Amyloid beta interacts with the amyloid precursor protein: a potential toxic mechanism in Alzheimer's disease

The association of GNB5 with Alzheimer disease revealed by genomic analysis restricted to variants impacting gene function

Disease-associated variants identified from genome-wide association studies (GWASs) frequently map to non-coding areas of the genome such as introns and intergenic regions. An exclusive reliance on gene-agnostic methods of genomic investigation could limit the identification of relevant genes associated with polygenic diseases such as Alzheimer disease (AD). To overcome such potential restriction, we developed a gene-constrained analytical method that considers only moderate- and high-risk variants that affect gene coding sequences. We report here the application of this approach to publicly available datasets containing 181,388 individuals without and with AD and the resulting identification of 660 genes potentially linked to the higher AD prevalence among Africans/African Americans. By integration with transcriptome analysis of 23 brain regions from 2,728 AD case-control samples, we concentrated on nine genes that potentially enhance the risk of AD: AACS, GNB5, GNS, HIPK3, MED13, SHC2, SLC22A5, VPS35, and ZNF398. GNB5, the fifth member of the heterotrimeric G protein beta family encoding Gβ5, is primarily expressed in neurons and is essential for normal neuronal development in mouse brain. Homozygous or compound heterozygous loss of function of GNB5 in humans has previously been associated with a syndrome of developmental delay, cognitive impairment, and cardiac arrhythmia. In validation experiments, we confirmed that Gnb5 heterozygosity enhanced the formation of both amyloid plaques and neurofibrillary tangles in the brains of AD model mice. These results suggest that gene-constrained analysis can complement the power of GWASs in the identification of AD-associated genes and may be more broadly applicable to other polygenic diseases.

The presymptomatic treatment with 3HFWC nanosubstance decreased plaque load in 5XFAD mouse model of Alzheimer's disease

INTRODUCTION

In the present study, we assessed the effects of the hyper-harmonized-hydroxylated fullerene-water complex (3HFWC) on Alzheimer's disease (AD) neuropathological hallmarks in 5XFAD mice, an AD animal model.

METHODS

The 3-week-old 5XFAD mice were exposed to 3HFWC water solution ad libitum for 3 months in the presymptomatic phase of pathology. The functional effects of the treatment were confirmed through near-infrared spectroscopy (NIRS) analysis through machine learning (ML) using artificial neural networks (ANNs) to classify the control and 3HFWC-treated brain tissue samples. The effects of 3HFWC treatment on amyloid-β (Aβ) accumulation, plaque formation, gliosis, and synaptic plasticity in cortical and hippocampal tissue were assessed.

RESULTS

The 3HFWC treatment significantly decreased the amyloid-β plaque load in specific parts of the cerebral cortex. At the same time, 3HFWC treatment did not induce the activation of glia (astrocytes and microglia) nor did it negatively affect synaptic protein markers (GAP-43, synaptophysin, and PSD-95).

CONCLUSION

The obtained results point to the potential of 3HFWC, when applied in the presymptomatic phase of AD, to interfere with amyloid plaque formation without inducing AD-related pathological processes such as neuroinflammation, gliosis, and synaptic vulnerability.

Re-Arranging the Puzzle between the Amyloid-Beta and Tau Pathology: An APP-Centric Approach

After several years of research in the field of Alzheimer's disease (AD), it is still unclear how amyloid-beta (Aβ) and Tau, two key hallmarks of the disease, mediate the neuropathogenic events that lead to AD. Current data challenge the "Amyloid Cascade Hypothesis" that has prevailed in the field of AD, stating that Aβ precedes and triggers Tau pathology that will eventually become the toxic entity in the progression of the disease. This perspective also led the field of therapeutic approaches towards the development of strategies that target Aβ or Tau. In the present review, we discuss recent literature regarding the neurotoxic role of both Aβ and Tau in AD, as well as their physiological function in the healthy brain. Consequently, we present studies suggesting that Aβ and Tau act independently of each other in mediating neurotoxicity in AD, thereafter, re-evaluating the "Amyloid Cascade Hypothesis" that places Tau pathology downstream of Aβ. More recent studies have confirmed that both Aβ and Tau could propagate the disease and induce synaptic and memory impairments via the amyloid precursor protein (APP). This finding is not only interesting from a mechanistic point of view since it provides better insights into the AD pathogenesis but also from a therapeutic point of view since it renders APP a common downstream effector for both Aβ and Tau. Subsequently, therapeutic strategies that act on APP might provide a more viable and physiologically relevant approach for targeting AD.

A computational study of fluid transport characteristics in the brain parenchyma of dementia subtypes

The cerebral environment is a complex system consisting of parenchymal tissue and multiple fluids. Dementia is a common class of neurodegenerative diseases, caused by structural damages and functional deficits in the cerebral environment. In order to better understand the pathology of dementia from a cerebral fluid transport angle and provide clearer evidence that could help differentiate between dementia subtypes, such as Alzheimer's disease and vascular dementia, we conducted fluid-structure interaction modelling of the brain using a multiple-network poroelasticity model, which considers both neuropathological and cerebrovascular factors. The parenchyma was further subdivided and labelled into parcellations to obtain more localised and detailed data. The numerical results were converted to computed functional images by an in-house workflow. Different cerebral blood flow (CBF) and cerebrospinal fluid (CSF) clearance abnormalities were identified in the modelling results, when comparing Alzheimer's disease and vascular dementia. This paper presents our preliminary results as a proof of concept for a novel clinical diagnostic tool, and paves the way for a larger clinical study.

Amyloidogenesis: What Do We Know So Far?

The study of protein aggregation, and amyloidosis in particular, has gained considerable interest in recent times. Several neurodegenerative diseases, such as Alzheimer's (AD) and Parkinson's (PD) show a characteristic buildup of proteinaceous aggregates in several organs, especially the brain. Despite the enormous upsurge in research articles in this arena, it would not be incorrect to say that we still lack a crystal-clear idea surrounding these notorious aggregates. In this review, we attempt to present a holistic picture on protein aggregation and amyloids in particular. Using a chronological order of discoveries, we present the case of amyloids right from the onset of their discovery, various biophysical techniques, including analysis of the structure, the mechanisms and kinetics of the formation of amyloids. We have discussed important questions on whether aggregation and amyloidosis are restricted to a subset of specific proteins or more broadly influenced by the biophysiochemical and cellular environment. The therapeutic strategies and the significant failure rate of drugs in clinical trials pertaining to these neurodegenerative diseases have been also discussed at length. At a time when the COVID-19 pandemic has hit the globe hard, the review also discusses the plausibility of the far-reaching consequences posed by the virus, such as triggering early onset of amyloidosis. Finally, the application(s) of amyloids as useful biomaterials has also been discussed briefly in this review.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

Effect of Selenium Treatment on Central Insulin Sensitivity: A Proteomic Analysis in β-Amyloid Precursor Protein/Presenilin-1 Transgenic Mice

Prior studies have demonstrated a close association between brain insulin resistance and Alzheimer’s disease (AD), while selenium supplementation was shown to improve insulin homeostasis in AD patients and to exert neuroprotective effects in a mouse model of AD. However, the mechanisms underlying the neuroprotective actions of selenium remain incompletely understood. In this study, we performed a label-free liquid chromatography-tandem mass spectrometry (LC–MS/MS) quantitative proteomics approach to analyze differentially expressed proteins (DEPs) in the hippocampus and cerebral cortex of Aβ precursor protein (APP)/presenilin-1 (PS1) mice following 2 months of treatment with sodium selenate. A total of 319 DEPs (205 upregulated and 114 downregulated proteins) were detected after selenium treatment. Functional enrichment analysis revealed that the DEPs were mainly enriched in processes affecting axon development, neuron differentiation, tau protein binding, and insulin/insulin-like growth factor type 1 (IGF1)-related pathways. These results demonstrate that a number of insulin/IGF1 signaling pathway-associated proteins are differentially expressed in ways that are consistent with reduced central insulin resistance, suggesting that selenium has therapeutic value in the treatment of neurodegenerative and metabolic diseases such as AD and non-alcoholic fatty liver disease (NAFLD).

Mechanistic Kinetic Model Reveals How Amyloidogenic Hydrophobic Patches Facilitate the Amyloid-β Fibril Elongation

Abnormal aggregation of amyloid β (Aβ) peptides into fibrils plays a critical role in the development of Alzheimer's disease. A two-stage "dock-lock" model has been proposed for the Aβ fibril elongation process. However, the mechanisms of the Aβ monomer-fibril binding process have not been elucidated with the necessary molecular-level precision, so it remains unclear how the lock phase dynamics leads to the overall in-register binding of the Aβ monomer onto the fibril. To gain mechanistic insights into this critical step during the fibril elongation process, we used molecular dynamics (MD) simulations with a physics-based coarse-grained UNited-RESidue (UNRES) force field and sampled extensively the dynamics of the lock phase process, in which a fibril-bound Aβ_(9-40) peptide rearranged to establish the native docking conformation. Analysis of the MD trajectories with Markov state models was used to quantify the kinetics of the rearrangement process and the most probable pathways leading to the overall native docking conformation of the incoming peptide. These revealed a key intermediate state in which an intra-monomer hairpin is formed between the central core amyloidogenic patch ₁₈VFFA₂₁ and the C-terminal hydrophobic patch ₃₄LMVG₃₇. This hairpin structure is highly favored as a transition state during the lock phase of the fibril elongation. We propose a molecular mechanism for facilitation of the Aβ fibril elongation by amyloidogenic hydrophobic patches.

Aβ Assemblies Promote Amyloidogenic Processing of APP and Intracellular Accumulation of Aβ42 Through Go/Gβγ Signaling

Alzheimer’s disease (AD) is characterized by the deposition of aggregated species of amyloid beta (Aβ) in the brain, which leads to progressive cognitive deficits and dementia. Aβ is generated by the successive cleavage of the amyloid precursor protein (APP), first by β-site APP cleaving enzyme 1 (BACE1) and subsequently by the γ-secretase complex. Those conditions which enhace or reduce its clearance predispose to Aβ aggregation and the development of AD. In vitro studies have demonstrated that Aβ assemblies spark a feed-forward loop heightening Aβ production. However, the underlying mechanism remains unknown. Here, we show that oligomers and fibrils of Aβ enhance colocalization and physical interaction of APP and BACE1 in recycling endosomes of human neurons derived from induced pluripotent stem cells and other cell types, which leads to exacerbated amyloidogenic processing of APP and intracellular accumulation of Aβ42. In cells that are overexpressing the mutant forms of APP which are unable to bind Aβ or to activate Go protein, we have found that treatment with aggregated Aβ fails to increase colocalization of APP with BACE1 indicating that Aβ-APP/Go signaling is involved in this process. Moreover, inhibition of Gβγ subunit signaling with βARKct or gallein prevents Aβ-dependent interaction of APP and BACE1 in endosomes, β-processing of APP, and intracellular accumulation of Aβ42. Collectively, our findings uncover a signaling mechanism leading to a feed-forward loop of amyloidogenesis that might contribute to Aβ pathology in the early stages of AD and suggest that gallein could have therapeutic potential.

Rab35 and glucocorticoids regulate APP and BACE1 trafficking to modulate Aβ production

Chronic stress and elevated glucocorticoids (GCs), the major stress hormones, are risk factors for Alzheimer’s disease (AD) and promote AD pathomechanisms, including overproduction of toxic amyloid-β (Aβ) peptides and intraneuronal accumulation of hyperphosphorylated Tau protein. The latter is linked to downregulation of the small GTPase Rab35, which mediates Tau degradation via the endolysosomal pathway. Whether Rab35 is also involved in Aβ overproduction remains an open question. Here, we find that hippocampal Rab35 levels are decreased not only by stress/GC but also by aging, another AD risk factor. Moreover, we show that Rab35 negatively regulates Aβ production by sorting amyloid precursor protein (APP) and β-secretase (BACE1) out of the endosomal network, where they interact to produce Aβ. Interestingly, Rab35 coordinates distinct intracellular trafficking steps for BACE1 and APP, mediated by its effectors OCRL and ACAP2, respectively. Finally, we demonstrate that Rab35 overexpression prevents the amyloidogenic trafficking of APP and BACE1 induced by high GC levels. These studies identify Rab35 as a key regulator of APP processing and suggest that its downregulation may contribute to stress-related and AD-related amyloidogenesis.

Oligomeric and Fibrillar Species of Aβ42 Diversely Affect Human Neural Stem Cells

Amyloid-β 42 peptide (Aβ_1-42 (Aβ42)) is well-known for its involvement in the development of Alzheimer’s disease (AD). Aβ42 accumulates and aggregates in fibers that precipitate in the form of plaques in the brain causing toxicity; however, like other forms of Aβ peptide, the role of these peptides remains unclear. Here we analyze and compare the effects of oligomeric and fibrillary Aβ42 peptide on the biology (cell death, proliferative rate, and cell fate specification) of differentiating human neural stem cells (hNS1 cell line). By using the hNS1 cells we found that, at high concentrations, oligomeric and fibrillary Aβ42 peptides provoke apoptotic cellular death and damage of DNA in these cells, but Aβ42 fibrils have the strongest effect. The data also show that both oligomeric and fibrillar Aβ42 peptides decrease cellular proliferation but Aβ42 oligomers have the greatest effect. Finally, both, oligomers and fibrils favor gliogenesis and neurogenesis in hNS1 cells, although, in this case, the effect is more prominent in oligomers. All together the findings of this study may contribute to a better understanding of the molecular mechanisms involved in the pathology of AD and to the development of human neural stem cell-based therapies for AD treatment.

Genetic deletion of α7 nicotinic acetylcholine receptors induces an age-dependent Alzheimer's disease-like pathology

The accumulation of amyloid-beta peptide (Aβ) and the failure of cholinergic transmission are key players in Alzheimer's disease (AD). However, in the healthy brain, Aβ contributes to synaptic plasticity and memory acting through α7 subtype nicotinic acetylcholine receptors (α7nAChRs). Here, we hypothesized that the α7nAChR deletion blocks Aβ physiological function and promotes a compensatory increase in Aβ levels that, in turn, triggers an AD-like pathology. To validate this hypothesis, we studied the age-dependent phenotype of α7 knock out mice. We found that α7nAChR deletion caused an impairment of hippocampal synaptic plasticity and memory at 12 months of age, paralleled by an increase of Amyloid Precursor Protein expression and Aβ levels. This was accompanied by other classical AD features such as a hyperphosphorylation of tau at residues Ser 199, Ser 396, Thr 205, a decrease of GSK-3β at Ser 9, the presence of paired helical filaments and neurofibrillary tangles, neuronal loss and an increase of GFAP-positive astrocytes. Our findings suggest that α7nAChR malfunction might precede Aβ and tau pathology, offering a different perspective to interpret the failure of anti-Aβ therapies against AD and to find novel therapeutical approaches aimed at restoring α7nAChRs-mediated Aβ function at the synapse.

The existence of Aβ strains and their potential for driving phenotypic heterogeneity in Alzheimer's disease

Reminiscent of the human prion diseases, there is considerable clinical and pathological variability in Alzheimer's disease, the most common human neurodegenerative condition. As in prion disorders, protein misfolding and aggregation is a hallmark feature of Alzheimer's disease, where the initiating event is thought to be the self-assembly of Aβ peptide into aggregates that deposit in the central nervous system. Emerging evidence suggests that Aβ, similar to the prion protein, can polymerize into a conformationally diverse spectrum of aggregate strains both in vitro and within the brain. Moreover, certain types of Aβ aggregates exhibit key hallmarks of prion strains including divergent biochemical attributes and the ability to induce distinct pathological phenotypes when intracerebrally injected into mouse models. In this review, we discuss the evidence demonstrating that Aβ can assemble into distinct strains of aggregates and how such strains may be primary drivers of the phenotypic heterogeneity in Alzheimer's disease.

The effects of antivenom administrations on the brain tissue of experimentally envenomed pregnant rats and their pups with Androctonus crassicauda scorpion venom during organogenesis period

This study aims to show the changing effects of Androctonus crassicauda venom and A. crasicauda specific antivenom during pregnancy in brain tissue of dams and their pups. Totally, 12 pregnant-Wistar Albino rats were randomly divided into two groups as venom-antivenom administration (n = 6) and control groups (n = 6). In venom-antivenom administration group (VAV), the sublethal dose of A. crassicauda venom dissolved in 1 mL physiological saline solution was subcutaneously (s.c.) injected into pregnant rats during organogenesis period (between 7 and 13 days of pregnancy). Four hours after each venom injection, 1 mL/s.c. dose of the specific anti-venom was administered to rats of VAV group. The rats in control group were given sterile saline solution 1 mL/s.c. In both groups, the fetuses were surgically delivered on the 21st day of pregnancy; dams and pups were sacrificed on postnatal 21 days, and their brain tissues were removed. The brain tissue of dams and their pups were evaluated histopathologically and immunohistochemically. To show the neuronal damages, 8-hydroxy-2-deoxyguanosine (8-OHDG) and amyloid beta precursor protein (ABPP) immunoexpressions were scored in cerebrum, cerebellum, pons and medulla oblongata of brain. To show the neuroprotection, reelin and beta-arrestin immunoexpressions were scored again in the same way. In this context, 8-OHDG immunoexpressions were increased in neocortex, hippocampus and nucleus accumbens when compared with that of control group. Amyloid beta precursor protein was negative in both groups. Reelin and beta-arrestin partly increased in fore and mid brain of VAV group as a reaction against neuronal damages when compared with that of control pups. The authors believe that prompt intervention using anti-venom to scorpion envenomation can partly stop neuronal damages. This neuroprotection may be increased to high and serial doses of anti-venom to save neonatal lives.

Bacterial Extracellular DNA Promotes β-Amyloid Aggregation

Alzheimer’s disease is associated with prion-like aggregation of the amyloid β (Aβ) peptide and the subsequent accumulation of misfolded neurotoxic aggregates in the brain. Therefore, it is critical to clearly identify the factors that trigger the cascade of Aβ misfolding and aggregation. Numerous studies have pointed out the association between microorganisms and their virulence factors and Alzheimer’s disease; however, their exact mechanisms of action remain unclear. Recently, we discovered a new pathogenic role of bacterial extracellular DNA, triggering the formation of misfolded Tau aggregates. In this study, we investigated the possible role of DNA extracted from different bacterial and eukaryotic cells in triggering Aβ aggregation in vitro. Interestingly, we found that the extracellular DNA of some, but not all, bacteria is an effective trigger of Aβ aggregation. Furthermore, the acceleration of Aβ nucleation and elongation can vary based on the concentration of the bacterial DNA and the bacterial strain from which this DNA had originated. Our findings suggest that bacterial extracellular DNA might play a previously overlooked role in the Aβ protein misfolding associated with Alzheimer’s disease pathogenesis. Moreover, it highlights a new mechanism of how distantly localized bacteria can remotely contribute to protein misfolding and diseases associated with this process. These findings might lead to the use of bacterial DNA as a novel therapeutic target for the prevention and treatment of Alzheimer’s disease.

Amyloid β and Amyloid Precursor Protein Synergistically Suppress Large-Conductance Calcium-Activated Potassium Channel in Cortical Neurons

Intracellular amyloid β (Aβ) injection suppresses the large-conductance calcium-dependent potassium (BK) channel in cortical pyramidal cells from wild-type (WT) mice. In 3xTg Alzheimer’s disease (AD) model mice, intraneuronal Aβ is genetically programed to accumulate, which suppresses the BK channel. However, the mode of BK channel suppression remained unclarified. The present report revealed that only one (11A1) of the three anti-Aβ-oligomer antibodies that we examined, but not anti-monomer-Aβ-antibodies, was effective in recovering BK channel activity in 3xTg neurons. Antibodies against amyloid precursor protein (APP) were also found to be effective, suggesting that APP plays an essential part in this Aβ-oligomer-induced BK channel suppression in 3xTg neurons. In WT neurons, by contrast, APP suppressed BK channels by itself, which suggests that either APP or Aβ is sufficient to block BK channels, thus pointing to a different co-operativity of Aβ and APP in WT and 3xTg neurons. To clarify this difference, we relied on our previous finding that the scaffold protein Homer1a reverses the BK channel blockade in both WT and 3xTg neurons. In cortical neurons from 3xTg mice that bear Homer1a knockout (4xTg mice), neither anti-APP antibodies nor 11A1, but only the 6E10 antibody that binds both APP and Aβ, rescued the BK channel suppression. Given that Homer1a expression is activity dependent and 3xTg neurons are hyperexcitable, Homer1a is likely to be expressed sufficiently in 3xTg neurons, thereby alleviating the suppressive influence of APP and Aβ on BK channel. A unique way that APP modifies Aβ toxicity is thus proposed.

Amyloid Metabolism and Amyloid-Targeting Blood-Based Biomarkers of Alzheimer's Disease

Amyloid-β (Aβ) is a key protein in Alzheimer's disease (AD) in that its accumulation induces complex pathological changes. Although there has been extensive research on the metabolism of Aβ in AD, new compelling results have recently emerged. Historically, the production and clearance of Aβ have been thought to originate in the central nervous system (CNS). However, recent evidence suggests that the production and clearance of Aβ can also occur in the peripheral system, and that the peripherally driven Aβ migrates to the CNS and induces amyloidopathy with subsequent AD pathologic changes in the brain. This concept implies that AD is not restricted to the CNS but is a systemic disease instead. As such, the development of blood-based biomarkers targeting Aβ is of great interest. Central and peripheral Aβ are both active contributors to the pathology of AD and interact bidirectionally. Measuring peripheral Aβ is not just observing the reflection of the residual Aβ removed from the CNS but also tracking the ongoing process of AD pathology. Additionally, blood-based biomarkers could be a more accessible tool in clinical and research settings. Through arduous research, several blood-based biomarker assays have demonstrated notable results. In this review, we describe the metabolism of Aβ and the amyloid-targeting blood-based biomarkers of AD.

Caspase Activation and Caspase-Mediated Cleavage of APP Is Associated with Amyloid β-Protein-Induced Synapse Loss in Alzheimer's Disease

Amyloid β-protein (Aβ) toxicity is hypothesized to play a seminal role in Alzheimer's disease (AD) pathogenesis. However, it remains unclear how Aβ causes synaptic dysfunction and synapse loss. We hypothesize that one mechanism of Aβ-induced synaptic injury is related to the cleavage of amyloid β precursor protein (APP) at position D664 by caspases that release the putatively cytotoxic C31 peptide. In organotypic slice cultures derived from mice with a knock-in mutation in the APP gene (APP D664A) to inhibit caspase cleavage, Aβ-induced synaptic injury is markedly reduced in two models of Aβ toxicity. Loss of dendritic spines is also attenuated in mice treated with caspase inhibitors. Importantly, the time-dependent dendritic spine loss is correlated with localized activation of caspase-3 but is absent in APP D664A cultures. We propose that the APP cytosolic domain plays an essential role in Aβ-induced synaptic damage in the injury pathway mediated by localized caspase activation.

Tau is not necessary for amyloid-β-induced synaptic and memory impairments

The amyloid hypothesis posits that the amyloid-beta (Aβ) protein precedes and requires microtubule-associated protein tau in a sort of trigger-bullet mechanism leading to Alzheimer's disease (AD) pathology. This sequence of events has become dogmatic in the AD field and is used to explain clinical trial failures due to a late start of the intervention when Aβ already activated tau. Here, using a multidisciplinary approach combining molecular biological, biochemical, histopathological, electrophysiological, and behavioral methods, we demonstrated that tau suppression did not protect against Aβ-induced damage of long-term synaptic plasticity and memory, or from amyloid deposition. Tau suppression could even unravel a defect in basal synaptic transmission in a mouse model of amyloid deposition. Similarly, tau suppression did not protect against exogenous oligomeric tau-induced impairment of long-term synaptic plasticity and memory. The protective effect of tau suppression was, in turn, confined to short-term plasticity and memory. Taken together, our data suggest that therapies downstream of Aβ and tau together are more suitable to combat AD than therapies against one or the other alone.

An overview of disease models for NLRP3 inflammasome over-activation

Introduction: Inflammatory reactions, including those mediated by the NLRP3 inflammasome, maintain the body's homeostasis by removing pathogens, repairing damaged tissues, and adapting to stressed environments. However, uncontrolled activation of the NLRP3 inflammasome tends to cause various diseases using different mechanisms. Recently, many inhibitors of the NLRP3 inflammasome have been reported and many are being developed. In order to assess their efficacy, specificity, and mechanism of action, the screening process of inhibitors requires various types of cell and animal models of NLRP3-associated diseases.Areas covered: In the following review, the authors give an overview of the cell and animal models that have been used during the research and development of various inhibitors of the NLRP3 inflammasome.Expert opinion: There are many NLRP3 inflammasome inhibitors, but most of the inhibitors have poor specificity and often influence other inflammatory pathways. The potential risk for cross-reaction is high; therefore, the development of highly specific inhibitors is essential. The selection of appropriate cell and animal models, and combined use of different models for the evaluation of these inhibitors can help to clarify the target specificity and therapeutic effects, which is beneficial for the development and application of drugs targeting the NLRP3 inflammasome.

Accumulation of Cerebrospinal Fluid Glycerophospholipids and Sphingolipids in Cognitively Healthy Participants With Alzheimer's Biomarkers Precedes Lipolysis in the Dementia Stage

Insight into lipids' roles in Alzheimer's disease (AD) pathophysiology is limited because brain membrane lipids have not been characterized in cognitively healthy (CH) individuals. Since age is a significant risk factor of AD, we hypothesize that aging renders the amyloid precursor protein (APP) more susceptible to abnormal processing because of deteriorating membrane lipids. To reflect brain membranes, we studied their lipid components in cerebrospinal fluid (CSF) and brain-derived CSF nanoparticle membranes. Based on CSF Aβ42/Tau levels established biomarkers of AD, we define a subset of CH participants with normal Aβ42/Tau (CH-NAT) and another group with abnormal or pathological Aβ42/Tau (CH-PAT). We report that glycerophospholipids are differentially metabolized in the CSF supernatant fluid and nanoparticle membrane fractions from CH-NAT, CH-PAT, and AD participants. Phosphatidylcholine molecular species from the supernatant fraction of CH-PAT were higher than in the CH-NAT and AD participants. Sphingomyelin levels in the supernatant fraction were lower in the CH-PAT and AD than in the CH-NAT group. The decrease in sphingomyelin corresponded with an increase in ceramide and dihydroceramide and an increase in the ceramide to sphingomyelin ratio in AD. In contrast to the supernatant fraction, sphingomyelin is higher in the nanoparticle fraction from the CH-PAT group, accompanied by lower ceramide and dihydroceramide and a decrease in the ratio of ceramide to sphingomyelin in CH-PAT compared with CH-NAT. On investigating the mechanism for the lipid changes in AD, we observed that phospholipase A2 (PLA2) activity was higher in the AD group than the CH groups. Paradoxically, acid and neutral sphingomyelinase (SMase) activities were lower in AD compared to the CH groups. Considering external influences on lipids, the clinical groups did not differ in their fasting blood lipids or dietary lipids, consistent with the CSF lipid changes originating from brain pathophysiology. The lipid accumulation in a prodromal AD biomarker positive stage identifies perturbation of lipid metabolism and disturbances in APP/Amyloid beta (Aβ) as early events in AD pathophysiology. Our results identify increased lipid turnover in CH participants with AD biomarkers, switching to a predominantly lipolytic state in dementia. This knowledge may be useful for targeting and testing new AD treatments.

Oligomer β-amyloid Induces Hyperactivation of Ras to Impede NMDA Receptor-Dependent Long-Term Potentiation in Hippocampal CA1 of Mice

The activity of Ras, a small GTPase protein, is increased in brains with Alzheimer’s disease. The objective of this study was to determine the influence of oligomeric Aβ_1-42 on the activation of Ras, and the involvement of the Ras hyperactivity in Aβ_1-42-induced deficits in spatial cognition and hippocampal synaptic plasticity. Herein, we show that intracerebroventricular injection of Aβ_1-42 in mice (Aβ-mice) enhanced hippocampal Ras activation and expression, while 60 min incubation of hippocampal slices in Aβ_1-42 (Aβ-slices) only elevated Ras activity. Aβ-mice showed deficits in spatial cognition and NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) in hippocampal CA1, but basal synaptic transmission was enhanced. The above effects of Aβ_1-42 were corrected by the Ras inhibitor farnesylthiosalicylic acid (FTS). ERK2 phosphorylation increased, and Src phosphorylation decreased in Aβ-mice and Aβ_1-42-slices. Both were corrected by FTS. In CA1 pyramidal cells of Aβ_1-42-slices, the response of AMPA receptor and phosphorylation of GluR1 were enhanced with dependence on Ras activation rather than ERK signaling. In contrast, NMDA receptor (NMDAR) function and GluN2A/2B phosphorylation were downregulated in Aβ_1-42-slices, which was recovered by application of FTS or the Src activator ouabain, and mimicked in control slices treated with the Src inhibitor PP2. The administration of PP2 impaired the spatial cognition and LTP induction in control mice and FTS-treated Aβ-mice. The treatment of Aβ-mice with ouabain rescued Aβ-impaired spatial cognition and LTP. Overall, the results indicate that the oligomeric Aβ_1-42 hyperactivates Ras and thereby causes the downregulation of Src which impedes NMDAR-dependent LTP induction resulting in cognitive deficits.

Association of microbiota-derived propionic acid and Alzheimer's disease; bioinformatics analysis

PURPOSE

Microbiota-derived metabolites could alter the brain tissue toward the neurodegeneration disease. This study aims to select the genes associated with Propionic acid (PPA) and compromise Alzheimer's disease (AD) to find the possible roles of PPA in AD pathogenesis.

METHODS

Microbiota-derived metabolites could alter the brain tissue toward the neurodegeneration disease. This study aims to select the genes associated with Propionic acid (PPA) and compromise Alzheimer's disease (AD) to find the possible roles of PPA in AD pathogenesis.

RESULTS

Amongst all genes associated with PPA and AD, 284 genes to be shared by searching databases and were subjected to further analysis. AD-PPA genes mainly involved in cancer, bacterial and virus infection, and neurological and non-neurological diseases. Gene Ontology and pathway analysis covered the most AD hallmark, such as amyloid formation, apoptosis, proliferation, inflammation, and immune system. Network analysis revealed hub and bottleneck genes. MCODE analysis also indicated the seed genes represented in the significant subnetworks. ICAM1 and CCND1 were the hub, bottleneck, and seed genes.

CONCLUSIONS

PPA interacted genes implicated in AD act through pathways initiate neuronal cell death. In sum up, AD-PPA shared genes exhibited evidence that supports the idea PPA secreted from bacteria could alter brain physiology toward the emerging AD signs. This idea needs to confirm by more future investigation in animal models.

Amyloid-β Protein Precursor Regulates Depolarization-Induced Calcium-Mediated Synaptic Signaling in Brain Slices

BACKGROUND

Coordinated calcium influx upon neuronal depolarization activates pathways that phosphorylate CaMKII, ERKs, and the transcription factor CREB and, therefore, expression of pro-survival and neuroprotective genes. Recent evidence indicates that amyloid-β protein precursor (AβPP) is trafficked to synapses and promotes their formation. At the synapse, AβPP interacts with synaptic proteins involved in vesicle exocytosis and affects calcium channel function.

OBJECTIVE

Herein, we examined the role of AβPP in depolarization-induced calcium-mediated signaling using acute cerebral slices from wild-type C57bl/6 mice and AβPP-/- C57bl/6 mice.

METHODS

Depolarization of acute cerebral slices from wild-type C57bl/6 and AβPP-/- C57bl/6 mice was used to induce synaptic signaling. Protein levels were examined by western blot and calcium dynamics were assessed using primary neuronal cultures.

RESULTS

In the absence of AβPP, decreased pCaMKII and pERKs levels were observed. This decrease was sensitive to the inhibition of N- and P/Q-type Voltage Gated Calcium Channels (N- and P/Q-VGCCs) by ω-conotoxin GVIA and ω-conotoxin MVIIC, respectively, but not to inhibition of L-type VGCCs by nifedipine. However, the absence of AβPP did not result in a statistically significant decrease of pCREB, which is a known substrate of pERKs. Finally, using calcium imaging, we found that down regulation of AβPP in cortical neurons results in a decreased response to depolarization and altered kinetics of calcium response.

CONCLUSION

AβPP regulates synaptic activity-mediated neuronal signaling by affecting N- and P/Q-VGCCs.

Effect of Aβ Oligomers on Neuronal APP Triggers a Vicious Cycle Leading to the Propagation of Synaptic Plasticity Alterations to Healthy Neurons

Alterations of excitatory synaptic function are the strongest correlate to the pathologic disturbance of cognitive ability observed in the early stages of Alzheimer's disease (AD). This pathologic feature is driven by amyloid-β oligomers (Aβos) and propagates from neuron to neuron. Here, we investigated the mechanism by which Aβos affect the function of synapses and how these alterations propagate to surrounding healthy neurons. We used complementary techniques ranging from electrophysiological recordings and molecular biology to confocal microscopy in primary cortical cultures, and from acute hippocampal and cortical slices from male wild-type and amyloid precursor protein (APP) knock-out (KO) mice to assess the effects of Aβos on glutamatergic transmission, synaptic plasticity, and dendritic spine structure. We showed that extracellular application of Aβos reduced glutamatergic synaptic transmission and long-term potentiation. These alterations were not observed in APP KO neurons, suggesting that APP expression is required. We demonstrated that Aβos/APP interaction increases the amyloidogenic processing of APP leading to intracellular accumulation of newly produced Aβos. Intracellular Aβos participate in synaptic dysfunctions as shown by pharmacological inhibition of APP processing or by intraneuronal infusion of an antibody raised against Aβos. Furthermore, we provide evidence that following APP processing, extracellular release of Aβos mediates the propagation of the synaptic pathology characterized by a decreased spine density of neighboring healthy neurons in an APP-dependent manner. Together, our data unveil a complementary role for Aβos in AD, while intracellular Aβos alter synaptic function, extracellular Aβos promote a vicious cycle that propagates synaptic pathology from diseased to healthy neurons.SIGNIFICANCE STATEMENT Here we provide the proof that a vicious cycle between extracellular and intracellular pools of Aβ oligomers (Aβos) is required for the spreading of Alzheimer's disease (AD) pathology. We showed that extracellular Aβos propagate excitatory synaptic alterations by promoting amyloid precursor protein (APP) processing. Our results also suggest that subsequent to APP cleavage two pools of Aβos are produced. One pool accumulates inside the cytosol, inducing the loss of synaptic plasticity potential. The other pool is released into the extracellular space and contributes to the propagation of the pathology from diseased to healthy neurons. Pharmacological strategies targeting the proteolytic cleavage of APP disrupt the relationship between extracellular and intracellular Aβ, providing a therapeutic approach for the disease.

Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disease characterized by progressive memory loss. Although AD neuropathological hallmarks are extracellular amyloid plaques and intracellular tau tangles, the best correlate of disease progression is synapse loss. What causes synapse loss has been the focus of several researchers in the AD field. Synapses become dysfunctional before plaques and tangles form. Studies based on early-onset familial AD (eFAD) models have supported that synaptic transmission is depressed by β-amyloid (Aβ) triggered mechanisms. Since eFAD is rare, affecting only 1% of patients, research has shifted to the study of the most common late-onset AD (LOAD). Intracellular trafficking has emerged as one of the pathways of LOAD genes. Few studies have assessed the impact of trafficking LOAD genes on synapse dysfunction. Since endocytic traffic is essential for synaptic function, we reviewed Aβ-dependent and independent mechanisms of the earliest synaptic dysfunction in AD. We have focused on the role of intraneuronal and secreted Aβ oligomers, highlighting the dysfunction of endocytic trafficking as an Aβ-dependent mechanism of synapse dysfunction in AD. Here, we reviewed the LOAD trafficking genes APOE4, ABCA7, BIN1, CD2AP, PICALM, EPH1A, and SORL1, for which there is a synaptic link. We conclude that in eFAD and LOAD, the earliest synaptic dysfunctions are characterized by disruptions of the presynaptic vesicle exo- and endocytosis and of postsynaptic glutamate receptor endocytosis. While in eFAD synapse dysfunction seems to be triggered by Aβ, in LOAD, there might be a direct synaptic disruption by LOAD trafficking genes. To identify promising therapeutic targets and biomarkers of the earliest synaptic dysfunction in AD, it will be necessary to join efforts in further dissecting the mechanisms used by Aβ and by LOAD genes to disrupt synapses.

Physiological and pathological effects of amyloid-β species in neural stem cell biology

Although amyloid-β peptide is considered neurotoxic, it may mediate several physiological processes during embryonic development and in the adult brain. The pathological function of amyloid-β peptide has been extensively studied due to its implication in Alzheimer's disease, but its physiological function remains poorly understood. Amyloid-β peptide can be detected in non-aggregated (monomeric) and aggregated (oligomeric and fibrillary) forms. Each form has different cytotoxic and/or physiological properties, so amyloid-β peptide and its role in Alzheimer's disease need to be studied further. Neural stem cells and neural precursor cells are good tools for the study on neurodegenerative diseases and can provide future therapeutic applications in diseases such as Alzheimer's disease. In this review, we provide an outline of the effects of amyloid-β peptide, in monomeric and aggregated forms, on the biology of neural stem cells/neural precursor cells, and discuss the controversies. We also describe the possible molecular targets that could be implicated in these effects, especially GSK3β. A better understanding of amyloid-β peptide (both physiological and pathological), and the signaling pathways involved are essential to advance the field of Alzheimer's disease.

Protofibrillar and Fibrillar Amyloid-β Binding Proteins in Cerebrospinal Fluid

Aggregation and deposition of misfolded amyloid-β (Aβ) peptide in the brain is central to Alzheimer's disease (AD). Oligomeric, protofibrillar, and fibrillar forms of Aβ are believed to be neurotoxic and cause neurodegeneration in AD, but the toxicity mechanisms are not well understood and may involve Aβ-interacting molecular partners. In a previous study, we identified potential Aβ42 protofibrillar-binding proteins in serum and cerebrospinal fluid (CSF) using an engineered version of Aβ42 (Aβ42CC) that forms protofibrils, but not fibrils. Here we studied binding of proteins to Aβ42 fibrils in AD and non-AD CSF and compared these with protofibrillar Aβ42CC-binding partners. Aβ42 fibrils sequestered 2.4-fold more proteins than Aβ42CC protofibrils. Proteins with selective binding to fibrillar aggregates with low nanomolar affinity were identified. We also found that protofibrillar and fibrillar Aβ-binding proteins represent distinct functional categories. Aβ42CC protofibrils triggered interactions with proteins involved in catalytic activities, like transferases and oxidoreductases, while Aβ42 fibrils were more likely involved in binding to proteoglycans, growth factors and neuron-associated proteins, e.g., neurexin-1, -2, and -3. Interestingly, 10 brain-enriched proteins were identified among the fibril-binding proteins, while protofibril-extracted proteins had more general expression patterns. Both types of Aβ aggregates bound several extracellular proteins. Additionally, we list a set of CSF proteins that might have potential to discriminate between AD and non-AD CSF samples. The results may be of relevance both for biomarker studies and for studies of Aβ-related toxicity mechanisms.

Genome-wide association study identifies CBFA2T3 affecting the rate of CSF Aβ42 decline in non-demented elders

Brain amyloid deposition is an early pathological event in Alzheimer's disease (AD), and abnormally low levels amyloid-β42 peptide (Aβ42) in cerebrospinal fluid (CSF) can be detected in preclinical AD. To identify the genetic determinants that regulate the rate of CSF Aβ42 decline among non-demented elders, we conducted a genome-wide association study involved 321 non-demented elders from Alzheimer's Disease Neuroimaging Initiative (ADNI) 1/GO/2 cohorts restricted to non-Hispanic Caucasians. A novel genome-wide significant association of higher annualized percent decline of CSF Aβ42 in the gene CBFA2T3 (CBFA2/RUNX1 translocation partner 3; rs13333659-T; p = 2.24 × 10-9) was identified. Besides displaying abnormal CSF Aβ42 levels, rs13333659-T carriers were more likely to exhibit a greater longitudinal cognitive decline (p = 0.029, β = 0.097) and hippocampal atrophy (p = 0.029, β = -0.160) in the non-demented elders, especially for the participants who were amyloid-positive at baseline. These findings suggest rs13333659 in CBFA2T3 as a risk locus to modulate the decline rate of CSF Aβ42 preceding the onset of clinical symptoms.

Neuromodulatory Action of Picomolar Extracellular Aβ42 Oligomers on Presynaptic and Postsynaptic Mechanisms Underlying Synaptic Function and Memory

Failure of anti-amyloid-β peptide (Aβ) therapies against Alzheimer's disease (AD), a neurodegenerative disorder characterized by high amounts of the peptide in the brain, raised the question of the physiological role of Aβ released at low concentrations in the healthy brain. To address this question, we studied the presynaptic and postsynaptic mechanisms underlying the neuromodulatory action of picomolar amounts of oligomeric Aβ42 (oAβ42) on synaptic glutamatergic function in male and female mice. We found that 200 pm oAβ42 induces an increase of frequency of miniature EPSCs and a decrease of paired pulse facilitation, associated with an increase in docked vesicle number, indicating that it augments neurotransmitter release at presynaptic level. oAβ42 also produced postsynaptic changes as shown by an increased length of postsynaptic density, accompanied by an increased expression of plasticity-related proteins such as cAMP-responsive element binding protein phosphorylated at Ser133, calcium-calmodulin-dependent kinase II phosphorylated at Thr286, and brain-derived neurotrophic factor, suggesting a role for Aβ in synaptic tagging. These changes resulted in the conversion of early into late long-term potentiation through the nitric oxide/cGMP/protein kinase G intracellular cascade consistent with a cGMP-dependent switch from short- to long-term memory observed in vivo after intrahippocampal administration of picomolar amounts of oAβ42 These effects were present upon extracellular but not intracellular application of the peptide and involved α7 nicotinic acetylcholine receptors. These observations clarified the physiological role of oAβ42 in synaptic function and memory formation providing solid fundamentals for investigating the pathological effects of high Aβ levels in the AD brains.SIGNIFICANCE STATEMENT High levels of oligomeric amyloid-β42 (oAβ42) induce synaptic dysfunction leading to memory impairment in Alzheimer's disease (AD). However, at picomolar concentrations, the peptide is needed to ensure long-term potentiation (LTP) and memory. Here, we show that extracellular 200 pm oAβ42 concentrations increase neurotransmitter release, number of docked vesicles, postsynaptic density length, and expression of plasticity-related proteins leading to the conversion of early LTP into late LTP and of short-term memory into long-term memory. These effects require α7 nicotinic acetylcholine receptors and are mediated through the nitric oxide/cGMP/protein kinase G pathway. The knowledge of Aβ function in the healthy brain might be useful to understand the causes leading to its increase and detrimental effect in AD.

Synaptic and memory dysfunction induced by tau oligomers is rescued by up-regulation of the nitric oxide cascade

BACKGROUND

Soluble aggregates of oligomeric forms of tau protein (oTau) have been associated with impairment of synaptic plasticity and memory in Alzheimer's disease. However, the molecular mechanisms underlying the synaptic and memory dysfunction induced by elevation of oTau are still unknown.

METHODS

This work used a combination of biochemical, electrophysiological and behavioral techniques. Biochemical methods included analysis of phosphorylation of the cAMP-responsive element binding (CREB) protein, a transcriptional factor involved in memory, histone acetylation, and expression immediate early genes c-Fos and Arc. Electrophysiological methods included assessment of long-term potentiation (LTP), a type of synaptic plasticity thought to underlie memory formation. Behavioral studies investigated both short-term spatial memory and associative memory. These phenomena were examined following oTau elevation.

RESULTS

Levels of phospho-CREB, histone 3 acetylation at lysine 27, and immediate early genes c-Fos and Arc, were found to be reduced after oTau elevation during memory formation. These findings led us to explore whether up-regulation of various components of the nitric oxide (NO) signaling pathway impinging onto CREB is capable of rescuing oTau-induced impairment of plasticity, memory, and CREB phosphorylation. The increase of NO levels protected against oTau-induced impairment of LTP through activation of soluble guanylyl cyclase. Similarly, the elevation of cGMP levels and stimulation of the cGMP-dependent protein kinases (PKG) re-established normal LTP after exposure to oTau. Pharmacological inhibition of cGMP degradation through inhibition of phosphodiesterase 5 (PDE5), rescued oTau-induced LTP reduction. These findings could be extrapolated to memory because PKG activation and PDE5 inhibition rescued oTau-induced memory impairment. Finally, PDE5 inhibition re-established normal elevation of CREB phosphorylation and cGMP levels after memory induction in the presence of oTau.

CONCLUSIONS

Up-regulation of CREB activation through agents acting on the NO cascade might be beneficial against tau-induced synaptic and memory dysfunctions.

Inflammasome-Mediated Inflammation in Neurodegenerative Diseases

Inflammasomes are protein platforms consisting of multiple proteins. The biological function includes the activation of caspase-1, leading to the maturation of IL-1β and IL-18. These pro-inflammatory cytokines promote fundamental inflammatory processes in numerous infectious diseases. The inflammasome-mediated inflammation has become increasingly important in central nervous system disorders. In neurodegenerative disorders, significant contributors to disease progression include neuroinflammation and inflammatory cascades initiated by the inflammasome protein complex. This review discusses the recent progress of research on inflammasome associated with neurodegenerative disorders.

Complex formation of APP with GABAB receptors links axonal trafficking to amyloidogenic processing

GABAB receptors (GBRs) are key regulators of synaptic release but little is known about trafficking mechanisms that control their presynaptic abundance. We now show that sequence-related epitopes in APP, AJAP-1 and PIANP bind with nanomolar affinities to the N-terminal sushi-domain of presynaptic GBRs. Of the three interacting proteins, selectively the genetic loss of APP impaired GBR-mediated presynaptic inhibition and axonal GBR expression. Proteomic and functional analyses revealed that APP associates with JIP and calsyntenin proteins that link the APP/GBR complex in cargo vesicles to the axonal trafficking motor. Complex formation with GBRs stabilizes APP at the cell surface and reduces proteolysis of APP to Aβ, a component of senile plaques in Alzheimer's disease patients. Thus, APP/GBR complex formation links presynaptic GBR trafficking to Aβ formation. Our findings support that dysfunctional axonal trafficking and reduced GBR expression in Alzheimer's disease increases Aβ formation.

Heterogeneous Association of Alzheimer's Disease-Linked Amyloid-β and Amyloid-β Protein Precursor with Synapses

Alzheimer’s disease (AD) is increasingly viewed as a disease of synapses. Loss of synapses correlates better with cognitive decline than amyloid plaques and neurofibrillary tangles, the hallmark neuropathological lesions of AD. Soluble forms of amyloid-β (Aβ) have emerged as mediators of synapse dysfunction. Aβ binds to, accumulates, and aggregates in synapses. However, the anatomical and neurotransmitter specificity of Aβ and the amyloid-β protein precursor (AβPP) in AD remain poorly understood. In addition, the relative roles of Aβ and AβPP in the development of AD, at pre- versus post-synaptic compartments and axons versus dendrites, respectively, remain unclear. Here we use immunogold electron microscopy and confocal microscopy to provide evidence for heterogeneity in the localization of Aβ/AβPP. We demonstrate that Aβ binds to a subset of synapses in cultured neurons, with preferential binding to glutamatergic compared to GABAergic neurons. We also highlight the challenge of defining pre- versus post-synaptic localization of this binding by confocal microscopy. Further, endogenous Aβ₄₂ accumulates in both glutamatergic and GABAergic AβPP/PS1 transgenic primary neurons, but at varying levels. Moreover, upon knock-out of presenilin 1 or inhibition of γ-secretase AβPP C-terminal fragments accumulate both pre- and post-synaptically; however earlier pre-synaptically, consistent with a higher rate of AβPP processing in axons. A better understanding of the synaptic and anatomical selectivity of Aβ/AβPP in AD can be important for the development of more effective new therapies for this major disease of aging.

Prion-like propagation of β-amyloid aggregates in the absence of APP overexpression

The amyloid cascade hypothesis posits that the initiating event in Alzheimer's disease (AD) is the aggregation and deposition of the β-amyloid (Aβ) peptide, which is a proteolytic cleavage product of the amyloid precursor protein (APP). Mounting evidence suggests that the formation and spread of prion-like Aβ aggregates during AD may contribute to disease progression. Inoculation of transgenic mice that overexpress APP with pre-formed Aβ aggregates results in the prion-like induction of cerebral Aβ deposition. To determine whether Aβ deposition can also be induced when physiological APP levels are present in the brain, we inoculated App^NL-F mice, a knock-in model of AD that avoids potential artifacts associated with APP overexpression, with Aβ aggregates derived from the brains of AD patients or transgenic mice. In all cases, induced Aβ deposition was apparent in the corpus callosum, olfactory bulb, and meningeal blood vessels of inoculated mice at 130-150 days post-inoculation, whereas uninoculated and buffer-inoculated animals exhibited minimal or no Aβ deposits at these ages. Interestingly, despite being predominantly composed of protease-resistant Aβ42 aggregates, the induced parenchymal Aβ deposits were largely diffuse and were unreactive to an amyloid-binding dye. These results demonstrate that APP overexpression is not a prerequisite for the prion-like induction of cerebral Aβ deposition. Accordingly, spreading of Aβ deposition may contribute to disease progression in AD patients.

Amyloid-Beta and Phosphorylated Tau Accumulations Cause Abnormalities at Synapses of Alzheimer's disease Neurons

Amyloid-beta (Aβ) and hyperphosphorylated tau are hallmark lesions of Alzheimer's disease (AD). However, the loss of synapses and dysfunctions of neurotransmission are more directly tied to disease severity. The role of these lesions in the pathoetiological progression of the disease remains contested. Biochemical, cellular, molecular, and pathological studies provided several lines of evidence and improved our understanding of how Aβ and hyperphosphorylated tau accumulation may directly harm synapses and alter neurotransmission. In vitro evidence suggests that Aβ and hyperphosphorylated tau have both direct and indirect cytotoxic effects that affect neurotransmission, axonal transport, signaling cascades, organelle function, and immune response in ways that lead to synaptic loss and dysfunctions in neurotransmitter release. Observations in preclinical models and autopsy studies support these findings, suggesting that while the pathoetiology of positive lesions remains elusive, their removal may reduce disease severity and progression. The purpose of this article is to highlight the need for further investigation of the role of tau in disease progression and its interactions with Aβ and neurotransmitters alike.

The Level of NMDA Receptor in the Membrane Modulates Amyloid-β Association and Perforation

Alzheimer's disease is a neurodegenerative disorder that affects mostly the elderly. The main histopathological markers are the senile plaques formed by amyloid-β peptide (Aβ) aggregates that can perforate the plasma membrane of cells, increasing the intracellular calcium levels and releasing synaptic vesicles that finally lead to a delayed synaptic failure. Several membrane proteins and lipids interact with Aβ affecting its toxicity in neurons. Here, we focus on NMDA receptors (NMDARs) as proteins that could be modulating the association and neurotoxic perforation induced by Aβ on the plasma membrane. In fact, our results showed that decreasing NMDARs, using enzymatic or siRNA approaches, increased the association of Aβ to the neurons. Furthermore, overexpression of NMDARs also resulted in an enhanced association between NMDA and Aβ. Functionally, the reduction in membrane NMDARs augmented the process of membrane perforation. On the other hand, overexpressing NMDARs had a protective effect because Aβ was now unable to cause membrane perforation, suggesting a complex relationship between Aβ and NMDARs. Because previous studies have recognized that Aβ oligomers are able to increase membrane permeability and produce amyloid pores, the present study supports the conclusion that NMDARs play a critical protective role on Aβ actions in hippocampal neurons. These results could explain the lack of correlation between brain Aβ burden and clinically observed dementia.

Role of Amyloid-β and Tau Proteins in Alzheimer's Disease: Confuting the Amyloid Cascade

The "Amyloid Cascade Hypothesis" has dominated the Alzheimer's disease (AD) field in the last 25 years. It posits that the increase of amyloid-β (Aβ) is the key event in AD that triggers tau pathology followed by neuronal death and eventually, the disease. However, therapeutic approaches aimed at decreasing Aβ levels have so far failed, and tau-based clinical trials have not yet produced positive findings. This begs the question of whether the hypothesis is correct. Here we have examined literature on the role of Aβ and tau in synaptic dysfunction, memory loss, and seeding and spreading of AD, highlighting important parallelisms between the two proteins in all of these phenomena. We discuss novel findings showing binding of both Aβ and tau oligomers to amyloid-β protein precursor (AβPP), and the requirement for the presence of this protein for both Aβ and tau to enter neurons and induce abnormal synaptic function and memory. Most importantly, we propose a novel view of AD pathogenesis in which extracellular oligomers of Aβ and tau act in parallel and upstream of AβPP. Such a view will call for a reconsideration of therapeutic approaches directed against Aβ and tau, paving the way to an increased interest toward AβPP, both for understanding the pathogenesis of the disease and elaborating new therapeutic strategies.

APP/Go protein Gβγ-complex signaling mediates Aβ degeneration and cognitive impairment in Alzheimer's disease models

Deposition of amyloid-β (Aβ), the proteolytic product of the amyloid precursor protein (APP), might cause neurodegeneration and cognitive decline in Alzheimer's disease (AD). However, the direct involvement of APP in the mechanism of Aβ-induced degeneration in AD remains on debate. Here, we analyzed the interaction of APP with heterotrimeric Go protein in primary hippocampal cultures and found that Aβ deposition dramatically enhanced APP-Go protein interaction in dystrophic neurites. APP overexpression rendered neurons vulnerable to Aβ toxicity by a mechanism that required Go-Gβγ complex signaling and p38-mitogen-activated protein kinase activation. Gallein, a selective pharmacological inhibitor of Gβγ complex, inhibited Aβ-induced dendritic and axonal dystrophy, abnormal tau phosphorylation, synaptic loss, and neuronal cell death in hippocampal neurons expressing endogenous protein levels. In the 3xTg-AD mice, intrahippocampal application of gallein reversed memory impairment associated with early Aβ pathology. Our data provide further evidence for the involvement of APP/Go protein in Aβ-induced degeneration and reveal that Gβγ complex is a signaling target potentially relevant for developing therapies for halting Aβ degeneration in AD.

Human Brain-Derived Aβ Oligomers Bind to Synapses and Disrupt Synaptic Activity in a Manner That Requires APP

Compelling genetic evidence links the amyloid precursor protein (APP) to Alzheimer's disease (AD) and several theories have been advanced to explain the relationship. A leading hypothesis proposes that a small amphipathic fragment of APP, the amyloid β-protein (Aβ), self-associates to form soluble aggregates that impair synaptic and network activity. Here, we used the most disease-relevant form of Aβ, protein isolated from AD brain. Using this material, we show that the synaptotoxic effects of Aβ depend on expression of APP and that the Aβ-mediated impairment of synaptic plasticity is accompanied by presynaptic effects that disrupt the excitatory/inhibitory (E/I) balance. The net increase in the E/I ratio and inhibition of plasticity are associated with Aβ localizing to synapses and binding of soluble Aβ aggregates to synapses requires the expression of APP. Our findings indicate a role for APP in AD pathogenesis beyond the generation of Aβ and suggest modulation of APP expression as a therapy for AD.SIGNIFICANCE STATEMENT Here, we report on the plasticity-disrupting effects of amyloid β-protein (Aβ) isolated from Alzheimer's disease (AD) brain and the requirement of amyloid precursor protein (APP) for these effects. We show that Aβ-containing AD brain extracts block hippocampal LTP, augment glutamate release probability, and disrupt the excitatory/inhibitory balance. These effects are associated with Aβ localizing to synapses and genetic ablation of APP prevents both Aβ binding and Aβ-mediated synaptic dysfunctions. Our results emphasize the importance of APP in AD and should stimulate new studies to elucidate APP-related targets suitable for pharmacological manipulation.

Tau and amyloid-related pathologies in the entorhinal cortex have divergent effects in the hippocampal circuit

The entorhinal cortex (EC) is affected early in Alzheimer's disease, an illness defined by a co-occurrence of tau and amyloid-related pathologies. How the co-occurrence of these pathologies in the EC affects the hippocampal circuit remains unknown. Here we address this question by performing electrophysiological analyses of the EC circuit in mice that express mutant human amyloid precursor protein (hAPP) or tau (hTau), or both in the EC. We show that the alterations in the hippocampal circuit are divergent, with hAPP increasing but hTau decreasing neuronal/circuit excitability. Most importantly, mice co-expressing hAPP and hTau show that hTau has a dominant effect, dampening the excitatory effects of hAPP. Additionally, compensatory synaptic downscaling, in response to increased excitability in EC was observed in subicular neurons of hAPP mice. Based on simulations, we propose that EC interneuron pruning can account for both EC hyperexcitability and subicular synaptic downscaling found in mice expressing hAPP.

The physiological role of the amyloid precursor protein as an adhesion molecule in the developing nervous system

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein better known for its participation in the physiopathology of Alzheimer disease as the source of the beta amyloid fragment. However, the physiological functions of the full length protein and its proteolytic fragments have remained elusive. APP was first described as a cell-surface receptor; nevertheless, increasing evidence highlighted APP as a cell adhesion molecule. In this review, we will focus on the current knowledge of the physiological role of APP as a cell adhesion molecule and its involvement in key events of neuronal development, such as migration, neurite outgrowth, growth cone pathfinding, and synaptogenesis. Finally, since APP is over-expressed in Down syndrome individuals because of the extra copy of chromosome 21, in the last section of the review, we discuss the potential contribution of APP to the neuronal and synaptic defects described in this genetic condition. Read the Editorial Highlight for this article on page 9. Cover Image for this issue: doi. 10.1111/jnc.13817.

LTP and memory impairment caused by extracellular Aβ and Tau oligomers is APP-dependent

The concurrent application of subtoxic doses of soluble oligomeric forms of human amyloid-beta (oAβ) and Tau (oTau) proteins impairs memory and its electrophysiological surrogate long-term potentiation (LTP), effects that may be mediated by intra-neuronal oligomers uptake. Intrigued by these findings, we investigated whether oAβ and oTau share a common mechanism when they impair memory and LTP in mice. We found that as already shown for oAβ, also oTau can bind to amyloid precursor protein (APP). Moreover, efficient intra-neuronal uptake of oAβ and oTau requires expression of APP. Finally, the toxic effect of both extracellular oAβ and oTau on memory and LTP is dependent upon APP since APP-KO mice were resistant to oAβ- and oTau-induced defects in spatial/associative memory and LTP. Thus, APP might serve as a common therapeutic target against Alzheimer's Disease (AD) and a host of other neurodegenerative diseases characterized by abnormal levels of Aβ and/or Tau.

DOI:http://dx.doi.org/10.7554/eLife.26991.001

Somatostatin binds to the human amyloid β peptide and favors the formation of distinct oligomers

The amyloid β peptide (Aβ) is a key player in the etiology of Alzheimer disease (AD), yet a systematic investigation of its molecular interactions has not been reported. Here we identified by quantitative mass spectrometry proteins in human brain extract that bind to oligomeric Aβ1-42 (oAβ1-42) and/or monomeric Aβ1-42 (mAβ1-42) baits. Remarkably, the cyclic neuroendocrine peptide somatostatin-14 (SST14) was observed to be the most selectively enriched oAβ1-42 binder. The binding interface comprises a central tryptophan within SST14 and the N-terminus of Aβ1-42. The presence of SST14 inhibited Aβ aggregation and masked the ability of several antibodies to detect Aβ. Notably, Aβ1-42, but not Aβ1-40, formed in the presence of SST14 oligomeric assemblies of 50 to 60 kDa that were visualized by gel electrophoresis, nanoparticle tracking analysis and electron microscopy. These findings may be relevant for Aβ-directed diagnostics and may signify a role of SST14 in the etiology of AD.

DOI:http://dx.doi.org/10.7554/eLife.28401.001

Treating Alzheimer’s disease and related dementias is one of the major challenges currently facing healthcare providers worldwide. A hallmark of the disease is the formation of large deposits of a specific molecule, known as amyloid beta (Aβ), in the brain. However, more and more research suggests that smaller and particularly toxic amyloid beta clumps – often referred to as oligomeric Aβ – appear as an early sign of Alzheimer’s disease.

To understand how the formation of these smaller amyloid beta clumps triggers other aspects of the disease, it is important to identify molecules in the human brain that oligomeric Aβ binds to. To this end, Wang et al. attached amyloid beta or oligomeric Aβ molecules to microscopically small beads. The beads were then exposed to human brain extracts in a test tube, which allowed molecules in the extracts to bind to the amyloid beta or oligomeric Aβ. The samples were then spun at high speed, meaning that the beads and any other molecules bound to them sunk and formed pellets at the bottom of the tubes. Each pellet was then analyzed to see which molecules it contained.

The experiments identified more than a hundred human brain proteins that can bind to amyloid beta. One of them, known as somatostatin, selectively binds to oligomeric Aβ. Wang et al. were able to determine the structural features of somatostatin that control this binding.

Finally, in further experiments performed in test tubes, Wang et al. noticed that smaller oligomeric Aβ clumps were more likely to form than larger amyloid beta deposits when somatostatin was present. This could signify a previously unrecognized role of somatostatin in the development of Alzheimer’s disease.

Further studies are now needed to confirm whether the presence of somatostatin in the brain favors the formation of smaller, toxic oligomeric Aβ clumps over large innocuous amyloid beta deposits. If so, new treatments could be developed that aim to reduce oligomeric Aβ levels in the brain by preventing somatostatin from interacting with amyloid beta molecules. Wang et al. also suggest that somatostatin could be used in diagnostic tests to detect abnormal levels of oligomeric Aβ in the brain or body fluids of people who have Alzheimer’s disease.

DOI:http://dx.doi.org/10.7554/eLife.28401.002

Activation of Ras-ERK Signaling and GSK-3 by Amyloid Precursor Protein and Amyloid Beta Facilitates Neurodegeneration in Alzheimer's Disease

It is widely accepted that amyloid β (Aβ) generated from amyloid precursor protein (APP) oligomerizes and fibrillizes to form neuritic plaques in Alzheimer’s disease (AD), yet little is known about the contribution of APP to intracellular signaling events preceding AD pathogenesis. The data presented here demonstrate that APP expression and neuronal exposure to oligomeric Aβ42 enhance Ras/ERK signaling cascade and glycogen synthase kinase 3 (GSK-3) activation. We find that RNA interference (RNAi)-directed knockdown of APP in B103 rat neuroblastoma cells expressing APP inhibits Ras-ERK signaling and GSK-3 activation, indicating that APP acts upstream of these signal transduction events. Both ERK and GSK-3 are known to induce hyperphosphorylation of tau and APP at Thr668, and our findings suggest that aberrant signaling by APP facilitates these events. Supporting this notion, analysis of human AD brain samples showed increased expression of Ras, activation of GSK-3, and phosphorylation of APP and tau, which correlated with Aβ levels in the AD brains. Furthermore, treatment of primary rat neurons with Aβ recapitulated these events and showed enhanced Ras-ERK signaling, GSK-3 activation, upregulation of cyclin D1, and phosphorylation of APP and tau. The finding that Aβ induces Thr668 phosphorylation on APP, which enhances APP proteolysis and Aβ generation, denotes a vicious feedforward mechanism by which APP and Aβ promote tau hyperphosphorylation and neurodegeneration in AD. Based on these results, we hypothesize that aberrant proliferative signaling by APP plays a fundamental role in AD neurodegeneration and that inhibition of this would impede cell cycle deregulation and neurodegeneration observed in AD.

Role of APP Interactions with Heterotrimeric G Proteins: Physiological Functions and Pathological Consequences

Following the discovery that the amyloid precursor protein (APP) is the source of β-amyloid peptides (Aβ) that accumulate in Alzheimer’s disease (AD), structural analyses suggested that the holoprotein resembles a transmembrane receptor. Initial studies using reconstituted membranes demonstrated that APP can directly interact with the heterotrimeric G protein Gαo (but not other G proteins) via an evolutionarily G protein-binding motif in its cytoplasmic domain. Subsequent investigations in cell culture showed that antibodies against the extracellular domain of APP could stimulate Gαo activity, presumably mimicking endogenous APP ligands. In addition, chronically activating wild type APP or overexpressing mutant APP isoforms linked with familial AD could provoke Go-dependent neurotoxic responses, while biochemical assays using human brain samples suggested that the endogenous APP-Go interactions are perturbed in AD patients. More recently, several G protein-dependent pathways have been implicated in the physiological roles of APP, coupled with evidence that APP interacts both physically and functionally with Gαo in a variety of contexts. Work in insect models has demonstrated that the APP ortholog APPL directly interacts with Gαo in motile neurons, whereby APPL-Gαo signaling regulates the response of migratory neurons to ligands encountered in the developing nervous system. Concurrent studies using cultured mammalian neurons and organotypic hippocampal slice preparations have shown that APP signaling transduces the neuroprotective effects of soluble sAPPα fragments via modulation of the PI3K/Akt pathway, providing a mechanism for integrating the stress and survival responses regulated by APP. Notably, this effect was also inhibited by pertussis toxin, indicating an essential role for Gαo/i proteins. Unexpectedly, C-terminal fragments (CTFs) derived from APP have also been found to interact with Gαs, whereby CTF-Gαs signaling can promote neurite outgrowth via adenylyl cyclase/PKA-dependent pathways. These reports offer the intriguing perspective that G protein switching might modulate APP-dependent responses in a context-dependent manner. In this review, we provide an up-to-date perspective on the model that APP plays a variety of roles as an atypical G protein-coupled receptor in both the developing and adult nervous system, and we discuss the hypothesis that disruption of these normal functions might contribute to the progressive neuropathologies that typify AD.

Elucidating Important Sites and the Mechanism for Amyloid Fibril Formation by Coarse-Grained Molecular Dynamics

Fibrils formed by the β-amyloid (Aβ) peptide play a central role in the development of Alzheimer's disease. In this study, the principles governing their growth and stability are investigated by analyzing canonical and replica-exchange molecular dynamics trajectories of Aβ_(9-40) fibrils. In particular, an unstructured monomer was allowed to interact freely with an Aβ fibril template. Trajectories were generated with the coarse-grained united-residue force field, and one- and two-dimensional free-energy landscapes (FELs) along the backbone virtual-bond angle θ and backbone virtual-bond-dihedral angle γ of each residue and principal components, respectively, were analyzed. Also, thermal unbinding (unfolding) of an Aβ peptide from the fibril template was investigated. These analyses enable us to illustrate the entire process of Aβ fibril elongation and to elucidate the key residues involved in it. Several different pathways were identified during the search for the fibril conformation by the monomer, which finally follows a dock-lock mechanism with two distinct locking stages. However, it was found that the correct binding, with native hydrogen bonds, of the free monomer to the fibril template at both stages is crucial for fibril elongation. In other words, if the monomer is incorrectly bound (with nonnative hydrogen bonds) to the fibril template during the first "docking" stage, it can remain attached to it for a long time before it dissociates and either attempts a different binding or allows another monomer to bind. This finding is consistent with an experimentally observed "stop-and-go" mechanism of fibril growth.

Potential role of PCTAIRE-2, PCTAIRE-3 and P-Histone H4 in amyloid precursor protein-dependent Alzheimer pathology

Amyloid Precursor Protein (APP) is regulated in a mitosis-specific manner and plays a role in proliferative signaling in cells. Though APP-derived Aβ generation has a well-established role in neurodegeneration, the mechanistic role of APP in this process is not fully understood. Here, we performed an unbiased, comprehensive analysis of the phosphoproteome signature in APP-null neuroblastoma cells (B103) compared to those expressing APP-695 isoform (B103-695) to determine if APP expression affects protein phosphorylation. Stable isotope labeling by amino acids in cell culture (SILAC) followed by mass spectrometry-based phosphoproteomic analysis with PolyMAC identified a total of 2,478 phosphopeptides in the B103 and B103-695 cell culture model system. We observed that phosphorylation of PCTAIRE-2 (CDK17), PCTAIRE-3 (CDK18), and Histone H4 are significantly elevated in B103-695 cells; western blot analysis confirmed overexpression of PCTAIREs and increased phosphorylation of Histone H4. More importantly, analysis of primary neurons treated with Aβ, as well as brain samples from MCI (mild cognitive impaired) and AD patients recapitulated these results, showing increased levels of PCTAIREs and P-Histone H4. These novel findings identify a hitherto uncharacterized mechanism by which APP and/or Aβ may promote AD neurodegeneration, and raises the possibility that their inhibition may protect against pathology development in AD.