Deleterious effects of amyloid beta oligomers acting as an extracellular scaffold for mGluR5

PET imaging of mGluR5 in Alzheimer's disease

BACKGROUND

Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and may constitute an important therapeutic target in Alzheimer's disease (AD) by mediating the synaptotoxic action of amyloid-β oligomers. We utilized the positron emission tomography (PET) radioligand [18F]FPEB to investigate mGluR5 binding in early AD.

METHODS

Sixteen individuals with amnestic mild cognitive impairment (MCI) due to AD or mild AD dementia who were positive for brain amyloid were compared to 15 cognitively normal (CN) participants who were negative for brain amyloid. Diagnostic groups were well balanced for age, sex, and education. Dynamic PET scans were acquired for 60 min, starting at 60 min after the initial administration of up to 185 MBq of [18F]FPEB using a bolus-plus-constant-infusion method (Kbol = 190 min). Equilibrium modeling with a cerebellum reference region was used to estimate [18F]FPEB binding (BPND) to mGluR5. Analyses were performed with and without corrections for gray matter atrophy and partial volume effects.

RESULTS

Linear mixed model analysis demonstrated a significant effect of group (p = 0.011) and the group × region interaction (p = 0.0049) on BPND. Post hoc comparisons revealed a significant reduction (43%) in mGluR5 binding in the hippocampus of AD (BPND = 0.76 ± 0.41) compared to CN (BPND = 1.34 ± 0.58, p = 0.003, unpaired t test) participants, and a nonsignificant trend for a reduction in a composite association cortical region in AD (BPND = 1.57 ± 0.25) compared to CN (BPND = 1.86 ± 0.63, p = 0.093) participants. Exploratory analyses suggested additional mGluR5 reductions in the entorhinal cortex and parahippocampal gyrus in the AD group. In the overall sample, hippocampal mGluR5 binding was associated with episodic memory scores and global function.

CONCLUSIONS

[18F]FPEB-PET revealed reductions in hippocampal mGluR5 binding in early AD. Quantification of mGluR5 binding in AD may expand our understanding of AD pathogenesis and accelerate the development of novel biomarkers and treatments.

Reversal of Calcium Dysregulation as Potential Approach for Treating Alzheimer's Disease

Despite decades of research and effort, there is still no effective disease-modifying treatment for Alzheimer's Disease (AD). Most of the recent AD clinical trials were targeting amyloid pathway, but all these trials failed. Although amyloid pathology is a hallmark and defining feature of AD, targeting the amyloid pathway has been very challenging due to low efficacy and serious side effects. Alternative approaches or mechanisms for our understanding of the major cause of memory loss in AD need to be considered as potential therapeutic targets. Increasing studies suggest that Ca2+ dysregulation in AD plays an important role in AD pathology and is associated with other AD abnormalities, such as excessive inflammation, increased ROS, impaired autophagy, neurodegeneration, synapse, and cognitive dysfunction. Ca2+ dysregulation in cytosolic space, Endoplasmic Reticulum (ER) and mitochondria have been reported in the context of various AD models. Drugs or strategies, to correct the Ca2+ dysregulation in AD, have been demonstrated to be promising as an approach for the treatment of AD in preclinical models. This review will discuss the mechanisms of Ca2+ dysregulation in AD and associated pathology and discuss potential approaches or strategies to develop novel drugs for the treatment of AD by targeting Ca2+ dysregulation.

Oligomeric Aβ in the monkey brain impacts synaptic integrity and induces accelerated cortical aging

As the average age of the population continues to rise, the number of individuals affected with age-related cognitive decline and Alzheimer's disease (AD) has increased and is projected to cost more than $290 billion in the United States in 2019. Despite significant investment in research over the last decades, there is no effective treatment to prevent or delay AD progression. There is a translational gap in AD research, with promising drugs based on work in rodent models failing in clinical trials. Aging is the leading risk factor for developing AD and understanding neurobiological changes that affect synaptic integrity with aging will help clarify why the aged brain is vulnerable to AD. We describe here the development of a rhesus monkey model of AD using soluble oligomers of the amyloid beta (Aβ) peptide (AβOs). AβOs infused into the monkey brain target a specific population of spines in the prefrontal cortex, induce neuroinflammation, and increase AD biomarkers in the cerebrospinal fluid to similar levels observed in patients with AD. Importantly, AβOs lead to similar dendritic spine loss to that observed in normal aging in monkeys, but so far without detection of amyloid plaques or tau pathology. Understanding the basis of synaptic impairment is the most effective route to early intervention and prevention or postponement of age-related cognitive decline and transition to AD. These initial findings support the use of monkeys as a platform to understand age-related vulnerabilities of the primate brain and may help develop effective disease-modifying therapies for treatment of AD and related dementias.

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.

The fruit of Crataegus pinnatifida ameliorates memory deficits in β-amyloid protein-induced Alzheimer's disease mouse model

ETHNOPHARMACOLOGICAL RELEVANCE

The fruit of Crataegus pinnatifida is a traditional medicine widely used as digestive drug in East Asia. Although Chinese herbal medicine used it for mental health, scientific evidence does not exist, yet.

AIMS OF STUDY

The aim of this study is to show that the ethanol extract of the fruit of Crataegus pinnatifida (CPE) has neuroprotective effect on Alzheimer' disease model mice.

MATERIALS AND METHODS

Intracerebroventricular injection of Aβ was used to induce Alzheimer's disease-like pathology. Passive avoidance and Y-maze tasks were used to examine the effect of CPE on memory impairments by Aβ. Immunohistochemistry was used to examine the effect of CPE on glial activation. ThT assay was used to observe the effect of CPE on Aβ aggregation. MTT and LDH release assays were utilized to examine effects of CPE on Aβ-induced cytotoxicity.

RESULTS

CPE prevented memory deficit in Aβ-induced memory impairment model. Moreover, CPE prevented glial activation in the hippocampus of Aβ-injected model. In in vitro test, CPE inhibited Aβ fibril formation in a concentration-dependent manner. CPE also caused disaggregation of Aβ fibrils. Along with this, CPE blocked neuronal cell death induced by Aβ.

CONCLUSIONS

Collectively, these experimental findings demonstrated that CPE could be a candidate for development of AD therapy.

Aβ oligomers induce sex-selective differences in mGluR5 pharmacology and pathophysiological signaling in Alzheimer mice.. [DOI: 10.1101/803262]

Sex is a key modifier of the prevalence and progression of Alzheimer’s disease (AD). β- Amyloid (Aβ) deposition is a pathological hallmark of AD and aberrant activation of metabotropic glutamate receptor 5 (mGluR5) by Aβ has been linked to AD progression. We find that mGluR5 exhibits distinct sex-dependent pharmacological profiles. Specifically, endogenous mGluR5 from male mouse cortex and hippocampus binds with high-affinity to Aβ oligomers whereas, female mGluR5 exhibits no affinity to Aβ oligomers. The binding affinity of mGluR5 to Aβ oligomer is dependent on its interaction with cellular prion protein (PrPC) as mGluR5 co-immunoprecipitates with PrPCfrom male, but not female, mouse brain. Aβ oligomers also bind with high-affinity to human mGluR5 in male, but not female, cortex. The mGluR5/Aβ oligomer/PrPCternary complex is essential to elicit mGluR5-dependent pathological signaling and as a consequence mGluR5-regulated GSK3β/ZBTB16 autophagic signaling is dysregulated in male, but not female, primary neuronal cultures. These sex-specific differences in mGluR5 signaling translate into in vivo differences in mGluR5-dependent pathological signaling between male and female AD mice. We show that the chronic inhibition of mGluR5 using a mGluR5-selective negative allosteric modulator reactivates GSK3β/ZBTB16-regulated autophagy, mitigates Aβ pathology and reverses cognitive decline in male, but not female, APPswe/PS1ΔE9 mice. Thus, it is evident that, unlike male brain, mGluR5 does not contribute to Aβ pathology in female AD mice. This study highlights the complexity of mGluR5 pharmacology and Aβ oligomer-activated pathological signaling and emphasizes the need for clinical trials redesign and analysis of sex-tailored treatment for AD.

A positive allosteric modulator of mGluR5 promotes neuroprotective effects in mouse models of Alzheimer's disease

Alzheimer's Disease (AD) is the most prevalent neurodegenerative disorder. Despite advances in the understanding of its pathophysiology, none of the available therapies prevents disease progression. Excess glutamate plays an important role in excitotoxicity by activating ionotropic receptors. However, the mechanisms modulating neuronal cell survival/death via metabotropic glutamate receptors (mGluRs) are not completely understood. Recent data indicates that CDPPB, a positive allosteric modulator of mGluR5, has neuroprotective effects. Thus, this work aimed to investigate CDPPB treatment effects on amyloid-β (Aβ) induced pathological alterations in vitro and in vivo and in a transgenic mouse model of AD (T41 mice). Aβ induced cell death in primary cultures of hippocampal neurons, which was prevented by CDPPB. Male C57BL/6 mice underwent stereotaxic surgery for unilateral intra-hippocampal Aβ injection, which induced memory deficits, neurodegeneration, neuronal viability reduction and decrease of doublecortin-positive cells, a marker of immature neurons and neuronal proliferation. Treatment with CDPPB for 8 days reversed neurodegeneration and doublecortin-positive cells loss and recovered memory function. Fourteen months old T41 mice presented cognitive deficits, neuronal viability reduction, gliosis and Aβ accumulation. Treatment with CDPPB for 28 days increased neuronal viability (32.2% increase in NeuN⁺ cells) and reduced gliosis in CA1 region (Iba-1⁺ area by 31.3% and GFAP⁺ area by 37.5%) in transgenic animals, without inducing hepatotoxicity. However, it did not reverse cognitive deficit. Despite a four-week treatment did not prevent memory loss in aged transgenic mice, CDPPB is protective against Aβ stimulus. Therefore, this drug represents a potential candidate for further investigations as AD treatment.

AD molecular: Molecular imaging of Alzheimer's disease: PET imaging of neurotransmitter systems

Current understanding of Alzheimer's disease (AD) pathogenesis relies on the observed accumulations of amyloid β and phosphorylated tau aggregates that are thought to play key roles in initiating or propagating disease. However, other processes including changes in synaptic proteins and neurotransmitter loss have been suggested as important etiologies or contributors. Positron emission tomography (PET) imaging allows in vivo investigations of molecular changes associated with AD. PET imaging with multiple radiotracers can be used in combination with other modalities such as magnetic resonance imaging (MRI), and with assessments of cognition and neuropsychiatric symptoms to investigate the molecular underpinnings of AD. Studies of synaptic protein changes may improve the understanding of disease mechanisms and provide valuable markers of disease progression and therapeutic efficacy. This chapter will illustrate the importance of in vivo molecular imaging in the study of AD with a specific emphasis on PET and radioligands for several non-amyloid targets.

Peripheral Amyloid Precursor Protein Derivative Expression in Fragile X Syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and is associated with increased risk for autism spectrum disorder (ASD), anxiety, ADHD, and epilepsy. While our understanding of FXS pathophysiology has improved, a lack of validated blood-based biomarkers of disease continues to impede bench-to-bedside efforts. To meet this demand, there is a growing effort to discover a reliable biomarker to inform treatment discovery and evaluate treatment target engagement. Such a marker, amyloid-beta precursor protein (APP), has shown potential dysregulation in the absence of fragile X mental retardation protein (FMRP) and may therefore be associated with FXS pathophysiology. While APP is best understood in the context of Alzheimer disease, there is a growing body of evidence suggesting the molecule and its derivatives play a broader role in regulating neuronal hyperexcitability, a well-characterized phenotype in FXS. To evaluate the viability of APP as a peripheral biological marker in FXS, we conducted an exploratory ELISA-based evaluation of plasma APP-related species involving 27 persons with FXS (mean age: 22.0 ± 11.5) and 25 age- and sex-matched persons with neurotypical development (mean age: 21.1 ± 10.7). Peripheral levels of both Aβ(1–40) and Aβ(1–42) were increased, while sAPPα was significantly decreased in persons with FXS as compared to control participants. These results suggest that dysregulated APP processing, with potential preferential β-secretase processing, may be a readily accessible marker of FXS pathophysiology.

Age dependency of mGluR5 availability in 5xFAD mice measured by PET

The major pathologies of Alzheimer's disease (AD) are amyloid plaques and hyperphosphorylated tau. The deposition of amyloid plaques leads to synaptic dysfunction, neuronal cell death, and cognitive impairment. Among the neurotransmitters, glutamate is the most abundant in the mammalian brain and plays an important role in synaptic plasticity. With respect to synaptic transmission, metabotropic glutamate receptor 5 (mGluR5) is highly affected by amyloid pathology. However, the neuropathologic changes in the protein expression of mGluR5 in AD remain unclear. Therefore, to elucidate the alteration in mGluR5 expression with the progression of AD, we performed serial behavioral tests, longitudinal imaging studies, and histopathological immunoassay for both 5xFAD (n = 14) mice and age-matched wild-type mice (n = 14). The 5xFAD mice started showing severe hyperactivity and memory impairment from 7 months of age. In addition, mGluR5 positron emission tomography revealed that while the binding values in the wild-type mice were similar over time, those in 5xFAD mice fluctuated from 5 months of age. Furthermore, the 5xFAD mice presented a 35% decrease in the binding values of their cortical and subcortical areas at 9 months of age compared with those at 3 months of age. Magnetic resonance spectroscopy and histopathological studies showed similar changes. In conclusion, mGluR5 availability changes with age, and mGluR5 positron emission tomography could successfully detect this synaptic change in the 5xFAD mice.

Prions, prionoids and protein misfolding disorders

Prion diseases are progressive, incurable and fatal neurodegenerative conditions. The term 'prion' was first nominated to express the revolutionary concept that a protein could be infectious. We now know that prions consist of PrP^Sc, the pathological aggregated form of the cellular prion protein PrP^C. Over the years, the term has been semantically broadened to describe aggregates irrespective of their infectivity, and the prion concept is now being applied, perhaps overenthusiastically, to all neurodegenerative diseases that involve protein aggregation. Indeed, recent studies suggest that prion diseases (PrDs) and protein misfolding disorders (PMDs) share some common disease mechanisms, which could have implications for potential treatments. Nevertheless, the transmissibility of bona fide prions is unique, and PrDs should be considered as distinct from other PMDs.

Protein-Protein Interactions and Aggregation Inhibitors in Alzheimer's Disease

BACKGROUND

Alzheimer's Disease (AD), a multifaceted disorder, involves complex pathophysiology and plethora of protein-protein interactions. Thus such interactions can be exploited to develop anti-AD drugs.

OBJECTIVE

The interaction of dynamin-related protein 1, cellular prion protein, phosphoprotein phosphatase 2A and Mint 2 with amyloid β, etc., studied recently, may have critical role in progression of the disease. Our objective has been to review such studies and their implications in design and development of drugs against the Alzheimer's disease.

METHODS

Such studies have been reviewed and critically assessed.

RESULTS

Review has led to show how such studies are useful to develop anti-AD drugs.

CONCLUSION

There are several PPIs which are current topics of research including Drp1, Aβ interactions with various targets including PrPC, Fyn kinase, NMDAR and mGluR5 and interaction of Mint2 with PDZ domain, etc., and thus have potential role in neurodegeneration and AD. Finally, the multi-targeted approach in AD may be fruitful and opens a new vista for identification and targeting of PPIs in various cellular pathways to find a cure for the disease.

Brain Inflammation Connects Cognitive and Non-Cognitive Symptoms in Alzheimer's Disease

Alzheimer's disease (AD) is the main form of dementia in the elderly and affects greater than 47 million people worldwide. Care for AD patients poses very significant personal and economic demands on individuals and society, and the situation is expected to get even more dramatic in the coming decades unless effective treatments are found to halt the progression of the disease. Although AD is most commonly regarded as a disease of the memory, the entire brain is eventually affected by neuronal dysfunction or neurodegeneration, which brings about a host of other behavioral disturbances. AD patients often present with apathy, depression, eating and sleeping disorders, aggressive behavior, and other non-cognitive symptoms, which deeply affect not only the patient but also the caregiver's health. These symptoms are usually associated with AD pathology but are often neglected as part of disease progression due to the early and profound impact of disease on memory centers such as the hippocampus and entorhinal cortex. Yet, a collection of findings offers biochemical insight into mechanisms underlying non-cognitive symptoms in AD, and indicate that, at the molecular level, such symptoms share common mechanisms. Here, we review evidence indicating mechanistic links between memory loss and non-cognitive symptoms of AD. We highlight the central role of the pro-inflammatory activity of microglia in behavioral alterations in AD patients and in experimental models of the disease. We suggest that a deeper understanding of non-cognitive symptoms of AD may illuminate a new beginning in AD research, offering a fresh approach to elucidate mechanisms involved in disease progression and potentially unveiling yet unexplored therapeutic targets.

The Amyloid-β Oligomer Hypothesis: Beginning of the Third Decade

The amyloid-β oligomer (AβO) hypothesis was introduced in 1998. It proposed that the brain damage leading to Alzheimer’s disease (AD) was instigated by soluble, ligand-like AβOs. This hypothesis was based on the discovery that fibril-free synthetic preparations of AβOs were potent CNS neurotoxins that rapidly inhibited long-term potentiation and, with time, caused selective nerve cell death (Lambert et al., 1998). The mechanism was attributed to disrupted signaling involving the tyrosine-protein kinase Fyn, mediated by an unknown toxin receptor. Over 4,000 articles concerning AβOs have been published since then, including more than 400 reviews. AβOs have been shown to accumulate in an AD-dependent manner in human and animal model brain tissue and, experimentally, to impair learning and memory and instigate major facets of AD neuropathology, including tau pathology, synapse deterioration and loss, inflammation, and oxidative damage. As reviewed by Hayden and Teplow in 2013, the AβO hypothesis “has all but supplanted the amyloid cascade.” Despite the emerging understanding of the role played by AβOs in AD pathogenesis, AβOs have not yet received the clinical attention given to amyloid plaques, which have been at the core of major attempts at therapeutics and diagnostics but are no longer regarded as the most pathogenic form of Aβ. However, if the momentum of AβO research continues, particularly efforts to elucidate key aspects of structure, a clear path to a successful disease modifying therapy can be envisioned. Ensuring that lessons learned from recent, late-stage clinical failures are applied appropriately throughout therapeutic development will further enable the likelihood of a successful therapy in the near-term.

Melatonin in Synaptic Impairments of Alzheimer's Disease

Alzheimer's disease (AD) underlies dementia for millions of people worldwide with no effective treatment. The dementia of AD is thought stem from the impairments of the synapses because of their critical roles in cognition. Melatonin is a neurohormone mainly released by the pineal gland in a circadian manner and it regulates brain functions in various manners. It is reported that both the melatonin deficit and synaptic impairments are present in the very early stage of AD and strongly contribute to the progress of AD. In the mammalian brains, the effects of melatonin are mainly relayed by two of its receptors, melatonin receptor type 1a (MT1) and 1b (MT2). To have a clear idea on the roles of melatonin in synaptic impairments of AD, this review discussed the actions of melatonin and its receptors in the stabilization of synapses, modulation of long-term potentiation, as well as their contributions in the transmissions of glutamatergic, GABAergic and dopaminergic synapses, which are the three main types of synapses relevant to the synaptic strength. The synaptic protective roles of melatonin in AD treatment were also summarized. Regarding its protective roles against amyloid-β neurotoxicity, tau hyperphosphorylation, oxygenation, inflammation as well as synaptic dysfunctions, melatonin may be an ideal therapeutic agent against AD at early stage.

Alteration of scaffold: Possible role of MACF1 in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a progressive neurodegenerative disease, with the sign of sensory or motor function loss, memory decline, and dementia. Histopathological study shows AD neuron has irregular cytoskeleton and aberrant synapse. Amyloid-β (Aβ) is believed as the trigger of AD, however, the detailed pathogenesis is not fully elucidated. Microtubule-actin crosslinking factor 1 (MACF1) is a unique giant molecule which can bind to all three types of cytoskeleton fibers, different linkers/adaptors, as well as various functional proteins. MACF1 is a critical scaffold for orchestrating the complex 3D structure, and is essential for correct synaptic function. MACF1's binding ability to microtubule depends on Glycogen synthase kinase 3 Bate (GSK3β) mediated phosphorylation. While GSK3β can be regulated by the binding of Aβ and the receptor Paired immunoglobulin-like receptor B (PirB), possibly via Protein phosphatase 2A (PP2A). So based on literature search and logic analysis, we propose a hypothesis: Aβ binds to its receptor PirB, and triggers cytosol PP2A, which might activate GSK3β. GSK3β might further phosphorylates microtubule-binding domain (MTBD) of MACF1, causes the separation of microtubule and MACF1. Thus MACF1 might lose the control of the whole cytoskeleton system, synapse might change and AD might develop. That is Aβ-PirB-PP2A-GSK3β-MACF1 axis might give rise to AD. We hope our hypothesis might provide new clue and evidence to AD pathogenesis.

Fragile X and APP: a Decade in Review, a Vision for the Future

Fragile X syndrome (FXS) is a devastating developmental disability that has profound effects on cognition, behavior, and seizure susceptibility. There are currently no treatments that target the underlying cause of the disorder, and recent clinical trials have been unsuccessful. In 2007, seminal work demonstrated that amyloid-beta protein precursor (APP) is dysregulated in Fmr1KO mice through a metabotropic glutamate receptor 5 (mGluR5)-dependent pathway. These findings raise the hypotheses that: (1) APP and/or APP metabolites are potential therapeutic targets as well as biomarkers for FXS and (2) mGluR5 inhibitors may be beneficial in the treatment of Alzheimer's disease. Herein, advances in the field over the past decade that have reproduced and greatly expanded upon these original findings are reviewed, and required experimentation to validate APP metabolites as potential disease biomarkers as well as therapeutic targets for FXS are discussed.

Aß Facilitates LTD at Schaffer Collateral Synapses Preferentially in the Left Hippocampus

Promotion of long-term depression (LTD) mechanisms by synaptotoxic soluble oligomers of amyloid-β (Aß) has been proposed to underlie synaptic dysfunction in Alzheimer's disease (AD). Previously, LTD was induced by relatively non-specific electrical stimulation. Exploiting optogenetics, we studied LTD using a more physiologically diffuse spatial pattern of selective pathway activation in the rat hippocampus in vivo. This relatively sparse synaptic LTD requires both the ion channel function and GluN2B subunit of the NMDA receptor but, in contrast to electrically induced LTD, is not facilitated by boosting endogenous muscarinic acetylcholine or metabotropic glutamate 5 receptor activation. Although in the absence of Aß, there is no evidence of hippocampal LTD asymmetry, in the presence of Aß, the induction of LTD is preferentially enhanced in the left hippocampus in an mGluR5-dependent manner. This circuit-selective disruption of synaptic plasticity by Aß provides a route to understanding the development of aberrant brain lateralization in AD.

Calcitonin native prefibrillar oligomers but not monomers induce membrane damage that triggers NMDA-mediated Ca2+-influx, LTP impairment and neurotoxicity

Amyloid protein misfolding results in a self-assembling aggregation process, characterized by the formation of typical aggregates. The attention is focused on pre-fibrillar oligomers (PFOs), formed in the early stages and supposed to be neurotoxic. PFOs structure may change due to their instability and different experimental protocols. Consequently, it is difficult to ascertain which aggregation species are actually neurotoxic. We used salmon Calcitonin (sCT) as an amyloid model whose slow aggregation rate allowed to prepare stable samples without photochemical cross-linking. Intracellular Ca²⁺ rise plays a fundamental role in amyloid protein-induced neurodegerations. Two paradigms have been explored: (i) the "membrane permeabilization" due to the formation of amyloid pores or other types of membrane damage; (ii) "receptor-mediated" modulation of Ca²⁺ channels. In the present paper, we tested the effects of native sCT PFOs- with respect to Monomer-enriched solutions in neurons characterized by an increasing degree of differentiation, in terms of -Ca²⁺-influx, cellular viability, -Long-Term Potentiation impairment, Post-Synaptic Densities and synaptophysin expression. Results indicated that PFOs-, but not Monomer-enriched solutions, induced abnormal -Ca²⁺-influx, which could only in part be ascribed to NMDAR activation. Thus, we propose an innovative neurotoxicity mechanism for amyloid proteins where "membrane permeabilization" and "receptor-mediated" paradigms coexist.

Amyloid β oligomers suppress excitatory transmitter release via presynaptic depletion of phosphatidylinositol-4,5-bisphosphate

Amyloid β (Aβ) oligomer-induced aberrant neurotransmitter release is proposed to be a crucial early event leading to synapse dysfunction in Alzheimer's disease (AD). In the present study, we report that the release probability (Pr) at the synapse between the Schaffer collateral (SC) and CA1 pyramidal neurons is significantly reduced at an early stage in mouse models of AD with elevated Aβ production. High nanomolar synthetic oligomeric Aβ₄₂ also suppresses Pr at the SC-CA1 synapse in wild-type mice. This Aβ-induced suppression of Pr is mainly due to an mGluR5-mediated depletion of phosphatidylinositol-4,5-bisphosphate (PIP₂) in axons. Selectively inhibiting Aβ-induced PIP₂ hydrolysis in the CA3 region of the hippocampus strongly prevents oligomeric Aβ-induced suppression of Pr at the SC-CA1 synapse and rescues synaptic and spatial learning and memory deficits in APP/PS1 mice. These results first reveal the presynaptic mGluR5-PIP₂ pathway whereby oligomeric Aβ induces early synaptic deficits in AD.

Systematic and standardized comparison of reported amyloid-β receptors for sufficiency, affinity, and Alzheimer's disease relevance

Oligomeric assemblies of amyloid-β (Aβ) peptide (Aβo) in the brains of individuals with Alzheimer's disease (AD) are toxic to neuronal synapses. More than a dozen Aβ receptor candidates have been suggested to be responsible for various aspects of the molecular pathology and memory impairment in mouse models of AD. A lack of consistent experimental design among previous studies of different receptor candidates limits evaluation of the relative roles of these candidates, producing some controversy within the field. Here, using cell-based assays with several Aβ species, including Aβo from AD brains obtained by autopsy, we directly compared the Aβ-binding capacity of multiple receptor candidates while accounting for variation in expression and confirming cell surface expression. In a survey of 15 reported Aβ receptors, only cellular prion protein (PrP^C), Nogo receptor 1 (NgR1), and leukocyte immunoglobulin-like receptor subfamily B member 2 (LilrB2) exhibited direct binding to synaptotoxic assemblies of synthetic Aβ. Both PrP^C and NgR1 preferentially bound synaptotoxic oligomers rather than nontoxic monomers, and the method of oligomer preparation did not significantly alter our binding results. Hippocampal neurons lacking both NgR1 and LilrB2 exhibited a partial reduction of Aβo binding, but this reduction was lower than in neurons lacking PrP^C under the same conditions. Finally, binding studies with soluble Aβo from human AD brains revealed a strong affinity for PrP^C, weak affinity for NgR1, and no detectable affinity for LilrB2. These findings clarify the relative contributions of previously reported Aβ receptors under controlled conditions and highlight the prominence of PrP^C as an Aβ-binding site.

Kalirin-7 prevents dendritic spine dysgenesis induced by amyloid beta-derived oligomers

Synapse degeneration and dendritic spine dysgenesis are believed to be crucial early steps in Alzheimer's disease (AD), and correlate with cognitive deficits in AD patients. Soluble amyloid beta (Aβ)-derived oligomers, also termed Aβ-derived diffusible ligands (ADDLs), accumulate in the brain of AD patients and play a crucial role in AD pathogenesis. ADDLs bind to mature hippocampal neurons, induce structural changes in dendritic spines and contribute to neuronal death. However, mechanisms underlying structural and toxic effects are not fully understood. Here, we report that ADDLs bind to cultured mature cortical pyramidal neurons and induce spine dysgenesis. ADDL treatment induced the rapid depletion of kalirin-7, a brain-specific guanine-nucleotide exchange factor for the small GTPase Rac1, from spines. Kalirin-7 is a key regulator of dendritic spine morphogenesis and maintenance in forebrain pyramidal neurons and here we show that overexpression of kalirin-7 prevents ADDL-induced spine degeneration. Taken together, our results suggest that kalirin-7 may play a role in the early events leading to synapse degeneration, and its pharmacological activation may prevent or delay synapse pathology in AD.

Clustering of Tau fibrils impairs the synaptic composition of α3-Na+/K+-ATPase and AMPA receptors

Tau assemblies have prion‐like properties: they propagate from one neuron to another and amplify by seeding the aggregation of endogenous Tau. Although key in prion‐like propagation, the binding of exogenous Tau assemblies to the plasma membrane of naïve neurons is not understood. We report that fibrillar Tau forms clusters at the plasma membrane following lateral diffusion. We found that the fibrils interact with the Na⁺/K⁺‐ATPase (NKA) and AMPA receptors. The consequence of the clustering is a reduction in the amount of α3‐NKA and an increase in the amount of GluA2‐AMPA receptor at synapses. Furthermore, fibrillar Tau destabilizes functional NKA complexes. Tau and α‐synuclein aggregates often co‐exist in patients’ brains. We now show evidences for cross‐talk between these pathogenic aggregates with α‐synuclein fibrils dramatically enhancing fibrillar Tau clustering and synaptic localization. Our results suggest that fibrillar α‐synuclein and Tau cross‐talk at the plasma membrane imbalance neuronal homeostasis.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Synaptic dysfunction in Alzheimer's disease: Mechanisms and therapeutic strategies

Alzheimer's disease (AD), the most prevalent neurodegenerative disease in the elderly population, is characterized by progressive cognitive decline and pathological hallmarks of amyloid plaques and neurofibrillary tangles. However, its pathophysiological mechanisms are poorly understood, and diagnostic tools and interventions are limited. Here, we review recent research on the amyloid hypothesis and beta-amyloid-induced dysfunction of neuronal synapses through distinct cell surface receptors. We also review how tau protein leads to synaptotoxicity through pathological modification, localization, and propagation. Finally, we discuss experimental therapeutics for AD and propose potential applications of disease-modifying strategies targeting synaptic failure for improved treatment of AD.

Amyloid toxicity in Alzheimer's disease

A major feature of Alzheimer's disease (AD) pathology is the plaque composed of aggregated amyloid-β (Aβ) peptide. Although these plaques may have harmful properties, there is much evidence to implicate soluble oligomeric Aβ as the primary noxious form. Aβ oligomers can be generated both extracellularly and intracellularly. Aβ is toxic to neurons in a myriad of ways. It can cause pore formation resulting in the leakage of ions, disruption of cellular calcium balance, and loss of membrane potential. It can promote apoptosis, cause synaptic loss, and disrupt the cytoskeleton. Current treatments for AD are limited and palliative. Much research and effort is being devoted to reducing Aβ production as an approach to slowing or preventing the development of AD. Aβ formation results from the amyloidogenic cleavage of human amyloid precursor protein (APP). Reconfiguring this process to disfavor amyloid generation might be possible through the reduction of APP or inhibition of enzymes that convert the precursor protein to amyloid.

Dysregulation of Intracellular Calcium Signaling in Alzheimer's Disease

SIGNIFICANCE

Calcium (Ca²⁺) hypothesis of Alzheimer's disease (AD) gains popularity. It points to new signaling pathways that may underlie AD pathogenesis. Based on calcium hypothesis, novel targets for the development of potential AD therapies are identified. Recent Advances: Recently, the key role of neuronal store-operated calcium entry (nSOCE) in the development of AD has been described. Correct regulation of nSOCE is necessary for the stability of postsynaptic contacts to preserve the memory formation. Molecular identity of hippocampal nSOCE is defined. Perspective nSOCE-activating molecule, prototype of future anti-AD drugs, is described.

CRITICAL ISSUES

Endoplasmic reticulum Ca²⁺ overload happens in many but not in all AD models. The nSOCE targeting therapy described in this review may not be universally applicable.

FUTURE DIRECTIONS

There is a need to determine whether AD is a syndrome with one critical signaling pathway that initiates pathology, or it is a disorder with many different signaling pathways that are disrupted simultaneously or one after each other. It is necessary to validate applicability of nSOCE-activating therapy for the development of anti-AD medication. There is an experimental correlation between downregulated nSOCE and disrupted postsynaptic contacts in AD mouse models. Signaling mechanisms downstream of nSOCE which are responsible for the regulation of stability of postsynaptic contacts have to be discovered. That will bring new targets for the development of AD-preventing therapies. Antioxid. Redox Signal. 29, 1176-1188.

Functional organization of postsynaptic glutamate receptors

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in the brain, responsible for mediating the vast majority of excitatory transmission in neuronal networks. The AMPA- and NMDA-type ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the fast synaptic responses, while metabotropic glutamate receptors (mGluRs) are coupled to downstream signaling cascades that act on much slower timescales. These functionally distinct receptor sub-types are co-expressed at individual synapses, allowing for the precise temporal modulation of postsynaptic excitability and plasticity. Intriguingly, these receptors are differentially distributed with respect to the presynaptic release site. While iGluRs are enriched in the core of the synapse directly opposing the release site, mGluRs reside preferentially at the border of the synapse. As such, to understand the differential contribution of these receptors to synaptic transmission, it is important to not only consider their signaling properties, but also the mechanisms that control the spatial segregation of these receptor types within synapses. In this review, we will focus on the mechanisms that control the organization of glutamate receptors at the postsynaptic membrane with respect to the release site, and discuss how this organization could regulate synapse physiology.

Neurotoxic Agent-Induced Injury in Neurodegenerative Disease Model: Focus on Involvement of Glutamate Receptors

Glutamate receptors play a crucial role in the central nervous system and are implicated in different brain disorders. They play a significant role in the pathogenesis of neurodegenerative diseases (NDDs) such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Although many studies on NDDs have been conducted, their exact pathophysiological characteristics are still not fully understood. In in vivo and in vitro models of neurotoxic-induced NDDs, neurotoxic agents are used to induce several neuronal injuries for the purpose of correlating them with the pathological characteristics of NDDs. Moreover, therapeutic drugs might be discovered based on the studies employing these models. In NDD models, different neurotoxic agents, namely, kainic acid, domoic acid, glutamate, β-N-Methylamino-L-alanine, amyloid beta, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1-methyl-4-phenylpyridinium, rotenone, 3-Nitropropionic acid and methamphetamine can potently impair both ionotropic and metabotropic glutamate receptors, leading to the progression of toxicity. Many other neurotoxic agents mainly affect the functions of ionotropic glutamate receptors. We discuss particular neurotoxic agents that can act upon glutamate receptors so as to effectively mimic NDDs. The correlation of neurotoxic agent-induced disease characteristics with glutamate receptors would aid the discovery and development of therapeutic drugs for NDDs.

The NMDA receptor antagonist Radiprodil reverses the synaptotoxic effects of different amyloid-beta (Aβ) species on long-term potentiation (LTP)

Aβ_1-42 is well accepted to be a primary early pathogenic agent in Alzheimer's disease (AD). However, other amyloid peptides are now gaining considerable attention as potential key participants in AD due to their proposed higher neuronal toxicity. Impairment of the glutamatergic system is also widely accepted to be associated with pathomechanisms underlying AD. There is ample evidence that Aβ_1-42 affects GLUN2B subunit containing N-methyl-D-aspartate receptor function and abolishes the induction of long term potentiation (LTP). In this study we show that different β-amyloid species, 1-42 Aβ_1-42 and 1-40 (Aβ_1-40) as well as post-translationally modified forms such as pyroglutamate-modified amyloid-(AβpE3) and nitrated Aβ (3NTyr10-Aβ), when applied for 90 min to murine hippocampal slices, concentration-dependently prevented the development of CA1-LTP after tetanic stimulation of the Schaffer collaterals with IC₅₀s of 2, 9, 2 and 35 nM, respectively whilst having no effect on baseline AMPA receptor mediated fEPSPs. Aβ_1-43 had no effect. Interestingly, the combination of all Aβ species did not result in any synergistic or additive inhibitory effect on LTP - the calculated pooled Aβ species IC₅₀ was 20 nM. A low concentration (10 nM) of the GLUN2B receptor antagonist Radiprodil restored LTP in the presence of Aβ_1-42, 3NTyr10-Aβ, Aβ_1-40, but not AβpE3. In contrast to AMPA receptor mediated fEPSPs, all different β-amyloid species tested at 50 nM supressed baseline NMDA-EPSC amplitudes. Similarly, all different Aβ species tested decreased spine density. As with LTP, Radiprodil (10 nM) reversed the synaptic toxicity of Aβ species but not that of AβpE3. These data do not support the enhanced toxic actions reported for some Aβ species such as AβpE3, nor synergistic toxicity of the combination of different Aβ species. However, whilst in our hands AβpE_3-42 was actually less toxic than Aβ_1-42, its effects were not reversed by Radiprodil indicating that the target receptors/subunits mediating such synaptotoxicity may differ between the different Aβ species tested.

Regulation of Neuronal Na,K-ATPase by Extracellular Scaffolding Proteins

Neuronal activity leads to an influx of Na⁺ that needs to be rapidly cleared. The sodium-potassium ATPase (Na,K-ATPase) exports three Na⁺ ions and imports two K⁺ ions at the expense of one ATP molecule. Na,K-ATPase turnover accounts for the majority of energy used by the brain. To prevent an energy crisis, the energy expense for Na⁺ clearance must provide an optimal effect. Here we report that in rat primary hippocampal neurons, the clearance of Na⁺ ions is more efficient if Na,K-ATPase is laterally mobile in the membrane than if it is clustered. Using fluorescence recovery after photobleaching and single particle tracking analysis, we show that the ubiquitous α1 and the neuron-specific α3 catalytic subunits as well as the supportive β1 subunit of Na,K-ATPase are highly mobile in the plasma membrane. We show that cross-linking of the β1 subunit with polyclonal antibodies or exposure to Modulator of Na,K-ATPase (MONaKA), a secreted protein which binds to the extracellular domain of the β subunit, clusters the α3 subunit in the membrane and restricts its mobility. We demonstrate that clustering, caused by cross-linking or by exposure to MONaKA, reduces the efficiency in restoring intracellular Na⁺. These results demonstrate that extracellular interactions with Na,K-ATPase regulate the Na⁺ extrusion efficiency with consequences for neuronal energy balance.

Store depletion-induced h-channel plasticity rescues a channelopathy linked to Alzheimer's disease

Voltage-gated ion channels are critical for neuronal integration. Some of these channels, however, are misregulated in several neurological disorders, causing both gain- and loss-of-function channelopathies in neurons. Using several transgenic mouse models of Alzheimer's disease (AD), we find that sub-threshold voltage signals strongly influenced by hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels progressively deteriorate over chronological aging in hippocampal CA1 pyramidal neurons. The degraded signaling via HCN channels in the transgenic mice is accompanied by an age-related global loss of their non-uniform dendritic expression. Both the aberrant signaling via HCN channels and their mislocalization could be restored using a variety of pharmacological agents that target the endoplasmic reticulum (ER). Our rescue of the HCN channelopathy helps provide molecular details into the favorable outcomes of ER-targeting drugs on the pathogenesis and synaptic/cognitive deficits in AD mouse models, and implies that they might have beneficial effects on neurological disorders linked to HCN channelopathies.

mGluR5 mediates post-radiotherapy fatigue development in cancer patients

Cancer-related fatigue (CRF) is a common burden in cancer patients and little is known about its underlying mechanism. The primary aim of this study was to identify gene signatures predictive of post-radiotherapy fatigue in prostate cancer patients. We employed Fisher Linear Discriminant Analysis (LDA) to identify predictive genes using whole genome microarray data from 36 men with prostate cancer. Ingenuity Pathway Analysis was used to determine functional networks of the predictive genes. Functional validation was performed using a T lymphocyte cell line, Jurkat E6.1. Cells were pretreated with metabotropic glutamate receptor 5 (mGluR5) agonist (DHPG), antagonist (MPEP), or control (PBS) for 20 min before irradiation at 8 Gy in a Mark-1 γ-irradiator. NF-κB activation was assessed using a NF-κB/Jurkat/GFP Transcriptional Reporter Cell Line. LDA achieved 83.3% accuracy in predicting post-radiotherapy fatigue. "Glutamate receptor signaling" was the most significant (p = 0.0002) pathway among the predictive genes. Functional validation using Jurkat cells revealed clustering of mGluR5 receptors as well as increased regulated on activation, normal T cell expressed and secreted (RANTES) production post irradiation in cells pretreated with DHPG, whereas inhibition of mGluR5 activity with MPEP decreased RANTES concentration after irradiation. DHPG pretreatment amplified irradiation-induced NF-κB activation suggesting a role of mGluR5 in modulating T cell activation after irradiation. These results suggest that mGluR5 signaling in T cells may play a key role in the development of chronic inflammation resulting in fatigue and contribute to individual differences in immune responses to radiation. Moreover, modulating mGluR5 provides a novel therapeutic option to treat CRF.

Autophagy is increased following either pharmacological or genetic silencing of mGluR5 signaling in Alzheimer's disease mouse models

Alzheimer's disease (AD) is characterized by neurotoxicity mediated by the accumulation of beta amyloid (Aβ) oligomers, causing neuronal loss and progressive cognitive decline. Genetic deletion or chronic pharmacological inhibition of mGluR5 by the negative allosteric modulator CTEP, rescues cognitive function and reduces Aβ aggregation in both APPswe/PS1ΔE9 and 3xTg-AD mouse models of AD. In late onset neurodegenerative diseases, such as AD, defects arise at different stages of the autophagy pathway. Here, we show that mGluR5 cell surface expression is elevated in APPswe/PS1ΔE9 and 3xTg-AD mice. This is accompanied by reduced autophagy (accumulation of p62) as the consequence of increased ZBTB16 expression and reduced ULK1 activity, as we have previously observed in Huntington's disease (HD). The chronic (12 week) inhibition of mGluR5 with CTEP in APPswe/PS1ΔE9 and 3xTg-AD mice prevents the observed increase in mGluR5 surface expression. In addition, mGluR5 inactivation facilitates the loss of ZBTB16 expression and ULK1 activation as a consequence of ULK-Ser757 dephosphorylation, which promotes the loss of expression of the autophagy marker p62. Moreover, the genetic ablation of mGluR5 in APPswe/PS1ΔE9 mice activated autophagy via similar mechanisms to pharmacological blockade. This study provides further evidence that mGluR5 overactivation contributes to inhibition of autophagy and can result in impaired clearance of neurotoxic aggregates in multiple neurodegenerative diseases. Thus, it provides additional support for the potential of mGluR5 inhibition as a general therapeutic strategy for neurodegenerative diseases such as AD and HD.

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.

Silent Allosteric Modulation of mGluR5 Maintains Glutamate Signaling while Rescuing Alzheimer's Mouse Phenotypes

Metabotropic glutamate receptor 5 (mGluR5) has been implicated in Alzheimer's disease (AD) pathology. We sought to understand whether mGluR5's role in AD requires glutamate signaling. We used a potent mGluR5 silent allosteric modulator (SAM, BMS-984923) to separate its well-known physiological role in glutamate signaling from a pathological role in mediating amyloid-β oligomer (Aβo) action. Binding of the SAM to mGluR5 does not change glutamate signaling but strongly reduces mGluR5 interaction with cellular prion protein (PrP^C) bound to Aβo. The SAM compound prevents Aβo-induced signal transduction in brain slices and in an AD transgenic mouse model, the APPswe/PS1ΔE9 strain. Critically, 4 weeks of SAM treatment rescues memory deficits and synaptic depletion in the APPswe/PS1ΔE9 transgenic mouse brain. Our data show that mGluR5's role in Aβo-dependent AD phenotypes is separate from its role in glutamate signaling and silent allosteric modulation of mGluR5 has promise as a disease-modifying AD intervention with a broad therapeutic window.

Interactions of amyloid-β peptides on lipid bilayer studied by single molecule imaging and tracking

The amyloid-β peptides (Aβ40 and Aβ42) feature prominently in the synaptic dysfunction and neuronal loss associated with Alzheimer's disease (AD). This has been proposed to be due either to interactions between Aβ and cell surface receptors affecting cell signaling, or to the formation of calcium-permeable channels in the membrane that disrupt calcium homeostasis. In both mechanisms the cell membrane is the primary cellular structure with which Aβ interacts. Aβ concentrations in human bodily fluids are very low (pM-nM) rendering studies of the size, composition, cellular binding sites and mechanism of action of the oligomers formed in vivo very challenging. Most studies, therefore, have utilized Aβ oligomers prepared at micromolar peptide concentrations, where Aβ forms oligomeric species which possess easily observable cell toxicity. Such toxicity has not been observed when nM concentrations of peptide are used in the experiment highlighting the importance of employing physiologically relevant peptide concentrations for the results to be of biological significance. In this paper single-molecule microscopy was used to monitor Aβ oligomer formation and diffusion on a supported lipid bilayer at nanomolar peptide concentrations. Aβ monomers, the dominant species in solution, tightly associate with the membrane and are highly mobile whereas trimers and higher-order oligomers are largely immobile. Aβ dimers exist in a mixture of mobile and immobile states. Oligomer growth on the membrane is more rapid for Aβ40 than for the more amyloidogenic Aβ42 but is largely inhibited for a 1:1 Aβ40:Aβ42 mixture. The mechanism underlying these Aβ40-Aβ42 interactions may feature in Alzheimer's pathology.

Synaptic Alterations in Mouse Models for Alzheimer Disease-A Special Focus on N-Truncated Abeta 4-42

This commentary reviews the role of the Alzheimer amyloid peptide Aβ on basal synaptic transmission, synaptic short-term plasticity, as well as short- and long-term potentiation in transgenic mice, with a special focus on N-terminal truncated Aβ4-42. Aβ4-42 is highly abundant in the brain of Alzheimer's disease (AD) patients. It demonstrates increased neurotoxicity compared to full length Aβ, suggesting an important role in the pathogenesis of AD. Transgenic Tg4-42 mice, a model for sporadic AD, express human Aβ4-42 in Cornu Ammonis (CA1) neurons, and develop age-dependent hippocampal neuron loss and neurological deficits. In contrast to other transgenic AD mouse models, the Tg4-42 model exhibits synaptic hyperexcitability, altered synaptic short-term plasticity with no alterations in short- and long-term potentiation. The outcomes of this study are discussed in comparison with controversial results from other AD mouse models.

Role of ASIC1a in Aβ-induced synaptic alterations in the hippocampus

Acid-sensing ion channels (ASICs) are widely expressed in the mammalian central nervous system where they play a key role in synaptic transmission and in specific forms of memory. On the other hand, ASICs can be persistently active under pathological conditions contributing to neuronal damage in ischemic stroke, brain trauma, epilepsy and Parkinson's disease. However, to date no experimental evidence has linked ASICs to Alzheimer's disease (AD). Aim of the present work was to investigate, in CA1 pyramidal neurons, the possible involvement of ASIC1a in the Aβ-mediated effect on metabotropic glutamate (mGlu) receptor dependent transmission. We found that, in slices pretreated with Aβ, the pharmacological blockade of ASIC1a restored the increased intrinsic excitability following group I mGlu receptor activation. This suggests that, under certain conditions, ASIC1a might further contribute to the Aβ-related depolarizing response. We have recently demonstrated that ASIC1a is also involved long-term depression (LTD) induced either by low-frequency stimulation or by application of the group I mGlu receptor agonist DHPG. Here, we have shown that psalmotoxin-1, a selective blocker of ASIC1a, rescued the DHPG-LTD facilitation associated with genetic and non-genetic models of AD. Overall, these results suggest that a functional coupling between ASIC1a and mGlu receptors occurs and might contribute to the synaptic alterations associated with AD.

An acute functional screen identifies an effective antibody targeting amyloid-β oligomers based on calcium imaging

Soluble amyloid β oligomers (AβOs) are widely recognized neurotoxins that trigger aberrant signaling in specific subsets of neurons, leading to accumulated neuronal damage and memory disorders in Alzheimer's disease (AD). One of the profound downstream consequences of AβO-triggered events is dysregulation of cytosolic calcium concentration ([Ca2+]i), which has been implicated in synaptic failure, cytoskeletal abnormalities, and eventually neuronal death. We have developed an in vitro/in vivo drug screening assay to evaluate putative AβO-blocking candidates by measuring AβO-induced real-time changes in [Ca2+]i. Our screening assay demonstrated that the anti-AβO monoclonal antibody ACU3B3 exhibits potent blocking capability against a broad size range of AβOs. We showed that picomolar concentrations of AβOs were capable of increasing [Ca2+]i in primary neuronal cultures, an effect prevented by ACU3B3. Topical application of 5 nM AβOs onto exposed cortical surfaces also elicited significant calcium elevations in vivo, which was completely abolished by pre-treatment of the brain with 1 ng/mL (6.67 pM) ACU3B3. Our results provide strong support for the utility of this functional screening assay in identifying and confirming the efficacy of AβO-blocking drug candidates such as the human homolog of ACU3B3, which may emerge as the first experimental AD therapeutic to validate the amyloid oligomer hypothesis.

Therapeutic role of rifampicin in Alzheimer's disease

Rifampicin exerts significant brain protective functions in multiple experimental models. Here we summarize the underlying mechanisms of the neuroprotective and pro-cognitive effects of rifampicin that are mediated by its anti-inflammatory, anti-tau, anti-amyloid, and cholinergic effects. Beyond suggesting that rifampicin shows strong brain protective effects in preclinical models of Alzheimer's disease, we also provide substantial clinical evidence for the neuroprotective and pro-cognitive effects of rifampicin. Future neuroimaging studies combined with clinical assessment scores are the following steps to be taken in this field of research.

Synaptic vesicle cycle and amyloid β: Biting the hand that feeds

The synaptic vesicle cycle (SVC) holds center stage in the biology of presynaptic terminals. Through recurrent exocytosis and endocytosis, it facilitates a sequence of events enabling chemical neurotransmission between functionally related neurons. As a fundamental process that links the interior of nerve cells with their environment, the SVC is also critical for signaling and provides an entry route for a range of pathogens and toxins, enabling detrimental effects. In Alzheimer's disease, the SVC is both the prime site of amyloid β production and toxicity. In this study, we discuss the emerging evidence for physiological and pathological effects of Aβ on various stages of the SVC, from postfusion membrane recovery to trafficking, docking, and priming of vesicles for fusion and transmitter release. Understanding of the mechanisms of Aβ interaction with the SVC within the unifying calcium hypothesis of aging and Alzheimer's disease should further elucidate the fundamental biology of the presynaptic terminal and reveal novel therapeutic targets for Alzheimer's disease and other age-related dementias.

Chronic low-dose-rate ionising radiation affects the hippocampal phosphoproteome in the ApoE-/- Alzheimer's mouse model

Accruing data indicate that radiation-induced consequences resemble pathologies of neurodegenerative diseases such as Alzheimer´s. The aim of this study was to elucidate the effect on hippocampus of chronic low-dose-rate radiation exposure (1 mGy/day or 20 mGy/day) given over 300 days with cumulative doses of 0.3 Gy and 6.0 Gy, respectively. ApoE deficient mutant C57Bl/6 mouse was used as an Alzheimer´s model. Using mass spectrometry, a marked alteration in the phosphoproteome was found at both dose rates. The radiation-induced changes in the phosphoproteome were associated with the control of synaptic plasticity, calcium-dependent signalling and brain metabolism. An inhibition of CREB signalling was found at both dose rates whereas Rac1-Cofilin signalling was found activated only at the lower dose rate. Similarly, the reduction in the number of activated microglia in the molecular layer of hippocampus that paralleled with reduced levels of TNFα expression and lipid peroxidation was significant only at the lower dose rate. Adult neurogenesis, investigated by Ki67, GFAP and NeuN staining, and cell death (activated caspase-3) were not influenced at any dose or dose rate. This study shows that several molecular targets induced by chronic low-dose-rate radiation overlap with those of Alzheimer´s pathology. It may suggest that ionising radiation functions as a contributing risk factor to this neurodegenerative disease.

Prion Protein as a Toxic Acceptor of Amyloid-β Oligomers

The initial report that cellular prion protein (PrPC) mediates toxicity of amyloid-β species linked to Alzheimer's disease was initially treated with scepticism, but growing evidence supports this claim. That there is a high-affinity interaction is now clear, and its molecular basis is being unraveled, while recent studies have identified possible downstream toxic mechanisms. Determination of the clinical significance of such interactions between PrPC and disease-associated amyloid-β species will require experimental medicine studies in humans. Trials of compounds that inhibit PrP-dependent amyloid-β toxicity are commencing in humans, and although it is clear that only a fraction of Alzheimer's disease toxicity could be governed by PrPC, a partial, but still therapeutically useful, role in human disease may soon be testable.

Aβ pathology downregulates brain mGluR5 density in a mouse model of Alzheimer

The aim of the present study was to evaluate functional changes of mGluR5 expression in advanced Alzheimer's disease (AD) using positron emission tomography (PET) with an mGluR5 specific radiotracer ([¹⁸F]FPEB) in 5xFAD AD model. Subsequently, in the same animal, mGluR5 expression was quantified by immunoassay techniques. The non-displaceable binding potential values for mGluR5 was estimated by the Logan's graphical analysis. Brain PET imaging revealed that radioactivities in the hippocampus and the striatum were significantly lower in 5xFAD mice compared to control animals. Binding values were also significantly lowered in 5xFAD mice. This decline was validated by immunoblotting of protein isolates from brain tissues, as the mean band density for 5xFAD mice had a lower mGluR5 intensity than for wild type mice. These results indicated that mGluR5 levels in 5xFAD mice were down regulated in the limbic system.

Mesenchymal stem cells and cell-derived extracellular vesicles protect hippocampal neurons from oxidative stress and synapse damage induced by amyloid-β oligomers

Alzheimer's disease (AD) is a disabling and highly prevalent neurodegenerative condition, for which there are no effective therapies. Soluble oligomers of the amyloid-β peptide (AβOs) are thought to be proximal neurotoxins involved in early neuronal oxidative stress and synapse damage, ultimately leading to neurodegeneration and memory impairment in AD. The aim of the current study was to evaluate the neuroprotective potential of mesenchymal stem cells (MSCs) against the deleterious impact of AβOs on hippocampal neurons. To this end, we established transwell cocultures of rat hippocampal neurons and MSCs. We show that MSCs and MSC-derived extracellular vesicles protect neurons against AβO-induced oxidative stress and synapse damage, revealed by loss of pre- and postsynaptic markers. Protection by MSCs entails three complementary mechanisms: 1) internalization and degradation of AβOs; 2) release of extracellular vesicles containing active catalase; and 3) selective secretion of interleukin-6, interleukin-10, and vascular endothelial growth factor to the medium. Results support the notion that MSCs may represent a promising alternative for cell-based therapies in AD.