GluN2B subunit-containing NMDA receptor antagonists prevent Abeta-mediated synaptic plasticity disruption in vivo

Soluble tau aggregates inhibit synaptic long-term depression and amyloid β-facilitated LTD in vivo

Soluble synaptotoxic aggregates of the main pathological proteins of Alzheimer's disease, amyloid β-protein (Aß) and tau, have rapid and potent inhibitory effects on long-term potentiation (LTP). Although the promotion of synaptic weakening mechanisms, including long-term depression (LTD), is posited to mediate LTP inhibition by Aß, little is known regarding the action of exogenous tau on LTD. The present study examined the ability of different assemblies of full-length human tau to affect LTD in the dorsal hippocampus of the anaesthetized rat. Unlike Aß, intracerebroventricular injection of soluble aggregates of tau (SτAs), but not monomers or fibrils, potently increased the threshold for LTD induction in a manner that required cellular prion protein. However, MTEP, an antagonist of the putative prion protein coreceptor metabotropic glutamate receptor 5, did not prevent the disruption of synaptic plasticity by SτAs. In contrast, systemic treatment with Ro 25-6981, a selective antagonist at GluN2B subunit-containing NMDA receptors, reduced SτA-mediated inhibition of LTD, but not LTP. Intriguingly, SτAs completely blocked Aß-facilitated LTD, whereas a subthreshold dose of SτAs facilitated Aß-mediated inhibition of LTP. Overall, these findings support the importance of cellular prion protein in mediating a range of, sometimes opposing, actions of soluble Aß and tau aggregates with different effector mechanisms on synaptic plasticity.

Hippocampal Mossy Fibers Synapses in CA3 Pyramidal Cells Are Altered at an Early Stage in a Mouse Model of Alzheimer's Disease

Early Alzheimer's disease (AD) affects the brain non-uniformly, causing hippocampal memory deficits long before wide-spread brain degeneration becomes evident. Here we addressed whether mossy fiber inputs from the dentate gyrus onto CA3 principal cells are affected in an AD mouse model before amyloid β plaque deposition. We recorded from CA3 pyramidal cells in a slice preparation from 6-month-old male APP/PS1 mice, and studied synaptic properties and intrinsic excitability. In parallel we performed a morphometric analysis of mossy fiber synapses following viral based labeling and 3D-reconstruction. We found that the basal structural and functional properties as well as presynaptic short-term plasticity at mossy fiber synapses are unaltered at 6 months in APP/PS1 mice. However, transient potentiation of synaptic transmission mediated by activity-dependent release of lipids was abolished. Whereas the presynaptic form of mossy fiber long-term potentiation (LTP) was not affected, the postsynaptic LTP of NMDAR-EPSCs was reduced. In addition, we also report an impairment in feedforward inhibition in CA3 pyramidal cells. This study, together with our previous work describing deficits at CA3-CA3 synapses, provides evidence that early AD affects synapses in a projection-dependent manner at the level of a single neuronal population.SIGNIFICANCE STATEMENT Because loss of episodic memory is considered the cognitive hallmark of Alzheimer's disease (AD), it is important to study whether synaptic circuits involved in the encoding of episodic memory are compromised in AD mouse models. Here we probe alterations in the synaptic connections between the dentate gyrus and CA3, which are thought to be critical for enabling episodic memories to be formed and stored in CA3. We found that forms of synaptic plasticity specific to these synaptic connections are markedly impaired at an early stage in a mouse model of AD, before deposition of β amyloid plaques. Together with previous work describing deficits at CA3-CA3 synapses, we provide evidence that early AD affects synapses in an input-dependent manner within a single neuronal population.

Clusterin in Alzheimer's Disease: Mechanisms, Genetics, and Lessons From Other Pathologies

Clusterin (CLU) or APOJ is a multifunctional glycoprotein that has been implicated in several physiological and pathological states, including Alzheimer's disease (AD). With a prominent extracellular chaperone function, additional roles have been discussed for clusterin, including lipid transport and immune modulation, and it is involved in pathways common to several diseases such as cell death and survival, oxidative stress, and proteotoxic stress. Although clusterin is normally a secreted protein, it has also been found intracellularly under certain stress conditions. Multiple hypotheses have been proposed regarding the origin of intracellular clusterin, including specific biogenic processes leading to alternative transcripts and protein isoforms, but these lines of research are incomplete and contradictory. Current consensus is that intracellular clusterin is most likely to have exited the secretory pathway at some point or to have re-entered the cell after secretion. Clusterin's relationship with amyloid beta (Aβ) has been of great interest to the AD field, including clusterin's apparent role in altering Aβ aggregation and/or clearance. Additionally, clusterin has been more recently identified as a mediator of Aβ toxicity, as evidenced by the neuroprotective effect of CLU knockdown and knockout in rodent and human iPSC-derived neurons. CLU is also the third most significant genetic risk factor for late onset AD and several variants have been identified in CLU. Although the exact contribution of these variants to altered AD risk is unclear, some have been linked to altered CLU expression at both mRNA and protein levels, altered cognitive and memory function, and altered brain structure. The apparent complexity of clusterin's biogenesis, the lack of clarity over the origin of the intracellular clusterin species, and the number of pathophysiological functions attributed to clusterin have all contributed to the challenge of understanding the role of clusterin in AD pathophysiology. Here, we highlight clusterin's relevance to AD by discussing the evidence linking clusterin to AD, as well as drawing parallels on how the role of clusterin in other diseases and pathways may help us understand its biological function(s) in association with AD.

NMDA receptors mediate synaptic depression, but not spine loss in the dentate gyrus of adult amyloid Beta (Aβ) overexpressing mice

Amyloid beta (Aβ)-mediated synapse dysfunction and spine loss are considered to be early events in Alzheimer’s disease (AD) pathogenesis. N-methyl-D-aspartate receptors (NMDARs) have previously been suggested to play a role for Amyloid beta (Aβ) toxicity. Pharmacological block of NMDAR subunits in cultured neurons and mice suggested that NMDARs containing the GluN2B subunit are necessary for Aβ-mediated changes in synapse number and function in hippocampal neurons. Interestingly, NMDARs undergo a developmental switch from GluN2B- to GluN2A-containing receptors. This indicates different functional roles of NMDARs in young mice compared to older animals. In addition, the lack of pharmacological tools to efficiently dissect the role of NMDARs containing the different subunits complicates the interpretation of their specific role. In order to address this problem and to investigate the specific role for Aβ toxicity of the distinct NMDAR subunits in dentate gyrus granule cells of adult mice, we used conditional knockout mouse lines for the subunits GluN1, GluN2A and GluN2B. Aβ-mediated changes in synaptic function and neuronal anatomy were investigated in several-months old mice with virus-mediated overproduction of Aβ and in 1-year old 5xFAD mice. We found that all three NMDAR subunits contribute to the Aβ-mediated decrease in the number of functional synapses. However, NMDARs are not required for the spine number reduction in dentate gyrus granule cells after chronic Aβ-overproduction in 5xFAD mice. Furthermore, the amplitude of synaptic and extrasynaptic NMDAR-mediated currents was reduced in dentate gyrus granule of 5xFAD mice without changes in current kinetics, suggesting that a redistribution or change in subunit composition of NMDARs does not play a role in mediating Amyloid beta (Aβ) toxicity. Our study indicates that NMDARs are involved in AD pathogenesis by compromising synapse function but not by affecting neuron morphology.

Soluble Aβ Oligomers Impair Dipolar Heterodendritic Plasticity by Activation of mGluR in the Hippocampal CA1 Region

Soluble Aβ oligomers (oAβs) contribute importantly to synaptotoxicity in Alzheimer disease (AD), but the mechanisms related to heterogeneity of synaptic functions at local circuits remain elusive. Nearly all studies of the effects of oAβs on hippocampal synaptic plasticity have only examined homosynaptic plasticity. Here we stimulated the Schaffer collaterals and then simultaneously recorded in stratum radiatum (apical dendrites) and stratum oriens (basal dendrites) of CA1 neurons. We found that the apical dendrites are significantly more vulnerable to oAβ-mediated synaptic dysfunction: the heterosynaptic basal dendritic long-term potentiation (LTP) remained unchanged, whereas the homosynaptic apical LTP was impaired. However, the heterosynaptic basal dendritic plasticity induced by either spaced 10-Hz bursts or low-frequency (1-Hz) stimulation was disrupted by oAβs in a mGluR5-dependent manner. These results suggest that different firing patterns in the same neurons may be selectively altered by soluble oAβs in an early phase of AD, before frank neurodegeneration.

Peripheral Interventions Enhancing Brain Glutamate Homeostasis Relieve Amyloid β- and TNFα- Mediated Synaptic Plasticity Disruption in the Rat Hippocampus

Dysregulation of glutamate homeostasis in the interstitial fluid of the brain is strongly implicated in causing synaptic dysfunction in many neurological and psychiatric illnesses. In the case of Alzheimer's disease (AD), amyloid β (Aβ)-mediated disruption of synaptic plasticity and memory can be alleviated by interventions that directly remove glutamate or block certain glutamate receptors. An alternative strategy is to facilitate the removal of excess glutamate from the nervous system by activating peripheral glutamate clearance systems. One such blood-based system, glutamate oxaloacetate transaminase (GOT), is activated by oxaloacetate, which acts as a co-substrate. We report here that synthetic and AD brain-derived Aβ-mediated inhibition of synaptic long-term potentiation in the hippocampus is alleviated by oxaloacetate. Moreover the effect of oxaloacetate was GOT-dependent. The disruptive effects of a general inhibitor of excitatory amino acid transport or TNFα, a pro-inflammatory mediator of Aβ action, were also reversed by oxaloacetate. Furthermore, another intervention that increases peripheral glutamate clearance, peritoneal dialysis, mimicked the beneficial effect of oxaloacetate. These findings lend support to the promotion of the peripheral clearance of glutamate as a means to alleviate synaptic dysfunction that is caused by impaired glutamate homeostasis in the brain.

Antibody Engineering for Optimized Immunotherapy in Alzheimer's Disease

There are nearly 50 million people with Alzheimer's disease (AD) worldwide and currently no disease modifying treatment is available. AD is characterized by deposits of Amyloid-β (Aβ), neurofibrillary tangles, and neuroinflammation, and several drug discovery programmes studies have focussed on Aβ as therapeutic target. Active immunization and passive immunization against Aβ leads to the clearance of deposits in humans and transgenic mice expressing human Aβ but have failed to improve memory loss. This review will discuss the possible explanations for the lack of efficacy of Aβ immunotherapy, including the role of a pro-inflammatory response and subsequent vascular side effects, the binding site of therapeutic antibodies and the timing of the treatment. We further discuss how antibodies can be engineered for improved efficacy.

Mechanism of NMDA receptor channel block by MK-801 and memantine

The NMDA (N-methyl-D-aspartate) receptor transduces the binding of glutamate and glycine, coupling it to the opening of a calcium-permeable ion channel ¹ . Owing to the lack of high-resolution structural studies of the NMDA receptor, the mechanism by which ion-channel blockers occlude ion permeation is not well understood. Here we show that removal of the amino-terminal domains from the GluN1-GluN2B NMDA receptor yields a functional receptor and crystals with good diffraction properties, allowing us to map the binding site of the NMDA receptor blocker, MK-801. This crystal structure, together with long-timescale molecular dynamics simulations, shows how MK-801 and memantine (a drug approved for the treatment of Alzheimer's disease) bind within the vestibule of the ion channel, promote closure of the ion channel gate and lodge between the M3-helix-bundle crossing and the M2-pore loops, physically blocking ion permeation.

HIPK2-Mediated Transcriptional Control of NMDA Receptor Subunit Expression Regulates Neuronal Survival and Cell Death

NMDA receptors are critical for neuronal communication. Dysfunction in NMDA receptors has been implicated in neuropsychiatric diseases. While it is well recognized that the composition of NMDA receptors undergoes a GluN2B-to-GluN2A switch in early postnatal life, the mechanism regulating this switch remains unclear. Using transcriptomic and functional analyses in brain tissues from male and female Hipk2^+/+ and Hipk2^-/- mice, we showed that the HIPK2-JNK-c-Jun pathway is important in suppressing the transcription of Grin2a and Grin2c, which encodes the GluN2A and GluN2C subunits of the NMDA receptors, respectively. Loss of HIPK2 leads to a significant decrease in JNK-c-Jun signaling, which in turn derepresses the transcription of Grin2a and Grin2c mRNA and upregulates GluN2A and GluN2C protein levels. These changes result in a significant increase of GluN2A/GluN2B ratio in synapse and mitochondria, a persistent activation of the ERK-CREB pathway and the upregulation of synaptic activity-regulated genes, which collectively contribute to the resistance of Hipk2^-/- neurons to cell death induced by mitochondrial toxins.SIGNIFICANCE STATEMENT We identify HIPK2-JNK-c-Jun signaling as a key mechanism that regulates the transcription of NMDA receptor subunits GluN2A and GluN2C in vivo Our results provide insights into a previously unrecognized molecular mechanism that control the switch of NMDA receptor subunits in early postnatal brain development. Furthermore, we provide evidence that changes in the ratio of NMDA subunits GluN2A/GluN2B can also be detected in the synapse and mitochondria, which contributes to a persistent activation of the prosurvival ERK-CREB pathway and its downstream target genes. Collectively, these changes protect HIPK2 deficient neurons from mitochondrial toxins.

Physiological activation of mGlu5 receptors supports the ion channel function of NMDA receptors in hippocampal LTD induction in vivo

Synaptic long-term depression (LTD) is believed to underlie critical mnemonic processes in the adult hippocampus. The roles of the metabotropic and ionotropic actions of glutamate in the induction of synaptic LTD by electrical low-frequency stimulation (LFS) in the living adult animal is poorly understood. Here we examined the requirement for metabotropic glutamate (mGlu) and NMDA receptors in LTD induction in anaesthetized adult rats. LTD induction was primarily dependent on NMDA receptors and required the involvement of both the ion channel function and GluN2B subunit of the receptor. Endogenous mGlu5 receptor activation necessitated the local application of relatively high doses of either competitive or non-competitive NMDA receptor antagonists to block LTD induction. Moreover, boosting endogenous glutamate activation of mGlu5 receptors with a positive allosteric modulator lowered the threshold for NMDA receptor-dependent LTD induction by weak LFS. The present data provide support in the living animal that NMDA receptor-dependent LTD is boosted by endogenously released glutamate activation of mGlu5 receptors. Given the predominant perisynaptic location of mGlu5 receptors, the present findings emphasize the need to further evaluate the contribution and mechanisms of these receptors in NMDA receptor-dependent synaptic plasticity in the adult hippocampus in vivo.

Local and Use-Dependent Effects of β-Amyloid Oligomers on NMDA Receptor Function Revealed by Optical Quantal Analysis

Beta amyloid (Aβ) triggers the elimination of excitatory synaptic connections in the CNS, an early manifestation of Alzheimer's disease. Oligomeric assemblies of Aβ peptide associate with excitatory synapses resulting in synapse elimination through a process that requires NMDA-type glutamate receptor activation. Whether Aβ affects synaptic NMDA receptor (NMDAR) function directly and acts locally at synapses to which it has bound and whether synaptic activity influences Aβ synaptic binding and synaptotoxicity have remained fundamental questions. Here, we used subcellular Ca²⁺ imaging in rat hippocampal neurons to visualize NMDAR function at individual synapses before and after Aβ application. Aβ triggered a robust impairment of NMDAR Ca²⁺ entry at most, but not all, synapses. NMDAR function was more severely impaired at highly active synapses and synapses with bound Aβ, but activity was not required for Aβ synapse binding. Blocking NMDARs during Aβ exposure prevented Aβ-mediated impairment. Finally, Aβ impaired NMDAR Ca²⁺ entry at doses much lower than those required for NMDAR internalization, revealing a novel, potent mode of NMDAR regulation by Aβ.

SIGNIFICANCE STATEMENT

Amyloid β (Aβ) is strongly implicated in Alzheimer's disease. Aβ triggers the elimination of excitatory synapses through a mechanism that requires NMDA receptors (NMDARs). However, little is known about how or whether Aβ influences synaptic NMDAR function. We used an imaging-based assay to investigate the relationship among Aβ binding, activity, and NMDAR function at individual synapses. Aβ triggered a robust impairment of NMDAR Ca²⁺ entry at most, but not all, synapses. NMDAR function was more severely impaired at highly active synapses and synapses with bound Aβ. Blocking NMDARs during Aβ exposure prevented Aβ-mediated impairment. Together, our experiments reveal a novel use-dependent, potent, and local mode of Aβ-mediated NMDAR impairment.

Neural changes in Alzheimer's disease from circuit to molecule: Perspective of optogenetics

Alzheimer's disease (AD), as a crucial neurodegenerative disorder, affects neural activities at many levels. Synaptic plasticity and neural circuits are most susceptible in AD, but the detailed mechanism is unclear. Optogenetic tools provide unprecedented spatio-temporal specificity to stimulate specific neural circuits or synaptic molecules to reveal the precise function of normal brain and mechanism of deficits in AD models. Furthermore, using optogenetics to stimulate neurons can rescue learning and memory loss caused by AD. It also has possibility to use light to control the Neurotransmitter receptors and their downstream signal pathway. These technical methods have broad therapeutic application prospect.

Posttranslational modification impact on the mechanism by which amyloid-β induces synaptic dysfunction

Oligomeric amyloid-β (Aβ) 1-42 disrupts synaptic function at an early stage of Alzheimer's disease (AD). Multiple posttranslational modifications of Aβ have been identified, among which N-terminally truncated forms are the most abundant. It is not clear, however, whether modified species can induce synaptic dysfunction on their own and how altered biochemical properties can contribute to the synaptotoxic mechanisms. Here, we show that a prominent isoform, pyroglutamated Aβ3(pE)-42, induces synaptic dysfunction to a similar extent like Aβ1-42 but by clearly different mechanisms. In contrast to Aβ1-42, Aβ3(pE)-42 does not directly associate with synaptic membranes or the prion protein but is instead taken up by astrocytes and potently induces glial release of the proinflammatory cytokine TNFα. Moreover, Aβ3(pE)-42-induced synaptic dysfunction is not related to NMDAR signalling and Aβ3(pE)-42-induced impairment of synaptic plasticity cannot be rescued by D1-agonists. Collectively, the data point to a scenario where neuroinflammatory processes together with direct synaptotoxic effects are caused by posttranslational modification of soluble oligomeric Aβ and contribute synergistically to the onset of synaptic dysfunction in AD.

Novel conjugates of aminoadamantanes with carbazole derivatives as potential multitarget agents for AD treatment

A new group of compounds, promising for the design of original multitarget therapeutic agents for treating neurodegenerative diseases, based on conjugates of aminoadamantane and carbazole derivatives was synthesized and investigated. Compounds of these series were found to interact with a group of targets that play an important role in the development of this type of diseases. First of all, these compounds selectively inhibit butyrylcholinesterase, block NMDA receptors containing NR2B subunits while maintaining the properties of MK-801 binding site blockers, exert microtubules stabilizing properties, and possess the ability to protect nerve cells from death at the calcium overload conditions. The leading compound C-2h has been shown the most promising effects on all analyzed parameters. Thus, these compounds can be regarded as promising candidates for the design of multi-target disease-modifying drugs for treatment of AD and/or similar neuropathologies.

Enhanced LTP in aged rats: Detrimental or compensatory?

Age-dependent memory deterioration has been well documented and yet an increase in rat hippocampal LTP upon aging has been reported. This poses the question of whether the enhanced LTP is a cause or an attempt to compensate the memory deficits described in aged rats. Hippocampal slices from young, adult and aged Wistar rats were pre-incubated, with an NMDA receptor (NMDAR) antagonist, memantine (1 μM, 4 h), and hippocampal LTP was evaluated. The results show that memantine significantly decreases the larger LTP magnitude recorded in hippocampal slices from aged rats without compromising LTP recorded in slices from young and adult animals. To unveil the impact of in vivo administration of memantine, different doses (1, 5 and 10 mg/kg/day) or saline vehicle solution were intraperitoneally administered, for 15-20 days, to both young and aged animals. Memantine did not significantly affect neither the place learning of young animals, evaluated by Morris Water Maze, nor LTP recorded from hippocampal slices from the same group of animals. However, memantine (5 and 10 mg/kg/day) significantly decreased the large LTP recorded in hippocampal slices from aged animals. Moreover, aged animals treated with memantine (10 mg/kg/day) showed a significantly compromised place learning when compared to aged control animals. Overall, these results suggest that the larger LTP observed in aged animals is a compensatory phenomenon, rather than pathological. The finding that age-dependent blockade of LTP by a NMDAR antagonist leads to learning deficits, implies that the increased LTP observed upon aging may be playing an important role in the learning process.

Opposite in vivo effects of agents that stimulate or inhibit the glutamate/cysteine exchanger system xc- on the inhibition of hippocampal LTP by Aß

Aggregated amyloid ß-protein (Aß) is pathognomonic of Alzheimer's disease and certain assemblies of Aß are synaptotoxic. Excess glutamate or diminished glutathione reserve are both implicated in mediating or modulating Aß-induced disruption of synaptic plasticity. The system xc- antiporter promotes Na⁺ -independent exchange of cystine with glutamate thereby providing a major source of extracellular glutamate and intracellular glutathione concentrations. Here we probed the ability of two drugs with opposite effects on system xc-, the inhibitor sulfasalazine and facilitator N-acetylcysteine, to modulate the ability of Aß1-42 to inhibit long-term potentiation (LTP) in the CA1 area of the anaesthetized rat. Whereas acute systemic treatment with sulfasalazine lowered the threshold for Aß to interfere with synaptic plasticity, N-acetylcysteine prevented the inhibition of LTP by Aß alone or in combination with sulfasalazine. Moreover acute N-acetylcysteine also prevented the inhibition of LTP by TNFα, a putative mediator of Aß actions, and repeated systemic N-acetylcysteine treatment for 7 days reversed the delayed deleterious effect of Aß on LTP. Since both of these drugs are widely used clinically, further evaluation of their potential beneficial and deleterious actions in early Alzheimer's disease seems warranted. © 2016 Wiley Periodicals, Inc.

Expression of Phenotypic Astrocyte Marker Is Increased in a Transgenic Mouse Model of Alzheimer's Disease versus Age-Matched Controls: A Presymptomatic Stage Study

Recent mouse studies of the presymptomatic stage of Alzheimer's disease (AD) have suggested that proinflammatory changes, such as glial activation and cytokine induction, may occur already at this early stage through unknown mechanisms. Because TNFα contributes to increased Aβ production from the Aβ precursor protein (APP), we assessed a putative correlation between APP/Aβ and TNFα during the presymptomatic stage as well as early astrocyte activation in the hippocampus of 3-month-old APPswe/PS1dE9 mice. While Western blots revealed significant APP expression, Aβ was not detectable by Western blot or ELISA attesting that 3-month-old, APPswe/PS1dE9 mice are at a presymptomatic stage of AD-like pathology. Western blots were also used to show increased GFAP expression in transgenic mice that positively correlated with both TNFα and APP, which were also mutually correlated. Subregional immunohistochemical quantification of phenotypic (GFAP) and functional (TSPO) markers of astrocyte activation indicated a selective and significant increase in GFAP-immunoreactive (IR) cells in the dentate gyrus of APPswe/PS1dE9 mice. Our data suggest that subtle morphological and phenotypic alterations, compatible with the engagement of astrocyte along the activation pathway, occur in the hippocampus already at the presymptomatic stage of AD.

Effects of Low Amyloid-β (Aβ) Concentration on Aβ1-42 Oligomers Binding and GluN2B Membrane Expression

Numerous studies have shown that amyloid-β (Aβ) modulate intracellular metabolic cascades and an intracellular Ca²⁺ homeostasis and a cell surface NMDA receptor expression alteration in Alzheimer’s disease (AD). However most of these findings have been obtained by using non-physiological Aβ concentrations. The present study deals with the effect of low Aβ concentrations on cellular homeostasis. We used nerve growth factor-differentiated PC12 cells and murine cortical neurons sequentially treated with low chronic monomeric or small oligomeric Aβ concentrations and high acute oligomeric Aβ concentrations to bring out a priming effect of chronic treatment on subsequently high Aβ concentrations-elicited cellular response. Both cell types indeed displayed an enhanced capacity to bind oligomeric Aβ after monomeric or small oligomeric Aβ application. Furthermore, the results show that monomeric Aβ_1–42 application to the cells induces an increase of the Ca²⁺-response and of the membrane expression of the extrasynaptic subunit of the NMDA receptor GluN2B in PC12 cells, while the opposite effects were observed in cultured neurons. This suggests a sequential interaction of Aβ with the cellular plasma membrane involving monomers or small Aβ oligomers which would facilitate the binding of the deleterious high molecular Aβ oligomers. This mechanism would explain the slow progression of AD in the human nervous system and the deep gradient of neuronal death observed around the amyloid plaques in the nervous tissue.

Dysfunction of NMDA receptors in Alzheimer's disease

N-methyl-D-aspartate receptors (NMDARs) play a pivotal role in the synaptic transmission and synaptic plasticity thought to underlie learning and memory. NMDARs activation has been recently implicated in Alzheimer's disease (AD) related to synaptic dysfunction. Synaptic NMDARs are neuroprotective, whereas overactivation of NMDARs located outside of the synapse cause loss of mitochondrial membrane potential and cell death. NMDARs dysfunction in the glutamatergic tripartite synapse, comprising presynaptic and postsynaptic neurons and glial cells, is directly involved in AD. This review discusses that both beta-amyloid (Aβ) and tau perturb synaptic functioning of the tripartite synapse, including alterations in glutamate release, astrocytic uptake, and receptor signaling. Particular emphasis is given to the role of NMDARs as a possible convergence point for Aβ and tau toxicity and possible reversible stages of the AD through preventive and/or disease-modifying therapeutic strategies.

The Role of Visinin-Like Protein-1 in the Pathophysiology of Alzheimer's Disease

Calcium ions are crucial in the process of information transmission and integration in the central nervous system (CNS). These ions participate not only in intracellular mechanisms but also in intercellular processes. The changes in the concentration of Ca2 + ions modulate synaptic transmission, whereas neuronal activity induces calcium ion waves. Disturbed calcium homeostasis is thought to be one of the main features in the pathophysiology of Alzheimer's disease (AD), and AD pathogenesis is closely connected to Ca2 + signaling pathways. The effects of changes in neuronal Ca2 + are mediated by neuronal calcium sensor (NCS) proteins. It has been revealed that NCS proteins, with special attention to visinin-like protein 1 (VILIP-1), might have a connection to the etiology of AD. In the CNS, VILIP-1 influences the intracellular neuronal signaling pathways involved in synaptic plasticity, such as cyclic nucleotide cascades and nicotinergic signaling. This particular protein is implicated in calcium-mediated neuronal injury as well. VILIP-1 also participates in the pathological mechanisms of altered Ca2 + homeostasis, leading to neuronal loss. These findings confirm the utility of VILIP-1 as a useful biomarker of neuronal injury. Moreover, VILIP-1 plays a vital role in linking calcium-mediated neurotoxicity and AD-type pathological changes. The disruption of Ca2 + homeostasis caused by AD-type neurodegeneration may result in the damage of VILIP-1-containing neurons in the brain, leading to increased cerebrospinal fluid levels of VILIP-1. Thus, the aim of this overview is to describe the relationships of the NCS protein VILIP-1 with the pathogenetic factors of AD and neurodegenerative processes, as well as its potential clinical usefulness as a biomarker of AD. Moreover, we describe the current and probable therapeutic strategies for AD, targeting calcium-signaling pathways and VILIP-1.

Early synaptic deficits in the APP/PS1 mouse model of Alzheimer's disease involve neuronal adenosine A2A receptors

Synaptic plasticity in the autoassociative network of recurrent connections among hippocampal CA3 pyramidal cells is thought to enable the storage of episodic memory. Impaired episodic memory is an early manifestation of cognitive deficits in Alzheimer's disease (AD). In the APP/PS1 mouse model of AD amyloidosis, we show that associative long-term synaptic potentiation (LTP) is abolished in CA3 pyramidal cells at an early stage. This is caused by activation of upregulated neuronal adenosine A2A receptors (A2AR) rather than by dysregulation of NMDAR signalling or altered dendritic spine morphology. Neutralization of A2AR by acute pharmacological inhibition, or downregulation driven by shRNA interference in a single postsynaptic neuron restore associative CA3 LTP. Accordingly, treatment with A2AR antagonists reverts one-trial memory deficits. These results provide mechanistic support to encourage testing the therapeutic efficacy of A2AR antagonists in early AD patients.

From Molecular Circuit Dysfunction to Disease: Case Studies in Epilepsy, Traumatic Brain Injury, and Alzheimer's Disease

Complex circuitry with feed-forward and feed-back systems regulate neuronal activity throughout the brain. Cell biological, electrical, and neurotransmitter systems enable neural networks to process and drive the entire spectrum of cognitive, behavioral, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits relies on hundreds, if not thousands, of unique molecular interactions. Even single molecule dysfunctions can be disrupting to neural circuit activity, leading to neurological pathology. Here, we sample our current understanding of how molecular aberrations lead to disruptions in networks using three neurological pathologies as exemplars: epilepsy, traumatic brain injury (TBI), and Alzheimer's disease (AD). Epilepsy provides a window into how total destabilization of network balance can occur. TBI is an abrupt physical disruption that manifests in both acute and chronic neurological deficits. Last, in AD progressive cell loss leads to devastating cognitive consequences. Interestingly, all three of these neurological diseases are interrelated. The goal of this review, therefore, is to identify molecular changes that may lead to network dysfunction, elaborate on how altered network activity and circuit structure can contribute to neurological disease, and suggest common threads that may lie at the heart of molecular circuit dysfunction.

Central role for NMDA receptors in redox mediated impairment of synaptic function during aging and Alzheimer's disease

Increased human longevity has magnified the negative impact that aging can have on cognitive integrity of older individuals experiencing some decline in cognitive function. Approximately 30% of the elderly will have cognitive problems that influence their independence. Impaired executive function and memory performance are observed in normal aging and yet can be an early sign of a progressive cognitive impairment of Alzheimer's disease (AD), the most common form of dementia. Brain regions that are vulnerable to aging exhibit the earliest pathology of AD. Senescent synaptic function is observed as a shift in Ca²⁺-dependent synaptic plasticity and similar mechanisms are thought to contribute to the early cognitive deficits associated with AD. In the case of aging, intracellular redox state mediates a shift in Ca²⁺ regulation including N-methyl-d-aspartate (NMDA) receptor hypofunction and increased Ca²⁺ release from intracellular stores to alter synaptic plasticity. AD can interact with these aging processes such that molecules linked to AD, β-amyloid (Aβ) and mutated presenilin 1 (PS1), can also degrade NMDA receptor function, promote Ca²⁺ release from intracellular stores, and may increase oxidative stress. Thus, age is one of the most important predictors of AD and brain aging likely contributes to the onset of AD. The focus of this review article is to provide an update on mechanisms that contribute to the senescent synapse and possible interactions with AD-related molecules, with special emphasis on regulation of NMDA receptors.

Emerging Link between Alzheimer's Disease and Homeostatic Synaptic Plasticity

Alzheimer's disease (AD) is an irreversible brain disorder characterized by progressive cognitive decline and neurodegeneration of brain regions that are crucial for learning and memory. Although intracellular neurofibrillary tangles and extracellular senile plaques, composed of insoluble amyloid-β (Aβ) peptides, have been the hallmarks of postmortem AD brains, memory impairment in early AD correlates better with pathological accumulation of soluble Aβ oligomers and persistent weakening of excitatory synaptic strength, which is demonstrated by inhibition of long-term potentiation, enhancement of long-term depression, and loss of synapses. However, current, approved interventions aiming to reduce Aβ levels have failed to retard disease progression; this has led to a pressing need to identify and target alternative pathogenic mechanisms of AD. Recently, it has been suggested that the disruption of Hebbian synaptic plasticity in AD is due to aberrant metaplasticity, which is a form of homeostatic plasticity that tunes the magnitude and direction of future synaptic plasticity based on previous neuronal or synaptic activity. This review examines emerging evidence for aberrant metaplasticity in AD. Putative mechanisms underlying aberrant metaplasticity in AD will also be discussed. We hope this review inspires future studies to test the extent to which these mechanisms contribute to the etiology of AD and offer therapeutic targets.

MT5-MMP is a new pro-amyloidogenic proteinase that promotes amyloid pathology and cognitive decline in a transgenic mouse model of Alzheimer's disease

Membrane-type 5-matrix metalloproteinase (MT5-MMP) is a proteinase mainly expressed in the nervous system with emerging roles in brain pathophysiology. The implication of MT5-MMP in Alzheimer's disease (AD), notably its interplay with the amyloidogenic process, remains elusive. Accordingly, we crossed the genetically engineered 5xFAD mouse model of AD with MT5-MMP-deficient mice and examined the impact of MT5-MMP deficiency in bigenic 5xFAD/MT5-MMP(-/-) mice. At early stages (4 months) of the pathology, the levels of amyloid beta peptide (Aβ) and its amyloid precursor protein (APP) C-terminal fragment C99 were largely reduced in the cortex and hippocampus of 5xFAD/MT5-MMP(-/-), compared to 5xFAD mice. Reduced amyloidosis in bigenic mice was concomitant with decreased glial reactivity and interleukin-1β (IL-1β) levels, and the preservation of long-term potentiation (LTP) and spatial learning, without changes in the activity of α-, β- and γ-secretases. The positive impact of MT5-MMP deficiency was still noticeable at 16 months of age, as illustrated by reduced amyloid burden and gliosis, and a better preservation of the cortical neuronal network and synaptophysin levels in bigenic mice. MT5-MMP expressed in HEKswe cells colocalized and co-immunoprecipitated with APP and significantly increased the levels of Aβ and C99. MT5-MMP also promoted the release of a soluble APP fragment of 95 kDa (sAPP95) in HEKswe cells. sAPP95 levels were significantly reduced in brain homogenates of 5xFAD/MT5-MMP(-/-) mice, supporting altogether the idea that MT5-MMP influences APP processing. MT5-MMP emerges as a new pro-amyloidogenic regulator of APP metabolism, whose deficiency alleviates amyloid pathology, neuroinflammation and cognitive decline.

Synaptic activity and Alzheimer's disease: a critical update

Synapses have been known for many years to be the crucial target of pathology in different forms of dementia, in particular Alzheimer's disease (AD). Synapses and their appropriate activation or inhibition are fundamental for the proper brain function. Alterations in synaptic/neuronal activity and brain metabolism are considered among the earliest symptoms linked to the progression of AD, and lead to a central question in AD research: what is the role played by synaptic activity in AD pathogenesis? Intriguingly, in the last decade, important studies demonstrated that the state of activation of synapses affects the homeostasis of beta-amyloid (Aβ) and tau, both of which aggregate and accumulate during AD, and are involved in neuronal dysfunction. In this review we aim to summarize the up-to-date data linking synaptic/neuronal activity with Aβ and tau; moreover, we also intend to provide a critical overview on brain activity alterations in AD, and their role in the disease's pathophysiology.

Soluble Aβ oligomers impair hippocampal LTP by disrupting glutamatergic/GABAergic balance

Epileptic activity may be more prevalent in early stage Alzheimer's disease (AD) than previously believed. Several studies report spontaneous seizures and interictal discharges in mouse models of AD undergoing age-related Aβ accumulation. The mechanism by which Aβ-induced neuronal excitability can trigger epileptiform activity remains unknown. Here, we systematically examined field excitatory postsynaptic potentials (fEPSP) in stratum radiatum and population spikes (PSs) in the adjacent stratum pyramidale of CA1 in wild-type mouse hippocampal slices. Soluble Aβ oligomers (oAβ) blocked hippocampal LTP and EPSP-spike (E-S) potentiation, and these effects were occluded by prior treatment with the glutamate uptake inhibitor TBOA. In accord, oAβ elevated glutamate levels in the hippocampal slice medium. Recording the PS revealed that oAβ increased PS frequency and reduced LTP, and this LTP deficit was occluded by pretreatment with the GABAA antagonist picrotoxin. Whole-cell recordings showed that oAβ significantly increased spontaneous EPSC frequency. Decreasing neuronal activity by increasing GABA tone or partially blocking NMDAR activity prevented oAβ impairment of hippocampal LTP. Finally, treating slices with two antiepileptic drugs rescued the LTP inhibition induced by oAβ. We conclude that soluble Aβ oligomers at the low nanomolar levels present in AD brain increase neuronal excitability by disrupting glutamatergic/GABAergic balance, thereby impairing synaptic plasticity.

Developmental attenuation of N-methyl-D-aspartate receptor subunit expression by microRNAs

BACKGROUND

N-methyl-D-aspartate receptors (NMDARs) are a subtype of ionotropic glutamate receptors and are expressed throughout the central nervous system (CNS). Their activity is required for excitatory synaptic transmission, the developmental refinement of neural circuits and for the expression of many forms of synaptic plasticity. NMDARs are obligate heterotetramers and the expression of their constituent subunits is developmentally and anatomically regulated. In rodent cortex and hippocampus, the GluN2B subunit is expressed at high levels early in development and decreases to plateau levels later while expression of the GluN2A subunit has a concomitant increase. Regulation of GluN2A and GluN2B expressions are incompletely understood. Here, we showed the influence of miRNAs in this process.

FINDINGS

Two miRNAs, miR-19a and miR-539 can influence the levels of NMDARs subunits, as they target the mRNAs encoding GluN2A and GluN2B respectively. MiR-539 also modified the expression of the transcription factor REST, a known regulator of NMDAR subunit expression.

CONCLUSIONS

miR-19a and miR-539, in collaboration with REST, serve to set the levels of GluN2A and GluN2B precisely during development. These miRNAs offer an entry point for interventions that affect plasticity and a novel approach to treat neurodegenerative diseases.

Longitudinal testing of hippocampal plasticity reveals the onset and maintenance of endogenous human Aß-induced synaptic dysfunction in individual freely behaving pre-plaque transgenic rats: rapid reversal by anti-Aß agents

Long before synaptic loss occurs in Alzheimer’s disease significant harbingers of disease may be detected at the functional level. Here we examined if synaptic long-term potentiation is selectively disrupted prior to extracellular deposition of Aß in a very complete model of Alzheimer’s disease amyloidosis, the McGill-R-Thy1-APP transgenic rat. Longitudinal studies in freely behaving animals revealed an age-dependent, relatively rapid-onset and persistent inhibition of long-term potentiation without a change in baseline synaptic transmission in the CA1 area of the hippocampus. Thus the ability of a standard 200 Hz conditioning protocol to induce significant NMDA receptor-dependent short- and long-term potentiation was lost at about 3.5 months of age and this deficit persisted for at least another 2–3 months, when plaques start to appear. Consistent with in vitro evidence for a causal role of a selective reduction in NMDA receptor-mediated synaptic currents, the deficit in synaptic plasticity in vivo was associated with a reduction in the synaptic burst response to the conditioning stimulation and was overcome using stronger 400 Hz stimulation. Moreover, intracerebroventricular treatment for 3 days with an N-terminally directed monoclonal anti- human Aß antibody, McSA1, transiently reversed the impairment of synaptic plasticity. Similar brief treatment with the BACE1 inhibitor LY2886721 or the γ-secretase inhibitor MRK-560 was found to have a comparable short-lived ameliorative effect when tracked in individual rats. These findings provide strong evidence that endogenously generated human Aß selectively disrupts the induction of long-term potentiation in a manner that enables potential therapeutic options to be assessed longitudinally at the pre-plaque stage of Alzheimer’s disease amyloidosis.

Chronic traumatic encephalopathy: a critical appraisal

Chronic traumatic encephalopathy (CTE) formerly known as dementia pugilistica is a long-term neurodegenerative disorder associated with repeated subconcussive head injuries in high-contact sports. We reviewed the existing literature on CTE and examined epidemiological trends, risk factors, and its temporal progression, and proposed the underlying pathophysiological mechanisms that may provide unique insights to clinicians with an in-depth understanding of the disease to aid in the diagnosis and prevention, and provide future perspectives for research via search of Medline and Cochrane databases as well as manual review of bibliographies from selected articles and monographs. The prevalence of CTE in recent years is on the rise and almost exclusively affects men, with pathologic signs characterized by progressive memory loss, behavioral changes, and violent tendencies with some patients demonstrating Parkinsonian-like symptoms and signs. Many patients with CTE die following suicide, accident, or complications of drug or alcohol use. Postmortem pathologic analysis is characterized by neurofibrillary tangles and Aβ plaques in 50 % of cases. Currently, there are no ante-mortem diagnostic criteria, but modern imaging techniques such as functional magnetic resonance (MR) imaging, MR spectroscopy, and diffusion tension imaging hold promise for delineating the future diagnostic criteria. Further long-term longitudinal studies are warranted to investigate risk factors that will enhance understanding of the disease progression and its pathogenesis.

Chronic GluN2B antagonism disrupts behavior in wild-type mice without protecting against synapse loss or memory impairment in Alzheimer's disease mouse models

Extensive evidence implicates GluN2B-containing NMDA receptors (GluN2B-NMDARs) in excitotoxic-insult-induced neurodegeneration and amyloid β (Aβ)-induced synaptic dysfunction. Therefore, inhibiting GluN2B-NMDARs would appear to be a potential therapeutic strategy to provide neuroprotection and improve cognitive function in Alzheimer's disease (AD). However, there are no reports of long-term in vivo treatment of AD mouse models with GluN2B antagonists. We used piperidine18 (Pip18), a potent and selective GluN2B-NMDAR antagonist with favorable pharmacokinetic properties, for long-term dosing in AD mouse models. Reduced freezing behavior in Tg2576 mice during fear conditioning was partially reversed after subchronic (17 d) Pip18 treatment. However, analysis of freezing behavior in different contexts indicated that this increased freezing likely involves elevated anxiety or excessive memory generalization in both nontransgenic (NTG) and Tg2576 mice. In PS2APP mice chronically fed with medicated food containing Pip18 for 4 months, spatial learning and memory deficits were not rescued, plaque-associated spine loss was not affected, and synaptic function was not altered. At the same time, altered open field activity consistent with increased anxiety and degraded performance in an active avoidance task were observed in NTG after chronic treatment. These results indicate that long-term treatment with a GluN2B-NMDAR antagonist does not provide a disease-modifying benefit and could cause cognitive liabilities rather than symptomatic benefit in AD mouse models. Therefore, these results challenge the expectation of the therapeutic potential for GluN2B-NMDAR antagonists in AD.

Current and emerging therapies for Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder, with a rapidly increasing worldwide prevalence. Although no cure for AD has yet been found, substantial progress has been made in our understanding of AD pathogenesis. This progress has led to the development of numerous promising compounds in various stages of clinical testing. In this review, the current pharmacologic treatments for AD are discussed in detail, followed by an overview of the main experimental strategies that will shape AD therapeutics over the next decade.

CaMKII: claiming center stage in postsynaptic function and organization

While CaMKII has long been known to be essential for synaptic plasticity and learning, recent work points to new dimensions of CaMKII function in the nervous system, revealing that CaMKII also plays an important role in synaptic organization. Ca(2+)-triggered autophosphorylation of CaMKII not only provides molecular memory by prolonging CaMKII activity during long-term plasticity (LTP) and learning but also represents a mechanism for autoactivation of CaMKII's multifaceted protein-docking functions. New details are also emerging about the distinct roles of CaMKIIα and CaMKIIβ in synaptic homeostasis, further illustrating the multilayered and complex nature of CaMKII's involvement in synaptic regulation. Here, I review novel molecular and functional insight into how CaMKII supports synaptic function.

mGlu5 receptors and cellular prion protein mediate amyloid-β-facilitated synaptic long-term depression in vivo

NMDA-type glutamate receptors (NMDARs) are currently regarded as paramount in the potent and selective disruption of synaptic plasticity by Alzheimer’s disease amyloid β-protein (Aβ). Non-NMDAR mechanisms remain relatively unexplored. Here we describe how Aβ facilitates NMDAR-independent long-term depression of synaptic transmission in the hippocampus in vivo. Synthetic Aβ and Aβ in soluble extracts of Alzheimer’s disease brain usurp endogenous acetylcholine muscarinic receptor-dependent long-term depression, to enable long-term depression that required metabotropic glutamate-5 receptors (mGlu5Rs). We also find that mGlu5Rs are essential for Aβ-mediated inhibition of NMDAR-dependent long-term potentiation in vivo. Blocking Aβ binding to cellular prion protein with antibodies prevents the facilitation of long-term depression. Our findings uncover an overarching role for Aβ-PrP^C-mGlu5R interplay in mediating both LTD facilitation and LTP inhibition, encompassing NMDAR-mediated processes that were previously considered primary.

In Alzheimer's disease, the soluble amyloid beta peptide is known to modulate synaptic function by forming a complex with prion proteins and metabotropic glutamate receptors. Here, Hu et al. show that amyloid beta signalling via this complex facilitates the induction of long-term depression at synapses.

Reciprocal disruption of neuronal signaling and Aβ production mediated by extrasynaptic NMDA receptors: a downward spiral

It is becoming increasingly clear that aberrant neuronal activity can be the cause and the result of amyloid beta production. Synaptic activation facilitates non-amyloidogenic processing of amyloid precursor protein (APP) and cell survival, primarily through synaptic NMDA receptors (NMDARs) and perhaps specifically those containing GluN2A-subunits. In contrast, extrasynaptic and GluN2B-containing NMDARs promote beta-secretase cleavage of APP into amyloid-beta (Aβ). The opposing nature of these NMDAR populations is reflected in their control over cell survival and death pathways. Subtle changes in glutamate homeostasis may shift the balance between these pathways and could play a role in Alzheimer's disease (AD). Indeed, Aβ production, regional loss of brain connectivity and neurodegeneration correlate with neuronal activity in AD patients. From another perspective, Aβ oligomers (Aβo) alter neuronal signaling through several mechanisms involving NMDARs and intracellular calcium mishandling. While Aβo affect multiple receptors, GluN2B-NMDARs have emerged as primary mediators of altered synaptic plasticity and neurotoxicity. Memantine and its successor, NitroMemantine, are efficient at blocking or reversing the deleterious actions of Aβo largely due to their selectivity for extrasynaptic NMDARs. Recently, Aβo were shown to trigger astrocytic release of glutamate to the extrasynaptic space where it activates NMDARs to promote further Aβ production and synaptic depression. Combined with the reciprocal regulation between neuronal activity and Aβ production, extrasynaptic glutamate release adds to a maladaptive model and ultimately results in synaptotoxicity and neurodegeneration of AD. Extrasynaptic NMDAR antagonists remain as a promising therapeutic avenue by interfering with this cascade.

Neurotransmitter receptor and time dependence of the synaptic plasticity disrupting actions of Alzheimer's disease Aβ in vivo

Many endogenous factors influence the time course and extent of the detrimental effects of amyloid β-protein (Aβ) on synaptic function. Here, we assessed the impact of varying endogenous glutamatergic and cholinergic transmission by pharmacological means on the disruption of plasticity at hippocampal CA3-to-CA1 synapses in the anaesthetized rat. NMDA receptors (NMDARs) are considered critical in mediating Aβ-induced inhibition of long-term potentiation (LTP). However, intracerebroventricular injection of Aβ1-42 inhibited not only NMDAR-dependent LTP but also voltage-activated Ca(2+)-dependent LTP induced by strong conditioning stimulation during NMDAR blockade. On the other hand, another form of NMDAR-independent synaptic plasticity, endogenous acetylcholine-induced muscarinic receptor-dependent long-term enhancement, was not hindered by Aβ1-42. Interestingly, augmenting endogenous acetylcholine activation of nicotinic receptors prior to the injection of Aβ1-42 prevented the inhibition of NMDAR-dependent LTP, whereas the same intervention when introduced after the infusion of Aβ was ineffective. We also examined the duration of action of Aβ, including water soluble Aβ from Alzheimer's disease (AD) brain. Remarkably, the inhibition of LTP induction caused by a single injection of sodium dodecyl sulfate-stable Aβ dimer-containing AD brain extract persisted for at least a week. These findings highlight the need to increase our understanding of non-NMDAR mechanisms and of developing novel means of overcoming, rather than just preventing, the deleterious synaptic actions of Aβ.

The preventive effect of NR2B and NR2D-containing NMDAR antagonists on Aβ-induced LTP disruption in the dentate gyrus of rats

Amyloid β-protein (Aβ) in the brain of Alzheimer's disease (AD) potently inhibits the synaptic plasticity subsequently causing the cognitive deficits. Long-term potentiation (LTP) of synaptic transmission is thought to be an important cellular mechanism underlying memory formation. Different NR2 subunits are involved in NMDA receptor-dependent LTP. In the present study, we investigated the roles of NR2B and NR2D-containing NMDAR on Aβ(1-42)-induced LTP deficits in the hippocampal slices of rats by using selective NMDAR antagonists. First, we found that Aβ(1-42) significantly inhibited the LTP in the dentate gyrus of slices as reported before. Following that the Aβ(1-42)-induced LTP inhibition was prevented by the pre-perfusion of the specific NR2B-containing NMDAR antagonists ifenprodil (approximately >200-fold selectivity for NR2B) and Ro25-6981 (>3,000-fold selectivity for NR2B), as well as PPDA, a specific NR2D receptor antagonist. Meanwhile, the antagonists on their own had no or only partial effects on the normal LTP in the same dose condition. These findings not only support the effects of NR2B and NR2D subunits on Aβ(1-42)-induced LTP deficits, but also imply that preferentially targeting NR2B- and NR2D-containing NMDARs may provide an effective means to prevent cognitive deficits in the early 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.

Prenatal activation of maternal TLR3 receptors by viral-mimetic poly(I:C) modifies GluN2B expression in embryos and sonic hedgehog in offspring in the absence of kynurenine pathway activation

Activation of the immune system during pregnancy is believed to lead to psychiatric and neurological disorders in the offspring, but the molecular changes responsible are unknown. Polyinosinic:polycytidylic acid (poly(I:C)) is a viral-mimetic double-stranded RNA complex which activates Toll-Like-Receptor-3 and can activate the metabolism of tryptophan through the oxidative kynurenine pathway to compounds that modulate activity of glutamate receptors. The aim was to determine whether prenatal administration of poly(I:C) affects the expression of neurodevelopmental proteins in the offspring and whether such effects were mediated via the kynurenine pathway. Pregnant rats were treated with poly(I:C) during late gestation and the offspring were allowed to develop to postnatal day 21 (P21). Immunoblotting of the brains at P21 showed decreased expression of sonic hedgehog, a key protein in dopaminergic neuronal maturation. Expression of α-synuclein was decreased, while tyrosine hydroxylase was increased. Disrupted in Schizophrenia-1 (DISC-1) and 5-HT2C receptor levels were unaffected, as were the dependence receptors Unc5H1, Unc5H3 and Deleted in Colorectal Cancer (DCC), the inflammation-related transcription factor NFkB and the inducible oxidative enzyme cyclo-oxygenase-2 (COX-2). An examination of embryo brains 5 h after maternal poly(I:C) showed increased expression of GluN2B, with reduced doublecortin and DCC but no change in NFkB. Despite altered protein expression, there were no changes in the kynurenine pathway. The results show that maternal exposure to poly(I:C) alters the expression of proteins in the embryos and offspring which may affect the development of dopaminergic function. The oxidation of tryptophan along the kynurenine pathway is not involved in these effects.

Dysfunctional synapse in Alzheimer's disease - A focus on NMDA receptors

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly. Alterations capable of causing brain circuitry dysfunctions in AD may take several years to develop. Oligomeric amyloid-beta peptide (Aβ) plays a complex role in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in this devastating disease. Moreover, N-methyl-D-aspartate (NMDA) receptors (NMDARs) activation has been recently implicated in AD-related synaptic dysfunction. Thus, in this review we focus on glutamatergic neurotransmission impairment and the changes in NMDAR regulation in AD, following the description on the role and location of NMDARs at pre- and post-synaptic sites under physiological conditions. In addition, considering that there is currently no effective ways to cure AD or stop its progression, we further discuss the relevance of NMDARs antagonists to prevent AD symptomatology. This review posits additional information on the role played by Aβ in AD and the importance of targeting the tripartite glutamatergic synapse in early asymptomatic and possible reversible stages of the disease through preventive and/or disease-modifying therapeutic strategies. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

NMDA receptor as a newly identified member of the metabotropic glutamate receptor family: clinical implications for neurodegenerative diseases

Recent reports have proposed a novel function for the N-methyl-D-aspartate (NMDA) receptor (NMDAR), a well-known excitatory, ionotropic receptor. A series of observations employing pharmacological techniques has proposed that upon ligand binding, this ionotropic receptor can actually function via signaling cascades independent of traditional ionotropic action. Moreover, the "metabotropic" action of NMDARs is suggested to mediate a form of synaptic plasticity, namely long-term synaptic depression (LTD), which shares cellular mechanisms with the synaptic deficits observed in Alzheimer's disease. Given that a growing body of clinical and preclinical evidence strongly recommends NMDAR antagonists for their therapeutic potentials and advantages in a variety of diseases, further investigation into their molecular and cellular mechanisms is required to better understand the "metabotropic" action of NMDARs.

NMDA-receptor activation but not ion flux is required for amyloid-beta induced synaptic depression

Alzheimer disease is characterized by a gradual decrease of synaptic function and, ultimately, by neuronal loss. There is considerable evidence supporting the involvement of oligomeric amyloid-beta (Aβ) in the etiology of Alzheimer’s disease. Historically, AD research has mainly focused on the long-term changes caused by Aβ rather than analyzing its immediate effects. Here we show that acute perfusion of hippocampal slice cultures with oligomeric Aβ depresses synaptic transmission within 20 minutes. This depression is dependent on synaptic stimulation and the activation of NMDA-receptors, but not on NMDA-receptor mediated ion flux. It, therefore, appears that Aβ dependent synaptic depression is mediated through a use-dependent metabotropic-like mechanism of the NMDA-receptor, but does not involve NMDA-receptor mediated synaptic transmission, i.e. it is independent of calcium flux through the NMDA-receptor.