The Role of Cholesterol Metabolism in Alzheimer’s Disease

Relationships of brain cholesterol and cholesterol biosynthetic enzymes to Alzheimer's pathology and dementia in the CFAS population-derived neuropathology cohort

Altered cholesterol metabolism is implicated in brain ageing and Alzheimer's disease. We examined whether key genes regulating cholesterol metabolism and levels of brain cholesterol are altered in dementia and Alzheimer's disease neuropathological change (ADNC). Temporal cortex (n = 99) was obtained from the Cognitive Function and Ageing Study. Expression of the cholesterol biosynthesis rate-limiting enzyme HMG-CoA reductase (HMGCR) and its regulator, SREBP2, were detected using immunohistochemistry. Expression of HMGCR, SREBP2, CYP46A1 and ABCA1 were quantified by qPCR in samples enriched for astrocyte and neuronal RNA following laser-capture microdissection. Total cortical cholesterol was measured using the Amplex Red assay. HMGCR and SREBP2 proteins were predominantly expressed in pyramidal neurones, and in glia. Neuronal HMGCR did not vary with ADNC, oxidative stress, neuroinflammation or dementia status. Expression of HMGCR neuronal mRNA decreased with ADNC (p = 0.022) and increased with neuronal DNA damage (p = 0.049), whilst SREBP2 increased with ADNC (p = 0.005). High or moderate tertiles for cholesterol levels were associated with increased dementia risk (OR 1.44, 1.58). APOE ε4 allele was not associated with cortical cholesterol levels. ADNC is associated with gene expression changes that may impair cholesterol biosynthesis in neurones but not astrocytes, whilst levels of cortical cholesterol show a weak relationship to dementia status.

Cholesterol 25-hydroxylase mediates neuroinflammation and neurodegeneration in a mouse model of tauopathy

Alzheimer's disease (AD) is characterized by amyloid plaques and neurofibrillary tangles, in addition to neuroinflammation and changes in brain lipid metabolism. 25-Hydroxycholesterol (25-HC), a known modulator of both inflammation and lipid metabolism, is produced by cholesterol 25-hydroxylase encoded by Ch25h expressed as a "disease-associated microglia" signature gene. However, whether Ch25h influences tau-mediated neuroinflammation and neurodegeneration is unknown. Here, we show that in the absence of Ch25h and the resultant reduction in 25-HC, there is strikingly reduced age-dependent neurodegeneration and neuroinflammation in the hippocampus and entorhinal/piriform cortex of PS19 mice, which express the P301S mutant human tau transgene. Transcriptomic analyses of bulk hippocampal tissue and single nuclei revealed that Ch25h deficiency in PS19 mice strongly suppressed proinflammatory signaling in microglia. Our results suggest a key role for Ch25h/25-HC in potentiating proinflammatory signaling to promote tau-mediated neurodegeneration. Ch25h may represent a novel therapeutic target for primary tauopathies, AD, and other neuroinflammatory diseases.

Normal-Tension Glaucoma and Potential Clinical Links to Alzheimer's Disease

Glaucoma is a group of optic neuropathies and the world's leading cause of irreversible blindness. Normal-tension glaucoma (NTG) is a subtype of glaucoma that is characterized by a typical pattern of peripheral retinal loss, in which the patient's intraocular pressure (IOP) is considered within the normal range (<21 mmHg). Currently, the only targetable risk factor for glaucoma is lowering IOP, and patients with NTG continue to experience visual field loss after IOP-lowering treatments. This demonstrates the need for a better understanding of the pathogenesis of NTG and underlying mechanisms leading to neurodegeneration. Recent studies have found significant connections between NTG and cerebral manifestations, suggesting NTG as a neurodegenerative disease beyond the eye. Gaining a better understanding of NTG can potentially provide new Alzheimer's Disease diagnostics capabilities. This review identifies the epidemiology, current biomarkers, altered fluid dynamics, and cerebral and ocular manifestations to examine connections and discrepancies between the mechanisms of NTG and Alzheimer's Disease.

Maternal history of Alzheimer's disease predisposes to altered serum cholesterol levels in adult offspring

Controversial findings regarding the association between serum cholesterol levels and Alzheimer's disease (AD) have been identified through observational studies. The genetic basis shared by both factors and the causality between them remain largely unknown. The objective of this study is to examine the causal impact of maternal history of AD on changes in serum cholesterol levels in adult offspring. By retrieving genetic variants from summary statistics of large-scale genome-wide association study of maternal history of AD (European-based: Ncase = 27 696, Ncontrol = 260 980). The causal association between genetically predicted maternal history of AD and changes in serum cholesterol levels in adult offspring was examined using the two-sample Mendelian randomization (MR) method. Causal impact estimates were calculated using single-nucleotide polymorphisms in both univariable MR (UMR) and multivariable MR (MVMR) analyses. Additionally, other approaches, such as Cochran's Q test and leave-one-out variant analysis, were employed to correct for potential biases. The results of UMR presented that genetically predicted maternal history of AD was positively associated with hypercholesterolemia (OR = 1.014; 95% CI: 1.009-1.018; p < 0.001), total cholesterol (OR = 1.29; 95% CI: 1.134-1.466; p < 0.001) and low-density lipoprotein (OR = 1.525; 95% CI: 1.272-1.828; p < 0.001) among adult offspring. Genetic predisposition for maternal history of AD to be negatively associated with high-density lipoprotein (OR = 0.889; 95% CI: 0.861-0.917; p < 0.001). The MVMR analysis remained robust and significant after adjusting for diabetes and obesity in offspring. Sufficient evidence was provided in this study to support the putative causal impact of maternal history of AD on the change of serum cholesterol profile in adult offspring. In clinical practice, priority should be given to the detection and monitoring of cholesterol levels in individuals with a maternal history of AD, particularly in the early stages.

A mediation analysis of the role of total free fatty acids on pertinence of gut microbiota composition and cognitive function in late life depression

BACKGROUND

Extensive evidence demonstrates correlations among gut microbiota, lipid metabolism and cognitive function. However, there is still a lack of researches in the field of late-life depression (LLD). This research targeted at investigating the relationship among gut microbiota, lipid metabolism indexes, such as total free fatty acids (FFAs), and cognitive functions in LLD.

METHODS

Twenty-nine LLD patients from the Cognitive Outcome Cohort Study of Depression in Elderly were included. Cognitive functions were estimated through the Chinese version of Montreal Cognitive Assessment (MoCA). Blood samples were collected to evaluate serum lipid metabolism parameters. Fecal samples were evaluated for gut microbiota determination via 16S rRNA sequencing. Spearman correlation, linear regression and mediation analysis were utilized to explore relationship among gut microbiota, lipid metabolism and cognitive function in LLD patients.

RESULTS

Spearman correlation analysis revealed significant correlations among Akkermansia abundance, total Free Fatty Acids (FFAs) and MoCA scores (P < 0.05). Multiple regression indicated Akkermansia and total FFAs significantly predicted MoCA scores (P < 0.05). Mediation analysis demonstrated that the correlation between decreased Akkermansia relative abundance and cognitive decline in LLD patients was partially mediated by total FFAs (Bootstrap 95%CI: 0.023-0.557), accounting for 43.0% of the relative effect.

CONCLUSION

These findings suggested a significant relationship between cognitive functions in LLD and Akkermansia, as well as total FFAs. Total FFAs partially mediated the relationship between Akkermansia and cognitive functions. These results contributed to understanding the gut microbial-host lipid metabolism axis in the cognitive function of LLD.

APP and Bace1: Differential effect of cholesterol enrichment on processing and plasma membrane mobility

High cholesterol levels are a risk factor for the development of Alzheimer's disease. Experiments investigating the influence of cholesterol on the proteolytic processing of the amyloid precursor protein (APP) by the β-secretase Bace1 and on their proximity in cells have led to conflicting results. By using a fluorescence bioassay coupled with flow cytometry we found a direct correlation between the increase in membrane cholesterol amount and the degree of APP shedding in living human neuroblastoma cells. Analogue results were obtained for cells overexpressing an APP mutant that cannot be processed by α-secretase, highlighting the major influence of cholesterol enrichment on the cleavage of APP carried out by Bace1. By contrast, the cholesterol content was not correlated with changes in membrane dynamics of APP and Bace1 analyzed with single molecule tracking, indicating that the effect of cholesterol enrichment on APP processing by Bace1 is uncoupled from changes in their lateral diffusion.

Inflammasome activation under high cholesterol load triggers a protective microglial phenotype while promoting neuronal pyroptosis

BACKGROUND

Persistent inflammatory response in the brain can lead to tissue damage and neurodegeneration. In Alzheimer's disease (AD), there is an aberrant activation of inflammasomes, molecular platforms that drive inflammation through caspase-1-mediated proteolytic cleavage of proinflammatory cytokines and gasdermin D (GSDMD), the executor of pyroptosis. However, the mechanisms underlying the sustained activation of inflammasomes in AD are largely unknown. We have previously shown that high brain cholesterol levels promote amyloid-β (Aβ) accumulation and oxidative stress. Here, we investigate whether these cholesterol-mediated changes may regulate the inflammasome pathway.

METHODS

SIM-A9 microglia and SH-SY5Y neuroblastoma cells were cholesterol-enriched using a water-soluble cholesterol complex. After exposure to lipopolysaccharide (LPS) plus muramyl dipeptide or Aβ, activation of the inflammasome pathway was analyzed by immunofluorescence, ELISA and immunoblotting analysis. Fluorescently-labeled Aβ was employed to monitor changes in microglia phagocytosis. Conditioned medium was used to study how microglia-neuron interrelationship modulates the inflammasome-mediated response.

RESULTS

In activated microglia, cholesterol enrichment promoted the release of encapsulated IL-1β accompanied by a switch to a more neuroprotective phenotype, with increased phagocytic capacity and release of neurotrophic factors. In contrast, in SH-SY5Y cells, high cholesterol levels stimulated inflammasome assembly triggered by both bacterial toxins and Aβ peptides, resulting in GSDMD-mediated pyroptosis. Glutathione (GSH) ethyl ester treatment, which recovered the cholesterol-mediated depletion of mitochondrial GSH levels, significantly reduced the Aβ-induced oxidative stress in the neuronal cells, resulting in lower inflammasome activation and cell death. Furthermore, using conditioned media, we showed that neuronal pyroptosis affects the function of the cholesterol-enriched microglia, lowering its phagocytic activity and, therefore, the ability to degrade extracellular Aβ.

CONCLUSIONS

Changes in intracellular cholesterol levels differentially regulate the inflammasome-mediated immune response in microglia and neuronal cells. Given the microglia-neuron cross-talk in the brain, cholesterol modulation should be considered a potential therapeutic target for AD treatment, which may help to block the aberrant and chronic inflammation observed during the disease progression.

White matter injury, cholesterol dysmetabolism, and APP/Abeta dysmetabolism interact to produce Alzheimer's disease (AD) neuropathology: A hypothesis and review

We postulate that myelin injury contributes to cholesterol release from myelin and cholesterol dysmetabolism which contributes to Abeta dysmetabolism, and combined with genetic and AD risk factors, leads to increased Abeta and amyloid plaques. Increased Abeta damages myelin to form a vicious injury cycle. Thus, white matter injury, cholesterol dysmetabolism and Abeta dysmetabolism interact to produce or worsen AD neuropathology. The amyloid cascade is the leading hypothesis for the cause of Alzheimer's disease (AD). The failure of clinical trials based on this hypothesis has raised other possibilities. Even with a possible new success (Lecanemab), it is not clear whether this is a cause or a result of the disease. With the discovery in 1993 that the apolipoprotein E type 4 allele (APOE4) was the major risk factor for sporadic, late-onset AD (LOAD), there has been increasing interest in cholesterol in AD since APOE is a major cholesterol transporter. Recent studies show that cholesterol metabolism is intricately involved with Abeta (Aβ)/amyloid transport and metabolism, with cholesterol down-regulating the Aβ LRP1 transporter and upregulating the Aβ RAGE receptor, both of which would increase brain Aβ. Moreover, manipulating cholesterol transport and metabolism in rodent AD models can ameliorate pathology and cognitive deficits, or worsen them depending upon the manipulation. Though white matter (WM) injury has been noted in AD brain since Alzheimer's initial observations, recent studies have shown abnormal white matter in every AD brain. Moreover, there is age-related WM injury in normal individuals that occurs earlier and is worse with the APOE4 genotype. Moreover, WM injury precedes formation of plaques and tangles in human Familial Alzheimer's disease (FAD) and precedes plaque formation in rodent AD models. Restoring WM in rodent AD models improves cognition without affecting AD pathology. Thus, we postulate that the amyloid cascade, cholesterol dysmetabolism and white matter injury interact to produce and/or worsen AD pathology. We further postulate that the primary initiating event could be related to any of the three, with age a major factor for WM injury, diet and APOE4 and other genes a factor for cholesterol dysmetabolism, and FAD and other genes for Abeta dysmetabolism.

The Association between Electronegative Low-Density Lipoprotein Cholesterol L5 and Cognitive Functions in Patients with Mild Cognitive Impairment

L5, the most electronegative subfraction of low-density lipoprotein cholesterol (LDL-C), may play a role in the pathogenesis of cerebrovascular dysfunction and neurodegeneration. We hypothesized that serum L5 is associated with cognitive impairment and investigated the association between serum L5 levels and cognitive performance in patients with mild cognitive impairment (MCI). This cross-sectional study conducted in Taiwan included 22 patients with MCI and 40 older people with normal cognition (healthy controls). All participants were assessed with the Cognitive Abilities Screening Instrument (CASI) and a CASI-estimated Mini-Mental State Examination (MMSE-CE). We compared the serum total cholesterol (TC), LDL-C, and L5 levels between the MCI and control groups and examined the association between lipid profiles and cognitive performance in these groups. The serum L5 concentration and total CASI scores were significantly negatively correlated in the MCI group. Serum L5% was negatively correlated with MMSE-CE and total CASI scores, particularly in the orientation and language subdomains. No significant correlation between the serum L5 level and cognitive performance was noted in the control group. Conclusions: Serum L5, instead of TC or total LDL-C, could be associated with cognitive impairment through a disease stage-dependent mode that occurs during neurodegeneration.

Sex and APOE Genotype Alter the Basal and Induced Inflammatory States of Primary Microglia from APOE Targeted Replacement Mice

The sex and APOE4 genotype are significant risk factors for Alzheimer’s disease (AD); however, the mechanism(s) responsible for this interaction are still a matter of debate. Here, we assess the responses of mixed-sex and sex-specific APOE3 and APOE4 primary microglia (PMG) to lipopolysaccharide and interferon-gamma. In our investigation, inflammatory cytokine profiles were assessed by qPCR and multiplex ELISA assays. Mixed-sex APOE4 PMG exhibited higher basal mRNA expression and secreted levels of TNFa and IL1b. In sex-specific cultures, basal expression and secreted levels of IL1b, TNFa, IL6, and NOS2 were 2−3 fold higher in APOE4 female PMG compared to APOE4 males, with both higher than APOE3 cells. Following an inflammatory stimulus, the expression of pro-inflammatory cytokines and the secreted cytokine level were upregulated in the order E4 female > E4 male > E3 female > E3 male in sex-specific cultures. These data indicate that the APOE4 genotype and female sex together contribute to a greater inflammatory response in PMG isolated from targeted replacement humanized APOE mice. These data are consistent with clinical data and indicate that sex-specific PMG may provide a platform for exploring mechanisms of genotype and sex differences in AD related to neuroinflammation and neurodegeneration.

Cholesterol as a key player in amyloid β-mediated toxicity in Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is one of the most devastating and widespread diseases worldwide, mainly affecting the aging population. One of the key factors contributing to AD-related neurotoxicity is the production and aggregation of amyloid β (Aβ). Many studies have shown the ability of Aβ to bind to the cell membrane and disrupt its structure, leading to cell death. Because amyloid damage affects different parts of the brain differently, it seems likely that not only Aβ but also the nature of the membrane interface with which the amyloid interacts, helps determine the final neurotoxic effect. Because cholesterol is the dominant component of the plasma membrane, it plays an important role in Aβ-induced toxicity. Elevated cholesterol levels and their regulation by statins have been shown to be important factors influencing the progression of neurodegeneration. However, data from many studies have shown that cholesterol has both neuroprotective and aggravating effects in relation to the development of AD. In this review, we attempt to summarize recent findings on the role of cholesterol in Aβ toxicity mediated by membrane binding in the pathogenesis of AD and to consider it in the broader context of the lipid composition of cell membranes.

Brain ethanolamine phospholipids, neuropathology and cognition: A comparative post-mortem analysis of structurally specific plasmalogen and phosphatidyl species

Reduced cognition in the elderly is associated with low levels of plasmalogens and high levels of lipid rafts, amyloid plaques, and neurofibrillary tangles in the temporal cortex. A systematic integrative analysis of key indices of these pathologies to determine their collective and independent contributions to cognition was performed. Levels of four phosphatidylethanolamines (PE) and four ethanolamine plasmalogens (PL) of identical sn-1 carbon length and desaturation (stearic, 18:0) and identical sn-2 fatty acid compositions of varying side chain lengths and degrees of unsaturation (oleic, 18:1; linoleic, 18:2; arachidonic, 20:4; docosahexaenoic, 22:6), flotillin-1 expression and amyloid plaque and neurofibrillary tangle densities were measured in inferior temporal cortex tissue from 100 elderly subjects (Rush University Memory and Aging Project, 88.5 ± 5.8 years old). Subjects were evenly distributed with respect to gender (52/48, F/M) and cognitive status (38/24/38, no cognitive impairment/mild cognitive impairment/Alzheimer’s dementia) proximate to death. Multivariate logistic regression analyses were used to determine the relative and collective associations of the neuropathological indices with cognition. Higher levels of tangles, amyloid, or flotillin and lower levels of PL 18:0/22:6 were significantly associated with lower cognition in the base model (adjusted for age, sex, education). Multivariate analysis revealed that only PL 18:0/22:6 (β = 0.506; p < 0.00001), tangles (−0.307; p < 0.01), and flotillin (−0.2027; p < 0.05) were independently associated with reduced cognition. PL 18:0/22:6 and PE 18:0/22:6 levels were independently associated with cognition in the presence of tangles, amyloid, and flotillin, but only PL 18:0/22:6 retained its association with cognition when both PL and PE 18:0/22:6 were included in the model indicating that PE 18:0/22:6 levels were associated with PL 18:0/22:6, not cognition. Only high brain levels of PL 18:0/22:6 (>mean+1SD) was predictive of normal cognition (coef = 1.67, p < 0.05) and non-demented state (coef = −2.73, p < 0.001), whereas low levels of PL 18:0/22:6 and high levels of tangles or flotillin were predictive of dementia. The association of high brain polyunsaturated (PUFA)-PL levels with better cognition was independent of amyloid plaque, neurofibrillary tangle, PE, and flotillin-1 expression. Maintenance or augmentation of brain docosahexaenoic (DHA)-PL levels warrants further investigation as a target for preventing cognitive decline or improving cognition in the elderly, respectively.

HDL-like-Mediated Cell Cholesterol Trafficking in the Central Nervous System and Alzheimer's Disease Pathogenesis

The main aim of this work is to review the mechanisms via which high-density lipoprotein (HDL)-mediated cholesterol trafficking through the central nervous system (CNS) occurs in the context of Alzheimer’s disease (AD). Alzheimer’s disease is characterized by the accumulation of extracellular amyloid beta (Aβ) and abnormally hyperphosphorylated intracellular tau filaments in neurons. Cholesterol metabolism has been extensively implicated in the pathogenesis of AD through biological, epidemiological, and genetic studies, with the APOE gene being the most reproducible genetic risk factor for the development of AD. This manuscript explores how HDL-mediated cholesterol is transported in the CNS, with a special emphasis on its relationship to Aβ peptide accumulation and apolipoprotein E (ApoE)-mediated cholesterol transport. Indeed, we reviewed all existing works exploring HDL-like-mediated cholesterol efflux and cholesterol uptake in the context of AD pathogenesis. Existing data seem to point in the direction of decreased cholesterol efflux and the impaired entry of cholesterol into neurons among patients with AD, which could be related to impaired Aβ clearance and tau protein accumulation. However, most of the reviewed studies have been performed in cells that are not physiologically relevant for CNS pathology, representing a major flaw in this field. The ApoE4 genotype seems to be a disruptive element in HDL-like-mediated cholesterol transport through the brain. Overall, further investigations are needed to clarify the role of cholesterol trafficking in AD pathogenesis.

A review ofimaging genetics in Alzheimer's disease

Determining the association between genetic variation and phenotype is a key step to study the mechanism of Alzheimer's disease (AD), laying the foundation for studying drug therapies and biomarkers. AD is the most common type of dementia in the aged population. At present, three early-onset AD genes (APP, PSEN1, PSEN2) and one late-onset AD susceptibility gene apolipoprotein E (APOE) have been determined. However, the pathogenesis of AD remains unknown. Imaging genetics, an emerging interdisciplinary field, is able to reveal the complex mechanisms from the genetic level to human cognition and mental disorders via macroscopic intermediates. This paper reviews methods of establishing genotype-phenotype to explore correlations, including sparse canonical correlation analysis, sparse reduced rank regression, sparse partial least squares and so on. We found that most research work did poorly in supervised learning and exploring the nonlinear relationship between SNP-QT.

A human tau seeded neuronal cell model recapitulates molecular responses associated with Alzheimer's disease

Cellular models recapitulating features of tauopathies are useful tools to investigate the causes and consequences of tau aggregation and the identification of novel treatments. We seeded rat primary cortical neurons with tau isolated from Alzheimer’s disease brains to induce a time-dependent increase in endogenous tau inclusions. Transcriptomics of seeded and control cells identified 1075 differentially expressed genes (including 26 altered at two time points). These were enriched for lipid/steroid metabolism and neuronal/glial cell development genes. 50 genes were correlated with tau inclusion formation at both transcriptomic and proteomic levels, including several microtubule and cytoskeleton-related proteins such as Tubb2a, Tubb4a, Nefl and Snca. Several genes (such as Fyn kinase and PTBP1, a tau exon 10 repressor) interact directly with or regulate tau. We conclude that this neuronal model may be a suitable platform for high-throughput screens for target or hit compound identification and validation.

Bioactive lipids and their metabolism: new therapeutic opportunities for Parkinson's disease

Parkinson's disease (PD) is a neurological disorder characterized by motor dysfunction, which can also be associated with non-motor symptoms. Its pathogenesis is thought to stem from a loss of dopaminergic neurons in the substantia nigra pars compacta and the formation of Lewy bodies containing aggregated α-synuclein. Recent works suggested that lipids might play a pivotal role in the pathophysiology of PD. In particular, the so-called "bioactive" lipids whose changes in the concentration may lead to functional consequences and affect many pathophysiological processes, including neuroinflammation, are closely related to PD in terms of symptoms, disease progression, and incidence. This study aimed to explore the molecular metabolism and physiological functions of bioactive lipids, such as fatty acids (mainly unsaturated fatty acids), eicosanoids, endocannabinoids, oxysterols, representative sphingolipids, diacylglycerols, and lysophosphatidic acid, in the development of PD. The knowledge of bioactive lipids in PD gained through preclinical and clinical studies is expected to improve the understanding of disease pathogenesis and provide novel therapeutic avenues.

Sex-specific accelerated decay in time/activity-dependent plasticity and associative memory in an animal model of Alzheimer's disease

Clinical studies have shown that female brains are more predisposed to neurodegenerative diseases such as Alzheimer's disease (AD), but the cellular and molecular mechanisms behind this disparity remain unknown. In several mouse models of AD, synaptic plasticity dysfunction is an early event and appears before significant accumulation of amyloid plaques and neuronal degeneration. However, it is unclear whether sexual dimorphism at the synaptic level contributes to the higher risk and prevalence of AD in females. Our studies on APP/PS1 (APPSwe/PS1dE9) mouse model show that AD impacts hippocampal long‐term plasticity in a sex‐specific manner. Long‐term potentiation (LTP) induced by strong tetanic stimulation (STET), theta burst stimulation (TBS) and population spike timing‐dependent plasticity (pSTDP) show a faster decay in AD females compared with age‐matched AD males. In addition, behavioural tagging (BT), a model of associative memory, is specifically impaired in AD females with a faster decay in memory compared with males. Together with the plasticity and behavioural data, we also observed an upregulation of neuroinflammatory markers, along with downregulation of transcripts that regulate cellular processes associated with synaptic plasticity and memory in females. Immunohistochemistry of AD brains confirms that female APP/PS1 mice carry a higher amyloid plaque burden and have enhanced microglial activation compared with male APP/PS1 mice. Their presence in the diseased mice also suggests a link between the impairment of LTP and the upregulation of the inflammatory response. Overall, our data show that synaptic plasticity and associative memory impairments are more prominent in females and this might account for the faster progression of AD in females.

An evidence-based review of neuronal cholesterol role in dementia and statins as a pharmacotherapy in reducing risk of dementia

INTRODUCTION

Dementia is a progressive neurodegenerative disorder impairing memory and cognition. Alzheimer's Disease, followed by vascular dementia - the most typical form. Risk factors for vascular dementia include diabetes, cardiovascular disease, hyperlipidemia. Lipids' levels are significantly associated with vascular changes in the brain.

AREAS COVERED

The present article reviews the cholesterol metabolism in the brain, which includes: the synthesis, transport, storage, and elimination process. Additionally, it reviews the role of cholesterol in the pathogenesis of dementia and statin as a therapeutic intervention in dementia. In addition to the above, it further reviews evidence in support of as well as against statin therapy in dementia, recent updates of statin pharmacology, and demerits of use of statin pharmacotherapy.

EXPERT OPINION

Amyloid-β peptides and intraneuronal neurofibrillary tangles are markers of Alzheimer's disease. Evidence shows cholesterol modulates the functioning of enzymes associated with Amyloid-β peptide processing and synthesis. Lowering cholesterol using statin may help prevent or delay the progression of dementia. This paper reviews the role of statin in dementia and recommends extensive future studies, including genetic research, to obtain a precise medication approach for patients with dementia.

A comprehensive review on RAGE-facilitated pathological pathways connecting Alzheimer’s disease, diabetes mellitus, and cardiovascular diseases

Background

Alzheimer’s disease (AD), cardiovascular disease (CVD), and diabetes are some of the most common causes of morbidity and mortality among the aging populations and cause a heavy burden on the worldwide healthcare system. In this review, we briefly highlighted cellular inflammation-based pathways of diabetes mellitus and CVD through receptor for advanced glycation end products AGEs or RAGE leading to Alzheimer’s disease and interrelation between these vascular and metabolic disorders. The articles were retrieved from Google Scholar, Science Direct, and PubMed databases using the following terms: Alzheimer’s; AGEs; RAGE; RAGE in Alzheimer’s; AGEs in Alzheimer’s; RAGE in diabetes; RAGE related pathways of CVD; RAGE in hypertension; RAGE and RAS system; RAGE and oxidative stress.

Main body of the abstract

AD is a neurodegenerative disease characterized by cognitive dysfunction and neuronal cell death. Vascular complications like hypertension, coronary artery disease, and atherosclerosis as well as metabolic syndromes like obesity and diabetes are related to the pathophysiology of AD. RAGE plays significant role in the onset and progression of AD. Amyloid plaques and neurofibrillary tangles (NFT) are two main markers of AD that regulates via RAGE and other RAGE/ligands interactions which also induces oxidative stress and a cascade of other cellular inflammation pathways leading to AD. Though AD and diabetes are two different disorders but may be inter-linked by AGEs and RAGE. In long-term hyperglycemia, upregulated AGEs interacts with RAGE and produces reactive oxygen species which induces further inflammation and vascular complications. Aging, hypercholesterolemia, atherosclerosis, hypertension, obesity, and inflammation are some of the main risk factors for both diabetes and dementia. Chronic hypertension and coronary artery disease disrupt the functions of the blood-brain barrier and are responsible for the accumulation of senile plaques and NFTs.

Short conclusion

RAGE plays a role in the etiology of Aβ and tau hyperphosphorylation, both of which contribute to cognitive impairment. So far, targeting RAGE may provide a potential sight to develop therapies against some metabolic disorders.

Regulation of cholesterol biosynthesis and lipid metabolism: A microRNA management perspective

Cellular disruption of lipid and cholesterol metabolism results in pathological processes linked to metabolic and cardiovascular diseases. Classically, at the transcription stages, the Cholesterol levels are controlled by two cellular pathways. First, the SREBP transcription factor family controls Cholesterol biosynthesis via transcriptional regulation of critical rate-limiting cholesterogenic and lipogenic proteins. Secondly, The LXR/RXR transcription factor family controls cholesterol shuttling via transcriptional regulation of cholesterol transport proteins. In addition, the posttranscriptional control of gene expression of various enzymes and proteins of cholesterol biosynthesis pathways is mediated by small non-coding microRNAs. Regulatory noncoding miRNAs are critical regulators of biological processes, including developmental and metabolic functions. miRNAs function to fine-tune lipid and cholesterol metabolism pathways by controlling the mRNA levels and translation of critical molecules in each pathway. This review discusses the regulatory roles of miRNAs in cholesterol and lipid metabolism via direct and indirect effects on their target genes, including SREBP, LXR, HDL, LDL, and ABCA transporters. We also discuss the therapeutic implications of miRNA functions and their purported role in the potentiation of small molecule therapies.

The Role of Cholesterol in Amyloidogenic Substrate Binding to the γ-Secretase Complex

Alzheimer’s disease is the most common progressive neurodegenerative disorder and is characterized by the presence of amyloid β (Aβ) plaques in the brain. The γ-secretase complex, which produces Aβ, is an intramembrane-cleaving protease consisting of four membrane proteins. In this paper we investigated the amyloidogenic fragments of amyloid precursor protein (substrates Aβ₄₃ and Aβ₄₅, leading to less amyloidogenic Aβ₄₀ and more amyloidogenic Aβ₄₂, respectively) docked to the binding site of presenilin, the catalytic subunit of γ-secretase. In total, we performed 9 μs of all-atom molecular dynamics simulations of the whole γ-secretase complex with both substrates in low (10%) and high (50%) concentrations of cholesterol in the membrane. We found that, at the high cholesterol level, the Aβ₄₅ helix was statistically more flexible in the binding site of presenilin than Aβ₄₃. An increase in the cholesterol concentration was also correlated with a higher flexibility of the Aβ₄₅ helix, which suggests incompatibility between Aβ₄₅ and the binding site of presenilin potentiated by a high cholesterol level. However, at the C-terminal part of Aβ₄₅, the active site of presenilin was more compact in the case of a high cholesterol level, which could promote processing of this substrate. We also performed detailed mapping of the cholesterol binding sites at low and high cholesterol concentrations, which were independent of the typical cholesterol binding motifs.

Abnormal brain cholesterol homeostasis in Alzheimer's disease-a targeted metabolomic and transcriptomic study

The role of brain cholesterol metabolism in Alzheimer's disease (AD) remains unclear. Peripheral and brain cholesterol levels are largely independent due to the impermeability of the blood brain barrier (BBB), highlighting the importance of studying the role of brain cholesterol homeostasis in AD. We first tested whether metabolite markers of brain cholesterol biosynthesis and catabolism were altered in AD and associated with AD pathology using linear mixed-effects models in two brain autopsy samples from the Baltimore Longitudinal Study of Aging (BLSA) and the Religious Orders Study (ROS). We next tested whether genetic regulators of brain cholesterol biosynthesis and catabolism were altered in AD using the ANOVA test in publicly available brain tissue transcriptomic datasets. Finally, using regional brain transcriptomic data, we performed genome-scale metabolic network modeling to assess alterations in cholesterol biosynthesis and catabolism reactions in AD. We show that AD is associated with pervasive abnormalities in cholesterol biosynthesis and catabolism. Using transcriptomic data from Parkinson's disease (PD) brain tissue samples, we found that gene expression alterations identified in AD were not observed in PD, suggesting that these changes may be specific to AD. Our results suggest that reduced de novo cholesterol biosynthesis may occur in response to impaired enzymatic cholesterol catabolism and efflux to maintain brain cholesterol levels in AD. This is accompanied by the accumulation of nonenzymatically generated cytotoxic oxysterols. Our results set the stage for experimental studies to address whether abnormalities in cholesterol metabolism are plausible therapeutic targets in AD.

Physical Exercise and Alzheimer's Disease: Effects on Pathophysiological Molecular Pathways of the Disease

Alzheimer’s disease (AD), the most common form of neurodegenerative dementia in adults worldwide, is a multifactorial and heterogeneous disorder characterized by the interaction of genetic and epigenetic factors and the dysregulation of numerous intracellular signaling and cellular/molecular pathways. The introduction of the systems biology framework is revolutionizing the study of complex diseases by allowing the identification and integration of cellular/molecular pathways and networks of interaction. Here, we reviewed the relationship between physical activity and the next pathophysiological processes involved in the risk of developing AD, based on some crucial molecular pathways and biological process dysregulated in AD: (1) Immune system and inflammation; (2) Endothelial function and cerebrovascular insufficiency; (3) Apoptosis and cell death; (4) Intercellular communication; (5) Metabolism, oxidative stress and neurotoxicity; (6) DNA damage and repair; (7) Cytoskeleton and membrane proteins; (8) Synaptic plasticity. Moreover, we highlighted the increasingly relevant role played by advanced neuroimaging technologies, including structural/functional magnetic resonance imaging, diffusion tensor imaging, and arterial spin labelling, in exploring the link between AD and physical exercise. Regular physical exercise seems to have a protective effect against AD by inhibiting different pathophysiological molecular pathways implicated in AD.

Cholesterol alters mitophagy by impairing optineurin recruitment and lysosomal clearance in Alzheimer's disease

BACKGROUND

Emerging evidence indicates that impaired mitophagy-mediated clearance of defective mitochondria is a critical event in Alzheimer's disease (AD) pathogenesis. Amyloid-beta (Aβ) metabolism and the microtubule-associated protein tau have been reported to regulate key components of the mitophagy machinery. However, the mechanisms that lead to mitophagy dysfunction in AD are not fully deciphered. We have previously shown that intraneuronal cholesterol accumulation can disrupt the autophagy flux, resulting in low Aβ clearance. In this study, we examine the impact of neuronal cholesterol changes on mitochondrial removal by autophagy.

METHODS

Regulation of PINK1-parkin-mediated mitophagy was investigated in conditions of acute (in vitro) and chronic (in vivo) high cholesterol loading using cholesterol-enriched SH-SY5Y cells, cultured primary neurons from transgenic mice overexpressing active SREBF2 (sterol regulatory element binding factor 2), and mice of increasing age that express the amyloid precursor protein with the familial Alzheimer Swedish mutation (Mo/HuAPP695swe) and mutant presenilin 1 (PS1-dE9) together with active SREBF2.

RESULTS

In cholesterol-enriched SH-SY5Y cells and cultured primary neurons, high intracellular cholesterol levels stimulated mitochondrial PINK1 accumulation and mitophagosomes formation triggered by Aβ while impairing lysosomal-mediated clearance. Antioxidant recovery of cholesterol-induced mitochondrial glutathione (GSH) depletion prevented mitophagosomes formation indicating mitochondrial ROS involvement. Interestingly, when brain cholesterol accumulated chronically in aged APP-PSEN1-SREBF2 mice the mitophagy flux was affected at the early steps of the pathway, with defective recruitment of the key autophagy receptor optineurin (OPTN). Sustained cholesterol-induced alterations in APP-PSEN1-SREBF2 mice promoted an age-dependent accumulation of OPTN into HDAC6-positive aggresomes, which disappeared after in vivo treatment with GSH ethyl ester (GSHee). The analyses in post-mortem brain tissues from individuals with AD confirmed these findings, showing OPTN in aggresome-like structures that correlated with high mitochondrial cholesterol levels in late AD stages.

CONCLUSIONS

Our data demonstrate that accumulation of intracellular cholesterol reduces the clearance of defective mitochondria and suggest recovery of the cholesterol homeostasis and the mitochondrial scavenging of ROS as potential therapeutic targets for AD.

27-Hydroxycholesterol, cognition, and brain imaging markers in the FINGER randomized controlled trial

BACKGROUND

27-Hydroxycholesterol (27-OH), the main circulating oxysterol in humans and the potential missing link between peripheral hypercholesterolemia and Alzheimer's disease (AD), has not been investigated previously in relation to cognition and neuroimaging markers in the context of preventive interventions.

METHODS

The 2-year Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability (FINGER) included older individuals (60-77 years) at increased risk for dementia but without dementia or substantial cognitive impairment from the general population. Participants were randomized to a multidomain intervention (diet, exercise, cognitive training, and vascular risk management) or control group (general health advice) in a 1:1 ratio. Outcome assessors were masked to group allocation. This FINGER exploratory sub-study included 47 participants with measures of 27-OH, cognition, brain MRI, brain FDG-PET, and PiB-PET. Linear regression models were used to assess the cross-sectional and longitudinal associations between 27-OH, cognition, and neuroimaging markers, considering several potential confounders/intervention effect modifiers.

RESULTS

27-OH reduction during the intervention was associated with improvement in cognition (especially memory). This was not observed in the control group. The intervention reduced 27-OH particularly in individuals with the highest 27-OH levels and younger age. No associations were found between changes in 27-OH levels and neuroimaging markers. However, at baseline, a higher 27-OH was associated with lower total gray matter and hippocampal volume, and lower cognitive scores. These associations were unaffected by total cholesterol levels. While sex seemed to influence associations at baseline, it did not affect longitudinal associations.

CONCLUSION

27-OH appears to be a marker not only for dementia/AD risk, but also for monitoring the effects of preventive interventions on cholesterol metabolism.

TRIAL REGISTRATION

ClinicalTrials.gov , NCT01041989 . Registered on 4 January 2010.

An Analysis of the Neurological and Molecular Alterations Underlying the Pathogenesis of Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by amyloid beta (Aβ) plaques, neurofibrillary tangles, and neuronal loss. Unfortunately, despite decades of studies being performed on these histological alterations, there is no effective treatment or cure for AD. Identifying the molecular characteristics of the disease is imperative to understanding the pathogenesis of AD. Furthermore, uncovering the key causative alterations of AD can be valuable in developing models for AD treatment. Several alterations have been implicated in driving this disease, including blood–brain barrier dysfunction, hypoxia, mitochondrial dysfunction, oxidative stress, glucose hypometabolism, and altered heme homeostasis. Although these alterations have all been associated with the progression of AD, the root cause of AD has not been identified. Intriguingly, recent studies have pinpointed dysfunctional heme metabolism as a culprit of the development of AD. Heme has been shown to be central in neuronal function, mitochondrial respiration, and oxidative stress. Therefore, dysregulation of heme homeostasis may play a pivotal role in the manifestation of AD and its various alterations. This review will discuss the most common neurological and molecular alterations associated with AD and point out the critical role heme plays in the development of this disease.

Favorable effects of dill tablets and Ocimum basilicum L. extract on learning, memory, and hippocampal fatty acid composition in hypercholesterolemic rats

OBJECTIVES

Hypercholesterolemia is correlated with brain amyloid-β (Aβ) deposition and impaired cognitive functions and contributes to Alzheimer's disease. Effects of cholesterol-lowering dill tablets and aqueous extract of Ocimum basilicum L. (basil) on learning and memory and hippocampus fatty acid composition were examined. mRNA levels of the genes involved in cholesterol homeostasis were also determined in high-cholesterol diet (HCD) fed rats.

MATERIALS AND METHODS

Forty male Wistar rats were allocated to 4 groups: rats fed chow diet (C); rats fed high-cholesterol (2%) diet (HCD); rats treated with HCD+300 mg/kg dill tablets (HCD+Dill); and finally, rats fed HCD and treated with 400 mg/kg basil aqueous extract (HCD+basil). Treatment was carried out for 16 weeks. Hippocampus Aβ(1-42) level was determined. Spatial and passive avoidance tests were used to examine cognitive functions. Hippocampal FA composition was assessed by gas chromatography. Basil aqueous extract was analyzed by GC-double mass spectroscopy (GC-MS/MS) and expression of LXR-α, LXR-β, and ABCA1 genes was assessed by qRT-PCR.

RESULTS

Dill tablets and basil extract remarkably ameliorated serum cholesterol (P<0.001), retarded hippocampal accumulation of Aβ, and attenuated HCD-induced memory impairment. Hippocampus FA composition did not change but serum cholesterol was found positively correlated with hippocampus Aβ(1-42) (P<0.001), total n 6 PUFA (P=0.013), and Aβ(1-42) showed correlation with the ratio of n6 to n3 PUFA. At least 70 components were identified in basil aqueous extract.

CONCLUSION

Dill tablets and aqueous extract of basil attenuated the hypercholesterolemia-induced memory impairment by lowering serum cholesterol and hippocampus amyloid deposits, and probably beneficial in AD adjuvant therapy.

Simvastatin impairs hippocampal synaptic plasticity and cognitive function in mice

Lipophilic statins which are blood brain barrier (BBB) permeable are speculated to affect the cholesterol synthesis and neural functions in the central nervous system. However, whether these statins can affect cholesterol levels and synaptic plasticity in hippocampus and the in vivo consequence remain unclear. Here, we report that long-term subcutaneous treatments of simvastatin significantly impair mouse hippocampal synaptic plasticity, reflected by the attenuated long-term potentiation of field excitatory postsynaptic potentials. The simvastatin administration causes a deficiency in recognition and spatial memory but fails to affect motor ability and anxiety behaviors in the mice. Mass spectrometry imaging indicates a significant decrease in cholesterol intensity in hippocampus of the mice receiving chronic simvastatin treatments. Such effects of simvastatin are transient because drug discontinuation can restore the hippocampal cholesterol level and synaptic plasticity and the memory function. These findings may provide further clues to elucidate the mechanisms of neurological side effects, especially the brain cognitive function impairment, caused by long-term usage of BBB-permeable statins.

The Impact of Air Pollution on Neurodegenerative Diseases

BACKGROUND

With the development of industrialization in human society, ambient pollutants are becoming more harmful to human health. Epidemiological and toxicological studies indicate that a close relationship exists between particulate matter with a diameter ≤2.5 µm (PM2.5) and neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). To further confirm the relationship, we focus on possible relevant mechanisms of oxidative stress and neuroinflammation underlying the association between PM2.5 and neurodegenerative diseases in the review.

METHODS

A literature search was performed on the studies about PM2.5 and neurodegenerative diseases via PubMed. A total of 113 articles published were selected, and 31 studies were included.

RESULTS

PM2.5 can enter the central nervous system through 2 main pathways, the blood-brain barrier and olfactory neurons. The inflammatory response and oxidative stress are 2 primary mechanisms via which PM2.5 leads to toxicity in the brain. PM2.5 abnormally activates microglia, inducing the neuroinflammatory process. Inflammatory markers such as IL-1β play an essential role in neurodegenerative diseases such as AD and PD. Moreover, the association between lipid mechanism disorders related to PM2.5 and neurodegenerative diseases has been gaining momentum.

CONCLUSIONS

In conclusion, PM2.5 could significantly increase the risk of neurological disorders, such as AD and PD. Furthermore, any policy aimed at reducing air-polluting emissions and increasing air quality would be protective in human beings.

Inhibition of microRNA-155 Alleviates Cognitive Impairment in Alzheimer's Disease and Involvement of Neuroinflammation

BACKGROUND

Neuroinflammation has important effects on cognitive functions in the pathophysiological process of Alzheimer's Disease (AD). In the current report, we determined the effects of microRNA-155 (miR-155) on the levels of IL-1β, IL-6 and TNF-α, and their respective receptors in the hippocampus using a rat model of AD.

METHODS

Real-time RT-PCR, ELISA and western blot analysis were used to examine the miR-155, PICs and PIC receptors. The Morris water maze and spatial working memory tests were used to assess cognitive functions.

RESULTS

miR-155 was increased in the hippocampus of AD rats, accompanied by amplification of IL-1β, IL-6 and TNF-α. Intracerebroventricular infusion of miR-155 inhibitor, but not its scramble attenuated the increases of IL-1β, IL-6 and TNF-α and upregulation of their receptors. MiR-155 inhibitor also attenuated upregulation of apoptotic Caspase-3 in the hippocampus of AD rats. Notably, inhibition of miR- 155 or PIC receptors largely recovered the impaired learning performance in AD rat.

CONCLUSION

We showed the critical role of miR-155 in regulating the memory impairment in AD rats likely via engagement of neuroinflammatory mechanisms, suggesting that miR-155 and its signaling molecules may present prospects in preventing and/or improving the development of the impaired cognitive functions in AD.

Evaluation of Insulin Resistance Induced Brain Tissue Dysfunction in Obese Dams and their Neonates: Role of Ipriflavone Amelioration

BACKGROUND

Nonalcoholic steatohepatitis (NASH) is associated with activation of liver fibrogenesis and predisposes to cirrhosis and associated morbi-mortality. A high fat high cholesterol diet (HFD) was provided to female albino rats to establish a NASH model. It is well known that the offspring of obese mothers have an increased risk of obesity and diabetes. The present study aimed at evaluating the ameliorative effects of ipriflavone (IP) as a natural food supplement on lipid metabolism, improving insulin sensitivity, reducing oxidative stress and inflammation, modifying metabolic risk factors and/or reduce brain damage, in both neonates and their dams.

MATERIALS AND METHODS

The present aim was achieved by evaluating the oxidative stress and antioxidant defense system biomarkers, as thiobarbituric acid reactive substances (TBARS) and reduced glutathione (GSH), catalase, and superoxide dismutase (SOD) activities. In addition, the neurotransmitter acetylcholine (Ach) and acetylcholine esterase (AchE) activities, as well as levels of the apolipoprotein E4 (APOE4); β-secretase, hyper phosphor-tau and β-amyloid 42; 3-hydroxy- 3-methyl glutaryl coenzyme A reductase (HMG CoA R)" and COX-II by immunoblotting assays in the brain tissue of neonates and their dams in all the studied groups.

RESULTS

A very significant amelioration in acetylcholine and acetylcholine esterase neurotransmitters, Alzheimer's makers (β-amyloid), antioxidants (reduced glutathione (GSH) contents, catalase (CAT) and superoxide dismutase (SOD); and inflammatory cytokines in NASH model is observed upon administrating ipriflavone (IP) as a natural food supplement. The multifunctional activities of ipriflavone as an antioxidant, anti-inflammatory and anti-insulin resistance drug were discussed and correlated with other investigations.

CONCLUSION

Regarding steatohepatitis, the present study confirmed the anti-inflammatory effects of the ipriflavone (IP). Therefore, future studies should focus on hepatic fatty acid uptake, hepatic lipogenesis, and fatty acid oxidation and the role of IP in regulating hepatic fat metabolism. In addition, natural products like IP could be combined with the highly used pharmaceutical drugs to reduce the side effects of nonalcoholic steatohepatitis, and minimize progression of dementia. Moreover, the present study supports further attempts to heal the neural dysfunction via antioxidant and anti-inflammatory cascade activities using ipriflavone (IP).

Sodium butyrate inhibits high cholesterol-induced neuronal amyloidogenesis by modulating NRF2 stabilization-mediated ROS levels: involvement of NOX2 and SOD1

The gut-brain axis is currently being studied as a therapeutic strategy for neurological diseases, especially Alzheimer's disease (AD). Obesity results in the gut microbiota dysbiosis, which includes butyrate-producing bacteria are reduced. Although sodium butyrate (NaB) has emerged as the potential therapeutic substance in AD, there is a lack of detailed results into what signaling pathways affect amyloidogenesis in AD induced by obesity. Thus, we investigated the regulatory role of NaB on amyloidogenesis in neuronal cells under high cholesterol. In our results, we verified that increased amyloid β peptide (Aβ) accumulation in the brain of obese mice and a reduction in butyrate-producing bacteria due to the gut microbiota dysbiosis induced by obesity. We showed that NaB decreased the expression levels of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) and Aβ accumulation induced by high cholesterol in SK-N-MC cells. We demonstrated that NaB was absorbed in cells through sodium-coupled monocarboxylate transporter 1 (SMCT1) and then inhibited high cholesterol-induced Aβ accumulation. Subsequently, we also observed that reactive oxygen species (ROS) were overproduced because of increased NADPH oxidase 2 (NOX2) expression under high cholesterol. Meanwhile, NaB decreased NOX2 levels through a reduction of NF-κB activity, which ultimately inhibited Aβ accumulation caused by high cholesterol. We demonstrated that NaB increased the expression levels of p21 under high cholesterol, contributing to p21/NRF2 (Nuclear factor erythroid 2-related factor 2) colocalization, which leads to NRF2 stabilization. NRF2 stabilization causes NF-κB inactivation, followed by NOX2 suppression and superoxide dismutase 1 (SOD1) upregulation. Thus, NaB with SOD1 silencing under high cholesterol did not eliminate excessive ROS, and eventually resulted in Aβ accumulation. In conclusion, we demonstrated that NaB prevents excessive ROS through NOX2 suppression and SOD1 upregulation by p21/NRF2 pathway, which is critical for inhibiting BACE1-dependent amyloidogenesis in neuronal cells exposed to high cholesterol environment.

Oxidative inactivation of amyloid beta-degrading proteases by cholesterol-enhanced mitochondrial stress

Familial early-onset forms of Alzheimer's disease (AD) are linked to overproduction of amyloid beta (Aβ) peptides, while decreased clearance of Aβ is the driving force leading to its toxic accumulation in late-onset (sporadic) AD. Oxidative modifications and defective function have been reported in Aβ-degrading proteases such as neprilysin (NEP) and insulin-degrading enzyme (IDE). However, the exact mechanisms that regulate the proteolytic clearance of Aβ and its deficits are largely unknown. We have previously showed that cellular cholesterol loading, by depleting the mitochondrial GSH (mGSH) content, stimulates Αβ-induced mitochondrial oxidative stress and promotes AD-like pathology in APP-PSEN1-SREBF2 mice. Here, using the same AD mouse model we examined whether cholesterol-enhanced mitochondrial oxidative stress affects NEP and IDE function. We found that brain extracts from APP-PSEN1-SREBF2 mice displayed increased presence of oxidatively modified forms of NEP and IDE, associated with impaired enzymatic activities. Both alterations were substantially recovered after an in vivo treatment with the cholesterol-lowering agent 2-hydroxypropyl-β-cyclodextrin. The recovery of the proteolytic activity after treatment was accompanied with a significant reduction of Aβ levels. Supporting these results, cholesterol-enriched SH-SY5Y cells were more sensitive to Aβ-induced impairment of IDE and NEP function in vitro. The rise of cellular cholesterol also stimulated the extracellular release of IDE by an unconventional autophagy-coordinated mechanism. Recovery of depleted pool of mGSH in these cells not only prevented the detrimental effect of Aβ on intracellular AβDPs activities but also had an impact on extracellular IDE levels and function, stimulating the extracellular Aβ degrading activity. Therefore, changes in brain cholesterol levels by modifying the mGSH content would play a key role in IDE and NEP-mediated proteolytic elimination of Aβ peptides and AD progression.

•

Cholesterol regulates IDE and NEP by enhancing the detrimental effect of Aβ on their proteolytic activities.

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Cholesterol-mediated mitochondrial GSH depletion is responsible for the oxidative impairment of IDE and NEP.

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High cholesterol levels induce the release of inactive IDE through secretory autophagy.

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A rise in cellular cholesterol affects the extracellular Aβ degradation, favoring oligomers formation.

•

Cholesterol lowering compounds and antioxidant therapy restore IDE and NEP activity.

Gene-environment interactions in Alzheimer's disease: A potential path to precision medicine

Alzheimer's disease (AD) is the leading cause of dementia in the United States and afflicts >5.7 million Americans in 2018. Therapeutic options remain extremely limited to those that are symptom targeting, while no drugs have been approved for the modification or reversal of the disease itself. Risk factors for AD including aging, the female sex, as well as carrying an APOE4 genotype. These risk factors have been extensively examined in the literature, while less attention has been paid to modifiable risk factors, including lifestyle, and environmental risk factors such as exposures to air pollution and pesticides. This review highlights the most recent data on risk factors in AD and identifies gene by environment interactions that have been investigated. It also provides a suggested framework for a personalized therapeutic approach to AD, by combining genetic, environmental and lifestyle risk factors. Understanding modifiable risk factors and their interaction with non-modifiable factors (age, susceptibility alleles, and sex) is paramount for designing personalized therapeutic interventions.

27-Hydroxycholesterol Alters Synaptic Structural and Functional Plasticity in Hippocampal Neuronal Cultures

This study aimed to explore the neurotoxic effects of 27-hydroxycholesterol (27-OHC), a major circulating cholesterol active derivative in brain on synaptic structural and functional plasticity in primary hippocampal neurons. Newborn SD rat primary hippocampal neurons were treated with 0, 1, 3, 10, and 30 μM 27-OHC for 24 hours. MTT and CCK-8 assays were used to monitor the cell viability of neurons with different treatments. Neurite morphology was assessed by staining for microtubule-associated protein-2 (MAP2) and analyzed by immunofluorescence. Synaptic ultrastructure was evaluated by transmission electron microscopy. Real-time polymerase chain reaction and Western blot analyses were used to evaluate the expression of key synaptic proteins: synaptophysin (SYP), postsynaptic density protein-95 (PSD-95), synaptosomal-associated protein 25 (SNAP-25), growth-associated protein-43 (GAP-43), MAP2, and activity-regulated cytoskeleton-associated protein (Arc). Treatment with 27-OHC at various doses stimulated cell death and resulted in significant decreases in neurite number and length, alteration of synaptic ultrastructure, and downregulated expression of synaptic proteins in a dose-dependent manner. These results suggest that 27-OHC is deleterious for synaptic structural and functional plasticity, which may partially account for its neurotoxic effects.

Mass Spectrometry Imaging of Brain Cholesterol and Metabolites with Trifluoroacetic Acid-Enhanced Desorption Electrospray Ionization

Imaging of cholesterol and other metabolites simultaneously by ambient mass spectrometry will greatly benefit biological studies, however, it still remains challenging. Herein, by adding acid into the desorption electrospray ionization (DESI) spray solvent, we achieved simultaneous mass spectrometry imaging of cholesterol and other metabolites directly from mouse brain sections. The introduction of acid increased the signal intensity of cholesterol in mouse brain tissues by approximately 21-fold. Additionally, the present strategy provided increased signal intensities for other metabolites up to 62-fold, as well as identification of seven more metabolites (23 vs 16 for acid-enhanced DESI vs DESI). Moreover, increased corelationships for alanine as well as putrescine and spermidine with cholesterol were discovered under acid-enhanced DESI. The potential of the present strategy in the fields of biological and medical research was demonstrated by investigating the level change for cholesterol, alanine, putrescine, and spermidine in Alzheimer's disease (AD) mouse brain.

Influence of membrane lipid composition on the structure and activity of γ-secretase

γ-Secretase (GS) is a multi-subunit membrane-embedded aspartyl protease that cleaves more than 80 integral membrane proteins, including the amyloid precursor protein (APP) to produce the amyloid-β (Aβ) peptide. Oligomerization and aggregation of the 42-amino acid length Aβ isoform in the brain has been associated with the development and progression of Alzheimer's disease (AD). Based on recent experimental structural studies and using multiscale computational modeling approaches, the conformational states and protein-membrane interactions of the GS complex embedded in six homogeneous and six heterogeneous lipid bilayers were characterized. In order to identify potential lipid and cholesterol binding sites, GS regions with high lipid/cholesterol occupancy values were analyzed using atomistic and coarse-grained simulations. Long lipid residence times were observed to be correlated with a large number of hydrogen bonds between the charged headgroups and key GS amino acids. This observation provides a plausible explanation for the inhibition of GS by charged lipids observed in previous experimental studies. Computed lateral pressure profiles suggest that higher transmembrane pressures favor active state conformations of the catalytic subunit. A probable mechanism for the regulation of the local stress response in cholesterol-rich multicomponent lipid bilayers is identified. Finally, it is demonstrated that interactions between the nicastrin extracellular domain and lipid headgroups leads to a compact structural conformation of the GS complex. Overall, this study provides valuable insight into the effect of bilayer lipid composition on the GS structural ensemble and its function.

Serum Ethanolamine Plasmalogen and Urine Myo-Inositol as Cognitive Decline Markers

Recent studies have suggested that metabolic disorders, particularly type 2 diabetes mellitus (T2DM), and dementia, including Alzheimer's disease (AD), were linked at the clinical and molecular levels. Brain insulin deficiency and resistance may be key events in AD pathology mechanistically linking AD to T2DM. Ethanolamine plasmalogens (PlsEtns) are abundant in the brain and play essential roles in neuronal function and myelin formation. As such, PlsEtn deficiency may be pathologically relevant in some neurodegenerative disorders such as AD. Decreased brain PlsEtn associated with dementia may reflect serum PlsEtn deficiency. We hypothesized that myo-inositol plays a role in myelin formation through its facilitation of PlsEtn biosynthesis. Excessive urinary myo-inositol (UMI) loss would likely result in PlsEtn deficiency potentially leading to demyelinating diseases such as dementia. Accordingly, measurement of both serum PlsEtn and baseline UMI excretion could improve the detection of cognitive impairment (CI) in a more specific and reliable manner. To verify our hypothesis, we conducted a clinical observational study of memory clinic outpatients (MCO) and cognitively normal elderly (NE) for nearly 4.5years. We demonstrated that serum PlsEtn concentration associated with UMI excretion was useful for predicting advancing dementia in patients with mild CI. Because hyperglycemia and associated insulin resistance might be a leading cause of increased baseline UMI excretion, serum PlsEtn quantitation would be useful in detecting CI among the elderly with hyperglycemia. Our findings suggest that myo-inositol is a novel candidate molecule linking T2DM to AD.

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.

Alterations in cholesterol metabolism-related genes in sporadic Alzheimer's disease

Genome-wide association studies have identified several cholesterol metabolism-related genes as top risk factors for late-onset Alzheimer's disease (LOAD). We hypothesized that specific genetic variants could act as disease-modifying factors by altering the expression of those genes. Targeted association studies were conducted with available genomic, transcriptomic, proteomic, and histopathological data from 3 independent cohorts: the Alzheimer's Disease Neuroimaging Initiative (ADNI), the Quebec Founder Population (QFP), and the United Kingdom Brain Expression Consortium (UKBEC). First, a total of 273 polymorphisms located in 17 cholesterol metabolism-related loci were screened for associations with cerebrospinal fluid LOAD biomarkers beta amyloid, phosphorylated tau, and tau (from the ADNI) and with amyloid plaque and tangle densities (from the QFP). Top polymorphisms were then contrasted with gene expression levels measured in 134 autopsied healthy brains (from the UKBEC). In the end, only SREBF2 polymorphism rs2269657 showed significant dual associations with LOAD pathological biomarkers and gene expression levels. Furthermore, SREBF2 expression levels measured in LOAD frontal cortices inversely correlated with age at death; suggesting a possible influence on survival rate.

Effects of thymol on amyloid-β-induced impairments in hippocampal synaptic plasticity in rats fed a high-fat diet

Obesity and a high-fat diet (HFD) are known to increase the incidence of Alzheimer's disease (AD). Oxidative stress, a major risk factor for AD, is increased with HFD consumption. Thymol (Thy) has antioxidant properties. Therefore, in the present study, we examined the protective and therapeutic effects of Thy on amyloid-β (Aβ)-induced impairments in the hippocampal synaptic plasticity of HFD-fed rats. In this study, 72 adult male Wistar rats were randomly assigned to 9 groups (n = 8 rats/group): Group 1 (control; standard diet); Group 2: Control + phosphate-buffered saline (PBS) + Oil (Thy vehicle); Group 3 (HFD + PBS); Group 4: (HFD + Aβ); Group 5: Control + PBS + Thy; Group 6: (HFD + Aβ + Oil); Group 7: Control + Aβ + Thy; Group 8: HFD + PBS + Thy; Group 9: (HFD + Aβ + Thy). After stereotaxic surgery, the field potentials were recorded after the implantation of the recording and stimulating electrodes in the dentate gyrus (DG) and perforant pathway, respectively. Following high-frequency stimulation, the long-term potentiation (LTP) of the population spike (PS) amplitude and the slope of the excitatory postsynaptic potentials (EPSPs) were measured in the DG. The HFD rats that received Aβ exhibited a significant decrease in their EPSP slope and PS amplitude as compared to the control group. In contrast, Thy administration in the HFD + Aβ rats reduced the decrease in the EPSP slope and PS amplitude. Thy decreased the Aβ-induced LTP impairments in HFD rats. The HFD significantly increased serum malondialdehyde levels and decreased total antioxidant capacity and total glutathione levels; whereas, Thy supplementation significantly reversed these parameters. Therefore, these results suggest that Thy, a natural antioxidant, can be therapeutic against high risk factors for AD, such as HFD.

A Meta-Analysis on the Relationship of the PON Genes and Alzheimer Disease

AIM

This study aimed to evaluate the association of the paraoxonase (PON) gene variants and Alzheimer disease (AD) using meta-analysis.

METHODS

Relevant studies were identified by searching English and Chinese databases extensively. Allele and genotype frequencies for each included study were extracted. Newcastle-Ottawa Scale (NOS) was employed to assess the quality of included studies. The odds ratio (OR) with 95% confidence interval (95% CI) was calculated using a random-effects or fixed-effects model. A Q statistic was used to evaluate homogeneity, and Egger test and funnel plot were used to assess publication bias.

RESULTS

A total of 15 studies (involving 5 polymorphisms) were included and identified for the current meta-analysis. The NOS scores ranged from 7 to 8, meaning good quality of studies. It was found that the SS genotype of PON2 S311C polymorphism had an significant association with AD in the studied population (OR = 0.82, 95% CI: 0.68-0.99, P = .04), and the A allele of PON1 rs705379 polymorphism was positively related to AD in Caucasian population (OR = 1.21, 95% CI: 1.05-1.39, P = .009) as well as the GG genotype decreased AD risk significantly in Caucasians (OR = 0.7, 95% CI: 0.56-0.88, P = .002). However, there was no significant relationship between other 3 genetic variants of PON genes (L55 M, Q192 R, and -161C/T of PON1 gene) and AD.

CONCLUSION

Existing evidence indicates that the S311C polymorphism (SS genotype) and the rs705379 (the A allele and GG genotype) are associated with risk of AD in studied population. Future studies with larger sample sizes will be necessary to confirm the present results.

Screening for Small Molecule Inhibitors of Statin-Induced APP C-terminal Toxic Fragment Production

Alzheimer's disease (AD) is characterized by neuronal and synaptic loss. One process that could contribute to this loss is the intracellular caspase cleavage of the amyloid precursor protein (APP) resulting in release of the toxic C-terminal 31-amino acid peptide APP-C31 along with the production of APPΔC31, full-length APP minus the C-terminal 31 amino acids. We previously found that a mutation in APP that prevents this caspase cleavage ameliorated synaptic loss and cognitive impairment in a murine AD model. Thus, inhibition of this cleavage is a reasonable target for new therapeutic development. In order to identify small molecules that inhibit the generation of APP-C31, we first used an APPΔC31 cleavage site-specific antibody to develop an AlphaLISA to screen several chemical compound libraries for the level of N-terminal fragment production. This antibody was also used to develop an ELISA for validation studies. In both high throughput screening (HTS) and validation testing, the ability of compounds to inhibit simvastatin- (HTS) or cerivastatin- (validation studies) induced caspase cleavage at the APP-D720 cleavage site was determined in Chinese hamster ovary (CHO) cells stably transfected with wildtype (wt) human APP (CHO-7W). Several compounds, as well as control pan-caspase inhibitor Q-VD-OPh, inhibited APPΔC31 production (measured fragment) and rescued cell death in a dose-dependent manner. The effective compounds fell into several classes including SERCA inhibitors, inhibitors of Wnt signaling, and calcium channel antagonists. Further studies are underway to evaluate the efficacy of lead compounds - identified here using cells and tissues expressing wt human APP - in mouse models of AD expressing mutated human APP, as well as to identify additional compounds and determine the mechanisms by which they exert their effects.

Dietary high cholesterol and trace metals in the drinking water increase levels of ABCA1 in the rabbit hippocampus and temporal cortex

BACKGROUND

Cholesterol-fed rabbits have been documented to show increased amyloid-β (Aβ) deposits in the brain that can be exacerbated by the quality of drinking water especially if rabbits drink tap water or distilled water containing copper. One mechanism of cholesterol and Aβ clearance may be through the ATP-binding cassette transporter A1 (ABCA1).

OBJECTIVE AND METHODS

Using an ABCA1 antibody, we determined the number of ABCA1-immunopositive neurons in three areas of rabbit brain as a function of feeding 2% cholesterol and providing tap water, distilled water, or distilled water to which aluminum, copper, or zinc was added.

RESULTS

The number of neurons with ABCA1 immunoreactivity was increased significantly as a result of dietary cholesterol in the rabbit hippocampus and inferior and superior temporal cortex. The number of neurons with ABCA1 immunoreactivity was further increased in all three areas as a result of cholesterol-fed rabbits drinking tap water or distilled water with copper. Finally, cholesterol-fed rabbits that drank distilled water with aluminum also showed an increased number of ABCA1-immunopositive neurons in inferior and superior temporal cortex.

CONCLUSIONS

These data suggest that ABCA1 levels increase in parallel with previously documented increases in Aβ levels as a result of high dietary cholesterol and copper in the drinking water. Addition of aluminum to distilled water may have a similar effect in the temporal cortex. ABCA1 has been proposed as a means of clearing Aβ from the brain and manipulations that increase Aβ also result in an increase of clearance machinery.

MicroRNAs in brain cholesterol metabolism and their implications for Alzheimer's disease

Cholesterol is important for various neuronal functions in the brain. Brain has elaborate regulatory mechanisms to control cholesterol metabolism that are distinct from the mechanisms in periphery. Interestingly, dysregulation of the cholesterol metabolism is strongly associated with a number of neurodegenerative diseases. MicroRNAs are short non-coding RNAs acting as post-transcriptional gene regulators. Recently, several microRNAs are demonstrated to be involved in regulating cholesterol metabolism in the brain. This article reviews the regulatory mechanisms of cellular cholesterol homeostasis in the brain. In addition, we discuss the role of microRNAs in brain cholesterol metabolism and their potential implications for the treatment of Alzheimer's disease. This article is part of a special issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

Serum ethanolamine plasmalogens improve detection of cognitive impairment among elderly with high excretion levels of urinary myo-inositol: A cross-sectional study

BACKGROUND

Several reports have implicated myo-inositol (MI) in myelin formation. We hypothesized that MI is involved in this process through facilitating the biosynthesis of ethanolamine plasmalogens (PlsEtns), which are the major component of myelin membranes, and essential for myelin formation and function. Excessive MI urinary excretion possibly causes PlsEtn deficiency, leading to demyelinating diseases including dementia.

METHODS

We examined the association between cognitive impairment, serum levels of PlsEtn, and baseline levels of urinary MI excretion, in the enrollment of 55 memory clinic outpatients and 107 cognitively normal elderly.

RESULTS

Serum PlsEtns were independently associated with cognitive impairment, and significantly reduced in memory clinic outpatients, especially in those with high urinary MI, as compared to normal elderly. On the other hand, there was no direct association between urinary MI and cognitive impairment, but urinary MI was significantly associated with serum hemoglobin A1c and amyloid β 1-40. The interaction between PlsEtn and urinary MI for cognitive impairment was statistically confirmed, and their combined usage improved diagnosis of cognitive impairment.

CONCLUSIONS

We proposed the involvement of MI and PlsEtn in cognitive impairment pathology. In conclusion, serum PlsEtn may be useful in detecting cognitive decline among elderly with hyperglycemia.