Ionotropic glutamate receptors & CNS disorders

Spider and Wasp Acylpolyamines: Venom Components and Versatile Pharmacological Leads, Probes, and Insecticidal Agents

Polyamines (PAs) are polycationic biogenic amines ubiquitously present in all life forms and are involved in molecular signaling and interaction, determining cell fate (e.g., cell proliferation, dif-ferentiation, and apoptosis). The intricate balance in the PAs' levels in the tissues will determine whether beneficial or detrimental effects will affect homeostasis. It's crucial to note that endoge-nous polyamines, like spermine and spermidine, play a pivotal role in our understanding of neu-rological disorders as they interact with membrane receptors and ion channels, modulating neuro-transmission. In spiders and wasps, monoamines (histamine, dopamine, serotonin, tryptamine) and polyamines (spermine, spermidine, acyl polyamines) comprise, with peptides and other sub-stances, the low molecular weight fraction of the venom. Acylpolyamines are venom components exclusively from spiders and a species of solitary wasp, which cause inhibition chiefly of iono-tropic glutamate receptors (AMPA, NMDA, and KA iGluRs) and nicotinic acetylcholine receptors (nAChRs). The first venom acylpolyamines ever discovered (argiopines, Joro and Nephila toxins, and philanthotoxins) have provided templates for the design and synthesis of numerous analogs. Thus far, analogs with high potency exert their effect at nanomolar concentrations, with high se-lectivity toward their ionotropic and ligand receptors. These potent and selective acylpolyamine analogs can serve biomedical purposes and pest control management. The structural modification of acylpolyamine with photolabile and fluorescent groups converted these venom toxins into use-ful molecular probes to discriminate iGluRs and nAchRs in cell populations. In various cases, the linear polyamines, like spermine and spermidine, constituting venom acyl polyamine backbones, have served as cargoes to deliver active molecules via a polyamine uptake system on diseased cells for targeted therapy. In this review, we examined examples of biogenic amines that play an essential role in neural homeostasis and cell signaling, contributing to human health and disease outcomes, which can be present in the venom of arachnids and hymenopterans. With an empha-sis on the spider and wasp venom acylpolyamines, we focused on the origin, structure, derivatiza-tion, and biomedical and biotechnological application of these pharmacologically attractive, chemically modular venom components.

Preclinical Evaluation of Azabenzimidazole-Based PET Radioligands for γ-8 Dependent Transmembrane AMPA Receptor Regulatory Protein Imaging

AMPA glutamate receptors (AMPARs) play a pivotal role in excitatory neurotransmission, particularly in the hippocampus where the TARP γ-8 subunit is enriched and serves as a target for emerging anti-epileptic drugs. To enable in vivo visualization of TARP γ-8 distribution and expression by positron emission tomography (PET), this study focuses on the development of novel 18 F-labeled TARP γ-8 inhibitors and their corresponding precursors, stemming from the azabenzimidazole scaffold. The resulting radioligands [18 F]TARP-2204 and [18 F]TARP-2205 were successfully synthesized with acceptable radiochemical yield, high molar activity, and excellent radiochemical purity. In vitro autoradiography demonstrates high level of specific binding of [18 F]TARP-2205 to TARP γ-8 in both rat and nonhuman primate brain tissues. However, unexpected radiodefluorination in PET imaging studies of rodents emphasizes the need for further structural refinement. This work serves as an excellent starting point for the development of future 18 F-labeled TARP γ-8 PET tracers, offering valuable insights into medicinal chemistry design, radiosynthesis and subsequent PET evaluation.

MCU complex: Exploring emerging targets and mechanisms of mitochondrial physiology and pathology

BACKGROUND

Globally, the onset and progression of multiple human diseases are associated with mitochondrial dysfunction and dysregulation of Ca2+ uptake dynamics mediated by the mitochondrial calcium uniporter (MCU) complex, which plays a key role in mitochondrial dysfunction. Despite relevant studies, the underlying pathophysiological mechanisms have not yet been fully elucidated.

AIM OF REVIEW

This article provides an in-depth analysis of the current research status of the MCU complex, focusing on its molecular composition, regulatory mechanisms, and association with diseases. In addition, we conducted an in-depth analysis of the regulatory effects of agonists, inhibitors, and traditional Chinese medicine (TCM) monomers on the MCU complex and their application prospects in disease treatment. From the perspective of medicinal chemistry, we conducted an in-depth analysis of the structure-activity relationship between these small molecules and MCU and deduced potential pharmacophores and binding pockets. Simultaneously, key structural domains of the MCU complex in Homo sapiens were identified. We also studied the functional expression of the MCU complex in Drosophila, Zebrafish, and Caenorhabditis elegans. These analyses provide a basis for exploring potential treatment strategies targeting the MCU complex and provide strong support for the development of future precision medicine and treatments.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The MCU complex exhibits varying behavior across different tissues and plays various roles in metabolic functions. It consists of six MCU subunits, an essential MCU regulator (EMRE), and solute carrier 25A23 (SLC25A23). They regulate processes, such as mitochondrial Ca2+ (mCa2+) uptake, mitochondrial adenosine triphosphate (ATP) production, calcium dynamics, oxidative stress (OS), and cell death. Regulation makes it a potential target for treating diseases, especially cardiovascular diseases, neurodegenerative diseases, inflammatory diseases, metabolic diseases, and tumors.

Modulatory mechanisms of TARP γ8-selective AMPA receptor therapeutics

AMPA glutamate receptors (AMPARs) mediate excitatory neurotransmission throughout the brain. Their signalling is uniquely diversified by brain region-specific auxiliary subunits, providing an opportunity for the development of selective therapeutics. AMPARs associated with TARP γ8 are enriched in the hippocampus, and are targets of emerging anti-epileptic drugs. To understand their therapeutic activity, we determined cryo-EM structures of the GluA1/2-γ8 receptor associated with three potent, chemically diverse ligands. We find that despite sharing a lipid-exposed and water-accessible binding pocket, drug action is differentially affected by binding-site mutants. Together with patch-clamp recordings and MD simulations we also demonstrate that ligand-triggered reorganisation of the AMPAR-TARP interface contributes to modulation. Unexpectedly, one ligand (JNJ-61432059) acts bifunctionally, negatively affecting GluA1 but exerting positive modulatory action on GluA2-containing AMPARs, in a TARP stoichiometry-dependent manner. These results further illuminate the action of TARPs, demonstrate the sensitive balance between positive and negative modulatory action, and provide a mechanistic platform for development of both positive and negative selective AMPAR modulators.

Psychedelic Targeting of Metabotropic Glutamate Receptor 2 and Its Implications for the Treatment of Alcoholism

Alcohol abuse is a leading risk factor for the public health burden worldwide. Approved pharmacotherapies have demonstrated limited effectiveness over the last few decades in treating alcohol use disorders (AUD). New therapeutic approaches are therefore urgently needed. Historical and recent clinical trials using psychedelics in conjunction with psychotherapy demonstrated encouraging results in reducing heavy drinking in AUD patients, with psilocybin being the most promising candidate. While psychedelics are known to induce changes in gene expression and neuroplasticity, we still lack crucial information about how this specifically counteracts the alterations that occur in neuronal circuits throughout the course of addiction. This review synthesizes well-established knowledge from addiction research about pathophysiological mechanisms related to the metabotropic glutamate receptor 2 (mGlu2), with findings and theories on how mGlu2 connects to the major signaling pathways induced by psychedelics via serotonin 2A receptors (2AR). We provide literature evidence that mGlu2 and 2AR are able to regulate each other's downstream signaling pathways, either through monovalent crosstalk or through the formation of a 2AR-mGlu2 heteromer, and highlight epigenetic mechanisms by which 2ARs can modulate mGlu2 expression. Lastly, we discuss how these pathways might be targeted therapeutically to restore mGlu2 function in AUD patients, thereby reducing the propensity to relapse.

Conserved gene signatures shared among MAPT mutations reveal defects in calcium signaling

Introduction: More than 50 mutations in the MAPT gene result in heterogeneous forms of frontotemporal lobar dementia with tau inclusions (FTLD-Tau). However, early pathogenic events that lead to disease and the degree to which they are common across MAPT mutations remain poorly understood. The goal of this study is to determine whether there is a common molecular signature of FTLD-Tau. Methods: We analyzed genes differentially expressed in induced pluripotent stem cell-derived neurons (iPSC-neurons) that represent the three major categories of MAPT mutations: splicing (IVS10 + 16), exon 10 (p.P301L), and C-terminal (p.R406W) compared with isogenic controls. The genes that were commonly differentially expressed in MAPT IVS10 + 16, p.P301L, and p.R406W neurons were enriched in trans-synaptic signaling, neuronal processes, and lysosomal function. Many of these pathways are sensitive to disruptions in calcium homeostasis. One gene, CALB1, was significantly reduced across the three MAPT mutant iPSC-neurons and in a mouse model of tau accumulation. We observed a significant reduction in calcium levels in MAPT mutant neurons compared with isogenic controls, pointing to a functional consequence of this disrupted gene expression. Finally, a subset of genes commonly differentially expressed across MAPT mutations were also dysregulated in brains from MAPT mutation carriers and to a lesser extent in brains from sporadic Alzheimer disease and progressive supranuclear palsy, suggesting that molecular signatures relevant to genetic and sporadic forms of tauopathy are captured in a dish. The results from this study demonstrate that iPSC-neurons capture molecular processes that occur in human brains and can be used to pinpoint common molecular pathways involving synaptic and lysosomal function and neuronal development, which may be regulated by disruptions in calcium homeostasis.

Willardiine and Its Synthetic Analogues: Biological Aspects and Implications in Peptide Chemistry of This Nucleobase Amino Acid

Willardiine is a nonprotein amino acid containing uracil, and thus classified as nucleobase amino acid or nucleoamino acid, that together with isowillardiine forms the family of uracilylalanines isolated more than six decades ago in higher plants. Willardiine acts as a partial agonist of ionotropic glutamate receptors and more in particular it agonizes the non-N-methyl-D-aspartate (non-NMDA) receptors of L-glutamate: ie. the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and kainate receptors. Several analogues and derivatives of willardiine have been synthesised in the laboratory in the last decades and these compounds show different binding affinities for the non-NMDA receptors. More in detail, the willardiine analogues have been employed not only in the investigation of the structure of AMPA and kainate receptors, but also to evaluate the effects of receptor activation in the various brain regions. Remarkably, there are a number of neurological diseases determined by alterations in glutamate signaling, and thus, ligands for AMPA and kainate receptors deserve attention as potential neurodrugs. In fact, similar to willardiine its analogues often act as agonists of AMPA and kainate receptors. A particular importance should be recognized to willardiine and its thymine-based analogue AlaT also in the peptide chemistry field. In fact, besides the naturally-occurring short nucleopeptides isolated from plant sources, there are different examples in which this class of nucleoamino acids was investigated for nucleopeptide development. The applications are various ranging from the realization of nucleopeptide/DNA chimeras for diagnostic applications, and nucleoamino acid derivatization of proteins for facilitating protein-nucleic acid interaction, to nucleopeptide-nucleopeptide molecular recognition for nanotechnological applications. All the above aspects on both chemistry and biotechnological applications of willardine/willardine-analogues and nucleopeptide will be reviewed in this work.

An Insight into Animal Glutamate Receptors Homolog of Arabidopsis thaliana and Their Potential Applications-A Review

Most excitatory impulses received by neurons are mediated by ionotropic glutamate receptors (iGluRs). These receptors are located at the apex and play an important role in memory, neuronal development, and synaptic plasticity. These receptors are ligand-dependent ion channels that allow a wide range of cations to pass through. Glutamate, a neurotransmitter, activates three central ionotropic receptors: N-methyl-D-aspartic acid (NMDA), -amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), and kainic acid (KA). According to the available research, excessive glutamate release causes neuronal cell death and promotes neurodegenerative disorders. Arabidopsis thaliana contains 20 glutamate receptor genes (AtGluR) comparable to the human ionotropic glutamate (iGluRs) receptor. Many studies have proved that AtGL-rec genes are involved in a number of plant growth and physiological activities, such as in the germination of seeds, roots, abiotic and biotic stress, and cell signaling, which clarify the place of these genes in plant biology. In spite of these, the iGluRs, Arabidopsis glutamate receptors (AtGluR), is associated with the ligand binding activity, which confirms the evolutionary relationship between animal and plant glutamate receptors. Along with the above activities, the impact of mammalian agonists and antagonists on Arabidopsis suggests a correlation between plant and animal glutamate receptors. In addition, these glutamate receptors (plant/animal) are being utilized for the early detection of neurogenerative diseases using the fluorescence resonance energy transfer (FRET) approach. However, a number of scientific laboratories and institutes are consistently working on glutamate receptors with different aspects. Currently, we are also focusing on Arabidopsis glutamate receptors. The current review is focused on updating knowledge on AtGluR genes, their evolution, functions, and expression, and as well as in comparison with iGluRs. Furthermore, a high throughput approach based on FRET nanosensors developed for understanding neurotransmitter signaling in animals and plants via glutamate receptors has been discussed. The updated information will aid in the future comprehension of the complex molecular dynamics of glutamate receptors and the exploration of new facts in plant/animal biology.

Identification of a Prognostic Microenvironment-Related Gene Signature in Glioblastoma Patients Treated with Carmustine Wafers

Despite the state-of-the-art treatment, patients diagnosed with glioblastoma (GBM) have a median overall survival (OS) of 14 months. The insertion of carmustine wafers (CWs) into the resection cavity as adjuvant treatment represents a promising option, although its use has been limited due to contrasting clinical results. Our retrospective evaluation of CW efficacy showed a significant improvement in terms of OS in a subgroup of patients. Given the crucial role of the tumor microenvironment (TME) in GBM progression and response to therapy, we hypothesized that the TME of patients who benefited from CW could have different properties compared to that of patients who did not show any advantage. Using an in vitro model of the glioma microenvironment, represented by glioma-associated-stem cells (GASC), we performed a transcriptomic analysis of GASC isolated from tumors of patients responsive and not responsive to CW to identify differentially expressed genes. We found different transcriptomic profiles, and we identified four genes, specifically down-regulated in GASC isolated from long-term survivors, correlated with clinical data deposited in the TCGA–GBM dataset. Our results highlight that studying the in vitro properties of patient-specific glioma microenvironments can help to identify molecular determinants potentially prognostic for patients treated with CW.

Structure, Function, and Regulation of the Kainate Receptor

Neural communication and modulation are complex processes. Ionotropic glutamate receptors (iGluRs) significantly contribute to mediating the fast-excitatory branch of neurotransmission in the mammalian brain. Kainate receptors (KARs), a subfamily of the iGluRs, act as modulators of the neuronal circuitry by playing important roles at both the post- and presynaptic sites of specific neurons. The functional tetrameric receptors are formed by two different gene families, low agonist affinity (GluK1-GluK3) and high agonist affinity (GluK4-GluK5) subunits. These receptors garnered attention in the past three decades, and since then, much work has been done to understand their localization, interactome, physiological functions, and regulation. Cloning of the receptor subunits (GluK1-GluK5) in the early 1990s led to recombinant expression of kainate receptors in heterologous systems. This facilitated understanding of the functional differences between subunit combinations, splice variants, trafficking, and drug discovery. Structural studies of individual domains and recent full-length homomeric and heteromeric kainate receptors have revealed unique functional mechanisms, which have answered several long-standing questions in the field of kainate receptor biology. In this chapter, we review the current understanding of kainate receptors and associated disorders.

Optical Control of N-Methyl-d-aspartate Receptors by Azobenzene Quaternary Ammonium Compounds

Azobenzene-based quaternary ammonium compounds provide optical control of ion channels and are considered promising agents for regulation of neuronal excitability and for restoration of the photosensitivity of retinal cells. However, the selectivity of the action of these compounds remains insufficiently known. We studied the action of DENAQ (diethylamine-azobenzene-quaternary ammonium) and DMNAQ (dimethylamine-azobenzene-quaternary ammonium) on ionotropic glutamate receptors in rat brain neurons. In the dark, both compounds applied extracellularly caused fast and reversible inhibition of NMDA (N-methyl-d-aspartate) receptor-mediated currents with IC₅₀ values of 10 and 5 μM, respectively. Light-induced transformation of DENAQ and DMNAQ to their cis forms caused the IC₅₀ values to increase to 30 and 27 μM, respectively. Detailed analysis of this action revealed a complex nature consisting of fast inhibitory and slower potentiating effects. The AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors were only weakly affected independently on illumination. We conclude that, in addition to their long-lasting intracellular action, which persists after washout, azobenzene-based quaternary ammonium compounds should affect glutamatergic transmission and synaptic plasticity during treatment. Our findings also extend the list of soluble photoswitchable inhibitors of NMDA receptors. While the site(s) and mechanisms of action are unclear, the effect of DENAQ demonstrates strong pH dependence. At acidic pH values, DENAQ potentiates both NMDA and AMPA receptors.

ELAVL4, splicing, and glutamatergic dysfunction precede neuron loss in MAPT mutation cerebral organoids

Frontotemporal dementia (FTD) because of MAPT mutation causes pathological accumulation of tau and glutamatergic cortical neuronal death by unknown mechanisms. We used human induced pluripotent stem cell (iPSC)-derived cerebral organoids expressing tau-V337M and isogenic corrected controls to discover early alterations because of the mutation that precede neurodegeneration. At 2 months, mutant organoids show upregulated expression of MAPT, glutamatergic signaling pathways, and regulators, including the RNA-binding protein ELAVL4, and increased stress granules. Over the following 4 months, mutant organoids accumulate splicing changes, disruption of autophagy function, and build-up of tau and P-tau-S396. By 6 months, tau-V337M organoids show specific loss of glutamatergic neurons as seen in individuals with FTD. Mutant neurons are susceptible to glutamate toxicity, which can be rescued pharmacologically by the PIKFYVE kinase inhibitor apilimod. Our results demonstrate a sequence of events that precede neurodegeneration, revealing molecular pathways associated with glutamate signaling as potential targets for therapeutic intervention in FTD.

Ca2+ -permeable AMPA receptors and their auxiliary subunits in synaptic plasticity and disease

AMPA receptors are tetrameric glutamate-gated ion channels that mediate a majority of fast excitatory neurotransmission in the brain. They exist as calcium-impermeable (CI-) and calcium-permeable (CP-) subtypes, the latter of which lacks the GluA2 subunit. CP-AMPARs display an array of distinctive biophysical and pharmacological properties that allow them to be functionally identified. This has revealed that they play crucial roles in diverse forms of central synaptic plasticity. Here we summarise the functional hallmarks of CP-AMPARs and describe how these are modified by the presence of auxiliary subunits that have emerged as pivotal regulators of AMPARs. A lasting change in the prevalence of GluA2-containing AMPARs, and hence in the fraction of CP-AMPARs, is a feature in many maladaptive forms of synaptic plasticity and neurological disorders. These include modifications of glutamatergic transmission induced by inflammatory pain, fear conditioning, cocaine exposure, and anoxia-induced damage in neurons and glia. Furthermore, defective RNA editing of GluA2 can cause altered expression of CP-AMPARs and is implicated in motor neuron damage (amyotrophic lateral sclerosis) and the proliferation of cells in malignant gliomas. A number of the players involved in CP-AMPAR regulation have been identified, providing useful insight into interventions that may prevent the aberrant CP-AMPAR expression. Furthermore, recent molecular and pharmacological developments, particularly the discovery of TARP subtype-selective drugs, offer the exciting potential to modify some of the harmful effects of increased CP-AMPAR prevalence in a brain region-specific manner.

AMPA Receptor Auxiliary Subunit GSG1L Suppresses Short-Term Facilitation in Corticothalamic Synapses and Determines Seizure Susceptibility

The anterior thalamus (AT) is critical for memory formation, processing navigational information, and seizure initiation. However, the molecular mechanisms that regulate synaptic function of AT neurons remain largely unexplored. We report that AMPA receptor auxiliary subunit GSG1L controls short-term plasticity in AT synapses that receive inputs from the cortex, but not in those receiving inputs from other pathways. A canonical auxiliary subunit stargazin co-exists in these neurons but is functionally absent from corticothalamic synapses. In GSG1L knockout mice, AT neurons exhibit hyperexcitability and the animals have increased susceptibility to seizures, consistent with a negative regulatory role of GSG1L. We hypothesize that negative regulation of synaptic function by GSG1L plays a critical role in maintaining optimal excitation in the AT.

Kamalova et al. report the phenotypes of GSG1L KO mice. The synaptic function of AMPAR auxiliary subunit GSG1L in the anterior thalamus is input specific. GSG1L suppresses short-term facilitation and decreases AMPAR activity specifically in corticothalamic synapses, where stargazin is functionally absent. GSG1L KO mice exhibit hyperexcitability and seizure susceptibility.

Kainate receptors in the developing neuronal networks

Kainate receptors (KARs) are highly expressed in the immature brain and have unique developmentally regulated functions that may be important in linking neuronal activity to morphogenesis during activity-dependent fine-tuning of the synaptic connectivity. Altered expression of KARs in the developing neural network leads to changes in glutamatergic connectivity and network excitability, which may lead to long-lasting changes in behaviorally relevant circuitries in the brain. Here, we summarize the current knowledge on physiological and morphogenic functions described for different types of KARs at immature neural circuitries, focusing on their roles in modulating synaptic transmission and plasticity as well as circuit maturation in the rodent hippocampus and amygdala. Finally, we discuss the emerging evidence suggesting that malfunction of KARs in the immature brain may contribute to the pathophysiology underlying developmentally originating neurological disorders.

Architecture and structural dynamics of the heteromeric GluK2/K5 kainate receptor

Kainate receptors (KARs) are L-glutamate-gated ion channels that regulate synaptic transmission and modulate neuronal circuits. KARs have strict assembly rules and primarily function as heteromeric receptors in the brain. A longstanding question is how KAR heteromer subunits organize and coordinate together to fulfill their signature physiological roles. Here we report structures of the GluK2/GluK5 heteromer in apo, antagonist-bound, and desensitized states. The receptor assembles with two copies of each subunit, ligand binding domains arranged as two heterodimers and GluK5 subunits proximal to the channel. Strikingly, during desensitization, GluK2, but not GluK5, subunits undergo major structural rearrangements to facilitate channel closure. We show how the large conformational differences between antagonist-bound and desensitized states are mediated by the linkers connecting the pore helices to the ligand binding domains. This work presents the first KAR heteromer structure, reveals how its subunits are organized, and resolves how the heteromer can accommodate functionally distinct closed channel structures.

GluN2B/N-methyl-D-aspartate Receptor Antagonists: Advances in Design, Synthesis, and Pharmacological Evaluation Studies

Selective GluN2B/N-methyl-D-aspartate receptor (NMDAR) antagonists have exposed their clinical effectiveness in a cluster of neurodegenerative diseases, such as epilepsy, Alzheimer's disease, Parkinson's disease, pain, and depression. Hence, GluN2B/NMDARs are considered to be a prospective target for the management of neurodegenerative diseases. Here, we have discussed the current results and significance of subunit selective GluN2B/NMDAR antagonists to pave the way for the establishment of new, safe, and economical drug candidates in the near future. By using summarized data of selective GluN2B/NMDAR antagonists, medicinal chemists are certainly a step closer to the goal of improving the therapeutic and side effect profile of selective antagonists. Outlined summary of designing strategies, synthetic schemes, and pharmacological evaluation studies reinvigorate efforts to identify, modify, and synthesize novel GluN2B/NMDAR antagonists for treating neurodegenerative diseases.

Multiplex gene and phenotype network to characterize shared genetic pathways of epilepsy and autism

It is well established that epilepsy and autism spectrum disorder (ASD) commonly co-occur; however, the underlying biological mechanisms of the co-occurence from their genetic susceptibility are not well understood. Our aim in this study is to characterize genetic modules of subgroups of epilepsy and autism genes that have similar phenotypic manifestations and biological functions. We first integrate a large number of expert-compiled and well-established epilepsy- and ASD-associated genes in a multiplex network, where one layer is connected through protein-protein interaction (PPI) and the other layer through gene-phenotype associations. We identify two modules in the multiplex network, which are significantly enriched in genes associated with both epilepsy and autism as well as genes highly expressed in brain tissues. We find that the first module, which represents the Gene Ontology category of ion transmembrane transport, is more epilepsy-focused, while the second module, representing synaptic signaling, is more ASD-focused. However, because of their enrichment in common genes and association with both epilepsy and ASD phenotypes, these modules point to genetic etiologies and biological processes shared between specific subtypes of epilepsy and ASD. Finally, we use our analysis to prioritize new candidate genes for epilepsy (i.e. ANK2, CACNA1E, CACNA2D3, GRIA2, DLG4) for further validation. The analytical approaches in our study can be applied to similar studies in the future to investigate the genetic connections between different human diseases.

Molecular mechanisms of action determine inhibition of paroxysmal depolarizing shifts by NMDA receptor antagonists in rat cortical neurons

N-methyl-d-aspartate glutamate receptors (NMDARs) are involved in numerous central nervous system (CNS) processes, including epileptiform activity. We used a picrotoxin-induced epileptiform activity model to compare the action of different types of NMDAR antagonists in rat brain slices. Paroxysmal depolarizing shifts (PDS) were evoked by external stimulation in the medial prefrontal cortex (mPFC) slices and recorded in pyramidal cells (PC) and in fast-spiking interneurons (FSI). The NMDAR antagonists APV and memantine reduced the duration of PDS. However, the competitive antagonist APV caused similar effects on the PC and FSI, while the open-channel blocker memantine had a much stronger effect on the PDS in the FSI than in the PC. This difference cannot be explained by a corresponding difference in NMDAR sensitivity to memantine because the drug inhibited the excitatory postsynaptic current (EPSC) similarly in both cell types. Importantly, the PDS were significantly longer in the FSI than in the PC. The degree of PDS inhibition by memantine correlated with individual PDS durations in each cell type. Computer modeling of a synaptic network in the mPFC suggests that the different effects of memantine on the PDS in the PC and FSI can be explained by use dependence of its action. An open-channel blocking mechanism and competition with Mg²⁺ ions for the binding site result in pronounced inhibition of the long PDS, whereas the short PDS are weakly sensitive. Our results show that peculiarities of kinetics and the mechanism of action largely determine the effects of NMDAR antagonists on physiological and/or pathological processes.

Transient Enhanced GluA2 Expression in Young Hippocampal Neurons of a Fragile X Mouse Model

AMPA-type glutamate receptors (AMPARs) are tetrameric ligand-gated channels made up of combinations of GluA1-4 subunits and play important roles in synaptic transmission and plasticity. Here, we have investigated the development of AMPAR-mediated synaptic transmission in the hippocampus of the Fmr1 knock-out (KO) mouse, a widely used model of Fragile X syndrome (FXS). FXS is the leading monogenic cause of intellectual disability and autism spectrum disorders (ASD) and it is considered a neurodevelopmental disorder. For that reason, we investigated synaptic properties and dendritic development in animals from an early stage when synapses are starting to form up to adulthood. We found that hippocampal CA1 pyramidal neurons in the Fmr1-KO mouse exhibit a higher AMPAR-NMDAR ratio early in development but reverses to normal values after P13. This increase was accompanied by a larger presence of the GluA2-subunit in synaptic AMPARs that will lead to altered Ca²⁺ permeability of AMPARs that could have a profound impact upon neural circuits, learning, and diseases. Following this, we found that young KO animals lack Long-term potentiation (LTP), a well-understood model of synaptic plasticity necessary for proper development of circuits, and exhibit an increased frequency of spontaneous miniature excitatory postsynaptic currents, a measure of synaptic density. Furthermore, post hoc morphological analysis of recorded neurons revealed altered dendritic branching in the KO group. Interestingly, all these anomalies are transitory and revert to normal values in older animals. Our data suggest that loss of FMRP during early development leads to temporary upregulation of the GluA2 subunit and this impacts synaptic plasticity and altering morphological dendritic branching.

Interplay between Gating and Block of Ligand-Gated Ion Channels

Drugs that inhibit ion channel function by binding in the channel and preventing current flow, known as channel blockers, can be used as powerful tools for analysis of channel properties. Channel blockers are used to probe both the sophisticated structure and basic biophysical properties of ion channels. Gating, the mechanism that controls the opening and closing of ion channels, can be profoundly influenced by channel blocking drugs. Channel block and gating are reciprocally connected; gating controls access of channel blockers to their binding sites, and channel-blocking drugs can have profound and diverse effects on the rates of gating transitions and on the stability of channel open and closed states. This review synthesizes knowledge of the inherent intertwining of block and gating of excitatory ligand-gated ion channels, with a focus on the utility of channel blockers as analytic probes of ionotropic glutamate receptor channel function.

The pore domain in glutamate-gated ion channels: Structure, drug binding and similarity with potassium channels

Ionotropic glutamate receptors in the CNS excitatory synapses of vertebrates are involved in numerous physiological and pathological processes. Decades of intensive studies greatly advanced our understanding of molecular organization of these transmembrane proteins. Here we focus on the channel pore domain, its selectivity filter and the activation gate, and the pore block by organic ligands. We compare findings from indirect experimental approaches, including site-directed mutagenesis, with recent crystal and cryo-EM structures of different channels in different functional states and complexed with different ligands. We summarize remaining uncertainties and unresolved problems related to the channel structure, function and pharmacology.

Characterizing the binding and function of TARP γ8-selective AMPA receptor modulators

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA)-type glutamate receptors (AMPARs) are the predominant excitatory neurotransmitter receptors in the brain, where they mediate synaptic transmission and plasticity. Excessive AMPAR activation leads to diseases such as epilepsy. AMPAR properties are modulated by auxiliary proteins and foremost by the transmembrane AMPAR regulatory proteins (TARPs). These distribute in unique expression patterns across the brain, rendering AMPAR/TARP complexes promising targets for region-specific therapeutic intervention. TARP γ8 is predominantly expressed in the forebrain and is enriched in the hippocampus, a region associated with temporal lobe epilepsy. Recent high-throughput medicinal chemistry screens have identified multiple promising compounds that selectively target AMPARs associated with γ8 and hold promise for epilepsy treatment. However, how these modulators target the receptor complex is currently unknown. Here, we use a combination of ligand docking, molecular dynamics simulations, and electrophysiology to address this question. We identify a conserved oxindole isostere, shared between three structurally diverse modulators (LY-3130481, JNJ-55511118, and JNJ-61432059) as the major module engaging γ8 by an H-bond to Asn-172 (γ8). The remaining variable region of each molecule likely targets the receptor complex in ligand-selective modes. Functional data reveal parallels in the underlying modulatory action of two prominent compounds. This work will aid development of refined AMPAR epilepsy therapeutics and facilitate to uncover the mechanisms by which TARPs modulate the receptor.

Binding of a negative allosteric modulator and competitive antagonist can occur simultaneously at the ionotropic glutamate receptor GluA2

Ionotropic glutamate receptors are ligand-gated ion channels governing neurotransmission in the central nervous system. Three major types of antagonists are known for the AMPA-type receptor GluA2: competitive, noncompetitive (i.e., negative allosteric modulators; NAMs) used for treatment of epilepsy, and uncompetitive antagonists. We here report a 4.65 Å resolution X-ray structure of GluA2, revealing that four molecules of the competitive antagonist ZK200775 and four molecules of the NAM GYKI53655 are capable of binding at the same time. Using negative stain electron microscopy, we show that GYKI53655 alone or ZK200775/GYKI53655 in combination predominantly results in compact receptor forms. The agonist AMPA provides a mixed population of compact and bulgy shapes of GluA2 not impacted by addition of GYKI53655. Taken together, this suggests that the two different mechanisms of antagonism that lead to channel closure are independent and that the distribution between bulgy and compact receptors primarily depends on the ligand bound in the glutamate binding site. DATABASE: The atomic coordinates and structure factors from the crystal structure determination have been deposited in the Protein Data Bank under accession code https://doi.org/10.2210/pdb6RUQ/pdb. The electron microscopy 3D reconstruction volumes have been deposited in EMDB (EMD-4875: Apo; EMD-4920: ZK200775/GYKI53655; EMD-4921: AMPA compact; EMD-4922: AMPA/GYKI53655 bulgy; EMD-4923: GYKI53655; EMD-4924: AMPA bulgy; EMD-4925: AMPA/GYKI53655 compact).

Ionotropic Glutamate Receptor GluA2 in Complex with Bicyclic Pyrimidinedione-Based Compounds: When Small Compound Modifications Have Distinct Effects on Binding Interactions

(S)-2-Amino-3-(5-methyl-3-hydroxyisoxazol-4-yl)propanoic acid (AMPA) receptors comprise an important class of ionotropic glutamate receptors activated by glutamate in the central nervous system. These receptors have been shown to be involved in brain diseases, for example, Alzheimer's disease and epilepsy. To understand the functional role of AMPA receptors at the molecular level and their potential as targets for drugs, development of tool compounds is essential. We have previously reported the synthesis of six bicyclic pyrimidinedione-based analogues of willardiine with differences limited to the pyrimidinedione-fused five-membered rings. Despite minor molecular differences, we observed >500-fold difference in binding affinity of the compounds at full-length GluA2. Here, we report binding affinities and the binding mode of these compounds at the ligand-binding domain of GluA2 using X-ray crystallography. The structures revealed similar binding modes, with distinct differences in the interaction between GluA2 and the compounds. The methylene (2) and sulfur (3) containing compounds showed the greatest binding affinities. Changing the dihydrothiophene (3) into pyrrolidine (4), N-methyl pyrrolidine (5), or dihydrofuran (6) induced flexibility in the position of a binding-site water molecule and changes in the hydrogen-bonding network between compound, water, and GluA2. This might be essential for explaining the reduced binding affinity of these compounds. The weakest binding affinity was observed when the aliphatic oxygen containing dihydrofuran (6) was changed into an aromatic furan system (7). Molecular docking studies revealed two possible orientations of 7, whereas only one binding mode was observed for the other analogues. This could likely contribute to the weakest binding affinity of 7 at GluA2.

Measurements of the Timescale and Conformational Space of AMPA Receptor Desensitization

Ionotropic glutamate receptors are ligand-gated ion channels that mediate excitatory synaptic transmission in the central nervous system. Desensitization of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype after glutamate binding appears critical for brain function and involves rearrangement of the ligand binding domains (LBDs). Recently, several full-length structures of ionotropic glutamate receptors in putative desensitized states were published. These structures indicate movements of the LBDs that might be trapped by cysteine cross-links and metal bridges. We found that cysteine mutants at the interface between subunits A and C and lateral zinc bridges (between subunits C and D or A and B) can trap freely desensitizing receptors in a spectrum of states with different stabilities. Consistent with a close approach of subunits during desensitization processes, the introduction of bulky amino acids at the A-C interface produced a receptor with slow recovery from desensitization. Further, in wild-type GluA2 receptors, we detected the population of a stable desensitized state with a lifetime around 1 s. Using mutations that progressively stabilize deep desensitized states (E713T and Y768R), we were able to selectively protect receptors from cross-links at both the diagonal and lateral interfaces. Ultrafast perfusion enabled us to perform chemical modification in less than 10 ms, reporting movements associated to desensitization on this timescale within LBD dimers in resting receptors. These observations suggest that small disruptions of quaternary structure are sufficient for fast desensitization and that substantial rearrangements likely correspond to stable desensitized states that are adopted relatively slowly on a timescale much longer than physiological receptor activation.

Structural dynamics of the GluK3-kainate receptor neurotransmitter binding domains revealed by cryo-EM

Kainate receptors belong to the ionotropic glutamate receptor family and play critical roles in the regulation of synaptic networks. The kainate receptor subunit GluK3 has unique functional properties and contributes to presynaptic facilitation at the hippocampal mossy fiber synapses along with roles at the post-synapses. To gain structural insights into the unique functional properties and dynamics of GluK3 receptor, we imaged them via electron microscopy in the apo-state and in complex with either agonist kainate or antagonist UBP301. Our analysis of all the GluK3 full-length structures not only provides insights into the receptor transitions between desensitized and closed states but also reveals a "non-classical" conformation of neurotransmitter binding domain in the closed-state distinct from that observed in AMPA and other kainate receptor structures. We show by molecular dynamics simulations that Asp759 influences the stability of the LBD dimers and hence could be responsible for the observed conformational variability and dynamics of the GluK3 via electron microscopy. Lower dimer stability could explain faster desensitization and low agonist sensitivity of GluK3. In overview, our work helps to associate biochemistry and physiology of GluK3 receptors with their structural biology and offers structural insights into the unique functional properties of these atypical receptors.

The structural arrangement and dynamics of the heteromeric GluK2/GluK5 kainate receptor as determined by smFRET

Kainate receptors, which are glutamate activated excitatory neurotransmitter receptors, predominantly exist as heteromers of GluK2 and GluK5 subunits in the mammalian central nervous system. There are currently no structures of the full-length heteromeric kainate receptors. Here, we have used single molecule FRET to determine the specific arrangement of the GluK2 and GluK5 subunits within the dimer of dimers configuration in a full-length receptor. Additionally, we have also studied the dynamics and conformational heterogeneity of the amino-terminal and agonist-binding domain interfaces associated with the resting and desensitized states of the full-length heteromeric kainate receptor using FRET-based methods. The smFRET data are compared to similar experiments performed on the homomeric kainate receptor to provide insight into the differences in conformational dynamics that distinguish the two functionally. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

Structures of the AMPA receptor in complex with its auxiliary subunit cornichon

In the brain, AMPA-type glutamate receptors (AMPARs) form complexes with their auxiliary subunits and mediate the majority of fast excitatory neurotransmission. Signals transduced by these complexes are critical for synaptic plasticity, learning, and memory. The two major categories of AMPAR auxiliary subunits are transmembrane AMPAR regulatory proteins (TARPs) and cornichon homologs (CNIHs); these subunits share little homology and play distinct roles in controlling ion channel gating and trafficking of AMPAR. Here, I report high-resolution cryo–electron microscopy structures of AMPAR in complex with CNIH3. Contrary to its predicted membrane topology, CNIH3 lacks an extracellular domain and instead contains four membrane-spanning helices. The protein-protein interaction interface that dictates channel modulation and the lipids surrounding the complex are revealed. These structures provide insights into the molecular mechanism for ion channel modulation and assembly of AMPAR/CNIH3 complexes.

Structural Mechanisms Underlying Activity Changes in an AMPA-type Glutamate Receptor Induced by Substitutions in Its Ligand-Binding Domain

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors produce postsynaptic current by transmitting an agonist-induced structural change in the ligand-binding domain (LBD) to the transmembrane channel. Receptors carrying T686S/A substitutions in their LBDs produce weaker glutamate-evoked currents than wild-type (WT) receptors. However, the substitutions induce little differences in the crystal structures of their LBDs. To understand the structural mechanism underlying reduced activities of these AMPAR variants, we analyzed the structural dynamics of WT, T686S, and T686A variants of LBD using nuclear magnetic resonance. The HD exchange studies of the LBDs showed that the kinetic step where the ligand-binding cleft closes was changed by the substitutions, and the substitution-induced population shift from cleft-closed to cleft-open structures is responsible for the reduced activities of the variants. The chemical shift analyses revealed another structural equilibrium between cleft-locked and cleft-partially-open conformations. The substitution-induced population shift in this equilibrium may be related to slower desensitization observed for these variants.

Structural and functional insights into transmembrane AMPA receptor regulatory protein complexes

Twomey et al. examine recent structural and functional data that have provided insight into AMPA receptor modulation by TARPs.

Fast excitatory neurotransmission is mediated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of ionotropic glutamate receptor (AMPAR). AMPARs initiate depolarization of the postsynaptic neuron by allowing cations to enter through their ion channel pores in response to binding of the neurotransmitter glutamate. AMPAR function is dramatically affected by auxiliary subunits, which are regulatory proteins that form various complexes with AMPARs throughout the brain. The most well-studied auxiliary subunits are the transmembrane AMPAR regulatory proteins (TARPs), which alter the assembly, trafficking, localization, kinetics, and pharmacology of AMPARs. Recent structural and functional studies of TARPs and the TARP-fold germ cell-specific gene 1-like (GSG1L) subunit have provided important glimpses into how auxiliary subunits regulate the function of synaptic complexes. In this review, we put these recent structures in the context of new functional findings in order to gain insight into the determinants of AMPAR regulation by TARPs. We thus reveal why TARPs display a broad range of effects despite their conserved modular architecture.

A Cold-Sensing Receptor Encoded by a Glutamate Receptor Gene

In search of the molecular identities of cold-sensing receptors, we carried out an unbiased genetic screen for cold-sensing mutants in C. elegans and isolated a mutant allele of glr-3 gene that encodes a kainate-type glutamate receptor. While glutamate receptors are best known to transmit chemical synaptic signals in the CNS, we show that GLR-3 senses cold in the peripheral sensory neuron ASER to trigger cold-avoidance behavior. GLR-3 transmits cold signals via G protein signaling independently of its glutamate-gated channel function, suggesting GLR-3 as a metabotropic cold receptor. The vertebrate GLR-3 homolog GluK2 from zebrafish, mouse, and human can all function as a cold receptor in heterologous systems. Mouse DRG sensory neurons express GluK2, and GluK2 knockdown in these neurons suppresses their sensitivity to cold but not cool temperatures. Our study identifies an evolutionarily conserved cold receptor, revealing that a central chemical receptor unexpectedly functions as a thermal receptor in the periphery.

Persistent Toxoplasma Infection of the Brain Induced Neurodegeneration Associated with Activation of Complement and Microglia

Toxoplasma gondii, a common neurotropic parasite, is increasingly being linked to neuropsychiatric disorders, including schizophrenia, Alzheimer's disease, and Parkinson's disease. However, the pathogenic mechanisms underlying these associations are not clear. Toxoplasma can reside in the brain for extensive periods in the form of tissue cysts, and this process requires a continuous immune response to prevent the parasite's reactivation. Because neuroinflammation may promote the onset and progression of neurodegenerative diseases, we investigated neurodegeneration-associated pathological changes in a mouse model of chronic Toxoplasma infection. Under conditions of high-grade chronic infection, we documented the presence of neurodegeneration in specific regions of the prefrontal cortex, namely, the anterior cingulate cortex (ACC) and somatomotor cortex (SC). Neurodegeneration occurred in both glutamatergic and GABAergic neurons. Neurons that showed signs of degeneration expressed high levels of CX3CL1, were marked by profoundly upregulated complement proteins (e.g., C1q and C3), and were surrounded by activated microglia. Our findings suggest that chronic Toxoplasma infection leads to cortical neurodegeneration and results in CX3CL1, complement, and microglial interactions, which are known to mediate the phagocytic clearance of degenerating neurons. Our study provides a mechanistic explanation for the link between Toxoplasma infection and psychiatric disorders.

Structural and Functional Insights into GluK3-kainate Receptor Desensitization and Recovery

GluK3-kainate receptors are atypical members of the iGluR family that reside at both the pre- and postsynapse and play a vital role in the regulation of synaptic transmission. For a better understanding of structural changes that underlie receptor functions, GluK3 receptors were trapped in desensitized and resting/closed states and structures analyzed using single particle cryo-electron microscopy. While the desensitized GluK3 has domain organization as seen earlier for another kainate receptor-GluK2, antagonist bound GluK3 trapped a resting state with only two LBD domains in dimeric arrangement necessary for receptor activation. Using structures as a guide, we show that the N-linked glycans at the interface of GluK3 ATD and LBD likely mediate inter-domain interactions and attune receptor-gating properties. The mutational analysis also identified putative N-glycan interacting residues. Our results provide a molecular framework for understanding gating properties unique to GluK3 and exploring the role of N-linked glycosylation in their modulation.

The structural arrangement at intersubunit interfaces in homomeric kainate receptors

Kainate receptors are glutamate-gated cation-selective channels involved in excitatory synaptic signaling and are known to be modulated by ions. Prior functional and structural studies suggest that the dimer interface at the agonist-binding domain plays a key role in activation, desensitization, and ion modulation in kainate receptors. Here we have used fluorescence-based methods to investigate the changes and conformational heterogeneity at these interfaces associated with the resting, antagonist-bound, active, desensitized, and ion-modulated states of the receptor. These studies show that in the presence of Na+ ions the interfaces exist primarily in the coupled state in the apo, antagonist-bound and activated (open channel) states. Under desensitizing conditions, the largely decoupled dimer interface at the agonist-binding domain as seen in the cryo-EM structure is one of the states observed. However, in addition to this state there are several additional states with lower levels of decoupling. Replacing Na+ with Cs+ does not alter the FRET efficiencies of the states significantly, but shifts the population to the more decoupled states in both resting and desensitized states, which can be correlated with the lower activation seen in the presence of Cs+.

Toward a Glutamate Hypothesis of Frontotemporal Dementia

Frontotemporal dementia (FTD) is a heterogenous neurodegenerative disorder, characterized by diverse clinical presentations, neuropathological characteristics and underlying genetic causes. Emerging evidence has shown that FTD is characterized by a series of changes in several neurotransmitter systems, including serotonin, dopamine, GABA and, above all, glutamate. Indeed, several studies have now provided preclinical and clinical evidence that glutamate is key in the pathogenesis of FTD. Animal models of FTD have shown a selective hypofunction in N-methyl D-aspartate (NMDA) and α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, while in patients, glutamatergic pyramidal neurons are depleted in several areas, including the frontal and temporal cortices. Recently, a selective involvement of the AMPA GluA3 subunit has been observed in patients with autoimmune anti-GluA3 antibodies, which accounted for nearly 25% of FTD patients, leading to a decrease of the GluA3 subunit synaptic localization of the AMPA receptor and loss of dendritic spines. Other in vivo evidence of the involvement of the glutamatergic system in FTD derives from non-invasive brain stimulation studies using transcranial magnetic stimulation, in which specific stimulation protocols have indirectly identified a selective and prominent impairment in glutamatergic circuits in patients with both sporadic and genetic FTD. In view of limited disease modifying therapies to slow or revert disease progression in FTD, an important approach could consist in targeting the neurotransmitter deficits, similarly to what has been achieved in Parkinson’s disease with dopaminergic therapy or Alzheimer’s disease with cholinergic therapy. In this review, we summarize the current evidence concerning the involvement of the glutamatergic system in FTD, suggesting the development of new therapeutic strategies.

Glutamate Receptor Antibodies in Autoimmune Central Nervous System Disease: Basic Mechanisms, Clinical Features, and Antibody Detection

Immune-mediated inflammation of the brain has been recognized for more than 50 years, although the initial descriptions were mainly thought to be secondary to an underlying neoplasm. Some of these paraneoplastic encephalitides express serum antibodies, but these were not thought to be pathogenic but instead have a T-cell-mediated pathophysiology. Over the last two decades, several pathogenic antibodies against neuronal surface antigens have been described in autoimmune encephalitis, which are amenable to immunotherapy. Several of these antibodies are directed against glutamate receptors (GluRs). NMDAR encephalitis (NMDARE) is the most common of these antibodies, and patients often present with psychosis, hallucinations, and reduced consciousness. Patients often progress on to develop confusion, seizures, movement disorders, autonomic instability, and respiratory depression. Although initially described as exclusively occurring secondary to ovarian teratoma (and later other tumors), non-paraneoplastic forms are increasingly common, and other triggers like viral infections are now well recognized. AMPAR encephalitis is relatively less common than NMDARE but is more likely to paraneoplastic. AMPAR antibodies typically cause limbic encephalitis, with patients presenting with confusion, disorientation, memory loss, and often seizures. The syndromes associated with the metabotropic receptor antibodies are much rarer and often can be paraneoplastic-mGluR1 (cerebellar degeneration) and mGluR5 (Ophelia syndrome) being the ones described in literature.With the advance in molecular biology techniques, it is now possible to detect these antibodies using cell-based assays with high sensitivity and specificity, especially when coupled with brain tissue immunohistochemistry and binding to live cell-based neurons. The rapid and reliable identification of these antibodies aids in the timely treatment (either in the form of identifying/removing the underlying tumor or instituting immunomodulatory therapy) and has significantly improved clinical outcome in this otherwise devastating group of conditions.

Druggability Simulations and X-Ray Crystallography Reveal a Ligand-Binding Site in the GluA3 AMPA Receptor N-Terminal Domain

Ionotropic glutamate receptors (iGluRs) mediate the majority of excitatory neurotransmission in the brain. Their dysfunction is implicated in many neurological disorders, rendering iGluRs potential drug targets. Here, we performed a systematic analysis of the druggability of two major iGluR subfamilies, using molecular dynamics simulations in the presence of drug-like molecules. We demonstrate the applicability of druggability simulations by faithfully identifying known agonist and modulator sites on AMPA receptors (AMPARs) and NMDA receptors. Simulations produced the expected allosteric changes of the AMPAR ligand-binding domain in response to agonist. We also identified a novel ligand-binding site specific to the GluA3 AMPAR N-terminal domain (NTD), resulting from its unique conformational flexibility that we explored further with crystal structures trapped in vastly different states. In addition to providing an in-depth analysis into iGluR NTD dynamics, our approach identifies druggable sites and permits the determination of pharmacophoric features toward novel iGluR modulators.

Differential modulation of NMDA and AMPA receptors by cellular prion protein and copper ions

N-Methyl-D-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are two major types of ionotropic glutamate receptors involved in synaptic transmission. However, excessive activity of these receptors can be cytotoxic and thus their function must be precisely controlled. We have previously reported that NMDA receptor activity is dysregulated following genetic knockout of cellular prion protein (PrP^C), and that PrP^C regulation of NMDA receptors is copper-dependent. Here, we employed electrophysiological methods to study NMDAR and AMPAR currents of cultured hippocampal neurons from PrP^C overexpresser mice. We show that NMDA receptor current amplitude and kinetics are differentially modulated by overexpression of human or mouse PrP^C. By contrast, AMPA receptor activity was unaffected. Nonetheless, AMPA receptor activity was modulated by copper ions in a manner similar to what we previously reported for NMDA receptors. Taken together, our findings reveal that AMPA and NMDA receptors are differentially regulated by PrP^C, but share common modulation by copper ions.

Mechanisms of Channel Block in Calcium-Permeable AMPA Receptors

AMPA receptors mediate fast excitatory neurotransmission and are critical for CNS development and function. Calcium-permeable subsets of AMPA receptors are strongly implicated in acute and chronic neurological disorders. However, despite the clinical importance, the therapeutic landscape for specifically targeting them, and not the calcium-impermeable AMPA receptors, remains largely undeveloped. To address this problem, we used cryo-electron microscopy and electrophysiology to investigate the mechanisms by which small-molecule blockers selectively inhibit ion channel conductance in calcium-permeable AMPA receptors. We determined the structures of calcium-permeable GluA2 AMPA receptor complexes with the auxiliary subunit stargazin bound to channel blockers, including the orb weaver spider toxin AgTx-636, the spider toxin analog NASPM, and the adamantane derivative IEM-1460. Our structures provide insights into the architecture of the blocker binding site and the mechanism of trapping, which are critical for development of small molecules that specifically target calcium-permeable AMPA receptors.

Biological Mechanisms of Cancer-Induced Depression

Patients with cancer are more likely to develop depression than the general population, which negatively impacts their quality of life and prognosis. In order to identify effective antidepressants catered toward cancer patients, the biology of depression in the context of cancer must be well-understood. Many theories have emerged postulating the mechanisms underlying the development of depressive disorder. Here, we review the role inflammation, a hyperactive hypothalamic-pituitary-adrenal (HPA) axis, and glutamate excitotoxicity may play in cancer-induced depression. Hopefully, novel therapeutics targeting these dysregulated pathways may be potent in ameliorating depressive symptoms in the cancer population.

LARGE, an intellectual disability-associated protein, regulates AMPA-type glutamate receptor trafficking and memory

Mutations in the human LARGE gene result in severe intellectual disability and muscular dystrophy. How LARGE mutation leads to intellectual disability, however, is unclear. In our proteomic study, LARGE was found to be a component of the AMPA-type glutamate receptor (AMPA-R) protein complex, a main player for learning and memory in the brain. Here, our functional study of LARGE showed that LARGE at the Golgi apparatus (Golgi) negatively controlled AMPA-R trafficking from the Golgi to the plasma membrane, leading to down-regulated surface and synaptic AMPA-R targeting. In LARGE knockdown mice, long-term potentiation (LTP) was occluded by synaptic AMPA-R overloading, resulting in impaired contextual fear memory. These findings indicate that the fine-tuning of AMPA-R trafficking by LARGE at the Golgi is critical for hippocampus-dependent memory in the brain. Our study thus provides insights into the pathophysiology underlying cognitive deficits in brain disorders associated with intellectual disability.