Activity regulates the synaptic localization of the NMDA receptor in hippocampal neurons

Aberrant activation of hippocampal astrocytes causes neuroinflammation and cognitive decline in mice

Reactive astrocytes are associated with neuroinflammation and cognitive decline in diverse neuropathologies; however, the underlying mechanisms are unclear. We used optogenetic and chemogenetic tools to identify the crucial roles of the hippocampal CA1 astrocytes in cognitive decline. Our results showed that repeated optogenetic stimulation of the hippocampal CA1 astrocytes induced cognitive impairment in mice and decreased synaptic long-term potentiation (LTP), which was accompanied by the appearance of inflammatory astrocytes. Mechanistic studies conducted using knockout animal models and hippocampal neuronal cultures showed that lipocalin-2 (LCN2), derived from reactive astrocytes, mediated neuroinflammation and induced cognitive impairment by decreasing the LTP through the reduction of neuronal NMDA receptors. Sustained chemogenetic stimulation of hippocampal astrocytes provided similar results. Conversely, these phenomena were attenuated by a metabolic inhibitor of astrocytes. Fiber photometry using GCaMP revealed a high level of hippocampal astrocyte activation in the neuroinflammation model. Our findings suggest that reactive astrocytes in the hippocampus are sufficient and required to induce cognitive decline through LCN2 release and synaptic modulation. This abnormal glial-neuron interaction may contribute to the pathogenesis of cognitive disturbances in neuroinflammation-associated brain conditions.

Purinergic Astrocyte Signaling Driven by TNF-α After Cannabidiol Administration Restores Normal Synaptic Remodeling Following Traumatic Brain Injury

Traumatic brain injury (TBI) is a prevalent form of cranial trauma that results in neural conduction disruptions and damage to synaptic structures and functions. Cannabidiol (CBD), a primary derivative from plant-based cannabinoids, exhibits a range of beneficial effects, including analgesic, sedative, anti-inflammatory, anticonvulsant, anti-anxiety, anti-apoptotic, and neuroprotective properties. Nevertheless, the effects of synaptic reconstruction and the mechanisms underlying these effects remain poorly understood. TBI is characterized by increased levels of tumor necrosis factor-alpha (TNF-α), a cytokine integral for the modulation of glutamate release by astrocytes. In the present study, the potential of CBD in regulating aberrant glutamate signal transmission in astrocytes following brain injury, as well as the underlying mechanisms involved, were investigated using immunofluorescence double staining, enzyme-linked immunosorbent assay (ELISA), western blot analysis, hematoxylin and eosin (H&E) staining, Nissl staining, transmission electron microscopy, and RT-qPCR. In this study, we examined the impact of CBD on neuronal synapses, focusing on the TNF-α-driven purinergic signaling pathway. Specifically, our research revealed that CBD pretreatment effectively reduced the secretion of TNF-α induced by astrocyte activation following TBI. This reduction inhibited the interaction between TNF-α and P2Y1 receptors, leading to a decrease in the release of neurotransmitters, including Ca2+ and glutamate, thereby initiating synaptic remodeling. Our study showed that CBD exhibits significant therapeutic potential for TBI-related synaptic dysfunction, offering valuable insights for future research and more effective TBI treatments. Further exploration of the potential applications of CBD in neuroprotection is required to develop innovative clinical strategies.

Imbalance of synaptic and extrasynaptic NMDA receptors induced by the deletion of CRMP1 accelerates age-related cognitive decline in mice

Collapsin response mediator protein 1 (CRMP1) is involved in semaphorin 3A signaling pathway, promoting neurite extension and growth cone collapse. It is highly expressed in the nervous system, especially the hippocampus. The crmp1 knockout (KO) mice display impaired spatial learning and memory, and this phenomenon seemingly tends to deteriorate with age. Here we investigated whether CRMP1 is involved in age-related cognitive decline in WT and crmp1 KO mice at adult, middle-aged and older stages. The results revealed that cognitive dysfunction in the Morris water maze task became more severe and decreased glutamate and glutamine level in middle-aged crmp1 KO mice. Additionally, increasing levels of extrasynaptic NMDA receptors and phosphorylation of Tau were observed in middle-aged crmp1 KO mice, leading to synaptic and neuronal loss in the CA3 regions of hippocampus. These findings suggest that deletion of CRMP1 accelerates age-related cognitive decline by disrupting the balance between synaptic and extrasynaptic NMDA receptors, resulting in the loss of synapses and neurons.

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

Deficits in integrative NMDA receptors caused by Grin1 disruption can be rescued in adulthood

Glutamatergic NMDA receptors (NMDAR) are critical for cognitive function, and their reduced expression leads to intellectual disability. Since subpopulations of NMDARs exist in distinct subcellular environments, their functioning may be unevenly vulnerable to genetic disruption. Here, we investigate synaptic and extrasynaptic NMDARs on the major output neurons of the prefrontal cortex in mice deficient for the obligate NMDAR subunit encoded by Grin1 and wild-type littermates. With whole-cell recording in brain slices, we find that single, low-intensity stimuli elicit surprisingly-similar glutamatergic synaptic currents in both genotypes. By contrast, clear genotype differences emerge with manipulations that recruit extrasynaptic NMDARs, including stronger, repetitive, or pharmacological stimulation. These results reveal a disproportionate functional deficit of extrasynaptic NMDARs compared to their synaptic counterparts. To probe the repercussions of this deficit, we examine an NMDAR-dependent phenomenon considered a building block of cognitive integration, basal dendrite plateau potentials. Since we find this phenomenon is readily evoked in wild-type but not in Grin1-deficient mice, we ask whether plateau potentials can be restored by an adult intervention to increase Grin1 expression. This genetic manipulation, previously shown to restore cognitive performance in adulthood, successfully rescues electrically-evoked basal dendrite plateau potentials after a lifetime of NMDAR compromise. Taken together, our work demonstrates NMDAR subpopulations are not uniformly vulnerable to the genetic disruption of their obligate subunit. Furthermore, the window for functional rescue of the more-sensitive integrative NMDARs remains open into adulthood.

A synaptic molecular dependency network in knockdown of autism- and schizophrenia-associated genes revealed by multiplexed imaging

The complex functions of neuronal synapses depend on their tightly interconnected protein network, and their dysregulation is implicated in the pathogenesis of autism spectrum disorders and schizophrenia. However, it remains unclear how synaptic molecular networks are altered biochemically in these disorders. Here, we apply multiplexed imaging to probe the effects of RNAi knockdown of 16 autism- and schizophrenia-associated genes on the simultaneous joint distribution of 10 synaptic proteins, observing several protein composition phenotypes associated with these risk genes. We apply Bayesian network analysis to infer hierarchical dependencies among eight excitatory synaptic proteins, yielding predictive relationships that can only be accessed with single-synapse, multiprotein measurements performed simultaneously in situ. Finally, we find that central features of the network are affected similarly across several distinct gene knockdowns. These results offer insight into the convergent molecular etiology of these widespread disorders and provide a general framework to probe subcellular molecular networks.

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.

GLAST versus GFAP as astroglial marker for the subcellular study of cannabinoid CB1 receptors in astrocytes

The cannabinoid CB₁ receptor-mediated functions in astrocytes are highly dependent on the CB₁ receptor distribution in these glial cells relative to neuronal sites, particularly at the nearby synapses under normal or pathological conditions. However, the portrait of the CB₁ receptor distribution in astroglial compartments remains uncompleted because of the scarce CB₁ receptor expression in these cells and the limited identification of astrocytes. The glial fibrillary acidic protein (GFAP) is commonly used as astroglial marker. However, because GFAP is a cytoskeleton protein mostly restricted to the astroglial cell bodies and their main branches, it seems not ideal for the localization of CB₁ receptor distribution in astrocytes. Therefore, alternative markers to decipher the actual astroglial CB₁ receptors are required. In this work, we have compared the glutamate aspartate transporter (GLAST) versus GFAP for the CB₁ receptor localization in astrocytes. We found by immunoelectron microscopy that GLAST reveals almost three-fold astroglial area and four-fold astroglial membranes compared to GFAP. In addition, this better visualization of astrocytes was associated with the detection of 12% of the total CB₁ receptor labeling in GLAST-positive astrocytes.

Activity-regulated E3 ubiquitin ligase TRIM47 modulates excitatory synapse development

The Ubiquitin Proteasome System (UPS) has been shown to regulate neuronal development and synapse formation. Activity-dependent regulation of E3 ligase, a component of the UPS that targets specific proteins for proteasome-mediated degradation, is emerging as a pivotal player for the establishment of functional synapses. Here, we identified TRIM47 as a developmentally regulated E3 ligase that is expressed in rat hippocampus during the temporal window of synapse formation. We have demonstrated that the expression of TRIM47 is regulated by the glutamate-induced synaptic activity of hippocampal neurons in culture. In addition, the activity-dependent enhancement of TRIM47 expression is recapitulated following the object location test, a hippocampus-dependent spatial memory paradigm. We observed that this enhancement of TRIM47 expression requires NMDA receptor activation. The knockdown of TRIM47 leads to an enhancement of spine density without affecting dendritic complexity. Furthermore, we observed an increase in excitatory synapse development upon loss of TRIM47 function. Comprehensively, our study identified an activity-regulated E3 ligase that drives excitatory synapse formation in hippocampal neurons.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

Convergence of distinct signaling pathways on synaptic scaling to trigger rapid antidepressant action

Ketamine is a noncompetitive glutamatergic N-methyl-d-aspartate receptor (NMDAR) antagonist that exerts rapid antidepressant effects. Preclinical studies identify eukaryotic elongation factor 2 kinase (eEF2K) signaling as essential for the rapid antidepressant action of ketamine. Here, we combine genetic, electrophysiological, and pharmacological strategies to investigate the role of eEF2K in synaptic function and find that acute, but not chronic, inhibition of eEF2K activity induces rapid synaptic scaling in the hippocampus. Retinoic acid (RA) signaling also elicits a similar form of rapid synaptic scaling in the hippocampus, which we observe is independent of eEF2K functioni. The RA signaling pathway is not required for ketamine-mediated antidepressant action; however, direct activation of the retinoic acid receptor α (RARα) evokes rapid antidepressant action resembling ketamine. Our findings show that ketamine and RARα activation independently elicit a similar form of multiplicative synaptic scaling that is causal for rapid antidepressant action.

Sigma-1 Receptor: A Potential Therapeutic Target for Traumatic Brain Injury

The sigma-1 receptor (Sig-1R) is a chaperone receptor that primarily resides at the mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) and acts as a dynamic pluripotent modulator regulating cellular pathophysiological processes. Multiple pharmacological studies have confirmed the beneficial effects of Sig-1R activation on cellular calcium homeostasis, excitotoxicity modulation, reactive oxygen species (ROS) clearance, and the structural and functional stability of the ER, mitochondria, and MAM. The Sig-1R is expressed broadly in cells of the central nervous system (CNS) and has been reported to be involved in various neurological disorders. Traumatic brain injury (TBI)-induced secondary injury involves complex and interrelated pathophysiological processes such as cellular apoptosis, glutamate excitotoxicity, inflammatory responses, endoplasmic reticulum stress, oxidative stress, and mitochondrial dysfunction. Thus, given the pluripotent modulation of the Sig-1R in diverse neurological disorders, we hypothesized that the Sig-1R may affect a series of pathophysiology after TBI. This review summarizes the current knowledge of the Sig-1R, its mechanistic role in various pathophysiological processes of multiple CNS diseases, and its potential therapeutic role in TBI.

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.

PSD-95 protects synapses from β-amyloid

Beta-amyloid (Aβ) depresses excitatory synapses by a poorly understood mechanism requiring NMDA receptor (NMDAR) function. Here, we show that increased PSD-95, a major synaptic scaffolding molecule, blocks the effects of Aβ on synapses. The protective effect persists in tissue lacking the AMPA receptor subunit GluA1, which prevents the confounding synaptic potentiation by increased PSD-95. Aβ modifies the conformation of the NMDAR C-terminal domain (CTD) and its interaction with protein phosphatase 1 (PP1), producing synaptic weakening. Higher endogenous levels or overexpression of PSD-95 block Aβ-induced effects on the NMDAR CTD conformation, its interaction with PP1, and synaptic weakening. Our results indicate that increased PSD-95 protects synapses from Aβ toxicity, suggesting that low levels of synaptic PSD-95 may be a molecular sign indicating synapse vulnerability to Aβ. Importantly, pharmacological inhibition of its depalmitoylation increases PSD-95 at synapses and rescues deficits caused by Aβ, possibly opening a therapeutic avenue against Alzheimer's disease.

Chronic and Acute Manipulation of Cortical Glutamate Transmission Induces Structural and Synaptic Changes in Co-cultured Striatal Neurons

In contrast to the prenatal topographic development of sensory cortices, striatal circuit organization is slow and requires the functional maturation of cortical and thalamic excitatory inputs throughout the first postnatal month. While mechanisms regulating synapse development and plasticity are quite well described at excitatory synapses of glutamatergic neurons in the neocortex, comparatively little is known of how this translates to glutamate synapses onto GABAergic neurons in the striatum. Here we investigate excitatory striatal synapse plasticity in an in vitro system, where glutamate can be studied in isolation from dopamine and other neuromodulators. We examined pre-and post-synaptic structural and functional plasticity in GABAergic striatal spiny projection neurons (SPNs), co-cultured with glutamatergic cortical neurons. After synapse formation, medium-term (24 h) TTX silencing increased the density of filopodia, and modestly decreased dendritic spine density, when assayed at 21 days in vitro (DIV). Spine reductions appeared to require residual spontaneous activation of ionotropic glutamate receptors. Conversely, chronic (14 days) TTX silencing markedly reduced spine density without any observed increase in filopodia density. Time-dependent, biphasic changes to the presynaptic marker Synapsin-1 were also observed, independent of residual spontaneous activity. Acute silencing (3 h) did not affect presynaptic markers or postsynaptic structures. To induce rapid, activity-dependent plasticity in striatal neurons, a chemical NMDA receptor-dependent “long-term potentiation (LTP)” paradigm was employed. Within 30 min, this increased spine and GluA1 cluster densities, and the percentage of spines containing GluA1 clusters, without altering the presynaptic signal. The results demonstrate that the growth and pruning of dendritic protrusions is an active process, requiring glutamate receptor activity in striatal projection neurons. Furthermore, NMDA receptor activation is sufficient to drive glutamatergic structural plasticity in SPNs, in the absence of dopamine or other neuromodulators.

Rapid exchange of synaptic and extrasynaptic NMDA receptors in hippocampal CA1 neurons

N-methyl-d-aspartate receptors (NMDARs) are fundamental coincidence detectors of synaptic activity necessary for the induction of synaptic plasticity and synapse stability. Adjusting NMDAR synaptic content, whether by receptor insertion or lateral diffusion between extrasynaptic and synaptic compartments, could play a substantial role defining the characteristics of the NMDAR-mediated excitatory postsynaptic current (EPSC), which in turn would mediate the ability of the synapse to undergo plasticity. Lateral NMDAR movement has been observed in dissociated neurons; however, it is currently unclear whether NMDARs are capable of lateral surface diffusion in hippocampal slices, a more physiologically relevant environment. To test for lateral mobility in rat hippocampal slices, we rapidly blocked synaptic NMDARs using MK-801, a use-dependent and irreversible NMDAR blocker. Following a 5-min washout period, we observed a strong recovery of NMDAR-mediated responses. The degree of the observed recovery was proportional to the amount of induced blockade, independent of levels of intracellular calcium, and mediated primarily by GluN2B-containing NMDA receptors. These results indicate that lateral diffusion of NMDARs could be a mechanism by which synapses rapidly adjust parameters to fine-tune synaptic plasticity.NEW & NOTEWORTHY N-methyl-d-aspartate-type glutamate receptors (NMDARs) have always been considered stable components of synapses. We show that in rat hippocampal slices synaptic NMDARs are in constant exchange with extrasynaptic receptors. This exchange of receptors is mediated primarily by NMDA receptors containing GluN2B, a subunit necessary to undergo synaptic plasticity. Thus this lateral movement of synaptic receptors allows synapses to rapidly regulate the total number of synaptic NMDARs with potential consequences for synaptic plasticity.

Kinesin Kif3b mutation reduces NMDAR subunit NR2A trafficking and causes schizophrenia-like phenotypes in mice

The transport of N-methyl-d-aspartate receptors (NMDARs) is crucial for neuronal plasticity and synapse formation. Here, we show that KIF3B, a member of the kinesin superfamily proteins (KIFs), supports the transport of vesicles simultaneously containing NMDAR subunit 2A (NR2A) and the adenomatous polyposis coli (APC) complex. Kif3b+/- neurons exhibited a reduction in dendritic levels of both NR2A and NR2B due to the impaired transport of NR2A and increased degradation of NR2B. In Kif3b+/- hippocampal slices, electrophysiological NMDAR response was found decreased and synaptic plasticity was disrupted, which corresponded to a common feature of schizophrenia (SCZ). The histological features of Kif3b+/- mouse brain also mimicked SCZ features, and Kif3b+/- mice exhibited behavioral defects in prepulse inhibition (PPI), social interest, and cognitive flexibility. Indeed, a mutation of KIF3B was specifically identified in human SCZ patients, which was revealed to be functionally defective in a rescue experiment. Therefore, we propose that KIF3B transports NR2A/APC complex and that its dysfunction is responsible for SCZ pathogenesis.

Long-term adult human brain slice cultures as a model system to study human CNS circuitry and disease

Most of our knowledge on human CNS circuitry and related disorders originates from model organisms. How well such data translate to the human CNS remains largely to be determined. Human brain slice cultures derived from neurosurgical resections may offer novel avenues to approach this translational gap. We now demonstrate robust preservation of the complex neuronal cytoarchitecture and electrophysiological properties of human pyramidal neurons in long-term brain slice cultures. Further experiments delineate the optimal conditions for efficient viral transduction of cultures, enabling ‘high throughput’ fluorescence-mediated 3D reconstruction of genetically targeted neurons at comparable quality to state-of-the-art biocytin fillings, and demonstrate feasibility of long term live cell imaging of human cells in vitro. This model system has implications toward a broad spectrum of translational studies, regarding the validation of data obtained in non-human model systems, for therapeutic screening and genetic dissection of human CNS circuitry.

Approaches and Limitations in the Investigation of Synaptic Transmission and Plasticity

The numbers and strengths of synapses in the brain change throughout development, and even into adulthood, as synaptic inputs are added, eliminated, and refined in response to ongoing neural activity. A number of experimental techniques can assess these changes, including single-cell electrophysiological recording which offers measurements of synaptic inputs with high temporal resolution. Coupled with electrical stimulation, photoactivatable opsins, and caged compounds, to facilitate fine spatiotemporal control over release of neurotransmitters, electrophysiological recordings allow for precise dissection of presynaptic and postsynaptic mechanisms of action. Here, we discuss the strengths and pitfalls of various techniques commonly used to analyze synapses, including miniature excitatory/inhibitory (E/I) postsynaptic currents, evoked release, and optogenetic stimulation. Together, these techniques can provide multiple lines of convergent evidence to generate meaningful insight into the emergence of circuit connectivity and maturation. A full understanding of potential caveats and alternative explanations for findings is essential to avoid data misinterpretation.

Neurogranin Expression Is Regulated by Synaptic Activity and Promotes Synaptogenesis in Cultured Hippocampal Neurons

Neurogranin (Ng) is a calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) and is highly enriched in the dendrites and spines of telencephalic neurons. It is proposed to be involved in regulating CaM availability in the post-synaptic environment to modulate the efficiency of excitatory synaptic transmission. There is a close relationship between Ng and cognitive performance; its expression peaks in the forebrain coinciding with maximum synaptogenic activity, and it is reduced in several conditions of impaired cognition. We studied the expression of Ng in cultured hippocampal neurons and found that both protein and mRNA levels were about 10% of that found in the adult hippocampus. Long-term blockade of NMDA receptors substantially decreased Ng expression. On the other hand, treatments that enhanced synaptic activity such as long-term bicuculline treatment or co-culture with glial cells or cholesterol increased Ng expression. Chemical long-term potentiation (cLTP) induced an initial drop of Ng, with a minimum after 15 min followed by a slow recovery during the next 2-4 h. This effect was most evident in the synaptosome-enriched fraction, thus suggesting local synthesis in dendrites. Lentiviral expression of Ng led to increased density of both excitatory and inhibitory synapses in the second and third weeks of culture. These results indicate that Ng expression is regulated by synaptic activity and that Ng promotes the synaptogenesis process. Given its relationship with cognitive function, we propose targeting of Ng expression as a promising strategy to prevent or alleviate the cognitive deficits associated with aging and neuropathological conditions.

The C-terminus of NMDAR GluN1-1a Subunit Translocates to Nucleus and Regulates Synaptic Function

NMDARs, the Ca²⁺ permeable channels, play central roles in synaptic plasticity, brain development, learning, and memory. NMDAR binding partners and associated signaling has been extensively studied in synapse-to-nucleus communications. However, whether NMDARs could directly regulate synapse-to-nucleus communications is largely unknown. Here, we analyze the four alternative splicing of the C-terminus isoforms of GluN1 (1a, 2a, 3a, and 4a), and find that C1 domain of GluN1 is necessary for nuclear localization. Besides, we find that the 10 basic amino acids in C1 domain determine the nuclear localization of GluN1 C-terminus. Further investigating the expression patterns of the full length of GluN1 four isoforms shows that only GluN-1a exhibits the cytoplasmic and nucleus distribution in primary hippocampal neurons. Electrophysiological analyses also show that over-expression of GluN1 C-terminus without C1 domain doesn't affect synaptic transmission, whereas GluN1 C-terminus containing C1 domain potentiates NMDAR-mediated synaptic transmission. Our data suggested that the 10 basic amino acids in C1 domain determine translocation of GluN1 C-terminus into nucleus and regulate synaptic transmission.

Memantine attenuated alcohol withdrawal-induced anxiety-like behaviors through down-regulating NR1-CaMKII-ERK signaling pathway

Alcohol abuse and anxiety disorders often occur concurrently, but their underlying cellular mechanisms remain unclear. N-methyl-D-aspartic acid receptors (NMDARs) have recently received attention from those interested in the neurobiology of anxiety. A chronic alcohol exposure rat model (28 consecutive days of 20% alcohol intake and 6 h of withdrawal) was established. Here, we investigated the NMDAR1 (NR1), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinases (ERK) pathway in the modulation of anxiety-like behaviors in rats exposed to an open field and elevated plus maze (EPM) through systematic injections of memantine (a NMDAR inhibitor). We found that the NR1-CaMKII-ERK signaling pathway was activated after alcohol withdrawal in medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAcSh) but not core (NAcC). Memantine treatment greatly ameliorated anxiety-like behavior in the rats experiencing alcohol withdrawal. Moreover, memantine uniformly suppressed the phosphorylation of NR1-CaMKII-ERK pathway induced by alcohol withdrawal. Our results suggest that activation of the NR1-CaMKII-ERK pathway in the mPFC and NAcSh is an important contributor to the molecular mechanisms underlying alcohol withdrawal-induced anxiety behaviors. NMDAR signaling pathway inhibitors are thus potential therapeutics for treating alcohol abuse.

NMDA receptor-dependent dephosphorylation of serine 387 in Argonaute 2 increases its degradation and affects dendritic spine density and maturation

Argonaute (AGO) proteins are essential components of the microRNA (miRNA) pathway. AGO proteins are loaded with miRNAs to target mRNAs and thereby regulate mRNA stability and protein translation. As such, AGO proteins are important actors in controlling local protein synthesis, for instance, at dendritic spines and synapses. Although miRNA-mediated regulation of dendritic mRNAs has become a focus of intense interest over the past years, the mechanisms regulating neuronal AGO proteins remain largely unknown. Here, using rat hippocampal neurons, we report that dendritic Ago2 is down-regulated by the proteasome upon NMDA receptor activation. We found that Ser-387 in Ago2 is dephosphorylated upon NMDA treatment and that this dephosphorylation precedes Ago2 degradation. Expressing Ser-387 phosphorylation-deficient or phosphomimetic Ago2 in neurons, we observed that this phosphorylation site is involved in modulating dendritic spine morphology and postsynaptic density protein 95 (PSD-95) expression in spines. Collectively, our results point toward a signaling pathway linking NMDA receptor-dependent Ago2 dephosphorylation and turnover to postsynaptic structural changes. They support a model in which NMDA receptor-mediated dephosphorylation of Ago2 and Ago2 turnover contributes to the de-repression of mRNAs involved in spine growth and maturation.

CDKL5 controls postsynaptic localization of GluN2B-containing NMDA receptors in the hippocampus and regulates seizure susceptibility

Mutations in the Cyclin-dependent kinase-like 5 (CDKL5) gene cause severe neurodevelopmental disorders accompanied by intractable epilepsies, i.e. West syndrome or atypical Rett syndrome. Here we report generation of the Cdkl5 knockout mouse and show that CDKL5 controls postsynaptic localization of GluN2B-containing N-methyl-d-aspartate (NMDA) receptors in the hippocampus and regulates seizure susceptibility. Cdkl5 -/Y mice showed normal sensitivity to kainic acid; however, they displayed significant hyperexcitability to NMDA. In concordance with this result, electrophysiological analysis in the hippocampal CA1 region disclosed an increased ratio of NMDA/α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated excitatory postsynaptic currents (EPSCs) and a significantly larger decay time constant of NMDA receptor-mediated EPSCs (NMDA-EPSCs) as well as a stronger inhibition of the NMDA-EPSCs by the GluN2B-selective antagonist ifenprodil in Cdkl5 -/Y mice. Subcellular fractionation of the hippocampus from Cdkl5 -/Y mice revealed a significant increase of GluN2B and SAP102 in the PSD (postsynaptic density)-1T fraction, without changes in the S1 (post-nuclear) fraction or mRNA transcripts, indicating an intracellular distribution shift of these proteins to the PSD. Immunoelectron microscopic analysis of the hippocampal CA1 region further confirmed postsynaptic overaccumulation of GluN2B and SAP102 in Cdkl5 -/Y mice. Furthermore, ifenprodil abrogated the NMDA-induced hyperexcitability in Cdkl5 -/Y mice, suggesting that upregulation of GluN2B accounts for the enhanced seizure susceptibility. These data indicate that CDKL5 plays an important role in controlling postsynaptic localization of the GluN2B-SAP102 complex in the hippocampus and thereby regulates seizure susceptibility, and that aberrant NMDA receptor-mediated synaptic transmission underlies the pathological mechanisms of the CDKL5 loss-of-function.

Assembly, Secretory Pathway Trafficking, and Surface Delivery of Kainate Receptors Is Regulated by Neuronal Activity

Ionotropic glutamate receptor (iGluR) trafficking and function underpin excitatory synaptic transmission and plasticity and shape neuronal networks. It is well established that the transcription, translation, and endocytosis/recycling of iGluRs are all regulated by neuronal activity, but much less is known about the activity dependence of iGluR transport through the secretory pathway. Here, we use the kainate receptor subunit GluK2 as a model iGluR cargo to show that the assembly, early secretory pathway trafficking, and surface delivery of iGluRs are all controlled by neuronal activity. We show that the delivery of de novo kainate receptors is differentially regulated by modulation of GluK2 Q/R editing, PKC phosphorylation, and PDZ ligand interactions. These findings reveal that, in addition to short-term regulation of iGluRs by recycling/endocytosis and long-term modulation by altered transcription/translation, the trafficking of iGluRs through the secretory pathway is under tight activity-dependent control to determine the numbers and properties of surface-expressed iGluRs.

Advances in Autoimmune Epilepsy Associated with Antibodies, Their Potential Pathogenic Molecular Mechanisms, and Current Recommended Immunotherapies

In this comprehensive article, we present an overview of some most common autoimmune antibodies believed to be potentially pathogenic for autoimmune epilepsies and elaborate their pathogenic mode of action in molecular levels based on the existing knowledge. Findings of the studies of immunemodulatory treatments for epilepsy are also discussed, and guidelines for immunotherapy are sorted out. We aim to summarize the emerging understanding of different pathogenic mechanisms of autoantibodies and clinical immunotherapy regimens to open up therapeutic possibilities for future optimum therapy. We conclude that early diagnosis of autoimmune epilepsy is of great significance, as early immune treatments have useful disease-modifying effects on some epilepsies and can facilitate the recovery.

The Role of Posttraumatic Hypothermia in Preventing Dendrite Degeneration and Spine Loss after Severe Traumatic Brain Injury

Posttraumatic hypothermia prevents cell death and promotes functional outcomes after traumatic brain injury (TBI). However, little is known regarding the effect of hypothermia on dendrite degeneration and spine loss after severe TBI. In the present study, we used thy1-GFP transgenic mice to investigate the effect of hypothermia on the dendrites and spines in layer V/VI of the ipsilateral cortex after severe TBI. We found that hypothermia (33 °C) dramatically prevented dendrite degeneration and spine loss 1 and 7 days after CCI. The Morris water maze test revealed that hypothermia preserved the learning and memory functions of mice after CCI. Hypothermia significantly increased the expression of the synaptic proteins GluR1 and PSD-95 at 1 and 7 days after CCI in the ipsilateral cortex and hippocampus compared with that of the normothermia TBI group. Hypothermia also increased cortical and hippocampal BDNF levels. These results suggest that posttraumatic hypothermia is an effective method to prevent dendrite degeneration and spine loss and preserve learning and memory function after severe TBI. Increasing cortical and hippocampal BDNF levels might be the mechanism through which hypothermia prevents dendrite degeneration and spine loss and preserves learning and memory function.

Colocalization of synapse marker proteins evaluated by STED-microscopy reveals patterns of neuronal synapse distribution in vitro

BACKGROUND

Quantification of synapses and their morphological analysis are extensively used in network development and connectivity studies, drug screening and other areas of neuroscience. Thus, a number of quantitative approaches were introduced so far. However, most of the available methods are highly tailored to specific applications and have limitations for widespread use.

NEW METHOD

We present a new plugin for the open-source software ImageJ to provide a modifiable, high-throughput and easy to use method for synaptic puncta analysis. Our approach is based on colocalization of pre- and postsynaptic protein markers. Structurally completed glutamatergic and GABAergic synapses were identified by VGLUT1-PSD95 and VGAT-gephyrin colocalization, respectively. By combining conventional confocal microscopy with stimulated emission depletion (STED) imaging, we propose a method to quantify the number of scaffolding protein clusters, recruited to a single postsynaptic density.

RESULTS

In a proof-of-concept study, we reveal the differential distribution of glutamatergic and GABAergic synapse density with reference to perineuronal net (PNN) expression. Using super-resolution STED imaging, we demonstrate that postsynaptic puncta of completed synapses are composed of significantly more protein clusters, compared to uncompleted synapses.

COMPARISON WITH EXISTING METHODS

Our Synapse Counter plugin for ImageJ offers a rapid and unbiased research tool for a broad spectrum of neuroscientists. The proposed method of synaptic protein clusters quantification exploits super-resolution imaging to provide a comprehensive approach to the analysis of postsynaptic density composition.

CONCLUSIONS

Our results strongly substantiate the benefits of colocalization-based synapse detection.

Surface expression of NMDA receptor changes during memory consolidation in the crab Neohelice granulata

The aim of the present study was to analyze the surface expression of the NMDA-like receptors during the consolidation of contextual learning in the crab Neohelice granulata Memory storage is based on alterations in the strength of synaptic connections between neurons. The glutamatergic synapses undergo various forms of N-methyl-D aspartate receptor (NMDAR)-dependent changes in strength, a process that affects the abundance of other receptors at the synapse and underlies some forms of learning and memory. Here we propose a direct regulation of the NMDAR. Changes in NMDAR's functionality might be induced by the modification of the subunit's expression or cellular trafficking. This trafficking does not only include NMDAR's movement between synaptic and extra-synaptic localizations but also the cycling between intracellular compartments and the plasma membrane, a process called surface expression. Consolidation of contextual learning affects the surface expression of the receptor without affecting its general expression. The surface expression of the GluN1 subunit of the NMDAR is down-regulated immediately after training, up-regulated 3 h after training and returns to naïve and control levels 24 h after training. The changes in NMDAR surface expression observed in the central brain are not seen in the thoracic ganglion. A similar increment in surface expression of GluN1 in the central brain is observed 3 h after administration of the competitive GABAA receptor antagonist, bicuculline. These consolidation changes are part of a plasticity event that first, during the down-regulation, stabilizes the trace and later, at 3-h post-training, changes the threshold for synapse activation.

On the Role of Glutamate in Presynaptic Development: Possible Contributions of Presynaptic NMDA Receptors

Proper formation and maturation of synapses during development is a crucial step in building the functional neural circuits that underlie perception and behavior. It is well established that experience modifies circuit development. Therefore, understanding how synapse formation is controlled by synaptic activity is a key question in neuroscience. In this review, we focus on the regulation of excitatory presynaptic terminal development by glutamate, the predominant excitatory neurotransmitter in the brain. We discuss the evidence that NMDA receptor activation mediates these effects of glutamate and present the hypothesis that local activation of presynaptic NMDA receptors (preNMDARs) contributes to glutamate-dependent control of presynaptic development. Abnormal glutamate signaling and aberrant synapse development are both thought to contribute to the pathogenesis of a variety of neurodevelopmental disorders, including autism spectrum disorders, intellectual disability, epilepsy, anxiety, depression, and schizophrenia. Therefore, understanding how glutamate signaling and synapse development are linked is important for understanding the etiology of these diseases.

The planar cell polarity protein Vangl2 is involved in postsynaptic compartmentalization

The excitatory postsynaptic region of the vertebrate hippocampus is usually compartmentalized into the postsynaptic density (PSD) and N-cadherin-rich domain, which is important for synaptic adhesion. However, the molecular mechanisms underlying the compartment formation are unknown. In the present report, we show that the planar cell polarity (PCP) protein Van Gogh-like 2 (Vangl2) plays a role in this regionalization. In cultured rat hippocampal neurons that were subjected to Vangl2 expression silencing, the formed clusters of PSD-95, one of the major scaffolding proteins in PSD, tended to overlap with those of N-cadherin. Further, in the dendrites of these neurons, the immunofluorescence of PSD-95 was to some extent diffused, without a significant change in the total signal. Because Vangl2 physically interacts with both PSD-95 and N-cadherin in vivo, these results suggest that a PCP-related direct molecular mechanism underlies the horizontal polarization of the postsynaptic regions.

Depolarization of Hippocampal Neurons Induces Formation of Nonsynaptic NMDA Receptor Islands Resembling Nascent Postsynaptic Densities

Depolarization of neurons in 3-week-old rat hippocampal cultures promotes a rapid increase in the density of surface NMDA receptors (NRs), accompanied by transient formation of nonsynaptic NMDA receptor clusters or NR islands. Islands exhibit cytoplasmic dense material resembling that at postsynaptic densities (PSDs), and contain typical PSD components, including MAGUKS (membrane-associated guanylate kinases), GKAP, Shank, Homer, and CaMKII detected by pre-embedding immunogold electron microscopy. In contrast to mature PSDs, islands contain more NMDA than AMPA receptors, and more SAP102 than PSD-95, features that are shared with nascent PSDs in developing synapses. Islands do not appear to be exocytosed or endocytosed directly as preformed packages because neurons lacked intracellular vacuoles containing island-like structures. Islands form and disassemble upon depolarization of neurons on a time scale of 2-3 min, perhaps representing an initial stage in synaptogenesis.

Organization, control and function of extrasynaptic NMDA receptors

N-methyl D-aspartate receptors (NMDARs) exist in different forms owing to multiple combinations of subunits that can assemble into a functional receptor. In addition, they are located not only at synapses but also at extrasynaptic sites. There has been intense speculation over the past decade about whether specific NMDAR subtypes and/or locations are responsible for inducing synaptic plasticity and excitotoxicity. Here, we review the latest findings on the organization, subunit composition and endogenous control of NMDARs at extrasynaptic sites and consider their putative functions. Because astrocytes are capable of controlling NMDARs through the release of gliotransmitters, we also discuss the role of the glial environment in regulating the activity of these receptors.

GluN2B-Containing NMDA Receptors Regulate AMPA Receptor Traffic through Anchoring of the Synaptic Proteasome

NMDA receptors play a central role in shaping the strength of synaptic connections throughout development and in mediating synaptic plasticity mechanisms that underlie some forms of learning and memory formation in the CNS. In the hippocampus and the neocortex, GluN1 is combined primarily with GluN2A and GluN2B, which are differentially expressed during development and confer distinct molecular and physiological properties to NMDA receptors. The contribution of each subunit to the synaptic traffic of NMDA receptors and therefore to their role during development and in synaptic plasticity is still controversial. We report a critical role for the GluN2B subunit in regulating NMDA receptor synaptic targeting. In the absence of GluN2B, the synaptic levels of AMPA receptors are increased and accompanied by decreased constitutive endocytosis of GluA1-AMPA receptor. We used quantitative proteomic analysis to identify changes in the composition of postsynaptic densities from GluN2B(-/-) mouse primary neuronal cultures and found altered levels of several ubiquitin proteasome system components, in particular decreased levels of proteasome subunits. Enhancing the proteasome activity with a novel proteasome activator restored the synaptic levels of AMPA receptors in GluN2B(-/-) neurons and their endocytosis, revealing that GluN2B-mediated anchoring of the synaptic proteasome is responsible for fine tuning AMPA receptor synaptic levels under basal conditions.

NMDA receptor triggered molecular cascade underlies compression-induced rapid dendritic spine plasticity in cortical neurons

Compression causes the reduction of dendritic spines of underlying adult cortical pyramidal neurons but the mechanisms remain at large. Using a rat epidural cerebral compression model, dendritic spines on the more superficial-lying layer III pyramidal neurons were found quickly reduced in 12h, while those on the deep-located layer V pyramidal neurons were reduced slightly later, starting 1day following compression. No change in the synaptic vesicle markers synaptophysin and vesicular glutamate transporter 1 suggest no change in afferents. Postsynaptically, N-methyl-d-aspartate (NMDA) receptor trafficking to synaptic membrane was detected in 10min and lasting to 1day after compression. Translocation of calcineurin to synapses and enhancement of its enzymatic activity were detected within 10min as well. These suggest that compression rapidly activated NMDA receptors to increase postsynaptic calcium, which then activated the phosphatase calcineurin. In line with this, dephosphorylation and activation of the actin severing protein cofilin, and the consequent depolymerization of actin were all identified in the compressed cortex within matching time frames. Antagonizing NMDA receptors with MK801 before compression prevented this cascade of events, including NR1 mobilization, calcineurin activation and actin depolymerization, in the affected cortex. Morphologically, MK801 pretreatment prevented the loss of dendritic spines on the compressed cortical pyramidal neurons as well. In short, we demonstrated, for the first time, mechanisms underlying the rapid compression-induced cortical neuronal dendritic spine plasticity. In addition, the mechanical force of compression appears to activate NMDA receptors to initiate a rapid postsynaptic molecular cascade to trim dendritic spines on the compressed cortical pyramidal neurons within half a day.

The role of cAMP in synaptic homeostasis in response to environmental temperature challenges and hyperexcitability mutations

Homeostasis is the ability of physiological systems to regain functional balance following environment or experimental insults and synaptic homeostasis has been demonstrated in various species following genetic or pharmacological disruptions. Among environmental challenges, homeostatic responses to temperature extremes are critical to animal survival under natural conditions. We previously reported that axon terminal arborization in Drosophila larval neuromuscular junctions (NMJs) is enhanced at elevated temperatures; however, the amplitude of excitatory junctional potentials (EJPs) remains unaltered despite the increase in synaptic bouton numbers. Here we determine the cellular basis of this homeostatic adjustment in larvae reared at high temperature (HT, 29°C). We found that synaptic current focally recorded from individual synaptic boutons was unaffected by rearing temperature (<15°C to >30°C). However, HT rearing decreased the quantal size (amplitude of spontaneous miniature EJPs, or mEJPs), which compensates for the increased number of synaptic releasing sites to retain a normal EJP size. The quantal size decrease is accounted for by a decrease in input resistance of the postsynaptic muscle fiber, indicating an increase in membrane area that matches the synaptic growth at HT. Interestingly, a mutation in rutabaga (rut) encoding adenylyl cyclase (AC) exhibited no obvious changes in quantal size or input resistance of postsynaptic muscle cells after HT rearing, suggesting an important role for rut AC in temperature-induced synaptic homeostasis in Drosophila. This extends our previous finding of rut-dependent synaptic homeostasis in hyperexcitable mutants, e.g., slowpoke (slo). In slo larvae, the lack of BK channel function is partially ameliorated by upregulation of presynaptic Shaker (Sh) IA current to limit excessive transmitter release in addition to postsynaptic glutamate receptor recomposition that reduces the quantal size.

ICAM-5 affects spine maturation by regulation of NMDA receptor binding to α-actinin

ICAM-5 is a negative regulator of dendritic spine maturation and facilitates the formation of filopodia. Its absence results in improved memory functions, but the mechanisms have remained poorly understood. Activation of NMDA receptors induces ICAM-5 ectodomain cleavage through a matrix metalloproteinase (MMP)-dependent pathway, which promotes spine maturation and synapse formation. Here, we report a novel, ICAM-5-dependent mechanism underlying spine maturation by regulating the dynamics and synaptic distribution of α-actinin. We found that GluN1 and ICAM-5 partially compete for the binding to α-actinin; deletion of the cytoplasmic tail of ICAM-5 or ablation of the gene resulted in increased association of GluN1 with α-actinin, whereas internalization of ICAM-5 peptide perturbed the GluN1/α-actinin interaction. NMDA treatment decreased α-actinin binding to ICAM-5, and increased the binding to GluN1. Proper synaptic distribution of α-actinin requires the ICAM-5 cytoplasmic domain, without which α-actinin tended to accumulate in filopodia, leading to F-actin reorganization. The results indicate that ICAM-5 retards spine maturation by preventing reorganization of the actin cytoskeleton, but NMDA receptor activation is sufficient to relieve the brake and promote the maturation of spines.

NMDA receptor activity determines neuronal fate: location or number?

It is widely believed that the proper activation of N-methyl-D-aspartate (NMDA) receptors (NMDARs) promotes neuronal survival, whereas an excessive activation of NMDARs leads to neuronal damage. NMDARs are found at both synaptic and extrasynaptic sites. One current prevailing theory proposes the dichotomy of NMDAR activity. The role of the two population receptors is mutual antagonism. The activation of synaptic NMDARs, such as synaptic activity at physiological levels, promotes neuronal survival. However, the activation of extrasynaptic NMDARs occurring during stroke, brain injury, and chronic neurological diseases contributes to neuronal death. Thus, the location of NMDARs determines the neuronal fate. However, the theory is greatly challenged. Several studies suggested that synaptic NMDARs are involved in neuronal death. Recently, our work further showed that the coactivation of synaptic and extrasynaptic NMDARs contributes to neuronal death under neuronal insults. Therefore, we propose that the magnitude and duration of NMDAR activation determines the neuronal fate. More interestingly, there appears to be some subtle differences in the affinity between synaptic and extrasynaptic NMDARs, shedding light on the development of selective drugs to block extrasynaptic NMDARs.

SAP97 blocks the RXR ER retention signal of NMDA receptor subunit GluN1-3 through its SH3 domain

SAP97 is directly involved in exporting NMDA receptors with a specific subunit composition from the endoplasmic reticulum (ER). Characterization of the interactions between SAP97 and an NMDA receptor splice variant, GluN1-3, and of the effects on forward trafficking revealed that an ER-level interaction blocked the RXR ER-retention motif in the GluN1-3 cytoplasmic C-terminus in the context of both reporter molecules and full-length receptors. Binding of SAP97 to the PDZ-binding domain of GluN1-3 was required, but the blockade of ER-retention was mediated by the SH3-GuK domains coupled with the action of the N-terminus of SAP97. While other domains of SAP97 were involved in forward trafficking of GluN1-3 out of the ER, the SH3 domain was necessary and sufficient to block the ER retention. This is the first direct evidence for the masking of ER-retention signals by PDZ domain-containing proteins, and provides detailed underlying mechanistic requirements. Such a mechanism could be central to modulating the ER exit of receptors into local, non-conventional or conventional, secretory pathways in neurons.

Positive feedback of NR2B-containing NMDA receptor activity is the initial step toward visual imprinting: a model for juvenile learning

Imprinting in chicks is a good model for elucidating the processes underlying neural plasticity changes during juvenile learning. We recently reported that neural activation of a telencephalic region, the core region of the hyperpallium densocellulare (HDCo), was critical for success of visual imprinting, and that N-Methyl-D-aspartic (NMDA) receptors containing the NR2B subunit (NR2B/NR1) in this region were essential for imprinting. Using electrophysiological and multiple-site optical imaging techniques with acute brain slices, we found that long-term potentiation (LTP) and enhancement of NR2B/NR1 currents in HDCo neurons were induced in imprinted chicks. Enhancement of NR2B/NR1 currents as well as an increase in surface NR2B expression occurred even following a brief training that was too weak to induce LTP or imprinting behavior. This means that NR2B/NR1 activation is the initial step of learning, well before the activation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors which induces LTP. We also showed that knockdown of NR2B/NR1 inhibited imprinting, and inversely, increasing the surface NR2B expression by treatment with a casein kinase 2 inhibitor successfully reduced training time required for imprinting. These results suggest that imprinting stimuli activate post-synaptic NR2B/NR1 in HDCo cells, increase NR2B/NR1 signaling through up-regulation of its expression, and induce LTP and memory acquisition. The study investigated the neural mechanism underlying juvenile learning. In the initial stage of chick imprinting, NMDA receptors containing the NMDA receptor subunit 2B (NR2B) are activated, surface expression of NR2B/NR1 (NMDA receptor subunit 1) is up-regulated, and consequently long-term potentiation is induced in the telencephalic neurons. We suggest that the positive feedback in the NR2B/NR1 activation is a unique process of juvenile learning, exhibiting rapid memory acquisition.

Leucine-rich repeat kinase 2 regulates Sec16A at ER exit sites to allow ER-Golgi export

Leucine-rich repeat kinase 2 (LRRK2) has been associated with Parkinson's disease (PD) and other disorders. However, its normal physiological functions and pathogenic properties remain elusive. Here we show that LRRK2 regulates the anterograde ER-Golgi transport through anchoring Sec16A at the endoplasmic reticulum exit sites (ERES). LRRK2 interacted and co-localized with Sec16A, a key protein in the formation of ERES. Lrrk2 depletion caused a dispersion of Sec16A from ERES and impaired ER export. In neurons, LRRK2 and Sec16A showed extensive co-localization at the dendritic ERES (dERES) that locally regulate the transport of proteins to the dendritic spines. A loss of Lrrk2 affected the association of Sec16A with dERES and impaired the activity-dependent targeting of glutamate receptors onto the cell/synapse surface. Furthermore, the PD-related LRRK2 R1441C missense mutation in the GTPase domain interfered with the interaction of LRRK2 with Sec16A and also affected ER-Golgi transport, while LRRK2 kinase activity was not required for these functions. Therefore, our findings reveal a new physiological function of LRRK2 in ER-Golgi transport, suggesting ERES dysfunction may contribute to the pathogenesis of PD.

P38 MAPK inhibition protects against glutamate neurotoxicity and modifies NMDA and AMPA receptor subunit expression

NMDA and AMPA receptors are thought to be responsible for Ca(++) influx during glutamate-induced excitotoxicity and, therefore, hippocampal neuronal death. We assessed whether excitotoxicity induced by neonatal treatment with monosodium glutamate in rats at postnatal age of 1, 3, 5, and 7 modifies the hippocampal expression of the NMDAR subunit NR1 and the AMPAR subunits GluR1/GluR2 at postnatal days 8, 10, 12, and 14. We also assessed the involvement of MAPK signaling by using the p38 inhibitor SB203580. Our results showed that monosodium glutamate induces neuronal death and alters the expression of the subunits evaluated in the hippocampus at all ages studied, which could be prevented by SB203580 treatment.Furthermore, expression of the NRSF gene silencing factor also increased in response to excitotoxicity, suggesting a relationship in suppressing GluR2-expression, which was regulated by the p38-MAPK pathway inhibitor SB203580. This result suggests that selectively blocking the pro-death signaling pathway may reduce neuronal death in some neurodegenerative diseases in which these neurotoxic processes are present and produce major clinical benefits in the treatment of these pathologies.

NMDA receptors and L-type voltage-gated Ca²⁺ channels mediate the expression of bidirectional homeostatic intrinsic plasticity in cultured hippocampal neurons

Homeostatic plasticity is engaged when neurons need to stabilize their synaptic strength and excitability in response to acute or prolonged destabilizing changes in global activity. Compared to the extensive studies investigating the molecular mechanisms for homeostatic synaptic plasticity, the mechanism underlying homeostatic intrinsic plasticity is largely unknown. Through whole-cell patch-clamp recording in low-density cultures of dissociated hippocampal neurons, we demonstrate here that prolonged activity blockade induced by the sodium channel blocker tetrodotoxin (TTX) leads to increased action potential firing rates. Conversely, prolonged activity enhancement induced by the A-type gamma-aminobutyric acid receptor antagonist bicuculline (BC) results in decreased firing rates. Prolonged activity enhancement also enhanced potassium (K(+)) current through Kv1 channels, suggesting that changes in K(+) current, in part, mediate stabilization of hippocampal neuronal excitability upon prolonged activity elevation. In contrast to the previous reports showing that L-type voltage-gated calcium (Ca(2+)) channels solely mediate homeostatic regulation of excitatory synaptic strength (Ibata et al., 2008; Goold and Nicoll, 2010), inhibition of N-Methyl-d-aspartate (NMDA) receptors alone mimics the elevation in firing frequency driven by prolonged TTX application, while the decrease in firing rates induced by prolonged BC treatment involves the activity of NMDA receptors and L-type voltage-gated Ca(2+) channels. These results collectively provide strong evidence that alterations in Ca(2+) influx through NMDA receptors and L-type voltage-gated Ca(2+) channels mediate homeostatic intrinsic plasticity in hippocampal neurons in response to prolonged activity changes.

Acute mechanisms underlying antibody effects in anti-N-methyl-D-aspartate receptor encephalitis

Objective

A severe but treatable form of immune-mediated encephalitis is associated with antibodies in serum and cerebrospinal fluid (CSF) against the GluN1 subunit of the N-methyl-D-aspartate receptor (NMDAR). Prolonged exposure of hippocampal neurons to antibodies from patients with anti-NMDAR encephalitis caused a reversible decrease in the synaptic localization and function of NMDARs. However, acute effects of the antibodies, fate of the internalized receptors, type of neurons affected, and whether neurons develop compensatory homeostatic mechanisms were unknown and are the focus of this study.

Methods

Dissociated hippocampal neuron cultures and rodent brain sections were used for immunocytochemical, physiological, and molecular studies.

Results

Patient antibodies bind to NMDARs throughout the rodent brain, and decrease NMDAR cluster density in both excitatory and inhibitory hippocampal neurons. They rapidly increase the internalization rate of surface NMDAR clusters, independent of receptor activity. This internalization likely accounts for the observed decrease in NMDAR-mediated currents, as no evidence of direct blockade was detected. Once internalized, antibody-bound NMDARs traffic through both recycling endosomes and lysosomes, similar to pharmacologically induced NMDAR endocytosis. The antibodies are responsible for receptor internalization, as their depletion from CSF abrogates these effects in hippocampal neurons. We find that although anti-NMDAR antibodies do not induce compensatory changes in glutamate receptor gene expression, they cause a decrease in inhibitory synapse density onto excitatory hippocampal neurons.

Interpretation

Our data support an antibody-mediated mechanism of disease pathogenesis driven by immunoglobulin-induced receptor internalization. Antibody-mediated downregulation of surface NMDARs engages homeostatic synaptic plasticity mechanisms, which may inadvertently contribute to disease progression. Ann Neurol 2014;76:108–119

Synaptic structure quantification in cultured neurons

Behavioral problems (e.g., learning and memory) following developmental exposure to toxicants suggests that dysregulation of the process of synapse formation and function may occur. The ability to assess these changes is thus of value. This unit describes a method to investigate toxicant-induced changes to synaptic structure formation in primary hippocampal neurons using immunocytochemical labeling of the pre- and post-synaptic markers synaptophysin and PSD-95, confocal imaging, and three-dimensional object analysis. Protocols for the long-term culturing of primary hippocampal neurons and of primary cortical astrocytes, as well as their co-culture, are included. While the described methods focus on how astrocytes influence synapse formation and how toxicants may interfere in this process, modifications to the experimental plan can easily be implemented. This would allow for the investigation of the effects of toxicants after treating neurons alone, or both astrocytes and neurons in co-culture. With the common endpoint of synapse structure formation, differences between varying treatment paradigms can expand the understanding of the influence of particular toxicants on these diverse cell types and provide insight into potential mechanisms of effect and the contributions of each to synapse formation.

Glutamate receptor abnormalities in schizophrenia: implications for innovative treatments

Schizophrenia is a devastating psychiatric illness that afflicts 1% of the population worldwide, resulting in substantial impact to patients, their families, and health care delivery systems. For many years, schizophrenia has been felt to be associated with dysregulated dopaminergic neurotransmission as a key feature of the pathophysiology of the illness. Although numerous studies point to dopaminergic abnormalities in schizophrenia, dopamine dysfunction cannot completely account for all of the symptoms seen in schizophrenia, and dopamine-based treatments are often inadequate and can be associated with serious side effects. More recently, converging lines of evidence have suggested that there are abnormalities of glutamate transmission in schizophrenia. Glutamatergic neurotransmission involves numerous molecules that facilitate glutamate release, receptor activation, glutamate reuptake, and other synaptic activities. Evidence for glutamatergic abnormalities in schizophrenia primarily has implicated the NMDA and AMPA subtypes of the glutamate receptor. The expression of these receptors and other molecules associated with glutamate neurotransmission has been systematically studied in the brain in schizophrenia. These studies have generally revealed region- and molecule-specific changes in glutamate receptor transcript and protein expression in this illness. Given that glutamatergic neurotransmission has been implicated in the pathophysiology of schizophrenia, recent drug development efforts have targeted the glutamate system. Much effort to date has focused on modulation of the NMDA receptor, although more recently other glutamate receptors and transporters have been the targets of drug development. These efforts have been promising thus far, and ongoing efforts to develop additional drugs that modulate glutamatergic neurotransmission are underway that may hold the potential for novel classes of more effective treatments for this serious psychiatric illness.