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

Potential Mechanism of S. baicalensis on Lipid Metabolism Explored via Network Pharmacology and Untargeted Lipidomics

Background

S. baicalensis, a traditional herb, has great potential in treating diseases associated with aberrant lipid metabolism, such as inflammation, hyperlipidemia, atherosclerosis and Alzheimer’s disease.

Aim of the Study

To elucidate the mechanism by which S. baicalensis modulates lipid metabolism and explore the medicinal effects of S. baicalensis at a holistic level.

Materials and Methods

The potential active ingredients of S. baicalensis and targets involved in regulating lipid metabolism were identified using a network pharmacology approach. Metabolomics was utilized to compare lipids that were altered after S. baicalensis treatment in order to identify significantly altered metabolites, and crucial targets and compounds were validated by molecular docking.

Results

Steroid biosynthesis, sphingolipid metabolism, the PPAR signaling pathway and glycerolipid metabolism were enriched and predicted to be potential pathways upon which S. baicalensis acts. Further metabolomics assays revealed 14 significantly different metabolites were identified as lipid metabolism-associated elements. After the pathway enrichment analysis of the metabolites, cholesterol metabolism and sphingolipid metabolism were identified as the most relevant pathways. Based on the results of the pathway analysis, sphingolipid and cholesterol biosynthesis and glycerophospholipid metabolism were regarded as key pathways in which S. baicalensis is involved to regulate lipid metabolism.

Conclusion

According to our metabolomics results, S. baicalensis may exert its therapeutic effects by regulating the cholesterol biosynthesis and sphingolipid metabolism pathways. Upon further analysis of the altered metabolites in certain pathways, agents downstream of squalene were significantly upregulated; however, the substrate of SQLE was surprisingly increased. By combining evidence from molecular docking, we speculated that baicalin, a major ingredient of S. baicalensis, may suppress cholesterol biosynthesis by inhibiting SQLE and LSS, which are important enzymes in the cholesterol biosynthesis pathway. In summary, this study provides new insights into the therapeutic effects of S. baicalensis on lipid metabolism using network pharmacology and lipidomics.

Plasma Lipids as Biomarkers for Alzheimer's Disease: A Systematic Review

Alzheimer's disease (AD) is caused by several risk factors leading to dementia. It's diagnosis usually depends on clinical presentation and certain biomarkers in the cerebrospinal fluid (CSF). The brain has a high content of cholesterol and the metabolism of cholesterol in the brain can be associated with beta-amyloid plaques formation, which is seen in Alzheimer's disease. Given these implications, we studied if plasma lipid levels can vary in Alzheimer's disease and if these can be used as biomarkers to diagnose and predict the progression of Alzheimer's disease. Certain mutations in the brain cholesterol transport receptors and proteins and their association with Alzheimer's were also studied. This systematic review abides by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. We searched multiple databases, such as Pubmed, Google Scholar, Pubmed central, ScienceDirect, Web of Science, and Medline with the help of keywords like Alzheimer's disease, cognitive impairment, plasma lipid biomarkers, cholesterol, brain cholesterol metabolism separately and in combination with each other. We collected 49 quality appraised articles on the association between plasma lipids and Alzheimer's disease and the genetic mutations in alleles related to cholesterol metabolism and Alzheimer's disease by applying the inclusion and exclusion criteria. Based on the finding of the studies reviewed, we found an association between plasma lipids, polymorphisms in genes associated with cholesterol transport, and Alzheimer's disease. Increased serum low-density lipoprotein (LDL-C), triglycerides (TG), total cholesterol (TC), sphingolipids, 24S hydroxycholesterol (24S-HC), 27O hydroxycholesterol (27O-HC) was associated with Alzheimer's. Decreased high-density lipoprotein (HDL-C) and phospholipids were noticed. Genetic mutations in apolipoprotein E (ApoE), apolipoprotein B (ApoB), apolipoprotein A (ApoA), ATP binding cassette transporter 1 (ABCA1), ATP binding cassette transporter 7 (ABCA7), amyloid precursor protein (APP), cytochrome P450 family 46 subfamilies A member 1 (CYP46A1), presenilin 1 (PSEN1), presenilin 2 (PSEN2) are also associated with increased risk of Alzheimer's disease. This study found an association between plasma lipids and Alzheimer's, proving that plasma lipids can be used as biomarkers for early diagnosis of Alzheimer's disease. It may also help predict the prognosis and stage the disease severity. Further studies are needed to find out the exact mechanism behind these changes.

Dietary DHA and health: cognitive function ageing

DHA is a key nutritional n-3 PUFA and needs to be supplied by the human diet. DHA is found in significant amounts in the retinal and neuronal cell membranes due to its high fluidity. Indeed, DHA is selectively concentrated in the synaptic and retinal membranes. DHA is deemed to display anti-inflammatory properties and to reduce the risk of CVD. Consumption of larger amounts of DHA appears to reduce the risk of depression, bipolar disorder, schizophrenia and mood disorders. Conversely, it has been shown that loss of DHA from the nerve cell membrane leads to dysfunction of the central nervous system in the form of anxiety, irritability, susceptibility to stress, dyslexia, impaired memory and cognitive functions, and extended reaction times. DHA plays an important role in ensuring a healthy ageing, by thwarting macular degeneration, Alzheimer's disease, and other brain disorders at the same time as enhancing memory and strengthening neuroprotection in general. A reduced level of DHA is associated with cognitive decline during ageing. Different mechanisms for this fundamental DHA role have been put forward. Namely, neuroprotectin D1, a DHA derivative, may support brain cell survival and repair through neurotrophic, anti-apoptotic, and anti-inflammatory signalling. Many of the effects of DHA on the neurological system may be related to signalling connections, thus leading to the study of the related signalolipidomics. Therefore, the present review will focus on the influence of DHA deficiency upon ageing, with specific emphasis upon neurological disorders related to cognitive function and mental health.

Cholesterol overload induces apoptosis in SH-SY5Y human neuroblastoma cells through the up regulation of flotillin-2 in the lipid raft and the activation of BDNF/Trkb signaling

Epidemiological investigations have shown that Alzheimer's disease (AD) is one of the most common neurodegenerative diseases. It has been indicated that the cholesterol concentration in the brain of AD patients is higher than that in normal people. In this study, we investigated the effects of cholesterol concentrations, 0, as the control, 3.125, 12.5, and 25μM, on cholesterol metabolism, neuron survival, AD-related protein expressions, and cell morphology and apoptosis using SH-SY5Y human neuroblastoma cells. We observed that expressions of cholesterol hydroxylase (Cyp46), flotillin-2 (a marker of lipid raft content), and truncated tyrosine kinase B (TrkBtc) increased, while expressions of brain-derived neurotrophic factor (BDNF) and full-length TrkB (TrkBfl) decreased as the concentration of cholesterol loading increased. Down-regulation of the PI3K-Akt-glycogen synthase kinase (GSK)-3β cascade and cell apoptosis were also observed at higher concentrations of cholesterol, along with elevated levels of β-amyloid (Aβ), β-secretase (BACE), and reactive oxygen species (ROS). In conclusion, we found that cholesterol overload in neuronal cells imbalanced the cholesterol homeostasis and increased the protein expressions causing cell apoptosis, which illustrates the neurodegenerative pathology of abnormally elevated cholesterol concentrations found in AD patients.

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.

Quercetin activates AMP-activated protein kinase by reducing PP2C expression protecting old mouse brain against high cholesterol-induced neurotoxicity

It is known that a high-cholesterol diet induces oxidative stress, inflammatory response, and beta-amyloid (Abeta) accumulation in mouse brain, resulting in neurodegenerative changes. Quercetin, a naturally occurring flavonoid, has been reported to possess numerous biological activities beneficial to health. Our previous studies have demonstrated that quercetin protects mouse brain against D-galactose-induced oxidative damage. Against this background, we evaluated the effect of quercetin on high-cholesterol-induced neurotoxicity in old mice and explored its potential mechanism. Our results showed that oral administration of quercetin significantly improved the behavioural performance of high-cholesterol-fed old mice in both a step-through test and the Morris water maze task. This is at least in part caused by decreasing ROS and protein carbonyl levels and restoring Cu--Zn superoxide dismutase (Cu, Zn-SOD) activity. Furthermore, quercetin also significantly activated the AMP-activated protein kinase (AMPK) via down-regulation of protein phosphatase 2C (PP2C), which reduced the integral optical density (IOD) of activated microglia cells and CD11b expression, down-regulated iNOS and cyclooxygenase-2 (COX-2) expression, and decreased IL-1beta, IL-6, and TNF-alpha expression in the brains of high-cholesterol-fed old mice through the suppression of NF-kappaB p65 nuclear translocation. Moreover, AMPK activation significantly increased 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acetyl-CoA carboxylase (ACC) phosphorylation and reduced fatty acid synthase (FAS) expression in the brains of high-cholesterol-fed old mice, which reduced cholesterol levels, down-regulated cholesterol 24-hydroxylase (CYP46A1) and beta-amyloid converting enzyme 1 (BACE1) expression, decreased eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation, and lowered Abeta deposits. However, the neuroprotective effect of quercetin was weakened by intraperitoneal injection of compound C, an AMPK inhibitor. These results suggest that AMPK activated by quercetin may be a potential target to enhance the resistance of neurons to age-related diseases.

Changes in cholesterol biosynthetic and transport pathways after excitotoxicity

The present study was carried out to elucidate changes in the gene expression and activity of cholesterol biosynthetic enzymes and transporters in the rat hippocampus after kainate excitotoxicity. Significantly increased cholesterol level was detected in the degenerating hippocampus, reaching double normal levels at 1 week after kainate injury. RT-PCR analyses of hippocampal homogenates showed significantly decreased mRNA expression of the transcription factor controlling cholesterol biosynthesis SREBP-2, and the rate-controlling enzyme HMG-CoA (3-hydroxy-3-methyl-glutaryl-CoA) reductase at all time points after kainate injection; and decreased lanosterol synthase and CYP51 at 1 and 2 weeks post-kainate injection respectively. GC-MS analyses showed a significant increase in cholesterol biosynthetic precursors lanosterol, desmosterol and 7-dehydrocholesterol at 1 day after kainate injection presumably reflecting biosysnthesis in injured neurons, and significant decreases in precursors at 1 and 2 weeks post-kainate injection, at time of gliosis in the degenerating hippocampus. Levels of cholesterol autooxidation including 7 ketocholesterol and cholesterol epoxides were elevated in the kainate lesioned hippocampus. Furthermore, loss of expression of the cholesterol transporter, ABCA1 was detected in neurons, but increased expression in astrocytes was detected after kainate lesions. The results suggest that increased cholesterol biosynthesis and loss of ABCA1 expression in injured neurons might result in increase in cholesterol in the degenerating hippocampus. The increased cholesterol might predispose to increased formation of cholesterol oxidation products which have been shown to be toxic to neurons.

Drug development for Alzheimer's disease: where are we now and where are we headed?

OBJECTIVE

The aim of this article was to provide a survey of the clinical development of pharmacotherapy for Alzheimer's disease (AD).

METHODS

A search of PubMed to identify pertinent English-language literature was conducted using the terms Alzheimer's disease AND clinical trials (2003-2008), dementia AND prevention AND clinical trials (2003-2008), and the chemical names of all compounds mentioned in articles on new drugs for AD published since 2005. www.ClinicalTrials.gov was searched for relevant trials. Abstracts of the 2008 International Conference on Alzheimer's Disease (ICAD) were reviewed for relevance, as were pharmaceutical company and AD advocacy Web sites. Articles selected for review were primary reports of data from preclinical studies and clinical trials.

RESULTS

A large number of drugs with differing targets and mechanisms of action are under development for the treatment of AD. Phase III trials of Ginkgo biloba, NSAIDs, phenserine, statins, tarenflurbil, tramiprosate, and xaliproden have been completed, none of them demonstrating adequate efficacy. Encouraging results from completed Phase II trials of dimebon, huperzine A, intravenous immunoglobulin, and methylthioninium chloride were reported at ICAD 2008. Nineteen compounds are currently in Phase II trials, and 3 compounds (AN1792, lecozotan SR, and SGS742) failed at this stage of development.

CONCLUSIONS

Despite disappointing results from recently completed Phase III trials of several novel compounds, the extent and breadth of activity at all phases of clinical development suggest that new pharmacotherapeutic options for the treatment of AD will become available within the next decade.

Docosahexaenoic acid and the aging brain

The dietary essential PUFA docosahexaenoic acid [DHA; 22:6(n-3)] is a critical contributor to cell structure and function in the nervous system, and deficits in DHA abundance are associated with cognitive decline during aging and in neurodegenerative disease. Recent studies underscore the importance of DHA-derived neuroprotectin D1 (NPD1) in the homeostatic regulation of brain cell survival and repair involving neurotrophic, antiapoptotic and antiinflammatory signaling. Emerging evidence suggests that NPD1 synthesis is activated by growth factors and neurotrophins. Evolving research indicates that NPD1 has important determinant and regulatory interactions with the molecular-genetic mechanisms affecting beta-amyloid precursor protein (betaAPP) and amyloid beta (Abeta) peptide neurobiology. Deficits in DHA or its peroxidation appear to contribute to inflammatory signaling, apoptosis, and neuronal dysfunction in Alzheimer disease (AD), a common and progressive age-related neurological disorder unique to structures and processes of the human brain. This article briefly reviews our current understanding of the interactions of DHA and NPD1 on betaAPP processing and Abeta peptide signaling and how this contributes to oxidative and pathogenic processes characteristic of aging and AD pathology.

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.

Update on lipid membrane microdomains

PURPOSE OF REVIEW

Lipid membrane microdomains are involved in major types of disease, ranging from vascular and metabolic diseases to neurodegeneration, autoimmunity, infectious and inflammatory diseases, and cancer. This review provides an update of membrane microdomain abnormalities.

RECENT FINDINGS

Lipid membrane microdomains are dynamic assemblies of sphingolipids, cholesterol and proteins that dissociate and associate rapidly and form functional clusters. Membrane microdomain clustering is the key to how membrane microdomains can form lipid-protein platforms in cell membranes, functioning in membrane trafficking, cell polarization and signalling. Clustering of membrane microdomains can be modified, for example by dietary lipids and pharmacological agents.

SUMMARY

Metabolic overload through a cholesterol-rich and fat-rich diet can trigger metabolic learning, which is associated with membrane microdomain persistence, persistent signalling and disturbed vesicular traffic. Detailed characterization of lipid membrane microdomains and dynamics at the molecular level is necessary and will help to identify new dietary and pharmacological therapeutic targets for the treatment and prevention of lipid membrane microdomain related diseases.

Reduction of sortilin-1 in Alzheimer hippocampus and in cytokine-stressed human brain cells

Sortilin 1 (SORL1) is a transmembrane sorting receptor that regulates the intracellular trafficking of beta-amyloid precursor protein (betaAPP). Interactions between SORL1 and betaAPP result in the decreased processing of betaAPP into toxic amyloid-beta42 (Abeta42) peptides that accumulate in Alzheimer's disease brain. Here, we report selectively decreased levels of SORL1 in limbic and occipital regions of Alzheimer brain that inversely correlate with amyloid plaque and neurofibrillary tangle density. Reduced SORL1, coupled to elevated beta-amyloid cleaving enzyme, presenilin-1 and increased Abeta42 peptide secretion, was observed after incubation of cultured human neural cells with the proinflammatory cytokine interleukin-1beta. The results suggest that SORL1 deficits may not only promote the pathogenic processing of betaAPP but may also contribute to Abeta42-mediated inflammatory signaling in stressed human brain cells.

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.

Apolipoprotein E epsilon4 offers protection against age-related macular degeneration

BACKGROUND

In many previous studies, age-related macular degeneration (ARMD) has been linked to a variety of different risk factors. The publications have debated whether apolipoprotein E (apoE) epsilon4 serves as a potential protective factor in the development of the disease. Other studies have classified the behavior of this protein in different pathologies, including Alzheimer's disease (AD) and cardiovascular disease. The general behavior of the epsilon4 isoform of ApoE is different than the predominant epsilon3 isoform.

HYPOTHESIS

We propose that the general characteristics and molecular behavior of apoE epsilon4 cause it to be a protective factor against the development of ARMD by preventing cumulative effects of oxidative retinal damage. EVALUATION OF HYPOTHESIS: Review of the literature related to ARMD and ApoE, using OVID as our main database, led to the development of several theories regarding ApoE epsilon4's behavior compared to epsilon3 and potential explanation of its protective characteristics. CONSEQUENCES OF HYPOTHESIS: We relate these theories to the potential behavior of ApoE epsilon4 in other situations including choroidal neovascularization, Alzheimer's Disease (AD), cardiovascular disease, herpes simplex virus infection, and smoking.

DISCUSSION

The potential implications of this theory could be used as a branching point for further studies that examine the role of the different apoE isoforms, in relation to the other risk factors for ARMD.

Can statins put the brakes on Alzheimer’s disease?

Statins inhibit 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, which initiates the syntheses of cholesterol and isoprenoid lipids that are needed to provide amyloid peptides for the amyloid cascade. This cascade is believed to induce sporadic or late-onset Alzheimer's disease, which accounts for 90 - 95% of Alzheimer's disease sufferers. Cholesterol is also the prime driver of cerebrovascular disease that (along with amyloid peptides) increasingly appears to be linked to the cognitive deterioration of Alzheimer's disease. Cholesterol is needed to make the lipid rafts that are the platforms for isoprenoid-dependent assembly and activation of raftophilic beta- and gamma-secretases that work in tandem to excise dangerous 40 and 42 amino acid amyloid-beta (Abeta) fragments from amyloid precursor protein, the transmembrane amyloid precursor glycoprotein. When they are excessively produced and can no longer be effectively destroyed or otherwise cleared from the hypoperfused ageing brain, the Abeta42 fragments released from the active synaptic terminals of normally busy neurons (and from stressed neurons unsuccessfully trying to proliferate and producing disruptive tangles of hyperphosphorylated tau-proteins) aggregate into neuritic plaques, which activate glial cells. The pro-inflammatory cytokines and growth factors from the glial cells further damage and kill neurons. As statins strike at several parts of the Alzheimer's disease mechanism (such as the infliction of cholesterol-dependent cerebrovascular damage) by inhibiting HMG-CoA reductase, their long-term use (starting as early as possible during Alzheimer's disease development) should slow or even prevent the progression of Alzheimer's disease. Indeed, there is some evidence of a significantly reduced incidence of Alzheimer's disease among people who have been using statins to reduce hypercholesterolaemia and its cardiovascular effects. To be certain of this, there must be more multi-year trials to specifically assess the effects of statins on sporadic Alzheimer's disease.