Interaction between 24-hydroxycholesterol, oxidative stress, and amyloid-β in amplifying neuronal damage in Alzheimer's disease: three partners in crime

Cholesterol Metabolism in Neurodegenerative Diseases

Significance: Cholesterol plays a crucial role in the brain, where it is highly concentrated and tightly regulated to support normal brain functions. It serves as a vital component of cell membranes, ensuring their integrity, and acts as a key regulator of various brain processes. Dysregulation of cholesterol metabolism in the brain has been linked to impaired brain function and the onset of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease, and Huntington's disease. Recent Advances: A significant advancement has been the identification of astrocyte-derived apoliprotein E as a key regulator of de novo cholesterol biosynthesis in neurons, providing insights into how extracellular signals influence neuronal cholesterol levels. In addition, the development of antibody-based therapies, particularly for AD, presents promising opportunities for therapeutic interventions. Critical Issues: Despite significant research, the association between cholesterol and neurodegenerative diseases remains inconclusive. It is crucial to distinguish between plasma cholesterol and brain cholesterol, as these pools are relatively independent. This differentiation should be considered when evaluating statin-based treatment approaches. Furthermore, assessing not only the total cholesterol content in the brain but also its distribution among different types of brain cells is essential. Future Direction: Establishing a causal link between changes in brain/plasma cholesterol levels and the onset of brain dysfunction/neurodegenerative diseases remains a key objective. In addition, conducting cell-specific analyses of cholesterol homeostasis in various types of brain cells under pathological conditions will enhance our understanding of cholesterol metabolism in neurodegenerative diseases. Manipulating cholesterol levels to restore homeostasis may represent a novel approach for alleviating neurological symptoms.

Which neuroimaging and fluid biomarkers method is better in theranostic of Alzheimer's disease? An umbrella review

Biomarkers are measured to evaluate physiological and pathological processes as well as responses to a therapeutic intervention. Biomarkers can be classified as diagnostic, prognostic, predictor, clinical, and therapeutic. In Alzheimer's disease (AD), multiple biomarkers have been reported so far. Nevertheless, finding a specific biomarker in AD remains a major challenge. Three databases, including PubMed, Web of Science, and Scopus were selected with the keywords of Alzheimer's disease, neuroimaging, biomarker, and blood. The results were finalized with 49 potential CSF/blood and 35 neuroimaging biomarkers. To distinguish normal from AD patients, amyloid-beta42 (Aβ42), plasma glial fibrillary acidic protein (GFAP), and neurofilament light (NFL) as potential biomarkers in cerebrospinal fluid (CSF) as well as the serum could be detected. Nevertheless, most of the biomarkers fairly change in the CSF during AD, listed as kallikrein 6, virus-like particles (VLP-1), galectin-3 (Gal-3), and synaptotagmin-1 (Syt-1). From the neuroimaging aspect, atrophy is an accepted biomarker for the neuropathologic progression of AD. In addition, Magnetic resonance spectroscopy (MRS), diffusion weighted imaging (DWI), diffusion tensor imaging (DTI), tractography (DTT), positron emission tomography (PET), and functional magnetic resonance imaging (fMRI), can be used to detect AD. Using neuroimaging and CSF/blood biomarkers, in combination with artificial intelligence, it is possible to obtain information on prognosis and follow-up on the different stages of AD. Hence physicians could select the suitable therapy to attenuate disease symptoms and follow up on the efficiency of the prescribed drug.

Cholesterol and oxysterols in retinal neuron-glia interactions: relevance for glaucoma

Cholesterol is an essential component of cellular membranes, crucial for maintaining their structural and functional integrity. It is especially important for nervous tissues, including the retina, which rely on high amounts of plasma membranes for the transmission of the nervous signal. While cholesterol is by far the most abundant sterol, the retina also contains cholesterol precursors and metabolites, especially oxysterols, which are bioactive molecules. Cholesterol lack or excess is deleterious and some oxysterols are known for their effect on neuron survival. Cholesterol homeostasis must therefore be maintained. Retinal glial cells, especially Müller cells, the principal glial cells of the vertebrate retina, provide mechanical, nutritional, and metabolic support for the neighboring neurons. Several pieces of evidence indicate that Müller cells are major actors of cholesterol homeostasis in the retina, as it is known for other glial cells in the brain. This process is based on a close cooperation with neurons, and sterols can be signaling molecules participating in glia-neuron interactions. While some implication of cholesterol in age-related macular degeneration is now recognized, based on epidemiological and laboratory data, evidence for its role in glaucoma is still scarce. The association between cholesterolemia and glaucoma is controversial, but experimental data suggest that sterols could take part in the pathological processes. It has been demonstrated that Müller glial cells are implicated in the development of glaucoma through an ambivalent reactive retinal gliosis process. The early steps contribute to maintaining retinal homeostasis and favor the survival of ganglion cells, which are targeted during glaucoma. If gliosis persists, dysregulation of the neuroprotective functions, cytotoxic effects of gliotic Müller cells and disruption of glia-neuron interactions lead to an acceleration of ganglion cell death. Sterols could play a role in the glial cell response to glaucomatous injury. This represents an understudied but attractive topic to better understand glaucoma and conceive novel preventive or curative strategies. The present review describes the current knowledge on i) sterol metabolism in retinal glial cells, ii) the potential role of cholesterol in glaucoma, and iii) the possible relationships between cholesterol and oxysterols, glial cells and glaucoma. Focus is put on glia-neuron interactions.

Impact of Oxysterols in Age-Related Disorders and Strategies to Alleviate Adverse Effects

Oxysterols or cholesterol oxidation products are a class of molecules with the sterol moiety, derived from oxidative reaction of cholesterol through enzymatic and non-enzymatic processes. They are widely reported in animal-origin foods and prove significant involvement in the regulation of cholesterol homeostasis, lipid transport, cellular signaling, and other physiological processes. Reports of oxysterol-mediated cytotoxicity are in abundance and thus consequently implicated in several age-related and lifestyle disorders such as cardiovascular diseases, bone disorders, pancreatic disorders, age-related macular degeneration, cataract, neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and some types of cancers. In this chapter, we attempt to review a selection of physiologically relevant oxysterols, with a focus on their formation, properties, and roles in health and disease, while also delving into the potential of natural and synthetic molecules along with bacterial enzymes for mitigating oxysterol-mediated cell damage.

Enzymatically Formed Oxysterols and Cell Death

The side-chain hydroxylation of cholesterol by specific enzymes produces 24(S)-hydroxycholesterol, 25-hydroxycholesterol, 27-hydroxycholesterol, and other products. These enzymatically formed side-chain oxysterols act as intermediates in the biosynthesis of bile acids and serve as signaling molecules that regulate cholesterol homeostasis. Besides these intracellular functions, an imbalance in oxysterol homeostasis is implicated in pathophysiology. Furthermore, growing evidence reveals that oxysterols affect cell proliferation and cause cell death. This chapter provides an overview of the pathophysiological role of side-chain oxysterols in developing human diseases. We also summarize our understanding of the molecular mechanisms underlying the induction of various forms of cell death by side-chain oxysterols.

Cholesterol Oxidation Products: Potential Adverse Effect and Prevention of Their Production in Foods

Cholesterol oxidation products (COPs) are a group of substances formed during food processing. COPs in diet is a health concern because they may affect human health in association with the risk of various diseases including atherosclerosis, Alzheimer's disease, age-related macular degeneration, diabetes, and chronic gastrointestinal inflammatory colitis. Production of COPs in foods can be affected by many factors such as temperature, pH, light, oxygen, water, carbohydrates, fatty acids, proteins, and metal cations. The key issue is preventing its generation in foods. Some COPs can also be produced in vivo by both nonenzymatic and enzymatic-catalyzed oxidation reactions. Currently, a number of natural antioxidants such as catechins, flavonoids, and other polyphenols have been proven to inhibit the generation of COPs. In addition, measures taken during food processing can also minimize the production of COPs, such as the Maillard reaction and marinating food with plant polyphenol-rich seasonings. In conclusion, a comprehensive approach encompassing the suppression on COPs generation and implementation of processing measures is imperative to safeguard human health against the production of COPs in the food chain.

Reexamining the Causes and Effects of Cholesterol Deposition in the Brains of Patients with Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. Numerous studies have shown that imbalances in cholesterol homeostasis in the brains of AD patients precede the onset of clinical symptoms. In addition, cholesterol deposition has been observed in the brains of AD patients even though peripheral cholesterol does not enter the brain through the blood‒brain barrier (BBB). Studies have demonstrated that cholesterol metabolism in the brain is associated with many pathological conditions, such as amyloid beta (Aβ) production, Tau protein phosphorylation, oxidative stress, and inflammation. In 2022, some scholars put forward a new hypothesis of AD: the disease involves lipid invasion and its exacerbation of the abnormal metabolism of cholesterol in the brain. In this review, by discussing the latest research progress, the causes and effects of cholesterol retention in the brains of AD patients are analyzed and discussed. Additionally, the possible mechanism through which AD may be improved by targeting cholesterol is described. Finally, we propose that improving the impairments in cholesterol removal observed in the brains of AD patients, instead of further reducing the already impaired cholesterol synthesis in the brain, may be the key to preventing cholesterol deposition and improving the corresponding pathological symptoms.

Tissue-Specific Oxysterols as Predictors of Antidepressant (Escitalopram) Treatment Response in Patients With Major Depressive Disorder

Background

There is growing evidence that disturbances in cholesterol metabolism may be involved in major depressive disorder (MDD). However, it is not known if cholesterol metabolites present in the brain and periphery can be used to diagnose and predict an MDD patient's response to antidepressant treatment.

Methods

A total of 176 subjects (85 patients with MDD and 91 healthy control subjects) were included in this study. The expression of peripheral and brain-specific oxysterols and related gene polymorphisms were investigated in all subjects. The severity of depression was measured using the 17-item Hamilton Depression Rating Scale, 16-item Quick Inventory of Depressive Symptoms-Self-Report, and Patient Health Questionnaire-9 for all patients with MDD before and after 12 weeks of antidepressant treatment.

Results

Patients with MDD expressed higher plasma levels of 24(S)-hydroxycholesterol (24OHC) (mainly secreted from the brain) compared with healthy control subjects, and the higher levels of 24OHC were associated with 24OHC synthetase (CYP46A1) gene polymorphisms. In patients with MDD, an improved response to the 12-week antidepressant treatment was associated with a reduction of both 24OHC and 27OHC (mainly secreted from the peripheral system) levels relative to baseline levels. Nonresponders exhibited increased levels of oxysterols at the end of treatment compared with baseline. The superior reduction in oxysterol levels correlated with better outcomes from the antidepressant treatment.

Conclusions

These data suggest a potential role for oxysterols as diagnostic and treatment response-related indicators for MDD.

Unraveling the impact of 27-hydroxycholesterol in autoimmune diseases: Exploring promising therapeutic approaches

The role of 27-hydroxycholesterol (27-OHC) in autoimmune diseases has become a subject of intense research in recent years. This oxysterol, derived from cholesterol, has been identified as a significant player in modulating immune responses and inflammation. Its involvement in autoimmune pathogenesis has drawn attention to its potential as a therapeutic target for managing autoimmune disorders effectively. 27-OHC, an oxysterol derived from cholesterol, has emerged as a key player in modulating immune responses and inflammatory processes. It exerts its effects through various mechanisms, including activation of nuclear receptors, interaction with immune cells, and modulation of neuroinflammation. Additionally, 27-OHC has been implicated in the dysregulation of lipid metabolism, neurotoxicity, and blood-brain barrier (BBB) disruption. Understanding the intricate interplay between 27-OHC and autoimmune diseases, particularly neurodegenerative disorders, holds promise for developing targeted therapeutic strategies. Additionally, emerging evidence suggests that 27-OHC may interact with specific receptors and transcription factors, thus influencing gene expression and cellular processes in autoimmune disorders. Understanding the intricate mechanisms by which 27-OHC influences immune dysregulation and tissue damage in autoimmune diseases is crucial for developing targeted therapeutic interventions. Further investigations into the molecular pathways and signaling networks involving 27-OHC are warranted to unravel its full potential as a therapeutic target in autoimmune diseases, thereby offering new avenues for disease intervention and management.

Molecular basis for the recognition of 24-(S)-hydroxycholesterol by integrin αvβ3

A growing body of evidence suggests that oxysterols such as 25-hydroxycholesterol (25HC) are biologically active and involved in many physiological and pathological processes. Our previous study demonstrated that 25HC induces an innate immune response during viral infections by activating the integrin-focal adhesion kinase (FAK) pathway. 25HC produced the proinflammatory response by binding directly to integrins at a novel binding site (site II) and triggering the production of proinflammatory mediators such as tumor necrosis factor-α (TNF) and interleukin-6 (IL-6). 24-(S)-hydroxycholesterol (24HC), a structural isomer of 25HC, plays a critical role in cholesterol homeostasis in the human brain and is implicated in multiple inflammatory conditions, including Alzheimer's disease. However, whether 24HC can induce a proinflammatory response like 25HC in non-neuronal cells has not been studied and remains unknown. The aim of this study was to examine whether 24HC produces such an immune response using in silico and in vitro experiments. Our results indicate that despite being a structural isomer of 25HC, 24HC binds at site II in a distinct binding mode, engages in varied residue interactions, and produces significant conformational changes in the specificity-determining loop (SDL). In addition, our surface plasmon resonance (SPR) study reveals that 24HC could directly bind to integrin αvβ3, with a binding affinity three-fold lower than 25HC. Furthermore, our in vitro studies with macrophages support the involvement of FAK and NFκB signaling pathways in triggering 24HC-mediated production of TNF. Thus, we have identified 24HC as another oxysterol that binds to integrin αvβ3 and promotes a proinflammatory response via the integrin-FAK-NFκB pathway.

Rise and fall of peroxisomes during Alzheimer´s disease: a pilot study in human brains

Peroxisomes are eukaryotic organelles that rapidly change in number depending on the metabolic requirement of distinct cell types and tissues. In the brain, these organelles are essential for neuronal migration and myelination during development and their dysfunction is associated with age-related neurodegenerative diseases. Except for one study analysing ABCD3-positive peroxisomes in neurons of the frontal neocortex of Alzheimer disease (AD) patients, no data on other brain regions or peroxisomal proteins are available. In the present morphometric study, we quantified peroxisomes labelled with PEX14, a metabolism-independent peroxisome marker, in 13 different brain areas of 8 patients each either with low, intermediate or high AD neuropathological changes compared to 10 control patients. Classification of patient samples was based on the official ABC score. During AD-stage progression, the peroxisome density decreased in the area entorhinalis, parietal/occipital neocortex and cerebellum, it increased and in later AD-stage patients decreased in the subiculum and hippocampal CA3 region, frontal neocortex and pontine gray and it remained unchanged in the gyrus dentatus, temporal neocortex, striatum and inferior olive. Moreover, we investigated the density of catalase-positive peroxisomes in a subset of patients (> 80 years), focussing on regions with significant alterations of PEX14-positive peroxisomes. In hippocampal neurons, only one third of all peroxisomes contained detectable levels of catalase exhibiting constant density at all AD stages. Whereas the density of all peroxisomes in neocortical neurons was only half of the one of the hippocampus, two thirds of them were catalase-positive exhibiting increased levels at higher ABC scores. In conclusion, we observed spatiotemporal differences in the response of peroxisomes to different stages of AD-associated pathologies.

Microglia and Cholesterol Handling: Implications for Alzheimer's Disease

Cholesterol is essential for brain function and structure, however altered cholesterol metabolism and transport are hallmarks of multiple neurodegenerative conditions, including Alzheimer's disease (AD). The well-established link between apolipoprotein E (APOE) genotype and increased AD risk highlights the importance of cholesterol and lipid transport in AD etiology. Whereas more is known about the regulation and dysregulation of cholesterol metabolism and transport in neurons and astrocytes, less is known about how microglia, the immune cells of the brain, handle cholesterol, and the subsequent implications for the ability of microglia to perform their essential functions. Evidence is emerging that a high-cholesterol environment, particularly in the context of defects in the ability to transport cholesterol (e.g., expression of the high-risk APOE4 isoform), can lead to chronic activation, increased inflammatory signaling, and reduced phagocytic capacity, which have been associated with AD pathology. In this narrative review we describe how cholesterol regulates microglia phenotype and function, and discuss what is known about the effects of statins on microglia, as well as highlighting areas of future research to advance knowledge that can lead to the development of novel therapies for the prevention and treatment of AD.

Characterization of three naturally occurring lignans, sesamol, sesamolin, and sesamin, as potent inhibitors of human cytochrome P450 46A1: Implications for treating excitatory neurotoxicity

CYP46A1 is a brain-specific enzyme responsible for cholesterol homeostasis. Inhibition of CYP46A1 activity serves as a therapeutic target for excitatory neurotoxicity. Sesame is a common medicine and food resource; its component lignans possess various pharmacological activities. In this study, the inhibitory effects of sesame lignans on CYP46A1 activity were investigated. Inhibition kinetics analyses revealed that sesamin and sesamolin produce mixed partial competitive inhibition of CYP46A1, while sesamol produces non-competitive inhibition. Notably, molecular simulations revealed that the sesame lignans have excellent orientations within the active cavity of CYP46A1. Importantly, the sesame lignans had high permeability coefficients and low efflux ratios. Furthermore, sesamin significantly reduced the levels of 24S-hydroxycholesterol in rat plasma and brain tissues, and down-regulated the protein expressions of CYP46A1, NMDAR2A, NMDAR2B, and HMGCR. Collectively, sesame lignans exhibit significant inhibitory effects on CYP46A1 activity, highlighting their potential therapeutic role in treating excitatory neurotoxicity.

Effect of Sublethal Copper Overload on Cholesterol De Novo Synthesis in Undifferentiated Neuronal Cells

Although copper (Cu) is an essential trace metal for cells, it can induce harmful effects as it participates in the Fenton reaction. Involuntary exposure to Cu overload is much more common than expected and has been linked with neurodegeneration, particularly with Alzheimer's disease (AD) evidenced by a positive correlation between free Cu in plasma and the severity of the disease. It has been suggested that Cu imbalance alters cholesterol (Chol) homeostasis and that high membrane Chol promotes the amyloidogenic processing of the amyloid precursor protein (APP) secreting the β-amyloid (Aβ) peptide. Despite the wide knowledge on the effects of Cu in mature brain metabolism, the consequence of its overload on immature neurons remains unknown. Therefore, we used an undifferentiated human neuroblastoma cell line (SH-SY5Y) to analyze the effect of sublethal concentrations of Cu on 1- de novo Chol synthesis and membrane distribution; 2-APP levels in cells and its distribution in membrane rafts; 3-the levels of Aβ in the culture medium. Our results demonstrated that Cu increases reactive oxygen species (ROS) and favors Chol de novo synthesis in both ROS-dependent and independent manners. Also, at least part of these effects was due to the activation of 3-hydroxy-3-methyl glutaryl CoA reductase (HMGCR). In addition, Cu increases the Chol/PL ratio in the cellular membranes, specifically Chol content in membrane rafts. We found no changes in total APP cell levels; however, its presence in membrane rafts increases with the consequent increase of Aβ in the culture medium. We conclude that Cu overload favors Chol de novo synthesis in both ROS-dependent and independent manners, being at least in part, responsible for the high Chol levels found in the cell membrane and membrane rafts. These may promote the redistribution of APP into the rafts, favoring the amyloidogenic processing of this protein and increasing the levels of Aβ.

Six genetically linked mutations in the CD36 gene significantly delay the onset of Alzheimer's disease

The risk of Alzheimer’s disease (AD) has a strong genetic component, also in the case of late-onset AD (LOAD). Attempts to sequence whole genome in large populations of subjects have identified only a few mutations common to most of the patients with AD. Targeting smaller well-characterized groups of subjects where specific genetic variations in selected genes could be related to precisely defined psychological traits typical of dementia is needed to better understand the heritability of AD. More than one thousand participants, categorized according to cognitive deficits, were assessed using 14 psychometric tests evaluating performance in five cognitive domains (attention/working memory, memory, language, executive functions, visuospatial functions). CD36 was selected as a gene previously shown to be implicated in the etiology of AD. A total of 174 polymorphisms were tested for associations with cognition-related traits and other AD-relevant data using the next generation sequencing. Several associations between single nucleotide polymorphisms (SNP’s) and the cognitive deficits have been found (rs12667404 with language performance, rs3211827 and rs41272372 with executive functions, rs137984792 with visuospatial performance). The most prominent association was found between a group of genotypes in six genetically linked and the age at which the AD patients presented with, or developed, a full-blown dementia. The identified alleles appear to be associated with a delay in the onset of LOAD. In silico studies suggested that the SNP’s alter the expression of CD36 thus potentially affecting CD36-related neuroinflammation and other molecular and cellular mechanisms known to be involved in the neuronal loss leading to AD. The main outcome of the study is an identification of a set of six new mutations apparently conferring a distinct protection against AD and delaying the onset by about 8 years. Additional mutations in CD36 associated with certain traits characteristic of the cognitive decline in AD have also been found.

Different functions of vitamin E homologues in the various types of cell death induced by oxysterols

24(S)-Hydroxycholesterol (24S-OHC) and 25-hydroxycholesterol (25-OHC) are produced by cholesterol 24-hydroxylase and cholesterol 25-hydroxylase, respectively. The purpose of the present study was to determine the type of cell death induced by these oxysterols in neuronal cells, hepatic cells, and keratinocytes, and to elucidate the inhibitory effect of vitamin E homologues on various types of cell death. In human neuronal cells (SH-SY5Y cells), 24S-OHC and 25-OHC caused a cell death that was independent of caspase activation. We reported previously that the esterification of 24S-OHC by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) and the resulting formation of a lipid droplet (LD)-like structure are responsible for the 24S-OHC-induced neuronal cell death. Here, we found that 25-OHC also induced ACAT1-mediated 25-OHC esterification and LD formation in neuronal cells. 25-OHC-induced cell death was inhibited by α-tocopherol (α-Toc) but not by α-tocotrienol (α-Toc3), as observed for 24S-OHC-induced cell death in SH-SY5Y cells. In human hepatic cells (HepG2 cells), these oxysterols caused a cell death that was caspase- and oxysterol-esterification-independent. This cell death was suppressed by both α-Toc and α-Toc3, suggesting the involvement of free-radical-mediated lipid peroxidation in the cell death induced by these oxysterols in hepatic cells. In human keratinocytes (HaCaT cells), these oxysterols caused a caspase-dependent but oxysterol-esterification-independent cell death that was inhibited by α-Toc but not by α-Toc3. These results suggest that α-Toc and α-Toc3 act as radical-scavenging antioxidants against oxysterol-induced cell death in the same way in hepatic cells, whereas their behavior is different in inhibition of cell death in neuronal cells and keratinocytes. Collectively, these results demonstrated that 24S-OHC and 25-OHC induced the same type of cell death in each of the cell types examined, and that α-Toc and α-Toc3 exerted different effects, depending on the type of cell death.

Serum Metabolites Differentiate Amnestic Mild Cognitive Impairment From Healthy Controls and Predict Early Alzheimer's Disease via Untargeted Lipidomics Analysis

Background and Aim: Alzheimer's disease (AD) is the most common type of dementia and presents with metabolic perturbations early in the disease process. In order to explore biomarkers useful in predicting early AD, we compared serum metabolites among patients suffering different stages of AD.

Methods: We recruited 107 participants including 23 healthy controls (HC), 21 amnestic mild cognitive impairment (aMCI), 24 non-amnestic mild cognitive impairment (naMCI) and 39 AD patients. Via liquid chromatography-mass spectrometry based serum untargeted lipidomics analysis, we compared differences in serum lipid metabolites among these patient groups and further elucidated biomarkers that differentiate aMCI from HC.

Results: There were significant differences of serum lipid metabolites among the groups, and 20 metabolites were obtained under negative ion mode from HC and aMCI comparison. Notably, 16:3 cholesteryl ester, ganglioside GM3 (d18:1/9z-18:1) and neuromedin B were associated with cognition and increased the predictive effect of aMCI to 0.98 as revealed by random forest classifier. The prediction model composed of MoCA score, 16:3 cholesteryl ester and ganglioside GM3 (d18:1/9z-18:1) had good predictive performance for aMCI. Glycerophospholipid metabolism was a pathway common among HC/aMCI and aMCI/AD groups.

Conclusion: This study provides preliminary evidence highlighting that 16:3 cholesteryl ester were useful for AD disease monitoring while ganglioside GM3 (d18:1/9z-18:1) and neuromedin B discriminated aMCI from HC, which can probably be applied in clinic for early predicting of AD.

Fatty acids and evolving roles of their proteins in neurological, cardiovascular disorders and cancers

The dysregulation of fat metabolism is involved in various disorders, including neurodegenerative, cardiovascular, and cancers. The uptake of long-chain fatty acids (LCFAs) with 14 or more carbons plays a pivotal role in cellular metabolic homeostasis. Therefore, the uptake and metabolism of LCFAs must constantly be in tune with the cellular, metabolic, and structural requirements of cells. Many metabolic diseases are thought to be driven by the abnormal flow of fatty acids either from the dietary origin and/or released from adipose stores. Cellular uptake and intracellular trafficking of fatty acids are facilitated ubiquitously with unique combinations of fatty acid transport proteins and cytoplasmic fatty acid-binding proteins in every tissue. Extensive data are emerging on the defective transporters and metabolism of LCFAs and their clinical implications. Uptake and metabolism of LCFAs are crucial for the brain's functional development and cardiovascular health and maintenance. In addition, data suggest fatty acid metabolic transporter can normalize activated inflammatory response by reprogramming lipid metabolism in cancers. Here we review the current understanding of how LCFAs and their proteins contribute to the pathophysiology of three crucial diseases and the mechanisms involved in the processes.

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

The Potential Effects of Phytoestrogens: The Role in Neuroprotection

Phytoestrogens are naturally occurring non-steroidal phenolic plant compounds. Their structure is similar to 17-β-estradiol, the main female sex hormone. This review offers a concise summary of the current literature on several potential health benefits of phytoestrogens, mainly their neuroprotective effect. Phytoestrogens lower the risk of menopausal symptoms and osteoporosis, as well as cardiovascular disease. They also reduce the risk of brain disease. The effects of phytoestrogens and their derivatives on cancer are mainly due to the inhibition of estrogen synthesis and metabolism, leading to antiangiogenic, antimetastatic, and epigenetic effects. The brain controls the secretion of estrogen (hypothalamus-pituitary-gonads axis). However, it has not been unequivocally established whether estrogen therapy has a neuroprotective effect on brain function. The neuroprotective effects of phytoestrogens seem to be related to both their antioxidant properties and interaction with the estrogen receptor. The possible effects of phytoestrogens on the thyroid cause some concern; nevertheless, generally, no serious side effects have been reported, and these compounds can be recommended as health-promoting food components or supplements.

The Controversial Role of 24-S-Hydroxycholesterol in Alzheimer's Disease

The development of Alzheimer’s disease (AD) is influenced by several events, among which the dysregulation of cholesterol metabolism in the brain plays a major role. Maintenance of brain cholesterol homeostasis is essential for neuronal functioning and brain development. To maintain the steady-state level, excess brain cholesterol is converted into the more hydrophilic metabolite 24-S-hydroxycholesterol (24-OHC), also called cerebrosterol, by the neuron-specific enzyme CYP46A1. A growing bulk of evidence suggests that cholesterol oxidation products, named oxysterols, are the link connecting altered cholesterol metabolism to AD. It has been shown that the levels of some oxysterols, including 27-hydroxycholesterol, 7β-hydroxycholesterol and 7-ketocholesterol, significantly increase in AD brains contributing to disease progression. In contrast, 24-OHC levels decrease, likely due to neuronal loss. Among the different brain oxysterols, 24-OHC is certainly the one whose role is most controversial. It is the dominant oxysterol in the brain and evidence shows that it represents a signaling molecule of great importance for brain function. However, numerous studies highlighted the potential role of 24-OHC in favoring AD development, since it promotes neuroinflammation, amyloid β (Aβ) peptide production, oxidative stress and cell death. In parallel, 24-OHC has been shown to exert several beneficial effects against AD progression, such as preventing tau hyperphosphorylation and Aβ production. In this review we focus on the current knowledge of the controversial role of 24-OHC in AD pathogenesis, reporting a detailed overview of the findings about its levels in different AD biological samples and its noxious or neuroprotective effects in the brain. Given the relevant role of 24-OHC in AD pathophysiology, its targeting could be useful for disease prevention or slowing down its progression.

Study on the neuroprotective effects of Genistein on Alzheimer's disease

Along with the aging of the world population, the incidence rate of Alzheimer's disease (AD) has been increasing. At present, AD has become one of the most serious problems faced by modern medicine. Studies have shown that estrogen has a positive effect on AD, but estrogen has the side effect of leading to tumors. Recent in vivo studies have shown that genistein, one of the selective estrogen receptor modulators (SERMs), can improve brain function through the blood-brain barrier (BBB), antagonize the toxicity of amyloid β-protein (Aβ), that is, to inhibit neurotoxicity due to aggregation of beta amyloid protein, and have neuroprotective effects. In addition, the use of Gen can avoid the risk of endometrial cancer and breast cancer caused by estrogen therapy while exerting an estrogen-like effect, which has some potential for the delay and treatment of AD.

Phytosterols: Targeting Neuroinflammation in Neurodegeneration

Plant-derived sterols, phytosterols, are well known for their cholesterol-lowering activity in serum and their anti-inflammatory activities. Recently, phytosterols have received considerable attention due to their beneficial effects on various non-communicable diseases, and recommended use as daily dietary components. The signaling pathways mediated in the brain by phytosterols have been evaluated, but little is known about their effects on neuroinflammation, and no clinical studies have been undertaken on phytosterols of interest. In this review, we discuss the beneficial roles of phytosterols, including their attenuating effects on inflammation, blood cholesterol levels, and hallmarks of the disease, and their regulatory effects on neuroinflammatory disease pathways. Despite recent advancements made in phytosterol pharmacology, some critical questions remain unanswered. Therefore, we have tried to highlight the potential of phytosterols as viable therapeutics against neuroinflammation and to direct future research with respect to clinical applications.

Oxysterols present in Alzheimer's disease brain induce synaptotoxicity by activating astrocytes: A major role for lipocalin-2

Among Alzheimer's disease (AD) brain hallmarks, the presence of reactive astrocytes was demonstrated to correlate with neuronal loss and cognitive deficits. Evidence indeed supports the role of reactive astrocytes as mediators of changes in neurons, including synapses. However, the complexity and the outcomes of astrocyte reactivity are far from being completely elucidated. Another key role in AD pathogenesis is played by alterations in brain cholesterol metabolism. Oxysterols (cholesterol oxidation products) are crucial for brain cholesterol homeostasis, and we previously demonstrated that changes in the brain levels of various oxysterols correlate with AD progression. Moreover, oxysterols have been shown to contribute to various pathological mechanisms involved in AD pathogenesis. In order to deepen the role of oxysterols in AD, we investigated whether they could contribute to astrocyte reactivity, and consequently impact on neuronal health. Results showed that oxysterols present in mild or severe AD brains induce a clear morphological change in mouse primary astrocytes, accompanied by the upregulation of some reactive astrocyte markers, including lipocalin-2 (Lcn2). Moreover, astrocyte conditioned media analysis revealed a significant increase in the release of Lcn2, cytokines, and chemokines in response to oxysterols. A significant reduction of postsynaptic density protein 95 (PSD95) and a concurrent increase in cleaved caspase-3 protein levels have been demonstrated in neurons co-cultured with oxysterol-treated astrocytes, pointing out that mediators released by astrocytes have an impact on neurons. Among these mediators, Lcn2 has been demonstrated to play a major role on synapses, affecting neurite morphology and decreasing dendritic spine density. These data demonstrated that oxysterols present in the AD brain promote astrocyte reactivity, determining the release of several mediators that affect neuronal health and synapses. Lcn2 has been shown to exert a key role in mediating the synaptotoxic effect of oxysterol-treated astrocytes.

24S-hydroxycholesterol: Cellular effects and variations in brain diseases

The adult brain exhibits a characteristic cholesterol homeostasis, with low synthesis rate and active catabolism. Brain cholesterol turnover is possible thanks to the action of the enzyme cytochrome P450 46A1 (CYP46A1) or 24-cholesterol hydroxylase, that transforms cholesterol into 24S-hydroxycholesterol (24S-HC). But before crossing the blood-brain barrier (BBB), this oxysterol, that is the most abundant in the brain, can act locally, affecting the functioning of neurons, astrocytes, oligodendrocytes, and vascular cells. The first part of this review addresses different aspects of 24S-HC production and elimination from the brain. The second part concentrates in the effects of 24S-HC at the cellular level, describing how this oxysterol affects cell viability, amyloid β production, neurotransmission, and transcriptional activity. Finally, the role of 24S-HC in Alzheimer, Huntington and Parkinson diseases, multiple sclerosis and amyotrophic lateral sclerosis, as well as the possibility of using this oxysterol as predictive and/or evolution biomarker in different brain disorders is discussed.

CD36 in Alzheimer's Disease: An Overview of Molecular Mechanisms and Therapeutic Targeting

CD36 is a membrane protein with wide distribution in the human body, is enriched in the monocyte-macrophage system and endothelial cells, and is involved in the cellular uptake of long chain fatty acids (LCFA) and oxidized low-density lipoproteins. It is also a scavenger receptor, binding hydrophobic amyloid fibrils found in the Alzheimer's disease (AD) brain. In neurobiology research, it has been mostly studied in relationship with chronic ischemia and stroke, but it was also related to amyloid clearance by microglial phagocytosis. In AD animal models, amyloid binding to CD36 has been consistently correlated with a pro-inflammatory response. Therapeutic approaches have two main focuses: CD36 blockade with monoclonal antibodies or small molecules, which is beneficial in terms of the inflammatory milieu, and upregulation of CD36 for increased amyloid clearance. The balance of the two approaches, centered on microglia, is poorly understood. Furthermore, CD36 evaluation in AD clinical studies is still at a very early stage and there is a gap in the knowledge regarding the impact of LCFA on AD progression and CD36 expression and genetic phenotype. This review summarizes the role played by CD36 in the pathogenic amyloid cascade and explore the translatability of preclinical data towards clinical research.

Simultaneous exposure to noise and carbon monoxide increases the risk of Alzheimer's disease: a literature review

Dementia is a syndrome of cognitive and functional decline, commonly occurring in later life as a result of neurodegenerative and cerebrovascular processes beginning earlier in the life course. An excess of free radicals has an essential role in neurodegenerative diseases and aging. This paper aims to review the effects of noise and carbon monoxide as a risk factor in Alzheimer's disease as well as the role of free radicals in the progress of Alzheimer's disease. Articles included in this review were identified through a search of the databases PubMed, Scopus, and Google Scholar using the search terms Alzheimer's disease, dementia, noise, reactive oxygen species, and Carbon Monoxide. The literature search was restricted to the years 1982 to 2020 and articles published in the English language. The metabolism rate of the body is very high when exposed to noise and carbon monoxide; this leads to overproduction of reactive oxygen species and oxidative stress conditions. Oxidative stress has an essential role in the mechanisms concerned in Alzheimer's disease. In addition to the consequences of noise and a chemical substance on the auditory system, they also have non-auditory effects that affect the brain and induced neurodegenerative disease.

25-hydroxycholesterol impairs neuronal and muscular development in zebrafish

Oxysterols have essential effects on brain homeostasis and their levels are often altered in neurodegenerative and neuroinflammatory diseases. Several studies have demonstrated the cytotoxic effects of 25-HC on different cell lines, however, not much is known about its effects on neurons in vivo. In this study, we examined the effects of 25-HC exposure on the nervous system development in the zebrafish. We showed that survival rate of zebrafish embryos/larvae is significantly decreased at doses of 25-HC above 40 μM. 25-HC was found to affect the motility of zebrafish larvae, primary motor axon and muscle morphology. Furthermore, larvae treated with 25-HC showed a reduced neuronal network and number of HuC-positive cells in the brain. An increased cell death was also observed in both the brain and spinal cord of zebrafish treated with 25-HC. Interestingly, administration of 25-HC at later stages of development (24 and 48 h post fertilization) had no detrimental effects on motor axons. Altogether, our findings show that elevated levels of 25-HC may have important consequences on neuronal development and cell survival.

The role of NOX inhibitors in neurodegenerative diseases

Oxidative stress is a key player in both chronic and acute brain disease due to the higher metabolic demand of the brain. Among the producers of free radicals, NADPH-oxidase (NOX) is a major contributor to oxidative stress in neurological disorders. In the brain, the superoxide produced by NOX is mainly found in leukocytes. However, recent studies have reported that it can be found in several other cell types. NOX has been reported to regulate neuronal signaling, memory processing, and central cardiovascular homeostasis. However, overproduction of NOX can contribute to neurotoxicity, CNS degeneration, and cardiovascular disorders. Regarding the above functions, NOX has been shown to play a crucial role in chronic CNS diseases like Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), multiple sclerosis (MS), and amyotrophic lateral sclerosis (ALS), and in acute CNS disorders such as stroke, spinal cord injury, traumatic brain injury (TBI), and related cerebrovascular diseases. NOX is a multi-subunit complex consisting of two membrane-associated and four cytosolic subunits. Thus, in recent years, inhibition of NOX activity has drawn a great deal of attention from researchers in the field of treating chronic and acute CNS disorders and preventing secondary complications. Mounting evidence has shown that NOX inhibition is neuroprotective and that inhibiting NOX in circulating immune cells can improve neurological disease conditions. This review summarizes recent studies on the therapeutic effects and pharmacological strategies regarding NOX inhibitors in chronic and acute brain diseases and focuses on the hurdles that should be overcome before their clinical implementation.

A Crosstalk Between Brain Cholesterol Oxidation and Glucose Metabolism in Alzheimer's Disease

In Alzheimer’s disease (AD), both cholesterol and glucose dysmetabolism precede the onset of memory deficit and contribute to the disease’s progression. It is indeed now believed that oxidized cholesterol in the form of oxysterols and altered glucose uptake are the main triggers in AD affecting production and clearance of Aβ, and tau phosphorylation. However, only a few studies highlight the relationship between them, suggesting the importance of further extensive studies on this topic. Recently, a molecular link was demonstrated between cholesterol oxidative metabolism and glucose uptake in the brain. In particular, 27-hydroxycholesterol, a key linker between hypercholesterolemia and the increased AD risk, is considered a biomarker for reduced glucose metabolism. In fact, its excess increases the activity of the renin-angiotensin system in the brain, thus reducing insulin-mediated glucose uptake, which has a major impact on brain functioning. Despite this important evidence regarding the role of 27-hydroxycholesterol in regulating glucose uptake by neurons, the involvement of other cholesterol oxidation products that have been clearly demonstrated to be key players in AD cannot be ruled out. This review highlights the current understanding of the potential role of cholesterol and glucose dysmetabolism in AD progression, and the bidirectional crosstalk between these two phenomena.

NF-κB-mediated inflammatory damage is differentially affected in SH-SY5Y and C6 cells treated with 27-hydroxycholesterol

Previous studies have demonstrated that 27-hydroxycholesterol (27-OHC), a cholesterol metabolite, was involved in the inflammatory process of Alzheimer's disease (AD). The present study aimed to investigate the 27-OHC-induced inflammatory damage to neurons and astrocytes and the underlying mechanism(s) accounting for this damage. Human neuroblastoma cells (SH-SY5Y cells) and rat glioma cells (C6 cells) were treated with vehicle or 27-OHC (5, 10, or 20 μM) for 24 hr. The levels of secreted interleukin-1β (IL-1β), interleukin-10 (IL-10), tumor necrosis factor alpha (TNF-α), and inducible nitric oxide synthase (iNOS) were determined by using an enzyme-linked immunosorbent assay (ELISA). Immunofluorescence staining was used to determine the cellular expression of toll-like receptor 4 (TLR4) and transforming growth factor-β (TGF-β). The mRNA and protein expression levels of nuclear factor-κB p65 (NF-κB p65), nuclear factor-κB p50 (NF-κB p50) and cyclooxygenase-2 (COX-2) in both SH-SY5Y and C6 cells were also detected by real-time PCR and Western blot, respectively. The results of this study showed that 27-OHC treatment increased secretion of TNF-α and iNOS and decreased secretion of IL-10, upregulated expression of TGF-β, NF-κB p65 and p50, and downregulated expression of COX-2 in SH-SY5Y cells. In C6 cells, treatment with 27-OHC resulted in decreased secretion of IL-1β, IL-10, TNF-α, and iNOS, and increased expression of TLR4 and TGF-β. These results suggest that 27-OHC may cause inflammatory damage to neurons by activating the TGF-β/NF-κB signaling pathway and to astrocytes by activating the TLR4/TGF-β signaling, which results in the subsequent release of inflammatory cytokines.

Oxysterols and nuclear receptors

Oxysterols are derivatives of cholesterol and an important regulator of cholesterol metabolism, in part due to their role as ligands for nuclear receptors, such as the liver X receptors. Oxysterols are also known to be ligands for the RAR-related orphan receptors, involved in normal T cell differentiation. However, increasing evidence supports a role for oxysterols in the progression of several diseases. Here, we review recent developments in oxysterol research, highlighting the biological functions that oxysterols exert through their target nuclear receptors: the liver X receptors, estrogen receptors, RAR-related orphan receptors and the glucocorticoid receptor. We also bring the regulation of the immune system into the context of interaction between oxysterols and nuclear receptors, discussing the effect of such interaction on the pro-inflammatory function of macrophages and the development of T cells. Finally, we examine the impact that oxysterols have on various disease models, including cancer, Alzheimer's disease and atherosclerosis, stressing the role of nuclear receptors if previously identified. This review underscores the need to consider the multifaceted roles of oxysterols in terms of multiple receptor engagements and selective modulation of these receptors.

Fatty acid transporting proteins: Roles in brain development, aging, and stroke

Polyunsaturated fatty acids are required for the brain development and significantly impact aging and stroke. Due to the hydrophobicity of fatty acids, fatty acids transportation related proteins that include fatty acid binding proteins (FABPs), long chain acyl-coA synthase (ACS), fatty acid transportation proteins (FATPs), fatty acid translocase (FAT/CD36) and newly reported major facilitator superfamily domain-containing protein (Mfsd2a) play critical roles in the uptake of various fatty acids, especially polyunsaturated fatty acids. They are not only involved in neurodevelopment, but also have great impact on neurological disease, such as aging related dementia and stroke.

A silver lining for 24-hydroxycholesterol in Alzheimer's disease: The involvement of the neuroprotective enzyme sirtuin 1

It is now established that cholesterol oxidation products (oxysterols) are involved in several events underlying Alzheimer's disease (AD) pathogenesis. Of note, certain oxysterols cause neuron dysfunction and degeneration but, recently, some of them have been shown also to have neuroprotective effects. The present study, which aimed to understand the potential effects of 24-hydroxycholesterol (24-OH) against the intraneuronal accumulation of hyperphosphorylated tau protein, stressed these latter effects. A beneficial effect of 24-OH was demonstrated in SK-N-BE neuroblastoma cells, and is due to its ability to modulate the deacetylase sirtuin 1 (SIRT1), which contributes to preventing the neurotoxic accumulation of the hyperphosphorylated tau protein. Unlike 24-OH, 7-ketocholesterol (7-K) did not modulate the SIRT1-dependent neuroprotective pathway. To confirm the neuroprotective role of 24-OH, in vivo experiments were run on mice that express human tau without spontaneously developing tau pathology (hTau mice), by means of the intracerebroventricular injection of 24-OH. 24-OH, unlike 7-K, was found to completely prevent the hyperphosphorylation of tau induced by amyloid β monomers. These data highlight the importance of preventing the loss of 24-OH in the brain, and of maintaining high levels of the enzyme SIRT1, in order to counteract neurodegeneration.

Graphical abstract

A hypothetical scheme of the molecular mechanisms underlying the effects of 24-OH on hyperphosphorylated tau accumulation.fx1

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24-OH, unlike 7-K, upregulates the neuroprotective enzyme SIRT1.

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24-OH induces a cell redox imbalance leading to SIRT1 activation.

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24-OH prevents the accumulation of hyperphosphorylated tau.

Strikingly different effects of cholesterol and 7-ketocholesterol on lipid bilayer-mediated aggregation of amyloid beta (1-42)

Oxidized cholesterol has been widely reported to contribute to the pathogenesis of Alzheimer's disease (AD). However, the mechanism by which they affect the disease is not fully understood. Herein, we aimed to investigate the effect of 7-ketocholesterol (7keto) on membrane-mediated aggregation of amyloid beta (Aβ-42), one of the critical pathogenic events in AD. We have shown that when cholesterol is present in lipid vesicles, kinetics of Aβ nuclei formation is moderately hindered while that of fibril growth was considerably accelerated. The partial substitution of cholesterol with 7keto slightly enhanced the formation of Aβ-42 nuclei and remarkably decreased fibril elongation, thus maintaining the peptide in protofibrillar aggregates, which are reportedly the most toxic species. These findings add in understanding of how cholesterol and its oxidation can affect Aβ-induced cytotoxicity.

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Cholesterol and 7-ketocholesterol membranes had different effects on Aβ aggregation.

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Cholesterol-containing membranes considerably accelerated fibril elongation of Aβ-42.

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7-ketocholesterol membranes remarkably decreased Aβ-42 fibril elongation.

The level of 24-hydroxycholesteryl esters decreases in plasma of patients with Parkinson's disease

24-hydroxycholesterol (24OH-C) is synthesized almost exclusively in neurons. This oxysterol is mostly present as ester form in both cerebrospinal fluid and plasma. The enzyme lecithin-cholesterol acyltransferase esterifies 24OH-C in the brain, and the level of 24OH-C esters in cerebrospinal fluid was found to be correlated with the level of 24OH-C esters in plasma. Decreased levels of 24OH-C esters levels were previously found in Alzheimer's disease and Amyotrophic Lateral Sclerosis. This finding was attributed to the inhibitory effect of oxidative stress on lecithin-cholesterol acyltransferase activity in neurodegenerative conditions. Data reported here show that the plasma level of 24OH-C esters is decreased also in Parkinson's disease. ROC analysis identified 69.0% of 24OH-C esterification as the threshold (AUC = 0.98) discriminating patients (N = 19) from healthy subjects (N = 19) with 100% specificity vs controls, 89.5% sensitivity, 94.7% accuracy, and 100% precision. The level of 24OH-C esters was not correlated with UPDRS I or UPDRS III when evaluated at the time of blood sampling. By contrast, it was negatively correlated with UPDRS I (r = -0.4984, p = 0.0299) after one year of follow up. Therefore, this level might represent a novel biomarker of neurodegeneration in Parkinson's disease. The biomarker level is here proposed as a measure to evaluate the severity of disease, as well as to monitor the progression of this pathology.

The level of 24-Hydroxycholesteryl Esters is an Early Marker of Alzheimer's Disease

Cholesterol (C) brain accumulation seems to play a role in the Alzheimer's disease (AD) pathogenesis. 24(S)-hydroxycholesterol (24OH-C) is the predominant metabolite of brain C and its synthesis is believed to represent a way to remove excess C from neurons. Previous studies showed that 24OH-C level is altered in patients with neurodegenerative diseases, including AD. Only one study demonstrated that 24OH-C esterification is altered in neurodegenerative diseases, i.e., amyotrophic lateral sclerosis. Herein we analyzed the level of 24OH-C esters (% 24OH-CE) in i) cerebrospinal fluid (CSF) and homologous serum of AD (n = 13) and controls (n = 8); ii) plasma from AD (n = 30), controls (n = 30), mild cognitive impairment (MCI) converting to AD (n = 34), and stable MCI (n = 40). The % 24OH-CE in CSF positively correlated with that in homologous serum and was lower in both CSF and blood from AD patients as compared to controls; moreover, the plasma value of % 24OH-CE was lower in MCI conv-AD than in non-converters. Kaplan Meier Survival curves revealed a significant anticipation of the disease onset in AD and MCI conv-AD subjects with the lowest % 24OH-CE values. In conclusion, the reduction of % 24OH-CE in AD and MCI conv-AD, as well as the anticipation of the disease in patients with the lowest % 24OH-CE, support a role of the cholesterol/lecithin-cholesterol acyltransferase axis in AD onset/progression. Thus, targeting brain cholesterol metabolism could be a valuable strategy to prevent AD associated cognitive decline.

Low-density lipoprotein oxidation biomarkers in human health and disease and effects of bioactive compounds

Based on the significance of oxidized low-density lipoprotein (LDL) in health and disease, this review focuses on human studies addressing oxidation of LDL, including three lines of biomarkers, (i) ex vivo LDL resistance to oxidation, a "challenge test" model, (ii) circulating oxidized LDL, indicating the "current in vivo status", and (iii) autoantibodies against oxidized LDL as fingerprints of an immune response to oxidized LDL, along with circulating oxysterols and 4-hydroxynonenal as biomarkers of lipid peroxidation. Lipid peroxidation and oxidized LDL are hallmarks in the development of various metabolic, cardiovascular and other diseases. Changes further occur across life stages from infancy to older age as well as in athletes and smokers. Given their responsiveness to targeted nutritional interventions, markers of LDL oxidation have been employed in a rapidly growing number of human studies for more than 2 decades. There is growing interest in foods, which, besides providing energy and nutrients, exert beneficial effects on human health, such as protection of DNA, proteins and lipids from oxidative damage. Any health claim, however, needs to be substantiated by supportive evidence derived from human studies, using reliable biomarkers to demonstrate such beneficial effects. A large body of evidence has accumulated, demonstrating protection of LDL from oxidation by bioactive food compounds, including vitamins, other micronutrients and secondary plant ingredients, which will facilitate the selection of oxidation biomarkers for future human intervention studies and health claim support.

Cholesterol-metabolizing enzyme cytochrome P450 46A1 as a pharmacologic target for Alzheimer's disease

Cytochrome P450 46A1 (CYP46A1 or cholesterol 24-hydroxylase) controls cholesterol elimination from the brain and plays a role in higher order brain functions. Genetically enhanced CYP46A1 expression in mouse models of Alzheimer's disease mitigates the manifestations of this disease. We enhanced CYP46A1 activity pharmacologically by treating 5XFAD mice, a model of rapid amyloidogenesis, with a low dose of the anti-HIV medication efavirenz. Efavirenz was administered from 1 to 9 months of age, and mice were evaluated at specific time points. At one month of age, cholesterol homeostasis was already disturbed in the brain of 5XFAD mice. Nevertheless, efavirenz activated CYP46A1 and mouse cerebral cholesterol turnover during the first four months of administration. This treatment time also reduced amyloid burden and microglia activation in the cortex and subiculum of 5XFAD mice as well as protein levels of amyloid precursor protein and the expression of several genes involved in inflammatory response. However, mouse short-term memory and long-term spatial memory were impaired, whereas learning in the context-dependent fear test was improved. Additional four months of drug administration (a total of eight months of treatment) improved long-term spatial memory in the treated as compared to the untreated mice, further decreased amyloid-β content in 5XFAD brain, and also decreased the mortality rate among male mice. We propose a mechanistic model unifying the observed efavirenz effects. We suggest that CYP46A1 activation by efavirenz could be a new anti-Alzheimer's disease treatment and a tool to study and identify normal and pathological brain processes affected by cholesterol maintenance.

The ROS-mediated activation of IL-6/STAT3 signaling pathway is involved in the 27-hydroxycholesterol-induced cellular senescence in nerve cells

The oxysterol 27-hydroxycholesterol (27HC) is a selective estrogen receptor modulator (SERMs), which like endogenous estrogen 17β-estradiol (E₂) induces the proliferation of ER-positive breast cancer cells in vitro. Interestingly, the observation that 27HC induces adverse effects in neural system, distinguishing it from E₂. It has been suggested that high levels of circulating cholesterol increase the entry of 27HC into the brain, which may induce learning and memory impairment. Based on this evidence, 27HC may be associated with neurodegenerative processes and interrupted cholesterol homeostasis in the brain. However, the biological events that participate in this process remain largely elusive. In the present study, we demonstrated that 27HC induced apparent cellular senescence in nerve cells. Senescence-associated β-galactosidase (SA-β-Gal) assay revealed that 27HC induced senescence in both BV2 cells and PC12 cells. Furthermore, we demonstrated that 27HC promoted the accumulation of cellular reactive oxygen species (ROS) in nerve cells and subsequently activation of IL-6/STAT3 signaling pathway. Notably, treatment with the ROS scavenger N-acetylcysteine (NAC) markedly blocked 27HC-induced ROS production and activation of IL-6/STAT3 signaling pathway. Either blocking the generation of ROS or inhibition of IL-6/STAT3 both attenuated 27HC-induced cellular senescence. In sum, these findings not only suggested a mechanism whereby 27HC induced cellular senescence in nerve cells, but also helped to recognize the 27HC as a novel harmful factor in neurodegenerative diseases.

Effect of physical exercise on brain and lipid metabolism in mouse models of multiple sclerosis

Multiple sclerosis (MS) is a central nervous demyelinating disease characterized by cyclic loss and repair of myelin sheaths associated with chronic inflammation and neuronal loss. This degenerative pathology is accompanied by modified levels of oxysterols (oxidative derivatives of cholesterol, implicated in cholesterol metabolism), highlighted in the brain, blood and cerebrospinal fluid of MS patients. The pathological accumulation of such derivatives is thought to participate in the onset and progression of the disease through their implication in inflammation, oxidative stress, demyelination and neurodegeneration. In this context, physical exercise is envisaged as a complementary resource to ameliorate therapeutic strategies. Indeed, physical activity exerts beneficial effects on neuronal plasticity, decreases inflammation and oxidative stress and improves blood-brain integrity in extents that could be beneficial for brain health. The present review attempts to summarize the available data on the positive effect of physical exercise to highlight possible links between physical activity and modulation of cholesterol/oxysterol homeostasis in MS.

Amyloidogenicity and toxicity of the reverse and scrambled variants of amyloid-β 1-42

β‐amyloid 1‐42 (Aβ1‐42) is a self‐assembling peptide that goes through many conformational and morphological changes before forming the fibrils that are deposited in extracellular plaques characteristic of Alzheimer's disease. The link between Aβ1‐42 structure and toxicity is of major interest, in particular, the neurotoxic potential of oligomeric species. Many studies utilise reversed (Aβ42‐1) and scrambled (AβS) forms of amyloid‐β as control peptides. Here, using circular dichroism, thioflavin T fluorescence and transmission electron microscopy, we reveal that both control peptides self‐assemble to form fibres within 24 h. However, oligomeric Aβ reduces cell survival of hippocampal neurons, while Aβ42‐1 and Aβs have reduced effect on cellular health, which may arise from their ability to assemble rapidly to form protofibrils and fibrils.

Contribution of cholesterol and oxysterols to the pathophysiology of Parkinson's disease

Neurodegenerative diseases are a major public health issue worldwide. Some countries, including France, have engaged in research into the causes of Parkinson's disease, Alzheimer's disease, and multiple sclerosis and the management of these patients. It should lead to a better understanding of the mechanisms leading to these diseases including the possible involvement of lipids in their pathogenesis. Parkinson's disease is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra and the accumulation of α-synuclein (Lewy bodies). Several in vivo studies have shown a relationship between the lipid profile [cholesterol, oxidized cholesterol products (oxysterols) formed either enzymatically or by auto-oxidation], the use of drugs regulating cholesterol levels, and the development of Parkinson's disease. Several oxysterols are present in the brain and could play a role in the development of this disease, particularly in the accumulation of α-synuclein, and through various side effects, such as oxidation, inflammation, and cell death. Consequently, in Parkinson's disease, some oxysterols could contribute to the pathophysiology of the disease and constitute potential biomarkers or therapeutic targets.

Current Knowledge about Oxysterols: A Review

For years food consumers have been warned that a cholesterol-rich diet may result in atherosclerosis. It is also well known that consumption of large amounts of phytosterols decreases concentration of low-density lipoproteins (LDLs) in blood (LDLs are regarded a key risk factor in development of cardiovascular diseases). However, no scientific evidence has unambiguously proved any direct connection between amount of consumed cholesterol and LDL level in blood. On the other hand, concentration of cholesterol oxidation products, oxysterols, seems to be indeed relevant; for example, they significantly impact appearance of atherosclerotic lesions (plaques). Phytosterols (like sitosterol or campasterol) decrease LDL level in blood, but on the other hand products of their oxidation are toxic. Therefore, it is worth to know influence of phytosterols on living organisms, processes which lead to their formation, and their levels in popular foodstuffs. This paper is an attempt to review literature data on the above aspects, as well as on impact on living organisms of oxidation products of popular sterols.

24(S)-Hydroxycholesterol as a Modulator of Neuronal Signaling and Survival

The major cholesterol metabolite in brain, 24(S)-hydroxycholesterol (24S-HC), serves as a vehicle for cholesterol removal. Its effects on neuronal function, however, have only recently begun to be investigated. Here, we review that nascent work. Our own studies have demonstrated that 24S-HC has potent positive modulatory effects on N-methyl-d-aspartate (NMDA) receptor (NMDAR) function. This could have implications not only for brain plasticity but also for pathological NMDAR overuse. Other work has demonstrated effects of 24S-HC on neuronal survival and as a possible biomarker of neurodegenerative disease. Depending on circumstances, both upregulation/mimicry of 24S-HC signaling and down-regulation/antagonism may have therapeutic potential. We are interested in the possibility that synthetic analogues of 24S-HC with positive effects at NMDARs may hold neurotherapeutic promise, given the role of NMDA receptor hypofunction in certain neuropsychiatric disorders.

Oxysterols and mechanisms of survival signaling

Oxysterols, a family of oxidation products of cholesterol, are increasingly drawing attention of scientists to their multifaceted biochemical properties, several of them of clear relevance to human pathophysiology. Taken up by cells through both vesicular and non-vesicular ways or often generated intracellularly, oxysterols contribute to modulate not only the inflammatory and immunological response but also cell viability, metabolism and function by modulating several signaling pathways. Moreover, they have been recognized as elective ligands for the most important nuclear receptors. The outcome of such a complex network of intracellular reactions promoted by these cholesterol oxidation products appears to be largely dependent not only on the type of cells, the dynamic conditions of the cellular and tissue environment but also on the concentration of the oxysterols. Here focus has been given to the cascade of molecular events exerted by relatively low concentrations of certain oxysterols that elicit survival and functional signals in the cells, with the aim to contribute to further expand the knowledge about the biological and physiological potential of the biochemical reactions triggered and modulated by oxysterols.