Complement and microglia mediate early synapse loss in Alzheimer mouse models.

AimsAmyloid‐β (Aβ) oligomers trigger synaptic degeneration that precedes plaque and tangle pathology. However, the signalling molecules that link Aβ oligomers to synaptic pathology remain unclear. Here, we addressed the potential role of RAPGEF2 as a novel signalling molecule in Aβ oligomer‐induced synaptic and cognitive impairments in human‐mutant amyloid precursor protein (APP) mouse models of Alzheimer's disease (AD).MethodsTo investigate the role of RAPGEF2 in Aβ oligomer‐induced synaptic and cognitive impairments, we utilised a combination of approaches including biochemistry, molecular cell biology, light and electron microscopy, behavioural tests with primary neuron cultures, multiple AD mouse models and post‐mortem human AD brain tissue.ResultsWe found significantly elevated RAPGEF2 levels in the post‐mortem human AD hippocampus. RAPGEF2 levels also increased in the transgenic AD mouse models, generating high levels of Aβ oligomers before exhibiting synaptic and cognitive impairment. RAPGEF2 upregulation activated the downstream effectors Rap2 and JNK. In cultured hippocampal neurons, oligomeric Aβ treatment increased the fluorescence intensity of RAPGEF2 and reduced the number of dendritic spines and the intensities of synaptic marker proteins, while silencing RAPGEF2 expression blocked Aβ oligomer‐induced synapse loss. Additionally, the in vivo knockdown of RAPGEF2 expression in the AD hippocampus prevented cognitive deficits and the loss of excitatory synapses.ConclusionsThese findings demonstrate that the upregulation of RAPGEF2 levels mediates Aβ oligomer‐induced synaptic and cognitive disturbances in the AD hippocampus. We propose that an early intervention regarding RAPGEF2 expression may have beneficial effects on early synaptic pathology and memory loss in AD.

In Alzheimer's disease, perturbations of glutamate neurotransmission lead to synaptic dysfunction and synapse loss. Several studies have used glutamate transport inhibitors to demonstrate that soluble oligomers of amyloid-β induce synaptic dysfunction by interrupting glutamate uptake mediated by glutamate transporter 1, the major glutamate transporter in the brain. The cellular targets of the synaptic effects of soluble amyloid-β oligomers, including the nature of any interaction with glutamate transporter 1, remain ill-defined. We have generated a conditional glutamate transporter 1 knockout mouse to investigate celltype specific functions of glutamate transporter 1. Field excitatory postsynaptic potentials were examined in the CA1 region of mouse hippocampal slices. We confirmed that hippocampal long-term potentiation impairment is induced by both soluble Aβ oligomers and glutamate uptake inhibitors. Amyloid-β oligomers, including those isolated directly from the cortex of patients with Alzheimer's disease, failed to inhibit hippocampal long-term potentiation in neuronal glutamate transporter 1 but not astrocytic glutamate transporter 1 knockout mice. The masking or occlusion of the effect of soluble Aβ oligomers by knockout of glutamate transporter 1 in neurons suggests that the metabolic or signaling consequences of knockout of glutamate transporter 1 in neurons and oAβ inhibition of synaptic plasticity show epistasis and thus share a similar molecular pathway. To extend these observations, we tested the effects of other types of manipulation of glutamate homeostasis on synaptic plasticity and the pathophysiology of soluble Aβ oligomers. Ceftriaxone, which upregulates glutamate transporter 1 levels, among other effects, prevented the impairment of long-term potentiation by soluble Aβ oligomers. Collectively, our findings suggest that the effects of amyloid-β on synaptic function are highly dependent on glutamate reuptake homeostasis and that the disruption of synaptic function by soluble Aβ oligomers is mediated by pathways linked to neuronal, not astrocytic, glutamate transporter 1. This study's findings highlight the translational potential of targeting neuronal glutamate transporter 1 to counteract amyloid-β-induced synaptic dysfunction in Alzheimer's disease. By showing that glutamate transporter 1 upregulation (e.g., via ceftriaxone) can prevent Aβ-related impairments, this research supports developing therapies aimed at modulating glutamate homeostasis to preserve synaptic function and combat cognitive decline in patients with Alzheimer's disease.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder and a major medical problem. Here, we have investigated the impact of amyloid-β (Aβ) oligomers, AD-related neurotoxins, in the brains of rats and adult nonhuman primates (cynomolgus macaques). Soluble Aβ oligomers are known to accumulate in the brains of AD patients and correlate with disease-associated cognitive dysfunction. When injected into the lateral ventricle of rats and macaques, Aβ oligomers diffused into the brain and accumulated in several regions associated with memory and cognitive functions. Cardinal features of AD pathology, including synapse loss, tau hyperphosphorylation, astrocyte and microglial activation, were observed in regions of the macaque brain where Aβ oligomers were abundantly detected. Most importantly, oligomer injections induced AD-type neurofibrillary tangle formation in the macaque brain. These outcomes were specifically associated with Aβ oligomers, as fibrillar amyloid deposits were not detected in oligomer-injected brains. Human and macaque brains share significant similarities in terms of overall architecture and functional networks. Thus, generation of a macaque model of AD that links Aβ oligomers to tau and synaptic pathology has the potential to greatly advance our understanding of mechanisms centrally implicated in AD pathogenesis. Furthermore, development of disease-modifying therapeutics for AD has been hampered by the difficulty in translating therapies that work in rodents to humans. This new approach may be a highly relevant nonhuman primate model for testing therapeutic interventions for AD.

Alzheimer disease (AD) is characterized by neurodegeneration marked by loss of synapses and spines associated with hyperphosphorylation of tau protein. Accumulating amyloid β peptide (Aβ) in brain is linked to neurofibrillary tangles composed of hyperphosphorylated tau in AD. Here, we identify β2-adrenergic receptor (β2AR) that mediates Aβ-induced tau pathology. In the prefrontal cortex (PFC) of 1-year-old transgenic mice with human familial mutant genes of presenilin 1 and amyloid precursor protein (PS1/APP), the phosphorylation of tau at Ser-214 Ser-262 and Thr-181, and the protein kinases including JNK, GSK3α/β, and Ca(2+)/calmodulin-dependent protein kinase II is increased significantly. Deletion of the β2AR gene in PS1/APP mice greatly decreases the phosphorylation of these proteins. Further analysis reveals that in primary PFC neurons, Aβ signals through a β2AR-PKA-JNK pathway, which is responsible for most of the phosphorylation of tau at Ser-214 and Ser-262 and a significant portion of phosphorylation at Thr-181. Aβ also induces a β2AR-dependent arrestin-ERK1/2 activity that does not participate in phosphorylation of tau. However, inhibition of the activity of MEK, an upstream enzyme of ERK1/2, partially blocks Aβ-induced tau phosphorylation at Thr-181. The density of dendritic spines and synapses is decreased in the deep layer of the PFC of 1-year-old PS1/APP mice, and the mice exhibit impairment of learning and memory in a novel object recognition paradigm. Deletion of the β2AR gene ameliorates pathological effects in these senile PS1/APP mice. The study indicates that β2AR may represent a potential therapeutic target for preventing the development of AD.

Alzheimer's disease (AD) is the most common cause of progressive cognitive impairment in the aged. The aggregation of the amyloid β-protein (Aβ) is a hallmark of AD and is linked to synapse loss and cognitive impairment. Capsaicin, a specific agonist of the transient receptor potential vanilloid 1 (TRPV1), has been proven to ameliorate stress-induced AD-like pathological and cognitive impairments, but it is unclear whether TRPV1 activation can affect cognitive and synaptic functions in Aβ-induced mouse model of AD. In this study, we investigated the effects of TRPV1 activation on spatial memory and synaptic plasticity in mice treated with Aβ. To induce AD-like pathological and cognitive impairments, adult C57Bl/6 mice were microinjected with Aβ42 (100 μM, 2.5 μl/mouse, i.c.v.). Two weeks after Aβ42 microinjection, spatial learning and memory as well as hippocampal long-term potentiation (LTP) were examined. The results showed that Aβ42 microinjection significantly impaired spatial learning and memory in the Morris water maze and novel object recognition tests compared with controls. These behavioral changes were accompanied by synapse loss and impaired LTP in the CA1 area of hippocampus. More importantly, daily capsaicin (1 mg/kg, i.p.) treatment throughout the experiment dramatically improved spatial learning and memory and synaptic function, as reflected by enhanced hippocampal LTP and reduced synapse loss, whereas the TRPV1 antagonist capsazepine (1 mg/kg, i.p.) treatment had no effects on cognitive and synaptic function in Aβ42-treated mice. These results indicate that TRPV1 activation by capsaicin rescues cognitive deficit in the Aβ42-induced mouse model of AD both structurely and functionally.

Wnt signaling is crucial for synapse and cognitive function. Indeed, deficient Wnt signaling is causally related to increased expression of DKK1, an endogenous negative Wnt regulator, and synapse loss, both of which likely contribute to cognitive decline in Alzheimer’s disease (AD). Increasingly, AD research efforts have probed the neuroinflammatory role of microglia, the resident immune cells of the CNS, which have furthermore been shown to be modulated by Wnt signaling. The DKK1 homolog DKK2 has been previously identified as an activated response and/or disease-associated microglia (DAM/ARM) gene in a mouse model of AD. Here, we performed a detailed analysis of DKK2 in mouse models of neurodegeneration, and in human AD brain. In APP/PS1 and APPNL-G-F AD mouse model brains as well as in SOD1G93A ALS mouse model spinal cords, but not in control littermates, we demonstrated significant microgliosis and microglial Dkk2 mRNA upregulation in a disease-stage-dependent manner. In the AD models, these DAM/ARM Dkk2+ microglia preferentially accumulated close to βAmyloid plaques. Furthermore, recombinant DKK2 treatment of rat hippocampal primary neurons blocked WNT7a-induced dendritic spine and synapse formation, indicative of an anti-synaptic effect similar to that of DKK1. In stark contrast, no such microglial DKK2 upregulation was detected in the postmortem human frontal cortex from individuals diagnosed with AD or pathologic aging. In summary, the difference in microglial expression of the DAM/ARM gene DKK2 between mouse models and human AD brain highlights the increasingly recognized limitations of using mouse models to recapitulate facets of human neurodegenerative disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by memory loss and multiple cognitive impairments. Current healthcare costs for over 50 million people afflicted with AD are about $818 million and are projected to be $2 billion by 2050. Unfortunately, there are no drugs currently available that can delay and/or prevent the progression of disease in elderly individuals and in AD patients. Loss of synapses and synaptic damage are largely correlated with cognitive decline in AD patients. Women are at a higher lifetime risk of developing AD encompassing two-thirds of the total AD afflicted population. Only about 1-2% of total AD patients can be explained by genetic mutations in APP, PS1, and PS2 genes. Several risk factors have been identified, such as Apolipoprotein E4 genotype, type 2 diabetes, traumatic brain injury, depression, and hormonal imbalance, are reported to be associated with late-onset AD. Strong evidence reveals that antioxidant enriched diets and regular exercise reduces toxic radicals, enhances mitochondrial function and synaptic activity, and improves cognitive function in elderly populations. Current available data on the use of antioxidants in mouse models of AD and antioxidant(s) supplements in diets of elderly individuals were investigated. The use of antioxidants in randomized clinical trials in AD patients was also critically assessed. Based on our survey of current literature and findings, we cautiously conclude that healthy diets, regular exercise, and improved lifestyle can delay dementia progression and reduce the risk of AD in elderly individuals and reverse subjects with mild cognitive impairment to a non-demented state.

BackgroundSynaptic dysfunction, characterized by synapse loss and structural alterations, emerges as a prominent correlate of cognitive decline in Alzheimer’s disease (AD). Actin cytoskeleton, which serves as the structural backbone of synaptic architecture, is observed to be lost from synapses in AD. Actin cytoskeleton loss compromises synaptic integrity, affecting glutamatergic receptor levels, neurotransmission, and synaptic strength. Understanding these molecular changes is crucial for developing interventions targeting synaptic dysfunction, potentially mitigating cognitive decline in AD.MethodsIn this study, we investigated the synaptic actin interactome using mass spectrometry in a mouse model of AD, APP/PS1. Our objective was to explore how alterations in synaptic actin dynamics, particularly the interaction between PSD-95 and actin, contribute to synaptic and cognitive impairment in AD. To assess the impact of restoring F-actin levels on synaptic and cognitive functions in APP/PS1 mice, we administered F-actin stabilizing agent, jasplakinolide. Behavioral deficits in the mice were evaluated using the contextual fear conditioning paradigm. We utilized primary neuronal cultures to study the synaptic levels of AMPA and NMDA receptors and the dynamics of PSD-95 actin association. Furthermore, we analyzed postmortem brain tissue samples from subjects with no cognitive impairment (NCI), mild cognitive impairment (MCI), and Alzheimer’s dementia (AD) to determine the association between PSD-95 and actin.ResultsWe found a significant reduction in PSD-95-actin association in synaptosomes from middle-aged APP/PS1 mice compared to wild-type (WT) mice. Treatment with jasplakinolide, an actin stabilizer, reversed deficits in memory recall, restored PSD-95-actin association, and increased synaptic F-actin levels in APP/PS1 mice. Additionally, actin stabilization led to elevated synaptic levels of AMPA and NMDA receptors, enhanced dendritic spine density, suggesting improved neurotransmission and synaptic strength in primary cortical neurons from APP/PS1 mice. Furthermore, analysis of postmortem human tissue with NCI, MCI and AD subjects revealed disrupted PSD-95-actin interactions, underscoring the clinical relevance of our preclinical studies.ConclusionOur study elucidates disrupted PSD-95 actin interactions across different models, highlighting potential therapeutic targets for AD. Stabilizing F-actin restores synaptic integrity and ameliorates cognitive deficits in APP/PS1 mice, suggesting that targeting synaptic actin regulation could be a promising therapeutic strategy to mitigate cognitive decline in AD.

Oxidative stress is related to the pathogenesis of Alzheimer's disease (AD) characterized by progressive memory impairment. Soluble amyloid-β (Aβ) oligomers cause cognitive loss and synaptic dysfunction rather than senile plaques in AD. The decline of the antioxidant status is associated with dementia in AD patients, especially low levels of vitamin C. Our group previously reported a relationship between anti-aging and supplementation of vitamin C derivatives. Here we report that vitamin C mitigated Aβ oligomer formation and behavioral decline in an AD mouse model treated with a vitamin C solution for 6 months. The attenuation of Aβ oligomerization was accompanied with a marked decrease in brain oxidative damage and in the ratio of soluble Aβ₄₂ to Aβ₄₀, a typical indicator of AD progression. Furthermore, the intake of vitamin C restored the declined synaptophysin and the phosphorylation of tau at Ser396. On the other hand, brain plaque deposition was not altered by the dietary intake of vitamin C. These results support that vitamin C is a useful functional nutrient for the prevention of AD.

BackgroundSoluble amyloid-β (Aβ) oligomers are the major toxic substances associated with the pathology of Alzheimer’s disease (AD). The ability to measure Aβ oligomer levels in the blood would provide simple and minimally invasive tools for AD diagnostics. In the present study, the recently developed Multimer Detection System (MDS) for AD, a new enzyme-linked immunosorbent assay for measuring Aβ oligomers selectively, was used to detect Aβ oligomers in the plasma of patients with AD and healthy control individuals.MethodsTwenty-four patients with AD and 37 cognitively normal control individuals underwent extensive clinical evaluations as follows: blood sampling; detailed neuropsychological tests; brain magnetic resonance imaging; cerebrospinal fluid (CSF) measurement of Aβ42, phosphorylated tau protein (pTau), and total tau protein (tTau); and 11C-Pittsburgh compound B (PIB) positron emission tomography. Pearson’s correlation analyses between the estimations of Aβ oligomer levels by MDS and other conventional AD biomarkers (CSF Aβ42, pTau, and tTau, as well as PIB standardized uptake value ratio [PIB SUVR]) were conducted. ROC analyses were used to compare the diagnostic performance of each biomarker.ResultsThe plasma levels of Aβ oligomers by MDS were higher in patients with AD than in normal control individuals, and they correlated well with conventional AD biomarkers (levels of Aβ oligomers by MDS vs. CSF Aβ42, r = −0.443; PIB SUVR, r = 0.430; CSF pTau, r = 0.530; CSF tTau, r = 0.604). The sensitivity and specificity of detecting plasma Aβ oligomers by MDS for differentiating AD from the normal controls were 78.3% and 86.5%, respectively. The AUC for plasma Aβ oligomers by MDS was 0.844, which was not significantly different from the AUC of other biomarkers (p = 0.250).ConclusionsPlasma levels of Aβ oligomers could be assessed using MDS, which might be a simple, noninvasive, and accessible assay for evaluating brain amyloid deposition related to AD pathology.

Microtubule (MT) stability is crucial for proper neuronal function. Understanding MT dysregulation is critical for connecting amyloid beta (Aβ) and tau-based degenerative events and early changes in presymptomatic Alzheimer's disease (AD). Herein we present positron emission tomography (PET) imaging properties of our MT-PET radiotracer, [11C]MPC-6827, in multiple established AD mouse models. Longitudinal PET, biodistribution, autoradiography, immunohistochemistry, and behavioral studies were conducted at multiple time points in APPswe/PSEN1dE9 (APP/PS1), P301S-PS19 (P301S), 5xFAD, and age-matched control mice. Longitudinal [11C]MPC-6827 brain imaging showed significant increases in APP/PS1, P301S, and 5xFAD mice compared to controls. Longitudinal MT-PET correlated positively with biodistribution, autoradiography, and immunohistochemistry results and negatively with behavior data. Our study demonstrated significant longitudinal [11C]MPC-6827 PET increases in multiple AD mouse models for the first time. Strong correlations between PET and biomarker data underscored the interplay of MT destabilization, amyloid, and tau pathology in AD. These results suggest [11C]MPC-6827 PET as a promising tool for monitoring MT dysregulation early in AD progression. Longitudinal positron emission tomography (PET) imaging studies using [11C]MPC-6827 in multiple established Alzheimer's disease (AD) mouse models revealed an early onset of microtubule dysregulation, with significant changes in brain radiotracer uptake evident from 2 to 4 months of age. Intra-group analysis showed a progressive increase in microtubule dysregulation with increasing AD burden, supported by significant correlations between PET imaging data and biodistribution, autoradiography, and molecular pathological markers. [11C]MPC-6827 PET imaging demonstrated its efficacy in detecting early microtubule alterations preceding observable behavioral changes in AD mouse models, suggesting its potential for early AD imaging. The inclusion of the 5xFAD mouse model further elucidated the impact of amyloid beta (Aβ) toxicity on inducing tau hyperphosphorylation-mediated microtubule dysregulation, highlighting the versatility of [11C]MPC-6827 in delineating various aspects of AD pathology. Our study provides immediate clarity on high uptake of the microtubule-based radiotracer in AD brains in a longitudinal setting, which directly informs clinical utility in Aβ/tau-based studies.

Cognitive decline in Alzheimer’s disease (AD) is associated with synaptic loss (1) and network dysfunction (2). The mechanisms involved are not fully understood; however, recent evidence indicate that the neurotoxicity of amyloid-β (Aβ) oligomers is mediated by interactions with synaptic receptors (3). In PNAS, Fu et al. demonstrate that soluble Aβ oligomers trigger synaptic loss and dysfunction via activation of erythropoietin-producing hepatocellular A4 receptor (EphA4) (4). Remarkably, blockade of EphA4 reversed the functional deficits in the hippocampus in animal models of AD. Moreover, based on molecular docking studies, they identify rhynchophylline as a novel EphA4 inhibitor that, following oral administration, rescued the Aβ oligomer-induced deficits in neurotransmission in amyloid precursor protein (APP)/presenilin 1 (PS1) transgenic mice. In agreement with these findings, a recent study by Vargas et al. showed that Eph4 activation via c-Abl mediates the synaptic damage triggered by Aβ oligomers (5). Together, these studies point to EphA4 as a novel mediator of AD-associated synaptic dysfunction and support the notion that Aβ oligomer receptors might be viable therapeutic targets for this disease.

During the past 2 decades, the elucidation of susceptibility and causative genes for Alzheimer disease as well as proteins involved in the pathogenic process has greatly facilitated the development of genetically altered mouse models. These models have played a major role in defining critical disease-related mechanisms and in evaluating novel therapeutic approaches, with many treatments currently in clinical trial owing their origins to studies initially performed in mice. This review discusses the utility of transgenic mice as a research tool and their contributions to our understanding of Alzheimer disease.

Genetic Ablation of Apolipoprotein A-IV Accelerates Alzheimer's Disease Pathogenesis in a Mouse Model

Alzheimer’s disease (AD) is characterized by neuronal network dysfunction. Functional imaging studies showed that oscillatory activity is altered in AD patients. Interestingly, network dysfunction can be observed at an early stage of the disease: subjects with mild cognitive impairment, or with subjective cognitive decline, already show alterations in network activity. The relative ease with which network dysfunction can be detected, as well as its presence during early stages of the disease, make it a valuable starting point for the development of treatments. To further facilitate the development of network-based therapies we need a better understanding of the mechanisms underlying network dysfunction as well as how disease progression in mice translates to the different stages along the AD continuum in humans. Chapter 1 provides an introduction into the field of neuronal network impairments in AD patients and mouse models of AD. In chapter 2, we explored the mechanisms underlying network dysfunction in AD mice with a focus on early disease stages. As a recent study found that hippocampal parvalbumin (PV) interneurons, which play an important role in the generation of network oscillations, are hyperactive at an early disease stage, and that restoring the activity of these cells can mitigate cognitive symptoms in the APP/PS1 mouse model of AD, we investigated the mechanisms underlying PV hyperexcitability. Sequencing data indicated that altered expression of genes related to potassium ion transport might underlie early PV hyperexcitability. In chapter 3, we took a closer look at the network alterations taking place in APP/PS1 mice. Local field potential (LFP) recordings, performed in 3-12-month-old mice, showed that network alterations occurred already at an early age and persisted over time. To obtain a better understanding of how AD-related changes in network activity translate between mice and humans, we also performed MEG recordings in pre-symptomatic human subjects carrying similar mutations in the APP and PSEN1 genes. The very different and often opposite nature of network changes observed in mice and humans raises questions regarding the underlying mechanisms. Findings in mice should be interpreted with care, and additional experiments, for example in AD mouse models bearing both amyloid and tau pathology, are needed to further establish potential translational links between both species. In chapter 4, we explored network alterations in the context of AD-relevant behavior. AD mouse models show a deficit in nesting behavior, an essential behavior that is shared by many species. Even though the nesting test is widely used in the AD field, relatively little is known about how a deficit in nesting behavior relates to AD. A detailed behavioral characterization of nesting behavior in APP/PS1 and wildtype mice showed that, even though APP/PS1 mice were capable of performing the different behavioral components of nesting behavior, they frayed their nestlet with a delay compared to wildtype mice and showed delayed and reduced arranging (pulling in and fluffing) of nest material. These findings suggested that the deficit in nesting behavior in APP/PS1 mice might be related to reduced motivation to build a nest. In chapter 5, we aimed to design a behavioral task that would allow to measure cognitive function and record network alterations in parallel in a longitudinal manner. To this end, we developed an automated figure-8-maze to test spatial working memory. However, we did not find clear differences in memory performance between APP/PS1 and wildtype mice from 2 to 6 months of age. In chapter 6 we provide a systematic review of neuronal network changes in mouse models of AD. This review will help to provide direction for future studies to improve translatability of AD-related network changes between mice and humans.