The interaction between apolipoprotein E and Alzheimer's amyloid beta-peptide is dependent on beta-peptide conformation

Multiple Roles of Apolipoprotein E4 in Oxidative Lipid Metabolism and Ferroptosis During the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide and has a great socio-economic impact. Modified oxidative lipid metabolism and dysregulated iron homeostasis have been implicated in the pathogenesis of this disorder, but the detailed pathophysiological mechanisms still remain unclear. Apolipoprotein E (APOE) is a lipid-binding protein that occurs in large quantities in human blood plasma, and a polymorphism of the APOE gene locus has been identified as risk factors for AD. The human genome involves three major APOE alleles (APOE2, APOE3, APOE4), which encode for three subtly distinct apolipoprotein E isoforms (APOE2, APOE3, APOE4). The canonic function of these apolipoproteins is lipid transport in blood and brain, but APOE4 allele carriers have a much higher risk for AD. In fact, about 60% of clinically diagnosed AD patients carry at least one APOE4 allele in their genomes. Although the APOE4 protein has been implicated in pathophysiological key processes of AD, such as extracellular beta-amyloid (Aβ) aggregation, mitochondrial dysfunction, neuroinflammation, formation of neurofibrillary tangles, modified oxidative lipid metabolism, and ferroptotic cell death, the underlying molecular mechanisms are still not well understood. As for all mammalian cells, iron plays a crucial role in neuronal functions and dysregulation of iron homeostasis has also been implicated in the pathogenesis of AD. Imbalances in iron homeostasis and impairment of the hydroperoxy lipid-reducing capacity induce cellular dysfunction leading to neuronal ferroptosis. In this review, we summarize the current knowledge on APOE4-related oxidative lipid metabolism and the potential role of ferroptosis in the pathogenesis of AD. Pharmacological interference with these processes might offer innovative strategies for therapeutic interventions.

Temporal Characterization of the Amyloidogenic APPswe/PS1dE9;hAPOE4 Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a devastating disorder with a global prevalence estimated at 55 million people. In clinical studies administering certain anti-beta-amyloid (Aβ) antibodies, amyloid-related imaging abnormalities (ARIAs) have emerged as major adverse events. The frequency of these events is higher among apolipoprotein ε4 allele carriers (APOE4) compared to non-carriers. To reflect patients most at risk for vascular complications of anti-Aβ immunotherapy, we selected an APPswe/PS1dE9 transgenic mouse model bearing the human APOE4 gene (APPPS1:E4) and compared it with the same APP/PS1 mouse model bearing the human APOE3 gene (APOE ε3 allele; APPPS1:E3). Using histological and biochemical analyses, we characterized mice at three ages: 8, 12, and 16 months. Female and male mice were assayed for general cerebral fibrillar and pyroglutamate (pGlu-3) Aβ deposition, cerebral amyloid angiopathy (CAA), microhemorrhages, apoE and cholesterol composition, astrocytes, microglia, inflammation, lysosomal dysfunction, and neuritic dystrophy. Amyloidosis, lipid deposition, and astrogliosis increased with age in APPPS1:E4 mice, while inflammation did not reveal significant changes with age. In general, APOE4 carriers showed elevated Aβ, apoE, reactive astrocytes, pro-inflammatory cytokines, microglial response, and neuritic dystrophy compared to APOE3 carriers at different ages. These results highlight the potential of the APPPS1:E4 mouse model as a valuable tool in investigating the vascular side effects associated with anti-amyloid immunotherapy.

Molecular modeling of apoE in complexes with Alzheimer's amyloid-β fibrils from human brain suggests a structural basis for apolipoprotein co-deposition with amyloids

Apolipoproteins co-deposit with amyloids, yet apolipoprotein-amyloid interactions are enigmatic. To understand how apoE interacts with Alzheimer's amyloid-β (Aβ) peptide in fibrillary deposits, the NMR structure of full-length human apoE was docked to four structures of patient-derived Aβ1-40 and Aβ1-42 fibrils determined previously using cryo-electron microscopy or solid-state NMR. Similar docking was done using the NMR structure of human apoC-III. In all complexes, conformational changes in apolipoproteins were required to expose large hydrophobic faces of their amphipathic α-helices for sub-stoichiometric binding to hydrophobic surfaces on sides or ends of fibrils. Basic residues flanking the hydrophobic helical faces in apolipoproteins interacted favorably with acidic residue ladders in some amyloid polymorphs. Molecular dynamics simulations of selected apoE-fibril complexes confirmed their stability. Amyloid binding via cryptic sites, which became available upon opening of flexibly linked apolipoprotein α-helices, resembled apolipoprotein-lipid binding. This mechanism probably extends to other apolipoprotein-amyloid interactions. Apolipoprotein binding alongside fibrils could interfere with fibril fragmentation and secondary nucleation, while binding at the fibril ends could halt amyloid elongation and dissolution in a polymorph-specific manner. The proposed mechanism is supported by extensive prior experimental evidence and helps reconcile disparate reports on apoE's role in Aβ aggregation. Furthermore, apoE domain opening and direct interaction of Arg/Cys158 with amyloid potentially contributes to isoform-specific effects in Alzheimer's disease. In summary, current modeling supported by prior experimental studies suggests similar mechanisms for apolipoprotein-amyloid and apolipoprotein-lipid interactions; explains why apolipoproteins co-deposit with amyloids; and helps reconcile conflicting reports on the chaperone-like apoE action in Aβ aggregation.

Molecular modeling of apoE in complexes with Alzheimer's amyloid-β fibrils from human brain suggests a structural basis for apolipoprotein co-deposition with amyloids

Apolipoproteins co-deposit with amyloids, yet apolipoprotein-amyloid interactions are enigmatic. To understand how apoE interacts with Alzheimer's amyloid-β (Aβ) peptide in fibrillary deposits, the NMR structure of full-length human apoE was docked to four structures of patient-derived Aβ1-40 and Aβ1-42 fibrils determined previously using cryo-electron microscopy or solid-state NMR. Similar docking was done using the NMR structure of human apoC-III. In all complexes, conformational changes in apolipoproteins were required to expose large hydrophobic faces of their amphipathic α-helices for sub-stoichiometric binding to hydrophobic surfaces on sides or ends of fibrils. Basic residues flanking the hydrophobic helical faces in apolipoproteins interacted favorably with acidic residue ladders in some amyloid polymorphs. Molecular dynamics simulations of selected apoE-fibril complexes confirmed their stability. Amyloid binding via cryptic sites, which became available upon opening of flexibly linked apolipoprotein α-helices, resembled apolipoprotein-lipid binding. This mechanism probably extends to other apolipoprotein-amyloid interactions. Apolipoprotein binding alongside fibrils could interfere with fibril fragmentation and secondary nucleation, while binding at the fibril ends could halt amyloid elongation and dissolution in a polymorph-specific manner. The proposed mechanism is supported by extensive prior experimental evidence and helps reconcile disparate reports on apoE's role in Aβ aggregation. Furthermore, apoE domain opening and direct interaction of Arg/Cys158 with amyloid potentially contributes to isoform-specific effects in Alzheimer's disease. In summary, current modeling supported by prior experimental studies suggests similar mechanisms for apolipoprotein-amyloid and apolipoprotein-lipid interactions; explains why apolipoproteins co-deposit with amyloids; and helps reconcile conflicting reports on the chaperone-like apoE action in Aβ aggregation.

Cross interactions between Apolipoprotein E and amyloid proteins in neurodegenerative diseases

Three common Apolipoprotein E isoforms, ApoE2, ApoE3, and ApoE4, are key regulators of lipid homeostasis, among other functions. Apolipoprotein E can interact with amyloid proteins. The isoforms differ by one or two residues at positions 112 and 158, and possess distinct structural conformations and functions, leading to isoform-specific roles in amyloid-based neurodegenerative diseases. Over 30 different amyloid proteins have been found to share similar characteristics of structure and toxicity, suggesting a common interactome. The molecular and genetic interactions of ApoE with amyloid proteins have been extensively studied in neurodegenerative diseases, but have not yet been well connected and clarified. Here we summarize essential features of the interactions between ApoE and different amyloid proteins, identify gaps in the understanding of the interactome and propose the general interaction mechanism between ApoE isoforms and amyloid proteins. Perhaps more importantly, this review outlines what we can learn from the interactome of ApoE and amyloid proteins; that is the need to see both ApoE and amyloid proteins as a basis to understand neurodegenerative diseases.

Imipramine and olanzapine block apoE4-catalyzed polymerization of Aβ and show evidence of improving Alzheimer’s disease cognition

Background

The apolipoprotein E (APOE) ε4 allele confers the strongest risk for late-onset Alzheimer’s disease (AD) besides age itself, but the mechanisms underlying this risk are debated. One hypothesis supported by evidence from multiple labs is that apoE4 binds to the amyloid-β (Aβ) peptide and catalyzes its polymerization into neurotoxic oligomers and fibrils. Inhibiting this early step in the amyloid cascade may thereby reduce or prevent neurodegeneration and AD.

Methods

Using a design of experiments (DOE) approach, we developed a high-throughput assay to identify inhibitors of apoE4-catalyzed polymerization of Aβ into oligomers and fibrils. We used it to screen the NIH Clinical Collection of small molecule drugs tested previously in human clinical trials. We then evaluated the efficacy and cytotoxicity of the hit compounds in primary neuron models of apoE4-induced Aβ and phosphorylated tau aggregation. Finally, we performed retrospective analyses of the National Alzheimer’s Coordinating Center (NACC) clinical dataset, using Cox regression and Cox proportional hazards models to determine if the use of two FDA-approved hit compounds was associated with better cognitive scores (Mini-Mental State Exam), or improved AD clinical diagnosis, when compared with other medications of the same clinical indication.

Results

Our high-throughput screen identified eight blood-brain barrier (BBB)-permeable hit compounds that reduced apoE4-catalyzed Aβ oligomer and fibril formation in a dose-dependent manner. Five hit compounds were non-toxic toward cultured neurons and also reduced apoE4-promoted Aβ and tau neuropathology in a dose-dependent manner. Three of the five compounds were determined to be specific inhibitors of apoE4, whereas the other two compounds were Aβ or tau aggregation inhibitors. When prescribed to AD patients for their normal clinical indications, two of the apoE4 inhibitors, imipramine and olanzapine, but not other (non-hit) antipsychotic or antidepressant medications, were associated with improvements in cognition and clinical diagnosis, especially among APOE4 carriers.

Conclusions

The critical test of any proposed AD mechanism is whether it leads to effective treatments. Our high-throughput screen identified two promising FDA-approved drugs, imipramine and olanzapine, which have no structural, functional, or clinical similarities other than their shared ability to inhibit apoE4-catalyzed Aβ polymerization, thus identifying this mechanism as an essential contribution of apoE4 to AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-01020-9.

In Silico Analysis of the Antagonist Effect of Enoxaparin on the ApoE4–Amyloid-Beta (Aβ) Complex at Different pH Conditions

Apolipoprotein E4 (ApoE4) is thought to increase the risk of developing Alzheimer’s disease. Several studies have shown that ApoE4-Amyloid β (Aβ) interactions can increment amyloid depositions in the brain and that this can be augmented at low pH values. On the other hand, experimental studies in transgenic mouse models have shown that treatment with enoxaparin significantly reduces cortical Aβ levels, as well as decreases the number of activated astrocytes around Aβ plaques. However, the interactions between enoxaparin and the ApoE4-Aβ proteins have been poorly explored. In this work, we combine molecular dynamics simulations, molecular docking, and binding free energy calculations to elucidate the molecular properties of the ApoE4-Aβ interactions and the competitive binding affinity of the enoxaparin on the ApoE4 binding sites. In addition, we investigated the effect of the environmental pH levels on those interactions. Our results showed that under different pH conditions, the closed form of the ApoE4 protein, in which the C-terminal domain folds into the protein, remains stabilized by a network of hydrogen bonds. This closed conformation allowed the generation of six different ApoE4-Aβ interaction sites, which were energetically favorable. Systems at pH5 and 6 showed the highest energetic affinity. The enoxaparin molecule was found to have a strong energetic affinity for ApoE4-interacting sites and thus can neutralize or disrupt ApoE4-Aβ complex formation.

Selective reduction of astrocyte apoE3 and apoE4 strongly reduces Aβ accumulation and plaque-related pathology in a mouse model of amyloidosis

Background

One of the key pathological hallmarks of Alzheimer disease (AD) is the accumulation of the amyloid-β (Aβ) peptide into amyloid plaques. The apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset AD and has been shown to influence the accumulation of Aβ in the brain in an isoform-dependent manner. ApoE can be produced by different cell types in the brain, with astrocytes being the largest producer of apoE, although reactive microglia also express high levels of apoE. While studies have shown that altering apoE levels in the brain can influence the development of Aβ plaque pathology, it is not fully known how apoE produced by specific cell types, such as astrocytes, contributes to amyloid pathology.

Methods

We utilized APOE knock-in mice capable of having APOE selectively removed from astrocytes in a tamoxifen-inducible manner and crossed them with the APP/PS1-21 mouse model of amyloidosis. We analyzed the changes to Aβ plaque levels and assessed the impact on cellular responses to Aβ plaques when astrocytic APOE is removed.

Results

Tamoxifen administration was capable of strongly reducing apoE levels in the brain by markedly reducing astrocyte apoE, while microglial apoE expression remained. Reduction of astrocytic apoE3 and apoE4 led to a large decrease in Aβ plaque deposition and less compact plaques. While overall Iba1⁺ microglia were unchanged in the cortex after reducing astrocyte apoE, the expression of the disease-associated microglial markers Clec7a and apoE were lower around amyloid plaques, indicating decreased microglial activation. Additionally, astrocyte GFAP levels are unchanged around amyloid plaques, but overall GFAP levels are reduced in the cortex of female apoE4 mice after a reduction in astrocytic apoE. Finally, while the amount of neuritic dystrophy around remaining individual plaques was increased with the removal of astrocytic apoE, the overall amount of cortical amyloid-associated neuritic dystrophy was significantly decreased.

Conclusion

This study reveals an important role of astrocytic apoE3 and apoE4 on the deposition and accumulation of Aβ plaques as well as on certain Aβ-associated downstream effects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-022-00516-0.

Innate Immune System Activation and Neuroinflammation in Down Syndrome and Neurodegeneration: Therapeutic Targets or Partners?

Innate immune system activation and inflammation are associated with and may contribute to clinical outcomes in people with Down syndrome (DS), neurodegenerative diseases such as Alzheimer's disease (AD), and normal aging. In addition to serving as potential diagnostic biomarkers, innate immune system activation and inflammation may play a contributing or causal role in these conditions, leading to the hypothesis that effective therapies should seek to dampen their effects. However, recent intervention studies with the innate immune system activator granulocyte-macrophage colony-stimulating factor (GM-CSF) in animal models of DS, AD, and normal aging, and in an AD clinical trial suggest that activating the innate immune system and inflammation may instead be therapeutic. We consider evidence that DS, AD, and normal aging are accompanied by innate immune system activation and inflammation and discuss whether and when during the disease process it may be therapeutically beneficial to suppress or promote such activation.

Apolipoprotein E isoform-dependent effects on the processing of Alzheimer's amyloid-β

Since the identification of the apolipoprotein E (apoE) *ε4 allele as a major genetic risk factor for late-onset Alzheimer's disease, significant efforts have been aimed at elucidating how apoE4 expression confers greater brain amyloid-β (Aβ) burden, earlier disease onset and worse clinical outcomes compared to apoE2 and apoE3. ApoE primarily functions as a lipid carrier to regulate cholesterol metabolism in circulation as well as in the brain. However, it has also been suggested to interact with hydrophobic Aβ peptides to influence their processing in an isoform-dependent manner. Here, we review evidence from in vitro and in vivo studies extricating the effects of the three apoE isoforms, on different stages of the Aβ processing pathway including synthesis, aggregation, deposition, clearance and degradation. ApoE4 consistently correlates with impaired Aβ clearance, however data regarding Aβ synthesis and aggregation are conflicting and likely reflect inconsistencies in experimental approaches across studies. We further discuss the physical and chemical properties of apoE that may explain the inherent differences in activity between the isoforms. The lipidation status and lipid transport function of apoE are intrinsically linked with its ability to interact with Aβ. Traditionally, apoE-oriented therapeutic strategies for Alzheimer's disease have been proposed to non-specifically enhance or inhibit apoE activity. However, given the wide-ranging physiological functions of apoE in the brain and periphery, a more viable approach may be to specifically target and neutralise the pathological apoE4 isoform.

Real-Time Analysis of Specific Binding between Apolipoprotein E Isoforms and Amyloid β-Peptide by Dual Polarization Interferometry

One of the pathogenesis hypotheses of Alzheimer's disease (AD) is amyloid depositions and neurofibrillary tangles. Apolipoprotein E (Apo E) acts a vital part in the development of AD by affecting the aggregation and clearance of amyloid-β (Aβ). In this paper, a dual polarization interferometry (DPI) technique was employed for a real-time investigation toward the binding events of Apo E isoforms, for instance, Apo E2, Apo E3, and Apo E4, with Aβ_1-40. By evaluation of detailed binding information provided by DPI, the affinities between Apo E isoforms and Aβ_1-40 follow the order of E4 > E3 > E2, and the dissociation constants (K_D) of Aβ_1-40 with Apo E2, Apo E3, and Apo E4 were determined to be 251 ± 37, 40 ± 0.65, and 24.6 ± 2.42 nM, respectively. Our findings reveal the isoform-specific binding behaviors from a kinetics perspective, which can help us understand that Apo E4 has a higher risk of causing AD because of its promoting effect on Aβ aggregation and fibrillation and inefficient clearance of Aβ. Remarkably, this work provides a promising method for exploring the dynamics of interactions between biomolecules and expectantly contributes to the development of AD drugs and therapies targeting Apo E and Aβ.

APOE-amyloid interaction: Therapeutic targets

Alzheimer's disease (AD) is a devastating neurodegenerative disorder that is growing in prevalence globally. It is the only major cause of death without any effective pharmacological means to treat or slow progression. Inheritance of the ε4 allele of the Apolipoprotein (APO) E gene is the strongest genetic risk factor for late-onset AD. The interaction between APOE and amyloid β (Aβ) plays a key role in AD pathogenesis. The APOE-Aβ interaction regulates Aβ aggregation and clearance and therefore directly influences the development of amyloid plaques, congophilic amyloid angiopathy and subsequent tau related pathology. Relatively few AD therapeutic approaches have directly targeted the APOE-Aβ interaction thus far. Here we review the critical role of APOE in the pathogenesis of AD and some of the most promising therapeutic approaches that focus on the APOE-Aβ interaction.

Apolipoprotein E and oxidative stress in brain with relevance to Alzheimer's disease

Inheritance of apolipoprotein E4 (APOE4) is a major risk factor for development of Alzheimer's disease (AD). This lipoprotein, in contrast to apoE2, has arginine residues at positions 112 and 158 in place of cysteines in the latter isoform. In apoE3, the Cys at residue 158 is replaced by an arginine residue. This differential amino acid composition of the three genotypes of APOE have profound influence on the structure, binding properties, and multiple functions of this lipoprotein. Moreover, AD brain is under a high degree of oxidative stress, including that associated with amyloid β-peptide (Aβ) oligomers. Lipid peroxidation produces the highly reactive and neurotoxic molecule, 4-hydroxynonenal (HNE) that forms covalent bonds with cysteine residues (Cys) [as well as with Lys and His residues]. Covalently modified Cys significantly alter structure and function of modified proteins. HNE bound to Cys residue(s) on apoE2 and apoE3 lessens the chance of HNE damage other proteins. apoE4, lacking Cys residues, is unable to scavenge HNE, permitting this latter neurotoxic molecule to lead to oxidative modification of neuronal proteins and eventual cell death. We posit that this lack of HNE scavenging activity in apoE4 significantly contributes to the association of APOE4 inheritance and increased risk of developing AD. Apoe knock-out mice provide insights into the role of this lipoprotein in oxidative stress. Targeted replacement mice in which the mouse gene of Apoe is separately replaced by the human APOE2, APOE3, or APOE4 genes, while keeping the mouse promoter assures the correct location and amount of the human protein isoform. Human APOE targeted replacement mice have been used to investigate the notion that oxidative damage to and death of neurons in AD and its earlier stages is related to APOE genotype. This current paper reviews the intersection of human APOE genotype, oxidative stress, and diminished function of this lipoprotein as a major contributing risk factor for development of AD. Discussion of potential therapeutic strategies to mitigate against the elevated risk of developing AD with inheritance of the APOE4 allele also is presented.

Morphological analysis of Apolipoprotein E binding to Aβ Amyloid using a combination of Surface Plasmon Resonance, Immunogold Labeling and Scanning Electron Microscopy

Background

Immunogold labeling in combination with transmission electron microscopy analysis is a technique frequently used to correlate high-resolution morphology studies with detailed information regarding localization of specific antigens. Although powerful, the methodology has limitations and it is frequently difficult to acquire a stringent system where unspecific low-affinity interactions are removed prior to analysis.

Results

We here describe a combinatorial strategy where surface plasmon resonance and immunogold labeling are used followed by a direct analysis of the sensor-chip surface by scanning electron microscopy. Using this approach, we have probed the interaction between amyloid-β fibrils, associated to Alzheimer’s disease, and apolipoprotein E, a well-known ligand frequently found co-deposited to the fibrillar form of Aβ in vivo. The results display a lateral binding of ApoE along the amyloid fibrils and illustrates how the gold-beads represent a good reporter of the binding.

Conclusions

This approach exposes a technique with generic features which enables both a quantitative and a morphological evaluation of a ligand-receptor based system. The methodology mediates an advantage compared to traditional immunogold labeling since all washing steps can be monitored and where a high stringency can be maintained throughout the experiment.

Apolipoprotein E impairs amyloid-β fibril elongation and maturation

Alzheimer's disease (AD) is strongly linked to amyloid depositions of the Aβ peptide (Aβ). The lipid-binding protein apolipoprotein E (ApoE) has been found to interfere with Aβ amyloid formation and to exert a strong clinical impact to the pathology of AD. The APOE gene exists in three allelic isoforms represented by APOE ε2, APOE ε3, and APOE ε4. Carriers of the APOE ε4 variant display a gene dose-dependent increased risk of developing the disease. Aβ amyloids are formed via a nucleation-dependent mechanism where free monomers are added onto a nucleus in a template-dependent manner. Using a combination of surface plasmon resonance and thioflavin-T assays, we here show that ApoE can target the process of fibril elongation and that its interference effectively prevents amyloid maturation. We expose a complex equilibrium where the concentration of ApoE, Aβ monomers, and the amount of already formed Aβ fibrils will affect the relative proportion and formation rate of mature amyloids versus alternative assemblies. The result illustrates a mechanism which may affect both the clearance rate of Aβ assemblies in vivo and the population of cytotoxic Aβ assemblies.

Apolipoprotein E4, inhibitory network dysfunction, and Alzheimer's disease

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease (AD), increasing risk and decreasing age of disease onset. Many studies have demonstrated the detrimental effects of apoE4 in varying cellular contexts. However, the underlying mechanisms explaining how apoE4 leads to cognitive decline are not fully understood. Recently, the combination of human induced pluripotent stem cell (hiPSC) modeling of neurological diseases in vitro and electrophysiological studies in vivo have begun to unravel the intersection between apoE4, neuronal subtype dysfunction or loss, subsequent network deficits, and eventual cognitive decline. In this review, we provide an overview of the literature describing apoE4's detrimental effects in the central nervous system (CNS), specifically focusing on its contribution to neuronal subtype dysfunction or loss. We focus on γ-aminobutyric acid (GABA)-expressing interneurons in the hippocampus, which are selectively vulnerable to apoE4-mediated neurotoxicity. Additionally, we discuss the importance of the GABAergic inhibitory network to proper cognitive function and how dysfunction of this network manifests in AD. Finally, we examine how apoE4-mediated GABAergic interneuron loss can lead to inhibitory network deficits and how this deficit results in cognitive decline. We propose the following working model: Aging and/or stress induces neuronal expression of apoE. GABAergic interneurons are selectively vulnerable to intracellularly produced apoE4, through a tau dependent mechanism, which leads to their dysfunction and eventual death. In turn, GABAergic interneuron loss causes hyperexcitability and dysregulation of neural networks in the hippocampus and cortex. This dysfunction results in learning, memory, and other cognitive deficits that are the central features of AD.

Apolipoprotein E/Amyloid-β Complex Accumulates in Alzheimer Disease Cortical Synapses via Apolipoprotein E Receptors and Is Enhanced by APOE4

Apolipoprotein E (apoE) colocalizes with amyloid-β (Aβ) in Alzheimer disease (AD) plaques and in synapses, and evidence suggests that direct interactions between apoE and Aβ are important for apoE's effects in AD. The present work examines the hypothesis that apoE receptors mediate uptake of apoE/Aβ complex into synaptic terminals. Western blot analysis shows multiple SDS-stable assemblies in synaptosomes from human AD cortex; apoE/Aβ complex was markedly increased in AD compared with aged control samples. Complex formation between apoE and Aβ was confirmed by coimmunoprecipitation experiments. The apoE receptors low-density lipoprotein receptor (LDLR) and LDLR-related protein 1 (LRP1) were quantified in synaptosomes using flow cytometry, revealing up-regulation of LRP1 in early- and late-stage AD. Dual-labeling flow cytometry analysis of LRP1- and LDLR positives indicate most (approximately 65%) of LDLR and LRP1 is associated with postsynaptic density-95 (PSD-95)-positive synaptosomes, indicating that remaining LRP1 and LDLR receptors are exclusively presynaptic. Flow cytometry analysis of Nile red labeling revealed a reduction in cholesterol esters in AD synaptosomes. Dual-labeling experiments showed apoE and Aβ concentration into LDLR and LRP1-positive synaptosomes, along with free and esterified cholesterol. Synaptic Aβ was increased by apoE4 in control and AD samples. These results are consistent with uptake of apoE/Aβ complex and associated lipids into synaptic terminals, with subsequent Aβ clearance in control synapses and accumulation in AD synapses.

APOE and Alzheimer's Disease: Evidence Mounts that Targeting APOE4 may Combat Alzheimer's Pathogenesis

Alzheimer's disease (AD) is an immutable neurodegenerative disease featured by the two hallmark brain pathologies that are the extracellular amyloid ß (Aß) and intraneuronal tau protein. People carrying the APOE4 allele are at high risk of AD concerning the ones carrying the ε3 allele, while the ε2 allele abates risk. ApoE isoforms exert a central role in controlling the transport of brain lipid, neuronal signaling, mitochondrial function, glucose metabolism, and neuroinflammation. Regardless of widespread indispensable studies, the appropriate function of APOE in AD etiology stays ambiguous. Existing proof recommends that the disparate outcomes of ApoE isoforms on Aβ accretion and clearance have a distinct function in AD pathogenesis. ApoE-lipoproteins combine diverse cell-surface receptors to transport lipids and moreover to lipophilic Aβ peptide, that is believed to begin deadly events that generate neurodegeneration in the AD. ApoE has great influence in tau pathogenesis, tau-mediated neurodegeneration, and neuroinflammation, as well as α-synucleinopathy, lipid metabolism, and synaptic plasticity despite the presence of Aβ pathology. ApoE4 shows the deleterious effect for AD while the lack of ApoE4 is defensive. Therapeutic strategies primarily depend on APOE suggest to lessen the noxious effects of ApoE4 and reestablish the protective aptitudes of ApoE. This appraisal represents the critical interactions of APOE and AD pathology, existing facts on ApoE levels in the central nervous system (CNS), and the credible active stratagems for AD therapy by aiming ApoE. This review also highlighted utmost ApoE targeting therapeutic tactics that are crucial for controlling Alzheimer's pathogenesis.

A flow cytometry-based in vitro assay reveals that formation of apolipoprotein E (ApoE)-amyloid beta complexes depends on ApoE isoform and cell type

Apolipoprotein E (ApoE) is a secreted apolipoprotein with three isoforms, E2, E3, and E4, that binds to lipids and facilitates their transport in the extracellular environment of the brain and the periphery. The E4 allele is a major genetic risk factor for the sporadic form of Alzheimer's disease (AD), and studies of human brain and mouse models have revealed that E4 significantly exacerbates the deposition of amyloid beta (Aβ). It has been suggested that this deposition could be attributed to the formation of soluble ApoE isoform-specific ApoE-Aβ complexes. However, previous studies have reported conflicting results regarding the directionality and strength of those interactions. In this study, using a series of flow cytometry assays that maintain the physiological integrity of ApoE-Aβ complexes, we systematically assessed the association of Aβ with ApoE2, E3, or E4. We used ApoE secreted from HEK cells or astrocytes overexpressing ApoE fused with a GFP tag. As a source of soluble Aβ peptide, we used synthetic Aβ40 or Aβ42 or physiological Aβ secreted from CHO cell lines overexpressing WT or V717F variant amyloid precursor protein (APP). We observed significant interactions between the different ApoE isoforms and Aβ, with E4 interacting with Aβ more strongly than the E2 and E3 isoforms. We also found subtle differences depending on the Aβ type and the ApoE-producing cell type. In conclusion, these results indicate that the strength of the ApoE-Aβ association depends on the source of Aβ or ApoE.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Accuracy of cerebrospinal fluid Aβ1-42 measurements: evaluation of pre-analytical factors using a novel Elecsys immunosassay

BACKGROUND

A decreased level of Aβ1-42 in cerebrospinal fluid (CSF) is characteristic of Alzheimer disease and often used to support clinical diagnosis. The measured concentration of CSF Aβ1-42, however, depends strongly on several pre-analytical and analytical "confounding" factors such as sample collection, material of testing tube, CSF handling and storage procedures (e.g. transfer to new tubes after centrifugation, freeze-thaw effects). As a consequence, substantial variations in the measured levels of this biomarker are observed even for the same sample. This study investigates whether the accuracy of quantitative analysis of CSF Aβ1-42 can be improved by pre-analytical treatment of CSF with agents that could potentially reduce a freeze-thaw and adhesion-related depletion of Aβ1-42 from CSF, including modulators of Aβ aggregation and cryoprotecting or anti-adhesion agents.

METHODS

The concentration of CSF Aβ1-42 was assessed with a novel Elecsys immunoassay developed for quantification of Aβ1-42 in human CSF.

RESULTS

Low-molecular weight Aβ oligomerization inhibitors, β-sheet breaker peptides, or the mid domain 4G8 antibody do not improve the stability of CSF Aβ1-42 during a repeated freeze-thaw treatment. Cryoprotecting agents reduce a freeze-thaw dependent loss of Aβ1-42 only when spiked to CSF to final concentration of 300 mM or higher. Adhesion of Aβ1-42 can be prevented by pre-treating CSF with Tween or by using tubes with a siliconized surface.

CONCLUSIONS

Between-center variability in measured level of CSF Aβ1-42 can be reduced only by standardized CSF collection into one specific tube that, without centrifugation, transfer or other types of pre-analytical processing, is directly analyzed after sample collection.

Targeting Apolipoprotein E/Amyloid β Binding by Peptoid CPO_Aβ17-21 P Ameliorates Alzheimer's Disease Related Pathology and Cognitive Decline

Inheritance of the apolipoprotein E4 (apoE4) genotype has been identified as the major genetic risk factor for late onset Alzheimer's disease (AD). Studies have shown that apoE, apoE4 in particular, binds to amyloid-β (Aβ) peptides at residues 12-28 of Aβ and this binding modulates Aβ accumulation and disease progression. We have previously shown in several AD transgenic mice lines that blocking the apoE/Aβ interaction with Aβ12-28 P reduced Aβ and tau-related pathology, leading to cognitive improvements in treated AD mice. Recently, we have designed a small peptoid library derived from the Aβ12-28 P sequence to screen for new apoE/Aβ binding inhibitors with higher efficacy and safety. Peptoids are better drug candidates than peptides due to their inherently more favorable pharmacokinetic properties. One of the lead peptoid compounds, CPO_Aβ17-21 P, diminished the apoE/Aβ interaction and attenuated the apoE4 pro-fibrillogenic effects on Aβ aggregation in vitro as well as apoE4 potentiation of Aβ cytotoxicity. CPO_Aβ17-21 P reduced Aβ-related pathology coupled with cognitive improvements in an AD APP/PS1 transgenic mouse model. Our study suggests the non-toxic, non-fibrillogenic peptoid CPO_Aβ17-21 P has significant promise as a new AD therapeutic agent which targets the Aβ related apoE pathway, with improved efficacy and pharmacokinetic properties.

High affinity binding of amyloid β-peptide to calmodulin: Structural and functional implications

Amyloid β-peptides (Aβ) are a major hallmark of Alzheimer's disease (AD) and their neurotoxicity develop with cytosolic calcium dysregulation. On the other hand, calmodulin (CaM), a protein which plays a major multifunctional role in neuronal calcium signaling, has been shown to be involved in the regulation of non-amyloidogenic processing of amyloid β precursor protein (APP). Using fluorescent 6-bromoacetyl-2-dimethylaminonaphthalene derivatives of CaM, Badan-CaM, and human amyloid β(1-42) HiLyte™-Fluor555, we show in this work that Aβ binds with high affinity to CaM through the neurotoxic Aβ25-35 domain. In addition, the affinity of Aβ for calcium-saturated CaM conformation is approximately 20-fold higher than for CaM conformation in the absence of calcium (apo-CaM). Moreover, the value of K_d of 0.98 ± 0.11 nM obtained for Aβ1-42 dissociation from CaM saturated by calcium points out that CaM is one of the cellular targets with highest affinity for neurotoxic Aβ peptides. A major functional consequence of Aβ-CaM interaction is that it slowdowns Aβ fibrillation. The novel and high affinity interaction between calmodulin and Aβ shown in this work opens a yet-unexplored gateway to further understand the neurotoxic effect of Aβ in different neural cells and also to address the potential of calmodulin and calmodulin-derived peptides as therapeutic agents in AD.

Cerebrospinal Fluid Proteins as Regulators of Beta-amyloid Aggregation and Toxicity

Amyloid disorders, such as Alzheimer's, are almost invariably late-onset diseases. One defining diagnostic feature of Alzheimer's disease is the deposition of beta-amyloid as extracellular plaques, primarily in the hippocampus. This raises the question: are there natural protective agents that prevent beta-amyloid from depositing, and is it loss of this protection that leads to onset of disease? Proteins in cerebrospinal fluid (CSF) have been suggested to act as just such natural protective agents. Here, we describe some of the early evidence that led to this suggestion, and we discuss, in greater detail, two CSF proteins that have garnered the bulk of the attention.

The Neurobiology and Age-Related Prevalence of the ε4 Allele of Apolipoprotein E in Alzheimer's Disease Cohorts

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterised by amyloid beta (Aβ) plaques and tau neurofibrillary tangles in the brain. Human apolipoprotein E (ApoE) is a lipid transport protein coded by the polymorphic APOE gene, with three major alleles: ε2, ε3 and ε4. After age, the ε4 allele is the greatest risk factor for developing sporadic AD, conferring an increased risk of 3-4 and 8-12 times for one or two copies of the allele, respectively. This risk is reported to vary by demographic factors including sex, ethnicity and geography. In order to understand the risk of ApoE ε4 in relation to age, the primary risk factor for developing AD, we need to understand how the prevalence of APOE genotypes changes with age. Here, we present the first data on age-related prevalence of APOE ε4 in AD in three AD cohorts in Australia and the USA. There is a significant association between age and ε4 prevalence, particularly for ε4 homozygotes, such that as age increases the prevalence of ε4 decreases. Further studies on a random, population-based sample of the population are needed to provide more generalizable data, particularly in the >90-year-old age group.

ApoE and Aβ in Alzheimer's disease: accidental encounters or partners?

Among the three human apolipoprotein E (apoE) isoforms, apoE4 increases the risk of Alzheimer's disease (AD). While transporting cholesterol is a primary function, apoE also regulates amyloid-β (Aβ) metabolism, aggregation, and deposition. Although earlier work suggests that different affinities of apoE isoforms to Aβ might account for their effects on Aβ clearance, recent studies indicate that apoE also competes with Aβ for cellular uptake through apoE receptors. Thus, several factors probably determine the variable effects apoE has on Aβ. In this Review, we examine biochemical, structural, and functional studies and propose testable models that address the complex mechanisms underlying apoE-Aβ interaction and how apoE4 may increase AD risk and also serve as a target pathway for therapy.

Apolipoprotein E, amyloid-beta, and neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the accumulation and deposition of amyloid-beta (Aβ) peptides in the brain. Neuroinflammation occurs in the AD brain and plays a critical role in the neurodegenerative pathology. Particularly, Aβ evokes an inflammatory response that leads to synaptic dysfunction, neuronal death, and neurodegeneration. Apolipoprotein E (ApoE) proteins are involved in cholesterol transport, Aβ binding and clearance, and synaptic functions in the brain. The ApoE4 isoform is a key risk factor for AD, while the ApoE2 isoform has a neuroprotective effect. However, studies have reached different conclusions about the roles of the isoforms; some show that both ApoE3 and ApoE4 have anti-inflammatory effects, while others show that ApoE4 causes a predisposition to inflammation or promotes an inflammatory response following lipopolysaccharide treatment. These discrepancies may result from the differences in models, cell types, experimental conditions, and inflammatory stimuli used. Further, little was known about the role of ApoE isoforms in the Aβ-induced inflammatory response and in the neuroinflammation of AD. Our recent work showed that ApoE isoforms differentially regulate and modify the Aβ-induced inflammatory response in neural cells, with ApoE2 suppressing and ApoE4 promoting the response. In this article, we review the roles, mechanisms, and interrelations among Aβ, ApoE, and neuroinflammation in AD.

Soluble apoE/Aβ complex: mechanism and therapeutic target for APOE4-induced AD risk

The APOE4 allele of apolipoprotein E (apoE) is the greatest genetic risk factor for Alzheimer's disease (AD) compared to APOE2 and APOE3. Amyloid-β (Aβ), particularly in a soluble oligomeric form (oAβ), is considered a proximal cause of neurodegeneration in AD. Emerging data indicate that levels of soluble oAβ are increased with APOE4, providing a potential mechanism of APOE4-induced AD risk. However, the pathway(s) by which apoE4 may increase oAβ levels are unclear and the subject of continued inquiry. In this editorial review, we present the hypothesis that apoE isoform-specific interactions with Aβ, namely apoE/Aβ complex, modulate Aβ levels. Specifically, we propose that compared to apoE3, apoE4-containing lipoproteins are less lipidated, leading to less stable apoE4/Aβ complexes, resulting in reduced apoE4/Aβ levels and increased accumulation, particularly of oAβ. Evidence that support or counter this argument, as well as the therapeutic significance of this pathway to neurodegeneration, are discussed.

Blocking the apolipoprotein E/amyloid β interaction in triple transgenic mice ameliorates Alzheimer's disease related amyloid β and tau pathology

Inheritance of the apolipoprotein E4 (apoE4) genotype has been identified as the major genetic risk factor for late-onset Alzheimer's disease (AD). Studies have shown that the binding between apoE and amyloid-β (Aβ) peptides occurs at residues 244-272 of apoE and residues 12-28 of Aβ. ApoE4 has been implicated in promoting Aβ deposition and impairing clearance of Aβ. We hypothesized that blocking the apoE/Aβ interaction would serve as an effective new approach to AD therapy. We have previously shown that treatment with Aβ12-28P can reduce amyloid plaques in APP/PS1 transgenic (Tg) mice and vascular amyloid in TgSwDI mice with congophilic amyloid angiopathy. In the present study, we investigated whether the Aβ12-28P elicits a therapeutic effect on tau-related pathology in addition to amyloid pathology using old triple transgenic AD mice (3xTg, with PS1M146V , APPSwe and tauP30IL transgenes) with established pathology from the ages of 21 to 26 months. We show that treatment with Aβ12-28P substantially reduces tau pathology both immunohistochemically and biochemically, as well as reducing the amyloid burden and suppressing the activation of astrocytes and microglia. These affects correlate with a behavioral amelioration in the treated Tg mice.

Levels of soluble apolipoprotein E/amyloid-β (Aβ) complex are reduced and oligomeric Aβ increased with APOE4 and Alzheimer disease in a transgenic mouse model and human samples

Human apolipoprotein E (apoE) isoforms may differentially modulate amyloid-β (Aβ) levels. Evidence suggests physical interactions between apoE and Aβ are partially responsible for these functional effects. However, the apoE/Aβ complex is not a single static structure; rather, it is defined by detection methods. Thus, literature results are inconsistent and difficult to interpret. An ELISA was developed to measure soluble apoE/Aβ in a single, quantitative method and was used to address the hypothesis that reduced levels of soluble apoE/Aβ and an increase in soluble Aβ, specifically oligomeric Aβ (oAβ), are associated with APOE4 and AD. Previously, soluble Aβ42 and oAβ levels were greater with APOE4 compared with APOE2/APOE3 in hippocampal homogenates from EFAD transgenic mice (expressing five familial AD mutations and human apoE isoforms). In this study, soluble apoE/Aβ levels were lower in E4FAD mice compared with E2FAD and E3FAD mice, thus providing evidence that apoE/Aβ levels isoform-specifically modulate soluble oAβ clearance. Similar results were observed in soluble preparations of human cortical synaptosomes; apoE/Aβ levels were lower in AD patients compared with controls and lower with APOE4 in the AD cohort. In human CSF, apoE/Aβ levels were also lower in AD patients and with APOE4 in the AD cohort. Importantly, although total Aβ42 levels decreased in AD patients compared with controls, oAβ levels increased and were greater with APOE4 in the AD cohort. Overall, apoE isoform-specific formation of soluble apoE/Aβ modulates oAβ levels, suggesting a basis for APOE4-induced AD risk and a mechanistic approach to AD biomarkers.

Apolipoprotein e: essential catalyst of the Alzheimer amyloid cascade

The amyloid cascade hypothesis remains a robust model of AD neurodegeneration. However, amyloid deposits contain proteins besides Aβ, such as apolipoprotein E (apoE). Inheritance of the apoE4 allele is the strongest genetic risk factor for late-onset AD. However, there is no consensus on how different apoE isotypes contribute to AD pathogenesis. It has been hypothesized that apoE and apoE4 in particular is an amyloid catalyst or “pathological chaperone”. Alternatively it has been posited that apoE regulates Aβ clearance, with apoE4 been worse at this function compared to apoE3. These views seem fundamentally opposed. The former would indicate that removing apoE will reduce AD pathology, while the latter suggests increasing brain ApoE levels may be beneficial. Here we consider the scientific basis of these different models of apoE function and suggest that these seemingly opposing views can be reconciled. The optimal therapeutic target may be to inhibit the interaction of apoE with Aβ rather than altering apoE levels. Such an approach will not have detrimental effects on the many beneficial roles apoE plays in neurobiology. Furthermore, other Aβ binding proteins, including ACT and apo J can inhibit or promote Aβ oligomerization/polymerization depending on conditions and might be manipulated to effect AD treatment.

Peptides for therapy and diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with devastating effects. The greatest risk factor to develop AD is age. Today, only symptomatic therapies are available. Additionally, AD can be diagnosed with certainty only post mortem, whereas the diagnosis "probable AD" can be established earliest when severe clinical symptoms appear. Specific neuropathological changes like neurofibrillary tangles and amyloid plaques define AD. Amyloid plaques are mainly composed of the amyloid-βpeptide (Aβ). Several lines of evidence suggest that the progressive concentration and subsequent aggregation and accumulation of Aβ play a fundamental role in the disease progress. Therefore, substances which bind to Aβ and influence aggregation thereof are of great interest. An enormous number of organic substances for therapeutic purposes are described. This review focuses on peptides developed for diagnosis and therapy of AD and discusses the pre- and disadvantages of peptide drugs.

Preferential interactions between ApoE-containing lipoproteins and Aβ revealed by a detection method that combines size exclusion chromatography with non-reducing gel-shift

The association between apolipoprotein E (apoE) and amyloid-β peptide (Aβ) may significantly impact the function of both proteins, thus affecting the etiology of Alzheimer's disease (AD). However, apoE/Aβ interactions remain fundamentally defined by the stringency of the detection method. Here we use size exclusion chromatography (SEC) as a non-stringent approach to the detection of apoE/Aβ interactions in solution, specifically apoE and both endogenous and exogenous Aβ from plasma, CSF and astrocyte conditioned media. By SEC analysis, Aβ association with plasma and CNS lipoproteins is apoE-dependent. While endogenous Aβ elutes to specific human plasma lipoproteins distinct from those containing apoE, it is the apoE-containing lipoproteins that absorb excess amounts of exogenous Aβ40. In human CSF, apoE, endogenous Aβ and phospholipid elute in an almost identical profile, as do apoE, exogenous Aβ and phospholipid from astrocyte conditioned media. Combining SEC fractionation with subsequent analysis for SDS-stable apoE/Aβ complex reveals that apoE-containing astrocyte lipoproteins exhibit the most robust interactions with Aβ. Thus, standardization of the methods for detecting apoE/Aβ complex is necessary to determine its functional significance in the neuropathology characteristic of AD. Importantly, a systematic understanding of the role of apoE-containing plasma and CNS lipoproteins in Aβ homeostasis could potentially contribute to identifying a plasma biomarker currently over-looked because it has multiple components.

Interactions of pathological hallmark proteins: tubulin polymerization promoting protein/p25, beta-amyloid, and alpha-synuclein

The disordered tubulin polymerization promoting protein (TPPP/p25) was found to be co-enriched in neuronal and glial inclusions with α-synuclein in Parkinson disease and multiple system atrophy, respectively; however, co-occurrence of α-synuclein with β-amyloid (Aβ) in human brain inclusions has been recently reported, suggesting the existence of mixed type pathologies that could result in obstacles in the correct diagnosis and treatment. Here we identified TPPP/p25 as an interacting partner of the soluble Aβ oligomers as major risk factors for Alzheimer disease using ProtoArray human protein microarray. The interactions of oligomeric Aβ with proteins involved in the etiology of neurological disorders were characterized by ELISA, surface plasmon resonance, pelleting experiments, and tubulin polymerization assay. We showed that the Aβ(42) tightly bound to TPPP/p25 (K(d) = 85 nm) and caused aberrant protein aggregation by inhibiting the physiologically relevant TPPP/p25-derived microtubule assembly. The pair-wise interactions of Aβ(42), α-synuclein, and tubulin were found to be relatively weak; however, these three components formed soluble ternary complex exclusively in the absence of TPPP/p25. The aggregation-facilitating activity of TPPP/p25 and its interaction with Aβ was monitored by electron microscopy with purified proteins by pelleting experiments with cell-free extracts as well as by confocal microscopy with CHO cells expressing TPPP/p25 or amyloid. The finding that the interaction of TPPP/p25 with Aβ can produce pathological-like aggregates is tightly coupled with unusual pathology of the Alzheimer disease revealed previously; that is, partial co-localization of Aβ and TPPP/p25 in the case of diffuse Lewy body disease with Alzheimer disease.

Blocking the apolipoprotein E/amyloid-β interaction reduces fibrillar vascular amyloid deposition and cerebral microhemorrhages in TgSwDI mice

The accumulation of amyloid-β (Aβ) peptides as toxic oligomers, amyloid plaques, and cerebral amyloid angiopathy (CAA) is critical in the pathogenesis of Alzheimer's disease (AD). The binding of Aβ peptides to apolipoprotein E (ApoE) plays an important role in modulation of amyloid deposition and clearance. We have shown that blocking the Aβ/ApoE interaction with Aβ(12-28P), a nontoxic blood-brain-barrier permeable and non-fibrillogenic synthetic peptide, constitutes a novel therapeutic approach for AD by reducing Aβ parenchymal deposition. In the present study, we investigate this therapeutic effect on CAA in the transgenic (Tg) AD mice model (TgSwDI), which expresses Swedish (K670N/M671L), Dutch (E693Q)/Iowa (D694N) AβPP mutations. These mice develop abundant CAA beginning at the age of 6 months. Behavioral results show that Aβ(12-28P) treated TgSwDI AD mice performed the same as wild-type mice, whereas vehicle treated TgSwDI were impaired in spatial memory. Furthermore, this treatment resulted in a significant reduction of total amyloid burden, especially the fibrillar vascular amyloid burden, which importantly was accompanied by a reduction in microhemorrhages and neuroinflammation. Measurement of Aβ levels in the brain homogenate revealed a significant decrease in both the total amount of Aβ and Aβ oligomer levels in Aβ(12-28P) treated TgSwDI mice. These findings suggest that blocking the Aβ/ApoE interaction is a highly effective therapeutic approach for vascular amyloid deposition, in contrast to some other therapeutic approaches.

Detection of amyloid plaques targeted by USPIO-Aβ1-42 in Alzheimer's disease transgenic mice using magnetic resonance microimaging

Amyloid plaques are one of the pathological hallmarks of Alzheimer's disease (AD). The visualization of amyloid plaques in the brain is important to monitor AD progression and to evaluate the efficacy of therapeutic interventions. Our group has developed several contrast agents to detect amyloid plaques in vivo using magnetic resonance microimaging (μMRI) in AD transgenic mice, where we used intra-carotid mannitol to enhance blood-brain barrier (BBB) permeability. In the present study, we used ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles, chemically coupled with Aβ1-42 peptide to detect amyloid deposition along with mannitol for in vivo μMRI by femoral intravenous injection. A 3D gradient multi-echo sequence was used for imaging with a 100μm isotropic resolution. The amyloid plaques detected by T2*-weighted μMRI were confirmed with matched histological sections. Furthermore, two different quantitative analyses were used. The region of interest-based quantitative measurement of T2* values showed contrast-injected APP/PS1 mice had significantly reduced T2* values compared to wild-type mice. In addition, the scans were examined with voxel-based morphometry (VBM) using statistical parametric mapping (SPM) for comparison of contrast-injected AD transgenic and wild-type mice. The regional differences seen in VBM comparing USPIO-Aβ1-42 injected APP/PS1 and wild-type mice correlated with the amyloid plaque distribution histologically, contrasting with no differences between the two groups of mice without contrast agent injection in regions of the brain with amyloid deposition. Our results demonstrated that both approaches were able to identify the differences between AD transgenic mice and wild-type mice, after injected with USPIO-Aβ1-42. The feasibility of using less invasive intravenous femoral injections for amyloid plaque detection in AD transgenic mice facilitates using this method for longitudinal studies in the pathogenesis of AD.

An apolipoprotein E4 fragment can promote intracellular accumulation of amyloid peptide beta 42

Apolipoprotein E (apoE) plays a crucial role in lipid transport in circulation and the brain. The apoE4 isoform is a major risk factor for Alzheimer's disease (AD). ApoE4 is more susceptible to proteolysis than other apoE isoforms and apoE4 fragments have been found in brains of AD patients. These apoE4 fragments have been hypothesized to be involved in the pathogenesis of AD, although the mechanism is not clear. In this study we examined the effect of lipid-free apoE4 on amyloid precursor protein processing and 40-amino-acid Aβ variant and 42-amino-acid Aβ variant levels in human neuroblastoma SK-N-SH cells. We discovered that a specific apoE4 fragment, apoE4[Δ(166-299)], can promote the cellular uptake of extracellular 40-amino-acid Aβ variant and 42-amino-acid Aβ variant either generated after amyloid precursor protein transfection or added exogenously. A longer length fragment, apoE4[Δ(186-299)], or full-length apoE4 failed to elicit this effect. ApoE4[Δ(166-299)] effected a 20% reduction of cellular sphingomyelin levels, as well as changes in cellular membrane micro-fluidity. Following uptake, approximately 50% of 42-amino-acid Aβ variant remained within the cell for at least 24 h, and led to increased formation of reactive oxygen species. Overall, our findings suggest a direct link between two early events in the pathogenesis of AD, apoE4 proteolysis and intraneuronal presence of amyloid beta peptide.

Abeta20-29 peptide blocking apoE/Abeta interaction reduces full-length Abeta42/40 fibril formation and cytotoxicity in vitro

A key event in the pathogenesis of Alzheimer's disease (AD) is the conversion of the peptide beta-amyloid (Abeta) from its soluble monomeric form into various aggregated morphologies in the brain. Apolipoprotein E (apoE) is known to act as a pathological chaperone of Abeta in this process, promoting its fibril formation from soluble Abeta by binding interaction between carboxy-terminal domain of apoE and residues 12-28 of full-length Abeta. Therefore, blocking apoE/Abeta interaction is being actively pursued as a primary therapeutic strategy for AD. Abeta20-29, a short peptide, contains the residues to competitively bind to apoE and may potentially block the interaction between apoE and full-length Abeta. However, little is known whether Abeta20-29 could block apoE/Abeta interaction to play an effective role in reducing full-length Abeta fibrillization and cytotoxicity. Utilizing fluorescence spectroscopic analysis with thioflavin T and electron microscopic study, we show here that Abeta20-29 alone was non-fibrillogenic, and had no direct effects on Abeta1-42 or Abeta1-40 aggregation. Moreover, apoE can directly promote both Abeta1-42 and Abeta1-40 aggregation and fibril formation, while this promoting effect was inhibited when adding Abeta20-29, with a dose-dependent manner. In the series of cell culture experiments, Abeta20-29 alone shows no cytotoxicity to PC12 cells as demonstrated by MTT assay, while co-incubation apoE isoforms and Abeta1-42 or Abeta1-40 shows stronger cytotoxicity as compared to Abeta1-42 or Abeta1-40 alone. When incubated with Abeta20-29, whereas such strong cytotoxic effect was concentration-dependently reduced. Taken together, we demonstrate for the first time that Abeta20-29 has no direct effect on full-length Abeta aggregation, and may competitively block the binding of full-length Abeta to apoE, resulting in an inhibitory effect on apoE's promoting full-length Abeta fibrillogenesis and Abeta-induced cytotoxicity. Our results raise the possibility that Abeta20-29 peptide blocking the interaction between full-length Abeta and apoE isoforms may be effective as a therapeutic agent for AD.

Pyrene fluorescence analysis offers new insights into the conformation of the lipoprotein-binding domain of human apolipoprotein E

The C-terminal domain (CT) of apolipoprotein E (apoE), a critical protein involved in cholesterol transport in the plasma and brain, plays an important role in high-affinity lipoprotein binding. Although high-resolution structural information is available for the N-terminal domain of apoE, the structural organization of the CT (residues 201-299) is largely unknown. In this study, we employ site-specific fluorescence labeling with pyrene maleimide to gain insight into the structure and conformation of apoE CT in its naturally self-associated state in buffer at physiologically relevant concentrations (5-50 microg/mL). Pyrene is a highly sensitive fluorophore that reports on spatial proximity between desired sites by displaying unique spectral features. Pyrene was covalently attached to single cysteine-containing recombinant human apoE CT at position 223 or 255 to probe the first predicted helical segment and at position 277 to monitor the terminal predicted helical segment. Regardless of the location of the probe, all three pyrene-labeled apoE CT variants display an intense and dramatic fluorescence excimer band at 460 nm, a signature feature of pyrene, which indicates that two pyrene moieties are within 10 A of each other. In addition, an intense peak at 387 nm (indicative of a highly hydrophobic environment) was noted in all cases. Fluorescence emission quenching by potassium iodide indicates that the accessibility to the probes was restricted at these locations. The possibility that the hydrophobicity of the pyrene moiety was the driving force for helix-helix interaction was excluded because pyrene located at position 209, which is predicted to be located in a nonhelical segment, did not display the above intense unique features. Lastly, denaturation studies suggest that the terminal helix unfolds prior to the first predicted helix in apoE CT. Our studies indicate that there are extensive intermolecular helix-helix contacts throughout the entire CT in the lipid-free state with two apoE CT molecules oriented parallel to each other to form a dimer, which dimerizes further to yield a tetramer. Such an organization allows helix-helix interactions to be replaced by helix-lipid interactions upon encountering a lipoprotein surface, with the terminal helix likely initiating the binding interaction. This study presents the possibility of employing pyrene fluorophores as powerful new alternatives to obtain conformational information of proteins at physiologically relevant concentrations.

Reversal of temperature-induced conformational changes in the amyloid-beta peptide, Abeta40, by the beta-sheet breaker peptides 16-23 and 17-24

BACKGROUND AND PURPOSE

Aggregates of the protein amyloid-beta (Abeta) play a crucial role in the pathogenesis of Alzheimer's disease (AD). Most therapeutic approaches to AD do not target Abeta, so determination of the factor(s) that facilitate aggregation and discovering agents that prevent aggregation have great potential therapeutic value.

EXPERIMENTAL APPROACH

We investigated ex vivo the temperature-sensitive regions of Abeta1-40 (Abeta40) and their interactions with octapeptides derived from sequences within Abeta40 -beta-sheet breaker peptides (betaSBP) - using enzyme-linked immunosorbent assay, and dot blot and far-UV circular dichroism (CD) spectroscopy. We measured changes within the physiological limits of temperature, using antibodies targeting epitopes 1-7, 5-10, 9-14 and 17-21 within Abeta40.

KEY RESULTS

Temperature-dependent conformational changes were observed in Abeta40 at epitopes 9-14 and 17-21 at 36-38 and 36-40 degrees C respectively. The betaSBPs 16-23 and 17-24, but not 15-22 and 18-25, could inhibit the changes. Moreover, betaSBPs 16-23 and 17-24 increased digestion of Abeta40 by protease K, indicating a decreased aggregation of Abeta40, whereas betaSBPs 15-22 and 18-25 did not increase this digestion. CD spectra revealed that beta-sheet formation in Abeta40 at 38 degrees C was reduced with betaSBPs 16-23 and 17-24.

CONCLUSIONS AND IMPLICATIONS

The epitopes 9-14 and 17-21 are the temperature-sensitive regions within Abeta40. The betaSBPs, Abeta16-23 and 17-24 reversed temperature-induced beta-sheet formation, and decreased Abeta40 aggregation. The results suggest that the 17-23 epitope of Abeta40 is crucially involved in preventing Abeta40 aggregation and consequent deposition of Abeta40 in AD brain.

Preventing beta-amyloid fibrillization and deposition: beta-sheet breakers and pathological chaperone inhibitors

Central to the pathogenesis of Alzheimer's disease (AD) is the conversion of normal, soluble beta-amyloid (sAbeta) to oligomeric, fibrillar Abeta. This process of conformational conversion can be influenced by interactions with other proteins that can stabilize the disease-associated state; these proteins have been termed 'pathological chaperones'. In a number of AD models, intervention that block soluble Abeta aggregation, including beta-sheet breakers, and compounds that block interactions with pathological chaperones, have been shown to be highly effective. When combined with early pathology detection, these therapeutic strategies hold great promise as effective and relatively toxicity free methods of preventing AD related pathology.

Interaction with amyloid beta peptide compromises the lipid binding function of apolipoprotein E

Apolipoprotein (apo) E is an exchangeable apolipoprotein that plays an integral role in cholesterol transport in the plasma and the brain. It is also associated with protein misfolding or amyloid proteopathy of the beta amyloid peptide (Abeta) in Alzheimer's disease (AD) and cerebral amyloid angiopathy. The C-terminal domain (CT) of apoE encompasses two types of amphipathic alpha helices: a class A helix (residues 216-266) and a class G* helix (residues 273-299). This domain also harbors high-affinity lipoprotein binding and apoE self-association sites that possibly overlap. The objective of this study is to examine if the neurotoxic oligomeric Abeta interacts with apoE CT and if this association affects the lipoprotein binding function of recombinant human apoE CT. Site-specific fluorescence labeling of single cysteine-containing apoE CT variants with donor probes were employed to identify the binding of Abeta bearing an acceptor probe by intermolecular fluorescence resonance energy-transfer analysis. A higher efficiency of energy transfer was noted with probes located in the class A helix than with those located in the class G* helix of apoE CT. In addition, incubation of apoE CT with Abeta severely impaired the lipid binding ability and the overall amount of lipid-associated apoE CT. However, when apoE CT is present in a lipid-bound state, Abeta appears to be localized within the lipid milieu of the lipoprotein particle and not associated with any specific segments of the protein. When our data are taken together, they suggest that Abeta association compromises the fundamental lipoprotein binding function of apoE, which may have implications not only in terms of amyloid buildup but also in terms of the accumulation of cholesterol at extracellular sites.

Prevention and promotion effects of apolipoprotein E4 on amylin aggregation

The misfolding of islet amyloid polypeptide (IAPP, amylin) results in the formation of islet amyloid, which is one of the most common pathological features of type 2 diabetes (T2D). Amylin, a 37-amino-acid peptide co-secreted with insulin and apolipoprotein E (ApoE) from the beta-cells of pancreatic islets, is thought to be responsible for the reduced mass of insulin-producing beta-cells. However, neither the relationship between amylin and ApoE nor the biological consequence of amylin misfolding is known. Here we have characterized the interaction between ApoE4 and amylin in vitro. We found that ApoE4 can strongly bind to amylin, and insulin can hardly inhibit amylin-ApoE binding. We further found that amylin fibrillization can be prevented by low concentration of ApoE4 and promoted by high concentration of ApoE4. Taken together, we propose that under physiological conditions ApoE4 efficiently binds and sequesters amylin, preventing its aggregation, and in T2D the enhanced ApoE4-amylin binding leads to the critical accumulation of amylin, facilitating islet amyloid formation.

Therapeutic approaches for prion and Alzheimer's diseases

Alzheimer's and prion diseases belong to a category of conformational neurodegenerative disorders [Prusiner SB (2001) N Eng J Med344, 1516-1526; Sadowski M & Wisniewski T (2007) Curr Pharm Des 13, 1943-1954; Beekes M (2007) FEBS J 274, 575]. Treatments capable of arresting or at least effectively modifying the course of disease do not yet exist for either one of these diseases. Alzheimer's disease is the major cause of dementia in the elderly and has become an ever greater problem with the aging of Western societies. Unlike Alzheimer's disease, prion diseases are relatively rare. Each year only approximately 300 people in the USA and approximately 100 people in the UK succumb to various forms of prion diseases [Beekes M (2007) FEBS J 274, 575; Sigurdsson EM & Wisniewski T (2005) Exp Rev Vaccines 4, 607-610]. Nevertheless, these disorders have received great scientific and public interest due to the fact that they can be transmissible among humans and in certain conditions from animals to humans. The emergence of variant Creutzfeld-Jakob disease demonstrated the transmissibility of the bovine spongiform encephalopathy to humans [Beekes M (2007) FEBS J 274, 575]. Therefore, the spread of bovine spongiform encephalopathy across Europe and the recently identified cases in North America have put a large human population at risk of prion infection. It is estimated that at least several thousand Britons are asymptomatic carriers of prion infections and may develop variant Creutzfeld-Jakob disease in the future [Hilton DA (2006) J Pathol 208, 134-141]. This delayed emergence of human cases following the near elimination of bovine spongiform encephalopathy in the UK may occur because prion disease have a very prolonged incubation period, ranging from months to decades, which depends on the amount of inoculum, the route of infection and the genetic predisposition of the infected subject [Hilton DA (2006) J Pathol 208, 134-141]. Therefore, there is a great need for effective therapies for both Alzheimer's disease and prion diseases.

Lipocalin-type prostaglandin D synthase/beta-trace is a major amyloid beta-chaperone in human cerebrospinal fluid

The conformational change in amyloid beta (Abeta) peptide from its monomeric form to aggregates is crucial in the pathogenesis of Alzheimer's disease (AD). In the healthy brain, some unidentified chaperones appear to prevent the aggregation of Abeta. Here we reported that lipocalin-type prostaglandin D synthase (L-PGDS)/beta-trace, the most abundant cerebrospinal fluid (CSF) protein produced in the brain, was localized in amyloid plaques in both AD patients and AD-model Tg2576 mice. Surface plasmon resonance analysis revealed that L-PGDS/beta-trace tightly bound to Abeta monomers and fibrils with high affinity (K(D) = 18-50 nM) and that L-PGDS/beta-trace recognized residues 25-28 in Abeta, which is the key region for its conformational change to a beta-sheet structure. The results of a thioflavin T fluorescence assay to monitor Abeta aggregation disclosed that L-PGDS/beta-trace inhibited the spontaneous aggregation of Abeta (1-40) and Abeta (1-42) within its physiological range (1-5 microM) in CSF. L-PGDS/beta-trace also prevented the seed-dependent aggregation of 50 microM Abeta with K(i) of 0.75 microM. Moreover, the inhibitory activity toward Abeta (1-40) aggregation in human CSF was decreased by 60% when L-PGDS/beta-trace was removed from the CSF by immunoaffinity chromatography. The deposition of Abeta after intraventricular infusion of Abeta (1-42) was 3.5-fold higher in L-PGDS-deficient mice and reduced to 23% in L-PGDS-overexpressing mice as compared with their wild-type levels. These data indicate that L-PGDS/beta-trace is a major endogenous Abeta-chaperone in the brain and suggest that the disturbance of this function may be involved in the onset and progression of AD. Our findings may provide a diagnostic and therapeutic approach for AD.