Subunit-specific NMDA receptor trafficking to synapses

Myosin Va-dependent Transport of NMDA Receptors in Hippocampal Neurons

N-methyl-D-aspartate receptor (NMDAR) trafficking is a key process in the regulation of synaptic efficacy and brain function. However, the molecular mechanism underlying the surface transport of NMDARs is largely unknown. Here we identified myosin Va (MyoVa) as the specific motor protein that traffics NMDARs in hippocampal neurons. We found that MyoVa associates with NMDARs through its cargo binding domain. This association was increased during NMDAR surface transport. Knockdown of MyoVa suppressed NMDAR transport. We further demonstrated that Ca2+/calmodulin-dependent protein kinase II (CaMKII) regulates NMDAR transport through its direct interaction with MyoVa. Furthermore, MyoVa employed Rab11 family-interacting protein 3 (Rab11/FIP3) as the adaptor proteins to couple themselves with NMDARs during their transport. Accordingly, the knockdown of FIP3 impairs hippocampal memory. Together, we conclude that in hippocampal neurons, MyoVa conducts active transport of NMDARs in a CaMKII-dependent manner.

GluN2A- and GluN2B-containing pre-synaptic N-methyl-d-aspartate receptors differentially regulate action potential-evoked Ca2+ influx via modulation of SK channels

N-methyl-d-aspartate receptors (NMDARs) play a pivotal role in synaptic plasticity. While the functional role of post-synaptic NMDARs is well established, pre-synaptic NMDAR (pre-NMDAR) function is largely unexplored. Different pre-NMDAR subunit populations are documented at synapses, suggesting that subunit composition influences neuronal transmission. Here, we used electrophysiological recordings at Schaffer collateral-CA1 synapses partnered with Ca2+ imaging and glutamate uncaging at boutons of CA3 pyramidal neurones to reveal two populations of pre-NMDARs that contain either the GluN2A or GluN2B subunit. Activation of the GluN2B population decreases action potential-evoked Ca2+ influx via modulation of small-conductance Ca2+-activated K+ channels, while activation of the GluN2A population does the opposite. Critically, the level of functional expression of the subunits is subject to homeostatic regulation, bidirectionally affecting short-term facilitation, thus providing a capacity for a fine adjustment of information transfer. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Half a century legacy of long-term potentiation

In 1973, two papers from Bliss and Lømo and from Bliss and Gardner-Medwin reported that high-frequency synaptic stimulation in the dentate gyrus of rabbits resulted in a long-lasting increase in synaptic strength. This form of synaptic plasticity, commonly referred to as long-term potentiation (LTP), was immediately considered as an attractive mechanism accounting for the ability of the brain to store information. In this historical piece looking back over the past 50 years, we discuss how these two landmark contributions directly motivated a colossal research effort and detail some of the resulting milestones that have shaped our evolving understanding of the molecular and cellular underpinnings of LTP. We highlight the main features of LTP, cover key experiments that defined its induction and expression mechanisms, and outline the evidence supporting a potential role of LTP in learning and memory. We also briefly explore some ramifications of LTP on network stability, consider current limitations of LTP as a model of associative memory, and entertain future research orientations.

Dynamic regulation of phosphorylation of NMDA receptor GluN2B subunit tyrosine residues mediates ketamine rapid antidepressant effects

The rapid antidepressant effects of ketamine depend on the N-methyl-D-aspartate (NMDA) receptor containing 2B subunit (NR2B), whose function is influenced by its phosphorylated regulation and distribution within and outside synapses. It remains unclear if ketamine's rapid onset of antidepressant effects relies on the dynamic phosphorylated regulation of NR2B within and outside synapses. Here, we show that ketamine rapidlyalleviated depression-like behaviors and normalized abnormal expression of pTyr1472NR2B and striatal-enriched protein tyrosine phosphatase (STEP) 61 within and outside synapses in the medial prefrontal cortex (mPFC) induced by chronic unpredictable stress (CUS) and conditional knockdown of STEP 61, a key phosphatase of NR2B, within 1 hour after administration Together, our results delineate the rapid initiation of ketamine's antidepressant effects results from the restoration of NR2B phosphorylation homeostasis within and outside synapses. The dynamic regulation of phosphorylation of NR2B provides a new perspective for developing new antidepressant strategies.

Ex Vivo Cortical Circuits Learn to Predict and Spontaneously Replay Temporal Patterns

It has been proposed that prediction and timing are computational primitives of neocortical microcircuits, specifically, that neural mechanisms are in place to allow neocortical circuits to autonomously learn the temporal structure of external stimuli and generate internal predictions. To test this hypothesis, we trained cortical organotypic slices on two specific temporal patterns using dual-optical stimulation. After 24-hours of training, whole-cell recordings revealed network dynamics consistent with training-specific timed prediction. Unexpectedly, there was replay of the learned temporal structure during spontaneous activity. Furthermore, some neurons exhibited timed prediction errors. Mechanistically our results indicate that learning relied in part on asymmetric connectivity between distinct neuronal ensembles with temporally-ordered activation. These findings further suggest that local cortical microcircuits are intrinsically capable of learning temporal information and generating predictions, and that the learning rules underlying temporal learning and spontaneous replay can be intrinsic to local cortical microcircuits and not necessarily dependent on top-down interactions.

Rescuing tri-heteromeric NMDA receptor function: the potential of pregnenolone-sulfate in loss-of-function GRIN2B variants

N-methyl-D-aspartate receptors (NMDARs emerging from GRIN genes) are tetrameric receptors that form diverse channel compositions in neurons, typically consisting of two GluN1 subunits combined with two GluN2(A-D) subunits. During prenatal stages, the predominant channels are di-heteromers with two GluN1 and two GluN2B subunits due to the high abundance of GluN2B subunits. Postnatally, the expression of GluN2A subunits increases, giving rise to additional subtypes, including GluN2A-containing di-heteromers and tri-heteromers with GluN1, GluN2A, and GluN2B subunits. The latter  emerge as the major receptor subtype at mature synapses in the hippocampus. Despite extensive research on purely di-heteromeric receptors containing two identical GRIN variants, the impact of a single variant on the function of other channel forms, notably tri-heteromers, is lagging. In this study, we systematically investigated the effects of two de novo GRIN2B variants (G689C and G689S) in pure, mixed di- and tri-heteromers. Our findings reveal that incorporating a single variant in mixed di-heteromers or tri-heteromers exerts a dominant negative effect on glutamate potency, although 'mixed' channels show improved potency compared to pure variant-containing di-heteromers. We show that a single variant within a receptor complex does not impair the response of all receptor subtypes to the positive allosteric modulator pregnenolone-sulfate (PS), whereas spermine completely fails to potentiate tri-heteromers containing GluN2A and -2B-subunits. We examined PS on primary cultured hippocampal neurons transfected with the variants, and observed a positive impact over current amplitudes and synaptic activity. Together, our study supports previous observations showing that mixed di-heteromers exhibit improved glutamate potency and extend these findings towards the exploration of the effect of Loss-of-Function variants over tri-heteromers. Notably, we provide an initial and crucial demonstration of the beneficial effects of GRIN2B-relevant potentiators on tri-heteromers. Our results underscore the significance of studying how different variants affect distinct receptor subtypes, as these effects cannot be inferred solely from observations made on pure di-heteromers. Overall, this study contributes to ongoing efforts to understand the pathophysiology of GRINopathies and provides insights into potential treatment strategies.

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamatergic transmission can be mediated by ionotropic glutamate receptors (iGluRs), which mediate rapid synaptic depolarization that can be associated with Ca2+ entry and activity-dependent change in the strength of synaptic transmission, as well as by metabotropic glutamate receptors (mGluRs), which mediate slower postsynaptic responses through the recruitment of second messenger systems. A wealth of evidence reported over the last three decades has shown that this dogmatic subdivision between iGluRs and mGluRs may not reflect the actual physiological signaling mode of the iGluRs, i.e., α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA) receptors (AMPAR), kainate receptors (KARs), and N-methyl-D-aspartate (NMDA) receptors (NMDARs). Herein, we review the evidence available supporting the notion that the canonical iGluRs can recruit flux-independent signaling pathways not only in neurons, but also in brain astrocytes and cerebrovascular endothelial cells. Understanding the signaling versatility of iGluRs can exert a profound impact on our understanding of glutamatergic synapses. Furthermore, it may shed light on novel neuroprotective strategies against brain disorders.

Early expression of GluN2A-containing NMDA receptors in a model of fragile X syndrome

NMDA-type glutamate receptors (NMDARs) play a crucial role in synaptogenesis, circuit development, and synaptic plasticity, serving as fundamental components in cellular models of learning and memory. Their dysregulation has been implicated in several neurological disorders and synaptopathies. NMDARs are heterotetrameric complexes composed of two GluN1 and two GluN2 subunits. The composition of GluN2 subunits determines the main biophysical properties of the channel, such as calcium permeability and gating kinetics, and influences the ability of the receptor to interact with postsynaptic proteins involved in normal synaptic physiology and plasticity, including scaffolding proteins and signaling molecules. During early development, NMDARs in the forebrain contain solely the GluN2B subunit, a necessary subunit for proper synaptogenesis and synaptic plasticity. As the animal matures, the expression of the GluN2A subunit increases, leading to a partial replacement of GluN2B-containing synaptic NMDARs with GluN2A-containing receptors. The switch in the synaptic GluN2A-to-GluN2B ratio has a significant impact on the kinetics of excitatory postsynaptic currents and diminishes the synaptic plasticity capacity. In this study, we present findings indicating that GluN2A expression occurs earlier in a mouse model of fragile X syndrome (FXS). This altered timing of GluN2A expression affects various important parameters of NMDAR-mediated excitatory postsynaptic currents, including maximal current amplitude, decay time, and response to consecutive stimuli delivered in close temporal proximity. These observations suggest that the early expression of GluN2A during a critical period when synapses and circuits are developing could be an underlying factor contributing to the formation of pathological circuits in the FXS mouse model.NEW & NOTEWORTHY NMDA receptors (NMDARs) play important roles in synaptic transmission and are involved in multiple neurological disorders. During development, GluN2A in the forebrain becomes incorporated into previously GluN2B-dominated NMDARs, leading to the "GluN2A/GluN2B ratio switch." This is a crucial step for normal brain development. Here we present findings indicating that GluN2A expression occurs earlier in the fragile X mouse and this could be an underlying factor contributing to the pathology found in the fragile X model.

Glutamate signaling and neuroligin/neurexin adhesion play opposing roles that are mediated by major histocompatibility complex I molecules in cortical synapse formation

Although neurons release neurotransmitter before contact, the role for this release in synapse formation remains unclear. Cortical synapses do not require synaptic vesicle release for formation 1-4 , yet glutamate clearly regulates glutamate receptor trafficking 5,6 and induces spine formation 7-11 . Using a culture system to dissect molecular mechanisms, we found that glutamate rapidly decreases synapse density specifically in young cortical neurons in a local and calcium-dependent manner through decreasing NMDAR transport and surface expression as well as co-transport with neuroligin (NL1). Adhesion between NL1 and neurexin 1 protects against this glutamate-induced synapse loss. Major histocompatibility I (MHCI) molecules are required for the effects of glutamate in causing synapse loss through negatively regulating NL1 levels. Thus, like acetylcholine at the NMJ, glutamate acts as a dispersal signal for NMDARs and causes rapid synapse loss unless opposed by NL1-mediated trans-synaptic adhesion. Together, glutamate, MHCI and NL1 mediate a novel form of homeostatic plasticity in young neurons that induces rapid changes in NMDARs to regulate when and where nascent glutamatergic synapses are formed.

Tonic NMDAR Currents of NR2A-Containing NMDARs Represent Altered Ambient Glutamate Concentration in the Supraoptic Nucleus

NMDA receptors (NMDARs) modulate glutamatergic excitatory tone in the brain via two complementary modalities: a phasic excitatory postsynaptic current and a tonic extrasynaptic modality. Here, we demonstrated that the tonic NMDAR-current (I NMDA) mediated by NR2A-containing NMDARs is an efficient biosensor detecting the altered ambient glutamate level in the supraoptic nucleus (SON). I NMDA of magnocellular neurosecretory cells (MNCs) measured by nonselective NMDARs antagonist, AP5, at holding potential (V holding) -70 mV in low concentration of ECF Mg2+ ([Mg2+]o) was transiently but significantly increased 1-week post induction of a DOCA salt hypertensive model rat which was compatible with that induced by a NR2A-selective antagonist, PEAQX (I PEAQX) in both DOCA-H2O and DOCA-salt groups. In agreement, NR2B antagonist, ifenprodil, or NR2C/D antagonist, PPDA, did not affect the holding current (I holding) at V holding -70 mV. Increased ambient glutamate by exogenous glutamate (10 mM) or excitatory amino acid transporters (EAATs) antagonist (TBOA, 50 mM) abolished the I PEAQX difference between two groups, suggesting that attenuated EAATs activity increased ambient glutamate concentration, leading to the larger I PEAQX in DOCA-salt rats. In contrast, only ifenprodil but not PEAQX and PPDA uncovered I NMDA at V holding +40 mV under 1.2 mM [Mg2+]o condition. I ifenprodil was not different in DOCA-H2O and DOCA-salt groups. Finally, NR2A, NR2B, and NR2D protein expression were not different in the SON of the two groups. Taken together, NR2A-containing NMDARs efficiently detected the increased ambient glutamate concentration in the SON of DOCA-salt hypertensive rats due to attenuated EAATs activity.

Disease-associated nonsense and frame-shift variants resulting in the truncation of the GluN2A or GluN2B C-terminal domain decrease NMDAR surface expression and reduce potentiating effects of neurosteroids

N-methyl-D-aspartate receptors (NMDARs) play a critical role in normal brain function, and variants in genes encoding NMDAR subunits have been described in individuals with various neuropsychiatric disorders. We have used whole-cell patch-clamp electrophysiology, fluorescence microscopy and in-silico modeling to explore the functional consequences of disease-associated nonsense and frame-shift variants resulting in the truncation of GluN2A or GluN2B C-terminal domain (CTD). This study characterizes variant NMDARs and shows their reduced surface expression and synaptic localization, altered agonist affinity, increased desensitization, and reduced probability of channel opening. We also show that naturally occurring and synthetic steroids pregnenolone sulfate and epipregnanolone butanoic acid, respectively, enhance NMDAR function in a way that is dependent on the length of the truncated CTD and, further, is steroid-specific, GluN2A/B subunit-specific, and GluN1 splice variant-specific. Adding to the previously described effects of disease-associated NMDAR variants on the receptor biogenesis and function, our results improve the understanding of the molecular consequences of NMDAR CTD truncations and provide an opportunity for the development of new therapeutic neurosteroid-based ligands.

Activity-Dependent Stabilization of Nascent Dendritic Spines Requires Nonenzymatic CaMKIIα Function

The outgrowth and stabilization of nascent dendritic spines are crucial processes underlying learning and memory. Most new spines retract shortly after growth; only a small subset is stabilized and integrated into the new circuit connections that support learning. New spine stabilization has been shown to rely upon activity-dependent molecular mechanisms that also contribute to long-term potentiation (LTP) of synaptic strength. Indeed, disruption of the activity-dependent targeting of the kinase CaMKIIα to the GluN2B subunit of the NMDA-type glutamate receptor disrupts both LTP and activity-dependent stabilization of new spines. Yet it is not known which of CaMKIIα's many enzymatic and structural functions are important for new spine stabilization. Here, we used two-photon imaging and photolysis of caged glutamate to monitor the activity-dependent stabilization of new dendritic spines on hippocampal CA1 neurons from mice of both sexes in conditions where CaMKIIα functional and structural interactions were altered. Surprisingly, we found that inhibiting CaMKIIα kinase activity either genetically or pharmacologically did not impair activity-dependent new spine stabilization. In contrast, shRNA knockdown of CaMKIIα abolished activity-dependent new spine stabilization, which was rescued by co-expressing shRNA-resistant full-length CaMKIIα, but not by a truncated monomeric CaMKIIα. Notably, overexpression of phospho-mimetic CaMKIIα-T286D, which exhibits activity-independent targeting to GluN2B, enhanced basal new spine survivorship in the absence of additional glutamatergic stimulation, even when kinase activity was disrupted. Together, our results support a model in which nascent dendritic spine stabilization requires structural and scaffolding interactions mediated by dodecameric CaMKIIα that are independent of its enzymatic activities.

ROR2 homodimerization is sufficient to activate a neuronal Wnt/calcium signaling pathway

Wnt signaling plays a key role in the mature CNS by regulating trafficking of NMDA-type glutamate receptors and intrinsic properties of neurons. The Wnt receptor ROR2 has been identified as a necessary component of the neuronal Wnt5a/Ca2+ signaling pathway that regulates synaptic and neuronal function. Since ROR2 is considered a pseudokinase, its mechanism for downstream signaling upon ligand binding has been controversial. It has been suggested that its role is to function as a coreceptor of a G-protein-coupled Wnt receptor of the Frizzled family. We show that chemically induced homodimerization of ROR2 is sufficient to recapitulate key signaling events downstream of receptor activation in neurons, including PKC and JNK kinases activation, elevation of somatic and dendritic Ca2+ levels, and increased trafficking of NMDARs to synapses. In addition, we show that homodimerization of ROR2 induces phosphorylation of the receptor on Tyr residues. Point mutations in the conserved but presumed nonfunctional ATP-binding site of the receptor prevent its phosphorylation, as well as downstream signaling. This suggests an active kinase domain. Our results indicate that ROR2 can signal independently of Frizzled receptors to regulate the trafficking of a key synaptic component. Additionally, they suggest that homodimerization can overcome structural conformations that render the tyrosine kinase inactive. A better understanding of ROR2 signaling is crucial for comprehending the regulation of synaptic and neuronal function in normal brain processes in mature animals.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

Uncovering the Significance of STEP61 in Alzheimer's Disease: Structure, Substrates, and Interactome

STEP (STriatal-Enriched Protein Tyrosine Phosphatase) is a brain-specific phosphatase that plays an important role in controlling signaling molecules involved in neuronal activity and synaptic development. The striatum is the main location of the STEP enzyme. An imbalance in STEP61 activity is a risk factor for Alzheimer's disease (AD). It can contribute to the development of numerous neuropsychiatric diseases, including Parkinson's disease (PD), schizophrenia, fragile X syndrome (FXS), Huntington's disease (HD), alcoholism, cerebral ischemia, and stress-related diseases. The molecular structure, chemistry, and molecular mechanisms associated with STEP61's two major substrates, Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPAr) and N-methyl-D-aspartate receptors (NMDARs), are crucial in understanding the relationship between STEP61 and associated illnesses. STEP's interactions with its substrate proteins can alter the pathways of long-term potentiation and long-term depression. Therefore, understanding the role of STEP61 in neurological illnesses, particularly Alzheimer's disease-associated dementia, can provide valuable insights for possible therapeutic interventions. This review provides valuable insights into the molecular structure, chemistry, and molecular mechanisms associated with STEP61. This brain-specific phosphatase controls signaling molecules involved in neuronal activity and synaptic development. This review can aid researchers in gaining deep insights into the complex functions of STEP61.

Synaptogenesis by Cholinergic Stimulation of Astrocytes

Astrocytes release numerous factors known to contribute to the process of synaptogenesis, yet knowledge about the signals that control their release is limited. We hypothesized that neuron-derived signals stimulate astrocytes, which respond to neurons through the modulation of astrocyte-released synaptogenic factors. Here we investigate the effect of cholinergic stimulation of astrocytes on synaptogenesis in co-cultured neurons. Using a culture system where primary rat astrocytes and primary rat neurons are first grown separately allowed us to independently manipulate astrocyte cholinergic signaling. Subsequent co-culture of pre-stimulated astrocytes with naïve neurons enabled us to assess how prior stimulation of astrocyte acetylcholine receptors uniquely modulates neuronal synapse formation. Pre-treatment of astrocytes with the acetylcholine receptor agonist carbachol increased the expression of synaptic proteins, the number of pre- and postsynaptic puncta, and the number of functional synapses in hippocampal neurons after 24 h in co-culture. Astrocyte secretion of the synaptogenic protein thrombospondin-1 increased after cholinergic stimulation and inhibition of the receptor for thrombospondins prevented the increase in neuronal synaptic structures. Thus, we identified a novel mechanism of neuron-astrocyte-neuron communication, where neuronal release of acetylcholine stimulates astrocytes to release synaptogenic proteins leading to increased synaptogenesis in neurons. This study provides new insights into the role of neurotransmitter receptors in developing astrocytes and into our understanding of the modulation of astrocyte-induced synaptogenesis.

Molecular basis of interactions between CaMKII and α-actinin-2 that underlie dendritic spine enlargement

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is essential for long-term potentiation (LTP) of excitatory synapses that is linked to learning and memory. In this study, we focused on understanding how interactions between CaMKIIα and the actin-crosslinking protein α-actinin-2 underlie long-lasting changes in dendritic spine architecture. We found that association of the two proteins was unexpectedly elevated within 2 minutes of NMDA receptor stimulation that triggers structural LTP in primary hippocampal neurons. Furthermore, disruption of interactions between the two proteins prevented the accumulation of enlarged mushroom-type dendritic spines following NMDA receptor activation. α-Actinin-2 binds to the regulatory segment of CaMKII. Calorimetry experiments, and a crystal structure of α-actinin-2 EF hands 3 and 4 in complex with the CaMKII regulatory segment, indicate that the regulatory segment of autoinhibited CaMKII is not fully accessible to α-actinin-2. Pull-down experiments show that occupation of the CaMKII substrate-binding groove by GluN2B markedly increases α-actinin-2 access to the CaMKII regulatory segment. Furthermore, in situ labelling experiments are consistent with the notion that recruitment of CaMKII to NMDA receptors contributes to elevated interactions between the kinase and α-actinin-2 during structural LTP. Overall, our study provides new mechanistic insight into the molecular basis of structural LTP and reveals an added layer of sophistication to the function of CaMKII.

Visualizing the triheteromeric N-methyl-D-aspartate receptor subunit composition

N-methyl-D-aspartate receptors (NMDARs) are one of three ligand-gated ionotropic channels that transduce the effects of neurotransmitter glutamate at excitatory synapses within the central nervous system. Their ability to influx Ca2+ into cells, unlike mature AMPA or kainate receptors, implicates them in a variety of processes ranging from synaptic plasticity to cell death. Many of the receptor's capabilities, including binding glutamate and regulating Ca2+ influx, have been attributed to their subunit composition, determined putatively using cell biology, electrophysiology and/or pharmacology. Here, we show that subunit composition of synaptic NMDARs can also be readily visualized in acute brain slices (rat) using highly specific antibodies directed against extracellular epitopes of the subunit proteins and high-resolution confocal microscopy. This has helped confirm the expression of triheteromeric t-NMDARs (containing GluN1, GluN2, and GluN3 subunits) at synapses for the first time and reconcile functional differences with diheteromeric d-NMDARs (containing GluN1 and GluN2 subunits) described previously. Even though structural information about individual receptors is still diffraction limited, fluorescently tagged receptor subunit puncta coalesce with precision at various magnifications and/or with the postsynaptic density (PSD-95) but not the presynaptic active zone marker Bassoon. These data are particularly relevant for identifying GluN3A-containing t-NMDARs that are highly Ca2+ permeable and whose expression at excitatory synapses renders neurons vulnerable to excitotoxicity and cell death. Imaging NMDAR subunit proteins at synapses not only offers firsthand insights into subunit composition to correlate function but may also help identify zones of vulnerability within brain structures underlying neurodegenerative diseases like Temporal Lobe Epilepsy.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Synaptogenesis by Cholinergic Stimulation of Astrocytes

Astrocytes release numerous factors known to contribute to the process of synaptogenesis, yet knowledge about the signals that control their release is limited. We hypothesized that neuron-derived signals stimulate astrocytes, which respond by signaling back to neurons through the modulation of astrocyte-released synaptogenic factors. Here we investigate the effect of cholinergic stimulation of astrocytes on synaptogenesis in co-cultured neurons. Using a culture system where primary rat astrocytes and primary rat neurons are first grown separately allowed us to independently manipulate astrocyte cholinergic signaling. Subsequent co-culture of pre-stimulated astrocytes with naïve neurons enabled us to assess how prior stimulation of astrocyte acetylcholine receptors uniquely modulates neuronal synapse formation. Pre-treatment of astrocytes with the acetylcholine receptor agonist carbachol increased the expression of synaptic proteins, the number of pre- and postsynaptic puncta, and the number of functional synapses in hippocampal neurons after 24 hours in co-culture. Astrocyte secretion of the synaptogenic protein thrombospondin-1 increased after cholinergic stimulation and the inhibition of the target receptor for thrombospondins prevented the observed increase in neuronal synaptic structures. Thus, we identified a novel mechanism of neuron-astrocyte-neuron communication, i.e. , neuronal release of acetylcholine stimulates astrocytes to release synaptogenic proteins leading to increased synaptogenesis in neurons. This study provides new insights into the role of neurotransmitter receptors in developing astrocytes and into our understanding of the modulation of astrocyte-induced synaptogenesis.

Pharmacological characterization and differential expression of NMDA receptor subunits in the chicken vestibular system during development

Previous studies demonstrated that in vitro preparations of the isolated vestibular system of diverse animal species still exhibit stable resting electrical activity and mechanically evoked synaptic transmission between hair cells and primary afferent endings. However, there are no reports related to their neurodevelopment. Therefore, this research aimed to examine whether NMDA receptors mediate these electrical signals in an isolated preparation of the chicken vestibular system at three developmental stages, E15, E18, and E21. We found that the spontaneous and mechanically evoked discharges from primary afferents of the posterior semicircular canal were modulated by agonists NMDA and glycine, but not by the agonist d-serine applied near the synapses. Moreover, the individually applied by bath perfusion of three NMDA receptor antagonists (MK-801, ifenprodil, and 2-naphthoic acid) or high Mg²⁺ decreased the resting discharge rate, the NMDA response, and the discharge rate of mechanically evoked activity from these primary afferents. Furthermore, we found that the vestibular ganglion shows a stage-dependent increase in the expression of NMDA receptor subunits GluN1, GluN2 (A-C), and GluN3 (A-B), being greater at E21, except for GluN2D, which was inversely related to the developmental stage. However, in the crista ampullaris, the expression pattern remained constant throughout development. This could suggest the possible existence of presynaptic NMDA receptors. Our results highlight that although the NMDA receptors are functionally active at the early embryonic stages of the vestibular system, NMDA and glycine reach their mature functionality to increase NMDA responses close to hatching (E21).

Roles and Sources of Calcium in Synaptic Exocytosis

Calcium ions (Ca2+) play a critical role in triggering neurotransmitter release. The rate of release is directly related to the concentration of Ca2+ at the presynaptic site, with a supralinear relationship. There are two main sources of Ca2+ that trigger synaptic vesicle fusion: influx through voltage-gated Ca2+ channels in the plasma membrane and release from the endoplasmic reticulum via ryanodine receptors. This chapter will cover the sources of Ca2+ at the presynaptic nerve terminal, the relationship between neurotransmitter release rate and Ca2+ concentration, and the mechanisms that achieve the necessary Ca2+ concentrations for triggering synaptic exocytosis at the presynaptic site.

How Are Synapses Born? A Functional and Molecular View of the Role of the Wnt Signaling Pathway

Synaptic transmission is a dynamic process that requires precise regulation. Early in life, we must be able to forge appropriate connections (add and remove) to control our behavior. Neurons must recognize appropriate targets, and external soluble factors that activate specific signaling cascades provide the regulation needed to achieve this goal. Wnt signaling has been implicated in several forms of synaptic plasticity, including functional and structural changes associated with brain development. The analysis of synapses from an electrophysiological perspective allows us to characterize the functional role of cellular signaling pathways involved in brain development. The application of quantal theory to principles of developmental plasticity offers the possibility of dissecting the function of structural changes associated with the birth of new synapses as well as the maturation of immature silent synapses. Here, we focus on electrophysiological and molecular evidence that the Wnt signaling pathway regulates glutamatergic synaptic transmission, specifically N-methyl-d-aspartate receptors (NMDARs), to control the birth of new synapses. We also focus on the role of Wnts in the conversion of silent synapses into functional synapses.

Activity-Dependent Induction of Younger Biological Phenotypes

In several mammalian species, including humans, complex stimulation patterns such as cognitive and physical exercise lead to improvements in organ function, organism health and performance, as well as possibly longer lifespans. A framework is introduced here in which activity-dependent transcriptional programs, induced by these environmental stimuli, move somatic cells such as neurons and muscle cells toward a state that resembles younger cells to allow remodeling and adaptation of the organism. This cellular adaptation program targets several process classes that are heavily implicated in aging, such as mitochondrial metabolism, cell-cell communication, and epigenetic information processing, and leads to functional improvements in these areas. The activity-dependent gene program (ADGP) can be seen as a natural, endogenous cellular reprogramming mechanism that provides deep insight into the principles of inducible improvements in cell and organism function and can guide the development of therapeutic approaches for longevity. Here, these ADGPs are analyzed, exemplary critical molecular nexus points such as cAMP response element-binding protein, myocyte enhancer factor 2, serum response factor, and c-Fos are identified, and it is explored how one may leverage them to prevent, attenuate, and reverse human aging-related decline of body function.

Wnt-5a induces the conversion of silent to functional synapses in the hippocampus

Synapse unsilencing is an essential mechanism for experience-dependent plasticity. Here, we showed that the application of the ligand Wnt-5a converts glutamatergic silent synapses into functional ones by increasing both α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) currents (IAMPA and INMDA, respectively). These effects were mimicked by the hexapeptide Foxy-5 and inhibited by secreted frizzled-related protein sFRP-2. INMDA potentiation was produced by increased synaptic potency, followed by an increase in the probability of release (Pr), even in the presence of 7-nitro-2,3-dioxo-1,4-dihydroquinoxaline-6-carbonitrile (CNQX). At a longer time of Wnt-5a exposure, the Pr increments were higher in INMDA than in IAMPA. In the presence of NMDAR inhibitors, Wnt-5a-induced conversion was fully inhibited in 69.0% of silent synapses, whereas in the remaining synapses were converted into functional one. Our study findings showed that the Wnt-5a-activated pathway triggers AMPAR insertion into mammalian glutamatergic synapses, unsilencing non-functional synapses and promoting the formation of nascent synapses during the early postnatal development of the brain circuits.

Moringa Oleifera Alleviates Aβ Burden and Improves Synaptic Plasticity and Cognitive Impairments in APP/PS1 Mice

Alzheimer's disease is a global public health problem and the most common form of dementia. Due to the failure of many single therapies targeting the two hallmarks, Aβ and Tau, and the multifactorial etiology of AD, there is now more and more interest in nutraceutical agents with multiple effects such as Moringa oleifera (MO) that have strong anti-oxidative, anti-inflammatory, anticholinesterase, and neuroprotective virtues. In this study, we treated APP/PS1 mice with a methanolic extract of MO for four months and evaluated its effect on AD-related pathology in these mice using a multitude of behavioral, biochemical, and histochemical tests. Our data revealed that MO improved behavioral deficits such as anxiety-like behavior and hyperactivity and cognitive, learning, and memory impairments. MO treatment abrogated the Aβ burden to wild-type control mice levels via decreasing BACE1 and AEP and upregulating IDE, NEP, and LRP1 protein levels. Moreover, MO improved synaptic plasticity by improving the decreased GluN2B phosphorylation, the synapse-related proteins PSD95 and synapsin1 levels, the quantity and quality of dendritic spines, and neurodegeneration in the treated mice. MO is a nutraceutical agent with promising therapeutic potential that can be used in the management of AD and other neurodegenerative diseases.

ATP-binding cassette protein ABCA7 deficiency impairs sphingomyelin synthesis, cognitive discrimination, and synaptic plasticity in the entorhinal cortex

Sphingomyelin (SM) is an abundant plasma membrane and plasma lipoprotein sphingolipid. We previously reported that ATP-binding cassette family A protein 1 (ABCA1) deficiency in humans and mice decreases plasma SM levels. However, overexpression, induction, downregulation, inhibition, and knockdown of ABCA1 in human hepatoma Huh7 cells did not decrease SM efflux. Using unbiased siRNA screening, here we identified that ABCA7 plays a role in the biosynthesis and efflux of SM without affecting cellular uptake and metabolism. Since loss of function mutations in the ABCA7 gene exhibit strong associations with late-onset Alzheimer's disease (LOAD) across racial groups, we also studied the effects of ABCA7 deficiency in the mouse brain. Brains of ABCA7-deficient (KO) mice, compared with wild type (WT), had significantly lower levels of several SM species with long chain fatty acids. In addition, we observed that older KO mice exhibited behavioral deficits in cognitive discrimination in the active place avoidance task. Next, we performed synaptic transmission studies in brain slices obtained from older mice. We found anomalies in synaptic plasticity at the intracortical layer II/III lateral entorhinal cortex synapse but not in the hippocampal synapses in KO mice. These synaptic abnormalities in KO brain slices were rescued with extracellular SM supplementation, but not by supplementation with phosphatidylcholine. Taken together, these studies identify a role of ABCA7 in brain SM metabolism and the importance of SM in synaptic plasticity and cognition, as well as provide a possible explanation for the association between ABCA7 and LOAD.

Trafficking of NMDA receptors is essential for hippocampal synaptic plasticity and memory consolidation

NMDA receptor (NMDAR) plays a vital role in brain development and normal physiological functions. Surface trafficking of NMDAR contributes to the modulation of synaptic functions and information processing. However, it remains unclear whether NMDAR trafficking is independent of long-term potentiation (LTP) and whether it regulates behavior. Here, we report that LTP of AMPAR and NMDAR can occur concurrently and that NMDAR trafficking can regulate AMPAR trafficking and AMPAR-mediated LTP. By contrast, AMPAR trafficking does not impact NMDAR-mediated LTP. Using SAP97-interfering peptide and SAP97 knockin (KI) rat, we show that the effect is mediated by GluN2A-subunit-containing NMDARs. At the behavior level, impaired NMDAR trafficking results in deficits in consolidation, but not acquisition, of fear memory. Collectively, our results suggest the essential role of NMDAR trafficking in LTP and memory consolidation.

Ginsenoside Rb1 attenuates methamphetamine (METH)-induced neurotoxicity through the NR2B/ERK/CREB/BDNF signalings in vitro and in vivo models

Aim

This study investigates the effects of ginsenoside Rb1 (GsRb1) on methamphetamine (METH)-induced toxicity in SH-SY5Y neuroblastoma cells and METH-induced conditioned place preference (CPP) in adult Sprague-Dawley rats. It also examines whether GsRb1 can regulate these effects through the NR2B/ERK/CREB/BDNF signaling pathways.

Methods

SH-SY5Y cells were pretreated with GsRb1 (20 μM and 40 μM) for 1 h, followed by METH treatment (2 mM) for 24 h. Rats were treated with METH (2 mg/kg) or saline on alternating days for 10 days to allow CPP to be examined. GsRb1 (5, 10, and 20 mg/kg) was injected intraperitoneally 1 h before METH or saline. Western blot was used to examine the protein expression of NR2B, ERK, P-ERK, CREB, P-CREB, and BDNF in the SH-SY5Y cells and the rats' hippocampus, nucleus accumbens (NAc), and prefrontal cortex (PFC).

Results

METH dose-dependently reduced the viability of SH-SY5Y cells. Pretreatment of cells with 40 μM of GsRb1 increased cell viability and reduced the expression of METH-induced NR2B, p-ERK, p-CREB and BDNF. GsRb1 also attenuated the expression of METH CPP in a dose-dependent manner in rats. Further, GsRb1 dose-dependently reduced the expression of METH-induced NR2B, p-ERK, p-CREB, and BDNF in the PFC, hippocampus, and NAc of rats.

Conclusion

GsRb1 regulated METH-induced neurotoxicity in vitro and METH-induced CPP through the NR2B/ERK/CREB/BDNF regulatory pathway. GsRb1 could be a therapeutic target for treating METH-induced neurotoxicity or METH addiction.

Amyloid β / PKC-dependent alterations in NMDA receptor composition are detected in early stages of Alzheimer´s disease

Amyloid beta (Aβ)-mediated synapse dysfunction is an early event in Alzheimer’s disease (AD) pathogenesis and previous studies suggest that NMDA receptor (NMDAR) dysregulation may contribute to these pathological effects. Although Aβ peptides impair NMDAR expression and activity, the mechanisms mediating these alterations in the early stages of AD are unclear. Here, we observed that NMDAR subunit NR2B and PSD-95 levels were aberrantly upregulated and correlated with Aβ₄₂ load in human postsynaptic fractions of the prefrontal cortex in early stages of AD patients, as well as in the hippocampus of 3xTg-AD mice. Importantly, NR2B and PSD95 dysregulation was revealed by an increased expression of both proteins in Aβ-injected mouse hippocampi. In cultured neurons, Aβ oligomers increased the NR2B-containing NMDAR density in neuronal membranes and the NMDA-induced intracellular Ca²⁺ increase, in addition to colocalization in dendrites of NR2B subunit and PSD95. Mechanistically, Aβ oligomers required integrin β1 to promote synaptic location and function of NR2B-containing NMDARs and PSD95 by phosphorylation through classic PKCs. These results provide evidence that Aβ oligomers modify the contribution of NR2B to NMDAR composition and function in the early stages of AD through an integrin β1 and PKC-dependent pathway. These data reveal a novel role of Aβ oligomers in synaptic dysfunction that may be relevant to early-stage AD pathogenesis.

Ion flux-independent NMDA receptor signaling

NMDA receptors play vital roles in a broad array of essential brain functions, from synaptic transmission and plasticity to learning and memory. Historically, the fundamental roles of NMDARs were attributed to their specialized properties of ion flux. More recently, it has become clear that NMDARs also signal in an ion flux-independent manner. Here, we review these non-ionotropic NMDAR signaling mechanisms that have been reported to contribute to a broad array of neuronal functions and dysfunctions including synaptic transmission and plasticity, cell death and survival, and neurological disorders.

Common synaptic phenotypes arising from diverse mutations in the human NMDA receptor subunit GluN2A

Dominant mutations in the human gene GRIN2A, encoding NMDA receptor (NMDAR) subunit GluN2A, make a significant and growing contribution to the catalogue of published single-gene epilepsies. Understanding the disease mechanism in these epilepsy patients is complicated by the surprising diversity of effects that the mutations have on NMDARs. Here we have examined the cell-autonomous effect of five GluN2A mutations, 3 loss-of-function and 2 gain-of-function, on evoked NMDAR-mediated synaptic currents (NMDA-EPSCs) in CA1 pyramidal neurons in cultured hippocampal slices. Despite the mutants differing in their functional incorporation at synapses, prolonged NMDA-EPSC current decays (with only marginal changes in charge transfer) were a common effect for both gain- and loss-of-function mutants. Modelling NMDA-EPSCs with mutant properties in a CA1 neuron revealed that the effect of GRIN2A mutations can lead to abnormal temporal integration and spine calcium dynamics during trains of concerted synaptic activity. Investigations beyond establishing the molecular defects of GluN2A mutants are much needed to understand their impact on synaptic transmission.

The cell-autonomous effect of five severe loss- or gain-of-function GluN2A (NMDA receptor) mutations is assessed on evoked NMDAR-mediated synaptic currents in CA1 pyramidal neurons in cultured mouse hippocampal slices. Data and modelling suggest that mutant-like NMDA-EPSCs can lead to abnormal temporal summation and spine calcium dynamics.

Nanoscale rules governing the organization of glutamate receptors in spine synapses are subunit specific

Heterotetrameric glutamate receptors are essential for the development, function, and plasticity of spine synapses but how they are organized to achieve this is not known. Here we show that the nanoscale organization of glutamate receptors containing specific subunits define distinct subsynaptic features. Glutamate receptors containing GluA2 or GluN1 subunits establish nanomodular elements precisely positioned relative to Synaptotagmin-1 positive presynaptic release sites that scale with spine size. Glutamate receptors containing GluA1 or GluN2B specify features that exhibit flexibility: GluA1-subunit containing AMPARs are found in larger spines, while GluN2B-subunit containing NMDARs are enriched in the smallest spines with neither following a strict modular organization. Given that the precise positioning of distinct classes of glutamate receptors is linked to diverse events including cell death and synaptic plasticity, this unexpectedly robust synaptic nanoarchitecture provides a resilient system, where nanopositioned glutamate receptor heterotetramers define specific subsynaptic regions of individual spine synapses.

Glutamate receptors comprise two obligate subunits and two subunits that confer distinct properties and functions to the specific tetramers, which also localize to distinct synaptic spines. Here, the authors use STimulated Emission Depletion nanoscopy (STED) to provide detailed insights into the spatial organization of glutamate receptor types.

Behind the scenes: Are latent memories supported by calcium independent plasticity?

N-methyl-D-aspartate receptors (NMDARs) can be considered to be the de facto "plasticity" receptors in the brain due to their central role in the activity-dependent modification of neuronal morphology and synaptic transmission. Since the 1980s, research on NMDARs has focused on the second messenger properties of calcium and the downstream signaling pathways that mediate alterations in neural form and function. Recently, NMDARs were shown to drive activity-dependent synaptic plasticity without calcium influx. How this "nonionotropic" plasticity occurs in vitro is becoming clearer, but research on its involvement in behavior and cognition is in its infancy. There is a partial overlap in the downstream signaling molecules that are involved in ionotropic and nonionotropic NMDAR-dependent plasticity. Given this, and prior studies of the cognitive impacts of ionotropic NMDAR plasticity, a preliminary model explaining how NMDAR nonionotropic plasticity affects learning and memory can be established. We hypothesize that nonionotropic NMDAR plasticity takes part in latent memory encoding in immature rodents through nonassociative depression of synaptic efficacy, and possibly shrinking of dendritic spines. Further, the late postnatal alteration in NMDAR composition in the hippocampus appears to reduce nonionotropic signaling and remove a restriction on memory retrieval. This framework substantially alters the canonical model of NMDAR involvement in spatial cognition and hippocampal maturation and provides novel and exciting inroads for future studies.

Regulation of glutamate signaling in the extended amygdala by adolescent alcohol exposure

Adolescence is a critical period for brain development and behavioral maturation, marked by increased risk-taking behavior and the initiation of drug use. There are significant changes in gray matter volume and pruning of synapses along with a shift in excitatory to inhibitory balance which marks the maturation of cognition and decision-making. Because of ongoing brain development, adolescents are particularly sensitive to the detrimental effects of drugs, including alcohol, which can cause long-lasting consequences into adulthood. The extended amygdala is a region critically implicated in withdrawal and negative affect such as anxiety and depression. As negative affective disorders develop during adolescence, the effects of adolescent alcohol exposure on extended amygdala circuitry needs further inquiry. Here we aim to provide a framework to discuss the existing literature on the extended amygdala, the neuroadaptations which result from alcohol use, and the intersection of factors which contribute to the long-lasting effects of this exposure.

Regulation of Phosphorylated State of NMDA Receptor by STEP61 Phosphatase after Mild-Traumatic Brain Injury: Role of Oxidative Stress

Traumatic Brain Injury (TBI) mediates neuronal death through several events involving many molecular pathways, including the glutamate-mediated excitotoxicity for excessive stimulation of N-methyl-D-aspartate receptors (NMDARs), producing activation of death signaling pathways. However, the contribution of NMDARs (distribution and signaling-associated to the distribution) remains incompletely understood. We propose a critical role of STEP₆₁ (Striatal-Enriched protein tyrosine phosphatase) in TBI; this phosphatase regulates the dephosphorylated state of the GluN2B subunit through two pathways: by direct dephosphorylation of tyrosine-1472 and indirectly via dephosphorylation and inactivation of Fyn kinase. We previously demonstrated oxidative stress’s contribution to NMDAR signaling and distribution using SOD2^+/− mice such a model. We performed TBI protocol using a controlled frontal impact device using C57BL/6 mice and SOD2^+/− animals. After TBI, we found alterations in cognitive performance, NMDAR-dependent synaptic function (decreased synaptic form of NMDARs and decreased synaptic current NMDAR-dependent), and increased STEP₆₁ activity. These changes are reduced partially with the STEP₆₁-inhibitor TC-2153 treatment in mice subjected to TBI protocol. This study contributes with evidence about the role of STEP₆₁ in the neuropathological progression after TBI and also the alteration in their activity, such as an early biomarker of synaptic damage in traumatic lesions.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Transcranial Magnetic Stimulation Exerts "Rejuvenation" Effects on Corticostriatal Synapses after Partial Dopamine Depletion

BACKGROUND

In experimental models of Parkinson's disease (PD), different degrees of degeneration to the nigrostriatal pathway produce distinct profiles of synaptic alterations that depend on progressive changes in N-methyl-D-aspartate receptors (NMDAR)-mediated functions. Repetitive transcranial magnetic stimulation (rTMS) induces modifications in glutamatergic and dopaminergic systems, suggesting that it may have an impact on glutamatergic synapses modulated by dopamine neurotransmission. However, no studies have so far explored the mechanisms of rTMS effects at early stages of PD.

OBJECTIVES

We tested the hypothesis that in vivo application of rTMS with intermittent theta-burst stimulation (iTBS) pattern alleviates corticostriatal dysfunctions by modulating NMDAR-dependent plasticity in a rat model of early parkinsonism.

METHODS

Dorsolateral striatal spiny projection neurons (SPNs) activity was studied through ex vivo whole-cell patch-clamp recordings in corticostriatal slices obtained from 6-hydroxydopamine-lesioned rats, subjected to a single session (acute) of iTBS and tested for forelimb akinesia with the stepping test. Immunohistochemical analyses were performed to analyze morphological correlates of plasticity in SPNs.

RESULTS

Acute iTBS ameliorated limb akinesia and rescued corticostriatal long-term potentiation (LTP) in SPNs of partially lesioned rats. This effect was abolished by applying a selective inhibitor of GluN2B-subunit-containing NMDAR, suggesting that iTBS treatment could be associated with an enhanced activation of specific NMDAR subunits, which are major regulators of structural plasticity during synapse development. Morphological analyses of SPNs revealed that iTBS treatment reverted dendritic spine loss inducing a prevalence of thin-elongated spines in the biocytin-filled SPNs.

CONCLUSIONS

Taken together, our data identify that an acute iTBS treatment produces a series of plastic changes underlying striatal compensatory adaptation in the parkinsonian basal ganglia circuit. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Sustained treatment with an α5 GABA A receptor negative allosteric modulator delays excitatory circuit development while maintaining GABAergic neurotransmission

α5 subunit GABA type A receptor (GABA_AR) preferring negative allosteric modulators (NAMs) are cognitive enhancers with antidepressant-like effects. α5-NAM success in treating mouse models of neurodevelopmental disorders with excessive inhibition have led to Phase 2 clinical trials for Down syndrome. Despite in vivo efficacy, no study has examined the effects of continued α5-NAM treatment on inhibitory and excitatory synapse plasticity to identify mechanisms of action. Here we used L-655,708, an imidazobenzodiazepine that acts as a highly selective but weak α5-NAM, to investigate the impact of sustained treatment on hippocampal neuron synapse and dendrite development. We show that 2-day pharmacological reduction of α5-GABA_AR signaling from DIV12-14, when GABA_ARs contribute to depolarization, delays dendritic spine maturation and the NMDA receptor (NMDAR) GluN2B/GluN2A developmental shift. In contrast, α5-NAM treatment from DIV19-21, when hyperpolarizing GABA_AR signaling predominates, enhances surface synaptic GluN2A while decreasing GluN2B. Despite changes in NMDAR subtype surface levels and localization, total levels of key excitatory synapse proteins were largely unchanged, and mEPSCs were unaltered. Importantly, 2-day α5-NAM treatment does not alter the total surface levels or distribution of α5-GABA_ARs, reduce the gephyrin inhibitory synaptic scaffold, or impair phasic or tonic inhibition. Furthermore, α5-NAM inhibition of the GABA_AR tonic current in mature neurons is maintained after 2-day α5-NAM treatment, suggesting reduced tolerance liability, in contrast to other clinically relevant GABA_AR-targeting drugs such as benzodiazepines. Together, these results show that α5-GABA_ARs contribute to dendritic spine maturation and excitatory synapse development via a NMDAR dependent mechanism without perturbing overall neuronal excitability.

Regulation of NMDA receptor trafficking and gating by activity-dependent CaMKIIα phosphorylation of the GluN2A subunit

NMDA receptor (NMDAR)-dependent Ca2+ influx underpins multiple forms of synaptic plasticity. Most synaptic NMDAR currents in the adult forebrain are mediated by GluN2A-containing receptors, which are rapidly inserted into synapses during long-term potentiation (LTP); however, the underlying molecular mechanisms remain poorly understood. In this study, we show that GluN2A is phosphorylated at Ser-1459 by Ca2+/calmodulin-dependent kinase IIα (CaMKIIα) in response to glycine stimulation that mimics LTP in primary neurons. Phosphorylation of Ser-1459 promotes GluN2A interaction with the sorting nexin 27 (SNX27)-retromer complex, thereby enhancing the endosomal recycling of NMDARs. Loss of SNX27 or CaMKIIα function blocks the glycine-induced increase in GluN2A-NMDARs on the neuronal membrane. Interestingly, mutations of Ser-1459, including the rare S1459G human epilepsy variant, prolong the decay times of NMDAR-mediated synaptic currents in heterosynapses by increasing the duration of channel opening. These findings not only identify a critical role of Ser-1459 phosphorylation in regulating the function of NMDARs, but they also explain how the S1459G variant dysregulates NMDAR function.

Regulation of the NMDA receptor by its cytoplasmic domains: (How) is the tail wagging the dog?

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is critical for synapse development, function, and plasticity in the brain. NMDARs are tetra-heteromeric cation-channels that mediate synaptic transmission and plasticity. Extensive human studies show the existence of genetic variants in NMDAR subunits genes (GRIN genes) that are associated with neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorders (ASD), epilepsy (EP), intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and schizophrenia (SCZ). NMDAR subunits have a unique modular architecture with four semiautonomous domains. Here we focus on the carboxyl terminal domain (CTD), also known as the intracellular C-tail, which varies in length among the glutamate receptor subunits and is the most diverse domain in terms of amino acid sequence. The CTD shows no sequence homology to any known proteins but encodes short docking motifs for intracellular binding proteins and covalent modifications. Our review will discuss the many important functions of the CTD in regulating NMDA membrane and synaptic targeting, stabilization, degradation targeting, allosteric modulation and metabotropic signaling of the receptor. This article is part of the special issue on 'Glutamate Receptors - NMDA Receptors'.

Mitochondrial ROS control neuronal excitability and cell fate in frontotemporal dementia

INTRODUCTION

The second most common form of early-onset dementia-frontotemporal dementia (FTD)-is often characterized by the aggregation of the microtubule-associated protein tau. Here we studied the mechanism of tau-induced neuronal dysfunction in neurons with the FTD-related 10+16 MAPT mutation.

METHODS

Live imaging, electrophysiology, and redox proteomics were used in 10+16 induced pluripotent stem cell-derived neurons and a model of tau spreading in primary cultures.

RESULTS

Overproduction of mitochondrial reactive oxygen species (ROS) in 10+16 neurons alters the trafficking of specific glutamate receptor subunits via redox regulation. Increased surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors containing GluA1 and NR2B subunits leads to impaired glutamatergic signaling, calcium overload, and excitotoxicity. Mitochondrial antioxidants restore the altered response and prevent neuronal death. Importantly, extracellular 4R tau induces the same pathological response in healthy neurons, thus proposing a mechanism for disease propagation.

DISCUSSION

These results demonstrate mitochondrial ROS modulate glutamatergic signaling in FTD, and suggest a new therapeutic strategy.

An Epilepsy-Associated GRIN2A Rare Variant Disrupts CaMKIIα Phosphorylation of GluN2A and NMDA Receptor Trafficking

Rare variants in GRIN genes, which encode NMDAR subunits, are strongly associated with neurodevelopmental disorders. Among these, GRIN2A, which encodes the GluN2A subunit of NMDARs, is widely accepted as an epilepsy-causative gene. Here, we functionally characterize the de novo GluN2A-S1459G mutation identified in an epilepsy patient. We show that S1459 is a CaMKIIα phosphorylation site, and that endogenous phosphorylation is regulated during development and in response to synaptic activity in a dark rearing model. GluN2A-S1459 phosphorylation results in preferential binding of NMDARs to SNX27 and a corresponding decrease in PSD-95 binding, which consequently regulates NMDAR trafficking. Furthermore, the epilepsy-associated GluN2A-S1459G variant displays defects in interactions with both SNX27 and PSD-95, resulting in trafficking deficits, reduced spine density, and decreased excitatory synaptic transmission. These data demonstrate a role for CaMKIIα phosphorylation of GluN2A in receptor targeting and implicate NMDAR trafficking defects as a link to epilepsy.

Neuropathic pain generates silent synapses in thalamic projection to anterior cingulate cortex

ABSTRACT

Pain experience can change the central processing of nociceptive inputs, resulting in persistent allodynia and hyperalgesia. However, the underlying circuit mechanisms remain underexplored. Here, we focus on pain-induced remodeling of the projection from the mediodorsal thalamus (MD) to the anterior cingulate cortex (ACC), a projection that relays spinal nociceptive input for central processing. Using optogenetics combined with slice electrophysiology, we detected in male mice that 7 days of chronic constriction injury (CCI; achieved by loose ligation of the sciatic nerve) generated AMPA receptor (AMPAR)-silent glutamatergic synapses within the contralateral MD-to-ACC projection. AMPAR-silent synapses are typically GluN2B-enriched nascent glutamatergic synapses that mediate the initial formation of neural circuits during early development. During development, some silent synapses mature and become "unsilenced" by recruiting and stabilizing AMPARs, consolidating and strengthening the newly formed circuits. Consistent with these synaptogenic features, pain-induced generation of silent synapses was accompanied by increased densities of immature dendritic spines in ACC neurons and increased synaptic weight of GluN2B-containing NMDA receptors (NMDARs) in the MD-to-ACC projection. After prolonged (∼30 days) CCI, injury-generated silent synapses declined to low levels, which likely resulted from a synaptic maturation process that strengthens AMPAR-mediated MD-to-ACC transmission. Consistent with this hypothesis, viral-mediated knockdown of GluN2B in ACC neurons, which prevented pain-induced generation of silent synapses and silent synapse-mediated strengthening of MD-to-ACC projection after prolonged CCI, prevented the development of allodynia. Taken together, our results depict a silent synapse-mediated mechanism through which key supraspinal neural circuits that regulate pain sensitivity are remodeled to induce allodynia and hyperalgesia.

AMPA and NMDA Receptor Trafficking at Cocaine-Generated Synapses

Cocaine experience generates AMPA receptor (AMPAR)-silent synapses in the nucleus accumbens (NAc), which are thought to be new synaptic contacts enriched in GluN2B-containing NMDA receptors (NMDARs). After drug withdrawal, some of these synapses mature by recruiting AMPARs, strengthening the newly established synaptic transmission. Silent synapse generation and maturation are two consecutive cellular steps through which NAc circuits are profoundly remodeled to promote cue-induced cocaine seeking after drug withdrawal. However, the basic cellular processes that mediate these two critical steps remains underexplored. Using a combination of electrophysiology, viral-mediated gene transfer, and confocal imaging in male rats as well as knock-in (KI) mice of both sexes, our current study characterized the dynamic roles played by AMPARs and NMDARs in generation and maturation of silent synapses on NAc medium spiny neurons after cocaine self-administration and withdrawal. We report that cocaine-induced generation of silent synapses not only required synaptic insertion of GluN2B-containing NMDARs, but also, counterintuitively, involved insertion of AMPARs, which subsequently internalized, resulting in the AMPAR-silent state on withdrawal day 1. Furthermore, GluN2B NMDARs functioned to maintain these cocaine-generated synapses in the AMPAR-silent state during drug withdrawal, until they were replaced by nonGluN2B NMDARs, a switch that allowed AMPAR recruitment and maturation of silent synapses. These results reveal dynamic interactions between AMPARs and NMDARs during the generation and maturation of silent synapses after cocaine experience and provide a mechanistic basis through which new synaptic contacts and possibly new neural network patterns created by these synapses can be manipulated for therapeutic benefit.SIGNIFICANCE STATEMENT Studies over the past decade reveal a critical role of AMPA receptor-silent, NMDA receptor-containing synapses in forming cocaine-related memories that drive cocaine relapse. However, it remains incompletely understood how AMPA and NMDA receptors traffic at these synapses during their generation and maturation. The current study characterizes a two-step AMPA receptor trafficking cascade that contributes to the generation of silent synapses in response to cocaine experience, and a two-step NMDA receptor trafficking cascade that contributes to the maturation of these synapses after cocaine withdrawal. These results depict a highly regulated cellular procedure through which nascent glutamatergic synapses are generated in the adult brain after drug experience and provide significant insight into the roles of glutamate receptors in synapse formation and maturation.

Leptin modulates pancreatic β-cell membrane potential through Src kinase-mediated phosphorylation of NMDA receptors

The adipocyte-derived hormone leptin increases trafficking of KATP and Kv2.1 channels to the pancreatic β-cell surface, resulting in membrane hyperpolarization and suppression of insulin secretion. We have previously shown that this effect of leptin is mediated by the NMDA subtype of glutamate receptors (NMDARs). It does so by potentiating NMDAR activity, thus enhancing Ca2+ influx and the ensuing downstream signaling events that drive channel trafficking to the cell surface. However, the molecular mechanism by which leptin potentiates NMDARs in β-cells remains unknown. Here, we report that leptin augments NMDAR function via Src kinase-mediated phosphorylation of the GluN2A subunit. Leptin-induced membrane hyperpolarization diminished upon pharmacological inhibition of GluN2A but not GluN2B, indicating involvement of GluN2A-containing NMDARs. GluN2A harbors tyrosine residues that, when phosphorylated by Src family kinases, potentiate NMDAR activity. We found that leptin increases phosphorylation of Tyr-418 in Src, an indicator of kinase activation. Pharmacological inhibition of Src or overexpression of a kinase-dead Src mutant prevented the effect of leptin, whereas a Src kinase activator peptide mimicked it. Using mutant GluN2A overexpression, we show that Tyr-1292 and Tyr-1387 but not Tyr-1325 are responsible for the effect of leptin. Importantly, β-cells from db/db mice, a type 2 diabetes mouse model lacking functional leptin receptors, or from obese diabetic human donors failed to respond to leptin but hyperpolarized in response to NMDA. Our study reveals a signaling pathway wherein leptin modulates NMDARs via Src to regulate β-cell excitability and suggests NMDARs as a potential target to overcome leptin resistance.

Psychostimulant-Induced Adaptations in Nucleus Accumbens Glutamatergic Transmission

Carrying different aspects of emotional and motivational signals, glutamatergic synaptic projections from multiple limbic and paralimbic brain regions converge to the nucleus accumbens (NAc), in which these arousing signals are processed and prioritized for behavioral output. In animal models of drug addiction, some key drug-induced alterations at NAc glutamatergic synapses underlie important cellular and circuit mechanisms that promote subsequent drug taking, seeking, and relapse. With the focus of cocaine, we review changes at NAc glutamatergic synapses that occur after different drug procedures and abstinence durations, and the behavioral impact of these changes.

GluN2B but Not GluN2A for Basal Dendritic Growth of Cortical Pyramidal Neurons

NMDA receptors are important players for neuronal differentiation. We previously reported that antagonizing NMDA receptors with APV blocked the growth-promoting effects evoked by the overexpression of specific calcium-permeable or flip-spliced AMPA receptor subunits and of type I transmembrane AMPA receptor regulatory proteins which both exclusively modify apical dendritic length and branching of cortical pyramidal neurons. These findings led us to characterize the role of GluN2B and GluN2A for dendritogenesis using organotypic cultures of rat visual cortex. Antagonizing GluN2B with ifenprodil and Ro25-6981 strongly impaired basal dendritic growth of supra- and infragranular pyramidal cells at DIV 5–10, but no longer at DIV 15–20. Growth recovered after washout, and protein blots revealed an increase of synaptic GluN2B-containing receptors as indicated by a enhanced phosphorylation of the tyrosine 1472 residue. Antagonizing GluN2A with TCN201 and NVP-AAM077 was ineffective at both ages. Dendrite growth of non-pyramidal interneurons was not altered. We attempted to overexpress GluN2A and GluN2B. However, although the constructs delivered currents in HEK cells, there were neither effects on dendrite morphology nor an enhanced sensitivity to NMDA. Further, co-expressing GluN1-1a and GluN2B did not alter dendritic growth. Visualization of overexpressed, tagged GluN2 proteins was successful after immunofluorescence for the tag which delivered rather weak staining in HEK cells as well as in neurons. This suggested that the level of overexpression is too weak to modify dendrite growth. In summary, endogenous GluN2B, but not GluN2A is important for pyramidal cell basal dendritic growth during an early postnatal time window.

Disease-associated GRIN protein truncating variants trigger NMDA receptor loss-of-function

De novo GRIN variants, encoding for the ionotropic glutamate NMDA receptor subunits, have been recently associated with GRIN-related disorders, a group of rare paediatric encephalopathies. Current investigational and clinical efforts are focused to functionally stratify GRIN variants, towards precision therapies of this primary disturbance of glutamatergic transmission that affects neuronal function and brain. In the present study, we aimed to comprehensively delineate the functional outcomes and clinical phenotypes of GRIN protein truncating variants (PTVs)-accounting for ~20% of disease-associated GRIN variants-hypothetically provoking NMDAR hypofunctionality. To tackle this question, we created a comprehensive GRIN PTVs variants database compiling a cohort of nine individuals harbouring GRIN PTVs, together with previously identified variants, to build-up an extensive GRIN PTVs repertoire composed of 293 unique variants. Genotype-phenotype correlation studies were conducted, followed by cell-based assays of selected paradigmatic GRIN PTVs and their functional annotation. Genetic and clinical phenotypes meta-analysis revealed that heterozygous GRIN1, GRIN2C, GRIN2D, GRIN3A and GRIN3B PTVs are non-pathogenic. In contrast, heterozygous GRIN2A and GRIN2B PTVs are associated with specific neurological clinical phenotypes in a subunit- and domain-dependent manner. Mechanistically, cell-based assays showed that paradigmatic pathogenic GRIN2A and GRIN2B PTVs result on a decrease of NMDAR surface expression and NMDAR-mediated currents, ultimately leading to NMDAR functional haploinsufficiency. Overall, these findings contribute to delineate GRIN PTVs genotype-phenotype association and GRIN variants stratification. Functional studies showed that GRIN2A and GRIN2B pathogenic PTVs trigger NMDAR hypofunctionality, and thus accelerate therapeutic decisions for this neurodevelopmental condition.