Amyloid-β Receptors: The Good, the Bad, and the Prion Protein

Revealing the Molecular Mechanisms of Alzheimer's Disease Based on Network Analysis

The complex pathology of Alzheimer's disease (AD) emphasises the need for comprehensive modelling of the disease, which may lead to the development of efficient treatment strategies. To address this challenge, we analysed transcriptome data of post-mortem human brain samples of healthy elders and individuals with late-onset AD from the Religious Orders Study and Rush Memory and Aging Project (ROSMAP) and Mayo Clinic (MayoRNAseq) studies in the AMP-AD consortium. In this context, we conducted several bioinformatics and systems medicine analyses including the construction of AD-specific co-expression networks and genome-scale metabolic modelling of the brain in AD patients to identify key genes, metabolites and pathways involved in the progression of AD. We identified AMIGO1 and GRPRASP2 as examples of commonly altered marker genes in AD patients. Moreover, we found alterations in energy metabolism, represented by reduced oxidative phosphorylation and ATPase activity, as well as the depletion of hexanoyl-CoA, pentanoyl-CoA, (2E)-hexenoyl-CoA and numerous other unsaturated fatty acids in the brain. We also observed that neuroprotective metabolites (e.g., vitamins, retinoids and unsaturated fatty acids) tend to be depleted in the AD brain, while neurotoxic metabolites (e.g., β-alanine, bilirubin) were more abundant. In summary, we systematically revealed the key genes and pathways related to the progression of AD, gained insight into the crucial mechanisms of AD and identified some possible targets that could be used in the treatment of AD.

β-Cleavage of the prion protein in the human eye: Implications for the spread of infectious prions and human ocular disorders

Recently, we reported β-cleavage of the prion protein (PrP^C) in human ocular tissues. Here, we explored whether this is unique to the human eye, and its functional implications. A comparison of the cleavage pattern of PrP^C in human ocular tissues with common nocturnal and diurnal animals revealed mainly β-cleavage in humans, and mostly full-length PrP^C in animal retinas. Soluble FL PrP^C and N-terminal fragment (N2) released from β-cleavage was observed in the aqueous and vitreous humor (AH & VH). Expression of human PrP^C in ARPE-19 cells, a human retinal pigmented epithelial cell line, also showed β-cleaved PrP^C. Surprisingly, β-cleavage was not altered by a variety of insults, including oxidative stress, suggesting a unique role of this cleavage in the human eye. It is likely that β-cleaved C- or N-terminal fragments of PrP^C protect from various insults unique to the human eye. On the contrary, β-cleaved C-terminus of PrP^C is susceptible to conversion to the pathological PrP-scrapie form, and includes the binding sites for β1-integrin and amyloid-β, molecules implicated in several ocular disorders. Considering the species and tissue-specific cleavage of PrP^C, our data suggest re-evaluation of prion infectivity and other ocular disorders of the human eye conducted in mouse models.

Distinct types of amyloid-β oligomers displaying diverse neurotoxicity mechanisms in Alzheimer's disease

Soluble oligomers of amyloid-β (Aβ) are recognized as key pernicious species in Alzheimer's disease (AD) that cause synaptic dysfunction and memory impairments. Numerous studies have identified various types of Aβ oligomers having heterogeneous peptide length, size distribution, structure, appearance, and toxicity. Here, we review the characteristics of soluble Aβ oligomers based on their morphology, size, and structural reactivity toward the conformation-specific antibodies and then describe their formation, localization, and cellular effects in AD brains, in vivo and in vitro. We also summarize the mechanistic pathways by which these soluble Aβ oligomers cause proteasomal impairment, calcium dyshomeostasis, inhibition of long-term potentiation, apoptosis, mitochondrial damage, and cognitive decline. These cellular events include three distinct molecular mechanisms: (i) high-affinity binding with the receptors for Aβ oligomers such as N-methyl- d-aspartate receptors, cellular prion protein, nerve growth factor, insulin receptors, and frizzled receptors; (ii) the interaction of Aβ oligomers with the lipid membranes; (iii) intraneuronal accumulation of Aβ by α7-nicotinic acetylcholine receptors, apolipoprotein E, and receptor for advanced glycation end products. These studies indicate that there is a pressing need to carefully examine the role of size, appearance, and the conformation of oligomers in identifying the specific mechanism of neurotoxicity that may uncover potential targets for designing AD therapeutics.

Role of Oxidative Damage in Alzheimer's Disease and Neurodegeneration: From Pathogenic Mechanisms to Biomarker Discovery

Alzheimer’s disease (AD) is a neurodegenerative disorder accounting for over 50% of all dementia patients and representing a leading cause of death worldwide for the global ageing population. The lack of effective treatments for overt AD urges the discovery of biomarkers for early diagnosis, i.e., in subjects with mild cognitive impairment (MCI) or prodromal AD. The brain is exposed to oxidative stress as levels of reactive oxygen species (ROS) are increased, whereas cellular antioxidant defenses are decreased. Increased ROS levels can damage cellular structures or molecules, leading to protein, lipid, DNA, or RNA oxidation. Oxidative damage is involved in the molecular mechanisms which link the accumulation of amyloid-β and neurofibrillary tangles, containing hyperphosphorylated tau, to microglia response. In this scenario, microglia are thought to play a crucial role not only in the early events of AD pathogenesis but also in the progression of the disease. This review will focus on oxidative damage products as possible peripheral biomarkers in AD and in the preclinical phases of the disease. Particular attention will be paid to biological fluids such as blood, CSF, urine, and saliva, and potential future use of molecules contained in such body fluids for early differential diagnosis and monitoring the disease course. We will also review the role of oxidative damage and microglia in the pathogenesis of AD and, more broadly, in neurodegeneration.

Multifaceted Alzheimer's Disease: Building a Roadmap for Advancement of Novel Therapies

Alzheimer's disease (AD) is one of the most prevailing neurodegenerative disorders of elderly humans associated with cognitive damage. Biochemical, epigenetic, and pathophysiological factors all consider a critical role of extracellular amyloid-beta (Aß) plaques and intracellular neurofibrillary tangles (NFTs) as pathological hallmarks of AD. In an endeavor to describe the intricacy and multifaceted nature of AD, several hypotheses based on the roles of Aß accumulation, tau hyperphosphorylation, impaired cholinergic signaling, neuroinflammation, and autophagy during the initiation and advancement of the disease have been suggested. However, in no way do these theories have the potential of autonomously describing the pathophysiological alterations located in AD. The complex pathological nature of AD has hindered the recognition and authentication of successful biomarkers for the progression of its diagnosis and therapeutic strategies. There has been a significant research effort to design multi-target-directed ligands for the treatment of AD, an approach which is developed by the knowledge that AD is a composite and multifaceted disease linked with several separate but integrated molecular pathways.

The Interplay of Tau Protein and β-Amyloid: While Tauopathy Spreads More Profoundly Than Amyloidopathy, Both Processes Are Almost Equally Pathogenic

Alzheimer's disease (AD) is a neurodegenerative disorder, in which amyloid precursor protein (APP) misprocessing and tau protein hyperphosphorylation are well-established pathogenic cascades. Despite extensive considerations, the central mediator of neuronal cell death upon AD remains under debate. Therefore, we examined the direct interplay between tauopathy and amyloidopathy processes. We employed primary culture neurons and examined pathogenic P-tau and Aβ oligomers upon hypoxia treatment by immunofluorescence and immunoblotting. We observed both tauopathy and amyloidopathy processes upon the hypoxia condition. We also applied Aβ1-42 or P-tau onto primary cultured neurons. We overexpressed P-tau in SH-SY5Y cells and found Aβ accumulation. Furthermore, adult male rats received Aβ1-42 or pathogenic P-tau in the dorsal hippocampus and were examined for 8 weeks. Learning and memory performance, as well as anxiety behaviors, were assessed by Morris water maze and elevated plus-maze tests. Both Aβ1-42 and pathogenic P-tau significantly induced learning and memory deficits and enhanced anxiety behavior after treatment 2 weeks. Aβ administration induced robust tauopathy distribution in the cortex, striatum, and corpus callosum as well as CA1. On the other hand, P-tau treatment developed Aβ oligomers in the cortex and CA1 only. Our findings indicate that Aβ1-42 and pathogenic P-tau may induce each other and cause almost identical neurotoxicity in a time-dependent manner, while tauopathy seems to be more distributable than amyloidopathy.

Endo-lysosomal Aβ concentration and pH trigger formation of Aβ oligomers that potently induce Tau missorting

Amyloid-β peptide (Aβ) forms metastable oligomers >50 kDa, termed AβOs, that are more effective than Aβ amyloid fibrils at triggering Alzheimer’s disease-related processes such as synaptic dysfunction and Tau pathology, including Tau mislocalization. In neurons, Aβ accumulates in endo-lysosomal vesicles at low pH. Here, we show that the rate of AβO assembly is accelerated 8,000-fold upon pH reduction from extracellular to endo-lysosomal pH, at the expense of amyloid fibril formation. The pH-induced promotion of AβO formation and the high endo-lysosomal Aβ concentration together enable extensive AβO formation of Aβ42 under physiological conditions. Exploiting the enhanced AβO formation of the dimeric Aβ variant dimAβ we furthermore demonstrate targeting of AβOs to dendritic spines, potent induction of Tau missorting, a key factor in tauopathies, and impaired neuronal activity. The results suggest that the endosomal/lysosomal system is a major site for the assembly of pathomechanistically relevant AβOs.

Aβ oligomers (AβO) are thought to represent the main toxic species in Alzheimer’s disease but very high Aβ concentrations are required to study them in vitro and it remains unknown what role these off-pathway oligomers play in vivo. Here, the authors use a dimeric variant of Aβ termed dimAβ, where two Aβ40 units are linked, which facilitates to study AβO formation kinetics and they observe that Aβ off-pathway oligomer formation is strongly accelerated at endo-lysosomal pH, while amyloid fibril formation is delayed. Furthermore, the authors demonstrate that dimAβ is a disease-relevant model construct for pathogenic AβO formation by showing that dimAβ AβOs target dendritic spines and induce AD-like somatodendritic Tau missorting.

Conformation-specific perturbation of membrane dynamics by structurally distinct oligomers of Alzheimer's amyloid-β peptide

The accumulation of toxic soluble oligomers of the amyloid-β peptide (Aβ) is a key step in the pathogenesis of Alzheimer's disease. There are mainly two conformationally distinct oligomers, namely, prefibrillar and fibrillar oligomers, that are recognized by conformation-specific antibodies, anti-amyloid oligomer antibody (A11) and anti-amyloid fibrillar antibody (OC), respectively. Previous studies have shown that the interaction of Aβ oligomers with the lipid membrane is one of the key mechanisms of toxicity produced by Aβ oligomers. However, the mechanism by which structurally distinct Aβ oligomers interact with the lipid membrane remains elusive. In this work, we dissect the molecular mechanism underlying the interaction of structurally distinct Aβ42 oligomers with the lipid membrane derived from the brain total lipid extract. Using picosecond time-resolved fluorescence spectroscopy, we show that the A11-positive Aβ42 oligomers undergo a membrane-induced conformational change that promotes the deeper immersion of these oligomers into the lipid hydrocarbon region and results in an increase in the membrane micro-viscosity. In sharp contrast, OC-positive Aβ42 oligomers interact with the lipid membrane via electrostatic interactions between the negatively-charged lipid headgroup and positively-charged residues of Aβ42 without perturbing the membrane dynamics. We show that the two structurally distinct Aβ42 oligomers demonstrating different interaction mechanisms with the lipid membrane eventually lead to the formation of typical amyloid fibrils. Our findings provide the mechanistic underpinning of the perturbation of lipid membranes by two conformationally distinct Aβ42 oligomers and can be of prime importance in designing anti-Alzheimer's therapeutic agents targeting Aβ-membrane interactions.

Relationship Between Amyloid-β Deposition and Blood-Brain Barrier Dysfunction in Alzheimer's Disease

Amyloid-β (Aβ) is the predominant pathologic protein in Alzheimer's disease (AD). The production and deposition of Aβ are important factors affecting AD progression and prognosis. The deposition of neurotoxic Aβ contributes to damage of the blood-brain barrier. However, the BBB is also crucial in maintaining the normal metabolism of Aβ, and dysfunction of the BBB aggravates Aβ deposition. This review characterizes Aβ deposition and BBB damage in AD, summarizes their interactions, and details their respective mechanisms.

Microglia show differential transcriptomic response to Aβ peptide aggregates ex vivo and in vivo

Aggregation and accumulation of amyloid-β (Aβ) is a defining feature of Alzheimer's disease pathology. To study microglial responses to Aβ, we applied exogenous Aβ peptide, in either oligomeric or fibrillar conformation, to primary mouse microglial cultures and evaluated system-level transcriptional changes and then compared these with transcriptomic changes in the brains of CRND8 APP mice. We find that primary microglial cultures have rapid and massive transcriptional change in response to Aβ. Transcriptomic responses to oligomeric or fibrillar Aβ in primary microglia, although partially overlapping, are distinct and are not recapitulated in vivo where Aβ progressively accumulates. Furthermore, although classic immune mediators show massive transcriptional changes in the primary microglial cultures, these changes are not observed in the mouse model. Together, these data extend previous studies which demonstrate that microglia responses ex vivo are poor proxies for in vivo responses. Finally, these data demonstrate the potential utility of using microglia as biosensors of different aggregate conformation, as the transcriptional responses to oligomeric and fibrillar Aβ can be distinguished.

A screened PirB antagonist peptide antagonizes Aβ42-mediated inhibition of neurite outgrowth in vitro

Alzheimer's disease (AD) is a type of progressive neurodegenerative disease, and amyloid β-protein 42 (Aβ₄₂) serves an important role in the pathological process of development of AD. Paired immunoglobulin-like receptor B (PirB) is a functional receptor for myelin inhibitors of neuron regeneration in the CNS, and it has also been identified to function as a high-affinity receptor for Aβ. Here, we used a phage display to identify a specific PirB antagonist peptide 11(PAP11, PFRLQLS), which could reverse Aβ₄₂-induced neurotoxicity and promote neurite outgrowth in vitro. Immunofluorescence analysis showed that PAP11 colocalized with PirB on the membrane of cortical neurons. Horseradish peroxidase-streptavidin-biotin assay further proved that PAP11 directly binds to PirB and the dissociation constant (K_d) was 0.128μM. PAP11 functionally antagonized the neurite outgrowth inhibitory effect induced by Aβ₄₂ in cortical neurons, and the underlying mechanism was associated with a PirB-ROCK2/CRMP2 signaling pathway. The novel PirB antagonist peptide PAP11 may be a promising candidate therapeutic agent for the treatment of AD and other neurodegenerative diseases. KEY POINTS: • PAP11 was the first PirB antagonist peptide screened by phage display technology. • PAP11 could protect neurons by blocking the binding of Aβ₄₂ and PirB. • PAP11 reverse inhibitory effect of neurite outgrowth through ROCK2/CRMP2 pathway.

VEGF counteracts amyloid-β-induced synaptic dysfunction

The vascular endothelial growth factor (VEGF) pathway regulates key processes in synapse function, which are disrupted in early stages of Alzheimer's disease (AD) by toxic-soluble amyloid-beta oligomers (Aβo). Here, we show that VEGF accumulates in and around Aβ plaques in postmortem brains of patients with AD and in APP/PS1 mice, an AD mouse model. We uncover specific binding domains involved in direct interaction between Aβo and VEGF and reveal that this interaction jeopardizes VEGFR2 activation in neurons. Notably, we demonstrate that VEGF gain of function rescues basal synaptic transmission, long-term potentiation (LTP), and dendritic spine alterations, and blocks long-term depression (LTD) facilitation triggered by Aβo. We further decipher underlying mechanisms and find that VEGF inhibits the caspase-3-calcineurin pathway responsible for postsynaptic glutamate receptor loss due to Aβo. These findings provide evidence for alterations of the VEGF pathway in AD models and suggest that restoring VEGF action on neurons may rescue synaptic dysfunction in AD.

Regulation of tau internalization, degradation, and seeding by LRP1 reveals multiple pathways for tau catabolism

In Alzheimer's disease (AD), pathological forms of tau are transferred from cell to cell and “seed” aggregation of cytoplasmic tau. Phosphorylation of tau plays a key role in neurodegenerative tauopathies. In addition, apolipoprotein E (apoE), a major component of lipoproteins in the brain, is a genetic risk determinant for AD. The identification of the apoE receptor, low-density lipoprotein receptor–related protein 1 (LRP1), as an endocytic receptor for tau raises several questions about the role of LRP1 in tauopathies: is internalized tau, like other LRP1 ligands, delivered to lysosomes for degradation, and does LRP1 internalize pathological tau leading to cytosolic seeding? We found that LRP1 rapidly internalizes ¹²⁵I-labeled tau, which is then efficiently degraded in lysosomal compartments. Surface plasmon resonance experiments confirm high affinity binding of tau and the tau microtubule-binding domain to LRP1. Interestingly, phosphorylated forms of recombinant tau bind weakly to LRP1 and are less efficiently internalized by LRP1. LRP1-mediated uptake of tau is inhibited by apoE, with the apoE4 isoform being the most potent inhibitor, likely because of its higher affinity for LRP1. Employing post-translationally–modified tau derived from brain lysates of human AD brain tissue, we found that LRP1-expressing cells, but not LRP1-deficient cells, promote cytosolic tau seeding in a process enhanced by apoE. These studies identify LRP1 as an endocytic receptor that binds and processes monomeric forms of tau leading to its degradation and promotes seeding by pathological forms of tau. The balance of these processes may be fundamental to the spread of neuropathology across the brain in AD.

Passive Immunization With a Novel Monoclonal Anti-PrP Antibody TW1 in an Alzheimer's Mouse Model With Tau Pathology

Neurofibrillary tangles (NFTs) are a major pathologic hallmark of Alzheimer’s disease (AD). Several studies have shown that amyloid β oligomers (Aβo) and tau oligomers mediate their toxicity, in part, via binding to cellular prion protein (PrP^C) and that some anti-PrP antibodies can block this interaction. We have generated a novel monoclonal anti-PrP antibody (TW1) and assessed the efficacy of passive immunization with it in a mouse model of AD with extensive tau pathology: hTau/PS1 transgenic (Tg) mice. These mice were injected intraperitoneally once a week with TW1 starting at 5 months of age. Behavior was assessed at 8 months of age and brain tissue was subsequently harvested for analysis of treatment efficacy at 9 months. Mice treated with TW1 did not show any significant difference in sensorimotor testing including traverse beam, rotarod, and locomotor activity compared to controls. Significant cognitive benefits were observed with the novel object recognition test (ORT) in the immunized mice (two-tailed, t-test p = 0.0019). Immunized mice also showed cognitive benefits on the closed field symmetrical maze (day 1 two-tailed t-test p = 0.0001; day 2 two-tailed t-test p = 0.0015; day 3 two-tailed t-test p = 0.0002). Reduction of tau pathology was observed with PHF-1 immunohistochemistry in the piriform cortex by 60% (two-tailed t-test p = 0.01) and in the dentate gyrus by 50% (two-tailed t-test p = 0.02) in animals treated with TW1 compared to controls. There were no significant differences in astrogliosis or microgliosis observed between treated and control mice. As assessed by Western blots using PHF-1, the TW1 therapy reduced phosphorylated tau pathology (two-tailed t-test p = 0.03) and improved the ratio of pathological soluble tau to tubulin (PHF1/tubulin; two-tailed t-test p = 0.0006). Reduction of tau pathology also was observed using the CP13 antibody (two-tailed t-test p = 0.0007). These results indicate that passive immunization with the TW1 antibody can significantly decrease tau pathology as assessed by immunohistochemical and biochemical methods, resulting in improved cognitive function in a tau transgenic mouse model of AD.

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

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

Protofibril-Fibril Interactions Inhibit Amyloid Fibril Assembly by Obstructing Secondary Nucleation

Amyloid-β peptides (Aβ) assemble into both rigid amyloid fibrils and metastable oligomers termed AβO or protofibrils. In Alzheimer's disease, Aβ fibrils constitute the core of senile plaques, but Aβ protofibrils may represent the main toxic species. Aβ protofibrils accumulate at the exterior of senile plaques, yet the protofibril-fibril interplay is not well understood. Applying chemical kinetics and atomic force microscopy to the assembly of Aβ and lysozyme, protofibrils are observed to bind to the lateral surfaces of amyloid fibrils. When utilizing Aβ variants with different critical oligomer concentrations, the interaction inhibits the autocatalytic proliferation of amyloid fibrils by secondary nucleation on the fibril surface. Thus, metastable oligomers antagonize their replacement by amyloid fibrils both by competing for monomers and blocking secondary nucleation sites. The protofibril-fibril interaction governs their temporal evolution and potential to exert specific toxic activities.

Preclinical and clinical biomarker studies of CT1812: A novel approach to Alzheimer's disease modification

INTRODUCTION

Amyloid beta (Aβ) oligomers are one of the most toxic structural forms of the Aβ protein and are hypothesized to cause synaptotoxicity and memory failure as they build up in Alzheimer's disease (AD) patients' brain tissue. We previously demonstrated that antagonists of the sigma-2 receptor complex effectively block Aβ oligomer toxicity. CT1812 is an orally bioavailable, brain penetrant small molecule antagonist of the sigma-2 receptor complex that appears safe and well tolerated in healthy elderly volunteers. We tested CT1812's effect on Aβ oligomer pathobiology in preclinical AD models and evaluated CT1812's impact on cerebrospinal fluid (CSF) protein biomarkers in mild to moderate AD patients in a clinical trial (ClinicalTrials.gov NCT02907567).

METHODS

Experiments were performed to measure the impact of CT1812 versus vehicle on Aβ oligomer binding to synapses in vitro, to human AD patient post mortem brain tissue ex vivo, and in living APP_Swe /PS1dE9 transgenic mice in vivo. Additional experiments were performed to measure the impact of CT1812 versus vehicle on Aβ oligomer-induced deficits in membrane trafficking rate, synapse number, and protein expression in mature hippocampal/cortical neurons in vitro. The impact of CT1812 on cognitive function was measured in transgenic Thy1 huAPP^Swe/Lnd+ and wild-type littermates. A multicenter, double-blind, placebo-controlled parallel group trial was performed to evaluate the safety, tolerability, and impact on protein biomarker expression of CT1812 or placebo given once daily for 28 days to AD patients (Mini-Mental State Examination 18-26). CSF protein expression was measured by liquid chromatography with tandem mass spectrometry or enzyme-linked immunosorbent assay in samples drawn prior to dosing (Day 0) and at end of dosing (Day 28) and compared within each patient and between pooled treated versus placebo-treated dosing groups.

RESULTS

CT1812 significantly and dose-dependently displaced Aβ oligomers bound to synaptic receptors in three independent preclinical models of AD, facilitated oligomer clearance into the CSF, increased synaptic number and protein expression in neurons, and improved cognitive performance in transgenic mice. CT1812 significantly increased CSF concentrations of Aβ oligomers in AD patient CSF, reduced concentrations of synaptic proteins and phosphorylated tau fragments, and reversed expression of many AD-related proteins dysregulated in CSF.

DISCUSSION

These preclinical studies demonstrate the novel disease-modifying mechanism of action of CT1812 against AD and Aβ oligomers. The clinical results are consistent with preclinical data and provide evidence of target engagement and impact on fundamental disease-related signaling pathways in AD patients, supporting further development of CT1812.

Amyloid binding and beyond: a new approach for Alzheimer's disease drug discovery targeting Aβo-PrPC binding and downstream pathways

Amyloid β oligomers (Aβo) are the main toxic species in Alzheimer's disease, which have been targeted for single drug treatment with very little success. In this work we report a new approach for identifying functional Aβo binding compounds. A tailored library of 971 fluorine containing compounds was selected by a computational method, developed to generate molecular diversity. These compounds were screened for Aβo binding by a combined 19F and STD NMR technique. Six hits were evaluated in three parallel biochemical and functional assays. Two compounds disrupted Aβo binding to its receptor PrPC in HEK293 cells. They reduced the pFyn levels triggered by Aβo treatment in neuroprogenitor cells derived from human induced pluripotent stem cells (hiPSC). Inhibitory effects on pTau production in cortical neurons derived from hiPSC were also observed. These drug-like compounds connect three of the pillars in Alzheimer's disease pathology, i.e. prion, Aβ and Tau, affecting three different pathways through specific binding to Aβo and are, indeed, promising candidates for further development.

Stromal-driven and Amyloid β-dependent induction of neutrophil extracellular traps modulates tumor growth

Tumors consist of cancer cells and a network of non-cancerous stroma. Cancer-associated fibroblasts (CAF) are known to support tumorigenesis, and are emerging as immune modulators. Neutrophils release histone-bound nuclear DNA and cytotoxic granules as extracellular traps (NET). Here we show that CAFs induce NET formation within the tumor and systemically in the blood and bone marrow. These tumor-induced NETs (t-NETs) are driven by a ROS-mediated pathway dependent on CAF-derived Amyloid β, a peptide implicated in both neurodegenerative and inflammatory disorders. Inhibition of NETosis in murine tumors skews neutrophils to an anti-tumor phenotype, preventing tumor growth; reciprocally, t-NETs enhance CAF activation. Mirroring observations in mice, CAFs are detected juxtaposed to NETs in human melanoma and pancreatic adenocarcinoma, and show elevated amyloid and β-Secretase expression which correlates with poor prognosis. In summary, we report that CAFs drive NETosis to support cancer progression, identifying Amyloid β as the protagonist and potential therapeutic target.

Development and characterization of a novel, megakaryocyte NF-κB reporter cell line for investigating inflammatory responses

Essentials An easily detectable readout in megakaryocyte cell lines will enhance inflammatory research in these cells. Here, we report the development and characterization of a novel megakaryocyte NF-κB-reporter cell line (Meg-01R). Multiple inflammatory molecules modulate NF-κB activity in Meg-01R cells. Meg-01R cells respond to small molecule inhibitors such as IMD0354 and C87 that are known to inhibit NF-κB activity upon stimulation with TNFα. ABSTRACT: Background Because of the difficulties in acquiring large numbers of megakaryocytes, the impact of inflammatory responses on these cells and their ability to produce fully functional platelets under various pathological conditions has not been investigated in detail. Objectives The primary objective of this study is to develop and functionally characterize a novel megakaryocyte nuclear factor κB (NF-κB) reporter cell line to determine the effects of various inflammatory molecules on megakaryocytes and their signalling pathways. Methods A Meg-01-NF-κB-GFP-Luc (Meg-01R) cell line was developed by inserting a reporter NF-κB-GFP-Luc cassette into normal Meg-01 cells to produce luciferase following activation of NF-κB to enable easy detection of pro-inflammatory and reparative signalling. Results and conclusions Meg-01 and Meg-01R cells have comparable characteristics, including the expression of both GPIbα and integrin β3 . Meg-01R cells responded to various inflammatory molecules as measured by NF-κB-dependent bioluminescence. For example, inflammatory molecules such as tumor necrosis factor-α and Pam3CSK4 increased NF-κB activity, whereas an antimicrobial peptide, LL37, reduced its activity. Meg-01R cells were also found to be sensitive to inhibitors (IMD0354 and C87) of inflammatory pathways. Notably, Meg-01R cells were able to respond to lipopolysaccharide (LPS; non-ultrapure), although it was not able to react to ultrapure LPS because of the lack of sufficient TLR4 molecules on their surface. For the first time, we report the development and characterization of a novel megakaryocyte NF-κB reporter cell line (Meg-01R) as a robust tool to study the inflammatory responses/signalling of megakaryocytes upon stimulation with a broad range of inflammatory molecules that can affect NF-κB activity.

BODIPY Dyes as Probes and Sensors to Study Amyloid-β-Related Processes

Amyloid formation plays a major role in a number of neurodegenerative diseases, including Alzheimer's disease. Amyloid-β peptides (Aβ) are one of the primary markers associated with this pathology. Aβ aggregates exhibit a diverse range of morphologies with distinct pathological activities. Recognition of the Aβ aggregates by using small molecule-based probes and sensors should not only enhance understanding of the underlying mechanisms of amyloid formation, but also facilitate the development of therapeutic strategies to interfere with amyloid neurotoxicity. BODIPY (boron dipyrrin) dyes are among the most versatile small molecule fluorophores. BODIPY scaffolds could be functionalized to tune their photophysical properties to the desired ranges as well as to adapt these dyes to various types of conditions and environments. Thus, BODIPY dyes could be viewed as unique platforms for the design of probes and sensors that are capable of detecting and tracking structural changes of various Aβ aggregates. This review summarizes currently available examples of BODIPY dyes that have been used to investigate conformational changes of Aβ peptides, self-assembly processes of Aβ, as well as Aβ interactions with various molecules.

The Aβ aggregation modulator MRZ-99030 prevents and even reverses synaptotoxic effects of Aβ1-42 on LTP even following serial dilution to a 500:1 stoichiometric excess of Aβ1-42, suggesting a beneficial prion-like seeding mechanism

MRZ-99030 (GAL-101) is a small molecule that promotes the formation of off-pathway, non-toxic amorphous clusters of Aβ thereby reducing the amount of toxic soluble oligomeric Aβ species. MRZ-99030 clearly prevents synaptotoxic effects of Aβ_1-42 oligomers on synaptic plasticity and cognition. Long lasting in vivo effects indicate that MRZ-99030 seeds a beneficial self-replication of non-toxic Aβ aggregates - "trigger effect". To test this, we prepared a serial dilution of MRZ-99030 starting with a 20:1 stoichiometric excess to Aβ_1-42. After incubating the Aβ_1-42/MRZ-99030 mixture for 20 min, 10% was transferred to a freshly prepared Aβ_1-42 solution. This dilution step was repeated 3 times finally resulting in a 500:1 stoichiometric excess of Aβ_1-42 over MRZ-99030. This solution was tested for its ability to impair long-term potentiation (LTP) in CA1 neurons. Even following serial dilution, MRZ-99030 prevented the synaptotoxic effect of Aβ_1-42 on CA1-LTP after tetanic stimulation of the Schaffer collaterals whereas incubation with MRZ-99030 (0.1 nM) without serial dilution did not prevent the synaptic deficits caused by Aβ_1-42 (50 nM). Time course experiments revealed that this protective effect was still evident even when the serially diluted Aβ_1-42/MRZ-99030 mixture was prepared up to 1 week before the LTP experiment. MRZ-99030, when serially diluted with Aβ_1-42, was also capable of detoxifying/reversing an already established neurotoxic process. In TEM experiments, Aβ oligomers/annular protofibrils were converted to amorphous Aβ clusters following incubation with serially diluted MRZ-99030 to a final concentration of MRZ-99030 (20 nM) and Aβ_1-42 (10 μM).

Single-molecule studies of amyloid proteins: from biophysical properties to diagnostic perspectives

In neurodegenerative diseases, a wide range of amyloid proteins or peptides such as amyloid-beta and α-synuclein fail to keep native functional conformations, followed by misfolding and self-assembling into a diverse array of aggregates. The aggregates further exert toxicity leading to the dysfunction, degeneration and loss of cells in the affected organs. Due to the disordered structure of the amyloid proteins, endogenous molecules, such as lipids, are prone to interact with amyloid proteins at a low concentration and influence amyloid cytotoxicity. The heterogeneity of amyloid proteinscomplicates the understanding of the amyloid cytotoxicity when relying only on conventional bulk and ensemble techniques. As complementary tools, single-molecule techniques (SMTs) provide novel insights into the different subpopulations of a heterogeneous amyloid mixture as well as the cytotoxicity, in particular as involved in lipid membranes. This review focuses on the recent advances of a series of SMTs, including single-molecule fluorescence imaging, single-molecule force spectroscopy and single-nanopore electrical recording, for the understanding of the amyloid molecular mechanism. The working principles, benefits and limitations of each technique are discussed and compared in amyloid protein related studies.. We also discuss why SMTs show great potential and are worthy of further investigation with feasibility studies as diagnostic tools of neurodegenerative diseases and which limitations are to be addressed.

Evidence for aggregation-independent, PrPC-mediated Aβ cellular internalization

Evidence linking amyloid beta (Aβ) cellular uptake and toxicity has burgeoned, and mechanisms underlying this association are subjects of active research. Two major, interconnected questions are whether Aβ uptake is aggregation-dependent and whether it is sequence-specific. We recently reported that the neuronal uptake of Aβ depends significantly on peptide chirality, suggesting that the process is predominantly receptor-mediated. Over the past decade, the cellular prion protein (PrP^C) has emerged as an important mediator of Aβ-induced toxicity and of neuronal Aβ internalization. Here, we report that the soluble, nonfibrillizing Aβ (1-30) peptide recapitulates full-length Aβ stereoselective cellular uptake, allowing us to decouple aggregation from cellular, receptor-mediated internalization. Moreover, we found that Aβ (1-30) uptake is also dependent on PrP^C expression. NMR-based molecular-level characterization identified the docking site on PrP^C that underlies the stereoselective binding of Aβ (1-30). Our findings therefore identify a specific sequence within Aβ that is responsible for the recognition of the peptide by PrP^C, as well as PrP^C-dependent cellular uptake. Further uptake stereodifferentiation in PrP^C-free cells points toward additional receptor-mediated interactions as likely contributors for Aβ cellular internalization. Taken together, our results highlight the potential of targeting cellular surface receptors to inhibit Aβ cellular uptake as an alternative route for future therapeutic development for Alzheimer's disease.

Smart treatment strategies for alleviating tauopathy and neuroinflammation to improve clinical outcome in Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disease leading to progressive loss of memory that mainly affects people above 60 years of age. It is one of the leading causes of deaths in the USA. Given its inherent heterogeneity and a still-incomplete understanding of its pathology, biomarkers, and targets available for therapy, it is a challenge to design an effective therapeutic strategy. Several hypotheses have been proposed to understand the disease and to identify reliable markers and targets for treatments. However, none have resulted in strong support from clinical trials. In this review, we objectively discuss the various therapeutic strategies and mechanistic approaches to improve the current clinical outcome of AD therapy.

Therapeutic Strategies Targeting Amyloid-β in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder linked to protein misfolding and aggregation. AD is pathologically characterized by senile plaques formed by extracellular Amyloid-β (Aβ) peptide and Intracellular Neurofibrillary Tangles (NFT) formed by hyperphosphorylated tau protein. Extensive synaptic loss and neuronal degeneration are responsible for memory impairment, cognitive decline and behavioral dysfunctions typical of AD. Amyloidosis has been implicated in the depression of acetylcholine synthesis and release, overactivation of N-methyl-D-aspartate (NMDA) receptors and increased intracellular calcium levels that result in excitotoxic neuronal degeneration. Current drugs used in AD treatment are either cholinesterase inhibitors or NMDA receptor antagonists; however, they provide only symptomatic relief and do not alter the progression of the disease. Aβ is the product of Amyloid Precursor Protein (APP) processing after successive cleavage by β- and γ-secretases while APP proteolysis by α-secretase results in non-amyloidogenic products. According to the amyloid cascade hypothesis, Aβ dyshomeostasis results in the accumulation and aggregation of Aβ into soluble oligomers and insoluble fibrils. The former are synaptotoxic and can induce tau hyperphosphorylation while the latter deposit in senile plaques and elicit proinflammatory responses, contributing to oxidative stress, neuronal degeneration and neuroinflammation. Aβ-protein-targeted therapeutic strategies are thus a promising disease-modifying approach for the treatment and prevention of AD. This review summarizes recent findings on Aβ-protein targeted AD drugs, including β-secretase inhibitors, γ-secretase inhibitors and modulators, α-secretase activators, direct inhibitors of Aβ aggregation and immunotherapy targeting Aβ, focusing mainly on those currently under clinical trials.

Tetrapeptide Ac-HAEE-NH2 Protects α4β2 nAChR from Inhibition by Aβ

The cholinergic deficit in Alzheimer’s disease (AD) may arise from selective loss of cholinergic neurons caused by the binding of Aβ peptide to nicotinic acetylcholine receptors (nAChRs). Thus, compounds preventing such an interaction are needed to address the cholinergic dysfunction. Recent findings suggest that the ¹¹EVHH¹⁴ site in Aβ peptide mediates its interaction with α4β2 nAChR. This site contains several charged amino acid residues, hence we hypothesized that the formation of Aβ-α4β2 nAChR complex is based on the interaction of ¹¹EVHH¹⁴ with its charge-complementary counterpart in α4β2 nAChR. Indeed, we discovered a ³⁵HAEE³⁸ site in α4β2 nAChR, which is charge-complementary to ¹¹EVHH¹⁴, and molecular modeling showed that a stable Aβ₄₂-α4β2 nAChR complex could be formed via the ¹¹EVHH¹⁴:³⁵HAEE³⁸ interface. Using surface plasmon resonance and bioinformatics approaches, we further showed that a corresponding tetrapeptide Ac-HAEE-NH₂ can bind to Aβ via ¹¹EVHH¹⁴ site. Finally, using two-electrode voltage clamp in Xenopus laevis oocytes, we showed that Ac-HAEE-NH₂ tetrapeptide completely abolishes the Aβ₄₂-induced inhibition of α4β2 nAChR. Thus, we suggest that ³⁵HAEE³⁸ is a potential binding site for Aβ on α4β2 nAChR and Ac-HAEE-NH₂ tetrapeptide corresponding to this site is a potential therapeutic for the treatment of α4β2 nAChR-dependent cholinergic dysfunction in AD.

Impact of Four Common Hydrogels on Amyloid-β (Aβ) Aggregation and Cytotoxicity: Implications for 3D Models of Alzheimer's Disease

The physiochemical properties of hydrogels utilized in 3D culture can be used to modulate cell phenotype and morphology with a striking resemblance to cellular processes that occur in vivo. Indeed, research areas including regenerative medicine, tissue engineering, in vitro cancer models, and stem cell differentiation have readily utilized 3D biomaterials to investigate cell biological questions. However, cells are only one component of this biomimetic milieu. In many models of disease such as Alzheimer's disease (AD) that could benefit from the in vivo-like cell morphology associated with 3D culture, other aspects of the disease such as protein aggregation have yet to be methodically considered in this 3D context. A hallmark of AD is the accumulation of the peptide amyloid-β (Aβ), whose aggregation is associated with neurotoxicity. We have previously demonstrated the attenuation of Aβ cytotoxicity when cells were cultured within type I collagen hydrogels versus on 2D substrates. In this work, we investigated the extent to which this phenomenon is conserved when Aβ is confined within hydrogels of varying physiochemical properties, notably mesh size and bioactivity. We investigated the Aβ structure and aggregation kinetics in solution and hydrogels composed of type I collagen, agarose, hyaluronic acid, and polyethylene glycol using fluorescence correlation spectroscopy and thioflavin T assays. Our results reveal that all hydrogels tested were associated with enhanced Aβ aggregation and Aβ cytotoxicity attenuation. We suggest that confinement itself imparts a profound effect, possibly by stabilizing Aβ structures and shifting the aggregate equilibrium toward larger species. If this phenomenon of altered protein aggregation in 3D hydrogels can be generalized to other contexts including the in vivo environment, it may be necessary to reevaluate aspects of protein aggregation disease models used for drug discovery.

Signaling via the p75 neurotrophin receptor facilitates amyloid-β-induced dendritic spine pathology

Synapse and dendritic spine loss induced by amyloid-β oligomers is one of the main hallmarks of the early phases of Alzheimer's disease (AD) and is directly correlated with the cognitive decline typical of this pathology. The p75 neurotrophin receptor (p75NTR) binds amyloid-β oligomers in the nM range. While it was shown that µM concentrations of amyloid-β mediate cell death, the role and intracellular signaling of p75NTR for dendritic spine pathology induced by sublethal concentrations of amyloid-β has not been analyzed. We describe here p75NTR as a crucial binding partner in mediating effects of soluble amyloid-β oligomers on dendritic spine density and structure in non-apoptotic hippocampal neurons. Removing or over-expressing p75NTR in neurons rescues or exacerbates the typical loss of dendritic spines and their structural alterations observed upon treatment with nM concentrations of amyloid-β oligomers. Moreover, we show that binding of amyloid-β oligomers to p75NTR activates the RhoA/ROCK signaling cascade resulting in the fast stabilization of the actin spinoskeleton. Our results describe a role for p75NTR and downstream signaling events triggered by binding of amyloid-β oligomers and causing dendritic spine pathology. These observations further our understanding of the molecular mechanisms underlying one of the main early neuropathological hallmarks of AD.

Sleep is bi-directionally modified by amyloid beta oligomers

Disrupted sleep is a major feature of Alzheimer’s disease (AD), often arising years before symptoms of cognitive decline. Prolonged wakefulness exacerbates the production of amyloid-beta (Aβ) species, a major driver of AD progression, suggesting that sleep loss further accelerates AD through a vicious cycle. However, the mechanisms by which Aβ affects sleep are unknown. We demonstrate in zebrafish that Aβ acutely and reversibly enhances or suppresses sleep as a function of oligomer length. Genetic disruptions revealed that short Aβ oligomers induce acute wakefulness through Adrenergic receptor b2 (Adrb2) and Progesterone membrane receptor component 1 (Pgrmc1), while longer Aβ forms induce sleep through a pharmacologically tractable Prion Protein (PrP) signaling cascade. Our data indicate that Aβ can trigger a bi-directional sleep/wake switch. Alterations to the brain’s Aβ oligomeric milieu, such as during the progression of AD, may therefore disrupt sleep via changes in acute signaling events.

Functionalized Mesoporous Silicas Direct Structural Polymorphism of Amyloid-β Fibrils

The aggregation of amyloid-β (Aβ) is associated with the onset of Alzheimer's disease (AD) and involves a complex kinetic pathway as monomers self-assemble into fibrils. A central feature of amyloid fibrils is the existence of multiple structural polymorphs, which complicates the development of disease-relevant structure-function relationships. Developing these relationships requires new methods to control fibril structure. In this work, we evaluated the effect that mesoporous silicas (SBA-15) functionalized with hydrophobic (SBA-PFDTS) and hydrophilic groups (SBA-PEG) have on the aggregation kinetics and resulting structure of Aβ_1-40 fibrils. The hydrophilic SBA-PEG had little effect on amyloid kinetics, while as-synthesized and hydrophobic SBA-PFDTS accelerated aggregation kinetics. Subsequently, we quantified the relative population of fibril structures formed in the presence of each material using electron microscopy. Fibrils formed from Aβ_1-40 exposed to SBA-PEG were structurally similar to control fibrils. In contrast, Aβ_1-40 incubated with SBA-15 or SBA-PFDTS formed fibrils with shorter crossover distances that were more structurally representative of fibrils found in AD patient derived samples. Overall, our results suggest that mesoporous silicas and other exogenous materials are promising scaffolds for the de novo production of specific fibril polymorphs of Aβ_1-40 and other amyloidogenic proteins.

The N-formyl peptide receptors: contemporary roles in neuronal function and dysfunction

N-formyl peptide receptors (FPRs) were first identified upon phagocytic leukocytes, but more than four decades of research has unearthed a plethora of non-myeloid roles for this receptor family. FPRs are expressed within neuronal tissues and markedly in the central nervous system, where FPR interactions with endogenous ligands have been implicated in the pathophysiology of several neurodegenerative diseases including Alzheimer's disease and Parkinson's disease, as well as neurological cancers such as neuroblastoma. Whilst the homeostatic function of FPRs in the nervous system is currently undefined, a variety of novel physiological roles for this receptor family in the neuronal context have been posited in both human and animal settings. Rapid developments in recent years have implicated FPRs in the process of neurogenesis and neuronal differentiation which, upon greater characterisation, could represent a novel pharmacological target for neuronal regeneration therapies that may be used in the treatment of brain/spinal cord injury, stroke and neurodegeneration. This review aims to summarize the recent progress made to determine the physiological role of FPRs in a neuronal setting, and to put forward a case for FPRs as a novel pharmacological target for conditions of the nervous system, and for their potential to open the door to novel neuronal regeneration therapies.

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.

Neurodegeneration in type 2 diabetes: Alzheimer's as a case study

INTRODUCTION

Rigorous research in the last few years has shown that in addition to the classical mechanism of neurodegeneration, certain unconventional mechanisms may also lead to neurodegenerative disease. One of them is a widely studied metabolic disorder: type 2 diabetes mellitus (T2DM). We now have a clear understanding of glucose-mediated neurodegeneration, mostly from studies in Alzheimer's disease (AD) models. AD is recognized to be significantly associated with hyperglycemia, even earning the term "type 3 diabetes." Here, we review first the pathophysiology of AD, both from the perspective of classical protein accumulation, as well as the newer T2DM-dependent mechanisms supported by findings from patients with T2DM. Secondly, we review the different pathways through which neurodegeneration is aggravated in hyperglycemic conditions taking AD as a case study. Finally, some of the current advances in AD management as a result of recent research developments in metabolic disorders-driven neurodegeneration are also discussed.

METHODS

Relevant literatures found from PubMed search were reviewed.

RESULTS

Apart from the known causes of AD, type 2 diabetes opens a new window to the AD pathology in several ways. It is a bidirectional interaction, of which, the molecular and signaling mechanisms are recently studied. This is our attempt to connect all of them to draw a complete mechanistic explanation for the neurodegeneration in T2DM. Refer to Figure 3.

CONCLUSION

The perspective of AD as a classical neurodegenerative disease is changing, and it is now being looked at from a zoomed-out perspective. The correlation between T2DM and AD is something observed and studied extensively. It is promising to know that there are certain advances in AD management following these studies.

The Important Interface Between Apolipoprotein E and Neuroinflammation in Alzheimer's Disease

Alzheimer’s disease (AD) is the most prevalent form of neurodegenerative disease, currently affecting over 5 million Americans with projections expected to rise as the population ages. The hallmark pathologies of AD are Aβ plaques composed of aggregated beta-amyloid (Aβ), and tau tangles composed of hyperphosphorylated, aggregated tau. These pathologies are typically accompanied by an increase in neuroinflammation as an attempt to ameliorate the pathology. This idea has pushed the field toward focusing on mechanisms and the influence neuroinflammation has on disease progression. The vast majority of AD cases are sporadic and therefore, researchers investigate genetic risk factors that could lead to AD. Apolipoprotein E (ApoE) is the largest genetic risk factor for developing AD. ApoE has 3 isoforms-ApoE2, ApoE3, and ApoE4. ApoE4 constitutes an increased risk of AD, with one copy increasing the risk about 4-fold and two copies increasing the risk about 15-fold compared to those with the ApoE3 allele. ApoE4 has been shown to play a role in Aβ deposition, tau tangle formation, neuroinflammation and many subsequent pathways. However, while we know that ApoE4 plays a role in these pathways and virtually all aspects of AD, the exact mechanism of how ApoE4 impacts AD progression is murky at best and therefore the role ApoE4 plays in these pathways needs to be elucidated. This review aims to discuss the current literature regarding the pathways and mechanisms of ApoE4 in AD progression with a focus on its role in neuroinflammation.

Leveraging preclinical models for the development of Alzheimer disease therapeutics

A large number of mouse models have been engineered, characterized and used to advance biomedical research in Alzheimer disease (AD). Early models simply damaged the rodent brain through toxins or lesions. Later, the spread of genetic engineering technology enabled investigators to develop models of familial AD by overexpressing human genes such as those encoding amyloid precursor protein (APP) or presenilins (PSEN1 or PSEN2) carrying mutations linked to early-onset AD. Recently, more complex models have sought to explore the impact of multiple genetic risk factors in the context of different biological challenges. Although none of these models has proven to be a fully faithful reproduction of the human disease, models remain essential as tools to improve our understanding of AD biology, conduct thorough pharmacokinetic and pharmacodynamic analyses, discover translatable biomarkers and evaluate specific therapeutic approaches. To realize the full potential of animal models as new technologies and knowledge become available, it is critical to define an optimal strategy for their use. Here, we review progress and challenges in the use of AD mouse models, highlight emerging scientific innovations in model development, and introduce a conceptual framework for use of preclinical models for therapeutic development.

A vicious cycle of β amyloid-dependent neuronal hyperactivation

β-amyloid (Aβ)-dependent neuronal hyperactivity is believed to contribute to the circuit dysfunction that characterizes the early stages of Alzheimer's disease (AD). Although experimental evidence in support of this hypothesis continues to accrue, the underlying pathological mechanisms are not well understood. In this experiment, we used mouse models of Aβ-amyloidosis to show that hyperactivation is initiated by the suppression of glutamate reuptake. Hyperactivity occurred in neurons with preexisting baseline activity, whereas inactive neurons were generally resistant to Aβ-mediated hyperactivation. Aβ-containing AD brain extracts and purified Aβ dimers were able to sustain this vicious cycle. Our findings suggest a cellular mechanism of Aβ-dependent neuronal dysfunction that can be active before plaque formation.

Molecular Mechanism of Apoptosis by Amyloid β-Protein Fibrils Formed on Neuronal Cells

Aggregational states of amyloid β-protein (Aβ) are critical for its neurotoxicity, although they are not well-characterized, particularly after binding to the cell membranes. This is one reason why the mechanisms of Aβ neurotoxicity are controversial and elusive. In this study, the effects of toxic Aβ-(1-42) fibrils formed in the membrane on cellular processes were investigated using human neuroblastoma SH-SY5Y cells. Consistent with previous observations, fibrillar Aβs formed on the membranes induced activation of caspase-3, the effector caspase for apoptosis. Knockdown analyses of the initiator caspases, caspase-8 and caspase-9, indicated that the apoptosis was induced via activation of caspase-8, followed by activation of caspase-9 and caspase-3. We also found that inflammation signaling pathways including Toll-like receptors and inflammasomes NOD-, LRR-, and pyrin domain-containing protein 3 are involved in the initiation of apoptosis by the Aβ fibrils. These inflammation-related molecules are promising targets for the prevention of apoptotic cell death induced by Aβ.

Early Effects of Aβ Oligomers on Dendritic Spine Dynamics and Arborization in Hippocampal Neurons

Alzheimer’s disease (AD) is a neurodegenerative disorder that leads to impaired memory and cognitive deficits. Spine loss as well as changes in spine morphology correlates with cognitive impairment in this neurological disorder. Many studies in animal models and ex vivo cultures indicate that amyloid β-peptide (Aβ) oligomers induce synaptic damage early during the progression of the disease. Here, in order to determine the events that initiate synaptic alterations, we acutely applied oligomeric Aβ to primary hippocampal neurons and an ex vivo model of organotypic hippocampal cultures from a mouse after targeted expression of EGFP to allow high-resolution imaging and algorithm-based evaluation of spine changes. Dendritic spines were classified as thin, stubby or mushroom, based on morphology. In vivo, time-lapse imaging showed that the three spine types were relatively stable, although their stability significantly decreased after treatment with Aβ oligomers. Unexpectedly, we observed that the density of total dendritic spines increased in organotypic hippocampal slices treated with Aβ compared to control cultures. Specifically, the fraction of stubby spines significantly increased, while mushroom and thin spines remained unaltered. Pharmacological tools revealed that acute Aβ oligomers induced spine changes through mechanisms involving CaMKII and integrin β1 activities. Additionally, analysis of dendritic complexity based on a 3D reconstruction of the whole neuron morphology showed an increase in the apical dendrite length and branching points in CA1 organotypic hippocampal slices treated with Aβ. In contrast to spines, the morphological changes were affected by integrin β1 but not by CaMKII inhibition. Altogether, these data indicate that the Aβ oligomers exhibit early dual effects by acutely enhancing dendritic complexity and spine density.

NOTCH3 is non-enzymatically fragmented in inherited cerebral small-vessel disease

The small-vessel disorder cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) arises from mutations in the human gene encoding NOTCH3 and results in vascular smooth muscle cell degeneration, stroke, and dementia. However, the structural changes in NOTCH3 involved in CADASIL etiology are unclear. Here, we discovered site-specific fragmentation of NOTCH3 protein in pathologically affected vessels of human CADASIL-affected brains. EM-based experiments to pinpoint NOTCH3 localization in these brains indicated accumulation of NOTCH3 fragmentation products in the basement membrane, collagen fibers, and granular osmiophilic material within the cerebrovasculature. Using antibodies generated against a disease-linked neo-epitope found in degenerating vascular medium of CADASIL brains, we mapped the site of fragmentation to the NOTCH3 N terminus at the peptide bond joining Asp⁸⁰ and Pro⁸¹ Cleavage at this site was predicted to separate the first epidermal growth factor (EGF)-like domain from the remainder of the protein. We found that the cleavage product from this fragmentation event is released into the conditioned medium of cells expressing recombinant NOTCH3 fragments. Mutagenesis of Pro⁸¹ abolished the fragmentation, and low pH and reducing conditions enhanced NOTCH3 proteolysis. Furthermore, substitution of multiple cysteine residues of the NOTCH3 N terminus activated proteolytic release of the first EGF-like repeat, suggesting that the elimination of multiple disulfide bonds in NOTCH3 accelerates its fragmentation. These characteristics link the signature molecular genetic alterations present in individuals with CADASIL to a post-translational protein alteration in degenerating brain arteries. The cellular consequences of these pathological NOTCH3 fragments are an important area for future investigation.

A Focused Chiral Mutant Library of the Amyloid β 42 Central Electrostatic Cluster as a Tool To Stabilize Aggregation Intermediates

Amyloidogenic peptides and proteins aggregate into fibrillary structures that are usually deposited in tissues and organs and are often involved in the development of diseases. In contrast to native structured proteins, amyloids do not follow a defined energy landscape toward the fibrillary state and often generate a vast population of aggregation intermediates that are transient and exceedingly difficult to study. Here, we employ chiral editing as a tool to study the aggregation mechanism of the Amyloid β (Aβ) 42 peptide, whose aggregation intermediates are thought to be one of the main driving forces in Alzheimer's disease (AD). Through the design of a focused chiral mutant library (FCML) of 16 chiral Aβ42 variants, we identified several point D-substitutions that allowed us to modulate the aggregation propensity and the biological activity of the peptide. Surprisingly, the reduced propensity toward aggregation and the stabilization of oligomeric intermediates did not always correlate with an increase in toxicity. In the present study, we show how chiral editing can be a powerful tool to trap and stabilize Aβ42 conformers that might otherwise be too transient and dynamic to study, and we identify sites within the Aβ42 sequence that could be potential targets for therapeutic intervention.

Preferential Recruitment of Conformationally Distinct Amyloid-β Oligomers by the Intrinsically Disordered Region of the Human Prion Protein

Soluble oligomeric species of the amyloid-β (Aβ) peptide exhibit pronounced neurotoxic effects in Alzheimer's disease. Recent studies have indicated that the prion protein (PrP) is one of the cell-surface receptors, so-called a bad receptor, of Aβ oligomers that mediates downstream cellular toxicity. A rational classification of Aβ oligomers on the basis of conformation indicates that there are two distinct types of oligomers, namely, prefibrillar and fibrillar oligomers that are positive to A11 and OC conformation-dependent antibodies, respectively. The mechanism of heterotypic assembly of conformationally distinct oligomers and PrP is poorly understood. In this work, using an array of biophysical and biochemical tools, we dissect the molecular mechanism of the interaction of A11- and OC-positive Aβ42 oligomers with human PrP. Using site-specific binding titrations, we show that the recruitment of Aβ oligomers primarily occurs via the electrostatic interaction between the N-terminal intrinsically disordered region of PrP and Aβ oligomers. Our results demonstrate that OC-positive fibrillar oligomers possessing in-register parallel β-sheet packing displayed ∼30 times stronger binding with PrP compared to A11-positive oligomers. We also show that these OC-positive oligomers exacerbate their toxic effects on mammalian cells upon binding to PrP. On the contrary, the addition of PrP does not alter the toxicity exhibited by A11-positive oligomers. Our findings suggest that strategies targeting the interaction between PrP and OC-positive oligomers, which have been shown to be highly concentrated in the vicinity of amyloid plaques, may have therapeutic potential against Alzheimer's disease.

A Coordinated Approach by Public Domain Bioinformatics Resources to Aid the Fight Against Alzheimer's Disease Through Expert Curation of Key Protein Targets

BACKGROUND

The analysis and interpretation of data generated from patient-derived clinical samples relies on access to high-quality bioinformatics resources. These are maintained and updated by expert curators extracting knowledge from unstructured biological data described in free-text journal articles and converting this into more structured, computationally-accessible forms. This enables analyses such as functional enrichment of sets of genes/proteins using the Gene Ontology, and makes the searching of data more productive by managing issues such as gene/protein name synonyms, identifier mapping, and data quality.

OBJECTIVE

To undertake a coordinated annotation update of key public-domain resources to better support Alzheimer's disease research.

METHODS

We have systematically identified target proteins critical to disease process, in part by accessing informed input from the clinical research community.

RESULTS

Data from 954 papers have been added to the UniProtKB, Gene Ontology, and the International Molecular Exchange Consortium (IMEx) databases, with 299 human proteins and 279 orthologs updated in UniProtKB. 745 binary interactions were added to the IMEx human molecular interaction dataset.

CONCLUSION

This represents a significant enhancement in the expert curated data pertinent to Alzheimer's disease available in a number of biomedical databases. Relevant protein entries have been updated in UniProtKB and concomitantly in the Gene Ontology. Molecular interaction networks have been significantly extended in the IMEx Consortium dataset and a set of reference protein complexes created. All the resources described are open-source and freely available to the research community and we provide examples of how these data could be exploited by researchers.

Beyond the neuron-cellular interactions early in Alzheimer disease pathogenesis

The symptoms of Alzheimer disease reflect a loss of neural circuit integrity in the brain, but neurons do not work in isolation. Emerging evidence suggests that the intricate balance of interactions between neurons, astrocytes, microglia and vascular cells required for healthy brain function becomes perturbed during the disease, with early changes likely protecting neural circuits from damage, followed later by harmful effects when the balance cannot be restored. Moving beyond a neuronal focus to understand the complex cellular interactions in Alzheimer disease and how these change throughout the course of the disease may provide important insight into developing effective therapeutics.

Amyloid Beta and Tau Cooperate to Cause Reversible Behavioral and Transcriptional Deficits in a Model of Alzheimer's Disease

A key knowledge gap blocking development of effective therapeutics for Alzheimer's disease (AD) is the lack of understanding of how amyloid beta (Aβ) peptide and pathological forms of the tau protein cooperate in causing disease phenotypes. Within a mouse tau-deficient background, we probed the molecular, cellular, and behavioral disruption triggered by the influence of wild-type human tau on human Aβ-induced pathology. We find that Aβ and tau work cooperatively to cause a hyperactivity behavioral phenotype and to cause downregulation of transcription of genes involved in synaptic function. In both our mouse model and human postmortem tissue, we observe accumulation of pathological tau in synapses, supporting the potential importance of synaptic tau. Importantly, tau reduction in the mice initiated after behavioral deficits emerge corrects behavioral deficits, reduces synaptic tau levels, and substantially reverses transcriptional perturbations, suggesting that lowering synaptic tau levels may be beneficial in AD.

Amyloid-β oligomers suppress subunit-specific glutamate receptor increase during LTP

Introduction

Amyloid-β oligomers (AβOs) are assumed to impair the ability of learning and memory by suppressing the induction of synaptic plasticity, such as long-term potentiation (LTP) in the early stage of Alzheimer's disease. However, the direct molecular mechanism of how AβOs affect excitatory synaptic plasticity remains to be elucidated.

Methods

In order to study the effects of AβOs on LTP-associated changes of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptor (AMPAR) movement, we performed live-cell imaging of fluorescently labeled AMPAR subunit GluA1 or GluA2 with total internal reflection fluorescence microscopy.

Results

Incubation of cultured hippocampal neurons with AβOs for 1–2 days inhibited the increase in GluA1 number and GluA1 exocytosis frequency in both postsynaptic and extrasynaptic membranes during LTP. In contrast, AβOs did not inhibit the increase in GluA2 number or exocytosis frequency.

Discussion

These results suggest that AβOs primarily inhibit the increase in the number of GluA1 homomers and suppress hippocampal LTP expression.

Amyloid Beta Oligomers Target to Extracellular and Intracellular Neuronal Synaptic Proteins in Alzheimer's Disease

Introduction: β-Amyloid protein (Aβ) putatively plays a seminal role in synaptic loss in Alzheimer's disease (AD). While there is no consensus regarding the synaptic-relevant species of Aβ, it is known that Aβ oligomers (AβOs) are noticeably increased in the early stages of AD, localizing at or within the synapse. In cell and animal models, AβOs have been shown to attach to synapses and instigate synapse dysfunction and deterioration. To establish the pathological mechanism of synaptic loss in AD, it will be important to identify the synaptic targets to which AβOs attach. Methods: An unbiased approach using far western ligand blots has identified three synaptic proteins to which AβOs specifically attach. These proteins (p100, p140, and p260) were subsequently enriched by detergent extraction, ultracentrifugation, and CHT-HPLC column separation, and sequenced by LC-MS/MS. P100, p140, and p260 were identified. These levels of AβOs targets in human AD and aging frontal cortexes were analyzed by quantitative proteomics and western-blot. The polyclonal antibody to AβOs was developed and used to block the toxicity of AβOs. The data were analyzed with one-way analysis of variance. Results: AβOs binding proteins p100, p140, and p260 were identified as Na/K-ATPase, synGap, and Shank3, respectively. α3-Na/K-ATPase, synGap, and Shank3 proteins showed loss in the postsynaptic density (PSD) of human AD frontal cortex. In short term experiments, oligomers of Aβ inhibited Na/K-ATPase at the synapse. Na/K-ATPase activity was restored by an antibody specific for soluble forms of Aβ. α3-Na/K-ATPase protein and synaptic β-amyloid peptides were pulled down from human AD synapses by co-immunoprecipitation. Results suggest synaptic dysfunction in early stages of AD may stem from inhibition of Na/K-ATPase activity by Aβ oligomers, while later stages could hypothetically result from disrupted synapse structure involving the PSD proteins synGap and Shank3. Conclusion: We identified three AβO binding proteins as α3-Na/K-ATPase, synGap, and Shank3. Soluble Aβ oligomers appear capable of attacking neurons via specific extracellular as well as intracellular synaptic proteins. Impact on these proteins hypothetically could lead to synaptic dysfunction and loss, and could serve as novel therapeutic targets for AD treatment by antibodies or other agents.

Family C G-Protein-Coupled Receptors in Alzheimer's Disease and Therapeutic Implications

Alzheimer’s disease (AD), particularly its sporadic or late-onset form (SAD/LOAD), is the most prevalent (96–98% of cases) neurodegenerative dementia in aged people. AD’s neuropathology hallmarks are intrabrain accumulation of amyloid-β peptides (Aβs) and of hyperphosphorylated Tau (p-Tau) proteins, diffuse neuroinflammation, and progressive death of neurons and oligodendrocytes. Mounting evidences suggest that family C G-protein-coupled receptors (GPCRs), which include γ-aminobutyric acid B receptors (GABA_BRs), metabotropic glutamate receptors (mGluR1-8), and the calcium-sensing receptor (CaSR), are involved in many neurotransmitter systems that dysfunction in AD. This review updates the available knowledge about the roles of GPCRs, particularly but not exclusively those expressed by brain astrocytes, in SAD/LOAD onset and progression, taking stock of their respective mechanisms of action and of their potential as anti-AD therapeutic targets. In particular, GABA_BRs prevent Aβs synthesis and neuronal hyperexcitability and group I mGluRs play important pathogenetic roles in transgenic AD-model animals. Moreover, the specific binding of Aβs to the CaSRs of human cortical astrocytes and neurons cultured in vitro engenders a pathological signaling that crucially promotes the surplus synthesis and release of Aβs and hyperphosphorylated Tau proteins, and also of nitric oxide, vascular endothelial growth factor-A, and proinflammatory agents. Concurrently, Aβs•CaSR signaling hinders the release of soluble (s)APP-α peptide, a neurotrophic agent and GABA_BR1a agonist. Altogether these effects progressively kill human cortical neurons in vitro and likely also in vivo. Several CaSR’s negative allosteric modulators suppress all the noxious effects elicited by Aβs•CaSR signaling in human cortical astrocytes and neurons thus safeguarding neurons’ viability in vitro and raising hopes about their potential therapeutic benefits in AD patients. Further basic and clinical investigations on these hot topics are needed taking always heed that activation of the several brain family C GPCRs may elicit divergent upshots according to the models studied.