APOE4-specific changes in Aβ accumulation in a new transgenic mouse model of Alzheimer disease

Synergistic effects of APOE and sex on the gut microbiome of young EFAD transgenic mice

Background

Alzheimer’s disease (AD) is a fatal neurodegenerative disease. APOE4 is the greatest genetic risk factor for AD, increasing risk up to 15-fold compared to the common APOE3. Importantly, female (♀) APOE4 carriers have a greater risk for developing AD and an increased rate of cognitive decline compared to male (♂) APOE4 carriers. While recent evidence demonstrates that AD, APOE genotype, and sex affect the gut microbiome (GM), how APOE genotype and sex interact to affect the GM in AD remains unknown.

Methods

This study analyzes the GM of 4-month (4 M) ♂ and ♀ E3FAD and E4FAD mice, transgenic mice that overproduce amyloid-β 42 (Aβ42) and express human APOE3^+/+ or APOE4^+/+. Fecal microbiotas were analyzed using high-throughput sequencing of 16S ribosomal RNA gene amplicons and clustered into operational taxonomic units (OTU). Microbial diversity of the EFAD GM was compared across APOE, sex and stratified by APOE + sex, resulting in 4-cohorts (♂E3FAD, ♀E3FAD, ♂E4FAD and ♀E4FAD). Permutational multivariate analysis of variance (PERMANOVA) evaluated differences in bacterial communities between cohorts and the effects of APOE + sex. Mann-Whitney tests and machine-learning algorithms identified differentially abundant taxa associated with APOE + sex.

Results

Significant differences in the EFAD GM were associated with APOE genotype and sex. Stratification by APOE + sex revealed that APOE-associated differences were exhibited in ♂EFAD and ♀EFAD mice, and sex-associated differences were exhibited in E3FAD and E4FAD mice. Specifically, the relative abundance of bacteria from the genera Prevotella and Ruminococcus was significantly higher in ♀E4FAD compared to ♀E3FAD, while the relative abundance of Sutterella was significantly higher in ♂E4FAD compared to ♂E3FAD. Based on 29 OTUs identified by the machine-learning algorithms, heatmap analysis revealed significant clustering of ♀E4FAD separate from other cohorts.

Conclusions

The results demonstrate that the 4 M EFAD GM is modulated by APOE + sex. Importantly, the effect of APOE4 on the EFAD GM is modulated by sex, a pattern similar to the greater AD pathology associated with ♀E4FAD. While this study demonstrates the importance of interactive effects of APOE + sex on the GM in young AD transgenic mice, changes associated with the development of pathology remain to be defined.

Evaluation of B0-correction of relative CBF maps using tagging distance dependent Z-spectrum (TADDZ)

Arterial spin labeling (ASL) MRI, based on endogenous contrast from blood water, is used in research and diagnosis of cerebral vascular conditions. However, artifacts due to imperfect imaging conditions such as B₀-inhomogeneity (ΔB₀) could lead to variations in the quantification of relative cerebral blood flow (CBF). In this study, we evaluate a new approach using tagging distance dependent Z-spectrum (TADDZ) data, similar to the ΔB₀ corrections in the chemical exchange saturation transfer (CEST) experiments, to remove the imaging plane B₀ inhomogeneity induced CBF artifacts in ASL MRI. Our results indicate that imaging-plane B₀-inhomogeneity can lead to variations and errors in the relative CBF maps especially under small tagging distances. Along with an acquired B₀ map, TADDZ data helps to eliminate B₀-inhomogeneity induced artifacts in the resulting relative CBF maps. We demonstrated the effective use of TADDZ data to reduce variation while subjected to systematic changes in ΔB₀. In addition, TADDZ corrected ASL MRI, with improved consistency, was shown to outperform conventional ASL MRI by differentiating the subtle CBF difference in Alzheimer's disease (AD) mice brains with different APOE genotypes.

ApoE4 Alters ABCA1 Membrane Trafficking in Astrocytes

The APOE ε4 allele is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE protein aggregation plays a central role in AD pathology, including the accumulation of β-amyloid (Aβ). Lipid-poor ApoE4 protein is prone to aggregate and lipidating ApoE4 protects it from aggregation. The mechanisms regulating ApoE4 aggregation in vivo are surprisingly not known. ApoE lipidation is controlled by the activity of the ATP binding cassette A1 (ABCA1). ABCA1 recycling and degradation is regulated by ADP-ribosylation factor 6 (ARF6). We found that ApoE4 promoted greater expression of ARF6 compared with ApoE3, trapping ABCA1 in late-endosomes and impairing its recycling to the cell membrane. This was associated with lower ABCA1-mediated cholesterol efflux activity, a greater percentage of lipid-free ApoE particles, and lower Aβ degradation capacity. Human CSF from APOE ε4/ε4 carriers showed a lower ability to induce ABCA1-mediated cholesterol efflux activity and greater percentage of aggregated ApoE protein compared with CSF from APOE ε3/ε3 carriers. Enhancing ABCA1 activity rescued impaired Aβ degradation in ApoE4-treated cells and reduced both ApoE and ABCA1 aggregation in the hippocampus of male ApoE4-targeted replacement mice. Together, our data demonstrate that aggregated and lipid-poor ApoE4 increases ABCA1 aggregation and decreases ABCA1 cell membrane recycling. Enhancing ABCA1 activity to reduce ApoE and ABCA1 aggregation is a potential therapeutic strategy for the prevention of ApoE4 aggregation-driven pathology.SIGNIFICANCE STATEMENT ApoE protein plays a key role in the formation of amyloid plaques, a hallmark of Alzheimer's disease (AD). ApoE4 is more aggregated and hypolipidated compared with ApoE3, but whether enhancing ApoE lipidation in vivo can reverse ApoE aggregation is not known. ApoE lipidation is controlled by the activity of the ATP binding cassette A1 (ABCA1). In this study, we demonstrated that the greater propensity of lipid-poor ApoE4 to aggregate decreased ABCA1 membrane recycling and its ability to lipidate ApoE. Importantly, enhancing ABCA1 activity to lipidate ApoE reduced ApoE and ABCA1 aggregation. This work provides critical insights into the interactions among ABCA1, ApoE lipidation and aggregation, and underscores the promise of stabilizing ABCA1 activity to prevent ApoE-driven aggregation pathology.

Lack of hepatic apoE does not influence early Aβ deposition: observations from a new APOE knock-in model

Background

The apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer disease (AD). ApoE is produced by both astrocytes and microglia in the brain, whereas hepatocytes produce the majority of apoE found in the periphery. Studies using APOE knock-in and transgenic mice have demonstrated a strong isoform-dependent effect of apoE on the accumulation of amyloid-β (Aβ) deposition in the brain in the form of both Aβ-containing amyloid plaques and cerebral amyloid angiopathy. However, the specific contributions of different apoE pools to AD pathogenesis remain unknown.

Methods

We have begun to address these questions by generating new lines of APOE knock-in (APOE-KI) mice (ε2/ε2, ε3/ε3, and ε4/ε4) where the exons in the coding region of APOE are flanked by loxP sites, allowing for cell type-specific manipulation of gene expression. We assessed these mice both alone and after crossing them with mice with amyloid deposition in the brain. Using biochemical and histological methods. We also investigated how removal of APOE expression from hepatocytes affected cerebral amyloid deposition.

Results

As in other APOE knock-in mice, apoE protein was present predominantly in astrocytes in the brain under basal conditions and was also detected in reactive microglia surrounding amyloid plaques. Primary cultured astrocytes and microglia from the APOE-KI mice secreted apoE in lipoprotein particles of distinct size distribution upon native gel analysis with microglial particles being substantially smaller than the HDL-like particles secreted by astrocytes. Crossing of APP/PS1 transgenic mice to the different APOE-KI mice recapitulated the previously described isoform-specific effect (ε4 > ε3) on amyloid plaque and Aβ accumulation. Deletion of APOE in hepatocytes did not alter brain apoE levels but did lead to a marked decrease in plasma apoE levels and changes in plasma lipid profile. Despite these changes in peripheral apoE and on plasma lipids, cerebral accumulation of amyloid plaques in APP/PS1 mice was not affected.

Conclusions

Altogether, these new knock-in strains offer a novel and dynamic tool to study the role of APOE in AD pathogenesis in a spatially and temporally controlled manner.

Electronic supplementary material

The online version of this article (10.1186/s13024-019-0337-1) contains supplementary material, which is available to authorized users.

Induced Pluripotent Stem Cell-Derived Astroglia: A New Tool for Research Towards the Treatment of Alzheimer's Disease

Despite over a century of research into Alzheimer's disease (AD), progress in understanding the complex aetiology has been hindered, in part, by a lack of human, disease relevant, cellular models, reflected in an inability to translate results from animal studies to successful human therapies. Induced pluripotent stem cell (iPSC) technology, in which somatic cells are reprogrammed to pluripotent stem cells, creates an ideal physiologically relevant model as they maintain the genetic identity of the donor. These iPSCs can self-renew indefinitely in vitro and have the capacity to differentiate into any cell type, opening up new discovery and therapeutic opportunities. Despite a plethora of publications indicating the generation and utility of iPSC-derived neurones for disease modelling to date, in comparison only a limited number of studies have described generation of enriched astroglia from patients with early- or late-stage onset of AD. We recently reported that iPSC-astroglia derived from these patients are capable of mimicking a wide variety of deficits in homeostatic molecular cascades, intimately associated with AD, that are routinely observed in vivo. This review examines the opportunities and limitations of this innovative technology in the context of AD modelling and uses for preclinical discovery to improve our success for an efficacious therapeutic outcome.

Dietary inulin alters the gut microbiome, enhances systemic metabolism and reduces neuroinflammation in an APOE4 mouse model

The apolipoprotein ε4 allele (APOE4) is the strongest genetic risk factor for Alzheimer's disease (AD). APOE4 carriers develop systemic metabolic dysfunction decades before showing AD symptoms. Accumulating evidence shows that the metabolic dysfunction accelerates AD development, including exacerbated amyloid-beta (Aβ) retention, neuroinflammation and cognitive decline. Therefore, preserving metabolic function early on may be critical to reducing the risk for AD. Here, we show that inulin increases beneficial microbiota and decreases harmful microbiota in the feces of young, asymptomatic APOE4 transgenic (E4FAD) mice and enhances metabolism in the cecum, periphery and brain, as demonstrated by increases in the levels of SCFAs, tryptophan-derived metabolites, bile acids, glycolytic metabolites and scyllo-inositol. We show that inulin also reduces inflammatory gene expression in the hippocampus. This knowledge can be utilized to design early precision nutrition intervention strategies that use a prebiotic diet to enhance systemic metabolism and may be useful for reducing AD risk in asymptomatic APOE4 carriers.

The role of APOE in transgenic mouse models of AD

Identified in 1993, APOE4 is the greatest genetic risk factor for Alzheimer's disease (AD), increasing risk up to 15-fold compared to the common variant APOE3. Since the mid 1990's, transgenic (Tg) mice have been developed to model AD pathology and progression, primarily via expression of the familial AD (FAD) mutations in the presence of mouse-APOE (m-APOE). APOE4, associated with enhanced amyloid-β (Aβ) accumulation, has rarely been the focus in designing FAD-Tg mouse models. Initially, FAD-Tg mice were crossed with human (h)-APOE driven by heterologous promoters to identify an APOE genotype-specific AD phenotype. These models were later supplemented with FAD-Tg mice crossed with APOE-knockouts (APOE-/- or APOE-KO) and h-APOE-targeted replacement (h-APOE-TR) mice, originally generated to study the role of APOE genotype in peripheral lipid metabolism and atherosclerotic lesion development. Herein, we compare the m- and h-APOE multi-gene clusters, and then critically review the relevant history and approaches to developing a Tg mouse model to characterize APOE-dependent AD pathology, in combination with genetic (sex, age) and modifiable (e.g., inflammation, obesity) risk factors. Finally, we present recent data from the EFAD mice, which express 5xFAD mutations with the expression of the human apoE isoforms (E2FAD, E3FAD and E4FAD). This includes a study of 6- and 18-month-old male and female E3FAD and E4FAD, a comparison that enables examination of the interaction among the main AD risk factors: age, APOE genotype and sex. While no single transgenic mouse can capture the effects of all modifiable and genetic risk factors, going forward, a conscious effort needs to be made to include the factors that most significantly modulate AD pathology.

Apolipoprotein E and Alzheimer disease: pathobiology and targeting strategies

Polymorphism in the apolipoprotein E (APOE) gene is a major genetic risk determinant of late-onset Alzheimer disease (AD), with the APOE*ε4 allele conferring an increased risk and the APOE*ε2 allele conferring a decreased risk relative to the common APOE*ε3 allele. Strong evidence from clinical and basic research suggests that a major pathway by which APOE4 increases the risk of AD is by driving earlier and more abundant amyloid pathology in the brains of APOE*ε4 carriers. The number of amyloid-β (Aβ)-dependent and Aβ-independent pathways that are known to be differentially modulated by APOE isoforms is increasing. For example, evidence is accumulating that APOE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. In addition, APOE4 is either pathogenic or shows reduced efficiency in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. Here, we review the recent progress in clinical and basic research into the role of APOE in AD pathogenesis. We also discuss how APOE can be targeted for AD therapy using a precision medicine approach.

Curcumin restores innate immune Alzheimer's disease risk gene expression to ameliorate Alzheimer pathogenesis

Alzheimer's disease (AD) genetics implies a causal role for innate immune genes, TREM2 and CD33, products that oppose each other in the downstream Syk tyrosine kinase pathway, activating microglial phagocytosis of amyloid (Aβ). We report effects of low (Curc-lo) and high (Curc-hi) doses of curcumin on neuroinflammation in APPsw transgenic mice. Results showed that Curc-lo decreased CD33 and increased TREM2 expression (predicted to decrease AD risk) and also increased TyroBP, which controls a neuroinflammatory gene network implicated in AD as well as phagocytosis markers CD68 and Arg1. Curc-lo coordinately restored tightly correlated relationships between these genes' expression levels, and decreased expression of genes characteristic of toxic pro-inflammatory M1 microglia (CD11b, iNOS, COX-2, IL1β). In contrast, very high dose curcumin did not show these effects, failed to clear amyloid plaques, and dysregulated gene expression relationships. Curc-lo stimulated microglial migration to and phagocytosis of amyloid plaques both in vivo and in ex vivo assays of sections of human AD brain and of mouse brain. Curcumin also reduced levels of miR-155, a micro-RNA reported to drive a neurodegenerative microglial phenotype. In conditions without amyloid (human microglial cells in vitro, aged wild-type mice), Curc-lo similarly decreased CD33 and increased TREM2. Like curcumin, anti-Aβ antibody (also reported to engage the Syk pathway, increase CD68, and decrease amyloid burden in human and mouse brain) increased TREM2 in APPsw mice and decreased amyloid in human AD sections ex vivo. We conclude that curcumin is an immunomodulatory treatment capable of emulating anti-Aβ vaccine in stimulating phagocytic clearance of amyloid by reducing CD33 and increasing TREM2 and TyroBP, while restoring neuroinflammatory networks implicated in neurodegenerative diseases.

Influence of Matrix Metallopeptidase 9 on Beta-Amyloid Elimination Across the Blood-Brain Barrier

Lipoprotein receptor transport across the blood-brain barrier (BBB) mediates beta-amyloid (Aβ) accumulation in the brain and may be a contributing factor in Alzheimer's disease (AD) pathogenesis. Lipoprotein receptors are susceptible to proteolytic shedding at the cell surface, which precludes the endocytic transport of ligands. A ligand that closely interacts with the lipoprotein receptors is apolipoprotein E (apoE), which exists as three isoforms (apoE2, apoE3, apoE4). Our prior work showed an inverse relationship between lipoprotein receptor shedding and Aβ transport across the BBB, which was apoE-isoform dependent. To interrogate this further, the current studies investigated an enzyme implicated in lipoprotein receptor shedding, matrix metalloproteinase 9 (MMP9). Treatment with MMP9 dose-dependently elevated lipoprotein receptor shedding in brain endothelial cells and freshly isolated mouse cerebrovessels. Furthermore, treatment with a MMP9 inhibitor (SB-3CT) mitigated Aβ-induced lipoprotein receptor shedding in brain endothelial cells and the brains of apoE4 animals. In terms of BBB transit, SB-3CT treatment increased the transport of Aβ across an in vitro model of the BBB. In vivo, administration of SB-3CT to apoE4 animals significantly enhanced Aβ clearance from the brain to the periphery following intracranial administration of Aβ. The current studies show that MMP9 impacts lipoprotein receptor shedding and Aβ transit across the BBB, in an apoE  isoform-specific manner. In total, MMP9 inhibition can facilitate Aβ clearance across the BBB, which could be an effective approach to lowering Aβ levels in the brain and mitigating the AD phenotype, particularly in subjects carrying the apoE4 allele.

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.

Potential Novel Approaches to Understand the Pathogenesis and Treat Alzheimer's Disease

There is growing genetic and proteomic data highlighting the complexity of Alzheimer's disease (AD) pathogenesis. Greater use of unbiased "omics" approaches is being increasingly recognized as essential for the future development of effective AD research, that need to better reflect the multiple distinct pathway abnormalities that can drive AD pathology. The track record of success in AD clinical trials thus far has been very poor. In part, this high failure rate has been related to the premature translation of highly successful results in animal models that mirror only limited aspects of AD pathology to humans. We highlight our recent efforts to increase use of human tissue to gain a better understanding of the AD pathogenesis subtype variety and to develop several distinct therapeutic approaches tailored to address this diversity. These therapeutic approaches include the blocking of the Aβ/apoE interaction, stimulation of innate immunity, and the simultaneous blocking of Aβ/tau oligomer toxicity. We believe that future successful therapeutic approaches will need to be combined to better reflect the complexity of the abnormal pathways triggered in AD pathogenesis.

Impaired memory-guided attention in asymptomatic APOE4 carriers

Attention and memory may be impaired in individuals at-risk for Alzheimer's disease (AD), though standard cognitive assessments typically study the two in parallel. In reality, attention and memory interact to facilitate information processing, and thus a more integrative approach is required. Here, we used a novel auditory paradigm to assess how long-term memory for auditory scenes facilitates detection of an auditory target in asymptomatic carriers of Apolipoprotein E4 (APOE4), the principle risk gene for late-onset AD. We tested 60 healthy middle-aged adults with varying doses of APOE4 - 20 APOE4 homozygotes (E4/E4), 20 heterozygotes (E3/E4) and 20 non-carriers (E3/E3) - to determine effect on memory-guided attention. While explicit memory was unaffected by genotype, APOE4 dose significantly impaired memory-guided attention. A relationship between explicit memory and memory-guided attention was observed in non-carriers, but this correlation was not significant in E3/E4 and E4/E4 carriers, suggesting that APOE4 carriers rely less on explicit memory to facilitate attention. Since memory-guided attention declined with age in APOE4 homozygotes, this impairment may reflect early disease rather than being a life-long trait. In sum, asymptomatic individuals at increased genetic risk of AD show an age-dependent decline in attention-memory interaction when memory alone is not impaired.

APOE genotype and sex affect microglial interactions with plaques in Alzheimer's disease mice

Microglia affect Alzheimer’s disease (AD) pathogenesis in opposing manners, by protecting against amyloid accumulation in early phases of the disease and promoting neuropathology in advanced stages. Recent research has identified specific microglial interactions with amyloid plaques that exert important protective functions including attenuation of early pathology. It is unknown how these protective microglial interactions with plaques are affected by apolipoprotein E (APOE) genotype and sex, two well-established AD risk factors that modulate microglial function. We investigated this question using quantitative confocal microscopy to compare microglial interactions with amyloid plaques in male and female EFAD mice across APOE3 and APOE4 genotypes at 6 months of age. We observed that microglial coverage of plaques is highest in male APOE3 mice with significant reductions in coverage observed with both APOE4 genotype and female sex. Plaque compaction, a beneficial consequence of microglial interactions with plaques, showed a similar pattern in which APOE4 genotype and female sex were associated with significantly lower values. Within the plaque environment, microglial expression of triggering receptor expressed on myeloid cells 2 (TREM2), a known regulator of microglial plaque coverage, was highest in male APOE3 mice and reduced by APOE4 genotype and female sex. These differences in plaque interactions were unrelated to the number of microglial processes in the plaque environment across groups. Interestingly, the pattern of amyloid burden across groups was opposite to that of microglial plaque coverage, with APOE4 genotype and female sex showing the highest amyloid levels. These findings suggest a possible mechanism by which microglia may contribute to the increased AD risk associated with APOE4 genotype and female sex.

Cyclic O3 exposure synergizes with aging leading to memory impairment in male APOE ε3, but not APOE ε4, targeted replacement mice

The etiology of late-onset Alzheimer's disease is unknown. Recent epidemiological studies suggest that exposure to high levels of ozone (O₃) may be a risk factor for late-onset Alzheimer's disease. Nonetheless, whether and how O₃ exposure contributes to AD development remains to be determined. In this study, we tested the hypothesis that O₃ exposure synergizes with the genetic risk factor APOE ε4 and aging leading to AD, using male apolipoprotein E (apoE)4 and apoE3 targeted replacement mice as men have increased risk exposure to high levels of O₃ via working environments and few studies have addressed APOE ε4 effects on males. Surprisingly, our results show that O₃ exposure impairs memory in old apoE3, but not old apoE4 or young apoE3 and apoE4, male mice. Further studies show that old apoE4 mice have increased hippocampal activities or expression of some enzymes involved in antioxidant defense, diminished protein oxidative modification, and neuroinflammation following O₃ exposure compared with old apoE3 mice. These novel findings highlight the complexity of interactions between APOE genotype, age, and environmental exposure in AD development.

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.

Intracellular metalloprotease activity controls intraneuronal Aβ aggregation and limits secretion of Aβ via exosomes

Accumulating evidence suggests that the abnormal aggregation of amyloid-β (Αβ) peptide in Alzheimer's disease (AD) begins intraneuronally, within vesicles of the endosomal-lysosomal pathway where Aβ is both generated and degraded. Metalloproteases, including endothelin-converting enzyme (ECE)-1 and -2, reside within these vesicles and normally limit the accumulation of intraneuronally produced Aβ. In this study, we determined whether disruption of Aβ catabolism could trigger Aβ aggregation within neurons and increase the amount of Aβ associated with exosomes, small extracellular vesicles derived from endosomal multivesicular bodies. Using cultured cell lines, primary neurons, and organotypic brain slices from an AD mouse model, we found that pharmacological inhibition of the ECE family of metalloproteases increased intracellular and extracellular Aβ levels and promoted the intracellular formation of Aβ oligomers, a process that did not require internalization of secreted Aβ. In vivo, the accumulation of intraneuronal Aβ aggregates was accompanied by increased levels of both extracellular and exosome-associated Aβ, including oligomeric species. Neuronal exosomes were found to contain both ECE-1 and -2 activities, suggesting that multivesicular bodies are intracellular sites of Aβ degradation by these enzymes. ECE dysfunction could lead to the accumulation of intraneuronal Aβ aggregates and their subsequent release into the extracellular space via exosomes.-Pacheco-Quinto, J., Clausen, D., Pérez-González, R., Peng, H., Meszaros, A., Eckman, C. B., Levy, E., Eckman, E. A. Intracellular metalloprotease activity controls intraneuronal Aβ aggregation and limits secretion of Aβ via exosomes.

APOE genotype affects metabolic and Alzheimer-related outcomes induced by Western diet in female EFAD mice

Development of Alzheimer's disease (AD) is regulated by interactive effects of genetic and environmental risk factors. The most significant genetic risk factor for AD is the ε4 allele of apolipoprotein E ( APOE4), which has been shown to exert greater AD risk in women. An important modifiable AD risk factor is obesity and its associated metabolic dysfunctions. Whether APOE genotype might interact with obesity in females to regulate AD pathogenesis is unclear. To investigate this issue, we studied the effects of Western diet (WD) on female EFAD mice, a transgenic mouse model of AD that includes human APOE alleles ε3 (E3FAD) and ε4 (E4FAD). EFAD mice were fed either control (10% fat, 7% sugar) or WD (45% fat, 17% sugar), and both metabolic and neuropathologic outcomes were determined. Although E4FAD mice generally exhibited poorer metabolic status at baseline, E3FAD mice showed greater diet-induced metabolic impairments. Similarly, E4FAD mice exhibited higher levels of AD-related pathology overall, but only E3FAD showed significant increases on select measures of β-amyloid pathology after exposure to WD. These data demonstrate a gene-environment interaction between APOE and obesogenic diets in females. Understanding how AD-promoting effects of obesity are modulated by genetic factors will foster the identification of at-risk populations and development of preventive interventions.-Christensen, A., Pike, C. J. APOE genotype affects metabolic and Alzheimer-related outcomes induced by Western diet in female EFAD mice.

Advances in developing novel therapeutic strategies for Alzheimer's disease

Alzheimer's Disease (AD), the most prevalent neurodegenerative disease of aging, affects one in eight older Americans. Nearly all drug treatments tested for AD today have failed to show any efficacy. There is a great need for therapies to prevent and/or slow the progression of AD. The major challenge in AD drug development is lack of clarity about the mechanisms underlying AD pathogenesis and pathophysiology. Several studies support the notion that AD is a multifactorial disease. While there is abundant evidence that amyloid plays a role in AD pathogenesis, other mechanisms have been implicated in AD such as tangle formation and spread, dysregulated protein degradation pathways, neuroinflammation, and loss of support by neurotrophic factors. Therefore, current paradigms of AD drug design have been shifted from single target approach (primarily amyloid-centric) to developing drugs targeted at multiple disease aspects, and from treating AD at later stages of disease progression to focusing on preventive strategies at early stages of disease development. Here, we summarize current strategies and new trends of AD drug development, including pre-clinical and clinical trials that target different aspects of disease (mechanism-based versus non-mechanism based, e.g. symptomatic treatments, lifestyle modifications and risk factor management).

A small molecule ApoE4-targeted therapeutic candidate that normalizes sirtuin 1 levels and improves cognition in an Alzheimer's disease mouse model

We describe here the results from the testing of a small molecule first-in-class apolipoprotein E4 (ApoE4)-targeted sirtuin1 (SirT1) enhancer, A03, that increases the levels of the neuroprotective enzyme SirT1 while not affecting levels of neurotoxic sirtuin 2 (SirT2) in vitro in ApoE4-transfected cells. A03 was identified by high-throughput screening (HTS) and found to be orally bioavailable and brain penetrant. In vivo, A03 treatment increased SirT1 levels in the hippocampus of 5XFAD-ApoE4 (E4FAD) Alzheimer’s disease (AD) model mice and elicited cognitive improvement while inducing no observed toxicity. We were able to resolve the enantiomers of A03 and show using in vitro models that the L-enantiomer was more potent than the corresponding D-enantiomer in increasing SirT1 levels. ApoE4 expression has been shown to decrease the level of the NAD-dependent deacetylase and major longevity determinant SirT1 in brain tissue and serum of AD patients as compared to normal controls. A deficiency in SirT1 level has been recently implicated in increased tau acetylation, a dominant post-translational modification and key pathological event in AD and tauopathies. Therefore, as a novel approach to therapeutic development for AD, we targeted identification of compounds that enhance and normalize brain SirT1 levels.

Amyloid toxicity in Alzheimer's disease

A major feature of Alzheimer's disease (AD) pathology is the plaque composed of aggregated amyloid-β (Aβ) peptide. Although these plaques may have harmful properties, there is much evidence to implicate soluble oligomeric Aβ as the primary noxious form. Aβ oligomers can be generated both extracellularly and intracellularly. Aβ is toxic to neurons in a myriad of ways. It can cause pore formation resulting in the leakage of ions, disruption of cellular calcium balance, and loss of membrane potential. It can promote apoptosis, cause synaptic loss, and disrupt the cytoskeleton. Current treatments for AD are limited and palliative. Much research and effort is being devoted to reducing Aβ production as an approach to slowing or preventing the development of AD. Aβ formation results from the amyloidogenic cleavage of human amyloid precursor protein (APP). Reconfiguring this process to disfavor amyloid generation might be possible through the reduction of APP or inhibition of enzymes that convert the precursor protein to amyloid.

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.

Pathological human astroglia in Alzheimer's disease: opening new horizons with stem cell technology

Pathological remodeling, degeneration and reactivity of astrocytes are fundamental astrogliopathies contributing to all neurological diseases. In neurodegenerative disorders (including Alzheimer's disease [AD]) astroglia undergo complex changes that range from atrophy with loss of function to accumulation of reactive cells around disease-specific lesions (senile plaques in the case of AD). The cellular pathology of astroglia in the context of human AD remains enigmatic; mainly because of the severe limitations of animal models, which, although reproducing some pathological features of the disease, do not mimic its progression in full. Human-induced pluripotent stem cells technology creates a novel and potentially revolutionizing platform for studying fundamental mechanisms of the disease and for screening to identify new therapeutic compounds.

Targeting the accomplice to thwart the culprit: a new target for the prevention of amyloid deposition

Inheritance of the E4 allele of the apolipoprotein E gene (APOE4) substantially increases the risk of developing late-onset Alzheimer disease (AD). A large body of evidence has firmly established a role for apoE in modulating the risk of developing the amyloid plaque pathology that is pathognomonic for AD. In this issue of the JCI, Liao and colleagues discovered that antibodies against a nonlipidated form of apoE4 are highly effective in delaying the deposition of amyloid β (Aβ) peptides in mouse models of AD pathology. Using a combination of passive immunization and viral-mediated expression of recombinant antibodies, the authors show that Fc receptor-mediated clearance of the nonlipidated apoE4 was critical in delaying Aβ deposition. Collectively, this study identifies a new therapeutic target that could be exploited to prevent, or possibly reverse, the Aβ pathology of AD.

Microglia-Mediated Neuroprotection, TREM2, and Alzheimer's Disease: Evidence From Optical Imaging

Recent genetic studies have provided overwhelming evidence of the involvement of microglia-related molecular networks in the pathophysiology of Alzheimer's disease (AD). However, the precise mechanisms by which microglia alter the course of AD neuropathology remain poorly understood. Here we discuss current evidence of the neuroprotective functions of microglia with a focus on optical imaging studies that have revealed a role of these cells in the encapsulation of amyloid deposits ("microglia barrier"). This barrier modulates the degree of plaque compaction, amyloid fibril surface area, and insulation from adjacent axons thereby reducing neurotoxicity. We discuss findings implicating genetic variants of the microglia receptor, triggering receptor expressed on myeloid cells 2, in the increased risk of late onset AD. We provide evidence that increased AD risk may be at least partly mediated by deficient microglia polarization toward amyloid deposits, resulting in ineffective plaque encapsulation and reduced plaque compaction, which is associated with worsened axonal pathology. Finally, we propose possible avenues for therapeutic targeting of plaque-associated microglia with the goal of enhancing the microglia barrier and potentially reducing disease progression.

Apolipoprotein E, Receptors, and Modulation of Alzheimer's Disease

Apolipoprotein E (apoE) is a lipid carrier in both the peripheral and the central nervous systems. Lipid-loaded apoE lipoprotein particles bind to several cell surface receptors to support membrane homeostasis and injury repair in the brain. Considering prevalence and relative risk magnitude, the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE4 contributes to AD pathogenesis by modulating multiple pathways, including but not limited to the metabolism, aggregation, and toxicity of amyloid-β peptide, tauopathy, synaptic plasticity, lipid transport, glucose metabolism, mitochondrial function, vascular integrity, and neuroinflammation. Emerging knowledge on apoE-related pathways in the pathophysiology of AD presents new opportunities for AD therapy. We describe the biochemical and biological features of apoE and apoE receptors in the central nervous system. We also discuss the evidence and mechanisms addressing differential effects of apoE isoforms and the role of apoE receptors in AD pathogenesis, with a particular emphasis on the clinical and preclinical studies related to amyloid-β pathology. Finally, we summarize the current strategies of AD therapy targeting apoE, and postulate that effective strategies require an apoE isoform-specific approach.

Application of Metabolomics in Alzheimer's Disease

Progress toward the development of efficacious therapies for Alzheimer’s disease (AD) is halted by a lack of understanding early underlying pathological mechanisms. Systems biology encompasses several techniques including genomics, epigenomics, transcriptomics, proteomics, and metabolomics. Metabolomics is the newest omics platform that offers great potential for the diagnosis and prognosis of neurodegenerative diseases as an individual’s metabolome reflects alterations in genetic, transcript, and protein profiles and influences from the environment. Advancements in the field of metabolomics have demonstrated the complexity of dynamic changes associated with AD progression underscoring challenges with the development of efficacious therapeutic interventions. Defining systems-level alterations in AD could provide insights into disease mechanisms, reveal sex-specific changes, advance the development of biomarker panels, and aid in monitoring therapeutic efficacy, which should advance individualized medicine. Since metabolic pathways are largely conserved between species, metabolomics could improve the translation of preclinical research conducted in animal models of AD into humans. A summary of recent developments in the application of metabolomics to advance the AD field is provided below.

Apolipoprotein E isotype-dependent modulation of microRNA-146a in plasma and brain

The Apolipoprotein E (ApoE) isotype ApoE4 is a prevalent genetic risk factor for Alzheimer's disease (AD) that can modulate systemic and central inflammation, independent of amyloid accumulation. Although disruption of innate immune toll receptor signaling is modulated by ApoE and observed in AD, ApoE isotype-specific effects remain poorly understood. Therefore, we examined the effect of the ApoE isotype on the brain levels of major regulators of TLR signaling including miR146a, a microRNA enriched in the brain. We used 6-month-old ApoE3 or ApoE4 targeted replacement mice with and without mutant familial AD transgenes. ApoE4 reduced the levels of miR146a compared with ApoE3, both in the brain (29%; P<0.0001) and in plasma (47%; P<0.05), which correlated with each other (r=0.74; P<0.05). The presence of 5xFAD transgenes increased brain miR146a in both ApoE3 (E3FAD) and ApoE4 (E4FAD) mice; however, miR146a levels in E4FAD mice remained lower than those in E3FAD mice (62%; P<0.05), despite increased amyloid and inflammation. Supporting these observations, ApoE4 brains showed increased expression of interleukin receptor-associated kinase-1 (160%; P<0.05) (normally downregulated by miR146) that correlated inversely with miR146a levels (r=0.637; P<0.0001). Reduced negative feedback of toll-like receptor signaling (by miRNA146a) can explain early-life hypersensitivity to innate immune stimuli (including Aβ) in ApoE4 carriers. Thus, ApoE4 causes early dysregulation of a central controller of the innate immune system both centrally and systemically. This defect persists with familial AD pathology and may be relevant to ApoE4 AD risk.

SLOH, a carbazole-based fluorophore, mitigates neuropathology and behavioral impairment in the triple-transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative dysfunction characterized by memory impairment and brings a heavy burden to old people both in developing and developed countries. Amyloid hypothesis reveals that aggregation and deposition of amyloid plaques are the cause of AD neurodegeneration. SLOH, a carbazole-based fluorophore, is reported to inhibit amyloid beta (Aβ) aggregation in vitro. In the current study, we intended to evaluate the protective effect of SLOH in a triple transgenic AD mouse model (3xTg-AD). 3xTg-AD (10-month-old) were treated with SLOH (0.5, 1 and 2 mg kg^-1) for one month via intraperitoneal injection. After treatment, cognitive function was assessed by Morris Water Maze (MWM) and Y-maze tasks. In addition, biochemical estimations were used to examine the degree of Aβ deposition, tau hyperphosphorylation and neuroinflammation in the brains of 3xTg-AD mice. An in vitro study was conducted on human neuroblastoma (SH-SY5Y) cells to determine the activity of SLOH on tau and GSK-3β using western blot and immunofluorescence staining. One month treatment with SLOH significantly ameliorated memory impairments in 3xTg-AD mice in MWM and Y-maze tests. Moreover, SLOH treatment mitigated the level of amyloid plaques, tau hyperphosphorylation and neuroinflammation in the mouse brain. SLOH also reduced tau hyperphosphorylation and down-regulated GSK-3β activity in Aβ induced neurotoxic SH-SY5Y cells. The promising results in mitigating amyloid plaques, tau hyperphosphorylation, neuroinflammation and ameliorating cognitive deficits following one-month treatment suggest that SLOH could be a potential multi-target molecule for the AD treatment.

ApoE and Neurodegenerative Diseases in Aging

Age and apolipoprotein E (ApoE) are the mightiest risk factors for dementia and cardiovascular diseases, but the underlying mechanisms remain unclear. In human, ApoE has three isoforms, ApoE2, ApoE3, and ApoE4, which are expressed by the polymorphic alleles: ɛ2, ɛ3, and ɛ4. Among the three polymorphic alleles, apoE ε4 is the most risk gene. ApoE is the main ligand for the low-density lipoprotein (LDL) receptor and the LDL receptor-related protein (LRP), functioning as the component of plasma lipoproteins in the transportation of lipids. Physiologically, ApoE is a multifunctional protein with central roles in lipid metabolism; it transports lipids, including cholesterol, through the cerebrospinal fluid (CSF) and plasma. ApoE expression regulation and apoE gene polymorphism have an important connection with neurological or neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), ischemic stroke, and other diseases.

Peripheral Inflammation, Apolipoprotein E4, and Amyloid-β Interact to Induce Cognitive and Cerebrovascular Dysfunction

Cerebrovascular dysfunction is rapidly reemerging as a major process of Alzheimer’s disease (AD). It is, therefore, crucial to delineate the roles of AD risk factors in cerebrovascular dysfunction. While apolipoprotein E4 (APOE4), Amyloid-β (Aβ), and peripheral inflammation independently induce cerebrovascular damage, their collective effects remain to be elucidated. The goal of this study was to determine the interactive effect of APOE4, Aβ, and chronic repeated peripheral inflammation on cerebrovascular and cognitive dysfunction in vivo. EFAD mice are a well-characterized mouse model that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce human Aβ42 via expression of 5 Familial Alzheimer’s disease (5xFAD) mutations. Here, we utilized EFAD carriers [5xFAD^+/−/APOE^+/+ (EFAD+)] and noncarriers [5xFAD^−/−/APOE^+/+ (EFAD−)] to compare the effects of peripheral inflammation in the presence or absence of human Aβ overproduction. Low-level, chronic repeated peripheral inflammation was induced in EFAD mice via systemic administration of lipopolysaccharide (LPS; 0.5 mg/kg/wk i.p.) from 4 to 6 months of age. In E4FAD+ mice, peripheral inflammation caused cognitive deficits and lowered post-synaptic protein levels. Importantly, cerebrovascular deficits were observed in LPS-challenged E4FAD+ mice, including cerebrovascular leakiness, lower vessel coverage, and cerebral amyloid angiopathy-like Aβ deposition. Thus, APOE4, Aβ, and peripheral inflammation interact to induce cerebrovascular damage and cognitive deficits.

APOE ε4 specific imbalance of arachidonic acid and docosahexaenoic acid in serum phospholipids identifies individuals with preclinical Mild Cognitive Impairment/Alzheimer's Disease

This study was designed to explore the influence of apolipoprotein E (APOE) on blood phospholipids (PL) in predicting preclinical Alzheimer's disease (AD). Lipidomic analyses were also performed on blood from an AD mouse model expressing human APOE isoforms (EFAD) and five AD mutations and from 195 cognitively normal participants, 23 of who converted to mild cognitive impairment (MCI)/AD within 3 years. APOE ε4-carriers converting to MCI/AD had high arachidonic acid (AA)/docosahexaenoic acid (DHA) ratios in PL compared to cognitively normal ε4 and non-ε4 carriers. Arachidonic acid and DHA containing PL species, ε4-status and Aβ42/Aβ40 ratios provided 91% accuracy in detecting MCI/AD. Fish oil/omega-3 fatty acid consumption was associated with lower AA/DHA ratios even among ε4 carriers. High plasma AA/DHA ratios were observed in E4FAD compared to EFAD mice with other isoforms. In particular, alterations in plasma AA and DHA containing PL species were also observed in the brains of E4FAD mice compared to E3FAD mice. Despite the small sample size and a short follow-up, these results suggest that blood PL could potentially serve as biomarkers of preclinical MCI/AD.

Alzheimer's Disease Genetic Risk Factor APOE-ε4 Also Affects Normal Brain Function

APOE-ε4 is the strongest genetic risk factor for Alzheimer's disease (AD), and is associated with an increase in the levels of amyloid deposition and an early age of onset. Recent data demonstrate that AD pathological changes occur decades before clinical symptoms, raising questions about the precise onset of the disease. Now a convergence of approaches in mice and humans has demonstrated that APOE-ε4 affects normal brain function even very early in life in the absence of gross AD pathological changes. Normal mice expressing APOE4 have task-specific spatial learning deficits, as well as reduced NMDAR-dependent signaling and structural changes to presynaptic and postsynaptic compartments in neurons, particularly in hippocampal regions. Young humans possessing APOE-ε4 are more adept than APOE-ε4 negative individuals at some behavioral tasks, and functional magnetic resonance imaging has shown that inheritance of APOE-ε4 has specific effects on medial temporal brain activities. These findings suggest that inheritance of APOE-ε4 causes life long changes to the brain that may be related to the late risk of AD. Several possible mechanisms of how APOE-ε4 could affect brain neurochemistry, structure, and function are reviewed.

EFAD transgenic mice as a human APOE relevant preclinical model of Alzheimer's disease

Identified in 1993, APOE4 is the greatest genetic risk factor for sporadic Alzheimer's disease (AD), increasing risk up to 15-fold compared with APOE3, with APOE2 decreasing AD risk. However, the functional effects of APOE4 on AD pathology remain unclear and, in some cases, controversial. In vivo progress to understand how the human (h)-APOE genotypes affect AD pathology has been limited by the lack of a tractable familial AD-transgenic (FAD-Tg) mouse model expressing h-APOE rather than mouse (m)-APOE. The disparity between m- and h-apoE is relevant for virtually every AD-relevant pathway, including amyloid-β (Aβ) deposition and clearance, neuroinflammation, tau pathology, neural plasticity and cerebrovascular deficits. EFAD mice were designed as a temporally useful preclinical FAD-Tg-mouse model expressing the h-APOE genotypes for identifying mechanisms underlying APOE-modulated symptoms of AD pathology. From their first description in 2012, EFAD mice have enabled critical basic and therapeutic research. Here we review insights gleaned from the EFAD mice and summarize future directions.

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.

Transcriptional regulation of APP by apoE: To boldly go where no isoform has gone before: ApoE, APP transcription and AD: Hypothesised mechanisms and existing knowledge gaps

Alzheimer's disease (AD) is the most common form of dementia that gradually disrupts the brain network to impair memory, language and cognition. While the amyloid hypothesis remains the leading proposed mechanism to explain AD pathophysiology, anti-amyloid therapeutic strategies have yet to translate into useful therapies, suggesting that amyloid β-protein and its precursor, the amyloid precursor protein (APP) are but a part of the disease cascade. Further, risk of AD can be modulated by a number of factors, the most impactful being the ɛ4 isoform of apolipoprotein E (apoE). A recent study reported a novel isoform-dependent transcriptional regulation of APP by apoE. These interesting new results add to the myriad of mechanisms that have been proposed to explain how apoE4 enhances AD risk, highlighting the complexities of not only apoE and AD pathophysiology, but also of disease itself. Also see the video abstract here: https://youtu.be/yd14MBdPkCY.

APOE ε4/ε4 diminishes neurotrophic function of human iPSC-derived astrocytes

The ε4 allele of the APOE gene encoding apolipoprotein E (apoE) is a strong genetic risk factor for aging-related cognitive decline as well as late-onset Alzheimer's disease (AD) compared to the common ε3 allele. In the central nervous system, apoE is produced primarily by astrocytes and functions in transporting lipids including cholesterol to support neuronal homeostasis and synaptic integrity. Although mouse models and corresponding primary cells have provided valuable tools for studying apoE isoform-dependent functions, recent studies have shown that human astrocytes have a distinct gene expression profile compare with rodent astrocytes. Human induced pluripotent stem cells (iPSCs) derived from individuals carrying specific gene variants or mutations provide an alternative cellular model more relevant to humans upon differentiation into specific cell types. Thus, we reprogramed human skin fibroblasts from cognitively normal individuals carrying APOE ε3/ε3 or ε4/ε4 genotype to iPSC clones and further differentiated them into neural progenitor cells and then astrocytes. We found that human iPSC-derived astrocytes secreted abundant apoE with apoE4 lipoprotein particles less lipidated compared to apoE3 particles. More importantly, human iPSC-derived astrocytes were capable of promoting neuronal survival and synaptogenesis when co-cultured with iPSC-derived neurons with APOE ε4/ε4 astrocytes less effective in supporting these neurotrophic functions than those with APOE ε3/ε3 genotype. Taken together, our findings demonstrate APOE genotype-dependent effects using human iPSC-derived astrocytes and provide novel evidence that the human iPSC-based model system is a strong tool to explore how apoE isoforms contribute to neurodegenerative diseases.

Epidermal growth factor prevents APOE4-induced cognitive and cerebrovascular deficits in female mice

Cerebrovascular dysfunction is re-emerging as a major component of aging, and may contribute to the risk of developing Alzheimer's disease (AD). Two important risk factors for cerebrovascular dysfunction are APOE and female sex, which are primarily researched in the context of high amyloid-β (Aβ) levels as found in AD. However, APOE4 and sex modulate Aβ-independent pathways that may induce cerebrovascular dysfunction as a downstream consequence. Therefore, testing the activity of factors that target cerebrovascular dysfunction in Aβ-independent models that incorporate APOE4 and female sex is crucial. We have previously demonstrated that peripheral administration of the epidermal growth factor (EGF) prevents cognitive dysfunction, cerebrovascular leakiness, and cerebrovascular coverage deficits in female mice that express APOE4 and overproduce Aβ, without affecting Aβ levels. These data raise the question of whether EGF protects the cerebrovasculature from general stress-induced damage. Therefore, the goal of this study was to determine whether EGF prevents Aβ-independent cerebrovascular dysfunction. In eight-month old mice that express human APOE, the interaction of APOE4 and female sex induced cognitive dysfunction, increased cerebrovascular leakiness and lowered vessel coverage. Importantly, in a prevention paradigm (from six to eight and a half months of age), EGF ameliorated cognitive decline and cerebrovascular deficits in female mice that express APOE4. Thus, developing treatment strategies based on EGF signaling could provide alternative therapeutic options for age-related cerebrovascular dysfunction and reduce AD risk.

Obesity Accelerates Alzheimer-Related Pathology in APOE4 but not APOE3 Mice

Alzheimer's disease (AD) risk is modified by both genetic and environmental risk factors, which are believed to interact to cooperatively modify pathogenesis. Although numerous genetic and environmental risk factors for AD have been identified, relatively little is known about potential gene-environment interactions in regulating disease risk. The strongest genetic risk factor for late-onset AD is the ε4 allele of apolipoprotein E (APOE4). An important modifiable risk factor for AD is obesity, which has been shown to increase AD risk in humans and accelerate development of AD-related pathology in rodent models. Potential interactions between APOE4 and obesity are suggested by the literature but have not been thoroughly investigated. In the current study, we evaluated this relationship by studying the effects of diet-induced obesity (DIO) in the EFAD mouse model, which combines familial AD transgenes with human APOE3 or APOE4. Male E3FAD and E4FAD mice were maintained for 12 weeks on either a control diet or a Western diet high in saturated fat and sugars. We observed that metabolic outcomes of DIO were similar in E3FAD and E4FAD mice. Importantly, our data showed a significant interaction between diet and APOE genotype on AD-related outcomes in which Western diet was associated with robust increases in amyloid deposits, β-amyloid burden, and glial activation in E4FAD but not in E3FAD mice. These findings demonstrate an important gene-environment interaction in an AD mouse model that suggests that AD risk associated with obesity is strongly influenced by APOE genotype.

Aβ propagation and strains: Implications for the phenotypic diversity in Alzheimer's disease

The progressive nature of Alzheimer's disease (AD) is thought to occur, at least in part, by the self-replication and spreading of Aβ and Tau aggregates through a prion mechanism. Evidence now exists that structural variants of Aβ prions can propagate their distinct conformations through template-directed folding of naïve Aβ peptides. This notion implicates that the first self-propagating Aβ assembly to emerge in the brain dictates the conformation, anatomical spread and pace of subsequently formed deposits. It is hypothesized that a prion mechanism defines the molecular basis underlying the diverse clinicopathologic phenotypes observed across the spectrum of AD patients. Thus, distinct AD strains might require further sub-classification based on biochemical and structural characterization of aggregated Aβ. Here, we review the evidence for distinct, self-propagating Aβ strains, and discuss potential cellular mechanisms that might contribute to their manifestation. From this perspective, we also explore the implications of Aβ strains for current FDA-approved medical imaging probes and therapies for amyloid. Ultimately, the discovery of new molecular tools to differentiate Aβ strains and dissect the heterogeneity of AD may lead to the development of more informative diagnostics and strain-specific therapeutics.

Alzheimer's disease: experimental models and reality

Experimental models of Alzheimer's disease (AD) are critical to gaining a better understanding of pathogenesis and to assess the potential of novel therapeutic approaches. The most commonly used experimental animal models are transgenic mice that overexpress human genes associated with familial AD (FAD) that result in the formation of amyloid plaques. However, AD is defined by the presence and interplay of both amyloid plaques and neurofibrillary tangle pathology. The track record of success in AD clinical trials thus far has been very poor. In part, this high failure rate has been related to the premature translation of highly successful results in animal models that mirror only limited aspects of AD pathology to humans. A greater understanding of the strengths and weakness of each of the various models and the use of more than one model to evaluate potential therapies would help enhance the success of therapy translation from preclinical studies to patients. In this review, we summarize the pathological features and limitations of the major experimental models of AD, including transgenic mice, transgenic rats, various physiological models of sporadic AD and in vitro human cell culture models.

Particulate air pollutants, APOE alleles and their contributions to cognitive impairment in older women and to amyloidogenesis in experimental models

Exposure to particulate matter (PM) in the ambient air and its interactions with APOE alleles may contribute to the acceleration of brain aging and the pathogenesis of Alzheimer's disease (AD). Neurodegenerative effects of particulate air pollutants were examined in a US-wide cohort of older women from the Women's Health Initiative Memory Study (WHIMS) and in experimental mouse models. Residing in places with fine PM exceeding EPA standards increased the risks for global cognitive decline and all-cause dementia respectively by 81 and 92%, with stronger adverse effects in APOE ɛ4/4 carriers. Female EFAD transgenic mice (5xFAD+/-/human APOE ɛ3 or ɛ4+/+) with 225 h exposure to urban nanosized PM (nPM) over 15 weeks showed increased cerebral β-amyloid by thioflavin S for fibrillary amyloid and by immunocytochemistry for Aβ deposits, both exacerbated by APOE ɛ4. Moreover, nPM exposure increased Aβ oligomers, caused selective atrophy of hippocampal CA1 neurites, and decreased the glutamate GluR1 subunit. Wildtype C57BL/6 female mice also showed nPM-induced CA1 atrophy and GluR1 decrease. In vitro nPM exposure of neuroblastoma cells (N2a-APP/swe) increased the pro-amyloidogenic processing of the amyloid precursor protein (APP). We suggest that airborne PM exposure promotes pathological brain aging in older women, with potentially a greater impact in ɛ4 carriers. The underlying mechanisms may involve increased cerebral Aβ production and selective changes in hippocampal CA1 neurons and glutamate receptor subunits.

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.

Rapamycin rescues vascular, metabolic and learning deficits in apolipoprotein E4 transgenic mice with pre-symptomatic Alzheimer's disease

Apolipoprotein E ɛ4 allele is a common susceptibility gene for late-onset Alzheimer's disease. Brain vascular and metabolic deficits can occur in cognitively normal apolipoprotein E ɛ4 carriers decades before the onset of Alzheimer's disease. The goal of this study was to determine whether early intervention using rapamycin could restore neurovascular and neurometabolic functions, and thus impede pathological progression of Alzheimer's disease-like symptoms in pre-symptomatic Apolipoprotein E ɛ4 transgenic mice. Using in vivo, multimodal neuroimaging, we found that apolipoprotein E ɛ4 mice treated with rapamycin had restored cerebral blood flow, blood-brain barrier integrity and glucose metabolism, compared to age- and gender-matched wild-type controls. The preserved vasculature and metabolism were associated with amelioration of incipient learning deficits. We also found that rapamycin restored the levels of the proinflammatory cyclophilin A in vasculature, which may contribute to the preservation of cerebrovascular function in the apolipoprotein E ɛ4 transgenics. Our results show that rapamycin improves functional outcomes in this mouse model and may have potential as an effective intervention to block progression of vascular, metabolic and early cognitive deficits in human Apolipoprotein E ɛ4 carriers. As rapamycin is FDA-approved and neuroimaging is readily used in humans, the results of the present study may provide the basis for future Alzheimer's disease intervention studies in human subjects.

Epidermal growth factor prevents APOE4 and amyloid-beta-induced cognitive and cerebrovascular deficits in female mice

Cerebrovascular (CV) dysfunction is emerging as a critical component of Alzheimer's disease (AD), including altered CV coverage. Angiogenic growth factors (AGFs) are key for controlling CV coverage, especially during disease pathology. Therefore, evaluating the effects of AGFs in vivo can provide important information on the role of CV coverage in AD. We recently demonstrated that epidermal growth factor (EGF) prevents amyloid-beta (Aβ)-induced damage to brain endothelial cells in vitro. Here, our goal was to assess the protective effects of EGF on cognition, CV coverage and Aβ levels using an AD-Tg model that incorporates CV relevant AD risk factors. APOE4 is the greatest genetic risk factor for sporadic AD especially in women and is associated with CV dysfunction. EFAD mice express human APOE3 (E3FAD) or APOE4 (E4FAD), overproduce human Aβ42 and are a well characterized model of APOE pathology. Thus, initially the role of APOE and sex in cognitive and CV dysfunction was assessed in EFAD mice in order to identify a group for EGF treatment. At 8 months E4FAD female mice were cognitively impaired, had low CV coverage, high microbleeds and low plasma EGF levels. Therefore, E4FAD female mice were selected for an EGF prevention paradigm (300 μg/kg/wk, 6 to 8.5 months). EGF prevented cognitive decline and was associated with lower microbleeds and higher CV coverage, but not changes in Aβ levels. Collectively, these data suggest that EGF can prevent Aβ-induced damage to the CV. Developing therapeutic strategies based on AGFs may be particularly efficacious for APOE4-induced AD risk.

Interactions between inflammation, sex steroids, and Alzheimer's disease risk factors

Alzheimer's disease (AD) is an age-related neurodegenerative disorder for which there are no effective strategies to prevent or slow its progression. Because AD is multifactorial, recent research has focused on understanding interactions among the numerous risk factors and mechanisms underlying the disease. One mechanism through which several risk factors may be acting is inflammation. AD is characterized by chronic inflammation that is observed before clinical onset of dementia. Several genetic and environmental risk factors for AD increase inflammation, including apolipoprotein E4, obesity, and air pollution. Additionally, sex steroid hormones appear to contribute to AD risk, with age-related losses of estrogens in women and androgens in men associated with increased risk. Importantly, sex steroid hormones have anti-inflammatory actions and can interact with several other AD risk factors. This review examines the individual and interactive roles of inflammation and sex steroid hormones in AD, as well as their relationships with the AD risk factors apolipoprotein E4, obesity, and air pollution.