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

Amyloid-β pathology and APOE genotype modulate retinoid X receptor agonist activity in vivo

Previous data demonstrate that bexarotene (Bex), retinoid X receptor (RXR) agonist, reduces soluble and insoluble amyloid-β (Aβ) in Alzheimer disease (AD)-transgenic mice either by increasing the levels of mouse apolipoprotein E (apoE) or increasing ABCA1/ABCG1-induced apoE lipoprotein association/lipidation. However, although the mechanism of action of RXR agonists remains unclear, a major concern for their use is human (h)-APOE4, the greatest AD genetic risk factor. If APOE4 imparts a toxic gain-of-function, then increasing apoE4 may increase soluble Aβ, likely the proximal AD neurotoxin. If the APOE4 loss-of-function is lipidation of apoE4, then induction of ABCA1/ABCG1 may be beneficial. In novel EFAD-Tg mice (overexpressing h-Aβ42 with h-APOE), levels of soluble Aβ (Aβ42 and oligomeric Aβ) are highest in E4FAD hippocampus (HP) > E3FAD-HP > E4FAD cortex (CX) > E3FAD-CX, whereas levels of lipoprotein-associated/lipidated apoE have the opposite pattern (6 months). In E4FAD-HP, short-term RXR agonist treatment (Bex or LG100268; 5.75-6 months) increased ABCA1, apoE4 lipoprotein-association/lipidation, and apoE4/Aβ complex, decreased soluble Aβ, and increased PSD95. In addition, hydrogel delivery, which mimics low sustained release, was equally effective as gavage for Bex and LG100268. RXR agonists induced no beneficial effects in the E4FAD-HP in a prevention protocol (5-6 months) and actually increased soluble Aβ levels in E3FAD-CX and E4FAD-CX with the short-term protocol, possibly the result of systemic hepatomegaly. Thus, RXR agonists address the loss-of-function associated with APOE4 and exacerbated by Aβ pathology, i.e. low levels of apoE4 lipoprotein association/lipidation. Further studies are vital to address whether RXR agonists are an APOE4-specific AD therapeutic and the systemic side effects that limit translational application.

APOE ε4: the most prevalent yet understudied risk factor for Alzheimer's disease

Brain pathology of Alzheimer's diseases (AD) and the genetics of autosomal dominant familial AD have been the "lamp posts" under which the AD field has been looking for therapeutic targets. Although this approach still remains valid, none of the compounds tested to date have produced clinically meaningful results. This calls for developing complementary therapeutic approaches and AD targets. The allele ε4 of apolipoprotein E4 (APOE ε4), is the most prevalent genetic risk factor for sporadic AD, and is expressed in more than half of the AD patients. However, in spite of its genetic prominence, the allele APOE ε4 and its corresponding protein product apoE4 have been understudied. We presently briefly discuss the reasons underlying this situation and review newly developed AD therapeutic approaches that target apoE4 and which pave the way for future studies.

A non-invasive, rapid method to genotype late-onset Alzheimer's disease-related apolipoprotein E gene polymorphisms

The apolipoprotein E gene ε4 allele is considered a negative factor for neural regeneration in late-onset Alzheimer's disease cases. The aim of this study was to establish a non-invasive, rapid method to genotype apolipoprotein E gene polymorphisms. Genomic DNA from mouth swab specimens was extracted using magnetic nanoparticles, and genotyping was performed by real-time PCR using TaqMan-BHQ probes. Genotyping accuracy was validated by DNA sequencing. Our results demonstrate 100% correlation to DNA sequencing, indicating reliability of our protocol. Thus, the method we have developed for apolipoprotein E genotyping is accurate and reliable, and also suitable for genotyping large samples, which may help determine the role of the apolipoprotein E ε4 allele in neural regeneration in late-onset Alzheimer's disease cases.

Effects of acidic oligosaccharide sugar chain on amyloid oligomer-induced impairment of synaptic plasticity in rats

Soluble amyloid-β protein (Aβ) oligomers have been recognized to be early and key intermediates in Alzheimer's disease-related synaptic dysfunction. In this study, using in vitro electrophysiology, we investigated interactions of the acidic oligosaccharide sugar chain (AOSC), a marine-derived acidic oligosaccharide, with oligomeric Aβ. We found that the inhibition of long-term potentiation (LTP) induced by Aβ oligomers can be dose dependently reversed by the application of AOSC, whereas AOSC alone did not alter normal LTP induction. Interestingly, treatment with Aβ monomers with or without AOSC did not affect LTP induction. Additionally, when fresh-made Aβ was co-incubated with AOSC before in vitro testing, there was no impairment of LTP induction. The results from Western blots demonstrated that AOSC prevent the aggregation of Aβ oligomers. These findings indicate that AOSC may reverse Aβ oligomer-mediated cytotoxicity by directly disrupting the amyloid oligomer aggregation, and this action is concentration dependent. Thus, we propose that AOSC might be a potential therapeutic drug for Alzheimer's disease due to its protection against oligomeric Aβ-induced dysfunction of synaptic plasticity.

The peculiar role of the A2V mutation in amyloid-β (Aβ) 1-42 molecular assembly

We recently reported a novel Aβ precursor protein mutation (A673V), corresponding to position 2 of Aβ1-42 peptides (Aβ1-42A2V), that caused an early onset AD-type dementia in a homozygous individual. The heterozygous relatives were not affected as an indication of autosomal recessive inheritance of this mutation. We investigated the folding kinetics of native unfolded Aβ1-42A2V in comparison with the wild type sequence (Aβ1-42WT) and the equimolar solution of both peptides (Aβ1-42MIX) to characterize the oligomers that are produced in the early phases. We carried out the structural characterization of the three preparations using electron and atomic force microscopy, fluorescence emission, and x-ray diffraction and described the soluble oligomer formation kinetics by laser light scattering. The mutation promoted a peculiar pathway of oligomerization, forming a connected system similar to a polymer network with hydrophobic residues on the external surface. Aβ1-42MIX generated assemblies very similar to those produced by Aβ1-42WT, albeit with slower kinetics due to the difficulties of Aβ1-42WT and Aβ1-42A2V peptides in building up of stable intermolecular interaction.

Reversal of apoE4-driven brain pathology and behavioral deficits by bexarotene

Apolipoprotein E4 (apoE4), the most prevalent genetic risk factor for Alzheimer's disease (AD), is less lipidated than its corresponding AD-benign form, apoE3, and it has been suggested that the pathological effects of apoE4 are mediated by lipid-related mechanisms. ATP-binding cassette transporters A1 and G1 (ABCA1 and ABCG1, respectively) are the most important apoE-lipidating proteins. The expression of these proteins, as well as that of apoE, is controlled by the transcription regulation retinoid X receptor (RXR)-liver X receptor (LXR) system. In the present study, we investigated the effects of the RXR agonist bexarotene on mRNA and protein levels of apoE, ABCA1, and ABCG1 in young, naive apoE3- and apoE4-targeted replacement mice and assessed the extent to which this reverses the apoE4-driven pathological phenotype. This investigation reveled that bexarotene increases the mRNA and protein levels of ABCA1 and ABCG1 in hippocampal neurons, but has no effect on the corresponding levels of apoE. These findings were associated with reversal of the lipidation deficiency of apoE4 and of the cognitive impairments of apoE4 mice in several tests. Furthermore, bexarotene reversed the apoE4-driven accumulation of Aβ42 and hyperphosphorylated tau in hippocampal neurons, as well as the apoE4-induced reduction in the levels of the presynaptic marker vesicular glutamatergic transporter 1 (VGluT1). In conclusion, the results show that treatment of apoE4 mice with the RXR agonist bexarotene reverses the apoE4-induced cognitive and neuronal impairments in vivo and suggest that this is due to reversal of the lipidation deficiency of apoE4. This puts forward the possibility that RXR activation and increased levels of ABCA1 and ABCG1 could be useful in the treatment of human apoE4 carriers.

Plasma-Enabled Carbon Nanostructures for Early Diagnosis of Neurodegenerative Diseases

Carbon nanostructures (CNs) are amongst the most promising biorecognition nanomaterials due to their unprecedented optical, electrical and structural properties. As such, CNs may be harnessed to tackle the detrimental public health and socio-economic adversities associated with neurodegenerative diseases (NDs). In particular, CNs may be tailored for a specific determination of biomarkers indicative of NDs. However, the realization of such a biosensor represents a significant technological challenge in the uniform fabrication of CNs with outstanding qualities in order to facilitate a highly-sensitive detection of biomarkers suspended in complex biological environments. Notably, the versatility of plasma-based techniques for the synthesis and surface modification of CNs may be embraced to optimize the biorecognition performance and capabilities. This review surveys the recent advances in CN-based biosensors, and highlights the benefits of plasma-processing techniques to enable, enhance, and tailor the performance and optimize the fabrication of CNs, towards the construction of biosensors with unparalleled performance for the early diagnosis of NDs, via a plethora of energy-efficient, environmentally-benign, and inexpensive approaches.

LCAT deficiency does not impair amyloid metabolism in APP/PS1 mice

A key step in plasma HDL maturation from discoidal to spherical particles is the esterification of cholesterol to cholesteryl ester, which is catalyzed by LCAT. HDL-like lipoproteins in cerebrospinal fluid (CSF) are also spherical, whereas nascent lipoprotein particles secreted from astrocytes are discoidal, suggesting that LCAT may play a similar role in the CNS. In plasma, apoA-I is the main LCAT activator, while in the CNS, it is believed to be apoE. apoE is directly involved in the pathological progression of Alzheimer's disease, including facilitating β-amyloid (Aβ) clearance from the brain, a function that requires its lipidation by ABCA1. However, whether apoE particle maturation by LCAT is also required for Aβ clearance is unknown. Here we characterized the impact of LCAT deficiency on CNS lipoprotein metabolism and amyloid pathology. Deletion of LCAT from APP/PS1 mice resulted in a pronounced decrease of apoA-I in plasma that was paralleled by decreased apoA-I levels in CSF and brain tissue, whereas apoE levels were unaffected. Furthermore, LCAT deficiency did not increase Aβ or amyloid in APP/PS1 LCAT(-/-) mice. Finally, LCAT expression and plasma activity were unaffected by age or the onset of Alzheimer's-like pathology in APP/PS1 mice. Taken together, these results suggest that apoE-containing discoidal HDLs do not require LCAT-dependent maturation to mediate efficient Aβ clearance.

Human APOE4 increases microglia reactivity at Aβ plaques in a mouse model of Aβ deposition

BACKGROUND

Having the apolipoprotein E4 (APOE-ϵ4) allele is the strongest genetic risk factor for the development of Alzheimer's disease (AD). Accumulation of amyloid beta (Aβ) in the brain is influenced by APOE genotype. Transgenic mice co-expressing five familial AD mutations (5xFAD) in the presence of human APOE alleles (ϵ2, ϵ3 or ϵ4) exhibit APOE genotype-specific differences in early Aβ accumulation, suggesting an interaction between APOE and AD pathology. Whether APOE genotype affects Aβ-plaque-associated neuroinflammation remains unclear. In the current study, we address the role of APOE genotype on Aβ-associated microglial reactivity in the EFAD transgenic mouse model.

METHODS

We analyzed Aβ-induced glial activation in the brains of 6-month-old EFAD transgenic mice (E2FAD, E3FAD and E4FAD). Region-specific morphological profiles of Aβ plaques in EFAD brain sections were compared using immunofluorescence staining. We then determined the degree of glial activation in sites of Aβ deposition while comparing levels of the inflammatory cytokine Interleukin-1β (IL-1β) by ELISA. Finally, we quantified parameters of Aβ-associated microglial reactivity using double-stained EFAD brain sections.

RESULTS

Characterization of Aβ plaques revealed there were larger and more intensely stained plaques in E4FAD mice relative to E2FAD and E3FAD mice. E4FAD mice also had a greater percentage of compact plaques in the subiculum than E3FAD mice. Reactive microglia and dystrophic astrocytes were prominent in EFAD brains, and primarily localized to two sites of significant Aβ deposition: the subiculum and deep layers of the cortex. Cortical levels of IL-1β were nearly twofold greater in E4FAD mice relative to E3FAD mice. To control for differences in levels of Aβ in the different EFAD mice, we analyzed the microglia within domains of specific Aβ deposits. Morphometric analyses revealed increased measures of microglial reactivity in E4FAD mice, including greater dystrophy, increased fluorescence intensity and a higher density of reactive cells surrounding cortical plaques, than in E3FAD mice.

CONCLUSIONS

In addition to altering morphological profiles of Aβ deposition, APOE genotype influences Aβ-induced glial activation in the adult EFAD cortex. These data support a role for APOE in modulating Aβ-induced neuroinflammatory responses in AD progression, and support the use of EFAD mice as a suitable model for mechanistic studies of Aβ-associated neuroinflammation.

Apolipoprotein E increases cell association of amyloid-β 40 through heparan sulfate and LRP1 dependent pathways

The increased risk of Alzheimer's disease (AD) associated with specific apolipoprotein E (ApoE) isoforms appears to relate to altered amyloid-β (Aβ) homeostasis. Clearance of Aβ from the brain is reduced in the presence of the AD-associated ApoE4 isoform, which may contribute to the accumulation of Aβ deposits in the parenchyma and vasculature. The low-density lipoprotein receptor-related protein 1 (LRP1) and heparan sulfate proteoglycans (HSPGs), both established ApoE receptors, are involved in Aβ uptake, with LRP1 additionally implicated in Aβ transcytosis across the blood-brain barrier. In this study, we detected the co-distribution of heparan sulfate (HS), ApoE and LRP1 in Aβ(1-40)-positive brain microvessels from individuals with Down's syndrome diagnosed with AD. In addition, ApoE was pulled-down from AD cerebrospinal fluid with anti-Aβ antibodies. Using Chinese hamster ovary cells deficient in HS or LRP1, we found that ApoE increases cell association of Aβ in a HSPG- and LRP1-dependent manner; and further, ApoE processing is altered in the absence of cellular HS. These interactions may facilitate Aβ clearance from the brain, but if overwhelmed could contribute to Aβ accumulation and the pathogenesis of AD.

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 in Alzheimer's disease: an update

The vast majority of Alzheimer's disease (AD) cases are late onset (LOAD), which is genetically complex with heritability estimates up to 80%. Apolipoprotein E (APOE) has been irrefutably recognized as the major genetic risk factor, with semidominant inheritance, for LOAD. Although the mechanisms that underlie the pathogenic nature of APOE in AD are still not completely understood, emerging data suggest that APOE contributes to AD pathogenesis through both amyloid-β (Aβ)-dependent and Aβ-independent pathways. Given the central role for APOE in the modulation of AD pathogenesis, many therapeutic strategies have emerged, including converting APOE conformation, regulating APOE expression, mimicking APOE peptides, blocking the APOE/Aβ interaction, modulating APOE lipidation state, and gene therapy. Accumulating evidence also suggests the utility of APOE genotyping in AD diagnosis, risk assessment, prevention, and treatment response.

Cerebrospinal fluid apolipoprotein E concentration and severity of cognitive impairment in patients with newly diagnosed Alzheimer's disease

BACKGROUND/OBJECTIVE

Apolipoprotein E (apoE) plays an important role in the pathogenesis of Alzheimer's disease (AD). Altered cerebrospinal fluid (CSF) and plasma levels have been previously reported in patients with AD. We hypothesized that CSF apoE levels of patients with newly diagnosed AD might be associated with their cognitive performance.

METHODS

Patients with AD (N = 71) enrolled into an observational study underwent neuropsychological testing (Consortium to Establish a Registry for AD [CERAD] plus) at time of diagnosis. The CSF was obtained, and apoE concentrations were determined. Generalized linear models were constructed to assess the associations of apoE and neuropsychological measures while adjusting for important potential confounders.

RESULTS

No association of CSF apoE levels and cognitive function could be demonstrated. Still, the use of neuroleptic drugs, female gender, preprogression time, and lower education were linked to worse cognitive function in some domains.

CONCLUSION

The CSF apoE appears not to be suitable as a biochemical surrogate of cognitive function in AD under the given circumstances. By means of longitudinal analyses, potential associations with the velocity of decline will be investigated in the near future.

Apolipoprotein E-low density lipoprotein receptor interaction affects spatial memory retention and brain ApoE levels in an isoform-dependent manner

Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a major genetic risk factor for cardiovascular disease (CVD) and Alzheimer's disease (AD). ApoE mediates cholesterol metabolism by binding various receptors. The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4>E3>>E2). Evidence suggests that a functional interaction between apoE and LDLR influences the risk of CVD and AD. We hypothesize that the differential cognitive effects of the apoE isoforms are a direct result of their varying interactions with LDLR. To test this hypothesis, we have employed transgenic mice that express human apoE2, apoE3, or apoE4, and either human LDLR (hLDLR) or no LDLR (LDLR(-/-)). Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.

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.

Concerning the structure of apoE

Apolipoprotein E (apoE), first described in 1973, is a truly fascinating protein. While studies initially focused on its role in cholesterol and lipid metabolism, one apoE isoform (apoE4) is a major risk factor for development of late onset Alzheimer's disease. Yet the difference between apoE3, the common form, and apoE4 is a single amino acid of the 299 in this 34 kDa protein. Structure determination of the two domain full length apoE3 protein was only accomplished in 2011 and supports the notion that mutations in the N-terminal domain can be propagated through the structure to the C-terminal domain. Understanding the structural differences between apoE3 and apoE4 is critical for finding ways to modulate the deleterious effect of apoE4.

Alzheimer's disease biomarkers in animal models: closing the translational gap

The rising prevalence of Alzheimer's disease (AD) is rapidly becoming one of the largest health and economic challenges in the world. There is a growing need for the development and implementation of reliable biomarkers for AD that can be used to assist in diagnosis, inform disease progression, and monitor therapeutic efficacy. Preclinical models permit the evaluation of candidate biomarkers and assessment of pipeline agents before clinical trials are initiated and provide a translational opportunity to advance biomarker discovery. Fast and inexpensive data can be obtained from examination of peripheral markers, though they currently lack the sensitivity and consistency of imaging techniques such as MRI or PET. Plasma and cerebrospinal fluid (CSF) biomarkers in animal models can assist in development and implementation of similar approaches in clinical populations. These biomarkers may also be invaluable in decisions to advance a treatment to human testing. Longitudinal studies in AD models can determine initial presentation and progression of biomarkers that may also be used to evaluate disease-modifying efficacy of drugs. The refinement of biomarker approaches in preclinical systems will not only aid in drug development, but may facilitate diagnosis and disease monitoring in AD patients.