Internalization of beta-amyloid peptide by primary neurons in the absence of apolipoprotein E

Rational Design of Dual-Functionalized Gd@C82 Nanoparticles to Relieve Neuronal Cytotoxicity in Alzheimer's Disease via Inhibition of Aβ Aggregation

The accumulation of amyloid-β (Aβ) peptides is a major hallmark of Alzheimer's disease (AD) and plays a crucial role in its pathogenesis. Particularly, the structured oligomeric species rich in β-sheet formations were implicated in neuronal organelle damage. Addressing this formidable challenge requires identifying candidates capable of inhibiting peptide aggregation or disaggregating preformed oligomers for effective antiaggregation-based AD therapy. Here, we present a dual-functional nanoinhibitor meticulously designed to target the aggregation driving force and amyloid fibril spatial structure. Leveraging the exceptional structural stability and facile tailoring capability of endohedral metallofullerene Gd@C82, we introduce desired hydrogen-binding sites and charged groups, which are abundant on its surface for specific designs. Impressively, these designs endow the resultant functionalized-Gd@C82 nanoparticles (f-Gd@C82 NPs) with high capability of redirecting peptide self-assembly toward disordered, off-pathway species, obstructing the early growth of protofibrils, and disaggregating the preformed well-ordered protofibrils or even mature Aβ fibrils. This results in considerable alleviation of Aβ peptide-induced neuronal cytotoxicity, rescuing neuronal death and synaptic loss in primary neuron models. Notably, these modifications significantly improved the dispersibility of f-Gd@C82 NPs, thus substantially enhancing its bioavailability. Moreover, f-Gd@C82 NPs demonstrate excellent cytocompatibility with various cell lines and possess the ability to penetrate the blood-brain barrier in mice. Large-scale molecular dynamics simulations illuminate the inhibition and disaggregation mechanisms. Our design successfully overcomes the limitations of other nanocandidates, which often overly rely on hydrophobic interactions or photothermal conversion properties, and offers a viable direction for developing anti-AD agents through the inhibition and even reversal of Aβ aggregation.

Differential accumulation of human β-amyloid and tau from enriched extracts in neuronal and endothelial cells

While Aβ and Tau cellular distribution has been largely studied, the comparative internalization and subcellular accumulation of Tau and Aβ isolated from human brain extracts in endothelial and neuronal cells has not yet been unveiled. We have previously demonstrated that controlled enrichment of Aβ from human brain extracts constitutes a valuable tool to monitor cellular internalization in vitro and in vivo. Herein, we establish an alternative method to strongly enrich Aβ and Tau aggregates from human AD brains, which has allowed us to study and compare the cellular internalization, distribution and toxicity of both proteins within brain barrier endothelial (bEnd.3) and neuronal (Neuro2A) cells. Our findings demonstrate the suitability of human enriched brain extracts to monitor the intracellular distribution of human Aβ and Tau, which, once internalized, show dissimilar sorting to different organelles within the cell and differential toxicity, exhibiting higher toxic effects on neuronal cells than on endothelial cells. While tau is strongly concentrated preferentially in mitochondria, Aβ is distributed predominantly within the endolysosomal system in endothelial cells, whereas the endoplasmic reticulum was its preferential location in neurons. Altogether, our findings display a picture of the interactions that human Aβ and Tau might establish in these cells.

Improvement of Key Molecular Events Linked to Alzheimer's Disease Pathology Using Postbiotics

In the past 50 years, life expectancy has increased by more than 20 years. One consequence of this increase in longevity is the rise of age-related diseases such as dementia. Alzheimer's disease (AD) is the most common form of dementia, accounting for 60-70% of cases. AD pathogenesis is not restricted to the neuronal compartment but includes strong interactions with other brain cells, particularly microglia triggering the release of inflammatory mediators, which contribute to disease progression and severity. There is growing evidence revealing the diverse clinical benefits of postbiotics in many prevalent conditions, including neurodegenerative diseases. Here, we tested the ability of bacterial conditioned media (BCM) derived from selected lactic acid bacteria (LAB) strains to regulate core mechanisms relevant to AD pathophysiology in the microglia cell line BV-2. Levilactobacillus brevis CRL 2013, chosen for its efficient production of the neurotransmitter GABA, and Lactobacillus delbrueckii subsp. lactis CRL 581, known for its anti-inflammatory properties, were selected alongside Enterococcus mundtii CRL 35, a LAB strain that can significantly modulate cytokine production. BCM from all 3 strains displayed antioxidant capabilities, reducing oxidative stress triggered by beta-amyloid oligomers (oAβ1-42). Additionally, BCM effectively mitigated the expression of inflammatory cytokines, namely, TNF-α, IL-1β, and IL-6 triggered by oAβ1-42. Furthermore, our study identified that BCM from CRL 581 inhibit the activity of acetylcholinesterase (AChE), a crucial enzyme in AD progression, in both human erythrocytes and mouse brain tissues. Notably, the inhibitory effect was mediated by low-molecular-weight components of the BCM. L. delbrueckii subsp. lactis CRL 581 emerged as a favorable candidate for production of postbiotics with potential benefits for AD therapy since it demonstrated potent antioxidant activity, reduction of cytokine expression, and partial AChE inhibition. On the other hand, E. mundtii CRL 35 showed that the antioxidant activity failed to inhibit AChE and caused induction of iNOS expression, rendering it unsuitable as a potential therapeutic for AD. This study unveils the potential benefits of LAB-derived postbiotics for the development of new avenues for therapeutic interventions for AD.

Interactions of amyloidogenic proteins with mitochondrial protein import machinery in aging-related neurodegenerative diseases

Most mitochondrial proteins are targeted to the organelle by N-terminal mitochondrial targeting sequences (MTSs, or "presequences") that are recognized by the import machinery and subsequently cleaved to yield the mature protein. MTSs do not have conserved amino acid compositions, but share common physicochemical properties, including the ability to form amphipathic α-helical structures enriched with basic and hydrophobic residues on alternating faces. The lack of strict sequence conservation implies that some polypeptides can be mistargeted to mitochondria, especially under cellular stress. The pathogenic accumulation of proteins within mitochondria is implicated in many aging-related neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases. Mechanistically, these diseases may originate in part from mitochondrial interactions with amyloid-β precursor protein (APP) or its cleavage product amyloid-β (Aβ), α-synuclein (α-syn), and mutant forms of huntingtin (mHtt), respectively, that are mediated in part through their associations with the mitochondrial protein import machinery. Emerging evidence suggests that these amyloidogenic proteins may present cryptic targeting signals that act as MTS mimetics and can be recognized by mitochondrial import receptors and transported into different mitochondrial compartments. Accumulation of these mistargeted proteins could overwhelm the import machinery and its associated quality control mechanisms, thereby contributing to neurological disease progression. Alternatively, the uptake of amyloidogenic proteins into mitochondria may be part of a protein quality control mechanism for clearance of cytotoxic proteins. Here we review the pathomechanisms of these diseases as they relate to mitochondrial protein import and effects on mitochondrial function, what features of APP/Aβ, α-syn and mHtt make them suitable substrates for the import machinery, and how this information can be leveraged for the development of therapeutic interventions.

A Genome-Wide Functional Screen Identifies Enhancer and Protective Genes for Amyloid Beta-Peptide Toxicity

Alzheimer's disease (AD) is known to be caused by amyloid β-peptide (Aβ) misfolded into β-sheets, but this knowledge has not yet led to treatments to prevent AD. To identify novel molecular players in Aβ toxicity, we carried out a genome-wide screen in Saccharomyces cerevisiae, using a library of 5154 gene knock-out strains expressing Aβ_1-42. We identified 81 mammalian orthologue genes that enhance Aβ_1-42 toxicity, while 157 were protective. Next, we performed interactome and text-mining studies to increase the number of genes and to identify the main cellular functions affected by Aβ oligomers (oAβ). We found that the most affected cellular functions were calcium regulation, protein translation and mitochondrial activity. We focused on SURF4, a protein that regulates the store-operated calcium channel (SOCE). An in vitro analysis using human neuroblastoma cells showed that SURF4 silencing induced higher intracellular calcium levels, while its overexpression decreased calcium entry. Furthermore, SURF4 silencing produced a significant reduction in cell death when cells were challenged with oAβ_1-42, whereas SURF4 overexpression induced Aβ_1-42 cytotoxicity. In summary, we identified new enhancer and protective activities for Aβ toxicity and showed that SURF4 contributes to oAβ_1-42 neurotoxicity by decreasing SOCE activity.

Distinct brain lipid signatures in response to low-level PM2.5 exposure in a 3xTg-Alzheimer's disease mouse inhalation model

Fine particulate matter (PM2.5) poses a significant risk to human health. The molecular mechanisms underlying low-level PM2.5-induced neurotoxicity in the central nervous system remain unclear. In addition, changes in lipids in response to PM2.5 exposure have not yet been fully elucidated. In this study, 3xTg-Alzheimer's disease (AD) mice experienced continuous whole-body exposure to non-concentrated PM2.5 for three consecutive months, while control mice inhaled particulate matter-filtered air over the same time span. A liquid chromatography-mass spectrometry-based lipidomic platform was used to determine the distinct lipid profiles of various brain regions. The average PM2.5 concentration during the exposure was 11.38 μg/m3, which was close to the regulation limits of USA and Taiwan. The partial least squares discriminant analysis model showed distinct lipid profiles in the cortex, hippocampus, and olfactory bulb, but not the cerebellum, of mice in the exposure group. Increased levels of fatty acyls, glycerolipids, and sterol lipids, as well as the decreased levels of glycerophospholipids and sphingolipids in PM2.5-exposed mouse brains may be responsible for the increased energy demand, membrane conformation, neuronal loss, antioxidation, myelin function, and cellular signaling pathways associated with AD development. Our research suggests that subchronic exposure to low levels of PM2.5 may cause neurotoxicity by changing the lipid profiles in a susceptible model. Lipidomics is a powerful tool to study the early effects of PM2.5-induced AD toxicity.

Membrane interaction to intercellular spread of pathology in Alzheimer’s disease

Progressive development of pathology is one of the major characteristic features of neurodegenerative diseases. Alzheimer’s disease (AD) is the most prevalent among them. Extracellular amyloid-β (Aβ) plaques and intracellular tau neurofibrillary tangles are the pathological phenotypes of AD. However, cellular and animal studies implicate tau as a secondary pathology in developing AD while Aβ aggregates is considered as a trigger point. Interaction of Aβ peptides with plasma membrane (PM) seems to be a promising site of involvement in the events that lead to AD. Aβ binding to the lipid membranes initiates formation of oligomers of Aβ species, and these oligomers are known as primary toxic agents for neuronal toxicities. Once initiated, neuropathological toxicities spread in a “prion-like” fashion probably through the mechanism of intercellular transfer of pathogenic aggregates. In the last two decades, several studies have demonstrated neuron-to-neuron transfer of neurodegenerative proteins including Aβ and tau via exosomes and tunneling nanotubes (TNTs), the two modes of long-range intercellular transfer. Emerging pieces of evidence indicate that molecular pathways related to the biogenesis of exosomes and TNTs interface with endo-lysosomal pathways and cellular signaling in connection to vesicle recycling-imposed PM and actin remodulation. In this review, we discuss interactions of Aβ aggregates at the membrane level and its implications in intercellular spread of pathogenic aggregates. Furthermore, we hypothesize how spread of pathogenic aggregates contributes to complex molecular events that could regulate pathological and synaptic changes related to AD.

Complex Inclusion Bodies and Defective Proteome Hubs in Neurodegenerative Disease: New Clues, New Challenges

A healthy physiological environment of cells represents the dynamic homeostasis of crowded molecules. A subset of cellular proteome forms protein quality control (PQC) machinery to maintain an uninterrupted synthesis of new polypeptides and targeted elimination of old or defective proteins. The process of PQC may get overwhelmed under specific genetic mutations, environmental stress conditions, and aging-associated perturbances. Many of these conditions may lead to the generation of various types of aberrant protein species that may or may not accumulate as large cellular inclusions. These proteinaceous formations, referred to as inclusion bodies (IBs), could be membrane-bound or membrane-less, cytoplasmic, or nuclear. Most importantly, they could either be toxic or protective. Under acute stress conditions, the formation of aggregates may cause proteostasis failure, leading to large-scale changes in the cellular proteome compositions. However, the large insoluble IBs may act as reservoirs for many soluble proteins with high aggregation propensities, which can overwhelm the cellular chaperoning capacity and protein degradation machinery. The kinetic equilibrium between folding and unfolding, misfolding, and refolding; aggregation and degradation is perturbed in one or many neurodegenerative disorders (NDDs) associated with dementia, cognitive impairments, movement, and behavioural losses. However, a detailed interplay of IBs into the manifestation of the NDDs is unknown, and a very primitive knowledge of structural compositions of amyloid inclusions is present. The present article presents a brief evolutionary background of IBs; their functional relevance for prokaryotes, plants, and animals; and associated involvement in neuronal proteostasis.

Correlation between Sialylation Status and Cell Susceptibility to Amyloid Toxicity

The interaction between the cell membrane and misfolded protein species plays a crucial role in the development of neurodegeneration. This study was designed to clarify the relationship between plasma membrane composition in terms of the differently linked sialic acid (Sia) content and cell susceptibility to toxic and misfolded Aβ-42 peptides. The sialylation status in different cell lines was investigated by lectin histochemistry and confocal immunofluorescence and then correlated with the different propensities to bind amyloid fibrils and with the relative cell susceptibility to amyloid damage. This study reveals that expressions of Sias α2,3 and α2,6 linked to galactose/N-acetyl-galactosamine, and PolySia are positively correlated with Aβ-42-induced cell toxicity. PolySia shows an early strong interaction with amyloid fibrils, favoring their binding to GM1 ganglioside containing α2,3 galactose-linked Sia and a loss of cell viability. Our findings demonstrate that cell lines with a prevailing plastic neuron-like phenotype and high monoSia and PolySia contents are highly susceptible to amyloid Aβ-42 toxicity. This toxicity may involve a change in neuron metabolism and promote a compensative/protective increase in PolySia, which, in turn, could favor amyloid binding to GM1, thus exacerbating cell dysmetabolism and further amyloid aggregation.

Amyloid-β induced membrane damage instigates tunneling nanotube-like conduits by p21-activated kinase dependent actin remodulation

Alzheimer's disease (AD) pathology progresses gradually via anatomically connected brain regions. Direct transfer of amyloid-β_1-42 oligomers (oAβ) between connected neurons has been shown, however, the mechanism is not fully revealed. We observed formation of oAβ induced tunneling nanotubes (TNTs)-like nanoscaled f-actin containing membrane conduits, in differentially differentiated SH-SY5Y neuronal models. Time-lapse images showed that oAβ propagate from one cell to another via TNT-like structures. Preceding the formation of TNT-like conduits, we detected oAβ_-induced plasma membrane (PM) damage and calcium-dependent repair through lysosomal-exocytosis, followed by massive endocytosis to re-establish the PM. Massive endocytosis was monitored by an influx of the membrane-staining dye TMA-DPH and PM damage was quantified by propidium iodide influx in the absence of Ca²⁺. The massive endocytosis eventually caused accumulation of internalized oAβ in Lamp1 positive multivesicular bodies/lysosomes via the actin cytoskeleton remodulating p21-activated kinase1 (PAK1) dependent endocytic pathway. Three-dimensional quantitative confocal imaging, structured illumination superresolution microscopy, and flowcytometry quantifications revealed that oAβ induces activation of phospho-PAK1, which modulates the formation of long stretched f-actin extensions between cells. Moreover, the formation of TNT-like conduits was inhibited by preventing PAK1-dependent internalization of oAβ using the small-molecule inhibitor IPA-3, a highly selective cell-permeable auto-regulatory inhibitor of PAK1. The present study reveals that the TNT-like conduits are probably instigated as a consequence of oAβ induced PM damage and repair process, followed by PAK1 dependent endocytosis and actin remodeling, probably to maintain cell surface expansion and/or membrane tension in equilibrium.

Synapsin-caveolin-1 gene therapy preserves neuronal and synaptic morphology and prevents neurodegeneration in a mouse model of AD

Alzheimer's disease (AD) is the most common form of neurodegeneration and cognitive dysfunction in the elderly. Identifying molecular signals that mitigate and reverse neurodegeneration in AD may be exploited therapeutically. Transgenic AD mice (PSAPP) exhibit learning and memory deficits at 9 and 11 months, respectively, with associated decreased expression of caveolin-1 (Cav-1), a membrane/lipid raft (MLR) scaffolding protein necessary for synaptic and neuroplasticity. Neuronal-targeted gene therapy using synapsin-Cav-1 cDNA (SynCav1) was delivered to the hippocampus of PSAPP mice at 3 months using adeno-associated virus serotype 9 (AAV9). Bilateral SynCav1 gene therapy was able to preserve MLRs profile, learning and memory, hippocampal dendritic arbor, synaptic ultrastructure, and axonal myelin content in 9- and 11-month PSAPP mice, independent of reducing toxic amyloid deposits and astrogliosis. Our data indicate that SynCav1 gene therapy may be an option for AD and potentially in other forms of neurodegeneration of unknown etiology.

The Relevance of Amyloid β-Calmodulin Complexation in Neurons and Brain Degeneration in Alzheimer's Disease

Intraneuronal amyloid β (Aβ) oligomer accumulation precedes the appearance of amyloid plaques or neurofibrillary tangles and is neurotoxic. In Alzheimer’s disease (AD)-affected brains, intraneuronal Aβ oligomers can derive from Aβ peptide production within the neuron and, also, from vicinal neurons or reactive glial cells. Calcium homeostasis dysregulation and neuronal excitability alterations are widely accepted to play a key role in Aβ neurotoxicity in AD. However, the identification of primary Aβ-target proteins, in which functional impairment initiating cytosolic calcium homeostasis dysregulation and the critical point of no return are still pending issues. The micromolar concentration of calmodulin (CaM) in neurons and its high affinity for neurotoxic Aβ peptides (dissociation constant ≈ 1 nM) highlight a novel function of CaM, i.e., the buffering of free Aβ concentrations in the low nanomolar range. In turn, the concentration of Aβ-CaM complexes within neurons will increase as a function of time after the induction of Aβ production, and free Aβ will rise sharply when accumulated Aβ exceeds all available CaM. Thus, Aβ-CaM complexation could also play a major role in neuronal calcium signaling mediated by calmodulin-binding proteins by Aβ; a point that has been overlooked until now. In this review, we address the implications of Aβ-CaM complexation in the formation of neurotoxic Aβ oligomers, in the alteration of intracellular calcium homeostasis induced by Aβ, and of dysregulation of the calcium-dependent neuronal activity and excitability induced by Aβ.

Expression of N-Terminal Cysteine Aβ42 and Conjugation to Generate Fluorescent and Biotinylated Aβ42

Fluorescent derivatives of the β-amyloid peptides (Aβ) are valuable tools for studying the interactions of Aβ with cells. Facile access to labeled expressed Aβ offers the promise of Aβ with greater sequence and stereochemical integrity, without impurities from amino acid deletion and epimerization. Here, we report methods for the expression of Aβ42 with an N-terminal cysteine residue, Aβ(C1-42), and its conjugation to generate Aβ42 bearing fluorophores or biotin. The methods rely on the hitherto unrecognized observation that expression of the Aβ(MC1-42) gene yields the Aβ(C1-42) peptide, because the N-terminal methionine is endogenously excised by Escherichia coli. Conjugation of Aβ(C1-42) with maleimide-functionalized fluorophores or biotin affords the N-terminally labeled Aβ42. The expression affords ∼14 mg of N-terminal cysteine Aβ from 1 L of bacterial culture. Subsequent conjugation affords ∼3 mg of labeled Aβ from 1 L of bacterial culture with minimal cost for labeling reagents. High-performance liquid chromatography analysis indicates the N-terminal cysteine Aβ to be >97% pure and labeled Aβ peptides to be 94-97% pure. Biophysical studies show that the labeled Aβ peptides behave like unlabeled Aβ and suggest that labeling of the N-terminus does not substantially alter the properties of the Aβ. We further demonstrate applications of the fluorophore-labeled Aβ peptides by using fluorescence microscopy to visualize their interactions with mammalian cells and bacteria. We anticipate that these methods will provide researchers convenient access to useful N-terminally labeled Aβ, as well as Aβ with an N-terminal cysteine that enables further functionalization.

Binding of Amyloid β(1-42)-Calmodulin Complexes to Plasma Membrane Lipid Rafts in Cerebellar Granule Neurons Alters Resting Cytosolic Calcium Homeostasis

Lipid rafts are a primary target in studies of amyloid β (Aβ) cytotoxicity in neurons. Exogenous Aβ peptides bind to lipid rafts, which in turn play a key role in Aβ uptake, leading to the formation of neurotoxic intracellular Aβ aggregates. On the other hand, dysregulation of intracellular calcium homeostasis in neurons has been observed in Alzheimer's disease (AD). In a previous work, we showed that Aβ(1-42), the prevalent Aβ peptide found in the amyloid plaques of AD patients, binds with high affinity to purified calmodulin (CaM), with a dissociation constant ≈1 nM. In this work, to experimentally assess the Aβ(1-42) binding capacity to intracellular CaM, we used primary cultures of mature cerebellar granule neurons (CGN) as a neuronal model. Our results showed a large complexation of submicromolar concentrations of Aβ(1-42) dimers by CaM in CGN, up to 120 ± 13 picomoles of Aβ(1-42) /2.5 × 106 cells. Using fluorescence microscopy imaging, we showed an extensive co-localization of CaM and Aβ(1-42) in lipid rafts in CGN stained with up to 100 picomoles of Aβ(1-42)-HiLyteTM-Fluor555 monomers. Intracellular Aβ(1-42) concentration in this range was achieved by 2 h incubation of CGN with 2 μM Aβ(1-42), and this treatment lowered the resting cytosolic calcium of mature CGN in partially depolarizing 25 mM potassium medium. We conclude that the primary cause of the resting cytosolic calcium decrease is the inhibition of L-type calcium channels of CGN by Aβ(1-42) dimers, whose activity is inhibited by CaM:Aβ(1-42) complexes bound to lipid rafts.

Metabolism of Glycosphingolipids and Their Role in the Pathophysiology of Lysosomal Storage Disorders

Glycosphingolipids (GSLs) are a specialized class of membrane lipids composed of a ceramide backbone and a carbohydrate-rich head group. GSLs populate lipid rafts of the cell membrane of eukaryotic cells, and serve important cellular functions including control of cell-cell signaling, signal transduction and cell recognition. Of the hundreds of unique GSL structures, anionic gangliosides are the most heavily implicated in the pathogenesis of lysosomal storage diseases (LSDs) such as Tay-Sachs and Sandhoff disease. Each LSD is characterized by the accumulation of GSLs in the lysosomes of neurons, which negatively interact with other intracellular molecules to culminate in cell death. In this review, we summarize the biosynthesis and degradation pathways of GSLs, discuss how aberrant GSL metabolism contributes to key features of LSD pathophysiology, draw parallels between LSDs and neurodegenerative proteinopathies such as Alzheimer's and Parkinson's disease and lastly, discuss possible therapies for patients.

Super-Resolution Infrared Imaging of Polymorphic Amyloid Aggregates Directly in Neurons

Loss of memory during Alzheimer's disease (AD), a fatal neurodegenerative disorder, is associated with neuronal loss and the aggregation of amyloid proteins into neurotoxic β-sheet enriched structures. However, the mechanism of amyloid protein aggregation is still not well understood due to many challenges when studying the endogenous amyloid structures in neurons or in brain tissue. Available methods either require chemical processing of the sample or may affect the amyloid protein structure itself. Therefore, new approaches, which allow studying molecular structures directly in neurons, are urgently needed. A novel approach is tested, based on label-free optical photothermal infrared super-resolution microspectroscopy, to study AD-related amyloid protein aggregation directly in the neuron at sub-micrometer resolution. Using this approach, amyloid protein aggregates are detected at the subcellular level, along the neurites and strikingly, in dendritic spines, which has not been possible until now. Here, a polymorphic nature of amyloid structures that exist in AD transgenic neurons is reported. Based on the findings of this work, it is suggested that structural polymorphism of amyloid proteins that occur already in neurons may trigger different mechanisms of AD progression.

Lipid and Lipid Raft Alteration in Aging and Neurodegenerative Diseases: A Window for the Development of New Biomarkers

Lipids in the brain are major components playing structural functions as well as physiological roles in nerve cells, such as neural communication, neurogenesis, synaptic transmission, signal transduction, membrane compartmentalization, and regulation of gene expression. Determination of brain lipid composition may provide not only essential information about normal brain functioning, but also about changes with aging and diseases. Indeed, deregulations of specific lipid classes and lipid homeostasis have been demonstrated in neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). Furthermore, recent studies have shown that membrane microdomains, named lipid rafts, may change their composition in correlation with neuronal impairment. Lipid rafts are key factors for signaling processes for cellular responses. Lipid alteration in these signaling platforms may correlate with abnormal protein distribution and aggregation, toxic cell signaling, and other neuropathological events related with these diseases. This review highlights the manner lipid changes in lipid rafts may participate in the modulation of neuropathological events related to AD and PD. Understanding and characterizing these changes may contribute to the development of novel and specific diagnostic and prognostic biomarkers in routinely clinical practice.

Solvent Composition Effects on the Structural Properties of the Aβ42 Monomer from the 3D-RISM-KH Molecular Theory of Solvation

Structural characterization of amyloid (A)β peptides implicated in Alzheimer's disease is a challenging problem due to their intrinsically disordered nature and their high propensity for aggregation. Only limited information is currently available from experiments on conformational properties and aggregation pathways of the peptides in cellular environments. In silico modeling complements experimental information, providing atomistic insight into structure and dynamics of different Aβ species. All-atom explicit solvent molecular dynamics (MD) simulations with a properly selected force field can deliver reliable structural and dynamic information. In the case of intrinsically disordered Aβ peptides, enhanced sampling simulations beyond the nanosecond time scale are required to obtain statistically meaningful results even for simple solvent conditions. To overcome the challenges of conformational sampling in crowded cellular environments, alternative approaches have to be used, including postprocessing of MD data. In this study, we employ the statistical-mechanical, three-dimensional reference interaction site model with the Kovalenko-Hirata closure integral equation molecular theory of solvation to describe solvent composition effects on the conformational equilibrium in a structural ensemble of the Aβ42 (covering residues 1-42) monomer based on a statistical reweighting technique. The methodology enables a computationally efficient prediction on how different factors in the cellular environment, such as solvent composition, nonpolar solvation, and macromolecular crowding, affect the structural properties of the monomer. Similarities have been identified between changes in the structural ensemble caused by nonpolar solvation and crowded environments modeled by ionic solution with large negative ions. In particular, both solvent conditions reduce the random coil content and enhance the helical structure content of the monomer. In contrast to the previous studies, which reported increased α-helical content of peptides in crowded environments, this work attributes these structural features to the difference in solvent exposure of hydrophilic residues of the monomer for different secondary structure elements, rather than to (entropic) excluded volume effects.

HIV-1 increases extracellular amyloid-beta levels through neprilysin regulation in primary cultures of human astrocytes

Since the success of combined antiretroviral therapy, HIV-1-infected individuals are now living much longer. This increased life expectancy is accompanied by a higher prevalence of HIV-1 associated neurocognitive disorders. Rising too is the incidence in these patients of pathological hallmarks of Alzheimer's disease such as increased deposition of amyloid beta protein (Aβ). Although neurons are major sources of Aβ in the brain, astrocytes are the most numerous glial cells, therefore, even a small level of astrocytic Aβ metabolism could make a significant contribution to brain pathology. Neprilysin (NEP) is a decisive/crucial regulator of Aβ levels. We evaluated the effects of HIV-1 on Aβ deposition and the expression and activity of NEP in primary human astrocytes. Specifically, no differences in intracellular amyloid deposits were found between infected and control cells. However, primary cultures of infected astrocytes showed more extracellular Aβ levels compared to controls. This was accompanied by reduced expression of NEP and to a significant decrease in its activity. These results indicate that the presence of HIV-1 in the brain could contribute to the increase in the total burden of cerebral Aβ.

Elevated Membrane Cholesterol Disrupts Lysosomal Degradation to Induce β-Amyloid Accumulation: The Potential Mechanism Underlying Augmentation of β-Amyloid Pathology by Type 2 Diabetes Mellitus

The endocytic membrane trafficking system is altered in the brains of early-stage Alzheimer disease (AD) patients, and endocytic disturbance affects the metabolism of β-amyloid (Aβ) protein, a key molecule in AD pathogenesis. It is widely accepted that type 2 diabetes mellitus (T2DM) is one of the strongest risk factors for development of AD. Supporting this link, experimentally induced T2DM enhances AD pathology in various animal models. Spontaneous T2DM also enhances Aβ pathology with severe endocytic pathology, even in nonhuman primate brains. However, it remains unclear how T2DM accelerates Aβ pathology. Herein, we demonstrate that cholesterol metabolism-related protein levels are increased and that membrane cholesterol level is elevated in spontaneous T2DM-affected cynomolgus monkey brains. Moreover, in vitro studies that manipulate cellular cholesterol reveal that elevated membrane cholesterol disrupts lysosomal degradation and enhances chemical-induced endocytic disturbance, resulting in great accumulation of Aβ in Neuro2a cells. These findings suggest that an alteration of cerebral cholesterol metabolism may be responsible for augmentation of Aβ pathology in T2DM-affected brains, which, in turn, may increase the risk for developing AD.

The Role of Irisin in Alzheimer's Disease

Alzheimer’s disease (AD) is characterized by progressive memory dysfunction, oxidative stress, and presence of senile plaques formed by amyloid beta (Aβ) accumulation in the brain. AD is one of the most important causes of morbidity and mortality worldwide. AD has a variety of risk factors, including environmental factors, metabolic dysfunction, and genetic background. Recent research has highlighted the relationship between AD and systemic metabolic changes such as glucose and lipid imbalance and insulin resistance. Irisin, a myokine closely linked to exercise, has been associated with glucose metabolism, insulin sensitivity, and fat browning. Recent studies have suggested that irisin is involved in the process in central nervous system (CNS) such as neurogenesis and has reported the effects of irisin on AD as one of the neurodegenerative disease. Here, we review the roles of irisin with respect to AD and suggest that irisin highlight therapeutic important roles in AD. Thus, we propose that irisin could be a potential future target for ameliorating AD pathology and preventing AD onset.

Fucosterol exerts protection against amyloid β-induced neurotoxicity, reduces intracellular levels of amyloid β and enhances the mRNA expression of neuroglobin in amyloid β-induced SH-SY5Y cells

Alzheimer's disease (AD) is a neurodegenerative disease that leads to progressive loss of neurons which often results in deterioration of memory and cognitive function. The development of AD is highly associated with the formation of senile plaques and neurofibrillary tangles. Amyloid β (Aβ) induces neurotoxicity and contributes to the development of AD. Recent evidences also highlighted the importance of neuroglobin (Ngb) in ameliorating AD. This study assessed the ability of fucosterol, a phytosterol found in brown alga, in protecting SH-SY5Y cells against Aβ-induced neurotoxicity. Its effects on the mRNA levels of APP and Ngb as well as the intracellular Aβ levels were also determined in Aβ-induced SH-SY5Y cells. SH-SY5Y cells were exposed to fucosterol prior to Aβ treatment. The effect on apoptosis was determined using Annexin V FITC staining and mRNA expression was studied using RT-PCR. Flow cytometry confirmed the protective effects of fucosterol on SH-SY5Y cells against Aβ-induced apoptosis. Pretreatment with fucosterol increased the Ngb mRNA levels but reduced the levels of APP mRNA and intracellular Aβ in Aβ-induced SH-SY5Y cells. These observations demonstrated the protective properties of fucosterol against Aβ-induced neurotoxicity in neuronal cells.

Evidence from ITIR-FCS Diffusion Studies that the Amyloid-Beta (Aβ) Peptide Does Not Perturb Plasma Membrane Fluidity in Neuronal Cells

The amyloid-beta (Aβ) peptide, commonly found in elevated levels in the brains of patients with Alzheimer's disease (AD) and in the cerebrospinal fluid of individuals presenting mild cognitive impairment, is thought to be one of the major factors resulting in the onset of AD. Although observed and studied at the molecular level for several decades, the exact disease pathology of AD is still not totally clear. One way in which Aβ is thought to affect neurons is by influencing cell membrane fluidity, which could result in abnormal synaptic or signaling function. The effects of Aβ on the fluidity of biological membranes have been studied using numerous membrane models such as artificial lipid bilayers and vesicles, living cells and membranes extracted from animal models of AD, yet there is still no consensus as to what effects Aβ has, if any, on membrane fluidity. As one of the most precise and accurate means of assaying membrane dynamics, we have thus chosen fluorescence correlation spectroscopy to investigate the issue, using fluorescent membrane-targeted probes on living cells treated with Aβ(1-42) oligomers and observing possible changes in membrane diffusion. Effects of Aβ on viability in different cell types varied from no detectable effect to extensive cell death by 72 h post-exposure. However, there was no change in the fluidity of either ordered membrane domains or the bulk membrane in any of these cells within this period. Our conclusion from these results is that perturbation of membrane fluidity is not likely to be a factor in acute Aβ-induced cytotoxicity.

Aβ inhibits SREBP-2 activation through Akt inhibition

We previously demonstrated that oligomeric amyloid β₄₂ (oAβ₄₂) inhibits the mevalonate pathway impairing cholesterol synthesis and protein prenylation. Enzymes of the mevalonate pathway are regulated by the transcription factor SREBP-2. Here, we show that in several neuronal types challenged with oAβ₄₂, SREBP-2 activation is reduced. Moreover, SREBP-2 activation is also decreased in the brain cortex of the Alzheimer's disease (AD) mouse model, TgCRND8, suggesting that SREBP-2 may be affected in vivo early in the disease. We demonstrate that oAβ₄₂ does not affect enzymatic cleavage of SREBP-2 per se, but may impair SREBP-2 transport from the endoplasmic reticulum (ER) to the Golgi. Trafficking of SREBP-2 from the ER to the Golgi requires protein kinase B (Akt) activation. oAβ₄₂ significantly reduces Akt phosphorylation and this decrease is responsible for the decline in SREBP-2 activation. Overexpression of constitutively active Akt prevents the effect of oAβ₄₂ on SREBP-2 and the downstream inhibition of cholesterol synthesis and protein prenylation. Our work provides a novel mechanistic link between Aβ and the mevalonate pathway, which will impact the views on issues related to cholesterol, isoprenoids, and statins in AD. We also identify SREBP-2 as an indirect target of Akt in neurons, which may play a role in the cross-talk between AD and diabetes.

Functional interplay between plasma membrane Ca2+-ATPase, amyloid β-peptide and tau

It is well known that dysregulation of Ca²⁺ homeostasis is involved in Alzheimeŕs disease (AD), a neurodegenerative disorder characterized by the presence of toxic aggregates of amyloid β-peptide (Aβ) and neurofibrillary tangles of tau. Alteration of calcium signaling has been linked to Aβ and tau pathologies, although the understanding of underlying molecular and cellular mechanisms is far from clear. This review summarizes the functional inhibition of plasma membrane Ca²⁺-ATPase (PMCA) by Aβ and tau, and its modulation by calmodulin and the ionic nature of phospholipids. The data obtained until now in our laboratory suggest that PMCA injury linked to Aβ and tau can be significantly involved in the cascade of events leading to intracellular calcium overload associated to AD.

Investigation of the Subcellular Neurotoxicity of Amyloid-β Using a Device Integrating Microfluidic Perfusion and Chemotactic Guidance

Alzheimer's disease (AD) is a neurodegenerative disorder with the histopathological hallmark of extracellular accumulation of amyloid-β (Aβ) peptide in brain senile plaques. Though many studies have shown the neural toxicity from various forms of Aβ peptides, the subcellular mechanisms of Aβ peptide are still not well understood, partially due to the technical challenges of isolating axons or dendrites from the cell body for localized investigation. In this study, the subcellular toxicity and localization of Aβ peptides are investigated by utilizing a microfluidic compartmentalized device, which combines physical restriction and chemotactic guidance to enable the isolation of axons and dendrites for localized pharmacological studies. It is found that Aβ peptides induced neuronal death is mostly resulted from Aβ treatment at cell body or axonal processes, but not at dendritic neurites. Simply applying Aβ to axons alone induces significant hyperactive spiking activity. Dynamic transport of Aβ aggregates is only observed between axon terminal and cell body. In addition to differential cellular uptake, more Aβ-peptide secretion is detected significantly from axons than from dendritic side. These results clearly demonstrate the existence of a localized mechanism in Aβ-induced neurotoxicity, and can potentially benefit the development of new therapeutic strategies for AD.

Apolipoprotein E and Alzheimer's disease: the influence of apolipoprotein E on amyloid-β and other amyloidogenic proteins

Alzheimer's disease (AD) is one of the fastest-growing causes of death and disability in persons 65 years of age or older, affecting more than 5 million Americans alone. Clinical manifestations of AD include progressive decline in memory, executive function, language, and other cognitive domains. Research efforts within the last three decades have identified APOE as the most significant genetic risk factor for late-onset AD, which accounts for >99% of cases. The apoE protein is hypothesized to affect AD pathogenesis through a variety of mechanisms, from its effects on the blood-brain barrier, the innate immune system, and synaptic function to the accumulation of amyloid-β (Aβ). Here, we discuss the role of apoE on the biophysical properties and metabolism of the Aβ peptide, the principal component of amyloid plaques and cerebral amyloid angiopathy (CAA). CAA is characterized by the deposition of amyloid proteins (including Aβ) in the leptomeningeal medium and small arteries, which is found in most AD cases but sometimes occurs as an independent entity. Accumulation of these pathologies in the brain is one of the pathological hallmarks of AD. Beyond Aβ, we will extend the discussion to the potential role of apoE on other amyloidogenic proteins found in AD, and also a number of diverse neurodegenerative diseases.

The Complex Role of Apolipoprotein E in Alzheimer's Disease: an Overview and Update

Apolipoprotein E (ApoE) plays a crucial role in the homeostatic control of lipids in both the periphery and the central nervous system (CNS). In humans, ApoE exists in three different isoforms: ε2, ε3 and ε4. ApoE ε3 is the most common isoform, while the ε4 isoform confers the greatest genetic risk for Alzheimer's disease (AD). However, the mechanisms underlying how ApoE contributes to the pathogenesis of AD are still debated. ApoE has been shown to impact amyloid β (Aβ) deposition and clearance in the brain. ApoE also has Aβ-independent pathways in AD, which has led to the discovery of new roles of ApoE ranging from mitochondria dysfunction to, most recently, iron metabolism. Here, we review the role of ApoE in health and in AD, with the view of identifying therapeutic approaches that could prevent the risk associated with the ε4 isoform.

3D compartmented model to study the neurite-related toxicity of Aβ aggregates included in collagen gels of adaptable porosity

Insoluble deposits of β-amyloid (Aβ) are associated to neurodegenerative pathologies, in particular Alzheimer's Disease (AD). The toxicity of synthetic amyloid-like peptides has been largely demonstrated and shown to depend upon their aggregation state. However, standard 2D cell culture conditions are not well suited to study the role of the close vicinity of Aβ aggregates and growing neurites in the degenerative process. Here, we have designed a compartmented set-up where model neural cells are differentiated on the surface of Aβ-containing collagen matrices. The average pore size can be modulated, from below 0.2μm to more than 0.5μm by simple treatment with collagenase, to respectively hamper or permit neurite outgrowth towards the depth of the matrix. Dense Aβ aggregates (Congo red and ThT-positive) were obtained inside the collagen matrix with a homogeneous distribution and dimensions similar to those observed in post-mortem brain slices from Alzheimer's patients. The aggregates are not toxic to cells when the pore size is small, in spite of relatively high concentrations of 0.05-0.62mg of peptide per gram of collagen (equivalent to 11.3-113μM). In contrast, on Aβ-containing matrices with large pores, massive neural death is observed when the cells are seeded in the same conditions. It is the first time to our knowledge that Aβ aggregates with a typical morphology of dense plaques are obtained within a porous biomimetic matrix, and are shown to be toxic only when accessible to differentiating cells.

STATEMENT OF SIGNIFICANCE

Insoluble deposits of β-amyloid (Aβ) are associated to neurodegenerative pathologies, in particular Alzheimer's Disease (AD). In this study, we have formed Aβ aggregates directly inside a biomimetic collagen matrix loaded with growth factors to induce the differentiation of PC12 or SH-SY6Y cells. For the first time, we show that when the contact between cells and Aβ aggregates is allowed by opening up the matrix porosity, the close vicinity with aggregates induces neurite dystrophy. The compartmented 3D culture model developed and used in this study is a valuable tool to study the cytotoxicity of preformed dense Aβ aggregates and proves that contact between the aggregates and neurons is required to induce neurodegenerative processes.

Amyloid beta receptors responsible for neurotoxicity and cellular defects in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease. Although a major cause of AD is the accumulation of amyloid-β (Aβ) peptide that induces neuronal loss and cognitive impairments, our understanding of its neurotoxic mechanisms is limited. Recent studies have identified putative Aβ-binding receptors that mediate Aβ neurotoxicity in cells and models of AD. Once Aβ interacts with a receptor, a toxic signal is transduced into neurons, resulting in cellular defects including endoplasmic reticulum stress and mitochondrial dysfunction. In addition, Aβ can also be internalized into neurons through unidentified Aβ receptors and induces malfunction of subcellular organelles, which explains some part of Aβ neurotoxicity. Understanding the neurotoxic signaling initiated by Aβ-receptor binding and cellular defects provide insight into new therapeutic windows for AD. In the present review, we summarize the findings on Aβ-binding receptors and the neurotoxicity of oligomeric Aβ.

Lipid rafts participate in aberrant degradative autophagic-lysosomal pathway of amyloid-beta peptide in Alzheimer's disease

Amyloid-beta peptide is the main component of amyloid plaques, which are found in Alzheimer's disease. The generation and deposition of amyloid-beta is one of the crucial factors for the onset and progression of Alzheimer's disease. Lipid rafts are glycolipid-rich liquid domains of the plasma membrane, where certain types of protein tend to aggregate and intercalate. Lipid rafts are involved in the generation of amyloid-beta oligomers and the formation of amyloid-beta peptides. In this paper, we review the mechanism by which lipid rafts disturb the aberrant degradative autophagic-lysosomal pathway of amyloid-beta, which plays an important role in the pathological process of Alzheimer's disease. Moreover, we describe this mechanism from the view of the Two-system Theory of fasciology and thus, suggest that lipid rafts may be a new target of Alzheimer's disease treatment.

Mechanisms of U87 astrocytoma cell uptake and trafficking of monomeric versus protofibril Alzheimer's disease amyloid-β proteins

A significant hallmark of Alzheimer’s disease is the formation of senile plaques in the brain due to the unbalanced levels of amyloid-beta (Aβ). However, although how Aβ is produced from amyloid precursor proteins is well understood, little is known regarding the clearance and metabolism of various Aβ aggregates from the brain. Similarly, little is known regarding how astrocytes internalize and degrade Aβ, although astrocytes are known to play an important role in plaque maintenance and Aβ clearance. The objective of this study is to investigate the cellular mechanisms that mediate the internalization of soluble monomeric versus oligomeric Aβ by astrocytes. We used a combination of laser confocal microscopy and genetic and pharmacological experiments to dissect the internalization of sAβ42 and oAβ42 and their postendocytic transport by U87 human brain astrocytoma cell line. Both Aβ42 species were internalized by U87 cells through fluid phase macropinocytosis, which required dynamin 2. Depleting LDL receptor-related protein 1 (LRP1) decreased sAβ42 uptake more significantly than that of oAβ42. We finally show that both Aβ42 species were rapidly transported to lysosomes through an endolytic pathway and subjected to proteolysis after internalization, which had no significant toxic effects to the U87 cells under relatively low concentrations. We propose that macropinocytic sAβ42 and oAβ42 uptake and their subsequent proteolytic degradation in astroglial cells is a significant mechanism underlying Aβ clearance from the extracellular milieu. Understanding the molecular events involved in astrocytic Aβ internalization may identify potential therapeutic targets for Alzheimer’s disease.

Clathrin-independent pathways of endocytosis

There are many pathways of endocytosis at the cell surface that apparently operate at the same time. With the advent of new molecular genetic and imaging tools, an understanding of the different ways by which a cell may endocytose cargo is increasing by leaps and bounds. In this review we explore pathways of endocytosis that occur in the absence of clathrin. These are referred to as clathrin-independent endocytosis (CIE). Here we primarily focus on those pathways that function at the small scale in which some have distinct coats (caveolae) and others function in the absence of specific coated intermediates. We follow the trafficking itineraries of the material endocytosed by these pathways and finally discuss the functional roles that these pathways play in cell and tissue physiology. It is likely that these pathways will play key roles in the regulation of plasma membrane area and tension and also control the availability of membrane during cell migration.

Nuclear translocation uncovers the amyloid peptide Aβ42 as a regulator of gene transcription

Although soluble species of the amyloid-β peptide Aβ42 correlate with disease symptoms in Alzheimer disease, little is known about the biological activities of amyloid-β (Aβ). Here, we show that Aβ peptides varying in lengths from 38 to 43 amino acids are internalized by cultured neuroblastoma cells and can be found in the nucleus. By three independent methods, we demonstrate direct detection of nuclear Aβ42 as follows: (i) biochemical analysis of nuclear fractions; (ii) detection of biotin-labeled Aβ in living cells by confocal laser scanning microscopy; and (iii) transmission electron microscopy of Aβ in cultured cells, as well as brain tissue of wild-type and transgenic APPPS1 mice (overexpression of amyloid precursor protein and presenilin 1 with Swedish and L166P mutations, respectively). Also, this study details a novel role for Aβ42 in nuclear signaling, distinct from the amyloid precursor protein intracellular domain. Chromatin immunoprecipitation showed that Aβ42 specifically interacts as a repressor of gene transcription with LRP1 and KAI1 promoters. By quantitative RT-PCR, we confirmed that mRNA levels of the examined candidate genes were exclusively decreased by the potentially neurotoxic Aβ42 wild-type peptide. Shorter peptides (Aβ38 or Aβ40) and other longer peptides (nontoxic Aβ42 G33A substitution or Aβ43) did not affect mRNA levels. Overall, our data indicate that the nuclear translocation of Aβ42 impacts gene regulation, and deleterious effects of Aβ42 in Alzheimer disease pathogenesis may be influenced by altering the expression profiles of disease-modifying genes.

Alzheimer amyloid beta inhibition of Eg5/kinesin 5 reduces neurotrophin and/or transmitter receptor function

The mechanism by which amyloid beta (Aβ) causes neuronal dysfunction and/or death in Alzheimer's disease (AD) is unclear. Previously, we showed that Aβ inhibits several microtubule-dependent kinesin motors essential for mitosis and also present in mature neurons. Here, we show that inhibition of kinesin 5 (Eg5) by Aβ blocks neuronal function by reducing transport of neurotrophin and neurotransmitter receptors to the cell surface. Specifically, cell-surface NGF/NTR(p75) and NMDA receptors decline in cells treated with Aβ or the kinesin 5 inhibitor monastrol, or expressing APP. Aβ and monastrol also inhibit NGF-dependent neurite outgrowth from PC12 cells and glutamate-dependent Ca++ entry into primary neurons. Like Aβ, monastrol inhibits long-term potentiation, a cellular model of NMDA-dependent learning and memory, and kinesin 5 activity is absent from APP/PS transgenic mice brain or neurons treated with Aβ. These data imply that cognitive deficits in AD may derive in part from inhibition of neuronal Eg5 by Aβ, resulting in impaired neuronal function and/or survival through receptor mislocalization. Preventing inhibition of Eg5 or other motors by Aβ may represent a novel approach to AD therapy.

Effect of apolipoprotein E genotype and diet on apolipoprotein E lipidation and amyloid peptides: randomized clinical trial

IMPORTANCE

Sporadic Alzheimer disease (AD) is caused in part by decreased clearance of the β-amyloid (Aβ) peptide breakdown products. Lipid-depleted (LD) apolipoproteins are less effective at binding and clearing Aβ, and LD Aβ peptides are more toxic to neurons. However, not much is known about the lipid states of these proteins in human cerebrospinal fluid.

OBJECTIVE

To characterize the lipidation states of Aβ peptides and apolipoprotein E in the cerebrospinal fluid in adults with respect to cognitive diagnosis and APOE ε4 allele carrier status and after a dietary intervention.

DESIGN

Randomized clinical trial.

SETTING

Veterans Affairs Medical Center clinical research unit.

PARTICIPANTS

Twenty older adults with normal cognition (mean [SD] age, 69 [7] years) and 27 with amnestic mild cognitive impairment (67 [6] years).

INTERVENTIONS

Randomization to a diet high in saturated fat content and with a high glycemic index (High diet; 45% of energy from fat [>25% saturated fat], 35%-40% from carbohydrates with a mean glycemic index >70, and 15%-20% from protein) or a diet low in saturated fat content and with a low glycemic index (Low diet; 25% of energy from fat [<7% saturated fat], 55%-60% from carbohydrates with a mean glycemic index <55, and 15%-20% from protein).

MAIN OUTCOMES AND MEASURES

Lipid-depleted Aβ42 and Aβ40 and apolipoprotein E in cerebrospinal fluid.

RESULTS

Baseline levels of LD Aβ were greater for adults with mild cognitive impairment compared with adults with normal cognition (LD Aβ42, P = .05; LD Aβ40, P = .01). These findings were magnified in adults with mild cognitive impairment and the ε4 allele, who had higher LD apolipoprotein E levels irrespective of cognitive diagnosis (P < .001). The Low diet tended to decrease LD Aβ levels, whereas the High diet increased these fractions (LD Aβ42, P = .01; LD Aβ40, P = .15). Changes in LD Aβ levels with the Low diet negatively correlated with changes in cerebrospinal fluid levels of insulin (LD Aβ42 and insulin, r = -0.68 [P = .01]; LD Aβ40 and insulin, r = -0.78 [P = .002]).

CONCLUSIONS AND RELEVANCE

The lipidation states of apolipoproteins and Aβ peptides in the brain differ depending on APOE genotype and cognitive diagnosis. Concentrations can be modulated by diet. These findings may provide insight into the mechanisms through which apolipoprotein E4 and unhealthy diets impart risk for developing AD.

Convergence of synapses, endosomes, and prions in the biology of neurodegenerative diseases

Age-related misfolding and aggregation of disease-linked proteins in selective brain regions is a characteristic of neurodegenerative diseases. Although neuropathological aggregates that characterize these various diseases are found at sites other than synapses, increasing evidence supports the idea that synapses are where the pathogenesis begins. Understanding these diseases is hampered by our lack of knowledge of what the normal functions of these proteins are and how they are affected by aging. Evidence has supported the idea that neurodegenerative disease-linked proteins have a common propensity for prion protein-like cell-to-cell propagation. However, it is not thought that the prion-like quality of these proteins/peptides that allows their cell-to-cell transmission implies a role for human-to-human spread in common age-related neurodegenerative diseases. It will be important to better understand the molecular and cellular mechanisms governing the role of these aggregating proteins in neural function, especially at synapses, how their propagation occurs and how pathogenesis is promoted by aging.

Neurotrophin receptor p75 mediates the uptake of the amyloid beta (Aβ) peptide, guiding it to lysosomes for degradation in basal forebrain cholinergic neurons

A fascinating yet perhaps overlooked trait of the p75 neurotrophin receptor (p75(NTR)) is its ability to bind ligands with no obvious neurotrophic function. Using cultured basal forebrain (BF) neurons, this study demonstrates selective internalization of amyloid β (Aβ) 1-42 in conjunction with p75(NTR) (labelled with IgG192-Cy3) by cholinergic cells. Active under resting conditions, this process was enhanced by high K(+) stimulation and was insensitive to inhibitors of regulated synaptic activity-tetrodotoxin or botulinum neurotoxins (BoNT type/A and/B). Blockade of sarco-endoplasmic reticulum (SERCA) Ca(2+) ATPase with thapsigargin and CPA or chelation of Ca(2+) with EGTA-AM strongly suppressed the endocytosis of p75(NTR), implicating the role of ER released Ca(2+). The uptake of IgG192-Cy3 was also reduced by T-type Ca(2+) channel blocker mibefradil but not Cd(2+), an indiscriminate blocker of high voltage-activated Ca(2+) currents. A strong co-localization of IgG192-Cy3 with late endosome (Rab7) or lysosome (Lamp1) qualifier proteins suggest these compartments as the primary destination for internalized IgG192 and Aβ. Selective uptake and labeling of BF cholinergic cells with IgG192-Cy3 injected into the prefrontal cortex was verified also in vivo. The significance of these findings in relation to Aβ clearance in the cerebral cortex and pathophysiology of Alzheimer's disease is discussed.

Blocking the interaction between apolipoprotein E and Aβ reduces intraneuronal accumulation of Aβ and inhibits synaptic degeneration

Accumulation of β-amyloid (Aβ) in the brain is a key event in Alzheimer disease pathogenesis. Apolipoprotein (Apo) E is a lipid carrier protein secreted by astrocytes, which shows inherent affinity for Aβ and has been implicated in the receptor-mediated Aβ uptake by neurons. To characterize ApoE involvement in the intraneuronal Aβ accumulation and to investigate whether blocking the ApoE/Aβ interaction could reduce intraneuronal Aβ buildup, we used a noncontact neuronal-astrocytic co-culture system, where synthetic Aβ peptides were added into the media without or with cotreatment with Aβ12-28P, which is a nontoxic peptide antagonist of ApoE/Aβ binding. Compared with neurons cultured alone, intraneuronal Aβ content was significantly increased in neurons co-cultured with wild-type but not with ApoE knockout (KO) astrocytes. Neurons co-cultured with astrocytes also showed impaired intraneuronal degradation of Aβ, increased level of intraneuronal Aβ oligomers, and marked down-regulation of several synaptic proteins. Aβ12-28P treatment significantly reduced intraneuronal Aβ accumulation, including Aβ oligomer level, and inhibited loss of synaptic proteins. Furthermore, we showed significantly reduced intraneuronal Aβ accumulation in APPSW/PS1dE9/ApoE KO mice compared with APPSW/PS1dE9/ApoE targeted replacement mice that expressed various human ApoE isoforms. Data from our co-culture and in vivo experiments indicate an essential role of ApoE in the mechanism of intraneuronal Aβ accumulation and provide evidence that ApoE/Aβ binding antagonists can effectively prevent this process.

Weak glycolipid binding of a microdomain-tracer peptide correlates with aggregation and slow diffusion on cell membranes

Organized assembly or aggregation of sphingolipid-binding ligands, such as certain toxins and pathogens, has been suggested to increase binding affinity of the ligand to the cell membrane and cause membrane reorganization or distortion. Here we show that the diffusion behavior of the fluorescently tagged sphingolipid-interacting peptide probe SBD (Sphingolipid Binding Domain) is altered by modifications in the construction of the peptide sequence that both result in a reduction in binding to ganglioside-containing supported lipid membranes, and at the same time increase aggregation on the cell plasma membrane, but that do not change relative amounts of secondary structural features. We tested the effects of modifying the overall charge and construction of the SBD probe on its binding and diffusion behavior, by Surface Plasmon Resonance (SPR; Biacore) analysis on lipid surfaces, and by Fluorescence Correlation Spectroscopy (FCS) on live cells, respectively. SBD binds preferentially to membranes containing the highly sialylated gangliosides GT1b and GD1a. However, simple charge interactions of the peptide with the negative ganglioside do not appear to be a critical determinant of binding. Rather, an aggregation-suppressing amino acid composition and linker between the fluorophore and the peptide are required for optimum binding of the SBD to ganglioside-containing supported lipid bilayer surfaces, as well as for interaction with the membrane. Interestingly, the strength of interactions with ganglioside-containing artificial membranes is mirrored in the diffusion behavior by FCS on cell membranes, with stronger binders displaying similar characteristic diffusion profiles. Our findings indicate that for aggregation-prone peptides, aggregation occurs upon contact with the cell membrane, and rather than giving a stronger interaction with the membrane, aggregation is accompanied by weaker binding and complex diffusion profiles indicative of heterogeneous diffusion behavior in the probe population.

Amyloid beta accumulation in HIV-1-infected brain: The role of the blood brain barrier

In recent years, we face an increase in the aging of the HIV-1-infected population, which is not only due to effective antiretroviral therapy but also to new infections among older people. Even with the use of the antiretroviral therapy, HIV-associated neurocognitive disorders represent an increasing problem as the HIV-1-infected population ages. Increased amyloid beta (Aβ) deposition is characteristic of HIV-1-infected brains, and it has been hypothesized that brain vascular dysfunction contributes to this phenomenon, with a critical role suggested for the blood-brain barrier in brain Aβ homeostasis. This review will describe the mechanisms by which the blood-brain barrier may contribute to brain Aβ accumulation, and our findings in the context of HIV-1 infection will be discussed.

p75NTR is mainly responsible for Aβ toxicity but not for its internalization: a primary study

Accumulating evidence indicates that the intraneuronal accumulation of beta-amyloid peptide (Aβ) is earlier than the formation of extraneuronal amyloid plaque but the mechanism of the accumulation remains unclear. p75NTR is a receptor for Aβ and interacts with Aβ in vitro and in vivo but whether p75NTR mediates Aβ internalization and intraneuronal accumulation is not known. In this study, we aim to determine if p75NTR mediates Aβ internalization, which might provide new insights into Aβ metabolism and toxicity. FRET analysis in PC12 cells showed that internalized Aβ was close to p75NTR. Aβ1-42 could be internalized in PC12 cells in a concentration-dependent manner but the antibody to the p75NTR extracellular domain did not prevent its internalization. Aβ1-42 could also be internalized in mouse neonatal cortical neurons and the deletion of p75NTR in these neurons did not prevent its internalization but prevented Aβ neurotoxicity. Cholesterol at 10 μM significantly increased Aβ1-42 internalization in PC12 cells. Internalized Aβ1-42 is mainly co-localized with Beclin-1 (a biomarker of autophagosomes) but not with endosomal and lysomal markers. p75NTR may not play a main role in Aβ internalization at the concentrations tested but is responsible for Aβ induced toxicity in primary neurons. Internalized Aβ is mainly sorted to autophagosomes for metabolism.

Role of cholesterol in APP metabolism and its significance in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disorder believed to be initiated by accumulation of amyloid β (Aβ)-related peptides derived from proteolytic processing of amyloid precursor protein (APP). Research over the past two decades provided a mechanistic link between cholesterol and AD pathogenesis. Genetic polymorphisms in genes regulating the pivotal points in cholesterol metabolism have been suggested to enhance the risk of developing AD. Altered neuronal membrane cholesterol level and/or subcellular distribution have been implicated in aberrant formation, aggregation, toxicity, and degradation of Aβ-related peptides. However, the results are somewhat contradictory and we still do not have a complete understanding on how cholesterol can influence AD pathogenesis. In this review, we summarize our current understanding on the role of cholesterol in regulating the production/function of Aβ-related peptides and also examine the therapeutic potential of regulating cholesterol homeostasis in the treatment of AD pathology.

Spreading of neurodegenerative pathology via neuron-to-neuron transmission of β-amyloid

Alzheimer's disease (AD) is the major cause of dementia. During the development of AD, neurofibrillary tangles progress in a fixed pattern, starting in the transentorhinal cortex followed by the hippocampus and cortical areas. In contrast, the deposition of β-amyloid (Aβ) plaques, which are the other histological hallmark of AD, does not follow the same strict spatiotemporal pattern, and it correlates poorly with cognitive decline. Instead, soluble Aβ oligomers have received increasing attention as probable inducers of pathogenesis. In this study, we use microinjections into electrophysiologically defined primary hippocampal rat neurons to demonstrate the direct neuron-to-neuron transfer of soluble oligomeric Aβ. Additional studies conducted in a human donor-acceptor cell model show that this Aβ transfer depends on direct cellular connections. As the transferred oligomers accumulate, acceptor cells gradually show beading of tubulin, a sign of neurite damage, and gradual endosomal leakage, a sign of cytotoxicity. These observations support that intracellular Aβ oligomers play a role in neurodegeneration, and they explain the manner in which Aβ can drive disease progression, even if the extracellular plaque load is poorly correlated with the degree of cognitive decline. Understanding this phenomenon sheds light on the pathophysiological mechanism of AD progression. Additional elucidation will help uncover the detailed mechanisms responsible for the manner in which AD progresses via anatomical connections and will facilitate the development of new strategies for stopping the progression of this incapacitating disease.

Amyloid-β acts as a regulator of neurotransmitter release disrupting the interaction between synaptophysin and VAMP2

BACKGROUND

It is becoming increasingly evident that deficits in the cortex and hippocampus at early stages of dementia in Alzheimer's disease (AD) are associated with synaptic damage caused by oligomers of the toxic amyloid-β peptide (Aβ42). However, the underlying molecular and cellular mechanisms behind these deficits are not fully understood. Here we provide evidence of a mechanism by which Aβ42 affects synaptic transmission regulating neurotransmitter release.

METHODOLOGY/FINDINGS

We first showed that application of 50 nM Aβ42 in cultured neurones is followed by its internalisation and translocation to synaptic contacts. Interestingly, our results demonstrate that with time, Aβ42 can be detected at the presynaptic terminals where it interacts with Synaptophysin. Furthermore, data from dissociated hippocampal neurons as well as biochemical data provide evidence that Aβ42 disrupts the complex formed between Synaptophysin and VAMP2 increasing the amount of primed vesicles and exocytosis. Finally, electrophysiology recordings in brain slices confirmed that Aβ42 affects baseline transmission.

CONCLUSIONS/SIGNIFICANCE

Our observations provide a necessary and timely insight into cellular mechanisms that underlie the initial pathological events that lead to synaptic dysfunction in Alzheimer's disease. Our results demonstrate a new mechanism by which Aβ42 affects synaptic activity.

β-amyloid inhibits protein prenylation and induces cholesterol sequestration by impairing SREBP-2 cleavage

Accumulation of β-amyloid (Aβ) inside brain neurons is an early and crucial event in Alzheimer's disease (AD). Studies in brains of AD patients and mice models of AD suggested that cholesterol homeostasis is altered in neurons that accumulate Aβ. Here we directly investigated the role of intracellular oligomeric Aβ(42) (oAβ(42)) in neuronal cholesterol homeostasis. We report that oAβ(42) induces cholesterol sequestration without increasing cellular cholesterol mass. Several features of AD, such as endosomal abnormalities, brain accumulation of Aβ and neurofibrillary tangles, and influence of apolipoprotein E genotype, are also present in Niemann-Pick type C, a disease characterized by impairment of intracellular cholesterol trafficking. These common features and data presented here suggest that a pathological mechanism involving abnormal cholesterol trafficking could take place in AD. Cholesterol sequestration in Aβ-treated neurons results from impairment of intracellular cholesterol trafficking secondary to inhibition of protein prenylation. oAβ(42) reduces sterol regulatory element-binding protein-2 (SREBP-2) cleavage, causing decrease of protein prenylation. Inhibition of protein prenylation represents a mechanism of oAβ(42)-induced neuronal death. Supply of the isoprenoid geranylgeranyl pyrophosphate to oAβ(42)-treated neurons recovers normal protein prenylation, reduces cholesterol sequestration, and prevents Aβ-induced neurotoxicity. Significant to AD, reduced levels of protein prenylation are present in the cerebral cortex of the TgCRND8 mouse model. In conclusion, we demonstrate a significant inhibitory effect of Aβ on protein prenylation and identify SREBP-2 as a target of oAβ(42), directly linking Aβ to cholesterol homeostasis impairment.

Critical role of intraneuronal Aβ in Alzheimer's disease: technical challenges in studying intracellular Aβ

AIMS

Multiple lines of evidence have implicated β-amyloid (Aβ) in the pathogenesis of Alzheimer's disease (AD). However, the mechanism(s) whereby Aβ is involved in the disease process remains unclear. The dominant hypothesis in AD has been that Aβ initiates the disease via toxicity from secreted, extracellular Aβ aggregates. More recently, an alternative hypothesis has emerged focusing on a pool of Aβ that accumulates early on within AD vulnerable neurons of the brain. Although the topic of intraneuronal Aβ has been of major interest in the field, technical difficulties in detecting intraneuronal Aβ have also made this topic remarkably controversial. Here we review evidence pointing to the critical role of intraneuronal Aβ in AD and provide insights both into challenges faced in detecting intracellular Aβ and the prion-like properties of Aβ.

MAIN METHODS

Immunoprecipitation and Western blot are used for Aβ detection.

KEY FINDINGS

We highlight that a standard biochemical method can underestimate intraneuronal Aβ and that extracellular Aβ can up-regulate intracellular Aβ. We also show that detergent can remove intraneuronal Aβ.

SIGNIFICANCE

There is a growing awareness that intraneuronal Aβ is a key pathogenic pool of Aβ involved in causing synapse dysfunction. Difficulties in detecting intraneuronal Aβ are an insufficient reason for ignoring this critical pool of Aβ.

Differences in the cellular uptake and intracellular itineraries of amyloid beta proteins 40 and 42: ramifications for the Alzheimer's drug discovery

Mounting evidence suggests that the pathological hallmarks of Alzheimer's disease (AD), neurofibrillary tangles and parenchymal amyloid plaques, are downstream reflections of neurodegeneration caused by the intraneuronal accumulation of amyloid-β proteins (Aβ), particularly Aβ42 and Aβ40. While the neurotoxicity of more amyloidogenic but less abundant Aβ42 is well documented, the effect of Aβ40 on neurons has been understudied. The Aβ40 expression in the presymptomatic AD brain is ten times greater than that of Aβ42. However, the Aβ40:42 ratio decreases with AD progression and coincides with increased amyloid plaque deposition in the brain. Hence, it is thought that Aβ40 protects neurons from the deleterious effects of Aβ42. The pathophysiological pathways involved in the neuronal uptake of Aβ40 or Aβ42 have not been clearly elucidated. Lack of such critical information obscures therapeutic targets and thwarts rational drug development strategies aimed at preventing neurodegeneration in AD. The current study has shown that fluorescein labeled Aβ42 (F-Aβ42) is internalized by neurons via dynamin dependent endocytosis and is sensitive to membrane cholesterol, whereas the neuronal uptake of F-Aβ40 is energy independent and nonendocytotic. Following their uptake, both F-Aβ40 and F-Aβ42 did not accumulate in early/recycling endosomes; F-Aβ42 but not F-Aβ40 accumulated in late endosomes and in the vesicles harboring caveolin-1. Furthermore, F-Aβ42 demonstrated robust accumulation in the lysosomes and damaged their integrity, whereas F-Aβ40 showed only a sparse lysosomal accumulation. Such regulated trafficking along distinct pathways suggests that Aβ40 and Aβ42 exercise differential effects on neurons. These differences must be carefully considered in the design of a pharmacological agent intended to block the neurodegeneration triggered by Aβ proteins.