Amyloid β oligomer induces cerebral vasculopathy via pericyte-mediated endothelial dysfunction

BACKGROUND

Although abnormal accumulation of amyloid beta (Aβ) protein is thought to be the main cause of Alzheimer's disease (AD), emerging evidence suggests a pivotal vascular contribution to AD. Aberrant amyloid β induces neurovascular dysfunction, leading to changes in the morphology and function of the microvasculature. However, little is known about the underlying mechanisms between Aβ deposition and vascular injuries. Recent studies have revealed that pericytes play a substantial role in the vasculopathy of AD. Additional research is imperative to attain a more comprehensive understanding.

METHODS

Two-photon microscopy and laser speckle imaging were used to examine cerebrovascular dysfunction. Aβ oligomer stereotactic injection model was established to explain the relationship between Aβ and vasculopathy. Immunofluorescence staining, western blot, and real-time PCR were applied to detect the morphological and molecular alternations of pericytes. Primary cultured pericytes and bEnd.3 cells were employed to explore the underlying mechanisms.

RESULTS

Vasculopathy including BBB damage, hypoperfusion, and low vessel density were found in the cortex of 8 to 10-month-old 5xFAD mice. A similar phenomenon accompanied by pericyte degeneration appeared in an Aβ-injected model, suggesting a direct relationship between Aβ and vascular dysfunction. Pericytes showed impaired features including low PDGFRβ expression and increased pro-inflammatory chemokines secretion under the administration of Aβ in vitro, of which supernatant cultured with bEND.3 cells led to significant endothelial dysfunction characterized by TJ protein deficiency.

CONCLUSIONS

Our results provide new insights into the pathogenic mechanism underlying Aβ-induced vasculopathy. Targeting pericyte therapies are promising to ameliorate vascular dysfunction in AD.

Amyloid-β Oligomer-Induced Electrophysiological Mechanisms and Electrical Impedance Changes in Neurons

Amyloid plays a critical role in the pathogenesis of Alzheimer's disease (AD) and can aggregate to form oligomers and fibrils in the brain. There is increasing evidence that highly toxic amyloid-β oligomers (AβOs) lead to tau protein aggregation, hyperphosphorylation, neuroinflammation, neuronal loss, synaptic loss, and dysfunction. Although the effects of AβOs on neurons have been investigated using conventional biochemical experiments, there are no established criteria for electrical evaluation. To this end, we explored electrophysiological changes in mouse hippocampal neurons (HT22) following exposure to AβOs and/or naringenin (Nar, a flavonoid compound) using electrical impedance spectroscopy (EIS). AβO-induced HT22 showed a decreased impedance amplitude and increased phase angle, and the addition of Nar reversed these changes. The characteristic frequency was markedly increased with AβO exposure, which was also reversed by Nar. The AβOs decreased intranuclear and cytoplasmic resistance and increased nucleus resistance and extracellular capacitance. Overall, the innovative construction of the eight-element CPE-equivalent circuit model further reflects that the pseudo-capacitance of the cell membrane and cell nucleus was increased in the AβO-induced group. This study conclusively revealed that AβOs induce cytotoxic effects by disrupting the resistance characteristics of unit membranes. The results further support that EIS is an effective technique for evaluating AβO-induced neuronal damage and microscopic electrical distinctions in the sub-microscopic structure of reactive cells.

Peptide aptamer targeting Aβ-PrP-Fyn axis reduces Alzheimer's disease pathologies in 5XFAD transgenic mouse model

Currently, no effective therapeutics exist for the treatment of incurable neurodegenerative diseases such as Alzheimer's disease (AD). The cellular prion protein (PrP^C) acts as a high-affinity receptor for amyloid beta oligomers (AβO), a main neurotoxic species mediating AD pathology. The interaction of AβO with PrP^C subsequently activates Fyn tyrosine kinase and neuroinflammation. Herein, we used our previously developed peptide aptamer 8 (PA8) binding to PrP^C as a therapeutic to target the AβO-PrP-Fyn axis and prevent its associated pathologies. Our in vitro results indicated that PA8 prevents the binding of AβO with PrP^C and reduces AβO-induced neurotoxicity in mouse neuroblastoma N2a cells and primary hippocampal neurons. Next, we performed in vivo experiments using the transgenic 5XFAD mouse model of AD. The 5XFAD mice were treated with PA8 and its scaffold protein thioredoxin A (Trx) at a 14.4 µg/day dosage for 12 weeks by intraventricular infusion through Alzet^® osmotic pumps. We observed that treatment with PA8 improves learning and memory functions of 5XFAD mice as compared to Trx-treated 5XFAD mice. We found that PA8 treatment significantly reduces AβO levels and Aβ plaques in the brain tissue of 5XFAD mice. Interestingly, PA8 significantly reduces AβO-PrP interaction and its downstream signaling such as phosphorylation of Fyn kinase, reactive gliosis as well as apoptotic neurodegeneration in the 5XFAD mice compared to Trx-treated 5XFAD mice. Collectively, our results demonstrate that treatment with PA8 targeting the AβO-PrP-Fyn axis is a promising and novel approach to prevent and treat AD.

Treadmill Exercise Reduces Neuroinflammation, Glial Cell Activation and Improves Synaptic Transmission in the Prefrontal Cortex in 3 × Tg-AD Mice

Physical exercise improves memory and cognition in physiological aging and Alzheimer's disease (AD), but the mechanisms remain poorly understood. Here, we test the hypothesis that Aβ oligomer accumulation, neuroinflammation, and glial cell activation may lead to disruption of synaptic transmission in the prefrontal cortex of 3 × Tg-AD Mice, resulting in impairment of learning and memory. On the other hand, treadmill exercise could prevent the pathogenesis and exert neuroprotective effects. Here, we used immunohistochemistry, western blotting, enzyme-linked immunosorbent assay, and slice electrophysiology to analyze the levels of GSK3β, Aβ oligomers (Aβ dimers and trimers), pro-inflammatory cytokines (IL-1β, IL-6, and TNFα), the phosphorylation of CRMP2 at Thr514, and synaptic currents in pyramidal neurons in the prefrontal cortex. We show that 12-week treadmill exercise beginning in three-month-old mice led to the inhibition of GSK3β kinase activity, decreases in the levels of Aβ oligomers, pro-inflammatory cytokines (IL-1β, IL-6, and TNFα), and the phosphorylation of CRMP2 at Thr514, reduction of microglial and astrocyte activation, and improvement of excitatory and inhibitory synaptic transmission of pyramidal neurons in the prefrontal cortex of 3 × Tg-AD Mice. Thus, treadmill exercise reduces neuroinflammation, glial cell activation and improves synaptic transmission in the prefrontal cortex in 3 × Tg-AD mice, possibly related to the inhibition of GSK3β kinase activity.

Recognition of Aβ oligomer by LilrB2 acceptor: a tetracoordinated zipper mechanism

Leukocyte immunoglobulin-like receptor B2 (LilrB2) is one of discovered cell surface β-amyloid (Aβ) receptors and taken as a promising therapeutic target for the treatment of Alzheimer's disease (AD). Aβ42 oligomer rather than monomer is toxic to neuronal cells and can directly bind to LilrB2, resulting in synaptic loss and cognitive impairment in the development of AD. Therefore, uncovering the mechanism of interaction between Aβ42 oligomer and LilrB2 becomes the first step to obtain a clear drug target and specific binding sites. Herein, a tetracoordinated mechanism for the Aβ oligomer-LilrB2 binding was first put forward by employing Aβ42 dimer mimic-antiparallel copies of Aβ42 core fragment ¹⁶KLVFFA²¹, to bind LilrB2 as models, in which four key residues (F5/F6/L12/F14) in the Aβ42 mimic are bound strongly with LilrB2 residue(s) or accommodated by four hydrophobic cavities in LilrB2 to generate a stable complex. Bi-dentate binding, however, cannot keep the complex Aβ42 mimic-LilrB2 stable. The inhibitor fluspirilene can disturb the binding of four key residues of Aβ42 to LilrB2, justifying the tetracoordinated zipper mechanism on the other hand.

Characterization of a Conformation-Restricted Amyloid β Peptide and Immunoreactivity of Its Antibody in Human AD brain

Characterization of amyloid β (Aβ) oligomers, the transition species present prior to the formation of Aβ fibrils and that have cytotoxicity, has become one of the major topics in the investigations of Alzheimer's disease (AD) pathogenesis. However, studying pathophysiological properties of Aβ oligomers is challenging due to the instability of these protein complexes in vitro. Here, we report that conformation-restricted Aβ42 with an intramolecular disulfide bond at positions 17 and 28 (SS-Aβ42) formed stable Aβ oligomers in vitro. Thioflavin T binding assays, nondenaturing gel electrophoresis, and morphological analyses revealed that SS-Aβ42 maintained oligomeric structure, whereas wild-type Aβ42 and the highly aggregative Aβ42 mutant with E22P substitution (E22P-Aβ42) formed Aβ fibrils. In agreement with these observations, SS-Aβ42 was more cytotoxic compared to the wild-type and E22P-Aβ42 in cell cultures. Furthermore, we developed a monoclonal antibody, designated TxCo-1, using the toxic conformation of SS-Aβ42 as immunogen. X-ray crystallography of the TxCo-1/SS-Aβ42 complex, enzyme immunoassay, and immunohistochemical studies confirmed the recognition site and specificity of TxCo-1 to SS-Aβ42. Immunohistochemistry with TxCo-1 antibody identified structures resembling senile plaques and vascular Aβ in brain samples of AD subjects. However, TxCo-1 immunoreactivity did not colocalize extensively with Aβ plaques identified with conventional Aβ antibodies. Together, these findings indicate that Aβ with a turn at positions 22 and 23, which is prone to form Aβ oligomers, could show strong cytotoxicity and accumulated in brains of AD subjects. The SS-Aβ42 and TxCo-1 antibody should facilitate understanding of the pathological role of Aβ with toxic conformation in AD.

Dysregulated CRTC1-BDNF signaling pathway in the hippocampus contributes to Aβ oligomer-induced long-term synaptic plasticity and memory impairment

Expression of CREB-regulated transcription coactivator 1 (CRTC1) in the hippocampus is impaired in Alzheimer's disease (AD). However, CRTC1 related mechanisms associated with long-term synaptic plasticity impairment and cognitive decline in the onset of AD are unknown. In this study, electrophysiological recordings indicated that lentivirus-mediated CRTC1 overexpression effectively ameliorates suppression of late-phase long-term potentiation (L-LTP) in rat hippocampal slices treated with oligomeric amyloid β(1-42) peptides (oAβ42) (200 nM). In addition, application of oAβ42 and genetic knockdown of CRTC1 by lentivirus-mediated CRTC1-shRNA inhibit L-LTP, whereas their combination does not further impair L-LTP. Brain-derived neurotrophic factor (BDNF), an important downstream protein confers protection of CRTC1 overexpression against oAβ42-induced L-LTP impairment as shown by administration of K252a (200 nM) and TrkB-FC (20 μg/ml). Furthermore, behavioral and western blotting analyses showed that CRTC1 overexpression reverses oAβ42-induced hippocampal-dependent cognitive deficits, downregulation of CRTC1 and BDNF expression. Notably, CRTC1-shRNA directly elicits cognitive deficits. In summary, these findings show that hippocampal CRTC1 signaling is affected by soluble oAβ, and CRTC1-BDNF pathway is involved in hippocampal L-LTP impairment and memory deficits induced by oAβ42.

Plasma Amyloid-β Oligomerization Tendency Predicts Amyloid PET Positivity

Purpose

Among other emerging amyloid-targeting blood-based biomarkers, Multimer Detection System-Oligomeric Amyloid-β (MDS-OAβ) measures dynamic changes in concentration of oligomeric amyloid-β (OAβ), which is considered the main pathogenic culprit of Alzheimer’s disease (AD), in plasma after spiking with synthetic amyloid-β (Aβ). We aimed to investigate the predictability of MDS-OAβ on amyloid positron emission tomography (PET) positivity.

Patients and Methods

A total of 96 subjects who visited Seoul National University Bundang Hospital for medical check-up complaining of cognitive decline and had undergone extensive medical assessment were recruited. Amyloid statuses were dichotomized into positive or negative based on visual assessment of amyloid PET. Plasma OAβ concentration was measured by MDS-OAβ. In the previous validation study, 0.78ng/mL was established as the cut-off value and the plasma OAβ concentration higher than or equal to the cut-off value was defined as MDS-OAβ positive.

Results

MDS-OAβ positivity could discriminate amyloid PET positivity with the AUC value of 0.855 (95% CI 0.776–0.933). Adding MDS-OAβ positivity to prediction models including age, MMSE score, and APOE ε4 status improved performance up to the AUC value of 0.926 (95% CI 0.871–0.980).

Conclusion

The Aβ oligomerization tendency in plasma could predict amyloid PET positivity with high performance, and, when it is combined with age, MMSE score, and APOE ε4 status, predictability was improved substantially. This suggests the potential of MDS-OAβ as a useful initial stage test in the clinical and research fields of AD.

Accumulation of cellular prion protein within β-amyloid oligomer plaques in aged human brains

Alzheimer’s disease (AD) is the main cause of dementia, and β‐amyloid (Aβ) is a central factor in the initiation and progression of the disease. Different forms of Aβ have been identified as monomers, oligomers, and amyloid fibrils. Many proteins have been implicated as putative receptors of respective forms of Aβ. Distinct forms of Aβ oligomers are considered to be neurotoxic species that trigger the pathophysiology of AD. It was reported that cellular prion protein (PrP^C) is one of the most selective and high‐affinity binding partners of Aβ oligomers. The interaction of Aβ oligomers with PrP^C is important to synaptic dysfunction and loss. The binding of Aβ oligomers to PrP^C has mostly been studied with synthetic peptides, cell culture, and murine models of AD by biochemical and biological methods. However, the molecular mechanisms underlying the relationship between Aβ oligomers and PrP^C remain unclear, especially in the human brain. We immunohistochemically investigated the relationship between Aβ oligomers and PrP^C in human brain tissue with and without amyloid pathology. We histologically demonstrate that PrP^C accumulates with aging in human brain tissue even prior to AD mainly within diffuse‐type amyloid plaques, which are composed of more soluble Aβ oligomers without stacked β‐sheet fibril structures. Our results suggest that PrP^C accumulating plaques are associated with more soluble Aβ oligomers, and appear even prior to AD. The investigation of PrP^C accumulating plaques may provide new insights into AD.

Stabilization Mechanism for a Nonfibrillar Amyloid β Oligomer Based on Formation of a Hydrophobic Core Determined by Dissipative Particle Dynamics

Neurotoxicity caused by nonfibrillar amyloid β (Aβ) oligomers in the brain is suggested to be associated with the onset of Alzheimer's disease (AD). Elucidating the structural features of Aβ oligomers is critical for promoting drug discovery research for AD. One of the Aβ oligomers, known as Aβ*56, is a dodecamer that impairs memory when injected into healthy rats, suggesting that Aβ*56 may contribute to cognitive deficits in AD patients. Another dodecamer structure, formed by 20-residue peptide segments derived from the Aβ peptide (Aβ_17-36), has been revealed by X-ray crystallography. The structure of the Aβ_17-36 dodecamer is composed of trimer units and shows the oligomer antibody A11 reactivity, which are characteristic of Aβ*56, indicating that Aβ*56 and the Aβ_17-36 dodecamer share a similar structure. However, the structure of the C-terminal regions (Aβ_37-42) remains unclear. The C-terminal region, which is abundant in hydrophobic residues, is thought to play a key role in stabilizing the oligomer structure by forming a hydrophobic core. In this study, we employed dissipative particle dynamics, a coarse-grained simulation method with soft core potentials, utilizing the crystal structure information to unravel Aβ dodecamer structures with C-terminal regions. The simulation results were validated by the reported experimental data. Hence, an analysis of the simulation results can provide structural insights into Aβ oligomers. Our simulations revealed the stabilization mechanism of the dodecamer structure at the molecular level. We showed that C-terminal regions spontaneously form a hydrophobic core in the central cavity, contributing to stabilizing the dodecamer structure. Furthermore, four consecutive hydrophobic residues in the C-terminal region (i.e., Val39-Ala42) are important for core formation.

Brains of rhesus monkeys display Aβ deposits and glial pathology while lacking Aβ dimers and other Alzheimer's pathologies

Cerebral amyloid beta (Aβ) deposits are the main early pathology of Alzheimer's disease (AD). However, abundant Aβ deposits also occur spontaneously in the brains of many healthy people who are free of AD with advancing aging. A crucial unanswered question in AD prevention is why AD does not develop in some elderly people, despite the presence of Aβ deposits. The answer may lie in the composition of Aβ oligomer isoforms in the Aβ deposits of healthy brains, which are different from AD brains. However, which Aβ oligomer triggers the transformation from aging to AD pathogenesis is still under debate. Some researchers insist that the Aβ 12‐mer causes AD pathology, while others suggest that the Aβ dimer is the crucial molecule in AD pathology. Aged rhesus monkeys spontaneously develop Aβ deposits in the brain with striking similarities to those of aged humans. Thus, rhesus monkeys are an ideal natural model to study the composition of Aβ oligomer isoforms and their downstream effects on AD pathology. In this study, we found that Aβ deposits in aged monkey brains included 3‐mer, 5‐mer, 9‐mer, 10‐mer, and 12‐mer oligomers, but not 2‐mer oligomers. The Aβ deposits, which were devoid of Aβ dimers, induced glial pathology (microgliosis, abnormal microglia morphology, and astrocytosis), but not the subsequent downstream pathologies of AD, including Tau pathology, neurodegeneration, and synapse loss. Our results indicate that the Aβ dimer plays an important role in AD pathogenesis. Thus, targeting the Aβ dimer is a promising strategy for preventing AD.

Sulforaphane Inhibits the Generation of Amyloid-β Oligomer and Promotes Spatial Learning and Memory in Alzheimer's Disease (PS1V97L) Transgenic Mice

Abnormal amyloid-β (Aβ) aggregates are a striking feature of Alzheimer's disease (AD), and Aβ oligomers have been proven to be crucial in the pathology of AD. Any intervention targeting the generation or aggregation of Aβ can be expected to be useful in AD treatment. Oxidative stress and inflammation are common pathological changes in AD that are involved in the generation and aggregation of Aβ. In the present study, 6-month-old PS1V97L transgenic (Tg) mice were treated with sulforaphane, an antioxidant, for 4 months, and this treatment significantly inhibited the generation and aggregation of Aβ. Sulforaphane also alleviated several downstream pathological changes that including tau hyperphosphorylation, oxidative stress, and neuroinflammation. Most importantly, the cognition of the sulforaphane-treated PS1V97L Tg mice remained normal compared to that of wild-type mice at 10 months of age, when dementia typically emerges in PS1V97L Tg mice. Pretreating cultured cortical neurons with sulforaphane also protected against neuronal injury caused by Aβ oligomers in vitro. These findings suggest that sulforaphane may be a potential compound that can inhibit Aβ oligomer production in AD.

Active full-length DNA Aβ42 immunization in 3xTg-AD mice reduces not only amyloid deposition but also tau pathology

Background

Alzheimer’s disease (AD) is the most well-known and most common type of age-related dementia. Amyloid deposition and hyperphosphorylation of tau protein are both pathological hallmarks of AD. Using a triple-transgenic mouse model (3xTg-AD) that develops plaques and tangles in the brain similar to human AD, we provide evidence that active full-length DNA amyloid-β peptide 1–42 (Aβ₄₂) trimer immunization leads to reduction of both amyloid and tau aggregation and accumulation.

Methods

Immune responses were monitored by enzyme-linked immunosorbent assay (ELISA) (antibody production) and enzyme-linked immunospot (cellular activation, cytokine production). Brains from 20-month-old 3x Tg-AD mice that had received DNA Aβ₄₂ immunotherapy were compared with brains from age- and gender-matched transgenic Aβ₄₂ peptide-immunized and control mice by histology, Western blot analysis, and ELISA. Protein kinase activation and kinase levels were studied in Western blots from mouse hemibrain lysates.

Results

Quantitative ELISA showed a 40% reduction of Aβ₄₂ peptide and a 25–50% reduction of total tau and different phosphorylated tau molecules in the DNA Aβ₄₂ trimer-immunized 3xTg-AD mice compared with nonimmunized 3xTg-AD control animals. Plaque and Aβ peptide reductions in the brain were due to the anti-Aβ antibodies generated following the immunizations. Reductions of tau were likely due to indirect actions such as less Aβ in the brain resulting in less tau kinase activation.

Conclusions

The significance of these findings is that DNA Aβ₄₂ trimer immunotherapy targets two major pathologies in AD—amyloid plaques and neurofibrillary tangles—in one vaccine without inducing inflammatory T-cell responses, which carry the danger of autoimmune inflammation, as found in a clinical trial using active Aβ₄₂ peptide immunization in patients with AD (AN1792).

A Bifunctional Anti-Amyloid Blocks Oxidative Stress and the Accumulation of Intraneuronal Amyloid-Beta

There is growing recognition regarding the role of intracellular amyloid beta (Aβ) in the Alzheimer’s disease process, which has been linked with aberrant signaling and the disruption of protein degradation mechanisms. Most notably, intraneuronal Aβ likely underlies the oxidative stress and mitochondrial dysfunction that have been identified as key elements of disease progression. In this study, we employed fluorescence imaging to explore the ability of a bifunctional small molecule to reduce aggregates of intracellular Aβ and attenuate oxidative stress. Structurally, this small molecule is comprised of a nitroxide spin label linked to an amyloidophilic fluorene and is known as spin-labeled fluorene (SLF). The effect of the SLF on intracellular Aβ accumulation and oxidative stress was measured in MC65 cells, a human neuronal cell line with inducible expression of the amyloid precursor protein and in the N2a neuronal cell line treated with exogenous Aβ. Super-resolution microscopy imaging showed SLF decreases the accumulation of intracellular Aβ. Confocal microscopy imaging of MC65 cells treated with a reactive oxygen species (ROS)-sensitive dye demonstrated SLF significantly reduces the intracellular Aβ-induced ROS signal. In order to determine the contributions of the separate SLF moieties to these protective activities, experiments were also carried out on cells with nitroxides lacking the Aβ targeting domain or fluorene derivatives lacking the nitroxide functionality. The findings support a synergistic effect of SLF in counteracting both the conformational toxicity of both endogenous and exogenous Aβ, its promotion of ROS, and Aβ metabolism. Furthermore, these studies demonstrate an intimate link between ROS production and Aβ oligomer formation.

The influences of E22Q mutant on solvated 3Aβ11-40 peptide: A REMD study

The residue E22 plays a critical role in the aggregation process of Amyloid beta (Aβ) peptides. The effect of E22Q mutant on the shapes of the solvated Aβ_11-40 trimer is clarified using a replica exchange molecular dynamics (REMD) simulation employing ∼20.6 μs of MD simulations with 48 disparate replicas. The increase of intramolecular polar contacts and salt bridge between the residue D23 to residues (24-29) was observed. The residual secondary structure of the mutated trimer is shifted in a similar way to the picture observed in previous investigations of F19W mutant. The free energy surface (FES) of the mutated E22Q system has a fewer number of minima in comparison with the wild-type trimer. The optimized shapes of the mutated E22Q form a significant increase in beta structure (47%) and serious decrease in coil content (46%) compared with the wild-type (of 36 and 56%, respectively). The binding affinity of constituting chains to the rest is of -43.7 ± 6.5 kcal/mol, implying that the representative structure of E22Q is more stable than the wild-type one. Furthermore, the E22Q mutant increases the size of stable structures due to larger collision cross section (CCS) and solvent accessible area (SASA). The observed results may enhance the Aβ inhibition throughout the contribution to the knowledge of the Aβ oligomerization/aggregation.

Bexarotene Does Not Clear Amyloid Beta Plaques but Delays Fibril Growth: Molecular Mechanisms

In 2012, it was reported that anticancer drug bexarotene reduced amyloid plaque and improved mental functioning in a small sample of mice engineered to exhibit Alzheimer's like symptoms. It has been suggested that bexarotene stimulates expression of apolipoprotein E (ApoE) leading to intracellular clearance of amyloid beta (Aβ). However, the effect of bexarotene on clearance of plaques has not been seen in some mouse models. Two interesting questions include whether bexarotene can destroy Aβ fibrils via direct interaction with them and how this compound impacts the lag phase in the fibril growth process. By the Thioflavin T fluorescence assay and atomic force microscopy, we have shown that bexarotene prolongs the lag phase, but it does not degrade Aβ fibrils. The impotence of bexarotene in destroying fibrils means that this compound is weakly bound to Aβ. On the other hand, the weak binding would prevent bexarotene from prolonging the lag phase. Thus, our two main in vitro observations seem to contradict each other. In order to settle this problem at the atomic level, we have performed all-atom molecular dynamics simulations in explicit water. We have demonstrated that bexarotene is not capable to reduce amyloid deposits due to weak binding to Aβ fibrils. However, it delays the self-assembly through reduction of the β-content of Aβ monomers at high enough ligand concentrations. Bexarotene is the first compound which displays such an unusual behavior. We have also shown that bexarotene has a low binding propensity to Aβ monomer and dimer.

Attenuation of β-Amyloid Toxicity In Vitro and In Vivo by Accelerated Aggregation

Accumulation and aggregation of β-amyloid (Aβ) peptides result in neuronal death, leading to cognitive dysfunction in Alzheimer's disease. The self-assembled Aβ molecules form various intermediate aggregates including oligomers that are more toxic to neurons than the mature aggregates, including fibrils. Thus, one strategy to alleviate Aβ toxicity is to facilitate the conversion of Aβ intermediates to larger aggregates such as fibrils. In this study, we designed a peptide named A3 that significantly enhanced the formation of amorphous aggregates of Aβ by accelerating the aggregation kinetics. Thioflavin T fluorescence experiments revealed an accelerated aggregation of Aβ monomers, accompanying reduced Aβ cytotoxicity. Transgenic Caenorhabditis elegans over-expressing amyloid precursor protein exhibited paralysis due to the accumulation of Aβ oligomers, and this phenotype was attenuated by feeding the animals with A3 peptide. These findings suggest that the Aβ aggregation-promotion effect can potentially be useful for developing strategies to reduce Aβ toxicity.

Solvent exposure of Tyr10 as a probe of structural differences between monomeric and aggregated forms of the amyloid-β peptide

Aggregation of amyloid-β (Aβ) peptides is a characteristic pathological feature of Alzheimer's disease. We have exploited the relationship between solvent exposure and intrinsic fluorescence of a single tyrosine residue, Tyr10, in the Aβ sequence to probe structural features of the monomeric, oligomeric and fibrillar forms of the 42-residue Aβ1-42. By monitoring the quenching of Tyr10 fluorescence upon addition of water-soluble acrylamide, we show that in Aβ1-42 oligomers this residue is solvent-exposed to a similar extent to that found in the unfolded monomer. By contrast, Tyr10 is significantly shielded from acrylamide quenching in Aβ1-42 fibrils, consistent with its proximity to the fibrillar cross-β core. Furthermore, circular dichroism measurements reveal that Aβ1-42 oligomers have a considerably lower β-sheet content than the Aβ1-42 fibrils, indicative of a less ordered molecular arrangement in the former. Taken together these findings suggest significant differences in the structural assembly of oligomers and fibrils that are consistent with differences in their biological effects.

Folic acid attenuates the effects of amyloid β oligomers on DNA methylation in neuronal cells

PURPOSE

Alzheimer's disease (AD) is a highly prevalent type of dementia. The epigenetic mechanism of gene methylation provides a putative link between nutrition, one-carbon metabolism, and disease progression because folate deficiency may cause hypomethylation of promoter regions in AD-relevant genes. We hypothesized that folic acid supplementation may protect neuron cells from amyloid β (Aβ) oligomer-induced toxicity by modulating DNA methylation of APP and PS1 in AD models.

METHODS

Primary hippocampal neuronal cells and hippocampal HT-22 cells were incubated for 24 h with a combination of folic acid and either Aβ oligomers or vehicle and were then incubated for 72 h with various concentrations of folic acid. AD transgenic mice were fed either folate-deficient or control diets and gavaged daily with various doses of folic acid (0 or 600 μg/kg). DNA methyltransferase (DNMT) activity, cell viability, methylation potential of cells, APP and PS1 expression, and the methylation of the respective promoters were determined.

RESULTS

Aβ oligomers lowered DNMT activity, increased PS1 and APP expression, and decreased cell viability. Folic acid dose-dependently stimulated methylation potential and DNMT activity, altered PS1 and APP promoter methylation, decreased PS1 and APP expression, and partially preserved cell viability. Folic acid increased PS1 and APP promoter methylation in AD transgenic mice.

CONCLUSION

These results suggest a mechanism by which folic acid may prevent Aβ oligomer-induced neuronal toxicity.

Coffee and caffeine potentiate the antiamyloidogenic activity of melatonin via inhibition of Aβ oligomerization and modulation of the Tau-mediated pathway in N2a/APP cells

There is an increasing prevalence of Alzheimer's disease (AD), which has become a public health issue. However, the underlying mechanisms for the pathogenesis of AD are not fully understood, and the current therapeutic drugs cannot produce acceptable efficacy in AD patients. Previous animal studies have shown that coffee (Coff), caffeine (Caff), and melatonin (Mel) have beneficial effects on AD. Disturbed circadian rhythms are observed in AD, and chronotherapy has shown promising effects on AD. In this study, we examined whether a combination of Coff or Caff plus Mel produced a synergistic/additive effect on amyloid-β (Aβ) generation in Neuro-2a (N2a)/amyloid precursor protein (APP) cells and the possible mechanisms involved. Cells were treated with Coff or Caff, with or without combined Mel, with three different chronological regimens. In regimen 1, cells were treated with Coff or Caff for 12 hours in the day, followed by Mel for 12 hours in the night. For regimen 2, cells were treated with Coff or Caff plus Mel for 24 hours, from 7 am to 7 am the next day. In regimen 3, cells were treated with Coff or Caff plus Mel with regimen 1 or 2 for 5 consecutive days. The extracellular Aβ40/42 and Aβ oligomer levels were determined using enzyme-linked immunosorbent assay (ELISA) kits. The expression and/or phosphorylation levels of glycogen synthase kinase 3β (GSK3β), Erk1/2, PI3K, Akt, Tau, Wnt3α, β-catenin, and Nrf2 were detected by Western blot assay. The results showed that regimen 1 produced an additive antiamyloidogenic effect with significantly reduced extracellular levels of Aβ40/42 and Aβ42 oligomers. Regimen 2 did not result in remarkable effects, and regimen 3 showed a less antiamyloidogenic effect compared to regimen 1. Coff or Caff, plus Mel reduced oxidative stress in N2a/APP cells via the Nrf2 pathway. Coff or Caff, plus Mel inhibited GSK3β, Akt, PI3K p55, and Tau phosphorylation but enhanced PI3K p85 and Erk1/2 phosphorylation in N2a/APP cells. Coff or Caff, plus Mel downregulated Wnt3α expression but upregulated β-catenin. However, Coff or Caff plus Mel did not significantly alter the production of T helper cell (Th)1-related interleukin (IL)-12 and interferon (IFN)-γ and Th2-related IL-4 and IL-10 in N2a/APP cells. The autophagy of cells was not affected by the combinations. Taken together, combination of Caff or Coff, before treatment with Mel elicits an additive antiamyloidogenic effects in N2a/APP cells, probably through inhibition of Aβ oligomerization and modulation of the Akt/GSK3β/Tau signaling pathway.

Elucidating molecular mass and shape of a neurotoxic Aβ oligomer

Alzheimer's disease (AD), the most prevalent type of dementia, has been associated with the accumulation of amyloid β oligomers (AβOs) in the central nervous system. AβOs vary widely in size, ranging from dimers to larger than 100 kDa. Evidence indicates that not all oligomers are toxic, and there is yet no consensus on the size of the actual toxic oligomer. Here we used NU4, a conformation-dependent anti-AβO monoclonal antibody, to investigate size and shape of a toxic AβO assembly. By using size-exclusion chromatography and immuno-based detection, we isolated an AβO-NU4 complex amenable for biochemical and morphological studies. The apparent molecular mass of the NU4-targeted oligomer was 80 kDa. Atomic force microscopy imaging of the AβO-NU4 complex showed a size distribution centered at 5.37 nm, an increment of 1.5 nm compared to the size of AβOs (3.85 nm). This increment was compatible with the size of NU4 (1.3 nm), suggesting a 1:1 oligomer to NU4 ratio. NU4-reactive oligomers extracted from AD human brain concentrated in a molecular mass range similar to that found for in vitro prepared oligomers, supporting the relevance of the species herein studied. These results represent an important step toward understanding the connection between AβO size and toxicity.