Amyloid-beta peptide induces temporal membrane biphasic changes in astrocytes through cytosolic phospholipase A2

Metabolic profiling of Alzheimer's disease: Untargeted metabolomics analysis of plasma samples

Alzheimer's disease (AD) is often not recognized or is diagnosed very late, which significantly reduces the effectiveness of available pharmacological treatments. Metabolomic analyzes have great potential for improving existing knowledge about the pathogenesis and etiology of AD and represent a novel approach towards discovering biomarkers that could be used for diagnosis, prognosis, and therapy monitoring. In this study, we applied the untargeted metabolomic approach to investigate the changes in biochemical pathways related to AD pathology. We used gas chromatography and liquid chromatography coupled to mass spectrometry (GC-MS and LC-MS, respectively) to identify metabolites whose levels have changed in subjects with AD diagnosis (N = 40) compared to healthy controls (N = 40) and individuals with mild cognitive impairment (MCI, N = 40). The GC-MS identified significant differences between groups in levels of metabolites belonging to the classes of benzene and substituted derivatives, carboxylic acids and derivatives, fatty acyls, hydroxy acids and derivatives, keto acids and derivatives, and organooxygen compounds. Most of the compounds identified by the LC-MS were various fatty acyls, glycerolipids and glycerophospholipids. All of these compounds were decreased in AD patients and in subjects with MCI compared to healthy controls. The results of the study indicate disturbed metabolism of lipids and amino acids and an imbalance of metabolites involved in energy metabolism in individuals diagnosed with AD, compared to healthy controls and MCI subjects.

Increased Expression and Activity of Brain Cortical cPLA2 Due to Chronic Lipopolysaccharide Administration in Mouse Model of Familial Alzheimer's Disease

Cytosolic phospholipase A2 (cPLA2) is an enzyme regulating membrane phospholipid homeostasis and the release of arachidonic acid utilized in inflammatory responses. It represents an attractive target for the treatment of Alzheimer's disease (AD). Previously, we showed that lipopolysaccharide (LPS)-induced systemic inflammation caused abnormal lipid metabolism in the brain of a transgenic AD mouse model (APdE9), which might be associated with potential changes in cPLA2 activity. Here, we investigated changes in cPLA2 expression and activity, as well as the molecular mechanisms underlying these alterations due to chronic LPS administration in the cerebral cortex of female APdE9 mice as compared to saline- and LPS-treated female wild-type mice and saline-treated APdE9 mice. The study revealed the significant effects of genotype LPS treatment on cortical cPLA2 protein expression and activity in APdE9 mice. LPS treatment resulted in nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB) activation in the cortex of APdE9 mice. The gene expressions of inflammation markers Il1b and Tnfa were significantly elevated in the cortex of both APdE9 groups compared to the wild-type groups. The study provides evidence of the elevated expression and activity of cPLA2 in the brain cortex of APdE9 mice after chronic LPS treatment, which could be associated with NFkB activation.

Alteration of plasma metabolic profile and physical performance combined with metabolites is more sensitive to early screening for mild cognitive impairment

Objective

Unbiased metabolic profiling has been initiated to identify novel metabolites. However, it remains a challenge to define reliable biomarkers for rapid and accurate diagnosis of mild cognitive impairment (MCI). Our study aimed to evaluate the association of serum metabolites with MCI, attempting to find new biomarkers and combination models that are distinct for MCI.

Methods

A total of 380 participants were recruited (mean age: 72.5 ± 5.19 years). We performed an untargeted metabolomics analysis on older adults who underwent the Mini-Mental State Examination (MMSE), the Instrumental Activities of Daily Living (IADL), and physical performance tests such as hand grip, Timed Up and Go Test (TUGT), and walking speed. Orthogonal partial least squares discriminant analysis (OPLS-DA) and heat map were utilized to distinguish the metabolites that differ between groups.

Results

Among all the subjects, 47 subjects were diagnosed with MCI, and methods based on the propensity score are used to match the MCI group with the normal control (NC) group (n = 47). The final analytic sample comprised 94 participants (mean age: 75.2 years). The data process from the metabolic profiles identified 1,008 metabolites. A cluster and pathway enrichment analysis showed that sphingolipid metabolism is involved in the development of MCI. Combination of metabolite panel and physical performance were significantly increased discriminating abilities on MCI than a single physical performance test [model 1: the area under the curve (AUC) = 0.863; model 2: AUC = 0.886; and model 3: AUC = 0.870, P < 0.001].

Conclusion

In our study, untargeted metabolomics was used to detect the disturbance of metabolism that occurs in MCI. Physical performance tests combined with phosphatidylcholines (PCs) showed good utility in discriminating between NC and MCI, which is meaningful for the early diagnosis of MCI.

The Impact of the hAPP695SW Transgene and Associated Amyloid-β Accumulation on Murine Hippocampal Biochemical Pathways

Background: Alzheimer’s disease (AD) is a neurodegenerative disease characterized by the accumulation of amyloid-β (Aβ) peptide in the brain. Objective: Gain a better insight into alterations in major biochemical pathways underlying AD. Methods: We compared metabolomic profiles of hippocampal tissue of 20-month-old female Tg2576 mice expressing the familial AD-associated hAPP695SW transgene with their 20-month-old wild type female littermates. Results: The hAPP695SW transgene causes overproduction and accumulation of Aβ in the brain. Out of 180 annotated metabolites, 54 metabolites differed (30 higher and 24 lower in Tg2576 versus wild-type hippocampal tissue) and were linked to the amino acid, nucleic acid, glycerophospholipid, ceramide, and fatty acid metabolism. Our results point to 1) heightened metabolic activity as indicated by higher levels of urea, enhanced fatty acid β-oxidation, and lower fatty acid levels; 2) enhanced redox regulation; and 3) an imbalance of neuro-excitatory and neuro-inhibitory metabolites in hippocampal tissue of aged hAPP695SW transgenic mice. Conclusion: Taken together, our results suggest that dysregulation of multiple metabolic pathways associated with a concomitant shift to an excitatory-inhibitory imbalance are contributing mechanisms of AD-related pathology in the Tg2576 mouse.

Evaluating membrane structure by Laurdan imaging: Disruption of lipid packing by oxidized lipids

Impact of different lipids on membrane structure/lipid order is critical for multiple biological processes. Laurdan microscopy provides a unique tool to assess this property in heterogeneous biological membranes. This review describes the general principles of the approach and its application in model membranes and cells. It also provides an in-depth discussion of the insights obtained using Laurdan microscopy to evaluate the differential effects of cholesterol, oxysterols and oxidized phospholipids on lipid packing of ordered and disordered domains in vascular endothelial cells.

Dynamic Role of Phospholipases A2 in Health and Diseases in the Central Nervous System

Phospholipids are major components in the lipid bilayer of cell membranes. These molecules are comprised of two acyl or alkyl groups and different phospho-base groups linked to the glycerol backbone. Over the years, substantial interest has focused on metabolism of phospholipids by phospholipases and the role of their metabolic products in mediating cell functions. The high levels of polyunsaturated fatty acids (PUFA) in the central nervous system (CNS) have led to studies centered on phospholipases A2 (PLA2s), enzymes responsible for cleaving the acyl groups at the sn-2 position of the phospholipids and resulting in production of PUFA and lysophospholipids. Among the many subtypes of PLA2s, studies have centered on three major types of PLA2s, namely, the calcium-dependent cytosolic cPLA2, the calcium-independent iPLA2 and the secretory sPLA2. These PLA2s are different in their molecular structures, cellular localization and, thus, production of lipid mediators with diverse functions. In the past, studies on specific role of PLA2 on cells in the CNS are limited, partly because of the complex cellular make-up of the nervous tissue. However, understanding of the molecular actions of these PLA2s have improved with recent advances in techniques for separation and isolation of specific cell types in the brain tissue as well as development of sensitive molecular tools for analyses of proteins and lipids. A major goal here is to summarize recent studies on the characteristics and dynamic roles of the three major types of PLA2s and their oxidative products towards brain health and neurological disorders.

Lipids: biomarkers of healthy aging

Life expectancy, and longevity have been increasing in recent years. However, this is, in most cases, accompanied by age-related diseases. Thus, it became essential to better understand the mechanisms inherent to aging, and to establish biomarkers that characterize this physiological process. Among all biomolecules, lipids appear to be a good target for the study of these biomarkers. In fact, some lipids have already been associated with age-related diseases. With the development of analytical techniques such as Mass Spectrometry, and Nuclear Magnetic Resonance, Lipidomics has been increasingly used to study pathological, and physiological states of an organism. Thus, the study of serum, and plasma lipidome in centenarians, and elderly individuals without age-related diseases can be a useful tool for the identification of aging biomarkers, and to understand physiological aging, and longevity. This review focus on the importance of lipids as biomarkers of aging, and summarize the changes in the lipidome that have been associated with aging, and longevity.

Plasma lipidomic biomarker analysis reveals distinct lipid changes in vascular dementia

Vascular dementia (VaD) is a complex neurocognitive disorder secondary to a variety of cerebrovascular lesions. Numerous studies have shown that lipid metabolism is involved in the pathobiology of the disease. We examined the plasma lipid profiles in VaD, with the expectation of identifying reliable lipid biomarkers for VaD. 49 VaD patients and 48 healthy controls were recruited from Bankstown-Lidcombe Hospital in Sydney, Australia. Lipids were extracted by single phase 1-butanol/methanol, and untargeted analysis was performed by liquid chromatography coupled-mass spectrometry (LC–MS/MS). Univariate analysis of variance was used to examine the differences in lipid classes and individual lipids between VaD and control groups. In an independent sample of 161 subjects from the Older Australian Twins Study (OATS), elastic net penalization for the generalized linear model (Glmnet) and Random Forest were applied to the lipid levels to subcategorise the sample into vascular cognitive impairment and controls. Most lipids belonging to the classes of ceramides (Cer), cholesterol esters (ChE) and phospholipids were significantly lower in VaD plasma, while glycerides were elevated compared to controls. Levels of ChE, Cer and the two lipid classes together achieved the best accuracy in discriminating VaD from controls, with more than 80% accuracy. The probable VaD group in the OATS sample predicted by the lipid levels showed greater impairment in most cognitive domains, especially attention and processing speed and executive function from controls but did not differ in white matter hyperintensities and DTI measures. As a conclusion, plasma lipids levels, in particular Cer and ChE, are abnormal in VaD and may help discriminate them from healthy controls. Understanding the basis of these differences may provide insights into the pathobiology of VaD.

Roles of microglial membranes in Alzheimer's disease

The majority of Alzheimer's disease (AD) risk genes are highly and selectively expressed by microglia in the brain. Several of these genes are related to lipid and cholesterol metabolism, lipid synthesis, lipid transport, endocytosis, exocytosis and phagocytosis. Therefore, studying the roles of cellular membrane biophysics in microglial function should improve our understanding of the AD pathology. In this chapter, we discuss how lipid rafts and membrane-cytoskeleton adhesion impact microglial-mediated oxidative stress and clearance of amyloid-β peptide (Aβ). We also discuss potential roles of lipid membrane-bound extracellular vesicles as carriers of pathological factors to promote inflammation and cytotoxicity.

UHPLC-QTOF/MS-based metabolomics investigation for the protective mechanism of Danshen in Alzheimer's disease cell model induced by Aβ1-42

INTRODUCTION

Alzheimer's disease (AD) is a chronic neurodegenerative disorder with neither definitive pathogenesis nor effective therapy so far. Danshen, the dried root and rhizome of Salvia miltiorrhiza Bunge, is used extensively in Alzheimer's disease treatment to ameliorate the symptoms, but the underlying mechanism remains to be clarified.

OBJECTIVES

To investigate potential biomarkers for AD and elucidate the protective mechanism of Danshen on AD cell model.

METHODS

An ultra high performance liquid chromatography-quadrupole time of flight mass spectrometry (UHPLC-QTOF/MS)-based approach combined with partial least squares discriminant analysis (PLS-DA) has been developed to discriminate the metabolic modifications between human brain microvascular endothelial cell (hBMEC) and AD cell model induced by amyloid-β protein (Aβ_1-42). To further elucidate the pathophysiology of AD, related metabolic pathways have been studied.

RESULTS

Thirty-three distinct potential biomarkers were screened out and considered as potential biomarkers corresponding to AD, which were mostly improved and partially restored back to normalcy in Danshen pre-protection group. It was found that AD was closely related to disturbed arginine and proline metabolism, glutathione metabolism, alanine aspartate and glutamate metabolism, histidine metabolism, pantothenate and CoA biosynthesis, phenylalanine tyrosine and tryptophan biosynthesis, citrate cycle and glycerophospholipid metabolism, and the protective mechanism of Danshen in AD cell model may be related to partially regulating the perturbed pathways.

CONCLUSIONS

These outcomes provide valuable evidences for therapeutic mechanism investigation of Danshen in AD treatment, and such an approach could be transferred to unravel the mechanism of other traditional Chinese medicine (TCM) and diseases.

Molecular rotors report on changes in live cell plasma membrane microviscosity upon interaction with beta-amyloid aggregates

Amyloid deposits of aggregated beta-amyloid Aβ(1-42) peptides are a pathological hallmark of Alzheimer's disease. Aβ(1-42) aggregates are known to induce biophysical alterations in cells, including disruption of plasma membranes. We investigated the microviscosity of plasma membranes upon interaction with oligomeric and fibrillar forms of Aβ(1-42). Viscosity-sensing fluorophores termed molecular rotors were utilised to directly measure the microviscosities of giant plasma membrane vesicles (GPMVs) and plasma membranes of live SH-SY5Y and HeLa cells. The fluorescence lifetimes of membrane-inserting BODIPY-based molecular rotors revealed a decrease in bilayer microviscosity upon incubation with Aβ(1-42) oligomers, while fibrillar Aβ(1-42) did not significantly affect the microviscosity of the bilayer. In addition, we demonstrate that the neuroprotective peptide H3 counteracts the microviscosity change induced by Aβ(1-42) oligomers, suggesting the utility of H3 as a neuroprotective therapeutic agent in neurodegenerative disorders and indicating that ligand-induced membrane stabilisation may be a possible mechanism of neuroprotection during neurodegenerative disorders such as Alzheimer's disease.

Cytosolic Phospholipase A2 Facilitates Oligomeric Amyloid-β Peptide Association with Microglia via Regulation of Membrane-Cytoskeleton Connectivity

Cytosolic phospholipase A₂ (cPLA₂) mediates oligomeric amyloid-β peptide (oAβ)-induced oxidative and inflammatory responses in glial cells. Increased activity of cPLA₂ has been implicated in the neuropathology of Alzheimer's disease (AD), suggesting that cPLA₂ regulation of oAβ-induced microglial activation may play a role in the AD pathology. We demonstrate that LPS, IFNγ, and oAβ increased phosphorylated cPLA₂ (p-cPLA₂) in immortalized mouse microglia (BV2). Aβ association with primary rat microglia and BV2 cells, possibly via membrane-binding and/or intracellular deposition, presumably indicative of microglia-mediated clearance of the peptide, was reduced by inhibition of cPLA₂. However, cPLA₂ inhibition did not affect the depletion of this associated Aβ when cells were washed and incubated in a fresh medium after oAβ treatment. Since the depletion was abrogated by NH₄Cl, a lysosomal inhibitor, these results suggested that cPLA₂ was not involved in the degradation of the associated Aβ. To further dissect the effects of cPLA₂ on microglia cell membranes, atomic force microscopy (AFM) was used to determine endocytic activity. The force for membrane tether formation (F_mtf) is a measure of membrane-cytoskeleton connectivity and represents a mechanical barrier to endocytic vesicle formation. Inhibition of cPLA₂ increased F_mtf in both unstimulated BV2 cells and cells stimulated with LPS + IFNγ. Thus, increasing p-cPLA₂ would decrease F_mtf, thereby increasing endocytosis. These results suggest a role of cPLA₂ activation in facilitating oAβ endocytosis by microglial cells through regulation of the membrane-cytoskeleton connectivity.

Effects of breviscapine on amyloid beta 1-42 induced Alzheimer's disease mice: A HPLC-QTOF-MS based plasma metabonomics study

Herba Erigerontis has long been used to cure apoplexy hemiplegia and precordial pain in China. In addition, the bioactivities of its total flavonoids-breviscapine included inhibiting amyloid beta (Aβ) fibril formation, antioxidation and metal chelating, which are beneficial to treat Alzheimer's disease (AD). Hence, A HPLC-QTOF-MS based plasma metabonomics approach was applied to investigate the neuroprotective effects of breviscapine on intracerebroventricular injection of aggregated Aβ 1-42 induced AD mice for the first time in the study. Ten potential biomarkers were screened out by multivariate statistical analysis, eight of which were further identified as indoleacrylic acid, C16 sphinganine, LPE (22:6), sulfolithocholic acid, LPC (16:0), PA (22:1/0:0), taurodeoxycholic acid, and PC (0:0/18:0). According to their metabolic pathways, it was supposed that breviscapine ameliorated the learning and memory deficits of AD mice predominantly by regulating phospholipids metabolism, elevating serotonin level and lowering cholesterols content in vivo.

The molecular behavior of a single β-amyloid inside a dipalmitoylphosphatidylcholine bilayer at three different temperatures: An atomistic simulation study: Aβ interaction with DPPC: Atomistic simulation

The behavior of a single Aβ40 molecule within a dipalmitoylphosphatidylcholine (DPPC) bilayer was studied by all-atom molecular dynamics simulations. The effect of membrane structure was investigated on Aβ40 behavior, secondary structure, and insertion depth. Simulations were performed at three temperatures (323, 310, and 300 K) to probe three different bilayer fluidities. Results show that at all above temperatures, the peptide contains two short helices, coil, bend, and turn structures. At 300 K, the peptide contains a region with β structure in C-terminal region. Our results also show that Aβ decreases the bilayer thickness and the order of lipids in its vicinity which leads to water insertion into the bilayer and concomitant increase in the local fluidity. The peptide remains embedded in the bilayer at all temperatures, and become inserted into the bilayer up to several residues at 323 and 310 K. At 310 and 300 K, the dominant interaction energy between Aβ and bilayer changes from electrostatic to van der Waals. It can be proposed that at higher temperatures (e.g., 323 K), Lys28 and the C-terminal region of the peptide play the role of two anchors that keep Aβ inside the top leaflet. This study demonstrates that Aβ molecule can perturb the integrity of cellular membranes. Proteins 2017; 85:1298-1310. © 2017 Wiley Periodicals, Inc.

Dietary arachidonic acid as a risk factor for age-associated neurodegenerative diseases: Potential mechanisms

Alzheimer's disease and associated diseases constitute a major public health concern worldwide. Nutrition-based, preventive strategies could possibly be effective in delaying the occurrence of these diseases and lower their prevalence. Arachidonic acid is the second major polyunsaturated fatty acid (PUFA) and several studies support its involvement in Alzheimer's disease. The objective of this review is to examine how dietary arachidonic acid contributes to Alzheimer's disease mechanisms and therefore to its prevention. First, we explore the sources of neuronal arachidonic acid that could potentially originate from either the conversion of linoleic acid, or from dietary sources and transfer across the blood-brain-barrier. In a second part, a brief overview of the role of the two main agents of Alzheimer's disease, tau protein and Aβ peptide is given, followed by the examination of the relationship between arachidonic acid and the disease. Third, the putative mechanisms by which arachidonic acid could influence Alzheimer's disease occurrence and evolution are presented. The conclusion is devoted to what remains to be determined before integrating arachidonic acid in the design of preventive strategies against Alzheimer's disease and other neurodegenerative diseases.

Signaling pathways regulating neuron-glia interaction and their implications in Alzheimer's disease

Astrocytes are the most abundant cells in the central nervous system. They play critical roles in neuronal homeostasis through their physical properties and neuron-glia signaling pathways. Astrocytes become reactive in response to neuronal injury and this process, referred to as reactive astrogliosis, is a common feature accompanying neurodegenerative conditions, particularly Alzheimer's disease. Reactive astrogliosis represents a continuum of pathobiological processes and is associated with morphological, functional, and gene expression changes of varying degrees. There has been a substantial growth of knowledge regarding the signaling pathways regulating glial biology and pathophysiology in recent years. Here, we attempt to provide an unbiased review of some of the well-known players, namely calcium, proteoglycan, transforming growth factor β, NFκB, and complement, in mediating neuron-glia interaction under physiological conditions as well as in Alzheimer's disease. This review discusses the role of astrocytic NFκB and calcium as well as astroglial secreted factors, including proteoglycans, TGFβ, and complement in mediating neuronal function and AD pathogenesis through direct interaction with neurons and through cooperation with microglia.

The ratio of phosphatidylcholines to lysophosphatidylcholines in plasma differentiates healthy controls from patients with Alzheimer's disease and mild cognitive impairment

BACKGROUND

Metabolomic processes have been identified as being strongly linked to the development of Alzheimer's disease (AD). Thus, lipid metabolites appear to be highly useful as diagnostic substrates for the diagnosis of AD and mild cognitive impairment (MCI) in plasma.

METHODS

We analyzed plasma samples from controls (n = 35), MCI (n = 33), and AD patients (n = 43) using the AbsoluteIDQ p180 Kit (Biocrates Life Sciences), which included quantitative analysis of 40 acylcarnitines, 21 amino acids, 19 biogenic amines, 15 sphingolipids, 90 glycerophospholipids, and sum of hexoses.

RESULTS

We found that individual lipid metabolites can differentiate controls from MCI and AD with relevant significance. However, the ratio between PC aa C34:4 and lysoPC a C18:2 differentiates controls from MCI (P = .0000007) and from AD (P = .0000009) with greater significance.

CONCLUSIONS

The results provide evidence that the ratio of these two lipid metabolites is useful for diagnosing MCI and AD with an accuracy of 82%-85%.

Role of membrane biophysics in Alzheimer's-related cell pathways

Cellular membrane alterations are commonly observed in many diseases, including Alzheimer's disease (AD). Membrane biophysical properties, such as membrane molecular order, membrane fluidity, organization of lipid rafts, and adhesion between membrane and cytoskeleton, play an important role in various cellular activities and functions. While membrane biophysics impacts a broad range of cellular pathways, this review addresses the role of membrane biophysics in amyloid-β peptide aggregation, Aβ-induced oxidative pathways, amyloid precursor protein processing, and cerebral endothelial functions in AD. Understanding the mechanism(s) underlying the effects of cell membrane properties on cellular processes should shed light on the development of new preventive and therapeutic strategies for this devastating disease.

Platelet-activating factor antagonists enhance intracellular degradation of amyloid-β42 in neurons via regulation of cholesterol ester hydrolases

INTRODUCTION

The progressive dementia that is characteristic of Alzheimer's disease is associated with the accumulation of amyloid-beta (Aβ) peptides in extracellular plaques and within neurons. Aβ peptides are targeted to cholesterol-rich membrane micro-domains called lipid rafts. Observations that many raft proteins undertake recycling pathways that avoid the lysosomes suggest that the accumulation of Aβ in neurons may be related to Aβ targeting lipid rafts. Here we tested the hypothesis that the degradation of Aβ by neurons could be increased by drugs affecting raft formation.

METHODS

Primary neurons were incubated with soluble Aβ preparations. The amounts of Aβ42 in neurons or specific cellular compartments were measured by enzyme-linked immunosorbent assay. The effects of drugs on the degradation of Aβ42 were studied.

RESULTS

Aβ42 was targeted to detergent-resistant, low-density membranes (lipid rafts), trafficked via a pathway that avoided the lysosomes, and was slowly degraded by neurons (half-life was greater than 5 days). The metabolism of Aβ42 was sensitive to pharmacological manipulation. In neurons treated with the cholesterol synthesis inhibitor squalestatin, less Aβ42 was found within rafts, greater amounts of Aβ42 were found in lysosomes, and the half-life of Aβ42 was reduced to less than 24 hours. Treatment with phospholipase A2 inhibitors or platelet-activating factor (PAF) antagonists had the same effects on Aβ42 metabolism in neurons as squalestatin. PAF receptors were concentrated in the endoplasmic reticulum (ER) along with enzymes that constitute the cholesterol ester cycle. The addition of PAF to ER membranes triggered activation of cholesterol ester hydrolases and the release of cholesterol from stores of cholesterol esters. An inhibitor of cholesterol ester hydrolases (diethylumbelliferyl phosphate) also increased the degradation of Aβ42 in neurons.

CONCLUSIONS

We conclude that the targeting of Aβ42 to rafts in normal cells is a factor that affects its degradation. Critically, pharmacological manipulation of neurons can significantly increase Aβ42 degradation. These results are consistent with the hypothesis that the Aβ-induced production of PAF controls a cholesterol-sensitive pathway that affects the cellular localization and hence the fate of Aβ42 in neurons.

Cellular membrane fluidity in amyloid precursor protein processing

The senile plaque is a pathologic hallmark of Alzheimer's disease (AD). Amyloid-β peptide (Aβ), the main constituent of senile plaques, is neurotoxic especially in its oligomeric form. Aβ is derived from the sequential cleavage of amyloid precursor protein (APP) by β- and γ-secretases in the amyloidogenic pathway. Alternatively, APP can be cleaved by α-secretases within the Aβ domain to produce neurotrophic and neuroprotective α-secretase-cleaved soluble APP (sAPPα) in the nonamyloidogenic pathway. Since APP and α-, β-, and γ-secretases are membrane proteins, APP processing should be highly dependent on the membrane composition and the biophysical properties of cellular membrane. In this review, we discuss the role of the biophysical properties of cellular membrane in APP processing, especially the effects of phospholipases A(2) (PLA(2)s), fatty acids, cholesterol, and Aβ on membrane fluidity in relation to their effects on APP processing.

Metabolic Dysfunction of Astrocyte: An Initiating Factor in Beta-amyloid Pathology?

Astrocytes, the most important energy regulator in the brain, support brain energy needs. In the meantime, numerous studies have demonstrated that impaired brain glucose metabolism is closely linked to abnormal astrocytic metabolism in AD. Indeed, the interaction between amyloid plaques and perturbed astrocytic homeostasis contributes to AD pathogenesis and astrocytic metabolic dysfunction is thought to be a trigger for Aβ pathology through oxidative stress and neuroinflammation Moreover, astrocytic metabolic dysfunction may regulate Aβ generation via modulating proteolytic processing of amyloid precursor protein (APP) by β-secretase, γ-secretase, and α-secretase, and may also modulate APP post-translational modifications such as glycosylation, phosphorylation, and tyrosine sulfation. While it is known that metabolic dysfunction of astrocytes contributes to the failure of Aβ clearance, it has also been reported that such dysfunction has neuroprotective property and exhibits no detrimental outcomes. Therefore, the exact role of astrocytic metabolic dysfunction in Aβ pathology remains to be further investigated.

Ca(2+) -dependent endoplasmic reticulum stress correlates with astrogliosis in oligomeric amyloid β-treated astrocytes and in a model of Alzheimer's disease

Neurotoxic effects of amyloid β peptides are mediated through deregulation of intracellular Ca(2+) homeostasis and signaling, but relatively little is known about amyloid β modulation of Ca(2+) homeostasis and its pathological influence on glia. Here, we found that amyloid β oligomers caused a cytoplasmic Ca(2+) increase in cultured astrocytes, which was reduced by inhibitors of PLC and ER Ca(2+) release. Furthermore, amyloid β peptides triggered increased expression of glial fibrillary acidic protein (GFAP), as well as oxidative and ER stress, as indicated by eIF2α phosphorylation and overexpression of chaperone GRP78. These effects were decreased by ryanodine and 2APB, inhibitors of ryanodine receptors and InsP3 receptors, respectively, in both primary cultured astrocytes and organotypic cultures of hippocampus and entorhinal cortex. Importantly, intracerebroventricular injection of amyloid β oligomers triggered overexpression of GFAP and GRP78 in astrocytes of the hippocampal dentate gyrus. These data were validated in a triple-transgenic mouse model of Alzheimer's disease (AD). Overexpression of GFAP and GRP78 in the hippocampal astrocytes correlated with the amyloid β oligomer load in 12-month-old mice, suggesting that this parameter drives astrocytic ER stress and astrogliosis in vivo. Together, these results provide evidence that amyloid β oligomers disrupt ER Ca(2+) homeostasis, which induces ER stress that leads to astrogliosis; this mechanism may be relevant to AD pathophysiology.

Mechanism of oxidative stress in neurodegeneration

Biological tissues require oxygen to meet their energetic demands. However, the consumption of oxygen also results in the generation of free radicals that may have damaging effects on cells. The brain is particularly vulnerable to the effects of reactive oxygen species due to its high demand for oxygen, and its abundance of highly peroxidisable substrates. Oxidative stress is caused by an imbalance in the redox state of the cell, either by overproduction of reactive oxygen species, or by dysfunction of the antioxidant systems. Oxidative stress has been detected in a range of neurodegenerative disease, and emerging evidence from in vitro and in vivo disease models suggests that oxidative stress may play a role in disease pathogenesis. However, the promise of antioxidants as novel therapies for neurodegenerative diseases has not been borne out in clinical studies. In this review, we critically assess the hypothesis that oxidative stress is a crucial player in common neurodegenerative disease and discuss the source of free radicals in such diseases. Furthermore, we examine the issues surrounding the failure to translate this hypothesis into an effective clinical treatment.

Role of Aβ-receptor for advanced glycation endproducts interaction in oxidative stress and cytosolic phospholipase A₂ activation in astrocytes and cerebral endothelial cells

Blood-brain barrier (BBB) dysfunctions have been implicated in the progression of Alzheimer's disease. Cerebral endothelial cells (CECs) and astrocytes are the main cell components of the BBB. Although amyloid-β oligomers (Aβ₄₂) have been reported to mediate oxidative damage to the CECs and astrocytes and trigger the downstream mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, the cell surface binding site for Aβ₄₂ and exact sequence of these events have yet to be elucidated. In this study, the receptor for advanced glycation endproducts (RAGE) was postulated to function as a signal transducing cell surface receptor for Aβ₄₂ to induce reactive oxygen species (ROS) generation from NADPH oxidase and trigger downstream pathways for the phosphorylation of extracellular signal-regulated kinases (ERK1/2) and cytosolic phospholipase A₂ (cPLA₂). We found that Aβ₄₂ competed with the anti-RAGE antibody (Ab(RAGE)) to bind to RAGE on the surfaces of CECs and primary astrocytes. In addition, Ab(RAGE) abrogate Aβ₄₂-induced ROS production and the colocalization between the cytosolic (p47-phox) and membrane (gp91-phox) subunits of NADPH oxidase in both cell types. Ab(RAGE) as well as NADPH oxidase inhibitor and ROS scavenger suppressed Aβ₄₂-induced ERK1/2 and cPLA₂ phosphorylation in CECs. At the same time, only Ab(RAGE), but neither NADPH oxidase inhibitor nor ROS scavenger, inhibited the ERK1/2 pathway and cPLA₂ phosphorylation in primary astrocytes. Therefore, this study demonstrates that NADPH oxidase complex assembly and ROS production are not required for Aβ₄₂ binding to RAGE at astrocytic surface leading to sequential phosphorylation of ERK1/2 and cPLA₂, and suggests the presence of two different RAGE-dependent downstream pathways in the CECs and astrocytes.

From brain to food: analysis of phosphatidylcholins, lyso-phosphatidylcholins and phosphatidylcholin-plasmalogens derivates in Alzheimer's disease human post mortem brains and mice model via mass spectrometry

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by extracellular senile plaques mainly consisting of Aβ, a 40-42 amino acid long peptide, and intracellular neurofibrillary tangles, accompanied by an excessive loss of synapses. Recently evidence accumulated that nutrition, especially polyunsaturated fatty acids, influences AD pathogenesis. Especially mid-life food habits with the consumption of specific fatty acids (FA) appear to influence the disease risk. The timely separation between food intake and disease makes a direct correlation with detailed analysis of eating habits combined with accurate food analysis nearly unattainable. A possible solution to circumvent these difficulties is to investigate the FA composition in human post mortem brain. In this study we focused on the main phospholipids phosphatidylcholin (PC), phosphatidylcholin-plasmalogen (PC-PL) and lyso-phosphatidylcholin (lyso-PC) in AD brains compared to control brains. Frontal cortices, temporal cortices and cerebellum of 30 AD (mean 78 years) and 14 control aged matched brains (mean 77.4 years) as well as APP transgenic mice compared to control mice were analyzed using an AB Sciex 4000 Qtrap mass spectrometer utilizing a FIA MS/MS method. PC, PC-PL and lyso-PC metabolites were analyzed in respect to saturation level and FA composition. As expected, the majority of the lipid species showed no significant differences, but interestingly a few species revealed a highly significant reduction in AD brains. These FAs are potential candidates for further food analysis in respect to AD pathology. Additionally, we show that the method applied with multiple reaction monitoring (MRM) used for this study is suitable for semi quantitative analysis of small amounts (10 μl) of brain tissue.

4-Hydroxy-2-nonenal upregulates and phosphorylates cytosolic phospholipase A(2) in cultured Ra2 microglial cells via MAPK pathways

Several studies have suggested the involvement of neuroinflammation in the pathomechanism of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). We recently demonstrated increased levels of protein-bound 4-hydroxy-2-nonenal (HNE) as a highly reactive lipid peroxidation product and cytosolic phospholipase A(2) (cPLA(2)) as a proinflammatory enzyme in glial cells as well as motor neurons in the spinal cord of sporadic ALS patients. However, a link between HNE and cPLA(2) in ALS remains to be determined. To address this issue, we investigated effects of HNE stimulation on the state of cPLA(2) expression in cultured microglial cell line (Ra2). Exposure of Ra2 cells to HNE significantly increased expression levels of cPLA(2) and its activated form phosphorylated at amino acid residue S(505) (p-cPLA(2)) on immunoblots. Pretreatment of Ra2 cells with the antioxidant N-acetylcysteine, the extracellular signal-regulated kinase (ERK) inhibitor PD98059 or the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 prevented the HNE-induced increased expression of cPLA(2) and p-cPLA(2). Immunocytochemical analysis revealed that staining for p-cPLA(2) in Ra2 cells was localized in the cytoplasm and more intense in the HNE-stimulated group than in the vehicle group. The present results provide in vitro evidence that HNE upregulates and phosphorylates cPLA(2) in microglia via the ERK and p38 MAPK pathways.

Impacts of membrane biophysics in Alzheimer's disease: from amyloid precursor protein processing to aβ Peptide-induced membrane changes

An increasing amount of evidence supports the notion that cytotoxic effects of amyloid-β peptide (Aβ), the main constituent of senile plaques in Alzheimer's disease (AD), are strongly associated with its ability to interact with membranes of neurons and other cerebral cells. Aβ is derived from amyloidogenic cleavage of amyloid precursor protein (AβPP) by β- and γ-secretase. In the nonamyloidogenic pathway, AβPP is cleaved by α-secretases. These two pathways compete with each other, and enhancing the non-amyloidogenic pathway has been suggested as a potential pharmacological approach for the treatment of AD. Since AβPP, α-, β-, and γ-secretases are membrane-associated proteins, AβPP processing and Aβ production can be affected by the membrane composition and properties. There is evidence that membrane composition and properties, in turn, play a critical role in Aβ cytotoxicity associated with its conformational changes and aggregation into oligomers and fibrils. Understanding the mechanisms leading to changes in a membrane's biophysical properties and how they affect AβPP processing and Aβ toxicity should prove to provide new therapeutic strategies for prevention and treatment of AD.

Phospholipases A2 and neural membrane dynamics: implications for Alzheimer's disease

Phospholipases A(2) (PLA(2)s) are essential enzymes in cells. They are not only responsible for maintaining the structural organization of cell membranes, but also play a pivotal role in the regulation of cell functions. Activation of PLA(2) s results in the release of fatty acids and lysophospholipids, products that are lipid mediators and compounds capable of altering membrane microdomains and physical properties. Although not fully understood, recent studies have linked aberrant PLA(2) activity to oxidative signaling pathways involving NADPH oxidase that underlie the pathophysiology of a number of neurodegenerative diseases. In this paper, we review studies describing the involvement of cytosolic PLA(2) in oxidative signaling pathways leading to neuronal impairment and activation of glial cell inflammatory responses. In addition, this review also includes information on the role of cytosolic PLA(2) and exogenous secretory PLA(2) on membrane physical properties, dynamics, and membrane proteins. Unraveling the mechanisms that regulate specific types of PLA(2)s and their effects on membrane dynamics are important prerequisites towards understanding their roles in the pathophysiology of Alzheimer's disease, and in the development of novel therapeutics to retard progression of the disease.

Membrane biophysics and mechanics in Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disorder characterized by neuronal loss, cerebrovascular inflammation, and accumulation of senile plaques in the brain parenchyma and cerebral blood vessels. Amyloid-beta peptide (Abeta), a major component of senile plaques, has been shown to exert multiple toxic effects to neurons, astrocytes, glial cells, and brain endothelium. Oligomeric Abeta can disturb the structure and function of cell membranes and alter membrane mechanical properties, such as membrane fluidity and molecular order. Much of these effects are attributed to their capability to trigger oxidative stress and inflammation. In this review, we discuss the effects of Abeta on neuronal cells, astrocytes, and cerebral endothelial cells with special emphasis on cell membrane properties and cell functions.

NOX enzymes in the central nervous system: from signaling to disease

Oxidative stress has been implicated in the pathogenesis of neurologic and psychiatric diseases. The brain is particularly vulnerable to oxidative damage due to high oxygen consumption, low antioxidant defense, and an abundance of oxidation-sensitive lipids. Production of reactive oxygen species (ROS) by mitochondria is generally thought to be the main cause of oxidative stress. However, a role for ROS-generating NADPH oxidase NOX enzymes has recently emerged. Activation of the phagocyte NADPH oxidase NOX2 has been studied mainly in microglia, where it plays a role in inflammation, but may also contribute to neuronal death in pathologic conditions. However, NOX-dependent ROS production can be due to the expression of other NOX isoforms, which are detected not only in microglia, but also in astrocytes and neurons. The physiologic and pathophysiologic roles of such NOX enzymes are only partially understood. In this review, we summarize the present knowledge about NOX enzymes in the central nervous system and their involvement in neurologic and psychiatric diseases.