Celebrating the Birthday of AMPA Receptor Nanodomains: Illuminating the Nanoscale Organization of Excitatory Synapses with 10 Nanocandles.

A decade ago, in 2013, and over the course of 4 summer months, three separate observations were reported that each shed light independently on a new molecular organization that fundamentally reshaped our perception of excitatory synaptic transmission (Fukata et al., 2013; MacGillavry et al., 2013; Nair et al., 2013). This discovery unveiled an intricate arrangement of AMPA-type glutamate receptors and their principal scaffolding protein PSD-95, at synapses. This breakthrough was made possible, thanks to advanced super-resolution imaging techniques. It fundamentally changed our understanding of excitatory synaptic architecture and paved the way for a brand-new area of research. In this Progressions article, the primary investigators of the nanoscale organization of synapses have come together to chronicle the tale of their discovery. We recount the initial inquiry that prompted our research, the preceding studies that inspired our work, the technical obstacles that were encountered, and the breakthroughs that were made in the subsequent decade in the realm of nanoscale synaptic transmission. We review the new discoveries made possible by the democratization of super-resolution imaging techniques in the field of excitatory synaptic physiology and architecture, first by the extension to other glutamate receptors and to presynaptic proteins and then by the notion of trans-synaptic organization. After describing the organizational modifications occurring in various pathologies, we discuss briefly the latest technical developments made possible by super-resolution imaging and emerging concepts in synaptic physiology.

Tritium and deuterium labelling of a kainate receptor antagonist and evaluation as a radioligand.

Kainate receptors play a crucial role in mediating synaptic transmission within the central nervous system. However, the lack of selective pharmacological tool compounds for the GluK3 subunit represents a significant challenge in studying these receptors. Recently presented compound 1 stands out as a potent antagonist of GluK3 receptors, exhibiting nanomolar affinity at GluK3 receptors and strongly inhibiting glutamate-induced currents at homomeric GluK1 and GluK3 receptors in HEK293 cells with Kb values of 65 and 39 nM, respectively. This study presents the synthesis of two potent GluK3-preferring iodine derivatives of compound 1, serving as precursors for radiolabelling. Furthermore, we demonstrate the optimisation of dehalogenation conditions using hydrogen and deuterium, resulting in [2H]-1, and demonstrate the efficient synthesis of the radioligand [3H]-1 with a specific activity of 1.48 TBq/mmol (40.1 Ci/mmol). Radioligand binding studies conducted with [3H]-1 as a radiotracer at GluK1, GluK2, and GluK3 receptors expressed in Sf9 and rat P2 membranes demonstrated its potential applicability for selectively studying native GluK3 receptors in the presence of GluK1 and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-blocking ligands.

5-Nitroisoxazoles in SNAr Reactions: A Novel Chemo- and Regioselective Approach to Isoxazole-Based Bivalent Ligands of AMPA Receptors.

An efficient regioselective approach to novel functionalized bis(isoxazoles) with a variety of aromatic and aliphatic linkers was elaborated, based on the heterocyclization reaction of electrophilic alkenes under the treatment with tetranitromethane-triethylamine complex affording 3-EWG-5-nitroisoxazoles. The subsequent SNAr reactions of 5-nitroisoxazoles with various O,O-, N,N- and S,S-bis(nucleophiles) provide a wide range of bis(isoxazole) derivatives in good isolated yields. Employing an elaborated method, a series of novel bis(3-EWG-isoxazoles) as the promising allosteric modulators of AMPA receptors were designed and synthesized. The effect of the compounds on the kainate-induced currents was studied in the patch clamp experiments, revealing modulator properties for several of them. The best positive modulator potency was found for dimethyl 5,5'-(ethane-1,2-diylbis(sulfanediyl))bis(isoxazole-3-carboxylate), which potentiated the kainate-induced currents in a wide concentration range (10-12-10-6 M) with maximum potentiation of 77% at 10-10 M. The results were rationalized using molecular docking and molecular dynamics simulations of modulator complexes with the dimeric ligand-binding domain of the GluA2 AMPA receptor. The predicted physicochemical, ADMET, and PAINS properties confirmed that the AMPA receptor modulators based on the bis(isoxazole) scaffold may serve as potential lead compounds for the development of neuroprotective drugs.

Differential regulation of tetramerization of the AMPA receptor glutamate-gated ion channel by auxiliary subunits.

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) auxiliary subunits are specialized, nontransient binding partners of AMPARs that modulate AMPAR channel gating properties and pharmacology, as well as their biogenesis and trafficking. The most well-characterized families of auxiliary subunits are transmembrane AMPAR regulatory proteins (TARPs), cornichon homologs (CNIHs), and the more recently discovered GSG1-L. These auxiliary subunits can promote or reduce surface expression of AMPARs (composed of GluA1-4 subunits) in neurons, thereby impacting their functional role in membrane signaling. Here, we show that CNIH-2 enhances the tetramerization of WT and mutant AMPARs, presumably by increasing the overall stability of the tetrameric complex, an effect that is mainly mediated by interactions with the transmembrane domain of the receptor. We also find CNIH-2 and CNIH-3 show receptor subunit-specific actions in this regard with CNIH-2 enhancing both GluA1 and GluA2 tetramerization, whereas CNIH-3 only weakly enhances GluA1 tetramerization. These results are consistent with the proposed role of CNIHs as endoplasmic reticulum cargo transporters for AMPARs. In contrast, TARP γ-2, TARP γ-8, and GSG1-L have no or negligible effect on AMPAR tetramerization. On the other hand, TARP γ-2 can enhance receptor tetramerization but only when directly fused with the receptor at a maximal stoichiometry. Notably, surface expression of functional AMPARs was enhanced by CNIH-2 to a greater extent than TARP γ-2, suggesting that this distinction aids in maturation and membrane expression. These experiments define a functional distinction between CNIHs and other auxiliary subunits in the regulation of AMPAR biogenesis.

Modulation of GluA2-γ5 synaptic complex desensitization, polyamine block and antiepileptic perampanel inhibition by auxiliary subunit cornichon-2.

Synaptic complexes of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) with auxiliary subunits mediate most excitatory neurotransmission and can be targeted to treat neuropsychiatric and neurological disorders, including epilepsy. Here we present cryogenic-electron microscopy structures of rat GluA2 AMPAR complexes with inhibitory mouse γ5 and potentiating human cornichon-2 (CNIH2) auxiliary subunits. CNIH2 appears to destabilize the desensitized state of the complex by reducing the separation of the upper lobes in ligand-binding domain dimers. At the same time, CNIH2 stabilizes binding of polyamine spermidine to the selectivity filter of the closed ion channel. Nevertheless, CNIH2, and to a lesser extent γ5, attenuate polyamine block of the open channel and reduce the potency of the antiepileptic drug perampanel that inhibits the synaptic complex allosterically by binding to sites in the ion channel extracellular collar. These findings illustrate the fine-tuning of synaptic complex structure and function in an auxiliary subunit-dependent manner, which is critical for the study of brain region-specific neurotransmission and design of therapeutics for disease treatment.

New Allosteric Modulators of AMPA Receptors: Synthesis and Study of Their Functional Activity by Radioligand-Receptor Binding Analysis.

The synthetic approaches to three new AMPA receptor modulators-derivatives of 1,11-dimethyl-3,6,9-triazatricyclo[7.3.1.13,11]tetradecane-4,8,12-trione-had been developed and all steps of synthesis were optimized. The structures of the compounds contain tricyclic cage and indane fragments necessary for binding with the target receptor. Their physiological activity was studied by radioligand-receptor binding analysis using [3H]PAM-43 as a reference ligand, which is a highly potent positive allosteric modulator of AMPA receptors. The results of radioligand-binding studies indicated the high potency of two synthesized compounds to bind with the same targets as positive allosteric modulator PAM-43 (at least on AMPA receptors). We suggest that the Glu-dependent specific binding site of [3H]PAM-43 or the receptor containing this site may be one of the targets of the new compounds. We also suggest that enhanced radioligand binding may indicate the existence of synergistic effects of compounds 11b and 11c with respect to PAM-43 binding to the targets. At the same time, these compounds may not compete directly with PAM-43 for its specific binding sites but bind to other specific sites of this biotarget, changing its conformation and thereby causing a synergistic effect of cooperative interaction. It can be expected that the newly synthesized compounds will also have pronounced effects on the glutamatergic system of the mammalian brain.

Desensitization dynamics of the AMPA receptor.

To perform their physiological functions, amino methyl propionic acid receptors (AMPARs) cycle through active, resting, and desensitized states, and dysfunction in AMPAR activity is associated with various neurological disorders. Transitions among AMPAR functional states, however, are largely uncharacterized at atomic resolution and are difficult to examine experimentally. Here, we report long-timescale molecular dynamics simulations of dimerized AMPAR ligand-binding domains (LBDs), whose conformational changes are tightly coupled to changes in AMPAR functional states, in which we observed LBD dimer activation and deactivation upon ligand binding and unbinding at atomic resolution. Importantly, we observed the ligand-bound LBD dimer transition from the active conformation to several other conformations, which may correspond with distinct desensitized conformations. We also identified a linker region whose structural rearrangements heavily affected the transitions to and among these putative desensitized conformations, and confirmed, using electrophysiology experiments, the importance of the linker region in these functional transitions.

Asymmetry and Ion Selectivity Properties of Bacterial Channel NaK Mutants Derived from Ionotropic Glutamate Receptors.

Ionotropic glutamate receptors are ligand-gated cation channels that play essential roles in the excitatory synaptic transmission throughout the central nervous system. A number of open-pore structures of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid (AMPA)-type glutamate receptors became available recently by cryo-electron microscopy (cryo-EM). These structures provide valuable insights into the conformation of the selectivity filter (SF), the part of the ion channel that determines the ion selectivity. Nonetheless, due to the moderate resolution of the cryo-EM structures, detailed information such as ion occupancy of monovalent and divalent cations as well as positioning of the side-chains in the SF is still missing. Here, in an attempt to obtain high-resolution information about glutamate receptor SFs, we incorporated partial SF sequences of the AMPA and kainate receptors into the bacterial tetrameric cation channel NaK, which served as a structural scaffold. We determined a series of X-ray structures of NaK-CDI, NaK-SDI and NaK-SELM mutants at 1.42-2.10 Å resolution, showing distinct ion occupation of different monovalent cations. Molecular dynamics (MD) simulations of NaK-CDI indicated the channel to be conductive to monovalent cations, which agrees well with our electrophysiology recordings. Moreover, previously unobserved structural asymmetry of the SF was revealed by the X-ray structures and MD simulations, implying its importance in ion non-selectivity of tetrameric cation channels.

Novel Nanomolar Allosteric Modulators of AMPA Receptor of Bis(pyrimidine) Series: Synthesis, Biotesting and SAR Analysis.

Positive allosteric modulators (PAMs) of AMPA receptors represent attractive candidates for the development of drugs for the treatment of cognitive and neurodegenerative disorders. Dimeric molecules have been reported to have an especially potent modulating effect, due to the U-shaped form of the AMPA receptor's allosteric binding site. In the present work, novel bis(pyrimidines) were studied as AMPA receptor modulators. A convenient and flexible preparative approach to bis(pyrimidines) containing a hydroquinone linker was elaborated, and a series of derivatives with varied substituents was obtained. The compounds were examined in the patch clamp experiments for their influence on the kainate-induced currents, and 10 of them were found to have potentiating properties. The best potency was found for 2-methyl-4-(4-((2-methyl-5,6,7,8-tetrahydroquinazolin-4-yl)oxy)phenoxy)-6,7,8,9-tetrahydro-5H-cyclohepta[d]pyrimidine, which potentiated the kainate-induced currents by up to 77% in all tested concentrations (10-12-10-6 M). The results were rationalized via the modeling of modulator complexes with the dimeric ligand binding domain of the GluA2 AMPA receptor, using molecular docking and molecular dynamics simulation. The prediction of ADMET, physicochemical, and PAINS properties of the studied bis(pyrimidines) confirmed that PAMs of this type may act as the potential lead compounds for the development of neuroprotective drugs.

Structural Investigation of the Interaction Mechanism between Chlorogenic Acid and AMPA Receptor via In Silico Approaches.

Chlorogenic acid (CGA), an important metabolite in natural plant medicines such as honeysuckle and eucommia, has been shown to have potent antinociceptive effects. Nevertheless, the mechanism by which CGA relieves chronic pain remains unclear. α-amino-3-hydroxy-5-methyl-4-isooxazolpropionic acid receptor (AMPAR) is a major ionotropic glutamate receptor that mediates rapid excitatory synaptic transmission and its glutamate ionotropic receptor AMPA type subunit 1 (GluA1) plays a key role in nociceptive transmission. In this study, we used Western blot, surface plasmon resonance (SPR) assay, and the molecular simulation technologies to investigate the mechanism of interaction between CGA and AMPAR to relieve chronic pain. Our results indicate that the protein expression level of GluA1 showed a dependent decrease as the concentration of CGA increased (0, 50, 100, and 200 µM). The SPR assay demonstrates that CGA can directly bind to GluA1 (KD = 496 µM). Furthermore, CGA forms a stable binding interaction with GluA1, which is validated by molecular dynamics (MD) simulation. The binding free energy between CGA and GluA1 is -39.803 ± 14.772 kJ/mol, where van der Waals interaction and electrostatic interaction are the major contributors to the GluA1-CGA binding, and the key residues are identified (Val-32, Glu-33, Ala-36, Glu-37, Leu-48), which play a crucial role in the binding interaction. This study first reveals the structural basis of the stable interaction between CGA and GluA1 to form a binding complex for the relief of chronic pain. The research provides the structural basis to understand the treatment of chronic pain and is valuable to the design of novel drug molecules in the future.

Expression and Development of TARP γ-4 in Embryonic Zebrafish.

Fast excitatory synaptic transmission in the CNS is mediated by the neurotransmitter glutamate, binding to and activating AMPA receptors (AMPARs). AMPARs are known to interact with auxiliary proteins that modulate their behavior. One such family of proteins is the transmembrane AMPAR-related proteins, known as TARPs. Little is known about the role of TARPs during development or about their function in nonmammalian organisms. Here, we report on the presence of TARP γ-4 in developing zebrafish. We find that zebrafish express 2 forms of TARP γ-4: γ-4a and γ-4b as early as 12 h post-fertilization. Sequence analysis shows that both γ-4a and γ-4b shows great level of variation particularly in the intracellular C-terminal domain compared to rat, mouse, and human γ-4. RT-qPCR showed a gradual increase in the expression of γ-4a throughout the first 5 days of development, whereas γ-4b levels were constant until day 5 when levels increased significantly. Knockdown of TARP γ-4a and γ-4b via either splice-blocking morpholinos or translation-blocking morpholinos resulted in embryos that exhibited deficits in C-start escape responses, showing reduced C-bend angles. Morphant larvae displayed reduced bouts of swimming. Whole-cell patch-clamp recordings of AMPAR-mediated currents from Mauthner cells showed a reduction in the frequency of mEPCs but no change in amplitude or kinetics. Together, these results suggest that γ-4a and γ-4b are required for proper neuronal development.

IN SILICO PROFILING OF THE NEW ALLOSTERIC MODULATOR OF AMPA RECEPTORS.

The aim of this study is to investigate the computer-aided prediction of pharmacological activity and mechanisms of action of 6-[4-methoxy-3-(1H-pyrazol-1-ylmethyl)benzyl]-1,11-dimethyl-3,6,9-triazatricyclo[7.3.1.1]tetradecane-4,8,12-trione (TCT-9). The compound was designed for modulation of ionotropic glutamate AMPA receptors, and its affinity for the receptor has been earlier proven experimentally. A cognitive stimulating effect of TCT-9 has been shown using a model of freezing behavior in mice. The drug candidate TCT-9 is now under the development process: it is intended for the treatment of cognitive impairments in case of brain injury. Following the existing requirements, the present study was carried out in the framework of secondary pharmacodynamic studies to determine possible off-target effects and interaction of the compound with regulatory signaling and metabolic networks/pathways. In silico study of the TCT-9 binding to pharmacologically significant targets and the new AMPA receptor modulator's effects on signaling pathways was carried out by the analysis of structure-activity relationships. Prediction of biological activity spectra was performed using PASS (Prediction of Activity Spectra for Substances), which estimates the probabilities for more than five thousand biological activities. The PharmaExpert program assessed information on the belonging of the targets predicted by the PASS program to the signaling and metabolic pathways. The prediction results are the basis for the experimental verification of the binding of the TCT-9 to the steroid hormone receptor ERR1 and further studies of the drug activity in animal models of diseases.

Amelioration of a neurodevelopmental disorder by carbamazepine in a case having a gain-of-function GRIA3 variant.

GRIA3 at Xq25 encodes glutamate ionotropic receptor AMPA type 3 (GluA3), a subunit of postsynaptic glutamate-gated ion channels mediating neurotransmission. Hemizygous loss-of-function (LOF) variants in GRIA3 cause a neurodevelopmental disorder (NDD) in male individuals. Here, we report a gain-of-function (GOF) variant at GRIA3 in a male patient. We identified a hemizygous de novo missense variant in GRIA3 in a boy with an NDD: c.1844C > T (p.Ala615Val) using whole-exome sequencing. His neurological signs, such as hypertonia and hyperreflexia, were opposite to those in previous cases having LOF GRIA3 variants. His seizures and hypertonia were ameliorated by carbamazepine, inhibiting glutamate release from presynapses. Patch-clamp recordings showed that the human GluA3 mutant (p.Ala615Val) had slower desensitization and deactivation kinetics. A fly line expressing a human GluA3 mutant possessing our variant and the Lurcher variant, which makes ion channels leaky, showed developmental defects, while one expressing a mutant possessing either of them did not. Collectively, these results suggest that p.Ala615Val has GOF effects. GRIA3 GOF variants may cause an NDD phenotype distinctive from that of LOF variants, and drugs suppressing glutamatergic neurotransmission may ameliorate this phenotype. This study should help in refining the clinical management of GRIA3-related NDDs.

Structural basis for positive allosteric modulation of AMPA and kainate receptors.

This paper summarizes the present knowledge on how positive allosteric modulators (PAMs) interact with the ligand-binding domain (LBD) of AMPA and kainate receptors, based on structure determinations. AMPA and kainate receptors belong to the family of ionotropic glutamate receptors that are responsible for mediating the majority of fast excitatory neurotransmission. These receptors have been related to brain disorders, e.g. Alzheimer's disease and attention deficit hyperactivity disorder. PAMs are small molecules that potentiate AMPA and kainate receptor currents by interfering with receptor desensitization. Therefore, PAMs are considered to be of interest for the development of pharmacological tools. Whereas PAMs for AMPA receptors have been known for several years, only recently have PAMs for kainate receptors been reported. Today, >80 structures are available for AMPA receptors with PAMs. These PAMs bind at the interface between two LBD subunits in the vicinity of residue 775, which is important for functional differences between flip and flop isoforms of AMPA receptors. PAMs can be divided into five classes based on their binding mode. The most potent PAM reported to date belongs to class 3, which comprises dimerized PAMs. Three structures of the kainate receptor GluK1 were determined with PAMs belonging to class 2. One PAM enhances kainate receptor currents 5- to 59-fold but shows 100-fold lower potency compared to AMPA receptors. Selective PAMs for kainate receptors will be of great use as pharmacological tools for functional investigations in vivo and might potentially prove useful as drugs in controlling the activity of neuronal networks.

Structure and desensitization of AMPA receptor complexes with type II TARP γ5 and GSG1L.

AMPA receptors (AMPARs) mediate the majority of excitatory neurotransmission. Their surface expression, trafficking, gating, and pharmacology are regulated by auxiliary subunits. Of the two types of TARP auxiliary subunits, type I TARPs assume activating roles, while type II TARPs serve suppressive functions. We present cryo-EM structures of GluA2 AMPAR in complex with type II TARP γ5, which reduces steady-state currents, increases single-channel conductance, and slows recovery from desensitization. Regulation of AMPAR function depends on its ligand-binding domain (LBD) interaction with the γ5 head domain. GluA2-γ5 complex shows maximum stoichiometry of two TARPs per AMPAR tetramer, being different from type I TARPs but reminiscent of the auxiliary subunit GSG1L. Desensitization of both GluA2-GSG1L and GluA2-γ5 complexes is accompanied by rupture of LBD dimer interface, while GluA2-γ5 but not GluA2-GSG1L LBD dimers remain two-fold symmetric. Different structural architectures and desensitization mechanisms of complexes with auxiliary subunits endow AMPARs with broad functional capabilities.

AMPA receptor anchoring at CA1 synapses is determined by N-terminal domain and TARP γ8 interactions.

AMPA receptor (AMPAR) abundance and positioning at excitatory synapses regulates the strength of transmission. Changes in AMPAR localisation can enact synaptic plasticity, allowing long-term information storage, and is therefore tightly controlled. Multiple mechanisms regulating AMPAR synaptic anchoring have been described, but with limited coherence or comparison between reports, our understanding of this process is unclear. Here, combining synaptic recordings from mouse hippocampal slices and super-resolution imaging in dissociated cultures, we compare the contributions of three AMPAR interaction domains controlling transmission at hippocampal CA1 synapses. We show that the AMPAR C-termini play only a modulatory role, whereas the extracellular N-terminal domain (NTD) and PDZ interactions of the auxiliary subunit TARP γ8 are both crucial, and each is sufficient to maintain transmission. Our data support a model in which γ8 accumulates AMPARs at the postsynaptic density, where the NTD further tunes their positioning. This interplay between cytosolic (TARP γ8) and synaptic cleft (NTD) interactions provides versatility to regulate synaptic transmission and plasticity.

Inhibition of AMPA receptors (AMPARs) containing transmembrane AMPAR regulatory protein γ-8 with JNJ-55511118 shows preclinical efficacy in reducing chronic repetitive alcohol self-administration.

BACKGROUND: A prominent therapeutic indication for alcohol use disorder (AUD) is reduction in chronic repetitive alcohol use. Glutamate α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) regulate chronic alcohol self-administration in preclinical models. Recent evidence indicates that the expression and function of AMPARs require the transmembrane AMPAR regulatory protein γ-8 (TARP γ-8). This study evaluated the preclinical efficacy of JNJ-55511118, a novel, selective, high-affinity inhibitor of TARP γ-8-bound AMPARs, in reducing chronic operant alcohol self-administration. METHODS: Separate groups of male and female C57BL/6J mice (n = 8/sex/group) were trained to lever press for sweetened alcohol (9% v/v + sucrose 2% w/v) or sucrose only (2% w/v) in operant conditioning chambers using an FR-4 schedule of reinforcement. After a 40-day baseline, JNJ-55511118 (0, 1, and 10 mg/kg, p.o.) was administered in randomized order 1 h before self-administration sessions. Parameters of operant behavior including response rate, total reinforcers, and head entries in the drinking troughs were computer recorded. RESULTS: During baseline, responding to alcohol, but not sucrose, was greater in female than male mice. In male mice, both doses of JNJ-55511118 decreased multiple parameters of alcohol self-administration but did not reduce behavior-matched sucrose-only self-administration. JNJ-55511118 had no effect on sweetened alcohol or sucrose self-administration in female mice. Subsequent tests of motor function showed that the lowest effective dose of JNJ-55511118 (1 mg/kg) had no effect on open-field activity in male mice. CONCLUSIONS: This study shows for the first time that TARP γ-8-bound AMPARs regulate a behavioral pathology associated with addiction. The preclinical efficacy of JNJ-55511118 in reducing alcohol self-administration in male mice suggests that inhibition of TARP γ-8-bound AMPARs is a novel and highly significant neural target for developing medications to treat AUD and other forms of addiction.

Gating and modulation of a hetero-octameric AMPA glutamate receptor.

AMPA receptors (AMPARs) mediate the majority of excitatory transmission in the brain and enable the synaptic plasticity that underlies learning1. A diverse array of AMPAR signalling complexes are established by receptor auxiliary subunits, which associate with the AMPAR in various combinations to modulate trafficking, gating and synaptic strength2. However, their mechanisms of action are poorly understood. Here we determine cryo-electron microscopy structures of the heteromeric GluA1-GluA2 receptor assembled with both TARP-γ8 and CNIH2, the predominant AMPAR complex in the forebrain, in both resting and active states. Two TARP-γ8 and two CNIH2 subunits insert at distinct sites beneath the ligand-binding domains of the receptor, with site-specific lipids shaping each interaction and affecting the gating regulation of the AMPARs. Activation of the receptor leads to asymmetry between GluA1 and GluA2 along the ion conduction path and an outward expansion of the channel triggers counter-rotations of both auxiliary subunit pairs, promoting the active-state conformation. In addition, both TARP-γ8 and CNIH2 pivot towards the pore exit upon activation, extending their reach for cytoplasmic receptor elements. CNIH2 achieves this through its uniquely extended M2 helix, which has transformed this endoplasmic reticulum-export factor into a powerful AMPAR modulator that is capable of providing hippocampal pyramidal neurons with their integrative synaptic properties.

Hippocampal AMPA receptor assemblies and mechanism of allosteric inhibition.

AMPA-selective glutamate receptors mediate the transduction of signals between the neuronal circuits of the hippocampus1. The trafficking, localization, kinetics and pharmacology of AMPA receptors are tuned by an ensemble of auxiliary protein subunits, which are integral membrane proteins that associate with the receptor to yield bona fide receptor signalling complexes2. Thus far, extensive studies of recombinant AMPA receptor-auxiliary subunit complexes using engineered protein constructs have not been able to faithfully elucidate the molecular architecture of hippocampal AMPA receptor complexes. Here we obtain mouse hippocampal, calcium-impermeable AMPA receptor complexes using immunoaffinity purification and use single-molecule fluorescence and cryo-electron microscopy experiments to elucidate three major AMPA receptor-auxiliary subunit complexes. The GluA1-GluA2, GluA1-GluA2-GluA3 and GluA2-GluA3 receptors are the predominant assemblies, with the auxiliary subunits TARP-γ8 and CNIH2-SynDIG4 non-stochastically positioned at the B'/D' and A'/C' positions, respectively. We further demonstrate how the receptor-TARP-γ8 stoichiometry explains the mechanism of and submaximal inhibition by a clinically relevant, brain-region-specific allosteric inhibitor.

Antibodies Against the NH2-Terminus of the GluA Subunits Affect the AMPA-Evoked Releasing Activity: The Role of Complement.

Antibodies recognizing the amino-terminal domain of receptor subunit proteins modify the receptor efficiency to controlling transmitter release in isolated nerve endings (e.g., synaptosomes) indirectly confirming their presence in these particles but also allowing to speculate on their subunit composition. Western blot analysis and confocal microscopy unveiled the presence of the GluA1, GluA2, GluA3, and GluA4 receptor subunits in cortical synaptosomes. Functional studies confirmed the presence of presynaptic release-regulating AMPA autoreceptors in these terminals, whose activation releases [3H]D-aspartate ([3H]D-Asp, here used as a marker of glutamate) in a NBQX-dependent manner. The AMPA autoreceptors traffic in a constitutive manner, since entrapping synaptosomes with the pep2-SVKI peptide (which interferes with the GluA2-GRIP1/PICK1 interaction) amplified the AMPA-evoked releasing activity, while the inactive pep2-SVKE peptide was devoid of activity. Incubation of synaptosomes with antibodies recognizing the NH2 terminus of the GluA2 and the GluA3 subunits increased, although to a different extent, the GluA2 and 3 densities in synaptosomal membranes, also amplifying the AMPA-evoked glutamate release in a NBQX-dependent fashion. We then analyzed the releasing activity of complement (1:300) from both treated and untreated synaptosomes and found that the complement-induced overflow occurred in a DL-t-BOA-sensitive, NBQX-insensitive fashion. We hypothesized that anti-GluA/GluA complexes in neuronal membranes could trigger the classic pathway of activation of the complement, modifying its releasing activity. Accordingly, the complement-evoked release of [3H]D-Asp from antiGluA2 and anti-GluA3 antibody treated synaptosomes was significantly increased when compared to untreated terminals and facilitation was prevented by omitting the C1q component of the immunocomplex. Antibodies recognizing the NH2 terminus of the GluA1 or the GluA4 subunits failed to affect both the AMPA and the complement-evoked tritium overflow. Our results suggest the presence of GluA2/GluA3-containing release-regulating AMPA autoreceptors in cortical synaptosomes. Incubation of synaptosomes with commercial anti-GluA2 or anti-GluA3 antibodies amplifies the AMPA-evoked exocytosis of glutamate through a complement-independent pathway, involving an excessive insertion of AMPA autoreceptors in plasma membranes but also affects the complement-dependent releasing activity, by promoting the classic pathway of activation of the immunocomplex. Both events could be relevant to the development of autoimmune diseases typified by an overproduction of anti-GluA subunits.