Amyloid beta-protein affects cholesterol metabolism in cultured neurons: implications for pivotal role of cholesterol in the amyloid cascade

Cellular cholesterol loss by DHCR24 knockdown leads to Aβ production by changing APP intracellular localization

For the past 20 years, the majority of cell culture studies reported that increasing cholesterol level increases amyloid-β (Aβ) production. Conversely, other studies and genetic evidences support that cellular cholesterol loss leads to Aβ generation. As a highly controversial issue in Alzheimer's disease (AD) pathogenesis，the apparent contradiction prompted us to again explore the role of cellular cholesterol in Aβ production. Here, we adopted new neuronal and astrocytic cell models induced by 3β-hydroxysterol-Δ24 reductase (DHCR24), which obviously differ from the widely used cell models with overexpressing amyloid precursor protein (APP) in the majority of previous studies. In neuronal and astrocytic cell model, we found that deficiency of cellular cholesterol by DHCR24 knock-down obviously increased intracellular and extracellular Aβ generation. Importantly, in cell models with overexpressing APP, we found that APP overexpression could disrupt cellular cholesterol homeostasis and affect function of cells, coupled with the increase of APP β-cleavage product, 99-residue transmembrane C-terminal domain (C99). Therefore, we suppose the results derived from the APP knock-in models will need to be re-evaluated. One rational explanation for the discrepancy between our outcomes and the previous studies could be attributed to the two different cell models. Mechanistically, we showed that cellular cholesterol loss obviously altered APP intracellular localization by affecting cholesterol-related trafficking protein of APP. Therefore, our outcomes strongly support cellular cholesterol loss by DHCR24 knockdown leads to Aβ production.

The Sigma Enigma: A Narrative Review of Sigma Receptors

The sigma-1 and sigma-2 receptors were first discovered in the 1960s and were thought to be a form of opioid receptors initially. Over time, more was gradually learned about these receptors, which are actually protein chaperones, and many of their unique or unusual properties can contribute to a range of important new therapeutic applications. These sigma receptors translocate in the body and regulate calcium homeostasis and mitochondrial bioenergetics and they also have neuroprotective effects. The ligands to which these sigma receptors respond are several and dissimilar, including neurosteroids, neuroleptics, and cocaine. There is controversy as to their endogenous ligands. Sigma receptors are also involved in the complex processes of cholesterol homeostasis and protein folding. While previous work on this topic has been limited, research has been conducted in multiple disease states, such as addiction, aging. Alzheimer's disease, cancer, psychiatric disorders, pain and neuropathic pain, Parkinson's disease, and others. There is currently increasing interest in sigma-1 and sigma-2 receptors as they provide potential therapeutic targets for many disease indications.

The role of DHCR24 in the pathogenesis of AD: re-cognition of the relationship between cholesterol and AD pathogenesis

Previous studies show that 3β-hydroxysterol-Δ24 reductase (DHCR24) has a remarked decline in the brain of AD patients. In brain cholesterol synthetic metabolism, DHCR24 is known as the heavily key synthetase in cholesterol synthesis. Moreover, mutations of DHCR24 gene result in inhibition of the enzymatic activity of DHCR24, causing brain cholesterol deficiency and desmosterol accumulation. Furthermore, in vitro studies also demonstrated that DHCR24 knockdown lead to the inhibition of cholesterol synthesis, and the decrease of plasma membrane cholesterol and intracellular cholesterol level. Obviously, DHCR24 could play a crucial role in maintaining cholesterol homeostasis via the control of cholesterol synthesis. Over the past two decades, accumulating data suggests that DHCR24 activity is downregulated by major risk factors for AD, suggesting a potential link between DHCR24 downregulation and AD pathogenesis. Thus, the brain cholesterol loss seems to be induced by the major risk factors for AD, suggesting a possible causative link between brain cholesterol loss and AD. According to previous data and our study, we further found that the reduced cholesterol level in plasma membrane and intracellular compartments by the deficiency of DHCR24 activity obviously was involved in β-amyloid generation, tau hyperphosphorylation, apoptosis. Importantly, increasing evidences reveal that the brain cholesterol loss and lipid raft disorganization are obviously linked to neuropathological impairments which are associated with AD pathogenesis. Therefore, based on previous data and research on DHCR24, we suppose that the brain cholesterol deficiency/loss might be involved in the pathogenesis of AD.

Quinovic Acid Impedes Cholesterol Dyshomeostasis, Oxidative Stress, and Neurodegeneration in an Amyloid-β-Induced Mouse Model

Alzheimer's disease (AD) is a progressive neurodegenerative disorder typified by several neuropathological features including amyloid-beta (Aβ) plaque and neurofibrillary tangles (NFTs). Cholesterol retention and oxidative stress (OS) are the major contributors of elevated β- and γ-secretase activities, leading to excessive Aβ deposition, signifying the importance of altered cholesterol homeostasis and OS in the progression of Aβ-mediated neurodegeneration and cognitive deficit. However, the effect of Aβ on cholesterol metabolism is lesser-known. In this study, we evaluated the effect of quinovic acid (QA; 50 mg/kg body weight, i.p.) against the intracerebroventricular (i.c.v.) injection of Aβ (1-42)-induced cholesterol dyshomeostasis, oxidative stress, and neurodegeneration in the cortex and hippocampal brain regions of wild-type male C57BL/6J mice. Our results indicated that Aβ (1-42)-treated mice have increased Aβ oligomer formation along with increased β-secretase expression. The enhanced amyloidogenic pathway in Aβ (1-42)-treated mice intensified brain cholesterol accumulation due to increased expressions of p53 and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) enzyme. Importantly, we further confirmed the p53-mediated HMGCR axis activation by using pifithrin-α (PFT) in SH-SY5Y cells. Furthermore, the augmented brain cholesterol levels were also associated with increased OS. However, the QA administration to Aβ (1-42)-injected mice significantly ameliorated the Aβ burden, p53 expression, and cholesterol accumulation by deterring the oxidative stress through upregulating the Nrf2/HO-1 pathway. Moreover, the QA downregulated gliosis, neuroinflammatory mediators (p-NF-κB and IL-1β), and the expression of mitochondrial apoptotic markers (Bax, cleaved caspase-3, and cytochrome c). QA treatment also reversed the deregulated synaptic markers (PSD-95 and synaptophysin) and improved spatial learning and memory behaviors in the Aβ-treated mouse brains. These results suggest that Aβ (1-42) induces its acute detrimental effects on cognitive functions probably by increasing brain cholesterol levels through a possible activation of the p53/HMGCR axis. However, QA treatment reduces the cholesterol-induced oxidative stress, neuroinflammation, and neurodegeneration, leading to the restoration of cognitive deficit after Aβ (1-42) i.c.v. injection in mice.

Sigma-2 Receptor-A Potential Target for Cancer/Alzheimer's Disease Treatment via Its Regulation of Cholesterol Homeostasis

The sigma receptors were classified into sigma-1 and sigma-2 receptor based on their different pharmacological profiles. In the past two decades, our understanding of the biological and pharmacological properties of the sigma-1 receptor is increasing; however, little is known about the sigma-2 receptor. Recently, the molecular identity of the sigma-2 receptor has been identified as TMEM97. Although more and more evidence has showed that sigma-2 ligands have the ability to treat cancer and Alzheimer’s disease (AD), the mechanisms connecting these two diseases are unknown. Data obtained over the past few years from human and animal models indicate that cholesterol homeostasis is altered in AD and cancer, underscoring the importance of cholesterol homeostasis in AD and cancer. In this review, based on accumulated evidence, we proposed that the beneficial roles of sigma-2 ligands in cancer and AD might be mediated by their regulation of cholesterol homeostasis.

A practical application of generative adversarial networks for RNA-seq analysis to predict the molecular progress of Alzheimer's disease

Next-generation sequencing (NGS) technology has become a powerful tool for dissecting the molecular and pathological signatures of a variety of human diseases. However, the limited availability of biological samples from different disease stages is a major hurdle in studying disease progressions and identifying early pathological changes. Deep learning techniques have recently begun to be applied to analyze NGS data and thereby predict the progression of biological processes. In this study, we applied a deep learning technique called generative adversarial networks (GANs) to predict the molecular progress of Alzheimer's disease (AD). We successfully applied GANs to analyze RNA-seq data from a 5xFAD mouse model of AD, which recapitulates major AD features of massive amyloid-β (Aβ) accumulation in the brain. We examined how the generator is featured to have specific-sample generation and biological gene association. Based on the above observations, we suggested virtual disease progress by latent space interpolation to yield the transition curves of various genes with pathological changes from normal to AD state. By performing pathway analysis based on the transition curve patterns, we identified several pathological processes with progressive changes, such as inflammatory systems and synapse functions, which have previously been demonstrated to be involved in the pathogenesis of AD. Interestingly, our analysis indicates that alteration of cholesterol biosynthesis begins at a very early stage of AD, suggesting that it is the first effect to mediate the cholesterol metabolism of AD downstream of Aβ accumulation. Here, we suggest that GANs are a useful tool to study disease progression, leading to the identification of early pathological signatures.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Aβ inhibits SREBP-2 activation through Akt inhibition

We previously demonstrated that oligomeric amyloid β₄₂ (oAβ₄₂) inhibits the mevalonate pathway impairing cholesterol synthesis and protein prenylation. Enzymes of the mevalonate pathway are regulated by the transcription factor SREBP-2. Here, we show that in several neuronal types challenged with oAβ₄₂, SREBP-2 activation is reduced. Moreover, SREBP-2 activation is also decreased in the brain cortex of the Alzheimer's disease (AD) mouse model, TgCRND8, suggesting that SREBP-2 may be affected in vivo early in the disease. We demonstrate that oAβ₄₂ does not affect enzymatic cleavage of SREBP-2 per se, but may impair SREBP-2 transport from the endoplasmic reticulum (ER) to the Golgi. Trafficking of SREBP-2 from the ER to the Golgi requires protein kinase B (Akt) activation. oAβ₄₂ significantly reduces Akt phosphorylation and this decrease is responsible for the decline in SREBP-2 activation. Overexpression of constitutively active Akt prevents the effect of oAβ₄₂ on SREBP-2 and the downstream inhibition of cholesterol synthesis and protein prenylation. Our work provides a novel mechanistic link between Aβ and the mevalonate pathway, which will impact the views on issues related to cholesterol, isoprenoids, and statins in AD. We also identify SREBP-2 as an indirect target of Akt in neurons, which may play a role in the cross-talk between AD and diabetes.

Oxidized cholesterol as the driving force behind the development of Alzheimer's disease

Alzheimer’s disease (AD), the most common neurodegenerative disorder associated with dementia, is typified by the pathological accumulation of amyloid Aβ peptides and neurofibrillary tangles (NFT) within the brain. Considerable evidence indicates that many events contribute to AD progression, including oxidative stress, inflammation, and altered cholesterol metabolism. The brain’s high lipid content makes it particularly vulnerable to oxidative species, with the consequent enhancement of lipid peroxidation and cholesterol oxidation, and the subsequent formation of end products, mainly 4-hydroxynonenal and oxysterols, respectively from the two processes. The chronic inflammatory events observed in the AD brain include activation of microglia and astrocytes, together with enhancement of inflammatory molecule and free radical release. Along with glial cells, neurons themselves have been found to contribute to neuroinflammation in the AD brain, by serving as sources of inflammatory mediators. Oxidative stress is intimately associated with neuroinflammation, and a vicious circle has been found to connect oxidative stress and inflammation in AD. Alongside oxidative stress and inflammation, altered cholesterol metabolism and hypercholesterolemia also significantly contribute to neuronal damage and to progression of AD. Increasing evidence is now consolidating the hypothesis that oxidized cholesterol is the driving force behind the development of AD, and that oxysterols are the link connecting the disease to altered cholesterol metabolism in the brain and hypercholesterolemia; this is because of the ability of oxysterols, unlike cholesterol, to cross the blood brain barrier (BBB). The key role of oxysterols in AD pathogenesis has been strongly supported by research pointing to their involvement in modulating neuroinflammation, Aβ accumulation, and cell death. This review highlights the key role played by cholesterol and oxysterols in the brain in AD pathogenesis.

Amyloid precursor protein α- and β-cleaved ectodomains exert opposing control of cholesterol homeostasis via SREBP2

Amyloid precursor protein (APP) is ubiquitously expressed. Studies in neuronal cells have implicated APP or its fragments as negative regulators of cholesterol metabolism. In the current study, APP acted, via its α-cleavage, as a positive regulator of sterol regulatory element-binding protein-2 (SREBP2) signaling in human astrocytic cells (U251MG), hepatic cells (HepG2), and primary fibroblasts, leading to an approximate 30% increase in SRE-dependent gene expression and, consequently, enhanced cholesterol biosynthesis and LDL receptor levels. This effect was mediated via the secretory ectodomain APPsα. The β-cleaved ectodomain, in turn, repressed SRE-dependent gene expression by up to ∼ 30%. This resulted in decreased cholesterol synthesis and LDL receptor content, establishing a physiological feedback loop in cholesterol-loaded cells, where APP undergoes preferential β-cleavage. Patients with familial Alzheimer's disease had decreased circulating lathosterol, reflecting hepatic cholesterol synthesis, and their fibroblasts had reduced LDL receptor content, which was alleviated by decreasing β-cleavage. These results show that APP regulates cholesterol metabolism in cells relevant for whole-body cholesterol balance and reveal that APP α- and β-cleavages produce opposing paracrine regulators of SREBP2 signaling.

The link between altered cholesterol metabolism and Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by the progressive loss of neurons and synapses, and by extracellular deposits of amyloid-β (Aβ) as senile plaques, Aβ deposits in the cerebral blood vessels, and intracellular inclusions of hyperphosphorylated tau in the form of neurofibrillary tangles. Several mechanisms contribute to AD development and progression, and increasing epidemiological and molecular evidence suggests a key role of cholesterol in its initiation and progression. Altered cholesterol metabolism and hypercholesterolemia appear to play fundamental roles in amyloid plaque formation and tau hyperphosphorylation. Over the last decade, growing evidence supports the idea that cholesterol oxidation products, known as oxysterols, may be the missing link between altered brain cholesterol metabolism and AD pathogenesis, as their involvement in neurotoxicity, mainly by interacting with Aβ peptides, is reported.

β-amyloid inhibits protein prenylation and induces cholesterol sequestration by impairing SREBP-2 cleavage

Accumulation of β-amyloid (Aβ) inside brain neurons is an early and crucial event in Alzheimer's disease (AD). Studies in brains of AD patients and mice models of AD suggested that cholesterol homeostasis is altered in neurons that accumulate Aβ. Here we directly investigated the role of intracellular oligomeric Aβ(42) (oAβ(42)) in neuronal cholesterol homeostasis. We report that oAβ(42) induces cholesterol sequestration without increasing cellular cholesterol mass. Several features of AD, such as endosomal abnormalities, brain accumulation of Aβ and neurofibrillary tangles, and influence of apolipoprotein E genotype, are also present in Niemann-Pick type C, a disease characterized by impairment of intracellular cholesterol trafficking. These common features and data presented here suggest that a pathological mechanism involving abnormal cholesterol trafficking could take place in AD. Cholesterol sequestration in Aβ-treated neurons results from impairment of intracellular cholesterol trafficking secondary to inhibition of protein prenylation. oAβ(42) reduces sterol regulatory element-binding protein-2 (SREBP-2) cleavage, causing decrease of protein prenylation. Inhibition of protein prenylation represents a mechanism of oAβ(42)-induced neuronal death. Supply of the isoprenoid geranylgeranyl pyrophosphate to oAβ(42)-treated neurons recovers normal protein prenylation, reduces cholesterol sequestration, and prevents Aβ-induced neurotoxicity. Significant to AD, reduced levels of protein prenylation are present in the cerebral cortex of the TgCRND8 mouse model. In conclusion, we demonstrate a significant inhibitory effect of Aβ on protein prenylation and identify SREBP-2 as a target of oAβ(42), directly linking Aβ to cholesterol homeostasis impairment.

Reciprocal regulation of cholesterol and beta amyloid at the subcellular level in Alzheimer's disease

Since the discovery that apolipoprotein E, a cholesterol transport protein, is a major risk factor for Alzheimer's disease (AD) development, there has been a remarkable interest in understanding the many facets of the relationship between cholesterol and AD. Several lines of evidence have demonstrated the importance of cholesterol in amyloid beta peptide (Aβ) production and metabolism, as well as the involvement of Aβ in cholesterol homeostasis. The emerging picture is complex and still incomplete. This review discusses findings that indicate that a reciprocal regulation exists between Aβ and cholesterol at the subcellular level. The pathological impact of such regulation is highlighted.

Sortilin is required for toxic action of Aβ oligomers (AβOs): extracellular AβOs trigger apoptosis, and intraneuronal AβOs impair degradation pathways

AIMS

We investigated the pathological relevance of the "Aβ oligomer (AβO) cascade hypothesis" in 3xTg-AD mice. This study was also designed to elucidate the molecular mechanism underlying the toxic action of AβOs.

MAIN METHODS

To target the extracellular AβOs in vivo, a monoclonal antibody specific for AβOs was developed using a novel method. Monoclonal 72D9 was intravenously administered to aged 3xTg-AD mice bearing the human AD pathology to investigate the relevance of the AβO cascade hypothesis. To further identify the AβO-binding molecule on the cell surface, small interfering RNA (siRNA) for sortilin was transfected into SH-SY5Y cells. The sortilin-dependent molecular mechanism underlying toxic action of AβOs and/or AβO endocytosis was also assessed in cultured cortical neurons forming synapses.

KEY FINDINGS

The 72D9 immunotherapy of aged 3xTg-AD mice revealed that extracellular and intraneuronal AβOs are related, and that intraneuronal AβOs act upstream of tau. We also found that extracellular AβOs first act as a sortilin ligand, and then induce p75(NTF)-mediated apoptosis, endocytosis-induced attenuation of autophagy, or accumulation of AβOs in autophagosomes.

SIGNIFICANCE

Taken together, these findings provide novel lines of evidence that sortilin governs the toxic action of extracellular AβOs, which affects the degradation and/or clearance of either intraneuronal AβOs or tau. Thus, therapeutic intervention targeting extracellular AβOs themselves or for preventing the interaction between intraneuronal AβOs and tau is a promising strategy to be developed for AD treatment.

Function and comorbidities of apolipoprotein e in Alzheimer's disease

Alzheimer's disease (AD)—the most common type of dementia among the elderly—represents one of the most challenging and urgent medical mysteries affecting our aging population. Although dominant inherited mutation in genes involved in the amyloid metabolism can elicit familial AD, the overwhelming majority of AD cases, dubbed sporadic AD, do not display this Mendelian inheritance pattern. Apolipoprotein E (APOE), the main lipid carrier protein in the central nervous system, is the only gene that has been robustly and consistently associated with AD risk. The purpose of the current paper is thus to highlight the pleiotropic roles and the structure-function relationship of APOE to stimulate both the functional characterization and the identification of novel lipid homeostasis-related molecular targets involved in AD.

Dissociation of β-amyloid from lipoprotein in cerebrospinal fluid from Alzheimer's disease accelerates β-amyloid-42 assembly

Monoclonal 2C3 specific to β-amyloid (Aβ) oligomers (AβOs) enabled us to test our hypothesis that the alteration of lipoprotein-Aβ interaction in the central nervous system (CNS) initiates and/or accelerates the cascade favoring Aβ assembly. Immunoprecipitation of frontal cortex employing 2C3 unequivocally detected soluble 4-, 8-, and 12-mers in Alzheimer's disease (AD) brains. Immunoblot analysis of the entorhinal cortex employing 2C3 revealed that the accumulation of soluble 12-mers precedes the appearance of neuronal loss or cognitive impairment and is enhanced as the Braak neurofibrially tangle (NFT) stages progress. The dissociation of soluble Aβ from lipoprotein particles occurs in cerebrospinal fluid (CSF), and the presence of lipoprotein-free oligomeric 2C3 conformers (4- to 35-mers) was evident, which mimic CNS environments. Such CNS environments may strongly affect conformation of soluble Aβ peptides, resulting in the conversion of soluble Aβ(42) monomers into soluble Aβ(42) assembly. The findings suggest that functionally declined lipoproteins may accelerate the generation of metabolic conditions leading to higher levels of soluble Aβ(42) assembly in the CNS.

Direct binding of cholesterol to the amyloid precursor protein: An important interaction in lipid-Alzheimer's disease relationships?

It is generally believed that cholesterol homoeostasis in the brain is both linked to and impacted by Alzheimer's disease (AD). For example, elevated levels of cholesterol in neuronal plasma and endosome membranes appear to be a pro-amyloidogenic factor. The recent observation that the C-terminal transmembrane domain (C99, also known as the beta-C-terminal fragment, or beta-CTF) of the amyloid precursor protein (APP) specifically binds cholesterol helps to tie together previously loose ends in the web of our understanding of Alzheimer's-cholesterol relationships. In particular, binding of cholesterol to C99 appears to favor the amyloidogenic pathway in cells by promoting localization of C99 in lipid rafts. In turn, the products of this pathway-amyloid-beta and the intracellular domain of the APP (AICD)-may down-regulate ApoE-mediated cholesterol uptake and cholesterol biosynthesis. If confirmed, this negative-feedback loop for membrane cholesterol levels has implications for understanding the function of the APP and for devising anti-amyloidogenic preventive strategies for AD.

Abeta(25-35) attenuated SREBP level in nuclear extracts of serum-deprived human neuroblastoma cells

Disturbance in cholesterol homeostasis appears to be an important factor in the pathogenesis of neurodegenerative disorders. The aim of the present study was to investigate sterol regulatory element binding protein (SREBP) levels in the nuclear extracts of human neuroblastoma cells and the possible interaction of beta-amyloid peptide (Abeta) and cholesterol with this transcription factor. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in serum-deprived media in the presence or absence of Abeta((25-35)) (1 microM) or cholesterol (300 microM) for 24 h. Nuclear extracts were subjected to SDS-PAGE, and SREBP cleavage product (68 kDa) was detected by immunoblotting. SREBP levels were elevated in the cells incubated 24 h in serum-deprived experimental media and were attenuated by Abeta or cholesterol-supplementation. It is likely that the ability of Abeta to release cholesterol into the medium and downregulate SREBP is due to a feedback mechanism.

Glia-induced neuronal differentiation by transcriptional regulation

There is increasing evidence that different phases of brain development depend on neuron-glia interactions including postnatal key events like synaptogenesis. To address how glial cells influence synapse development, we analyzed whether and how glia-derived factors affect gene expression in primary cultures of immunoisolated rat retinal ganglion cells (RGCs) by oligonucleotide microarrays. Our results show that the transcript pattern matched the developmental stage and characteristic properties of RGCs in vitro. Glia-conditioned medium (GCM) and cholesterol up- and downregulated a limited number of genes that influence the development of dendrites and synapses and regulate cholesterol and fatty acid metabolism. The oligonucleotide microarrays detected the transcriptional regulation of neuronal cholesterol homeostasis in response to GCM and cholesterol treatment. Surprisingly, our study revealed neuronal expression and glial regulation of matrix gla protein (Mgp). Together, our results suggest that glial cells promote different aspects of neuronal differentiation by regulating transcription of distinct classes of genes.

Role of amyloid precursor protein, amyloid-beta and gamma-secretase in cholesterol maintenance

Lipids play an important part as risk factors for Alzheimer's disease. This article summarizes the current understanding of the molecular mechanism by which amyloid-beta (Abeta) peptides regulate cholesterol and sphingomyelin metabolism, and how in return cholesterol and sphingomyelin regulate Abeta peptide production. An understanding of the physiological function of amyloid precursor protein processing and Abeta function is critical for the development of future therapeutic approaches, e.g. statin treatment.

Effect of docosahexaenoic acid on brain 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in male ICR mice

We investigated the influence of docosahexaenoic acid ethyl ester (DHA-EE) on 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity in the brains of adult and aged mice. Male mice (Crlj:CD-1) were fed diets containing 3% lard plus 2% linoleic acid ethyl ester (LA-EE), or 2% DHA-EE, for 3 months. The brain HMG-CoA reductase activity of 8-month-old (adult) mice was not significantly influenced by dietary intake of DHA-EE. However, in 18-month-old (aged) mice, its activity was enhanced with dietary intake of DHA-EE. Brain HMG-CoA reductase activity and brain cholesterol content significantly increased with age. Hepatic HMG-CoA reductase activity and the cholesterol content of both adult and aged mice were reduced in DHA-EE diet groups, compared with LA-EE diet groups. The DHA percentages of brain and liver microsomal fractions increased with the intake of DHA-EE in adult and aged mice. These results suggest that DHA may enhance brain HMG-CoA reductase activity in aged mice.

Role of cholesterol in amyloid cascade: cholesterol-dependent modulation of tau phosphorylation and mitochondrial function

Apolipoprotein E (apoE) alleles are important genetic risk factors for Alzheimer's disease (AD), with the epsilon4 allele increasing and the epsilon2 allele decreasing the risk of developing AD. ApoE is the major apolipoprotein that modulates cholesterol transport in the central nervous system, cholesterol being an essential component of membranes for maintaining their structure and functions. Epidemiological studies have suggested a link between serum cholesterol levels and AD development and the potential therapeutic effectiveness of statins for AD; and furthermore, biological studies have shown that amyloid beta-protein (Abeta) secretion is modulated by cellular cholesterol level. However, other lines of evidence show controversial results. In addition to the role of cholesterol in Abeta generation, different interactions of cholesterol with Abeta and its role in AD pathogenesis have been shown, i.e. Abeta affects cholesterol dynamics in neurons, and altered cholesterol metabolism in turn leads to neurodegeneration with abnormally phosphorylated tau (tauopathy). In this review, the reciprocal interactions between cholesterol and Abeta, and the role of cholesterol in tauopathy are discussed. The isoform-specific involvement of apoE in this cascade, in which high-density lipoprotein-like particles are generated and supplied to neurons to maintain cholesterol homeostasis, is also discussed.

Induction of vascular amyloidosis-β by oxidative stress depends on APOE genotype

The reduced antioxidant defense in apolipoprotein E epsilon4/epsilon4 carriers may contribute to beta-amyloidosis. Previously we found that Fe(2+)-induced oxidative stress caused greater protein oxidation in epsilon4/epsilon4 than in epsilon3/epsilon3 human brain vascular smooth muscle cells. Moreover, Fe(2+) induced lysosomal accumulation of endogenous Abeta and APOE in cultured cells, and Abeta deposition in vascular tunica media in organotypic cultures of brain vessels. Here we demonstrated that Fe(2+) enhanced an uptake of exogenous Abeta 1-40 and its deposition together with APOE in lysosomes in myocytes. Abeta deposits were associated with lipid-peroxidation and protein ubiquitination, and were more abundant and stable in epsilon4/epsilon4 than in epsilon3/epsilon3 cells. In organotypic cultures of brain vessels Fe(2+) induced deposition of non-fibrillar and fibrillar Abeta 1-40 in vascular tunica media. We hypothesize that locally increased concentrations of iron induce accumulation of exogenous and endogenous Abeta in SMCs, triggering beta-amyloid angiopathy. The greater susceptibility of epsilon4 carriers to Fe(2+) ions may result in an increased risk of beta-amyloidosis.

Feedback Regulation of SREBP and Aromatase in A β(25-35)-Supplemented Human Neuroblastoma Cells

1. The analogies between the processing of amyloid precursor protein (APP) and other transmembrane sterol regulatory element binding proteins (SREBPs) inspired us to conduct further studies on whether beta-amyloid (Abeta) affects aromatase by interacting with APP and SREBP. 2. In this study, cultured human neuroblastoma cells (SHSY-5Y) were incubated in experimental media (media without FBS, the main cholesterol source) in the presence or absence of Abeta (1 microM) for 24 h. 3. Cellular extracts were subjected to immunoblot analysis using anti-APP, anti-aromatase and anti-SREBP-1. In these cell lines, we detected aromatase (55 kDa), SREBP cleavage product (68 kDa) and APP precursor (100-95 kDa) and cleavage product (60 kDa) by immunoblotting. Aromatase and SREBP levels were elevated in the cells incubated 24 h in experimental media and were attenuated in Abeta-supplemented experimental media. 4. The disturbance of cholesterol homeostasis appears to be an important factor in the pathogenesis of Alzheimer's disease. These findings may have important implications for understanding the mechanisms of the aromatase enzyme gene in disease states such as Alzheimer's.

Lipid homeostasis and apolipoprotein E in the development and progression of Alzheimer's disease

Extracellular amyloid plaques, intracellular neurofibrillary tangles, and loss of basal forebrain cholinergic neurons in the brains of Alzheimer's disease (AD) patients may be the end result of abnormalities in lipid metabolism and peroxidation that may be caused, or exacerbated, by beta-amyloid peptide (Abeta). Apolipoprotein E (apoE) is a major apolipoprotein in the brain, mediating the transport and clearance of lipids and Abeta. ApoE-dependent dendritic and synaptic regeneration may be less efficient with apoE4, and this may result in, or unmask, age-related neurodegenerative changes. The increased risk of AD associated with apoE4 may be modulated by diet, vascular risk factors, and genetic polymorphisms that affect the function of other transporter proteins and enzymes involved in brain lipid homeostasis. Diet and apoE lipoproteins influence membrane lipid raft composition and the properties of enzymes, transporter proteins, and receptors mediating Abeta production and degradation, tau phosphorylation, glutamate and glucose uptake, and neuronal signal transduction. The level and isoform of apoE may influence whether Abeta is likely to be metabolized or deposited. This review examines the current evidence for diet, lipid homeostasis, and apoE in the pathogenesis of AD. Effects on the cholinergic system and response to cholinesterase inhibitors by APOE allele carrier status are discussed briefly.

Altered cholesterol metabolism in Niemann-Pick type C1 mouse brains affects mitochondrial function

Niemann-Pick type C1 (NPC1) disease is a fatal hereditary disorder characterized by a defect in cholesterol trafficking and progressive neurodegeneration. Although the NPC1 gene has been identified, the molecular mechanism responsible for neuronal dysfunction in brains of patients with NPC1 disease remains unknown. This study demonstrates that the amount of cholesterol within mitochondria membranes is significantly elevated in NPC1 mouse brains and neural cells. In addition, the mitochondrial membrane potential, the activity of ATP synthase, and henceforth the level of ATP are markedly decreased in NPC1 mouse brains and neurons. Importantly, reducing the level of cholesterol within mitochondrial membranes using methyl-beta-cyclodextrin can restore the activity of ATP synthase. Finally, NPC1 neurons show an impaired neurite outgrowth, which can be rescued by exogenous ATP. These results suggest that mitochondrial dysfunctions and subsequent ATP deficiency, which are induced by altered cholesterol metabolism in mitochondria, may be responsible for neuronal impairment in NPC1 disease.

Coenzyme Q10 protects SHSY5Y neuronal cells from beta amyloid toxicity and oxygen-glucose deprivation by inhibiting the opening of the mitochondrial permeability transition pore

Coenzyme Q10 (CoQ10) is an essential biological cofactor which increases brain mitochondrial concentration and exerts neuroprotective effects. In the present study, we exposed SHSY5Y neuroblastoma cells to neurotoxic beta amyloid peptides (Abeta) and oxygen glucose deprivation (OGD) to investigate the neuroprotective effect of 10 microM CoQ10 by measuring (i) cell viability by the MTT assay, (ii) opening of the mitochondrial permeability transition pore via the fluorescence intensity of calcein-AM, and (iii) superoxide anion concentration by hydroethidine. Cell viability (mean +/- S.E.M.) was 55.5 +/- 0.8% in the group exposed to Abeta + OGD, a value lower than that in the Abeta or OGD group alone (P < 0.01). CoQ10 had no neuroprotective effect on cell death induced by either Abeta or OGD, but increased cell survival in the Abeta + OGD group to 57.3 +/- 1.7%, which was higher than in the group treated with vehicle (P < 0.05). The neuroprotective effect of CoQ10 was blocked by administration of 20 microM atractyloside. Pore opening and superoxide anion concentration were increased in the Abeta + OGD group relative to sham control (P < 0.01), and were attenuated to the sham level (P > 0.05) when CoQ10 was administered. Our results demonstrate that CoQ10 protects neuronal cells against Abeta neurotoxicity together with OGD by inhibiting the opening of the pore and reducing the concentration of superoxide anion.

Apolipoprotein Abeta: black sheep in a good family

Amyloid-beta (Abeta) has for a long time been thought to play a central role in the pathogenesis of Alzheimer disease (AD). Analysis of available data indicates that Abeta possesses properties of a metal-binding apolipoprotein influencing lipid transport and metabolism. Protection of lipoproteins from oxidation by transition metals, synaptic activity and role in the acute phase response represent plausible physiological functions of Abeta. However, these important biochemical qualities which may critically influence the development of AD, have been largely ignored by mainstream AD researchers, making Abeta appear to be a "black sheep" in a "good apolipoprotein" family. New studies are needed to shed further light on the physiological role of Abeta in lipid metabolism in the brain.

Patients with Alzheimer's disease may be particularly susceptible to adverse effects of statins

In epidemiological, cross-sectional studies, treatment with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) prevented to a large extent the development of Alzheimer's disease (AD), but the results of randomized, placebo-controlled studies, focused on statin therapy in patients with ischemic heart disease (IHD), are at variance. Nonetheless, data from epidemiological, longitudinal studies in humans as well as studies on transgenic mouse models and cultured neuronal cell lines indicate that cholesterol may contribute to the pathogenesis of AD. Statins have proven therapeutic and preventive effects in IHD and other vascular diseases in man. They generally are well tolerated, but some adverse effects, probably due to antiproliferative and proapoptotic properties of the statins, are matters of concern. AD patients may be extrasusceptible to adverse effects of statins due to preexisting aberrations in signal transduction and energy metabolism in the neurons and a perturbed cholesterol metabolism in the brain. This problem might be addressed in randomized, double-blind studies with statins in AD. The statins differ from each other in several aspects, and they are not considered to be therapeutically interchangeable. It could be fruitful to use both a placebo and two different types of statins, i.e. an essentially hydrophilic statin and a lipophilic statin, in a double-blinded fashion, and to compare the effects on the cognitive decline in AD.

Membrane cholesterol interferes with neuronal apoptosis induced by soluble oligomers but not fibrils of amyloid-beta peptide

Neuronal cell death in Alzheimer's disease (AD) is partly induced by the interaction of the amyloid-beta peptide (Abeta) with the plasma membrane of target cells. Accordingly, recent studies have suggested that cholesterol, an important component of membranes that controls their physical properties and functions, plays a critical role in neurodegenerative diseases. We report here that the enrichment of the neuronal plasma membrane with cholesterol protects cortical neurons from apoptosis induced by soluble oligomers of the Abeta(1-40) peptide. Conversely, cholesterol depletion using cyclodextrin renders cells more vulnerable to the cytotoxic effects of the Abeta-soluble oligomers. Increasing the cholesterol content of small unilamellar liposomes also decreases Abeta-dependent liposome fusion. We clearly demonstrate that cholesterol protection is specific to the soluble conformation of Abeta, because we observed no protective effects on cortical neurons treated by amyloid fibrils of the Abeta(1-40) peptide. This may provide a new opportunity for the development of an effective AD therapy as well as elucidate the impact of the cholesterol level during AD development.

Interactions of Alzheimer amyloid peptides with cultured cells and brain tissue, and their biological consequences

The Alzheimer amyloid peptides are the main constituent of the diagnostic hallmark of Alzheimer disease, the senile plaque. A halo of neurodegeneration surrounds the senile plaques observed in the brains of Alzheimer patients. Significant efforts are under way to determine whether the Alzheimer peptides are the causal agents of this neurodegeneration. We review the developments in identifying the putative interaction sites of Alzheimer amyloid peptides on cells and intact brain tissue. We focus on the specificity of this interaction and on the molecular nature of potential receptors. These studies form the bases for developing therapeutics that target potential interaction sites and inhibit Alzheimer amyloid peptide deposition.

Amyloid beta-protein (Abeta)1-40 protects neurons from damage induced by Abeta1-42 in culture and in rat brain

Previously, we found that amyloid beta-protein (Abeta)1-42 exhibits neurotoxicity, while Abeta1-40 serves as an antioxidant molecule by quenching metal ions and inhibiting metal-mediated oxygen radical generation. Here, we show another neuroprotective action of nonamyloidogenic Abeta1-40 against Abeta1-42-induced neurotoxicity in culture and in vivo. Neuronal death was induced by Abeta1-42 at concentrations higher than 2 microm, which was prevented by concurrent treatment with Abeta1-40 in a dose-dependent manner. However, metal chelators did not prevent Abeta1-42-induced neuronal death. Circular dichroism spectroscopy showed that Abeta1-40 inhibited the beta-sheet transformation of Abeta1-42. Thioflavin-T assay and electron microscopy analysis revealed that Abeta1-40 inhibited the fibril formation of Abeta1-42. In contrast, Abeta1-16, Abeta25-35, and Abeta40-1 did not inhibit the fibril formation of Abeta1-42 nor prevent Abeta1-42-induced neuronal death. Abeta1-42 injection into the rat entorhinal cortex (EC) caused the hyperphosphorylation of tau on both sides of EC and hippocampus and increased the number of glial fibrillary acidic protein (GFAP)-positive astrocytes in the ipsilateral EC, which were prevented by the concurrent injection of Abeta1-40. These results indicate that Abeta1-40 protects neurons from Abeta1-42-induced neuronal damage in vitro and in vivo, not by sequestrating metals, but by inhibiting the beta-sheet transformation and fibril formation of Abeta1-42. Our data suggest a mechanism by which elevated Abeta1-42/Abeta1-40 ratio accelerates the development of Alzheimer's disease (AD) in familial AD.

Promotion of tau phosphorylation by MAP kinase Erk1/2 is accompanied by reduced cholesterol level in detergent-insoluble membrane fraction in Niemann-Pick C1-deficient cells

Niemann-Pick type C (NPC) disease is a cholesterol-storage disease accompanied by neurodegeneration with the formation of neurofibrillary tangles, the major component of which is the hyperphosphorylated tau. Here, we examined the mechanism underlying hyperphosphorylation of tau using mutant Chinese hamster ovary (CHO) cell line defective in NPC1 (CT43) as a tool. Immunoblot analysis revealed that tau was hyperphosphorylated at multiple sites in CT43 cells, but not in their parental cells (25RA) or the wild-type CHO cells. In CT43 cells, mitogen-activated protein (MAP) kinase Erk1/2 was activated and the specific MAPK inhibitor, PD98059, attenuated the hyperphosphorylation of tau. The amount of protein phosphatase 2A not bound to microtubules was decreased in CT43 cells. CT43 cells but not 25RA cells were amphotericin B-resistant, indicating that cholesterol level in the plasma membrane of CT43 is decreased. In addition, the level of cholesterol in the detergent-insoluble, low-density membrane (LDM) fraction of CT43 cells was markedly reduced compared with the other two types of CHO cells. As LDM domain plays critical role in signaling pathways, these results suggest that the reduced cholesterol level in LDM domain due to the lack of NPC1 may activate MAPK, which subsequently promotes tau phosphorylation in NPC1-deficient cells.

The role of cholesterol in pathogenesis of Alzheimer's disease: dual metabolic interaction between amyloid beta-protein and cholesterol

The implication that cholesterol plays an essential role in the pathogenesis of Alzheimer's disease (AD) is based on the 1993 finding that the presence of apolipoprotein E (apoE) allele epsilon;4 is a strong risk factor for developing AD. Since apoE is a regulator of lipid metabolism, it is reasonable to assume that lipids such as cholesterol are involved in the pathogenesis of AD. Recent epidemiological and biochemical studies have strengthened this assumption by demonstrating the association between cholesterol and AD, and by proving that the cellular cholesterol level regulates synthesis of amyloid beta-protein (Abeta). Yet several studies have demonstrated that oligomeric Abeta affects the cellular cholesterol level, which in turn has a variety of effects on AD related pathologies, including modulation of tau phosphorylation, synapse formation and maintenance of its function, and the neurodegenerative process. All these findings suggest that the involvement of cholesterol in the pathogenesis of AD is dualistic-it is involved in Abeta generation and in the amyloid cascade, leading to disruption of synaptic plasticity, promotion of tau phosphorylation, and eventual neurodegeneration. This review article describes recent findings that may lead to the development of a strategy for AD prevention by decreasing the cellular cholesterol level, and also focuses on the impact of Abeta on cholesterol metabolism in AD and mild cognitive impairment (MCI), which may result in promotion of the amyloid cascade at later stages of the AD process.

A novel function of monomeric amyloid beta-protein serving as an antioxidant molecule against metal-induced oxidative damage

Aggregated and oligomeric amyloid beta-protein (Abeta) is known to exhibit neurotoxicity. However, the action of Abeta monomers on neurons is not fully understood. We have studied aggregation state-dependent actions of Abeta and found an oligomer-specific effect of Abeta on lipid metabolism in neurons (Michikawa et al., 2001). Here, we show a novel function of monomeric Abeta1-40, which is the major species found in physiological fluid, as a natural antioxidant molecule that prevents neuronal death caused by transition metal-induced oxidative damage. Monomeric Abeta1-40, which is demonstrated by SDS-PAGE after treatment with glutaraldehyde, protects neurons cultured in a medium containing 1.5 microm Fe(II) without antioxidant molecules. Metal ion chelators such as EDTA, CDTA (trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid), and DTPA (diethylenetriamine-N,N,N',N",N"-penta-acetic acid, an iron-binding protein, transferrin, and antioxidant scavengers such as catalase, glutathione, and vitamin E also inhibit neuronal death under the same conditions. Monomeric Abeta1-40 inhibits neuronal death caused by Cu(II), Fe(II), and Fe(III) but does not protect neurons against H2O2-induced damage. Monomeric Abeta1-40 inhibits the reduction of Fe(III) induced by vitamin C and the generation of superoxides and prevents lipid peroxidation induced by Fe(II). Abeta1-42 remaining as a monomer also exhibits antioxidant and neuroprotective effects. In contrast, oligomeric and aggregated Abeta1-40 and Abeta1-42 lose their neuroprotective activity. These results indicate that monomeric Abeta protects neurons by quenching metal-inducible oxygen radical generation and thereby inhibiting neurotoxicity. Because aggregated Abeta is known to be an oxygen radical generator, our results provide a novel concept that the aggregation-dependent biological effects of Abeta are dualistic, being either an oxygen radical generator or its inhibitor.