Exclusively targeting beta-secretase to lipid rafts by GPI-anchor addition up-regulates beta-site processing of the amyloid precursor protein

Membrane Lipid Nanodomains Modulate HCN Pacemaker Channels in Nociceptor DRG Neurons

Cell membranes consist of heterogeneous lipid nanodomains that influence key cellular processes. Using FRET-based fluorescent assays and fluorescence lifetime imaging microscopy (FLIM), we found that the dimension of cholesterol-enriched ordered membrane domains (OMD) varies considerably, depending on specific cell types. Particularly, nociceptor dorsal root ganglion (DRG) neurons exhibit large OMDs. Disruption of OMDs potentiated action potential firing in nociceptor DRG neurons and facilitated the opening of native hyperpolarization-activated cyclic nucleotide-gated (HCN) pacemaker channels. This increased neuronal firing is partially due to an increased open probability and altered gating kinetics of HCN channels. The gating effect on HCN channels was likely due to a direct modulation of their voltage sensors by OMDs. In animal models of neuropathic pain, we observed reduced OMD size and a loss of HCN channel localization within OMDs. Additionally, cholesterol supplementation inhibited HCN channels and reduced neuronal hyperexcitability in pain models. These findings suggest that disturbances in lipid nanodomains play a critical role in regulating HCN channels within nociceptor DRG neurons, influencing pain modulation.

Cholesterol imbalance and neurotransmission defects in neurodegeneration

The brain contains the highest concentration of cholesterol in the human body, which emphasizes the importance of cholesterol in brain physiology. Cholesterol is involved in neurogenesis and synaptogenesis, and age-related reductions in cholesterol levels can lead to synaptic loss and impaired synaptic plasticity, which potentially contribute to neurodegeneration. The maintenance of cholesterol homeostasis in the neuronal plasma membrane is essential for normal brain function, and imbalances in cholesterol distribution are associated with various neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and Huntington's disease. This review aims to explore the molecular and pathological mechanisms by which cholesterol imbalance can lead to neurotransmission defects and neurodegeneration, focusing on four key mechanisms: (1) synaptic dysfunction, (2) alterations in membrane structure and protein clustering, (3) oligomers of amyloid beta (Aβ) protein, and (4) α-synuclein aggregation.

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 and Lipid Rafts in the Biogenesis of Amyloid-β Protein and Alzheimer's Disease

Cholesterol has been conjectured to be a modulator of the amyloid cascade, the mechanism that produces the amyloid-β (Aβ) peptides implicated in the onset of Alzheimer's disease. We propose that cholesterol impacts the genesis of Aβ not through direct interaction with proteins in the bilayer, but indirectly by inducing the liquid-ordered phase and accompanying liquid-liquid phase separations, which partition proteins in the amyloid cascade to different lipid domains and ultimately to different endocytotic pathways. We explore the full process of Aβ genesis in the context of liquid-ordered phases induced by cholesterol, including protein partitioning into lipid domains, mechanisms of endocytosis experienced by lipid domains and secretases, and pH-controlled activation of amyloid precursor protein secretases in specific endocytotic environments. Outstanding questions on the essential role of cholesterol in the amyloid cascade are identified for future studies.

Brain cholesterol and Alzheimer's disease: challenges and opportunities in probe and drug development

Cholesterol homeostasis is impaired in Alzheimer's disease; however, attempts to modulate brain cholesterol biology have not translated into tangible clinical benefits for patients to date. Several recent milestone developments have substantially improved our understanding of how excess neuronal cholesterol contributes to the pathophysiology of Alzheimer's disease. Indeed, neuronal cholesterol was linked to the formation of amyloid-β and neurofibrillary tangles through molecular pathways that were recently delineated in mechanistic studies. Furthermore, remarkable advances in translational molecular imaging have now made it possible to probe cholesterol metabolism in the living human brain with PET, which is an important prerequisite for future clinical trials that target the brain cholesterol machinery in Alzheimer's disease patients-with the ultimate aim being to develop disease-modifying treatments. This work summarizes current concepts of how the biosynthesis, transport and clearance of brain cholesterol are affected in Alzheimer's disease. Further, current strategies to reverse these alterations by pharmacotherapy are critically discussed in the wake of emerging translational research tools that support the assessment of brain cholesterol biology not only in animal models but also in patients with Alzheimer's disease.

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

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

Direct regulation of the voltage sensor of HCN channels by membrane lipid compartmentalization

Ion channels function within a membrane environment characterized by dynamic lipid compartmentalization. Limited knowledge exists regarding the response of voltage-gated ion channels to transmembrane potential within distinct membrane compartments. By leveraging fluorescence lifetime imaging microscopy (FLIM) and Förster resonance energy transfer (FRET), we visualized the localization of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in membrane domains. HCN4 exhibits a greater propensity for incorporation into ordered lipid domains compared to HCN1. To investigate the conformational changes of the S4 helix voltage sensor of HCN channels, we used dual stop-codon suppression to incorporate different noncanonical amino acids, orthogonal click chemistry for site-specific fluorescence labeling, and transition metal FLIM-FRET. Remarkably, altered FRET levels were observed between VSD sites within HCN channels upon disruption of membrane domains. We propose that the voltage-sensor rearrangements, directly influenced by membrane lipid domains, can explain the heightened activity of pacemaker HCN channels when localized in cholesterol-poor, disordered lipid domains, leading to membrane hyperexcitability and diseases.

Cholesterol-dependent amyloid β production: space for multifarious interactions between amyloid precursor protein, secretases, and cholesterol

Amyloid β is considered a key player in the development and progression of Alzheimer's disease (AD). Many studies investigating the effect of statins on lowering cholesterol suggest that there may be a link between cholesterol levels and AD pathology. Since cholesterol is one of the most abundant lipid molecules, especially in brain tissue, it affects most membrane-related processes, including the formation of the most dangerous form of amyloid β, Aβ42. The entire Aβ production system, which includes the amyloid precursor protein (APP), β-secretase, and the complex of γ-secretase, is highly dependent on membrane cholesterol content. Moreover, cholesterol can affect amyloidogenesis in many ways. Cholesterol influences the stability and activity of secretases, but also dictates their partitioning into specific cellular compartments and cholesterol-enriched lipid rafts, where the amyloidogenic machinery is predominantly localized. The most complicated relationships have been found in the interaction between cholesterol and APP, where cholesterol affects not only APP localization but also the precise character of APP dimerization and APP processing by γ-secretase, which is important for the production of Aβ of different lengths. In this review, we describe the intricate web of interdependence between cellular cholesterol levels, cholesterol membrane distribution, and cholesterol-dependent production of Aβ, the major player in AD.

Inhibition of ACAT as a Therapeutic Target for Alzheimer's Disease Is Independent of ApoE4 Lipidation

APOE4, encoding apolipoprotein E4 (apoE4), is the greatest genetic risk factor for Alzheimer's disease (AD), compared to the common APOE3. While the mechanism(s) underlying APOE4-induced AD risk remains unclear, increasing the lipidation of apoE4 is an important therapeutic target as apoE4-lipoproteins are poorly lipidated compared to apoE3-lipoproteins. ACAT (acyl-CoA: cholesterol-acyltransferase) catalyzes the formation of intracellular cholesteryl-ester droplets, reducing the intracellular free cholesterol (FC) pool. Thus, inhibiting ACAT increases the FC pool and facilitates lipid secretion to extracellular apoE-containing lipoproteins. Previous studies using commercial ACAT inhibitors, including avasimibe (AVAS), as well as ACAT-knock out (KO) mice, exhibit reduced AD-like pathology and amyloid precursor protein (APP) processing in familial AD (FAD)-transgenic (Tg) mice. However, the effects of AVAS with human apoE4 remain unknown. In vitro, AVAS induced apoE efflux at concentrations of AVAS measured in the brains of treated mice. AVAS treatment of male E4FAD-Tg mice (5xFAD+/-APOE4+/+) at 6-8 months had no effect on plasma cholesterol levels or distribution, the original mechanism for AVAS treatment of CVD. In the CNS, AVAS reduced intracellular lipid droplets, indirectly demonstrating target engagement. Surrogate efficacy was demonstrated by an increase in Morris water maze measures of memory and postsynaptic protein levels. Amyloid-beta peptide (Aβ) solubility/deposition and neuroinflammation were reduced, critical components of APOE4-modulated pathology. However, there was no increase in apoE4 levels or apoE4 lipidation, while amyloidogenic and non-amyloidogenic processing of APP were significantly reduced. This suggests that the AVAS-induced reduction in Aβ via reduced APP processing was sufficient to reduce AD pathology, as apoE4-lipoproteins remained poorly lipidated.

Dual role of Apolipoprotein D as long-term instructive factor and acute signal conditioning microglial secretory and phagocytic responses

Microglial cells are recognized as very dynamic brain cells, screening the environment and sensitive to signals from all other cell types in health and disease. Apolipoprotein D (ApoD), a lipid-binding protein of the Lipocalin family, is required for nervous system optimal function and proper development and maintenance of key neural structures. ApoD has a cell and state-dependent expression in the healthy nervous system, and increases its expression upon aging, damage or neurodegeneration. An extensive overlap exists between processes where ApoD is involved and those where microglia have an active role. However, no study has analyzed the role of ApoD in microglial responses. In this work, we test the hypothesis that ApoD, as an extracellular signal, participates in the intercellular crosstalk sensed by microglia and impacts their responses upon physiological aging or damaging conditions. We find that a significant proportion of ApoD-dependent aging transcriptome are microglia-specific genes, and show that lack of ApoD in vivo dysregulates microglial density in mouse hippocampus in an age-dependent manner. Murine BV2 and primary microglia do not express ApoD, but it can be internalized and targeted to lysosomes, where unlike other cell types it is transiently present. Cytokine secretion profiles and myelin phagocytosis reveal that ApoD has both long-term pre-conditioning effects on microglia as well as acute effects on these microglial immune functions, without significant modification of cell survival. ApoD-triggered cytokine signatures are stimuli (paraquat vs. Aβ oligomers) and sex-dependent. Acute exposure to ApoD induces microglia to switch from their resting state to a secretory and less phagocytic phenotype, while long-term absence of ApoD leads to attenuated cytokine induction and increased myelin uptake, supporting a role for ApoD as priming or immune training factor. This knowledge should help to advance our understanding of the complex responses of microglia during aging and neurodegeneration, where signals received along our lifespan are combined with damage-triggered acute signals, conditioning both beneficial roles and limitations of microglial functions.

Specific Mutations in the Cholesterol-Binding Site of APP Alter Its Processing and Favor the Production of Shorter, Less Toxic Aβ Peptides

Excess brain cholesterol is strongly implicated in the pathogenesis of Alzheimer's disease (AD). Here we evaluated how the presence of a cholesterol-binding site (CBS) in the transmembrane and juxtamembrane regions of the amyloid precursor protein (APP) regulates its processing. We generated nine point mutations in the APP gene, changing the charge and/or hydrophobicity of the amino-acids which were previously shown as part of the CBS. Most mutations triggered a reduction of amyloid-β peptides Aβ40 and Aβ42 secretion from transiently transfected HEK293T cells. Only the mutations at position 28 of Aβ in the APP sequence resulted in a concomitant significant increase in the production of shorter Aβ peptides. Mass spectrometry (MS) confirmed the predominance of Aβx-33 and Aβx-34 with the APPK28A mutant. The enzymatic activity of α-, β-, and γ-secretases remained unchanged in cells expressing all mutants. Similarly, subcellular localization of the mutants in early endosomes did not differ from the APPWT protein. A transient increase of plasma membrane cholesterol enhanced the production of Aβ40 and Aβ42 by APPWT, an effect absent in APPK28A mutant. Finally, WT but not CBS mutant Aβ derived peptides bound to cholesterol-rich exosomes. Collectively, the present data revealed a major role of juxtamembrane amino acids of the APP CBS in modulating the production of toxic Aβ species. More generally, they underpin the role of cholesterol in the pathophysiology of AD.

Structural Determinant of β-Amyloid Formation: From Transmembrane Protein Dimerization to β-Amyloid Aggregates

Most neurodegenerative diseases have the characteristics of protein folding disorders, i.e., they cause lesions to appear in vulnerable regions of the nervous system, corresponding to protein aggregates that progressively spread through the neuronal network as the symptoms progress. Alzheimer's disease is one of these diseases. It is characterized by two types of lesions: neurofibrillary tangles (NFTs) composed of tau proteins and senile plaques, formed essentially of amyloid peptides (Aβ). A combination of factors ranging from genetic mutations to age-related changes in the cellular context converge in this disease to accelerate Aβ deposition. Over the last two decades, numerous studies have attempted to elucidate how structural determinants of its precursor (APP) modify Aβ production, and to understand the processes leading to the formation of different Aβ aggregates, e.g., fibrils and oligomers. The synthesis proposed in this review indicates that the same motifs can control APP function and Aβ production essentially by regulating membrane protein dimerization, and subsequently Aβ aggregation processes. The distinct properties of these motifs and the cellular context regulate the APP conformation to trigger the transition to the amyloid pathology. This concept is critical to better decipher the patterns switching APP protein conformation from physiological to pathological and improve our understanding of the mechanisms underpinning the formation of amyloid fibrils that devastate neuronal functions.

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.

Cathepsin B Gene Knockout Improves Behavioral Deficits and Reduces Pathology in Models of Neurologic Disorders

Cathepsin B (CTSB) is a powerful lysosomal protease. This review evaluated CTSB gene knockout (KO) outcomes for amelioration of brain dysfunctions in neurologic diseases and aging animal models. Deletion of the CTSB gene resulted in significant improvements in behavioral deficits, neuropathology, and/or biomarkers in traumatic brain injury, ischemia, inflammatory pain, opiate tolerance, epilepsy, aging, transgenic Alzheimer's disease (AD), and periodontitis AD models as shown in 12 studies. One study found beneficial effects for double CTSB and cathepsin S KO mice in a multiple sclerosis model. Transgenic AD models using amyloid precursor protein (APP) mimicking common sporadic AD in three studies showed that CTSB KO improved memory, neuropathology, and biomarkers; two studies used APP representing rare familial AD and found no CTSB KO effect, and two studies used highly engineered APP constructs and reported slight increases in a biomarker. In clinical studies, all reports found that CTSB enzyme was upregulated in diverse neurologic disorders, including AD in which elevated CTSB was positively correlated with cognitive dysfunction. In a wide range of neurologic animal models, CTSB was also upregulated and not downregulated. Further, human genetic mutation data provided precedence for CTSB upregulation causing disease. Thus, the consilience of data is that CTSB gene KO results in improved brain dysfunction and reduced pathology through blockade of CTSB enzyme upregulation that causes human neurologic disease phenotypes. The overall findings provide strong support for CTSB as a rational drug target and for CTSB inhibitors as therapeutic candidates for a wide range of neurologic disorders. SIGNIFICANCE STATEMENT: This review provides a comprehensive compilation of the extensive data on the effects of deleting the cathepsin B (CTSB) gene in neurological and aging mouse models of brain disorders. Mice lacking the CTSB gene display improved neurobehavioral deficits, reduced neuropathology, and amelioration of neuronal cell death and inflammatory biomarkers. The significance of the compelling CTSB evidence is that the data consilience validates CTSB as a drug target for discovery of CTSB inhibitors as potential therapeutics for treating numerous neurological diseases.

Lipid Rafts Act as a Common Platform for Amyloid-β Oligomer-Induced Alzheimer’s Disease Pathology

Background: Amyloid-β (Aβ) oligomers induce the overproduction of phosphorylated tau and neurodegeneration. These cascades gradually cause cognitive impairment in Alzheimer’s disease (AD). While each pathological event in AD has been studied in detail separately, the spatial and temporal relationships between pathological events in AD remain unclear. Objective: We demonstrated that lipid rafts function as a common platform for the pathological cascades of AD. Methods: Cellular and synaptosomal lipid rafts were prepared from the brains of Aβ amyloid model mice (Tg2576 mice) and double transgenic mice (Tg2576 x TgTauP301L mice) and longitudinally analyzed. Results: Aβ dimers, the cellular prion protein (PrPc), and Aβ dimer/PrPc complexes were detected in the lipid rafts. The levels of Fyn, the phosphorylated NR2B subunit of the N-methyl-D-aspartate receptor, glycogen synthase kinase 3β, total tau, phosphorylated tau, and tau oligomers increased with Aβ dimer accumulation in both the cellular and synaptosomal lipid rafts. Increases in the levels of these molecules were first seen at 6 months of age and corresponded with the early stages of Aβ accumulation in the amyloid model mice. Conclusion: Lipid rafts act as a common platform for the progression of AD pathology. The findings of this study suggest a novel therapeutic approach to AD, involving the modification of lipid raft components and the inhibition of their roles in the sequential pathological events of AD.

Neuronal KCNQ2/3 channels are recruited to lipid raft microdomains by palmitoylation of BACE1

β-Secretase 1 (β-site amyloid precursor protein [APP]-cleaving enzyme 1, BACE1) plays a crucial role in the amyloidogenesis of Alzheimer’s disease (AD). BACE1 was also discovered to act like an auxiliary subunit to modulate neuronal KCNQ2/3 channels independently of its proteolytic function. BACE1 is palmitoylated at its carboxyl-terminal region, which brings BACE1 to ordered, cholesterol-rich membrane microdomains (lipid rafts). However, the physiological consequences of this specific localization of BACE1 remain elusive. Using spectral Förster resonance energy transfer (FRET), BACE1 and KCNQ2/3 channels were confirmed to form a signaling complex, a phenomenon that was relatively independent of the palmitoylation of BACE1. Nevertheless, palmitoylation of BACE1 was required for recruitment of KCNQ2/3 channels to lipid-raft domains. Two fluorescent probes, designated L10 and S15, were used to label lipid-raft and non-raft domains of the plasma membrane, respectively. Coexpressing BACE1 substantially elevated FRET between L10 and KCNQ2/3, whereas the BACE1-4C/A quadruple mutation failed to produce this effect. In contrast, BACE1 had no significant effect on FRET between S15 probes and KCNQ2/3 channels. A reduction of BACE1-dependent FRET between raft-targeting L10 probes and KCNQ2/3 channels by applying the cholesterol-extracting reagent methyl-β-cyclodextrin (MβCD), raft-disrupting general anesthetics, or pharmacological inhibitors of palmitoylation, all supported the hypothesis of the palmitoylation-dependent and raft-specific localization of KCNQ2/3 channels. Furthermore, mutating the four carboxyl-terminal cysteines (4C/A) of BACE1 abolished the BACE1-dependent increase of FRET between KCNQ2/3 and the lipid raft–specific protein caveolin 1. Taking these data collectively, we propose that the AD-related protein BACE1 underlies the localization of a neuronal potassium channel.

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.

Suppression of amyloid-β secretion from neurons by cis-9, trans-11-octadecadienoic acid, an isomer of conjugated linoleic acid

Two common conjugated linoleic acids (LAs), cis-9, trans-11 CLA (c9,t11 CLA) and trans-10, cis-12 CLA (t10,c12 CLA), exert various biological activities. However, the effect of CLA on the generation of neurotoxic amyloid-β (Aβ) protein remains unclear. We found that c9,t11 CLA significantly suppressed the generation of Aβ in mouse neurons. CLA treatment did not affect the level of β-site APP-cleaving enzyme 1 (BACE1), a component of active γ-secretase complex presenilin 1 amino-terminal fragment, or Aβ protein precursor (APP) in cultured neurons. BACE1 and γ-secretase activities were not directly affected by c9,t11 CLA. Localization of BACE1 and APP in early endosomes increased in neurons treated with c9,t11 CLA; concomitantly, the localization of both proteins was reduced in late endosomes, the predominant site of APP cleavage by BACE1. The level of CLA-containing phosphatidylcholine (CLA-PC) increased dramatically in neurons incubated with CLA. Incorporation of phospholipids containing c9,t11 CLA, but not t10,c12 CLA, into the membrane may affect the localization of some membrane-associated proteins in intracellular membrane compartments. Thus, in neurons treated with c9,t11 CLA, reduced colocalization of APP with BACE1 in late endosomes may decrease APP cleavage by BACE1 and subsequent Aβ generation. Our findings suggest that accumulation of c9,t11 CLA-PC/LPC in neuronal membranes suppresses production of neurotoxic Aβ in neurons.

Cholesterol and Alzheimer's Disease; From Risk Genes to Pathological Effects

While the central nervous system compromises 2% of our body weight, it harbors up to 25% of the body's cholesterol. Cholesterol levels in the brain are tightly regulated for physiological brain function, but mounting evidence indicates that excessive cholesterol accumulates in Alzheimer's disease (AD), where it may drive AD-associated pathological changes. This seems especially relevant for late-onset AD, as several of the major genetic risk factors are functionally associated with cholesterol metabolism. In this review we discuss the different systems that maintain brain cholesterol metabolism in the healthy brain, and how dysregulation of these processes can lead, or contribute to, Alzheimer's disease. We will also discuss how AD-risk genes might impact cholesterol metabolism and downstream AD pathology. Finally, we will address the major outstanding questions in the field and how recent technical advances in CRISPR/Cas9-gene editing and induced pluripotent stem cell (iPSC)-technology can aid to study these problems.

Delta- and beta- secretases crosstalk amplifies the amyloidogenic pathway in Alzheimer's disease

Asparagine endopeptidase (AEP), a newly identified delta-secretase, simultaneously cleaves both APP and Tau, promoting Alzheimer's disease (AD) pathologies. However, its pathological role in AD remains incompletely understood. Here we show that delta-secretase cleaves BACE1, a rate-limiting protease in amyloid-β (Aβ) generation, escalating its enzymatic activity and enhancing senile plaques deposit in AD. Delta-secretase binds BACE1 and cuts it at N294 residue in an age-dependent manner and elevates its protease activity. The cleaved N-terminal motif is active even under neutral pH and associates with senile plaques in human AD brains. Subcellular fractionation reveals that delta-secretase and BACE1 reside in the endo-lysosomes. Interestingly, truncated BACE1 enzymatic domain (1-294) augments delta-secretase enzymatic activity and accelerates Aβ production, facilitating AD pathologies and cognitive impairments in APP/PS1 AD mouse model. Uncleavable BACE1 (N294A) inhibits delta-secretase activity and Aβ production and decreases AD pathologies in 5XFAD mice, ameliorating cognitive dysfunctions. Hence, delta- and beta- secretases' crosstalk aggravates each other's roles in AD pathogenesis.

Relationships Between Plasma Lipids Species, Gender, Risk Factors, and Alzheimer's Disease

Background:

Lipid metabolism is altered in Alzheimer’s disease (AD); however, the relationship between AD risk factors (age, APOEɛ4, and gender) and lipid metabolism is not well defined.

Objective:

We investigated whether altered lipid metabolism associated with increased age, gender, and APOE status may contribute to the development of AD by examining these risk factors in healthy controls and also clinically diagnosed AD individuals.

Methods:

We performed plasma lipidomic profiling (582 lipid species) of the Australian Imaging, Biomarkers and Lifestyle flagship study of aging cohort (AIBL) using liquid chromatography-mass spectrometry. Linear regression and interaction analysis were used to explore the relationship between risk factors and plasma lipid species.

Results:

We observed strong associations between plasma lipid species with gender and increasing age in cognitively normal individuals. However, APOEɛ4 was relatively weakly associated with plasma lipid species. Interaction analysis identified differential associations of sphingolipids and polyunsaturated fatty acid esterified lipid species with AD based on age and gender, respectively. These data indicate that the risk associated with age, gender, and APOEɛ4 may, in part, be mediated by changes in lipid metabolism.

Conclusion:

This study extends our existing knowledge of the relationship between the lipidome and AD and highlights the complexity of the relationships between lipid metabolism and AD at different ages and between men and women. This has important implications for how we assess AD risk and also for potential therapeutic strategies involving modulation of lipid metabolic pathways.

The interplay between lipid and Aβ amyloid homeostasis in Alzheimer's Disease: risk factors and therapeutic opportunities

Alzheimer's Diseases (AD) is characterized by the accumulation of amyloid deposits of Aβ peptide in the brain. Besides genetic background, the presence of other diseases and an unhealthy lifestyle are known risk factors for AD development. Albeit accumulating clinical evidence suggests that an impaired lipid metabolism is related to Aβ deposition, mechanistic insights on the link between amyloid fibril formation/clearance and aberrant lipid interactions are still unavailable. Recently, many studies have described the key role played by membrane bound Aβ assemblies in neurotoxicity. Moreover, it has been suggested that a derangement of the ubiquitin proteasome pathway and autophagy is significantly correlated with toxic Aβ aggregation and dysregulation of lipid levels. Thus, studies focusing on the role played by lipids in Aβ aggregation and proteostasis could represent a promising area of investigation for the design of valuable treatments. In this review we examine current knowledge concerning the effects of lipids in Aβ aggregation and degradation processes, focusing on the therapeutic opportunities that a comprehensive understanding of all biophysical, biochemical, and biological processes involved may disclose.

Amyloid Oligomers: A Joint Experimental/Computational Perspective on Alzheimer's Disease, Parkinson's Disease, Type II Diabetes, and Amyotrophic Lateral Sclerosis

Protein misfolding and aggregation is observed in many amyloidogenic diseases affecting either the central nervous system or a variety of peripheral tissues. Structural and dynamic characterization of all species along the pathways from monomers to fibrils is challenging by experimental and computational means because they involve intrinsically disordered proteins in most diseases. Yet understanding how amyloid species become toxic is the challenge in developing a treatment for these diseases. Here we review what computer, in vitro, in vivo, and pharmacological experiments tell us about the accumulation and deposition of the oligomers of the (Aβ, tau), α-synuclein, IAPP, and superoxide dismutase 1 proteins, which have been the mainstream concept underlying Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes (T2D), and amyotrophic lateral sclerosis (ALS) research, respectively, for many years.

Is It the Twilight of BACE1 Inhibitors?

β-secretase (BACE1) has been regarded as a prime target for the development of amyloid beta (Aβ) lowering drugs in the therapy of Alzheimer´s disease (AD). Although the enzyme was discovered in 1991 and helped to formulate the Aβ hypothesis as one of the very important features of AD etiopathogenesis, progress in AD treatment utilizing BACE1 inhibitors has remained limited. Moreover, in the last years, major pharmaceutical companies have discontinued clinical trials of five BACE1 inhibitors that had been strongly perceived as prospective. In our review, the Aβ hypothesis, the enzyme, its functions, and selected substrates are described. BACE1 inhibitors are classified into four generations. Those that underwent clinical trials displayed adverse effects, including weight loss, skin rashes, worsening of neuropsychiatric symptoms, etc. Some inhibitors could not establish a statistically significant risk-benefit ratio, or even scored worse than placebo. We still believe that drugs targeting BACE1 may still hide some potential, but a different approach to BACE1 inhibition or a shift of focus to modulation of its trafficking and/or post-translational modification should now be followed.

Molecular Dysfunctions of Mitochondria-Associated Membranes (MAMs) in Alzheimer's Disease

Alzheimer’s disease (AD) is a multifactorial neurodegenerative pathology characterized by a progressive decline of cognitive functions. Alteration of various signaling cascades affecting distinct subcellular compartment functions and their communication likely contribute to AD progression. Among others, the alteration of the physical association between the endoplasmic reticulum (ER) and mitochondria, also referred as mitochondria-associated membranes (MAMs), impacts various cellular housekeeping functions such as phospholipids-, glucose-, cholesterol-, and fatty-acid-metabolism, as well as calcium signaling, which are all altered in AD. Our review describes the physical and functional proteome crosstalk between the ER and mitochondria and highlights the contribution of distinct molecular components of MAMs to mitochondrial and ER dysfunctions in AD progression. We also discuss potential strategies targeting MAMs to improve mitochondria and ER functions in AD.

Preferred Endocytosis of Amyloid Precursor Protein from Cholesterol-Enriched Lipid Raft Microdomains

Amyloid precursor protein (APP) at the plasma membrane is internalized via endocytosis and delivered to endo/lysosomes, where neurotoxic amyloid-β (Aβ) is produced via β-, γ-secretases. Hence, endocytosis plays a key role in the processing of APP and subsequent Aβ generation. β-, γ-secretases as well as APP are localized in cholesterol-enriched lipid raft microdomains. However, it is still unclear whether lipid rafts are the site where APP undergoes endocytosis and whether cholesterol levels affect this process. In this study, we found that localization of APP in lipid rafts was increased by elevated cholesterol level. We also showed that increasing or decreasing cholesterol levels increased or decreased APP endocytosis, respectively. When we labeled cell surface APP, APP localized in lipid rafts preferentially underwent endocytosis compared to nonraft-localized APP. In addition, APP endocytosis from lipid rafts was regulated by cholesterol levels. Our results demonstrate for the first time that cholesterol levels regulate the localization of APP in lipid rafts affecting raft-dependent APP endocytosis. Thus, regulating the microdomain localization of APP could offer a new therapeutic strategy for Alzheimer’s disease.

Alkyl-Acylglycerols and the Important Clinical Ramifications of Raising Plasmalogens in Dementia and Alzheimer's Disease

A critical factor involved in the pathophysiology of Alzheimer's disease (AD) and related dementias is the decline of plasmalogens, a key glycerophospholipid required for normal neuron function. An accumulating body of evidence correlates low blood and brain plasmalogens with higher levels of AD pathology and lower cognition scores and indicates that declines in these phospholipids begin years before clinical symptoms develop. Furthermore, it has been recently reported that high blood plasmalogen levels neutralize the increased risk of dementia in persons who carry the APOE epsilon 4 allele, the most significant genetic risk factor for AD. There are over 30 common species of plasmalogens in the human body with different plasmalogen species playing different roles, depending on the organ and cell type. Accordingly, there is great interest in understanding how to selectively target plasmalogen augmentation for specific health needs. For example, brain white matter is comprised of plasmalogens containing monounsaturated fatty acids, whereas gray matter is comprised of plasmalogens containing polyunsaturated fatty acids. Fortunately, the structure-activity and biochemistry of plasmalogen augmentation has been extensively studied in cell and animal models. Restoring and augmenting levels of selective plasmalogens can be achieved with dietary supplementation of 1-O-alkyl-2-acyl glycerol oils containing the desired fatty acid type at the 2-acyl position. Neuron-targeted 1-O-alkyl-2-acyl glycerol containing DHA has been shown to be neuroprotective and neuroactive in animal models of neurodegeneration. This review will discuss the mechanisms by which plasmalogen deficiency leads to Alzheimer's and/or dementia and the critical role that 1-O-alkyl-2-acyl glycerol oils can play in patients with those disorders.

Lesch-Nyhan disease: I. Construction of expression vectors for hypoxanthine-guanine phosphoribosyltransferase (HGprt) enzyme and amyloid precursor protein (APP)

Lesch-Nyhan disease (LND) is a rare X-linked inherited neurogenetic disorder of purine metabolism in which the enzyme, hypoxanthine-guanine phosphoribosyltransferase (HGprt) is defective. Despite having been characterized over 50 years ago, it remains unclear precisely how deficits in HGprt enzyme activity can lead to the neurological syndrome, especially the self-injury of LND. Several studies have proposed different hypotheses regarding the etiology of this disease, and several treatments have been tried in patients. However, up to now, there is no satisfactory explanation of the disease and for many LND patients, efficacious treatment for persistent self-injurious behavior remains unreachable. A role for epistasis between mutated hypoxanthine phosphoribosyltransferase 1 (HPRT1) and amyloid precursor protein (APP) genes has been recently suggested. This finding may provide new directions not only for investigating the role of APP in neuropathology associated with HGprt-deficiency in LND but also for the research in neurodevelopmental and neurodegenerative disorders in which the APP gene is involved in the pathogenesis of diseases and may pave the way for new strategies applicable to rational antisense drugs design. It is therefore necessary to study the HGprt enzyme and APP using expression vectors for exploring their impacts on LND as well as other human diseases, especially the ones related to APP such as Alzheimer's disease in which the physiologic function and the structure of the entire APP remain largely unclear until now. For such a purpose, we report here the construction of expression vectors as the first step (Part I) of our investigation.

Aβ-ganglioside interactions in the pathogenesis of Alzheimer's disease

It is widely accepted that the abnormal self-association of amyloid β-protein (Aβ) is central to the pathogenesis of Alzheimer's disease, the most common form of dementia. Accumulating evidence, both in vivo and in vitro, suggests that the binding of Aβ to gangliosides, especially monosialoganglioside GM1, plays an important role in the aggregation of Aβ. This review summarizes the molecular details of the binding of Aβ to ganglioside-containing membranes and subsequent structural changes, as revealed by liposomal and cellular studies. Furthermore, mechanisms of cytotoxicity by aggregated Aβ are also discussed.

Focusing on cellular biomarkers: The endo-lysosomal pathway in Down syndrome

Down syndrome (DS) is the most frequent chromosomal disorder. It is caused by the triplication of human chromosome 21, leading to increased dosage of a variety of genes including APP (Amyloid Precursor Protein). Mainly for this reason, individuals with DS are at high risk to develop Alzheimer's disease (AD). Extensive literature identified various morphological and molecular abnormalities in the endo-lysosomal pathway both in DS and AD. Most studies in this field investigated the causative role of APP (Amyloid Precursor Protein) in endo-lysosomal dysfunctions, thus linking phenotypes observed in DS and AD. In DS context, several lines of evidence and emerging hypotheses suggest that other molecular players and pathways may be implicated in these complex phenotypes. In this review, we outline the normal functioning of endosomal trafficking and summarize the research on endo-lysosomal dysfunction in DS in light of AD findings. We emphasize the role of genes of chromosome 21 implicated in endocytosis to explain endosomal abnormalities and set the limitations and perspectives of models used to explore endo-lysosomal dysfunction in DS and find new biomarkers. The review highlights the complexity of endo-lysosomal dysfunction in DS and suggests directions for future research in the field.

Simvastatin Efficiently Reduces Levels of Alzheimer's Amyloid Beta in Yeast

A large-scale epidemiology study on statins previously showed that simvastatin was unique among statins in reducing the incidence of dementia. Since amyloid beta (Aβ42) is the protein that is most associated with Alzheimer's disease, this study has focused on how simvastatin influences the turnover of native Aβ42 and Aβ42 fused with green fluorescent protein (GFP), in the simplest eukaryotic model organism, Saccharomyces cerevisiae. Previous studies have established that yeast constitutively producing Aβ42 fused to GFP offer a convenient means of analyzing yeast cellular responses to Aβ42. Young cells clear the GFP fusion protein and do not have green fluorescence while the older population of cells retains the fusion protein and exhibits green fluorescence, offering a fast and convenient means of studying factors that affect Aβ42 turnover. In this study the proportion of cells having GFP fused to Aβ after exposure to simvastatin, atorvastatin and lovastatin was analyzed by flow cytometry. Simvastatin effectively reduced levels of the cellular Aβ42 protein in a dose-dependent manner. Simvastatin promoted the greatest reduction as compared to the other two statins. A comparison with fluconazole, which targets that same pathway of ergosterol synthesis, suggests that effects on ergosterol synthesis do not account for the reduced amounts of Aβ42 fused to GFP. The levels of native Aβ42 following treated with simvastatin were also examined using a more laborious approach, quantitative MALDI TOF mass spectrometry. Simvastatin efficiently reduced levels of native Aβ42 from the population. This work indicates a novel action of simvastatin in reducing levels of Aβ42 providing new insights into how simvastatin exerts its neuroprotective role. We hypothesize that this reduction may be due to protein clearance.

Elevated cellular cholesterol in Familial Alzheimer's presenilin 1 mutation is associated with lipid raft localization of β-amyloid precursor protein

Familial Alzheimer’s disease (FAD)-associated presenilin 1 (PS1) serves as a catalytic subunit of γ-secretase complex, which mediates the proteolytic liberation of β-amyloid (Aβ) from β-amyloid precursor protein (APP). In addition to its proteolytic role, PS1 is involved in non-proteolytic functions such as protein trafficking and ion channel regulation. Furthermore, postmortem AD brains as well as AD patients showed dysregulation of cholesterol metabolism. Since cholesterol has been implicated in regulating Aβ production, we investigated whether the FAD PS1-associated cholesterol elevation could influence APP processing. We found that in CHO cells stably expressing FAD-associated PS1 ΔE9, total cholesterol levels are elevated compared to cells expressing wild-type PS1. We also found that localization of APP in cholesterol-enriched lipid rafts is substantially increased in the mutant cells. Reducing the cholesterol levels by either methyl-β-cyclodextrin or an inhibitor of CYP51, an enzyme mediating the elevated cholesterol in PS1 ΔE9-expressing cells, significantly reduced lipid raft-associated APP. In contrast, exogenous cholesterol increased lipid raft-associated APP. These data suggest that in the FAD PS1 ΔE9 cells, the elevated cellular cholesterol level contributes to the altered APP processing by increasing APP localized in lipid rafts.

The gender- and age- dependent relationships between serum lipids and cognitive impairment: a cross-sectional study in a rural area of Xi'an, China

Background

Serum lipids [total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), high density lipoprotein cholesterol (HDL-C) and triglyceride (TG)] are risk factors for stroke, but the relationships between serum lipids and cognitive impairment have not been verified completely. In this study, we studied the relationships between serum lipids and cognitive impairment and explored whether gender and age had effects on the relationships.

Methods

In this cross-sectional study, we collected serum lipids and cognitive function information from 1762 participants (aged 40–85). Univariate analysis, multivariate analysis, and both gender- and age-based stratified multivariate analysis were used.

Results

In the entire sample set, there was no significant correlation between serum lipid parameters (TC, LDL-C, HDL-C and TG) and cognitive impairment. In both gender- and age-based stratified multivariate analysis, high serum TC was positively associated with cognitive impairment in the elderly (> 55) male participants (OR = 4.404, 95% CI = 1.264–15.344, p = 0.02), and high serum LDL-C was positively correlated with cognitive impairment in the elderly female subjects (OR = 2.496, 95% CI = 1.057–5.896, p = 0.037), while high serum TG was negatively associated with cognitive impairment in the middle-aged (≤ 55) male participants (OR = 0.157, 95% CI = 0.051–0.484, p = 0.001).

Conclusions

The relationships between serum lipids and cognitive impairment are gender- and age- dependent, with high serum TC and LDL-C may be risk factors of cognitive impairment in the elderly male and female subjects respectively, while high serum TG may be protector of cognitive impairment in the middle-aged male participants.

Research Progress in the Pathogenesis of Alzheimer's Disease

Objective:

Alzheimer's disease (AD) is a kind of chronic degenerative disease of the central nervous system, characteristics of cognitive dysfunction, and behavioral disability. The pathological changes include the formation of senile plaques-containing beta-amyloid (Aβ), neurofibrillary tangles (NFTs), loss of neurons, and synapses. So far, the pathogenesis of AD is still unclear. This study was aimed to review the major pathogenesis of AD-related to the published AD studies in recent 20 years.

Data Sources:

The author retrieved information from the PubMed database up to January 2018, using various search terms and their combinations, including AD, Aβ, NFTs, pathogenesis, and genetic mutation.

Study Selection:

The author included data from peer-reviewed journals printed in English and Chinese on pathophysiological factors in AD. He organized these informations to explain the possible pathogenesis in AD.

Results:

There are many amounts of data supporting the view that AD pathogenesis so far there mainly are Aβ toxicity, tau protein, gene mutation, synaptic damages, intermediate neurons and network abnormalities, changes in mitochondrial function, chemokines, etc., Its nosogenesis may be involved in multiple theories and involved in multiple molecular signaling pathways, including Aβ, tau protein, and synaptic anomaly; mutual relationship between the mechanisms urge jointly neuronal degeneration.

Conclusions:

This review highlights the research advances in the pathogenesis of AD. Future research has needed to fully disclose the association between multiple pathogenesis at the same time to interdict multiple signaling pathways, etc.

Regimes of Complex Lipid Bilayer Phases Induced by Cholesterol Concentration in MD Simulation

Cholesterol is essential to the formation of phase-separated lipid domains in membranes. Lipid domains can exist in different thermodynamic phases depending on the molecular composition and play significant roles in determining structure and function of membrane proteins. We investigate the role of cholesterol in the structure and dynamics of ternary lipid mixtures displaying phase separation using molecular dynamics simulations, employing a physiologically relevant span of cholesterol concentration. We find that cholesterol can induce formation of three regimes of phase behavior: 1) miscible liquid-disordered bulk, 2) phase-separated, domain-registered coexistence of liquid-disordered and liquid-ordered domains, and 3) phase-separated, domain-antiregistered coexistence of liquid-disordered and newly identified nanoscopic gel domains composed of cholesterol threads we name "cholesterolic gel" domains. These findings are validated and discussed in the context of current experimental knowledge, models of cholesterol spatial distributions, and models of ternary lipid-mixture phase separation.

MicroRNA in Alzheimer's disease revisited: implications for major neuropathological mechanisms

Pathology of Alzheimer's disease (AD) goes far beyond neurotoxicity resulting from extracellular deposition of amyloid β (Aβ) plaques. Aberrant cleavage of amyloid precursor protein and accumulation of Aβ in the form of the plaque or neurofibrillary tangles are the known primary culprits of AD pathogenesis and target for various regulatory mechanisms. Hyper-phosphorylation of tau, a major component of neurofibrillary tangles, precipitates its aggregation and prevents its clearance. Lipid particles, apolipoproteins and lipoprotein receptors can act in favor or against Aβ and tau accumulation by altering neural membrane characteristics or dynamics of transport across the blood-brain barrier. Lipids also alter the oxidative/anti-oxidative milieu of the central nervous system (CNS). Irregular cell cycle regulation, mitochondrial stress and apoptosis, which follow both, are also implicated in AD-related neuronal loss. Dysfunction in synaptic transmission and loss of neural plasticity contribute to AD. Neuroinflammation is a final trail for many of the pathologic mechanisms while playing an active role in initiation of AD pathology. Alterations in the expression of microRNAs (miRNAs) in AD and their relevance to AD pathology have long been a focus of interest. Herein we focused on the precise pathomechanisms of AD in which miRNAs were implicated. We performed literature search through PubMed and Scopus using the search term: ('Alzheimer Disease') OR ('Alzheimer's Disease') AND ('microRNAs' OR 'miRNA' OR 'MiR') to reach for relevant articles. We show how a limited number of common dysregulated pathways and abnormal mechanisms are affected by various types of miRNAs in AD brain.

Lipidomic Profiles in Diabetes and Dementia

Lipids are a diverse class of hydrophobic and amphiphilic molecules which make up the bulk of most biological systems and are essential for human life. The role of lipids in health and disease has been recognized for many decades, as evidenced by the early identification of cholesterol as an important risk factor of heart disease and the development and introduction of statins as a one of the most successful therapeutic interventions to date. While several studies have demonstrated an increased risk of dementia, including Alzheimer's disease (AD), in those with diabetes mellitus, the nature of this risk is not well understood. Recent developments in the field of lipidomics, driven primarily by technological advances in high pressure liquid chromatography and particularly mass spectrometry, have enabled the detailed characterization of the many hundreds of individual lipid species in mammalian systems and their association with disease states. Diabetes mellitus and AD have received particular attention due to their prominence in Western societies as a result of the ongoing obesity epidemic and the aging populations. In this review, we examine how these lipidomic studies are informing on the relationship between lipid metabolism with diabetes and AD and how this may inform on the common pathological pathways that link diabetes risk with dementia.

Localization and Processing of the Amyloid-β Protein Precursor in Mitochondria-Associated Membranes

Alteration of mitochondria-associated membranes (MAMs) has been proposed to contribute to the pathogenesis of Alzheimer’s disease (AD). We studied herein the subcellular distribution, the processing, and the protein interactome of the amyloid-β protein precursor (AβPP) and its proteolytic products in MAMs. We reveal that AβPP and its catabolites are present in MAMs in cellular models overexpressing wild type AβPP or AβPP harboring the double Swedish or London familial AD mutations, and in brains of transgenic mice model of AD. Furthermore, we evidenced that both β- and γ-secretases are present and harbor AβPP processing activities in MAMs. Interestingly, cells overexpressing APP_swe show increased ER-mitochondria contact sites. We also document increased neutral lipid accumulation linked to Aβ production and reversed by inhibiting β- or γ-secretases. Using a proteomic approach, we show that AβPP and its catabolites interact with key proteins of MAMs controlling mitochondria and ER functions. These data highlight the role of AβPP processing and proteomic interactome in MAMs deregulation taking place in AD.

Not Oligomers but Amyloids are Cytotoxic in the Membrane-Mediated Amyloidogenesis of Amyloid-β Peptides

The formation of neurotoxic aggregates by amyloid-β peptide (Aβ) is considered to be a key step in the onset of Alzheimer's disease. It is widely accepted that oligomers are more neurotoxic than amyloid fibrils in the aqueous-phase aggregation of Aβ. Membrane-mediated amyloidogenesis is also relevant to the pathology, although the relationship between the aggregate size and cytotoxicity has remained elusive. Here, aggregation processes of Aβ on living cells and cytotoxic events were monitored by fluorescence techniques. Aβ formed amyloids after forming oligomers composed of ≈10 Aβ molecules. The formation of amyloids was necessary to activate apoptotic caspase-3 and reduce the ability of the cell to proliferate; this indicated that amyloid formation is a key event in Aβ-induced cytotoxicity.

The lipid raft-dwelling protein US9 can be manipulated to target APP compartmentalization, APP processing, and neurodegenerative disease pathogenesis

The trafficking behavior of the lipid raft-dwelling US9 protein from Herpes Simplex Virus strikingly overlaps with that of the amyloid precursor protein (APP). Both US9 and APP processing machinery rely on their ability to shuttle between endosomes and plasma membranes, as well as on their lateral accumulation in lipid rafts. Therefore, repurposing US9 to track/modify these molecular events represents a valid approach to investigate pathological states including Alzheimer's disease and HIV-associated neurocognitive disorders where APP misprocessing to amyloid beta formation has been observed. Accordingly, we investigated the cellular localization of US9-driven cargo in neurons and created a US9-driven functional assay based on the exogenous enzymatic activity of Tobacco Etch Virus Protease. Our results demonstrate that US9 can direct and control cleavage of recombinant proteins exposed on the luminal leaflet of transport vesicles. Furthermore, we confirmed that US9 is associated with lipid-rafts and can target functional enzymes to membrane microdomains where pathologic APP-processing is thought to occur. Overall, our results suggest strongly that US9 can serve as a molecular driver that targets functional cargos to the APP machinery and can be used as a tool to study the contribution of lipid rafts to neurodegenerative disease conditions where amyloidogenesis has been implicated.

Recruitment of the amyloid precursor protein by γ-secretase at the synaptic plasma membrane

Γ-secretase is a membrane-embedded protease that cleaves single transmembrane helical domains of various integral membrane proteins. The amyloid precursor protein (APP) is an important substrate due to its pathological relevance to Alzheimer's disease. The mechanism of the cleavage of APP by γ-secretase that leads to accumulation of Alzheimer's disease causing amyloid-β (Aβ) is still unknown. Coarse-grained molecular dynamics simulations in this study reveal initial lipids raft formation near the catalytic site of γ-secretase as well as changes in dynamic behavior of γ-secretase once interacting with APP. The results suggest a precursor of the APP binding mode and hint at conformational changes of γ-secretase in the nicastrin (NCT) domain upon APP binding.

Therapeutic Effects of Anthocyanins and Environmental Enrichment in R6/1 Huntington's Disease Mice

BACKGROUND

Huntington's disease (HD) is a progressive neurodegenerative disease with no effective treatment or cure. Environmental enrichment has been used to slow processes leading to ageing and neurodegenerative diseases including HD. Phenolic phytochemicals including anthocyanins have also been shown to improve brain function in ageing and neurodegenerative diseases.

OBJECTIVE

This study examined the effects of anthocyanin dietary supplementation and environmental enrichment on behavioural phenotypes and brain cholesterol metabolic alterations in the R6/1 mouse model of HD.

METHODS

R6/1 HD mice and their wild-type littermate controls were randomised into the different experimental conditions, involving either environmentally enriched versus standard housing conditions, or anthocyanin versus control diet. Motor dysfunction was assessed from 6 to 26 weeks using the RotaRod and the hind-paw clasping tests. Gas chromatography - tandem mass spectrometry was used to quantify a broad range of sterols in the striatum and cortex of R6/1 HD mice.

RESULTS

Anthocyanin dietary supplementation delayed the onset of motor dysfunction in female HD mice. Environmental enrichment improved motor function and the hind paw clasping phenotype in male HD mice only. These mice also had lower levels of cholesterol oxidation products in the cortex compared to standard-housed mice.

CONCLUSION

Both anthocyanin supplementation and environmental enrichment are able to improve the motor dysfunction phenotype of R6/1 mice, however the effectiveness of these interventions was different between the two sexes. The interventions examined did not alter brain cholesterol metabolic deficits that have been reported previously in this mouse model of HD.

Full-length cellular β-secretase has a trimeric subunit stoichiometry, and its sulfur-rich transmembrane interaction site modulates cytosolic copper compartmentalization

The β-secretase (BACE1) initiates processing of the amyloid precursor protein (APP) into Aβ peptides, which have been implicated as central players in the pathology of Alzheimer disease. BACE1 has been described as a copper-binding protein and its oligomeric state as being monomeric, dimeric, and/or multimeric, but the native cellular stoichiometry has remained elusive. Here, by using single-molecule fluorescence and in vitro cross-linking experiments with photo-activatable unnatural amino acids, we show that full-length BACE1, independently of its subcellular localization, exists as trimers in human cells. We found that trimerization requires the BACE1 transmembrane sequences (TMSs) and cytoplasmic domains, with residues Ala⁴⁶³ and Cys⁴⁶⁶ buried within the trimer interface of the sulfur-rich core of the TMSs. Our 3D model predicts that the sulfur-rich core of the trimeric BACE1 TMS is accessible to metal ions, but copper ions did not trigger trimerization. The results of functional assays of endogenous BACE1 suggest that it has a role in intracellular copper compartmentalization by transferring cytosolic copper to intracellular compartments, while leaving the overall cellular copper concentration unaltered. Adding to existing physiological models, our results provide novel insight into the atypical interactions between copper and BACE1 and into its non-enzymatic activities. In conclusion, therapeutic Alzheimer disease prevention strategies aimed at decreasing BACE1 protein levels should be regarded with caution, because adverse effects in copper homeostasis may occur.

Dysregulation of intracellular trafficking and endosomal sorting in Alzheimer's disease: controversies and unanswered questions

Alzheimer's disease (AD) is characterized by the accumulation of amyloid plaques in the brain consisting of an aggregated form of amyloid β-peptide (Aβ) derived from sequential amyloidogenic processing of the amyloid precursor protein (APP) by membrane-bound proteases β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase. The initial processing of APP by BACE1 is re-gulated by intracellular sorting events of the enzyme, which is a prime target for therapeutic intervention. GWAS (genome-wide sequencing studies) have identified several AD-susceptibility genes that are associated with the regulation of membrane trafficking, and substantial evidence now indicates that AD is likely to arise from defective membrane trafficking in either or both of the secretory and endocytic pathways. Considerable progress has been made in defining the intracellular trafficking pathways of BACE1 and APP and the sorting signals of these membrane proteins that define their itineraries. In this review we highlight recent advances in understanding the regulation of the intracellular sorting of BACE1 and APP, discuss how dysregulation of these trafficking events may lead to enhanced generation of the neurotoxic Aβ products in AD and highlight the unresolved questions in the field.

A diet enriched with plant sterols prevents the memory impairment induced by cholesterol loss in senescence-accelerated mice

Cholesterol reduction at the neuronal plasma membrane has been related to age-dependent cognitive decline. We have used senescent-accelerated mice strain 8 (SAMP8), an animal model for aging, to examine the association between cholesterol loss and cognitive impairment and to test strategies to revert this process. We show that the hippocampus of SAMP8 mice presents reduced cholesterol levels and enhanced amount of its degrading enzyme Cyp46A1 (Cyp46) already at 6 months of age. Cholesterol loss accounts for the impaired long-term potentiation in these mice. Plant sterol (PSE)-enriched diet prevents long-term potentiation impairment and cognitive deficits in SAMP8 mice without altering cholesterol levels. PSE diet also reduces the abnormally high amyloid peptide levels in SAMP8 mice brains and restores membrane compartmentalization of presenilin1, the catalytic component of the amyloidogenic γ-secretase. These results highlight the influence of cholesterol loss in age-related cognitive decline and provide with a noninvasive strategy to counteract it. Our results suggest that PSE overtake cholesterol functions in the brain contributing to reduce deleterious consequences of cholesterol loss during aging.

The pleiotropic effects of omega-3 docosahexaenoic acid on the hallmarks of Alzheimer's disease

Among omega-3 polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA, 22:6n-3) is important for adequate brain development and cognition. DHA is highly concentrated in the brain and plays an essential role in brain functioning. DHA, one of the major constituents in fish fats, readily crosses the blood-brain barrier from blood to the brain. Its critical role was further supported by its reduced levels in the brain of Alzheimer's disease (AD) patients. This agrees with a potential role of DHA in memory, learning and cognitive processes. Since there is yet no cure for dementia such as AD, there is growing interest in the role of DHA-supplemented diet in the prevention of AD pathogenesis. Accordingly, animal, epidemiological, preclinical and clinical studies indicated that DHA has neuroprotective effects in a number of neurodegenerative conditions including AD. The beneficial effects of this key omega-3 fatty acid supplementation may depend on the stage of disease progression, other dietary mediators and the apolipoprotein ApoE genotype. Herein, our review investigates, from animal and cell culture studies, the molecular mechanisms involved in the neuroprotective potential of DHA with emphasis on AD.