogenerated with Abeta, during gamma-secretase cleavage of betaAPP. Neprilysin gene promoters are transactivated by AICDs from APP-like proteins (APP, APLP1, and APLP2), but not by Abeta or by the gamma-secretase cleavage products of Notch, N- or E- cadherins. The presenilin-dependent regulation of neprilysin, mediated by AICDs, provides a physiological means to modulate Abeta levels with varying levels of gamma-secretase activity.

Time-dependent reduction in Abeta levels after intracranial LPS administration in APP transgenic mice

Inflammation has been argued to play a primary role in the pathogenesis of Alzheimer's disease (AD). Lipopolysaccharide (LPS) activates the innate immune system, triggering gliosis and inflammation when injected in the central nervous system. In studies described here, APP transgenic mice were injected intrahippocampally with 4 or 10 microg of LPS and evaluated 1, 3, 7, 14, or 28 days later. Abeta load was significantly reduced at 3, 7, and 14 days but surprisingly returned near baseline 28 days after the injection. No effects of LPS on congophilic amyloid deposits could be detected. LPS also activated both microglia and astrocytes in a time-dependent manner. The GFAP astrocyte reaction and the Fcgamma receptor microglial reaction peaked at 7 days after LPS injection, returning to baseline by 2 weeks postinjection. When stained for CD45, microglial activation was detected at all time points, although the morphology of these cells transitioned from an ameboid to a ramified and bushy appearance between 7 and 14 days postinjection. These results indicate that activation of brain glia can rapidly and transiently clear diffuse Abeta deposits but has no effect on compacted fibrillar amyloid.

Discovery of a new class of synthetic protein kinase inhibitors that suppress selective aspects of glial activation and protect against beta-amyloid induced injury: a foundation for future medicinal chemistry efforts focused on targeting Alzheimer's disease progression

A prevailing hypothesis in Alzheimer's disease (AD) research is that chronically activated glia may contribute to neuronal dysfunction, through generation of a detrimental state of neuroinflammation. This raises the possibility in drug discovery research of targeting the cycle of untoward glial activation and neuronal dysfunction that characterizes neuroinflammation. Success over the past century with effective anti-inflammatory drug development, in which the molecular targets are intracellular enzymes involved in signal transduction events and cellular homeostasis, demands that a similar approach be tried with neuroinflammation. Suggestive clinical correlations between inflammation markers and AD contribute to the urgency in addressing the hypothesis that targeting selective glial activation processes might be a therapeutic approach complementary to existing drugs and discovery efforts. An academic collaboratorium initiated a rapid inhibitor discovery effort 2 yr ago, focused on development of novel compounds with new mechanisms of action in AD-relevant cellular processes, in order to obtain the small-molecule compounds required to address the neuroinflammation hypothesis and provide a proof of concept for future medicinal chemistry efforts. We summarize here our progress toward this goal in which novel pyridazine-based inhibitors of gene-regulating protein kinases have been discovered. Feasibility studies indicate their potential utility in current medicinal chemistry efforts focused on improvement in molecular properties and the longer term targeting of AD-related pathogenic processes.

Thiorphan-induced neprilysin inhibition raises amyloid beta levels in rabbit cortex and cerebrospinal fluid

Studies on the pathogenesis of Alzheimer's disease (AD) suggest overproduction of amyloid beta (Abeta) may not be the only pathogenic route to AD. Decreased degradation of Abeta is another possible disease mechanism. Neprilysin is a neutral endopeptidase that has been proposed to be the major enzyme responsible for Abeta degradation. Studies have reported correlations between Abeta deposition and neprilysin activity in the human brain. This study shows that intracerebroventricular infusion of thiorphan, a neprilysin inhibitor, raises cortical and cerebrospinal fluid (CSF) Abeta concentrations in rabbits. Rabbits treated with thiorphan for 5 days had levels of CSF and cortical Abeta40 that were 147 and 142% of the control group, respectively. Results for Abeta42 showed a similar trend. The results indicate that age-related decreases of neprilysin could lead to increased brain concentrations of Abeta, plaque formation, and AD.

Human Abeta1-42 reduces iron-induced toxicity in rat cerebral cortex

Senile plaques, the major neuropathological lesions of Alzheimer's disease (AD), are composed primarily of amyloid-beta (Abeta) peptide and contain high concentrations of iron (1.0 mM). We have previously shown that intracortical injections of 1.0 mM iron to adult rats produce significantly more neuronal loss than control injections of saline vehicle, whereas injections of Abeta do not. Because iron has been shown to increase the in vitro toxicity of Abeta, the present study was undertaken to determine whether iron can make Abeta neurotoxic in vivo. Abeta and 1.0 mM iron (as ferric ammonium citrate) were coinjected into rat cerebral cortex, and the neuronal loss was compared with that produced by pure Abeta or pure iron. The human and rat variants of Abeta(1-42) were compared to determine whether they produce the same amount of neuronal loss when combined with iron. Coinjection of iron with either Abeta variant caused significantly more neuronal loss than Abeta peptide alone, suggesting that iron may contribute to the toxicity associated with senile plaques. Rat Abeta(1-42) combined with iron was as toxic as iron alone, whereas iron combined with human Abeta(1-42) was significantly less toxic. This latter finding indicates that fibrillar human Abeta is able to reduce iron-induced neurotoxicity in vivo and raises the interesting possibility that senile plaques in AD may represent a neuroprotective response to the presence of elevated metal ions.

Microglial activation induces cell death, inhibits neurite outgrowth and causes neurite retraction of differentiated neuroblastoma cells

Activation of glial cells has been proposed to contribute to neuronal dysfunction and neuronal cell death in Alzheimer's disease. In this study, we attempt to determine some of the effects of secreted factors from activated murine N-11 microglia on viability and morphology of neurons using the differentiated neuroblastoma cell line Neuro2a. Microglia were activated either by lipopolysaccharide (LPS), bacterial cell wall proteoglycans, or advanced glycation endproducts (AGEs), protein-bound sugar oxidation products. At high LPS or AGE concentrations, conditioned medium from microglia caused neuronal cell death in a dose-dependent manner. At sublethal LPS or AGE concentrations, conditioned media inhibited retinoic acid-induced neurite outgrowth and stimulated retraction of already extended neurites. Among the many possible secreted factors, the contribution of NO or NO metabolites in the cytotoxicity of conditioned medium was investigated. Cell death and changes in neurite morphology were partly reduced when NO production was inhibited by nitric oxide synthase inhibitors. The results suggest that even in the absence of significant cell death, inflammatory processes, which are partly transmitted via NO metabolites, may affect intrinsic functions of neurons such as neurite extension that are essential components of neuronal morphology and thus may contribute to degenerative changes in Alzheimer's disease.