Alzheimer's disease--a dysfunction in cholesterol and lipid metabolism

A cholesterol-centric outlook on steroidogenesis

Cholesterol, an essential and versatile lipid, is the precursor substrate for the biosynthesis of steroid hormones, and a key structural and functional component of organelle membranes in eukaryotic cells. Consequently, the framework of steroidogenesis across main steroidogenic cell types is built around cholesterol, including its cellular uptake, mobilization from intracellular storage, and finally, its transport to the mitochondria where steroidogenesis begins. This setup, which is controlled by different trophic hormones in their respective target tissues, allows steroidogenic cells to meet their steroidogenic need of cholesterol effectively without impinging on the basic need for organelle membranes and their functions. However, our understanding of the basal steroidogenesis (i.e., independent of trophic hormone stimulation), which is a cell-intrinsic trait, remains poor. Particularly, the role that cholesterol itself plays in the regulation of steroidogenic factors and events in steroid hormone-producing cells remains largely unexplored. This is likely because of challenges in selectively targeting the steroidogenic intracellular cholesterol pool in studies. New evidence suggests that cholesterol plays a role in steroidogenesis. These new findings have created new opportunities to advance our understanding in this field. In this book chapter, we will provide a cholesterol-centric view on steroidogenesis and emphasize the importance of the interplay between cholesterol and the mitochondria in steroidogenic cells. Moreover, we will discuss a novel mitochondrial player, prohibitin-1, in this context. The overall goal is to provide a stimulating perspective on cholesterol as an important regulator of steroidogenesis (i.e., more than just a substrate for steroid hormones) and present the mitochondria as a potential cell-intrinsic factor in regulating steroidogenic cholesterol homeostasis.

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.

Discovery and development of brain-penetrant 18F-labeled radioligands for neuroimaging of the sigma-2 receptors

We have discovered and synthesized a series of indole-based derivatives as novel sigma-2 (σ ₂) receptor ligands. Two ligands with high σ ₂ receptor affinity and subtype selectivity were then radiolabeled with F-18 in good radiochemical yields and purities, and evaluated in rodents. In biodistribution studies in male ICR mice, radioligand [¹⁸F]9, or 1-(4-(5,6-dimethoxyisoindolin-2-yl)butyl)-4-(2-[¹⁸F]fluoroethoxy)-1H-indole, was found to display high brain uptake and high brain-to-blood ratio. Pretreatment of animals with the selective σ ₂ receptor ligand CM398 led to significant reductions in both brain uptake (29%-54%) and brain-to-blood ratio (60%-88%) of the radioligand in a dose-dependent manner, indicating high and saturable specific binding of [¹⁸F]9 to σ ₂ receptors in the brain. Further, ex vivo autoradiography in male ICR mice demonstrated regionally heterogeneous specific binding of [¹⁸F]9 in the brain that is consistent with the distribution pattern of σ ₂ receptors. Dynamic positron emission tomography imaging confirmed regionally distinct distribution and high levels of specific binding for [¹⁸F]9 in the rat brain, along with appropriate tissue kinetics. Taken together, results from our current study indicated the novel radioligand [¹⁸F]9 as the first highly specific and promising imaging agent for σ ₂ receptors in the brain.

Alzheimer's Disease, a Lipid Story: Involvement of Peroxisome Proliferator-Activated Receptor α

Alzheimer's disease (AD) is the leading cause of dementia in the elderly. Mutations in genes encoding proteins involved in amyloid-β peptide (Aβ) production are responsible for inherited AD cases. The amyloid cascade hypothesis was proposed to explain the pathogeny. Despite the fact that Aβ is considered as the main culprit of the pathology, most clinical trials focusing on Aβ failed and suggested that earlier interventions are needed to influence the course of AD. Therefore, identifying risk factors that predispose to AD is crucial. Among them, the epsilon 4 allele of the apolipoprotein E gene that encodes the major brain lipid carrier and metabolic disorders such as obesity and type 2 diabetes were identified as AD risk factors, suggesting that abnormal lipid metabolism could influence the progression of the disease. Among lipids, fatty acids (FAs) play a fundamental role in proper brain function, including memory. Peroxisome proliferator-activated receptor α (PPARα) is a master metabolic regulator that regulates the catabolism of FA. Several studies report an essential role of PPARα in neuronal function governing synaptic plasticity and cognition. In this review, we explore the implication of lipid metabolism in AD, with a special focus on PPARα and its potential role in AD therapy.

The Novel Perspectives of Adipokines on Brain Health

First seen as a fat-storage tissue, the adipose tissue is considered as a critical player in the endocrine system. Precisely, adipose tissue can produce an array of bioactive factors, including cytokines, lipids, and extracellular vesicles, which target various systemic organ systems to regulate metabolism, homeostasis, and immune response. The global effects of adipokines on metabolic events are well defined, but their impacts on brain function and pathology remain poorly defined. Receptors of adipokines are widely expressed in the brain. Mounting evidence has shown that leptin and adiponectin can cross the blood–brain barrier, while evidence for newly identified adipokines is limited. Significantly, adipocyte secretion is liable to nutritional and metabolic states, where defective circuitry, impaired neuroplasticity, and elevated neuroinflammation are symptomatic. Essentially, neurotrophic and anti-inflammatory properties of adipokines underlie their neuroprotective roles in neurodegenerative diseases. Besides, adipocyte-secreted lipids in the bloodstream can act endocrine on the distant organs. In this article, we have reviewed five adipokines (leptin, adiponectin, chemerin, apelin, visfatin) and two lipokines (palmitoleic acid and lysophosphatidic acid) on their roles involving in eating behavior, neurotrophic and neuroprotective factors in the brain. Understanding and regulating these adipokines can lead to novel therapeutic strategies to counteract metabolic associated eating disorders and neurodegenerative diseases, thus promote brain health.

Docosanoids and elovanoids from omega-3 fatty acids are pro-homeostatic modulators of inflammatory responses, cell damage and neuroprotection

The functional significance of the selective enrichment of the omega-3 essential fatty acid docosahexaenoic acid (DHA; 22C and 6 double bonds) in cellular membrane phospholipids of the nervous system is being clarified by defining its specific roles on membrane protein function and by the uncovering of the bioactive mediators, docosanoids and elovanoids (ELVs). Here, we describe the preferential uptake and DHA metabolism in photoreceptors and brain as well as the significance of the Adiponectin receptor 1 in DHA retention and photoreceptor cell (PRC) survival. We now know that this integral membrane protein is engaged in DHA retention as a necessary event for the function of PRCs and retinal pigment epithelial (RPE) cells. We present an overview of how a) NPD1 selectively mediates preconditioning rescue of RPE and PR cells; b) NPD1 restores aberrant neuronal networks in experimental epileptogenesis; c) the decreased ability to biosynthesize NPD1 in memory hippocampal areas of early stages of Alzheimer's disease takes place; d) NPD1 protection of dopaminergic circuits in an in vitro model using neurotoxins; and e) bioactivity elicited by DHA and NPD1 activate a neuroprotective gene-expression program that includes the expression of Bcl-2 family members affected by Aβ42, DHA, or NPD1. In addition, we highlight ELOVL4 (ELOngation of Very Long chain fatty acids-4), specifically the neurological and ophthalmological consequences of its mutations, and their role in providing precursors for the biosynthesis of ELVs. Then we outline evidence of ELVs ability to protect RPE cells, which sustain PRC integrity. In the last section, we present a summary of the protective bioactivity of docosanoids and ELVs in experimental ischemic stroke. The identification of early mechanisms of neural cell survival mediated by DHA-synthesized ELVs and docosanoids contributes to the understanding of cell function, pro-homeostatic cellular modulation, inflammatory responses, and innate immunity, opening avenues for prevention and therapeutic applications in neurotrauma, stroke and neurodegenerative diseases.

Oleic Acid and Hydroxytyrosol Inhibit Cholesterol and Fatty Acid Synthesis in C6 Glioma Cells

Recently, the discovery of natural compounds capable of modulating nervous system function has revealed new perspectives for a healthier brain. Here, we investigated the effects of oleic acid (OA) and hydroxytyrosol (HTyr), two important extra virgin olive oil compounds, on lipid synthesis in C6 glioma cells. OA and HTyr inhibited both de novo fatty acid and cholesterol syntheses without affecting cell viability. The inhibitory effect of the individual compounds was more pronounced if OA and HTyr were administered in combination. A reduction of polar lipid biosynthesis was also detected, while triglyceride synthesis was marginally affected. To clarify the lipid-lowering mechanism of these compounds, their effects on the activity of key enzymes of fatty acid biosynthesis (acetyl-CoA carboxylase-ACC and fatty acid synthase-FAS) and cholesterologenesis (3-hydroxy-3-methylglutaryl-CoA reductase-HMGCR) were investigated in situ by using digitonin-permeabilized C6 cells. ACC and HMGCR activities were especially reduced after 4 h of 25 μM OA and HTyr treatment. No change in FAS activity was observed. Inhibition of ACC and HMGCR activities is corroborated by the decrease of their mRNA abundance and protein level. Our results indicate a direct and rapid downregulatory effect of the two olive oil compounds on lipid synthesis in C6 cells.

Alzheimer's disease and metabolic syndrome: A link from oxidative stress and inflammation to neurodegeneration

Alzheimer's disease (AD) is the most common cause of dementia and one of the most important causes of morbidity and mortality among the aging population. AD diagnosis is made post-mortem, and the two pathologic hallmarks, particularly evident in the end stages of the illness, are amyloid plaques and neurofibrillary tangles. Currently, there is no curative treatment for AD. Additionally, there is a strong relation between oxidative stress, metabolic syndrome, and AD. The high levels of circulating lipids and glucose imbalances amplify lipid peroxidation that gradually diminishes the antioxidant systems, causing high levels of oxidative metabolism that affects cell structure, leading to neuronal damage. Accumulating evidence suggests that AD is closely related to a dysfunction of both insulin signaling and glucose metabolism in the brain, leading to an insulin-resistant brain state. Four drugs are currently used for this pathology: Three FDA-approved cholinesterase inhibitors and one NMDA receptor antagonist. However, wide varieties of antioxidants are promissory to delay or prevent the symptoms of AD and may help in treating the disease. Therefore, therapeutic efforts to achieve attenuation of oxidative stress could be beneficial in AD treatment, attenuating Aβ-induced neurotoxicity and improve neurological outcomes in AD. The term inflammaging characterizes a widely accepted paradigm that aging is accompanied by a low-grade chronic up-regulation of certain pro-inflammatory responses in the absence of overt infection, and is a highly significant risk factor for both morbidity and mortality in the elderly.

Food-induced changes of lipids in rat neuronal tissue visualized by ToF-SIMS imaging

Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to image the lipid localization in brain tissue sections from rats fed specially processed cereals (SPC). An IonTof 5 instrument equipped with a Bi cluster ion gun was used to analyze the tissue sections. Data from 15 brain samples from control and cereal-fed rats were recorded and exported to principal components analysis (PCA). The data clearly show changes of certain lipids in the brain following cereal feeding. PCA score plots show a good separation in lipid distribution between the control and the SPC-fed group. The loadings plot reveal that the groups separated mainly due to changes in cholesterol, vitamin E and c18:2, c16:0 fatty acid distribution as well as some short chain monocarboxylic fatty acid compositions. These insights relate to the working mechanism of SPC as a dietary supplement. SPC is thought to activate antisecretory factor (AF), an endogenous protein with regulatory function for inflammation and fluid secretion. These data provide insights into lipid content in brain following SPC feeding and suggest a relation to activating AF.

Metal-sulfate induced generation of ROS in human brain cells: detection using an isomeric mixture of 5- and 6-carboxy-2',7'-dichlorofluorescein diacetate (carboxy-DCFDA) as a cell permeant tracer

Evolution of reactive oxygen species (ROS), generated during the patho-physiological stress of nervous tissue, has been implicated in the etiology of several progressive human neurological disorders including Alzheimer’s disease (AD) and amylotrophic lateral sclerosis (ALS). In this brief communication we used mixed isomers of 5-(and-6)-carboxy-2′,7′-dichlorofluorescein diacetate (carboxy-DCFDA; C₂₅H₁₄C_l2O₉; MW 529.3), a novel fluorescent indicator, to assess ROS generation within human neuronal-glial (HNG) cells in primary co-culture. We introduced pathological stress using the sulfates of 12 environmentally-, industrially- and agriculturally-relevant divalent and trivalent metals including Al, Cd, Cu, Fe, Hg, Ga, Mg, Mn, Ni, Pb, Sn and Zn. In this experimental test system, of all the metal sulfates analyzed, aluminum sulfate showed by far the greatest ability to induce intracellular ROS. These studies indicate the utility of using isomeric mixtures of carboxy-H₂DCFDA diacetates as novel and highly sensitive, long-lasting, cell-permeant, fluorescein-based tracers for quantifying ROS generation in intact, metabolizing human brain cells, and in analyzing the potential epigenetic contribution of different metal sulfates to ROS-generation and ROS-mediated neurological dysfunction.

Amyloid beta (Aβ) peptide modulators and other current treatment strategies for Alzheimer's disease (AD)

Introduction: Alzheimer's disease (AD) is a common, progressive neurological disorder whose incidence is reaching epidemic proportions. The prevailing "amyloid cascade hypothesis," which maintains that the aberrant proteolysis of beta-amyloid precursor protein (βAPP) into neurotoxic amyloid beta (Aβ) peptides is central to the etiopathology of AD, continues to dominate pharmacological approaches to the clinical management of this insidious disorder. This review is a compilation and update on current pharmacological strategies designed to down-regulate Aβ42 peptide generation in an effort to ameliorate the tragedy of AD. Areas covered: This review utilized online data searches at various open online-access websites including the Alzheimer Association, Alzheimer Research Forum; individual drug company databases; the National Institutes of Health (NIH) Medline; Pharmaprojects database; Scopus; inter-University research communications; and unpublished research data. Expert opinion: Anti-acetylcholinesterase-, chelation-, N-methyl-D-aspartate (NMDA) receptor antagonist-, statin-, Aβ immunization-, β-secretase-, γ-secretase-based, and other strategies to modulate βAPP processing, have dominated pharmacological approaches directed against AD-type neurodegenerative pathology. Cumulative clinical results of these efforts remain extremely disappointing, and have had little overall impact on the clinical management of AD. While a number of novel approaches are in consideration and development, to date there is still no effective treatment or cure for this expanding healthcare concern.

Effects of diet and behavioral enrichment on free fatty acids in the aged canine brain

Despite several recent studies suggesting that dysregulation of brain lipid metabolism might contribute to the mechanisms of aging and Alzheimer's disease (AD), lipid metabolism has not been evaluated extensively in the aging brain. Here, we use a lipidomic approach to demonstrate that antioxidants plus mitochondrial cofactors treatment, either alone or in combination with behavioral enrichment, attenuates lipid abnormalities in the frontal cortices of aged canine in a manner correlated with cognitive scores. Our analyses revealed that the levels of free palmitoleic acid and nervonic acid were decreased in frontal cortices of aged dogs (n=5-6/group) treated with antioxidant compared with the control group. The monounsaturated/saturated fatty acid ratio, also known as "desaturation index"-an ex-vivo indicator of stearoyl-CoA desaturase activity, was also reduced in the frontal cortex of dogs treated with antioxidants compared with control groups. Increased palmitoleic acid levels and desaturation index were positively correlated with increased reversal learning errors and decreased cognitive performance. In conclusion, our study indicates that the addition of antioxidants and mitochondrial cofactors to the regular diet alters the composition of free fatty acids in the aged brain. Together with data showing increased palmitoleic acid levels in AD patients, our data suggest that reducing palmitoleic acid levels and desaturation index in the brain may be associated with improved cognitive performance.

Elevated stearoyl-CoA desaturase in brains of patients with Alzheimer's disease

The molecular bases of Alzheimer's disease (AD) remain unclear. We used a lipidomic approach to identify lipid abnormalities in the brains of subjects with AD (N = 37) compared to age-matched controls (N = 17). The analyses revealed statistically detectable elevations in levels of non-esterified monounsaturated fatty acids (MUFAs) and mead acid (20:3n-9) in mid-frontal cortex, temporal cortex and hippocampus of AD patients. Further studies showed that brain mRNAs encoding for isoforms of the rate-limiting enzyme in MUFAs biosynthesis, stearoyl-CoA desaturase (SCD-1, SCD-5a and SCD-5b), were elevated in subjects with AD. The monounsaturated/saturated fatty acid ratio (‘desaturation index’) – displayed a strong negative correlation with measures of cognition: the Mini Mental State Examination test (r = −0.80; P = 0.0001) and the Boston Naming test (r = −0.57; P = 0.0071). Our results reveal a previously unrecognized role for the lipogenic enzyme SCD in AD.

The effects of cholesterol on learning and memory

Cholesterol is vital to normal brain function including learning and memory but that involvement is as complex as the synthesis, metabolism and excretion of cholesterol itself. Dietary cholesterol influences learning tasks from water maze to fear conditioning even though cholesterol does not cross the blood brain barrier. Excess cholesterol has many consequences including peripheral pathology that can signal brain via cholesterol metabolites, pro-inflammatory mediators and antioxidant processes. Manipulations of cholesterol within the central nervous system through genetic, pharmacological, or metabolic means circumvent the blood brain barrier and affect learning and memory but often in animals already otherwise compromised. The human literature is no less complex. Cholesterol reduction using statins improves memory in some cases but not others. There is also controversy over statin use to alleviate memory problems in Alzheimer's disease. Correlations of cholesterol and cognitive function are mixed and association studies find some genetic polymorphisms are related to cognitive function but others are not. In sum, the field is in flux with a number of seemingly contradictory results and many complexities. Nevertheless, understanding cholesterol effects on learning and memory is too important to ignore.

Minocycline recovers MTT-formazan exocytosis impaired by amyloid beta peptide

Minocycline, a tetracycline antibiotic, has been reported to exert beneficial effects in models of Alzheimer's disease (AD). To characterize the mechanisms underlying the putative minocycline-related neuroprotection, we studied its effect in an in vitro model of AD. Primary hippocampal cultures were treated with β-amyloid peptide (Aβ) and cell viability was assessed by standard MTT-assay. Incubation with 10 μM Aβ for 24 h significantly inhibits cellular MTT-reduction without inducing morphological signs of enhanced cell death or increase in release of lactate dehydrogenase. This indicates that cell viability was not affected. The inhibition of MTT-reduction by Aβ was due to an acceleration of MTT-formazan exocytosis. Intriguingly, the Aβ-triggered increase in MTT-formazan exocytosis was abolished by co-treatment with minocycline. In vehicle-treated cells minocycline had no effect on formazan exocytosis. This hitherto unrecognized property of minocycline has to be noticed in the elucidation of the underlying mechanism of this promising neuroprotectant.

Phosphorylated extracellular signal-regulated kinase 1/2 immunoreactivity and its protein levels in the gerbil hippocampus during normal aging

Phosphorylated extracellular signal-regulated kinase (pERK) mediates neuronal synaptic plasticity, long-term potentiation, and learning and memory in the hippocampus. In this study, we examined pERK1/2 immunoreactivity and its protein level in the gerbil hippocampus at various ages. In the postnatal month 1 (PM 1) group, very weak pERK1/2 immunoreactivity was detected in the hippocampus. In the CA1 region, pERK1/2 immunoreactivity was considerably increased in the stratum pyramidale in the PM 6 group. Thereafter, pERK1/2 immunoreactivity was decreased. In the CA2/3 region, pERK1/2 immunoreactivity increased in an age-dependent manner until PM 12. Thereafter, numbers of pERK1/2-immunoreactive neurons were decreased. However, in the mossy fiber zone, pERK1/2 immunostaining became stronger with age. In the dentate gyrus, a few pERK1/2-immunoreactive cells were observed until PM 12. In the PM 18 and 24 groups, numbers of pERK1/2-immunoreactive cells were increased, especially in the polymorphic layer. In Western blot analysis, pERK1/2 level in the gerbil hippocampus was increased with age. These results indicate that total pERK1/2 levels are increased in the hippocampus with age. However pERK1/2 immunoreactivity in subregions of the gerbil hippocampus was changed with different pattern during normal aging.

Cardiovascular dementia - a different perspective

The number of dementia patients has been growing in recent years and dementia represents a significant threat to aging people all over the world. Recent research has shown that the number of people affected by Alzheimer's disease (AD) and dementia is growing at an epidemic pace. The rapidly increasing financial and personal costs will affect the world's economies, health care systems, and many families. Researchers are now exploring a possible connection among AD, vascular dementia (VD), diabetes mellitus (type 2, T2DM) and cardiovascular diseases (CD). This correlation may be due to a strong association of cardiovascular risk factors with AD and VD, suggesting that these diseases share some biologic pathways. Since heart failure is associated with an increased risk of AD and VD, keeping the heart healthy may prove to keep the brain healthy as well. The risk for dementia is especially high when diabetes mellitus is comorbid with severe systolic hypertension or heart disease. In addition, the degree of coronary artery disease (CAD) is independently associated with cardinal neuropathological lesions of AD. Thus, the contribution of T2DM and CD to AD and VD implies that cardiovascular therapies may prove useful in preventing AD and dementia.

Cholesterol in Alzheimer's disease and other amyloidogenic disorders

The complex association of cholesterol metabolism and Alzheimer's disease is presented in depth, including the possible benefits to be gained from cholesterol-lowering statin therapy. Then follows a survey of the role of neuronal membrane cholesterol in Abeta pore formation and Abeta fibrillogenesis, together with the link with membrane raft domains and gangliosides. The contribution of structural studies to Abeta fibrillogenesis, using TEM and AFM, is given some emphasis. The role of apolipoprotein E and its isoforms, in particular ApoE4, in cholesterol and Abeta binding is presented, in relation to genetic risk factors for Alzheimer's disease. Increasing evidence suggests that cholesterol oxidation products are of importance in generation of Alzheimer's disease, possibly induced by Abeta-produced hydrogen peroxide. The body of evidence for a link between cholesterol in atherosclerosis and Alzheimer's disease is increasing, along with an associated inflammatory response. The possible role of cholesterol in tau fibrillization, tauopathies and in some other non-Abeta amyloidogenic disorders is surveyed.

Gender- and age-dependent gamma-secretase activity in mouse brain and its implication in sporadic Alzheimer disease

Alzheimer disease (AD) is an age-related disorder. Aging and female gender are two important risk factors associated with sporadic AD. However, the mechanism by which aging and gender contribute to the pathogenesis of sporadic AD is unclear. It is well known that genetic mutations in γ-secretase result in rare forms of early onset AD due to the aberrant production of Aβ42 peptides, which are the major constituents of senile plaques. However, the effect of age and gender on γ-secretase has not been fully investigated. Here, using normal wild-type mice, we show mouse brain γ-secretase exhibits gender- and age-dependent activity. Both male and female mice exhibit increased Aβ42∶Aβ40 ratios in aged brain, which mimics the effect of familial mutations of Presenilin-1, Presenlin-2, and the amyloid precursor protein on Aβ production. Additionally, female mice exhibit much higher γ-secretase activity in aged brain compared to male mice. Furthermore, both male and female mice exhibit a steady decline in Notch1 γ-secretase activity with aging. Using a small molecule affinity probe we demonstrate that male mice have less active γ-secretase complexes than female mice, which may account for the gender-associated differences in activity in aged brain. These findings demonstrate that aging can affect γ-secretase activity and specificity, suggesting a role for γ-secretase in sporadic AD. Furthermore, the increased APP γ-secretase activity seen in aged females may contribute to the increased incidence of sporadic AD in women and the aggressive Aβ plaque pathology seen in female mouse models of AD. In addition, deceased Notch γ-secretase activity may also contribute to neurodegeneration. Therefore, this study implicates altered γ-secretase activity and specificity as a possible mechanism of sporadic AD during aging.

Emerging amyloid beta (Ab) peptide modulators for the treatment of Alzheimer's disease (AD)

BACKGROUND

According to the 'amyloid cascade hypothesis' of Alzheimer's disease (AD), abnormal processing of beta-amyloid precursor protein (betaAPP) into toxic amyloid beta (Abeta)-peptides is central to the etiopathology of this uniquely human brain disorder.

OBJECTIVE

To review current AD drugs, pharmacological approaches and strategies aimed at modulating Abeta-peptide generation and/or aggregation in the treatment of AD.

METHODS

Data searches at various websites: Alzheimer Research Forum; individual drug company databases; Medline; Pharmaprojects database; unpublished research; inter-University research communications.

RESULTS/CONCLUSION

Considerable research effort has focused on secretase-mediated mechanisms of betaAPP processing, and the latest pharmacological strategies have used selective Abeta-peptide-lowering agents (SALA) to provide therapeutic benefit against Abeta-initiated neurodegenerative pathology. Currently, dedicated anticholinesterase, glutamatergic agonist and Abeta-peptide immunization have had little impact in the clinical treatment of AD. One unexpected benefit of statins (HMG-CoA inhibitors), besides their cholesterol lowering abilities, has been their ancillary effects in potentiating the enzymatic mechanisms that generate Abeta-peptides. The long-term benefits or complications of statin-based therapies for use in the clinical management of AD are not known.

miRNAs: from neurogeneration to neurodegeneration

miRNAs are reported to sequence-specifically control the translation of target mRNAs by binding to 3 UTRs. The abundant expression of miRNAs in the brain highlights their biological significance in neurodevelopment. Many studies have shown that miRNAs are involved in a variety of functions, including developmental transitions and neuronal patterning, apoptosis, fat metabolism and regulation of hematopoietic lineage differentiation in different organisms. miRNAs act as regulatory switches in the determination of developmental fate through their distinct patterns of expression. The tissue-specific expression of miRNAs during brain development could possibly direct the development of cells in different subtypes. Several miRNAs are localized to neuronal subtypes and exhibit a more diverse or specific expression pattern within various neuronal cell types such as glial cells and neuronal progenitor cells. Perturbations in the expression pattern of miRNAs could lead to defects in human brain development and neurological disorders. The bioinformatic prediction tools suggest that some genes involved in synaptic formations and mental retardation are putative targets for miRNAs. miRNAs have been shown to specify cell fates in the nervous system of worms and brain morphogenesis in fish, and their distinct expression patterns during mammalian brain development. This suggests a potential role of miRNAs in neurodevelopment of mammals and other organisms. In this review, I have focused on the role of miRNAs in brain development and possible neurological disorders.

Polyunsaturated fatty acids induce alpha-synuclein-related pathogenic changes in neuronal cells

The misfolding and aggregation of normally soluble proteins has emerged as a key feature of several neurodegenerative diseases. In Parkinson's disease, progressive loss of dopaminergic neurons is accompanied by polymerization of the cytoplasmic protein alpha-synuclein (alphaS) into filamentous inclusions found in neuronal somata (Lewy bodies) and dendrites (Lewy neurites). Similar alphaS aggregates occur in cortical neurons in dementia with Lewy bodies. Numerous reports now indicate that alphaS can interact with lipids. We previously found that treating dopaminergic cells expressing alphaS with polyunsaturated fatty acids (PUFAs) induced the formation of soluble, sodium dodecyl sulfate-stable oligomers whereas treatment with saturated fatty acids did not. Here, we examine the relevance of alphaS-PUFA interactions to the development of Parkinson's disease-like cytopathology. Exposure of alphaS-overexpressing dopaminergic or neuronal cell lines to physiological levels of a PUFA induced the formation of proteinaceous inclusions in the cytoplasm. Kinetic experiments indicated that PUFA-induced soluble oligomers of alphaS precede these Lewy-like inclusions. Importantly, we found that alphaS oligomers were associated with cyto-toxicity, whereas the development of Lewy-like inclusions appeared to be protective. We conclude that alterations in PUFA levels can lead to aggregation of alphaS and subsequent deposition into potentially cyto-toxic oligomers that precede inclusions in dopaminergic cells.

Hypercholesterolemia is associated with visual field alterations detectable with computerized perimetry

Computerized perimetry analysis of the visual field enables the capacity of the visual cortex to process the electric impulse received from the eyeball to be detected. Considering that hypercholesterolemia can cause cognitive worsening by altering the functioning of the cerebral cortex, we attempted to determine whether it can affect the cortical processing of the visual stimuli, as detected by computerized perimetry. We undertook computerized perimetry analysis of 208 male and female patients (mean age, 48.5 years) with plasma low density lipoprotein (LDL) cholesterol concentrations above 3.36 mmol/L and compared the results with the same number of matched controls with LDL cholesterol concentrations below 3.36 mmol/L. No subject had any neuro-ophthalmological disease that could have affected the results. A Humphrey 745 computerized perimetry device was used to measure the mean deviation, the pattern standard deviation, the corrected pattern standard deviation, short-term fluctuations and the foveal threshold. Compared with the control subjects, the patients with hypercholesterolemia showed worsening in all the perimetry variables analyzed. Moreover, we also found a direct association between plasma LDL cholesterol concentrations and the perimetry alterations, and an inverse association between these alterations and the figures for HDL cholesterol. Our data show that computerized perimetry can detect alterations in the visual field of persons with hypercholesterolemia, suggesting that increased plasma concentrations of cholesterol may have a negative influence on the visual neuron cells.

Oleic acid is a potent inhibitor of fatty acid and cholesterol synthesis in C6 glioma cells

Glial cells play a pivotal role in brain fatty acid metabolism and membrane biogenesis. However, the potential regulation of lipogenesis and cholesterologenesis by fatty acids in glial cells has been barely investigated. Here, we show that physiologically relevant concentrations of various saturated, monounsaturated, and polyunsaturated fatty acids significantly reduce [1-(14)C]acetate incorporation into fatty acids and cholesterol in C6 cells. Oleic acid was the most effective at depressing lipogenesis and cholesterologenesis; a decreased label incorporation into cellular palmitic, stearic, and oleic acids was detected, suggesting that an enzymatic step(s) of de novo fatty acid biosynthesis was affected. To clarify this issue, the activities of acetyl-coenzyme A carboxylase (ACC) and FAS were determined with an in situ digitonin-permeabilized cell assay after incubation of C6 cells with fatty acids. ACC activity was strongly reduced ( approximately 80%) by oleic acid, whereas no significant change in FAS activity was observed. Oleic acid also reduced the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR). The inhibition of ACC and HMGCR activities is corroborated by the decreases in ACC and HMGCR mRNA abundance and protein levels. The downregulation of ACC and HMGCR activities and expression by oleic acid could contribute to the reduced lipogenesis and cholesterologenesis.

Natural secretory products of human neural and microvessel endothelial cells: Implications in pathogenic "spreading" and Alzheimer's disease

Neurons, glia, and endothelial cells of the cerebral microvasculature co-exist in intimate proximity in nervous tissues, and their homeostatic interactions in health, as well as coordinated response to injury, have led to the concept that they form the basic elements of a functional neurovascular unit. During the course of normal cellular metabolism, growth, and development, each of these brain cell types secrete various species of potentially neurotoxic peptides and factors, events that increase in magnitude as brain cells age. This article reviews contemporary research on the secretory products of the three primary cell types that constitute the neurovascular unit in deep brain regions. We provide some novel in vitro data that illustrate potentially pathogenic paracrine effects within primary cells of the neurovascular unit. For example, the pro-inflammatory cytokine interleukin (IL)-1beta was found to stimulate amyloid-beta (Abeta) peptide release from human neural cells, and human brain microvessel endothelial cells exposed to transient hypoxia were found to secrete IL-1beta at concentrations known to induce Abeta42 peptide release from human neural cells. Hypoxia and excessive IL-1beta and Abeta42 abundance are typical pathogenic stress factors implicated in the initiation and development of common, chronic neurological disorders such as Alzheimer's disease. These data support the hypothesis that paracrine effects of stressed constituent cells of the neurovascular unit may contribute to "spreading effects" characteristic of progressive neurodegenerative disorders.

Proteomic analysis of human very low-density lipoprotein by two-dimensional gel electrophoresis and MALDI-TOF/TOF

Biochemical studies of lipoproteins have shed light on their composition, highly contributing to the comprehension of their function. Due to the complexity of their structure, however, an in-depth structural analysis, in terms of components and PTMs, may still unravel important players in physiological and pathological processes of lipid metabolism. In this study, we performed a protein map of very low-density lipoprotein (VLDL) using a 2-DE MALDI-TOF/TOF proteomic approach. Several VLDL-associated apolipoproteins were identified, including five isoforms of apoE, three isoforms of apoC-IV, and one isoform each of apoC-III, apoM, apoA-I, and apoA-IV. Notably, we also identified seven isoforms of apoL-I and two isoforms of prenylcysteine lyase as new VLDL-associated proteins. Furthermore, we were able to identify PTM of apoE, which was found to be differently O-glycosylated at Thr212 residue, and PTM of apoL-I which we described, for the first time, to be phosphorylated at Ser296. While the physiological relevance of our finding remains to be assessed, we believe that our results will be useful as reference for future studies of VLDL structure in specific physiopathological conditions.

Survival signalling in Alzheimer's disease

Significant advancements in our understanding of cell-survival signalling in AD (Alzheimer's disease) stem from recent investigations into the metabolism, trafficking and fate of the essential omega-3 fatty acid DHA (docosahexaenoic acid) (C(22:6), n=3). Brain synaptic terminals and neuronal plasma membranes are highly enriched in DHA, and deficiencies in this polyunsaturated fatty acid are characteristic of AD-affected brain. Oxidative stress, targeting phospholipids containing DHA, and age-related DHA depletion are associated with the progressive erosion of normal cognitive function in AD. Current studies support the idea that DHA itself and novel DHA-derived neural synapse- and membrane-derived lipid messengers have considerable potential to modulate cell survival signalling in stressed cultured neural cell models in vitro and in mammalian models of learning, memory and AD in vivo. Key players in this intrinsic rescue system include the alpha-secretase-processed neurotrophin sAPPalpha [soluble APPalpha (amyloid precursor protein alpha)] peptide, the DHA-derived 10,17S-docosatriene NPD1 (neuroprotectin D1), a tandem brain cytosolic phospholipase A(2) and 15-lipoxygenase enzymatic system that biosynthesizes NPD1, and a small family of anti-apoptotic neuroprotective genes that encode Bcl-2, Bcl-X(L) and Bfl-1 (A1). This paper reviews current ideas regarding DHA and the oxygenated DHA derivative NPD1, intrinsically triggered biolipid neuroprotectants that along with their associated rescue pathways, contribute to life-or-death decisions of brain cells during homoeostasis, aging and neurodegenerative disease.

Micro-RNA speciation in fetal, adult and Alzheimer's disease hippocampus

Micro-RNAs constitute a family of small noncoding ribonucleic acids that are posttranscriptional regulators of messenger RNA activity. Although micro-RNAs are known to be dynamically regulated during neural development, the role of micro-RNAs in brain aging and neurodegeneration is not known. This study examined micro-RNA abundance in the hippocampal region of fetal, adult and Alzheimer's disease brain. The data indicate that micro-RNAs encoding miR-9, miR-124a, miR-125b, miR-128, miR-132 and miR-219 are abundantly represented in fetal hippocampus, are differentially regulated in aged brain, and an alteration in specific micro-RNA complexity occurs in Alzheimer hippocampus. These data are consistent with the idea that altered micro-RNA-mediated processing of messenger RNA populations may contribute to atypical mRNA abundance and neural dysfunction in Alzheimer's disease brain.

Chromium picolinate positively influences the glucose transporter system via affecting cholesterol homeostasis in adipocytes cultured under hyperglycemic diabetic conditions

Since trivalent chromium (Cr(3+)) enhances glucose metabolism, interest in the use of Cr(3+)as a therapy for type 2 diabetes has grown in the mainstream medical community. Moreover, accumulating evidence suggests that Cr(3+) may also benefit cardiovascular disease (CVD) and atypical depression. We have found that cholesterol, a lipid implicated in both CVD and neurodegenerative disorders, also influences cellular glucose uptake. A recent study in our laboratory shows that exposure of 3T3-L1 adipocytes to chromium picolinate (CrPic, 10 nM) induces a loss of plasma membrane cholesterol. Concomitantly, accumulation of intracellularly sequestered glucose transporter GLUT4 at the plasma membrane was dependent on the CrPic-induced cholesterol loss. Since CrPic supplementation has the greatest benefit on glucose metabolism in hyperglycemic insulin-resistant individuals, we asked here if the CrPic effect on cells was glucose-dependent. We found that GLUT4 redistribution in cells treated with CrPic occurs only in cells cultured under high glucose (25 mM) conditions that resemble the diabetic-state, and not in cells cultured under non-diabetic (5.5 mM glucose) conditions. Examination of the effect of CrPic on proteins involved in cholesterol homeostasis revealed that the activity of sterol regulatory element-binding protein (SREBP), a membrane-bound transcription factor ultimately responsible for controlling cellular cholesterol balance, was upregulated by CrPic. In addition, ABCA1, a major player in mediating cholesterol efflux was decreased, consistent with SREBP transcriptional repression of the ABCA1 gene. Although the exact mechanism of Cr(3+)-induced cholesterol loss remains to be determined, these cellular responses highlight a novel and significant effect of chromium on cholesterol homeostasis. Furthermore, these findings provide an important clue to our understanding of how chromium supplementation might benefit hypercholesterolemia-associated disorders.

Cholesterol and 24S-hydroxycholesterol trafficking in Alzheimer's disease

Cholesterol and the cholesterol oxide 24S-hydroxycholesterol (24S-HC) are highly enriched in the human CNS. Clinical, genetic, neurochemical and epidemiological evidence continue to support dysfunctional cholesterol metabolism as an important contributing factor driving the development and/or progression of Alzheimer's disease (AD) neuropathology. Cholesterol overabundance in the brain plasma membrane lipid- raft domains, appears to be fundamental to the generation of the more neurotoxic forms of amyloid-beta (Abeta) peptide from beta-amyloid holoprotein precursor. 24S-HC may have a pivotal role in promoting altered inflammatory signaling, apoptotic genetic responses and AD-type change. In clinical studies, cholesterol-lowering statins, nonsteroidal anti-inflammatory drugs, cholesterol absorption/transport inhibitors and related modulators of cholesterol trafficking have demonstrated some pharmacological benefit for the treatment of AD, but overall their efficacy at slowing the cognitive decline and the progression of AD remains controversial and open to question.

Docosahexaenoic acid prevents neuronal apoptosis induced by soluble amyloid-β oligomers

A growing body of evidence supports the notion that soluble oligomers of amyloid-beta (Abeta) peptide interact with the neuronal plasma membrane, leading to cell injury and inducing death-signalling pathways that could account for the increased neurodegeneration occurring in Alzheimer's disease (AD). Docosahexaenoic acid (DHA, C22:6, n-3) is an essential polyunsaturated fatty acid in the CNS and has been shown in several epidemiological and in vivo studies to have protective effects against AD and cognitive alterations. However, the molecular mechanisms involved remain unknown. We hypothesized that DHA enrichment of plasma membranes could protect neurones from apoptosis induced by soluble Abeta oligomers. DHA pre-treatment was observed to significantly increase neuronal survival upon Abeta treatment by preventing cytoskeleton perturbations, caspase activation and apoptosis, as well as by promoting extracellular signal-related kinase (ERK)-related survival pathways. These data suggest that DHA enrichment probably induces changes in neuronal membrane properties with functional outcomes, thereby increasing protection from soluble Abeta oligomers. Such neuroprotective effects could be of major interest in the prevention of AD and other neurodegenerative diseases.