AD synapses contain abundant Aβ monomer and multiple soluble oligomers, including a 56-kDa assembly

The concept of resilience to Alzheimer's Disease: current definitions and cellular and molecular mechanisms

Some individuals are able to maintain their cognitive abilities despite the presence of significant Alzheimer's Disease (AD) neuropathological changes. This discrepancy between cognition and pathology has been labeled as resilience and has evolved into a widely debated concept. External factors such as cognitive stimulation are associated with resilience to AD, but the exact cellular and molecular underpinnings are not completely understood. In this review, we discuss the current definitions used in the field, highlight the translational approaches used to investigate resilience to AD and summarize the underlying cellular and molecular substrates of resilience that have been derived from human and animal studies, which have received more and more attention in the last few years. From these studies the picture emerges that resilient individuals are different from AD patients in terms of specific pathological species and their cellular reaction to AD pathology, which possibly helps to maintain cognition up to a certain tipping point. Studying these rare resilient individuals can be of great importance as it could pave the way to novel therapeutic avenues for AD.

Aβ∗56 is a stable oligomer that impairs memory function in mice

Amyloid-β (Aβ) oligomers consist of fibrillar and non-fibrillar soluble assemblies of the Aβ peptide. Aβ∗56 is a non-fibrillar Aβ assembly that is linked to memory deficits. Previous studies did not decipher specific forms of Aβ present in Aβ∗56. Here, we confirmed the memory-impairing characteristics of Aβ∗56 and extended its biochemical characterization. We used anti-Aβ(1-x), anti-Aβ(x-40), anti-Aβ(x-42), and A11 anti-oligomer antibodies in conjunction with western blotting, immunoaffinity purification, and size-exclusion chromatography to probe aqueous brain extracts from Tg2576, 5xFAD, and APP/TTA mice. In Tg2576, Aβ∗56 is a ∼56-kDa, SDS-stable, A11-reactive, non-plaque-dependent, water-soluble, brain-derived oligomer containing canonical Aβ(1-40). In 5xFAD, Aβ∗56 is composed of Aβ(1-42), whereas in APP/TTA, it contains both Aβ(1-40) and Aβ(1-42). When injected into the hippocampus of wild-type mice, Aβ∗56 derived from Tg2576 mice impairs memory. The unusual stability of this oligomer renders it an attractive candidate for studying relationships between molecular structure and effects on brain function.

Nascent Aβ42 Fibrillization in Synaptic Endosomes Precedes Plaque Formation in a Mouse Model of Alzheimer's-like β-Amyloidosis

Accumulation of amyloid-β peptide (Aβ) aggregates in synapses may contribute to the profound synaptic loss characteristic of Alzheimer's disease (AD). The origin of synaptic Aβ aggregates remains elusive, but loss of endosomal proteostasis may trigger their formation. In this study, we identified the synaptic compartments where Aβ accumulates, and performed a longitudinal analysis of synaptosomes isolated from brains of TgCRND8 APP transgenic mice of either sex. To evaluate the specific contribution of Aβ-degrading protease endothelin-converting enzyme (ECE-1) to synaptic/endosomal Aβ homeostasis, we analyzed the effect of partial Ece1 KO in brain and complete ECE1 KO in SH-SY5Y cells. Global inhibition of ECE family members was used to further assess their role in preventing synaptic Aβ accumulation. Results showed that, before extracellular amyloid deposition, synapses were burdened with detergent-soluble Aβ monomers, oligomers, and fibrils. Levels of all soluble Aβ species declined thereafter, as Aβ42 turned progressively insoluble and accumulated in Aβ-producing synaptic endosomal vesicles with characteristics of multivesicular bodies. Accordingly, fibrillar Aβ was detected in brain exosomes. ECE-1-deficient mice had significantly increased endogenous synaptosomal Aβ42 levels, and protease inhibitor experiments showed that, in TgCRND8 mice, synaptic Aβ42 became nearly resistant to degradation by ECE-related proteases. Our study supports that Aβ accumulating in synapses is produced locally, within endosomes, and does not require the presence of amyloid plaques. ECE-1 is a determinant factor controlling the accumulation and fibrillization of nascent Aβ in endosomes and, in TgCRND8 mice, Aβ overproduction causes rapid loss of Aβ42 solubility that curtails ECE-mediated degradation.SIGNIFICANCE STATEMENT Deposition of aggregated Aβ in extracellular plaques is a defining feature of AD. Aβ aggregates also accumulate in synapses and may contribute to the profound synaptic loss and cognitive dysfunction typical of the disease. However, it is not clear whether synaptotoxic Aβ is mainly derived from plaques or if it is produced and aggregated locally, within affected synaptic compartments. Filling this knowledge gap is important for the development of an effective treatment for AD, as extracellular and intrasynaptic pools of Aβ may not be equally modulated by immunotherapies or other therapeutic approaches. In this manuscript, we provide evidence that Aβ aggregates building up in synapses are formed locally, within synaptic endosomes, because of disruptions in nascent Aβ proteostasis.

Synapse pathology in Alzheimer's disease

Synapse loss and damage are central features of Alzheimer's disease (AD) and contribute to the onset and progression of its behavioural and physiological features. Here we review the literature describing synapse pathology in AD, from what we have learned from microscopy in terms of its impacts on synapse architecture, to the mechanistic role of Aβ, tau and glial cells, mitochondrial dysfunction, and the link with AD risk genes. We consider the emerging view that synapse pathology may operate at a further level, that of synapse diversity, and discuss the prospects for leveraging new synaptome mapping methods to comprehensively understand the molecular properties of vulnerable and resilient synapses. Uncovering AD impacts on brain synapse diversity should inform therapeutic approaches targeted at preserving or replenishing lost and damaged synapses and aid the interpretation of clinical imaging approaches that aim to measure synapse damage.

Aβ*56 is a stable oligomer that correlates with age-related memory loss in Tg2576 mice

Amyloid-β (Aβ) oligomers consist of fibrillar and non-fibrillar soluble assemblies of the Aβ peptide. Tg2576 human amyloid precursor protein (APP)-expressing transgenic mice modeling Alzheimer's disease produce Aβ*56, a non-fibrillar Aβ assembly that has been shown by several groups to relate more closely to memory deficits than plaques. Previous studies did not decipher specific forms of Aβ present in Aβ*56. Here, we confirm and extend the biochemical characterization of Aβ*56. We used anti-Aβ(1-x), anti-Aβ(x-40), and A11 anti-oligomer antibodies in conjunction with western blotting, immunoaffinity purification, and size-exclusion chromatography to probe aqueous brain extracts from Tg2576 mice of different ages. We found that Aβ*56 is a ∼56-kDa, SDS-stable, A11-reactive, non-plaque-related, water-soluble, brain-derived oligomer containing canonical Aβ(1-40) that correlates with age-related memory loss. The unusual stability of this high molecular-weight oligomer renders it an attractive candidate for studying relationships between molecular structure and effects on brain function.

Metabotropic Glutamate Receptors Modulate Exocytotic Tau Release and Propagation

Using synaptosomes purified from the brains of two transgenic mouse models overexpressing mutated human tau (TgP301S and Tg4510) and brains of patients with sporadic Alzheimer's disease, we showed that aggregated and hyperphosphorylated tau was both present in purified synaptosomes and released in a calcium- and synaptosome-associated protein of 25 kDa (SNAP25)-dependent manner. In all mouse and human synaptosomal preparations, tau release was inhibited by the selective metabotropic glutamate receptor 2/3 (mGluR2/3) agonist LY379268, an effect prevented by the selective mGlu2/3 antagonist LY341495. LY379268 was also able to block pathologic tau propagation between primary neurons in an in vitro microfluidic cellular model. These novel results are transformational for our understanding of the molecular mechanisms mediating tau release and propagation at synaptic terminals in Alzheimer's disease and suggest that these processes could be inhibited therapeutically by the selective activation of presynaptic G protein-coupled receptors. SIGNIFICANCE STATEMENT: Pathological tau release and propagation are key neuropathological events underlying cognitive decline in Alzheimer's disease patients. This paper describes the role of regulated exocytosis, and the soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) protein SNAP25, in mediating tau release from rodent and human synaptosomes. This paper also shows that a selective mGluR2/3 agonist is highly effective in blocking tau release from synaptosomes and tau propagation between neurons, opening the way to the discovery of novel therapeutic approaches to this devastating disease.

Emerging Role of Neuron-Glia in Neurological Disorders: At a Glance

Based on the diverse physiological influence, the impact of glial cells has become much more evident on neurological illnesses, resulting in the origins of many diseases appearing to be more convoluted than previously happened. Since neurological disorders are often random and unknown, hence the construction of animal models is difficult to build, representing a small fraction of people with a gene mutation. As a result, an immediate necessity is grown to work within in vitro techniques for examining these illnesses. As the scientific community recognizes cell-autonomous contributions to a variety of central nervous system illnesses, therapeutic techniques involving stem cells for treating neurological diseases are gaining traction. The use of stem cells derived from a variety of sources is increasingly being used to replace both neuronal and glial tissue. The brain's energy demands necessitate the reliance of neurons on glial cells in order for it to function properly. Furthermore, glial cells have diverse functions in terms of regulating their own metabolic activities, as well as collaborating with neurons via secreted signaling or guidance molecules, forming a complex network of neuron-glial connections in health and sickness. Emerging data reveals that metabolic changes in glial cells can cause morphological and functional changes in conjunction with neuronal dysfunction under disease situations, highlighting the importance of neuron-glia interactions in the pathophysiology of neurological illnesses. In this context, it is required to improve our understanding of disease mechanisms and create potential novel therapeutics. According to research, synaptic malfunction is one of the features of various mental diseases, and glial cells are acting as key ingredients not only in synapse formation, growth, and plasticity but also in neuroinflammation and synaptic homeostasis which creates critical physiological capacity in the focused sensory system. The goal of this review article is to elaborate state-of-the-art information on a few glial cell types situated in the central nervous system (CNS) and highlight their role in the onset and progression of neurological disorders.

The HSP40 chaperone Ydj1 drives amyloid beta 42 toxicity

Amyloid beta 42 (Abeta42) is the principal trigger of neurodegeneration during Alzheimer's disease (AD). However, the etiology of its noxious cellular effects remains elusive. In a combinatory genetic and proteomic approach using a yeast model to study aspects of intracellular Abeta42 toxicity, we here identify the HSP40 family member Ydj1, the yeast orthologue of human DnaJA1, as a crucial factor in Abeta42-mediated cell death. We demonstrate that Ydj1/DnaJA1 physically interacts with Abeta42 (in yeast and mouse), stabilizes Abeta42 oligomers, and mediates their translocation to mitochondria. Consequently, deletion of YDJ1 strongly reduces co-purification of Abeta42 with mitochondria and prevents Abeta42-induced mitochondria-dependent cell death. Consistently, purified DnaJ chaperone delays Abeta42 fibrillization in vitro, and heterologous expression of human DnaJA1 induces formation of Abeta42 oligomers and their deleterious translocation to mitochondria in vivo. Finally, downregulation of the Ydj1 fly homologue, Droj2, improves stress resistance, mitochondrial morphology, and memory performance in a Drosophila melanogaster AD model. These data reveal an unexpected and detrimental role for specific HSP40s in promoting hallmarks of Abeta42 toxicity.

Molecular Insight into Cu2+-Induced Conformational Transitions of Amyloid β-Protein from Fast Kinetic Analysis and Molecular Dynamics Simulations

Cu²⁺-mediated amyloid β-protein (Aβ) aggregation is implicated in the pathogenesis of Alzheimer's disease, so it is of significance to understand Cu²⁺-mediated conformational transitions of Aβ. Herein, four Aβ mutants were created by using the environment-sensitive cyanophenylalanine to respectively substitute F4, Y10, F19, and F20 residues of Aβ₄₀. By using stopped-flow fluorescence spectroscopy and molecular dynamics (MD) simulations, the early stage conformational transitions of the mutants mediated by Cu²⁺ binding were investigated. The fast kinetics unveils that Cu²⁺ has more significant influence on the conformational changes of N-terminal (F4 and Y10) than on the central hydrophobic core (CHC, F19, and F20) under different pH conditions (pH 6.6-8.0), especially Y10. Interestingly, lag periods of the conformational transitions are observed for the F19 and F20 mutants at pH 8.0, indicating the slow response of the two mutation sites on the conformational transitions. More importantly, significantly longer lag periods for F20 than for F19 indicate the conduction of the transition from F19 to F20. The conduction time (difference in lag period) decreases from 4.5 s at Cu²⁺ = 0 to undetectable (<1 ms) at Cu²⁺ = 10 μM. The significant difference in the response time of F19 and F20 and the fast local conformational changes of Y10 imply that the conformational transitions of Aβ start around Y10. MD simulations support the observation of hydrophobicity increase at N-terminal during the conformational transitions of Aβ-Cu²⁺. It also reveals that Y10 is immediately approached by Cu²⁺, supporting the speculation that the starting point of conformational transitions of Aβ is near Y10. The work has provided molecular insight into the early stage conformational transitions of Aβ₄₀ mediated by Cu²⁺.

Exosomal tau with seeding activity is released from Alzheimer's disease synapses, and seeding potential is associated with amyloid beta

Synaptic transfer of tau has long been hypothesized from the human pathology pattern and has been demonstrated in vitro and in vivo, but the precise mechanisms remain unclear. Extracellular vesicles such as exosomes have been suggested as a mechanism, but not all tau is exosomal. The present experiments use a novel flow cytometry assay to quantify depolarization of synaptosomes by KCl after loading with FM2-10, which induces a fluorescence reduction associated with synaptic vesicle release; the degree of reduction in cryopreserved human samples equaled that seen in fresh mouse synaptosomes. Depolarization induced the release of vesicles in the size range of exosomes, along with tetraspanin markers of extracellular vesicles. A number of tau peptides were released, including tau oligomers; released tau was primarily unphosphorylated and C-terminal truncated, with Aβ release just above background. When exosomes were immunopurified from release supernatants, a prominent tau band showed a dark smeared appearance of SDS-stable oligomers along with the exosomal marker syntenin-1, and these exosomes induced aggregation in the HEK tau biosensor assay. However, the flow-through did not seed aggregation. Size exclusion chromatography of purified released exosomes shows faint signals from tau in the same fractions that show a CD63 band, an exosomal size signal, and seeding activity. Crude synaptosomes from control, tauopathy, and AD cases demonstrated lower seeding in tauopathy compared to AD that is correlated with the measured Aβ42 level. These results show that AD synapses release exosomal tau that is C-terminal-truncated, oligomeric, and with seeding activity that is enhanced by Aβ. Taken together with previous findings, these results are consistent with a direct prion-like heterotypic seeding of tau by Aβ within synaptic terminals, with subsequent loading of aggregated tau onto exosomes that are released and competent for tau seeding activity.

Effect of Different Aβ Aggregates as Antigen on the Measure of Naturally Occurring Autoantibodies against Amyloid-β40/42 in IVIG

BACKGROUND

The specific Intravenous Immunoglobulin (IVIG) for Alzheimer's Disease (AD) is developing, which contains a high level of naturally occurring autoantibodies against amyloid-β (nAbs-Aβ), and the measure of nAbs-Aβ content is greatly essential. Though Enzyme-Linked Immunosorbent Assay (ELISA) has been widely used in detecting the nAbs-Aβ content, the impact of Aβ aggregates species chosen as antigen in ELISA on this measure has not been evaluated.

OBJECTIVE

To clarify the influence of different Aβ40/42 aggregates as antigen during ELISA on the content of nAbs-Aβ40/42 measured in IVIG.

METHOD

Preparation of various Aβ40/42 aggregates was performed by different aggregation solutions and various lengths of time, and analyzed by western blot. Different Aβ40/42 aggregates as antigen were adopted to measure the nAbs-Aβ40/42 content in IVIG by ELISA, and the control was carried out to reduce interference of nonspecific binding. The Bonferroni and Dunnett's T3 were used for statistical analysis.

RESULTS

The duration for the formation of Aβ40/42 aggregates had more effect on detecting nAbs-Aβ40/42 content in IVIG than the aggregation solution. Higher content of nAbs-Aβ40/42 in the same IVIG was displayed when measured with Aβ40/42 aggregates at day 3, instead of at day 0.5 and day 7.0. The nAbs- Aβ40/42 contents in the same IVIG measured with Aβ40/42 aggregates prepared in different solutions were obviously different, but there was no significant regularity among them.

CONCLUSION

The nAbs-Aβ40/42 content in the same IVIG is significantly different when measured with Aβ40/42 aggregated under different conditions. The nAbs-Aβ40/42 content in IVIG by antigen-dependent measures, like ELISA, is uncertain.

X-ray Crystallography Reveals Parallel and Antiparallel β-Sheet Dimers of a β-Hairpin Derived from Aβ16-36 that Assemble to Form Different Tetramers

High-resolution structures of oligomers formed by the β-amyloid peptide, Aβ, are important for understanding the molecular basis of Alzheimer's disease. Dimers of Aβ are linked to the pathogenesis and progression of Alzheimer's disease, and tetramers of Aβ are neurotoxic. This paper reports the X-ray crystallographic structures of dimers and tetramers, as well as an octamer, formed by a peptide derived from the central and C-terminal regions of Aβ. In the crystal lattice, the peptide assembles to form two different dimers-an antiparallel β-sheet dimer and a parallel β-sheet dimer-that each further self-assemble to form two different tetramers-a sandwich-like tetramer and a twisted β-sheet tetramer. The structures of these dimers and tetramers derived from Aβ serve as potential models for dimers and tetramers of full-length Aβ that form in vitro and in Alzheimer's disease-afflicted brains.

Microglia Do Not Take Up Soluble Amyloid-beta Peptides, But Partially Degrade Them by Secreting Insulin-degrading Enzyme

Microglia play important roles in the pathogenesis of Alzheimer's disease (AD), in part, by affecting the clearance of amyloid-β (Aβ) peptides. Most studies, however, used synthetic soluble Aβ (sAβ) at higher concentrations. The exact mechanisms underlying microglia-mediated clearance of physiological sAβ at very low concentrations remain unclear. Here we reported that there were much more Iba-1- and CD68-positive microglia and significantly less sAβ left in the brain of adult mice 5 days after the surgery of sAβ microinjection compared to 2 h after the surgery (p < 0.05). However, very few Iba-1- and CD68-positive microglia co-localized with microinjected fluorescently labeled sAβ (FLsAβ₄₂) 5 days after the surgery. Also, there was no co-localization of FLsAβ₄₂ with a lysosomal marker (LAMP-1) 5 days after the surgery. There was no significant difference in the percentage of Aβ⁺/PE-CD11b⁺/APC-CD45^low microglia between the control group and the group microinjected with TBS-soluble Aβ extracted from the brains of AD patients (p > 0.05). The degradation of physiological sAβ was prevented by a highly selective insulin-degrading enzyme inhibitor (Ii1) but not by a phagocytosis inhibitor (polyinosinic acid) or pinocytosis inhibitor (cytochalasin B) in vitro. Furthermore, the reduction of synthetic and physiological sAβ in the brain was partially prevented by the co-injection of Ii1 in vivo (p < 0.05). Our results demonstrate that microglia do not take up synthetic or physiological sAβ, but partially degrade it via the secretion of insulin-degrading enzyme, which will be beneficial for understanding how sAβ is removed from the brain by microglia.

Synaptosome as a tool in Alzheimer's disease research

Synapse dysfunction is an integral feature of Alzheimer's disease (AD) pathophysiology. In fact, prodromal manifestation of structural and functional deficits in synapses much prior to appearance of overt pathological hallmarks of the disease indicates that AD might be considered as a degenerative disorder of the synapses. Several research instruments and techniques have allowed us to study synaptic function and plasticity and their alterations in pathological conditions, such as AD. One such tool is the biochemically isolated preparations of detached and resealed synaptic terminals, the "synaptosomes". Because of the preservation of many of the physiological processes such as metabolic and enzymatic activities, synaptosomes have proved to be an indispensable ex vivo model system to study synapse physiology both when isolated from fresh or cryopreserved tissues, and from animal or human post-mortem tissues. This model system has been tremendously successful in the case of post-mortem tissues because of their accessibility relative to acute brain slices or cultures. The current review details the use of synaptosomes in AD research and its potential as a valuable tool in furthering our understanding of the pathogenesis and in devising and testing of therapeutic strategies for the disease.

Expedition into Taurine Biology: Structural Insights and Therapeutic Perspective of Taurine in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are characterized by the accumulation of misfolded proteins. The hallmarks of protein aggregation in NDs proceed with impairment in the mitochondrial function, besides causing an enhancement in endoplasmic reticulum (ER) stress, neuroinflammation and synaptic loss. As accumulation of misfolded proteins hampers normal neuronal functions, it triggers ER stress, which leads to the activation of downstream effectors formulating events along the signaling cascade—referred to as unfolded protein response (UPRER) —thereby controlling cellular gene expression. The absence of disease-modifying therapeutic targets in different NDs, and the exponential increase in the number of cases, makes it critical to explore new approaches to treating these devastating diseases. In one such approach, osmolytes (low molecular weight substances), such as taurine have been found to promote protein folding under stress conditions, thereby averting aggregation of the misfolded proteins. Maintaining the structural integrity of the protein, taurine-mediated resumption of protein folding prompts a shift in folding homeostasis more towards functionality than towards aggregation and degradation. Together, taurine enacts protection in NDs by causing misfolded proteins to refold, so as to regain their stability and functionality. The present study provides recent and useful insights into understanding the progression of NDs, besides summarizing the genetics of NDs in correlation with mitochondrial dysfunction, ER stress, neuroinflammation and synaptic loss. It also highlights the structural and functional aspects of taurine in imparting protection against the aggregation/misfolding of proteins, thereby shifting the focus more towards the development of effective therapeutic modules that could avert the development of NDs.

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.

Glial Contribution to Excitatory and Inhibitory Synapse Loss in Neurodegeneration

Synapse loss is an early feature shared by many neurodegenerative diseases, and it represents the major correlate of cognitive impairment. Recent studies reveal that microglia and astrocytes play a major role in synapse elimination, contributing to network dysfunction associated with neurodegeneration. Excitatory and inhibitory activity can be affected by glia-mediated synapse loss, resulting in imbalanced synaptic transmission and subsequent synaptic dysfunction. Here, we review the recent literature on the contribution of glia to excitatory/inhibitory imbalance, in the context of the most common neurodegenerative disorders. A better understanding of the mechanisms underlying pathological synapse loss will be instrumental to design targeted therapeutic interventions, taking in account the emerging roles of microglia and astrocytes in synapse remodeling.

Improved synaptic and cognitive function in aged 3 × Tg-AD mice with reduced amyloid-β after immunotherapy with a novel recombinant 6Aβ15-TF chimeric vaccine

Alzheimer's disease (AD) is the most common progressive neurodegenerative disorder impairing memory and cognition. In this study, we describe the immunogenicity and protective efficacy of the novel recombinant 6Aβ15-TF chimeric antigen as a subunit protein vaccine for AD. Recombinant 6Aβ15-TF chimeric vaccine induced strong Aβ-specific humoral immune responses without Aβ-specific T cell immunity in C57/BL6 and 3 × Tg-AD mice at different ages. As an early immunotherapy model for AD, this vaccine induced high titers of long-lasting anti-Aβ42 antibodies in aged 3 × Tg-AD mice, which led to improve behavioral performance and markedly reduced the levels of insoluble and soluble Aβ and Aβ oligomers. In agreement with these findings, immunotherapy with 6Aβ15-TF prevented the Aβ-induced decrease of presynaptic and postsynaptic proteins in aged 3 × Tg-AD mice. Our results suggest that this novel and highly immunogenic recombinant 6Aβ15-TF chimeric vaccine provides neuroprotection in AD mice and can be considered an effective AD candidate vaccine.

Human Striatal Dopaminergic and Regional Serotonergic Synaptic Degeneration with Lewy Body Disease and Inheritance of APOE ε4

Cognitive impairment in older individuals is a complex trait that in population-based studies most commonly derives from an individually varying mixture of Alzheimer disease, Lewy body disease, and vascular brain injury. We investigated the molecular composition of synaptic particles from three sources: consecutive rapid autopsy brains from the Adult Changes in Thought Study, a population-based cohort; four aged nonhuman primate brains optimally processed for molecular investigation; and targeted replacement transgenic mice homozygous for APOE ε4. Our major goal was to characterize the molecular composition of human synaptic particles in regions of striatum and prefrontal cortex. We performed flow cytometry to measure six markers of synaptic subtypes, as well as amyloid β 42 and paired helical filament tau. Our results showed selective degeneration of dopaminergic terminals throughout the striatum in individuals with Lewy body disease, and serotonergic degeneration in human ventromedial caudate nucleus from individuals with an APOE ε4 allele. Similar results were seen in mouse caudate nucleus homozygous for APOE ε4 via targeted replacement. Together, extension of these clinical, pathologic, and genetic associations from tissue to the synaptic compartment of cerebral cortex and striatum strongly supports our approach for accurately observing the molecular composition of human synapses by flow cytometry.

Oligomeric Amyloid-β Toxicity Can Be Inhibited by Blocking Its Cellular Binding in Cortical Neuronal Cultures with Addition of the Triphenylmethane Dye Brilliant Blue G

Accumulation of soluble amyloid β (Aβ) oligomers in the brain has been suggested to cause neurodegeneration associated with Alzheimer's disease (AD). Our previous findings showed that the binding of Aβ trimer and tetramer to neurons is significantly correlated with Aβ-induced neuronal cell death. We propose blocking of neuronal binding of these neurotoxic Aβ oligomers as a therapeutic strategy for preventing this disease. To test this, a nontoxic triphenylmethane dye, Brilliant Blue G (BBG), which has been reported to modulate Aβ aggregation and neurotoxicity, was investigated using mouse primary cortical neuronal cultures treated with photoinduced cross-linked toxic Aβ40 oligomers as well as soluble Aβ40 and Aβ42 peptides. We found that the BBG-induced decrease in Aβ binding resulted in a significant decrease in its neurotoxicity. These findings support our hypothesis that disruption of cellular Aβ binding is a promising therapeutic strategy for combating AD.

How many amyloid-β peptides can a neuron bind before it dies?

This Editorial highlights a study by Jana and coworkers published in the current issue of Journal of Neurochemistry, in which the authors performed a detailed, quantitative analysis to identify the Aβ oligomer causing neuronal cell death. While most studies so far aimed to determine the Aβ oligomer with highest toxicity using preformed and characterized Aβ oligomers added to cell cultures, this study established an approach to analyze Aβ oligomers bound to primary neurons. This may shed new light on how oligomeric status changes at the cell surface and if minor oligomeric species may account for measured effects. The authors' procedure allows to monitor the effects of different Aβ oligomers in parallel, constituting an important advancement in the research field. Read the highlighted article 'Membrane bound tetramer and trimer Aβ oligomeric species correlate with toxicity towards cultured neurons' on page 594.

Inflammation Combined with Ischemia Produces Myelin Injury and Plaque-Like Aggregates of Myelin, Amyloid-β and AβPP in Adult Rat Brain

Background: Ischemia, white matter injury, and Alzheimer’s disease (AD) pathologies often co-exist in aging brain. How one condition predisposes to, interacts with, or perhaps causes the others remains unclear.

Objectives: To better understand the link between ischemia, white matter injury, and AD, adult rats were administered lipopolysaccharide (LPS) to serve as an inflammatory stimulus, and 24 h later subjected to 20-min focal cerebral ischemia (IS) followed by 30-min hypoxia (H).

Methods: Myelin and axonal damage, as well as amyloid-β (Aβ) and amyloid-β protein precursor (AβPP) deposition were examined by Western blot and immunocytochemistry following LPS/IS/H. Findings were compared to the 5XFAD mouse AD brain.

Results: Myelin/axonal injury was observed bilaterally in cortex following LPS/IS/H, along with an increase in IL-1, granzyme B, and LPS. AβPP deposition was present in ischemic striatum in regions of myelin loss. Aβ_1-42 and AβPP were deposited in small foci in ischemic cortex that co-localized with myelin aggregates. In the 5XFAD mouse AD model, cortical amyloid plaques also co-localized with myelin aggregates.

Conclusions: LPS/IS/H produce myelin injury and plaque-like aggregates of myelin. AβPP and Aβ co-localize with these myelin aggregates.

A novel recombinant 6Aβ15-THc-C chimeric vaccine (rCV02) mitigates Alzheimer's disease-like pathology, cognitive decline and synaptic loss in aged 3 × Tg-AD mice

Alzheimer's disease (AD) is a neurodegenerative disorder that impairs memory and cognition. Targeting amyloid-β (Aβ) may be currently the most promising immunotherapeutic strategy for AD. In this study, a recombinant chimeric 6Aβ15-THc-C immunogen was formulated with alum adjuvant as a novel Aβ B-cell epitope candidate vaccine (rCV02) for AD. We examined its efficacy in preventing the cognitive deficit and synaptic impairment in 3 × Tg-AD mice. Using a toxin-derived carrier protein, the rCV02 vaccine elicited robust Aβ-specific antibodies that markedly reduced AD-like pathology and improved behavioral performance in 3 × Tg-AD mice. Along with the behavioral improvement in aged 3 × Tg-AD mice, rCV02 significantly decreased calpain activation concurrent with reduced soluble Aβ or oligomeric forms of Aβ, probably by preventing dynamin 1 and PSD-95 degradation. Our data support the hypothesis that reducing Aβ levels in rCV02-immunized AD mice increases the levels of presynaptic dynamin 1 and postsynaptic PSD-95 allowing functional recovery of cognition. In conclusion, this novel and highly immunogenic rCV02 shows promise as a new candidate prophylactic vaccine for AD and may be useful for generating rapid and strong Aβ-specific antibodies in AD patients with pre-existing memory Th cells generated after immunization with conventional tetanus toxoid vaccine.

Synaptic Amyloid-β Oligomers Precede p-Tau and Differentiate High Pathology Control Cases

Amyloid-β (Aβ) and hyperphosphorylated tau (p-tau) aggregates form the two discrete pathologies of Alzheimer disease (AD), and oligomeric assemblies of each protein are localized to synapses. To determine the sequence by which pathology appears in synapses, Aβ and p-tau were quantified across AD disease stages in parietal cortex. Nondemented cases with high levels of AD-related pathology were included to determine factors that confer protection from clinical symptoms. Flow cytometric analysis of synaptosome preparations was used to quantify Aβ and p-tau in large populations of individual synaptic terminals. Soluble Aβ oligomers were assayed by a single antibody sandwich enzyme-linked immunosorbent assay. Total in situ Aβ was elevated in patients with early- and late-stage AD dementia, but not in high pathology nondemented controls compared with age-matched normal controls. However, soluble Aβ oligomers were highest in early AD synapses, and this assay distinguished early AD cases from high pathology controls. Overall, synapse-associated p-tau did not increase until late-stage disease in human and transgenic rat cortex, and p-tau was elevated in individual Aβ-positive synaptosomes in early AD. These results suggest that soluble oligomers in surviving neocortical synaptic terminals are associated with dementia onset and suggest an amyloid cascade hypothesis in which oligomeric Aβ drives phosphorylated tau accumulation and synaptic spread. These results indicate that antiamyloid therapies will be less effective once p-tau pathology is developed.

A Novel Aβ B-Cell Epitope Vaccine (rCV01) for Alzheimer's Disease Improved Synaptic and Cognitive Functions in 3 × Tg-AD Mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive amyloid-β accumulation, loss of cognitive abilities, and synaptic alterations. Given the remarkable recovery of cognition in AD models of targeting-Aβ immunotherapy, we sought to determine the molecular correlate(s) associated with improvement. We evaluated the efficacy of a recombinant chimeric 6Aβ15-T antigen formulated with alum adjuvant as a novel Aβ B-cell epitope vaccine (rCV01) in 3 × Tg-AD mice. rCV01 elicited robust Th2-polarized Aβ-specific antibodies without autoimmune T cell responses in 3 × Tg-AD mice. The long-lasting anti-Aβ42 antibodies were associated with markedly reduced AD-like pathology, enhanced synaptic function, and improved cognitive performance in aged 3 × Tg-AD mice. This is the first report to provide one hypothesis for the improved outcomes following vaccination is a reduction in the levels of active calpain in rCV01-immunized AD mice, which is likely attributable to preventing dynamin 1 and PSD-95 degradation allowing functional recovery of cognition. rCV01 is a highly immunogenic recombinant chimeric 6Aβ15-T vaccine that shows clear neuroprotective properties in preclinical mouse models of AD and is a candidate for an effective AD vaccine.

Tau Oligomers: The Toxic Player at Synapses in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive disorder in which the most noticeable symptoms are cognitive impairment and memory loss. However, the precise mechanism by which those symptoms develop remains unknown. Of note, neuronal loss occurs at sites where synaptic dysfunction is observed earlier, suggesting that altered synaptic connections precede neuronal loss. The abnormal accumulation of amyloid-β (Aβ) and tau protein is the main histopathological feature of the disease. Several lines of evidence suggest that the small oligomeric forms of Aβ and tau may act synergistically to promote synaptic dysfunction in AD. Remarkably, tau pathology correlates better with the progression of the disease than Aβ. Recently, a growing number of studies have begun to suggest that missorting of tau protein from the axon to the dendrites is required to mediate the detrimental effects of Aβ. In this review we discuss the novel findings regarding the potential mechanisms by which tau oligomers contribute to synaptic dysfunction in AD.

Cerebrospinal Fluid Particles in Alzheimer Disease and Parkinson Disease

Human cerebrospinal fluid (CSF) contains diverse lipid particles, including lipoproteins that are distinct from their plasma counterparts and contain apolipoprotein (apo) E isoforms, apoJ, and apoAI, and extracellular vesicles, which can be detected by annexin V binding. The aim of this study was to develop a method to quantify CSF particles and evaluate their relationship to aging and neurodegenerative diseases. We used a flow cytometric assay to detect annexin V-, apoE-, apoAI-, apoJ-, and amyloid (A) β42-positive particles in CSF from 131 research volunteers who were neurologically normal or had mild cognitive impairment (MCI), Alzheimer disease (AD) dementia, or Parkinson disease. APOE ε4/ε4 participants had CSF apoE-positive particles that were more frequently larger but at an 88% lower level versus those in APOE ε3/ε3 or APOE ε3/ε4 patients; this finding was reproduced in conditioned medium from mouse primary glial cell cultures with targeted replacement of apoE. Cerebrospinal fluid apoE-positive and β-amyloid (Aβ42)-positive particle concentrations were persistently reduced one-third to one-half in middle and older age subjects; apoAI-positive particle concentration progressively increased approximately 2-fold with age. Both apoAI-positive and annexin V-positive CSF particle levels were reduced one-third to one-half in CSF of MCI and/or AD dementia patients versus age-matched controls. Our approach provides new methods to investigate CNS lipid biology in relation to neurodegeneration and perhaps develop new biomarkers for diagnosis or treatment monitoring.

Tau immunotherapy modulates both pathological tau and upstream amyloid pathology in an Alzheimer's disease mouse model

In Alzheimer's disease (AD), the pathological accumulation of tau appears to be a downstream effect of amyloid β protein (Aβ). However, the relationship between these two proteins and memory loss is unclear. In this study, we evaluated the specific removal of pathological tau oligomers in aged Tg2576 mice by passive immunotherapy using tau oligomer-specific monoclonal antibody. Removal of tau oligomers reversed memory deficits and accelerated plaque deposition in the brain. Surprisingly, Aβ*56 levels decreased, suggesting a link between tau and Aβ oligomers in the promotion of cognitive decline. The results suggest that tau oligomerization is not only a consequence of Aβ pathology but also a critical mediator of the toxic effects observed afterward in AD. Overall, these findings support the potential of tau oligomers as a therapeutic target for AD.

Loss of endophilin-B1 exacerbates Alzheimer's disease pathology

Endophilin-B1, also known as Bax-interacting factor 1 (Bif-1, and encoded by SH3GLB1), is a multifunctional protein involved in apoptosis, autophagy and mitochondrial function. We recently described a unique neuroprotective role for neuron-specific alternatively spliced isoforms of endophilin-B1. To examine whether endophilin-B1-mediated neuroprotection could be a novel therapeutic target for Alzheimer's disease we used a double mutant amyloid precursor protein and presenilin 1 (APPswe/PSEN1dE9) mouse model of Alzheimer's disease and observed that expression of neuron-specific endophilin-B1 isoforms declined with disease progression. To determine if this reduction in endophilin-B1 has a functional role in Alzheimer's disease pathogenesis, we crossed endophilin-B1(-/-) mice with APPswe/PSEN1dE9 mice. Deletion of endophilin-B1 accelerated disease onset and progression in 6-month-old APPswe/PSEN1dE9/endophilin-B1(-/-) mice, which showed more plaques, astrogliosis, synaptic degeneration, cognitive impairment and mortality than APPswe/PSEN1dE9 mice. In mouse primary cortical neuron cultures, overexpression of neuron-specific endophilin-B1 isoforms protected against amyloid-β-induced apoptosis and mitochondrial dysfunction. Additionally, protein and mRNA levels of neuron-specific endophilin-B1 isoforms were also selectively decreased in the cerebral cortex and in the synaptic compartment of patients with Alzheimer's disease. Flow sorting of synaptosomes from patients with Alzheimer's disease demonstrated a negative correlation between amyloid-β and endophilin-B1 levels. The importance of endophilin-B1 in neuronal function was further underscored by the development of synaptic degeneration and cognitive and motor impairment in endophilin-B1(-/-) mice by 12 months. Our findings suggest that endophilin-B1 is a key mediator of a feed-forward mechanism of Alzheimer's disease pathogenesis where amyloid-β reduces neuron-specific endophilin-B1, which in turn enhances amyloid-β accumulation and neuronal vulnerability to stress.

MRZ-99030 - A novel modulator of Aβ aggregation: I - Mechanism of action (MoA) underlying the potential neuroprotective treatment of Alzheimer's disease, glaucoma and age-related macular degeneration (AMD)

Therapeutic approaches addressing β-amyloid1-42 (Aβ1-42) aggregation represent a promising neuroprotective strategy for the treatment of Alzheimer's disease, dry age-related macular degeneration (AMD) and glaucoma. MRZ-99030 is a dipeptide containing d-tryptophan and 2-amino-2-methylpropionic acid in clinical development for the topical treatment of glaucoma and AMD. MRZ-99030 is an Aβ aggregation modulator, previously reported to prevent the formation of soluble toxic oligomeric Aβ species. The present study confirmed that MRZ-99030 prevents the formation of oligomeric Aβ species using similar SDS-PAGE experiments. However, additional data from TR-FRET, DLS and AFM experiments revealed that MRZ-99030 does not directly prevent early protein/protein interactions between monomeric Aβ, but rather promotes the formation of large, non-amyloidogenic, amorphous Aβ aggregates and thereby reduces the amount of intermediate toxic soluble oligomeric Aβ species. The affinity of MRZ-99030 to Aβ1-42 determined by SPR was 28.4 nM but the ratio of compound to Aβ is also important: a 10-20 fold excess of MRZ-99030 over Aβ is probably required for effective modulation of protein/protein interactions. For example, in glaucoma, assuming a maximal Aβ concentration of 1-15 nM in the retina, up to 150 nM MRZ-99030 could be required at the protein target. In line with this consideration, MRZ-99030 was able to prevent Aβ-induced toxicity on PC12 cells, retinal ganglion cells and retinal pigment epithelium cells when present at a 10-20 fold stoichiometric excess over Aβ. Moreover, in vivo studies demonstrate the neuroprotective potential of MRZ-99030 after systemic and topical administration in animal models of Alzheimer's disease and glaucoma/AMD respectively.

Pre-synaptic C-terminal truncated tau is released from cortical synapses in Alzheimer's disease

The microtubule-associated protein tau has primarily been associated with axonal location and function; however, recent work shows tau release from neurons and suggests an important role for tau in synaptic plasticity. In our study, we measured synaptic levels of total tau using synaptosomes prepared from cryopreserved human postmortem Alzheimer's disease (AD) and control samples. Flow cytometry data show that a majority of synaptic terminals are highly immunolabeled with the total tau antibody (HT7) in both AD and control samples. Immunoblots of synaptosomal fractions reveal increases in a 20 kDa tau fragment and in tau dimers in AD synapses, and terminal-specific antibodies show that in many synaptosome samples tau lacks a C-terminus. Flow cytometry experiments to quantify the extent of C-terminal truncation reveal that only 15-25% of synaptosomes are positive for intact C-terminal tau. Potassium-induced depolarization demonstrates release of tau and tau fragments from pre-synaptic terminals, with increased release from AD compared to control samples. This study indicates that tau is normally highly localized to synaptic terminals in cortex where it is well-positioned to affect synaptic plasticity. Tau cleavage may facilitate tau aggregation as well as tau secretion and propagation of tau pathology from the pre-synaptic compartment in AD. Results demonstrate the abundance of tau, mainly C-terminal truncated tau, in synaptic terminals in aged control and in Alzheimer's disease (AD) samples. Tau fragments and dimers/oligomers are prominent in AD synapses. Following depolarization, tau release is potentiated in AD nerve terminals compared to aged controls. We hypothesize (i) endosomal release of the different tau peptides from AD synapses, and (ii) together with phosphorylation, fragmentation of synaptic tau exacerbates tau aggregation, synaptic dysfunction, and the spread of tau pathology in AD. Aβ = amyloid-beta.

An alkaline phosphatase transport mechanism in the pathogenesis of Alzheimer's disease and neurodegeneration

Systemic inflammation is associated with loss of blood-brain barrier integrity and neuroinflammation that lead to the exacerbation of neurodegenerative diseases. It is also associated specifically with the characteristic amyloid-β and tau pathologies of Alzheimer's disease. We have previously proposed an immunosurveillance mechanism for epithelial barriers involving negative feedback-regulated alkaline phosphatase transcytosis as an acute phase anti-inflammatory response that hangs in the balance between the resolution and the progression of inflammation. We now extend this model to endothelial barriers, particularly the blood-brain barrier, and present a literature-supported mechanistic explanation for Alzheimer's disease pathology with this system at its foundation. In this mechanism, a switch in the role of alkaline phosphatase from its baseline duties to a stopgap anti-inflammatory function results in the loss of alkaline phosphatase from cell membranes into circulation, thereby decreasing blood-brain barrier integrity and functionality. This occurs with impairment of both amyloid-β efflux and tau dephosphorylating activity in the brain as alkaline phosphatase is replenished at the barrier by receptor-mediated transport. We suggest systemic alkaline phosphatase administration as a potential therapy for the resolution of inflammation and the prevention of Alzheimer's disease pathology as well as that of other inflammation-related neurodegenerative diseases.

Oligomeric Aβ-induced synaptic dysfunction in Alzheimer's disease

Alzheimer’s disease (AD) is a devastating disease characterized by synaptic and neuronal loss in the elderly. Compelling evidence suggests that soluble amyloid-β peptide (Aβ) oligomers induce synaptic loss in AD. Aβ-induced synaptic dysfunction is dependent on overstimulation of N-methyl-D-aspartate receptors (NMDARs) resulting in aberrant activation of redox-mediated events as well as elevation of cytoplasmic Ca²⁺, which in turn triggers downstream pathways involving phospho-tau (p-tau), caspases, Cdk5/dynamin-related protein 1 (Drp1), calcineurin/PP2B, PP2A, Gsk-3β, Fyn, cofilin, and CaMKII and causes endocytosis of AMPA receptors (AMPARs) as well as NMDARs. Dysfunction in these pathways leads to mitochondrial dysfunction, bioenergetic compromise and consequent synaptic dysfunction and loss, impaired long-term potentiation (LTP), and cognitive decline. Evidence also suggests that Aβ may, at least in part, mediate these events by causing an aberrant rise in extrasynaptic glutamate levels by inhibiting glutamate uptake or triggering glutamate release from glial cells. Consequent extrasynaptic NMDAR (eNMDAR) overstimulation then results in synaptic dysfunction via the aforementioned pathways. Consistent with this model of Aβ-induced synaptic loss, Aβ synaptic toxicity can be partially ameliorated by the NMDAR antagonists (such as memantine and NitroMemantine). PSD-95, an important scaffolding protein that regulates synaptic distribution and activity of both NMDA and AMPA receptors, is also functionally disrupted by Aβ. PSD-95 dysregulation is likely an important intermediate step in the pathological cascade of events caused by Aβ. In summary, Aβ-induced synaptic dysfunction is a complicated process involving multiple pathways, components and biological events, and their underlying mechanisms, albeit as yet incompletely understood, may offer hope for new therapeutic avenues.

Memantine prevents memory consolidation failure induced by soluble beta amyloid in rats

It has been well documented that β-amyloid (Aβ) peptide accumulation and aggregation in the brain plays a crucial role in the pathophysiology of Alzheimer's disease (AD). However, a new orientation of the amyloid cascade hypothesis has evidenced that soluble forms of the peptide (sAβ) are involved in Aβ-induced cognitive impairment and cause rapid disruption of the synaptic mechanisms underlying memory. The primary aim of this study was to elucidate the effects of sAβ, acutely injected intracerebrally (i.c.v., 4 μM), on the short term and long term memory of young adult male rats, by using the novel object recognition task. Glutamatergic receptors have been proposed as mediating the effect of Aβ on synaptic plasticity and memory. Thus, we also investigated the effects of sAβ on prefrontal cortex (PFC) glutamate release and the specific contribution of N-methyl-D-aspartate (NMDA) receptor modulation to the effects of sAβ administration on the cognitive parameters evaluated. We found that a single i.c.v. injection of sAβ 2 h before testing did not alter the ability of rats to differentiate between a familiar and a novel object, in a short term memory test, while it was able to negatively affect consolidation/retrieval of long term memory. Moreover, a significant increase of glutamate levels was found in PFC of rats treated with the peptide 2 h earlier. Interestingly, memory deficit induced by sAβ was reversed by a NMDA-receptor antagonist, memantine (5 mg/kg i.p), administered immediately after the familiarization trial (T1). On the contrary, memantine administered 30 min before T1 trial, was not able to rescue long term memory impairment. Taken together, our results suggest that an acute i.c.v. injection of sAβ peptide interferes with the consolidation/retrieval of long term memory. Moreover, such sAβ-induced effect indicates the involvement of glutamatergic system, proposing that NMDA receptor inhibition might prevent or lead to the recovery of early cognitive impairment.

IVIg protects the 3xTg-AD mouse model of Alzheimer's disease from memory deficit and Aβ pathology

Background

Intravenous immunoglobulin (IVIg) is currently in clinical study for Alzheimer’s disease (AD). However, preclinical investigations are required to better understand AD-relevant outcomes of IVIg treatment and develop replacement therapies in case of unsustainable supply.

Methods

We investigated the effects of IVIg in the 3xTg-AD mouse model, which reproduces both Aβ and tau pathologies. Mice were injected twice weekly with 1.5 g/kg IVIg for 1 or 3 months.

Results

IVIg induced a modest but significant improvement in memory in the novel object recognition test and attenuated anxiety-like behavior in 3xTg-AD mice. We observed a correction of immunologic defects present in 3xTg-AD mice (−22% CD4/CD8 blood ratio; −17% IL-5/IL-10 ratio in the cortex) and a modulation of CX3CR1⁺ cell population (−13% in the bone marrow). IVIg treatment led to limited effects on tau pathology but resulted in a 22% reduction of the soluble Aβ42/Aβ40 ratio and a 60% decrease in concentrations of 56 kDa Aβ oligomers (Aβ*56).

Conclusion

The memory-enhancing effect of IVIg reported here suggests that Aβ oligomers, effector T cells and the fractalkine pathway are potential pharmacological targets of IVIg in AD.

Environmental enrichment strengthens corticocortical interactions and reduces amyloid-β oligomers in aged mice

Brain aging is characterized by global changes which are thought to underlie age-related cognitive decline. These include variations in brain activity and the progressive increase in the concentration of soluble amyloid-β (Aβ) oligomers, directly impairing synaptic function and plasticity even in the absence of any neurodegenerative disorder. Considering the high social impact of the decline in brain performance associated to aging, there is an urgent need to better understand how it can be prevented or contrasted. Lifestyle components, such as social interaction, motor exercise and cognitive activity, are thought to modulate brain physiology and its susceptibility to age-related pathologies. However, the precise functional and molecular factors that respond to environmental stimuli and might mediate their protective action again pathological aging still need to be clearly identified. To address this issue, we exploited environmental enrichment (EE), a reliable model for studying the effect of experience on the brain based on the enhancement of cognitive, social and motor experience, in aged wild-type mice. We analyzed the functional consequences of EE on aged brain physiology by performing in vivo local field potential (LFP) recordings with chronic implants. In addition, we also investigated changes induced by EE on molecular markers of neural plasticity and on the levels of soluble Aβ oligomers. We report that EE induced profound changes in the activity of the primary visual and auditory cortices and in their functional interaction. At the molecular level, EE enhanced plasticity by an upward shift of the cortical excitation/inhibition balance. In addition, EE reduced brain Aβ oligomers and increased synthesis of the Aβ-degrading enzyme neprilysin. Our findings strengthen the potential of EE procedures as a non-invasive paradigm for counteracting brain aging processes.

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.

Synaptic protein α1-takusan mitigates amyloid-β-induced synaptic loss via interaction with tau and postsynaptic density-95 at postsynaptic sites

The synaptic toxicity of soluble amyloid-β (Aβ) oligomers plays a critical role in the pathophysiology of Alzheimer's disease (AD). Here we report that overexpressed α1-takusan, which we previously identified as a protein that enhances synaptic activity via interaction with PSD-95, mitigates oligomeric Aβ-induced synaptic loss. In contrast, takusan knockdown results in enhanced synaptic damage. α1-Takusan interacts with tau either directly or indirectly, and prevents Aβ-induced tau hyperphosphorylation and mitochondrial fragmentation. Deletion analysis identified the second domain (D2) within the takusan protein that is required for PSD-95 clustering and synaptic protection from Aβ. A 51 aa sequence linking D2 to the PDZ-binding C terminus was found to be as effective as full-length takusan in protecting synapses from Aβ-induced damage. Moreover, a sequence containing the D2 from the human protein discs large homolog 5, when linked to a C-terminal PDZ-binding motif, can also increase the clustering of PSD-95 in cortical dendrites. In summary, α1-takusan protects synapses from Aβ-induced insult via interaction with PSD-95 and tau. Thus, takusan-based protein sequences from either mouse or human may be of potential therapeutic benefit in AD.

New ELISAs with high specificity for soluble oligomers of amyloid β-protein detect natural Aβ oligomers in human brain but not CSF

BACKGROUND

Soluble oligomers of amyloid ß-protein (Aß) have been increasingly linked to synaptic dysfunction, tau alteration, and neuritic dystrophy in Alzheimer's disease (AD) and mouse models. There is a great need for assays that quantify Aß oligomers with high specificity and sensitivity.

METHODS

We designed and validated two oligomer-specific (o-) enzyme-linked immunoassays (ELISAs) using either an Aß aggregate-selective monoclonal for capture and a monoclonal to the free N-terminus for detection, or the latter antibody for both capture and detection.

RESULTS

The o-ELISAs specifically quantified pure oligomers of synthetic Aß with sizes from dimers up to much larger assemblies and over a wide dynamic range of concentrations, whereas Aß monomers were undetectable. Natural Aß oligomers of similarly wide size and concentration ranges were measured in extracts of AD and control brains, revealing >1000-fold higher concentrations of Aß oligomers than monomers in the soluble fraction of AD cortex. The assays quantified the age-related rise in oligomers in hAPP transgenic mice. Unexpectedly, none of 90 human cerebrospinal fluid (CSF) samples gave a specific signal in either o-ELISA.

CONCLUSIONS

These new o-ELISAs with rigorously confirmed specificity can quantify oligomer burden in human and mouse brains for diagnostic and mechanistic studies and for AD biomarker development. However, our data raise the likelihood that the hydrophobicity of Aß oligomers makes them very low in number or absent in aqueous CSF.

Alzheimer's disease, β-amyloid, glutamate, NMDA receptors and memantine--searching for the connections

β-amyloid (Aβ) is widely accepted to be one of the major pathomechanisms underlying Alzheimer's disease (AD), although there is presently lively debate regarding the relative roles of particular species/forms of this peptide. Most recent evidence indicates that soluble oligomers rather than plaques are the major cause of synaptic dysfunction and ultimately neurodegeneration. Soluble oligomeric Aβ has been shown to interact with several proteins, for example glutamatergic receptors of the NMDA type and proteins responsible for maintaining glutamate homeostasis such as uptake and release. As NMDA receptors are critically involved in neuronal plasticity including learning and memory, we felt that it would be valuable to provide an up to date review of the evidence connecting Aβ to these receptors and related neuronal plasticity. Strong support for the clinical relevance of such interactions is provided by the NMDA receptor antagonist memantine. This substance is the only NMDA receptor antagonist used clinically in the treatment of AD and therefore offers an excellent tool to facilitate translational extrapolations from in vitro studies through in vivo animal experiments to its ultimate clinical utility.

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

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

Extensive p-tau pathology and SDS-stable p-tau oligomers in Alzheimer's cortical synapses

Like amyloid beta (Aβ) oligomers, tau aggregates are increasingly recognized as potential key toxic intermediates in Alzheimer's disease (AD) and as therapeutic targets. P-tau co-localizes with Aβ in cortical AD synapses and may contribute to synapse dysfunction and loss. Flow cytometry analysis of synaptosomes from AD compared with aged cognitively normal cortex demonstrates increased immunolabeling for three p-tau antibodies (AT8, PHF-1 and pS422), indicating phosphorylation at multiple tau epitopes. Sequential extraction experiments show increased soluble p-tau in AD synapses, but a sizable pool of p-tau requires detergent solubilization, suggesting endosomal/lysosomal localization. P-tau is co-localized with Aβ in individual synaptosomes in dual labeling experiments, and flow cytometry sorting of Aβ-positive synaptosomes from an AD case reveals a marked enrichment of p-tau aggregates. The p-tau enrichment, a 76-fold increase over the initial homogenate, is consistent with sequestration of p-tau in internal synaptic compartments. Western analysis of a series of AD and normal cases shows SDS-stable tau oligomers in the dimer/trimer size range in AD samples. These results indicate that widespread synaptic p-tau pathology accompanies Aβ accumulations in surviving synaptic terminals, particularly in late-stage AD.

Apolipoprotein E level and cholesterol are associated with reduced synaptic amyloid beta in Alzheimer's disease and apoE TR mouse cortex

The apolipoprotein E4 allele (APOE4) contributes to Alzheimer's disease (AD) risk and APOE2 is protective, but the relevant cellular mechanisms are unknown. We have used flow cytometry analysis to measure apolipoprotein E (apoE) and amyloid beta peptide (Aβ) levels in large populations of synaptic terminals from AD and aged cognitively normal controls, and demonstrate that modest but significant increases in soluble apoE levels accompany elevated Aβ in AD cortical synapses and in an APP/PS1 rat model of AD. Dual labeling experiments document co-localization of apoE and Aβ in individual synapses with concentration of Aβ in a small population of apoE-positive synapses in both AD and controls. Consistent with a clearance role, the apoE level was higher in Aβ-positive synapses in control cases. In aged targeted replacement mice expressing human apoE, apoE2/4 synaptic terminals demonstrated the highest level of apoE and the lowest level of Aβ compared to apoE3/3 and apoE4/4 lines. In apoE2/4 terminals, the pattern of immunolabeling for apoE and Aβ closely resembled the pattern in human control cases, and elevated apoE was accompanied by elevated free cholesterol in apoE2/4 synaptic terminals. These results are consistent with a role for APOE in Aβ clearance in AD synapses, and suggest that optimal lipidation of apoE2 compared to E3 and E4 makes an important contribution to Aβ clearance and synaptic function.

Abnormal accumulation of autophagic vesicles correlates with axonal and synaptic pathology in young Alzheimer's mice hippocampus

Dystrophic neurites associated with amyloid plaques precede neuronal death and manifest early in Alzheimer's disease (AD). In this work we have characterized the plaque-associated neuritic pathology in the hippocampus of young (4- to 6-month-old) PS1(M146L)/APP(751SL) mice model, as the initial degenerative process underlying functional disturbance prior to neuronal loss. Neuritic plaques accounted for almost all fibrillar deposits and an axonal origin of the dystrophies was demonstrated. The early induction of autophagy pathology was evidenced by increased protein levels of the autophagosome marker LC3 that was localized in the axonal dystrophies, and by electron microscopic identification of numerous autophagic vesicles filling and causing the axonal swellings. Early neuritic cytoskeletal defects determined by the presence of phosphorylated tau (AT8-positive) and actin-cofilin rods along with decreased levels of kinesin-1 and dynein motor proteins could be responsible for this extensive vesicle accumulation within dystrophic neurites. Although microsomal Aβ oligomers were identified, the presence of A11-immunopositive Aβ plaques also suggested a direct role of plaque-associated Aβ oligomers in defective axonal transport and disease progression. Most importantly, presynaptic terminals morphologically disrupted by abnormal autophagic vesicle buildup were identified ultrastructurally and further supported by synaptosome isolation. Finally, these early abnormalities in axonal and presynaptic structures might represent the morphological substrate of hippocampal dysfunction preceding synaptic and neuronal loss and could significantly contribute to AD pathology in the preclinical stages.

Isolation of synaptic terminals from Alzheimer's disease cortex

Amyloid beta (Aβ) oligomers and phosphorylated tau (p-tau) aggregates are increasingly identified as potential toxic intermediates in Alzheimer's disease (AD). In cortical AD synapses, p-tau co-localizes with Aβ, but the Aβ and p-tau peptide species responsible for synaptic dysfunction and demise remains unclear. The present experiments were designed to use high-speed cell sorting techniques to purify synaptosome population based on size, and then extend the method to physically isolate Aβ-positive synaptosomes with the goal of understanding the nature of Aβ and tau pathology in AD synapses. To examine the purity of size-gated synaptosomes, samples were first gated on size; particles with sizes between 0.5 and 1.5 microns were collected. Electron microscopy documented a homogenous population of spherical particles with internal vesicles and synaptic densities. Next, size-gated synaptosomes positive for Aβ were collected by fluorescence activated sorting and then analyzed by immunoblotting techniques. Sorted Aβ-positive synaptosomes were enriched for amyloid precursor protein (APP) and for Aβ oligomers and aggregates; immunolabeling for p-tau showed a striking accumulation of p-tau aggregates compared to the original homogenate and purified synaptosomes. These results confirm co-localization of Aβ and p-tau within individual synaptic terminals and provide proof of concept for the utility of flow sorting synaptosomes.

Preferential accumulation of amyloid-beta in presynaptic glutamatergic terminals (VGluT1 and VGluT2) in Alzheimer's disease cortex

Amyloid-beta (Aβ) is thought to play a central role in synaptic dysfunction (e.g. neurotransmitter release) and synapse loss. Glutamatergic dysfunction is involved in the pathology of Alzheimer's disease (AD) and perhaps plays a central role in age-related cognitive impairment. Yet, it is largely unknown whether Aβ accumulates in excitatory boutons. To assess the possibility that glutamatergic terminals are lost in AD patients, control and AD synaptosomes were immunolabeled for the most abundant vesicular glutamate transporters (VGluT1 and VGluT2) and quantified by flow cytometry and immunoblot methods. In post-mortem parietal cortex from aged control subjects, glutamatergic boutons are fairly abundant as approximately 40% were immunoreactive for VGluT1 (37%) and VGluT2 (39%). However, the levels of these specific markers of glutamatergic synapses were not significantly different among control and AD cases. To test the hypothesis that Aβ is associated with excitatory terminals, AD synaptosomes were double-labeled for Aβ and for VGluT1 and VGluT2, and analyzed by flow cytometry and confocal microscopy. Our study demonstrated that Aβ immunoreactivity (IR) was present in glutamatergic terminals of AD patients. Quantification of Aβ and VGluT1 in a large population of glutamatergic nerve terminals was performed by flow cytometry, showing that 42% of VGluT1 synaptosomes were immunoreactive for Aβ compared to 9% of VGluT1 synaptosomes lacking Aβ-IR. Percentage of VGluT2 synaptosomes immunoreactive for Aβ (21%) was significantly higher than VGluT2 synaptosomes lacking Aβ-IR (9%). Moreover, Aβ preferentially affects VGluT1 (42% positive) compared to VGluT2 terminals (21%). These data represent the first evidence of high levels of Aβ in excitatory boutons in AD cortex and support the hypothesis that Aβ may play a role in modulating glutamate transmission in AD terminals.