Amyloid precursor protein overexpression depresses excitatory transmission through both presynaptic and postsynaptic mechanisms

Metformin Improves Spatial Memory and Reduces Seizure Severity in a Rat Model of Epilepsy and Alzheimer's Disease comorbidity via PI3K/Akt Signaling Pathway

Emerging evidence suggests a bidirectional relationship between Alzheimer's disease (AD) and epilepsy. In our previous studies, we identified a partial AD-like phenotype associated with central insulin resistance in the Wistar audiogenic rat (WAR), a genetic model of epilepsy. We also found that intracerebroventricular administration of streptozotocin, a compound used to model diabetes and AD, exacerbates seizure susceptibility. Given the role of insulin signaling in both AD and epilepsy, we hypothesized that metformin (MET), an anti-diabetic drug known for enhancing insulin sensitivity, could be a potential therapeutic agent for both conditions. Our objective was to investigate MET's effects on brain insulin signaling, seizure activity, and AD-like pathology in WARs. Adult male WARs received oral MET (250 mg/kg) for 21 days. Audiogenic seizures were assessed using the Categorized Severity Index and Racine's scale. Spatial memory was tested with the Morris water maze (MWM), followed by Western blot analysis of hippocampal proteins. MET significantly reduced seizure severity and improved MWM performance. Although MET did not affect insulin receptor levels or activation, it increased phosphoinositide 3-kinase (PI3K), activated Akt, and increased glycogen synthase kinase-3α/β (GSK-3α/β) levels. MET also decreased amyloid β precursor protein (AβPP) levels but did not affect Tau phosphorylation. These results suggest that chronic MET treatment alleviates behaviors related to both AD and epilepsy in WARs and modulates insulin signaling independently of insulin receptor activation. Our findings highlight MET's potential as a therapeutic agent for managing comorbid AD and epilepsy, warranting further investigation into its mechanisms of action.

Familial Alzheimer's disease mutations in amyloid precursor protein impair calcineurin signaling to NMDA receptors

Familial Alzheimer's disease (FAD) is frequently associated with mutations in the amyloid precursor protein (APP), which are thought to lead to cognitive deficits by impairing NMDA receptor (NMDAR)-dependent forms of synaptic plasticity. Given the reliance of synaptic plasticity on NMDAR-mediated Ca2+ entry, shaping of NMDAR activity by APP and/or its disease-causing variants could provide a basis for understanding synaptic plasticity impairments associated with FAD. A region of APP (residues 639-644 within APP695) processed by the γ-secretase complex, which generates amyloid-β peptides, is a hotspot for FAD mutations. This region bears similarity to a binding motif for calcineurin (CaN), a Ca2+/calmodulin-dependent phosphatase. Interaction assays confirm that APP associates with CaN in native tissue as well as in a heterologous expression system. This capacity to bind CaN extends to APP family members amyloid precursor-like protein 1 and amyloid precursor-like protein 2 (APLP1 and APLP2, respectively). Electrophysiological analysis demonstrates that APP and its family members limit NMDAR activity, in a manner consistent with CaN-dependent regulation of NMDAR desensitization. FAD mutations, in this region of APP, impair this regulation and consequently enhance NMDAR activity. Thus, by altering the landscape for CaN regulation of NMDA receptors, FAD mutations in APP may contribute to faulty information processing by modifying the dynamic range and temporal window of a critical signal for synaptic plasticity.

A novel peptide derived from vascular endothelial growth factor prevents amyloid beta aggregation and toxicity

Amyloid-β oligomers (Aβo) are the most pathologically relevant Aβ species in Alzheimer's disease (AD), because they induce early synaptic dysfunction that leads to learning and memory impairments. In contrast, increasing VEGF (Vascular Endothelial Growth Factor) brain levels have been shown to improve learning and memory processes, and to alleviate Aβ-mediated synapse dysfunction. Here, we designed a new peptide, the blocking peptide (BP), which is derived from an Aβo-targeted domain of the VEGF protein, and investigated its effect on Aβ-associated toxicity. Using a combination of biochemical, 3D and ultrastructural imaging, and electrophysiological approaches, we demonstrated that BP strongly interacts with Aβo and blocks Aβ fibrillar aggregation process, leading to the formation of Aβ amorphous aggregates. BP further impedes the formation of structured Aβo and prevents their pathogenic binding to synapses. Importantly, acute BP treatment successfully rescues long-term potentiation (LTP) in the APP/PS1 mouse model of AD, at an age when LTP is highly impaired in hippocampal slices. Moreover, BP is also able to block the interaction between Aβo and VEGF, which suggests a dual mechanism aimed at both trapping Aβo and releasing VEGF to alleviate Aβo-induced synaptic damage. Our findings provide evidence for a neutralizing effect of the BP on Aβ aggregation process and pathogenic action, highlighting a potential new therapeutic strategy.

Protective Effects of Cirsium japonicum var. maackii Flower on Amyloid Beta25-35-Treated C6 Glial Cells

Amyloid beta (Aβ) is a neurotoxic peptide and a key factor causing Alzheimer's disease. Cirsium japonicum var. maackii (CJM) has neuroprotective effects, but the protective effects of the flower from CJM (FCJM) on the neural system remain unclear. This study aimed to identify the fraction of FCJM with the highest neuroprotective potential and investigate its protective mechanisms against Aβ_25-35-induced inflammation in C6 glial cells. The cell viability and generation of reactive oxygen species (ROS) were measured to investigate the positive effect of FCJM on oxidative stress. Treatment with the FCJM extract or fractions increased the cell viability to 60-70% compared with 52% in the Aβ_25-35-treated control group and decreased ROS production to 84% compared with 100% in the control group. The ethyl acetate fraction of FCJM (EFCJM) was the most effective among all the extracts and fractions. We analyzed the protective mechanisms of EFCJM on Aβ_25-35-induced inflammation in C6 glial cells using Western blot. EFCJM downregulated amyloidogenic pathway-related proteins, such as Aβ precursor protein, β-secretase, presenilin 1, and presenilin 2. Moreover, EFCJM attenuated the Bax/Bcl-2 ratio, an index of apoptosis, and upregulated the oxidative stress-related protein, heme oxygenase-1. Therefore, this study demonstrated that FCJM improves cell viability and inhibits ROS in Aβ_25-35-treated C6 glial cells. Furthermore, EFCJM exhibits neuroprotective effects in Aβ_25-35-induced inflammation in C6 glial cells by modulating oxidative stress and amyloidogenic and apoptosis signaling pathways. FCJM, especially EFCJM, can be a promising agent for neurodegenerative disease prevention.

Physiology and pharmacology of amyloid precursor protein

Amyloid precursor protein (APP) is an evolutionarily conserved transmembrane protein and a well-characterized precursor protein of amyloid-beta (Aβ) peptides, which accumulate in the brains of individuals with Alzheimer's disease (AD)-related pathologies. Aβ has been extensively investigated since the amyloid hypothesis in AD was proposed. Besides Aβ, previous studies on APP and its proteolytic cleavage products have suggested their diverse pathological and physiological functions. However, their roles still have not been thoroughly understood. In this review, we extensively discuss the evolutionarily-conserved biology of APP, including its structure and processing pathway, as well as recent findings on the physiological roles of APP and its fragments in the central nervous system and peripheral nervous system. We have also elaborated upon the current status of APP-targeted therapeutic approaches for AD treatment by discussing inhibitors of several proteases participating in APP processing, including α-, β-, and γ-secretases. Finally, we have highlighted the future perspectives pertaining to further research and the potential clinical role of APP.

The Effects of Physical Running on Dendritic Spines and Amyloid-beta Pathology in 3xTg-AD Male Mice

Memory loss is the key symptom of Alzheimer's disease (AD). As successful drug treatments have not yet been identified, non-pharmaceutical interventions such as physical exercise and training have been employed to improve the memory function of people with dementia. We investigated the effect of prolonged physical running on hippocampal-dependent spatial memory and its underlying mechanisms using a well-established rodent model of AD. 3xTg-AD transgenic mice and non-transgenic mice were subjected to voluntary wheel running for 5 months (1 hour per day, 5 days per week), followed by spatial memory testing. After the behavioral testing, dendritic spines, synapses, and synaptic proteins as well as amyloid-beta (Aβ) pathology were analyzed in the dorsal hippocampi. Running improved hippocampal-dependent spatial memory in 3xTg-AD mice. This running strategy prevented both thin and mushroom-type spines on CA1 pyramidal cells in 3xTg-AD mice, whereas the effects of running in non-transgenic mice were limited to thin spines. The enormous effects of running on spines were accompanied by an increased number of synapses and upregulated expression of synaptic proteins. Notably, running downregulated the processing of amyloid precursor protein, decreasing intracellular APP expression and extracellular Aβ accumulation, and spatial memory performance correlated with levels of Aβ peptides Aβ_1-40 and Aβ_1-42. These data suggest that prolonged running may improve memory in preclinical AD via slowing down the amyloid pathology and preventing the loss of synaptic contacts.

Prolonged Volatile Anesthetic Exposure Exacerbates Cognitive Impairment and Neuropathology in the 5xFAD Mouse Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disease which shows a set of symptoms involving cognitive changes and psychological changes. Given that AD is the most common form of dementia in aging population and the increasing demand for anesthesia/surgery with aging, there has been significant interest in the exact impact of volatile anesthetics on cognitive function and pathological alterations in AD population.

OBJECTIVE

This study aimed to investigate behavioral changes and neuropathology in the 5xFAD mouse model of Alzheimer's disease with short-term exposure or long-term exposure to desflurane, sevoflurane, or isoflurane.

METHODS

In this study, we exposed 5xFAD mouse model of AD to isoflurane, sevoflurane, or desflurane in two different time periods (30 min and 6 h), and the memory related behaviors as well as the pathological changes in 5xFAD mice were evaluated 7 days after the anesthetic exposure.

RESULTS

We found that short-term exposure to volatile anesthetics did not affect hippocampus dependent memory and the amyloid-β (Aβ) deposition in the brain. However, long-term exposure to sevoflurane or isoflurane significantly increased the Aβ deposition in CA1 and CA3 regions of hippocampus, as well as the glial cell activation in amygdala. Besides, the PSD-95 expression was decreased in 5xFAD mice with exposure to sevoflurane or isoflurane and the caspase-3 activation was enhanced in isoflurane, sevoflurane, and desflurane groups.

CONCLUSION

Our results demonstrate the time-dependent effects of common volatile anesthetics and implicate that desflurane has the potential benefits to prolonged anesthetic exposure in AD patients.

Regulation of PPARα by APP in Alzheimer disease affects the pharmacological modulation of synaptic activity

Among genetic susceptibility loci associated with late-onset Alzheimer disease (LOAD), genetic polymorphisms identified in genes encoding lipid carriers led to the hypothesis that a disruption of lipid metabolism could promote disease progression. We previously reported that amyloid precursor protein (APP) involved in Alzheimer disease (AD) physiopathology impairs lipid synthesis needed for cortical networks' activity and that activation of peroxisome proliferator-activated receptor α (PPARα), a metabolic regulator involved in lipid metabolism, improves synaptic plasticity in an AD mouse model. These observations led us to investigate a possible correlation between PPARα function and full-length APP expression. Here, we report that PPARα expression and activation were inversely related to APP expression both in LOAD brains and in early-onset AD cases with a duplication of the APP gene, but not in control human brains. Moreover, human APP expression decreased PPARA expression and its related target genes in transgenic mice and in cultured cortical cells, while opposite results were observed in APP-silenced cortical networks. In cultured neurons, APP-mediated decrease or increase in synaptic activity was corrected by a PPARα-specific agonist and antagonist, respectively. APP-mediated control of synaptic activity was abolished following PPARα deficiency, indicating a key function of PPARα in this process.

Illuminating Neural Circuits in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and there is currently no cure. Neural circuit dysfunction is the fundamental mechanism underlying the learning and memory deficits in patients with AD. Therefore, it is important to understand the structural features and mechanisms underlying the deregulated circuits during AD progression, by which new tools for intervention can be developed. Here, we briefly summarize the most recently established cutting-edge experimental approaches and key techniques that enable neural circuit tracing and manipulation of their activity. We also discuss the advantages and limitations of these approaches. Finally, we review the applications of these techniques in the discovery of circuit mechanisms underlying β-amyloid and tau pathologies during AD progression, and as well as the strategies for targeted AD treatments.

The Implication of STEP in Synaptic Plasticity and Cognitive Impairments in Alzheimer's Disease and Other Neurological Disorders

STriatal-Enriched protein tyrosine Phosphatase (STEP) is a tyrosine phosphatase that has been implicated in Alzheimer’s disease (AD), the most common form of dementia, and many other neurological diseases. The protein level and activity of STEP have been found to be elevated in most of these disorders, and specifically in AD as a result of dysregulation of different pathways including PP2B/DARPP32/PP1, PKA as well as impairments of both proteasomal and lysosomal systems. The upregulation in STEP leads to increased binding to, and dephosphorylation of, its substrates which are mainly found to be synaptic plasticity and thus learning and memory related proteins. These proteins include kinases like Fyn, Pyk2, ERK1/2 and both NMDA and AMPA receptor subunits GluN2B and GluA2. The dephosphorylation of these molecules results in inactivation of these kinases and internalization of NMDA and AMPA receptor complexes leading to synapse loss and cognitive impairments. In this study, we aim to review STEP regulation and its implications in AD as well as other neurological disorders and then summarize data on targeting STEP as therapeutic strategy in these diseases.

VEGF counteracts amyloid-β-induced synaptic dysfunction

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

STEP inhibition prevents Aβ-mediated damage in dendritic complexity and spine density in Alzheimer's disease

Loss of dendritic spines and decline of cognitive function are hallmarks of patients with Alzheimer's disease (AD). Previous studies have shown that AD pathophysiology involves increased expression of a central nervous system-enriched protein tyrosine phosphatase called STEP (STriatal-Enriched protein tyrosine Phosphatase). STEP opposes the development of synaptic strengthening by dephosphorylating substrates, including GluN2B, Pyk2, and ERK1/2. Genetic reduction of STEP as well as pharmacological inhibition of STEP improve cognitive function and hippocampal memory in the 3×Tg-AD mouse model. Here, we show that the improved cognitive function is accompanied by an increase in synaptic connectivity in cell cultures as well as in the triple transgenic AD mouse model, further highlighting the potential of STEP inhibitors as a therapeutic agent.

Inhibition of Histone Methyltransferases EHMT1/2 Reverses Amyloid-β-Induced Loss of AMPAR Currents in Human Stem Cell-Derived Cortical Neurons

Emerging evidence suggests that epigenetic dysregulation of gene expression is one of the key molecular mechanisms of neurodegeneration and Alzheimer's disease (AD). However, little is known about the role of epigenetic dysregulation on synaptic dysfunction in humans, because of the difficulties of obtaining live human neurons. Here we generated mature human cortical neurons differentiated from human embryonic stem cells, and exposed them to amyloid-β (Aβ). We found that the histone methyltransferase, EHMT1, which catalyzes histone lysine 9 dimethylation (H3K9me2, a mark for gene repression), was significantly elevated in Aβ-treated human stem cell-derived neurons. Aβ treatment led to a significant reduction of AMPAR-mediated whole-cell current and excitatory postsynaptic current. Application of BIX01294, a selective inhibitor of EHMT1/2, restored AMPAR currents and glutamatergic synaptic transmission in Aβ-treated human cortical neurons. These results suggest that inhibition of the aberrant histone methylation is a novel approach to reverse Aβ-induced synaptic deficits in human neurons.

Acetylation of AMPA Receptors Regulates Receptor Trafficking and Rescues Memory Deficits in Alzheimer's Disease

In Alzheimer's disease (AD), decreases in the amount and synaptic localization of AMPA receptors (AMPARs) result in weakened synaptic activity and dysfunction in synaptic plasticity, leading to impairments in cognitive functions. We have previously found that AMPARs are subject to lysine acetylation, resulting in higher AMPAR stability and protein accumulation. Here we report that AMPAR acetylation was significantly reduced in AD and neurons with Aβ incubation. We identified p300 as the acetyltransferase responsible for AMPAR acetylation and found that enhancing GluA1 acetylation ameliorated Aβ-induced reductions in total and cell-surface AMPARs. Importantly, expression of acetylation mimetic GluA1 (GluA1-4KQ) in APP/PS1 mice rescued impairments in synaptic plasticity and memory. These findings indicate that Aβ-induced reduction in AMPAR acetylation and stability contributes to synaptopathy and memory deficiency in AD, suggesting that AMPAR acetylation may be an effective molecular target for AD therapeutics.

Novel huperzine A based NMDA antagonists: insights from molecular docking, ADME/T and molecular dynamics simulation studies

Huperzine A (HupA) is an alkaloidal natural product and drug isolated from Chinese herb Huperzia serrata, which is a potent selective anticholinesterase inhibitor. HupA has symptomatic, cognitive-enhancing and protective effect on neurons against amyloid beta-induced oxidative injury and antagonizing N-methyl-d-aspartate receptors by blocking the ion channels. The present study aimed to identify the docking, ADME/T and molecular dynamics simulation parameters of a library of 40 analogues which can correlate the binding affinity, conformational stability and selectivity of the ligands towards NMDA receptor through in silico approach. Glide molecular docking analysis was performed for the designed analogues to understand the binding mode and interactions. MD simulations were performed to explain the conformational stability and natural dynamics of the interaction in physiological environmental condition of protein-ligand complex affording a better understanding of chemical-scale interactions between HupA and its analogues with NMDA channel that could potentially benefit the development of new drugs for neurodegenerative diseases involving NMDA receptors.

Amyloid-beta1-42 induced glutamatergic receptor and transporter expression changes in the mouse hippocampus

Alzheimer's disease (AD) is the leading type of dementia worldwide. With an increasing burden of an aging population coupled with the lack of any foreseeable cure, AD warrants the current intense research effort on the toxic effects of an increased concentration of beta-amyloid (Aβ) in the brain. Glutamate is the main excitatory brain neurotransmitter and it plays an essential role in the function and health of neurons and neuronal excitability. While previous studies have shown alterations in expression of glutamatergic signaling components in AD, the underlying mechanisms of these changes are not well understood. This is the first comprehensive anatomical study to characterize the subregion- and cell layer-specific long-term effect of Aβ_1-42 on the expression of specific glutamate receptors and transporters in the mouse hippocampus, using immunohistochemistry with confocal microscopy. Outcomes are examined 30 days after Aβ_1-42 stereotactic injection in aged male C57BL/6 mice. We report significant decreases in density of the glutamate receptor subunit GluA1 and the vesicular glutamate transporter (VGluT) 1 in the conus ammonis 1 region of the hippocampus in the Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected and naïve controls, notably in the stratum oriens and stratum radiatum. GluA1 subunit density also decreased within the dentate gyrus dorsal stratum moleculare in Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected controls. These changes are consistent with findings previously reported in the human AD hippocampus. By contrast, glutamate receptor subunits GluA2, GluN1, GluN2A, and VGluT2 showed no changes in expression. These findings indicate that Aβ_1-42 induces brain region and layer specific expression changes of the glutamatergic receptors and transporters, suggesting complex and spatial vulnerability of this pathway during development of AD neuropathology. Read the Editorial Highlight for this article on page 7. Cover Image for this issue: https://doi.org/10.1111/jnc.14763.

Amyloid-β oligomers in cellular models of Alzheimer's disease

Amyloid-β (Aβ) dysmetabolism is tightly associated with pathological processes in Alzheimer's disease (AD). Currently, it is thought that, in addition to Aβ fibrils that give rise to plaque formation, Aβ aggregates into non-fibrillar soluble oligomers (AβOs). Soluble AβOs have been extensively studied for their synaptotoxic and neurotoxic properties. In this review, we discuss physicochemical properties of AβOs and their impact on different brain cell types in AD. Additionally, we summarize three decades of studies with AβOs, providing a compelling bulk of evidence regarding cell-specific mechanisms of toxicity. Cellular models may lead us to a deeper understanding of the detrimental effects of AβOs in neurons and glial cells, putatively shedding light on the development of innovative therapies for AD.

Presynaptic failure in Alzheimer's disease

Synaptic loss is the best correlate of cognitive deficits in Alzheimer's disease (AD). Extensive experimental evidence also indicates alterations of synaptic properties at the early stages of disease progression, before synapse loss and neuronal degeneration. A majority of studies in mouse models of AD have focused on post-synaptic mechanisms, including impairment of long-term plasticity, spine structure and glutamate receptor-mediated transmission. Here we review the literature indicating that the synaptic pathology in AD includes a strong presynaptic component. We describe the evidence indicating presynaptic physiological functions of the major molecular players in AD. These include the amyloid precursor protein (APP) and the two presenilin (PS) paralogs PS1 or PS2, genetically linked to the early-onset form of AD, in addition to tau which accumulates in a pathological form in the AD brain. Three main mechanisms participating in presynaptic functions are highlighted. APP fragments bind to presynaptic receptors (e.g. nAChRs and GABA_B receptors), presenilins control Ca²⁺ homeostasis and Ca²⁺-sensors, and tau regulates the localization of presynaptic molecules and synaptic vesicles. We then discuss how impairment of these presynaptic physiological functions can explain or forecast the hallmarks of synaptic impairment and associated dysfunction of neuronal circuits in AD. Beyond the physiological roles of the AD-related proteins, studies in AD brains also support preferential presynaptic alteration. This review features presynaptic failure as a strong component of pathological mechanisms in AD.

Intracellular Trafficking Mechanisms of Synaptic Dysfunction in Alzheimer's Disease

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

Modulation of the MAPKs pathways affects Aβ-induced cognitive deficits in Alzheimer's disease via activation of α7nAChR

Cognitive impairment in Alzheimer's disease (AD) is characterized by being deficient at learning and memory. Aβ_1-42 oligomers have been shown to impair rodent cognitive function. We previously demonstrated that activation of α7nAChR, inhibition of p38 or JNK could alleviate Aβ-induced memory deficits in Y maze test. In this study, we investigated whether the effects of α7nAChR and MAPKs on Y maze test is reproducible with a hippocampus-dependent spatial memory test such as Morris water maze. We also assessed the possible co-existence of hippocampus-independent recognition memory dysfunction using a novel object recognition test and an alternative and stress free hippocampus-dependent recognition memory test such as the novel place recognition. Besides, previous research from our lab has shown that MAPKs pathways regulate Aβ internalization through mediating α7nAChR. In our study, whether MAPKs pathways exert their functions in cognition by modulating α7nAChR through regulating glutamate receptors and synaptic protein, remain little known. Our results showed that activation of α7nAChR restored spatial memory, novel place recognition memory, and short-term and long-term memory in novel object recognition. Inhibition of p38 restored spatial memory and short-term and long-term memory in novel object recognition. Inhibition of ERK restored short-term memory in novel object recognition and novel place recognition memory. Inhibition of JNK restored spatial memory, short-term memory in novel object recognition and novel place recognition memory. Beside this, the activation of α7nAChR, inhibition of p38 or JNK restored Aβ-induced levels of NMDAR1, NMDAR2A, NMDAR2B, GluR1, GluR2 and PSD95 in Aβ-injected mice without influencing synapsin 1. In addition, these treatments also recovered the expression of acetylcholinesterase (AChE). Finally, we found that the inhibition of p38 or JNK resulted in the upregulation of α7nAChR mRNA levels in the hippocampus. Our results indicated that inhibition of p38 or JNK MAPKs could alleviate Aβ-induced spatial memory deficits through regulating activation of α7nAChR via recovering memory-related proteins. Moreover, p38, ERK and JNK MAPKs exert different functions in spatial and recognition memory.

Single amyloid-beta injection exacerbates 4-aminopyridine-induced seizures and changes synaptic coupling in the hippocampus

Accumulation of amyloid-beta (Aβ) in temporal lobe structures, including the hippocampus, is related to a variety of Alzheimer's disease symptoms and seems to be involved in the induction of neural network hyperexcitability and even seizures. Still, a direct evaluation of the pro-epileptogenic effects of Aβ in vivo, and of the underlying mechanisms, is missing. Thus, we tested whether the intracisternal injection of Aβ modulates 4-aminopyridine (4AP)-induced epileptiform activity, hippocampal network function, and its synaptic coupling. When tested 3 weeks after its administration, Aβ (but not its vehicle) reduces the latency for 4AP-induced seizures, increases the number of generalized seizures, exacerbates the time to fully recover from seizures, and favors seizure-induced death. These pro-epileptogenic effects of Aβ correlate with a reduction in the power of the spontaneous hippocampal network activity, involving all frequency bands in vivo and only the theta band (4-10 Hz) in vitro. The pro-epileptogenic effects of Aβ also correlate with a reduction of the Schaffer-collateral CA1 synaptic coupling in vitro, which is exacerbated by the sequential bath application of 4-AP and Aβ. In summary, Aβ produces long-lasting pro-epileptic effects that can be due to alterations in the hippocampal circuit, impacting its coordinated network activity and its synaptic efficiency. It is likely that normalizing synaptic coupling and/or coordinated neural network activity (i.e., theta activity) may contribute not only to improve cognitive function in Alzheimer's disease but also to avoid hyperexcitation in conditions of amyloidosis.

Endophilin 1 knockdown prevents synaptic dysfunction induced by oligomeric amyloid β

Amyloid β (Aβ) has been reported to have an important role in the cognitive deficits of Alzheimer's disease (AD), as oligomeric Aβ promotes synaptic dysfunction and triggers neuronal death. Recent evidence has associated an endocytosis protein, endophilin 1, with AD, as endophilin 1 levels have been reported to be markedly increased in the AD brain. The increase in endophilin 1 levels in neurons is associated with an increase in the activation of the stress kinase JNK, with subsequent neuronal death. In the present study, whole-cell patch-clamp recording demonstrated that oligomeric Aβ caused synaptic dysfunction and western blotting revealed that endophilin 1 was highly expressed prior to neuronal death of cultured hippocampal neurons. Furthermore, RNA interference and electrophysiological recording techniques in cultured hippocampal neurons demonstrated that knockdown of endophilin 1 prevented synaptic dysfunction induced by Aβ. Thus, a potential role for endophilin 1 in Aβ-induced postsynaptic dysfunction has been identified, indicating a possible direction for the prevention of postsynaptic dysfunction in cognitive impairment and suggesting that endophilin may be a potential target for the clinical treatment of AD.

Autaptic cultures of human induced neurons as a versatile platform for studying synaptic function and neuronal morphology

Recently developed technology to differentiate induced pluripotent stem cells (iPSCs) into human induced neurons (iNs) provides an exciting opportunity to study the function of human neurons. However, functional characterisations of iNs have been hampered by the reliance on mass culturing protocols which do not allow assessment of synaptic release characteristics and neuronal morphology at the individual cell level with quantitative precision. Here, we have developed for the first time a protocol to generate autaptic cultures of iPSC-derived iNs. We show that our method efficiently generates mature, autaptic iNs with robust spontaneous and action potential-driven synaptic transmission. The synaptic responses are sensitive to modulation by metabotropic receptor agonists as well as potentiation by acute phorbol ester application. Finally, we demonstrate loss of evoked and spontaneous release by Unc13A knockdown. This culture system provides a versatile platform allowing for quantitative and integrative assessment of morphophysiological and molecular parameters underlying human synaptic transmission.

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.

Mechanisms of AMPA Receptor Endosomal Sorting

The regulation of synaptic AMPA receptors (AMPARs) is critical for excitatory synaptic transmission, synaptic plasticity and the consequent formation of neural circuits during brain development and their modification during learning and memory processes. The number of synaptic AMPARs is regulated through endocytosis, exocytosis and endosomal sorting that results in recycling back to the plasma membrane or degradation in the lysosome. Hence, endo-lysosomal sorting is vitally important in maintaining AMPAR expression at the synapse, and the dynamic regulation of these trafficking events is a key component of synaptic plasticity. A reduction in synaptic strength such as in long-term depression (LTD) involves AMPAR sorting to lysosomes to reduce synaptic AMPAR number, whereas long-term potentiation (LTP) involves an increase in AMPAR recycling to increase the number of AMPARs at synapses. Here, we review our current understanding of the endosomal trafficking routes taken by AMPARs, and the mechanisms involved in AMPAR endosomal sorting, focussing on the numerous AMPAR associated proteins that have been implicated in this complex process. We also discuss how these events are dysregulated in brain disorders.

Neuronal mechanisms mediating pathological reward-related behaviors: A focus on silent synapses in the nucleus accumbens

The compulsive drive to seek drugs despite negative consequences relies heavily on drug-induced alterations that occur within the reward neurocircuit. These alterations include changes in neuromodulator and neurotransmitter systems that ultimately lock behaviors into an inflexible and permanent state. To provide clinicians with improved treatment options, researchers are trying to identify, as potential targets of therapeutic intervention, the neural mechanisms mediating an "addictive-like state". Here, we discuss how drug-induced generation of silent synapses in the nucleus accumbens may be a potential therapeutic target capable of reversing drug-related behaviors.

Amyloid fibrils induce dysfunction of hippocampal glutamatergic silent synapses

Silent glutamatergic synapses lacking functional AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate) receptors exist in several brain regions including the hippocampus. Their involvement in the dysfunction of hippocampal glutamatergic transmission in the setting of Alzheimer's disease (AD) is unknown. This study demonstrated a decrease in the percentage of silent synapses in rats microinjected with amyloid fibrils (Aβ1-40 ) into the hippocampal CA1. Also, pairing low-frequency electric stimuli failed to induce activation of the hippocampal silent synapses in the modeled rats. Immunoblotting studies revealed a decreased expression of GluR1 subunits in the hippocampal CA1 synaptosomal preparation, indicating a potential reduction in the GluR1 subunits anchoring in postsynaptic density in the modeled rats. We also noted a decreased expression of phosphorylated cofilin, which regulates the function of actin cytoskeleton and receptor trafficking, and reduced expression of the scaffolding protein PSD95 in the hippocampal CA1 synaptosome in rats injected with Aβ1-40 . Taken together, this study illustrates dysfunction of hippocampal silent synapse in the rodent model of AD, which might result from the impairments of actin cytoskeleton and postsynaptic scaffolding proteins induced by amyloid fibrils.

Alleviation of Hippocampal Endoplasmic Reticulum Stress by Allomyrina dichotoma Larvae Extract

In the brain, endoplasmic reticulum (ER) stress results in synaptic dysfunction and eventually leads to neurodegeneration. Allomyrina dichotoma larvae are a Chinese ethnomedicine and are widely used in East Asia. In the present study, we investigated the ability of ethanol extract of A. dichotoma larvae (ADE) to improve synaptic structure and function by activating unfolded protein response (UPR) under ER stress in animal and neuron culture models. ER stress was induced in obese mice fed a high fat diet (HFD) or by treating dissociated cultures of rat embryonic (E19) hippocampal neurons with tunicamycin (TM). Western blot and real-time or conventional RT-PCR were performed to analyze the expressions of ER stress marker proteins. In dissociated hippocampal cultures, immunocytochemistry was performed for synaptic proteins, and cultures were stained with styryl dye FM1-43 to assess presynaptic activities. In HFD-fed obese mice, ADE efficiently reduced the expressions of ER stress markers, such as, xbp-1, chop, atf4, erdi4, and eIf2a, and those of the ER chaperone/foldases Bip/grp78, Ero-1l, and PDI. Unconventionally spliced xbp-1s mRNA was not detected. In primary rat hippocampal cultures under ER stress, ADE significantly lowered the nuclear expression of CHOP, inhibited the downregulations of postsynaptic proteins, such as, GluN2A, GluN2B, and PSD-95, and maintained the pool size of recycling presynaptic vesicles. The study shows that ADE potently suppressed the induction of ER stress and maintained the structure and function of hippocampal neurons, and suggests that ADE is a potentially valuable food supplement and preventive therapeutic for ER stress-related nervous disorders.

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.

Endophilin2 Interacts with GluA1 to Mediate AMPA Receptor Endocytosis Induced by Oligomeric Amyloid-β

Amyloid-β (Aβ) plays an important role in Alzheimer's disease (AD), as oligomeric Aβ induces loss of postsynaptic AMPA receptors (AMPARs) leading to cognitive deficits. The loss of postsynaptic AMPARs is mediated through the clathrin-dependent endocytosis pathway, in which endophilin2 is one of the important regulatory proteins. Endophilin2, which is enriched in both the pre- and postsynaptic membrane, has previously been reported to be important for recycling of synaptic vesicles at the presynaptic membrane. However, the role of endophilin2 in oligomeric Aβ-induced postsynaptic AMPAR endocytosis is not well understood. In this study, we show that endophilin2 does not affect constitutive AMPAR endocytosis. Endophilin2 knockdown, but not overexpression, resisted oligomeric Aβ-induced AMPAR dysfunction. Moreover, endophilin2 colocalized and interacted with GluA1, a subunit of AMPAR, to regulate oligomeric Aβ-induced AMPAR endocytosis. Thus, we have determined a role of endophilin2 in oligomeric Aβ-induced postsynaptic AMPAR dysfunction, indicating possible directions for preventing the loss of AMPARs in cognitive impairment and providing evidence for the clinical treatment of AD.

Overexpression of Swedish mutant APP in aged astrocytes attenuates excitatory synaptic transmission

Amyloid precursor protein (APP), a type I transmembrane protein, has different aspects, namely, performs essential physiological functions and produces β‐amyloid peptide (Aβ). Overexpression of neuronal APP is responsible for synaptic dysfunction. In the central nervous system, astrocytes – a major glial cell type – have an important role in the regulation of synaptic transmission. Although APP is expressed in astrocytes, it remains unclear whether astrocytic overexpression of mutant APP affects synaptic transmission. In this study, the effect of astrocytic overexpression of a mutant APP on the excitatory synaptic transmission was investigated using coculture system of the transgenic (Tg) cortical astrocytes that express the human APP695 polypeptide with the double mutation K670N + M671L found in a large Swedish family with early onset Alzheimer's disease, and wild‐type hippocampal neuron. Significant secretion of Aβ 1–40 and 1–42 was observed in cultured cortical astrocytes from the Tg2576 transgenic mouse that genetically overexpresses Swedish mutant APP. Under the condition, Tg astrocytes did not affect excitatory synaptic transmission of cocultured wild‐type neurons. However, aged Tg astrocytes cultured for 9 weeks elicited a significant decrease in excitatory synaptic transmission in cocultured neurons. Moreover, a reduction in the number of readily releasable synaptic vesicles accompanied a decrease in the number of excitatory synapses in neurons cocultured with aged Tg astrocytes. These observations indicate that astrocytic expression of the mutant APP is involved in the downregulation of synaptic transmission with age.

Amyloid Plaque-Associated Oxidative Degradation of Uniformly Radiolabeled Arachidonic Acid

Oxidative stress is a frequently observed feature of Alzheimer's disease, but its pathological significance is not understood. To explore the relationship between oxidative stress and amyloid plaques, uniformly radiolabeled arachidonate was introduced into transgenic mouse models of Alzheimer's disease via intracerebroventricular injection. Uniform labeling with carbon-14 is used here for the first time, and made possible meaningful quantification of arachidonate oxidative degradation products. The injected arachidonate entered a fatty acid pool that was subject to oxidative degradation in both transgenic and wild-type animals. However, the extent of its degradation was markedly greater in the hippocampus of transgenic animals where amyloid plaques were abundant. In human Alzheimer's brain, plaque-associated proteins were post-translationally modified by hydroxynonenal, a well-known oxidative degradation product of arachidonate. These results suggest that several recurring themes in Alzheimer's pathogenesis, amyloid β proteins, transition metal ions, oxidative stress, and apolipoprotein isoforms, may be involved in a common mechanism that has the potential to explain both neuronal loss and fibril formation in this disease.

Amyloid-β Impairs Synaptic Inhibition via GABA(A) Receptor Endocytosis

Amyloid β (Aβ) is thought to play an important role in the pathogenesis of Alzheimer's disease. Aβ may exert its neurotoxic effects via multiple mechanisms and in particular through degradation of excitatory synaptic transmission associated with impaired synaptic plasticity. In contrast, much less is known about Aβ effects at inhibitory synapses. This study investigates the impact of acute Aβ1-42 application on GABAergic synaptic transmission in rat somatosensory cortex in vitro. Whole-cell voltage-clamp recordings were obtained from layer V pyramidal cells, and monosynaptic GABA(A) receptor-mediated IPSCs were elicited. Bath-applied Aβ (1 μm) depressed the IPSCs on average to 60% of control, whereas a reversed sequence control peptide was ineffective. Paired-pulse stimuli indicated a postsynaptic site of action. This was further corroborated by a decreased postsynaptic responsiveness to local puffs of the GABAA receptor agonist isoguvacine. The Aβ-induced IPSC decline could be prevented with intracellular applications of p4, a blocker of GABA(A) receptor internalization. It is concluded that Aβ weakens synaptic inhibition via downregulation of GABA(A) receptors.

Osmotin attenuates amyloid beta-induced memory impairment, tau phosphorylation and neurodegeneration in the mouse hippocampus

The pathological hallmarks of Alzheimer's disease (AD) include amyloid beta (Aβ) accumulation, neurofibrillary tangle formation, synaptic dysfunction and neuronal loss. In this study, we investigated the neuroprotection of novel osmotin, a plant protein extracted from Nicotiana tabacum that has been considered to be a homolog of mammalian adiponectin. Here, we observed that treatment with osmotin (15 μg/g, intraperitoneally, 4 hr) at 3 and 40 days post-intracerebroventricular injection of Aβ1-42 significantly ameliorated Aβ1-42-induced memory impairment in mice. These results revealed that osmotin reverses Aβ1-42 injection-induced synaptic deficits, Aβ accumulation and BACE-1 expression. Treatment with osmotin also alleviated the Aβ1-42-induced hyperphosphorylation of the tau protein at serine 413 through the regulation of the aberrant phosphorylation of p-PI3K, p-Akt (serine 473) and p-GSK3β (serine 9). Moreover, our western blots and immunohistochemical results indicated that osmotin prevented Aβ1-42-induced apoptosis and neurodegeneration in the Aβ1-42-treated mice. Furthermore, osmotin attenuated Aβ1-42-induced neurotoxicity in vitro.To our knowledge, this study is the first to investigate the neuroprotective effect of a novel osmotin against Aβ1-42-induced neurotoxicity. Our results demonstrated that this ubiquitous plant protein could potentially serve as a novel, promising, and accessible neuroprotective agent against progressive neurodegenerative diseases such as AD.

Oxidative and nitrative stress in neurodegeneration

Aerobes require oxygen for metabolism and normal free radical formation. As a result, maintaining the redox homeostasis is essential for brain cell survival due to their high metabolic energy requirement to sustain electrochemical gradients, neurotransmitter release, and membrane lipid stability. Further, brain antioxidant levels are limited compared to other organs and less able to compensate for reactive oxygen and nitrogen species (ROS/RNS) generation which contribute oxidative/nitrative stress (OS/NS). Antioxidant treatments such as vitamin E, minocycline, and resveratrol mediate neuroprotection by prolonging the incidence of or reversing OS and NS conditions. Redox imbalance occurs when the antioxidant capacity is overwhelmed, consequently leading to activation of alternate pathways that remain quiescent under normal conditions. If OS/NS fails to lead to adaptation, tissue damage and injury ensue, resulting in cell death and/or disease. The progression of OS/NS-mediated neurodegeneration along with contributions from microglial activation, dopamine metabolism, and diabetes comprise a detailed interconnected pathway. This review proposes a significant role for OS/NS and more specifically, lipid peroxidation (LPO) and other lipid modifications, by triggering microglial activation to elicit a neuroinflammatory state potentiated by diabetes or abnormal dopamine metabolism. Subsequently, sustained stress in the neuroinflammatory state overwhelms cellular defenses and prompts neurotoxicity resulting in the onset or amplification of brain damage.

Alzheimer's disease-related amyloid-β induces synaptotoxicity in human iPS cell-derived neurons

Human induced pluripotent stem cell (iPSC)-derived neurons have been proposed to be a highly valuable cellular model for studying the pathomechanisms of Alzheimer's disease (AD). Studies employing patient-specific human iPSCs as models of familial and sporadic forms of AD described elevated levels of AD-related amyloid-β (Aβ). However, none of the present AD iPSC studies could recapitulate the synaptotoxic actions of Aβ, which are crucial early events in a cascade that eventually leads to vast brain degeneration. Here we established highly reproducible, human iPSC-derived cortical cultures as a cellular model to study the synaptotoxic effects of Aβ. We developed a highly efficient immunopurification procedure yielding immature neurons that express markers of deep layer cortical pyramidal neurons and GABAergic interneurons. Upon long-term cultivation, purified cells differentiated into mature neurons exhibiting the generation of action potentials and excitatory glutamatergic and inhibitory GABAergic synapses. Most interestingly, these iPSC-derived human neurons were strongly susceptible to the synaptotoxic actions of Aβ. Application of Aβ for 8 days led to a reduction in the overall FM4–64 and vGlut1 staining of vesicles in neurites, indicating a loss of vesicle clusters. A selective analysis of presynaptic vesicle clusters on dendrites did not reveal a significant change, thus suggesting that Aβ impaired axonal vesicle clusters. In addition, electrophysiological patch-clamp recordings of AMPA receptor-mediated miniature EPSCs revealed an Aβ-induced reduction in amplitudes, indicating an impairment of postsynaptic AMPA receptors. A loss of postsynaptic AMPA receptor clusters was confirmed by immunocytochemical stainings for GluA1. Incubation with Aβ for 8 days did not result in a significant loss of neurites or cell death. In summary, we describe a highly reproducible cellular AD model based on human iPSC-derived cortical neurons that enables the mechanistic analysis of Aβ-induced synaptic pathomechanisms and the development of novel therapeutic approaches.

Intracellular accumulation of amyloid-β (Aβ) protein plays a major role in Aβ-induced alterations of glutamatergic synaptic transmission and plasticity

Intracellular accumulation of amyloid-β (Aβ) protein has been proposed as an early event in AD pathogenesis. In patients with mild cognitive impairment, intraneuronal Aβ immunoreactivity was found especially in brain regions critically involved in the cognitive deficits of AD. Although a large body of evidence demonstrates that Aβ42 accumulates intraneuronally ((in)Aβ), the action and the role of Aβ42 buildup on synaptic function have been poorly investigated. Here, we demonstrate that basal synaptic transmission and LTP were markedly depressed following Aβ42 injection into the neuron through the patch pipette. Control experiments performed with the reverse peptide (Aβ42-1) allowed us to exclude that the effects of (in)Aβ depended on changes in oncotic pressure. To further investigate (in)Aβ synaptotoxicity we used an Aβ variant harboring oxidized methionine in position 35 that does not cross the neuronal plasma membrane and is not uploaded from the extracellular space. This Aβ42 variant had no effects on synaptic transmission and plasticity when applied extracellularly, but induced synaptic depression and LTP inhibition after patch-pipette dialysis. Finally, the injection of an antibody raised against human Aβ42 (6E10) in CA1 pyramidal neurons of mouse hippocampal brain slices and autaptic microcultures did not, per se, significantly affect LTP and basal synaptic transmission, but it protected against the toxic effects of extracellular Aβ42. Collectively, these findings suggest that Aβ42-induced impairment of glutamatergic synaptic function depends on its internalization and intracellular accumulation thus paving the way to a systemic proteomic analysis of intracellular targets/partners of Aβ42.

Rescue of impaired late-phase long-term depression in a tau transgenic mouse model

Cognitive decline, the hallmark of Alzheimer's disease, and accompanying neuropsychiatric symptoms share dysfunctions of synaptic processes as a common cellular pathomechanism. Long-term potentiation has proven to be a sensitive tool for the "diagnosis" of such synaptic dysfunctions. Much less, however, is known about how long-term depression (LTD), an alternative mechanism for the storage of memory, is affected by Alzheimer's disease progression. Here, we demonstrate that impaired late LTD (>3 hours) in THY-Tau22 mice can be rescued by either inhibition of glycogen synthase kinase-3 (GSK3β) activity or by application of the protein-phosphatase 2A agonist selenate. In line with these findings, we observed increased phosphorylation of GSK3β at Y216 and reduced total phosphatase activity in biochemical assays of hippocampal tissue of THY-Tau22 mice. Interestingly, LTD induction and pharmacologic inhibition of GSK3β appeared to downregulate GSK3ß activity via a marked upregulation of phosphorylation at the inhibitory Ser9 residue. Our results point to alterations in phosphorylation and/or dephosphorylation homeostasis as key mechanisms underlying the deficits in LTD and hippocampus-dependent learning found in THY-Tau22 mice.

Mitochondria-Targeted Antioxidant SS31 Prevents Amyloid Beta-Induced Mitochondrial Abnormalities and Synaptic Degeneration in Alzheimer's Disease

In neuronal systems, the health and activity of mitochondria and synapses are tightly coupled. For this reason, it has been postulated that mitochondrial abnormalities may, at least in part, drive neurodegeneration in conditions such as Alzheimer’s disease (AD). Mounting evidence from multiple Alzheimer’s disease cell and mouse models and postmortem brains suggest that loss of mitochondrial integrity may be a key factor that mediates synaptic loss. Therefore, the prevention or rescue of mitochondrial dysfunction may help delay or altogether prevent AD-associated neurodegeneration. Since mitochondrial health is heavily dependent on antioxidant defenses, researchers have begun to explore the use of mitochondria-targeted antioxidants as therapeutic tools to prevent neurodegenerative diseases. This review will highlight advances made using a model mitochondria-targeted antioxidant peptide, SS31, as a potential treatment for AD.

AMPA receptor activation promotes non-amyloidogenic amyloid precursor protein processing and suppresses neuronal amyloid-β production

Soluble oligomeric amyloid β peptide (Aβ) generated from processing of the amyloid precursor protein (APP) plays a central role in the pathogenesis of Alzheimer's Disease (AD) and through actions at glutamatergic synapses affects excitability and plasticity. The physiological control of APP processing is not fully understood but stimulation of synaptic NMDA receptors (NMDAR) can suppress Aβ levels through an ERK-dependent increase in α-secretase activity. AMPA-type glutamate receptors (AMPAR) couple to ERK phosphorylation independently of NMDAR activation raising the possibility that stimulation of AMPAR might similarly promote non-amyloidogenic APP processing. We have tested this hypothesis by investigating whether AMPAR directly regulate APP processing in cultured mouse cortical neurons, by analyzing APP C-terminal fragments (CTFs), soluble APP (sAPP), Aβ levels, and cleavage of an APP-GAL4 reporter protein. We report that direct stimulation of AMPAR increases non-amyloidogenic α-secretase-mediated APP processing and inhibits Aβ production. Processing was blocked by the matrix metalloproteinase inhibitor TAPI-1 but was only partially dependent on Ca²⁺ influx and ERK activity. AMPAR can therefore, be added to the repertoire of receptors that couple to non-amyloidogenic APP processing at glutamatergic synapses and thus pharmacological targeting of AMPAR could potentially influence the development and progression of Aβ pathology in AD.

Dysregulation of synaptic and extrasynaptic N-methyl-D-aspartate receptors induced by amyloid-β

The toxicity of amyloid-beta (Aβ) is strongly associated with Alzheimer's disease (AD), which has a high incidence in the elderly worldwide. Recent evidence showed that alteration in the activity of N-methyl-D-aspartate receptors (NMDARs) plays a key role in Aβ-induced neurotoxicity. However, the activation of synaptic and extrasynaptic NMDARs has distinct consequences for plasticity, gene regulation, neuronal death, and Aβ production. This review focuses on the dysregulation of synaptic and extrasynaptic NMDARs induced by Aβ. On one hand, Aβ downregulates the synaptic NMDAR response by promoting NMDAR endocytosis, leading to either neurotoxicity or neuroprotection. On the other hand, Aβ enhances the activation of extrasynaptic NMDARs by decreasing neuronal glutamate uptake and inducing glutamate spillover, subsequently causing neurotoxicity. In addition, selective enhancement of synaptic activity by low doses of NMDA, or reduction of extrasynaptic activity by memantine, a non-competitive NMDAR antagonist, halts Aβ-induced neurotoxicity. Therefore, future neuroprotective drugs for AD should aim at both the enhancement of synaptic activity and the disruption of extrasynaptic NMDAR-dependent death signaling.

Early enriched environment exposure protects spatial memory and accelerates amyloid plaque formation in APP(Swe)/PS1(L166P) mice

Enriched environment exposure improves several aspects of cognitive performance in Alzheimer’s disease patients and in animal models and, although the role of amyloid plaques is questionable, several studies also assessed their response to enriched environment, with contrasting results. Here we report that rearing APP^Swe/PS1^L166P mice in an enriched environment since birth rescued the spatial memory impairment otherwise present at 6 months of age. At the same time, the exposure to the enriched environment caused a transient acceleration of plaque formation, while there was no effect on intracellular staining with the 6E10 antibody, which recognizes β-amyloid, full length amyloid precursor protein and its C-terminal fragments. The anticipation of plaque formation required exposure during early development, suggesting an action within critical periods for circuits formation. On the other hand, chronic neuronal activity suppression by tetrodotoxin decreased the number of plaques without affecting intracellular amyloid. These results indicate that enriched environment exposure since early life has a protective effect on cognitive deterioration although transiently accelerates amyloid deposition. In addition, the effects of the enriched environment might be due to increased neuronal activity, because plaques were reduced by suppression of electrical signaling by tetrodotoxin.

Aβ induces acute depression of excitatory glutamatergic synaptic transmission through distinct phosphatase-dependent mechanisms in rat CA1 pyramidal neurons

Beta-amyloid peptide (Aβ) has a causal role in the pathophysiology of Alzheimer's disease (AD). Recent studies indicate that Aβ can disrupt excitatory glutamatergic synaptic function at synaptic level. However, the underlying mechanisms remain obscure. In this study, we recorded evoked and spontaneous EPSCs in hippocampal CA1 pyramidal neurons via whole-cell voltage-clamping methods and found that 1 μM Aβ can induce acute depression of basal glutamatergic synaptic transmission through both presynaptic and postsynaptic dysfunction. Moreover, we also found that Aβ-induced both presynaptic and postsynaptic dysfunction can be reversed by the inhibitor of protein phosphatase 2B (PP2B), FK506, whereas only postsynaptic disruption can be ameliorated by the inhibitor of PP1/PP2A, Okadaic acid (OA). These results indicate that PP1/PP2A and PP2B have overlapping but not identical functions in Aβ-induced acute depression of excitatory glutamatergic synaptic transmission of hippocampal CA1 pyramidal neurons.

Metabotropic NMDA receptor function is required for β-amyloid-induced synaptic depression

The mechanisms by which β-amyloid (Aβ), a peptide fragment believed to contribute to Alzheimer's disease, leads to synaptic deficits are not known. Here we find that elevated oligomeric Aβ requires ion flux-independent function of NMDA receptors (NMDARs) to produce synaptic depression. Aβ activates this metabotropic NMDAR function on GluN2B-containing NMDARs but not on those containing GluN2A. Furthermore, oligomeric Aβ leads to a selective loss of synaptic GluN2B responses, effecting a switch in subunit composition from GluN2B to GluN2A, a process normally observed during development. Our results suggest that conformational changes of the NMDAR, and not ion flow through its channel, are required for Aβ to produce synaptic depression and a switch in NMDAR composition. This Aβ-induced signaling mediated by alterations in GluN2B conformation may be a target for therapeutic intervention of Alzheimer's disease.

Glutamate system, amyloid ß peptides and tau protein: functional interrelationships and relevance to Alzheimer disease pathology

Alzheimer disease is the most prevalent form of dementia globally and is characterized premortem by a gradual memory loss and deterioration of higher cognitive functions and postmortem by neuritic plaques containing amyloid ß peptide and neurofibrillary tangles containing phospho-tau protein. Glutamate is the most abundant neurotransmitter in the brain and is essential to memory formation through processes such as long-term potentiation and so might be pivotal to Alzheimer disease progression. This review discusses how the glutamatergic system is impaired in Alzheimer disease and how interactions of amyloid ß and glutamate influence synaptic function, tau phosphorylation and neurodegeneration. Interestingly, glutamate not only influences amyloid ß production, but also amyloid ß can alter the levels of glutamate at the synapse, indicating that small changes in the concentrations of both molecules could influence Alzheimer disease progression. Finally, we describe how the glutamate receptor antagonist, memantine, has been used in the treatment of individuals with Alzheimer disease and discuss its effectiveness.

Differential requirement for protein synthesis in presynaptic unmuting and muting in hippocampal glutamate terminals

Synaptic function and plasticity are crucial for information processing within the nervous system. In glutamatergic hippocampal neurons, presynaptic function is silenced, or muted, after strong or prolonged depolarization. This muting is neuroprotective, but the underlying mechanisms responsible for muting and its reversal, unmuting, remain to be clarified. Using cultured rat hippocampal neurons, we found that muting induction did not require protein synthesis; however, slow forms of unmuting that depend on protein kinase A (PKA), including reversal of depolarization-induced muting and forskolin-induced unmuting of basally mute synapses, required protein synthesis. In contrast, fast unmuting of basally mute synapses by phorbol esters was protein synthesis-independent. Further studies of recovery from depolarization-induced muting revealed that protein levels of Rim1 and Munc13-1, which mediate vesicle priming, correlated with the functional status of presynaptic terminals. Additionally, this form of unmuting was prevented by both transcription and translation inhibitors, so proteins are likely synthesized de novo after removal of depolarization. Phosphorylated cyclic adenosine monophosphate response element-binding protein (pCREB), a nuclear transcription factor, was elevated after recovery from depolarization-induced muting, consistent with a model in which PKA-dependent mechanisms, possibly including pCREB-activated transcription, mediate slow unmuting. In summary, we found that protein synthesis was required for slower, PKA-dependent unmuting of presynaptic terminals, but it was not required for muting or a fast form of unmuting. These results clarify some of the molecular mechanisms responsible for synaptic plasticity in hippocampal neurons and emphasize the multiple mechanisms by which presynaptic function is modulated.

Monoacylglycerol lipase is a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia among older people. There are no effective medications currently available to prevent and treat AD and halt disease progression. Monoacylglycerol lipase (MAGL) is the primary enzyme metabolizing the endocannabinoid 2-arachidonoylglycerol in the brain. We show here that inactivation of MAGL robustly suppressed production and accumulation of β-amyloid (Aβ) associated with reduced expression of β-site amyloid precursor protein cleaving enzyme 1 (BACE1) in a mouse model of AD. MAGL inhibition also prevented neuroinflammation, decreased neurodegeneration, maintained integrity of hippocampal synaptic structure and function, and improved long-term synaptic plasticity, spatial learning, and memory in AD animals. Although the molecular mechanisms underlying the beneficial effects produced by MAGL inhibition remain to be determined, our results suggest that MAGL, which regulates endocannabinoid and prostaglandin signaling, contributes to pathogenesis and neuropathology of AD, and thus is a promising therapeutic target for the prevention and treatment of AD.

Long-term culture of astrocytes attenuates the readily releasable pool of synaptic vesicles

The astrocyte is a major glial cell type of the brain, and plays key roles in the formation, maturation, stabilization and elimination of synapses. Thus, changes in astrocyte condition and age can influence information processing at synapses. However, whether and how aging astrocytes affect synaptic function and maturation have not yet been thoroughly investigated. Here, we show the effects of prolonged culture on the ability of astrocytes to induce synapse formation and to modify synaptic transmission, using cultured autaptic neurons. By 9 weeks in culture, astrocytes derived from the mouse cerebral cortex demonstrated increases in β-galactosidase activity and glial fibrillary acidic protein (GFAP) expression, both of which are characteristic of aging and glial activation in vitro. Autaptic hippocampal neurons plated on these aging astrocytes showed a smaller amount of evoked release of the excitatory neurotransmitter glutamate, and a lower frequency of miniature release of glutamate, both of which were attributable to a reduction in the pool of readily releasable synaptic vesicles. Other features of synaptogenesis and synaptic transmission were retained, for example the ability to induce structural synapses, the presynaptic release probability, the fraction of functional presynaptic nerve terminals, and the ability to recruit functional AMPA and NMDA glutamate receptors to synapses. Thus the presence of aging astrocytes affects the efficiency of synaptic transmission. Given that the pool of readily releasable vesicles is also small at immature synapses, our results are consistent with astrocytic aging leading to retarded synapse maturation.

Effects of different amyloid β-protein analogues on synaptic function

Perisynaptic accumulations of amyloid β-protein (Aβ) play a critical role in the synaptic dysfunction underlying the cognitive impairment observed in Alzheimer's disease. The methionine residue at position 35 (Met35) in Aβ is highly subject to oxidation in Alzheimer's disease brains. In hippocampal brain slices we found that long-term potentiation at CA3-CA1 synapses was significantly inhibited by wild type Aβ42 in which Met35 is reduced, but not by Aβ42 harboring Met35 sulfoxide. Similar differences were observed when basal synaptic transmission was investigated in autaptic hippocampal neurons. The significant decreases in excitatory postsynaptic current amplitude, vesicle release probability and miniature excitatory postsynaptic current frequency caused by 20-minute exposure to wild type Aβ42 were not observed after exposure to Aβ42 harboring Met35 sulfoxide. With longer (24-hour) Aβ treatments, this early impairment of the presynaptic terminal function extended to involve the postsynaptic side as well. The Met35 oxidation also affected Aβ42 negative impact on dendritic spine density and expression of pre- and postsynaptic proteins (synaptophysin and postsynaptic density protein-95). Our findings suggest that oxidation of Met35 is critical for molecular, structural, and functional determinants of Aβ42 synaptotoxicity.